Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. However, most nanowire lasers currently suffer from emitting at multiple frequencies, arising from the longitudinal modes native to simple Fabry-Pérot cavities. In this work, we demonstrate spectral manipulation of lasing modes by axially coupling two nanowire cavities through a nanoscale gap. Lasing operation at a single ultraviolet wavelength at room temperature was achieved in cleaved-coupled GaN nanowires. The agreement between the theory and the experiment presents design principles for rational control over the lasing modes. The highly monochromatic light from the cleaved-coupled nanowire lasers is anticipated to provide ultra-compact photon sources for laser-based remote sensing, high-density optical data storage, and long-distance optical communication.

Theoretically, GaxIn1-xP can reach a direct band gap of up to 2.2 eV at room temperature,[1] but at the corresponding composition there is no suitable substrate for thin-film growth. Axial NW heterostructures, however, can be grown with large lattice mismatch. Here we demonstrate GaInP devices with a bandgap close to the theoretical limit. First, the growth of Au-seeded undoped GaInP NWs was investigated, using TMGa, TMIn and PH3 in a MOVPE system. The composition could be controlled throughout the GaxIn1-xP range by tuning the TMGa and TMIn flows.[2] The radial growth was prevented by in situ use of HCl, as previously demonstrated for InP NWs.[3] The non-capped nanowires emitted room-temperature photoluminescence strongly in the energy range of 1.43 to 2.16 eV, correlated with the band gap expected from the materials composition. Next, we investigated in situ doping of GaInP NWs using DEZn for p-doping and H2S for n-doping.[4] TEM revealed that both dopants affected the crystal structure, similar to InP NWs. Both dopants decreased the Ga fraction. Electrical measurements using NW-FETs demonstrated p- and n-type doping. Finally, we made single NW p-i-n junctions. The devices showed typical diode behavior with an ideality factor of 2.1. Photocurrent spectroscopy indicated a bandgap of around 2.1 eV. In forward bias, the diodes showed room temperature electroluminescence at 2.18 eV (569 nm), in good agreement with photoluminescence and photocurrent spectroscopy measurements. There was also lower-energy emission, presumably due to recombination in the lower-bandgap p- and n- regions. Our results demonstrate that GaInP NWs are promising components for optoelectric devices such as light-emitting diodes and solar cells. [1] J.W. Nicklas, J.W. Wilkins: Accurate ab initio predictions of III-V direct-indirect band gap crossovers. Applied Physics Letters 97, 091902. (2010) [2] D. Jacobsson, J.M. Persson, D. Kriegner, T. Etzelstorfer, J. Wallentin, J.B. Wagner, J. Stangl, L. Samuelson, K. Deppert, M.T. Borgström: Particle-assisted GaxIn1-xP nanowire growth for designed band gap structures. Nanotechnology 23, 245601. (2012) [3] M.T. Borgström, J. Wallentin, J. Trägaring;rdh, P. Ramvall, M. Ek, L.R. Wallenberg, L. Samuelson, K. Deppert: In Situ Etching for Total Control Over Axial and Radial Nanowire Growth. Nano Research 3, 264. (2010) [4] J. Wallentin, L. Barrutia Poncela, A.M. Jansson, K. Mergenthaler, M. Ek, D. Jacobsson, L.R. Wallenberg, K. Deppert, L. Samuelson, D. Hessman, M.T. Borgström: Single GaInP nanowire p-i-n junctions near the direct to indirect bandgap crossover point. Applied Physics Letters In press. (2012)

The optoelectronic properties of one-dimensional CdS nanowires and nanobelts have attracted a great deal of attention during the last few decades owing to its visible range bandgap, excellent optical properties and large surface-to-volume ratio. In this report, the photoconductivity (PC) spectroscopy and photoluminescence (PL) spectroscopy are utilized to investigate the electro-absorption and the confinement effect beyond the quantum confinement regime in the one-dimensional CdS nanostructures. The photoconductive characteristics of the individual CdS nanobelts show that the gain can reach ~ 1000 with response time of ~ 100 mu;s. The electric field dependent PC near the band edge region has been investigated in detail. The Franz-Keldysh effect, Stark effect and exciton ionization have been observed. A large Stark shift up to 48 meV has been achieved in CdS nanowires due to the confinement and surface depletion. The temperature and thickness dependence of PL spectra have been carried out to investigate the confinement effect beyond the quantum confinement regime due to surface depletion. From room temperature to 77 K, the emission energy of free exciton A scales linearly versus 1/ L2 when the thickness L is less than 100 nm, while a deviation occurs for those belts thicker than 100 nm due to the reabsorption effect. The 1/ L2 dependence can be explained by the surface depletion induced quantum confinement, which modifies the confinement potential leading to a quasi-square potential well smaller than the geometric thickness of nanobelts, giving rise to the confinement effect to exciton emission beyond the quantum confinement regime. As the temperature decreases, the decrease of the electrostatic potential drop in the surface depletion region leads to a weaker confinement due to the decrease of carrier concentration. The surface depletion induced quantum confinement is further supported by the redshift of emission spectra after surface passivation. References: [1] Dehui Li; Jun Zhang; Qing Zhang; Qihua Xiong. Electric Field-Dependent Photoconductivity in Cds Nanowires and Nanobelts: Exciton Ionization, Franz-Keldysh and Stark Effects. Nano Lett. DOI: 10.1021/nl300749z. [2] Dehui Li; Jun Zhang; Qihua Xiong. Surface Depletion Induced Quantum Confinement in CdS Nanobelts. ACS Nano. DOI: 10.1021/nn301053r.

We have used photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy along with structural information from high resolution transmission electron microscopy of nanowire cross-sections to illuminate the physics of quantum well tubes formed in GaAs/AlGaAs core/multi-shell nanowires. The nanowires were grown using the Au-catalyst assisted MOCVD method, starting with a 50 nm GaAs core and followed in turn by shells of AlGaAs, GaAs (the quantum well tube), AlGaAs and finally a thin GaAs shell to prevent oxidation. EDX measurements (confirmed by PLE measurements) indicate an Al concentration of ~40% which provides confinement of ~200 meV for electrons and ~100 meV for holes in the GaAs quantum well tube. Two quantum well tube samples with widths of 4 nm and 7 nm were studied. PL from the 7 nm structures displays a peak at 1.59 eV with a lifetime of ~300 ps reflecting a direct transition with modest confinement. PL from the 4 nm structure displays a broad response with multiple narrow peaks centered at 1.71 eV indicating both significant confinement and the presence of localized states due to spatial fluctuations in the quantum well tube width. PLE measurements provide in each case evidence for excited states. A simple theoretical model of a cylindrical quantum well is used to compare to both the Pl and PLE results and allows identification of both direct and indirect transitions. We acknowledge the financial support of the NSF through grants DMR-0806700, 0806572, 1105362, 1105121, and ECCS-1100489, and the Australian Research Council. The Australian National Fabrication Facility is acknowledged for access to the growth facilities used in this research.

Semiconductor nanowires present unique properties, such as a large surface/volume ratio, with possibility of engineering novel axial and core/shell heterostructures, which make them potential candidates for the fabrication of sensors [1], photonic [2] and even phononic-controlled photonic devices, as we have recently demonstrated [3]. Indium arsenide and phosphide nanowires (NWs) were grown by chemical beam epitaxy in the vapor-liquid-solid mode using gold as a catalyst. The NWs were characterized by scanning and transmission electron microscopy (SEM and TEM). The TEM measurements show that the samples are basically grown in the wurtzite phase although they may contain small regions with stacking faults or, in other words, of the zincblende phase. Raman and resonant Raman scattering (RRS) measurements have been performed in single nanowires. From Raman scattering using different light polarization configurations we identified vibration modes of InAs and InP in wurtzite phase. From RRS measurements on the InAs NWs we were able to extract the electronic E1-transition energy in the wurtzite phase [4], which fits very well with our ab-initio electronic band calculations. Using photoluminescence spectroscopy we observed optical emissions in the InAs NWs which are attributed to the gap energy of the wurtzite phase. For InP in wurtzite phase, using photoluminescence excitation (PLE) experiments, we extracted the energy separation between A, B and C valence bands and obtained good agreement with the ab-initio calculation values [5]. Finally, the comparison of the optical and vibration properties between zincblende and wurtzite phase is discussed. [1] J. D. Prades, R. Jimenez-Diaz, F. Hernandez-Ramirez, A. Cirera, A. Romano-Rodriguez, and J. R. Morante, Sensors and Actuators B: Chemical144, 1 (2010). [2] R. Yang, D. Gargas, and P. Yang, Nature Photon. 3, 569 (2009). [3] A. Hernández-Mínguez, M. Möller, S. Breuer, C. Pfüller, C. Somaschini, S. Lazic, O. Brandt, A. García-Cristoacute;bal, M. M. de Lima Jr., A. Cantarero, L. Geelhaar, H. Riechert, and P. V. Santos, Nano Lett. 12, 252 (2012). [4] M. Möller, M. M. de Lima Jr., A. Cantarero, L. C. O. Dacal, J. R. Madureira, T. Chiaramonte, M. A. Cotta, and F. Iikawa, Phys. Rev. B 84, 085318 (2011). [5] E. G. Gadret, G. O. Dias, L. C. O. Dacal, M. M. de Lima Jr., C. V. R. S. Ruffo, F. Iikawa, M. J. S. P. Brasil, T. Chiaramonte, M. A. Cotta, L. H. G. Tizei, D. Ugarte, and A. Cantarero, Phys. Rev. B 82, 125327 (2010).

The assembly of group-III nitride nanowires into optoelectronics offers a promising approach to improve the performance of light-emitting devices. Two dimensional quantum confinement effects, created by coaxial band structure engineering, lead large spectral tunability and high luminescence quantum yields. Sophisticated core/multishell nanowires have already been designed to produce a large variety of size-dependent phenomena for advanced light-emitting diodes. Although theory suggests that the carrier distributions in nanowires exhibit two dimensional confinement under a cross-section of hexagonal geometry, its direct observation has never been addressed. By combining synchrotron excited optical luminescence with simultaneous energy-disperse X-ray spectroscopy using a nanometre-sized hard X-ray beam, here we show experimental evidence for these carrier localization effects. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our hyperspectral imaging method reveals a stronger transition at the hexagon corners, matching theoretical predictions. Based on core-level excitation processes, our experiment opens new avenues for further local structure, and time-resolved studies with both nanometre resolution and optical sensitivity. We anticipate that this methodology will contribute to a greater understanding of the underlying design concepts of photonic nanodevices.

Bulk InSb is an important semiconductor material due to its advantageous intrinsic properties. When compared to other III - V material systems InSb demonstrates the narrowest direct band gap ( asymp; 0.17 eV ), the highest electron mobility ( asymp; 7.7 x 104 cm2V-1s-1 ) and the lowest effective electron mass ( asymp; 0.015me ). InSb is known to have the large Landé g-factor ( asymp; 51 )[1] and strong spin-orbit interaction which makes it an attractive material for spin-based applications. Consequently, InSb nanowires are the most promising III-V material system with potential applications in high speed nanoelectronic devices and quantum information processing. Recently, high-quality InSb nanowires have been used to demonstrate the possible existence of Majorana fermions in solid state. [2] In a recent paper by Plissard et al. the highest electron mobilities measured so far in InSb nanowires have been reported. [3] These values (2.0 x 104 - 3.5 x 104 cm2V-1s-1 ) are still significantly lower than the values measured in the bulk material. In this work, we are investigating the upper limit on electron mobility in InSb nanowires. We are trying to improve the mobility by growing an InSb/GaInSb core-shell heterostructure in order to passivate the surface of the InSb nanowire while confining the carriers in the InSb core. [1]H. A. Nilsson; P. Caroff; C. Thelander; M. Larsson; J. B. Wagner; L. E. Wernersson; L. Samuelson; H. Q. Yu Nano Lett. 2009, 9, 3151 [2] V. Mourik; K. Zuo; S. M. Frolov; S.R. Plissard; E. P. A. M. Bakkers; L. P. Kouwenhoven Science 2012, 336, 1003 [3] S.R. Plissard; D. R. Slapak, M. A. Verheijen; M. Hocevar; G. W. G. Immink; I. van Weperen; S. Nadj-Perge; S. M. Frolov; L. P. Kouwenhoven; E. P. A. M. Bakkers Nano Lett. 2012, 12, 1794

In this work we show strong (up to 500%) photoluminescence (PL) heterogeneity along individual MOCVD grown Indium Phosphide Nanowires (InP NWs) via next generation near field optical microscopy. InP NWs have shown fascinating optical properties and the potential as an ultimate nanomaterial for light harvesting due to a bandgap around 1.4 eV. Trap states are believed to be responsible for many optical phenomena in nanocrystals and wires including surface-state-mediated luminescence modification in InP NWs, but are not well-understood due to optical resolution limitations. Gaining this crucial insight requires both local optical excitation and local luminescence collection well below the diffraction limit, which is provided by or next generation near field probes called “campanile” tip. These unique, background-free, broadband near field enhancing campanile tips enabled us to hyperspectrally map local PL heterogeneity both in terms of intensity and spectral shape with ~40nm resolution, that is not possible to observe with other existing methods. Correlating the local PL with the parallel measured local surface potential measurements via Kelvin Probe Microscopy we shed light on the influence of local trap states on the local excitation energies and exciton recombination rates in InP NWs and considerable local surface potential modifications while optically excited compared to the unexcited state. These results support the notion on the influence of surface states on the optical properties of InP NWs, crucial to understand photo to electron energy transfer processes at the critical length scales in these systems.

The vapor-liquid-solid (VLS) mechanism of semiconductor nanowire (NW) growth provides a means to fabricate one-dimensional structures with control over doping and aspect ratio provided in situ during growth. Developing deep understanding and precise control of the structure and chemical composition of VLS-grown NWs is crucial, as phenomena such as polytype formation¹ or surface doping² can have a dramatic impact on the optoelectronic properties. The ability to control the distribution of dopants within NWs is particularly important for photovoltaic applications, where broadened axial and/or radial doping junctions lower efficiencies. Most models of VLS growth assume a planar interface between the catalyst and NW, with dopant species distributed homogeneously throughout the liquid and within the NW. However, recent in situ TEM observations have shown that growth actually proceeds from a multiply faceted liquid-solid interface, a feature that is general to all VLS-grown NWs³. We report the experimental observation of radially inhomogeneous dopant distributions within semiconducting NWs, which are a direct consequence of the multiply faceted growth interface. Atom probe tomographic analysis reveals enhancements in dopant concentrations of as much as ~100 times near the VLS trijunction in both B-doped Si and P-doped Ge nanowires. Finite element modeling of the doping process established that radially inhomogeneous dopant distribution is a direct consequence of the faceted geometry of the liquid-solid interface, distinct from the previously identified vapor-solid doping². As the same segregation behavior was observed in two distinct semiconductors, with two different dopants, the inhomogeneity is likely general to all NW systems, and should be taken into consideration when interpreting the optoelectronic properties of doped nanowire devices. At the same time, such a distribution is particularly favorable for the formation of ohmic contacts. 1. Lopez, F. J., Givan, U., Connell, J. G. & Lauhon, L. J. ACS Nano5, 8958-8966 (2011). 2. Perea, D. E. et al. Nat. Nanotechnol.4, 315-319 (2009). 3. Wen, C.-Y. et al. Phys. Rev. Lett.107, 025503 (2011).

Semiconductor nanowires (NWs) are an ideal platform for optoelectronic applications with minimal space requirements. Their unique properties are readily exploited to create single NW chemical/biosensors, LEDs, lasers, and photodetectors. These technologies can be used as building blocks to create nanoscale assemblies with an extremely wide range of functionalities. In recent years, significant progress has been made towards a microscopic picture of phenomena that give NWs their unique properties. NW optical properties, particularly absorption of light and its polarization sensitivity to light polarization are well understood. At the same time, processes that occur after photoexcitation and define photoluminescence quantum yields as well as carrier lifetimes are still challenging to understand. In particular, quantum yields on the order of 1% suggest that defects play a key role in dictating the NWs&’ optical and electrical response. This connection between a system&’s optical/electrical properties and defects was established more than 50 years ago. In today&’s nanomaterials, defect investigations are even more relevant due to the large fraction of atoms that reside at the surface of nanostructures given their enhanced surface-to-volume ratios. The advent of heterostructures such as core/shell species and mixed metal/semiconductor hybrid systems means the existence of additional interfaces, possessing large surface areas. In all cases, little is known about the nature of associated defects since limited means exist for probing them. In this study, we monitor individual NW emission intensities in parallel plate capacitor-like structure. CdSe nanowires show reversible emission intensity enhancements of ~14±7% when subjected to electric field strengths ranging from 5 MV/m to 22 MV/m. As a consequence, alternating positive and negative biases yield emission intensity modulation depths of ~15%. These emission sensitivities are rationalized by the field-induced modulation of carrier detrapping rates from NW defect sites responsible for nonradiative relaxation processes. The exclusion of these states from subsequent photophysics leads to observed photoluminescence quantum yield enhancements. We explain the phenomenon by developing a kinetic model to account for field-induced variations of carrier detrapping rates through a nanoscale Poole-Frenkel effect. The observed effect allows direct visualization of trap state behavior in individual CdSe nanowires and represents a first step towards developing new optical techniques that can probe defects in low dimensional materials.

Controlling the optical properties of semiconductors with an engineered surface plasmon nanocavity is important for understanding the underlying physics and designing new nanoscale optoelectronic devices. In this talk we will demonstrate highly enhanced absorption and emission from single CdS-SiO2-Ag core-shell plasmonic nanowires, which are significantly different from simple photonic CdS nanowires. We will demonstrate that by fabricating a complete nanoplasmonic cavity, drastically enhanced absorption and emission occurs in such hybrid systems due to the optical antenna effect, which can be tuned completely by controlling the nanowire size. Likewise, by tuning the plasmonic cavity size to match the whispering gallery mode resonances, an almost complete transition from thermalized excitonic to hot-excitonic emission can be achieved, which reflects exceptionally high radiative rate enhancement. Time-resolved measurements for the plasmonic nanowires showed the excited-state lifetime shortening by a factor of >1000, resulting in sub-picosecond lifetimes. Numerical calculations also confirmed that the electromagnetic field enhancement by the whispering gallery plasmon nanocavity is as high as 40000 in these structures. In addition, we also demonstrate bright visible emission from Si nanowires with large diameters (~100 nm) coupled to plasmonic nanocavity due to hot carrier recombination with a quantum efficiency of ~5%, showing many orders of magnitude enhancement over their photonic counterparts. These observations indicate that the intrinsic optical properties of semiconductors can be engineered by their interaction with nanocavity plasmons and is important for understanding and designing nanoscale optoelectronic devices with novel properties.

Copper-indium disulfide, CuInS₂, CIS, is a very promising material in photovoltaic and photoelectrochemical solar cells. Since the conversion of sunlight into electricity is associated with surface photoreactions, micro- or nanostructured surfaces exposed to incident light may significantly enhance the small-area solar cell efficiency. For this reason, synthesis and examination of CuInS₂ nanorods, nanotubes, or nanowires is of great interest. Nanorods are expected to have better sunlight absorption characteristics than isometric particles, as well as superior charge carrier mobility in hybrid solar cells. Herein we report on a template assisted single source precursor approach for the preparation of highly uniform CuInS₂ (CIS) nanorods (chalcopyrite phase) using molecular ketoacid-oximato complexes of In and Cu and thiourea as a sulphur source. Such oximato-complexes are readily available for a variety of transition-, rare earth- and main group metals, and typically decompose under mild conditions in a controlled manner. Our solution approach is versatile and thus of more general importance for the synthesis of non-oxide functional nanomaterials for such potential application areas as electronics, photovoltaics or photochemical water splitting with semiconductors.

Much attention has been given to the fabrication of zinc oxide (ZnO) nanostructures because of their potential applications in optoelectronic devices. Among various growth techniques, chemical bath deposition (CBD) method permits low growth temperatures suited for the fabrication of the nanostructures on the flexible substrates, such as metal foils and polymers. The aim of this study is to discuss the possibility of the shape-controlled growth of ZnO nanorods (NRs) by the CBD method. The effect of hexamethylenetetramine (HMT) on the formation of the NRs and the fabrication of the ZnO-CuO hetero-nanostructures have also been studied. Zinc acetate dihydrate [(Zn(CH₃COO)₂2H₂O] (denoted hereafter by "ZnAc") and zinc chloride (ZnCl₂) were used as precursors. The pH value of the aqueous solution was adjusted to 10 by adding ammonia water. The Au layer with 20 nm in thickness was used as a seed layer. The temperature of the water bath was kept at 90 °C during the deposition process. Growth time was varied in the range from 15 to 240 min. .The surfaces of the samples grown on the Au seed layers were covered with the vertically aligned NRs. The XRD patternes of these samples were dominated by the (002) diffraction peak of ZnO with the hexagonal phase, suggesting that the NRs grow preferentially in the c-axis direction. The shapes of the NRs grown from the ZnAc solution assisted by HMT exhibited hexagonal prisms. For the NRs grown from the ZnCl₂ solution, the shapes of the NRs changed from cones to hexagonal prisms with the increase in ZnCl₂ concentration. When the ZnCl₂ concentration rises from 0.03 to 0.17 M, the averaged diameter increases from ~200 to ~1000 nm, and the averaged length increases from ~1.5 to ~5.5 mu;m. Photolumienscence (PL) spectra of the ZnO NRs grown from the aqueous solution of ZnCl₂ were dominated by the orange band (OB) emission associated with the interstitial oxgen atoms. Moreover, photoluminescence excitaion (PLE) measurements revealed that the NRs contain the secondary phase Zn(OH)₂ on their surface regions. On the other hand, PL spectra of the NRs grown from the ZnAc solutions assisted by HMT composed of the strong near-band-edge emission and the OB emission. The appearance of the former emission suggests that the surfaces of the NRs have high crystalline quality. It was also confirmed that the OB emission of the NRs grown from the ZnAc solution is effectively excited at the photon energy of the A free exciton. The direct depoition of the CuO layer on the ZnO NRs layer by the CBD method using CuCl as a precursor resulted in the formation of the needle-like ZnO NRs with the flower-like CuO nanostructures on their tips (cauliflower-like nanostructures). After the thermal annealing in air at 500 or 600 °C for 30 min, the CuO layers exhibited p-type conduction. These results provide useful information on the realization of ZnO nanostructure based optoelectronic devices.

Hybridization of organic and inorganic materials is a powerful strategy to realize highly efficient nonlinear optical devices by taking on the advantages of the two systems. Here we report enhanced optical nonlinearity in hybrid nanocomposites of ZnO nanowires, and the 3,4,7,8 naphthalene tetracarboxylic dianhydride (NTCDA). ZnO nanowires were firstly grown on Si substrate by chemical vapor deposition, and removed into ethanol solution by sonication, and finally spin-coated onto Si or quartz substrates. NTCDA with various thicknesses was then deposited on top of ZnO nanowires by thermal evaporation. Steady-state and time-resolved photoluminescence measurements were performed to verify the Forster resonant energy transfer between the two systems. The nonlinear optical absorption coefficients and nonlinear refractive indices were extracted from the open-aperture and closed-aperture Z-scan measurements in two-photon absorption regime. The results show enhancement in both the nonlinear absorption coefficient and nonlinear refractive index in the hybrid composite. This is attributed to the Forster energy transfer occurring from the higher energy vibronic states of NTCDA to ZnO thus providing an exciton funneling mechanism and hence higher nonlinear optical absorption.

Liquid crystal-inorganic semiconductor hybrid photovoltaics (HPVs) present an exciting opportunity to combine the flexibility and processability of organic photovoltaics (OPVs) with the efficiency of inorganic photovoltaics (IPVs). Our recent work has shown that the presence of ZnO nanoparticles improves the alignment of octylcyanobiphenyl (8CB) liquid crystal arrays, improving the mobility of the electron-holes' dissociation pathway. Electron conduction through the inorganic nanoparticles is limited however, due to their discrete nature. To overcome this limitation, we have proposed combining ZnO nanowire arrays with 8CB liquid crystal, requiring reliable and scalable fabrication of these arrays. This is obtained via chemical vapor deposition (CVD) within a horizontal tube furnace. Zn powder (carried by Ar gas) is reacted with O₂ and deposited onto an Au-coated Si substrate via the vapor-liquid-solid (VLS) mechanism. We have characterized and optimized the effect of numerous growth parameters within our system, including total gas flow, relative precursor concentrations, timing of gas release, and geometry of interior components. Growth results are characterized structurally by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD), as well as compositionally by energy dispersive x-ray spectroscopy (EDS). We have been able to obtain uniform ZnO nanowire arrays with relatively monodisperse diameters for an Ar:O₂ flow ratio of 20:1, 700°C furnace temperature and small amount of Zn powder source The nanowires are single crystalline with the wurtzite structure and the (0001) growth direction aligned along the nanowire axis. Our nanowires are poorly aligned, however, and indicate growth via a mechanism other than VLS. Nevertheless, preliminary I-V results show promising photovoltaic behavior. Promoting vertical alignment within these arrays remains a challenge, but achieving this will allow us to characterize the effect of mixing and the performance of HPV devices fabricated from these materials. * Supported in part by NSF under MRSEC DMR 0520471

Phase change material (PCM) nanowires (NWs) can serve as model systems for investigating the scaling characteristics of phase-change random access memory (PRAM) at the nanometer scale. Here, we report the scaling behaviors on the PRAM NW switching, primarily driven by NW diameter dependent contact resistance. Specifically, it was found that the reset current, arising from local amorphization of PCM NWs, decreases with decreasing NW diameter from 250 nm to 20 nm, and is also inversely proportional to the contact resistance of PRAM NW devices. Our observations suggest that the power consumption in the switching of PRAM cells for the unit programmable volume in the sub-lithographic regime can be simultaneously achieved by the cell diameter and related contact resistance.

Nanowire growing using vapor-liquid-solid (VLS) and pulsed laser ablation becomes a widely used technique in spite of the still rather limited knowledge and control possibilities over VLS elementary processes. Particularly for the pulsed growing case, competition between thin film and VLS growing could sometimes be challenging to predict and control. In this work we used a high repetition laser with different pulse trains and powers in an 'eclipse' setup for growing one-dimensional nanostructures. For a relatively wide catalyst size range distribution over the substrate surface, quasi-uniform nanowire morphology could still be selectively grown by longer low energy laser pulse trains. Influences of the pulse duration and repetition rate over VLS elementary processes were indirectly investigated by analysing ablation plume propagation and resulting morphology. Based on the VLS elementary processes, some theoretical considerations on the morphology optimisations are also included.

ZnO nanostructures have been extensively investigated during the last years due to their application in different optoelectronic devices. In particular, doping ZnO nanostructures with rare earth ions is of special interest since the optical properties of the nanostructures can be modified. In this work, Tb doped ZnO micro- and nanowires have been grown by a catalyst free vapor-solid method under constant N₂ flux. Two set of samples were obtained. One was grown from a mixture of pure ZnO and Tb₄ O₇ powders (treatment temperature of 1270^oC). The other set was obtained using a mixture of ZnS and Tb₄ O₇ powders as precursors, in this way the growth temperature is decreased up to 950^oC. In both cases, three different dopant proportions (0.1, 0.5 and 1 at. %) were used in the initial mixture. The samples were then characterized by SEM, EDS and photoluminescence (PL). Waveguiding behavior of the nanostructures was investigated by means of micro-PL. In all samples, a large amount of nanostructures have been grown, however sizes and morphologies depend on the precursor used. The structures grown from ZnO powder are rods and wires with lengths of several microns and diameters ranging between a few microns and hundreds of nanometers. In the case of ZnS precursor, nanowires and nanobelts with lengths from hundreds of microns to several cm are obtained. The diameter of the nanowires is a few hundreds of nm while nanobelts have a width of tens of microns and hundreds of nm of thickness. EDS spectra recorded on the samples show the incorporation of Tb dopants into the structures with a concentration of about 2-3 at. %. The amount of dopant does not depend neither on the content of Tb₄ O₇ in the initial powder nor on the precursor used and it is homogenous through the nanostructures. PL spectra performed on individual nanostructures show narrow emission lines superimposed to the ZnO luminescence bands. The lines are located at 490, 545, 580 and 620 nm and correspond to ⁵D₄ to ⁷F_x intraionic transitions of Tb³⁺. Waveguiding properties of the nanowires were investigated. Micro-PL images show that luminescence is guided along the nanowires and exits through the edges of the structures. PL spectra of the transmitted light demonstrate that both ZnO related bands and Tb³⁺ emission lines are guided. However, the relative intensity of Tb lines is higher than that of ZnO bands as the light travels longer distances inside the nanowire.

Zinc oxide (ZnO) nanostructures are promising for different applications such as optoelectronic, photovoltaic, gas and bio-sensing, photocatalysis [1]. Moreover this versatility can be further increased through the tailoring of material properties by surface functionalization: typically a lower band gap material is coupled with ZnO to extend the absorption threshold into the visible region. That&’s why ZnO nanostructures are widely used in many energy conversion devices. In particular, vertically aligned ZnO nanorods (NRs), and nanoneedles (NN) grown directly on a transparent conducting oxide are employed in a 3D photo-anode for fast and efficient electron collection in DSSC. Self-catalyzed ZnO NRs and NNs have been grown by this group on aluminium-doped ZnO layers by an optimized vapor phase process, without using any metal catalyst or precursor. In this work we will present the morphological and optical properties of the obtained ZnO-NRs. The morphological properties are studied by high resolution scanning electron microscopy, and they show that the NR diameter ranges from 50 nm to 150 nm. It is worth noting that the nanorods shape changes from round to hexagonal when the diameter exceeds the 70 nm. The nanoneedles are hexagonal shaped with diameters up to hundreds of nanometers with the tip ranging in few tens of nanometers. The optical properties are studied by means of cathodoluminescence (CL) spectroscopy and imaging. Room temperature CL spectroscopy reveals two different emissions, one set at 3.26 eV related to the ZnO near-band edge (NBE) and a peak at 2.45 eV, normally attributed to point defects in particular to oxygen vacancies [2]. CL monochromatic maps show that two different behaviours for round and hexagonal shaped NRs. The round shaped nanorod have an homogeneous defect related emissions, meanwhile the hexagonal shaped nanorod shows that the NBE emission is strongly localized at the tip of the nanorod and the defect-related emission is localized on the lateral surfaces. Point-by-point CL spectroscopy analysis of the single round shaped NR shows an increase of defect-related band intensity and a concurrent decrease of the NBE band intensity along the NR. This analysis reveals the presence of a distribution of the oxygen vacancy along the nanorod. The defect distribution induces a quenching of the NBE emission. In the case of the ZnO-NNs the increase of the diameters induces an increase of the NBE emission in the main body of the nanostructure, but the NNs tip show an high intense defect related emission. In conclusion the analyses concerning ZnO-NRs show a size distribution in which it is possible to find a critical diameter that affects the shape of the NR, in addition optical studies reveal that the different shaped NRs have different luminescence distributions. [1] Wang ZL, Song J (2006) Science 312:242-246. [2] Foley M, Ton-That C, Phillips MR, (2008) Appl. Phys. Lett. 93:243104-1-3

Synthesis of epitaxial structures has been of interest during the last few decades for the development of electronic and photonic devices. Here we report the application of van der Waals epitaxy to synthesize nonplanar epitaxial tripod nanocrystals of II-VI compounds (ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe). Using vapor transport setup with (001) muscovite mica as the substrate, large quantity of nanotripods could be produced. The tripods exhibited well-faceted legs with temperature-dependent length. Preferential orientation of the arms with respect to the substrate was noted, which is strongly suggestive of the epitaxy at the tripod centre. The epitaxy is attributed to the van der Waals interactions between the tripod base and the mica surface, instead of to the covalent chemical bonding which would require lattice matching between the epilayer and the substrate. With CdS as a case study, we conclude via Raman spectroscopy and electron microscopy studies that the tripods, which have polytypic structure, followed a seeded growth mechanism. Our results could have widespread immediate implications, including the potential of van der Waals epitaxy to be applicable in producing ordered arrays of more complex nanoarchitectures from various classes of compounds, which may be utilized toward a mass production of ordered nanocrystal array-based devices.

Zinc oxide (ZnO), with a large exciton binding energy of 60 meV and a direct band-gap of 3.37 eV at room temperature, is an attractive material with numerous applications in areas such as solar cells, varistors, LEDs, nanogenerators, chemical and gas sensors and spintronics. In order to fully realize the potential of ZnO for these applications, great efforts have been put into the study of fundamental electronic and optoelectronic properties of p-type ZnO since 1980s. However, the carrier transport properties and conductivities of p-type ZnO are still lacking, especially that of nanostructured ZnO due to the non-applicability of conventional characterization methods. As a contactless characterization method, terahertz time domain spectroscopy (THz-TDS) can give insightful information on the physical properties of semiconductor materials without the need of Kramers-Kronig analysis as the carrier scattering rates of semiconductors lies in the terahertz frequency range. In this work, we have investigated carrier transport properties and conductivity of potassium doped ZnO film and nanowires in the frequency range from 0.1 to 3 THz. K-doped p-type ZnO film and nanorods with different doping concentrations are grown by aqueous solution method by varying the amount of potassium acetate. The measured absorption, dispersion and conductivity are well-fitted by Drude-Smith model. The carrier mobility (0.29~56 cm2V-1s-1) and concentration (1016~1017cm-3) of as-grown and annealed samples obtained from THz-TDS method are consistent with our previous results obtained from Hall measurement, SIMS and XPS data.

Vertically aligned, highly ordered and large area nanorod arrays are essential building blocks for multifunctional devices. Understanding of the growth mechanism is crucial for tailoring the morphologies and therefore the physical properties. A wafer-scale solution growth of ZnO nanorods was demonstrated. From detailed studies on the morphologies and interfaces of ZnO nanorods with transmission electron microscopy (TEM), an atomistic model of step edge barrier induced mounds formation is proposed for the first time, which unifies both the two dimensional nucleation driven wedding cakes growth and the screw dislocation driven spiral growth. The growth mechanism is analysed with continuum theory of dynamic roughening at different growth stages. The nanorods were grown through the holes in the resist formed by nanoimprinting and considerations of non-local effect, namely, shadowing and steering, which drastically change the nanorod morphologies, are discussed. Optical properties of the nanorod arrays subjected to post-growth thermal treatment are presented. Polariton induced lasing behavior is observed under high power optically pumping with the faceted ZnO nanorods acting as Fabry-Perot like waveguide cavities.

In this work, we enhanced the sensitivity by using piezoelectric effect of ZnO nanowires(NWs) for nanosensor devices. By applying the mechanical strain on ZnO NWs, the carriers transported channel will be reduced due to the formation of depletion region by piezo-effect. For UV sensing, the detection of the device is more sensitive because of the piezoelectric property, and also maintains the stability of the output signal. For gas sensing, comparing with non-strained ZnO nanosensor, the piezoelectric ZnO nanosensor also can get better sensitivity for O2 and CO sensing. By using the piezoelectric properties to manipulate the width of carriers transported channel, we not only can have the enhanced sensitivity, but also can maintain the signal output level.

ZnO is envisioned to have promising applications in UV optoelectronics because of their fundamental advantages over GaN [1]. Although ZnO nanowires have received lots of research attention due to their unprecedented properties compared to their bulk couterparts [2], solution-grown ZnO is always suffered from poor near-band edge (NBE) emission and process-dependent performance variations [3]. Annealing has been found to be effective to enhance the NBE emission from solution-grown ZnO [3], but the underlying mechanism is unclear. Recently, we did a detailed study on the photoluminescence (PL) of solution-grown ZnO nanowires with respect to post-growth annealing duration, temperature, and atmosphere, and found a universal PL evolution behavior [4]. After analyzing the Raman and PL spectra of the nanowires, we proposed hydrogen doping and dissociation are responsible to the PL evolution of the ZnO nanowires with annealing temperature. Through optimizing the annealing parameters, the intensity of the UV luminescence of solution-grown ZnO nanowires can be enhanced by two orders of magnitude compared to the as-synthesized sample, which is largely ascribed to increased internal quantum efficiency as a result of hydrogen doping [5]. And the presence of hydrogen donor within ZnO nanowires is directly probed by solid state nuclear magnetic resonance [5]. Furthermore, the direct correlation between the optical properties of an individual ZnO nanowire and its morphology as well as structure is established through cathodoluminescence characterization. References [1] D. C. Look, Mater. Sci. Eng. B 80: 383-387 (2001). [2] D. J. Gargas, H. Gao, H. Wang, P. D. Yang, Nano Lett. 11: 3792-3796 (2011). [3] B. S. Ong, C. S. Li, Y. N. Li, Y. L. Wu, R. Loutfy, J. Am. Chem. Soc. 129: 2750-2751 (2007). [4] X. H. Huang, C. B. Tay, Z. Y. Zhan, C. Zhang, L. X. Zheng, T. Venkatesan, S. J. Chua, CrystEngComm 13: 7032-7036 (2011). [5] X. H. Huang, Z. Y. Zhan, K. P. Pramoda, C. Zhang, L. X. Zheng, S. J. Chua, CrystEngComm DOI: 10.1039/C2CE25518A (2012).

Metal silicide nanowires (NWs) have attracted much research interest due to their interesting electrical, optical and catalytic properties. The formation of metal silicide NWs typically does not conform to the well established VLS growth mechanism and is inherently more challenging than the formation of pure Si NWs due to the multitude of silicide phases which can exist for a given metal. Synthetic methods for the formation of metal silicide NWs to date have included; evaporation and subsequent annealing of a metal layer on a preformed NW backbone, the reaction of Si vapour with a metal substrate (and vice versa) and finally, the co-evaporation of both metal and Si source to a non-reactive substrate. Of these subsets, the reaction of Si vapour is an attractive route due to the well established nature of Si CVD processes and the ready availability of cheap metal foils. However, to date this method has only been exploited for the formation of Ni silicide NWs of various phases (i.e. NiS2, Ni2Si etc.). Herein we report on the formation of copper silicide NWs (of the cubic Cu15Si4 phase) using a high boiling point organic solvent based CVD system within standard glassware. The system involves the reaction of in situ formed Si vapour with a Cu foil substrate and allows the formation of dense arrays of Cu15Si4 NWs with average diameters of approximately 100 nm. The NWs were found to be compositionally homogenous along their length and grow from an underlying Cu15Si4 phase which is formed on the substrate surface as a prerequisite for NW growth. Compositional analysis of the substrates and the NWs using EBSD analysis showed clearly that the NWs originated from much larger crystallites of the same underlying phase. Electrical analysis of the NWs showed low resistivity values for the NWs, highlighting their potential for use as interconnect materials.

Significant progress has been achieved in implementing flexible pixel-array pressure sensors for mimicking the tactile sensing capabilities of human skin. Notably in these demonstrated systems, external mechanical stimulations directly interface with pressure-sensitive media while electronic components like field-effect-transistors (FETs) function as read-out elements for passively detecting pressure-induced characteristic change in the media. Intensive efforts have been devoted to minimize the effect of induced strain on performance of these electronic components. This sensing scheme not only requires complicated system integration but also lacks proficiency in directly interfacing/controlling electronics with mechanical actions. Moreover, electronic components are incorporated into existing flexible systems in planar configuration, with pixel dimension around hundreds of microns to even tens of millimeters, severely limiting the pixel-density and spatial resolving capability. The concept of extending electronics into vertical dimension with wrap-gate presents an attractive approach to achieve high-density assembly of nanomaterials; whereas, it is cumbersome to fabricate the wrap-gate and manage interconnect layout for addressing and controlling individual FET effectively. Meanwhile, the coupling of semiconducting and piezoelectric properties in ZnO NWs have been recently investigated and utilized for implementing novel applications, based on modulation of local contact characteristics as well as charge carrier transport by strain-induced piezoelectric polarization charges at the metal-semiconductor interface, which is the fundamental of piezotronics. Driven by the above challenges and concepts, here we demonstrate by far the largest integration of 3D piezotronic NW transistors circuitry as active unit for integrated pressure-sensor, capable of monitoring profiles of applied small pressures (< 10 kPa) with the highest spatial resolution as well as tactile sensitivity. The piezotronic NW transistors array directly interface with mechanical actions while the changes in electrical characteristics of NW transistors, modulated by strain-induced polarization charges at the semiconductor/metal interface due to piezotronic effect, are simultaneously detected to reflect variations in applied deformation. Capabilities of multi-dimensional and self-adaptive sensing have also been demonstrated, showing the potential for future applications such as artificial/prosthetic skin in smart biomedical treatments. The feasibility and scalability of this platform together with its demonstrated compatibility with state-of-art microfabrication techniques paves routes towards future large-scale integration of nanomaterials for applications in functional micro/nano-systems capable for active/adaptive and self-sufficient operations.

In this work, two different mechanisms of current transportation have been discovered by focusing on ZnO schottky nanodevices, which have 2-D and 1-D contact areas, namely thermionic emission and field emission, respectively. With the same schottky barrier height (around 0.5eV), current density of 1-D device reveals about 2.5 times higher than that of 2-D device, because the current transportation is field emission. From the measurement result, we can clarify and improve the size effect and the sensitivity of ZnO nanosensor, respectively.

Nanodevices based on one-dimensional inorganic nanostructures have become one of the focal points of research in applications ranging from electronics, photonics, energy harvesting and storage, to sensors due to their ultra-small volume, high performance and low energy consumption. Among them, as one kind of significant application, photodetectors are essential elements in high-resolution imaging techniques, light wave communications, and future optical memory and circuits as well. Indium selenide (In2Se3), an n-type III-VI compound semiconductor with narrow direct band gap, has attracted substantial attention as a promising semiconductor material for several different applications such as photodetector, photovoltaic solar cell, optoelectronics, phase-change memory, and ionic battery. Here we report the fabrication of single In2Se3 nanowire (NW) photodetectors from high-quality In2Se3 NWs synthesized by simple thermal evaporation approach and the systematic investigation of the performance characteristics of the NW devices. The single In2Se3 NW photodetectors show high and stable photoresponse over a wide light wavelength (254-800 nm) and temperature range (7-300 K). The device current responsivity (Rlambda;) and external quantum efficiency (EQE) are about 150 A/W and 37000% with light wavelength of 500 nm at 5 V, respectively and much higher than most reported photodetector parameters. The spectral response curve indicates the absorption coefficient of the In2Se3 NWs at certain wavelength dominates the performance of the devices. The good linearity of the photocurrents with the incident irradiation over a wide wavelength range has been obtained, indicating the In2Se3 NW photodetector works under a typical light-dependent resistor mode. The size dependent effect of In2Se3 NW on the photodetector performance has also been investigated. The excellent performance of the In2Se3 NW photodetectors opens up the possibilities of using In2Se3 NWs for next-generation photodetection and photosensing applications.

We report low voltage, low-hysteresis, ambipolar PbSe nanowire (NW) field-effect transistors (FETs) using electrolyte-gating with ionic liquids. The working principle of electrolyte-gating relies on the formation of an electric double layer with very high capacitance (above 10 µF/cm²) at the semiconductor surface due to redistribution of the electrolyte ions when a gate field is applied [1]. Thus, ionic liquids can replace traditional gate dielectrics in FETs and allow for accumulation of very high charge carrier densities at low voltages (below 2 V). In addition, the improved contact between the semiconductor NWs and the gate dielectric in case of a liquid electrolyte enhances charge injection and results in low-hysteresis devices compared to nanowire FETs with traditional dielectrics, such as SiO₂ or Al₂O₃ [2]. High-mobility, ambipolar FETs are of particular interest for the realization of low power complementary circuits. In this work, we investigate ambipolar electrolyte-gated FETs based on single-crystalline, 6-7 nm diameter PbSe nanowires synthesized in a colloidal solution. The NWs are aligned on pre-patterned source-drain gold electrodes by means of slow solvent evaporation or by applying an electric field during drop-casting. The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide ([EMIM][TFSI]) serves as the electrolyte. The produced FETs exhibit current-voltage characteristics with very low hysteresis and high and balanced electron and hole mobilities of 0.4 cm²V^-1s^-1 at gate voltages of less than 1 V. The technique of electrolyte-gating can be applied to various nanoscale semiconductors in order to study their transport physics at high charge carrier concentrations. [1] M. S. Kang, J. Lee, D. J. Norris, and C. D. Frisbie. Nano Lett., 2009, 9, 3848-3852 [2] D. K. Kim, Y. Lai, T. R. Vemulkar, and C. R. Kagan. ACS Nano, 2011, 5, 10074-10083

In this study, we prepared vertically oriented and dense ZnO nanostructures (nanorods and nanoneedles) on both nylon fabric and electrospun nylon nanofibers by a simple, two-step wet chemical route at low temperature. The physicochemical properties of the samples prepared by each production techniques were analyzed. Secondary ZnO nanorods were hydrothermally grown on electrospun nylon nanofibers. The samples were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UV-vis transmission spectroscopy, and AC impedance measurements. Nylon fabric was also used to grow vertically oriented ZnO nanostructures. It was found that the morphology of the resulting ZnO nanostructures strongly depended on the hydrothermal growth conditions. Excellent UV blocking activities were observed for nanorods and nanoneedles on both nylon textiles and electrospun nylon nanofibers in the wavelength region of 280-400 nm. Superhydrophobic behavior was achieved, for both nylon fabric electrospun nylon nanofibers containing ZnO nanorods, upon 1-dodecanethiol treatment. ZnO nanostructures on nylon fabric was found to be durable after one hour stirring in DI water.

Fabrication of metal-oxide nanostructure has been increased continuously for numerous applications in microelectronic electronics, sensors and energy devices. Among various kinds of nanostructures, one dimensional (1D) nanostructure received huge interest due to its large-surface-volume ratio and high electron mobility. While the metal-oxide nanowire can be synthesized by various methods including evaporation, chemical or physical vapor deposition and hydrothermal method, its patterning process to date - photolithography, dielectrophoresis, microfluidics, inkjet printing, imprinting and so on - either presents imperfect results or requires multi-steps for growth, harvesting and placement of nanowires that increase the overall cost and process complexity. In this study, we introduce rapid and one step selective growth of metal-oxide nanowires, ZnO nanowires in particular, using laser induced hydrothermal growth to achieve the growth and the patterning of nanowire simultaneously. The laser is focused at an arbitrary position on the substrate to raise the temperature photothermally and grow the nanowires only at the focused spot. The laser focus is then carefully controlled by translational stage and the entire growth process is observed through CCD. The resultant ZnO nanowires array after a complete growth has a hemispherical shape in the aggregate with an urchin-like semicircular ZnO nanowires array as a whole. Through this process, the growth of ZnO nanowires array is complete within less than 30min where the vertical length of fully grown ZnO nanowires is as long as 10um. The growth rate of ZnO nanowires is therefore as high as 20um/h which is almost one order higher compared to the standard hydrothermal methods using convection oven. This process is further applied to the fabrication of UV sensor by making ZnO nanowires array network on desired metal pattern. The resultant UV sensor showed a clear response to UV exposure under a constant bias voltage. These results indicate that laser induced hydrothermal growth process is a novel process which offers rapid and one step selective growth of ZnO nanowires at very fast rate, low cost and eco-friendly environment for various applications in electronics and sensors.

Cadmium chalcogenide quantum dot-sensitized photoanode has a serious problem in photochemical stability owing to hole-induced anodic corrosion of cadmium chalcogenides. Here, IrO_x nH₂O loading on cadmium chalcogenide quantum dot-sensitized ZnO nanowire photoanode induces an enhanced photocurrent generation and a cathodic shift of the onset potential, as well as substantially improved photochemical stability. IrO_x nH₂O-modified CdSe/CdS/ZnO nanowire photoanode shows 13.9 mA cm^-2 (at 0.6 V) and -0.277 V vs. the reversible hydrogen electrode (RHE), which are the highest photocurrent density and the lowest onset potential attained with ZnO-based electrode, respectively. The incident photon to current conversion efficiency (IPCE) is observed up to 730 nm, with a maximum IPCE of 80% at 480~520 nm. Finally, an average hydrogen evolution rate is 240 mu;mol h^-1 cm^-2 at 0.6 V vs. RHE, with almost 100% of faradaic efficiency.

One-dimensional (1D) semiconductor nanostructures, such as rod, wires, wall, and tube, have attracted considerable interest for their potential applications in sensors, light emitting diode(LED), field-effect transistor (FET), ultraviolet laser diodes(LD), and detector. In particular, Also, ZnO nanostructure also has lots of advantages such as nanoscale dimension, quantum confinement effect, coulomb blockade effect, self assembly, and high surface-to-volume ratio. Various ZnO nanostructures, such as nanowire, rod, belt, brides, nail, and nanocomb, have been produced by various method. In this work, we investigated focused on one and two-dimensional(1-2D) ZnO nanostructure multi-complex, including nanorod and nanowalls ZnO nanostructures. we synthesized a ZnO nanorod/nanowall multi-complex structure on transparent conducting oxide layer at low pressure using thermal CVD system with tree zone types. Mixed ZnO and graphite powder were used as source material and we use Ar gas and oxygen gas as a carrier and reaction gas respectively. An Au thin film of 3nm thickness was using catalyst for the growth of the ZnO nanostructures. By optimizing the growth conditions, we synthesized high-quality ZnO nanorod on a ZnO nanowall with patterning substrate. Multi-complex ZnO nanorod/wall structures and straw dummy were synthesized using a vapor-liquid-solid method. The ZnO nanorod show excellent alignment, crystal quality and optical properties which are independent of the substrates. The diameter and length of nanowall was about 3.2mu;m and 4mu;m, respectively. This nanostructure in crystalline form have hexagonal shapes. We proposed the 4-steps of the ZnO nanostructure growth mechanism via a VLS mechanism at the critical point. We was investigated And using by energy dispersion spectrometry (EDS), we were analyzed the chemical composition of the ZnO nanostructure. The shapes and morphology of ZnO nanostructures were analyzed using field-emission scanning electron microscopy (FESEM) and also was investigated the multi-complex ZnO nanostructures by using HRTEM. In addition, we perfectly growth to the nanorod/wall and straw dummy forming ideal selected-area which are very important many technical applications of ZnO devices. More detailed characteristics and the fabrication steps will be discussed later.

Resistive switching memory devices are promising candidates for emerging memory technologies because they yield outstanding device performances. Storage mechanisms for achieving high-density memory applications have been developed; however, many of them exhibit typical resistive switching behavior from the limited controlling conditions so far. In this study, we introduce photon as an unconventional stimulus for activating resistive switching behaviors. First, we compare the resistive switching behavior in the light and dark conditions to describe how resistive switching memories can benefit from photon. Second, we drive the repeatable switching of material&’s resistance states not by the electrical stimulus but by only the modulation of photon irradiating condition. As ZnO nanorods were employed as a model system to demonstrate photo-stimulated resistive switching in high-surface-area nanomaterials, metal-insulator-metal of FTO/ZnO Nano Rods/Au device structure was fabricated. Our ZnO nanorods resistive switching system exhibits stable, bipolar behaviors with tri-level resistance states dependent upon the electrical and photonic conditions. It exhibits photo-driven surface states strongly affect their photoconductivity and resistance states. Resistive switching attributed to annihilation and generation of oxygen vacancy assisted filament path was intimately involved with the conditions relating to filament dissipation and reappearance caused by chemisorbed oxygen ions on the ZnO NR surfaces under various photo-illumination conditions.

We study on electrical and luminescent characteristics of ZnO nanorods array when an electric field is applied in lateral direction to the axis of ZnO nanorods. ZnO nanorods are grown by chemical vapor deposition after laser ablation of gold on glass substrate. This growth method was developed by us. Two thin film electrodes with thickness of 100 nm are deposited in advance of ZnO nanorod growth by thermal deposition of carbon on the glass substrate as anode and cathode. Shape of anode is rectangular (4 mm x 4 mm) and that of cathode is triangular with 30 degree vertical angle and 3 mm base, and the separation between the side of anode rectangle and the vertex of cathode triangle is 1.5 mm. The length of ZnO nanorods is about 1.5 micron meter and the diameter is from 50 nm to 80 nm. ZnO nanorods grow simultaneously on the carbon electrodes and on glass substrate. And the number density of nanorods on carbon electrodes is generally higher than that on the surface of glass substrate. The number density of ZnO nanorods can be controlled to some extent by laser ablation and CVD conditions. The mean separation between nanorods on the area between electrodes is about 1.5 micron meter, and nanorods are isolated with each other. This is confirmed by SEM images and the measurement of electrical conduction between electrodes. DC electric voltage which is gradually increased from 0 volt is applied between carbon electrodes in vacuum. When the voltage is low, electric current does not flow. And the applied voltage reaches about 2500 V, however, the current of about 10**-6 [A] initiates abruptly to flow and blue green light emission is observed at several points along the cathode edge. And the applied voltage is further increased, the value of current does not increase and is maintained at the almost same value and the number of light emission point increases along the cathode edge. The intensity of emitted light is very high. The color of emitted light is blue when the light emission initiates, then changes soon to green. We consider that the blue light is due to band-to-band transition and green light is due to oxygen vacancy in top region of ZnO nanorods when electrons emitted by electric field from this region. On the other hand, no light emission is observed in the gap area between electrodes and on anode edge. The mechanism of electron conduction and light emission is not clear at present. We are studying on this mechanism and on the role of ZnO nanorods in the gap area between electrodes.

One-dimensional nano-structure materials have garnished great attention in the past decade. Among them, semiconducting zinc oxide (ZnO) is an especially attractive inorganic material, as it shows interesting characteristics such as large exciton binding energy, as well as piezoelectric and optical properties that can potentially be used for optical, electronic, sensing and energy harvesting applications. Despite of the practical importance of the ZnO material and early reports of the phonon properties in such confined nanostructures, current knowledge of vibrational (phonon) properties of ZnO vertical nanowires is rather limited. Understanding the details of phonon spectra of ZnO nanostructures and their evolution with sizes by micro-Raman investigation can help understand the underlying physics and help in the development of ZnO nanostructure based devices. In this work, ZnO vertical nanowires have been synthesized on silicon substrates that were coated with ZnO seeding layer by using single zone quartz tube furnace with chemical vapor deposition technique. Vertical ZnO nanowires grown at different substrate temperatures varying from 1000°C to 800 °C in a single growth run show distinctively different morphology, ranging from sharp-tipped vertical needles at highest temperature to well-developed pillars at intermediate temperatures to almost continuous films at lowest temperature of 800°C. The morphology, chemical composition, crystal structure and orientation of as-synthesized ZnO vertical nanowires were examined by field-emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDAX), and high-resolution transmission electron microscopy (HRTEM). Especially, micro-Raman spectroscopy was employed to study how the morphology of ZnO nanowires, grown under different temperatures, relates to the Raman peak intensity and possible shifts and their corresponding peak symmetry. A number of Raman peaks were identified and compared to bulk Raman spectrum, and their peak intensity variations due to different substrate growth temperature were observed and accounted for. In particular, it was observed that one set of phonon peak shifted significantly from 403 cm-1 for vertical wires grown at 940 °C to 387 cm-1 for wires grown at 1000 °C. The width of this peak is also broader than other peaks from the ZnO wires. This observed phonon softening was attributed to the phonon confinement effect in such small structures.

ZnO nanowires (NWs) have been attracting much interest due to their potential use as building blocks for the fabrication of electronic and optoelectronic devices. ZnO is a II-VI direct wide band gap (3.37 eV) semiconductor with large exciton energy (60 meV). At room temperature ZnO nanowires present luminescence bands in both UV (near band emission) and visible regions. The visible (400-750 nm) and the near-infrared (750 -900 nm) emission spectra are tentatively attributed to deep levels such as oxygen vacancies (VO), oxygen interstitials (OI), zinc antisites (ZnO), oxygen antisites (OZn) or zinc vacancies (VZn) [1]. Nevertheless, the emission in the visible range is still far from full understanding. In this way, ion irradiation/implantation may be useful to tailor electronic and optical properties of ZnO NWs through defect passivation or creation [2]. Depending on the ion species, energy and fluence, different types of damages can be produced or not. In this work, we report on the photoluminescence (PL) of ZnO nanowires subjected to high energy ion irradiation. The nanowires were grown by standard VLS method and irradiated with He ions at 1.2 MeV in several fluences. PL measurements were performed at room and low (down to 12 K) temperatures using a 266nm CW Laser. The results show a relative intensity decrease of whole visible band concern the UV one for higher ion fluencies, indicating an enhanced dynamic annealing effect during irradiation. On the other hand, using low temperature PL it is possible to observe a relative increase of the 1.7 eV emission with the ion fluence. This short-infrared emission starts to rise at 100K (enhanced by the freezeout regime) and it is believed to be a deep donor state directly related to oxygen vacancies (VO). A detailed model describing the effect of high energy ion irradiation on the atomic structure of the ZnO nanowires and its PL is presented. The NWs atomic structure was also analyzed by HRTEM. [1] A. F. Kohan. Phys. Rev. B 61, 15019 (2000). [2] Materials Research Bulletin 44 (2009) 41 - 44.

Organic/inorganic hybrid solar cells based on the integration of conductive polymers into semiconducting nanowire arrays offer opportunities for the development of hybrid devices with increased power conversion efficiencies due to high charge carrier collection, one-dimensional transport pathways and large interfacial area. However, control of the nanowire density and effective infiltration of conductive polymers into nano-sized gaps within semiconducting nanowire arrays have been challenging. Here, an inverted device structure of a hybrid solar cell has been realized utilizing ZnO nanowire arrays hydrothermally grown directly on an ITO electrode and by effective polymer coating. The hydrothermal growth technique enables realization of highly uniform, vertically-aligned ZnO nanowires over large areas on electrode. The photovoltaic device performance was investigated by controlling the size and density of ZnO nanowires, thickness of the conductive polymer consistent with carrier diffusion lengths, and an interfacial layer. The size and density of ZnO nanowires were successfully controlled by the concentration of precursor solution (zinc nitrate hexahydrate, hexamethylenetetramine), growth time, and temperature. Effective infiltration of P3HT was achieved using a vacuum annealing technique which eliminated voids and maximized the interfacial area between ZnO and P3HT. Enhanced P3HT crystallinity was observed through UV-Vis absorption and PL quenching showed effective charge separation in vacuum-annealed samples. ZnO nanowire arrays/P3HT nanofibers hybrid devices with interfacial layer, Dibenzo{[f,fprime;]-4,4prime;,7,7prime;-tetraphenyl}diindeno[1,2,3-cd:1prime;,2prime;,3prime;-lm]perylene (DBP) exhibit increased short circuit current (Jsc) and open circuit voltage (Voc). The interfacial layers provide cascade charge transfer from P3HT nanofibers to ZnO nanowires. This scalable, cost-effective approach for fabricating ZnO nanowire arrays and effective coating of organic electron donor materials over the arrays are promising for applications in hybrid solar cells device fabrication.

Cadmium sulfide nanostructures have potential applications in light emitting and light absorbing devices including lasers, photodectors and solar cells. Understanding the effects of size, shape and surface passivation on carrier dynamics in these materials is critical to improving device performance. In this work, we use broadband ultrafast transient absorption spectroscopy to study carrier dynamics in CdS nanowires and nanosheets grown by physical vapor deposition. The nanowires (diameters 50 - 100nm, lengths up to 20 microns) and nanosheets (width 1 - 5 microns, thickness 10 - 25 nm) were all determined by electron backscattered diffraction to posses wurtzite structures, but the orientation of the crystalline c-axis with respect to the long axis of the nanostructure varied. Nanostructures exhibited strong band edge photoluminescence, with lifetimes shorter than the 2 ns detector resolution, and no measurable emission from defect states at energies below the bandgap. Transient absorption measurements using a supercontinuum (425 nm - 780 nm) probe were performed on isolated nanowires and ensembles of nanowires transferred from the silicon wafer growth substrate to quartz cover slip by sonication in ethanol. The cover slip was laser scribed near measurement sites to allow direct comparison between optical and electron microscopy data. In addition to a bleaching near the bandgap energy, a broad induced absorption between 600 and 720 nm is observed in all samples studied. The rise time of this feature, which can be attributed to population of defect states, is faster than the time resolution (~ 250 fs) of the experiment. The evolution of the induced absorption is well described by a bi-exponential decay function, with a fast (< 10 ps) and a slow (> 10 ps) component. The relative weights and time constants of the bi-exponential decay depend on nanowire width, with smaller diameter nanowire exhibiting faster decays, and on polarization of the excitation pulse. The effects of various surface passivation methods on carrier lifetimes will also be presented. This experimental data ,confirms the critical role of surface mediated processes in the dynamics of excited carriers in semiconductor nanowires.

Real-time early detection of HCl gas detection is one of the emerging issues for fire protection since this gas is produced by burning halogenated polymers such as polyvinyl chloride, incinerating waste, and home electronic appliances. Especially, early detection of this gas may play a great role in preventing big fire caused by an electrical short in home appliances. To achieve this goal, the gas sensors require fast response time, ultra-sensitivity, high selectivity, low power consumption, and low cost. However, the conventional gas sensors are difficult to address these all requirement. To achieve this goal, we present novel HCl gas sensor using porphyrin modified amorphous carbon (a-C)/ZnO core-shell nanowire device. This sensor platform was designed to unify the advantages of both fast ultra-violet (UV) response characteristic of a-C/ZnO core-shell nanowire and optical properties of porphyrin with HCl gas. For this work, a-C/ZnO core-shell nanowire was fabricated by sequential thermal evaporation and plasma enhanced chemical vapor deposition. The a-C/ZnO core-shell nanowire devices can be formed by the conventional electric-field-assisted assembly. Then, the effective functionalization of porphyrin on the completed nanodevice was performed with wet solution methods. Finally, the porphyrin modified a-C/ZnO core-shell nanowire gas sensor showed fast and ultra-sensitive detection of HCl gas up to ppm level under UV illumination, due to the significant changes in the absorption spectrum of porphyrin by the protonation and deprotonation with HCl gas. Furthermore, we investigated the overall gas sensing performance of this nanosensor systematically in terms of low level detection limits, selectivity, response time and humidity dependence. This work opens a new way to meet key requirements such as ultra-fast response time, small size, and low power consumption.

Nowadays Chemist are request to construct more and more complex architectures bearing multifunctionalities enable to respond to external stimuli. The construction of such complex architectures will be addressed through strong interplay in chemical science, as proposed recently with the concept of integrative Chemistry.[1] We synthesized PVA/ZnO-nanorods composite fibers using co-axial flux extrusion.[2] These fibers exhibit higher anisotropic photonic properties, both in absorption and emission, as a result of the collective alignment of the ZnO nanorods along the main axis of the PVA fiber. This photonic anisotropy is triggered by a synergistic interaction between the PVA matrix, stretched above Tg, and cooled down under strain. Compared with non-elongated fibers that present an isotropic emission, composite fibers previously submitted to a tensile stress absorb selectively UV emission when the polarized laser beam is parallel to the main axis of the fiber. In addition, their photolumincescence is also anisotropic, with a waveguide behavior along the main axis of the fiber. Mechanical properties of these composite fibers are also drastically improved, compared with pure PVA fibers: the longitudinal Young modulus of these fibers is increased from 2 to 6 GPa upon ZnO addition, a value similar to those already observed for composite fibers, prepared either with carbon nanotubes, or V2O5 macroscopic fibers. [1] N. Brun, S. Ungureanu, H. Deleuze and R. Backov. Chem. Soc. Rev., 2011, 40, 771 [2] Kinadjian, N., Achard, M.-F., Julián-Loacute;pez, B., Maugey, M., Poulin, P., Prouzet, E. and Backov, R. (2012). Adv. Funct. Mater.. doi: 10.1002/adfm.201200360

Phase change memory (PCM) is one of the most promising non-volatile resistive memory technologies. These devices work based on the resistivity contrast between amorphous (high resistivity) and crystalline (low resistivity) phases of phase change materials. Amorphization and crystallization can be achieved reversibly by suitable electrical pulses. We study the crystallization and amorphization of nano-scale patterned Ge₂Sb₂Te₅ wires in this work. The metastable amorphous, crystalline and liquid Ge₂Sb₂Te₅ resistivities are measured on the device level using nanosecond pulse measurements on a large number of devices with varying dimensions at elevated temperatures (300 K-625 K). Slow (~1-100 K/min) thin film resistivity vs. temperature measurements on phase change materials show transitions between the amorphous and crystalline phases. However, phase change memory devices operate in nanosecond time scales, and these phase change materials do not have enough time to change phase until melting due to crystallization times that are much longer than the typical reset pulse durations; hence phase transitions are not seen at metastable state.

Phase change memory is a non-volatile memory technology that uses a small volume of phase change material that can be rapidly and reversibly changed from amorphous and crystalline phases by using different electrical pulses to define electrically resistive (amorphous) and conductive (crystalline) states. Phase change from crystalline to amorphous occurs when the material is heated to at least melting temperature by a voltage pulse and quenched. Transition from amorphous to crystalline state is achieved by heating the material to just above the crystallization temperature or melting and slowly cooling the material. Crystallization dynamics are key in understanding phase-change memory technology. Crystallization times of nanoscale Ge₂Sb₂Te₅ wires are studied through amorphization of the wires using single voltage pulses ranging from 50-1000 ns followed by crystallization at various chuck temperatures (425 K to 625 K) under high-vacuum. Current through the wire is measured for long periods after the voltage pulse using an offset signal to determine crystallization time. The crystallization time of Ge₂Sb₂Te₅ decreases with increasing temperature, remaining in the order of seconds at high temperatures.

Semiconductor nanowires (NWs) possess intrinsic polarization sensitivities, making them ideal candidates for a multitude of applications. Such applications include single NW polarization sensitive photodiodes and sensors. However, to fully utilize this property, the absorption polarization dependence of individual NWs must be sufficiently understood. Towards this end, excitation polarization anisotropies (ρ_exc) have previously been observed in single NWs, where the photoluminescence is monitored as a function of excitation polarization. Sizable anisotropies have been recorded (ρ_exc~0.9) in single NWs, where ρ_exc=(I^#8741;-I^perp;)/(I^#8741;+I^perp;) and I^#8741; (I^perp;) is the NW&’s emission intensity under parallel (perpendicularly) polarized excitation (relative the NW&’s long axis). However, ρ_exc measurements only indirectly measure the NW&’s absorption polarization dependence and often lack the ability to probe band edge sensitivities. In this work, we directly probe the extinction polarization anisotropy (ρ_ext) of individual CdSe NWs throughout the visible (480-750 nm) using single particle extinction spectroscopy. Here, ρ_ext=(σ_ext^#8741;-σ_ext^perp;)/(σ_ext^#8741;+σ_ext^perp;), where σ_ext^#8741; (σ_ext^perp;) is the NW&’s extinction cross section under parallel (perpendicularly) polarized illumination. Using a sample modulation method, the ~0.1-0.01% change in a transmitted beam&’s intensity due to NW extinction can be measured, giving typical NW cross sections of σ_ext^#8741;~10^-11 and σ_ext^perp;~10^-12 cm²mu;m^-1. In contrast with ρ_exc spectra, observed ρ_ext spectra have distinct wavelength dependencies and spectral features that correlate well with theoretical transition rules derived from a 6-band k.p theory. This is the first time the absorption polarization sensitivities of an individual NW&’s electronic transitions have been resolved at room temperature. The spectral features, along with ρ_ext magnitudes, suggest that the absorption polarization anisotropy of individual CdSe NWs originates from both confinement induced valance band mixing and dielectric contrast effects. This provides insight into the electronic transitions that dominate the optical properties of single CdSe nanowires and further elucidates their absorption polarization sensitivities.

In this work, the schottky and ohmic contacted nanodevices were fabricated by using TiO2 nanowires (NWs). Using schottky contact as a detection mechanism, for UV sensing, the response and reset time were great improved. The response and reset time of the schottky/ohmic contacted nanosensor were 2.7s/40.9s and 15.8s/343.9s, respectively. Compared to the ohmic contacted nanosensor, the schottky contacted nanosensor has great enhanced sensitivity too, the sensitivity of the schottky contacted nanosensors was 3 times better than ohmic contacted nanosensor. Using schottky contacted mechanism, the TiO2 nanowire can have great enhancement for sensing applications.

Ion exchange in nanostructures has proven to be a successful method to transform them into unique compositions and morphologies, which cannot be obtained otherwise. Our group previously studied cation exchange at the nanoscale in Cadmium Sulfide with Zinc in the vapor phase (Bin Zhang et al., Nano Lett. 10, 149 (2010)). It motivated us to further study nanoscale transformation in II-VI semiconductors under anion exchange in vapor phase. Anion exchange is kinetically more difficult to achieve than cation exchange owing to the large size of an anion and the its role in forming the crystal framework. Performing these reactions at elevated temperatures acts as an added stimulus which in itself leads to formation of interesting nanostructures. We performed anion exchange on a variety of II-VI nanowires and nanobelts with Selenium and Sulfur in the vapor phase at different temperatures. It was carried out inside a tube furnace with precise control over the precursor concentration in the reaction zone. The resulting products, a variety of nanostructures, included core-shell, alloyed and completely transformed nanostructures. Photoluminescence (PL) measurements on these structures indicated a shift in band gap as a function of compositional variation. Another interesting morphology was single crystalline, periodically branched nanobelts with a preferential growth orientation. PL experiments showed waveguiding and Fabry-Perot resonance in these belts with periodic scattering from the branches. Besides the chemical composition, often the crystal structure transformation from wurtzite to zinc blend was observed. This was accompanied by formation of defects such as stacking faults that made the products even more interesting. To better understand the mechanism behind formation of these novel structures, real-time structural evolution of nanowires and nanobelts as a function of temperature (without precursor) was studied via in situ electron microscopy. The latter helped us better understand sublimation of these materials along certain crystallographic planes at elevated temperature and extremely low pressure which hinted towards possible mechanisms behind branched nanobelt formation. We are currently performing real-time in-situ selected area electron diffraction experiments while heating the nanostructures (without precursor) to further understand phase transformation in our system. The eventual goal is to complete our understanding of the underlying mechanisms behind all these transformations as a function of the various stimuli such as temperature and anionic precursor so as to make novel structures of desired composition and morphology. Such products can be utilized for a variety of applications ranging from quantum wells to precisely engineered band gap materials. In our talk, we would like to present the above results and explain them with rigorous physical reasoning supplemented with concrete proof gathered from numerous characterization techniques.

We have extracted absorption coefficients of GaN nanowires (0001) grown on Si (111), having density of about 10¹⁰cm^-2, from 25K to 295K, based on their respective reflectivity spectra. To achieve this, a code is written to model the Fabry Perot region of the reflectivity spectrum. Considering the nanowires to be immersed in air, the effective permittivity of the GaN nanowire-air matrix is first calculated as a function of wavelength using Bruggeman effective medium approximation. The obtained values in turn are used to simulate the reflectivity spectrum, and estimate the reflectivities of the top and bottom interface of the nanowires, which returned values, less than 0.3% for the top, and nearly 35% for the bottom interface. These values can be attributed to the small or large difference in permittivity of the nanowire-air composite medium with air or Si, respectively. Thus, light incident on GaN nanowire-air matrix, hardly gets reflected from the top interface but instead, most of it gets transmitted through the nanowires, suffers reflection at the bottom interface and is transmitted back again, helping us to conclude that the reflectivity spectra observed experimentally, can be technically referred to as, transmission. Based on the average heights and filling factors of the nanowires-air dielectric matrix, an estimate for the film factor is also calculated from the model, which is used to project the final absorption coefficients at the exciton bandgap to 65000cm^-1 at 25K and 50000cm^-1 at 295K. These values should be compared to 140000cm^-1 at 25K and 100000cm^-1 at 295K, reported for high crystal quality, strain free bulk GaN films. A possible explanation for the lower absorption coefficients of GaN nanowires involves the hypothesis of a “dead” layer due to band bending at the nanowire surface, not contributing to absorption, and reducing the effective nanowire diameter entering in the absorption coefficient estimates. To sum up: we present here a new method to measure the absorption coefficients in any family of nanowires, provided they are grown on a substrate having considerable difference in permittivity with the nanowire-air matrix. Secondly, the high crystal quality, strain-free GaN nanowires, with about 10¹⁰cm^-2 density, does not show any indication of enhancement in absorption compared to bulk values. This may be due to the relatively high nanowire density in our samples. On-going experiments focus on samples with considerably smaller nanowire densities.

The recent discovery of topological insulators, small gap semiconductors possessing exotic metallic states on their surfaces, initiated a number of studies predicting interesting properties and their potential applications. However, the ideal topological insulator device to realize such novel proposals is not fully developed yet, mainly because of the challenges in materials. For example, in Bismuth Selenide (Bi₂Se₃) - the representative topological insulator with a single Dirac surface band - both bulk carriers and surface degradation often make it difficult to observe the characteristic signature of surface electron transport. In this presentation, we report a synthesis and transport measurements of Bi₂Se₃ core - Se shell heterostructure nanowire. In-situ passivation by Se layer protects the surface of Bi₂Se₃ nanowire and thus enables high mobility electron transport via the topological surface states. In addition, anomalous quantum interferences from coherent surface electrons clearly manifest the signature of topological surface states, proposing the ideal platform to realize “topological electronics.”

It is known that semiconductor nanowires with high refractive index could support Mie resonances. These resonances give rise to scattering and absorption of nanowire structures, making them ideal candidate for compact and efficient light sources, photodetectors and solar cells. In this talk, we would discuss some novel optical responses arise from the excitation of optical resonances in semiconductor nanowires (Si nanowire). We will show for certain geometry, Si nanowire scattering shows strong Fano-like resonance, which is previously mainly observed and studied from metallic nanostructures. And with access to scattering and absorption properties of Si nanostructure separately, more understanding about Fano resonance could be gained compared to metallic nanostructures. We will also show the magnetic nature of some of the optical response from Si nanowires, an expansion from previous focus on the electric nature of these resonances, which could lead to novel applications such as metamaterials with low loss, negative refraction with Si nanostructures.

Nanoscale electronics have attracted much attention, wherein the contact engineering is of paramount importance leading to reliable and high device performance at this scale. In microelectronics, silicide contacts form through solid state reaction in 2-D and 3-D structures, where the first formed silicide phase was predicted by two hypothetic models, the effective heat of formation and the transformation rate competition. Very interestingly, the phase transformation sequence in Si nanostructures such as nanowires have been found to deviate from that of the bulk and the thin film system, for which no consistent explanations are currently available. Our studies focus on systematic investigation and understanding of the silicide formation process at nanoscale, which are prescribed in order to form predictable and reliable contacts for high performance nanodevices. With systematic studies, we rule out the effective heat difference and Si/silicide interfacial energies as the key contributors in the nanowire sizes above 20-30 nm, but confirm that the kinetic competition among all potential silicide phases determines the first phase formation in Si nanowire templates. On top of the understanding, kinetic competition process within the nanowire templates can be modulated to render different silicide phases as the first contact phase.

Scaling down the semiconductor lasers in all three dimensions holds the key to the development of compact, low-threshold, and fast coherent light sources/amplifiers, which are critically important for emerging applications in nanophotonics, integrated optics, and information technology. However, the minimum size of conventional semiconductor lasers utilizing dielectric cavity resonators is ultimately governed by the diffraction limit. Recently, it has been proposed and experimentally demonstrated that the use of plasmonic cavities based on metallodielectric structures can break this limit. But it remains to be seen whether one can indeed overcome the high losses in three-dimensional (3D)-confined, deep-subwavelength plasmonic cavity with the currently available metals and semiconductor gain materials. Here, we report an important breakthrough in the growth of atomically smooth, epitaxial Ag films on Si substrates, which can become the ultimate building materials to fabricate low-loss plasmonic cavities due to their superior plasmonic, structural, and material properties. In particular, by using single, shape-controlled InGaN/GaN core-shell nanorods as the gain media in the full visible range, we are able to realize spaser-enabled metal-oxide-semiconductor (MOS) nanolasers that can be operated with an ultralow continuous-wave threshold power above liquid nitrogen temperature and a nearly 100% polarized lasing mode. In these nanolasers, the record-small cavity and mode volumes are much smaller than the 3D diffraction limit and the feature sizes are comparable with that of the state-of-the-art complementary MOS transistors used in nanoelectronics.

We present our work on the characterization of the light absorption properties of single silicon nanowires (NW) using photocurrent spectroscopy along with the dielectric coating induced absorption enhancement in core shell nanocylinders. First, we start with a short description of the growth methods used to synthesize active NWs for photocurrent generation, with recent results obtained on the use of hydrogen chloride in the CVD VLS growth of doped NWs. This method offers highly straight structures, widened process temperatures allowing in particular very efficient boron incorporation—NA up to 5 1019 cm-3—and inhibited gold diffusion, thereby greatly reducing elemental contamination from the catalyst. In a second part, we present the photocurrent response of a set of different diameter active Si NWs and correlate our results with an analytical treatment of the photon absorption at the nanoscale using the Lorentz Mie theory adapted to the cylindrical geometry. Very good agreement is found between experiment and theory for TE-TM polarization spectra. Absorption resonances are resolved, and can display absorption efficiencies higher than one, making downscaling an efficient tool to increase energy harvesting capabilities. In a last part, we adapt the antireflective coating strategy used in the bulk to improve the coupling of the incoming light to Si NWs. For this, different dielectric films are deposited using PECVD, providing a set of structures coated with a high level of conformability. Based on the new set of spectra we obtain the relative gain curves and compare them with analytical calculations specifically derived for getting the absorption in the core of coaxial nanocylinders only. The latter is shown to be substantially different from the absorption by the overall core-shell structure, especially in the blue-ultra violet region where optical power dissipation in the deposited dielectrics and the “antireflective absorption gain” in the core concurrently contribute to the core shell absorption gain. This approach allows discriminating both components of the absorption spectra which can&’t be done using more conventional calculations based on the knowledge of the scattered and incoming fields, only providing the global absorption in the multilayer.

Electron transport in composites made of metal nanoparticles embedded in dielectric material has attracted substantial interest in both fundamental and applied research. In our previous studies demonstrated that the Au nanoparticles embedded silica nanowires devices with wavelength-dependent and reversible photoresponse behavior associated with surface plasmon resonance (SPR) effect, which can potentially be applied for wavelength selective nanoswitch. However, despite all the growth and properties reported, the origins of the electron transport behavior in the Au-peapodded silica nanowire are still unsolved. Furthermore, the effect of Au nanoparticles on the electronic properties of the nanowire and its accompanying transport mechanism is still not investigated. In this study, we report the enhanced electrical conductivity properties of the single gold-peapodded amorphous silica nanowires synthesized using microwave plasma enhanced chemical vapor deposition. Dark conductivity of the gold-peapodded silica nanowire can be adjusted by controlling the number of metal nanoparticles incorporated. The temperature-dependent conductivity measurement reveals that band tail hopping mechanism dominates the electron transport in the gold-peapodded silica nanowires. The high conductivity in the nano-peapodded nanowire with more embedded gold-nanoparticles can be explained by the higher density of hopping states and shorter hopping distance. The significant enhancement of the conductivity is also demonstrated can improve the chemical sensor performance. The Au embedded amorphous silica nanowire has provided new approach to enhance not only the electron conduction, but also the chemical-sensor response/sensitivity.

Doping transparent conducting elongated micro and nanostructures is an active field of research that enables to control their physical and chemical properties in order to widen the field of potential applications. In this work a catalyst free thermal evaporation method has been used to grow different Zn doped In2O3 and IZO (Indium-Zinc-Oxide) micro- and nanostructures, such as micro-rods, needles and hierarchical arrangements of them. In order to realize the range of potential device applications, as contacts or sensors in nanoscale material where the surface to bulk ratio is much higher than in films, it is crucial to understand the surface electronic properties of the material. Microrods, microneedles and larger hierarchical structures are obtained, with an inhomogeneous incorporation of Zn ranging from few at.% to values that double the In content, which enables to probe with spatial resolution the compositional dependence of several properties of the structures such as electronic surface properties, work function, band gap and luminescence. XPS shows that IZO microstructures are degenerated at room temperature, with carrier concentration of the order of 1020 cm-3 estimated from the plasmon peak of In (3d) core levels. Reverse shift of Zn (3d) peaks relative to In (3d) and O (1s) peaks and VB edge with increasing the [Zn]/[Zn+In] ratio (also known as M ratio), is explained by a lattice expansion in Zn2+cations environment respect to pure ZnO. Surface energy band diagrams for undoped and IZO structures have been obtained. Accumulation of electrons takes place for undoped (100) and (111) surfaces whereas depletion of carriers at the surface is observed in IZO samples. The Fermi level position correlates with the Zn concentration at the surface, which taking into account the surface dependence of the ionization potentials, work functions and band gaps, could lead to tunable material properties for device applications.

Nanoscale semiconductor structures have been used and studied intensively for electronic and optoelectronic applications. Germanium nanowires and nanostructures are of particular interest due to their high carrier mobilities, compatibility with silicon-based integrated circuits, and the favorable bulk Ge band structure for photonic applications. The latter property provides the ability to use such nanostructures in light sources and detectors operating at standard telecommunication wavelengths. Photoluminescence (PL) measurements of Ge nanostructures can provide valuable information about their electronic structure and interface characteristics. However, due to the difficulty of detecting emission from unpassivated Ge nanowires, which results from carrier trapping and nonradiative recombination, there are few reports on PL detected for band-to-band transitions in such nanowires. Among these, coherently-strained Ge core/SiGe shell nanowires exhibiting strong and bulk-like band-edge PL have been reported recently [1]. Also, we have recently observed near-infrared PL from Ge nanowires originating from direct-band-gap recombination. It is known that Ge forms a native oxide of somewhat ill-defined composition when exposed to the air. In addition, thermal oxidation at moderate temperatures will also produce a GeO2 layer on the Ge surface. In this work, we have studied the effect of surface oxide formation on band-edge photoluminence measured from Ge nanowires. Epitaxial nanowires were grown using colloidal gold catalyst particles of 40 nm diameter via the vapor-liquid-solid mechanism on a Si (111) substrate. Both furnace annealing under controlled oxygen partial pressures and ambient air exposure were used to form the oxide layers. We have observed two intense PL peaks very close to the direct-band-gap (1.55um) and indirect-band-gap (1.77um) of bulk crystalline Ge. With increasing oxidation of the wires, the indirect band gap PL becomes more intense. This suggests that oxide formation passivates carrier recombination centers initially present on the nanowire surface. Low temperature PL measurements on the same samples illustrate that peaks are shifted and also their intensities are affected by thermally-induced changes in carrier occupancy of the Γ valley and L valley of Ge. X-ray photoelectron spectroscopy and transmission electron microscopy have also been performed to correlate changes in surface oxide thickness and stoichiometry with the measure photoluminescence. [1] S. Hu, Y. Kawamura, K.C.Y. Huang, Y. Li, A.F. Marshall, K.M. Itoh, M.L. Brongersma, and P.C. McIntyre, “Thermal Stability and Surface Passivation of Ge Nanowires Coated by Epitaxial SiGe Shells,” Nano Lett. 12, 1385minus;91 (2012).

The physical properties of semiconductor nanowires are intimately connected to their crystal structure, which must be controlled with atomic-level precision. Unfortunately, the accessible design space remains limited by a reliance on chemistries that were originally developed for 2-D thin film growth. The 3-D nature of nanowires, and nanoscale materials in general, requires new strategies with which to manipulate growth processes and engineer structure. This talk will provide an overview of our recent efforts to advance semiconductor nanowire complexity and function by expanding the available synthetic “toolkit.” Our experimental approach couples the real-time in-situ spectroscopic interrogation of nanowire chemistry with post-growth structural characterization. We connect nanowire crystal structure with the specific chemical bonds present during synthesis and, in doing so, provide a robust foundation from which to rationally achieve novel structural motifs. The role of hydrogen as the root cause of well-known phenomena in Si nanowire growth will be discussed in detail. In particular, our studies show that transiently adsorbed hydrogen atoms play an important, and previously unrecognized role, in kinking and defect formation. We subsequently leverage this fundamental knowledge to generate new types of superstructures with user-defined periodicity.

Self-assembled one-dimensional “Nanowires” have attracted much attention due to not only the fundamental interests in nanoscale-confided physical properties but also novel nano-device applications. Metal oxide is a promising material to incorporate novel functionality into nanowire-based devices because they exhibit a rich variety of physical properties including ferroelectric, ferromagnetic, high-Tc superconducting, memristive switching, which are hardly attainable to other materials. [1-3] However the feasibility of functional oxide nanowires has been strongly limited due to the absence of fabrication strategy to design various metal oxide nanowires. Here we propose a strategy to fabricate and design various metal oxide nanowires using vapor-liquid-solid (VLS) mechanisms. First, we have compared the experimental trends of existing VLS grown metal oxide nanowires [4-10] with MD simulations [11-12] to extract the intrinsic feature of VLS mechanisms in oxide nanowires. The comparisons have highlighted that the difference between LS interface and VS interface on the critical nucleation size essentially determines the occurrence of VLS nanowire growth for metal oxides. [11-12] Based on this VLS principle of metal oxides, we have succeeded to fabricate novel single crystalline metal oxide nanowires, including MnO, NiO, CoO, Sm2O3, Eu2O3, CaO and SrO, which had been impossible to be formed by existing VLS techniques. The major factor to succeed such fabrications of new oxide nanowire family is the precise control of the competition between VLS and VS growths by tuning the surrounding pressure, flux and temperature. We hope that the presented strategy will open up a new research field of functional nanowires. References: [1] Kawai et al., Nature 349, 200 (1991), [2] Kawai et al., Science 267, 71 (1995), [3] Kawai et al., Science 280, 1064 (1998), [4] Kawai et al., J. Am. Chem. Soc., 130, 5378 (2008), [5] Kawai et al., J. Am. Chem. Soc., 131, 3434 (2009), [6] Kawai et al., Nano Lett., 10, 1359 (2010), [7] Kawai et al., J. Am. Chem. Soc., 132, 6634 (2010), [8] Kawai et al., Nano Lett., 11, 2114 (2011), [9] Kawai et al., J. Am. Chem. Soc., 133, 12482 (2011), [10] Kawai et al., J. Am. Chem. Soc., 134, 134567 (2012) [11] Kawai et al., Phys. Rev. E, 82, 011605 (2010), [12] Kawai et al., Phys. Rev. E, 83, 061606 (2011).

Tandem photovoltaic devices based on lattice matched GaAsP and GaInP conformal coatings on SiGe microwire arrays have the potential for high conversion efficiencies with advantages including (1) inherently efficient light trapping, (2) mechanical flexibility and (3) potential for a low-cost waferless tandem cell design. Si microwire solar cells have been demonstrated to exhibit promising photovoltaic and photoelectrochemical characteristics. Furthermore, the wire arrays offer geometric benefits, orthogonalizing light absorption and carrier collection directions along the short radial and long axial directions respectively, thereby mitigating the deleterious effects of defects in absorber layers, and assisting in defect annihilation or gettering at free surfaces due to the high surface area to volume ratio. SiGe wire arrays have been prepared with Cu-catalyzed vapor-liquid -solid growth on oxide-masked Si(111) substrates in a hot wall reactor with chlorosilane and chlorogermane precursors over a wide range of compositions, as verified by energy dispersive spectroscopy and scanning Auger microscopy. High resolution x-ray diffraction confirms their epitaxial nature, and transmission electron microscopy is used to quantify the lattice mismatch and growth induced defect density and distribution in individual wires along the radial and axial directions. Single wire spectral response and light beam induced current measurements are used to evaluate the quantum efficiency and minority carrier diffusion length. Strained and lattice matched GaP and GaInP conformal coatings are grown with metallorganic chemical vapor deposition on Si and SiGe microwire arrays. A two step growth process is used, whereby a thin nucleation layer is deposited at low temperatures before a high temperature growth step. Selective epitaxy allows control of the contact area at the III-V and IV interface and the geometry of the III-V layer by height variation of an oxide mask. Transmission electron microscopy confirms that careful control of the nucleation parameters is necessary to minimize the density of growth defects, manifest as stacking faults and twins, on the predominantly (110) facets of the wires. A pulsed approach, whereby the group III and V precursors are independently introduced to the growth in succession, similar to atomic layer deposition or migration enhanced epitaxy, greatly reduces the defect density and improves the morphology of the wire coatings. The optoelectronic properties of the wire coatings are evaluated with time-resolved photoluminescence spectroscopy and spectral response. [1] Kelzenberg, et al. Nat. Mater., vol. 9, no. 3, pp. 239-244 (2010). [2] Plass, et al. Adv. Mater., vol. 21, no. 3, pp. 325-328 (2009). [3] Turner-Evans et al. 37th PVSC, 2011 IEEE, pp. 2669-2673 (2011). [4] Putnam, et al. E. Environ. Sci., vol. 3, no. 8, pp. 1037-1041 (2010). [5] Boettecher, et al. Science, vol. 327, no. 5962, pp. 185-187 (2010).

While semiconductor nanowires present exciting new opportunities to modulate electronic and photonic function, the constant diameter of strictly 1-D materials limits their application when axial variations in properties are required. To this end, we demonstrate diameter-modulated nanowire superstructures by systematically modifying sidewall surface chemistry during vapor-liquid-solid (VLS) synthesis. Our approach relies on the addition of “molecular resists” that inhibit radial deposition at user-defined points during growth and opens the door to complex, previously inaccessible morphologies. As a proof of concept, arrays of Ge nanowires were synthesized with GeH₄/H₂ while temporarily varying the partial pressure of tetramethyltin. Temperature (325 - 475 °C) dependent growth studies suggest that Sn-C bond breaking delivers methyl groups to the nanowire sidewall, which block GeH₄ adsorption up to 450 °C. Electron microscopy reveals that nanowires retain their <111> crystal orientation, confirming that tetramethyltin does not adversely impact the growth front (i.e. triple phase line). Compositional analysis indicates no significant bulk alloying, although gradual Sn incorporation into the catalyst tip is observed. The rates of axial vs. radial growth are adjustable by varying catalyst composition and temperature, which enables the diameter expansion to be rationally tuned.

One-dimensional semiconductor nanostructures with tuneable morphologies, dimensions, crystallographic phases, and orientations have gained tremendous attention due to their vast number of applications, including electronics, sensing, energy harvesting, etchellip; Several techniques have been successfully employed for the growth of high quality semiconducting nanowires [1]; however, most of the processes using metal supported methodologies require high temperatures of an entire substrate for an effective formation of single crystalline nanowires. Our studies are based on small heating elements such as micromembranes and microhotplates for a localized growth of high quality semiconducting nanowires. The investigated model systems are SnO2 and Ge. The nanowires are grown via LPCVD techniques employing molecular sources. Key features of such microsystems are extremely fast cooling and heating processes due to their low mass and low power consumption. The selectively heated elements allow a site-specific formation of nanowires in sub-mm2 areas. The growing nanowire bundles bridge the gap between a set of interdigital electrodes located on top of the heated membranes und thus leading to in situ contact formation. This approach allows us to perform heating and measuring operations independently, which is mandatory for thermally supported devices such as metal oxide gas sensors.[2] An additional advantage of our approach is the in situ contact formation replacing cost and time consuming procedures. To the best of our knowledge, this is the first report for the growth of these materials using such a technique. [1] S. Barth, F. Hernandez-Ramirez, J. D. Holmes, A. Romano-Rodriguez. Prog. Mater. Sci. 2010, 55, 563. [2] S. Barth, R. Jimenez-Diaz, J. Sama, J. D. Prades, I. Gracia, J. Santander, C. Cane, A. Romano-Rodriguez. Chem. Commun. 2012, 48, 4734.

One-dimensional III-nitride (AlN, GaN, InN and their alloys) nanorod structures are well known to have great prospects for fundamental studies and novel technological applications. GaN nanorods can be doped as n- or p-type semiconducting materials, which are prerequisite for electrically driven photonic devices,such as light-emitting diodes, lasers, and solar cells. The ability to quantitatively determine the dopant distributions and electrostatic potentials of semiconductor junctions is therefore crucial for understanding the properties and performances of functional nanodevices. Herein, we report an in-situ technique to measure the electrical properties and dopant profiling of single GaN nanorod p-n junctions by using a scanning electron microscope (SEM). The GaN nanorod p-n junctions were grown by plasma-assisted molecular-beam epitaxy (PAMBE). Excellent crystallographic alignments and no presence of dislocations or any extended defects throughout the entire length of nanorods are confirmed by transmission electron microscopy (TEM) analysis. The atomic arrangement of p-n GaN nanorods was investigated by annular bright field scanning transmission electron microscopy (ABF-STEM) under aberration-corrected condition. High-resolution ABF STEM images of single GaN nanorod reveal the ABABAB stacking order of structure, The Ga-N dumbbell pairs can be clearly identified indicating that GaN nanorods have the nitrogen polarity. The electrical measurements were carried out by probing single GaN nanorods in an SEM. Due to perfect crystallographic alignment of GaN nanorod p-n junctions and no detectable reverse-bias leakage current in I-V measurements, the contrasts across the p-n junction can be distinctly observed under reverse biasing conditions. Consequently, We performed in-situ quantitative measurements of single GaN nanorod p-n junctions under different reverse biases revealing that contrast differences result from the electrostatic potential variation during the measurements. We find that increasing reverse bias enhances both junction potential barrier and depletion width across the p-n junction, the depletion widths as well as carrier concentrations of GaN nanorod p-n junctions can be quantitatively determined using our technique.

Silicon (Si) and germanium (Ge) are promising replacements for graphite in lithium ion batteries (LIBs) to enable the significantly higher charge storage capacity needed for higher performance portable electronic devices, electrical vehicle and large-scale energy storage. Si and Ge both have much higher lithium capacity than graphite, yet expand significantly upon lithiation. They also have poor electrical conductivity. The use of Si and Ge nanomaterials helps to circumvent these problems, but new formulations of active material, binder, electrolyte and conductive additives are needed to enable good performance. In thicker-film (>10 micrometer) electrode materials, good results have been obtained with Si powder, but nanowires might provide added benefit in terms of the mechanical integrity needed for extended battery life. Solution-phase synthetic routes can provide the necessary large amounts of nanowires needed for these applications. Thick film anodes of solution-grown Si and Ge nanowire anodes were studied. These require conductive carbon, binder (in most cases) and stable solid/electrolyte interface (SEI). Various binders and electrolytes have been tested and sodium alginate binder and added fluoroethylene carbonate (FEC) provide some of the best cycle stability with high capacity. Excess gold seeds in the nanowires lead to large irreversible capacity loss. Tin-seeded Si nanowires were made and found to provide very good performance with stable capacities of nearly 2,000 mA h g-1. Ge nanowires have lower theoretical capacity than Si; however, Ge nanowires have been found to perform much better at faster cycle rates than Si nanowires. For example, Ge nanowire anode capacities of 900 mA h g-1 at cycle rates of 10C have been measured for thick-film Ge nanowire electrodes. The much faster lithium diffusion in Ge than in Si appears to be very important.

A combinatory approach of Step-and-Flash Imprint Lithography (SFIL) and Metal-Assisted Chemical Etching (MacEtch) was used to generate near perfectly-ordered, high aspect ratio silicon nanowires (SiNWs) on 4" silicon wafers. Optimization of the MacEtch conditions allowed us to obtain uniform etching over the wafer surface. The ordering of SiNWs depends only on the SFIL nanoimprinting template used, thereby enabling arbitary SiNW arrangements not possible with nanosphere lithography to be generated. Very densely packed SiNWs with periodicity finer than that permitted by conventional photolithography can in theory be produced. The height of SiNWs is, in turn, controlled by the etching duration. However, it was found that very high aspect ratio SiNWs tend to be bent during processing. Hexagonal and square SiNW arrays were produced and their optical characteristics were measured by UV-Vis-NIR spectrometry. In summary, this approach provides a route of generating highly-ordered SiNWs on a wafer-level basis suitable for semiconductor device manufacturing.

The growth of alloyed NWs is receiving an increasing attention because of the continuous tunability of their physical properties, which opens the possibility of fabricating a wide range of heterostructured NWs, necessary for the design of advanced devices. SiGe NWs were grown by the Vapour-Liquid-Solid method using a LPCVD reactor, Si2H6 and GeH4 as precursor gases for Si and Ge respectively and Ga-Au metallizations of different compositions as catalysts. The Ge fraction of the NWs, in the 0 - 0.15 range, was controlled by the gas flow ratio and the growth temperature. The NWs were separated from the substrate by sonication and collected in a methanol bath for their analysis. The NWs were structurally characterized by Transmission Electron Microscopy, revealing an overall good crystalline quality, with a cubic diamond structure and no defects. In some of the NWs analyzed, localized regions with wurtzite structure as well as arrays of stacking faults along the main NW axis are observed. A microRaman spectrometer was used for the optical characterization of the NWs. A drop of methanol containing NWs was deposited onto a thick Al film evaporated on top of a Si wafer. The NWs were observed by Scanning Electron Microscopy and located. Raman maps were subsequently obtained in the selected single NWs. The intensity of the Raman signal arising from the individual NWs was found to be high enough to allow their detailed analysis. The Raman maps reveal the presence of NWs zones with ordered polytypes, in particular the wurtzite 2H polytype. The NWs composition homogeneity, the formation of the wurtzite phase, the influence of the composition of the catalyst and the possible causes of the high intensity of the Raman signal are discussed.

The importance of charge injection and extraction through metal contacts has been recognized since the early days of semiconductor devices; the alignment of electronic energy levels at interfaces between the metal and the nanostructure is essential for engineering high performance electronic devices. For example, the presence of a Schottky barrier limits the charge injection and, thus, key metrics in many types of devices. Conventionally, internal photoemission spectroscopy (IPS) is used to measure the barrier height of Schottky contacts [1]. However, the application of IPS has been limited to bulk and thin film semiconductor/dielectrics. As existing models of electrical contacts are often complicated for one-dimensional nanostructures (i.e. nanowires) due to significant deviations in geometries and electrostatics from conventional planar devices [2], new techniques are needed to measure barrier heights at reduced dimensions. We have therefore used scanning photocurrent microscopy (SPCM) to measure the barrier height between metal contacts and a Si nanowire with varying doping levels. Devices were fabricated on nanowires with n+/i junctions. A Schottky contact was formed on the intrinsic portion, whereas an Ohmic contact was formed on the doped portion. Sub-bandgap SPCM induces internal photoemission at the metal-semiconductor junction, enabling separation of the contacts response from that of the semiconducting channel. The scaling of the photocurrent with photon energy is consistent with internal photoemission, enabling extraction of a quantitative barrier height. Furthermore, we investigated the effect of near-surface doping, which is the primary method to overcome Schottky barriers to achieve an Ohmic contact. As expected, near-surface doping induces tunneling by reducing the barrier width, and the effective barrier height is lowered [3]. The measurement of the barrier height of Schottky as well as Ohmic contacts on nanowires is an important step in understanding energy levels at nanoscale interfaces, and it can play a critical role in analysis of nanoscale devices. References [1] J. C. Brewer, R. J. Walters, L. D. Bell, D. B. Farmer, R. G. Gordon, H. A. Atwater, Appl. Phys. Lett.2004, 85, 4133-4135. [2] F. Léonard, A. A. Talin, Nature Nanotechnology2011, 6, 773-783. [3] J. M. Shannon, Appl. Phys. Lett.1974, 24, 369.

Mesoporous silicon nanowires (mp-SiNWs) were exclusively fabricated by metal-assisted chemical etching (MACE) of heavily doped silicon wafers under ambient conditions. The etching mechanism was systematically studied with respect to the HF concentration, etching time, ambient temperature, intrinsic semiconductor properties of silicon wafers (i.e. dopant elements, doping level and crystalline orientation). Optical properties of the mp-SiNW arrays were investigated by measuring total and diffuse reflection and transmission via an integrated sphere over wavelength of 400-800 nm. It was found that compared to the solid SiNWs without pores, the porosification in SiNWs effectively enhances optical scattering from the rough surfaces of the mesoporous silicon skeleton, leading to a deterioration of optical trapping in the NW array. mp-SiNWs emit blue-green and red light under laser excitation, and the photoluminescent mechanism was studied in terms of laser power, and ambient temperature and pressure.

I will first discuss a nanowire formation mechanism that is different from the well-known vapor-liquid-solid (VLS) growth, in which axial screw dislocations provide the self-perpetuating steps to enable 1-dimensional (1D) crystal growth. This mechanism was initially found in hierarchical nanowire structures with helically rotating branches resembling “pine trees”. Dislocations can further result in the spontaneous formation of nanotubes, 2D plates, and other morphologies. Many recently established new examples show that dislocation-driven growth is general to many materials grown in vapor or solution phase. We have used classical crystal growth theory to guide the rational design of dislocation-driven nanowire growth. These discoveries can create a new dimension in the rational design and synthesis of nanomaterials. Furthermore, it will enable the scalable and low-cost synthesis of earth-abundant nanomaterials for large scale renewable energy applications, such as in solar and thermoelectric energy conversion, and nanostructured battery electrodes. This will be illustrated by the growth of 1D nanomaterials of earth-abundant and inexpensive semiconductors, such as hematite (α-Fe2O3), pyrite (FeS2), and cuprous oxide (Cu2O). The photoelectrochemical investigations of these nanomaterials and various doping and nanostructuring strategies using 3D hierarchical nanocomposites or branching nanostructures are investigated to overcome the conflicting requirements by light harvesting and carrier collection.

Searching for renewable and green energy is one of the most urgent challenges to the sustainable development of human civilization owing to the threat of global warming and energy crises. Solar is probably the most abundant clean and renewable energy. Semiconductor nanowires (NWs) have a lot of advantages as candidates for photovoltaic (PV) applications due to their large surface-to-volume ratio, better charge collection and the possibility of enhanced absorption through light trapping; at the other side, nanowires will cause large surface and interface recombination, which could be overcome by surface passivation and epitaxial growth of p-n junctions. Nanowire solar cells are promising candidates for powering nanosystems and flexible electronics. The strain in the nanowires, introduced during growth, device fabrication and/or application, is an important issue for piezoelectric semiconductor (like CdS, ZnO and CdTe) based photovoltaic. In this work, we firstly demonstrated largely enhanced performance of n-CdS/p-Cu2S coaxial NW PV devices using the piezo-phototronics effect when the PV device is subjected to an external strain. Piezo-phototronics effect could control the electron-hole pair generation, transport, separation and/or recombination, thus enhanced the performance of the PV devices by as high as 70%. This effect offers a new concept for improving solar energy conversation efficiency by designing the orientation of the nanowires and the strain to be purposely introduced in the packaging of the solar cells. This study shed light on the enhanced flexible solar cells for applications in self-powered technology, environmental monitoring and even defensive technology.[1] Reference: [1] Pan, Caofeng; Niu, Simiao; Ding, Yong; Dong, Lin; Yu, Ruomeng; Liu, Ying; Zhu, Guang; Wang, Zhong Lin, Enhanced Cu2S/CdS coaxial nanowire solar cells by piezo-phototronic effect, Nano Letters, (2012),DOI: 10.1021/nl3014082

III-V semiconductor nanowire (NW) array solar cells with core-shell type p-n junction have attracted much attention in the last decade for higher efficiency and the cost-effective photovoltaic applications because of low reflectance, more efficient charge collection and less materials are required than planar based one. We have grown precisely controlled GaAs and InP NW array on masked (111) oriented substrates by selective-area metal organic vapor phase epitaxy (SA-MOVPE), and already reported InP core-shell NW array solar cells [1]. On the other hand, GaAs core-shell (CS) NW array solar cells have some problems such as high surface recombination velocity and high contact resistance between GaAs and ITO deposited by RF sputtering. In this study, we report on GaAs NW array solar cells with n-type InGaP passivation thin-layer and thin-film Ti protection layer between ITO and NW to obtain good rectifying characteristic for photovoltaic applications. First, a 20-nm-thick SiO₂ thin-film was deposited on p-type GaAs(111)B substrate by RF sputtering and hexagonal openings were formed by electron beam lithography and wet chemical etching. Growth of GaAs/InGaP core-multishell (CMS) NWs was carried out on this masked substrate in a horizontal MOVPE system working at 0.1 atm. The source materials were TMGa, TBP, TMIn and AsH₃ to grow NW, and DEZn and SiH₄ as p-type and n-type dopant sources, respectively. The average height, the diameter and the pitch of NWs were about 1 µm, 250 nm and 400 nm, respectively. Therefore, the geometric fill factor as the ratio between the area of NW and the area of the unit cell is 0.25. After the growth, the space between NWs was filled with transparent resin (BCB) to obtain an electrical separation layer by spin coating. Next, the top of resin was removed by reactive ion etching. Finally, thin-film Ti layer and transparent indium tin oxide (ITO) film electrode were deposited on NW top by vacuum evaporation method and plasma sputtering method, respectively. GaAs/InGaP CMS NW array solar cells with thin-film Ti layer realized 11.5 mA/cm² of short-circuit current density, 0.50 V of open-circuit voltage, 0.63 of fill factor and 3.64% of conversion efficiency under Air Mass 1.5 Global illumination. The internal quantum efficiency of GaAs/InGaP CMS NW array solar cells at higher energy side was higher than GaAs CS NW array solar cells. That means n-type InGaP thin layer worked as passivation layer to decrease surface recombination. [1] Takashi Fukui et al., AMBIO 41 suppl. 2 (2012) 119.

We detect a systematic redshift of the band edge of GaAs/AlGaAs core-shell nanowires grown on Si(111) substrates, with increasing shell thickness from 0 to 100 nm. The redshift is detected both in emission and absorption experiments, and reaches values up to 14 meV in emission and 10 meV in absorption. It is interpreted as due to a small but finite strain imposed to the GaAs core by the AlGaAs shell material. This interpretation is supported by theoretical calculations of the strain field profile in these core-shell nanowires. A direct consequence of this strain field is the development of a sizable piezoelectric field reaching values of tens of kV/cm. In order to take advantage of this piezoelectric field for enhanced solar cell applications we have re-designed the doping and alloy concentration profile of GaAs/AlGaAs core-shell nanowires. Solar cell devices made of such piezoelectrically-engineered nanowire heterostructures will be discussed.

Recently, nanostructure-based hybrid photovoltaic structures have gained significant interest because of their potential to achieve one-dimensional charge transport pathways and large interfacial areas of well-ordered bulk heterojunction geometry. Graphene, due to its outstanding electrical, mechanical, optical, and chemical properties, can be used as an effective front-window electrode in an inverted device configuration. However, building nanostructures directly on the pristine graphene surface has been challenging due to the stable nature of sp2 hybridized graphene. In this work, we have demonstrated efficient inverted graphene-based hybrid solar cells by using two different photoactive materials - conjugated polymers and PbS quantum dots - and ZnO nanowires as an electron selecting/transporting layer. Highly uniform, well-aligned ZnO nanowire arrays were successfully grown on the graphene surface via hydrothermal method. The growth was enabled by a simple interfacial treatment of graphene that preserves its structural and electrical properties. Efficient graphene cathode-based cells were fabricated with power conversion efficiencies of 4.18% and 0.52% for PbS quantum dot and P3HT polymer structures, respectively. These relatively high power conversion efficiencies are comparable to similar ITO-based devices and can be attributed to the interfacial modification of the graphene electrode which facilitates conformal, smooth wetting of ZnO seed layer and subsequent ordered growth of ZnO nanowires. The advances demonstrated by this work suggest that graphene serves as a viable replacement/alternative for ITO in various photovoltaic device configurations.

Storing energy electrochemically involves electronic and ionic processes and chemical transformation inside and at the interface of materials. The ability to understand, design and test nanostructures and their interfaces afford the great opportunities for controlling these fundamental processes, which can ultimately lead to high performance energy storage devices. Here I will present several exciting examples on designing nanostructures and their interfaces to realize high performance energy storage devices. One example is on designing nanowires and heterostructured nanowires for ultrahigh capacity storage of lithium ions in silicon anodes and sulfur cathodes. The challenges associated with large volume expansion, electron and ion transport, and solid-electrolyte-interphase (SEI) have been addressed. Long cycle life has been demonstrated. Another example is to design open framework structure of nanocrystals, which facility facile insertion of sodium and potassium ions. The high power, high-energy efficiency, ultralong cycle life and low-cost aqueous batteries can be enabled for grid scale stationary storage.

Nanowires are filamentary crystals with a diameter in the order of few to hundred nanometers. In the last few years it has been shown that their particular morphology leads to novel properties and opens new avenues for applications in the area of optoelectronics and energy harvesting. To industry, it is particularly appealing the possibility of integrating defect-free III-V materials on silicon, which his possible in the form of nanowire. For nanowires to become a reality in device applications, the synthesis of nanowires, complex heterostructures and doping should be mastered. Moreover, for a realistic integration in the silicon industry, the use of gold as nucleation seed should be avoided. It has been shown that Molecular Beam Epitaxy (MBE) is able to produce gold-free III-V nanowires with excellent optoelectronic properties [1,2]. We believe this technique offers a model system to understand growth phenomena and to provide materials with high purity and atomically sharp heterostructures. We will present our results in the growth of GaAs nanowires by the gallium-assisted method [3]. We will explain the growth mechanisms and how they can be adapted to mismatched substrates such as silicon [4]. Then we will proceed on how the optical properties of GaAs nanowires can be modified by adding heterostructures, such as quantum wells and quantum dots [5]. The optical properties will be elucidated by confocal photoluminescence spectroscopy. Finally, we will show how these nanowires can be obtained in ordered arrays, opening new perspectives of III-V nanowires in photovoltaics [6,7]. We will present our recent results in this area. [1] A. Fontcuberta i Morral et al. Small 4, 899 (2008) [2] D. Spirkoska et al., Phys. Rev. B 80, 245325 (2009); [3] C. Colombo et al, Phys. Rev. B, 77, 155326 (2008); [4] E. Uccelli et al, Nano Lett. 11, 3827 (2011), E. Russo-Averchi et al, Nanoscale 4, 1486 (2012) [5] E. Uccelli et al, ACS Nano 4, 5985 (2010). [6] C. Colombo et al., Appl. Phys. Lett. 94, 173108 (2009) [7] A. Dalmau-Mallorqui et al, PSSA asap (2012)

III-nitride nanowire-based light-emitting diodes (LEDs) are promising alternatives to thin-film LEDs for solid-state lighting. Facile strain relaxation in nanowires allows epitaxial growth on mismatched substrates, such as silicon. This study investigates general design rules for light extraction in large-scale nanowire-based LEDs on silicon for experimentally observed GaN nanowire polar and non-polar growth directions. We focus on axial and radial heterostructures, with an InGaN quantum well embedded in GaN, along the length and radius of the nanowire, respectively. We demonstrate that m-directional quantum wells (QWs) have higher internal quantum efficiency than c-directional QWs, due to the absence of the quantum confined Stark effect, favoring m- and c-directional nanowires for axial and radial heterostructures, respectively. Next, the extraction efficiency of individual nanowires was investigated using finite difference time domain photonic simulations and excited by point sources. In thicker nanowires, extraction efficiency is dependent on coupling of the excitation source into the existing modes of the nanowire waveguide and their reflection at the extraction and substrate interfaces. Below a cutoff diameter, nanowires no longer act as waveguides and the extraction efficiency depends approximately linearly on the distance of the emission source from the silicon substrate. These results are extended to actual plane sources, representing axial heterostructures. In radial heterostructures whispering gallery modes become significant, especially when considering trends of heterostructures with varying diameters. Finally, square and hexagonal unit cells of photonic crystal arrays of vertically aligned nanowires were considered in both regimes of light extraction: waveguide and non-waveguide nanowires. These arrangements are especially promising for increasing the extraction efficiency of non-waveguide nanowires. We show that both active area and extraction efficiency can be increased in nanowire-based architectures as compared to thin-films limited by total internal reflection. These design rules will guide experimental work in the field toward nanowire-based LEDs with higher efficiencies than current thin-film technologies.

Integration of III/V semiconducting nanowires on silicon opens up new possibilities for the fabrication and design of nanoelectronic, optoelectronic and thermoelectric devices that are not possible by conventional thin film growth methods. Here, we demonstrate a catalyst-free selective area growth technique for the fabrication of axial III/V nanowire homo- and hetero-structures on silicon that is based on a nanowire template. In the current implementation, the template has the shape of a vertical open tube made out of SiOx with a wall thickness of 25 nm and a diameter of 100 - 150nm standing on top of the Si substrate. The hollow tube can be selectively filled from the bottom up by metal organic chemical vapor deposition (MOCVD) with the oxide layers of the tube effectively suppressing deposition of material on its surface. We show a proof of concept by demonstrating the successful filling of the templates to create single crystal InAs/InAs axial homo-structures and InSb/InAs axial hetero-structures with atomically sharp interfaces. We investigated the morphology of the nanowires by scanning electron microscopy (SEM), the structure by high resolution transmission electron microscopy (HRTEM), the chemical composition by energy dispersive x-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Further, we extracted strain maps from the HRTEM images by geometric phase analysis (GPA). Compared to nanowires fabricated using metal-catalyzed or self-catalyzed growth methods this approach has the benefit of greater flexibility of growth materials and process parameters, well defined geometry, and notably the absence of intermixing or contamination effects often accompanying catalyzed nanowire growth.

The monolithic integration of III-V optoelectronic devices operating in the infrared range of the electromagnetic spectrum on the Si CMOS platform has been envisioned as a pathway to fast optical data transfer within a chip. However, the differences in lattice constant and symmetry lead during the growth of planar thin films to the formation of defects that prevent device operation. In contrast, this integration is feasible when the nanowire geometry is employed, where the constraint of the lattice mismatch between the III-V semiconductors and Si can be surmounted. Here, we demonstrate for the first time light emitting diodes based on vertical GaAs/(In,Ga)As core-shell nanowire ensembles, grown epitaxially and catalyst-free on Si(111) substrates. The growth was performed by solid-source molecular beam epitaxy (MBE) in the self-assisted mode. A single 10-nm-thick In_0.15Ga_0.85As quantum well grown in a core/multi-shell configuration constitutes the active region in each nanowire, while free carriers are injected from the p-type substrate through the p-type GaAs:Be nanowire core, and from the n-type GaAs:Si outer shell. All nanowires in each device are contacted in parallel configuration. Various aspects of the doping, the device fabrication, and the emission properties will be presented. Regarding the doping, we had to overcome the challenge that Si, the standard n-type dopant in arsenide MBE systems, is during nanowire growth typically incorporated on both Ga and As lattice sites, thus not giving rise to the desired n-type conductivity. Hence, we investigated the incorporation site of Si and Be atoms as a function of the growth conditions and the growth mode (vapor-liquid-solid or vapor-solid) using Raman spectroscopy. We developed a procedure to control the amphoteric behaviour of Si and incorporate it on Ga sites as donor only by growing the GaAs:Si shell under kinetically limited conditions under excess of As. For Be, its incorporation on Ga sites as acceptor is possible despite the extremely Ga-rich conditions during the vapor-liquid-solid growth of the GaAs nanowire cores. The growth of the In_0.15Ga_0.85As quantum well was performed under conditions that ensured a homogeneous and conformal shell growth around the p-type GaAs core. Finally, light emitting diodes were fabricated by first planarizing the nanowire ensembles with an electrical insulator, and second fabricating contacts on the n-type GaAs outer shell layer and the p-type Si substrate. Current-voltage measurements demonstrated the rectifying behavior of the diodes. The devices were characterized by photo-, cathodo-, and electroluminescence spectroscopy at temperatures between 10 and 300 K. The device characteristics are discussed in terms of the core-shell geometry and the doping profile of these heterostructure nanowires.

Ternary In_1-xGa_xAs nanowires (NWs) hold great promise for a variety of compound semiconductor based device technologies such as nanoelectronics, integrated photonics, as well as photodetection and solar cell applications. To utilize NWs within these technologies, it is essential to optimize the growth of In_1-xGa_xAs NWs comprising high uniformity in terms of composition, structure, and site-selectivity. Here, recent insights into growth, microstructural and optical properties of self-induced, catalyst-free In_1-xGa_xAs NW arrays on Si (111) substrate are reported. Using nanoimprint lithography (NIL) for large-scale pre-patterned SiO₂/Si (111) templates and subsequent high-purity molecular beam epitaxy (MBE), we realized the spontaneous non-catalytic growth of well-alignedIn_1-xGa_xAs NWs was realized with very high-yield (> 90%) and pristine non-tapered morphological homogeneity [1]. Several material and physical properties, as composition and structure, were investigated using energy dispersive X-ray spectroscopy (EDX), (scanning) transmission electron microscopy (STEM), high-resolution X-ray diffraction (HRXRD), photoluminescence (PL) and µ-Raman spectroscopy. Based on careful growth optimization we first demonstrate that for the composition-tuned In_1-xGa_xAs NW arrays the Ga content can be varied over a large band gap region (0.4-1.4 eV), while surprisingly emission (PL) linewidths remain largely unmodified. This remarkable finding points to the superiority of site-selective growth (SAG) on NIL- SiO₂/Si (111) substrates giving very low degree of phase separation, as confirmed by STEM and EDX. In contrast, self-assembled, spatially uncorrelated In_1-xGa_xAs NW arrays show larger compositional inhomonot;gennot;eity across the entire NW array with increased peakwidths in 2theta;minus;omega; HRXRD scans as well as broadened Raman modes [1]. Moreover, for both site-selective and self-assembled In_1-xGa_xAs NW arrays the alloy composition along the length of individual NWs was quite uniform as a result of the catalyst-free growth mode, surpassing well-known limitations of catalyst-driven growth. In addition, the structural properties of the In_1-xGa_xAs NWs were found to be very sensitive to the Ga content within a given set of growth conditions. While at low Ga content (<30%) the NW microstructure is predominated by wurtzite phase (WZ) with distinct segments and stacking defects, higher Ga content (70%) yielded higher degree of stacking disorder along the growth direction (intermixing of WZ and Zincblende (ZB) twins). These findings were confirmed by µ-Raman spectroscopy, which displays specific features such as scaling of phonon mode broadening and disorder-activated longitudinal optical (LO) and acoustical modes (DALA) in dependence of Ga content [2]. [1] S. Hertenberger et al., submitted (2012). [2] S. Morkötter et al., in preparation (2012).

Randomly assembled semiconductor (i.e., ZnO, ZnMgO, SnO2, ITO, ZnS, AlN, etc) nanowires grow on lattice mismatched substrates via vapor transport technique exhibits good interfacial properties (i.e., significant reduction of interface defects and nonradiative recombination) due to the small in size as well as high-crystal-quality of the nanowires. Therefore, the use of nanowires can realize optoelectronic heterostructures with large lattice-mismatch. Furthermore, the possibility to modify the optical properties of semiconductor nanowires (e.g. surface modification of SnO2 and ITO nanowires, which are dipole-forbidden nature - intrinsic band-edge transition is forbidden, can allow intensive emission from the surface defect states) is an advantage to obtain wide bandwidth emission. In addition, the random distribution of nanowires can enhance light extraction from the active region of the light-emitting devices. Hence, the use of semiconductor nanowires has potential to achieve novel high-performance light-emitting diodes. On the other hand, careful design of random structures can also simplify the realization of optical feedback mechanism via the formation of random optical cavities. This implies that it is possible to realize random lasers with closed-loop optical cavities of sub-wavelength dimensions using randomly assembled nanowires. Therefore, randomly assembled semiconductor nanowires can have unique electrical and optical characteristics suitable to achieve high-performance optoelectronic devices over the conventional thin-film technologies. In this presentation, detailed information on the construction of light-emitting diodes and lasers by using randomly assembled semiconductor nanowires will be discussed.

Recently, III-nitride nanorods grown by plasma-assisted molecular-beam epitaxy have been demonstrated to be dislocation-free single crystals, which can be used as strain-free growth templates due to their unique nanoscale geometry. Many interesting device structures are currently under development based on the nanorod geometry. In principle, III-nitride alloys such as In_xGa_1-xN (InGaN) can be tuned continuously with emission colors spanning from ultraviolet to the near-infrared region by adjusting the ratio of indium to gallium content. However, the conventional polar InGaN light-emitting devices are efficient only in low-In-content regime due to the adverse polarization effects. It has been anticipated that nanorod devices can overcome this difficulty, especially for the piezoelectric effects. The commonly observed blueshift in emission wavelength and the reduction of efficiency (efficiency droop) with increasing excitation power density have been proposed to originate from the polarization effects. Here, we study the high crystalline quality InGaN@GaN core-shell nanorods, which have definite wurtzite crystallographic planes, as revealed by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and transmission electron microscopy (TEM) images. The side facets of the hexagonal nanorods are formed by six smooth m-planes. When the nanorods are dispersed on a supporting substrate, we can measure the nonpolar emission characteristics of InGaN@GaN core-shell nanorods from the nonpolar side facets. Depending on the In content in the InGaN core, we can observe intense full-color emissions from single nanorods ranging from 468 nm to 640 nm. The blue, green, and red emission intensities increase monotonically with increasing pumping power densities at a constant slope. No emission efficiency droop can be observed even at pump power density as high as 140 kW/cm². Concurrently, the photoluminescence spectra of the three emission bands do not show any significant spectral shifts. Using these samples, we are able to measure intrinsic optical properties of InGaN alloy without the influence of spontaneous and piezoelectric polarization effects. Furthermore, the complete tunability of InGaN@GaN core-shell nanorod in the full color spectrum is demonstrated for device application requiring highly efficient full color nanoemitters.

Strain engineering has proven to be a powerful strategy to enhance the performance of electronic and optoelectronic devices and promises to play an important role on nanowire (NW) based devices. Such devices can benefit of the favorable scaling between the small cross-section and the force necessary to induce a crystal deformation. The increased yield strength and facile accommodation of strain on such 1D nano-structures offer exciting opportunities to tune the electronic properties. Furthermore, nanowires allow III/V materials like GaAs to be grown with wurtzite structure (WZ), otherwise unstable crystal structure for bulk GaAs. The lack of cubic symmetry has interesting consequences on the energy band structure of such crystals: the valence band degeneracy is lifted by the crystal field, and the conduction band becomes composed of two distinct bands with different symmetries. We have investigated the effect of uniaxial strain on the electronic band structure and phonon energies of GaAs/AlGaAs core/shell WZ nanowires, performing Photoluminescence (PL) and Raman spectroscopy simultaneously while applying uniaxial mechanical stress in both tensile and compressive way. The NWs, grown in the c-axis direction by Au-catalyzed molecular beam epitaxy, were transferred onto flexible stainless steel substrates coated with a transparent polymer. Mechanical clamping of NWs aligned in strain direction was achieved by e-beam lithography and lift-off technique. Oxygen plasma etching was used to remove the polymer underneath the NW segment left uncovered by the metal, thus decoupling it mechanically from the substrate. Tensile and compressive uniaxial stress was applied to the nanowire by bending the underlying substrate, using a three-point bending mechanism. We show that uniaxial stress can be used to tune continuously and reversibly the PL of the WZ nanowires over 160 meV and the valence band splitting over 50 meV, by elongating or compressing the wire along its axis. Performing polarization-dependent Raman and PL measurements, we can univocally identify the distinct optical phonon contributions as well as the valence band symmetry involved in the radiative processes. We will provide models to explain the observed PL and Raman peak shifts, and show how this information can be used to determine the axial strain, infer about the mechanical properties of the nanowire and extract band structure and phonon deformation potentials. Finally, we will compare our results with recent ab-initio calculations that model uniaxial strain effects in a bulk GaAs WZ crystal.

The fabrication of vertically-aligned ZnO electrodes by the low-temperature hydrothermal synthesis method (LT-HM), also known as chemical bath deposition (CBD), has received enormous attention for many years due to the possibility to be carried out at low temperatures, on almost any substrate and relatively large areas (something not achievable with other semiconductor oxides like TiO2). The latter makes the technique highly appealing for the scale up fabrication of semiconductor oxide electrodes useful in many optoelectronic devices. Nevertheless, up to now, 1D-nanostructures of ZnO obtained by this methodology has exhibited serious problems since their application in dye sensitized solar cells or organic solar cells have resulted in low power conversion efficiencies. Thus, the aim of this work is to analyse the properties of the ZnO NRs obtained by two similar hydrothermal synthesis methods: the low-temperature hydrothermal method (LT-HM), also known as the chemical bath deposition, and the similar method applying an pressurized Autoclave reactor (A-HM). Our results demonstrate that, for the same growth time, the vertically-aligned ZnO NRs obtained by the A-HM show NRs that are five times shorter in length, but present 60% enhancement in DSC power conversion efficiency. The latter response is attributed to the effect of temperature and pressure that modify the growth mechanism of the NRs. As a result, we can obtain reproducible, highly homogeneous and thin ZnO NR electrodes with good packing density and higher dye loading capacity. Analyses by photoluminescence (PL) and time-resolved photoluminescence (TRPL) reveal a decrease on the surface defects of the vertically-aligned ZnO NRs synthesized by the A-HM. Reducing surface defects, such as oxygen vacancies (Ovac), leads to lower electron traps and less recombination centres improving DSC performance. Thus, higher dye loading capacity, higher FF and higher Voc is obtained due to the reduced surface defects in the A-HM. These results could explain some of the reasons behind the limited power conversion efficiency, not higher than 2.5%, reported up to date for DSC when vertically-aligned ZnO NRs are obtained by the LT-HM is applied.

Heterostructures of GaSb and InAs represent an exotic case of semiconductor interfaces for which the bottom of the conduction band in one material (InAs) is below the top of the valence band in the other (GaSb).¹ This broken gap leads to very efficient band-to-band transport, with high tunnel diode current densities. With recent focus on developing low-power devices, and so-called tunnel-field-effect transistors (T-FETs), there has been a strong, renewed interest in this particular junction. We have investigated junctions of GaSb-InAs(Sb) realized within nanowires,² for which we observe pronounced Esaki tunnel diode behavior. Boosting of the band-to-band tunnel current through both p- and n-doping of either side of the junction was studied. Measured current densities at room temperature represent state-of-the-art for tunnel diodes, with reverse current density of 3.5 MA/cm² (-0.50 V), peak current density of 0.4 MA/cm² (0.16 V), and a peak-to-valley ratio of 4. Zn-doping (p) of the InAs(Sb)-side of the junction was observed to shift the onset of band-to-band tunneling to positive gate voltages, thus providing means to having threshold voltage control in future T-FET concepts based on this material system. (1) H. Sakaki, L.L. Chang, R. Ludeke, C.-A. Chang, G. Sai-Halasz, L. Esaki Appl. Phys. Lett. 31, 211 (1977). (2) B. Ganjipour, A.W. Dey, B.M. Borg, M.Ek, M.-E. Pistol, K.A. Dick, L.-E. Wernersson, and C. Thelander, Nano Lett. 11, 4222 (2011).

ZnO is a wide band gap semiconductor with a high exciton binding energy, which is of significant interest for optoelectronic applications. In addition, ZnO nanostructures can be grown by a variety of methods, including low-temperature, low-cost, environmentally friendly aqueous solution-based techniques such as hydrothermal growth and electrodeposition. ZnO also has a complex defect chemistry, which results in a wide range of properties obtained for different synthesis methods, growth conditions, and post-growth treatments. Defect concentrations are typically higher for nanostructures grown at low temperatures. Nevertheless, significant variation in nanorod properties can be obtained by changing the growth conditions, such as precursors used, substrate pretreatment (seed layer deposition), bias applied etc., as well as changing the post-deposition treatments (plasma treatments, annealing). We will discuss the influence of the growth conditions and post-deposition treatments on the properties of ZnO nanostructures grown by different methods and under different conditions. The properties of ZnO nanostructures will be examined by photoluminescence spectra and electron microscopy, with other techniques (I-V curves, C-V curves, ESR etc.) applied to those structures with suitable morphologies. Finally, we will discuss the effect of the properties of ZnO nanorods (grown by different methods and subjected to different post-deposition treatments) on the performance of ZnO-based heterojunction light emitting diodes (LEDs). For ZnO nanorod/p-GaN heterojunction, the effect of properties of both materials on LED performance will be discussed.

Zinc oxide (ZnO) is considered a very interesting material owing to its direct wide band gap (3.37 eV at room temperature (RT)) and high exciton binding energy (60 meV). Its use in several applications, such as electronic, optoelectronic, electrochemical and electromechanical devices, has been already demonstrated, and interest is increasing due to the possibility to synthesize ZnO in a large number of different nanostructures that allow novel devices to be achieved. Recently an increasing attention has been devoted to the fabrication of porous ZnO nanostructures since large surface areas and a high surface-to-volume ratio make porous materials interesting for a variety of applications such as catalysts, nanosieve filters, solar cells and gas sensors. In this communication we report about the processes that allow monocrystalline porous ZnO nanobelts to be obtained from the ZnS(en)0.5 (en=ethylenediamine) hybrid organic-inorganic precursor synthesized by a solvothermal route. In particular it is shown that porous ZnO nanostructures are obtained through the topotactic transformation ZnS(en)0.5 > ZnS > ZnO induced by proper thermal treatments [1]. The properties of ZnO porous nanostructures are thoroughly investigated by SEM, TEM, cathodoluminescence spectroscopy and electrical measurements. The morphological and light-emission properties are compared for three different families of nanobelts: ZnS, mixed ZnO/ZnS and porous ZnO. TEM analyses reveal that the oxidation process develops through the formation of ZnO grains that gradually spread all over the nanostructures as the annealing time is increased. The lattice contraction associated to the ZnS→ZnO transition is thought to be the mechanism leading the highly porous morphology. Cathodoluminescence spectroscopy of porous ZnO NBs reveals an intense emission in the visible range. This emission peaked at 530 nm (2.34 eV) is related to sulfur doping in ZnO nanostructures [2]. Meanwhile the porous ZnS NBs has en emission peaked at 590 nm (2.10 eV), probably related to ZnS point defects [3]. Finally it is shown that the porous ZnO nanobelts can be obtained as supported on proper substrates and that their distribution can be patterned by patterning the growth of the hybrid precursor. Possible applications are envisaged for these nanostructures due to the possibility to combine porosity, mono-crystallinity and patterned growth. [1] L. Zhang et al. Inorganic Chemistry, 2008, Vol. 47, 11950-11957 [2] J.V. Foreman et al. NANO LETTERS 2006 Vol. 6, 1126-1130 [3] T. Zhai et al. J. Phys. Chem. C 2007, Vol. 111, 11604-11611

Monoclinic Ga2O3 is a very promising transparent conductive oxide (TCO) which is being considered for optoelectronic and photonic applications, due to its high structural, thermal and chemical stability, wide band gap and controllable conductivity. In order to tailor the electronic and optical properties of the nanowires, doping with appropriate impurities is a key issue. In particular, doping with Sn has been demonstrated to be effective to enhance the n-type conductivity of β-Ga2O3 [1]. On the other hand, chromium ions are responsible for a very intense red luminescence of monoclinic gallium oxide [2, 3]. In this work, we show effective co-doping of gallium oxide micro- and nanostructures with tin and chromium ions by a thermal evaporation method. The growth of different morphologies as a function of the experimental conditions is demonstrated. The influence of Sn on the growth of the structures is discussed in the light of morphological, structural and compositional studies. The obtained structures include Sn and Cr co-doped gallium oxide nanowires and branched nanostructures, as well as heterostructures in the form of core-shell Ga2O3/SnO2 microrods and Ga2O3 trunks with SnO2 nano-branches. The luminescence has been assessed by cathodoluminescence (CL) in the SEM, including images, spectra and depth-resolved CL. These results are correlated with EDX analysis in the core-shell structures. In addition, light modulation due to longitudinal resonant modes within the Cr doped nanowires has been demonstrated by means of confocal micro-photoluminescence (PL) analysis. [1] S.I. Maximenko, L. Mazeina, Y.N. Picard, J.A. Freitas Jr., V.M. Bermudez, S.M. Prokes, Nano Lett. 9, 3245 (2009) [2] E. Nogales, J. A. García, B. Méndez, and J. Piqueras, J. Appl. Phys. 101, 033517 (2007) [3] E. Nogales, J. A. García, B. Méndez, and J. Piqueras, Appl. Phys. Lett. 91, 133108 (2007)

The exciton-phonon coupling in high-quality cubic phase zinc telluride (ZnTe) nanorods (NRs) is investigated by resonant micro-Raman spectroscopy near the direct bandgap of ZnTe. The scattering cross section of longitudinal optical (LO) phonon is enhanced extensively in the resonant process and the enhancement factor of LO modes is much higher than that of the transverse optical (TO) modes, indicating a dominant Fröhlich electron-phonon interaction mechanism. Up to fifth-order LO phonons are observed by resonant Raman scattering at room temperature. The Huang-Rhys factor of individual NRs thus the exciton-LO coupling strengths is evaluated, showing an increasing with the NR diameter. Surface phonon (SO) and its high-order overtones are observed between nLO and (n-1)LO +TO for the first time, whose positions are consistent with a dielectric continuum model. Strong acoustic phonon-exciton coupling induces a high-frequency shoulder above each nLO peaks with two maxima located around 14 cm-1 and 32 cm-1, which are assigned to transverse acoustic (TA) and longitudinal acoustic (LA) phonons, respectively. The resonant multi-phonon scattering process involving acoustic and LO phonons is discussed based on an exciton-mediated cascade model, where a scattering sequence of acoustic phonon followed by LO phonons is favorable. These results advance the understanding of electron-phonon coupling and exciton scattering in quasi-one-dimensional systems, especially in the scarcely documented ZnTe compound, facilitating the development and optimization of NR-based optoelectronic devices.

Modern vapor-liquid-solid (VLS) growth based on alloy catalysts has recently demonstrated the possibility of growing SiGe nanowires with controlled axial heterojunction abruptness [1] combined with simultaneous control of both material composition (Si and Ge) and doping profile. This advance opens up many potential applications for photovoltaic devices. In this presentation, we report on our first steps towards employing Si/Ge axial heterostructured nanowires (hetero-NWs) for high-efficiency tandem solar cells. The integration of Ge with Si in a hetero-NW structure is promising for broad-spectrum and high absorption efficiency solar cells. The hetero-NW material consists of a Si axial pin junction grown on top of a Ge pin junction, all grown on a Si (111) substrate. The heavy doping in the n-Si/p-Ge junction between the Si and Ge sections ensures a low-resistance tunneling contact (analogous to heavily-doped nanowire TFETs reported previously [2]) and the i-Ge and i-Si regions can be scaled to ensure approximate short-circuit current ISC match under solar illumination. Simulations were carried out using Synopsys TCAD under 1 W/cm2 illumination at lambda; = 500 nm. In the ideal case, the total open-circuit voltage VOC would simply be the sum of the two pin VOC values, and the resulting tandem structure with identical currents at the maximum power operating point would significantly enhance the overall efficiency. In the final device, the VLS-grown vertical hetero-NWs array is embedded in a thick spin-on-glass matrix, with transparent metal contacts deposited on top of the array for measurements under AM 1.5 solar illumination. Our preliminary optical measurements on p-Ge/i-Si/n-Si NWs provide important ingredients for a working hetero-NW photovoltaic device. We achieved good rectification with ~103 ratio between forward and reverse bias currents at moderate voltage. Under laser illumination (lambda; = 532 nm), thanks to low reverse-bias current and the inserted i-Si section, we measured a large open circuit voltage VOC ~ 0.54 V and a very high short-circuit current density JSC ~ 4x103 A/cm2, comparable to state-of-the-art reported single NW results [3]. We will report on the additional optimization of single Ge and Si pin NW structures, including I-V charateristics, diffusion lengths of photogenerated carriers, and dicuss progress towards their integration in a combined Ge/Si tandem hetero-NW solar cell. Finite Diffence Time Domain (FDTD) combines with Synopsys TCAD simulation are performed to characterized the optical properties of SiGe NWs and used to optimize NWs diameter, densities, length for improved quantum efficiency. [1] D. E. Perea, N. Li, R. M. Dickerson, A. Misra, and S. T. Picraux, Nano Lett. 11, 3117 (2011). [2] S. T. Le, P. Jannaty, A. Zaslavsky, S. A. Dayeh, and S. T. Picraux, Appl. Phys. Lett. 96, 262102 (2010). [3] B. Tian, T. J. Kempa, and C. M. Lieber, Chem. Soc. Rev. 38, 16 (2009).

Scanning cathodoluminescence (CL) spectroscopy has the capacity to determine the effect of dopants, impurities as well as point and extended defects on the optical and electrical properties of semiconductor nanostructures at high spatial resolution. CL spectral imaging, where a complete CL spectrum is measured at each image pixel, significantly extends these capabilities by facilitating correlative statistical analysis of the entire CL spectral data cube. This approach has been used to investigate ZnO nanowires (NWs) grown by the vapor liquid solid technique using carbothermic reduction of ZnO powder and gold nano-dot catalysts on silicon (111) and sapphire (11-20) substrates. Using this synthesis route, the morphology as well as the bulk and surface defect chemistry of the ZnO NWs can be controlled by varying the growth temperature (973 - 1173K), heating and cooling rates, the type of carrier gas and its flow rate and the growth time. At 300 K the near band edge (NBE) of ZnO exhibits a featureless broad asymmetric band centered at 3.25 eV, consisting of the free exciton (FX) peak and a red-extended tail comprising series of broad overlapping longitudinal optical phonon replicas. All specimens exhibited deep level emission (DLE), made up of a green luminescence (GL) and the yellow luminescence (GL) centered at 2.4 eV and 2.1 eV respectively. Spatially-resolved CL intensity-ratio imaging of the cross-section of ZnO nanowires were measured using the deep level GL, YL bands and the NBE. This analysis revealed that the DLE and NBE can be correlated, anti-correlated and uncorrelated in different NWs. However, most NWs exhibited increased GL in well-defined margins at the a-plane surface of the NW along the <0001> growth axis. This enhanced GL surface emission has been attributed to upward surface band bending due to oxygen chemisorption generating Vo+ centers within a surface depletion layer. In this work CL spectral maps clearly show regions of GL that appear to be remnants of margins inside larger ZnO NWs that merged during growth, suggesting that surface band bending may not be the only explanation for the a-plane enhanced GL. Heating the ZnO NWs in 25 K steps to 700 K causes the NBE to red shift due to band gap shrinkage from lattice expansion and of the electron-phonon interaction. Varshni&’s relation as well as the Bose-Einstein phonon model can be used to describe the observed temperature dependent shift of the NBE. These thermal parameters are equivalent with CL data from bulk ZnO but both differ from Varshni data from photoluminescence measurements. The NBE also red-shift and broaden above 300 K due to greater scattering into LO-phonon replicas because of the higher average FX momentum. The NBE CL of ZnO can be measured up to ~ 800K before the tail of the incandescence starts to noticeably spread into the visible spectral region. Both the NBE and DLE exhibited Arrhenius-like thermal quenching behaviour at elevated temperatures.

The formation of atomic pairs with opposite partial charge due to the chemical bonding ionicity within a semiconductor material has huge impact in many physical properties. Thus, it is worth to determine the orientation of these atomic pairs, so called dumbbells, to assign the polarity of the structure. Polarity influences the growth mechanism driving the formation of different architectures, and it is strongly correlated with the optoelectronic properties. Going further, the polarity preservation along the entire nanostructure must be confirmed, especially at the twin boundaries where polarity can be inversed. In this context, aberration-corrected High Angle Annular Dark Field (HAADF) and the recently developed Annular Bright Field (ABF) Scanning Transmission Electron Microscopy (STEM) techniques have shown to be very useful for direct visualization of dumbbells and hence, direct polarity assignment [1,2]. The present study analyses the polarity in binary semiconductor nanowires (NWs) in two extreme cases: i) When the dumbbell constituents are atoms of similar mass, as can be GaAs (Z_Ga= 31, Z_As= 33). Aberration-corrected HAADF provides images with contrast nearly proportional to the squared atomic number, allowing straightforward identification of the dumbbell components. ii) Compounds containing one extremely light atom, such as O or N in ZnO or GaN/AlN. The use of an annular detector located at the bright field region allows the visualization of light atoms by ABF . The combined use of both techniques, aberration-corrected HAADF- and ABF-STEM, is shown as powerful tool for achieving the fine characterization of semiconductor NWs that enables a better understanding of their growth mechanisms and physical properties. References [1] E Uccelli, J Arbiol, C Magen, P Krogstrup, E Russo-Averchi, M Heiss, G Mugny, F Morier-Genoud, J Nygard, J R Morante, A Fontcuberta i Morral, Nano Letters 11, 3827-3832 (2011) [2] M de la Mata, C Magen, J Gazquez, M I B Utama, M Heiss, S Lopatin, F Furtmayr, C J Fernández-Rojas, B Peng, J R Morante, R Rurali, M Eickhoff, Anna Fontcuberta i Morral, Q Xiong, J Arbiol, Nano Letters 12, 2579-2586 (2012)

One of the most important advantages of nanowires for future applications in a variety of fields is the potential to form heterostructures of lattice-mismatched materials, due to efficient lateral strain relaxation. Elimination of the barrier of lattice-mismatch opens up this technology for a virtually unlimited number of possible combinations of material properties within single devices and a great freedom in bandstructure engineering. The demonstration more than a decade ago of dislocation-free nanowires heterostructures with 3% lattice mismatch acted as a major driving force for the enormous growth of nanowire research into the mature field of today. However, nanowire heterointerfaces suffer from several pervasive challenges unique to this structure, which have to date limited their application in both fundamental physical studies and devices. The first challenge is persistent interface grading, which makes it difficult to achieve the abrupt changes necessary for many applications. However, when interfaces are sufficiently sharp, kinking at the heterointerface is common for most material combinations. We have investigated the causes of both of these issues, interface grading and kinking, and developed a fundamental understanding of the processes underlying these effects for III-V nanowires grown from Au seed particles. The specific processes and challenges in group III and group V interfaces will be discussed and compared (InAs-GaAs-InAs and InAs-InP-InAs heterostructures), along with structures where both III and V materials change at the interface (InAs-GaSb-InAs). In general, group III species result in more interfacial grading than As and P species, which can be attributed primarily to the higher solubilities of the former in the alloy seed particle. Memory effects of the precursor molecules also play a role, however, especially for Sb precursors. Kinking is shows to be a challenge for all combinations, but the details of the kinking process, and its causes, are unique to the materials and growth parameters. Characterization of interface sharpness and morphology for kinked, graded and optimized interface is performed by a combination of high resolution scanning transmission electron microscopy (HR-STEM) and energy dispersive X-ray spectroscopy (EDX). Statistical analysis of morphology to elucidate the relevant parameters is performed using scanning electron microscopy (SEM) images. Based on the understanding of the processes involved, specific techniques will be demonstrated to minimize both challenges of interface grading and kinking. By optimization of the growth procedure, we show that it is possible to achieve straight and sharp interfaces in all investigated systems, in both switching directions.

One of the most powerful tools to achieve atomic characterization is transmission electron microscopy (TEM). The dramatic progress in hardware in recent years allows for the first time such analysis, if new acquisition modes, simulation aids and data analysis schemes are developed in parallel. The ultimate goal is to extend nanoscience into the atomic scale, where the atomic structure can be correlated with the overall physical and chemical properties. A specific case is the incorporation of single Au atoms into GaAs nanowires during the growth process. One-dimensional nanowires (NWs) made of III-V compound semiconductors attract increasing interest for their use in studying fundamental physics problems as well as for potential applications. In general, such nanowires are nucleated and grown by the vapour-liquid-solid (VLS) method assisted by a metal droplet (normally Au). The utilization of III-V nanowires is hindered by three major questions: the occurrence of stacking faults, the large surface area covered by an oxide layer as well as impurities, and the possibility of incorporation of the catalyst (Au) atoms during the growth process, all of which are deleterious to ballistic transport through the nanowires. Verifying the level of incorporation of gold in III-V nanowires has turned into one of the leading challenges, mainly because of the small difference in the atomic number Z between the Au and the GaAs. High-angle annular dark-field (HAADF) image calculations are required to find the best conditions for the detection of a single Au atom either on the surface or in the bulk of a NW. The minute concentration (10 ppm) can be directly imaged and the implication on the ballistic transport in the wires could be detrimental. The application of the method to other dopes semiconductors will be discussed.

Group III-nitrides have received much interest in the application of light-emitting diodes with wavelengths that can span the entire visible spectrum, due to band gap energies of 0.7 eV for InN and 3.4 eV for GaN. The high lattice mismatch in the film/substrate interface is a major roadblock in the growth of high quality planar thin films due to high density of threading dislocations that can degrade device properties. Hence the growth of III-N nanowires (NWs) has been a promising alternative, as these one-dimensional nanostructures give rise to free surfaces for radial strain relaxation preventing the formation of dislocations at the interface. Unique InGaN/GaN (quantum) dot-in-a-wire nanostructures were recently developed to achieve more effective lateral carrier confinement and controlled light emission across the entire visible spectrum by varying properties of the dots. With the successful achievement of high quality, almost strain- and defect-free III-N heteroepitaxy with the NW geometry, further characterization with electron energy-loss spectroscopy (EELS) is necessary to further understand the electronic states and optical properties of the III-N materials. In this work, multiple InGaN/GaN dot-in-a-wire nanostructures were grown on Si(111) substrates by molecular beam epitaxy. The structural properties of the NWs have been characterized by aberration-corrected scanning transmission electron microscopy (STEM) using high-angle annular dark-field (HAADF) imaging providing atomic number Z-contrast. The NWs are grown in the wurtzite crystal structure, with the polar c-axis aligned perpendicular to the substrate, as confirmed using Fourier Transforms (FT) of the HAADF-STEM images. EELS spectra acquired from the GaN NW reference sample show similar peaks to bulk hexagonal-GaN thin film in the low-loss region, including the bandgap region, the bulk plasmon and the Ga 3d transition. The surface plasmon excitation is more pronounced in the NWs, where its contributions from the NW surfaces are dominant. The relative In-content in the InGaN/GaN dot-in-a-wire structures is evaluated using a multiple linear least squares fitting routine using an internal reference to fit the N K-edge (399 eV) and In M4,5-edge (443 eV, in close proximity to the N K-edge) in the EELS spectra. A surface plot of the relative In-content clearly illustrates the systematic non-uniformity in In-content within the dots. The phenomena can be attributed to the evolution of the underlying strain distribution within the NW during growth, which plays a dominant role in the incorporation of In as well as the morphology of the InGaN layer. Geometric phase analysis of high-resolution TEM images was used to effectively extract the local strain components at the nanoscale from the image FT. Examining the strain distribution in and around the quantum dots allows to better understand the local strain relaxation. These effects and the role of the incorporation of In will be discussed.

Phase change materials based on chalcogenide alloys, in particular the pseudobinary GeTe-Sb2Te3 compounds (GST), are interesting for phase change memory (PCM) devices. A very attractive option involves GST nanowires (NWs) because they offer a defect-free downscaling to achieve high performance and low-power PCMs, thanks to the reduced volume to be programmed. The GST atomic arrangement is not yet assessed, despite the considerable interest and the great effect it has on the phase switching properties. The relative stability of the several structures proposed for bulk GST are well discussed in the literature but no data are available about NWs, for which a different behavior is expected to due to their growth out of the thermodynamical equilibrium and to size effects. In this work we study GST NWs grown by MOCVD via the Au assisted Vapour-Liquid-Solid mechanism. They have been found to crystallize in two stable compounds with different stoichiometry present on the pseudobinary line of the phase diagram: Ge1Sb2Te4 and Ge2Sb2Te5. Both have hexagonal/rhombohedral crystal structure with respectively 21 and 9 different atomic layers stacked along the c axis direction. Different stacking sequences have been hypotized for each phase in the literature, in order to account for some material properties, but no one has been verified. We propose a method to identify the stacking sequence in GST NWs by combining the direct observation with High Angle Annular Dark Field Scanning TEM (HAADF-STEM) and simulations. We find out that the simulated structures best fitting the experimental images are the ones with the Ge and Sb atoms randomly sharing the same lattice sites, although this configuration is considered not stable by the existing theoretical models.

The development of nanotechnology in photonics offers significant scientific and technological potentials. It fosters the substantial efforts for exploring novel photonic materials, developing easy device fabrication techniques, reducing the size of photonic devices, improving device integration density, and fabricating low-cost nanowaveguides (nanoWGs) and nanodevices. Since nanometer-scale optical waveguides are highly desirable for applications in high density and miniaturized photonic integrated circuits, so subwavelength-diameter waveguides have been fabricated in semiconductors, silica fibers, bulk glasses, compound-glass fibers, etc. Meanwhile, miniaturization of photonic devices is being intensively focused because they drive great developments in the fields of optical communications, biophotonics, engineered structures, and photonic integrations. In this talk, we present a type of nanoWGs (also called nanowires or nanofibers) which were drawn by a one-step drawing process from the melt of novel poly(trimethylene terephthalate) (PTT), followed by presenting a number of nanodevices assembled by flexible PTT nanoWGs. As a promising application of nanoWGs in optofluidics which is a new research field emerged in 2003 that combined microfluidic and optical (specifically nanophotonics), optical trapping and manipulation of particles using the PTT nanoWGs together with silica nanoWGs will also be presented including assembly, migration, separation, arrangement, delivery, release, etc.

The piezo-phototronic effect is a new field of research that utilizes the three-way coupling of piezoelectricity, photoexcitation and semiconductor properties of the same material to enhance its performance for optoelectronic devices. Its core physics relies on the modification effect of the piezoelectric potential (piezopotential) on the band structure at a contact or junction for effective tuning/controlling of the charge transport. Materials that exhibit the piezo-phototronic effect are mainly in the Wurtzite family. The one-dimensional structures of these materials are ideal for fabricating strain-controlled piezo-phototronic devices. The strain applied to cause the deformation of the nanowires is mainly through shape change of the flexible substrate that supports the device. Such devices can be the basis for active flexible electronics, which uses the mechanical actuation from the substrate for inducing new electronic/optoelectronic effects. Here we have constructed a theoretical model for describing the characteristics of a metal-nanowire-metal structured piezo-phototronic photodetector. We discussed the influences of photogeneration and piezoelectric charges on photocurrent and their coupling in dictating the performance of the photodetectors with a single Schottky contact and double Schottky contacts. Numerical simulations fit well to the experimental results of ZnO nanowire based UV detector. Such study is extended to CdS nanowire based photodetectors for visible light detection. As conclusion, we proposed three criteria for identifying the presence of piezo-phototronic effect in a general photodetector. Reference: Y. Liu, Q. Yang, Y. Zhang, Z. Yang, Z. L. Wang, Adv. Mater. 2012, 14, 1410 - 1417

SiO2/SiC coaxial nanowires (NWs) are intriguing as novel nanostructured to be functionalized because of the 3C-SiC biocompatibility and of the presence of a SiO2 native shell that favours surface functionalization. Here, we report on preliminary results of the functionalization of CVD grown mats of 3C-SiC NWs catalyzed by metal(Ni, Fe, Fe3O4) nanoparticles with partially-fluorinated tetraphenylporphyrins (H2TPP(F)) and of the effects of the NWs on test cells in in-vitro experiments. The morphological, structural and optical properties of the nanostructures are analyzed by means of Field Emission-SEM, AFM, HRTEM, Fluorescence and Cathodoluminescence Spectroscopy and imaging and XPS, before and after the functionalization. The surface functionalization is performed by means of Supersonic Molecular Beam Deposition in order to kinetically activate bonding formation between the molecules and the nanostructure surface. Peak Chemical shifts and lineshape modification for XPS spectra of C1s and F1s, with respect to the corresponding molecular bulk signals, have been found pushing towards the proof of NWs functionalization. After SuMD functionalization, the NWs present, around the Q absorption band of the porphyrin, three order of magnitude higher Cathodoluminescence integrated intensity in comparison with the molecules deposited on bulk substrates. This effect is very attractive in view of deep solid tumor treatments since it is correlated to an efficient energy transfer between the NWs and the molecules and in turn to an enhanced process of oxidation of the prophyrin [1]. Analytical transmission electron microscopy elemental mapping confirms an homogeneous 20 nm thick H2TPP(F) shell around each single NW. Surprisingly the porphyrin deposited by SuMBD onto the SiC/SiO2 NWs are not affected both by electron (30-200 keV) and X-ray beams (10 keV). This is extremely important for future use in self lighted PDT for cancer treatments when the singlet oxygen generation is induced by X-rays [1,2]. In-vitro tests on cell lung and human brest adenocarcinoma celllines and normal human fibroblasts have been also carried out. After 72 hs a dose-dependent inhibited proliferation of the cells and a mainly necrotic cellular death after NWs concentrations ge; 50-100 mu;g/ml have been found. No toxic effects are found. Those findings are encouraging in the prospective to employ this functionalized system for different nano-medical applications such as targeted therapy against deep tumor cells. Aknowledgements: This work has been supported by the European Project Nanowiring and by the Italianl Project Bionimed [1] G. Salviati et al., MRS Fall Meeting, PM BB4.2, Boston, MA Nov 27-Dec 2, (2011) [2] P. Yuzenas et al., Advanced Drug Delivery Reviews 60, (2008) 1600-1614 [3] Yuanfang Liu et al., Appl. Phys. Lett. 92, 043901 (2008); doi:10.1063/1.2835701

Silicon-based transistor integration confront with huge power dissipation. In order to reduce the power dissipation, reduction in subthreshold slope (SS) of transistors is effective approach. Here, we investigate electrical properties in InGaAs NW/Si heterojunction and firstly demonstrate tunnel field-effect transistor (TFET) using an InGaAs NW/Si with surrounding-gate architecture to achieve a steep-slope switching. The substrate was p-Si (p ~ 1 × 10¹⁸cm^-3) and p⁺-type (p ~ 2 × 10¹⁹cm^-3) Si(111). After 20 nm-thick SiO₂ was formed by thermal oxidation, openings were formed using electron beam (EB) lithography and wet etching. InGaAs NWs were grown in low-pressure horizontal MOVPE system. The ratio of TMIn in vapor phase was 0.61. The growth temperature was 670°C and growth time was 10 min. Before the InGaAs NW growth, Si(111) substrate was annealed at 925°C in H₂ and treated in AsH₃ gas to form Si(111) 1×1:As surface. At last, flow-rate modulation epitaxy (FME) was introduced to completely aligned the vertical InGaAs NWs on Si(111). After the specific growth sequence, vertically aligned InGaAs NWs were grown on Si(111). The average diameter of the InGaAs NWs was 80 nm, and the average height was about 1 mu;m. EDX line scan profile showed the In composition of the InGaAs NW was 70 ± 2%. Next, we fabricated a TFET. A single vertically aligned n+-InGaAs/undoped-InGaAs axial NW was grown on a p⁺-Si substrate that has lithographically defined mask opening-area inside a 50 × 50 mu;m square SiO₂ mask. The device processes for the TFET were the same as previously reported^1,2. A 20-nm-thick Hf_0.8Al_0.2O_x was served as high-k gate dielectric. Photolithography-defined tungsten (W) was used as gate-metal. The gate length was 200 nm, which corresponds to the length of the undoped InGaAs region. A 20 nm of Ti, 10 nm of Pd and 100 nm of Au were evaporated to create drain and source electrodes. The fabricated InGaAs NW/Si TFET at drain-source voltage (V_DS) of 0.05 - 1.00 V showed a switching behavior with a SS of 80 mV/dec under reverse bias direction. This switching characterization appeared at the V_DS of 0.05 V. The output characteristic of the TFET exhibits saturation region under very low drain voltage. The current is thought to be based on Zener tunneling current occurred at the InGaAs NW/Si heterojunction. The Also, the SS became slightly steeper with increasing the V_DS. The ratio of the ON/OFF current was approximately ~ 10⁵ at V_DS of 0.10V. Further improvements are re-quired for attaining higher ON current and steeper SS. Although the switching behaviors of the TFET were based on Zener tunnel transport across the III-V/Si heterojunction, the SS showed a large value. Further improvements are required for attaining higher ON current and steeper SS. [1] T. Tanaka et al., Appl. Phys. Exp. 3 (2010) 025003. [2] K. Tomioka et al., 98 (2011) 083114.

The synthesis of an axial p-n homojunction within a single nanowire using a facile technique is still rare. Here, we illustrate a simple method to synthesize highly ordered ZnO axial p-n homojunction-containing nanowires using a low temperature method, and on a variety of substrates. X-ray diffraction, scanning transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy are used to reveal high quality single-crystalline wires with a [001] growth direction. The study of electrical transport through single nanowire based device and cathodoluminescence via scanning transmission electron microscopy demonstrates that an axial p-n junction exists within each ZnO nanowire. This represents the first low temperature synthesis of axial p-n homojunction-containing ZnO nanowires with uniform and controllable diameters.

The possibility of achieving large thermoelectric efficiencies in one-dimensional conductors was first predicted in 1993 [1] Recent reports of a figure of merit ZT approaching 1 in Si nanowires (NWs) [2] boosted a resurgent interest in this field [3,4]. Since these works, many theoretical studies attempted to propose novel approaches to further increase those efficiencies. All these theoretical proposals, however, focused on the decrease of the thermal conductivity, mostly neglecting the effects on the electrical conductivity. Such an approach is rigorous only in the idealized case of isotope blends, where different isotopes are mixed on purpose. In these systems the ballistic electrical conductance is not affected because the electronic structure does not change, while phonon transmission can suffer additional scattering, leading to a drop in the thermal conductivity. The use of different isotopes is not practical, however, and similar schemes have been explored in SiGe systems, where a significant difference in the mass offers the possibility of engineering phonon scattering, while the similar electronic structure allows expecting a limited degradation of the electrical conductivity. Here we study from first-principles the electrical conductance of Si1minus;xGex alloy nanowires in the ballistic regime [5]. On the basis of single-impurity scattering the assumption that Ge alloying does not affect significantly the conductance seems sound: (i) only interstitial defects act as efficient scattering centers, but their concentration is expected to be negligible; (ii) substitutional defects are easily incorporated in the Si lattice and the transport channels of the pristine wire are only marginally affected. Yet, in SiGe alloy NWs Ge concentrations of up to 70% can be reached, thus we addressed explicitely the study of Si1minus;xGex , with x ranging from 0.1 to 0.7. The calculated conductances show that in all the cases the concentration turns out to be sufficiently high so that the alloy NW behaves in practice as an abrupt Ge NW inclusion, with scattering concentrated at the interface. This is an important result, because abrupt junctions are difficult to obtain, while (random) concentration gradients can be obtained in an easier way. Peculiarly, distributing the impurities along a longer wire section yields a larger scattering, thus lower concentrations can result in lower conductances. This happens because in the limit of low Ge concentration the conductance results from a series of individual scattering events, while in heavily alloyed wires scattering only occurs at the interface and the scattering is reduced. [1] L. D. Hicks, and M. S. Dresselhaus, Phys. Rev. B 47, 16631 (1993). [2] R. Rurali, Rev. Mod. Phys. 82, 427 (2010). [3] A. I. Boukai et al., Nature 451, 168 (2008). [4] A. I. Hochbaum et al., Nature 451, 163 (2008). [5] M. Amato, S. Ossicini, and R. Rurali, Nano Lett. 12, 2717 (2012).

We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as photoanodes for solid-state dye-sensitized solar cells. In general, higher photovoltaic performance can be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit currents. To further characterize the fabricated devices, the electronic properties of the different networks were measured via the transient photocurrent and photovoltage decay technique. Nanowire-based devices exhibit extremely high, light independent electron transport rates while recombination dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually considered electron-limited due to the poor electron transport through nanocrystalline titania networks. In the case of the nanowire-based devices, however, the system has become limited by the organic hole transporter used. In the case of polycrystalline TiO2 nanotubes, lower transport rates and higher recombination dynamics than its nanoparticle-based counterpart were measured, suggesting that in order to improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline structures should be used. These findings should aid future design of photoanodes based on nanowire semiconductors to be used in dye-sensizited solar cells.

The superior properties of InP, such as its direct bandgap, high electron mobility and low surface recombination velocity, point to the promise of InP nanowires for future electronic devices. For the development of such devices, a detailed understanding of charge carrier dynamics in InP nanowires is essential. However, determination of nanowire electrical properties has proved challenging, because making direct electrical contacts to nanoscale structures is technically difficult and measurement artefacts arise from these electrical contacts. In contrast, optical pump-terahertz probe (OPTP) spectroscopy is ideal for nanowire studies, because it is a non-contact probe of room temperature conductivity with sub-picosecond resolution. In this study, OPTP spectroscopy is used to assess critical electrical properties of InP nanowires. These terahertz photoconductivity measurements were performed on a number of samples hosting nanowires of well-defined diameters. InP nanowires exhibited long photoconductivity lifetimes of over 1 ns. The photoconductivity lifetime showed only a weak dependence on nanowire diameter: nanowires of 50 nm diameter exhibited 1.2 ns lifetimes whereas nanowires of 165 nm diameter exhibited marginally longer lifetimes of 1.4 ns. This suggests that carrier lifetimes are relatively insensitive to surface traps, even in narrow diameter nanowires with very high surface area-to-volume ratios. This prolonged photoconductivity and insensitivity to surface states is promising for many electronic applications which require long carrier lifetimes, including photovoltaics. Photoconductivity spectra of InP nanowires exhibited a pronounced surface plasmon mode, and from these spectra electron scattering rates and mobilities were extracted. Typical mobilities were between 200 and 600 cm²/Vs, significantly lower than typical values for high quality bulk InP. The measurements, together with modelling, indicate the carrier mobility is strongly degraded by the high density of stacking faults in these predominantly wurtzite nanowires. In conclusion, OPTP measurements of InP nanowires demonstrated that carrier lifetime is relatively insensitive to surface states, showing the potential of InP nanowires for optoelectronics and photovoltaics. However, crystallographic defects severely limit carrier mobility, which points to the importance of engineering InP nanowires with high crystal quality for future device applications.

Chemical sensing based on conductance change of one-dimensional (1D) nanostructures such as nanowires and/or nanotubes have attracted intensive research efforts due to the rapid, label-free and highly sensitive detection provided by these new and powerful class of sensors. In this sense, ultrathin metal nanowires (diameter < 10 nm) free of grain boundaries can be considered as almost ideal one-dimensional (1D) conductors that can be applied as sensing nanostructures. Since the diameter of such wires approach molecular dimensions, these ultrathin conductors are considered ultimate building blocks for fabricating highly sensitive chemiresistors. However, the application of such metallic 1D nanostructures as chemical sensors had not previously been demonstrated. In this work, we describe the fabrication of chemiresistors using ultrathin Au nanowires, which were produced through a wet chemical synthesis and sequentially assembled over a Si/SiO2 surface. The wires were electrically contacted by electrodes fabricated by conventional photolithography and metal deposition steps. In order to avoid interferences from the contacts, the electrodes were passivated. By isolating these undesired effects, we restricted the electrical response of the sensors only to the nanowires surface. We then investigated how surface chemistry can influence the electrical behavior of such ultrathin nanowires in air and water by immobilizing short-chain alkanethiol molecules with different functional groups on their surface. In doing so, we choose thiolpropane (TP), cysteamine (CA) and mercapto-propionic acid (MPA) molecules. In solution the MPA molecules immobilized on Au nanowires generated a net of negative charges due to deprotonation of their carboxyl groups, while CA yielded a positively charged surface due to protonation of their primary amine groups and TP served as a non-charged or neutral molecule. Our results demonstrate that besides the Au-binding moiety of the molecules, their interfacial charge due to ionization in solution has a significant impact in the electrical resistance of the wires, which depending on the chemical functionality can be drastically enhanced (> 100%). These data provide new insights into the electrical sensitivity of ultrathin metallic nanowires upon molecular adsorption.

We utilize the new technique of transient Rayleigh scattering spectroscopy to probe the band structure and carrier dynamics of both zincblende InP nanowires and wurtzite InP nanowires. The nanowires were grown using Au-catalyst assisted MOCVD growth with 50 nm gold nanoparticles for these growths. For the zincblende (cubic) nanowires we observe an energy gap of 1.35 eV at room temperature, and a value of 1.42 eV at low temperatures, close to the values expected from previous photoluminescence and photocurrent spectroscopy experiments on these nanowires. For the case of the wurtzite nanowires with hexagonal symmetry, we observe three excitonic resonances associated with electrons in the lowest conduction band and holes from the valence bands, namely A at 1.50 eV, B at 1.54 eV and C at1.65 eV. In addition, we also see responses associated with the same A, B and C hole bands and the second conduction band (~200 meV higher in energy than the first) resulting from zone folding of the L-valleys. This clearly demonstrates the power of this technique in the case of nanowires with more complex symmetries than the simple cubic symmetry. In the WZ InP nanowires the lifetimes of the A exciton are ~800 ps, while the higher lying B and C-excitons exhibit shorter lifetimes of ~400ps and ~50ps reflecting their ability to relax through phonon emission to the valence band ground state. We acknowledge the financial support of the NSF through grants DMR-0806700, 0806572, 1105362, 1105121, and ECCS-1100489, and the Australian Research Council. The Australian National Fabrication Facility is acknowledged for access to the growth facilities used in this research.

Majorana fermions are particles identical to their own antiparticles, first introduced by Majorana as hypothetical elementary particles. Subsequently they were predicted in topological superconductors as quasiparticles of zero energy and zero charge. We have implemented a recipe for Majoranas that has four simple ingredients: 1) one-dimensional wires 2) spin-orbit interaction 3) superconductivity and 4) magnetic field. I will present electrical measurements on nanowires grown from indium antimonide, a material with strong spin-orbit interaction. Nanowires are contacted by a superconducting electrode. In the presence of magnetic field of order 100 mT we observe bound states at zero bias voltage. These bound states remain fixed to zero bias even when magnetic field and gate voltages are changed over considerable ranges. At the same time, such robust peaks are not observed when we remove magnetic field, align it with spin-orbit field or replace a superconductor with a normal metal. Our observations support the hypothesis of Majorana fermions. Reference: Science 336, 1003-1007 (2012) This work is done in collaboration with V. Mourik, K. Zuo, S. R. Plissard, E. P. A. M. Bakkers and L. P. Kouwenhoven.

III-V nanowires have shown great promises in the recent years thanks to the ability to control their dimensions, position, doping and to design heterostructures, both axially and laterally. This make them ideally suited for fundamental transport physics studies and future electronic nanodevices. Over all the III-V semiconductors, InSb is the one having the highest electron mobility, the highest G-factor and the lowest band gap. These unique properties made them ideal material for detection of Majorana quasiparticles [1]. However, for advanced Qubit and Majorana manipulations, new high quality structures are needed. Here we propose a study of branched structures based on InSb nanowires. The influence of the growth conditions is studied, advanced structural characterizations are performed and first electrical properties are presented. Hall effect measurements at low temperature prove the high quality of this new structures. 1 - “Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices” V. Mourik, K. Zuo, S. M. Frolov, S. R. Plissard, E. P. A. M. Bakkers, and L. P. Kouwenhoven, 2012 Science 336, 1003-1007 - DOI: 10.1126/science.1222360

Remarkable results have been achieved in the development of (In,Ga)N alloys as active layer materials of ultraviolet (UV) and visible light-emitting diodes (LEDs) as well as of laser diodes (LDs). This class of materials is also very promising for solar water splitting because their bandgap energy can be tuned from the UV to the near infrared region, thus covering the entire solar spectrum. In addition, their conduction and valence band edges can straddle the H+/H2 and O2/H2O redox potentials, and they are non-toxic and chemically stable. However, it is hard to grow (In,Ga)N bulk layers in high structural quality because of the lack of lattice-matched substrates. On the other hand, nanowires are known to accommodate lattice mismatch by lateral elastic relaxation without the formation of defects. Moreover, the nanowire geometry offers many other benefits such as enhanced light absorption, high surface area for electrochemical reactions, and improved carrier collection efficiency. In fact, recently overall water splitting by using GaN nanowires was reported [1]. However, GaN absorbs only the UV part of the solar spectrum. Therefore, we report here the growth and investigation of (In,Ga)N nanowire photoelectrodes for solar water splitting. Both n- and p-type (In,Ga)N nanowires with varying indium compositions up to 41% were grown on n- and p-type Si(111) substrates by molecular beam epitaxy, respectively. The photoelectrochemical properties were analyzed with electrochemical mass spectrometry (EMS), which allows the on-line detection of volatile products like H2 and O2 generated from electrochemical reactions during cyclic voltammetry. 0.5 M H2SO4 was used as the electrolyte solution. For all n-type (In,Ga)N nanowires on n-Si(111), a positive photocurrent was observed at positive potential under white light illumination. However, instead of oxygen evolution the formation of nitrogen was observed, indicating that the photocurrent is related to photocorrosion. We managed to suppress this photocorrosion by the electrochemical deposition of Pt on the nanowires, but the photocurrent was also reduced to very low level. Current work aims at developing a surface treatment of the nanowires that prevents photocorrosion while enabling photochemical reactions. In contrast to n-(In,Ga)N nanowires, p-(In,Ga)N nanowires showed hydrogen evolution at negative potential without photocorrosion. Compared to pure p-(In,Ga)N nanowires, the deposition of Pt shifted the onset potential for hydrogen evolution by about 400 mV to +250 mV vs NHE. At zero bias a photocurrent of 0.5 mA/cm2 was observed under illumination and the evolution of hydrogen was confirmed by EMS. This is the first demonstration of water splitting using (In,Ga)N nanowires by the detection of the evolved gas and thus paves the way for the development of a new class of photoelectrodes. Reference:[1] D. Wang, et al., Nano Lett. 11, 2353 (2011).

The hydrogen production through photocatalytic water splitting under visible light is an attractive environmental-friendly method, which utilizes two renewables: water as a raw material and solar light as an abundant energy source. Semiconducting metal chalcogenides (such as CdSe, CdS, etc.) possess band gap, conduction and valence band values suitable for photocatalytic H₂ generation under visible. The present work is thus focused on utilization of one-dimensional (1D) high-quality CdSe nanowires (NWs) in fabrication of quasi-type-II CdSe/CdS core/shell photocatalytic system toward efficient H₂ generation from aqueous solution. The obtained heterostructures have been additionally decorated with noble metal nanoparticles (NPs) so as to improve charge separation and extend exciton lifetimes. CdSe NWs have been obtained through the solution-liquid-solid (SLS) method using Bi salt as a catalyst. To obtain core/shell heterostructures, CdSe NWs have been overcoated with CdS at mild temperature (sim;120 °C) using elemental sulfur and dimethylcadmium precursors. The decoration of CdSe or CdSe/CdS core/shell with gold NPs has been accomplished via photodeposition method. The photocatalytic H₂ generation of all NWs obtained and their heterostructures has been performed under visible light in the presence of Na₂S/Na₂SO₃ as sacrificial electron donors, which also prevent photocorrosion of NWs contributing materials stability during photocatalytic reactions. The observed photocatalytic activity for H₂ generation increases in the order: CdSe< CdSe/Au< CdSe/CdS/Au< CdSe/CdS. A maximum H₂ generation rate of 56 mu;mol h^-1 g^-1 for CdSe/CdS core/shell heterostructure is 40 times larger than that for CdSe NWs. This large improvement is explained in terms of better charge separation in core/shell structure under irradiation, where the photogenerated electrons are delocalized between the CdSe core and CdS shell conduction bands, while the photogenerated holes migrate to the valence band of CdSe core.

In previous work, for the application to solar cells, we successfully fabricated 30 nm-diameter SiNW arrays by metal assisted chemical etching using silica nanoparticles (MACES), which enables us to control the diameter and density of SiNWs. However the net surface recombination velocity of SiNWs is very high due to the large surface area. To obtain high efficiency, a passivation film must be applied to the SiNW array in order to reduce the surface recombination. In this study, we analyzed the influence of surface recombination on the performance of SiNW solar cells using the two-dimensional device simulator ATLAS. Moreover, to confirm the effect of a passivation layer, Al2O3 was deposited on n-type SiNW arrays with the diameter of 30 nm by Atomic Layer Deposition (ALD), which enables us to cover the structure with high aspect ratio such as a SiNW array. The simulation structure of SiNW solar cells is transparent conductive oxide (TCO)/n-type hydrogenated amorphous silicon (n-a-Si:H) (40 nm)/intrinsic a-Si:H (i-a-Si:H) (4 nm)/SiNW(x mu;m)/i-a-Si:H (4 nm)/p-type a-Si:H (p-a-Si:H) (40 nm)/electrode. The diameter of SiNW is fixed at 30 nm and SiNW was embedded in Al2O3. Taking account of surface recombination, 1 nm-thick recombination layers were addressed on both side surfaces of the SiNW. The length of the SiNWs was changed from 1 to 6 mu;m. Radiative, Shockley-Read-Hall, and Auger recombination were taken in account. On the experimental side, Al2O3 was deposited on the prepared SiNW arrays using trimethylaluminium (Al(CH3)3) and H2O by ALD and the samples were annealed in a forming gas at various temperature. Minority carrier lifetime in SiNW arrays with Al2O3 was measured by the mu;-PCD method. Simulation results revealed that SiNW solar cells have the maximum efficiency at 3 mu;m. Most of the generated minority carriers have to diffuse to a p-n junction in the SiNW because the depletion layer width is only about 200 nm. Therefore, if the length of SiNWs increased, the possibility of surface recombination will also be increased, which leads to the degradation of the open circuit voltage (Voc) due to increasing reverse saturation current. On the other hand, the short circuit current (Isc) has the maximum value at 3 mu;m. Although the photo-absorption should increase with increasing the length, the possibility of carrier collection decreases due to surface recombination. That is why Isc has a peak on the length of SiNWs, suggesting that the length is very important parameter to obtain high efficiency. On the experimental side, the carrier lifetime of bare, Al2O3 as-deposited, and post-annealed SiNW arrays was measured, and 1.8, 4.7, and 26.2 mu;sec were obtained, respectively. The post-annealing was conducted at 350 oC. From this result, it is expected that the Voc and efficiency will be improved by the deposition of Al2O3 on SiNW arrays.

Epitaxial non-planar nanostructures such as semiconductor nanowire array may serve as a valuable platform to study various nanoscale phenomena and to develop novel device concepts with augmented performance and functionalities. However, despite the improved ability of nanostructures to relax strain elastically, a well-crystallized epitaxy of nanostructure is still strongly dependent on the lattice matching of the material with the substrate. The requirement for a suitable substrate restricts the material-substrate variations into homoepitaxy and limited combinations of heteroepitaxy. Hence, the explorations for the development of various nanowire array-based technologies are severely hampered since a myriad of interesting material properties unique to a particular compound cannot be harvested. An epitaxy mechanism called van der Waals epitaxy circumvents such limitation. By utilizing layered substrates, the heterointerface is connected via weak van der Waals interactions instead of covalent chemical bonds, as typically occurred in a conventional heteroepitaxy. Here we show that, by utilizing only (001) muscovite mica substrates in a catalyst-free vapor transport technique, the van der Waals epitaxy enabled the growth of well-crystallized nanowire arrays from various semiconductor compounds (ZnO, ZnS, ZnTe, CdS, CdSe, CdTe, PbS, and GaN) irrespective of the lattice mismatch with the substrate. We then establish the characteristics of the van der Waals epitaxy in nanowire array as a case study of non-planar nanostructures. Cross-sectional electron microscopy analysis revealed that the epitaxy is of incommensurate nature, which we believe to be responsible in alleviating the lattice matching requirement. We then extend the applicability of the van der Waals epitaxy by synthesizing non-planar, polytypic tripod nanocrystals epitaxially from II-VI semiconductors for the first time. We also recently apply the concept of van der Waals epitaxy in liquid phase synthesis, where the growth of ZnO nanowire array on phlogopite mica via hydrothermal growth was achieved despite the large lattice mismatch within the system. Our results effectively illustrate the versatility of the van der Waals epitaxy to serve as a universal epitaxy strategy and the possibility for the preparation of other materials and other nanoarchitectures with higher complexity.

Group III-nitrides as wide band gap semiconductors have attracted considerable interest during recent years. Moreover, combination of such materials with diamond as substrate can create an efficient hetero-diode system with high carrier mobility [1, 2]. The main idea of our work is to create a heterojunction diode with n-type silicon-doped group III-nitride nanowires (NWs) on a p-type boron-doped diamond substrate. Self-assembled high-quality catalyst-free GaN NWs were nucleated and grown on Single-Crystal Diamond (SCD) substrates by plasma-assisted molecular beam epitaxy [3]. To characterize the samples, advanced microscopy techniques were applied. The crystal structure and faceting was investigated by means of high-resolution transmission electron microscopy (HRTEM). We also used aberration-corrected high annular dark-field (HAADF) and annular bright field (ABF) scanning transmission microscopy (STEM) imaging techniques. The latter is a newly developed technique in aberration-corrected STEM that allows us to visualize the light elements and thus measure the polarity in GaN directly. 3D atomic models were created to compare the theoretical mismatch with the experimental one. Moreover, photoluminescence (PL) and cathodoluminescence (CL) measurements were performed to show structural imperfections. In the case of these GaN NWs grown on diamond we observe a lower density of defects than e.g. for GaN nanowires on silicon. This corroborates the HRTEM and ABF-STEM results. References [1] G. Vogg et al, Journal of Applied Physics 96 (2004), 895. [2] C.R. Miskys et al, Applied Physics Letters 82 (2003), 290. [3] F. Schuster et al, Nano Letters 12 (2012), 2199.

A novel approach for the formation of coaxially heterostructured In(x)Ga(1-x)As/InAs nanowires (NWs), through catalyst-free van der Waals epitaxy on single layer graphene (SLG), is presented. Monolayer graphene sheets were synthesized by chemical vapor deposition (CVD) on Cu foils and transferred to SiO(2)-coated Si substrates, resulting in a high-quality and large-area platform for the subsequent assembly of NWs, via metal-organic chemical vapor deposition (MOCVD). Distinct growth regimes were observed on pristine SLG surfaces, in comparison to regions containing linear-defects or edge-state imperfections. Growth of densely packed and vertically oriented InAs NWs, with diameters ranging between 35 - 115 nm and lengths exceeding 5 µm, was achieved on pristine SLG regions; while growth of randomly oriented NWs and polycrystalline islands took place along substrate imperfections. In contrast, growth of InGaAs on SLG, under a constant trimethyl-indium (TMI) to trimethyl-gallium (TMG) molar flow ratio, resulted in a self-organized core-shell heterostructured NW geometry. Through scanning transmission electron microscopy (STEM) experiments involving high-angle annular dark-field (HAADF) imaging and energy-dispersive X-ray (EDX) spectrometry of individual NWs, compositional variances along both axial and radial directions were observed, indicating a distinct InAs-InGaAs core-shell architecture. The presence of both InAs and InGaAs segments was further confirmed through X-ray diffraction (XRD) analysis. Variation of the group-III metal-organic precursor molar flow ratios resulted in controlled assembly of In(x)Ga(1-x)As NW shell segments throughout a wide compositional range of 0.2 < x < 1, while the NW core segments remained composed of InAs. A mechanism based on van der Waals epitaxy on 2D materials is presented to elucidate the heterostructure self-organization phenomenon. This mechanism is further exploited for the growth of patterned NW arrays on graphene substrates. This work is presented as the foundation for a research path toward the fabrication of flexible opto-electronic devices, including large-area light emitters and solar cells, based on the novel nanowire-graphene hybrid material.

The large-scale assembly of nanowires with controlled orientation on surfaces remains one of the most critical challenges toward their integration into practical devices. We report the vapor-liquid-solid growth of perfectly aligned, millimeter-long, horizontal GaN (1) and ZnO (2) nanowires with controlled crystallographic orientations on different planes of sapphire and other substrates. The growth directions, crystallographic orientation and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. Guided GaN nanowires and ZnO nanowires present general similarities and a few interesting differences, which shed light into the guided growth mechanism. The controlled horizontal growth of nanowires of different materials on different substrates proves the generality of the guided growth approach. This paves the way to a variety of highly controlled nanowire structures with potential applications not available by other means. (1) D. Tsivion, M. Schvartzman, R. Popovitz-Biro, P. von Huth, E. Joselevich, Science 333, 1003 (2011). (2) D. Tsivion, M. Schvartzman, R. Popovitz-Biro, P. E. Joselevich, ACS Nano, in press.

Semiconductor nanowires (NWs) have today advanced to a level beyond thin films with respect to design freedom, including structuring of both material composition and crystal phase with high spatial precision, making them promising for a number of electronic and optoelectronic device applications [1]. Graphene on the other hand, a zero-bandgap semiconductor, has some unique and complementary properties to conventional semiconductors that is believed to revolutionize future devices [2]. Apart from being an excellent electrical and thermal conductor, graphene is also transparent and flexible and thus has the potential to become an ideal electrode material for especially optoelectronic devices [3, 4]. Thereby, if these complementary materials classes can be combined, various unique hybrid systems can be envisioned. In this work, we show that by utilizing the reduced contact area of NWs, epitaxial growth of semiconductors on graphene can be achieved. Highly uniform vertical GaAs NWs were grown both on graphite and few-layer graphene using molecular beam epitaxy. Scanning electron microscopy and cross-sectional transmission electron microscopy studies revealed the epitaxial relationship of the NWs with the graphitic substrates in spite of a lattice mismatch of 6.3%. In addition, we present a generic atomic model which describes the epitaxial growth configurations of the semiconductor atoms on graphene and should in principle be applicable to all conventional semiconductor materials. Finally, a prototype of a single GaAs nanowire photodetector was also fabricated which demonstrates a high-quality material essential for successful optoelectronic device applications. References: 1. Yang, P.; Yan, R.; Fardy, M., Nano Lett. 2010, 10, 1529-1536. 2. Geim, A. K.; Novoselov, K. S., Nature Mater. 2007, 6, 183-191. 3. Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C., Nature Photon. 2010, 4, 611-622. 4. Chung, K.; Lee, C.-H.; Yi, G.-C., Science 2010, 330, 655-657.

One dimensional nanostructures have potential applications in photovoltaics owing to their excellent axial conductivity and directed charge transport. One of the particular advantages of NWs and CNTs is that they can be readily processed into thin films consisting of interconnected networks with tailored optical transparency and electrical conductivity, as well as good mechanical flexibility. These merits have been explored extensively in devices such as field effect transistors, sensors, flexible transparent electrodes, super capacitors. Despite those promises, there remain challenges in employing nanowire networks as active layer for light absorption in solar cells. High surface area of nanowires provides more recombination centers which limits the diffusion lengths of minority carriers. The resistance of the nanowire network (although continuous) could be large due to high contact resistance between nanowires. Bulk heterojunction is an effective strategy to enhance charge separation and carrier transport in solar cells, and has been adopted in polymeric and colloidal nanoparticle solar cells to improve energy conversion efficiency. Here, we propose a bulk heterojunction structure based on solution synthesized CdS nanowires and single walled canbon nanotubes (SWNTs). The CdS NWs and SWNTs are mixed in solution and filtered to form a homogeneous composite thin film. Within the composite, CdS nanowires (CdS NWs) and carbon nanotubes (CNTs) form mutually interpenetrating networks through the simple solution filtration process. The CNT network boosts charge separation by extracting holes generated from CdS NWs and also forms the transport path for carrier collection to external electrode. At an optimized CNT loading of about 5 wt.%, the CdS NWs/CNTs bulk heterojunction solar cells show 3 orders of magnitude increase in photocurrent and cell efficiency comparing to that of the same materials arranged in a plain heterojunction configuration. The CdS NWs/SWNTs bulk heterojunction solar cells showed efficiencies up to 0.33% under AM 1.5, 100 mW/cm2 illumination. External quantum efficiency and photoluminescence studies reveal efficient charge transfer process from photo excited CdS NWs to CNTs in mixed form. This demonstration shows bulk heterojunction is a useful architecture for nanowires network based solar cell. The present nanowire/nanotube network based solar cell is facile in fabrication with low cost. The solution process is compatible with well developed ink-jet technique for scale-up production. With future optimization on heterojunction material combination and device design, the nanowire network solar cells, which are flexible and bendable, will show great potential applications as energy supplies for paper electronics and electronics on fabrics.

Nanotechnology and the different methods for the preparation of nanostructures are in continuous evolution, easy and cheap growth techniques for the production of nanostruc-tures in a variety of morphologies are constantly proposed by the research community. Key features are the capability to control the composition, the particle shape and size dis-tribution since in chemical sensing and many other applications these nanostructures ex-ploit properties related to crystallographic features. In 2002, the field of metal oxide nanowires underwent a significant expansion and became one of the most active research areas in nanoscience. Stimulating advances have been made at an extraordinarily fast rate in different laboratories all over the world following cu-riosity, discovery or hypothesis driven research. Nowadays it is almost a decade from the first presentation of metal oxide nanowires as chemical sensors. Significant advances have been made both in terms of preparation procedures and their integration into func-tional sensing devices, while progress in fundamental understanding of their functional properties is slow-moving. In fact, the full integration still remains a challenge that has been wisely approached in different ways. The most recent developments in bottom up and top down approaches for chemical sens-ing application will be reviewed and the recent achievements obtained at SENSOR labora-tory will be presented. N-type metal oxide such as tin, titanium, niobium and zinc oxide and p-type metal oxide such as copper oxide were prepared using different techniques starting from metal/ metal oxide powders and films and their chemical sensing properties were studied in different operating conditions. The prepared materials resulted sensitive to gas-eous polluting species like CO and NO2, as well as to alcohols. Furthermore, NW-based chemical sensors have been tested in different fields, such as the detection of Chemical Warfare Agents, where high sensitivity is mandatory. Acknowledgements The research leading to these results has received funding from the European Community&’s 7th Framework Programme, under the grant agreement n° 247768, and from the Russian Federation Government, under the State Contract 02.527.11.0008, within the collaborative Europe-Russia S3 project.

Metal oxide nanowires are a broad class of materials which are gaining a growing interest due to their potential in gas sensing, optoelectronics and energy applications. Although preliminary studies revealed promising outcomes, further research is necessary in order to reach complete control on their properties. Working into the direction of nanowire-based systems, the integration of active characteristics in a single nanostructure by combinatorial materials architectures is a promising approach to develop diodes, transistors or any other active element equivalent to those employed in standard microelectronics. Specifically, radial p-n nanowire heterojunction devices represent a favorable geometry to maximize the interfacial area and carrier separation due to the built-in field established across the junction. In this contribution, we present the synthesis, fabrication and functional characterization of a heterojunction device based on a single coaxial n-ZnO/p-Si nanowire that was integrated in a circuit by FIB nanolithography to study the electrical properties. The photovoltaic and gas sensing performances of these single p-n heterojunctions were preliminary assessed, showing for the first time that the sensing response could be modulated by changing the bias current through the device, showing a complementary functionality of these nanoarchitectured devices; with the maximum gas sensing response obtained at low reverse values. This working principle could be applied to many other one-dimensional p-n junction metal oxide nanostructures and used to tailor their gas sensor responses.

Nanowires (NWs), and especially ZnO NWs have been attracting great attention due to their amazing properties for electronic and optical applications as well as the broad spectrum of sensing applications. The detection of a number of different physical phenomena was reported such as field effect transistors in various configurations, chemical and biological sensors, gas sensors, pH sensors (i.e. for intracellular detection), photo detectors or temperature sensors. The integration of NW based devices with standard microsystems technology (MEMS) processes capable for mass production of various sensor platforms is still a lacking point and motivated us to conduct an extended study exploring the detection of various physical phenomena based on a multifunctional NW device. We will demonstrate a simple fabrication route to integrate ZnO nanowires with microsystem structures and their implementation as pH sensors, temperature sensors, and photo detectors [1]. The here presented multifunctional ZnO multi-nanowire sensor platform contains an Au finger structure realized by photolithography on SiO2 substrate. The ZnO NWs are grown directly on the metal contacts using thermal vapor-solid chemical vapor deposition (VS CVD) resulting in a metal-semiconductor-metal (MSM) structure. The obtained platform is versatile to detect physical signals both in air and fluidic environments by just small modifications to the device. In order to detect the respective physical signals, changes in electrical signals were measured (conductance and current). The current measured in the range between 90 K and 380 K under vacuum conditions exhibit a tunneling behavior between spaced and overlapped ZnO nanowires and is used for the temperature sensing. The results suggest operating the sensor device both at low and high temperatures. For photo sensing the current response between the “on” and “off” states of light were measured when exposed to different wavelengths ranging from UV to visible light. Near band edge (NBE) response for UV and the broad emission peak for visible light were used for the detection of various wavelengths. It will be demonstrated that even high quality ZnO NWs with a strongly reduced green luminescence can be used for a photo response in the visible. Finally, the integration into a microfluidic chamber is realized for chemical sensing devices. We will show pH sensing while ZnO NWs were protected from chemical attacks by a thin layer of a C4F8 plasma based coating. [1] A. Menzel, K. Subannajui, F. Güder, D. Moser, O. Paul, M. Zacharias, Adv. Funct. Materials 2011, 21, 4342-4348.

Nanowires have been a topic of intense research in the last years due to their possibility to be part of new devices and circuit architectures thanks to their performance derived from their reduced size and well-controlled chemical and physical properties [1]. Several research papers dealing with the optimization of growth conditions and with their structural and chemical characterisation have been published in literature, but few report on the manipulation and contact fabrication to the individual nanowires to allow the complete electrical characterisation and the implementation of functional devices. Here we will present our most recent activity on the fabrication of contacts by Focused Electron Beam Induced Deposition (FEBID) of metallic materials onto individual metal-oxide nanowires in combination with microsystem-fabricated microhotplates for the production and test of prototypes of advanced low power consumption gas nanosensors. The basic material employed in this work has been defect-free monocrystalline tin dioxide nanowires, with radii in the range from few tens to few hundred nm, were synthesized by chemical vapor deposition (CVD), using a molecular precursor, published in detail elsewhere [2]. The principle of operation of the gas sensors based on metal oxides is that of a chemiresistor, i.e., a resistor whose value changes in the presence of gas. This phenomenon, which has been reported more than 40 years ago, has been used for the fabrication of commercial devices for quite some time, especially when the materials are in micro- and nanoparticle forms [3]. Because the phenomenon takes place at the surface of the material, nanomaterials, which present a high surface-to-volume ratio, are expected to present an enhanced effect and, thus, an enhanced sensing behaviour. The fabrication strategy and electrical properties of these device prototypes, as well as their gas sensing behaviour towards gases of interest in environmental applications will be presented and discussed. Hints for the development of mass-production methods that exploit the benefits of the here-presented prototypes will be given. References: [1] S. Barth, F. Hernandez-Ramirez, J.D. Holmes and A. Romano-Rodriguez, Prog. Mater. Sci. 55 (2010) 563. [2] S. Mathur, S. Barth, H. Shen, J. C. Pyun and U. Werner, Small 1 (2005) 713. [3] P.T. Moseley and B.C. Tofield, Solid State Gas Sensors, Adam Hilger, Bristol, 1987.

We report on a novel all-optical approach for chemical sensors for application in gaseous and liquid environment based on the photoluminescence (PL) response of group III-nitride (III-N) nanowire (NW) ensembles. We assess the PL response of III-N NWs to oxidizing and reducing gases and show that the results can be explained in terms of changes in the non-radiative surface recombination rate. The photoelectrochemical properties of (Al,In)GaN NWs in contact with electrolyte solutions are determined by the position of the band edges with respect to the redox levels of the electrolyte. We will show that bias-control of charge transfer to the electrolyte gives rise to a pronounced pH sensitivity of the PL intensity of n-type III-N NWs that can be used in optochemical sensors. We will also demonstrate the effect of Si- and Mg-doping both on the pH-sensitivity and the photoelectrochemical stability of III-N NWs as well as the realization of stable pH-sensitive devices with optical readout for application in acidic and basic regime. The immobilization of biomolecules allows the realization of novel biochemical sensors based on III-N nanophotonic probes.

A novel sensing concept was developed; namely solar diode sensor (SDS), based on the integration and correlation of complementary functionalities originating from multiple junctions in a singular nanostructure to palliate the current issues in gas sensor technologies such as low power consumption, low operating temperature and cost effective production. In this work, the gas sensing and solar energy harvesting abilities of metal oxide semiconductors were utilized to deliver a self-sustained gas sensing signal without any external power sources. The proof of concept was demonstrated in our previous work. The generality of the concept was demonstrated by extending the new sensing approach to other nanomaterial based systems such as thin-film based heterojunctions and core-shell radial nanowire heterojunctions. The electrical properties of obtained systems were evaluated by measuring current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The sensing properties were tested under solar simulated light, without using any other energy sources. The fabricated sensors were capable of detecting oxidising and reducing gases with reproducible response at room temperature by solely using solar illumination. The core-shell radial heterojunction showed better sensitivities toward oxidising and reducing gases as compared to their thin-film heterojunction counterparts. These results showed the impact of the material design on the sensor performance making these gas sensors promising candidates for the development of a new generation of commercially appealing, technically feasible and easy to use self-powered gas sensor nanodevices.

Magnetite (Fe₃O₄ ) exhibits distinct electrical and magnetic properties due to its strong electronic correlation. It was in 1939 when Verwey discovered that magnetite undergoes a first order phase transition at a temperature of T_V = 117 K. Ever since then Fe₃O₄ and in particular the so-called Verwey transition has been an object of extensive research and great controversy. Nanowire Field-effect transistor (FET) measurements provide new information about Fe₃O₄ and lead to a better understanding of the transition mechanisms. Fe₃O₄ nanowires with diameters from 100 to 300 nm were grown in a cold-wall CVD reactor on an alumina substrate seeded by Gold (Au) particles, which have been formed by annealing a presputtered Gold film. Iron(III)-tert-butoxide was thermally decomposed at 750-850 °C to germinate the growth. The nanowires were directly transferred from the growth substrate onto a highly doped silicon chip with a 200 nm toplayer of SiO₂. Electrical contacts were defined by using standard laser beam lithography. In this work we present resistance measurements on single crystalline Fe₃O₄ nanowires as a function of gate voltage. Without applied gate voltage the nanowire&’s resistance increases monotonically as temperature is decreased from 200 to 50 K. At 108 K the resistance shows a sharp increase by two orders of magnitudes. Below the Verwey Temperature T_V we observe that high electric fields trigger the break down of the insulating phase into a high conductive stage. We propose to attribute this resistive switching to a re-ordering of the ionic crystal under electrical fields.

One of the greatest challenges of nanotechnology is high throughput assembly of nanostructures into large scale functional devices. Nanowires are distinguished among all nanoscale building blocks regarding their unprecedented physical and material properties.¹ However, despite considerable efforts towards large scale utilization of nanowires, devices produced at industrial scale are still far from being realized. While it is not plausible to propose a generic assembly strategy due to the diversity of the methods for obtaining nanowires, self assembly or top-down device architecture are two broad categories, impeded by physical limitations of nanoscale manipulation or material composition, respectively. Nevertheless, progress in functional nanowire assembly is closely related to progress in nanowire fabrication methods. Recently, we have demonstrated a novel top to bottom fabrication approach enabling production of nanostructures with extraordinary characteristics. By iterative thermal drawing of macroscopic composite preforms, it becomes possible to produce thousands of uniform and ordered nanowires with indefinite lengths, embedded inside a protecting polymer fiber.² The technique could be used with a diverse material set in order to obtain nanostructures having different geometries, compositions and tunable characteristics.³ Here, we devised a large area nanowire photodetection device by manual alignment of fibers containing semiconducting selenium nanowires, which are produced by this fabrication technique.⁴ The intrinsic order of these nanowires was preserved throughout the assembly process. Polymer encapsulation around the nanowires was removed by an organic solvent in a controlled manner, while the centimeter long nanowires remained intact and aligned over the predefined electrical contacts, which constitute a circuitry within an area of 1 cm². The final device having 10X10 photoconductive pixels is able to identify alphabetic characters. The assembly strategy presented here is applicable in order to devise extremely large nanowire devices on planar, curved or flexible substrates with diverse functionalities such as phase change memory, solar cell or artificial skin. ¹ Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32 (2), 99-108. ² Yaman, M.; Khudiyev, T.; Ozgur, E.; Kanik, M.; Aktas, O.; Ozgur, E. O.; Deniz, H.; Korkut, E.; Bayindir, M. Nat. Mater. 2011, 10, 494-501. ³ Khudiyev, T.; Ozgur, E.; Yaman, M.; Bayindir, M. Nano Lett. 2011, 11, 4661-4665. ⁴ Ozgur, E.; Aktas, O.; Kanik, M.; Yaman, M.; Bayindir, M. Nano Lett. 2012, 12, 2483-2487.

Real-Time Investigation of NanoFET Current Surge Capability During Heavy Ion Irradiation The real-time I-V characteristics of a gallium nitride nanowire-based nanoFET were investigated at 5-minute intervals during 30 minutes of continuous irradiation by Xenon-124 relativistic heavy ions. The nanoFET maintained compliance set at one micro-Amp for the duration of the experiment. Additionally a current surge was observed immediately upon exposure to the heavy ion beam. Similar current surges immediately upon Xenon-124 beam exposure have been reported for conventional planar silicon carbide Schottky barrier transistors and are attributed to immediate disturbance of the electric field near the contact interfaces with resulting increase in tunneling. In the case of the conventional Schottky barrier transistor, the current surge resulted in significantly increased leakage current and device failure. In the case of the nanowire-based nanoFET, the current surge resulted in improved device performance. An analysis of the observed behaviors in terms of possible influences of the Xenon-124 on the contacts, the oxide layer and within the nanowire is presented. The pre-characterized gallium nitride nanowires were shown to have highly crystalline regions extending the full nanowire length that supported large current densities. Radiative situations, such as those encountered in space, collider and nuclear reactors, are extreme environments in which capability beyond current state of the art silicon electronics is needed, and for which nanowire-based nanocircuits may offer improvements. Present affiliation BW Jacobs: Protochips, Inc., Raleigh, NC 27606, USA

The dissipative absorption of electromagnetic energy by one-dimensional nanoscale structures at optical frequencies is applicable to several important phenomena including biomedical photothermal theranostics, nanoscale photovoltaic materials, atmospheric aerosols, and integrated photonic devices. Closed-form analytical calculations are presented for the absolute temperature rise within infinite circular cylinders with nanometer-scale diameters (nanowires) that are irradiated at right angles by a continuous-wave laser source polarized along the nanowire&’s axis. Solutions for the heat source are compared with both numerical finite-difference time domain (FDTD) simulations and well-known Mie scattering cross sections for infinite cylinders. The analysis predicts that the maximum temperature increase is affected not only by the cylinder's composition and porosity, but also by morphology-dependent-resonances (MDRs) that lead to significant spikes in the local temperature at particular diameters. Furthermore, nanowires with high thermal conductivities are observed to exhibit extremely uniform internal temperatures during electromagnetic heating, including cases where there are substantial fluctuations of the internal electric-field source-term that generates the Joule-heat. Although the internal heat source can be highly non-uniform, a photophoretic force cannot be expected to occur when the temperature is uniform which has significant implications for the dynamics of nanoscale atmospheric aerosols.

Morphology of catalytically grown 1-D nanostructures is often controlled by the crystallographic orientation of substrate. Like in the case of thin film growth, the wire can be grown epitaxially. When the substrate&’s orientation is chosen properly the epitaxial relation will force the wire to grow at a specific angle to the surface normal. In the GaN system often the choice of substrate will also affect the growth axis of the wire. The most common growth axis are the <001>, <110> and <1-10> directions. The <001> nanowires are hexagonal with 6 equivalent nonpolar faces. On the other hand the <110> and <1-10> nanowires are triangular each with one (001) surface and two (112) or (1-11) surfaces respectively. This enables us to study optical properties of GaN surfaces. Correlating photo and cathodoluminescence data with structure defined by transmission electron microscopy from individual GaN nanowires allow us to deduce information about the optical and electronic structure of the various GaN surfaces

Currently, a lot of effort is put into the investigation of the growth of semi-polar and non-polar heterostructures in GaN for optoelectronic devices. In comparison with polar GaN the electric field present due to polarization can be reduced or even eliminated for these structures, with an expected increase of the radiative transition probability. However the growth of non-polar planar heterostructures turns out to be quite challenging. GaN based nanocolumns might represent an alternative for the realization of high-quality non-polar heterostructures for photonic devices, as already shown for GaN-based nanocolumns grown along the polar direction [1]. In this study, we report on the growth of GaN/AlN quantum wells (QWs) with different polarity at the pyramidal or flat tips of GaN nanocolumns (NCs) by molecular beam epitaxy (MBE). Non-catalytic selective area growth (SAG) was achieved on Mo pre-patterned GaN(0001) MOCVD templates on sapphire as well as on 6H-SiC(0001) substrate. GaN nanocolumns with pyramidal tips showing semi-polar {1-10n} (n = 1.5 - 2) facets were grown onto GaN(0001), whereas pyramids with semi-polar {11-2n} (n = 2.5 - 4) facets are obtained for 6H-SiC(0001) substrates. Under certain growth conditions nanocolumns with c-plane tips (polar) grow onto unpatterned surface areas due to a self-organized growth mechanism. This allows the comparison of the structural and optical properties of semi-polar and polar GaN/AlN quantum wells under nominally identical growth conditions. For optimization of the heterostructures, samples containing multi quantum wells (MQWs) have been grown to get an insight into the growth rates along the semi-polar as well as the polar directions. Position and size control of the nanocolumns is achieved by patterningarrays of apertures into a thin Mo layer by electron beam lithography. The mechanism of selective area growth is studied by statistical analysis of scanning electron microscopy (SEM) images. Using a wide range of diameters and pitches the influence of these parameters on the axial growth rate of the nanocolumns is investigated and a contribution from the Ga diffusion on the Mo mask is put in evidence. The structural properties of the nanocolumns and the quantum wells are characterized by means of scanning and transmission electron microscopy (TEM). An increased density of basal plane stacking faults is observed at the pyramidal region in comparison with the rest of the nanocolumns. The polarity of the nanocolumns is studied by convergent beam electron diffraction (CBED). GaN nanocolumns with pyramidal tips were characterized by Raman spectroscopy. The Raman shift of the E_2h mode reveals strain free GaN nanocolumns on a slightly compressed GaN MOCVD template. Photoluminescence properties of the semi-polar and polar GaN/AlN QWs will be presented and discussed. [1] H. Sekiguchi, K. Kishino, and A. Kikuchi, Appl. Phys. Lett. 96, 231104 (2010)

One-dimensional conducting polymer nanofibers provide significant promise for miniaturized electronic devices. The inherent properties of polymeric nanofibers are particularly suited for flexible electronics because of their mechanical flexibility, reliability, and potential for high density arrays. Although various methods have been investigated for fabrication of polymeric nanofibers, they have not been appropriate methode to align the polymer nanofibers at the desired position and orientation while controlling the number of wires precisely. We realized highly aligned semiconducting polymer nanofibers by using an electrohydrodynamic (EHD) nozzle printing method which can control the position, orientation, and dimension of polymer nanofibers. In addition to these advantages, we can control the number of nanofibers which have the uniform electrical properties. To investigate the properies of printed polymer nanofibers, we fabricated poly(3-hexylthiopehene) (P3HT) nanofiber transistors which showed a high mobility of 0.0145 cm2/Vs. The performance of P3HT nanofiber transistors are improved as compare with same composition film transistors. To achieve good electrical contact properties between organic semiconductor nanofiber and electrodes, we employed the conducting polymer as the source-drain electrodes which can be formed by using selective wetting. We reduced the contact resistance of bottom-contact field-effect transistors by single spin-coating of conducting polymer poly (3,4-ethylene-dioxythiophene)/ poly(styrene sulfonate) (PEDOT:PSS) compositions with a perfluorinated ionomer (PFI). In addition, we fabricated flexible polymer nanofiber transistors on PET substrates and device performance turned out to be very stable. With our nanofibers and polymeric electrodes patterning techniques, we eventually achieved remarkably improved performance of polymer nanofiber transistors compared with conventional Au electrodes as well as conventional conducting polymers (PEDOT:PSS).

In this work, systematic compositional study of ternary InGaAs nanowires (NWs) synthesized by metalorganic chemical vapor deposition (MOCVD) using a pulsed laser local electrode atom probe LEAP (Cameca 3000X Si) is carried out. The three dimensional atom probe tomography (APT) elemental mappings not only image individual atoms of the InGaAs NWs, but also provide precise radial chemical information of the NWs. We show clear evidence that there is a core/shell structure of the ternary InGaAs NWs, and the core growth is attributed to the vapor-liquid-solid (VLS) mechanism through Au catalyst while shell growth is attributed to vapor-solid (VS) mechanism through sidewall facets. Semiconductor NWs have shown their remarkable electronic, photonic, mechanical and thermal properties, and some NWs based optoelectronics devices, such as light-emitting diodes [1], lasers [2], solar cells [3], photodetectors [4], biosensors [5], and transistors [6] have been demonstrated. The most common growth technique for group III-V semiconductor NWs is MOCVD via a VLS mechanism [7]. This approach offers a means to precisely control the NWs composition and morphology to create axial and radial core/shell heterostructures and superlattices [8-10]. The versatility of the VLS process to synthesis NWs provides an approach to engineer semiconductor NWs with novel functionality. Two types of InGaAs NWs grown under a same condition have been found on the same substrate. The first type is Ga rich InGaAs NWs grown using Au catalysts with pure zincblende phase. A core/shell structure has also been found in this type NWs with a Ga rich core and In rich shell. {112} sidewall facets have been observed Ga rich while {532} sidewall facets have been found In rich. The In/Ga ratio of the core remains the same (0.13) along the axial direction while the In/Ga ratio of the shell gradually increases from 0.75 to 1 in 2µm length from the top towards the bottom of the NW. The In/Ga ratio of the In rich facets remains the same (1.7) while the In/Ga ratio of the Ga rich facets increases from 0.59 to 0.83 in 2 µm length along the axial direction. The second type is In rich InGaAs NWs grown through In self catalysts with pure wurtzite phase. The total In/Ga ratio 4.5 has been determined in 1.3µm length. Detail analysis of this type NWs is undergoing. The APT results of InGaAs NWs clearly illustrate that there is a core/shell structure of the ternary NWs. We infer that the VLS growth through Au catalyst leads to the core growth while the VS growth through sidewall facets results in the shell growth. We will continue this investigation to determine accurate composition values at the atomic resolution, and compare the structure and composition of the two type NWs synthesized under the same condition but different growth mechanisms to direct the growth of high quality semiconductor NWs.

The improvement of sensors towards higher sensitivity, better selectivity and long term stability has become a highly active field both in basic and applied research. In particular, humidity sensing is receiving significant attention in diverse areas of industrial processing, environmental control and medical uses. Nb2O5 is a promising ceramic material for sensing applications that exhibits good response to moisture even at room temperature. At low temperatures, water molecules condense on the NbOx semiconductor surface, inducing proton conduction and a change in the electrical conductivity related to the surface covering. Thus, morphology of sensing materials based on ionic conduction strongly influences the humidity detection and response. In general, materials sensing on ionic conduction mechanisms promise higher sensitivity to water vapour than sensors based on electronic conduction mechanisms. Also, the reduction in operating temperature allows the simultaneous detection of water besides other gaseous species reducing the cross-sensitivity effects. In this work, a detailed TEM structural characterization of single crystalline Nb2O5 nanorods and Nb2O5@SnO2 heterostructures synthesized by the chemical vapour deposition method is presented. Morphology and structure are correlated to the observed sensing properties. For pure Nb2O5 nanorods, with thickness in the 30-50 nm range, the presence of contrast fringes along their growth direction, and the streaking patterns in the corresponding power spectra, could be misunderstood as a high density of twin defects. Indexation of the power spectra was carried out using two different space groups for the monoclinic Nb2O5. Computer simulation of the HRTEM images was carried out in order to obtain a better understanding of the structure of the nanorods, including the defects expected from the power spectra. Computer models were generated for both space groups C2 and C2/m and HRTEM images of these were simulated for different defocus in order to systematically compare them with the experimental images. Simulation supported the structural characterization in the C2/m space group, with the defect-free models also presenting changes in contrast without any interface involved. This points out that, although the presence of defects is still clear, the nanorods present a low defect density, which is supported by the observed high resistivity of NbOx nanostructures. In order to optimize the sensing behavior, architectured Nb2O5@SnO2 heterostructures were designed to use the good sensing properties of Nb2O5 and the good charge transport of SnO2. In these systems, interfaces were studied by means of HRTEM. A variety of defects due to the lattice mismatch was detected. Strain propagation from dislocations in the interfaces was quantitatively measured by Geometric Phase Analysis (GPA). The existence of a characteristic length for the C2 to C2/m structural transition was pointed out.

Nanowires (NWs) are promising materials for future electronics, optics and sensor device applications. Typically, ZnO NWs are grown in a tube furnace which includes that the respective material necessary for the desired NW materials is evaporated and transported toward a substrate. Hence evaporation efficiency, diffusion and gas flow are mandatory for a successful growth. When the transported species approach the substrate nucleation, diffusion and growths occurs. A huge amount of work reports about growth of NWs by thermal CVD, however, the role and influence of growth parameters still remains unclear. Different parameters, different geometries (diameters, inner and outer tube configurations etc.) as well as growth parameters (temperature, carrier gas and pressure) are reported, but cannot really compared. The consequence is an insufficient understanding and a limited control. We systematically studied and simulated the distribution of the growth species in such tube processes and evaluate their effects on the NW growth. Numerical simulations will be presented to get a deeper insight how the respective growth species behaves in the growth reactor. The time evolution of the species distribution is analyzed at different carrier gas pressures and flow rates. We thereby demonstrate that diffusion and convection can be tuned in a controlled way, thus, a balance can be achieved to achieve steady growth conditions. To support our calculations we compare our results with systematic growth experiments for Si and ZnO NWs, which demonstrate the good agreement between simulation and experiment [1]. The substrate growth position with respect to the species source both in upstream and downstream direction can be tuned over a large area [2]. With the here presented method we offer new insight into the appropriate design of nanowire growth experiments which can be used to achieve a better control over the NW growth process. The data offer for the first time a systematic and deep understanding into the growth process which can be transferred to other growth systems. [1] A. Menzel, R. Goldberg, G. Burshtein, V. Lumelsky, K. Subannajui, M. Zacharias, Y. Lifshitz, J. Phys. Chem. C 2012, 116, 5524-5530. [2] K. Subannajui, N. Ramgir, R. Grimm, R. Michiels, Y. Yang, S. Müller, M. Zacharias, Crystal Growth & Design 2010, 10, 1585-1589

Here we present a method for fabricating high-density, well-ordered arrays of extremely high-aspect-ratio nanowires through thermal drawing of multi-material fibers. We investigate the lower limit of nanowire diameters stably produced by the process of thermal fiber drawing and fiber tapering. A centimeter-scale macroscopic cylindrical preform containing the nanowire material in the core encased in a polymer scaffold cladding is thermally drawn in the viscous state to a fiber. This fiber is used as the core of another preform which is drawn into a fiber whose core is now further reduced in diameter. By cascading several iterations of the process, continuous reduction of the diameter of an amorphous semiconducting chalcogenide glass is demonstrated. Starting from a 10-mm-diameter rod, we thermally draw hundreds of meters of continuous sub-5-nm-diameter nanowires. The high density and well-ordered nature of the nanowires in a single fiber is a consequence of the stacking process which is carried out at a macroscopic scale prior to fiber drawing. The initial fiber is drawn with a single, large diameter As2Se3 glass core and a polyethersulfone (PES) cladding. These fibers are then stacked in a precise fashion into a subsequent preform and drawn, resulting in a fiber with multiple cores whose transverse ordering is maintained during the thermal drawing process. By repeating this process, we have fabricated a 1 mm diameter fiber with ~ 3x10⁵ continuous, well-ordered nanowires each with a diameter of 20 nm. This density may be improved by increasing the fill factor, which at 25% would result in 10⁸ 50-nm diameter nanowires in a single 1-mm diameter fiber. By further tapering these fibers, we have shown a reduction of the nanowire diameters to below 5 nm. Because fiber drawing results in a continuous cross sectional geometry throughout the entire length, the resulting nanowires can be potentially kilometers in length, resulting in an unprecedented aspect ratio in nanowire fabrication.