Symposium NM03 : Nanowires and Related 1D Nanostructures—New Opportunities and Grand Challenges

Nanowire photodetectors attract broad interest due to their low dimensionality, small electrical cross-section, and ultrahigh photocurrent gain. In the ultraviolet region, ZnO and GaN nanowires have been intensively studied as spectrally-selective photodetectors. For this application, III-nitride nanowires present advantages in terms of heterostructuring possibilities and stability against chemical, mechanical or electrical stress. In a single-GaN-nanowire UV photodetector, the efficiency can be enhanced by the insertion of a GaN/AlN heterostructure [1,2], which leads to an increase of the responsivity by about two orders of magnitude, improved linearity, and the possibility to select the detected wavelength, while maintaining a UV/visible contrast larger than six orders of magnitude. Furthermore, devices with a linear photoresponse to the optical power can be implemented by using nanowires with a thickness below a certain threshold [3]. On the other hand, the insertion of quantum dots in nanowires is also interesting for infrared photodetection using intraband transitions. Therefore, intraband transitions in GaN/AlN nanowire heterostructures have been investigated, varying the geometry and doping level of the GaN insertions [4]. Based on this research, we present the first single-nanowire quantum well infrared photodetector (NW-QWIP), operating at the 1.55 µm telecom band [5]. Finally, the study has been extended to cover the mid-infrared spectral range, up to around 6 µm, using intraband transitions in GaN/Al_0.4Ga_0.6N dots-in-a-wire.
[1] J. Lähnemann et al., Nano Lett. 16, 3260 (2016)
[2] M. Spies et al., Nano Lett. 17, 4231 (2017).
[3] M. Spies et al., Nanotechnology 29, 255204 (2018).
[4] A. Ajay et al., Nanotechnology 28, 405204 (2017).
[5] J. Lähnemann et. al., Nano Lett. 17, 6954 (2017).

Semiconductor nanowires with reduced diameters enable high-performance chemical sensors and photodetectors owing to their large surface-to-volume ratios and enhanced surface band bending. Synthesis of nanowires and their device integration by CMOS (complementary metal-oxide-semiconductor)-compatible processes however remain a formidable challenge. Here we report fully CMOS-compatible synthesis and ultraviolet (UV)-photodetector-integration of ultrathin (~30 nm diameter), perfectly parallel-aligned, polycrystalline ZnO nanowire arrays by using infiltration synthesis, a new type of material hybridization technique derived from atomic layer deposition (ALD), where vapor-phase organometallic precursors are infiltrated into polymer templates, forming inorganic-infiltrated hybrid nanocomposites that can be directly converted into monolithic inorganic nanostructures inheriting the positional registry and morphological features of starting polymer templates by ashing the polymer matrix. Specifically, the ultrathin ZnO nanowire array is generated by infiltrating diethylzinc and water vapors into lithographically patterned polymer nanowire template made of a negative-tone photoresist SU-8. The integrated ZnO nanowire array photodetectors feature ultralow dark currents <20 fA invariant with the number of nanowires, over 6-decade photocurrent on-off ratios leading to >120 dB dynamic range, and unusual superlinear photoconductive responses, enabling increasing photodetector performance parameters for a higher incident light power. Considering the temperature-dependent field-effect transistor characteristics of the ZnO nanowire arrays, the observed superlinear photoconductivity can be explained by a new type of photoelectrochemical thermionic charge emission mechanism involving the reaction of chemisorbed oxygen and photo-generated charge carriers at grain boundaries. The demonstrated ultrathin nanowire synthesis and device fabrication methods have potentials for fully CMOS-compatible integration of nanowire sensor devices and circuitries. The identified photoelectrochemical grain boundary thermionic emission mechanism provides an improved understanding on the superlinear photoconductivity observed in nanostructured materials.

Plasmonic Au-ZnO nanostructures with a size less than the incident light wavelength have been found to exhibit a localized surface plasmon resonance (LSPR) that may lead to strong absorption, scattering, and local field enhancement.¹ These resonances, associated with noble metal nanostructures create sharp spectral absorption and scattering peaks as well as strong electromagnetic near-field enhancements.² However, operation of ZnO gas sensors is limited to elevated temperature, which leads to enhanced energy consumption and large sensor size. Thus, reducing operating temperature or room temperature gas detection become significant in future sensor development. In this work, by utilizing the wavelength tunable photo-irradiation, selective gas detection has been demonstrated at room temperature based on Au/ZnO nanowire arrays. The Au/ZnO nanowires were synthesized by the microwave-assisted hydrothermal deposition of ZnO nanowires followed by Au nanoparticle (NP) dip-coating process. Compared to pristine ZnO, the Au-ZnO nanowire sensor performance was enhanced in both UV and visible regions, especially with highly enhanced sensitivity observed at 550 nm. The sensitivity towards 20 ppm NO2 could reach as high as 250%, and the detection limit is determined to be around 1 ppm at 25 °C. The sensitivity enhancement resulted from UV is due to the migration of photo-generated electrons from Au NPs to ZnO. On the hand, the sensing mechanism in the visible region is primarily due to the LSPR effect of Au. The oscillated electrons become more sensitive to the charge density and dielectric environment of Au. Besides, a large selectivity was found for NO2 gas over CO, NH3 and O2 at 330 nm UV irradiation. The ratio of cross sensitivity towards target gas and interfering gases is larger than 300. It is clear that with tunable light irradiation, room temperature NO2 gas detection could be achieved using plasmonic Au/ZnO nanowires with high sensitivity and selectivity.
Reference
1. Gogurla, Narendar, et al. "Multifunctional Au-ZnO plasmonic nanostructures for enhanced UV photodetector and room temperature NO sensing devices." Scientific reports 4 (2014): 6483.
2. Mayer, K. M. & Hafner, J. H. Localized surface plasmon resonance sensors. Chemical reviews 111 (2011): 3828-3857.

Zinc Oxide is a nontoxic material, with a wide direct band gap (3.4 eV), high exciton binding energy (60 meV) and good thermal stability, making it suitable for UV LEDs, photo catalysts, UV sensors and gas sensors. At the surface of zinc oxide, a layer of positive space charge is usually formed to cancel out the charge of surface oxygen species. Removal and addition of said surface oxygen species may alter the electrical properties at the surface. Utilizing this mechanism, applications such as UV and gas sensors were studied by others. Here, we explored the various factors that may affect the UV absorbing and sensing properties of ZnO 1-D nanostructured devices, including crystallinity, contact junctions, and morphology. Regarding morphology, 1-D nanostructures have high surface area ratios and can exhibit surface properties in larger scales, as the structures can be viewed as wrapping surfaces around lines.

In this study, 1-D nanostructured ZnO UV sensors were developed on silicon dioxide using an atomic layer deposition (ALD) seed and chemical vapor deposition (CVD) nanostructure growth to investigate nanostructure properties. An ALD process was selected to deposit the seed layer due to the capability of uniform thickness growth and excellent crystallinity. The CVD process with vapor-solid (VS) growth provided a clean method to grow zinc oxide nanostructures without introducing unnecessary additives, by using a source of zinc metal powder and high purity oxygen. Compared to wet chemical methods and vapor-liquid-solid (VLS) growth of ZnO nanostructures, the VS processes would not have salts and metal particles, which may affect devices in undesired ways, such as salt interactions with humidity, Ohmic or Schottky contacts by metal particles with ZnO and stability issues of metal catalysts.

Different 1-D nanostructures such as crossing nanowires and free-standing nanorods were deposited through the control of seed layer quality, zinc source temperature and substrate undercooling. Characterization of ZnO structures was done by SEM, X-ray reflection (XRR) and XRD techniques for morphology, thickness and crystallinity; UV-VIS, Photoluminescence spectroscopy and a Keithley 2400 instrument were used for the absorption, emission spectrums and UV irradiated conductivity change. The ALD seed ZnO layer showed highly preferred (002) plane crystal growth by a strong XRD peak with d-spacing approximately 2.6 angstroms, while 1-D nanostructure-grown samples exhibited similar or slightly lower (002) preference, indicated by minor ZnO (100) and (101) peaks. Structures with conductivity responses after 5 minutes of 3V UVA LED (345nm to 425nm, < 0.5 mWatts) irradiation from 3% to 226% were observed, with diagonally crossing nanowires showing lower responses, and vertical free-standing nanorods of higher responses. Comparison of such materials showed the dependence of 1-D nanostructured device UV response properties on ZnO morphologies.

The scientific community has devoted an increasing interest to quantum confinement materials. In particular, silicon nanowires (Si NWs) are considered one of the most appealing resource to be employed in nanoscaled devices. Si NWs with an efficient room temperature (RT) light emission would represent a great industrial advancement, opening the route to a wide range of unexpected photonic applications. Nevertheless, to achieve a good control on quantum confined Si NWs fabrication is complex and challenging with the current technology. The most diffuses approaches such as lithography or Vapor-Liquid-Solid techniques suffers of different limits restraining the realization of quantum confined Si NWs. We demonstrated the realization of an ultradense array (10¹² NWs/cm²) of light emitting Si NWs by using a modified metal assisted chemical etching without any type of mask or lithography. This method is fast, cheap and compatible with the standard Si technology. NWs achieved by this technique exhibited a very bright RT PL and EL tunable with NWs size in agreement with the occurrence of quantum confinement effect. With this method we demonstrated the realization of a 2D random fractal array of aligned Si NWs without any lithographic process or mask and by using a fractal gold layer realized by a Si technology compatible approach. We were able to control and tune the optical properties of the system by changing the fractal morphology of the Si NWs array [1]. In-plane multiple scattering and very strong light trapping with diffuse reflectance below 0.1% related to the fractal structure were observed overall the visible range [1-2]. An innovative generation of Si NW-based optical biosensor is realized, which exploits the PL properties for the ultrasensitive and selective detection of proteins [3] in a wide range of concentrations. The occurrence of non radiative phenomena introduced by the target analyte on the NWs surface determines the quenching of the PL signal. In particular, we realized a sensor for C-reactive protein (CRP), which is crucial for heart-failure pathology. Cardiovascular problems are some of the major cause of death for both men and women. The availability of high sensitivity, low cost and reliable CRP sensors is a priority demand in clinical diagnosis for cardiovascular diseases. Si NWs sensors are fast, highly selective and offer a broad concentration dynamic range. Moreover, these sensors reach a fM sensitivity permitting non-invasive analysis in saliva [3]. Si NWs open the route towards new optical label-free cheap sensors and a full compatible with the standard Si technology for primary health care diagnosis of biomarkers. Moreover, by changing the functionalization the use of Si NW sensors opens the route towards a new class of promising label free optical sensors for different application fields.
1. Light: Science & Applications 5 (4), e16062, 2016
2. Nature Photonics 11,170-176, 2017
3. ACS Photonics 5 (2), 471–479, 2018

Metal halide perovskites exhibit favorable properties for optoelectronic devices. This talk will summarize results on metal halide perovskite wires illuminating fundamental properties of metal-halide perovskites.
Intermittency effects in the photoluminescence suggest the existence of photo-induced dynamic state in metal-halide wires that lead to non-radiative recombination.[1] For wires longer than 10 µm, photoluminescence quenching at defined positions along the wire suggest localized states that periodically become activated, leading to efficient charge carrier quenching. From the gradient in photoluminescence, the charge carrier diffusion length can be estimated from this one-dimensional model system.
Structural defects also play a role in the observed phase coexistence and hysteresis during the tetragonal to orthorhombic phase transition in methylammonium lead iodide wires.[2] The phase transition temperature appears to be dependent on the local defect concentration with more defective domains more readily transforming from the tetragonal to orthorhombic domain. This leads to charge carriers being funneled to lower energy tetragonal sites during the phase transition shown by photoluminescence microscopy and super-resolution imaging.

[1] Merdasa et al., ACS Nano, 11, 5391 (2017)
[2] Dobrovolsky et al., Nat. Comm. 8, 34 (2017)

Organic-inorganic hybrid perovskites, such as methylammonium lead iodide (CH₃NH₃PbI₃) have shown outstanding optoelectronic properties, promising extensive application in solar harvesting. Although most hybrid perovskite materials have been synthesized under solution process, recent demonstrations of the vapor phase synthesis of the hybrid perovskite nanostructures suggest there is an emerging interest in controlling their properties under highly-confined structures. In particluar, single-crystalline 1D lead halide nanowires can be prepared via vapor-liquid-solid (VLS) growth mechanism through the self-catalyzed growth mechanism, which then converted to hybrid perovskite. Since the conversion occurs on the preformed nanowire, electronic properties 1D perovskite depend on the original nanowire structure. In this study, we report the VLS growth of lead iodide (PbI₂) nanowires on a c-sapphire (0001) substrate followed by conversion to CH₃NH₃PbI₃ using methylammonium iodide, and confirm that the degree of perovskite conversion depends on the growth orientations of PbI₂ nanowires. We observe two different growth directions; vertically-oriented [0001] nanowires and kinked nanowires. Photoluminescence (PL) measurements on each growth direction suggest that the oriented nanowires exhibit a higher degree of conversion compared to the [0001] oriented nanowires. In addition, [0001] oriented nanowires exhibit the position-dependent degree of conversion, depending on the presence of the catalyst tip on top of the nanowire. In particular, the conversion is observed both on the catatlyst tip and the base of nanowire, suggesting methylammonium iodide incorporates into the nanowire either by vapor trasport and surface diffusion. Our observation indicates that the vapor phase conversion of PbI₂ to CH₃NH₃PbI₃ is a diffusion-limited process. This finding is an important step towards an structure engineering of perovskite nanowires.

Low dimensional materials have been one of the focus areas in materials science for more than three decades. In particular, the low dimensionality and reduced dielectric screening lead to pronouced optoelectronic properties such as excitons and polaritons. Here we demonstrate that quasi-one dimensional (1D) materials exhibit unique electronic structure and optical properties promising for low-cost photovoltaics and novel optoelectronics. First, we will present our first-principles study of a few quasi-1D van der Waals crystals with non-toxic and earth-abundant elements, and will elaborate the role of 1D structure and defects on their highly anisotropic optical and electronic properties. Our results shed light on new thin-film photovoltaic systems with excellent defect tolerance. Second, we will show that quasi-1D van der Waals crystals exhibit exciting nonlinear optical effects, and a microscopic picture is provided based on first-principles theory. We believe the theoretical findings presented here will open up many exciting opportunities in quasi-1D materials and nanostructures.

The remarkable performance of lead halide perovskites in solar cells can be attributed to the excellent photophysical properties that are also ideal for lasers and light-emitting devices (LEDs). The chemical and structural characteristics of halide perovskites make their crystal growth behaviors very different from conventional inorganic semiconductor materials. Here we first report new insights on the crystal growth of halide perovskites and developed the solution growth of single crystal nanowires, nanorods, and nanoplates of methylammonium (MA), formamidinium (FA) and all-inorganic cesium (Cs) lead halides perovskites (APbX₃) via a dissolution-recrystallization pathway. We also developed the vapor phase epitaxial growth of aligned CsPbX₃ perovskite nanowires and single-crystal thin films. Moreover, nanostructures of metastable perovskite phases, such as FAPbI₃ and CsPbI₃, can be stabilized via new chemical strategies by using surface ligands. These single-crystal nanowires are excellent model systems to study the intrinsic properties of perovskite materials, such as carrier transport and ionic interdiffusion. We also demonstrated high performance room temperature lasing with broad tunability of emission color from 420 nm to 824 nm from single-crystal lead halide perovskite nanowires with estimated lasing quantum yields approaching 100%. LEDs can also be fabricated with nanoscale structures of 3D or 2D perovskites. The excellent properties of these single-crystal perovskite nanowires of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance semiconductor lasers, LEDs, and other optoelectronic applications.

Interfaces are key elements in nanostructured device architectures. The quality of interfaces is particularly important for quantum devices where the device performance depends on the order, uniformity and purity of the interfaces that takes part in the quantum device structure. Hybrid nanowire materials with semiconducting, superconducting and magnetic components properties constitute some of the most promising candidates in the search for materials suitable for topological quantum computing [i]. I will discuss the mechanisms of hybrid epitaxy by Molecular Beam Epitaxy [ii] that lead to well defined interfaces between crystals of different structural and electronic properties.
I will present on new characterization schemes of how to extract information from the hybrid epitaxial materials and discuss the challenges and material requirements needed for realizing and eventually manipulating topological protected quantum states.

[i] Nayak et al. Rev. Mod. Phys. 80, 1083 (2008)
[ii] Krogstrup et al. Nature Mater. 14, 400-406 (2015)

We present electrical and structural characterisation of superconductor/semiconductor hybrid nanowires consisting of previously unexplored materials combinations. Each superconductor was deposited in-situ after semiconductor growth, without breaking vacuum. This ensures a clean and transparent interface between the semiconductor and superconductor; a crucial requirement for inducing a ‘hard’ BCS superconducting gap in the nanowires. Additionally, we developed a shadow mask method for patterning the superconductor layer during deposition. This allowed us to fabricate functional devices without need to develop specific etches and/or lithography techniques for each material. The method thus streamlines growth and fabrication while minimizing the risk of materials degradation during post-processing. We have demonstrated several key architectures including tunnel probes, Josephson Junctions and Majorana islands. These devices facilitated characterization of induced superconductivity properties in InAs nanowires, and provided insights into the requirements for obtaining a hard gap. Also of interest were the nanoscale superconducting properties of each material since the critical temperatures and critical magnetic fields differ strongly from bulk values. In particular, the materials we studied show dramatically increased out-of-plane critical field compared to aluminium, which is highly desirable for many proposed applications in topological superconductivity. The ability to fabricate devices based on a wide variety of interfaces broadens the scope for future applications of superconductor/semiconductor hybrid devices.

Semiconducting nanowires offer a unique platform to study fundamental physical effects. Recent advances in crystal phase engineering enable switching between zinc-blende and wurtzite crystal phases in InAs nanowire growth. Using this knowledge, a quantum dot (QD) can be created consisting of a thin zinc-blende section between two wurtzite tunnel barriers [1]. Furthermore, the QD can be split into two parallel coupled QDs using two local sidegates and a global backgate [2]. This system can be considered an artificial molecule, for which the electron population on the two dots can be changed separately, and the tunnel coupling between the two dots can be tuned. This offers a great flexibility to study for example the interaction between electrons and spins located on the two dots [3].

In this work, we use the crystal phase InAs QD system to investigate Kondo transport. The Kondo effect is an extensively studied many-body phenomenon which can be observed in QDs with two degenerate levels containing only one particle. Experimentally, the spin-degeneracy is often used to study the Kondo-effect. Recently the degeneracy of two orbitals in parallel coupled QDs has also been used to explore the so-called orbital-Kondo effect [4-6]. Special interest has arisen in the condition where both the spin- and the orbital-Kondo effects are present, resulting in a SU(4) symmetry. The challenge in experimental studies is to distinguish between the spin- and orbital-Kondo effects.

The InAs parallel QD system allows us to study the crossing of two spin-degenerate orbitals, one from each QD. The large single QD orbital spacing results in even-odd level spacing, making it evident whether the electron population on the dot is even or odd. At zero magnetic field, we observe enhanced conductance at zero bias due to spin-Kondo transport when one (or both) of the QDs contains an odd number of electrons. We further observe a Kondo peak when two orbitals from the QDs are aligned and contain only one electron. The zero-bias peak due to the spin degeneracy splits in magnetic field (Zeeman effect). Similarly, we demonstrate that the zero-bias peak due to the orbital degeneracy splits when detuning the orbital energies. The large g-factor of InAs allows us to study the orbital-Kondo effect independently of the spin-Kondo effect, and to demonstrate that the zero-bias peak at the orbital degeneracy persists, even though the spin-degeneracy is lifted by applying a magnetic field.

In conclusion we demonstrate both spin- and orbital-Kondo effects in InAs nanowire QDs formed by crystal phase engineering, and clearly show how the system can be tuned to have either one or both effects present at zero bias.

[1] M. Nilsson et al. PRB 93, 195423 (2016)
[2] M. Nilsson et al. Nano Letters 17, 7847 (2017)
[3] M. Nilsson et al. arXiv:1803.00326 (2018)
[4] T. Delattre et al. Nature Physics 5, 208 (2009)
[5] Y. Okazaki et al. PRB 84, 161305(R) (2011)
[6] A. J. Keller et al. Nature Physics 10, 145 (2013)

Nanoscale hybrid III-V semiconductor/superconductor heterostructures have demonstrated potential to advanced quantum transport physics, in particular to host the elusive Majorana quasiparticles. Virtually all progress on the materials side in the last 6 years has relied on high quality vapor liquid solid growth of III-V free-standing nanowires functionalized at a later stage by ex-situ deposited Al. Despite these early successes a more robust, scalable materials platform is still missing to enable the future topological quantum computer.
Here I will report on our progress in using molecular beam epitaxy (MBE) to grow both III-V semiconductor nanowire networks and a high quality epitaxial s-wave metal superconductor, using selective area epitaxy. I will first introduce the current state of the art, advantages and challenges of the selective area approach and MBE with respect to other growth techniques and geometries. Then a new method to successfully map the selective area epitaxy window in the ultra-high purity MBE environment will be detailed. Fundamental knowledge gained by growth studies will be applied to obtain reproducible, high yield advanced high spin-orbit III-V nanowire/superconductor networks. The materials properties and quality are assessed by a combination of morphological, compositional and structural analyses.

Among the III-V group semiconductors, InAs and InSb nanostructures have attracted much attention since they exhibit narrow-band gaps, high electron mobilities, strong spin-orbit couplings and giant g factors. These unique properties make them ideal materials for applications in high-speed and low-power electronics, infrared optoelectronics and especially topological quantum computing. All these applications need not only a high crystal quality but also a high degree control of the material morphology. In my talk, I will firstly give a brief introduction about our work on the successful growth of free-standing single-crystalline InSb nanosheets/nanowires on one-dimensional InAs nanowires stems on Si (111) substrates by molecular-beam epitaxy (MBE) [1,2]. Then I will present the growth of the wafer-scale free-standing high-quality two-dimensional InAs nanosheets also by MBE. These InAs nanosheets show the outstanding electrical and optical properties. Finally, I will forward to our recent progress on the heterostructures composed with the narrow-band gap semiconductor nanostructure and a superconducting aluminum fabricated by an MBE system without exposing to the air during all the growth process [3].

References:
[1] D. Pan, M. Q. Fu, X. Z. Yu, X. L. Wang, L. J. Zhu, S. H. Nie, S. L. Wang, Q. Chen, P. Xiong, S. von Molnár, and J. H. Zhao, Nano Lett., 14 (2014) 1214.
[2] D. Pan, D. X. Fan, N. Kang, J. H. Zhi, X. Z. Yu, H. Q. Xu, and J. H. Zhao, Nano Lett., 16(2016) 834.
[3] D. Pan, J. H. Zhao et al., to be published.

Group IV-Semiconducting materials, especially Ge- rich alloys, with a hexagonal crystal structure have been theoretically predicted to exhibit a direct band gap nature^1,2. Density functional theory (DFT) calculations predict a 0.3 eV bandgap for hexagonal Ge and a range from 0.3 to 0.8 eV for hexagonal Ge-rich SiGe alloys. This opens new frontiers towards uniting the electronic and optoelectronic functionalities on a single chip. However, experimentally, hexagonal Ge is an unexplored territory structurally, since Ge and its alloys crystallize naturally in the cubic structure which is optically inactive due to its indirect bandgap nature.

Remarkably, the Nanowire (NW) geometry offers a unique platform for realizing new crystal structures which are inaccessible except under extreme conditions³. Recently, we have developed generic technique in which a core/shell NW template is utilized for transferring the crystal structure in achieving new crystal phases⁴.
Here, we explore the unique structural and optical properties of Hex- Ge achieved in the WZ-GaAs/Hex-Ge core/shell NW geometry. We demonstrate photoluminescence from pure hexagonal Ge showing clear emission at 3.4 µm at low temperatures and it is measurable up to room temperature suggesting first indications of the direct band gap nature. Further, we demonstrate the tunability of the bandgap via alloying Ge with Si. Also, we explore its interesting crystal structure properties that reveal a new type of crystal defects which have not been observed in a group IV material in the literature before.

References
¹ J. Joannopoulos et al., Phys. Rev. B 1973, 7 (6), 2644−2657.
² A. D and C. E. Pryor, J. Phys.: Condens. Matter 2014, 26 (4), 045801.
³ L. Vincent et al., Nano Lett. 2014, 14 (8), 4828− 4836
⁴ I. Hauge et al., Nano Lett., 2017, 17 (1), pp 85–90

Germanium nanowires and nanorods have a broad spectrum of potential applications including electronic devices, lithium ion batteries, sensors etc. We present in this contribution the growth of highly crystalline Ge nanowires and nanorods at temperatures as low as 170 °C.[1] These structures grow either via the solution-liquid-solid (SLS) or the vapor-liquid-solid (VLS) mechanism depending on the growth conditions employed. The decomposition of the Ge precursor is catalyzed by the presence of Ga seeds as suggested by the growth temperatures below the onset of the thermal composition of the pure precursor. The compositional and structural characterization of the anisotropic Ge nanostructures has been carried out by different analytical methods including TEM, EDX as well as XRD. The analyses demonstrated the incorporation of unusually high Ga contents of up to 3-4 at% in the Ge. Unusually high metal incorporation in group IV nanowires has been observed for other semiconductor/metal combinations, but the effect on the electronic properties typically not very significant.[2] The electrical characterization at different temperatures of individual Ge nanowires demonstrates a very low resistivity and a quasi-metal like behavior.[3] Temperature treatment at slightly higher temperatures can be used to induce phase separation of this material with metastable composition leading to Ga segregation. We will also demonstrate how to switch between thermodynamically controlled Ge NW growth and the kinetically controlled formation of Ga-hyperdoped Ge.
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[1] P. Pertl, M. S. Seifner, C. Herzig, A. Limbeck, M. Sistani, A. Lugstein, S. Barth Monatsh. Chem. 2018, DOI: 10.1007/s00706-018-2191-1
[2] O. Moutanabbir, D. Insheim, et.al. Nature 2013, 496, 78.
[3] M. S. Seifner, M. Sistani, F. Porrati, G. DiPrima, P. Pertl, M. Huth, A. Lugstein, S. Barth ACS Nano 2018, 12, 1236.

Functional devices (e.g., transistors) contain one or more features with nanoscale dimensions. When such devices are to be produced at very large manufacturing rates (e.g., for large-area integrated circuitry), an alternative to top-down patterning is necessary to define key features. In this study, we show how Si and Ge surfaces can be selectively masked using the surface-initiated growth of polymer films. Our approach is particularly useful for patterning axially-encoded Si/Ge nanowire heterostructures. Surface masking of Si, but not Ge, is accomplished in a two-step procedure. Atom transfer radical polymerization (ATRP) of polymethylmethacrylate (PMMA) first occurs from a surface-tethered initiator. The selectivity of initiator attachment leads to thick and thin PMMA layers on Si and Ge surfaces, respectively. Unwanted PMMA on the Ge surface is subsequently removed by a clean-up etch that targets GeO_x, but not SiO_x, to yield nearly 100% polymerization selectivity. We investigate the role of Si/Ge surface pre-treatment, PMMA polymerization, and post-polymerization cleaning on the resulting polymer properties and surface selectivity with a suite of spectroscopy and microscopy techniques. We also show that selective polymerization is possible on Si/Ge nanowire heterostructures. The ability to mask nanoscale objects in a bottom-up fashion opens up the possibility of nanoscale patterning in a truly scalable manner.

Semiconductor nanowires are often grown from metal nanoparticle seeds via the solution-liquid-solid (SLS) or the vapor-liquid-solid (VLS) mechanisms. Although SLS or VLS growth of semiconductor nanowires is common, related wire growth in all metal systems is rare. Here, we report a new synthesis method that results in the spontaneous growth of nano- and microwires from Li-rich bulk alloys containing Au, Ag, or In at relatively low temperatures (<300 °C). Wire growth was induced by heating metal foil bilayers in an argon environment to cause alloying, followed by cooling. Optical microscopy of samples during this heating and cooling procedure showed that wires grew only during cooling. Scanning electron microscopy (SEM), cryo-transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) showed that the wires consisted of Au-, Ag-, or In-rich metal tips and polycrystalline LiOH shafts. Based on these results and systematic experiments to determine growth parameters by varying heating temperature, metal molar ratio, and cooling time, we conclude that the wires grow from alloy seed particles as lithium metal and are converted to LiOH during and/or after growth due to exposure to H₂O and O₂. The growth mechanism involves the traverse of the two-phase region containing Li metal and liquid alloy on the phase diagram, which causes Li growth from the Li-rich alloy particles. The Li needed for growth is supplied to the seed particles via surface diffusion of Li from the Li-rich alloy reservoir below. It is also thought that the growth of passivating LiOH plays a key role in determining the 1-D morphology. While there are a few important differences, such as the source of the growth species, this mechanism is similar to VLS or SLS growth of semiconductor nanowires. These experiments revealing wire growth from Li/metal alloys demonstrate a new, simple, low-temperature method for nanostructure synthesis, and the results indicate that nanowire growth in other all-metal systems is also possible.

The synthesis of bimetallic nanoparticles supported at metal oxides surfaces represents an emerging strategy to maximize catalytic performances and increase stability of heterogenous catalysts. However, the controllable synthesis of these nanohybrids as well as the systematic role played by the addition of a second metal in their composition over catalytic performances remains unclear. Here, we present an approach based on the optimization of both the support material and active phase to achieve superior catalytic performances in the oxygen reduction reaction, consisting in small PdPt NPs deposited onto TiO₂ nanowires. The composition could be systematically tuned by varying the molar ratios between Pd and Pt metal precursors. They displayed an uniform 1D morphology, monodisperse PdPt NPs sizes, no agglomeration, strong metal-support interactions, and high concentration of oxygen vacancies at their surface. These features led to improved activities and stabilities for the Pd₂₅Pt₇₅-TiO₂ sample towards the oxygen reduction reaction (ORR) relative to the commercial Pd/C and Pt/C catalysts as well as other reported materials. Moreover, a volcano-type relationship between the activity and the PdPt loading was detected. We suggest that the optimized catalytic activities observed for the Pd₂₅Pt₇₅-TiO₂ catalyst are a result of the higher concentrations of oxygen vacancies and Pd(0)/Pt(0) species investigated by XPs analysis. We believe that the catalytic activities and stabilities described herein for the ORR because of the optimization of both the support and active phase may inspire the development of novel catalytic systems towards a wealth of sustainable transformations.

Bundles of twisted conductive wires are used as effective inductors in communication devices that operate in the lower MHz frequencies. To make such a bundle be useful in the GHz frequency range, the constituent wires need to be around a micron or less in diameter. They also need to be sufficiently long and strong for the twisting process, and each individual wire needs to be coated with an insulator. Nanowires prepared with most conventional methods do not meet these requirements.
We present a new method for the preparation of flexible and centimeter long nanowires using a polymer fiber scaffold. Poly(m-phenylene isophthalamide) fibers are electrospun onto a rotating collector such that single strands of centimeter long fibers with diameters in the range of 300 – 600 nm can be collected. These fibers are sufficiently strong when suspended on frame-shaped holders that they can be coated with various deposition methods. With physical vapor deposition methods, since the deposition follows line-of-sight coverage, the frames can be coated from two opposite sides to maximize the conformality of the layer. Highly conformal layers can be deposited onto the fibers with chemical vapor or atomic layer deposition, and the polymer fibers are thermally stable enough (> 350°C) to survive most such methods.
We have demonstrated coated fibers with a conductive metal layer and an outer insulating layer to produce wires that can be twisted into a bundle to serve in telecommunication devices. We have also demonstrated adding multiple alternating layers of metal and insulator to make core-multishell structures that could have applications in photonics and related fields. Being compatible with a large range of deposition methods, this polymer fiber scaffold offers a new way making centimeter long nanowires with high materials generality.

High-mobility carrier transport channel at nanoscale is an important platform for electronic devices and quantum transport studies. Recent interest on implementation of topological quantum computation with semiconductor nanomaterials requires fabrication of high-mobility conduction channel in nanowires. Group-IV semiconductors (Si and Ge) have been considered candidates for nanowire-based topological quantum computers due to spin-orbit interaction, especially for Ge. However, Si/Ge-based heterostructures were not suitable materials for high-mobility conduction channel, such as III-As-based heterostructures. Recent advances in group-IV materials growth show that strained Ge quantum well embedded in graded Si_1-xGe_xheterostructures in thin film architecture can have high-mobility enough to observe quantum Hall effect. Here, we present the growth and transport characterization of Si_xGe_1-x/Si_1-xGe_x(0<x<1) graded multishell nanowire heterostructures. The nanowire heterostructures show structural coherency. Moreover, the nanowire heterostructures can have several carrier transport paths: core Si wire, sandwiched strained shell, outmost shell by changes of experimental conditions.
The coherent graded nanowire heterostructures were prepared by low-pressure chemical vapor deposition. The core Si_xGe_1-xnanowires were grown by Au-catalyzed vapor-liquid-solid process with silane and germane. Subsequently, Si and SiGe multishell layers were epitaxially grown by chemical vapor deposition. By controlling the ratio of silane and germane the composition of SiGe was controlled. The electrical conductivity type and doping concentration of core SiGe NWs and outmost SiGe shells were controlled by introducing diborane and phosphine. Top-gated single nanowire heterostructure devices with multi-terminals were fabricated by e-beam lithography and metallization processs. Transmission electron microscopy was employed to characterize the structural properties of the interfaces between adjacent layers. Fully single crystalline coherent graded multishell nanowire heterostructures were successfully prepared. The transport characteristics at various temperatures will be discussed in detail.

III-V semiconductor nanowire solar cells have attracted significant attention in recent years due to their suitable bandgaps, superior optical and electrical properties, as well as small footprints. Several III-V semiconductor materials such as GaAs, InP, InGaP and GaAsP have been used as single horizontal nanowire solar cells. In particular, because of their very low surface recombination velocity, InP nanowire solar cells have been studied intensively. In this work, axial n-i-p InP nanowires (growth with n-segment first) were fabricated as single horizontal nanowire solar cells, to suppress Zn dopant diffusion leading to better performance than previous reported p-i-n InP nanowire solar cells grown under similar conditions.

The axial n-i-p junction InP nanowires were grown by selective-area metal organic vapor phase epitaxy (MOVPE) technique targeting at the nanowire diameter of 180 nm and array pitch of 800 nm. The growth was carried out in a horizontal flow MOVPE reactor (AIXTRON 200/4) where trimethylindium and phosphine were used as precursors for In and P, respectively. Also silane and diethylzinc were used as n-type and p-type dopant, respectively. The growth starts with n-segment for 10 min and i-segment for 10 min followed by p-segment for 10 min. Single horizontal nanowire solar cells were fabricated by first mechanical transfer to a thermally oxidized Si substrate followed by EBL patterning to define electrodes and wet etching to remove the surface native oxide. Finally, Ti/Au electrodes on nanowires were formed by electron beam evaporation and lift-off.

A series of nano-scale characterization techniques such as photoluminescence, cathodoluminescence, photocurrent mapping and electron beam induced current on the solar cells were employed to characterize the p-n junction configuration and understand the device behavior. It is found that due to the suppression of Zn-diffusion, an improved efficiency up to 7.73% has been achieved in nanowire solar cells with the n-i-p growth sequence compared with the efficiency up to 6.5% achieved in the axial p-i-n nanowire solar cells.

Exposing a porous Si segment to light can trigger a current in a Si nanowire (NW) with a high on/off ratio. Using this unique property, photon-triggered NW transistors, photon-triggered NW logic gates, and a single NW photodetection system have been recently demonstrated¹. In this work, we develop photon-triggered NW transistors using a more reliable and simple fabrication procedure. We employed the 100-nm-diameter bottom-up Si NWs that possess the n-type high doping level and extremely smooth surface. First, the Si NWs were dispersed on a Si₃N₄ substrate and PMMA was uniformly coated on the NWs. The PMMA with a length of 360 nm was opened on a NW by electron-beam lithography. Then, the exposed NW region became porous by metal-assisted chemical etching method in which Ag nanoparticles are used as catalyst. Next, the electrical contacts on both ends of the NWs were fabricated by another electron-beam lithography and thermal evaporation. To characterize the device properties, the electrical current was measured while the laser was focused on the porous Si segment. The current level was controlled by the power of incident laser as well. The measured on/off ratio was ~10⁵ at a forward bias of 5 V. In addition, we investigated the porous-segment-length dependent responsivity of the NW device with the porous Si segment. When the porous segment length is shorter than 360 nm, the responsivity decreased because of the high dark current level. Furthermore, we demonstrated the NW transistor device with ten porous Si segments in a single Si NW, using our new fabrication procedure. We believe that our photon-triggered NW transistors offer a new venue towards programmable logic elements and ultrasensitive photodetectors.


1. Jungkil Kim et al. Photon-triggered nanowire transistors. Nat. Nanotech. 12, 963–968 (2017).

Motivation:
Single crystalline, metal oxide semiconductor nanowires (NWs) loaded with metal oxide nanoparticles (NPs) are very promising for developing a new generation of inexpensive, yet highly sensitive and more stable gas sensors.¹ By supporting p-type metal oxide NPs on n-type metal oxide NWs, both chemical and electronic sensitization effects can be obtained, which can dramatically tune the response to target gases of the resulting hybrid nanomaterials, thus enabling the engineering of selectivity. Here we show that the aerosol-assisted chemical vapor deposition (AACVD) is a technique that enables growing a wide range of nanostructures. In particular, the synthesis of single crystalline metal oxide NWs supporting homogeneously distributed, mono-modal metal oxide NPs in a wide range of loading levels is studied (from few sparse NPs to a complete coverage leading to core-shell nanostructures). SEM, TEM, XRD, XPS, Raman and PL are used to analyze results. Gas sensing properties (response, selectivity and stability) are obtained and sensing mechanisms are discussed in detail.
Results:
AACVD is run at moderate temperatures and atmospheric pressure, which enables the direct and fast (matter of minutes, rather than hours in high-vacuum) growth of nanomaterials in a wide range of application substrates including MEMS, ceramic or flexible polymeric transducers for achieving chemoresistors. We demonstrate the growth of single crystalline WO₃ n-type NWs, loaded with metal oxide NPs of p-type late transition metals (Pt, Pd, Ni, Co or Ir). Single-step or two-step AACVD methods are implemented to achieve a wide range of metal oxide loadings onto WO₃ NWs (from under 1% at. to higher than 15% at.), as confirmed by XPS. NWs are up to 20 microns in length and about 50 nm in diameter and NPs are 2 to 5 nm in diameter. While in a single-step AACVD the precursors for NWs and NPs are mixed together, in the two-step AACVD two independent processes are conducted (WO₃ NWs are grown at first and then loaded with metal oxide NPs). A two-step process is needed for correctly achieving high loading levels and core-shell nanowires. The selection of late transition metals and the loading levels has enabled us to design highly selective materials for H₂, NO₂ or H₂S with moderate humidity interference and low operating temperatures (from 250^oC down to room temperature). The reasons for this superior performance are due to chemical (catalytic) and electronic (modulation of potential barriers at the p-n heterojunction interfaces) sensitization effects that will be discussed in detail.
AACVD is revealed as an excellent approach for growing high-quality 1-D nanomaterials in a flexible and scalable way. The technique allows for fine tuning the gas-sensing properties of nanomaterials, particularly selectivity, paving the way for realizing inexpensive, highly performing devices.

^[1] Navarrete, E., Bittencourt, C., Umek, P., & Llobet, E. (2018). J. Mat. Chem. C, 6(19), 5181-5192.

Because of their complementary optical properties, plasmonic nanoparticles (NPs) and semiconductor nanostructures when combined may enhance their opto-electronic properties making them important heterostructures with high potentialities in photocatalytic and photovoltaic applications.¹ When the two materials are in close vicinity, indeed, many interactions between the electronic states of the semiconductor and the surface electrons within the metal NPs occur, with interaction enhancement when the electronic states of the two materials are close in energy. Examples of materials with this property are silver NPs and ZnSe, with the localized surface plasmon resonance (LSPR) of silver resonant with the ZnSe band-gap (2.7 eV at RT).² In this work we investigate how Ag NPs affect the optical properties of ZnSe nanowires (NWs) and give rise to charge carrier transfer from the NPs to the NWs. ZnSe NWs were grown by molecular beam epitaxy at low temperatures (300-350°C) in order to assure a good optical quality.³ A thin Ag film (5 nm) is evaporated on the NW sidewalls and the Ag NPs are obtained by thermal dewetting. Photoluminescence (PL) and fast transient absorption spectroscopy (FTAS) in pump-probe mode with a tempora