Symposium NT1 : Functional Nanostructures and Metamaterials for Solar Energy and Novel Optical Phenomena

The intense research activity of the past two decades focused on the collective electronic oscillations in high-electron-density media, known as surface plasmons, has led to multiple breakthroughs in fields ranging from chemical sensing and catalysis, to active optical devices, solar light harvesting, even nanomedicine. For many of these applications, the original focus on noble metals may ultimately limit their transition from the research laboratory to widely used commercial technologies. We will describe several research directions that, as they point towards more sustainable materials, open up new research opportunities. Aluminum, the most abundant metal on earth, opens the door to new colorimetric sensing applications and opportunities for active devices. Graphene in its smallest form, that of polycyclic aromatic hydrocarbon molecules, can support intense, optical frequency plasmon oscillations with the addition or removal of a single electron from the neutral molecule. In applications that directly address sustainability, we will discuss how plasmonic nanoparticles can be used for solar distillation of liquid mixtures, providing insight into the mechanism of nanoparticle-based distillation that in certain cases allows the distillation fraction to deviate dramatically from conventional thermal distillation processes.

This talk will emphasize recent theory and experiments in collaboration with Chad Mirkin in which DNA-functionalized nanoparticle superlattice structures are used to generate plasmonic metamaterials with a variety of properties. This involves a bottom-up assembly technique in which DNA hybridization drives the self-assembly of nanoparticle superlattices with exquisite control over the atomic, nano and micron level structure of the materials. We show that the array structures lead to new kinds of hybrid optical modes in which localized surface plasmon resonances in the nanoparticles are coupled with photonic modes of the lattices, including Bragg modes, Fabry-Perot modes and other modes. These hybrid modes are often much narrower than the isolated particle plasmons, and films composed of these superlattices have unusual metamaterials properties related to strong interactions between photonic modes and plasmons, and between excitons and plasmons.

Plasmon nanolasers, or spasers (surface plasmon amplification by stimulated emission of radiation) are devices based on plasmonic cavities and gain media that can compensate loss and achieve amplification of nano-localized electromagnetic fields. Several nanocavity architectures have been reported for spasers, such as a metal film-dielectric spacer-semiconductor nanowire configuration or arrays of plasmonic cavities, where the unit cells are nanoparticles or nanoholes. Although the array cavities exhibit directional far-field emission normal to the surface, such plasmonic crystals show bi-directional lasing, where half the emitted light is not collected and is essentially wasted. This talk will discuss a platform for unidirectional, tunable lasing from template-stripped 2D plasmonic crystals. 2D plasmonic crystals combine the advantages of a metal film and nanoparticle arrays and can show lasing in a single emission direction. We will describe the design principles for an optimized unidirectional lasing device by examining different plasmonic materials, unit cell shapes, and gain materials.

Plasmon resonances with their dramatically enhanced cross sections for light harvesting can serve as efficient generators of hot electrons and holes. Such hot carriers can be exploited in a wide range of photophysical and photochemical processes. The physical mechanism for plasmon-induced hot carrier generation is plasmon decay. Plasmons can decay either radiatively or non-radiatively. The branching ratio between these two decay channels can be controlled by tuning the radiance of the plasmon mode. Non-radiative plasmon decay is a quantum mechanical process in which one plasmon quantum is transferred to the conduction electrons of the nanostructure by excitation of an electron below the Fermi level into a state above the Fermi level but below the vacuum level. In my talk I will discuss the basic mechanism of plasmon-induced hot carrier formation, hot carrier relaxation, and how hot carriers can be exploited in a variety of applications ranging from photodetection, photocatalysis, and to dope or induce phase changes in nearby media.

Progress in nanomedicine will be driven by the ability to detect and manipulate the living matter at the molecular scale in order to cure cancers or fix genetic anomalies. One of the most promising way is the use of confined optical source in the ultra-violet wavelengths to image by self fluorescence, to analyse by enhanced Raman spectroscopy and to repair the wrong molecular sequences by inducing local chemical reaction. Metallic nanoparticles are widely recognized as local sources of energy that resolve the above issues thanks to their optical properties based to the plasmon resonance. To achieve UV plasmonics, aluminum appears as the best candidate [1]. This metal has a negative dielectric constant combined with a low loss coefficient at UV wavelengths down to 100 nm, matching all the criteria to obtain high energy Localized Surface Plasmon Resonances (LSPR) [2].
UV Localized Surface Plasmon Resonances (LSPRs) are very attracting because their energy matches with most of the electronic transition energies of molecules or solids. In this scope, the development of efficient and low-cost techniques for the synthesis of reproducible Al nanostructures with very good crystalline quality and optical properties has to be investigated [3,4].

In this presentation, we describe various methods for the growth of crystalline Al-NPs. The nanoparticles are made using very reproducible synthesis routes. The first approach is based on the reduction of aluminum ions. The second approach relies on the use of sono-chemistry of aluminum foils. Particles as small as 2nm have been synthesized and characterized with a transmission electron microscope, extinction spectroscopy and other methods. By playing on various the medium of synthesis and the temperature of reaction, it appears to be possible to tune under control the size of the nanoparticles. We completed the characterizations by investigating the optical properties of the synthesized Al NPs.
To summarize, we described in this presentation various chemical method for the growth of aluminum nanoparticle. AL-NPs present a very good homogeneity and reproducibility. They exhibit sharp localized surface plasmon resonances (LSPRs) in the UV region as it has been showed by extinction spectroscopy characterization.

JP acknowledge the Région Champagne-Ardennes, the Conseil général de l'Aube, and the FEDER funds through their support of the regional platform Nanomat. JP thanks the ANR projects NATO for the financial support.


[1]. D. Gerard and S.K. Gray, Journal of Physics D: Applied Physics 48, 184001 (2015).
[2]. J. Martin, M. Kociak, Z. Mahfoud, J. Proust, D. Gerard, and J. Plain, Nano Letters 14, 5517-5523 (2014).
[3]. J. Martin, J. Proust, D. Gerard, and J. Plain, Optical Materials Express 3, 954 (2013).
[4]. J. Martin and J. Plain, Journal of Physics D: Applied Physics 48, 184002 (2015).

Over the past decades, significant experimental and theoretical advances were made in the field of light manipulation with hybrid plasmonic nanostructures ^[1]. In the integrated plasmonic nanodevices, noble metal nanostructures are commonly used as building blocks because they possess geometry-dependent localized plasmon resonances which can be tuned easily ^[2].

In this context, we are designing a new path selective plasmonic switching device by combining molecular dipoles with V- or Y-shape gold nanostructures. In such a system, aligned molecules with well-defined dipoles will trigger the selective propagation of a plasmonic signal generated at the entrance of the nanostructure, into one of the nanostructure's branches. Basic designs and predictions of the device's properties have been performed owing to a computational FDTD simulation approach.
The Y-shape nanostructure consists of three gold nanorods linked in a controlled geometry via dedicated nanometric organic assembler. The V-shape nanostructure is a hollow gold triangle synthesized via a two step-method using silver nanoprism as a seed ^[3].

In this work, we have designed and fabricated new functional nanodevices and investigated the transport control and routing of the light, thereby establishing an approach to nanoscale plasmonic switching and light manipulation.

^[1] O. L. Berman et al. ACS Nano 2014, 8, 10437–10447.
^[2] H. Wang et al. Nano Lett. 2006, 6, 827–832.
^[3] M. M. Shahjamali et al. Small 2013, 9, 2880-2886.

Fossil energy consumption becomes a more and more significant issue because of the limited source in the Earth and the great impact on environment. Photovoltaic technologies have been regarded as a promising route to converting solar energy into electrical power as an alternative energy source. Due to the simple process and low threshold, dye-sensitized solar cells (DSSCs) have received great attentions since they are introduced in 1991. DSSCs mainly consist of several components: a transparent conductive photoanode, a mesoporous oxide layer (typically, TiO₂), photo-excitable dye, an electrolyte, a catalyst (typically, Pt), and a conductive photocathode (typically, transparent). For this study, it was the first time to apply electrodes of silver nanowires (AgNWs) as working electrodes for dye-sensitized solar cells (DSSCs). The performance of DSSCs with the AgNW working electrodes has been investigated. To avoid AgNW corrosion by iodine electrolyte, graphene oxide (GO) was used as a protective layer to isolate the AgNWs, revealing effective prevention of corrosion due to the electrostatic interaction between the electrolyte and the charges of the protective layers. With incorporation of 25% PEDOT:PSS into GO, the GO/PEDOT:PSS-AgNW DSSCs showed an 324% improvement of power conversion efficiency. The PEDOT:PSS doping can reduce the work function of GO from 5.63 to 5.24 eV due to alleviating the strength of the C-O dipole moment on GO. As a result, the enhancement on the power conversion efficiency may be attributed to the reduction of Schottky barrier between AgNWs and GO due to the descent of the work function of the GO.

Stress-grown carbon nanotubes (SG-CNTs) are carbon-based nanomaterials that formed in the presence of an external stressor. The stressor creates a unique thermodynamic condition during molecular development and has been shown to manifest itself in the chemical and physical properties of the resulting material. SG-CNTs have not been previously explored as active components in optical mixtures or composite materials, but their similarities to (and differences from) conventional carbon nanotubes (CNTs) suggest that this is an appealing area to pursue. To this end, the interaction of SG-CNTs with electromagnetic radiation in the ultraviolet (UV), visible and near infrared (NIR) regime has been examined. Experimental methodology, material performance and future implications of this work will be presented.

Localized surface plasmon resonance of metallic nanostructures receives noticeable attention in photonics, electronics, catalysis and so on. Core-shell nanostructures are particularly attractive owing to the versatile tunability of plasmonic properties along with the independent control of core size, shell thickness and corresponding chemical composition, but commonly suffer from difficult synthetic procedures. We present reliable and controllable route to highly ordered uniform Au@Ag core-shell nanoparticle array via block copolymer lithography and subsequent seeded-shell growth. Size-tunable monodisperse Au nanodot arrays are generated by block copolymer self-assembly and used as seed layers to grow Ag shells with variable thickness. The resultant Au@Ag core-shell nanoparticle arrays exhibit widely tunable broadband enhancement of plasmonic resonance that greatly surpass single element nanoparticle or homogeneous alloy nanoparticle arrays. Surface-enhanced Raman scattering of the core-shell nanoparticle arrays showed an enhancement factor over 270 from Au nanoparticle arrays.

With feature sizes shrinking and established designs reaching their limits, semiconductor nanowires have become one of the main candidates for the next generation of electronic and optoelectronic devices. A staple of the field, selective area growth through the use of pre-patterned oxides is often employed when growing nanowires because of its effectiveness at controlling uniformity and areal density for device applications. Unfortunately, the lithographical processes required for patterning oxides introduce an added layer of cost and complexity that inhibits the rapid study of nanowire device designs during the prototype stage. As a solution to this problem, our group has previously reported on the lithography-free fabrication of radial p-n junction tunnel diodes and the single nanowire characterization of an ensemble of devices without removal from the substrate [1]. Here, we expand our study to the impacts of oxide choice and annealing temperature on this novel nanowire growth technique. Gold seed particles are used to initiate vapor-liquid-solid (VLS) growth of GaAs nanowires. Short GaAs ‘pedestals’ are then coated in thin conformal films of SiO₂, Al₂O₃ or Ga₂O₃ by means of atomic layer deposition (ALD). These oxides are generally stable at the growth temperature of 400°C, but some will selectively weaken around the gold particle at the tip of the nanowire pedestals, allowing for continued VLS nanowire ‘regrowth’. Additional axial and radial nanowire segments are then electrically isolated from the substrate by the oxide layer which inhibits any planar growth that would result in a parasitic junction. Initially, only Ga₂O₃–coated wires exhibited regrowth at 400°C, but it was determined that an annealing step before regrowth would also allow Al₂O₃–coated wires to continue VLS growth. Annealing steps at 580°C and 640°C led to some nanowire regrowth for Al₂O₃–coated wires, but annealing SiO₂–coated wires at 550°C and 640°C resulted in no nanowire regrowth. This suggests that the oxide quality and the oxide bond strength play a role in this process. Nanowire morphology and mechanical deformation of the oxide coating were assessed using transmission electron microscopy. Electrical characterization of individual nanowires and dielectric oxides was carried out using a tungsten probe inside a scanning electron microscope.
[1] A. Darbandi, K. L. Kavanagh, and S. P.Watkins, Nano Letters, 15 (2015) 5408.

Coherent coupling between an optical-transition and confined optical mode, when sufficiently strong, gives rise to new modes separated by the vacuum Rabi splitting. Such systems have been investigated for electronic-state transitions, however, only very recently have vibrational transitions been considered. Here, we bring strong polaritonic-coupling in cavities from the visible into the infrared, where a new range of static and dynamic vibrational processes await investigation.
We demonstrate both static and dynamic results for vibrational bands strongly coupled to optical cavities. First, we experimentally and numerically describe coupling between a Fabry-Pérot cavity and carbonyl stretch (~1730 cm^-1) in poly-methylmethacrylate. As is requisite for “strong coupling”, the measured vacuum Rabi splitting of 132 cm^-1 is much larger than the full width of the cavity (34 cm^-1) and the inhomogeneously broadened carbonyl-stretch (24 cm^-1). Agreement with classical theories provides evidence that the mixed-states are relatively immune to inhomogeneous broadening. Next, we investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of urethane chromophore. Rabi splittings increase from 0 to ~104 cm^-1 with concentrations from 0-74 vol% and are in excellent agreement with an analytical description using no fitting parameters. Ultra-fast pump-probe measurements reveal transient absorption signals over a frequency range well-separated from the vibrational band, as well as drastically modified relaxation rates. We speculate these modified relaxation rates are a consequence of the energy separation between the vibration-cavity polariton modes and excited state transitions.
Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied statically and dynamically.

GaAs nanowires’ direct band gap and large surface-to-volume ratio have attracted considerable attention for their potential use in high efficiency solar cells, energy storage, and lasers. Measuring the concentrations of the dopants in III-V nanowires and specifically GaAs nanowires is an important problem since it governs the mobility, minority carrier diffusion length/lifetime, and conductivity. Previously, both contact and non-contact methods have been employed for estimating carrier concentrations in III-V nanowires. Here, we present a systematic study of the carrier concentration of n-type doped GaAs nanowires grown by MOCVD using low-temperature photoluminescence spectroscopy. The advantage of this method, especially for n-type GaAs nanowires with large diameters is that it is contact-less and does not require complex lithographic processing. Our measurements indicate that an increase in carrier concentration leads to an increase in the complexity of the doping mechanism, which we attribute to the formation of different recombination centers. At high carrier concentrations, we observe a blueshift of the effective band gap energies by up to 25meV due to the Burstein-Moss shift. Based on the FWHM of the photoluminescence peaks, we estimate the carrier concentrations for these nanowires, which varies from 6x10¹⁷ cm^-3 (lightly doped), to 1.5x10¹⁸ cm^-3 (moderately doped), to 3.5x10¹⁸ cm^-3 (heavily doped) as the partial pressure of the disilane is varied from 0.01 sccm to 1 sccm during the growth process.

Due to their remarkable physical properties, metallic nanowires emerge as the most promising alternatives to indium tin oxide. Percolatively connected Ag nanowire (AgNW) networks can be spin-coated over nearly any substrate at room temperature and show the potential for realization of cheap and flexible electrodes. Due to the increase of electrical resistivity and the decrease of thermal conductivity related with the size effects on the nanoscale, external heating or self-heating of AgNWs may cause melting of NWs at a much lower temperature than the melting temperature of bulk silver. Indeed, when the AgNW film is annealed at 200 °C for 30 min, some of the NW junctions are fused or broken to form Ag droplets. Then the comprehensive understanding of long-term thermal stability of the AgNW network is a crucial issue for AgNW-based devices. In this work, the thermal stability of AgNW films is investigated through the in situ measurements of sheet resistance and terahertz (THz) conductivity.
Thermal heating of AgNWs at 90–150 °C could remove the polymer protection coating on AgNWs and improve the ohmic contact between NWs. Annealing at 200 °C further reduced the resistance in the AgNW film, but prolonged annealing led to the breakage of NW junctions and the formation of silver droplets. To improve the thermal stability of AgNW film, we fabricated graphene-AgNW (GAgNW) films by stamp transferring a monolayer graphene onto the precoated AgNW film. It was found that when coupled with subpercolative AgNWs, stamp-transferred graphene reduces the resistance of the heated GAgNW film up to ~300 W and dramatically improves the thermal stability. THz spectroscopy showed that near-percolative AgNW network can transit to the percolative network by thermal heating to 200 °C. THz time-domain spectroscopy (THz-TDS) also exhibited that the physical parameters of the GAgNW film show only little variation upon thermal heating and annealing, consistent with the results of two-point probe measurement. Since a single-layer graphene is sufficient to improve the thermal stability in the GAgNW film, optical transparency of the GAgNW film is only 2–3 % lower than that of AgNW films. Our results demonstrate that highly transparent conductive metal NW electrodes with excellent thermal stability can be realized from these graphene-incorporated AgNW hybrid films. Finally, this work provides a full understanding on thermal behavior of metallic films and an easy and effective way to improve thermal and electrical properties of the metal nanostructure networks.

Transition metal dichalcogenides (TMDs) have drawn much attention as two-dimensional (2D) materials due to their unique properties for solar energy conversion. In particular, the band offsets of monolayer molybdenum and tungsten dichalcogenides are suitable for water-splitting and CO2 reduction in electrocatalysis based on theoretical studies. However, practical applications in solar energy conversion devices using these monolayer materials are hindered by the poor absorption of incident solar illumination. Here, we describe initial demonstrations and future prospects of three-dimensional photoelectrode architectures, which can largely increase absorption within the monolayer [1]. The improved light absorption is achieved by a planar cavity composed of a dielectric spacer and a reflector. Based on our previous studies, NiOx is an excellent candidate as a dielectric spacer layer. These photoelectrode architectures have been shown to achieve enhanced light absorption compared to MoS2 monolayer alone, which should translate into an increased photocurrent for driving the water splitting reaction and CO2 reduction in future work.

[1] Photon management strategies for monolayer MoS2, Shah Mohammad Bahauddin, Hossein Robatjazi and Isabell Thomann, submitted

The radiative decay of rate of quantum emitters can be significantly enhanced in close proximity to a metal surface. This enhancement in the radiative decay rate arises from a higher local density of optical states due to surface plasmon modes near the metal. Since SPPs are lossy bounded waves, coupling of radiated light to SPPs can degrade the efficiency of thin film emitting devices such as organic light emitting diodes (OLED), which have the emitter layer close to a metallic cathode. It has been shown for OLEDs that SPP coupling losses and waveguide coupling losses have opposite dependence on the distance between the emitter layer and the cathode/mirror. Thus, there is an optimal thickness for the optical spacer layer that is conventionally put between the metal cathode and emitter layer to afford the highest light extraction efficiency. In this work, we illustrate how a metamaterial mirror fabricated with subwavelength periodic structures can be used to suppress the coupling of light to SPPs even when the emitter layer is very close to a metallic back reflector. We have looked at the emission lifetime and extraction efficiency of a 20 nm R6G dye emitter layer for two different structures, one with a flat mirror and one with a metamaterial mirror, and showed that the coupling to SPPs can be significantly reduced for the metamaterial reflector. This type of metamaterial mirror offers a new approach to increase the light extraction/absorption efficiency of thin film light emitting devices.

Coupled with the growing interest in carbon nanotubes (CNT), significant progress has been made in the synthesis of wide bandgap semiconductor boron nitride nanotubes (BNNT) for advanced nanotechnology applications. In spite to CNT, growing quality BNNT up to few atomic walls at low temperature remains a challenge. Here, we report on the low temperature (350 ^oC) synthesis of BNNT in the presence of nickel and cobalt nanopowder as catalytic support. Synthesis process was carried out by irradiating solid hexagonal boron nitride (h-BN) target with short laser pulses. Entire surface of substrate was covered with large number of BNNT distributed randomly, twisted and scrolled evidenced by SEM and TEM microscopic outcomes. The average diameter of BNNT is estimated ~ 0.25 µm, while due to the complicated geometry, length of nanotubes was unpredictable. Raman together with XRD clearly identified hexagonal structure for BNNT. Due to their wide band-gap, the BNNT could provide bright future of highly efficient deep-UV photo-luminescent device and because of their unique properties; they can play a fundamental role in the rapidly developing field of nanoscience and nanotechnology.

TiN nanoparticles are well-known for their interesting plasmonic properties. They have a localized surface plasmon resonance (LSPR) located at the biological transparency window [1]. They have been proposed as a a substitute of gold nanoparticles for biomedical applications with special focus on laser ablation of carcinogen tissues [2]. TiN also has good chemical stability that enables its use as a catalyst in diverse chemical reactions [3]. Most of the methods involving TiN nanopowder synthesis use effective but complicated chemical routes [4,5]. In this contribution, we will present a one-step method for the production of TiN nanoparticles using a non-thermal plasma process. Ar gas flows into a bubbler system containing TiCl4 and carries the precursor gas into a quartz plasma reactor, where an RF power source with a radiofrequency of 13.56MHz is connected to a cylindrical copper electrode wrapped around the quartz tube. NH₃ is then added from an independent gas line at the entrance of the system. The system is operated in the 1-5 Torr pressure range, and the gas residence time in the reactor is of the order of few tens of milliseconds. Under this conditions, TiN particles with a size of 5-10 nm are produced. TEM and XRD confirm the production of crystalline TiN particles with cubic structure. The particle size can be controlled by modifying the NH₃ flow rate and the applied power. Higher power and a lower NH₃-to-TiCl₄ ratio yield larger particles. Absorption measurements of the as-synthesized particles show clear plasmonic resonance in the near-infrared region, between 800 and 1000nm. The role of process parameters on the surface of the particles, which in turn affects their plasmonic properties, will be discussed extensively.
REFERENCES
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As a good semiconducting with a direct energy band gap, bismuth sulfide (Bi₂S₃) whit band gap of 1.3 eV has received attention of many researchers because of its potential applications in areas of photovoltaic devices, photocatalysis, thermoelectric devices, and so on. On the other hand, the application of microwave (MW) heating in synthesis of materials is a fast growing research area due to its advantages, in comparison to conventional heating methods, of rapid volumetric heating, high reaction rate and selectivity that can reduce reaction time by orders of magnitude and increase yield of products. In recent years, various methods with MW have been developed to synthesize Bi₂S₃ with different sources of Bi and S, solvents, temperature and reaction time to obtain Bi₂S₃ with different morphologies. Since the hydrolysis of the sulfur source, thiourea or thioacetamide, depends on the solution pH, the reaction kinetics of Bi₂S₃ as a function of solution pH has not been analyzed till now. Another factor that influences directly on the synthesis of Bi₂S₃ is the reaction pressure, which is the result of a combination between the type of solvent, the reaction temperature and the MW power used for synthesis. Many reports on MW synthesis mentioned the numbers of those three parameters without the analysis of the pressure in the solution. In fact, the real experimental conditions change from one MW reactor to another, it is important to know the effect of reaction pressure on the final products. In this work Bi₂S₃ nanocrystals were prepared by the MW irradiation method with bismuth nitrate (Bi(NO₃)₃5H₂O) and thiourea ((NH₂)₂CS) as raw materials and distilled water as solvent. Different morphologies were obtained by varying the solution pH. The MW power of the reaction was varied from 600 to 800W, and the reaction temperature from 100 to 140°C. The crystal phase, morphology, purity and optical properties of the as-synthesized Bi₂S₃ products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS), UV–vis diffuse reflection spectroscopy (UV–vis DRS) and photoluminescence (PL) spectroscopy. The XRD results give an orthorhombic phase of Bi₂S₃ (PDF: 17-320). The SEM results show that at 100°C and 800 W, corresponding to a solution pressure of 20 psi, morphologies of nanorods in form of flowers were obtained with rods of 100-500nm in length. For 140°C and 1000W, a solution pressure of 430 psi, only deformed bars from 100 to 2000 nm in length were obtained. Generating high pressure during synthesis facilitates the crystal growth and separation. The asymmetrical morphology of Bi₂S₃ nanocrystals may encourage their application in hybrid solar cells.

Due to the low cost, nontoxicity and special physico-chemical properties of titanium oxides, their application were vastly investigated in areas such as pigments, environmental remediation, photocatalysis and so on.^[1,2] These applications rely on wide band-gap semiconductor titanium dioxides TiO₂, but those are not the only titanium–oxo compounds of potential wide interest. Recently, a long known family of titanium oxides, namely, Ti_nO_2n-1 (4 ≤ n ≤9) phases, has attracted considerable attention due to its surprising electronic and phononic properties. Metal Ti₄O₇ nanofilaments were evidenced to play a major role in TiO₂ resistive switching memory,^[3] while light-triggered metal-semiconductor transition was demonstrated for Ti₃O₅.^[4] Different bulk Ti_nO_2n-1 phases were also reported as efficient thermoelectric materials.^[5] These examples show the potential of these ordered substoichiometric titanium oxides for the design of novel information storage and energy conversion devices. Several techniques were proposed for fabricating Ti_nO_2n−1 materials.^[6,7] However, processed Magnéli phases generally contain large grains with mixtures of stoichoimetries. As a result, the transport and chemical properties of the separate phases are difficult to evaluate with a high degree of reliability. In this study we present a process for preparing Magnéli phases thin films of optical quality, with efficient control on phase, grain size and morphology. The thickness and properties of the films can be adjusted through chemical and processing conditions. The films were prepared using molecular precursor under reducing environment. The thickness, the optical properties (refractive index) and the porosity of the films were assessed using spectroscopic ellipsometry and suggest that such materials may pave the way toward energy conversion devices.

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[4] S.-I. Ohkoshi, Y. Tsunobuchi, T. Matsuda, K. Hashimoto, A. Namai, F. Hakoe, H. Tokoro, Nat. Chem. 2010, 2, 539.
[5] D. Portehault, V. Maneeratana, C. Candolfi, N. Oeschler, I. Veremchuk, Y. Grin, C. Sanchez, M. Antonietti, ACS Nano 2011, 5, 9052.
[6] R. Tu, G. Huo, T. Kimura, T. Goto, Thin Solid Films 2010, 518, 6927.
[7] F. C. Walsh, R. G. A Wills, Electrochim. Acta 2010, 55, 6342.

Porous hierarchically structured microspheres of bismuth sulfide (Bi₂S₃)/bismuth tungstate (Bi₂WO₆) have shown promising photocatalytic activity by reducing highly toxic and carcinogenic Cr (VI) in comparison to various heterostructured photocatalysts in different morphologies. Co-synthesis of the composites Bi₂S₃/Bi₂WO₆ were carried out in a ternary solvent medium (ethanol/acetic acid/water), ethyl acetate in water acted as soft template for microspheres. Composites were synthesized on basis of different Bi₂S₃ content and characterized using different spectroscopic, microscopic and surface area analysis techniques. Photoexcitation mechanism of composite materials was explained using photoluminescence spectrometry. Bi₂S₃/Bi₂WO₆ heterocatalysts were used to remove toxic Cr(VI) ions via reduction to water insoluble Cr(III) utilizing visible-light irradiation. Similarly, role of citric acid as a hole scavenger in the reduction of Cr(VI) with minimizing the rate of electron-hole recombination during photocatalysis was also investigated. Enhanced activity was observed under appropriate balance between hierarchical structure of catalysts and the amount of hole scavenger, which highlights hierarchically heterostructured materials as a promising photocatalysts.

Three-dimensional (3D) semiconducting nanostructured arrays decorated with plasmonic metal nanostructures provide giant surface-enhanced Raman Scattering (SERS) effects due to high density of SERS ‘hot spots’ on large area 3D nanostructures, increased light absorption through the 3D light pathway, and the charge transfer interactions between the semiconductors and noble metals. Here, we introduce 3D SERS sensors based on ultra-sharp ZnO nanocones arrays hybridized with particle-on-film plasmonic systems. In this design, in addition to efficient light trapping and wave-guiding properties of ultra-sharp ZnO nanocones arrays, charge transfer effects between the ZnO and noble metals, and gap plasmons between the metal nanoparticles and the metal film provide large electric field enhancements, resulting in significantly improved SERS effects. To study the effects of various morphologies of ZnO and plasmonic systems, we compared the SERS performance of three different ZnO morphologies (nanocones, nanonails, and nanorods) with Au nanoparticles and Ag film based on the finite-difference time-domain (FDTD) calculation of E-fields and the experimental SERS intensities. Finally, we demonstrated that the multiple field enhancement in optimized SERS sensor can be utilized for the molecular-level detection (100 zeptomole) of target molecules.

Flexible and transparent photodetectors have recently attracted great attentions for various potential applications. Silver nanowires (AgNWs) can provide a high optical transparency and an outstanding mechanical flexibility for the electrodes in flexible and transparent photodetectors, but the high-resolution patterning of AgNWs electrodes is a great challenge for applications in photodetector arrays. In this study, we introduce a simple solution-based technique for the high-resolution AgNWs patterning based on the filtration of AgNWs solution on the patterned polyimide shadow mask. In this method, the pattern size of AgNWs network can be easily controllable via the pattern size of polyimide mask and can be reduced down to 5 μm width. We also demonstrate the highly flexible and transparent ZnO-based UV photodetector arrays based on the patterned AgNWs electrodes. Here, the AgNWs electrodes can be embedded in ZnO film to enhance the photocurrent by light scattering and plasmon resonance effects. The optical performances of ZnO UV photodetector embedded with AgNWs electrode were greatly enhanced about 800% for photocurrent and over 1000% for On/Off ratio compared to the stacked AgNWs electrode type. In addition, the flexible photodetectors can be operated under extremely bent state (bending radius of ~770 μm) with slight decrease of performances. The patterning technique of AgNWs electrodes can be potentially employed for applications in photodetectors and thin-film transistor arrays.

Colloidal metal-semiconductor hybrid nanomaterials have recently gained much attention due to synergistic properties originating from different combinations of metal and semiconductor within a single nanocrystal structure. Particularly, platinum-incorporated cadmium chalcogenide hybrid nanocrystals have extensively been studied for photocatalytic hydrogen generation due to semiconductors as light absorbers and metals serving as cocatalysts. Here, we report on the direct decoration of Pt nanoparticles on cadmium chalcogenide (CdX) nanocrystals with different shapes (i.e., rods and tetrapods) and their effects on the photocatalytic hydrogen generation. In the presence of alkyl halide ligands at CdSe or CdSe@CdS semiconductor nanorods and tetrapods, we noted that Pt nanoparticles directly nucleated and grew uniformly at the surface of semiconductor nanocrystals. Controlled size of Pt nanoparticles decorated on the CdX nanocrystals showed different photocatalytic hydrogen generation efficiency, which is believed to be due to the controlled reduction potential of Pt clusters. Detailed structural analysis and carrier dynamics are also discussed.

Ordered nanoarchitectures have attracted an intense research interest recently because of their promising device applications. They are always fabricated by self-assembling building blocks such as nanowires, nanodots. This kind of bottom up approaches is limited in poor control over height, lateral resolution, aspect ratio, and patterning. Here, we break these limits and realize 3D sculpturing of vertical ZnO nanowire arrays (NAs) based on the conventional photolithography approach. These are achieved by immersing nanowire NAs in thick PR layers, which enable the cutting and patterning of ZnO NAs as well as the tailoring of NAs. Our strategy of 3D sculpturing of NAs promisingly paves the way for designing novel NAs-based nanoarchitectures.

Organic thin film solar cells is one of the solar device which are light in weight, thin and flexible. However, it showed the poor power conversion efficiency (PCE). Recently, many researcher are focusing on to improve the PCE of organic thin film solar cell. It has been widely reported that plasmonic effect of metallic nanoparticles can enhance the photon absorption in polymer solar cells (PSCs). Herein, we demonstrated the immobilization of gold nanoparticles on the self-assembled monolayer (SAM) modified ZnO electron transport layer. The amount of gold nanoparticles can be easily controlled by fixing the immersion time and concentration of Au nanoparticle solution and it was confirmed by absorption and scanning electron microscope. Moreover, the electron generated from the active layer could pass through the surfactant-capped gold nanoparticles to the ZnO surface with the help of SAM whereas the electrons were trapped in the Au nanoparticles for without using SAM modified Au/ZnO due to the physisorption of surfactant-capped Au nanoparticles on the surface of ZnO. As a result, the short circuit current density could be enhanced effectively for the Au nanoparticles incorporated SAM modified ZnO device when comparing the without SAM modified An/ZnO device.

This work demonstrates approaches to fabricate multicolored photopatterns in mixed QD-polymer films which significantly extend previous photopatterning approaches that utilized only a single QD component. Two approaches are presented that allow for either selective or collective modification of specific QD photoluminescent (PL) peaks during photopattern development, yielding novel photopatterns and unprecedented control over how the color of the photopattern evolves. Applying selective or collective modification of selected QDs makes it possible to change or maintain the color of the photopattern as it is developed. These results demonstrate that the evolution of the PL spectrum of a multicolor QD film during pattern development can be controlled either by careful consideration of the development wavelength or through combinations of unstable and stable QDs. The flexibility and capability of the selective or collective modification approaches greatly expand the capabilities of photopatterns, particularly in the areas of sensing, imaging, and lighting systems where it is important to have control over the intensity of selected colors within specific spatial regions.

Embedding or dispersing quantum dots (QDs) in photovoltaic devices is a continuing challenge where there is a need to have high concentration of QDs to facilitate efficient solar absorption. However, if they are too closely packed, increased Forester resonant energy transfer (FRET) can serve to destabilize QD ensembles by enhancing photo-degradation. We aim to reduce this photo-degradation by performing a ligand exchange with a custom made molecule that demonstrates liquid crystalline (LC) behavior. During self-assembly of the QDs, the LC-like ligand increases the inter-particle distance between the dots, thereby reducing FRET efficiency. To determine if the close-packing with the new ligands is optimal with respect to photo-stability, we study photo-induced static and dynamic spectral changes in self-assembled CdSe/ZnS core-shell QD thin films under ambient conditions. We observe that photoluminescence (PL) quenching of a QD ensemble is arrested by up to 25% when the red-shift due to FRET is limited to approximately 5 nm, corresponding to an inter-particle spacing of approximately 10.5 nm. With the octadecylamine (ODA) ligands, the observed red-shift is 8 nm and the inter-particle separation is approximately 9.4 nm, which decreases photo-stability. These inter-particle distances were obtained by performing small angle X-ray scattering experiments. TEM images indicate that the LC-like molecules form a loose 3-D network comprised of quasi-linear chains, reducing average nearest neighbor distances. Dynamically, the radiative lifetime of the QD ensembles is measured using time-resolved spectroscopy. As the ensembles are photo-excited, the radiative lifetime of the larger (acceptor) QDs increase until the ensemble PL quenches, at which point the radiative lifetime begins to decrease, indicative of preferential photo-oxidization. This process allows for uniform, macroscopic self-assembly of QDs and may provide an inexpensive option for improving the stability of QD-based photovoltaic devices.
This work was funded by NSF DMR 1056860.

Titania (TiO₂) is a wide band gap semiconductor that exhibits photocatalytic activity, high resistance to photocorrosion, and stability when exposed to light. The appropriate valence and conduction band energies of TiO₂ are best suited to overcome the thermodynamic and the electrochemical potential required for photoelectrolysis of water. However, TiO₂ as a photo anode material faces some significant challenges such as poor absorption of visible light, high carrier recombination, and limited charge-carrier transport. To overcome these limitations, we propose the synthesis of a composite material using carbon based graphene quantum dots (GQDs) and TiO₂ nano particles. The GQDs are synthesized by an inexpensive wet chemical method using bird charcoal as a precursor. GQD nanostructures exhibit band gap tunability based on their size and has the potential to enhance the photo absorption in TiO₂. In particular, the hybrid combination of the nano materials is expected to decrease the recombination of charge carriers, increase charge carrier mobility and aids to improve the overall photo-conversion efficiency. The synthesized composite is characterized using scanning electron microscope (SEM) image, atomic force microscope (AFM) and dynamic light scattering (DLS). Electrical/electronic performance of the composite is investigated by photocurrent density measurements. Further, optoelectronic properties are studied using photoluminescence (PL) spectrum and UV-visible transmission spectrum. The use of this combination of nano materials is non-toxic, inexpensive, and novel for photo electrochemical (PEC) water splitting application and has implications for cost effective solar fuel cell developments.

The photophysics of two types of nanoparticles with intense optical emission in the blue and green spectral region will be discussed. Nanoplatelets made of organometal halide perovskites show high quantum efficiencies, pronounced quantum size effects and strong excitonic characteristics [1]. We have also addressed the striking optical properties of blue/green-emitting carbon dots. Their properties can be understood as a consequence of a unique fluorescent cocktail of polycyclic aromatic hydrocarbons [2].

J. Sichert, Y. Tong, N. Mutz, M. Vollmer, S. Fischer, K. Milowska, R. Cortadella, B. Nickel, C. Cardenas-Daw, J. Stolarczyk, A. Urban and J. Feldmann, Nano Letters 15, 6521 (2015)
M. Fu, F. Ehrat, Y. Wang, K. Milowska, C. Reckmeier, A. Rogach, J. Stolarczyk, A. Urban and J. Feldmann, Nano Letters 15, 6030 (2015)

Glutathione stabilized few atom gold clusters (Au-GSH) exhibit red emission with a 5-10 % quantum efficiency. The excited state lifetime of ~1ms arises from ligand-metal charge transfer state. These clusters are capable of sensitizing TiO₂ and deliver a photoconversion efficiency of 2% when employed in the DSSC mode. The high open circuit voltage (0.9 V) observed in these cells makes them interesting candidate as sensitizers and cosensitizers. Their relatively high reduction and oxidation potential also makes them suitable for water splitting reaction. Furthermore we have also evaluated the metal cluster-plasmonic Ag (or Au) nanoparticle interactions by coupling glutathione protected gold clusters (Au-GSH) with larger size Ag nanoparticles. By employing transient absorption spectroscopy we have succeeded in demonstrating the synergy arising from optical interactions (i.e. plasmon enhancement) between the two coupled systems. The role of metal clusters in light harvesting systems will be discussed.

The bottom–up approach is considered a potential alternative for low cost manufacturing of nanostructured materials [1]. It is based on the concept of self–assembly of nanostructures on a substrate, and is emerging as an alternative paradigm for traditional top down fabrication used in the semiconductor industry. We demonstrate various strategies to control nanostructure assembly (both organic and inorganic) at the nanoscale. We study, in particular, multifunctional materials, namely materials that exhibit more than one functionality, and structure/property relationships in such systems, including for example: (i) control of size and luminescence properties of semiconductor nanostructures, synthesized by reactive laser ablation [2]; (ii) we developed new experimental tools and comparison with simulations are presented to gain atomic scale insight into the surface processes that govern nucleation, growth and assembly [3-7]; (iii) we devised new strategies for synthesizing multifunctional nanoscale materials to be used for electronics and photovoltaics [8-25].

References
[1] F. Rosei, J. Phys. Cond. Matt. 16, S1373 (2004); [2] D. Riabinina et al., Phys. Rev. B 74, 075334 (2006); [3] K. Dunn et al., Phys. Rev. B 80, 035330 (2009); [4] F. Ratto et al., Small 2, 401 (2006); [5] F. Ratto et al., Phys. Rev. Lett. 96, 096193 (2006); [6] F. Ratto et al., Nanotechnology 19, 265703 (2008); [7] F. Ratto et al., Surf. Sci., 602, 249 (2008); [8] C. Yan et al., Adv. Mater. 22, 1741 (2010); [9] C. Yan et al., J. Am. Chem. Soc. 132, 8868 (2010); [10] R. Nechache et al., Adv. Mater. 23, 1724–1729 (2011); [11] R. Nechache et al., Appl. Phys. Lett. 98, 202902 (2011); [12] G. Chen et al., Chem. Comm. 48, 8009–8011 (2012); [13] G. Chen et al., Adv. Func. Mater. 22, 3914–3920 (2012); [14] R. Nechache et al., Nanoscale 4, 5588–5592 (2012); [15] J. Toster et al., Nanoscale 5, 873–876 (2013); [16] T. Dembele et al., J. Power Sources 233, 93–97 (2013); [17] S. Li et al., Chem. Comm. 49, 5856–5858 (2013); [18] T. Dembele et al., J. Phys. Chem. C 117, 14510–14517 (2013); [19] R. Nechache et al., Nature Photonics 9, 61 (2015).

While the non-radiative decay of surface plasmons was once thought to be only a parasitic process which limits the performance of plasmonic devices, it has recently been shown that it can be harnessed in the form of hot electrons for use in photocatalysis, photovoltaics, and photodetectors. Here, we will discuss metamaterials and plasmonic antennas that have been specifically designed to maximize and harness hot electron injection at near-infrared wavelengths in both traditional semiconductors such as silicon as well as 2D semiconductors such as MoS₂. We will first discuss metamaterial perfect absorbers that allow one to achieve near-unity optical absorption in metal films that are thinner than the hot electron diffusion length. The metamaterials are integrated with a silicon substrate forming a Schottky barrier with a height that is smaller than the silicon band gap, allowing electrons with sub-band gap energy to be collected. This configuration was used to experimentally demonstrate a broadband and omnidirectional hot electron photodetector with a photoresponsivity >3mA/W at wavelengths >1200 nm. We also show how metamaterial perfect absorbers can be utilized to realize hot electron photodetectors that are sensitive to circularly polarized light. The detectors have among the highest polarization discrimination that has been demonstrated to date in an integrated detector. Lastly, we will discuss hot electron injection in bilayer MoS₂. One of the unique features of MoS₂ is the presence of traps that in turn lead to photoamplifaction. The amplification allows the realization of photoresponsivities in excess of 1 A/W.

In this talk, we present investigations of vapor phase synthesis of purely inorganic or organic-inorganic perovskites nanoplatelets by a van der Waals epitaxy mechanism, and their excitoic properties. Those crystals exhibit 2D well-faceted triangular, hexagonal or square geometry with thickness range of tens to hundreds of nanometers. Optical spectroscopy investigations suggest that the crystals have large exciton binding energy, high external quantum efficiency and long diffusion lengths. The naturally formed high-quality planar whispering-gallery mode cavities ensure adequate gain and efficient optical feedback for low-threshold optically pumped in-plane nanolasers ranging from ultraviolet and near-infrared, with a very high quality factor (>4000) in purely inorganic perovskite square-shaped crystals. Our findings open up a new class of wavelength tunable nanomaterials potentially suitable for on-chip integration and flexible optoelectronic devices. Progress in light-emitting diode and waveguiding will also be discussed.
References:
[1] Q. Zhang, S.T. Ha, X.F. Liu, T.C. Sum* and Q.H. Xiong*, "Room-Temperature Near-Infrared High-Q Perovskite Whispering-Gallery Planar Nanolasers", Nano Lett. 14, 5995–6001 (2014)
[2] S.T. Ha, X.F. Liu, Q. Zhang, D. Giovanni, T. C. Sum and Q.H. Xiong*, "Synthesis of Organic–Inorganic Lead Halide Perovskite Nanoplatelets: Towards High-Performance Perovskite Solar Cells and Optoelectronic Devices”, Adv. Opt. Mater. 2, 838-844 (2014)
[3] J. Xing, X. F. Liu, Q. Zhang and Q. H. Xiong*, “Vapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature Nanolasers”, Nano Lett. 15, 4571 - 4577 (2015).

Photoinduced free-carrier generation in semiconducting single-walled carbon nanotubes has been controversial because of the substantial exciton binding energy (hundreds of meV). Here, we report the direct measurement of long-lived free-carrier generation in a chirality-pure single-walled carbon nanotube in a low dielectric solvent. Frequency-resolved solution-phase flash-photolysis time-resolved microwave conductivity (fp-TRMC) provides a contactless and quantitative measurement of the real and imaginary photoconductance of individually suspended nanotubes. This solution-phase fp-TRMC as well as low excitation fluences, allow us to avoid complications from tube-tube/tube-electrode contact, dielectric screening by nearby-excitons or many-body interactions. Even under these mild conditions, we unambiguously probe a photoconductance.

Absorption and propagation of light at the sub-wavelength length scale is the key process for many technologies such as light management in modern photovoltaics. Similarly, heat exchange and transfer at length scales shorter than the phonon mean free path is critically important for nanoscale thermal management. For such near-field energy conversion and transfer, gauging the energy flow has been either indirect or requires complicated tools.^{[1, 2]} Here we demonstrate a multi-functional powermeter that directly quantifies light absorption and heat transfer at the near-field length scales, such as in a single nanowire. The mechanism is based on the metal-insulator phase transition (MIT) in single-crystal vanadium dioxide (VO₂) microbeams,^[3] where the domain wall moves free of kinetic obstruction and exhibits distinct optical contrast between the two phases. The powermeter is contactless and optically readable, allowing quick determination of local temperature, optical absorbance, thermal conductivity, and contact thermal resistance of single nanostructures.

[1] L. Cao, J. S. White, J. Park, J. A. Schuller, B. M. Clemens, M. L. Brongersma, Nat. Mater. 2009, 8, 643.
[2] P. Kim, L. Shi, A. Majumdar, P. McEuen, Phys. Rev. Lett. 2001, 87, 215502.
[3] J. Wu, Q. Gu, B. S. Guiton, L. Ouyang, N. de Leon, H. Park, Nano Lett. 2006, 6, 2313.

III-V semiconductor nanowires offer the possibility of high density/three dimensional optoelectronic device integration and growth of III-V semiconductors on cheap substrates. The shape anisotropy of nanowires that makes the above applications possible also means that nanowires have a large free surface area. The free surface area of nanowires increases the probability of surface states assisted non-radiative recombination processes, especially in materials like GaAs that have very large surface recombination velocity (~10⁶ cm/s). High non-radiative recombination rates result in low radiative efficiency in nanowires, defined as, where and are the probability of radiative recombination and non-radiative recombination event, respectively, compromising the performance of optoelectronic devices fabricated from these nanowires.
Combating the detrimental effects of surface states on the radiative efficiency of nanowires is thus essential for device applications. The conventional approach for enhancing the radiative efficiency in semiconductors involves reducing the surface state assisted non-radiative recombination probability by employing surface passivation. AlGaAs is widely used for passivating the surface of GaAs, and with this approach we have demonstrated radiative efficiency of ~2% in GaAs nanowires.
An alternative approach to increase the radiative efficiency of nanowires is to enhance the probability of radiative recombination processes. This can be achieved by coupling the nanowires to resonant cavities. We have recently demonstrated ten-fold increase in the radiative efficiency of surface passivated GaAs nanowires using this approach. In my presentation, I will discuss the challenges of this approach, design and characteristics of different resonant cavities for enhancing the radiative efficiency of nanowires, the effect of these cavities on the emission properties of nanowires and its implications for nanowire based optoelectronic devices.

Group III-V compound semiconductor nanowire arrays are promising candidates for energy harvesting and sensing applications, including photovoltaics, solar fuels, and photodetectors, due to their high volumetric absorption. Nevertheless, like many resonant optical structures, scalability is a prominent concern. Widely-practiced nanowire array fabrication methods include e-beam lithography and MOCVD, making them prohibitively expensive and time-intensive for large-scale applications in, e.g., photovoltaics, large-area detectors or thermal management layers.
We report here a scalable and economic fabrication procedure for achieving broadband near-unity absorption in sparse arrays of InP nanowires that function by strong coupling into resonant waveguide modes. Large area nanowire arrays are fabricated without epitaxy, but instead from bulk InP substrates by mechanical exfoliation of InP nanowire arrays, defined via nanoimprint and ICP-RIE.
Uniform nanowire arrays exhibit high absorption at certain wavelengths due to strong coupling into resonant waveguide modes. Simulations indicate that it is possible to achieve near-unity broadband absorption in sparse semiconductor nanowire arrays. Multi-radii nanowire arrays and tapered nanowire arrays were predicted to achieve near-unity broadband absorption in sparse arrays (<5% fill fraction) and exhibited ~25% absorption enhancements compared to arrays with uniform wire radius [1].
In this work, we experimentally demonstrate scalable, epitaxy-free fabrication of InP nanowire arrays using nanoimprint lithography for pattern transfer and inductively-coupled plasma, reactive-ion etching to define nanowire arrays in bulk InP wafers. Polymer-embedded wires are removed from the bulk InP substrate by a mechanical method that facilitates extensive reuse of a single bulk InP wafer to synthesize many polymer-embedded nanowire array thin films. In addition to mask pattern definition (wire radius and spacing) and etch chemistry (wire taper), appropriate selection of a hard mask material and thickness for the InP etch is found to be critical to attaining precise dimension control and reproducibility. After embedding in PDMS and peeling-off the substrate, the resulting arrays achieve ~80% broadband absorption (λ=400-900 nm) in less than 100 nm planar equivalence of InP. Depositing a silver back reflector increases this broadband absorption to ~90%. The repeatable process of imprinting, etching and peeling to obtain many nanowire arrays from one single wafer represents an economical manufacturing route for high efficiency III-V photovoltaics and photodetectors.

[1] K.T. Fountaine, C.G. Kendall, Harry A. Atwater, “Near-unity broadband absorption designs for semiconducting nanowire arrays via localized radial mode excitation,” Opt. Exp. (2014).

Exploring semiconductor nanowires with mid-infrared band gaps has received much less attention than the prototypical III-V nanowires with band gaps in the visible range like GaAs. One reason is that the mid-infrared poses certain technical challenges. We demonstrate that the recent technique of transient Rayleigh scattering (TRS) can be utilized to probe these materials in fruitful ways. Recently, high quality growth of InGaAs and GsAsSb nanowires whose band gaps are in the mid-infrared range has been carried out [1,2]. Here we present Raman spectroscopy and (TRS) spectroscopy results to characterize the energy structure, strain, and carrier dynamics of individual InGaAs and GaAsSb and individual core/shell nanowires with an InP shell. Specifically, individual wurtzite In_0.65 Ga_0.35 As nanowires and In_0.65 Ga_0.35 As/InP core/shell nanowires as well as zincblende GaAs_0.70 Sb_0.30 nanowires and GaAs_0.70 Sb_0.30/InP core/shell nanowires were investigated.
Raman measurements indicate that in the core/shell structures the InGaAs core is under compressive strain. TRS measurements on single nanowires at room temperature show a clear resonance at 1440 nm or 0.86 eV for the unstrained InGaAs that shifts up in energy in the core/shell structure to 1400 nm or 0.886 eV. At room temperature, the time decay is found to be quite long, ~500 ps in the core/shell structure in contrast to the bare InGaAs core where the response is weak suggesting a much larger nonradiative recombination rate.
For the GaAsSb core/shell nanowires, Raman scattering suggests that the GaAsSb core is under tensile strain. TRS measurements on the bare GaAsSb again find recombination times of less than the instrument response of 50 ps at all temperatures. In sharp contrast, lifetimes from the core/shell structures are measure to be 130 ps at room temperature and 820 ps at 10K. A shift in the bandgap between bare and core/shell nanowires is also observed.
These measurements demonstrate that the TRS technique is suitable for probing mid-infrared optical properties including band gaps and carrier dynamics of single nanowires.

We acknowledge the support of NSF through DMR-1507841, DMR 1531373, and ECCS-1509706. We also acknowledge the support of the Australian Research Council (ARC) and the Australian National Fabrication Facility.

[1] A. Ameruddin, H.A. Fonseka, P. Caroff, J. Wong-Leung, R.L.M. Veld, J. Boland, M.B. Johnston, H.H. Tan, and C. Jagadish, Nanotechnology 26, 205604 (2015).
[2] X. M. Yuan, P. Caroff, F. Wang, Y.N. Guo. Y.D. Wang. H.E. Jackson, L. M. Smith, H.H. Tan, and C. Jagadish, Advanced Functional Materials 24, 5300 (2015).

Modern information and communication technologies require higher bandwidth and greater energy efficiency in order to face the challenge of the increasing amount of data traffic. In this context, optical data is the key: the fast development of integrated optics and nanophotonics has steadily produced more optical functionality in a small chip. This continued development of nanophotonics requires integration of efficient optical nanosources (including multicolor sources for future nanoscale multiplexing) that are able to address and activate specific parts of the circuit. In this regard, hybrid plasmonic nanosources, including plasmon lasers, are a recent promising solution.
We introduce a new type of hybrid plasmonic nano-emitter. We demonstrate two-color nano-emitters that enable the selection of the dominant emitting wavelength by varying the polarization of excitation light. The nano-emitters were fabricated via surface plasmon-triggered two-photon polymerization. By using two polymerizable solutions with different quantum dots, emitters of different colors can be positioned selectively in different orientations in the close vicinity of the metal nanoparticles. The dominant emission wavelength of the metal/polymer anisotropic hybrid nano-emitter can thus be selected by altering the incident polarization.

Luminescent solar concentrators (LSCs) could become an important element of future net zero-energy-consumption buildings as semitransparent photovoltaic windows. Colloidal quantum dots (QDs) are promising materials for LSCs, as in addition to a tunable emission wavelength they can be engineered in such a way as to provide strong absorption across much of the solar spectrum while having a negligibly small re-absorption at the emission wavelength. This is important for suppressing losses for wave-guided light in large-area devices. Here, we study LSCs utilizing two types of QDs. The first is based on II-VI core/shell heterostructures (e.g., CdSe/CdS and CdSe/CdZnS), in which the absorption and emission functions are separated between a wider-gap shell and a narrower-gap core. This allows for almost independent control of absorption cross-sections responsible for light harvesting and re-absorption losses. The second system, ternary I-III-VI₂ semiconductors (CuInE₂, where E = S or Se or their mixture), provides a heavy-metal-free alternative to Cd-based QDs. In these materials, re-absorption losses are considerably reduced compared to II-VI counterparts due to the involvement of an intra-gap hole state in the emission process, which leads to the spectral displacement of the photoluminescence with respect to the onset of inter-band absorption. Using QD-solution-based LSCs, we investigate the dependence of the light collection efficiency on micro-structural parameters of the QDs, their concentration, and LSC dimensions. To analyze the experimental data, we develop an analytical model (validated by Monte Carlo simulations), which allows us to predict the LSC performance based on the QD optical spectra and device geometry. In addition, we demonstrate prototype solid-state devices by incorporating QDs into polymeric matrices, which results in freestanding, almost scattering-free slabs with negligible reabsorption losses over distances of a few tens of centimeters. Measurements under simulated solar illumination indicate high light-collection efficiencies of more than 3% with semi-transparent devices that transmit 80 to 90% of incident radiation. These studies demonstrate the significant promise of engineered QDs for applications in solar window technologies.

Solution-processed semiconductor nanocrystals have attracted great interest in photonics including color conversion and enrichment in quality lighting and display backlighting [1]. Optical properties of these colloidal nanocrystals can be conveniently tuned and controlled by tailoring the dimensionality, size, and composition of these nanostructures in an effort to realize high performance in light generation and lasing [2]. These span different types and heterostructures of colloidal semiconductors in the form of quantum dots and rods to more recently emerging quantum wells. Based on the rational design and control of excitonic processes in these nanocrystals, it is possible to achieve highly efficient light-emitting diodes [3] and optically pumped lasers [4,5]. In this talk, we will present all-colloidal solid lasers developed by incorporating nanocrystal emitters as the optical gain media in a fully colloidal cavity for the first time [5]. As an extreme case of solution-processed highly-confined quasi-2D colloids, we showed that the atomically flat heteronanoplatelets uniquely combine ultra-low threshold stimulated emission and record high optical gain coefficients. In addition, the controlled stacking of these nanoplatelets further allows us to tune their excitonic properties [7]. The recent progress in the colloidal optoelectronics suggests that solution-processed quantum materials hold great promise to challenge epitaxial counterparts in the near future.


References:
[1] H. V. Demir et al., Nano Today 6, 632 (2011); T. Erdem and H. V. Demir, Nature Photonics 5, 126 (2011).
[2] B. Guzelturk et al., Laser & Photonics Reviews 8, 73 (2014); and J. Phys. Chem. Lett. 5, 2214 (2014).
[3] X. Yang et al., Advanced Materials 24, 4180 (2012); Advanced Functional Materials 24, 5977 (2014); ACS Nano 8, 8224 (2014); and Small 10, 246 (2014).
[4] Y. Wang et al., Advanced Materials 27, 169 (2015).
[5] B. Guzelturk et al., Advanced Materials 27, 2678 (2015).
[6] B. Guzelturk et al. ACS Nano 8, 6599 (2014); and ACS Nano 8, 12524 (2014).

We study plasmon-exciton interaction by using topological singularities to spatially confine, selectively deliver, co-trap and optically probe colloidal semiconductor and plasmonic nanoparticles. The interaction is monitored in a single quantum system in the bulk of a liquid crystal medium where nanoparticles are manipulated and nanoconfined far from dielectric interfaces using laser tweezers and topological configurations containing singularities. When quantum dot-in-a-rod particles are spatially co-located with a plasmonic gold nanoburst particle in a topological singularity core, its fluorescence increases because blinking is significantly suppressed and the radiative decay rate increases by nearly an order of magnitude owing to the Purcell effect. We argue that the blinking suppression is the result of the radiative rate change that mitigates Auger recombination and quantum dot ionization, consequently reducing nonradiative recombination. Our work demonstrates that topological singularities are an effective platform for studying and controlling plasmon-exciton interactions.

For photovoltaic and solar cell applications, it is desirable for quantum dots to have a high density in order to increase the absorption of light. Epitaxial nanodots are typically fabricated by the so-called Stranski-Krastanov (S-K) growth method, which is an energy minimization process driven by the relaxation of accumulated strain energy. The quantum dots by the S-K technique have been studied for their application of solar cells such as intermediate band solar cells. High density quantum dots were sought because it is crucial to have high absorption of light for photovoltaics and other optoelectronic device applications. However, large cluster defects begin to show up as the dot density is increased over the certain value, a critical density of about 7x10¹⁰ dots/cm², over which the increase in dot density does not increase the performance of solar cells. We report that direct laser fabrication produce quantum dots with their density higher than the critical density without appearance of large clumps. Atomic force microscopy is used to image GaAs(001) surfaces that are irradiated by high power laser pulses interferentially, while the stoichiometric analysis indicate that their composition is the same as that of the substrate. The formation mechanism of high density quantum dots will be discussed along with their optical properties, examined by photoluminescence.

For optoelectronic applications, quantum wires may provide unique physical properties. However, conventional approaches such as the self-assembly via the Stranski-Krastanov (S-K) technique have a limited success in their applications toward optoelectronic devices including photovoltaics and solar cells. A novel mechanism for quality quantum wires will be been discovered. The laser fabricated nanowires on the GaAs(001) surfaces has that the width and height of nanowire can be as small as 30 and 5 nm, respectively while the density is one per 200 nm as indicated in atomic force microscope (AFM) images. The examinations by low voltage of 3kV, electron energy dispersive X-ray spectroscopy (EDS), suggest that their chemical composition is the same as that of substrate. The formation mechanism of nanowires fabricated directly by laser will be discussed along with their optical properties, examined by photoluminescence.

In this study, a new CO₂ utilization strategy is developed via the hydrogenation of CO₂ derived bicarbonate to produce value-added chemicals such as formate. A high yield of formate (production rate 160 micromol/gm TiO₂.hr.), was achieved after reducing sodium bicarbonate in glycerol–water solution at room temperature with the TiO₂ nano-catalyst (