Surface-Guided Core-Shell ZnSe@ZnTe Nanowires as Radial p-n Heterojunctions with Photovoltaic Behavior

Lattice-mismatch-free construction of III-V/chalcogenide core-shell heterostructure nanowires

Growing high-quality core-shell heterostructure nanowires is still challenging due to the lattice mismatch issue at the radial interface. Herein, a versatile strategy is exploited for the lattice-mismatch-free construction of III-V/chalcogenide core-shell heterostructure nanowires by simply utilizing the surfactant and amorphous natures of chalcogenide semiconductors. Specifically, a variety of III-V/chalcogenide core-shell heterostructure nanowires are successfully constructed with controlled shell thicknesses, compositions, and smooth surfaces. Due to the conformal properties of obtained heterostructure nanowires, the wavelength-dependent bi-directional photoresponse and visible light-assisted infrared photodetection are realized in the type-I GaSb/GeS core-shell heterostructure nanowires. Also, the enhanced infrared photodetection is found in the type-II InGaAs/GeS core-shell heterostructure nanowires compared with the pristine InGaAs nanowires, in which both responsivity and detectivity are improved by more than 2 orders of magnitude. Evidently, this work paves the way for the lattice-mismatch-free construction of core-shell heterostructure nanowires by chemical vapor deposition for next-generation high-performance nanowire optoelectronics.

Carrier separation in type-II quantum dots inserted in (Zn,Mg)Te/ZnSe nanowires

Quantum dots consisting of an axial Zn_0.97Mg_0.03Te insertion inside a large-bandgap Zn_0.9Mg_0.1Te nanowire core are fabricated in a molecular-beam epitaxy system by employing the vapor-liquid-solid growth mechanism. In addition, this structure is coated with a thin ZnSe radial shell that forms a type-II interface with the dot semiconductor. The resulting radial electron-hole separation is evidenced by several distinct effects that occur in the presence of the ZnSe shell, including the optical emission redshift of about 250 meV, a significant decrease in emission intensity, an increase in the excitonic lifetime by one order of magnitude, and an increase in the biexciton binding energy. The type-II nanowire quantum dots where electrons and holes are radially separated constitute a promising platform for potential applications in the field of quantum information technology.

Real-Time Study of Surface-Guided Nanowire Growth by In Situ Scanning Electron Microscopy

Surface-guided growth has proven to be an efficient approach for the production of nanowire arrays with controlled orientations and their large-scale integration into electronic and optoelectronic devices. Much has been learned about the different mechanisms of guided nanowire growth by epitaxy, graphoepitaxy, and artificial epitaxy. A model describing the kinetics of surface-guided nanowire growth has been recently reported. Yet, many aspects of the surface-guided growth process remain unclear due to a lack of its observation in real time. Here we observe how surface-guided nanowires grow in real time by in situ scanning electron microscopy (SEM). Movies of ZnSe surface-guided nanowires growing on periodically faceted substrates of annealed M-plane sapphire clearly show how the nanowires elongate along the substrate nanogrooves while pushing the catalytic Au nanodroplet forward at the tip of the nanowire. The movies reveal the timing between competing processes, such as planar vs nonplanar growth, catalyst-selective vapor-liquid-solid elongation vs nonselective vapor-solid thickening, and the effect of topographic discontinuities of the substrate on the growth direction, leading to the formation of kinks and loops. Contrary to some observations for nonplanar nanowire growth, planar nanowires are shown to elongate at a constant rate and not by jumps. A decrease in precursor concentration as it is consumed after long reaction time causes the nanowires to shrink back instead of growing, thus indicating that the process is reversible and takes place near equilibrium. This real-time study of surface-guided growth, enabled by in situ SEM, enables a better understanding of the formation of nanostructures on surfaces.

Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al₂O₃ substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Planar growth of nanowire arrays involves interactions between materials that affect the electronic behavior of the effective heterojunction. Here, authors show how core curvature and cross-section morphology affect shell growth, demonstrating how strain at the core-shell interface induces electronic band modulations in ZnSe@ZnTe nanowires.

Real-Time Observation of Temperature-Induced Surface Nanofaceting in M-Plane α-Al2O3

The spontaneous crystal surface reconstruction of M-plane α-Al₂O₃ is employed for nanopatterning and nanofabrication in various fields of research including, among others, magnetism, superconductivity, and optoelectronics. In this reconstruction process the crystalline surface transforms from a planar morphology to one with a nanoscale ripple patterning. However, the high sample temperature required to induce surface reconstruction made in situ studies of the process seem unfeasible. The kinetics of ripple pattern formation therefore remained uncertain, and thus production of templates for nanofabrication could not advance beyond a trial-and-error stage. We present an approach combining in situ real-time grazing incidence small-angle X-ray scattering experiments (GISAXS) with model-based analysis and with ex situ atomic force microscopy (AFM) to observe this morphological transition in great detail. Our approach provides time-resolved information about all relevant morphological parameters required to trace the surface topography on the nanometer scale during reconstruction, i.e., the time dependence of the pattern wavelength, the ripple length, width, and height, and thus their facet angles. It offers a comprehensive picture of this process exemplified by a M-plane α-Al₂O₃ surface annealed at 1325 °C for 930 min. Fitting the model parameters to the experimental GISAXS data revealed a Johnson-Mehl-Avrami-Kolmogorov type of behavior for the pattern wavelength and a predominantly linear time dependence of the other parameters. In this case the reconstruction resulted in a crystalline surface fully patterned with asymmetric ripple-shaped nanostructures of 75 nm periodicity, 15 nm in height, and 630 nm in length. By elucidating the time dependence of these morphological parameters, this study shows a powerful way to significantly advance the predictability of annealing outcome and thus to efficiently customize nanopatterned α-Al₂O₃ templates for improved nanofabrication routines.

Nanofabrication Techniques: Challenges and Future Prospects

Nanofabrication of functional micro/nano-features is becoming increasingly relevant in various electronic, photonic, energy, and biological devices globally. The development of these devices with special characteristics originates from the integration of low-cost and high-quality micro/nano-features into 3D-designs. Great progress has been achieved in recent years for the fabrication of micro/nanostructured based devices by using different imprinting techniques. The key problems are designing techniques/approaches with adequate resolution and consistency with specific materials. By considering optical device fabrication on the large-scale as a context, we discussed the considerations involved in product fabrication processes compatibility, the feature's functionality, and capability of bottom-up and top-down processes. This review summarizes the recent developments in these areas with an emphasis on established techniques for the micro/nano-fabrication of 3-dimensional structured devices on large-scale. Moreover, numerous potential applications and innovative products based on the large-scale are also demonstrated. Finally, prospects, challenges, and future directions for device fabrication are addressed precisely.

Near-infrared emission from spatially indirect excitons in type II ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor-liquid-solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron-hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure.

Polarity-dependent nonlinear optics of nanowires under electric field

Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. Among these properties are piezoelectricity, ferroelectricity, pyroelectricity, and the bulk photovoltaic effect. Here we report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively. This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. EFM-SHG may be observed in any type of polar nanostructures and used to map the absolute polarity of materials at the nanoscale.

Finding dipole orientation of nanostructures is a challenge. Here the authors report a method to determine the sign of the polarity of a single nanowire using electric-field-modulated second-harmonic generation from surface-guided ZnO nanowire.

Novel synthesis of Mn: ZnSe@ZnS core-shell quantum dots based on photoinduced fluorescence enhancement

A novel Type-I Mn: ZnSe@ZnS core-shell quantum dots (QDs) was reported through a two-step procedure by using low-cost inorganic salts and naturalbiomacromolecule as raw materials. Based on a designed structure of L-cysteine-capped Mn: ZnSe QDs in aqueous media with the controllable surface, Mn: ZnSe@ZnS core-shell QDs were formed due to photoactive ions and defect curing under continuous constant light. The influences of experimental variables, including synthesis conditions of Mn: ZnSe QDs, different types and affecting factors of photo irradiation had been systematically investigated. Under the effect of photoinduced fluorescence enhancement, the photoluminescence (PL) intensity increases significantly by about 5-10 times after 1-3 h of UV irradiation. The position of the fluorescence peak was red-shifted by about 17 nm, emitting orange-red fluorescence. The photoluminescence quantum yield (PL QY) was markedly improved (up to 35%). The structure and morphology of Mn: ZnSe@ZnS core-shell QDs were also confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) in detail. The mechanism of photoinduced fluorescence enhancement was attributed to L-cysteine allowed to release S^2- to form a ZnS shell, and the passivated surface non-radiative relaxation centers of Mn: ZnSe@ZnS QDs was successfully synthesized with highuniform size, excellent photoluminescence performance, and good stability, all ofwhichmakethemgood potential candidates for white LEDs, and biological labels.

Kinetics of Guided Growth of Horizontal GaN Nanowires on Flat and Faceted Sapphire Surfaces

The bottom-up assembly of nanowires facilitates the control of their dimensions, structure, orientation and physical properties. Surface-guided growth of planar nanowires has been shown to enable their assembly and alignment on substrates during growth, thus eliminating the need for additional post-growth processes. However, accurate control and understanding of the growth of the planar nanowires were achieved only recently, and only for ZnSe and ZnS nanowires. Here, we study the growth kinetics of surface-guided planar GaN nanowires on flat and faceted sapphire surfaces, based on the previous growth model. The data are fully consistent with the same model, presenting two limiting regimes-either the Gibbs-Thomson effect controlling the growth of the thinner nanowires or surface diffusion controlling the growth of thicker ones. The results are qualitatively compared with other semiconductors surface-guided planar nanowires materials, demonstrating the generality of the growth mechanism. The rational approach enabled by this general model provides better control of the nanowire (NW) dimensions and expands the range of materials systems and possible application of NW-based devices in nanotechnology.

Understanding homoepitaxial growth of horizontal kinked GaN nanowires

Epitaxial horizontal nanowires (NWs) have attracted much attention due to their easily large-scale integration. From the reported literature, epitaxial growth is usually driven by minimization of strain between NW and substrate, which governs the growth along with specific crystallographic orientation. Here, we report the first homoepitaxial growth of horizontal GaN NWs from a surface-directed vapor-liquid-solid growth method. The NWs grow along with six symmetry-equivalent 〈1-100〉 (m-axis) directions, exhibiting a random 60°/120° kinked configuration. Owing to homoepitaxial growth, strain could be eliminated. From the obtained results, we suggest that the formation the horizontal NWs, and their growth direction /orientation is not directly related to the strain minimization. A general rule based on the epitaxial relationship and potential low-index growth orientation is proposed for understanding the arrangement of epitaxial horizontal NWs. It is deduced that kinking of the horizontal NWs was attributed to unintentional guided growth determined by the roughness of the substrates' surface. This study provides an insight for a better understanding of the evolution of epitaxial horizontal NWs, especially for the growth direction/orientation.

Planar Growth, Integration, and Applications of Semiconducting Nanowires

Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high-performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More-than-Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line-shape of NWs on planar or out-of-plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in-plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line-shape engineering are reviewed, followed by their latest device applications in building high-performance field-effect transistors, photodetectors, stretchable electronics, and 3D stacked-channel integration.

Large lattice distortions and size-dependent bandgap modulation in epitaxial halide perovskite nanowires

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. However, despite ongoing research from multiple perspectives, some fundamental questions regarding their optoelectronic properties remain controversial. One reason is the high-variance of data collected from, often unstable, polycrystalline thin films. Here we use ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr₃) nanowires grown by surface-guided chemical vapor deposition to study fundamental properties of these semiconductors in a one-dimensional model system. Specifically, we uncover the origin of an unusually large size-dependent luminescence emission spectral blue-shift. Using multiple spatially resolved spectroscopy techniques, we establish that bandgap modulation causes the emission shift, and by correlation with state-of-the-art electron microscopy methods, we reveal its origin in substantial and uniform lattice rotations due to heteroepitaxial strain and lattice relaxation. Understanding strain and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

The structural properties of silicon-doped DBrTBT/ZnSe solar cell materials: a theoretical study

A new molecular design for solar cell materials is reported for the silicon-doped 4,7-di(5-bromothiophen-2-yl)-2,1,3-benzothiadiazole adsorbed on ZnSe(100) and ZnSe(111) surfaces.

Kinetics and mechanism of planar nanowire growth

Surface-guided growth of planar nanowires offers the possibility to control their position, direction, length, and crystallographic orientation and to enable their large-scale integration into practical devices. However, understanding of and control over planar nanowire growth are still limited. Here, we study theoretically and experimentally the growth kinetics of surface-guided planar nanowires. We present a model that considers different kinetic pathways of material transport into the planar nanowires. Two limiting regimes are established by the Gibbs-Thomson effect for thinner nanowires and by surface diffusion for thicker nanowires. By fitting the experimental data for the length-diameter dependence to the kinetic model, we determine the power exponent, which represents the dimensionality of surface diffusion, and results to be different for planar vs. nonplanar nanowires. Excellent correlation between the model predictions and the data is obtained for surface-guided Au-catalyzed ZnSe and ZnS nanowires growing on both flat and faceted sapphire surfaces. These data are compared with those of nonplanar nanowire growth under similar conditions. The results indicate that, whereas nonplanar growth is usually dominated by surface diffusion of precursor adatoms over the nanowire walls, planar growth is dominated by surface diffusion over the substrate. This mechanism of planar nanowire growth can be extended to a broad range of material-substrate combinations for higher control toward large-scale integration into practical devices.

Understanding semiconductor nanostructures via advanced electron microscopy and spectroscopy

Transmission electron microscopy (TEM) offers an ample range of complementary techniques which are able to provide essential information about the physical, chemical and structural properties of materials at the atomic scale, and hence makes a vast impact on our understanding of materials science, especially in the field of semiconductor one-dimensional (1D) nanostructures. Recent advancements in TEM instrumentation, in particular aberration correction and monochromation, have enabled pioneering experiments in complex nanostructure material systems. This review aims to address these understandings through the applications of the methodology for semiconductor nanostructures. It points out various electron microscopy techniques, in particular scanning TEM (STEM) imaging and spectroscopy techniques, with their already-employed or potential applications on 1D nanostructured semiconductors. We keep the main focus of the paper on the electronic and optoelectronic properties of such semiconductors, and avoid expanding it further. In the first part of the review, we give a brief introduction to each of the STEM-based techniques, without detailed elaboration, and mention the recent technological and conceptual developments which lead to novel characterization methodologies. For further reading, we refer the audience to a handful of papers in the literature. In the second part, we highlight the recent examples of application of the STEM methodology on the 1D nanostructure semiconductor materials, especially III-V, II-V, and group IV bare and heterostructure systems. The aim is to address the research questions on various physical properties and introduce solutions by choosing the appropriate technique that can answer the questions. Potential applications will also be discussed, the ones that have already been used for bulk and 2D materials, and have shown great potential and promise for 1D nanostructure semiconductors.

In-Plane Nanowires with Arbitrary Shapes on Amorphous Substrates by Artificial Epitaxy

The challenge of nanowire assembly is still one of the major obstacles toward their efficient integration into functional systems. One strategy to overcome this obstacle is the guided growth approach, in which the growth of in-plane nanowires is guided by epitaxial and graphoepitaxial relations with the substrate to yield dense arrays of aligned nanowires. This method relies on crystalline substrates which are generally expensive and incompatible with silicon-based technologies. In this work, we expand the guided growth approach into noncrystalline substrates and demonstrate the guided growth of horizontal nanowires along straight and arbitrarily shaped amorphous nanolithographic open guides on silicon wafers. Nanoimprint lithography is used as a high-throughput method for the fabrication of the high-resolution guiding features. We first grow five different semiconductor materials (GaN, ZnSe, CdS, ZnTe, and ZnO) along straight ridges and trenches, demonstrating the generality of this method. Through crystallographic analysis we find that despite the absence of any epitaxial relations with the substrate, the nanowires grow as single crystals in preferred crystallographic orientations. To further expand the guided growth approach beyond straight nanowires, GaN and ZnSe were grown also along curved and kinked configurations to form different shapes, including sinusoidal and zigzag-shaped nanowires. Photoluminescence and cathodoluminescence were used as noninvasive tools to characterize the sine wave-shaped nanowires. We discuss the similarities and differences between in-plane nanowires grown by epitaxy/graphoepitaxy and artificial epitaxy in terms of generality, morphology, crystallinity, and optical properties.

Horizontal GaN nanowires grown on Si (111) substrate: the effect of catalyst migration and coalescence

Here, we demonstrate the growth of horizontal GaN nanowires (NWs) on silicon (111) by a surface-directed vapor-liquid-solid growth. The influence of the Au/Ni catalysts migration and coalescence on the growth of the NWs has been systematically studied. 2D root-like branched NWs were gown spontaneously through catalyst migration. Furthermore, a novel phenomenon that a catalyst particle is embedded in a horizontal NW was observed and attributed the destruction of growth steady state due to the catalysts coalescence. The transmission electron microscopy and photoluminescence, cathodoluminescence measurement demonstrated that the horizontal NWs exhibit single crystalline structures and good optical properties. Our work sheds light on the horizontal NWs growth and should facilitate the development of highly integrated III-V nanodevices on silicon.

Controllable Epitaxial Growth of Core-Shell PbSe@CsPbBr3 Wire Heterostructures

1D semiconductor core-shell wire heterostructures are crucial for high-performance optical and optoelectronic device applications, but they are limited to the traditional semiconductor families. Here, the conformal epitaxy of CsPbBr₃ shell on PbSe wire core is realized to form the core-shell PbSe@CsPbBr₃ wire heterostructures via a chemical vapor deposition route. The Pb-particle catalysts at the tips of the PbSe wires grown by vapor-liquid-solid provide the nucleation sites for the in situ rapid growth of CsPbBr₃ cube crystals, which serve as the adatom collector for the following shell growth due to the faster adsorption of the evaporated source atoms on them than on the sidewalls of PbSe wires. This determines the directional growth of the shell along the PbSe wires from the tip to bottom. The spectral and transient photoluminescence reveals the efficient photogenerated carrier transfer from the shell to the core. Importantly, the photodetectors (PDs) based on the heterostructures show responsivity up to 4.7 × 10⁴ A W^-1 under 405 nm light illumination, and a wavelength-dependent photocurrent polarity with the excitation of the light from near- to mid-infrared (IR), which indicates potential applications in IR PDs and novel optoelectronic logical circuits.

High-Gain 200 ns Photodetectors from Self-Aligned CdS-CdSe Core-Shell Nanowalls

1D core-shell heterojunction nanostructures have great potential for high-performance, compact optoelectronic devices owing to their high interface area to volume ratio, yet their bottom-up assembly toward scalable fabrication remains a challenge. Here the site-controlled growth of aligned CdS-CdSe core-shell nanowalls is reported by a combination of surface-guided vapor-liquid-solid horizontal growth and selective-area vapor-solid epitaxial growth, and their integration into photodetectors at wafer-scale without postgrowth transfer, alignment, or selective shell-etching steps. The photocurrent response of these nanowalls is reduced to 200 ns with a gain of up to 3.8 × 10³ and a photoresponsivity of 1.2 × 10³ A W^-1 , the fastest response at such a high gain ever reported for photodetectors based on compound semiconductor nanostructures. The simultaneous achievement of sub-microsecond response and high-gain photocurrent is attributed to the virtues of both the epitaxial CdS-CdSe heterojunction and the enhanced charge-separation efficiency of the core-shell nanowall geometry. Surface-guided nanostructures are promising templates for wafer-scale fabrication of self-aligned core-shell nanostructures toward scalable fabrication of high-performance compact photodetectors from the bottom-up.

Surface-Guided CsPbBr3 Perovskite Nanowires on Flat and Faceted Sapphire with Size-Dependent Photoluminescence and Fast Photoconductive Response

All-inorganic lead halide perovskite nanowires have been the focus of increasing interest since they exhibit improved stability compared to their hybrid organic-inorganic counterparts, while retaining their interesting optical and optoelectronic properties. Arrays of surface-guided nanowires with controlled orientations and morphology are promising as building blocks for various applications and for systematic research. We report the horizontal and aligned growth of CsPbBr₃ nanowires with a uniform crystallographic orientation on flat and faceted sapphire surfaces to form arrays with 6-fold and 2-fold symmetries, respectively, along specific directions of the sapphire substrate. We observed waveguiding behavior and diameter-dependent photoluminescence emission well beyond the quantum confinement regime. The arrays were easily integrated into multiple devices, displaying p-type behavior and photoconductivity. Photodetectors based on those nanowires exhibit the fastest rise and decay times for any CsPbBr₃-based photodetectors reported so far. One-dimensional arrays of halide perovskite nanowires are a promising platform for investigating the intriguing properties and potential applications of these unique materials.

Directional Growth of Ultralong CsPbBr3 Perovskite Nanowires for High-Performance Photodetectors

Directional growth of ultralong nanowires (NWs) is significant for practical application of large-scale optoelectronic integration. Here, we demonstrate the controlled growth of in-plane directional perovskite CsPbBr₃ NWs, induced by graphoepitaxial effect on annealed M-plane sapphire substrates. The wires have a diameter of several hundred nanometers, with lengths up to several millimeters. Microstructure characterization shows that CsPbBr₃ NWs are high-quality single crystals, with smooth surfaces and well-defined cross section. The NWs have very strong band-edge photoluminescence (PL) with a long PL lifetime of ∼25 ns and can realize high-quality optical waveguides. Photodetectors constructed on these individual NWs exhibit excellent photoresponse with an ultrahigh responsivity of 4400 A/W and a very fast response speed of 252 μs. This work presents an important step toward scalable growth of high-quality perovskite NWs, which will provide promising opportunities in constructing integrated nanophotonic and optoelectronic systems.

Hot electron-hole plasma dynamics and amplified spontaneous emission in ZnTe nanowires

Key to optimizing and tailoring the optoelectronic properties of semiconductor nanostructures for practical applications is a clear understanding of their carrier interactions and recombination dynamics. Herein, the electron-hole (e-h) plasma dynamics and the electron-phonon coupling interactions in zincblende ZnTe nanowires (NWs) were systematically investigated by time-resolved photoluminescence (TRPL) spectroscopy over a wide range of lattice temperatures (4-300 K) and pump densities. Following intense, non-resonant femtosecond (fs) laser pulse excitation, the excited carriers thermalize to quasi-equilibrium distribution through carrier-carrier and carrier-phonon scattering within a few picoseconds. The peak temperature of the hot electron gas (T_e0) is much higher than the lattice temperature and increases sub-linearly with the pump fluence. The hot electron gas thermalizes in two characteristic carrier density-dependent regimes - i.e., within 35 ps under high carrier densities (e-h plasma) while persisting to 360 ps under low carrier densities (exciton). Temperature-dependent studies of the ZnTe NWs revealed that the acoustic phonons play a significant role in the cooling of the hot e-h plasma in these NWs and the emission band broadening arises from the interplay of the contributions from crystal imperfections, LA and LO phonon scattering and most importantly, from the hot carrier thermalization. For demonstration, e-h plasma-amplified spontaneous emission in ZnTe NWs at room temperature by one- and two-photon excitation was realized. The results provide new insights into carrier interactions and recombination dynamics of ZnTe NWs and highlight their potential for high-efficiency e-h plasma light emitters, sensors and in plasma photochemotherapy.

Heavy-Metal-Free Fluorescent ZnTe/ZnSe Nanodumbbells

For visible range emitting particles, which are relevant for display and additional applications, Cd-chalcogenide nanocrystals have reached the highest degree of control and performance. Considering potential toxicity and regulatory limitations, there is a challenge to successfully develop Cd-free emitting nanocrystals and, in particular, heterostructures with desirable properties. Herein, we report a colloidal synthesis of fluorescent heavy-metal-free Zn-chalcogenide semiconductor nanodumbbells (NDBs), in which ZnSe tips were selectively grown on the apexes of ZnTe rods, as evidenced by a variety of methods. The fluorescence of the NDBs can be tuned between ∼500 and 585 nm by changing the ZnSe tip size. The emission quantum yield can be greatly increased through chloride surface treatment and reaches more than 30%. Simulations within an effective-mass-based model show that the hole wave function is spread over the ZnTe nanorods, while the electron wave function is localized on the ZnSe tips. Quantitative agreement for the red-shifted emission wavelength is obtained between the simulations and the experiments. Additionally, the changes in radiative lifetimes correlate well with the calculated decrease in electron-hole overlap upon growth of larger ZnSe tips. The heavy-metal-free ZnTe/ZnSe NDBs may be relevant for optoelectronic applications such as displays or light-emitting diodes.