Compressibility Studies of Copper Selenides Obtained by Cation Exchange Reaction Revealing the New CsCl Phase.

In this study, we conducted a high-pressure investigation of Cu2-xSe nanostructures with pyramid- and plate-like morphologies, created through cation exchange from zinc-blende CdSe nanocrystals and wurtzite CdSe nanoplatelets respectively. Using a diamond anvil cell setup at the APS synchrotron, we observed the phase transitions in the Cu2-xSe nanostructures up to 40 GPa, identifying a novel CsCl-type lattice with Pm3̅m symmetry above 4 GPa. This CsCl-type structure, previously unreported in copper selenides, was partially retained after decompression. Our results indicate that the initial crystalline structure of CdSe does not affect the stability of Cu2-xSe nanostructures formed via cation exchange. Both morphologies of Cu2-xSe sintered under compression, potentially contributing to the stabilization of the high-pressure phase through interfacial defects. These findings are significant for discovering new phases with potential applications in future technologies.

Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well.

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities-the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density.

Concurrent Zinc-Blende and Wurtzite Film Formation by Selection of Confined Growth Planes.

Recent research on nanowires (NWs) demonstrated the ability of III-V semiconductors to adopt a different crystallographic phase when they are grown as nanostructures, giving rise to a novel class of materials with unique properties. Controlling the crystal structure however remains difficult and the geometrical constraints of NWs cause integration challenges for advanced devices. Here, we report for the first time on the phase-controlled growth of micron-sized planar InP films by selecting confined growth planes during template-assisted selective epitaxy. We demonstrate this by varying the orientation of predefined templates, which results in concurrent formation of zinc-blende (ZB) and wurtzite (WZ) material exhibiting phase purities of 100% and 97%, respectively. Optical characterization revealed a 70 meV higher band gap and a 2.5× lower lifetime for WZ InP in comparison to its natural ZB phase. Further, a model for the transition of the crystal structure is presented based on the observed growth facets and the bonding configuration of InP surfaces.

Distinct Excitonic Circular Dichroism between Wurtzite and Zincblende CdSe Nanoplatelets.

Nanocrystals (NCs) with identical components and sizes but different crystal structures could not be distinguished by conventional absorption and emission spectra. Herein, we find that circular dichroism (CD) spectroscopy can easily distinguish the CdSe nanoplatelets (NPLs) with different crystal structures of wurtzite (WZ) and zincblende (ZB) with the help of chiral l- or d-cysteine ligands. In particular, the CD signs of the first excitonic transitions in WZ and ZB NPLs capped by the same chiral cysteine are opposite. Theoretic calculation supports the viewpoint of different crystal structures and surfaces arrangements between WZ and ZB NPLs contributing to this significant phenomenon. The CD peaks appearing at the first excitonic transition band of WZ or ZB CdSe NPLs are clearly assigned to the different transition polarizations along 4p( x,y,z),Se → 5sCd or 4p( x,y),Se → 5sCd. This work not only provides a deep insight into the origin of the optical activity inside chiral semiconductor nanomaterials but also proposes the design principle of chiral semiconductor nanocrystals with high optic activity.

Anion Exchange in II-VI Semiconducting Nanostructures via Atomic Templating.

Controlled chemical transformation of nanostructures is a promising technique to obtain precisely designed novel materials, which are difficult to synthesize otherwise. We report high-temperature vapor-phase anion-exchange reactions to chemically transform II-VI semiconductor nanostructures (100-300 nm length scale) while retaining the single crystallinity, crystal structure, morphology, and even defect distribution of the parent material via atomic templating. The concept of atomic templating is employed to obtain kinetically controlled, thermodynamically metastable structural phases such as zincblende CdSe and CdS from zincblende CdTe upon complete chemical replacement of Te with Se or S. The underlying transformation mechanisms are explained through first-principles density functional theory calculations. Atomic templating is a unique path to independently tune materials' phase and composition at the nanoscale, allowing the synthesis of novel materials.

Growth of Pure Zinc-Blende GaAs(P) Core-Shell Nanowires with Highly Regular Morphology.

The growth of self-catalyzed core-shell nanowires (NWs) is investigated systematically using GaAs(P) NWs. The defects in the core NW are found to be detrimental for the shell growth. These defects are effectively eliminated by introducing beryllium (Be) doping during the NW core growth and hence forming Be-Ga alloy droplets that can effectively suppress the WZ nucleation and facilitate the droplet consumption. Shells with pure zinc-blende crystal quality and highly regular morphology are successfully grown on the defect-free NW cores and demonstrated an enhancement of one order of magnitude for room-temperature emission compared to that of the defective shells. These results provide useful information on guiding the growth of high-quality shell, which can greatly enhance the NW device performance.

Donor Impurity States in Semiconductor Zincblende Nitride Quantum Systems as a Source of Nonlinear Optical Response.

The optical absorption and the optical rectification coefficients associated to hydrogenic impurity interstate transitions in zincblende GaN-based nanostructures of the quantum wire type are investigated. The system is assumed to have cylindrical shape and the influence of external tuning probes such as hydrostatic pressure and static electric fields is particularly taken into account. The electron states are obtained within the effective mass approximation, via the exact diagonalization of the donor-impurity Hamiltonian with parabolic confinement. The nonlinear optical coefficients are calculated using a nonperturbative solution of the density-matrix Bloch equation. Our results show that the resonance-related features of the optical response become shifted in the frequency range of the incident radiation due to the effect of the hydrostatic pressure, the strength of the applied field and the change in the impurity center position.

GaAsP Nanowires Grown by Aerotaxy.

We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III-V semiconductor based nanowires. By varying the PH3/AsH3 ratio and growth temperature, size selected GaAs1-xPx nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ x ≤ 0.43) were grown, which is the energy range needed for creating tandem III-V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III-V semiconductor based ternary nanowires.

Long-Lived Hot Carriers in III-V Nanowires.

Heat management mechanisms play a pivotal role in driving the design of nanowire (NW)-based devices. In particular, the rate at which charge carriers cool down after an external excitation is crucial for the efficiency of solar cells, lasers, and high-speed transistors. Here, we investigate the thermalization properties of photogenerated carriers by continuous-wave (cw) photoluminescence (PL) in InP and GaAs NWs. A quantitative analysis of the PL spectra recorded up to 310 K shows that carriers can thermalize at a temperature much higher than that of the lattice. We find that the mismatch between carrier and lattice temperature, ΔT, increases exponentially with lattice temperature and depends inversely on the NW diameter. ΔT is instead independent of other NW characteristics, such as crystal structure (wurtzite vs zincblende), chemical composition (InP vs GaAs), shape (tapered vs columnar NWs), and growth method (vapor-liquid-solid vs selective-area growth). Remarkably, carrier temperatures as high as 500 K are reached at the lattice temperature of 310 K in NWs with ∼70 nm diameter. While a population of nonequilibrium carriers, usually referred to as "hot carriers", is routinely generated by high-power laser pulses and detected by ultrafast spectroscopy, it is quite remarkable that it can be observed in cw PL measurements, when a steady-state population of carriers is established. Time-resolved PL measurements show that even in the thinnest NWs carriers have enough time (∼1 ns) after photoexcitation to interact with phonons and thus to release their excess energy. Nevertheless, the inability of carriers to reach a full thermal equilibrium with the lattice points to inhibited phonon emission primarily caused by the large surface-to-volume ratio of small diameter NWs.

Kinetic Growth of Ultralong Metastable Zincblende MnSe Nanowires Catalyzed by a Fast Ionic Conductor via a Solution-Solid-Solid Mechanism.

The metastable semiconductor phase allows for the exploration of unusual properties and functionalities of abnormal structures, although it is often difficult to prevent thermodynamic transformations to lower energy structures from higher, unfavored energy states. Here, we show for the first time the preparation of high-quality ultralong metastable zincblende (ZB)-MnSe nanowires with a four-coordinate structure via solution-solid-solid (SSS) growth in a mild solution-phase synthetic environment (120-220 °C) in the presence of a trace amount of Ag(I). The metastable ZB-MnSe nanowires are stabilized kinetically due to the catalysis of early formed body-centered cubic (bcc) fast-ionic (superionic) Ag2Se nanocrystals from the Ag(I) source, and the ZB-MnSe nanowires grow epitaxially along the ⟨110⟩ axis rather than the ⟨111⟩ axis, as commonly observed for typical four-coordinate Grimm-Sommerfeld bonding solids. Our method provides a new route for the growth of metastable nanostructures.

Exploring Crystal Phase Switching in GaP Nanowires.

The growth of wurtzite/zincblende (WZ and ZB, respectively) superstructures opens new avenues for band structure engineering and holds the promise of digitally controlling the energy spectrum of quantum confined systems. Here, we study growth kinetics of pure and thus defect-free WZ/ZB homostructures in GaP nanowires with the aim to obtain monolayer control of the ZB and WZ segment lengths. We find that the Ga concentration and the supersaturation in the catalyst particle are the key parameters determining growth kinetics. These parameters can be tuned by the gallium partial pressure and the temperature. The formation of WZ and ZB can be understood with a model based on nucleation either at the triple phase line for the WZ phase or in the center of the solid-liquid interface for the ZB phase. Furthermore, the observed delay/offset time needed to induce WZ and ZB growth after growth of the other phase can be explained within this framework.

Carrier thermalization dynamics in single zincblende and wurtzite InP Nanowires.

Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band-to-band transition theory is developed to extract the electron-hole-plasma density and temperature as a function of time from TRS measurements of single nanowires, which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs InP NW) and less strongly on crystal structure (ZB vs WZ). The thermalization dynamics of ZB and WZ InP NWs are similar. But a comparison of the thermalization dynamics in ZB and WZ InP NWs with ZB GaAs NWs reveals more than an order of magnitude slower relaxation for the InP NWs. We interpret these results as reflecting their distinctive phonon band structures that lead to different hot phonon effects. Knowledge of hot carrier thermalization dynamics is an essential component for effective incorporation of nanowire materials into electronic devices.

Crystal phase-dependent nanophotonic resonances in InAs nanowire arrays.

Nanostructures have many material, electronic, and optical properties that are not found in bulk systems and that are relevant for technological applications. For example, nanowires realized from III-V semiconductors can be grown into a wurtzite crystal structure. This crystal structure does not naturally exist in bulk where these materials form the zinc-blende counterpart. Being able to concomitantly grow these nanowires in the zinc-blende and/or wurtzite crystal structure provides an important degree of control for the design and optimization of optoelectronic applications based on these semiconductor nanostructures. However, the refractive indices of this new crystallographic phase have so far not been elucidated. This shortcoming makes it impossible to predict and utilize the full potential of these new nanostructured materials for optoelectronics applications: a careful design and optimization of optical resonances by tuning the nanostructure geometry is needed to achieve optimal performance. Here, we report and analyze striking differences in the optical response of nanophotonic resonances in wurtzite and zinc-blende InAs nanowire arrays. Specifically, through reflectance measurements we find that the resonance can be tuned down to λ ≈ 380 nm in wurtzite nanowires by decreasing the nanowire diameter. In stark contrast, a similar tuning to below λ ≈ 500 nm is not possible in the zinc-blende nanowires. Furthermore, we find that the wurtzite nanowires can absorb twice as strongly as the zinc-blende nanowires. We attribute these strikingly large differences in resonant behavior to large differences between the refractive indices of the two crystallographic phases realized in these nanostructures. We anticipate our findings to be relevant for other III-V materials as well as for all material systems that manifest polytypism. Taken together, our results demonstrate crystal phase engineering as a potentially new design dimension for optoelectronics applications.

State of the Art of Continuous and Atomistic Modeling of Electromechanical Properties of Semiconductor Quantum Dots.

The main intent of this paper is to present an exhaustive description of the most relevant mathematical models for the electromechanical properties of heterostructure quantum dots. Models are applied both to wurtzite and zincblende quantum dot due to the relevance they have shown for optoelectronic applications. In addition to a complete overview of the continuous and atomistic models for the electromechanical fields, analytical results will be presented for some relevant approximations, some of which are unpublished, such as models in cylindrical approximation or a cubic approximation for the transformation of a zincblende parametrization to a wurtzite one and vice versa. All analytical models will be supported by a wide range of numerical results, most of which are also compared with experimental measurements.

Tuning the Photoluminescence Anisotropy of Semiconductor Nanocrystals.

Semiconductor nanocrystals are promising optoelectronic materials. Understanding their anisotropic photoluminescence is fundamental for developing quantum-dot-based devices such as light-emitting diodes, solar cells, and polarized single-photon sources. In this study, we experimentally and theoretically investigate the photoluminescence anisotropy of CdSe semiconductor nanocrystals with various shapes, including plates, rods, and spheres, with either wurtzite or zincblende structures. We use defocused wide-field microscopy to visualize the emission dipole orientation and find that spheres, rods, and plates exhibit the optical properties of 2D, 1D, and 2D emission dipoles, respectively. We rationalize the seemingly counterintuitive observation that despite having similar aspect ratios (width/length), rods and long nanoplatelets exhibit different defocused emission patterns by considering valence band structures calculated using multiband effective mass theory and the dielectric effect. The principles are extended to provide general relationships that can be used to tune the emission dipole orientation for different materials, crystalline structures, and shapes.

Analytic description of nanowires II: morphing of regular cross sections for zincblende- and diamond-structures to match arbitrary shapes. Corrigendum.

Corrections to the article by König and Smith [Acta Cryst. (2022), B78, 643-664] are given.

Analytic description of nanowires II: morphing of regular cross sections for zincblende- and diamond-structures to match arbitrary shapes.

Setting out from our recent publication [König & Smith (2021). Acta Cryst. B77, 861], we extend our analytic description of the regular cross sections of zincblende- and diamond-structure nanowires (NWires) by introducing cross section morphing to arbitrary convex shapes featuring linear interfaces as encountered in experiment. To this end, we provide add-on terms to the existing number series with their respective running indices for zinc-blende- (zb-) and diamond-structure NWire cross sections. Such add-on terms to all variables yield the required flexibility for cross section morphing, with main variables presented by the number of NWire atoms NWire(dWire[i]), bonds between NWire atoms Nbnd(dWire[i]) and interface bonds NIF(dWire[i]). Other basic geometric variables, such as the specific length of interface facets, as well as widths, heights and total area of the cross section, are given as well. The cross sections refer to the six high-symmetry zb NWires with low-index faceting frequently occurring in the bottom-up and top-down approaches of NWire processing. The fundamental insights into NWire structures revealed here offer a universal gauge and thus enable major advancements in data interpretation and the understanding of all zb- and diamond-structure-based NWires with arbitrary convex cross sections. We corroborate this statement with an exact description of irregular Si NWire cross sections and irregular InGaAs/GaAs core-shell NWire cross sections, where a radially changing unit-cell parameter can be included.

Experimental and Theoretical Study of Stable and Metastable Phases in Sputtered CuInS2.

The chalcopyrite Cu(In,Ga)S2 has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS2 (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn5 S8 and NaInS2 accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.

Positive longitudinal magnetoconductivity induced by chiral magnetic effect in mercury selenide.

A negative longitudinal magnetoresistance without any sign of saturation was found in a non-centrosymmetric Weyl semimetal (WSM) candidate mercury selenide in an electron concentration range of 5.5 × 1015-1.7 × 1017cm-3and a temperature range of 0.33-150 K. The magnitude of the effect varies with a sample from≈10% up to≈30% in a magnetic field of 12 T atT= 150 K. Moreover, the positive contribution to magnetoconductivity has a characteristic quadratic dependence on the magnetic field, increasing with a charged center concentration atT= 150 K. The most likely explanation for the discovered longitudinal magnetoconductivity feature lies in the chiral magnetic effect, which is inherent to WSMs. The role of the Dyakonov-Perel mechanism in inter-nodal spin relaxation is discussed in regard to HgSe.

Fully Active Bimetallic Phosphide Zn0.5Ge0.5P: A Novel High-Performance Anode for Na-Ion Batteries Coupled with Diglyme-Based Electrolyte.

Metal phosphides are promising candidates for sodium-ion battery (SIB) anode owing to their large capacities with suitable redox potential, while the reversibility and rate performances are limited due to some electrochemically inactive transition-metal components and sluggish reaction kinetics. Here, we report a fully active bimetallic phosphide Zn0.5Ge0.5P anode and its composite (Zn0.5Ge0.5P-C) with excellent performance attributed to the Zn, Ge, and P components exerting their respective Na-storage merit in a cation-disordered structure. During Na insertion, Zn0.5Ge0.5P undergoes an alloying-type reaction, along with the generation of NaP, Na3P, NaGe, and NaZn13 phases, and the uniform distribution of these phases ensures the electrochemical reversibility during desodiation. Based on this reaction mechanism, excellent electrochemical properties such as a high reversible capacity of 595 mAh g-1 and an ultrafast charge-discharge capability of 377.8 mAh g-1 at 50C for 500 stable cycles were achieved within the Zn0.5Ge0.5P-C composite in a diglyme-based electrolyte. This work reveals the Na-storage reaction mechanism within Zn0.5Ge0.5P and offers a new perspective on designing high-performance anodes.