Migration,Formation, and Growth of Pure Cd Whiskers in Cd-Based Compounds

The migration, formation, and growth of Cd islands and long whiskers were studied in single crystals of Cd₈₄Yb₁₆, Cd₅₁Yb₁₄, and Cd₁₇Ca₃ and polycrystals of Cd₈₅Ni₁₅ and high-purity Cd. It was found that Cd moves rapidly to the sample surface in quasi-crystal Cd₈₄Yb₁₆, forming islands in less than 2 h. The Cd whiskers that have submicron diameters and lengths of several hundred microns to millimeters can grow in several days under high vacuum. Evidence was presented to demonstrate that the whiskers grow from the base instead of the tip. While whiskers are easily observed in hex-CdYb, i-CdYb, and CdCa single crystals, no Cd whisker formation or migration of any kind was present on the surfaces of Cd₈₅Ni₁₅ and pure Cd samples, subjected to similar environmental conditions for even longer durations. This result suggests that oxidation of the reactive element is essential for the enhanced whisker growth kinetics observed. Because Cd islands and whiskers grow spontaneously in a matter of hours instead of months required for tin or zinc whiskers, CdYb alloys would be good candidates for detailed whisker formation and growth mechanisms studies.     

(Pb1–xCdx)S Nanoparticles Embedded in a Conjugated Organic Matrix,as Studied by Photoluminescence and Light‐Induced X‐ray Photoelectron Spectroscopy

Elliptically shaped (Pb_1–xCd_x)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p‐phenylene‐ethynylene) dicarboxylate]Pb_0.9Cd_0.1 salt matrix, with gaseous H₂S. A significantly long emission lifetime, with multi‐exponential behavior, is detected in time‐resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co‐existence of Cd and Pb within the same particle is provided by light‐induced X‐ray photoelectron spectroscopy.     

Hydrothermal Growth of Manganese Dioxide into Three‐Dimensional Hierarchical Nanoarchitectures

Novel three‐dimensional (3D) hierarchical nanoarchitectures of ?‐MnO₂ have been synthesized by a simple chemical route without the addition of any surfactants or organic templates. The self‐organized crystals consist of a major ?‐MnO₂ dipyramidal single crystal axis and six secondary branches, which are arrays of single‐crystal ?‐MnO₂ nanobelts. The growth directions of the nanobelts are perpendicular to the central dipyramidal axis, which shows sixfold symmetry. The shape of the ?‐MnO₂ assembly can be controlled by the reaction temperature. The morphology of ?‐MnO₂ changes from a six‐branched star‐like shape to a hexagonal dipyramidal morphology when the temperature is increased from 160 to 180 °C. A possible growth mechanism is proposed. The synthesized ?‐MnO₂ shows both semiconducting and magnetic properties. These materials exhibit ferromagnetic behavior below 25 K and paramagnetic behavior above 25 K. The ?‐MnO₂ system may have potential applications in areas such as fabrication of nanoscale spintronic materials, catalysis, and sensors.     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

Single‐Crystalline p‐Type Zn3As2 Nanowires for Field‐Effect Transistors and Visible‐Light Photodetectors on Rigid and Flexible Substrates

Zn₃As₂ is an important p‐type semiconductor with the merit of high effective mobility. The synthesis of single‐crystalline Zn₃As₂ nanowires (NWs) via a simple chemical vapor deposition method is reported. High‐performance single Zn₃As₂ NW field‐effect transistors (FETs) on rigid SiO₂/Si substrates and visible‐light photodetectors on rigid and flexible substrates are fabricated and studied. As‐fabricated single‐NW FETs exhibit typical p‐type transistor characteristics with the features of high mobility (305.5 cm² V^?1 s^?1) and a high I_on/I_off ratio (10⁵). Single‐NW photodetectors on SiO₂/Si substrate show good sensitivity to visible light. Using the contact printing process, large‐scale ordered Zn₃As₂ NW arrays are successfully assembled on SiO₂/Si substrate to prepare NW thin‐film transistors and photodetectors. The NW‐array photodetectors on rigid SiO₂/Si substrate and flexible PET substrate exhibit enhanced optoelectronic performance compared with the single‐NW devices. The results reveal that the p‐type Zn₃As₂ NWs have important applications in future electronic and optoelectronic devices.     

Cd2+/NH3 treatment of Cu(In,Ga)(S,Se)2 thin‐film solar cell absorbers: a model for the performance‐enhancing processes in the partial electrolyte

To obtain highly efficient chalcopyrite‐based thin‐film solar cells where the conventionally used CdS buffer is replaced by a ZnO layer prepared by the ILGAR (ion layer gas reaction) process, the Cu(In,Ga)(S,Se)₂ absorber has to be pretreated in a Cd²⁺/NH₃ solution. Based on the measured characteristics of the pH‐value in the Cd²⁺/NH₃ solution during the treatment, a model of the processes in the bath can be established. The conclusions are correlated with results from X‐ray‐photoelectron and X‐ray‐excited Auger electron spectroscopy of the Cd²⁺/NH₃‐treated Cu(In,Ga)(S,Se)₂ surface, giving an explanation for the observed formation of Cd‐compounds on the surface of the absorber. Copyright © 2005 John Wiley & Sons, Ltd.     

Arsenic incorporation in MBE grown Hg1−xCdxTe

The p-type doping of Hg_1−xCd_xTe (MCT) has proven to be a significant challenge in present day MCT-based detector technology. One of the most promising acceptor candidates, arsenic, behaves as an amphoteric dopant which can be activated as an acceptor during Hg-rich, low temperature annealing of as-grown molecular beam epitaxy (MBE) samples. This study focuses on developing an understanding of the microscopic behavior of arsenic incorporation during MBE growth. In particular, the question of whether arsenic incorporates as individual As atoms, as As₂ dimers, or as As₄ tetramers is addressed for MBE growth with an As₄ source. A quasithermodynamical model is employed to describe the MCT growth and As incorporation, with parameters fitted to an extensive database of samples grown at the Microphysics Laboratory. The best fits for growth temperatures between 175 and 185°C are obtained for arsenic incorporation as As₄ or possibly as As₄ clusters, with lower probabilities for As₂ and individual As atoms. Based on these results, we investigate the relaxed atomic configurations of As₄ and As₂ in bulk HgTe by ab initio total energy calculations. The calculations are performed in the pseudopotential density-functional framework within the local density approximation, employing supercells with periodic boundary conditions. The lattice distortions due to As₄ and As₂ in bulk HgTe are predicted to be modest due to the small size of these arsenic clusters.     

Fast Detection of Precipitates and Oxides on CdZnTe Surfaces by Spectroscopic Ellipsometry

We study the surface chemistry of Cd_0.96Zn_0.04Te(211)B by X-ray photoelectron spectroscopy and ellipsometry. We obtain first the dielectric functions of amorphous Te and Cd on Cd_0.96Zn_0.04Te(211)B by in␣situ ellipsometry after growing thin films of each material by molecular beam epitaxy. We then study their oxidation in air and show that Cd oxidizes primarily as a hydroxide whereas Te is present as TeO₂. In neither case is the oxidation of the films complete, as a substantial amount of either metallic Te or Cd remains even after several days of oxidation. We subsequently exploit an electroless chemical etchant based on Ce⁴⁺ to produce Te-rich Cd_0.96Zn_0.04Te(211)B surfaces which oxidize very little in air. Another etchant containing KCN reduces the amount of α-Te substantially. Finally, we construct a tentative optical model for the Cd_0.96Zn_0.04Te(211)B surface that yields the abundance of amorphous Te on the surface.     

Atomic‐Scale Insights into the Low‐Temperature Oxidation of Methanol over a Single‐Atom Pt1‐Co3O4 Catalyst

Heterogeneous catalysts with single‐atom active sites offer a means of expanding the industrial application of noble metal catalysts. Herein, an atomically dispersed Pt₁‐Co₃O₄ catalyst is presented, which exhibits an exceptionally high efficiency for the total oxidation of methanol. Experimental and theoretical investigations indicate that this catalyst consists of Pt sites with a large proportion of occupied high electronic states. These sites possess a strong affinity for inactive Co²⁺ sites and anchor over the surface of (111) crystal plane, which increases the metal–support interaction of the Pt₁‐Co₃O₄ material and accelerates the rate of oxygen vacancies regeneration. In turn, this is determined to promote the coadsorption of the probe methanol molecule and O₂. Density functional theory calculations confirm that the electron transfer over the oxygen vacancies reduces both the methanol adsorption energy and activation barriers for methanol oxidation, which is proposed to significantly enhance the dissociation of the C? H bond in the methanol decomposition reaction. This investigation serves as a solid foundation for characterizing and understanding single‐atom catalysts for heterogeneous oxidation reactions.     

Reconstruction transition (4×2) → (2×4) on the (001) surfaces of InAs and GaAs

Phase transitions involving various atomic configurations on the (001) surfaces of GaAs and InAs were studied by RHEED. A kinetic scheme of the interaction between the As₄ flow and the surface is proposed and the main equations describing the transitions are modified so as to correspond to the As₄ (rather than As₂) flux. A model of the (4×2) → (2×4) transition is suggested for reconstruction of a layer of metal atoms with subsequent stabilization by the adsorption of arsenic atoms. A considerable difference of the surface transitions in GaAs from that in InAs consists in greater force constants (more rigid bonds) in the former case. A significant role in the continuous evolution from (2×4)β to (4×2) phase in GaAs belongs to metastable disordered phases.     

In Situ Regulating the Order–Disorder Phase Transition in Cs2AgBiBr6 Single Crystal toward the Application in an X‐Ray Detector

The double perovskite Cs₂AgBiBr₆ single crystal holds great potential for detecting applications because of its low minimum detectable dose rate and toxic‐free merit. Nevertheless, the disordered arrangement of Ag⁺/Bi³⁺ usually gives rise to unexpected structural distortion and thereafter heavily influences the photoelectric properties of the Cs₂AgBiBr₆ single crystal. Herein, phenylethylamine bromide is demonstrated to be capable of in situ regulation of the order–disorder phase transition in the Cs₂AgBiBr₆ single crystal. The improved ordering extent of alternatively arranged [AgX₆]^5? and [BiX₆]^3? octahedra is theoretically and experimentally proven to decrease the defect density and suppress self‐trapped exciton formation, and thereby tune the band gap and enhance the carrier mobility, which consequently promotes its application in an X‐ray detector. The performance of a corresponding detector based on PEA‐Cs₂AgBiBr₆ single crystal displays superior performances, e.g., longer carrier drift distance, higher photoconductive gain, and faster current response (13 vs 3190 µs). Prominently, the as‐fabricated PEA‐Cs₂AgBiBr₆ single‐crystal X‐ray detector has an extremely high sensitivity with a value of 288.8 µC Gy_air^?1 cm^?2 under a bias of 50 V (22.7 V mm^?1), which largely outperforms those of their counterparts with lower ordering structure.     

A new magnetic semiconductor Cd1−x MnxGeP2

A new semiconductor material, which comprises a solid solution of a ternary diamond-like semiconductor and transition element Mn, was grown and investigated. According to X-ray diffraction data, the crystal structure of the material is similar to that of the CdGeP₂ host substance with a chalcopyrite-type crystal structure. The interplanar distances and the unit cell parameter decrease with an increase in Mn content: a=5.741 ? → 5.710 ? → 5.695 ? in the series of CdGeP₂ → Cd_1−x Mn_xGeP₂ → Cd_1−y Mn_yGeP₂ compounds (x<y). The surface composition and in-depth concentration profiles for elements of a Cd-Mn-Ge-P quaternary system were investigated using electron microscopy and energy dispersive X-ray spectroscopy. The molecular concentration ratio for Mn and Cd at a depth of 0.4 μm is Mn/Cd=0.2. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 3, 2001, pp. 305–309. Original Russian Text Copyright ? 2001 by Medvedkin, Ishibashi, Nishi, Sato.     

Nanodiamond‐Palladium Core–Shell Organohybrid Synthesis: A Mild Vapor‐Phase Procedure Enabling Nanolayering Metal onto Functionalized sp3‐Carbon

A novel approach for the bottom‐up construction of hybrid organic–inorganic nanocomposites with an intimate arrangement between sp³‐carbon 3D molecular‐size nanodiamonds (diamondoids) and a coated palladium surface as nanolayer is reported. The construction process is conducted stepwisely from the gas phase, using first controlled vapor‐phase self‐assembly of tailor‐made functionalized diamantane derivatives, followed by low‐temperature (45 °C) chemical vapor deposition of an organometallic complex in a reducing H₂ atmosphere over the self‐assembled diamondoid scaffold. The use of self‐assemblies of primary diamantane phosphine and phosphine oxide, which are produced with high structural uniformity and reproducibility, yields new hybrid diamondoid‐palladium materials incorporating Pd? O? PH? diamantane bonding motifs. Additional investigations provide evidence for a very challenging issue in the intimate construction of sp³‐C/metal scaffolds. Scanning electron microscopy and transmission electron microscopy microscopies combined with X‐ray photoelectron spectroscopy surface analysis and EDX bulk analysis confirm the formation of diamondoid‐palladium organohybrids with unique surface layering. The vapor phase‐controlled mild synthetic process allows excellent control over nanocomposite formation and morphology from molecular‐level modifications. As such, this bottom‐up composite building process bridges scales from the molecular (functionalized diamondoids) over nanoscopic (self‐assemblies) to microscopic regime (hybrids), in the challenging association of transition metals with an electronically saturated sp³‐carbon organic host material.     

MgSiAs: An Unexplored System with Promising Nonlinear Optical Properties

Two new non‐centrosymmetric ternary compounds, MgSiAs₂ and Mg₃Si₆As₈, are discovered via metal flux and solid‐state synthetic methods. MgSiAs₂ belongs to the well‐known II‐IV‐V₂ family, which is extensively studied experimentally and computationally for their optical properties. MgSiAs₂ is computationally predicted but not experimentally known prior to this work. Mg₃Si₆As₈ crystallizes in a new non‐centrosymmetric cubic chiral structure type with the Pearson symbol cP68. The syntheses, crystal structure, thermal and chemical stabilities, electronic structures, and optical properties of these two new compounds are investigated in this work. Optical absorption measurements and electronic structure calculations reveal the two compounds to be direct or pseudo‐direct bandgap semiconductors (1.8–2 eV). The crystal structures of both compounds are non‐centrosymmetric, though Mg₃Si₆As₈ belongs to the 432 chiral crystal class, which is optically active but does not exhibit second harmonic generation (SHG) behavior. The SHG response of MgSiAs₂ is 60% of that for AgGaS₂, but MgSiAs₂ exhibits a higher laser damage threshold than AgGaS₂ at 33.2 MW cm^?2.     

Mg?Si?As: An Unexplored System with Promising Nonlinear Optical Properties

Two new non‐centrosymmetric ternary compounds, MgSiAs₂ and Mg₃Si₆As₈, are discovered via metal flux and solid‐state synthetic methods. MgSiAs₂ belongs to the well‐known II‐IV‐V₂ family, which is extensively studied experimentally and computationally for their optical properties. MgSiAs₂ is computationally predicted but not experimentally known prior to this work. Mg₃Si₆As₈ crystallizes in a new non‐centrosymmetric cubic chiral structure type with the Pearson symbol cP68. The syntheses, crystal structure, thermal and chemical stabilities, electronic structures, and optical properties of these two new compounds are investigated in this work. Optical absorption measurements and electronic structure calculations reveal the two compounds to be direct or pseudo‐direct bandgap semiconductors (1.8–2 eV). The crystal structures of both compounds are non‐centrosymmetric, though Mg₃Si₆As₈ belongs to the 432 chiral crystal class, which is optically active but does not exhibit second harmonic generation (SHG) behavior. The SHG response of MgSiAs₂ is 60% of that for AgGaS₂, but MgSiAs₂ exhibits a higher laser damage threshold than AgGaS₂ at 33.2 MW cm^?2.     

Raman scattering in As-Se-S and As-Se-Te Chalcogenide vitreous semiconductors

Chalcogenide vitreous semiconductors of As-Se-S and As-Se-Te compositions are synthesized, with thin films produced by thermal evaporation in vacuum. X-ray phase analysis and Raman spectroscopy are used to confirm their amorphous state and examine their structural specific features. It is shown that the matrices of both materials have a network-chain structure with covalent bonds between As, Se, S, and Te atoms. Structural units of the type of AsSe₃ and AsS₃ pyramids, AsSe_3/2 bipyramids, and α(β)-As₄S₄ molecules exist in the As-Se-S matrix, and AsSe₃, AsSe_3/2, As₂Se₃, and As₂Te₃ units, in As-Se-Te. The vibrational modes of Se-Se, Te-Te, As-As, As-S, Se-As-Se, As-Se-As, Se-As-S, Se-As-Te, and Se-Te bonds constituting both separate molecules and amorphous matrices are determined.     

Epitaxy of Single‐Crystalline GaN Film on CMOS‐Compatible Si(100) Substrate Buffered by Graphene

Fabricating single‐crystalline gallium nitride (GaN)‐based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal‐oxide‐semiconductor circuits, is a prerequisite for next‐generation high‐performance electronics and optoelectronics. However, the direct epitaxy of single‐crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two‐domain structure. Here, by utilizing single‐crystalline graphene as a buffer layer, the epitaxy of a single‐crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH₃ can generate sp³ C? N bonds, which then triggers the nucleation of nitrides. The one‐atom‐thick single‐crystalline graphene provides an in‐plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first‐principle calculations. This work lays the foundation for the integration of GaN‐based devices into Si‐based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.     

Features of obtaining diluted magnetic semiconductors in the system of narrow-gap GaInAsSb alloys

Nanotechnology of obtainment of diluted magnetic semiconductors based on the GaInAsSb compounds is developed using the laser deposition of Mn atoms on the surface of the epitaxial layer of a quaternary alloy obtained by liquid-phase epitaxy. Fabricated heterostructures were studied using high-resolution X-ray diffraction for the Bragg and grazing diffraction geometries, and the layer-by-layer analysis is performed by secondary-ion mass spectrometry. It is established that the near-boundary region of the Ga_0.96In_0.04As_0.11Sb_0.89 layer near the deposition surface of atomic Mn exhibits the presence of a quinary compound with Mn atoms in the lattice and Mn₃As₂-type binary inclusions. Saturation of the GaIn(Mn)AsSb multicomponent diluted semiconductor with the Mn compounds makes it possible to specify the concentration of the magnetic impurity in the crystal and control the magnetic properties of the heterostructure.     

Anatase (101) Reconstructed Surface with Novel Functionalities: Desired Bandgap for Visible Light Absorption and High Chemical Reactivity

Unreconstructed surfaces of anatase TiO₂ are known to have two main limitations for their application as photocatalysts, namely, low efficiency for sun‐light absorption due to the wide bandgap, and low chemical reactivity. Strategies to overcoming the two limitations and to enhancing TiO₂'s photocatalytic efficiency have been highly sought. To this end, a global search of anatase reconstructed surfaces is performed based on the evolutionary method. It is found that the newly predicted anatase (101) reconstructed surface possesses a desired bandgap whose value is within the energy domain of visible light as well as notably high chemical reactivity compared to the unreconstructed anatase (101) surface. In particular, it is predicted that under Ti‐richness condition, the anatase (101) reconstructed surface is energetically very stable. The anatase (101) reconstructed surface exhibits similar topmost surface structure as the unreconstructed anatase (101) surface but different subsurface structure. Not only the fivefold coordinated Ti atoms (Ti_5c) in the topmost surface layer but also the sixfold coordinated Ti atoms in the subsurface layer contribute to the desirable gap states. The high chemical reactivity of anatase (101) reconstructed surface can be attributed to the extra electrons drawn by the surface Ti_5c atoms and subsurface Ti_6c atoms.     

Strong Electronic Interaction of Amorphous Fe2O3 Nanosheets with Single‐Atom Pt toward Enhanced Carbon Monoxide Oxidation

Platinum‐based catalysts are critical to several chemical processes, but their efficiency is not satisfying enough in some cases, because only the surface active‐site atoms participate in the reaction. Henceforth, catalysts with single‐atom dispersions are highly desirable to maximize their mass efficiency, but fabricating these structures using a controllable method is still challenging. Most previous studies have focused on crystalline materials. However, amorphous materials may have enhanced performance due to their distorted and isotropic nature with numerous defects. Here reported is the facile synthesis of an atomically dispersed catalyst that consists of single Pt atoms and amorphous Fe₂O₃ nanosheets. Rational control can regulate the morphology from single atom clusters to sub‐nanoparticles. Density functional theory calculations show the synergistic effect resulted from the strong binding and stabilization of single Pt atoms with the strong metal‐support interaction between the in situ locally anchored Pt atoms and Fe₂O₃ lead to a weak CO adsorption. Moreover, the distorted amorphous Fe₂O₃ with O vacancies is beneficial for the activation of O₂, which further facilitates CO oxidation on nearby Pt sites or interface sites between Pt and Fe₂O₃, resulting in the extremely high performance for CO oxidation of the atomic catalyst.