Preparation of Uncapped CdSe x Te1−x Nanocrystals with Strong Near-IR Tunable Absorption

Semiconducting nanocrystals with near-infrared (NIR) photosensitivity are appealing materials for application as photodetectors and in medical diagnostics. Herein, we report the preparation of composition-tunable, uncapped CdSe _x Te_1?x (x = 0 to 1) nanocrystals by simple mechanical alloying. The resulting ternary CdSe _x Te_1?x (x = 0.25, 0.5, 0.75) nanocrystals with average sizes smaller than 10 nm have zincblende crystal structure, instead of the wurtzite structure commonly obtained by wet chemical routes, and show strong NIR absorption even beyond 1400 nm. While a linear relationship between the lattice parameter and the chemical composition (Se/Te ratio) is observed, indicating the formation of homogeneous alloys, the bandgap energy of the three ternary samples is found to be substantially lower than that of binary CdSe or CdTe nanocrystals, and lower than any ternary CdSeTe reported so far. Existence of a small number of tellurium metal defects in the CdSe _x Te_1?x (x = 0.25, 0.5, 0.75) nanocrystals is confirmed by x-ray diffraction and Raman spectroscopy. Both the optical bowing effect and tellurium metal-induced defects of the mechanically alloyed samples are believed to cause the strong NIR photosensitivity.     

Highly Stable Colloidal “Giant” Quantum Dots Sensitized Solar Cells

Colloidal quantum dots (QDs) are widely studied due to their promising optoelectronic properties. This study explores the application of specially designed and synthesized “giant” core/shell CdSe/(CdS)_x QDs with variable CdS shell thickness, while keeping the core size at 1.65 nm, as a highly efficient and stable light harvester for QD sensitized solar cells (QDSCs). The comparative study demonstrates that the photovoltaic performance of QDSCs can be significantly enhanced by optimizing the CdS shell thickness. The highest photoconversion efficiency (PCE) of 3.01% is obtained at optimum CdS shell thickness ≈1.96 nm. To further improve the PCE and fully highlight the effect of core/shell QDs interface engineering, a CdSe_x S_1?x interfacial alloyed layer is introduced between CdSe core and CdS shell. The resulting alloyed CdSe/(CdSe_x S_1?x )₅/(CdS)₁ core/shell QD‐based QDSCs yield a maximum PCE of 6.86%, thanks to favorable stepwise electronic band alignment and improved electron transfer rate with the incorporation of CdSe_x S_1?x interfacial layer with respect to CdSe/(CdS)₆ core/shell. In addition, QDSCs based on “giant” core/CdS‐shell or alloyed core/shell QDs exhibit excellent long‐term stability with respect to bare CdSe‐based QDSCs. The giant core/shell QDs interface engineering methodology offers a new path to improve PCE and the long‐term stability of liquid junction QDSCs.     

Composition‐ and Shape‐Controlled Synthesis and Optical Properties of ZnxCd1–xS Alloyed Nanocrystals

Composition‐tunable Zn_xCd_1–xS alloyed nanocrystals have been synthesized by a new approach consisting of thermolyzing a mixture of cadmium ethylxanthate (Cd(exan)₂) and zinc ethylxanthate (Zn(exan)₂) precursors in hot, coordinating solvents at relatively low temperatures (180–210 °C). The composition of the alloyed nanocrystals was accurately adjusted by controlling the molar ratio of Cd(exan)₂ to Zn(exan)₂ in the mixed reactants. The alloyed Zn_xCd_1–xS nanocrystals prepared in HDA/TOP (HDA: hexadecylamine; TOP: trioctylphosphine) solution exhibit composition‐dependent shape and phase structures as well as composition‐dependent optical properties. The shape of the Zn_xCd_1–xS nanocrystals changed from dot to single‐armed rod then to multi‐armed rod with a decrease of Zn content in the ternary nanoparticles. The alloying nature of the Zn_xCd_1–xS nanocrystals was consistently confirmed by the results of high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), and UV‐vis absorption and photoluminescence (PL) spectroscopy. Further, the shape‐controlled synthesis of the ternary alloyed nanocrystals was realized by selecting appropriate solvents. Uniform nanodots in the whole composition range were obtained from TOPO/TOP solution, (TOPO: trioctylphosphine oxide) and uniform nanorods in the whole composition range were prepared from HDA/OA solution (OA: octylamine). The effect of the reaction conditions, such as solvent, reaction temperature, and reaction time, on the PL spectra of the alloyed Zn_xCd_1–xS nanocrystals was also systematically studied, and the reaction conditions were optimized for improving the PL properties of the nanocrystals.     

Optical properties of ultradisperse CdSexTe1−x (0≤x≤1) particles in a silicate glass matrix

Ultradisperse particles of CdSe_xTe_1?x (0≤x≤1) were formed in the matrix of a silicate glass by introduction of corresponding prepared compounds into a mixture of SiO₂ and Ca, Na, K, and Li oxides. Initially produced particles with an average size of 10–15 nm grow by a factor of 2–3 after additional thermal treatment of the glasses; the optical absorption spectra are noticeably changed only in the case of CdSe_xTe_1?x (x=0.8 or 0.4) solid solutions, but not in the case of CdSe and CdTe binary compounds. All glasses synthesized exhibit similar luminescence bands in the visible region of the spectrum. The features observed are interpreted under the assumption that two crystalline modifications (wurtzite and sphalerite) are separated in the process of thermal treatment of glasses activated by solid solutions.     

Limitations in Producing Nanocrystals Using Reverse Micelles as Nanoreactors

In this paper the synthesis and characterization of CdTe nanocrystals are presented. Opposite to what has been observed with Cd_1–yMn_yS, it has been impossible to include manganese ions in CdTe matrices. Only large rods of telluride were seen. ZnTe could also not be formed and again only Te rods were perceived. This clearly indicates that chemistry in colloidal self‐assemblies is not always similar to that in homogeneous solutions.     

High‐Performance Hybrid Solar Cell Made from CdSe/CdTe Nanocrystals Supported on Reduced Graphene Oxide and PCDTBT

Core/shell tetrapods synthesized from CdSe and CdTe exhibit a type II band offset that induces separation of charge upon photoexcitation and localizes carriers to different regions of the tetrahedral geometry. CdSe/CdTe nanocrystals immobilized on oleylamine‐functionalized reduced graphene oxide (rGO) sheets can be homogeneously mixed with an organic dye (PCDTBT) to form donor–acceptor dispersed heterojunctions and exhibit a high power conversion efficiency of ～3.3% in solar cell devices. The near‐IR light absorbing type II nanocrystals complement the absorption spectrum of the visible light‐absorbing organics. The high efficiency is attributed to the amine‐functionalized rGO sheets, which allow intimate contact with the nanocrystals and efficient dispersal in the organic matrix, contributing to highly efficient charge separation and transfer at the nanocrystal, rGO, and polymer interfaces.     

Structure of cadmium selen-telluride alloy films grown by the thermal-screen method under highly nonequilibrium conditions

The results of technological experiments and structural investigations of films of CdSe _x Te_{1 ? x} alloys synthesized by the thermal-screen method on heated and cooled substrates (under highly nonequilibrium conditions) are presented. It is shown that the synthesis of the entire range of compositions of alloy films with the structure from epitaxial to amorphous is possible from the mechanical mixture of CdSe and CdTe powders of the same composition under highly nonequilibrium conditions. The electron diffraction patterns and the microphotographs of film surfaces are reported.     

Characteristics of slurry coated CdSeTe films

CdSe_xTe_1?x films were deposited by the slurry coating technique using CdSe_xTe_1?x powders synthesized in the laboratory. X-ray diffraction studies indicated the formation of a hexagonal phase. From EDAX compositional analysis, the individual concentrations of Se and Te in the films were estimated. Analysis of the optical data indicate the band gap to vary from 1.44 to 1.68 eV as the value of ‘x’ changes from 0 to 1. XPS analysis was also carried out on the films. The films were used as photoanodes in polysulphide electrolyte and it was observed that the films with composition CdSe_0.6Te_0.4 exhibited the maximum photoactivity. The Mott–Schottky plot indicated an n-type behaviour. Spectral response measurements were made and the electrodes exhibited a quantum efficiency of 0.6.     

Molecular-beam epitaxial growth of CdSe<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> on Si(211)

We report on the first successful growth of the ternary-alloy CdSe_xTe_1−x(211) on 3-in. Si(211) substrates using molecular-beam epitaxy (MBE). The growth of CdSeTe was performed using a compound CdTe effusion source and an elemental Se effusion source. The alloy composition (x) of the CdSe_xTe_1−x ternary compound was controlled through the Se:CdTe flux ratios. Our results indicated that the crystalline quality of CdSeTe decreases as the alloy composition increases, which is possibly due to an alloy-disordering effect. A similar trend was observed for the CdZnTe ternary-alloy system. However, the alloy-disordering effect in CdSeTe was found to be less severe than that in CdZnTe. We have carried out the growth of CdSeTe on Si at different temperatures. An optimized growth window was established for CdSeTe on Si(211) to achieve high-crystalline-quality CdSeTe/Si layers with 4% Se. The as-grown layers exhibited excellent surface morphology, low surface-defect density (less than 500 cm⁻²), and low x-ray full width at half maximum (FWHM) values near 100 arcsec. Additionally, the CdSeTe/Si layer exhibited excellent lateral uniformity and the best etched-pit density (EPD) value on a 4% CdSeTe, measured to be as low as 1.4 × 10⁵ cm⁻².     

Ultralong CdTe Nanowires: Catalyst‐Free Synthesis and High‐Yield Transformation into Core–Shell Heterostructures

A novel catalyst‐free synthetic strategy for producing high‐quality CdTe nanowires in solution is proposed. A special reaction condition is intentionally constructed in the reaction system to induce the formation of nanowires through oriented in situ assembly of tiny particles. To establish such special synthetic conditions in the CdTe system, not only are its typical features and possible solutions deeply analyzed, but also related factors, such as the ligand environment, injection and growth temperature, and Cd‐to‐Te precursor ratio, are systemically investigated. High‐quality ultralong (up to 10 μm) and ultrathin (less than 10 nm) CdTe nanowires are produced in solution under optimal reaction conditions. Morphological, spectral, and compositional analyses are performed to examine the products formed at different reaction stages in order to clarify the formation mechanism of the CdTe nanowires. Furthermore, the transformation of the CdTe nanowires into CdTe/CdSe core–shell heterostructures is intensively explored, and the CdSe epitaxial growth process is specially tracked by morphological and spectral characterization techniques. Finally, CdTe nanowires coated with a continuous and dense CdSe shell are successfully fabricated by using a proper coating protocol.     

Martensitic Phase Transformation of Isolated HfO2, ZrO2, and HfxZr1 – xO2 (0 < x < 1) Nanocrystals

We previously reported that, during the reactions to make nanocrystals of HfO₂ and Hf‐rich Hf_xZr_{1 – x}O₂, a tetragonal‐to‐monoclinic phase transformation occurs that is accompanied by a shape change of the particles (faceted spherical to nanorods) when the temperature at which the reaction is conducted is changed from 340 to 400 °C. We now conclude that this concomitant phase and shape change is a result of the martensitic transformation of isolated nanocrystals in a hot liquid, where twinning plays a crucial role in accommodating the shape‐change‐induced strain. That such change was not observed during the reactions forming ZrO₂ and Zr‐rich Hf_xZr_{1 – x}O₂ nanocrystals is attributed to the higher driving force needed in those instances compared to that needed for producing HfO₂ and Hf‐rich Hf_xZr_{1 – x}O₂ nanocrystals. We also report here the post‐synthesis, heat‐induced phase transformation of Hf_xZr_{1 – x}O₂ (0 < x < 1) nanocrystals. As temperature increases, all the tetragonal nanocrystals transform to the monoclinic phase accompanied by an increase in particle size (as evidenced by X‐ray diffraction and transmission electron microscopy), which confirms that there is a critical size for the phase transformation to occur. When the monoclinic nanorods are heated above a certain temperature the grains grow considerably; under certain conditions a small amount of tetragonal phase appears.     

Photoacoustic study of optical and thermal properties of alloyed CdTexS1−x nanocrystals

Photoacoustic (PA) technique has been applied to study the optical and thermal properties of the alloyed CdTe_xS_1−x (nanocrystals (NCs) with different (Te/(S+Te)) molar ratio (x=0, 0.2, 0.4, 0.6, 0.8 and 1). Increasing x value causes clearly observed red shift of the corresponding exciton peak in PA spectra. The same spectra were compared to those obtained by a regular UV–Vis. absorption. The effective mass approximation (EMA) model was applied to determine the size of the NCs. The calculated sizes of the alloyed NCs are in a good agreement with the directly measured values obtained using high resolution transmission electron microscopy (HRTEM). The values of thermal diffusivity and thermal conductivity obtained using PA technique show at least an order of magnitude larger than that of the bulk values.     

One‐Step Preparation of Coaxial CdS–ZnS and Cd1–xZnxS–ZnS Nanowires

Preparation of coaxial (core–shell) CdS–ZnS and Cd_1–xZn_xS–ZnS nanowires has been achieved via a one‐step metal–organic chemical vapor deposition (MOCVD) process with co‐fed single‐source precursors of CdS and ZnS. Single‐source precursors of CdS and ZnS of sufficient reactivity difference were prepared and paired up to form coaxial nanostructures in a one‐step process. The sequential growth of ZnS on CdS nanowires was also conducted to demonstrate the necessity and advantages of the precursor co‐feeding practice for the formation of well‐defined coaxial nanostructures. The coaxial nanostructure was characterized and confirmed by high‐resolution transmission electron microscopy and corresponding energy dispersive X‐ray spectrometry analyses. The photoluminescence efficiencies of the resulting coaxial CdS–ZnS and Cd_1–xZn_xS–ZnS nanowires were significantly enhanced compared to those of the plain CdS and plain Cd_1–xZn_xS nanowires, respectively, owing to the effective passivation of the surface electronic states of the core materials by the ZnS shell.     

Mg3+δSbxBi2−x Family: A Promising Substitute for the State‐of‐the‐Art n‐Type Thermoelectric Materials near Room Temperature

The Bi₂Te_3?xSe_x family has constituted n‐type state‐of‐the‐art thermoelectric materials near room temperature (RT) for more than half a century, which dominates the active cooling and novel heat harvesting application near RT. However, the drawbacks of a brittle nature and Te‐content restricts the possibility for exploring potential applications. Here, it is shown that the Mg_3+δSb_xBi_2?x family ((ZT)_avg = 1.05) could be a promising substitute for the Bi₂Te_3?xSe_x family ((ZT)_avg = 0.9–1.0) in the temperature range of 50–250 °C based on the comparable thermoelectric performance through a synergistic effect from the tunable bandgap using the alloy effect and the suppressible Mg‐vacancy formation using an interstitial Mn dopant. The former is to shift the optimal thermoelectric performance to near RT, and the latter is helpful to partially decouple the electrical transport and thermal transport in order to get an optimal RT power factor. The positive temperature dependence of the bandgap suggests this family is also a superior medium‐temperature thermoelectric material for the significantly suppressed bipolar effect. Furthermore, a two times higher mechanical toughness, compared with the Bi₂Te_3?xSe_x family, allows for a promising substitute for state‐of‐the‐art n‐type thermoelectric materials near RT.     

Facile Controlled Synthesis and Spectroscopy of CdS1−xSex Alloy and (CdS)1−x@(CdSe)x Core–Shell Nanotetrapods

Nanotetrapods of alloy CdS_1?xSe_x and core–shell (CdS)_1?x@(CdSe)_x are fabricated easily in water using ethylenediamine as a solvent‐coordinating molecular template, and then their optical properties are investigated using diverse static and time‐resolved spectroscopic methods. The arms of the alloy nanotetrapods have single‐crystalline structures of CdS_1?xSe_x without showing staking faults, while the arms of the core–shell nanotetrapods display polycrystalline shell structures of CdSe. The optical properties of CdS_1?xSe_x, where Se atoms are isolated in the CdS lattice, are very different from those of (CdS)_1?x@(CdSe)_x, where banded CdSe passivates the CdS core. Compared with pure CdS nanotetrapods, the photoluminescence of CdS_0.9Se_0.1 shifts to the red by 40 nm, whereas that of (CdS)_0.9@(CdSe)_0.1 does so only by 5 nm. Although the mean luminescence lifetime of alloy CdS_1?xSe_x is shorter than that of pure CdS, it is still much longer than that of core‐shell (CdS)_1?x@(CdSe)_x.     

Optical and Structural Properties of CdTe Grown by Molecular Beam Epitaxy at Low Temperature for Resonant-Cavity-Enhanced HgCdTe Detectors

Investigation into resonant-cavity-enhanced (RCE) HgCdTe detectors has revealed a discrepancy in the refractive index of the CdTe layers grown by molecular beam epitaxy (MBE) for the detectors, compared with the reported value for crystalline CdTe. The refractive index of the CdTe grown for RCE detectors was measured using ellipsometry and matches that of CdTe with an inclusion of approximately 10% voids. X-ray measurements confirm that the sample is crystalline and strained to match the lattice spacing of the underlying Hg_(1−x)Cd_(x)Te, while electron diffraction patterns observed during growth indicate that the CdTe layers exhibit some three-dimensional structure. Secondary ion mass spectroscopy results further indicate that there is enhanced interdiffusion at the interface between Hg_(1−x)Cd_(x)Te and CdTe when the Hg_(1−x)Cd_(x)Te is grown on CdTe, suggesting that the defects are nucleated within the CdTe layers.     

Optical properties of CdSe<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> epitaxial films studied by spectroscopic ellipsometry

We have determined the dielectric constants for a series of CdSe_xTe_1−x thin films grown on Si substrates using a rotating-analyzer spectroscopic ellipsometer. Initially, the alloy concentration and the sample quality were determined using x-ray diffraction. A standard inversion technique was then used to obtain the dielectric constants from the measured ellipsometric spectra. Using these calculated absorption spectra, we were able to estimate the fundamental bandgap for these CdSe_xTe_1−x alloys. In addition, we also determined the dispersion of the indices of refraction as well as the critical points related to the higher-order electronic transitions for this alloy.     

Thermoelectric Characteristics of <Emphasis Type="Italic">p</Emphasis>-Type (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript>/(Pb,Sn)Te Functional Gradient Materials with Variation of the Segment Ratio

The p-type (Bi,Sb)₂Te₃/(Pb,Sn)Te functional gradient materials (FGMs) were fabricated by hot-pressing mechanically alloyed (Bi_0.2Sb_0.8)₂Te₃ and 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te powders together at 500°C for 1 h in vacuum. Segment ratios of (Bi,Sb)₂Te₃ to (Pb,Sn)Te were varied as 3:1, 1.3:1, and 1:1.6. A reaction layer of about 350-μm thickness was formed at the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGM interface. Under temperature differences larger than 340°C applied across a specimen, superior figures of merit were predicted for the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs to those of (Bi_0.2Sb_0.8)₂Te₃ and (Pb_0.7Sn_0.3)Te. With a temperature difference of 320°C applied across a specimen, the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs with segment ratios of 3:1 and 1.3:1 exhibited the maximum output powers of 72.1 mW and 72.6 mW, respectively, larger than the 63.9 mW of (Bi_0.2Sb_0.8)₂Te₃ and the 26 mW of 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te.     

A Family of Far‐Infrared‐Transmitting Glasses in the Ga–Ge–Te System for Space Applications

Conditions of formation of bulk tellurium chalcogenide glasses, containing 70–80 at % Te associated to Ga and Ge are described. The characteristic temperatures are measured: the glass‐transition temperature, T_g, is situated in the range 140–185 °C and the difference T_x – T_g, where T_x is the onset crystallization temperature, is in the range 76–113 °C. Moreover, the optical transmission window is exceptional, ranging from 1.99 μm in the bandgap up to 28 μm in the phonon region. Developed within the framework of requirements for the Darwin mission (a search for and study of extrasolar planets), feasibility of infrared optical fibers formed from these glasses is studied. Drawing experiments are conducted with the glass Ga₂‐Ge₃Te₁₅, which possesses the maximum T_x – T_g, and a first optical fiber is presented.     

PbSe/PbS and PbSe/PbSexS1–x Core/Shell Nanocrystals

The synthesis of PbSe/PbS and PbSe/PbSe_xS_1–x core/shell nanocrystals (NCs) with luminescence quantum efficiencies of 45–55 % is reported. PbSe/PbS NCs are prepared via a two‐stage process, while the PbSe/PbSe_xS_1–x NCs are formed in a single‐stage procedure. The core/shell NCs exhibit an energy tuning of the exciton transitions, with respect to that of the core NC, that is dependent on the core diameter, shell thickness, and composition.