High‐Yield Fabrication and Electrochemical Characterization of Tetrapodal CdSe,CdTe, and CdSexTe1–x Nanocrystals

The high‐yield fabrication of tetrapodal CdSe, CdTe, and CdSe_xTe_1–x nanocrystals is systematically studied. CdSe nanocrystals are prepared by first controlling the synthesis of high‐quality wurtzite CdSe and zinc blende CdSe nanocrystals at a relatively high temperature (260 °C) by selecting different ligands. Then, based on the phase control of the CdSe nanocrystals, two nanoparticle‐tailoring routes (i.e., a seed‐epitaxial route and ligand‐dependent multi‐injecting route) are used, and a high yield of CdSe tetrapods is obtained. CdTe nanocrystals are prepared by adjusting the ligand composition and the ratio of Cd to Te; CdTe tetrapods are synthesized in high yield using a mixed ligand that does not contain alkylphosphonic acids. Moreover, the nanoscale Te powder (Te nanowires/nanorods), which is highly soluble in the ligand solvent, is first used as a Te source to synthesize CdTe nanocrystals, which remarkably enhanced the output of the CdTe nanocrystals in one reaction. Furthermore, composition‐tunable ternary CdSe_xTe_1–x alloyed tetrapods are synthesized on a large scale, for the first time, by thermolyzing the mixture of the organometallic Cd precursor and the mixed (Se + Te) source in a mixed‐ligand solution. The CdSe, CdTe, and CdSe_xTe_1–x nanocrystals are characterized by transmission electron microscopy (TEM), high‐resolution TEM, selected‐area electron diffraction, X‐ray diffraction, and UV‐vis and photoluminescence (PL) spectroscopy. Interesting nonlinear, composition‐dependent absorption and PL spectra are observed for the ternary CdSe_xTe_1–x alloyed nanocrystals. The band‐edge positions of the nanocrystals of CdSe, CdSe_xTe_1–x, and CdTe are systematically studied by cyclic voltammetry.     

High‐Efficient Deep‐Blue Light‐Emitting Diodes by Using High Quality ZnxCd1‐xS/ZnS Core/Shell Quantum Dots

High‐quality violet‐blue emitting Zn_xCd_1‐xS/ZnS core/shell quantum dots (QDs) are synthesized by a new method, called “nucleation at low temperature/shell growth at high temperature”. The resulting nearly monodisperse Zn_xCd_1‐xS/ZnS core/shell QDs have high PL quantum yield (near to 100%), high color purity (FWHM) <25 nm), good color tunability in the violet‐blue optical window from 400 to 470 nm, and good chemical/photochemical stability. More importantly, the new well‐established protocols are easy to apply to large‐scale synthesis; around 37 g Zn_xCd_1‐xS/ZnS core/shell QDs can be easily synthesized in one batch reaction. Highly efficient deep‐blue quantum dot‐based light‐emitting diodes (QD‐LEDs) are demonstrated by employing the Zn_xCd_1‐xS/ZnS core/shell QDs as emitters. The bright and efficient QD‐LEDs show a maximum luminance up to 4100 cd m^?2, and peak external quantum efficiency (EQE) of 3.8%, corresponding to 1.13 cd A^?1 in luminous efficiency. Such high value of the peak EQE can be comparable with OLED technology. These results signify a remarkable progress, not only in the synthesis of high‐quality QDs but also in QD‐LEDs that offer a practicle platform for the realization of QD‐based violet‐blue display and lighting.     

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.     

One‐Step Solvent‐Free Synthesis and Characterization of Zn1−xMnxSe@C Nanorods and Nanowires

The carbon‐encapsulated, Mn‐doped ZnSe (Zn_1−xMn_xSe@C) nanowires, nanorods, and nanoparticles are synthesized by the solvent‐free, one‐step RAPET (reactions under autogenic pressure at elevated temperature) approach. The aspect ratio of the nanowires/nanorods is altered according to the Mn/Zn atomic ratio, with the maximum being observed for Mn/Zn = 1:20. A 10–20 nm amorphous carbon shell is evidenced from electron microscopy analysis. The replacement of Zn by Mn in the Zn_1−xMn_xSe lattice is confirmed by the hyperfine splitting values in the electron paramagnetic resonance (EPR) experiments. Raman experiments reveal that the Zn_1−xMn_xSe core is highly crystalline, while the shell consists of disordered graphitic carbon. Variable‐temperature cathodoluminescence measurements are performed for all samples and show distinct ZnSe near‐band‐edge and Mn‐related emissions. An intense and broad Mn‐related emission at the largest Mn alloy composition of 19.9% is further consistent with an efficient incorporation of Mn within the host ZnSe lattice. The formation of the core/shell nanowires and nanorods in the absence of any template or structure‐directing agent is controlled kinetically by the Zn_1−xMn_xSe nucleus formation and subsequent carbon encapsulation. Mn replaces Zn mainly in the (111) plane and catalyzes the nanowire growth in the [111] direction.     

Synthesis and Characterization of Discrete Nickel Phosphide Nanoparticles: Effect of Surface Ligation Chemistry on Catalytic Hydrodesulfurization of Thiophene

Discrete, unsupported nanoparticles of Ni₂P have been prepared by using a solution‐phase method with bis(1,5‐cyclooctadiene)nickel(0) [Ni(COD)₂] as the nickel source and trioctylphosphine (TOP) as the phosphorus source in the presence of the coordinating solvent trioctylphosphine oxide (TOPO). Ni₂P nanoparticles prepared at 345 °C have an average crystallite size of 10.2 ± 0.7 nm and are capped with TOP and/or TOPO coordinating agents. The surface of the Ni₂P nanoparticles can be modified by washing with CHCl₃ or by exchanging TOP/TOPO groups with mercaptoundecanoic acid (MUA). The surface areas of these nanoparticles are on the order of 30–70 m² g^–1. As‐prepared and MUA‐capped nanoparticles undergo a phase transformation at 370 °C under reducing conditions, but CHCl₃‐washed Ni₂P nanoparticles retain the Ni₂P structure. CHCl₃‐washed and MUA‐capped nanoparticles exhibit higher HDS catalytic activity than as‐prepared nanoparticles or unsupported Ni₂P prepared by temperature‐programmed reduction of a phosphate precursor. The surface modifications have a clear effect on the catalytic activity as well as the thermal stability of Ni₂P nanoparticles under reducing conditions.     

Effect of window layer composition in Cd1−xZnxS/CdTe solar cells

To improve CdS/CdTe cell/module efficiencies, CdS window layer thinning is commonly applied despite the risk of increased pin‐hole defects and shunting. An alternative approach is to widen the band gap of the window layer (2.42 eV for CdS) via alloying, for example, by forming compositions of Cd_1−xZn_xS. In this study, the performance of Cd_1−xZn_xS/CdTe thin‐film solar cells has been studied as a function of x (from x = 0 to 0.9), widening the window layer band gap up to and over 3.4 eV. Optimum Cd_1−xZn_xS compositions were clearly identified to be around x = 0.7, and limitations to the achievable photocurrent and conversion efficiencies have been addressed. Copyright © 2012 John Wiley & Sons, Ltd.     

Aluminum‐doped Zn1 − xMgxO as transparent conductive oxide of Cu(In,Ga)(S,Se)2‐based solar cell for minimizing surface carrier recombination

A 19.5%‐efficient Cu(In,Ga)(S,Se)₂ (CIGSSe)‐based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O buffer layers for increasing short‐circuit current density because band‐gap energies of Cd_0.75Zn_0.25S and Zn_0.79Mg_0.21O are wider than those of CdS and ZnO, respectively. This yields the increase in external quantum efficiency in a short wavelength range of approximately 320 to 550 nm. Moreover, difference of conduction band minimum (E _C) between Zn_1 − x Mg_x O:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg] + [Zn]), x . It is revealed that Zn_1 − x Mg_x O:Al films with [Mg]/([Mg] + [Zn]) in a range of 0.10 to 0.12, enhancing E _g from 3.72 to 3.76 eV, are appropriate as TCO because of their enhanced mobility and decreased carrier density. Addition of 12% Mg into ZnO:Al to form Zn_0.88Mg_0.12O:Al as TCO layer effectively decreases surface carrier recombination and improves photovoltaic parameters, especially open‐circuit voltage and fill factor. This is the first experimental proof of the concept for optimizing E _C difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency (η ) of CIGSSe solar cell with alternative Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O/Zn_0.88Mg_0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total E _C alignment, which is higher η up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.     

Band gap tailoring and yellow band emission of Cd0.9−xMnxZn0.1S (x=0 to 0.05) nanoparticles: Influence of Mn concentration

Cd_0.9-xMn_xZn_0.1S nanoparticles with x=0 to 0.05 were prepared by a simple chemical co-precipitation method at room temperature. Crystal structure and optical properties of the synthesized nanoparticles have been analyzed by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirmed the phase singularity of the synthesized material, which also confirmed the formation of Cd–Mn–Zn–S solid solution rather than secondary phase formation. Energy dispersive X-ray spectra showed the presence of Cd, Zn, Mn and S in the synthesized samples. The observed higher absorbance and lower transmittance of Mn-doped Cd_0.9Zn_0.1S than Cd_0.9Zn_0.1S is due to the size effect and also the defect states induced by Mn. The decrease in energy gap at Mn=0.01 is due the ‘sp–d’ exchange interactions between band electrons in CdS and the localized ‘d’ electrons of the Mn²⁺ ions. The increase in energy gap after Mn=0.01 can be explained by the excessive carriers generated by the impurity atoms. Fourier transform infrared spectroscopy (FTIR) illustrated the vibration modes of Cd–Zn–Mn–S between the wave number 530 cm⁻¹ and 780 cm⁻¹. Mn=0.01 doped sample exhibits a relatively high PL intensity and covers most of the visible region than the other samples; so desirable for LED application. The intensity ratio of the green band (GB) to Mn-related yellow band (YB) is decreased after Mn=0.01 which may be due the size effect or reduction of surface defect at higher doping concentrations.     

Gradient Conduction Band Energy Engineering Driven High-Efficiency Solution-Processed Cu2ZnSn(S,Se)4/ZnxCd1–x S Solar Cells

The photovoltaic performance of the environmentally friendly Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is lower than its predecessor Cu(In,Ga)Se₂ solar cells. Severe carrier recombination at the CZTSSe/CdS interface is one major reason that results in a large open-circuit voltage loss. Doping zinc into CdS is a feasible strategy to modifying the CdS buffer layer film, but the present methods are not satisfactory. In this study, novel zinc incorporation strategy is developed to deposit a gradient composition ternary Zn_xCd_1–xS buffer layer for optimizing the heterojunction interface. The application of gradient composition Zn_xCd_1–xS buffer layer constructs a gradient conduction band energy configuration in the CZTSSe/buffer layer interface, which highly reduces the interface recombination. The suppressed interface recombination contributes to the enhanced open circuit voltage and device performance. Consequently, the CZTSSe solar cell based on gradient composition Zn_xCd_1–xS buffer layers achieves champion efficiency of 12.35% with V_OC of 504.81 mV, J_SC of 36.90 mA cm⁻², and FF of 66.28%. It is worth noting that flammable and the toxic hydrazine solvent are replaced by the safe and low-toxic 2-methoxyethanol, making it more promising for the future commercialization of CZTSSe solar cells.     

Synthesis and Assembly of Core–Shell Nanorods with High Quantum Yield and Linear Polarization

The seeded growth method offers an efficient way to design core–shell semiconductor nanocrystals in the liquid phase. The combination of seed and shell materials offers wide tunability of morphologies and photophysical properties. Also, semiconductor nanorods (NRs) exhibit unique polarized luminescence which can potentially break the theoretical limit of external quantum efficiency in light emitting diodes based on spherical quantum dots. Although rod-in-rod core–shell NRs present higher degree of polarization, most studies have focused on dot-in-rod core–shell NRs due to the difficulties in achieving uniform NR seeds. Here, this study prepares high-quality uniform CdSe NRs by improving the reactivity of the Se source, using a secondary phosphine, namely diphenylphosphine, to dissolve the Se power, along with the conventional tertiary phosphine, namely trioctylphosphine. Starting from these high-quality NR seeds, this study synthesizes CdSe/Cd_xZn_1−xS/ZnS core–shell NRs with narrow emission bandwidth (29 nm at 620 nm), high PLQY (89%) and high linear polarization (p = 0.90). This study then assembles these core–shell NRs using the confined assembly method and fabricates long-range-ordered microarrays with programmable patterns and displaying highly polarized emission (p = 0.80). This study highlights the great potential of NRs for application in liquid crystal displays and full-color light emitting diodes displays.     

(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.     

A study of polycrystalline Cd(Zn,Mn)Te/Cds films and interfaces

Polycrystalline films of Cd_1-x Zn _x Te (x = 0–0.4) and Cd_1-x Mn _x Te (x = 0–0.25) were grown by MBE and MOCVD, respectively, on CdS/SnO₂/glass substrates to investigate their feasibility for solar cell applications. The compositional uniformity and interface quality of the films were analyzed by x-ray diffraction, surface photovoltage, and Auger depth profile measurements to establish a correlation between growth conditions and lattice constant, atomic concentration, and bandgap of the ternary films. MBE-grown polycrystalline Cd_1-x Zn _x Te films showed a linear dependence between Zn/(Cd + Zn) beam flux ratio, Zn concentration in the film, and the bandgap. Polycrystalline Cd_1-x Zn _x Te films grown at 300° C showed good compositional uniformity in contrast to compositionally non-uniform Cd_1-x Mn _x Te films grown by MOCVD in the temperature range of 420–450° C. The MBE-grown Cd_1-x Zn _x Te interface also showed significantly less interdiffusion compared to the MOCVD-grown Cd_1-x Mn _x Te/CdS interface, where preferential exchange between Cd from the CdS layer and Mn from the Cd_1-x Mn _x Te film was observed. The compositional uniformity of MOCVD-grown polycrystalline Cd_1-x Mn _x Te films grown on CdS/SnO₂/glass substrates was found to be a strong function of the growth conditions as well as the Mn source.     

Dependence of the properties of Cd1−x ZnxTe crystals on the type of intrinsic point defect formed by oxygen

Coordinated investigations of cathodoluminescence spectra, microstructure, specific resistance and presence of oxygen in Cd_1−x Zn_xTe crystals are used to identify how the electrical properties and degree of perfection of the crystal lattice of these materials are affected by the form in which oxygen is present as an intrinsic point defect. It is found that oxygen, which is the main background impurity in Cd_1−x Zn_xTe, forms different types of point defects at different positions in the lattice, depending on the ratio [Cd]/[Zn]. The optimum composition for making detectors of ionizing radiation, for which the crystal resistance is highest, is Cd_0.77Zn_0.23Te. Fiz. Tekh. Poluprovodn. 33, 569–573 (May 1999)     

Melt growth and stoichiometry control of (Cd1−xZnx)1+yTe single crystals

Growth of (Cd_1−xZn_x)_1+yTe(CZT) single crystals is tried by a modified Bridgman method using a reservoir chamber containing Cd and Zn metals with a fixed mole ratio. The aim of this method is to obtain the single crystals with controlled deviation y from stoichiometry and homogeneous target composition x. A suitable growth condition was examined experimentally and the effectiveness of this method for controlling the deviation y from stoichiometry and the composition x is confirmed.     

Vapor phase equilibria in the Cd1−xZnxTe alloy system

Cd_1−xZn_xTe compounds of different compositions have been prepared at temperatures ranging from 400 to 1000°C by annealing elemental Te in sealed quartz ampoules, in an atmosphere comprising vapors of Cd and Zn whose partial pressures were varied by varying the composition of the binary Cd_1−yZn_y alloys which provided the Cd and Zn vapors in these annealing experiments. The chemical compositions of the resulting Cd_1−xZn_xTe compounds have been analyzed using electron probe microanalytical techniques. Results indicate that presence of a 0.5%Zn along with Cd in a closed or semi-closed system may prove to be beneficial in preventing decomposition and/or formation of a metal/non metal phase during annealing of Cd_0.96Zn_0.04 Te substrates. Using the thermodynamic data in the literature for the binary Cd_1−yZn_y alloys and with the assumption that the activities of the Cd and Zn components are weakly dependent on temperature, the partial pressures of Cd and Zn in equilibrium with the Cd_1−xZn_xTe compounds at various temperatures have been evaluated.     

On the origin of the 1-eV band in photoluminescence from Cd1−x Zn x Te

Photoluminescence (PL) at 77 K from Cd_1?x Zn _x Te samples (x = 0, 0.005 and 0.01) annealed at 900°C and cadmium vapor pressure P _Cd = 3 × 10⁴?2 × 10⁵ Pa has been studied. It was found that the contribution of the 1-eV band to the spectrum-integrated PL from these samples is independent of P _Cd, in contrast to Cd_0.95Zn_0.05Te samples in which this contribution increases up to ～90% as P _Cd grows. The band is not shifted to shorter wavelengths as x becomes larger. The conclusion that Zn vacancies are involved in the formation of Cd_1?x Zn _x Te properties is confirmed. The 1-eV band is attributed to capture of free holes to acceptor levels related to vacancies of both cadmium and zinc. These levels are closely spaced and, therefore, are difficult to resolve.     

Cd1–xMnxS Diluted Magnetic Semiconductors as Nanostructured Guest Species in Mesoporous Thin‐Film Silica Host Media

Recently, we demonstrated the possibility of synthesizing ordered nanowires of diluted magnetic II/VI semiconductors inside the channels of mesoporous silica host structures. Here, we expand this procedure from mesoporous powders to mesoporous thin films. Diluted magnetic semiconductors Cd_1–xMn_xS were synthesized within the pores of mesoporous thin‐film silica host structures by a wet‐impregnation technique using an aqueous solution of the respective metal acetates, followed by drying steps and a conversion to sulfide by thermal H₂S treatment. The presence of Cd_1–xMn_xS nanoparticles inside the pores was proved by powder X‐ray diffraction, infrared and Raman spectroscopy, and transmission electron microscopy. Photoluminescence excitation measurements clearly demonstrate the quantum size effect of the incorporated nanostructured guest species. The quality of the nanoparticles incorporated into the mesoporous films is comparable to that of those inside the mesoporous powders.     

Size‐Controlled Synthesis of Cu2‐xE (E = S,Se) Nanocrystals with Strong Tunable Near‐Infrared Localized Surface Plasmon Resonance and High Conductivity in Thin Films

A facile method for preparing highly self‐doped Cu_2‐xE (E = S, Se) nanocrystals (NCs) with controlled size in the range of 2.8–13.5 nm and 7.2–16.5 nm, for Cu_2‐xS and Cu_2‐xSe, respectively, is demonstrated. Strong near‐infrared localized surface plasmon resonance absorption is observed in the NCs, indicating that the as‐prepared particles are heavily p‐doped. The NIR plasmonic absorption is tuned by varying the amount of oleic acid used in synthesis. This effect is attributed to a reduction in the number of free carriers through surface interaction of the deprotonated carboxyl functional group of oleic acid with the NCs. This approach provides a new pathway to control both the size and the cationic deficiency of Cu_2‐xSe and Cu_2‐xS NCs. The high electrical conductivity exhibited by these NPs in metal‐semiconductor‐metal thin film devices shows promise for applications in printable field‐effect transistors and microelectronic devices.     

Tunable properties of cadmium substituted ZnS nanocrystals

Tailored Zn_1−xCd_xS (x = 0, 0.25, 0.5, 0.75 and 1) nanoparticles, synthesized by co-precipitation method under ultrasonic irradiation, were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), UV–Vis and photoluminescence (PL) spectroscopy measurements. According to the XRD results, substitution of Zn²⁺ by Cd²⁺ ion leads to an increase in the lattice parameters and the average size of zinc blended nanoparticles are in the range of 3–4 nm. Transmission electron microscopy image revealed the formation of nano-sized particles with dimension of 5 nm confirming that the samples are quantum dots. The shift observed in the absorption edges by increasing Cd²⁺ ion substitution is ascribed by the alloying effect but the enhancement of band gap energy compared to that of the corresponding bulk value is attributed to the nanometric grain size and quantum confinement effects. The position and intensity of PL emission peaks are tuned with Cd²⁺/Zn²⁺ ion content.     

Over 30% External Quantum Efficiency Light‐Emitting Diodes by Engineering Quantum Dot‐Assisted Energy Level Match for Hole Transport Layer

In the study of hybrid quantum dot light‐emitting diodes (QLEDs), even for state‐of‐the‐art achievement, there still exists a long‐standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn_1?x Cd_xSe/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn_1?xCd_xSe‐ based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m^?2, very low efficiency roll‐off at high current density (EQE ≈25% @ current density of 150 mA cm^?2), and operational lifetime extended to ≈1 800 000 h at 100 cd m^?2. These extraordinary performances make this work the best among all solution‐processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2‐ethylhexane‐1‐thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.