Wurtzite CdTe Research Articles

Synthesis of Atomically Thin CdTe Nanoplatelets by Using Polytelluride Tellurium Precursors

Colloidal two-dimensional (2D) semiconductor nanocrystals are of great importance due to their remarkable optical and electronic properties. Herein, shape-controllable synthesis of 2D wurtzite CdTe nanoplatelets (NPLs) by simply tailoring the reactivity of a tellurium (Te) precursor is reported. Ribbon-, shield-, and bullet-like 2D CdTe NPLs were prepared by a stepwise conversion from CdTe magic-size nanoclusters (MSNCs) by using Te32–, Te22–, and Te2– polytellurides as the tellurium precursor, respectively. This work not only develops a synthetic strategy capable of synthesising wurtzite CdTe nanoplatelets with controlled shapes by tailoring the reactivity of tellurium precursors but also gives insights into the growth mechanisms of colloidal 2D semiconductor nanocrystals.

A study of structural, electronic, elastic, phonon properties, and transition mechanism of wurtzite CdTe under high pressure

Ab initio computational methods based on density functional theory to study the structural, electronic, elastic and phonon properties of Cadmium Telluride (CdTe) were applied. SIESTA method was used for calculations with the generalized gradient approximation (GGA) for the exchange-correlation functional and norm-conserving Troullier-Martins pseudopotentials. Our calculations are carried out to investigate the high-pressure behavior of the hexagonal wurtzite structured CdTe corresponding to the space group of P63mc. When increased hydrostatic pressure was applied on this structure of CdTe, phase transformation was obtained to a cubic structure with space group Fm3‾m at 10 GPa. During this phase transformation, the transition path was predicted as follows: P63mc→P21→Cmc21→P21→P21/m→Pmn21→Pmmn→Fm3‾→Fm3‾m. As the pressure continued to increase, phase transformation to an orthorhombic structure with space group Cmmm at 140 GPa occurred. During this transformation, the transition path was predicted as follows. Fm3‾m→R3‾m→P1‾→P4/mmm→Cmmm. As a result of the literature studies, the transition paths obtained in the study were estimated for the first time. In addition, the electronic properties of CdTe such as band structure and density of states for the obtained high-pressure phases were examined. Although the wurtzite structure of CdTe has a semiconductor character, the other obtained cubic and orthorhombic structures have a metallic character. In addition, in order to determine whether all phases of CdTe are mechanically and dynamically stable, we also examined the elastic and phonon properties respectively. As a result, the wurtzite and cubic structures of CdTe were determined to be both mechanically and dynamically stable, whereas the orthorhombic structure was unstable.

Polarization and magneto-optical properties of excitonic emission from wurtzite CdTe/(Cd,Mg)Te core/shell nanowires

Wurtzite CdTe and (Cd,Mn)Te nanowires embedded in (Cd,Mg)Te shells are grown by employing vapour–liquid–solid growth mechanism in a system for molecular beam epitaxy. A combined study involving cathodoluminescence, transmission electron microscopy and micro-photoluminescence is used to correlate optical and structural properties in these structures. Typical features of excitonic emission from individual wurtzite nanowires are highlighted including the emission energy of 1.65 eV, polarization properties and the appearance B-exciton related emission at high excitation densities. Angle dependent magneto-optical study performed on individual (Cd,Mn)Te nanowires reveals heavy-hole-like character of A-excitons typical for wurtzite structure and allows to determine the crystal field splitting, ΔCR. The impact of the strain originating from the lattice mismatched shell is discussed and supported by theoretical calculations.

Core-Shell Cadmium Telluride Quantum Platelets with Absorptions Spanning the Visible Spectrum.

CdS and CdSe shells are deposited on wurtzite CdTe quantum platelets (nanoplatelets) by exchanging the initial primary-amine ligation to Cd(OAc)2 ligation, with subsequent reaction of the Cd(OAc)2 ligand shell and thiourea or selenourea, respectively. Shell deposition is conducted in a cyclic manner, with 0.21-0.34 monolayers of CdS and 0.99-1.20 monolayers of CdSe being deposited in each cycle. The CdTe quantum platelets having an initial thickness of 1.9 nm are converted to CdTe-CdS and CdTe-CdSe core-shell quantum platelets having maximum thicknesses of 3.0 and 6.3 nm, respectively. The morphologies and wurtzite structure of the initial CdTe quantum platelets are retained upon shell deposition. The absorption spectrum of the CdTe quantum platelets is progressively shifted to lower energy with increasing shell thickness, across the entire visible spectrum. The spectral shifts observed scale with the inverse square of the total core-shell thickness.

Growth of wurtzite CdTe nanowires on fluorine-doped tin oxide glass substrates and room-temperature bandgap parameter determination

The growth of CdTe nanowires, catalyzed by Sn, was achieved on fluorine-doped tin oxide glass by physical vapor transport. CdTe nanowires grew along the 〈0001〉 direction, with a very rare and phase-pure wurtzite structure, at 290 °C. CdTe nanowires grew under Te-limited conditions by forming SnTe nanostructures in the catalysts and the wurtzite structure was energetically favored. By polarization-dependent and power-dependent micro-photoluminescence measurements of individual nanowires, heavy and light hole-related transitions could be differentiated, and the fundamental bandgap of wurtzite CdTe at room temperature was determined to be 1.562 eV, which was 52 meV higher than that of zinc-blende CdTe. From the analysis of doublet photoluminescence spectra, the valence band splitting energy between heavy hole and light hole bands was estimated to be 43 meV.

Electrodeposition of wurtzite CdTe and the potential dependence of the phase structure

CdTe solar cell is promising in high efficiency and low cost. Wurtzite CdTe is metastable and hard to prepare at high temperature. Herein, we electrodeposited CdTe film from an alkaline solution. The wurtzite structure was firstly found in the electrodeposited films. The film composition and phase structure varied much with deposition potential. Annealing causes the phase transformation from wurtzite to zinc blende. The crystallinity, density, and interface quality of CdTe films after annealing depend heavily on component and phase structure of the as-prepared films.

Cyclic arginyl–glycyl–aspartic acid (RGD) peptide-induced synthesis of uniform and stable one-dimensional CdTe nanostructures in aqueous solution

Aqueous synthesis of one dimensional (1D) semiconductor nanostructures presents a great challenge. By using a cyclic RGD peptide as the ligand, 1D wurtzite CdTe nanocrystals with uniform size and excellent storage stability were prepared in aqueous solution. The integration of one dimensionality with the specific RGD motif makes the CdTe nanocrystals attractive in constructing optical and electrical devices or sensors for biotechnological applications.

Fabrication and growth mechanism of zinc blende and wurtzite CdTe nanowire arrays with different photoelectric properties

Large-scale, well-aligned CdTe nanowire arrays with different crystallographic phases have been prepared simply by regulating the basic growth parameters of the sputtering method. Pure zinc blende phase nanowire arrays that propagate in the (111) direction are achieved using a high growth temperature coupled with a low deposition rate. Conversely, wurtzite phase nanowire arrays with growth in the (002) direction have also been prepared by using a lower growth temperature and higher deposition rate. A Gibbs free energy nucleation model is use to explain the formation of these different crystal phases under the growth conditions employed. The differences in crystal structure are shown to exhibit different energy bands, defects and carrier transport properties. The wurtzite phase, with a narrower band gap (1.64 eV), is found to have better photoelectric properties than those of the zinc blende phase.

CdTe nanoparticles and clusters

Precipitation from solution under highly nonequilibrium conditions can be used to prepare cadmium telluride nanocrystals with a stable cubic (sphalerite) structure or clusters with a metastable structure close to that of wurtzite CdTe, depending on the process conditions.

The role of substrate surface alteration in the fabrication of vertically aligned CdTenanowires

Previously we have described the deposition of vertically aligned wurtzite CdTe nanowiresderived from an unusual catalytically driven growth mode. This growth mode could onlyproceed when the surface of the substrate was corrupted with an alcohol layer, althoughthe role of the corruption was not fully understood. Here, we present a studydetailing the remarkable role that this substrate surface alteration plays in thedevelopment of CdTe nanowires; it dramatically improves the size uniformityand largely eliminates lateral growth. These effects are demonstrated to arisefrom the altered surface’s ability to limit Ostwald ripening of the catalytic seedmaterial and by providing a surface unable to promote the epitaxial relationshipneeded to sustain a lateral growth mode. The axial growth of the CdTe nanowiresis found to be exclusively driven through the direct impingement of adatomsonto the catalytic seeds leading to a self-limiting wire height associated with thesublimation of material from the sidewall facets. The work presented furthers thedevelopment of the mechanisms needed to promote high quality substrate-basedvertically aligned CdTe nanowires. With our present understanding of the growthmechanism being a combination of selective area epitaxy and a catalytically drivenvapour–liquid–solid growth mode, these results also raise the intriguing possibility ofemploying this growth mode in other material systems in an effort to produce superiornanowires.

From CdTe Nanoparticles Precoated on Silicon Substrate to Long Nanowires and Nanoribbons: Oriented Attachment Controlled Growth

This manuscript describes a simple, environmentally friendly strategy for the rapid and large-scale growth of ultralong nanowires and nanoribbons of wurtzite CdTe. The nanowires and nanoribbons were prepared through the direct hydrothermal treatment of CdTe nanoparticles precoated on ⟨100⟩ Czochralski silicon, which did not involve complicated reactions. The well-crystalline nanowires and nanoribbons were grown along the [102] direction and were up to 100 μm long. The growth of the nanowires and nanoribbons was dominated by the mechanism of oriented attachment, which was clarified through the tracing of the temporal evolution of CdTe nanoparticles coated on the silicon substrate in the process of hydrothermal treatment. Furthermore, the proposed strategy was also effective in the preparation of anisotropic nanostructures of other II−IV group compounds (e.g., ZnO and CdSe).

Vertically aligned wurtzite CdTe nanowires derived from a catalytically driven growthmode

Substrate-based catalytic growth modes have been widely used to fabricate verticallyaligned nanowires for most technologically relevant semiconducting systems, with CdTebeing a notable exception. The catalysts that promote a one-dimensional nanoscale growthmode in other systems seem to fail, creating the need for an alternative approach. Here, wedemonstrate how nanowire structures can be derived from a newly developedcatalytically driven process. The vertically aligned nanowires produced are highly facetedand share an epitaxial relationship with the underlying substrate. The nanowirestructures could also be described as nanorods; they show a high degree of sizeuniformity over large areas with heights of 300 nm imposed by growth dynamics.Two-dimensional x-ray diffraction techniques indicate that the CdTe exists in the wurtzitecrystal structure instead of the zinc blende structure normally associated withthe bulk material. The work presented here adds these substrate-based wurtziteCdTe nanowires to the growing list of potential building blocks for nano-baseddevices.

Evolution of wurtzite CdTe through the formation of cluster assembled films

An approach has been developed to produce CdTe films with a high proportion of the metastable wurtzite phase. It involves the use of pulsed laser deposition to produce CdTe nanoparticles which are then collected on a substrate. Electron microscopy indicates that the nanoparticles produced have a relatively minor fraction of the wurtzite phase coexisting with the stable zinc blende phase. If these same nanoparticles arrive at a heated substrate they form a relatively dense cluster assembled film with an obvious wurtzite signature. Quite remarkable is that higher temperatures promote the metastable structure at the expense of the stable zinc blende phase.

Manipulation of Aqueous Growth of CdTe Nanocrystals To Fabricate Colloidally Stable One-Dimensional Nanostructures

The present article is devoted to systematically exploring the influence of various experimental variables, including the precursor concentration, the ligand nature, the counterion type, the Cd-to-Te molar ratio, pH, and temperature, on the aqueous growth of CdTe nanocrystals. The growth may be divided into two stages: the early fast growth stage and the later slow growth stage. The later stage is found to be dominated by Ostwald ripening (OR), being strongly dependent on all experimental conditions. In contrast, the early stage is dominated by adding monomers to nanocrystals, which may be dramatically accelerated by lowering precursor concentrations and using ligands with a molecular structure similar to that of thioglycolic acid (TGA). This fast growth stage is similar to that observed during organometallic growth of nanocrystals in hot organic media. On the basis of this finding, one-dimensional wurtzite CdTe nanostructures can be directly prepared in aqueous media by storing rather dilute precursor solution (2.4 mM with reference to ligand) in the presence of TGA at lower temperature (from room temperature to 80 degrees C). A low growth temperature is used to suppress OR during crystal growth. In addition, the simultaneous presence of both TGA-like ligand and 1-thioglycerol or 2-mercaptoethylamine leads to formation of colloidally stable 1D CdTe nanostructures with controlled aspect ratios.

Observation and first-principles calculation of buried wurtzite phases in zinc-blende CdTe thin films

We report direct observation of the existence of buried thin wurtzite CdTe layers in nominally pure zinc-blende CdTe thin films using high-resolution transmission electron microscopy. The formation of the buried wurtzite layers is a result of the formation of high density of planar defects in the zinc-blende films—the wurtzite layers are formed by closely spaced lamellar twins. First-principles calculations reveal that the presence of the buried wurtzite layers may be responsible for the poor electrical properties of the polycrystalline zinc-blende CdTe films.

Measurement of the optical band gap and crystal-field splitting in wurtzite CdTe.

CdTe nanocrystals embedded in a sodium borosilicate matrix, obtained by a sol-gel process, present a wurtzite (hexagonal) structure. Low-temperature photoluminescence and absorption spectra are measured for samples containing bulklike nanocrystals large enough to suppress confinement effects. Thus, the fundamental free-exciton gap and the splitting between the A (${\mathrm{\ensuremath{\Gamma}}}_{9}^{\mathit{v}}$-${\mathrm{\ensuremath{\Gamma}}}_{7}^{\mathit{c}}$) and B (${\mathrm{\ensuremath{\Gamma}}}_{7}^{\mathit{v}}$-${\mathrm{\ensuremath{\Gamma}}}_{7}^{\mathit{c}}$) excitons of bulk wurtzite CdTe are measured. We find ${\mathit{E}}_{\mathit{g}}^{\mathit{A}}$=1653\ifmmode\pm\else\textpm\fi{}1 meV and ${\mathit{E}}_{\mathit{g}}^{\mathit{A}}$-${\mathit{E}}_{\mathit{g}}^{\mathit{B}}$=46.5\ifmmode\pm\else\textpm\fi{}1.0 meV, which corresponds to a crystal-field splitting ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{cr}}$=71.6\ifmmode\pm\else\textpm\fi{}1.5 meV. \textcopyright{} 1996 The American Physical Society.