Structure Of CdSe Research Articles

Ab Initio Molecular Dynamics of CdSe Quantum-Dot-Doped Glasses.

We have probed the local atomic structure of the interface between a CdSe quantum dot (QD) and a sodium silicate glass matrix. Using ab initio molecular dynamics simulations, we determined the structural properties and bond lengths, in excellent agreement with previous experimental observations. On the basis of an analysis of radial distribution functions, coordination environment, and ring structures, we demonstrate that an important structural reconstruction occurs at the interface between the CdSe QD and the glass matrix. The incorporation of the CdSe QD disrupts the Na-O bonds, while stronger SiO4 tetrahedra are reformed. The existence of the glass matrix breaks the stable 4-membered (4MR) and 6-membered (6MR) Cd-Se rings, and we observe a disassociated Cd atom migrated in the glass matrix. Besides, the formation of Se-Na and Cd-O linkages is observed at the CdSe QD/glass interface. These results significantly extend our understanding of the interfacial structure ofCdSe QD-doped glasses and provide physical and chemical insight into the possible defect structure origin of CdSe QD, of interest to the fabrication of the highly luminescent CdSe QD-doped glasses.

Structures of CdSe and CdS Nanoclusters from Ab Initio Random Structure Searching

The possible structures of (CdSe)n and (CdS)n (n up to 34) nanoclusters were systematically studied using the ab initio random structure searching (AIRSS) method. We have been able to find stable, ...

Mechanical properties and band structure of CdSe and CdTe nanostructures at high pressure - a first-principles study

First-principles calculations for CdSe and CdTe nanostructures were carried out to study their mechanical properties and band structure under the uniaxial pressure range of 0 to 50GPa. It was presumed that the CdSe and CdTe nanostructures exist in the zinc-blende phase under high pressure. The mechanical properties, such as elastic constants, bulk modulus, shear modulus and Young?s modulus, were explored. Furthermore, Cauchy pressure, Poisson?s ratio and Pugh?s criterion were studied under high pressure for both CdSe and CdTe nanostructures, and the results show that they exhibit ductile property. The band structure studies of CdSe and CdTe were also investigated. The findings show that the mechanical properties and the band structures of CdSe and CdTe can be tailored with high pressure.

Role of Surface Morphology on Exciton Recombination in Single Quantum Dot-in-Rods Revealed by Optical and Atomic Structure Correlation.

The physical structure of colloidal quantum dot (QD) nanostructures strongly influences their optical and electronic behavior. A fundamental understanding of this interplay between structure and function is crucial to fully tailor the performance of QDs and their assemblies. Here, by directly correlating the atomic and chemical structure of single CdSe-CdS quantum dot-in-rods with time-resolved fluorescence measurements on the same structures, we identify morphological irregularities at their surfaces that moderate photoluminescence efficiencies. We find that two nonradiative exciton recombination mechanisms are triggered by these imperfections: charging and trap-assisted nonradiative processes. Furthermore, we show that the proximity of the surface defects to the CdSe core of the core-shell structures influences whether the charging or trap-assisted nonradiative channel dominates exciton recombination. Our results extend to other QD nanostructures and emphasize surface roughness as a crucial parameter when designing colloidal QDs with specific excitonic fates.

Electronic energy levels and electrochemical properties of co-electrodeposited CdSe thin films

CdSe semiconductor thin films were grown on indium tin oxide (ITO) coated glass substrates by co-electrochemical deposition method. Deposition potential was kept at − 0.95 V vs. Ag/AgCl reference electrode for ten minutes. Deposition electrolyte includes an aqueous solution of 10 mM CdCl2, 20 mM H2SeO3 as precursors, 200 mM LiCl as complexing agent, and HCl for adjusting of pH. Deposited CdSe thin film was annealed at 500 °C for 30 min in air medium. Precursor and annealed CdSe thin films were characterized using a number of techniques, including SEM, EDX, XRD, UV–vis spectroscopy, and electrochemical impedance spectroscopy. SEM studies show that annealing alters the surface of precursor CdSe film from smooth to granular appearance. According to EDX analyses, the ratio of Cd/Se is close to 1.07 and 1.04 for the precursor and annealed CdSe thin film, respectively. XRD analysis shows that each film has polycrystalline structure. Precursor film has only cubic structure of CdSe, while annealed film has hexagonal structure of CdSe and cubic crystal phase of CdO. Optical energy band gap of the as-deposited CdSe film increases from 1.64 to 1.71 eV after annealing due to the mixture of the two phases. Refractive index against wavelength changes between 2.0 and 3.3. Calculations performed by using the data of Mott-Schottky measurements show that precursor CdSe film has 1.72 × 1016 cm−3, while annealed film is of 3.65 × 1017 cm−3 carrier concentration. The prepared films exhibit n-type semiconductor character. The study reports energy level diagrams of the produced semiconductor CdSe thin films by using the Mott-Schottky and Tauc's approximations. The carrier transport properties in the interface between active CdSe thin film and electrolyte are discussed based on an equivalent electronic circuit simulated to the Nyquist data of the CdSe/electrolyte system.

Electronic band structure and optical absorption of CdSe doped with interstitial oxygen molecules

We have investigated the electronic structure and optical properties of CdSe doped with molecular oxygen by using first-principle local density-functional theory (DFT) based on a hybrid functional. We find that interstitial oxygen molecule introduces two kinds of states in CdSe: filled states which are degenerate and strongly hybridized with the top of the valence bands and one empty state lying in the middle of the gap. The origin of these states can be traced to the 1πg molecular orbitals of O2. We compared the optical absorption spectra of CdSe with and without molecular oxygen. States introduced by oxygen molecules lower the absorption onset of CdSe to ∼1.0eV while hybridization causes redistribution of the oscillator strengths of CdSe transitions from the ultraviolet to the infrared. Our results demonstrate that doping small molecules into semiconductors can alter the electronic and optical properties of the host in ways different from atomic dopants.

Synthesis and Characterization of Controlled Size CdSe Quantum Dots by Colloidal Method.

Monodispersed and highly luminescence cadmium selenide (CdSe) quantum dots (QDs) have been prepared in a single pot by colloidal reaction method. The QDs were characterized using X-ray diffraction (XRD), Raman Spectroscopy, transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy to study the structural, morphological, compositional and optical properties. The growth temperature played an important role to control the particle size. The optical wavelength was found to be shifted systematically from 460 nm to 575 nm upon increasing the reaction temperature from 110 °C to 260 °C. The size of CdSe QDs, ~2-4 nm was estimated from absorption data. The emission tail exhibited at higher wavelength in PL measurement for the QDs synthesized for lower reaction temperature revealed the presence of surface trap-states. A cubic crystal structure of CdSe QDs was revealed by XRD analysis. The spherical QDs of size 2 to 4.5 nm were observed from TEM analysis for the samples prepared at 140 °C, 200 °C and 260 °C. The sizes of QDs obtained by TEM are in good agreement with the results obtained from optical and XRD data. High resolution transmission electron microscopy (HRTEM) confirmed the cubic crystal structure of CdSe QDs. The Selected area diffraction (SAD) pattern exhibited diffused ring corresponds to (111), (220) and (311) reflections of cubic structure of CdSe. The compositional analysis studied by EDS revealed the growth of nearly stoichiometric CdSe QDs. The LO1 vibrational mode observed about 202-205 cm-1 decreases the broadening systematically upon increasing the reaction temperature.

Encapsulation of Cadmium Selenide Nanocrystals in Biocompatible Nanotubes: DFT Calculations, X-ray Diffraction Investigations, and Confocal Fluorescence Imaging.

The encapsulation of CdSe nanocrystals within single‐walled carbon nanotube (SWNT) cavities of varying dimensions at elevated temperatures under strictly air‐tight conditions is described for the first time. The structures of CdSe nanocrystals under confinement inside SWNTs was established in a comprehensive study, combining both experimental and DFT theoretical investigations. The calculated binding energies show that all considered polymorphs [(3:3), (4:4), and (4:2)] may be obtained experimentally. The most thermodynamically stable structure (3:3) is directly compared to the experimentally observed CdSe structures inside carbon nanotubes. The gas‐phase DFT‐calculated energy difference between “free” 3:3 and 4:2 structures (whereby 3:3 models a novel tubular structure in which both Cd and Se form three coordination, as observed experimentally for HgTe inside SWNT, and 4:2 is a motif derived from the hexagonal CuI bulk structure in which both Cd and Se form 4 or 2 coordination) is surprisingly small, only 0.06 eV per formula unit. X‐ray powder diffraction, Raman spectroscopy, high‐resolution transmission electron microscopy, and energy‐dispersive X‐ray analyses led to the full characterization of the SWNTs filled with the CdSe nanocrystals, shedding light on the composition, structure, and electronic interactions of the new nanohybrid materials on an atomic level. A new emerging hybrid nanomaterial, simultaneously filled and beta‐d‐glucan coated, was obtained by using pristine nanotubes and bulk CdSe powder as starting materials. This displayed fluorescence in water dispersions and unexpected biocompatibility was found to be mediated by beta‐d‐glucan (a biopolymer extracted from barley) with respect to that of the individual inorganic material components. For the first time, such supramolecular nanostructures are investigated by life‐science techniques applied to functional nanomaterial characterization, opening the door for future nano‐biotechnological applications.

Enhancement of photoconductive performance of CdSe via Yb nanosandwiching

In this work, we have explored the effects of the ytterbium nanosandwiching on the structural and optoelectronic properties of CdSe thin films. A ytterbium layer of thickness of 200 nm was sandwiched between two layers of CdSe (1.0 μm). The structural investigations have shown that the hexagonal structure of CdSe is deformed by 1.9% and 0.8% along the a and c-axes, respectively, by the inclusion of the Yb in the structure of CdSe. In addition, a systematic redshifts in the interference patterns of the optical transmittance and reflectance were observed. Accordingly, the optical absorption coefficient spectra of CdSe which displayed two energy gap values of 1.62 and 1.36 eV, are also redshifted. On the other hand, when exposed to blue light irradiation, the steady state and time dependent photocurrent analysis revealed a remarkable enhancement presented by a three orders of magnitude increase in the responsivity of CdSe upon Yb sandwiching. The photocurrent was observed to be limited by the saturation of the surface states and traps that causes longer relaxation constant compared to that of pure CdSe.

Atomistic Modeling of Quantum Dots at Experimentally Relevant Scales Using Charge Equilibration.

Quantum dots (QDs) have been successfully employed within a vast array of fundamental and applied studies spanning all subdisciplines of chemistry. However, ab initio models of QD behavior are inherently limited by computational cost due to the large number of atoms within QDs of experimentally relevant size. This work builds upon the method of charge equilibration (qEQ) to account for system interactions unique to QDs (QD-qEQ) and demonstrates accuracy through calculated per-QD energies and dipole moments that agree generally with ab initio calculations and experimental observation, respectively. By forgoing electronic structure information, QD-qEQ exhibits a distinct advantage in its exceptionally low computational cost, which affords consideration of over 35,000 unique spherical wurtzite CdSe structures with radii ≤12.5 Å. A comparison of QD-qEQ calculations with experimental data relating to the phenomenon of CdSe magic size crystals (MSCs) affords statistical and structural insight into why MSCs are observed. Consideration of structures ≤12.5 Å reveals QD sizes corresponding with local minima in QD energy, correlating closely with experimentally observed MSCs. The physical origin of observed energy minima is assigned to QD structures with surfaces exhibiting large fractions of highly coordinated atoms, a physical trait postulated to yield fewer reaction sites for stepwise growth, resulting in MSC stability. The low computational cost along with the per-atom and per-structure electrostatic data afforded by QD-qEQ makes this method an enticing approach to address dynamic QD behavior and enables potential applications within a broad range of fields concomitant to those in which QD inclusion has already proven useful.

Electrochemical Evaluation of Dopant Energetics and the Modulation of Ultrafast Carrier Dynamics in Cu-Doped CdSe Nanocrystals

Cyclic voltammetric and femtosecond transient absorption (TA) measurements on Cu+-doped CdSe nanocrystals (NCs) were utilized to reveal the energetics of the electroactive Cu+ dopant with respect to the band energies of CdSe NC host and the influence of Cu in tuning the carrier dynamics, respectively. Oxidation–reduction peaks due to an electroactive dopant within CdSe NC host have been traced to determine its energy level which was correlated to the dopant emission energy and Stokes shift. The low doping density of Cu does not significantly alter the band structure of CdSe as the shape of the TA spectra remains similar before and after doping. However, Cu+ acts as a hole localizing center decoupling the electronic wave function from the hole leading to slower Auger-assisted electron cooling in doped NCs. As hole localization to Cu+ is the primary step for dopant emission, in the presence of hole quenchers (aminophenols) the dopant emission gets drastically quenched. Interestingly, once hole is captured b...

Hydrothermal assisted growth of CdSe nanoparticles and study on its dielectric properties

In this work, we have synthesized cadmium selenide (CdSe) nanoparticles by using cadmium chloride (CdCl2) as cadmium ion and sodium selenosulfate (Na2SeSO3) as selenium ion sources through a simple, convenient and cost-effective hydrothermal route at 180 °C temperature for 24 h. Aqueous ammonia was employed as a complex reagent to adjust the pH of the solution. Structural analysis of the obtained product was carried out by using x-ray diffractometer, which revealed that the final product has a cubic structure of CdSe with average crystallite size 13.15 nm. The cauliflower-like CdSe nanostructures were confirmed from the scanning electron microscopy and high-resolution transmission electron microscopy. EDS analysis indicates that the obtained product has a good elemental stoichiometric ratio. The electron diffraction pattern reveals the polycrystalline nature of CdSe. From UV–visible absorption spectral analysis, the optical energy bandgap of CdSe nanoparticles was found to be 1.90 eV. XPS spectra presented Cd 3d3/2, Cd 3d5/2 and Se 3d3/2 peaks at 411.04, 404.29 and 53.52 eV respectively. The CdSe nanoparticles exhibit photoluminescence with two distinct emission bands at 632 nm and 720 nm. FTIR study was used towards the understanding of the formation mechanism and bonding on the surface of the resulting nanoparticles. The dielectric properties of a pelletized sample of CdSe nanoparticles were carried out at room temperature.

Two-dimensional cadmium selenide electronic and optical properties: first principles studies

Structural, electronic and optical properties of two-dimensional (2D) cadmium selenide (CdSe) structures with $$2\times 2$$ periodicities are investigated. First principles total energy calculations are performed within the periodic density functional theory. Initially, the structural properties are determined using the local density approximation as implemented in the PWscf code of quantum ESPRESSO package. To investigate the electronic properties, the GW method is applied to determine the energy gap within the plasmon pole and the random phase approximations. Optical properties are investigated to determine the dielectric constant and the Bethe–Salpeter theory is used to calculate the exciton binding energies. Zinc blende and wurtzite phases are considered to calculate the bulk energy gaps, which are compared to the experimental values, finding good agreement. The 2D structure exhibits an energy gap larger than that of the bulk, indicating the effects of reduction in dimensionality; these changes can be attributed to the dangling bonds that are present in the 2D layer.

Synthesis of CdSe quantum dots decorated SnO2 nanotubes as anode for photo-assisted electrochemical degradation of hydrochlorothiazide: Kinetic process

Pharmaceutical residues have been increasingly detected in the aquatic environment and are considered important contaminants of emerging concern. This study examines the photo assisted electrochemical degradation of the Hydrochlorothiazide by using CdSe quantum dots decorated SnO2 nanotubes. The characteristic devices such as Scanning electron microscopy, X-ray diffraction, UV–vis diffuse reflectance Transmission electron Microscopy were used to analyze information structure of CdSe QDs/SnO2 nanotubes. All the experiments were perform with influence of the current density (10–60mAcm−2) and sodium chloride (0.02–0.10molL−1) in the supporting electrolyte composition was analyzed. The results showed that the Hydrochlorothiazide and TOC removal was achieved in the current density range used. As expected, the degradation kinetics presented a pseudo first order behavior. Comparison of the efficiencies of the photocatalytic, electrochemical (EC) and photo-assisted electrochemical (PAEC) techniques verified that the combined process showed a synergism for HCT and TOC removal.

Size dependent properties of one dimensional CdSe micro/nanostructures

Cadmium Selenide (CdSe) micro/nanowires of different diameters were synthesized in polycarbonate membranes on copper substrate by template assisted electro-deposition method at room temperature. The structure, morphology, elemental, optical and electrical properties of CdSe micro/nanowires were investigated using X-ray Diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible (UV–vis) spectroscopy, photoluminescence (PL) spectroscopy and probe station assisted with Keithley source meter. Structural analysis by X-ray diffraction depicted polycrystalline nature with hexagonal phase of micro/nanowires at all diameters. An increase in the value of average crystallite size was seen with increase in diameter. Morphological investigation confirmed the micro/nanowire synthesis and revealed an ordered array of prepared CdSe micro/nanowires of uniform diameters and having high aspect ratios. Optical band gap is found to increase from 1.89eV to 2.24eV as the diameter is decreased from 1µm to 50nm. Using the band gap values, refractive index and optical dielectric constants (static and high frequency) were also evaluated. The electrical conductivity of CdSe micro/nanowires varied from 21.58 × 102Ω−1m−1to 1.93 × 102Ω−1m−1 when diameter was decreased. These semiconducting micro/nanowires arrays of different diameters may have potential applications in future nanoscale based electronic and optoelectronic devices.

Ferromagnetism and the Optical Properties of Mn-Doped CdSe with the Wurtzite Structure

The geometrical structure of CdSe was optimized by using the ultrasoft pseudopotential method of a total energy plane wave based on density functional theory. The band structure, density of states, and optical properties were calculated and discussed in detail. The Mn-doped CdSe is found to be a half-metallic ferromagnet with 100% carrier spin polarization at the Fermi level. At a Mn concentration of 12.5%, the calculated total energy of the spin-polarized state is 614 meV lower than that of the nonspin-polarized state. The net magnetic moment of 5 μ B is found per supercell for 12.5% Mn-doped CdSe. The estimated Curie temperature of 748.6 K for Mn-doped CdSe is above room temperature. The ferromagnetic ground state in Mn-doped CdSe can be explained in terms of the p − d hybridization mechanism. These results suggest that Mn-doped CdSe may present a promising dilute magnetic semiconductor, and may have potential applications in the field of spintronics.

Effect of quantum confinement in CdSe/Se multilayer thin films prepared by PVD technique

Quantum confined nanostructures were prepared by depositing alternate CdSe and Se thin layers. The structural and optical characterizations of the prepared samples were carried out using X-ray diffractometer (XRD), Field emission scanning electron microscope (FE-SEM), UV–visible and photoluminescence spectrophotometers. XRD studies revealed that CdSe nanocrystals are polycrystalline in nature with hexagonal phase. The crystallite size of CdSe nanoparticles was found to be in the range of 8–14nm. FE-SEM images also confirmed that the embedded nanocrystalline CdSe particles are a few nanometers in dimension having a spherical morphology. The quantum confinement of charge carriers in the multilayer (ML) films is evident from the shifting of absorption edge to lower wavelengths in the UV–visible spectra. An increase in the energy band gap with decreasing thickness of the CdSe sub-layer has been ascribed to quantum confinement effect and the subsequent crystallite size calculated from Brus approximation method is ~3.5nm. Hence, the results indicate that the quantum confinement effect could be realized in CdSe nanocrystallites by ML stacking structure of CdSe and Se in appropriate thickness ratio.

Design of Heterogeneous Chalcogenide Nanostructures with Pressure-Tunable Gaps and without Electronic Trap States.

Heterogeneous nanostructures, such as quantum dots (QDs) embedded in solid matrices or core-shell nanoparticles, are promising platforms for a wide variety of applications, including phosphors with increased quantum yield, photocatalysis, and solar energy conversion. However, characterizing and controlling their interfacial morphology and defects, which greatly influence their electronic properties, have proven difficult in numerous cases. Here we carried out atomistic calculations on chalcogenide nanostructured materials, i.e., PbSe QDs in CdSe matrices and CdSe embedded in PbSe, and we established how interfacial and core structures affect their electronic properties. In particular, we showed that defects present at interfaces of PbSe nanoparticles and CdSe matrices give rise to detrimental intragap states, degrading the performance of photovoltaic devices. Instead, the electronic gaps of the inverted system (CdSe dots in PbSe) are clean, indicating that this material has superior electronic properties for solar applications. In addition, our calculations predicted that the core structure of CdSe and in turn its band gap may be tuned by applying pressure to the PbSe matrix, providing a means to engineering the properties of new functional materials.

Effect of P-Containing Ligands on the Structural and Optical Properties of (CdSe)n (n = 3, 6, 10) Clusters

The effect of phosphorus-containing ligands on the structure, energetics and properties of the (CdSe)n clusters (n = 3, 6, and 10) with different number of PH3 and PMe3 ligands were studied by using density functional theory calculations. The P atom in the ligand interacts with Cd and forms a strong Cd–P coordination bond. The introduction of ligands does not change the cluster architecture, but leads to considerable changes in Cd–Se bondlength, charge distribution, binding energy, HOMO–LUMO gap and optical absorption. The ligand influence is enhanced with increasing ligand coverage. A blueshift in absorption band was predicted for the clusters with increasing ligands, resulting from the electron donating characteristics of the ligands that hamper electron transition from Se to Cd. As P-containing ligands are often used in the preparation of CdSe nanocrystals, our calculations reveal the influence of ligand-cluster interaction on the cluster geometrical and electronic properties, which would be helpful for the nanocrystal design and synthesis.

Phase discrimination in CdSe structures by means of Raman scattering

Raman spectra of epitaxial layers of CdSe grown by molecular beam epitaxy have been measured for the cubic (zincblende) and hexagonal (wurtzite) phases. The Raman spectra are examined in the light of density functional calculations for these two highly similar structures. Characteristic Raman frequencies and spectral features associated with the different symmetry are discussed and reliable criteria for phase discrimination based on Raman spectroscopy are proposed. Although LO frequencies are virtually identical in both structures and may be affected by size effects, the observation of a low energy E2 mode at 33 cm−1 unambiguously identifies the wurtzite structure and can be used as a specific fingerprint to distinguish between these two phases in CdSe‐based nanostructures. The slightly lower LO frequency measured in the zincblende epitaxial layer is ascribed to residual tensile strain.