Bulk CdSe Research Articles

Density functional theory study on the electronic structures and piezoelectric properties of hydrogenated monolayer II-VI semiconductor XYH2 (X = Zn, Cd; Y = S, Se, Te)

Piezoelectric materials have been widely used in various fields such as sensing and energy catalysis. This paper primarily investigates the structural, electronic and piezoelectric properties of hydrogenated II-VI monolayer semiconductor XYH2 (X = Zn, Cd; Y = S, Se, Te) by density functional theory. Results reveal that XYH2 monolayers exhibit stable hexagonal structures and tunable bandgaps ranging from 3.27 eV to 4.30 eV. Density functional perturbation theory is employed to investigate the out-of-plane piezoelectric coefficients of XYH2 monolayers. CdSH2 exhibits a value of −1.8 pm/V of out-of-plane piezoelectric coefficients which surpasses typical piezoelectric materials such as bulk ZnS (0.076 pm/V), ZnO (0.21 pm/V), CdS (0.143 pm/V), CdSe (0.104 pm/V), BN (0.33 pm/V) and GaN (0.96 pm/V). Our results indicate that hydrogenated II-VI monolayers can be a new type of novel piezoelectric semiconductors and hold a potential application for the piezoelectric devices.

Stimulated Emission and Lasing from Bulk CdSe Nanocrystals.

Nanocrystals with a size in the regime of vanishing quantum confinement, or bulk nanocrystals (BNCs), have emerged recently as viable solution processable optical gain materials in the green part of the spectrum. Here, we show that these properties can be extended to the crucial red region using CdSe BNCs. Through quantitative time-resolved spectroscopy, we can model these nanocrystals as bulk semiconductors, thereby revealing that the gain originates from an unbound electron-hole plasma state. The gain is broadband in nature and is not capped by Auger processes, but by a slower second-order recombination resulting in nanosecond gain lifetimes. Finally, optically pumped lasers under femtosecond pulsed and quasi-continuous wave operation are demonstrated using a photonic crystal surface emitting laser cavity, thereby stretching from 635 to 720 nm. Our results indicate that compositional variation can indeed provide spectral versatility to the BNC concept, while preserving the excellent gain metrics associated with it.

Investigations on the defect structures for Mn2+ in CdSe nanocrystals and bulk materials and the criterion of occupation for Mn2+ in CdX (X = S, Se, Te) nanocrystals.

The spin Hamiltonian parameters and defect structures are theoretically studied for the substitutional Mn2+ at the core of CdSe nanocrystals and in the bulk materials from the perturbation calculations of spin Hamiltonian parameters for trigonal tetrahedral 3d5 clusters. Both the crystal-field and charge transfer contributions are taken into account in the calculations from the cluster approach. The impurity-ligand bond angles are found to be about 1.84° larger and 0.10° smaller in the CdSe:Mn2+ nanocrystals and bulk materials, respectively, than those (≈109.37°) of the host Cd2+ sites. The quantitative criterion of occupation (at the core or surface) for Mn2+ in CdX (X = S, Se, Te) nanocrystals is presented for the first time based on the inequations of hyperfine structure constants (HSCs). This criterion is well supported by the experimental HSCs data of Mn2+ in CdX nanocrystals. The previous assignments of signals SI as Mn2+ at the core of CdS nanocrystals are renewed as Mn2+ at the surface based on the above criterion. The present studies would be helpful to achieve convenient determination of occupation for Mn2+ impurities in CdX semiconductor nanocrystals by means of spectral (e.g., HSCs) analysis.

Pressure Impact on Lattice Thermal Conductivity and Its Related Parameters in CdSe Bulk, Nanowires, and Thin Film

The hydrostatic pressure effect on lattice thermal conductivity (LTC) in CdSe bulk, nanowires, and thin films was calculated. Values of LTC obtained through Callaway with that of Clapeyron equations are used to correlate those of the experimentally measured curves. Frequency‐dependent relaxation time approximation of phonon‐scattering processes was considered, which scattering due to boundary, imperfections, dislocations, electron, and other phonons via Umklapp and normal processes were used and clarified their effects. The impact of pressure on LTC is clarified according to both sample boundary dislocation concentrations. The influence of pressure is described, along with how the size dependence of the melting temperature, Debye temperature, and phonon group velocity of CdSe nanocrystals is affected. These parameters decrease as pressure increases.

Pure spin current injection of single-layer monochalcogenides

We compute the spectrum of pure spin current injection in ferroelectric single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the coherent spin dynamics of optically excited conduction states split in energy by spin–orbit coupling. The velocity of the electron’s spins is calculated as a function of incoming photon energy and angle of linearly polarized light within a full electronic band structure scheme using density functional theory. We find peak speeds of 520, 360, 270 and 370 Km s−1 for SnS, SnSe, GeS and GeSe, respectively which are an order of magnitude larger than those found in bulk semiconductors, e.g., GaAs and CdSe. Interestingly, the spin velocity is almost independent of the direction of polarization of light in a range of photon energies. Our results demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce on demand spin-current in spintronics applications.

Tracing spatial confinement in semiconductor quantum dots by high-order harmonic generation

We report here on results of systematic experimental-theoretical investigation of high-order harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference bulk CdSe thin film. We observe a strong decrease in the efficiency, up to complete suppression of HHG with energies of quanta above the band gap for the smallest dots, whereas the intensity of below band gap harmonics remains weakly affected by the dot size. In addition, it is observed that the suppression of the above gap harmonics is enhanced with increasing the driving wavelength. These systematic investigations allow us to develop a simple physical picture explaining the observed suppression of the highest harmonics: the discretization of electronic energy levels seems to be not the predominant contribution to the observed suppression but rather the confined dot size itself, causing field-driven electrons to scatter off the dot's walls. The reduction in the dot size below the classical electron oscillatory radius and the corresponding scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency of the emission of highest-order harmonics. Our results demonstrate a regime of intense laser-nanoscale solid interaction, intermediate between the bulk and single-molecule response, and are crucial for nanophotonic platforms aiming at control over high-order harmonic properties and efficiency.

Effect of Variation of pH and Shell Thickness upon the Optical and Structural Characteristics of PVA Capped CdSe and CdSe/ZnO Nanoparticles

Poly-Vinyl Alcohol (PVA) capped Cadmium Selenide (CdSe) and Cadmium Selenide/Zinc Oxide (CdSe/ZnO) core/shell semiconductor/semiconductor nanoparticles have been synthesized using wet chemical precipitation method. The pH of each solution was varied ranging from 9.5 to 11.5. Particle size of the CdSe nanoparticles was estimated using Brus Equation. The results of UV-Visible spectroscopy of core CdSe samples show blue shifting of absorption edges in a range of 342-377 nm compared to that of the bulk CdSe 712 nm and red shifting of the absorption edges of the core /shell samples in a range (346-357) nm in comparison to the core sample, over which the shell is deposited. For CdSe core nanoparticles, the band gap values were found to be in the range of 3.70–3.90 eV, which is larger than the bulk CdSe of 1.74 eV. Also the band gap values for the core/shell nanoparticles were in the range of 3.75-3.80 eV. The positions of excitonic emission peak obtained from photoluminescence spectra for the core is around 323 nm and for the core/shell samples is around 324nm. The average crystallite size of the core/shell CdSe/ZnO sample was obtained from XRD spectra in the range of 62-69 nm and for the core CdSe sample it was of 11-31 nm. The core and the core/shell samples were more or less spherical as obtained from the SEM analysis. Some of the core nanoparticles were 3-8 nm in size whereas the core/shell nanoparticles were 20-50 nm in size as obtained from HRTEM analysis.

Thermal Stability of Semiconductor Nanocrystal Solids: Understanding Nanocrystal Sintering and Grain Growth

Nanomaterials are naturally metastable with respect to bulk solids. This raises the very important fundamental problem of their morphological stability, especially when nanoscale crystallites are touching or nearly touching each other, such as in thin-film devices. In some cases, nanostructuring must be preserved under operational conditions (e.g., in quantum dot LEDs, lasers, photodetectors, and nanogranular thermoelectric devices). In other cases, we use nanocrystalline particles as precursors to a material with large crystalline grains and aim to sinter them as efficiently as possible (e.g., in polycrystalline thin-film solar cells). We carried out a systematic study of sintering and grain growth in materials composed of various sub-10 nm semiconductor grains. The boundaries between individual semiconductor grains have been chemically engineered using inorganic surface ligands. We found that the early stages of sintering and grain growth of nanocrystalline semiconductors are controlled by the ion mobility at the nanocrystal surfaces, while the late stages of grain growth are controlled by the mobility of the grain boundaries. This appears to be a general phenomenon for semiconductor nanocrystals, and it leads to several interesting and counterintuitive trends. For example, III–V InAs nanocrystals are generally much more resilient against sintering and grain growth compared to II–VI CdSe nanocrystals even though bulk CdSe has significantly higher melting point temperature than InAs (1268 °C vs 942 °C). Grain growth can be dramatically accelerated when coupled to solid−solid phase transitions. These findings expand our toolbox for rational design of nanocrystal materials for different applications.

New hybrid semiconducting CdSe and Fe doped CdSe quantum dots based electrochemical capacitors

In this study, pure and Fe-doped CdSe (Fe@CdSe) quantum dots (QDs) in various concentrations have been prepared via the chemical co-precipitation method. The PXRD, FT-IR, UV–vis spectroscopy, and HR-TEM micrographs have been utilized to confirm the structural, optical, and morphological features of as-synthesized QDs. The optical bandgap values of pure and Fe@CdSe QDs have been found to vary from 2.12 eV to 1.82 eV, as determined by the Tauc relation, and the absorption spectrum of pure and Fe@CdSe QDs exhibits a blue shift compared to the bulk CdSe. Moreover, three-electrode configuration systems have been employed to characterize the electrochemical properties of pure and Fe@CdSe QDs. Notably, the areal capacitance of pure CdSe is 47 mFcm−2, which considerably increases to ∼73 mFcm−2 for 6 mol.% Fe@CdSe QDs QDs at a scan rate of 5 mVs−1.

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Biocompatible Ni-doped CdSe/ZnS semiconductor nanocrystals for cellular imaging and sorting.

Quantum dots (QD) with chemical composition were successfully synthesized using a hydrothermal method and chemical precipitation. The nanocrystalline phase of the nanostructures was isolated and characterized using X-ray diffraction (XRD). The mean crystalline size doped core/shell Ni-dopant range was 9.0 ± 2.0 nm. The ferromagnetic data revealed the magnetic behaviour of . The optical absorption measurements of these QDs were in the UV-visible light range 200-800 nm for a band gap value of 2.11 eV for . This means that pure and underwent a redshift when compared with bulk CdSe. For there was successful uptake by cell lines including HeLa and MCF-7 for bioimaging and sorting applications.

Exciton Binding Energy in CdSe Nanoplatelets Measured by One- and Two-Photon Absorption

Colloidal semiconductor nanoplatelets exhibit strong quantum confinement for electrons and holes as well as excitons in one dimension, while their in-plane motion is free. Because of the large dielectric contrast between the semiconductor and its ligand environment, the Coulomb interaction between electrons and holes is strongly enhanced. By means of one- and two-photon photoluminescence excitation spectroscopy, we measure the energies of the 1S and 1P exciton states in CdSe nanoplatelets with thicknesses varied from 3 up to 7 monolayers. By comparison with calculations, performed in the effective mass approximation with account of the dielectric enhancement, we evaluate exciton binding energies of 195–315 meV, which is about 20 times greater than that in bulk CdSe. Our calculations of the effective Coulomb potential for very thin nanoplatelets are close to the Rytova-Keldysh model, and the exciton binding energies are comparable with the values reported for monolayer-thick transition metal dichalcogenides.

Synthesis and characterisation of CdSe QDs by using a chemical solution route

An efficient synthesis process approach based on a chemical solution route is developed for the cadmium selenide quantum dots (CdSe QDs) that utilise photonic and optoelectronic device manufacturing. The developed route consists of dissolving the cadmium chloride (CdCl2.H2O), 2-mercaptoethanol and sodium selenide anhydrous (Na2SeO3). The different characterisation parameters such as ultraviolet (UV) absorbance, x-ray diffraction (XRD), scanning electron microscopy, energy dispersive x-ray and transmission electron microscopy (TEM) were employed in order to develop the CdSe QDs. When the sample was analysed from the UV–visible studies, the bandgap was about 2.16 eV, whereas the bulk CdSe bandgap was about 1.78 eV. The developed CdSe QDs possessed a cubic crystal structure with crystalline dimensions of about 4.86 nm. Its surface morphology and structure showed the smooth appearance of the surface. The result indicated agglomerated spheres. Ultimately, according to XRD and TEM results, the crystalline dimension was determined in good agreements.

Investigation of Microwave Irradiation Procedure for Synthesizing CdSe Quantum Dots

In recent years, microwave heating techniques for quantum dot (QD) synthesis have come to supplement the typical hot‐injection methods. In addition to increasing control and replicability, microwave synthesis can be up‐scaled to industry standards, an advantage that increases its lucrativeness. This study depicts a strategy to take a hot‐injection procedure for cadmium selenide (CdSe) QD synthesis that is safe enough for undergraduate research labs and adapt it to an easier, more energy‐efficient microwave synthesis method. Additionally, this study details successes in synthesizing these QDs, along with some challenges, limitations, and peculiarities. For future users of this method, it is recommended to keep holding temperatures between 170°C and 240°C to achieve the highest monodispersity of CdSe QDs while also avoiding confounding effects, such as wide‐spectrum photoluminescence and bulk CdSe precipitation.

Size- and Charge-Dependent Optoelectronic Properties of CdSe Clusters: Evolution of the Optical Gap from Molecular to Bulk Behavior.

In the present work, the optical response of isolated (CdSe)n+ clusters with n = 3-6 is probed by measuring the photodissociation cross section in the photon energy range ℏω = 1.9-4.9 eV. In this joint experimental and theoretical study, the experimental observations are analyzed with time-dependent density functional theory and equation-of-motion coupled cluster theory. Structural candidates for the time-dependent excited-state calculations are obtained via global optimization by employing a genetic algorithm. The combined experimental and theoretical approach allows the discrimination of cluster geometries in the molecular beam experiments. From n ≥ 5, three-dimensional structures are found. Already for n = 6, light absorption in the red spectral range is observed. This observation is discussed with respect to the size dependence of the optical behavior of finite systems taking experimental and theoretical work on bare and ligated CdSe clusters and nanoparticles into account. Particularly, the influence of the net charge and ligands is considered. This allows a detailed discussion of the size-dependent evolution of the optical properties starting from molecular species over to nanoclusters and nanoparticles and finally to bulk CdSe.

Explaining observed stability of excitons in highly excited CdSe nanoplatelets

Two-dimensional electron-hole gases in colloidal semiconductors have a wide variety of applications. Therefore, a proper physical understanding of these materials is of great importance. In this paper we present a detailed theoretical analysis of the recent experimental results by Tomar et al. [J. Phys. Chem. C 123, 9640 (2019)] that show an unexpected stability of excitons in CdSe nanoplatelets at high photoexcitation densities. Including the screening effects by free charges on the exciton properties, our analysis shows that CdSe nanoplatelets behave very differently from bulk CdSe, and in particular do not show a crossover to an electron-hole plasma in the density range studied experimentally, even though there is substantial overlap between the excitons at the highest densities achieved. From our results we also conclude that a quantum degenerate exciton gas is realized in the experiments, which opens the prospect of observing superfluidity in CdSe nanoplatelets in the near future.

The effects of the intrinsic defects on the electronic, magnetic and optical properties for bulk and monolayer CdSe: first-principles GGA + U investigations

In the paper, the formation energies of the intrinsic defects as well as the effects of the intrinsic defects on the electronic, magnetic and optical properties for bulk and monolayer CdSe are investigated using first-principles calculations. In contrast to the defects VSe, CdSe and SeCd, it is found that the VCd defect in bulk CdSe under Se-rich conditions is the easiest to form, this is consistent with the experiments; while under Cd-rich conditions, the VSe defect is the easiest to form both in bulk and monolayer CdSe. With the present GGA + U calculations, the reasonable direct band gaps for the pure bulk and monolayer CdSe are 1.7 and 2.3 eV, respectively. These different intrinsic defects do not change the nature of the direct band gap of the bulk or monolayer CdSe. However, these different intrinsic defects make the corresponding band gap have a more or less increase or decrease; and almost all defects will introduce the impurity energy levels into the band gap, these could be responsible for the changes of some optical properties. In particular, the VCd defect can induce the nearest two Se atoms to produce the similar magnetic moment (around 1 μB) in the bulk or monolayer CdSe, while the anti-site defect CdSe as well as other intrinsic defects can’t induce any magnetic moment using the GGA + U calculations. Yet both electronic and magnetic properties in the bulk and monolayer CdSe with the VCd defect are influenced by the Hubbard U value of the Se 4 p orbital. If the Hubbard U value is set to zero, then there are no any magnetic moments for the bulk or monolayer CdSe supercell with any intrinsic defect.

Negative thermal expansion in CdSe quasi-two-dimensional nanoplatelets

The in-plane coefficient of thermal expansion (CTE) for CdSe nanoplatelets with the zinc-blende structure containing from two to five monolayers is calculated from first principles within the quasiharmonic approximation. A comparison of the obtained results with those for bulk CdSe with both the zinc-blende and wurtzite structures finds a significant increase in the magnitude of negative CTE and the temperature range of its observation in nanoplatelets. The main contribution to the negative thermal expansion in CdSe nanoplatelets is given by the out-of-plane flexural ZA mode and in-plane optical $E$ modes that arise from the folding of TA phonon of bulk CdSe.

Effect of Sm concentration on optical and electrical properties of CdSe nanocrystalline thin film

Abstract Present paper reports optical and electrical properties of samarium doped CdSe nanocrystalline thin film which was grown on a glass substrate by chemical bath deposition method (CBD). X-ray diffraction (XRD) analysis revealed that the deposited films were nanocrystalline with sphalerite cubic structure. The average crystallite size calculated from FWHM of XRD peaks was found to be 10.11 nm. The bandgap of the Sm doped CdSe nanocrystalline thin films was calculated to be 1.91 eV to 2.22 eV. The optical absorption edge of undoped (pure) and Sm doped CdSe films was obtained between 650 nm to 640 nm showing blue shift as compared to bulk CdSe. Sm doping further enhanced the photoconductivity of these films. The I-V characteristic confirmed the suitability of prepared films for photosensor applications.

Chemical bonding in initial building blocks of semiconductors: Geometrical structures and optical absorption spectra of isolated and Cd2 species.

We present the first experimental optical absorption spectra of isolated and Cd2 species in the photon energy range ℏω = 1.9-4.9 eV. We probe the optical response by measuring photodissociation cross sections and combine our results with time-dependent density functional theory and equation-of-motion coupled cluster calculations. Structural candidates for the time-dependent excited state calculations are generated by a density functional theory based genetic algorithm as a global geometry optimization tool. This approach allows us to determine the cluster geometries present in our molecular beams by a comparison of experimental spectra with theoretical predictions for putative global minimum candidates. For , an excellent agreement between the global minimum and the experimental results is presented. We identify the global minimum geometry of Cd2 as a trapezium, which is built up of a neutral Se2 and a cationic unit, in contrast to what was previously proposed. We find an excellent overall agreement between experimental spectra and excited state calculations. We further study the influence of total and partial charges on the optical and geometric properties of Cd2Se2 and compare our findings to CdSe quantum dots and to bulk CdSe.