Bulk CdSe Research Articles

Stimulated two-photon emission in bulk CdSe

While two-photon emission processes are firmly established in atomic physics, their observation and use in semiconductor physics remains elusive. Here, we experimentally investigate stimulated two-photon emission in photoexcited bulk CdSe and identify requirements for the observation of stimulated two-photon emission. In particular, this process requires population inversion as well as two-photon transition energies close to the bandgap energy. In any regime investigated in the present study, net optical gain is not achieved, as the free-carrier absorption intrinsically linked to the photoexcitation completely masks the two-photon gain. The results are well in line with a recent study on nondegenerate versions of two-photon emission in GaAs and place clear limits for the practical use of two-photon emission in optically excited semiconductors.

Thermochemical Measurements of Cation Exchange in CdSe Nanocrystals Using Isothermal Titration Calorimetry.

Among the various reported post synthetic modifications of colloidal nanocrystals, cation exchange (CE) is one of the most promising and versatile approaches for the synthesis of nanostructures that cannot be directly synthesized from their constitutive precursors. Numerous studies have reported on the qualitative analysis of these reactions, but rigorous quantitative study of the thermodynamics of CE in colloidal nanoparticles is still lacking. We demonstrate using isothermal titration calorimetry (ITC), the thermodynamics of the CE between cadmium selenide (CdSe) nanocrystals and silver in solution can be quantified. We survey the influence of CdSe nanocrystal diameter, capping ligands and temperature on the thermodynamics of the exchange reaction. Results obtained from ITC provide a detailed description of overall thermodynamic parameters-equilibrium constant ( K eq), enthalpy (Δ H), entropy (Δ S) and stoichiometry ( n)-of the exchange reaction. We compared the free energy change of reaction (Δ G) between CdSe and Ag+ obtained directly from ITC for both CdSe bulk and nanoparticles with values calculated from previously reported methods. While the calculated value is closer to the experimentally obtained Δ G rxn for bulk particles, nanocrystals show an additional Gibbs free energy stabilization of ∼-14 kJ/mol Se. We discuss a thermochemical cycle elucidating the steps involved in the overall cation exchange process. This work demonstrates the application of ITC to probe the thermochemistry of nanoscale transformations under relevant solution conditions.

Effect of ligand exchange on photocurrent enhancement in cadmium selenide (CdSe) quantum dot water splitting cells

We report photocurrent enhancement through ligand exchange in cadmium selenide (CdSe) photoelectrochemical (PEC) cells. CdSe quantum dot (QD) layers were used as a light absorption layer in combination with titanium dioxide (TiO2) layers. For a shorter ligand exchange, pristine CdSe QDs with oleic acid (OA) ligands were treated with 1,2-ethanedithiol (EDT) and CdSe QD PEC cells exhibited enhanced photocatalytic properties in comparison with pristine CdSe PEC cells. To articulate the origin of enhanced photocurrent, ligand dependent energy band bending at the CdSe/TiO2 interface was probed from thickness dependent highest occupied molecular orbital level measurements using photoelectron spectroscopy in air. Increase in the photocurrent also originates from enhancement in charge transport through ligand exchange to shorter EDT ligands in the bulk CdSe film with OA ligands suppressing back diffusion of carriers from TiO2 to CdSe.

Wave Function Engineering in CdSe/PbS Core/Shell Quantum Dots.

The synthesis of epitaxial CdSe/PbS core/shell quantum dots (QDs) is reported. The PbS shell grows in a rock salt structure on the zinc blende CdSe core, thereby creating a crystal structure mismatch through additive growth. Absorption and photoluminescence (PL) band edge features shift to lower energies with increasing shell thickness, but remain above the CdSe bulk band gap. Nevertheless, the profiles of the absorption spectra vary with shell growth, indicating that the overlap of the electron and hole wave functions is changing significantly. This leads to over an order of magnitude reduction of absorption near the band gap and a large, tunable energy shift, of up to 550 meV, between the onset of strong absorption and the band edge PL. While the bulk valence and conduction bands adopt an inverse type-I alignment, the observed spectroscopic behavior is consistent with a transition between quasi-type-I and quasi-type-II behavior depending on shell thickness. Three effective mass approximation models support this hypothesis and suggest that the large difference in effective masses between the core and shell results in hole localization in the CdSe core and a delocalization of the electron across the entire QD. These results show the tuning of wave functions and transition energies in CdSe/PbS nanoheterostructures with prospects for use in optoelectronic devices for luminescent solar concentration or multiexciton generation.

Spectroscopic study of oscillator strength and radiative decay time of colloidal CdSe quantum dots

Characterization of samples of cadmium selenide quantum dots (CdSe) QDs dissolved in toluene colloidal solutions at a concentration of 1.4 mg/ml was carried out through UV–Vis absorption and photoluminescence (PL) spectroscopy. The size-dependent absorption and red-shifted PL emission peak wavelengths could be tuned between 510–576 and 545–606 nm respectively. Optical absorption spectral measurements yielded CdSe QDs having diameters about ~ 2.44–3.69 nm with energy gaps 2.32–2.08 eV which are higher than the bulk CdSe (1.74 eV) reminiscent of quantum confinement. This is found to be in good agreement with the semi-empirical pseudopotential model. In addition, the first excitonic absorption transition 1S(e)1S3/2(h) oscillator strength and the corresponding fluorescence radiative decay time of CdSe QDs are assessed using relevant Einstein relations for absorption and emission in a two-level system. The elaborated calculations would anticipate that the transition oscillator scale with the CdSe QD radius as ~ R2.54. Correspondingly, the calculated radiative decay times decrease from 56.4 to 23.2 ns which scale with CdSe QDs radius as ~ R−2.155 in fairly good agreement with experimental values reported in the literature.

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.

Dielectric Properties of Photo-Luminescent CdSe/CdS Mono-Shell and CdSe/CdS/ZnS Multi-Shell Nanocrystals Studied by TEM-EELS

Semiconductor nanocrystals (NCs) of CdSe/CdS (core/shell) mono-shell and CdSe/CdS/ZnS (core/shell/shell) multi-shell having bright and stable luminescence were investigated using high energy-resolution electron energy-loss spectroscopy (EELS) based on transmission electron microscopy to evaluate their dielectric properties in the visible light energy region. EELS spectra of the mono-shell and multi-shell NCs showed two-step structures attributed to interband transitions near band gaps of individual CdSe core and CdS shell. EELS simulations based on the classical dielectric theory revealed that the two-step structures were results of dielectric response from CdSe core and CdS shell. The intensity profiles of the two-step structures depend on a thickness of CdS shells and mono/multi-shell structures of NCs. Besides, the quantum confinement effect was observed in the spectra of mono- and multi-shell NCs, where onset energy of the CdSe core was larger than that of bulk CdSe. The EELS spectra of multi-shell NCs, however, did not present an evident onset due to a bandgap of ZnS outer-shell suggesting defects or amorphous state in ZnS outer-shell. Such dielectric properties of NCs obtained from EELS analysis could provide valuable information to evaluate the crystallinity of individual core and shells in NCs.

Exciton binding energies in CdSe/MgSe quantum well structures

Binding energies of heavy- and light-hole excitons in CdSe/MgSe quantum wells have been determined in terms of quantum well width. Hydrogenic wave functions are assumed to represent exciton wave functions. Contribution of the polarization charges originating from the dielectric mismatch between the well and the barrier materials to the Coulomb interaction is calculated by means of image charge method. The effect of the exciton-phonon interaction on the exciton binding energies are included by means of an effective potential obtained through an exciton-bulk-optical-phonon Hamiltonian. The results indicate that both the modified Coulomb interaction due to the dielectric mismatch and the exciton-phonon interaction considerably change the exciton binding energy. The heavy-hole exciton binding energy in a 10-Å quantum well is seen to be enhanced as much as 87% compared to the value found when only the Coulomb interaction without dielectric mismatch is taken into account. The resulting binding energies of heavy-hole excitons range approximately between 57 and 41 meV for the quantum well width changing between 10 and 35 Å. Compared to the bulk CdSe exciton binding energy 15 meV, the exciton binding energies in CdSe/MgSe quantum wells are increased almost four times.

Lattice dynamics of quasi-two-dimensional CdSe nanoplatelets and their Raman and infrared spectra

Phonon spectra of CdSe nanoplatelets (2-6 ML) with the zinc-blende structure were calculated from first principles within the density-functional theory. It turned out that the Lamb modes in nanoplatelets are in fact optical rather than acoustic vibrations. Phonon spectra of the nanoplatelets show the appearance of a large number of low-frequency modes inherited from TA phonons in bulk CdSe. Calculations of the Raman spectra indicate a need to revise the interpretation of available experimental data. The largest contribution to the Raman spectra is provided by the quasi-Lamb modes with the $A_1$ symmetry. The $B_2$ modes whose frequencies depend on the environment of nanoplatelets and whose properties are closest to the properties of LO phonons explain the results obtained in the "nanoparticle-on-mirror" geometry. The features in Raman spectra previously attributed to surface optical (SO) modes should be interpreted as a manifestation of lower-order quasi-Lamb $A_1$ modes. Calculations of the infrared spectra find, in addition to the TO phonon line, the appearance of intense lines from surface modes originating from terminating F(Cl) atoms on the surface of nanoplatelets and true SO-modes.

Photoluminescence characteristics of CdSe quantum dots: role of exciton–phonon coupling and defect/trap states

In this paper, we report temperature dependent photoluminescence (PL) characteristics of CdSe colloidal QDs with average diameter ~2.8 nm. Temperature dependence of strongly confined exciton PL peak position, linewidth and intensity were investigated in 30 K to 300 K temperature range. Our studies reveal nearly four times weaker exciton-LO phonon coupling than bulk CdSe crystal. Theoretically, it should be vanishingly small due to near identical electron and hole charge distributions in strongly confined QDs. On the other hand, exciton-acoustic phonon coupling is an order of magnitude larger than its bulk counterpart. Observed finite value of exciton-LO phonon coupling and enhanced exciton-acoustic phonon coupling are due to piezoelectric strain fields. PL intensity exhibits anomalous behavior in the temperature range 100–230 K. This has been explained by thermally activated detrapping of the charge carriers trapped in the potential wells formed at the interface adjoining dislocations/stacking faults developed during the synthesis process. Above 230 K, PL is partially quenched by thermal escape of charge carriers from luminescing exciton state to higher lying nonluminescing states.

CdSe quantum dots using selenourea as selenium source in polymer matrix

In this paper we have synthesized cadmium selenide quantum dots in aqueous medium at low temperature of 80 °C through chemical route. Three different concentrations of polyvinyl alcohol (PVA) 6, 8 and 10 wt% was used as a capping agent to prevent coagulation in quantum dots. An alternative precursor selenourea was used in place of selenium powder as a selenium source. Thin film of CdSe/PVA nanocomposite was made by spin coating and characterized by UV–Vis spectroscopy; Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Fourier transform infra red spectroscopy. For electrical measurements temperature dependent DC conductivity has been performed. From UV–Visible absorption measurement it has been observed that on increasing PVA concentration the absorption edge shifts and bandgap calculated comes out to be more than the bulk CdSe material due to quantum confinement effect. This indicates blue shift and reduction in crystallite size with increase in PVA concentration. This tunability of bandgap with size is very effective in solar cell application. SEM/TEM micrograph confirms the zero dimensional shape of the prepared quantum dot with average size in the range of 6 nm. The DC conductivity measurement shows increase in conductivity with increase in temperature which confirms semiconducting behaviour of CdSe quantum dot.

Lattice Thermal Conductivity of Wurtzite Bulk and Zinc Blende CdSe Nanowires and Nanoplayer

By using Morelli-Callaway model and some structure dependent parameters, theoretical calculations of LTC for wurtzite Bulk CdSe, zinc blende CdSe nanowire and nanolayer are performed. The theoretical and experimental correlation for CdSe NWs with diameter 41, 52 and 88nm and nanolayer with thickness of 4.3nm are investigated. While, the direction of growth of ZB CdSe is one equation for longitudinal and two different equation for transverse mode are used for calculating acoustic group velocity. Therefore, Morelli-Callaway model splits to three branches. There are six phonon transfer scattering rate, which are umklapp, normal, boundary impurity, dislocation, and phonon-electron scattering rate. In different temperatures, different phonon scattering process appeal. The shape of LTC as a function of temperature has a bell shape that all phonon scattering rate configured this shape. The peak of lattice thermal conductivity shift to higher temperature with decreasing the size of CdSe nanostructure. In summarize, the LTC for a particular temperature depends on the size and crystal structure. At 300K thermal conductivity of WZ bulk CdSe has less value than all ZB CdSe NWs mentioned in this work. Also quantum confinement effect cause mechanical and thermal parameters change with decreasing the size and dimension of CdSe semiconductor.

Comparison of three empirical force fields for phonon calculations in CdSe quantum dots.

Three empirical interatomic force fields are parametrized using structural, elastic, and phonon dispersion data for bulk CdSe and their predictions are then compared for the structures and phonons of CdSe quantum dots having average diameters of ˜2.8 and ˜5.2 nm (˜410 and ˜2630 atoms, respectively). The three force fields include one that contains only two-body interactions (Lennard-Jones plus Coulomb), a Tersoff-type force field that contains both two-body and three-body interactions but no Coulombic terms, and a Stillinger-Weber type force field that contains Coulombic interactions plus two-body and three-body terms. While all three force fields predict nearly identical peak frequencies for the strongly Raman-active "longitudinal optical" phonon in the quantum dots, the predictions for the width of the Raman peak, the peak frequency and width of the infrared absorption peak, and the degree of disorder in the structure are very different. The three force fields also give very different predictions for the variation in phonon frequency with radial position (core versus surface). The Stillinger-Weber plus Coulomb type force field gives the best overall agreement with available experimental data.

Reversible ultrafast melting in bulk CdSe

In this work, transient reflectivity changes in bulk CdSe have been measured with two-color femtosecond pump-probe spectroscopy under a wide range of pump fluences. Three regions of reflectivity change with pump fluences have been consistently revealed for excited carrier density, coherent phonon amplitude, and lattice temperature. For laser fluences from 13 to 19.3 mJ/cm2, ultrafast melting happens in first several picoseconds. This melting process is purely thermal and reversible. A complete phase transformation in bulk CdSe may be reached when the absorbed laser energy is localized long enough, as observed in nanocrystalline CdSe.

Effect of Ni doping on the structural, optical and morphological properties of CdSe QDs by chemical precipitation method

This article reports the synthesis of undoped and Ni (1–5%) doped CdSe quantum dots (QDs) through chemical precipitation method. The structure and size of synthesized QDs were studied by X-ray diffraction pattern. The absorption wavelength of both samples were blue shifted with respect to the bulk CdSe phase due to quantum confinement effect. The enhanced photoluminescence was observed for Ni (2%) doped CdSe QDs. The morphology of the undoped and Ni (2%) doped CdSe QDs was studied using FE-TEM analysis.

A Rapid and Cost-Effective Laser Based Synthesis of High Purity Cadmium Selenide Quantum Dots.

A rapid and cost effective method is developed to synthesize high purity cadmium Selenide (CdSe) quantum dots in acetone medium using second harmonic of Nd:YAG nanosecond pulsed laser of 532 nm wavelength. The thermal agglomeration due the nanosecond pulse duration of the laser was successfully eliminated by using unfocussed laser beam and thereby providing a favorable conditions for the synthesis of quantum dots having the grain size of 3 nm. The morphological and optical characterizations like XRD, HRTEM, optical absorption of the synthesized CdSe quantum dots, reveal that the material possesses the similar characteristics of the one synthesized through cumbersome wet chemical methods. Relative to the CdSe bulk material, the synthesized CdSe quantum dots showed a blue shift in the measured band gap energy from near infrared spectral region to visible region, making this material very attractive for many solar energy harvesting applications like photo-catalysis and solar cells.

Studies on cadmium telluride–cadmium selenide bilayer

Group II–VI elements of the periodic table are suitable for electronic and optoelectronic applications. Cadmium selenide (CdSe) and cadmium telluride (CdTe) bilayer films were deposited onto well-cleaned glass substrates using vacuum evaporation technique. These films were analyzed using X-ray diffraction, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDAX) and UV-visible spectrophotometry to study their microstructural, morphological and optical properties. Crystallinity of CdSe film increased with increase in film thickness, which in turn favored the crystallinity of the CdTe layer deposited over CdSe. The CdTe/CdSe bilayer showed globular surface morphology, and the grain size of the films measured by FE-SEM micrographs range from 16 to 31 nm. EDAX analysis confirmed the presence of cadmium (Cd), selenium (Se) and tellurium (Te). Two electronic transitions were observed in the optical band gap of the CdTe/CdSe bilayer whose energy levels lie in between the energy band gap of the bulk CdTe and CdSe semiconductors. These electronic transitions are mainly due to the spin orbit split of valence band energy levels.

Structure and Properties of a (CdSe)6@(CdSe)30Cluster Doped with Mn Atoms

The structure and properties of a (CdSe)6@(CdSe)30 cluster doped with one or two Mn atoms were studied using density functional theory. Geometrical structure optimizations were performed using relativistic effective core potentials for Se and Cd atoms and an all-electron description for Mn atoms. The (CdSe)6 core corresponds to a bulk-type structure, and each shell atom is connected to three atoms of the opposite type. We considered different Mn substitutions of a single Se and Cd atom, substitutions of two Cd atoms with two Mn atoms, and additions of one and two Mn atoms to the (CdSe)6@(CdSe)30 cluster. We found that antiferromagnetic singlet and high-spin ferromagnetic states of the Mn2Cd4Se6@(CdSe)30 clusters are nearly degenerate in total energy. In the Cd-substituted clusters, the binding energies of Mn atoms were found to be appreciably larger than the binding energies of Cd atoms in the initial cluster, which explains the high miscibility of Mn in bulk CdSe. Experimental observations of dramatic changes in the magnetic behavior of Mn-doped QDs after annealing can be explained from the results of our calculations of total energy as a function of total spin.

Ultrafast carrier dynamics and coherent acoustic phonons in bulk CdSe.

The femtosecond pump-probe technique is used to study the dynamics of photoexcited carriers and coherent acoustic phonons in bulk CdSe semiconductor. A turning point from fast to slow decay is observed, whose amplitude decreases with pump fluences and eventually flips the sign of differential reflectivity. The maximum change of differential reflectivity shows a saturation at high pump fluences, which is attributed to the optical energy gap dependent on carrier density. Long-lasting coherent oscillations of acoustic phonons have also been detected, and their amplitude and lifetime have a strong dependence on pump fluences. Our results can facilitate the understanding of ultrafast carrier and phonon dynamics in CdSe nanocrystals.

Investigation of the bulk and surface properties of CdSe: insights from theory.

CdSe, in the form of quantum dots, is a semiconductor material extensively used for photovoltaic applications. We have performed a comprehensive density functional theory investigation of the geometrical and electronic properties of CdSe bulk crystals (for both the wurtzite and zinc blende phases) as well as their wurtzite (10-10) surface. Several combinations of Gaussian-type orbital basis sets and exchange-correlation functionals have been tested with a periodic formalism. The computational method for further studies was selected on the basis of the comparison of computed geometrical and electronic properties to available experimental data for the bulk crystals of CdSe, as well as to additional projector-augmented wave calculations. In a second step, the so-defined protocol was used to successfully simulate CdSe wurtzite nanocrystals exposing their nonpolar (10-10) surface. Most importantly, we have shown that the presented computational protocol is accurate to model not only bulk crystals but also surfaces, thus providing a powerful theoretical tool to simulate the light harvesting components of quantum dot sensitized solar cells.