CdSe Clusters Research Articles

Hybrid Au–CdSe and Ag–CdSe Nanoflowers and Core–Shell Nanocrystals via One‐Pot Heterogeneous Nucleation and Growth

A general approach, based on heterogeneous nucleation and growth of CdSe nanostructures on Au or Ag nanocrystals, for the synthesis of Au-CdSe and Ag-CdSe hybrid nanostructures is developed. The new approach provides a versatile one-pot route for the synthesis of hybrid nanoflowers consisting of a gold or silver core and multipod CdSe rods or an intact CdSe shell with controlled thickness, depending on the nucleation and growth parameters. At lower growth temperatures such as 150 °C, the CdSe clusters are adsorbed on the surface of the metal cores in their surface defects, then multiple arms and branches form, resulting in nanoflower-shaped hybrid structures. Increasing the size of the metal core through the choice of the reducing and capping agents results in an improvement of the interface between the metal and CdSe domains, producing core-shell structures. The growth temperature appears to be the most important factor determining the nature of the interface between the metal and CdSe domains. At relatively high temperatures such as 300 °C, the formation of large, faceted Au cores creates preferential growth sites for the CdSe nanocrystalline shell, thus resulting in well-defined Au-CdSe core-shell structures with large interfaces between the Au and CdSe domains. The present approach is expected to foster systematic studies of the electronic structures and optical properties of the metal-semiconductor hybrid materials for potential applications in photovoltaic and nanoelectronic devices.

Electronic Structure of Ligated CdSe Clusters: Dependence on DFT Methodology

Simulations of ligated semiconductor quantum dots (QDs) and their physical properties, such as morphologies, QD–ligand interactions, electronic structures, and optical transitions, are expected to be very sensitive to computational methodology. We utilize Density Functional Theory (DFT) and systematically study how the choice of density functional, atom-localized basis set, and a solvent affects the physical properties of the Cd33Se33 cluster ligated with a trimethylphosphine oxide ligand. We have found that qualitative performance of all exchange-correlation (XC) functionals is relatively similar in predicting strong QD–ligand binding energy (∼1 eV). Additionally, all functionals predict shorter Cd–Se bond lengths on the QD surface than in its core, revealing the nature and degree of QD surface reconstruction. For proper modeling of geometries and QD–ligand interactions, however, augmentation of even a moderately sized basis set with polarization functions (e.g., LANL2DZ* and 6-31G*) is very important. A...

Influence of Surfactants and Charges on CdSe Quantum Dots

Surface effects significantly influence the functionality of semiconductor nanocrystals. High quality nanocrystals can be achieved with good control of surface passivation by various hydrophobic ligands. In this work, the chemistry between CdSe quantum dots and common surface capping ligands is investigated using density functional theory (DFT). We discuss the electronic structures and optical properties of small CdSe clusters controlled by their size of particle, self-organization, capping ligands, and positive charges. The chosen model ligands reproduce good structural and energetic description of the interactions between the ligands and quantum dots. In order to capture the chemical nature and energetics of the interactions between the capping ligands and CdSe quantum dots, we found that PMe3 is needed to adequately model trioctylphosphine (TOP), NH3 is sufficient for amines, while OPH2Me could be used to model trioctylphosphine oxide. The relative binding interaction strength between ligands was found to decrease in order Cd–O > Cd–N > Cd–P with average binding energy per ligand being −25 kcal/mol for OPH2Me, −20 kcal/mol for NH3 and −10 kcal/mol for PMe3. Charges on studied stoichiometric clusters were found to have a significant effect on their structures, binding energies, and optical properties.

Solution–Liquid–Solid Synthesis of Semiconductor Nanowires Using Clusters as Single‐Source Precursors

CdSe clusters react with liquid Bi- nanocatalysts to form one-dimensional CdSe nanowires in solution. The use of cluster precursors as compared to molecular precursors leads to a high local CdSe concentration, which favors the growth of nanowires with thin diameters and a narrow diameter distribution.

Low-temperature synthesis of CdSe nanocrystal quantum dots

A method for fabricating colloidal CdSe nanocrystals at low reaction temperatures was developed. The transition from CdSe clusters to continuously-growing nanocrystals was found to be crucial in the formation of high-quality quantum dots with narrow size distribution and efficient, tunable optical properties.

Giant Zeeman splitting in nucleation-controlled doped CdSe:Mn2+ quantum nanoribbons

Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

Theoretical investigation of CdSe clusters: influence of solvent and ligand on nanocrystals

Based on experimental zinc blende and wurtzite models of CdSe nanocrystals, four clusters of CdSe, seven Se-Cd-ligand structures, and their characters are studied at DFT/B3LYP/Lanl2dz theoretical level. Cd(3)Se(3), (Cd(3)Se(3))(2) and (Cd(3)Se(3))(3) clusters which have a ring with six atoms are similar to wurtzite structures, Cd(4)Se(4) have resemblant conformation with zinc blende for they are all composed of tetrahedron. Calculated Raman spectra of Cd(3)Se(3), Cd(4)Se(4), (Cd(3)Se(3))(2) and (Cd(3)Se(3))(3) are about 175 cm(-1) which is consistent with the experimental result. Then, through investigation of Se-Cd-ligand clusters, we find that all Se-Cd-ligand structures have similar characters because main influence of ligands on nanocrystals comes from thiol. Finally, we testify that both solvent and ligand make absorption peaks shift to blue, compared with those in gas phase and without ligand. Under these conditions, calculated data of four clusters are almost identical with the absorption peaks of CdSe nanocrystals. Besides, we also prove that the absorption peaks of four clusters are the transitions from HOMO to LUMO or from d to p orbitals. And HOMO-LUMO gaps reduce in order of Cd(3)Se(3), (Cd(3)Se(3))(2) and (Cd(3)Se(3))(3), which is induced by the quantum size effect.

Effect of Surface Ligands on Optical and Electronic Spectra of Semiconductor Nanoclusters

We investigate the impact of ligands on the morphology, electronic structure, and optical response of the Cd(33)Se(33) cluster, which overlaps in size with the smallest synthesized CdSe nanocrystal quantum dots (QDs). Our density functional theory calculations demonstrate significant surface reorganization for both the bare cluster and the cluster capped with amine or phosphine oxide model ligands. We observe strong surface-ligand interactions leading to substantial charge redistribution and polarization effects on the surface. These effects result in the development of hybridized states, for which the electronic density is spread over the cluster and the ligands. The loss of one of the passivating ligands leads to either optically dark or bright additional states inside of the band gap, depending on the position of the leaving ligand on the QD surface. However, for fully ligated QDs, neither the ligand-localized nor hybridized molecular orbitals appear as trap states inside or near the band gap of the QD. Instead, being mostly optically dark, dense hybridized states could open new relaxation channels for high-energy photoexcitations. Comparing QDs passivated by different ligands, we also found that hybridized states are denser at the edge of the conduction band of the cluster ligated with phosphine oxide molecules than that with primary amines. Such a different manifestation of ligand binding may potentially lead to faster electron relaxation in QDs passivated by phosphine oxide than by amine ligands.

Many-beam dynamical simulation for multilayer structures without a superlattice cell

A many-beam dynamical theory for plan-view high-resolution transmission electron microscopy (HRTEM) images of multilayer systems without the limitation of a superlattice cell is proposed. The accuracy of our method is examined by comparing convergent-beam electron-diffraction calculations of Si(011) and HRTEM calculations of a system of epitaxial Al(100) on GaAs(100). Furthermore, this method is applied to CdSe clusters embedded in MgO, where it is revealed that the relative shift of their crystal-lattice planes produces moiré-like fringes.

Structures and Electronic Spectra of CdSe−Cys Complexes: Density Functional Theory Study of a Simple Peptide-Coated Nanocluster

We present density functional theory (DFT) structures and time-dependent DFT electronic excitation energies of several small CdSe nanoclusters with the composition Cd(n)Se(n) (n = 3, 6, 10, 13). We examine the effects on the geometries and excitation spectra of the nanoclusters induced by two chemical changes: peptide-binding and ligand passivation. We use cysteine (Cys) and cysteine-cysteine dipeptide (Cys-Cys) as model peptides and hydrogen atoms as surface-bound solvent ligands (or stabilizing agents). By comparing the results calculated for bare, hydrogen-passivated (Cd(n)Se(n)H(2n)), as well as the corresponding Cys- and Cys-Cys- bound clusters (Cd(n)Se(n)-, Cd(n)Se(n)H(2n), -Cys, -Cys-Cys), we find that peptide-binding blue shifts the electronic excitations of bare nanoclusters, but red shifts those of hydrogen-passivated nanoclusters. The carboxyl oxygen and the sulfur atom tend to form a four-centered ring with adjacent two Cd atoms when the CdSe cluster forms covalent bonds with Cys or Cys-Cys. Further, this type of bonds may be distinguishable by significant red shifts of the excitation energies.

Static and dynamic polarizabilities of (CdSe) n ( n = 1–16) clusters

Density functional theoretical calculations have been performed on small and medium-sized (CdSe) n ( n = 1–16) clusters in order to evaluate the variation in electric polarizability and anisotropy in polarizability with increase in the cluster size at both static and Nd:YAG laser frequencies. The electric polarizability values at static frequency show a rapid decrease up to heptamer only with the exception observed for (CdSe) 5 clusters where a sudden increase in the polarizability value is observed. An even–odd oscillating behavior is observed in the anisotropy values between the dimer and heptamer CdSe clusters. Apart from polarizability and anisotropy calculations, the variation in the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital and second difference of energies with the cluster size have also been calculated. Just like the anisotropy values, an even–odd oscillating behavior is also observed in the second difference value for the smaller CdSe clusters.

First principle study of free and surface terminated CdTe nanoparticles

Density functional calculations of structural and electronic properties of stoichiometric and nonstoichiometric CdTe clusters, containing up to few tens of atoms, are carried out using projector augmented wave method. Molecular dynamics has been performed for Cd12Te12 and Cd15Te15 to predict the structure corresponding to global energy minimum. Cage type structures and bulk fragments, both in zinc blende and wurtzite structures, are used as starting geometries and conjugate gradient method is used to locate the local energy minima for other clusters. The aim of these calculations is to get the energetically favorable probable structures, to be compared with the experimentally known structures. Clusters are relaxed both in vacuum and in the presence of surface passivating ligands and the resulting structural rearrangement is analyzed. As expected, passivation increases the stability of an individual cluster, as indicated by specific properties like binding energy, vertical detachment energy, electron affinity etc. Passivation also locks the symmetry for three-dimensional structures but the small CdnTen (1 ≤ n ≤ 6) clusters, which are planar, attain higher symmetry structures on passivation. We observe `self-healing' mechanism viz., opening of optical gap on relaxation without the aid of passivating ligand, in CdTe clusters as observed in CdSe clusters [A. Puzder et al., Phys. Rev. Lett. 92, 217401 (2004)]. However, we note that 'self-healing' is a stoichiometry dependent phenomenon. Te atoms are found to achieve a total coordination of 4 on passivation, a fact useful in chemical synthesis of nanoclusters.

Preparation of CdSe Quantum Dots with Full Color Emission Based on a Room Temperature Injection Technique

High quality CdSe quantum dots are synthesized through a room temperature injection technique by using CdAc2 and Na2SeSO3 as precursors. In this synthesis approach, small CdSe clusters are formed after the injection at room temperature. Thereafter, CdSe quantum dots with emissions from the green to the red region can be obtained by transferring these clusters to different temperatures (40-150 degrees C) for particle growth. Meanwhile, CdSe quantum dots with emission in the blue-violet region (500-430 nm) are gained by an oxidation etching approach using H2O2 as oxidant. The advantage of this method is the natural separation of the nucleation and the growth process, which can provide a longer time for the preparation of the nuclei in simple operations and a well controlled fluorescence of the products, as the evolution of the fluorescence is slow at this low particle growth temperature.

Ab initiomethods for the optical properties of CdSe clusters

We have performed time-dependent density functional theory and $GW$/Bethe-Salpeter calculations of the optical properties of a series of CdSe clusters ranging in size from 10 to 82 atoms and passivated by fictitious atoms of half-integer charge. The two methods predict a different character of the optical excitations of the CdSe clusters. In time-dependent density functional theory, the lowest-energy excitation is mainly due to a single-level to single-level transition. In $GW$/Bethe-Salpeter, there is a strong mixture of several different transitions, which is attributed to excitonic effects. Furthermore, $GW$/Bethe-Salpeter calculations predict the presence of dark transitions (optically forbidden) before the first bright transition for all but one of the clusters studied, whereas time-dependent density functional theory predicts the presence of dark transitions for only the two largest clusters. In this paper, we plot and analyze the effective valence and empty state charge densities of these clusters. We determine that the mixing in transitions observed in the $GW$/Bethe-Salpeter calculations is mostly due to the Bethe-Salpeter kernel and not from the fact that quasiparticle wave functions are a linear combination of wave functions obtained from density functional theory. We calculate the radiative decay lifetime of the excitations, and we explain the selection rules that lead to the presence of dark transitions in two of the clusters: ${\mathrm{Cd}}_{17}{\mathrm{Se}}_{28}$ and ${\mathrm{Cd}}_{32}{\mathrm{Se}}_{50}$. Finally, we compare time-dependent density functional theory and $GW$/Bethe-Salpeter absorption spectra to that of Mie theory, which has recently been shown to yield surprisingly accurate results for Si clusters.

Synthesis and bandgap variation of molecular-size CdSe clusters via electroporation ofvesicles

Molecular-size uncapped CdSe clusters are synthesized via the electroporationof synthetic dioleoylphosphatidylcholine vesicles of diameter 178 nm.Cd2+ ions, originally entrapped in the vesicles, are ejected through the transientpores of the vesicle membrane into the bulk where they can react withSeSO32− ions in basic condition to form CdSe clusters at room temperature. Growth andself-aggregation of CdSe clusters is slowed down to timescales of days by their adsorption atthe exterior surface of the vesicle, which permits spectral monitoring of the continuousgrowth process over a long period of time. Growth in the molecular size regimeis found to entail novel blue-shifts of the characteristic absorption band. Theblue-shift of the transition energy is in agreement with an analogous oscillation ofthe HOMO–LUMO gap calculated by the use of density functional theory for(CdSe)n clusters. The reason for the blue-shift absorption spectra of molecular-size CdSe clusters isinterpreted as the higher stability and bigger gaps of magic number. On the basis of thesimilarity between the experimental and theoretical trends, the characteristic absorptionpeaks of 256 nm, 215 nm can be assigned to the monomer or dimer, trimer clusters ofCdSe, respectively. Any further growth beyond trimers is associated with thecustomary monotonic red-shift of the absorption band, due to quantum size effects.

Evaporation and deposition of alkyl-capped silicon nanocrystals in ultrahigh vacuum

Nanocrystals are under active investigation because of their interesting size-dependent properties and potential applications. Silicon nanocrystals have been studied for possible uses in optoelectronics, and may be relevant to the understanding of natural processes such as lightning strikes. Gas-phase methods can be used to prepare nanocrystals, and mass spectrometric techniques have been used to analyse Au and CdSe clusters. However, it is difficult to study nanocrystals by such methods unless they are synthesized in the gas phase. In particular, pre-prepared nanocrystals are generally difficult to sublime without decomposition. Here we report the observation that films of alkyl-capped silicon nanocrystals evaporate upon heating in ultrahigh vacuum at 200 degrees C, and the vapour of intact nanocrystals can be collected on a variety of solid substrates. This effect may be useful for the controlled preparation of new quantum-confined silicon structures and could facilitate their mass spectroscopic study and size-selection.

Blue light emitting diodes based on fluorescent CdSe∕ZnS nanocrystals

The authors report on the blue electroluminescence from CdSe∕ZnS core/shell nanocrystals prepared from ultrasmall, magic size CdSe clusters that have a diameter of less than 2nm. The light emitting device consists of an active layer of nanocrystals blended with 4,4′,N,N′- diphenylcarbazole and an evaporated electron transporting/hole blocking layer made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. A blue, stable electroluminescence at 485nm from the hybrid device was observed, in good agreement with the photoluminescence spectra of a solid film of the same nanocrystals used for the device.

Manipulation of the Morphology of Semiconductor‐Based Nanostructures from Core–Shell Nanoparticles to Nanocables: The Case of CdSe/SiO2

The morphology of CdSe/SiO(2) was manipulated from core-shell-structured nanoparticles to nanocables by using a chemical vapor deposition (CVD) process. The growth of nanocables, with cores no more than 20 nm in diameter, is initiated by the formation of core-shell nanoparticles with SiO(2) as matrix and CdSe clusters dispersed inside. After the subsequent vaporization of the SiO(2) matrix, the follow-up CdSe vapor crystallizes with the remaining CdSe clusters as nuclei to form CdSe nanowires as the furnace was cooled to 1200 degrees C. During the controlled cooling of the furnace, the SiO vapor re-deposits to sheathe the nanowires. The thickness of the shell and the diameter of core were successfully controlled. The photoluminescence measurements show that the CdSe/SiO(2) nanocables have strong visible-light emission bands located at 590 and 688 nm, which are attributed to the defects induced by SiO(2) sheaths nanowires and the quantum confinement effect of the CdSe, respectively. The UV/Vis absorption spectra of the naked CdSe nanowires further validate the above-mentioned quantum confinement effect. The deterministic growth of these nanocables is very important for the design of the nanodevices based on them.

Identification of fullerene-like CdSe nanoparticles from optical spectroscopy calculations

Semiconducting nanoparticles are the building blocks of optical nanodevices as their electronic states, and therefore light absorption and emission, can be controlled by modifying their size and shape. CdSe is perhaps the most studied of these nanoparticles, due to the efficiency of its synthesis, the high quality of the resulting samples, and the fact that the optical gap is in the visible range. In this article, we study light absorption of CdSe nanostructures with sizes up to 1.5 nm within density functional theory. We study both bulk fragments with wurtzite symmetry and novel fullerene-like core-cage structures. The comparison with recent experimental optical spectra allows us to confirm the synthesis of these fullerene-like CdSe clusters.

Frequency-dependent nonlinear optical properties of CdSe clusters

Nonlinear optical properties of CdSe clusters have been investigated using time dependent density functional theory. Finite and negative values of static and frequency dependent first order hyperpolarizability have been observed in case of CdSe and ${\mathrm{Cd}}_{3}{\mathrm{Se}}_{3}$ while the same is absent in ${\mathrm{Cd}}_{2}{\mathrm{Se}}_{2}$ and ${\mathrm{Cd}}_{4}{\mathrm{Se}}_{4}$. At specific frequencies, sudden increase in the frequency dependent components of first order hyperpolarizability tensor (second harmonic generation) has been attributed to the near resonance of the input energy. Three components of second order hyperpolarizability tensor have been calculated by combining analytical and finite difference techniques. The results indicate that these clusters have a high degree of nonlinear optical properties.