ZnSe Clusters Research Articles

Effect of One-Coordinated Atoms on the Electronic and Optical Properties of ZnSe Clusters.

To understand the influence of one-coordinated Zn and Se atoms on the structures, electronic, and optical properties of ZnSe clusters, we investigate the Zn37Se20 clusters employing first-principles theoretical calculations. The Zn37Se20 cluster, constructed from the InP nanocrystal structure, possesses a Zn21Se20 core and 16 one-coordinated surface atoms. The effect of one-coordinated atoms is studied by adding or removing one-coordinated atoms of the Zn37Se20 cluster. The calculations show that the modifications of one-coordinated atoms change slightly the coordination states and bond lengths of the atoms on the cluster surface. The clusters with the same core structure and different amounts of one-coordinated atoms have similar optical spectra, suggesting the importance of the cluster core structure in their optical properties.

Theoretical Investigation on Structural and Magnetic Properties of Mn-doped and C co-doped Zn12Se12 Nanoclusters

Background: Mn doped ZnSe low dimensional materials are attractive for different biological labels, gene silencing and dilute-magnetic device. ZnSe clusters are one of the basic building blocks of quantum dots and even cluster-assembled nanodevices, stable structures of undoped ZnSe clusters were established by previous pioneering work, and the Mn doped ZnSe clusters had been investigated, but the stable clusters in the ferromagnetic state have not been found yet. Objective: Our work is mainly based on Mn doped clusters (Mn2Zn10Se12) and C codoped clusters (Mn2Zn10Se11) structure, magnetic properties through theoretical calculations. Methods: First principle density functional theory calculation with Dmol3 is used to execute all calculations. Results: Mn atoms prefer to substitute the nearest neighbor Zn atom sites in the rhombi part, and C atom prefers to occupy Se atom sites with shortest Mn-C bond length in Zn12Se12 nanocluster doping. Mn doped clusters (Mn2Zn10Se12) are in antiferromagnetic states and the most stable C codoped clusters (Mn2Zn10Se11) are in ferromagnetic states. Magnetic behavior localized at the 3d orbitals of transitional metal Mn, 4p orbital of atom Se and 2p orbital of C atom. Mn2Zn10Se12 clusters are in antiferromagnetic states as the p-d hybridization introduced Mn-Mn superexchange mechanism. For the ferromagnetism of Mn2Zn10Se12 nanocluster, hole mediated double exchange mechanism introduced by C atom p-d hole state hybridization has been suggested. Conclusion: The codoping of C atom can stabilize the ferromagnetism of clusters through hole mediated double exchange mechanism, which may be meaningful for the exploring materials for cluster- assembled spin-electronic devices.

Radiative and non-radiative exciton recombination rate constants in ZnSe clusters

Understanding the origin of photoluminescence intermittency and its correlation with microstructure is crucial for the design and preparation of quantum dots (QDs) with high fluorescence quantum yield. ZnSe clusters provide a typical model for studying the effect of their size, geometrical and electronic structures on their radiative and non-radiative process of II-VI QDs. The rate constants of radiative and non-radiative processes, kr and knr, of the (ZnSe)n clusters were computed by using first-principles calculations, Einstein spontaneous radiation theory and Fermi’s golden rule. The kr and knr variations with cluster size were analyzed in term of a number of quantities. Emission energy and reorganization energy were identified to play dominant roles in the determination of kr and knr for the studied clusters. Furthermore, a correlation between these two quantities and the geometric rigidity of the ZnSe clusters was revealed. The clusters with greater geometric rigidity tend to possess larger emission energy, and smaller reorganization energy. While radiative and non-radiative electron–hole recombination rates of the ZnSe clusters vary in a complicated way because of their diverse structures and prominent quantum size effect, our study highlights the correlation between recombination rate and cluster structure, which would be helpful for the design of QD materials with high fluorescence quantum yields.

Reprint of “Structure and optical properties of SiO2 films with ZnSe nanocrystals formed by ion implantation”

ZnSe nanocrystals have been formed in the silicon dioxide matrix by the sequential high-fluence implantation of Zn+ and Se+ ions at 500 °C. After implantation a part of samples was annealed at 1000 °C for 3 min using rapid thermal annealing. Structural and optical properties of ZnSe/SiO2 nano-composite films were analyzed by means of Rutherford Backscattering Spectrometry, cross-sectional Transmission Electron Microscopy, Raman scattering and photoluminescence techniques. It was shown that a sequence of implantation affects structural and optical properties of synthesized ZnSe clusters. Based on the Raman scattering and photoluminescence data the samples for which Zn ions were implanted first exhibited a better ZnSe crystalline quality than those of reverse sequence of implantation, i.e. with Se ions implanted at the beginning. The bands of blue ZnSe band edge emission and green-red ZnSe deep defect level emission were revealed in the PL spectra of the as-implanted and annealed nano-composites. The PL spectral features observed in the blue region are due to the quantum-size effects in the ZnSe nanocrystals embedded into the silicon dioxide matrix. The PL intensity ratio of the deep defect band to the near edge emission is higher in the samples first implanted with Se ions, and Zn ions implanted next. The effect of rapid thermal annealing on structural and light-emitting properties was discussed.

Structure and optical properties of SiO2 films with ZnSe nanocrystals formed by ion implantation

ZnSe nanocrystals have been formed in the silicon dioxide matrix by the sequential high-fluence implantation of Zn+ and Se+ ions at 500 °C. After implantation a part of samples was annealed at 1000 °C for 3 min using rapid thermal annealing. Structural and optical properties of ZnSe/SiO2 nano-composite films were analyzed by means of Rutherford Backscattering Spectrometry, cross-sectional Transmission Electron Microscopy, Raman scattering and photoluminescence techniques. It was shown that a sequence of implantation affects structural and optical properties of synthesized ZnSe clusters. Based on the Raman scattering and photoluminescence data the samples for which Zn ions were implanted first exhibited a better ZnSe crystalline quality than those of reverse sequence of implantation, i.e. with Se ions implanted at the beginning. The bands of blue ZnSe band edge emission and green-red ZnSe deep defect level emission were revealed in the PL spectra of the as-implanted and annealed nano-composites. The PL spectral features observed in the blue region are due to the quantum-size effects in the ZnSe nanocrystals embedded into the silicon dioxide matrix. The PL intensity ratio of the deep defect band to the near edge emission is higher in the samples first implanted with Se ions, and Zn ions implanted next. The effect of rapid thermal annealing on structural and light-emitting properties was discussed.

Stokes shifts of small ZnSe clusters from first-principles calculations

ABSTRACTThe large Stokes shifts of II-VI semiconductor quantum dots favour their application in bioimaging. Using a cluster model, we revealed that ZnSe clusters have large Stokes shifts at small cluster sizes by means of density functional theory calculations. Both the optical absorption and emission wavelengths of the clusters are size-dependent. While some trends were noted for the absorption spectra, the emission wavelength varies in a complicated way, leading to large fluctuations in their Stokes shifts with cluster size. These fluctuations, as well as their variations with solvent, were rationalised in terms of changes in their geometrical and electronic structures from size to size, and from ground- to excited-state.

Simulation-Based Optical Spectra Analyses and Synthesis of Highly Monodispersed Mn-Doped ZnSe Nanocrystal

Geometrical structures of (ZnSe)n, [Formula: see text], ([Formula: see text] 1–4) and (MnxZn[Formula: see text]Se[Formula: see text], ([Formula: see text] clusters were calculated using density functional theory (DFT). Optical/absorption spectra, Raman spectra, HOMO–LUMO gap energy and binding energy of each cluster were calculated. The calculated results show the red shift of the optical/absorption spectra band caused by the manganese atoms doped in ZnSe clusters, and the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap energy value is decreased. Furthermore, we realized highly monodispersed manganese-doped zinc selenide quantum dots (Mn:ZnSe d-dots) experimentally by using a convenient route. The as-synthesized Mn:ZnSe d-dots were characterized by UV-Vis absorption, photoluminescence (PL), X-ray diffraction (XRD), TEM and HRTEM. The experimental results revealed that the as-prepared Mn:ZnSe d-dots with zinc-blende structure have an average size of about 3.9 nm.

Theoretical and experimental investigation of doping M in ZnSe (M = Cd, Mn, Ag, Cu) clusters: optical and bonding characteristics.

The optical and bonding characteristics of doping ZnSe quantum dots (QDs) were investigated. Cd-, Mn-, Ag- and Cu-doped ZnSe were synthesized in aqueous solution. Theoretically, the intensity of the Cd-Se bond was similar to that of the Zn-Se bond, which illustrates that Cd can be doped into ZnSe materials at any ratio. We found that Mn-Se bonding was stronger than Zn-Se bonding. Ag-doped ZnSe nanoclusters show the same bonding and configuration as Cu-doped ZnSe. Moreover, Cd can be doped into ZnSe using both the substitution- and vacancy-doping method. For Mn-doped ZnSe clusters, small amounts of Mn impurity lead to stronger bonding with Se, but larger amounts of Mn impurity led to the formation of a Mn-Mn metal bond. The theoretical results show that it is difficult to form a vacancy-doping cluster for Mn-doped ZnSe materials. In experiments, the absorption and photoluminescence (PL) spectral wavelengths of Mn-doped ZnSe nanocrystals were the same as those of pure ZnSe nanocrystals, showing that the Mn impurity is not doped into ZnSe nanocrystals. Ag- and Cu-doped ZnSe nanocrystals have the same PL characteristics. The doping of an impurity is related to the solubility product, and not the bonding intensity.

Modeling and Simulation of Bonding and Optical Characters of Ternary Nanocrystals

In this paper, conformations of the ternary structures ZnSe(Te), ZnS(Te), ZnS(Se), CdSe(Te), CdS(Te) and CdS(Se) were optimized, and their orbital, spectra have been investigated at the B3LYP/LANL2DZ level. First, we have found some rules which agreed with experimental results. HOMO–LUMO gaps, WBI values and the wavelengths of the absorption peaks of ternary structures changed gradually with the ratio of Te, Se and S atoms in ternary structures. Second, analysis of Raman spectra revealed that doped structures had the spectra of two relevant binary clusters. Namely, Raman spectra of ternary ZnSe(Te) clusters had the Raman peaks of ZnSe and ZnTe. In this way, with the help of the Raman spectra, the formations of the ternary structures can be determined in experiment. This was important to estimate during synthesis progress. Finally, calculated results have proved that ZnS and CdS structures had the shorter wavelengths of the absorption peaks, the higher excited energies of singlets, and good stability.

Zinc selenide nanoclusters: Static dipole polarizability and electronic properties

The structural stability and electronic properties of stoichiometric zinc selenide (ZnSe) nanoclusters up to 20 atoms have been systematically investigated in the framework of density functional theory. The similarity function has been used to investigate the extent that the clusters are similar to the pure ZnSe or to each other. Energetically, the relative stability of ZnSe clusters at different sizes is studied by calculating the second-order difference in total energy. The size evolution of several reactivity descriptors such as static dipole polarizability and chemical hardness has been determined for ZnSe clusters. In addition, the minimum polarizability principle and principle of maximum hardness are used to characterize the magic clusters. Moreover, it is shown that the differential mean polarizability can be also a useful quantity to characterize the stability of the studied clusters. It is found that there is a strong inverse correlation between the static dipole polarizability and the ionization potential of the ZnSe clusters. Similarly, the softness has also been shown to mostly correlate with the static dipole polarizability of these clusters. This work thus will have some important implications for the calculation of polarizability of ZnSe clusters in terms of the corresponding ionization potentials and softness. For each property, a comparison between studied clusters here and the zinc sulfide clusters from our recent article is presented. V C 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3888-3896, 2011

Theoretical investigation for ZnSe clusters: Influence of solvent and ligand on their characteristics

On the basis of experimental zinc blende and wurtzite of ZnSe nanocrystals, four structures Zn 3Se 3, Zn 4Se 4, (Zn 3Se 3) 2, (Zn 3Se 3) 3 and Se–Zn–SR clusters were studied at DFT/B3LYP/LANL2DZ theoretical level. First, Zn 3Se 3, (Zn 3Se 3) 2 and (Zn 3Se 3) 3 structures have the same units (hexagon) with wurtzite. Zn 4Se 4 has resemblant conformation with zinc blende for they are all composed of tetrahedron. Second, calculated Raman peaks of Zn 3Se 3, Zn 4Se 4, (Zn 3Se 3) 2 and (Zn 3Se 3) 3 are about 180 cm −1 that is consistent with the experimental result. Third, through investigation of Se–Zn–SR clusters, we have found that their stability, bond lengths and the wavelengths of the absorption peaks do not have essential difference in theory. It illuminates that the main influence of ligand comes from thiol. Finally, we detect that both solvent and ligand make the wavelengths of the absorption peaks shift to blue. With solvent and ligand, our wavelengths of the absorption peaks will be almost identical with experimental ZnSe nanocrystals. Moreover, the wavelengths of the absorption peaks shift to red in order Zn 3Se 3, (Zn 3Se 3) 2 and (Zn 3Se 3) 3, which is induced by quantum size effect. Besides, we also prove that the calculated absorption peaks of four structures are the transitions from HOMO to LUMO or from d to p orbital.

Synthesis of ZnSe nanocrystalline powders by mechanochemical reaction

The ZnSe nanocrystalline powders were synthesized by mechanochemical reaction method. The powders milled at air atmosphere have the second phase ZnO, but that milled above 2 h at high N2 atmosphere have not second phase ZnO. The phase structure of the ZnSe powder milled for 10 h at N2 atmosphere is sphalerite structure. The average size of the as-milled and annealed ZnSe nanocrystalline powders is 5.17 and 8.88 nm respectively. The TEM analysis demonstrated that the size of agglomerated ZnSe crystalline is less than 100 nm, but the electron diffraction pattern showed that ZnSe clusters were a polycrystalline structure. The absorption spectrum of ZnSe samples was measured by UV–VIS–NIR Photometer Spectrum. It showed that the absorption edge of the ZnSe nanocrystalline powders by mechanochemical reaction comparing to ZnSe bulk materials had a red-shift. This result can be attributed to the residual stress and lattice aberrance of ZnSe nanocrystalline powders in milling process. The absorption edge of the annealed ZnSe powders was recovered owing to the residual stress was eliminated by annealing for 2 h containing nitrogen.

Mechanism of Cluster Formation on Zinc Selenium/Gallium Arsenic Prepared by Metallorganic Vapor Phase Epitaxy

Atomic force microscopy is used to investigate the surface morphology of zinc selenium clusters on gallium arsenic substrate prepared by low-pressure metallorganic vapor phase epitaxy. The density of cluster can reach . The average sizes are 50 nm in height and 112 nm in diameter. From the relationship between the density of cluster and hydrogen selenide flow rate, and the surface and depth compositional variation of zinc and selenium, the ZnSe clusters on GaAs are formed by selenium segregation. The dislocations enhance the selenium segregation. The nonspherical shaped cluster is formed from the mergence of two clusters. The spherical shaped cluster is formed by long-range potential from the nonstoichiometry epi layer.

Cluster formation in UV laser ablation plumes of ZnSe and ZnO studied by time-of-flight mass spectrometry

The composition of nitrogen laser (wavelength 337 nm, pulse width 3 ns) induced ablation plumes from ZnSe and ZnO targets was studied at different laser fluences using a time-of-flight mass spectrometer. In the case of ZnSe, abundant ion signals corresponding to Se and ZnSe clusters, some of which were Se-rich, were detected with fluence-dependent distributions. At a laser fluence of 1250 J m-2, clusters with elevated intensity were observed at sizes of 6, 13, 19, 23 and 33 ZnSe molecules (‘magic numbers’), which match quite well with the earlier observation by others of magic numbers of chemically similar ZnS produced by a conventional vaporization and quenching scheme. In the case of ZnO, we detected the presence of atomic Zn and molecular species of ZnO, as well as a series of (ZnO)n-type clusters with fluence-dependent distributions. Unlike the case of ZnSe, no magic numbers were observed for ZnO.

Quartz-crystal microbalance study of the growth of Zn(Se,O) thin-films in a chemical bath. A sequential electroless-chemical process

The chemical bath deposition of Zn(Se,O) thin films is studied with a quartz crystal microbalance (QCM). The deposition velocity is measured with QCM under different experimental conditions, including substrate properties, bath temperature and bath composition. The plots of the growth velocity vs. time reflect the sequence of processes for the deposition of the film. At the beginning, an induction period takes place, which ends up with the formation of first ZnSe particles. These particles activate an electroless reaction induced by a reducer, hydrazine or hydroxide, which gives rise to deposition of a film with ZnO and ZnSe composition. This step is necessary to attain compact and thicker films with appropriate conditions for solar cell application. The electroless mechanism is decelerated after deposition of some nanometers of film, hence, a second growth mechanism starts to predominate consisting in the chemical reaction of soluble selenide with hydroxide and Zn 2+ cations on the substrate surface. The layers resulting from this second mechanism have higher proportion of ZnSe than the previously electroless generated layers. At longer times, the predominant growth mechanism is the deposition of ZnSe clusters formed in the solution, which produces less compact and poorly adherent top layers. This sequence of mechanisms gives rise to films with heterogeneous morphology and composition in depth.

High-efficiency Cd-free CIGSS thin-film solar cells with solution grown zinc compound buffer layers

Zn-compounds Zn( X,OH) ( X=S,Se) buffer layers have been deposited by chemical bath (CBD) process on Cu(In,Ga)(S,Se) 2 (CIGSS) with the aim of developing Cd-free CIGSS-based devices. The films are produced in alkaline aqueous solution containing ZnSO 4, ammonia NH 3 and XC(NH 2) 2. Optimum deposition conditions were established. The temperature ( T sub) of the chemical bath is found to be critical for the device quality. The thickness and good surface coverage were controlled by XPS-UPS photoemission spectroscopy. SEM study showed that the growth of ZnSe nuclei on CIGSS proceeds in lateral direction. Once the surface is covered the growth takes place in vertical direction . The ZnSe clusters grow in size and their elongated shapes cover the CIGSS surface. High efficiency of over 13% was obtained for both CIGSS/Zn(S,OH) and CIGSS/Zn(Se,OH)-based solar cells. Solar cells with CIGSS/Zn(Se,OH) x /ZnO/MgF 2 structure show an active area efficiency up to 15.7%. Using Zn(Se,OH) buffer layer, efficiency of 11.7% was achieved with a 20 cm 2 aperture-area monolithic minimodule.

Morphological and compositional study of CBD-ZnSe thin films by microscopy techniques and angle resolved XPS

A morphological and compositional study is carried out on ZnSe thin films obtained by the chemical bath deposition (CBD) method. SEM, TEM and AFM images are shown for the morphological characterisation. Angle-resolved XPS (ARXPS) measurements are used for the study of surface and subsurface composition of the films. The composition underneath is studied with XPS analysis of films eroded by sputtering. It is found that films have a mixed ZnSe-ZnO (or Zn(OH) 2) composition. The Zn/Se ratio in the film increases with depth, indicating that Zn is preferentially as ZnO-Zn(OH) 2 close to the film-substrate interface, and that the ZnSe proportion increases above. Such composition inhomogeneity is attributed to a change in the deposition mechanism during film growth. At first, it proceeds via reaction of adsorbed Zn and Se precursors, and then by deposition of ZnSe clusters formed in the bulk of the solution. Apparently the first mechanism is less efficient for the formation of a pure ZnSe film, at least under the experimental conditions used here, hence gives rise to higher concentration of Zn oxides close to the film-substrate interface. At longer times during the deposition process, the cluster precipitation mechanism predominates and, consequentially, the top layers of the film become richer in ZnSe but less compact. Other compounds detected by ARXPS are Se 0 occluded in the bulk of the film, and some SeO 2 at the surface. Annealing at 300°C results in structural and compositional changes which involve compaction of the films, the loss of the occluded Se 0, the increment of the SeO 2 overlayer and the transformation of Zn(OH) 2 into ZnO and/or Zn(O, Se) compounds.

Investigation of properties of porous silicon embedded with ZnSe and CdSe

Three-dimensional arrays of a few nanometer size clusters have been realized using a porous silicon (PS) matrix by filling its pores with CdSe or ZnSe. From the photoluminescence behavior it is concluded that the average size of the CdSe, ZnSe clusters is about 3–5 nm. We compare the PL and Raman scattering spectra of the pure PS area of samples with those obtained from the embedded areas on the same wafer. The results are consistent with the stabilization of the PL peak in the case of CdSe in spite of the different PL peak positions of the pure PS. PL spectra of the PS were examined as function of laser irradiation time and laser intensities.

Luminescence properties of ZnSe/ZnS( h 1 1)A low dimensional structures

The power dependence and the temperature dependence of the luminescence intensity of ultra-thin pseudomorphic ZnSe quantum wells, grown on high index GaP substrates were investigated. A blue shift of PL with power excitation is mainly related to the band filling of interfacial ZnSe clusters while it is found that the piezoelectric field has only a small influence. Luminescence spectra of ultra-thin ZnSe/ZnS single quantum wells are characterized by an excitation dependent excitonic localization phenomenon.

Quantum confinement effects in chemically grown, stable ZnSe nanoclusters

Stable ZnSe quantum dots have been prepared by a wet chemical route. Sodium Hexameta Phosphate, an inorganic polymer, is used to encapsulate the ZnSe quantum dots and to enhance the stability of quantum dots. X-ray Photoelectron Spectroscopic analysis confirms the high purity of ZnSe clusters. The optical absorption spectra exhibit a blue shift of about 0.6 eV, suggesting the existence of quantum size effects. Room temperature photoluminescence measurements show a strong feature which originates from the radiative recombination of the quantum confined electron-hole pairs.