Semiconductor Clusters Research Articles

Reducing the propensity for unphysical wavefunction symmetry breaking in multireference calculations of the excited states of semiconductor clusters

Unphysical spatial symmetry breaking in multiconfigurational self-consistent field calculations can lead to undesirable artifacts in the potential energy surfaces and electronic properties of molecules. Herein, we report several examples of such symmetry breaking in calculations of the excited states of small semiconductor clusters and related molecules at the state-averaged complete active space self-consistent field (SA-CASSCF) level of theory. A multireference approach is proposed to reduce its incidence: the singly excited active space complete active space configuration interaction (SEAS-CASCI) method. In SEAS-CASCI, the orbitals are determined by variationally minimizing an energy expression that does not depend on the off-diagonal Hamiltonian matrix elements which drive symmetry breaking at the SA-CASSCF level of theory. By application to several highly symmetric molecules, SEAS-CASCI is demonstrated to reduce the propensity for unphysical spatial symmetry breaking and eliminate resulting errors in the potential energy surfaces and molecular properties relative to the SA-CASSCF description. The SEAS method is also found to eliminate unphysical wavefunction distortion in asymmetric molecules. Finally, SEAS-CASCI is demonstrated to accurately describe the biradicaloid region of the potential energy surface of ethylene.

Excited-State and Reactive Clusters: Synthesis, Structure, Reactivities, and Properties

This is a special issue on ‘‘Excited-State and Reactive Clusters: Synthesis, Structure, Reactivities, and Properties.’’ It contains two reviews and 13 original papers. The subject matters range from experimental to theoretical studies of excited-state or reactive clusters as they relate to, or are involved in, interesting chemical (such as catalysis) and physical properties (e.g., luminescence). The materials involved range from zero-dimensional (0-D) quantum dots to one-dimensional (1-D) nanowires or nanotubes to two-dimensional (2-D) hydrogen bonded systems to three-dimensional (3-D) polyhedral structures. New synthetic methods, novel structures, and unusual chemical bonding of clusters, especially those related to excited or reactive clusters, are also reported. The subject matters are highlighted below: In the invited review entitled ‘‘Excited State Relaxation and Stabilization of Hydrogen-Terminated Silicon Quantum Dots,’’ Wu, Wang, Li, and Zhang provide a comprehensive review of recent advances in the theoretical investigation of the excited state properties of silicon quantum dots (QDs). The theoretical calculations revealed that the size-dependent excited-state relaxation due to surface passivation of silicon QDs by hydrogen can critically impact physical properties such as photoluminescence and hence their ultimate technological applications. The invited review entitled ‘‘Excited Semiconductor Clusters: Magic Source,’’ by Yang, Wang, and Zhang deals with the preparation and properties of CdS-like clusters as well as their applications. The formation of nanostructures, including 1-D solid and tubular fibers, 2-D dendritic morphology, and 3-D crystals are reviewed and their photoluminescence discussed. A specific example is provided in a related

Binary Superlattices from Colloidal Nanocrystals and Giant Polyoxometalate Clusters

We report a new kind of long-range ordered binary superlattices comprising atomically defined inorganic clusters and colloidally synthesized nanocrystals. In a model system, we combined surfactant-encapsulated, nearly spherical giant polyoxometalate clusters containing 2.9 nm polyoxomolybdate or 2.5 nm polyoxovanadomolybdate cores with monodisperse colloidal semiconductor nanocrystals (PbS, CdSe, PbS/CdS; 4-11 nm). The results are rationalized on the basis of dense packing principles of sterically stabilized particles with predominantly hard-spherelike interparticle interactions. By varying the size-ratios and relative concentrations of constituents, we obtained known thermodynamically stable binary packings of hard-spheres such as NaCl, AlB2, and NaZn13 lattices and also CaCu5-type lattice and aperiodic quasicrystals with 12-fold symmetry. These results suggest that other kinds of cluster materials such as fullerenes and magic-sized metallic and semiconductor clusters can also be integrated into supramolecular assemblies with nanocrystals. Furthermore, synergistic effects are expected from the combination of redox and catalytic properties of polyoxometalates with excitonic and plasmonic properties of inorganic nanocrystals.

Excited Semiconductor Clusters: Magic Source

In this paper, we review the preparation and properties of CdS-like clusters as well as their important applications in several nanostructures. The clusters created by the reaction of S2− ions generated by the decomposition of thioglycolic acid (TGA) and Cd2+ ions play important roles for the construction of novel nanocrystals (NCs), the assembly of the NCs, and the formation of nanostructures including 1D solid and tubal fibers, 2D dendritic morphology, and 3D crystals. The formation and property of the clusters depended strongly on the molar ratio of TGA and Cd2+ ions in solutions. When aqueous CdTe NCs coated with a hybrid SiO2 shell containing the clusters, they revealed a drastic increase in photoluminescence (PL) efficiency (from 28 to 80 %) and temperature-dependent PL. These excellent PL properties were ascribed to the clusters very closed to CdTe cores. This is confirmed through the observation of a lengthening of the Auger recombination lifetime by a factor of ~3.5 in the presence of the clusters. The size of the clusters determined the PL properties of the hybrid SiO2-coated NCs and those fibers. Because of their high PL, these biofunctional materials could provide a platform for various applications.

What one can learn about clusters using the unique tools of x-ray photoelectron spectroscopy

This presentation is intended to illustrate various types of collisional processes relevant for free clusters probed by x-ray photoelectron spectroscopy using our own examples ranging from free-electron-metal clusters, through half-metal and semiconductor clusters to dielectrics, the latter from van-der-Waals to ionic clusters.

Surfactant-Controlled Polymerization of Semiconductor Clusters to Quantum Dots through Competing Step-Growth and Living Chain-Growth Mechanisms

This article reports control of the competition between step-growth and living chain-growth polymerization mechanisms in the formation of cadmium chalcogenide colloidal quantum dots (QDs) from CdSe(S) clusters by varying the concentration of anionic surfactant in the synthetic reaction mixture. The growth of the particles proceeds by step-addition from initially nucleated clusters in the absence of excess phosphinic or carboxylic acids, which adsorb as their anionic conjugate bases, and proceeds indirectly by dissolution of clusters, and subsequent chain-addition of monomers to stable clusters (Ostwald ripening) in the presence of excess phosphinic or carboxylic acid. Fusion of clusters by step-growth polymerization is an explanation for the consistent observation of so-called "magic-sized" clusters in QD growth reactions. Living chain-addition (chain addition with no explicit termination step) produces QDs over a larger range of sizes with better size dispersity than step-addition. Tuning the molar ratio of surfactant to Se(2-)(S(2-)), the limiting ionic reagent, within the living chain-addition polymerization allows for stoichiometric control of QD radius without relying on reaction time.

Positron lifetime spectroscopy with optical excitation: a case study of natural diamond

In this paper, we report positron lifetime results obtained under high-power steady-state and transient optical excitation. We present a model for analysing the results. The method has been applied to vacancy clusters in natural diamond, for which we self-consistently analyse optoelectronic constants such as optical absorption cross-section and hole recombination cross-section. The temperature dependences of transient and steady-state measurements are studied, suggesting the possibility of analysing the positron trapping to extended defects and vacancy clusters in semiconductors.

Mechanism of conductivity in molecular systems and polymer nanocomposites

A model is considered in which the characteristics of polymer nonocomposites based on poly(p-xylylene) are observed due to the formation of large percolation clusters consisting of semiconductor or metal nonoparticles, and to electron transfer from the surfaces of clusters on -C6H4- phenyl rings. It is assumed that an electrical double layer is formed near a cluster’s surface, accompanied by the appearance of excess electrons in the polymer matrix as components of -C6H 4 − - ion resonance. It is shown that a metallic cluster in the polymer matrix is oxidized following an increase in its electrostatic potential. This could be explained by the notable reduction in the oxidation of nanocomposite metallic clusters by atmospheric oxygen and water molecules. Upon the formation of semiconductor clusters in poly(p-xylylene) nanocomposite, conductivity is observed due to the hopping mechanism. The extra electron of the −C6H 4 − - anion resonance in the electric field jumps to another ring. The hopping character of nanocomposite conductivity is explained for the case of small nanoparticle concentrations using the Mott model. Precise equations from percolation cluster theory are used to explain the nonmonotonic dependency of nanocomposite photoconductivity on the concentration of semiconductor nanoparticles.

Electric dipole moments and polarizabilities of small Bin (n = 2–24, 40, 80) clusters

AbstractThe electric dipole moments (EDMs) and polarizabilities of small Bin (n = 2–24, 40, 80) clusters are investigated by the finite field method within density functional theory (DFT). The results show that both dipole moments and polarizabilities have even–odd oscillation behaviors, and they strongly depend on geometrical structures and electronic structures. High symmetry structure prohibits the occurrence of EDMs on Bi clusters. The increasing polarizabilities of Bi clusters are attributed to the inherent novel chain‐like geometrical evolution, which is significantly different from the characters observed in metal clusters or semiconductor clusters.

Self-Assembly of Nanosized 0D Clusters: CdS Quantum Dot-Polyoxotungstate Nanohybrids with Strongly Coupled Electronic Structures and Visible-Light-Active Photofunctions

Nanohybrids of CdS-polyoxotungstate with strongly coupled electronic structures and visible-light-active photofunctions can be synthesized by electrostatically derived self-assembly of very small CdS quantum dots, or QDs, (particle size ≈ 2.5 nm) and polyoxotungstate nanoclusters (cluster size ≈1 nm). The formation of CdS-polyoxotungstate nanohybrids is confirmed by high-resolution transmission electron microscopy, elemental mapping, and powder X-ray diffraction analysis. Due to the strong electronic coupling between two semiconductors, the CdS-polyoxotungstate nanohybrids show a narrow bandgap energy of around 1.9-2.7 eV, thus reflecting their ability to harvest visible light. Time-resolved photoluminescence experiments indicate that the self-assembly between nanosized CdS and polyoxotungstate is very effective in increasing the lifetime of holes and electrons, thus indicating an efficient electron transfer between two-component semiconductors. The hybridization results not only in a significant improvement in the photostability of CdS QD but also in the creation of visible-light-induced photochromism. Of particular importance is that the present nanohybrids show visible-light-driven photocatalytic activity to produce H(2) and O(2) , which is superior to those of the unhybridized CdS and polyoxotungstate. The self-assembly of nanometer-level semiconductor clusters can provide a powerful way of optimizing the photoinduced functionalities of each component (i.e., visible-induced photochromism and photocatalysis) by means of strong electronic coupling.

The Importance of the DFT method on the computation of the second hyperpolarizability of semiconductor clusters of increasing size: A critical analysis on prolate aluminum phosphide clusters

AbstractThe importance of the density functional theory (DFT) methods on the computation of cluster hyperpolarizabilities is discussed. The performance of the conventional BLYP, BP86, BPW91, B3LYP, B3PW91, and B3P86 functionals in the computation of the second hyperpolarizability of aluminum phosphide prolate clusters up to 60 atoms is compared with the “half and half functionals” BHandH and BHandHLYP and to the long‐range corrected functionals LC‐(BLYP, BP86, BPW91), CAM‐B3LYP, and wB97XD. The presented results demonstrate that when long‐range corrections are incorporated on pure and hybrid functionals their performance is vastly affected. What is more, the obtained DFT results are compared with second‐order Møller–Plesset perturbation theory (MP2) all electron calculations. It is shown that all the long‐range outcomes are bracketed by the MP2 and Hatree–Fock (HF) values. The relative ordering of the obtained longitudinal hyperpolarizabilities follows strictly the trend MP2 > CAM‐B3LYP > wB97XD > LC‐(BLYP, BP86, BPW91) > HF. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

New Insights into Applicability of Electron-Counting Rules in Transition Metal Encapsulating Ge Cage Clusters

The relative stability of Sc, Ti, and V encapsulating Ge(n) clusters in the size range n = 14-20 has been studied through first-principles electronic structure calculations based on density functional theory. Variations of the embedding energy, gap between the highest occupied and the lowest occupied molecular orbitals, ionization potential, vertical detachment energy, and electron affinity with cluster size have been calculated to identify clusters with enhanced stability. The enhanced stability of some clusters can be very well explained as due to the formation of a filled shell free-electron gas inside the Ge cages. For the first time, direct evidence of the formation of a free-electron gas is also presented. In some other clusters, enhanced stability is found to originate from geometric effects. Some clusters that may be expected to have enhanced stability from simple electron counting rules do not show that. These results provide new insights into the long-standing question of whether electron counting rules can explain the relative stability of transition metal encapsulated semiconductor clusters and show that these clusters are too complex for such simple generalizations.

Stopping of energetic argon cluster ions in graphite: Role of cluster momentum and charge

We show that the implantation depth for argon clusters in graphite scales linearly with cluster momentum. A plot of implantation depth versus the momentum scaled with the projected surface area of the cluster falls on the same universal plot as that shown for semiconductor and metallic clusters, thus providing a universal scaling law for cluster implantation. Molecular dynamics simulations provide some insight to the mechanisms behind the empirical observation.

Giant cross section of resonant electromagnetic wave scattering by electron in metal and semiconductor clusters

A new formula is obtained for the cross section of light scattering by electron under conditions where the damping of its oscillations is related entirely to the spontaneous re-radiation of photon. The cross section for resonant light scattering can become macroscopic, which is only possible in a nanocluster where the phonon energy is greater than the photon energy.

A Chemical Bond Theory of Quantum Size Effects of Semiconductor Clusters

Size dependence effects in semiconductor clusters have been a subject of extensive studies for the last two decades. However, it is still difficult to employ the existing theoretical models to give reliable results of energies for clusters in the whole nanometer region. Here we offer a new theoretical method for the quantum size effects based on the idea that the energy gap shift of the cluster arises from the sum of the surface effect shift and quantum effect shift parts. We express the effects through algebraic relations rather than through variational solutions of the wave equation, without the use of any special adjustable parameter. Results reveal for the first time that the shape of the energy gap shift curve is dominated by the surface energy shift. Our method can also predict quantitatively the size dependence of dielectric constant. The new theoretical findings in the ultrasmall (<1 nm) anatase TiO(2) and the silicon clusters cannot be explained using previous theories.

Mn-rich clusters in GeMn magnetic semiconductors: Structural evolution and magnetic property

The structural evolution and magnetic property of Mn-rich clusters in GeMn thin films grown at 70 °C by molecular beam epitaxy has been systematically investigated by TEM, EDS and SQUID. It was discovered that, by controlling the Mn concentration, the morphology of coherent Mn-rich clusters can be manipulated from tadpole to nanocolumn shapes. For a particular Mn concentration, the critical thickness to form coherent Mn-rich clusters at 70 °C has been identified. Magnetic measurements show a complex magnetic behavior, which is mainly related to the Mn-rich clusters or precipitates. Our results also reveal that a flat surface of the Ge buffer layer plays an important role in producing vertically aligned tadpoles and nanocolumns.

On the shape dependence of cluster (hyper)polarizabilities. A combined ab initio and DFT study on large fullerene–like gallium arsenide semiconductor clusters

AbstractThe effect of the cluster shape on the magnitudes of the static dipole (hyper)polarizabilities of large gallium arsenic clusters built from 72 atoms is presented and discussed. Also, the performance of conventional and long range corrected density functionals is assessed on the prediction of the static electric dipole hyperpolarizabilities. The reported results obtained at the Hartree‐Fock, (LC‐) BLYP, (LC‐) BPW91, (CAM)‐B3LYP, and WB97XD levels of theory demonstrate that the cluster shape not only dominates the magnitude of the second hyperpolarizabilities of clusters but also affects dramatically the performance of the density functional theory functionals used. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

ChemInform Abstract: Semiconductor Clusters in the Sol-Gel Process: Quantized Aggregation, Gelation, and Crystal Growth in Concentrated ZnO Colloids.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A Study on the Cooperation between Korean-Japanese Semiconductor Clusters

A Study on the Cooperation between Korean-Japanese Semiconductor Clusters

Nature of vacancy defect clusters in chalcogenic semiconductors of AIIBVI group irradiated by neutrons

AbstractAn experimentally proven model is proposed stating that vacancies of cadmium atoms are dominating in kernels of the defect clusters (DCs) formed in single CdS crystals by the irradiation with fast 1 MeV reactor neutrons. The so‐called subthreshold effects play the essential role in cluster formation. The subthreshold effects consequently lead to the excitation and ionization of the K‐shell of Cd atoms followed by an increase of charge of Cd ions. This finally result in an ejection of these ions from the kernels to periphery of DCs due to the electrostatic force (electrostatic explosion). (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)