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Optical measurements on ZnS nanoclusters have been carried out to investigate surface effects along with quantum size effects. ZnS nanocrystals have been synthesized in the range of 1.5–2.5 nm, using different chemical methods as well as electronic passivating procedures. The size of nanoparticles has been estimated from empirical pseudopotential calculations. We have obtained a significantly narrower size distribution of ZnS nanocrystals than reported in earlier published results. We observed band gap luminescence in mercaptoethanol capped ZnS nanocrystals. Effects of various defect levels on the luminescent behavior of ZnS nanoparticles have been examined

The band gap bowing effect in oleic acid-stabilized CdS x Se 1−x alloy quantum dots (Q-dots) with varying composition has been studied experimentally by means of cyclic voltammetry and theoretically using density functional theory based calculations. Distinct cathodic and anodic peaks observed in the cyclic voltammograms of diffusing quantum dots alloy are attributed to the respective conduction and valence band edges. The quasi-particle gap values determined from voltammetric measurements are compared with interband transition peaks in UV−vis and PL spectra. Electronic structure for alloy Q-dots is determined computationally with projector augmented wave method for a particular size of dots. The band gap bowing is observed predominantly in the conduction band states. The bowing parameter determined experimentally (0.45 eV) has been found to be in good agreement with the one estimated from DFT (0.43 eV).

Computational detailsElectronic structure calculations based on density functional theory (DFT) [1] are carried out using accurate plane augmented wave (PAW) method [2] as implemented in Vienna Ab initio Simulations Package (VASP) [3]. The exchange-correlation energy functional as given by Perdew, Burke and Ernzerhof (PBE) [4] is used, since it is known to provide descent estimates for electronic properties. The valence electronic configurations used for Zn and S are 4s 2 3d 10

We present results of a study of small stoichiometric CdnT en (1≤n≤6) clusters and few medium sized non-stoichiometric CdmT en [(m, n = 13, 16, 19); (m =n)] clusters using the Density Functional formalism and projector augmented wave method within the generalized gradient approximation. Structural properties viz. geometry, bond length, symmetry and electronic properties like HOMO-LUMO gap, binding energy, ionization potential and nature of bonding etc. have been analyzed. Medium sized non-stoichiometric clusters were considered as fragments of the bulk with T d symmetry. It was observed that upon relaxation, the symmetry changes for the Cd-rich clusters whereas the Te-rich clusters retain their symmetry. The Cd-rich clusters develop a HOMO-LUMO gap due to relaxation whereas there is no change in the HOMO-LUMO gap of the Te-rich clusters. Thus, the symmetry of a cluster seems to be an important factor in determining the HOMO-LUMO gap.

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The authors present theoretical results describing the adsorption of H2 and H2S molecules on small neutral and cationic gold clusters (Aun(0∕+), n=1–8) using density functional theory with the generalized gradient approximation. Lowest energy structures of the gold clusters along with their isomers are considered in the optimization process for molecular adsorption. The adsorption energies of H2S molecule on the cationic clusters are generally greater than those on the corresponding neutral clusters. These are also greater than the H2 adsorption energies on the corresponding cationic and neutral clusters. The adsorption energies for cationic clusters decrease with increasing cluster size. This fact is reflected in the elongations of the Au–S and Au–H bonds indicating weak adsorption as the cluster grows. In most cases, the geometry of the lowest energy gold cluster remains planar even after the adsorption. In addition, the adsorbed molecule gets adjusted such that its center of mass lies on the plane of the gold cluster. Study of the orbital charge density of the gold adsorbed H2S molecule reveals that conduction is possible through molecular orbitals other than the lowest unoccupied molecular orbital level. The dissociation of the cationic AunSH2+ cluster into AunS+ and H2 is preferred over the dissociation into AumSH2+ and Aun−m, where n=2–8 and m=1−(n−1). H2S adsorbed clusters with odd number of gold atoms are more stable than neighboring even n clusters.

Two-dimensional ultrasound arrays hold great promise for 3-D imaging; however, wiring of each channel becomes impractical for large arrays or for small-footprint catheter probes for which the number of wires must be limited. Capacitive micromachined ultrasound transducers offer a promising solution for such 2-D array applications, but channel routing is still non-trivial. A top-orthogonal-to-bottom-electrode (TOBE) 2-D CMUT array architecture is presented along with row-column addressing schemes for low-channel-count 3-D ultrasound imaging. An N × N TOBE array is capable of obtaining 3-D images using only 2N channels. An interfacing scheme is presented in which transmit-receive signals are routed along rows while bias voltages are applied along columns, effectively allowing for single-element transmit/receive control. Simulations demonstrated potentially finer resolution and improved side lobe suppression over a previously published row-column-based imaging method. Laser vibrometer testing was done to measure membrane displacement in air and confirmed that single-element air-coupled actuation in transmit mode could be achieved using our proposed interfacing scheme. Acoustic testing was also performed in both transmit and receive modes to characterize the ability of the proposed interfacing scheme to achieve dominant-element transmission and reception in immersion operation. It was seen that membrane displacement in both modes was indeed largely confined to the active area.