Although the stoichiometry of bulk lead sulfide (PbS) is exactly 1:1, that of quantum dots (QDs) can be considerably different from this crystalline limit. Employing first-principles calculations, we show that the impact of PbS QD stoichiometry on the electronic structure can be enormous, suggesting that control over the overall stoichiometry in the QD will play a critical role for improving the efficiency of optoelectronic devices made with PbS QDs. In particular, for bare PbS QDs, we find that: (i) stoichiometric PbS QDs are free from midgap states even without ligand passivation and independent of shape, (ii) off stoichiometry in PbS QDs introduces new states in the gap that are highly localized on certain surface atoms, and (iii) further deviations in stoichiometry lead to QDs with "metallic" behavior, with a dense number of energy states near the Fermi level. We further demonstrate that this framework holds for the case of passivated QDs by considering the attachment of ligand molecules as stoichiometry variations. Our calculations show that an optimal number of ligands makes the QD stoichiometric and heals unfavorable electronic structure, whereas too few or too many ligands cause effective off stoichiometry, resulting in QDs with defect states in the gap.

Room-temperature thermoelectric properties of n-type crystalline Si with periodically arranged nanometer-sized pores are computed using a combination of classical molecular dynamics for lattice thermal conductivity and ab initio density functional theory for electrical conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The electrical conductivity is found to decrease by a factor of 2-4, depending on doping levels, compared to that of bulk due to confinement. The Seebeck coefficient S yields a 2-fold increase for carrier concentrations less than 2 x 10(19) cm(-3), above which S remains closer to the bulk value. Combining these results with our calculations of lattice thermal conductivity, we predict the figure of merit ZT to increase by 2 orders of magnitude over that of bulk. This enhancement is due to the combination of the nanometer size of pores which greatly reduces the thermal conductivity and the ordered arrangement of pores which allows for only a moderate reduction in the power factor. We find that while alignment of pores is necessary to preserve power factor values comparable to those of bulk Si, a symmetric arrangement is not required. These findings indicate that nanoporous semiconductors with aligned pores may be highly attractive materials for thermoelectric applications.

We present molecular and lattice dynamics calculations of the thermal conductivity of nanoporous silicon, and we show that it may attain values 10-20 times smaller than in bulk Si for porosities and surface-to-volume ratios similar to those obtained in recently fabricated nanomeshes. Further reduction of almost an order of magnitude is obtained in thin films with thickness of 20 nm, in agreement with experiment. We show that the presence of pores has two main effects on heat carriers: appearance of non-propagating, diffusive modes and reduction of the group velocity of propagating modes. The former effect is enhanced by the presence of disorder at the pore surfaces, while the latter is enhanced by decreasing film thickness.

We investigate the effects of two-dimensional (2D) periodic patterns of functional groups on the thermal transport in a graphene monolayer by employing molecular and lattice dynamics simulations. Our calculations show that the use of patterned 2D shapes on graphene reduces the room temperature thermal conductivity, by as much as 40 times lower than that of the pristine monolayer, due to a combination of boundary and clamping effects. Lattice dynamics calculations elucidate the correlation between this large reduction in thermal conductivity and two dynamical properties of the main heat carrying phonon modes: (1) decreased phonon lifetimes by an order of magnitude due to scattering, and (2) direction-dependent group velocities arising from phonon confinement. Taken together, these results suggest that patterned graphene nanoroads provide a method for tuning the thermal conductivity of graphene without the introduction of defects in the lattice, opening an important possibility for thermoelectric applications.

A layer-by-layer deposition of two conducting polymers, each layer of which is a few tenths of nanometer thick, has been successfully performed to enhance the thermoelectric power factor of organic thin films.

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Robust detection of small targets is very important in IRST (Infrared Search and Track). This paper presents a novel mathematical method for the incoming target detection problem in cluttered background motivated from the robust properties of human visual system (HVS). The HVS shows the best efficiency and robustness for an object detection task. The robust properties of the HVS are contrast mechanism, multi-resolution representation, size adaptation, and pop-out phenomena. Based on these facts, a plausible computational model integrating these facts is proposed using Laplacian scale-space theory and Tune-Max based optimization method. Simultaneous target signal enhancement and background clutter suppression is achieved by tuning and maximizing the signal-to-clutter ratio (TMSCR) in Laplacian scale-space. At the first stage, the Tune-Max of the signal to background contrast produces candidate targets with adapted scale. At the second stage, the Tune-Max of the signal-to-clutter ratio (SCR) produces maximal SCR which is used to popout detections. Experimental evaluation results for the incoming target sequence validate the upgraded detection capability of the proposed method compared with the Top-hat method at the same false alarm rate. Experimental results for the six kinds of cluttered background images show that the proposed TMSCR produces less false alarms (4.3 times reduction) compared to the Top-hat at the same detection rate.

We investigate the effect of O impurities on the thermoelectric properties of ZnSe from a combination of first-principles and analytic calculations. It is demonstrated that dilute amounts of O impurities introduce peaks in the density of states (DOS) above the conduction band minimum, and that the charge density near the DOS peaks is substantially attracted toward O atoms due to their high electronegativity. The impurity-induced peaks in the DOS result in a sharp increase of the room-temperature Seebeck coefficient and power factor from those of O-free ZnSe by a factor of 30 and 180, respectively. Furthermore, this effect is found to be absent when the impurity electronegativity well matches the host that it substitutes. The results suggest that highly electronegativity-mismatched alloys can be designed for high performance thermoelectric applications.