Michael I. Haftel

Doping semiconductor nanocrystals

Doping—the intentional introduction of impurities into a material—is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7–9), but not into CdSe (ref. 12)—despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to ‘self-purification’, an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption—and therefore doping efficiency—is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals—for applications from solar cells to spintronics—can be anticipated.

Nuclear saturation and the smoothness of nucleon-nucleon potentials

Abstract The two-nucleon and nuclear matter problems are solved by matrix inversion in momentum space. Direct matrix inversion of the Lippman-Schwinger and Brueckner equations is shown to be useful for general nuclear potentials including ones that are local, nonlocal, weak, strong, central, or noncentral. This flexibility is employed to study the relationship between nuclear saturation and the smoothness of the two-nucleon interaction. Five potentials are considered that give approximately equivalent phase shifts but differ in their smoothness. Two examples of smooth potentials with very weak nonlocal tensor terms are given. The potentials are classified according to their smoothness by calculating the wave function defect and the wound integral for each case. The binding energy of nuclear matter is calculated for each potential using the effective mass and angle-averaged Pauli operator approximations. A self-consistent hole spectrum and a free particle spectrum are used. A systematic dependence of saturation on the smoothness of the two-nucleon interaction is found. Only strong potentials with strong tensor terms yield correct saturation, whereas very smooth potentials produce overbinding and large equilibrium densities.

Compact nanomechanical plasmonic phase modulators

Researchers exploit the strong dependence of gap-plasmon phase velocity on gap width to make a compact phase-modulator. An electromechanically variable gap size enables a 23-μm-long non-resonant modulator with moderate losses.

Tetragonal Phase Transformation in Gold Nanowires

First principle, tight binding, and semi-empirical embedded atom calculations are used to investigate a tetragonal phase transformation in gold nanowires. As wire diameter is decreased, tight binding and modified embedded atom simulations predict a surface-stress-induced phase transformation from a face-centered-cubic (fcc) nanowire into a body-centered-tetragonal (bet) nanowire. In bulk gold, all theoretical approaches predict a local energy minimum at the bet phase, but tight binding and first principle calculations predict elastic instability of the bulk bct phase. The predicted existence of the stable bet phase in the nanowires is thus attributed to constraint from surface stresses. The results demonstrate that surface stresses are theoretically capable of inducing phase transformation and subsequent phase stability in nanometer scale metallic wires under appropriate conditions.

Dual resonance mechanisms facilitating enhanced optical transmission in coaxial waveguide arrays

We experimentally and computationally demonstrate high transmission through arrays of coaxial apertures with different geometries and arrangements in silver films. By studying both periodic and random arrangements of apertures, we were able to isolate transmission enhancement phenomena owing to surface plasmon effects from those owing to the excitation of cylindrical surface plasmons within the apertures themselves.

Extraordinary optical transmission with coaxial apertures

This work was partially supported by the Office of Naval Research. Computations were carried out under the Department of Defense High Performance Computation Modernization Project. The support of the Australian Research Council through its Centers of Excellence, Federation Fellow and Discovery programs is gratefully acknowledged.

Goal-orientated computational steering

Computational steering is a newly evolving paradigm for working with simulation models. It entails integration of model execution, observation and input data manipulation carried out concurrently in pursuit of rapid insight and goal achievement. Keys to effective computational steering include advanced visualization, high performance processing and intuitive user control. The Naval Research Laboratory (NRL) has been integrating facilities in its Virtual Reality Lab and High Performance Computing Center for application of computational steering to study effects of electromagnetic wave interactions using the HASP (High Accuracy Scattering and Propagation) modeling technique developed at NRL. We are also investigating automated inverse steering which involves incorporation of global optimization techniques to assist the user with tuning of parameter values to produce desired behaviors in complex models.

Off-shell effects in 16O

Abstract We calculate the binding energy of 16 O for a set of phase-shift-equivalent potentials previously studied in nuclear matter. Off-shell variations of up to 2.8 MeV per particle occur compared with about a 10 MeV per particle variation in nuclear matter. As in nuclear matter calculations a nearly linear relation exists between the variations in the binding-energy results and the wound integral k . We compare the 16 O results with a nuclear matter calculation at the “equivalent” nuclear matter density of k F = 1.13 fm −1 . This “equivalent” density reflects the fact that 16 O has a surface and hence a lower average density than nuclear matter. The 16 O and nuclear matter off-shell variations are comparable once one takes into account the lower average density of 16 O and the suppression of the relative D-wave interaction — also a surface effect. We present a method of computing the correlated wave functions of finite nuclear systems and display such wave functions for 16 O. The correlated wave functions of 16 O and of nuclear matter are strikingly similar for all of the potentials studied.

New ballistically and thermally activated exchange processes in the vapor deposition of Au on Ag(111) : a molecular dynamics study

Abstract Diffusion processes of Au on Ag(111) are examined by molecular dynamics calculations using surface-embedded-atom potentials. We find that complex forms of exchange diffusion dominate the early deposition. The usual direct AuAg exchange is unlikely, but more complicated exchange mechanisms involving multiple substrate atoms and catalyzed by nearby adatoms or clusters have very low activation barriers. These highly collective exchange processes can be activated ballistically by an incoming adatom or thermally on surfaces well below room temperature.

The 2H(p, 2p)n reaction at Einc = 45 MeV and its sensitivity to various separable S-wave N-N potentials

Abstract This paper presents and analyses the data for the 2 H(p, pp)n reaction at E inc = 44.9 MeV. Kinematic conditions including the quasi-free scattering region and the regions far from quasi-two-body processes are considered. The experimental results are compared with the predictions of several separable S-wave models of the N-N interaction. Potential models that differ only off-shell, as well as models that predict different N-N scattering results are included to help to isolate aspects of the break-up reaction sensitive to off-shell and on-shell differences. The regions far from quasi-free scattering are generally much more sensitive to the off-shell features of the interaction than are quasi-free scattering results. On-shell differences in the potential affect predictions in all regions of phase space with the potentials best representing the free N-N data giving the best overall fit to the data over most regions of phase space.