Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

▪ Abstract Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 A, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 A, exchange interactions cause otherwise insulating ass...

Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies

Colloidal-quantum-dot (CQD) optoelectronics offer a compelling combination of solution processing and spectral tunability through quantum size effects. So far, CQD solar cells have relied on the use of organic ligands to passivate the surface of the semiconductor nanoparticles. Although inorganic metal chalcogenide ligands have led to record electronic transport parameters in CQD films, no photovoltaic device has been reported based on such compounds. Here we establish an atomic ligand strategy that makes use of monovalent halide anions to enhance electronic transport and successfully passivate surface defects in PbS CQD films. Both time-resolved infrared spectroscopy and transient device characterization indicate that the scheme leads to a shallower trap state distribution than the best organic ligands. Solar cells fabricated following this strategy show up to 6% solar AM1.5G power-conversion efficiency. The CQD films are deposited at room temperature and under ambient atmosphere, rendering the process amenable to low-cost, roll-by-roll fabrication.

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Colloidal-quantum-dot photovoltaics using atomic-ligand passivation

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Electronic Transport In Mesoscopic Systems

The emerging field of bionanotechnology aims at revolutionizing biomedical research and clinical practice via introduction of nanoparticle-based tools, expanding capabilities of existing investigative, diagnostic, and therapeutic techniques as well as creating novel instruments and approaches for addressing challenges faced by medicine. Quantum dots (QDs), semiconductor nanoparticles with unique photo-physical properties, have become one of the dominant classes of imaging probes as well as universal platforms for engineering of multifunctional nanodevices. Possessing versatile surface chemistry and superior optical features, QDs have found initial use in a variety of in vitro and in vivo applications. However, careful engineering of QD probes guided by application-specific design criteria is becoming increasingly important for successful transition of this technology from proof-of-concept studies towards real-life clinical applications. This review outlines the major design principles and criteria, from general ones to application-specific, governing the engineering of novel QD probes satisfying the increasing demands and requirements of nanomedicine and discusses the future directions of QD-focused bionanotechnology research (critical review, 201 references).

Designing multifunctional quantum dots for bioimaging, detection, and drug delivery

Confined many-electron systems

The size dependence of the electronic spectrum of InAs nanocrystals ranging in radius from 10–35 A has been studied by size-selective spectroscopy An eight-band effective mass theory of the quantum size levels has been developed which describes the observed absorption level structure and transition intensities very well down to smallest crystal size using bulk band parameters This model generalizes the six-band model which works well in CdSe nanocrystals and should adequately describe most direct semiconductor nanocrystals with band edge at the Γ-point of the Brillouin zone

Size-dependent electronic level structure of InAs nanocrystal quantum dots: Test of multiband effective mass theory

We present an atomistic tight-binding study of the electronic structure and optical properties of vertically stacked, double, self-assembled, $\mathrm{InAs}∕\mathrm{GaAs}$ quantum dots. The investigated dots are lens-shaped and are situated on wetting layers. We study coupling and strain effects for closely spaced dots. For intermediate separation distances between the dots, the tight-binding theory confirms the effect of strain-induced localization of the ground hole state in the lower dot, as predicted in other approaches. However, the tight-binding calculations predict weaker localization at large separation distances and no localization for closely spaced and overlapping dots, which have not been investigated so far. An anomalous reversal of the bonding character of the ground hole state for large separation distances, found previously by us for unstrained systems, is present also for strained dots. We also show that in double quantum dots there may exist bound and localized electron and hole states with energies above the edge of the wetting layer continuum. The calculated redshift of the lowest optical transition for decreasing distance between the interacting dots agrees qualitatively with experimental data.

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Strain effects on the electronic structure of strongly coupled self-assembled InAs/GaAs quantum dots : Tight-binding approach

Surface effects significantly influence the functionality of semiconductor nanocrystals. A theoretical understanding of these effects requires an atomic-scale description of the surface. We present an atomistic tight-binding theory of the electronic and optical properties of passivated and unpassivated CdS nanocrystals and CdS/ZnS core/shell nanocrystals. Fully passivated dots, with all dangling bonds saturated, have no surface states in the fundamental band gap, and all near-band-edge states are quantum-confined internal states. When surface anion dangling bonds are unpassivated, an anion-derived, narrow (bandwidth 0.05 eV), surface-state band lies 0.5 eV above the valence band edge, and a broader (0.2 eV) band of back-bonded surface states exists in the gap just above the valence band edge. When surface cation dangling bonds are unpassivated, a broad band of mixed surface/internal states exists above the conduction band edge. Partial passivation can push internal levels above the internal levels of a fu...

Surface effects on capped and uncapped nanocrystals.

Near-infrared spectra of self-assembled quantum rings are calculated using the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ method with rectangular band-offset potentials in three dimensions. The effective-mass model with energy-dependent mass is employed for the electron states and the four-band Hamiltonian for the hole states. Two different ways to include the magnetic-field interaction term in the four-band Hamiltonian are compared. Our calculations describe well, qualitatively, the absorption spectrum measured by Petterson et al. [H. Petterson, R. J. Warburton, A. Lorke, K. Karrai, J. P. Kotthaus, J. M. Garcia, and P. M. Petroff, Physica E 6, 510 (2000)]. and the photoluminescence magnetoresonances recently achieved by Haft et al. [D. Haft, C. Schulhauser, A. O. Govorov, R. J. Warburton, K. Karrai, J. M. Garcia, W. Schoenfeld, and P. M. Petroff, Physica E 13, 165 (2002)]. The possibility of finding the ``optical'' Aharanov-Bohm effect in the state-of-the-art self-assembled InGaAs quantum rings is also discussed.

Wlodzimierz Jaskolski

Papers

Confined many-electron systems

Size-dependent electronic level structure of InAs nanocrystal quantum dots: Test of multiband effective mass theory

Strain effects on the electronic structure of strongly coupled self-assembled InAs/GaAs quantum dots : Tight-binding approach

Surface effects on capped and uncapped nanocrystals.

Magneto-optical transitions in nanoscopic rings