Session J31: Focus Session: Computational Nanoscience IV: Nanocrystals

Atomistic design of semiconductor nanostructures with optimal thermoelectric properties

The search for novel materials with optimal thermoelectric properties (for either thermoelectric power generation or heat dissipation) is an active field of research. We present atomistic and \textit{ab-initio} simulations of selected nanomaterials, aimed at predicting thermal conductivities and electronic transport properties, and ultimately at designing materials with optimal thermoelectric figure of merit. In particular we focus on carbon nanotubes [1], silicon wires [2] and nanoporous silicon [3] and we discuss both strategies and algorithms to optimize thermoelectric properties at the nanoscale. \newline [1] D. Donadio and G.Galli, Phys. Rev. Lett. 2007 (in press). \newline [2] T.Vo, A.Williamson, V.Lordi and G.Galli (submitted) and J.Reed, A.Williamson, E.Schwegler and G.Galli (submitted). \newline [3] J.-H. Lee, J.C.Grossman, J.Reed and G.Galli, Appl. Phys. Lett. 2007 (in press).   

Ab initio method for the electron-phonon scattering times in semiconducting nanostructures

The interaction of excited electrons with phonons plays a central role for electronic and transport properties at the nanoscale. It is the dominant process limiting the excitation lifetime at medium excitation energies. Despite its importance, a reliable approach within ab initio methods for phonon interaction with excited carriers was still lacking. We present in this work our fully ab initio approach to calculate the electron-phonon scattering times for collisions of carriers in the conduction band with short-wavelength phonons. We apply it to the deexcitation of hot electrons in GaAs [1,2], and to the lifetime of the direct exciton in GaP and GaAs [2,3], all in excellent agreement with experiments. Finally, we discuss the effect of nanostructuring on the electron-phonon coupling constants in GaAs/AlAs superlattices. \newline [1] J. Sjakste, N. Vast, V. Tyuterev, 2007, accepted in Phys. Rev. Lett. \newline [2] J. Sjakste, V. Tyuterev, N. Vast, Appl. Phys. A 86 (2007) 301. \newline [3] J. Sjakste, V. Tyuterev, N. Vast, Phys. Rev. B 74, 235216 (2006).   

Quantum-size-induced phase transitions in quantum dots: Indirect-band gap GaAs nanostructures

Quantum nanostructures are often advertised as having stronger absorption than the bulk material from which they are made, to the potential benefit of nanotechnology. However, nanostructures made of direct gap materials such as GaAs can convert to indirect-gap, weakly-aborbing systems when the quantum size becomes small. This is the case for spherical GaAs dots of radius 15 {\AA} or less (about 1000 atoms) embedded in a wide-gap matrix. The nature of the transition: $\Gamma$-to-X or $\Gamma$-to-L is however, controversial. The distinction can not be made on the basis of electronic structure techniques that misrepresent the magnitude of the various competing effective mass tensors (e.g, LDA or GGA) or wavefunction coupling (e.g, tight-binding). Using a carefully fit screened pseudopotential method we show that the transition occurs from $\Gamma$ to X, and, more importantly, that the transition involves a finite V ($\Gamma$-X) interband coupling, manifested as an ``anti-crossing'' between the confined electron states of GaAs as the dot size crosses 15 {\AA}. The physics of this reciprocal-space $\Gamma$-X transition, as well as the real-space (type II) transition in GaAs/AlGaAs will be briefly discussed.   

Co-doping of Boron and Phosphorus in Silicon Nanoclusters

The effect of cluster size on the interaction between impurity atoms is studied using the first-principles calculations, i.e. pseudopotentials in real space. We calculate the stable configurations of B and P co-doped silicon nanoclusters as a function of size. We evaluate the evolution of interactions between impurity atoms by comparing the stable configurations. The evolution of photoluminescence is discussed.   

Quantum Confinement and Non-Magnetic-Doped Dilute Magnetic Semiconductors

Dilute magnetic semiconductors are of interest for their unique magnetic properties and their promising role in development of ``spintronic'' semiconductor devices. Recently, a new dimension has been brought to this class of material by observing room temperature ferromagnetism in non-magnetic doped semiconductors and insulators. Using real-space pseudopotential applied to nitrogen-doped ZnO nanowires and nanocrystals, we report the theoretical evidence of magnetism in spatially confined non-magnetic doped semiconductor nanocrystals. Detailed electronic structures and magnetic properties are examined by comparing the total energy of different spin orderings and defect configurations. Besides the prediction of high Curie temperature, our results show that the ferromagnetic order becomes more stable when the nitrogen defects experience strong quantum confinement.   

Impurity Doping in PbSe Nanocrystals

We recently proposed that impurity doping in colloidally grown nanocrystals is controlled primarily by kinetics, rather than by thermodynamics.\footnote{S.C. Erwin, L. Zu, M.I. Haftel, Al.L. Efros, T.A. Kennedy, and D.J. Norris. Doping semiconductor nanocrystals. Nature 436, 91 (2005).} In this ``trapped dopant'' model, the diffusion of an impurity through a nanocrystal is negligible at colloidal growth temperatures. Therefore, an impurity can only be incorporated into a growing nanocrystal if it first adsorbs on the surface and is then overgrown. But this simple surface adsorption process is complicated by a competing process: the binding of the impurity by surfactant molecules, which are used in the growth solution to passivate the nanocrystal and control its growth. Here we use density-functional theory to study the interplay and outcome of these two processes for the doping of PbSe nanocrystals by a variety of candidate impurities (Mn, Cl, In, Cd, Tl) in the presence of several widely used growth surfactants (oleic acid, trioctylphosphine, hexadecylamine).   

Size Limits on Doping Phosphorous into Silicon Nanocrystals

The evolution of the semiconductor industry requires continued miniaturization. As this trend continues, devices will ultimately approach the nanometer-scale and it is expected that device construction based on macroscopic laws will start to fail. Using a real-space first-principles pseudopotential method, we study doping in the nano-regime using phosphorus-doped Si nanocrystals as the prototypical system. We simulate phosphorus-doped Si nanocrystals with diameter up to 6 nm and study the evolution of the defect state with the size of the nanocrystal. Our calculated size dependence of hyperfine splitting is in excellent agreement with experimental data. The effect of quantum confinement is also manifested in the higher binding energy of the dopant electron, we estimate that phosphorus in Si nanocrystals of less than 20 nm in diameter will not be a shallow donor. We also find that for Si nanocrystals smaller than 2 nm in diameter, the phosphorus atom will be energetically expelled to the surface, leading to a self-purification mechanism that hinders the incorporation of impurity atoms into nanocrystals.   

Interface chemistry of silicon nanocrystals embedded in silica

Molecular dynamics simulations of realistic-sized silicon nanocrystals (NCs) with the diameters in the range from 1~nm to 3~nm embedded in amorphous oxide are carried out till steady state conditions with the chemical environment described by the reactive force field model. We identify different types of three-coordinated oxygen (3cO) complexes, previously not noted, on the oxide interface. No double bonds were observed. We reveal that the interface bond topology evolves among different oxygen bridges through these 3cO complexes. The abundance and the charge distribution of each oxygen complex is determined as a function of the NC size as well as the transitions among them. The number of bridge bonds is observed to scale with surface area, thus the curvature has a small effect on the number of bridges. Among the three bonds of 3cO, the weaker bond is more susceptible to bond breaking which is also likely to take part in an optical activity through bond breaking and reformation. Our results indicate that the Si NC-oxide interface is more complicated than the previously proposed schemes which were based on solely double and bridge bonds.   

Structure and electronic properties of gold-tipped CdSe nanorods

We investigate CdSe nanorods capped by gold contacts and passivated by phospho-organic molecules of varying chain length. The geometry is optimized and the electronic structure obtained using first-principles quantum mechanical methods. We discuss the formation of Schottky barriers, the development of interfacial dipoles, the presence and extent of gap states induced by the metallic contact and of states in the semiconductor energy gap associated with the passivant carbon chains.   

F{\"{o}}rster resonant energy transfer between CdSe nanocrystals: An empirical pseudopotential/transition density cube approach

We study the energy transfer between semiconductor nanocrystal dots and rods of CdSe using a semiempirical pseudopotential method (SEPM) description of the electronic structure of the nanocrystals, followed by evaluation of the Coulombic contribution to the energy transfer evaluated using the transition density cube (TDC) method. Our results are compared to the dipole-dipole theory of F{\"{o}}rster to characterize the effects of nanocrystal shape, distance, and orientation. In agreement with previous effective-mass and tight-binding studies, we find that the coupling between spherical nanocrystals is well described by the F{\"{o}}rster model. In contrast, we find that rod-shaped nanocrystals display more complicated behavior, which may be relevant to exciton migration in all-inorganic nanorod-based photovoltaic devices.   

Polar properties of ZnO nanostructures.

The advent of nanostructured devices critically enhances the role of surface and interface effects on bulk properties and determines the physical characteristics of the material: in particular, the understanding of the electronic properties of nanosized structures requires a proper accurate treatment. Here we report on first principles density functional calculations of the structural and electronic properties of the (1-100) ``non-polar'' surface of hexagonal zinc oxide (ZnO) and compare the results with those of ZnO nanowires grown along the [0001] direction and analogous exposed cuts, with a diameter range of about 9-23 {\AA}. We discuss the changes in the nanostructures polarity in terms of two contributions, one related to changes in equilibrium lattice parameters at the nanoscale and the other related to surface effects. Variations of other relevant observables such as the piezoelectric response of the nanowire are addressed. We compare our results with those obtained for nanostructures based on materials with different polarity (e.g. SiC and InN).   

Softening of ultra-nanocrystalline diamond at small grain sizes

Ultra-nanocrystalline diamond is a polycrystalline material, having crystalline diamond grains of sizes in the nanometer regime. We study the structure and mechanical properties of this material as a function of the average grain size, employing atomistic simulations. Using the bulk and Young's moduli as probes of stiffness, we observe softening of the material as the size of its grains decreases, similar to the reverse Hall-Petch effect observed for nanocrystalline metals. This softening is attributed to the enhanced fraction of interfacial atoms. The calculated scaling of the cohesive energy and bulk modulus with respect to average grain size agrees very well with this picture. Our results suggest that softening at very small grain sizes might be a generic property of nanocrystalline materials.   

A Molecular Dynamics Study of the Melting and Nucleation of Iron Nanoparticles

The melting and nucleation of iron nanoparticles were investigated by molecular dynamics simulation using a Finnis-Sinclair potential. The nanoparticle of the bcc single-crystal was uniformly melted from the surface at a melting point during heating, whereas a nucleus was generated near one side of an undercooled liquid droplet and the solidification spread toward another side at a lower temperature during cooling. The melting point and nucleation temperature decreased with particle radius. Moreover, the solid-liquid interfacial energy was estimated to be 0.101 J/m$^{2}$ using a Gibbs-Thomson equation, which is of the same order as the experimental value based on Turnbull-Fusher's classical nucleation theory.