Softening of ultra-nanocrystalline diamond at low 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. From the calculated elastic constants and the estimated hardness, we observe softening of the material as the size of its grains decreases. We attribute the observed softening to the enhanced fraction of interfacial atoms as the average grain size becomes smaller. We provide a fitting formula for the scaling of the cohesive energy and bulk modulus with respect to the average grain size. We find that they both scale as quadratic polynomials of the inverse grain size. Our formulae yield correct values for bulk diamond in the limit of large grain sizes.Comment: 5 pages, 3 figures, to be published in Acta Materiali

Hydrogen dynamics and light-induced structural changes in hydrogenated amorphous silicon

We use accurate first principles methods to study the network dynamics of hydrogenated amorphous silicon, including the motion of hydrogen. In addition to studies of atomic dynamics in the electronic ground state, we also adopt a simple procedure to track the H dynamics in light-excited states. Consistent with recent experiments and computer simulations, we find that dihydride structures are formed for dynamics in the light-excited states, and we give explicit examples of pathways to these states. Our simulations appear to be consistent with aspects of the Staebler-Wronski effect, such as the light-induced creation of well separated dangling bonds.Comment: 9 pages, 8 figures, submitted to PR

Extreme events in two dimensional disordered nonlinear lattices

Spatiotemporal complexity is induced in a two dimensional nonlinear disordered lattice through the modulational instability of an initially weakly perturbed excitation. In the course of evolution we observe the formation of transient as well as persistent localized structures, some of which have extreme magnitude. We analyze the statistics of occurrence of these extreme collective events and find that the appearance of transient extreme events is more likely in the weakly nonlinear regime. We observe a transition in the extreme events recurrence time probability from exponential, in the nonlinearity dominated regime, to power law for the disordered one.Comment: 5 figures, 5 page

Magnetized Iron Atmospheres for Neutron Stars

Using a Hartree-Fock formalism, we estimate energy levels and photon cross sections for atomic iron in magnetic fields B ~ 10^13 G. Computing ionization equilibrium and normal mode opacities with these data, we construct LTE neutron star model atmospheres at 5.5 < Log(T_eff) < 6.5 and compute emergent spectra. We examine the dependence of the emergent spectra on T_eff and B. We also show the spectral variation with the angle between the magnetic field and the atmosphere normal and describe the significant limb darkening in the X-ray band. These results are compared with recent detailed computations of neutron star H model atmospheres in high fields and with low field Fe and H model atmospheres constructed from detailed opacities. The large spectral differences for different surface compositions may be discernible with present X-ray data; we also note improvements needed to allow comparison of Fe models with high quality spectra.Comment: 18 pages with 5 eps figures, accepted for publication in ApJ Replaced due to clerical error only: one more author, no new conten

Do the Defects Make It Work? Defect Engineering in Pi-Conjugated Polymers and Their Solar Cells: Preprint

The charged defect density in common pi-conjugated polymers such as poly(3-hexylthiophene), P3HT, is around 1018 cm-3. Despite, or perhaps because of, this huge defect density, bulk heterojunction solar cells made from these polymers and a C60 derivative such as PCBM exhibit some of the highest efficiencies (~5%) yet obtained in solid state organic photovoltaic cells. We discuss defects in molecular organic semiconductors and in pi-conjugated polymers. These defects can be grouped in two categories, covalent and noncovalent. Somewhat analogous to treating amorphous silicon with hydrogen, we introduce chemical methods to modify the density and charge of the covalent defects in P3HT by treating it with electrophiles such as dimethyl sulfate and nucleophiles such as sodium methoxide. The effects of these treatments on the electrical and photovoltaic properties and stability of organic PV cells is discussed in terms of the change in the number and chemical properties of the defects. Finally, we address the question of whether the efficiency of OPV cells requires the presence of these defects which function as adventitious p-type dopants. Their presence relieves the resistance limitations usually encountered in cleaner organic semiconductors and can create built-in electric fields at junctions

Do the Defects Make it Work? Defect Engineering in π - Conjugated Polymer Films and Their Solar Cells

The summaries of this report are: (1) charged defects produce 10{sup 15}-10{sup 17} cm{sup -3} free carriers; (2) treatment with nucleophiles decreases p{sub f} and {sigma} while treatment with electrophiles does not change p{sub f} but increases {sigma}; (3) both treatments increase {mu}{sub p}, L{sub ex} and stability against photo-degradation; (4) charged defects can improve OPV by increasing conductivity and creating interfacial electric fields but they hurt {mu}{sub p}, L{sub ex} and chemical stability; and (5) a better way--synthesize materials without covalent defects and dope with purposely added, bound dopants

Effect of a Coadsorbent on the Performance of Dye-Sensitized TiO2 Solar Cells: Shielding versus Band-Edge Movement

The objective of this research is to determine the operational characteristics key to efficient, low-cost, stable solar cells based on dye-sensitized mesoporous films (in collaboration with DOE's Office of Science Program). Toward this end, we have investigated the mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, improves the open-circuit photovoltage (VOC) and short-circuit photocurrent density (JSC). We find that adding chenodeoxycholate not only shifts the TiO2 conduction-band edge to negative potentials but also accelerates the rate of recombination. The net effect of these opposing phenomena is to produce a higher photovoltage. It is also found that chenodeoxycholate reduces the dye loading significantly but has only a modest effect on JSC. Implications of these results to developing more efficient cells are discussed

Oscillatory Instabilities of Standing Waves in One-Dimensional Nonlinear Lattices

In one-dimensional anharmonic lattices, we construct nonlinear standing waves (SWs) reducing to harmonic SWs at small amplitude. For SWs with spatial periodicity incommensurate with the lattice period, a transition by breaking of analyticity versus wave amplitude is observed. As a consequence of the discreteness, oscillatory linear instabilities, persisting for arbitrarily small amplitude in infinite lattices, appear for all wave numbers Q not equal to zero or \pi. Incommensurate analytic SWs with |Q|>\pi/2 may however appear as 'quasi-stable', as their instability growth rate is of higher order.Comment: 4 pages, 6 figures, to appear in Phys. Rev. Let

Photoionization of hydrogen in atmospheres of magnetic neutron stars

The strong magnetic fields (B ~ 10^{12} - 10^{13} G) characteristic of neutron stars make all the properties of an atom strongly dependent on the transverse component K_\perp of its generalized momentum. In particular, the photoionization process is modified substantially: (i) threshold energies are decreased as compared with those for an atom at rest, (ii) cross section values are changed significantly, and (iii) selection rules valid for atoms at rest are violated by the motion so that new photoionization channels become allowed. To calculate the photoionization cross sections, we, for the first time, employ exact numerical treatment of both initial and final atomic states. This enables us to take into account the quasi-bound (autoionizing) atomic states as well as coupling of different ionization channels. We extend the previous consideration, restricted to the so-called centered states corresponding to relatively small values of K_\perp, to arbitrary states of atomic motion. We fold the cross sections with the thermal distribution of atoms over K. For typical temperatures of neutron star atmospheres, the averaged cross sections differ substantially from those of atoms at rest. In particular, the photoionization edges are strongly broadened by the thermal motion of atoms; this "magnetic broadening" exceeds the usual Doppler broadening by orders of magnitude. The decentered states of the atoms give rise to the low-energy component of the photoionization cross section. This new component grows significantly with increasing temperature above 10^{5.5} K and decreasing density below 1 g/cm^3, i.e., for the conditions expected in atmospheres of middle-aged neutron stars.Comment: 19 pages including 8 figures, LaTeX (using aas2pp4.sty and epsf.sty). Accepted for publication in ApJ. PostScript available also at http://www.ioffe.rssi.ru/dtastrop.htm

Particle Path Correlations in a Phonon Bath

The path integral formalism is applied to derive the full partition function of a generalized Su-Schrieffer-Heeger Hamiltonian describing a particle motion in a bath of oscillators. The electronic correlations are computed versus temperature for some choices of oscillators energies. We study the perturbing effect of a time averaged particle path on the phonon subsystem deriving the relevant temperature dependent cumulant corrections to the harmonic partition function and free energy. The method has been applied to compute the total heat capacity up to room temeperature: a low temperature upturn in the heat capacity over temperature ratio points to a glassy like behavior ascribable to a time dependent electronic hopping with variable range in the linear chain.Comment: To be published in J.Phys.:Condensed Matte