Cation/Anion Substitution in Cu2ZnSnS4 for Improved Photovoltaic Performance.

Cations and anions are replaced with Fe, Mn, and Se in CZTS in order to control the formations of the secondary phase, the band gap, and the micro structure of Cu2ZnSnS4. We demonstrate a simplified synthesis strategy for a range of quaternary chalcogenide nanoparticles such as Cu2ZnSnS4 (CZTS), Cu2FeSnS4 (CFTS), Cu2MnSnS4 (CMTS), Cu2ZnSnSe4 (CZTSe), and Cu2ZnSn(S0.5Se0.5)4 (CZTSSe) by thermolysis of metal chloride precursors using long chain amine molecules. It is observed that the crystal structure, band gap and micro structure of the CZTS thin films are affected by the substitution of anion/cations. Moreover, secondary phases are not observed and grain sizes are enhanced significantly with selenium doping (grain size ~1 µm). The earth-abundant Cu2MSnS4/Se4 (M = Zn, Mn and Fe) nanoparticles have band gaps in the range of 1.04-1.51 eV with high optical-absorption coefficients (~104 cm-1) in the visible region. The power conversion efficiency of a CZTS solar cell is enhanced significantly, from 0.4% to 7.4% with selenium doping, within an active area of 1.1 ± 0.1 cm2. The observed changes in the device performance parameters might be ascribed to the variation of optical band gap and microstructure of the thin films. The performance of the device is at par with sputtered fabricated films, at similar scales.

Energetics and kinetics of li intercalation in irradiated graphene scaffolds.

In the present study, we investigate the irradiation-defects hybridized graphene scaffold as one potential building material for the anode of Li-ion batteries. Designating the Wigner V2(2) defect as a representative, we illustrate the interplay of Li atoms with the irradiation defects in graphene scaffolds. We examine the adsorption energetics and diffusion kinetics of Li in the vicinity of a Wigner V2(2) defect using density functional theory calculations. The equilibrium Li adsorption sites at the defect are identified and shown to be energetically preferable to the adsorption sites on pristine (bilayer) graphene. Meanwhile, the minimum energy paths and corresponding energy barriers for Li migration at the defect are determined and computed. We find that, while the defect is shown to exhibit certain trapping effects on Li motions on the graphene surface, it appears to facilitate the interlayer Li diffusion and enhance the charge capacity within its vicinity, because of the reduced interlayer spacing and characteristic symmetry associated with the defect. Our results provide critical assessment for the application of irradiated graphene scaffolds in Li-ion batteries.

Enhanced quantum confinement due to nonuniform composition in alloy quantum dots.

Strain and nanoscale variations in composition can significantly alter the electronic and optical properties of self-assembled alloy quantum systems. Using a combination of finite element and first-principles methods, we have developed an efficient and accurate technique to study the influence of strain and composition on the quantum confinement behavior in alloy quantum dots. Interestingly, we find that a nonuniform distribution of alloy components can lead to an enhanced confinement potential that allows a large quantum dot to behave electronically in a manner similar to a much smaller dot. The approach presented here provides a general means to quantitatively predict the influence of strain and composition variations on the performance characteristics of various small-scale alloy systems.

Substrate-induced magnetism in epitaxial graphene buffer layers.

Magnetism in graphene is of fundamental as well as technological interest, with potential applications in molecular magnets and spintronic devices. While defects and/or adsorbates in freestanding graphene nanoribbons and graphene sheets have been shown to cause itinerant magnetism, controlling the density and distribution of defects and adsorbates is in general difficult. We show from first principles calculations that graphene buffer layers on SiC(0001) can also show intrinsic magnetism. The formation of graphene-substrate chemical bonds disrupts the graphene pi-bonds and causes localization of graphene states near the Fermi level. Exchange interactions between these states lead to itinerant magnetism in the graphene buffer layer. We demonstrate the occurrence of magnetism in graphene buffer layers on both bulk-terminated as well as more realistic adatom-terminated SiC(0001) surfaces. Our calculations show that adatom density has a profound effect on the spin distribution in the graphene buffer layer, thereby providing a means of engineering magnetism in epitaxial graphene.

Stress-enhanced pattern formation on surfaces during low energy ion bombardment.

Ion-induced surface patterns (sputter ripples) are observed to grow more rapidly than predicted by current models, suggesting that additional sources of roughening may be involved. Using a linear stability analysis, we consider the contribution of ion-induced stress in the near surface region to the formation rate of ripples. This leads to a simple model that combines the effects of stress-induced roughening with the curvature-dependent erosion model of Bradley and Harper. The enhanced growth rate observed on Cu surfaces appears to be consistent with the magnitude of stress measured from wafer curvature measurements.

Spontaneous formation and growth of a new polytype on SiC(0001).

Using in situ electron microscopy, we have measured the structure of SiC(0001)-4H during annealing in vacuum. Above 1000 degrees C, an additional SiC bilayer forms on the surface that changes the polytype from hexagonal (4H) to cubic (3C). The interaction with surface steps prevents the cubic layer from growing thicker: the new phase does not wet the steps of the underlying 4H substrate. Instead, the cubic layer expands laterally, accelerating step bunching in the surrounding hexagonal regions. During SiC homoepitaxy, this lack of step edge wetting leads to the growth of 3C twins separated by deep grooves.

Composition maps in self-assembled alloy quantum dots.

Nanoscale variations in composition arising from the competition between chemical mixing effects and elastic relaxation can substantially influence the electronic and optical properties of self-assembled alloy quantum dots. Using a combination of finite element and quadratic programming optimization methods, we have developed an efficient technique to compute the equilibrium composition profiles in strained quantum dots. We find that the composition profiles depend strongly on the morphological features such as the slopes and curvatures of their surfaces and the presence of corners and edges as well as the ratio of the strain and chemical mixing energy densities. More generally, our approach provides a means to quantitatively model the interplay among the composition variations, the temperature, the strain, and the shapes of small-scale lattice-mismatched structures.

Metastability in 2D self-assembling systems.

We show that 2D self-assembled domains can remain trapped in a large variety of long-lived and metastable shapes that arise from an interplay of crystalline anisotropy and relaxation of elastic strain. On commonly used cubic (111) substrates, these shapes include extended or stacked structures made up of triangular domains connected at their corners, compact shapes with both convex and concave curvatures and others with narrow and elongated arms. We show that all of these distinct experimentally observed shapes can be explained within a unified framework and present a phase diagram that systematically classifies the metastable shapes as a function of their size.