Session V7: Focus Session: Computational Design of Materials - Engineering of Electronic Structure Materials

Band-edge engineering of Silicon by Surface Functionalization: a Combined Ab-initio and Photoemission Study

The electrode material choice is limited in solar to fuel formation devices because of the requirement of band-edge matching to the fixed fuel formation potential. This limitation can be relieved via band-edge engineering. The changes of band-edge positions of Si electrodes induced by the adsorption of H-, Cl-, Br- and short-chain alkyl groups were investigated by combining density functional (DFT), many-body perturbation theory (MBPT), and ultraviolet photoelectron spectroscopy. The band edge shifts are related to the formation of surface dipole moments, and determine the barrier height of electrons and holes in doped silicon surfaces. We find that the trends of the sign and magnitude of the computed surface dipoles as a function of the adsorbate may be explained by simple electronegative rules. We show that quasi-particle energies obtained within MBPT are in good agreement with experiment, while DFT values may exhibit substantial errors. However computed band edge differences are in good agreement with spectroscopic and electrical measurements even at the DFT level of theory. [1] Y. Li and G. Galli, Phys. Rev. B 82, 045321 (2010). [2] Y. Li, L. O'Leary, N. Lewis and G. Galli, to be submitted.   

Genetic engineering of band-egde optical absorption in Si/Ge superlattices

Integrating optoelectronic functionalities directly into the mature Silicon-Germanium technology base would prove invaluable for many applications. Unfortunately, both Si and Ge display indirect band-gaps unsuitable for optical applications. It was previously shown (Zachai \textit{et al.} PRL \textbf{64} (1990)) that epitaxially grown [(Si)$_n$(Ge)$_m$]$_p$ (i.~e.~ a single repeat unit) grown on Si can form direc-gap heterostructures with weak optical transitions as a result of zone folding and quantum confinement. The much richer space of \emph{multiple-period} superlattices [(Si)$_{n_1}$(Ge)$_{n_2}$(Si)$_{n_3}$(Ge)$_{n_4}$\ldots$Ge_{n_N}$]$_p$ has not been considered. If $M=\sum n_i$ is the total number of monolayers, then there are, roughly, $2^M$ different possible superlattices. To explore this large space, we combine a (i) genetic algorithm for effective configurational search with (ii) empirical pseudopotential designed to accurately reproduce the inter-valley and spin-orbit splittings, as well as hydrostatic and biaxial strains. We will present multiple-period SiGe superlattices with large electric dipole moments and direct gaps at $\Gamma$ yielded by this search. We show this pattern is robust against known difficulties during experimental synthesis.   

New mechanism for work-function tuning: ZnO surfaces modified by a strong organic electron acceptor

A key task for optimizing optoelectronic devices comprising hybrid inorganic/organic systems is to control the energy level alignment at interfaces. The use of interlayers provides a pathway to solve this challenge. To demonstrate the concept we investigated the polar surfaces of ZnO, modified by the prototypical organic electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) theoretically and experimentally. DFT-PBE $\Delta$SCF calculations of the O 1s surface core-level shifts combined with XPS measurements suggest that ZnO(000-1)/ZnO(0001) are H/OH covered. Depositing F4TCNQ on these surfaces considerably increases the work function that is shown to be insensitive to the doping level in PBE+vdW. F4TCNQ on ZnO(0001) exhibits an extraordinary high work function due to the appearance of upright adsorption. The PBE+vdW results are in line with the UPS data that shows work-function increases up to 1.4/2.8 eV on ZnO(000-1)/ZnO(0001). In contrast to F4TCNQ on metals, where pronounced bidirectional charge transfer occurs, the charge transfer from ZnO to F4TCNQ is small, pinning the LUMO close to the Fermi level. The polarization of the system, caused by strong charge rearrangement within the adsorbate, is the main mechanism for the large work-function increase.   

Quantum Monte Carlo Characterization of Excited States and Energy-Level Alignment at Oligomer/Quantum-Dot Interfaces

Charge separation of excitons in materials is one of the most important physical processes that need to take place in excitonic solar cells and in photocatalytic devices. Heterogeneous interfaces with the so-called type-II character are often employed for inducing the exciton dissociation through interfacial charge transfer. As the simplest criterion for designing such an interface, the energy alignment of the quasi-particle states is often discussed in literature, together with the exciton binding energy of electron-donating materials. Therefore, accurate characterization of the interfacial energy-level alignment and the exciton binding energy using first principles calculations is important for making systematic progresses in designing better materials for solar energy conversion. However, Density Functional Theory calculations need to be employed with caution in this context. First principles calculations such as Many-Body Perturbation Theory and Quantum Monte Carlo are promising alternatives for accurate characterization, but much more work is needed in this area to assess how well these methods perform in practice. In this talk, we will discuss our preliminary results using diffusion Quantum Monte Carlo on calculating the excited states and energy-level alignment of popular Oligomer/Quantum-Dot interfaces.   

Shape managing the cross-section of a semiconductor nanowire as a method for fine tuning electro-optical properties

Geometry design in nanowire and nanopillar arrays is used to create photon managing structures in a number of optical applications ranging from photovoltaics to field emission devices. The diameter dependence of the intrinsic properties of the material at the nanoscale provides further tunability and significantly improved performance. Using ab-initio methods (TDDFT), we explore the additional possibility of tuning and enhancing the optical properties by fine structuring the cross-sectional geometry.   

Dimensional Reduction: A design tool for new semiconductor compounds for radiation detection

To address the need for new wide gap semiconductors for efficient radiation detection, we present a new design tool called \textit{dimensional reduction }(DR). The method is based on reducing the dimensionality of highly dense but low bandgap ($<$1 eV) compounds to create comparably dense wide bandgap ($>$1.6 eV) semiconductors without compromises on their mass density. We utilize electronic band structure calculations of bandgap and effective mass to aid in the selection among the wide variety of compounds that can be formed by DR. As a proof of the concept, we report on computational design as well as optical characterization of three such dimensionally reduced materials based on \textit{$\beta $}-HgS, HgSe, and CdTe compounds. These compounds, namely, Cs$_{2}$Hg$_{6}$S$_{7}$, Cs$_{2}$Hg$_{3}$Se$_{4}$, and Cs$_{2}$Cd$_{3}$Te$_{4}$, show great promise as detector materials. Band structure calculations show that they have direct bandgaps of 1.28, 1.97, and 2.35 eV, respectively, in good agreement with experimental values of 1.63, 2.2 and 2.5 eV, respectively. Measured electrical resistivity values of $\sim $10$^{6}$, 10$^{7}$, and 10$^{9} \quad \Omega $-cm, respectively, are high enough for further evaluation of these materials for hard radiation detection. Computational design of other wide gap semiconductors is underway.   

First principles study of LaGaO$_3$/MgAl$_2$O$_4$ (001) polar interfaces

Materials with high oxygen ion conductivity have been the center of much attention due to both fundamental interest and technological applications. One of the most remarkable ionic conductors and an excellent candidate for future solid oxide fuel cells is LaGaO$_3$ (LGO), as it exhibits very high ionic conductivity when doped with Sr or Mg. To achieve enhanced ionic transport in this system, where oxygen vacancies (V$_{\rm O}$) are the dominant carriers, we propose a negatively charged interface as a way of inducing a V$_{\rm O}$ enrichment layer. In this study, the interface is comprised of LGO and the spinel MgAl$_2$O$_4$, both of which exhibit nominally charged (001) planes. We consider an interface where the (GaO$_2$)$^{1-}$ layer of LGO is in contact with the (AlO$_2$)$^{1-}$ layer of the spinel. Such negatively charged interfaces require compensating defects, providing a strong driving force for enhancing the V$_{\rm O}$ concentration, and hence, the in-plane ionic conductivity in the space charge region adjacent to the boundary. We report results from first-principles calculations which provide information on the structure and relative stability of these polar interfaces, compensation mechanisms, and defect formation energies as a function of distance from the interface.   

Theory-driven design of hole-conducting transparent oxides

The design of {\em p}-type transparent conducting oxides (TCOs) aims at {\em simultaneously} achieving transparency and high hole concentration and hole conductivity in one compound. Such design principles (DPs) define a multi-objective optimization problem that is to be solved by {\em searching} a large set of compounds for optimum ones. Here, we screen a large set of ternary compounds, including Ag and Cu oxides and chalcogenides, by calculating via first-principles methods the design properties of each compound, in order to search for optimum {\em p}-type TCOs. We first select Ag$_{3}$VO$_{4}$ as a case study of the application of {\em ab-initio} methods to assess a compound as a candidate {\em p}-type TCO. We predict Ag$_{3}$VO$_{4}$ (i) to have a hole concentration of $\approx 10^{14}$ $cm^{-3}$ at room temperature, (ii) to be at the verge of transparency, and (iii) to have lower hole effective mass than the prototype {\em p}-type TCO CuAlO$_{2}$. We then map the hole effective mass $vs.$ the band gap in the selected compounds and determine those that best meet the DPs by having simultaneously minimum effective mass and a band gap large enough for transparency.   

Screw Dislocated ZnO and Si Nanostructures Studied with Objective Molecular Dynamics

Objective molecular dynamics [1] coupled with tight-binding density functional-based models makes it possible to investigate the stability and electronic structure of ZnO and Si nanotubes [2] and nanowires [3] containing axial screw dislocations. The dislocated structures adopt twisted configurations that stabilize the dislocation at the center despite the close vicinity of surfaces, in excellent agreement with Eshelby's elasticity model of cylinders containing an axial screw dislocation. Coupled to this elasticity model, our simulations represent a new efficient method of calculating the core energy of a dislocation and allow to rationalize the stability of chiral hollow nanowires. The uncovered mechanical and electronic behaviors have implications for a broad class of nanomaterials grown by engaging a screw dislocation. 1. T. Dumitrica and R.D. James, J. Mech. Phys. Sol. 55, 2206-2236 (2007). 2. D.-B. Zhang, E. Akatyeva, and T. Dumitrica, Phys. Rev. B 84, 115431 (2011). 3. I. Nikiforov, D.-B. Zhang, and T. Dumitrica, J. Phys. Chem. Lett. 2, 2544 (2011).   

Tunable band gaps in transition metal dichalcogenides

We investigate band-gap tuning in transition-metal dichalcogenide bilayers by external electric fields applied perpendicular to the layers. Using density functional theory, we show that the fundamental band gap of MoS$_{2}$, MoSe$_{2}$, MoTe$_{2}$, and WS$_{2}$ bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic. We interpret our results in the light of the Giant Stark Effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of change of band gap with applied external field. Our study expands the known space of layered materials with widely tunable band gaps beyond the classic example of bilayer graphene and suggests potential directions for fabrication of novel electronic and photonic devices.   

Growth of Monolayer Boron Sheet on Metal and Metal Boride Surface

Monolayer boron (B) sheet has attracted lots of interests recently due to its metallic conductivity. However, their experimental synthesis has not been achieved so far, which calls for theoretical investigation. Using first principles calculations, we study the possibility of growing monolayer B sheets on metal (Ag, Au) and metal boride (MgB2, TiB2) surface as catalytic substrate. It is shown that after decomposition from precursor, B atoms will aggregate to cluster, then to sheets, while three dimensional bulky B is prohibited due to high nucleation barrier. Charge transfer between substrate and B sheet shifts its stability dependence on hexagon vacancy density. B sheet with specific vacancy density can have cleavage energy as small as graphite thus should be easily peeled off. This work suggests promising approach to synthesize B sheets and would possibly pave the way towards their applications to electronic, optic, and mechanic nano-devices.   

Understanding the properties of hexagonal Semiconduncting Nanomembranes

Namomembranes are an interesting material with novel applications, such as their integration into electronic devices. We can highlight the high degree of bendability of nanomembranes that can be important to device integration and the possibility of modifying electronic properties by changing the roughness. Using density functional theory (DFT) combined with non-equilibrium green's function (NEGF) theory, we investigate different hexagonal semiconducting nanomembranes (e.g. BN, AlN and GaN). We will show the stability, electronic and transport properties of these nitride membranes and look into their possible integration with graphene.   

Single-atom Magnetic Anisotropy on a Surface

Studying single-atom magnetic anisotropy on surfaces enables the exploration of the smallest magnetic storage bit that can be built. In this work, magnetic anisotropy of a single rare-earth atom on a surface is studied for the first time, both computationally and theoretically. The substrate surface is chosen to be a copper-nitrite surface, where single transition-metal magnetic atoms on the same surface were previously studied one atom at a time by STM.\footnote{C. F. Hirjibehedin, C.-Y. Lin, A. F. Otte, M. Ternes, C. P. Lutz, B. A. Jones, A. J. Heinrich, Science 317, 1199 (2007).} We propose unconventional $f$ and $d$ subshell symmetries so that following first-principles calculations, simple pictorial analyses of such anisotropy can be performed for the first time, independently for both rare-earth and transition-metal adatoms. The analyses explain the spin-density distribution of a single adatom, and derive the spin orientation of its largest spin-orbit coupling. The magnetic anisotropy energy of the present study is calculated to be a factor of five larger than the previous highest one.   

Computer Simulated Cold Welding of Metal Nanowires

Metallic contacts are of great importance in electronic devives, the ability of welding them without temperature change is quite remarkable and of interest. Recently cold welding was achieved in gold and silver nanowires (NWs) with diameters in the range of 4 to 10 nm [1]. In the present work we use computer simulations to produce cold welding in gold, silver and silver-gold NWs at room temperature. We used molecular dynamics with many body effective potentials based on the embedded-atom method EAM using the LAMMPS code to simulate first the braking of gold and silver NWs. The two produced NWs are then cold welded and similarly as occurred in the experiments the newly welded NWs showed fcc structures as the printine samples. The structural analysis is done with two independent methods [2] and strain stress curves of the breaking and welding are present. Our computer simulation compare very well with the experiments. This work is supported by CNPq CAPES and FAPESP and FAEPEX. ZSP is supported by CAPES. CENAPAD-SP and IFGW are acknowledged for computer time. \\[4pt] [1] Y. Lu, \textit{et al}. Nature Nanotechnology 5, 218 - 224 (2010)\\[0pt] [2] E. Z. da Silva and Z.S. Pereira, Phys. Rev. B \textbf{81}, 195417 (2010).