Quantum Theory of Advanced Materials

Abstract Band structure by design in 2D layered semiconductors is highly desirable, with the goal to acquire the electronic properties of interest through the engineering of chemical composition, structure, defect, stacking, or doping. For atomically thin transition metal dichalcogenides, substitutional doping with more than one single type of transition metals is the task for which no feasible approach is proposed. Here, the growth of WS2 monolayer is shown codoped with multiple kinds of transition metal impurities via chemical vapor deposition controlled in a diffusion-limited mode. Multielement embedment of Cr, Fe, Nb, and Mo into the host lattice is exemplified. Abundant impurity states thus generate in the bandgap of the resultant WS2 and provide a robust switch of charging/discharging states upon sweep of an electric filed. A profound memory window exists in the transfer curves of doped WS2 field-effect transistors, forming the basis of binary states for robust nonvolatile memory. The doping technique presented in this work brings one step closer to the rational design of 2D semiconductors with desired electronic properties.

The phase stability is studied for the beta-phase YH2+x system based on first-principles total energy calculations. Our study predicts that the D0(22), ''40'', and D1a structures are stable near x = 0. 25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtained from the first-principles total-energy data, the phase diagram for the D0(22) and ''40'' ordered phases is calculated by the cluster variational method. The calculated order-disorder transition temperature at x = 0.1 for the D0(22) structure is around 280 K, which is consistent with the recent observation of the metal-semiconductor transition near 230-280 K and resistivity anomalies near 200-250 K for the system with x near 0.1 [Daou and Vajda, Phys. Rev. B 45, 10 907 (1992)].

The asymmetry in the phase diagram of the H/Ru(001) system is studied by assuming a lattice gas model for the chemisorbed hydrogen and using the cluster variation method. Ground state analysis of the ordered structures shows that the effective pair interaction for the next-nearest neighbors has to be repulsive. We also found that the order-disorder transition temperatures and hence the phase diagram are very sensitive to upsilon3, the ratio of the effective next-nearest to nearest neighbor interactions of H adatoms. The asymmetry in the phase diagram, which cannot be accounted for by the adsorbate relaxation model by Persson [Surf. Sci. 258 (1991) 451], is attributed to the coverage dependence of the effective pair interactions. By assuming a simple piecewise linear dependence of upsilon3 on the chemical potential, we constructed an asymmetric phase diagram which is in excellent agreement with the experimental data. The model studied can be applied to the H/Pd(111) system directly and can be easily generalized for other close-packed metal surfaces.

A p-T phase diagram of graphene nanoribbons (GNRs) terminated by hydrogen atoms is established based on first-principles calculations, where the stable phase at standard conditions (25 degrees C and 1 bar) is found to be a zigzag GNR (zzGNR). The stability of this new GNR is understood based on an electron-counting model, which predicts semiconducting nonmagnetic zzGNRs. Quantum confinement of Dirac fermions in the stable zzGNRs is found to be qualitatively different from that in ordinary semiconductors. Bifurcation of the band gap is predicted to take place, leading to the formation of polymorphs with distinct band gaps but equal thermodynamic stability. A tight-binding model analysis reveals the role of edge symmetry on the band-gap bifurcation.

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Atomically uniform Pb films are successfully prepared on Si(111), despite a large lattice mismatch. Angle-resolved photoemission measurements of the electronic structure show layer-resolved quantum well states which can be correlated with dramatic variations in thermal stability. The odd film thicknesses N=5, 7, and 9 monolayers show sharp quantum well states. The even film thicknesses N=6 and 8 do not, but are much more stable than the odd film thicknesses. This correlation is discussed in terms of a total energy calculation and Friedel-like oscillations in properties.

The cubic YH2+x system with an extended hydrogen composition is studied using the pseudopotential method and the local-density-functional approximation with a plane-wave basis. The study focuses on the beta phase with the metal atoms forming a face-centered-cubic lattice and the octahedral sites partially occupied by hydrogen for 0 < x < 1. The self-consistent total-energy calculation is performed by employing the supercell modeling method. The structural property, in particular, the volume contraction with increasing x, is investigated by analyzing the energy changes for different site occupation. It is found that the lattice contracts mainly to increase the interaction of the additional electron and the metal d potential. In addition, the (420)-plane ordering of the x-excess hydrogen is examined for YH2.25 and is confirmed by energetics studies.

Aluminum hydride (alane) AlH(3) is an important material in hydrogen storage applications. It is known that AlH(3) exists in multiply forms of polymorphs, where alpha-AlH(3) is found to be the most stable with a hexagonal structure. Recent experimental studies on gamma-AlH(3) reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in alpha-AlH(3) or other polymorphs. Using density functional theory, we have investigated the energetics, and the structural, electronic, and phonon vibrational properties for the newly reported gamma-AlH(3) structure. The current calculation concludes that gamma-AlH(3) is less stable than alpha-AlH(3) by 1.2 KJ/mol, with the zero-point energy included. Interesting binding features associated with the unique geometry of gamma-AlH(3) are discussed from the calculated electronic properties and phonon vibrational modes. The binding of H-s with higher energy Al-p,d orbitals is enhanced within the double-bridge arrangement, giving rise to a higher electronic energy for the system. Distinguishable new features in the vibrational spectrum of gamma-AlH(3) were attributed to the double-bridge and hexagonal-ring structures.

A pseudopotential local-density calculation is performed for YH3 to study the unusual hydrogen displacements previously found in neutron diffraction. These displacements are identified as Peierls distortions associated with (hydrogen) lattice instability in this 3D system. The wave vector of these displacements is close to the vector connecting the electron and hole pockets in the undistorted system. With other electron and hole pockets at GAMMA that still overlap after distortion, the possibility of the existence of an excitonic insulator phase will be discussed.

The Hofstadter butterfly spectrum for Landau levels in a two-dimensional periodic lattice is a rare example exhibiting fractal properties in a truly quantum system. However, the observation of this physical phenomenon in a conventional material will require a magnetic field strength several orders of magnitude larger than what can be produced in a modern laboratory. It turns out that for a specific range of rotational angles twisted bilayer graphene serves as a special system with a fractal energy spectrum under laboratory accessible magnetic field strengths. This unique feature arises from an intriguing electronic structure induced by the interlayer coupling. Using a recursive tight-binding method, we systematically map out the spectra of these Landau levels as a function of the rotational angle. Our results give a complete description of LLs in twisted bilayer graphene for both commensurate and incommensurate rotational angles and provide quantitative predictions of magnetic field strengths for observing the fractal spectra in these graphene systems.

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The structural properties of hexagonal-close-packed yttrium are studied by using the plane-wave basis within the pseudopotential method and local-density-functional approximation. By employing a "soft" pseudopotential proposed by Troullier and Martins, satisfactory convergence is achieved with a plane-wave energy cutoff of 30-40 Ry for this early-transition-metal element. The overall results for the structural properties are in good agreement with experiment. It is found that the charge overlap between core and valence electrons has a substantial effect on the accuracy of the calculated structural properties. Two different calculations are performed with and without the outer-core 4p orbital included as a valence state. In addition, as found in some other local-density calculations, the uncertainty in the results due to different exchange-correlation energy functionals may not be negligible in transition metals.

A small amount of catalyst, such as Ti, was found to greatly improve the kinetics of hydrogen reactions in the prototypical hydrogen storage compound sodium alanate (NaAlH(4)). We propose a near-surface alloying mechanism for the rehydrogenation cycle based on a detailed analysis of available experimental data as well as first-principles calculations. The calculated results indicate that the catalyst remains at subsurface sites near the Al surface, reducing the dissociation energy barrier of H(2). The binding between Ti and Al modifies the surface charge distribution, which facilitates hydrogen adsorption and enhances hydrogen mobility on the surface.

The structural and electronic properties of hydrogen in yttrium are studied using the pseudopotential method within the local-density-functional approximation (LDA). Different concentration regions are considered for the alpha and beta phases. The binding energies associated with different interstitial sites are evaluated as well as the diffusion energy barrier and local vibrational modes. It is found that the occupation of the tetrahedral site is energetically more favorable than that of the octahedral site in the alpha phase. The calculated vibrational frequency is in excellent agreement with the value observed in neutron scattering experiments. Possibility of pairing is also examined from the consideration of energetics.

The solid-solution phase of hydrogen in hexagonal close-packed yttrium (a-YH(x)) is studied using the pseudopotential method within the local-density-functional approximation with a plane-wave basis. The binding energies associated with different interstitial sites are evaluated for several ordered structures: YH0.5, YH0.25, and YH0.167. It is found that the occupation of the tetrahedral site is always energetically favorable. The hydrogen potential-energy curves around the tetrahedral sites along the c axis and along the path connecting the adjacent octahedral sites are also calculated for YH0.25. In particular, the local vibrational mode along the c axis is estimated to be 100 meV, in excellent agreement with that measured in neutron-scattering experiments. Finally, the intriguing pairing phenomenon is investigated by calculating the total energy for various pairing configurations. The possibility of pairing between nearest-neighbor tetrahedral sites is excluded due to the high energy. It is found that the pairing of hydrogen across a metal atom is indeed energetically favorable compared with other kinds of pairs considered and also with isolated tetrahedral hydrogen atoms. The connection with the electronic structure of the system is also examined.

Recently it was discovered that a total of 5.6 wt. % H-2 could be released from the 1:2 mixture of lithium amide and magnesium hydride at temperatures as low as 150 degrees C. With a reaction enthalpy of 44 KJ/mol H-2, this system has high potential for on-board hydrogen storage applications. The fully desorbed product is believed to be a mixed lithium and magnesium imide Li2Mg(NH)(2). In this work, the crystal structure of this mixed imide is studied from total-energy density-functional calculations. Based on a recent experimentally established space group, possible ordered configurations are examined. Important local orderings are identified for the experimentally observed disordered phase at room temperature. These unique local arrangements are also connected with the observed structural transitions above room temperature. In addition, the local ordering in Mg(NH2)(2) is analyzed. The similarity and difference of local arrangements among hydrogen, cations, and vacancies are discussed for the three amide (imide) systems: LiNH2, Mg(NH2)(2), and Li2Mg(NH)(2). The identification of the cation and hydrogen local orderings are expected to facilitate the design of new mixed imides and amides as hydrogen storage materials with desired physical properties.

We studied total energies of various ordered structures of PdHx (in which hydrogen occupies the octahedral sites within the fee Pd lattice) using the pseudopotential method and a plane-wave basis within the local-density-functional approximation. The structures considered include the (420)-plane ordering of hydrogen atoms at different concentrations. For x greater than or equal to 1/2 we found that the NiMo- and Ni4Mo (D1(a))-type structures at x=1/2 and x=4/5, respectively, were energetically favored phases, in agreement with the superlattice reflections found in previous neutron-scattering measurements. For the intermediate concentrations, linear variation of the formation energy as a function of x in several (420)-ordered structures explained the observed short-range order. In contrast to an earlier proposal, we did not find the Fermi surface imaging effect responsible in this case. The overall energy variation in different phases indicates the importance of going beyond pairwise interactions between interstitial hydrogen atoms in this system.

We report first-principles calculations of Pb (100) films up to 22 monolayers to study variations in the surface energy and work function as a function of film thickness. An even-odd oscillation is found in these two quantities, while a jelliumlike model for this s-p metal predicts a periodicity of about three monolayers. This unexpected result is explained by considering a coherent superposition of contributions from quantum-well states centered at both the Gamma and M points in the two-dimensional Brillouin zone, demonstrating the importance of crystal band structure in studying the quantum size effect in metal thin films.

We report the first implementation of orthonormal wavelet bases in self-consistent electronic structure calculations within the local-density approximation. These local bases of different scales efficiently describe localized orbitals of interest. As an example, we studied two molecules, H-2 and O-2, using pseudopotentials and supercells. Considerably fewer bases are needed compared with conventional plane-wave approaches, yet calculated binding properties are similar. Our implementation employs fast wavelet and Fourier transforms, avoiding evaluating any three-dimensional integral numerically.

We propose a simple method to evaluate the energies of ideal metal surfaces as a function of orientation based on cluster energy expansion. By symmetry only clusters with even-number corners will be present. It is found that the energy expansion converges rapidly and in most cases can be truncated at the pair interaction level. The parameters can be determined from a limited number of low-index surface energies obtained from first-principles calculations. The equilibrium crystal shape at T = O is then predicted and the step energy on major facets is derived for some fee metals.