arXiv subjects

Zhenyu Li

Publications and source records attributed to Zhenyu Li.

191 records · Page 11Linked to original sources

arXiv2007

Dithiocarbamate Anchoring in Molecular Wire Junctions: A First Principles Study

Recent experimental realization [J. Am. Chem. Soc., 127 (2005) 7328] of various dithiocarbamate self assembly on gold surface opens the possibility for use of dithiocarbamate linkers to anchor molecular wires to gold electrodes. In this paper, we explore this hypothesis computationally. We computed the electron transport properties of 4,4'-bipyridine (BP), 4,4'-bipyridinium-1,1'-bis(carbodithioate) (BPBC), 4-(4'-pyridyl)-peridium-1-carbodithioate (BPC) molecule junctions based on the density functional theory and non-equilibrium Green's functions. We demonstrated that the stronger molecule-electrode coupling associated with the conjugated dithiocarbamate linker broadens transmission resonances near the Fermi energy. The broadening effect along with the extension of the $π$ conjugation from the molecule to the gold electrodes lead to enhanced electrical conductance for BPBC molecule. The conductance enhancement factor is as large as 25 at applied voltage bias 1.0 V. Rectification behavior is predicted for BPC molecular wire junction, which has the asymmetric anchoring groups.

cond-mat.mtrl-sci↗

arXiv2007

Orbital Interaction Mechanisms of Conductance Enhancement and Rectification by Dithiocarboxylate Anchoring Group

We study computationally the electron transport properties of dithiocarboxylate terminated molecular junctions. Transport properties are computed self-consistently within density functional theory and nonequilibrium Green's functions formalism. A microscopic origin of the experimentally observed current amplification by dithiocarboxylate anchoring groups is established. For the 4,4'-biphenyl bis(dithiocarboxylate) junction, we find that the interaction of the lowest unoccupied molecular orbital (LUMO) of the dithiocarboxylate anchoring group with LUMO and highest occupied molecular orbital (HOMO) of the biphenyl part results in bonding and antibonding resonances in the transmission spectrum in the vicinity of the electrode Fermi energy. A new microscopic mechanism of rectification is predicted based on the electronic structure of asymmetrical anchoring groups. We show that the peaks in the transmission spectra of 4'-thiolato-biphenyl-4-dithiocarboxylate junction respond differently to the applied voltage. Depending upon the origin of a transmission resonance in the orbital interaction picture, its energy can be shifted along with the chemical potential of the electrode to which the molecule is more strongly or more weakly coupled.

cond-mat.mtrl-sci↗

arXiv2007

Nature of well-defined conductance of amine anchored molecular junctions

Amine terminated molecules show well behaved conductance in the scanning tunneling microscope break-junction experimental measurements. We performed density functional theory based electron transport calculations to explain the nature of this phenomenon. We find that amines can be adsorbed only on apex Au atom, while thiolate group can be attached equally well to undercoordinated and clean Au surfaces. Our calculations show that only one adsorption geo metry is sterically and energetically possible for amine anchored junction whereas three different adsorption geometries with very distinct transport pro perties are almost equally probable for thiolate anchored junction. We calculated the conductance as a function of the junction stretching when the molecules are pulled by the scanning tunneling microscope tip from the Au electrode. Our calculations show that the stretching of the thiolate anchored junction during its formation is accompanied by significant electrode geometry distortio n. The amine anchored junctions exhibit very different behavior -- the electrode remains intact when the scan ning tunneling microscope tip stretches the junction.

cond-mat.mtrl-sci↗

arXiv2006

Electron transport through dipyrimidinyl-diphenyl diblock molecular wire: protonation effect

Recently, rectifying direction inversion has been observed in dipyrimidinyl-diphenyl (PMPH) diblock molecular wire [J. Am. Chem. Soc. (2005) 127, 10456], and a protonation mechanism was suggested to explain this interesting phenomena. In this paper, we study the protonation effect on transport properties of PMPH molecule by first principles calculations. No significant rectification is found for the pristine diblock molecular wire. Protonation leads to conductance enhancement and rectification. However, for all considered junctions with rectifying effect, the preferential current directions are samely from dipyrimidinyl side to diphenyl side. Effect of molecule-electrode anchoring geometry is studied, and it is not responsible for the discrepancy between experiment and theory.

cond-mat.mtrl-sci↗

arXiv2006

Linear scaling calculation of maximally-localized Wannier functions with atomic basis set

We have developed a linear scaling algorithm for calculating maximally-localized Wannier functions (MLWFs) using atomic orbital basis. An O(N) ground state calculation is carried out to get the density matrix (DM). Through a projection of the DM onto atomic orbitals and a subsequent O(N) orthogonalization, we obtain initial orthogonal localized orbitals. These orbitals can be maximally localized in linear scaling by simple Jacobi sweeps. Our O(N) method is validated by applying it to water molecule and wurtzite ZnO. The linear scaling behavior of the new method is demonstrated by computing the MLWFs of boron nitride nanotubes.

cond-mat.mtrl-sci↗

arXiv2005

Self-similarity of single-channel transmission for electron transport in nanowires

We demonstrate that the single-channel transmission in the resonance tunneling regime exhibits self-similarity as a function of the nanowire length and the energy of incident electrons. The self-similarity is used to design the nonlinear transformation of the nanowire length and energy which, on the basis of known values of transmission for a certain region on the energy-length plane, yields transmissions for other regions on this plane. Test calculations with a one-dimensional tight-binding model illustrate the described transformations. Density function theory based transport calculations of Na atomic wires confirm the existence of the self-similarity in the transmission.

cond-mat.mes-hall↗

arXiv2005

First principle calculations of conductance within plane wave basis set via nonorthogonal Wannier-type atomic orbitals

We present a plane wave/pseudopotential implementation of the method to calculate electron transport properties of nanostructures. The conductance is calculated via the Landauer formula within formalism of Green's functions. Nonorthogonal Wannier-type atomic orbitals are obtained by the sequential unitary rotations of virtual and occupied Kohn-Sham orbitals, which is followed by two-step variational localization. We use these non-orthogonal Wannier type atomic orbitals to partition the Kohn-Sham Hamiltonian into electrode-contact-electrode submatrices. The electrode parts of the system are modeled by two metal clusters with additional Lorentzian broadening of discrete energy levels. We examined our implementation by modeling the transport properties of Na atomic wires. Our results indicate that with the appropriate level broadening the small cluster model for contacts reproduces odd-even oscillations of the conductance as a function of the nanowire length.

cond-mat.mtrl-sci↗

arXiv2004

Electron-Phonon Coupling in Boron-Doped Diamond Superconductor

The electronic structure, lattice dynamics, and electron-phonon coupling of the boron-doped diamond are investigated using the density functional supercell method. Our results indicate the boron-doped diamond is a phonon mediated superconductor, con rming previous theoretical conclusions deduced from the calculations employing the virtual crystal approximation. We show that the optical phonon modes involving B vibrations play an important role in the electron-phonon coupling. Di erent from previous theoretical results, our calculated electron-phonon coupling constant is 0.39 and the estimated superconducting transition temperature Tc is 4.4 K for the boron doped diamond with 2.78% boron content using the Coulomb pseudopotential μ*= 0.10, in excellent agreement with the experimental result.

cond-mat.supr-con↗

arXiv2004

Geometrical, electronic and magnetic properties of Na$_{0.5}$CoO$_2$ from first principles

We report a first-principles projector augmented wave (PAW) study on Na$_{0.5}$CoO$_2$. With the sodium ion ordered insulating phase being identified in experiments, pure density functional calculations fail to predict an insulating ground state, which indicates that Na ordering alone can not produce accompanying Co charge ordering, if additional correlation is not properly considered. At this level of theory, the most stable phase presents ferromagnetic ordering within the CoO$_2$ layer and antiferromagnetic coupling between these layers. When the on-site Coulomb interaction for Co 3d orbitals is included by an additional Hubbard parameter $U$, charge ordered insulating ground state can be obtained. The effect of on-site interaction magnitude on electronic structure is studied. At a moderate value of $U$ (4.0 eV for example), the ground state is antiferromagnetic, with a Co$^{4+}$ magnetic moment about 1.0 $μ_B$ and a magnetic energy of 0.12 eV/Co. The rehybridization process is also studied in the DFT+U point of view.

cond-mat.mtrl-sci↗

arXiv2004

First principles lattice dynamics of NaCoO$_2$

We report first principles linear response calculations on NaCoO$_2$. Phonon frequencies and eigenvectors are obtained throughout the Brillouin zone for two geometries with different Na site occupancies. While most of the phonon modes are found to be unsensitive to the Na site occupancy, there are two modes dominated by out-of-plane vibrations of Na giving very different frequencies for different geometries. One of these two modes, the A$_{2u}$ mode, is infrared-active, and can be used as a suitable sensor of Na distribution/ordering. The longitudinal-transverse splitting of the zone-center optical-mode frequencies, Born effective charges and the dielectric constants are also reported, showing considerable anisotropy. The calculated frequencies of Raman-active modes generally agree with the experimental values of corresponding Na de-intercalated and/or hydrated compounds, while it requires better experimental data to clarify the infrared-active mode frequencies.

cond-mat.mtrl-sci↗

arXiv2001

A first-principles study of MgB2 (0001) surfaces

We report self-consistent {\it ab initio} calculations of structural and electronic properties for the B- and Mg-terminated MgB$_{2}$ (0001) surfaces. We employ ultra-soft pseudopotentials and plane wave basis sets within the generalized gradient approximation. The surface relaxations are found to be small for both B- and Mg-terminated surfaces. For the B-terminated surface, both B $σ$ and $π$ surface bands appear, while only one B $π$ surface band exists near the Fermi level for the Mg-terminated surface. The superconductivity of the MgB$_2$ surfaces is discussed. The work function is predicted to be 5.95 and 4.25 eV for the B- and Mg-terminated surfaces respectively. The simulated scanning tunneling microscopy images of the surfaces are not sensitive to the sign and value of the bias voltages, but depend strongly on the tip-sample distance. An image reversal is predicted for the Mg-terminated surface.

cond-mat.mtrl-sci↗