Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability and systematic convergence properties. However, as a unique feature GPAW also facilitates a localized atomic-orbital basis set in addition to the grid. The efficient atomic basis set is complementary to the more accurate grid, and the possibility to seamlessly switch between the two representations provides great flexibility. While DFT allows one to study ground state properties, time-dependent density-functional theory (TDDFT) provides access to the excited states. We have implemented the two common formulations of TDDFT, namely the linear-response and the time propagation schemes. Electron transport calculations under finite-bias conditions can be performed with GPAW using non-equilibrium Green functions and the localized basis set. In addition to the basic features of the real-space PAW method, we also describe the implementation of selected exchange-correlation functionals, parallelization schemes, ΔSCF-method, x-ray absorption spectra, and maximally localized Wannier orbitals.

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We report a first principles investigation of photocurrent generation by graphene PN junctions. The junctions are formed by either chemically doping with nitrogen and boron atoms, or by controlling gate voltages. Non-equilibrium Green's function (NEGF) formalism combined with density functional theory (DFT) is applied to calculate the photo-response function. The graphene PN junctions show a broad band photo-response including the terahertz range. The dependence of the response on the angle between the light polarization vector and the PN interface is determined. Its variation against photon energy E ph is calculated in the visible range. The essential properties of chemically doped and gate-controlled PN junctions are similar, but the former shows fingerprints of dopant distribution.

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The ability to control over the quantum interference (QI) effect in single molecular junctions is attractive in the application of molecular electronics. Herein we report that the QI effect of meta-benzene based molecule with dihydrobenzo[b]thiophene as the anchoring group (meta-BT) can be controlled by manipulating the electrode potential of the junctions in electrolyte while the redox state of the molecule does not change. More than 2 orders of magnitude conductance change is observed for meta-BT ranging from <10 −6.0 to 10 −3.3 G 0 with varying the electrode potential, while the upper value is even larger than the conductance of para-BT (para-benzene based molecule with anchoring group of dihydrobenzo[b]thiophene). This phenomenon is attributed to the shifting of energy level alignment between the molecule and electrodes under electrode potential control. Calculation is carried out to predict the transmission function of single molecular junction and the work function of Au surface in the presence of the molecule, and good agreement is found between theory and experiments, both showing sharpvalley featured destructive QI effect for the meta-BT. The present work demonstrates that the QI effect can be tuned through electrochemical gating without change of molecular redox states, which provides a feasible way toward realization of effective molecular switches.

Recently, an emergent layered material T d -WTe 2 was explored for its novel electron-hole overlapping band structure and anisotropic inplane crystal structure. Here, the photoresponse of mechanically exfoliated WTe 2 flakes is investigated. A large anomalous current decrease for visible (514.5 nm), and mid-and far-infrared (3.8 and 10.6 µm) laser irradiation is observed, which can be attributed to light-induced surface bandgap opening from the first-principles calculations. The photocurrent and responsivity can be as large as 40 µA and 250 A W −1 for a 3.8 µm laser at 77 K. Furthermore, the WTe 2 anomalous photocurrent matches its in-plane crystal structure and exhibits light polarization dependence, maximal for linear laser polarization along the W atom chain a direction and minimal for the perpendicular b direction, with the anisotropic ratio of 4.9. Consistently, first-principles calculations confirm the angle-dependent bandgap opening of WTe 2 under polarized light irradiation. The anomalous and polarization-sensitive photoresponses suggest that linearly polarized light can significantly tune the WTe 2 surface electronic structure, providing a potential approach to detect polarized and broadband lights up to far infrared range.

Transition voltage spectroscopy (TVS) is a promising spectroscopic tool for molecular junctions. The principles in TVS is to find the minimum on a Fowler-Nordheim plot where ln(I/V 2 ) is plotted against 1/V and relate the voltage at the minimum, Vmin, to the closest molecular level. Importantly, Vmin, is approximately half the voltage required to see a peak in the dI/dV curve. Information about the molecular level position can thus be obtained at relatively low voltages. In this work we show that the molecular level position can be determined at even lower voltages, V (α) min by finding the minimum of ln(I/V α ) with α < 2. On the basis of a simple Lorentzian transmission model we analyze theoretical ab initio as well as experimental I − V curves and show that the voltage required to determine the molecular levels can be reduced by ∼ 30% as compared to conventional TVS. As for conventional TVS, the symmetry/asymmetry of the molecular junction needs to be taken into account in order to gain quantitative information. We show that the degree of asymmetry may be estimated from a plot of V (α) min vs. α.