Band gap engineering of atomically thin two-dimensional (2D) materials is the key to their applications in nanoelectronics, optoelectronics, and photonics. Here, for the first time, we demonstrate that in the 2D system, by alloying two materials with different band gaps (MoS2 and WS2), tunable band gap can be obtained in the 2D alloys (Mo1–xWxS2 monolayers, x = 0–1). Atomic-resolution scanning transmission electron microscopy has revealed random arrangement of Mo and W atoms in the Mo1–xWxS2 monolayer alloys. Photoluminescence characterization has shown tunable band gap emission continuously tuned from 1.82 eV (reached at x = 0.20) to 1.99 eV (reached at x = 1). Further, density functional theory calculations have been carried out to understand the composition-dependent electronic structures of Mo1–xWxS2 monolayer alloys.

Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys

We study the relaxations, surface energies, and work functions of low-index metallic surfaces using pseudopotential plane-wave density-functional calculations within the generalized gradient approximation. We study here the (100), (110), and (111) surfaces of Al, Pd, Pt, and Au and the (0001) surface of Ti, chosen for their use as contact or lead materials in nanoscale devices. We consider clean, mostly nonreconstructed surfaces in the slab-supercell approximation. Particular attention is paid to the convergence of these quantities with respect to slab thickness; furthermore, different methodologies for the calculation of work functions and surfaces energies are compared. We find that the use of bulk references for calculations of surface energies and work functions can be detrimental to convergence unless numerical grids are closely matched, especially when surface relaxations are being considered. Our results and comparison show that calculated values often do not quantitatively match experimental values. This may be understandable for the surface relaxations and surface energies, where experimental values can have large error but even for the work functions, neither local nor semilocal functionals emerge as an accurate choice for every case.

Surface energies, work functions, and surface relaxations of low index metallic surfaces from first principles

A method for calculating and subsequently tuning the electrochemical potential of a half cell using periodic plane-wave density functional theory and a homogenous counter-charge is presented and evaluated by comparison to simulations which explicitly model the countercharge by a plane of ions. The method involves the establishment of two reference potentials, one related to the potential of the free electron in vacuo, and the other related to the potential of ${\mathrm{H}}_{2}\mathrm{O}$ species far from the electrode. The surface potential can be specifically adjusted by the explicit introduction of excess or deficit surface charges in the simulation cell and the application of periodic boundary conditions. We demonstrate the absence of field emission from the electrode over the range of realistic electrochemical potentials covered and confirm that the method can explicitly determine reaction energies and adsorption geometries as a function of electrochemical potential. This latter point is most useful as it asserts the viability of this method to model electrochemical and electrocatalytical systems of academic as well as applied interest. We present two case studies. The first examines the changes in the structure of water at the metal interface as a function of potential over $\mathrm{Cu}(111)$. At cathodic potential, we observe the repulsion of ${\mathrm{H}}_{2}\mathrm{O}$ from the interface and the rotation of the water dipole toward the interface. The second study follows the initial pathways for the electrocatalytical activation of methanol over $\mathrm{Pt}(111)$ and the corresponding potential dependent reaction energetics for these paths. The results demonstrate that changes in the electrochemical potential can significantly alter the reaction energetics as well as the overall reaction selectivity. While the case studies presented herein described equilibrium geometries (i.e., the ideal forms at zero kelvin), the method is also suitable for application to ensembles of thermally activated systems.

First principles reaction modeling of the electrochemical interface: Consideration and calculation of a tunable surface potential from atomic and electronic structure

Tip-enhanced Raman spectroscopy in conjunction with scanning tunnelling microscopy can be used to correlate chemical properties and surface topography of bimetallic catalysts with high spatial resolution. An atomic- and molecular-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts1,2,3. Achieving this level of characterization requires techniques that can correlate catalytic performances to specific surface structures, so as to avoid averaging effects1. Tip-enhanced Raman spectroscopy4,5,6,7 combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topographical and chemical information at the nano/atomic scale from ambient8,9,10 to ultrahigh-vacuum11,12 and electrochemical environments13,14. Therefore, it has been used to monitor catalytic reactions15,16,17,18 and is proposed to correlate the local structure and function of heterogeneous catalysts19. Bimetallic catalysts, such as Pd–Au, show superior performance in various catalytic reactions20,21, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chemically and spatially probe the site-specific chemical (electronic and catalytic) and physical (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3 nm resolution in real space using phenyl isocyanide as a probe molecule (
 Fig. 1a
 ). We observe a weakened N≡C bond and enhanced reactivity of phenyl isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. Density functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3 nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.

Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution

On the HSAB based estimate of charge transfer between adsorbates and metal surfaces

Quantum-size effects have been shown to influence significantly the determination of work functions from thin-slab calculations. We show here that a technique based on macroscopic averages can be used to reduce such effects and determine more precisely the work functions of metals from ab initio thin-film calculations. The technique combines the mean electrostatic potential step across the slab surface with the Fermi energy of a bulk crystal. The method is applied to Al(100) slabs containing 1-14 atomic layers.

https://iopscience.iop.org/article/10.1088/0953-8984/11/13/006/pdf

Deriving accurate work functions from thin-slab calculations

Work-function anisotropy in noble metals: Contributions from d states and effects of the surface atomic structure

Ab initio calculations combined with a recently developed macroscopic-averaging technique have been used to examine the anisotropy of the aluminum work function. Our results indicate that the Al(111) work function is substantially lower than the Al(100) work function, and slightly below the Al(110) work function. This is in contrast to other fcc metals, for which a $(110)\ensuremath{\rightarrow}(100)\ensuremath{\rightarrow}(111)$ increasing trend is observed experimentally and predicted by Smoluchowski's rule. We show that the aluminum work-function anomaly, which is in agreement with experiment, is due to the increased $p$-atomic-like character of the density of states at the Fermi energy, as compared to most other fcc metals.

Anomaly in the anisotropy of the aluminum work function

We perform ab initio pseudopotential calculations for metal crystals with finite facets of different crystallographic orientation to investigate the work function profile near crystal edges We examine local edge effects, and address the problem of the coexistence of different face-dependent local work functions at crystal edges By modeling the electronic dipoles at the metal surface, we show how nonvanishing surface charges spontaneously appear on metals with inequivalent facets Our studies of Al crystal nanowires with (100) and (111) facets are extended to derive the dependence of the work function on the crystal morphology in the macroscopic limit

Work functions at facet edges.

Work functions of stepped metallic surfaces are modeled to generate work-function anisotropy maps. We show how the work function of any stepped surface can be accurately predicted by interpolating between the work functions of a small number of low-index facets using a set of physically motivated symmetry-adapted basis functions. This technique is applied to the work-function anisotropy of tungsten, where we study the W(110), W(100), W(211), W(310), W(111), and W(321) surfaces from first principles. The subsequently modeled work-function anisotropy map is found to be in excellent agreement with recent experimental maps over the full range of surface orientations.

C. J. Fall

Papers

Deriving accurate work functions from thin-slab calculations

Work-function anisotropy in noble metals: Contributions from d states and effects of the surface atomic structure

Anomaly in the anisotropy of the aluminum work function

Work functions at facet edges.

Theoretical maps of work-function anisotropies