Talat S. Rahman

2-Dimensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2(1–x)Se2x Monolayers

MoS2(1-x) Se2x single-layer films are prepared using a mixture of organic selenium and sulfur precursors as well as a solid molybdenum source. The direct bandgaps are found to scale nearly linearly with composition in the range of 1.87 eV (pure single-layer MoS2 ) to 1.55 eV (pure single-layer MoSe2 ) permitting straightforward bandgap engineering.

Molecular adsorption on graphene.

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H(2)O, H(2), O(2), CO, NO(2), NO, and NH(3)), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH(2), An-CH(3), An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphenes electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

On the dynamics of the associative desorption of H2

The dynamics of activated associative desorption is discussed with particular reference to the system H2–Cu and to the partitioning of the energy released among the various product degrees of freedom. It is argued that a simple theory based on transition‐state concepts should hold for this system because the potential energy surface (PES) divides naturally into reactant and product regions, separated by a ‘‘seam’’ or ‘‘ridge’’ at which it is reasonable to assume a thermal distribution of desorbing trajectories. Using a PES constructed in accordance with available electronic structure calculations we consider the angular distributions and translational, vibrational, and rotational energy distributions of the desorbing molecules. It is shown that, whereas the rotational energy reflects the surface temperature, the vibrational energy is markedly enhanced because the energetically low‐lying regions of the ridge in the PES correspond to an H–H bond distance that is distended as compared with the gas‐phase equi...

A Surface Coordination Network Based on Substrate‐Derived Metal Adatoms with Local Charge Excess

In the quest for increased control and tuneability of organic patterns at metal surfaces, more and more systems emerge that rely upon coordination of metal adatoms by organic ligands using endgroups such as carbonitriles, amines, and carboxylic acids. Such systems promise great flexibility in the size and geometry of the surface pattern through choice of the ligand shape, the number and arrangement of ligating endgroups, and the nature of the metal centers. Planar (trigonal or square) arrangements of ligands around metal centers occur most commonly as a result of attractive interactions of the ligands with the substrate. In contrast, in the solution phase planar, and in particular trigonal planar, arrangements are quite rare and generally require ligands whose nature (for example bidentate, pincer shape) forces planarity. Given the relatively short history of the field of surface coordination chemistry, compared to its solution-phase counterpart, it is of great interest to know which information can be gleaned from the latter to predict that for the former. Aspects of coordination chemistry at surfaces that have attracted very little attention to date are the effective oxidation state of the metal atom, which is much more straightforward to define in the solution phase, and the response of the coordination center to the presence of ligands at a surface. This study details an effort at gaining some insight into these two aspects, using a coordination system which is particularly facile to prepare, as it relies on substrate atoms as coordination centers, rather than requiring their separate deposition. In particular, this study describes the formation of a hexagonal network of 9,10-anthracenedicarbonitrile (DCA) on Cu(111) by titration of a nearly square molecular arrangement with copper atoms released from the substrate by annealing. We apply a combination of experimental and theoretical methods and juxtapose their results with the molecular patterns formed in the absence of a substrate. Individual DCA molecules adsorb flat onto Cu(111) with the anthracene moiety parallel to the high-symmetry direction of the substrate. Figure 1 shows an STM image of DCA

The Quantum Magnetism of Individual Manganese-12-Acetate Molecular Magnets Anchored at Surfaces

The high intrinsic spin and long spin relaxation time of manganese-12-acetate (Mn(12)) makes it an archetypical single molecular magnet. While these characteristics have been measured on bulk samples, questions remain whether the magnetic properties replicate themselves in surface supported isolated molecules, a prerequisite for any application. Here we demonstrate that electrospray ion beam deposition facilitates grafting of intact Mn(12) molecules on metal as well as ultrathin insulating surfaces enabling submolecular resolution imaging by scanning tunneling microscopy. Using scanning tunneling spectroscopy we detect spin excitations from the magnetic ground state of the molecule at an ultrathin boron nitride decoupling layer. Our results are supported by density functional theory based calculations and establish that individual Mn(12) molecules retain their intrinsic spin on a well chosen solid support.

Physisorption of nucleobases on graphene: a comparative van der Waals study

The physisorption of the nucleobases adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) on graphene is studied using several variants of the density functional theory (DFT): the generalized gradient approximation with the inclusion of van der Waals interaction (vdW) based on the TS approach (Tkatchenko and Scheffer 2009 Phys. Rev. Lett. 102 073005) and our simplified version of this approach (here called sTS), the van der Waals density functional vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92 246401) and vdW-DF2 (Lee et al 2010 Phys. Rev. B 82 081101), and DFT-D2 (Grimme 2006 J. Comput. Chem. 27 1787) and DFT-D3 (Grimme et al 2010 J. Chem. Phys. 132 154104) methods. The binding energies of nucleobases on graphene are found to be in the following order: G > A > T > C > U within TS, sTS, vdW-DF, and DFT-D2, and in the following order: G > A > T ~ C > U within DFT-D3 and vdW-DF2. The binding separations are found to be different within different methods and in the following order: DFT-D2 < TS < DFT-D3 ~ vdW-DF2 < vdW-DF. We also comment on the efficiency of combining the DFT-D approach and vdW-DF to study systems with van der Waals interactions.

Theory of depolarization of fluorescence in molecular pairs

Abstract Depolarization of fluorescence as a result of energy transfer is studied phenomenologically for a model pair of electronically coupled molecules. The usual rate equations are replaced by the Stochastic Liouville Equations and new radiative terms are included. An expression for the fluorescence polarization for all strengths of electronic coupling between the molecules is obtained. The inclusion of off-diagonal density matrix elements is essential for resolving a paradox arising in the Forster theory of depolarization. The calculation points the way toward using a previously untapped source of information on coherence in complex systems.

Toward the growth of an aligned single-layer MoS2 film.

Molybdenum disulfide (molybdenite) monolayer islands and flakes have been grown on a copper surface at comparatively low temperature and mild conditions through sulfur loading of the substrate using thiophenol (benzenethiol) followed by the evaporation of Mo atoms and annealing. The MoS(2) islands show a regular Moiré pattern in scanning tunneling microscopy, attesting to their atomic ordering and high quality. They are all aligned with the substrate high-symmetry directions providing for rotational-domain-free monolayer growth.

Self-learning kinetic Monte Carlo method: Application to Cu(111)

We present a method of performing kinetic Monte Carlo simulations that does not require an a priori list of diffusion processes and their associated energetics and reaction rates. Rather, at any time during the simulation, energetics for all possible single- or multiatom processes, within a specific interaction range, are either computed accurately using a saddle-point search procedure, or retrieved from a database in which previously encountered processes are stored. This self-learning procedure enhances the speed of the simulations along with a substantial gain in reliability because of the inclusion of many-particle processes. Accompanying results from the application of the method to the case of two-dimensional Cu adatom-cluster diffusion and coalescence on Cu111 with detailed statistics of involved atomistic processes and contributing diffusion coefficients attest to the suitability of the method for the purpose.

Electronic properties and charge transfer phenomena in Pt nanoparticles on γ-Al2O3: size, shape, support, and adsorbate effects.

This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and γ-Al(2)O(3)-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H(2)) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP-adsorbate (H(2)) and NP-support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1 : 1 ratio previously reported for bulk Pt surfaces.