Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance ( ), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers -the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multi-dimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.

We report on two novel routes to generating a train of attosecond pulses from a broad discrete spectrum in the near-infrared-visibleultraviolet range. One extends an integer-temporal-Talbot (ITT) concept to include high-order spectral dispersions and generates a pulse train that completely satisfies the transform-limited condition. The other numerically explores the optimum conditions under which we can obtain an attosecond pulse train that approximately satisfies the transform-limited condition. The second method is more practical than the first. Either of these methods is extremely simple and robust; we need only to place a few thin dispersive materials in the optical path and to adjust their thicknesses without spatially dispersing the frequency components. We numerically demonstrate the generation of a train of attosecond pulses with a transform-limited pulse duration of 728 as and a repetition period of 8.03 fs in gaseous parahydrogen.

The nonlinear absorption in nanostructured Ni-Ti alloys, fabricated by electrochemical deposition, was investigated at 532 and 1064 nm. The type of nonlinear absorption (saturable or reverse saturable absorption) was observed to depend on the laser intensity as well as on the nanoparticle size. The nanostructured Ni-Ti alloys comprising particle mean diameters of 20 and 30 nm exhibited large three-photon absorption (3PA coefficient approximately 10(6) cm(3)/GW(2)) and large two-photon absorption (2PA coefficient approximately 10(5) cm/GW) at 532 nm, respectively. The observed change over from reverse saturable absorption to saturable absorption at high peak intensities has qualitatively been analyzed by the excited-state theory of conduction electrons.

Enhanced ultrafast optical nonlinearities of porous anodized aluminum oxide (AAO) nanostructures, well-known templates for quantum dots fabrication, have been investigated using the differential optical Kerr gate technique at 800 nm. The optical nonlinearity is strongly influenced by the pore number density, the pore size and the shape. Large values of the third-order nonlinear optical susceptibility (chi((3))) of the order of 10(-10)esu are measured. The nonlinear response time is faster than or comparable to the laser pulse width (90 fs) used. The origin and variation of such remarkable optical nonlinearities has been discussed by considering the nanoporous AAO as an effective medium and utilizing the extended Maxwell Garnet theory, and by considering the additional influence from pore diameter, pore shape and surface states.

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Metallic iron exposed to air even at room temperature is instantly passivized by a thin layer of oxide, as what happens on the surface of metallic utensils we use daily. This layer of oxide has a thickness of 2 -3 nm, and the formation process of this thin oxide layer is normally referred to as initial oxidation. Relative to the abundant experimental observation and theoretical derivation on the growth of thick oxidation layer on metal surface at high temperature, atomic level understanding of the initial oxidation is rather limited. This is related to the fact that the initial oxide layer normally forms at an uncontrollable manner. The kinetic theory for the initial oxidation has been established by Caberra and Mott more than half a century ago [1]. In this theory, they made two assumptions: (1) tunneling of electrons from the metal to the surface absorbed oxygen, leading to the ionization of the surface absorbed oxygen, thus inducing an electrical field, and (2) the electric field drives outward transport of metal ions to combine with the surface chemically adsorbed oxygen ions. No experimental observation has ever been made to demonstrate that the cations outwards transport indeed happens. We report in this letter our observation of injection of vacancies into single crystal α-Fe nanoparticle during the initial oxidation of the iron at room temperature and condensation of those vacancies leads to the formation of void which is subsequently enclosed in the Fecore/oxide-shell structured nanoparticles. These observations provide direct evidence that, during the initial oxidation of iron at room temperature, the outward diffusion of iron is indeed the dominant process. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particle larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation also adds new dimensions for designing of metal/metal-oxide/quantum antidotes (vacancy cluster) nanoclusters for applications related to optical, magnetic, and electrical properties of the core-shell structured nanoparticles.Contrasted with the understanding of high temperature growth of thick oxide layer, less is known about the initial oxidation process of iron and the structural nature of their product. The initial oxidation process has two features: the formed oxide layer is normally just a few nanometer; and the growth rate is pretty fast. Depending on the type of migrating lattice defects, the reaction of oxide either can be at the metal-oxide interface (oxygen ions inward transport) or at the oxide-gas interface (metal ions outward transport). Under the frame of the Caberra-Mott theory of oxidation of metal [1], the oxidation process of iron can be described as the following: upon initial attachment of oxygen onto the surface of metal and formation of a thin layer of oxide, electron tunnels through the thin oxide layer and ...