Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

http://nanotubes.rutgers.edu/PDFs/nchem_TMDs_2013.pdf

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the nonlinear response regime at low temperatures. We find that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes.

Energy band-gap engineering of graphene nanoribbons.

We developed a chemical route to produce graphene nanoribbons (GNR) with width below 10 nanometers, as well as single ribbons with varying widths along their lengths or containing lattice-defined graphene junctions for potential molecular electronics. The GNRs were solution-phase-derived, stably suspended in solvents with noncovalent polymer functionalization, and exhibited ultrasmooth edges with possibly well-defined zigzag or armchair-edge structures. Electrical transport experiments showed that, unlike single-walled carbon nanotubes, all of the sub-10-nanometer GNRs produced were semiconductors and afforded graphene field effect transistors with on-off ratios of about 10(7) at room temperature.

http://science.sciencemag.org/content/sci/319/5867/1229.full.pdf

Chemically Derived, Ultrasmooth Graphene Nanoribbon Semiconductors

Graphene-Like Two-Dimensional Materials

Sub-10 nm wide graphene nanoribbon field-effect transistors (GNRFETs) are studied systematically. All sub-10 nm GNRs afforded semiconducting FETs without exception, with ${I}_{\mathrm{on}}/{I}_{\mathrm{off}}$ ratio up to ${10}^{6}$ and on-state current density as high as $\ensuremath{\sim}2000\text{ }\text{ }\ensuremath{\mu}\mathrm{A}/\ensuremath{\mu}\mathrm{m}$. We estimated carrier mobility $\ensuremath{\sim}200\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{V}\text{ }\mathrm{s}$ and scattering mean free path $\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$ in sub-10 nm GNRs. Scattering mechanisms by edges, acoustic phonon, and defects are discussed. The sub-10 nm GNRFETs are comparable to small diameter ($d\ensuremath{\le}\ensuremath{\sim}1.2\text{ }\text{ }\mathrm{nm}$) carbon nanotube FETs with Pd contacts in on-state current density and ${I}_{\mathrm{on}}/{I}_{\mathrm{off}}$ ratio, but have the advantage of producing all-semiconducting devices.

https://web.stanford.edu/group/dailab/Reprint/135.%20Room-Temperature%20All-Semiconducting%20Sub-10-nm%20Graphene%20Nanoribbon%20Field-Effect%20Transistors.pdf

Room-temperature all-semiconducting sub-10-nm graphene nanoribbon field-effect transistors.

The performance limits of monolayer transition metal dichalcogenide ( MX2) transistors are examined with a ballistic MOSFET model. Using an ab initio theory, we calculate the band structures of 2-D transition MX2. We find the lattice structures of monolayer MX2 remain the same as the bulk MX2. Within the ballistic regime, the performances of monolayer MX2 transistors are better compared with those of the silicon transistors if a thin high-κ gate insulator is used. This makes monolayer MX2 promising 2-D materials for future nanoelectronic device applications.

Performance Limits of Monolayer Transition Metal Dichalcogenide Transistors

The performance limits of monolayer transition metal dichalcogenide transistors are examined with a ballistic MOSFET model. Using ab-initio theory, we calculate the band structures of two-dimensional (2D) transition metal dichalco-genide (MX2). We find the lattice structures of monolayer MX2 remain the same as the bulk MX2. Within the ballistic regime, the performances of monolayer MX2 transistors are better compared to the silicon transistors if thin high-{\kappa} gate insulator is used. This makes monolayer MX2 promising 2D materials for future nanoelectronic device applications.

We investigate the thermoelectric properties of graphene nanoribbons (GNRs) by solving atomistic electron and phonon transport equations in the nonequilibrium Green’s function formalism. The dependence of thermopower on temperature and chemical potential is compared to that of graphene, which shows the important role of quasi-one-dimensional geometry in determining the thermoelectric properties of a GNR. The edge roughness and lattice vacancy are found to increase the thermopower but decrease the thermoelectric ZT factor because the decrease in the electronic conductance outweighs the decrease in the thermal conductance and the increase in the thermopower.

A theoretical study on thermoelectric properties of graphene nanoribbons

We investigated the chlorine plasma reaction with graphene and graphene nanoribbons and compared it with the hydrogen and fluorine plasma reactions. Unlike the rapid destruction of graphene by hydrogen and fluorine plasmas, much slower reaction kinetics between the chlorine plasma and graphene were observed, allowing for controlled chlorination. Electrical measurements on graphene sheets, graphene nanoribbons, and large graphene films grown by chemical vapor deposition showed p-type doping accompanied by a conductance increase, suggesting nondestructive doping via chlorination. Ab initio simulations were performed to rationalize the differences in fluorine, hydrogen, and chlorine functionalization of graphene.

Yijian Ouyang

Papers

Room-temperature all-semiconducting sub-10-nm graphene nanoribbon field-effect transistors.

Performance Limits of Monolayer Transition Metal Dichalcogenide Transistors

Performance Limits of Monolayer Transition Metal Dichalcogenide Transistors

A theoretical study on thermoelectric properties of graphene nanoribbons

Controlled chlorine plasma reaction for noninvasive graphene doping.