One sentence summary:We describe a general liquid-phase method to exfoliate layered compounds to give monoand few-layer flakes in large quantities. TMDs consist of hexagonal layers of metal atoms, M, sandwiched between two layers of chalcogen atoms, X, with stoichiometry MX 2 . While the bonding within these tri-layer sheets is covalent, adjacent sheets stack via van der Waals interactions to form a 3D crystal. TMDs occur in more than 40 different types (2, 3) depending on the combination of chalcogen (S, Se or Te) and transition metal(3). Depending on the co-ordination and oxidation state of the metal atoms, TMDs can be metallic, semi-metallic or semiconducting(2, 3), e.g. WS 2 is a semiconductor while NbSe 2 is a metal(3). In addition, superconductivity(4) and charge density wave effects(5) have been observed in some TMDs. This versatility makes them potentially useful in many areas of electronics.However, like graphene(6), layered materials must be exfoliated to fulfil their full potential. For example, films of exfoliated Bi 2 Te 3 should display enhanced thermoelectric efficiency by suppression of thermal conductivity(7). Exfoliation of 2D topological insulators such as Bi 2 Te 3 and Bi 2 Se 3 would reduce residual bulk conductance, 4 highlighting surface effects. In addition, we can expect changes in electronic properties as the number of layers is reduced e.g. the indirect bandgap of bulk MoS 2 becomes direct in few-layer flakes(8). Although exfoliation can be achieved mechanically on a small scale(9, 10), liquid phase exfoliation methods are required for many applications(11).Critically, a simple liquid exfoliation method would allow the formation of novel hybrid and composite materials. While TMDs can be chemically exfoliated in liquids(12-14), this method is time consuming, extremely sensitive to the environment and incompatible with most solvents.We demonstrate exfoliation of bulk TMD crystals in common solvents to give mono-and few layer nano-sheets. This method is insensitive to air and water and can potentially be scaled up to give large quantities of exfoliated material. In addition, we show that this procedure allows the formation of hybrid films with enhanced properties.We initially sonicated commercial MoS 2 , WS 2 and BN (15, 16) powders in a number of solvents with varying surface tensions. The resultant dispersions were centrifuged and the supernatant decanted (Section S3). Optical absorption spectroscopy showed that the amount of material retained (characterised by / A l C α = , where A/l is the absorbance per length, α is the extinction coefficient and C is the concentration) was maximised for solvents with surface tension close to 40 mJ/m 2 (17, 18) ( Fig. 1A-C). Detailed analysis, within the framework of Hansen solubility parameter theory(19), shows successful solvents to be those with dispersive, polar and H-bonding components of the cohesive energy density within certain well-defined ranges (Section S4, Figs. S2-S3). This can be interpreted to mean that successful solvents are those w...

All-printed transistors consisting of interconnected networks of various types of two-dimensional nanosheets are an important goal in nanoscience. Using electrolytic gating, we demonstrate all-printed, vertically stacked transistors with graphene source, drain, and gate electrodes, a transition metal dichalcogenide channel, and a boron nitride (BN) separator, all formed from nanosheet networks. The BN network contains an ionic liquid within its porous interior that allows electrolytic gating in a solid-like structure. Nanosheet network channels display on:off ratios of up to 600, transconductances exceeding 5 millisiemens, and mobilities of >0.1 square centimeters per volt per second. Unusually, the on-currents scaled with network thickness and volumetric capacitance. In contrast to other devices with comparable mobility, large capacitances, while hindering switching speeds, allow these devices to carry higher currents at relatively low drive voltages.

We demonstrate the controlled incorporation of P dopant atoms in Si (001) presenting a new path toward the creation of atomic-scale electronic devices. We present a detailed study of the interaction of PH3 with Si (001) and show that it is possible to thermally incorporate P atoms into Si (001) below the H desorption temperature. Control over the precise spatial location at which P atoms are incorporated was achieved using STM H-lithography. We demonstrate the positioning of single P atoms in Si with ∼ 1 nm accuracy and the creation of nanometer wide lines of incorporated P atoms.PACS numbers: 03.67. Lx, 68.37.Ef, The ability to control the location of individual dopant atoms within a semiconductor has enormous potential for the creation of atomic-scale electronic devices, including recent proposals for quantum cellular automata [1], single electron transistors [2] and solid-state quantum computers [3]. Current techniques for controlling the spatial extent of dopant atoms in Si rely on either ion implantation techniques, or dopant diffusion through optical or electron-beam patterned mask layers. While the resolution of these techniques continues to improve they have inherent resolution limits as we approach the atomicscale [4]. The work presented here looks beyond conventional techniques to position P dopant atoms with atomic-precision by using scanning tunneling microscopy (STM) based lithography on H passivated Si (001) surfaces [5,6] to control the adsorption and subsequent incorporation of single P dopant atoms into the Si (001) surface.First, we show the controlled adsorption of PH 3 molecules to STM-patterned areas of H-terminated Si (001) surfaces. In these studies, we have used the H-terminated surface as a reference where the intrinsic surface periodicity can be observed to identify both adsorbed PH 3 molecules [7] and the previously unobserved room temperature dissociation product, PH 2 . We then show, using low PH 3 dosed clean Si (001) surfaces, that both of these room temperature adsorbates can be completely dissociated using a critical anneal, and more importantly, that this results in the substitutional incorporation of individual P atoms into the top layer of the substrate. Finally, we combine these two results to demonstrate the spatially controlled incorporation of individual P dopant atoms into the Si (001) surface with atomicscale precision. Of crucial importance to this final result is that the anneal temperature for P atom incorporation lies below the H-desorption temperature, so that the Hresist layer effectively blocks any surface diffusion of P atoms before their incorporation into the substrate surface.Figures 1(a) -1(c) demonstrate the flexibility of STM H-lithography to create different sized regions of bare Si (001) surface. As we will show, these regions can be used not only as a template for dopant incorporation but also to aid in fundamental studies of surface reactions. Figures 1(a) and 1(b) show the creation of both large areas (200 × 30 nm 2 ) and parallel, nanometer-wide lines...

High‐performance sensors based on molybdenum disulfide (MoS2) grown by sulfurization of sputtered molybdenum layers are presented. Using a simple integration scheme, it is found that the electrical conductivity of MoS2 films is highly sensitive to NH3 adsorption, consistent with n‐type semiconducting behavior. A sensitivity of 300 ppb at room temperature is achieved, showing the high potential of 2D transition metal‐dichalcogenides for sensing.

and duesberg@tcd.ie Platinum diselenide (PtSe 2 ) is a newly discovered 2D material which is of great interest for applications in electronics and catalysis. PtSe 2 films were synthesized by thermally-assisted selenization of predeposited platinum films and scanning transmission electron microscopy revealed the crystal structure of these films to be 1T. Raman scattering of these films was studied as a function of film thickness, laser wavelength and laser polarization. E g and A 1gRaman active modes were identified using polarization measurements in the Raman setup.These modes were found to display a clear position and intensity dependence with film thickness, for multiple excitation wavelengths, and their peak positions agree with simulated phonon dispersion curves for PtSe 2 . These results highlight the practicality of using Raman spectroscopy as a prime characterization technique for newly-synthesized 2D materials.

Please cite this article in press as: R. Gatensby, et al., Controlled synthesis of transition metal dichalcogenide thin films for electronic applications, Appl. Surf. Sci. (2014) Two dimensional transition metal dichalcogenides (TMDs) are exciting materials for future applications in nanoelectronics, nanophotonics and sensing. In particular, sulfides and selenides of molybdenum (Mo) and tungsten (W) have attracted interest as they possess a band gap, which is important for integration into electronic device structures. However, the low throughput synthesis of high quality TMD thin films has thus far hindered the development of devices, and so a scalable method is required to fully exploit their exceptional properties. Within this work a facile route to the manufacture of devices from MoS 2 and WS 2 , grown by vapour phase sulfurisation of pre-deposited metal layers, is presented. Highly homogenous TMD films are produced over large areas. Fine control over TMD film thickness, down to a few layers, is achieved by modifying the thickness of the pre-deposited metal layer. The films are characterised by Raman spectroscopy, electron microscopy and X-ray photoelectron spectroscopy. The thinnest films exhibit photoluminescence, as predicted for monolayer MoS 2 films, due to confinement in two dimensions. By using shadow mask lithography, films with well-defined geometries were produced and subsequently integrated with standard microprocessing process flows and electrically characterised. In this way, MoS 2 based sensors were produced, displaying sensitivity to NH 3 down to 400 ppb. Our device manufacture is versatile, and is adaptable for future nanoscale (opto-) electronic devices as it is reproducible, cost effective and scalable up to wafer scale.

We present a complete fabrication process for the creation of robust nano-and atomic-scale devices in silicon using a scanning tunneling microscope (STM). In particular we develop registration markers which, in combination with a custom-designed STM-scanning electron microscope (SEM) system, solve one of the key fabrication problems − connecting the STM-patterned buried phosphorus-doped devices, fabricated in the ultrahigh vacuum environment, to the outside world. The first devices demonstrate the feasibility of this technology and confirm the presence of quantum confinement in devices as electron propagation is laterally constricted by STM patterning.

For many applications, in particular in electronics, where chemical vapor deposited (CVD) graphene is used, it needs to be transferred from the growth substrate to the device substrate, generally employing a polymer film as a support layer. This process step is crucial for the overall integrity and electronic performance of the graphene. In this work we will investigate the effects of the transfer using various polymers with atomic force, Raman spectroscopy and X‐ray photoelectron spectroscopy combined with field dependent transport measurements to build up a complete picture regarding the morphology, structural integrity and electrical performance of CVD graphene. Further, we introduce nitrocellulose based polymer alternatives to the most commonly used poly(methyl methacrylate) resist, outperforming the latter in most respects.