Meeghage Madusanka Perera

Improved carrier mobility in few-layer MoS2 field-effect transistors with ionic-liquid gating

We report the fabrication of ionic liquid (IL)-gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS2. Our transport measurements indicate that the electron mobility μ ≈ 60 cm(2) V(-1) s(-1) at 250 K in IL-gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from ≈ 100 cm(2) V(-1) s(-1) at 180 K to ≈ 220 cm(2) V(-1) s(-1) at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio >10(7) (10(4)) for electrons (holes), and a near ideal subthreshold swing of ≈ 50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin IL dielectric layer.

Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors.

We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ∼0.3 kΩ μm, high on/off ratios up to >10(9), and high drive currents exceeding 320 μA μm(-1). These favorable characteristics are combined with a two-terminal field-effect hole mobility μFE ≈ 2 × 10(2) cm(2) V(-1) s(-1) at room temperature, which increases to >2 × 10(3) cm(2) V(-1) s(-1) at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in postsilicon electronics.

Mobility Improvement and Temperature Dependence in MoSe2 Field-Effect Transistors on Parylene-C Substrate

We report low-temperature scanning tunneling microscopy characterization of MoSe2 crystals and the fabrication and electrical characterization of MoSe2 field-effect transistors on both SiO2 and parylene-C substrates. We find that the multilayer MoSe2 devices on parylene-C show a room-temperature mobility close to the mobility of bulk MoSe2 (100-160 cm(2) V(-1) s(-1)), which is significantly higher than that on SiO2 substrates (≈50 cm(2) V(-1) s(-1)). The room-temperature mobility on both types of substrates are nearly thickness-independent. Our variable-temperature transport measurements reveal a metal-insulator transition at a characteristic conductivity of e(2)/h. The mobility of MoSe2 devices extracted from the metallic region on both SiO2 and parylene-C increases up to ≈500 cm(2) V(-1) s(-1) as the temperature decreases to ≈100 K, with the mobility of MoSe2 on SiO2 increasing more rapidly. In spite of the notable variation of charged impurities as indicated by the strongly sample-dependent low-temperature mobility, the mobility of all MoSe2 devices on SiO2 converges above 200 K, indicating that the high temperature (>200 K) mobility in these devices is nearly independent of the charged impurities. Our atomic force microscopy study of SiO2 and parylene-C substrates further rules out the surface roughness scattering as a major cause of the substrate-dependent mobility. We attribute the observed substrate dependence of MoSe2 mobility primarily to the surface polar optical phonon scattering originating from the SiO2 substrate, which is nearly absent in MoSe2 devices on parylene-C substrate.

Rectification in Nanoscale Devices Based on an Asymmetric Five‐Coordinate Iron(III) Phenolate Complex

Rectification consists of an asymmetric flow of electric current. In a macroscopic electrical circuitry, rectifiers, such as vacuum tubes or solid-state diodes, control the mobility of current, enabling it to flow in one direction and preventing reversibility. This directionality is fundamental to the conversion of alternating into direct current. Molecular rectification, which was proposed in the celebrated Aviram–Ratner ansatz, anticipates the feasibility of a current flow in one direction that takes place in an electrode jmolecule j electrode junction. Central to this paradigm is the existence of asymmetric molecules that incorporate electron-donor and electron-acceptor moieties, [DA], with an excited state [DA ] of higher, but accessible, energy. Usually, donor and acceptor are separated by a sor p-bridge to decrease electronic coupling, and if the requirements are fulfilled, rectification occurs with contributions from Schottky, asymmetric, and/or unimolecular mechanisms. Schottky rectification is based on interfacial dipoles from electrode contact or on covalent bonding between the molecule and the electrode. Asymmetric and unimolecular mechanisms rely on the use of frontier molecular orbitals of the molecule; whereas the former relies on an asymmetric placement of the HOMO or the LUMO in the electrode jmolecule j electrode assembly, the latter is based on small HOMO–LUMO gaps that allow for through-molecule current flow. Although experimental distinction between asymmetric and unimolecular contributions can be ambiguous, there is consensus that electroactive molecules with local low symmetry constitute good candidates for this enterprise, and welldocumented cases of molecular rectification heavily rely on the formation of high-quality Langmuir–Blodgett (LB) films. Although it has been shown that self-assembled monolayers of polypyridine–cobalt(II) complexes in octahedral environments can act as single-electron transistors and induce increased resistance (Coulomb blockade) at cryogenic temperatures, the incorporation of transition-metal complexes into electrode jmolecule junctions has generally employed symmetric molecules and has been rather slow in development. Examples involve assemblies based on metalloporphyrins, terpyridine–ruthenium(II) complexes, as well as trivalent cobalt and rhodium azo-containing species in octahedral environments that are capable of the symmetric conductance that is relevant for memory-switching devices. An example of rectification based on an octahedral bipyridine/acac–ruthenium(II) system has been reported (acac = acetylacetonate), but the effect of lowering global symmetries around the metal center is yet to be tested. Our group is engaged in an effort to integrate bioinspired asymmetry principles into new molecular materials, with the aim of developing redox-responsive metallosurfactants with topologies that display unique structural, spectroscopic, and surface patterning behavior. We recently reported on the redox and electronic behavior of five-coordinate complexes where the iron(III) ion is bound to low-symmetry, phenolaterich, [N2O3] environments, and it was shown that geometric and electronic constraints determine the sequence by which the metal and each of the phenolate substituents gets oxidized. Herein, a new iron(III) species [FeL] (1; Scheme 1) is reported. This species shows marked local asymmetries and is based on a newly synthesized amphiphilic and redox-active [N2O3] ligand. The complex takes advantage of the presence of the phenylenediamino–metal and phenyl/ phenolate moieties that can act as electron-acceptors and electron-donors, respectively. The viability of 1 as a precursor

The Mechanisms of Rectification in Au|Molecule|Au Devices Based on Langmuir–Blodgett Monolayers of Iron(III) and Copper(II) Surfactants†

Langmuir-Blodgett films of metallosurfactants were used in Au|molecule|Au devices to investigate the mechanisms of current rectification.