Electron transport in bilayer graphene nano constrictions patterned using AFM nanolithography.

Here we report on low temperature transport measurements of encapsulated bilayer graphene nano constrictions fabricated employing AFM-based local anodic oxidation (LAO) nanolithography. This technique allows for the creation of constrictions as narrow as 20 nm much smaller than previous studies. In wider constrictions, we observe bulk transport characteristics.

Distinct Contact Scaling Effects in MoS Transistors Revealed with Asymmetrical Contact Measurements.

2D semiconducting materials have immense potential for future electronics due to their atomically thin nature, which enables better scalability. While the channel scalability of 2D materials has been extensively studied, the current understanding of contact scaling in 2D devices is inconsistent and oversimplified. Here physically scaled contacts and asymmetrical contact measurements (ACMs) are combined to investigate the contact scaling behavior in 2D field-effect transistors.

Are 2D Interfaces Really Flat?

Two-dimensional (2D) van der Waals materials are subject to mechanical deformation and thus forming bubbles and wrinkles during exfoliation and transfer. A lack of interfacial "flatness" has implications for interface properties, such as those formed by metal contacts or insulating layers. Therefore, an understanding of the detailed properties of 2D interfaces, especially their flatness under different conditions, is of high importance.

Optimal field-effect transistor operation for high-resolution biochemical measurements.

Field-effect transistors (FETs) are powerful tools for sensitive measurements of numerous biomarkers (e.g., proteins, nucleic acids, and antigen) and gaseous species.

Substrate-mediated hyperbolic phonon polaritons in MoO.

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent lattice vibrations that exist in strongly optically anisotropic media, including two-dimensional materials (e.g., MoO).

Rapid, quantitative therapeutic screening for Alzheimer's enzymes enabled by optimal signal transduction with transistors.

We show that commercially sourced n-channel silicon field-effect transistors (nFETs) operating above their threshold voltage with closed loop feedback to maintain a constant channel current allow a pH readout resolution of (7.2 ± 0.3) × 10 at a bandwidth of 10 Hz, or ≈3-fold better than the open loop operation commonly employed by integrated ion-sensitive field-effect transistors (ISFETs).

Quantum capacitance-limited MoS biosensors enable remote label-free enzyme measurements.

We have demonstrated atomically thin, quantum capacitance-limited, field-effect transistors (FETs) that enable the detection of pH changes with 75-fold higher sensitivity (≈4.4 V per pH) over the Nernst value of 59 mV per pH at room temperature when used as a biosensor. The transistors, which are fabricated from monolayer films of MoS, use a room temperature ionic liquid (RTIL) in place of a conventional oxide gate dielectric and exhibit very low intrinsic noise resulting in a pH resolution of 92 × 10 at 10 Hz.

Reproducible Performance Improvements to Monolayer MoS Transistors through Exposed Material Forming Gas Annealing.

Metal-mediated exfoliation has been demonstrated as a promising approach for obtaining large-area flakes of two-dimensional (2D) materials to fabricate prototypical nanoelectronics. However, several processing challenges related to organic contamination at the interface of a 2D material and gate oxide must be overcome to realize robust devices with high yields. Here, we demonstrate an optimized process to realize high-performance field-effect transistor (FET) arrays from large-area (∼5000 μm), monolayer MoS with a yield of 85%.

Improved contacts to p-type MoS transistors by charge-transfer doping and contact engineering.

MoS is known to show stubborn n-type behavior due to its intrinsic band structure and Fermi level pinning. Here, we investigate the combined effects of molecular doping and contact engineering on the transport and contact properties of monolayer (ML) MoS devices. Significant p-type (hole-transport) behavior was only observed for chemically doped MoS devices with high work function palladium (Pd) contacts, while MoS devices with low work function metal contacts made from titanium showed ambipolar behavior with electron transport favored even after prolonged p-doping treatment.

Contact and Noncontact Measurement of Electronic Transport in Individual 2D SnS Colloidal Semiconductor Nanocrystals.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made toward the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products' inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements.

Controllable, Wide-Ranging n-Doping and p-Doping of Monolayer Group 6 Transition-Metal Disulfides and Diselenides.

Developing processes to controllably dope transition-metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS , WS , MoSe , and WSe . Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n-dopant) and "Magic Blue," [N(C H -p-Br) ]SbCl , as the oxidant (p-dopant).

Quantifying atom-scale dopant movement and electrical activation in Si:P monolayers.

Advanced hydrogen lithography techniques and low-temperature epitaxial overgrowth enable the patterning of highly phosphorus-doped silicon (Si:P) monolayers (ML) with atomic precision. This approach to device fabrication has made Si:P monolayer systems a testbed for multiqubit quantum computing architectures and atomically precise 2-D superlattice designs whose behaviors are directly tied to the deterministic placement of single dopants. However, dopant segregation, diffusion, surface roughening, and defect formation during the encapsulation overgrowth introduce large uncertainties to the exact dopant placement and activation ratio.

The influence of isomer purity on trap states and performance of organic thin-film transistors.

Organic field-effect transistor (OFET) performance is dictated by its composition and geometry, as well as the quality of the organic semiconductor (OSC) film, which strongly depends on purity and microstructure. When present, impurities and defects give rise to trap states in the bandgap of the OSC, lowering device performance. Here, 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene is used as a model system to study the mechanism responsible for performance degradation in OFETs due to isomer coexistence.

Contacts to solution-synthesized SnS nanoribbons: dependence of barrier height on metal work function.

Tin(ii) monosulfide (SnS) is a layered, anisotropic material that is of interest as a two-dimensional semiconductor for opto-electronic, thermoelectric, and piezoelectric applications. In this study, the effect of work function on contact behavior was investigated. Ni/Au, Pd/Au, Cr/Au, and Ti/Au contacts were fabricated onto individual, solution-synthesized, p-type SnS nanoribbons.

Electrical Stabilization of Surface Resistivity in Epitaxial Graphene Systems by Amorphous Boron Nitride Encapsulation.

Homogeneous monolayer epitaxial graphene (EG) is an ideal candidate for the development of millimeter-sized devices with single-crystal domains. A clean fabrication process was used to produce EG-based devices, with n-type doping level of the order of 10 cm. Generally, electrical properties of EG, such as longitudinal resistivity, remain unstable when devices are exposed to air due to adsorption of molecular dopants, whose presence shifts the carrier density close to the Dirac point (<10 cm) or into the p-type regime.

Interacting nanoscale magnetic superatom cluster arrays in molybdenum oxide bronzes.

In this study, we examine several reduced ternary molybdates in the family of yellow rare earth molybdenum bronzes produced by electrochemical synthesis with composition LnMoO. These compounds contain an array of electrically isolated but magnetically interacting multi-atom clusters with composition MoO. These arrayed superatom clusters support a single hole shared among the eight molybdenum atoms in the unit, corresponding to a net spin moment of 1μ, and exhibit magnetic exchange between the units via the MoO tetrahedra (containing Mo ions) and the LnO cubes (containing Ln ions).

Preservation of Surface Conductivity and Dielectric Loss Tangent in Large-Scale, Encapsulated Epitaxial Graphene Measured by Noncontact Microwave Cavity Perturbations.

Regarding the improvement of current quantized Hall resistance (QHR) standards, one promising avenue is the growth of homogeneous monolayer epitaxial graphene (EG). A clean and simple process is used to produce large, precise areas of EG. Properties like the surface conductivity and dielectric loss tangent remain unstable when EG is exposed to air due to doping from molecular adsorption.

Organic Spin-Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering.

Since the first observation of the spin-valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin-polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin-flip mechanisms in organic semiconductors and the role of hybrid metal-organic interfaces in spin injection.

Fe-catalyzed etching of exfoliated graphite through carbon hydrogenation.

We present an investigation on Fe-catalyzed etching of graphite by dewetting Fe thin films on graphite in forming gas. Raman mapping of the etched graphite shows thickness variation in the etched channels and reveals that the edges are predominately terminated in zigzag configuration. X-ray diffraction and photoelectron spectroscopy measurements identify that the catalytic particles are Fe with the presence of iron carbide and iron oxides.

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired.