Michal J. Mleczko

Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy

Molybdenum disulfide (MoS2), with its active edge sites, is a proposed alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Recently, the inert basal plane of MoS2 was successfully activated and optimized with excellent intrinsic HER activity by creating and further straining sulfur (S) vacancies. Nevertheless, little is known about the HER kinetics of those S vacancies and the additional effects from elastic tensile strain. Herein, scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies (formal potential Ev0 = −0.53 VAg/AgCl, electron-transfer coefficient αv = 0.4, electron-transfer rate constant kv0 = 2.3 × 10(–4) cm/s) and strained S vacancies (Esv0= −0.53 VAg/AgCl, αsv = 0.4, ksv0 = 1.0 × 10(–3) cm/s) on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the unstrained S vacancy. This study provides a general platform for measuring the kinetics of two-dimensional material-based catalysts.

High Current Density and Low Thermal Conductivity of Atomically Thin Semimetallic WTe2

Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomically thin limit. Here, we introduce a facile growth mechanism for semimetallic WTe2 crystals and then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 μm long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe2 devices studied to date (3-20 layers thick). High-field electrical measurements and electrothermal modeling demonstrate that ultrathin WTe2 can carry remarkably high current density (approaching 50 MA/cm(2), higher than most common interconnect metals) despite a very low thermal conductivity (of the order ∼3 Wm(-1) K(-1)). These results suggest several pathways for air-stable technological viability of this layered semimetal.

HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides

Atomically thin two-dimensional semiconductors feature silicon-like band gaps and native high-κ metal oxides. The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO2 as a high-quality “native” insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable “high-κ” native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio > 106; on current, ~30 μA/μm), with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high-κ dielectrics, and scaling benefits from their atomically thin nature.

Cross plane thermal conductance of graphene-metal interfaces

We present an experimental study of cross-plane thermal conductance in metal-graphene-oxide stacks employing time-domain thermoreflectance (TDTR) measurements on Al/m/graphene/SiO2 structures with monolayer graphene for varying adhesion metals m. Thermal conductance G in the range of 15-60 MWm-2K-1 are found across several metals - a two-to-fourfold decrease over Al/m/SiO2 reference values, charted against metal Debye temperature over a 274-630 K range, as well as electronic work function. The results of this study help with a better understanding of the roles of electron and phonon transport in thermal conduction across graphene-metal interfaces, revealing potential trade-offs between electrical contact resistance and heat management in graphene devices.

Probing the Optical Properties and Strain-Tuning of Ultrathin Mo1–xWxTe2

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe2 and Mo0.91W0.09Te2 extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.

Unipolar n-Type Black Phosphorus Transistors with Low Work Function Contacts

Black phosphorus (BP) is a promising two-dimensional (2D) material for nanoscale transistors, due to its expected higher mobility than other 2D semiconductors. While most studies have reported ambipolar BP with a stronger p-type transport, it is important to fabricate both unipolar p- and n-type transistors for low-power digital circuits. Here, we report unipolar n-type BP transistors with low work function Sc and Er contacts, demonstrating a record high n-type current of 200 μA/μm in 6.5 nm thick BP. Intriguingly, the electrical transport of the as-fabricated, capped devices changes from ambipolar to n-type unipolar behavior after a month at room temperature. Transmission electron microscopy analysis of the contact cross-section reveals an intermixing layer consisting of partly oxidized metal at the interface. This intermixing layer results in a low n-type Schottky barrier between Sc and BP, leading to the unipolar behavior of the BP transistor. This unipolar transport with a suppressed p-type current is favorable for digital logic circuits to ensure a lower off-power consumption.

Electronic, thermal, and unconventional applications of 2D materials

This invited talk will present recent highlights from our research on two-dimensional (2D) materials including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The results span from fundamental measurements and simulations, to device- and several unusual system-oriented applications which take advantage of unique 2D material properties. Basic electrical, thermal, and thermoelectric properties of 2D materials will also be discussed.

Electrons, phonons, and unconventional applications of 2D materials

This invited talk will present recent highlights from our research on two-dimensional (2D) materials including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The results span from fundamental measurements and simulations, to device- and several unusual system-oriented applications which take advantage of unique 2D material properties. Basic electrical, thermal, and thermoelectric properties of 2D materials will also be discussed.

Thermal limitations of two-dimensional semi-metallic WTe 2 devices

Layered semi-metallic materials other than graphene have not received much attention. Here, we examine electrical and thermal transport in WTe2 devices by experiments, analytical and finite element simulations. We uncover that few-layer WTe2 has remarkably high current density but ultra-low thermal conductivity, suggesting potential use as phase-change memory electrodes.

WTe 2 as a two-dimensional (2D) metallic contact for 2D semiconductors

In this paper, WSe₂ FET contacted with thin metallic WTe₂ is demonstrated, at channel lengths down to 90 nm and current saturation up to 60 μA/μm. A temperature-dependent study suggests the presence of an intrinsic vdWg at the 2D-2D contact. Thus, ultra-thin WTe₂ could be preferred to graphene as a contact depinning layer.