Thermal transport in MoS2 from molecular dynamics using different empirical potentials

Thermal properties of molybdenum disulfide (MoS2) have recently attracted attention related to fundamentals of heat propagation in strongly anisotropic materials, and in the context of potential applications to optoelectronics and thermoelectrics. Multiple empirical potentials have been developed for classical molecular dynamics (MD) simulations of this material, but it has been unclear which provides the most realistic results. Here, we calculate lattice thermal conductivity of single- and multilayer pristine MoS2 by employing three different thermal transport MD methods: equilibrium, nonequilibrium, and homogeneous nonequilibrium ones. We mainly use the Graphics Processing Units Molecular Dynamics code for numerical calculations, and the Large-scale Atomic/Molecular Massively Parallel Simulator code for crosschecks. Using different methods and computer codes allows us to verify the consistency of our results and facilitate comparisons with previous studies, where different schemes have been adopted. Our results using variants of the Stillinger-Weber potential are at odds with some previous ones and we analyze the possible origins of the discrepancies in detail. We show that, among the potentials considered here, the reactive empirical bond order (REBO) potential gives the most reasonable predictions of thermal transport properties as compared to experimental data. With the REBO potential, we further find that isotope scattering has only a small effect on thermal conduction in MoS2 and the in-plane thermal conductivity decreases with increasing layer number and saturates beyond about three layers. We identify the REBO potential as a transferable empirical potential for MD simulations of MoS2 which can be used to study thermal transport properties in more complicated situations such as in systems containing defects or engineered nanoscale features. This work establishes a firm foundation for understanding heat transport properties of MoS2 using MD simulations

Resistive Random Access Memory Enabled by Carbon Nanotube Crossbar Electrodes

We use single-walled carbon nanotube (CNT) crossbar electrodes to probe sub-5 nm memory domains of thin AlO_<i>x</i> films. Both metallic and semiconducting CNTs effectively switch AlO_<i>x</i> bits between memory states with high and low resistance. The low-resistance state scales linearly with CNT series resistance down to ∼10 MΩ, at which point the ON-state resistance of the AlO_<i>x</i> filament becomes the limiting factor. Dependence of switching behavior on the number of cross-points suggests a single channel to dominate the overall characteristics in multi-crossbar devices. We demonstrate ON/OFF ratios up to 5 × 10⁵ and programming currents of 1 to 100 nA with few-volt set/reset voltages. Remarkably low reset currents enable a switching power of 10–100 nW and estimated switching energy as low as 0.1–10 fJ per bit. These results are essential for understanding the ultimate scaling limits of resistive random access memory at single-nanometer bit dimensions

Spectral decomposition of thermal conductivity: Comparing velocity decomposition methods in homogeneous molecular dynamics simulations

The design of applications, especially those based on heterogeneous integration, must rely on detailed knowledge of material properties, such as thermal conductivity (TC). To this end, multiple methods have been developed to study TC as a function of vibrational frequency. Here, we compare three spectral TC methods based on velocity decomposition in homogenous molecular dynamics simulations: Green-Kubo modal analysis (GKMA), the spectral heat current (SHC) method, and a method we propose called homogeneous nonequilibrium modal analysis (HNEMA). First, we derive a convenient per-atom virial expression for systems described by general many-body potentials, enabling compact representations of the heat current, each velocity decomposition method, and other related quantities. Next, we evaluate each method by calculating the spectral TC for carbon nanotubes, graphene, and silicon. We show that each method qualitatively agrees except at optical phonon frequencies, where a combination of mismatched eigenvectors and a large density of states produces artificial TC peaks for modal analysis (MA) methods. Our calculations also show that the HNEMA and SHC methods converge much faster than the GKMA method, with the SHC method being the most computationally efficient. Finally, we demonstrate that our MA implementation in the Graphics Processing Units Molecular Dynamics code on a single graphics processing unit is over 1000 times faster than the existing implementation in the Large-scale Atomic/Molecular Massively Parallel Simulator code on one central processing unit

High-Velocity Saturation in Graphene Encapsulated by Hexagonal Boron Nitride

We measure drift velocity in monolayer graphene encapsulated by hexagonal boron nitride (hBN), probing its dependence on carrier density and temperature. Due to the high mobility (>5 × 10⁴ cm²/V/s) of our samples, the drift velocity begins to saturate at low electric fields (∼0.1 V/μm) at room temperature. Comparing results to a canonical drift velocity model, we extract room-temperature electron saturation velocities ranging from 6 × 10⁷ cm/s at a low carrier density of 8 × 10¹¹ cm^–2 to 2.7 × 10⁷ cm/s at a higher density of 4.4 × 10¹² cm^–2. Such drift velocities are much higher than those in silicon (∼10⁷ cm/s) and in graphene on SiO₂, likely due to reduced carrier scattering with surface optical phonons whose energy in hBN (>100 meV) is higher than that in other substrates

High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS₂

Two-dimensional semiconductors such as monolayer MoS₂ are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, <i>v</i>_sat. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS₂. We find that in typical device geometries (e.g. on SiO₂ substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron <i>v</i>_sat = (3.4 ± 0.4) × 10⁶ cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS₂ could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal management

Improved Contacts to MoS₂ Transistors by Ultra-High Vacuum Metal Deposition

The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (<i>R</i>_C). Here we present a systematic study of scaling MoS₂ devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (10¹² to 10¹³ cm^–2), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (∼10^–9 Torr) yields three times lower <i>R</i>_C than under normal conditions, reaching 740 Ω·μm and specific contact resistivity 3 × 10^–7 Ω·cm², stable for over four months. Modeling reveals separate <i>R</i>_C contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS₂ contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the “14 nm” technology node

High-Field Electrical and Thermal Transport in Suspended Graphene

We study the intrinsic transport properties of suspended graphene devices at high fields (≥1 V/μm) and high temperatures (≥1000 K). Across 15 samples, we find peak (average) saturation velocity of 3.6 × 10⁷ cm/s (1.7 × 10⁷ cm/s) and peak (average) thermal conductivity of 530 W m^–1 K^–1 (310 W m^–1 K^–1) at 1000 K. The saturation velocity is 2–4 times and the thermal conductivity 10–17 times greater than in silicon at such elevated temperatures. However, the thermal conductivity shows a steeper decrease at high temperature than in graphite, consistent with stronger effects of second-order three-phonon scattering. Our analysis of sample-to-sample variation suggests the behavior of “cleaner” devices most closely approaches the intrinsic high-field properties of graphene. This study reveals key features of charge and heat flow in graphene up to device breakdown at ∼2230 K in vacuum, highlighting remaining unknowns under extreme operating conditions

Self-Aligned Cu Etch Mask for Individually Addressable Metallic and Semiconducting Carbon Nanotubes

Two means to achieve high yield of individually addressable single-walled carbon nanotubes (CNTs) are developed and examined. The first approach matches the effective channel width and the density of horizontally aligned CNTs. This method can provide single CNT devices and also allows control over the average number of CNTs per channel. The second and a more deterministic approach uses self-aligned Cu-filled trenches formed in a photoresist (after Joule heating of the underlying CNT) to protect and obtain a large number of single CNT devices. Unlike electrical breakdown methods, which preserve the least conducting CNT and can leave behind CNT fragments, our approach allows the selection of the single most conducting metallic CNT from an array of as-grown CNTs with average resistance ∼14 times lower than that of as-fabricated single metallic CNTs. This method can also be used to select the best semiconducting CNT from an array and yields, on average, devices that are 15 times more conductive with 40 times higher ON/OFF ratio than those selected through electrical breakdown alone

Scanning Tunneling Microscopy Study and Nanomanipulation of Graphene-Coated Water on Mica

We study interfacial water trapped between a sheet of graphene and a muscovite (mica) surface using Raman spectroscopy and ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at room temperature. We are able to image the graphene–water interface with atomic resolution, revealing a layered network of water trapped underneath the graphene. We identify water layer numbers with a carbon nanotube height reference. Under normal scanning conditions, the water structures remain stable. However, at greater electron energies, we are able to locally manipulate the water using the STM tip

Li Intercalation in MoS₂: In Situ Observation of Its Dynamics and Tuning Optical and Electrical Properties

Two-dimensional layered materials like MoS₂ have shown promise for nanoelectronics and energy storage, both as monolayers and as bulk van der Waals crystals with tunable properties. Here we present a platform to tune the physical and chemical properties of nanoscale MoS₂ by electrochemically inserting a foreign species (Li⁺ ions) into their interlayer spacing. We discover substantial enhancement of light transmission (up to 90% in 4 nm thick lithiated MoS₂) and electrical conductivity (more than 200×) in ultrathin (∼2–50 nm) MoS₂ nanosheets after Li intercalation due to changes in band structure that reduce absorption upon intercalation and the injection of large amounts of free carriers. We also capture the first in situ optical observations of Li intercalation in MoS₂ nanosheets, shedding light on the dynamics of the intercalation process and the associated spatial inhomogeneity and cycling-induced structural defects