Nonnegative magnetoresistance in hydrodynamic regime of electron fluid transport in two-dimensional materials

Two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have attracted increased interest due to their potential applications in electronics and optoelectronics. Thermal transport in two-dimensional materials could be quite different from three-dimensional bulk materials. This article reviews the progress on experimental measurements and theoretical modeling of phonon transport and thermal conductivity in two-dimensional materials. We focus our review on a few typical two-dimensional materials, including graphene, boron nitride, silicene, transition metal dichalcogenides, and black phosphorus. The effects of different physical factors, such as sample size, strain and defects, on thermal transport in Two-dimensional materials are summarized. We also discuss the environmental effect on the thermal transport of two-dimensional materials, such as substrate and when two-dimensional materials are presented in heterostructures and intercalated with inorganic components or organic molecules.

Heat vortices of ballistic and hydrodynamic phonon transport in two-dimensional materials

Thermal transport in two-dimensional (2D) materials has attracted great attention since the discovery of high thermal conductivity in graphene, which is closely related to the hydrodynamic phonon transport. In this Perspective, we briefly summarize the recent progresses in studying hydrodynamic phonon transport in 2D materials, including both theoretical and experimental works. First, the criterion and numerical methods for studying hydrodynamic phonon transport are reviewed. We then discuss the physical mechanism and peculiar phenomena related to hydrodynamic phonon transport in 2D materials and finally present the challenge for future studies. This Perspective aims to provide the physical understanding of the hydrodynamic phonon transport, which might be beneficial to the exploration of novel thermal transport behaviors in 2D materials.

The single mode relaxation time approximation has been demonstrated to greatly underestimate the lattice thermal conductivity of two-dimensional materials due to the collective effect of phonon normal scattering. Callaway's dual relaxation model represents a good approximation to the otherwise ab initio solution of the phonon Boltzmann equation. In this work we develop a discrete-ordinate-method (DOM) scheme for the numerical solution of the phonon Boltzmann equation under Callaway's model. Heat transport in a graphene ribbon with different geometries is modeled by our scheme, which produces results quite consistent with the available molecular dynamics, Monte Carlo simulations, and experimental measurements. Callaway's lattice thermal conductivity model with empirical boundary scattering rates is examined and shown to overestimate or underestimate the direct DOM solution. The length convergence of the lattice thermal conductivity of a rectangular graphene ribbon is explored and found to depend appreciably on the ribbon width, with a semiquantitative correlation provided between the convergence length and the width. Finally, we predict the existence of a phonon Knudsen minimum in a graphene ribbon only at a low system temperature and isotope concentration so that the average normal scattering rate is two orders of magnitude stronger than the intrinsic resistive one. The present work will promote not only the methodology for the solution of the phonon Boltzmann equation but also the theoretical modeling and experimental detection of hydrodynamic phonon transport in two-dimensional materials.

Strong light-matter interaction of functional materials is emerging as a promising area of research. Recent experiments suggest that material properties like charge transport can be controlled by coupling to a vacuum electromagnetic field. Here, we explored the design of a Fabry-Perot cavity in a field-effect transistor configuration and studied the charge transport in two-dimensional materials. The optical and electrical measurements of strongly coupled WS2 suggest an enhancement of electron transport at room temperature. Electron mobility is enhanced more than 50 times at ON resonance conditions. Similarly, Ion/Ioff ratio of the device increased by 2 orders of magnitude without chemical modification of the active layer. Cavity tuning and coupling strength-dependent studies support the evidence of modifying the electronic properties of the coupled system. A clear correlation in the effective mass of the polaritonic state and Schottky barrier height indicates a collective nature of light-matter interaction.

Two-dimensional (2D) materials have captured the attention of the scientific community due to the wide range of unique properties at nanometer-scale thicknesses. While significant exploratory research in 2D materials has been achieved, the understanding of 2D electronic transport and carrier dynamics remains in a nascent stage. Furthermore, because prior review articles have provided general overviews of 2D materials or specifically focused on charge transport in graphene, here we instead highlight charge transport mechanisms in post-graphene 2D materials, with particular emphasis on transition metal dichalcogenides and black phosphorus. For these systems, we delineate the intricacies of electronic transport, including band structure control with thickness and external fields, valley polarization, scattering mechanisms, electrical contacts, and doping. In addition, electronic interactions between 2D materials are considered in the form of van der Waals heterojunctions and composite films. This review concludes with a perspective on the most promising future directions in this fast-evolving field.

We discuss some basic physical properties of electron transport in two-dimensional materials. First, we discuss how the predicted thermodynamic instability of 2D crystals may influence charge transport via the coupling of electrons with acoustic flexural modes. We then review the properties of suspended and supported graphene and its ribbons and consider the problem of evaluating correctly the electron-phonon coupling in the case of phosphorene. Finally, we discuss the main features of 2D topological insulators and their possible use in transistors.

Novel two-dimensional (2D) materials show unusual physical properties which combined with strain engineering open up the possibility of new potential device applications in nanoelectronics. In particular, transport properties have been found to be very sensitive to applied strain. In the present work, using a density-functional based tight-binding (DFTB) method in combination with Green's function (GF) approaches, we address the effect of strain engineering of the transport setup (contact-device(scattering)-contact regions) on the electron and phonon transport properties of two-dimensional materials, focusing on hexagonal boron-nitride (hBN), phosphorene, and MoS2 monolayers. Considering unstretched contact regions, we show that the electronic bandgap displays an anomalous behavior and the thermal conductance continuously decreases after increasing the strain level in the scattering region. However, when the whole system (contact and device regions) is homogeneously strained, the bandgap for hBN and MoS2 monolayers decreases, while for phosphorene it first increases and then tends to zero with larger strain levels. Additionally, the thermal conductance shows specific strain dependence for each of the studied 2D materials. These effects can be tuned by modifying the strain level in the stretched contact regions.

Today, we live in a hi-tech world filled with electronic gadgets whose building blocks are field-effect transistors (FETs). FETs are made of semiconductors which are mostly silicon based. Over the past half-century, semiconductor industry has continually scaled down the semiconductor in FETs to make our electronic gadgets faster and smaller. However, we are nearing the physical scaling limit down to the size of individual atoms at which the semiconductor becomes unstable. To further continue scaling and increase performance of FETs, we need to explore new channel materials or new device concepts alternate to FETs, or a combination of these both. As part of my doctoral research, I have explored both the approaches: showcasing FETs of two-dimensional (2D) materials, and presenting an alternate device concept harnessing the spin property of electron. I report in my dissertation, for the first time, a FET fabricated of germanane – a new 2D semiconductor. Experimental results reveal unique electrical and optical properties for germanane with great potential for optoelectronics applications. To integrate the spin property of electron in the realm of FETs, I investigated heterostructures of 2D materials. In a heterostructure of WSe¬2 on single-layer graphene, the spins travelling in graphene along the in-plane direction were tuneable by applying an external electric field. And, in a heterostructure of WS2 and bi-layer graphene, the spins travelling in graphene along the in-plane and the out-of-plane directions showed different spin lifetimes. Both these observations are relevant in realising next generation of FETs, called Spin-FETs, to compute binary logic.

Thermal transport in graphene

The approach of solving the Boltzmann transport equation (BTE) is widely used to evaluate the thermal conductivity and screen low thermal conductivity materials for thermoelectric applications, where phonon transport is approximated as particlelike propagation. Phonon transport through a wavelike tunneling channel, as described by the Wigner transport equation (WTE), will have a notable effect in some two-dimensional (2D) materials due to the parabolic out-of-plane acoustic modes and lower phonon energy, which is usually neglected, inducing an underestimation of thermal conductivity. Here, we investigate the phonon transport of four representative 2D structures by the WTE approach. Both the low-symmetry unit cell with heavy atoms and the strong anharmonicity will lead to a higher contribution from the tunneling channel. The total lattice thermal conductivity of low-symmetry KAgSe is only 0.34 ${\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 800 K, of which 26% is contributed by the wavelike tunneling. The strong lattice anharmonicity of 2D InSe with lone-pair electrons induces wide phonon linewidths for both acoustic and optical phonon modes, suppressing the conductivity through particlelike propagation channel. The coherence conductivity through wavelike tunneling accounts for 58% of the total one at 800 K. Our work helps to gain a better understanding of the dual-channel phonon transport in complex 2D structures. The strong anharmonic acoustic modes are crucial to achieve ultralow lattice thermal conductivity.

Thermoelectric effects allow the generation of electrical power from waste heat and the electrical control of cooling and heating. Remarkably, these effects are also highly sensitive to the asymmetry in the density of states around the Fermi energy and can therefore be exploited as probes of distortions in the electronic structure at the nanoscale. Here we consider two-dimensional graphene as an excellent nanoscale carbon material for exploring the interaction between electronic and thermal transport phenomena, by presenting a direct and quantitative measurement of the Peltier component to electronic cooling and heating in graphene. Thanks to an architecture including nanoscale thermometers, we detected Peltier component modulation of up to 15 mK for currents of 20 μA at room temperature and observed a full reversal between Peltier cooling and heating for electron and hole regimes. This fundamental thermodynamic property is a complementary tool for the study of nanoscale thermoelectric transport in two-dimensional materials.

Given the present interest of the VLSI industry on two-dimensional materials, theoretical studies of their charge-transport properties may help us to identify the most promising materials and device structures. Here we focus on three main problems that we must face when undertaking this theoretical task: 1. How to assess critically the accuracy of results obtained using ab initio methods. 2. How to account for the effect that the dielectric environment (substrate, gate insulator, metallic gates) may have on the carrier transport. 3. How to deal with quantum-mechanical effects that may control transport in nanometer-scale devices.

We study electron transport in two-dimensional materials with parabolic and linear (graphene) dispersions of the carriers in the presence of surface acoustic waves and an external magnetic field using semiclassical Boltzmann equations approach. We observe an oscillatory behavior of both the longitudinal and Hall electric currents as functions of the surface acoustic wave frequency at a fixed magnetic field and as functions of the inverse magnetic field at a fixed frequency of the acoustic wave. We explain the former by the phenomenon of geometric resonances, while we relate the latter to the Weiss-like oscillations in the presence of the dynamic superlattice created by the acoustic wave. Thus we demonstrate the dual nature of the acoustomagnetoelectric effect in two-dimensional electron gas.

Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and non-equilibrium molecular dynamics simulations of thermal conduction in suspended single-layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene, and provides fundamental understanding of thermal transport in two-dimensional materials.