Session A32: Devices from 2D Materials

Switching between hidden charge density wave phases in 1T-TaS$_{2}$

Recent experimental progress studying the multitude of correlated electronic properties of the layered material 1T-TaS$_{2}$ has revealed peculiar electronic phases which exist out of thermal equilibrium yet remain stable for surprisingly long time periods. TaS$_{2}$ is a 2D material that has attracted much attention due to its rich electronic spectrum exhibiting several charge density wave phases accessible through varying temperature as well as a low temperature superconducting phase at high pressure or extreme carrier doping. Applying voltage pulses across the sample at low temperatures suddenly switches the sample from an insulating commensurate-charge density wave state into a spectrum of thermally inaccessible metallic phases. These newly observed hidden phases hold promising device applications such as electronic oscillators, memristors, and Landau switches. In this work, we explore the electronic transport properties of 1T-TaS$_{2}$ in these hidden phases and the dynamics of switching between them using voltage pulses. This study provides insight into the microscopic details of these processes and motivates further investigation of such details.   

Thermal transports in two-dimensional materials.

As atomically thick two-dimensional (2D) materials, Graphene and Boron nitride (BN) exhibits extraordinary optical and mechanical properties, and extremely high thermal conductivity. Being very stable nanometer-thick membrane that can be suspended between two leads, graphene and BN provide a perfect test platform for studying thermal transport in 2D systems. Here, we report experimental measurements of thermal conduction in suspended single layer graphene and few-layer BN. a) We found that thermal conductivity in single layer graphene increases with sample length (L) and remains length-dependent with logL at T$=$ 300K even for lengths much larger than the averaged phonon mean free path, providing experimental evidence of the Breakdown of Fourier's law in thermal conduction. b) We observed a thickness-dependent thermal conductivity in bilayer suspended h-BN with the room temperature value reaching as high as 484 W/(mK), exceeding that in bulk h-BN. These results are the consequence of the two-dimensional nature of phonons and provide fundamental understanding into thermal transports in two-dimensional materials.   

2D Materials: Science and Technology.

Two-dimensional (2D) materials is one of the fastest growing research areas in science. In the last 5 years there has been amazing developments in the area of synthesis, characterization, manipulation and development of new van der Waals heterostructures with these materials. These developments are allowing a deeper scientific understanding of the physics of 2D materials and also creating an immense industrial interest. In this seminar I will cover some of these develpments and point out new opportunities.   

Demonstration of Persistent Optical Gating Effect in MoS$_2$ and Graphene

Two-dimensional layered materials (2DLMs) offer a wide range of emergent properties beyond those of the bulk species, making them attractive for novel technologies. One important example is the direct modulation of the electrical and other physical properties of 2DLMs using their strong interactions with the supporting substrate. Recently, we demonstrated a persistent, all-optical gating effect in thin films of topological insulators, an effect which was based on the optical modulation of space-charge in an underlying SrTiO$_3$ substrate [1]. Here, we show that this optical gating effect can be utilized as a means of locally controlling the chemical potential in other ultra-thin electronic systems. We will present systematic optical and electrical transport measurements on monolayer graphene and the transition metal dichalcogenide MoS$_2$, showing a persistent, bidirectional optical effect on the carrier concentration of these materials when they are grown or placed on SrTiO$_3$. We will also discuss the outlook for potential extensions of this research, such as the creation of dynamically-configurable electronics that can be written, erased, and rewritten using light. 1] A. L. Yeats et al., Sci. Adv. 1, e1500640 (2015).   

Quantum Hall drag of exciton condensation in bilayer graphene double layer

Excitons are pairs of electrons and holes bound together by the Coulomb interaction. At low temperatures, excitons can form a Bose-Einstein condensate (BEC), enabling macroscopic phase coherence and superfluidity. We report exciton condensation under magnetic field in bilayer graphene double layers separated by a few atomic layers of hexagonal boron nitride (hBN). Driving current in one graphene layer generates a quantized Hall voltage in the other layer, signifying coherent exciton transport. Owing to the strong Coulomb coupling across the atomically thin dielectric, the observed $\nu_{tot} = 1$ exciton BEC state exhibits $T_c$ of 8K, an order of magnitude higher than previously reported in GaAs systems. With the wide-range gate tunability, we surveyed the parameter space and discovered new exciton BEC phases selectively appearing at $\nu_{tot} = 3$ and $-3$, while many other integer $\nu_{tot}$ states are missing. We also discovered that changing displacement fields through each bilayer graphene can induce phase transitions of the exciton BEC. By comparing the exciton BEC phase transitions with symmetry-breaking quantum Hall phase transitions of each bilayer graphene, the selection rule for establishing exciton BEC phases was inferred.   

Epsilon-near-zero behavior from plasmonic Dirac point: Theory and realization using two-dimensional materials

The electromagnetic response of a two-dimensional metal embedded in a periodic array of a dielectric host can give rise to a plasmonic Dirac point that emulates Epsilon-Near-Zero (ENZ) behavior. This theoretical result is extremely sensitive to structural features like periodicity of the dielectric medium and thickness imperfections. We propose that such a device can actually be realized by using graphene as the 2D metal and materials like the layered semiconducting transition-metal dichalcogenides or hexagonal boron nitride as the dielectric host. We propose a systematic approach, in terms of design characteristics, for constructing metamaterials with linear, elliptical and hyperbolic dispersion relations which produce ENZ behavior, normal or negative diffraction.   

Electronic structure of bilayer graphene on transition metal dichalcogenides

Graphene on transition-metal dichalcogenides (TMDCs) opens new venues for optospintronics [1], as well as for investigating giant proximity spin-orbit effects [2]. We have predicted that graphene on WSe2 exhibits robust helical edge states within a 7 K Rashba gap [2]. However, for an experimental observation, ultraclean graphene would be required to see such effects, due to electron-hole puddles which in monolayer graphene mask spectral features at such levels. The cure is bilayer graphene, in which the energy fluctuations are much weaker and meV features could be well resolved. Here we present our first-principles results for the electronic band structures of bilayer graphene on TMDCs, and discuss the orbital and spin-orbital proximity effects with phenomenological symmetry-based Hamiltonians that we use to fit the first-principles data. A fascinating perspective is to have a gating tenability of the proximity effects, which we will also discuss by presenting calculations in the presence of transverse electric fields. [1] M. Gmitra, J. Fabian, Phys. Rev. B 92, 155403 (2015). [2] M. Gmitra, D. Kochan, P. H\"{o}gl, J. Fabian, Phys. Rev. B 93, 155104 (2016).   

Scaling and carrier transport properties of monolayer MoS$_{2}$ transistors

2D crystals of layered~transition metal dichalcogenides~such as MoS$_{2}$ are ideal candidates for aggressive miniaturization of field-effect transistors (FETs) to the single digit nanometer scale. This class of materials can benefit from their atomically thin body with dangling-bond-free surfaces. In particular, monolayer-MoS$_{2}$, because of its bandgap of 1.8 eV yields high I$_{on}$/I$_{off\, \, }$ratio FETs, while its atomically thin body, t $\approx $ 0.7 nm, facilitate the reduction of characteristic scaling length. In this work, we first demonstrate the fabrication and electrical characteristics of a MoS$_{2}$ FET using single-layer graphene as the source/drain contacts and a channel length of 15 nm. The MoS$_{2}$ FET had an I$_{on}$/I$_{of\, \, }$of $\approx $10$^{6}$ with an I$_{on}$ \textasciitilde 50 $\mu $A/$\mu $m and minimum subthreshold slope of 90 mV/dec. Next, by exploiting the semiconducting to metallic phase transition in MoS$_{2}$, we demonstrate a 7.5 nm transistor channel length by patterning of MoS$_{2}$ in a periodic chain of semiconducting and metallic-phase MoS$_{2}$ regions. The transistor chain shows I$_{on}$/I$_{off} \quad \approx $10$^{5}$ with I$_{off} \quad \approx $100 pA/$\mu $m. Modeling of the resulting characteristics reveals that the 2H/1T' MoS$_{2}$ homojunction has a resistance of 75 $\Omega $.$\mu $m while the 2H-MoS$_{2}$ exhibits low-field mobility of \textasciitilde 25 cm$^{2}$/V.s and carrier injection velocity of \textasciitilde 10$^{6}$ cm/s.   

Demonstration of High-performance Transistors with Narrow Bandgap High-Mobility Ultrathin 2D Films

The scaling trend of transistors has triggered a thirst of seeking for ultrathin 2D semiconductors with excellent electrical transport properties. Here we present a promising solution of high-performance transistors on ultrathin 2D semiconducting films with unprecedented transport properties and stability. Similar to black phosphorus or phosphorene, it is a p-type semiconductor with a direct band gap ranging from 0.35eV (bulk) to \textgreater 1.2eV (monolayer). Large scale (\textgreater 100$\mu $m) atomic-flat 2D films with controllable thickness were achieved by hydrothermal method. Great scaling potential was demonstrated by aggressively shrinking the device dimensions, including film thickness (from over 30nm to 4nm), channel length (from 5 $\mu $m to sub-100nm), and gate dielectric (EOT from 300nm to 5nm). Other methods such as buried gate and contact engineering were also applied to optimize the device performance. Transistors display high performance with on/off ratio over 106, maximum on-current over 500 mA/mm, field-effect mobility over 700cm2/Vs, contact resistance 0.6k$\Omega $*mm and great air stability.   

Nanoscale control of the charge neutrality point of graphene

Nano-engineered graphene devices can exhibit novel and useful electronic and optical properties, many of which depend critically on controlling the chemical potential relative to the charge-neutrality point. Complex-oxide heterostructures enable reconfigurable control of conductive nanostructures{REFs}, making them an interesting platform for controlling the electronic properties of graphene at nanoscale dimensions. Here we report the fabrication of graphene/LaAlO$_3/SrTiO$_3$ heterostructures with nanoscale programmable control of the charge-neutrality point. Magnetotransport measurements of superlattice structures show characteristic interference features that can be associated with the electronically patterned interface. We discuss possible new directions based on this highly versatile hybrid platform.