Session A53: 2D Semiconductors: Transport and Devices

Electron-hole liquid in a van der Waals heterostructure photocell at room temperature

Condensation - the familiar process underlying the formation of clouds and the distillation of ethyl alcohol into whiskey - is the phase transition of gas into liquid droplets. In semiconductors, at sufficiently high electron-hole (e-h) densities or low temperatures, the gas of non-equilibrium electrons and holes may undergo condensation into one of several potential liquid-like phases. In this talk, I present recent results on the gas-to-liquid phase transition of electrons and holes in ultrathin van der Waals heterostructure photocells revealed through multi-parameter dynamic photoresponse microscopy (MPDPM). By combining rich visualization with comprehensive analysis of very large data sets acquired through MPDPM, we find that ultrafast laser excitation at a graphene-molybdenum ditelluride-graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power. These patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional e-h liquid. While our imaging experiments mark the first observation (in over 50 years of study) of an e-h liquid at room temperature, I will discuss our results within the greater context of strongly correlated electronic condensates.   

Intrinsic disorder effects on device performance of few-layer MoS2 field-effect transistors

Field-effect transistors (FETs) made by two dimensional (2D) materials attract attention because of high on-off ratios and negligible short-channel effects. The 2D material of MoS₂ has been studied for years while problems from contact, interface, and structural disorders have not been solved. There are a lot of reports to give a wide range of electron mobility and the best performance of a single-layer MoS2 FET is still in debate. We present a large variation of disorders in MoS2 flakes exfoliated from the same bulk. The disorder dominate the low performance of FETs. In particular, for high performance devices, the mobility increases with decreasing temperature and the device reveals metallic behaviors. Setting the FET in an on state, the conductivity of high-performance devices is close to e2/h, giving a standard to check the quality. There are some disorders resulting in a large decrease in conductance as well as in the FET mobility. On the other hand, the subthreshold swings are independent from the mobility. The disorder effects dominate to low performance thus growth of perfect 2D crystals is essential for the application of 2D FETs.   

Control of valley-polarized exciton currents in 2D heterostructures

Valleytronics is an alternative to charge-based electronics aiming at encoding data in the valley degree of freedom, which could enable new paradigms for computing devices. Transition metal dichalcogenides (TMDCs) are an ideal platform for this due to the combination of unique spin–valley physics and direct bandgap, allowing optical initialization and readout of the valley state. Recent developments in the control of interlayer excitons in heterostructures of these materials offer an effective way to realize valley-optoelectronic devices [1]. Here, we show the generation and transport over mesoscopic distances of valley-polarized excitons in a device based on a type-II TMDC heterostructure. Engineering of the interlayer coupling results in enhanced diffusion of valley-polarized excitons, which can be controlled and switched electrically [2]. Furthermore, using electrostatic traps we can increase the exciton concentration by an order of magnitude, reaching densities higher than 10¹² cm^-2, a promising approach to obtain coherent quantum states of excitons.

[1] Ciarrocchi*, Unuchek* et al. Nat. Photon., 13, 2, (2019)
[2] Unuchek*, Ciarrocchi* et al. Nat. Nanotechnol. (2019)   

Frequency Stability of MoS2 drum resonators at Room Temperature

Nanomechanical resonators hold great promise for ultrasensitive mass and force sensing. The discovery of two-dimensional (2D) materials having ultralow mass and exceptional mechanical properties have made them an ideal choice for NEMS. 2D resonators tend to be a great platform for sensing applications since the minimum detectable mass is proportional to mass of the resonator itself. The limit of detection is also, directly dependent on the measurement uncertainty of resonant frequency (δf/f). Thus, frequency stability is an important performance metric for such resonators. We report frequency stability at room temperature on MoS₂ drum resonators. The Allan deviation (AD) was observed to be of the order of 10^-5 at 150ms integration time for 24V VgDC and AC drive of 70mV. This corresponds to a mass sensitivity of few attograms. We observe that increasing VgAC leads to more improvement in AD as compared to that of VgDC drive. This is due to an enhanced signal to noise ratio. We estimate various types of noise associated with MoS₂ resonator. This work holds promise for ultrasensitive sensing and realizing ultrastable oscillators from 2D materials.   

Probing anisotropic transport in atomically thin ReS2 via ferroelectric domain controlled nanowire patterning

The layered van der Waals material ReS₂ exhibits highly anisotropic band structure in the 1T' phase. In this work, we probe the effect of the band anisotropy on the transport properties of single- and few-layer ReS₂ via ferroelectric field effect combined with ferroelectric domain patterning. We fabricated mechanically exfoliated ReS₂ flakes into two-point transistor devices sandwiched between a SiO₂/Si back gate and a ferroelectric polymer PVDF-TrFE thin film top layer. The polarization of PVDF was then controlled at the nanoscale using conductive atomic force microscopy. By uniformly polarizing the ferroelectric top layer into the up (P_up) and down (P_down) directions, we induced up to 10⁵ current switching in bilayer ReS₂ channel at 300 K. We then polarized the entire channel into the insulating state, and created a nanoscale line-domain across the channel, leading to a conductive nanowire. By creating nanowires at different orientations, we mapped out the angular dependence of ReS₂ conductivity, which reveals more than one order of magnitude difference between the directions along and perpendicular to the b-axis. We compared the results with DFT calculated band structure of ReS₂.   

Electronic transport in two-dimensional MXenes for energy storage

MXenes are a large family of two-dimensional transition metal carbides and carbonitrades, which is attracting great research attention in the energy storage devices, including supercapacitor, due to their excellent electrical conductivity. In order to assist the design of supercapacitor electrodes based on these materials, we study the electronic conductivity of selected MXenes from first principles, and we aim to understand how their conductivity depends on their chemical composition and surface termination. We use the Abinit software to perform first-principles calculations. First, the total energy, the electronic banecutsmd structure and the density of state (DOS) are investigated using density functional theory (DFT). Second, we employed density functional perturbation theory (DFPT) to obtain the phonon band structure and electron-phonon coupling. Finally, we solve the linearized Boltzmann equation in order to obtain the phonon-limited electronic mobility. We will show preliminary results for Ti3C2F2 and Ti3C2(OH)2.   

Anisotropic carrier transport of black phosphorus encapsulated by h-BN

Anisotropic two-dimensional (2D) materials are promising candidates for novel electronics and optoelectronics because in-plane anisotropy in 2D materials can be used for polarization controlled infrared sensors, directionally controlled heat dissipators, and the angle control between adjacent 2D layered materials. Black phosphorus (BP), one of the anisotropic 2D materials, is known to have strong p-type in-plane anisotropic properties [1]. But, researches of anisotropic properties of BP in the tightly controlled fabrication environment and device structure are still elusive. Here, we report the anisotropic electrical properties of the BP field effect transistor (FET) by pricisely aligning the FET channel along its pre-determined in-plane directions. Using angle-resolved polarized Raman spectroscopy (ARPR), we were able to identify BP into zigzag (ZZ) and armchair (AC) directions. Field effect mobility and resistances of AC direction was found to be better than that of ZZ direction at room and low temperature respectively.   

Characterization of the metal/MoS2 top contact based on first-principles quantum transport calculations

We have recently reported the experimental achievement of Ohmic van der Waals (vdW) contacts at the indium (In)/MoS₂ devices thorough a novel thermal evaporation process, and based on the combined density functional theory and matrix Green’s function calculations concluded that the nature of the ideal Ohmic contact is characterized by the abrupt and rigid shift of TMDC bands across the In/MoS₂ interface (i.e. no band bending) together with the formation of metal-induced gap states (MIGS) [1]. In this work, we extend the analysis by considering four other metal species, Ag, Au, Sc, and Pd. Systematically considering how the electrode contact-region MoS₂ electronic structure develops into that of the channel region in different metal cases, we conclude that the identification of MIGS (rather than band bending) as the main determining factor for the clean metal/MoS₂ contacts is generally valid irrespective of the differences in the metal work function and wettability.
[1] B. K. Kim, D. H. Kim, and T. H. Kim et al. arXiv:1904.10295 [cond-mat.mes-hall]   

Photo-Induced Anomalous Hall Effect in 2D Transition-Metal Dichalcogenide

Illumination of a circularly polarized a.c. pump field induces a valley polarization in two-dimensional transition metal dichalcogenides (TMDs), which results in an anomalous Hall voltage in response to a d.c. bias. In this work, we develop a theory for this photovoltaic valley-resolved anomalous Hall effect in undoped TMDs. A quantum kinetic equation is used to study the non-equilibrium carrier populations and time-averaged transport currents under the simultaneous influence of the strong a.c pump field and the weak d.c probe field. Our results for the photo-induced longitudinal and Hall conductivities show strong resonant features when the pump field frequency reaches the spin-split band-to-band transition energies.   

Density-functional prediction of a strong Orbital Hall effect in the monolayer WX2 (X = Te, S)

The orbital Hall effect (OHE) is the transverse flow of the orbital angular momentum in response to the applied electric field, analogous to the charge current flow in the standard Hall effect. Even though the OHE has been proposed about a decade ago, it has not been directly observed experimentally to our knowledge. We have recently proposed that the 2D transition metal dichalcogenides (TMDC) with non-centrosymmetric crystal structure may be a good candidate for a robust OHE. In this talk, we evaluate the effect for the 2D-TMDC materials WTe₂ and WS₂ where there is a strong spin-orbit coupling present, both for the non-centrosymmetric (2H) and the centrosymmeteic (1T') cases. As anticipated, the OHE in the 2H structure is much stronger, due to the existence of the intrinsic orbital moment in different parts of the Brillouin zone, which flow in different directions, as opposed to the centrosymmeteic (1T') structure, where the OHE occurs due to the induced orbital moment in the presence of the Hall electric field. Our results are based on density-functional calculations as well as minimal tight-binding models.   

Heterojunctions from Coulomb-Engineered Transition Metal Dichalcogenides

The band structure and the band gap of semiconducting layered materials are strongly affected by the Coulomb interaction between carriers within the layer. At the same time we can externally modify this interaction by means of dielectric substrates or dielectric coating. This Coulomb-engineering can thus be used to tailor the fundamental electronic properties of layered materials.

Here we use a combination of the GdW [1] and WFCE [2] approaches to systematically study the environmental-screening effects to monolayers of semiconducting transition metal dichalcogenides on a material-realistic level. We compare static and dynamic screening effects of homogeneous substrates and derive an effective modeling scheme for spatially-varying heterogeneous substrates. The latter allows for the external and non-invasive induction of heterojunctions within the otherwise homogeneous mononalyer. Our calculations show that spatial band gap modulations on the length scale of a few lattice constants are possible and are just limited by the heterogeneous substrate.

[1] M. Rohlfing, Phys. Rev. B 82, 205127 (2010)
[2] M. Rösner et al., Phys. Rev. B 92, 085102 (2015)   

Quasi-1D TiS3 Nanoribbons: Mechanical Exfoliation, Thickness-Dependent Raman Spectroscopy and Electronic Properties

Quasi-one-dimensional (quasi-1D) materials enjoy growing interest due to their unusual physical properties and promise for miniature electronic devices. We investigated the micromechanical exfoliation of representative quasi-1D crystals, TiS₃ whiskers, and demonstrate that they typically split into narrow nanoribbons with very smooth edges and clear signatures of 1D TiS₃ chains. Theoretical calculations show that the energies required for breaking weak interactions between the 2D layers and between 1D chains within the layers are comparable, and in turn are considerably lower than those required for breaking the covalent bonds within the chains. We systematically studied the exfoliated TiS₃ crystals by Raman spectroscopy and identified the Raman peaks whose spectral positions were most dependent on the crystals' thickness. Finally, we fabricated TiS₃-based electronic devices and tested their transport properties. The conclusions established in this study for the exfoliated TiS₃ crystals can be extended to a variety of transition metal trichalcogenide materials as well as other quasi-1D crystals. The possibility of exfoliation of TiS₃ into few-nm wide crystals with smooth edges is important for realization of miniature device channels with reduced edge scattering.   

Tuning Electronic Properties of Monolayer Molybdenum Disulfide

Molybdenum disulfide (MoS₂) has emerged as a prototypical material among the 2D transition metal dichalcogenides for its stability, low cost and unique electronic, optical and mechanical properties. Its electronic properties can be tuned using different control parameters. This great sensitivity presents an opportunity to functionalize its properties through defect engineering, strain or by proximity to another material. We use high resolution low temperature STM/STS to study the local electronic properties of monolayer MoS₂. We were able to induce strains up to 3% before slipping effects take place and relaxation mechanisms prevail. We found a reduction of the quasiparticle bandgap of about 400 meV per percent local strain with a minimum gap of 1.2 eV. Heterostructures based on MoS₂ offer another viable possibility to tune its electronic properties. In this case, interactions between the planes of different materials are expected to modify the electronic properties of the constituent materials and open unprecedented possibilities of combining them for technological use.