Gate-Defined One-Dimensional Channel and Broken Symmetry States in MoS2 van der Waals Heterostructures

Ballistic Electron Source with Magnetically Controlled Valley Polarization in Bilayer Graphene

The achievement of valley-polarized electron currents is a cornerstone for the realization of valleytronic devices. Here, we report on ballistic coherent transport experiments where two opposite quantum point contacts (QPCs) are defined by electrostatic gating in a bilayer graphene (BLG) channel. By steering the ballistic currents with an out-of-plane magnetic field we observe two current jets, a consequence of valley-dependent trigonal warping. Tuning the BLG carrier density and number of QPC modes (m) with a gate voltage we find that the two jets are present for m=1 and up to m=6, indicating the robustness of the effect. Semiclassical simulations confirm the origin of the signals by quantitatively reproducing the jet separations without fitting parameters. In addition, our model shows that the ballistic current jets have opposite valley polarization. As a consequence, by steering each jet toward the detector using a magnetic field, we achieve full control over the valley polarization of the collected currents, envisioning such devices as ballistic current sources with tunable valley polarization. We also show that collimation experiments are a sensitive probe to the trigonal warping of the Fermi surface.

Quantum transport in InSb quantum well devices: progress and perspective

InSb, a narrow-band III-V semiconductor, is known for its small bandgap, small electron effective mass, high electron mobility, large effectiveg-factor, and strong spin-orbit interactions. These unique properties make InSb interesting for both industrial applications and quantum information processing. In this paper, we provide a review of recent progress in quantum transport research on InSb quantum well devices. With advancements in the growth of high-quality heterostructures and micro/nano fabrication, quantum transport experiments have been conducted on low-dimensional systems based on InSb quantum wells. Furthermore, ambipolar operations have been achieved in undoped InSb quantum wells, allowing for a systematic study of the band structure and quantum properties of p-type narrow-band semiconductors. Additionally, we introduce the latest research on InAsSb quantum wells as a continuation of exploring physics in semiconductors with even narrower bandgaps.

From Bulk Molybdenum Disulfide (MoS2) to Suspensions of Exfoliated MoS2 in an Aqueous Medium and Their Applications

Two-dimensional (2D) layered materials like graphene, transition-metal dichalcogenides (TMDs), boron nitrides, etc., exhibit unique and fascinating properties, such as high surface-to-volume ratio, inherent mechanical flexibility and robustness, tunable bandgap, and high carrier mobility, which makes them an apt candidate for flexible electronics with low consumption of power. Because of these properties, they are in tremendous demand for advancement in energy, environmental, and biomedical sectors developed through various technologies. The production and scalability of these materials must be sustainable and ecofriendly to utilize these unique properties in the real world. Here, in this current review, we review molybdenum disulfide (MoS2 nanosheets) in detail, focusing on exfoliated MoS2 in water and the applicability of aqueous MoS2 suspensions in various fields. The exfoliation of MoS2 results in the formation of single or few-layered MoS2. Therefore, this Review focuses on the few layers of exfoliated MoS2 that have the additional properties of 2D layered materials and higher excellent compatibility for integration than existing conventional Si tools. Hence, a few layers of exfoliated MoS2 are widely explored in biosensing, gas sensing, catalysis, photodetectors, energy storage devices, a light-emitting diode (LED), adsorption, etc. This review covers the numerous methodologies to exfoliate MoS2, focusing on the various published methodologies to obtain nanosheets of MoS2 from water solutions and their use.

Observation of interlayer plasmon polaron in graphene/WS2 heterostructures

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of single-layer WS2. By using micro-focused angle-resolved photoemission spectroscopy during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the single-layer WS2 conduction band minimum. Our results are explained by an effective many-body model in terms of a coupling between single-layer WS2 conduction electrons and an interlayer plasmon mode. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides.

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS2

Fermi-level pinning caused by the kinetic damage during metallization has been recognized as one of the major reasons for the non-ideal behavior of electrical contacts, forbidding reaching the Schottky-Mott limit. In this manuscript, we present a scalable technique wherein Indium, a low-work-function metal, is diffused to contact a few-layered MoS2flake. The technique exploits a smooth outflow of Indium over gold electrodes to make edge contacts to pre-transferred MoS2flakes. We compare the performance of three pairs of contacts made onto the same MoS2flake, the bottom-gold, top-gold, and Indium contacts, and find that the Indium contacts are superior to other contacts. The Indium contacts maintain linearI-Vcharacteristics down to cryogenic temperatures with an extracted Schottky barrier height of ∼2.1 meV. First-principle calculations show the induced in-gap states close to the Fermi level, and the damage-free contact interface could be the reason for the nearly Ohmic behavior of the Indium/MoS2interface.

Spin-Valley Locking for In-Gap Quantum Dots in a MoS2 Transistor

Spins confined to atomically thin semiconductors are being actively explored as quantum information carriers. In transition metal dichalcogenides (TMDCs), the hexagonal crystal lattice gives rise to an additional valley degree of freedom with spin-valley locking and potentially enhanced spin life and coherence times. However, realizing well-separated single-particle levels and achieving transparent electrical contact to address them has remained challenging. Here, we report well-defined spin states in a few-layer MoS₂ transistor, characterized with a spectral resolution of ∼50 μeV at T_el = 150 mK. Ground state magnetospectroscopy confirms a finite Berry-curvature induced coupling of spin and valley, reflected in a pronounced Zeeman anisotropy, with a large out-of-plane g-factor of g_⊥ ≃ 8. A finite in-plane g-factor (g_∥ ≃ 0.55-0.8) allows us to quantify spin-valley locking and estimate the spin-orbit splitting 2Δ_SO ∼ 100 μeV. The demonstration of spin-valley locking is an important milestone toward realizing spin-valley quantum bits.

Direct Patterning of Optoelectronic Nanostructures Using Encapsulated Layered Transition Metal Dichalcogenides

Direct top-down nanopatterning of semiconductors is a powerful tool for engineering properties of optoelectronic devices. Translating this approach to two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) is challenging because of both the small scales required for confinement and the degradation of electronic and optical properties caused by high-energy and high-dose electron radiation used for high-resolution top-down direct electron beam patterning. We show that encapsulating a TMD monolayer with hexagonal boron nitride preserves the narrow exciton linewidths and emission intensity typical in such heterostructures after electron beam lithography, allowing direct patterning of functional optical monolayer nanostructures on scales of a few tens of nanometers. We leverage this fabrication method to study size-dependent effects on nanodot arrays of MoS2 and MoSe2 as well as laterally confined electrical transport devices, demonstrating the potential of top-down lithography for nanoscale TMD optoelectronics.

Signature of Spin-Resolved Quantum Point Contact in p-Type Trilayer WSe2 van der Waals Heterostructure

In this study, an electrostatically induced quantum confinement structure, so-called quantum point contact, has been realized in a p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at a step of e²/h in addition to quarter plateaus such as 0.25 × 2e²/h at a finite bias voltage condition indicating the signature of intrinsic spin-polarized quantum point contact.

Evolution of high-frequency Raman modes and their doping dependence in twisted bilayer MoS2

Twisted van der Waals heterostructures provide a new platform for studying strongly correlated quantum phases. The interlayer coupling in these heterostructures is sensitive to the twist angle (θ) and key to controllably tuning several interesting properties. Here, we demonstrate the systematic evolution of the interlayer coupling strength with twist angle in bilayer MoS₂ using a combination of Raman spectroscopy and classical simulations. At zero doping, we observe a monotonic increase in the separation between the A_1g and E_2g¹ mode frequencies as θ decreases from 10°→ 1°, and the separation approaches that of a bilayer at small twist angles. Furthermore, using doping dependent Raman spectroscopy, we reveal the θ dependent softening and broadening of the A_1g mode, whereas the E_2g¹ mode remains unaffected. Using first principles based simulations, we demonstrate large (weak) electron-phonon coupling for the A_1g (E_2g¹) mode, which explains the experimentally observed trends. Our study provides a non-destructive way to characterize the twist angle and the interlayer coupling and establishes the manipulation of phonons in twisted bilayer MoS₂ (twistnonics).

Investigation of laser-induced-metal phase of MoTe2 and its contact property via scanning gate microscopy

Although semiconductor to metal phase transformation of MoTe₂ by high-density laser irradiation of more than 0.3 MW cm^-2 has been reported, we reveal that the laser-induced-metal (LIM) phase is not the 1T' structure derived by a polymorphic-structural phase transition but consists instead of semi-metallic Te induced by photo-thermal decomposition of MoTe₂. The technique is used to fabricate a field effect transistor with a Pd/2H-MoTe₂/LIM structure having an asymmetric metallic contact, and its contact properties are studied via scanning gate microscopy. We confirm that a Schottky barrier (a diffusion potential) is always formed at the Pd/2H-MoTe₂ boundary and obstacles a carrier transport while an Ohmic contact is realized at the 2H-MoTe₂/LIM phase junction for both n- and p-type carriers.

Landau-Quantized Excitonic Absorption and Luminescence in a Monolayer Valley Semiconductor

We investigate Landau-quantized excitonic absorption and luminescence of monolayer WSe_{2} under magnetic field. We observe gate-dependent quantum oscillations in the bright exciton and trions (or exciton polarons) as well as the dark trions and their phonon replicas. Our results reveal spin- and valley-polarized Landau levels (LLs) with filling factors n=+0, +1 in the bottom conduction band and n=-0 to -6 in the top valence band, including the Berry-curvature-induced n=±0 LLs of massive Dirac fermions. The LL filling produces periodic plateaus in the exciton energy shift accompanied by sharp oscillations in the exciton absorption width and magnitude. This peculiar exciton behavior can be simulated by semiempirical calculations. The experimentally deduced g factors of the conduction band (g∼2.5) and valence band (g∼15) exceed those predicted in a single-particle model (g=1.5, 5.5, respectively). Such g-factor enhancement implies strong many-body interactions in gated monolayer WSe_{2}. The complex interplay between Landau quantization, excitonic effects, and many-body interactions makes monolayer WSe_{2} a promising platform to explore novel correlated quantum phenomena.

Absence of Interlayer Tunnel Coupling of K-Valley Electrons in Bilayer MoS_{2}

In Bernal stacked bilayer graphene interlayer coupling significantly affects the electronic band structure compared to monolayer graphene. Here we present magnetotransport experiments on high-quality n-doped bilayer MoS_{2}. By measuring the evolution of the Landau levels as a function of electron density and applied magnetic field we are able to investigate the occupation of conduction band states, the interlayer coupling in pristine bilayer MoS_{2}, and how these effects are governed by electron-electron interactions. We find that the two layers of the bilayer MoS_{2} behave as two independent electronic systems where a twofold Landau level's degeneracy is observed for each MoS_{2} layer. At the onset of the population of the bottom MoS_{2} layer we observe a large negative compressibility caused by the exchange interaction. These observations, enabled by the high electronic quality of our samples, demonstrate weak interlayer tunnel coupling but strong interlayer electrostatic coupling in pristine bilayer MoS_{2}. The conclusions from the experiments may be relevant also to other transition metal dichalcogenide materials.

Carrier control in 2D transition metal dichalcogenides with Al2O3 dielectric

We report transport measurements of dual gated MoS₂ and WSe₂ devices using atomic layer deposition grown Al₂O₃ as gate dielectrics. We are able to achieve current pinch-off using independent split gates and observe current steps suggesting possible carrier confinement. We also investigated the impact of gate geometry and used electrostatic potential simulations to explain the observed device physics.

Mapping multi-valley Lifshitz transitions induced by field-effect doping in strained MoS2 nanolayers

Gate-induced superconductivity at the surface of nanolayers of semiconducting transition metal dichalcogenides (TMDs) has attracted a lot of attention in recent years, thanks to the sizeable transition temperature, robustness against in-plane magnetic fields beyond the Pauli limit, and hints to a non-conventional nature of the pairing. A key information necessary to unveil its microscopic origin is the geometry of the Fermi surface hosting the Cooper pairs as a function of field-effect doping, which is dictated by the filling of the inequivalent valleys at the K/K[Formula: see text] and Q/Q[Formula: see text] points of the Brillouin zone. Here, we achieve this by combining density functional theory calculations of the bandstructure with transport measurements on ion-gated 2H-MoS₂ nanolayers. We show that, when the number of layers and the amount of strain are set to their experimental values, the Fermi level crosses the bottom of the high-energy valleys at Q/Q[Formula: see text] at doping levels where characteristic kinks in the transconductance are experimentally detected. We also develop a simple 2D model which is able to quantitatively describe the broadening of the kinks observed upon increasing temperature. We demonstrate that this combined approach can be employed to map the dependence of the Fermi surface of TMD nanolayers on field-effect doping, detect Lifshitz transitions, and provide a method to determine the amount of strain and spin-orbit splitting between sub-bands from electric transport measurements in real devices.

Intrinsic valley Hall transport in atomically thin MoS2

Electrons hopping in two-dimensional honeycomb lattices possess a valley degree of freedom in addition to charge and spin. In the absence of inversion symmetry, these systems were predicted to exhibit opposite Hall effects for electrons from different valleys. Such valley Hall effects have been achieved only by extrinsic means, such as substrate coupling, dual gating, and light illuminating. Here we report the first observation of intrinsic valley Hall transport without any extrinsic symmetry breaking in the non-centrosymmetric monolayer and trilayer MoS₂, evidenced by considerable nonlocal resistance that scales cubically with local resistance. Such a hallmark survives even at room temperature with a valley diffusion length at micron scale. By contrast, no valley Hall signal is observed in the centrosymmetric bilayer MoS₂. Our work elucidates the topological origin of valley Hall effects and marks a significant step towards the purely electrical control of valley degree of freedom in topological valleytronics.

Electrons hopping in two-dimensional honeycomb lattices possess a valley degree of freedom. Here, the authors observe room-temperature valley Hall transport without any extrinsic symmetry breaking in the non-centrosymmetric monolayer and trilayer MoS₂ by purely electronic means, whereas no valley signal is detected for centrosymmetric bilayer MoS₂.

Interactions and Magnetotransport through Spin-Valley Coupled Landau Levels in Monolayer MoS_{2}

The strong spin-orbit coupling and the broken inversion symmetry in monolayer transition metal dichalcogenides results in spin-valley coupled band structures. Such a band structure leads to novel applications in the fields of electronics and optoelectronics. Density functional theory calculations as well as optical experiments have focused on spin-valley coupling in the valence band. Here we present magnetotransport experiments on high-quality n-type monolayer molybdenum disulphide (MoS_{2}) samples, displaying highly resolved Shubnikov-de Haas oscillations at magnetic fields as low as 2 T. We find the effective mass 0.7m_{e}, about twice as large as theoretically predicted and almost independent of magnetic field and carrier density. We further detect the occupation of the second spin-orbit split band at an energy of about 15 meV, i.e., about a factor of 5 larger than predicted. In addition, we demonstrate an intricate Landau level spectrum arising from a complex interplay between a density-dependent Zeeman splitting and spin- and valley-split Landau levels. These observations, enabled by the high electronic quality of our samples, testify to the importance of interaction effects in the conduction band of monolayer MoS_{2}.

Stable and scalable 1T MoS2 with low temperature-coefficient of resistance

Monolithic realization of metallic 1T and semiconducting 2H phases makes MoS₂ a potential candidate for future microelectronic circuits. A method for engineering a stable 1T phase from the 2H phase in a scalable manner and an in-depth electrical characterization of the 1T phase is wanting at large. Here we demonstrate a controllable and scalable 2H to 1T phase engineering technique for MoS₂ using microwave plasma. Our method allows lithographically defining 1T regions on a 2H sample. The 1T samples show excellent temporal and thermal stability making it suitable for standard device fabrication techniques. We conduct both two-probe and four-probe electrical transport measurements on devices with back-gated field effect transistor geometry in a temperature range of 4 K to 300 K. The 1T samples exhibit Ohmic current-voltage characteristics in all temperature ranges without any dependence to the gate voltage, a signature of a metallic state. The sheet resistance of our 1T MoS₂ sample is considerably lower and the carrier concentration is a few orders of magnitude higher than that of the 2H samples. In addition, our samples show negligible temperature dependence of resistance from 4 K to 300 K ruling out any hoping mediated or activated electrical transport.

Gate-Defined Quantum Confinement in InSe-Based van der Waals Heterostructures

Indium selenide, a post-transition metal chalcogenide, is a novel two-dimensional (2D) semiconductor with interesting electronic properties. Its tunable band gap and high electron mobility have already attracted considerable research interest. Here we demonstrate strong quantum confinement and manipulation of single electrons in devices made from few-layer crystals of InSe using electrostatic gating. We report on gate-controlled quantum dots in the Coulomb blockade regime as well as one-dimensional quantization in point contacts, revealing multiple plateaus. The work represents an important milestone in the development of quality devices based on 2D materials and makes InSe a prime candidate for relevant electronic and optoelectronic applications.

Quantum transport through MoS2 constrictions defined by photodoping

We present a device scheme to explore mesoscopic transport through molybdenum disulfide (MoS₂) constrictions using photodoping. The devices are based on van-der-Waals heterostructures where few-layer MoS₂ flakes are partially encapsulated by hexagonal boron nitride (hBN) and covered by a few-layer graphene flake to fabricate electrical contacts. Since the as-fabricated devices are insulating at low temperatures, we use photo-induced remote doping in the hBN substrate to create free charge carriers in the MoS₂ layer. On top of the device, we place additional metal structures, which define the shape of the constriction and act as shadow masks during photodoping of the underlying MoS₂/hBN heterostructure. Low temperature two- and four-terminal transport measurements show evidence of quantum confinement effects.