Charge Transport in Dual Gated Bilayer Graphene with Corbino Geometry

The resistance of dual-gated bilayer graphene is measured as a function of temperature and gating electric fields in the Corbino geometry which precludes edge transport. The temperature-dependent resistance is quantitatively described by a two-channel conductance model including parallel thermal activation and variable range hopping channels, which gives the electric-field-dependent band gap whose magnitude is found to be in good agreement with infrared absorption experiments. Low-temperature transport is similar to that seen in previous studies of dual-gated bilayer graphene with edges, suggesting that edge transport does not play an important role

Charge Inhomogeneity Determines Oxidative Reactivity of Graphene on Substrates

Single-layer graphene (SLG) supported on SiO2 shows anomalously large chemical reactivity compared to thicker graphene, with charge inhomogeneity-induced potential fluctuations or topographic corrugations proposed as the cause. Here we systematically probe the oxidative reactivity of graphene supported on substrates with different surface roughnesses and charged impurity densities: hexagonal boron nitride (hBN), mica, thermally grown SiO2 on Si, and SiO2 nanoparticle thin films. SLG on low charge trap density hBN is not etched and shows little doping after oxygen treatment at temperatures up to 550 °C, in sharp contrast with oxidative etching under similar conditions of graphene on high charge trap density SiO2 and mica. Furthermore, bilayer graphene shows reduced reactivity compared to SLG regardless of its substrate-induced roughness. Together the observations indicate that graphene’s reactivity is predominantly controlled by charge inhomogeneity-induced potential fluctuations rather than surface roughness

Anisotropic Etching of Atomically Thin MoS₂

Exposure to oxygen at 300–340 °C results in triangular etch pits with uniform orientation on the surfaces of atomically thin molybdenum disulfide (MoS₂), indicating anisotropic etching terminating on lattice planes. The triangular pits grow laterally with oxidation time. The density of pits scarcely depends on oxidation time, temperature, and MoS₂ thickness but varies significantly from sample to sample, indicating that etching is initiated at native defect sites on the basal plane surface rather than activated by substrate effects such as charged impurities or surface roughness. Raman spectroscopy confirms that oxygen treatment produces no molybdenum oxide (MoO₃) below 340 °C. However, upon oxidation above 200 °C, the Raman A_1g mode upshifts and the linewidth decreases, indicating p-type doping of MoS₂. Oxidation at 400 °C results in complete conversion to MoO₃

Ambipolar Surface State Thermoelectric Power of Topological Insulator Bi₂Se₃

We measure gate-tuned thermoelectric power of mechanically exfoliated Bi₂Se₃ thin films in the topological insulator regime. The sign of the thermoelectric power changes across the charge neutrality point as the majority carrier type switches from electron to hole, consistent with the ambipolar electric field effect observed in conductivity and Hall effect measurements. Near the charge neutrality point and at low temperatures, the gate-dependent thermoelectric power follows the semiclassical Mott relation using the expected surface state density of states but is larger than expected at high electron doping, possibly reflecting a large density of states in the bulk gap. The thermoelectric power factor shows significant enhancement near the electron–hole puddle carrier density ∼0.5 × 10¹² cm^–2 per surface at all temperatures. Together with the expected reduction of lattice thermal conductivity in low-dimensional structures, the results demonstrate that nanostructuring and Fermi level tuning of three-dimensional topological insulators can be promising routes to realize efficient thermoelectric devices

Passivating Graphene and Suppressing Interfacial Phonon Scattering with Mechanically Transferred Large-Area Ga₂O₃

We demonstrate a large-area passivation layer for graphene by mechanical transfer of ultrathin amorphous Ga2O3 synthesized on liquid Ga metal. A comparison of temperature-dependent electrical measurements of millimeter-scale passivated and bare graphene on SiO2/Si indicates that the passivated graphene maintains its high field effect mobility desirable for applications. Surprisingly, the temperature-dependent resistivity is reduced in passivated graphene over a range of temperatures below 220 K, due to the interplay of screening of the surface optical phonon modes of the SiO2 by high-dielectric-constant Ga2O3 and the relatively high characteristic phonon frequencies of Ga2O3. Raman spectroscopy and electrical measurements indicate that Ga2O3 passivation also protects graphene from further processing such as plasma-enhanced atomic layer deposition of Al2O3

Hybridized hyperbolic surface phonon polaritons at {\alpha}-MoO3 and polar dielectric interfaces

Surface phonon polaritons (SPhPs) in polar dielectrics offer new opportunities for infrared nanophotonics due to sub-diffraction confinement with low optical losses. Though the polaritonic field confinement can be significantly improved by modifying the dielectric environment, it is challenging to break the fundamental limits in photon confinement and propagation behavior of SPhP modes. In particular, as SPhPs inherently propagate isotropically in these bulk polar dielectrics, how to collectively realize ultra-large field confinement, in-plane hyperbolicity and unidirectional propagation remains elusive. Here, we report an approach to solve the aforementioned issues of bulk polar dielectric's SPhPs at one go by constructing a heterostructural interface between biaxial van der Waals material (e.g., MoO3) and bulk polar dielectric (e.g., SiC, AlN, and GaN). Due to anisotropy-oriented mode couplings at the interface, the hybridized SPhPs with a large confinement factor (>100) show in-plane hyperbolicity that has been switched to the orthogonal direction as compared to that in natural MoO3. More interestingly, this proof of concept allows steerable, angle-dependent and unidirectional polariton excitation by suspending MoO3 on patterned SiC air cavities. Our finding exemplifies a generalizable framework to manipulate the flow of nano-light and engineer unusual polaritonic responses in many other hybrid systems consisting of van der Waals materials and bulk polar dielectrics

Self-Limiting Layer-by-Layer Oxidation of Atomically Thin WSe₂

Growth of a uniform oxide film with a tunable thickness on two-dimensional transition metal dichalcogenides is of great importance for electronic and optoelectronic applications. Here we demonstrate homogeneous surface oxidation of atomically thin WSe₂ with a self-limiting thickness from single- to trilayers. Exposure to ozone (O₃) below 100 °C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe₂. With further O₃ exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide film on top of unoxidized WSe₂. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide films formed on WSe₂ are nearly atomically flat. Using photoluminescence and Raman spectroscopy, we find that the underlying single-layer WSe₂ is decoupled from the top oxide but hole-doped. Our findings offer a new strategy for creating atomically thin heterostructures of semiconductors and insulating oxides with potential for applications in electronic devices

Electrochemically Prepared Polycrystalline Copper Surface for the Growth of Hexagonal Boron Nitride

The controlled and reproducible growth of hexagonal boron nitride (h-BN) by chemical vapor deposition on polycrystalline copper foil substrates remains a challenge as typical growth surfaces contain microscopic ridges (height ≈ 100 μm) arising from the foil manufacturing process. In this work, we report a method to prepare commercially cold-rolled polycrystalline copper substrates for greatly improved growth of h-BN by a combination of thermal annealing in a reducing environment and electrochemical polishing to create an excellent surface that enables control of BN nucleation sites. We report a root mean square roughness of ∼1.2 nm for the Cu substrate after electropolishing and a reduction of nucleation sites along with enlargement of h-BN crystals with this combined approach. We also assess the potential role of surface features that exist on the Cu surface as nucleation sites. The development of an electrochemical process to prepare two-dimensional (2D) material growth substrates and demonstration of greatly improved growth of 2D materials directly point to more pragmatic large scale processing of 2D materials since such techniques are already utilized in large scale industrial processing

Oxidation of Monolayer WS2_{2} in Ambient is a Photoinduced Process

We have studied the ambient air oxidation of chemical vapor deposition (CVD) grown monolayers of the semiconducting transition metal dichalcogenide (S-TMD) WS$_{2}$ using optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM). Monolayer WS$_{2}$ exposed to ambient conditions in the presence of light (typical laboratory ambient light for weeks, or typical PL spectroscopy map), exhibits damage due to oxidation which can be detected with the LSCM and AFM; though may not be evident in conventional optical microscopy due to poorer contrast and resolution. Additionally, this oxidation was not random, and correlated with 'high-symmetry' and red-shifted areas in the PL spectroscopy map - areas thought to contain a higher concentration of sulfur vacancies. In contrast, samples kept in ambient and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence experiments showed that samples subjected to excitation energies at or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88 eV) oxidized in as little as 7 days, even for irradiances and fluences eight and four orders of magnitude lower (respectively) than previously reported. No significant oxidation was observed for 760 nm/1.63 eV light exposure, which lies below the trion excitation energy in WS$_{2}$. The strong wavelength dependence and apparent lack of irradiance dependence suggests that ambient oxidation of WS$_{2}$ is initiated by photon-mediated electronic band transitions, that is, photo-oxidation. These findings have important implications for prior, present and future studies concerning S-TMDs measured, stored or manipulated in ambient conditions