van der Waals Heterostructures with High Accuracy Rotational Alignment

We describe the realization of van der Waals (vdW) heterostructures with accurate rotational alignment of individual layer crystal axes. We illustrate the approach by demonstrating a Bernal-stacked bilayer graphene formed using successive transfers of monolayer graphene flakes. The Raman spectra of this artificial bilayer graphene possess a wide 2D band, which is best fit by four Lorentzians, consistent with Bernal stacking. Scanning tunneling microscopy reveals no moiré pattern on the artificial bilayer graphene, and tunneling spectroscopy as a function of gate voltage reveals a constant density of states, also in agreement with Bernal stacking. In addition, electron transport probed in dual-gated samples reveals a band gap opening as a function of transverse electric field. To illustrate the applicability of this technique to realize vdW heterostructuctures in which the functionality is critically dependent on rotational alignment, we demonstrate resonant tunneling double bilayer graphene heterostructures separated by hexagonal boron-nitride dielectric.

Band Alignment in WSe2-Graphene Heterostructures

Using different types of WSe2 and graphene-based heterostructures, we experimentally determine the offset between the graphene neutrality point and the WSe2 conduction and valence band edges, as well as the WSe2 dielectric constant along the c-axis. In a first heterostructure, consisting of WSe2-on-graphene, we use the WSe2 layer as the top dielectric in dual-gated graphene field-effect transistors to determine the WSe2 capacitance as a function of thickness, and the WSe2 dielectric constant along the c-axis. In a second heterostructure consisting of graphene-on-WSe2, the lateral electron transport shows ambipolar behavior characteristic of graphene combined with a conductivity saturation at sufficiently high positive (negative) gate bias, associated with carrier population of the conduction (valence) band in WSe2. By combining the experimental results from both heterostructures, we determine the band offset between the graphene charge neutrality point, and the WSe2 conduction and valence band edges.

Field effect transistors with current saturation and voltage gain in ultrathin ReS2

We report the fabrication and device characteristics of exfoliated, few-layer, dual-gated ReS2 field effect transistors (FETs). The ReS2 FETs display n-type behavior with a room temperature Ion/I(off) of 10(5). Many devices were studied with a maximum intrinsic mobility of 12 cm(2) · V(-1) · s(-1) at room temperature and 26 cm(2) · V(-1) · s(-1) at 77 K. The Cr/Au-ReS2 contact resistance determined using the transfer length method is gate-bias dependent and ranges from 175 kΩ · μm to 5 kΩ · μm, and shows an exponential dependence on back-gate voltage indicating Schottky barriers at the source and drain contacts. Dual-gated ReS2 FETs demonstrate current saturation, voltage gain, and a subthreshold swing of 148 mV/decade.

Oxidized titanium as a gate dielectric for graphene field effect transistors and its tunneling mechanisms

We fabricate and characterize a set of dual-gated graphene field effect transistors using a novel physical vapor deposition technique in which titanium is evaporated onto the graphene channel in 10 Å cycles and oxidized in ambient to form a top-gate dielectric. A combination of X-ray photoemission spectroscopy, ellipsometry, and transmission electron microscopy suggests that the titanium is oxidizing in situ to titanium dioxide. Electrical characterization of our devices yields a dielectric constant of κ = 6.9 with final mobilities above 5500 cm(2)/(V s). Low temperature analysis of the gate-leakage current in the devices gives a potential barrier of 0.78 eV in the conduction band and a trap depth of 45 meV below the conduction band.