Sherman J. R. Tan

Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus

Two-dimensional black phosphorus (BP) has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic properties. BP is one of the few two-dimensional materials where it is possible to tune the bandgap over a wide energy range from the visible up to the infrared. In this article, we report the observation of a giant Stark effect in electrostatically gated few-layer BP. Using low-temperature scanning tunnelling microscopy, we observed that in few-layer BP, when electrons are injected, a monotonic reduction of the bandgap occurs. The injected electrons compensate the existing defect-induced holes and achieve up to 35.5% bandgap modulation in the light-doping regime. When probed by tunnelling spectroscopy, the local density of states in few-layer BP shows characteristic resonance features arising from layer-dependent sub-band structures due to quantum confinement effects. The demonstration of an electrical gate-controlled giant Stark effect in BP paves the way to designing electro-optic modulators and photodetector devices that can be operated in a wide electromagnetic spectral range.

Oscillating edge states in one-dimensional MoS2 nanowires.

Reducing the dimensionality of transition metal dichalcogenides to one dimension opens it to structural and electronic modulation related to charge density wave and quantum correlation effects arising from edge states. The greater flexibility of a molecular scale nanowire allows a strain-imposing substrate to exert structural and electronic modulation on it, leading to an interplay between the curvature-induced influences and intrinsic ground-state topology. Herein, the templated growth of MoS2 nanowire arrays consisting of the smallest stoichiometric MoS2 building blocks is investigated using scanning tunnelling microscopy and non-contact atomic force microscopy. Our results show that lattice strain imposed on a nanowire causes the energy of the edge states to oscillate periodically along its length in phase with the period of the substrate topographical modulation. This periodic oscillation vanishes when individual MoS2 nanowires join to form a wider nanoribbon, revealing that the strain-induced modulation depends on in-plane rigidity, which increases with system size.

Analyzing Dirac Cone and Phonon Dispersion in Highly Oriented Nanocrystalline Graphene.

Chemical vapor deposition (CVD) is one of the most promising growth techniques to scale up the production of monolayer graphene. At present, there are intense efforts to control the orientation of graphene grains during CVD, motivated by the fact that there is a higher probability for oriented grains to achieve seamless merging, forming a large single crystal. However, it is still challenging to produce single-crystal graphene with no grain boundaries over macroscopic length scales, especially when the nucleation density of graphene nuclei is high. Nonetheless, nanocrystalline graphene with highly oriented grains may exhibit single-crystal-like properties. Herein, we investigate the spectroscopic signatures of graphene film containing highly oriented, nanosized grains (20-150 nm) using angle-resolved photoemission spectroscopy (ARPES) and high-resolution electron energy loss spectroscopy (HREELS). The robustness of the Dirac cone, as well as dispersion of its phonons, as a function of graphenes grain size and before and after film coalescence, was investigated. In view of the sensitivity of atomically thin graphene to atmospheric adsorbates and intercalants, ARPES and HREELS were also used to monitor the changes in spectroscopic signatures of the graphene film following exposure to the ambient atmosphere.

Large Area Synthesis of 1D-MoSe2 Using Molecular Beam Epitaxy.

Large area synthesis of 1D-MoSe2 nanoribbons on both insulating and conducting substrates via molecular beam epitaxy is presented. Dimensional controlled growth of 2D, 1D-MoSe2 , and 1D-2D-MoSe2 hybrid heterostructure is achieved by tuning the growth temperature or Mo:Se precursor ratio.

Homoepitaxial Growth of Large‐Scale Highly Organized Transition Metal Dichalcogenide Patterns

Controllable growth of highly crystalline transition metal dichalcogenide (TMD) patterns with regular morphology and unique edge structure is highly desired and important for fundamental research and potential applications. Here, single-crystalline MoS2 flakes are reported with regular trigonal symmetric patterns that can be homoepitaxially grown on MoS2 monolayer via chemical vapor deposition. The highly organized MoS2 patterns are rhombohedral (3R)-stacked with the underlying MoS2 monolayer, and their boundaries are predominantly terminated by zigzag Mo edge structure. The epitaxial MoS2 crystals can be tailored from compact triangles to fractal flakes, and the pattern formation can be explained by the anisotropic growth rates of the S and Mo edges under low sulfur chemical potential. The 3R-stacked MoS2 pattern demonstrates strong second and third-harmonic-generation signals, which exceed those reported for monolayer MoS2 by a factor of 6 and 4, correspondingly. This homoepitaxial growth approach for making highly organized TMD patterns is also demonstrated for WS2 .

Resolving the Spatial Structures of Bound Hole States in Black Phosphorus

Understanding the local electronic properties of individual defects and dopants in black phosphorus (BP) is of great importance for both fundamental research and technological applications. Here, we employ low-temperature scanning tunnelling microscope (LT-STM) to probe the local electronic structures of single acceptors in BP. We demonstrate that the charge state of individual acceptors can be reversibly switched by controlling the tip-induced band bending. In addition, acceptor-related resonance features in the tunnelling spectra can be attributed to the formation of Rydberg-like bound hole states. The spatial mapping of the quantum bound states shows two distinct shapes evolving from an extended ellipse shape for the 1s ground state to a dumbbell shape for the 2px excited state. The wave functions of bound hole states can be well-described using the hydrogen-like model with anisotropic effective mass, corroborated by our theoretical calculations. Our findings not only provide new insight into the many-body interactions around single dopants in this anisotropic two-dimensional material but also pave the way to the design of novel quantum devices.

Tailoring sample-wide pseudo-magnetic fields on a graphene–black phosphorus heterostructure

Spatially tailored pseudo-magnetic fields (PMFs) can give rise to pseudo-Landau levels and the valley Hall effect in graphene. At an experimental level, it is highly challenging to create the specific strain texture that can generate PMFs over large areas. Here, we report that superposing graphene on multilayer black phosphorus creates shear-strained superlattices that generate a PMF over an entire graphene–black phosphorus heterostructure with edge size of tens of micrometres. The PMF is intertwined with the spatial period of the moiré pattern, and its spatial distribution and intensity can be modified by changing the relative orientation of the two materials. We show that the emerging pseudo-Landau levels influence the transport properties of graphene–black phosphorus field-effect transistor devices with Hall bar geometry. The application of an external magnetic field allows us to enhance or reduce the effective field depending on the valley polarization with the prospect of developing a valley filter.Pseudo-Landau levels in strained graphene modify not only the local density of states, but also the transport through a Hall bar device.