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Low carrier mobility and high electrical contact resistance are two major obstacles prohibiting explorations of quantum transport in TMDCs. Here, we demonstrate an effective method to establish low-temperature Ohmic contacts in boron nitride encapsulated TMDC devices based on selective etching and conventional electron-beam evaporation of metal electrodes. This method works for most extensively studied TMDCs in recent years, including MoS2, MoSe2, WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K. All of the few-layerTMDCdevices studied show excellent performance with remarkably improved field-effect mobilities ranging from 2300 to 16 000 cm 2 V-1 s(-1), as verified by the high carrier mobilities extracted from Hall effect measurements. Moreover, both high-mobility n-type and p-typeTMDCchannels can be realized by simply using appropriate contact metals. Prominent Shubnikov-de Haas oscillations have been observed and investigated in these high-quality TMDC devices.

Atomically thin black phosphorus (BP) is a promising two-dimensional material for fabricating electronic and optoelectronic nano-devices with high mobility and tunable bandgap structures. However, the charge-carrier mobility in few-layer phosphorene (monolayer BP) is mainly limited by the presence of impurity and disorders. In this study, we demonstrate that vertical BP heterostructure devices offer great advantages in probing the electron states of monolayer and few-layer phosphorene at temperatures down to 2 K through capacitance spectroscopy. Electronic states in the conduction and valence bands of phosphorene are accessible over a wide range of temperature and frequency. Exponential band tails have been determined to be related to disorders. Unusual phenomena such as the large temperature-dependence of the electron state population in few-layer phosphorene have been observed and systematically studied. By combining the first-principles calculation, we identified that the thermal excitation of charge trap states and oxidation-induced defect states were the main reasons for this large temperature dependence of the electron state population and degradation of the on-off ratio in phosphorene field-effect transistors.

Atomically thin black phosphorus (BP) is a promising two-dimensional material for fabricating electronic and optoelectronic nano-devices with high mobility and tunable bandgap structures. However, the charge-carrier mobility in few-layer phosphorene (monolayer BP) is mainly limited by the presence of impurity and disorders. In this study, we demonstrate that vertical BP heterostructure devices offer great advantages in probing the electron states of monolayer and few-layer phosphorene at temperatures down to 2 K through capacitance spectroscopy. Electronic states in the conduction and valence bands of phosphorene are accessible over a wide range of temperature and frequency. Exponential band tails have been determined to be related to disorders. Unusual phenomena such as the large temperature-dependence of the electron state population in few-layer phosphorene have been observed and systematically studied. By combining the first-principles calculation, we identified that the thermal excitation of charge trap states and oxidation-induced defect states were the main reasons for this large temperature dependence of the electron state population and degradation of the on-off ratio in phosphorene field-effect transistors.

Crystalline defects in MoS2 may induce midgap states, resulting in low carrier mobility. These midgap states are usually difficult to probe by conventional transport measurement. The quantum capacitance of single-layer graphene is sensitive to defect-induced states near the Dirac point, at which the density of states is extremely low. It is reported that the hexagonal-boron nitride/graphene/MoS2 sandwich structure facilitates the exploration of the properties of those midgap states in MoS2. Comparative results of the quantum capacitance of pristine graphene indicate the presence of several midgap states with distinct features. Some of these states donate electrons while some states lead to localization of electrons. It is believed that these midgap states originate from intrinsic point defects such as sulfur vacancies, which have a significant impact on the property of the MoS2/graphene interface. They are responsible for the contact problems of metal/MoS2 interfaces.

Graphene is a distinct 2D material which attracts scientists since its discovery in 2004. However, the presence of extrinsic disorders has been a major problem for fabricating high quality graphene devices. In this thesis, a dry transfer technique was developed to fabricate high quality monolayer graphene heterostructures (BN/Graphene/BN) and their electronic properties were subsequently investigated. Unique methods and structural designs were employed to protect graphene from direct contact with solvent and polymers, which are contaminants commonly introduced using traditional transfer techniques. Besides, preparation and identification procedures were optimized to shorten the fabrication period. Device structures of FETs and QC capacitances on graphite were designed and fabricated to test the reliability of these techniques. The ballistic transport verified the effectiveness of these methods (carrier mobility reached 195,000 cm2V-1s-1 and 500,000 cm2V-1s-1 at 300K and 2K, respectively). Landau Level splitting in Quantum Hall Effect phenomenon was observed in both FET and quantum capacitance devices at low magnetic field. Furthermore, negative compressibility in quantum capacitance was detected. Collectively, the pick-up dry transfer technique and the sandwich configuration are reliable and effective to fabricate high quality graphene devices. The ultra-clean system offers an opportunity to unravel physical phenomena, which were previously interfered by impurities and defects. With the successfulness in fabricating high quality graphene, the general use of the dry transfer technique in manufacturing potential electronic devices can be expected.

Bernal-stacked few-layer graphene has been investigated by analyzing its Landau-level spectra through quantum capacitance measurements. We find that surface relaxation, which is insignificant in trilayer graphene, starts to manifest in Bernal-stacked tetralayer graphene. In trilayer graphene, the interlayer interaction parameters are generally similar to those of graphite. However, in tetralayer graphene, the hopping parameters of the two bulk layers are quite different from those of the two outer layers. This represents direct evidence of the surface relaxation phenomenon. Traditionally, the van der Waals interaction between the carbon layers is thought to be insignificant. However, we suggest that the interlayer interaction is an important factor in explaining the observed results, and the symmetry-breaking effects in graphene sublattice are not negligible.

Two-dimensional materials such as graphene and transition metal dichalcogenides have attracted great attention because of their rich physics and potential applications in next-generation nanoelectronic devices. The family of two-dimensional materials was recently joined by atomically thin black phosphorus which possesses high theoretical mobility and tunable bandgap structure. However, degradation of properties under atmospheric conditions and high-density charge traps in black phosphorus have largely limited its actual mobility thus hindering its future applications. Here, we report the fabrication of stable sandwiched heterostructures by encapsulating atomically thin black phosphorus between hexagonal boron nitride layers to realize ultra-clean interfaces that allow a high field-effect mobility of similar to 1,350 cm(2)V(-1) s(-1) at room temperature and on-off ratios exceeding 105. At low temperatures, the mobility even reaches similar to 2,700 cm(2)V(-1) s(-1) and quantum oscillations in black phosphorus two-dimensional hole gas are observed at low magnetic fields. Importantly, the sandwiched heterostructures ensure that the quality of black phosphorus remains high under ambient conditions.

Two-dimensional (2D) atomic-layered heterostructures stacked by van der Waals interactions recently introduced new research fields, which revealed novel phenomena and provided promising applications for electronic, optical, and optoelectronic devices. In this study, we report the van der Waals epitaxial growth of high-quality atomically thin Bi2Se3 on single crystalline hexagonal boron nitride (h-BN) by chemical vapor deposition. Although the in-plane lattice mismatch between Bi2Se3 and h-BN is approximately 65%, our transmission electron microscopy analysis revealed that Bi2Se3 single crystals epitaxially grew on h-BN with two commensurate states; that is, the ((1) over bar2 (1) over bar0) plane of Bi2Se3 was preferably parallel to the ((1) over bar 100) or ((1) over bar2 (1) over bar0) plane of h-BN. In the case of the Bi2Se3 ((2) over bar 110) parallel to h-BN (1 (1) over bar 00) state, the Moire pattern wavelength in the Bi2Se3/h-BN superlattice can reach 5.47 nm. These naturally formed thin crystals facilitated the direct assembly of h-BN/Bi2Se3/h-BN sandwiched heterostructures without introducing any impurity at the interfaces for electronic property characterization. Our quantum capacitance (QC) measurements showed a compelling phenomenon of thickness-dependent topological phase transition, which was attributed to the coupling effects of two surface states from Dirac Fermions at/or above six quintuple layers (QLs) to gapped Dirac Fermions below six QLs. Moreover, in ultrathin Bi2Se3 (e.g., 3 QLs), we observed the midgap states induced by intrinsic defects at cryogenic temperatures. Our results demonstrated that QC measurements based on h-BN/Bi2Se3/h-BN sandwiched structures provided rich information regarding the density of states of Bi2Se3, such as quantum well states and Landau quantization. Our approach in fabricating h-BN/Bi2Se3/h-BN sandwiched device structures through the combination of bottom-up growth and top-down dry transferring techniques can be ex

The metal-insulator transition is one of the remarkable electrical properties of atomically thin ​molybdenum disulphide. Although the theory of electron–electron interactions has been used in modelling the metal-insulator transition in ​molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin ​molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of ​molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer ​molybdenum disulphide. In addition, the valence band of thin ​molybdenum disulphide layers and their intrinsic properties are accessed.

High-quality BN-Graphene-BN nanoribbon capacitors with double side-gates of graphene have been experimentally realized. The double side-gates can effectively modulate the electronic properties of graphene nanoribbon capacitors. By applying anti-symmetric side-gate voltages, we observed significant upward shifting and flattening of the V-shaped capacitance curve near the charge neutrality point. Symmetric side-gate voltages, however, only resulted in tilted upward shifting along the opposite direction of applied gate voltages. These modulation effects followed the behavior of graphene nanoribbons predicted theoretically for metallic side-gate modulation. The negative quantum capacitance phenomenon predicted by numerical simulations for graphene nanoribbons modulated by graphene side-gates was not observed, possibly due to the weakened interactions between the graphene nanoribbon and side-gate electrodes caused by the Ga+ beam etching process. (C) 2014 AIP Publishing LLC.