Topological insulators display unique properties, such as the quantum spin Hall effect, because time-reversal symmetry allows charges and spins to propagate along the edge or surface of the topological insulator without scattering. However, the direct manipulation of these edge/surface states is difficult because they are significantly outnumbered by bulk carriers. Here, we report experimental evidence for the modulation of these surface states by using a gate voltage to control quantum oscillations in Bi(2)Te(3) nanoribbons. Surface conduction can be significantly enhanced by the gate voltage, with the mobility and Fermi velocity reaching values as high as ~5,800 cm(2) V(-1) s(-1) and ~3.7 × 10(5) m s(-1), respectively, with up to ~51% of the total conductance being due to the surface states. We also report the first observation of h/2e periodic oscillations, suggesting the presence of time-reversed paths with the same relative zero phase at the interference point. The high surface conduction and ability to manipulate the surface states demonstrated here could lead to new applications in nanoelectronics and spintronics.

Atomically thin 2D-layered transition-metal dichalcogenides have been studied extensively in recent years because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe2 is an emerging material that exhibits a stable distorted 1T phase and strong in-plane anisotropy due to its reduced crystal symmetry. Here, the anisotropic nature of ReSe2 is revealed by Raman spectroscopy under linearly polarized excitations in which different vibration modes exhibit pronounced periodic variations in intensity. Utilizing high-quality ReSe2 nanosheets, top-gate ReSe2 field-effect transistors were built that show an excellent on/off current ratio exceeding 10(7) and a well-developed current saturation in the current-voltage characteristics at room temperature. Importantly, the successful synthesis of ReSe2 directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 500 times and the hole mobility over 100 times at low temperatures. Strikingly, corroborating with our density-functional calculations, the ReSe2-based photodetectors exhibit a polarization-sensitive photoresponsivity due to the intrinsic linear dichroism originated from high in-plane optical anisotropy. With a back-gate voltage, the linear dichroism photodetection can be unambiguously tuned both in the electron and hole regime. The appealing physical properties demonstrated in this study clearly identify ReSe2 as a highly anisotropic 2D material for exotic electronic and optoelectronic applications.

Atomically thin 2D layered transition metal dichalcogenides (TMDs) have been extensively studied in recent years because of their appealing electrical and optical properties. Here, the fabrication of ReS2 field‐effect transistors is reported via the encapsulation of ReS2 nanosheets in a high‐κ Al2O3 dielectric environment. Low‐temperature transport measurements allow to observe a direct metal‐to‐insulator transition originating from strong electron–electron interactions. Remarkably, the photodetectors based on ReS2 exhibit gate‐tunable photoresponsivity up to 16.14 A W−1 and external quantum efficiency reaching 3168%, showing a competitive device performance to those reported in graphene, MoSe2, GaS, and GaSe‐based photodetectors. This study unambiguously distinguishes ReS2 as a new candidate for future applications in electronics and optoelectronics.

The carrier recombination processes in ZnO quantum dots ͑ϳ4 nm in diameter͒, ZnO nanocrystals ͑ϳ20 nm in diameter͒ and bulk ZnO crystal have been studied using photoluminescence ͑PL͒ spectroscopy in the temperature range from 8.5 to 300 K. The obtained experimental data suggest that the ultraviolet PL in ZnO quantum dots originates from recombination of the acceptor-bound excitons for all temperatures. In the larger size ZnO nanocrystals, the recombination of the acceptor-bound excitons is the dominant contribution to PL only at low temperature ͑T Ͻ 150 K͒. For higher temperatures ͑T Ͼ 150 K͒, PL is mostly due to recombination of the donor-bound excitons. Recombination processes in ZnO quantum dots and nanocrystals differ from those in bulk ZnO mainly because of the large surface-to-volume ratio in both types of nanoparticles and, consequently, a large number of acceptor defects near the surface. No strong inhomogeneous broadening has been observed in ultraviolet PL from ZnO quantum dots. Our results shed light on the carrier-recombination processes in ZnO quantum dots and nanocrystals, and can be used for the ZnO nanostructure optimization for the proposed optoelectronic and spintronic applications.

Dirac semimetals have attracted extensive attentions in recent years. It has been theoretically suggested that many-body interactions may drive exotic phase transitions, spontaneously generating a Dirac mass for the nominally massless Dirac electrons. So far, signature of interaction-driven transition has been lacking. In this work, we report high-magnetic-field transport measurements of the Dirac semimetal candidate ZrTe5. Owing to the large g factor in ZrTe5, the Zeeman splitting can be observed at magnetic field as low as 3 T. Most prominently, high pulsed magnetic field up to 60 T drives the system into the ultra-quantum limit, where we observe abrupt changes in the magnetoresistance, indicating field-induced phase transitions. This is interpreted as an interaction-induced spontaneous mass generation of the Dirac fermions, which bears resemblance to the dynamical mass generation of nucleons in high-energy physics. Our work establishes Dirac semimetals as ideal platforms for investigating emerging correlation effects in topological matters.

Charge-trap memory with high-κ dielectric materials is considered to be a promising candidate for next-generation memory devices. Ultrathin layered twodimensional (2D) materials like graphene and MoS2 have been receiving much attention because of their novel physical properties and potential applications in electronic devices. Here, we report on a dual-gate charge-trap memory device composed of a few-layer MoS2 channel and a three-dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. Owing to the extraordinary trapping ability of both electrons and holes in HfO2, the MoS2 memory device exhibits an unprecedented memory window exceeding 20 V. More importantly, with a back gate the window size can be effectively tuned from 15.6 to 21 V; the program/erase current ratio can reach up to 10 4 , far beyond Si-based flash memory, which allows for multi-bit information storage. Furthermore, the device shows a high mobility of 170 cm 2 V -1 s -1 , a good endurance of hundreds of cycles and a stable retention of ~28% charge loss after 10 years which is drastically lower than ever reported MoS2 flash memory. The combination of 2D materials with traditional high-κ charge-trap gate stacks opens up an exciting field of novel nonvolatile memory devices. KEYWORDS. Charge-trap memory, MoS 2 , Memory window, Dual gate, Memory characteristics 3 Atomically thin 2D materials like graphene and MoS 2 has been extensivelystudied recently because of their promising applications in optoelectronics 1, 2 , spintronics 3-7 , transparent and flexible devices [8][9][10][11][12] . Due to its remarkable properties, such as high carrier mobility and mechanical flexibility, graphene has been incorporated into nonvolatile memory structures serving as a floating gate 13,14 or a transparent channel 15 . However, owing to its zero band gap 16 , the graphene channeled memory devices typically possess a low program/erase current ratio, which significantly hinders its application in nonvolatile memory devices. Unlike graphene, MoS 2 has a transition from indirect band gap (1.2 eV) to a direct band gap (1.8 eV) in monolayer 17,18 . Its field effect transistors 19 show a high mobility of 200 cm 2 V -1 s -1 with a high on/off ratio approximately 10 8 . To potentially enhance the program/erase current ratio, attempts were made to replace graphene with MoS 2 as a channel material in a ferroelectric memory 20 or as a charge-trap layer in a graphene flash memory 21 . It was demonstrated that the monolayer MoS 2 is very sensitive to the presence of charges 14 . However, the relatively small memory window, the degraded mobility, and the insufficient trap capability in those devices require further improvement of the chargetrap stack in the MoS 2 memory device.

Graphene has unique electronic properties1,2 and graphene nanoribbons are of particular interest because they exhibit a conduction band gap, which arises due to size confinement and edge effects3-11. Theoretical studies have suggested that graphene nanoribbons could have interesting magneto-electronic properties with very large magnetoresistance predicted4,12-20. Here we report the experimental observation of a significant enhancement in the conductance of a graphene nanoribbon field-effect transistor in a perpendicular magnetic field. A negative magnetoresistance of nearly 100% was observed at low temperatures, with over 50% remaining at room temperature. This magnetoresistance can be tuned by varying the gate or source-drain bias. We also find that the charge transport in the nanoribbons is not significantly modified by an in-plane magnetic field. The large values of the magnetoresistance we observe may be attributed to the reduction of quantum confinement by the formation of cyclotron orbits and the delocalization effect under the perpendicular magnetic field15-20.

We report the first experimental demonstration of electrical spin injection, transport and detection in bulk germanium (Ge). The non-local magnetoresistance in n-type Ge is observable up to 225K. Our results indicate that the spin relaxation rate in the n-type Ge is closely related to the momentum scattering rate, which is consistent with the predicted Elliot-Yafet spin relaxation mechanism for Ge. The bias dependence of the nonlocal magnetoresistance and the spin lifetime in n-type Ge is also investigated. a these authors contributed equally to this work