We report the magnetotransport properties of individual Bi2Se3 nanoplates. The carrier Hall mobility is up to 104 cm2/Vs. A large positive linear magnetoresistance (MR) approaching to 400% without sign of saturation was observed at 14 T. By angular dependence measurements, we demonstrate that the linear MR originates from a two-dimensional transport. Furthermore, by comparing the Hall mobility and longitudinal resistance under different temperatures, we give very clear evidence that reveals the close relationship between magnetoresistance and mobility.

The photothermoelectric effect in topological insulator Bi2Se3 nanoribbons is studied. The topological surface states are excited to be spin-polarized by circularly polarized light. Because the direction of the electron spin is locked to its momentum for the spin-helical surface states, the photothermoelectric effect is significantly enhanced as the oriented motions of the polarized spins are accelerated by the temperature gradient. The results are explained based on the microscopic mechanisms of a photon induced spin transition from the surface Dirac cone to the bulk conduction band. The as-reported enhanced photothermoelectric effect is expected to have potential applications in a spin-polarized power source.

Bi2Se3 nanocrystals with various morphologies, including nanotower, nanoplate, nanoflake, nanobeam and nanowire, have been synthesized. Well-distinguished Shubnikov-de Haas (SdH) oscillations were observed in Bi2Se3 nanoplates and nanobeams. Careful analysis of the SdH oscillations suggests the existence of Berry's phase π, which confirms the quantum transport of the surface Dirac fermions in both Bi2Se3 nanoplates and nanobeams without intended doping. The observation of the singular quantum transport of the topological surface states implies that the high-quality Bi2Se3 nanostructures have superiorities for investigating the novel physical properties and developing the potential applications.

In this study, a multiple kernel learning support vector machine algorithm is proposed for the identification of EEG signals including mental and cognitive tasks, which is a key component in EEG-based brain computer interface (BCI) systems. The presented BCI approach included three stages: (1) a pre-processing step was performed to improve the general signal quality of the EEG; (2) the features were chosen, including wavelet packet entropy and Granger causality, respectively; (3) a multiple kernel learning support vector machine (MKL-SVM) based on a gradient descent optimization algorithm was investigated to classify EEG signals, in which the kernel was defined as a linear combination of polynomial kernels and radial basis function kernels. Experimental results showed that the proposed method provided better classification performance compared with the SVM based on a single kernel. For mental tasks, the average accuracies for 2-class, 3-class, 4-class, and 5-class classifications were 99.20%, 81.25%, 76.76%, and 75.25% respectively. Comparing stroke patients with healthy controls using the proposed algorithm, we achieved the average classification accuracies of 89.24% and 80.33% for 0-back and 1-back tasks respectively. Our results indicate that the proposed approach is promising for implementing human-computer interaction (HCI), especially for mental task classification and identifying suitable brain impairment candidates.

Evident spin valve signals are observed in Co/graphene/Co sandwich structures with both monolayer and two-layer graphene stacks at temperatures from 1.5 K to 300 K. All the devices demonstrate linear current-voltage curves, indicating that an Ohmic property is dominating rather than a tunneling effect. The vertical graphene spin valves have potential applications in high-density non-volatile memories.

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure with spin locked to momentum, which is interesting in physics and may also have important applications in spintronics. In this work, by measuring the tunable helicity-dependent photocurrent (HDP), we present an identification of the HDP from the Dirac-like surface states at room temperature. It turns out that the total HDP has two components, one from the Dirac-like surface states, and the other from the surface accumulation layer. These two components have opposite directions. The clear gate tuning of the electron density as well as the HDP signal indicates that the surface band bending and resulted surface accumulation are successfully modulated by the applied ionic liquid gate, which provides a promising way to the study of the Dirac-like surface states and also potential applications in spintronic devices.

Magnetotransport measurements of topological insulators are very important to reveal the exotic topological surface states for spintronic applications. However, the novel properties related to the surface Dirac fermions are usually accompanied by a large linear magnetoresistance under perpendicular magnetic field, which makes the identification of the surface states obscure. Here, we report prominent Shubnikov-de Haas (SdH) oscillations under an in-plane magnetic field, which are identified to originate from the surface states in the sidewalls of topological insulator Bi2Se3 nanoplates. Importantly, the SdH oscillations appear with a dramatically weakened magnetoresistance background, offering an easy path to probe the surface states directly when the coexistence of surface states and bulk conduction is inevitable. Moreover, under a perpendicular magnetic field, the oscillations in Hall conductivity have peak-to-valley amplitudes of 2 e2/h, giving confidence to achieve a quantum Hall effect in this system. A cross-section view of the nanoplate shows that the sidewall is (015) facet dominant and therefore forms a 58° angle with regard to the top/bottom surface instead of being perpendicular; this gives credit to the surface states' behavior as two-dimensional transport.

As model Dirac materials, graphene 1,2 and topological insulators, 3,4 have attracted vast attention due to their unique physical properties. Graphene based van der Waals heterojunctions, formed by stacking graphene with other materials, have been a prosperous avenue of research, which exhibit many interesting physical phenomena, including Coulomb drag of massless fermions, 5 metal-insulator transitions 6 and enhancement of spin-orbit coupling. 7 The transport of Dirac fermions through potential barriers in graphene has been revealed to demonstrate the well-known Klein tunneling. [8][9][10] Meanwhile, grapheme-based vertical devices have shown promising properties for Schottky diode and tunneling junction applications. [11][12][13][14][15][16][17][18][19] Through tuning the tunneling barrier, a high on-off conductance ratio in graphene field-effect tunneling transistors has been achieved. 11 Another type of Dirac material, bismuth selenide (Bi 2 Se 3 ), known as a prototype topological insulator, possesses conducting surface states (SS) protected by the time-reversal symmetry. [3][4] Recently, theories have predicted significant modification of energy band structures induced by a proximity effect in graphene-topological insulator hybrid systems. [20][21][22][23] However, although the graphene-Bi 2 Se 3 heterojunctions have been fabricated by vapor-phase deposition 24 and molecular beam epitaxy, 25,26 the vertical electronic transport properties in such heterostructures remain unstudied.For Bi 2 Se 3 grown on graphene, it is difficult to fabricate multi-terminal electrodes separately on graphene and Bi 2 Se 3 to form the vertical transport devices. Fortunately, layer-by-layer stacking of graphene has been demonstrated to be an effective method to construct graphene-based vertical devices. 27 Here we report on the fabrication of 4 the graphene-Bi 2 Se 3 vertical devices using layer-by-layer stacking of graphene and RESULTS AND DISCUSSIONDevice Configurations. The hybrid devices were fabricated by transferring high quality Bi 2 Se 3 nanoplates onto monolayer graphene flakes, followed by the deposition of patterned Cr/Au (50/170 nm) electrodes ( Figure S1). As shown in Figure 1a, the applied bias current I flows from the bottom graphene (Electrode 1) to the upper Raman spectrum in Figure 2e excited by a 514 nm laser clearly indicates the nature of monolayer graphene, as the 2D peak to G peak intensity ratio is larger than 3 : 1.The Raman spectrum of a Bi 2 Se 3 nanoplate is shown in Figure 2f. The pronounced two peaks located at ~131 cm -1 and ~174 cm -1 are assigned to 2 and 1 2 phonon vibrational modes, respectively, which is consistent with the former report. 31Transport through the Graphene-Bi 2 Se 3 Interface. The Cr/Au electrodes form ohmic contacts with both Bi 2 Se 3 and graphene; however there is a potential barrier at the graphene/Bi 2 Se 3 interface ( Figure S2). Figure 3a shows the junction resistance R j increases with decreasing temperature, while the graphene resistance R g shows a very weak tem...