Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi 2 Se 3 ) is the most spotlighted threedimensional TI material; it has a Dirac cone at each top and bottom surface and a relatively wide bandgap. For application, suppression of the bulk effect is crucial, but in ultrathin TI materials, with thicknesses less than 3 QL, the finite size effect works on the linear dispersion of the surface states, so that the surface band has a finite bandgap because of the hybridization between the top and bottom surface states and Rashba splitting, resulting from the structure inversion asymmetry. Here, we studied the gapless top surface Dirac state of strained 3 QL Bi 2 Se 3 /graphene heterostructures. A strain caused by the graphene layer reduces the bandgap of surface states, and the band bending resulting from the charge transfer at the Bi 2 Se 3 −graphene interface induces localization of surface states to each top and bottom layer to suppress the overlap of the two surface states. In addition, we verified the independent transport channel of the top surface Dirac state in Bi 2 Se 3 /graphene heterostructures by measuring the magneto-conductance. Our findings suggest that the strain and the proximity effect in TI/non-TI heterostructures may be feasible ways to engineer the topological surface states beyond the physical and topological thickness limit.

This study used a spatially controlled boron-doping technique that enables a p–n junction diode to be realized within a single 2D black phosphorus (BP) nanosheet for high-performance photovoltaic application. The reliability of the BP surface and state-of-the-art 2D p–n heterostructure’s gated junctions was obtained using the controllable pulsed-plasma process technique. Chemical and structural analyses of the boron-doped BP were performed using X-ray photoelectron spectroscopy, transmission electron microscopy, and first-principles density functional theory (DFT) calculations, and the electrical characteristics of a field-effect transistor based on the p–n heterostructure were determined. The incorporated boron generated high electron density at the BP surface. The electron mobility of BP was significantly enhanced to ∼265 cm2/V·s for the top gating mode, indicating greatly improved electron transport behavior. Ultraviolet photoelectron spectroscopy and DFT characterizations revealed the occurrence of significant surface charge transfer in the BP. Moreover, the pulsed-plasma boron-doped BP p–n junction devices exhibited high-efficiency photodetection behavior (rise time: 1.2 ms and responsivity: 11.3 mA/W at V g = 0 V). This study’s findings on the tunable nature of the surface-transfer doping scheme reveal that BP is a promising candidate for optoelectronic devices and advanced complementary logic electronics.

Owing to their remarkable spin−charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin−orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi 2 Se 3 was enhanced by up to ∼2.4 times.

Topological insulators (TIs) have become popular in the field of optoelectronic devices because of their broadband and high-sensitivity properties, which are attributed to the narrow band gap of the bulk state and high mobility of the Dirac surface state. Although perfectly grown TIs are known to exhibit strong stability against oxidation, in most cases, the existence of vacancy defects in TIs reacts to air and the characteristics of TIs is affected by oxidation. Therefore, changes in the band structure and electrical characteristics by oxidation should be considered. A significant change occurs because of the oxidation; however, the dependence of the photoresponse of TIs on oxidation has not been studied in detail. In this study, the photoresponsivity of oxidized Bi2Se3 films is enhanced, rather than degraded, after oxidation in air for 24 h, resulting in a maximum responsivity of 140 mA W–1. This responsivity is substantially higher than previously reported values for Bi2Se3. Furthermore, a change in the photoresponse time of Bi2Se3 due to air exposure is systematically observed. Based on variations in the Fermi level and work function, using photoelectron spectroscopy, it is confirmed that the responsivity is improved from the junction effect of the Bi-based surface oxidized layer.

The topological surface state (TSS), as a new quantum state of matter protected by time-reversal symmetry (TRS), has a unique electronic band structure of linear energy momentum dispersion and spiral helical spin texture. [1][2][3][4][5] Thus, the electrons on the topological insulator (TI) surface are protected from backscattering and weak nonmagnetic perturbations because of their strong spin-momentum interaction. [6,7] Since the successful