Minhao Liu

Synthesis of Nitrogen‐Doped Graphene Using Embedded Carbon and Nitrogen Sources

Graphene is the two-dimensional crystalline form of carbon whose extraordinary charge carrier mobility and other unique features hold great promise for nanoscale electronics. [ 1 ] Because graphene has no bandgap, however, its electrical conductivity cannot be completely controlled like classical semiconductor. Theoretical and experimental studies on graphene doping show the possibility of opening the bandgap and modulating conducting types by substituting carbon atoms with foreign atoms. [ 2 ] Graphene is easily p-doped by adsorbates like physisorbed oxygen molecules, but complementary doping (both n-type and p-type doping) is essential for functional device applications like complementary metal-oxidesemiconductor (CMOS) circuits. [ 3 ] Recently, a number of approaches have been proposed to synthesize nitrogen-doped graphene (NG), such as chemical vapor deposition (CVD), [ 2 a, 4 ] arc-discharge, [ 2 b, 5 ] and post treatments. [ 6 ] Here, we report a new approach which makes use of embedded nitrogen and carbon atoms in metal substrate to prepare NG. As doping is accompanied with the combination of carbon atoms into graphene during annealing process, N atoms can be substitutionally doped into the graphene lattice. Our method provides not only a better control over the doping density but also a potential advantage to precisely control the solid dopants at desired locations to achieve patterned doping. Our approach for NG synthesis is actually the enthusiastic utilization of the very common segregation phenomenon to turn the trace amount of carbon and nitrogen dissolved in bulk metals into NG. [ 7 ] Metals usually contain a trace amount of carbon impurities, which could be brought into evaporated metal fi lm during the electron beam deposition process. [7a, 8 ]

Band structure engineering in (Bi 1− x Sb x ) 2 Te 3 ternary topological insulators

Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is to establish the existence of significant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi(1-x)Sb(x))(2)Te(3) ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi(1-x)Sb(x))(2)Te(3), indicating the robustness of bulk Z(2) topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behaves like one-quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on well-established semiconductor technology.

Intrinsic Topological Insulator Bi2Te3 Thin Films on Si and Their Thickness Limit

High-quality Bi2Te3 films can be grown on Si by the state-of-art molecular beam epitaxy technique. In situ angle-resolved photo-emission spectroscopy measurement reveals that the as-grown films are intrinsic topological insulators and the single-Dirac-cone surface state develops at a thickness of two quintuple layers. The work opens a new avenue for engineering of topological materials based on well-developed Si technology.

Crossover between Weak Antilocalization and Weak Localization in a Magnetically Doped Topological Insulator

We report transport studies on magnetically doped Bi(2)Se(3) topological insulator ultrathin films grown by molecular beam epitaxy. The magnetotransport behavior exhibits a systematic crossover between weak antilocalization and weak localization with the change of magnetic impurity concentration, temperature, and magnetic field. We show that the localization property is closely related to the magnetization of the sample, and the complex crossover is due to the transformation of Bi(2)Se(3) from a topological insulator to a topologically trivial dilute magnetic semiconductor driven by magnetic impurities. This work demonstrates an effective way to manipulate the quantum transport properties of the topological insulators by breaking time-reversal symmetry.

Electron interaction-driven insulating ground state in Bi2Se3 topological insulators in the two-dimensional limit

We report a transport study of ultrathin Bi

Thin films of magnetically doped topological insulator with carrier-independent long-range ferromagnetic order.

{}_{2}

GROWTH OF QUANTUM WELL FILMS OF TOPOLOGICAL INSULATOR Bi2Se3 ON INSULATING SUBSTRATE

Se

Evidence for Berezinskii-Kosterlitz-Thouless transition in atomically flat two-dimensional Pb superconducting films

{}_{3}

Field-effect modulation of anomalous Hall effect in diluted ferromagnetic topological insulator epitaxial films

topological insulators with thickness from one quintuple layer to six quintuple layers grown on sapphire by molecular beam epitaxy. At low temperatures, the film resistance increases logarithmically with decreasing temperature, revealing an insulating ground state. The insulating behavior becomes more pronounced in thinner films. The sharp increase of resistance with magnetic field, however, indicates the existence of weak antilocalization originated from the topological protection. We show that this unusual insulating ground state in the two-dimensional limit of topological insulators is induced by the combined effect of strong electron interaction and topological delocalization.

Growth dynamics and thickness-dependent electronic structure of topological insulator Bi2Te3 thin films on Si

Thin films of magnetically doped topological insulators Cr(0.22) (Bi(x) Sb(1-x) )(1.78) Te(3) are found to possess carrier-independent long-range ferromagnetic order with perpendicular magnetic anisotropy. The anomalous Hall resistance is greatly enhanced, up to one quarter of quantum Hall resistance, by depletion of the carriers. The results demonstrate this material as a promising system to realize the quantized anomalous Hall effect.