Large-Area and Transferred High-Quality Three-Dimensional Topological Insulator Bi2-xSbxTe3-ySey Ultrathin Film by Catalyst-Free Physical Vapor Deposition.

Uniform and large-area synthesis of bulk insulating ultrathin films is an important subject toward applications of a surface of three-dimensional topological insulators (3D-TIs) in various electronic devices. Here we report epitaxial growth of bulk insulating three-dimensional topological insulator (3D-TI) Bi2-xSbxTe3-ySey (BSTS) ultrathin films, ranging from a few quintuple to several hundreds of layers, on mica in a large-area (1 cm2) via catalyst-free physical vapor deposition. These films can nondestructively be exfoliated using deionized water and transferred to various kinds of substrates as desired. The transferred BSTS thin films show good ambipolar characteristics as well as well-defined quantum oscillations arising from the topological surface states. The carrier mobility of 2500-5100 cm2/(V s) is comparable to the high-quality bulk BSTS single crystal. Moreover, tunable electronic states from the massless to the massive Dirac fermion were observed with a decrease in the film thickness. Both the feasible large-area synthesis and the reliable film transfer process can promise that BSTS ultrathin films will pave a route to many applications of 3D-TIs.

In-plane topological p-n junction in the three-dimensional topological insulator Bi2-xSbxTe3-ySey.

A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three-dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi2-xSbxTe3-ySey thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of Bi2-xSbxTe3-ySey on its top half by using tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics.

Electric Properties of Dirac Fermions Captured into 3D Nanoporous Graphene Networks.

Nanoporous graphene- based electric-double-layer transistors (EDLTs) are successfully fabricated. Transport measurements of the EDLTs demonstrate that the ambipolar electronic states of massless Dirac fermions with a high carrier mobility are well preserved in 3D nanoporous graphene along with anomalous nonlinear Hall resistance and exceptional transistor on/off ratio. This study may open a new avenue for device applications of graphene.

High-quality three-dimensional nanoporous graphene.

We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.