Article pubs.acs.org/JPCC
Molecular Doping Control at a Topological Insulator Surface: F4‑TCNQ on Bi2Se3 J. Wang,† A. S. Hewitt,† R. Kumar,‡ J. Boltersdorf,§ T. Guan,† F. Hunte,‡ P. A. Maggard,§ J. E. Brom,∥ J. M. Redwing,∥,⊥ and D. B. Dougherty*,† †
Department of Physics, ‡Department of Materials Science and Engineering, and §Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ∥ Department of Materials Science and Engineering and ⊥Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
ABSTRACT: Recent electrical measurements have accessed transport in the topological surface state band of thin exfoliated samples of Bi2Se3 by removing the bulk n-type doping by contact with thin films of the molecular acceptor F4-TCNQ. Here we report on the film growth and interfacial electronic characterization of F4-TCNQ grown on Bi2Se3. Atomic force microscopy shows wetting layer formation followed by 3D island growth. X-ray photoelectron spectroscopy is consistent with this picture and also shows that charge transferred to the molecular layer is localized on nitrogen atoms. Ultraviolet photoelectron spectroscopy shows a work function increase and an upward shift of the valence band edge that suggest significant reduction in carrier density at the Bi2Se3 surface. Different kinds of impurity dopants12−14 have been used to move the Fermi level into the bulk band gap. Another promising approach to control the carrier density near the surface involves charge transfer to a strong acceptor species adsorbed on the surface. A recent transport measurement showed a change of carrier density on the Bi2Se3 surface by depositing tetrafluorotetracyanoquinodimethane (F 4 TCNQ).15 Ambipolar electrical transport was observed that is characteristic of the Dirac-like TSS band on Bi2Se3 with F4TCNQ on the surface.15 The strong electron acceptor F4-TCNQ has been widely used for contact doping control due to its high electron affinity (5.24 eV).16 It has been reported to reduce the hole injection barrier in organic light-emitting diodes17,18 and has been successfully used to tune the carrier type and concentration in graphene,18,19 carbon nanotubes,20,21 coinage metals,22,23 as well as organic semiconductors.24,25 The molecule is a strong candidate p-type dopant for Bi2Se3 and has the advantage that it
I. INTRODUCTION Topological insulators (TIs) have attracted significant attention due to the presence of two-dimensional Dirac-like surface states protected by a bulk topological invariant.1,2 This protection of topological surface states (TSSs) makes them insensitive to perturbations that do not change the symmetries required to establish the invariant (typically time-reversal symmetry, though others are also known3,4). The properties of this topological surface state (TSS) such as unique spin-momentum correlations5 suggest possible spintronic and quantum computing applications. Recently, Bi2Se3 has become the prototypical material to study the properties of three-dimensional topological insulators6−8 because of its relatively large bulk band gap and single Dirac-like topological surface state. Despite the bulk band gap, it remains a major challenge to experimentally deconvolute bulk and surface transport since almost all Bi2Se3 is found to be heavily n-doped owing to Se vacancies.9,10 Angle-resolved photoelectron spectroscopy (ARPES) shows that the Fermi level is typically located well within the bulk conduction band. This indicates that significant bulk and surface transport occur simultaneously in most measurements.6,11 © 2014 American Chemical Society
Received: December 27, 2013 Revised: June 11, 2014 Published: June 12, 2014 14860
dx.doi.org/10.1021/jp412690h | J. Phys. Chem. C 2014, 118, 14860−14865
The Journal of Physical Chemistry C
Article
can be easily vapor deposited directly onto the substrate surface with very precise thickness control. This contrasts with the often complicated solid-state chemistry required to use bulk compensating dopants.12−14 In this paper, we report on the characterization of the film morphology and interfacial electronic structure of Bi2Se3 surfaces in contact with thermally deposited F4-TCNQ films. We present direct electronic structure evidence that F4-TCNQ accepts negative charge from the surface using work function measurements and ultraviolet photoelectron spectroscopy of the surface valence bands. Atomic force microscopy (AFM) shows a layer-plus-island growth mode that suggests most doping effects occur in the first molecular layer that wets the surface. X-ray photoelectron spectroscopy shows that transferred charge is localized on the nitrogen atoms on the F4TCNQ molecule. These observations apply to molecular films on both Bi2Se3 single crystals grown from a binary melt and Bi2Se3 films grown on sapphire using hybrid physical chemical vapor deposition.26
II. EXPERIMENTAL METHODS Bi2Se3 single crystals were prepared in a molten-flux reaction by heating elemental Bi beads (Aldrich, 99.999%; 1−5 mm particle size) and Se pellets (Aldrich, 99.999%;
