Molecular Doping Control at a Topological Insulator Surface: F 4 ‑TCNQ on Bi 2 Se 3 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 Bi 2 Se 3 by removing the bulk n-type doping by contact with thin films of the molecular acceptor F 4 -TCNQ. Here we report on the film growth and interfacial electronic characterization of F 4 -TCNQ grown on Bi 2 Se 3 . 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 Bi 2 Se 3 surface. 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 known 3,4 ). The properties of this topological surface state (TSS) such as unique spin-momentum correlations 5 suggest possible spintronic and quantum comput- ing applications. Recently, Bi 2 Se 3 has become the prototypical material to study the properties of three-dimensional topological insu- lators 6−8 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 Bi 2 Se 3 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 Different kinds of impurity dopants 12−14 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 Bi 2 Se 3 surface by depositing tetra fl uorotetracyanoquinodimethane (F 4 - TCNQ). 15 Ambipolar electrical transport was observed that is characteristic of the Dirac-like TSS band on Bi 2 Se 3 with F 4 - TCNQ on the surface. 15 The strong electron acceptor F 4 -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 diodes 17,18 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 Bi 2 Se 3 and has the advantage that it Received: December 27, 2013 Revised: June 11, 2014 Published: June 12, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 14860 dx.doi.org/10.1021/jp412690h | J. Phys. Chem. C 2014, 118, 14860−14865