LEE ET AL . VOL. 9 ’ NO. 11 ’ 10861–10866 ’ 2015 www.acsnano.org 10861 November 07, 2015 C 2015 American Chemical Society Proximity Effect Induced Electronic Properties of Graphene on Bi 2 Te 2 Se Paengro Lee, † Kyung-Hwan Jin, † Si Jin Sung, † Jin Gul Kim, † Min-Tae Ryu, † Hee-Min Park, † Seung-Hoon Jhi, † Namdong Kim, * ,‡ Yongsam Kim, ‡ Seong Uk Yu, § Kwang S. Kim, ^ Do Young Noh, ) and Jinwook Chung * ,† † Department of Physics, ‡ Pohang Accelerator Laboratory, and § Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea, ^ Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea, and ) Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea T opological insulators (TIs) 1À4 and graphene, 5À7 despite their different dimensionality and structure, share a common feature revealing the linear Dirac bands with a unique chirality associated with the asymmetry of electron spin and of charge distribution, respectively. Various heterostructures have been studied to cre- ate or improve functionality of graphene, for example, a better surface flatness with an improved carrier mobility as reported in the heterostructure of graphene on hex- agonal boron nitride (hBN) 8,9 and also for the interface of grapheneÀTI. 10 Adding a spin degree of freedom to the π-electrons of graphene, in particular, has been a challenge to explore any spin-driven feature that can be exploited in future spintronics. 11À24 The heterostructural interfacing of graphene with TI may bring about new exotic properties mainly stemming from the hybridization of their respective Dirac fermions and the in- version symmetry breaking at the interface to enhance the spinÀorbit coupling (SOC) in graphene. 11À13 As is well known, the SOC in graphene through the mixing of π- and σ-electrons is extremely weak, 14À19 and there have been several efforts to enhance the SOC in graphene, for example, by using adatoms such as thallium, indium, and atomic hydrogen. 20À24 Here we have devised a new technical means to form a grapheneÀTI interface and to enhance the SOC in graphene by bring- ing the two Dirac points (DPs) together, i.e., the DP-induced proximity effect, without sacrificing the intrinsic nature of graphene. We have grown a three-dimensional (3D) TI film of Bi 2 Te 2 Se (BTS) onto a chemical vapor deposition (CVD) graphene, which becomes remarkably flattened upon the growth of BTS with its epitaxial domains increased enough to produce the π-band in our pho- toemission data. We observe both the to- pological surface state (TSS) of BTS and the linear π-band of graphene coexisting at the center (Γ-point) of the surface Brillouin zone using angle-resolved photoemission spec- troscopy (ARPES) with synchrotron photons. We demonstrate that the two DPs of the π-band of graphene and of the TSS of BTS can be fine-tuned by adjusting the adsorption * Address correspondence to (J. Chung) jwc@postech.ac.kr, (N. Kim) east@postech.ac.kr. Received for review June 23, 2015 and accepted November 7, 2015. Published online 10.1021/acsnano.5b03821 ABSTRACT We report that the π-electrons of graphene can be spin-polarized to create a phase with a significant spinÀorbit gap at the Dirac point (DP) using a graphene- interfaced topological insulator hybrid material. We have grown epitaxial Bi 2 Te 2 Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands from both the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, Δ DÀD , can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as Δ DÀD = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows the opening of a spinÀorbit gap of ∼20 meV in the π-band, enhanced by 3 orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semimetallic to a quantum spin Hall phase when Δ DÀD e 0.20 eV. We thus present a practical means of spin- polarizing the π-band of graphene, which can be pivotal to advance graphene-based spintronics. KEYWORDS: grapheneÀtopological insulator interface . control of Dirac point . proximity effect . enhanced spinÀorbit coupling . spinÀorbit gap ARTICLE