Engineering Ultra-Low Work Function of Graphene Hongyuan Yuan,* ,† Shuai Chang, ‡ Igor Bargatin, # Ning C. Wang, ‡ Daniel C. Riley, † Haotian Wang, ∥ Jared W. Schwede, † J. Provine, ‡ Eric Pop, ‡ Zhi-Xun Shen, †,∥ Piero A. Pianetta, ‡,⊥ Nicholas A. Melosh, § and Roger T. Howe ‡ † Department of Physics, ‡ Department of Electrical Engineering, § Department of Material Science and Engineering, and ∥ Department of Applied Physics, Stanford University, Stanford, California 94305, United States ⊥ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, MS31, Menlo Park, California 94205, United States # Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States * S Supporting Information ABSTRACT: Low work function materials are critical for energy conversion and electron emission applications. Here, we demonstrate for the first time that an ultralow work function graphene is achieved by combining electrostatic gating with a Cs/O surface coating. A simple device is built from large-area monolayer graphene grown by chemical vapor deposition, transferred onto 20 nm HfO 2 on Si, enabling high electric fields capacitive charge accumulation in the graphene. We first observed over 0.7 eV work function change due to electrostatic gating as measured by scanning Kelvin probe force microscopy and confirmed by conductivity measurements. The deposition of Cs/O further reduced the work function, as measured by photoemission in an ultrahigh vacuum environment, which reaches nearly 1 eV, the lowest reported to date for a conductive, nondiamond material. KEYWORDS: Graphene, work function, electrostatic gating, transistor, photoemission, scanning Kelvin probe force microscopy T he work function (Φ) of a material is the energy difference between its vacuum level and Fermi level (E F ). It is not a fundamental constant but can be tuned through doping or surface engineering. Materials with very low work function can significantly improve many electronic device technologies including organic electronics 1−4 and electron emission devices. 5−11 Similarly, recently proposed solar energy conversion technologies are predicted to have very high efficiencies if sufficiently low work function anodes can be produced. 12 To lower the work function of a material, one typical approach is to lower the vacuum level by surface engineering, particularly by depositing a very thin layer (about one monolayer) of alkali metal, such as Cs, Li, Sr, or Ba, 13−16 that are sometimes combined with a proper amount of oxygen. Among these approaches, Cs/O coated materials typically have the lowest work function between 1.1 and 1.4 eV. 17−19 The underlying mechanism for the work function reduction through the application of a thin layer of Cs/O has been extensively studied in the 1960s and 1970s, primarily driven by the development of negative electron affinity (NEA) photo- cathodes. 17,20−22 Another approach to lowering the work function is to raise the material’s Fermi level. In contrast to conventional three- dimensional materials, whose Fermi-level is normally “pinned” at the surface due to surface defects and traps, 23 the Fermi level of graphene (a two-dimensional material) can be effectively controlled by doping due to a lack of dangling bonds and surface states. By voltage biasing graphene relative to a gate, compensating charges build up in the graphene. This excess population of carriers shifts the Fermi level relative to its equilibrium value, thereby directly changing the graphene work Received: May 15, 2015 Revised: September 22, 2015 Published: September 24, 2015 Letter pubs.acs.org/NanoLett © 2015 American Chemical Society 6475 DOI: 10.1021/acs.nanolett.5b01916 Nano Lett. 2015, 15, 6475−6480