FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 7) 1400378 wileyonlinelibrary.com Formation of Air Stable Graphene p–n–p Junctions Using an Amine-Containing Polymer Coating Hossein Sojoudi, Jose Baltazar, Laren Tolbert, Clifford Henderson, and Samuel Graham* Dr. H. Sojoudi, Prof. S. Graham Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, Georgia 30332, USA E-mail: sgraham@gatech.edu Dr. J. Baltazar, Prof. C. Henderson School of Chemical & Biomolecular Engineering Georgia Institute of Technology Atlanta, Georgia 30332, USA Prof. L. Tolbert School of Chemistry & Biochemistry Georgia Institute of Technology Atlanta, Georgia 30332, USA DOI: 10.1002/admi.201400378 of graphene junctions have used multiple electrostatic gates, [7,8] electrical stress- induced doping, [9] chemical treatment by gas exposure, [10] molecular modifications on top of the graphene, [11–13] ionic liquid gating, [14] local doping by focused laser irradiation [15] and deep UV radiation, [16] and modification of the substrate by changing the local electrostatic potential in the vicinity of one of the contacts. [17,18] However, these methods might not be stable and can degrade carrier mobility by introducing defects. A low-temperature method was recently developed to fabri- cate junctions in graphene by modifying the interface between graphene and its support substrate with covalently bonded self-assembled monolayers (SAMs). [19,20] P- and n-type SAMs were patterned on a field effect transistor (FET) device, resulting in thermally stable graphene junctions while minimizing the introduction of defects. [19,20] However, these junctions are not stable upon expo- sure to air requiring hermetic packaging and multiple lithog- raphy steps to pattern a graphene FET channel. Here, we utilize an ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), on a back-gated FET device to obtain graphene p–n–p junctions. Recently, PEIE was employed as a universal method to lower the work function (WF) of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. [21] Amine-terminated compounds were shown to be effective molecules to control doping in graphene films. [19,20] In contrast to π-conjugated amine-containing small molecules and polymers, PEIE is an insulator, and it should not be regarded as a charge injection layer but rather as a surface modifier. The intrinsic molecular dipole moments associated with the neu- tral amine groups contained in such an insulating polymer layer, and the charge-transfer character of their interaction with the conductor surface, together reduce the WF of graphene. [21] In contrast, gold, a well-known electron acceptor, [22] results in p-doping of graphene. Charge transfer at a metal–graphene interface results in doping of the graphene sheet due to differ- ences in the WFs. [23–27] In physisorption interfaces such as those in graphene/gold contact, Fermi level pinning and Pauli-exclu- sion-induced energy-level shifts are shown to be two primary fac- tors determining graphene’s doping types and densities. [17,28–30] Thus, adding a layer of PEIE on a back-gated graphene FET device with gold contacts results in formation of graphene regions with n- and p-type characteristics on a single FET device. An ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), is deposited on a back-gated field effect graphene device to form graphene p–n–p junctions. Characteristic I–V curves indicate the superposition of two separate Dirac points, which confirms an energy separation of neutrality points within the complementary regions. This is a simple approach for making graphene p–n–p junctions without a need for multiple lithography steps or electrostatic gates and, unlike, the destruc- tive techniques such as substitutional doping or covalent functionalization, it induces a minor defect, if any, as there is no discernible D peak in the Raman spectra of the graphene films after creating junctions and degradation in the charge carrier mobilities of the graphene devices. This method can be easily processed from dilute solutions in environmentally-friendly solvents such as water or methoxyethanol and does not suffer any change upon exposure to air or heating at temperatures below 100 °C. 1. Introduction Graphene has many unique electrical properties, including its nearly linear energy dispersion relation, which results in electric-field-induced generation of electrons and holes in the material. These electrons theoretically travel as massless Dirac fermions with very high velocities. [1–5] Due to the zero-gap in single-layer graphene, both carrier type and concentration can be controlled through an electrostatic gate, making graphene a promising material for semiconductor applications. [1,3] Gra- phene junctions in which exciting phenomena such as Klein tunneling [6] and fractional quantum Hall transport [7] have been studied and observed, can be formed using this electrostatic gating. Most of the studies reported so far on the formation Adv. Mater. Interfaces 2014, 1400378 www.advmatinterfaces.de www.MaterialsViews.com