AVS 59th Annual International Symposium and Exhibition
    Surface Science Thursday Sessions
       Session SS-ThM

Paper SS-ThM11
Atomic Study for P-type Doping Process of CuPc Molecules with STM

Thursday, November 1, 2012, 11:20 am, Room 22

Session: Molecular Films: Chirality & Electronic Features
Presenter: J.H. Park, University of California San Diego
Authors: J.H. Park, University of California San Diego
K. Tyler, University of California San Diego
C.T. William, University of California San Diego
A.C. Kummel, University of California San Diego
Correspondent: Click to Email
Metal phthalocyanines (MPc) have been widely employed as channel materials in organic thin film transistor (OTFT) for chemical vapor sensing, due to their novel gas adsorption properties. Theoretically, MPc molecules act electron donors during reaction with oxidative analytes and are the basis of their chemical sensing. However, this sensing reaction has not been understood fully. This study presents molecular scale observation of NO adsorption onto CuPc monolayers using ultra-high vacuum (UHV) scanning tunneling microscopy (STM). CuPc monolayers were deposited on Au (111) surfaces by organic molecular beam epitaxy in ultra-high vacuum (UHV) and subsequently dosed at 150 K substrate temperature with NO (5% : diluted by He of 95 %) via a supersonic molecular beam source (MBS). After dosing NO for 1 min, STM images reveal small NO chemisorption sites on the CuPc metal centers and ~4 % of CuPc molecules are reacted with NO. Un-reacted central Cu2+ ion of CuPc appears as dark hole in both of empty and filled states STM images. Conversely, after chemisorption of NO, the topographic appearance of core metal ion is modified into a bright spot, while the 4-leaf pattern of ring still remain. As dosing duration increases to 10 min, the coverage of NO chemisorption sites increases to ~7 %. However this coverage increase is sublinear and further dosing does not increase the coverage consistent with a chemisorption induced change in electronic structure. In order to study the electronic structure of NO chemisorption onto CuPc molecules, scanning tunneling spectroscopy (STS) data was also obtained. Unreacted CuPc has a Fermi level (EF) almost in the middle of the band gap. However, after NO chemisorption, this central EF shifts to the highest occupied molecular orbital (HOMO) and this transition indicates CuPc molecules is doped to ‘p-type’ by NO. This ‘doping’ process of NO is in good agreement with previously published theoretical predictions and is consistent with the high sensitivity of CuPc film to strong oxidants in CuPc OTFT chemical sensors.