Department: Mechanical & Aerospace Engineering
Research Institute Affiliation: Graduate Program in Materials Science and Engineering
Faculty Advisor(s): Andrew Kummel

Primary Student
Name: Jun Hong Park
Email: jhp032@ucsd.edu
Phone: 858-534-3498
Grad Year: 2014

Student Collaborators
James Royer, jeroyer@ucsd.edu | Tyler Kent, tjkent@ucsd.edu

Copper phthalocyanine (CuPc) thin film devices have been widely studied for use as chemical vapor sensors; however, the molecular scale sensing mechanism remains undetermined. This study presents molecular scale observation of NO2 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 exposed to different NO2 concentrations at atmospheric pressure. After annealing at 50 ˚C to improve STM imaging, for low NO2 doses (1 ppm for 5min), the STM images reveal small NO2 chemisorption sites on the CuPc metal centers primarily along the domain boundaries. These weak chemisorbates almost completely desorb from the CuPc monolayer after annealing at 100 ˚C for 1 hr consistent with reversible chemisorption. Conversely, for high NO2 doses (10 ppm for 5 min), the NO2 induces irreversible effects due to strong adsorbate-surface interaction: The NO2 dosing induces a fracture of the CuPc domains. As new domain boundaries are generated, each domain size becomes smaller. After dosing 10 ppm NO2 for 2 hr, the domains are raised 1-2 above the original CuPc monolayer consistent with subsurface adsorption at the CuPc-Au(111) interface. The domain fracture can only be reversed by annealing above 150 ˚C consistent with irreversible chemisorption at near ambient temperatures. The behavior of NO2 dosed CuPc monolayers suggests that NO2 undergoes simple weak chemisorption with the CuPc metal centers at low exposure, but at high exposures, NO2 also induces a strong subsurface adsorption inducing fracture of the CuPc domains. The STM data is consistent with two chemisorption mechanisms of a strong oxidant on a single molecule inducing two different classes of sensor response: reversible sensing at low exposure and dosiometric (irreversible) sensing at high exposures. While nearly all weakly bonding analytes give reversible mobility sensor responses on CuPc chemical field-effect transistors (chemFETs), some strong oxidants are observed to give reversible mobility responses at short exposures and dosimetric irreversible threshold voltage responses for longer exposures. The domain fracturing is likely the source of the highly sensitive dosiometric sensing of NO2 by CuPc chemFETs. The reversal of domain fractures in the CuPc monolayers was not readily observed when held at 300 K, consistent with the macroscopic sensing experiments.

Related Files:

  1. Abstractfigure.jpg

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