Investigation of New Sensor Concept Using Non-Adiabatic Energy Transfer and "Chemicurrent" Production from Gas Adsorption and Reaction on Ultrathin Metal Films

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The detailed mechanisms and process dynamics by which the transfer of energy occurs during adsorption and reaction on catalytic surfaces are of fundamental interest. In experiments investigating energy transfer during chemisorption we were able to show experimentally that electronic excitations were a significant component of chemisorption energy transfer associated with atomic hydrogen or atomic deuterium on Ag and Cu ultrathin films (Phys. Rev. Letters 82 (2), 446-449 (1999)). With subsequent NSF support we demonstrated the ubiquitous presence of chemically induced charge transport for a variety of chemical interactions on a variety of different metal surfaces (Science 294 (5551) 2521-2523 (2001)). That energy transfer from chemisorption can proceed by direct electronic excitation is a departure from the conventional dogma which holds that multiple phonon excitation is the primary means through which reaction energy is dissipated. We observe the electronic excitations using ultra-thin metal films (~10 nm) deposited onto semiconductors in a Schottky diode detector structure as a "chemicurrent" resulting from reaction induced excited charge carriers which travel ballistically across the metal film and traverse the Schottky barrier. Though many questions were answered during this initial grant period many others were raised with significant implications for our understanding of surface catalyzed reactions and how we might link chemical processes at surfaces and electronics ("chemielectronics"). In our continuing investigations we will work towards: i) measuring the energy distribution of electrons excited in the metal surface, ii) characterizing the dependence of the electronic excitation probability and energy distribution on the kinetic energy of the adsorbate, iii) investigating the role of electronic excitations in surface mediated energy transfer, iv) investigate new device structures with applications to sensors, and v) developing a comprehensive theoretical model to understand and interpret the electronic excitation probabilities and energy distributions. The intellectual merit of our work is in developing a better understanding of the reaction associated electronic excitation phenomena. Broader impacts include potential applications for new sensors and chemielectronic devices as well as in the general area of heterogeneous catalysis. This project will impact basic surface science by helping to clarify our view of surface energy transfer and the connections between chemical and electronic observables. The immediate application will be in the practical area of sensor technology where we will have introduced a new class of solid-state sensor and photovoltaic device while the long-term significance will be in helping to improve our understanding of a basic process in nature. Most importantly, funds from this project will allow the continued high quality education of outstanding young scientists and engineers.