Ab Initio Studies of Work Function Changes of CO Adsorption on Clean and Pd-Doped ZnGa\({}_{2}\)O\({}_{4}\)(111) Surfaces for Gas Sensors  
[PERSON]  
1 Center for General Education, Chang Gung University, Taoyuan 33302, Taiwan; [EMAIL_ADDRESS]  
[PERSON]  
2 Department of Surgery, Show Chwan Memorial Hospital, Changhua City 50008, Taiwan; [EMAIL_ADDRESS]  
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3 Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung 402, Taiwan; [EMAIL_ADDRESS] (C.-A.P. ); [EMAIL_ADDRESS] (D.-R.H.)  
Institute of Electronics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan; [EMAIL_ADDRESS]  
Intelligent Minimally-Invasive Device Center, National Chung Hsing University, Taichung 402, Taiwan; [EMAIL_ADDRESS]  
Graduate Institute of Biomedical Engineering, National Chung Hsing University, Taichung 402, Taiwan  
Tomon and Development Center of Sustainable Agriculture, National Chung Hsing University, Taichung 402, Taiwan  
Tailchung 402, Taiwan  
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surface band bending is the difference between the work function and electron affinity of target gases on ZnGa\({}_{2}\)O\({}_{4}\)(111) surfaces. The surface energy band bending makes it difficult for electrons inside the semiconductor to migrate to the surface, thereby increasing the surface resistance of the semiconductor. In contrast, when the reducing gas contacts the \(n\)-type ZnGa\({}_{2}\)O\({}_{4}\) semiconductor, electrons flow from the reducing gas to the \(n\)-type semiconductor until the Fermi level is balanced, which results in ohmic contact formation [2; 3; 4; 5]. Ohmic contact means that the current can flow in both directions at the junction between the reducing gas and the semiconductor, and the ohmic contact make the electrons inside the semiconductor easily migrate to the surface, thereby reducing the surface resistance of the semiconductor. Recently, we have demonstrated that an NO\({}_{2}\)-oxidizing and H\({}_{2}\)S-reducing molecule adsorbed on the Ga-Zn-O-terminated ZnGa\({}_{2}\)O\({}_{4}\)(111) surface exhibit the highest work function change of +0.97 eV and \(-\)1.66 eV, respectively [2]. Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111) sensors also exhibit high selectivity to NO\({}_{2}\) gas detectors, which have the maximum work function change of +1.37 eV and +2.37 eV for one and two NO\({}_{2}\) molecules to a Pd atom, respectively [5]. In contrast, the work function change of one H\({}_{2}\)S molecule on the Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111) surface is reduced to \(-\)0.90 eV, resulting in the decrease in sensitivity to a palladium adsorbed on the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface [5]. In this regard, the work function changes of two H\({}_{2}\)S molecules on the Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111) surface changes significantly to \(-\)1.82 eV or shows a trend of increasing sensitivity with increasing gas molecule concentration [5]. The gas sensing sensitivity is not only dependent on the work function change, but also related to the adsorption energy of gas molecules. The adsorption energy of gas molecules with exothermic reaction can spontaneously react to cause gas molecule adsorption, so that the gas sensing sensitivity can be estimated by the change of the work function [5]. However, the adsorption energy of an NO\({}_{2}\) (H\({}_{2}\)S) molecule adsorbed to the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface is 0.64 eV (2.32 eV), showing that the positive adsorption energy or the corresponding endothermic process can be supported by an increase in temperature [5]. High positive adsorption energy means that more external energy is required, suggesting that the film is not easy to attract gas molecules and the sensitivity will be limited even under large work function changes.