Effect of Surface State Density on Oxygen Chemisorption, Grain Potential and Carrier Concentration for Different Grain Sizes of Nanocrystallite Metal Oxide Semiconductors: A Numerical Modelling Approach

摘要

Metal oxide semiconductor-based gas sensors with nanosized grain have significantly high gas sensitivity as compared withthe larger grain counterparts. A numerical model has been presented to recognize the role of different sizes of nanocrystallitesof n-type SnO_2 semiconductors as gas sensors. The potential profile inside the spherical grains as a function of grain size,temperature and density of surface states has been calculated. Wolkenstein approach for dissociative chemical adsorptionof environmental oxygen has been adopted for all calculations. Both full and partial depletion approximations have beenconsidered to review the potential in spherical nanograins. This model is based on the analytical solution of Poisson’sequation in spherical coordinates and numerical solution of the electroneutrality condition. Effect of normalized potential onthe charge transfer at the available surface states has been calculated. Present work discusses the phenomenon of energy bandflattening for smaller nanocrystals. In spherical nanosized grains, the sharp change in the potential at the grain centre at a fixedvalue of surface state density has been explained. Calculations for carrier concentrations and normalized gas sensor responseas a function of nanosized grains have been carried out and explained on the basis of different experimental observations.