(242b) Understanding Adsorption Hysteresis In Porous Silicon

Mesoporous silicon (PSi) is made by electrochemically etching a silicon wafer. The mesoscale dimensions of the silicon framework left after etching combined with high specific surface area makes it useful for applications ranging from photonics to biosensors. Pores of a given mean diameter and geometry can be synthesized by adjusting the etching conditions. The electrochemical etching method does introduce some mesoscale disorder in the pore structure. This disorder seems to play a crucial role on the nature of the condensation and evaporation processes in these materials. Recent experiments in which nitrogen adsorption and desorption was studied in linear [Coasne et al., Phys. Rev. Lett., 88, 256102 (2002)] and inkbottle [Wallacher et al., Phys. Rev. Lett., 92, 195704 (2004)] pores of PSi opened to the bulk gas at one or both ends have revealed some unexpected features in the behavior. In particular:

a) Linear pores of Psi, closed at one end exhibit hysteresis, and behave similar to a linear pore open at both ends, contrary to conventional understanding from classical theories and molecular simulations;

b) Inkbottle pores opened at both end, behave very similar to closed inkbottles. In other words the wider section of pore, behaves the same regardless of its accessibility to bulk vapor. Typically when the wider section is denied access to the bulk, desorption happens via pore blocking or cavitation mechanisms. Since both open and closed pores exhibit similar sorption isotherms, the desorption mechanism is also unclear.

Using new measurements of nitrogen and argon adsorption, together with Monte Carlo simulations and density functional theory for a lattice gas model, we have investigated the role of pore size inhomogeneities and surface roughness on capillary condensation in porous silicon with linear and inkbottle pores. Our results resolve some puzzling features of earlier experimental work. We describe how quenched disorder makes the open and closed pores behave similarly. We also describe how slow uptake dynamics, a consequence of quenched disorder, may result in trapping of gas bubbles during the filling of the pores. This may determine the mechanism of the desorption in the closed inkbottle pore.