Understanding Capillary Condensation and Hysteresis in Porous Silicon: Network Effects within Independent Pores

Porous silicon exhibits very interesting properties with a wealth of potential applications, especially in photonics. The ability to produce porous silicon materials via electrochemical etching of bulk silicon that have tailor-made pores of varying diameters and constrictions has made them attractive systems for investigations of the relationship between pore structure, capillary condensation and hysteresis. Some surprising observations emerged from this work, including the result that linear pores closed at one end exhibited hysteresis in nitrogen adsorption experiments at 77K. Classical theory and recent molecular simulations indicate that idealized smooth-walled linear pores closed at one end should not exhibit hysteresis - the closed pore end promotes condensation during adsorption while contact with the bulk at the open end promotes evaporation upon desorption.

Using both experimental measurements and a lattice gas model in mean field theory, we have investigated the role of pore size inhomogeneities and surface roughness on capillary condensation of nitrogen at 77 K in porous silicon with linear pores. Our results resolve some puzzling features of earlier experimental work. We find that this material has more in common with disordered materials such as Vycor glass than the idealized smooth-walled cylindrical pores discussed in the classical adsorption literature. We provide strong evidence that this behavior comes from the complexity of the processes within independent linear pores, arising from the pore size inhomogeneities along the pore axis, rather than from cooperative effects between different pores.