▁around ▁ 1 9 9 6 . ▁After ▁the ▁formation ▁of ▁por ous ▁sil icon , ▁the ▁sample ▁is ▁fro zen ▁at ▁a ▁temperature ▁of ▁about ▁ 2 0 0 ▁K ▁and ▁sub lim ed ▁under ▁vac u um . ▁ ▁Pent ane ▁dry ing ▁The ▁technique ▁uses ▁pent ane ▁as ▁the ▁dry ing ▁liquid ▁instead ▁of ▁water . ▁In ▁doing ▁so ▁the ▁cap ill ary ▁stress ▁is ▁reduced ▁because ▁pent ane ▁has ▁a ▁lower ▁surface ▁t ension ▁than ▁water . ▁ ▁S low ▁ev ap oration ▁S low ▁ev apor ating ▁technique ▁can ▁be ▁implemented ▁following ▁the ▁water ▁or ▁eth an ol ▁r ins ing . ▁It ▁was ▁found ▁that ▁slow ▁ev ap oration ▁decre ased ▁the ▁trap ▁density ▁ ▁Sur face ▁modification ▁of ▁por ous ▁sil icon ▁ ▁The ▁surface ▁of ▁por ous ▁sil icon ▁may ▁be ▁modified ▁to ▁exhib it ▁different ▁properties . ▁O ften , ▁fresh ly ▁et ched ▁por ous ▁sil icon ▁may ▁be ▁un stable ▁due ▁to ▁the ▁rate ▁of ▁its ▁ox id ation ▁by ▁the ▁atmosphere ▁or ▁un su itable ▁for ▁cell ▁attachment ▁purposes . ▁Therefore , ▁it ▁can ▁be ▁surface ▁modified ▁to ▁improve ▁stability ▁and ▁cell ▁attachment ▁ ▁Sur face ▁modification ▁impro ving ▁stability ▁ ▁Following ▁the ▁formation ▁of ▁por ous ▁sil icon , ▁its ▁surface ▁is ▁covered ▁with ▁c oval ently ▁bond ed ▁hydro gen . ▁Although ▁the ▁hydro gen ▁co ated ▁surface ▁is ▁sufficiently ▁stable ▁when ▁exposed ▁to ▁in ert ▁atmosphere ▁for ▁a ▁short ▁period ▁of ▁time , ▁prolong ed ▁expos ure ▁render