Porous silicon was accidentally discovered by Uhlir at Bell
Laboratories in the mid 1950s.[1] He was tryin todevelo! a means to
electrochemically machine silicon wa"ers "or use in microelectronic
circuits. He "oundthat under the a!!ro!riate conditions o" a!!lied
current and solution com!osition# the silicon did not
dissolveuni"ormly but instead "ine holes were !roduced# which
!ro!aated !rimarily in the $100% direction in thewa"er. &ince
this did not !rovide the smooth !olish desired# these curious
results were re!orted in a Bell labstechnical note[1] and the
material was !retty much inored. 'n the 19(0s and 19)0s a moderate
level o"interest arose because the hih sur"ace area o" !orous
&i was "ound to be use"ul as a model o" thecrystalline &i
sur"ace in s!ectrosco!ic studies#[*+5] as a !recursor to enerate
thic, o-ide layers on &i# andas a dielectric layer in
ca!acitance+based chemical sensors.[] /hen# in the late 19)0s# Leih
anham at thee"ense 2esearch 3ency in 4nland reasoned that the
dia!hanous &i "ilaments enerated when the !oresbecome lare and
numerous enouh to overla! miht dis!lay uantum con"inement e""ects.
/his intuitionturned out to be correct# and the electrochemically
etched material was re!orted to "luoresce with a brihtred+orane
color.[(# )] 3s e-!ected "rom the uantum con"inement
relationshi!#[9] this color is at an enerythat is sini"icantly
larer than the banda! enery "or bul, &i# which occurs in the
in"rared reion o" thes!ectrum.6ith the discovery o" e""icient
visible liht emission "rom !orous &i came an e-!losion o" wor,
"ocused oncreatin &i+based o!toelectronic switches# dis!lays#
and lasers. Problems with the material7s chemical andmechanical
stability and its disa!!ointinly low electroluminescence e""iciency
led to a wanin o" interest bythe mid 1990s. 'n the same time
!eriod# the uniue "eatures o" the material+ its lare sur"ace area
within asmall volume# its controllable !ore si8es# its convenient
sur"ace chemistry# and its com!atibility withconventional silicon
micro"abrication technoloies+ins!ired research into a!!lications
"ar outside o"o!toelectronics. " !articular relevance to our
research are the biomedical[10+1)] and sensor !ro!erties o"
