1
Supporting Information
Primary Corrosion Processes for Polymer Embedded Free
-
Standing or Substrate
-
Supported Silicon Microwire Arrays in Aqueous Alkaline Electrolytes
Kathleen
M.
Kennedy
a
, Paul
A
ndrew
Kempler
b
, Miguel Cab
á
n
-
Acevedo
b
, Kimberly M.
Papadantona
kis
b
,
Nathan S. Lewis
b,c
*
a
Division of Engineering and Applied Science, California Institute of Technology, Pasadena,
California 91125, United States
b
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena
,
Californi
a 91125, United States
c
Beckman Institute, California Institute of Technology, Pasadena, California 91125, United
States
*
Corresponding author:
nslewis@caltech.edu
Sample cleaning procedure
Chips of approximatel
y 5 cm x 2 cm were scribed from this wafer of wire arrays.
The
wire arrays were cleaned using a standard Radio Corporation of America (RCA) procedure
, in
which
the samples were
first
cleaned in a Standard Clean 1 bath, 5:1:1 H
2
O/NH
4
OH/
H
2
O
2
at 80
°C for >10
min.
The samples were then dipped into
Buffered oxide etchant (6:1 (v/v) 40% NH
4
F
to 49% HF
;
Transene
Inc.)
for
5 min at 20 °C
,
and were
then removed and immediately rinsed
with
>18
M
ohm
-
cm resistivity deionized water and blown dry un
der a stream of N
2
(g)
.
A
RCA
Standard Clean 2 bath, 6:1:1 H
2
O/HCl/H
2
O
2
at 70 °C for >10 min was used to remove SiO
2
,
Al
2
O
3
and trace metal impurities.
Hydrogen Peroxide 30% (w/w) Solution GR ACS
was
obtained from Millipore Sigma, and
Ammonia solution 28.0
-
30.0%
(w/w) was obt
ained from
2
J.T. Baker.
Hydrochloric Acid GR ACS
36.5
-
38.0%
(w/w) was obtained from Millipore Sigma.
All chemicals were used as received.
Sample assembly
The on
-
substrate samp
les were placed in a Falcon polystyrene petri dish, and Loctite
EA9460 epoxy was u
sed to seal all exposed edges and secure the sample to the bottom of the
petri dish. The bottom
of
another four Falcon polystyrene petri dishes
was covered with a
10:10:1 mix
ture by weight of polydimethylsiloxane (PDMS) from Sylguard® elastomer silicone
ba
se, toluene (Millipore Sigma
≥
99.5%, GR ACS), and Sylguard® 184 silicone elastomer
curing agent,
and cured on glass slides using a VWR hot plate at 75
°
C for 24 h. The four
free
-
standing
samples were placed in these petri dishes, and again were placed on g
lass slides on a
VWR hot plate at 75
°
C for 1 h
to bind the PDMS layers.
Fifty milliliters of 1.0 M KOH(aq) (Sigma
-
Aldrich
≥
85% KOH basis, pellets) was
prepared and poured i
nto each of the 8 petri dishes, such that each sample was well covered by
the liqui
d. All samples were then left in the dark, with pairs of one on
-
substrate sample and one
free
-
standing
sample removed after 24 h, 48 h, 168 h, and 240 h of immersion time.
R
emoval
of sample
from KOH
As each sample was removed, the KOH(aq) was poured out a
nd the sample was rinsed
thoroughly with 18.3 M
Ω
-
cm deionized H
2
O. The samples were then blown dry under a stream
of N
2
(g) and placed in a Napco 5831 Vacuum Oven for 1 h at
room temperature.
3
Statistics for Optically Identified Etched Wires
Figure S1
. Percent of etched wires seen with the optical
microscope
, with
averages given across
multiple fields of view in the optical microscope
. These data do not take into account incomplete
bottom
-
up etching, which cannot be seen optical
ly
. A data point is not given for the free
-
standing
sample after 248 h i
n 1 M KOH(aq) because no unetched wires were observed.
4
Figure
S
2
. Pair distribution
functions
(PDF’s)
for
etched wires identified in optical
images
for
a
n
on
-
sample (a) and
free
-
standing sample
(b) after
7 days
in
1 M
KOH
(aq)
.
Panel
(c) shows the
PDF for the
free
-
standing
image with the most
optically visible
etched wires, compared to a
random distribution of the same number of wires in (d).
The optical images used to generate
panels (b) and (c) were taken from different locations on
the same sample.
Th
e
analysis
in Fig S2
shows that the top
-
down etching visible under the optical microscope
approaches a random distribution,
whereas
the top
-
down to bottom
-
up corrosion seen in the on
-
substrate samples results in growing clusters of etch
ed wires.