of 13
S
1
Su
pplementary
Information for:
O
PTICALLY
T
UNABLE
M
ESOSCALE
C
D
S
E
M
ORPHOLOGIES
V
IA
I
NORGANIC
P
HOTOTROPIC
G
ROWTH
K
ATHRYN
R.
H
AMANN
,
a
A
ZHAR
I.
C
ARIM
,
a
M
ADELINE
C.
M
EIER
,
a
J
ONATHAN
R.
T
HOMPSON
,
b
N
ICOLAS
A.
B
ATARA
,
b
I
VAN
S.
Y
ERMOLENKO
,
c
H
ARRY
A.
A
TWATER
b
AND
N
ATHAN
S.
L
EWIS
*
a
a
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA 91125, USA. E
-
mail: nslewis@caltech.edu
b
Division of Engineering and Applied Sciences, Califo
rnia Institute of Technology, Pasadena,
CA 91125, USA
c
Bruker Corporation, Nano Surfaces Division, San Jose, CA 95134, USA
These authors contributed equally (KRH and AIC)
Electronic
Supplementary
Material
(ESI)
for
Journal
of
Materials
Chemistry
C.
This
journal
is
©
The
Royal
Society
of
Chemistry
2020
S
2
S1.
Contents
This document contains
a description of the
experimental
and
modeling/simulation
methods
utilized in this work
(Section
s
S
2
and S3
)
, additional scanning
-
electron micrographs
and
two
-
dimensional Fourier transform data
(Section S4),
additional computer simulation data
(Section S5)
,
and a list of asso
ciated references (Section
S
6
).
S
3
S2.
Experimental
Methods
Materials and Chemicals
H
2
SO
4
(ACS Reagent, J. T. Baker),
buffered
HF
improved
etchant
(
Transene
)
, In (99.999 %, Alfa Aesar), Ga (99.999 %, Alfa Aesar), SeO
2
(99.
999
%,
Acros
Organics
),
CdSO
4
(
99
+
%, Sigma
-
Aldrich)
,
and CS
2
(
99.9
+ %, Alfa Aesar)
were
used as received.
H
2
O with a resistivity ≥ 18.2 M
Ω
cm (Barnstead Nanopure System) was used throughout.
Au
-
coated
n
+
-
Si(1
00
) (
< 0.005
Ω
cm, As
-
doped,
525 ± 2
5 μm, single
-
side polished, Addison
Engineering
) was used as a substrate for deposition. Flash
-
Dry
Ag
Paint (SPI Supplies),
EP21ARHTND Epoxy
(
MasterBond
)
,
and nitrocellulose
-
based nail polish were used to assemble
the working electrodes.
Substrate Preparation
n
+
-
Si wafers were etched with bu
ffered HF
(aq)
for 30 s, rinsed with
H
2
O,
dried under a stream of N
2
(g)
, and then immediately transferred to an electron
-
beam metal
evaporator with a base pressure < 10
-
5
torr. Using an accelerating voltage of 10 kV, a 10 nm Ti
adhesion layer was deposited
on the polished side of the wafer
with
a 50 mA deposition current
and a 50 nm Pt capping layer was then deposited
with
a 150 mA current. The wafers were then
transferred to a RF sputterer in which 100 nm of Au was deposited on top of the Pt using a RF
po
wer of 80 W. The Au
-
topped Si sections were cut into square 0.50
by 0.50 cm sections for use
as deposition substrates.
Electrode Preparation
One end of a Sn
-
coated Cu wire (22 AWG) was bent to form a small,
flat coil
and the wire was threaded through glass tubing (6 mm O. D.) such that the coil was just
outside the tubing. Epoxy was applied to seal the end of the tube from which the coil protruded.
A
eutectic mixture of Ga and In was scratched
with a carbide
-
tipped scr
ibe
into the unpolished back
surfaces of the Au
-
topped Si sections.
The wire coil was
then contacted to the unpolished surface
and affixed with
Ag
paint. Nail polish was applied to insulate the unpolished face, the wire
-
coil
S
4
contact
,
and the exposed wire b
etween the coil and epoxy seal.
Immediately before deposition,
the
surface of each electrode was briefly cleaned
u
sing
a stream of N
2
(g)
.
Electrode Illumination
Illumination for photoelectro
chemical
growth
was provided by
light
-
emitting diode (LED
)
sources (Thorlabs) including
narrowband source
s
with
respective
intensity
-
weighted average wavelength
,
λ
avg
, values
and spectral bandwidth
s
(FWHM) of
458 nm
and 20 nm (SOLIS
-
460), and
528 nm and 32 nm (SOLIS
-
525
B or SOLIS
-
525C
)
.
Additionally,
a
broadband L
ED source (SOLIS
-
3C)
was utilized in conjunction with
a 550 nm bandpass filter
(Edmund Optics 33
-
330)
to
produce
a spectral distribution with
λ
avg
and FWHM
values
of 550 nm
and 93 nm.
The output of
each
LED
source was collected,
collimated
, and
condensed
using
a
bi
-
convex lens (Ø50.8 mm, f = 60 mm)
in conjunction with
two
aspheric condenser lens
es
25.4
mm, f =
16
mm
; Ø
50.8
mm, f =
32
mm
).
A
dichroic film polarizer (Thorlabs LPVISE
200
-
A
) was
inserted
before the final lens
to effect linear
polarization
.
For experiments involving simultaneous
illumination with two
sources, a polka dot beam splitter (Thorlabs BPD508
-
G) was utilized to
combine the outputs; both sources were incident upon the beamsplitter at an angle of 45° from the
surface norm
al and thus generated coaxial output.
For all experiments
,
a
1500 grit ground
-
glass
(N
-
BK7) diffuser was placed immediately in front of the photoelectrochemical cell to ensure
spatial homogeneity of the illumination.
The light intensity incident on the el
ectrode was measured by placing a calibrated Si
photodiode (Thorlabs FDS100)
in the photoelectrochemical cell with electrolyte
in place of an
e
lectrode assembly
, and the steady
-
state current response of that Si photodiode was measured.
Depositions were pe
rforme
d with a
total
light intensity of
I
0
=
0.25
0 W cm
-
2
unless otherwise
indicated.
For
experiments involving simultaneous illumination with two orthogonally polarized
S
5
sources, an intensity of 0.7
·
I
0
was provided by one source and an intensity of
0.3
·
I
0
was
provided
by
the orthogonally polarized source.
Photoelectrochemical
Growth
P
hotoelectrochemical growth
was
performed
using
a Bio
-
Logic SP
-
200 potentiostat
and
a single
-
compartment glass cell with
a
quartz
window.
A
three
-
electrode configuration was utilized with a graphite
-
rod counter electrode (99.999 %, Sigma
-
Aldrich) and a Ag/AgCl reference electrode (3
.00
M KCl
,
Bioanalytical Systems). Films were
grown
from an aqueous solution of 0.0
050
0 M SeO
2
, 0.
2
0
0
M
CdSO
4
,
and
0
.
10
0
M
H
2
S
O
4
.
Growth
was effected by
supplying
a current density of
-
0.
50
mA
cm
-
2
at
the
Au
-
coated
electrode
,
illuminated as detailed under the above subheading (
Electrode Illumination
)
,
at room temperature
until
a charge of
-
1.0
C
cm
-
2
had
passed
.
After
growth
, the electrode was immed
iately removed
from the cell,
rinsed with H
2
O
, and then
dried under a stream of N
2
(g)
.
The
Au
-
coated
substrate
with
top
-
facing
Se
Cd
film was
mechanically
separated from the
rest of the electrode assembly.
The nitrocellulose
-
based insulation
and
the majority of the
Ag
paint and In
-
Ga eutectic were then
removed mechanically.
Chemical
Post
-
Processing
Following
photoelectrochemical growth
as detailed in the above
subheading, th
e substrate and Se
-
Cd film were transferred to a round
-
bottom glass flask
and
immersed in CS
2
(l)
. The
CS
2
(l)
was maintained at the boiling point (46° C) under reflux
for 15 h
,
to
effect the
elimination
of excess Se
from
the
film
s
and
thereby
produce
stoichiometric
CdSe. The
electrode was then removed from solution and subsequently rinsed with
H
2
O
and dried under a
stream of
N
2
(g)
.
Microscopy
Scanning
-
electron micrographs (SEMs) were obtained with a FEI Nova
NanoSEM 450 at an accelerating voltage of 5.00 kV with a working distance of 5 mm and an in
-
lens secondary electron detector.
Scanning
-
elect
ron m
icrographs obtained for quantitative analysis
S
6
were ac
quired with a resolution of 172 pixels μm
-
1
over
~
120 μm
2
area whereas
m
icrographs
that
were used
to produce display figures were acquired with a resolution of 344 pixels μm
-
1
over
~
2
μm
2
areas.
Atomic
-
force
micrographs were
collected with a Dimension Ic
on Atomic Force
Microscope (Bruker Nano Surfaces)
using ScanAsyst
-
Air probes (Bruker AFM Probes).
Micrographs
were
collected with a
resolution of
342
pixel
s
μm
-
1
over ~
1
μm
2
areas
.
Energy
-
D
ispersive X
-
ray Spectroscopy
Energy dispersive X
-
ray
(EDX) spectroscopy
was
performed in the SEM using an accelerating voltage of 15.00 kV
with a
working distance of
5
mm.
An Oxford Instruments X
-
Max
Si
drift detector was utilized. Spectra were collected in the range
of 0 to 10 keV and quantitative film compositions were derived from these spectra using the
“INCA” software package (Oxford Instruments).
X
-
ray
Diffraction
Grazing incidence X
-
ray diffracti
on
(GIXRD)
was performed using a
Bruker D8 Discover diffractometer
with a
Cu K
α
source and a 2
-
dimensional Vantec detector.
The
X
-
rays were directed at a grazing angle
ω = 0.3 °
above th
e plane of the sample surface and the
detector was swept throughout th
e entire 2
θ
range.
S
7
S
3
.
Modeling and Simulation
Methods
Simulation of Film Morphology
The
growth
s
of
the photoelectrochemically
deposited
films
w
ere
simulated
with an iterative
growth
model
in which
electromagnetic simulations were
first
used to calculate the local photocarrier
-
generation rate
s at the film surface.
Then, mass addition
was simulated via a Monte Carlo method wherein the local photocarrier
-
generation rate weighted
the local rate of mass
ad
dition along the film surface.
Growth simulations began
with a bare, semi
-
infinite planar
substrate.
In the first step, the
light
-
absorption
profile under
a
linearly polarized, plane
-
wave illumination
source
was calculated
using
full
-
wave finite
-
differe
nce time
-
domain (FDTD) simulations
(“FDTD Solutions” software
package, Lumerical)
with periodic boundary conditions along the substrate interface
.
In the second
step
, a Monte Carlo simulation
was
perfor
med in which an amount of mass, equaling that of a 1
5
nm planar layer covering the simulation area, was
added to the upper surface of the structure with
a probability
F
:
=
3
1
=
)
(
i
r
x
G
G
F
i
i
(
Equation 1
)
where
G
is the
spatially
dependent
photocarrier
-
generation rate
at the deposit
/
solution
interface
,
x
i
is the fraction of
i
th
nearest neighbors occupied in the cubic lattice, and
r
i
is the distance to the
i
th
nearest
neighbor.
The multiplicative sum
in
the definition of this probability (Equation 1) serves
to
reduce
the surface roughness of the film so as to mimic the experimenta
lly observed surface
roughness.
A
value of n = 1.33 was used as the refractive index of the
growth solution
, regardless
of wav
elength.
1
Simulations of the film morphology utilized the intensity
-
weighted average
wavelengths,
λ
avg
, of the experimental sources described
in Section S2. The electric
-
field vector of
the illumination was oriented parallel to the substrate.
A t
hree
-
dimensional
cubic
mesh with a
lattice constant of
7
nm was used for the simulations.
After
the initial Monte Carlo simulation, t
he
S
8
absorbance of t
he new, structured film was then calculated in the same manner as
for
the initial
planar film
,
and
an
additional
Monte Carlo
simulation of mass addition was performed.
Th
is
process of
absorbance calculation
and mass addition
was
repeated for a total of
1
8
iterations.
Dipole Simulations
Dipole simulations were performed using a FDTD method. A two
-
dimensional square simulation plane was utilized. Dipoles were arranged with a separation of
twice the emission wavelength and the oscillation axis was set perpen
dicular to the separation
axis.
S
9
S
4
.
Additional Scanning
-
Electron Micrographs
and
Two
-
Dimensional
F
ourier
T
ransform
Data
Figure
S
1.
(a) Low
-
magnification SEM of a
film
generated using vertically polarized
λ
avg
= 528 nm illumination
.
(
b
)
-
(
d
)
High
-
magnification
SEMs acquired from the areas indicated in the SEM presented in (a
).
S
10
Figure S
2
.
SEM representative of a film generated using
vertically
polarized
λ
avg
= 528 nm illumination.
S
11
Figure
S
3
.
(a) SEM
representative of a film
generated using
diagonally
polarized
(aligned θ = 45° counterclockwise
from the horizontal)
λ
avg
= 528 nm illumination
.
(
b
)
2D FT
derived
from a SEM of the film depicted in (a).
Figure
S
4
.
SEMs representative of
films generated using vertically polarized
λ
avg
= 528 nm illumination
at the
indicated intensity (
I
0
= 250 mW cm
-
2
).
S
12
S
5
.
Additional Computer Simulation Data
Figure S5.
Normalized time
-
average of the E
-
field magnitude from two dipoles emitting radiation with a free space
wavelength of λ = 528 nm in a medium of refractive index
n
= 1.33. Dipoles separated by
the indicated distance d
along the horizontal
with the
oscillation axis perpendicular to the axis of separation.
Figure
S
6
.
Normalized time
-
average of the E
-
field magnitude from two dipoles emitting radiation with a free space
wavelength of λ = 528 nm in a medium of
refractive
index
n
= 1.33. Dipoles separat
ed by a distance of twice the
wavelength
along an axis θ = 45
° counterclockwise from the
horizontal with the oscillation axis perpendicular to the
axis of separation.
Figure
S
7
.
Normalized time
-
average
of the E
-
field magnitude from two dipoles emitting radiation with
the indicated
free space wavelength in a medium of
refractive
index
n
= 1.33. Dipoles separated by a distance of twice the
wavelength along the horizontal with the oscillation axis perpendi
cular to the axis of separation.
S
13
S
6
.
References
1.
G. M. Hale and M. R. Querry,
Appl. Opt.
, 1973,
12
, 555
-
563.