of 15
S
1
Supp
orting
Information for:
P
ATH
-
D
EPENDENT
M
ORPHOLOGICAL
E
VOLUTION OF
S
E
-
T
E
M
ESOSTRUCTURES
P
REPARED
B
Y
I
NORGANIC
P
HOTOTROPIC
G
ROWTH
K
ATHRYN
R.
H
AMANN
,
A
ZHAR
I.
C
ARIM
,
M
ADELINE
C.
M
EIER
,
N
ATHAN
S.
L
EWIS
,
*
Division of Chemistry and Chemical
Engineering
California Institute of Technology
Pasadena, CA 91125
S
2
S1.
Contents
Th
is
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
(Section S4),
Fourier transform data (Section S5),
microstructural analysis data (Section S
6
)
,
additional
computer simulation
data (Section S
7
),
and a list of associated references (Section S
8
).
S
3
S2.
Experimental
Methods
Mate
rials and Chemicals
H
2
SO
4
(ACS Reagent, J. T. Baker),
buffered
HF
improved
etchant
(
Transene
), SeO
2
(
99.
4
%,
Alfa Aesar
), and
TeO
2
(99
+
%,
Sigma
-
Aldrich
) were used as
received. H
2
O with a resistivity ≥ 18.2 M
Ω
cm (Barnstead Nanopure System) was used throughout.
Au
-
coated
n
+
-
S
i(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 buffered HF 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
-
6
torr. Using an accelerating voltage of 10 kV, a 10 nm Ti
adhesion layer was deposited on the polished side of the wafer u
sing a
4
0 mA
deposition current
and then
50 nm of Au
was deposited on top
of the Ti using a
1
5
0 mA deposition current
.
20
nm
of
Ti was
deposited on the unpolished side of the wafer to serve as a back
-
contact. The Au
-
topped Si
sections were then cut into square
0.
5
0
c
m by
0.50
c
m sections for use a
s
deposition substrates.
Electrode Preparation
Electrode assemblies were prepared by applyin
g epoxy to the flat
sides of each of two Al half
-
round bars (0.25 in diameter). The two bars were then joined together,
with an ~ 10 mm offset in the axial dimension to form a cylinder with two half
-
round ends.
Polytetrafluoroethylene heat
-
shrink tubing wa
s used to insulate the cylindrical section and epoxy
was used to insulate the rounded side of one of the half
-
round ends.
Ag paint was applied to the
Ti
-
coated b
ack surfaces of the Au
-
topped Si sections
and the sections were
affixed to the flat
surface of
the epoxied half
-
round end. Nail polish was used to insulate the remaining uncovered
area on the flat surface that surrounded the Au
-
topped Si section. Figure S1 presents a schematic
S
4
of an electrode assembly with an attached Au
-
topped Si section. Immediate
ly before deposition,
the surface of each electrode was briefly cleaned using a stream of N
2
(g).
Figure S1.
Schematic of an electrode assembly with an attached Au
-
topped Si section.
Electrode Illumination
Illumination for photoelectrochemical
growth
was
generated using
two
narrowband diode (LED) sources
.
The
first
(Thorlabs, M970L4)
provided an
intensity
-
weight
ed
average wavelength,
λ
avg
,
value
of 955 nm
with
a spectral bandwidth (FWHM) of 60 nm.
The second (Thorlabs, SOLIS
-
850C) was utilized in conjunction with a long pass filter with a 875
nm cut
-
on wavelength (Edmund Optics, 64
-
684) to
provide a
λ
avg
value of 885 nm with a FWHM
of 11 nm
.
The
λ
avg
= 955 nm illumination was collected,
condensed and collimated
using a single
aspheric
lens
(
Ø50.8 mm, f = 32 mm
)
whereas
for
λ
avg
= 885 nm illumination
an aspheric lens
(Ø25.4 mm, f = 1
6 mm
) followed by two
bi
-
convex lenses (Ø50.8 mm, f = 60
mm
and
Ø50.8 mm,
f = 1
0
0 mm
) w
ere
utilized for illumination.
A
broadband
film polarizer (LPNIRE200
-
B) was
inserted
after the lenses
to
effect vertical linear polarization
.
A 1500 grit ground
-
glass (
UV Fused
Silica
) diffuser was placed immediately in front of the photoelectrochemical cell to ensure spatial
homogeneity of the illumination.
S
5
The light intensity incident on the electrode was measured by placing a calibrated Si
photodiode (Thorlabs FDS100)
,
instead
of
a
n electrode
assembly
,
in the
photoelectrochemical cell
with electrolyte
,
and the stea
dy
-
state current response of that
Si photo
diode
was measured
.
Depositions
with
λ
avg
= 885 nm and 955 nm were performed with light intensities of
I
0
=
1
1
0
mW
cm
-
2
, and 130 mW cm
-
2
, respectively.
Photoelectrochemical Deposition
P
hotoelectrochemical deposition was
performed
using
a
Bio
-
Logic SP
-
200 potentiostat.
Deposition was performed in a two
-
compartment glass cell
with
a
quartz
window.
A
three
-
electrode configuration was utilized with a
n
Ir wire
counter electrode
(99.999 %, Sigma
-
Aldrich)
isolated behind a porous glass frit
and a Ag/AgCl
reference electrode
(3
.00
M KCl
,
Bioanalytical Systems). Films were deposited from an aqueous solution
of 0.0
2
00
M SeO
2
, 0.0
10
0 M
TeO
2
,
and
2.00
M
H
2
S
O
4
.
D
eposition was effected by
supplying a current
density
of
j
=
-
8
.0
0
mA
cm
-
2
to the
Au
-
coated
electrode
for
t
0
=
2.00 min
(charge density of
Q
0
=
0.96 mC cm
-
2
)
,
illuminated as detailed under the above
subheading (
Electrode Illumination
)
.
For
depositions using two sequential, discrete illumination inputs, the electrode was
transiently floated
to
open circuit following initial deposition as the illumination input w
as changed, and then
deposition
was
continued
with new illumination input
by
supplying
j
=
-
8.00
mA cm
-
2
for an
additional
t
1
=
t
0
=
2.00 min
(
additional
charge density of
Q
1
=
Q
0
=
0.96
mC cm
-
2
)
unless otherwise
noted
.
After deposition, 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
-
Te
film was
mechanically
separated from the
rest of the electrode assembly.
The nitrocellulose
-
based insulation
and
the majority of the
Ag
paint
were
then removed mechanically.
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
-
S
6
lens secondary electron detector. Micrographs obtained for quantitative analysis were acquired
with a reso
lution of 172 pixels μm
-
1
over
~
120 μm
2
areas. Micrographs
that were used
to produce
display figures were
acquired with a resolution of 344 pixels μm
-
1
over
~
2
μm
2
areas.
Raman
Spectroscopy
Raman
spectra were collected with a Renishaw
inVia
Raman
microprobe equipped with a Leica DM 2500 M microscope, a Leica N Plan 50x
objective (numerical aperture = 0.75), an 1800 lines mm
-
1
grating, and a CCD detector configured
in a 180° backscatter geometry.
A 532 nm diode
-
pumped solid
-
state (DPSS) laser (Renis
haw
RL532C50) was used as the excitation source and a 10 μW radiant flux was incident on the surface
of the sample. A
λ/4
plate was used to circularly polarize the incident excitation. No polarizing
collection optic was used.
X
-
Ray Diffraction
X
-
ray diffr
action (XRD)
data
was
collected
using a Bruker D8 Discover
diffractometer with a Cu K
α
source and a
two
-
dimensional detector
in a Bragg
-
Brentano geometry.
S
7
S
3
.
Modeling and Simulation
Methods
Simulation of Film Morphology
The growths of the photoelectrochemically deposited films
were simulated
with an iterative
growth
model
in which electromagnetic simulations were first
used to calculate the local photocarrier
-
generation rates at the film surface. Then, mass addition
was s
imulated via a Monte Carlo method wherein the local photocarrier
-
generation rate weighted
the local rate of mass addition 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
-
difference time
-
domain (FDTD) simulations
(“FDTD Solutions” software
package, Lumerical)
with periodic boundary conditions along the substrate interfac
e
. In the second
step, a Monte Carlo simulation
was
perfor
med in which an amount of mass, equaling that of a 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.
After
the initial Monte Carlo simulation, t
he absorbance of the 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 perfor
med.
Th
is process of
absorbance calculation
and mass
addition
was
repeated
until the
simulated
morphologies
achieved heights equivalent to
those
exhibited by the experimentally generated
deposits.
To
model growth
using two sequential,
S
8
discrete illumination inputs, the
computational process was
also first
iterated
using the initial
illumination input
until the simulated
morphology
height
achieved heights equivalent to those
exhibited by the experimentally generated
deposits using that initial input
.
Then,
the
simulated
illumination was updated to
represent the new optical input
and the
computational process was
further iterated
until
the simulated morphologies achieved
heights
equivalent to those exhibited by
the ex
perimentally generated deposits for growth with
sequential, discrete illumination inputs.
Previously measured values of the wavelength
-
dependent complex refractive index of Se
-
Te w
ere
utilized.
A
value of n = 1.33 was used as the refractive index of the e
lectrolyte
,
regardless
of wavelength.
1
Simulations of the film morphology utilized the
intensity
-
weighted average
wavelength
s,
λ
avg
,
of the experimental sources described in Section S2.
A two
-
dimensional square
mesh with a lattice constant of
5
nm
was used for the simulations.
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 perpendicular to the separation axis.
S
9
S
4
.
Additional Scanning
-
Electron Micrographs
Figure
S
2
.
Top
-
down
SEMs representative of
films
generated with the indicated λ
0
illumination (a) and (b) and then
extended with the indicated λ
1
illumination (c) and (d).
Figure S3.
Representative cross
-
sectional
, intermediate magnification
SEMs of deposits generated first with the
indicated λ
0
illumination and then extended with the indicated λ
1
illumination.
S
10
Figure S
4
.
Representative cross
-
sectional SEMs of deposits generated using λ = 955 nm illumination with an intensity
of
I
=
I
0
for the indicated time
(
j
=
j
0
)
.
Q
= 0.75 ·
Q
0
for (a) and 1.25·
Q
0
for (b).
Figure S5.
Representative cross
-
sectional SEMs of deposits generated using λ = 955 nm illumination with the
indicated intensity for
t
=
t
0
(
j
=
j
0
;
Q
=
Q
0
).
S
11
S5.
Fourier Transform Data
Figure S
6
.
(a) and (b)
2D FTs
derived from top
-
down SEM data of deposits
generated with the indicated λ
illumination
. (c) Fourier spectra derived from the 2D FT data in (a) and (b).
S
12
S
6
.
Deposit Microstructural Analysis
Figure S
7
presents a representative Raman spectrum of a deposit generated via inorganic
phototropic growth. The
spectrum exhibit
s
two principal modes centered at 144 cm
-
1
and 175 cm
-
1
. The
se
modes
are
consistent with a substitutional alloy of Se and Te in
a trigonal
crystal
structure
common to each element in the pure phase
.
2
Figure S
8
presents a representative X
-
ray diffraction
(XRD) data of a deposit generated via inorganic phototropic growth. The observed reflections are
consistent with the
crystalline
phase
suggested
by the Raman analysis an
d
indicate a
polycrystalline character and lack of preferred orientation.
3
,
4
Figure S
7
.
Representative Raman spectrum of a deposit generated via inorganic phototropic growth.
Figure S
8
.
Representative XRD data of a deposit generated via inorganic phototropic growt
h with reflections
characteristic of Se
-
Te and the Au substrate indicated.
S
13
S
7
.
Additional
Computer Simulation Data
Figure
S
9
.
Normalized time
-
average
s
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.
Point dipole sources were used to model the amplitude of the electric field modulation at
the growth interface caused by scattering of the incident illumination by the
by the topology of the
nascent electrochemically genera
ted deposit
. Figure S
3
a and b
present simulations of normalized
time
-
averaged electric field magnitude
s
from two dipoles emitting radiation with free space
wavelengths of λ =
885
nm
and
955
nm
, respectively,
in a medium of index
n
= 1.3
3
(
reflective of
the
growth electrolyte
)
and separated by a distance of two wavelengths perpendicular to the
oscillation axis. In each case,
periodic bands of elevated field intensity were
observed. The spacing
and width of the
se
regions of
elevated
field intensity
were proportional to the wavelength
; periods
of 333 nm and 359 nm were observed for
λ =
885
nm
and
955
nm
, respectively
. Th
ese
data
agree
with the
experimental observation
of
scaling of lamellar pitch
with
the input
λ
avg
.
This
behavior
is
consistent with the notion that photoelectrochemical growth occurs preferentially in areas of
localized field elevation and thereby physica
l
l
y
records the input illumination.
S
14
Figure
S
10
.
From left to right: successive
iterations of simulated
morphologies generated initially generated using λ
0
= 885 nm illumination and progressively extended the indicated λ
0
illumination and progressively extended with λ
1
=
955 nm illumination.
Figure
S
11
.
From left to right: successive iterations of simul
ated morphologies generated initially generated using λ
0
= 955 nm illumination and progressively extended the indicated λ
0
illumination and progressively extended with λ
1
=
885 nm illumination.
S
15
S
8
.
References
(1) Hale, G. M.; Querry, M. R. Optical Constants of Water in the 200
-
nm to 200
-
μm
Wavelength Region.
Appl. Opt.
1973
,
12
, 555
-
563.
(2) Geick, R.; Steigmeier, E. F.; Auderset, H. Raman Effect in Selenium
-
Tellurium Mixed
Crystals.
Phys. Status Solidi B
197
2
,
54
, 623
-
630.
(3) Smith, T. W.; Smith, S. D.; Badesha, S. S. Chemical Alloying, a Novel Method for the
Preparation of Homogenous Se
x
Te
1
-
x
Alloys.
J. Am. Chem. Soc.
1984
,
106
, 7247
-
7248.
(4) Mayers, B.; Gates, B.; Yin, Y.; Xia, Y. Large
-
Scale Synthesis
of Monodisperse Nanorods of
Se/Te Alloys Through a Homogenous Nucleation and Solution Growth Process.
Adv. Mater.
2001
,
13
, 1380
-
1384.