of 15
Supp
orting
Information for:
MORPHOLOGICAL EXPRES
SION OF
THE COHERENCE
AND RELATIVE PHASE O
F OPTICAL INPUTS
TO
THE
PHOTOELECTRODEPOSITI
ON
OF NANOPATTERNED
SE
-
TE FILMS
A
ZHAR
I.
C
ARIM
1
,
N
ICOLAS
A.
B
ATARA
2
,
A
NJALI
P
REMKUMAR
2
,
R
ICHARD
M
AY
1
,
H
ARRY
A.
A
TWATER
2
,3
AND
N
ATHAN
S.
L
EWIS
1
,
3,4
*
1
Division of Chemistry and Chemical Engineering
2
Division of Engineering and Applied Sciences
3
Kavli Nanoscience Institute
4
Beckman Institute
California Institute of Technology
Pasadena, CA 91125
*Corresponding Author:
nslewis@caltech.edu
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
),
cross
-
sectional scanning
-
electron micrographs
(Section S4), analyses of
the
elemental composition and the structure
of the photoelectrodeposits
(Sections S5 and S6)
,
and a list of associated references (Section S
7
).
S2.
Experimental
Methods
Materials
and Chemicals
(CH
3
)
2
CO (ACS Grade, BDH), H
2
SO
4
(ACS Reagent, J. T.
Baker), HF (49 %, Semiconductor Grade, Puritan Products), In (99.999 %, Alfa Aesar), Ga (99.999
%
, Alfa Aesar), SeO
2
(99.4 %, Alfa Aesar), and TeO
2
(99+ %
,
Sigma
-
Aldrich) were used as
recei
ved. H
2
O with a resistivity
≥ 18.2
M
Ω
cm (Barnstead Nanopure System) was used throughout.
n
+
-
Si(111) (0.004
0.006 Ω
cm, As
-
doped, 400 ± 15 μm, single
-
side polished, Addison
Engineering) was used as a substrate for deposition. Flash
-
Dry Silver Paint (SPI
Supplies),
Double/Bubble Epoxy (Hardman) and nitrocellulose
-
based nail polish were used to assemble the
Si working electrodes.
Electrode Preparation
One end of a Sn
-
coated Cu wire (22 AWG) was bent to form a small,
flat coil and the wire was then 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.
Square Si wafer sections (ca. 5 mm by 5 mm) were cut and a eutectic mixture of Ga and In was
scra
tched into the unpolished surfaces with a carbide
-
tipped
scribe. 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 contact
,
and the exposed wire between the coil and epoxy seal.
Immediately before deposition, the Si surface of each electrode was cleaned with
(CH
3
)
2
CO
, and
then the Si section of the electrode was immersed in a 49 wt.
%
solution of HF(aq) for ~ 10 s
to
remove an
y surficial SiO
x
from the Si. The electrode was then rinsed
with H
2
O
and
dried under a
stream of N
2
(g)
.
Electrode Illumination
Illumination for the photoelectrochemical depositions was provided
by narrowband diode (LED) sources with an intensity
-
weighted
λ
avg
value of 630 nm and a spectral
bandwidth (FWHM) of 18 nm (Thorlabs M625L2 and M625L3).
The output of each diode source
was collected and collimated with an aspheric condenser lens (Ø30 mm, f = 26.5 mm).
For
experiments involving simultaneous
illuminati
on with
two LED
sources,
a polka dot beam splitter
(Thorl
abs BPD508
-
G) was utilized to combine the outputs. Both sources were incident upon the
beam
splitter at an angle of 45 ° from the surface normal
,
and thus generated coaxial output. A
dichroic film po
larizer (Thorlabs LPVISE2X2 or LPNIRE200
-
B) was placed between each source
and the
beam splitter
to enable independent control of the polarization of each source.
A 1500 grit
ground
-
glass (N
-
BK7) diffuser was placed immediately in front of the
photoelectrochemical cell
to ensure spatial homogeneity of the illumination.
Additionally, a HeNe laser (Aerotech LSR5P) emitting at 632.8 nm in a TEM
00
mode with
linear polarization was also used
an illumination source
. The HeNe laser was fitted with a 1
0x
beam expander (Melles
-
Griot) to create a spot that overfilled
the working electrode. The output
from the HeNe laser was directed at normal incidence through a zero
-
order
λ/
4
plate (Thorlabs
WPQ
10
E
-
633). The
λ/4
plate was rotated a
bout
the optical axis s
uch that the fast axis of the plate
was orie
nted at angles between 0 and 45
° clockwise from the polarization axis of the laser
. The
presence of the
λ/4
plate
generated a
φ
= 90
° phase angle between
the orthogonal components of
the laser illumination and pr
ovided for the generation of defined elliptical polarizations.
The light intensity incident on the electrode was measured by placing a calibrated Si
photodiode (Thorlabs FDS100)
instead
of
an electrode
assembly
in the
photoelectrochemical cell
with electr
olyte
,
and the stea
dy
-
state current response of that
Si photo
diode
was measured
.
All
depositions were performed with an intensity of 13.7 mW cm
-
2
at the electrode.
Photoelectrochemical Deposition
P
hotoelectrochemical deposition was
performed
using
a
Bio
-
Logic SP
-
200 potentiostat.
Deposition was performed in a single
-
compartment glass cell with
a
pyrex
window.
A three
-
electrode configuration was utilized with a graphite
-
rod counter electrode
(99.999 %, Sigma
-
Aldrich) and a Ag/AgCl reference electrod
e (3 M KCl
,
Bioanalytical Systems).
Films were deposited from an aqueous solution of 0.0200 M SeO
2
, 0.0100 M TeO
2
,
and 2.00 M
H
2
SO
4
.
D
eposition was effected by biasing the n
+
-
Si electrode
,
illuminated as detailed under the
above subheading (
Electrode Illum
ination
)
,
potentiostatically
at
-
0.40 V vs. Ag/AgCl for
5.0
0
min
at room temperature.
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
Si 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 and In
-
Ga eutectic 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
-
lens secondary electron detector. Micrographs obtained for quantitative analysis were acquired
with a reso
lution of 172 pixels μm
-
1
over ca. 120 μm
2
areas. Micrographs utilized to produce
display figures were acquired with a resolution of 344 pixels μm
-
1
over ca. 8 μm
2
areas.
Energy
-
dispersive X
-
ray Spectroscopy
Energy dispersive X
-
ray spectroscopy (EDS) was
p
erformed in a Zeiss 1550VP SEM with an accelerating voltage of 15.00 kV and a working
distance of 12 mm. An Oxford Instruments X
-
Max silicon 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).
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), a 1800 lines mm
-
1
grating, and a CCD detector configured
in a 180° backscatter geometry.
A 532 nm diode
-
pumped solid
-
state (DPSS) laser (Renishaw
RL532C50) was used as the excitation source and a 10 μW radiant flux was incident
on the surface
of the sample. A line focus lens was utilized to transform the circular incident beam in one
dimension to generate a ca. 50 μm line at the sample. A
λ/4
plate was used to circularly polarize
the incident excitation. No polarizing collection
optic was used.
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
wherein 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
Si
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
wi
th 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
n
+
)
p
+
(n
+
1
=
)
(
2
2
0
0
i
r
x
G
G
G
F
i
i
i
n
n
p
p
(
Equation 1
)
where
G
is the
spatially
dependent
photocarrier
-
generation rate
at the deposit
/
solution interface
,
n
i
is the intrinsic carrier concentration,
n
0
is the electron concentration,
p
0
is the hole concentration,
τ
n
is the electron lifetime,
τ
p
is the hole lifetime,
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 performed.
Th
is process of
absorbance calculation
and mass
addition
was
repeated for a total of
2
0 iterations.
General Parameters
Se
Te films were assumed to be undoped (i.e.
n
0
=
p
0
=
n
i
) and a value
of
n
i
= 10
10
cm
-
3
was used for the in
trinsic carrier concentration.
1
A value of 1
μ
s was us
ed for
both the elec
tron and hole
lifetimes
.
2
Previously measured
values of the c
omplex index of
refraction
for Se
-
Te w
ere
utilized.
3
A value of n = 1.33 was used as the refractive index of the
electrolyte
,
regardless of wavelength.
4
Illumination intensities identical to those used
experimentally (see Section S2) were
used
in the simulations.
Simulations of the film morphology
utilized the peak intensity wavelength of the experimental sources described in Section S2.
The
electr
ic field vector of the illumination was oriented parallel to the substrate.
A two
-
dimensional
square mesh with a lattice constant of 1 nm was used for the simulations.
All FDTD simulations
were performed using the
“FDTD Solutions” software package (Lumeric
al)
.
S4.
Cross
-
Sectional Scanning
-
Electron Micrographs
Figure S
1
.
(a) Plot of the E
-
field vector of a LED source with λ
avg
= 630 nm linearly polarized 45
°
clockwise from
the vertical
,
and (b)
cross
-
sectional
SEM representative of a photoelectrode
posit generated with this source (cleaved
parallel to the long axis of the anisotropic lamellar
-
type pattern)
. (c) Plot illustrative of the many E
-
field vectors
characteristic of the same source as in
panel a
when unpo
larized
,
and (d)
cross
-
sectional
SEM representative of a
photoelectrodeposit generated with such
a
source in the unpolarized state.
Figure S
2
.
Two
-
source illumination polarization effect on photoelectrodeposit
morphology for near
-
orthogonal and
orthogonal polarizations. (a)
-
(d) Plots of the E
-
field vectors, E
0
and E
1
, of two incoherent LED sources with λ
avg
= 630
nm and equal intensity, the first source polarized vertically (θ
0
= 0 °) and the second at the indi
cated rotation (θ
1
)
clockwise from the vertical
,
and (e)
-
(h)
cross
-
sectional
SEMs representative of photoelectrodeposits generated using
these sources
(cleaved parallel to the long axis of the anisotropic lamellar
-
type pattern)
.
Figure S3.
(a)
-
(
d) Plots of the E
-
field vector traced over time at a fixed point for illumination provided by a HeNe
laser λ
avg
= 632.8 nm with
defined elliptical polarizations. ψ indicates the orientation of the major axis of the ellipse
measured clockwise from the verti
cal. χ represents the angle between the major axis and a line connecting a vertex on
the major axis with one on the minor axis and relates the eccentricity and asymmetry of the ellipse. (e)
-
(h)
C
ross
-
sectional
SEMs representative of photoelectrodeposits ge
nerated with the elliptical illumination profiles indicated in
panels a
-
d
respectively
(cleaved parallel to the long axis of the anisotropic lamellar
-
type pattern for (e)
-
(g))
.
S5.
Elemental Composition Analysis of Photoelectrodeposits
The elemental composition of all of the photoelectrodeposits was analyzed using energy
-
dispersive X
-
ray spectroscopy (EDS). All analyzed films were found to be wholly composed of
Se and Te. Photoelectrodeposits generated using
a single incoherent LED source with
λ
avg
= 633
nm
were found to on average
have compositions of
56
atomic % Se (remainder Te)
both when the
illumination was polarized and when the illumination was unpolarized
.
Figure S4
presents a plot
of the elemental com
position (in terms of atomic % Se) of the photoelectrodeposits
generated
by
simultaneously using two incoherent LED sources that had λ
avg
= 630 nm and equal intensities,
with the first source polarized vertically (θ
0
= 0°) and the second source offset cloc
kwise from the
vertical by θ
1
, as a function of θ
1
.
Figure S
5
presents analogous data pertaining to the
photoelectrodeposits generated
using a HeNe laser with λ
avg
= 632.8 nm with defined elliptical
polarizations wherein ψ = χ, as a function of ψ.
In
all
c
ases,
the average compositions of the
photoelectrodeposits
were found to range between
53
and
56
atomic % Se.
Figure S4.
Plot
of the elemental composition
, in terms of atomic % of Se, of
photoelectrodeposit
s generated using
two incoherent LED sources with λ
avg
= 630 nm and equal intensity, the first source polarized vertically (θ
0
= 0 °) and
the second at a defined rotation (θ
1
) clockwise from the vertical
,
as a function of θ
1
.
Photoelectrodeposits were
composed wholly of Se
and Te.
Figure S5.
Plot
of the elemental composition, in terms of atomic % of Se, of photoelectrodeposits
,
generated using a
HeNe laser with λ
avg
= 632.8 nm with defined elliptical polarizations
wherein ψ = χ, as a function of ψ
.
ψ indicates the
orienta
tion of the major axis of the ellipse measured clockwise from the vertical. χ represents the angle between the
major axis and a line connecting a vertex on the major axis with one on the minor axis and relates the eccentricity and
asymmetry of the ellipse.
Photoelectrodeposits were composed wholly of Se and Te.
S
6
.
S
tructural Analysis of Photoelectrodeposits
Figure S6 presents a Raman spectrum representative of the
Se
-
Te
photoelectrodeposits
generated in this work.
The spectrum displays modes centered at
96 cm
-
1
, 120 cm
-
1
, 170 cm
-
1
, 201
cm
-
1
,
and 240 cm
-
1
.
The presence of these modes is consistent with the presence of a substitutional
alloy of Se and Te in a hexagonal (trigonal) structure common to both elements in their pure
phases.
5
Figure S6.
Raman spectrum of
a Se
-
Te photoelectrodeposit
generated using an incoherent LED source with λ
avg
=
630 nm.
S
7
.
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El
-
Korashy, A.; El
-
Zahed, H.; Zayed, H. A.; Kenawy, M. A.
Solid State Commun.
1995
,
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-
339.
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Mott, N. F.; Davis, E. A.,
Electronic Processes in
Non
-
Crystalline Materials
. 2 ed.;
Oxford University Press: New York, 1971.
3.
Sadtler, B.; Burgos, S. P.; Batara, N. A.; Beardslee, J. A.; Atwater, H. A.; Lewis, N. S.
Proc. Natl. Acad. Sci. U. S. A.
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, 19707
-
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Hale, G. M.; Querry, M. R.
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,
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Geick, R.; Steigmeier, E. F.; Auderset, H.
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