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Plastic Morphological Response to Spectral Shifts during Inorganic
Phototropic Growth
Kathryn R. Hamann, Madeline C. Meier, Nathan S. Lewis,
*
and Azhar I. Carim
*
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JACSAu
2022, 2, 865
874
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Supporting Information
ABSTRACT:
Plants exhibit phototropism in which growth is directed toward
sunlight and demonstrate morphological plasticity in response to changes in the
spectral distribution of the incident illumination. Inorganic phototropic growth via
template-free, light-directed electrochemical deposition of semiconductor material
can spontaneously generate highly ordered mesostructures with anisotropic,
nanoscale lamellar features that exhibit a pitch proportional to the wavelength (
λ
)
of the stimulating illumination. In this work, Se
Te
fi
lms were generated via a
two-step inorganic phototropic growth process using a series of narrowband light-
emitting diode sources with discrete output wavelengths (
λ
0
λ
1
). Analogous to
the plasticity observed in plants, changes in illumination wavelength from
λ
0
to
λ
1
resulted in morphological changes including
feature branching, termination, and/or fusion along the growth direction. The interfacial feature pitch changed with the growth
duration, in some cases in a notably nonmonotonic fashion, and eventually matched that obtained for growth using only
λ
1
.
Simulated morphologies generated by modeling light
material interactions at the growth interface closely matched the evolved
structures observed experimentally, indicating that the characteristics of the optical stimulation produce the observed plastic
response during inorganic phototropic growth. Examination of the interfacial electric
fi
eld modulation for
λ
1
illumination of
simpli
fi
ed structures, representative of those generated experimentally, revealed the interfacial light scattering and concentration
behavior that directed phototropic growth away from equilibrium, as well as the emergent nature of the phenomena that reestablish
equilibrium.
KEYWORDS:
photoelectrodeposition, photoelectrochemistry, mesostructure, template-free, maskless, optical
B
iological systems demonstr
ate phenotypic plasticity
whereby an organism expresses di
ff
erent phenotypes in
response to changes in the local environment.
1
This biological
plasticity concept encompasses a wide set of responsive
phenomena beyond the physics and materials science
de
fi
nition of plasticity that speci
fi
cally describes a change in
the shape of a material as a result of an applied force. Biological
plasticity provides for the exhibition of a diversity of
morphologies, among other properties, and thus enables an
organism to maximize
fi
tness in variable environments. Plants
exhibit phenotypic plasticity to compensate in part for the
intrinsic inability to move physically to favorable locations.
2
Moreover, resources that are essential for plant growth are
usually distributed heterogeneously within a habitat.
3
Plants
thus actively control the construction of resource gathering
organs, such as root tips and leaves, and are capable of placing
these structures nonrandomly to modify the potential for
resource acquisition.
4
Plants are predominantly photosynthetic
organisms that depend on sunlight for energy and demonstrate
marked morphological plasticity to manage this resource.
5
The
process of phototropism, in which the addition of biomass is
directed in a feedback loop to optimize harvesting of solar
energy, allows plants to exhibit directed growth in response to
illumination.
6
Cucumber plants, which are shade intolerant,
can colonize horizontally patchy environments by preferen-
tially projecting leaf area into light gaps.
7
In addition to
responding to changes in light intensity, plants can display a
photomorphogenic response
to changes in the spectral
distribution of the available illumination. Green plant leaves
contain chlorophyll pigment and absorb red light in the
600
to 700 nm range but transmit and re
fl
ect far-red wavelengths in
the
700 to 800 nm regime. The red : far-red intensity ratio of
a speci
fi
c optical stimulus to green leaves provides a metric that
enables plants to sense the degree of shading and proximity to
neighboring organisms in a speci
fi
c location.
5
In response to a
diminished red : far-red intensity ratio when overtopped by
woody neighbors, open-habitat tree ferns principally produce
vertical frond growth.
8
Increases in the red : far-red intensity
ratio provided to geraniums and snapdragons produce
photomorphogenesis in which the plant height extension
ceases and the leaf area increases.
9
Thus, the expressed
Received:
December 31, 2021
Revised:
March 3, 2022
Accepted:
March 4, 2022
Published:
April 4, 2022
Article
pubs.acs.org/jacsau
© 2022 The Authors. Published by
American Chemical Society
865
https://doi.org/10.1021/jacsau.1c00588
JACSAu
2022, 2, 865
874
morphologies of plants are critically dependent on the
environmental light conditions.
In analogy to the natural phototropism exhibited by plants,
inorganic phototropic growth has been demonstrated in the
light-directed electrochemical deposition of chalcogen-based
semiconductor materials, including Se
Te, Se
Pb, and Se
Cd.
10
13
Inorganic phototropic growth produces light-de
fi
ned
mesostructures in which the feature size, orientation, and
anisotropies of the morphologies are functions of the input
illumination characteristics including the wavelength, polar-
ization, phase, coherence, and direction of the incident
illumination.
10
,
14
16
Growth using linearly polarized illumina-
tion has been demonstrated to generate highly anisotropic and
periodic lamellar structures. The long axes of these structures
grow parallel to the electric
fi
eld (E-
fi
eld) vector of the input
illumination and are characterized by a feature pitch (i.e., the
distance between nominally identical points on adjacent
lamellae) that is proportional to the input wavelength.
Analogous to natural phototropism, spatially directed inorganic
phototropic growth is a response to local light absorption that
promotes local deposition of additional absorber mass. This
photoelectrochemical process is compatible with semiconduc-
tor materials amenable to solution-phase electrodeposition.
Inorganic phototropic growth is modulated by inherent
interfacial scattering and absorption processes that de
fi
ne the
subwavelength scale distribution of optical
fi
eld intensity. The
evolution of ordered lamellar mesostructures is governed by an
emergent, synergistic process in which an individual feature
spontaneously scatters the incident illumination and con-
sequently produces spatially anisotropic, concentrated optical
intensity at the growth interface of the nearest-neighbor
features, promoting asymmetric absorption and addition of
new material.
17
Inorganic phototropic growth thus does not
require the use of structured light
fi
elds, high light intensities,
or chemical and physical templating agents because the process
exploits inherent optical anisotropies. Moreover, as an
emergent process with constituent interactions at the
subwavelength scale, high coherence length sources (e.g.,
lasers) are not required to produce long-range order. Rather,
mesoscale morphologies can be generated over macroscopic
areas using light-emitting diode (LED) sources and even
broadband incandescent (i.e., heated
fi
lament) sources.
Analogous to the growth of plant systems, inorganic
phototropic growth has the potential to demonstrate
phenotypic plasticity in response to a change in the
characteristics of the stimulating illumination. Moreover,
such an adaptive growth response may enable straightforward,
directed generation of complex three-dimensional morpholo-
gies in inorganic
fi
lms.
11
Certain temporal changes in the input
wavelength during inorganic phototropic growth can direct
mesostructural evolution that results in a new feature pitch at
the interface.
11
,
18
However, such adaptation may be inhibited
by a
history
e
ff
ect wherein the responsiveness depends not
only on the nature of the new optical input but also on the
nature of the preexisting structure and thus the prior optical
input.
18
This work aims to characterize any plastic morpho-
logical responsiveness, with a time-resolved assessment of the
transitory growth dynamics including any associated non-
linearities, to a broad series of changes in input wavelength
during inorganic phototropic growth. In tandem, this work
examines in detail changes in interfacial light scattering and
optical
fi
eld concentration, how these processes mechanisti-
cally direct the growth response, and the time-dependent
evolution of both phenomena. Here, inorganic phototropic
growth was e
ff
ected via a two-step process that used a series of
di
ff
erent LEDs with discrete output wavelengths. The resulting
morphologies were evaluated by scanning electron microscopy
and Fourier analysis. Modeling of the light-directed deposition
and generation of simulated growth morphologies were
performed by considering light
material interactions at the
growth interface. A comprehensive set of electromagnetic
simulations with idealized structure models was then utilized to
assess the optical processes that underpin the mechanism of
the plastic response.
Figure 1.
(a
c) Representative top-view and (d
f) cross-sectional SEMs of
fi
lms generated using the indicated
λ
avg
illumination for
t
= 2.00 min.
JACS Au
pubs.acs.org/jacsau
Article
https://doi.org/10.1021/jacsau.1c00588
JACSAu
2022, 2, 865
874
866
RESULTS
Se
Te
fi
lms were grown from an aqueous solution of oxidized
precursors by light-mediated electrodeposition using unstruc-
tured, vertically polarized illumination from low-power,
narrowband LED sources with values of the intensity-weighted
average of the emission spectrum,
λ
avg,
of 528, 727, and 955
nm. The structures produced by deposition for
t
= 2.00 min
using these values of
λ
avg
are designated herein as
Λ
528
,
Λ
727
,
and
Λ
955.
Se
Te
fi
lm morphologies were determined using
scanning electron microscopy.
Figure 1
a
c presents repre-
sentative top-view (from above the sample along the surface
normal) scanning electron micrographs (SEMs) of
Λ
528
,
Λ
727
,
and
Λ
955
fi
lms. Highly anisotropic and periodic lamellar
features were observed in which the long axes of the lamellae
were oriented vertically, parallel to the input polarization
vector. The feature width and feature pitch scaled with
λ
avg
(
Figure S1
presents SEMs of
Λ
528
,
Λ
727
, and
Λ
955
fi
lms with the
pitch denoted graphically). Two-dimensional Fourier trans-
form (2D FT) analysis of top-view SEM data was used to
quantify the pitch at the top interface and values of 203
±
6,
263
±
6, and 371
±
6 nm were measured for
Λ
528
,
Λ
727
, and
Λ
955
fi
lms, respectively;
Figure S2
presents a representative set
of these 2D FT data.
Figure 1
d
f presents representative
cross-sectional (view perpendicular to the substrate normal)
SEMs complementary to the top-view data presented in
Figure
1
a
c. The cross-sectional analysis was performed by physically
cleaving the substrates and top-facing
fi
lms perpendicular to
the direction of the input polarization vector. These cleaved
fi
lms were then viewed at a near grazing angle to assess the
out-of-plane morphology of individual features at single points
along the long axes of the lamellae. The cross-sectional SEM
data revealed that the features were oriented along a vector
normal to the substrate and exhibited substantial anisotropy in
this direction. Films generated with extended growth durations
using constant illumination at a given wavelength exhibited
linear extension of features along the out-of-plane direction but
did not exhibit a change in the in-plane morphology (
Figure
S3
). Thus, lamellar mesostructures were characterized by long,
anisotropic axes in one in-plane direction, nanoscale feature
widths in the orthogonal in-plane direction, and tunable
Figure 2.
(a
d) Representative top-view and (e
h) cross-sectional SEMs of
fi
lms generated initially using
λ
0
= 528 nm for
t
0
= 2.00 min and then
extended in a subsequent deposition step using
λ
1
= 955 nm for the indicated
t
1
.
Figure 3.
(a
d) Representative top-view and (e
h) cross-sectional SEMs of
fi
lms generated initially using
λ
0
= 727 nm for
t
0
= 2.00 min and then
extended in a subsequent deposition step using
λ
1
= 955 nm for the indicated
t
1
.
JACS Au
pubs.acs.org/jacsau
Article
https://doi.org/10.1021/jacsau.1c00588
JACSAu
2022, 2, 865
874
867