Bio-inspired nanostructures for enhanced light managementHonglei Wang, Yuchen Liang, Samantha Cheng, Benjamin Li, Andrew Li, George Du, and Walter Hu

Citation: Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials,Processing, Measurement, and Phenomena 35, 06GJ02 (2017); doi: 10.1116/1.4995472View online: http://dx.doi.org/10.1116/1.4995472View Table of Contents: http://avs.scitation.org/toc/jvb/35/6Published by the American Vacuum Society

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Bio-inspired nanostructures for enhanced light management

Honglei Wang and Yuchen LiangDepartment of Electrical and Computer Engineering, The University of Texas at Dallas, 800 W. Campbell Rd.,Richardson, Texas 75080

Samantha Cheng, Benjamin Li, Andrew Li, and George DuSTEM Bridge Summer Camp, CAST-TX, Richardson, Texas 75080

Walter Hua)

Department of Electrical and Computer Engineering, The University of Texas at Dallas, 800 W. Campbell Rd.,Richardson, Texas 75080 and ASIC and System State-Key Lab, Microelectronics Department, FudanUniversity, Shanghai 200433, China

(Received 11 July 2017; accepted 26 September 2017; published 11 October 2017)

The biological surface has developed functional structures during long-term evolution, which

inspires the development of biomimic materials for optical and optoelectronic applications. For

example, the micropapillae and nanofolding structures of rose petals could enhance light absorption

and color saturation. Here, the authors report a successful replication of rose hierarchical surface

structures by simple and cost-effective processes. A variety of rose structured surfaces were investi-

gated, which confirmed the diversity of functional surface architecture. The polydimethylsiloxane

(PDMS) negative replica was formed by casting PDMS solution on top of a rose petal followed by

a temperature-assisted curing process. The hierarchical structure was further transferred into photo-

resist films by ultraviolet nanoimprint using the PDMS replica as molds. The imprinted photoresist

films demonstrated uniform replications of rose microconvex cells with nanofolding details in the

scale of a square centimeter. Super-hydrophobicity was demonstrated on both PDMS negative rep-

lica and photoresist positive replica. The incorporation of photoresist replica on the surface of pho-

todetectors improved the responsivity by 35% to 42% due to enhanced light management effect.

This bio-inspired transfer imprint process with PDMS provided a high-fidelity and cost-effective

method to reproduce functional structures from biological surfaces. This study also demonstrated

the potential of utilizing rose structures in photovoltaic and optoelectronic applications. VC 2017American Vacuum Society. https://doi.org/10.1116/1.4995472

I. INTRODUCTION

Nature has offered us a lot of inspirations for designing

and fabricating new functional materials and structures.1,2

The biological surface, as the first interface between the

nature creatures and surrounding environment, has devel-

oped multiple functions during uninterrupted evolutions.3–6

The moth eyes, for example, have subwavelength cornel nip-

ple array structures, which act as an effective medium with a

gradually changed refractive index.7 The butterfly wing

scales have multilayer ridge array structures and show beau-

tiful iridescence due to the photonic crystal effects.8 Both

moth eyes and butterfly wings have been extensively studied

and imitated to form antireflection and light trapping

layers.6–9 Besides animal surfaces, the surface of plants

including, flowers10–12 and leaves,3,13 also feature intelligent

and interesting structures and surface architectures. While the

color of plants is normally pigment-based, the micro- and

nanohierarchical surface structures strengthen the color satu-

ration by reducing light reflection and redirecting photons to

strike pigments.14 With such light trapping mechanism, flow-

ers appear more attractive for pollinators, while leaves could

sustain metabolism in low-light environment via photosyn-

thesis.15,16 Reproducing such functional surface structures

would enable a lot of applications such as forming super-

hydrophobic surface for water repellent or self-cleaning,3 and

light harvesting element for photovoltaic and optoelectronic

devices.10 However, the research on plant surfaces and their

inspired microfabrication are still limited to date compared

with animal counterparts.

Rose, as one of the most popular flowers over the world, is

known for its beauty and fragrance. Previously, Feng et al.12

discovered rose structure-based super-hydrophobic property

with high water-adhesive force called “petal effect.” Direct

mimic of the three-dimensional hierarchical structures is

challenging and relies on an expensive microfabrication

technique such as interference lithography or electron-beam

lithography.17,18 Replication via nanoimprint19–21 or mold-

ing,11,22 on the other hand, provides advantages of low-cost

and high-throughput. A two-step transfer process by first gen-

erating a negative replica and then forming a positive replica

is usually involved. Successful replication of rose surface

structures have been reported with polystyrene,7 epoxy

resin,11 and UV-curing resist.10 H€unig et al.10 also demon-

strated reduced broadband reflection and improved solar cells

performance by coating resist replica of the Rosa “El Toro.”

Inspired by their work, here we studied the diverse surface

structures of rose petals, and successfully replicated the

hierarchical structures by a simple and low-cost process. The

polydimethylsiloxane (PDMS) negative replica was first syn-

thesized via solution mixing and curing. The photoresista)Electronic mail: walter.hu@utdallas.edu

06GJ02-1 J. Vac. Sci. Technol. B 35(6), Nov/Dec 2017 2166-2746/2017/35(6)/06GJ02/6/$30.00 VC 2017 American Vacuum Society 06GJ02-1

positive replica was then formed by the UV-imprint with the

PDMS mold. The imprinted rose structures were further incor-

porated on photodetectors, which demonstrate improved

sensitivity.

II. EXPERIMENT

A. Fabrication of rose replicas

1. Rose petals preparation

Roses with a variety of species and colors were collected

to study their relation with surface structures. Fresh roses

without pretreatment were used to avoid collapse of surface

structures due to dehydration. Rose samples were prepared

by cutting off from the flat part of the rose petals, which is

usually in the size of a few square centimeters.

2. PDMS negative replicas

In order to replicate the rose hierarchical structures, the

PDMS mold with negative replica was first prepared as

shown in the schematic of Fig. 1. First, the PDMS solution

was synthesized from the prepolymer (Momentive RTV

615A) and cross-linking agent (Momentive RTV 615B) in a

10:1 volume ratio in a glass beaker. Then, the glass beaker

was placed in a low-vacuum chamber for 1 h to evacuate the

air bubbles that may cause defects in replication. After that,

the rose samples were placed in a petri-dish with front-side

facing up and the PDMS solution was transferred into the

petri-dish. The petri-dish was then placed back in the low-

vacuum chamber for another 20 min, which further evacuate

air bubbles that introduced during PDMS solution transfer.

The rose hierarchical structures were conserved in the low-

vacuum environment under shelter of PDMS solutions. After

that, the PDMS/rose assembly was heated for 1 h at 60 �C.

The temperature was chosen to facilitate the PDMS curing

process while reserving the shape and structures of the rose

petals. Finally, the hardened PDMS/rose assembly was care-

fully removed from the petri-dish with razor blade after cool-

ing down to room temperature. In order to separate the

PDMS mold from rose petals, the PDMS/rose assembly was

soaked in heated piranha solution (mixture of sulfuric acid

and hydrogen peroxide in 2:1 volume ratio) at 110 �C for 10

min. The PDMS negative replica was completed after rinsing

with deionized water and blow dried with nitrogen.

3. Photoresist positive replicas

Positive replica of rose structures in photoresist films

were created by UV-imprint with the PDMS molds, as

shown in Fig. 2. A droplet of SU-8 2002 from MicroChem

was placed on the substrate, and carefully pressed with the

PDMS mold to ensure contact without air bubbles. The pho-

toresist was then cured with 350 nm UV at an irradiance of

30 mW/cm2 for 10 min in an Electro-Lite Electro-Cure 500

light exposure system. Finally, the PDMS mold was care-

fully separated from the SU-8 2002 coated substrate, leaving

positive replication of rose structures.

B. Characterizations

1. Surface characterization

The surface morphology of the rose petals, PDMS nega-

tive replicas or molds, and photoresist positive replicas were

investigated with scanning electron microscopy (SEM). The

SEM measurements were conducted with Zeiss Supra–40

SEM systems with a chamber vacuum below 1 � 10�4 Torr.

All samples were sputtered with �10 nm gold layer by

Hummer VII sputter deposition system prior to SEM

characterization.

FIG. 1. (Color online) Schematic of process to create PDMS mold with negative replication of rose petal structures.

FIG. 2. (Color online) Schematic of UV-nanoimprint process to create photoresist positive replica of rose structures.

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The contact angles of negative replicated PDMS and flat

pristine PDMS were measured using Ram�e-Hart Contact

Angle Goniometer with 20 ll deionized water droplet on the

sample surface.

2. Optoelectronic characterization

Optoelectronic performance of two sets of Si photodiodes

was characterized. One set of photodiodes was coated with

photoresist films of positive rose replica while the other set

was coated with flat photoresist films. A Keithley 4200 SCS

and a Cascade probe station were used for photodetector’s I-

V measurements. The devices under dark environment and

different illumination conditions (0.05 to 530 lW/cm2) were

tested. A 150 W halogen lamp was used for illumination for

all devices.

III. RESULTS AND DISCUSSIONS

The rose hierarchical structures were reported to be capa-

ble of reducing light reflection and increasing light absorp-

tion for color strengthen.23 The morphology of a variety of

rose petals was investigated with SEM. Figure 3(a) shows

the rose featuring uniform and closely packed microconvex

cells, which is referred as micropapillae,4 with diameter

around 20–30 lm. The individual micropapillae also demon-

strated cuticular foldings with irregular ridges in a few hun-

dred nanometers to a few micrometers [Fig. 3(a), inset].

More folding details were observed on the top of micropapil-

lae compared with the sidewalls. Figure 3(b) shows the rose

with only microconvex structures but without nanofoldings.

The aspect ratio of its micropapillae, defined as the ratio of

vertical depth to base diameter, is much higher than that of

most flower petals reported by the literature11,23 [Fig. 3(b),

inset]. The two different rose morphologies represent the

diversity of functional surface developed during evolution. It

was reported that both micropapillae with high aspect ratio

and nanofoldings would benefit the light trapping.23 In par-

ticular, specular reflection is largely reduced ascribed to

multiple reflections between micropapillaes. Figures 3(c)

and 3(d) show the hierarchical structures of same rose

Rosoideae species with yellow and pink colors, respectively.

No obvious micropapillae and cuticular folding structure dif-

ference were observed, which indicated the colors are chemi-

cal pigment-based. However, the color intensity could be

enhanced with surface hierarchical structures as more light

could be absorbed by the pigments ascribed to reduced reflec-

tion and photon redirection. Therefore, the roses appear more

optically attractive for the pollinators. Note that the micropa-

pillaes and nanofolding structure shrinkage were observed in

all SEM characterized rose petals, with Rosoideae rosa show-

ing most severe shrinkage or collapse, ascribing to the dehy-

dration of cells in high-vacuum chamber.4

The PDMS negative replica was characterized with SEM

and a typical micrograph of the surface morphology was

shown in Fig. 4(a). The homogeneous and uniform micro-

concave cells are negative replication of the rose micropapil-

laes. The nanofolding structures are also conserved on each

concave cell. The ridges on the bottom and sidewall are cor-

related with the grooves structures on rose micropapillae

cuticle [Fig. 4(a), inset]. Using fresh rose petals as well as

optimizing PDMS synthesis is important for successful repli-

cation. A major difficulty for PDMS replication is to achieve

macroscopically surface flatness, as flower petal has a natu-

ral shape. Preprocessing the rose petals by depressing or

sandwiching between glasses slides, although improving the

sample flatness, could simultaneously damages the micropa-

pillae structures. Nevertheless, replicating flat and high-

fidelity rose structures are generally much easier than that

FIG. 3. SEM micrographs of rose petal surface morphology. (a) Rose hierarchical structure with micropapillae and cuticular nanofoldings; inset shows nano-

folding structures with higher magnification. (b) Rose with only micropapillae structures; inset shows high aspect ratio micropapillae with tilted view at 45�.(c) and (d) Rosoideae rosa hierarchical structure with (c) yellow and (d) pink colors, respectively.

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JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

based on animals such as moth eyes and butterfly wings, as

their structures are only homogeneously distributed in scale

of square millimeter without discontinuity. Figure 4(b)

shows a picture of PDMS/rose assembly in petri-dish before

curing. Reasonable flatness in �1 in.2 is achieved by cutting

rose sample from the center of petal. The optimum curing

temperature is found at 60 �C, which conserves the shape

and structures of rose petals while facilitating the PDMS

cross-linking process. An example view of the cured PDMS/

rose assembly is shown in Fig. 4(c). The shape and color of

the yellow rose petal are conserved in the solidified PDMS.

The appearance of the final PDMS mold after removing rose

residues by piranha cleaning is demonstrated in Fig. 4(d).

The structure-replicated PDMS possess white color with

reduced transparency. The appearing color of PDMS replica

does not correlate with rose color, as rose color is chemical

pigment-based. The PDMS mold retains a reasonable flat-

ness which is beneficial for transfer imprint to create rose

positive replicas. The contact angle was measured on both

the PDMS negative replicas and the pristine PDMS without

structures. The pristine PDMS demonstrated hydrophobic

property with a contact angle of 112� [Fig. 4(e)], which

agrees with the literature.24,25 The structured PDMS replica,

on the other hand, shows super-hydrophobicity with a con-

tact angle of 150� [Fig. 4(f)], demonstrating the impact of

micro- and nanostructures on macroscopic surface hydro-

phobicity. Such a super-hydrophobic surface could be used

for various applications such as water-repelling and self-

cleaning.

The photoresist positive replica was created by imprinting

PDMS negative replica into UV-curing photoresist and cur-

ing with UV. We found at that no external pressure is

required between PDMS mold and the substrate during the

curing step, which simplifies the process and does not

require any instrument. However, the contact of the PDMS

molds with the photoresist films plays an important role for

defect control. Before imprinting, it is typical to pretreat the

PDMS with oxygen plasma to reduce the mold surface

hydrophobicity (e.g., 150� before treatment to 86� after treat-

ment) for better contact with resist. Almost perfect contact

of the PDMS mold with the photoresist films in large scale

without any air gap or bubbles could be achieved with the

treated PDMS surface. However, such treatment introduced

difficulty during demolding process, even when PDMS sur-

face recovers the hydrophobic property after a few hours of

imprinting. Therefore, an optimum process was developed

with untreated flat PDMS molds, by gently pressing into suf-

ficient liquid resist droplet on the substrate to remove air

bubbles before curing, and the PDMS super-hydrophobicity

yields easy mold release. SEM was used to characterize the

morphology of imprinted SU-8 2002 thin films, and the hier-

archical structures in Figs. 5(a) and 5(b) were corresponded

with the rose structures shown in Figs. 3(a) and 3(c), respec-

tively. The uniform and plump microconvex cells with an

average diameter around 20 to 30 lm were observed without

visible defects or structure shrinkage, suggesting high reli-

ability of replication process. The insets of Figs. 5(a) and

5(b) illustrate the zoom-in view of microconvex cells, which

FIG. 4. (Color online) SEM and photographs of the synthesized PDMS replicas. (a) SEM micrographs of PDMS negative replica showing microconcave cells

with nanofoldings; inset shows nanofolding structures at higher magnification. Photographs of (b) a PDMS/rose assembly before curing, (c) a cured PDMS/

rose assembly, and (d) a final PDMS mold after rose petals removed by piranha clean. Photographs of water droplets on a PDMS surface (e) without structures,

showing a contact angle of 112� and (f) with negative replicated rose structures, showing a contact angle of 150�.

FIG. 5. (a) and (b) SEM micrographs of imprinted photoresist positive replicas of rose hierarchical structures in Figs. 3(a) and 3(c), respectively. The uniform

micropapillae with nanofoldings are positive replication of rose surface structures. The inset shows nanofolding structures at higher magnification. (c) and (d)

Photographs of water droplets on photoresist positive replica surface of (a) showing a contact angle of 150�, and (b) showing a contact angle of 120�.

06GJ02-4 Wang et al.: Bio-inspired nanostructures 06GJ02-4

J. Vac. Sci. Technol. B, Vol. 35, No. 6, Nov/Dec 2017

demonstrated detailed cuticular folding structures on top

and sidewall with excellent reproduction of original rose

structures. The SU-8 2002 thin films imprinted with PDMS

negative replica also demonstrated hydrophobicity. The

water contact angle of 150� [Fig. 5(c)] and 120� [Fig. 5(d)]

were correlated with photoresist surface shown in Figs. 5(a)

and 5(b), respectively, while a reference SU-8 2002 film

imprinted with a flat pristine PDMS mold shows a contact

angle of 90�. The higher contact angle of Fig. 5(a) compared

with that of Fig. 5(b) was possibly ascribed to more nano-

folding structures. The results demonstrated that large-scale,

high-fidelity positive replication of rose hierarchical structures

including micropapillae and nanofoldings could be success-

fully created on the photoresist thin-film by this simple imprint

process. In addition, such functional structures could be

imprinted repeatedly with the same PDMS mold, given the

reliable demolding process due to its super-hydrophobicity.

The micro- and nanohierarchical structures of rose pet-

als can improve light absorption and color saturation.23

Successful replication of rose functional structures has

potential application on photovoltaic or optoelectronic

devices for performance enhancement. Here, photodetec-

tors with incorporated rose functional structures were stud-

ied. Two sets of identical Si photodiodes were coated with

15 ll of SU-8 2002. One was imprinted by a PDMS mold

with negative rose structure replications while the other

was imprinted by a flat pristine PDMS mold without struc-

tures for reference. The photoelectrical performance of

these two photodiodes was then characterized in the dark

and under halogen light illumination. Their corresponding

I-V characteristics under reverse junction bias were illus-

trated in Fig. 6(a). In dark state, both photodiodes show

comparable currents at the level of tens of nanoampere.

Under illumination, the currents significantly increased

due to the generation of electron–hole pairs upon photon

absorption, and charge carrier extraction under applied

electrical field. The illuminated currents increased gradu-

ally with increasing irradiance. Under the same irradiance,

the photodiode with replicated rose structures demon-

strated higher current compared with the one without rose

structures. Such behavior improvement was also illustrated

in the plot of photodiode current versus irradiance at a 10

V reverse bias in Fig. 6(b). The responsivity commonly

used to evaluate the efficiency of a photodetector respond-

ing to an optical signal is defined by

R ¼ Iilluminated � Idark

Llight � S; (1)

where Llight is the irradiance, S is the effective area, and

Iilluminated and Idark are the current with and without illumina-

tion, respectively. Here, the relative responsivity is calcu-

lated by the ratio of responsivity with rose structure to that

with referenced device of flat surface. The plot of relative

responsivity versus irradiance, as expressed in Fig. 6(b),

demonstrated �35% to 42% improvement under 0.05 to

530 lW/cm2 illumination by incorporating replicated rose

structures on the photodetectors. Similar behaviors are

observed on multiple devices. The improved performance is

ascribed to the light management effect of replicated rose

hierarchical structures. The high-density and high-aspect

ratio micropapillae function similar to the microlens array

which offers antireflection and photon redistribution.26 Such

light trapping effect is further enhanced with the irregular

cuticular nanofolding structures.10

IV. SUMMARY AND CONCLUSIONS

In summary, the diverse surface hierarchical structures of

roses were investigated. The micropapillae and nanofoldings

were observed on the rose petals, which improve light

absorption and color saturation. Inspired by the rose func-

tional structures, the PDMS molds with negative structure

replication were synthesized in a simple solution casting and

molding process. Uniform photoresist positive replicas of

rose structures were formed successfully in the scale of

square centimeter by UV-nanoimprint with the PDMS molds

without an instrument. Super-hydrophobicity was demon-

strated on both PDMS negative replicas and photoresist

positive replicas, which could be used for water-repelling

and self-cleaning applications. The super-hydrophobicity of

FIG. 6. (Color online) Optoelectronic characteristics of Si photodetectors with replicated rose structures and reference flat device. (a) Logarithm I-V plot of Si

photodiodes under reverse junction bias tested in the dark and under halogen light illumination conditions (0.05–530 lW/cm2). The solid line represents the

photodiode with photoresist rose replica while the dashed line represents the reference device without structures. (b) Plot of photodetector illuminated current

vs irradiance (left Y-axis), and calculated photodetector relative responsivity vs irradiance (right Y-axis). The photodetector responsivity improved �35% to

42% by incorporating replicated rose structures. The error bars represent standard deviation in multiple tests.

06GJ02-5 Wang et al.: Bio-inspired nanostructures 06GJ02-5

JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

PDMS replica also improved mold releasing and repeated

patterning. The replicated rose structures were incorporated

on the photodetectors, contributing to a 35% to 42%

improvement on the device responsivity due to light man-

agement effect. The results demonstrated the potential of

utilizing plant surface structures in photovoltaic and opto-

electronic applications by a simple and cost-effective repli-

cation process.

ACKNOWLEDGMENTS

This work was partially supported by the National Science

Foundation (Grant Nos. CBET-1606141 and ECCS-0955027).

W. Hu thanks support from the 1000 Talent Program of

Shanghai, China. S. Cheng, B. Li, A. Li, and G. Du thank the

CAST-TX’s STEM Bridge program for supporting their high

school summer camp.

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