Volum

e 19 | Num

ber 27 | 2009 Journal of M

aterials Chem

istry Pages 4641–4860 0959-9428(2009)19:27;1-P

www.rsc.org/materials Volume 19 | Number 27 | 21 July 2009 | Pages 4641–4860

ISSN 0959-9428COMMUNICATIONShu Yang et al.Direct fabrication of 3D silica-like microstructures from epoxy-functionalized polyhedral oligomeric silsesquioxane (POSS)

FEATURE ARTICLEParas N. Prasad et al.Multifunctional nanoparticles as biocompatible targeted probes for human cancer diagnosis and therapy

As featured in:

see Yuan Zhang, Qun Xiang, Jiaqiang Xu, Pengcheng Xu, Qingyi Pan and Feng Li, J. Mater. Chem., 2009, 19, 4701.

www.rsc.org/materialsRegistered Charity Number 207890

Using self-assembly method, ZnO nanowires have been modified with monodispersed Pd nanoparticles for fabricating H2S chemical sensors with highly enhanced performances. The approach has opened a new pathway for tailoring the functionalities of semiconductor gas sensors.

Title: Self-assemblies of Pd nanoparticles on the surfaces of single crystal ZnO nanowires for chemical sensors with enhanced performances

Showcasing research from the collaborative efforts of Professor Jiaqiang Xu’s group at Laboratory of Functional Materials, College of Science, Shanghai University, China, and Dr. Feng Li at Department of Chemistry, Rice University, Houston, USA.

Volum

e 19 | Num

ber 27 | 2009 Journal of M

aterials Chem

istry Pages 4641–4860 0959-9428(2009)19:27;1-P

www.rsc.org/materials Volume 19 | Number 27 | 21 July 2009 | Pages 4641–4860

ISSN 0959-9428COMMUNICATIONShu Yang et al.Direct fabrication of 3D silica-like microstructures from epoxy-functionalized polyhedral oligomeric silsesquioxane (POSS)

FEATURE ARTICLEParas N. Prasad et al.Multifunctional nanoparticles as biocompatible targeted probes for human cancer diagnosis and therapy

As featured in:

see Yuan Zhang, Qun Xiang, Jiaqiang Xu, Pengcheng Xu, Qingyi Pan and Feng Li, J. Mater. Chem., 2009, 19, 4701.

www.rsc.org/materialsRegistered Charity Number 207890

Using self-assembly method, ZnO nanowires have been modified with monodispersed Pd nanoparticles for fabricating H2S chemical sensors with highly enhanced performances. The approach has opened a new pathway for tailoring the functionalities of semiconductor gas sensors.

Title: Self-assemblies of Pd nanoparticles on the surfaces of single crystal ZnO nanowires for chemical sensors with enhanced performances

Showcasing research from the collaborative efforts of Professor Jiaqiang Xu’s group at Laboratory of Functional Materials, College of Science, Shanghai University, China, and Dr. Feng Li at Department of Chemistry, Rice University, Houston, USA.

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COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry

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Direct fabrication of 3D silica-like microstructuresfrom epoxy-functionalized polyhedral oligomeric silsesquioxane (POSS)†

Jun Hyuk Moon,‡a Jin Seok Seo,‡b Yongan Xub and Shu Yang*b

Received 20th January 2009, Accepted 14th April 2009

First published as an Advance Article on the web 23rd April 2009

DOI: 10.1039/b901226e

We have fabricated 2D and 3D structured organosilcates from

epoxy functionalized polyhedral oligomeric silsesquioxanes (POSS)

cage materials using holographic lithography, which can be conve-

niently converted to silica structures by thermal removal of the

organic moieties.

Three-dimensional (3D) periodic structures offer many unique

properties, including high surface-to-volume ratio, superior

mechanical strength yet low density, and recently, photonic and

phononic bandgap properties.1–7 Among many 3D fabrication tech-

niques,1,2,4,8–16 holographic lithography (HL) is a versatile approach

that can produce periodic structures in micrometre scale defect-free

over a large area (up to a few cm2). It uses the interference pattern of

multiple coherent laser beams in contrast to a photomask in

conventional photolithography processes to create periodic two-

dimensional (2D) and three-dimensional (3D) structures by exposing

a set of interference beams to a photoresist film.2,4 HL allows for

precise control over the size and shape of the resulting structures, and

has the flexibility to access a variety of lattice symmetries through the

proper arrangement of laser beams.5,17–19 Of particular interest is the

fabrication of 3D photonic crystals with controlled symmetries for

large photonic bandgaps (PBGs).

Most 3D photonic structures fabricated by HL are from conven-

tional organic photoresists, which are originally designed for photo-

lithography in the UV regions. The resists typically possess epoxy

(negative-tone) or (meth)acrylate (positive-tone) functionalities,

allowing for chemically amplified reactions between the functional

groups and photoacid generators (PAGs), therefore, exhibiting high

lithographic resolution (sub-microns), photospeed, and contrast.

However, the organic photoresists also have several unfavorable

characteristics, including low refractive index, high shrinkage, and

low thermal and mechanical stability,3,20,21 to achieve a high quality

photonic crystal with a complete PBG. Thus, the directly fabricated

polymeric 3D structures are often used as sacrificial templates for

conversion to high refractive index inorganic materials (e.g. Si).22–25

The deposition of high refractive index inorganic materials, for

example, through the chemical vapor deposition (CVD) process,

aDepartment of Chemical and Biomolecular Engineering, SogangUniversity, 1 Shinsu-dong, Mapo-gu, Seoul, Korea. E-mail: junhyuk@sogang.ac.kr; Fax: +82 2 711 0439; Tel: +82 2 705 8921bMaterials Science and Engineering, University of Pennsylvania, PA19104, USA. E-mail: shuyang@seas.upenn.edu; Fax: +1 215 573 2128;Tel: +1 215 898 9645

† Electronic Supplementary Information (ESI) available:Thermogravimetric analysis of the epoxy-POSS photoresist in air. SeeDOI: 10.1039/b901226e/

‡ These authors contributed equally to this work.

This journal is ª The Royal Society of Chemistry 2009

however, is often accompanied by a high processing temperature

(>400 �C), above which the polymer template is decomposed.22,24

Hence, a double templating procedure via a silica replica of the

polymer template is often utilized for high temperature deposition.

The extra processing steps could complicate the photonic crystal

fabrication, more importantly, our previous studies suggest that it is

nearly impossible to completely fill the HL patterned 3D templates

through conformal coating by CVD process,26 and the double tem-

plating procedure will further aggravate the problem.

Several groups have attempted to use hybrid materials that

combine organic and inorganic components to improve the thermal

and mechanical stability of the 3D structures. For example, the

inorganic nanoparticles, such as silica, have been dispersed in the

photoresist formation as an reinforcement.27 A number of organic–

inorganic hybrid materials have been synthesized for photo-

lithography using sol–gel reactions.15,28–30 Another possibility is a

combination of hard and soft chemistries to assemble thermally

stable 3D opals.31 However, the high loading of inorganic materials

could significantly lower the resist photosensitivity and make the film

crack-prone as film thickness increases. The higher refractive index of

the resists could also cause lattice distortion during the HL fabrica-

tion due to refraction effect at the air–film interface.21 When heated

above the thermal decomposition temperature of the organic

components, cracks often occur due to the residue strain generated

from the large weight loss and film shrinkage, and the brittleness of

pure inorganic materials, which could further degrades the optical

quality of the resulting photonic crystals.

Here, we investigate the direct fabrication of 3D silica-like struc-

tures from epoxy-functionalized polyhedral oligomeric silsesquiox-

anes (POSS) using holographic lithography, their thermal stability

and related chemical nature. The 3D POSS structures can be

conveniently converted to 3D silica structures for infiltration of

a wide range of materials, which can be subsequently removed using

aqueous hydrofluoric acid (HF) solution at room temperature.

Sandhage et al., however, have shown that it is possible to

convert biosilica (diatom) microshells to their ceramic32,33 and Si34

replicas through gas displacement reaction and chemical reduction,

respectively.

POSS is a unique class of hybrid materials, which possess the

structure of cube-octameric frameworks with eight organic corner

groups.35 It has chemical composition of RSiO1.5 with thermal and

mechanical properties intermediate between SiO2 and organic poly-

mers. Unlike SiO2 each POSS molecule contains nonreactive organic

moieties, making it compatible with various polymer systems and

enhancing its crack resistance.35 POSS with various numbers of epoxy

functionalities (n ¼ 1–8), and different organic substituents, R, have

been synthesized and used as nanosized inorganic building blocks to

enhance the thermal and mechanical properties of rubbery epoxy

J. Mater. Chem., 2009, 19, 4687–4691 | 4687

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network, based on diglycidyl ether of bisphenol A (DGEBA).36–38

POSS-bearing methacrylate copolymers have been demonstrated to

be positive-tone resists for e-beam lithography, deep UV lithography

and 157 nm lithography.39–41 They show similar lithographic char-

acteristics to conventional photoresists, including high contrast (>10),

high sensitivity (<10 mJ/cm2), and resolution up to sub-100 nm, but

possess higher glass transition temperatures (Tg) and etch resistance.

Recently, a ladder-structured methyl silsequioxane (MSQ) presursor

has been used to fabricate thermally stable woodpile structures via

direct laser writing (DLW).15 Despite the success, questions remain

about the resolution and contrast using MSQ as photoresist and its

applicability to HL since the photocrosslinking of MSQ is based on

polycondensation of silanol groups to generate –O–Si–O– bonds

upon exposure to the light and the subsequent post-exposure bake

(PEB). It is known that silanol groups are susceptible to temperature

and environmental effects (e.g. moisture and pH), which could cause

unwanted thermal crosslinking and self-condensing in the unexposed

regions, therefore, lowering the pattern resolution and contrast. It will

be advantageous if we can introduce photocrosslinkable epoxy

groups in POSS resists for high resolution and high contrast

patterning, at the same time build upon the knowledge from 3D

patterning using commercially available, negative-tone SU8 resists to

create thermally stable 3D silica-like structures.

Here, we prepared the negative-tone photoresists for HL from

epoxy cyclohexyl POSS cage mixtures. Since each epoxy-functional-

ized POSS molecule possesses an average of eight epoxy groups,

which can be photocrosslinked through chemically amplified reac-

tions in the same fashion as the SU8 resists, it offers an efficient

molecular building block for high-resolution photopatterning. The

loading of inorganic components can be conveniently controlled by

modification of the corner organic moieties as well as by varying the

degree of crosslinking. By using a single component resist system, we

avoid possible macroscopic separation between inorganic and

organic constituents when mixing epoxy resin and POSS. Three- and

four-beam interference patterns were employed to fabricate 2D

and 3D POSS structures, respectively, which were converted into

silica-like structures when heated above 400 �C (Scheme 1).

We investigated the relative volume, weight, and structural change

in the resultant 2D and 3D patterns at different thermal treatment

temperatures, as well as the related chemical nature, which in turn

influences the thermal and mechanical stability of the film. The 3D

Scheme 1 Photopolymerization reaction of epoxy cyclohexyl

4688 | J. Mater. Chem., 2009, 19, 4687–4691

structure was found well maintained without major cracks when

heated to 400 �C in air even though large volume shrinkage was

suggested according to TGA measurement. SEM image showed

decrease of the thickness of the 3D skeleton, however, the lattice

spacing in the (111) plane remained nearly unchanged. When the

calcination temperature was raised to 500 �C, major cracks occurred

in the film along with additional 21% weight loss.

The epoxy-POSS photoresist was prepared by dissolving epoxy

cyclohexyl POSS cage mixture [EP0408, (C8H13O)n(SiO1.5)n, n¼ 8,10

or 12, FW ¼ 1772.73, from Hybrid Plastics] and 2–4 wt% Irgacure

261 (a UV and visible photoacid generator from Ciba Specialty

Chemicals) in propylene glycol methyl ether acetate (PGMEA) to

obtain a concentration of 50–60 wt%. After soft bake at 65 �C, the

resist film was exposed to three or four interfering beams to fabricate

2D and 3D patterns, respectively, using the same optical assembly for

fabrication of SU8 structures.22,42 During exposure to the interference

beams, photoacids were generated and their diffusion within the

epoxy-POSS photoresist was accelerated when the film was post-

exposure baked at 95 �C, well above its glass transition temperature

to initiate cationic polymerization of the epoxy groups. SEM images

suggested that epoxy-POSS had similar photosensitivity as SU8 with

nearly the same volume fraction of 3D structures in both resist

systems when the concentration of photoacid generators, exposure

time and intensity, and baking conditions were kept the same. The

unexposed film was removed by PGMEA and the resulting pattern

was characterized by AFM and SEM, followed by calcination in

order to remove the organic moieties and form the pure silica

network structure. Since the interference laser beam has a Gaussian

distribution of intensity, for comparison of weight loss and film

shrinkage at different temperatures we kept the characterization in

the same region (e.g. center) of the films.

The photopolymerization was monitored by comparing the FT-IR

spectra of epoxy POSS before and after laser exposure. As shown in

Fig. 1, the decrease of the absorption peak around 830 cm�1 (epoxy

ring vibration) and the increase of the peak around 937 cm�1 (C–O–C

stretching) and 3450 cm�1 (hydroxyl group stretching) can be

attributed to the ring-opening polymerization of cycloaliphatic

epoxide.43 The absorption at 2853 cm�1–2988 cm�1 can be attributed

to the C–H stretching of the cyclohexane ring,43,44

Fig. 2 shows typical SEM and AFM images of 2D hexagonal

cylinders by three-beam interference pattern. The calcination was

POSS cage mixture and its subsequent thermal treatment.

This journal is ª The Royal Society of Chemistry 2009

Fig. 1 FT-IR spectra of epoxy cyclohexyl POSS films before and after

photocrosslinking and calcination at various temperatures in air for 1 h.

Fig. 2 SEM (a) and AFM (b) images of a typical 2D holo-

graphic pattern. Scale bar in SEM ¼ 5 mm. Distance to dotted line in

AFM ¼ 1.6 mm.

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carried out at 400 �C, 500 �C, and 600 �C in air for 1 h. Thermog-

ravimetric analysis (TGA) was conducted in air to quantify the

weight loss of epoxy-POSS during the heat treatment (Fig. S1†). The

crosslinked epoxy-POSS was found thermally stable up to 350 �C in

air,45 above which the organic moieties started to be decomposed,

showing linear weight loss vs. temperature (Table 1). The initial

weight loss presumably is due to the residual solvent in the film. The

neat POSS film showed no weight loss below 300 �C in air. At 600 �C,

however, only 30 wt% residue remained, close to the inorganic

fraction of the epoxy-POSS, 27 wt%. On the basis of the relative

weight loss from TGA measurement and the relative volume change

from AFM study of 2D patterns (Table 1), we estimated the relative

density of the film at different temperatures. Up to 400 �C, the

volume and weight of epoxy-POSS decreased by the same rate,

therefore, less then 10% relative change of density was observed. In

contrast, film densification below 300 �C occurs in MSQ films due to

Table 1 Relative weight, volume and density of the matrix of 2Dpatterns at various heat-treatment temperatures

Temp/�CRelativeweight (%)

Relativevolume (%)a

RelativeDensity b

25 100.0 100 1.0400 72.0 63–67 1.1500 51.0 30–35 1.5–1.7600 32.0 16–18 1.8–2.0

a The 2D patterns were considered as cylinders, whose diameter andheight were measured by SEM and AFM, respectively, and averaged toestimate the volume. b Relative density was calculated by the ratio ofrelative weight to relative volume.

This journal is ª The Royal Society of Chemistry 2009

condensation of unreacted silanol groups after photo-

polymerization.15 The relative film density of epoxy-POSS was found

increased more than 50% at 500 �C, which could be attributed to

removal of the organic moieties and collapse of the POSS cage to

denser SiO2-like network.

The FT-IR spectra of crosslinked epoxy-POSS treated at different

calcination temperatures were measured and compared to that of

photocrosslinked film to monitor the change of chemical structure

(Fig. 1). It is clearly seen that the C–H stretching peak was highly

intact at 300 �C, however, nearly disappeared above 400 �C. Because

of the broad hydroxyl peak (3000–3700 cm�1) appeared in the same

region, it is difficult to draw a conclusion whether all organic

components are completely decomposed above 400 �C or not. The

peak at 1054 cm�1 was attributed to O–Si–O long chain vibration due

to the presence of cage structure of POSS, which gradually dis-

appeared at higher temperatures. The absorption at�1165 cm�1 (O–

Si–O ladder structure or short chain vibration) broadened, suggesting

the collapse of the silica cage structure.44,46 This result is consistent to

the above analysis of relative film density. The absorption at 1232–

1252 cm�1 (Si–C stretching) appeared in all films, indicating the

existence of residual carbon in the film after calcinations in air. In

addition, absorption at 1630–1720 cm�1 (C]O stretching) appeared

after heating the film above 300 �C, indicating the formation of acid

and/or acid anhydride due to oxidation of the organic contents in air.

In comparison to the IR spectrum of SiO2 as a reference,44 it seems

that all films contain carbon content to a certain extent. To further

investigate the chemical nature of the POSS films treated above

400 �C, we performed compositional analysis using energy dispersive

X-ray (EDX) spectroscopy mapping technique. The film was trans-

ferred to Si wafers before EDS mapping. While there were substantial

carbons left in the film treated at 400 �C and below, it was clear that

residual carbon remained in the film after heated at 500 �C in air (see

Table 2). Meanwhile, there is a deviation from the theoretical value of

compositional weight ratio between Si and O (Si : O ¼ 41.2 : 58.8) if

the POSS film is fully oxidized at 500 �C. It is likely that a surface

oxide is formed in air, which passivates against further oxidation

below the surface, leaving residual carbon trapped.

In the case of 3D patterning, the umbrella-like 4-beam interfer-

ence pattern was transferred onto the epoxy-POSS resist film. In

Fig. 3a and 3b, the SEM images show 3D face-centered cubic

(FCC) lattice structure in POSS with a facing (111) plane. The

average period in the (111) surface is�1 mm according to SEM. The

3D POSS structures were then heated at 400 �C and 500 �C in air for

1 h, respectively, to study their thermomechanical stability. As seen

in Fig. 3c, the 3D network was surprisingly well-maintained

without major cracks during the heat treatment at 400 �C even

though large volume shrinkage occurred due to the weight loss,

�30 wt%, according to the TGA measurement from the isolated

2D patterns (see Table 1). A closer investigation of the SEM image

(inset of Fig. 3c) in comparison to non-calcined POSS structure

(inset of Fig. 3b) suggests that the thickness of shell connecting unit

atoms did shrink by �23%, however, in the (111) plane only �5%

shrinkage of the lattice spacing was observed at 400 �C. Such

skeleton structure has been reported,47 during backfilling of poly-

styrene opal template with titania precursor, followed by calcina-

tion. The authors note that it is possible to achieve two photonic

bandgaps simultaneously by varying the radius of cylinders.

In our system, when the calcination temperature was raised to

500 �C, major cracks occurred in the film (Fig. 3d), which could be

J. Mater. Chem., 2009, 19, 4687–4691 | 4689

Table 2 Elemental analysis of epoxy POSS films treated at different temperatures in air for 1 h

ElementBare filma If fully oxidized

At 400 �C At 500 �C

Relative weight (%) Relative weight (%) Relative weight (%) Relative atomic (%) Relative weight (%) Relative atomic (%)

C 58.5 0 20.31 27.59 2.28 3.45O 24.4 58.8 59.44 60.64 67.74 77.11Si 17.0 41.2 20.26 11.77 29.98 19.44Total 100.0 100.0 100.00 100.00 100.00 100.00

a The composition was calculated based on chemical fomulation of epoxy POSS provided by Hybrid Plastics, (C8H13O)n(SiO1.5)n.

Fig. 3 (a) A tilted SEM image of 3D patterns and a simulated image

(inset). Top-view SEM images of the central area in 3D patterns before

(b) and after heat-treatment at 400 �C (c) and 500 �C (d) in air for 1 h.

Insets in (b) and (c) show high-magnification images. (Scale bar ¼ 2 mm.)

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attributed to the tensile stress within the 3D network since the film was

attached to the substrate and undergoing large weight loss and volume

shrinkage at 500 �C. The results agree well with the TGA, FTIR and

EDS studies, confirming that above a critical temperature, here 400�C, the removal of carbon is large enough to cause collapse of O–Si–O

network, leading to the formation of cracks in the films. Currently we

investigate different film treatment conditions to maintain certain

carbon content in the film without major crack formation.

We have demonstrated the direct patterning of 2D and 3D orga-

nosilcates from epoxy-POSS cage materials using holographic

lithography. The patterned epoxy-POSS structures can be conve-

niently converted to silica structures by thermal removal of organic

moieties above 400 �C. In order to investigate the fidelity (both

structural and compositional) of the epoxy-POSS structures during

the heat-treatment, we measured FT-IR and estimated the variation

of relative density by using the TGA and AFM measurements.

Furthermore, we showed that the 3D POSS FCC structures could be

maintained without global volume shrinkage during the heat treat-

ment (up to 400 �C) by thinning the struts that connect atoms. The

study of the thermal and mechanical integrity of the POSS 3D

structures at different temperatures will offer important insights to

the design of ultrastrong yet light-weight engineering materials. The

direct conversion to 3D silica or silica-like structures will allow us to

simplify the backfilling procedure for high refractive index 3D

4690 | J. Mater. Chem., 2009, 19, 4687–4691

photonic crystals, therefore, minimizing problems such as incomplete

filling. We are especially encouraged by a recent report of creating 3D

silicon photonic crystals from a colloidal template by backfilling

silane at 325 �C through CVD process,48 suggesting that possibility of

directly backfilling silicon into our POSS 3D structures without

compromising the structural integrity may exist. The ability to

remove the template at room temperature using aqueous HF solution

will enable infiltration of a broader range of materials (e.g. inorganic,

polymers, nanoparticles) for many other applications. We believe

that our POSS resists can be easily adapted to other 3D fabrication

processes, such as multi-photon polymerization. Along the lines, the

ability to fabricate 3D structures from hybrid materials will broaden

the range of materials for study of nano- micromechanical properties

in 3D.3,49 Further, it will be intriguing to see whether structure

collapse or any pattern transformation could be observed during

backfilling or other surface functionalization of the POSS templates

in comparison to backfilled glassy polymeric structures.50

Experimental

Preparation of epoxy-POSS photoresists

Epoxy cyclohexyl POSS cage mixture [(C8H13O2)n(SiO1.5)n, n¼ 8,10

or 12] and Irgacure 261 (a visible photoacid generator, 2–4 wt%) were

dissolved in propylene glycol methyl ether acetate (PGMEA, 50–60

wt%). By varying the spin coating speed from 4000 to 6000 rpm, we

obtained film thickness ranging from 2 to 5 mm. After soft baking at

65 �C for 3 min and 95 �C for 3 min, respectively, the resist film was

exposed to three- or four-interfering beams.

Fabrication of 2D and 3D structures by holographic lithography

A laser beam source (Nd:YVO4 laser, l ¼ 532 nm) was expanded

by achromatic lens pair. For 2D holographic patterns, the three

beams possess wave vectors of 2p=a�12

ffiffiffi3p

=20�, 2p=a

�12

ffiffiffi3p

=20�

and 2p/a[1 0 0]. The polarization vectors of three-beams are equal

to each other to produce circular cylinders with hexagonal

symmetry. In the case of 3D patterns, the central beam was circu-

larly polarized with normal incidence to the photoresist film, while

the other three surrounding beams were linearly polarized and

oblique at 39� relative to the central one. The wave vector of each

beam is p/a[333], p/a [511], p/a [151], and p/a [115], respectively.

The polarization vectors of the obliqued beams are [�0.250 0.345

0.905], [0.905 �0.250 0.345], and [0.345 0.905 �0.250], respectively.

The intensity ratio was 1.8 : 1 : 1 : 1. The circular polarization of the

central beam distributes the intensity equally to the surrounding

beams. The exposure dose of interference pattern was 0.2–0.5 J/cm2

This journal is ª The Royal Society of Chemistry 2009

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for 2D and 0.1–0.3 J/cm2 for 3D. After the exposure to the epoxy-

POSS photoresist, the film was baked at 95 �C. The unexposed film

was removed by PGMEA.

Characterizations

The Fourier Transform Infrared (FT-IR) spectra were acquired using

Nicolet 8700 equipped with a Nicolet Continuum Infrared Micro-

scope. The samples were measured in a reflection mode with a MCT

detector, and the aperture size used was�80 mm� 80 mm. All epoxy-

POSS films were heated in air for 1 h at a specific temperature.

For thermogravimetric analysis (TGA, TA Instruments), the

photoresist film was lifted off from the substrate, and ground to

powder. The relative weight was measured when the sample was

heated in air up to 700 �C at a heating rate of 10 �C/min. For the

measurement of the volume and morphology in 2D and 3D patterns,

we only scanned the central part of each samples by AFM (Digital

instrument Dimension 3000) and SEM (FEI Strata). This is because

the volume fraction decreases from centre to the sample boundary

due to the Gaussian laser beam profile.

EDX analysis was performed on a high resolution field emission

SEM (JEOL 7500F) coupled to an Oxford Si/Li detector and INCA

software to study the overall chemical composition and the distri-

bution of the chemical elements of interest in the POSS films treated

at different temperatures. The spectra were acquired and collected at

an acceleration voltage 15 keV.

Acknowledgements

This research is supported in part by the Office of Naval Research

(ONR), Grant # N00014-05-0303, and Air Force of Scientific

Research (AFOSR), Grant # FA9550-06-1-0228. JHM thanks the

Manpower Development Program for Energy & Resources of the

Ministry of Knowledge and Economy (2008-E-AP-HM-P-23-0000)

and the Korea Research Foundation Grant funded by the Korean

Government (KRF-2008-313-D00295). We thank Prof. Chris Mur-

ray for helping with the access to the FT-IR instrument. We also

acknowledge the Penn Regional Nanotechnology Facility (PRNF)

and Korea Basic Science Institute (KBSI) for access to SEM, EDX,

and AFM analysis.

Notes and references

1 G. A. Ozin and S. M. Yang, Adv. Funct. Mater., 2001, 11, 95.2 J. H. Moon, J. Ford and S. Yang, Polym. Adv. Tech., 2006, 17, 83.3 J. H. Jang, C. K. Ullal, T. Y. Choi, M. C. Lemieux, V. V. Tsukruk and

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