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Oxygen insensitive thiol–ene photo-click chemistry for direct imprint lithography of oxides

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UV-nanoimprint lithography (UV-NIL) is a promising technique for direct fabrication of functional oxide nanostructures. Since it is mostly carried out in aerobic conditions, the free radical polymerization during imprinting is retarded due to the radical scavenging ability of oxygen. Therefore, it is highly desirable to have an oxygen-insensitive photo-curable resin that not only alleviates the requirement of inert conditions but also enables patterning without making substantial changes in the process. Here we demonstrate the formulation of metal-containing resins that employ oxygen-insensitive thiol–ene photo-click chemistry. Allyl acetoacetate (AAAc) has been used as a bifunctional monomer that, on one hand, chelates with the metal ion, and on the other hand, offers a reactive alkene group for polymerization. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a four-arm thiol derivative, is used as a crosslinker as well as an active component in the thiol–ene photo-click chemistry. The FT-IR analyses on the metal-free and metal-containing resin formulations revealed that the optimum ratio of alkene to thiol is 1 : 0.5 for an efficient photo-click chemistry. The thiol–ene photo-click chemistry has been successfully demonstrated for direct imprinting of oxides by employing TiO2 and Ta2O5 as candidate systems. The imprinted films of metal-containing resins were subjected to calcination to obtain the corresponding patterned metal oxides. This technique can potentially be expanded to other oxide systems as well.
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Oxygen insensitive thiolene photo-click
chemistry for direct imprint lithography of oxides
Ravikiran Nagarjuna,
a
Mohammad S. M. Saifullah
b
and Ramakrishnan Ganesan *
a
UV-nanoimprint lithography (UV-NIL) is a promising technique for direct fabrication of functional oxide
nanostructures. Since it is mostly carried out in aerobic conditions, the free radical polymerization during
imprinting is retarded due to the radical scavenging ability of oxygen. Therefore, it is highly desirable to
have an oxygen-insensitive photo-curable resin that not only alleviates the requirement of inert
conditions but also enables patterning without making substantial changes in the process. Here we
demonstrate the formulation of metal-containing resins that employ oxygen-insensitive thiolene
photo-click chemistry. Allyl acetoacetate (AAAc) has been used as a bifunctional monomer that, on one
hand, chelates with the metal ion, and on the other hand, oers a reactive alkene group for
polymerization. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a four-arm thiol derivative, is
used as a crosslinker as well as an active component in the thiolene photo-click chemistry. The FT-IR
analyses on the metal-free and metal-containing resin formulations revealed that the optimum ratio of
alkene to thiol is 1 : 0.5 for an ecient photo-click chemistry. The thiolene photo-click chemistry has
been successfully demonstrated for direct imprinting of oxides by employing TiO
2
and Ta
2
O
5
as
candidate systems. The imprinted lms of metal-containing resins were subjected to calcination to
obtain the corresponding patterned metal oxides. This technique can potentially be expanded to other
oxide systems as well.
1. Introduction
Due to its simplicity, versatility, and specic reactivity, click
chemistry has gained signicant interest in various elds
including organic transformations, surface functionalization,
polymeric materials, and biological applications.
16
Several
reactions including alkyneazide, DielsAlder reactions, thiol
ene, thiolyne, etc., have been classied under click chemistry,
as these reactions directly yield an adduct without giving any
other byproduct.
7
Among them, thiolene click is an interesting
reaction occurring between commonly available reactive
alkenes and thiol groups, which is known to be tolerant to the
presence of oxygen and moisture when the oxygen concentra-
tion is less than that of thiol.
812
Thiolene reaction is also called
as hydrothiolation, in which the thiol group adds across the
carboncarbon double bond in anti-Markovnikov orientation.
Generally, the hydrothiolation reaction can proceed under
a variety of conditions such as thermal catalytic processes, and
photo-addition.
1316
Hydrothiolation via photo-clicking is
attractive as it proceeds well at room temperature reaction
conditions. In a typical thiolene click reaction, the photo-
generated electron from a photo-initiator is transferred to the
thiol group to produce a thiyl radical, which adds across
a carboncarbon double bond and thus transferring the radical
to the alkene. Abstraction of a hydrogen radical from the
neighborhood thiol to this carbon radical accomplishes the
hydrothiolation, while simultaneously propagating the chain
reaction.
9,17
Fabrication of micro/nanostructures of oxides is of para-
mount importance in various applications including photo-
electrochemical cells,
18
photovoltaic cells,
19
sensors,
20
and
catalysis.
21
Several techniques including photolithography,
22,23
electron-beam lithography,
24,25
electro-hydrodynamic lithog-
raphy,
26,27
direct write assembly,
28
selective surface wetting,
29
dip-pen nanolithography,
30,31
nanoimprint lithography
(NIL)
3234
among others
35,36
have been utilized towards fabrica-
tion of metal oxide micro/nanostructures. Each of these tech-
niques has its own merits as well as limitations. Among the
various available techniques, NIL has been regarded as the next
generation fabrication approach due to its advantages such as
simplicity, low-cost, high-delity, high-resolution, non-
dependence on the optical diraction limit, compatibility with
various substrates and ability to imprint arbitrary structures
over at and curved surfaces.
3740
The descendants of NIL like
a
Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani,
Hyderabad Campus, Jawahar Nagar, Shameerpet Mandal, Hyderabad, 500078,
India. E-mail: ram.ganesan@hyderabad.bits-pilani.ac.in
b
Institute of Materials Research and Engineering, A*STAR (Agency for Science,
Technology, and Research), 2 Fusionopolis Way, #08-03 Innovis, Singapore 138634,
Republic of Singapore
Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra01688g
Cite this: RSC Adv.,2018,8,11403
Received 26th February 2018
Accepted 16th March 2018
DOI: 10.1039/c8ra01688g
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step-and-ash,
4143
roll-to-roll
44
and roll-to-at
45
imprint lithog-
raphy techniques have augmented its high-throughput capa-
bility while maintaining high-precision. The NIL process can be
broadly classied into two categories: thermal and UV. Thermal
NIL requires temperature cycling, which reduces the
throughput, aects the mold life, and leads to line-width vari-
ation owing to the dierences in thermal expansion coecients
of the mold, substrate, and resist.
46
On the other hand, UV-NIL
is an attractive room temperature process that does not require
any temperature cycling, and thereby allows high-throughput
patterning.
As opposed to the fabrication of polymeric micro/
nanostructures, fabrication of oxides has inherent challenges
in the materials design as the metal-containing polymers and
ceramics do not possess workable characteristics for direct
patterning. Therefore, the material design has to be customized
as per the requirement of each technique. In the case of
nanoimprinting, solgel approach was initially employed that
utilized hydrolysis followed by condensation of metal alkoxides
or halides.
33,4749
The conventional solgel approach suered
low yield due to the lack of mechanical strength of the imprints
during the demolding step. Nanoparticle suspensions have also
been used for direct nanoimprinting with a soPDMS mold.
50
This approach has been used to directly fabricate three-
dimensional structures of multicomponent oxides like indium
tin oxide and nickel ferrite.
51,52
Photosensitive ethylhexanoate-
based oxide patterning has been reported that utilizes photo-
breakable precursors rather than photo-curable monomers.
34
Recently, polymerizable solgel approach (PSG) has emerged as
a promising method for rapid direct nanoimprinting of a host
of metal oxides such as TiO
2
, ZrO
2
,Hf
2
O
5
amongst others.
46,53,54
The PSG approach utilizes metal-methacrylate liquid precursors
that undergo in situ thermal- or photo-induced free radical
polymerization during the nanoimprinting step. One of the
major advantages of this approach is the spin-coatable liquid
phase metal-containing resin that can easily ll up the pattern
features of the mold at near-ambient pressure due to capillary
action. Polymerization solidies the metal-containing precursor
and decreases the surface energy to facilitate easy and clean
demolding. This approach has been found to harness the
advantages and obviates the limitations of the conventional sol
gel approach. The PSG approach has been shown to be suitable
for high-throughput step-and-ash imprint lithography for
various oxides
55
and to synthesize supported oxide photo-cata-
lysts.
56,57
Despite the high potential, the PSG approach based on
free radical polymerization of methacrylates is vulnerable to
radical scavenging by oxygen that could aect the degree of
polymerization, which in turn could eect on the quality of
imprinting. This warrants purging/bubbling of inert gas into
the metal-containing resin before patterning in order to elimi-
nate any dissolved oxygen.
As thiolene click chemistry can full the requirements such
as oxygen-insensitivity, spin-coatability, and room temperature
photo-crosslinking, it is deemed to be attractive for UV-NIL.
5860
This chemistry has recently been shown for its potential for
imprinting polymeric structures over at and curved surfaces.
61
However, the merits of the thiolene photo-click chemistry have
so far not been realized for oxide nanoimprinting. Here, we
show the augmentation of the PSG approach by employing the
oxygen-insensitive thiolene click chemistry towards fabrica-
tion of metal oxide micro/nanostructures (Scheme 1). The
design of the photo-curable metal-containing resin, its photo-
curability and patternability are discussed by choosing tita-
nium alkoxide as the precursor to fabricate TiO
2
nano-
structures. TiO
2
was chosen as the principal oxide candidate, as
it is one of the most studied semiconducting oxides that
possesses huge potential for various applications including
energy, health, and environment.
6264
Finally, the thiolene
approach was also studied for patterning Ta
2
O
5
in order to
ascertain its potential towards structuring other metal oxides.
2. Experimental section
2.1 Materials and methods
Titanium(IV) isopropoxide, tantalum(V) ethoxide, allyl acetoa-
cetate (AAAc), pentaerythritol tetrakis(3-mercaptopropionate)
(PETMP), 2-(methacryloyloxy)ethyl acetoacetate (MAEAA), 2-
hydroxy-2-methylpropiophenone (HMP), 1H,1H,2H,2H-per-
uorodecyltrichlorosilane, and toluene were purchased from
Sigma Aldrich and used as-received. Polydimethylsiloxane
(PDMS) stamps were fabricated from Sylgard 184 purchased
from Sigma Aldrich.
Scheme 1 Schematic representation of the overall process involving thiolene click chemistry for imprinting of metal oxides.
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2.2 Resin formulation
The TiO
2
resists were prepared using requisite amounts of
precursors as mentioned in Table 1. Briey, one equivalent of
Ti(i-PrO)
4
was added and thoroughly stirred with 4 equivalents
of AAAc in a glass vial inside a nitrogen glove box. The color of
the solution rapidly turned into red that indicated the forma-
tion of Ti(AAAc)
4
complex. To this complex, either half equiva-
lent or one equivalent of the PETMP cross-linker was added and
the resultant mixture was shaken well on a vortex stirrer for
2 min to obtain a clear solution. To this mixture, 3 wt% (with
respect to the AAAc and PETMP) of HMP photo-initiator was
added to formulate the imprintable resin, which was diluted
with toluene in 1 : 1 (wt/wt) ratio to decrease the viscosity. The
Ta
2
O
5
resist was also prepared in a similar manner by taking
tantalum(V) ethoxide as the alkoxide source.
2.3 Imprinting of metal oxide
First, PDMS stamps were fabricated following the standard
procedure from Sylgard 184. Briey, the silicon rubber base pre-
polymer and cross-linker of Sylgard 184 were thoroughly mixed
in 10 : 1 weight ratio and the resulting mixture was poured over
polycarbonate master mold kept in a Petri dish. This assembly
was degassed in a vacuum desiccator to remove any trapped air
bubbles. The assembly was the subjected to 70 C for 1 h to
induce cross-linking of the precursor. Finally, the PDMS elas-
tomeric stamp was peeled ofrom the master mold. The
fabricated PDMS stamps were treated with 1H,1H,2H,2H-per-
uorodecyltrichlorosilane for 5 h inside an evacuated desiccator
in order to decrease the surface energy that facilitates easy
demolding. The water contact angle of the uorinated PDMS
stamp was found to be 130as opposed to 105for freshly
prepared PDMS, indicating the reduction in surface energy.
In a typical nanoimprinting process, the metal-containing
resin solution was spin-coated over a silicon wafer (20 mm
20 mm) at 1000 rpm for 45 s. The PDMS mold was then placed
on top of the wet thin lm and a slight pressure was applied.
This assembly was subjected to UV irradiation with a Hg vapor
lamp (125 W) for 20 min to induce crosslinking of the mono-
mers. Aer UV exposure, the PDMS stamp was carefully
demolded and the imprints were subjected to calcination at
designated temperatures for 1 h to obtain metal oxide micro/
nanostructures.
2.4 Characterization
The thiolene photoreaction was following using FT-IR spec-
trophotometer (JASCO FTIR 4200). TGA analyses were carried
out using Shimadzu DTG-60 dierential thermal analyzer in air
atmosphere to follow the degradation prole and the residual
oxide mass present in the metal-containing resins. XRD
patterns of the thin lm samples calcined at dierent temper-
atures were measured at a scan rate of 1min
1
using Rigaku
Ultima IV X-ray diractometer equipped with Cu K
a
(l¼1.5418
˚
A). Diuse reectance spectroscopy analysis of the thin lm
samples calcined at dierent temperatures was performed
using JASCO V-670 UV-visible spectrophotometer. The surface
morphology of the imprints before and aer calcination was
characterized using a Carl-Zeiss ULTRA-55 eld emission
scanning electron microscope (FE-SEM).
3. Results and discussion
A judicious choice of precursors is the key to successful
implementation of the thiolene click chemistry to realize oxide
nanofabrication. In this regard, we chose pentaerythritol
tetrakis(3-mercaptopropionate) (PETMP) as the cross-linker
that has four thiol arms. Allyl acetoacetate (AAAc) was chosen
as the bifunctional reagent that on one hand chelates with the
metal center through the acetoacetate group and on the other
hand possesses the reactive alkene group for the click reaction.
When the metal center is chelated with more than one AAAc, it
could also act as a cross-linker, which would lead to the
formation of highly branched polymeric network. The chemical
structures of the various components used in this study are
shown in Fig. 1.
To probe the thiolene click reaction under UV irradiation,
we formulated a resin containing PETMP and AAAc in 1 : 4 ratio
mixed with 3 wt% of 2-hydroxy-2-methylpropiophenone (HMP)
photo-initiator. This metal-free resin was spin-coated over a pre-
cleaned silicon wafer and studied with the Fourier transform
infrared (FT-IR) spectroscopy to follow the thiolene photo-click
chemistry (Fig. 2a). As seen from the gure, the characteristic
SH stretching peak was observed at 2570 cm
1
, conrming the
presence of free thiol groups in the resin formulation. In
addition, an intense peak at 1738 cm
1
due to the carbonyl
group stretching and a weak peak at 1632 cm
1
due to the
alkene group were also observed. Aer subjecting the lm to
20 min of UV irradiation, it remained wet, indicating poor
Table 1 Proportions of various components used in the dierent
formulations
Sample
code
TIPO
(mmol)
AAAc
(mmol)
PETMP
(mmol)
HMP
(mmol)
TA 1.0 4.0 0.0 0.156
TAP-0.5 1.0 4.0 0.50 0.208
TAP-1 1.0 4.0 1.00 0.277 Fig. 1 Chemical structures of the various components used for
imprinting of metal oxides.
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eciency of the thiolene click-reaction in this system. The FT-
IR spectrum obtained aer irradiation was almost identical to
that before UV irradiation. We postulated that this could be due
to the unfavorably positioned functional groups or reduced
probability of stoichiometric encountering of the allyl and thiol
groups in the thin lm. Therefore, we decreased the amount of
PETMP to half (PETMP and AAAc in 1 : 8 ratio) to provide more
alkene double bonds per SH group so as to enhance the
probability of the reaction at the thiol group sites. Interestingly,
the irradiated lm became dry to yield a smooth photo-cured
lm. The FT-IR corresponding to this composition revealed
the complete disappearance of the SH stretching peak,
strongly corroborating the occurrence of thiolene click reac-
tion (Fig. 2b).
To enable the thiolene chemistry for oxide nanoimprinting,
a photo-curable metal-containing resin formulation was
prepared by mixing titanium-allyl acetoacetate complex
[Ti(AAAc)
4
] with PETMP and HMP. Table 1 shows the three resin
formulations that were used for photo-curability studies. The
rst composition (TA), which does not contain PETMP could
undergo [2 + 2] photo-addition of alkene double bonds to yield
cyclobutane derivative. This composition yielded a dry lm aer
UV irradiation. However, the lm was found to be brittle and
easily aked oof the silicon wafer. This could be due to the
excessive photo-curing of the four-arm alkene-functionalized
Ti(AAAc)
4
that caused residual stress in the thin lm.
53
In the
second composition (TAP-0.5), 1/8 equivalent of PETMP with
respect to Ti(AAAc)
4
was introduced to keep the eective avail-
able thiol groups per alkene as 0.5. This formulation aer UV
irradiation was found to yield a dry and smooth lm that did
not suer any peeling obehavior. This could be attributed to
the cushioning eect provided by the optimum cross-linking of
PETMP that circumvented cracking of the lm.
In the third composition (TAP-1), 1/4 equivalent of PETMP
with respect to Ti(AAAc)
4
was used in order to keep the thiol to
alkene ratio 1 : 1. Interestingly, the lm obtained with this
composition was found not fully dry aer 20 min of UV irradi-
ation. This is similar to the poor photo-curing observed with the
metal-free resin containing stoichiometric ratio of allyl to thiol.
Thus, for the remaining characterization and imprinting
studies, we utilized the optimized composition of TAP-0.5.
Fig. 3 shows the FT-IR analyses using TAP-0.5 to probe the
thiolene photo-click reaction. As seen from the spectra, the
characteristic ester and keto carbonyl groups are observed at
1745 and 1718 cm
1
, respectively. Compared to the metal-free
resin, pristine Ti(AAA)
4
showed two additional characteristic
peaks at 1610 and 1532 cm
1
that can be attributed to the
bidendate chelation of the enol form of AAAc.
54
In addition, the
carboncarbon double bond of allyl group appeared at
1633 cm
1
. In the case of TAP-0.5, in addition to the above
mentioned stretching peaks, a new peak at 2572 cm
1
corre-
sponding to the thiol appeared. Aer UV light irradiation, the
thiol peak became nearly invisible, while the intensity of allyl
double bond peak diminished. These changes clearly indicate
the occurrence of click reaction between the thiol and allyl
Fig. 2 FT-IR analyses of thiolene photo-curing of resins containing
PETMP and AAAc in the ratio of (a) 1 : 4 (TAP-1) and (b) 1 : 8 (TAP-0.5)
showing that the latter composition is amenable to the click chemistry.
Fig. 3 FT-IR study of thiolene photo-addition in the titanium-con-
taining resin TAP-0.5.
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groups in this system. It is noteworthy that the chelation with
the metal remains intact even aer the UV irradiation, which
conrms that the metal is entrapped inside the cross-linked
polymeric network.
Fig. 4a shows the thermogravimetric analysis (TGA) of TAP-
0.5 performed in air. A steady mass loss was observed
between 100 and 500 C due to the removal of residual solvent,
low molecular weight monomers, and polymers. No further
mass loss was observed above 500 C, suggesting the complete
removal of organics present in the resin. The residual inorganic
mass corresponding to TiO
2
at 500 C was found to be 10.4%
which is in close agreement with the theoretical estimated value
of 11.9%.
To conrm the phases obtained aer calcination of the UV-
cured TAP-0.5 lms, X-ray diraction (XRD) and Raman spec-
troscopy were performed. The calcination temperature was
varied from 450 to 650 C at a constant duration of 1 h to
monitor the evolution of the phases and the corresponding XRD
patterns are presented in Fig. 4b. As seen from the gure, the
TAP-0.5 lms heat-treated at 450 and 500 C revealed the
formation of phase pure anatase. The broad peaks indicate the
nanocrystalline nature of TiO
2
obtained at these temperatures.
Above 500 C, the (101) anatase peak was found to intensify with
narrowing of the peak, indicating the growth of nanocrystallites
to larger crystals (JCPDS 89-4921). At and above 600 C,
formation of a small amount of rutile phase is indicated by the
appearance of (110) plane at 2q¼27.4. Applying the Scherrer's
formula to the (101) plane of anatase TiO
2
, the crystallite size of
the samples calcined at 450, 500, 550, 600 and 650 C are
calculated to be 6.7, 7.1, 12.0, 15.7, and 15.6 nm, respectively.
The Raman spectra of the TiO
2
samples calcined at dierent
Fig. 4 (a) Thermogravimetric analysis of TAP-0.5 resin. (b) XRD patterns of TiO
2
obtained by calcination of photo-cured TAP-0.5 thin lms at
various temperatures. (c) Diuse reectance spectra and (d) the corresponding KubelkaMunk plots of TiO
2
thin lms prepared at dierent
temperatures.
Fig. 5 FE-SEM images of imprinted lines of TAP-0.5 (a and b) before
and (c and d) after calcination.
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temperatures looked identical (Fig. S4). A major peak at
121 cm
1
along with weak peaks at 365, 482, and 603 cm
1
were
observed in all cases that can be attributed to the respective
vibrational symmetries of E
g
,B
1g
,A
1g
, and E
g
Raman-active
modes of anatase TiO
2.
57,65
Due to the presence in minute
quantity, the Raman peaks of rutile TiO
2
were not observed in
the samples calcined at 600 and 650 C.
The semiconducting nature of the oxide thin lms obtained
through the thiolene approach has been studied using diuse
reectance spectroscopy. The percentage reectance and their
corresponding KubelkaMunk plots of TiO
2
calcined at various
temperatures are shown in Fig. 4c and d, respectively. The
KubelkaMunk factor (K) is given by K¼(1 R)
2
/2R, where Ris
the % reectance. The intersection of the extrapolated linear
portion of the curve in (Kenergy)
n
versus energy plot with the
energy axis yields the band gap of the semiconductor material.
For direct band gap materials, the value of nis taken as 2, while
for indirect band gap material the value is taken as 1/2. Since
TiO
2
is an indirect band gap material, the value of nused was
1/2. The KubelkaMunk plots show the band gaps of TiO
2
calcined at various temperatures to be in the range of 3.3 to
3.45 eV. These values agree well with the band gap of anatase
TiO
2
reported in the literature (3.2 to 3.4 eV) and thereby con-
rming the semiconducting nature of the obtained thin
lms.
66,67
The metal-containing resin formulations were studied for
their suitability for imprinting under laboratory conditions. A
polydimethylsiloxane (PDMS) stamp was pressed over the spin-
coated TAP-0.5 lm and irradiated with UV light. Aer
demolding, the morphology of the photo-cured imprints before
and aer calcination was characterized using eld-emission
scanning electron microscopy (FE-SEM) and the images are
shown in Fig. 5. The PDMS somold used in this study
possessed a feature size of 2.1 mm line and 1.8 mm space
(Fig. S5). Aer patterning of TAP-0.5, the width of the
imprinted line and space was found to be 1.7 and 2.3 mm,
respectively. The shrinkage of the imprinted lines may be
attributed to the absorption of the residual solvent by PDMS
mold and polymerization. Post-calcination, the lines were
further shrunk to 420 nm and the spaces widened to 3.5 mm.
On the whole, the TiO
2
lines were shrunk to 25% compared to
the as-imprinted lines, which is similar to the values previously
reported PSG-based literature on TiO
2
nanoimprinting.
46,5355
The shelf-life of the AAAc-based titanium resin was poor and
thereby warrants immediate imprinting of the formulation. We
speculated that this could be due to the vulnerability of tita-
nium for hydrolysis, which was not averted by the chelated
AAAc. Therefore, we briey studied the shelf-life and imprint-
ability of a titanium resin in which AAAc was replaced with 2-
(methacryloyloxy)ethyl acetoacetate (MAEAA). This formulation
Fig. 6 (a) Thermogravimetric analysis of tantalum-containing resin. (b) XRD patterns, (c) diuse reectance spectra and (d) KubelkaMunk plots
of Ta
2
O
5
obtained at 450 and 700 C.
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was found to have a good shelf life of more than a month.
Furthermore, the successful direct imprinting using this
MAEAA-based thiolene resin conrmed that the approach can
be extended to other chelating monomers as well (Fig. S6).
To substantiate the applicability of this thiolene approach
for patterning other oxides, Ta
2
O
5
was chosen as the candidate
due to its potential applications in semiconducting devices.
68,69
Although tantalum is a pentavalent metal, we used 4 equiva-
lents of AAAc in order to match with the corresponding titanium
precursor. The resultant resin was found to have a good shelf
life of more than a month. The PETMP ratio to the AAAc was
xed in the similar fashion to that used in TAP-0.5 and the FT-IR
analyses before and aer UV light exposure were performed
(Fig. S7). The results revealed the disappearance of the SH
stretching peak, indicating the occurrence of thiolene photo-
click chemistry. Fig. 6 shows the TGA, XRD and diuse reec-
tance spectra of the tantalum-containing resin. The TGA prole
was observed to be in similar nature to that of TAP-0.5. The
complete mass loss was found to occur at 530 C and the
residual inorganic mass corresponding to Ta
2
O
5
was observed
to be 20.1%. The XRD pattern of Ta
2
O
5
was found to be broad,
indicating its amorphous nature at this temperature. Therefore,
the UV-cured lm was calcined at 700 C to obtain crystalline
Ta
2
O
5
. The XRD analysis of this sample revealed well-dened
orthorhombic crystalline phase of Ta
2
O
5
(JCPDS no. 79-
1375).
70
The diuse reectance and the corresponding Kubelka
Munk plots revealed a shallow spectrum for the 450 C calcined
sample, conrming the amorphous nature of the Ta
2
O
5
. The
thin lm calcined at 700 C was found to exhibit a band gap of
4.26 eV, which is similar to that reported in the literature.
71
UV imprint lithography of the tantalum-containing resin was
performed and the FE-SEM images corresponding to the as-
imprinted and calcined samples are shown in Fig. 7. The line
and space feature sizes of the as-imprinted patterns were found
to be 1.7 and 2.1 mm, respectively. Aer calcination, the lines
were shrunk to 470 nm and the spaces extended to 3.4 mm.
The cross-sectional view of FE-SEM images of the as-imprinted
and calcined samples showed the heights to be 1.7 mm and
410 nm, respectively. Thus, the lateral and vertical shrinkages
were found to be 75% that is similar to the case of TiO
2
. The
very similar imprinting behavior of tantalum-containing resin
shows the capability of the thiolene approach to be a potential
route for fabricating nanostructures of a host of oxides that
could nd applications in various devices.
4. Conclusion
In summary, thiolene photo-click chemistry has been
successfully applied to the fabrication of metal oxide micro/
nanostructures via imprinting. When PETMP and AAAc were
mixed to keep the allyl to thiol ratio as 1 : 1, the FT-IR studies
revealed poor eciency of the thiolene photo-click chemistry.
When allyl to thiol ratio was kept as 1 : 0.5, the thiolene photo-
click chemistry was found to be ecient under ambient
conditions, possibly due to the enhanced probability of the
reaction between SH group and the abundant alkene. The
metal-containing resin consisted of chelated titanium alkoxide
with AAAc (or MAEAA) with suitable proportion of PETMP cross-
linker, and HMP photo-initiator. With the optimal amount of
allyl to thiol ratio as 1 : 0.5 (TAP-0.5), the resin exhibited
excellent photo-curability to yield stable, crack-free, and dry
thin lm. The XRD and Raman analyses revealed the formation
of anatase TiO
2
phase at temperatures below 550 C, above
which a small amount of rutile formation was conrmed with
XRD. The diuse reectance spectroscopy revealed the semi-
conducting nature of the thin lm TiO
2
. The imprinting studies
demonstrated the patternability of the titanium-containing
resin to fabricate micro/nanostructures of TiO
2
. Successful
extension of this thiolene photo-click chemistry for patterning
Ta
2
O
5
conrmed the leeway of this approach towards fabrica-
tion of various functional oxide nanostructures.
Conicts of interest
There are no conicts to declare.
Acknowledgements
R. G. thanks the Department of Science & Technology (SERB/F/
825/2014-15) for the nancial aid. He also thanks Department of
Science and Technology fund for improvement of science and
technology infrastructure (DST FIST; SR/FST/CSI-240/2012) to
procure FT-IR, Raman microscopy, and UV-visible spectroscopy.
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