LETTER
pubs.acs.org/Langmuir
Patterned Removal of Molecular Organic Films by Diffusion
Corinne E. Packard,* Katherine E. Aidala,* Sulinya Ramanan, and Vladimir Bulovic
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, United States
bS Supporting Information
ABSTRACT: We demonstrate that “contact patterning” subtractively patterns a wide range of
molecular organic films of nanoscale thickness with nanometer-scale accuracy. In “contact patterning”,
an elastomeric stamp with raised features is brought into contact with the organic film and
subsequently removed, generating patterns by the diffusion of the film molecules into the stamp.
The mechanism of material removal via diffusion is documented over spans of minutes, hours, and days
and is shown to be consistently repeatable. Contact patterning provides a photolithography-free,
potentially scalable approach to subtractive patterning of a wide range of molecular organic films.
M
icrocontact printing-based methods offer viable approaches
for achieving microscale feature definition for a wide range of
materials, including vapor-deposited molecular organic solids.17 We
recently demonstrated that a variety of molecular organic thin films
can be patterned by using relief-patterned polydimethylsiloxane
(PDMS) stamps to remove a controlled thickness of vapor-deposited
material.8 In contrast to earlier studies, our “contact-patterning”
process eliminates the need for elevated processing temperatures,
partially cured polymer stamps, increased applied pressures, or
combinations of these.1,4,7 The present study investigates the
mechanism underlying the contact-patterning method. We find that
the patterning process is time dependent, with more material being
removed for longer contact times between the stamp and deposited
film. Data from this study show that patterning occurs by diffusion of
the organic chromophores into the PDMS stamp during contact.
The diffusive nature of the patterning process makes possible
repeated patterning with a single stamp and nanometer-scale control
over the removed film thickness.
Figure 1a shows a schematic of the contact-patterning technique.
A PDMS stamp is brought into contact with a thermally evaporated
molecular organic film, left in contact for a predetermined amount of
time, and then removed (experimental details are included in
Supporting Information). As detailed in ref 8, patterning may completely remove the organic thin film, revealing the substrate, or it may
remove a finite thickness of the film, depending on the material.
Figure 1b shows that the amount of material removed exhibits a time
dependence for the five different organic materials studied here:
3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ),
N,N0 -bis(3-methylphenyl)-N,N0 -bis(phenyl)-benzidine (TPD),
N,N0 -bis(3-methylphenyl)-N,N0 -bis-(phenyl)-9,9-spiro-bifluorene
(spiro-TPD), 2,20 ,200 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), and tris(8-hydroxy-quinolinato)aluminum (Alq3).
Each point in the data series is the average thickness removed, as
calculated from six ellipsometry measurements from two different
contact-patterned areas on the same organic film and three measurements of the as-deposited thickness. The error bars reflect the standard
r 2011 American Chemical Society
deviation and include the propagated error from the as-deposited and
the postpatterning measurements, neglecting covariance between the
two. Additional data (not shown) reveals that while the trend remains
consistent, the quantitative amount of material removed was dependent on the age of the stamp (which is defined as time between the
casting of the PDMS and use of the stamp for contact-print patterning), with stamps removing less organic material with age. The
present study controls for this aging effect by using stamps prepared
in a single batch and by completing each experiment over a comparatively short period of time. Figure 1b shows a large disparity in
the amount of material removed by the contact-patterning of
different thin films. After 60 min of stampfilm contact time,
(229 ( 25) nm of TAZ and (94 ( 6) nm of TPD are removed,
while only (39 ( 1) nm of spiro-TPD, (12 ( 1) nm of Alq3, and
(19 ( 1) nm of TPBi are removed. For all of the films, however,
patterning occurs to greater depths the longer the PDMS stamp
remains in contact with the organic film.
In an earlier study by Choi et al. employing polymeric stamps
to pattern N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)-benzidine (NPB) and Alq3,4 the proposed mechanism for the organic
thin-film removal was based on adhesion. There, a favorable
static work of adhesion between the surfaces of the polymer
stamp and organic film, calculated based on contact angle
measurements, was presented as evidence of adhesion-based
lift-off. In our study, the time-dependence of the material removal
suggests that the patterning mechanism is not strictly due to
adhesion of the organic film molecules to the PDMS stamp.
An alternative explanation is that organic thin film removal is
controlled by diffusion of molecules from the organic thin film
into the PDMS matrix during the PDMSstamp/film contact.
The driving force for diffusion of solutes into a matrix is
proportional to the concentration gradient that develops within
Received: May 6, 2011
Revised:
June 23, 2011
Published: June 23, 2011
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Figure 1. (a) Schematic of the contact-patterning process. A PDMS
stamp is brought into contact with a molecular organic film, left in
contact for a prescribed amount of time, and delaminated to subtractively pattern the film. (b) Prolonged contact with the PDMS stamp
results in a greater thickness of material removed by stamping for each of
the five organic materials studied here. Error bars represent the standard
deviation of thickness for multiple sample measurements.
the PDMS,9,10 which is consistent with the trend observed in
Figure 1b. The rate of removal declines over time, presumably
due to the decreasing concentration gradient as the organic dye
molecules continue to diffuse into the PDMS polymer network
with time. Differences in material removal rates and final saturation values are likely linked to differences in diffusivity between
various organic molecules.
To test whether the organic dye molecules diffuse into the
PDMS stamp, we prepared a series of samples with thin films of
organic molecules on PDMS, glass, and silicon substrates. All
samples underwent exposure to oxygen plasma for a period of 30
s, after which the photoluminescent intensity of their chromophores
was tested. While oxygen plasma is known to cause removal or
photooxidation of organic dye molecules on exposed surfaces,1114
a previous study15 that employed structurally similar organic dyes
showed that these molecules remained protected from oxidation
when embedded within a polymer matrix. Similarly, if the organic
dye molecules in our experiments diffuse into the bulk of a PDMS
stamp, they could become protected from the caustic effects of the
oxygen plasma exposure, remaining luminescent.
Two sets of samples were prepared, with each set including a
glass substrate, a flat piece of PDMS, and a silicon substrate. All glass
and flat PDMS substrates were first covered with shadow masks to
define a 5 mm 5 mm window in the center of each. We evaporated
a 20 nm thick film of spiro-TPD on one set of samples and a 7.5 nm
thick film of Alq3 on the other. The organic thin films on the silicon
substrates are patterned with a set of PDMS stamps molded with a
pattern consisting of interpenetrating arrays of square features 100
μm, 500 μm, and 1 mm in size. These molded PDMS stamps are
placed in contact with the organic thin films on silicon for a period of
20 min, which is sufficient time to pattern through the deposited
organic film thickness for each material. The molded PDMS stamp is
then delaminated from the substrates and retained for this study.
LETTER
Figure 2. Photographs of the effect of oxygen plasma exposure for
spiro-TPD (ac) and Alq3 (df) organic dyes shown on various
substrates under UV illumination. See text for full description of
samples. (a, d) Photoluminescence of the organic dye molecules prior
to the oxygen plasma treatment. (b, e) Photoluminescence from the
organics ceases on the glass substrates (left-most in each image), but
persists in the PDMS samples (middle and right) after 30 s long oxygen
plasma exposure. (c, f) Photographs of the PDMS samples in (b) and (e)
after 3 weeks of storage in a low vacuum (>0.1 Torr) environment. All
samples are approximately 1 cm 1 cm in size.
Photographs in Figure 2 show the glass substrate with evaporated organic thin film, the molded-PDMS stamp used for
patterning of films on silicon, and the flat-PDMS substrate with
evaporated organic film, arranged from left to right in each
photograph. In all photographs, the samples are illuminated by a
hand-held ultraviolet lamp (emitting λ = 365 nm wavelength
light) such that photoluminescence from the organic material is
apparent. The exact positioning of the samples and lighting was
not maintained for all of the photographs, but each image clearly
shows the luminescence of the samples, with Figure 2ac and
df pertaining to spiro-TPD and Alq3, respectively. Before
oxygen plasma treatment, the photoluminescence indicates the
presence of the organic dye molecules in all three samples for
each dye material. In the case of the molded-PDMS stamps
(center samples in the photographs), the photoluminescent
organic dye has transferred to the raised PDMS features that
were in contact with the organic films on silicon substrates. After
exposure to oxygen plasma, the glass substrates no longer show
photoluminescence, a result consistent with reports in the literature that intentionally employ oxygen plasma treatments to
remove organic species from surfaces. In both flat and molded
PDMS samples, however, the photoluminescence persists despite the plasma treatment, indicating that organic dye molecules
remain and have not become oxidized. This result provides
evidence that the dye molecules are not located on the surface
of the PDMS, but that they diffuse into the bulk of the PDMS
stamp where they are protected from the reactive plasma species.
Molecular diffusion is also evident in the Figure 2c photograph of
the spiro-TPD film on PDMS which shows that, after 3 weeks of
storage in a dry vacuum environment, the patterned squares of
spiro-TPD molecules on the molded-PDMS stamp are no longer
discernible. Instead, the entire stamp faintly glows the bluish
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color of spiro-TPD. Similarly, the shadow-masked square pattern
on the flat-PDMS has lost its well-defined boundaries, revealing
diffusion of molecules away from their original positions. For the
same time period, Figure 2f shows that Alq3 molecules do not
diffuse far enough to be visibly different from the photograph of
Figure 2e, consistent with the slower diffusion of Alq3 in comparison to spiro-TPD (cf. the shallower contact-patterning depth of
Alq3 in Figure 1b).
In rubbery polymers, such as the PDMS used in this study, a
prevailing theory for diffusion of molecules in the polymer matrix
is based on the concept of free volume.9,10 Free volume in the
network of the polymer chains consists of permanent void spaces
between the chains of the polymer backbone as well as temporary
openings that arise from chain movement above the glass transition
temperature.9,10 Since uptake of molecules by diffusion relies on the
movement of molecules through the free volume in the polymer,
diffusion constants are generally smaller for bigger molecules and for
stronger interactions of the molecules with the polymer chains.9
Lacking known diffusion constants for these organic materials in
PDMS, we can instead look at sublimation temperature (data in
Table S1 of the Supporting Information), which reflects a combination of the physical properties that are used as inputs in polymer
diffusion models, with larger and more strongly interacting molecules
having a higher sublimation temperature.16 The connection between
sublimation temperature and diffusion rate, however, is not a direct
correlation; the results shown in Figure 1 are consistent with trends
for the organic film removal in ref 8, but we find that trends in
sublimation temperature alone are insufficient in capturing the
subtleties of organic material removal by diffusion into PDMS.
The results of an earlier study by Lange et al.17 provide further
evidence that diffusion of organic small molecules in PDMS is
influenced by free volume within the polymer matrix, which we
tested through modification of the structure of our PDMS stamps
by extracting residual low molecular weight oligomers. The
oligomer extraction was accomplished by exposing the cured
PDMS stamps to a series of solvents that swell the polymer chain
network and allow oligomers to diffuse out of the PDMS matrix.
Lange et al. find that diffusion of organic dye molecules is
enhanced in the oligomer-extracted PDMS and attribute this
result to a greater degree of free volume in the extracted
polymer.17 Similarly, our results on contact-patterned spiroTPD thin films show that the amount of material removed by
stamping with a PDMS stamp that has been cured and extracted
according to the procedures in ref 18 results in a 2-fold
enhancement in material removal after a 60 min stamp application ((45 ( 3) nm for extracted stamps versus (19 ( 10) nm for
as-cast PDMS).
It is instructive that these early results show that control over
material removal rates can be tailored by manipulating the
polymer network and by material selection based on the sublimation temperature. Further study is necessary to determine
the precise dynamics at the interface of the PDMS and the film,
the exact role of the oligomers, and to gain predictive power over
diffusion rates of organic small molecules. We note that all of our
contact-patterning results have been performed on smooth, vapor
deposited surfaces (cf. Figure S1 of the Supporting Information).
We expect, however, that this patterning technique may be extended
to spun-cast materials, but additional study is required to confirm
this point, especially regarding the effect of spun-cast film roughness
on the fidelity of contact-patterning.
Though the industrial feasibility of patterning over large areas
via microcontact printing methods remains to be demonstrated,
LETTER
Figure 3. Fluorescence micrograph of repeated stamping trials on three
separate areas of a silicon substrate coated with a 20 nm thick film of
spiro-TPD. Dark areas indicate where material has been removed after
the first (a), second (b), and third (c) patterning attempts with a single
PDMS stamp. Scale bars represent a distance of 500 μm.
several schemes for scaling up have been suggested, including roll-toroll and wave printing.19,20 In these processes, the issue of stamp
reusability is critical. In adhesion-based printing methods, a cleaning
step must be included between each patterning attempt to refresh the
surface of the stamp. Given the diffusive nature of the patterning
process described in this paper, reusing the same stamp for repeated
patterning is a possibility. Figure 3 shows the results of repeated
patterning attempts using a single stamp that was placed in contact for
20 min sequentially on three different regions on a silicon substrate
with an initial layer thickness of 20 nm of deposited spiro-TPD.
Minimal loss in pattern fidelity is observed over the course of three
patterning attempts with the same stamp, suggesting that repeated
contact-patterning is possible, although the stamp may gradually saturate. Optimizing the process by altering the stamp, time, temperature,
or thickness of the films could result in complete material removal
across repeated uses.
In summary, we investigated the mechanism for subtractive
contact-patterning of a range of molecular organic thin films. Material
removal by PDMS stamps is found to be a time-dependent process
that occurs due to diffusion of organic small molecules from the
organic thin film into the polymer stamp. Results concerning material
removal rates and oligomer extraction are consistent with existing
free-volume models for diffusion of molecules into rubbery polymers
such as the PDMS we employ as patterning stamps. We expect that
the contact-patterning process should be scalable to patterning over
large areas by adopting roll-to-roll processing geometries.
’ ASSOCIATED CONTENT
bS
Supporting Information. Table of properties, AFM
images, and experimental details for PDMS, stamp, and film
preparation, characterization techniques, and oligomer extraction procedure. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*(C.E.P.) Mailing address: Department of Metallurgical and
Materials Engineering, Colorado School of Mines, Golden,
Colorado 80401, United States. E-mail: cpackard@mines.edu.
(K.E.A.) Mailing address: Department of Physics, Mount Holyoke College, South Hadley, Massachusetts 01075, United
States. E-mail: kaidala@mtholyoke.edu.
’ ACKNOWLEDGMENT
The authors gratefully acknowledge support from the Center
for Excitonics, an Energy Frontier Research Center funded by the U.
S. Department of Energy under Grant Number DE-SC0001088,
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LETTER
Office of Science and Office of Basic Energy Sciences (C.E.P.), the
Clare Boothe Luce Foundation (K.E.A.), and the NSF Electronics,
Photonics, and Magnetic Devices program (Grant Number
1001994). The authors also acknowledge the use of the facilities
of the NSF MRSEC Center for Materials Science and Engineering.
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