Bifunctional patterning of mixed monolayer surfaces using scanning probe lithography for multiplexed directed assembly

Article abstract of DOI:10.1021/ja101627e

The patterning of a surface with more than one type of functionality in spatially resolved fashion is described. A scanning probe was used to create patterns composed of two orthogonal types of functionality within a dense, electrochemically active mixed

Full text of DOI:10.1021/ja101627e

Published on Web 05/03/2010
Bifunctional Patterning of Mixed Monolayer Surfaces Using Scanning Probe
Lithography for Multiplexed Directed Assembly
David A. Unruh, Clayton Mauldin, Stefan J. Pastine, Marco Rolandi, and Jean M. J. Fre´chet*
College of Chemistry, UniVersity of California, Berkeley, California 94720
Received February 24, 2010; E-mail: frechet@berkeley.edu
The development of methods for the production of substrates
with different spatially resolved functionalities is highly desirable,
as their impact may range from semiconductor fabrication to
biology.1,2 Surface-bound functionalities can serve as templates for
the directed assembly of molecules or nano-objects, allowing the
bottom-up fabrication of sensors, arrays, and devices. Several
multicomponent patterning strategies have been reported, including
orthogonal solvent treatments, plasma treatments, photolithography,
and capillary force lithography.3 An issue with some of these
techniques is that they may not extend to the sub-100 nm feature
size regime or may require harsh treatments or complicated
syntheses. The use of scanning probe lithography (SPL) for nanoscale
patterning is attractive because it allows high-resolution direct-write
patterning,4 real-time imaging of patterns, serial on-chip patterning of
multiple functionalities, and mild processing conditions. We have
previously reported an SPL technique for the local deprotection of
surface-bound amines that utilizes the electric field produced by a
Figure 1. (a) Schematic of orthogonal AFM patterning on the benzo-
positively biased sample and a grounded atomic force microscope
quinone/propyltrichlorosilane mixed monolayer 3. (b) Syntheses of ben-
zoquinone-modified surface 2 and mixed monolayer 3.
(AFM) tip.5 The ability to pattern a second functionality chemically
distinct from amines in the same AFM patterning step would allow
surface coverage of 1 on surface 2 calculated from the CV data
amounted to only ∼18% of the available Si(100) surface binding
sites, likely as a result of steric interactions between the bulky
quinoid groups of 1.
the assembly of complementary combinations of molecules with sub-
100 nm features. Herein we report an electrochemically based SPL
method where, depending upon the surface bias applied (Figure 1a),
a redox-sensitive surface-bound molecule can be electrochemically
reduced to yield an amine or the surface can be locally oxidized to
produce an oxygen-rich species.
Having confirmed macroscopic reduction, we next investigated
SPL on 2. Patterning experiments were performed in contact mode
with doped silicon tips in an environment with 70 ( 5% relative
humidity. The grounded tip was translated at 1-4 µm/s across
the surface with a -12 V bias applied to the surface, and lateral
force microscopy was used to verify the surface modification.
The darkened vertical lines in Figure 2a have a width of 80 ( 4
nm and correspond to the modified areas. The vertical deflection
of the tip did not change during patterning, and the corresponding
postpatterned topography images do not show significant height
changes; therefore, it is unlikely that the observed lines are due to
scratching of the substrate. The difference in reduction bias between
SPL and CV may be due to the lack of a supporting electrolyte in
the water meniscus during SPL. The width of the patterned lines
was found to depend upon the relative humidity, applied surface
bias, and patterning speed (see the SI). Microscale patterning was
also achievable using electrically conductive copper grids,10 and
It has been established that the nanoscale water meniscus that
forms between the surface and the tip of the scanning probe can
mediate the controlled transfer of molecules from tip to surface,6
the localized oxidation of both inorganic and organic surfaces,7
and the electrochemical reduction of metals.8 In comparison with
oxidative methods, reductive SPL methods have not been as
extensively investigated. We envisioned that through careful
monolayer design we could use this water meniscus as a “nanoscale
electrochemical cell” capable of performing local surface oxidation
or reduction, depending upon the patterning conditions. To this end,
we designed a triethoxysilane derivative (1) featuring a redox-
active9 trimethylbenzoquinone moiety. Amine patterns could thus
be generated in monolayers based on 1 via the local reduction and
subsequent lactonization of the departing benzoquinone moiety
under the stimulus provided by the tip.
Figure 1b summarizes the reductively active surfaces used and
their preparation. Benzoquinone-modified surface 2 had a typical
thickness of 0.63 ( 0.27 nm, as measured by ellipsometry, and a
contact angle of 70 ( 4°. In line with the reduction of other
trimethylbenzoquinone-based surfaces,9b irreversible blanket reduc-
tion of 2 was demonstrated using cyclic voltammetry [CV; see the
Supporting Information (SI)]. Support for this blanket modification
was provided by surface contact angle measurements both before
and after CV reduction. The contact angle after reduction (55°)
approached that of an amine-terminated monolayer (51°). The
Figure 2. Lateral force microscopy images of (a) SPL-patterned and (b)
copper-grid-patterned surfaces.
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6890 J. AM. CHEM. SOC. 2010, 132, 6890–6891
10.1021/ja101627e  2010 American Chemical Society

C O M M U N I C A T I O N S
the lateral force microscopy image in Figure 2b clearly shows
pattern transfer from the grid to the substrate. No evidence of
patterning was observed under dry conditions (relative humidity
<5%), which suggests that an electrochemical process is responsible
for patterning, in contrast to other high electric field-based SPLs
on organic monolayers.5,11
These patterned areas can act as templates for the directed
assembly of various nano-objects through both ionic and covalent
chemistries. For example, incubation of surfaces patterned either
by SPL or on the microscale in a solution of C60 provided raised
regions ∼0.8 nm in height, which is close to the expected diameter
of an amine-linked fullerene (1 nm). These assembled features
withstood 3 days of Soxhlet extraction in toluene, as would be
expected for covalent binding. Placing the patterned substrates in
a solution of a pentathiophene butyric acid chloride and a carboxyl-
terminated third-generation benzyl ether dendron also resulted in
raised lines corresponding roughly to the height of the newly
attached molecule (see the SI). The lines formed by assembly of
the dendron onto the amine were stable toward repeated washings
with water, but they could be disrupted by immersion in a 1 M
NaCl solution or removed entirely with 1 M aqueous acetic acid.
Collectively, these results suggest that the electrochemically reduced
regions display typical amine chemistry.
Having successfully demonstrated the ability to reductively
pattern the benzoquinone-modified surface, we next explored
oxidative patterning of the same surface. Initial attempts with
existing AFM-based oxidative methods7a,c were unsuccessful, as
the underlying Si(100) was oxidized instead of the monolayer itself,
resulting in SiO2 patterns that were not amenable to directed
assembly. This observation was rationalized by the incomplete
passivation of the underlying Si(100) due to the low coverage of
the surface with 1. To better passivate the surface, the initial
benzoquinone-modified surface was treated with propyltrichlorosi-
lane (4) to “backfill” the exposed Si(100) binding sites. Propyl-
trichlorosilane has a carbon chain length similar to that of the tether
of 1, so it was expected that a mixed monolayer consisting of 1
and 4 would be sufficiently dense to facilitate constructive oxidative
patterning while still rendering the benzoquinone reductively active.
Consistent with propyltrichlorosilane modification, the mixed
monolayer 3 was thicker (0.75 ( 0.24 nm) and had a substantially
increased contact angle (92 ( 4°) relative to surface 2. Evidence
for constructive oxidation of the alkane-based monolayer was
provided by replication of a common contact-mode-imaging artifact
encountered for oxidized organic monolayers12 as well as by the
reactivity of the oxidized patterns to chlorosilanes, which afforded
raised lines roughly corresponding to the length of the molecule
(see the SI). Together, these results indicate constructive oxidation
of the alkane-based monolayer to form oxygen-rich functionalities
such as carboxylic acids in preference to oxidation of the underlying
silicon to form SiO2.
Figure 3. Multiplexed directed assembly onto bifunctionally patterned 3.
(left) AFM height image (Z axis ) 10 nm). (right) Graphical representation
of the assembled surface, with C60 deposited on the reductive patterns
(-12 V, 0.25 µm/s) and 5 deposited on the oxidative patterns (7 V, 1
µm/s). All of the patterns were drawn at 70% relative humidity.
two different functionalities onto the same surface for subsequent
use as separate templates in directed assembly. The sub-100 nm
feature sizes of both the reductive and oxidative lines are some of
the smallest to date for a bifunctional system. Bifunctional
patterning strategies may provide a general platform for applications
in hybrid top-down/bottom-up fabrication, directed surface-based
construction of elegant supramolecular assemblies,13,14 or produc-
tion of sensors and devices that may require complementary
combinations of organic or inorganic materials.
Acknowledgment. Financial support for this research by the
National Science Foundation (Award CMMI-0751621) under UC
Berkeley’s SINAM is gratefully acknowledged.
Supporting Information Available: Detailed syntheses, experi-
mental methods, and additional figures. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Because the combination of oxidative and reductive patterns on
3 generated two orthogonal functionalities on the same surface,
the directed assembly of two different molecules through bifunc-
tional patterning could be attempted, as shown in Figure 3.
Therefore, a pentathiophene dimethylchlorosilane, 5, was assembled
on the oxidized patterns while C60 was assembled onto the reduced
patterns. The observed height difference of ∼2.5 nm between 5
and C60 corresponds to the difference in the sizes of the two different
objects (see the SI). Directed assembly of both p-type and n-type
materials on the same surface highlights the possibility of surface
assembly of complementary heterogeneous structures in the con-
struction of more complex nanoarchitectures.
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In summary, we have developed a patterning methodology using
SPL that enables the patterning in a single lithography session of
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