DOI: 10.1007/s00339-005-3413-z
Appl. Phys. A 82, 15–18 (2006)
Rapid communication
Materials Science & Processing
Applied Physics A
t. balgar
s. franzka
e. hasselbrink
n. hartmann
✉
Laser-assisted fabrication
of submicron-structured hydrophilic/
hydrophobic templates
for the directed self-assembly
of alkylsiloxane monolayers
into confined domains
Fachbereich Chemie, Universität Duisburg-Essen, Universitätsstr. 5, 45141 Essen, Germany
Received: 2 September 2005/Accepted: 12 September 2005
Published online: 22 October 2005 • © Springer-Verlag 2005
ABSTRACT
A highly focused laser beam at a wavelength of 514 nm is used to pat-
tern hydrophilic, surface-oxidized silicon substrates. Irradiation converts the affected
surface areas from hydrophilic to hydrophobic and hence provides a means to pre-
pare hydrophilic/hydrophobic patterns. The patterns are stable for weeks or longer,
even if the samples are stored at ambient conditions. Contrary to ordinary avenues
to hydrophilic/hydrophobic patterns no coating is required. This makes the proced-
ure extremely simple and robust. Routine patterning can be carried out over large
areas at fast writing speeds under ambient conditions. At a focal spot diameter of
about 2.5 µm, hydrophobic lines with a width down to 0.4 µm are prepared indicat-
ing a highly superlinear dependence of the overall process on the laser intensity. The
patterned substrates might be useful in a broad variety of applications. In particular, as
shown here, they represent suitable templates for the directed self-assembly of organic
monolayers.
PACS
61.80.Ba; 68.08.Bc; 81.07.Pr
1 Introduction
Patterns with alternating hy-
drophilic and hydrophobic domains are
widely used to control and study pro-
cesses at surfaces in many fields of
material science [1–10]. Applications
include the formation of liquid pat-
terns [1], the phase separation of poly-
meric films [2] and the local deposition
of nanoparticles [3] and proteins [4] into
ordered structures as well as the directed
growth of crystals [5], thin films [6–8]
and living cells [9, 10]. The preparation
of suitable substrates usually is achieved
through coatings with laterally varying
hydrophilicity. Perhaps most prominent
are organic monolayers with alternating
domains of hydrophilic and hydropho-
bic termination. Coatings of this type
can be prepared using lithographic pro-
✉ Fax: +49 201 183 3228, E-mail: nils.hartmann@uni-essen.de
cedures [6, 7, 10–20]. In some cases,
however, patterning can complicate
the successive functionalization of dis-
tinct areas, particularly if submicron-
sized domains are addressed [18–20].
Also, some reagents easily form dis-
ordered coatings where the functional
terminal groups to some extent are
buried within the monolayer [21]. In
this contribution we present a new
approach towards submicron-struc-
tured hydrophilic
/
hydrophobic patterns,
which requires no coating and hence
inherently allows to circumvent such
complications. In particular, we demon-
strate a simple procedure, which uses
the opportunity to locally change the
wettability of oxidic silicon surfaces
upon exposure to laser light.
The impact of light on the wettabil-
ity of oxidic surfaces has been studied
quite extensively [22–28]. Most contri-
butions in this area focus on titania sur-
faces, which exhibit an increased hy-
drophilicity after irradiation with ultra-
violet (UV) light [22–27]. The underly-
ing cause of this effect is still under dis-
cussion. Recent studies, however, pro-
vide ample evidence that UV irradiation
results in a photocatalytic decompos-
ition and removal of organic contami-
nants, which rapidly adsorb on the pre-
viously hydrophilic surface when the
samples are left in a conventional lab-
oratory environment [24–26]. In con-
trast, the hydrophilicity of silica sur-
faces is determined by the density of
hydroxyl groups [29]. Exposure to high
power UV light here initiates dehydrox-
ylation and results in a decreased hy-
drophilicity [26, 28].
Overall, surprisingly few studies in
this research area address the opportu-
nity to prepare hydrophilic
/
hydrophobic
patterns [27, 28]. Direct patterning of
silica surfaces certainly appears very
promising as this approach requires no
coating at all. Recently, Halfpenny et al.
used a focused UV laser beam to locally
modify the hydrophilicity of silica glass
plates [28]. At a beam diameter of about
100 µm
, though, patterning was limited
to a mesoscopic length scale. Respec-
tive studies which prove the feasibility
of this routine on a microscopic length
scale are still awaiting.
In this contribution, we present a new
laser-assisted procedure for the fabri-
cation of hydrophobic patterns on hy-
drophilic, surface-oxidized silicon sub-
strates. As demonstrated, patterning of
these substrates can conveniently be car-
ried out at a wavelength in the visible