Formation and Organization of Amino Terminated Self-assembled Layers on Si(001) Surface
G. Demirel
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M. O. C aglayan
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B. Garipcan
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M. Duman
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E. Piskin
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G. Demirel (&) M. O. C aglayan B. Garipcan M. Duman E. Piskin Department of Chemical Engineering and Division of Bioengineering, Hacettepe University
, Beytepe, Ankara 06800,
Turkey
We have investigated the effects of dipping time, solution concentration and solvent type on the formation of self-assembled monolayers with aminosiloxane molecules (i.e., N-(3 trimethoxysilylpropyl)diethylenetriamine (TPDA)) on the Si(001) surface. Studies performed with an ellipsometer showed that monolayers with a thickness of about 1.2 nm were formed when the dipping time is about 2 h, while multilayer were observed for longer time periods. The effect of the TPDA concentration on the thickness of the deposited layer was not very profound, however, the contact angle data exhibit importance of concentration on the surface coverage. The type of the solvent used in the formation of the monolayers was found an important parameter. Monolayers were formed with solvent having larger dielectric constants. Relatively thick multilayer was observed when benzene was used as the solvent, due to its quite low dielectric constant (hydrophobicity).
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Self-assembly has recently emerged as a new approach in
chemical synthesis, nanotechnology, polymer science,
materials science, and engineering. Molecular
self-assembly systems lie at the interface of these disciplines and
many self-assembling systems have been developed.
Self-assembled monolayers (SAMs) are a class of
molecular assemblies that are typically prepared by exposure of a
surface to molecules with chemical groups that possess
strong affinities for the substrate. The driving force for the
formation of the monolayer includes chemisorption of
functionalized molecules on the substrate surface, and the
intermolecular interactions. Due to their ease of
preparation and controllable surface chemical functionality, SAMs
represent suitable model systems for studying wetting
[13], corrosion [4, 5], adhesion [6, 7], tribology [812],
charge transfer through molecules [13], and model surfaces
for biochemistry and cell biology [14]. Other applications
(resistance to etchants [15] and protein adsorption,
modified electrodes for electrochemistry) rely on the ability of
SAMs to prevent diffusion of other molecules to the
surface of the underlying substrate [16].
The final morphology and thickness of a SAMs are
reported to be extremely sensitive to experimental
parameters including the type of precursor molecule,
concentration, type of solvent and its quality, temperature and
reaction time, etc. Despite several experimental
investigations dedicated to the grafting of organic molecules to the
silicon surface, there are only few description of such
grafting and fewer attempts to understand the self-assembly
formation. In the present work, SAMs with amino end
group were prepared by using
N-(3-trimethoxysilylpropyl)diethylenetriamine (TPDA) molecule on the Si(001)
surface. Effects of dipping time, solution concentration and
solvent type on the formation of TPDA on Si(001) have
been investigated. Subsequently, thicknesses and water
contact angle of each film were measured using imaging
ellipsometry and contact angle goniometer, respectively.
The substrates used in these experiments were Si(001)
wafers (n-type, obtained from Shin-etsu, Handoutai,
Japan). The substrates were cut into 5 5 mm pieces for
further modification. The substrates were first cleaned by
repeated rinsing with deionized water and ethanol. They
were then further cleaned a mixture of NH3 (25%, v/v),
H2O2 (30%, v/v), and deionized water having a volume
ratio of 1:1:5 at the temperature of 70 C during 20 min.
Afterward, the substrates were washed with ethanol and
dried under nitrogen stream. Finally, these substrates
were exposed in UV/ozone chamber (Irvine, CA: Model
42, Jelight Company Inc. USA) for 15 min prior to
modification in order to remove hydrocarbon and to
produce a hydrophilic surface. For this cleaned surface,
the water contact angle was about 3 . The lower contact
angle obtained is consistent with the presence of
increased number of hydroxyl groups on the cleaned
surface [17].
Unless otherwise stated, freshly prepared TPDA
(Aldrich USA) solutions (0.25, 0.5, 1.0, 2.0, 4.0, and 8.0%, v/
v) in absolute ethanol (Aldrich USA) were used for the
monolayer formation. Silicon wafers were dipped in the
TPDA solution of particular concentrations and were
removed from solution after selected time intervals. Static
water contact angles of the sample surfaces were measured
at 25 C in ambient air using an automatic contact angle
goniometer equipped with a flash camera (model DSA 100,
Kruss, Germany) applying the sessile drop method. The
volume of the drop used was always 1 lL in all
measurements. The contact angles are calculated by using the
software of the instrument. All reported values herein are
the averages of at least nine measurements taken at three
different (...truncated)