1. Contact Angle Measurements
Acknowledgments
Hydrophobic Substrate Patterning
David Barton, Anna Swan
Boston University, Electrical and Computer Engineering, 8 St. Mary’s St, Boston, MA
Contact: swan@bu.edu
• This work was funded by Boston University’s Undergraduate Research
Opportunities Program (UROP) and the National Science Foundation’s Division
of Materials Research (NSF DMR) 1411008.
• Thanks to Bruker for the AFM images. Special thanks to Anlee Krupp, Paul Mak,
Xuanye Wang, Jason Christopher, Bennett Goldberg, and Anna Swan for all their
time and support.
Interface Between regions
Strain engineering is the process of changing the physical properties of a
material by applying strain. To lay the groundwork for measuring strain, it is
necessary to pattern a substrate to have varying coefficients of friction. This
was accomplished by altering the substrate’s hydrophobicity. In theory, the
more hydrophobic the substrate, the less friction a two-dimensional (2D)
crystal will experience, as this is due to a decrease in the van der Waals forces
between the crystal and the substrate.
The patterning of the substrates was achieved through the use of
photolithography and atmospheric chemical vapor deposition (CVD) of a
silane called octadecyltrimethoxysilane (OTMS). Photolithography is the
process of using light to transfer geometric patterns to a substrate covered in
a photopatternable chemical called a photoresist. Atmospheric CVD is the
process used to deposit a thin film of gaseous OTMS at atmospheric pressure
in order to alter the substrate’s hydrophobicity. Not only did the OTMS
increase the substrate’s hydrophobicity, but it was also experimentally
verified to be photodegradable. This infers that the OTMS can be degraded
with shorter wavelength UV light, meaning that the hydrophobicity of the
OTMS-coated substrate would decrease as the substrate is exposed to more
UV light. Thus, the combination of these processes will aid in discovering
intrinsic properties of 2D crystals such as graphene, MoS2, and hBN. These
materials may one day be used in various applications such as touch-screen
technology, LEDs, and improved solar cells.
• A basic patterning procedure for creating varying areas of friction on a substrate
through hydrophobic manipulation was achieved.
• In order to create these patterns, it is necessary to perform photolithography,
then atmospheric CVD of a silane (OTMS works very well), and to clean the
substrate to ensure optimal conditions for eventual transfers of 2D crystals.
• UV degradation was explored for future manipulation of the difference in
hydrophobicity between the silanized and non-silanized sections.
• Macroscale patterns were sufficiently studied through optical measurements of
contact angle as well as through water coverage.
• Microscale patterns require AFM measurements to determine the sharpness of
the interface between the two regions with and without silane.
• The next steps in the microscale patterning include verifying that silane was
successfully deposited as well as determining its locations with an AFM.
• Photolithography: This procedure contains the following steps: pre-baking,
spinning of photoresist, post-baking, exposure, and development.
• Atmospheric CVD: Liquid silane is heated into a gaseous form, and
deposited onto the substrate. This is also known as silanization.
• Cleaning: The substrate is sonicated in acetone and isopropanol, and blow-
dried with nitrogen. A piranha cleaning may be performed as well.
• UV Degradation: 190nm wavelength UV light is used to degrade the silane
covering the substrate. OTMS was used as it is photodegradable.
• Contact Angle Measurements: All contact angle (CA) measurements are
taken with a contact angle goniometer.
• Atomic Force Microscopy (AFM): An atomic force microscope is used to
detect sub-nanometer height differences on the surface of a substrate.
~10º ~50º ~100º
Macroscale Patterning
Microscale Patterning
UV Degradation
Semi-circles Quarter-circles
Image of Photomask Image of Photomask
Implementation on Substrate Implementation on Substrate
Photomask Substrate Photomask Substrate
vs. vs.
Abstract
Methods
Patterned with Photoresist Covered with Water
Before CVD After CVD and Sonication
Patterned with Photoresist After CVD and Sonication After Piranha Cleaning
Using Atomic Force Microscopy To Locate Silane Patterns
• The substrate patterned into quadrants
demonstrates the patterning accuracy.
• The substrate patterned into halves
demonstrates the difference in hydrophobicity
between the silanized and non-silanized
sections. Performing a piranha cleaning alters
the contact angles of each section, but the
difference in contact angle appears to be
maintained.
Lateral Force Microscopy (Contact Mode)
(These images are courtesy of Bruker.)
Optical Measurement
Non-silanized Silanized
• It is possible to determine the locations of microscopic patterns of silane due to the step height between the
silanized and non-silanized sections of the substrate.
• Microscale patterns were achieved, but more adjustments need to be made
to ensure maximum conformity.
Conclusions
Increasing Hydrophilicity Increasing Hydrophobicity
Before CVD