1. Molecular Self-assembled Monolayers on Au{111} surface
By: Sung Chou
Advisors: Dr. Lloyd A. Bumm, Dr. Abhijit Biswas

2. Outline Introduction Motivation What are Alkanethiols?
What are Self-Assembled Monolayers (SAM)?
Formation of Hybrid SAM
Preparation of PTCDI/Melamine SAM Networks
Initial results
UV visible test
Current results
Conclusion
Plans/Future
Self assembled monolayer of alkanethiols. Source:

3. Rafael Madueno et. al. Nature 454, 618 (2008)
Introduction
Self assembled monolayers (SAM) can act as a robust platform for developing nanostructures
Combine PTCDI/Melamine and alkanethiols to create a hybrid SAM system
PTCDI/Melamine provides nano-scale precision
Alkanethiols provide versatile functionalization
PTCDI: perylene-3,4,9,10-tetracarboxylic di-imide
Melamine: 1,3,5-triazine-2,4,6-triamine
PTCDI/Melamine SAM network on Au {111} surface
Alkanethiol added to form hybrid SAM system
Rafael Madueno et. al. Nature 454, 618 (2008)

4. Motivation
SAMs provide a functional and versatile platform for nanostructures of an extended length scale
Nano and Microelectromechanical systems (NEMS and MEMS)
Modeling attachments to cell membranes
Surface modifications
Hydrophobic coatings, e.g. for automobile windshields
Controlling electron transfer on electrodes in electrochemistry
Protecting metals from harsh substances
Could function as a chemical sensor
SAMs could potentially provide an easy to develop platform for MEMS, such as this ratcheting mechanism ~50μm across

5. What are Alkanethiols? Alkanes Thiol Alkanethiols
Simple single-bonded hydrocarbons
Examples: propane, octane, decane
Thiol
Functional group composed of a sulfur and a hydrogen
Alkanethiols
Alkanes with a thiol head group
Example: 1-decanethiol, CH3(CH2)8CH2SH
C10H22, Decane
General thiol functional group
1-decanethiol, SH as thiol head group

6. What are Self-Assembled Monolayers (SAM)?
Spontaneously produced, single-molecule thick, layers which coat a substrate surface
Alkanethiols bind onto gold surface
Thiol functional group loses hydrogen
Molecule converts to alkanethiolate (R―S-)

7. Formation of Hybrid SAM
PTCDI (blue rectangle) and melamine (red triangle) fit together to form hexagonal network structure
Alkanethiols (black circle) bond to gold surface, framed by PTCDI/melamine network

8. Preparation of PTCDI/Melamine SAM Networks

9. Preparation of PTCDI/Melamine SAM Networks
Step 1:
The PTCDI and Melamine are both mixed with DMF (Dimethylformamide) in separate test tubes
Both solutions shaken over night by the shaker machine
Centrifuge both solutions about 30 minutes
Extract the clear liquid, leaving behind undissolved solids
Repeat the process until little or no solid substance can be seen in either solution
Uniformly dissolved solution
Undissolved PTCDI or Melamine

10. Preparation of PTCDI/Melamine SAM Networks
Step 2:
Mix both solutions in the ratio of 1:4 PTCDI:Melamine
The Au/mica substrate is immersed in the PFA vial containing the PTCDI/Melamine solution
Heat the substrate and solution at 100°C inside an oven for 5 minutes
Rinse the substrate with DMF and blow dry with nitrogen
Gold substrate
PTCDI/Melamine Solution
Network formation

11. Initial results

12. Scanning Tunneling Microscopy (STM)
Atomically sharp tip
DC bias between tip & surface
Bring tip close to surface
Current very sensitive to gap distance
Negative feedback (z)
Current too high = tip raised
Current too low = tip lowered
Feedback always active
Tip rastered across surface (x, y)
Grayscale image displayed
Dark = lower
Light = higher

13. STM Micrograph Some hexagonal structure can be seen
Alkanethiols not added yet
Dark region indicates deep areas
40 nm x 40 nm
STM image of the PTCDI/Melamine SAM in the Nature article
STM image of our prepared PTCDI/Melamine SAM

14. Fourier Transform Image
Image mean in center
Points away from center represent higher frequencies
Distance between each cell is ~0.5 nm
Distance is supposed to be ~3.5nm according to the article
Suspect Melamine was not fully dissolved in DMF
Impurity in solutions
Size is too small to allow addition of alkanethiols
2nd and 3rd attempts yielded the same result
Fourier transform of the STM image

15. The Ultra violet visible test

16. UV Visible Test
UV-Vis test performed to check concentration of dissolved solids
UV light shined on sample
Sample
Light source
Separate detector
Diffraction grating
mirrors
Detector

17. UV Visible Test
Chromophores in an organic substance absorb characteristic wavelengths
Chromophore is a part of a molecule responsible for its color
Resulting absorption spectrum used to determine properties of a sample
Type and amount of substance
Structure

18. UV Visible Test
Each sample should have approximately 2x the concentration as the previous, or 2x the emitted intensity
Test results show that PTCDI is dissolving the way we expect
Successive samples scaled by factors of 2
K. Balakrishnan et. al. J. Am. Chem. Soc. 128, 7390 (2006)

19. Current results

20. STM image of the prepared PTCDI/Melamine SAM
STM Micrograph
4th attempt of PTCDI/Melamine network
40 nm x 40 nm
STM image of the prepared PTCDI/Melamine SAM

21. Fourier Transform Image
The measured distance is 0.488nm
FFT image[pm]
Distance[Gm-1]

22. Conclusion
We were able to produce saturated solutions of PTCDI-Melamine along with their optical characterization
The experiment was performed 4 times
Our results are fairly consistent despite our effort to obtain the original result
Some vital information might have been left out in the paper
Efforts to contact the research group failed
Trial
Spacing (nm) 1
0.500 2
0.503 3
0.497 4
0.488 ?

23. Plans/Future ü
UV-Vis characterization of saturated solutions of PTCDI and Melamine in order to examine the absorption properties
Optimization of the SAM preparation conditions:
ratio of mixing
growth temperature
time, etc.
Insertion of alkanethiol and other less-studied thiols and functional molecules (e.g. octylthiocyanates, Azides) in the PTCDI/Melamine network
Putting PTCDI/Melamine SAM network on Flat Gold Nano Particles (FGNPs) and study optical properties (absorption/fluorescence). ü

24. Acknowledgements THANK YOU
Dr. Lloyd A Bumm, Dr. Abhijit Biswas, Daminda Dahanayaka, Chris Schroeder, Matt Whiteway
THANK YOU