Molecular Self-assembled Monolayers on Au{111} surface

By: Sung Chou Advisors: Dr. Lloyd A. Bumm, Dr. Abhijit Biswas

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: http://www.nd.edu/~djacobs/sam.jpg

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/Melamine SAM network on Au {111} surface Alkanethiol added to form hybrid SAM system PTCDI: perylene 3,4,9,10-tetracarboxylic di-imide

Rafael Madueno et. al. Nature 454, 618 (2008)

Melamine: 1,3,5 triazine-2,4,6 triamine

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, windshields e.g. for automobile 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

What are Alkanethiols?

► Alkanes   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, CH 3 (CH 2 ) 8 CH 2 SH C 10 H 22 , Decane General thiol functional group 1-decanethiol, SH as thiol head group

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-)

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

Preparation of PTCDI/Melamine SAM Networks

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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 Undissolved PTCDI or Melamine Uniformly dissolved solution

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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

Initial results

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

STM Micrograph

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

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 ► 2 nd and 3 rd attempts yielded the same result Fourier transform of the STM image

The Ultra violet visible test

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

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

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 K. Balakrishnan et. al. J. Am. Chem. Soc. 128, 7390 (2006) Successive samples scaled by factors of 2

Current results

STM Micrograph

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

Fourier Transform Image

► The measured distance is 0.488nm

Distance[Gm-1]

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

1 2 3 4

Spacing (nm)

0.500

0.503

0.497

0.488

Plans/Future

► ►  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 ( network e.g.

octylthiocyanates, Azides) in the PTCDI/Melamine ► Putting PTCDI/Melamine SAM network on Flat Gold Nano Particles (FGNPs) and study optical properties (absorption/fluorescence). 

Acknowledgements

► Dr. Lloyd A Bumm, Dr. Abhijit Biswas, Daminda Dahanayaka, Chris Schroeder, Matt Whiteway

THANK YOU