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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

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molecular self assembled monolayers on au 111 surface

Molecular Self-assembled Monolayers on Au{111} surface

By: Sung Chou

Advisors: Dr. Lloyd A. Bumm, Dr. Abhijit Biswas

outline
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

PTCDI: perylene-3,4,9,10-tetracarboxylic di-imide

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

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

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

motivation
Motivation

SAMs could potentially provide an easy to develop platform for MEMS, such as this ratcheting mechanism ~50μm across

  • 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
what are alkanethiols
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, CH3(CH2)8CH2SH

C10H22, Decane

General thiol functional group

1-decanethiol, SH as thiol head group

what are self assembled monolayers sam
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
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 networks9
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

preparation of ptcdi melamine sam networks10
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

scanning tunneling microscopy stm
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
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

fourier transform image
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

uv visible test
UV Visible Test

Sample

Light source

Separate detector

Diffraction grating

mirrors

Detector

  • UV-Vis test performed to check concentration of dissolved solids
  • UV light shined on sample
uv visible test17
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 test18
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)

stm micrograph20
STM Micrograph
  • 4th attempt of PTCDI/Melamine network

40 nm x 40 nm

STM image of the prepared PTCDI/Melamine SAM

fourier transform image21
Fourier Transform Image
  • The measured distance is 0.488nm

FFT image[pm]

Distance[Gm-1]

conclusion
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

?

plans future
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

acknowledgements
Acknowledgements
  • Dr. Lloyd A Bumm, Dr. Abhijit Biswas, Daminda Dahanayaka, Chris Schroeder, Matt Whiteway

THANK YOU