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Self Assembled Monolayers

Self Assembled Monolayers. Growth On Gold Plated Silicon Wafers. How It’s Done. Gold coated wafer – (100) orientation Cut into 1.5 cm x 1.5 cm squares Etched in acid-base bath (2 hours)

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Self Assembled Monolayers

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  1. Self Assembled Monolayers Growth On Gold Plated Silicon Wafers

  2. How It’s Done • Gold coated wafer – (100) orientation • Cut into 1.5 cm x 1.5 cm squares • Etched in acid-base bath (2 hours) • [sulfuric acid (H2S04)/hydrogen peroxide (H2O2) 70:30 vol. %] - creates sites for monolayer growth • Immersion in 1 mM solution of organic thiols (12 hours) Sample 1) HS(CH2)10COOH (11-mercapto-undecanic acid) Sample 2) HS(CH2)15COOH (16-mercapto-hexadecanoic acid) Sample 3) HS(CH2)10CH3 (1-undecanoic acid) Sample 4) HS(CH2)15CH3 (1-hexadecanethiol) • Must take every precaution to keep experiment uncontaminated and steady – as soon as the wafer is placed into solution monolayers begin to form

  3. Forming a self-assembled monolayer The process of forming a self-assembled monolayer. A substrate is immersed into a dilute solution of a surface-active material that adsorbs onto the surface and organizes via a self-assembly process. The result is a highly ordered and well-packed molecular monolayer. (Adapted from Ref. 9 by permission of American Chemical Society.)

  4. CH2 Sample 1) HS(CH2)10COOH Sample 2) HS(CH2)15COOH Sample 3) HS(CH2)10CH3 Sample 4) HS(CH2)15CH3 Gold Coating Silicon Wafer

  5. Procedures For Testing Monolayers 1) Fourier Transform Infrared (FT-IR) Spectroscopy • Infrared spectroscopy detects the vibration characteristics of chemical functional groups in a sample. When an infrared light interacts with the matter, chemical bonds will stretch, contract and bend. • Consists of 2 mirrors: 1 moves, 1 is stationary. Infrared beam is split between the 2 mirrors, then recombined where then the computer produces an interferogram

  6. (FT-IR) Spectroscopy Instrument Inferred Beam TLiq. Nit. = -196 C

  7. How Do We Know We Have A Monolayer? • Noise occurs throughout interferogram due to the presence of CO2 and H20 vapor during testing. • Signs of highly ordered monolayers: • Wave-number peaks at 2850 cm (symmetric stretch) and 2918 cm (asymmetric stretch) – organized CH2 peaks CO2 CH2 H2O Organized Monolayer H2O

  8. (FT-IR) Interferogram Results: Sample 4) HS(CH2)15CH3 (1-hexadecanethiol) Plain Sample Background CH2 Sample 1) HS(CH2)15COOH (11-mercapto-undecanic acid) Sample 2) HS(CH2)10COOH (16-mercapto-hexadecanoic acid) CH2 CH2

  9. 2) Contact Angle Measurement • De-ionized water drops are placed on samples • Tester runs several contact angle scans, and outputs the average Water drop Needle Place Sample Angle Camera

  10. Surface Energy, Adhesion, Cohesion Young-Dupree Equation

  11. Contact Angle Results: Plain Sample: θ ≈ 120° (mostly hydrophobic) Sample 1 - HS(CH2)10COOH Sample 2 - HS(CH2)15COOH θ ≈ 32.1° θ ≈ 70.8° Sample 3 - HS(CH2)10CH3 Sample 4 - HS(CH2)15CH3 θ ≈ 99.1° θ ≈ 103°

  12. Problems with Testing Methods • FT-IR Spectroscopy: • Impurities in the testing chamber • Contact Angle Measurement: • Hard to make consistent sized drops • Have to work fast (water drop evaporates quickly) • Monolayers have both hydrophobic and hydrophilic properties – causes mixed results

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