Selfassembled monolayers SAMs on gold surfaces Paul Frank
Self-assembled monolayers (SAMs) on gold surfaces Paul Frank Institute of Solid State Physics, Graz University of Technology Financially supported by the Austrian Science Fund
Outline Introduction & Motivation Preparation of SAMs Characterization of SAMs
Introduction & Motivation Self assembled monolayers (SAMs) J. C. Love et al. , Chem. Rev. 2005, 1103 -1169
Introduction & Motivation SAMs can be used to: Modify wetting properties (e. g. water) Selective adhesion „Bio-functionalizing“[1] Prepare functional films Lubricants for hard discs Corrosion protection Photo patterning Electronic devices [1] E. V. Romanova et al. , Biomaterials, 2006, Vol. 27, 1665
Introduction & Motivation Our SAM: MUA on gold surfaces MUA: Mercaptoundecanoic acid Substrates: Recrystallized gold foils gold (111) on mica
Preparation of SAMs Ex situ preparation (solution) Simple preparation by immersion High affinity of sulfur to metals thiolate formation on gold In particular popular gold surfaces: do not oxidize under ambient conditions Physisorbed contaminations are removed by thiolate (self cleaning)
Preparation of SAMs In situ preparation (PVD in UHV) (UHV): ultra-high vacuum (PVD): physical vapor deposition Stainless steel tube (150 °C) Glass container (50 °C) MUA (liquid above 45 °C) Heated valve (150 °C)
Characterization of SAMs Alkanethiol-SAMs on Au(111): monolayer structure STM LEED (27 e. V) NEXAFS
Characterization of SAMs Alkanethiol-SAMs on Au(111): striped phase Dosing from gas phase: low-coverage phase (striped phase) Direct observation by STM: Formation of the SAM: [2] [1] G. E. Poirier, Langmuir, 1999, 15, 1167 [2] R. Staub et al. , Surf. Sci. , 2000, 445, 368
Characterization of SAMs Our experimental method: TDS = Thermal desorption spectroscopy QMS = Quadrupole mass spectrometer Sample attached to heatable steel plate (90 K up to 1000 K) 1) MUA is deposited on gold substrate 2) Gold substrate is heated and MUA desorbs into QMS filament QMS: incoming MUA molecules are ionized cracking
Characterization of SAMs Cracking pattern of MUA = (HS-(CH 2)10–COOH): m = 218 amu Desorption from MUA-multilayer (Tad = 200 K) no surface reactions in cracking pattern mass / amu cracking product 27, 41, 55, … Cx H Y 34 H 2 S 45 COOH 199, 200 S-(CH 2)10–CO
Characterization of SAMs TDS of MUA on recrystallized gold foil Tad = 200 K, Tsource = 50 °C Evaporation time: 30 min a……Multilayer-peak b 1, 2 …Monolayer-peaks b 1 and b 2 show different cracking pattern e. g. m 199: only in b 1 not in b 2 m 34: only in b 2 not in b 1
Characterization of SAMs TDS of MUA on recrystallized gold foil: Influence of waiting time Tad = 200 K, Tsource = 50 °C Evaporation time: 30 min TDS directly after film preparation TDS 115 h after film preparation Monolayer-peaks separated more clearly: Monolayer not stable?
Characterization of SAMs TDS: Binding energies Polanyi-Wigner equation: β: heating rate N: number of adsorbed particles : pre-exponential factor x: desorption order Edes: desorption energy : coverage k: Boltzmann factor x = 0 (multilayer):
Characterization of SAMs TDS: Binding energies: Arrhenius-plot ln(R) vs. 1/T Arrhenius plot: Edes = 25. 4 kcal/mol ~ 1. 1 e. V (multilayer) monolayer: 5 x 1014± 1 molecules/cm² pre-exponential factor n = 7. 5 x 1017± 1 s-1 „Redhead“ formula[1] for first order desorption (monolayer): Edes = 43. 9 kcal/mol ~ 1. 9 e. V (monolayer) [1] Redhead, Vacuum, 1962, 12, 203.
Characterization of SAMs In-situ vs. Ex-situ preparation TDS of PVD grown MUA film TDS of MUA film grown by immersion in solution “New“ peak @ ~ 700 K!
Characterization of SAMs Literature: [1] [2] hexanethiol [3] [1] D. Käfer et al. , J. Am. Chem. Soc. , 2006, 128, 1723 [2] D. J. Lavrich et al. , J. Phys. Chem. B, 1998, 102, 3456 [3] C. Kodama et al. , Appl. Surf. Sci. , 2001, 169, 264
Outlook • MUA on Au(111)/mica: In situ preparation + TDS • STM • Polarization modulation-infrared reflectionadsorption spectroscopy (PM-IRRAS) • Replacement of functional end group • Alkanethiols / gold
Acknowledgements: Franz Nussbacher Johanna Stettner Adolf Winkler Financially supported by the Austrian Science Fund
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