Paul Frank Institute of Solid State Physics, Graz University of Technology Financially supported by the Austrian Science Fund

Introduction & MotivationPreparation of SAMsCharacterization of SAMs

Introduction & Motivation Self assembled monolayers (SAMs) J. C. Love et al., Chem. Rev. 2005, 105,

Introduction & Motivation Modify wetting properties (e.g. water) Prepare functional films Selective adhesion „Bio-functionalizing“ [1] [1] E. V. Romanova et al., Biomaterials, 2006, Vol. 27, 1665 Lubricants for hard discs Corrosion protection Photo patterning Electronic devices

Introduction & Motivation MUA: Mercaptoundecanoic acid Substrates: Recrystallized gold foils gold (111) on mica

Preparation of SAMs Ex situ preparation (solution) 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) Simple preparation by immersion

Preparation of SAMs In situ preparation (PVD in UHV) (UHV): ultra-high vacuum (PVD): physical vapor deposition Stainless steel tube (150 °C) Heated valve (150 °C) Glass container (50 °C) MUA (liquid above 45 °C)

Characterization of SAMs Alkanethiol-SAMs on Au(111): monolayer structure STM LEED (27eV) NEXAFS

Characterization of SAMs Alkanethiol-SAMs on Au(111): striped phase [1] G. E. Poirier, Langmuir, 1999, 15, 1167 [2] R. Staub et al., Surf. Sci., 2000, 445, 368 Direct observation by STM: [1] [2] Dosing from gas phase: low-coverage phase (striped phase) Formation of the SAM:

Characterization of SAMs Our experimental method: TDS TDS = Thermal desorption spectroscopy QMS = Quadrupole mass spectrometer Sample attached to heatable steel plate (90K up to 1000K) 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 MUA = (HS-(CH 2 ) 10 –COOH): m = 218 amu Desorption from MUA-multilayer (T ad = 200K) no surface reactions in cracking pattern mass / amucracking product 27, 41, 55, …CxHYCxHY 34H2SH2S 45COOH 199, 200 S-(CH 2 ) 10 –CO

Characterization of SAMs TDS of MUA on recrystallized gold foil T ad = 200K, T source = 50 °C Evaporation time: 30 min  ……Multilayer-peak  1,2  …Monolayer-peaks  1 and  2 show different cracking pattern e.g. m199: only in  1 not in  2 m34: only in  2 not in  1

Characterization of SAMs TDS of MUA on recrystallized gold foil: Influence of waiting time T ad = 200K, T source = 50 °C Evaporation time: 30 min TDS directly after film preparationTDS 115h 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 E des : desorption energy  : coverage k: Boltzmann factor x = 0 (multilayer):

Characterization of SAMs TDS: Binding energies: Arrhenius-plot ln(R) vs.1/T Arrhenius plot: E des = 25.4 kcal/mol ~ 1.1eV (multilayer) monolayer: 5 x 10 14±1 molecules/cm² pre-exponential factor = 7.5 x 10 17±1 s -1 „Redhead“ formula [1] for first order desorption (monolayer): [1] Redhead, Vacuum, 1962, 12, 203. E des = 43.9 kcal/mol ~ 1.9eV (monolayer)

Characterization of SAMs In-situ vs. Ex-situ preparation TDS of PVD grown MUA filmTDS of MUA film grown by immersion in solution “New“ ~ 700K!

Characterization of SAMs Literature: [1] [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 hexanethiol [2] [3] hexanethiol

Outlook MUA on Au(111)/mica: In situ preparation + TDS STM Polarization modulation-infrared reflection- adsorption spectroscopy (PM-IRRAS) Replacement of functional end group Alkanethiols / gold

Acknowledgements: Franz Nussbacher Johanna Stettner Adolf Winkler Financially supported by the Austrian Science Fund