Transcript Self-assembled monolayers on gold surfaces

Paul Frank
Institute of Solid State Physics, Graz University of Technology
Financially supported by the
Austrian Science Fund
Introduction & Motivation
Preparation of SAMs
Characterization of SAMs
Introduction & Motivation
Self assembled monolayers (SAMs)
J. C. Love et al., Chem. Rev. 2005, 105, 1103-1169
Introduction & Motivation
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
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 (27eV)
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]
[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
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-(CH2)10–COOH): m = 218 amu
Desorption from MUA-multilayer (Tad = 200K)
no surface reactions in cracking pattern
mass / amu
cracking product
27, 41, 55, …
CxHY
34
H2S
45
COOH
199, 200
S-(CH2)10–CO
Characterization of SAMs
TDS of MUA on recrystallized gold foil
Tad = 200K, Tsource = 50 °C
Evaporation time: 30 min
a……Multilayer-peak
b1,2 …Monolayer-peaks
b1 and b2 show different cracking pattern
e.g. m199: only in b1 not in b2
m34: only in b2 not in b1
Characterization of SAMs
TDS of MUA on recrystallized gold foil:
Influence of waiting time
Tad = 200K, Tsource = 50 °C
Evaporation time: 30 min
TDS directly after film preparation
TDS 115h after film preparation
Monolayer-peaks separated more clearly: Monolayer not stable?
Characterization of SAMs
TDS: Binding energies
Polanyi-Wigner equation:
Rdes  
dN
dN

b  x  N x  exp(  Edes () / kT )
dt
dT
β: heating rate
N: number of adsorbed particles
: pre-exponential factor
x: desorption order
Edes: desorption energy
: coverage
k: Boltzmann factor
x = 0 (multilayer):
ln Rdes  ln  ln Nmax  Edes / kT
Characterization of SAMs
TDS: Binding energies: Arrhenius-plot
ln(R) vs.1/T
Arrhenius plot: Edes = 25.4 kcal/mol ~ 1.1eV (multilayer)
monolayer: 5 x 1014±1 molecules/cm²
pre-exponential factor  = 7.5 x 1017±1 s-1
„Redhead“ formula[1] for first order desorption (monolayer):
  T
E des  kTm  ln m
  b


  3.64


Edes = 43.9 kcal/mol ~ 1.9eV (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 @ ~ 700K!
Characterization of SAMs
Literature:
[1]
[2]
hexanethiol
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