Post on 31-Dec-2015
description
STRUCTURE DEPENDENT IRRADIATON-INDUCED DESORPTION
OF BIPHENYL ALKANETHIOL SELF-ASSEMBLED MONOLAYERS
S.Wyczawska1, M.Buck3, P. Cyganik2, P. Lievens1, R. E. Silverans1, E. Vandeweert1, F. Vervaecke1,
and Z. Postawa2
1 Laboratory of Solid State Physics and Magnetism, K.U.Leuven, Celestijnenlaan 200D, B 3001 Leuven, Belgium2 Institute of Physics, Jagiellonian University, ul. Reymonta 4, PL 30-059 Krakow, Poland3School of Chemistry, St Andrews University, North Haugh, St Andrews, KY16 9ST, United Kingdom
LAP 2006, 10-15 Sep 2006
2
Outline
1. Introduction
2. Experimental setup
3. Polymorphism in biphenyl-based SAMs
4. Odd/even effect in BPn/Au
5. Conclusions
3
Introduction
Self-Assembled Monolayer: (SAM)
Highly ordered and oriented assemblies that are formed spontaneously by the adsorption of a
surfactant with a specific affinity of the headgroup to a substrate.
Substrate
Spacer
Head
Tail
Tail : surface propertiesSpacer : intermolecular interactions
ordening and orientation moleculesHead : bound to substrate atom
Typical mM concentrations in solvent (ethanol)
4
Introduction
Aim: to investigate the fundamental influence ofthe detailed geometric and electronic structure
of SAMs on projectile-induced desorption
Interaction of energetic projectiles with self-assembled monolayers:
Characterization: damage induced during standard characterization techniques such as SIMS and AES
Controlled modification: SAMs are promising to be used as ultrathin resist in lithographic patterning
5
Introduction
We investigated biphenyl-based SAMs:
BPn, n=1, 2, 3, 4, 5, 6
4,4’-biphenyl-substituted alkanethiol
BP2
S
(CH2)nPhenyl chromophore
CH3
S
C
R
SC
Rsp3
Au
sp
Au
2 stable hybridization:
=180°
=104°
6
Experimental setup
charged particles
substrate
SAM
7
Experimental setup
laser pulse
++
+
+
+ +
TOF and iondetector
substrate
SAM
charged particles
8
Experimental setup
Ionization of neutral molecules
Detection of neutral molecules Photo-ionization but also photofragmentation
M0
M+
259
nm
259
nm
9
Experimental setup
Resonance-enhanced multiphoton ionization
M0
M*
M+
25
9 n
m25
9 n
m
Detection of neutral molecules Photo-ionization but also photofragmentation Introduction of a suitable chromophore M* resonance enhanced
increase of the ionization efficiency reduction of laser intensity
reduction of the photofragmentation
Aromatic rings act as chromophores
1. Vandeweert et al., Nucl. Instrum. and Meth. in Phys. Res. B 164-165 (2000)
10
Experimental setup
P = 10-10 Torr
15 keV Ar+
ion1011ions/cm²
= 259nmphoton = 1017ph/cm2 (m/m)lin = 200
(m/m)ref = 800
1 keV Ar+
ion1015electrons/cm²
Spectra Physics
11
0 20 40 60 80 100 120 140 160
Ar+ irradiation BP2/Aum/z = 168
Flight time (µs)
Ion
sign
al (
arb.
u.)
1. Flight-time distributions: probing ion signal in function of flight time ( difference between ion and laser pulse)
Experimental observables
kinetic energy of desorbing particles
2 ejection mechanisms: [2]
1. Ballistic ejection: direct momentum
transfer (~ 1 eV) 2. Thermal-like ejection:
bond cleavage byreactive species
(~ 0.02 eV)B
T
2. Riederer et al., J. Am. Chem. Soc. 119 (1997)
12
40 80 120 160 200 240 280 320 360 400
Mass (a.m.u.)
Ion
Sig
nal (
arb.
u.)
Ar+ irradiation
Experimental observables
2. Desorption fragmentation pattern: probing which particles are desorbed
168
165
167181
181
194
195
228
227
CH 3 CH2SCH2 Au
BP2/Au:
parent molecule desorbed desulphurized fragment desorbed m/z = 181 photofragment m/z = 168 desorbed m/z = 165 photofragment
13
Detailed geometric and electronic structure depends on growth parameters [3]
Polymorphism in BPn/Au
(b)
grown at 295 K 27.01 Ų/molecule C-S-Au bond angle
> 130°
BP4/Au:(c)
post-annealed at 423 K 32.4 Ų/molecule C-S-Au bond angle
< 130°
BP4/Au:
Only one phase –no changes when prepared at elevated temperature
(a)
BP3/Au:
3. Cyganik et al., J. Am. Chem. Soc. 126 (2004)
14
Polymorphism in BPn/Au
Au
n = even
• prepared at 295 K• angle C-S-Au > 130°
α-BPn
Au
β- & -BPn
• prepared at 423 K• angle C-S-Au < 130°
15
difference in desorption behavior of BP4 and BP4-S
0 40 80 120 160 200 240 280 320 360 400
Mass (a.m.u.)
Ion
sign
al (
arb.
u.)
BP4/Au
BP4/Au
168
168
181
181
BP4-S
BP4-S
BP4
BP4
Polymorphism in BPn/Au
Ion-induced desorption:Probing desorption mass spectra during 15-keV Ar+ irradiation of and even BPn/Au
16
Desorption probability of the parent molecule is larger for than for even BPn/Au
Desorption probability of the BPn-S molecule is larger for than for even BPn/Au
bond scission efficiency of the S-Au bond is larger for than for even BPn/Au
bond scission efficiency of the C-S bond is larger for than for even BPn/Au
2 3 4 5 60.0
0.1
0.2
0.3
n
BPn-S
Polymorphism in BPn/Au
Ion-induced desorption:
phase
2 3 4 5 60.0
0.1
0.2
0.3
Nor
mal
ized
ion
sign
al (
arb.
u.)
n
BPn
phase
17
RT BP5/Au and HT BP5/Au: no change in desorption probability
2 3 4 5 60.0
0.1
0.2
0.3
n
BPn-S
Polymorphism in BPn/Au
Ion-induced desorption:
phase
2 3 4 5 60.0
0.1
0.2
0.3
Nor
mal
ized
ion
sign
al (
arb.
u.)
n
BPn
phase
changes in bond scission efficiency between and even BPn/Au are related to structural change and not to annealing
18
Polymorphism in BPn/Au
Au
Au
Discussion:
even BPn/Au: even BPn/Au:
optimization bond geometry
optimization 2 dimensional
packingintermolecular interactions suppressed due to energy addition
Au
Au
19
Au
S-Au weakest bond
Polymorphism in BPn/Au
Discussion:
even BPn/Au:
even BPn/Au:
Au
C-S weakest bond
20
Detailed geometric and electronic structure depends on growth parameters and alkane chain:
Odd/even effect in BPn/Au
(b)
grown at 295 K 27.01 Ų/molecule C-S-Au bond angle
> 130°
BP4/Au:
grown at 295 K 21.6 Ų/molecule C-S-Au bond angle ~ 109°
[211]
(a)
BP3/Au:
3. Cyganik et al., J. Am. Chem. Soc. 126 (2004) 4. Azzam et al., Langmuir, 19 (2003)
[3,4]
21
Odd/even effect in BPn/Au
n = odd n = even
Au
• prepared at 295 K• angle C-S-Au ~ 109°
• prepared at 295 K• angle C-S-Au > 130°
α-BPn
Au
22
difference in desorption behavior of BPn and BPn-S
Odd/even effect in BPn/Au
Ion-induced desorption:
0 40 80 120 160 200 240 280 320 360 400
168
181
BP4-SBP4
Mass (a.m.u.)
Ion
sign
al (
arb.
u.)
BP3/Au
BP4/Au
168181 BP3-S
BP3
Probing desorption mass spectra during 15-keV Ar+ irradiation of odd and even BPn/Au
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1 2 3 4 5 60.00
0.04
0.08
0.12
0.16
0.20
bond scission efficiency of the S-Au bond is larger for eventhan for odd BPn/Au
bond scission efficiency of the C-S bond is larger for odd than for even BPn/Au
Desorption probability of BPn-S molecule is larger for odd than for even BPn/Au
Desorption probability of the parent molecule is larger for even than for odd BPn/Au
Odd/even effect in BPn/Au
Ion-induced desorption: N
orm
aliz
ed io
n si
gnal
(ar
b.u.
)
n
BPn
1 2 3 4 5 60.0
0.1
0.2
0.3
0.4
0.5
n
BPn-S
24
Au
Odd BPn/Au:
Au
Discussion:
Odd/even effect in BPn/Au
Even BPn/Au
S-Au weakest bondC-S weakest bond
conflict between interactions
both interactions result in same structure
25
Conclusions
competition between molecule-substrate bondand intermolecular interactions
difference in geometric and electronic structure
changes in bond scission efficiency induced by projectile irradiation
difference in desorption probability of different molecular fragments
26
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