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

23

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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