Effect of nanoscale geometry on molecular conformation:

Vibrational Sum-Frequency Generation of alkaynethiols on metal nanoparticles

Achani Yatawara, Andrey N. Bordenyuk and Alexander V. Benderskii*

Department of ChemistryWayne State University

Detroit, MI 48202

by

Champika N. Weeraman

Nanoscale geometry – molecular conformation

►Expect a relationship between the substrate geometry (nano scale) and conformation of the adsorbed molecule

vs.vs.

Nanoscale geometry – molecular conformation

Flat surface

Curved surface

Molecular conformation in nanostructured materials

Bio sensors-bio markers

Aerosols

Gratzel-solar cells

P E ENL ( ) ( , , ) : ( ) ( )( ) 1 22

1 2 1 2

I PSFGNL

2 2 2 ( )

( )2 0

Sum-frequency generation (SFG) originates from induced polarization

g

e

Virtual State

2 ( )IR

1 ( )VIS ( )SFG

In media without inversion symmetry

SFG= IR+ vis

S()

SFG - CCD image

Spectrally narrow (temporally long) Vis pulse

IR

IR

vis

vis

Broad-band IR pulse

(temporally short)

+

( )2 0 In media with inversion symmetry

Vibrational sum frequency generation (VSFG)

►Broad band VSFG

Case II:

gauche defects

Case I:

all trans (zig-zag)

strong CH3 transitions,weak CH2 transitions

Flat Surface

Experiment

More CH2 transitions

Curved surface

Increase surface curvature

Change of Molecular conformationSFG sensitivity ?

8000

6000

4000

2000

0S

FG

inte

nsity

(a.

u.)

30002950290028502800IR frequency (cm

-1)

d+ r+ d- r+FR r-

►Strong CH3 stretch transitions (r+,r+FR,r-)

►Weak CH2 stretch transitions (d+,d-)►predominantly trans zig-zag molecule arrangement with few gauche defects

Chain length~1.6 nm

TEM images:1-DDT capped Au and AgNPs(I) Gold nanoparticles

(II) Silver nanoparticles

20 nm

(b)

20 nm

(c)

100 nm

(d)

20 nm

(a)

<d>=1.8 nm<d>=1.8 nm <d>=2.9 nm<d>=2.9 nm <d>=7.4 nm<d>=7.4 nm <d>=23 nm<d>=23 nm

20 nm

(a)

20 nm

(b)

100 nm

(c)

100 nm

(d)

20 nm

(a)

20 nm

(b)

<d>=1.8 nm<d>=1.8 nm <d>=3.6 nm<d>=3.6 nm <d>=7.9 nm<d>=7.9 nm <d>=24.6 nm<d>=24.6 nm

Meliorum Technologies, Inc.

d+ r+ d- r-op

VSFG 1-DDTcapped AuNps (SSP polarization)

► Both CH2 stretch and CH3 stretch transitions easily observable

► Increase of relative intensity of CH2 stretch vs. CH3 stretch with decreasing particle size

4

3

2

1

0

VS

FG

inte

nsity

(a.

u)

30002950290028502800

Frequency (cm-1

)

23 nm

7.4 nm

2.9 nm

1.8 nm

SFG Spectral Fitting: Quantitative analysis of SFG spectra

Bj = amplitudeΓj = Lorentzian line widthωj = transition frequencyANR= nonresonant contribution

Intensity of mode j; X BSSP j j,( )2 2 2

I X I ISFG SSP IR SSP IR IR IR vis vis ( ) ( ) ( ) ( )( ) 2 2

Intensity of SFG signal (SSP polarization, SFG-Vis-IR)

Effective non linear susceptibility: Multi-Lorentzian app.

X A eB

iSSP IR NRi j

j

( ) ( )( )

2

I R Гj

Гj

Size dependent SFG spectragauche defects

Chain length ~1.6 nm

d+/r+

1.1

1.0

0.9

0.8

0.7

0.6

Am

plitu

de r

atio

2520151050Mean Diameter (nm)

Chain length ~1.6 nm

d-/r-

3.0

2.5

2.0

1.5

1.0

0.5

0.0A

mpl

itude

rat

io2520151050

Mean Diameter (nm)

VSFG 1-DDT capped AgNPs (SSP polarization)VSFG 1-DDT capped AgNPs (SSP polarization)

4

3

2

1

0

Inte

nsit

y (a

.u)

300029502900285028002750

Frequency (cm-1

)

dd++ rr--(op)(op)dd--rr++

3.6 nm

7.9 nm

24.6 nm

1.8 nm►Transitions are broader than AuNPs(AuNPs: 10-12cm-1 AgNPs: 15-23cm-1)

► Weaker thiol-Ag bond compared to thiol-Au bond

2.0

1.5

1.0Am

plitu

de r

atio

2520151050Mean diameter (nm)

2.5

2.0

1.5

1.0

0.5d+/r+

d-/r-

R

L

RΦ

V aL0

x L R /

Cylindrical volumeCylindrical volume for alkyl chain for alkyl chain

Conical volumeConical volume between between spheres of radii spheres of radii RR and and R+LR+L

Additional volume Additional volume for for gauchegauche defects defects

ΦΦ = solid angle= solid angleLL = chain length= chain lengthRR = particle radius = particle radiusaa = area per head = area per head groupgroup

Size dependentSize dependent gauchegauche defectsdefects

V R L R 3

3 3( )Φ

( )RV V

Vx x

0

0

21

3

Trans- gaucheTrans- gauche free energy difference and Isomerization barrier are comparable to kT free energy difference and Isomerization barrier are comparable to kT

Geometrical modelGeometrical model

Size dependent gauche defects

R

L

RΦ20

0

1( )

3

V VR x x

V

/x L R

d+/r+

1.1

1.0

0.9

0.8

0.7

0.6

Am

plitu

de r

atio

2520151050Mean Diameter (nm)

d-/r-

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Am

plitu

de r

atio

2520151050

Mean Diameter (nm)

0.6

0.4

0.2

Am

plitu

de r

atio

2520151050Mean diameter (nm)

0.5

0.4

0.3

0.2

0.1

0.0

d+/r+

d-/r-

dd++

rr--(op)(op)

dd--rr++

0.6

0.5

0.4

0.3

0.2

0.1

Am

plit

ude

rati

o

2520151050Mean diameter (nm)

1.2

1.0

0.8

0.6

0.4

0.2

d+/r+

d-/r-

VSFG spectra: Au and AgNPs (PPP polarization)

Gold nanoparticles Silver nanoparticles AuNPs

AgNPs

dd++

rr--(op)(op)dd--rr++

4

3

2

1

0

VS

FG

inte

nsit

y (a

.u.)

300029002800

Frequency (cm-1

)

1.8 nm

3.6 nm

7.9 nm

24.6 nm2.0

1.0

0.0

VS

FG

Int

ensi

ty (

a.u)

30002950290028502800

Frequency (cm-1

)

1.8 nm

2.9 nm

7.4 nm

23 nm

z

y

C3vθ

α

ψIRvis

SFG P

S

φ

x

ijk ijk lm nlm n

lm nN U( ),

( )( , , )2 2

X N d d d fijk ijk( ) ( )( , , ) ( , , )2 2

X N d fijk ijk( ) ( )( ) ( )2 2

I X eff ( )2 2

Euler matrix Molecularhyperpolarizability

Orientation angle (θ) Azimuthal angle (ψ) Torsional angle (φ)

Distribution function

Assumed random distribution for φ and ψ angles

SFG intensity

From experimental geometry and beam polarization

Molecular orientation analysis

1.2

1.0

0.8

0.6

0.4

0.2

0.0

d+

/r+

am

plitu

de ra

tio1801501209060300

c (deg)

A. SSP

B. PPP

Molecular orientation analysis

1.0

0.8

0.6

0.4

0.2

0.0

r+in

tens

ity

(SS

P)

1801501209060300

c (deg)

1.0

0.8

0.6

0.4

0.2

0.0

d+in

tens

ity

(SS

P)

1801501209060300

c (deg)

0 π

f(θ)

θθc

z

θc

θC3

z

AuNPsAuNPs

AgNPsAgNPs1.6

1.2

0.8Am

plit

ude

rati

o

2520151050Mean diameter (nm)

0.6

0.5

0.4

0.3

0.2

0.1d

+/r

+(SSP)

d+/r

+(PPP)

1.6

1.4

1.2

1.0

0.8

0.6Am

plit

ude

rati

o

2520151050Mean Diameter (nm)

0.7

0.6

0.5

0.4

0.3

0.2

0.1

d+/r

+(SSP)

d+/r

+(PPP)

0 π

f(θ)

θθc

z

θc

θC3

z

Conclusions

►Alkylthiols on nanoscale materials (gold and silver Nps) possess significant amount of gauche defects comparing with the SAM on planar gold

► Increasing amount of gauche defects with decreasing particle size can be understood in terms of the conical volume available for the chain on a curved surface

► Vibrational sum frequency generation is a powerful spectroscopic tool to characterized the molecular conformation on nanostructured materials

AcknowledgementAcknowledgementss

The GroupThe GroupAdvisor: Advisor: Prof. Alex V. BenderskiiProf. Alex V. Benderskii

Dr. Andrey N. BordenyukDr. Andrey N. Bordenyuk

Dr. Igor StiopkinDr. Igor StiopkinHimali D. JayethilakeHimali D. JayethilakeAchani K. YatawaraAchani K. YatawaraFadel Y. ShalhoutFadel Y. ShalhoutAdib J. SaminAdib J. Samin

WSU start-up grantWSU research grantNano@Wayne

ACS-PRFGrant No.40868-G6

NSF CAREER Grant No. 0449720

Funding

Professor Winters’ group-WSUDr. Charles Dezalah

ECE-WSUProf. G. AunerDr. J. Smolinski

CIF-WSUDr. Yi LiuDr. Sam Shinozaki

Surface curvature Chain lengthVSFG spectra: 5nm AuNPs VSFG spectra: 50nm AuNPs

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Inte

nsit

y (a

.u)

2960292028802840

Frequency (cm-1

)

C2-thiol

C12-thiol

C18-thiol

C6-thiolC6-thiol

C2-thiol

2.5

2.0

1.5

1.0

0.5

0.0

Inte

nsit

y (a

.u)

2960292028802840

Frequency (cm-1

)

C12-thiol

C18-thiol

1.0

0.8

0.6

0.4

0.2

0.0

r+in

tens

ity

(SS

P)

1801501209060300

c (deg)

1.0

0.8

0.6

0.4

0.2

0.0

d+in

tens

ity

(SS

P)

1801501209060300

c (deg)

1.2

1.0

0.8

0.6

Am

plit

ude

rati

o

2520151050Mean Diameter (nm)

d+/r

+

1.6

1.2

0.8

Am

plit

ude

rati

o

252015105Mean diameter (nm)

d+/r

+

AuNPsAuNPs

AgNPsAgNPs

dd++rr++

rr--(op)(op)

dd--

2.0

1.5

1.0

0.5

0.0

Abs

orba

nce

(a.u

)

30002950290028502800Frequency (cm

-1)

1.8 nm

2.9 nm

7.4 nm

23 nm

dd++ rr--(op)(op)

rr++dd--

2.5

2.0

1.5

1.0

0.5

0.0

Ram

an I

nten

sity

(a.

u)

30002950290028502800Raman Shift (cm

-1 )

1.8 nm

2.9 nm

7.4 nm

23 nm

IR and Raman spectra: 1-DDT gold nanoparticlesIR and Raman spectra: 1-DDT gold nanoparticlesIR spectraIR spectraRaman spectraRaman spectra

No size dependent spectral featuresNo size dependent spectral features

Possible interpretations: Possible interpretations: 1.1. Increasing fraction of Increasing fraction of gauchegauche

defects (SFG propenisty rules)defects (SFG propenisty rules)

2. Heterogeneity of local fields 2. Heterogeneity of local fields E(E(ωω))

E(ω, r)

r

No size-dependence in Raman spectra (should have same EM enhancement) Far off resonance:

λSPR~520 nm λSFG=665 nm

SFG Raman

IR

vis SFGpump Stokes

|v=0

|v=1

SSP polarization

1000

500

0

VS

FG

Inte

nsity

(a.

u.)

30002950290028502800Frequency (cm

-1)

CdSe

aac bbc ccc( ) ( ) ( ). , .2 2 23 4 0 1 0 0 aac bbc ccc

( ) ( ) ( ). , , .2 2 22 0 0 0 1 0 0

X L L L Xeffssp

y sfg y vis z IR yyz( ) , ( )( ) ( ) ( ) sin( )2 2

L L L Xz sfg x vis x IR sfg vis IR zxx( ) ( ) ( ) sin( ) co s( ) co s( )

X L L L Xeffppp

x sfg x vis z IR sfg vis IR xxz( ) , ( ) ( ) ( ) co s( ) co s( ) sin( )2

L L L Xx sfg z vis x IR sfg vis IR xzx( ) ( ) ( ) co s( ) sin( ) co s( )

L L L Xz sfg z vis z IR sfg vis IR zzz( ) ( ) ( ) sin( ) sin( ) sin( )

Molecular orientation analysis

Non-vanishing molecular hyperpolarizability C3v (r+)

Non-vanishing molecular hyperpolarizability C2v (d+)

Effective for polarization combination sfg-P, vis-P and IR-P X ( )2

Effective for polarization combination sfg-S, vis-S and IR-P X ( )2

Molecular orientation analysis

0.110

0.105

0.100

0.095

0.090

0.085

Inte

nsi

ty r

atio

(d

+/r

+)

150100500θc degrees

SSP polarization PPP polarization

0.070

0.060

0.050

0.040

0.030

0.020In

tens

ity r

atio

(d

+/r

+)

15010050θc degrees

►Qualitatively explains less intense d+ modes in PPP spectra for broad distribution of tilt angle θ.

800 nm40 nm bandwidth

803 nm26 nm bandwidth40 fs2 mJ/pulse, 1 kHz

Reflection geometry

IR output: 3-8 m 65-75 fs300 cm-1 bandwidth1-2 J/pulse

0.1 m precision

Shaped vis pulse:bandwidth 6 cm-1 Chirp control

Fs oscillator

Regenerative amplifier

2-pass amplifier

OPA with DFM

Optical delay stages

Sample

Monochromator

LN2-cooled CCD

Vis pulse shaping

Experimental Setup

OPA

Delay stages

Vis pulse shaping

Sample

CCD