Laboratoire de Physique des Interfaces et des Couches Minces (LPICM)

CNRS, Ecole Polytechnique

91128 Palaiseau

France

Gennaro Picardi

Polarization properties of oblique incidence

Tip enhanced Raman spectroscopy

Experimental set-up

Raman (HORIBA JY)SPM (PSIA) Optical coupling

Piezos x, y

Piezo z

MicroscopeLaser

Grating

Notch filter

Detector

Half-wave plate

Analyzer

Confocal Raman

Feedback control

Oblique back-scattering configuration

Side illumination of the tip

STM Au tip etching

1-2 min

Electrochemical etching of a gold wire (0.125 mm) in a solution of 1:1 ethanol and conc. HCl (37%).

Applied Voltage: ~2.4 V

7-8 minBreak of the tip(circuit closes)

Rtip = 20-30 nm

L. Billot, L. Berguiga, M.L. de la Chapelle, Y. Gilbert and R. Bachelot; Eur. Phys. J. Appl. Phys. 31 (2005) 139B. Pettinger, B. Ren, G. Picardi, R. Schuster and G. Ertl; Rev. Sci. Instrum. 75 (4) (2004) 837

Anodic oxidation(passivation)

AuOHe-

Au dissolution

H+, 2 Cl-

DisproportionCl-

½ e-, Cl-

AuCl4-

AuCl2- + H20

Au deposition

H20, Cl-

Au*-Cl-

Au*-H20

Oscillatory electrodissolution of gold

Passiveregion

Activeregion

Au electrodissolution in HCl is diffusion limited

Z.L. Li,T.H. Wu, Z.J. Niu, W. Huang, H.D. Nie ; Electrochem. Comm. 6 (2004) 44

X. Wang, Z. Liu, H. Zhang, X. Wang, Z. Xie, D. Wu, B. Ren and Z. Tian; Appl. Phys. Lett. 91 (2007) 101105

Brilliant Cresyl Blue

on Au (111)

Au tip(oblique incidence)

585 cm-1 exc 633 nm

20 s

D1 filter

STM-TERS on dyes

400 600 800 1000 1200 1400 1600

0

50

100

150

200

Ram

an i

nte

nsi

ty (

a. u

.)

Raman shift ( cm-1 )

G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Europ. Phys. J. Appl. Phys. in press (2007)

TERS for structured materials nano-characterization

W.X. Sun and Z.X. ShenNear-field scanning Raman microscopy using apertureless probes

J. Raman spectrosc. 2003; 34: 668-676

Micro-Raman Spectroscopy provide mapping of stresses in Si structures ( well defined Raman shift due to strain )

3700 nm

Near-field Raman mapping at 521 cm-1 ( Si-Si )

(using aperture-less probe)

380 nm300 nm

Introducing polarized TERS

(p) - pol

no analyzer analyzer at 90°

« … polarization of the ligth scattered by the particle will differ from the polarization of the incident light. The light partly depolarized by the particle then inelastically scatters with the optical phonon in Si thereby producing allowed Raman signal. The allowed Si Raman signal should be associated with a local area around the particle. »

V. Poborchii, T. Tada and T. Kanayama, Jpn. J. Appl. Phys. 44 (2005)

(p) - pol

Half-wave plate

AnalyzerSample

incident light-tip interaction

scattered light- tip interaction

Incident light Scattered light

Polarization control may become an important parameter in the TERS experiment

I. The tip modifies the polarization state of the incident and scattered radiation.

II. The far field signal can be reduced by using an analyzer.

Polarization control in TERS

Rj : Raman tensor of j-phonon

ei : incident polarization state (polarizer P)

es : scattered polarization state (analyzer A)

The scattered intensity depends on the polarization states ei, es as well as on the sample orientation S

Raman intensity

000

00d

0d0

R

0d0

d00

000

R

00d

000

d00

R 321

Sample orientation S

Analyzer

n

ie

se

Polarizer

j

2

ijTs e . R .eI

Calculation of the scattered intensity in the far field

Far field: experimental verification

Calculation of the scattered intensity in the far field

The sample orientation (or azimuth) S modulates the scattered intensity.

(100) c-Si (111) c-Si

P || A

P ┴ A

Sample orientation S (deg) Sample orientation S (deg)

Inte

nsi

ty (

arb

. un

.)

Inte

nsi

ty (

arb

. un

.)

The tip-enhancement tensor A describes the field enhancing and polarization properties of the tip

a and b : tip-dependent TERS parameters

The tip tensor A transforms the sample Raman tensor R to an effective  scattering tensor R’

Far field (tip withdrawn): R (R : Raman polarizability tensor)

Near field (light-tip intaction): R’ = AT R A (A : tip-enhancement tensor)

Total field (tip in contact): Far field + Near field

(the tip-enhancement tensor)

Calculation of the scattered intensity in the near field

b00

0b0

00a

A

R. Ossikovski, Q. Nguyen and G. Picardi, Phys. Rev. B 75, 045412 (2007)

NH4F etched Si (111)

NC-AFM STM

(b) Etch-pit initiation by dissolved oxygen on terraces of Si (111)C.P. Wade and C.E.D. ChidseyAppl. Phys. Lett. 71 (12) 1997

Ut = -1.5 V

It = 50 pA

Pt/Ir tip

Ut = -1.0 V

It = 100 pA

Au tip

750 nm

Opticalmicroscope

10 m

TERS on Si(111) with polarization control (I)

Raman intensity of the 1st order Si phonon peak ( 521 cm-1 ) tip down tip up

a : b = 1.6 : 1

Analyzer fixed at 90°

a : b = 5.5 : 1

= 20° = 71°

20

40

60

80

100

120

140

0 20 40 60 802.4

2.8

3.2

3.6

4.0

4.4

480 500 520 540 5600

30

60

90

120

150

Inte

nsit

y (c

ps) (b)

(d)

Con

tras

t

Incident polarization (deg)

Raman shift (cm-1)

0 20 40 60 80

2.0

2.4

2.8

3.2

20

30

40

50

60

70

80

Con

tras

t

Incident polarization (deg)

Inte

nsit

y (c

ps)

(c)

(a)

( s ) ( p ) ( p ) ( s )

Tip #1 Tip #2

G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Appl. Spectr. 61 (12), 2007

Oblique incidence: p - polarization or s - polarization ?

400 600 800 1000 1200 1400

20

40

60

80

100

120

Ram

an in

tens

ity

(a.u

.)

Raman shift (cm-1)

400 600 800 1000 1200 1400

20

40

60

80

100

120

Ram

an in

tens

ity

(a.u

.)

Raman shift (cm-1)

(with p-pol ) (with s-pol )

Higher intensity with incident (p) polarizationbut strong TERS also with incident (s) light

BCB on Au (111)No analyzer

Tip #2

G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Appl. Spectr. 61 (12), 2007

Influence of the incident polarization in TERS ( I )

( p ) – polarization (in-plane) of the incident electric field

EFE up to 35

EFE up to 10

( s ) – polarization (out-of-plane) of the incident electric field

‘ Understanding TERS ’

A. L. Demming, F. Festy and D. RichardsJ. Chem. Phys. 122, 1847 (2005)

‘ Finite Element simulations of TERS ’

A. Downes, D. Salter and A. ElfickJ. Phys. Chem. B 110, 6692 (2006)

Influence of the incident polarization in TERS ( II )

1. Overall higher field enhancement with p – polarized excitation,

but field enhancement also with s – polarized ligth.

Near-field Raman spectra of SWCNT measured under p- and s- polarization conditions.

Polarization measuraments in TERS applied to SWCNTY. Saito, H. Hayazawa, H. Kataura, T. Murakami, T. Tsukagoshi, Y. Inouye and S. Kawata

Chem. Phys. Lett. 410, 136 (2005)

2 bis.

2. With p – polarized light illumination also enhancement of the field

component in the substrate plane (i.e. out of the plane of incidence).

With s – polarized light illumination also enhancement of the field

component normal to the substrate plane (i.e. in the plane of incidence).

Cross polarization effect(depolarized enhancement)3.

Imaging should be (?) different.

S. Foteinopoulou, J. P. Vigneron and C. VandenbemOpt. Expr. 15, 4253 (2007)

‘Under p - pol or s - pol light illumination the charge is differently

concentrated.’

Far field artifacts when TERS probing bulk samples

‘ The (tip in) contact signal may include a component that is unrelated to the plasmon resonance enhancement due to reflection and scattering from the tip … leading to additional unlocalized Raman signal.’

‘ … significant enhancement of Raman scattering from silicon substrates can be achieved without a field enhancement effect by plasmon resonces. Pure laser deflection and near field scattering cause similar effects which are difficult to distinguish.’

C. Georgi, M. Hecker and E. Zschech, Appl. Phys. Lett. 90, 171102 (2007).

N. Lee, R. D. Hartschuh, D. Methani, …and A. P. Sokolov , J. Raman Spectrosc. 38, 789 (2007).

ACKNOWLEDGEMENTS

Quang Nguyen

Razvigor Ossikovski

Bernard Drevillon(LPICM director)