Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SURFACE STRUCTURE ANALYSIS BY RAMAN SPECTROSCOPY AT VIBRATIONAL EXCITATIONS

04.07.2017

Norbert Esser1, Eugen Speiser1,Sandhya Chandola1, Svetlana Suchkova1,

Conor Hogan2, Simone Sanna3, Wolf-Gero Schmidt4

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V., Berlin, Germany2 CNR-ISM, Via del Fosso del Calvaliere, Roma, Italy3 Institut für Theoretische Physik, Universität Gießen, Giessen, Germany4 Department Physik, Universität Paderborn, Germany

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

Dr. S. Chandola

Dr. J. Räthel

A. Baumann

Theory: W.G. Schmidt, Uni PaderbornS. Sanna, Uni GießenS. Wippermann, MPIE Düsseldorf

Raman: Uni WürzburgJ. Geurts

Dr. E. Speiser

J. Plaickner

Dr. S. Suchkova

ACKNOWLEDGEMENTSISAS

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

OUTLINE

04.07.2017 Surface Optical Spectroscopy at Metal Nanowires on Si(hhk)

Introduction:

semiconductor surfaces, surface phonons

Analysis of surfaces by Raman and DFT calculations:

- Si(111) (7x7)

- In-nanowires on Si(111): structure, phase transition

- Surface Resonant Raman scattering

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

STRUCTURE FORMATION AT SURFACES

04.07.2017 Surface Optical Spectroscopy at Metal Nanowires on Si(hhk)

bulk truncatedstructure

unsaturatedSi-bonds relaxation

reconstruction

adsorbate stabilization

π-bondsa

a

2a

Surface: few atomic planes with structural, electronic and vibrational

properties distinct from bulk

Semiconductor: covalent bonding

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

IN/SI(111): SURFACE CONFINED VIBRATIONS

ω=42 cm-1 ω=21 cm-1

Surface confined vibrational modes (surface phonons) ↔ atomic structure of uppermost few atomic planes (surface layer)

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SURFACE CONTRIBUTION IN RAMAN SPECTRA

frequency shift

bulk phononssurface phonons

Ra

ma

n in

ten

sity

ws

ws

wi

Raman spectrumScattering geometry

surface bulk

• surface and bulk phonons spectrally separated

• surface layer of few atomic planes: small Raman intensity

jis www

jis kkk

Energy Conservation:

Momentum Conservation:

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

CLEAN SI(111) (7X7) SURFACE

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

RAMAN SPECTRUM OF BULK SI

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

0,5

1,0

1,5

2,0

2,5

3,0

3,5

z(yx)z

z(yx)z

z(yy)z

*

*

*

Inte

nsity [cts

/s]

Si(111) 7x7 reconstruction

l = 647 nm, T = 100 K z(yy)z

0,0

0,1

0,2

0,3

I [c

ts/s

]

50 100 150 200 250 300 350 400 450

0,0

0,1

*

*

*

*

I [c

ts/s

]

Raman Shift [cm-1]

surface phonons

Surface Raman spectrum: clean Si(111)(7x7) – oxidized Si(111)M. Liebhaber et al, Phys. Rev. B 89 (2014) 045313: Surface phonons of the Si(111)-(7x7) reconstruction observed by Raman spectroscopy

difference spectra

RAMAN IN UHV: SURFACE PHONONS OF SI(111)(7X7)

clean and oxidized Si(111) surface

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SI ADATOM VIBRATIONAL MODES

J. Kim et al, Phys. Rev. B 52 (1995) 14709

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

1D METAL ATOMIC NANOWIRES ON SURFACES

11

6 nm wire

1.7 ML Pb

M. Strozak et al, Vacuum 74, 241 (2004)

J. Kuntze et al, Appl. Phys. Lett. 81, 2463, (2002) J. Schaefer et al, Phys. Rev. Lett. 101, 236802 (2008)

Gold on Ge(001)Silver on Si(111)

J. N. Crain et al, Phys. Rev. B 69, 125401 (2004)

Gold wires and dots on vicinal Si

557 755

335 553

25 nm

1 nm wires

Lead on Si(533) Indium on Si(111)

H.W. Yeom et al, Phys. Rev. Lett. 82, 4898 (1999)

Exotic Electrons in 1D; Correlated electron- system:

Charge Density Waves (CDW), Spin Density Waves (SDW), Luttinger Liquid

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V. 04.07.2017 Surface Optical Spectroscopy at Metal Nanowires on Si(hhk)

1D ATOMIC INDIUM NANOWIRES ON SI(111)

Scanning Tunneling Microscopy Atomic Structure Model

J.H. Cho et al, Phys. Rev. B (2001)

Kumpf et al, Phys. Rev. Lett. (2000)

structural phase transition (Peierls):

RT 1D metallic ↔ LT insulating

H.W. Yeom et al, Phys. Rev. Lett. (1999)

T. Uchihashi et al, Appl. Phys. Lett. (2002)

300K 60K

RT LT

Indium sub-monolayer on Si(111) surface

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

In-Si

RAMAN SPECTRA FROM (4X1)-RT PHASE

12 100 200 300 400 500

1,2

2,4

3,6

4,8

*

2T

A b

ulk

Si

Ra

ma

n in

ten

sity

(ct

s/m

in/m

W)

wavenumber (cm-1)

TO

bu

lk S

i

Si-

Si, I

n-S

i

su

bla

ye

r m

od

es

*

lase

r p

lasm

alin

e

In-In

Si double acoustical Si-Si

A´

A´´

surface phonon intensities ~ 10-2 of bulk phonons → much larger than expected! Surface enhancement by electronic resonant Raman scattering (see later)

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

LOW- / HIGH-ENERGY SURFACE MODES

ω=421 cm-1ω=21 cm-1

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

RAMAN SPECTRA FROM (4X1) RT PHASE

12 100 200 300 400 500

1,2

2,4

3,6

4,8

*

2T

A b

ulk

Si

Ra

ma

n in

ten

sity

(ct

s/m

in/m

W)

wavenumber (cm-1)

TO

bu

lk S

i

Si-

Si, I

n-S

i

su

bla

ye

r m

od

es

*

lase

r p

lasm

alin

e

In-Si

In-InSi double acoustical

Si-Si

A´

A´´

ω=42 cm-1ω=21 cm-1

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

IN/SI(111) PEIERLS TRANSITION

(4x2)/(8x2)

(4x1)

20 30 40 50 60 70

167 K

138 K

105 K

90 K

73K

Ra

ma

n in

ten

sity

Raman shift (cm-1)

44 K

STM structure model

E. Jeckelmann, S. Sanna, W.G. Schmidt, E. Speiser, N. EsserPhys. Rev.B 93 (2016) 241407: Grand canonical Peierls transition in In/Si(111)

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SURFACE TEMPERATURE DETERMINATION BYSTOKES/ANTISTOKES RAMAN

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

Ramansellection

rules

ASSIGNMENT OF SURFACE MODES BY DFT-CALCULATIONS

Vibrational patterns of surface structure model verified bymode frequency and symmetry

E. Speiser, N. Esser, S. Wippermann, W.G. Schmidt Phys. Rev. B 94 (2016) 75417: Surface vibrational Raman modes of In:Si(111)(4x1) and (8x2) nanowires

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

RAMAN ANALYSIS OF IN/SI(111) NANOWIRES

04.07.2017 Title of the presentation

- Raman lines of In and Si related surface vibrational modes

- Raman selection rules ↔ phonon symmetry

- correlation between Raman modes and atomic structure by

ab-initio DFT calculations

resonance enhancement of Surface Raman Scattering?

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

RESONANCE RAMAN SCATTERING: MICROSCOPIC THEORY

(time-dependent perturbation theory)

R. Loudon, Proc. Royal Soc. A275, 218 (1963).A. Pinczuk et al., Topics in Appl. Phys. Vol. 8, Springer (1975).

e-- phonon interactionConductionBand

Valence Band

q

Ener

gy

e-- phonon interaction

e-

l 'l

0Resonance

2

)()(

0||||||0~

ilsl

LE

EE

pHllpS

Raman cross section large if incoming or outgoing light in resonance with an electronic interband transition

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SURFACE ELECTRONIC TRANSITIONS: EXPERIMENT AND AB-INITIO CALCULATIONS

Optical absorption:

In-Si surface bonds: absorption at 2 eV

Si-Si bulk crystal: absorption at 3.6 eV

S. Wang et al, Phys. Rev. B (2003)

Surface Resonant Raman for laser excitation at 1.9-2.3eV

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SURFACE ELECTRONIC RESONANCE

RT (4x1) LT (4x2)

J.H. Cho et al, Phys. Rev. B (2001)

Surface phonon

Surface Raman scattering due to coupling between surface phonons and surface excitons

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SUMMARY

Raman spectroscopy on surfaces:

microscopic surface phonon modes (typically few atomic layers)

deformation potential scattering involving surface electronic band structure surface resonant Raman

temperature determination by Stokes/Antistokes ratio

Raman ↔ DFT :

atomic surface structure, surface reconstruction, phase transitions

Outlook: interaction of metal nanowire structures with molecules

DFT modelling of full Raman scattering process,i.e. vibrations and electronic transitions

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

AB-INITIO CALCULATED RAMAN SPECTRA

DFT-based calculations of surface atomic and electronic structure, vibrational and electronic excitations:W.G. Schmidt, Uni PaderbornS. Sanna, Uni Siegen

In-PhaseAnti-phase

In-Phase

2

)()(

0||||||0~

ilsl

LE

EE

pHllpS

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.Bunsen-Kirchhoff-Straße 1144139 Dortmund

T: +49 (0)231 1392-0F: +49 (0)231 1392-200info@isas.dewww.isas.de

THANK YOUNorbert Esser

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

CHARGE DENSITY WAVE AND PEIERLSINSTABILITY

1D METALLIC CHAIN

LATTICE PERIODICITY

1D

BAND

CHARGE-DENSITY-WAVE

(wavevector k=2kF)

± kFNEW PERIODICITY

GAPPEIERLS

INSTABILITY

METALLIC STATE:Constant charge distribution

Parabolic energy bands

Filled up to the Fermi wavevector

Metallic conductivity

CDW STATE:Spatially modulated charge density

Energy gap at the Fermi energy

Semiconducting

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

ASSIGNMENT OF VIBRATIONS IN (4X1)

27

Ramansellection

rules

• Low frequency modes localizedin top layers with mainlyIndium displacements

• Distinct selection rules

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V. 04.07.2017 Surface Optical Spectroscopy at Metal Nanowires on Si(hhk)

AU ON SI(553)

28

• (553) tilted by 12.3° with respect to (111) in [11-2]-direction• Regular array of (111) terrces and double layer steps

Crain et al, PRB 69, 2004

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V. 04.07.2017 Surface Optical Spectroscopy at Metal Nanowires on Si(hhk)

UHV IN-SITU OPTICS SETUP

laser sourcesFresnel

rombs

neutral

density

filter

dielectric

bandpass

(laser)filter

CCD

photo-

multiplier

spec

trom

eter

G1

G2

G3

subtractive/aditive

double monochromator

sample

ES

kondensor

intermediat diafragm

with laser line filter

Fresnel

rombs

infr

ared

el

lips

omet

ryin

terf

erro

met

er

optical anisotropy

measurement set-up

infrared detector

LEED

surface analysis

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

LANDAU MODEL OF 1ST ORDER PHASE TRANSITION

• strong coupling of shearand rotational mode to CDW

• Weak coupling of thesagittal mode to CDW

(meV

)

strong strong

weak

Phonons

electrons elastic energy

ph.-el.interaction

xm

x

T

TTTTf

C

cC

2),,(

22

0642

0

0

Freeenergy

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SURFACE ANALYSIS: ELECTRONS VS. PHOTONS

- electron spectroscopy: surface specific, typical 1nm

- optical spectroscopy: surface to bulk, typical 10-100nm

epilayer

substrate

surface (d ~ 1nm)

ws

wsws

wi

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

HYDROGEN TERMINATED SI(111) (1X1) SURFACE

04.07.2017

STM imagetop view

side view

no reconstruction, bulk like structure up to the topmost Si layer

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

STRUCTURE OF SI(553)-AU

J. Aulbach et al., PRL 111 (2013)

STM image

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

0

100

200 A` z(yy)z50K

300K

Raman shift (cm-1)20 50 100 150 200 250 300 350 400 450

0

50

100 A`` z(yx)z50K

300K

Inte

nsity (

cts

/W/m

in)

04.07.2017 Surface Optical Spectroscopy at Metal Nanowires on Si(hhk)

COMPARISON TO AB-INITIO CALCULATIONS

Dienstag, 4. Juli 2017

Translation In-Phase Anti-phase In-Phase

• Calculation for Krawiec-model• Vibrations strongly localised (75%)

in first and second layer• Good quantitative agreement to

surface Raman modes (average

deviation 2cm-1)

S. Sanna, private communikation

Honey combAu-chain

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

SYMMETRY AND RAMAN TENSOROF (4X1) AND (8X2) PHASES

2D space group pm: (rectangular)m (mirror plane, additional setof m generated)

2D space group pg: (rectangular)glide reflection g (additional set of g generated)

3D space group: pc Point group m

3D space group pm Point group m

Mirror plane

(010)

(00

1)

Both structures belongto point group m !

Cs symmetry

AA

ig

h

f

c

be

da

Raman tensor: Monoclinic

35

A´´

A´

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

(4X1) – (8X2) PHASE TRANSITION

(8x2)(4x1)

+

+

28 cm-1

20 cm-1

28 cm-1

17 cm-1

Back foldingof BZ edge phonons

Can appear inRaman spectra

Combined displacements

- backfolding of zone edge phonons + restructuring within unit cell- phonons assisted phase transition

S. Wippermann W.G. Schmidt, PRL 105 126102 (2010)E. Speiser, N. Esser, S. Wippermann, W.G. Schmidt, PRB (2016) in print

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

40 60 80 100 120 140 160 180 200

-12

-10

-8

-6

-4

-2

0

2

28 cm-1

(w(T

) - w

(44K

))/w

(44K

) (%

)

temperature (K)

CTTT )(2w

42 cm-1

20 cm-1

20 30 40 50 60 70

(8x2)

167 K

138 K

105 K

90 K

73K

Ra

ma

n in

ten

sity

Raman shift (cm-1)

44 K

(4x1)

-2

-1

0

1

55 cm-1

Phonon softening

INVESTIGATION OF PHASE TRANSITION BY RAMAN

partial softening of phononsinvolved in the phase transition → first order phase transition

Kohn anomaly

amplitudon

phason

T/TC

Ph

on

on

en

ergy

Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

RAMAN ANALYSIS OF SURFACES: CLEAN SI(111)

Raman lines of surface confined vibrational modes visible with „standard“ Raman (no E-field enhancement like SERS)

Surface modes: weak Raman signal superimposed on multiphonon scattering from Si bulk

Separation of surface and bulk signal by surface modification (oxidation)