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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 [email protected]
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)