Spectroscopy of
Hybrid Inorganic/Organic Interfaces
Vibrational Spectroscopy
Dietrich RT Zahn
The Application of Raman Spectroscopy
in the DIODE Project
The Overall Device Performance
(iv) The Interface between the
Organic Molecules and the Metal
Metal
I
V
(iii) The Organic
Molecular Film
Organic Interlayer
GaAs(100)
(ii) The Interface between
GaAs Substrate and
Organic Molecules
(i) The GaAs Substrate Surface
Dietrich RT Zahn, TU Chemnitz
Molecular Vibrational Properties
PTCDA: 3,4,9,10- Perylenetetracarboxylic diAnhydride
DiMe-PTCDI: 3,4,9,10- Perylenetetracarboxylic diImide
C24H8O6
Symmetry
D2h
Raman active: 19Ag+18B1g+10B2g+7B3g
IR active:
+10B1u+18B2u+18B3u
Silent:
+ 8Au
108 internal vibrations
C26H14O4N2
C2h
44Ag+22Bg
+23Au+43Bu
+ 8Au
132 internal vibrations
Monoclinic crystallographic system in thin films:
• PTCDA: - and -phases:
S. R. Forrest, Chem. Rev. 97 (1997), 1793.
• DiMe-PTCDI:
Cambridge Structural Database.
Dietrich RT Zahn, TU Chemnitz
Raman-active vibrations
of PTCDA (C24H8O6):
Effect of crystal formation
internal molecular modes:
Symmetry:
D2h
Davydov
Splitting
C- C- O
Bg
C-C
Bg
200 300 400 500 600 700
C-H
x2
C-C
1200 1300 1400 1500 1600 1700
-1
Raman shift / cm
6 rotational
vibrations:
3Ag+3Bg
Ag
Intensity / a.u.
2-fold
Intensity / a. u.
19Ag+18B1g
+10B2g+7B3g
external molecular modes (phonons):
C2h (monoclinic)
Ag
Bg
Ag
25
50
75
100
125
-1
Raman shift / cm
Dietrich RT Zahn, TU Chemnitz
Vibration modes of
PTCDA molecule
B3LYP / 3-21G
DFTB
Mode
freq
/ cm-1
freq
/ cm-1
freq
/ cm-1
12
19
25
29
33
41
49
64
67
75
76
79
83
86
92
94
100
103
107
233
389
476
537
624
727
858
1054
1150
1305
1340
1381
1389
1544
1572
1590
1774
233
383
474
550
639
728
863
1070
1140
1285
1304
1347
1393
1527
1616
1623
1723
3173
3190
233
389
476
539
627
732
853
1059
1151
1269
1303
1346
1389
1472
1611
1620
1800
3227
3253
Raman
Activity
0,99
6,43
0,88
122,86
72,23
12,01
2,63
346,71
161,30
1030,13
0,00
5362,37
551,12
1083,97
0,01
1395,17
1284,66
173,49
236,78
Intensity / a. u.
Exp
C-C
C-O B
g
200
300
C-H
400
500
600
700
x2
C-C
1200 1300 1400 1500 1600 1700
-1
Raman shift / cm
 19 Ag breathing modes
 very good agreement
between experimental and
calculated frequencies !
Dietrich RT Zahn, TU Chemnitz
Intensity / arb. units
Raman Spectra of a PTCDA Crystal
x0.1
200
400
600
1200
1350
Raman shift / cm
1500
1650
-1
• assignment of modes and their relative atomic contribution using
Gaussian `98 (B3LYP, 3-21G)
Dietrich RT Zahn, TU Chemnitz
Intensity / arb. units
Ag Raman Modes of PTCDA
with In
x0.1
200
400
600 1200
1350
Raman shift / cm
1500
1650
-1
Dietrich RT Zahn, TU Chemnitz
Raman Spectra of a PTCDA Crystal
and a DiMe-PTCDI
Intensity / arb. units
C-H
ring
C=O
x0.5
x0.1
200
400
600
1200
1350
Raman shift / cm
1500
1650
-1
• assignment of modes and their relative atomic contribution using
Gaussian `98 (B3LYP:3-21G).
Dietrich RT Zahn, TU Chemnitz
Intensity / arb. units
Raman Spectra of a PTCDA Crystal
and a DiMe-PTCDI
ωDiMe-PTCDI
mPTCDA
=
=0.97
ωPTCDA
mDiMe-PTCDI
200
400
600
1200
1350
ωDiMe-PTCDI 1500
221
= 1650
=0.95


ω
233
PTCDA
 -1
experimental
Raman shiftRaman
/cm-1 shift / cm
• assignment of modes and their relative atomic contribution using
Gaussian `98 (B3LYP:3-21G).
Dietrich RT Zahn, TU Chemnitz
Raman-active vibrations of PTCDA:
Effect of crystal formation
external molecular modes (phonons): 6 rotational vibrations: 3Ag+3Bg
Symmetry: C2h (monoclinic)
Bg
Intensity / a.u.
Ag
Bg
25
50
75
100
125
-1
Raman shift / cm
Dietrich RT Zahn, TU Chemnitz
Infrared Modes in Films on S-GaAs
Reflection, s-polarized light.
C=O
Rsample/Rsubstrate
1.4
C-O+
C-C
1.3
1.2
ring
C-H+
C-N-C
C-O-C
C-H
1.1 (oop)
1.0
750
1000
1250
1500
-1
1750
Wavenumber / cm
•Assignment of modes using Gaussian `98 (B3LYP, 3-21G).
Dietrich RT Zahn, TU Chemnitz
Epi-ready GaAs (100)
Sample Preparation
Degreasing
Acetone, Ethanol, Di-Water
Wet Chemical Treatment
S2Cl2:CCl4=1:3 (10 sec)
OMBD deposition:
PTCDA, DiMe-PTCDI
Thickness: 0.1 nm ÷15 nm
Rinsing
(CCl4, Acetone, Ethanol, Di-Water)
Annealing at 620 K, 30 min
Metal deposition:
Ag, In
Thickness: 0.1 nm ÷260 nm
S-GaAs(100):2x1
Dietrich RT Zahn, TU Chemnitz
Ex Situ (Micro-) and
In Situ (Macro- Configuration)
Raman Spectroscopy
Absorbtion coefficient *10
5
Wavelength / nm
Dilor XY 800 Spectrometer
800 700 600
6
500
S0-S1 transition
400
Ar+ line
4
4
S0-S2 transition
2
2
DiMe-PTCDI
0
PTCDA
0
1.5
2.0
2.5
3.0
3.5
Energy / eV
Monochromatic light source: Ar+ Laser (2.54eV), Detector: CCD
• resonance condition with the absorption band of the organic crystalline material.
• resolution: 1.2 cm-1 to 3.5 cm-1.
Dietrich RT Zahn, TU Chemnitz
4.0
E = 2.54 eV
Normalized Intensity
Monitoring of PTCDA Film Growth
on S-GaAs
1.0
0.8
0.6
-1
431 cm
-1
386 cm
-1
233 cm
simulation, k= 0.99
0.4
0.2
0.0
0
20
40
60
80
100 120
Thickness / nm
M. Ramsteiner et al.,
Appl. Opt. 28 (18) (1989), 4017.
• The relative intensity of internal
modes does not change upon
deposition.
 weak interaction of the molecules with the S-passivated substrate.
• Phonons are well resolved as soon as 20 nm of PTCDA are deposited.
Dietrich RT Zahn, TU Chemnitz
40 nm
x 0.01
0.002
-1
Intensity / cts mW s
-1
Chemistry at Organic/S-GaAs(100):2x1
Vibrational Properties:
PTCDA
Annealing at 623 K for 30
0.45 nm
(x 0.6)
0.18 nm
ann.
x 4.4
1300
1400
1500
-1
Raman shift / cm
1600
min:
• Molecules remaining at
the surface:
NPTCDA(0.04nm)~1013 cm-2
NdSi ~ 1012 cm-2
• Spectrum of annealed film
similar to that of an
annealed PTCDA film on
Si(100).
 The strongest
interaction: between the
PTCDA molecules and
defects due to Si at the
GaAs surface.
Dietrich RT Zahn, TU Chemnitz
Calculated Vibrational Properties: PTCDA
30
2.7 cm
-1
20
1000
4
Intensity / A amu
-1
1500
1340
1350
10
500
0
0
300
600
900 1200
1400
1600
-1
Raman shift / cm
Dietrich RT Zahn, TU Chemnitz
Calculated Vibrational Properties:
PTCDA
1500
1000
• significant spectral changes
predicted for the C=C modes
around 1600 cm-1
4
Intensity /A amu
-1
Molecular charging with one
elementary charge:
500
negative
positive
0
 fractional charge transfer
between the PTCDA and the
defects at the GaAs surface.
neutral
1300 1400 1500 1600 1700
Raman shift / cm
-1
Dietrich RT Zahn, TU Chemnitz
In Situ Raman: Monitoring of Indium
Deposition onto PTCDA (15 nm)
0.05
Intensity / cts mW s
-1 -1
0.005
In thickness
/ nm
/5
43
26.0
/10
/58
15.0
/33
8.0
/28
5.0
/13
2.8
/1.5
1.1
/0.7
200
400
600 1200
1400
Raman shift / cm
-1
0.4
0
1600
Dietrich RT Zahn, TU Chemnitz
Influence of Indium on Vibrational
Spectra of PTCDA
0.0025
-1 -1
Ag
Ag
Intensity /cts mW s
B3g
Ag B
B
Ag 2u
B1u B3g Ag
(B3g)
B3g
B1u
0.0025
Ag
Ag
2g
B2u
Ag
In
15 nm
GaAs
B3g
B3g
+ In
PTCDA
200
400
600
1200
Raman shift / cm
-1
1400
1600
Dietrich RT Zahn, TU Chemnitz
Influence of Indium on Vibrational
Spectra of PTCDA
R/Rsubstrate
1.6
C-H(z)
C-H,
C-O
1.4
• organic films grown on
S-GaAs(100):2x1
In(15nm)/
PTCDA(15 nm)
C-H
C-H
C-C,
C=C
• reflection
measurements at 20°
incidence.
C=O
C-O
1.0
PTCDA(130 nm)
800
1000
1200
1400
1600
Wavenumber/ cm
1800
2000
-1
• all PTCDA modes are preserved in the spectrum of In/PTCDA.
• observation of C=O modes (around 1730-1770cm-1)
 In does not react with the O of PTCDA !
Dietrich RT Zahn, TU Chemnitz
Indium/PTCDA:
Separation of Chemical
and Structural Properties
In: 1 nm/min
PTCDA
S-GaAs(100)
PTCDA
(15 nm)
In: 0 100 nm
PTCDA
Intensity / cts mW s
-1 -1
~0.4 nm
(~1 ML)
PTCDA
(0.4 nm)
0.03
0.001
x 0.017
x0.045
~15 nm
(~50ML)
+ In
PTCDA
S-GaAs(100)
1200
1350
1500
1650
1350
Raman shift / cm
1500
1650
-1
Dietrich RT Zahn, TU Chemnitz
Comparison of Indium and Silver
Deposition on PTCDA and DiMe-PTCDI
In: 1 nm/min
Ag:1.6 ÷ 5.5 nm/min
-1 -1
Intensity / cts mW s
S-GaAs(100)
DiMe-PTCDI
(15 nm)
PTCDA
(15 nm)
1200
0.03
0.01
S-GaAs(100)
Ag
+ In
1400
1600 1200
Raman shift / cm
1400
-1
1600
Dietrich RT Zahn, TU Chemnitz
Comparison of Indium and Silver
Deposition on PTCDA and DiMe-PTCDI
PTCDA
0.4 nm In
DiMe-PTCDI
0.4 nm In
Intensity /a.u.
x4
0.3 nm Ag
/2
0.4 nm Ag
/2
100
200
100
• the PTCDA external modes:
 are preserved broadened after
0.3 nm Ag deposition.
 disappear after 0.4 nm In.
• the DiMe-PTCDI external modes:
 less affected compared to
PTCDA.
 probably due to less compact
crystalline structure.
200
-1
Raman shift / cm
Dietrich RT Zahn, TU Chemnitz
Mg,PTCDA
In, Ag
on PTCDA
(15 nm)
0.03
Intensity / cts.mW s
-1 -1
0.01
/5
/10
+
Mg
+
In
+ Ag
PTCDA
200
300
400
500
600
1300
1400
1500
1600
-1
Raman shift / cm
Dietrich RT Zahn, TU Chemnitz
Mg, In, DiMe-PTCDI
Ag
on DiMe-PTCDI
(15 nm)
5x10
-3
-2
5x10
cts mW
-1 -1
cts.mW s
-1 -1
s
Intensity
++Mg
In
+
In
+ Ag
DiMe-PTCDI
200
300
400
500
600
1300
Raman shift / cm
-1
1400
1500
1600
Dietrich RT Zahn, TU Chemnitz
Indium and Silver Deposition:
Enhancement Factors
PTCDA (15 mn)
DiMe-PTCDI (15 nm)
Ag Thickness / nm
Ag Thickness / nm
10
20
30
0
40
100
100
10
10
Relative Area
Relative Area
0
-1
1244 cm
1
-1
1570 cm
-1
1615 cm
1000
100
20
30
40
-1
1236 cm
1
-1
1568 cm
-1
1610 cm
100
10
10
1
10
1
0
10
20
30
40
In thickness / nm
0
10
20
30
40
In thickness / nm
Dietrich RT Zahn, TU Chemnitz
Determination of Molecular Orientation:
=0°: x II [011]GaAs

DiMe-PTCDI
=90°:
x II [0-11]
phonons
phonons
 Azimuthal rotation of a 120 nm thick film; normal incidence.
 Periodic variation of signal in crossed and parallel polarization.
M. Friedrich, G. Salvan, D. Zahn et al., J. Phys. Cond. Mater. submitted.
Dietrich RT Zahn, TU Chemnitz
Determination of Molecular Orientation:
DiMe-PTCDI
Breathing mode at 221 cm-1
Depolarization Ratio/ a.u.
2.5
2.0
1.5
1.0
0.5
0.0
0
60
120
180
240
300
360
Experimental angle ()/°
I =  es  Ag  ei 
Dep =
Ag = R  , , ,   Agm  R -1  , , , 
Iyx
I xx
  56  4;
, 
 Good agreement with
IR and NEXAFS results
Dietrich RT Zahn, TU Chemnitz
Molecular Orientation with respect to
GaAs substrate:
PTCDA:  ~ 9°

Dietrich RT Zahn, TU Chemnitz

[-110]

DiMe-PTCDI:
 ~ 6°
 ~ 60°
Dietrich RT Zahn, TU Chemnitz
Raman Characterization of
Organic Thin Films:
Achievements and Outlook
Interface reactions 
Film thickness 
Growth Mode 
Crystallinity 
Crystalline
Order
Crystal modifications 
Orientation
Internal Modes:
Shifts, Intensities
Intensity modulations
Intensity modulations
Occurrence of Phonon-like Modes,
FWHM
Phonons,
Davydov Splitting of Internal Modes
Further investigations
Dietrich RT Zahn, TU Chemnitz
Raman Spectroscopy Team:
Dietrich RT Zahn, TU Chemnitz