spectroscopy of hybrid inorganic/organic interfaces vibrational spectroscopy dietrich rt zahn
TRANSCRIPT
Spectroscopy of Hybrid Inorganic/Organic Interfaces
Vibrational Spectroscopy
Dietrich RT Zahn
Dietrich RT Zahn, TU Chemnitz
The Overall Device Performance
GaAs(100)
Organic Interlayer
Metal
V
I
(iv) The Interface between the Organic Molecules and the Metal
(iii) The Organic Molecular Film
(ii) The Interface between GaAs Substrate and Organic Molecules
(i) The GaAs Substrate Surface
The Application of Raman Spectroscopy in the DIODE Project
Dietrich RT Zahn, TU Chemnitz
PTCDA: 3,4,9,10- Perylenetetracarboxylic diAnhydride DiMe-PTCDI: 3,4,9,10- Perylenetetracarboxylic diImide
Symmetry D2h Raman active: 19Ag+18B1g+10B2g+7B3g
IR active: +10B1u+18B2u+18B3u
Silent: + 8Au
108 internal vibrations
Molecular Vibrational Properties
CC2424HH88OO66
Monoclinic crystallographic system in thin films:
• PTCDA: - and -phases: S. R. Forrest, Chem. Rev. 97 (1997), 1793.
• DiMe-PTCDI: Cambridge Structural Database.
CC2626HH1414OO44NN22
C2h
44Ag+22Bg
+23Au+43Bu
+ 8Au
132 internal vibrations
Dietrich RT Zahn, TU Chemnitz
2-fold
Davydov Splitting
internal molecular modes: external molecular modes (phonons):
CC- - CC- - OOBBgg
CC--HH CC--CC
CC--CC
SymmetrySymmetry: : DD2h2h CC2h2h (monoclinic) (monoclinic)
6 rotational vibrations: 3Ag+3Bg
19Ag+18B1g+
10B2g+7B3g
BBgg
AAgg
AAgg
BBgg
AAgg
Raman-active vibrations of PTCDA (C24H8O6):Effect of crystal formation
Dietrich RT Zahn, TU Chemnitz
19 Ag breathing modes
very good agreement
between experimental and
calculated frequencies !
Vibration modes of Vibration modes of PTCDAPTCDA molecule molecule
Exp DFTB
freq / cm-1
freq / cm-1
freq / cm-1
Raman Activity
12 233 233 233 0,99
19 389 383 389 6,43
25 476 474 476 0,88
29 537 550 539 122,86
33 624 639 627 72,23
41 727 728 732 12,01
49 858 863 853 2,63
64 1054 1070 1059 346,71
67 1150 1140 1151 161,30
75 1305 1285 1269 1030,13
76 1340 1304 1303 0,00
79 1381 1347 1346 5362,37
83 1389 1393 1389 551,12
86 1544 1527 1472 1083,97
92 1572 1616 1611 0,01
94 1590 1623 1620 1395,17
100 1774 1723 1800 1284,66
103 3173 3227 173,49
107 3190 3253 236,78
B3LYP / 3-21GMode
CC--OO BBgg
CC--HH CC--CC
CC--CC
Dietrich RT Zahn, TU Chemnitz
200 400 600 1200 1350 1500 1650
Inte
nsi
ty /
arb
. un
its
Raman shift / cm-1
Raman Spectra of a PTCDA Crystal
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP, 3-21G)
x0.1
Dietrich RT Zahn, TU Chemnitz
200 400 600 1200 1350 1500 1650
Inte
nsi
ty /
arb
. un
its
Raman shift / cm-1
x0.1
Ag Raman Modes of PTCDAwith In
Dietrich RT Zahn, TU Chemnitz
x0.1
200 400 600 1200 1350 1500 1650
Inte
nsi
ty /
arb
. un
its
Raman shift / cm-1
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).
x0.5
C=O
ring
C-H
and a and a DiMe-PTCDIDiMe-PTCDIRaman Spectra of a Raman Spectra of a PTCDAPTCDA Crystal Crystal
Dietrich RT Zahn, TU Chemnitz
200 400 600 1200 1350 1500 1650
Inte
nsi
ty /
arb
. un
its
Raman shift / cm-1
Raman Spectra of a Raman Spectra of a PTCDAPTCDA Crystal Crystal
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).
Raman shift /cm-1
and a and a DiMe-PTCDIDiMe-PTCDI
DiMe-PTCDI PTCDA
PTCDA DiMe-PTCDI
DiMe-PTCDI
PTCDA experimental
ω m= =0.97
ω m
ω 221= =0.95
ω 233
Dietrich RT Zahn, TU Chemnitz
external molecular modes (phonons): 6 rotational vibrations: 3Ag+3Bg
SymmetrySymmetry: : CC2h2h (monoclinic) (monoclinic)
Raman-active vibrations of PTCDA:Effect of crystal formation
BBgg
AAgg
BBgg
Dietrich RT Zahn, TU Chemnitz
750 1000 1250 1500 1750
1.0
1.1
1.2
1.3
1.4
Rsa
mp
le/R
sub
stra
te
Wavenumber / cm-1
Infrared Modes in Films on S-GaAs
•Assignment of modes using Gaussian `98 (B3LYP, 3-21G).
Reflection, s-polarized light.
C=O
ring
C-O-C
C-O+
C-C
C-H
(oop)
C-H+C-N-C
Dietrich RT Zahn, TU Chemnitz
Sample PreparationEpi-ready GaAs (100)
DegreasingAcetone, Ethanol, Di-Water
Wet Chemical TreatmentS2Cl2:CCl4=1:3 (10 sec)
Rinsing(CCl4, Acetone, Ethanol, Di-Water)
Annealing at 620 K, 30 min
OMBD deposition:PTCDA, DiMe-PTCDI
Thickness: 0.1 nm ÷15 nm
S-GaAs(100):2x1
Metal deposition:Ag, In
Thickness: 0.1 nm ÷260 nm
Dietrich RT Zahn, TU Chemnitz
Ex Situ (Micro-) and In Situ (Macro- Configuration)
Raman Spectroscopy
Dilor XY 800 SpectrometerMonochromatic 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.
1.5 2.0 2.5 3.0 3.5 4.0
0
2
4
6
Abs
orbt
ion
coef
ficie
nt *
105
Abs
orbt
ion
coef
ficie
nt *
105
S0-S
2 transition
S0-S
1 transition
DiMe-PTCDI
PTCDA
Energy / eV
800 700 600 500 400
0
2
4
Wavelength / nm
Ar+ line
Dietrich RT Zahn, TU Chemnitz
Monitoring of PTCDA Film Growth on S-GaAs
• Phonons are well resolved as soon as 20 nm of PTCDA are deposited.
• The relative intensity of internal modes does not change upon deposition.
E = 2.54 eV
M. Ramsteiner et al., Appl. Opt. 28 (18) (1989), 4017.
weak interaction of the molecules with the S-passivated substrate.
0 20 40 60 80 100 120
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Inte
nsity
Thickness / nm
431 cm-1
386 cm-1
233 cm-1
simulation, k= 0.99
Dietrich RT Zahn, TU Chemnitz
1300 1400 1500 1600
Inte
nsity
/ ct
s m
W-1 s
-1
Raman shift / cm-1
0.0
02
Annealing at 623 K for 30 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.
Chemistry at Organic/S-GaAs(100):2x1Vibrational Properties:
PTCDA
40 nmx 0.01
0.45 nm(x 0.6)
0.18 nm
ann.x 4.4
Dietrich RT Zahn, TU Chemnitz
300 600 900
0
10
20
30
1200 1400 1600
0
500
1000
1500
Inte
nsity
/ A
4 am
u-1
Raman shift / cm-1
Calculated Vibrational Properties: PTCDA
1340 1350
2.7 cm-1
Dietrich RT Zahn, TU Chemnitz
1300 1400 1500 1600 1700
0
500
1000
1500
Inte
nsity
/A4 a
mu-1
Raman shift / cm-1
neutral
negative
Calculated Vibrational Properties: PTCDA
Molecular charging with one elementary charge:
positive fractional charge transfer between the PTCDA and the defects at the GaAs surface.
• significant spectral changes predicted for the C=C modes around 1600 cm-1
Dietrich RT Zahn, TU Chemnitz
In Situ Raman: Monitoring of Indium Deposition onto PTCDA (15 nm)
1200 1400 1600
0.05
Raman shift / cm-1200 400 600
Inte
nsity
/ c
ts m
W-1s-1
0.005
43/5
In thickness / nm
00.4/0.71.1/1.52.8/135.0
/288.0/3315.0
/5826.0/10
Dietrich RT Zahn, TU Chemnitz
Influence of Indium on Vibrational Spectra of PTCDA
1200 1400 1600
0.0025
+ InB
3g
B1u
Ag
Ag
B3g
B2u
Ag
B3g
(B3g
)B
1uB
3gA
g
Raman shift / cm-1
PTCDA
200 400 600
B2uA
g
B3g
Ag
Ag
In15 nm
Inte
nsi
ty /c
ts m
W-1s-1
0.0025Ag
B2g
GaAs
Dietrich RT Zahn, TU Chemnitz
800 1000 1200 1400 1600 1800 2000
1.0
1.4
1.6
C-H,
C-O
C-H
R/R
subs
trat
e
Wavenumber/ cm-1
In(15nm)/PTCDA(15 nm)
PTCDA(130 nm)
C=C
C=OC-H
C-C,
C-O
C-H
(z)
Influence of Indium on Vibrational Spectra of PTCDA
• 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 !
• organic films grown on S-GaAs(100):2x1
• reflection measurements at 20° incidence.
Dietrich RT Zahn, TU Chemnitz
Indium/PTCDA: Separation of Chemical
and Structural Properties
1200 1350 1500 1650
Inte
nsity
/ ct
s m
W-1s-1 0.03
PTCDA
PTCDA(15 nm)
Raman shift / cm-11350 1500 1650
PTCDA(0.4 nm)
0.001
x 0.017+ Inx0.045
In: 0 100 nm
In: 1 nm/min
PTCDA~0.4 nm(~1 ML)
S-GaAs(100)
~15 nm(~50ML)
PTCDA
S-GaAs(100)
Dietrich RT Zahn, TU Chemnitz
S-GaAs(100)
In: 1 nm/min Ag:1.6 ÷ 5.5 nm/min
Comparison of Indium and Silver Deposition on PTCDA and DiMe-PTCDI
1200 1400 1600
0.03
In
tens
ity /
cts
mW
-1s-1
Raman shift / cm-11200 1400 1600
0.01
DiMe-PTCDI (15 nm)
PTCDA(15 nm)
S-GaAs(100)
+ In+ Ag
Dietrich RT Zahn, TU Chemnitz
Comparison of Indium and Silver Deposition on PTCDA and DiMe-PTCDI
• 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.
100 200 100 200
0.4 nm In
PTCDA
x 4
0.4 nm In
DiMe-PTCDI
/ 2
0.3 nm Ag
Inte
nsity
/a.u
.
Raman shift / cm-1
/ 2
0.4 nm Ag
Dietrich RT Zahn, TU Chemnitz
1300 1400 1500 1600
/5
/10
+ Mg
PTCDA
+ In
+ Ag
Inte
nsi
ty /
cts.
mW
-1s-1
Raman shift / cm-1
200 300 400 500 600
0.03
0.01
PTCDA (15 nm)Mg, In, Ag on PTCDA
Dietrich RT Zahn, TU Chemnitz
200 300 400 500 600
1300 1400 1500 1600
Inte
nsi
ty
Raman shift / cm-1
5x10-2
cts.mW-1s-1
5x10-3
cts mW-1
s-1
DiMe-PTCDI (15 nm)
+ In
DiMe-PTCDI
+ In
+ Ag
Mg, In, Ag on DiMe-PTCDI
+Mg
Dietrich RT Zahn, TU Chemnitz
PTCDA (15 mn) DiMe-PTCDI (15 nm)
Indium and Silver Deposition:Enhancement Factors
0 10 20 30 401
10
100
Rel
ativ
e A
rea
In thickness / nm
0 10 20 30 40
1
10
100
1236 cm-1
1568 cm-1
1610 cm-1
Ag Thickness / nm
0 10 20 30 401
10
100
1000
In thickness / nm
Rel
ativ
e A
rea
0 10 20 30 40
1
10
100
1244 cm-1
1570 cm-1
1615 cm-1
Ag Thickness / nm
Dietrich RT Zahn, TU Chemnitz
Determination of Molecular Orientation:Determination of Molecular Orientation:DiMe-PTCDIDiMe-PTCDI
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.
=0°: x II [011]GaAs
=90°:x II [0-11]
phononsphonons phononsphonons
Dietrich RT Zahn, TU Chemnitz
Determination of Molecular Orientation:Determination of Molecular Orientation:DiMe-PTCDIDiMe-PTCDI
yx
xx
IDep =
I
56 4 ;
,
g
-1g
m= R , ,A ,A, R , , Good agreement with IR and NEXAFS results
s igAI = e e
0 60 120 180 240 300 3600.0
0.5
1.0
1.5
2.0
2.5
De
po
lari
zatio
n R
atio
/ a.u
.
Experimental angle ()/°
BreathingBreathing mode at 221 cm mode at 221 cm-1-1
Dietrich RT Zahn, TU Chemnitz
Molecular Orientation with respect to GaAs substrate:
PTCDA: ~ 9°
Dietrich RT Zahn, TU Chemnitz
DiMe-PTCDI: ~ 6° ~ 60°
[-110][-110]
Dietrich RT Zahn, TU Chemnitz
Interface reactions Internal Modes:
Shifts, Intensities
Film thickness Intensity modulations
Crystalline
Order
Growth Mode Intensity modulations
Crystallinity Occurrence of Phonon-like Modes, FWHM
Crystal modifications Phonons,
Davydov Splitting of Internal Modes
Orientation Further investigations
Raman Characterization of Organic Thin Films:
Achievements and Outlook
Dietrich RT Zahn, TU Chemnitz
Raman Spectroscopy Team: