1. Spectroscopy of Hybrid Inorganic/Organic Interfaces Vibrational Spectroscopy
Dietrich RT Zahn

2. The Application of Raman Spectroscopy in the DIODE Project
The Overall Device Performance
(iv) The Interface between the Organic Molecules and the Metal
GaAs(100)
Organic Interlayer
Metal V I
(iii) The Organic Molecular Film
(ii) The Interface between GaAs Substrate and Organic Molecules
(i) The GaAs Substrate Surface

3. Molecular Vibrational Properties
DiMe-PTCDI: 3,4,9,10- Perylenetetracarboxylic diImide
PTCDA: 3,4,9,10- Perylenetetracarboxylic diAnhydride
C24H8O6
C26H14O4N2
Symmetry D2h
Raman active: 19Ag+18B1g+10B2g+7B3g
IR active: B1u+18B2u+18B3u
Silent: Au
108 internal vibrations
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.

4. Raman-active vibrations
of PTCDA (C24H8O6): Effect of crystal formation
2-fold
Davydov Splitting
internal molecular modes: external molecular modes (phonons):
C- C- O Bg
C-H
C-C
Symmetry: D2h C2h (monoclinic)
6 rotational vibrations: 3Ag+3Bg
19Ag+18B1g+10B2g+7B3g Ag

5. Vibration modes of PTCDA molecule
C-O Bg
C-H
C-C
19 Ag breathing modes
very good agreement between experimental and calculated frequencies !

6. Raman Spectra of a PTCDA Crystal
x0.1
assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP, 3-21G)

7. Ag Raman Modes of PTCDA with In
x0.1

8. Raman Spectra of a PTCDA Crystal
x0.5
C=O
ring
C-H
and a DiMe-PTCDI
x0.1
assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).

9. Raman Spectra of a PTCDA Crystal
and a DiMe-PTCDI
assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).
Raman shift /cm-1

10. Raman-active vibrations of PTCDA: Effect of crystal formation
external molecular modes (phonons): 6 rotational vibrations: 3Ag+3Bg
Symmetry: C2h (monoclinic) Bg Ag Bg

11. Infrared Modes in Films on S-GaAs
C-H+
C-N-C
Reflection, s-polarized light.
C=O
C-O+
C-C
ring
C-O-C
C-H
(oop)
Assignment of modes using Gaussian `98 (B3LYP, 3-21G).

12. Sample Preparation Epi-ready GaAs (100) Degreasing
Acetone, Ethanol, Di-Water
OMBD deposition:
PTCDA, DiMe-PTCDI
Thickness: 0.1 nm ÷15 nm
Wet Chemical Treatment
S2Cl2:CCl4=1:3 (10 sec)
Rinsing
(CCl4, Acetone, Ethanol, Di-Water)
Metal deposition:
Ag, In
Thickness: 0.1 nm ÷260 nm
Annealing at 620 K, 30 min
S-GaAs(100):2x1

13. Ex Situ (Micro-) and In Situ (Macro- Configuration) Raman Spectroscopy
Ar+ line
Dilor XY 800 Spectrometer
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.

14. Monitoring of PTCDA Film Growth on S-GaAs
E = 2.54 eV
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.

15. Chemistry at Organic/S-GaAs(100):2x1 Vibrational Properties:
PTCDA
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.
0.45 nm
(x 0.6)
0.18 nm
ann.
x 4.4
40 nm
x 0.01

16. Calculated Vibrational Properties: PTCDA

17. Calculated Vibrational Properties: PTCDA
Molecular charging with one elementary charge:
significant spectral changes predicted for the C=C modes around 1600 cm-1
fractional charge transfer between the PTCDA and the defects at the GaAs surface.

18. In Situ Raman: Monitoring of Indium Deposition onto PTCDA (15 nm)
/28
/10
/58
/13
/33 /5
/0.7
/1.5

19. Influence of Indium on Vibrational Spectra of PTCDA

20. Influence of Indium on Vibrational Spectra of PTCDA
organic films grown on S-GaAs(100):2x1
reflection measurements at 20° incidence.
all PTCDA modes are preserved in the spectrum of In/PTCDA.
observation of C=O modes (around cm-1)
In does not react with the O of PTCDA !

21. Indium/PTCDA: Separation of Chemical and Structural Properties
In: 0 100 nm
In: 1 nm/min
PTCDA
~0.4 nm
(~1 ML)
S-GaAs(100)
~15 nm
(~50ML)

22. Comparison of Indium and Silver Deposition on PTCDA and DiMe-PTCDI
In: 1 nm/min
Ag:1.6 ÷ 5.5 nm/min

23. 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.

24. Mg, In, Ag on PTCDA

25. Mg, In, Ag on DiMe-PTCDI
+Mg

26. Indium and Silver Deposition:
Enhancement Factors
PTCDA (15 mn)
DiMe-PTCDI (15 nm)

27. Determination of Molecular Orientation: DiMe-PTCDI
=0°: x II [011]GaAs
=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.

28. Determination of Molecular Orientation: DiMe-PTCDI
Breathing mode at 221 cm-1
Good agreement with IR and NEXAFS results

29. Molecular Orientation with respect to GaAs substrate:
PTCDA:  ~ 9° 

30. 
[-110] 
DiMe-PTCDI:
 ~ 6°
 ~ 60°

31. Raman Characterization of Organic Thin Films: Achievements and Outlook
Interface reactions
Internal Modes:
Shifts, Intensities
Film thickness
Intensity modulations
Crystalline
Order
Growth Mode
Crystallinity
Occurrence of Phonon-like Modes, FWHM
Crystal modifications
Phonons,
Davydov Splitting of Internal Modes
Orientation
Further investigations

32. Raman Spectroscopy Team: