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 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

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: +10B1u+18B2u+18B3u Silent: + 8Au 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.

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

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

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

Ag Raman Modes of PTCDA with In x0.1

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).

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

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

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).

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

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.

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.

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

Calculated Vibrational Properties: PTCDA

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.

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

Influence of Indium on Vibrational Spectra of PTCDA

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 1730-1770cm-1) In does not react with the O of PTCDA !

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)

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

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.

Mg, In, Ag on PTCDA

Mg, In, Ag on DiMe-PTCDI +Mg

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

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.

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

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

 [-110]  DiMe-PTCDI:  ~ 6°  ~ 60°

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

Raman Spectroscopy Team: