1. Electron-phonon coupling in alpha-hexathiophene single crystals
Resonant Raman Measurements of an Organic Semiconducting Single Crystal
Electron-phonon coupling in
alpha-hexathiophene single crystals
Jennifer Weinberg-Wolf
Department of Physics and Astronomy
University of North Carolina at Chapel Hill
SESAPS Conference: November 2003

2. Why Organics? Cheap(er) Easily Processable Environmentally Friendly
Flexible
Chemically tailor molecules for desired physical properties (emission energy, melting point, etc.)
Some materials used:
Oligoacenes, Oligothiophenes, Polyphenylene Vinylene (PPV), etc.
Devices made so far:
OFETS, OLEDS, Photovoltaic devices, etc.
J.W.W SESAPS 2003

3. Organic Molecular Crystals
Pi-conjugated materials
Energy transitions in the visual wavelengths
Tunable for applications
High stability
Characterized by weak van der Waals intermolecular bonds (10-2 – 10-3 eV) and strong covalent intramolecular bonds (2-3 eV)
Retain many of the molecular characteristics in the solid state
Primary photoexcitations are Frenkel excitons
J.W.W SESAPS 2003

4. Alpha-Hexathiophene (6-aT)b ˆ
Macroscopic single crystals from Lucent Technologies
Most previous studies by other groups done with polycrystalline thin films
Typical Scale  mm
Monoclinic crystal
C2h point group
4 molecules per unit cell
Close packed/herringbone arrangement
Rigid Rod with <1° deviation from a plane
~2.2 eV band gap
J.W.W SESAPS 2003
PRB , 1999.

5. Raman Spectroscopy
Inelastic scattering process that measures vibrational energies
Non-invasive, non-destructive tool to probe phonon modes, electronic structure and the coupling of the e--phonon states
J.W.W SESAPS 2003

6. Raman Spectroscopy Instrumentation
Excitation source
Coherent Ar+ pump laser
Continuously tunable dye laser
Rhodamine 6G dye that lases from 640 to 590 nm (1.94 to 2.1 eV)
Spectrometer
Dilor XY Triple monochromator
High rejection ratio
High resolution (1 cm-1)
Detector
LN2 cooled CCD
Detector
Spectrometer
Sample
Dye
Laser
Ar+ laser
J.W.W SESAPS 2003

7. Raman Spectrum of α6T at 300K lexe=607nm (2.043 eV)
C-C stretching modes
In-plane C-S-C bending
Intermolecular
vibration
In-plane C-C-H bending
J.W.W SESAPS 2003

8. Resonant Raman Spectroscopy
Vary excitation energy (with dye laser) to approach an electronic excitation
Some electronic excitations can couple to vibrational modes
Excitations must have the same symmetry to couple
End result is a large enhancement of a vibrational Raman mode
Can appear that “new” lines emerge from the noise level
J.W.W SESAPS 2003

9. Resonant Raman Spectroscopy at 33K
(b)
Raman Shift (cm -1 )
900
1000
1100
1200
1300
1400
1500
1600
Intensity (arb. units) *
Off Resonance ( l
exe
=602 nm, eV)
On Resonance (
= nm, eV)
: Resonant Lines
J.W.W SESAPS 2003

10. Exciton Identification
Resonance peaks at excitation energies of eV and eV.
Each peak has a FWHM of 2 meV.
J.W.W SESAPS 2003

11. Frenkel Excitons
Previously identified lowest singlet exciton (Frolov et al. 2001)
2.3 eV
Au symmetry
Claim we have the two Davydov components of the triplet exciton state associated with the previously measured singlet state
Symmetry Considerations
In centrosymmetric molecules (like 6-aT), all Raman modes have gerade type symmetry.
Coupling of electronic and vibrational modes can only occur if they have the same symmetry.
J.W.W SESAPS 2003

12. Frenkel Excitons cont. Energy Considerations
Plausible down-shift in energy for a triplet state
Other organic crystals have a shift of ~0.5 eV, here DES-T=0.23 eV
Order of magnitude of typical Davydov splits for triplet and singlet states
Singlet States
Typically ’s cm-1.
Measured splitting energy of 0.32 eV gives DED= 2580 cm-1.
Triplet States
DED for triplet states is approximately 10 cm-1.
Measured splitting energy of 2 meV gives DED=16 cm-1.
Or maybe two different binding locations for the previously recorded singlet excitation
Singlet binding energy of ~0.4 eV reported in literature.
J.W.W SESAPS 2003

13. Acknowledgments Dr. Laurie McNeil
Dr. Christian Kloc at Lucent Technologies
The rest of my group:
Mr. Eric Harley
Mr. Chris Lawyer
Mr. Kris Capella
Jonathan Miller
J.W.W SESAPS 2003

14. Temperature effects - ˆ b Explicit Effect Implicit Effect
First term: reflects change in phonon occupation numbers
Implicit Effect
Second term: reflects the change in interatomic spacing due to thermal expansion or contraction -
is the compressibility
Where
is the expansivity
and

15. Resonant Raman Spectroscopy
and : x
Electronic transition freq.
Photon frequency
Oscillator strength tensor
Width
Normal modes
Coupling of the electronic and phonon states
Electronic state has to have the same symmetry as the vibrational state
Large enhancement of the vibrational term, amounts to “new” lines appearing in the spectrum
Also can change the lineshape of the Raman signal (no longer symmetric Lorentzian distribution)