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

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

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

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 59 10651, 1999.

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

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

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

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

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, 2.059 eV) On Resonance ( =599.43 nm, 2.0683 eV) : Resonant Lines J.W.W SESAPS 2003

Exciton Identification Resonance peaks at excitation energies of 2.066 eV and 2.068 eV. Each peak has a FWHM of 2 meV. J.W.W SESAPS 2003

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

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 100-1000’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

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

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

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)