High Pressure UV Raman Spectroscopy of Carbon Materials

1. High Pressure UV Raman Spectroscopy of Carbon Materials Thomas Fitzgibbons, John Badding Department of Chemistry Penn State University Reinhard Boehler Geophysical Laboratory How do we design and perfect atom- and energy-efficient syntheses of revolutionary new forms of matter with tailored properties? How do we characterize and control matter away – especially very far away – from equilibrium?

2. Carbon: Unsolved Problems Transparent Carbon in Diamond Anvil Cell 18 GPa 0 GPA, <70 K 0 GPa, 298 K Badding, J.V., Lueking, A.L., Reversible High Pressure sp2-sp3 Transformations in Carbon, Phase Transitions, 80, 1033 (2007) Miller, E. D., Nesting, D. C. & Badding, J. V. Quenchable Transparent Phase of Carbon. Chemistry of Materials 9, 18 (1997). WL Mao, HK Mao, PJ Eng, TP Trainor, M Newville, CC Kao, DL Heinz, JF Shu, Y Meng, RJ Hemley, Science 302, 425 (2003). Details of crystal structure not yet definitively established.

3. 514 nm Visible Raman Unsatisfactory Miller, E. D., Nesting, D. C. & Badding, J. V. Quenchable Transparent Phase of Carbon. Chemistry of Materials 9, 18 (1997). No evidence for sp3 bonding: cross section for sp2 bonding 50 to 100 times higher than sp3 with visible excitation.

4. Hydrogen Storage Implications? • Hydrogen storage requires reversible binding to carbon • A better knowledge of reversible sp2/sp3 carbon transitions may help us to design better hydrogen storage materials. Badding, J. & Lueking, A., Reversible high pressure sp2-sp3 transformations in carbon. Phase Transitions 80 (2007).

5. Crushing C60 to Diamond at Room Temperature Regueiro, M., Monceau, P., & Hodeau, J., Crushing C60 To Diamond At Room-temperature. Nature 355 (6357), 237-239 (1992). Ravindran, T. & Badding, J., Ultraviolet Raman analysis of the formation of diamond from C60. Solid State Communications 121 (6-7), 391-393 (2002). 257 nm DUV Raman Spectrum How does diamond form from non-hydrostatically compressed C60?

6. Advantages of MultiwavelengthDUV/Visible Raman Casiraghi, C., Ferrari, A., & Robertson, J., Raman spectroscopy of hydrogenated amorphous carbons. Phys Rev B 72 (8), 085401 (2005). DUV Raman: freedom from fluoresence in carbon materials and more equal cross section for sp3 carbon vs. sp2 carbon.

7. Soft Polymeric a-C:H from Compressed Benzene Raman with DUV 257 nm excitation on sample recovered after compression to 30 GPa Jackson, B., Trout, C., & Badding, J., UV Raman analysis of the C: H network formed by compression of benzene. Chem. Mater 15 (9), 1820-1824 (2003). Visible Raman spectrum is not visible due to background.

8. Higher Order Diamond Modes 244 nm excitation 514 nm excitation Synthetic diamond Resonance enhancement of sp3 bonding increases as gap is approached (226 nm).

9. Sapphire Anvils are Attractive Xu, J., Mao, H., & Hemley, R., The gem anvil cell: high-pressure behaviour of diamond and related materials. J Phys-Condens Mat 14 (44), 11549-11552 (2002). Sapphire: good UV transparency in principle. Diamond: issues with sp3 carbon mode overlap.

10. Challenge: Sapphire Impurities Backscattering geometry: substantial impurity fluoresence.

11. Sapphire Background Varies

12. Plans • Improve Raman spatial filtering by using 45 degree excitation • Characterize sapphire anvil UV fluoresence and impurities • Increase maximum pressure of sapphire anvil cell. • Build improved UV Raman microscope • Investigate transparent phase of carbon, high pressure collapse of C60 to diamond • Investigate high pressure carbon UV photochemistry: photoactivated vs thermal processes