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Probing Single Molecules with a Tunable Femtosecond Laser Coupled RF-STM

  • Author(s): Cao, Weicai
  • Advisor(s): Ho, Wilson;
  • Mecartney, Martha L
  • et al.

Scanning Tunneling Microscope (STM) has become a powerful tool in nanoscience for imaging, manipulation and electronic spectroscopy. STM inelastic electron tunneling spectroscopy (IETS) first achieved chemical identification of molecular species by characterizing vibrational energies. Recently, with the STM itProbe and H2 rotational spectromicroscopy, molecular structure and chemical bonds are observed with the STM. Despite these successes in spatial resolution, various efforts have been made to combine fs laser with STM to overcome the temporal resolution limitation of STM, there is so far no clear evidence of simultaneous fs and Å resolution.

Electronic properties of organic molecules are of central importance to applications such as molecular electronics, organic LEDs, and solar cells. Properties of these molecules can be probed by the scanning tunneling microscope (STM) at the single molecule level and with sub-Å spatial resolution. The molecular orbital of 4, 7-Di ([2, 20-bithiophen]-5-yl) benzo[c] [1, 2, 5] thiadiazole (4T-BTD) with intramolecular donor-acceptor-donor sites is probed with the electronic state dI/dV imaging and H2 rotational and vibrational spectromicroscopy. 1, 4-Phenylene Diisocyanide (PDI) is probed by imaging with a CO-terminated tip and H2. PDI can self-assemble on noble metal surfaces to form nanostructures, which could have potential applications in molecular electronics and catalysis.

Further combination of a RF-STM with a tunable femtosecond laser enables the investigation of light-molecule interactions. In this dissertation, efforts are spent to setup a new tunable fs laser (220 nm~1040 nm) to couple with the RF-STM. The effects of the femtosecond laser are followed by detecting photo induced electron emission and photochemistry. A new double lock-in technique is applied to detect the weak laser-induced signal in the tunneling regime. To sharpen the energy width and increase the lifetime of the excited states of molecules, thin aluminum oxide and copper oxide are grown on metal surfaces to provide electronic isolation of the metal substrate and adsorbed molecules. Metal nanoclusters are grown on metal and oxide to improve laser-induced signal through plasmonic enhancements.

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