Single molecule optical absorption at room temperature detected by scanning tunneling microscopy

The high spatial resolution of the scanning tunneling microscope makes it a powerful tool to investigate single molecules deposited on a variety of conductive or semi-conductive surfaces. By adding laser absorption, we are able to simultaneously examine single molecules with high spatial and high energy resolution. Our method of single molecule absorption detected by scanning tunneling microscopy (SMA-STM) relies on backside illumination to cut down on tip heating effects. The evanescent wave of a laser undergoing total internal reflection nearly saturates excitation of molecules on the surface, thus changing the net local density of states enough for STM detection. The excitation laser is amplitude modulated, allowing for simultaneous detection of the STM current (image) and its derivative (absorption signal) by a lock-in amplifier. Although this approach for the most part overcomes the junction heating effects, a new problem arises. It is no longer possible to use arbitrary substrates - apart from being atomically flat and conductive they must also be optically transparent at the wavelength of excitation. Previous studies involved E11 absorption spectroscopy of carbon nanotubes (CNT) on silicon substrates. For further studies of SMA we have chosen molecules with a more defined absorption band in the visible region: organic fluorophores and quantum dots, as well as higher excited states of CNTs. 15 nm thick platinum gold hybrid films deposited by electron beam evaporation onto c-plane sapphire substrates serve as substrates. Low resistance, sufficient light transmission and atomically flat island surfaces make these strong candidates for optical experiments. As expected, SMA-STM performed on quantum dots and carbon nanotubes deposited by dry contact transfer onto a Pt-Au film, resulted in a strong, phase dependent absorption signal. Results from a collaboration with a theoretical group at the University of Washington aid in the explanation of the observed shapes of excited electron density in PbS quantum dots Stark-shifted so that different electronic states contribute to the absorption signal. Semiconducting-to-metallic transitions in CNTs have been imaged and identified directly by SMA-STM, and also characterized by I-V curves. Finally the synthesis, optical and surface characterization of a dendron functionalized with two green donor dyes (Cy3) and one red acceptor dye (Cy5) through flexible linkers, that will be used in the next generation of SMA-STM experiments on metal films, is presented. Single-molecule experiments based on Förster resonant energy transfer (FRET) or on single molecule absorption spectroscopy (SMA) are now capable of studying energy funneling, exciton blockade, singlet fission, and a variety of other processes that involve multiple photoactive groups interacting on a single molecular backbone. Characterization of the dendron and of control molecules with fewer donors or no acceptor by ensemble absorption and emission spectroscopy shows that the system is capable of light harvesting, producing an intramolecular FRET signal from the acceptor greater than expected from a single donor. Additionally, intramolecular energy transfer upon UV excitation of the conjugated backbone is investigated. The photophysical behavior of this light harvesting dendron can be rationalized by a simple Förster/superexchange model. Simulations and scanning tunneling microscopy of single dendron molecules show that the dyes can fold over onto the dendron, creating a heterogeneous distribution of conformations suitable for single molecule studies of light harvesting