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The goal of the studies presented in this dissertation is to continuously expand the capability of a scanning tunneling microscope (STM) by improving its chemical sensitivity and temporal resolution. It has been demonstrated that the combination of STM with other techniques gains insight into the physical and chemical properties of single molecules. Single molecule rotational spectroscopy and microscopy is demonstrated using STM inelastic tunneling spectroscopy (IETS). We conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on surfaces at 10 K. The j = 0 to j = 2 rotational transition for para-H2 and HD were observed by STM-IETS. It is also found that the rotational energy is very sensitive to its local environment, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Due to this high sensitivity, the spatial variation in the potential energy surface can be quantified by the rotational and vibrational energies of the trapped H2. The ability of the tip to drag along a hydrogen molecule as it scans over another adsorbed molecule combined with the sensitivity of the hydrogen rotational excitation recorded by IETS to its immediate environment lead to the implementation of rotational spectromicroscopy, which helps us reveal the intermolecular interaction and charge transfer between H2 and a large molecule. Furthermore, we demonstrate that the H 2 in the STM junction can be dissociated by the mechanical motion of the STM tip. Hydrogen rotational spectroscopy and microscopy provides novels approach toward visualizing and quantifying the local potential energy surface as well as the potential landscape of chemical reactions.Joint Angstrom-femtosecond resolution is achieved by the combination of an STM with a femtosecond laser. We demonstrate the bond-selected, photo-assisted activation of a single C-H bond in an azulene molecule adsorbed on a Ag(110) surface. When the junction is illuminated by femtosecond laser, the electrons in the tip can be photo-excited into higher energy states and dissociate the molecule through a photo-assisted tunneling process. The photon-electron coupling at the junction enables the investigation of coherence molecular transformation with joint fs-A resolution. We also show the band bending and laser induced band flattening at a molecule-semiconductor interface. More importantly, we observe the photo-induced, reversible conformational change between two structures for a single pyrrolidine molecule on a Cu(001) surface. The conductance changes of the STM junction associated with the structural transitions exhibits oscillates in time with periods corresponding to specific molecular vibrations. The vibrational frequencies and decay time are observed in real-space and real-time. Our laser-STM technique enables the investigation of inhomogeneous environmental effect on the molecular dynamics. We have found that the intermolecular interaction between two pyrrolidine molecules can increase the vibrational period while shortening its decay time. We anticipate that this novel technique would lead to a broad impact in physics and chemistry through direct visualization of coherently driven reactions resolved in space and time.
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.
Scanning tunneling microscopy (STM) has given the scientific community a method to view, characterize, and manipulate the world at the atomic scale. Thirty years after the Nobel Prize in Physics was awarded for its invention, the remarkable instrument is still being used to deepen our understanding of physical and chemical processes. Tantamount to this has been the development of new techniques to expand its capabilities allowing STMs to answer increasingly more difficult scientific questions. This dissertation describes three technological thrusts in expanding the STMs capabilities in studying physics at the single molecule level.First, I have helped developed a new technique called the RF-STM which has the potential to snapshot femtosecond and picosecond processes by locking into the high frequency tunneling component generated from the 80MHz laser pulse train. This technique solves the problem of low frequency thermal oscillations when choppers are used in the beam line and if only tunneling signal is monitored, sub-angstrom spatial resolution should be simultaneously possible.Second, I have helped develop the itProbe technique by increasing its ability to map out the interaction potential energy surface (iPES) between a tip-CO molecule and a surface adsorbed molecule. I present a study conducted on the bridge-like 1,4 phenylene diisocyanide molecule where the iPES is probed at different heights and different energies. The result is an ability to 3-dimensionally map out the iPES and provide reliable insight into developing itProbe simulations.Third, I have developed a new technique called Energy Resolved Laser Action STM (ERLA-STM) where we can observe the change in molecular dynamics as a function of the illumination wavelength. In our pyrrolidine study, we demonstrated the kinetic changes that occur when an overtone of the CH stretch mode is excited by a near-IR laser pulse. By sweeping the excitation energy, we can characterize and control single molecule switches for use in potential molecular electronics applications.All three approaches mentioned above are driven by the goal of understanding chemical processes at the atomic level. Such studies are integral to increasing our fundamental knowledge and providing technological foundations for further development.
Single Molecule Spectroscopy is one of the hottest topics in today's chemistry. It brings us close to the the most exciting vision generations of chemists have been dreaming of: To observe and examine single molecules! While most of chemistry deals with myriads of molecules, this books presents the latest developments for the detection and investigation of single entities. Written by internationally renowned authors, it is a thorough and comprehensive survey of current methods and their applications.
Scientists and engineers have long relied on the power of imaging techniques to help see objects invisible to the naked eye, and thus, to advance scientific knowledge. These experts are constantly pushing the limits of technology in pursuit of chemical imagingâ€"the ability to visualize molecular structures and chemical composition in time and space as actual events unfoldâ€"from the smallest dimension of a biological system to the widest expanse of a distant galaxy. Chemical imaging has a variety of applications for almost every facet of our daily lives, ranging from medical diagnosis and treatment to the study and design of material properties in new products. In addition to highlighting advances in chemical imaging that could have the greatest impact on critical problems in science and technology, Visualizing Chemistry reviews the current state of chemical imaging technology, identifies promising future developments and their applications, and suggests a research and educational agenda to enable breakthrough improvements.
The investigation and manipulation of matter on the atomic scale have been revolutionised by scanning tunnelling microscopy and related scanning probe techniques. This book is the first to provide a clear and comprehensive introduction to this subject. Beginning with the theoretical background of scanning tunnelling microscopy, the design and instrumentation of practical STM and associated systems are described in detail, as are the applications of these techniques in fields such as condensed matter physics, chemistry, biology, and nanotechnology. Containing 350 illustrations, and over 1200 references, this unique book represents an ideal introduction to the subject for final-year undergraduates in physics or materials science. It will also be invaluable to graduate students and researchers in any branch of science where scanning probe techniques are used.
The topics range from single molecule experiments in quantum optics and solid-state physics to analogous investigations in physical chemistry and biophysics.
Written by the leading experts in the field, this book describes the development and current state of the art in single molecule spectroscopy. The application of this technique, which started 1989, in physics, chemistry and biosciences is displayed.
Deals with both the ultrashort laser-pulse technology in the few- to mono-cycle region and the laser-surface-controlled scanning-tunneling microscopy (STM) extending into the spatiotemporal extreme technology. The former covers the theory of nonlinear pulse propagation beyond the slowly-varing-envelope approximation, the generation and active chirp compensation of ultrabroadband optical pulses, the amplitude and phase characterization of few- to mono-cycle pulses, and the feedback field control for the mono-cycle-like pulse generation. In addition, the wavelength-multiplex shaping of ultrabroadband pulses, and the carrier-phase measurement and control of few-cycle pulses are described. The latter covers the CW-laser-excitation STM, the femtosecond-time-resolved STM and atomic-level surface phenomena controlled by femtosecond pulses.