Niv Levy

Measuring reversible photomechanical switching rates for a molecule at a surface

We have used single-molecule-resolved scanning tunneling microscopy to measure the photomechanical switching rates of azobenzene-derived molecules at a gold surface during exposure to UV and visible light. This enables the direct determination of both the forward and reverse photoswitching cross sections for surface-mounted molecules at different wavelengths. In a dramatic departure from molecular behavior in solution-based environments, visible light does not efficiently reverse the reaction for azobenzene-derived molecules at a gold surface.

Self-patterned molecular photoswitching in nanoscale surface assemblies

Photomechanical switching (photoisomerization) of molecules at a surface is found to strongly depend on molecule-molecule interactions and molecule-surface orientation. Scanning tunneling microscopy was used to image photoswitching behavior in the single-molecule limit of tetra-tert-butyl-azobenzene molecules adsorbed onto Au(111) at 30 K. Photoswitching behavior varied strongly with surface molecular island structure, and self-patterned stripes of switching and nonswitching regions were observed having approximately 10 nm pitch. These findings can be summarized into photoswitching selection rules that highlight the important role played by a molecules nanoscale environment in determining its switching properties.

Electric Field Tuning of the Surface Band Structure of Topological Insulator Sb2Te3 Thin Films

We measured the response of the surface state spectrum of epitaxial Sb(2)Te(3) thin films to applied gate electric fields by low temperature scanning tunneling microscopy. The gate dependent shift of the Fermi level and the screening effect from bulk carriers vary as a function of film thickness. We observed a gap opening at the Dirac point for films thinner than four quintuple layers, due to the coupling of the top and bottom surfaces. Moreover, the top surface state band gap of the three quintuple layer films was found to be tunable by a back gate, indicating the possibility of observing a topological phase transition in this system. Our results are well explained by an effective model of 3D topological insulator thin films with structure inversion asymmetry, indicating that three quintuple layer Sb(2)Te(3) films are topologically nontrivial and belong to the quantum spin Hall insulator class.

Scanning Tunneling Microscopy of Gate Tunable Topological Insulator Bi2Se3 Thin Films

Electrical-field control of the carrier density of topological insulators (TIs) has greatly expanded the possible practical use of these materials. However, the combination of low-temperature local probe studies and a gate tunable TI device remains challenging. We have overcome this limitation by scanning tunneling microscopy and spectroscopy measurements on in situ molecular-beam epitaxy grown Bi2Se3 films on SrTiO3 substrates with prepatterned electrodes. Using this gating method, we are able to tune the Fermi level of the top surface states within a range of ≈250 meV on a 3-nm-thick Bi2Se3 device. We report field effect studies of the surface-state dispersion, band gap, and electronic structure at the Fermi level.

Single-molecule-resolved structural changes induced by temperature and light in surface-bound organometallic molecules designed for energy storage

We have used scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory calculations to investigate thermal and photoinduced structural transitions in (fulvalene)tetracarbonyldiruthenium molecules (designed for light energy storage) on a Au(111) surface. We find that both the parent complex and the photoisomer exhibit striking thermally induced structural phase changes on Au(111), which we attribute to the loss of carbonyl ligands from the organometallic molecules. Density functional theory calculations support this conclusion. We observe that UV exposure leads to pronounced structural change only in the parent complex, indicative of a photoisomerization reaction.

Surface anchoring and dynamics of thiolated azobenzene molecules on Au(111)

We have investigated the temperature-dependent behavior of thiolated azobenzene molecules on Au(111) using scanning tunneling microscopy. The addition of a thiol functional group to azobenzene molecules leads to increased surface anchoring of single azobenzene molecules to gold. Thiolated azobenzene shows diverse surface morphology and does not form well-ordered structures at low coverage. At elevated temperatures, anchored molecules are observed to spin in place via hindered rotation. By measuring the number of rotating molecules as a function of temperature and using a simple model, we are able to estimate the energy barrier and attempt frequency for thermally induced hindered rotation to be 102+/-3 meV and 110+/-2 GHz, respectively.