We are an experimental research group in condensed matter physics. Using the uniquely designed scanning probe microscopes, we investigate the vibrational, electronic, magnetic, and optical properties of single molecules and nano-structures, as well as the related ultrafast dynamics processes. The current fields of research include:

1. Construction of the optical scanning tunneling microscope (STM)

 

 

It is well known that the spatial resolution of the optical excitation and detection is limited by diffraction to about half of the optical wavelength (hundreds of nanometers). In our group, we are trying to incorporate various optical methods into STM, such as the weak light detector and the femto-second laser system.

 

By taking the advantages of the ultra-high spatial resolution of STM, we hope to defeat diffraction limited resolution of the optical methods, and study the optical properties and the ultra-fast dynamics processes of the single molecules and nano-structures down to atomic scale.

2. Single molecules on the surface

The Single molecules/atoms are the basic building blocks of all materials and devices, and have been attracting increasing interests due to their essential roles in nanoscience and nanotechnology. The investigation of single molecules/atoms allows the intrinsic properties of molecules to be probed, which are usually buried in the ensemble of molecules due to the average effects. Many basic problems of quantum mechanics can be directly touched and demonstrated in single molecules/atoms, such as spin excitation, electron-phonon coupling, formation and cleavage of the chemical bond, charge transfer, and so on. The study of single molecules/atoms not only implicates the understanding of quantum mechanics from new perspectives, but also opens up the possibility of engineering the quantum effect with atomic resolution. One of our important directions focuses on the physics and chemistry of single molecules on the surface:

 

 

 

Bond-selective chemistry in single functionalized molecules: we induced a sequence of targeted bond dissociation and formation steps in single thiol-based π-conjugated molecules (DSB-2S-2Ac) adsorbed on NiAl(110) surface. A and B: Schematics and STM topographies for different resultants at each dissociation step, respectively. C: Corresponding molecular orbitals of the resultants, measured by STM. D: dI/dV mappings taken at key locations (1, 2, 3, 4) on the molecules, as noted in B, showing the evolution of the molecular electronic structure.

3. 2D systems on the surfaces

Although single molecules/atoms provide ideal platforms for probing quantum mechanics, remarkably rich emergent behaviors can appear when individual atoms/molecules are brought into a spatially ordered lattice. Such collective quantum phenomenon can be very different from the case of single molecules/atoms and requires an understanding of the complex interplay between single-site effect, orbital ordering, and long-range coherence. Two-dimensional (2D) periodic systems prepared on the surfaces are peculiar in that they are accessible to various surface sensitive probes such as scanning tunneling microscope (STM), which can provide new insights into the 2D systems. Furthermore, the electron-electron correlation and quantum fluctuation are believed to be more significant in reduced spatial dimensions due to the quantum confinement effects. Therefore, 2D systems on the surfaces are expected to exhibit many-body effects that are not observed in three dimensions. We study different types of 2D systems on surfaces, such as 2D metal, 2D semiconductor, and 2D strongly correlated system.

 

 

2D molecule-based Kondo lattice: we imaged in the real space the Kondo resonance in a 2D Kondo lattice formed by self-assembled O2 molecules, which are paramagnetic, on the Au (110)-1´2 reconstructed surface. The spatial interplay between the intersite coupling and the onsite Kondo effect was revealed. A: STM topography of the 2D O2 lattice. B: The I, dI/dV, and d2I/dV2 spectra taken on the O2 lattice, showing a prominent Kondo resonance. C: Spatial distribution of the Kondo resonance in the O2 lattice.

4. Atomic engineering of photon emission from low-dimensional nanostructures

 

 

Along with the rapid development of the microelectronic technique, the properties of future nanoscale optoelectronic devices will be mainly determined by the microscopic structures and the local environment. It is thus imperative to be able to characterize and control the optical properties in low-dimensional nanostructures at the atomic scale, such as electronic excitation via charge injection and conversion of this excitation into a photon. Due to the highly localized tunneling current, photon emission excited by the tunneling current of a STM shows atomic-scale spatial resolution, and spectroscopy of this emission can be a useful tool for characterize the electroluminescence in low-dimensional nanostructures. Meanwhile, single atom/molecule manipulation with a STM tip is an innovative experimental technique of nanoscience, which makes STM very desirable for achieving optical control at the atomic scale. By combining the atomic-scale manipulation and the tip-induced light emission of STM, we aim at controlling and tuning the optical properties of various low-dimensional nanostructures at atomic scale.

5. Nanoscale ultra-fast dynamics

 

 

The understanding and control of quantum dynamics, such as carrier transport and relaxation of the hot electrons in single molecule and nano-structures, are key factors for continuing the advancement of nanoscience and nanotechnology. Such dynamics process usually happen in time scale ranging from pico-second to femto-second, which can be analyzed by laser-based pump-probe technique. However, since the spatial resolution of the laser technique is limited by diffraction to about half of the optical wavelength, the pump-probe technique does not provide a sufficient description for the nanoscale ultrafast dynamics process.

 

Scanning tunneling microscopy (STM) has realized atomic-scale spatial resolution using the sensitivity of a tunneling current on as small a change in the tip-sample distance. The spatial resolution of STM in parallel to the sample surface reaches 1 Ångström and that perpendicular to the sample surface reaches 0.1 Ångström. Combining STM and ultra-fast laser is expected to achieve both Ångström spatial resolution and femtosecond temporal resolution simultaneously. Such femtosecond-Ångström technology can significantly advance our understanding of many elementary processes at surfaces. Our current interest is focused on the development of ultrafast scanning tunneling microscopy, by combining STM with a femto-second laser (<100fs). The time-resolved-atom-resolved microscopy will be very powerful to investigate various ultrafast dynamics processes in single molecule and nano-structures down to atomic scale, such as Coulomb blockade, Kondo effect, spin excitation, charge transfer, and so on.