We are an experimental research group in condensed matter physics. We focus on the investigation of not only steady states but also various non-equilibrium states (e.g., electronic excited states, charge transfer dynamics, spin dynamics, etc) of single molecules, 2D and strongly correlated materials with simultaneous ultrahigh spatial resolution and ultrahigh temporal resolution (“ångström-femtosecond” level) by developing cutting-edge time-resolved scanning probe microscopes including scanning tunneling microscope (STM) and atomic force microscope (AFM).

1. Development of time-resolved and optical spectroscopy combined scanning probe microscopies

Non-equilibrium states of molecules or solids play crucial roles across a broad range of fields such as photovoltaics, chemical reactions, phase transition, etc. The detection and control of non-equilibrium states are extremely crucial for understanding and tuning many basic processes involving electron and energy transfer, but they remain a great challenge to date. Optical methods with the ultrafast laser have proven to be very powerful in detecting various non-equilibrium states, but they suffer from a poor spatial resolution which is about half the wavelength due to the optical diffraction limit. Scanning probe microscopies (SPM) such as the STM and AFM have the advantage of ultrahigh spatial resolution down to the atomic level. However, they are usually only accessible to the equilibrium ground states due to their poor temporal resolution (~us).

To break this constraint, we will develop cutting-edge SPM-based techniques with simultaneously high spatial resolution and high temporal resolution. Examples include the electric pump-probe STM/AFM and optical/Terahertz pump-probe STM/AFM. Besides, we are interested in combining the STM/AFM with various optical spectroscopy techniques such as electron/photon induced luminescence and tip-enhanced Raman spectroscopy, etc. These techniques will provide unprecedented information about the molecules and solids.

2. Ultrahigh-resolution imaging and spectroscopy of single molecules, 2D and strongly correlated materials

Nanometer or atomic scale disorders like vacancies, dopants, domain boundaries, edges, moiré patterns exist widely in materials. These disorders may heavily affect the electronic, optical, magnetic, and thermal properties of materials. Therefore, it is of great importance to reveal their role in these processes which, however, requires nanometer and even atomic spatial resolution. With the state of art STM and qPlus AFM techniques, we will focus on investigating, with atomic or even single-bond resolution, how various disorders affect the electronic as well as the optical properties of single molecules, 2D and strongly correlated materials such as 2D transition metal sulfides (TMDs), magic-angle graphene, high-temperature superconductivity, topological insulators, etc.

3. Probing the dynamics of various non-equilibrium states at the atomic scale

With the cutting-edge techniques mentioned above, we will study the dynamics of various non-equilibrium states with nanosecond or femtosecond temporal resolution and atomic spatial resolution directly in real space. Examples include the excited states of single molecules, spin dynamics of single atoms and molecules, carrier dynamics in 2D semiconductors, charge/spin/phonon/polaron phase transitions in materials such as superconductors, topological insulators, etc.