| By combining scanning tunneling microscope (STM) with optical excitation, the laser-assisted STM envisions the possibility of investigating photon-excited states at the atomic spatial resolution. Such novel characterization technique is expected to help researchers acquire new information from functionalized materials such as graphene. Although significant progress has been made after years of exploration, challenging problems still exist and application of laser-assisted STM is in great need. One of the critical factors that influence the development progress is the thermal effect under the laser illumination. In addition, upgrading and modification of electronics control system are necessary to incorporate the operation of laser excitation system.This study focuses on the development and application of a low-temperature ultrafast laser assisted scanning tunneling microscope. The main content includes the fabrication of electronics for STM control and data acquisition; investigation of thermal expansion of STM tip under ultrafast laser pulse illumination; and exploration of novel properties of graphene on copper surface, with detailed characterization in its micro-structure and electronic states. The thesis is divided into three main parts as follows:1. Design and fabrication of ultrafast laser assisted STM. The STM data acquisition and control system has been built, and home-built16-bit AD/DA conversion electronics has been fabricated and tested. Digital signal processor (DSP) driver for AD/DA(Analog to Digital and Digital to Analog) communication has been programmed and tested in the CCS development environment. The normal error and noise in AD/DA output as demonstrated is less than lmV, meets the design objective and the system is expected to satisfy the operation requirements. The new STM control system based on such core circuits is expected to significantly improve its compatibility and scalability, achieving high efficient coupling and synchronization with laser excitation. 2. Thermal expansion of STM tip under the ultrafast laser illumination. By using ultrafast lasers at various wavelengths (177nm-800nm), we have investigated the dynamics behavior of a STM tip, as well as the theoretical simulations. It has been illustrated that the amplitude of tip expansion is proportional to the incidence power, and the thermal diffusion length at the cut-off frequency has a linear dependence on the illumination spot. Due to the plasmon excitation in the tip-sample junction, tip expansion under illumination of visible laser is much more significant than that under the near-infrared and deep ultraviolet laser. It is concluded that the thermal effect can be significantly reduced by employing the laser source with minimal plasmon excitation, such as deep ultraviolet laser, suggesting a new possible direction for future development in the field.3. Atomic-scale investigation of graphene on copper surface. By using low-temperature STM, we have studied the structure and electronic property of graphene on copper surface. A new maze-like reconstruction in graphene on copper surface has been observed. Resolved with functionalized STM tips, the reconstruction pattern is composed of a "three-for-six" triangular lattice and an orthogonal lattice, both of which are successfully visualized. The presence of partial sp3hybridization in the triangular lattice is verified by the appearance of a midgap state near the Fermi level in the local electronic density of states. The orthogonal lattice is illustrated as a mixed (2(?)2×(?)2)R45°reconstruction of Cu(100) surface as supported by X-ray diffraction data. Our results suggest a relatively strong interaction can exist between graphene and copper surface, results in partial sp3hybridization in graphene, and thus tuning of its electronic properties. |
PDF Full Text Request