Tien-Tzou Tsong

Mechanisms and energetics of site hopping and chemical reactions of O2 molecules at Si(111)-7 × 7 surfaces

Abstract With continuous-time imaging using a variable-temperature scanning tunneling microscope (STM), we observe the dynamic behavior of a bright species, which we believe to be single O 2 molecules on Si(111)-7 × 7 surfaces. We have found the hopping motion of this molecular species between neighboring adatom sites, which is mediated by two intermediate states. A model is proposed to explain the detailed mechanism of the site hopping. Using Arrhenius plots, we determine the activation energies of different hopping paths and the relative binding energies of different states. We also observe the chemical reaction process of two adsorbed O 2 molecules to form an atomic species. Our results reveal that the hopping motion and chemical reactions of oxygen molecules depend strongly on the configuration of dangling bonds at the surface and the oxygen bonding. This study provides a direct view of molecular dynamics at a solid surface.

In situ tailoring and manipulation of carbon nanotubes.

Multiwalled carbon nanotubes (MWNTs), with their excellent properties, have long been considered as a model material for realizing the potential of nanotechnology. Several techniques have been developed to tailor and manipulate MWNTs, and they have shown promise in the construction of nanobalances, nanosensors, SPM probes, nanoelectronics, fuel cells, and so on. In order to achieve their full capability, MWNTs need to be modified in length, diameter, and shape to atomic-scale precision. Therefore, the synergy of various techniques and sequential operations in real time is often required. Here we report on an in situ method that combines several tailoring and manipulation techniques using an ultra-high-vacuum transmission electron microscope/scanning tunneling microscope (UHV TEM/STM) system. We demonstrate nanoscale precision in the engineering and fabricating of a MWNT with two examples; a probe for investigating single nanoparticles and a balance with singleatom mass resolution. We carried out our experiments in an ultra-high-vacuum transmission electron microscope (UHV-TEM, JEOL JEM2000V) combined with a scanning tunneling microscope (STM; built inhouse) riding on a nanopositioning system. The nanopositioning system provided coarse mechanical motions in three dimensions and the fine adjustments were performed with a piezo-tube scanner. The MWNTs (Alfa, 3–24 nm in diameter, stock #43197) were first attached to a gold knife edge electrode using the electrophoresis technique (applying a 6 MHz AC voltage of 8 V). The assembly was then loaded into the electron microscope. A pre-inserted gold STM tip was aligned and connected with a chosen MWNT under TEM observation at an operating voltage of 200 kV. The bottom of Figure 1a shows such an initial MWNT with 15 walls and about 1.87 nm inner diameter attached to the STM

Dynamical Study of Single Silver Atoms on Si(111)-7 ×7 Surfaces

The dynamics and interactions of silver atoms on Si(111)-7 ×7 surfaces are investigated using variable-temperature scanning tunneling microscopy (STM). The activation energies and pre-exponential factors upon the room temperature are estimated to be 0.81 and 0.9 eV, and 109.65 and 1010.68 for the hopping out of the faulted halves and for hopping out of unfaulted halves, respectively. The behavior exhibited by two silver atoms that jump into a single half-unit cell is also addressed. Details of the tracking of single silver atoms that hop within a 7 ×7 half-unit cell are reexamined at ~80 K using low-temperature STM. The analytical data revealed that silver atoms diffuse within a 7 ×7 unit cells via a corner adatom–rest atom–center adatom–rest atom–corner adatom diffusion pathway in a faulted half under the stress associated with 7 ×7 reconstruction.

Determining the Thickness of Pb Film Similar to Bulk with Energy Dispersion Derived from Quantum Well States

It is known that the energy spacing between adjacent empty quantum well (QW) states in Pb islands on Cu(111) would reveal the shrinking characteristic originating from the effect of the image potential. Using the phase accumulation model, including a phase factor contributed from the image potential, the shrinking energy spacing can be quantitatively explained with the assumption of the parabolic energy versus wave vector (E–k) dispersion. However, an experimental dispersion acquired from analyzing the energies of the QW state reveals a linear E–k relationship corresponding to the Pb bulk band structure, implying the assumed parabolic dispersion is not appropriate. By combining the linear dispersion with the image potential effect in the calculation, it is found that the calculated values of energy spacing of island thickness below eight atomic layers are not in agreement with the experimental measurements. This implies that the electronic structure of Pb islands would be similar to that of the bulk when their thicknesses reach eight-atomic layers. # 2013 The Japan Society of Applied Physics