Hou Jianguo, Yang Jinlong, and Zhu Qingshi
School of Science, USTC
The recent rapid advances in nanotechnology and single molecule science are due in large part to the newly acquired tools in measuring and manipulating nanostructures, even in atomic and molecular scales. Scanning probe microscopy, especially scanning tunneling microscopy (STM) has fundamentally changed the way that we deal with the nano-world by its abilities of "seeing" and "moving" atoms and individual molecules directly. Concomitant developments in experiments and theories have allowed an extended range of nanostructures to be studied, and have demonstrated exciting physical, chemical, mechanical and electronic phenomena. On the basis of study identifying the orientation and local density of states (LDOS) of individual C60 molecules on Si(111) surface in 1999, and by using the ultra-high vacuum low-temperature STM technology, the research group led by Professor Hou Jianguo, Yang Jinlong and Zhu Qingshi, have further made the following significant progress and breakthrough in the field of the characterization and manipulation of single molecules:
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1. Imaging the native cage structure of single C60 molecules on inert alkanethiol self-assembly monolayers (SAMs), the research group has observed the difference of brightness between the stripes corresponding to C-C single and double bond (Fig. 1(a)). At the same time, a novel C60 domain structure of molecular orientations is found (Fig. 1(b)), where the correlation function of the molecular orientation within a domain is constant anywhere, but changes abruptly at domain boundaries, i.e. both the positional and bond-orientational orders are preserved across domain boundaries. This work has been selected as one of the top 10 achievements of science and technology of China in 2001.
Fig. 1
2. The negative differential-resistance (NDR) molecular device is realized involving two C60 molecules, one is adsorbed on the STM tip and the other is on the surface of alkanethiol SAMs (Fig. 2). The narrow LDOS features near the Fermi energy of the C60 molecules lead to the obvious NDR effect. Such controllable tunneling structures and the associated known electronic states ensure the stability and reproducibility of the NDR device.
Fig. 2
3. The insulating alkanethiol SAMs chemisorbed on Au(111) substrates has been investigated by STM. The images show clear intramolecular patterns, which are voltage- and site-dependent (Fig. 3(a)). Theoretical simulations using the density functional theory reproduce quantitatively the experimental STM images (Fig. 3(b)), and the orientational configuration can be identified (Fig. 3(c)).
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