Property and High-resolution Characterization of Special Nanostructures

    In nanometer scale, most discoveries of new functional materials come out from their structural characterization. The properties and behaviors of these materials are deeply related with their structures. So the structural characterization is a very important aspect in nano-material research. It is the basis of functional nano-device design and application of nano-materials. In the past years, continuously supported by NSFC, we got major achievements in the high-resolution characterization and manipulation of single molecules, and property characterization of nano-materials. More than 50 papers have been published in journals indexed in SCI, and three results have been applied for patent. We were invited to give talks in five international conferences and won a first class prize of CAS Natural Science Award. The main achievements are as follows:
Direct observation of C₆₀ cage structure and the discovery of 2D orientation domain
    We have developed a new method to directly observe the cage structure of a nearly free C₆₀ on alkylthiol self-assembled monolayer and distinguished the C-C bond and C=C bond of the C₆₀. This technology would have great potential on the exploration of the structure and property of other bulk molecules, even organic molecules. Using high-resolution scanning tunneling microscopy, we have observed that C₆₀ array on a self-assembled monolayer of alkylthiol forms an ideal two dimensional system which has another novel topological order originating from the orientational degrees of freedom. At the temperature of 5K, the two-dimensional C₆₀ forms a domain structure in which the correlation function of the molecular orientation within a domain is constant anywhere but changes abruptly at domain boundaries. Remarkably, the positional order and the bond-orientational order are both fully preserved across domain boundaries. The paper was published in Nature, and the refsxerees highly commented it "The paper by Hou et al. is a clear description of a well conceived and well executed experimental study…".

     Identifying molecular orientation and local electronic states of individual C60 molecules adsorbed on Si(111)-7×7 surface
By using low temperature scanning tunneling microscopy (STM) and combining with the local density approximation method with cluster models, we are the first in the world to identify molecular orientation and local electronic states of individual C60 on silicon surface at different adsorbed sites. This work was published in Phys. Rev. Lett., and was selected as one of the top ten news in basic science of China in 1999.

     Orientational configurations of the C60 molecules in the (2×2) superlattice on a solid C60 (111) surface at low temperature
    The orientational configurations of C60 molecules on a solid C60(111) were studied at 78 K by using UHV scanning tunneling microscope. A real space image of 2×2 superlattice due to the orientational ordering of the C60 molecules, was observed for the first time. This work was published in Phys. Rev. B.

    Formation of single molecule device using C60 and negative differential resistance (NDR The negative differential resistance (NDR) effect, which is characterized by the phenomenon of decreasing current with increasing voltage in the current-voltage (I-V)curves has wide applications in fast switches, oscillators and frequency-locking circuits. It has been shown experimentally and theoretically that atomic-scale NDR could arise in the tunneling structures with a scanning tunneling microscopy (STM) tip and localized surface site when the local density of states (LDOS) of both the tip and the surface site have adequately narrow features. We got the realization of the NDR molecular device involving two C60 molecules. This work was published in Appl. Phys. Lett., and was highly interested by the world academic community. We have been invited to write a review article about NDR molecular device for TRANS RESEARCH NETWORK.

Single-electron tunneling effect in a double-barrier tunnel junction (DBTJ) formed by a metal nano-cluster coupled two electrodes
    Single-electron tunneling (SET) effect is fundamental for the research in single-molecular device and nano-device design. By using scanning tunneling microscopy, we have studied the current-voltage characteristics of two special clusters: one is the two-dimensional Au cluster which is thermally deposited on self-assembled alkanethiol monolayer, the other is three-dimensional Au cluster which is ligand-stabilized by alkanethiol. The curves displayed Coulomb blockade and staircase with asymmetric behavior, and the quantum capacitance phenomena were observed . This work was published in Phys. Rev. Lett. and Appl. Phys. Lett.

    Novel nano-structure of C60-metal composite thin film
We have synthesized C60-metal composite thin films with special interface structure, and found a novel orthogonal structure of C60-metal composite and an interlayered structure of C60 lattice. Using the self-constraint growth effect of metal clusters, we got C60-metal nano-composite clusters with nearly mono-size distribution. Our work showed that the interaction across the interfaces between metal and C60 not only provides us a way to fabricate uniform and mono-dispersed nano-materials, but also gives a clue in the synthesis of novel metal-fullerene compounds. These works were published in Phys. Rev. B, Appl. Phys. Let., Adv. Mater., etc.

    Metal/ organism nano-composite multilayer films
We synthesized metal/organism nano-composite multilayer films from ligand-stabilized silver nanoparticles (LSSN) by the method of photochemical induction and experimentally investigated the electrophoretic deposition of LSSN. This work developed a new method of synthesizing metal/ organism multilayer films through liquid-phase procedure and was published in the Journal of Crystal Growth, etc.