Studies of nanostructure fabrication and properties by low-temperature ultrahigh-vacuum scanning tunneling microscopy

A low-temperature (LT), ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been used to fabricate nanocrystals of T-TaSe{dollar}sb2{dollar} with sizes ranging from 7 nm to above 100 nm within the surface layers of 2H-TaSe{dollar}sb2{dollar} single crystals at both liquid N{dollar}sb2{dollar} and liquid He temperatures. We have shown that the formation of nanocrystals is through an STM tip-induced solid-solid phase transformation. Atomically resolved images of the interfaces of T-phase nanocrystals and H-phase surroundings have been used to develop an atomic model of this structural transformation which illustrates that the formation of T-phase nanocrystals involves the collective shift of the atoms in the topmost layer of the H-phase crystal.; The study of the modification voltage dependence of the nanocrystal formation reveals that the electric field generated from the STM tip apex plays an important role in this nanofabrication process. A mechanism of this solid-solid phase transformation was proposed to illustrate the growth of nanocrystals under the intense electric field generated by a STM tip. This proposal was supported by further experimental evidences, such as, the effect of the modification voltage polarity on the structure of the interface, the effect of the modification temperature on the probability of the nanocrystal formation and the correlation of the nanocrystal shape to the interface morphologies.; In addition, we have carried out extensive studies of the CDW physics in nanocrystals. We have probed the CDW nucleation pathway in nanocrystals at low temperatures, investigated the electronic properties of different CDW defects in nanocrystals, and evaluated the pinning strengths of the T/H interface to both the T- and H-phase CDW lattices. Furthermore, we have studied the size effect on the properties of CDW states in nanocrystals and the preliminary results have been interpreted by our proposed rough Fermi surface model.; Finally, we have applied the nanocrystal fabrication method to other layered materials. It has been demonstrated that the T-phase TaS{dollar}sb2{dollar} nanocrystals can be created by this method and the consistency of the characterizations of the T-TaS{dollar}sb2{dollar} nanocrystals supports both the structural transformation model and the mechanism of the nanocrystal formation we proposed in the study of TaSe{dollar}sb2{dollar} system. However, applying the same nanofabrication method to T-TaSe{dollar}sb2{dollar} and T-TaS{dollar}sb2{dollar} leads to dramatically different results. In particular, a nano-size defect (ND) of atomic lattice and isolated CDW discommensuration (DC) lines were formed at the surface of a T-TaSe{dollar}sb2{dollar} crystal, while a CDW domain network was created at the surface of a T-TaS{dollar}sb2{dollar} crystal. The investigation of mechanisms of the nanostructure formation on both T-TaSe{dollar}sb2{dollar} and T-TaS{dollar}sb2{dollar} crystals suggests that the isolated CDW DC lines in the CDW lattice of T-TaSe{dollar}sb2{dollar} is mediated by atomic lattice dislocations, while the CDW domain network in the CDW lattice of T-TaS{dollar}sb2{dollar} was resulted from the CDW sliding under an external electric field generated by a STM tip.