More than 17 years have passed since the invention of the scanning tunneling microscopy, and the development of scanning probe microscopies is now so rapid and remarkable. Moreover the impact has been so great to activate the novel researches in the fields of surface science and nano-structure science. In this article we briefly review present status of the theoretical bases of SPM and future problems. In particular we focus on the theory of quantum transport of atom/molecular bridges and the basic mechanism of the noncontact atomic force microscopy.
Nowadays, we can fairly easily observe and manipulate atoms and molecules on solid surfaces with the help of scanning probe microscopy (SPM). However, we cannot yet identify the chemical species of atoms and molecules we observe or manipulate just by using the present SPM technique. Towards establishment of such a technique based on the SPM, there are many researches going on throughout the world. Here we shall overview these researches very briefly, by emphasizing our own research on “Atomic manipulation and identification techniques by combining SPM ability of atomic manipulation with atom probe (AP) ability of atomic mass analysis”;.
Recent development in STM measurements allows us to look into the detailed mechanism of catalytic reactions at metal surfaces on the atomic level. The topics for the surface elementary steps such as adsorption, dissociation, diffusion, surface reaction and reconstruction are reviewed in conjunction with surface heterogeneity and molecular interaction, which are essential for the understanding of catalytic reactions at surfaces. As for the heterogeneous character, dissociation of molecules at step edges, surface reactions at the perimeter of island, and precursor state of molecular adsorption are visualized and proved by STM. The other aspects of the topics are related to molecular or atomic diffusion, existence of hot atom, and chemical reconstruction. These insights obtained by STM will provide the basis of a new type of theory for catalytic reactions.
The recent trend in STM research on silicon materials is reviewed using the INSPECA database of Kyushu Univ. As examples, our recent works on Sn epitaxy on silicon surfaces and the formation of nickel-silicides on hydrogenterminated surfaces are described. In the former work, atomic arrangements in real space were clarified, and the new structural phases were also found. In the latter works, it was confirmed that the hydrogen termination technique was useful to fabricate high-quality silicide thin films.
Coupling of laser light into a scanning tunneling microscope is expected to open a new intriguing field. Namely, de-velopment of the photo-assisted scanning tunneling microscopy allows us to study surface physics with the excellently high spatial and time resolutions. In this paper, the recent progress and future potential of the new technique are overviewed.
The principles of aperture- and scattering (apertureless)-type near-field optical microscopies (NOM), their related techniques, and their applications are summarized. In terms of a key device of the aperture-type NOM, the functional performance of an optical fiber probe with a small metal aperture is demonstrated through measurement of the photoluminescence image of single quantum dots. In the range of 50-100nm spatial resolution, the aperture-type NOM is an established and reliable technique for the spectroscopy of highly localized structures: spatially resolved spectroscopy of the surface of semiconductor materials or spectroscopic analysis of single particles, such as single molecules, single quantum dots, single metal particles, and so on. The scattering-type NOM, on the other hand, is a promising device to be applied to the surface enhanced spectroscopy, nano-manufacturing, and high density optical memory with higher spatial accuracy and higher efficiency.
In 1995, a true atomic resolution was achieved for the first time, using an ultrahigh-vacuum noncontact-atomic force microscope (AFM) with frequency modulation (FM) detection method that enables to measure change in mechanical resonant frequency (frequency shift) of an atomic force probe (AFM cantilever). At present, noncontact AFM method is established as a novel microscopy with true atomic resolution, which can observe even insulator. Here, to make clear the next development on AFM, we introduced a force mapping of atomic force on an atomic scale, i.e., atomic force mi-crospectroscopy and control of atomic force by change of atom on tip apex of an atomic force probe. AFM, which utilizes atomic force itself based on the atomic interaction, can provide observation, spectroscopy, discrimination, identification, control and manipulation of individual atomic force and atom, so that AFM has large possibility as the coming generation of atomic and molecular technique and is expected to develop in very wide fields of science and engineering.
GaAs(001) is one of the most commonly used substrates in fabrication of wireless and opto-electronic devices based on III-V compound semiconductors by using molecular beam epitaxy (MBE), metallorganic chemical vapor deposition (MOCVD) and related techniques. The surface structure of GaAs(001) has been disputed since the beginning of the development of the techniques as to which of these materials are artificially prepared. The invention of scanning tunneling microscopy (STM) has revolutionized the situation. This paper reviews the STM studies of principal reconstructions, from As-rich c(4 × 4), 2 × 4, 2 × 6 to Ga-rich 4 × 2 and 4× 6, found on the GaAs(001) surface. These studies, together with advanced theoretical efforts, have eventually resulted in establishment of a unified model for various reconstructions, with which we could explain most of the observation and long-standing controversies in atomic structures and surface stoichiometries.
Using a scanning electron microscope-molecular beam epitaxy system, detailed near-equilibrium growth processes of island nucleation, coalescence, and step motion are clearly observed for the growth of GaAs on (111)A substrates. These observations allow the quantitative analysis of the growth processes on the basis of the BCF theory that provides the standard model for crystal growth. As an example, the Ga adatom surface diffusion length is directly measured from the dependence of measured step velocity on the Ga arrival rate. The presence of denuded zone in the distribution of two-dimensional nuclei is clearly confirmed, quantitativelly showing good agreement with the diffusion length obtained from the step velocity.