Nano- and Micromaterials

In a fundamental part of the field of nano- and microscale science, revolutional progress has been made since last two decades, in a way highly respected by the society, politics, and economics. In this stream, scientists and engineers from different fields of physics, chemistry, materials science, and information technology, including experimentalists, theorists, and researchers doing computer simulations, have collaborated to form a new interdisciplinary field called nanotechnology.

Nanostructured materials have been extensively studied for more than 10 years because of tremendous potential to application in a variety of technology, such as electronics, materials science, and biotechnology. Although a large part of these studies concerns nanostructures in three dimensions, this section focuses on nanostructures in rather lower dimensions, on solid surfaces. Moreover, “nano-” usually means the range of a few to hundreds of nm; however, we concentrate on the structures one order smaller than usual, in other words, the structures in “atomic scale” rather than “nanometer scale,” especially those formed on well-characterized surfaces under ultrahigh vacuum (UHV) conditions. Even in this scale, nanostructures can be formed both by selforganization and by ultrafine machining. Before we present our latest studies, some categories of this kind of nanostructures are introduced in this section. We do not attempt to present a detailed review with reference to huge number of articles, but give a few examples of each category with emphasis on the initial works or fundamental studies.

Observation of transient behavior of excited states at different frequencies is essential and of great importance in studying ultrafast photochemical and photophysical processes, such as photosynthesis, energy transfer, photochromic reaction, and carrier dynamics, of nano- and micromaterials. To investigate such phenomena, femtosecond laser spectroscopy has been extensively utilized for the last two decades [1]. In this review, we pay attention to three types of new femtosecond laser spectroscopy: optical Kerr gate (OKG) luminescence spectroscopy, transient grating spectroscopy combined with a phase mask, and real-time pump-probe imaging spectroscopy, which are applicable to the observation of transient behavior of excited states and the evaluation of optical functional properties of nano- and micromaterials.

Transition metal oxides have been attracting fundamental and technological interests because their various properties are influenced by many factors such as the number of d-electrons on transition metal ions, crystalline structures, oxygen defects and doped impurities. Elucidation of influence on properties from the factors will lead us to discovery of novel materials. From this standpoint of view, titanium dioxide is one of the prototypes. Titanium dioxide has no d-electron by itself, so that the number of d-electron can be controllable by doping of transition metals. Three crystalline modifications of titanium dioxide, rutile, anatase, and brookite are known well, so that structural dependence on properties can be discussed. It is expected that defects and impurities are also dominant over optical and electrical responses, such as transmittance, photoluminescence and conductivity, which is the same as conventional semiconductors. Titanium dioxide, moreover, is a material which has been used for a long time in a wide range of common and high technique applications because of its moderate price, chemical stability and nontoxicity. Recent topical application is a photocatalyst [1]. Photocatalytic reaction of titanium dioxide is a redox reaction of reactants adsorbed on the surface and it involves photogeneration, migration, and trapping of charge carriers. In these processes, the photogeneration and the migration of carriers are crucial processes to govern inherent activity of the material as a photocatalyst. It can be easily imagined that the factors in nanoscale size have some influence on the behavior of the carriers and the catalytic activity. In fact, anatase has higher photocatalytic activity than rutile because of a difference in Fermi energy [2] and the charge carrier in anatase thin film has a higher mobility than that of rutile [3]. Many approaches to raise photocatalytic activity under visible light have been done; for example transition metal doping [4,5], nitrogen doping [6], and oxygen defect [7]. Their results indicate that optical absorption in the visible region is controllable by doping of impurities. Among three crystalline modifications of titanium dioxide, only rutile crystals have been obtained by crystallization of the substance from its own melt or from a solution in a melt [8]. In contrast to extensive studies for rutile, fundamental optical and electronic properties of anatase which is low temperature modification have not been well understood. To reveal fundamental properties of anatase titanium dioxide, it is indispensable to investigate defect states in it. Recently, anatase titanium dioxide single crystals can be grown by gas phase reaction [9]. As-grown anatase crystals generally exhibit pale blue color in spite of the wide bandgap of about 3.3eV. This suggests the presence of some defects in the as-grown crystals. On this report, it is shown that several colors in anatase can be available by defects controlled in nanoscale, and some electrical properties are controllable by photoirradiation, which implies the possibility of nanoscale doping.

Recently, organic radicals in solid state have attracted much current interests with regard to photoinduced phase transitions, Peierls transitions, molecular Mott insulators, synthetic metals, and so on. They are expected to play a major role in novel materials research because of their potential in future applications to molecular magnetic and optical devices. In photoinduced phenomena, light irradiation stimulates the macroscopic phase transition between the ground state and a metastable state, and may control various properties including, e.g., magnetic and chromic properties of materials.

Investigating excited states of materials are of much interest from the viewpoint of both the theoretical and experimental physics because experiments are done in the presence of some field to excite materials and measure some quantity. However, calculating excited states of materials using first principles is nowadays very difficult. In the present chapter, we focus on ab initio calculations to investigate excited states. Especially we focus on state-ofthe- art first-principles calculations within the GW approximation (GWA), introduced by Hedin [1] from the viewpoint of the many-body perturbation theory (MBPT) in the quantum field theory. we discuss clusters rather than crystals because clusters have many interesting properties such as magic number, size dependence of optical gap, and so on and some reviews of GW calculations of crystals have already published.

Since the density functional theory (DFT) was established by Horhenberg and Kohn, the ab initio methods have been rapidly developed together with the improvements of the computer facilities. The local density approximation (LDA) or generalized gradient approximation (GGA) based on the DFT, in particular, have been widely applied to various real systems and have succeeded in describing the ground state properties.

Electronic transport is among the unique physical phenomena whose applications have shaped human civilization as we know it today. From old telegrams and light bulbs to modern televisions, mobile phones, laptops, and super-computers, all make use of electronic transport. In fact, nowadays we can hardly find any significant technological product in which electronic transport is not used one way or another. This enormous technological impact is a result of basic scientific research on electron tunneling and scattering, in different environments and including various levels of interactions and correlations. The basic research in the field of electronic transport is expected to yield equally unique, and even more important, fruits in future, as the challenges of this vibrant field are ever increasing. One of the main areas of interest which has been the focus of numerous scholarly works is electronic transport at nanometer length scales. The reason is that miniaturization of electronic components has caused the device dimensions to reach nanoscale. At nanoscale, the atomistic character of the systems can no longer be treated using rather rough models applicable at micrometer length scales. Therefore, fundamentally new approaches are necessary, in both theory and experiment, to deal with electronic transport at nanoscale.

Self-assembled quantum dot (QD) composed of III—V semiconductors is one of the most promising materials for the devices of next generation optical telecommunication and future single-photon quantum computation [1–8]. The QD structure is grown by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD), and has been investigated to exploit the delta-function-like state density which arises from the three-dimensional quantum confinement of carriers. Among other methods such as the electrostatical definition of QDs in a two-dimensional electron gas and the colloidal clustering using solution chemistry, the self-assembled epitaxial growth provides the best means of incorporating QDs into a variety of devices with high quantum efficiency.

Semiconductor quantum well (QW) and related structures have been studied intensively since the concept of the superlattice was proposed [1]. The fabrication of QWs became possible owing to remarkable progress in semiconductor crystal growth techniques such as molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). The QW structures are utilized for various kinds of photonic devices and electron devices. These days quantum wires (QWRs) and quantum dots (QDs) [2, 3] also attract much attention as advanced low-dimensional confinement structures for higher performance devices.

To study thermodynamic properties of materials, lattice model simulation such as lattice Monte Carlo (MC) simulation is one of the simple and fast method. One advantage of the method is that it can treat larger systems both in time scale and in spatial size compared with atomic-scale molecular dynamics (MD) simulations so that it can treat thermodynamic equilibrium or diffusion phase transition phenomena. However, it has limitation in the description of disordered or liquid phases because displacement of atoms from regular lattice points that may be important at high temperatures could not be considered. That is, lattice models neglect the vibration entropy as well as the elastic energy. The shortcomings lead to overestimation of the phase transition temperatures and underestimation of the width of single-phase fields.

In the last 1980s, microelectromechanical systems (MEMS) such as microgears and microactuators have been developed by using silicon-based micromachining techniques [1]. Most of MEMS utilize electrostatic actuators as a major driving source. The electrostatic actuators have been used in practical microdevices, including digital micromirror device (DMD) and RF MEMS. However, it is difficult to utilize electrostatic force in liquids such as electrolytes. For this reason, electrostatic actuators are not suitable for biological applications such as microfluidic devices and micromanipulation tools for cells.

Micron-sized dust particles in laboratory plasmas have large negative charges (ǀQǀ = 10³ ~ 10⁵e) as a result of interaction with ambient plasma. They form structures known as Coulomb crystals when the electrostatic energy is much more than the thermal energy of dust particles. In this section, fundamental physics of a complex plasma is described. Numerical simulation shows particle dynamics forming Coulomb clusters and experiments show the change of the charges of dust particles by irradiating the electron beam and Coulomb cluster formation in cryogenic environment.