Nano-probe assisted techniques for site-controlled quantum dots (SCQDs) with a nm-level site precision have been developed for suppression of dot-size fluctuation, thus achieving an excellent optical device performance such as femtosecond-level optical switching. An SCQD was first demonstrated by using an in situ EB (electron beam) process combined to a self assembled (S-K mode InAs/GaAs epitaxy. A two-dimensional sub-pm-pitch array of InAs SCQDS with sub-100 nm diameters was successfully formed. Existence of partially missing SCQDS was suppressed by a new technique of sulfur passivation prior to the S-K mode epitaxy. The site precision of the SCQD was drastically improved to a nm level for the first time by using an in situ STM scanning tunneling microscope probe technique. Implementation of an array-pitch and dotdiameter reduction to several tens nm by this technique was attributed to novel nano-deposit and nano-hole formation processes. As a novel technique for a shape-control of QDs, height-control of InAs quantum dots on GaAs surfaces by in-situ AsBr_3 etching and MBE was also introduced and results at the preliminary experiment have been described. This work was supported by NEDO within the framework of the Femtosescond Technology Project.
We have found a new route for fabricating self-organized semiconductor-insulator nanostructures by utilizing periodic instability in wetting property at nano-interface. Via an extension of the vapor-liquid-solid process with gold catalysts, we have grown chains of crystallinesilicon nanospheres: crystalline-silicon nanospheres are connected by and covered with amorphous silicon oxide at a nearly equal spacing forming chain-like structures. Special attention was paid to controlling impurities and we found that the yield of the chains growth increased remarkably when we used complex catalysts of such as gold/lead. The growth mechanism of the chains was revealed by means of transmission electron microscopy observations and computer simulations: the diameter of a silicon nano-wire was modulated due to the periodic instability and then the array of silicon nanocrystallites was formed by the surface oxidation.
We report silicon (Si) nanoparticles prepared by pulsed laser ablation in constant pressure inert background gas (PLA-IBG) . Size distribution of the Si nanoparticles depended on the inert background gas pressure. The relation between the average size and the background gas pressure, can be explained by an inertia fluid model. Crystallinity of the nanoparticles was crystalline similar to that of bulk Si.Furthermore, we demonstrate the synthesis of mono-dispersed, nonagglomerated Si nanocrystallites, using a novel integrated process system where a classification unit of a low-pressure-operating differential mobility analyzer (LP-DMA) was combined to the PLA-IBG unit. The LP-DMA has been designed to operate under pressures less than 5.0Torr. We have successfully synthesized and deposited the nonagglomerated Si nanocrystallites of 3.7 nm mean diameter and 1.2 geometrical standard deviation.
Fullerenes and Carbon Nanotubes are well known nanostructures, formed only of carbon. These materials display a variety of physical properties, such as superconductivity, magnetism, or various quantum effects. A great advantage of these carbon nanostructures is that the unit structures are well defined, in sharp contrast to the broad size distributions of inorganic clusters. In this article, solid state properties of fullerenes are reviewed with particular interest in the correlation between the molecular rotation and electric/magnetic states. Firstly, dielectric properties of metal encapsulating fullerene La@C_<82> is presented. Because of the polar nature of the molecule and the molecular rotations, the La@C_<82> solids are dielectric active. Dipolar functionality gives an important handle that maybe of use in developing potential molecular or solid state devices. Secondly, we show that the intermolecular magnetic interactions in the C_<60> magnets are controlled by the relative orientations of neighboring molecules. The correlation between magnetic interaction and molecular orientation provides an important analogy to the transition metal oxide magnets.
A simple method of producing aligned carbon nanotube (CNT) films and a mechanism for their formation are described. The CNT film is self-organized by surface decomposition of SiC heated in a vacuum at a temperature between 1500℃ to 1700℃. It was found that the length of CNTS was controlled with adjusting heating time and temperature. Formation of the CNT films depends on the surface orientation of SiC crystal. Especially on C(0001^^-) face, an aligned CNT film O.25 Pm in thickness was formed perpendicular to the surface after heating at 1700℃ for half an hour. On the contrary, a very thin layer of graphite sheets 5 nm in thickness parallel to the surface was formed on the Si face under the same condition. To clarify the formation tmechanism of the CNTs, the initial stage of the surface decomposition of SiC was investigated by high-resolution electron microscopy along the cross-sectional direction. As the results, an initial-nanocap mechanism was proposed.
The fabrication of oxide nanocrystals was investigated by using the advanced oxide heteroepitaxy technique based on the nanoscale surface engineering as well as the atomic layer control via laser molecular beam epitaxy. The atornic-stepped single-crystal oxide substrates developed by thermal annealing have been proved to be useful for the nanoscale growth control of films as well as for the nanoscale observation. The atomic-step decoration epitaxy resulting in nanowires or nanodots of oxide crystals was presented for Mn, Zn ferrite, Fe_3O_4 and NiO on the ultrasmooth sapphire substrates with the 0.2nm-high straight steps. The nanowires 0.5 nm high, about 20 nm wide show uniform configurations reflecting the straight step edges of the ultrasmooth substrates. Nanodots of Fe_3O_4 were also fabricated on the 2 nm-high bunching-stepped substrates.
New-type nanostructures of organic molecules and inorganic semiconductors have been successfully fabricated on 'inactive' substrate surfaces such as layered material surfaces and dangling-bond terminated semiconductor surfaces. In this article we introduce two examples of our methods. One is the fabrication of C_<60> molecular nanostructures on a GaSe/MoS2 heterostructure substrate by the selective growth method. The other is the fabrication of self-organized compound semiconductor quantum dots on a bilayer-GaSe terminated Si(111) substrate. The use of the inactive substrate surfaces opens a new way to fabricate position-controlled nanostructures, which have been difficult to form by conventional self-organization methods.
DNA is one of the most promising molecules as the scaffold for molecular nanotechnology and nanoelectronics. The investigations of DNA on the nanostructure, electrical conductivity and electronic states have significant implications for the application of DNA in electronic devices and m DNA based electrochemical biosensors. The direct measurements of the intrinsic electrical characteristics of polynucleotides using a conducting probe atomic force microscope have been performed using self-assembled DNA network. Poly[d(G-C)]_2 and poly(dG)・poly(dC) construct the uniform two-dimensional reticulate structure and show the p-type rectified behaviors in the atmospheric condition, presumably due to the different redox potentials of DNA bases. The conductivity of these molecules has DNA been successfully controlled by chemical doping. It is found that the poly(dG) 'poly(dC) has the best conductivity and can act as a conducting nanowire. The conductive mechanism is discussed by the charge hopping model based on the SPM observation of DNA nanostructure.
Two dimensional metallo-protein crystals will provide a way to realizing important break-through in nanometerscale engineering. We employ ferritin molecule to realize an array of quantum nano-dots. Ferritin has a protein shell with a diameter of 1 2 nm and a core of iron oxide with a diameter of 6 nm. Two-dimensional crystals of ferritin molecule were made at air/water interface and transferred onto a Si wafer with little deformation. The protein portion of ferritin was eliminated by the heat-treatment, which left two-dimensional arrays of inorganic iron oxide dots on the Si wafer. The size and repeat distance of the dots were 6 and 12 nm, respectively. Feasibility study of the application of this nano-dot array to the quantum devices is now in progress.