Soichiro Saita

Theoretical assessment of FePt nanoparticles as heating elements for magnetic hyperthermia

FePt magnetic nanoparticles (MNPs) are expected to be a high-performance nanoheater for magnetic hyperthermia because of their high Curie temperature, high saturation magnetization, and high chemical stability. Here, we present a theoretical performance assessment of chemically disordered fcc-phase FePt MNPs. We calculate heat generation and heat transfer in the tissue when an MNP-loaded tumor is placed on an external alternating magnetic field. For comparison, we estimate the performances of magnetite, maghemite, FeCo, and L10-phase FePt MNPs. We find that an fcc FePt MNP has a superior ability in magnetic hyperthermia

The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix

The first comprehensive study on the kinetics of nanocrystal growth in a hot amphiphile medium is presented. An example is given with CdSe semiconductor nanocrystals grown after the injection of precursor (a mixture of Cd- and Se-reagents) in concentrated tri-octylphosphine oxide matrix (heated to more than 300 degrees C). The particle size distribution is reconstructed as a function of time from the absorption and photoluminescence spectra collected during the synthesis process. For this purpose a new expression is used relating the exciton energy due to quantum confinement with the nanocrystal radius. The growth kinetics is considered as a two-stage process in order to describe the time variation of nanoparticle size. During the first stage, called reaction-limited growth, the size of initial nucleus rapidly increases due to a sort of surface reaction exhausting the precursor in the nanoparticle vicinity. The growth in such conditions favors also a remarkable narrowing of the size distribution. The nanocrystal develops further on account of a slow precursor transfer from a distant space driven by the concentration gradient--classical diffusion-limited growth. The width of size distribution also increases proportional to the average particle size. Any growth will stop after the precursor concentration reaches a minimum value defining the limit for the final nanocrystal size in a batch. Solving the kinetic equations for the growth rate in each case of kinetics derives analytical expressions for the mean radius and variance of size distribution. Then the respective expressions are matched in a uniform solution valid during the entire synthesis. The theoretical model is in a good quantitative agreement with the experimental data for independent syntheses. Important characteristic scales of the processes (time-constant and length) and microscopic parameters of the reacting system (interfacial energy and reaction rate constant) are estimated from the data. It turns out that the fast reaction-limited growth is important to obtain well-defined nanocrystals of high optical quality by using less energy, time and consumable. However, to make them reproducibly uniform one should control also the ultra-fast nucleation process preceding the nanocrystal growth, which is still unknown. Nevertheless, our current findings allow the conceptual design of a new continuos-flow reactor for the manufacturing of a large amount of uniform nanocrystals.

Optical Memory Media Based on Excitation-Time Dependent Luminescence from a Thin Film of Semiconductor Nanocrystals

We describe the increase of photoluminescence intensity from densely packed semiconductor nanocrystals (quantum dots) with excitation time, which is clearly observed by the naked eye under ambient conditions. This enabled the invention of the first luminescence-based optical memory media feasible for practical applications. Bright luminescent images are stored and then read out by exciting the medium, a thin film of cadmium selenide nanocrystals, with blue or UV light. The increase in emission intensity is attributed to the trapping and accumulation of photo-generated electrons in the matrix of organic molecules capping the nanocrystals.

Large-Scale Production of CdSe Nanocrystal by a Continuous Flow Reactor

Organically capped CdSe nanocrystals were successfully produced by a continuous flow reactor in 13 g/h rate as isolated CdSe nanocrystal, using trioctylphosphine oxide (TOPO) both as the capping organic reagent and the high-temperature reaction solvent. Relatively high reaction temperature (e.g. 350°C) was necessary for matured crystal growth. The quality of TOPO (i.e. impurity composition such like phosphonic acids) was also influential on the quality of the resulting CdSe nanocrystal. The continuous flow reactor was able to produce highly-luminescence, monodispersed CdSe nanocrystals, confirmed by transmission electron microscope observation. The production rate was stable at least 1 h to allow over 10 g production.

Chemical ordering of FePt nanoparticles by pulsed laser annealing

An irradiation with the third-harmonic wave of a Nd:yttrium–aluminium–garnet (YAG) laser with a 5 ns pulse width successfully transformed colloid–chemically synthesized FePt nanoparticles (NPs) from disordered face-centred cubic (fcc) into chemically ordered L 10 (face-centred tetragonal: fct) phase. Granular films and hexane dispersions of fcc-FePt NPs were irradiated by the pulsed laser beam. In the case of the granular film, a coercivity of 1 kOe was achieved after the laser irradiation; meanwhile, the dispersion sample exhibited superparamagnetism, although the crystalline structure was transformed into fct phase. X-ray diffractometry and transmission electron microscopy revealed that coalescence between NPs was fairly well suppressed in the case of the dispersion sample. The superparamagnetism observed in the dispersion sample was due to insignificant coalescence and the resulting small magnetic domain.

Influence of surface ligands on saturation magnetization of FePt nanoparticles

The influence of organic ligands on the saturation magnetization (MS) of chemically disordered face-centered-cubic (fcc) FePt nanoparticles (NPs) was investigated. By increasing the basicity and/or surface coverage of ligands, the MS of fcc-phase FePt NPs decreases due to an increase in electron donation from the ligand to the Fe d bands. FePt NPs capped with 1-octanethiol or 1-dodecanethiol show larger MS than as-synthesized NPs capped with oleic acid due to a thinning of the nonmagnetic shell.

Spatially controlled CdSe nanocrystal distribution in phase separated polymer blend films

We report the fabrication and characterization of the self-assembled structure of tri- n -octylphosphine oxide (TOPO)-capped CdSe nanocrystals in binary polymer phase separated films of polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(2-vinylpyridine) (P2VP). These structures are formed via demixing of CdSe nanocrystals and binary polymer during spin coating. The nanocrystals preferentially segregate to the PS-rich phase in phase separated PS/PMMA and to the P2VP-rich phase in phase separated PS/P2VP, as shown by optical microscopy and photoluminescence images. For CdSe/PS/PMMA, we attribute the driving force for CdSe segregating to the PS-rich domain to the stronger attractive interaction of TOPO on PS with respect to PMMA due to polarity, and in the case of CdSe/PS/P2VP, segregating to the P2VP-rich domain to the interaction such as coordinate bonds being formed between pyridine groups and surface Cd atoms.

Observation of CdSe Colloidal Nano-Dot Films by Scanning Probe Microscopy

We observed the surface topography of CdSe colloidal nano-dot film by cyclic contact mode atomic force microscopy. The observed structure changes with cantilever oscillation amplitude, and non-uniform images with long-range corrugations are obtained with relatively large oscillation amplitude while fine structures are revealed with smaller oscillation amplitude. When the amplitude is larger and the surface is weakly ‘tapped’, the topography of the soft organic matrix of the film dominates, and when the tapping force is increased, the hard CdSe dots begin to reveal.