Jan-Olle Malm

Gold nanoparticles: Production, reshaping, and thermal charging

Gold nanoparticles are of great interest for various nanoelectronic applications, e.g., for making single electron transistors or very fine leads to molecular size entities. For this and other applications, it is important that all particles have controllable size and shape. In this paper, we describe the production of size-selected gold aerosol particles in the 20 nm range made by evaporation in a high-temperature tube furnace and subsequent size selection. To obtain spherical particles, it was necessary to reshape the particles at high temperature, which was investigated for temperatures between 25°C and 1200°C. High-resolution transmission electron microscopy showed that the degree of crystallinity became higher for higher reshaping temperature. During reshaping at high temperature, an anomalous charging behavior was discovered, whereby negatively as well as positively charged particles became multiply negatively charged. Possible mechanisms for explaining this thermally activated phenomenon are discussed.

Crystal field, phonon coupling and emission shift of Mn2+ in ZnS:Mn nanoparticles

The Mn2+ emission wavelengths are at 591, 588, 581 and 570 nm, respectively, for the ∼10, ∼4.5, ∼3.5 nm sized nanoparticles and the ZnS:Mn nanoparticles formed in an ultrastable zeolite-Y. To reveal the cause for the shift, the crystal field and phonon coupling were investigated. The results show that the predominant factor for the shift is the phonon coupling, whose strength is size dependent and is determined by both the size confinement and the surface modification of the nanoparticles. Although the crystal field strength decreases with the decreasing of the particle size, its change has little contribution to the emission shift of Mn2+ in ZnS:Mn nanoparticles.

Sub-Ångstrom high-resolution transmission electron microscopy at 300 keV

Sub-Angstrom transmission electron microscopy has been achieved at the National Center for Electron Microscopy (NCEM) by a one-Angstrom microscope (OAM) project using software and enhanced hardware developed within a Brite-Euram project (Ultramicroscopy 64 (1996) 1). The NCEM OAM provides materials scientists with transmission electron microscopy at a resolution better than 1 A by using extensive image reconstruction to exploit the significantly higher information limit of an FEG-TEM over its Scherzer resolution limit. Reconstruction methods chosen used off-axis holograms and focal series of underfocused images. Measured values of coherence parameters predict an information limit of 0.78 A. Images from a [1 1 0] diamond test specimen show that sub-Angstrom resolution of 0.89 A has been achieved with the OAM using focal series reconstruction.

Size-selected gold nanoparticles by aerosol technology

Abstract We have produced size-selected gold particles with sizes below 30 nm by means of aerosol technology. The method allows for particles of many different materials to be produced with high purity and narrow size distribution, and it allows easy tranport and deposition of the particles onto any substrate. The particle quality has been investigated by transmission electron microscopy, revealing a high degree of crystallinity, especially after sintering at high temperatures.

Novel, Spongelike Ruthenium Particles of Controllable Size Stabilized Only by Organic Solvents

Soluble ruthenium nanoparticles of uniform size (see picture) with a porous spongelike structure were obtained by the reaction of [Ru(C(8)H(10))(C(8)H(12))] with H(2) in methanol or THF/methanol. The particle size can be controlled in the range 15-100 nm by varying the MeOH/THF ratio. The particles catalyze benzene hydrogenation without modification of their size or structure. Their formation is proposed to occur in the droplets of a nanosized emulsion, which act as nanoreactors.

Temperature and pressure dependences of the Mn2+ and donor-acceptor emissions in ZnS : Mn2+ nanoparticles

Temperature and pressure dependent measurements have been performed on 3.5 nm ZnS:Mn2+ nanoparticles. As temperature increases, the donor-acceptor (DA) emission of ZnS:Mn2+ nanoparticles at 440 nm shifts to longer wavelengths while the Mn2+ emission (T-4(1)-(6)A(1)) shifts to shorter wavelengths. Both the DA and Mn2+ emission intensities decrease with temperature with the intensity decrease of the DA emission being much more pronounced. The intensity decreases are fit well with the theory of thermal quenching. As pressure increases, the Mn2+ emission shifts to longer wavelengths while the DA emission wavelength remains almost constant. The pressure coefficient of the DA emission in ZnS:Mn2+ nanoparticles is approximately -3.2 meV/GPa, which is significantly smaller than that measured for bulk materials. The relatively weak pressure dependence of the DA emission is attributed to the increase of the binding energies and the localization of the defect wave functions in nanoparticles. The pressure coefficient of Mn2+ emission in ZnS:Mn2+ nanoparticles is roughly -34.3 meV/GPa, consistent with crystal field theory. The results indicate that the energy transfer from the ZnS host to Mn2+ ions is mainly from the recombination of carriers localized at Mn2+ ions

Deceptive “lattice spacings” in high-resolution micrographs of metal nanoparticles

Abstract Experimental HRTEM images of randomly oriented nanocrystals typically show fringes within the nanocrystals. Often these fringes are one-dimensional, although many appear to show two-dimensional resolution. In order to determine the tilt conditions necessary for the formation of one- and two-dimensional fringes in metal nanocrystals, simulations of HRTEM images of a 561-atom palladium nanoparticle were systematically carried out over an extensive range of tilts. The resultant images contained fringes that are deceptively like one- and two-dimensional lattice fringes. These fringes are, however, not simply related to the crystal structure of the particle, and “lattice spacings” measured from them will be in error. In addition, depending upon particle orientation, HRTEM images of a perfect nanoparticle can show details that may be erroneously interpreted as “relaxations”, “bent planes”, and even “twinned” areas.

Size dependence of Eu2+ fluorescence in ZnS:Eu2+ nanoparticles

The emission bands of the 4.2, 3.2 and 2.6 nm sized ZnS:Eu2+ nanoparticles are peaking at 670, 580 and 520 nm, respectively. The emission of the 4.2 nm sized nanoparticles originates from the recombination of the Eu2+-bound exciton, while the emission of the 3.2 and 2.6 nm sized nanoparticles is from the Eu2+ intra-ion transition of 4f65d1(t2g)–4f7. Possible mechanisms for the size dependence of the 4f65d1(t2g)–4f7 transition of Eu2+ in ZnS:Eu2+ nanoparticles were investigated, and it was concluded that the decreases in the electron–phonon coupling and in crystal field strength upon a decrease in size are the two major factors responsible for the shift.

Upconversion luminescence of Eu3+ and Mn2+ in ZnS : Mn2+, Eu3+ codoped nanoparticles

Strong upconversion luminescence of both Mn2+ and Eu3+ is observed in ZnS:Mn2+, Eu3+ codoped nanoparticles. Laser power dependencies and spectroscopic data show that the upconversion emission is due to two-photon excitation of each specific dopant ion. The relative differences in two-photon excitation cross section result in different relative intensities for the Eu3+ and Mn2+ upconversion at different wavelengths. Spectroscopic data and luminescence lifetime data indicate no evidence of energy transfer between the Mn2+ and Eu3+ ions.

Pressure dependence of Mn2+ fluorescence in ZnS : Mn2+ nanoparticles

The photoluminescence of Mn2+ in ZnS:Mn2+ nanoparticles with an average size of 4.5 nm has been measured under hydrostatic pressure from 0 to 6 GPa. The emission position is red-shifted at a rate of -33.3+/-0.6meV/GPa, which is in good agreement with the calculated value of -30.4meV/GPa using the crystal field theory