Size- and temperature-dependent structural transitions in gold nanoparticles were revealed with morphology statistics obtained by high-resolution electron microscopic observations for thousands of particles annealed in a helium heat bath. We found that gold nanoparticles over a wide size range, 3-14 nm, undergo a structural transformation from icosahedral to decahedral morphology just below the melting points. It was also clarified that the formation of bulk crystalline structures from the decahedral morphology requires the melt-freeze process due to an insurmountable high free-energy barrier.

Size- and Temperature-Dependent Structural Transitions in Gold Nanoparticles

Dispersion of Co3O4 nanoparticles within SBA-15 using alkane solvents

Acoustic vibrations of assemblies of gold nanoparticles were investigated using ultralow frequency micro-Raman scattering and finite element simulations. When exciting the assemblies resonantly with the surface plasmon resonance of electromagnetically coupled nanoparticles, Raman spectra present an ultralow frequency band whose frequency lies below the lowest Raman active Lamb mode of single nanoparticles that was observed. This feature was ascribed to a Raman vibration mode of gold nanoparticle “supermolecules”, that is, nanoparticles mechanically coupled by surrounding polymer molecules. Its measured frequency is inversely proportional to the nanoparticle diameter and sensitive to the elastic properties of the interstitial polymer. The latter dependence as well as finite element simulations suggest that this mode corresponds to the out-of-phase semirigid translation (l = 1 Lamb mode) of each nanoparticle of a dimer inside the matrix, activated by the mechanical coupling between the nanoparticles. These ...

/pdf/mechanical-coupling-in-gold-nanoparticles-supermolecules-3idy2mu0z9.pdf

Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy

The structural and morphological changes occurring in an ensemble of vapor deposited palladium nanoclusters have been studied after several hydrogenation cycles with x-ray diffraction, extended x-ray-absorption fine structure spectroscopy, Rutherford backscattering spectrometry, and STM. Initial hydrogenation increased the cluster size, a result that is attributed to hydrogen-induced Ostwald ripening. This phenomenon originates from the higher mobility of palladium atoms resulting from the low sublimation energy of the palladium hydride as compared to that of the palladium metal. The universality of this phenomenon makes it important for the application of future nanostructured hydrogen storage materials.

/pdf/hydrogen-induced-ostwald-ripening-at-room-temperature-in-a-589ecx5tl2.pdf

Hydrogen-induced Ostwald ripening at room temperature in a Pd nanocluster film.

We report on the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in an amorphous zirconia matrix. The deposition method allows one to control the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber (from 2 × 10 −5 to 10 −1 mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoOx , with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. For an oxygen pressure of about 10 −3 mbar the ratio between the FM and AFM phases is optimum with an exchange bias field of about 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also at the origin of the induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase. (Some figures in this article are in colour only in the electronic version)

/pdf/controlling-exchange-bias-in-co-coox-nanoparticles-by-oxygen-14znqvcero.pdf

Controlling exchange bias in Co-CoOx nanoparticles by oxygen content.

We report the preparation of a new type of nanocomposite containing cobalt and silver nanoparticles organized in parallel layers with a well controlled separation. This arrangement allows the observation of an enhanced low-frequency Raman signal at the vibration frequency of cobalt nanoparticles excited through the surface plasmons of silver nanoparticles. Numerical simulations of the electric field confirm the emergence of hot spots when the separation between silver and cobalt nanoparticles is small enough.

https://www.uni-ulm.de/fileadmin/website_uni_ulm/hrem/publications/2008/paper/surface_enhanced_raman_scattering.pdf

Surface enhanced Raman scattering of silver sensitized cobalt nanoparticles in metal–dielectric nanocomposites

Ensembles of nanometer-sized Au, Co, Er, and FePt clusters were generated by a laser vaporization source and embedded in a MgO matrix grown on a mica substrate. The size distribution of the clusters was measured before sample preparation by time-of-flight mass spectrometry and afterwards by transmission electron microscopy. These well-characterized samples were used for investigations by small-angle X-ray scattering (SAXS). The scattering data was evaluated by Guinier analysis providing the average radii of the embedded clusters. The results are in good agreement with the cluster sizes obtained from the mass spectra as well as the dimensions determined from the transmission electron micrographs. Furthermore, other samples which were produced at an elevated substrate temperature of 500 °C exhibited an increased average cluster radius in the SAXS measurements. This behavior is attributed to diffusion and coalescence emerging at higher deposition temperatures.

Measurement of the size of embedded metal clusters by mass spectrometry, transmission electron microscopy, and small-angle X-ray scattering

We compare the magnetic properties of Co cluster assembled films with different degrees of oxidation. Clusters with grain size $(2.3\ifmmode\pm\else\textpm\fi{}0.7)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are produced in a laser vaporization cluster source and soft-landed in ultrahigh vacuum conditions, forming highly porous nanogranular films. After exposure to air for different periods of time, the Co clusters oxidize and the sample may be considered as a thin antiferromagnetic Co oxide matrix containing ferromagnetic Co clusters. Magnetization measurements were performed in a temperature range from 300 down to $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at applied magnetic fields up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. The exchange bias value at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the strongly oxidized sample is $4.8\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ against the value of $0.75\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ for the less oxidized sample. The mean values of the thicknesses of the Co oxide layers are estimated to be 0.6 and $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the more and less oxidized sample, respectively. We propose a method of measuring the exchange bias inducing temperature, i.e., the temperature at which exchange anisotropy is established. We determined the mean inducing temperatures for both samples, which are 55 and $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for the more and less oxidized samples. Both temperatures are well below the bulk CoO N\'eel temperature of $292\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A low value of the inducing temperature of the Co oxide layer is a consequence of its subnanometer thickness, while a large exchange bias value is a consequence of different dimensionality of Co clusters and Co oxide matrix.

Influence of finite size effects on exchange anisotropy in oxidized Co nanocluster assembled films

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Surface enhanced Raman scattering of silver sensitized cobalt nanoparticles in metal–dielectric nanocomposites

Measurement of the size of embedded metal clusters by mass spectrometry, transmission electron microscopy, and small-angle X-ray scattering

Influence of finite size effects on exchange anisotropy in oxidized Co nanocluster assembled films