It has been shown experimentally that the thermal dissolution of silver and gold thin films in silicate glass is accompanied by the formation of a monolayer of silver and gold micro- and nanocrystals, respectively, on the surface of the glass. These processes occur at temperatures well below the melting temperature of the metal. Microcrystals are formed predominantly at the edges of islands of the metal film, where there is a sufficient amount of the material for their growth. Silver and gold nanocrystals are formed in the case when atoms of the metal dissolved in the glass repeatedly emerge on the glass surface. Silver and gold dissolved in the glass exist not only in the form of atoms and ions but also in the form of charged and neutral molecular clusters.
It is shown that metallic nanowires (5–8 nm in diameter) that form during laser ablation of Ni, Pb, In, and Sn targets embedded
in HeII contain extended single-crystal segments, while spherical clusters (about 2 μm in diameter) that form under these
conditions have a regular shape and an atomically smooth surface. Such structures are explained by melting of metal ablation
products under their coalescence in HeII. The short-term action of a low-intensity beam of electrons with an energy of 200
keV initiates the explosion in metallic spheres preserved in the vacuum chamber of a transmission electron microscope, which
is accompanied with the formation of thousands of clusters with a diameter of a few nanometers. This effect is due to metastability
of internal mechanical stresses produced upon sharp cooling of molten spheres by liquid helium. A mechanism of condensation
of atoms and nanoparticles in quantized vortices of superfluid helium is proposed. 相似文献
The yield and energy distributions of sodium atoms upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a gold film are investigated for the first time as functions of the thickness of the gold film, the concentration of deposited sodium, and the surface temperature. It is found that the energy distributions exhibit two peaks, namely, a narrow peak with a maximum at about 0.15 eV, whose intensity continuously increases with increasing temperature, and a broad peak with a maximum at about 0.35 eV, whose intensity either decreases or remains constant with increasing temperature. It is shown that both peaks arise as a result of the same excitation, which gives rise to different channels of electron-stimulated desorption of sodium atoms. Possible mechanisms of electron-stimulated desorption and the kinetics of destruction of the surface coating are discussed. 相似文献
A new model has been developed and calculations have been performed for the formation of nanodroplets after action of an ultrashort laser pulse on a thin (10–100 nm) gold film deposited on a glass substrate. The action of a laser results in the melting of the film in the region of a laser spot and in its thermomechanical separation from the substrate. The separated film acquires a dome shape because of a decrease in the temperature in the direction from the center of the laser spot. This theoretical model provides the explanation of the formation of nanodroplets. It has been established that, first, the separation speed of a gold film from glass decreases sharply because the acoustic impedance of gold is much larger than that of glass. Second, nanodroplets are formed owing to the capillary focusing of the substance, which is manifested in the appearance of the drag component directed toward the axis of symmetry of the dome. The surface tension becomes dynamically significant because of the indicated sharp decrease in the separation speed from glass and of the smallness of the diameter of the focal spot (D ～ 1 μm), which is determined by the diffraction limit of optical radiation. 相似文献
Laser micromachining on 1000 nm-thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz. Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e2 diameter). The sample was translated at a linear speed of 400 μm/s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime. 相似文献
A set of molten gold clusters, each with 1157 gold atoms, was studied by molecular dynamics simulations as the clusters underwent
freezing at three different temperatures. Most of the clusters attained an icosahedral structure upon freezing, a structure
found to be stable to mild annealing. Other structures observed were imperfect truncated decahedral, truncated octahedral
and hexagonal close packed structures. The role of kinetics in the process of cluster solidification is discussed.
Received 6 November 2000 相似文献
Strong surface (metal) enhanced fluorescence (SEF or MEF) is observed from clusters and single E coli bacteria cells labeled with Carbon nanodots (CDs), which were synthesized from date pits. The enhancement factor (EF) for SEF of the cell clusters were close to 50 for both 533 and 633 nm laser excitation wavelength. Those EFs are ratios of emission peak areas from CD labeled cell clusters on gold film to the peak areas of the same batch cell clusters on glass substrate. SEF with 633 nm excitation performed better than SEF with 532 nm excitation, achieving higher fluorescence intensity and much higher contrast. The contrast as high as 66 for cell clusters on gold film is a ratio of fluorescent emission peak area measured at the CD labeled cell clusters to the fluorescent peak area measured at unlabeled cell clusters (autofluorescence) on the same substrate. The contrast with the background (S/N) or the ratio of fluorescent peak area measured at bacteria cells to area measured at bare substrate was as high as 200. This report may pave a way for the broader application of surface enhanced fluorescence and especially metal enhanced fluorescence imaging of CD labeled cells and other biological objects.
Graphical abstract Carbon dots, synthesized from dates, are used for direct staining of E coli cells. Emission fluorescent spectroscopy of those CD labelled cells on gold film and glass, demonstrated enhancement factor about 50 for emission on gold as compared to glass, Excitation at 633 nm appears far superior to excitation at 532 nm in terms of contrast (up to 67) with unlabeled cells /control due to decrease in auto fluorescence of cells. Maximum Signal to noise ratio is 200.
Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO2(1 1 0) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C2H2, C2H4, H2, and hydrogen atoms. Although, C2H4 and H2 did not show any indications of reduction of oxidized gold clusters, CO, C2H2, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400 nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters. 相似文献
