Meng-Fan Luo

Patterning Co nanoclusters on thin-film Al2O3/NiAl(100)

Self-organized patterning of supported nanoclusters by virtue of low cost and readiness for mass production is considered as one of the most promising methods; however, this approach is challenging, since the capability of controlling the patterns relies on a suitable combination of clusters and templates. In this paper we demonstrate that Co nanoclusters grown from vapour deposition over Al2O3 thin films on NiAl(100) substrate make a perfect combination for self-organized patterning. Uniform and sizeable Co nanoclusters are formed only on crystalline Al2O3 films and they are highly aligned by protrusion structures of the crystalline Al2O3. Through simple thermal treatments we can pattern the crystalline Al2O3 films and consequently the grown Co nanoclusters. The patterns are robust as they are sustained even when the Co nanoclusters are flashed to 750 K, exposed to atmosphere or the coverage is increased to coalescence. Moreover, the patterns can be further refined by using STM tips. The results imply potential applications in both fundamental and applied researches for electronic and magnetic nanodevices as well as catalysis.

Self-aligned Co nanoparticle chains supported by single-crystalline Al2O3∕NiAl(100) template

We present Co nanoparticle chains grown by vapor deposition over a single-crystalline Al2O3 layers on NiAl(100) with such features as self-limiting size distribution with the average size of ∼2.7nm, well-ordered alignment, and high thermal stability. We attribute these features to peculiar one-dimensional long stripes with ∼4nm interdistance on the surface of the ultrathin Al2O3 template. This nanostructure may open the door to numerous applications, such as catalysis and nanostorage, where large area well-ordered nanodots are desired.

Low-temperature decomposition of methanol on Au nanoclusters supported on a thin film of Al2O3/NiAl100.

With a variety of surface probe techniques, we investigated low-temperature decomposition of methanol on Au nanoclusters formed by vapor deposition onto an ordered Al(2)O(3)/NiAl(100) thin film. Upon adsorption of methanol on the Au clusters (with mean diameter 1.5-3.8 nm and height 0.45-0.85 nm) at 110 K, some of the adsorbed methanol dehydrogenates directly into carbon monoxide (CO); the produced hydrogen atoms (H) begin to desorb near 125 K whereas most of the CO desorbs above 240 K. The reaction exhibits a significant dependence on the Au coverage: the produced CO increases in quantity with the Au coverage, reaches a maximum at about 1.0-1.5 ML Au, whereas decreases with further increase of the Au coverage. The coverage-dependence is rationalized partly by an altered number of reactive sites associated with low-coordinated Au in the clusters. At least two kinds of reactive sites for the low-temperature decomposition are distinguished through distinct C-O stretching frequencies (2050 cm(-1) and 2092 cm(-1)) while the produced CO co-adsorbs with H and methanol.

Growth of Co clusters on thin films Al2O3∕NiAl(100)

We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al(2)O(3) thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570 K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6 nm and a height of 0.7 nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3-4-nm-wide crystalline Al(2)O(3) strips where the deposited Co atoms are confined, the same features sustain. Only at 570 K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5 nm and height of 1.4 nm) appears for growth at 450 K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300 K and annealed to 750 K, in which the coalescence is eliminated and the two preferable geometries appear and coexist.

Initial stages of C60 thin film growth on graphite

Abstract The growth mechanism of a C 60 thin film on graphite substrate has been investigated by a quantitative analysis of electron energy-loss spectra, together with low-energy electron diffraction and thermal annealing measurements. It is found that the early stages of growth at room temperature are best described through two-dimensional island ripening in a fixed structure and predominantly by a layer-by-layer fashion.

Physisorption of molecular oxygen on C60 thin films

The interaction of oxygen molecules with a fullerene surface has been studied using high resolution electron energy loss spectroscopy and temperature programmed desorption. Vibrational excitation of the adsorbed oxygen is observed at 190 meV, an energy value comparable with that for molecular oxygen in the gas phase. We take this to indicate physisorption of molecular oxygen on the C(60) surface. Thermal desorption results also show that the bonding of oxygen molecules to the C(60) overlayer is comparable to that on a graphite surface. A detailed study of the energy dependence of the vibrational excitation reveals an inelastic electron resonance scattering process. The angular dependence of the resonant vibrational excitation exhibits features distinctively different from those for molecular oxygen physisorbed on the related graphite surface, at a comparable coverage. One possible reason is that the corrugated surface potential, due to the curvature of the C(60) molecules, promotes the preferential ordering of the physisorbed oxygen molecules perpendicular to the surface plane of the C(60) overlayer.

Dependence on size of supported Rh nanoclusters for CO adsorption

We studied the adsorption and lateral interactions of CO molecules on Rh nanoclusters supported on an ordered thin film of Al2O3/NiAl(100) with varied surface probe techniques under ultra-high vacuum conditions and with density-functional-theory (DFT) calculations. The Rh clusters were grown with vapor deposition onto the Al2O3/NiAl(100) surface at 300 K; with increasing deposition, their mean diameter evolved from 1.0 to 3.5 nm and their height from 0.4 to 0.8 nm. The initial adsorption energy (for sparse CO coverage) and the number of adsorbed CO per surface site on the Rh clusters increased with decreasing cluster size. The former effect results from the surface structure and expanded lattice parameter of small Rh clusters, whereas the latter effect involves not only the initial adsorption energy but also altered lateral interactions among CO molecules. In contrast with CO on Rh single crystals, CO on small clusters adsorbed with their axes tilted from the surface normal, weakening the CO–CO repulsive interactions for CO coverage over a wide range. The saturated density of CO on clusters of diameter near 1.0 nm and height near 0.4 nm is 2–3 times that of large clusters (diameter ≥ 3.5 nm) or a Rh(100) surface. The CO–CO repulsive interactions on small clusters became effective at large CO densities, given that the onset of desorption of CO at saturation was 100 K lower than that of large clusters.

The decomposition of methanol on Au–Pt bimetallic clusters supported by a thin film of Al2O3/NiAl(100)

With various techniques to probe a surface, we studied the decomposition of methanol on Au–Pt bimetallic clusters, of diameter ≤6.0 nm, formed by sequential deposition of Au and Pt evaporated onto thin-film Al2O3/NiAl(100). The surface of the bimetallic clusters comprised both Au and Pt, but the decomposition, through dehydrogenation to CO and scission of the C–O bond, proceeded primarily on the surface Pt. Alloying of Pt with Au altered little the dehydrogenation on the Pt sites. The CO and hydrogen produced from dehydrogenated methanol increased with the extent of Pt sites; the production per surface Pt was comparable to that of Pt clusters. The temperature of the onset of dehydrogenation resembled that of Pt clusters. Little methanol decomposed to CO on the Au sites. Varying the surface structure and composition of the bimetallic clusters affected these properties insignificantly. In contrast to the dehydrogenation, scission of the C–O bond in methanol did not depend exclusively on the concentration of Pt atoms at the surface, given that production of methane from this second channel did not increase with the extent of Pt surface sites. The modified electronic structure of the alloyed Pt controlled the probability of the C–O bond scission. The bimetallic clusters restructured during the reaction such that the Au atoms in the clusters aggregated and decorated the Pt surface, leading to fewer surface Pt and increased mean coordination of surface Au.

Engineering patterns of Co nanoclusters on thin film Al2O3∕NiAl(100) using scanning tunneling microscopy manipulation techniques

We present precise engineering of patterns of Co nanoclusters grown on ordered Al2O3∕NiAl(100) surface using the scanning tunneling microscopy (STM) manipulation technique. The clusters are attracted to the STM tip by lowering the bias below a threshold value and translated and relocated to another position by reversing the polarity. This facile manipulation technique in combination with the self-organized patterning on this system reported earlier might play a decisive role in nanotechnology for various applications where patterned nanoclusters are desired.

An atomic force microscope study of thermal behavior of phospholipid monolayers on mica

We observed by using atomic force microscope (AFM) phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers on mica being annealed and cooled to a selection of temperatures through steps of 2-4 degrees C/min. The annealed phospholipid monolayers started to disappear at 45-50 degrees C and disappeared completely above 60-63 degrees C under AFM observation. The phospholipid monolayers reformed when the samples were cooled below 60 degrees C and developed from fractal into compact monolayer films with decreasing temperatures. Simultaneously the height of the reformed phospholipid films also increased with decreasing temperatures from 0.4 nm to the value before annealing. The observed thermal features are attributed to a phase-transition process that upon heating to above 45-50 degrees C, the lipids condensed in the monolayers transform into a low-density expanded phase in which the lipids are invisible to AFM, and the transformation continues and completes at 60-63 degrees C. The lipid densities of the expanded phase inferred from the dissociated area of the condensed phase are observed to be a function of the temperature. The behavior contrasts with a conventional first-order phase transition commonly seen in the Langmuir films. The temperature-dependent height and shape of the reformed phospholipid films during cooling are argued to arise from the adjustment of the packing and molecular tilting (with respect to the mica surface) of the phospholipids in order to accommodate more condensed phospholipids.