Soft-land size-selected clusters

Harbich W (1999) Soft landing of size-selected clusters in chemically inert substrates. Phil Mag B 79 1307... 

A further less commonly employed but very classical method for mass-selection is the Wien-filter which combines an electrostatic and a magnetic field for mass-selective cluster deflection. This device is applied in particular with the purpose to obtain large cluster currents for cluster-surface interaction and deposition experiments at moderate resolution [75, 76]. Another approach for size-selection in combination with a sputter source ]47] or a laser vaporization source [61,77,78] has been chosen recently. In both cases, a magnetic dipole field has achieved the size-selection. Subsequent deceleration of the size-selected clusters to less than 1 eV per atom enables soft-landing under UHV conditions [47]. Monodispersed chromium cluster beam densities range from 0.1 to 5nA mm, depending on the cluster size ]47]. 

Many hypotheses have been proposed to explain the peculiar behaviour of gold nanoparticles. One of the first explanations attributes these observations to a quantum size effect. Indeed, Goodman and co-workers [74] observed on gold supported on Ti02 by STS the opening of a band gap for particles of 4nm or smaller (see Fig. 3.16). For size-selected clusters soft-landed on MgO... 

Figure 14 CO oxidation on size-selected Pt clusters soft-landed on MgO. CO2 production per Pt atom as a function of n. (From U. Heiz et al. [41].)...

Fig. 1.48. Absorption spectra of small gold clusters Au2, Au4, Aug, and Au2o in comparison to the spectra of gold atoms (lower left frame) and nanometer size gold islands (upper right frame). AU spectra were obtained with the cavity ring-down method. The mass-selected clusters were soft-landed onto an a-Si02 substrate. The gold nanoparticles were grown on the same substrate by thermal evaporation...

The reaction has been studied on size-selected, soft-landed Pd atoms and clusters deposited on MgO thin films [214,215]. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [222]. Here, we restrict the analysis to the reactivity of isolated Pd atoms. On a clean MgO(lOO) surface, no benzene is produced at the same experimental conditions. However, even 0.2 monolayers of deposited Pd atoms produce benzene for temperatures of about 215 K. This is an important result which has allowed to investigate the activity of the Pd atoms as function of the support where it is deposited or of the sites where it is bound. 

In the reduction of NO by CO at low temperature, on Pd/MgO(100) model catalysts, the reaction rate is independent of CO pressure but increases with NO pressure [64,65], then the TON of the reaction is modified by the reverse-spillover effect and, in particular, depends on particle size. In that case, it is no longer possible to compare the TON value measured on different particle sizes, it is more appropriated to calculate the reaction probability [64], which takes into account for the real flux of molecules that reach the clusters which can be measured by molecular beam experiments. Reverse-spillover effects have also been recently observed for the CO oxidation on size-selected Pd clusters soft-landed on MgO epitaxial films [66]. 

In the actual experiment, size-selected silver ions are softly landed on a transparent conductive glass plate covered with fine grains of silver bromide with a diameter of about 1 fan the cluster ions are rapidly neutralized to... 

Experiments on size-selected gold clusters, Au (2soft-landed on MgO(lOO) thin film [169] showed that the activity in CO oxidation depends on the number of cluster atoms, and is higher when the clusters are supported on defect-rich MgO rather than on defect-poor MgO. Ah initio calculations performed with Aug showed that the F centers on MgO induce a partial electron transfer to the cluster, which promotes the binding of O2 and CO, and activates the 0—0 bond to a peroxo-Uke adsorbate state that can react with gas-phase CO or adsorbed CO. 

Therefore, the cluster catalysts need to be deposited onto the support under so called soft landing conditions at deposition energies lower than 1 eV/atom [6, 11, 28]. These conditions can be ensured by a retarding field analysis (RFA), where the cluster current is measured as function of an applied retarding potential on the samples. Figure 3.4 shows a representative RFA for size-selected Ft/j clusters. 

The deposition of size-selected Si clusters on Ag(lll) [32] and Pd clusters on graphene [33] using a similar cluster source has been investigated using local probe experiments and supports the previous assumption of soft landing conditions. In order to focus the cluster beam onto the support an additional (attractive) potential of -2 V is applied for the UHV experiments, however no potential is applied for the cluster deposition of ambient samples. 

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