Introducción de datos

As the MACS is concerned with the increase of cluster flux towards the industrial R & D or batch production scale, this section aims to contextualise the source within the specific area of high flux nanocluster sources. The cluster beam sources, pre- sented above, have demonstrated numerous improvements, not only in understand- ing, but also in the control of size, composition (binary materials, core-shell clusters), deposition, purity and even structure.82, 83, 90, 103–105 However, size-controlled cluster production rates, remain relatively low (<10-100µg/h).86, 106 _{Interesting develop-} ments are taking place currently across several research groups towards attaining

the goal of high nanocluster production rates from cluster beam sources (i.e mg to g).

A pioneer in this field is the Micro Plasma Cluster Source (MPCS).107–110 De- veloped in Prof. Paolo Milani’s lab, this couples micro plasma vaporisation with supersonic expansion and aerodynamic focusing.111 _{A rod of cluster material is} used as feedstock, which is sputtered in a pulsed fashion (∼10 Hz) (similar to laser ablation or PAICS). This is performed by injecting a small pulse of gas (normally helium). As the gas reaches the rod, a bias is applied (102 _{V), igniting a plasma} and sputtering a localised area.

Plasma Anode Cathode Vacuum Pulse Valve Gas inlet Exit Nozzle b) a)

Figure 2.13: a) Schematic of an MPCS. b) Shows the effect of discharge voltage on material yield. Reproduced from [109].

The design of the pulsed micro plasma source is illustrated in figure 2.13 a), alongside the dependence of material yield on bias voltage, which is shown in Fig. 2.13 b). Initially, the increase is linear but diminishing returns are observed at higher voltages. The output can be further enhanced by careful choice of the pulse timings. In figures 2.14 a) and b) the effect of the ratio between the delay from gas pulse to voltage pulse (ti−d_{) and the gas pulse duration (t}

on) is investigated. The average size is almost an exact inverse of the yield, which is unexpected compared with the metal concentration in gas aggregation. As such, gas dynamics must have a significant effect on the cluster formation.

c) d) a) µm µm 0.2 0.6 0.2 0.6 1 b)

Figure 2.14: The intrinsic control over cluster size a) and yield b) produced is demonstrated by varying the ratio between the gas and voltage pulse delay (∆ti−d) and gas pulse duration (ton). Size can also be selected aerodynamically. Carbon films deposited from aerodynamically separated small c) and large d) nanoclusters show a change in roughness and grain size indicative separation. Images have been reproduced from [108, 109].

filtering.108 _{Figures 2.14 c) and d) show the films made by depositing light and} heavy clusters, respectively. The effectiveness can be seen in the grain size of the thin films produced (∼8 nm and 15 nm, respectively). Focusing has also been used to control the landing of particles leading to ”site selection”. The 2D position on the surface can be controlled by the aerodynamics of the source. In the case of soft materials, the velocity can be used to dictate the implantation depth.112–115

The MPCS is capable of high-scale production (of the order of 5 nm of material per minute) and relies on aerodynamic focusing and plasma sputtering parameters to provide coarse cluster size selection.108, 111 _{Since the original studies, a range of} materials has been investigated.113, 116–118

ter production techniques: a thermal supersonic seeded expansion source,119 _and a dual and tri-magnetron gas aggregation sources.84, 85, 120 _{The thermal supersonic} expansion-based source takes this high-flux source and deposits the clusters into a room temperature ionic liquid, providing an effective storage medium that limits aggregation (comparatively). The ionic liquid may provide a way to overcome the size limitation (discussed in section 3.3) as a controllable synthetic medium which both stores and grows the nanoclusters.

The dual magnetron gas aggregation source developed at Teer Coatings Ltd, removes the mass filtering step usually employed in order to gain greater flux. The charged fraction is directly deposited onto the sample achieving beam currents of around 100-200 nA, which compares with cluster beam currents of 0.1-1 nA which are normally realised using mass-filtered sources.86 The system has two other interesting features, the dual magnetron configuration, which enables the production of binary materials with tunable gas phase material ratios, and the ability to deposit onto catalytic support powders using the novel vibrating cup deposition stage.

The schematic in figure 2.15 shows the configuration of the system. After aggre- gation, an octopole lens deflects the beam down to deposit on an agitated powder. The beam can also be allowed to pass through the beam deflector in order to reach a mass-filter that is used to take mass spectra of the cluster beam or perform size selected deposition. Binary clusters of PdSn and PdTi have been deposited onto catalytic support powders at weight loadings of ∼0.1 %. The catalytic properties of the produced catalyst powders have been compared to similar reference materials made by chemical routes (prepared by Johnson Matthey).

A summary of the results for 1-pentene hydrogenation are given in figure 2.15 b). The results for the chemically prepared samples are indicated by solid symbols, while the cluster beam-deposited samples are indicated by open symbols. The selectivity of the catalyst is plotted against the reactivity, the region of optimum catalyst performance is shaded in yellow ( i.e. >80 % selectivity and activity). A large

a)

b)

Figure 2.15: a) An illustration of the dual magnetron gas aggregation cluster source developed at Teer Coatings Ltd. This has been designed for deposition onto cat- alyst support powders. b) shows the comparison of the activity and selectivity of catalysts for 1-pentene hydrogenation. The cluster beam materials (open symbols) are compared with chemically synthesised clusters (solid symbols). Samples have been made with comparable size and stoichiometry. The yellow box indicates the location of ideal catalysts. Images are reproduced from [84].

number of the beam deposited cluster materials are found in or near this region. This is thought to be due to a more monodispersed end product from the cluster beam route i.e. controlled size, cluster surface interaction and no residual active site from production.

The three magnetron gas aggregation system is a commercial system produced by Oxford Instruments. The running parameters (relative magnetron positions, total and relative magnetron gas flows and condensation length) have been investigated in detail by researchers in the Institute of Material Science in Madrid.121 _This

configuration allows better control of the gas flow within the condensation chamber, allowing the extraction of a larger fraction of the formed clusters (growth rate ∼90 nm/min). Furthermore, the three magnetron setup allows the production of ternary materials with a controllable average stoichiometry.120

A schematic diagram of the three magnetron source is shown in figure 2.16 a). A second highly interesting property of the source is the ability to produce binary and ternary clusters with ordered shells. This is done by changing the order of the magnetrons (e.g. staggered in vacuum lengths). An example of a CoAu core shell cluster produced in this way is given in Figure 2.16 b). The shell ordering can be reversed by changing the position of the magnetrons and the core-shell ratio modified by the relative magnetron powers. The production of a ternary core-shell- shell particle has also been demonstrated.

a)

b)

Figure 2.16: a) is a Schematic illustration of the Oxford Instruments three mag- netron gas aggregation cluster source (b). An example of a binary core-shell cluster produced from the source. In this case, a CoAu cluster, the shell order and thickness can be controlled via the source parameters. Reproduced from [85, 120].

Each source has a very different specific niche role; however, for a general com- parison the amounts of material produced by each system can be roughly compared based on their published numbers. An overview of the amount of material produced by each source is summarised in table 2.3.

Table 2.3: This table summarises the amount of material produced from various high-flux cluster beam sources. The values here are based on published results re- viewed in section 2.3. For comparison, the measured films (MPCS, Tri-Magnetron) have been treated as having bulk density. In the case of the MPCS, the true density is known to be 40-60 % of bulk (the corrected number is given in brackets).The dual magnetron average cluster mass is estimated at 7×105 amu from reported mass spectra.

Source Thickness Area Current Material

(nm/min) (mm2_{) (nA) (mg/h)}

Thermal Cluster Source 3.3 700 - 1.5

PMCS 120 28 - 46(23)

Dual Magnetron - - 200 5

Tri-Magnetron 90 1200 - 90

There is another system that has recently been reported with the potential to deliver a significant scale-up in cluster flux, the MACS, which is the main topic of this thesis. The MACS produces clusters via co-condensing a rare gas and cluster material film on to a mesh, which is then sputtered in transmission geometry to form, ripen and extract clusters.91 _{A schematic outline of the MACS system can be} seen in figure 2.17.

Figure 2.17: Scheme of the MACS1 system, a transmission mode Matrix Assembly Cluster Source (MACS). Reproduced from reference [91].

Figure 2.18 shows how the mean nanocluster size depends on the silver loading in the matrix. As the silver loading changes (1-10% by number of atoms) the mean cluster size varies from <100 to 1000 silver atoms (measured by HAADF STEM). In the best case, an intrinsic size control of ±10 % in the cluster diameter is achieved. This is equivalent to a mass resolution of >1.5 (Peak over FWHM). A 950 eV 70 nA ion beam was incident on the matrix for 60 s to deposit the clusters seen in the STEM image in figure 2.18.

Figure 2.18: The left hand image shows the average size of nanoclusters created by sputtering argon matrices containing different concentrations of silver. The size is obtained by comparing the integrated HAADF intensity of clusters to that of a single atom. The right hand side is an HAADF STEM image of nanoclusters produced from a matrix with a 2.3% loading of silver atoms. Reproduced from [91].

The ultimate aim of the MACS is to increase the production rate of nanoclus- ters to an industrially meaningful level, i.e. g/h. The conversion efficiency of ions hitting the matrix to clusters is ∼3 % (i.e. excluding ions hitting the support grid). Commercial ion sources producing 1 A of ion current are available. If the process can be effectively scaled this would equate to a nanocluster production rate of ∼1 g/s (assuming a cluster size of Ag200).