Graphite coatingof metallic clusters

T. ShirokaT. Shiroka11, M. Riccò, M. Riccò11, O. , O. LigabueLigabue11

D. PontiroliD. Pontiroli11 and G. Longoni and G. Longoni22

1. Dipartimento di Fisica e Sezione INFM, Università degli Studi di Parma, PARMA

2. Dipartimento di Chimica Fisica e Inorganica, Università degli Studi di Bologna, BOLOGNAF

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Preliminaryresults

Overview

Difficulties of traditional cluster preparation methods

Non conventional ways to cluster preparation

Modications of standard DC arc discharge reactor

Conclusions and outlook

What is a cluster?

Work plan for the activity 2 The aim of this activity is to enhance the magnetic

response of metallic clusters by substituting their ligands (usually a carbonyl cage) with a fullerene-like graphitic coating.

This will be achieved in a plasma discharge reactor for fullerene production by injecting the molecular clusters in the discharge zone.

It is expected that the substitution of the carbonyl cage with an inert graphitic shell will recover the metallic properties of the metal cluster to the detriment of its molecular character which depresses magnetism.

Metallic clusters

Metallic clusters consist of a nucleus of a few metallic atoms bound to one another by direct metal-metal bonds, surrounded by a non magnetic shell (either organic or inorganic), as e.g. Pt3Fe3(CO)15.

As far as the magnetic properties are concerned, in these clusters the magnetism is due to unpaired electrons occupying unfilled delocalised cluster orbitals, although the total magnetic moment is quite reduced as compared to that of the single magnetic ions (e.g. Co55 has only 5µB).

Although many properties of metallic clusters are known, the devolopment of new, technologically important nanoscale materials requires higher quality samples, especially with regard to size dispersion.

Metallic cluster synthesis issues

The chemical synthesis of large poly-nuclear molecular clusters using organic ligands provides a solution to the mono dispersion problem.

When it comes to practical applications however some difficulties arise:

The weak ligand bond will break even at temp. as low as 150ºC. The electrons “engaged” in the bonds cannot contribute to the

magnetic and/or metallic properties of the cluster. The ligand is often chemically reactive. Once the ligand is removed the clusters coalesce giving poly-

dispersion.

The proposed solution involves the carbon coating (inert shell) of clusters through the use of a modified Huffman-Krätchmer fullerene preparation method (arc discharge).

Clusters through carbon coating

Two ways for obtaining the carbon coating of metallic clusters: Arc discharge using carbon electrodes with metal-oxide cores. Introduction of already prepared molecular clusters in an

carbon arc discharge chamber through the microspray technique.

The first method cannot produce monodisperse metallic clusters):

But what about the microspray technique?

M. Riccò, et al., Fullerenes 3 (1996) 794.

DC arc for production of C60

Reactor operating conditions:

DC current: 100 A Voltage: 32 V Temp.: ~ 4000 K Pressure: 200 Torr He

Huffman KrätschmerW. Krätschmer, L.D. Lamb,

K. Fostiropoulus, D.R. Huffman,

Nature 347 (1990) 354.

Fullerene generator design factors

Higher yields when using helium pressures of ~150 mbar (i.e. ~1/7 atm).

Large chambers imply a lower pressure variation during high temp. arc operation. A better solution involves feedback controlled dynamic vacuum.

The carbon arc involves high temperatures (4000 K) => a leak proof vacuum system and water cooling of the stainless steel chamber are needed.

Apparatus easy to dismantle for removing soot and/or replacing rods.

A DC arc in a rod-block arrangement is more convenient than an AC arc.

Soot quality independent from carbon rod purity (at least for purity >1%).

A low voltage high current supply is needed (welding kit) => 30 V - 120 A.

Reactor components

Pumping station

Arc reactor

Power, control, and

cooling

Discharge control

unit

Generator control unit

Modifications for cluster injection

Modified arc reactor

Cluster injector

Three-way electrical

valve

Pinch valve operationFluid pressure:0.20.5 atm (N2).

Spraying pressure:36 atm (He).

Solvent:C3H6O – Acetone

Chamber pressure:200 ± 50 Torr He

Advantage: Starting from identical molecular clusters => no size dispersion.

Conclusions and Outlook Modifications of the arc discharge reactor for using the

microspray technique for metallic cluster production.

Cluster injection is still to be performed (different starting molecules, and different solvents to be tested).

Testing the produced cluster properties (electric, magnetic, size dispersion, etc).

Optimize synthesis condition for improving and enhancing selected properties.