3/31/98

Fellow microscopists:



We're trying to make a deposition of 2 micron gold particles on a silica

substrate.

We cannot use an evaporator because we don't want a thin film: we want to

deposit the particles directly on the substrate, keeping them isolated and

without forming aggregates.

At first we dispersed the powder directly on the silica, but SEM analysis

showed they formed aggregates.

Then we've tried making a suspension on etanol and then putting a droplet

of the solution directly on the substrate. But still it formed aggregates.



Does anyone know of a simple method to make such a deposit (without forming

aggregates) ?



Thanks in advance.





Isabel Nogueira

Instituto Superior Técnico

Departamento de Engenharia de Materiais

Av. Rovisco Pais

1096 Codex

Tel.: 351 - 1 - 8418124/0

Fax.: 351 - 1 - 8418120

E-mail: isabeln@alfa.ist.utl.pt



Isabel,

Some years ago we needed a high surface area Al target for laser induced

X-ray generation. The approach we used was to evaporate aluminum through a few

Torr of N2. This produced a very granular surface. The particles were very

well defined. Their nominal dimension was 100mn. It was a film but then that

is what we wanted. I suspect that grain size is a function of distance &

pressure while density is a function of time. Perhaps you can tweak this

approach to you need. Fair warning, this hi pressure deposition made our

evaporator very dirty.



Bruce Brinson

Rice U.

Bruce Brinson <brinson@rice.edu>

There are reliable methods to make coloidal gold in suspension. The gold

particles are very regular in size and show little aggregation. Also

aggregates can be removed by centrifugation.

Your problem would seem to be to deposit these particles onto the silica

substrate without aggregation. I guess it should not be impossible by

adjusting concentration and charge of the particles in suspension. Also,

they probably adsorb to the film if you put the suspension briefly in

contact with the substrate. If everything fails, may be they can be

centrifuged into the silica.

It is possible that the coloidal gold is more stable in solution than

the gold you are using, but I do not know if it is possible to make it

as large as you want. Usually the particles have up to 15nm.

May be they can be grown after they are deposited in the silica.



Good luck



A.P. Alves de Matos

Electron Microscopy Unit

Curry Cabral Hospital

Lisbon

A.P. Alves de Matos" <mtlopes@fc.ul.pt>

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