The JapaneseAssociationJapanese Association for Crystal Growth (JACG){JACG) Small Metallic ParticlesProduced by Evaporation in lnert Gas at Low Pressure Size distributions, crystal morphologyand crystal structures KazuoKimoto physiosLaboratory, Department of General Educatien, Nago)ra University Various experimental results ef the studies on fine 1. Introduction particles produced by evaperation and subsequent conden$ation in inert gas at low pressure are reviewed. Small particles of metals and semi-metals can A brief historical survey is given and experimental be produced by evaporation and subsequent arrangements for the production of the particles are condensation in the free space of an inert gas at described. The structure of the stnoke, the qualitative low pressure, a very simple technique, recently particle size clistributions, small particle statistios and "gas often referred to as evaporation technique"i) the crystallographic aspccts ofthe particles are consid- ered in some detail. Emphasis is laid on the crystal (GET). When the pressure of an inert is in the inorphology and the related crystal structures ef the gas range from about one to several tens of Torr, the particles efsome 24 elements. size of the particles produced by GET is in thc range from several to several thousand nm, de- pending on the materials evaporated, the naturc of the inert gas and various other evaporation conditions. One of the most characteristic fea- tures of the particles thus produced is that the particles have, generally speaking, very well- defined crystal habits when the particle size is in the range from about ten to several hundred nm. The crystal morphology and the relevant crystal structures of these particles greatly interested sDme invcstigators in Japan, 4nd encouraged them to study these propenies by means of elec- tron microscopy and electron difliraction. the crystallographic Apart from interest,there has recently been a considerable number of studies ofsmall metal particles, from the point ef view of solid state physios, many of which may be traced back to the work of Kubo. In 1962 Kubo published a paper2) on thc electronic properties Physics Laboratory, Department ef General Education, of small mctallic His approach to the Nagoya Univcrsity particles. Fure-cho, Chilcusaku, Nagoya 464, Japan problem was quite diflbrent from the classical 88 (122> Hlgkeftdift7kts NII-Electronic Library Service The JapaneseAssociationJapanese Association for Crystal Growth (JACG){JACG) to several Torr was a traditional one where the smal1 particles were at low pressures of a tenth usually regarded as a system having larger sur- very good absorber of infrared radiation. "black" face-to-volume ratio compared with the bulk Pfund5-'} first applied bismuth evapo- materials. His theory is based on the simple fact rated from a hot tungsten spiral in covering the that the numbers of free electrons in a metal delicate receiving area of his bolometer for use in a bell at a particle becomc fewer when the particle size in infrared spectroscopy placed jar O.25Torr air. Gradually becomes srnaller. Kubo pointed out clearly for pressure of about relatcd to infrared spectros- the first time that many physical properties of studies, not always the ef these metal blacks small metal particles at helium temperature copy, on properties investigators. difler radically from those of the bulk matcrials (or smokes) were begun by various SiegelB) studied smoke when the diameter of the particlc is less than Harris,JeflViesand gold rang- about 10 nm where the numbers of atoms con- deposits prepared in nitrogen at a pressure ing from O.3 to 3 Torr by electron microscopy stituting a particle are of the order of 104. found that the mean size of the individual Metal particles produced by GET have been and extensively used as materials suitable for testing colloidal geld particles increascd with the in- crease of the of nitrogen. The increase the predictions by Kubo and for other purposes pressure also had the same of solid state physics and, recently, evcn for of the rate of evaporation size distributions. industrial purposesS'4). Unfbrtunately, howcver, eflbct resulting in the broader They also confirmed that the smoke deposits thc particles produced by GET have a greater gold showed no evidenee of orientation and that they range of size distributions than those of islands in spacing of bulk Uyeda the discontinuous films produced by means of gave only the lattice gold. zinc black evaporated in vacuum deposition. Although this does not and Kimoto9) studied air at 1 rv 80 Torr by electron neccssarily imply that the particles by GET are pressuresbetween and found that the zinc black cen- inferior to those made by vacuum deposition, it diffVaction not only metallic zinc and zinc oxide ZnO is important to know the accurate size distribu- tained tungsten trioxide WO, due to the tung- tions and the expcrimenta1 conditions which but also filament used for the evaporation source. govern the distributions in order to obtain a stcn replaced air by nitrogen and confirmed correct understanding of the experimental re- Kimoto* that the zinc black in nitrogen sults on the propcrties of small particles prepared prepared gave electron Debye rings consistent with by GET. diffiraction of the zinc and the crystal zinc and that the grain size The particle size distributions pure decreased with the decrease of the morphology and the relevant crystal structures black pressure the nitrogen. The diameter of the ef these particles (mainly metals) will be the of particles was less than 10nm when the of thc main subjects of this review paper. However, pressure was about 1 Torr; howevcr thesc results before considering the individual items a very nitrogen not been MamiyaiO) also brief historical survey will be given. have published. pre- metal of nickel, zinc, silver, lead, pared blacks 2. BriefHistoricalSurvey tin and bismuth in air at reduced pressures and 1930'sthat that their lattice were censis- It was already known in the early found pararneters "metal metals or tent with those of the corresponding bulk the soot-like substance called black" "metal accuracy of O.5% by smoke" produced by evaporating metals within the er[perimental - -- .- * such as bismuth, zmc etc. in air or in nitrogcn K. Kimoto: unpublished data. 89 VoL 6 No. 3&4 (J23) NII-Electronic Library Service The JapaneseAssociationJapanese Association for Crystal Growth (JACG){JACG) means of X-ray Debye rings. Fritsche, Wolf A conical basket or a boat made of tungsten and Schaberii} first demonstrated the fact, by (or molybdenum) wire or foil is generally used means of Tyndall scattering, that thc particles as an evaporation source into which scveral to are formed in the free space of an ambient gas sevcral tens of mg of the evaporant is placed. before they deposit en the wall of the evaporation For the purpose of obtaining a Iarge amount of chambcr. Based upon their own experiments on particles, of the order of grams, oven made of bismuth particles and on the results of other incrt refractory material (sintered aluminai`}) investigators so far obtained, thcy made clear is used at a controlled temperature. Wadai5} the relations between the particle size and thc dcveloped the method by means of a plasma jet pressure of the gas and the evaporation tempera- flame in helium containing hydrogen and this ture. They also made clear the influenoeon the method has been used for industrial purposes of the distance between particle the evaporation to obtain a large number of particles having high source and the places where the particles were melting temperature. Laser light is also used to caught. Although their results were of a rather cvaporate particular substances such as silicatei6) nature, it is qualitative seen that they have ar- for geophysical study or olivine* for cosmogonic rived at a correct understanding, in outline, of study. To collect the particles, for elcctron mi- the important of phenomenon particlegrowth in eroscopic studies, grid meshes covered with a GET. In l963 Kimoto et al.i2) published results suitable substrate arc placed anywhere in the on finc of some fifteen metals particles produced evaporation chamber; when a Iarge number is by GET in argon. In this study they first paid needed the particles are often deposited on a attention te the shapes, crystal i.e.the morphol- largc metal block, the inside of which is cooled ogy, of the small thus particles produced by by running water or liquid nitrogen, which is taking fu11advantage ef the modern techniques placed around or above the evaporation sourcei4). of electron microscopy. Fig. 1 shows, example, the for evaporation Before closing this brief historical survey, the units which are being used by the prcscnt author. author would like to cite the review papers by They consist of three main parts: an ultrahigh Comsa and Henseli3), by Granqvist and vacuum evaporation chamber with oil-free Buhrmani4) and by UyedaBi). The former two pumping systerns, high purity inert gas reservoir deal with the results on the small metallic par- and a quadrupole mass anal)rzer. The mass ana- ticles produced by GET which appeared up to lyzer is provided with a small, interchangeable l975 and the last one deals with the crystal orifice and is installed vertically at the top ofthe morpho]ogical aspects of the particles. evaporation chamber. It is used for the examina- tion of residual gas or for the detection of very 3. Production ofParticles small clusters. The experimental procedures arc 3.1 Experimentalarrangement as follows : after carefu1 outgassing ofthc evapora- The techniques and the equipment necessary tion source and then the evaporant by melting, for producing the particles by GET have been the evaporation chamber is further evacuated simple since the time of Pfund5-') when he first to a pressure of 10'9'Torr. Immediately after evaporated bismuth in air at reducod pressure the ion pump is switched ofl; the inert gas is in a an glassbellljar.When inertgas is used a introduced into the evaporation chamber through gas inlet system becomes necessary besides the * B.