The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as ‘anomalous’. Our present electron-microscope studies indicate that this anomalous behaviour usually includes the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of...

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides

Data on the experimental structure of small particles is reviewed, the emphasis being an attempt to correlate experimental information with theoretical models. First, a general discussion of some of the controlling factors is presented, primarily equilibrium shapes of small particles, the effect of surface stresses, kinetics, and the role of chemisorption and the substrate. Experimental techniques for obtaining information about small particles are then described, primarily electron microscopy approaches. Experimental data on the static structure of small particles is then reviewed, both single crystals and the many, complicated twinned structures in face-centred cubic materials. An overview is then given of some of the more recent results on dynamic phenomena in small particles. Finally, a general model merging thermodynamic and kinetic factors is presented to attempt to rationalize the available data, followed by a brief discussion.

Experimental studies of small particle structures

Specimen size dependences of the dielectric constant and of its variations with temperature have been investigated by measuring the optical plasma resonance absorption of small silver particles embedded in glass. The following relations hold at the wavelength 405 nm: epsilon 2 (imaginary part at 300K)=0.23+2.64/R, and epsilon 2(300K)- epsilon 2(1.5K) approximately=0.07-0.08/R where R is the particle radius in nm. Agreement has been found with the free path effect but not with the quantum size effect theories of Kawabata and Kubo (1966) and of Glauberman and Adamyan (1969). The behaviour of the temperature dependence can partly be explained by thermal lattice contraction. The ratio of electron density to the optical effective mass shows no distinct changes as R varies from 10.5 nm to 1.1 nm. It may hence be concluded that normal metallic properties are present in silver particles embedded in glass which contain more than about 400 atoms.

https://iopscience.iop.org/article/10.1088/0305-4608/4/7/007/pdf

Electronic properties of small silver particles: the optical constants and their temperature dependence

The orientation and structure of gold particles formed by evaporation on rocksalt cleaved in ultrahigh vacuum have been studied at various stages of the nucleation and growth process. Besides four 200 and twenty-four 220 spots which indicate the presence of the (001) orientation and (111) orientation respectively, twenty-four abnormal 111 spots have been observed in electron diffraction patterns taken at earlier stages of growth, and various unusual contrasts have been observed in dark field electron micrographs using one of 111 spots. A multiply twinned particle model is proposed by the present author, and the appearance of the 111 spots and the dark field image contrasts can satisfactorily be explained by this model. With increasing thickness particles with the (111) orientation and without the multiple twinning exclusively grow up, while multiply twinned particles and those with the (001) orientation seem to stop their growth; and a continuous gold film with the (111) fibrous structure is formed at a t...

Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. II. Orientation and Structure of Gold Particles Formed in Ultrahigh Vacuum

We have synthesized Co fine particles with the average diameter $(D)$ of less than 500 \AA{} by sputtering Co in a somewhat high inert-gas pressure. It has been found that there is a close relationship between the particle size and the crystal phase; that is, pure fcc (\ensuremath{\beta}) phase for $Dl~200\AA{},$ a mixture of hcp (\ensuremath{\alpha}) and \ensuremath{\beta} phases for $D\ensuremath{\sim}300\AA{},$ and \ensuremath{\alpha} phase with inclusion of a very small amount of \ensuremath{\beta} phase for $Dg~400\AA{}.$ Precise structural characterizations have revealed that the \ensuremath{\beta} particles are multiply twinned icosahedrons and the \ensuremath{\alpha} particles are perfect single crystals with external shape of a Wulff polyhedron. In order to explain the size effect on the crystal phase of Co fine particles, we have performed theoretical calculations for total free energies of an \ensuremath{\alpha} single crystal, a \ensuremath{\beta} single crystal, and a multiply twinned \ensuremath{\beta} icosahedron. The present calculations well explain the size dependence of the crystal phase of the Co fine particles, and have revealed that the stabilization of \ensuremath{\beta} phase, confirmed by previous studies, is the intrinsic effect caused by the small dimensionality of fine particles. Moreover, the phase transformations that occurred in annealing experiments can also be explained by the theory.

Size effect on the crystal phase of cobalt fine particles

Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. I

Succeeding to previous studies (Ino, 1966; Ogawa et al. , 1966), the orientations and the structures of thin gold films formed in ultrahigh vacuum by evaporation on to NaCl and KCl have been studied at earlier stages of film formation by electron diffraction and electron microscopy, especially by the dark field technique. The present paper describes some results obtained after the previous reports. Some particles which show rhombic shape in the bright field image and various contrasts in the dark field images have newly been observed. In order to explain such contrasts, the “multiply twinned particle model with a nucleus of (001) orientation” is proposed. Abnormal diffraction spots and image contrasts observed from thin gold films have satisfactorily been explained by using this model in some cases and by using the multiply twinned particle models with a nucleus of (111) orientation in other cases.

Multiply Twinned Particles at Earlier Stages of Gold Film Formation on Alkalihalide Crystals

The epitaxial growth of Ni, Cu, Ag, Au and Al on NaCl crystals cleaved in vacuum has been studied. When the crystals are cleaved in ordinary high vacuum (10 -4 –10 -5 mmHg), a considerable lowering of the epitaxial temperatures for the parallel epitaxy takes place for the first four metals, as compared with the case in which the crystals are cleaved in air. The temperature differences due to this lowering are 270°C for Ni, 250°C for Cu, 150°C for Ag, and 120°C for Au. Al has been found for the first time to form single crystal films with the (111)-plane parallel to the substrate at the substrate temperature of 100°C. When the degree of vacuum is improved to ultrahigh vacuum (10 -7 –10 -9 mmHg), on the other hand, the (001) preferred orientation deteriorates and the (111) preferred orientation appears for Cu, Ag and Au, the latter orientation becoming conspicuous with rising temperature for Au and Ag, while no considerable change arises for Ni and Al with the improvement of the degree of vacuum.

Epitaxial Growth of Metals on Rocksalt Faces Cloven in Vacuum

The ordered state in the gold-copper alloy of the atomic ratio 1 to 1 was investigated by electron: diffraction with well orientated thin films. Below 340°C the pattern corresponded with a tetragonal superlattice the c axis of which is distributed into three original cubic axes, while above 360°C sharp crosses appeared at some of the positions of superlattice reflection, being accompanied with subsidiary maxima, and at the same time regularly spaced satellites arose around normal reflections, especially, conspicuously around the incident beam position. The former phenomenon could be clearly interpreted by antiphase domains which occur in the tetragonal superlattice and which lead to the orthorhombic CuAu II structure, and the latter which was firstly found by this experiment proved that a definite expansion of lattice appears periodically in each boundary of antiphase domains, due to the asymmetrical distribution of gold and copper atoms at this place and to the tetragonality. Among the satellites around ...

Electron Diffraction Study On the Ordered Alloy CuAu

Ordered structures of copper-palladiurn alloys, chiefly of a '' phase (18∼28 atomic per cent Pd) were investigated by electron diffraction, using orientated, evaporated films. The elucidated structure of a '' phase can be thought of as formed from the elementary face-centred tetragonal lattice, in which the atoms arrange themselves as in the ordered lattice of Cu 3 Au but the two kinds of step-shift occur at every definite distance along two directions parallel to the crystallographical axes, respectively. Thus, a two-dimensional anti-phase domain structure takes place, which is considered to have a large orthorhombic unit cell. The present model seems to be connected, not with that proposed by Geisler and Newkirk, but closely with that proposed by Schubert et al.

Shiro Ogawa

Papers

Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. I

Multiply Twinned Particles at Earlier Stages of Gold Film Formation on Alkalihalide Crystals

Epitaxial Growth of Metals on Rocksalt Faces Cloven in Vacuum

Electron Diffraction Study On the Ordered Alloy CuAu

On the Superstructure of the Ordered Alloy Cu 3 Pd I. Electron Diffraction Study