Size-induced metal-insulator transition

Composites containing nanometer-sized metal particles of a controllable and uniform size in an insulating ceramic matrix are very interesting materials for use as heterogeneous catalysts and for magnetic and electronic applications. They show quantum size effects, particularly the size-induced metal-insulator transition (SIMIT) [1],... 

Size- Induced Metal-Insulator Transition Insulator... 

On the Size-Induced Metal-Insulator Transition in Clusters and Small Particles... 

Figure 1. A representation of the successive fragmentation, or division, of a single grain of metal. The inevitable size-induced metal-insulator transition now effectively links the macroscopic (metal) and the microscopic (insulator) regimes. In all cases we identify the approximate particle diameter and nuclearity, and the accompanying (Kubo) average electronic energy gap separating occupied and unoccupied energy levels. This figure is modified from Edwards. ...

Gor kov and Eliashberg investigated the problem of a size-induced metal-insulator transition in terms of the location of the gas of conduction electrons in a metal through the (finite) size-induced confinement of the electron wave packet. The de Broglie wavelength X of electrons is given by ... 

Within the context of the metal-insulator transition in metal particles and clusters, two closely interwoven problems can be identified. The first is the inevitable transition to insulating (non-metallic) behavior within an individual grain or partiele of metal as its geometric dimensions are continuously reduced. Throughout this process, the electron wavefunction is assumed to be completely confined within the single particle this is the size-induced metal-insulator transition (Fig. 1). 

Figure 6. The variation in the measured ionization potential of mercury clusters as a function of cluster size. The work function for bulk Hg (4.49 eV) is indicated. The dashed line is a plot of the ionization potential calculated for the classical (liquid drop) electrostatic model for a metalUc sphere of diameter d. Region III contains clusters which are classified as insulating. Region II denotes the size-induced metal-insulator transition, in which overlap of the 6s and 6p states sets in at around Hgn. The larger clusters, located in Region I, have valence electronic structures that closely resemble the band structures of liquid and crystalline mercury. Adapted from Rademann. i...

Measuring the d.c. conductivity of isolated metal particles, of course, is arguably the most direct method of investigating the size-induced metal-insulator transition within a single particle. Marquardt, Nimtz and coworkers believe they have succeeded in measuring the quasi-A.c. conductivity of individual indium nanoparticles... 

Figure 10. The size-induced metal-insulator transition in mesoscopic crystals of indium. The size-dependent [quasi) d.c. conductivity versus particle diameter. For comparison the bulk conductivity and the classical (surface) size-effect are also displayed. Modified from Nimtz et alP° ...

Figure 11. (a) The conduction gap observed in small clusters of gold, palladium, cadmium, and silver as a function of cluster volume, (b) Normalized slope of the /-K curves (the conductance) as a fraction of cluster volume for the four metals studied. Above a critical volume of ca 4nm the slope becomes size-independent. This is possibly a direct indication of a size-induced-metal-insulator transition, as indicated. Taken from Vinod et a/. ... 

If an individual grain of a metallic element or compound is sufficiently reduced in physical size, we expect a size-induced-metal-insulator transition (SIMIT). We have attempted here to highlight certain key features of the problem in terms of the... 

On the Size-Induced Metal—Insulator Transition in Clusters 1479... 

In 1988, the so-called size induced metal insulator transition (SIMIT) was described. [97] It will be observed if the volume of a metal partide is reduced to such an extent that size dependent quantization effects occur. This is the beginning of the metal to semiconductor transition found at the very end of metallic behavior or, coming from the molecular state, the onset of metallic behavior. Hie SIMIT is already effective in 20 nm particles. Due to this quantization effect, standing electron waves with discrete energy levels are formed. 

Electronic properties of nanocrystals critically depend on size. This aspect is aptly put forth in the quest How many atoms make a metal . It is clear that as the size of metal nanocrystals is reduced, the accompanjung changes in the electronic structure render them insulating. This transition, called the size-induced metal-insulator transition (SIMIT), has evoked much interest from chemists and physicists alike. A SIMIT is manifested in experiments that measure the electronic band structure and atomistic properties such as ionization energy. 

P.P. Edwards, R.L. Johnston, C.N.R. Rao The size-induced metal insulator transition in clusters and metal particles in Metal clusters in Chemistry, ed by P. Braunstein, G. Oro, P.R. Raithbay (Wiley-VCH, Weinheim 1998)... 

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