Size-induced Metal-insulator Transition Research Articles

Size-induced metal–insulator transition temperature decrease of VO2 nanoparticles obtained by laser irradiation in water

Vanadium dioxide (VO2) is a highly attractive and multifunctional material for technological applications due to its metal–insulator transition, which is accompanied by several orders of magnitude changes in electrical resistivity and optical transmissivity. In this work, we report on the metal–insulator transition of VO2 nanoparticles which are produced by a simple, efficient, and low-cost two-step preparation method based on infrared laser irradiation of V2O5 powder dispersed in water and subsequent stoichiometric stabilization by vacuum annealing at 773 K. VO2 nanoparticles have an average size of 36 nm with facet-like contours and undergo a structural phase transition from a monoclinic to a tetragonal phase, accompanied by a metal–insulator transition occurring around 323 K, which is about 17 K lower than bulk VO2. No evidence of the formation of intermediary metastable phases is observed. Our findings reveal that the size of VO2 nanoparticles is a key parameter to moving the transition closer to room temperature, which is important for many applications.

Size-dependent electronic-transport mechanism and sign reversal of magnetoresistance in Nd0.5Sr0.5CoO3

A detailed investigation of electronic-transport properties of Nd0.5Sr0.5CoO3 has been carried out as a function of grain size ranging from micrometre order down to an average size of 28 nm. Interestingly, we observe a size induced metal–insulator transition in the lowest grain-size sample while the bulk-like sample is metallic in the whole measured temperature regime. An analysis of the temperature dependent resistivity in the metallic regime reveals that the electron–electron interaction is the dominating mechanism while other processes like electron–magnon and electron–phonon scatterings are also likely to be present. The fascinating observation of enhanced low temperature upturn and minimum in resistivity on reduction of grain size is found due to electron–electron interaction (quantum interference effect). This effect is attributed to enhanced disorder on reduction of grain size. Interestingly, we observe a cross over from positive to negative magnetoresistance in the low temperature regime as the grain size is reduced. This observed sign reversal is attributed to enhanced phase separation on decreasing the grain size of the cobaltite.

Size-induced metal insulator transition and glassy magnetic behavior in La0.5Sr0.5CoO3 nanoparticles

Structural, electrical, and magnetic properties of La0.5Sr0.5CoO3 nanoparticles prepared by sol-gel technique are investigated and the results reveal a size-induced metal insulator transition in the electrotransport behavior. The field cooled and zero field cooled magnetizations display a broad paramagnetic to ferromagnetic transition at Tc with a large magnetic irreversibility. Attempts are made to get an idea about the spin states in the nanoparticles. The observed frequency dependent shoulder in the in-phase (χ′) component and the peak in the out of phase (χ″) component of the ac susceptibility in the low temperature region indicate the glassy nature of the samples.

Size induced metal–insulator transition in nanostructured niobium thin films:intra-granular and inter-granular contributions

With a reduction in the average grain size in nanostructured films of elemental Nb, weobserve a systematic crossover from metallic to weakly insulating behaviour. An analysis ofthe temperature dependence of the resistivity in the insulating phase clearly indicates theexistence of two distinct activation energies corresponding to inter-granular andintra-granular mechanisms of transport. While the high temperature behaviour isdominated by grain boundary scattering of the conduction electrons, the effect ofdiscretization of energy levels due to quantum confinement shows up at low temperatures.We show that the energy barrier at the grain boundary is proportional to the widthof the largely disordered inter-granular region, which increases with a decreasein the grain size. For a metal–insulator transition to occur in nano-Nb due tothe opening up of an energy gap at the grain boundary, the critical grain size is≈8 nm and the corresponding grain boundary width is≈1.1 nm.

Dc conductivity of nanoparticles of CdS and ZnS

The dc conductivity of consolidated nanoparticles of ZnS and CdS has been studied over the temperature range from 300 to 525 K and the conductivity has been found to be much larger than that of single crystals. Nanoparticles of Ag of different concentration were dispersed in nanoparticles of ZnS and CdS and the conductivity of composite pellets was also studied. The conductivity has been found to be drastically enhanced compared to that of pellets not containing Ag and that of bulk crystals. The enhancement in conductivity may be attributed to a reversal of the size-induced metal-insulator transition of Ag nanoparticles permitting charges to pass through them thereby increasing the conductivity.

Counterion and dopant induced effects on the structure of electropolymerized polyaniline thin films

We have used the technique of scanning probe microscopy in the noncontact mode to investigate the nanometer-scale domain structure of electropolymerized polyaniline thin films. While undoped polyaniline films are structurally amorphous, doped, conductive polyaniline films exhibit a structure consisting of close-packed nanometer-scale domains. Using modified effective medium theory, it is possible to accurately predict the conductivity of these films using a model based on the size distribution of individual conducting domains within the material. Dipole–dipole interactions between conducting domains as well as size-induced metal insulator transition effects must be included in this model in order to fully account for the observed macroscopic conductivity.

Size-limited conductivity in submicrometre metal particles. Similarities with conducting polymers?

We report on the conductivity of submicrometre metal particles, which experience a size-induced metal-insulator transition (SIMIT). Also, their frequency behaviour and their dependence on temperature are quite different from bulk metal properties. Investigations of the conductivity of thin polyaniline layers have revealed that the two systems have some prominent properties in common; for instance both systems exhibit a pronounced relaxation process. A comparison and discussion of the similarities are presented.

Dielectric Properties of Percolating Nanocrystals

ABSTRACTMicrowave spectroscopy revealed a non-metallic effective dielectric function of non- supported nanocrystal networks. This new result is in agreement with the size-induced metal-insulator transition (SIMIT) observed in isolated sub-μm conductors. Separating the particle contribution ε = ε1 + iε2 from the measured effective dielectric response leads to the conclusion that percolating networks of SIMIT particles are characterized by an enormous positive value of El and a conductivity σ ∼ ε2 much smaller than expected for metallic networks. Another non-metallic feature of the networks is the weak thermal influence on ε1 and ε2. All properties change sensitively with the filling factor f of the network. Their unusual dielectric properties make low- density nanocrystal networks candidates for applications e.g. as high permittivity capacitor materials or temperature-independent resistors.

Size-governed electromagnetic absorption by metal particles.

Electromagnetic absorption by submicrometer conductors is qualitatively different from Drude absorption by bulk metals. Measurements performed between radio and far-infrared frequencies with metals having diameters of about 20 nm are now analyzed with the Landau-Lifshitz-Looyenga effective-medium formula. This procedure showed that the experimental data are consistent with a size-induced metal-insulator transition. Microwave measurements rule out a significant magnetic-dipole contribution to the absorption by mesoscopic conductors.

On the quasi-static conductivity of sub-micrometer crystals

We report that both size (d) and dimensionality of the confinement imposed on an electron gas govern its conductivity. A dramatic decay of the quasi-static conductivity by more than 8 orders of magnitude has been discovered in the sub-micrometer (so-called mesoscopic) size range of metal particles when their sizes are reduced down to the nm regime. This size-induced metal-insulator transition is of universal character and can only be understood as a quantum-size effect.

Microwave Evidence for a Size-Induced Metal-Insulator Transition in Mesoscopic Conductors

ABSTRACTMicrowave investigations on sub-pm (“mesoscopic”) metal crystals revealed a size-induced metal-insulator transition (SIMIT). The microwaves were applied to determine the sizedependent quasi-static conductivity of metal crystals dispersed in an insulating matrix. Choosing an indium colloid for a model system allowed an in-situ particle size variation from 10 nm up to about 1μm. The experiments were carried out in a microwave bridge at 10 GHz, a frequency where the oscillation time is much longer than the elastic scattering time of electrons in a metal (quasi-static limit). The conductivity of metal crystals was found to decrease approximately with their volumes in the above size range. The 3-dimensional confinement of the electron wave packets gives rise to quantum-mechanical interference effects leading to the SIMIT as the crystals become smaller than 1μm. Experimental details and results including the sizeaffected temperature dependence of the conductivity are presented. The universal significance of the SIMIT and its consequences for the engineering of novel materials and the ultimate size-reduction in microelectronic devices are discussed as well.,

Size-induced metal-insulator transition in metals and semiconductors

We report on the size-induced metal-insulator transition discovered in tiny metal crystals. Here the electric conductivity drastically decreases with particle diameter below a few μm. On the other hand, in semiconductor crystallites a size-correlated inter-band absorption blue shift was observed. The effects are a consequence of the quantum confinement in all three dimensions (“quantum dots”) and hence are of universal character. In addition to their fundamental importance, the observations now allow the design of the ultimate microelectronic device and permit the engineering of novel devices as well.