Granular Metal Research Articles

Tunnel percolation and current path switching in a granular metal

By means of electron beam induced deposition we have prepared a series of granular metals consisting of tungsten nanocrystallites embedded in an insulating matrix. The metal content varied between 19 at% and 34 at%. The samples have been analyzed by voltage-current and temperature-dependent conductivity measurements. Within the insulating matrix the nm-sized metallic nanocrystallites are irregularly distributed so that the classical percolation threshold can be assumed to be at approximately 50 at% metal volume fraction. Consequently, within the classical limit no current transport at low temperatures would be expected. Nevertheless, for samples with metal volume fraction above 24 at% the temperature-dependent conductivity extrapolates to a finite value for T = 0. We argue that clustering of nanocrystallites to larger aggregates of enhanced tunneling propability between the cluster members may be the reason for the observed behaviour. The transport can then be considered as a tunnel percolation process between clusters. As a consequence the current is inhomogeneously distributed in the samples. From analyzing different voltage regions of the voltage-current characteristics, separated by a sample-dependent threshold voltage VC, showing either reversible (at lower voltages) or irreversible (at higher voltages) behaviour, we conclude that only one current path is active at any given moment and that this current path need not be that of the highest possible conductance. In particular, in some instances it proved possible to increase the conductance by a factor of 106 after having passed VC.

ChemInform Abstract: Characterization of Granular Metal Oxide Semiconductor Gas Sensitive Layers by Using Hall Effect Based Approaches

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Thermal degradation mechanisms of PEDOT:PSS

The thermal stability of thin (50 nm) PEDOT:PSS films, was investigated by dc conductivity measurements, X-ray and UV photoelectron spectroscopies as a function of heating temperature and heating time. The mechanism of electrical conductivity as a function of temperature is consistent with a hopping type carrier transport. The electrical conductivity decreased, as a function of time, in agreement with a granular metal type structure, in which aging is due to the shrinking of the PEDOT conductive grains. XPS and UPS spectra indicate that conformational changes of the PEDOT:PSS film are responsible for this behaviour and a model for these modifications is proposed.

Magnetoconductance oscillations at a nanoparticle film–superconductor interface: a meansfor probing flux penetration depth

Interfaces between disordered normal materials and superconductors (S) can exhibit‘reflectionless tunnelling’ (RT)—a phenomenon that arises from repeated disorder-drivenelastic scattering, multiple Andreev reflections, and electron/hole interference. RT has beenused to explain zero-bias conductance peaks (ZBCPs) observed using doped semiconductorsand evaporated granular metal films as the disordered normal materials. Recently, inaddition to ZBCPs, magnetoconductance oscillations predicted by RT theory have beenobserved using a novel normal disordered material: self-assembled nanoparticle films. In thepresent study, we find that the period of these oscillations decreases as temperature(T) increases. This suggests that the magnetic flux associated with interfering pathwaysincreases accordingly. We propose that the increasing flux can be attributed to magneticfield penetration into S as . This model agrees remarkably well with knownT dependence of penetration depth predicted by Bardeen–Cooper–Schrieffer theory. Ourstudy shows that this additional region of flux is significant and must be considered inexperimental and theoretical studies of RT.

Intercluster conduction in lightly doped La1 − x Ca x MnO3 manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La 1 − xCaxMnO3 polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature TC. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ TC is fitted by the relation logρ ∼ T−1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.

Microstructural properties and local order around iron in granular metal–insulator Fe/Si3N4 systems prepared by magnetron sputtering

[Fe/Si3N4] multilayers have been prepared by magnetron sputtering with a layer thickness of 3 nm for silicon nitride and the iron layer ranging from 1 to 10 nm. A variety of techniques for determining the microstructure, such as x-ray reflectivity, transmission electron microscopy and Rutherford backscattering spectrometry techniques, have been performed to unveil the microstructure of such samples. Also, an analysis of the local environment around iron, by means of extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopies performed at the Fe K-edge, confirms the presence of two phases forming the iron layers, the metallic Fe and the FeN phase. Special emphasis has been placed on the x-ray absorption analysis for multilayers with smaller Fe layer thicknesses, which present features of granular systems.

The relationship between quantum transport and microstructure evolution incarbon-sheathed Pt granular metal nanowires

The carbon-sheathed Pt nanowires fabricated by focused ion beam induced deposition werePt grains in a Ga-doped carbon matrix. The dimensions of the Pt quantum dots and theintergrain coupling strength were modified through annealing the nanowires, and thereforeour investigation demonstrates a tunable ‘quantum metal’ system by experiment viaadjusting both the effects of electron–electron (e–e) interaction and the quantuminterference. At low temperatures, the as-deposited samples display a temperature dependence of resistivity, while the500 °C annealedsamples exhibit a ln(T) temperature dependence of conductivity. For the900 °C annealed samples, the conductivity was enhanced by one order of magnitude, accompaniedby a metallic temperature dependence of resistivity and a negative magnetoresistance. Theinteresting transitions from e–e interactions to local voltage fluctuations, from electronlocalization to delocalization, from tunneling transport between isolated Pt grains todiffusive transport in continuously conductive metal, and from weak antilocalization withspin–orbital scattering to weak localization, are discussed with reference to the evolution ofthe sample microstructures. The results and discussions may be valuable for understandinghow the microstructures influence the manner of electron transport through controlling theintergrain coupling strength, intragrain confinements, and the degree of disorder.

Preparation and Electrical Properties of Cobalt−Platinum Nanoparticle Monolayers Deposited by the Langmuir−Blodgett Technique

The Langmuir-Blodgett technique was utilized and optimized to produce closed monolayers of cobalt-platinum nanoparticles over vast areas. It is shown that sample preparation, "dipping angle", and subphase type have a strong impact on the quality of the produced films. The amount of ligands on the nanoparticle's surface must be minimized, the dipping angle must be around 105 degrees , while the glycol subphase is necessary to obtain nanoparticle monolayers. The achieved films were characterized by scanning electron microscopy (SEM) and grazing incidence X-ray scattering (GISAXS). The electrical properties of the deposited films were studied by direct current (DC) measurements, showing a discrepancy to the variable range hopping transport from the granular metal model and favoring the simple thermal activated charge transport. SEM, GISAXS, as well as DC measurements confirm a narrow size distribution and high ordering of the deposited films.

Using granular film to suppress charge leakage in a single-electron latch

A single-electron latch is a device that can be used as a building block for Quantum-dot Cellular Automata (QCA) circuits. It consists of three nanoscale metal "dots" connected in series by tunnel junctions; charging of the dots is controlled by three electrostatic gates. One very important feature of a single-electron latch is its ability to store ("latch") information represented by the location of a single electron within the three dots. To obtain latching, the undesired leakage of charge during the retention time must be suppressed. Previously, to achieve this goal, multiple tunnel junctions were used to connect the three dots. However, this method of charge leakage suppression requires an additional compensation of the background charges affecting each parasitic dot in the array of junctions. We report a single-electron latch where a granular metal film is used to fabricate the middle dot in the latch which concurrently acts as a charge leakage suppressor. This latch has no parasitic dots, therefore the background charge compensation procedure is greatly simplified. We discuss the origins of charge leakage suppression and possible applications of granular metal dots for various single-electron circuits.

Mesoscopic effects in macroscopic granular systems

Granular metals (systems of discontinuous metallic thin films) characterized by large distributions ofgrain sizes and inter-grain coupling were prepared by quench condensation. These samples, which are2 mm × 2 mm in size, exhibit a number of experimental findings characteristic of low dimensionalsamples including magnetoresistance oscillations, sample-to-sample fluctuations andresistance switches. Such ‘mesoscopic’ effects are observed in a variety of metals suchas Ni, Pb and Au. The results indicate that the transport in these systems isgoverned by a very small number of grains even though the sample contains about109 grains. We interpret the findings as indications that the large distribution of resistances inthe percolation network of the conduction trajectories facilitates the situationin which a small number of grains dominate the transport of the macroscopicsystem.

Hall transport in granular metals

We present a theory of the Hall effect in dense-packed granular systems at large tunneling conductance ${g}_{T}⪢1$ (metallic regime). The Hall transport is essentially determined by the intragrain electron dynamics which, as we find using the Kubo formula and diagrammatic technique, can be described by nonzero diffusion modes inside the grains. We show that in the absence of quantum effects the Hall resistivity ${\ensuremath{\rho}}_{xy}$ depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula ${\ensuremath{\rho}}_{xy}=H∕({n}^{*}ec)$, where ${n}^{*}$ differs from the carrier density $n$ inside the grains by a numerical coefficient determined by the shape of the grains and type of granular lattice. We then study the quantum effects of the Coulomb interaction and weak localization by calculating the first order in $1∕{g}_{T}$ corrections and find that (i) in a wide range of temperatures $T\ensuremath{\gtrsim}\ensuremath{\Gamma}$ exceeding the tunneling escape rate $\ensuremath{\Gamma}$, the Coulomb interaction gives rise to the logarithmic-in-$T$ correction to ${\ensuremath{\rho}}_{xy}$, which is of local origin and absent in conventional disordered metals; (ii) the large-scale ``Altshuler-Aronov'' correction to the Hall conductivity ${\ensuremath{\sigma}}_{xy}$ vanishes, $\ensuremath{\delta}{\ensuremath{\sigma}}_{xy}^{\mathit{AA}}=0$; (iii) the weak localization correction to the Hall resistivity ${\ensuremath{\rho}}_{xy}$ vanishes, $\ensuremath{\delta}{\ensuremath{\rho}}_{xy}^{\mathit{WL}}=0$. The results (ii) and (iii) are in agreement with the theory of conventional disordered metals.

Investigation of oxygen growth pressure effects on TiO2−δ:Co

We find that depending on the oxygen pressure during growth (PO2), the TiO2−δ:Co films show dramatically different magnetic behaviors. The magnetic properties are dominated by cobalt nanoparticles and are sensitive to the nanoparticle size. Hopping transport behavior expected in multiphase granular metal systems is observed for most of the samples in the measured temperature range and the linear I-V regime. Concomitantly, voltage induced tunneling conduction is observed in high electrical fields (nonlinear I-V regime) and at very low temperature (0.35K). Cross sectional transmission electron microscopy images provide further corroboration of the multiphase structure of these materials.

Relaxation Dynamics in Quantum Electron Glasses

It is experimentally shown that, depending on the carrier concentration of the system n, the dynamics of electron glasses either slows down with increasing temperature or it is independent of it. This also correlates with the dependence of a typical relaxation time (or "viscosity") on n. These linked features are argued to be consistent with a model for dissipative tunneling. The slow relaxation of the electron glass may emerge then as a manifestation of friction in a many-body quantum system. Our considerations may also explain why strongly localized granular metals are likely to show electron-glass effects while semiconductors are not.

Characterization of granular metal oxide semiconductor gas sensitive layers by using Hall effect based approaches

The paper provides a critical assessment of the progress made in the field of gas sensing metal oxide semiconductors (MOX) based on the Hall effect measurement as the main experimental data source. At the beginning, the focus is on the morphologic and the functional peculiarities of this type of material that differentiate them among the larger class of polycrystalline, granular and porous semiconductors. On this basis the complications in defining and extracting the electrical parameters of MOX, especially under operating conditions, are discussed. Different experimental solutions addressing the Hall effect measurements on gas sensing MOX reported in the literature are then briefly presented as the foundation of the discussions about the most utilized modelling procedures. Two main trends are identified: the solid-state and the chemo-physical ones. The solid-state methods focus on the crystallite energy structure, introducing the external influences as model parameters describing the interface trap distribution. The chemo-physical models firstly give a physical picture of the chemical interaction of the ambient with the solid-state surface by connecting the target gases partial pressures with the surface trap concentration and energy and, afterwards, follow the formalism used by the solid-state models. For each type of approach, representative examples and applicative illustrations are considered. An extended overview of the accessible data is included at the end of the paper in a tabulated form.

Hall Resistivity of Granular Metals

We calculate the Hall conductivity sigma(xy) and resistivity rho(xy) of a granular system at large tunneling conductance g(T)>>1. We show that in the absence of Coulomb interaction the Hall resistivity depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula rho(xy)=H/(n*ec), where n* differs from the carrier density n inside the grains by a numerical coefficient determined by the shape of the grains. The Coulomb interaction gives rise to logarithmic in temperature T correction to rho(xy) in the range Gamma less or similar T less or similar min(g(T)E(c), E(Th)), where Gamma is the tunneling escape rate, E(c) is the charging energy, and E(Th) is the Thouless energy of the grain.

Study of optical absorption differences of doped polyaniline films by photothermal spectroscopies

In this work, we studied the optical absorption spectral differences between doped and slightly doped polyaniline films and a derivative by photopyroelectric and photoacoustic spectroscopies. There exist great spectral differences between doped and slightly doped samples as shown by conventional optical absorption spectroscopy, but not greatly evidenced by photopyroelectric spectroscopy. The latter was applied to obtain thermal parameters such as thermal diffusivity and thermal conductivity, and as a result it showed that these thermal properties of polyaniline films are very similar for doped and slightly doped samples. Also it showed that for thicker films (about 20 μm), there are no significant optical absorption differences between them. However, for thin films both techniques showed greater optical absorption differences for doped and slightly doped samples, mainly detected by photoacoustic spectroscopy. These behaviors are in accordance with published results which is the granular metal model for polyaniline. This model explains the polyaniline polymeric matrix as formed by conductive islands in the insulating bulk material.

On the dissipative effects in the electron transport through conducting polymer nanofibers

Here, the author studies the effects of stochastic nuclear motions on the electron transport in doped polymer fibers assuming the conducting state of the material. The author treats conducting polymers as granular metals and applies the quantum theory of conduction in mesoscopic systems to describe the electron transport between metalliclike granules. To analyze the effects of nuclear motions, the author mimics them by a phonon bath and includes electron-phonon interactions in consideration. The results show that the phonon bath plays a crucial part in the intergrain electron transport at moderately low and room temperatures, suppressing the original intermediate state for the resonance electron tunneling and producing new states which support the electron transport. Also, the temperature dependence of the magnitudes of the peaks in the electron transmission corresponding to these new states is analyzed.

Granular electronic systems

A granular metal is an array of metallic nano-particles imbedded into an insulating matrix. Tuning the intergranular coupling strength a granular system can be transformed into either a good metal or an insulator and, in case of superconducting particles, experience superconductor-insulator transition. The ease of adjusting electronic properties of granular metals makes them most suitable for fundamental studies of disordered solids and assures them a fundamental role for nanotechnological applications. This Review discusses recent important theoretical advances in the study of granular metals, emphasizing on the interplay of disorder, quantum effects, fluctuations and effects of confinement in formation of electronic transport and thermodynamic properties of granular materials.

Anomalous electric-field effect and glassy behaviour in granular aluminium thin films: electron glass?

We present a study of non-equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films. An anomalous field effect and its slow relaxation are studied in some detail. The phenomenology is very similar to the one already observed in indium oxide. The origin of the phenomena is discussed. In granular systems, the present experiments can naturally be interpreted along two different lines. One relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one relies on a purely electronic mechanism: the formation of an electron Coulomb glass in the granular metal. More selective experiments and/or quantitative predictions about the Coulomb glass properties are still needed to definitely distinguish between the two scenarios.

Electronic conduction in (Bi, Pb)–Sr–Ca–Cu–O glass-ceramics

Electrical properties of (Bi,Pb)4Sr3Ca3Cu4Ox granular metals and superconductors obtained by the solid state crystallization method were studied. The studied materials may be considered as 3D systems of small oval granules of the (Bi,Pb)2Sr2CuOx and (Bi,Pb)2Sr2CaCu2Ox phases embedded in the insulating matrix. They may by divided into two main groups: these in which the granules below the critical temperature transform into the superconducting state and those in which they remain in the normal state throughout the entire temperature range studied. The electronic transport in the (Bi,Pb)Sr–Ca–Cu–O granular system of the granules in the normal state occurs by the variable range hopping mechanism either with or without Coulomb interactions. The materials, which below the transition temperature contain the granules in the superconducting state, show different types of temperature dependence of resistivity. Those composed of very small granules (10nm) of the superconducting phase exhibit an exponential dependence typical of the strong localization regime. In samples containing relatively large granules of the 2201 phase (30nm), the bulk superconducting state was achieved. The intermediate materials show unusual temperature dependence of resistivity, not compatible with the known models of electronic transport in granular materials.