Electrical transport properties

Although the superconducting properties of these materials are of major interest at this time, future applications of these materials may depend upon utilization of the dramatic changes in electrical properties accompanying stoichiometric changes in these solid solutions. Particularly interesting are the ferro-, ferri-, piezo- and paraelectric properties displayed by the insulating phases in BaPbx  

In the region of superconducting compositions in BaPbx.x-B Og, oxygen vacancies have been found to decrease Tc (46)(47). This has definite implications for preparation of high Tc material, but also indicates that the Te may be tuned to any desired transition temperature below the maximum Tc by careful technological treatments. 

One of the features first observed in BaPbj B Og, and now known as a hallmark of bulk ceramic superconductors is the appearance of grain boundary Josephson junctions. One of the most 

Activated, or hopping conductivity would, of course not be expected to be an intrinsic property of these metallic materials, as demonstrated for single crystalline films (39). 

Compositional homogeneity may strongly impact the observed properties of bulk material. Often the fact that Tc is a sensitive function of composition (the maximum Tc for BaPb1 xBixOs is observed at 13K (13) for x = 0.25, and a Tc of 34K is found for x =. 6 in Ba K BiOg (11)) means that the width of the resistive transition can be used to determine the uniformity of the product. Use of the resistive Tc onset to characterize these solid solutions, especially in bulk form can lead to errors due to stoichiometric nonuniformity. The results of Tomeno and Ando (50) who have reported that Tc remains constant at 29K for x values from 0.28 to 0.44 in Ba RbjjBiOg, are 

Two main methods have been used to measure the charge carrier mobility in electroluminescent polymers time of flight (TOF) carrier transit time measurements and analysis of the current-voltage (I-V) characteristics of single carrier devices in the space charge-limited current (SCLC) regime. A summary of die results for die hole mobility of PPV and PPV-related polymers is given in Table 11-1 [24, 

The rest of tliis chapter will focus primarily on results using the soluble PPV-based polymer MEH-PPV. The results obtained for MEH-PPV are typical of the conjugated polymers used in LEDs. The models and results presented are generally applicable. They describe the operation of a wide range of polymer LEDs if the appropriate polymer film properties are used. 

In tliis section the electronic structure of metal/polymer/metal devices is considered. This is the essential starting point to describe the operating characteristics of LEDs. The first section describes internal photoemission measurements of metal/polymer Schottky energy barriers in device structures. The second section presents measurements of built-in potentials which occur in device structures employing metals with different Schottky energy barriers. The Schottky energy barriers and the diode built-in potential largely determine the electrical characteristics of polymer LEDs. 

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Thin Films of Electroluminescent Polymers 335 Electronic Eneigy Structure 336 Optical Properties 336 Electrical Transport Properties 338... 

It is difficult to measure metal/polymer Schottky energy barriers smaller than about 0.5 eV using internal pholoemission. Small Schotiky energy barriers lead to thermal emission currents produced by the absorption of light in the metal which are difficult to separate from true photocurrents 134]. If the structure is cooled to try to improve this contrast, it is often found that the significant decrease in the electrical transport properties of the polymer [27 [ makes it difficult to measure the internal photoemission current. To overcome this limitation, internal photoemission and built-in potential measurements are combined to measure small Schottky energy barriers, as described below. 

To study the electrical transport properties of this double-barrier system Pd nanoclusters have been trapped in this gap. Figure 14 shows a typical l(U) curve. The most pronounced feature at 4.2 K is the Coulomb gap at a voltage of about 55 mV, which disappears at 295 K. Above the gap voltage, the l(U) curve is not linear, but increases exponentially, which was explained by a suppression of the effective tunnel barrier by the applied voltage. 

In the previous chapters, examples of ID arrays of nanoclusters have been given, where self-assembly or ET were used to address the arrays for electrical transport measurements. So far it is evident that these methods did not lead to strictly ID defect-free arrangements. Furthermore, inherent disorder cannot be avoided. This means that the electrical transport properties through a perfect array could only be studies theoretically up to now. 

Figure 11.2. Nanowire electronic and optical properties, (a) Schematic of an NW-FET used to characterize electrical transport properties of individual NWs. (inset) SEM image of an NW-FET two metal electrodes, which correspond to source and drain, are visible at the left and right sides of the image, (b) Current versus voltage for an n-type InP NW-FET. The numbers inside the plot indicate the corresponding gate voltages (Vg). The inset shows current versus Vg for Fsd of 0.1 V. (c) Real-color photoluminescence image of various NWs shows different color emissions, (d) Spectra of individual NW photoluminescence. All NW materials show a clean band-edge emission spectrum with narrow FWHM around 20nm. (See color insert.)...

The nanotechnology report issued in February 2004 by the UK Royal Society makes the general observation that Electrical transport properties across interfaces remain poorly understood in terms of science/predictive capability. This affects all nanomaterials . This observation most keenly summarizes the present state of play for Gbit level random access memories (RAMs), and it is our view that the electrode interface issues may dominate the device physics. Within the nanotech roadmap , high-dielectric ( high-K ) materials are strongly emphasized, as are nanotubes and new interconnects. 

A comprehensive report which focussed on the La2 xSrxCu O4-x/2+S ser es was published (139) in 1983 by this research group. In this broad review they reported the magnetic and electrical transport properties of these mixed-valent copper oxides in the temperature range 120-650 K. They concluded that the original semiconducting behavior in La2Cu04 transformed to semi-metallic behavior as the Cu3+ content increased with Sr-substitution. No experiments were conducted below 50 K, and therefore superconductivity was not observed. Three series of compounds, with 0.00 < x < 1.20 were... 

While complexes in which the metal is coordinatively unsaturated frequently oligomerize utilizing bridging jS-diketonate ligands, a different mode of aggregation is believed to occur in M(CO)2-(MeCOCHCOMe) (M = Rh, Ir), The near planarity of the acetylacetonate permits these flat molecules to stack so that short (3.20 A for Ir) metal-metal contacts are formed. This leads to highly anisotropic DC electrical transport properties and these compounds are semiconductors. 

Electrical Transport Properties of Metal-Ammonia and Metal-Amine Solutions... 

The electrical transport properties of alkali metals dissolved in ammonia and primary amines in many ways resemble the properties of simple electrolytes except that the anionic species is apparently the solvated electron. The electrical conductance, the transference number, the temperature coefficient of conductance, and the thermoelectric effect all reflect the presence of the solvated electron species. Whenever possible the detailed nature of the interactions of the solvated electrons with solvent and solute species is interpreted by mass action expressions. 

Sanchez-Herencia, A.J., Moreno, R., Jurado, J., (2000), Electrical transport properties in zirconia/alumina functionally graded materials , J. Eur. Ceram. Soc., 20, 1611— 1620. 

Seuter, A.M.J.H. (1974) Defect chemistry and electrical transport properties of barium titanate. Philips Research Reports, Supplement No. 3. 

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