P.P. Edwards

Metal nanoparticles and their assemblies

Metal nanoparticles of varying sizes can be prepared by physical as well as chemical methods. They exhibit many fascinating properties, the size-dependent metal to nonmetal transition being an important one. Metal nanoparticles capped by thiols can be organized into ordered one-, two- and three-dimensional structures and these structures have potential applications in nanodevices. In this context, organization of arrays of metal nanoparticles with a fixed number of atoms assumes significance.

Size-Dependent Chemistry: Properties of Nanocrystals

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.

Structural studies of trigonal lamellar particles of gold and silver

The results of detailed structural studies of trigonal lamellar particles of both gold and silver are presented. The particles have been characterized both in sol by means of optical spectroscopy and powder X-ray diffraction and ex sol using high resolution electron microscopy in both plan view and profile imaging modes. The results of these studies have indicated that the particles have a trigonal outline and are shortened along a ≺111≻ direction to give a plate-like morphology. The presence of small numbers of parallel {111} twin planes has also been confirmed and used to explain the presence of the formally forbidden ⅓{422} reflections observed in plan view. The precise structural requirements for the observation of such reflections has also been confirmed using multislice calculations. Possible growth mechanisms for these particles are also discussed.

High critical‐current densities in (Tl0.5Pb0.5)Sr2Ca2Cu3O9 with Tc up to 115 K

A high critical‐current density was observed in the material (Tl0.5Pb0.5)Sr2Ca2Cu3O9. Based on magnetic‐hysteresis measurements, this single (insulating) Tl‐O layer material has a Jc value of 1.24×105 A/cm2 when measured at a temperature of 77 K and a field of 1 T. This value of Jc is comparable to that measured here for YBa2Cu3O7 (1.04×105 A/cm2), having single‐conducting Cu‐O chains, but considerably greater than the value for the material Tl2Ba2Ca2Cu3O10 with double (insulating) Tl‐O layers in which no hysteresis was observed under these same conditions. Our results from specific heat measurements indicate that the (Tl0.5Pb0.5)Sr2Ca2Cu3O9 compound has a three‐dimensional character due to the enhancement of the superconducting coupling along the c axis between the Cu‐O planes and thus results in an increase in Jc compared to the two‐dimensional Bi‐ and Tl‐2212 and 2223 [e.g., (Bi or Tl)2(Sr or Ba)2CaCu2O8 and (Bi or Tl)2(Sr or Ba)2Ca2Cu3O10, respectively] compounds. We believe that the high Jc is associ...

Ionic and metallic clusters of the alkali metals in zeolite Y

Abstract Dehydrated samples of zeolite Y containing alkali-metal cations have been reacted with alkali-metal vapor in sealed silica tubes, and the products studied by electron-spin resonance (ESR) spectroscopy. Two distinct species were detected following the reaction of sodium-exchanged zeolite Y with sodium, potassium, or rubidium vapor. Exposure to a low concentration of metal vapor resulted in brightly colored samples with ESR signals characteristic of a stable ionic cluster species Na3+4, in which an unpaired electron is trapped on four equivalent sodium cations in the sodalite cages of the zeolite. Exposure to higher concentrations of metal vapor resulted in dark-colored samples with ESR signals characteristic of small metallic particles located inside the zeolite cavities. A similar ionic cluster species K3+4 was detected following the reaction of potassium-exchanged zeolite Y with sodium or potassium vapor although the potassium cluster was less stable than its sodium counterpart and an ESR signal from small metallic particles was observed at the same time. The corresponding Rb3+4 ionic cluster species was not detected following the reaction of rubidium-exchanged zeolite Y with rubidium vapor; only an ESR signal from small metallic particles was observed. The narrow linewidths of the ESR signals from the small metallic particles suggest an inhibition of the spin-relaxation mechanisms in the bulk metals.

Superconductivity and the metal-semiconductor transition in the septenary oxide system, (Tl0.5Pb0.5)(Ca1−yYy)Sr2Cu2O7−δ

Abstract The septenary compound (Tl0.5Pb0.5)(Ca1−yYy)Sr2Cu2O7−δ exhibits the highest superconducting transition temperature (110 K) yet observed in the TlCaBa2Cu2O7−δ (1122) structure type. This complex, six-blend combination of metallic elements which make up the material is, however, compensated by a relatively simply crystal structure, which bears many similarities to that of 90 K superconductor YBa2 Cu3O7−δ. In this note we report some important features of the cation-substitution chemistry of the title compound, drawing attention to the fundamental changes in the electronic properties of the (Tl0.5Pb0.5) (Ca1−yYy)Sr2Cu2O7−δ system as Ca2+ is replaced by Y3+. Superconductivity is observed over the homogeneity range y = 0–0.5, with the superconducting transition temperature showing a maximum (108 K) at y = 0.2. Across the homogeneity range y = 0.6–1.0, the material undergoes a metal-semiconductor transition.

An efficient and reproducible approach for attaining superconductivity at 128 K in Tl2Ba2Ca2Cu3O10-δ

Abstract We report an efficient, and highly reproducible, method for the preparation of almost single phase Tl-2223 material, which is characterized by high-temperature superconductivity with T c midpoint = 128 K and T c zero = 126 K as measured by electrical resistivity, and a diamagnetic onset temperature [ T c mag ] of 128 K as measured by AC susceptibility. This procedure involves synthesizing a material with nominal stoichiometry Tl 1.6 Ba 2 Ca 2.4 Cu 3 O 10-δ , which was sintered at 910°C for 3 h, annealed at 750°C (10 days) in an evacuated quartz tube, and finally annealed at 600°C (2 h) in a 0.2% oxygen/nitrogen gas mixture. Based on these results, it appears that the optimization of the hole concentration in Tl-2223, achieved here by controlling the oxidizing/reducing power of the annealing atmosphere, appears to be crucially important in attaining such a high transition temperature.

Superconductivity up to 64 K in the copper oxyfluorides Sr2−xAxCuO2F2+δ(A = Ca, Ba) prepared using NH4F as a fluorinating reagent

Abstract We report here a simple and efficient synthetic route to high-temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2 CuO 3 with NH 4 F at relatively low temperatures produces superconducting Sr 2 CuO 2 F 2+δ ; this simple preparation route eliminates the need for F 2 gas. Importantly, as-synthesised samples from the NH 4 F route appear to have the optimally doped value of the superconducting transition temperature, T c , of 46 K. Moreover, Ba doping for Sr results in an increase in T c , for both F 2 gas and NH 4 F routes, to a maximum value of 64 K for Sr 1.4 Ba 0.6 CuO 2 F 2+δ ; this represents the highest ever recorded value for a material with the confirmed La 2 CuO 4 structure.

A new 92 K high-Tc superconductor: Hg-containing Tl-based 1212 phase

Abstract Bulk superconductivity up to 92 K has been achieved in the mercury-containing curprate (Tl 0.5 Hg 0.5 )Sr 2 (Ca 1− x Y x )Cu 2 O 7−δ when x =0.3. Based on X-ray and electron diffraction and electron probe microanalyzer results, we identify the phase responsible for the superconductivity to be similar to that of (Tl 0.5 Pb 0.5 )Sr 2 CaCu 2 O 7 (so-called 1212 phase) with a space group of P4/mmm and lattice constants of a ∼ 3.8 A and c ∼ 12.0 A. Moreover, neither superlattice nor intergrowth along the a ∗ and c ∗ directions were observed by electron diffraction. An increase in Ca 2+ doping, 0.5⩽ x ⩽0.3, into the Y 3+ sites results in a maximum superconducting transition temperature ( T c ) at around 92 K for x =0.3. For compositions x T c was observed.

An improved route to the synthesis of superconducting copper oxyfluorides Sr2−xAxCuO2F2+δ (ACa, Ba) using transition metal difluorides as fluorinating reagents

Abstract We report here a simple and efficient synthetic route to high temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2− x A x CuO 3 (ACa, 0 ⩽ x ⩽ 2.0; ABa, 0 ⩽ x ⩽ 0.6) with the transition metal difluorides, CuF 2 , ZnF 2 , AgF 2 , NiF 2 , at relatively low temperatures (225–250°C) produces superconducting Sr 2− x A x CuO 2 F 2+δ with a maximum T c of 64 K (for ABa, x = 0.6). This simple preparation route eliminates the need for F 2 gas, and at the same time produces negligible (Sr/A)F 2 impurity, unlike the corresponding fluorination with NH 4 F. Using this route the synthesis of the oxyfluoride, Ba 2 CuO 2 F 2+δ , is also reported for the first time. These studies and the examination of the fluorination of other cuprate systems have shown that this is a powerful and versatile fluorination route.