Search

Nonstoichiometric (Cu 2− x Se) and stoichiometric (CuSe, β-Cu 2 Se and Cu 2 Se) copper selenide hexagonal nanoplates have been synthesized using different general and convenient copper sources, e.g. copper chloride, copper sulphate, copper nitrate, copper acetate, elemental copper with elemental selenium, friendly ethylene glycol and hydrazine hydrate in a defined amount of water at 100 °C within 12 h adopting the solvothermal method. Phase analysis, purity and morphology of the product have been well studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray diffraction (EDAX) techniques. The structural and compositional analysis revealed that the products were of pure phase with corresponding atomic ratios. SEM, TEM and HRTEM analyses revealed that the nanoplates were in the range 200–450 nm and the as-prepared products were uniform and highly crystallized. The nanoplates consisted of {0 0 1} facets of top–bottom surfaces and {1 1 0} facets of the other six side surfaces. This new approach encompasses many advantages over the conventional solvothermal method in terms of product quality (better morphology control with high yield) and reaction conditions (lower temperatures). Copper selenide hexagonal nanoplates obtained by the described method could be potential building blocks to construct functional devices and solar cell. This work may open up a new rationale on designing the solution synthesis of nanostructures for materials possessing similar intrinsic crystal symmetry. On the basis of the carefully controlled experiments mentioned herein, a plausible formation mechanism of the hexagonal nanoplates was suggested and discussed. To the best of our knowledge, this is the first report on nonstoichiometric (Cu 2− x Se) as well as stoichiometric (CuSe, β-Cu 2 Se and Cu 2 Se) copper selenide hexagonal nanoplates with such full control of morphologies and phases by this method under mild conditions.

Gallium nitride (GaN) nanocrystals have been grown on the tip of aligned carbon nanotubes (CNTs) substrate by chemical vapour transport method. It was found that GaN nanocrystals were formed on the tip and outermost shells of CNT bundles in the form of nanorods. X-ray diffraction (XRD) pattern shows that the GaN nanocrystals are of wurtzite structure. Scanning electron microscopy (SEM) images show the nanocrystals ranging from 10 to 100 nm has been realized under different experimental conditions exhibiting different shapes. Heterojunctions between CNTs and GaN nanocrystals, depending on their unique property, can be utilized for nanosized optical and optoelectronic devices. UV–optical absorption spectrum show the band edge at 360 nm, which is related to the bandgap energy of GaN.

The growth features of bulk single crystals of YBa2Cu3O7 (Y-123) heve been further investigated using the scanning tunnelling microscopy (STM) technique and the step heights of terraces associated with the growth spirals, observed in the a-b plane, have been measured. The results show that a growth step of height equal to the c-parameter of the unit cell comprises substeps and, in the ultimate resolution, six such substeps are observed. This provides the first piece of evidence of the crystals growing, layer by layer, in the framework of the spiral growth mechanism.

The STM observations of growth spirals in bulk single crystals of Y-123 and Bi-2122 high Tc cuprates are reported for the first time. The density of screw dislocations present is of the same order as recently reported for sputtered epitaxial thin films of Y-123. Growth features are briefly discussed and it is argued that the observed dislocation density is still too low for realizing optimum flux pinning in these HTSC systems.

Thin platelet-shaped (≈2×2×0.1 mm 3 ) YBa 2 Cu 3 O 7− x single crystals have been grown using the flux technique. BaCO 3 and CuO in the molar ratio 7:18 have been used as the flux material. The as-grown YBCO single crystals were annealed at 900°C for 100 h, followed by a very slow cooling rate under controlled oxygen (10 −2 to 10 −1 atm oxygen partial pressure). These crystals were found to be superconducting with T c (onset) ≈ 91 K and T c ( R = 0) ≈ 88 K. The microstructural features comprising growth spiral and microtwinning have been observed. Interesting microstructural characteristics have been obtained for the crystals which have been subjected to radiation damage through irradiation to neutron beams (≈ 1 MeV, fluence 10 16 cm −2 ). These characteristics consists of spike-like features with an initial melted core segment about 100 A wide. This is followed by spike manifesting atom knock on and resulting displacements and defects. The defects are thought to be oxygen atom/vacancy clusters (∼ 300 A) and these may represent flux pinning centres leading enhancement of the critical current density J c .

The single crystal films of CdTe have been found to exhibit polytypism. Three different polytypic series have been found to result in the films prepared from the as supplied and the homogenized CdTe fluxes. The polytypes are formed when the as grown amorphous films get crystallized on electron beam pulse annealing in the electron microscope. The polytypes which are the first known polytypes of CdTe correspond to (i) 2H, 3C, 5H, 6H, 6R, 15R, (ii) 2H', 4H', 5H', and (iii) 2H''. A feasible mechanism for the polytype formation has been suggested. This is based on the interaction between the constant energy surface and the Brilllouin zone leading to a reduction in the electronic energy.

Studies of transformations of 20H polytypic crystal of cadmium iodide have been described. Two curious intrapolytypic transformations 20H 7 →20H 1 and 20H 8 →20H 1 have been observed. The structural sequences of these polytypes have been determined and the transformations have been analysed in terms of the determined sequences. It has been shown that the two independent transformations lead to the same common transformed structure since this structure is the most stable out of the various structures.

Phase transformation studies on an eightyfour layered rhombohedral polytype of cadmium iodide are reported. The detailed atomic structure of the initial and transformed polytypes is determined and based on this, structural geometrical schemes embodying the transformation are put forward. Also the stacking fault energies of the structures are evaluated by the pair interaction and central force models. Evidence and arguments are advanced to show that the transformation of one polytype into another polytype takes place in such a way so as to lower the stacking fault energy of the polytype.

Lead iodide polytypic crystals have been found to undergo phase transformation when annealed under vacuum (10 -4 torr) at temperatures ranging between 290 and 310 °C. Structural transformations involving several low identity period polytypes have been reported. The possible mechanism for phase transformation based on the nucleation, propagation and rearrangement of stacking faults has been suggested.

Growth of large grain form of CuInSe 2 has been described and discussed. It has been shown that specific treatments such as Cd/Bi doping of the as-synthesized polycrystalline CuInSe 2 bulk flux and localized annealing (∼600°C) of the as-deposited polycrystalline CuInSe 2 thin films lead to large grain (bulk: 1000 μm, thin film: 10 μm) CuInSe 2 .

X-ray diffraction studies of single crystals of SnSe 2 grown by closed tube vapour transport technique reveal the existence of the following polytypes: 6H, 18R, 10H, 18H, 22H, 30H, 36H, 42H and 42R in addition to the basic 2H structure. Cases of syntactic coalescence and disordered structures have also been encountered in the present investigation.