Synthesis, crystal structure and blue electroluminescence of a new zinc complex based on 2,6-bis(benzimidazolyl)pyridine

A very stable and blue luminescent complex Zn(bbp)Cl₂ (bbp: 2,6-bis(benzimidazolyl)pyridine) was synthesized. X-ray crystal structural analysis for the complex revealed that there are intermolecular π?π interactions in the solid state. The fluorescence properties for this complex were studied. The similar devices with the structure of [ITO/CuPc(31 nm)/NPB(80 nm)/[Zn(bbp)Cl₂] (or L) (85 nm)/LiF15/Al] were constructed to investigate their electroluminescent performance. Both the complex and the ligand can be fabricated as blue-emitting materials. The complex shows emission peak at 555 nm, electroluminescent efficiency 0.017 cd A⁻¹ and turn-on voltage 7 V, compared to 470 nm, 0.036 cd A⁻¹ and 9 V for the ligand.     

Electrical conductance from the Fowler-Nordheim tunneling

Electrical conductance, including its normalized version, is discussed quantitatively in the context of the Fowler-Nordheim tunneling by considering ballistic electron transport through a generic insulating layer. This discussion is applicable to several nanostructures as, for example, nanowires as well as to specific problems in electron optics.     

Mn-doped GaN/AlN heterojunction for spintronic devices

We present first principles calculations of Mn-doped GaN/AlN(0001) heterostructures obtained within the framework of density functional theory by using plane wave pseudopotential techniques. We found that for diluted Mn concentration this system present an integer magnetization that is a fingerprint of half-metallic property; this suggests the possibility to use this junction as a spin injector.     

The unusual magnetic and electronic properties of Gd0.5Ba0.5CoO2.9

Magnetic and electrical properties of well-characterized Gd_0.5Ba_0.5CoO_2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln_0.5A_0.5CoO₃ (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd_0.5Ba_0.5CoO_2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.     

Preparation and study of silver added La0.67Ca0.33MnO3 film

La_0.67Ca_0.33MnO₃ (LCMO) and Ag admixed La_0.67Ca_0.33MnO₃ (Ag-LCMO) polycrystalline films have been prepared on SrTiO₃ single crystal (100) substrates by ultrasonic spray pyrolysis technique. These films are characterized using XRD, SEM, and temperature dependence of resistivity (ρ-T) and ac susceptibility (χ-T). The films are having cubic structure with lattice parameters as 3.890 and 3.885 Å for LCMO and Ag-LCMO films, respectively. The peek in ρ-T curve (T_p) and the ferromagnetic transition temperature (T_C) for the Ag-LCMO film is higher than that of LCMO film. The stability of both the films was tested by repeated measurements of its characteristics over a period of one week after several thermal cycling from room temperature to 77 K. In the LCMO film, the peak in the ρ-T curve (T_p) is found to shift towards lower value and conduction noise of the film increases in the subsequent measurements. In the case of Ag-LCMO the value of T_p, T_C and conduction noise of the film did not change even after several measurements. Silver segregating at the grain boundaries in Ag-LCMO polycrystalline film seems to be responsible for improving the characteristics of Ag-LCMO films.     

On imaging obstacles inside inhomogeneous media

We show that an obstacle inside a known inhomogeneous medium can be determined from measurements of the scattering amplitude at one frequency, without a priori knowledge of the boundary condition. We also show that an obstacle inside a known inhomogeneous anisotropic conducting medium can be determined from electrostatic current and voltage measurements on the boundary of a domain containing the obstacle. Moreover, two obstacles with boundary measurements which are merely comparable as operators must be identical. The first part of the paper gives an extension of the factorization method which may be of independent interest and also yields a new reconstruction procedure.     

Effect of intercalation on electrical and mechanical properties of C/C composites

The work is concerned with modification of C/C composites by intercalation of copper chloride. The samples of composites were made from graphite fibres and carbon matrix derived from mesophase pitch and from phenol-formaldehyde resin. The samples were prepared by impregnating graphite fibres with a liquid pitch or polymer solution to obtain unidirectional laminates. The laminates were used to prepare the composites which were then subjected to carbonization and graphitization up to 2150 °C. The work discusses the problem relevant to the effect of intercalation on mechanical and electrical properties of composites. The studies indicated that both mesophase pitch-based composites and phenolic-derived carbon-carbon composites changed their electrical and mechanical properties upon intercalation with copper chloride. Electrical conductivity of both types of composites decreased as a result of the damages formed during intercalation.     

UV–VIS and mid-IR ellipsometer characterization of layers used in OLED devices

We report on the analysis of layer materials commonly used for the fabrication of organic light-emitting diodes (OLEDs) by means of spectroscopic ellipsometry operating in the ultraviolet–visible (UV–VIS) and in the mid-infrared (mid-IR) range of the electromagnetic spectrum. On covering a wide spectral range, these non-destructive, non-contact techniques offer a huge potential in characterizing thin films in terms of layer thickness values, optical indices, absorption properties modified by incorporating optically active dopant molecules, and, finally, electrical layer properties. Individual absorption bands of single organic films can significantly affect the luminance and performance efficiency of OLEDs when building multi-layer stacks. Using mid-IR spectroscopic ellipsometry, the resistance of as-deposited and annealed indium–tin oxide (ITO) layers on glass is determined and compared with values obtained by four-point probe techniques.     

Creating room temperature Ohmic contacts to 4H-SiC: studied by specific contact resistance measurements and X-ray photoelectron spectroscopy

The use of a silicon interface pre-treatment to produce low resistance Ohmic nickel contacts to 4H-SiC, circumventing the need for contact post annealing, is reported. The effects of two different SiC pre-metal deposition surface preparation techniques: RCA cleaning (control sample) and a silicon interlayer pre-treatment (SIP), are discussed. Electrical characterization of contacts on treated surfaces, using circular transfer length measurements (CTLM), revealed that contacts to RCA cleaned samples were Schottky in nature, unless annealed at temperatures greater than 700 °C. In contrast, contacts formed on SIP SiC surfaces exhibited Ohmic behaviour directly after fabrication, without the need for post metallisation annealing. Average contact resistances as low as 1.3E−05 Ω cm² have been recorded for SIP samples. This fabrication process has distinct technological advantages compared to standard techniques for forming Ohmic contacts to SiC. To consolidate our findings the chemical and electrical nature of the SIP nickel-SiC interface, as it was sequentially formed and annealed, was examined using X-ray photoelectron spectroscopy (XPS). Based on these results, a model is proposed to explain the as-deposited Ohmic contact nature of the SIP sample.     

Kinetics of Relaxation of Stored Light Sums in Silver Chloride

Experimental investigation of the time and temperature dependences of the process of relaxation of excited silver chloride has been carried out by the method of a photostimulated burst of luminescence. It is found that the relaxation process can be divided into two components, the rates of which differ by an order of magnitude. The activation energies of these processes of 0.03 and 0.06 eV correspond to the thermal ionization energies of electrons from shallow traps. Thermally activated relocalization of charges from shallow to deep traps in the process of relaxation has been revealed. The depth of a corresponding trap appeared equal to 0.17 eV. A model of a crystal phosphor has been suggested that contains five levels in the forbidden band: a hole center of recombination (luminescence), a deep electron trap, two shallow electron traps located at a depth of 0.03 and 0.06 eV from the bottom of the conductivity band, and a hole trap located at a height of 0.17 eV from the valence band top. Within the framework of this model, we can qualitatively explain the time and temperature dependences of stored light sums. We have shown the possibility of applying the method of a photostimulated burst of luminescence to investigation of the processes of relaxation of electron excitations in crystals to establish the relaxation mechanisms.