Phase transition and triplet exciton mobility in anthracene—tetracyanobenzene charge-transfer single crystals

Triplet-Excitons in a mixed 1 : 1 anthracene—tetracyanobenzene single crystal are established by ESR-spectrocopy. These excitons (lifetime 0.5 msec) are characterized by a hopping frequency of v = 1.3 × 10⁶ K at 300K. The temperature dependence of the ESR line shape indicates a phase transition of the crystal at T_t = 205K. Above and below the phase transition the exciton mobility is nearly temperature independent.     

Measurements of electron energy losses and VUV-reflectivity of anthracene single crystals

Anisotropic behaviour of the electron energy loss spectra is discussed in terms of the oriented gas model. These measurements are compared with electron energy loss spectra in the vapour phase and normal incidence VUV-reflectivity from the (ab) plane for 2 directions of polarization. The VUV measurements were made at DESY using synchrotron radiation.     

Hole mobility in GeTe single crystals

The mobility of GeTe indicates anisotropic behaviour. The data based on a band structure compatible with the experimental rhombohedral C⁵_3y class, show scattering due to antistructure defects in the Te rich region, while scattering from neutral centers in the Ge side.     

Optical study of anthracene molecules doped in fluorene single crystals

Properties of two luminescence centers, which are observed in anthracene-doped fluorene crystal as concentration increases, have been investigated to clarify their origin. Excitation spectra and concentration dependence of absorption spectra were measured and two types of luminescence are attributed to anthracene molecule pair centers with weak and strong interaction. Dipole-dipole interactions between two anthracence molecules of various configurations are estimated and possible configuration for two types of luminescence is proposed.     

Positron mobility in anthracene

We have measured the positron mobility in a sample of scintillation grade anthracene at two temperatures. We obtain at 300 K: =(26.0±0.9±2.6) cm²V^–1s^–1 and at 77 K: =(33.4±1.1±3.3) cm²V^–1s^–1, where the first error estimate is statistical and the second is systematic. We have also made preliminary measurements on a highly purified sample that yields =(130±3±20) cm² V^–1 s^–1 at 300 K. The data are consistent with the hypothesis that the positron is scattered from both impurities and acoustic phonons in the first sample, and predominantly from photons in the second. It appears that positrons in pure anthracene crystals are delocalized and have a mean free path of about 85 Å at room temperature.     

Influence of deformation on the mobility and lifetimes of charge-carriers in anthracene crystals

Direct determination of the mobilities and lifetimes of electrons and holes in undeformed and in dislocated single-crystals of ultra pure anthracene reveals fresh insights into the mechanism of carrier trapping. The deformation introduced electron traps at depths of approximately 1.0 eV but was found not to affect hole transport. Such traps are considered to be due to impurities preferentially introduced into the lattice as a result of the generation of the additional dislocations.     

Surface morphologies of anthracene single crystals grown from vapor phase

The morphologies and lattice structures of anthracene single crystals grown from the vapor phase were investigated using optical microscopy, phase contrast microscopy, atomic force microscopy (AFM), and X-ray diffraction analysis. Common morphologies with hexagonal large planes were observed irrespective of crystal size. The observation of certain surface morphologies with a phase contrast microscopy revealed that the spiral steps originated from screw dislocations present on the (0 0 1) planes. Moreover, the center and edge of the (0 0 1) planes had large curvatures, similar to hills. Resultantly, quarter-monolayer (ML) steps were observed on the large and flat planes between both hills.     

Space-charge-limited-trap-limited-currents in anthracene crystals

Space-charge-limited-trap-limited hole currents in anthracene crystals are interpreted in terms of a trap distribution which starts at a discrete trap level at 0·6–0·8 eV and in which the trap density falls off exponentially with energy relative to this level. It is suggested that exponential hole trap distributions are produced by physical perturbations of the crystal lattice introduced by the same impurities which give rise to the discrete hole trap level at 0·6–0·8 eV (revealed by thermally stimulated current studies). The results indicate that a perturbed molecule is introduced for every 10,000 impurity molecules and the total number of perturbed molecules is constant at 10¹⁷ m^?3 for melt-grown and vapor-grown crystals. The parameter kT_c varies from crystal to crystal, indicating that the relative contributions of highly perturbed and slightly perturbed molecules is influenced by crystal growth conditions.     

Intersystem crossing mechanisms and single molecule fluorescence: Terrylene in anthracene crystals

Single molecule spectroscopy requires molecules with low triplet yields and/or short triplet lifetimes. The intersystem crossing (ISC) rate may be dramatically enhanced by the host matrix. Comparing the fluorescence intensity of single terrylene molecules in para-terphenyl, naphthalene, and anthracene crystals, we found a reduction of the saturation intensity by three orders of magnitude in the latter case. The fluorescence autocorrelation function indicates that the bottleneck state is the terrylene triplet. We propose a ping-pong mechanism between host and guest. This intermolecular ISC mechanism, which can open whenever the host triplet lies lower than the guest singlet, was overlooked in previous single molecule investigations.     

Well-thickness dependent electron transport effective mass and mobility in Sb-based quantum wells

We propose a special kind of Sb-based III–V semiconductor quantum wells, which, in contrary to conventional ones, are able to provide novel transport properties: smaller electron transport effective mass and higher mobility with thinner well width. By properly choosing the barrier materials with smaller electron effective mass and proper band lineups, the novel transport property could be achieved because of the penetration effect of the wave-function when interface roughness and alloy disorder scatterings are taken into consideration.     

Field dependent nuclear spin-lattice relaxation of cobalt impurities in iron single crystals

Nuclear spin-lattice relaxation (NSLR) of guest atoms in a ferromagnetic host metal is studied by means of nuclear orientation of radioactive isotopes at low temperatures. The sample, a ferromagnetic, tron single crystal containing⁵⁸Co or⁶⁰Co impurities, is cooled to about 20 mK and the lattice is warmed up by r.f. heating, whereafter the nuclear polarization is monitored by gamma-ray intensity measurements versus time. The results show a considerable influence of external magnetic fields and, in particular, also isotope effects. These will be discussed on the basis of second-order magnon processes.     

The electron paramagnetic resonance of Dy in single crystals of Pd

We report the observation of the electron paramagnetic resonance spectrum of Dy³⁺ in Pd single crystals. The observed resonance spectrum contains four main lines, one of which varies strongly with crystal orientation suggesting the ion ground state to be a Γ₈ quartet. The weak hyperfine lines due to the isotopes Dy¹⁶¹ and Dy¹⁶³ have also been observed. The results are discussed in terms of Bleaney's Hamiltonian for a Γ₈ quartet.     

Vacancy mobility in polymer crystals

A molecular-dynamics model of the behavior of a vacancy in the chain of an equilibrium polymer crystal (the “collective atom” approximation for polyethylene) is developed for the first time. It is shown that a defect of this type in a polymer crystal has a soliton mobility, as opposed to vacancies in crystals of low-molecular substances. Zh. éksp. Teor. Fiz. 115, 1063–1069 (March 1999)     

Impedance and electron diffraction studies on single crystals of BINIVOX

Single crystals of the nominal composition Bi₂V_0.9Ni_0.1O_5.35 were prepared. They were studied by TGA/DTA, impedance spectroscopy and electron diffraction. The impedance of the crystals with Pt electrodes was measured in two directions: parallel and perpendicular to the layers of the intergrowth structure in the frequency range 10⁻²−10⁷ Hz at temperatures between 300 and 940 K. Arrhenius type plots for both directions show two linear regions with different activation energies. In the low temperature region the conductivity in the direction parallel to the layers was by a factor of about 1000 larger than in the perpendicular direction. In the high temperature region the ratio of conductivities was about 100. A time dependence of the conductivity in the perpendicular direction was observed during heating and cooling in limited temperature ranges. The electron diffraction experiments gave evidence that the superlattice spots present in the diffraction patterns at low temperature, disappear reversibly at temperatures higher than 650 K. The transition temperatures observed in the DTA and in the electron diffraction experiments correspond to the temperature of changes of the activation energy. An order-disorder transition of oxygen ions is considered to be responsible for this phenomenon. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Electroluminescence in anthracene crystals under time-varying electric fields

Electroluminescence in anthracene crystals has been systematically measured under time varying electric fields at various temperatures using a sodium electrode as the electron-injecting contact and a silver electrode as the hole-injecting contact. The results show that the electroluminescent brightness decreases with increasing frequency over the temperature range from ? 20 to 60°C; and that for a given frequency the brightness increases with increasing temperature, reaches a peak at a critical temperature, and then decreases with increasing temperature. The electroluminescence under a half-wave rectified sinusoidal a.c. field is brighter than that under a sinusoidal a.c. field of the same peak amplitude. There is a delay time between the application of the field and the appearance of electroluminescence, and this delay time decreases with increasing frequency and saturates at a value which may be associated with the transit time of the injected electrons across the specimen. All these results can be explained on the basis of the model that electroluminescence occurs when an electron space charge built up near the anode is sufficient to enhance hole injection from the anode.     

Anomalous electron emission from lithium niobate and lithium tantalate single crystals

Anomalous electron emission from the surface of LiNbO₃ and LiTaO₃ single crystals excited by soft x radiation has been discovered and is investigated. The absence of anomalous emission from the face of these single crystals is established. The experimental results confirm the theoretical conclusions that the anomalous emission is caused by the presence of a maximum in the distribution of the potential in the near-surface layer. The dependence of the lifetime τ of the anomalous emission on the dielectric constant ɛ of the ferroelectric is calculated. Fiz. Tverd. Tela (St. Petersburg) 39, 679–682 (April 1997)