IR photoconductivity of C60 fullerene single crystals

The photoconductivity in the range 0.3–0.9 eV was investigated in a high-quality C₆₀ fullerene single crystal. It was concluded that the crystal investigated is an extrinsic semiconductor. Fiz. Tverd. Tela (St. Petersburg) 41, 1113–1114 (June 1999)     

Photoinduced transformation of luminescence centers in C60 crystals at high pressure

The influence of laser irradiation on the photoluminescence spectra of perfect C₆₀ crystals in the orientationally disordered phase is investigated. It is shown that irradiation of the crystals with low-power light for short durations at T=200 K produces radical changes in the luminescence spectrum. The pressure dependences of the spectral bands of the phototransformed and initial (without irradiation) spectra differ significantly, indicating a photoinduced structural transformation of the X centers responsible for the luminescence of C₆₀. The phototransformed C₆₀ crystals are stable against further exposure to light irradiation and pressure. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 784–788 (25 December 1999) Deceased.     

Trapped-hole centers in MgO single crystals

The properties of the majority trapped-hole centers in MgO, such as g-factors, positions of absorption and luminescence bands, and temperatures of thermal destruction, have been analyzed with the emphasis on the observed regular trends and interrelations between the properties of these centers. Particular emphasis has been placed on the positively charged [Be]⁺ and [Ca]⁺ trapped-hole centers, which have a large cross section for recombination with conduction electrons. In these centers, a hole is localized at an oxygen ion near the impurity Be²⁺ or Ca²⁺ ion located at a regular cation site. The generation and transformation of defects due to the recombination of either relaxed conduction electrons with OH⁻-containing hole centers or cold and hot electrons with [Be]⁺ and [Ca]⁺ centers have been considered. Using the interrelation of the characteristics of hole centers and taking into account that the recombination emission band revealed at ∼6.8 eV is due to the Ca²⁺-containing centers that are stable below 50 K, the prospects for the EPR detection of the [Ca]⁺ center at T < 4.2 K have been discussed.     

Field-dependent charge-carrier trapping process in disordered systems

We study the diffusive spreading of excees carriers in trapping systems to which a bias electric field is applied. Since for the intermediate time range the present model appears to be identical to the electronic disordered system (the Anderson model), we have been able to derive the time and field dependencies of carrier survival probability. The results of the exact calculations obtained might provide significant insight into the transport phenomena occurring in disordered systems. It is shown that in the long-time limit the coherent potential approximation can serve as an exact solution of the problem.     

Shift of the optical absorption edge in C60 clathrate single crystals

We measured the transmission between 1.6–2.2 eV of pure C₆₀ and three clathrate single crystals containing different aliphatic guest molecules. The absorption edge in the clathrates shows a blue shift of 0.1 eV irrespective of the guest molecule, indicating that C₆₀ is electronically isolated in these compounds.     

Rb掺杂C60单晶的相衍变和电子态

在C60单晶超高真空解理面上制备C60的Rb填隙化合物薄膜.用同步辐射光电子能谱研究了相衍变过程.观察到对应于固溶相、Rb1C60和Rb3C60的电子态密度分布.当数纳米厚Rb3C60薄膜在C60单晶(111)解理面形成后,室温条件下进一步沉积Rb至样品表面不产生fcc到bct或bcc结构相变.C60分子的大尺寸提供了表面填隙位置使得样品表面形成Rb4C60和Rb5C60吸附相.价带电子能谱结果表明这两种表面相为金属性.Rb 3d芯态电子能谱测量进一步证实了表面Rb4C60和Rb5C60吸附相的存在.     

Impurity Cu+ centers in LiF single crystals

LiF single crystals with copper impurity (0.0004–0.002%) have been grown by the Czochralski method and investigated. The luminescence, excitation, and optical absorption spectra have been recorded. The luminescence spectrum contains a band at 450 nm upon 250-nm excitation. This band is attributed to Cu⁺ centers in the samples grown. The mechanisms of capture and recombination during thermoluminescence are considered.     

Trapping centers in undoped GaS layered single crystals

Nominally undoped p-GaS layered single crystals were grown using the Bridgman technique. Thermally stimulated current measurements in the temperature range 10–300 K were performed at a heating rate of 0.10 K/s. The analysis of the data revealed six trap levels at 0.05, 0.06, 0.12, 0.63, 0.71, and 0.75 eV. The calculations for these traps yielded 1.2×10^-21, 2.9×10^-23, 2.4×10^-21, 8.0×10^-9, 1.9×10^-9 and 4.3×10^-10 cm² for the capture cross sections and 1.6×10¹³, 5.0×10¹², 7.3×10¹², 1.2×10¹⁴, 8.9×10¹³ and 2.6×10¹³ cm^-3 for the concentrations, respectively. PACS 71.55.-i; 72.20.Jv; 72.80.Jc     

On the role of hole trapping centers in the interactive mechanism of the trap interaction in anion-defect alumina single crystals

The effect of deep hole traps on the intensity and shape of the dosimetric peak of thermoluminescence (TL) has been studied at 450 K in anion-defect alumina single crystals. It has been shown that filling of deep hole centers leads to a decrease in the sensitivity to radiation of crystals with a small half-width of the TL peak and has no effect on the sensitivity of crystals with a broadened peak. It has been assumed that traps responsible for the TL dosimetric peak broadening are of hole nature, which can be caused by the presence of Ti3+ ions in the corundum lattice. The results obtained have been interpreted within the modified model of the interactive system of traps.     

Electronic properties of C60 single crystals doped with lithium by electrodiffusion

Diffusion of lithium cations in C₆₀ single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C₆₀ single crystal” heterojunction has been suggested. It has been found that lithium doping of C₆₀ single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in Li_xC₆₀ crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T _1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm⁻¹ to 1413 cm⁻¹), which indicates that one electron is present at the C₆₀ molecule, and this fact may be treated as evidence that the LiC₆₀ phase is generated in a C₆₀ crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)     

Long-lived excited states and photoluminescence excitation spectra in single crystals of fullerene C60

Photoluminescence, optical absorption spectra, and photoluminescence excitation spectra were measured on large (2–3 mm), very pure crystals of fullerene C₆₀ at 5 K. It is shown that the main contribution to the photoluminescence of these crystals is from singlet and triplet excitons captured on crystal defects. The concentration of these defects does not exceed 10¹⁸ cm⁻²³, and the lifetime of triplet excitons on these defects is about 3 ms. It is shown that the symmetry distortion of the C₆₀ molecules at the defects is rather large and causes the oscillator strength of the zero-phonon optical transitions to be comparable to the most intense optical transitions with the participation of intramolecular vibrations. Zh. éksp. Teor. Fiz. 113, 734–746 (February 1998)     

Luminescence centers in SnO2 single crystals

The luminescence spectrum observed in SnO₂ has been analyzed based on the vibroelectronic model. However, additional measurements of the photoconductivity and the spectral distribution of recombination probability cannot be explained by this model, which only takes into account strong phonon coupling. Arguments are presented in favor of Birman's model which is more consistent with all the present results.