First-principles study of transport properties of endohedral Li@C20 metallofullerene

The transport properties of the endohedral Li@C₂₀ metallofullerene are studied using density functional non-equilibrium Green’s function method. The equilibrium conductance of Li@C₂₀ metallofullerene becomes larger than that of the empty C₂₀ fullerene molecule. The I–V curve under low-bias voltage shows the characteristic of metallic behavior; another, the novel negative differential resistance behavior is also observed. It is found that the doping effect of Li atom significantly changes the transport properties of C₂₀ fullerene.     

Photopolymerization of C<Subscript>60</Subscript> and Li@C<Subscript>60</Subscript> studied by second-harmonic generation and infrared spectroscopy

The photopolymerization of C₆₀ and Li@C₆₀ films was investigated by means of optical second-harmonic generation. The films were deposited under ultra-high-vacuum conditions and irradiated in situ with an Ar⁺ laser at 514 nm. The second harmonic generated by a Nd:YAG laser working at 1064 nm was monitored after different steps of irradiation. Photopolymerization was observed after very low irradiation doses, of the order of 10²⁰ photons/cm², and confirmed with infrared absorption spectroscopy. Similar kinetics for C₆₀ and Li@C₆₀ were observed. The measurements give evidence for photopolymerization of the endohedral fullerene Li@C₆₀. PACS 78.30.Na; 82.50.Hp; 81.05.Tp     

Effect of the encapsulation of Li atom on the electronic transport properties of C20F20 cage

Carrying out theoretical calculations using a self-consistent ab initio approach that combines the non-equilibrium Green′s function formalism with density functional theory, we investigate the effect of the center encapsulation of Li atom on the electronic transport properties of C₂₀F₂₀ cage sandwiched between two bulk gold electrodes. The results show that the electrical conductivity of the endohedral complex Li@C₂₀F₂₀ becomes better than that of the empty C₂₀F₂₀ in the bias voltages ranging from 0 to 1.2 V. The novel negative differential resistance behavior in the I-V characteristic curves can be observed by inserting Li atom into C₂₀F₂₀ cage. The mechanism for the negative differential resistance behavior of Li@C₂₀F₂₀ is suggested.     

Features of low-frequency spin dynamics in manganite LaMnO3 according to 139La NMR data

The spin-lattice and spin-spin relaxation times of ¹³⁹La are measured in manganite LaMnO₃. Analysis of the frequency dependence of the spin-lattice relaxation rate in the paramagnetic temperature range shows that this quantity is determined by magnetic fluctuations. The magnitude of the fluctuating field is estimated. It is shown that the correlation time for spin fluctuations varies with temperature in accordance with the Arrhenius law. The high value of the spin-spin relaxation rate in the paramagnetic region can be due to strong anisotropy of fluctuating magnetic fields at La nuclei.     

Electrical and electroluminescence properties of ITO/PEDOT:PSS/TPD:Alq3:C60/Al organic light emitting diodes

Organic light emitting diodes (OLEDs) of ITO/PEDOT:PSS/TPD:Alq₃:C₆₀/Al with different C₆₀ concentrations (0-6.0 wt.%) have been fabricated. The physical parameters including electrical and optical properties of the samples have been measured by Luminance-current-voltage (L-I-V) characteristics and optical absorbance. The current-voltage characteristics indicate that field-emission tunneling injection dominates in the diodes at high applied voltages. It is found that with increasing the concentration of C₆₀, the injection barrier for holes slightly reduces and the hole’s mobility increases over two orders of magnitude. Also, electroluminescence enhances with the presence of C₆₀ in the blend; optimum current efficiency occurs at 3 wt% C₆₀. The method provides a simple way of increasing the efficiency of OLEDs.     

Effect of oxygen and iodine on the optical and magnetic properties of fullerite C<Subscript>60</Subscript>

The effect of oxygen and iodine on the optical and magnetic properties of fullerite C₆₀ is studied by luminescence and EPR spectroscopy within widely varied experimental conditions (temperature of the medium, oxygen or buffer gas pressure, concentration of iodine vapor). It is demonstrated that the efficiency of the singlet oxygen formation when a fullerene sample is irradiated by a neodymium laser at a wavelength of 532 nm and the amplitude of the EPR signal emitted from the unirradiated sample are strongly affected by the concentrations of both oxygen and iodine vapor sorbed by the fullerene sample, as well as by its surface temperature. The spin-spin and spin-lattice relaxation times of paramagnetic centers in fullerite samples studied in the presence of molecular oxygen are determined by the method of microwave radiation absorption saturation.     

Optical properties of endohedral Li@C60

UV–VIS, IR and Raman investigations have been carried out on purified Li@C₆₀ produced by low energy ion implantation of C₆₀. Two endohedral fractions can be isolated that have very different optical properties and stabilities. Here we show that the first endohedral fraction is a dimer species and the second fraction is the monomer. The IR spectra of sublimed monomer films show a prominent peak at around 1.1 μm corresponding to the first electronic transition of the C₆₀⁻ anion. Raman spectra show the vibrational–rotational movement of the Li inside the carbon cage.     

F NMR and EPR study of fluorinated buckminsterfullerene C60F58

Solid state ¹⁹F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of fluorinated buckminsterfullerene C₆₀F₅₈ have been carried out. The temperature dependence of the line width and the spin-lattice relaxation time show hindered molecular motion with the activation energy of ΔE_a=1.9 kcal/mol. Neither phase transition nor random rotation of C₆₀F₅₈ have been obtained. The spin-lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C-C bonds yielding localized unpaired electrons. Such broken bonds are caused by C-C bond rupture in a cage-opened structure of hyperfluorinated species.     

199Hg and63,65Cu NMR studies of mercury based high-T c superconductors

Hg-oxide ceramic high temperature superconductors were studied by¹⁹⁹Hg and^63,65Cu NMR spectroscopy. Room temperature spectra, spin-spin and spin-lattice relaxation times of samples with different superconducting transition temperatures are presented. A spin-lattice relaxation time ofT ₁=35 msec and a spin-spin relaxation time ofT ₂=1.6 msec were found for the¹⁹⁹Hg NMR. All samples exhibit similar characteristic powder spectra caused by an axially symmetric¹⁹⁹Hg spin interaction. The isotropic value and the anisotropy of the tensor relative to solid HgCl₂ as a standard substance is estimated. Furthermore, results of^63,65Cu NMR measurements at a temperature of 4.2 K which exhibit a typical powder line shape (forI=3/2) are presented.     

Ab initio calculations of endo-and exohedral C60 fullerene complexes with Li+ ion and the endohedral C60 fullerene complex with Li2 dimer

The results of ab initio Hartree-Fock calculations of endo-and exohedral C₆₀ fullerene complexes with the Li⁺ ion and Li₂ dimer are presented. The coordination of the Li⁺ ion and the Li₂ dimer in the endohedral complexes and the coordination of Li⁺ ion in the exohedral complex of C₆₀ fullerene are determined by the geometry optimization using the 3–21G basis set. In the endohedral Li⁺C₆₀ complex, the Li⁺ ion is displaced from the center of the C₆₀ cage to the centers of carbon hexa-and pentagons by 0.12 nm. In the Li₂ dimer encapsulated inside the C₆₀ cage, the distance between the lithium atoms is 0.02 nm longer than that in the free molecule. The calculated total and partial one-electron densities of states of C₆₀ fullerene are in good agreement with the experimental photoelectron and X-ray emission spectra. Analysis of one-electron density of states of the endohedral Li⁺@C₆₀ complex indicates an ionic bonding between the Li atoms and the C₆₀ fullerene. In the Li⁺C₆₀ and Li⁺@C₆₀ complexes, there is a strong electrostatic interaction between the Li⁺ ion and the fullerene.     

Activation of fullerene coatings on field emitters by potassium atom and ion fluxes

The activation of tip field emitters with a fullerene coating by the atomic and ionic fluxes of potassium is studied. The deposition of atomic potassium decreases characteristic voltage U ₁ generating fixed fieldemission current I by a factor of 3.5–4.0. However, field emitters activated by potassium atoms are rapidly deactivated and the resulting decrease in U ₁ after storage in a vacuum does not exceed 25–30%. A stable approximately twofold decrease in the characteristic voltage can be reached if the fullerene coating is exposed to a potassium ion flux. The enhanced efficiency of emitter activation by a potassium ion flux is explained by the formation of KC ₆₀ endohedral and/or C₆₀ K exohedral molecules in the fullerene coating.     

Nuclear magnetic relaxation by regularly distributed dense paramagnetic ions in a quasi-two-dimensional system

The ¹H nuclear spin-lattice relaxation behavior was characterized in the perovskite-type layered structure quasi-two-dimensional Heisenberg paramagnets, (C_nH_2n+1NH₃)₂MnCl₄,with different chain lengths (n=8, 10, and 12). In contrast to the case of the short-chain compound with n=8, the nuclear spin diffusion to the electron spin system alone is not able to fully account for the spin-lattice relaxation in the compounds with longer chain lengths. Our results are discussed in light of the nuclear magnetic relaxation by the regularly distributed dense paramagnetic ions.     

Short-circuiting in fullerene devices studied by in situ electrical measurement in high vacuum and infrared imaging analysis

In the present work, current–voltage (I–V) characteristics of fullerene devices (ITO⧹C₆₀⧹Al) are reexamined by in situ electrical measurement in high vacuum and by infrared imaging analysis. Two kinds of I–V curves are detected: ‘ohmic’ and nonohmic. Degradation processes of the two different devices are measured, and ‘ohmic’ degradation processes are ascribed to short-circuiting. ITO⧹C₆₀⧹Al devices in high vacuum are confirmed to be intrinsically nonohmic. Surface temperature distribution of the two different devices is measured and localized heat is detected further confirming the existence of short-circuiting in ‘ohmic’ devices. To avoid short-circuit, organic buffers are inserted between fullerene layer and cathode and this is found to be effective.     

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T_c and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above T_c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.     

The effect of interface states, excess capacitance and series resistance in the Al/SiO2/p-Si Schottky diodes

The current-voltage (I-V) characteristics of Al/SiO₂/p-Si metal-insulator-semiconductor (MIS) Schottky diodes were measured at room temperature. In addition the capacitance-voltage (C-V) and conductance-voltage (G-V) measurements are studied at frequency range of 10 kHz-1 MHz. The higher value of ideality factor of 3.25 was attributed to the presence of an interfacial insulator layer between metal and semiconductor and the high density of interface states localized at Si/SiO₂ interface. The density of interface states (N_ss) distribution profile as a function of (E_ss − E_v) was extracted from the forward bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_e) at room temperature for the Schottky diode on the order of ≅4 × 10¹³ eV⁻¹ cm⁻²_. These high values of N_ss were responsible for the non-ideal behaviour of I-V and C-V characteristics. Frequency dispersion in C-V and G-V can be interpreted only in terms of interface states. The N_ss can follow the ac signal especially at low frequencies and yield an excess capacitance. Experimental results show that the I-V, C-V and G-V characteristics of SD are affected not only in N_ss but also in series resistance (R_s), and the location of N_ss and R_s has a significant on electrical characteristics of Schottky diodes.     

Manifestation of liquid-state and solid-state properties in electron relaxation of paramagnetic ions in solutions: Non-Markovian processes

The linewidth δH and the spin-spin relaxation time T ₂ for Gd³⁺, Mn²⁺, and Cr³⁺ ions in aqueous, water-glycerol, and water-poly(ethylene glycol) solutions at paramagnetic ion concentrations providing the dipole-dipole mechanism of spin relaxation are measured using two independent methods, namely, electron paramagnetic resonance (EPR) and nonresonance paramagnetic absorption in parallel fields. Analysis of the experimental results indicates a gradual crossover from pure liquid-state (diffusion) to quasi-solid-state (rigid lattice) spin relaxation. It is demonstrated that the limiting cases are adequately described by standard, universally accepted formulas for dipole-dipole interactions in the liquid-state (the correlation time of translational motion satisfies the condition τ _c ?τ₂) and solid-state (τ _c ?τ₂) approximations. A complete theoretical treatment of the experimental dependences (including the observed gradual crossover of spin relaxation) is performed in the framework of the non-Markovian theory of spin relaxation in disordered media, which is proposed by one of the authors. Within this approach, the collective memory effects for spin and molecular (lattice) variables are taken into account using the first-order and second-order memory functions for spin-spin and spin-lattice interactions. A correlation between the spin magnitude and the temperature-viscosity conditions corresponding to the crossover to non-Markovian relaxation is revealed, and the situations in which structural transformations occurring in the solutions favor the crossover to solid-state spin relaxation are analyzed.     

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi₄Ge₃O₁₂ doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T ₂* occurred upon doping the pure Bi₄Ge₃O₁₂ sample. Unlike T ₂*, the effective spin-lattice relaxation time T ₁* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.     

A femtosecond laser study of the endohedral fullerenes Li@C60 and La@C82

The endohedral fullerenes La@C₈₂ and Li@C₆₀ have been studied in the gas phase with femtosecond laser excitation. The two molecules show qualitatively the same behaviour with respect to ion yield vs. pulse energy, but markedly different fragmentation patterns, with La@C₈₂ fragmenting via the shrink-wrap mechanism and Li@C₆₀ predominantly losing the metal atom in the first fragmentation step. The ion yields and electron energy distributions of La@C₈₂ agree well with a recently developed model for fs laser ionisation of C₆₀.     

Proton NMR study of α-MnH0.06

Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH_0.06 have been measured over the temperature range 11-297 K and the resonance frequency range 20-90 MHz. A considerable shift and broadening of the proton NMR line and a sharp peak of the spin-lattice relaxation rate are observed near 130 K. These effects are attributed to the onset of antiferromagnetic ordering below the Néel temperature T_N≈130 K. The proton NMR line does not disappear in the antiferromagnetic phase; this suggests a small magnitude of the local magnetic fields at H-sites in α-MnH_0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations.     

Composition spread metal thin film fabrication technique based on ion beam sputter deposition

A composition spread metal thin film fabrication technique based on ion beam sputter deposition method was developed. The technique enables us to fabricate any desired part or a complete binary/ternary composition spread metal thin films onto a single substrate by sequentially sputtering different target materials. Composition spread metal thin films can be deposited directly on a dielectric film in patterned electrode shape for C-V and I-V measurements. The system could be especially useful in the search for new multi-component metal gate materials.