Relaxation Properties Of Electrons Research Articles

1H Spin-Lattice Relaxation Processes in Solutions of H2N-Fe3O4 Nanoparticles: Insights from NMR Relaxometry.

1H spin-lattice relaxation experiments have been performed for water and glycerol/water solutions of H2N-Fe3O4 superparamagnetic nanoparticles (NPs) of about 7 nm diameter. The experiments encompass a broad frequency range covering 3 orders of magnitude, from 10 kHz to 10 MHz (referring to 1H resonance frequency), and have been performed in the temperature range from 298 to 313 K, varying the concentration of the superparamagnetic species. This extensive dataset has been used for twofold purposes. The first one is to serve as a challenge for thorough tests of theoretical models describing nuclear relaxation in solutions of superparamagnetic NPs, depending on their magnetic properties and dynamics of the solvent molecules. The challenge is posed by the wish to reproduce the data in a broad range of magnetic fields (not only at high fields) and by the need to explain the differences in the relaxation scenarios for water and glycerol/water solutions by varying only the solvent parameters. The second purpose is to get insights into the magnetic properties (electronic relaxation properties) of the nanoparticles due to their high applicational potential.

Efficient Dynamic Nuclear Polarization up to 230 K with Hybrid BDPA-Nitroxide Radicals at a High Magnetic Field.

Pairing the spectral resolution provided by high magnetic fields at ambient temperature with the enhanced sensitivity offered by dynamic nuclear polarization (DNP) is a major goal of modern solid-state NMR spectroscopy, which will allow one to unlock ever-challenging applications. This study demonstrates that, by combining HyTEK2, a hybrid BDPA-nitroxide biradical polarizing agent, with ortho-terphenyl (OTP), a rigid DNP matrix, enhancement factors as high as 65 can be obtained at 230 K, 40 kHz magic angle spinning (MAS), and 18.8 T. The temperature dependence of the DNP enhancement and its behavior around the glass transition temperature (Tg) of the matrix is investigated by variable-temperature EPR measurements of the electron relaxation properties and numerical simulations. A correlation is suggested between the decrease in enhancement at the passage of the Tg and the concomitant drop of both transverse electron relaxation times in the biradical.

Modulating lifetimes and relaxation times of phenoxyl radicals through their incorporation into different hybrid nanostructures.

The development of new open shell systems is essential for advances in spin science. In this work, we report the synthesis and characterization of three nanostructured materials, namely SBA-15 silicas, periodic mesoporous organosilicas (PMOs) and lamellar polysilsesquioxanes, all functionalized with the same diazene-based phenoxyl radical precursor. The impact of the nature of the material, i.e. loading of radical precursor and structure, on half-lifetimes (t1/2) and relaxation times of phenoxyl radicals was investigated. Although phenoxyl radicals are transient in solution, their t1/2 range from hours to years at room temperature (RT) when they are embedded in nanostructured materials. The above mentioned functionalized materials were used to generate the corresponding phenoxyl radicals and their relaxation times were measured (〈T1e〉 and Tm) from 50 K to RT. The results were rationalized in terms of limited mobility of the radical as a result of supramolecular interactions and structure rigidity. All these data show that it is possible to design functionalized nanostructured material with radicals possessing specific electronic relaxation properties which can be of interest in fields like DNP, organic magnetism or spin qubit.

Electron relaxation properties of Ar magnetron plasmas

An understanding of electron relaxation properties in plasmas is of importance in the application of magnetrons. An improved multi-term approximation of the Boltzmann equation is employed to study electron transport and relaxation properties in plasmas. Elastic, inelastic and nonconservative collisions between electrons and neutral particles are considered. The expressions for the transport coefficients are obtained using the expansion coefficients and the collision operator term. Numerical solutions of the matrix equations for the expansion coefficients are also investigated. Benchmark calculations of the Reid model are presented to demonstrate the accuracy of the improved multi-term approximation. It is shown that the two-term approximation is generally not accurate enough and the magnetic fields can reduce the anisotropy of the velocity distribution function. The electron relaxation properties of Ar plasmas in magnetrons for various magnetic fields are studied. It is demonstrated that the energy parameters change more slowly than the momentum parameters.

Computation of electron transport and relaxation properties in gases based on improved multi-term approximation of Boltzmann equation

An understanding of electron kinetics is of importance in various applications of low temperature plasmas. We employ a series of model and real gases to investigate electron transport and relaxation properties based on improved multi-term approximation of the Boltzmann equation. First, a comparison of different methods to calculate the interaction integrals has been carried out; the effects of free parameters, such as vmax, lmax, and the arbitrary temperature Tb, on the convergence of electron transport coefficients are analyzed. Then, the modified attachment model of Ness et al. and SF6 are considered to investigate the effect of attachment on the electron transport properties. The deficiency of the pulsed Townsend technique to measure the electron transport and reaction coefficients in electronegative gases is highlighted when the reduced electric field is small. In order to investigate the effect of external magnetic field on the electron transport properties, Ar plasmas in high power impulse sputtering devices are considered. In the end, the electron relaxation properties of the Reid model under the influence of electric and magnetic fields are demonstrated.

Improving Sensitivity of Solid-state NMR Spectroscopy by Rational Design of Polarizing Agents for Dynamic Nuclear Polarization.

We review our recent efforts to optimize the efficiency of polarizing agents for Dynamic Nuclear Polarization (DNP) in solid-state MAS NMR spectroscopy. We elucidate the links between DNP performance, molecular structure and electronic relaxation properties of dinitroxide biradicals. We show that deuteration and increased bulkiness lead to slower electronic relaxation and in turn to higher DNP enhancements. We also show that the incorporation of solid dielectric particles into the sample is a general method of amplifying DNP enhancements by about a factor of two.

A simple method for large scale synthesis of highly monodisperse gold nanoparticles atroom temperature and their electron relaxation properties

Here we demonstrate a simple method for large scale preparation ofmonodisperse gold nanoparticles by simple mixing of chloroauricacid (HAuCl4) with oleylamine (OA) at room temperature. The as-prepared gold nanoparticles havehigh monodispersity with an average diameter of 13 nm and can self-organize intotwo-dimensional (2D) hexagonal close-packed arrays. The size of the gold nanoparticles canbe experimentally controlled. The capping agent, oleylamine, can be easily replaced withother capping agents such as thiol groups for further functionalization. The electronrelaxation dynamics of these gold nanoparticles in toluene was studied by femtosecondpump–probe measurements, in comparison with the citrate-stabilized gold nanoparticles inwater. The phonon–phonon relaxation time of gold nanoparticles in toluene is slower thanthat of citrate-capped gold nanoparticles in water, due to the lower thermal conductivity oftoluene than water. The electron–phonon relaxation of the gold nanoparticlesin toluene was found to display weaker pump energy dependence, compared tothat of citrate-capped gold nanoparticles in water. The different electron–phononrelaxation dynamics is ascribed to the extra vibrational states provided by gold-NH2, which serves as an extra nonradiative relaxation pathway for the e–ph relaxation inoleylamine-capped gold nanoparticles in toluene.

Lanthanide Complexes of a Picolinate Ligand Derived from 1,4,7‐Triazacyclononane with Potential Application in Magnetic Resonance Imaging and Time‐Resolved Luminescence Imaging

The new potentially octadentate ligand, 1-(carboxymethyl)-4,7-bis[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H(3)bpatcn), in which two picolinate arms and one acetate arm are connected to the 1,4,7-triazacyclonane core, has been prepared. Potentiometric studies show an increased stability of the Gd(III) complex of H(3)bpatcn (logK(GdL)=15.8(2)) with respect to the Gd(III) complex of the analogous ligand 1,4,7-triazacyclononane-N,N',N''-triacetic acid (H(3)nota) (logK(GdL)=13.7), associated with an increased selectivity of H(3)bpatcn for gadolinium over calcium. The H(3)bpatcn ligand sensitises the terbium ion very efficiently, leading to a long-lived and highly luminescent terbium complex (quantum yield=43 %), in spite of the presence of a coordinated water molecule. (1)H proton NMR studies indicate that the metal ion is rigidly encapsulated by the three arms of the octadentate ligand H(3)bpatcn and that the macrocycle framework remains bound (through the five nitrogen and the three oxygen atoms) even at high temperature. A new theoretical method for interpreting the water proton relaxivity is presented. It is based on recent progresses in the description of the electronic spin relaxation and on an auxiliary probe solute. It replaces the Solomon, Bloembergen and Morgan (SBM) framework, which is questionable at low field, while avoiding resorting to simulations and/or sophisticated theories with additional unknown zero-field splitting (ZFS) parameters. The inclusion of two picolinate groups on a triazacyclononane framework affords the mono-aquo gadolinium complex [Gd(bpatcn)(H(2)O)] with favourable electron-relaxation properties (tau(eff)(S0)=125 ps). The optimisation of the electronic relaxation by ligand design is especially important to achieve high relaxivity in the new generation macromolecular complexes with long rotational correlation times.

Chromium(III) complexes as intermolecular probes

Metal ion complexes provide flexible paramagnetic centers that may be used to define intermolecular contacts in a variety of solution phase environments because both the charge and electronic relaxation properties of the complex may be varied. For most complex ions, there are several proton equilibria that may change the effective charge on the complex as a function of pH which in turn affects the efficacy of application for defining the electrostatic surfaces of co-solute molecules. We report here spectrophotometric and nuclear spin relaxation studies on aqueous solutions of chromium(III) complexes of EDTA, DTPA, and bis-amides of both. The effective charges available from these paramagnetic centers range from −3 to +1 and we report the pH ranges over which the effective charge is defined with confidence for application in magnetic relaxation experiments.

Self-sustained acoustoelectric and photoelectric oscillations in semiconductor–piezoelectric structures

Existence and possibility of self-sustained oscillations of acoustoelectric and photoelectric voltages in layered piezoelectric–semiconductor structures without any negative differential conductivity and acoustoelectric domains are demonstrated experimentally and theoretically. A joint experimental study of acoustoelectric and photoelectric phenomena in the same structure permits to establish similar electronic processes. Experimental and theoretical results show that nonlinear acoustoelectric and photoelectric voltages with electronic relaxation properties and external feedback are necessary for observing the phenomena. The theory presented explains the experimental results obtained. A criterion for excitation in a soft mode and dynamic characteristics of the self-sustained oscillations under various external conditions, as temperature, illumination and humidity are discussed. Periodic and stochastic modes of oscillation are noted. The quasi-harmonic mode of oscillations described by the Van der Pol equation is very useful and attractive for practical application because of the fact that the oscillation frequency is proportional to the square root of the relaxation time of electronic processes in semiconductors and that it can be precisely measured and digitized. Due to the precise method presented for measuring the relaxation time, new fundamental possible applications to methods of investigation of physical and chemical sensors with a frequency output are noted.

Dispersion of optical and acoustical phonons in the zinc-blende group III-nitride superlattices

A comprehensive theoretical study of phonons in cubic GaN, AlN, Ga1−xAlxN, and (GaN)m/(AlN)n superlattices (SLs) is reported using both simple and microscopic models. The new periodicity introduced by the SL structure causes zone folding of the acoustical modes allowing phonons with large q vectors, normally inactive in light scattering, to become Raman active. We predicted the frequencies of such modes in GaN/AlN SLs using elastic continuum, diatomic linear-chain, and rigid-ion models. In the rigid-ion model (RIM), the short range forces are optimized in terms of elastic constants and phonon energies at critical points while the long-range Coulomb forces are evaluated exactly via Ewald summation. The advantage of using RIM is that it permits the construction of a dynamical matrix entering into the secular equation for the SL vibrations of a given wavevector directly from the dynamical matrices of the bulk materials. For short period (GaN)m/(AlN)n SLs, the dependence of phonons on wavevectors both parallel and perpendicular to the growth direction [001] is investigated. Theoretical results for the GaN/AlN superlattices are compared and discussed with the GaAs/AlAs system as well as with the existing model calculation of Grille and Bechstedt. Unlike GaAs/AlAs, the larger optical phonon values and partially changed confinement characteristics in GaN/AlN superlattices are believed to have significant affects on the relaxation properties of electrons in group III-nitride material systems.

Phonons in cubic GaN/AlN semiconductor superlattices

Rigid-ion model calculations are reported for the lattice dynamics of cubic GaN, AlN, GaAlN and GaN/AlN superlattices. The short-range forces in the rigid-ion model are optimized by incorporating phonons at critical-points as an input and using the existing elastic constants as constraints on the values of the parameters. The long-range Coulomb forces are obtained exactly by using the Ewalds transformation method. For ultrathin superlattices, the dependence of phonons on wavevectors both parallel and perpendicular to the growth direction [001] are investigated. Theoretical results for the GaN/AlN superlattices are compared and discussed with the GaAs/AlAs system as well as with the model calculations of Grille and Bechstedt. Unlike GaAs/AlAs, the larger optical phonon values and partially changed confinement characteristics in GaN/AlN superlattices are believed to have significant affects on the relaxation properties of electrons in group III-nitride material systems.

Electron Relaxation Properties and Transient Spectroscopy of Hydrogenated Amorphous Silicon

The isothermal-capacitance-transient-spectroscopy (ICTS) and the deep-level-transient-spectroscopy (DLTS) data of a-Si:H are analyzed using a model based on the electronic relaxation theory of Ngai and coworkers. The electron capture and emission processes are taken into account in a selfconsistent manner, and are described by relaxation functions with the same fractional exponent, i.e., φ(t)αexp [-(t/τ*)1-λ], where λ=0.2 and τ* is energy dependent in both processes. This approach (i) explains the anomalous energy and temperature dependence of the effective relaxation rate obtained by the ICTS method and (ii) yields an explanation for the energy difference of the D- states between ICTS and DLTS measurements.