NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k _p/(R−s) with ak _p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek _p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T ₁), spin-spin relaxation times (T ₂), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT ₂ measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T ₁ of the narrow line is significantly shorter thanT ₁ of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.     

Dynamic NMR investigation of plastic crystals in bulk and confined in porous materials

In this work, the molecular dynamics of four organic compounds confined in silica pores of nominal diameter 6 and 20 nm are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substances. By using organic compounds forming soft plastic crystals on freezing as adsorbates, damage to the pore structures can be avoided. NMR lineshapes, spin-lattice relaxation times (T ₁), spin-spin relaxation timesT ₂ and diffusivities are reported as a function of temperature. Since the porous grains are much greater than the distance travelled by the molecules during the experiment, intracrystalline NMR parameters were obtained. However, the shortT ₂ (∼1 ms) encountered in both the bulk and confined samples prohibited measurements ofT ₂ and the diffusivity in the low-temperature ordered phases. The confinement in the pores gives rise to substantial changes in the phase behavior and molecular dynamics. Thus, the¹H lineshape observations of the confined samples clearly reveal a narrow-line component superimposed on a broad resonance at temperatures well below the transition point of the bulk material. In the freezing region, the narrow-line component is attributed to the surface layer and the undercooled liquid in the smaller pores that remains unfrozen. In the two-component, low-temperature region, the narrow component corresponds to the surface layer, while the broad component originates from the crystalline phase at the center of the pores.     

Influence of surface interactions on the dynamics of the glass former ortho-terphenyl confined in nanoporous silica

We present results on investigations of the dynamics of the glass forming ortho-terphenyl (oTP) confined in nanoporous silica. Calorimetry experiments showed that the glass transition temperature of the confined liquid, T_gconf, has a non-trivial pore size dependence and is strongly affected by surface interactions. Fluid-wall interactions introduce gradients of structural relaxation times in the pores. The molecules at the surface of the pores are slowed down compared to those at the center of the pores. We focus here on a pore diameter range (7 σ< d < 12 σ, where σ is the molecular diameter), where a large variety of dynamical behavior were observed. Depending on surface properties of the confined media, T _gconf may be smaller or larger than the bulk one. In a quite attractive matrix with a pore size of around 7 nm, the structural relaxation times gradient is important enough to allow the observation of two glass transitions for the same liquid. Effects of fluid wall interactions on the short time dynamics at high temperature were also investigated by quasielastic neutron scattering. The self and collective motions exhibit well above the bulk melting point the same dependence on fluid-wall interactions as at T_g.     

Size-dependent freezing of n-alcohols in silicon nanochannels

We present a study on the phase behavior of several linear n-alcohols (heptanol, nonanol and undecanol) in their bulk state as well as confined in mesoporous silicon. We were able to vary the mean pore radii of the nanochannels from r = 3.5  nm to 7 nm and to determine the respective temperatures of the freezing and melting transitions using infrared and dielectric spectroscopy. The smaller the chain length the lower the freezing point, both in the bulk and in the confined state. Under confinement the freezing temperature decreases by up to 28 K compared to the bulk value. In accordance with the Gibbs-Thompson model the lowering is proportional to the inverse pore radius, ΔT _fr ∝ 1/r. Moreover, the ratio of freezing temperature depression to melting temperature depression is close to the theoretical value of ΔT _fr /ΔT _melt = 3/2. The spectra also indicate a structural change: while the solid bulk alcohols are a polycrystalline mixture of the orthorhombic β- and monoclinic γ-form, geometrical confinement forces the alcohol-chains into the more simple orthorhombic structure. In addition, a part of the material does not crystallize. Such an additional amorphous phase seems to be a logical consequence of the size mismatch between molecular crystals and irregular shaped pores.     

Changes in dynamical behavior of ionic liquid in silica nano-pores

Dielectric relaxation measurement has been carried out on an ionic liquid (1-butyl-3-methyl imidazolium hexafluorophosphate, [BMIM][PF₆]) confined in nano-porous silica matrix. Two dielectric relaxation peaks have been observed in the confined ionic liquid (IL) while there is only one relaxation peak for bulk IL. Confinement results in layering of some IL molecules near the pore wall while other molecules, less affected by pore wall interaction, remain in the central core. The two relaxation peaks are assigned to the different dynamical behaviors of the central core and layered IL molecules.     

Polar optical phonon limited energy and momentum relaxation of hot electrons in a GaAs-quantum well (QW)

The average energy loss rate, the energy- as well as the momentum relaxation time of hot electrons confined in a GaAs-square quantum well are calculated as a function of the external controllable parametersn _s (electron density),T (lattice temperature) andT _e (electron temperature) for the interaction of the charge carriers with bulk- and surface polar optical phonons. Analytical expressions are derived in the limit of vanishing quantum well width at non-degeneracy and degeneracy of the electron system. Both energy-and momentum relaxation time are found to be complicated functions of the ratiosT _D /T _e andT _D /T withT _D being the Debye-temperature of the polar optical phonon involved in the scattering. In a thick (very thin) QW the energy loss rate to bulk PO-phonons is found to be larger (smaller) than the corresponding loss rate to surface modes. The energy- (momentum-) relaxation times are found to be constant (increasing) functions ofn _s at non-degeneracy (degeneracy) of the electron system. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

A field-cycling NMR study of nematic 4-pentyl-4′-cyanobiphenyl confined in porous glasses

The proton spin-lattice relaxation time T₁, in the nematic liquid crystal 4-pentyl-4′-cyanobiphenyl confined in a glassy porous matrix has been measured in a wide Larmor frequency range of 1 · 10²?2 · 10⁷ Hz employing the fast field-cycling NMR technique. A strong influence of the restricted geometry on the character of the T₁ dispersion was found. Our investigation clearly demonstrates the importance of the translationally induced molecular reorientations in inhomogeneous director field for the relaxation in the samples with 200 and 80 nm mean pore size. The experimental results are in a good agreement with the theoretical predictions. In the sample with 7 nm pore size the main contribution to the relaxation is ascribed to the slowing down of the molecular motion in the near-surface layer. Zero-field 1H NMR spectra of a microconfined liquid crystal are reported for the first time.     

Saturation spectroscopy of coherent raman scattering in molecular gases

We present the theory of two-photon Raman saturation of a two-level Raman transition studied by an independent CARS process. The main goal here is to probe the saturated homogeneous Raman line shape. It is shown that there appears a saturation dip with a width determined by the relaxation timeT ₁. In the case of Doppler-broadened line the coherent Raman saturation spectroscopy may be used to determine both theT ₁ andT ₂ relaxation times.     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT ₁ process and double quantum filtered (DQF) spectral analyses.²H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.     

Effect of surface modification on the pore condensation of fluids: experimental results and density functional theory

The physisorption and pore condensation of a polar fluid (CHF₃) in a series of MCM-41 type mesoporous silica materials with native and chemically modified pore walls has been studied over the temperature range 168–293 K, corresponding to reduced temperatures T/T_c in the range 0.56-0.98, where T_c is the critical temperature of the fluid. Chemical modification of the pore walls by attachment of Si(CH₃)₃ groups causes a shift in pore condensation to higher relative pressures p/p0. This effect is most pronounced for materials with narrow pores (2.9 nm) at low temperatures. In the theoretical part of the work density functional theory based on a simple cubic lattice model of the confined fluid has been used to analyse the combined effect of a reduced pore width and weaker fluid-wall interaction caused by the surface coating. For realistic values of the model parameters it is found that the effect of the lower pore width is outweighed by the opposing effect of the lower fluid-wall interactions. The weaker temperature dependence of the pore existence curve observed experimentally for the surface modified materials can be traced back to a crossover from a two-step to a single-step process of pore filling predicted by the model.     

Freezing and melting of binary mixtures confined in a nanopore

This paper reports on a Grand Canonical Monte Carlo study of the freezing and melting of Lennard–Jones Ar/Kr mixtures confined in a slit pore composed of two strongly attractive structureless walls. For all molar compositions and temperatures, the pore, which has a width of 1.44?nm, accommodates two contact layers and one inner layer. Different wall/fluid interactions are considered, corresponding to pore walls that have a larger affinity for either Ar or Kr. The solid/liquid phase diagram of the confined mixture is determined and results compared with data for the bulk mixture. The structure of the confined mixture is studied using 2D order parameters and both positional g(r) and bond orientational G₆(r) pair correlation functions. It is found that in the confined solid phase, both the contact and inner layers have a hexagonal crystal structure. It is shown that the freezing temperature of the Ar/Kr confined mixture is higher than the bulk freezing point for all molar compositions. Also, it is found that the freezing temperature becomes larger as the ratio α of the wall/fluid to the fluid/fluid interactions increases, in agreement with previous simulation studies on pure substances confined in nanopores. In the case of pore walls having a stronger affinity for Kr atoms (ε _Ar/W<ε _Kr/W), it is observed that both the contact and inner layers of the confined mixture undergo, at the same temperature, a transition from the liquid phase to the crystal phase. The freezing of Ar/Kr mixtures confined between the walls having a stronger affinity for Ar (ε _Ar/W?>?ε _Kr/W) is more complex: for Kr molar concentration lower than 0.35, we observe the presence of an intermediate state between all layers being 2D hexagonal crystals and all the layers being liquid. This intermediate state consists of a crystalline contact layer and a liquid-like inner layer. It is also shown that the qualitative variations of the increase of freezing temperature with the molar composition depend on the affinity of the pore wall for the different components. These results confirm that, in addition to the parameter α the ratio of the wall/fluid interactions for the two species, η=?_Ar/W/?_Kr/W, is a key variable in determining the freezing and melting behaviour of the confined mixture.     

Magnetic properties of (Fe, Fe–B)/γ-Fe2O3 core shell nanostructure

Temperature-dependent magnetic properties of a core/shell nanostructure are reported employing magnetometry and electron magnetic resonance (EMR) spectroscopy. Structural characterization of the sample synthesized by NaBH₄ reduction of FeCl₃ was done by x-ray diffraction, TEM and Mössbauer spectroscopy and showed a core/shell nanostructure with a core of diameter D?20 nm consisting of α-Fe and amorphous Fe–B alloy and a shell of 7 nm thickness made up of principally γ-Fe₂O₃. Temperature-dependent EMR studies at 9.28 GHz show a narrow line with g?2.01 superimposed on a broad line with g?2.20. The narrow line assigned to the oxide shell disappears below about 60 K, in agreement with a blocking temperature T_B?30 K measured in SQUID magnetometry. The EMR parameters of the broad EMR line are similar to those reported for α-Fe nanoparticles imbedded in amorphous SiO₂ matrix. The magnitude of the saturation magnetization M_S=70 emu/g of the nanostructure is smaller than that of bulk α-Fe (M_S=220 emu/g) and bulk γ-Fe₂O₃ (M_S=88 emu/g). Size dependence is used to interpret the absence of exchange-bias in the field-cooled sample of the nanostructure.     

Multiferroic and exchange bias in La0.85Sr0.15FeO3−δ perovskite nanoparticles

Multiferroic and exchange bias was observed in La_0.85Sr_0.15FeO_3?δ nanoparticles sample which was prepared by co-precipitation method. These nanoparticles have an average size of about 18.49?nm and orthorhombic phase (Pbnm) with small impurities. The anomalous peaks of the dielectric as a function of temperature indicate that the Neel temperature at T_N?=?321?K and a ferroelectric to paraelectric transition at T_C?=?555?K which confirmed by differential scanning calorimetric (DSC). The dielectric response at low frequency is dominated by Debye relaxation behaviour and the resonance behaviour is dominated above 1.7?GHz. Mössbauer spectrum revealed the presence of magnetic ordering temperature (T_N) lies above room temperature (RT). Moreover, the oxygen deficient (δ?=?0.038) was calculated from Mössbauer spectrum. The hysteresis loop (B-H) of the nanoparticle sample exhibits exchange bias as a result of the exchange coupling at the interface between the ferromagnetic surface with canted spins and the antiferromagnetic core of the particles.     

La0.82Te0.18MnO3薄膜的输运特性和光诱导效应

用脉冲激光沉积法制备了非金属Te掺杂的钙钛矿锰氧化物La_0.82Te_0.18MnO₃单晶薄膜.该薄膜从83 K升温至373 K过程中发生金属-绝缘体相变，转变点温度为283 K.其电阻率在T<T_MI时符合电子-电子、电子-磁振子散射公式；在T>T_MI时为小极化子输运.薄膜在低温段连续激光(波长为532 nm，40 mW)作用下电阻率显著增大，电阻变化率在253 K达到最大值51.1%,该变化率远大于相同条件下的空穴掺杂材料；在高温段产生了较小的光电导，电阻变化率小于10%.这些现象主要与激光激励下自旋系统和小极化子的变化有关.La_0.82Te_0.18MnO₃薄膜在激光诱导下具有明显的与自旋相关的弛豫现象.激光开始作用时薄膜电阻率随时间的变化符合指数关系. 关键词： 0.82Te_0.18MnO₃薄膜')" href="#">La_0.82Te_0.18MnO₃薄膜 光诱导 输运特性 电子掺杂     

Enthalpy recovery of a glass-forming liquid constrained in a nanoporous matrix: Negative pressure effects

The T _g of organic liquids confined to nanoporous matrices and that of thin polymer films can decrease dramatically from the bulk value. One possible explanation for this phenomenon is the development of hydrostatic tension during vitrification under confinement that results in a concomitant increase in the free volume. Here we present experimental evidence and modeling results for ortho-terphenyl (o-TP) confined in pores as small as 11.6 nm that indicate that, although there is an important hydrostatic tension in the liquid in the pores, it does not develop until near the reduced T _g of the constrained material --well below the bulk T _g. Enthalpy recovery for the o-TP in the nanopores exhibits accelerated physical aging relative to the bulk, as well as a leveling off of the fictive temperature at equilibrium values greater than the aging temperature. An adaptation of the structural recovery model that incorporates vitrification under isochoric conditions is able to provide a quantitative explanation for the apparently anomalous aging observed in nanopore confined liquids and in thin polymeric films. The results strongly support the existence of an intrinsic size effect as the cause of the reduced T _g. Received 3 September 2001     

Molecular motion in the plastic phase of pivalic acid studied by nuclear magnetic resonance

Nuclear magnetic spin relaxation times T ₁, T ₂ and T _1ρ have been measured for methyl and acid protons in pivalic acid throughout the plastic crystalline phase, 6·9 to 36·5°C. T _1ρ and T ₂ of the methyl protons are interpreted in terms of translational self-diffusion of molecules. Diffusion coefficients are deduced which are in fair agreement with previous radiotracer measurements. T ₁ measurements at 10 MHz and 20·8 MHz indicate molecular reorientation and the temperature dependence of this process is investigated. T _1ρ and T ₂ measurements for the acid protons show that they move more rapidly through the lattice than the molecules. This is in agreement with previously reported tracer studies. A mechanism is proposed connecting the anomalously fast hydrogen atom diffusion and molecular reorientation, which is consistent with the N.M.R. and tracer measurements.     

The “corset effect” of spin-lattice relaxation in polymer melts confined in nanoporous media

Linear polyethylene oxides with molecular weightsM _w of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycling nuclear magnetic resonance relaxometry. The pore diameter was varied in the range of 9–57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close toT ₁∞ v^3/4 in the range ofv=3·10^?1-2·10¹ MHz as predicted by the tube-reptation model. This protonT ₁ dispersion essentially reproduces that found in a previous deuteron study (R. Kimmich, R.-O. Seitter, U. Beginn, M. Möller, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). As a feature particularly characteristic for reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indicate that the diameter of the effective tubes in which reptation occurs is much smaller than the pore diameters on the time scale of spin-lattice relaxation experimens. An estimation leads to a valued ^*～0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains (“excluded volume”) and the low value of the compressibility in polymer melts create the “corset effect” which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer.     

35Cl N.M.R. relaxation in aqueous sodium perchlorate solutions

The relaxation time, T _1ρ, for ³⁵Cl in perchlorate ion in aqueous solutions is found to be 0·25 s. T _1ρ is insensitive to the presence of most first row transition metals; however, a weak complex with manganous ion is demonstrated, and the perchlorate ion-manganous exchange rate is shown to be between 3 × 10⁴ s^-1 and 3·6 × 10⁷ s^-1. In the absence of manganous ion, the relaxation is dominated by the nuclear electric quadrupole interaction; however, a dipolar contribution is observed with manganous ion present.     

Temperature dependences of the order parameter for sodium nitrite embedded into porous glasses and opals

The temperature dependences of the order parameter η(T) for sodium nitrite NaNO₂ embedded in porous glasses with average pore diameters of 320 and 20 nm, as well as in artificial opals, have been investigated. It has been demonstrated that the dependence η(T) for sodium nitrite in the porous glass almost coincides with that for the bulk material, whereas this dependence for NaNO₂ in opals differs substantially from that observed in the bulk material and from those previously determined for sodium nitrite in porous glasses with average pore diameters of 3 and 7 nm. It has been revealed that the dependence of the order parameter for sodium nitrite in opals exhibits a temperature hysteresis (approximately equal to 8 K). The temperature dependence η(T) has been described using a simple model, which takes into account the nanopore diameter distribution existing in artificial opals.