Low-temperature Samples Research Articles

Helium-cooling and -spinning dynamic nuclear polarization for sensitivity-enhanced solid-state NMR at 14 T and 30 K

We describe a 1H polarization enhancement via dynamic nuclear polarization (DNP) at very low sample temperature T≈30K under magic-angle spinning (MAS) conditions for sensitivity-enhanced solid-state NMR measurement. Experiments were conducted at a high external field strength of 14.1T. For MAS DNP experiments at T≪90K, a new probe system using cold helium gas for both sample-cooling and -spinning was developed. The novel system can sustain a low sample temperature between 30 and 90K for a period of time >10h under MAS at νR≈3kHz with liquid He consumption of ≈6L/h. As a microwave source, we employed a high-power, continuously frequency-tunable gyrotron. At T≈34K, 1H DNP enhancement factors of 47 and 23 were observed with and without MAS, respectively. On the basis of these observations, a discussion on the total NMR sensitivity that takes into account the effect of sample temperature and external field strength used in DNP experiments is presented. It was determined that the use of low sample temperature and high external field is generally rewarding for the total sensitivity, in spite of the slower polarization buildup at lower temperature and lower DNP efficiency at higher field. These findings highlight the potential of the current continuous-wave DNP technique also at very high field conditions suitable to analyze large and complex systems, such as biological macromolecules.

Conformational Analysis of Fluorinated Pyrrolidines Using 19F–1H Scalar Couplings and Heteronuclear NOEs

A series of 3-fluoropyrrolidines have been studied to investigate the influence of the stereoelectronic fluorine gauche effect on ring conformations in the solid state by single-crystal X-ray analysis and in solution phase by NMR spectroscopy. As part of these studies 1D (19)F-(1)H heteronuclear NOE (HOESY) experiments have been optimised for applications to small molecules and are described in detail. These have been employed to estimate (19)F-(1)H internuclear distances and were combined with vicinal (3)J(F,H) and (3)J(H,H) scalar coupling constants to analyse the ring conformations. Where possible the derived solution-phase structural data have been compared with those of the crystalline state. The results demonstrate the influence of the gauche effect in stabilising C(γ)-exo conformations of the fluorinated pyrrolidines. It was further shown that when steric interactions were also present, this conformational bias was diminished and the contribution of the alternative C(γ)-endo conformation was seen to increase in solution at lower sample temperatures.

Metallic state induced in Eu2Ru2O7 by hole creation and orbital overlap in the t2g bands

Two different series of ruthenium pyrochlores with the compositions Eu2−xCaxRu2O7 (0 ≤ x ≤ 0.6) and Eu2Ru2−xRexO7 (0 ≤ x ≤ 0.4) were prepared via solid state reaction, and their transport and magnetic properties were investigated. In both systems, the resistivity is shifted from 106 Ω cm to 10−3 Ω cm at low temperatures with increasing doping. The series correspond to two different approaches for inducing a metal–insulator transition in the Eu2Ru2O7 parent compound: (i) the substitution of Eu3+ by Ca2+, resulting in hole creation within the Ru t2g bands, and (ii) the partial substitution of Ru4+ (4d4) by Re4+ (5d3), leading to hole creation and an increase in the orbital overlap. Increasing the Ca or Re concentration suppresses the anomalies observed for Eu2Ru2O7 in dc magnetic susceptibility under zero-field-cooled and field-cooled conditions at 118 K and 23 K. In addition, a small negative magnetoresistance was obtained at low temperatures for doped samples with 2% Ca and Re.

Unconventional photoluminescence effect of SrTiO 3 thin film under UV-illumination

We find that PL intensity I ( t ) of SrTiO 3 thin film measured under UHV condition increases with UV-laser illumination over long time scale of ∼ 2 h. The intensity increase takes place at lower sample temperature as well, 200, 100 K, and 20 K. When O 2 and N 2 gas are introduced into the sample chamber the PL intensity decreases with the UV-illumination time, opposite to the UHV-case. We consider a quantitative thermal energy flow model of the laser-power and heat absorption by the sample, but find that temperature change of the sample is not large enough to account for the time dependent I ( t ) . We propose photo-catalysis effect on STO surface as possible scenario of the PL intensity change.

Physical properties of FeSe0.5Te0.5 single crystals grown under different conditions

We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe0.5Te0.5 single crystals grown by self-flux and Bridgman methods. The lowest values of the susceptibility in the normal state, the highest transition temperature T c of 14.4 K, and the largest heat-capacity anomaly at T c were obtained for pure (oxygen-free) samples. The critical current density j c of 8.6 × 104 A/cm2 (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the anion sites. The samples containing an impurity phase of Fe3O4 show increased j c up to 2.3 × 105 A/cm2 due to additional pinning centers. The upper critical field $H_{c2}$ of ~500 kOe is estimated from the resistivity study in magnetic fields parallel to the c-axis using a criterion of a 50% drop of the normal state resistivity R n . The anisotropy of the upper critical field γ H c2 = H ab c2/H c2 c reaches a value ~6 at $T\longrightarrow T_c$ . Extremely low values of the residual Sommerfeld coefficient $\gamma_r$ of about 1 mJ/mol K2, compared to the normal state Sommerfeld coefficient γ n = 25 mJ/mol K2 for pure samples indicate a high volume fraction of the superconducting phase (up to 97%). The electronic contribution to the specific heat in the superconducting state is well described within a single-band BCS model with a temperature dependent gap Δ(0 K) = 27(1) K. A broad cusp-like anomaly in the electronic specific heat observed at low temperatures in samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the Fe2+ ions at the 2c sites. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.

Iron Oxide Nanoparticles Embedded onto 3D Mesochannels of KIT‐6 with Different Pore Diameters and Their Excellent Magnetic Properties

Here, we report the results of our detailed study on the fabrication of iron oxide magnetic nanoparticles confined in mesoporous silica KIT-6 with a 3D structure and large, tunable pore diameters. It was confirmed by XRD, nitrogen adsorption, high-resolution (HR) TEM, and magnetic measurements that highly dispersed iron oxide nanoparticles are occupied inside the mesochannels of KIT-6. We also demonstrated that the size of the iron oxide nanoparticle can be controlled by simply changing the pore diameter of the KIT-6 and the weight percentage of the iron oxide nanoparticles. The effect of the weight percentage and size of the iron oxide nanoparticles, and the textural parameters of the support on the magnetic properties of iron oxide/KIT-6 has been demonstrated. The magnetization increases with decreasing iron content in the pore channels of KIT-6, whereas coercivity decreases for the same samples. Among the KIT-6 materials studied, KIT-6 with 7.5 wt % of iron showed the highest saturation magnetic moment and magnetic remanence. However, all the samples register a coercivity of around 2000 Oe, which is generally observed for the hard magnetic materials. In addition, we have found a paramagnetic-to-superparamagnetic transition at low temperature for samples with different iron content at low temperature. The cause for this exciting transition is also discussed in detail. Magnetic properties of the iron oxide loaded KIT-6 were also compared with pure iron oxide and iron oxide loaded over SBA-15. It was found that iron oxide loaded KIT-6 showed the highest magnetization due to its 3D structure and large pore volume. The pore diameter of the iron oxide loaded KIT-6 support also plays a critical role in controlling the magnetization and the blocking temperature, which has a direct relation to the particle diameter and increases from 48 to 63 K with an increase in the pore diameter of the support from 8 to 11.3 nm.

Thin tantalum films on crystalline silicon – a metallic glass

AbstractThin amorphous tantalum films are prepared on Si(111) substrates in a metallic glassy state. The amorphous monoatomic state of the film is characterized by X‐ray diffraction studies. The glassy state leads to a negative temperature coefficient of the resistivity (TCR) for low sample temperatures <200 K which is attributed to incipient localization. Above 200 K a positive TCR is observed as expected for a normal Boltzmann transport regime. Upon heating the Si substrate to 1200 K TaSi2 is formed out of the amorphous tantalum film and the silicon substrate. The TaSi2 layer is crystalline as evident from X‐ray diffraction data.magnified imageSchematic drawing of the evaporation setup on either glass or silicon samples. Scheme of annealing effects.(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Surface Recombination on Exciton Dynamics in GaN Nanorods

We have investigated the optical properties of GaN nanorods with different diameters by micro-photoluminescence (PL) and time-resolved PL measurements grown on (111) Si substrates by using radio-frequency plasma-assisted molecular-beam epitaxy. In order to confirm the formation of the GaN nanorods and their physical dimension, we employed high resolution scanning electron microscopy. As the diameters of the nanorods decreased, the donor bound exciton (D 0 X) emission, which is mainly observed at low temperature for samples with larger diameters, rapidly quenched compared with the free exciton (FX). The activation energy of the FX for GaN nanorods with different diameters was also evaluated. The decay times are a few tens of ps for both the D 0 X and the FX transitions. The overall decay times decrease linearly with decreasing average diameter of the nanorods. Time-resolved PL studies show that surface recombination is dominant in determining the excitonic dynamics.

The Magnetic and Structural Properties of SiC-Doped MgB2 Bulks Prepared by the Standard Ceramic Processing

According to general formula MgB2−xSiCx (x = 0, 0.05, 0.1, 0.2), MgB2 and SiC-doped bulk superconduc- tors were prepared by the standard ceramic processing. The mixtures of the corresponding powders were sintered at 750 °C for 0.5 h under pressure of 8 bar Argon. X-ray dif- fraction patterns show that all the samples have MgB2 as the main phase with a very small amount of MgO; further, with SiC-doped, the presence of Mg2Si is also noted. The magnetization-temperature measurements showed a transi- tion temperature of 37.5 K for the undoped sample which indicates the typical transition temperature of MgB2. When the content of SiC increased in the sample, the transition temperatures decreased to the lower temperatures system- atically. The M-H loops measured using a VSM showed very large magnetization value at low temperature for SiC doped samples. The largest M-H loops were taken from the sample contains 5% SiC. The critical current density of samples calculated from M-H loops indicated a value of around 4 × 10 5 A/cm 2 , which is in good agreement with the literature.

Magnetic and thermodynamic properties of cobalt-doped iron pyrite: Griffiths phase in a magnetic semiconductor

Doping of the band insulator FeS$_2$ with Co on the Fe site introduces a small density of itinerant carriers and magnetic moments. The lattice constant, AC and DC magnetic susceptibility, magnetization, and specific heat have been measured over the $0\le x\le 0.085$ range of Co concentration. The variation of the AC susceptibility with hydrostatic pressure has also been measured in a small number of our samples. All of these quantities show systematic variation with $x$ including a paramagnetic to disordered ferromagnetic transition at $x=0.007\pm 0.002$. A detailed analysis of the changes with temperature and magnetic field reveal small power law dependencies at low temperatures for samples near the critical concentration for magnetism, and just above the Curie temperature at higher $x$. In addition, the magnetic susceptibility and specific heat are non-analytic around H=0 displaying an extraordinarily sharp field dependence in this same temperature range. We interpret this behavior as due to the formation of Griffiths phases that result from the quenched disorder inherent in a doped semiconductor.

Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel

The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)−1 with corresponding sheet carrier densities of 2.3–1.3 × 1013 cm−2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)−1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm−2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.

1,4-Dioxane biodegradation at low temperatures in Arctic groundwater samples

1,4-Dioxane biodegradation was investigated in microcosms prepared with groundwater and soil from an impacted site in Alaska. In addition to natural attenuation conditions (i.e., no amendments), the following treatments were tested: (a) biostimulation by addition of 1-butanol (a readily available auxiliary substrate) and inorganic nutrients; and (b) bioaugmentation with Pseudonocardia dioxanivorans CB1190, a well-characterized dioxane-degrading bacterium, or with Pseudonocardia antarctica DVS 5a1, a bacterium isolated from Antarctica. Biostimulation enhanced the degradation of 50 mg L −1 dioxane by indigenous microorganisms (about 0.01 mg dioxane d −1 mg protein −1) at both 4 and 14 °C, with a simultaneous increase in biomass. A more pronounced enhancement was observed through bioaugmentation. Microcosms with 50 mg L −1 initial dioxane (representing source-zone contamination) and augmented with CB1190 degraded dioxane fastest (0.16 ± 0.04 mg dioxane d −1 mg protein −1) at 14 °C, and the degradation rate decreased dramatically at 4 °C (0.021 ± 0.007 mg dioxane d −1 mg protein −1). In contrast, microcosms with DVS 5a1 degraded dioxane at similar rates at 4 °C and 14 °C (0.018 ± 0.004 and 0.015 ± 0.006 mg dioxane d −1 mg protein −1, respectively). DVS 5a1 outperformed CB1190 when the initial dioxane concentration was low (500 μg L −1, which is representative of the leading edge of plumes). This indicates differences in competitive advantages of these two strains. Natural attenuation microcosms also showed significant degradation over 6 months when the initial dioxane concentration was 500 μg L −1. This is the first study to report the potential for dioxane bioremediation and natural attenuation of contaminated groundwater in sensitive cold-weather ecosystems such as the Arctic.

Effect of air voids on salt scaling and internal freezing

By combining calorimetric measurements with dilatometry, it has been possible to calculate the contributions of thermal expansion, pore pressure, and crystallization pressure of ice to the strain observed in a mortar during freezing/thawing cycles. Air-entrained mortars contract upon freezing, while non-air-entrained mortars expand. The expansion of the latter is attributed primarily to hydraulic pressure, owing to the rapid growth of ice, which nucleates at low temperatures in laboratory samples. Poromechanical calculations account quantitatively for the contraction of samples with air entrainment, assuming that ice crystals form in the air voids. As originally proposed by Powers and Helmuth, those crystals create suction in the pore liquid that offsets the crystallization pressure of ice in the mesopores of the paste, resulting in a net contraction. Ice in the matrix also contributes significantly to the increase in the thermal expansion coefficient of the mortar. The magnitude of the contraction in air-entrained mortar is shown to account for a reduction of salt scaling damage. According to the glue-spall theory, the damage results from cracking of the ice on the surface of concrete, when the thermal expansion mismatch stress exceeds the strength of the ice. The contraction of the mortar caused by air entrainment offsets the thermal expansion mismatch sufficiently to prevent cracking. Based on observations of the nucleation temperature of ice in laboratory samples of various sizes, it is estimated that there is one site capable of nucleating ice at − 1 °C in a cube of mortar roughly 34 cm on an edge (or, one per square meter in a slab 3 cm thick). This suggests that ice nucleates in the field at high temperatures, compared to what is typically seen in the laboratory, and propagates slowly through the pores as the temperature drops. This mode of growth may lead to fatigue damage over many cycles, owing to local stresses from crystallization pressure, where the contribution of hydraulic pressure is insignificant.

A comparative study on the magnetic properties of Fe-doped CuO nanopowders prepared by sol–gel and co-precipitation method

The Fe-doped CuO nanopowders (Cu 0.95Fe 0.05O) were prepared by the sol–gel and the co-precipitation methods. The effects of synthesis methods on the structure, magnetic property and exchange bias effect of Cu 0.95Fe 0.05O nanopowders were investigated. The microstructure and magnetic properties of the two types of nanopowders were characterized by X-ray diffraction and physics property measuring systems, respectively. It was found that the sol–gel method tended to lead the formation of CuFe 2O 4 phase, which dominated the magnetic behaviors of the sample; the co-precipitation tended to lead larger exchange bias field at low temperature in samples, comparing with sol–gel method.

Magnetotransport Behavior and the Phase Diagram of Nd$_{1 - {\rm x}}$Ca$_{\rm x}$MnO$_{3}$ Manganites

In this work, the temperature dependent magnetization M(T) and the electrical resistivity rho (T) of Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (x=0.1-0.5) are measured. From the M(T) curves, the ferromagnetic (FM) phase is observed for samples of x=0.1 and 0.2. However, the canted-antiferromagnetic (CAF) phase which coexists with the charge-ordering (CO) phase occurs at low temperature for samples x=0.3-0.5. By systematically changing the Ca concentration from x=0.1 to 0.5, a detailed phase diagram is established. Based on the result of magnetic field dependent rho (T), the magnetoresistance ratio is calculated, which shows a maximum in the x=0.4 sample. These results are helpful for improving the MR effect in these manganites.

A route to continuous ultra-thin cerium oxide films on Cu(1 1 1)

The growth and morphology of ultra-thin CeO 2(1 1 1) films on a Cu(1 1 1) substrate were investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The films were grown by physical vapor deposition of cerium in an oxygen atmosphere at different sample temperatures. The preparation procedure is based on a modification of a previous method suggested by Matolin and co-workers [1], involving growth at elevated temperature (520 K). Here, LEED shows good long range ordering with a “(1.5 × 1.5)” superstructure, but STM reveals a three-dimensional growth mode (Vollmer–Weber) with formation of a closed film only at larger thickness. Using a kinetically limited growth process by reactive deposition at low sample temperatures (100 K) and subsequent annealing, we show that closed layers of ceria with atomically flat terraces can be prepared even in the regime of ultra-thin films (1.5 ML). Closed and atomically flat ceria films of larger thickness (3 ML) are obtained by applying a multistep preparation procedure, in which successive ceria layers are homoepitaxially grown on this initial film. The resulting overlayers show strong similarities with the morphology of CeO 2(1 1 1) single crystal surfaces, suggesting the possibility to model bulk ceria by thin film systems.

Plasma treatment methods to improve indium bump bonding via indium oxide removal

Flip chip hybridization, also known as bump bonding, is a packaging technique for microelectronic devices which directly connects an active element or a detector to a substrate readout face down, eliminating the need for wire bonding. Indium bump technology has been a part of hybridization for many years and has been extensively employed in the infrared imager industry. However, obtaining a reliable, high yield process for high density patterns of bumps can be quite difficult in part due to the tendency of the indium bumps to oxidize during exposure to air. In this study, plasma, thermal, and wet chemical methods were screened to determine their ability to remove indium oxide from indium bumps. A novel two-step plasma process using methane, argon, and hydrogen was developed that removes indium oxide from indium bumps after prolonged air exposure while maintaining a low sample temperature. This method was tested by fabricating a fully hybridized scientific grade visible complementary metal oxide semiconductor detector and imaging a standard test pattern.

Electrical Resistivity in Ferromagnetic TM–Al–O (TM=Fe, Co) Granular Films: Scattering by Spin Waves and Kondo Like Behavior

We investigate the electrical resistivity of the ferromagnetic granular thin films TM–Al–O (TM=Fe, Co). The resistivity shows T 2 behavior at low temperatures in all metallic samples. This strongly...

Nanocoating hybrid polymer films on large quantities of cohesive nanoparticles by molecular layer deposition

AbstractThe conformal coating of ultra‐thin aluminum alkoxide (alucone) polymer films on primary silica and titania nanoparticles using molecular layer deposition (MLD) in a fluidized bed reactor from 100 to 160 °C is described. In situ mass spectrometry revealed that the growth of alucone MLD films was self‐limiting as a function of the individual trimethylaluminum and ethylene glycol exposures. The composition and highly conformal alucone films throughout the surface of both silica and titania nanoparticles were confirmed. The highest growth rate was observed at the lowest sample temperature. Primary nanoparticles were coated individually despite their strong tendency to aggregate during fluidization. Based on the results of chemical and thermogravimetric analysis, the value of x in the formula of Al(OCH2CH2O)x was estimated to be 1.9. The calculated film density slightly increased from 2.0 ± 0.1 to 2.2 ± 0.1 g/cm3 with the increasing of temperature from 100 to 160 °C. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Photoexcitation of Adenine Cation Radical [A•+] in the near UV−vis Region Produces Sugar Radicals in Adenosine and in Its Nucleotides

In this study, we report the formation of ribose sugar radicals in high yields (85-100%) via photoexcitation of adenine cation radical (A*+) in Ado and its ribonucleotides. Photoexcitation of A*+ at low temperatures in homogeneous aqueous glassy samples of Ado, 2'-AMP, 3'-AMP, and 5'-AMP forms sugar radicals predominantly at C5'- and also at C3'-sites. The C5'* and C3'* sugar radicals were identified employing Ado deuterated at specific carbon sites: C1', C2', and C5'. Phosphate substitution is found to deactivate sugar radical formation at the site of substitution. Thus, in 5'-AMP, C3'* is observed to be the main radical formed via photoexcitation at ca. 143 K, whereas, in 3'-AMP, C5'* is the only species found. These results were supported by results obtained employing 5'-AMP with specific deuteration at the C5'-site (i.e., 5',5'-D,D-5'-AMP). Moreover, contrary to the C5'* observed in 3'-dAMP, we find that C5'* in 3'-AMP shows a clear pH-dependent conformational change as evidenced by a large increase in the C4' beta-hyperfine coupling on increasing the pH from 6 to 9. Calculations performed employing DFT (B3LYP/6-31G*) for C5'* in 3'-AMP show that the two conformations of C5'* result from strong hydrogen bond formation between the O5'-H and the 3'-phosphate dianion at higher pHs. Employing time-dependent density functional theory [TD-DFT, B3LYP/6-31G(d)], we show that, in the excited state, the hole transfers to the sugar moiety and has significant hole localization at the C5'-site in a number of allowed transitions. This hole localization is proposed to lead to the formation of the neutral C5'-radical (C5'*) via deprotonation.