Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy

Many of the functions and applications of advanced materials result from their interfacial structures and properties. However, the difficulty in characterizing the surface structure of these materials at an atomic level can often slow their further development Solid-state NMR can probe surface structure and complement established surface science techniques, but its low sensitivity often limits its application. Many materials have low surface areas and/or low concentrations of active/surface sites. Dynamic nuclear polarization (DNP) is one Intriguing method to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude. In a DNP experiment, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of similar to 658 for H-1 NMR. In this Account, we discuss the application of DPIP to enhance surface NMR signals, an approach known as DNP surface enhanced NMR spectroscopy (DNP SENS). Enabling DNP for these systems requires bringing an exogeneous radical solution into contact with surfaces without diluting the sample. We proposed the incipient wetness impregnation technique (IWI) a well-known method in materials science, to impregnate porous and particulate materials with just enough radical containing solution to fill the porous volume. IWI offers several advantages: it is extremely simple, provides a uniform wetting of the surface, and does not increase the sample volume or substantially reduce the concentration of the sample. This Account describes the basic principles behind DNP SENS through results obtained for mesoporous and nanoparticulate samples impregnated with radical solutions. We also discuss the quantification of the overall sensitivity enhancements obtained with DNP SENS and compare that with ordinary room temperature NMR spectroscopy. We then review the development of radicals and solvents that give the best possible enhancements today. With the best polarizing mixtures, DNP SENS enhances sensitivity by a factor of up to 100, which decreases acquisition time by five orders of magnitude. Such enhancement enables the detailed and expedient atomic level characterization of the surfaces of complex materials at natural isotopic abundance and opens new avenues for NMR. To illustrate these improvements, we describe the successful application of DNP SENS to characterize hybrid materials, organometallic surface species, and metal-organic frameworks.

Scanning Tunneling Microscopy using Superconducting Tips to Probe Absolute Spin Polarization

Matthias Alexander Eltschka

The interplay of superconductivity and magnetism is investigated for systems with dimensions ranging from the mesoscopic to the atomic scale by scanning tunneling microscopy (STM) at millikelvin temperatures and by numerical calculations. Based on geometrically confined superconductors in magnetic fields, a novel STM approach is introduced to quantitatively probe the spin polarization of tunneling electrons. In the first part of this work, the effects of magnetic fields and geometrical confinement are probed for superconducting vanadium STM tips. Due to the unique confinement ranging from the atomic to the mesoscopic scale, the superconducting properties of the STM tips vary considerably from their bulk counterparts. To analyze the experimentally determined magnetic field dependence of several V tips, the superconductivity is numerically calculated for modeled cone geometries with various opening angles. The numerical approach based on a one-dimensional Usadel equation leads to a direct correlation between the opening angle ¿ and the order of the superconducting phase transition. First order phase transitions occur when the opening angle is smaller than a critical value (¿ < ¿c), whilst larger opening angles (¿ > ¿c) result in second order phase transitions. The comparison of experimental findings and numerical results reveals the existence of first and second order quantum phase transitions in the V STM tips. In addition, the numerical calculations also explain experimentally observed broadening effects of the superconducting spectra by the specific tip geometry. In the second part, the superconducting V tips are employed in a novel approach to quantitatively probe the spin polarization of tunneling electrons on the nanoscale. For this purpose, the Meservey-Tedrow-Fulde technique is transferred to STM in order to combine their virtues, such as the quantitative probing capability of the spin polarization, the precise control at the atomic scale and the well-defined vacuum tunnel barrier. To demonstrate the capabilities of the new technique, the local spin structure is resolved for a magnetic Co nanoisland, where spin polarizations ranging from -56% up to 65% were found, depending on the local position. Furthermore, the spin polarization P strongly varies with the tip-to-sample distance z (dP/dz ¿ 10%/Å), which is described by the different decays of the spin-up and spin-down wave functions into the vacuum tunnel barrier. The final part describes the local interaction between isolated magnetic moments and the superconducting ground state. Copper phthalocyanine molecules on the superconducting V(100) surface induce bound states within the superconducting gap due to the magnetic coupling and the Coulomb potentials. Spatially resolved measurements reveal the non-isotropic structure of the spectral weights that is explained by the adsorption site on the 5x1 reconstruction of the V(100) surface. The quasi-particle excitations are not only observed on the magnetic molecule but also occur in its close vicinity. With increasing distance from the molecular structure, the intensities of the bound states decay within the distance x ¿ ±30Å and show periodic oscillations at the same time. Comparing the experimental findings to a one-dimensional model suggests the presence of a complicated scattering potential, which can be simplified by assuming two point scatterers within the molecular structure.

EPFL2015

Phenylazide Hybrid-Silica - Polarization Platform for Dynamic Nuclear Polarization at Cryogenic Temperatures

David Baudouin, Geoffrey Bodenhausen, Aurélien Bornet, David Benjamin Roger Antoine Gajan, Sami Jannin, Jonas Milani, Basile Vuichoud

Hyperpolarization of NMR-active nuclei is key to gather high quality spectra of rare species and insensitive isotopes. We have recently established that silica-based materials containing regularly distributed nitroxyl radicals connected to the silica matrix by flexible linkers can serve as promising polarization matrices for dynamic nuclear polarization (DNP). Here we investigate the influence of the linker on the efficiency of the polarization. The materials were fully characterized and exhibit high surface areas and narrow pore size distributions with a tunable amount of phenyl azide groups over a broad range of concentrations. The phenyl azide groups can be easily functionalized via a two-step procedure with 4-carboxy-2,2,6,6-tetramethyl-1-oxylpiperidine (TEMPO) to give polarizing matrices with controllable radical content. The DNP efficiency was found to be similar as in materials with flexible linkers, both for magic angle spinning at 105 K and dissolution DNP at 4 K.

Wiley-V C H Verlag Gmbh2017

Extending the lifetime of hyperpolarisation

Xiao Ji

In nuclear magnetic resonance (NMR), one of the most serious inherent challenges is the low sensibility of nuclei. In order to enhance the signal of NMR experiments, the technique of dynamic nuclear hyperpolarisation (DNP) was introduced in the 1950s, leading to a high field prototype apparatus in the 1990s for solid-state applications under MAS conditions. In 2003, the coupling between low-temperature polarisation and room temperature detection also came into reality, which is known as dissolution-DNP (d-DNP). In this thesis, we present a new sample formulation for d-DNP experiments. Also, a new scheme for separating the preparation of hyperpolarised materials and detection apparatus is proposed, which would allow us to overcome the usual requirement that the apparatus for hyperpolarisation and the device for signal detection have to be in close proximity. A numerical model based on the finite element method (FEM) is proposed to study the dynamics of nuclear magnetic energy in both conventional spin glasses and biphasic d-DNP samples. This method first permits a visualisation of the process of the build-up of polarisation and of the relaxation of magnetisation during a DNP experiment. Based on such calculations, we could estimate the optimal experimental parameters for the preparation of hyperpolarised samples, including but not limited to the chemical composition of the samples, the length of spin locking pulses, and the duration of intervals between spin locking pulses in pulse sequences. An interesting conclusion from our simulations is that, under suitable conditions of storage, the lifetime of the hyperpolarisation can be extended to hours or even days in carefully designed biphasic samples. This allows us to propose two experimental approaches aiming at drawing benefit from such an attenuated relaxation. One method deals with the molecules of interest (MOIs) that are in crystalline form at room temperature and normal pressure. Such powders can be impregnated with radical-doped organic glass-forming solvents that have an orthogonal solubility. Under the DNP conditions, the magnetisation is transferred from the single electron of the polarising agent (PA) to the nuclei in the glassy organic solvents. The nuclear polarisation is then relayed to the particles of MOI. We can further confine the nuclear hyperpolarisation overnight in a moderate static magnetic field in a liquid helium bath. On the other hand, for the MOIs that are liquid or even gaseous at room temperature and normal pressure, we propose a spin-labelled solid matrix with interconnecting pores. The fluids can be filled into the pores of the matrix. Under the DNP conditions, the DNP process begins from inside the polarising matrix, and then passes to the fluids that are frozen in the pores.