NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY – Research

Research is focused in the study of strong electron correlated systems, topological matter, as well as the study of the electronic properties of various nanocatalysts, such as transition-metal based phosphide nanoparticles, supported 2D dichalcogenides (e.g. MoS2), nanozeolites, etc.

Other areas of interest are NMR studies of nanofluidic processes in restricted geometries (e.g. water and ionic liquid motion in carbonaceous and silica nanoporous structures), as well as gelation processes of industrial interest, such as cements, and self-healing coatings for the aerospace technology.

Topological Materials

In the past decade there has been an explosion of interest regarding the role of topology in condensed matter physics. Undoubtedly, the most topical of the studied materials are topological insulators, hosting protected metallic electron states on their surface, as well as Dirac and Weyl semi-metals with the protected electron states residing in their bulk interior. In all these materials, both experiments and theory unveiled striking similarities with states of matter in High Energy Physics, such as Axions (particles that do not obey classical Maxwell electrodynamics and are expected to explain the missing dark matter of the Universe), or Majorana Fermions (particles with a dual particle-antiparticle nature) that have been predicted but never observed. This unique topology grants to the topological materials a number of distinct quantum properties related with unthinkable until today applications.
Currently, the state-of-the-art method to uncover the topological character of electrons in all classes of Topological Materials is Angle Resolved Photoemission Spectroscopy (ARPES), which provides exact information on the electron energy band structure at the materials surface. However, ARPES fails to detect Dirac and Weyl nodal points if they are located at energy higher than the Fermi level.

In the last year great part of our efforts have been devoted in the study of Topological Materials with state-of-the-art NMR methods, combined with advanced DFT calculations and sub-Angstrom resolved   Transmission Electron Microscopy. This work is accomplished in collaboration with the Stockholm university (Sweden), the university of Lyon (France), the Korea Basic Science Institute (S. Korea), and the Khalifa university of Science and Technology at Abu Dhabi (UAE).

Figure 1. Dirac surface electron states detected with 125Te NMR nanocrystallography methods on Bi2Te3 nanoplatelets. “Resolving Dirac Electrons with broadband high resolution NMR”, Papawassiliou, W., et al., Nature Communications 11, 1285 (2020). https://doi.org/10.1038/s41467-020-14838-4

Figure 2. HR-TEM, 209Bi NMR and electron spin resonance (ESR) vs. temperature study of TI Bi2Se3 nanoplatelets. “Unexpected orbital magnetism in Bi-rich Bi2Se3 nanoplatelets”. Kim, H. J., et al., Nature Publishing Group, npg Asia Materials 8, e271; doi:10.1038/am.2016.56 (2016). https://doi.org/10.1038/am.2016.56

Crystal and electronic facet analysis with solid-state NMR nanocrystallography

Acquiring the crystal and electronic structure of material surfaces and interfaces is at the heart of Modern Surface Science, especially after the dawn of the era of novel 2D materials. It is also of significant importance in the study of materials related with major industrial processes such as heterogeneous catalysis and renewable energy sources. An impressive number of state-of-the-art methods ranging from Angle Resolved Photoemission Spectroscopy to Aberration Corrected HRTEM and STM have been successfully engaged in resolving the complicated issues related with the crystal and electronic structure of surfaces and nanostructured materials. However, by reducing the material size down to nanoscale, atomically resolved information is most of the time obscured, especially in the case of light elements.

In a recent Nature Communications publication, we demonstrated how NMR Nanocrystallography, i.e. combination of Nuclear Magnetic Resonance (NMR) with Density Functional Theory (DFT) calculations, implemented on ultrafine Ni2P nanoparticles (down to 5nm), succeeds in a unique way to visualize at atomic scale resolution the size-induced structural and electronic changes taking place in this exceptional Hydrogen Evolution Reaction (HER) nanocatalyst.

Figure 3. TEM, and 31P ssNMR analysis of Ni2P nanoparticles grown on reduced graphene oxide. a. TEM image of the nanoparticles. The upper inset shows the experimental EDP of the highlighted nanoparticle. The lower inset shows the EDP of the DFT calculated TEM of the  facet slab in panel d. b. Experimental (black) and calculated (red) 31P ssNMR spectrum of the nanoparticles. c. HRTEM of the highlighted Ni2P nanoparticle in panel a. The magnified region showcases the  facet crystal structure along the [0001] zone axis. d. The calculated TEM image of the DFT relaxed facet slab. Red color spots are P atoms (not observed experimentally). Papawassiliou, W., et al., Nature Communications 12, 4334 (2021). https://doi.org/10.1038/s41467-021-24589-5.

Polaron Dynamics in Mott Insulators

Broad-line NMR is an indispensable tool in the study of strongly electron correlated systems (SECS), as it provides atomic scale information on the electron-electron spin correlations and electron spin ordering. On the basis of 139La NMR combined with HRTEM measurements in the temperature range 3K-1000K, we have monitored the formation of zig-zaged Mn3+-Mn4+ polarons, the so called CE-type polarons in the paramagnetic phase (PM) of the prototype manganite La0.67Ca0.33MnO3. By lowering temperature, correlated polarons are shown to dominate the electron spin dynamics in the Ferromagnetic (FM) phase, leading to a bad metallic FM phase; however, at very low temperatures they appear to be organized into a unique quantum spin liquid-crystal (SLC) phase. This is new insight into the physics of doped Mott insulators, which becomes possible only with the use of NMR.

“Polaron freezing and the quantum liquid-crystal phase in the ferromagnetic metallic La0.67Ca0.33MnO3. A NMR and HRTEM study in the temperature range 3.2K to 1000K”, Panopoulos, N., et al. npj Quantum Materials 3, 20 (2018).

Figure 4. 139La NMR spectra for La0.67Ca0.33MnO3 in magnetic field 9.4 T. The yellow points show the NMR signal frequency. The upper left inset in the left panel, demonstrates the spin-echo signal at 57.782 MHz. The bottom right inset in the same panel shows satellite frequency distribution (SFD) as a function of temperature.

Figure 5. (Left Panel) Formation of static JT polarons affects the inverse 139La  NMR linewidth 1/w, which is proportional to the magnetic susceptibility. Presented are 1/w vs T curves for LCMO (x = 0.25, 0.33, 0.41) and LSMO (x = 0.33) in the temperature range 80 K – 900 K and 9.4 T external magnetic field. (LCMO: La1-xCaxMnO3, LSMO: La1-xSrxMnO3).  (Right Panel) HRTEM image parallel to the [113] Pnma zone axis at T=296 K and  the corresponding Fast Fourier Transform. The signal intensity profile along the [1-10] direction is shown in yellow color. Two inequivalent Mn sites are observed in a particular area, showing the presence of alternative JT active Mn3+ and JT non-active Mn4+. This is strong indication about the formation of static short ranged polarons.

Confined in a narrow channel, water separates into sheets with peculiar properties

by Anashe Bandari
https://aip.scitation.org/doi/10.1063/10.0001492

(devoted  by Scilight of the American Institute of Physics to our publication: “The peculiar size and temperature dependence of water diffusion in carbon nanotubes studied with 2D NMR diffusion-relaxation D – T2eff spectroscopy,” by L. Gkoura, G. Diamantopoulos, M. Fardis, D. Homouz, S. Alhassan, M. Beazi-Katsioti, M. Karagianni, A. Anastasiou, G. Romanos, J. Hassan, and G. Papavassiliou, Biomicrofluidics (2020). The article can be accessed at https://doi.org/10.1063/5.0005398.)

Water dynamics in hydrophobic nanochannels play a role in drug delivery and water treatment technologies. The dependence of this microscopic behavior on the channel size can have important nanofluidic effects but has previously been studied mostly theoretically. By conducting NMR diffusion-relaxation measurements, Gkoura et al. compared water’s diffusion behavior in carbon nanotubes (CNTs) of various sizes.
“Monitoring water diffusion in hydrophobic nanochannels is important in many applications,” said author Jamal Hassan. “Until now, there has been no experimental methods with atomic scale resolution to monitor diffusion of water molecules into mostly hydrophobic nanochannels.”
For CNTs with diameters between 3.0 and 4.5 nanometers, the group observed that water separates into concentric sheets of different diffusion coefficients with the outer sheet more rigid and the central sheet more liquid-like. Further, the diffusion coefficient in the central sheet can reach astonishingly high values, up to four times that of bulk water. As the diameter of the nanotube increases beyond this range, the diffusion coefficients of the separate sheets begin to converge, leading to uniform dynamics similar to bulk water.
“The stratified water arrangement inside the CNT channels is difficult to explain,” said author George Papavassiliou. He and Hassan conjectured the interplay between two opposing effects – hydrogen bonding between water molecules, and repulsive Coulomb forces, which become important when water squeezes into nanoscale channels – may be the root of these extraordinary water molecule configurations and dynamics.
CNTs have uses in applications like water desalination, molecular sensing and nanopore DNA sequencing. Combined with molecular dynamics simulations, the present diffusion-relaxation experiments can help to elucidate their molecular-scale nanofluidic properties for such applications.