Nuclear magnetic resonance (NMR) utilizes radio wave excitation to probe atomic-scale structural and dynamical properties in molecules and materials. NMR is applied in many fields of science as well as engineering, medicine and industry, but the technique is inherently insensitive due to the weak alignment of the magnetic nuclei and low efficiency of radio wave (Faraday Law) detection. Since 1994, the Bowers research group has actively advanced multiple hyperpolarization methods and their applications.
Materials
- Advanced catalytic nanomaterials
- Nanotubes and nanoporous media
- Semiconductor thin films and heterostructures
- Advanced polymers
Techniques
- Parahydrogen enhanced NMR in liquids and gases
- Solid-state NMR Spectroscopy
- Low temperatures NMR and ESR (down to 20mk)
- DNP MAS NMR
- Optical Overhauser Effect and optically pumped NMR
- Rb-Xe Spin Exchange Optical Pumping (SEOP)
- Conductivity detected NMR and ESR in semiconductors
- DNP in n-doped semiconductor heterostructures
Chemistry
Heterogeneous Catalysis. Hydrogenation reactions on supported metals and oxides are crucial to the economy, yet many aspects of the catalytic processes are still not completely clear. Parahydrogen enhanced polarization (PEP), which exploits the quantum mechanical properties of molecular H2 , is an established tool for the study of homogeneous hydrogenation reactions catalyzed by transition metal complexes in solution. PEP involves symmetry-breaking chemical conversion of the pure singlet nuclear spin order inherent to parahydrogen into NMR-observable hyperpolarization. Interest in PEP rapidly grew after the demonstration of its use in cellular metabolic flux studies and cancer imaging. PEP by heterogeneous hydrogenation catalysis was initiated in the Bowers lab with funding support from NSF and ACS-PRF. This work has probed the surface chemistry associated with pairwise hydrogen addition over supported metal nanoparticles and oxide materials PEP-NMR studies are aimed at determining the factors governing the pairwise proton transfer pathway of hydrogenation, wherein the two H atoms of the same H2 molecule are added to the same substrate molecule to yield the product.
Solid-State NMR and DNP MAS NMR Spectroscopy. Solid-state magic angle spinning (MAS) NMR is utilized to characterize adsorbate interactions and surface structure of catalytic materials. The use of dynamic nuclear polarization (DNP) can provided the needed sensitivity enhancement and surface selectivity for these studies. High-resolution solid-state NMR is a useful technique for structural characterization of solids lacking long-range crystalline order. We have applied it to a diverse range materials, including proton conductors (for fuel cells), thermoelectric materials, melanins, and novel synthetic polymers (branched polyethylene, high density polyethylene, cyclic polyacetylene). Recently we applied MAS DNP NMR to the atomic-scale structure of the mesoporous silica shell encapsulating the Pt-Sn intermetallic nanoparticles used in the SWAMP effect.
Physics
Optically pumped NMR in semiconductor nanostructures. OPNMR involves NMR spectroscopy under conditions of in-situ optical irradiation in high DC magnetic fields and cryogenic temperatures (down to ~1.5 K). The optical pumping effect provides up to a 10,000-fold NMR signal amplification in III-V semiconductors such as GaAs and InP, thereby extending the applicability of NMR to nanostructured systems that would otherwise be out-of-reach. Using the quadrupole splitting, we have explored the effects of strain and Mn-doping in GaAs on the OPNMR mechanism. The research has led to detailed information about the electronic energy band structure, including spin, and its correlation to OPNMR photon energy dependence.