Alec Kirkeminde

Synthesis and optoelectronic properties of two-dimensional FeS2 nanoplates.

There is a growing interest in the earth abundant and nontoxic iron disulfide (FeS(2)) photovoltaic materials. Here, we report the synthesis of FeS(2) nanoplates with different spectral features which we have associated with thicknesses and crystallization. The structure and crystalline order of ultrathin FeS(2) nanoplates have a strong influence on the carrier lifetime, electronic and optical properties. We demonstrate that two-dimensional FeS(2) nanoplates show great promise for fabrication of hybrid bulk heterojunction solar cells. This opens up a host of applications of these materials as inexpensive solar cells and photocatalysts.

Symmetry-Defying Iron Pyrite (FeS2) Nanocrystals through Oriented Attachment

Iron pyrite (fools gold, FeS2) is a promising earth abundant and environmentally benign semiconductor material that shows promise as a strong and broad absorber for photovoltaics and high energy density cathode material for batteries. However, controlling FeS2 nanocrystal formation (composition, size, shape, stoichiometry, etc.) and defect mitigation still remains a challenge. These problems represent significant limitations in the ability to control electrical, optical and electrochemical properties to exploit pyrites full potential for sustainable energy applications. Here, we report a symmetry-defying oriented attachment FeS2 nanocrystal growth by examining the nanostructure evolution and recrystallization to uncover how the shape, size and defects of FeS2 nanocrystals changes during growth. It is demonstrated that a well-controlled reaction temperature and annealing time results in polycrystal-to-monocrystal formation and defect annihilation, which correlates with the performance of photoresponse devices. This knowledge opens up a new tactic to address pyrites known defect problems.

Extraordinary photocurrent harvesting at type-II heterojunction interfaces: toward high detectivity carbon nanotube infrared detectors.

Despite the potentials and the efforts put in the development of uncooled carbon nanotube infrared detectors during the past two decades, their figure-of-merit detectivity remains orders of magnitude lower than that of conventional semiconductor counterparts due to the lack of efficient exciton dissociation schemes. In this paper, we report an extraordinary photocurrent harvesting configuration at a semiconducting single-walled carbon nanotube (s-SWCNT)/polymer type-II heterojunction interface, which provides highly efficient exciton dissociation through the intrinsic energy offset by designing the s-SWCNT/polymer interface band alignment. This results in significantly enhanced near-infrared detectivity of 2.3 × 10(8) cm·Hz(1/2)/W, comparable to that of the many conventional uncooled infrared detectors. With further optimization, the s-SWCNT/polymer nanohybrid uncooled infrared detectors could be highly competitive for practical applications.

Interdiffusion induced exchange coupling of L10-FePd/α-Fe magnetic nanocomposites.

One-pot synthesis of FePd and FePd/Fe2O3 (core/shell) nanoparticles via interdiffusion is reported for the first time. It was found that the size of FePd particles and Fe2O3 shell thickness could be controlled by the ligand and iron precursor amounts, respectively. These FePd/Fe2O3 particles can be reductively annealed at 500 °C to produce exchanged coupled L10-FePd/α-Fe magnetic nanocomposites. The effect of the phosphine ligand on magnetic characteristics of synthesized particles and final annealed nanocomposite is discussed. Finally, it was found that the magnetic properties of the final L10-FePd/α-Fe nanocomposites could be tuned by Fe2O3 shell thickness and can reach a coercivity (Hc) of up to 2.4 kOe and a saturation magnetization (Ms) of 141 emu/g.

Phase Transformation-Induced Tetragonal FeCo Nanostructures

Tetragonal FeCo nanostructures are becoming particularly attractive because of their high magnetocrystalline anisotropy and magnetization achievable without rare-earth elements, . Yet, controlling their metastable structure, size and stoichiometry is a challenging task. In this study, we demonstrate AuCu templated FeCo shell growth followed by thermally induced phase transformation of AuCu core from face-centered cubic to L10 structure, which triggers the FeCo shell to transform from the body-centered cubic structure to a body-centered tetragonal phase. High coercivity, 846 Oe, and saturation magnetization, 221 emu/g, are achieved in this tetragonal FeCo structure. Beyond a critical FeCo shell thickness, confirmed experimentally and by lattice mismatch calculations, the FeCo shell relaxes. The shell thickness and stoichiometry dictate the magnetic characteristics of the tetragonal FeCo shell. This study provides a general route to utilize phase transformation to fabricate high performance metastable nanomagnets, which could open up their green energy applications.

Thermodynamic control of iron pyrite nanocrystal synthesis with high photoactivity and stability

Non-toxic, earth abundant nanostructured semiconductors have received extensive attention recently. One of the more highly studied materials has been iron pyrite (FeS2) due to its many different promising applications. Herein, we report the thermodynamically-controlled synthesis of FeS2 nanocrystals, dependent on the reaction temperature and chemical precursors, and a Lewis acid/base model to explain the shape-controlled synthesis. The surface facet-controlled photocatalytic activity and photostability were studied and explained. This work further advances the synthesis with pyrite structure control and surface facet-dictated applications, such as photovoltaics, photocatalysts and photoelectrochemical cells.

Metal redox processes for the controlled synthesis of metal alloy nanoparticles.

Nanocrystalline metals have received widespread interest and found various applications owing to their magnetic and catalytic properties and in energy-related fields. A flexible approach for the growth of nanoalloys with controlled properties and well-defined structures on the atomic scale is thus greatly desired. A new synthetic method that avoids incompatible reduction potentials and rates would be critical to grow metal nanostructures with high purities and the desired stoichiometries. A metal-redox strategy that employs spontaneous oxidation/reduction reactions to grow nanocrystalline alloys using molecular-scale zerovalent metal precursors is now described. The selection of suitable zerovalent metal species allows for thermodynamic control of the compositional stoichiometry during the temperature-dependent formation of the metal alloy nanoparticles. A practical and scalable strategy for nanoalloy growth that can potentially produce key metal components of superior metallurgical quality for catalytic and magnetic systems has thus been developed.

Synthesis and characterization of rare-earth-free magnetic manganese bismuth nanocrystals

Earth abundant manganese bismuth (MnBi) has long been of interest due to its large magnetocrystalline anisotropy and high energy density for advanced permanent magnet applications. However, solution synthesis of MnBi phase is challenging due to the reduction potential mismatch between Mn and Bi elements. In this study, we show a versatile MnBi synthesis method involving the metal co-reduction followed by thermal annealing. The magnetically hard MnBi crystalline phase is then exchange coupled with magnetically soft cobalt coating. Our processing approach offers a promising strategy for manufacturing rare-earth-free magnetic nanocrystals.

Iron sulfide ink for the growth of pyrite crystals

Iron pyrite (FeS2, Fools Gold) is a non-toxic, earth abundant semiconductor that exhibits promise for use in energy conversion and storage devices, such as the cathode material for batteries, thermoelectrics and optoelectronics. However, pyrites potential as an energy-critical material is being curbed due to problems with controlling composition, stoichiometry and bulk and surface defects. To overcome these problems, simple and scalable methods to grow high quality crystalline pyrite for in-depth studies are necessary. In this study, we report a facile approach to create high quality, micron sized pyrite crystals from the FeS wire molecular ink. Growth of high quality pyrite crystals is examined and a model for growth and surface facet dependent activation energy is proposed. Unique thermal measurements are preformed that allow for insight into the pyrites crystallinity and thermoconductive properties. It is shown that as made pyrite crystals exhibit high crystallinity which will be vital for future in-depth studies and device fabrication.

Template-Directed FeCo Nanoshells on AuCu

Schematic AuCu/FeCo core-shell magnetic nanoparticles: FeCo shell is precisely synthesized on non-magnetic AuCu core to form the core/shell nanostructures. Due to the non-magnetic AuCu core, the FeCo shell exhibits a transition from single domain to magnetic vortex state.