Jon K. Baldwin

Nitrogen-Doped Graphene-Rich Catalysts Derived from Heteroatom Polymers for Oxygen Reduction in Nonaqueous Lithium–O2 Battery Cathodes

In this work, we present a synthesis approach for nitrogen-doped graphene-sheet-like nanostructures via the graphitization of a heteroatom polymer, in particular, polyaniline, under the catalysis of a cobalt species using multiwalled carbon nanotubes (MWNTs) as a supporting template. The graphene-rich composite catalysts (Co-N-MWNTs) exhibit substantially improved activity for oxygen reduction in nonaqueous lithium-ion electrolyte as compared to those of currently used carbon blacks and Pt/carbon catalysts, evidenced by both rotating disk electrode and Li-O(2) battery experiments. The synthesis-structure-activity correlations for the graphene nanostructures were explored by tuning their synthetic chemistry (support, nitrogen precursor, heating temperature, and transition metal type and content) to investigate how the resulting morphology and nitrogen-doping functionalities (e.g., pyridinic, pyrrolic, and quaternary) influence the catalyst activity. In particular, an optimal temperature for heat treatment during synthesis is critical to creating a high-surface-area catalyst with favorable nitrogen doping. The sole Co phase, Co(9)S(8), was present in the catalyst but plays a negligible role in ORR. Nevertheless, the addition of Co species in the synthesis is indispensable for achieving high activity, due to its effects on the final catalyst morphology and structure, including surface area, nitrogen doping, and graphene formation. This new route for the preparation of a nitrogen-doped graphene nanocomposite with carbon nanotube offers synthetic control of morphology and nitrogen functionality and shows promise for applications in nonaqueous oxygen reduction electrocatalysis for Li-O(2) battery cathodes.

Room-temperature single-photon generation from solitary dopants of carbon nanotubes

On-demand single-photon sources capable of operating at room temperature and the telecom wavelength range of 1,300-1,500 nm hold the key to the realization of novel technologies that span from sub-diffraction imaging to quantum key distribution and photonic quantum information processing. Here, we show that incorporation of undoped (6,5) single-walled carbon nanotubes into a SiO2 matrix can lead to the creation of solitary oxygen dopant states capable of fluctuation-free, room-temperature single-photon emission in the 1,100-1,300 nm wavelength range. We investigated the effects of temperature on photoluminescence emission efficiencies, fluctuations and decay dynamics of the dopant states and determined the conditions most suitable for the observation of single-photon emission. This emission can in principle be extended to 1,500 nm by doping of smaller-bandgap single-walled carbon nanotubes. This easy tunability presents a distinct advantage over existing defect centre single-photon emitters (for example, diamond defect centres). Our SiO2-encapsulated sample also presents exciting opportunities to apply Si/SiO2-based micro/nano-device fabrication techniques in the development of electrically driven single-photon sources and integration of these sources into quantum photonic devices and networks.

Role of interfaces in shock-induced plasticity in Cu/Nb nanolaminates

We investigate deformation of pure Cu, pure Nb and 30 nm Cu/30 nm Nb nanolaminates induced by high strain rate shock loading. Abundant dislocation activities are observed in shocked pure Cu and Nb. In addition, a few deformation twins are found in the shocked pure Cu. In contrast, in shocked Cu/Nb nanolaminates, abundant deformation twins are found in the Cu layers, but only dislocations in the Nb layers. High resolution transmission electron microscopy reveals that the deformation twins in the Cu layers preferentially nucleate from the Cu(112)//Nb(112) interface habit planes rather than the predominant Cu(111)//Nb(110) interface planes. Our comparative study on the shock-induced plastic deformation of the pure metals (Cu and Nb) and the Cu/Nb nanolaminates underscores the critical role of heterogeneous phase interfaces in the dynamic deformation of multilayer materials.

Trapping of implanted He at Cu/Nb interfaces measured by neutron reflectometry

Neutron reflectometry is used to characterize physical vapor deposited [Cu/Nb]x/Si layered nanocomposites exposed to extreme helium ion doses. The effects of He ions on the interfacial roughness, layer swelling, and chemical mixing have been measured. Regions of high He concentration were localized at Cu/Nb interfaces while bulk Cu and Nb layers remained intact. This remarkable behavior is attributed to the efficient trapping and storage of He at interfaces as compared to bulk.

Synthesis and characterization of nanoporous Pt–Ni alloys

Two nanoporous Pt–Ni alloys were synthesized by dealloying ternary amorphous Si–Pt–Ni precursors. Both foams have nearly the same composition, ligament diameter size, and density. However, their ligament patterns are different, depending on the microstructure of precursors. The difference in morphology is shown to have a profound effect on mechanical properties. The structure with well-aligned long nanoligaments exhibited over 50% higher hardness and stiffness than the structure with short random-oriented nanoligaments. These nanoporous Pt–Ni structures are thermally stable at 300 °C.

Detection of helium bubble formation at fcc-bcc interfaces using neutron reflectometry

United States. Dept. of Energy. Office of Basic Energy Sciences (Center for Materials in Irradiation and Mechanical Extremes. Award 2008LANL 1026)

A study of the effect of iron island morphology and interface oxidation on the magnetic hysteresis of Fe-MgO (001) thin film composites

Fe-MgO tunnel junctions have received much attention for their use in hard drive read heads and other spintronic applications. The system is particularly interesting because of its magnetoresistive behavior and the abundance and low cost of its constituent elements. However, many questions remain about how the structure and chemistry of the Fe-MgO interface mediates magnetic behavior. In this study, we report on transmission electron microscopy, electron energy loss spectroscopy, and magnetic characterization of Fe-MgO composite films with various morphologies. We explore relationships between film morphology, intermixing, and the resulting effects on magnetic structure. We find the presence of oxidation at the Fe-MgO interface, with a detrimental impact on the saturation magnetization of the composite. We also observe changes in coercivity and magnetocrystalline anisotropy with film morphology and thickness. These results will inform the design of MgO-based tunnel junctions and improve our understanding ...

Texture evolution in nanocrystalline iron films deposited using biased magnetron sputtering

Fe thin films were deposited on sodium chloride (NaCl) substrates using magnetron sputtering to investigate means of texture control in free standing metal films. The Fe thin films were studied using transmission electron microscopy equipped with automated crystallographic orientation microscopy. Using this technique, the microstructure of each film was characterized in order to elucidate the effects of altering deposition parameters. The natural tendency for Fe films grown on (100) NaCl is to form a randomly oriented nanocrystalline microstructure. By careful selection of substrate and deposition conditions, it is possible to drive the texture of the film toward a single (100) orientation while retaining the nanocrystalline microstructure.

Synthesis and mechanical behavior of nanoporous nanotwinned copper

We synthesize nanoporous copper (NP Cu) through electrochemical dealloying of amorphous Cu0.41Si0.59 under compressive residual stress. Transmission Electron Microscopy reveals that struts are nanocrystalline with grain size equal to the strut thickness. Moreover, a significant population of twins with spacing ∼7 nm is present within each imaged grain. The hardness of this nanocrystalline, nanotwinned NP Cu is approximately one order of magnitude greater than reports on NP Cu in the literature. The yield strength of individual struts inferred through dimensional analysis is approximately an order of magnitude greater than bulk copper and compares well with other nanostructured copper systems.

Characterization of a Fe/Y2O3 metal/oxide interface using neutron and x-ray scattering

The structure of metal/oxide interfaces is important to the radiation resistance of oxide dispersion-strengthened steels. We find evidence of gradual variations in stoichiometry and magnetization across a Fe/Y2O3 metal/oxide heterophase interface using neutron and x-ray reflectometry. These findings suggest that the Fe/Y2O3 interface is a transitional zone approximately ∼64 A-thick containing mixtures or compounds of Fe, Y, and O. Our results illustrate the complex chemical and magnetic nature of Fe/oxide interfaces and demonstrate the utility of combined neutron and x-ray techniques as tools for characterizing them.