Jeffrey B. Kortright

Depth-resolved photoemission spectroscopy from surface and buried layers with soft X-ray standing waves

Abstract A new type of depth-resolved soft X-ray photoemission spectroscopy (SXPS) is carried out in which depth sensitivity is provided by means of a periodic multilayer {[B 4 C(22.5xa0A)–W(17.1xa0A)] 40 } that possesses sizeable standing wave effects of ±∼50–60% for soft X-rays at hν ≈750xa0eV. The photoelectron intensities of each element in the sample, including impurity overlayers, are sharply altered when the incidence angle is tuned over the Bragg condition, in excellent agreement with theoretical simulations. The angular dependence of photoelectron intensity also shows unique chemical-state-resolved behavior depending on the location of a given species in the multilayer structure. Depth-resolved SXPS with soft X-ray standing wave excitation should thus be a very useful tool for studying surface and interface chemical, electronic, and magnetic properties.

Probing buried interfaces with soft x-ray standing wave spectroscopy: Application to the Fe/Cr interface

The nanometre-scale materials and devices that are under ever more intense study at present often depend for their unique properties on buried interfaces between two phases. Yet the number of experimental techniques that can specifically probe such interfaces, particularly with magnetic sensitivity, is limited. We here report a novel type of non-destructive methodxa0for spectroscopically studying buried nanometre-scale interfaces and other nanostructures with soft x-ray standing waves. Strong standing waves with a period of 4.0xa0nm and approximately 3:1 contrast ratios are created via Bragg reflection from a synthetic multilayer of form [B4C/W]40. By growing a wedge-shaped Fe/Cr bilayer on top of this multilayer, the mechanical translation of the bilayer exposed to a fixed and finely focused synchrotron radiation beam is converted into a translation of the standing wave through the interface. Analysing various core photoelectron intensities as a function of angle and beam position permits derivation of layer thicknesses and interface mixing/roughness scales. Magnetic circular dichroism in photoemission from the 2p and 3p levels of Fe and Cr further permits derivation of the positions and widths of regions with decreased (increased) ferromagnetic alignment for Fe (Cr), showing that normally antiferromagnetic Cr becomes ferromagnetic just below the centre of the interface but with antiparallel alignment with respect to Fe, and that the equal-concentration region in the centre of the interface strongly inhibits magnetic alignment for both species along the direction of net magnetizations that is probed. The magnetically altered regions in both metals are only 1-2 atomic layers in thickness. 3s spectra from Fe and Cr further indicate that the local spin moments on both atoms do not change on crossing the interface. This standing-wave-plus-wedge method should have a range of applications for the characterization of nanostructures and their interfaces.

High resolution electron microscopy study of as-prepared and annealed tungsten-carbon multilayers

A series of sputtered tungsten-carbon multilayer structures with periods ranging from 2 to 12 nm in the as-prepared state and after annealing at 500/degree/C for 4 hours has been studied with high resolution transmission electron microscopy. The evolution with annealing of the microstructure of these multilayers depends on their period. As-prepared structures appear predominantly amorphous from TEM imaging and diffraction. Annealing results in crystallization of the W-rich layers into WC in the larger period samples, and less complete or no crystallization in the smaller period samples. X-ray scattering reveals that annealing expands the period in a systematic way. The layers remain remarkably well-defined after annealing under these conditions. 12 refs., 4 figs., 1 tab.

Microstructure and Stability Comparison of Nanometer Period W/C, Wc/C, and Ru/C Multilayer Structures

Multilayer structures of W/C, WC/C, and Ru/C, of various periods were prepared and studied by high-resolution transmission electron microscopy. Comparison of the phases in the layered structures is made for as-prepared and annealed samples. Both as-prepared and annealed WC/C multilayers are predominantly amorphous, while the phases in the W/C depend on the periods. The 2 nm period W/C multilayer remains amorphous after annealing, and the longer periods recrystallize to form W{sub 2}C. The layered microstructures of W/C and WC/C are stable on annealing at all periods, while the amorphous Ru-rich layers in the 2 nm period Ru/C multilayer agglomerate upon annealing to form elemental hexagonal Ru crystallites. Larger period Ru/C multilayers show stable layered structures, and indicate hexagonal Ru in the Ru-rich layers. X-ray measurements show that the multilayer periods expand on annealing for all metal-carbon multilayers studied. 15 refs., 5 figs., 1 tab.

Depth-resolved photoemission spectroscopy with soft X-ray standing waves

Abstract Depth-resolved soft X-ray photoemission spectroscopy (SXPS) is performed in which depth sensitivity is provided by use of a periodic multilayer [B4C (22.5 A)/W (17.1 A)40] that shows sizeable standing wave effects of ±∼50–60% for soft X-rays at hν≈750 eV. The photoelectron intensities of each element in the sample, including impurity overlayers, are sharply altered when the incidence angle is tuned over the Bragg condition, in excellent agreement with theoretical simulations. The angular dependence of the photoelectron intensity also shows unique chemical-state resolved behavior depending on the location of a given species in the multilayer structure. Depth-resolved SXPS with soft X-ray standing wave excitation should thus be a very useful tool for studying surface and interface chemical, electronic and magnetic properties.

Cooperative insertion of CO2 in diamineappended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

Cooperative insertion of CO[subscript 2] in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.