53 research outputs found
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Quasi-ternary nanoparticle superlattices through nanoparticle design
Individual nanoscale building blocks exhibit a wide range of size-dependent properties, since their size can be tuned over known characteristic length scales of bulk materials. In the last several years, the possibility of combining different materials in the form of two and three component nanoparticles (NPs) has been extensively explored. Also multi-component materials can be obtained via self-assembly of NPs from their binary colloidal mixtures. These new nanocrystal solids may possess tunable collective properties that originate from interactions between size and composition controlled building blocks. Exchange coupling between neighboring NPs of magnetically soft and hard materials enhances the magnetic energy product of the nanocomposite material. Randomly mixed solids of small and large semiconducting CdSe NPs revealed enhancement of photoluminescence intensity of large semiconductor particles accompanied by quenching of photoluminescence of the small particles because of long-range resonant transfer of electronic excitations from the more electronically confined small particles to higher excited states of the large particles. Recently, it was demonstrated that binary semiconducting composite materials can show strongly enhanced electronic properties with about 100-fold higher conductance as compared to the sum of individual conductances of single-component films. Creation of highly periodic superlattices is expected not just provide the control of the homogeneity of the sample but also affect their properties. It was shown that silver nanocrystals organized into periodic cubic structures vibrated coherently [20] and demonstrated a change in electronic transport properties
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Bonding, Backbonding, and Spin-Polarized Molecular Orbitals:Basis for Magnetism and Semiconducting Transport in V[TCNE]x~;;2
X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L2,3 and C and N K edges reveal bonding/backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE]x~;;2. In V spectra, dxy-like orbitals are modeled assuming V2+ in an octahedral ligand field, while dz2 and dx2-y2 orbitals involved in strong covalent bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE]x~;;2 associated with backbonding interactions that yield its novel properties
Probing Adsorption Interactions in Metal-Organic Frameworks using X-ray Spectroscopy
We explore the local electronic signatures of molecular adsorption at coordinatively unsaturated binding sites in the metal-organic framework Mg-MOF-74 using X-ray spectroscopy and first-principles calculations. In situ measurements at the Mg K-edge reveal distinct pre-edge absorption features associated with the unique, open coordination of the Mg sites which are suppressed upon adsorption of CO2 and N,N′-dimethylformamide. Density functional theory shows that these spectral changes arise from modifications of local symmetry around the Mg sites upon gas uptake and are strongly dependent on the metal-adsorbate binding strength. The expanded MOF Mg2(dobpdc) displays the same behavior upon adsorption of CO 2 and N,N′-dimethylethylenediamine. Similar sensitivity to local symmetry is expected for any open metal site, making X-ray spectroscopy an ideal tool for examining adsorption in such MOFs. Qualitative agreement between ambient-temperature experimental and 0 K theoretical spectra is good, with minor discrepancies thought to result from framework vibrational motion. © 2013 American Chemical Society
Condensed Matter Astrophysics: A Prescription for Determining the Species-Specific Composition and Quantity of Interstellar Dust using X-rays
We present a new technique for determining the *quantity and composition* of
dust in astrophysical environments using <6keV X-rays. We argue that high
resolution X-ray spectra as enabled by the Chandra and XMM-Newton gratings
should be considered a powerful and viable new resource for delving into a
relatively unexplored regime for directly determining dust properties:
composition, quantity, and distribution. We present initial cross-section
measurements of astrophysically likely iron-based dust candidates taken at the
Lawrence Berkeley National Laboratory Advanced Light Source synchrotron
beamline, as an illustrative tool for the formulation of our methodology.
Focused at the 700eV Fe LIII and LII photoelectric edges, we discuss a
technique for modeling dust properties in the soft X-rays using L-edge data, to
complement K-edge X-ray absorption fine structure analysis techniques discussed
in Lee & Ravel 2005. This is intended to be *a techniques paper* of interest
and usefulness to both condensed matter experimentalists and astrophysicists.
For the experimentalists, we offer a new prescription for normalizing
relatively low S/N L-edge cross section measurements. For astrophysics
interests, we discuss the use of X-ray absorption spectra for determining dust
composition in cold and ionized astrophysical environments, and a new method
for determining *species-specific gas-to-dust ratios*. Possible astrophysical
applications of interest, are offered. Prospects for improving on this work
with future X-ray missions with higher throughput and spectral resolution are
presented in the context of spectral resolution goals for gratings and
calorimeters, for proposed and planned missions such as Astro-H and the
International X-ray Observatory.Comment: 37 pages; 8 figures; accepted for publication in Ap
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Resonant Soft X-Ray Contrast Variation Methods as Composition-Specific Probes of Thin Polymer Film Structure
We have developed complementary soft x-ray scattering and reflectometry techniques that allow for the morphological analysis of thin polymer films without resorting to chemical modification or isotopic 2 labeling. With these techniques, we achieve significant, x-ray energy-dependent contrast between carbon atoms in different chemical environments using soft x-ray resonance at the carbon edge. Because carbon-containing samples absorb strongly in this region, the scattering length density depends on both the real and imaginary parts of the atomic scattering factors. Using a model polymer film of poly(styrene-b-methyl methacrylate), we show that the soft x-ray reflectivity data is much more sensitive to these atomic scattering factors than the soft x-ray scattering data. Nevertheless, fits to both types of data yield useful morphological details on the polymer?slamellar structure that are consistent with each other and with literature values
Interface properties and built-in potential profile of a LaCrO/SrTiO superlattice determined by standing-wave excited photoemission spectroscopy
LaCrO (LCO) / SrTiO (STO) heterojunctions are intriguing due to a
polar discontinuity along (001), two distinct and controllable interface
structures [(LaO)/(TiO) and (SrO)/(CrO)], and
interface-induced polarization. In this study, we have used soft- and hard
x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to generate a
quantitative determination of the elemental depth profiles and interface
properties, band alignments, and the depth distribution of the
interface-induced built-in potentials in the two constituent oxides. We observe
an alternating charged interface configuration: a positively charged
(LaO)/(TiO) intermediate layer at the
LCO/STO interface and a negatively charged
(SrO)/(CrO) intermediate layer at the
STO/LCO interface. Using core-level SW data,
we have determined the depth distribution of species, including through the
interfaces, and these results are in excellent agreement with scanning
transmission electron microscopy and electron energy loss spectroscopy
(STEM-EELS) mapping of local structure and composition. SW-XPS also enabled
deconvolution of the LCO-contributed and STO- contributed
matrix-element-weighted density of states (MEWDOSs) from the valence band (VB)
spectra for the LCO/STO superlattice (SL). Monitoring the VB edges of the
deconvoluted MEWDOS shifts with a change in probing profile, the alternating
charge- induced built-in potentials are observed in both constituent oxides.
Finally, using a two-step simulation approach involving first core-level
binding energy shifts and then valence-band modeling, the built-in potential
gradients across the SL are resolved in detail and represented by the depth
distribution of VB edges.Comment: Main text: 29 pages, 5 figures; Supplementary Information: 20 pages,
10 figure
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Probing buried interfaces with soft x-ray standing wave spectroscopy: Application to the Fe/Cr interface
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