Thermo-mechanical behavior of low-dimensional systems: The local bond average approach

Over the last three decades low-dimensional systems have attracted increasing interest both from the fundamental and technological points of view due to their unique physical and chemical properties. X-ray absorption spectroscopy (XAS) is a powerful tool for the characterization of such kinds of systems, owing to its chemical selectivity and high sensitivity in interatomic distance determination. Moreover, XAS does not require long-range ordering, that is usually absent in low-dimensional systems. Finally, this technique can simultaneously provide information on electronic and local structural properties of the nanomaterials, significantly contributing to clarify the relation between their atomic structure and their peculiar physical properties. This review provides a general introduction to XAS, discussing the basic theory of the technique, the most used detection modes, the related experimental setups and some complementary relevant characterization techniques (diffraction anomalous fine structure, extended energy-loss fine structure, pair distribution function, x-ray emission spectroscopy, high-energy resolution fluorescence detected XAS and x-ray Raman scattering). Subsequently, a selection of significant applications of XAS to two-, one- and zero-dimensional systems will be presented. The selected low-dimensional systems include IV and III–V semiconductor films, quantum wells, quantum wires and quantum dots; carbon-based nanomaterials (epitaxial graphene and carbon nanotubes); metal oxide films, nanowires, nanorods and nanocrystals; metal nanoparticles. Finally, the future perspectives for the application of XAS to nanostructures are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/46/42/423001/pdf

Low-dimensional systems investigated by x-ray absorption spectroscopy: a selection of 2D, 1D and 0D cases

Simultaneously optimizing electrical and thermal transport properties of bulk thermoelectric materials remains a key challenge due to the conflicting combination of material traits. Here, we have explored the electrical and thermal transport features of In-filled CoSb3 through X-ray absorption fine structure, X-ray photoemission spectra, transport measurement and theoretical calculation. The results provide evidence of three types of coexisting multi-localization transport behaviours in the material; these are heat-carrying phonon-localized resonant scattering, accelerated electron movement and increase in density of states near the Fermi level. The 5p-orbital hybridization between In and Sb is discovered in the In-filled CoSb3 compound, which results in a charge transfer from Sb to In and the enhancement of p-d orbital hybridization between Co and Sb. Our work demonstrates that the electrical and thermal properties of filled skutterudite bulk thermoelectric materials can be simultaneously optimized through the three types of coexisting multi-localization transport behaviours in an independent way.

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Multi-localization transport behaviour in bulk thermoelectric materials

The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

The one phonon raman spectrum in microcrystalline silicon

The luminance degradation of BaMgAl10O17:Eu2+(BAM) caused by baking at 600°C was studied. The dissolution of Si–N into BAM lattice lead to only a reduction in the lattice parameters. Under UV excitation, the Si–N codoping enhanced the luminescent intensity by a factor of 110% for the as-received phosphors and by a factor of 122% for phosphors baked at 600°C for 1 h. This could be attributed to the stable local structure surrounding the Eu2+ ions and the lower electronegativity of nitrogen. The proposed method is expected to be potentially applicable to other aluminate phosphors with higher stability and photoluminescence.

High Thermal Stability and Photoluminescence of Si–N-Codoped BaMgAl10O17:Eu2+ Phosphors

The interface effect of InSb quantum dots (QDs) embedded in $\mathrm{Si}{\mathrm{O}}_{2}$ matrix has been investigated by Raman scattering spectroscopy, x-ray diffraction (XRD), and x-ray absorption fine structure (both of EXAFS and XANES). The EXAFS and XRD results show clearly that the bond length of the Sb-In first shell of the InSb QDs contracts slightly about 0.02 \AA{} compared with that of the bulk InSb. The Raman scattering spectrum of the InSb QDs reveals that the lattice contraction partly weakens the phonon confinement effect. The coordination geometry at the interface of the InSb QDs is mainly Sb (In)-O covalent bridge bonds. The Sb $K$-XANES calculations of InSb QDs embedded in $\mathrm{Si}{\mathrm{O}}_{2}$ matrix based on FEFF8 indicate that the intensity increase and the broadening of the white line peak of Sb atoms are essentially attributed to both the increase of Sb $p$-hole population and the change of Sb intra-atomic potential ${\ensuremath{\mu}}_{0}(E)$ affected by the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. Our results show that the interface effect between the InSb QDs and the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix leads not only to the slight lattice contraction of InSb QDs and the large structural distortion in the interface area of InSb QDs, but also to the significant change of the Sb intra-atomic potential and the obvious charge redistribution around Sb atoms.

Interface effect of InSb quantum dots embedded in Si O 2 matrix

The local structures of the immiscible Fe100−xCux alloys (x=0, 10, 20, 40, 60, 80 and 100) produced by mechanical alloying have been investigated by X-ray absorption fine structure technique. For the Fe100−xCux solid solutions with x⩾40, the local environment around the Fe atoms changes from bcc to fcc structure and the Cu atoms maintain the original coordination geometry after milling for 160 h. In contrast, the local structures around the Cu atoms in both Fe80Cu20 and Fe90Cu10 alloys exhibit a transition from fcc to bcc structure. Furthermore, we found that the coordination numbers N in the first shell of the Fe and Cu atoms were largely deviated from the stoichiometric composition for the Fe100−xCux solid solutions with x⩾40. The Debye–waller factor σ of the fcc Fe–Cu phase is larger than that of the bcc Fe–Cu phase, and the σ (0.099 A) around Fe atoms is larger than that around Cu atoms (0.089 A) in the Fe100−xCux solid solutions with x⩾40. This indicates that the mechanically alloyed Fe100−xCux supersaturated solid solutions with x⩾40 is not a homogeneous alloy, but consists of fcc Fe-rich and fcc Cu-rich regions. However, In Fe100−xCux solid solutions with x⩽20, the Cu atoms were almost homogeneously solved into the bcc Fe–Cu phase. A possible mechanism for bcc-to-fcc and fcc-to-bcc changes in Fe100−xCux solid solutions is discussed in relation to the interdiffusion and the transition induced by the ball milling.

Structural transitions of mechanically alloyed Fe100−xCux systems studied by X-ray absorption fine structure

The local structures of the immiscible Fe100-xCux alloys (x= 0, 10, 20, 40, 60, 80 and 100) produced by mechanical alloying have been investigated by XAFS. For the Fe100-xCux (x 40) solid solutions, the local structures around Fe atoms change from bcc structure to fcc one and the Cu atoms maintain the original coordination geometry after milling for 160 hours. On the contrary, the local structures around Cu atoms in both of Fe80Cu20 and Fe90Cu10 alloys appear a transition from fcc to bcc structure. We found that the Debye-waller factor of fcc Fe-Cu phase is larger than that of bcc Fe-Cu phase, and the (0.099 A) around Fe atoms is larger than that (0.089 A) of Cu in the Fe100-xCux (x 40) solid solutions. This suggests that the mechanically alloyed Fe100-xCux supersaturated solid solution is not a homogeneous alloy, and consists of Fe-rich and Cu-rich regions for various compositions. A possible mechanism for bcc-to-fcc and fcc-to-bcc changes in Fe100-xCux solid solutions is discussed in relation to the interdiffusion and transition induced by the ball milling.

Local structures of mechanically alloyed Fe100-xCux solid solutions studied by X-ray absorption fine structure.

Local structures of nanocomposite Co-C60 and Co-SiO2 thin films have been studied by fluorescence XAFS. The results indicate that the bond length R and coordination number N of the first Co–Co shell of Co-SiO2 thin film are 2.49 A and 11.8, respectively. It indicates that the local structure around Co atoms in the nanocomposite Co-SiO2 thin film is similar to that of the metallic Co thin film. However, the local structure and the XANES spectrum of the Co-C60 thin film are much different from those of the Co-SiO2 thin film. There are two Co–C and Co–Co coordination shells in the first neighbor of Co atoms of the Co-C60 thin film. For the Co-C60 thin film, the R, N and σ of the first Co–C and Co–Co shells are 1.88 A, 1.6, 0.050 A and 2.47 A, 7.8, 0.110 A respectively. The decrease (4.0) of the coordination number and the contraction (0.04 A) of the average band length for the first Co–Co shell in the Co-C60 thin film show that there is a strong interaction between the Co nano-particles and the C60 matrix. The XANES results further confirm that there is an electron transferring from Co atoms to C60 molecules in the Co-C60 thin film.

http://iopscience.iop.org/article/10.1238/Physica.Topical.115a00733/pdf

Jiangwei Fan

Papers

Interface effect of InSb quantum dots embedded in Si O 2 matrix

Structural transitions of mechanically alloyed Fe100−xCux systems studied by X-ray absorption fine structure

Local structures of mechanically alloyed Fe100-xCux solid solutions studied by X-ray absorption fine structure.

Local Structures of Nanocomposite CoC60 and CoSiO2 Thin Films Studied by Fluorescence XAFS

The "Hand as Foot" teaching method in the cyst of the seminal vesicle.