Determination of the atomic structure is mainly based on the measurement of Bragg intensities and yields the average structure of the infinite crystalline material. However, this approach ignores any defects or local structural deviations that manifest themselves as diffuse scattering. It also fails in case of disordered materials, badly crystalline such as many nano-materials, or not crystalline at all, such as glasses. In some cases crystalline and amorphous phases coexist making the traditional crystallographic structure refinement difficult or incomplete. The total scattering pattern or the derived atomic pair distribution function (PDF), however, contains structural information over all length scales [1] and can be used to obtain a complete structural picture of complex materials.
One of the great advantaged of the PDF is the fact that one can limit the range on atom-atom distance over which the structural model is refined. Focusing on small distances up to a few Angstroms will illuminate the local structure where as refinements over a wide range will yield the medium and long range structure. It is interesting to consider, that on high resolution powder diffractometers one can obtain PDFs up to distances in excess of 200Å or 20nm. As a result one can obtain a ‘complete&’ structural fingerprint of nanoparticles that are frequently smaller in size as demonstrated. In this presentation, recent advances on total scattering experiments as well as modeling approaches for nanomaterials will be presented.

Many of the useful materials that make modern life possible are crystalline. Quartz keeps our watches on time, perovskites are widely used in consumer electronics and solid oxide fuel cells may help to power the future.
The importance of local structure and disorder in crystalline materials is being recognised more and more as a key property of many functional materials. From negative thermal expansion to solid state amorphisation and the 'nanoscale' problem to improved fuel cell technology, a clear picture of the local atomic structure is essential to understanding these phenomena and solving the associated problems.
Total scattering, an extension of the powder diffraction method, is increasingly being used to study crystalline materials. The unique combination of Bragg and diffuse scattering can be used to determine both the average structure and the short-range fluctuations from this average within a single experiment. To maximise the structural information from such data, three-dimensional atomic models consistent with all aspects of the data are required.
RMCProfile [1] expands the reverse Monte Carlo (RMC) modelling technique [2] to take explicit account of the Bragg intensity profile from crystalline materials. Analysis of the RMCProfile-generated atomic models gives more detailed information than is available directly from the data alone. I will give several examples where RMCProfile has been used to successfully study the structure and disorder of crystalline materials to illustrate its potential.
References
[1] - see www.rmcprofile.org; M G Tucker, D A Keen, M T Dove, A l Goodwin and Q Hui, J. Phys. Condens. Matter 19, 335218 (2007)
[2] - R L McGreevy amd L Pusztai, Mol. Simul. 1, 359 (1988)

The uniqueness of nanocrystalline semiconductors lies, primarily, in the dependence of their luminescence properties on the grain size, and this feature is extensively explored in quantum dots (QD). It is commonly accepted that the knowledge about atomic structure of QDs, in particular about their surface, is the key to a better understanding and interpretation of their optical properties, and to further development of respective applications. Experience with applications of quantum dots shows, that a serious impediment in the progress is in a limited knowledge about the processes occurring on the nanograin surface during the synthesis and further technological processing. There is no doubt that, both for a better control of the syntheses as well as for a better interpretation of the properties of quantum dots, a good knowledge of the real atomic structure of the materials is of a paramount importance. So far, attempts to determine structural differences between the nanograin interior and its surface using powder diffraction methods and presented in the literature have failed [1-3]. A systematic study of the atomic structure of CdSe nanocrystals performed with application of PDF analysis [4,5] was a real progress and indicated a presence of internal strains in grains of different grain sizes, but neither their origin nor an appropriate atomic model of the individual nanograins was offered.
We developed a new methodology of elaboration of PDF data based on the fact, that for nanocrystals the values of individual interatomic distances, r _i &’s, derived with the G(r) analysis, are not strictly related to each other, as opposed to structures with a uniform crystal lattice represented by the unit cell. The function describing differences between average inter-atomic distances in nanograins and its parent bulk crystal, δ(r) [6], shows some distinctive features characteristic for the unique atomic architecture of a given material. Based on experimental PDFs of CdSe QDs with grains from 2.5 up to 3.5 nm in diameter, we elaborated atomistic models composed of the grain core surrounded by spherical density waves with the average thickness of about 2-3Å; the density varies about that of bulk CdSe by up to 10%. The number of shels, their thicknesses and densities depend on the grain diameter, chemical composition and environment.
[1] M.G. Bawendi et al.,J. Chem. Phys. 91 (1989) 7282-7290; [2] B.O. Dabbousi et al., J. Phys. Chem. B, 101 (1997) 9463-9475; [3] C.B. Murray et al., J. Am. Chem. Soc. 115 (1993) 8706-8715; [4] A.S. Masadeh et al., Phys. Rev. B76 (2007) 115413; [5] S. K. Pradhan et al., J.Appl.Phys. (2007) 102, 044304, [6] B.Palosz, Denver X-ray Conference Proceedings, Advances in X-ray Analysis, (2011), Volume 55

Nanosized materials are made with increasing complexity requiring improved techniques for atomic-scale structure characterization that can deliver both excellent spatial resolution and chemical specificity. In the talk recent advances in resonant high-energy x-ray diffraction coupled to differential atomic pair distribution function analysis will be presented. The power of the technique will be illustrated with examples of recent studies on PtPd and PtAu nanoparticles exhibiting chemical order-disorder effects such as core-shell morphology that go beyond the single unit cell of crystalline lattices.

Elucidating the role that fluctuations play in complex systems with competing interactions enables comprehensive understanding of their physical properties. Spinels with general formula AB2X4 provide a unique playground for studying the interplay of orbital, charge, spin, and lattice degrees of freedom. Metal-insulator transition (MIT) and nontrivial charge ordering on cooling are often observed in mixed-valent spinels. Examples of this are found in novel CuIr2S4 and LiRh2O4 spinels displaying nominal B-site charge of +3.5 on the pyrochlore sublattice. Metallic and cubic at room temperature, CuIr2S4 exhibits on cooling simultaneous orbital and charge ordering, spin dimerization, and corresponding lowering of the crystal symmetry below ~230 K. While isostructural LiRh2O4 also undergoes MIT on cooling, in contrast to CuIr2S4, it appears to be a distinctive two-step process via the intermediate phase, with orbital ordering occurring at 230 K, followed by a charge-ordered valence-bond solid formation below 170 K. Characteristic structural signature of the B4+-B4+ dimers in the insulating phases is a formation of a very short metallic bond. This can be directly observed using the x-ray total scattering based atomic pair distribution function (PDF) method. Since the PDF is sensitive to the presence of dimers irrespective of the presence/absence of their long range order, it provides unique insights beyond crystallographic approach important for better understanding of the detailed mechanisms behind the MIT. Comparative PDF mapping of the dimers across the MIT transitions in CuIr2S4 and LiRh2O4 will be presented and a possibility of presence of fluctuating dimers in the high temperature phases will be addressed. Further, comprehensive charting of the fluctuating Ir4+-dimer map across the phase diagram of Cu(Ir1-xCrx)2S4 (0le;xle;0.6) will be shown. While, as mentioned, CuIr2S4 exhibits complex phenomenology, CuCr2S4 displays ferromagnetic metallic behavior (TC~377K) understood within the double exchange model. Intermediate composition range sees suppression of end-member properties, with broad features observed in susceptibility around 180 K attributed in earlier studies to Cr3+ low-spin to high-spin crossover. Robust fluctuating Ir4+ dimers are detected and their evolution examined across the phase diagram using the PDF. Although their long range order is destroyed already by x~0.05, Ir4+ dimers exist locally at low temperature at all compositions studied. Detailed account will be provided of the Cr-doping and temperature dependence of the local dimers, and characteristic length-scale on which they are observable estimated. Such fluctuating dimers disappear on heating, for intermediate compositions at temperatures above 180 K. Their relation to the transport properties will also be discussed. The results emphasize the wealth of important information available on a nanoscale via detailed phase-diagram studies of complex materials.

Crystallization of noble metal atoms usually adopts the highly symmetric face-centered cubic (f.c.c.) phase that represents the thermodynamically stable structure. Introducing defective microstructures into the metal crystal lattices possibly induces lattice distortions to form non-f.c.c. phases when the lateral dimensions of objects decrease down to nanometer scale. In this presentation, the silver nanoparticles including fivefold twinning are discussed. Due to the existence of fivefold twinning planes, strong internal lattice strains are created in the nanoparticles, leading to a distortion of the cubic lattice symmetry. High-resolution of x-ray diffractions reveal that the lattice distortion results in a tetragonal symmetry that is stable in ambient environment. High-resolution transmission electron microscopy studies on the cross-sectional samples show that the lattice distortion in the center of such a nanoparticle is larger than that in the surfaces, indicating that the nanoparticle is composed of a highly strained core encapsulated in a less strained sheath that helps stabilize the strained core.
Reference: Y. Sun, Y. Ren, Y. Liu, J. Wen, J. S. Okasinski, D. J. Miller, Nature Communications 2012, 3, 971.
This work has been performed at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357. Use of Advanced Photon Source and Electron Microscopy Center for Materials Research at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357.

In this presentation, we will first shortly review our previous work of quantitative nano-organized structural evolution for high efficiency bulk heterojunction (BHJ) solar cell studied by simultaneous grazing-incidence small/wide angle X-ray scattering (GISAXS/GIWAXS) technique.[1-2] We have developed an improved GISAXS modeling and analysis methodology to quantitatively evaluate the nanostructures of conventional P3HT/PCBM BHJ. The resolved structural parameters and established indexes can be quantitatively evaluated and well correlated to the solar cell performances.

In recent years, BHJ solar cells have achieved promising efficiency over 10% which can be attributed to the development of (D-A) type copolymer/fullerene bulk heterojunctions (BHJs) and manipulation of BHJ nanostructure by solvent additives. However, to date qualitative microscopic observations reveal discrepant results on the effects of solvent additives. By employing the powerful tool (GISAXS/GIWAXS) technique, we present quantitative evolutions of bi-hierarchical nanostructure of D-A copolymers and fullerenes (i.e. a ternary phase system with surrounding matrix) herein.[3] The D-A type copolymer PCPDTBT reveals distinctively different crystallization behavior from that of conventional P3HT polymer. One of the characteristic length scales of aggregated polymer crystals is comparable to that of PCBM aggregation clusters. Namely, in the present quantitatively multi-length-scale GISAXS analysis, we resolve the comparable morphological structures that are contributed by crystalline polymer PCPDTBT and PCBM aggregations, respectively, which are in a ternary phase system (relative to the surrounding matrix). The SAXS analysis model of a ternary phase is a challenge and has been accurately established herein. Both structures reveal the characteristics of self-similar geometry: fractal systems. An insight into the structural BHJ model and bi-hierarchical structural evolution of both fractal-network-aggregated polymer crystals and fractal-structure fullerene clusters is provided. The other microscopic techniques including TEM, atomic force microscope (AFM) and Kelvin probe force microscope (KPFM) were complementally conducted to further evidence the resolved nanomorphology. The mechanism of bi-hierarchical formation and mutual influence between PCPDTBT and PCBM mediated by different additive contents and PCBM amounts can be rationally proposed. It can reasonably interpret the discrepancy among the reported literatures and be extended to other D-A type copolymers and other fullerene derivatives. It is helpful for optimum structural design of polymer solar cells and associated SAXS modeling.
1.Liao, H. C. et al., Journal of the American Chemical Society, 2011, vol.133, page 13064-13073
2.Liao, H. C. et al., ACS Nano, 2012, vol. 6, page 1657-1666
3.Liao, H. C. et al., Nano Letters, (under revision)

A chromonic electrospun nanofiber with the self-assembled Sunset-Yellow FCF (H-SSY) was fabricated by the combination of electrospinning and self-assembling methods. Based on the thermal, scattering and microscopic analyses, it was realized that H-SSY formed a glassy columnar nematic (N) phase in the PVP/H-SSY nanofiber. From the 2D WAXD and polarized FTIR results, it was realized that the nanocolumns were aligned parallel to the long axis of the nanofiber. The anisotropically oriented nanofiber mats embedding the self-assembled nanocolumns may open new doors for the practical applications in electro-optical devices. This work was mainly supported by the Human Resource Training Project for Regional Innovation and the Converging Research Center Program (2012K001428) of Korean government.

We have investigated the crystal structure of cellulose III (Cell III) by using first principal density functional theory (DFT) calculation. Cellulose is one of the most abundant renewable resources and has high potential for the use as a future energy and materials for the chemicals. For this purpose, it is important to know the accurate knowledge of the basic information of the crystal structure and the phase transition mechanism of the polymorph of the cellulose. In this study, we try to apply the density functional calculation method to investigate the crystal structure of cellulose crystal III-I and III-II. The geometry optimization was performed with variable-cell relaxation with the Quantum ESPRESSO program package. We used Perdew-Burke-Ernzerhof (PBE) functional and compared the results of the PBE with long-range van der Waals type correction term approach (PBE-D). The results are in good agreement with the experimentally obtained crystal structure III-I when we used the PBE with the inclusion of dispersion correction. However, the cell parameters were calculated slightly smaller than the experimental one. The smaller cell parameter can be considered to be come from the effect of the thermal expansion in the experimental condition of ambient temperature. The optimization of the structure of cellulose III-II are now investigating and the results will be presented in the meeting. In this work, we want to show that the density functional calculation methods are one of the powerful methods to investigate the detail structure and the arrangement in the crystal and the nano-structured materials.

It is generally agreed that in a nanograin the crystalline core is surrounded by a surface layer of similar atomic structure but different bond lengths [1, 2]. Experimental PDFs of nanodiamonds showed, that the atomic structure of individual diamond nanograins is more complex than such a simple model [3]. It was recently suggested that a more adequate description of nanodiamond can be provided by a model with multi-shell structure, where tensile and compressive strains are distributed in the whole grain volume. Preliminary analyses of PDF data showed also, that differently strained parts of the diamond grain volume have different thermal expansion coefficients [3]. Here we present a quantitative description of a model of nanodiamond featuring spherical density modulation waves present between the surface and the grain center.
The set of average inter-atomic distances, i > , derived from the G(r) function, might be considered as a projection of the unique atomic architecture of a given material. For nanocrystals the values of individual r _i &’s, derived with the G(r) analysis, are not strictly related to each other, as opposed to structures with uniform crystal lattice represented by the unit cell. Based on our comparison of interatomic distances in real materials with those of related perfect crystal, we are able to describe and evaluate modulation of density in nanodiamonds. To evaluate relative differences between specific inter-atomic distances in the material under examination, and corresponding distances in the reference perfect lattice, we introduced the function δ(r) equiv; Δ r_i/r_i,0, where Δ r_i=(i>-r_i,0) is the deviation of a given (volume averaged) interatomic distance <r_i> from the corresponding distance in the perfect crystal lattice, r_i,0.The r_i values vary between the shortest interatomic bond r₁and the longest distance which, for spherical grains, is the grain diameter 2R ( r₁ le; r_ile; 2R). Neutron diffraction data collected at the NPDF Station, LANSCE at LANL, Los Alamos, USA, allowed to measure interatomic distances (up to about 20-25 Å) with a reasonable resolution.
Here we show, that the structure of diamond grains of about 4.6 nm in diameter can be approximated by a model consisting of the grain core (of about 2 nm in diameter) surrounded by four spherical shells (density waves) with the average thickness of about 3Å. The shells have different densities varying about that of the bulk diamond. The bond length in the surface layer is larger than that in the shell next to the surface. Thicknesses and densities of individual shells change depending on the surface treatment and sample thermal history.
[1] Pantea, C. at al., (2006)., Technical Proceedings of the 2006 Nanotechnology Conference and Trade Show, Vol. 1, 823-826; [2] B. Palosz et al., Diamond and Related Materials, (2006)15, 1813-1817; [3] S. Stelmak et al., Advances in X-ray Analysis, (2011) 55, 175.

Measurements of local atomic arrangements by x-ray diffraction are necessary to determine the kinetics of spinodal decomposition. The transformation proceeds without nucleation as affected by several factors including the alloy composition and the depression of the coherent spinodal below chemical spinodal within the miscibility gap. Within the spinodal, phase separation leads to the formation of characteristic composition wavelengths from a solid solution. In several nickel-based alloy systems, nanolaminates were used to initially create an artificial composition fluctuation with unique nanoscale wavelengths. Although the decomposition reaction has been well documented in the literature, the direct measurement of the diffusivities at low temperatures requires sensitivity to nanoscale fluctuations and/or time-scale durations on the order of several decades. For this purpose, the results of x-ray diffraction scans to assess the state of changes in short-range order are evaluated at grazing incidence angles and at high angle where the composition fluctuation effects are compounded by lattice distortions.

Several examples for probing the structural, electrical and mechanical behaviors of nano-materials by various advanced TEM techniques have been introduced as follows.
The homogeneous Zincblende/Wurtzite heterostructural junctions in ZnSe Nanobelts have been fabricated and the polarity continuity across the junctions has been verified by the aberration-corrected HAADF-STEM imaging. The nanoscale electrostatic fields across the interfaces have been obtained through electron holography. The spontaneous polarization value for Wurtzite ZnSe has been calculated from holography results, which agrees with the ab initio calculation. [1]
Gold nanowires have been pulled in the TEM. The in situ experiments show that the dislocations produce from the surface and glide through the nanowires and then disappeared at another surface. The necking phenomenon has also been observed. After the fracture, the nanowires may experience an FCC to BCT phase transformation. [2] If pulled by different direction, the dislocations will form a small angle grain boundary. After the fracture, the dislocations move out to the surfaces immediately and the grain boundary will disappear. [3] If Li2O nanowires are being pulled in the TEM with the electron beam on, they will show much more plasticity with the electron beam assistance. [4]
[1] L. Li, L. Jin, J. Wang, D. J. Smith, W. J. Yin, Y. Yan, H. Sang, W. C. H. Choy and M. R. McCartney, Advanced Materials 24, 1328 (2012).
[2] H. Zheng, A. Cao, C. R. Weinberger, J. Y. Huang, K. Du, J. Wang, Y. Ma, Y. Xia and S. X. Mao, Nature Communications 1, 1149 (2010).
[3] H. Zheng, J. Wang, J. Y. Huang, A. Cao, S. X. Mao, Physical Review Letters (in press, 2012).
[4] H. Zheng, Y. Liu, S. X. Mao, J. Wang and J. Y. Huang, Scientific Reports 2, 542 (2012).

An important issue in the physics of amorphous materials is the possible presence of structural order at the nm length scale - beyond the second nearest neighbor - and whether such order varies as a function of glass-forming ability. Conventional diffraction techniques provide only limited information on the presence of nm scale order; the existence of a “first sharp diffraction peak” at low k values is the most salient feature, but the structural origins of this peak remain controversial. Here, we utilize fluctuation transmission electron microscopy (FTEM) to probe the order in Ge_xSe_1-x films as a function of the composition x. This alloy is a poor glass former on the Ge-rich side (x > 0.4) but a good glass former when Se-rich, hence, it allows for a systematic study of order vs. glass-forming ability.
In FTEM, the normalized statistical variance of scattered intensity from nano-volumes of material has been shown to originate in three- and four-body (pair-pair) atomic correlations across a length scale determined by the electron beam coherence. While these data cannot be inverted directly into a structural description, they are very sensitive. Previous work has shown that nm-scale order is ubiquitous in poor glass formers such as a-Si and a-Ge, exists to variable degrees in marginal glass formers such as the phase change materials Ge₂Sb₂Te₅ and doped Sb₂Te₃, and is weak to absent in good glasses such as SiO₂.
In Ge_xSe_1-x, FTEM reveals that there are two different spectral characteristics that behave independently as a function of composition. In all Ge-rich compositions, the variance is large at scattering vectors k = 0.30 and 0.55 Å^-1, similar to the peaks found in the phase-change materials. This signature diminishes for x = 0.30 and vanishes in samples with high selenium content, in parallel with increasing glass-forming ability. However, for compositions that straddle the glass-forming transition (our samples have x = 0.53 and 0.30), a new FTEM feature appears at k = 0.15 Å^-1, the position associated with the first sharp diffraction peak. This feature disappears for samples with either high Ge or high Se content. Thus, we conclude that two forms of nm scale order exist in the Ge_xSe_1-x system. We suggest that theory groups explore the possible structural configurations that could account for these observations and also be consistent with glass-forming ability. A possible starting point is consideration of the population and arrangement of GeSe₄ tetrahedra in the structure.

Modern electron microscopy has evolved into a real analytical technique, able to provide quantitative data on structure, composition, chemical bonding and magnetic properties. Using lens corrected instruments it is now possible to determine atom shifts at interfaces with a precision of a few picometer; chemical diffusion at these interfaces can be imaged down to atomic scale. Even the bonding state of the elements (e.g. Mn2+ versus Mn3+) can be detected on an atomic scale.
Most information though is still 2D projected information. Recently we have been able to extend the classical electron tomography and to count the number of atoms in 3D nanoparticles, determine the (surface) facets as well as their composition and measure the strain involved. We will report recent results on metallic nanoparticles as well as on core-shell nanoparticles. Ultra small Ge clusters (< 25 atoms) are unstable under the electron beam and the configuration of the cluster changes continuously. Using quantitative scanning transmission electron microscopy in combination with ab initio calculations, we have been able to characterize the transition between different equilibrium geometries such clusters. Seven-membered rings, trigonal prisms and some smaller subunits are identified as possible building blocks that stabilize the structure.
Three-dimensional atomic imaging of colloidal core-shell nanocrystals
Bals,S., Casavola,M., van Huis,A., Van Aert,S., Batenburg,K.J., Van Tendeloo,G., Vanmaekelbergh,D.
Nano Letters, 11, 3420 - 3424, 2011.
Three-dimensional atomic imaging of crystalline nanoparticles
Van Aert,S., Batenburg,K.J., Rossell,M.D., Erni,R., Van Tendeloo,G.
Nature, 470, 374 - 377, 2011.
"Atomic-scale determination of surface facets in gold nanorods",
B. Goris, S. Bals, W. Van den Broek, E. Carboacute;-Argibay, S. Goacute;mez-Graña, L. M. Liz-Marzán & G. Van Tendeloo,
Nature Materials doi: 10.1038/nmat3462, (2012)
“Atomic Scale Dynamics of Ultra-Small Ge Clusters”,
S. Bals, S. Van Aert, C. Romero, K. Lauwaet, M. Van Bael, B. Schoeters, B. Partoens, E. Yucelen, P. Lievens, G. Van Tendeloo
Nature Communications, ISSN 2041 - 1723 - 3, 887 (2012)
“Advanced Electron Microscopy for Advanced Materials”
G. Van Tendeloo , S. Bals , S. Van Aert , J. Verbeeck , and D. Van Dyck
Advanced Materials, ISSN 0935 - 9648, (2012)

The last six years saw unprecedented growth in real-space structural studies with aberration corrected electron microscopy, when improved resolution, stability and signal-to-noise ratio was brought to bear on the studies of interfaces, defects and bulk compounds. The field of complex oxides, where the multiple functional properties are enabled by minute displacements of atoms from high symmetry positions, proved to be especially fruitful. Parameters such as strain, polarization in ferroelectrics, and octahedral tilts could be quantitatively examined for every unit cell. For example, for BFO-LSMO interfaces we observe exciting interplay between octahedral tilting distortions, dielectric properties, and polarization that in some cases can stabilize antiferroelectric form of BFO. This demonstrates a paradigm of octahedral tilt engineering of interface behavior, complementing the well-established strategies based on misfit strain and polarization. The crystallography concept can be extended further than interatomic spacings, with possibilities for extracting information from column shapes and interpreting column widths in terms of variations of Debye-Waller factor at BFO domain walls.
At the same time, precise monitoring of interatomic spacings enables quantitative mapping of oxygen vacancies in functional oxides, as can be conclusively demonstrated on the example of vacancy ordered cobaltites. The intrinsic non-stoichiometry and high diffusional mobility of vacancies in oxides make them an active player in the physics of oxide interfaces, extending the implications of these studies from such model systems to more generic interfaces. Interestingly, vacancy dynamic and ordering can be described by the effective non-conserved order parameter fields, and the corresponding free energy parameters can be extracted from STEM data. Finally, integrating local crystallography with spectroscopic data is the key for decoupling physical and vacancy-mediated behaviors at interfaces via atomic-scale observations.
*Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division, and through user projects supported by ORNL&’s Shared Research Equipment (ShaRE) User Program, which is also sponsored by DOE-BES.

Recent studies of semiconducting materials aimed and developing advanced thermoelectrics revealed that systems which in the past were typically thought of being solid solutions are in fact nanostructured showing well defined compositional fluctuations. For example, extensive investigations in the PbTe-PbS system show that there are extensive nanoscale phase separation and it is determined by the addition of dopands. In once case the faceted shapes of precipitated nanocrystals of PbS in PbTe were found to be controllable by adding Na. The usual Vegard&’s law criterion used for solid solutions fails in these cases. Several systems including PbSe-PbS, NaSbTe2-PbTe, PbTe-PbSe were closely scrutinized using electron transmission microscopy and were found to be phase separated on the nanoscale. This discovery has profound implications in the performance, optimization and in-depth understanding of these materials. It also raises new questions regarding our understanding of other systems where solid solutions are used, for example in photovoltaics and ferroelectrics. In this context we examined two other systems for evidence of phase separation namely Sb1-xAsx and CuGaSe2-CuInSe2 using TEM and pair distribution function analysis and these results will be presented.

The functionality of dilute ferromagnetic semiconductors (DMS) depends on the incorporation of the magnetic elements into the semiconductor and their spatial distribution. Theory and experiments have shown that DMS generally exhibit a strong tendency to phase separation by means of spinodal decomposition. The GaN:Gd system have been attracting special interest because the ferromagnetism persists above room temperature even in the dilute limit of 10^16 cm^-3. The extremely low concentration, which is beyond the detectability limit of most spectroscopic and microscopic methods, makes data interpretation ambiguous and no convincing results have been reported about the distribution of Gd atoms in samples with content lower than 10^20 cm^-3.
In this study, we present a complete in-depth investigation about the incorporation and distribution of Gd atoms in epitaxial GaN thin films grown by molecular beam epitaxy. The Gd concentration is varied from 10^16 to 10^19 cm^-3. Tiny coherent GdN platelet clusters in GaN are unambiguously revealed by advanced transmission electron microscopy (TEM) methods combined with structure modeling based on density-functional theory (DFT).
HRTEM investigations of (11-20) cross-sectional specimen reveal no extended second-phase precipitates but rather local lattice distortions of dimensions of a few atom planes only. Quantitative evaluation of these coherent lattice distortions by geometric phase analysis of images yields a maximum distortion value of 0.5 Å of basal planes being equivalent to a huge strain of about 20%. Two-dimensional displacement mapping indicates that the lattice distortions are mainly along [0001] direction while no distortions are measured in the perpendicular in-plane directions. These results are explainable by means of thin coherently strained Gd-containing clusters with platelet shape being located along the basal plane. Including our strain contrast dark-field and Z-contrast STEM measurements the size of the platelets is estimated to be of 2 nm lateral diameter and a thickness of 1-2 ML. The exact atomic structure of these platelets is provided by DFT calculations. Here, Ga atoms are replaced by Gd atoms, which are shifted towards the octahedral interstitial positions due to bonding and relaxation processes. The resulting structure model is then used to simulate the HRTEM contrast and to determine the corresponding displacement field for matching with the experiment. Best fit is achieved in case of a coherent GdN bi-layer that strikingly reflects the minimum total energy configuration.
Taking the displacement field extracted from the DFT as input for TEM strain contrast simulation based on dynamic scattering theory, an excellent agreement is obtained for the observed dark-field coffee bean contrast. Due to this correlation, we have the ability to estimate the cluster size distribution and average distance, which are basic parameters for deeper understanding of the magnetic properties.

Nanoparticles exhibit a largely enhanced surface-to-volume ratio as compared to their bulk counterparts. Accordingly, many of their novel properties and functionalities are intimately related to the details of the atomic structure of these surfaces. Besides the symmetry breaking that occurs at surfaces and interfaces, surface atoms have a reduced coordination which not only alters their physical and chemical properties but also reduces their structural integrity resulting in enhanced diffusivities and a (surface-related) reduction of the melting temperatures. A thorough understanding of the mobility of surface atoms is thus mandatory in order to better understand and control the stability of the surface atom arrangements and in turn the surface-related properties of the particles.
This is all the more important as the atomic-resolution characterization of such surfaces is frequently conducted by means of transmission electron microscopy. Here, the imaging high energy electron beam may interact inelastically with the particle under investigation which in turn may result in a modification of the particle surfaces. It will be exemplarily demonstrated that adjacent and initially well separated Au particles coalesce upon irradiation with 300 keV electrons through enhanced surface diffusion towards the sintering neck.
Time resolved delocalization-free imaging of surface atom configurations through aberration-corrected high-resolution transmission electron microscopy (HR-TEM), however, also allows for the characterization of the atomic diffusion on the surfaces of nanoparticles and thus at the same time provides the potential to measure and control this impact. We present a statistical investigation of temporal sequences of HR-TEM images where the mobility of surface atoms is characterized by measuring the temporal fluctuation in the occupation of surface lattice sites. Based on the formalism used to describe tracer diffusion, a model is developed that allows for the quantification of the surface diffusion coefficient from the observed fluctuations [1]. The present study is focused on characterizing (multiply twinned) icosahedral and (single crystalline) octahedral Au nanoparticles which are prepared by inert gas condensation and are thus free of any organic ligands which would otherwise modify the native Au surfaces. The particles are supported by amorphous carbon films which are either continuous or holey which allows us to account for the effect of the substrate on the image analysis and diffusion, respectively. The coefficient of the surface self-diffusion determined through this novel approach is in very good agreement with the results of atomic force and scanning tunneling microscopy studies.
[1] A. Surrey, D. Pohl, L. Schultz, and B. Rellinghaus, Nano Lett. (2012), accepted.

Detecting the existence of, and quantifying, nanoscale ordered regions (1-3 nm) in amorphous materials is a challenging problem in the physics of disordered solids. Fluctuation Transmission Electron Microscopy (FTEM) is directly sensitive to order on this length scale. In FTEM the statistical variance of the scattered intensity from nanovolumes of material is directly related to the three- and four-body (pair-pair) atomic correlations within the region. However, there is no direct method to invert FTEM data into a structural model. Most of the materials studied appear to have discrete ordered regions embedded in an amorphous network. We therefore focus our attention on means to distinguish the size vs. the volume fraction of ordered regions. Both contribute to the magnitude of the variance in a spectrally similar fashion. One approach is to simulate the FTEM spectra from high quality atomistic models and compare with experimental data. However, this approach is not general. Another approach is to use the variable resolution mode of FTEM, in which the size of the coherent electron probe is modulated and changes in the variance are plotted under the assumption of a single-valued correlation length for the order. However, the interpretation of the correlation length is not yet obvious. Here we introduce a new approach, in which the scattering covariance is defined and extracted from the FTEM data. The concept is to determine whether the electron beam is interacting with few ordered regions, or with many. We seek information in the event that a particular nanovolume will simultaneously excite two Bragg conditions (the covariance). The endpoint cases are clear: a single large ordered region in the electron beam is most likely to excite a single reflection, hence produce little covariance; whereas a large collection of small regions will reliably excite many reflections, hence a large covariance. Intermediate cases are subtler since multiple reflections may be excited from a single particle. We examine covariance trends on 20 nm thick sputter-deposited Ge₂Sb₂Te₅ films in a JEOL 2200FS (S)TEM. The nanoscale order can be controllably increased by exposing the sample to an un-focused electron beam for times up to 60 minutes. As the material approaches the crystalline transition, a significant covariance signal emerges. We developed a mathematical interpretation that takes account of the reciprocal lattice broadening for small crystal sizes and the probability for multiple ordered regions to exist within the volume probed by the electron beam. Monte-Carlo simulations revealed that the effects of size and of volume density interact to produce a covariance map that is sensitive to the details of nanoscale order. But neither the covariance data reduction nor the simulation requires an atomistic model; hence, this method is directly applicable to a broad spectrum of materials.

Calcium carbonate, one of the most abundant minerals on the earth&’s crust, is present both geologically and in biominerals. Calcite, aragonite, and vaterite are the three anhydrous polymorphs of CaCO3, in order of decreasing thermodynamic stability. The crystal structure of vaterite has been elusive for almost a century, with conflicting results and differing interpretations continuing to be produced. Using state-of-the-art aberration-corrected high-resolution transmission electron microscopy and synchrotron radiation, we show here that the reason for the failure to explain all the experimental findings by attributing a single and unique crystallographic structure to vaterite is that this polymorph is composed not of one but at least two different crystallographic structures, which coexist within a pseudo-single crystal. The major structure demonstrates a hexagonal symmetry; the minor structure, present as different nano-domains (1-2 nm) within the major matrix, is as yet unknown.
These findings present how state-of-the-art characterization techniques are able to solve years old conundrums.

Phosphors play a key role in solid-state lighting, converting a part of the exciting blue radiation from a solid state device into longer wavelengths in a such a way that the total scattered light appears white. The three aspects of phosphor function that require research and development are quantum yield and efficacy, color rendition, and issues associated with the thermal stability of the luminescence. All of these issues rely on knowing the structure and dynamics of phosphor lattices from a very local perspective, of the few % of activation (usually trivalent Ce or divalent Eu) ions in the lattice. I will present our recent work on the use of total scattering, EXAFS, and NMR to probe these, and to correlate them to the phosphor properties.

A classic system of gas-metal interactions is hydrogen and palladium. It
lies at the heart of applications ranging from purifying hydrogen to
catalysis. The interplay of H and carbon monoxide on Pd is also of great
interest due their role in gas sensors, hydrogen purification, and heterogeneous
catalysis.
We have studied this system via low energy-electron microscopy/diffraction
(LEEM/LEED) and total energy calculations. First, LEEM is used to
image in real time how CO displaces H from the Pd(111) surface. Next we
address the H uptake by the Pd under different conditions, either dosing
atomic (or molecular) hydrogen on a CO precovered surface or co-dosing both
at the same time. The dynamical LEED analysis reveals that only upon codosing
atomic H and CO the latter assits the absorption of hydrogen which, surprisingly,
ends up trapped between the second and third layers thus blocking its absorption.
Total energy profiles (EPs) were calculated under the Density Functional Theory
for various unit cells with different H and CO coverages in order to rationalize
these findings. The trapping arises from the CO-H repulsion so that when an
adsorbed H has two COs at nearest neighbor sites, it destabilizes in favor of
an interstial site[1].
[1] J.I. Cerda et al, J. Phys. Chem. Lett. 3, (2012) 87.

EMSL, a national scientific user facility of the DOE, is developing a comprehensive chemical imaging capability combining atom probe tomography (APT) with high resolution scanning transmission electron microscopy (STEM) to provide solutions to problems pertaining to energy and environmental applications. We will emphasize a chemical imaging effort aimed at atomically-resolved composition and structural analysis of complex energy materials such as nanocomposites of embedded metal nanoparticles in oxides and multilayer thin films with applications in areas ranging from catalysis and solid oxide fuel cells. A unique benefit of APT is the ability to characterize the ppm level composition of buried interfaces and nanoscale volumes with sub nanometer spatial resolution in three dimensions. However the currently existing artifacts possible during APT analysis of heterogeneous materials need to be accounted for and solved by combining cross correlative microscopy with computational modeling. Therefore we have combined APT with HRSTEM and x-ray based capabilities along with computational modeling to gain quantitative understanding of these energy materials composition and structure at atomic scale.

Measuring and understanding heteroatom (e.g., boron or aluminum) incorporation in silicate frameworks are key to controlling and improving the properties of solid acid catalysts, such as technologically important zeolites. Macroscopic catalytic properties are known to be governed by the positions and local environments of heteroatoms in silicate frameworks, though molecular-level (sub-nm) information has been challenging to obtain. Experimental characterization of such heteroatom sites is often limited by the complex distributions of order and disorder in heteroatom-containing frameworks, which complicate their analyses by scattering or other conventional techniques. High-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy, however, overcomes many of the challenges by providing detailed information on the local bonding environments and interactions of heteroatoms, including disordered or surface species to which diffraction analyses are insensitive. In particular, powerful two-dimensional (2D) NMR methods can resolve and identify interactions between distinct framework moieties by measuring their through-space (dipole-dipole) interactions or through-bond scalar (J) couplings. Here, we report new applications of 2D solid-state NMR techniques that exploit heteronuclear dipole-dipole interactions and J couplings between 11B or 27Al heteroatoms and nearby 29Si species to probe their local framework environments, notably at surfaces. By measuring directly the molecular proximities and site connectivities of 11B or 27Al heteroatoms to nearby 29Si moieties, the respective distributions of heteroatoms are, for the first time, established in semi-crystalline boro- or aluminosilicate frameworks. Furthermore, the 2D NMR results, in conjunction with density functional theory (DFT) calculations, establish that boron atoms are incorporated selectively into specific four-coordinate Q3 29Si sites within the borosilicate frameworks. By comparison, aluminum atoms are more broadly distributed within otherwise similar aluminosilicate frameworks. The insights provided by the combination of state-of-the-art 2D NMR and DFT analyses are expected to enable the optimization of syntheses, compositions, and structures of zeolites and other heteroatom-containing siliceous materials, aimed at enhancing their catalytic reaction or adsorption properties.

The scattering of neutrons in the eV energy range gives access to the momentum distributions and kinetic energies of individual atomic species in a condensed matter system. This so called Deep Inelastic Neutron Scattering (DINS) is thus analogous to the well known Compton scattering of photons from electrons and is also known as Neutron Compton Scattering (NCS). To date, DINS has been used to determine quantum effects in systems of the lightest elements, H/D and He.
However, within the framework of DINS, the momentum width/kinetic energies of individual atomic species can be determined, making DINS a truly localised probe of the dynamical state of a system. Recent developments of the VESUVIO spectrometer have allowed, for the first time, the kinetic energies of masses heavier than He to be determined simultaneously in multi-mass systems, including lithium.
It is shown that the current form of DINS is able to give accurate results when determining the momentum widths of multiple atomic species simultaneously. We demonstrate the accuracy for a binary system, 7LiF, with the resulting momentum widths for 7Li and 19F found to be within ~5% of those calculated using a quasiharmonic density functional approach, across a range of temperatures between 4-300K. We also demonstrate the ability of DINS to examine nanostructured materials within an amorphous host, comparing results for NaH in bulk, and as nanocrystallites within a silica gel matrix (SiGNaH). We also compare this to an analogous system of Na/SiGNa.

Segregation of solute atoms in dilute alloys to grain boundaries has been shown to stabilize nanostructured solids against grain growth. At present, most theories of the stabilization of nanostructured alloys do not account for the detailed atomic structure of the interfaces, but instead rely on averaged segregation energies. One of the reasons for this is the daunting task of determining specific structures, including substitution sites for solute atoms, and their contribution to thermodynamic ensemble averages. We are developing a new approach to predicting and organizing interface structures in alloys that takes advantage of a disclination structural units model developed previously for grain boundaries in pure systems. The fundamental idea is to extend the geometrical structural units to include the concentration and placement of solute atoms. Based on comparison to molecular modeling results, it is found that using elastic energies and disclination core terms from the pure solvent in combination with the extended definition of structural units provides an accurate prediction of interface energies and resulting thermodynamic stabilization provided that a “bonding” term between inequivalent disclination units is included. After parameterization, this approach provides an efficient method in which to explore a large range of interface types and solute concentrations, including irregular structures.
This method will be demonstrated using symmetric tilt grain boundaries in Cu-Zr, Cu-Nb, Al-Zr, and Al-Pb alloys. Simulations of the influence of solute atoms on grain boundary mobilities will also be presented, in particular the influence of the interfacial solute atoms on the mechanisms of both shear-induced and diffusion-driven grain boundary motion.
This work was supported by a grant from the Office of Naval Research.

The structural and electronic properties of Co islands on Ge(111)-c(2x8) surfaces have been studied using scanning tunneling microscopy and spectroscopy. Room temperature deposition of a sub-monolayer amount of Co and subsequent annealing at 500 K leads to the formation of (radic;13xradic;13)R13.9° reconstructed islands, whereas annealing at 750 K results in (2x2) reconstructed islands. The latter type of islands have heights up to several atomic layers. We find that, in analogy with Co on Si(111), the (2x2) islands consist of CoGe2, condensed in a fluorite structure. Each Co(111) layer is encapsulated between two close packed Ge layers, while the outermost trilayer is terminated by an additional layer of Ge adatoms in a (2x2) registry. The (radic;13xradic;13)R13.9° structure can be considered as a precursor to the CoGe2 crystallites with triangular trilayer patches, composed of three embedded Co atoms terminated by six Ge adatoms. The remaining dangling bonds are saturated by one sp2 hybridized Ge-trimer per unit cell. The different domains in the (2x2) islands are separated by three types of boundaries. Two types of domain boundaries are dynamic, whereas the third type of domain boundary is static. The (2x2) domains show a faint (6x2) superstructure, which is tentatively attributed to aligned Co defects at the interface. Scanning tunneling spectroscopy measurements indicate the presence of Co atoms in the (2x2) and (radic;13xradic;13)R13.9° reconstructed islands.

Ice and frost formation is a major problem affecting ground and air transportation, power generation and transmission, and agriculture. Recently, Kim et al.[1] reported that ice as well as frost formation can be significantly decreased by impregnating nanostructured surfaces with a water-immiscible liquid. However, the underlying physical mechanism of icing and frosting mitigation on such Lubricant Impregnated Surfaces (LIS) is not well understood. A major obstacle in developing such understanding has been the lack of an imaging method capable of visualizing the complex ice-liquid-nanostructure interfacial region. Here, we resolve this issue by adapting our recently developed cryo-Focus Ion Beam/SEM (cryo-FIB/SEM) imaging method to visualization of such interfaces.[2] The method relies on cryostabilization of frosted LIS samples in liquid nitrogen slush, cryogenic temperature selective FIB milling, and SEM imaging. We demonstrate that our method clearly reveals the nanoscale details of the cross sectional geometry of ice nucleated on LIS. We use this novel method to study ice nucleation on LIS consisting of a variety of impregnating liquids and nanostructures.
References:
1. Kim et al., ACS Nano, 2012, 6 (8), pp 6569-6577.
2. Rykaczewski et al., ACS Nano, 2012, Articles ASAP.

Lead Telluride (PbTe) is a well characterized thermoelectric material frequently used for applications above room temperature. When doped with a few percent Tl, the figure of merit, ZT= TS²σ_e/Κ is significantly improved (here S is the Seebeck coefficient, σ_e the electrical conductivity and Κ the thermal conductivity). Tl has possible valence states of +1 and +3, Tl(+1) has a lone electron pair; consequently, the atom is asymmetric and large local distortions may occur if Tl(+1) is present. Additionally recent neutron diffraction and total neutron scattering experiments have suggested that Pb moves off-center along the 100 axis as T increases which makes the Pb site disordered. The off-center displacement is about 0.18 A at 300 K. To investigate the local structure around the dopant and host atoms we present an Extended X-ray Absorption Fine Structure (EXAFS) analysis for un-doped PbTe and for 1-3% Tl concentrations, at the Tl and Pb L_III edges and at the Te K edge. The Pb and Te EXAFS are nearly identical in both doped and un-doped samples; the presence of thallium appears to have little effect on the host atom structure. At 10K the local structure about Pb is well ordered; however as temperature increases, the Pb-Te (Te-Pb) pair distribution function (PDF) broadens rapidly. Attempts to model the increase in σ²(T) for the Pb-Te pair (sigma is the width of the PDF) with a 100 Pb off-center displacement, as proposed by neutron PDF experiments, were not successful. In particular, the interference effects expected for a split peak were not observed. However σ²(T) for the Pb-Te pair is well described by a correlated Debye model with a low correlated Debye temperature, theta;_cD = 115 K. The Te edge shows increased disorder for the the Te-Te pair and later peaks which may be caused by a structural change around the Te atom, but is not easily modeled by a simple off-center displacement. For Tl, the environment is distorted even at 10K within the host material. This indicates a large variation of the Tl-Te bond lengths, presumably as a result of the presence of Tl(+1). This increased local disorder will result in additional electron scattering and may contribute to the decrease in the figure of merit above 2% Tl. We discuss possible models for the disorder about Tl, Pb, and Te in PbTe:Tl.

Many phenomena of interest to materials science take place on the submicron scale in extended structures. Information on the nano-scale can be obtained by various methods, both by directly probing the sample in real space (via electron or X-ray microscopy) or indirectly measuring global nano-scale properties (e.g. crystallography, microstructure characterization or pair distribution function analysis). Interestingly, these techniques meet in a resolution range on the length scale of 10s of nm, a scale now becoming available in situ experiments carried out by X-ray microscopy beamlines at 3rd generation synchrotron sources. We report on the application of high spatial resolution techniques in to characterize diverse samples from microelectronics (1), metallic glasses (2), geophysics (3), and many other fields (4). The combination of directly probing sample inhomeogenity via the use of X-ray micro- or nano-beams with the application of local structure characterization techniques allows a uniquely complete characterization of bulk samples, both ex- or in-situ.
(1) N. Vaxelaire, P. Gergaud, and G.B. M. Vaughan, submitted to J. of Appl. Cryst.
(2) A.R. Yavari, K. Georgarakis, J. Antonowicz, M. Stoica, N. Nishiyama, G.B.M. Vaughan, M. Chen, M. Pons, Phys. Rev. Lett, 109(8) (2012) 085501-085504
(3) C. Nisr, G. Ribarik, T. Ungar, G.B.M. Vaughan, P. Cordier, and S. Merkel, J. Geophys. Res-Solid Earth, 117 (2012)
(4) H.O. Sorensen, S. Schmidt, J.P. Wright, G.B.M. Vaughan, S. Techert, E.F. Garman, J. Oddershede, J. Davaasambu, K.S. Paithankar, C. Gundlach, H.F. Poulsen, Z. Krist. 227 (2012) 63-78.

Nanoscale chemical imaging of catalysts under working conditions is possible with Transmission X-ray Microscopy. We have shown that both soft and hard x-ray TXM can image a catalytic system under relevant reaction conditions and provides detailed information on the morphology and composition of the catalyst material in situ [1]. The 20 nanometer resolution combined with powerful chemical speciation by XAS [2] and the ability to image materials under reaction conditions opens up new opportunities to study many chemical processes. I will discuss the present status of in-situ TXM, with an emphasis on the abilities of the 20 nm resolution TXM technique in comparison with 0.1 nm STEM-EELS [3,4]. Hard X-ray TXM allows the measurement of chemical images and tomographs, using a capillary reactor at 10 bar Fischer-Tropsch conditions [5].
[1] E. de Smit et al. Nature 456, 222 (2008).
[2] Core Level Spectroscopy of Solids, F. de Groot and A. Kotani (Taylor & Francis, 2008)
[3] M. van Schooneveld et al. Nature Nanotechnology 5, 538 (2010)
[4] F.M.F. de Groot et al. ChemPhysChem 11, 951 (2010);
[5] I. Gonzalez-Jimenez et al. Angew. Chem, DOI: 10.1002/anie.201204930 (2012)

Current remediation efforts for U-contaminated plumes in subsurface environments focus on limiting contaminant migration by sequestrating U as a solid in the subsurface. Hexavalent U, U(VI), is relatively mobile in oxidizing subsurface environments. In contrast, tetravalent U, U(IV), is typically present as sparingly soluble mineral phases such as uraninite (UO2) and is considered to be immobile relative to the U(VI) species. Therefore, the fate of U in the environment is determined by a complex array of complexation, adsorption, reduction, and precipitation reactions. Efforts to limit the solubility and mobility of U(VI) in the subsurface have generally focused on reducing U(VI) to U(IV) by stimulating the activity of indigenous dissimilatory metal-reducing bacteria. Although reduction of soluble U(VI) species to U(IV) by direct or indirect bacterial activity often results in the precipitation of a U(IV) species, we have shown that the U(IV) ions can be incorporated in solid phases ranging from macro-scale uraninite minerals to nanocrystalline uraninite to single U(IV) ions associated with phosphate and iron (hydr)oxid minerals. Understanding the atomic structure of U in these sub 10-nm structures is paramont for predicting post-remediation stability of U plumes because the solubility of uranium in these different forms may be drastically different. We have performed many U LIII-edge x-ray absorption spectroscopy and electron microscopy experiments to characterize uranium from the atomic scale to the nanoscale to the microscale in a variety of biogeochemically perturbed systems to understand the interplay between the biotic and abiotic reactions that control partitioning of uranium into the solid phase in environmental samples. In addition to demonstrating the control some of these biogeochemical interactions can have on the partitioning of uranium into the solid phase, we will show the utility of combining the x-ray and electron probe techniques to characterize the physical chemistry of uranium in nanostructures.

Studies of electronic confinement and other related effects in nanomaterials are often hampered by experiments that require bulk quantities of nanoparticles or rely on certain structural properties maintaining their character from the bulk, since their sub-Angstrom structural details are not typically known. An alternative is to use small molecules instead of nanoparticles, since their sizes and electronic structure are much more easily understood, allowing for a detailed understanding of electronic size effects. We have used this approach to study the molecular analogue to the Kondo effect in quantum-confined complexes of pentamethylcyclopentadienyl ytterbium and have successfully explained the magnetic and electronic properties of these molecules, strongly aided by CASSCF calculations. In particular, we found that magnetic coupling strengths and intermediate valence on Yb are related to molecular size, as are such effects in their excited states. A complete picture is thus provided from nearly Yb(II) to nearly Yb(III), including changes to the local chemical bonding.

Measurements of displacements and strains are of great importance for characterization of the electronic materials, as they may inform about quality of the sample and its mechanical or electrical properties. Displacements can be measured by analyzing the spectral content of the image of sample crystallic structure (obtained with, e.g., transmission electron microscope), strains can be calculated knowing the displacements. If the displacements of periodic crystal lattice are considered, a discrete set of peaks is present in the Fourier space (Bragg spots). Displacements (or, more precisely, local departures from the average reciprocal lattice) are coded by values of the spectral components in the close neighborhood of each peak and can be decoded with signal processing techniques such as geometric phase method, typically using a single peak. However, depending on their order, peaks possess different properties, as signal to noise ratio is diminishing with the peak order. On the other hand, peaks of the higher order may code more delicate variations of phase, allowing for more sensitive measurements of displacements and strains. This is because of the so-called phase multiplication phenomenon, well known in optical interferometry, but similarly present in measurements based on the electron microscopy. We will deliver a system for microscopic image processing, which combines phase measurements from many Bragg spots to benefit from both high signal to noise ratio of low order peaks and increased sensitivity of the higher order ones. Moreover, as the information content of the microscopic image spectrum is highly redundant, by combining measurements obtained with different Bragg spots we are able to reduce the spurious influence of the random noise. Proposed system provides more accurate results of displacement and strain measurements than the regular geometric phase method, which is corroborated by numerical simulations. Results of analysis of real microscopic data will be shown as well.

The accurate manipulation of strain in silicon nanowires can unveil new fundamental properties and enable novel or enhanced functionalities. To exploit these potentialities, it is essential to overcome major challenges at the fabrication and characterization levels. With this perspective, we have investigated the strain behavior in nanowires fabricated by patterning and etching of 15 nm thick tensile strained silicon (100) membranes. To this end, we have developed [1] a method to excite the “forbidden” transverse-optical (TO) phonons in single tensile strained silicon nanowires using high-resolution polarized Raman spectroscopy. Detecting this phonon is critical for precise analysis of strain in nanoscale systems. The intensity of the measured Raman spectra is analyzed based on three-dimensional field distribution of radial, azimuthal and linear polarizations focused by a high numerical aperture lens. The effects of sample geometry on the sensitivity of TO measurement are addressed. A significantly higher sensitivity is demonstrated for nanowires as compared to thin layers. In-plane and out-of-plane strain profiles in single nanowires are obtained through the simultaneous probe of local TO and LO (longitudinal-optical) phonons. New insights into strained nanowires mechanical properties are inferred from the measured strain profiles.
[1] A. Tarun, N. Hayazawa, H. Ishitobi, S. Kawata, M. Reiche, and O. Moutanabbir, Nano Letters Vol. 11, 4780 (2011).