The microstructure is defined by the type, crystal structure, number, shape and topological arrangement of phases and defects such as point defects, dislocations, stacking faults or grain boundaries in a crystalline material. Nanocrystalline materials are polycrystalline solids with grain sizes below 100 nm. Grains as well as pores, interfaces and other defects are of similar dimensions. This nano-specific microstructure (nanostructure) leads to chemical and physical size effects. It is a prerequisite for the understanding of properties of nanomaterials to have a detailed knowledge of the structure from the atomic/molecular (local) to the crystal structure (long range order) and to the microstructure (mesoscopic scale and defect structure). Consequently, various analytical techniques are required to characterize the nanomaterials on all length scales.

Particle size and morphology
Scattering and imaging methods such as line broadening in X-ray diffraction, small angle X-ray and neutron scattering and transmission electron microscopy, are employed to determine average particle and grain size, their distributions and the morphology of the particles. Additionally, nitrogen adsorption, light scattering, atomic probe techniques and mass spectroscopy provide complementary information on these size related parameters. The method of choice depends on the type of material (powder, solid and liquid dispersion, consolidated and sintered ceramic, etc.). Care should be taken in comparing the average sizes determined by these methods because different size related parameters are determined (e.g. column length in XRD line broadening) and the average values are based on different weight functions.

Local structure
Information on the local structure can be provided by spectroscopic methods such as NMR or EXAFS spectroscopy but is also contained in diffuse scattering. Nanocrystalline materials are heterogeneously disordered with a large fraction of atoms located in surfaces and interfaces. In case of pure, tetragonal nanocrystalline zirconia with a particle size of 5 nm it was shown that atoms located at particle surfaces have a higher degree of disorder compared to atoms in the core of the particles due to more degrees of freedom. Primary processes during the particle formation by CVS and the evolution of the microstructure during sintering occur on the local, molecular level. Segregation of aluminium atoms was identified as the mechanism for the grain growth inhibition during sintering of zirconia doped with alumina by Reverse Monte Carlo analysis of EXAFS spectra. Similarly, an inhomogeneous distribution of yttrium in zirconia was found and correlated to a low sinterability.

Crystal structure
X-ray, electron and neutron diffraction are extensively used to characterize the crystal structure which has been observed to be size dependent in many cases. The line broadening at very small crystallite sizes limits the accuracy of crystal structure determination, i.e. difficulty in distinguishing between tetragonal and cubic zirconia. In general it is observed for CVS nanopowders that all diffraction reflexes are present and that the background is very low. This indicates a high degree of crystallinity and low defect density within the individual particles. An exemption is nanocrystalline silicon carbide which exhibits stacking faults and twinning, especially when synthesized at low temperature. From Rietveld refinement not only the phase composition, lattice parameters and positions of atoms in the unit cell, but also crystallite size and the microstrain can be extracted.

Microstructure
On a larger length scale electron microscopy (both scanning and transmission) is extensively used to characterize defects such as dislocations, grain boundaries, agglomerate size and structure, distribution of phases, grain and phase boundaries, pores etc. which are important for the properties of the nanoceramics. In case of CVS nanopowders, the primary nanoparticles are single crystalline with lattice fringes extending to the surface. In addition, it is frequently found that the nanoparticles exhibit crystallographic habitus planes. For the characterization of porosity in nanoceramics, nitrogen adsorption and small angle scattering are used to determine surface area, total pore volume and pore size distributions. Powders exhibit fractal structures typically observed for aerogels.

• M. Winterer, xafsX: a program to process, analyse and reduce X-ray absorption fine structure spectra (XAFS) , International Tables of Crystallography I, X-ray Absorption Spectroscopy and Related Techniques (2024), ch.6.23, 834-847
• C. Gorynski, J. Geiß, U. Anselmi-Tamburini, and M. Winterer, Structural and compositional gradients in alternating current sintered aluminum-doped zinc oxide, Acta Materialia 270 (2024) 119855
• V. Mackert, T. Winter, S. Jackson, R. Kalia, A. Levish, S. Lukic, J. Geiss, and M. Winterer, Very Small Nanocrystalline Tin Dioxide Particles: Local-, Crystal-, and Micro-Structure, J. Phys. Chem. C 127 (2023) 17389–17405
• C. Gorynski, M. Frei, F. E. Kruis, M. Winterer, Machine learning based quantitative characterization of microstructures, Acta Mater. 256 (2023) 119106, 17pp.
• J. Geiss, J., Bueker, J., Schulte, J., Peng, B., Muhler, M., Winterer, M., LaCo1-xFexO3 Nanoparticles in Cyclohexene Oxidation, J. Phys. Chem. C 127 (2023) 5029–5038
• M. Winterer and J. Geiß, Combining reverse Monte Carlo analysis of X-ray scattering and extended X-ray absorption fine structure spectra of very small nanoparticles, J. Appl. Cryst. 56 (2023) pp. 7
• J. Geiss, T. Falk, S. Ognjanovic, S. Anke, B. Peng, M. Muhler, and M. Winterer, Atom Pair Frequencies as a Quantitative Structure–Activity Relationship for Catalytic 2-Propanol Oxidation over Nanocrystalline Cobalt–Iron–Spinel, J. Phys. Chem. C  126 (2022) 10346−10358
• V. Mackert, M. A. Schroer, M. Winterer, Unraveling agglomeration and deagglomeration in aqueous colloidal dispersions of very small tin dioxide nanoparticles, J. Colloid and Interface Science 608 (2022) 2681–2693
• S. Lukic, G. W. Busser, S. Zhang, J. Menze, M. Muhler, C. Scheu and M. Winterer, Nanocrystalline Ga-Zn Oxynitride Materials: Minimized Defect Density for Improved Photocatalytic Activity? Z. Phys. Chem. 234 (2020) 1133-1153
• S. M. Ognjanovic, M. Zähres, C. Mayer, and M. Winterer, Local Structure of Nanocrystalline Aluminum Nitride, J. Phys. Chem. C 122 (2018), 23749-23757
• S. M. Ognjanović, M. Winterer, Optimizing particle characteristics of nanocrystalline aluminum nitride, Powder Technol. 326 (2018), 488-497
• S. Lukic, J. Menze, P. Weide, G. W. Busser, M. Winterer, and M. Muhler, Decoupling the Effects of High Crystallinity and Surface Area on the Photocatalytic Overall Water Splitting over ß-Ga2O3 nanoparticles by Chemical Vapor Synthesis, ChemSusChem, 10 (2017), 4190 – 4197
• A. Kompch, A. Sahu, Chr. Notthoff, F. Ott, D. J. Norris, and M. Winterer, Localization of Ag Dopant Atoms in CdSe Nanocrystals by Reverse Monte Carlo Analysis of EXAFS Spectra, J. Phys. Chem. 119 (2015), 18762-18772
• A. Sandmann, A. Kompch, V. Mackert, Chr. Liebscher, M. Winterer, Interaction of L-Cysteine with ZnO: Structure, Surface Chemistry, and Optical Properties, Langmuir 31 (2015), 5701-5711
• C. Klein, M. Vyshnepolsky, A. Kompch, F. Klasing, A. Hanisch-Blicharski, M. Winterer, and M. Horn-von Hoegen, Strain state, film and surface morphology of epitaxial topological insulator Bi2Se3 films on Si(111), Thin. Sol. Films 564 (2014), 241-245
• C. Schilling, M. Zähres, C. Mayer, and M. Winterer, Aluminum-doped ZnO nanoparticles: gas-phase synthesis and dopant location, J. Nanopart. Res. 16 (2014), 2506, 15pp
• C. Schilling, and M. Winterer, Preserving Particle Characteristics at Increasing Production Rate of ZnO Nanoparticles by Chemical Vapor Synthesis, Chem. Vap. Dep. 20 (2014), 138-145
• C. Notthoff, M. Winterer, A. Beckel, M. Geller, and J. Heindl, Spatial high resolution energy dispersive X-ray spectroscopy on thin lamellas, Ultramicroscopy 129 (2013), 30-35
• C. Schilling, R. Theissmann, C. Notthoff, and M. Winterer, Synthesis of Small Hollow ZnO Nanospheres from the Gas Phase, Particle & Particle Systems Charakterization 30 (2013), 434-437
• R. Djenadic and M. Winterer, chapter 2, Chemical Vapor Synthess of Nanocrystalline Oxides, in Axel Lorke, Markus Winterer, Roland Schmechel, und Christof Schulz (eds.), Nanoparticles from the Gas Phase – Formation, Structure, Properties, Springer Berlin 2012, ISBN 978-3-642-28546-2
• G. Akgul, F.A. Akgul, K. Attenkofer, and M. Winterer, A Structural properties of zinc oxide and titanium dioxide nanoparticles prepared by chemical vapor synthesis; Journal of Alloys and Compounds 554 (2013), 177-181
• R. Djenadic, G. Akgul, K. Attenkofer, and M. Winterer, Chemical Vapor Synthesis and Structural Characterization of Nanocrystalline Zn1-xCoxO (x=0-0.50) Particles by X-ray Diffraction and X-ray Absorption Spectroscopy, Journal of Physical Chemistry 114 (2010), 9207-9215
• W. Jin, I. K. Lee, A. Kompch, U. Dörfler, and M. Winterer, Chemical vapor synthesis and characterization of chromium doped zinc oxide nanoparticles, J. Eur. Ceram. Soc. 27 (2007), 4333-4337
• J. U. Brehm, M. Winterer, and H. Hahn, Synthesis and local structure of doped nanocrystalline zinc oxides, J. Appl. Phys. 100 (2006), 064311
• Th. Enz, M. Winterer, B. Stahl, S. Bhattacharya, G. Miehe, K. Foster, C. Fasel, and H. Hahn, Structure and Magnetic Properties of Iron Nanoparticles Stabilized in Carbon, Journal of Applied Physics 99 (2006), 044306
• I. Berrodier, F. Farges, M. Benedetti, M. Winterer, G. E. Brown, M. Deveughele, Adsorption mechanisms of trivalent gold on iron- and aluminum-(oxy)hydroxides. Part 1: X-ray absorption and Raman scattering spectroscopic studies of Au(III) adsorbed on ferrihydrite, goethite, and boehmite, Geochim. Cosmochim. Acta 68 (2004), 3019
• V. V. Srdic and M. Winterer, Aluminum-Doped Zirconia Nanopowders: Chemical Vapor Synthesis and Structural Analysis by Rietveld Refinement of X-ray Diffraction Data, Chem. Mater. 15 (2003), 2668-2674
• Markus Winterer, Nanocrystalline Ceramics – Synthesis and Structure, Springer, Heidelberg 2002, Springer Series in Materials Science, Volume 53, ISBN 3-540-43433-X
• Th. E. Weirich, S. Seifried, M. Winterer and J. Mayer, Structure of Nanocrystalline Anatase Solved and Refined from Electron Powder Data, Acta Cryst. A58 (2002), 308
• M. Winterer, B. Delaplane, R. McGreevy, X-Ray Diffraction, Neutron Scattering and EXAFS Spectroscopy of Monoclinic Zirconia: Analysis by Rietveld Refinement and Reverse Monte Carlo Simulations, J. Appl. Cryst. 35 (2002), 434
• M. Winterer, Reverse Monte Carlo Analysis of EXAFS Spectra of Amorphous and Monoclinic Zirconia, J. Appl. Phys. 88 (2000), 5635
• T. Weirich, M. Winterer, S. Seifried, H. Hahn and H. Fueß, Rietveld Analysis of Electron Powder Diffraction Data from Nanocrystalline Anatase, TiO2, Ultramicroscopy 81 (2000), 263
• U. Keiderling, A. Möller, A. Wiedenmann, M. Winterer, and H. Hahn, Nanocrystalline Al2O3 and ZrO2 powders in aerogels and in aqueous solutions measured with SANS and Photon Correlation Spectroscopy, Physica B 276-278 (2000), 278
• U. Keiderling, A. Wiedenmann, V. Srdic, M. Winterer, and H. Hahn, Nano-sized Ceramics of Coated Alumina and Zirconia Analyzed with SANS, J. Appl. Cryst. 33 (2000), 483
• M. Winterer, R. Nitsche, and H. Hahn, Local Structure in Nanocrystalline ZrO2 and Y2O3 by EXAFS, Nanostructured Materials 9 (1997), 397
• M. Winterer, R. Nitsche, S.A.T. Redfern, W.W. Schmahl, and H. Hahn, Phase Stability in Nanostructured and Coarse Grained Zirconia under High Pressures, Nanostructured Materials 5 (1995), 679
• R. Nitsche, M. Winterer, M. Croft, H. Hahn, X-Ray Absorption Study on Nanostructured Zirconia and Yttria, Nucl. Instr. Methods in Physics Research B 97 (1995), 127
• R. Nitsche, M. Winterer, and H. Hahn, Structure of Nanocrystalline Zirconia and Yttria, Nanostructured Materials, 6 (1995), 679