B ogdan Palosz

1. "Nano-materials and Powder Diffraction" EPDIC-10 Nano-Workshop 31 August 2006, Geneve, Switzerland Chair: Robert Snyder & Bogdan Palosz Introduction Structure of nano-crystals as the key to understanding the unique properties of nano-materials Bogdan Palosz Institute of High Pressure Physics Polish Academy of Sciences Warsaw, Poland

2. "Future Directions and Research Priorities: . . . Greater emphasis needs to be placed on the fundamental understanding of materials rather than on applied science and product development. Naturally, application of materials is the ultimate goal, but this needs to be built on a firm theoretical basis so that improvements can be made more efficiently and reliably. Particular attention should be given to understanding a material's behavior from the atomic/nano-level via microstructure to macrostructure levels using advanced analytical techniques and computer modeling . . ." [EuropeanWhite Book on Fundamental Research in Materials Science, http://www.mpg.de/doku/wb_materials/wb_materials_010_013.pdf

3. "Nano-materials and Powder Diffraction" Different than for micro-to-macro materials, the major role in nanocrystals is played by surface phenomena which lead to most of the unique properties of nanomaterials. A meaningful understanding and interpretation of those properties has to be based on reliable information about the underlying atomic structure. The basic and essentially only preeminent technique for determination of the atomic structure of matter is diffraction. The complex structure of nanomaterials causes the standard methods of structural analysis to be of limited use so that non-standard and new approaches and tools for structural analysis are necessary. Iuliana Dragomir-Cernatescu & Robert L. Snyder will describe XRD techniques and instrumentation for specifically designed for nano-materials characterization. Relative to standard crystalline materials a nanocrystal is not a single crystal: (i), its structure is different in the bulk than at the surface, and (ii), the grain size limits long-range interactions to the characteristic dimension(s) of the grain only. (In other words, each nanocrystal constitutes a unique object.) For the above reasons, the model of an ideal crystal lattice cannot approximate the structure of a nanocrystal. In practice that means, that any structural analysis of such materials has to be based on “total scattering”, which comprises both coherent an incoherent components. This topic will be covered by Simon Billinge. Certainly, it is not always necessary to examine total scattering since useful information about the structure of nanocrystals can be obtained from the analysis of characteristic Bragg scattering also. It should, however, be realized that such information is definitely incomplete since it refers only to coherently scattering part of the crystal. No specific criterion for selection of the optimum type of radiation source for diffraction experiments on nanomaterials can be recommended, since each one has its advantages and limitations. The application of the less commonly used ones, electrons and neutrons will be presented by Stavros Nicolopoulus and Thomas E. Proffen, respectively. A particular role in investigating the structure of nanomaterials is played by low angle scattering (Arnt Kern) and determination of the grain size distribution in the sample (Paolo Scardi). Those studies are very helpful to find the characteristic nano-dimensions of nanomaterials. As it turns out those techniques, when applied to nanocrystals, require a non-standard approach. An alternative to powder diffractometry is diffraction on a single grain. Unfortunately, the radiation sources available today allow for such an analysis of grains of about 100 nm in size or larger only (Ian Robinson). We may expect that a real breakthrough in characterization of nanocrystals will come with the X-ray laser. Such an excellent research tool may be expected to become available in a few years from now (Jacek Krzywinski). We should start getting prepared for such a research opportunity today! It is quite natural that, working in the field of experimental powder diffraction, we are well aware of the shortcomings imposed by diffraction methods, both by their technical limitations and by the available methods of elaboration of the experimental data. Those limitations affect our ability to set novel tasks and envision new research horizons. Therefore it is worthwhile to go beyond those limitations and embrace a different point of view, where “impossible is possible”. Numerical simulations bring about such opportunity, i.e. a virtual experiment without the technical limitations of experimental reality. These techniques have a great potential and their application to nanomaterials is on the rise. It can be a tremendous inspiration for us, experimenters, and a great opportunity for the modelers to verify their theoretical predictions. The potential of MD simulations will be presented by Izabela Szlufarska. We believe that the above outlined program of the workshop will provide a good insight into the current state-of-the-art and the future of structural investigation of nanocrystals.

4. Introduction: Structure of nano-crystals as the key to understanding the unique properties of nano-materials Bogdan PALOSZ Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences 01 142 Warszawa, ul. Sokołowska 29/37 A very fast progress in nanomaterials' technologies in the last 20 years is stimulated mostly by the perspectives of immediate applications. The need for basic research in this field has been somehow overlooked, although it is obvious that for very small objects the knowledge about the structure at the atomic level is the key to understanding the materials behavior. The interest in such studies is rapidly increasing but standards for characterization of these types of materials have not been established yet. Labeling a group of materials as "nano" is justified if their properties are unique in the sense that they cannot be described or explained based on the rules (theories, formulas, etc.) which apply to "ordinary" materials. Distinction between "nano" and "non-nano" materials emerged and became common only in the last two decades, and that happened not because "nano-materials" did not exist before, but because the scientific community always demands new research directions which are worth to concentrate on and which require solving specific problems. Development of technologies of fabrication of nano-materials and of the methods of their characterization reached the point where examination of nano-sized objects became possible. This concerns in particular the resolution/accuracy level required for (i), technologies of the synthesis of objects with well controlled dimensions in the range of a few nanometers; (ii), direct observations of such nano-objects; and (iii), measurements of the materials properties associated with their small volume and/or physical dimensions. One of the key problems in "nanoscience" is designing appropriate experiments to measure specific size-dependencies of physical properties, what is necessary for understanding the origin of differences in the properties between nano- and larger crystals. Nanocrystal is a unique "piece of material" with its individual atomic structure and properties which are closely related to its characteristic dimensions. Nanoscience begins when properties of the material can no longer be described using conventional tools, in particular using relationships derived for ordinary materials. In other words, we deal with nanoscience when the property is clearly related to the small characteristic dimension(s) of the object. This statement is valid for any nano-property of materials, and it should also concern application of diffraction methods. As a matter of fact, typical problems which one faces searching for experiments "adequate" for nanocrystals and for proper methods of description and elaboration of the experimental data are well demonstrated in diffraction experiments which serve for structural studies on solids. Evaluation of powder diffractograms is usually done using standard numerical procedures (like the Rietveld program which is used for refinement of structural parameters and provides satisfactory results for conventional-size materials). However, all such methods have an inherent limitation which follows from the fact that they are based on the assumption that the material under examination forms an ideal, infinite lattice. A nano-crystal cannot be regarded as a single crystal due to, (i), its limited dimensions, and (ii), a large fraction of atoms associated with the grain surface which have different environment than the interior atoms. Because the assumption of a uniform crystal structure of nano-materials is not justified, application of routine procedures of collection and elaboration of diffraction data may lead to misinterpretation of the experiments and to incorrect conclusions about the atomic structure of nanocrystals. Tentative methods of elaboration of powder diffraction data of nanocrystals using reciprocal- (Bragg) and real-space (PDF) analysis, which permit for overcoming the limitations of conventional methods of elaboration of powder diffration data due to nano-dimensions and non-uniform structure of nano-particles, will be discussed.

5. nano- dimension and nano-material layers quantum dots polycrystals lamellas nano-crystals

6. ? is(why) a nano-crystal(not) a small single crystal ? ?

7. the dimension of a diamond nano-crystal is only several times larger than that of the unit cell

8. what is a single crystal ? a lattice is an array of points in space in which the environment of eachpointis identical the unit cell of the lattice and the motif thereof define the whole pattern or structure the number of unit cells in a crystal is infinite!

9. criteria of selection of „nano-projects” (NSF) • it concerns materials with (at least one) characteristic dimension • smaller than100 nm • and, simultaneously, • the expected result of the project is finding a material property which is • closely connected withnano-dimension • providing that • 3. conditions of fabrication of the material are reproducible

10. Navrotsky, 2001

11. Internal Energy (per mole) Enano-crystalEsingle-crystal (smalllarge : n)

12. unique properties of nano-crystals ? dimension surface area density melting temperature phase transformation bond strength interior atomic structure surface atomic structure internal pressure internal strains energy band gap luminescence hardness plasticity etc. effect origin origin origin effect effect origin effect of an individualnano-crystal ? an assembly ofnano-crystals ?

13. searching for novel materials’ properties ..... nano- materials „ordinary” materials nano-science new qualities

14. unique „diffraction properties” of nano-crystals ?

15. nano- materials „ordinary” materials nano-science new qualities crystalline ! ?

16. Calculated, neutron diffraction the same integral intensity

17. dimension(s) of a nano-crystal ? size ?

18. ZnS TEM

19. number of particles & grain boundaries volume in a unit weight of a nano-material from micro- to nano-scale Vgb= Ps x hs

20. tentative classification of materials based on surface area (effect of surface on phase stability) mono- nano- G = G(p,T,surface area, +environment) Navrotsky Reviews in Mineralogy % Geochemistry vol.44 (2001)

21. size effect – which size ? log-normal grain size distribution volume abundance a little change of (volume) grain size distribution may mean a very strong change of relative abundance of grains with specific sizes

22. model of a nano-crystal ? [111]

23. type of bonding < metallic < ionic < covalent > molecular clearly: bulk and surface atoms are not equivalent

24. what one observes by TEM ? ? ? ? ?

25. urgently needed: an atomic model of a nano-crystal ? available only in a virtual experiment e.g. by(MD) Simulations

26. the structure of the surface is always different than the bulk TiO2 theoretical MD simulation

27. MD simulation ZnS, 2r = 4 nm [111] starting model (a perfect lattice) calculated model Marcin Wojdyr, UNIPRESS

28. core shell a tentative model of a nanocrystal ZnS

29. nano-dimension ? ZnS physical dimension / Scherrer(FWHM)

30. ZnS, 3 nm effect of environment on atomic structure of nanocrystals in vacuum in water (H2O) theoretically calculated (molecular dynamics) 3 nm ZnS crystallites Hengzhong Zhang et al. Nature, 424 (2003)

31. dimension(s) of a nano-crystal ? there are at least two characteristic dimensionsneeded

32. ? the purpose of structural analysis ?

33. Internal energy of a crystal E = E0 + ∫ cv(T,v0) dT - v0∫ (Ta/ - p) dp cv – specific heat(cv = f(qD/T), qD – Debye temperature) - compressibility a – thermal expansion coefficient

34. the primary purpose of structural studies ? examination of inter-atomic potentials through measurements of interatomic distances

35. questions faced by a crystallographer: a crystalline material

36. specific problems of structural studies of nano-crystals „relaxed lattice” ? ? it looks like a two-phase system ! under pressure ? lattice parameters no ! at different temperatures ? B0(core) ≠ B0(shell) BT(core)≠BT(shell) T(core)≠T(shell)

37. questions faced by a crystallographer: a nano-crystalline material core core core shell shell shell

38. questions faced by a crystallographer: a nano-crystalline material & environment ! core core core shell shell shell

39. do not ignore the surface ! core shell this may be a very significant part of the sample volume !

40. ! the Conclusion: a nano-crystalis not a single crystal ......not only to its small size... !

41. ? how to approach the problem of structural analysis of a nano-crystal ?

42. ? ? output input the diffraction data (the reciprocal Q space) a set of parameters: atomic positions physical properties characterizing the model information on the sample (chemical composition, p, T, .. a model !

43. structural information contained in a diffraction pattern

44. PDF Bragg „Bragg” ? „Bragg” ?!

45. relationship between & real space reciprocal space ? ? ? unit cell inter-atomic distances coherent scattering ONLY ! information on long-range atomic order total scattering = „total” structural information

46. ? is Bragg scattering still applicable to structural analysis of nano-crystals ? ?

48. „refined” Lattice Parameters of a nano-crystal .... Rietveld !

49. (spherical particles with a perfect lattice, no strains !) CuK MoK what one measures is “at best” anapparent lattice parameter (alp)

50. limitations of the Bragg equation for nanocrystalline materials