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Characterizing nanomaterials with XRD
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X-ray diffraction (XRD) is a powerful method for the study of nanomaterials (materials with structural features of at least one dimension in the range of 1-100 nm). Nanomaterials have a characteristic microstructure length comparable with the critical length scales of physical phenomena, giving them unique mechanical, optical and electronic properties. X-ray diffractograms of nanomaterials provide a wealth of information - from phase composition to crystallite size, from lattice strain to crystallographic orientation. The Georgia Institute of Technology's Center for Nanoscience and Nanotechnology (Georgia Tech; Atlanta, Georgia, USA) is well equipped for nanomaterial research with PANalytical X'Pert PRO MRD, X'Pert PRO MPD and Alpha-1 systems, each used for particular applications. Growth orientation of ZnO nanobelts Nanobelts are single crystal, defect-free, shape-specific semiconductors requiring no protection against oxidation - properties that have many potential applications. At Georgia Tech, the X'Pert PRO MRD was used to study the growth orientation of nanobelts with respect to the single crystal Al2O3 substrate. Professor Z L Wang's research group, working with ZnO nanobelts, has shown that under certain conditions these can be grown in different crystallographic orientations with specific dimensions and shapes. Figure 1 shows the TEM image of a typical ZnO nanobelt. Figure 1: TEM micrograph of a ZnO nanobelt. The sphere at the tip of the nanobelt is the Au catalyst used in this specific study. Figure 2 shows nanobelts with a definite preferred growth orientation, with respect to a single crystal substrate. The Fourier transform of the SEM micrograph (Figure 3) confirms the XRD findings. While microscopy techniques and XRD are complementary, in this study XRD has the advantage of being able to sample a large volume of material, providing an average representation of the microstructure. Figure 2: XRD pole figure of 0002 Figure 3a: SEM micrograph of ZnO nanobelts Figure 3b: Fourier transform of SEM micrograph Studying the defects of nano-structured metals A high-resolution diffractometer such as the X'Pert PRO Alpha-1 may not be an obvious choice for studying dislocation type and density evolution in nano-structured Cu by deformation at liquid nitrogen temperature, considering that materials with small crystallite size will have correspondingly broad, low intensity diffraction peak profiles. However, for some materials, broadening may be due to more than just crystallite size and the Alpha-1's ability to eliminate the redundant Ka2 contribution to the profile means less error in the final analysis. Figure 4 shows crystallite size distributions at four levels of deformation by rolling copper under liquid nitrogen temperature. The total dislocation density and types of dislocations were then extracted from the XRD line profile analysis (figure 5). Studies like these help researchers understand the unique mechanical properties of nano-structured metals. Figure 4: Crystallite size distribution at different deformation levels Figure 5: Dislocation character and densities in deformed nano-structured Cu as a function of deformation In situ high temperature analysis of 3D nanostructures Georgia Tech's X'Pert PRO MPD is fitted with an HTK1200 oven - an in situ diffraction tool used in this case to convert silica-based diatom nanostructures to materials such as MgO, while preserving their complex shapes. Diatoms are single-celled aquatic microorganisms, which assemble complex silica microshells (frustules) containing channels, pores or other intricate features. Although the sizes of diatoms vary, typical frustule dimensions are around 100 micrometers. Such 3D assemblies of magnesia nanocrystals could have agricultural, pharmaceutical, petrochemical, environmental and structural applications. Diffractograms collected every five minutes during the isothermal annealing process document the progress of the chemical conversion of the diatoms in the presence of Mg vapor (Figure 6). Silica diatom Diatom converted to MgO Figure 6: X-ray diffraction patterns collected during annealing Many thanks to the researchers at Georgia Tech for their assistance with this report: ZnO nanobelts: R. Yang, I. Dragomir-Cernatescu, Z.L. Wang and R. L. Snyder MgO Nanodevices: M. S. Haluska, I. Dragomir-Cernatescu, K. H. Sandhage and R. L. Snyder Deformed Nano-structured Cu: I. Dragomir-Cernatescu, M. Gheorghe, N. Thadhani and R. L. Snyder More detailed descriptions of the above studies can be found in: 1. Dragomir-Cernatescu I., Gheorghe M., Thadhani N. and Snyder R. L.: "Dislocation densities and character evolution in copper deformed by rolling under liquid nitrogen from X-ray peak profile analysis"; Powder Diffraction pp. 109-111, 20(2), (2005). 2. M. S. Haluska, I. Dragomir-Cernatescu, K. H. Sandhage and R. L. Snyder: "X-ray diffraction Analysis of 3-D MgO Diatom Replicas Synthesized by Low-Temperature Gas/Solid Displacement Reaction"; Powder Diffraction pp. 306-310, 20(4), (2005). This is an X'Press article. Please see the overview of all X'Press articles of this issue. http://www.panalytical.com/index.cfm?pid=866&itemid=146&contentitemid=329 |
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