Dr. Q. Xu (Qianq)

Postdoc

Room: B 216
Phone: + 31 15 27 81201
E-mail: q.xu@tudelft.nl

Research interest

Electron crystallography (solving unknown structure using electron microscopy); Dynamical scattering effect; HEELS; Applying aberration corrected electron microscopy in valence electron distribution.

Selected paper

J.H. Chen, E. costan, M.A. van Huis, Q. Xu, H. W. Zandbergen “Atomic Pillar-Based Nanoprecipitates Strengthen AlMgSi Alloys” Science 21 312. 416 2006

Qiang Xu, Tomasz Klimezuk, Jacob Jansen, Robert J.Cava, and Henny W. Zandbergen, “Structure and Magnetic Properties of Eu2CaCu2O6”, Chem. Mater. 18 (2006), 4585.

Qiang Xu, Tomasz Klimczuk, Ton Cortnmulder, Jacob Jansen, Robert J.Cava, and Henny W.Zandbergen, “Ab initio structure determination of novel superconductor Mg10Ir19B16”, Chem.Mater.21(12),2499 2009

Arnold, M, Xu, Q & Tichelaar, FD “ Local charge disproportion in a high-performance perovskite.” Chem. Mater., 21(4), 635-64, .2009

Qiang Xu, Dirk van Dyck H.W.Zandbergen “From thickness dependent exit waves to projected potential: Thickness derivative approach”, Ultramicroscopy, accepted, available online, 2009

Q. Xu, D.V.Dyck, H.W.Zandbergen, “Towards direct imaging electron distribution”, to be submitted to Phys.Rev.Lett. (2010)

Selected image Galley

Figure 1: Experimental Reconstructed Exit Wave obtained from Philips 30 UT: amplitude (a); phase (b); Projected structure, estimated by Direct methods (MICE) (c); Simulated projected potential (d) and exit wave (amp) for the thickness of 1nm (e); 9nm (f); 25nm (g); 39nm (h) respectively; Display of the structure (with Mg as big light blue ball, Ir, big orange and B, small purple. (j). Experimental reconstructed exit wave (amplitude and phase) shows the structure of heavy atoms (Mg and Ir), providing a basis to choose the best estimated projected potential (c), among multi-solutions obtained from MICE. This estimated projected structure provides the structure information with higher resolution than REW, but it denotes the exit wave, instead of the projected potential (reflects projected structure only), thus, still thickness dependent. Note that some extra small peaks exist, not corresponding to any atoms. Comparing to simulated exit wave (f), these extra peaks are actually induced by bonding effects (marked with white cross). Dynamical scattering enhances these bonding effect as evident as B atoms. These bring the difficulty for retrieving light atoms structure directly from (C).

Figure 2: Symmetry determination of Nb12O29 using Convergent Beam Electron Diffraction. (J.Sol.St.Chem. 18, 2864,2007)

Figure 3. Evolution of the electron exit wave as function of thickness series simulated for a hypothetic structure model (See Table 1 for structure details). The model, contains seven types of atoms with increasing atomic numbers. The distance between with the neighboring atom distance 8 Ǻ, as shown in (a). The simulation of the EW was calculated for thicknesses in the range from 10 to 800 Ǻ with stepsize 1 Ǻ and shown in (b). The amplitude of the exit waves were aligned with increasing thickness to create a 3-d map (c), which indicates how the electron wave propagates through the sample. It can be observed that most of electrons propagate by channeling in the atom columns, being periodically focused and defocused by the potential of the atoms. The cross-section of the 3-d map at b=0.5 (d) demonstrates the channeling effect in a two dimensional way. It can be easily seen that for light atoms B only the 1s state is excited for the whole thickness range whereas for the other heavy atoms, the 2s state or even other higher states are excited. The projected potential and several exit waves at thicknesses z=5, 50,150, 400 Ǻ are respectively shown in a 1D graph (e-j) It can be noted that only when sample is extremely thin (z=5 Ǻ), the peak positions in (f) correspond to the atom positions and the height (intensity) of peaks is proportional to the corresponding projected potential (e); but for other thicknesses (z=50, 150, 400 Ǻ), the straightforward relation between the peak intensity of exit wave and the object (project potential) is lost. Furthermore, at certain thicknesses, the 1s state of some atoms (for instance for Nb at z=50 Ǻ, Ge at 150 Ǻ, Ti at 400 Ǻ), might be less excited than their 2s state such that double peaks will be observed on the both side of the atom positions, which destroys the straightforward relationship between the peak position shown in the exit wave and the corresponding atom positions.

Figure 4. Thickness derivative exit wave of the same hypothetical model as in Figure 3 was simulated for thicknesses 10 to 800 Ǻ, with step-size 1 Ǻ using with =10Ǻ . (a) shows the simulated TDEW in a two dimensional representation similar to that in Figure 1d. In (a) the TDEW shows much less thickness dependency as compared to the EW in Figure 1d, though slight variation can be still visualized on the edge of the heavy atom columns (Nb, Sb, Eu) in (a) due to the relatively strong exited high states. The projected potential, TDEW at thicknesses (z=5, 50, 150, 400Ǻ) are respectively shown in (b)-(f) one dimensionally. Note that all the peak positions and intensities in all TDEW keep the simple relation to the corresponding projected potential. Note that the peaks of light atom B Mg showing in TDEW are broader that those in projected potential.

Figure 5: Interpretation of high resolution Image formation in Electron microscope. The formation of image in electron microscope can be classified into two steps of information transmission through sub-channels. In the first step, the information of electron density of an object is transferred into an exit wave through multiple scattering processes. The multiple scattering processes act as a series transmission sub-channels. Each channel transfers the information of one certain electron state with a rate dependent on the corresponding energy of the transferred electron state and sample thickness. In the second step, the information of exit wave is transferred into image waves through electron optic system. This system acts as a series transmission sub-channels related to spatial frequencies, which has been well interpreted. In such a way, the information of electron density has been coded into an image in a complicated way, however, can be retrieved back by applying approximate information processing techniques. (in “Towards direct imaging electron distribution”)

Figure 6: Aberration corrected electron microscopy of Nb12O29. Oxygen atoms can be characterized in a high accuracy (See arrowed pointed oxygen positions).

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