X-ray diffraction and Crystal Structure Solutions from Thin Films

Transcription

1 X-ray diffraction and Crystal Structure Solutions from Thin Films Ingo Salzmann Humboldt-Universität zu Berlin Institut für Physik

2 Overview Experimental technique X-ray diffraction The principal phenomenon Specular x-ray diffraction Grazing incidence x-ray diffraction (GIXRD) Reciprocal Space Mapping (RSM) Practical: unit cell determination Some results Pentacenequinone (PQ) thin-film phase Perfluoropentacene () thin-film phase Mixed films of with pentacene () Correlation of structure and energetics: Tuning the ionization energy of mixed films PQ

3 X-ray diffraction Diffraction of electromagnetic radiation Elastic scattering:, wave vectors Vector of momentum transfer: net planes (hkl) Is oriented perpendicular to the scattering net planes (hkl) Bragg s law Path difference: Constructive interference: lattice spacing

4 X-ray diffraction experimental techniques Specular x-ray diffraction (XRD) D Substrate Investigation of periodicities perpendicular to the sample surface

5 X-ray diffraction experimental techniques Specular x-ray diffraction (XRD) D Investigation of periodicities perpendicular to the sample surface Position of the Bragg-peaks: Lattice spacing Integral breadth of the Bragg-peaks: Microstructure information (size, strain) Laue-oscillations: coherently scattering film thickness Interference pattern at low q z : film thickness, roughness and electron density

6 X-ray diffraction experimental techniques Grazing incidence x-ray diffraction (GIXRD) Substrate Investigation of periodicities parallel to the sample surface Enhanced surface sensitivity due to below angle of total reflection

7 X-ray diffraction experimental techniques Grazing incidence x-ray diffraction (GIXRD) Substrate Investigation of periodicities parallel to the sample surface Enhanced surface sensitivity due to below angle of total reflection Increase of detector angle α f out-of-plane periodicities accessible Mapping of the reciprocal space in terms of tuples q, q

8 Beamline HASYLAB W1

9 X-ray diffraction experimental techniques Reciprocal Space Maps (RSM) Point-detector to record intensity at detector angles (δ, ν) Calculation: (δ, ν) (q, q ) = Reciprocal Space Map Structure solution of thin film structures From (q, q ) d hkl of the reflection (h, k, l Miller indices) d hkl is a function of unit cell parameters a, b, c, α, β, γ From lin. independent h, k, l of 6 reflections 6 equations for a, b, c, α, β, γ But: Indexing of the peaks is a-priori unknown!

10 RSM finding the unit cell parameters Orientation (specular peak) Initial guess of unit cell parameters Manual variation of unit cell parameters for visual comparison

11 RSM finding the unit cell parameters Orientation (specular peak) Initial guess of unit cell parameters Manual variation of unit cell parameters for visual comparison Final step: Orientation of the molecules in the unit cell directly (from intensity values) or indirectly through force field calculations

12 Overview Experimental technique X-ray diffraction Principal phenomenon Specular x-ray diffraction Grazing incidence XRD Reciprocal Space Mapping Crystal structure determination Some results Pentacenequinone (PQ) thin-film phase Perfluoropentacene () thin-film phase Mixed films of with pentacene () Correlation of structure and energetics: Tuning the ionization energy of mixed films PQ

13 Pentacenequinone (PQ) thin-film phase Specular x-ray diffraction PQ on SiO x Atomic Force Microscopy (AFM) (020) of the single crystal phase 30 nm nominal thickness Polymorphism of PQ: single crystal phase (known) and thin-film phase (unknown) (00l)-series of thin-film phase: d = 1.31 nm AFM: Polymorphs show different morphology Thin-film phase shows island growth with steps of 1.4±0.2 nm standing molecules

14 Perfluoropentacene on SiO x Specular x-ray diffraction Atomic Force Microscopy (AFM) Log(Intensity) (arbritrary units) (001) (100) (002) (200) (003) (004) (300) (400) 30 nm nominal thickness 0.5 μm q z (Å -1 ) Pure / films: and : standing molecules : unknown thin-film polymorph Inoue, Sakamoto, Suzuki, Kobayashi, Gao, Tokito. Jpn. J. Appl. Phys. 44:3663, Salzmann, Duhm, Heimel, Rabe, Koch, Oehzelt, Sakamoto, Suzuki. Langmuir 24:7294, 2008.

15 Perfluoropentacene on SiO x Specular x-ray diffraction Atomic Force Microscopy (AFM) Log(Intensity) (arbritrary units) (001) (100) (002) (200) (003) (004) (300) (400) 30 nm nominal thickness μm q z (Å -1 ) Pure / films: and : standing molecules : unknown thin-film polymorph + + (1:1) mixed film: mixed crystal structure 2 orientations (oder polymorphs) Inoue, Sakamoto, Suzuki, Kobayashi, Gao, Tokito. Jpn. J. Appl. Phys. 44:3663, Salzmann, Duhm, Heimel, Rabe, Koch, Oehzelt, Sakamoto, Suzuki. Langmuir 24:7294, 2008.

16 Perfluoropentacene thin-film phase Single crystal phase: monoclinic P2 1 /c RSM: a = Å b = 4.49 Å c = Å α = 90 β = γ = 90 Z=2 V = Å 3 Inoue et al. Jpn. J. Appl. Phys. 44: 3663, Thin-film phase: monoclinic P2 1 /c a = Å b = 4.51 Å c = Å α = 90 β = 90.4 γ = 90 Z=2 V = Å 3 Polymorphs are very similar Molecules more upright standing in thin film phase Salzmann, Duhm, Heimel, Rabe, Koch, Oehzelt, Sakamoto, Suzuki. Langmuir 24:7294, 2008.

17 Correlation of structure and energetics Orientation dependence of the ionization energy (IE) Intramolecular polar bonds (IPBs): Approximation with point charges 1 Molecule E.g.: α-sexithiophene flat lying molecule (L): π-system (δ - ) standing molecule (S): hydrogen termination (δ (+) ) IE of the films different Duhm, Heimel, Salzmann, Glowatzki, Johnson, Vollmer, Rabe, Koch, Nat. Mater. 7:326, 2008.

18 Correlation of structure and energetics Orientation dependence of the ionization energy (IE) Intramolecular polar bonds: Approximation with point charges Intramolecular polar bonds of and C[δ ]-H[δ + ] and C[δ + ]-F[δ ]: & E.g.: α-sexithiophene Top view View along the long molecular axes flat lying molecule (L): π-system (δ - ) standing molecule (S): hydrogen termination (δ (+) ) Different IE of the films Mixing of components with different dipolar termination? Can the IE be tuned via the mixing ratio? Duhm, Heimel, Salzmann, Glowatzki, Johnson, Vollmer, Rabe, Koch, Nat. Mater. 7:326, 2008.

19 Correlation of structure and energetics Specular x-ray diffraction Fourier Transform Infrared spectroscopy (IR) Log(Intensity) (arb. units) Absorption (arb. units) C-F C-H C-H C-F Mixed + films: 2 orientations/polymorphs standing, lying Davydov-splits (D1, D2) in pure -film Vanish in mixed films! Mixing on molecular length scale Surface area dominated by film portion comprising standing molecules Salzmann, Duhm, Heimel, Oehzelt, Kniprath, Johnson, Rabe, Koch, J. Am. Chem. Soc. 130:12870, 2008.

20 UV-photoelectron spectroscopy (UPS) Principal setup photon spectrometer sample Surface sensitivity 0.5 nm Secondary electron cutoff (SECO): Determination of the IE Valence electron region: Emission from the highest occupied molecular orbital (HOMO)

21 Intramolecular polar bonds Ultraviolet-photoelectron spectroscopy results (UPS) Photoemissionsintensität (arb. units) S L L S S Standing molecules on SiO 2 L Lying molecules on gold Ionization energy (ev) Bindungsenergie (ev) lying/standing: ΔIE = ev lying/standing: ΔIE = ev IE can be tuned via the mixing ratio between the maxima of the pure films Koch, Vollmer, Duhm, Sakamoto, Suzuki, Adv. Mater. 19:112, Salzmann, Duhm, Heimel, Oehzelt, Kniprath, Johnson, Rabe, Koch, J. Am. Chem. Soc. 130:12870, 2008.

22 Intramolecular polar bonds Large-range phase separation Phase separation on molecular scale Individual vacuum-level alignment Collective electrostatic potential in distance d Salzmann, Duhm, Heimel, Oehzelt, Kniprath, Johnson, Rabe, Koch, J. Am. Chem. Soc. 130:12870, 2008.

23 Intramolecular polar bonds - simulation 60 z (a.u.) 0 +z distance d: fluctuations of potential < 20 mev (exp. Limit) D = 4 d d E pot (ev) SiO x d d SiO x V(r) -2 ln(r) E pot = -q e V -z Individual vacuum-level alignment Collective electrostatic potential in distance d

24 Acknowledgements Humboldt-Universität zu Berlin Steffen Duhm Georg Heimel Jürgen P. Rabe Norbert Koch JK-Universität Linz Martin Oehzelt Technische Universität Graz Roland Resel Montanuniversität Leoben Dmitrii Nabok Peter Puschnig Universität Hamburg / DESY Robert L. Johnson Financial support: Sonderforschungsbereich 448 (DFG) Emmy-Noether Programm (DFG)