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Pre-Nucleation Dynamics of Organic Molecule Self-Assembly Investigated by PEEM
(Result of the month 06/2010) |
Understanding the pre-nucleation dynamics of molecules will be important for the controlled growth of highly ordered molecular structures for organic electronics. Sexiphenyl (6P) molecules (C36H26) are linear conjugated oligomer chains of six phenyl rings that electroluminescence blue light. These molecules are prototypical molecules for organic electronics and are ideal candidates for the study of controlled growth. 6P molecules are known to assemble into ordered anisotropic crystallites when deposited on a surface, however until now the pre-nucleation kinetic and energetic mechanisms that control growth have not been fully determined.
Here it will be demonstrated that the pre-nucleation dynamics of 6P, when deposited on Cu (110) and Cu (110) 2 x 1 – O anisotropic substrates in UHV, can be effectively observed and measured by PEEM [1]. The in-situ deposition and real-time image acquisition by PEEM allows metastable layer formation, dynamic layer reconstructions, spontaneously induced layer dewetting and the anisotropy of molecular surface diffusion to be quantitatively measured by PEEM. Comparison of area-averaged photoemission intensity versus deposition time curves for both substrates indicates that more 6P molecules must be deposited on Cu (110) to initiate 6P critical nuclei formation. 6P deposited on Cu (110) fills three layers: the first is a flat-lying permanent wetting layer and the second/third layers together form a double metastable tilted-lying-down layer that partially dewets upon critical nucleation to form a permanent second layer reconstruction.
On the other hand, 6P deposited on Cu (110) 2 x 1 – O fills two layers with tilted-lying-down molecules: the first layer is a permanent tilted wetting layer and the second is a meta-stable layer that dewets upon critical nucleation. Despite clear differences in layer filling and crystallite alignment/geometry (6P (20-3) versus 6P (21-3)), critical nuclei formed on both substrates are demonstrated to be (20-3) rectangular unit cell 6P crystallites. These rectangular unit cell crystallites grow into long (> 20µm) straight needles on the tilted wetting layer on Cu (110) 2 x 1 – O, however on top of the flat lying wetting layer on Cu (110), the spontaneous partial dewetting of the double metastable tilted layer on Cu (110) relaxes by forming an energetically favourable reconstructed permanent second layer that includes tilted and flat-lying oriented molecules.
This mixed orientation layer, confirmed by STM, facilitates the change in growth from rectangular (20-3) to oblique (21-3) unit cell 6P crystallites during dewetting. The (21-3) crystallite contact plane is more energetically stable since it also has a mixed molecular orientation. These differences in molecular orientation within a layer are clearly observed when the temperature dependence of the surface diffusion anisotropy of 6P molecules on top of the reconstruction (6P deposited on Cu (110)) and tilted wetting layer (6P deposited on Cu (110) 2 x 1 – O) are compared. In the former, diffusion is anisotropic, whereas in the latter case it changes from 1D to pseudo 2D with increasing temperature. In conclusion, PEEM demonstrably allows kinetically and energetically driven mechanisms of pre-nucleation and molecular self-assembly to be distinctly observed and measured.
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PEEM images of 6P crystallites grown in-situ on Cu (110) (left image) and Cu (110) 2 x 1 – O (right image) acquired with an Omicron IS-PEEM instrument with a camera acquisition time of 500ms and 2Hz repetition rate under Hg lamp (4.9 eV) illumination. Scale bar and Cu (110) crystal direction are indicated. Insets: Schematic top-view of 6P (21-3) and 6P (20-3) crystallites grown on Cu (110) and Cu (110) 2 x 1 – O, respectively. Note on both substrates the molecules align along the surface corrugation. |
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Highly ordered uni-axially aligned molecular crystallites form when 6P (also known as hexaphenyl and p6P) molecules are deposited on Cu (110) and Cu (110) 2 x 1 – O. The templating of the first molecular layer by the surface corrugation into an ordered and aligned layer, results in anisotropic crystallite growth and anisotropic surface diffusion of molecules. Straight (20-3) needles form when 6P is deposited on Cu (110) 2 x 1 – O, whereas crossed (21-3) needles form when 6P is deposited on Cu (110). The binding/sticking anisotropy of plano-linear 6P molecules results in a preferred co-facial stacking of 6P molecules. Hence upon stacking, crystallites grow perpendicular to the molecular orientation within the molecular crystal. |
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Plots of area-averaged photoemission intensity versus normalised deposition time. Time starts when the molecular beam shutter is opened (the shutter remains continuously open). Peak maxima indicate the completion of the permanent 6P wetting layers, where the time ordinate is normalised to this time. For 6P deposited on Cu (110) (coloured orange) and Cu (110) 2 x 1 – O (coloured blue) the first layers are flat-lying and tilted lying-down, respectively. Kinks in the plot indicate a change in layer density (which arises from in changes in molecular tilt angles) or layer completion. An example of the former can be found at 0.2 ML for 6P deposited on Cu (110) 2 x 1 – O which indicates the re-arrangement of molecules from flat-lying to tilted lying-down orientation. An example of the latter can be found at 2.4 ML (6P deposited on Cu (110)). Cusps in the plot at 2.0 and 3.8 ML indicate the time at which critical nucleation spontaneously induced layer dewetting occurs. This coincides, in both cases, with the completion of the uppermost layer filling and the appearance of 3D crystallites. Note that due to the different molecular orientations in the wetting layers, a tilted lying-down layer is a factor 1.4 denser than a flat-lying layer (i.e. when comparing the normalised plots note that the second and third layers of 6P on Cu (110) have the same density as second layer of 6P on Cu (110) 2 x 1 – O). Insets: PEEM images showing metastable layer dewetting, after two tilted layers of 6P are deposited on Cu (110) 2 x 1 – O, and metastable double tilted layer partial dewetting, after 3 layers (one flat, two tilted) are deposited on Cu (110). |
Photoemission intensities from Cu (110), or Cu (110) 2 x 1 – O surfaces increase when 6P molecules are deposited since the layer of dielectric material reduces the surface dipole component of the workfunction. Minima in workfunction coincide with maxima in photoemission intensity when the first 6P layer is complete at 1 ML. Since no photoemission originates from 6P (its ionisation potential is above the 4.9 eV photons from Hg lamp), 6P molecules deposited after 1 ML only attenuate the photoemission. In PEEM, 6P second (or higher) layer filling and layer completions are observed as decreases and kinks in the intensity versus time plot, respectively (a cusp is observed when spontaneous nucleation induces layer dewetting). |
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The anisotropy of molecular surface diffusion on dewetted surfaces is illustrated by the photoemission intensity versus square-root-of-time plots for various growth temperatures. The diffusion of 6P molecules after dewetting process is complete is used to probe the symmetry of the reconstructed and dewetted surfaces. The anisotropy of molecular surface diffusion is examined by closing the molecular beam shutter. The ensuing rise in photoemission intensity observed is due to a reduction in the surface density as molecules diffuse to crystallites and attach. Molecules diffusing anisotropically in one dimension will give rise to a linear slope when plotted against the square-root-of-time. As can be seen, the one-dimensional diffusion of 6P molecules on the dewetting induced reconstructed layer that forms when 6P is deposited on Cu (110) is temperature independent (left image). However 6P diffusing on top of the single orientation tilted wetting layer (when 6P deposited on Cu (110) 2 x 1 – O) has a temperature dependence that changes the diffusion from 1D to pseudo 2D (right image). This is due the weaker surface corrugation of the tilted layer allowing molecules to hop between rows in a pseudo 2D diffusion at elevated temperatures. This difference strongly indicates that the reconstructed layer is not a single orientation tilted layer. Instead the reconstruction must have larger surface corrugation features that act as effective 1D transport channels to the nuclei. These conclusions about the nature of the reconstructed surface are confirmed by STM measurements. |
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Authors:
A. J. Fleming(1), F. P. Netzer (1), M. G. Ramsey (1)
(1)Surface and Interface Physics, Experimental Physik Institut, Karl-Franzens Universität Graz, Österreich.
Publications:
A. J. Fleming, F. P. Netzer, M. G. Ramsey, J. Phys: Condens. Matter 21 (2009)
Journal:
Journal of Physics: Condensed Matter
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