Correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. II. Experimental results

Abstract

While the crystal structure of the polymorph phase can be studied in three dimensions conveniently by X-ray methods like grazing-incidence X-ray diffraction (GIXD), the first monolayer is only accessible by surface-sensitive methods that allow the determination of a two-dimensional lattice. Here, GIXD measurements with sample rotation are compared with distortion-corrected low-energy electron diffraction (LEED) experiments on conjugated molecules: 3,4;9,10-perylenetetracarboxylic dianhydride (PTCDA), 6,13-pentacenequinone (P2O), 1,2;8,9-dibenzopentacene (trans-DBPen) and dicyanovinyl-quaterthiophene (DCV4T-Et2) grown by physical vapor deposition on Ag(111) and Cu(111) single crystals. For these molecular crystals, which exhibit different crystallographic lattices and crystal orientations as well as epitaxial properties, the geometric parameters of the three-dimensional lattice are compared with the corresponding geometry of the first monolayer. A comparison of the monolayer lattice from LEED investigations with the multilayer lattices determined by rotated GIXD experiments reveals a correlation between the first monolayer and the epitaxial growth of three-dimensional crystals together with lattice distortions and re-alignment of molecules. The selected examples show three possible scenarios of crystal growth on top of an ordered monolayer: (i) growth of a single polymorph, (ii) growth of three different polymorphs; in both cases the first monolayer serves as template. In the third case (iii) strong lattice distortion and distinct molecular re-alignments from the monolayer to epitaxially grown crystals are observed. This is the second part of our work concerning the correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. In the first part, the theoretical basis has been derived which provides a mathematical relationship between the six lattice parameters of the three-dimensional case and the three parameters obtained for the two-dimensional surface unit cell, together with their orientation to the single-crystalline substrate. In this work, a combined experimental approach of GIXD and LEED is introduced which can be used to investigate the effect of the epitaxial monolayer on the structural properties of molecular crystals grown on top.

Correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. I. Theory

“…Accordingly, a rectangular surface unit cell can be observed in the multilayer (see Table 3). However, as in the LEED experiment the angle 0 is about 89 (Kilian et al, 2004;Simbrunner et al, 2022), the two-dimensional unit cell in the monolayer is not rectangular. This demonstrates that the monolayer structure differs qualitatively from the monoclinic bulk lattice, although the quantitative difference is subtle.…”

“…In a forthcoming paper (Simbrunner et al, 2022) we will check our theoretically derived results by indexing only the x and y components of the reciprocal vectors obtained research papers Table 5 Correlations between the diagonals in the three-dimensional lattice and the parameters of the twodimensional unit cell for the AE(122) and AE(211) orientations in DCV4T-Et2/Ag(111) (Simbrunner et al, 2021a).…”

“…In Table 1, the general equations ( 18)-( 20) for calculating the cell parameters a 0 , b 0 and 0 from the corresponding parameters of the three-dimensional unit cell are applied to the four molecules. The coefficients i and i are chosen such that equation ( 15) is fulfilled for the corresponding contact planes, indicated by the Miller indices u, v and w. As we have shown, the positive (uvw) and the negative (Àu À v À w) orientations of the contact plane correspond to the observed mirror symmetry of the two-dimensional unit cells (Kilian et al, 2004;Simbrunner et al, 2022). If the contact plane is perpendicular to one of the main axes of the Cartesian system, the two-dimensional unit cell is spanned by two vectors of the three-dimensional unit cell.…”

“…Comparing the values of the areas with those in Table 3 [calculated with equation ( 21)] shows their accordance within the experimental error. In the second part of our work (Simbrunner et al, 2022), we will determine the parameters of the surface unit cells experimentally by indexing our GIXD data again, using only the x and y components of the reciprocal-space vectors. We will see that such obtained results are in good accordance with the theoretically predicted data.…”

“…In the second part of our work (Simbrunner et al, 2022), the analysis will be based upon the indexing of the two-dimensional diffraction patterns incorporated in the GIXD data, i.e. the x and y components of the reciprocal-lattice vectors, of the examples listed above, and obtained in LEED experiments on the same molecules.…”