The final sample, prepared at a substrate temperature of 300 K (Fig. 4, middle), again shows a different behavior. The peaks observed in the specular diffraction pattern (Fig. 2) are not observed here (regions close to q xy = 0 cannot be observed in GIXD reciprocal space maps), suggesting reduced film mosaicity (or a stronger preferred orientation) compared with the other samples. A strong rod-like feature is present at q xy ≈ 1.9 Å–1 (d = 3.3 Å), showing evidence of a disordered layer structure with molecules approximately upright-standing on the substrate surface, similar to the 350 K sample. However, in the case of the samples prepared at 300 K, Bragg peaks can also be observed at q xy values of ∼1.1 and 1.6 Å–1 and various q z. These Bragg peaks show that a three-dimensionally ordered species is also present with a preferred orientation on the substrate. Using the unit cell parameters from the previously determined structures of the 2,8-disubstituted chlorinated and fluorinated EPI derivatives (2F-EPI and 2Cl-EPI; CSD codes DUXYIG and DUXYOM, respectively)11 as a starting point (chemical structures are given in Fig. S3 of the ESI†), the Bragg peaks can be indexed and the unit cell parameters for EPI in thin films can be determined. The unit cell is given in Table 1, with EPI adopting a 001 orientation (the 001 lattice plane parallel to the substrate) in the 300 K thin film. This orientation of the unit cell means that EPI molecules are approximately upright-standing on the substrate surface, in agreement with the specular X-ray diffraction and XRR data.