The XRR curve and fit of the data for the sample prepared at 300 K are shown in Fig. 3(a). The data show the rapid decrease in intensity above the critical angle of the substrate (∼0.04 Å–1 in q z), typical for an XRR curve, followed by Kiessig fringes which give information on the thickness of the organic layer. Finally, at ∼0.47 Å–1 in q z, a Bragg peak can be observed which corresponds to the most intense Bragg peak observed in the specular X-ray diffraction measurements of the ∼100 nm thick sample (see Fig. 2). The fit of the data is shown superimposed over the experimental data in red in Fig. 3(a) and the electron density profile generated from the fit is shown in Fig. 3(b). The electron density profile shows regular oscillations which correspond to the periodic nature of the crystal packing perpendicular to the substrate surface, again, indicative of a well ordered film. The lower electron density of the first layer suggests that it is less ordered, and a better ordered film develops as subsequent layers are added. The decreasing electron density at higher z values (>60 Å) arises from the surface roughness of the film. The distance, d, determined from these oscillations gives a value of ∼12 Å for the thickness of one molecular layer. This again suggests approximately upright-standing molecules, though it is not clear which of the two standing phases observed in the specular X-ray diffraction data is present in the thinner films. The shape of the electron density plot for layers above the first layer suggests that the curve could be the result of a superposition of the electron density profiles for the two different phases; this would also explain the slightly irregular peak shapes (e.g. for the third layer, marked by an asterisk in Fig. 3(b)). The influence of the individual phases cannot be ascertained from the electron density plot, however, from the specular diffraction data of the thicker film (Fig. 2), the peak corresponding to the less tilted phase is more intense. The XRR curves of the films prepared at 200 and 350 K do not show the same degree of densely packed periodic ordering perpendicular to the substrate and have a more disordered structure (see Fig. S2 in the ESI†).