X-ray diffraction shows recrystallization during storage

The crystallographic phases in pentacene films can be discriminated by their (001)-spacing of the molecular layers. For the TFP, the pentacene molecules are oriented almost perpendicularly on the substrate and the (001)-spacing amounts to 15.4 Å, while for the BP it is 14.4 Å9,16,17 as the molecules are slightly tilted, shown in the inset of Fig. 1. The sample studied in Fig. 1 was grown at 65 °C to obtain large crystalline grains. After deposition, X-ray diffraction measurements showed that the film was dominated by TFP, with some weak fraction of BP (Fig. 1, yellow curve) as expected for deposition temperatures above 50 °C5,18. However, surprisingly, the BP fraction was found considerably increased at the expense of TFP (red curve) after the sample had been stored over 20 months in a desiccator at room temperature (23 °C). This observation by itself is an alarming signal for pentacene-based device development and calls for immediate attention.

Figure 1: Recrystallization in highly ordered pentacene film. X-ray diffraction of a 40-nm thick pentacene layer acquired right after fabrication (yellow curve), and after storage at room temperature for 20 months (red curve, vertically offset). The blue and green dashed lines indicate the positions of the (00L) Bragg peaks due to TFP and BP pentacene, respectively, corresponding to their well-known (001)-spacings of 15.4 and 14.4 Å (refs 5, 9). Aging decreases TFP peaks and increases BP peaks. This observation clearly indicates a recrystallization from TFP to BP within the pentacene film. The two different molecular arrangements for TFP and BP, as well as their layer spacing d(001) perpendicular to the substrate are depicted in the inset. Full size image

Infrared nano-imaging reveals elliptic BP inclusions

As the X-ray experiment geometry does not provide lateral information, additional methods are needed to determine the domain geometries and to understand how the phase conversion takes place in detail. An atomic force microscopy (AFM) image of the stored sample is shown in Fig. 2a. The film exhibits a grainy structure on the expected characteristic lateral length scale of 2–5 μm6. On the one hand, there is the argument that larger grains are beneficial for high charge-carrier mobility, simply as any kind of grain boundary will naturally impose a barrier due to defects, lowering the mobility6,7,13. On the other hand, film deposition at increased substrate temperatures to obtain large grains led to diverging results of increased1, almost unchanged11, and even reduced5 carrier mobility. In addition, even for pentacene films deposited at constant substrate temperature, the mobility tends to saturate with increasing grain size12,19. In a similar direction, Shtein et al.20 have reported that in some cases smaller grains allow for higher mobility, while others observed that post-annealing reduces mobility due to structure variation and nucleation of BP pentacene21,22. Unfortunately in the experimental studies, it remains unclear whether apparent individual grains as observed by AFM are single crystals, and in particular, whether and on what scale BP and TFP domains are arranged. Thus, resolving the lateral crystallinity and the organization of BP and TFP pentacene might resolve the conflicting reports on the interplay between growth temperature, apparent grain size and charge transport in pentacene films.

Figure 2: Grain morphology and lateral distribution of two coexisting phases. (a) AFM topography (13.5 μm × 13.5 μm) showing a 40-nm thick pentacene film on SiO 2 /Si substrate, after storage at room temperature for 20 months. (b) s-SNOM amplitude image at 907.1 cm−1, recorded simultaneously, proves the coexistence of two phases of pentacene, which obviously persist across grain boundaries. The dashed square marks the section shown in Fig. 4. Scale bar, 2 μm. Full size image

To map and contrast BP and TFP domains in pentacene thin-film devices, we employ scattering-type scanning near-field optical microscopy (s-SNOM) with mid-infrared illumination, as it is known from far-field measurements that the different packing of TFP and BP pentacene results in a small shift of infrared vibrational frequencies23,24,25. s-SNOM enhances AFM probing by an additional channel of local infrared spectroscopy and should allow imaging the local distribution of both phases with a lateral resolution given by the probing tip diameter of typically 20 nm (see Methods)10. The near-field infrared image of the stored pentacene film shown in Fig. 2b was recorded using mid-infrared illumination at 907.1 cm−1, which is close to the resonance of BP pentacene23. Strikingly, the infrared map is not homogeneous at all but exhibits ubiquitous nanoscale features, at strong contrast, in the form of bright and highly elongated ellipsoids, which, according to the choice of the infrared frequency, originate from BP domains. These domains appear uncorrelated to the topographic morphology recorded simultaneously (Fig. 2a), as they even persist across grain boundaries. Many BP ellipsoids are oriented approximately perpendicularly with respect to their neighbours and appear to stay mutually aligned within larger domains comprising several grains.

Crystalline packing of molecules shifts local infrared resonance

To investigate the spectral origin of this strong infrared contrast, we employ a fully spectroscopic mode of the s-SNOM, recently termed nano-FTIR (nanoscale Fourier transform infrared spectroscopy)26,27 that registers broad near-field spectra at each sample position, here one on a BP ellipsoid and one on TFP next to it (Fig. 3, as marked in the left inset). A Lorentzian fit to each spectrum confirms the two distinctly different vibrational resonances on the BP ellipsoid, peaked at 906 cm−1 (green curve), and on the surrounding TFP, peaked at 904 cm−1 (blue curve). These frequencies agree well with literature values of TFP and BP pentacene23,24. A vertical, dashed line in the spectra of Fig. 3 indicates the specific frequency used for monochromatic imaging in Fig. 2b (907.1 cm−1, CO 2 laser line P08). At this frequency, the nano-FTIR absorption on the BP ellipsoid (green curve, Fig. 3) is more than twice as high compared with the surrounding TFP material (blue curve), explaining the bright contrast observed in Fig. 2b.

Figure 3: Structural phases can be distinguished by an infrared resonance shift. Nano-FTIR spectra (data points) of a 40-nm thick pentacene film on SiO 2 /Si substrate taken at two different positions that are marked in the left inset, taken from Fig. 4b (on ellipsoid—green and off ellipsoid—blue). For each spectrum, 20 interferograms with 5 min acquisition time each were averaged. We chose to record 4.8-mm long interferograms to attain 2.1 cm−1 spectral resolution (interferometer configuration as in ref. 26, Fig. 1). As seen from the Lorentzian fits (curves) to the data, the spectrum taken on the BP ellipsoid (green curve) clearly shows a higher resonance frequency (906 cm−1) than that of surrounding TFP (904 cm−1, blue curve). The CO 2 laser lines (P08, P12) used for monochromatic imaging (Figs 2 and 4) are indicated by vertical dashed lines. The right inset sketches the principle of back-scattering SNOM. Full size image

For a final verification of the optical response of the BP features observed in Fig. 2b, we applied the monochromatic imaging mode of the s-SNOM to a smaller sample area, with both the previous (Fig. 4a–c) and a slightly shifted illumination frequency (Fig. 4e,f). As expected, changing from 907.1 cm−1 to 903.7 cm−1, near the TFP resonance23,24, results in a reversion of the infrared amplitude contrast (cf. Fig. 4e). This invertible contrast proves that (i) infrared s-SNOM can selectively highlight either BP or TFP pentacene simply by choice of the infrared frequency, and that (ii) BP and TFP domains coexist on a ca. 300-nm length scale and seem to be not at all correlated to the well-known grain structure. This experimental finding is very surprising, as it allows us to conjecture that BP pentacene nucleates in the form of nanometre inclusions within, rather than on top of, TFP pentacene. BP seeds seem hidden in the morphology of the TFP and grow with storage time, as indicated by our X-ray analysis above (Fig. 1). In addition, s-SNOM imaging of a thick pentacene film (120 nm average thickness) shows that pentacene BP aggregates forming on top of the thick film18 act as growth seeds for BP nucleation throughout the film underneath (see Supplementary Fig. 1). The strong infrared contrasts of our s-SNOM images appear homogeneous within BP ellipses as well as among ellipses. As the probing depth of s-SNOM is of the order of the tip diameter10, here 20–30 nm, the observed homogeneity suggests that BP material in the ellipses is not mixed with TFP and extends through the full film depth.