Asymmetric grain distribution in phthalocyanine thin films

Many electronic and optical properties of organic thin films depend on the precise morphology of grains. Iron phthalocyanine thin films are grown on sapphire substrates at different temperatures to study the effect of grain growth kinematics and to experimentally quantify the grain size distribution in organic thin films. The grain size is measured with an atomic force microscope and the data is processed and analyzed with well-known image segmentation algorithms. For relevant statistics, over 3000 grains are evaluated for each sample. The data show pronounced asymmetric grain growth with increasing deposition temperature from almost spherical grains at room temperature to elongated needlelike shapes at $260°\text{C}$. The average size along the major axis increases from 35 to 200 nm and along the minor axis from 25 to 90 nm. The distribution is almost symmetric at low-deposition temperatures, but becomes lognormal at higher temperatures. Strikingly, the major axis and minor axis of the elliptically shaped grains have different distributions at all temperatures due to the planar asymmetry of the molecule.

Figure 1

(Color online) Evolution of AFM images for six iron phthalocyanine thin films with deposition temperatures from 25 to $260°\text{C}$. Each AFM image is $1\mu \text{m}$ in size. All samples have the same thickness of 27nm, which is less than the average grain size for all samples. A 20-fold increase in grain area is observed over this temperature range.

Figure 2

(Color online) (a) AFM image with overlaid red contours of grain edges as found by the Watershed algorithm. (b) Ellipses of the same size are fit to the grains and edge grains are removed from the image.

Figure 3

(Color online) The peak positions of the AFM frequency height plot are graphed for a 27 nm thick iron phthalocyanine film deposited at $260°\text{C}$. The frequency distribution (bottom right) shows uniformly spaced peaks that correspond to molecular planes. From a linear fit, the average peak spacing is determined to be 1.36 nm reflecting the size of one monolayer of iron phthalocyanine molecule with the $b$-axis parallel to the substrate plane. The inset (top left) shows the stacking angle $\varphi$ that the molecular plane makes with respect to the $b$-axis. This is a top view of the molecular arrangement onto the substrate.

Figure 4

(Color online) The unit cell of $\alpha$-phase iron phthalocyanine contains 4 molecules. The lattice parameters are $a=25.9\text{Å}$, $b=3.8\text{Å}$, $c=24.1\text{Å}$, and $\beta =90°$. In the top view the frontal plane of molecules is drawn only for better clarity. The top view shows the $ac$ plane and the bottom view the $ab$ plane of the unit cell.[21, 22]

Figure 5

(Color online) Grain size distributions for two iron phthalocyanine thin films deposited at (a) $25°\text{C}$ and (b) $260°\text{C}$. The diameter corresponds to a nominally circular grain with equal area. For the room temperature sample, the Gaussian fit has a lower ${\chi }^2$ value, and for the high-temperature sample, a lognormal distribution follows the experimental data more closely.

Figure 6

(Color online) Grain size distributions extracted for over 3000 grains for six samples deposited at different temperatures are normalized with respect to the mean grain diameter. The two distinct distributions stem from the $\alpha$ to $\beta$ phase transition that occurs above $200°\text{C}$.

Figure 7

(Color online) Fitted grain size for minor and major axis versus deposition temperature. The circularity decreases continuously with higher deposition temperatures. The error bars signify the standard deviations of the best fits to the normal or lognormal distributions. Thus, they include 68% of all grains.

Figure 8

(Color online) Grain size distributions for a 27 nm iron phthalocyanine sample deposited at room temperature. The grains were fitted to ellipses and their distributions are plotted for the (a) minor and (b) major axis. Using a least ${\chi }^2$ fit the normal distribution (dashed) fits better to the minor axis, and the lognormal distribution (line) fits better to the major axis.