Epitaxy of highly ordered organic semiconductor crystallite networks supported by hexagonal boron nitride

Abstract

This study focuses on hexagonal boron nitride as an ultra-thin van der Waals dielectric substrate for the epitaxial growth of highly ordered crystalline networks of the organic semiconductor parahexaphenyl. Atomic force microscopy based morphology analysis combined with density functional theory simulations reveal their epitaxial relation. As a consequence, needle-like crystallites of parahexaphenyl grow with their long axes oriented five degrees off the hexagonal boron nitride zigzag directions. In addition, by tuning the deposition temperature and the thickness of hexagonal boron nitride, ordered networks of needle-like crystallites as long as several tens of micrometers can be obtained. A deeper understanding of the organic crystallites growth and ordering at ultra-thin van der Waals dielectric substrates will lead to grain boundary-free organic field effect devices, limited only by the intrinsic properties of the organic semiconductors.

Figure 1

(a) 20 × 20 μm² AFM image of 18.1 nm thick bulk hBN flake covered with 6P needles (T_D = 363 K, lateral scale bar 4 μm, logarithmic z scale 65 nm). (b) Total needle length in a particular direction with ±0.5° tolerance, considering x axis of the AFM scanner as 0°. Solid red lines and dashed green lines in (a) and (b) indicate zigzag and armchair edges of the hBN flake, respectively (the procedure to index flake edges is presented at the end of the first subsection). Dot-dashed red lines in (b) represent ±5° splitting from zigzag directions of hBN flakes. (c) The fraction of needles that are in the preferred growth directions, as a function of T_D. The preferred growth directions are marked as shaded areas in (b). The dashed line set to 33.3% corresponds to the case of a completely disordered network.

Figure 2

Scheme of the preferred adsorption site for an isolated 6P molecule on a single layer hBN as obtained by DFT calculations, (a) top view, (b) side view. The molecule prefers to be face-on, oriented along armchair direction with the center of the phenyl rings above N and an adsorption height of about 0.33 nm. (c) A relative change of the adsorption energy with a slight rotation of 6P molecule from the preferred adsorption site. (d) Schematic representation of the rotated molecule for the case of 5° angle.

Figure 3

A schematic presentation of the 6P contact plane with hBN basal plane, (a) side view of the interface, and (b) top view. Molecules closest to the hBN surface are labeled with the numbers 1 and 2. Red arrows in (b) represent the 6P [130] direction that corresponds to the long axis of the resulting 6P needles.

Figure 4. The influence of hBN substrate thickness on 6P needle length and orientation, at T_D = 363 K.

(a–c) 10 × 10 μm² AFM topography images of 6P crystallite on hBN flakes with varied hBN thickness. Logarithmic z scales are 15 nm (a,b), and 30 nm (c). The arrows in (b) and (c) indicate high-symmetry directions of the supporting hBN flakes. (d) The dependence of the average 6P needle length on hBN thickness, each point in (d) represents a different hBN flake and over 100 needles were considered on each flake. Points denoted in (d) by i, ii, and iii are further analyzed in (e) with respect to the needle orientation, and are respectively shown in (a–c). (e) Angular distribution of 6P needles for the three selected hBN thicknesses. The x-axis in (e) is normalized for each sample to align the zigzag directions of supporting hBN flakes (dashed vertical lines).

Figure 5

(a–c) AFM topography images showing the dependence of 6P morphology with increasing T_D. All images show 10 × 10 μm² areas of bulk hBN flakes (logarithmic z scales: 50 nm (a), 25 nm (b,c)). (d) Shows the height profile of the red line denoted in (c), which intersects several 6P needles. The average length of 6P needles (e), and 6P volume ratio hBN/SiO₂ (f) as a function of T_D. Solid lines represent the data for 6P grown on bulk hBN flakes (over 3 nm thick), while dashed lines represents the data for 6P grown on hBN flakes that are less than 1.5 nm thick. Each data point is obtained as an average value of more than 400 measured 6P needles including several flakes. Shaded areas indicate standard deviations.