Tailoring the Optical Response of Pentacene Thin Films via Templated Growth on Hexagonal Boron Nitride

Abstract

The optoelectronic properties of organic thin films are strongly dependent on their molecular orientation and packing, which in turn is sensitive to the underlying substrate. Hexagonal boron nitride (hBN) and other van der Waals (vdW) materials are known to template different organic thin film growth modalities from conventional inorganic substrates such as SiO2. Here, the morphology and temperature-dependent optical properties of pentacene films grown on hBN are reported. Pentacene deposited on hBN forms large-grain films with a molecular π-face-on orientation unlike the dendritic edge-on thin-film phase on SiO2. Pentacene/hBN films exhibit a 40 meV lower free exciton emission than pentacene/SiO2 and an unconventional emission energy temperature dependence. Time-resolved photoluminescence (PL) decay measurements show a long-lived signal in the π-face-on phase related to delayed emission from triplet–triplet fusion. This work demonstrates that growth on vdW materials provides a pathway for controlling optoelectronic functionality in molecular thin films.

Interlayer exciton landscape in WS$_2$/tetracene heterostructures

“…We focus on heterostruc-tures comprising of a WS 2 monolayer and a thin crystalline film of tetracene molecules. The latter attach to the surface of the TMD via van der Waals bonds and form a molecular lattice, as has been shown in previous studies 26,27 . Tetracene molecules tend to form periodic lattices rather than distribute themselves randomly on the surface [28][29][30][31][32] .…”

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

“…As noted above, pentacene grows π-face-on for hBN substrates versus edge-on for SiO 2 substrates. 18,35,47 Thus, hBN templates a unique thin-film phase for pentacene that exhibits a needle-like morphology in contrast to the dendritic motif typical of the thinfilm phase of pentacene on SiO 2 (Figure 3c). 18 Interestingly, the optical response of the π-face-on phase is notably different from the edge-on phase, showing PL more closely resembling that of single-crystal pentacene than that of thin-film pentacene (Figure 3d), as well as different temperature dependencies and decay times.…”

“…18,35,47 Thus, hBN templates a unique thin-film phase for pentacene that exhibits a needle-like morphology in contrast to the dendritic motif typical of the thinfilm phase of pentacene on SiO 2 (Figure 3c). 18 Interestingly, the optical response of the π-face-on phase is notably different from the edge-on phase, showing PL more closely resembling that of single-crystal pentacene than that of thin-film pentacene (Figure 3d), as well as different temperature dependencies and decay times. 18 At thicknesses greater than 10−20 nm, pentacene growth on hBN can transition from π-face-on to an edge-on phase similar to its bulk crystalline phase.…”

“…17 This orientational difference affects recombination times upon photoexcitation, with excitons more readily dissociating into long-lived free carriers in the CuPc-MoS 2 case. Many other large π-conjugated molecules have been shown to orient face-on to 2D materials including pentacene 18 and perylenes. 19 This orientation is often in contrast to the edge-on orientations on more common surfaces such as SiO 2 and sapphire.…”

“…19 This orientation is often in contrast to the edge-on orientations on more common surfaces such as SiO 2 and sapphire. 17,18 This qualitative difference in thin-film growth has led to the emergence of vdW materials, particularly hexagonal boron nitride (hBN), as promising substrates for templating the epitaxial growth of functional organic molecules. 20 The morphology and molecular orientation of organic−2D heterojunctions is often dependent on how they are created.…”