Abstract
The fluorescence of a two-dimensional supramolecular network of 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) adsorbed on hexagonal boron nitride (hBN) is red-shifted due to, primarily, adsorbate-substrate van der Waals interactions. TCPP is deposited from solution on hBN and forms faceted islands with typical dimensions of 100 nm and either square or hexagonal symmetry. The molecular arrangement is stabilized by in-plane hydrogen bonding as determined by a combination of molecular-resolution atomic force microscopy performed under ambient conditions and density functional theory; a similar structure is observed on MoS2 and graphite. The fluorescence spectra of submonolayers of TCPP on hBN are red-shifted by ∼30 nm due to the distortion of the molecule arising from van der Waals interactions, in agreement with time-dependent density functional theory calculations. Fluorescence intensity variations are observed due to coherent partial reflections at the hBN interface, implying that such hybrid structures have potential in photonic applications.
Highlights
There have been great advances over the past decade in the understanding of the interactions that stabilize the two-dimensional organization of organic molecules on surfaces.1À4 This body of research provides a route to combine chemical functionality with spatial organization at the nanometer scale through the formation of templates and networks stabilized by hydrogen bonding, metal coordination, and/or covalent coupling.5À16 Many of the advances in the field have arisen from studies of supramolecular arrangements on noble metal or highly oriented pyrolytic graphite (HOPG) surfaces, but it is clear that these substrates have some properties that are not compatible with investigations of the functional properties of the molecular arrays
VOL. 9 ’ NO. 10 ’ 10347–10355 ’ 2015 www.acsnano.org widely studied previously on metallic surfaces.26À29 The molecules are deposited from solution in an immersion process, and we have investigated the formation of networks on hexagonal boron nitride, an insulator, and, for comparison, the surfaces of highly oriented pyrolytic graphite and MoS2
High-resolution atomic force microscopy (AFM) images acquired under ambient conditions show TCPP molecules in two distinct phases on hexagonal boron nitride (hBN), with, respectively, square (Figure 1aÀc) and hexagonal (Figure 1d) symmetry
Summary
There have been great advances over the past decade in the understanding of the interactions that stabilize the two-dimensional organization of organic molecules on surfaces.1À4 This body of research provides a route to combine chemical functionality with spatial organization at the nanometer scale through the formation of templates and networks stabilized by hydrogen bonding, metal coordination, and/or covalent coupling.5À16 Many of the advances in the field have arisen from studies of supramolecular arrangements on noble metal or highly oriented pyrolytic graphite (HOPG) surfaces, but it is clear that these substrates have some properties that are not compatible with investigations of the functional properties of the molecular arrays. The metallic nature of the substrates precludes subsequent investigations of the optical and electrical properties of self-assembled networks, limiting the potential to exploit the control of organization and functionality of the arrays. To extend these studies beyond the conventional choice of substrate, for example to include molecular organization on insulators, requires the use of atomic force microscopy (AFM), which has been used previously to acquire highresolution images of 2D molecular and supramolecular organization under ultra-highvacuum conditions,17À22 but less widely23À25 at atmospheric pressure due to the limitations of ambient AFM. We observe a significant variation in the emission intensity, which depends on the thickness of the hBN layers due to coherent reflections at the upper and lower surfaces of the exfoliated flakes
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