van der Waals-Induced Chromatic Shifts in Hydrogen-Bonded Two-Dimensional Porphyrin Arrays on Boron Nitride

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.

Coherent acoustic phonons in van der Waals nanolayers and heterostructures

“…The AFM data show that the roughness of the InSe surface (up to 1 nm) is significantly larger than for the sapphire substrate (∼ ±0.1 nm). Also, it is larger than that of hBN, which has an abrupt and atomically smooth surface [82,83]. Thus the most likely reason for the imperfectness of the interface revealed in our experiments is the surface roughness of the InSe layer.…”

Optical absorption signature of a self-assembled dye monolayer on graphene

“…2D ordered aggregation of similar molecules on metals have been reported to induced drastic changes in optical spectra, attributed to new electronic transitions [ 16 ]. Less pronounced rigid spectral shifts have been reported previously for molecules deposited on crystalline dielectrics such as hexagonal boron nitride (h-BN) [ 9 , 19 ]. The cited possible origins of such shifts are optical interactions between molecules or between molecules and the substrate and the deformation of molecules induced by van der Waals interactions between molecules and substrate.…”

“…Interactions between a dye and its surroundings at the molecular scale may also induce drastic changes in its photonic properties. Structural planarization of the adsorbed molecules [ 9 ], or the immersion inside a polarizable medium [ 10 ] can induce uniform bathochromic shifts of the vibronic peaks constituting the absorption spectrum. Finally, when distances between π-conjugated systems are small enough to permit electron tunnelling, quantum effects can also come into the play, at the origin, for instance, of new intermolecular charge-transfer absorption peaks [ 11 ].…”

Supramolecular heterostructures formed by sequential epitaxial deposition of two-dimensional hydrogen-bonded arrays

“…These molecular networks, which can be formed through selfassembly processes on a variety of different substrates including semiconductors 5,6 , metals 7,8 , insulators [9][10][11] and layered materials [12][13][14][15] , are, in almost all cases, limited to monolayer thickness. Progress towards the growth of higher layers has so far been much more limited, with demonstrations of bilayer growth 16,17 and site-specific molecular adsorption 5,[18][19][20][21][22][23][24] .…”

“…The layers are deposited via sequential immersion in solutions, and we have investigated heterostructure formation on the surface of hexagonal boron nitride (hBN) flakes which are exfoliated from mm-scale crystals; we have recently demonstrated that these substrates support molecular self-assembly. 13,14 Since both hBN and, potentially, the supramolecular heterostructures are insulating, we use AFM to acquire images of the molecular arrangements in adsorbed networks. This work represents an advance in the application of AFM to imaging such networks through the acquisition of images, under ambient conditions, with sufficiently high resolution to identify the relative placement of molecules in different layers of the resulting heterostructures.…”