From 2D to 3D: Bridging Self-Assembled Monolayers to a Substrate-Induced Polymorph in a Molecular Semiconductor

Abstract

In this study, a new bottom-up approach is proposed to predict the crystal structure of the substrate-induced polymorph (SIP) of an archetypal molecular semiconductor. In spite of intense efforts, the formation mechanism of SIPs is still not fully understood, and predicting their crystal structure is a very delicate task. Here, we selected lead phthalocyanine (PbPc) as a prototypical molecular material because it is a highly symmetrical yet nonplanar molecule and we demonstrate that the growth and crystal structure of the PbPc SIPs can be templated by the corresponding physisorbed self-assembled molecular networks (SAMNs). Starting from SAMNs of PbPc formed at the solution/graphite interface, the structural and energetic aspects of the assembly were studied by a combination of in situ scanning tunneling microscopy and multiscale computational chemistry approach. Then, the growth of a PbPc SIP on top of the physisorbed monolayer was modeled without prior experimental knowledge, from which the crystal structure of the SIP was predicted. The theoretical prediction of the SIP was verified by determining the crystal structure of PbPc thin films using X-ray diffraction techniques, revealing the formation of a new polymorph of PbPc on the graphite substrate. This study clearly illustrates the correlation between the SAMNs and SIPs, which are traditionally considered as two separate but conceptually connected research areas. This approach is applicable to molecular materials in general to predict the crystal structure of their SIPs.

Exploring the effects of graphene and temperature in reducing electron beam damage: A TEM and electron diffraction-based quantitative study on Lead Phthalocyanine (PbPc) crystals

“…Lead phthalocyanine (PbPc) (Fig. 1a) (Hao et al, 2022) is of special interest because of its excellent properties such as thermal and chemical stability, (Collins and Belghachi, 1989) photoconductivity, (Mohan Kumar and Achar, 2006) electrical conductivity, (Melville et al, 2015) and sensing abilities for various toxic gases (Ho et al, 2005). Although most structural investigations on PbPc or other phthalocyanines are done using X-ray diffraction (XRD), the information obtained is usually from a large area and not locally from a single nanometer-sized crystal.…”

STM Study of the Self-Assembly of Biphenyl-3,3′,5,5′-Tetracarboxylic Acid and Its Mixing Behavior with Coronene at the Liquid–Solid Interface

“…Due to the tunneling requirement of STM, three layers are the maximal stacking that has been reported for the liquid–solid interface condition , although more layers have been revealed under UHV . Recently, bilayer and trilayer structures have attracted much attention because of vertically strong intermolecular interactions and in turn tailored electronic properties such as molecular energy-level splitting and thus reduced band gap. ,− …”

Conformation-Dependent Monolayer and Bilayer Structures of an Alkylated TTF Derivative Revealed using STM and Molecular Modeling

“…33−35 On conductive substrates, techniques like STM may provide valuable input and allow the characterization of up until few-layer-thin structures with submolecular resolution. 31 However, relying only on STM for the structural characterization of thin films on conductive substrates has its limitations as well, and often complementary atomistic simulations 36,37 are required to achieve deep understanding.…”

“…One of the bottlenecks for the characterization of thin SIPs with techniques such as X-ray diffraction is the low signal-to-noise ratio posing constraints on the detection limit. − On conductive substrates, techniques like STM may provide valuable input and allow the characterization of up until few-layer-thin structures with submolecular resolution . However, relying only on STM for the structural characterization of thin films on conductive substrates has its limitations as well, and often complementary atomistic simulations , are required to achieve deep understanding. A good starting point is the study of the molecular conformation and structural organization of multilayer SAMNs, which acts as bridging stage between monolayer SAMNs and SIPs.…”