Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion

Abstract

Among organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption, and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, nonplanarity can be an additional parameter to modulate their properties without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with "cove"-type edges, having an identical aromatic core but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the "cove"-type edges can "bend" the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and nonplanar GNRs through UV-vis-near-infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.

Emerging Bottom‐Up Strategies for the Synthesis of Graphene Nanoribbons and Related Structures

“…Thep ioneering work of Müllen and his group,b ased on the preparation of soluble polyphenylene precursors followed by aromatization using the cyclodehydrogenation (Scholl) reaction, set the tone in this area of research. [27][28][29][30][31][32][33][34][35][36] Using this strategy,s everal examples of GNRs with different sizes,shapes and edge topologies have been prepared and thoroughly characterized, especially in regard to their electronic properties.A lthough the Scholl reaction is still the method of choice to prepare well-defined GNRs,o ther research groups have been interested in developing alternative methods to further expand the diversity of GNR structures that could be prepared.…”

Spiers Memorial Lecture : Carbon nanostructures by macromolecular design – from branched polyphenylenes to nanographenes and graphene nanoribbons

“…The synthesis of GNRs can be modified for incorporation of functional substituents to build in chromophores or stable free radicals, which will be useful later. [146][147][148][149][150] Additionally, GNRs can be subjected to further transformations by employing chlorination 58 and potentially hydrogenation, 151 as already established for NGs. Regarding scalability, the abovedescribed synthesis has been performed on a gram scale and can certainly be developed for even larger quantities.…”

Synthetic Engineering of Graphene Nanoribbons with Excellent Liquid-Phase Processability

“…By tailoring the position of the peripheral alkyl chains, GNRs 3 and 4, both with widths of 1.5 nm and lengths of ca. 100 nm, were also synthesized by a Diels-Alder approach from monomers 2 and 3, respectively ( Figure 1B) [49]. GNR 3 possesses a planar geometry, while GNR 4 adopts a nonplanar conformation due to the steric repulsion between the aromatic protons and the introduced alkyl chains ( Figure 1B).…”

“…In this respect, structurally defined GNRs with a width of 1.7 nm and lengths of 15-60 nm were synthesized from monomer 3 by the Yamamoto polymerization approach, and flexible poly(ethylene oxide) (PEO) chains were and GNR 2, respectively, on highly oriented pyrolytic graphite. Adapted, with permission, from [34,41,49]. then grafted onto the ribbons (Figure 2A, GNR-PEO) [55].…”

“…Substantial efforts have recently been devoted to the solution-phase synthesis of GNRs and several elegant protocols have been developed. Several types of all-carbon or heteroatom-doped GNRs with excellent liquid-phase processability have been synthesized [11,12,[24][25][26][43][44][45][46][47][48][49][50][51][52]. In this opinion article, we focus our discussion on our recent contributions in this emerging research area.…”