Modulation of waveguide behaviour of an ICT 2H-Benzo[d][1,2,3]Triazole derivative with graphene

Highlights

• Synthesis of 2H-benzo[d][1,2,3]triazole derivative and graphene aggregate to form a new supramolecular assembly.

• This aggregate shows properties as optical waveguide.

• The graphene-modified aggregate acts as a selective optical waveguide for green light.

• These new materials may be of crucial importance for the development of high-performance optoelectronic devices.

Abstract

Needle-like supramolecular structures prepared by the organized aggregation of a 2H-benzo[d][1,2,3]triazole derivative with few-layer graphene using the slow diffusion technique are described. The as-prepared aggregate shows, by using scanning electron microscopy (SEM), the formation of needles with graphene attached on the surface, which was corroborated by Raman spectroscopy and optical microscopy. The graphene-modified aggregate acts as a selective optical waveguide for green light, in contrast to the pristine aggregate which emits green and red light. These new materials may be of crucial importance for the development of high-performance optoelectronic devices.

Introduction

Among the most promising materials which are currently under investigation, well-defined nano- and micro-structured materials have received increasing attention due to their potential applications in the electronic and photonic fields [[1], [2], [3]]. Their individual molecular structures make them excellent building blocks to obtain supramolecular aggregates with potential applications as device elements and inter-connects [4].

Within this framework, intramolecular charge transfer (ICT) molecules which exhibit multifunctional properties, such as multiple electroactivity or photoactivity, offer a wide range of applications in organic electronic devices [5] due to their semiconductor character and high charge carrier mobilities [6].

Supramolecular self-assembling [[7], [7]a), [7]b), [7]c)] processes offer wonderful synthetic advantages for tuning the size, shape and distribution of the formed aggregates, hence exploiting the supramolecular interaction for controlling the growth of ICT compounds as microtubes, microrods and cubic structures in large quantities and with high morphological and chemical purity.

In particular, in the area of organic nanophotonics, the confinement of optical waves in shape-shifting organic nanostructures is really interesting because of their potential use in “smart” nano-optical devices, such as optical waveguides [[8], [8]a), [8]b)], photonic detectors and optical sensors [9]. Currently, a new technology named optical networks-on-chips (ONoCs) has emerged in the last years [[10]a), [10]b), [10]c), [10]], to be placed at the forefront with the objective of providing higher bandwidths and shorter latencies with reduced energy consumption. The most important component is the waveguide, which is the photonic equivalent of a wire. Although optical networks have already proved themselves to be useful for long distances and chip-to-chip communication, their establishment as on-chip networks still requires an intensive research effort, both in designing new required mechanisms and devices, and in successfully proving their superiority against their electronic counterparts in compensating for the migration cost towards the new technology.

Nowadays, an increasing focus of attention has been placed on optical waveguides fabricated from low-molecular-weight organic compounds [[11]e), [11]f), [11]g), [11], [11]a), [11]b), [11]c), [11]d)]. This kind of waveguides contain different types of light-active materials which can be excited by an external light source that guides the emitted light instead of the incident light. Furthermore, the waveguides are made of transparent media with higher refractive indexes than their environment, hence they can keep light transmitting internally by total reflection at the interface.

In these days, attractive modifications have been described in the aggregates with waveguide behaviour. It is noteworthy that among them is the epitaxial self-assembly of binary molecular components into branched nanowire heterostructures with potential applications as high-speed data highways in optical communication [12], or the hybridization of organic semiconducting microrods with functionalized gold nanoparticles [13].

Graphene has emerged as a promising alternative material for novel photonic and optoelectronic devices due to its interesting properties, especially its high carrier mobility and zero bandgap [[14]a), [14]b), [14]c), [14]]. In comparison with the conventional metal film, the conductivity of graphene can be modified by means of chemical doping, electric or magnetic fields, and/or gate bias voltage [[15]a), [15]b), [15]].

Systems based on graphene embedded into an optical waveguide have been an object of great interest in the last years, motivated by the fact that highly localized photons of the waveguide can interact very efficiently with graphene [[16]a), [16]b), [16]c), [16]]. This stronger confinement of plasmons in graphene can create strong light-matter interactions and it can potentially be used to build different types of optical devices. For these reasons, optical waveguides incorporating graphene as the electro-optical element are a promising platform for future electro-optical modulators. Their operational principle is based on tuning the optical conductivity ϭ(ω) of graphene by the application of a gate voltage, thus changing the modal propagation constants Ky.

Finally, it is noteworthy that one of the most intriguing optical properties of graphene is its universal linear absorption coefficient of 2.3% for normal incident light, but within a short interaction length [[17]a), [17]b), [17]]. In a recent study, it is described that complete absorption can be achieved by integrating graphene on a waveguide [18].

To our knowledge, studies in the body of literature to date on the modulation of waveguides with graphene, use polymer [19] or inorganic [20] waveguides with graphite formed by vapour phase deposition, or there are purely theoretical articles [[16]a), [16]b), [16]c), [17]a), [17]b), [17], [18], [19], [20], [21]]. However, no waveguide studies based on discrete graphene-doped molecules have been reported.

In this context, and based on our ongoing research line in small organic molecules with waveguiding behaviour [[22]b), [22]c), [22]d), [22], [22]a)], in this paper we describe the doping of 4,7-bis(benzo[b]thiophen-2-ylethynyl)-2-(3,5-bis(trifluoromethyl)phenyl)-2H-benzo[d][1,2,3]triazole (1) with graphene.

In this sense, our objective was to design new optical waveguide structures in order to eventually apply them in the recent optical technology.