Thermoelectric properties of organic thin films enhanced by π–π stacking

Abstract

Thin films comprising synthetically robust, scalable molecules have been shown to have major potential for thermoelectric en-ergy harvesting. Previous studies of molecular thin-films have tended to focus on massively parallel arrays of discrete but iden-tical conjugated molecular wires assembled as a monolayer perpendicular to the electrode surface and anchored via a covalent bond, know as self-assembled monolayers. In these studies, to optimise the thermoelectric properties of the thin-film there has been a trade-off between synthetic complexity of the molecular components and the film performance, limiting the opportuni-ties for materials integration into practical thermoelectric devices. In this work, we demonstrate an alternative strategy for en-hancing the thermoelectric performance of molecular thin-films. We have built up a series of films, of controlled thickness, where the basic units – here zinc tetraphenylporphyrin – lie parallel to the electrodes and are linked via π-π stacking. We have compared three commonly used fabrications routes and characterised the resulting films with scanning probe and computation-al techniques. Using a Langmuir-Blodgett fabrication technique, we successfully enhanced the thermopower perpendicular to the plane of the ZnTPP multilayer film by a factor of 10, relative to the monolayer, achieving a Seebeck coefficient of -65 μV/K. Furthermore, the electronic transport of the system, perpendicular to the plane of the films, was observed to follow the tunnel-ling regime for multi-layered films, and the transport efficiency was comparable with most conjugated systems. Furthermore, scanning thermal microscopy characterisation shows a factor of 7 decrease in thermal conductance with increasing film thick-ness from monolayer to multilayer, indicating enhanced thermoelectric performance in a π-π stacked junction.

Regulating the Single-Molecule Conductance of Corroles by the Substituents on the B-Site

“…Tetrapyrrolic macrocycles have been comprehensively utilized in reactions such as the activation and transport of molecular oxygen in mammals and the harvesting of sunlight in plant photosynthetic systems due to a highly conjugated and rigid structural backbone, which is also an important feature for building molecular devices with high performance. Porphyrins, which stand for a sort of natural tetrapyrrolic macrocycle system, provide a good starting point for building molecular devices − and have been studied extensively as molecular devices in the form of the complex monomer, oligomer, fused structures and through-space π–π stacked structures . However, to our knowledge, despite fascinating features such as low conductance attenuation at long molecular length, , the tunable conductance by the image-charge effect and the central-coordinated metal, the systematic investigation on the influence of substituents, which is a primary effect in regulating energy level of molecular junctions and further the single-molecule conductance, − on the side groups of the conductive backbones in tetrapyrrolic macrocycles, has not been reported.…”

High Seebeck coefficient from isolated oligo-phenyl arrays on single layered graphene via stepwise assembly

“…In this study, it was shaped into a cone-shaped tip, and the tip retained its shape due to the formation of a ∼0.7 nm native oxide outer shell (GaOx) upon contact with air. 49,50 It has been proved that this type of tip can contact non-destructively with SAMs4, 8–52 and form Cu/Gr/SAMs/GaOx/EGaIn junction.…”

“…Conversely, the packing density of Gr/ 4/5 was determined based on the coverage area of molecule 4 , which is established to be approximately 10–20 times larger than that of alkyl chains. 23,61 Consequently, the number of parallel molecules per unit area formed with an EGaIn tip for Gr/ 4/5 was about 10–20 times lower than that of Gr/ 1 . (2) For Gr/ 1 , electrons tunnel through the amine–graphene interface directly, but for Gr/ 4/5 there were two interfacial tunnel barriers: the barrier between graphene and molecule 4 , and the barrier between molecule 4 and molecule 5 , which decreases the electron transmission probability.…”

Robust large area molecular junctions based on transparent and flexible electrodes

“…Although single-molecule electrode|molecule| electrode junctions have proven to be remarkably versatile research tools, [1][2][3][4][5][6][7][8][9] and led to many proposals for applications beyond mimicry of conventional electronic components, 10 the translation of molecular electronic science to a practical molecular electronic technology arguably requires junctions in which monolayer assemblies are reliably contacted between larger-area electrodes to better enable integration with existing technologies and interface with the macroscopic world. [11][12][13][14][15][16] The fabrication of these large-area devices typically involves the deposition of a molecular high-quality monolayer by selfassembly, [17][18][19][20] Langmuir-Blodgett, [21][22][23][24][25] electrografting, [26][27][28][29][30] etc. onto a bottom electrode, usually formed from a conductive material deposited onto a rigid substrate and subsequent deposition of a top electrode.…”