Multi-component self-assembled molecular-electronic films: towards new high-performance thermoelectric systems

Orientational Effects and Molecular-Scale Thermoelectricity Control

The orientational effect concept in a molecular-scale junction is established for asymmetric junctions, which requires the fulfillment of two conditions: (1) design of an asymmetric molecule with strong distinct terminal end groups and (2) construction of a doubly asymmetric junction by placing an asymmetric molecule in an asymmetric junction to form a multicomponent system such as Au/Zn-TPP+M/Au. Here, we demonstrate that molecular-scale junctions that satisfy the conditions of these effects can manifest Seebeck coefficients whose sign fluctuates depending on the orientation of the molecule within the asymmetric junction in a complete theoretical investigation. Three anthracene-based compounds are investigated in three different scenarios, one of which displays a bithermoelectric behavior due to the presence of strong anchor groups, including pyridyl and thioacetate. This bithermoelectricity demonstration implies that if molecules with alternating orientations can be placed between an asymmetric source and drain, they can be potentially utilized for increasing the thermovoltage in molecular-scale thermoelectric energy generators (TEGs).

Signatures of Topological States in Conjugated Macrocycles

Single-molecule electrical junctions possess a molecular core connected to source and drain electrodes via anchor groups, which feed and extract electricity from specific atoms within the core. As the distance between electrodes increases, the electrical conductance typically decreases, which is a feature shared by classical Ohmic conductors. Here we analyze the electrical conductance of cycloparaphenylene (CPP) macrocycles and demonstrate that they can exhibit a highly nonclassical increase in their electrical conductance as the distance between electrodes increases. We demonstrate that this is due to the topological nature of the de Broglie wave created by electrons injected into the macrocycle from the source. Although such topological states do not exist in isolated macrocycles, they are created when the molecule is in contact with the source. They are predicted to be a generic feature of conjugated macrocycles and open a new avenue to implementing highly nonclassical transport behavior in molecular junctions.

Large area arrays of discrete single-molecule junctions derived from host-guest complexes

The desire to continually reduce the lower limits of semiconductor integrated circuit (IC) fabrication methods continues to inspire interest in unimolecular electronics as a platform technology for the realization of future (opto)electronic devices. However, despite successes in developing methods for the construction and measurement of single-molecule and large-area molecular junctions, exercising control over the precise junction geometry remains a significant challenge. Here, host-guest complexes of the wire-like viologen derivative 1,1'-bis(4-(methylthio)-phenyl)-[4,4'-bipyridine]-1,1'-diium chloride ([1][Cl]2) and cucurbit[7]uril (CB[7]) have been self-assembled in a regular pattern over a gold substrate. Subsequently, ligandless gold nanoparticles (AuNPs) synthesized in situ are deposited over the host-guest array. The agreement between the conductance of individual mono-molecular junctions, appropriately chosen as a function of the AuNP diameter, within this array determined by conductive probe atomic force microscope (c-AFM) and true single-molecule measurements for a closely similar host-guest complex within a scanning tunneling microscope break-junction (STM-BJ) indicates the formation of molecular junctions derived from these host-guest complexes without deleterious intermolecular coupling effects.

A convenient synthesis of ferrocene-(ethynylphenyl)thioacetates

Ferrocene is popular within the field of molecular electronics due to its well-defined electronic properties. However, where the conductance of highly-conjugated oligophenylethylenes has been widely studied, work on analogous ferrocenyl systems has been relatively rare, possibly due to difficulties associated with the synthesis of molecules containing terminal thioacetates, which are often used to bind molecules to metallic electrodes. Herein, a widely applicable synthetic methodology is demonstrated which can be used to synthesize a variety of conjugated ferrocene-alkyne systems terminated with thioacetates, including symmetric, asymmetric and multi-ferrocene systems. Conjugation of the ferrocene units to their terminal atoms is then shown through the use of both UV/Vis spectroscopy and cyclic voltammetry. This work paves the way for future studies and applications of conjugated ferrocene systems in the field of nanoelectronics.

20-State Molecular Switch in a Li@C60 Complex

A substantial potential advantage of industrial electric and thermoelectric devices utilizing endohedral metallofullerenes (EMFs) is their ability to accommodate metallic moieties inside their empty cavities. Experimental and theoretical studies have elucidated the merit of this extraordinary feature with respect to developing electrical conductance and thermopower. Published research studies have demonstrated multiple state molecular switches initiated with 4, 6, and 14 distinguished switching states. Through comprehensive theoretical investigations involving electronic structure and electric transport, we report 20 molecular switching states that can be statistically recognized employing the endohedral fullerene Li@C₆₀ complex. We propose a switching technique that counts on the location of the alkali metal that encapsulates inside a fullerene cage. The 20 switching states correspond to the 20 hexagonal rings that the Li cation energetically prefers to reside close to. We demonstrate that the multiswitching feature of such molecular complexes can be controlled by taking advantage of the off-center displacement and charge transfer from the alkali metal to the C₆₀ cage. The most energetically favorable optimization suggests 1.2-1.4 Å off-center displacement, and Mulliken, Hirshfeld, and Voronoi simulations articulate that the charge migrates from the Li cation to C₆₀ fullerene; however, the amount of the charge transferred depends on the nature and location of the cation within the complex. We believe that the proposed work suggests a relevant step toward the practical application of molecular switches in organic materials.

Quantum Interference and Contact Effects in the Thermoelectric Performance of Anthracene-Based Molecules

We report on the single-molecule electronic and thermoelectric properties of strategically chosen anthracene-based molecules with anchor groups capable of binding to noble metal substrates, such as gold and platinum. Specifically, we study the effect of different anchor groups, as well as quantum interference, on the electric conductance and the thermopower of gold/single-molecule/gold junctions and generally find good agreement between theory and experiments. All molecular junctions display transport characteristics consistent with coherent transport and a Fermi alignment approximately in the middle of the highest occupied molecular orbital/lowest unoccupied molecular orbital gap. Single-molecule results are in agreement with previously reported thin-film data, further supporting the notion that molecular design considerations may be translated from the single- to many-molecule devices. For combinations of anchor groups where one binds significantly more strongly to the electrodes than the other, the stronger anchor group appears to dominate the thermoelectric behavior of the molecular junction. For other combinations, the choice of electrode material can determine the sign and magnitude of the thermopower. This finding has important implications for the design of thermoelectric generator devices, where both n- and p-type conductors are required for thermoelectric current generation.

Impact of the terminal end-group on the electrical conductance in alkane linear chains

This research presents comprehensive theoretical investigations of a series of alkane-based chains using four different terminal end groups including amine -NH₂, thiomethyl -SMe, thiol -SH and direct carbon contact -C. It is widely known that the electrical conductance of single molecules can be tuned and boosted by chemically varying their terminal groups to metal electrodes. Here, we demonstrate how different terminal groups affect alkane molecules' electrical conductance. In general, alkane chain conductance decreases exponentially with length, regardless of the anchor group types. In these simulations the molecular length varies from 3 to 8 -CH₂ units, with 4 different linker groups; these simulations suggest that the conductances follow the order G _C > G _SH > G _SMe > G _NH2 . The DFT prediction order of the 4 anchors is well supported by STM measurements. This work demonstrates an excellent correlation between our simulations and experimental measurements, namely: the percent difference ΔG, exponential decay slopes, A constants and β factors at different molecular alkane chain lengths.

Controlling Charge Transport in Molecular Wires through Transannular π-π Interaction

This paper describes the influence of the transannular π-π interaction in controlling the carrier transport in molecular wires by employing the STM break junction technique. Five pentaphenylene-based molecular wires that contained [2.2]paracyclophane-1,9-dienes (PCD) as the building block were prepared as model compounds. Functional substituents with different electronic properties, ranging from strong acceptors to strong donors, were attached to the top parallel aromatic ring and used as a gate. It was found that the carrier transport features of these molecular wires, such as single-molecule conductance and a charge-tunneling barrier, can be systematically controlled through the transannular π-π interaction.

Orientational control of molecular scale thermoelectricity

Through a comprehensive theoretical study, we demonstrate that single-molecule junctions formed from asymmetric molecules with different terminal groups can exhibit Seebeck coefficients, whose sign depends on the orientation of the molecule within the junction. Three anthracene-based molecules are studied, one of which exhibits this bi-thermoelectric behaviour, due to the presence of a thioacetate terminal group at one end and a pyridyl terminal group at the other. A pre-requisite for obtaining this behaviour is the use of junction electrodes formed from different materials. In our case, we use gold as the bottom electrode and graphene-coated gold as the top electrode. This demonstration of bi-thermoelecricity means that if molecules with alternating orientations can be deposited on a substrate, then they form a basis for boosting the thermovoltage in molecular-scale thermoelectric energy generators (TEGs).