Innovative Molecular Junctions for Terahertz Switches and Detectors
The Nanostructures, nano-Components and Molecules (NCM) group, in collaboration with the Linear Conjugate Systems (SCL) group at Moltech-Anjou (Angers) and the New Materials Chemistry Laboratory (U. Mons, Belgium), has developed molecular junctions designed to function as switches in the terahertz (THz) frequency range, a significant advance in molecular-scale electronics.
These molecular junctions consist of self-assembled monolayers (SAMs) of molecules with a π-σ-π structure. This structure comprises two conjugated (π) moieties linked by a non-conjugated (σ) spacer, a configuration that allows the device to exhibit negative differential conductance (NDC) – a phenomenon in which an increase in voltage across a device results in a decrease in current.
NDC occurs when the applied electrical voltage aligns the energy levels of the two conjugated subunits, generating a current peak, followed by a decrease as these levels become misaligned. This property is essential for the operation of molecular switches, playing a crucial role in the transition between high- and low-conductance states under the influence of terahertz irradiation.
The experimental set-up involved creating molecular junctions by carefully assembling SAMs on ultra-flat gold electrodes, then measuring their electronic transport properties using conductive atomic force microscopy (C-AFM) (figure 1.a). This technique, combined with terahertz laser irradiation (at 30THz and 2.5THz), enabled the switching behaviour to be observed and analysed in real time and at room temperature (Figure 1.b). It was found that at 30 THz, the NDC effect could be completely and reversibly suppressed as a function of terahertz wave power and frequency, which is not only a major experimental achievement but also a validation of theoretical predictions (figure 1.c). This behaviour indicates a highly sensitive interaction between molecular junctions and the terahertz field, underlining the potential of these junctions for use in advanced terahertz detectors.
The mechanism for suppressing NDC effects under terahertz irradiation is based on a dynamic interaction between the THz wave and the molecular orbitals of the two subunits of the molecular system. This interaction induces a resonant charge transfer between the energy levels of the molecules, leading to a reduction in the current at the conductance peaks. In addition, the suppression effect is enhanced by a phenomenon known as ‘coherent tunneling destruction’, where terahertz irradiation temporarily disables the passage of electrons across the molecular junction, thereby reducing the overall conductance. The presence of surface plasmon polaritons, interacting with the molecular levels, also contributes to this suppression.
However, when the devices were irradiated at 2.5 THz, suppression of the NDC behaviour was not observed. This indicates that the photon energy of 2.5 THz (around 10 meV) is insufficient to induce the same suppression effect as that observed at 30 THz. These results highlight the importance of the terahertz irradiation frequency in the operation of the molecular switch, demonstrating that the response of the device is strongly dependent on the incident photon energy.
Looking to the future, the integration of these molecular switches into practical terahertz detectors seems possible. The team proposes to use transparent electrodes with a large surface area, such as monolayer graphene, which would be almost transparent to terahertz waves, thus facilitating the detection process. In addition, the nature of the molecules, which can be optimized by chemical engineering, would allow the frequency response of these devices to be tuned by modifying the conjugated π groups, thus extending their applicability across a range of terahertz frequencies.
For further information :
Nano Letters : https://pubs.acs.org/doi/10.1021/acs.nanolett.3c04602
ArXiv : https://arxiv.org/abs/2310.08916
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