The use of electric fields is a powerful approach to manipulate molecular electronic states, which can subsequently influence the optical properties, adsorption structures, oxidation states, vibrational frequencies, and chemical reactivity of molecules.
Researchers used electricity to control the internal states of molecules. Credit: Jose Lado/Aalto University
A recent study published in the journal Advanced Materials focuses on developing a novel platform for studying the effects of electric fields on molecular electronic states using two-dimensional (2D) ferroelectric materials.
This could be a major breakthrough in the field of molecular electronics and quantum science, as it offers a new way of influencing the quantum states of deposited molecules via the proximity effect.
2D Ferroelectric Materials
2D ferroelectric materials provide a promising platform for the electrical control of quantum states because of their unique combination of ferroelectric and quantum mechanical properties.
Ferroelectricity refers to the ability of certain materials to have a spontaneous electric polarization, which can be reversed by an applied electric field. In 2D materials, this polarization is confined to a single plane, which makes them particularly attractive for electronic device applications.
Quantum mechanical properties, on the other hand, refer to the behavior of materials at the atomic or subatomic level. These properties are important for controlling and manipulating the behavior of electrons in a material.
In 2D ferroelectric materials, the coupling between the ferroelectric polarization and the quantum mechanical properties of the electrons allows for the electrical control of quantum states. This can be achieved by applying an electric field, which can be used to control the material's polarization and, thus, the behavior of the electrons in it.
Current Challenges in Studying Electric Field Effects
The use of electric fields to manipulate molecular electronic states has been challenging. Controlled experimentation has proved very difficult, and scanning tunneling microscopy (STM) has appeared as a leading technique in this difficult field.
However, with STM, a considerable electric field exists between the STM tip and the sample surface, which may trigger a radical change in the observed electronic properties in the tunneling spectra. By raising the set-point tunneling current, the distance between the tip and sample can be reduced, resulting in an enhancement in electric field strength.
The limitations of this approach are that molecules must be decoupled from a metallic substrate because of the strong perturbation of their electronic states. In addition, large tunneling currents often result in an unstable connection between the sample, molecule, and tip.
Highlights of the Current Study
In the current study, these constraints were overcome by connecting single monomers with two-dimensional ferroelectric (2D-FE) materials. Because of their 2D character, 2D ferroelectric materials present a viable platform for the electronic control of quantum states.
Using a monolayer of tin telluride (SnTe) as the FE substrate, the researchers concentrated on iron-phthalocyanine (FePc) molecules deposited on a 2D-FE SnTe substrate.
Low-temperature STM and scanning tunneling spectroscopy (STS) were utilized to investigate how an in-plane electric field from the 2D-FE SnTe substrate influenced the molecular states. Density-functional theory (DFT) calculations were also conducted to support the experimental results.
"In our experiments, we demonstrated how two-dimensional ferroelectrics allow us to realize electrically switchable quantum states. Controlling quantum states electrically is a major milestone in quantum materials, and here we demonstrated one strategy for doing it at the deepest level of individual molecules," said Ph.D. researcher Mohammad Amini, the first author of the study.
Important Findings and Prospects
The researchers found that the orbital filling and degeneracy of d-orbitals of a singular FePc molecule change due to the presence of an electric field from the SnTe substrate.
This phenomenon is induced by the unique metal d-orbital occupancy resulting from the electron transfer and energy level change connected with the polarization shift of the SnTe monolayer. In addition, researchers were able to change the molecular states by manipulating the polarization of the FE region using STM.
This technology has great potential for practical applications in molecular electronic and spintronic devices. Understanding the close connection between molecules and their electrostatic surroundings requires the capability to examine these effects at the level of a single molecule.
The novel platform developed by the researchers allows for the controlled modification of the electronic states of molecules using an electric field.
In summary, researchers have proposed a novel strategy for probing the effect of an electric field on molecular electronic states by coupling single molecules with two-dimensional ferroelectric (2D-FE) materials, thus overcoming the current limitations faced by traditional approaches like scanning tunneling microscopy.
This platform offers a powerful and controllable way to manipulate molecular electronic states, which could have far-reaching implications for molecular electronic and spintronic devices.
Amini, M. et al. (2022). Control of Molecular Orbital Ordering Using a van der Waals Monolayer Ferroelectric. Advanced Materials. Available at: https://doi.org/10.1002/adma.202206456
Source: Aalto University