There are 1.2 million different molecules, and only one can control resistance!

Recently, researchers have achieved the goal of making a varistor using a single molecule.

Written by Qu Lijian

Recently, researchers from Shanghai University and several universities in Australia have collaborated to realize a single-molecule piezoresistor in nanoelectromechanical systems (NEMS) [1]. By applying forces of different magnitudes to this molecule, different resistances can be obtained, which shows the piezoresistive effect.

Piezoresistive effect and its applications

The piezoresistive effect was first discovered by Sir Kelvin (William Thomson) in metal materials in 1856. It is characterized by the change of resistivity when external force is applied to it.

Devices made using the piezoresistive effect have a wide range of applications. The most common application is the accelerometer in a smartphone, which enables us to count steps; it is also used in cars to sense external impacts and trigger airbags or anti-lock braking systems. Materials with the piezoresistive effect have also been developed into piezoresistive pressure sensors. When the pressure changes, the circuit outputs a signal proportional to the pressure, which converts mechanical force into other measurable physical quantities. For example, it is used to measure blood pressure in the medical field and to measure oil pressure and air pressure in car engines in the automotive industry.

Piezoresistive pressure sensor. Image source: wiki

How did the joint team from Shanghai University and Australia discover that single molecules can also exhibit piezoresistive effect?

First, we need to connect a single molecule into a circuit, but the molecule is too small, only about 10 nanometers. How do we connect such a small thing into a circuit?

The researchers achieved their goal by applying the scanning tunneling microscope-junction technique (STM-BJ, see Science 301, 1221–1223 (2003). They transformed the scanning tunneling microscope (STM) probe into an electrode, and the other electrode into a gold-plated silicon wafer; the probe electrode was repeatedly pushed forward and pulled backward, so that a nanometer-sized gap would appear between the probe and the gold-plated silicon wafer substrate. If the target molecule happened to enter this nanometer gap, it would connect the two electrodes, thus forming a "junction" and connecting the single-molecule circuit, so that relevant measurements could be made. [2]

Schematic diagram of STM-BJ technology. Source: Reference [2]

There are 1.2 million different molecules

The molecule that achieved the single-molecule piezoresistive effect this time is called Bullvalene (molecular formula C10H10). Bullvalene, as its name suggests, is a molecule that changes rapidly - carbon atoms are constantly exchanging positions with each other, constantly changing their structures, and can have up to 1.2 million structures. [3]

Five structures of the quencene molecule. The quencene molecule can have up to 1.2 million structures. Image source: wiki

The experimental principle is shown in the figure below. The researchers modified the instantene molecule with two aromatic ring groups and prepared the solution. They moved the STM probe close to the gold-coated silicon wafer until the distance was appropriate (0.7-1.5 nanometers), waited for the molecule to enter the gap between the probe and the gold-coated silicon wafer, and grabbed the two rings modified on the molecule. At this time, the circuit was connected.

Then, the probe is slowly moved to stretch or compress the molecules while measuring the conductivity (the inverse of resistivity). It is found that the conductivity varies with the position of the probe, which shows the piezoresistive effect.

Schematic diagram of the experimental principle. Source: Reference [1]

Why does conductivity change?

When molecules are stretched or compressed, they will be deformed differently and present different structures. We call molecules with the same molecular formula but different structures isomers. Different isomers have different electrical conductivities. Isomers have two sources: constitutional isomerism and conformational isomerism. The former comes from different ways of connecting atoms, that is, forming different chemical bonds; the latter comes from different ways of distributing atoms in space, which is due to the rotation of chemical bonds.

For this result, the researchers also conducted theoretical simulations to clarify the mechanism of the single-molecule piezoresistance effect of icine: the structural isomers affect the conductivity by affecting the interference of electron waves, while the effect of conformational isomers on conductivity comes from the interaction between molecules and electrodes. The two isomers also affect the binding of aromatic ring groups to electrodes.

Potential applications

This interesting work, which relies on single-molecule control of resistance, has attractive potential applications. For example, it can be made into a new type of piezoresistors for biomechanical measurements and the study of mechanical problems at the subcellular scale [4]. It is expected to be used to detect biological macromolecules such as chemicals, proteins and enzymes, and it may be technically applied to human-machine interface technology and health detection equipment [5].

Furthermore, the ability to control conductivity at the single-molecule level through mechanical forces has the potential to be used to construct molecular circuits and thus develop highly miniaturized devices (as small as 3–100 square nanometers).[6]

Of course, there is still a long way to go from this basic research to the realization of these exciting applications. The most direct one is to reduce the cost, get rid of the expensive scanning tunneling microscope, and develop a low-cost experimental platform.

References

[1] Reimers, JR, Li, T., Birvé, AP et al. Controlling piezoresistance in single molecules through the isomerisation of bullvalenes. Nat Commun 14, 6089 (2023). https://doi.org/10.1038/s41467-023-41674-z

[2] Acta Physico-Chimica Sinica, 2019, 35(8): 829-839 doi: 10.3866/PKU.WHXB201811027

[3] Molecule of the Week Archive, 2005-1-4, ACS, https://www.acs.org/molecule-of-the-week/archive/b/bullvalene.html

[4] Norman, J., Mukundan, V., Bernstein, D. et al. Microsystems for Biomechanical Measurements. Pediatr Res 63, 576–583 (2008). https://doi.org/10.1203/PDR.0b013e31816b2ec4

[5] https://www.curtin.edu.au/news/media-release/electronic-sensor-the-size-of-a-single-molecule-a-potential-game-changer/

[6]Chem. Soc. Rev., 2014, 43,7378, https://pubs.rsc.org/en/content/articlepdf/2014/cs/c4cs00143e

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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