Nanotechnology Now - Press Release: Scientists build the smallest cable containing a spin switch

Abstract: *IMDEA Nanociencia researchers report the first encapsulation ofspin-crossover (SCO) molecules inside carbon nanotubes.*

�*SCO molecules present a radical spin change with temperature, relevantfor magnetic switches and spintronics devices. *

�*This is a fundamental research result that helps to understand thebehaviour of magnetic molecules confined in very small spaces.*

A research work published in /Nature Communications/, involvingresearchers from the Madrid Institute for Advanced Studies inNanoscience (IMDEA) and the University of Sevilla, has measured for thefirst time the electrical conductivity of a single carbon nanotube withspin-crosslinked molecules inside it.

As electronic devices continue to shrink to meet the demands of themarket, scientists are working to develop the minute components thatmake them work. There is a persistent demand for fast and efficientprocesses, and spin-logic (Spintronics) devices could be the solution toshape the future of computing. Here, magnetic molecules could add a newtwist to conventional electronics. In particular, spin-crossover (SCO)molecules conform a family of zero-dimensional (0D) functional unitsthat display a radical spin switch triggered by an electro-structuralchange activatable by external stimulus such as light, pressure ortemperature. The spin switch confers SCO molecules excellentcapabilities and functionalities for implementation in nano-electronics.However, their insulating character prevent these molecules to be fullyexploited so far. Several groups have embedded SCO molecules intomatrices of conductive material but the results are not fully compatiblewith the requirements of nanoscale devices.

A ground-breaking system for effectively incorporating SCO molecules toconductive materials is to introduce them inside conductive carbonnanotubes. Carbon nanotubes are one-dimensional (1D) materials, strong,lightweight and, most importantly, highly electrically conductingminiature wires, typically 1-5 nanometres in diameter, but up tocentimetres in length. For the first time, a group of researchers atIMDEA Nanociencia have encapsulated Fe-based SCO molecules inside carbonnanotubes. The single-walled carbon nanotubes act as conductingbackbones that carry, protect and sense the SCO spin state of themolecules, and overcomes their insulating drawbacks.

Iron-based SCO molecules encapsulated in a single carbon nanotube.Credit: Nature Communications 2021.

The researchers, led by Prof. Emilio M. P�rez, Dr. Jos� Sanchez Costaand Dr. Enrique Burzur�, studied the electron transport throughindividual carbon nanotubes embedded in nanoscale transistors bydielectrophoresis. They found a change in the nanotube�s electricalconductance that is modified by the spin state of the encapsulated SCOmolecules. The transition between the two conducting states is triggeredby a thermal switch that turns out to be not symmetric: the transitiontemperature point is not the same going down than up the thermometer.This fact opens a hysteresis not present in crystalline samples, andmany interesting potential applications for the hybrid system arise:�These systems are like mini-memory elements at the nanoscale, as theypresent a hysteresis cycle with temperature variation. They could alsoserve as a filter of spin (a demand for spintronic devices) because thenanotube �feels� if the molecule has spin or not� Dr. Burzur� comments.

The experimental results are supported by theory calculations byresearchers at Universidad de Sevilla. During the switching, theorbitals of the SCO molecules change and hence their hybridization withthe carbon nanotube, that in turn modifies the electrical conductivityof the latter. The SCO molecules in their low spin state have a strongerinteraction with the nanotubes; it is more difficult for them to changetheir spin state and this is translated to a �jump� in the nanotubeconductivity at a certain temperature, depending on the initial spin state.

This first encapsulation of SCO molecules inside single-walled carbonnanotubes is a fundamental research result that helps to understand thebehaviour of these molecules when confined in very small spaces, andprovides a backbone for their readout and positioning into nanodevices.The authors hope that such mix-dimensional (0D-1D) hybrid can leveragethe best properties of their constituent materials, exploiting the spinstate as another degree of freedom. This miniscule wire and switch canbe produced on a preparative scale and may represent a relevant step inthe development of nanoscale magnetic systems.

This research outcome is the result of a collaboration amongstresearchers at IMDEA Nanociencia and Departamento de Qu�mica F�sica atUniversidad de Sevilla. Prof. Emilio M. P�rez is the leader of the Groupof Chemistry of Low Dimensional Materials; Dr. Jos� S�nchez Costa is theleader of the Group of Switchable Nanomaterials; Dr. Enrique Burzur� isthe leader of the group Functional Nanoscale Materials and Devices; allthree groups at IMDEA Nanociencia. The three main authors acknowledgethe funding from the Spanish Ministry of Science and Innovation and theSevero Ochoa Excellence in R&D award to IMDEA Nanociencia (2017-2021).