Counting electronic states in a single molecule

News - 08 December 2021 - Communication TNW

A team of researchers from the UK, the Netherlands, Switzerland and Belgium has managed to measure changes in the energy levels of a free radical molecule in a magnetic field. They were able to measure directly the thermoelectric currents inside the molecule, using a new method called thermocurrent spectroscopy. This method, published in Nano Letters, is an important tool for both chemical synthesis and single-molecule electronics.

The entropy – degree of disorder – of a single molecule can reveal the presence of microscopic electronic transfers, which are currently difficult to observe. The researchers developed a tool they call thermocurrent spectroscopy, to measure entropy via the thermoelectric currents present when a molecule is in a magnetic field. They achieved this by linking the molecule to a nanometer sized gap between two gold contacts. This single-molecule device heats up one side of the molecule and measures the electric current that flows through the molecule in response to this ‘thermal bias’.

Chunwei Hsu

Left: electron microscopy image (false coloured) of the molecular device. A molecular junction is formed in the constriction area (red circle) of an electro-migrated gold bridge (yellow). Right: Schematic of the molecular junction. A radical molecule is connected to a hot and a cold gold lead. This leads to a thermoelectric current which contains information about the entropy of the molecule. Figure: Chunwei Hsu

Measuring entropy

As each electron passes through the nanometer gap, it has to choose which of the molecular states it will occupy during the time it spends on the molecule, the number of states and their relative probabilities of occupation – all parameters of entropy. By measuring this entropy directly and its dependence on the magnetic field, the group of researchers gained information about the energy levels and the spin states in the molecule. Unlike existing methods, this new method can be readily applied to any kind of nano-scale system.

Chunwei Hsu, who performed the experiment, explains: ‘’Entropy is a thermodynamic quantity, describing the disorder in a classical system. In a quantum system, it essentially quantifies the occupancy of states and thus is important for understanding chemical processes and exotic physical systems, such as Kondo states and Majorana fermions. While researchers can rarely address entropy in these nanoscale systems, we managed to make a tool to measure entropy via the thermocurrent.” Thermocurrent spectroscopy may be useful in both chemical synthesis and single-molecule electronics, for example in the design new of OLED applications used in the creation of digital displays, and more efficient thermoelectric power generators.