A bulk material is more than just the sum of its parts. Because of interactions and quantum effects, atoms or molecules in a group have different electronic and thermal transport properties than a single atom or molecule. Isolated measurements are necessary to understand how energy flows through one atom or molecule. That understanding will also enable the engineering of nanoelectric devices.
Single-molecule transport properties are often measured using a scanning tunneling microscope. A metal STM tip is lowered onto a metal substrate coated with the molecule of interest. That contact creates a molecular bridge between the tip and the surface. As the tip slowly lifts away from the surface, the number of molecules in the bridge decreases until only a single molecule is left.
Although it can measure electron transport, a standard STM probe lacks the sensitivity needed for single-atom and single-molecule thermal conduction measurements. In 2017 Longji Cui (now at the University of Colorado Boulder) and coworkers at the University of Michigan solved that problem for metallic atoms by replacing the tip with their calorimetric scanning thermal microscopy (C-SThM) probe, shown in the figure. The probe’s T-shaped silicon nitride beams, which have a high stiffness and low thermal conductance, and a high-sensitivity platinum thermometer enabled the researchers to measure heat transport through both platinum and gold single-atom junctions. That transport process is primarily due to electron flow.
Now Cui and collaborators have used their probe to measure the thermal conductance of single thiol-terminated alkane molecules. Because the molecules’ heat transport is dominated by phonons instead of electrons, their thermal conductance—just tens of picowatts per kelvin—is an order of magnitude smaller than that of a metal. The researchers drove heat transfer through the molecules by keeping the substrate at 295 K and heating the STM tip to 320–340 K. As long as they remained connected, heat would flow from the hot tip to the cooler substrate; once the last molecular connection broke, the heat flow stopped. By averaging over a few hundred breaking events, the researchers measured a thermal conductance of about 20 pW/K for a single molecule, as shown in the graph below.
The single-molecule experiments confirmed a counterintuitive theoretical prediction: Although the electrical conductance decreased with molecular length, the thermal conductance was nearly independent of length. Now that the technique exists, C-SThM can be used to study thermal transport in polymer chains and other molecules of interest. (L. Cui et al., Nature, 2019, doi:10.1038/s41586-019-1420-z.)