Understanding energy dissipation at the nanoscale requires the ability to probe temperature fields with nanometer resolution. Here, we describe an ultra-high vacuum (UHV)-based scanning thermal microscope (SThM) technique that is capable of quantitatively mapping temperature fields with ∼15 mK temperature resolution and ∼10 nm spatial resolution. In this technique, a custom fabricated atomic force microscope (AFM) cantilever, with a nanoscale Au–Cr thermocouple integrated into the tip of the probe, is used to measure temperature fields of surfaces. Operation in an UHV environment eliminates parasitic heat transport between the tip and the sample enabling quantitative measurement of temperature fields on metal and dielectric surfaces with nanoscale resolution. We demonstrate the capabilities of this technique by directly imaging thermal fields in the vicinity of a 200 nm wide, self-heated, Pt line. Our measurements are in excellent agreement with computational results—unambiguously demonstrating the quantitative capabilities of the technique. UHV-SThM techniques will play an important role in the study of energy dissipation in nanometer-sized electronic and photonic devices and the study of phonon and electron transport at the nanoscale.Keywords: nanoscale thermal contact; quantitative temperature profiling; scanning thermal microscopy; thermocouple probe; ultrahigh vacuum

We analyze triphenyl molecules coupled to gold electrodes through five different end groups to understand the effect of end groups on the thermoelectric properties of molecular junctions. Our investigation suggests that end-group-mediated charge transfer between the molecule and electrodes plays an important role in the resulting thermoelectric properties. We find that the direction of charge transfer, which is governed by the electronegativity of the end-group functionalized molecule, is strongly correlated to the degree of reorganization of frontier molecular orbitals (HOMO–LUMO). In particular, isocyanide, nitrile, and amine end-group molecular junctions, with charge (electron) transfer out of the molecule, exhibit a strong overall downward shift in the energies of frontier molecular orbitals, whereas thiol and hydroxyl end-group molecular junctions, with charge transfer into the molecule, exhibit a smaller overall downward shift. Finally, our study shows that the sign of the thermopower of molecular junctions is closely related to the HOMO–LUMO energies and electronegativity of isolated molecules.Keywords: electronegativity; energy conversion; molecular electronics; molecular thermoelectrics; orbital alignment;

We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold electrodes. The end groups (−SH, −NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from −SH to −H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol-terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital, as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecular orbital.

Probing temperature fields with nanometer resolution is critical to understanding nanoscale thermal transport as well as dissipation in nanoscale devices. Here, we demonstrate an atomic force microscope (AFM)-based technique capable of mapping temperature fields in metallic films with ∼10 mK temperature resolution and <100 nm spatial resolution. A platinum-coated AFM cantilever placed in soft mechanical contact with a metallic (gold) surface is used to sequentially create point contact thermocouples on a grid. The local temperature at each point contact is obtained by measuring the thermoelectric voltage of the platinum−gold point contact and relating it to the local temperature. These results demonstrate a direct measurement of the temperature field of a metallic surface without using specially fabricated scanning temperature-probes.