Wonho Jeong

Heat dissipation in atomic-scale junctions

Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits. They are also ideal platforms for testing quantum transport theories that are required to describe charge and energy transfer in novel functional nanometre-scale devices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized owing to experimental challenges. Here we use custom-fabricated scanning probes with integrated nanoscale thermocouples to investigate heat dissipation in the electrodes of single-molecule (‘molecular’) junctions. We find that if the junctions have transmission characteristics that are strongly energy dependent, this heat dissipation is asymmetric—that is, unequal between the electrodes—and also dependent on both the bias polarity and the identity of the majority charge carriers (electrons versus holes). In contrast, junctions consisting of only a few gold atoms (‘atomic junctions’) whose transmission characteristics show weak energy dependence do not exhibit appreciable asymmetry. Our results unambiguously relate the electronic transmission characteristics of atomic-scale junctions to their heat dissipation properties, establishing a framework for understanding heat dissipation in a range of mesoscopic systems where transport is elastic—that is, without exchange of energy in the contact region. We anticipate that the techniques established here will enable the study of Peltier effects at the atomic scale, a field that has been barely explored experimentally despite interesting theoretical predictions. Furthermore, the experimental advances described here are also expected to enable the study of heat transport in atomic and molecular junctions—an important and challenging scientific and technological goal that has remained elusive.

Ultra-High Vacuum Scanning Thermal Microscopy for Nanometer Resolution Quantitative Thermometry

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.

Radiative heat transfer in the extreme near field

Radiative transfer of energy at the nanometre length scale is of great importance to a variety of technologies including heat-assisted magnetic recording, near-field thermophotovoltaics and lithography. Although experimental advances have enabled elucidation of near-field radiative heat transfer in gaps as small as 20–30 nanometres (refs 4, 5, 6), quantitative analysis in the extreme near field (less than 10 nanometres) has been greatly limited by experimental challenges. Moreover, the results of pioneering measurements differed from theoretical predictions by orders of magnitude. Here we use custom-fabricated scanning probes with embedded thermocouples, in conjunction with new microdevices capable of periodic temperature modulation, to measure radiative heat transfer down to gaps as small as two nanometres. For our experiments we deposited suitably chosen metal or dielectric layers on the scanning probes and microdevices, enabling direct study of extreme near-field radiation between silica–silica, silicon nitride–silicon nitride and gold–gold surfaces to reveal marked, gap-size-dependent enhancements of radiative heat transfer. Furthermore, our state-of-the-art calculations of radiative heat transfer, performed within the theoretical framework of fluctuational electrodynamics, are in excellent agreement with our experimental results, providing unambiguous evidence that confirms the validity of this theory for modelling radiative heat transfer in gaps as small as a few nanometres. This work lays the foundations required for the rational design of novel technologies that leverage nanoscale radiative heat transfer.

Heat dissipation and its relation to thermopower in single-molecule junctions

Motivated by recent experiments, we present here a detailed theoretical analysis of the joule heating in current-carrying single-molecule junctions. By combining the Landauer approach for quantum transport with ab initio calculations, we show how the heating in the electrodes of a molecular junction is determined by its electronic structure. In particular, we show that in general heat is not equally dissipated in both electrodes of the junction and it depends on the bias polarity (or equivalently on the current direction). These heating asymmetries are intimately related to the thermopower of the junction as both these quantities are governed by very similar principles. We illustrate these ideas by analyzing single-molecule junctions based on benzene derivatives with different anchoring groups. The close relation between heat dissipation and

Characterization of nanoscale temperature fields during electromigration of nanowires

Quantitative studies of nanoscale heat dissipation (Joule heating) are essential for advancing nano-science and technology. Joule heating is widely expected to play a critical role in accelerating electromigration induced device failure. However, limitations in quantitatively probing temperature fields—with nanoscale resolution—have hindered elucidation of the role of Joule heating in electromigration. In this work, we use ultra-high vacuum scanning thermal microscopy to directly quantify thermal fields in nanowires during electromigration. Our results unambiguously illustrate that electromigration begins at temperatures significantly lower than the melting temperature of gold. Further, we show that during electromigration voids predominantly accumulate at the cathode resulting in both local hot spots and asymmetric temperature distributions. These results provide novel insights into the microscopic details of hot spot evolution during electromigration and are expected to guide the design of reliable nanoscale functional devices.

Quantized thermal transport in single-atom junctions

Calorimetry reaches an atomic junction Electrical and thermal conductivity in metals are linked at the macroscopic length scale because electrons carry both heat and current. Cui et al. found that this relationship, the Wiedemann-Franz law, holds down to the atomic scale in gold and platinum (see the Perspective by Segal). They made thermal and electrical conductance measurements through a point contact only one atom thick. In gold, the thermal and electrical conductance was quantized, owing to the electronic band structure of the metal. The experiments pave the way for high-resolution calorimetry and other thermal measurements at the nanoscale. Science, this issue p. 1192; see also p. 1125 Thermal and electrical conductance remain tightly connected across gold and platinum single-atom junctions. Thermal transport in individual atomic junctions and chains is of great fundamental interest because of the distinctive quantum effects expected to arise in them. By using novel, custom-fabricated, picowatt-resolution calorimetric scanning probes, we measured the thermal conductance of gold and platinum metallic wires down to single-atom junctions. Our work reveals that the thermal conductance of gold single-atom junctions is quantized at room temperature and shows that the Wiedemann-Franz law relating thermal and electrical conductance is satisfied even in single-atom contacts. Furthermore, we quantitatively explain our experimental results within the Landauer framework for quantum thermal transport. The experimental techniques reported here will enable thermal transport studies in atomic and molecular chains, which will be key to investigating numerous fundamental issues that thus far have remained experimentally inaccessible.

Study of radiative heat transfer in Ångström- and nanometre-sized gaps

Radiative heat transfer in Ångström- and nanometre-sized gaps is of great interest because of both its technological importance and open questions regarding the physics of energy transfer in this regime. Here we report studies of radiative heat transfer in few Å to 5 nm gap sizes, performed under ultrahigh vacuum conditions between a Au-coated probe featuring embedded nanoscale thermocouples and a heated planar Au substrate that were both subjected to various surface-cleaning procedures. By drawing on the apparent tunnelling barrier height as a signature of cleanliness, we found that upon systematically cleaning via a plasma or locally pushing the tip into the substrate by a few nanometres, the observed radiative conductances decreased from unexpectedly large values to extremely small ones—below the detection limit of our probe—as expected from our computational results. Our results show that it is possible to avoid the confounding effects of surface contamination and systematically study thermal radiation in Ångström- and nanometre-sized gaps.

Quantification of thermal and contact resistances of scanning thermal probes

Scanning thermal probes are widely used for imaging temperature fields with nanoscale resolution, for studying near-field radiative heat transport and for locally heating samples. In all these applications, it is critical to know the thermal resistance to heat flow within the probe and the thermal contact resistance between the probe and the sample. Here, we present an approach for quantifying the aforementioned thermal resistances using picowatt resolution heat flow calorimeters. The measured contact resistance is found to be in good agreement with classical predictions for thermal contact resistance. The techniques developed here are critical for quantitatively probing heat flows at the nanoscale.

A platform to parallelize planar surfaces and control their spatial separation with nanometer resolution

Parallelizing planar surfaces and manipulating them into close proximity with spatial separation of nanoscale dimensions is critical for probing phenomena such as near-field radiative heat transport and Casimir forces. Here, we report on a novel platform, with an integrated reflected light microscope, that is capable of parallelizing two planar surfaces such that the angular deviation is <6 μrad, while simultaneously allowing control of the gap from 15 μm down to contact with ∼0.15 nm resolution. The capabilities of this platform were verified by using two custom-fabricated micro-devices with planar surfaces, 60 × 60 μm(2) each, whose flatness and surface roughness were experimentally quantified. We first parallelized the two micro-devices by using the developed platform in conjunction with a simple optical approach that relies on the shallow depth of field (∼2 μm) of a long working distance microscope objective. Subsequently, we experimentally tested the parallelism achieved via the optical alignment procedure by taking advantage of electrodes integrated into the micro-devices. Our measurements unambiguously show that the simple depth-of-field based optical approach enables parallelization such that the angular deviation between the two surfaces is within ∼500 μrad. This ensures that the separation between any two corresponding points on the parallel surfaces deviate by ∼30 nm or less from the expected value. Further, we show that improved parallelization can be achieved using the integrated micro-electrodes which enable surface roughness limited parallelization with deviations of ∼5 nm from parallelism.

Scanning Probe Microscopy for Thermal Transport Measurements

Understanding energy transport and dissipation in nanoscale devices is of increasing importance. Novel transport and dissipation phenomena are expected to arise in these devices as the characteristic dimensions become comparable to the wavelength, coherence length, and mean free path (MFP) of charge and energy carriers—electrons and phonons. Of particular importance are irreversible processes that occur in all nanoscale devices under nonequilibrium conditions and are associated with Joule heating and resultant temperature gradients. Therefore, the ability to measure local temperatures with nanometer resolution is critical for characterizing and understanding irreversibility, heat dissipation, and transport in nanoscale devices. A novel approach for high-resolution characterization of temperature fields is scanning thermal microscopy (SThM), which leverages special scanning probes with integrated temperature sensors. This review discusses various SThM techniques and highlights recent advances in ultra-high-vacuum SThM techniques for quantitative nanoscale thermometry. Further, we discuss applications of SThM to the elucidation of energy transport and dissipation in functional devices as well as prototypical low-dimensional devices.