Controlling the flow of thermal energy is crucial to numerous applications ranging from microelectronic devices to energy storage and energy conversion devices. Here, we report ultralow lattice thermal conductivities of solution-synthesized, single-crystalline all-inorganic halide perovskite nanowires composed of CsPbI (0.45 ± 0.05 W·m·K), CsPbBr (0.42 ± 0.04 W·m·K), and CsSnI (0.38 ± 0.04 W·m·K). We attribute this ultralow thermal conductivity to the cluster rattling mechanism, wherein strong optical-acoustic phonon scatterings are driven by a mixture of 0D/1D/2D collective motions. Remarkably, CsSnI possesses a rare combination of ultralow thermal conductivity, high electrical conductivity (282 S·cm), and high hole mobility (394 cm·V·s). The unique thermal transport properties in all-inorganic halide perovskites hold promise for diverse applications such as phononic and thermoelectric devices. Furthermore, the insights obtained from this work suggest an opportunity to discover low thermal conductivity materials among unexplored inorganic crystals beyond caged and layered structures.

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 4-6), 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.

Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits 1 . They are also ideal platforms to test quantum transport theories that are required to describe charge and energy transfer in novel functional nanodevices.Recent work has successfully probed electric and thermoelectric phenomena 2-8 in atomicscale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized due to experimental challenges. Here, using custom-fabricated scanning probes with integrated nanoscale thermocouples, we show that heat dissipation in the electrodes of molecular junctions, whose transmission characteristics are strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both the bias polarity and the identity of majority charge carriers (electrons vs. holes). In contrast, 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. 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 9-11 . 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 12,13 .Charge transport is always accompanied by heat dissipation (Joule heating). This process is well understood at the macroscale where the power dissipation (heat dissipated per unit time) is volumetric and is given by j 2 ρ, where j is the magnitude of the current density and ρ is the In this work, we overcome this challenging experimental hurdle by leveraging custom- Seebeck coefficient of the thermocouple. We note that R P and S TC were experimentally determined to be 72800 ± 500 K/W and 16.3 ± 0.2 µV/K, respectively (see SI).We began our experimental studies, at room temperature, by trapping single molecules of 1,4-benezenediisonitrile (BDNC, see Fig. 1c) between the Au electrodes of the NTISTP and the substrate using a break junction technique 5,20 . We first obtained electrical conductance versus displacement traces by monitoring the electrical current under an applied bias while the NTISTPsubstrate separation was systematically varied. In order to probe heat dissipation we created stable Au-BDNC-Au junctions with a conductance that is within 10% of the most probable low-bias conductance 20 . We studied heat dissipation in 100 distinct Au-BDNC-Au junctions, at each bias, to obtain the average temperature rise (ΔT TC, Avg ) and the time-averaged power dissipation in the NTISTP (Q P, Avg ) for both positive and negative biases. Here,...

Molecular junctions hold significant promise for efficient and high-power-output thermoelectric energy conversion. Recent experiments have probed the thermoelectric properties of molecular junctions. However, electrostatic control of thermoelectric properties via a gate electrode has not been possible due to technical challenges in creating temperature differentials in three-terminal devices. Here, we show that extremely large temperature gradients (exceeding 1 × 10(9) K m(-1)) can be established in nanoscale gaps bridged by molecules, while simultaneously controlling their electronic structure via a gate electrode. Using this platform, we study prototypical Au-biphenyl-4,4'-dithiol-Au and Au-fullerene-Au junctions to demonstrate that the Seebeck coefficient and the electrical conductance of molecular junctions can be simultaneously increased by electrostatic control. Moreover, from our studies of fullerene junctions, we show that thermoelectric properties can be significantly enhanced when the dominant transport orbital is located close to the chemical potential (Fermi level) of the electrodes. These results illustrate the intimate relationship between the thermoelectric properties and charge transmission characteristics of molecular junctions and should enable systematic exploration of the recent computational predictions that promise extremely efficient thermoelectric energy conversion in molecular junctions.