Seid Sadat

Effect of Length and Contact Chemistry on the Electronic Structure and Thermoelectric Properties of Molecular Junctions

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.

Enhancement of near-field radiative heat transfer using polar dielectric thin films

Thermal radiative emission from a hot surface to a cold surface plays an important role in many applications, including energy conversion, thermal management, lithography, data storage and thermal microscopy. Recent studies on bulk materials have confirmed long-standing theoretical predictions indicating that when the gap between the surfaces is reduced to tens of nanometres, well below the peak wavelength of the blackbody emission spectrum, the radiative heat flux increases by orders of magnitude. However, despite recent attempts, whether such enhancements can be obtained in nanoscale dielectric films thinner than the penetration depth of thermal radiation, as suggested by theory, remains experimentally unknown. Here, using an experimental platform that comprises a heat-flow calorimeter with a resolution of about 100 pW (ref. 7), we experimentally demonstrate a dramatic increase in near-field radiative heat transfer, comparable to that obtained between bulk materials, even for very thin dielectric films (50-100 nm) when the spatial separation between the hot and cold surfaces is comparable to the film thickness. We explain these results by analysing the spectral characteristics and mode shapes of surface phonon polaritons, which dominate near-field radiative heat transport in polar dielectric thin films.

Nanoscale thermometry using point contact thermocouples.

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 approximately 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.

Measurement of thermopower and current-voltage characteristics of molecular junctions to identify orbital alignment

We report an experimental technique that concurrently measures the Seebeck coefficient and the current-voltage (I-V) characteristics of a molecular junction to determine the identity and the effective energetic separation of the molecular orbital closest to the electrodes’ Fermi level. Junctions created by contacting a gold-coated atomic force microscope tip with a monolayer of molecules assembled on a gold substrate were found to have a Seebeck coefficient of (+16.9±1.4) μV/K. This positive value unambiguously shows that the highest occupied molecular orbital (HOMO) dominates charge transport. Further, by analyzing the (I-V) characteristics, the HOMO level is estimated to be ∼0.69 eV with respect to the Fermi level.

Room temperature picowatt-resolution calorimetry

Picowatt-resolution calorimetry is necessary for fundamental studies of nanoscale energy transport. Here, we report a microfabricated device capable of <4 pW resolution—an order of magnitude improvement over state-of-the-art room temperature calorimeters. This is achieved by the incorporation of two important features. First, the active area of the device is thermally isolated by thin and long beams with a total thermal conductance (G) of ∼600 nW/K. Further, a bimaterial cantilever thermometer capable of a temperature resolution (ΔTres) of ∼4 μK is integrated into the microdevice. The small thermal conductance and excellent temperature resolution enable measurements of heat currents (q = G × ΔTres) with a resolution <4 pW.

Resistance thermometry-based picowatt-resolution heat-flow calorimeter

We demonstrate a microfabricated heat-flow calorimeter capable of measuring modulated heat currents with ∼5 pW resolution. This is achieved by combining the excellent thermal isolation of a microdevice suspended by thin and long beams (conductance ∼150 nW/K) with a high-resolution resistance thermometer that enables temperature measurements with 10–50 μK resolution [Sadat et al., Rev. Sci. Instrum. 83(8), 084902 (2012)). The calorimeter described here has a resolution comparable to state-of-the-art bimaterial cantilever-based calorimeters but surpasses previous designs by dissipating an order of magnitude lower power in the measurement process.

High resolution resistive thermometry for micro/nanoscale measurements

High resolution thermometry plays an important role in several micro/nanoscale studies. Here, we present a detailed analysis of the resolution of resistance thermometry schemes that employ an electrical sensing current to monitor the temperature-dependent resistance. Specifically, we theoretically and experimentally analyze four different schemes where modulated or unmodulated temperatures in microdevices are measured using modulated or unmodulated sensing currents. Our analysis and experiments suggest that measurement of unmodulated temperatures using a modulated sensing current improves the resolution in comparison to a scenario where an unmodulated sensing current is used. However, depending on the exact measurement conditions, such improvements might be modest as the overall resolution may be limited by random low frequency environmental temperature fluctuations. More importantly, we find that high-resolution thermometry can be achieved in the measurement of modulated temperatures. Specifically, we show that by using appropriate instrumentation and a 10 kΩ platinum resistance thermometer it is possible to measure modulated temperatures (0.5-20 Hz) with a resolution of about 20-100 μK. The advances described here will enable a dramatic improvement in the heat-current resolution of resistive thermometry based microdevices that are used for probing nanoscale phonon and photon transport.

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.

Hundred-fold enhancement in far-field radiative heat transfer over the blackbody limit

Radiative heat transfer (RHT) has a central role in entropy generation and energy transfer at length scales ranging from nanometres to light years1. The blackbody limit2, as established in Max Planck’s theory of RHT, provides a convenient metric for quantifying rates of RHT because it represents the maximum possible rate of RHT between macroscopic objects in the far field—that is, at separations greater than Wien’s wavelength3. Recent experimental work has verified the feasibility of overcoming the blackbody limit in the near field4–7, but heat-transfer rates exceeding the blackbody limit have not previously been demonstrated in the far field. Here we use custom-fabricated calorimetric nanostructures with embedded thermometers to show that RHT between planar membranes with sub-wavelength dimensions can exceed the blackbody limit in the far field by more than two orders of magnitude. The heat-transfer rates that we observe are in good agreement with calculations based on fluctuational electrodynamics. These findings may be directly relevant to various fields, such as energy conversion, atmospheric sciences and astrophysics, in which RHT is important.Rates of radiative heat transfer between sub-wavelength planar membranes are experimentally and theoretically shown to exceed the blackbody limit in the far field by more than two orders of magnitude.