While fundamental understanding of thermal radiation (Nobel prize-1911) and explanation of its spectral characteristics (Nobel prize-1918) have provided a clear picture of radiative energy transfer between bodies in the far-field, this picture totally collapses in the near-field regime when the two interacting bodies are placed at sub-wavelength distances (i.e. distances smaller than the peak wavelength given by Wien’s law). The breakdown of Stefan-Boltzmann law promises remarkable potential for different applications in the field of energy conversion and sensing technologies. Theoretical studies predict that radiative heat flow between parallel surfaces increases by orders of magnitude when the separation between the surfaces decreases from a couple of micrometer to tens of nanometers. This enhanced heat flux is ascribed to an increased density of electromagnetic states in the near-field of the thermal sources. Moreover, at sources supporting surface modes this enhanced density of states is attributed to surface plasmons or surface phonon polaritons which are predicted to have high spatial and temporal coherence in the near-field of emitting surfaces. Interestingly and in contrast with the broadband nature of far-field radiation, near-field radiation has a monochromatic spectrum where the frequency of dominant photon depends on the material and its surface structure. In my graduate studies, I developed micro-mesa shaped thermal emitter devices alongside with heat flow calorimeters and studied Near-Field Radiative Heat Transport (NFRHT) between SiO2-SiO2 and SiO2-Au surfaces in sphere-plane and plane-plane configurations [Song & Sadat et al., In Preparation, 2013].
While Fourier’s law describes diffusive transport of energy carriers in bulk materials, breakdown of this picture at microscales where the wave-nature of the energy carriers comes into the play, poses serious limitations on long-established engineering design procedures. Study of energy transport at micro/nanometer length scales where material dimensions are comparable with or smaller than the wavelength or mean free path of energy carriers, promises groundbreaking scientific progress and revolutionary applications from the cooling of hot electronic devices to the design of more efficient thermoelectric energy harvesters.

However, the most prominent hurdle to study heat transport at nanoscales is manifested as the lack of instruments and measurement techniques to accurately quantify flux of heat at these length scales. This challenge arises from the fact that the small physical size and miniscule signals that need to be recorded from these systems require unprecedented precisions and resolutions. To address this need, I have studied fundamental principles governing high-resolution calorimetry experiments and took these understandings to developed two types of calorimeters [Sadat et al, App. Phys. Lett., 2011 and Sadat et al, App. Phys. Lett., 2013], suitable for study of energy transport at nanoscales. Both devices feature low thermal conductance in conjunction with the high resolution Bimaterial Cantilever (BMC) and Resistance thermometers that correspondingly enabled me to perform calorimetry measurements with picowatts resolutions. While the BMC version successfully demonstrated the feasibility of such high-resolution calorimetry measurements, I developed second device envisioning a device tailored to the specific goal of studying heat transport in the limiting one dimensional case of single molecule entities. For this purpose, after detailed analysis of the noise characteristics of resistive thermometers, I developed techniques to improve the resolution of generally renowned as low-resolution resistance thermometers by an order of magnitude [Sadat et al, Rev. Sci. Inst., 2012].

Probing temperature fields with nanometer resolution is critical to understanding nanoscale thermal transport as well as heat dissipation in nanoscale devices. I demonstrated 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 [Sadat et al, App. Phys. Lett., 2010]. In this technique, a platinum-coated AFM cantilever is sequentially placed in soft mechanical contact with a metallic (gold) surface to 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. The results of this study demonstrate a direct measurement of the temperature field of metallic surfaces without using specially fabricated scanning thermal microscopy probes.
Understanding the charge and energy transport properties of metal-molecule-metal junctions (MMMJs) is essential to the creation of molecular electronic, photovoltaic, and thermoelectric devices. On this way and before such devices can be created, it is necessary to answer several fundamental questions regarding the electronic structure of molecular junctions. We developed an experimental technique to concurrently measure Seebeck coefficient and current-voltage (I-V) characteristics of a molecular junction and identify the nature (electron or hole-dominated) and effective energetic separation of the molecular orbital closest to the electrodes’ Fermi level [Tan* & Sadat* et al, App. Phys. Lett., 2010]. Understanding transport properties of metal-molecule-metal junctions (MMMJs) helps to answer fundamental questions necessary for realizing molecular and organic devices. By placing a gold-coated atomic force microscope tip in contact with a monolayer of molecules assembled on a gold substrate, I measured their Seebeck coefficient and analyzed the (I-V) characteristics of the junction to estimate the separation between frontier orbital and Fermi levels. Further, we used this technique to perform a systematic study on the relation between the electronic structure of molecules and their thermoelectric properties [Tan, Balachandran & Sadat et al, JACS, 2011].

Research Directions

Research Portfolio


Experimental Study of Near-Field Radiative Heat Transfer

Ultra-High-Resolution Heat Flow Calorimetry for Micro/Nanoscale Studies

Nanoscale Thermometry Using Point Contact Thermocouples

Concurrent Study of Charge Transport and Thermopower in Metal-Molecule-Metal Junctions

Large Deflection Spiral-Shaped Micro-Mirror Actuator for Free Space Optical Switching

I developed a novel two-axis spiral-shaped micro-mirror manipulator suitable for free-space optical switching applications [Sadat et al., Journal of Microelectromechanical Systems, 2009; Sadat et al., Journal of Advanced Mechanical Design, Systems and Manufacturing, 2007]. This microfabricated device is actuated using electrostatic force operate based on a so called “zipping-effect”.

The interacting plates of the capacitor are comprised of two electrodes, one called fixed and the other moving electrode. The fixed electrode is realized using a metallic path, patterned on the substrate and the moving electrode is a spiral-shaped thin film silicon oxide beam. The two plates  are fabricated on two different chips which subsequently, were aligned and placed in proper orientation using a micro-manipulator system and fixed using adhesive.

The design utilizes the enhanced flexibility of the spiral-shaped electrode while keeping a small overall footprint of 600 μm × 600 μm for the assembled device. I modeled the device using a coupled electromechanical model and experimentally characterized its performance using a reflection measurement approach. A continuous rotational actuation of 17 has been achieved with an actuation voltage of 235 V.