Milad Yarali

Physisorbed versus chemisorbed oxygen effect on thermoelectric properties of highly organized single walled carbon nanotube nanofilms

The effect of physisorbed vs. chemisorbed oxygen on highly organized single walled carbon nanotube (SWCNT) ultrathin films is investigated by correlating the thermoelectric properties measured by a suspended micro-device to the SWCNT structure characterized by Raman spectroscopy and transmission electron microscopy. The results show that SWCNTs with weakly bonded oxygen molecules on the surface were determined to be initially p-type with metallic behavior and after annealing in vacuum they transition to n-type with semiconducting behavior where the charge transport is dominated by a 2D Mott variable range hopping mechanism due to molecular desorption. The structural characterization reveals that there is no change in the structure of the SWCNT network, indicating that the source of the drastic change in electrical properties is due to the molecule interaction with the surface of the SWCNT. Even though there is a significant change in the electrical properties, the thermal conductivity remains unchanged. On the other hand, the oxidized SWCNT sample with stronger C–O bonds exhibits purely p-type metallic behavior that is insensitive to annealing conditions and shows lower thermal conductivity values because of the enhanced phonon scattering due to the absorbed oxygen molecules and residual poly-methyl-methacrylate (PMMA).

In-Plane Thermal Conductivity of Radial and Planar Si/SiOx Hybrid Nanomembrane Superlattices

Silicon, although widely used in modern electronic devices, has not yet been implemented in thermoelectric applications mainly due to its high thermal conductivity, κ, which leads to an extremely low thermoelectric energy conversion efficiency (figure of merit). Here, we present an approach to manage κ of Si thin-film-based nanoarchitectures through the formation of radial and planar Si/SiOx hybrid nanomembrane superlattices (HNMSLs). For the radial Si/SiOx HNMSLs with various numbers of windings (1, 2, and 5 windings), we observe a continuous reduction in κ with increasing number of windings. Meanwhile, the planar Si/SiOx HNMSL, which is fabricated by mechanically compressing a five-windings rolled-up microtube, shows the smallest in-plane thermal conductivity among all the reported values for Si-based superlattices. A theoretical model proposed within the framework of the Born-von Karman lattice dynamics to quantitatively interpret the experimental data indicates that the thermal conductivity of Si/SiOx HNMSLs is to a great extent determined by the phonon processes in the SiOx layers.

Electrical and optical properties of sub-10 nm nickel silicide films for silicon solar cells

Highly conductive and transparent films of ultra-thin p-type nickel silicide films have been prepared by RF magnetron sputtering of nickel on silicon substrates followed by rapid thermal annealing in an inert environment in the temperature range 400–600 °C. The films are uniform throughout the wafer with thicknesses in the range of 3–6 nm. The electrical and optical properties are presented for nickel silicide films with varying thickness. The Drude–Lorentz model and Fresnel equations were used to calculate the dielectric properties, sheet resistance, absorption and transmission of the films. These ultrathin nickel silicide films have excellent optoelectronic properties for p-type contacts with optical transparencies up to 80% and sheet resistance as low as ~0.15 µΩ cm. Furthermore, it was shown that the use of a simple anti-reflection (AR) coating can recover most of the reflected light approaching the values of a standard Si solar cell with the same AR coating. Overall, the combination of ultra-low thickness, high transmittance, low sheet resistance and ability to recover the reflected light by utilizing standard AR coating makes them ideal for utilization in silicon based photovoltaic technologies as a p-type transparent conductor.

Using Mosaicity to Tune Thermal Transport in Polycrystalline Aluminum Nitride Thin Films

The effect of controlling the c-axis alignment (mosaicity) to the cross-plane thermal transport in textured polycrystalline aluminum nitride (AlN) thin films is experimentally and theoretically investigated. We show that by controlling the sputtering conditions we are able to deposit AlN thin films with varying c-axis grain tilt (mosaicity) from 10° to 0°. Microstructural characterization shows that the films are nearly identical in thickness and grain size, and the difference in mosaicity alters the grain interface quality. This has a significant effect to thermal transport where a thermal conductivity of 4.22 vs 8.09 W/mK are measured for samples with tilt angles of 10° versus 0° respectively. The modified Callaway model was used to fit the theoretical curves to the experimental results using various phonon scattering mechanisms at the grain interface. It was found that using a non-gray model gives an overview of the phonon scattering at the grain boundaries, whereas treating the grain boundary as an array of dislocation lines with varying angle relative to the heat flow, best describes the mechanism of the thermal transport. Lastly, our results show that controlling the quality of the grain interface provides a tuning knob to control thermal transport in polycrystalline materials.

Effect of Metal Doping and Vacancies on the Thermal Conductivity of Monolayer Molybdenum Diselenide

It is well understood that defect engineering can give rise to exotic electronic properties in transition-metal dichalcogenides, but to this date, there is no detailed study to illustrate how defects can be engineered to tailor their thermal properties. Here, through combined experimental and theoretical approaches based on the first-principles density functional theory and Boltzmann transport equations, we have explored the effect of lattice vacancies and substitutional tungsten (W) doping on the thermal transport of the suspended molybdenum diselenide (MoSe2) monolayers grown by chemical vapor deposition (CVD). The results show that even though the isoelectronic substitution of the W atoms for Mo atoms in CVD-grown Mo0.82W018Se2 monolayers reduces the Se vacancy concentration by 50% compared to that found in the MoSe2 monolayers, the thermal conductivity remains intact in a wide temperature range. On the other hand, Se vacancies have a detrimental effect for both samples and more so in the Mo0.82W018Se2 monolayers, which results in thermal conductivity reduction up to 72% for a vacancy concentration of 4%. This is because the mass of the W atom is larger than that of the Mo atom, and missing a Se atom at a vacancy site results in a larger mass difference and therefore kinetic energy and potential energy difference. Furthermore, the monotonically increasing thermal conductivity with temperature for both systems at low temperatures indicates the importance of boundary scattering over defects and phonon-phonon scattering at these temperatures.