Anastassios Mavrokefalos

Thermoelectric and structural characterizations of individual electrodeposited bismuth telluride nanowires

The thermoelectric properties and crystal structure of individual electrodeposited bismuth telluride nanowires (NWs) were characterized using a microfabricated measurement device and transmission electron microscopy. Annealing in hydrogen was used to obtain electrical contact between the NW and the supporting Pt electrodes. By fitting the measured Seebeck coefficient with a two-band model, the NW samples were determined to be highly n-type doped. Higher thermal conductivity and electrical conductivity were observed in a 52 nm diameter monocrystalline NW than a 55 nm diameter polycrystalline NW. The electron mobility of the monocrystalline NW was found to be about 19% lower than that of bulk crystal at a similar carrier concentration and about 2.5 times higher than that of the polycrystalline NW. The specularity parameter for electron scattering by the NW surface was determined to be about 0.7 and partially specular and partially diffuse, leading to a reduction in the electron mean-free path from 61 nm in ...

Four-probe measurements of the in-plane thermoelectric properties of nanofilms

Measuring in-plane thermoelectric properties of submicron thin films has remained a challenging task. Here we report a method based on a suspended microdevice for four-probe measurements of the Seebeck coefficient, thermal conductivity, electrical conductivity, and thermoelectric figure of merit of patterned indium arsenide (InAs) nanofilms assembled on the microdevice. The contact thermal resistance and intrinsic thermal resistance of the 40 nm thick InAs nanofilm sample were measured by using the nanofilm itself as a differential thermocouple to determine the temperature drops at the contacts. The microdevice was also used to measure a 190 nm thick silicon dioxide (SiO(2)) film and the results were compared with those reported in the literature. A through-substrate hole under the suspended microdevice allows for transmission electron microscopy characterization of the nanofilm sample assembled on the device. This capability enables one to correlate the measured thermoelectric properties with the crystal structures of the nanofilm.

Nanoscale thermal radiation between two gold surfaces

In this letter, we measured the nanoscale thermal radiation between a microsphere and a substrate which were both coated with thick gold films. Although gold is highly reflective for thermal radiation, the radiative heat transfer between two gold surfaces was demonstrated to be significantly enhanced at nanoscale gaps beyond the blackbody radiation limit due to the tunneling of non-resonant evanescent waves. The measured heat transfer coefficient between two gold surfaces agreed well with theoretical prediction. At a gap d = 30 nm ± 5 nm, the heat transfer coefficient between two gold surfaces was observed to be as large as ∼400 W/m2·K, much greater than the blackbody radiation limit (∼5 W/m2·K).

In-plane thermal conductivity of disordered layered WSe2 and (W)x(WSe2)y superlattice films

It was recently reported that misoriented layered WSe2 and (W)x(WSe2)y films possess extremely low cross-plane thermal conductivity. Here, we report that the in-plane thermal conductivity results for WSe2 and W4(WSe2)10 films measured by using a suspended device are about 30 times higher than the cross-plane values because of the in-plane ordered and cross-plane disordered structures and about six times lower than that of compacted single-crystal WSe2 platelets. The additional W layers in the W4(WSe2)10 films were found to greatly increase the in-plane electrical conductivity relative to the WSe2 films, but reduce the in-plane lattice thermal conductivity assuming the Wiedemann-Franz law.

In-plane thermal and thermoelectric properties of misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films

The in-plane thermal conductivity is measured to be three times lower in misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films than disordered-layered WSe2 because of interface scattering despite a higher cross-plane value in the former than the latter. While having little effect on the in-plane thermal conductivity, annealing the p-type [(PbSe)0.99]2(WSe2)2 films in Se increases the in-plane Seebeck coefficient and electrical conductivity because of decreased defect and hole concentrations. Increasing interface density of the annealed films by decreasing x from 4 to 2 has weak influence on the in-plane thermal conductivity but increases the Seebeck coefficient and decreases the room-temperature electrical conductivity.

Probing electronic properties of molecular engineered zinc oxide nanowires with photoelectron spectroscopy.

ZnO nanowires (NWs) are emerging as key elements for new lasing, photovoltaic and sensing applications but elucidation of their fundamental electronic properties has been hampered by a dearth of characterization tools capable of probing single nanowires. Herein, ZnO NWs were synthesized in solution and integrated into a low energy photoelectron spectroscopy system, where quantitative optical measurements of the NW work function and Fermi level location within the band gap were collected. Next, the NWs were decorated with several dipolar self-assembled monolayers (SAMs) and control over the electronic properties is demonstrated, yielding a completely tunable hybrid electronic material. Using this new metrology approach, a host of other extraordinary interfacial phenomena could be explored on nanowires such as spatial dopant profiling or heterostructures.

Visible Flip-Chip Light-Emitting Diodes on Flexible Ceramic Substrate With Improved Thermal Management

We demonstrate flip-chip light-emitting diodes (FC-LEDs) on a flexible yttria-stabilized zirconia (YSZ) substrate and compare them with FC-LEDs on a polymeric substrate. Degradation of luminescence intensity and red-shift of peak wavelength are not observed for the LED on the flexible YSZ, unlike one on the polyimide substrate, due to improved capability to remove the generated heat from the chip to the substrate. Thermal distribution measurements and finite-element simulations show improved thermal management by the flexible ceramic as compared with previously developed flexible LEDs on polymeric substrates. The results present an improved solution to high power operation of flexible LEDs.

Combined Thermoelectric and Structure Characterizations of Patterned Nanowires

Theoretical studies have suggested that Bi-based and III-V nanowire structures may have high thermoelectric figure of merit (ZT). It was found in a previous measurement that the thermoelectric properties of individual electro-deposited bismuth telluride nanowires are largely influenced by the crystal structure including crystalline quality, chemical composition, doping concentration, and surface roughness, all of which cannot be controlled readily in various bottom-up nanowire synthesis method. We have developed a top-down fabrication process of suspended indium arsenide (InAs) nanowires. Based on nanolithography and reactive ion etching, the nanowires are patterned from an epitaxial thin film deposited by molecular beam epitaxy with well-controlled doping concentration, which can be determined from Hall measurement. The thermoelectric properties of these top-down patterned III-V nanowires have been characterized using a new design of a suspended microdevice. The new device allows for transmission electron microscopy and energy dispersive X-ray spectroscopy analysis of the same nanowire assembled on the microdevice so as to establish the structure-thermoelectric properties relationships. This paper reports the measured thermoelectric properties of a patterned InAs nanowire with a rectangular cross section of 150 nm in width and 40 nm thickness in a temperature range between 100 K and 400 K. The obtained Seebeck coefficient, thermal conductivity, electrical conductivity, and ZT are -57.2 muV/K, 4.11 W/m K, 1350 S/m, and 0.00032, respectively, at temperature 300 K

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.