Single-phase vanadium dioxide (VO2) thin films have been grown on Si3N4∕Si substrates by means of a well-controlled magnetron sputtering process. The deposited VO2 films were found to exhibit a semiconductor-to-metal transition (SMT) at ∼69°C with a resistivity change as high as 3.2 decades. A direct and clear-cut correlation is established between the SMT characteristics (both amplitude and abruptness of the transition) of the VO2 films and their crystallite size.
The UV‐assisted direct‐write fabrication of microcoils is shown using a UV‐photocurable carbon nanotube nanocomposite ink. The method employs the robotically controlled microextrusion of a filament combined with a UV exposure that follows the extrusion point. Upon curing, the increased rigidity of the extruded filament enables the creation of complex multi‐directional shapes.
We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.
A solution‐processed nanoarchitecture based on PbS quantum dots (QDs) and multi‐walled carbon nanotubes (MWCNTs) is synthesized by simply mixing the pre‐synthesized high‐quality PbS QDs and oleylamine (OLA) pre‐functionalized MWCNTs. Pre‐functionalization of MWCNTs with OLA is crucial for the attachment of PbS QDs and the coverage of QDs on the surface of MWCNTs can be tuned by varying the ratio of PbS QDs to MWCNTs. The apparent photoluminescence (steady‐state emission and fluorescence lifetime) “quenching” effect indicates efficient charge transfer from photo‐excited PbS QDs to MWCNTs. The as‐synthesized PbS‐QD/MWCNT nanoarchitecture is further incorporated into a hole‐conducting polymer poly(3‐hexylthiophene)‐(P3HT), forming the P3HT:PbS‐QD/MWCNT nanohybrid, in which the PbS QDs act as a light harvester for absorbing irradiation over a wide wavelength range of the solar spectrum up to near infrared (NIR, ≈1430 nm) range; whereas, the one‐dimensional MWCNTs and P3HT are used to collect and transport photoexcited electrons and holes to the cathode and anode, respectively. Even without performing the often required “ligand exchange” to remove the long‐chained OLA ligands, the built nanohybrid photovoltaic (PV) device exhibits a largely enhanced power conversion efficiency (PCE) of 3.03% as compared to 2.57% for the standard bulk hetero‐junction PV cell made with P3HT and [6,6]‐Phenyl‐C61‐Butyric Acid Methyl Ester (PCBM) mixtures. The improved performance of P3HT:PbS‐QD/MWCNT nanohybrid PV device is attributed to the significantly extended absorption up to NIR by PbS QDs as well as the effectively enhanced charge separation and transportation due to the integrated MWCNTs and P3HT. Our research results suggest that properly integrating QDs, MWCNTs, and polymers into nanohybrid structures is a promising approach for the development of highly efficient PV devices.
Terahertz time-domain spectroscopy is used to measure the complex terahertz conductivity of a nanogranular vanadium dioxide (VO2) thin film as a function of temperature through the metal-insulator transition. The Drude–Smith model provides a good fit to the observed terahertz conductivity, revealing a metallic state that forms via switching of individual nanograins and strong carrier confinement within the nanograins due to scattering off grain boundaries. Furthermore, the directly applied Drude–Smith model provides a more accurate description of the measured terahertz conductivity in this material than either Bruggeman or Maxwell–Garnett effective medium theories.
A newly designed photoactive nanohybrid structure based on the combination of near-infrared PbS quantum dots (QDs) as light harvester and onedimensional TiO 2 nanobelts (NBs) to guide the flow of photogenerated charge carriers is reported. Efficient electron transfer from photoexcited PbS QDs to TiO 2 NBs has been demonstrated to occur in the developed PbS-QD/TiO 2 -NB nanohybrids, and the charge-transfer property can be tuned through the size quantization effect of PbS QDs. Moreover, the use of TiO 2 NBs instead of TiO 2 NPs permits a larger critical size of PbS QDs capable of injecting electrons into TiO 2 NBs, which, in turn, markedly extends the "effective" absorption of the PbS-QD/TiO 2 -NB nanohybrids to a longer wavelength region up to 1400 nm. Such an extension of the "effective" absorption is a major asset for improving the overall photoconversion efficiency of PbS-QD/TiO 2 -NB nanohybrids-based photovoltaic devices.SECTION Nanoparticles and Nanostructures S emiconductor quantum dots (QDs) as light-harvesting assemblies have been attracting continued research interest for the development of next-generation photovoltaic (PV) devices 1-6 because they offer the possibility to tune their light response through the size quantization effect. Among the various semiconductor materials, PbS QDs have been paid special attention due to their specific advantages including the narrow band gap (e.g., 0.41 eV for bulk) and large excitonic Bohr radius (∼18 nm). 7 These specific advantages give more latitude to effectively use the size quantization effect for extending the absorption into the infrared range that comprises ∼40% of the solar spectrum. 7,8 More interestingly, the recent discovery of the multiple exciton generation (MEG) effect in some lead salts including PbS QDs opens up the possibility of overcoming the ordinary thermodynamic limits of solar energy conversion. 9-11 However, in the various investigations aiming at using PbS QD films for solar cells, 12-16 fast capture of electrons at the quantum dot interface remains a major challenge for efficient harvesting of light energy. The photogenerated electrons in mesoscopic films, for example, encounter many grain boundaries and/or interfaces in their pathways through the random network of semiconductor nanoparticles (NPs), thus increasing the probability of their recombination with the photogenerated holes. In fact, attaining high photoconversion efficiency in such nanostructured materials is still hindered by the rather poor transport of electrons across the NPs network.In order to improve the separation of photogenerated charge carriers, semiconductor QDs have been often combined with another appropriate semiconductor to form a hybrid structure. [17][18][19][20][21] In the prior aqueous solution route to synthesize PbS QDs in situ in a porous TiO 2 film, 17-19 the photoconversion efficiency was found to be fairly low because of the broad size distribution and poor crystal surface quality of QDs. Recently, the colloidal synthetic route has been proven to be ab...
This paper deals with the design and microfabrication of two three-dimensional (3D) freestanding patterned strain sensors made of single-walled carbon nanotube (SWCNT) nanocomposites with the ultraviolet-assisted direct-write (UV-DW) technique. The first sensor consisted of three nanocomposite microfibers suspended between two rectangular epoxy pads. The flexibility of the UV-DW technique enables the sensor and its housing to be manufactured in one monolithic structure. The second sensor was composed of a nanocomposite network consisting of four parallel microsprings, which demonstrates the high capability of the technique when compared to conventional photolithographic technologies. The performances of the sensors were assessed under tension and compression, respectively. The sensors' sensitivities were evaluated by correlating their measured resistivities to the applied displacements/strains. Electrical conductivity measurements revealed that the manufactured sensors are highly sensitive to small mechanical disturbances, especially for lower nanotube loadings when compared to traditional metallic or nanocomposite films. The present manufacturing method offers a new perspective for manufacturing highly sensitive 3D freestanding microstructured sensors.
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