Time-Resolved Microwave Photoconductivity (TRMC) Using Planar Microwave Resonators: Application to the Study of Long-Lived Charge Pairs in Photoexcited Titania Nanotube Arrays

Abstract

Steady-state (SRMC) and time-resolved microwave photoconductivity (TRMC) are key techniques used to perform the contact-less determination of carrier density, transport, trapping, and recombination parameters in charge transport materials such as organic semiconductors and dyes, inorganic semiconductors, and metal−insulator composites, which find use in conductive inks, thin film transistors, lightemitting diodes, photocatalysts, and photovoltaics. We present the theory, design, simulation, and fabrication of a planar microwave ring resonator operating at 5.25 GHz with a quality factor of 224, to perform SRMC and TRMC measurements. Our method consists of measuring the resonance frequency (f 0 ) and Q-factor of the microwave resonator with the sample to be probed placed in a defined sensitive region of the resonator where the microwave field is highly concentrated. We also provide proof of concept measurements of the time-resolved microwave photoresponse of anatase-phase TiO 2 nanotube array membranes (TNTAMs) using the planar microstrip resonator. An unusual observation was the persistence of charged pair states in TNTAMs for several hours at room temperature under ambient conditions. Fast (120−220 s), slow (1300−2850 s), and very slow (6−26 h) components were extracted from the long-lived photoconductive decays of TNTAMs in response to 365, 250, and 405 nm illumination and assigned to various trap-mediated processes in TiO 2 nanotubes.

Multi-Channel Capacitive Sensor Arrays

“…This shows that a higher quality factor can indeed be achieved with the metamaterial approach. Furthermore, the SRR-based design agrees with most of the metamaterial-based microwave sensor designs toward a higher quality factor, with typical quality factors varying between 50 and 100 [ 9 , 10 , 11 , 12 , 13 ]. The resonances from the SRR coupled sensor array are within the band between 2.6 and 3.3 GHz, with a fractional bandwidth of about 12%, while for the design with surface-mounted inductors, the fractional bandwidth is over 100% (frequency band between 0.25 and 0.85 GHz).…”

“…It has been shown previously that a target can be detected from the reflected signal of a transmission line with multiple capacitive elements [ 6 , 7 ]. A planar structure based on resonators, in particular metamaterial-based ring resonators, has been proposed for various microwave sensing applications [ 9 , 10 , 11 , 12 , 13 ]. For resonator-based sensors, the quality factor of the resonance is a critical parameter in determining the sensitivity of the sensor.…”

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

“…It was considered by them that the hydration dependent rate is important in the ultra-slow recombination. The ultra-long life-time, on a timescale of hours, of charge carriers in photoexcited TiO 2 nanotubes arrays prepared from an anodization were also revealed by a time-resolved TRMC method, which is ascribed to the thermally-assisted re-emission of holes and the recombination processes involving carriers trapped in deep gap states located 1.0-1.2 eV above the VB edge [375].…”

Interfacial band alignment for photocatalytic charge separation in TiO2nanotube arrays coated with CuPt nanoparticles

“…[8][9][10][11] Indeed, the dynamics of carrier recombination in TiO 2 nanotubes and nanorod arrays have been found to be significantly different from nanoparticles of comparable size. [12][13][14][15][16] The interactions of light with spherical and rod-type structures are different owing to the scattering of nanoparticles governed by the exact solutions given by Mie theory 17 and that of nanorods/nanotubes affected by optical anisotropy and approximately described by Gans theory. 18 While the co-catalysts in nanoparticulate photocatalysts are also nanoparticles of roughly the same size and present a single length scale, co-catalyst NPs are much smaller than the length of NRs/NTs resulting in multiple length scales, a desirable feature for high performance photocatalysis.…”