Observation of Charge Transport in Single Titanium Dioxide Nanotubes by Micro-Photoluminescence Imaging and Spectroscopy

We present the first report of photoluminescence spectra and images of single TiO2 (anatase) nanotubes. In previous work using ensembles of conventional TiO2 nanoparticles, we interpreted the broad photoluminescence (PL) spectrum to be a superposition of hole trap emission, peaking in the green, and broad red PL arising from electron traps. PL spectra of individual nanotubes in inert environment show a similar broad emission, with peaks at around 560-610 nm. The PL from single nanotubes differs from the more blue-shifted PL of ordered nanotube films. The intensity of PL is found to be larger for single nanotubes than for ordered arrays, as a result of competition from transport in the contiguous samples and from introduction of additional trap states when the nanotubes are dispersed. PL images of single nanotubes show the emission to be concentrated in the area of excitation, but the peaks in the red and green components of the PL are not spatially coincident. Remote PL, occurring away from the excitation point, is observed in the green (similar to 510 nm), showing the possible contribution of charge transport to the observed PL. While the PL from ensembles of TiO2 nanotubes is fairly insensitive to contacting media, exposure of single nanotubes to air and ethanol changes the shape and intensity of the PL spectrum. Our results point to a very different trap state distribution in TiO2 nanotubes compared to that of conventional TiO2 nanoparticles, which we attribute to differences in exposed crystal facets. In addition, separation of nanotubes introduces additional photoluminescent trap states and changes the character of the emission from excitonic in the array to trap-mediated in single nanotubes.