Fritz J. Knorr

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 TiO(2) (anatase) nanotubes. In previous work using ensembles of conventional TiO(2) 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 (∼510 nm), showing the possible contribution of charge transport to the observed PL. While the PL from ensembles of TiO(2) 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 TiO(2) nanotubes compared to that of conventional TiO(2) 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.

Quality classification via Raman identification and SEM analysis of carbon nanotube bundles using artificial neural networks

One of the major obstacles for successful mass production of carbon nanotubes (CNTs) is performing quick and precise characterization of the properties of a given batch of nanotubes. In this paper, we have identified a set of intermediate steps that will lead to a comprehensive, scalable set of procedures for analyzing nanotubes. The proposed methodology was originated with data processing of Raman spectra of multi-wall carbon nanotubes (MWCNT) turfs and image enhancement of SEM micrographs. Image analysis techniques of SEM images were employed and stereological relations were determined for SEM images of CNT structures; these results were utilized to estimate the morphology of the turf (i.e. CNTs alignment and curvature) using an artificial neural networks (ANN) classifier. This model was also used to investigate the link between Raman spectra of CNTs and the quality of the turf morphology. This novel methodology will improve our capability to control the quality of the grown nanotubes through the use of this system in a supervised growth environment.

Temperature-dependent absorption spectrum of betaine-30 in methanol

The absorption spectrum of the strongly solvatochromic dye betaine-30 was examined in methanol as a function of temperature. In contrast to previous studies of betaine-30 in acetonitrile, the temperature-dependent spectrum in methanol displays an isosbestic point, suggestive of two forms of betaine-30 in equilibrium. We propose that the two species are betaine hydrogen-bonded to one and two methanol molecules, and we discuss the implications of the isosbestic point in view of the expected thermo-solvatochromic behavior. The absorption spectrum of betaine-30 in several binary solvent mixtures is examined to investigate the importance of specific interactions between betaine-30 and methanol and ethanol.

Equilibrium constants and optical spectra of 1:1 and 2:1 electron donor—acceptor complexes with overlapping bands

Abstract Equilibrium constants and molar absorptivities of the 1:1 and 2: 1 electron donor—acceptor complexes of a previously unstudied system ( p -xylene/ p -bromanil) have been determined. Since the optical spectrum of the acceptor ( p -bromanil) severely overlaps the charge-transfer spectra of the complexes, a parameterized matrix modeling method was used to resolve the experimental absorption spectra into contributions from the acceptor (A) the 1:1 complex (DA), and the 2:1 complex (D 2 A). Association constants for the bimolecular and termolecular complexes were found to be 3.696 ± 0.521 mol −1 and 1.095 ± 0.0061 mol −1 , respectively.

Aromatic Electron Acceptors Change the Chirality Dependence of Single-Walled Carbon Nanotube Oxidation

Single-walled carbon nanotubes (SWNTs) dispersed in sodium dodecyl sulfate (SDS) suspensions exhibit diameter-dependent protonation and oxidative quenching of their E11 fluorescence. This nanotube-diameter-based difference in solution chemistry is substantially changed when complexed with aromatic electron-accepting compounds such as nitrobenzene, o-nitrotoluene, 2,4-dinitrotoluene, and 9-nitroanthracene. SWNTs were suspended in aqueous SDS, and their emission spectra were measured as a function of pH and concentration of oxidizing agent (hypochlorite or hydrogen peroxide) to observe their protonation and oxidation behavior. The chirality dependence of the protonation and oxidation behavior became substantially reduced upon the addition of nitroaromatic compounds to the aqueous suspension. This suggests the possibility of forming an electron donor-acceptor (EDA) complex, where the SWNT is the electron donor and nitroaromatic compounds are the acceptor, and the resulting supramolecular complex exhibits different redox behavior than the uncomplexed SWNT.

Trap state photoluminescence of nanocrystalline and bulk TiO2: Implications for carrier transport

The visible photoluminescence of nanocrystalline TiO 2 is examined in the presence of surface binding agents and as a function of vacuum annealing in order to probe the molecular nature of surface defects. The photoluminesence (PL) of bulk crystals of anatase TiO 2 from (101) and (001) planes is also reported in order to test the hypothesis that electron and hole traps are spatially isolated on different crystal planes. We find that a number of hole scavengers are capable of quenching the PL associated with trapped electrons, while the ability of oxygen to quench PL through electron scavenging varies with the nature of the sample. We conclude that hole scavengers exert their influence on the PL through reaction with valence band holes rather than with spatially isolated trapped holes. Scavenging of electrons by O 2 , on the other hand, depends on adsorption at oxygen vacancies and varies with TiO 2 sample.

Two-electron photo-oxidation of betanin on titanium dioxide and potential for improved dye-sensitized solar energy conversion

The plant pigment betanin is investigated as a dye-sensitizer on TiO2 with regard to its potential to undergo twoelectron oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide intermediate which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on nanocrystalline TiO2 and ZrO2 films were performed on femtosecond and nanosecond time-scales and provide evidence for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dyesensitized solar cells.

Adaptive Neuro-Fuzzy Modeling of Mechanical Behavior for Vertically Aligned Carbon Nanotube Turfs

Several characterization methods have been developed to investigate the mechanical and structural properties of vertically aligned carbon nanotubes (VACNTs). Establishing analytical models at nanoscale to interpret these properties is complicated due to the nonuniformity and irregularity in quality of as-grown samples. In this paper, we propose a new methodology to investigate the correlation between indentation resistance of multi-wall carbon nanotube (MWCNT) turfs, Raman spectra and the geometrical properties of the turf structure using adaptive neuro-fuzzy phenomenological modeling. This methodology yields a novel approach for modeling at the nanoscale by evaluating the effect of structural morphologies on nanomaterial properties using Raman spectroscopy.

Empirical Modeling of Nanoindentation of Vertically Aligned Carbon Nanotube Turfs using Intelligent Systems

Establishing analytical models at the nanoscale to interpret the mechanical and structural properties of vertically aligned carbon nanotubes (VACNTs) is complicated due to the nonuniformity in quality of as-grown samples and the lack of an accurate procedure to evaluate structural properties of nanotubes in these samples. In this paper, we present a comparative study of empirical methodologies to investigate the correlation between indentation resistance of multi-wall carbon nanotube (MWCNT) turfs, Raman features and the morphological properties of the turf structure using adaptive neuro-fuzzy system and probabilistic neural networks. Both methodologies provide comprehensive and innovative approaches for phenomenological modeling of VACNTs morphologies, mechanical properties and Raman Spectra using intelligent-based systems.

Spectroelectrochemical photoluminescence of titanium dioxide nanosheets and nanoparticles in aqueous and nonaqueous environments

Spectroelectrochemical photoluminescence (SEPL) is used to investigate surface electron and hole traps on anatase nanoparticles, anatase nanosheets and rutile nanowires in aqueous and nonaqueous environments. In aqueous environment there is an overvoltage for occupying surface electron traps in rutile and anatase samples. For anatase, this overvoltage is larger on (101) nanoparticles than on (001) nanosheets. The electrochemical energy levels of electron traps determined by SEPL in acetonitrile are consistent with emitted photon energies as determined by the photoluminescence spectrum. Our results show how the contacting solvent and particle morphology can influence the redox potential of surface electron traps and thus guide further research on improving the performance of nano- TiO2 in applications such as dye–sensitized solar cells and solar water splitting.