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Measuring the sp2/sp3 carbon content ratio in a single nanodiamond using quantitative optical microscopy

Hamilton, Samuel 2023. Measuring the sp2/sp3 carbon content ratio in a single nanodiamond using quantitative optical microscopy. PhD Thesis, Cardiff University.
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The uses for dielectric nanoparticles, in particular nanodiamond (ND), cover a wide variety of fields including drug delivery and biomarkers in medicine. This is due to the properties of ND, being non-toxic, mechanically hard, and offering facile functionalisation chemistry. An important aspect modulating these properties is the presence of sp2 hybridisation at the surface, which changes the surface chemistry and the optical absorption properties. Treatments to reduce surface sp2 have been developed, however, a non-destructive method to accurately measure the sp2/sp3 ratio of single NDs is lacking. This thesis presents such a method, determining this ratio combining the optical microscopy methods of quantitative differential interference contrast (qDIC), extinction, and photothermal microscopy. By converting an optical phase gradient to an intensity change, DIC enables the detection of dielectric nanoparticles. A method to determine the size of these particles from DIC has been developed, for which a calibration has been carried out using polystyrene beads of 100 nm radius. This showed the technique to be accurate to better than 10% in volume, with the smallest detectable particles being limited by background and shot noise. Measurements of milled nanodiamonds with nominal sizes of below 50 nm, showed a nearly exponential size distribution with a mean size of 28 ± 1.4 nm. When illuminated, nanoparticles (NPs) cause attenuation of the transmitted light due to scattering and absorption, the sum of which is the extinction caused by the NP. A wide-field microscopy method by which the extinction cross section of NPs can be obtained has previously been developed. Since pure sp2 samples are absorbing under illumination by visible wavelengths, and scattering can be neglected, measurements of the extinction cross section could be obtained. Photothermal background has been shown to be present in stimulated Raman scattering (SRS) measurements. The absorption of the Stokes beam causes a temperature change, which by the formation of a thermal lens causes a modulation of the pump beam, detected as stimulated Raman loss (SRL). Using this photothermal imaging on individual nanodiamonds, by using a Stokes wavelength which is absorbed by surface sp2 and delaying the pump by a set time, photothermal signal from the NDs can be used to determine the absorption by sp2. Correlative qDIC measurements of the same nanodiamonds, along with correlative optical extinction and photothermal measurements of a pure sp2 sample, allowed for quantitative determination of the sp2/sp3 ratio of individual NDs. Investigations into NDs that had been commercially purchased, and subsequently treated by a combination of temperature annealing and acid etching to reduce the amount of surface sp2, were carried out. It was found that the treatment procedure reduced the surface sp2 significantly, from up to 150% coverage (meaning a layer and a half of sp2 surface coverage around the ND) when untreated to around 30% when treated. The small-est area of sp2 detectable using this technique was 52 nm2, corresponding to a full surface layer coverage of a diamond with an edge length of 10nm assuming cubic geometry. Similarly to NDs, carbon nanotubes (CNTs) also have a wide range of uses in medical and biotechnology applications due to their tensile strength, and their electrical properties. The electrical properties of a given CNT depend on a property known as its chiral index. Techniques by which this property of individual CNTs can be determined are available in the form of atomic force microscopy and Raman spectroscopy, however these require specialised setups to carry out. While the progress of work here was limited due to the Coronavirus pandemic, an investigation to determine the best sample preparation procedure to deposit CNTs onto a coverslip ensuring the presence of individual nanotubes has been carried out. Subsequent initial measurements of individual CNTs to determine their extinction cross section using a wide field microscope were started. It was found that a nanotube concentration of 2 μg/ml provided an adequate distribution of CNTs on the coverslip surface such that individual nanotubes could be identified. Given the respective properties of carbon allotropes such as NDs, making them desirable in many applications, are heavily dependant on the structures of individual NPs, it is important to have the ability to accurately characterise them prior to use. Presented here is a method by which the surface sp2 content in individual NDs can be measured using non-destructive techniques. This would allow for such characterisation of samples prior to use, providing the ability to screen samples which contain either too much or too little sp2 for a given application.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Biosciences
Subjects: Q Science > Q Science (General)
Date of First Compliant Deposit: 9 November 2023
Last Modified: 10 Nov 2023 09:59

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