Diagnosis of the redox levels of TCNQF4 compounds using vibrational spectroscopy

The vibrational spectroscopy of TCNQF4, TCNQF41- and TCNQF42- has been investigated by means of density functional theory. Band assignments in infrared and Raman spectra have been clarified and a series of diagnostics developed for redox level characterisation of TCNQF4 compounds. In the C£C stretching region (1460-1600 cm-1), TCNQF40 and TCNQF 41- show two bands, with the more energetic being at 1600 cm-1 in TCNQF40 and at approximately 1535 cm-1 in TCNQF41-; in TCNQF42- both modes absorb below 1500 cm-1, often merging to give a single band. In the C-F and endocyclic C-C stretching region (1290 and 1360 cm-1), TCNQF40 and TCNQF41- show strong bands, whereas TCNQF42- absorbs weakly or not at all. (Additional bands, e.g. from co-crystallised solvent molecules, may complicate this region.) In the nitrile stretching region (2000-2250 cm-1), modes are highly sensitive to nitrile coordination by metal cations. All three redox levels can produce bands above 2200 cm -1, however bands below 2150 cm-1 are usually due to TCNQF42-. This sensitivity to coordination is likely to affect the spectra of many organic molecular ions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A new Raman metric for the characterisation of graphene oxide and its derivatives

Raman spectroscopy is among the primary techniques for the characterisation of graphene materials, as it provides insights into the quality of measured graphenes including their structure and conductivity as well as the presence of dopants. However, our ability to draw conclusions based on such spectra is limited by a lack of understanding regarding the origins of the peaks. Consequently, traditional characterisation techniques, which estimate the quality of the graphene material using the intensity ratio between the D and the G peaks, are unreliable for both GO and rGO. Herein we reanalyse the Raman spectra of graphenes and show that traditional methods rely upon an apparent G peak which is in fact a superposition of the G and D' peaks. We use this understanding to develop a new Raman characterisation method for graphenes that considers the D' peak by using its overtone the 2D'. We demonstrate the superiority and consistency of this method for calculating the oxygen content of graphenes, and use the relationship between the D' peak and graphene quality to define three regimes. This has important implications for purification techniques because, once GO is reduced beyond a critical threshold, further reduction offers limited gain in conductivity.

Boron nitride nanosheet-veiled gold nanoparticles for surface-enhanced raman scattering

Atomically thin boron nitride (BN) nanosheets have many properties desirable for surface-enhanced Raman spectroscopy (SERS). BN nanosheets have a strong surface adsorption capability toward airborne hydrocarbon and aromatic molecules. For maximized adsorption area and hence SERS sensitivity, atomically thin BN nanosheet-covered gold nanoparticles have been prepared for the first time. When placed on top of metal nanoparticles, atomically thin BN nanosheets closely follow their contours so that the plasmonic hot spots are retained. Electrically insulating BN nanosheets also act as a barrier layer to eliminate metal-induced disturbances in SERS. Moreover, the SERS substrates veiled by BN nanosheets show an outstanding reusability in the long term. As a result, the sensitivity, reproducibility, and reusability of SERS substrates can be greatly improved. We also demonstrate that large BN nanosheets produced by chemical vapor deposition can be used to scale up the proposed SERS substrate for practical applications.