Raman G band in double-wall carbon nanotubes combining p doping and high pressure

We use sulfuric acid as pressure medium to extrapolate the G-band position of the inner and outer tubes of double-wall carbon nanotubes. Keeping the G-band position of the inner and outer tubes constant, we can determine the fraction of double-wall and single-wall tubes in samples containing a mixture of the two. A-band-related electronic interwall interaction at 1560 cm−1 is observed, which is associated with the outer tube walls. This band is observed to shift with pressure at the same rate as the G band of outer tubes and is not suppressed with chemical doping. Differences in the interwall interaction is discussed for double-wall carbon nanotubes grown by the catalytic chemical-vapor method and double-wall carbon nanotubes obtained through transformation of peapods

Charge transfer between carbon nanotubes and sulfuric acid as determined by Raman spectroscopy

The spontaneous interaction between sulfuric acid and carbon nanotubes is studied using Raman spectroscopy. We are able to determine the charge transfer without any additional parameter using the spectral signature of inner and outer walls of double-wall carbon nanotubes. While for the outer wall both the lattice contraction and the nonadiabatic effects contribute to the phonon shift, only the lattice contraction contributes for the inner nanotube. For the outer nanotube, we are able to separate these two contributions of the Raman G-band shift as a function of the charge transfer. We have carried out density functional theory calculations on graphene to see how different chemical species (HSO4-, H2SO4, H+) affect the electronic band structure and electron-phonon coupling. The Raman G band shift for the outer nanotube, Δω as a function of hole harge transfer per carbon atom, fC, is found to be Δω (cm−1) = (350 ± 20)fC + (101 ± 8)√fC

Quantifying Intuition : Bayesian Approach to Figures of Merit in EXAFS Analysis of Magic Size Clusters

Analysis of the extended X-ray absorption fine structure (EXAFS) can yield local structural information in magic size clusters even when other structural methods (such as X-ray diffraction) fail, but typically requires an initial guess - an atomistic model. Model comparison is thus one of the most crucial steps in establishing atomic structure of nanoscale systems and relies critically on the corresponding figures of merit (delivered by the data analysis) to make a decision on the most suitable model of atomic arrangements. However, none of the currently used statistical figures of merit take into account the significant factor of parameter correlations. Here we show that ignoring such correlations may result in a selection of an incorrect structural model. We then report on a new metric based on Bayes theorem that addresses this problem. We show that our new metric is superior to the currently used in EXAFS analysis as it reliably yields correct structural models even in cases when other statistical criteria may fail. We then demonstrate the utility of the new figure of merit in comparison of structural models for CdS magic-size clusters using EXAFS data

Engineering Nitrogen‐Doped Carbon Quantum Dots: Tailoring Optical and Chemical Properties through Selection of Nitrogen Precursors

The process of N‐doping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing high‐quality N‐doped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, eco‐ friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating convention‐related issues. Citric acid is used as the carbon source, and urea, trizma base, beta‐alanine, L‐arginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitation‐independent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and time‐dependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDs’ electronic structure than nitrogen‐containing functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling single‐layer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications

Spectroscopic super-resolution fluorescence cell imaging using ultra-small Ge quantum dots

QMUL/CSC scholarship (2011611045); BBSRC grant (BB/J001473/1)