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Nitrogen (N)-rich organic thin films were deposited using both low-pressure plasma- and vacuum-ultraviolet-based techniques, from mixtures of ammonia (NH3) and ethylene (C2H4). These films were investigated using angle-resolved and excitation energy resolved X-ray photoelectron spectroscopy (ARXPS and ERXPS, respectively) in order to determine their sub-surface chemical profiles. These two techniques enable one to tune the “XPS 95%” information depth, z 95%, by varying either the angle or the excitation energy. Using a combination of both techniques, z 95% can be varied continuously from 0.7 to 11 nm. The surface-near chemistry is investigated using both high-resolution C 1s spectra and elemental concentrations derived from elemental peak intensities. Results show that while laboratory XPS, and even ARXPS, suggest homogenous surface chemistries, the novel combination of ARXPS and ERXPS points to the existence of a compositional profile in the extreme outer surface layer. Our conclusions are supported by simulations using SESSA software.
Fluorescent carbon nanodots (CNDs) are very promising nanomaterials for a broad range of applications because of their high photostability, presumed selective luminescence, and low cost at which they can be produced. In this respect, CNDs are superior to well-established semiconductor quantum dots and organic dyes. However, reported synthesis protocols for CNDs typically lead to low photoluminescence quantum yield (PLQY) and low reproducibility, resulting in a poor understanding of the CND chemistry and photophysics. Here, we report a one-step synthesis of nitrogen-doped carbon nanodots (N-CNDs) from various carboxylic acids, Tris, and ethylenediaminetetraacetic acid resulting in high PLQY of up to 90%. The reaction conditions in terms of starting materials, temperature, and reaction time are carefully optimized and their influence on the photophysical properties is characterized. We find that citric acid-derived N-CNDs can result in a very high PLQY of 90%, but they do not Show selective luminescence. By contrast, acetic acid-derived N-CNDs show selective luminescence but a PLQY of 50%. The chemical
composition of the surface and core of these two selected N-CND types is characterized among others by high-Resolution synchrotron X-ray photoelectron spectroscopy using single isolated N-CND clusters. The results indicate that photoexcitation occurs in the N-CND core, whereas the emission properties are determined by the N-CND surface groups.