Photoluminescent carbon nanodots (CNDs) have received great interest because of their photostability, low toxicity, and excellent biocompatibility, compared with organic dyes and other semiconductor nanodots. CNDs can be used in a broad range of optoelectronic application fields, such as bioimaging, photocatalysis, sensing, light-emitting diodes, and energy conversion/storage devices. However the understanding of fluorescence origins of CNDs is still remaining unclear. Nevertheless, the involvement of surface states in the radiative transition of CNDs has been widely proposed to origin of fluorescence. Surface groups can introduce trapping states with different energy levels, making CND’s photoluminescence(PL) varies with excitation wavelength. The excitation-dependent emission was frequently observed, whereas excitation-independent emission was seldom observed in CNDs.
If all surface states are completely passivated, the emission will take place only through the radiative transition of sp2 carbon, so that CND’s PL will show excitation independence. The surface states of the CNDs could be engineered so that their photoluminescence was either excitation-dependent or distinctly independent. This was achieved by changing the density of amino-groups on the CND surface.
We synthesized fluorescent CNDs via a “water-in-oil” emulsion as a self-assembled-soft template. The PL spectrum of CNDs synthesized with citric acid and oleylamine shows strong peaks between 400-600 nm. The emission peaks of CNDs-oleylamine are dependent of excitation wavelength. The PL spectrum of CNDs synthesized with citric acid and urea also shows PL peaks between 400-500 nm, but emission peaks are independent of excitation wavelength. There is a series of single peaks spaced by ~1450 cm-1 corresponding to the average of D band and G band.
Further study is in progress including picosecond time-resolved fluorescence, femtosecond transient absoption spectroscopy which may allow more detailed structures and origin of fluorescence of CNDs.
Fig. 1 PL emission peak positions are either independent (a) or dependent(b) of excitation wavelength for the urea and oleylamine for capping molecule, respectively.