The emission properties of carbon dots (CDs) have already found many potential applications, from bio-imaging and cell labelling, to optical imaging and drug delivery, and are largely investigated in technological fields, such as lighting and photonics. Besides their high efficiency emission, CDs are also virtually nontoxic and can be prepared through many green chemistry routes. Despite these important features, the very origin of their luminescence is still debated. In this paper, we present an overview of sounding data and the main models proposed to explain the emission properties of CDs and their tunability.

The light-induced phase transition of TiO2\ud nanoparticles from anatase to rutile structure is reported\ud depending on the surrounding environment, the transition\ud being accomplished under oxygen-poor conditions. The\ud transition mechanism is interpreted in the framework of oxygen\ud adsorption and desorption phenomena with the involvement of\ud surface oxygen vacancies and F centers. It is shown that the\ud observed phase transition is not thermally driven because the\ud local temperature of the nanoparticles during irradiation is\ud about 370 K (estimated through the Stokes to anti-Stokes\ud Raman peaks ratio). On the contrary, the phase transition is\ud initiated by intragap irradiation (with the exception of the red\ud light one) that acts as TiO2 surface sensitizer, promoting the\ud activation of the surface and the nucleation of rutile crystallites starting from two activated anatase neighboring nanoparticles

COMMUNICATIONRecent studies have shown amplifi ed spontaneous emission (ASE) under short pulse excitation conditions, with lasing threshold densities comparable to or even lower than those observed in state-of-the-art organic materials. [ 18,19,[30][31][32] Short pulse excitation is a very favorable regime for light amplifi cation, as carrier densities well above the threshold required for lasing can be easily injected. Yet light amplifi cation disappears just a few picoseconds after excitation, a transient regime that is far away from the working conditions of interest for applications. How long light amplifi cation can last in perovskite materials is the open question we address in this Communication.Here, we demonstrate ASE sustained over transients two orders of magnitude longer than the excited state lifetime. Through optical spectroscopy, we measure threshold densities for ASE as a function of the temperature of the environment and the duration of the exciting laser pulse. Particularly, we employ an optical thermometry technique to track the dynamics of the temperature of the electron-hole plasma and identify the runaway heating mechanism limiting the maximum achievable duration of light amplifi cation. We then discuss the conditions needed for true continuous wave operation of light amplifi cation.Light emission in methylammonium lead iodide (MAPbI 3 ) and methylammonium lead bromide (MAPbBr 3 ) thin fi lms has fi rstly been analyzed under 130 fs pulsed laser excitation (392 nm in wavelength), using a streak camera to detect timeresolved photoluminescence and a cooled charge-coupled device (CCD) camera for the time-integrated spectra. The resulting photoluminescence decays, shown in Figure 1 a, had characteristic decay times much longer than the pulse duration, few nanoseconds for MAPbBr 3 , tens of nanoseconds for MAPbI 3 , meaning that such an excitation regime can be considered as impulsive. ASE was demonstrated by a sharp peak (Figure 1 b,c) appearing in the low energy side of the emission spectrum for both fi lms once the excitation laser fl uence reached a threshold value; such a value turned out to be 26 µJ cm −2 per pulse for MAPbI 3 , a factor of two lower than for MAPbBr 3 (54 µJ cm −2 per pulse). The corresponding average excited population densities, as calculated by averaging the laser fl uence times the fi lm absorption coeffi cient over the fi lm thickness, were 4 × 10 18 cm −3 and 7 × 10 18 cm −3 for MAPbI 3 and MAPbBr 3 , respectively (see absorption coeffi cients in the Supporting Information paragraph). Films realized from different solution batches and with different age showed variations in ASE threshold fl uence, mainly due to the different optical losses occurring as a result of different morphology. All samples were therefore stored in vacuum and typically measured within the

Thermal decomposition of citric acid is one of the most common synthesis methods for fluorescent carbon dots; the reaction pathway is, however, quite complex and the details are still far from being understood. For instance, several intermediates form during the process and they also give rise to fluorescent species. In the present work, the formation of fluorescent C‐dots from citric acid has been studied as a function of reaction time by coupling infrared analysis, X‐ray photoelectron spectroscopy, liquid chromatography/mass spectroscopy (LC/MS) with the change of the optical properties, absorption and emission. The reaction intermediates, which have been identified at different stages, produce two main emissive species, in the green and blue, as also indicated by the decay time analysis. C‐dots formed from the intermediates have also been synthesised by thermal decomposition, which gave an emission maximum around 450 nm. The citric acid C‐dots in water show short temporal stability, but their functionalisation with 3‐aminopropyltriethoxysilane reduces the quenching. The understanding of the citric acid thermal decomposition reaction is expected to improve the control and reproducibility of C‐dots synthesis.

Surface in porous media has a key role for both theoretical and technological aspects. A comparative analysis of the surface vibrational properties of sol−gel synthesized porous silica monoliths with different porosity is presented. Raman spectroscopy investigation of the fundamental O−H stretching range (3000−3800 cm-1) reveals a dependence of the surface reactivity on pore dimensions. The pore surface curvature plays an important role affecting the hydrogen-bonding interaction between surface hydroxyls and modifying the distribution of the OH species at the surface; the relative contribution of interacting hydroxyls and adsorbed water with respect to isolated species is larger in smaller pores. Water and silanol vibrations were singled out as a function of porosity.

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Highly fluorescent blue and green-emitting carbon dots have been designed to be integrated into solgel processing of hybrid organic-inorganic materials through surface modification with an organosilane, 3-(aminopropyl)triethoxysilane (APTES). The carbon dots have been synthesised using citric acid and urea as precursors; the intense fluorescence exhibited by the nanoparticles, among the highest reported in the scientific literature, has been stabilised against quenching by APTES. When the modification is carried out in an aqueous solution, it leads to the formation of silica around the C-dots and an increase of luminescence, but also to the formation of large clusters which do not allow the deposition of optically transparent films. On the contrary, when the C-dots are modified in ethanol, the APTES improves the stability in the precursor sol even if any passivating thin silica shell does not form. Hybrid films containing APTES-functionalized C-dots are transparent with no traces of C-dots aggregation and show an intense luminescence in the blue and green range.Carbon dots (C-dots) are fluorescent nanomaterials with optical properties comparable to semiconductor quantum dots. C-dots, however, have a much lower cost and environmental impact, which make them a hot topic of research 1,2 . A major advantage is the possibility to produce C-dots from an almost endless variety of precursors and methods. On the other hand, strict control of the properties through the process is still challenging to achieve, and the main efforts are now dedicated to obtaining reliable and reproducible synthesis.The citric acid (CA) alone or in combination with other compounds is one of the most popular precursors for C-dots 3 . CA-based C-dots have on their surface different carboxy-groups, which increase the solubility and allow surface passivation or functionalization with organic molecules 4-6 or polymers 7 . In general, pure CA C-dots, without any modification, show a weak emission, and doping 1 with B, N, S, Si and P atoms is a possible solution to improve their quantum yield. Most of the CA C-dots are doped with nitrogen that enhances the luminescence by producing azo-compounds through the reaction between the carboxylic and amino groups; after carbonisation, they form water-dispersible and highly emitting C-dots 8 . Different amines have been used for this purpose, such as ethylenediamine (EDA) 2,5,9 , hexamethylenetetramine 6 , o-phenylenediamine (o-PD) 10 , triethylenetetramine 11 , hexadecylamine (HDA) 8 , and triethanolamine 6 . Quantum yields (QY) under 8% for most of the amines, with the exception of EDA 6 which gives a QY of 86%, have been obtained. Urea, because of the high nitrogen content, can be used for doping CA C-dots 1,5,12-17 and different methods have been developed so far. Hydrothermal treatment in an autoclave and microwave exposure are simple synthesis for producing luminescent C-dots from citric acid and urea. Low QY (16%) 1 have been obtained by processing the dots via oven treatments, with even lower QY value...