Luminescent semiconductor quantum dots (QDs) are nanometer-sized particles with size-dependent optical and electronic properties that have been under intensive research and development for a broad range of applications, such as energy-efficient displays and lighting, photovoltaic devices, and biological markers. [1][2][3][4][5][6] The estimated market size for QD-based products is $721 million by 2013. 2 However, the intrinsic toxicity and potential environmental hazards associated with many of these materials represent considerable challenges to their practical applications. The emerging light-emitting, quantum-sized carbon dots (CDs) 7 appear to be a promising alternative to semiconductor QDs in many of applications because of their advantages in low toxicity and cheaper cost. 6 Following the laser ablation method, 7 various methods have been established to produce quantum-sized carbon nanoparticles, 8-12 like electrochemical release or exfoliation from a graphitic source, separation of combusted carbon soot, 13,14 carbonizing polymerized resols on silica spheres, 15 thermal oxidation of suitable molecular precursors, 16-18 and dehydration of carbohydrates using concentrated sulfuric acid. 19 However, most of these synthesis methods need several steps. 7-14 In addition, the photoluminescent quantum yields (QY) of these resultant carbon dots are very low, 7-14,16-18 usually below 6%. The highest quantum yield 6 reported until now reached 20%, where the laser ablated carbon dots were conjugated with PEG 1500N in neat SOCl 2 . In view of the significant potential of this zero-dimension carbon nanomaterial in various fields, a facile and scalable synthetic approach for luminescent carbon dots is highly desired.Here, we report a novel strategy to synthesize highly luminescent oil-soluble carbon dots (OCDs) by carbonizing carbon precursors, e.g., citric acid, in hot noncoordinating solvent. This one-step method is inspired by the synthesis of various monodispersed and size-controlled semiconductor and magnetic nanocrystals. 20-22 The obtained OCDs possess a maximum QY at room temperature up to 53% (excited at 360 nm), which is the best result reported so far. Furthermore, as a natural extension of this facile method, we also synthesized water-soluble CDs (WCDs) via changing the reaction solvent and capping agent. The QY of the WCDs is lower than the OCDs, yet it still can reach 17% (excited at 360 nm). In this communication, we focus on the synthesis of OCDs, whereas the details of WCDs are presented in the Supporting Information.The OCDs was prepared using octadecene (ODE) as the noncoordinating solvent, 1-hexadecylamine (HDA) as the surface passivation agent, and citric acid anhydrous as the carbon precursor. Typically, a mixture of 15 mL ODE and 1.5 g of HDA loaded in a three-neck flask was heated to 300°C under argon flow, and then 1 g of citric acid was quickly injected into the reaction flask. Aliquots were taken at different time intervals for the optical and Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Scie...

LAMOST DR5 released more than 200,000 low resolution spectra of early-type stars with S/N>50. Searching for metallic-line (Am) stars in such a large database and study of their statistical properties are presented in this paper. Six machine learning algorithms were experimented with using known Am spectra, and both the empirical criteria method (Hou et al. 2015) and the MKCLASS package(Gray et al. 2016) were also investigated. Comparing their performance, the random forest (RF) algorithm won, not only because RF has high successful rate but also it can derives and ranks features. Then the RF was applied to the early type stars of DR5, and 15,269 Am candidates were picked out. Manual identification was conducted based on the spectral features derived from the RF algorithm and verified by experts. After manual identification, 9,372 Am stars and 1,131 Ap candidates were compiled into a catalog. Statistical studies were conducted including temperature distribution, space distribution, and infrared photometry. The spectral types of Am stars are mainly between F0 and A4 with a peak around A7, which is similar to previous works. With the Gaia distances, we calculated the vertical height Z from the Galactic plane for each Am star. The distribution of Z suggests that the incidence rate of Am stars shows a descending gradient with increasing |Z|. On the other hand, Am stars do not show a noteworthy pattern in the infrared band. As wavelength gets longer, the infrared excess of Am stars decreases, until little or no excess in W1 and W2 bands.

a b s t r a c tIn this paper, the dynamical behaviors of an SEIR epidemic system governed by differential and algebraic equations with seasonal forcing in transmission rate are studied. The cases of only one varying parameter, two varying parameters and three varying parameters are considered to analyze the dynamical behaviors of the system. For the case of one varying parameter, the periodic, chaotic and hyperchaotic dynamical behaviors are investigated via the bifurcation diagrams, Lyapunov exponent spectrum diagram and Poincare section. For the cases of two and three varying parameters, a Lyapunov diagram is applied. A tracking controller is designed to eliminate the hyperchaotic dynamical behavior of the system, such that the disease gradually disappears. In particular, the stability and bifurcation of the system for the case which is the degree of seasonality β 1 = 0 are considered. Then taking isolation control, the aim of elimination of the disease can be reached. Finally, numerical simulations are given to illustrate the validity of the proposed results.

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In this work, we present a catalog of 2651 carbon stars from the fourth Data Release (DR4) of the Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAM-OST). Using an efficient machine-learning algorithm, we find out these stars from more than seven million spectra. As a by-product, 17 carbon-enhanced metal-poor (CEMP) turnoff star candidates are also reported in this paper, and they are preliminarily identified by their atmospheric parameters. Except for 176 stars that could not be given spectral types, we classify the other 2475 carbon stars into five subtypes including 864 C-H, 226 C-R, 400 C-J, 266 C-N, and 719 barium stars based on a series of spectral features. Furthermore, we divide the C-J stars into three subtypes of C-J(H), C-J(R), C-J(N), and about 90% of them are cool N-type stars as expected from previous literature. Beside spectroscopic classification, we also match these carbon stars to multiple broadband photometries. Using ultraviolet photometry data, we find that 25 carbon stars have FUV detections and they are likely to be in binary systems with compact white dwarf companions.