Search

An attractive feature of nanomaterials is the possibility of tuning their properties through controlling their size and surface morphology, and understanding the effects of various parameters on thermal transport properties of nanostructures has been an active research topic in the past two decades. Through systematic studies of kinked silicon nanoribbons, we show how the kink morphology, a newly recognized degree of freedom for tuning thermal transport in nanostructures, modulates the thermal conductivity of these nanoribbons. For kinked Si nanoribbons that are 34 nm thick and 141 nm wide, the measured thermal conductivity first decreases as the period length reduces from 2 μm to 0.5 μm, reaching a 21% thermal conductivity reduction as compared to that of a straight counterpart at 300 K. However, as the period length drops to a level at which a straight heat transfer channel opens between the heat source and the sink, the thermal conductivity exhibits a steep increasing trend. Moreover, the comparison of thermal conductivity reduction for kinked ribbons along different crystalline directions indicates that phonon focusing could be exploited to further suppress thermal transport in kinked silicon nanoribbons. These results provide important guidelines on modulating heat transfer in nanostructures using kinks, which could be adopted to tune the thermal properties of nanostructures for different applications, such as thermoelectrics, microelectronic device thermal management, and functional thermal regulators. [ABSTRACT FROM AUTHOR]

Abstract: Rare earth ions (Ln3+) doped optical materials manifest sharp and tunable emission, high photostability, long luminescent lifetime and low cytotoxicity, and find diverse applications in optics, optoelectronic devices and bio-labelling. Herein, we employed a conventional hydrothermal approach to control the size and tune the phase as well as the morphology of GdF3:Tb3+,Yb3+ nanocrystals, wherein the effects of the synthetic conditions, such as pH, time, the ratio of the Gd to F and the surfactant, have been detailed investigated, producing sphere, platelet, rice, flower-shaped GdF3 nanoparticles. Diffuse reflectance spectra, photoluminescence emission and excitation spectra, and photoluminescence lifetime measurements were employed to characterize the optical spectroscopic properties of GdF3:Tb3+,Yb3+ nanocrystals. No efficient energy transfer from Tb3+ to Yb3+ is observed in GdF3:Tb3+,Yb3+ nanocrystals and the Yb3+ shows a concentration quenching effect at high doping levels. [Copyright &y& Elsevier]

Abstract: Eu2+ doped silicate glasses were prepared of the system 52SiO2–48MO: xEu2+(in molar ratio, M=Ca, Sr, Ba; x=1, 3, 5, 7, 9) by a high temperature melt-quenching method in a reducing atmosphere. Glass ceramics containing MSiO3:Eu2+(M=Ca, Sr, Ba) nano-phosphors were obtained after the heat treatment of the glass samples. The excitation, emission spectra and lifetime decay curves of 4f65d1→4f7 of Eu2+ were measured and interpreted with respect to their crystal structures and multi-site occupations of divalent europium in the hosts. Their excitation bands mainly extend from 450 to 250nm, which is adaptable to the main emission region of the UV LED chip. With UV light excitation, the Eu2+ emission in CaSiO3, SrSiO3 and BaSiO3 shows blue, green and yellow colors centered at 440, 505 and 555nm, respectively. The critical Eu2+concentration was studied and determined to be x=5 for both CaSiO3 and SrSiO3 and x=7 for BaSiO3 phosphors. The results show that the Eu2+ doped glass ceramic phosphors containing MSiO3 (M=Ca, Sr, Ba) nano-crystals can be used as potential matrix materials for a high power white LED pumped by the UV LED chip. [Copyright &y& Elsevier]

Abstract: A novel Eu2+ activated 60SiO2–40BaO (mol%) glass ceramics phosphor was prepared and the optical properties were investigated. X-ray diffraction (XRD) and Raman spectra confirmed the formation of Ba2Si3O8 nano-crystals in the glass matrix. The Eu2+ activated glass ceramics exhibited broad emission band centered at 518nm due to the 4f65d1→4f7 transition of Eu2+. Compared with the glass, the emission intensity of Eu2+ activated glass ceramics was much stronger, and the peak wavelength shifted toward shorter wavelength. The photoluminescence excitation (PLE) spectra of the glass ceramics showed an overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED). According to the photoluminescence (PL) spectra, the CIE chromaticity coordinates of the Eu2+ activated glass and glass ceramics were calculated. The results indicated that the Eu2+ activated glass ceramics containing Ba2Si3O8 nano-crystals can be used as a potential green emitting phosphor under UV-LED excitation. [Copyright &y& Elsevier]

Abstract: In the present work, transparent Eu2+/Dy3+ co-doped white light-emitting glass ceramics containing CaF2 were successfully prepared under reducing atmosphere. Their luminescence properties have been studied by excitation and emission spectra. A combination of blue, yellow and red emissions has emerged in Eu2+/Dy3+ co-doped glass and glass ceramics, which allows the observation of bright white light when the samples are excited by the ultraviolet light. Energy transfer (ET) from Eu2+ to Dy3+ is discovered by directly observing the luminescence intensity of Dy3+ in the Eu2+/Dy3+ co-doped glass which is much stronger than that in the Dy3+-doped glass. ET from Eu2+ to Dy3+ in glass is further confirmed by fluorescence studies performed on the samples with various activator (Dy3+) concentrations. The color of luminescence could be adjusted by varying the proportions of europium and dysprosium. The optimal composition generates white light with chromaticity coordinates (0.296, 0.311). The results indicate that Eu2+/Dy3+ co-doped glass ceramics containing CaF2 nano-crystals is a potential material for white LED. [Copyright &y& Elsevier]

Abstract: Eu2+/Sm3+ co-doped silicate glass was prepared by high temperature melting under reducing atmosphere and the Eu2+/Sm3+ co-doped SrSiO3 transparent glass-ceramics were obtained after heat-treatment. X-ray diffraction (XRD) and Raman spectra confirmed the formation of SrSiO3 nano-crystals in the glass matrix. The photoluminescence excitation (PLE) spectra and photoluminescence (PL) spectra of the samples were measured. A broad emission band from 400nm to 550nm due to the 4f65d1 →4f7 transitions of Eu2+ was observed, as well as several sharp emission peaks at 563nm, 600nm, 646nm and 713nm ascribed to the 4f→4f transitions of Sm3+. The luminescence properties of the glass ceramics with different molar ratio of Eu2+/Sm3+ were studied and the corresponding chromaticity coordinates were calculated. The ultraviolet light-emitting diode (UV-LED) excitable glass-ceramics emitting white light were obtained by tuning the relative emission intensity of Eu2+ and Sm3+. The results indicate that the Eu2+/Sm3+ co-doped SrSiO3 transparent glass-ceramics can be used as a potential matrix material for White LED under UV-LED excitation. [Copyright &y& Elsevier]

Van der Waals (vdW) crystals with covalently bonded building blocks assembled together through vdW interactions have attracted tremendous attention recently because of their interesting properties and promising applications. Compared to the explosive research on two-dimensional (2D) vdW materials, quasi-one-dimensional (quasi-1D) vdW crystals have received considerably less attention, while they also present rich physics and engineering implications. Here we report on the thermal conductivity of exfoliated quasi-1D Ta 2 Pd 3 Se 8 vdW nanowires. Interestingly, even though the interatomic interactions along each molecular chain are much stronger than the interchain vdW interactions, the measured thermal conductivity still demonstrates a clear dependence on the cross-sectional size up to >110 nm. The results also reveal that partial ballistic phonon transport can persist over 13 μm at room temperature along the molecular chain direction, the longest experimentally observed ballistic transport distance with observable effects on thermal conductivity so far. First-principles calculations suggest that the ultralong ballistic phonon transport arises from the highly focused longitudinal phonons propagating along the molecular chains. These data help to understand phonon transport through quasi-1D vdW crystals, facilitating various applications of this class of materials.