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Identifying Clusters and/or Small-Size Quantum Dots in Colloidal CdSe Ensembles with Optical Spectroscopy
摘要： It is well-known that optical absorption and photoluminescence (PL) provide information that is sensitive to the size and size distribution of colloidal binary semiconductor quantum dots (QDs). To explore the nature of reaction products, clusters, and/or small-size QDs, we show that it is important to perform as well photoluminescence excitation (PLE) spectroscopy. For two non-hot-injection reactions of cadmium oleate (Cd(OA)2) and selenium (Se) in 1-octadecene (ODE), we show that sequentially extracted products displayed a similar apparent red shift in both absorption and PL with a full width at half-maximum (fwhm) of ～30 nm. We demonstrate that one reaction (with the presence of diphenyl phosphine (HPPh2)) produced multiple types of clusters (with slightly different optical properties) in one ensemble, while the other reaction (without HPPh2) yielded primarily small-size QDs. Our findings provide evidence for the probable existence of clusters within small-size CdSe QD products, the existence of which complicates the size determination of small-size CdSe QDs.
关键词： optical absorption,photoluminescence,photoluminescence excitation spectroscopy,clusters,CdSe quantum dots
更新于2025-09-16 10:30:52
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A novel multi-center activated single-component white light-emitting phosphor for deep UV chip-based high color-rendering WLEDs
摘要： Single-component white light-emitting phosphors for phosphors-converted WLEDs are a hot spot of solid-state lighting field. Dy3+ is a promising candidate for white light generation in single-component hosts; however, the inappropriate ratio of yellow light to blue and the lack of red light restrict its applications in full-color display and indoor lighting. Here, a novel multi-center activated single-component white light-emitting phosphor, Sr2LaGaO5:Dy3+,Sm3+, has been designed and synthesized. It is surprisingly found that there is no evident energy transfer between the host and the dopants even though they satisfy the rule of spectral overlap, which can be explained from the special structure of the phosphor. The co-excitation of the host and the dopants would be a facile strategy to finely tune the luminescent spectra of phosphors. By adjusting the doping concentrations of Dy3+ and Sm3+, we can control the relative intensities of the emissions peaked at 493 nm and 574 nm from Dy3+, 604 nm from Sm3+ and the broadband within the range of 400 - 500 nm from the host. Therefore, the emission color of the co-doped phosphors can be tuned from white light to orangish red region with correlated color temperature decreasing from 5708 K to 1971 K. A WLED device with extremely high color-rendering index (Ra) up to 91.4 was fabricated by coating the synthesized phosphor with a deep UV LED chip. And it is the first time to use a single-component white light-emitting phosphor co-activated with Dy3+ and Sm3+ in deep UV chip excited WLEDs with high Ra.
关键词： Co-excitation,Sr2LaGaO5:Dy3+,Sm3+,Deep UV-LED chip,White light-emitting phosphor,High color-rendering index
更新于2025-09-16 10:30:52
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Corea??shell polymeric nanoparticles co-loaded with photosensitizer and organic dye for photodynamic therapy guided by fluorescence imaging in near and short-wave infrared spectral regions
摘要： Background: Biodistribution of photosensitizer (PS) in photodynamic therapy (PDT) can be assessed by fluorescence imaging that visualizes the accumulation of PS in malignant tissue prior to PDT. At the same time, excitation of the PS during an assessment of its biodistribution results in premature photobleaching and can cause toxicity to healthy tissues. Combination of PS with a separate fluorescent moiety, which can be excited apart from PS activation, provides a possibility for fluorescence imaging (FI) guided delivery of PS to cancer site, followed by PDT.
关键词： Polymeric nanoparticles,Poly-N-isopropylacrylamide,Electronic excitation energy transfer,Photodynamic therapy,Short wave infrared fluorescence bioimaging
更新于2025-09-16 10:30:52
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Relative phase effect of nonsequential double ionization in Ar by two-color elliptically polarized laser field
摘要： We investigated the nonsequential double ionization (NSDI) in Ar by two-color elliptically polarized laser field with a 3D classical ensemble method. We study the relative phase effect of NSDI and distinguish two particular recollision channels in NSDI, which are recollision-impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI), according to the delay-time between the recollision and the final double ionization. The numerical results indicate that the ion momentum distribution is changed and the triangle structure is more obvious with the decrease of the relative phase. We also demonstrate that the RESI process always dominates in the whole double ionization process and the ratio of the RESI and RII channel can be influenced by the relative phase.
关键词： nonsequential double ionization,recollision-induced excitation with subsequent ionization,two-color elliptically polarized laser field,recollision-impact ionization
更新于2025-09-12 10:27:22
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<i>Ab initio</i> study of temperature- and laser-induced phase transitions in
摘要： We analyze the bulk phase transitions of the three titanium dioxide polymorphs anatase, rutile, and brookite in thermodynamical equilibrium and after ultrafast laser excitation using the generalized solid-state nudged elastic band (G-SSNEB) method. For calculating the forces and stresses needed for G-SSNEB, we use electronic-temperature dependent density functional theory calculations. Our results show that the anatase-rutile and brookite-rutile phase transitions have a considerably lower energy barrier than the anatase-brookite phase transition in the thermodynamical equilibrium and upon laser excitation. The obtained transition temperatures in thermodynamical equilibrium are consistent with experimentally observed ones. Furthermore, our calculations show that the pathways of the phase transition are not affected considerably by the applied laser pulse. Although the pathways do not change considerably we see that the rutile phase is stabilized by the laser pulse.
关键词： phase transitions,density functional theory,G-SSNEB method,titanium dioxide,laser excitation
更新于2025-09-12 10:27:22
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Investigation of excitation interference in laser-induced breakdown spectroscopy assisted with laser-induced fluorescence for chromium determination in low-alloy steels
摘要： Chromium determination in steels suffered from excitation interference in laser-induced breakdown spectroscopy assisted with laser-induced fluorescence (LIBS-LIF). In this work, excitation interference between target chromium atoms and matrix iron atoms was investigated in detail. Relative absorption strength (RAS) of chromium and iron was mathematically deduced and utilized to estimate the excitation interference theoretically. Standard calibration curves of LIBS and LIBS-LIF for chromium determination in low-alloy steels were experimentally established. The chromium line 359.35 nm with less interference in the RAS map provided better analytical performance (slope of 1.99 ×10 5 counts/ wt%, detection limit of 8.64 μg g ? 1 , R 2 of 0.992, average RSD of 2.89%), in accordance with the theoretical estimation. This work realized sensitive and accurate determination of trace chromium in low-alloy steels, even influenced by excitation interference. An effective estimation method was provided for evaluating excitation interference to choose a suitable excitation line.
关键词： Laser-induced fluorescence,Steel,Excitation interference,Chromium,Laser-induced breakdown spectroscopy
更新于2025-09-12 10:27:22
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Formation Dynamics of Excited Neutral Nitrogen Molecules inside Femtosecond Laser Filaments
摘要： Nitrogen molecules are promoted to excited neutral states during femtosecond laser pulse filamentary propagation in atmosphere, leading to a characteristic UV fluorescence. Using a laser-induced fluorescence depletion technique, we measure the formation dynamics of these excited neutral nitrogen molecules with femtosecond time resolution. We find that the excited neutral molecules are formed in an unexpected ultrafast timescale of ～4 ps at 1 bar and ～120 ps at 30 mbar pressure. From this observation we deduce that the excitation of neutral N2 occurs via multiple collisions with hot free electrons. Numerical simulations based on rate equations reproduce well this ultrafast formation time and its dependence on gas pressure, and thus support this interpretation.
关键词： electron impact excitation,femtosecond laser,plasma filament,UV fluorescence,nitrogen molecules
更新于2025-09-12 10:27:22
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Magnetosonic wave-aided terahertz emission by nonlinear mixing of lasers in plasmas
摘要： A scheme of phase-matched terahertz generation by beating two co-propagating lasers in magnetized plasma, in the presence of a magnetosonic wave (MSW), is developed. The beat frequency ponderomotive force of the lasers imparts an oscillatory drift to electrons. The electron drift velocity couples with the electron density perturbation associated with the MSW to produce an irrotational nonlinear current ? × (cid:1)J NL = 0. The beat current density resonantly excites a THz (Terahertz) radiation when the phase-matching conditions are satisfied. The MSW mediates the phase matching. At 9.6 and 10.6 μm wavelengths, and background magnetic field of 285 kG, one may achieve normalized THz wave amplitude of the order of 10?6 and one obtains the ratio of THz power to pump power of the order of 10?3.
关键词： terahertz radiation,phase-matching condition,Magnetized plasma,ponderomotive nonlinearity,resonant excitation
更新于2025-09-12 10:27:22
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Excitation-wavelength- and size-dependent photo-darkening and photo-brightening of photoluminescence from PbS quantum dots in glasses
摘要： PbS quantum dots (QDs) with mean radii of 3.7 nm to 9.0 nm are precipitated in silicate glasses. Upon above-band-gap excitation, photoluminescence from QDs is strongly dependent on their size and excitation wavelength, exhibiting photo-darkening (PD) or photo-brightening (PB). Photoluminescence of PbS QDs exhibits strong darkening by short excitation wavelength but the darkening gradually mitigated as the excitation wavelength increased and even turns to be photo-brightening at room temperature. But PD and PB show a much more complicated variation tendency under the same excitation condition when the size of QDs increased. The dependence of PD and PB on the QDs’ size and excitation wavelength indicates that electron/hole trap states of PbS QDs, defect states in surrounding glass matrix as well as on the interface between the glass matrix and PbS QDs all have strong effects on the photoluminescence properties of PbS QDs. These findings are important to modulate the photoluminescence and promote the potential applications of PbS QDs embedded glasses towards various optoelectronic devices.
关键词： photoluminescence,PbS quantum dots,photo-darkening,excitation wavelength,size-dependent,photo-brightening
更新于2025-09-12 10:27:22
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Frequency and energy characteristics of a Cu–Ne laser at different durations of the leading edge of the excitation pulse
摘要： The lasing characteristics of a copper vapour laser in a tube with forced heating, having a length of 50 cm and a diameter of 2 cm, excited by a train of pulses are investigated. Comparative studies of the frequency and energy characteristics of the laser are performed with a leading edge duration of the excitation pulse of ~25 ns (when the capacitance discharges through a thyratron and a magnetic compression line) and ~3 and 1 ns (using circuits with a high-speed switch – kivotron). It is shown that a decrease in the leading edge duration gives rise to an increase in the optimal pulse repetition rate up to ~30 kHz, the generation efficiency up to 3.2 % and the generation power per unit length over 100 W m–1. The results obtained confirm the concept of limiting the frequency and energy characteristics of a copper vapour laser due to the insufficient rate of energy input into the plasma at high prepulse electron concentrations.
关键词： duration of the excitation pulse leading edge,frequency and energy characteristics,copper vapour laser
更新于2025-09-12 10:27:22