Enhancement of optical emission generated from femtosecond double-pulse laser-induced glass plasma at different sample temperatures in air

In double-pulse laser-induced breakdown spectroscopy(DP-LIBS), the collinear femtosecond double-pulse laser configuration is experimentally investigated with different initial sample temperatures using a Ti:sapphire laser. The glass sample is ablated to produce the plasma spectroscopy. During the experiment, the detected spectral lines include two Na(I) lines(589.0 nm and 589.6 nm) and one Ca(I) line at the wavelength of 585.7 nm. The emission lines are measured at room temperature(22 ℃) and three higher initial sample temperatures(T_s?=?100 ℃, 200 ℃, and 250 ℃). The inter-pulse delay time ranges from-250 ps to 250 ps.The inter-pulse delay time and the sample temperature strongly influence the spectral intensity,and the spectral intensity can be significantly enhanced by increasing the sample temperature and selecting the optimized inter-pulse time. For the same inter-pulse time of 0 ps(single-pulse LIBS), the enhancement ratio is approximately 2.5 at T_s?=?200 ℃ compared with that obtained at T_s?=?22 ℃. For the same inter-pulse time of 150 ps, the enhancement ratio can be up to 4 at T_s?=?200 ℃ compared with that obtained at T_s?=?22 ℃. The combined enhancement effects of the different initial sample temperatures and the double-pulse configuration in femtosecond LIBS are much stronger than that of the different initial sample temperatures or the double-pulse configuration only.

Breaking Mirror Circularly Polarized Luminescence of Chiral Metal-Organic Frameworks by High-Pressure Stimulation

Understanding high-pressure-stimulated circularly polarized luminescence(CPL)of enantiomers remains a challenging but significant task in fundamental research and optical applications.Here,we combined in situ high-pressure photoluminescence with circularly polarized light to study how high pressure stimulated the CPL of crystalline enantiomers.Chiral lanthanide(Ln)-tartrate(Tar)metal-organic frameworks(MOFs;Ln=Eu,Tb)were synthesized to study their CPL from atmospheric pressure to 10 GPa.Under atmospheric pressure,D-and L-Eu(Tar)showed strong andmirror CPL.CPL intensity changes,emission wavelength shifts,and signal inversions were induced by increasing pressure.Note that the D-Eu(Tar)enantiomer showed strong CPL with a maximal dissymmetric factor(g_(lum))of 0.69 under 3 GPa,which is much higher than that under atmospheric pressure and of other reported MOF-based CPL materials.More interestingly,D-and L-Ln(Tar)enantiomers display obvious asymmetric CPL signals with increasing pressure,demonstrating that high pressure can break the mirror CPL of the MOF enantiomers.Our findings provide a critical understanding on in situ high-pressure CPL of chiral materials and establish a new optical phenomenon where high pressure can break the mirror CPL of enantiomers.

Red-emitting,self-oxidizing carbon dots for the preparation of white LEDs with super-high color rendering index

Carbon dots(CDs),as a kind of carbon nanomaterials,have attracted widespread attention due to their unique structure and excellent optical properties,and they are low-cost,environmentally friendly and biocompatible.However,the development of near-infrared(NIR)emission CDs remains a challenge.In this study,we successfully prepared CDs with a maximum emission of714 nm using citric acid as the carbon source,thiourea and ammonium fluoride as the dopant source,and N,N-dimethylformamide as the solvent.The quantum yield(QY)is as high as 22.64%.Interestingly,the prepared CDs self-oxidize in the presence of oxygen,resulting in a blue shift of their emission.Therefore,they can be used to prepare white light-emitting diodes(WLEDs)without adding other fluorescent substances.Notably,the work presented herein constitutes the first report of WLEDs preparation from single CDs.

Photoluminescence mechanisms of red-emissive carbon dots derived from non-conjugated molecules

Red-emissive carbon dots(R-CDs)have been widely studied because of their potential application in tissue imaging and optoelectronic devices.At present,most R-CDs are synthesized by using aromatic precursors,but the synthesis of R-CDs from non-aromatic precursors is challenging,and the emission mechanism remains unclear.Herein,different R-CDs were rationally synthesized using citric acid(CA),a prototype non-aromatic precursor,with the assistance of ammonia.Their structural evolution and optical mechanism were investigated.The addition of NH_(3)·H_(2)O played a key role in the synthesis of CA-based R-CDs,which shifted the emission wavelength of CA-based CDs from 423 to 667 nm.Mass spectrometry(MS)analysis indicated that the amino groups served as N dopants and promoted the formation of localized conjugated domains through an intermolecular amide ring,thereby inducing a significant emission redshift.The red-emissive mechanism of CDs was further confirmed by control experiments using other CA-like molecules(e.g.,aconitic acid,tartaric acid,aspartic acid,malic acid,and maleic acid)as precursors.MS,nuclear magnetic resonance characterization,and computational modeling revealed that the main carbon chain length of CA-like precursors tailored the cyclization mode,leading to hexatomic,pentatomic,unstable three/four-membered ring systems or cyclization failure.Among these systems,the hexatomic ring led to the largest emission redshift(244 nm,known for CA-based CDs).This work determined the origin of red emission in CA-based CDs,which would guide research on the controlled synthesis of R-CDs from other non-aromatic precursors.

Electron-phonon coupling-assisted universal red luminescence of o-phenylenediamine-based carbon dots

Due to the complex core-shell structure and variety of surface functional groups,the photoluminescence(PL)mechanism of carbon dots(CDs)remain unclear.o-Phenylenediamine(oPD),as one of the most common precursors for preparing red emissive CDs,has been extensively studied.Interestingly,most of the red emission CDs based on oPD have similar PL emission characteristics.Herein,we prepared six different oPD-based CDs and found that they had almost the same PL emission and absorption spectra after purifiication.Structural and spectral characterization indicated that they had similar carbon core structures but diffferent surface polymer shells.Furthermore,single-molecule PL spectroscopy confirmed that the multi-modal emission of those CDs originated from the transitions of different vibrational energy levels of the same PL center in the carbon core.In addition,the phenomenon of"spectral splitting"of single-particle CDs was observed at low temperature,which confirmed these oPD-based CDs were unique materials with properties of both organic molecules and quantum dots.Finally,theoretical calculations revealed their potential polymerization mode and carbon core structure.Moreover,we proposed the PL mechanism of red-emitting CDs based on oPD precursors;that is,the carbon core regulates the PL emission,and the polymer shell regulates the PL intensity.Our work resolves the controversy on the PL mechanism of oPD-based red CDs.These findings provide a general guide for the mechanism exploration and structural analysis of other types of CDs.