Exciton radiative lifetime in CdSe quantum dots

Colloidal CdSe quantum dots(QDs)are promising materials for solar cells because of their simple preparation pro-cess and compatibility with flexible substrates.The QD radiative recombination lifetime has attracted enormous attention as it affects the probability of photogenerated charges leaving the QDs and being collected at the battery electrodes.However,the scaling law for the exciton radiative lifetime in CdSe QDs is still a puzzle.This article presents a novel explanation that recon-ciles this controversy.Our calculations agree with the experimental measurements of all three divergent trends in a broadened energy window.Further,we proved that the exciton radiative lifetime is a consequence of the thermal average of decays for all thermally accessible exciton states.Each of the contradictory size-dependent patterns reflects this trend in a specific size range.As the optical band gap increases,the radiative lifetime decreases in larger QDs,increases in smaller QDs,and is weakly depend-ent on size in the intermediate energy region.This study addresses the inconsistencies in the scaling law of the exciton life-time and gives a unified interpretation over a widened framework.Moreover,it provides valuable guidance for carrier separa-tion in the thin film solar cell of CdSe QDs.

Spectroscopic properties and radiative lifetimes of SiTe:A high-level multireference configuration interaction investigation

The 18 A-S states correlated to the lowest dissociation limit of SiTe were calculated by using a high-level multirefer-ence configuration interaction （MRCI） method, including scalar relativistic and spin-orbit coupling effects. Based on the calculated potential energy curves, the spectroscopic constants of bound states were determined, which are well consistent with previous experimental results. The spin-orbit matrix elements between the A-S states were computed, which lead to an in-deoth understanding, of oerturbations on the electronic state a^3∏. Finally. the transition dioole moments of allowed transitionsA^1∏-X^1∑^＋,E^1∑^＋-X^1∑^＋,a^3∏-d^3△,a^3∏-d^3△,a^∏-a′^3∑^＋,a^3∏-e^3∑^-,and the radiative lifetimes of A^1∏,E^1∑^＋,and a^3∏ were evaluated.

Radiative lifetime measurements of odd-parity highly-excited levels of Sn I by time-resolved laser spectroscopy

Natural radiative lifetimes of five higher-lying odd-parity levels 5pTs ^3P1^0, 5p5d 1P1^0, 5p6d 3F2^0, 3D1^0 and 3F3^0 in neutral tin are measured by the time-resolved laser-induced fluorescence （TR-LIF） technique and the atomic beam method. All these lifetimes are not longer than 100 ns and they are found to be shorter than the lifetimes of evenparity levels in the same energy region. The results reported in this paper provide important transition parameters for highly-excited atomic Sn, which may be useful for theoretically calculating excited heavy atoms.

Relativistic calculations on the transition electric dipole moments and radiative lifetimes of the spin-forbidden transitions in the antimony hydride molecule

Calculations on the spectroscopic constants and transition properties of the first three states(a^(1)△,b^(1)Σ^(+),and X3Σ-)of the Sb H molecule were performed under the relativistic framework using the exact two-component Hamiltonian(X2C).The potential energy curves in the Franck-Condon region were computed and compared with the previous values.Furthermore,the transition dipole moments for the weak spin-forbidden transitions(b0^(+)-X10^(+),b0^(+)-X_(2)1,X10^(+)-X_(2)1,and X_(2)1-a2)were reported.The spontaneous radiative lifetime of the b1Σ^(+)(υ′=0)state was calculated as 163.5±7.5μs,which is in reasonable agreement with the latest experimental value of 173±3μs.The spontaneous radiative lifetimes of the X_(2)1(υ′=0)state and the a2(υ′=0)state were calculated to be 48.6 s and~8 ms,respectively.Our study is expected to be a benchmark transition property computation for comparison with other theoretical and experimental results.The datasets presented in this paper,including the transition dipole moments,are openly available at https://dx.doi.org/10.11922/sciencedb.j00113.00018.

Energy levels and radiative lifetimes of 3pns ~3p_0 and 3pnd ~3P_0 series of SiⅠ

The energy levels and lifetimes of 3pns ^3po （n = 7 - 35） and 3pnd ^3p0 （n = 6 - 17） series of neutral silicon are calculated and predicted by means of multichannel quantum defect theory （MQDT）. In addition, the perturbation caused by core-excited state 3s3p^3 is discussed. The 3pnd ^3p0 series, especially 3p4d ^3p0, 3p5d ^3P0, and 3p6d ^3P0 are perturbed strongly by the core-excited state 3s3p3 ^3P0. These cause the lifetime of 3pnd ^3P0 （n = 5 - 7） to be less than that of 3p4d 3Po. The lifetimes of 3p14d ^3P0 （65479.14 cm^-1） and 3p16d ^3p0 （65608.77 cm^-1） are less than that of their frontal states respectively, because these states are perturbed by 3p22s ^3p0 （65476.48 cm-1） and 3p30s ^3p0 （65608.99 cm^-1） respectively.

Strain tunable excitonic optical properties in monolayer Ga_(2)O_(3)

Two-dimensional(2D)Ga_(2)O_(3)has been confirmed to be a stable structure with five atomic layer thickness configuration.In this work,we study the quasi-particle electronic band structures and then access the excitonic optical properties through solving the Bethe-Salpeter equation(BSE).The results reveal that the exciton dominates the optical absorption in the visible light region with the binding energy as large as~1.0 eV,which is highly stable at room temperature.Importantly,both the dominant absorption P_(1)and P_(2)peaks are optically bright without dark exciton between them,and thus is favorable for luminescence process.The calculated radiative lifetime of the lowest-energy exciton is 2.0×10^(-11)s at 0 K.Furthermore,the radiative lifetime under+4%tensile strain is one order of magnitude shorter than that of the strainfree case,while it is less insensitive under the compressive strain.Our findings set the stage for future theoretical and experimental investigation on monolayer Ga_(2)O_(3).

Excitonic optical properties in monolayer SnP_(2)S_(6)

Quantum confinement effect and reduced dielectric screening in two-dimensional(2D)dramatically enhance theelectron-hole interactions.In this work,we use many-body perturbation theory and Bethe-Salpeter equation(BSE)toinvestigate the electronic and excitonic optical properties of monolayer SnP_(2)S_(6).Our findings reveal that the excitoniceffect dominates the optical absorption spectra in the visible light range,and the lowest-energy exciton X0 in monolayerSnP_(2)S_(6)is optically bright with the binding energy of 0.87 eV and the radiative lifetime of~10^(-11)s,which is highly advantageousto the photo-luminescence.Most importantly,the absence of optically forbidden states below the bright statesX0 would give rise to a high quantum efficiency of 2D SnP_(2)S_(6).We also find that applied biaxial strain can further shortenthe radiative lifetime of the bright states.These results imply that 2D SnP_(2)S_(6)is a promising candidate for the optoelectronicdevices.

Effect of lithium-potassium mixed alkali on spectroscopic properties of Er^3＋-doped aluminophosphate glasses

Er^3＋-doped lithium-potassium mixed alkali aluminophosphate glasses belonging to the oxide system xK2O-（15-x）Li2O-4B2O3-11Al2O3-5BaO-65P2O5 are obtained in a semi-continuous melting quenching process. Spectroscopic properties of Er^3＋-doped glass matrix have been analysed by fitting the experimental data with the standard Judd-Ofelt theory. It is observed that Judd-Ofelt intensity parameters Ωt（t = 2, 4 and 6） of Er^3＋ change when the second alkali is introduced into glass matrix. The variation of line strength Sed[^4I13/2,^4I15/2] follows the same trend as that of the/26 parameter. The effect of mixed alkali on the spectroscopic properties of the aluminophosphate glasses, such as absorption cross-section, stimulated emission cross-section, spontaneous emission probability, branching ratio and the radiative lifetime, has also been investigated in this paper.

Theoretical study on the transition properties of AlF

Potential energy curves of the X^(1)Σ+and A;Πstates of the Al F molecule are studied through the combination of the multi-reference configuration interaction(MRCI)approach and Davidson corrections(MRCI+Q).The AWCV5 Z basis set is employed in the calculations.The transition dipole moments(TDMs)of the A;Π■X^(1)Σ+transition are explored based on the AWCV5 Z basis set and(4,2,2,0)active space.The Schrödinger equation is solved via the LEVEL 8.2 program,and the vibrational levels and rotational constants of the X^(1)Σ+and A1Πstates are calculated.It is shown that the Al F molecule has high diagonal Franck-Condon factors(f00=0.9949 and f11=0.9854)and large Einstein coefficients for the transition of A;Π(ν′=0)■X^(1)Σ+(ν′′=0).In addition,the radiative lifetimes of the vibrational levels are close to 10-9 s for the A;Πstate.The line intensities of the A;Π(ν′=4-15)■X^(1)Σ+(ν′′=0)transitions are also calculated.The calculated TDMs and transition probabilities in this work are credible and provide some guidance for the study of similar transitions,particularly for exploring interstellar space.

Thermal and radiative characteristics of oxyfluoride glass singly doped with lanthanide ions

Rare earths-doped oxyfluoride glasses based on germanium oxide and lead fluoride were prepared from commercial raw materials. The glasses with general composition of 50GeO2-(50-x-y)PbO-yPbF2-xLnF3 (Ln=Pr3+-Yb3+), contained different concentrations of optically active dopants (x=0.2 mol.% and 2 mol.%) and PbF2 (y≤15 mol.%). The differential thermal analysis (DTA) was used to determine both thermal characteristic and thermal stability properties of the glasses in the function of the kind of dopant, its concentration, and a glass composition. Characteristic glass temperatures such as glass transition temperature (Tg), glass crystallization temperature (Tc) and temperature corresponding to the maximum of the crystallization rate (Tpc) were evaluated. On the basis of obtained results, the thermal stabilities of glasses under study were evaluated using various thermal stability criteria (Dietzel factor ?T, Saad-Poulain factors H’ and S). It was found that the increase in rare earth fluoride contents influenced thermal characteristics when the characteristic temperatures of the individual glass was shifted towards higher values. The effect of the PbF2 content and the kind of rare earth impurity on the glass stability was observed. Absorption spectra of lanthanide-doped glasses were measured at room temperature and used to determine the phenomenological intensity parameters Ωt and next, to estimate radiative properties of lanthanide ions in this matrix. Radiative transition probabilities of luminescent states of Ln3+, branching ratios and radiative lifetimes were determined. The variation of the Ωt along the lanthanide series was presented and discussed.

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/Mg_x Zn_(1-x) O Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite （WZ） ZnO/MgxZn1-xO cylindrical quantum dots （QDs） for four different Mg compositions： x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters （height and radius） are calculated in detail The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has of ZnO/MgxZn1-x 0 QDs. The ground-state exciton a significant influence on the exciton states and optical properties binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L 〈 3.5 nm, but the interband emission wavelength has a red-shift when L 〉 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L 〉 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.