Additional Non-radiative Channel Research Articles

Substrate Ferroelectric Proximity Effects Have a Strong Influence on Charge Carrier Lifetime in Black Phosphorus.

By stacking monolayer black phosphorus (MBP) with nonpolarized and ferroelectric polarized bilayer hexagonal boron nitride (h-BN), we demonstrate that ferroelectric proximity effects have a strong influence on the charge carrier lifetime of MBP using nonadiabatic (NA) molecular dynamics simulations. Through enhancing the motion of phosphorus atoms, ferroelectric polarization enhances the overlap of electron-hole wave functions that improves NA coupling and decreases the bandgap, resulting in a rapid electron-hole recombination completing within a quarter of nanoseconds, which is two times shorter than that in nonpolarized stackings. In addition to the dominant in-plane Ag2 mode in free-standing MBP, the out-of-plane high-frequency Ag1 and low-frequency interlayer breathing modes presented in the heterojunctions drive the recombination. Notably, the resonance between the breathing mode within bilayer h-BN and the B1u mode of MBP provides an additional nonradiative channel in ferroelectric stackings, further accelerating charge recombination. These findings are crucial for charge dynamics manipulation in two-dimensional materials via substrate ferroelectric proximity effects.

A comparative photophysical study on the structurally related coumarin 102 and coumarin 153 dyes

Photophysical properties of two 7-aminocoumarin dyes, namely, coumarin 102 (C102) and coumarin 153 (C153), have been investigated in different aprotic solvents to understand their polarity dependent behaviors. Both the dyes have the same closed-ring amine (julolidyl) substituent at 7-th position of their 1,2-benzopyrone moiety, but differ in regard to their 4-CH3 and 4-CF3 substituents, respectively. For both the dyes, absorption and fluorescence spectra undergo red shift, peak energies of these transitions decrease and the Stokes’ shifts follow linear correlations with the increasing solvent polarity parameter Δf, as defined by Lippert-Mataga. While fluorescence quantum yield (Φf) and lifetime values (τf) of C102 dye undergo linear changes with Δf, both these parameters for C153 display quite unusual deviations from linearity in the lower polarity solvents. For C102, the estimated fluorescence decay rates (kf) are significantly higher than C153 dye. The nonradiative decay rates (knr) are also substantially higher for C102 dye than C153 in most of the solvents. Interestingly, while kf values for both the dyes change linearly with Δf, the knr values show complete linearity with Δf for C102 dye only, but these values for C153 dye show quite substantial enhancements and an unusual positive deviation from linearity in lower polarity solvent. Quantum chemical studies indicate that for C153 dye in lower polarity solvents, especially, there is a possible involvement of intersystem crossing process, which might induce an additional nonradiative channel (kadd-nr) for the dye. Further, though we could not obtain any direct evidence in the present study, yet it is felt that some kind of fluorous effect, as might be arising due to large negative charge density on bulky perfluomethyl group of C153 dye, can also introduce additional kadd-nr, specifically in the nonpolar solvents, for which further investigations are continuing in our group.

Time evolution of the solvated and conformationally relaxed emissive excited state of the anionic form of salophen, a Schiff base

Salophen is a weakly fluorescent Schiff base which forms emissive co-ordination complexes with Zn2+ and Al3+. The complex with Al3+ is significantly more fluorescent than that with Zn2+, presumably because the dimeric complex with Zn2+ is associated with additional nonradiative channels. This contention has been put to test, through a careful investigation of excited state dynamics of the anionic form of salophen (Sal2−), which is the form in which the ligand exists in the complexes. The emissive excited state of the anion (Sal2−) has been found to be solvated and conformationally relaxed, over tens of picosecond. It is significantly more fluorescent than the neutral compound, with fluorescence lifetime that is longer by almost two orders of magnitude. Fluorescence lifetime of the anion is in fact longer than that of the complex with Zn2+ and slightly less than that of the complex with Al3+. So, the earlier hypothesis about additional nonradiative deactivation pathways in the Zn2+ complex gains credence from the present study.

Influence of morphology on the blinking mechanisms and the excitonic fine structure of single colloidal nanoplatelets.

Colloidal semiconductor nanoplatelets with a similar electronic structure as quantum wells have recently emerged as exciting materials for optoelectronic applications. Here we investigate how morphology affects important photoluminescence properties of single CdSe and core/shell CdSe/CdZnS nanoplatelets. By analyzing photoluminescence intensity-lifetime correlation and second-order photon correlation results, we demonstrate that, irrespective of the morphology, Auger recombination plays only a minor role in dictating the blinking behavior of the nanoplatelets. We find that a rough shell induces additional nonradiative channels presumably related to defects or traps of an imperfect shell. Furthermore, polarization-resolved spectroscopy analysis reveals exciton fine-structure splitting of the order of several tens of meV in rough-shell nanoplatelets at room temperature, which is attributed to exciton localization and is substantiated by theoretical calculations taking into account the nanoplatelet shape and electron-hole exchange interaction.

Tracking emission rate dynamics of nitrogen vacancy centers in nanodiamonds

Spontaneous emission from crystal centers is influenced by both the photonic local density of states and non-radiative processes. Here, we monitor the spontaneous emission of single nitrogen vacancy (NV) centers as their host diamond is reduced in size from a large monolithic crystal to a nanocrystal by successive cycles of oxidation. The size reduction induces a quenching of the NV radiative emission. Additional non-radiative channels lead to a decrease of the fluorescence intensity and the excited state lifetime. In one case, we observe the onset of blinking which may provide a route to understand these additional non-radiative decay channels.

Phonon-induced dephasing of chromium color centers in diamond

We report on the coherence properties of single photons from chromium-based colour centres in diamond. We use field-correlation and spectral lineshape measurements to reveal the interplay between slow spectral wandering and fast dephasing mechanisms as a function of temperature. We show that the zero-phonon transition frequency and its linewidth follow a power-law dependence on temperature indicating that the dominant fast dephasing mechanisms for these centres are direct electron-phonon coupling and phonon-modulated Coulomb coupling to nearby impurities. Further, the observed reduction in the quantum yield for photon emission as a function of temperature is consistent with the opening of additional nonradiative channels through thermal activation to higher energy states predominantly and indicates a near-unity quantum efficiency at 4 K.

Thermal quenching of photoluminescence in ZnO/ZnMgO multiple quantum wells following oxygen implantation and rapid thermal annealing

The temperature-dependent photoluminescence in oxygen-implanted and rapid thermally annealed ZnO/ZnMgO multiple quantum wells is investigated. A difference in the thermal quenching of the photoluminescence is found between the implanted and unimplanted quantum wells. Oxygen implantation and subsequent rapid thermal annealing results in the diffusion of magnesium atoms into quantum wells and thus, leads to an increased fluctuation in the potential of the quantum wells and the observation of a large thermal activation energy. However, a high dose of implantation results in large defect clusters and thus an additional nonradiative channel, which leads to a flat potential fluctuation and a small thermal activation energy.

Synthesis, characterisation and luminescence properties of MCM-41 impregnated with an Eu 3+ β-diketonate complex

A luminescent material has been prepared by formation of the Eu 3+ β-diketonate complex Eu(NTA) 3L″ {NTA = 1-(2-naphthoyl)-3,3,3-trifluoroacetonate; L″ = ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate} within the channels of MCM-41-type ordered mesoporous silica. Using simple wet impregnation methods, the ligand L″ was first encapsulated followed by treatment with the complex Eu(NTA) 3(H 2O) 2. An analogous Gd 3+ supported material was also prepared, as well as the model complexes Ln(NTA) 3L′ {Ln = Eu, Gd; L′ = 2-[3(5)-pyrazolyl]pyridine}. The materials were characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), FTIR and FT Raman spectroscopies, 13C and 29Si MAS NMR, and photoluminescence spectroscopy. The spectroscopic studies showed that exchange of the H 2O ligands for the bidentate ligand L″ occurred to give the desired 8-coordinate species Ln(NTA) 3L″, with a local Ln 3+ coordination environment similar to that for the model complexes. The low (−259 °C) and room temperature emission spectra of the Eu-modified mesoporous material displayed the typical Eu 3+ intra-4f 6 lines ascribed to transitions between the 5D 0,1 excited states and the ground multiplet ( 7F 0–4). Negligible emission from the organic part of the encapsulated species was observed, indicating that energy transfer from the ligands to the Eu 3+ ion was quite efficient. The 5D 0 quantum efficiency was estimated to be 21.1%, compared with 46.1% for the model complex Eu(NTA) 3L′. This difference is attributed to the presence of an excited state of the organic ligands in the supported material that lies approximately resonant with the 5D 1 intra-4f 6 level, opening up an additional non-radiative channel that is not present in the corresponding model complexes.