Nonradiative Electron Research Articles

Highly fluorescent carbon dots as selective and visual probes for sensing copper ions in living cells via an electron transfer process

As an integral part of many important enzymes, Cu2+ is involved in a number of vital biological processes, which is linked to the oxidative damage and environmental contamination when Cu2+ is excessive. In this work, Cu2+ can be captured by the amino groups of carbon dots (CDs) to form complexes, resulting in a strong fluorescence quenching of CDs via a nonradiative electron transfer process, which offered a rapid, visual, and selective methodology for Cu2+ detection. The probe exhibited a wide response concentration range (0.01–2μM) to Cu2+ with a detection limit of 6.7nM. Significantly, the CDs presented excellent biocompatibility and high photostability, which were applicable for the visualization of Cu2+ dynamic invasion into living cells and Tilapia mossambica. Furthermore, the toxicity of Cu2+ ions to living cells could be inhibited with CDs by the formation of complexes.

Brightening Quinolineimines by Al3+ and Subsequent Quenching by PPi/PA in Aqueous Medium: Synthesis, Crystal Structures, Binding Behavior, Theoretical and Cell Imaging Studies

Recent years have witnessed an upsurge of Al3+ selective optical sensors involving simple Schiff bases to other complex organic frameworks. However, more than ∼95% of such reports lack crystallographic evidence, and proposals of binding sites for Al3+ are based upon spectroscopic evidence only. We herein synthesized and fully characterized a quinolineimine derivative (CMO) and explored its potential toward efficient detection of Al3+ with crystallographic evidence. The ongoing nonradiative photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) processes in CMO got inhibited via the chelation enhanced fluorescence (CHEF) effects induced by Al3+, and consequently turn-on fluorescence response was observed with 18-fold emission enhancements. The theoretical calculations performed were in good consonance with experimental results. We also explored further the applicability of the CMO·Al3+ complex toward highly sensitive and selective detection of inorganic phosphate (PPi) and an explosive picric acid (PA) via fluorescence quenching processes through two different chemical routes. The bioimaging of Al3+ and PPi were carried out in the living human cancer cells (MCF-7).

Temperature-Driven Enhancement of the Stimulated Emission Rate in Terahertz Quantum Cascade Lasers

We analyze the temperature dependence of the light output power of terahertz quantum cascade lasers (THz-QCLs) and demonstrate an enhancement of the stimulated emission rate triggered by thermally activated electron leakage from the lower laser level into the continuum. Contrary to common sense expectation, we find that this leakage channel contributes positively to the temperature performance of THz-QCLs. We show that this leakage mechanism is the missing component to explain the full temperature dependence of the light output power as it counteracts the population inversion reduction that arises from non-radiative electron scattering from the upper into the lower laser state. We analyze experimental light output power versus temperature data for a 3.87-THz-emitting device with highly diagonal optical transition over a wide temperature range (10-180 K) and show how thermally activated leakage of electrons from the lower laser level leads to a nearly constant output power up to a temperature of ~120 K. These results open the question if new design approaches that exploit this effect can be developed in order to demonstrate devices with higher maximum operating temperature.

Surface decoration of cadmium-sulfide quantum dots with 3-mercaptopropionic acid as a fluorescence probe for determination of ciprofloxacin in real samples

A sensitive fluorescence probe based on 3-mercaptopropionic acid (MPA) capped cadmium-sulfide quantum dots (CdS QDs) was prepared for determination of ciprofloxacin (CIP) in real samples. The synthesized MPA capped CdS QDs was characterized using fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) measurements. Based on the results, the fluorescence of MPA capped CdS QDs was quenched in presence of Cu2+ that it could have occurred because of the nonradiative electron–hole recombination annihilation through an electron transfer process. Subsequently, the fluorescence of MPA capped CdS QDs mixed with the Cu2+ could be recovered after the addition of CIP due to its strong affinities for Cu2+. The recovered fluorescence intensity of capped CdS QDs was proportional to the concentration of CIP in the ranges of 0.13–150.0μmolL−1 and the detection limit was 0.04μmolL−1. The established method showed a good selectivity for CIP among several kinds of ions and biomolecules. The fluorescent probe was successfully applied to determine CIP in pharmaceutical preparation and human serum samples.

Nonradiative Relaxation of Photoexcited Black Phosphorus Is Reduced by Stacking with MoS2: A Time Domain ab Initio Study.

Black phosphorus (BP) is an appealing material for applications in electronics and optoelectronics because of its tunable direct band gap and high charge carrier mobility. For real optoelectronic device utilization, nonradiative electron-hole recombination should be slow because it constitutes a major pathway for charge and energy losses. Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that nonradiative electron-hole recombination occurs within several tens of picoseconds in bilayer BP, agreeing well with experimental data. When a single layer of BP is stacked with monolayer MoS2, the recombination is reduced because of the increased band gap and reduced electron-phonon NA coupling compared to bilayer BP. The slow electron-phonon energy losses in BP-MoS2 van der Waals heterojunction relative to bilayer BP indicate that rationally stacking BP with other two-dimensional materials is an attractive route for designing novel and efficient photovoltaic materials.

Towards clarifying what distinguishes cyanobacteria able to resurrect after desiccation from those that cannot: The photosynthetic aspect

Organisms inhabiting biological soil crusts (BSCs) are able to cope with extreme environmental conditions including daily hydration/dehydration cycles, high irradiance and extreme temperatures. The photosynthetic machinery, potentially the main source of damaging reactive oxygen species during cessation of CO2 fixation in desiccating cells, must be protected to avoid sustained photodamage. We compared certain photosynthetic parameters and the response to excess light of BCS-inhabiting, desiccation-tolerant cyanobacteria Leptolyngbya ohadii and Nostoc reinholdii with those observed in the “model” organisms Nostoc sp. PCC 7120, able to resurrect after mild desiccation, and Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803 that are unable to recover from dehydration. Desiccation-tolerant strains exhibited a transient decline in the photosynthetic rate at light intensities corresponding to the inflection point in the PI curve relating the O2 evolution rate to light intensity. They also exhibited a faster and larger loss of variable fluorescence and profoundly faster QA− re-oxidation rates after exposure to high illumination. Finally, a smaller difference was found in the temperature of maximal thermoluminescence signal in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) than observed in “model” cyanobacteria. These parameters indicate specific functional differences of photosystem II (PSII) between desiccation tolerant and sensitive cyanobacteria. We propose that exposure to excess irradiation activates a non-radiative electron recombination route inside PSII that minimizes formation of damaging singlet oxygen in the desiccation-tolerant cyanobacteria and thereby reduces photodamage.

The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photodamage resistance.

Excess illumination damages the photosynthetic apparatus with severe implications with regard to plant productivity. Unlike model organisms, the growth of Chlorella ohadii, isolated from desert soil crust, remains unchanged and photosynthetic O2 evolution increases, even when exposed to irradiation twice that of maximal sunlight. Spectroscopic, biochemical and molecular approaches were applied to uncover the mechanisms involved. D1 protein in photosystem II (PSII) is barely degraded, even when exposed to antibiotics that prevent its replenishment. Measurements of various PSII parameters indicate that this complex functions differently from that in model organisms and suggest that C.ohadii activates a nonradiative electron recombination route which minimizes singlet oxygen formation and the resulting photoinhibition. The light-harvesting antenna is very small and carotene composition is hardly affected by excess illumination. Instead of succumbing to photodamage, C.ohadii activates additional means to dissipate excess light energy. It undergoes major structural, compositional and physiological changes, leading to a large rise in photosynthetic rate, lipids and carbohydrate content and inorganic carbon cycling. The ability of C.ohadii to avoid photodamage relies on a modified function of PSII and the dissipation of excess reductants downstream of the photosynthetic reaction centers. The biotechnological potential as a gene source for crop plant improvement is self-evident.

Nonradiative Electron–Hole Recombination Rate Is Greatly Reduced by Defects in Monolayer Black Phosphorus: Ab Initio Time Domain Study

We report ab initio time-domain simulations of nonradiative electron-hole recombination and electronic dephasing in ideal and defect-containing monolayer black phosphorus (MBP). Our calculations predict that the presence of phosphorus divacancy in MBP (MBP-DV) substantially reduces the nonradiative recombination rate, with time scales on the order of 1.57 ns. The luminescence line width in ideal MBP of 150 meV is 2.5 times larger than MBP-DV at room temperature, and is in excellent agreement with experiment. We find that the electron-hole recombination in ideal MBP is driven by the 450 cm(-1) vibrational mode, whereas the recombination in the MBP-DV system is driven by a broad range of vibrational modes. The reduced electron-phonon coupling and increased bandgap in MBP-DV rationalize slower recombination in this material, suggesting that electron-phonon energy losses in MBP can be minimized by creating suitable defects in semiconductor device material.

Fluorescent carbon dots sensor for highly sensitive detection of guanine

In the present work, unmodified carbon nanodots (CDs) can act as fluorescent probes for detection and measurement of guanine concentration. CDs were synthesized by using a facile one-step microwave-assisted method with dl-malic acid and ethylenediamine as the precursors. The fluorescence of CDs can be obviously quenched by copper(II) ions through nonradiative electron transfer. Meanwhile, due to the coordination of copper(II) ions with guanine, the fluorescence of the CDs was restored reversibly by the addition of guanine. Under optimized conditions, the prepared CDs demonstrate a linear range from 1.31×10−8 to 7.27×10−7molL−1 with the detection limit of 0.67×10−8molL−1. The new fluorescence system has been applied for the determination of guanine in urine and DNA samples with the recoveries from 98.1% to 103.0%.

Plight of Mn Doping in Colloidal CdS Quantum Dots To Boost the Efficiency of Solar Cells

One of the strategies to boost the efficiency of highly promising yet struggling quantum dot solar cells (QDSCs) is to dope the QDs with optically active Mn2+ ions to create long-lived charge carriers and reduce the electron–hole recombination. The in situ deposition of QDs although offers good surface coverage and better charge separation; in “doped” QDSCs the type of doping responsible for higher efficiencies cannot be ascertained. In the electrophoretically deposited Mn:CdS QDSCs, highest lattice doping of Mn2+ ions in presynthesized colloidal CdS QDs results in the highest efficiency, whereas exchange coupled Mn2+–Mn2+ pairs increase nonradiative electron–hole recombination and decrease the efficiency of QDSCs. With Mn:Cd at. % of 1.44, the efficiency could be increased to 2.1% as compared to 1.3% for CdS QDSC. The increase in efficiency by 66.4% is due to slower charge recombination in the photoanode and the electrolyte interface and higher electron lifetime in the doped QDSCs.

One-pot green synthesis of oxygen-rich nitrogen-doped graphene quantum dots and their potential application in pH-sensitive photoluminescence and detection of mercury(II) ions

Nitrogen doping has been a powerful method to modulate the properties of carbon materials for various applications, and N-doped graphene quantum dots (GQDs) have gained remarkable interest because of their unique chemical, electronic, and optical properties. Herein, we introduce a facile one-pot solid-phase synthesis strategy for N-doped GQDs using citric acid (CA) as the carbon source and 3,4-dihydroxy-l-phenylalanine (l-DOPA) as the N source. The as-prepared N-GQDs with oxygen-rich functional groups are uniform with an average diameter of 12.5nm. Because of the introduction of nitrogen atoms, N-GQDs exhibit excitation-wavelength-independent fluorescence with the maximum emission at 445nm, and a high quantum yield of 18% is achieved at an excitation wavelength of 346nm. Furthermore, a highly efficient fluorosensor based on the as-prepared N-GQDs was developed for the detection of Hg2+ because of the effective quenching effect of metal ions via nonradiative electron transfer. This fluorosensor exhibits high sensitivity toward Hg2+ with a detection limit of 8.6nM. The selectivity experiments reveal that the fluorescent sensor is specific for Hg2+. Most importantly, the practical use of the sensor based on N-GQDs for Hg2+ detection was successfully demonstrated in river-water samples.

A steady-state and time-resolved photophysical study of CdTe quantum dots in water.

The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitive to small variations in dimensions, shape and surface capping. In the present work CdTe quantum dots are synthesized in water using 3-mercaptopropionic acid and 1-thioglycerol as stabilizers. Nanocrystals with an average dimension of 4.0 ± 1.0 and 3.7 ± 0.9 nm were obtained, when 3-mercaptopropionic acid or 1-thioglycerol, respectively, was used as a capping agent. The steady-state characterization shows that the two types of colloids have different luminescence behavior. In order to investigate the electronic structure and the dynamics of the exciton state, a combined study in the time domain has been carried out by using fluorescence time-correlated single photon counting and femtosecond transient absorption techniques. The electron-hole radiative recombination follows the non-exponential decay law for both colloids, which results in different average decay time values (of the order of tens of nanoseconds) for the two samples. The data demonstrate that the process is slower for 1-thioglycerol-stabilized colloids. The ultrafast transient absorption measurements are performed at two different excitation wavelengths (at the band gap and at higher energies). The spectra are dominated in both types of samples by the negative band-gap bleaching signals although transient positive absorption bands due to the electrons in the conduction band are observable. The analysis of the signals is affected by the different interactions with the defect states, due to ligand capping capacities. In particular, the data indicate that in 1-thioglycerol-stabilized colloids the non-radiative recombination processes are kinetically more competitive than the radiative recombination. Therefore the comparison of the data obtained from the two samples is interpreted in terms of the effects of the capping agents on the electronic relaxation of the colloids.

The influence of the crystal structure on aggregation-induced luminescence of derivatives of aminobenzoic acid

The luminescence of three derivatives of 2-(phenylamino)-benzoic acid (N-phenylanthranilic, mefenamic, and niflumic acids) in benzene solution, in the polycrystalline state, and in the hexamethylbenzene matrix is studied. In the crystalline state, these compounds exhibit intense aggregation-induced luminescence. An increase in luminescence is also observed in the impurity crystal. The hexamethylbenzene crystal lattice restricts the mobility of molecules, thus ensuring the rigidity of the molecular structure of acids, which decreases the efficiency of nonradiative electron energy degradation. The main reason for the increase in the luminescence intensity in the case of fixation in a crystalline matrix is the formation of intramolecular hydrogen bonds and dimers of acid molecules.

Simulation of sublinear dose dependence of thermoluminescence with the inclusion of the competitive interaction of trapping centers

The sublinearity of dose characteristics of the thermoluminescence has been calculated in terms of the model of competitive interaction of electron and hole trapping centers. It has been found that the sub-linearity is caused by the nonradiative electron recombination with hole deep traps during irradiation and recording of the thermoluminescence upon linear heating of a dosimetric phosphor. It has been shown that the proposed sublinearity mechanism can be used to explain the dose characteristics of the thermoluminescence of anion-defective aluminum oxide.

Photophysical Properties of Doped Carbon Dots (N, P, and B) and Their Influence on Electron/Hole Transfer in Carbon Dots–Nickel (II) Phthalocyanine Conjugates

Doping in carbon nanomaterial with various hetero atoms draws attention due to their tunable properties. Herein, we have synthesized nitrogen containing carbon dots [C-dots (N)], phosphorus co-doped nitrogen containing carbon dots [C-dots (N, P)], and boron co-doped nitrogen containing carbon dots [C-dots (N, B)]; and detailed elemental analysis has been unveiled by X-ray photoelectron spectroscopy (XPS) measurements. Our emphasis is given to understand the effect of doping on the photophysical behavior of carbon dots by using steady-state and time-resolved spectroscopy. Nitrogen containing carbon dots have quantum yield (QY) of 64.0% with an average decay time of 12.8 ns. Photophysical properties (radiative decay rate and average decay time) are found to be increased for phosphorus co-doping carbon dots due to extra electron incorporation for n-type doping (phosphorus dopant) to carbon dots which favors the radiative relaxation pathways. On the contrary, boron (p-type dopant) co-doping with nitrogen containing carbon dots favors the nonradiative electron–hole recombination pathways due to incorporation of excess hole; as a result QY, radiative rate, and average decay time are decreased. To understand the effect of doping on charge transfer phenomena, we have attached nickel (II) phthalocyanine on the surface of C-dots. It is seen that phosphorus co-doping carbon dots accelerates the electron transfer process from carbon dots to phthalocyanine. In contrast, after boron co-doping in carbon dots, the electron transfer process slows down and a simultaneous hole transfer process occurs.

MATLAB-based program for optimization of quantum cascade laser active region parameters and calculation of output characteristics in magnetic field

A strong magnetic field applied along the growth direction of a quantum cascade laser (QCL) active region gives rise to a spectrum of discrete energy states, the Landau levels. By combining quantum engineering of a QCL with a static magnetic field, we can selectively inhibit/enhance non-radiative electron relaxation process between the relevant Landau levels of a triple quantum well and realize a tunable surface emitting device. An efficient numerical algorithm implementation is presented of optimization of GaAs/AlGaAs QCL region parameters and calculation of output properties in the magnetic field. Both theoretical analysis and MATLAB implementation are given for LO-phonon and interface roughness scattering mechanisms on the operation of QCL. At elevated temperatures, electrons in the relevant laser states absorb/emit more LO-phonons which results in reduction of the optical gain. The decrease in the optical gain is moderated by the occurrence of interface roughness scattering, which remains unchanged with increasing temperature. Using the calculated scattering rates as input data, rate equations can be solved and population inversion and the optical gain obtained. Incorporation of the interface roughness scattering mechanism into the model did not create new resonant peaks of the optical gain. However, it resulted in shifting the existing peaks positions and overall reduction of the optical gain. Program summaryProgram title: QCLCatalogue identifier: AERL_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AERL_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 37763No. of bytes in distributed program, including test data, etc.: 2757956Distribution format: tar.gzProgramming language: MATLAB.Computer: Any capable of running MATLAB version R2010a or higher.Operating system: Any platform supporting MATLAB version R2010a or higher.RAM: Minimum required is 1 GB. Memory usage increases for less intense magnetic fields.Classification: 15.Nature of problem:The nature of the problem is to provide an efficient numerical algorithm implementation for optimization of GaAs/AlGaAs QCL active region parameters and calculation of output properties in the magnetic field.Solution method:The optimization of the QCL laser performance at selected wavelength is performed at entire free-parameters space using simulated annealing algorithm. The scattering rates are calculated in the presence and without magnetic field and used as coefficients in rate equations. The standard MATLAB procedures were used to solve iteratively this system of equations and obtain distribution of electron densities over electronic states.Restrictions:The machine must provide the necessary main memory which decreases roughly quadratically with the increase of the magnetic field intensity.Running time:Optimization time on Intel 3 GHz processor is about 2×104 s. The calculation time of laser output properties for values set automatically in GUI is 5×104 s.

Review of recent progress of III-nitride nanowire lasers

One-dimensional compound semiconductor nanolasers, especially nanowire (NW)-based nanolasers utilizing III-nitride (AlGaInN) materials system, are an emerging and promising area of research. Significant achievements have been made in developing III-nitride NW lasers with emission wavelengths from the deep ultraviolet (UV) to the near-infrared spectral range. The types of lasers under investigation include Fabry-Pérot, photonic crystal, plasmonic, ring resonator, microstadium, random, polariton, and two-dimensional distributed feedback lasers. The lasing thresholds vary by several orders of magnitude, which are a direct consequence of differing NW dimensions, quality of the NWs, characteristics of NW cavities, and coupling with the substrate. For electrically injected, such as ultralow-threshold and continuous-wave III-nitride NW lasers that can operate at room temperature, the following obstacles remain: carrier loss mechanisms including defect-related nonradiative surface recombination, electron overflow, and poor hole transport; low radiative recombination efficiency and high surface recombination; poor thermal management; and highly resistive ohmic contacts on the p -layer. These obstacles must be overcome to fully realize the potential of these lasers.

Synthetic Control Over Photoinduced Electron Transfer in Phosphorescence Zinc Sensors

Despite the promising photofunctionalities, phosphorescent probes have been examined only to a limited extent, and the molecular features that provide convenient handles for controlling the phosphorescence response have yet to be identified. We synthesized a series of phosphorescence zinc sensors based on a cyclometalated heteroleptic Ir(III) complex. The sensor construct includes two anionic cyclometalating ligands and a neutral diimine ligand that tethers a di(2-picolyl)amine (DPA) zinc receptor. A series of cyclometalating ligands with a range of electron densities and band gap energies were used to create phosphorescence sensors. The sensor series was characterized by variable-temperature steady-state and transient photoluminescence spectroscopy studies, electrochemical measurements, and quantum chemical calculations based on time-dependent density functional theory. The studies demonstrated that the suppression of nonradiative photoinduced electron transfer (PeT) from DPA to the photoexcited Ir(IV) species provided the underlying mechanism that governed the phosphorescent response to zinc ions. Importantly, the Coulombic barrier, which was located on either the cyclometalating ligand or the diimine ligand, negligibly influenced the PeT process. Phosphorescence modulation by PeT strictly obeyed the Rehm-Weller principle, and the process occurred in the Marcus-normal region. These findings provide important guidelines for improving sensing performance; an efficient phosphorescence sensor should include a cyclometalating ligand with a wide band gap energy and a deep oxidation potential. Finally, the actions of the sensor were demonstrated by visualizing the intracellular zinc ion distribution in HeLa cells using a confocal laser scanning microscope and a photoluminescence lifetime imaging microscope.

An Antilock Molecular Braking System

A light-driven molecular brake displaying an antilock function is constructed by introducing a nonradiative photoinduced electron transfer (PET) decay channel to compete with the trans (brake-off) → cis (brake-on) photoisomerization. A fast release of the brake can be achieved by deactivating the PET process through addition of protons. The cycle of irradiation-protonation-irradiation-deprotonation conducts the brake function and mimics the antilock braking system (ABS) of vehicles.

Ultrafast Charge Transfer Dynamics in Polycrystalline CdSe/TiO2 Nanorods Prepared by Oblique Angle Codeposition

Ultrafast exciton dynamics of aligned polycrystalline nanorod arrays composed of CdSe or CdSe/TiO2 grown on conductive glass substrates using oblique angle deposition/codeposition have been studied using femtosecond transient absorption (TA) spectroscopy. Scanning electron microscopy images show that the morphology of the two samples are comparable in height, width, and tilt angle. X-ray diffraction and Raman spectroscopy indicate that the as-deposited CdSe nanorod arrays are in the hexagonal phase, while the TiO2 is amorphous. In the TA studies, a pump wavelength of 580 nm was used to determine the exciton lifetimes of CdSe in the two samples. Transient bleach dynamics probed at 695 nm can be fit with triple exponential functions with lifetimes of 7 ps, 84 ps, and ∼1.0 ns for CdSe nanorods versus 0.5 ps, 3 ps, and 24 ps for the CdSe/TiO2 composite-nanorods. These lifetimes are independent of the pump power, indicating that nonlinear processes are not involved. For CdSe nanorods, the two fast decays are mainly due to nonradiative electron–hole recombination or exciton relaxation mediated by trap states. The overall much faster decay in CdSe/TiO2 nanorods is due to electron transfer from the conduction band of CdSe to the conduction band of TiO2. The electron injection rate from CdSe into TiO2 was calculated to be 1.7 × 1011 s–1 based on the average lifetime measured for CdSe with and without TiO2. This very high rate of electron injection is attributed to the large interfacial area and strong coupling between the two materials in CdSe/TiO2 composite-nanorods. Such strongly coupled semiconductor–metal oxide heterostructures are desired for applications in solar energy conversion.