Halide Mixing Research Articles

Thermoresponsive Emission Switching via Lower Critical Solution Temperature Behavior of Organic–Inorganic Perovskite Nanoparticles

Lead halide perovskites have shown much promise for high-performing solar cells due to their inherent electronic nature, and though the color of bright-light emitters based on perovskite nanoparticles can be tuned by halide mixing and/or size control, dynamic switching using external stimuli remains a challenge. This article reports an unprecedented lower critical solution temperature (LCST) for toluene solutions containing methylammonium lead bromide (MAPbBr3 ), oleic acid, alkylamines, and dimethylformamide. The delicate interplay of these molecules and ions allows for the reversible formation and decomposition of MAPbBr3 nanoparticles upon heating and cooling, which is accompanied by green and blue photoemissions at each state. An intermediate 1D crystal with PbBr2 -amine coordination is found to play pivotal role in this, and a mechanistic insight is provided based on a three-state model. In addition to a high quantum yield (up to 85%), this system allows for control over the cloud point (30-80 °C) through compositional engineering and the luminescent color (blue to red) via halogen exchange, thus making it a versatile solution for developing functional molecular organic-inorganic LCST quantum dots.

Enhancing the carrier thermalization time in organometallic perovskites by halide mixing.

Hybrid metal-organic halide perovskites have recently attracted a great deal of attention because of their interesting electronic, optical and transport properties, which make them promising materials for high-performance, low-cost solar cells. Fundamental understanding of the formation mechanisms and dynamics of photoinduced charge carriers is essential for improving the performance of perovskite solar cell devices. For example, a significant amount of absorbed solar energy is lost as a result of carrier thermalization. This energy could be harnessed by extracting hot carriers before they cool down to the band edges. Although such hot carrier collection is experimentally challenging, theoretical investigations based on time-dependent methods can guide future experimental research by providing insights into the thermalization process. Here, we perform ab initio nonadiabatic molecular dynamics simulations to study non-radiative relaxation dynamics of charge carriers in hybrid halide perovskites. We find that the carrier relaxation time can be considerably increased by mixing halogen atoms in the perovskite materials. These findings show that simple approaches could be adopted to slow down the thermalization process of hot carriers in perovskite materials.

Trace gas and particulate emissions from the 2003 southern California wildfires

In October 2003, thirteen major wildfires in southern California burned more than 300,000 hectares of mainly chaparral biome. High‐precision in situ trace gas and particle measurements of the wildfire plumes in La Jolla, California, showed a high degree of correlation among carbon dioxide (CO2), methane (CH4), nonmethane hydrocarbons, and methyl halide mixing ratios, as well as with particle number concentrations (10–300 nm and 500–2500 nm aerodynamic diameter). Aerosol time‐of‐flight mass spectrometry of individual aerosol particles (50–2500 nm range) showed that 70–85% had typical biomass burning signatures (levoglucosan coupled with potassium). Only 5–18% of particles in the 50–300 nm range had vehicle signatures. Molar trace gas enhancement ratios (ERs) versus ethane and CO2 were calculated and showed a narrow age distribution, consistent with the short distance from the wildfires. ERs for N2O and CH3CCl3 versus CO2 were determined, but correlations were poor. No significant CH2Cl2 or CHCl3 emissions were detected. CO2 emissions from the nearby Cedar fire were estimated both with a simple Lagrangian atmospheric transport model and a burned area approach and extrapolated to 11 Tg CO2 for the total burned area in southern California. Total CO2, CH4, C2‐hydrocarbons, benzene, toluene, methyl chloride, methyl iodide, and PM2.5 emissions were ∼0.2–3.5% of yearly global extratropical forest fire emissions and more than 28% of CH4, C6H6, and PM2.5 2003 San Diego and South Coast Air Basins anthropogenic emissions. Particle distributions and single particle chemistry are discussed. PM2.5 considerably exceeded the EPA short‐term exposure limit.

Are methyl halides produced on all ice surfaces? Observations from snow-laden field sites

We present data collected from a number of snow-covered environments including two polar locations (Summit, Greenland and the South Pole) and two mid-latitude regions (a remote site in northern Michigan, and Niwot Ridge, Colorado). At each site, concentrations of CH 3 I and C 2 H 5 I were enhanced within the interstitial air near the snow surface, compared to levels in boundary layer air. Fluxes of CH 3 Br from surface snow to the atmosphere were observed at each site except Niwot Ridge, where CH 3 Br appeared to have a sink. The mid-latitude sites showed significant emissions of CH 3 Cl , mostly originating at the ground surface and traveling up through the snow, while at the polar locations CH 3 Cl emissions from firn air were relatively small. In general, methyl halide mixing ratios in firn air were significantly greater at Summit than at the South Pole, with Summit showing a strong diurnal cycle in the production of alkyl halides that was well correlated with actinic radiation and firn temperature. We suggest that the most likely route to alkyl halide formation is through an acid catalyzed nucleophilic substitution of an alcohol type function by a halide, both of which should be preferentially segregated to the quasi-liquid layer at the surface of the snow grains. A series of experiments using a snow-filled quartz chamber irradiated by natural sunlight allowed estimation of emission trends that were hard to measure in the natural snowpack. These static chamber experiments confirmed significant production of the primary alkyl halides, following the order CH 3 Cl > CH 3 Br > C 2 H 5 Cl > CH 3 I > C 2 H 5 Br > C 2 H 5 I > 1 - C 3 H 7 Br > 1 - C 3 H 7 I . Our observations at all four locations, including polar and mid-latitude sites, imply that alkyl halide production may be associated with all surface snows.

Nonideal mixing of dodecyltrimethylammonium halides and nonionic surfactant in adsorbed films and micelles

The ion–dipole interaction between dodecyltrimethylammonium cations and nonionic surfactant molecules in adsorbed films and micelles was investigated by concentrating on the difference in the degree of counterion binding by employing dodecyltrimethylammonium chloride (DTAC)–octyl methyl sulfoxide (OMS) and dodecyltrimethylammonium bromide (DTAB)–OMS mixtures. The phase diagrams of adsorption and micelle formation were constructed and then the nonideal mixing of different species of surfactants was demonstrated in terms of the excess Gibbs free energies of adsorption and micelle formation, and the surface excess areas. Furthermore the dependence of them on the counterion was clearly shown. All these results were found to support our previous view that the direct interaction between surfactant cation and the dipole of the hydrophilic part of a nonionic surfactant is essential in cationic-nonionic surfactant mixtures, i.e., the DTAC system with a lower counterion biding has more negative excess thermodynamic quantities than the DTAB system with a higher one.

Dissociation of Cyanogen Azide: An Alternative Route to Synthesis of Carbon Nitride

Solid films composed principally of carbon and nitrogen were grown on a variety of substrates at ambient temperature in a flow-tube reactor by upstream mixing of cyanogen azide, cyanogen, or cyanogen halides with active nitrogen obtained from an electrical discharge. Ab initio calculations and dependence of deposition rates on both choice of donor and N atom production suggests that NCN radicals are a critical growth species. The films obtained are electrically insulating with a refractive index of 2.3 at visible wavelength and are optically transparent from 550 nm out to at least 14 μm with the exception of two broad absorption bands centered at 1550 and 3250 cm-1, the latter band growing in upon exposure of the film to atmospheric moisture. Film analysis by X-ray photoelectron spectroscopy revealed comparable concentrations of both carbon-to-nitrogen bonds (with approximate C3N4 stoichiometry) and diamond-like carbon-to-carbon bonds as well as minority bonding of carbon to impurities.

Enthalpies of mixing of some aqueous electrolytes at 373.15 K

The enthalpies of mixing of a variety of aqueous electrolytes at an ionic strength of 1.000 mol·kg −1 have been measured at 373.15 K. The mixtures studied were LiBr + KBr, NaBr + KBr, KBr + (C 4H 9) 4NBr, NaCl + Na 2SO 4, NaCl + MgCl 2, MgSO 4 + Na 2SO 4, MgCl 2 + MgSO 4, NaCl + MgSO 4, and MgCl 2 + Na 2SO 4. In contrast to most previous enthalpies of mixing of alkali halides, the mixtures with multivalent ions studied here often have large temperature dependences. The results for the mixtures involving MgSO 4 are consistent with a higher degree of ion pairing as the temperature increases. The present measurements, together with Gibbs energies of mixing at 298.15 K, allow the calculation of Gibbs energies of mixing at the higher temperature. This appears to be the most accurate and convenient method available for determining Gibbs energies of mixing at high temperatures.

The volume changes of five gases under high pressures

All-inorganic halide double perovskite has recently emerged as a potential substitute for lead halide perovskite capable of solving the problem of toxicity associated with lead content and its inherent instability issue. However, most double perovskites possess wide bandgaps having drastic consequences on their efficiency, hindering their usage as solar absorbers in the photovoltaic industry. Halide mixing has been identified as a viable means of engineering bandgaps of all-inorganic halide double perovskite. This study investigates material properties of all-inorganic double perovskite Cs2NaM(IxBr1−x)6 [M=Bi,In] for optoelectronic applications. Structural, electronic, and optical properties of the materials are investigated using first-principles density functional theory with the help of virtual crystal approximation for modeling the virtual atoms IxBr1−x. The results show a linear increase in lattice parameters of the compounds as iodine content is increased while the bandgaps decrease following a quadratic function. For M=Bi, indirect band nature is observed while for M=In, direct band nature is observed all through the range. With increased iodine content, the optical absorption of the perovskites is observed to enhance weakly. The static dielectric constants increase quadratically with increased iodine content. Phase decomposition analysis of the materials shows that CNBX materials remain stable up to x=0.2 while CNIX remains stable up to x=0.6 against phase decomposition. Thus, halide mixing of iodine with bromine may be a viable means of bandgap reduction and enhancing optical properties of double perovskites for optoelectronic applications.