Lattice Strain Limit for Uniform Shell Deposition in Zincblende CdSe/CdS Quantum Dots

The effects of lattice strain on the spectroscopy and photoluminescence quantum yields of zincblende CdSe/CdS core/shell quantum dots are examined. The quantum yields are measured as a function of core size and shell thickness. High quantum yields are achieved as long as the lattice strain energy density is below ∼0.85 eV/nm², which is considerably greater than the limiting value of 0.59 eV/nm² for thermodynamic stability of a smooth, defect-free shell, as previously reported (<i>J. Chem. Phys.</i> <b>2014</b>, <i>141</i>, 194704). Thus, core/shell quantum dots having strain energy densities between 0.59 and 0.85 eV/nm² can have very high PL QYs but are metastable with respect to surface defect formation. Such metastable core/shell QDs can be produced by shell deposition at comparatively low temperatures (<140 °C). Annealing of these particles causes partial loss of core pressure and a red shift of the spectrum

Nonuniform Excitonic Charge Distribution Enhances Exciton–Phonon Coupling in ZnSe/CdSe Alloyed Quantum Dots

Zinc to cadmium cation exchange of ZnSe quantum dots has been used to produce a series of alloyed Zn_1–<i>x</i>Cd_<i>x</i>Se quantum dots. As <i>x</i> increases and the lowest-energy exciton shifts to the red, the peak initially broadens and then sharpens as <i>x</i> approaches 1. Resonance Raman spectra obtained with excitation near the lowest excitonic absorption peak show a gradual shift of the longitudinal optical phonon peak from 251 cm^–1 in pure ZnSe to 210 cm^–1 in nearly pure CdSe with strong broadening at intermediate compositions. The LO overtone to fundamental intensity ratio, a rough gauge of exciton–phonon coupling strength, increases considerably for intermediate compositions compared with those of either pure ZnSe or pure CdSe. The results indicate that partial localization of the hole in locally Cd-rich regions of the alloyed particles increases the strengths of local internal electric fields, increasing the coupling between the exciton and polar optical phonons

Resonance Raman Spectroscopy and Electron–Phonon Coupling in Zinc Selenide Quantum Dots

Resonance Raman spectra, including absolute scattering cross sections, depolarization ratios, and overtone to fundamental intensity ratios, have been measured for three sizes of ZnSe quantum dots between 3.8 and 4.9 nm diameter (first excitonic peak maximum at 402–417 nm) using excitation wavelengths between 400 and 425 nm. Spectra were obtained both in cyclohexane solution and in thin films in order to quantitate the exciton–phonon coupling strength. The Raman data and optical absorption spectra were simulated using a particle in a sphere effective mass model for the excitonic transitions similar to that previously employed for CdSe [Lin, C.; Gong, K.; Kelley, D. F.; Kelley, A. M. <i>J. Phys. Chem. C</i> <b>2015</b>, <i>119</i>, 7491]. The Huang–Rhys parameter of the longitudinal optical phonon in the lowest excitonic transition is in the range <i>S</i> = 0.3–0.5, about a factor of 2 larger than for CdSe quantum dots of similar size. Smaller ZnSe quantum dots (∼3.0 nm diameter, first excitonic maximum at 379 nm) measured in films show stronger overtones, suggesting an increase in Huang–Rhys parameter at smaller sizes

Extinction Coefficients, Oscillator Strengths, and Radiative Lifetimes of CdSe, CdTe, and CdTe/CdSe Nanocrystals

In this paper we critically examine the literature and provide new data on fundamental optical properties of II–VI quantum dots (QDs). Specifically, we examine the integrated extinction coefficients and radiative lifetimes of different sizes of CdSe and CdTe QDs and different shell thicknesses in CdTe/CdSe core/shell QDs. We have synthesized particles having very high quantum yields and find that the measured radiative lifetimes are considerably longer and have a very different size dependence than what has been previously reported. In a simple two-level system the integrated extinction coefficients (or oscillator strengths) are related to the radiative lifetimes through the Einstein relations. The situation is more complicated in the case of II–VI QDs because of the thermal accessibility of dark states resulting from the valence band fine structure. There are significant but not equal populations in both bright and dark sublevels of the 1S_e-1S_3/2 exciton and in the dark 1S_e-1P_3/2 exciton. These Boltzmann populations depend on the QD size and shape. We find that in all three cases, quantitative or semiquantitative agreement between the measured radiative lifetimes and values calculated from the integrated extinction coefficients is obtained only if Boltzmann populations in all of the thermally accessible bright and dark states are considered. We also find that the shell thickness dependence of the radiative lifetimes of the CdTe/CdSe core/shell particles can be quantitatively understood in terms of overlap of calculated electron and hole wave functions. The results and analyses presented here clarify several discrepancies in the literature

Accurate Measurements of NH₃ Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites

Accurate Measurements of NH₃ Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air–liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]− complex (named PC–). Subsequently, PC– undergoes reduction and the C–N bond dissociates to generate COOH– and [Tz-H]− (m/z = 69). COOH– easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air–water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air–liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]− complex (named PC–). Subsequently, PC– undergoes reduction and the C–N bond dissociates to generate COOH– and [Tz-H]− (m/z = 69). COOH– easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air–water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air–liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]− complex (named PC–). Subsequently, PC– undergoes reduction and the C–N bond dissociates to generate COOH– and [Tz-H]− (m/z = 69). COOH– easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air–water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air–liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]− complex (named PC–). Subsequently, PC– undergoes reduction and the C–N bond dissociates to generate COOH– and [Tz-H]− (m/z = 69). COOH– easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air–water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets