Colloidal CsPbBr3 perovskite nanocrystals (NCs) have emerged as an excellent light emitting material in last one year. Using time domain and time-resolved THz spectroscopy and density functional theory based calculations, we establish 3-fold free carrier recombination mechanism, namely, nonradiative Auger, bimolecular electron-hole recombination, and inefficient trap-assisted recombination in 11 nm sized colloidal CsPbBr3 NCs. Our results confirm a negligible influence of surface defects in trapping charge carriers, which in turn results into desirable intrinsic transport properties, from the perspective of device applications, such as remarkably high carrier mobility (∼4500 cm(2) V(-1) s(-1)), large diffusion length (>9.2 μm), and high luminescence quantum yield (80%). Despite being solution processed and possessing a large surface to volume ratio, this combination of high carrier mobility and diffusion length, along with nearly ideal photoluminescence quantum yield, is unique compared to any other colloidal quantum dot system.

a b s t r a c tA numerical methodology is proposed in this paper to simulate the degradation of cementitious materials under external sulfate attack. The methodology includes diffusion of ions in and out of the structure, chemical reactions which lead to dissolution and precipitation of solids, and mechanical damage accumulation using a continuum damage mechanics approach. Diffusion of ions is assumed to occur under a concentration gradient as well as under a chemical activity gradient. Chemical reactions are assumed to occur under a local equilibrium condition which is considered to be valid for diffusion controlled reaction mechanisms. A macro-scale representation of mechanical damage is used in this model which reflects the cracking state of the structure. The mechanical and diffusion properties are modified at each time step based on the accumulated damage. The model is calibrated and validated using experimental results obtained from the literature. The usefulness of the model in evaluating the mineralogical evolution and mechanical deterioration of the structure is demonstrated.

Bridging the concepts of homogeneous and heterogeneous reactions is an important challenge in modern chemistry. Toward that end, here, we connect the homogeneous chemistry concept of the Hammett parameter, used by organic and organometallic chemists to quantify the electron-withdrawing capability of a functional group, to the electrochemical concept of polarization induced by a biased electrode. Because these two effects share similar origins, a theoretically motivated and experimentally verifiable link between them can be established. A convenient experiment that links the two is measuring the shift of vibrational frequency that is induced by these factors. To achieve this, first, we have measured the vibrational frequency of the nitrile stretch of 4-R-benzonitrile for a series of functional groups R spanning the Hammett parameter range −0.83 ≤ σp ≤ +1.11. Because the nitrile stretch is sensitive to molecular polarization, its frequency depends on the Hammett parameter of the polarizing functional groups. Second, we have measured the nitrile vibrational frequency of 4-mercaptobenzonitrile tethered on a gold electrode and polarized in an electrochemical cell as a function of potential from −1.4 to +0.6 V versus Ag/AgCl. Comparison of the nitrile-stretch frequency between the two experiments allows us to correlate the polarization caused by a functional group to that induced by the electrode. The data suggest equivalence between the Hammett parameter σp and the local electric field at the electrode interface, therefore allowing a polarizing electrode to be treated as a functional group. Computational work supports the experimental results and allows for a quantitative relation between the interfacial electric field and σp. We anticipate the benefits of this correlation, in particular, in linking concepts between homogeneous and heterogeneous reactions.

Surfactants modulate interfacial processes. In electrochemical CO 2 reduction, cationic surfactants promote carbon product formation and suppress hydrogen evolution. The interfacial field produced by the surfactants affects the energetics of electrochemical intermediates, mandating their detailed understanding. We have used two complementary toolsvibrational Stark shift spectroscopy which probes interfacial fields at molecular length scales and electrochemical impedance spectroscopy (EIS) which probes the entire double layerto study the electric fields at metal−surfactant interfaces. Using a nitrile as a probe, we found that at open-circuit potentials, cationic surfactants produce larger effective interfacial fields (∼−1.25 V/nm) when compared to anionic surfactants (∼0.4 V/nm). At a high bulk surfactant concentration, the surface field reaches a terminal value, suggesting the formation of a full layer, which is also supported by EIS. We propose an electrostatic model that explains these observations. Our results help in designing tailored surfactants for influencing electrochemical reactions via the interfacial field effect.

Colloidal all inorganic CsPbX (X = Cl, Br, I) nanocrystals (NCs) have emerged to be an excellent material for applications in light emission, photovoltaics, and photocatalysis. Efficient interfacial transfer of photogenerated electrons and holes are essential for a good photovoltaic and photocatalytic material. Using time-resolved terahertz spectroscopy, we have measured the kinetics of photogenerated electron and hole transfer processes in CsPbBr NCs in the presence of benzoquinone and phenothiazine molecules as electron and hole acceptors, respectively. Efficient hot electron/hole transfer with a sub-300 fs time scale is the major channel of carrier transfer thus overcomes the problem related to Auger recombination. A secondary transfer of thermalized carriers also takes place with time scales of 20-50 ps for electrons and 137-166 ps for holes. This work suggests that suitable interfaces of CsPbX NCs with electron and hole transport layers would harvest hot carriers, increasing the photovoltaic and photocatalytic efficiencies.

This study reports the formation of self-assembled transmembrane anion channels by small-molecule fumaramides. Such artificial ion channel formation was confirmed by ion transport across liposomes and by planar bilayer conductance measurements. The geometry-optimized model of the channel and Cl ion selectivity within the channel lumen was also illustrated.

Intramolecular vibrational energy relaxation (IVR) is fundamentally important to chemical dynamics. We show that externally applied electric fields affect IVR and vibrational line widths by changing the anharmonic couplings and frequency detunings between modes. We demonstrate this effect in benzonitrile for which prior experimental results show a decrease in vibrational line width as a function of applied electric field. We identify three major channels for IVR that depend on electric field. In the dominant channel, the electric field affects the frequency detuning, while in the other two channels, variation of anharmonic couplings as a function of field is the underlying mechanism. Consistent with experimental results, we show that the combination of all channels gives rise to reduced line widths with increasing electric field in benzonitrile. Our results are relevant for controlling IVR with external or internal fields and for gaining a more complete interpretation of line widths of vibrational Stark probes.

No abstract