Acid gases (e.g., NO x and SO x ), commonly found in complex chemical and petrochemical streams, require material development for their selective adsorption and removal. Here, we report the NO x adsorption properties in a family of rare earth (RE) metal–organic frameworks (MOFs) materials. Fundamental understanding of the structure–property relationship of NO x adsorption in the RE-DOBDC materials platform was sought via a combined experimental and molecular modeling study. No structural change was noted following humid NO x exposure. Density functional theory (DFT) simulations indicated that H2O has a stronger affinity to bind with the metal center than NO2, while NO2 preferentially binds with the DOBDC ligands. Further modeling results indicate no change in binding energy across the RE elements investigated. Also, stabilization of the NO2 and H2O molecules following adsorption was noted, predicted to be due to hydrogen bonding between the framework ligands and the molecules and nanoconfinement within the MOF structure. This interaction also caused distinct changes in emission spectra, identified experimentally. Calculations indicated that this is due to the adsorption of NO2 molecules onto the DOBDC ligand altering the electronic transitions and the resulting photoluminescent properties, a feature that has potential applications in future sensing technologies.

Methylammonium lead iodide perovskite (MAPbI) is a promising material for photovoltaic devices. A modification of MAPbI into confined nanostructures is expected to further increase efficiency of solar energy conversion. Photoexcited dynamic processes in a MAPbI quantum dot (QD) have been modeled by many-body perturbation theory and nonadiabatic dynamics. A photoexcitation is followed by either exciton cooling (EC), its radiative (RR) or nonradiative recombination (NRR), or multiexciton generation (MEG) processes. Computed times of these processes fall in the order of MEG < EC < RR < NRR, where MEG is on the order of a few femtoseconds, EC is in the picosecond range, while RR and NRR are on the order of nanoseconds. Computed time scales indicate which electronic transition pathways can contribute to increase in charge collection efficiency. Simulated mechanisms of relaxation and their rates show that quantum confinement promotes MEG in MAPbI QDs.

A comparison of two first-principles methodologies, (1) molecular dynamics (MD) sampling and (2) time integration along progression of subsequently occupied excited states for computing emission spectra of semiconductors at different levels of accuracy is presented. Photoluminescence (PL) linewidth broadening is calculated to account for the motion of ionic positions. (1) In the MD sampling method, excited state lifetimes are assumed to be very short due to quick cascade thermalization, leading to intense PL peaks in semiconductors at transition energies corresponding to the bandgap energy of the materials, according to Kasha's rule. Nuclear motion is modeled via adiabatic MD within VASP software. During MD, electronic orbital energies fluctuate through time according to electron−phonon coupling terms. A sampling of possible radiative transition energies along the MD trajectory contributes to PL line width inhomogeneous broadening σ 1 . Optically allowed transitions along fluctuating transition energies found at each MD time step model the emission spectra. (2) In the time integration method, nonadiabatic dynamics of a photoexcitation provides details of cascade thermalization and allows description of several channels of emission from long-lived intermediate states, taking into account non-radiative pathways, providing possible exceptions from Kasha's rule and demonstrating relatively small PL line broadening σ 2 , so that σ 1 ≫ σ 2 . PL spectra calculated using different methods are applied to nanostructured TiO 2 semiconducting materials in different quantum confinement regimes, showing optical gap narrowing due to confinement. Ti(OH) 4 , which is a precursor in TiO 2 synthesis, wet TiO 2 ⟨001⟩ nanowire (NW), and TiO 2 anatase thin film models are presented and compared with experimental PL results. Computed time-integrated PL spectra for the TiO 2 ⟨001⟩ NW accurately describe the relevant PL energy and, when used in conjunction with the MD sampling PL method, provide the realistic PL line width.

Detection and capture of toxic nitrogen oxides (NO x) is important for emissions control of exhaust gases and general public health. The ability to directly electrically detect trace (0.5-5 ppm) NO 2 by a metal-organic framework (MOF)-74-based sensor at relatively low temperatures (50 °C) is demonstrated via changes in electrical properties of M-MOF-74, M = Co, Mg, Ni. The magnitude of the change is ordered Ni > Co > Mg and explained by each variant's NO 2 adsorption capacity and specific chemical interaction. Ni-MOF-74 provides the highest sensitivity to NO 2 ; a 725× decrease in resistance at 5 ppm NO 2 and detection limit <0.5 ppm, levels relevant for industry and public health. Furthermore, the Ni-MOF-74-based sensor is selective to NO 2 over N 2 , SO 2 , and air. Linking this fundamental research with future technologies, the high impedance of MOF-74 enables applications requiring a near-zero power sensor or dosimeter, with the active material drawing <15 pW for a macroscale device 35 mm 2 with 0.8 mg MOF-74. This represents a 10 4-10 6 × decrease in power consumption compared to other MOF sensors and demonstrates the potential for MOFs as active components for long-lived, near-zero power chemical sensors in smart industrial systems and the internet of things.

The magnetic susceptibility of NO x -loaded RE-DOBDC (rare earth (RE): Y, Eu, Tb, Yb; DOBDC: 2,5-dihydroxy­terephthalic acid) metal–organic frameworks (MOFs) is unique to the MOF metal center. RE-DOBDC samples were synthesized, activated, and subsequently exposed to humid NO x . Each NO x -loaded MOF was characterized by powder X-ray diffraction, and the magnetic characteristics were probed by using a VersaLab vibrating sample magnetometer (VSM). Lanthanide-containing RE-DOBDC (Eu, Tb, Yb) are paramagnetic with a reduction in paramagnetism upon adsorption of NO x . Y-DOBDC has a diamagnetic moment with a slight reduction upon adsorption of NO x . The magnetic susceptibility of the MOF is determined by the magnetism imparted by the framework metal center. The electronic population of orbitals contributes to determining the extent of magnetism and change with NO x (electron acceptor) adsorption. Eu-DOBDC results in the largest mass magnetization change upon adsorption of NO x due to more available unpaired f electrons. Experimental changes in magnetic moment were supported by density functional theory (DFT) simulations of NO x adsorbed in lanthanide Eu-DOBDC and transition metal Y-DOBDC MOFs.