Synthesis and Characterization of New Hybrid Organic-Inorganic Metal Halides [(CH3)3SO]M2I3 (M = Cu and Ag).

Recently, all-inorganic copper(I) metal halides have emerged as promising optical materials due to their high light emission efficiencies. This work details the crystal structure of the two hybrid organic-inorganic metal halides [(CH3)3SO]M2I3 (M = Cu and Ag) and their alloyed derivatives [(CH3)3SO]Cu2-xAgxI3 (x = 0.2; 1.25), which were obtained by incorporating trimethylsulfoxonium organic cation (CH3)3SO+ in place of Cs+ in the yellow-emitting all-inorganic CsCu2I3. These compounds are isostructural and centrosymmetric with the space group Pnma, featuring one-dimensional edge-sharing [M2I3]- anionic double chains separated by rows of (CH3)3SO+ cations. Based on density functional theory calculations, the highest occupied molecular orbitals (HOMOs) of [(CH3)3SO]M2I3 (M = Cu and Ag) are dominated by the Cu or Ag d and I p orbitals, while the lowest unoccupied molecular orbitals (LUMOs) are Cu or Ag s and I p orbitals. [(CH3)3SO]Cu2I3 single crystals exhibit a semiconductor resistivity of 9.94 × 109 Ω·cm. Furthermore, a prototype [(CH3)3SO]Cu2I3 single-crystal-based X-ray detector with a detection sensitivity of 200.54 uCGy-1 cm-2 (at electrical field E = 41.67 V/mm) was fabricated, indicating the potential use of [(CH3)3SO]Cu2I3 for radiation detection applications.

The effect of solvent on determining highest occupied molecular orbital energies of semiconducting organic molecules: Insight from a combined computational approach.

Although cyclic voltammetry (CV) measurements in solution have been widely used to determine the highest occupied molecular orbital energy (EHOMO ) of semiconducting organic molecules, an understanding of the experimentally observed discrepancies due to the solvent used is lacking. To explain these differences, we investigate the solvent effects on EHOMO by combining density functional theory and molecular dynamics calculations for four donor molecules with a common backbone moiety. We compare the experimental EHOMO values to the calculated values obtained from either implicit or first solvation shell theories. We find that the first solvation shell method can capture the EHOMO variation arising from the functional groups in solution, unlike the implicit method. We further applied the first solvation shell method to other semiconducting organic molecules measured in solutions for different solvents. We find that the EHOMO obtained using an implicit method is insensitive to solvent choice. The first solvation shell, however, produces EHOMO values that are sensitive to solvent choices and agrees with published experimental results. The solvent sensitivity arises from a hierarchy of three effects: (1) the solute electronic state within a surrounding dielectric continuum, (2) ambient temperature or solvent atoms changing the solute geometry, and (3) electronic interactions between the solute and solvents. The implicit method, on the other hand, only captures the effect of a dielectric environment. Our findings suggest that EHOMO obtained by CV measurements should account for the influence of solvent when the results are reported, interpreted, or compared to other molecules.

Effects of inter-radical interactions and scavenging radicals on magnetosensitivity: spin dynamics simulations of proposed radical pairs.

Although the magnetosensitivity to weak magnetic fields, such as the geomagnetic field, which was exhibited by radical pairs that are potentially responsible for avian navigation, has been previously investigated by spin dynamics simulations, understanding this behavior for proposed radical pairs in other species is limited. These include, for example, radical pairs formed in the single-cell green alga Chlamydomonas reinhardtii (CraCRY) and in Columba livia (ClCRY4). In addition, the radical pair of FADH• with the one-electron reduced cyclobutane thymine dimer that was shown to be sensitive to weak magnetic fields has been of interest. In this work, we investigated the directional magnetosensitivity of these radical pairs to a weak magnetic field by spin dynamics simulations. We find significant reduction in the magnetosensitivity by inclusion of dipolar and exchange interactions, which can be mitigated by a scavenging radical, as demonstrated for the [FAD•- TyrD•] radical pair in CraCRY, but not for the [FADH• T□T•-] radical pair because of the large exchange coupling. The directional magnetosensitivity of the ClCRY4 [FAD•- TyrE•] radical pair can survive this adverse effect even without the scavenging reaction, possibly motivating further experimental exploration.

Raman spectroscopy study of pressure-induced phase transitions in single crystal CuInP2S6.

Two-dimensional ferroic materials exhibit a variety of functional properties that can be tuned by temperature and pressure. CuInP2S6 is a layered material that is ferrielectric at room temperature and whose properties are a result of the unique structural arrangement of ordered Cu+ and In3+ cations within a (P2S6)4- anion backbone. Here, we investigate the effect of hydrostatic pressure on the structure of CuInP2S6 single crystals through a detailed Raman spectroscopy study. Analysis of the peak frequencies, intensities, and widths reveals four high pressure regimes. At 5 GPa, the material undergoes a monoclinic-trigonal phase transition. At higher pressures (5-12 GPa), we see Raman peak sharpening, indicative of a change in the electronic structure, followed by an incommensurate phase between 12 and 17 GPa. Above 17 GPa, we see evidence for bandgap reduction in material. The original state of the material is fully recovered upon decompression, showing that hydrostatic pressure could be used to tune the electronic and ferrielectric properties of CuInP2S6.

Calculated linear and nonlinear optical absorption spectra of phosphine-ligated gold clusters.

Although prediction of optical excitations of ligated gold clusters by time-dependent density functional theory (TDDFT) is relatively well-established, limitations still exist, for example in the choice of the exchange-correlation functional. In aiming to improve on the accuracy of the calculated linear absorption, we report a theoretical study on phosphine-ligated gold clusters, specifically Au9(PR3)83+ and Au8(PR3)72+ characterized by highly resolved UV/Vis spectra, using mass-selective electronic absorption photofragmentation spectroscopy (A. Cirri, H. M. Hernández and C. J. Johnson, J. Phys. Chem. A, 2020, 124, 1467-1479, and references therein). The optical absorption spectra of the Au9(PR3)83+ and Au8(PR3)72+ clusters were calculated using TDDFT and the many-body GW (G-Green's function, and W-screened Coulomb interaction)-BSE (Bethe Salpeter Equation) method, and compared to the experimental measurements. The evGW-BSE results demonstrated fair agreement with the experimental data, comparable to the TDDFT results, but with less dependence on the reference exchange-correlation functional. Experimentally observed ligand-effects in these materials were reproduced in our calculations as well. Finally, to assess the utility of the materials for nonlinear optical absorption, a theoretical evaluation of two-photon absorption cross-sections is included.

Theoretical Investigation of the Electronic Spectra of Cadmium Chalcogenide 2D Nanoplatelets.

Although colloidal cadmium chalcogenide 2D nanoplatelets (NPLs) have recently demonstrated strongly enhanced one- and two-photon absorption (OPA, TPA) spectra, an understanding of the effects of quantum confinement in the lateral and vertical (thickness) dimensions is mostly lacking. In this work, we investigate theoretically CdS and CdSe NPLs passivated with formate and acetate ligands with thicknesses of two and three monolayers (MLs) and different lateral dimensions. Initial structures for CdS nanoplatelets were obtained using our recently developed deep neural network potential, and the low-energy geometries were subsequently optimized using density functional theory (DFT). Linear- and nonlinear-response calculations using time-dependent DFT (TDDFT) and the simplified Tamm-Dancoff approximation (sTDA) demonstrated good agreement between measured spectra and calculated TDDFT and sTDA spectra for 2 and 3 ML NPLs. The OPA red-shifts from 2 to 3 ML NPLs can be attributed to the electron delocalization in the lateral and vertical directions. TPA responses for CdS and CdSe NPLs were found to be dominated by weakly absorbing and forbidden OPA states.

Fano plasmonics goes nonlinear.

We investigate the process of the second harmonic generation by plasmonic nano-antennas that exhibit Fano-like resonances. A rigorous fully vectorial Maxwell-hydrodynamics approach is employed to directly calculate the second order susceptibilities as a function of the pump frequency, considering a periodic array of nanodolmens comprised of three Au nanorods. The results of the numerical simulations demonstrate a noticeable enhancement of the second harmonic efficiency by the antisymmetric mode. Additionally, a simple analytical model based on two coupled nonlinear oscillators is proposed. It is shown that the second order optical response can be significantly enhanced at the frequency of the antisymmetric normal mode, thus supporting our numerical results.

Surface Functionalization of Ti3C2Tx MXene Nanosheets with Catechols: Implication for Colloidal Processing.

Precise tailoring of two-dimensional nanosheets with organic molecules is critical to passivate the surface and control the reactivity, which is essential for a wide range of applications. Herein, we introduce catechols to functionalize exfoliated MXenes (Ti3C2Tx) in a colloidal suspension. Catechols react spontaneously with Ti3C2Tx surfaces, where binding is initiated from a charge-transfer complex as confirmed by density functional theory (DFT) and UV-vis. Ti3C2Tx sheet interlayer spacing is increased by catechol functionalization, as confirmed by X-ray diffraction (XRD), while Raman and atomic force microscopy-infrared spectroscopy (AFM-IR) measurements indicate binding of catechols at the Ti3C2Tx surface occurs through metal-oxygen bonds, which is supported by DFT calculations. Finally, we demonstrate immobilization of a fluorescent dye on the surface of MXene. Our results establish a strategy for tailoring MXene surfaces via aqueous functionalization with catechols, whereby colloidal stability can be modified and further functionality can be introduced, which could provide excellent anchoring points to grow polymer brushes and tune specific properties.

Theoretical analysis of structures and electronic spectra of molecular colloidal cadmium sulfide clusters and nanoplatelets.

In the present study, we systematically examine structures and absorption spectra for CdS nanoplatelets (NPLs) with thicknesses of two and three monolayers (2 MLs and 3 MLs) and extended lateral dimensions. These nanoplatelet model systems, passivated with formate and acetate ligands, are used to analyze the effects of quantum confinement in the lateral dimension within an extended monolayer and the effects of thickness when changing from two to three monolayers. Based on the computed cubic structures using density functional theory (DFT), we found good agreement between observed and time-dependent DFT-calculated spectra, revealing little ligand participation to influence the color and intensity of low-energy absorption bands as the structures are laterally extended to eight and seven monolayers for 2-ML and 3-ML systems, respectively. The spectral redshift for 3-ML CdS NPLs is attributed to the electron delocalization due to expansion of the nanoplatelet in the lateral and vertical directions.

Two-dimensional MoS2 2H, 1T, and 1T' crystalline phases with incorporated adatoms: theoretical investigation of electronic and optical properties.

Although there has been progress in studying the electronic and optical properties of monolayer and near-monolayer (two-dimensional, 2D) MoS2 upon adatom adsorption and intercalation, understanding the underlying atomic-level behavior is lacking, particularly as related to the optical response. Alkali atom intercalation in 2D transition metal dichalcogenides (TMDs) is relevant to chemical exfoliation methods that are expected to enable large scale production. In this work, focusing on prototypical 2D MoS2, the adsorption and intercalation of Li, Na, K, and Ca adatoms were investigated for the 2H, 1T, and 1T' phases of the TMD by the first principles density functional theory in comparison to experimental characterization of 2H and 1T 2D MoS2 films. Our electronic structure calculations demonstrate significant charge transfer, influencing work function reductions of 1-1.5 eV. Furthermore, electrical conductivity calculations confirm the semiconducting versus metallic behavior. Calculations of the optical spectra, including excitonic effects using a many-body theoretical approach, indicate enhancement of the optical transmission upon phase change. Encouragingly, this is corroborated, in part, by the experimental measurements for the 2H and 1T phases having semiconducting and metallic behavior, respectively, thus motivating further experimental exploration. Overall, our calculations emphasize the potential impact of synthesis-relevant adatom incorporation in 2D MoS2 on the electronic and optical responses that comprise important considerations toward the development of devices such as photodetectors or the miniaturization of electroabsorption modulator components.

Systematic Study of the Properties of CdS Clusters with Carboxylate Ligands Using a Deep Neural Network Potential Developed with Data from Density Functional Theory Calculations.

Although structures of the inorganic core of CdS atomically precise quantum dots were reported, characterizing the nature of the metal-carboxylate coordination in these materials remains a challenge due to the large number of possible isomers. The computational cost imposed by first-principles methods is prohibitive for such a configurational search, and empirical potentials are not available. In this work, we applied deep neural network algorithms to train a potential for CdS clusters with carboxylate ligands using a database of energies and gradients obtained from density functional theory calculations. The derived potential provided energies and gradients based on a set of reference structures. Our trained potential was then used to accelerate genetic algorithm and molecular dynamics simulations searches of low-energy structures, which in turn, were used to compute the X-ray diffraction and electronic absorption spectra. Our results for CdS clusters with carboxylate ligands, analyzed and compared with experimental findings, demonstrated that the structure of a cluster whose properties agree better with experiment may deviate from the one previously assumed.

Electron transfer and spin dynamics of the radical-pair in the cryptochrome from Chlamydomonas reinhardtii by computational analysis.

In an effort to elucidate the origin of avian magnetoreception, it was postulated that a radical-pair formed in a cryptochrome upon light activation provided the basis for the mechanism that enables an inclination compass sensitive to the geomagnetic field. Photoreduction in this case involves formation of a flavin adenine dinucleotide (FAD)-tryptophan (TRP) radical-pair, following electron transfer within a conserved TRP triad in the cryptochrome. Recently, an animal-like cryptochrome from Chlamydomonas reinhardtii (CraCRY) was analyzed, demonstrating the role of a fourth aromatic residue, which serves as a terminal electron donor in the photoreduction pathway, resulting in the creation of a more distal radical-pair and exhibiting fast electron transfer. In this work, we investigated the electron transfer in CraCRY with a combination of free energy molecular dynamics (MD) simulations, frozen density functional theory, and QM/MM MD simulations, supporting the suggestion of a proton coupled electron transfer mechanism. Spin dynamics simulations discerned details on the dependence of the singlet yield on the direction of the external magnetic field for the [FAD•- TYRH•+] and [FAD•- TYR•] radical-pairs in CraCRY, in comparison with the previously modeled [FAD•- TRPH•+] radical-pair.

Theoretical Prediction of Optical Absorption and Emission in Thiolated Gold Clusters.

Although the photoluminescence of gold clusters has been extensively studied so far, there are still questions on the origin of the emission in these materials. In this work, we report time-dependent density functional theory calculations on the absorption and emission spectra of the well-studied Au25(SR)18- cluster, the lowest energy isomer of the Au38(SR)24 cluster, and five isomers of the Au22(SR)18 cluster. Good agreement between the calculated and measured absorption spectra, as well as with the lowest-energy emission values for these clusters, was demonstrated, verifying the accuracy of the theoretical methods employed. Our results for Au25(SR)18- explain a newly observed feature in the absorption peak, also rationalizing the optical response in terms of the superatom model. The analysis of the absorption and emission characteristics of the Au25(SR)18- and Au38(SR)24 clusters provides an estimate of the spectral regions, where fluorescence or phosphorescence is predicted to occur. Interestingly, we find that for Au22(SR)18, one of the five proposed structures could be present at a significant concentration in the sample, even though it is not the lowest in energy structure, which can be explained, in part, by solvent effects.

Systematic Study of Structure, Stability, and Electronic Absorption of Tetrahedral CdSe Clusters with Carboxylate and Amine Ligands.

In this work, we carried out a systematic investigation to assess the effects of ligands on the structure, stability, and absorption spectra of ultrasmall CdSe tetrahedral quantum dots, where the cores of small tetrahedral quantum dots have been postulated to be stabilized by amine and carboxylate ligands. We found that amine and carboxylate ligands form extensive hydrogen bonding networks, which provide thermodynamic stability to the clusters. On the basis of the optimized structures, good agreement between observed and computed spectra was obtained. The ligands were also found to have a large influence on the color and intensity of the electronic absorption spectra, particularly for the small clusters, which were previously monitored with in situ UV-visible absorbance spectroscopy. Our work provides an understanding of the effect of ligands that influence thermodynamic stability and electronic absorption of ultrasmall quantum dots, thus potentially motivating further experimental exploration.

Theoretical Analysis of Optical Absorption and Emission in Mixed Noble Metal Nanoclusters.

In this work, we studied theoretically two hybrid gold-silver clusters, which were reported to have dual-band emission, using density functional theory (DFT) and linear and quadratic response time-dependent DFT (TDDFT). Hybrid functionals were found to successfully predict absorption and emission, although explanation of the NIR emission from the larger cluster (cluster 1) requires significant vibrational excitation in the final state. For the smaller cluster (cluster 2), the Δ H(0-0) value calculated for the T1 → S0 transition, using the PBE0 functional, is in good agreement with the measured NIR emission, and the calculated T2 → S0 value is in fair agreement with the measured visible emission. The calculated T1 → S0 phosphorescence Δ H(0-0) for cluster 1 is close to the measured visible emission energy. In order for the calculated phosphorescence for cluster 1 to agree with the intense NIR emission reported experimentally, the vibrational energy of the final state (S0) is required to be about 0.7 eV greater than the zero-point vibrational energy.

A Theoretical Investigation of the Structure and Optical Properties of a Silver Cluster in Solid Form and in Solution.

Using density functional theory (DFT) and linear and quadratic response time-dependent DFT, we investigated the structure and optical properties of a silver sulfide cluster with the interesting property of dual emission that was observed when in crystal form but not in solution. Since the dual fluorescence is observed only in the crystal, a supposition of stabilization of a higher-energy excited state by an excimer-like complex was analyzed by calculations for a cluster dimer, formed through π-stacking of aromatic groups bonded to the sulfur atoms. However, because of the complexity of the system, a simple one-dimensional method for dimer optimization, which works moderately well in predicting the red-shifted fluorescence compared to its absorption in a naphthalene dimer, predicts only partially the red shift for the emission energy. Interestingly, calculations of the two-photon absorption (TPA) cross-section on the optimized isolated cluster as well as the crystal structure geometry indicate significant off-resonance TPA. While some materials have significantly larger TPA cross-sections, such a TPA cross-section off-resonance could be useful. The high density of states in the dimer system results in a higher probability for significant resonance enhancement and thus much larger TPA cross-sections.

Calculations of One- and Two-Photon Absorption Spectra for Molecular Metal Chalcogenide Clusters with Electron-Acceptor Ligands.

We present calculated one- and two-photon absorption (OPA, TPA) spectra for molecular neutral, cation, and anion cadmium chalcogenide nonstoichiometric clusters [CdnE'm'(ER)m, E = S and Se, R = hydrogen, methyl, phenyl, para-nitrophenyl, para-cyanophenyl], ranging from less than 1 nm to more than 2 nm in size with well-defined structures. A systematic treatment of the clusters is carried out to assess the effects of size and ligand on their linear and nonlinear optical properties. Ligands and cluster size were found to have a large influence on the color and intensity of the electronic absorption spectra. TPA cross sections were found to increase linearly with cluster size. Electron-accepting ligands were also found to induce linear enhancement in TPA cross sections. Blue shifts of TPA maxima were observed for the first band with reduced molecular size. The effects of phenyl, para-nitrophenyl, and para-cyanophenyl substitutions, as well as changes in the chalcogenide atom, have been analyzed in detail.

Theoretical analysis of the combined effects of sulfur vacancies and analyte adsorption on the electronic properties of single-layer MoS2.

We report a first-principles theoretical investigation on the electronic structure and electron transport of defective single-layer (SL) MoS2, as well as of corresponding structures adsorbed with benzyl viologen (BV), which was shown to provide improved performance of a field effect transistor. O2 adsorption was included to gain an understanding of the response upon air-exposure. Following analysis of the structure and stability of sulfur single vacancy and line defects in SL MoS2, we investigated the local transport at the adsorbed sites via a transport model that mimics a scanning tunneling spectroscopy experiment. Distinct current-voltage characteristics were indicated for adsorbed oxygen species at a sulfur vacancy. The electronic structures of defective MoS2 indicated the emergence of impurity states in the bandgap due to sulfur defects and oxygen adsorption. Electron transport calculations for the MoS2 surface with an extended defect in a device setting demonstrated that physisorption of BV enhances the output current, while facile chemisorption by O2 upon air-exposure causes degradation of electron transport.

Linear and Nonlinear Optical Response in Silver Nanoclusters: Insight from a Computational Investigation.

We report a density functional theory (DFT) and time-dependent DFT (TDDFT) investigation of the thiolated silver nanoclusters [Ag44(SR)30](4-), Ag14(SR)12(PR'3)8, Ag31(SG)19, Ag32(SG)19, and Ag15(SG)11, which were synthesized and for which one-photon absorption (OPA) characterization is available. Our computational investigation based on careful examination of the exchange-correlation functional used in DFT geometry optimization and for the linear optical properties predictions by TDDFT, demonstrated good agreement with the measured linear absorption spectra, however dependent on the applied functional. Following the benchmarking, we evaluated the two-photon absorption (TPA) response using TDDFT, noting that accurate prediction of OPA is important for suppositions on the spectral range for TPA enhancement because of the sensitivity to the excitation energies. Although the TPA cross-section results are complicated by resonance effects and quantifying TPA cross sections for these systems is difficult, our results indicate that the nanoclusters Ag15 and Ag31/32 are likely to have large TPA cross sections. The spherical symmetry of the Ag44 and Ag14 nanoclusters leads to applicability of superatom theory, while it is not as useful for the more oblate geometries of the Ag15 and Ag31/32 systems.