Overcoming Thermal Quenching in X-ray Scintillators through Multi-Excited State Switching.

X-ray scintillators have gained significant attention in medical diagnostics and industrial applications. Despite their widespread utility, scintillator development faces a significant hurdle when exposed to elevated temperatures, as it usually results in reduced scintillation efficiency and diminished luminescence output. Here we report a molecular design strategy based on a hybrid perovskite (TpyBiCl5) that overcomes thermal quenching through multi-excited state switching. The structure of perovskite provides a platform to modulate the luminescence centers. The rigid framework constructed by this perovskite structure stabilized its triplet states, resulting in TpyBiCl5 exhibiting an approximately 12 times higher (45 % vs. 3.8 %) photoluminescence quantum yield of room temperature phosphorescence than that of its organic ligand (Tpy). Most importantly, the interactions between the components of this perovskite enable the mixing of different excited states, which has been revealed by experimental and theoretical investigations. The TpyBiCl5 scintillator exhibits a detection limit of 38.92 nGy s-1 at 213 K and a detection limit of 196.31 nGy s-1 at 353 K through scintillation mode switching between thermally activated delayed fluorescence and phosphorescence. This work opens up the possibility of solving the thermal quenching in X-ray scintillators by tuning different excited states.

Three-Component Donor-π-Acceptor Covalent-Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution.

Two-dimensional covalent-organic frameworks (2D COFs) have recently emerged as great prospects for their applications as new photocatalytic platforms in solar-to-hydrogen conversion; nevertheless, their inefficient solar energy capture and fast charge recombination hinder the improvement of photocatalytic hydrogen production performance. Herein, two photoactive three-component donor-π-acceptor (TCDA) materials were constructed using a multicomponent synthesis strategy by introducing electron-deficient triazine and electron-rich benzotrithiophene moieties into frameworks through sp2 carbon and imine linkages, respectively. Compared with two-component COFs, the novel TCDA-COFs are more convenient in regulating the inherent photophysical properties, thereby realizing outstanding photocatalytic activity for hydrogen evolution from water. Remarkably, the first sp2 carbon-linked TCDA-COF displays an impressive hydrogen evolution rate of 70.8 ± 1.9 mmol g-1 h-1 with excellent reusability in the presence of 1 wt % Pt under visible-light illumination (420-780 nm). Utilizing the combination of diversified spectroscopy and theoretical prediction, we show that the full π-conjugated linkage not only effectively broadens the visible-light harvesting of COFs but also enhances charge transfer and separation efficiency.

Cu(I)/Cu(II) Creutz-Taube Mixed-Valence 2D Coordination Polymers.

Graphene-like 2D coordination polymers (GCPs) have been of central research interest in recent decades with significant impact in many fields. According to classical coordination chemistry, Cu(II) can adopt the dsp2 hybridization to form square planar coordination geometry, but not Cu(I); this is why so far, there has been few 2D layered structures synthesized from Cu(I) precursors. Herein a pair of isostructural GCPs synthesized by the coordination of benzenehexathiol (BHT) ligands with Cu(I) and Cu(II) ions, respectively, is reported. Spectroscopic characterizations indicate that Cu(I) and Cu(II) coexist with a near 1:1 ratio in both GCPs but remain indistinguishable with a fractional oxidation state of +1.5 on average, making these two GCPs a unique pair of Creutz-Taube mixed-valence 2D structures. Based on density functional theory calculations, an intramolecular pseudo-redox mechanism is further uncovered whereby the radicals on BHT ligands can oxidize Cu(I) or reduce Cu(II) ions upon coordination, thus producing isostructures with distinct electron configurations. For the first time, it is demonstrated that using Cu(I) or Cu(II), one can achieve 2D isostructures, indicating an unusual fact that a neutral periodic structure can host a different number of total electrons as ground states, which may open a new chapter for 2D materials.

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1-xAgxBiCl6 Double Perovskites.

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn2+-doped Cs2Na1-xAgxBiCl6 double perovskites that exhibit dual emission modes. Introducing Ag+ ions to Cs2NaBiCl6 samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn2+ dopants induce a yellow-orange emission. Importantly, Mn2+ doping into Cs2Na1-xAgxBiCl6 double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag+ ions into Cs2NaBiCl6 can localize wave function to the [AgCl6]5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn2+ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn2+ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn2+-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

Photocatalytic Hydrogen Production on a sp2 -Carbon-Linked Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D-COFs) have emerged as attractive platforms for solar-to-chemical energy conversion. In this study, we have implemented a gradient heating strategy to synthesize a sp2 -carbon-linked triazine-based COF, COF-JLU100, exhibiting high crystallinity, large surface area, good durability and carrier mobility for solar-driven photocatalytic hydrogen evolution. The Pt-doped COF-JLU100 demonstrated a high hydrogen evolution rate of over 100 000 µmol g-1 h-1 for water splitting under visible-light illumination (λ>420 nm). Experimental and theoretical studies corroborate that the cyano-vinylene segments in COF-JLU100 extend the π-delocalization and enable fast charge transfer and separation rates as well as good dispersion in water. Moreover, COF-JLU100 can be prepared by low-cost and easily available monomers and has excellent stability, which is desirable for practical solar-driven hydrogen production.

Neutral Antiaromatic Bis(1,2-dithiolene)-Chelated Nickel Complexes Bearing Multiradical Characters.

Metal-organic complexes with radical characteristics are unique species attracting immense attention in recent years due to their peculiar properties and promising applicability in a wide variety of innovative research fields. However, the reported complexes typically do not exceed diradicality. This study systematically investigates a series of square planar neutral Ni-bis(1,2-dithiolene) and Ni-bis(1,2-dioxolene) complexes with linear, branched, and macrocyclic configurations via ab initio calculations. The linear Ni-complexes display strong singlet diradical characters, while their branched counterparts can also exhibit moderate singlet multiradical characters. Importantly, the macrocyclic Ni-complexes can possess extremely strong singlet multiradical characters up to dodeca-radicality along with their global antiaromaticity and hence strong induced ring current in the presence of an external magnetic field, ascribed to the localization of unpaired α and ß electrons residing in the highest few molecular orbitals at different molecular sites, minimizing their coupling and annihilation. Our work represents the first indication in the rational design of novel multiradical neutral antiaromatic macrocyclic complexes for potential applications in molecular machines and electronic devices.

Self-assembled molecular nanowires on prepatterned Ge(001) surfaces.

It is a long-standing goal to fabricate conductive molecular nanowires (NWs) on semiconductor surfaces. Anchoring molecules to pre-patterned surface nanostructures is a practical approach to construct molecular NWs on semiconductor surfaces. Previously, well-ordered inorganic Ge NWs were deduced to spontaneously grow onto Pt/Ge(001) surfaces after annealing at an elevated temperature. In this work, we further demonstrate that organic 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecular NWs can self-assemble onto the atomic NWs on Pt/Ge(001) surfaces. The outer nitrogen atoms in TCNQ molecules hybridize with under-coordinated Ge atoms in Ge NWs with an energy release of ∼1.14 eV per molecule, and electrons transfer from Ge NWs to the frontier orbitals of anchored TCNQs resulting in a negatively charged state. This largely tailors the electronic configurations of TCNQs and Pt/Ge(001) surfaces, enhancing the electron transport along the dimer row direction. The TCNQ molecular NWs coupled with the Ge NWs represent an exemplary showcase for the fabrication of molecular NWs on semiconductor surfaces.

Polaron Delocalization Dependence of the Conductivity and the Seebeck Coefficient in Doped Conjugated Polymers.

Conjugated polymers are promising materials for thermoelectrics as they offer good performances at near ambient temperatures. The current focus on polymer thermoelectric research mainly targets a higher power factor (PF; a product of the conductivity and square of the Seebeck coefficient) through improving the charge mobility. This is usually accomplished via structural modification in conjugated polymers using different processing techniques and doping. As a result, the structure-charge transport relationship in conjugated polymers is generally well-established. In contrast, the relationship between the structure and the Seebeck coefficient is poorly understood due to its complex nature. A theoretical framework by David Emin (Phys. Rev. B, 1999, 59, 6205-6210) suggests that the Seebeck coefficient can be enhanced via carrier-induced vibrational softening, whose magnitude is governed by the size of the polaron. In this work, we seek to unravel this relationship in conjugated polymers using a series of highly identical pro-quinoid polymers. These polymers are ideal to test Emin's framework experimentally as the quinoid character and polaron delocalization in these polymers can be well controlled even by small atomic differences (<10 at. % per repeating unit). By increasing the polaron delocalization, that is, the polaron size, we demonstrate that both the conductivity and the Seebeck coefficient (and hence PF) can be increased simultaneously, and the latter is due to the increase in the polaron's vibrational entropy. By using literature data, we also show that this phenomenon can be observed in two closely related diketopyrrolopyrrole-conjugated polymers as well as in p-doped P3HT and PANI systems with an increasing molecular order.

Electric Field-Induced Phase Transition of Nanowires on Germanium(001) Surfaces.

The manipulation of conductive nanowires (NWs) on semiconductor platforms provides important insights into the fabrication of nanoscale electronic devices. In this work, we directly observed the electric field-induced phase transitions in atomic Au-NWs self-assembled on Ge(001) surfaces using scanning tunneling microscopy (STM). The tunneling electrons and electric fields underneath a STM tip apex can effectively trigger a phase transition in Au-NWs on Ge(001) surfaces. Such phase transitions are associated with a remarkable atomic rearrangement in the Au-NWs, thereby modifying their band structures. Moreover, directly monitoring the dynamic reconstruction of Au-NWs on Ge(001) surfaces helps us to understand the NWs' intricate atomic configurations and their electronic properties. The spatially controlled phase transition at the nanometer scale using STM shows the possibility of modulating NWs' properties at an atomic scale.

Gate-Defined Quantum Confinement in CVD 2D WS2.

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS2 single- and bilayer devices before and after atomic layer deposition (ALD) of HfO2 . This isolates the influence of HfO2 deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS2 and ALD-grown HfO2 . The high dielectric constant and low leakage current enabled by HfO2 allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications.

Electronic transport descriptors for the rapid screening of thermoelectric materials.

The discovery of novel materials for thermoelectric energy conversion has potential to be accelerated by data-driven screening combined with high-throughput calculations. One way to increase the efficacy of successfully choosing a candidate material is through its evaluation using transport descriptors. Using a data-driven screening, we selected 12 potential candidates in the trigonal ABX2 family, followed by charge transport property simulations from first principles. The results suggest that carrier scattering processes in these materials are dominated by ionised impurities and polar optical phonons, contrary to the oft-assumed acoustic-phonon-dominated scattering. Using these data, we further derive ground-state transport descriptors for the carrier mobility and the thermoelectric powerfactor. In addition to low carrier mass, high dielectric constant was found to be an important factor towards high carrier mobility. A quadratic correlation between dielectric constant and transport performance was established and further validated with literature. Looking ahead, dielectric constant can potentially be exploited as an independent criterion towards improved thermoelectric performance. Combined with calculations of thermal conductivity including Peierls and inter-branch coherent contributions, we conclude that the trigonal ABX2 family has potential as high performance thermoelectrics in the intermediate temperature range for low grade waste heat harvesting.

Designing Intrinsic Topological Insulators in Two-Dimensional Metal-Organic Frameworks.

The connection between electronic structures of metal-organic frameworks (MOFs) and their building subunits is a key cornerstone for rational MOF material design. Some two-dimensional conjugated MOFs were reported to be topological insulators. However, many of them are not intrinsic as the Fermi levels are far from the topological gaps. The subunit-to-MOF electronic orbital correspondence should be established to bridge their chemical structure and physical properties, thus understanding the design rules toward intrinsic topological insulators. Herein we reveal the fundamental role of the subunit-to-MOF symmetry relation in determining their orbital interaction and hybridization and, consequently, topological characteristics. In particular, such honeycomb-kagome MOFs possess delocalized symmetry-enforced nonbonding electronic states with the topological spin-orbit gap. The nonbonding nature of these states allows tailored band structure modulation through molecular structure and strain engineering, with the potential realization of an intrinsic metal-organic topological insulator.

Boosting Electrocatalytic Activity of 3d-Block Metal (Hydro)oxides by Ligand-Induced Conversion.

The 3d-transition-metal (hydro)oxides belong to a group of highly efficient, scalable and inexpensive electrocatalysts for widespread energy-related applications that feature easily tailorable crystal and electronic structures. We propose a general strategy to further boost their electrocatalytic activities by introducing organic ligands into the framework, considering that most 3d-metal (hydro)oxides usually exhibit quite strong binding with reaction intermediates and thus compromised activity due to the scaling relations. Involving weakly bonded ligands downshifts the d-band center, which narrows the band gap, and optimizes the adsorption of these intermediates. For example, the activity of the oxygen evolution reaction (OER) can be greatly promoted by ≈5.7 times over a NiCo layered double hydroxide (LDH) after a terephthalic acid (TPA)-induced conversion process, arising from the reduced energy barrier of the deprotonation of OH* to O*. Impressively, the proposed ligand-induced conversion strategy is applicable to a series of 3d-block metal (hydro)oxides, including NiFe2 O4 , NiCo2 O4 , and NiZn LDH, providing a general structural upgrading scheme for existing high-performance electrocatalytic systems.

Enabling Superior Sodium Capture for Efficient Water Desalination by a Tubular Polyaniline Decorated with Prussian Blue Nanocrystals.

The application of electrochemical energy storage materials to capacitive deionization (CDI), a low-cost and energy-efficient technology for brackish water desalination, has recently been proven effective in solving problems of traditional CDI electrodes, i.e., low desalination capacity and incompatibility in high salinity water. However, Faradaic electrode materials suffer from slow salt removal rate and short lifetime, which restrict their practical usage. Herein, a simple strategy is demonstrated for a novel tubular-structured electrode, i.e., polyaniline (PANI)-tube-decorated with Prussian blue (PB) nanocrystals (PB/PANI composite). This composite successfully combines characteristics of two traditional Faradaic materials, and achieves high performance for CDI. Benefiting from unique structure and rationally designed composition, the obtained PB/PANI exhibits superior performance with a large desalination capacity (133.3 mg g-1 at 100 mA g-1 ), and ultrahigh salt-removal rate (0.49 mg g-1 s-1 at 2 A g-1 ). The synergistic effect, interfacial enhancement, and desalination mechanism of PB/PANI are also revealed through in situ characterization and theoretical calculations. Particularly, a concept for recovery of the energy applied to CDI process is demonstrated. This work provides a facile strategy for design of PB-based composites, which motivates the development of advanced materials toward high-performance CDI applications.

Correlating charge and thermoelectric transport to paracrystallinity in conducting polymers.

The conceptual understanding of charge transport in conducting polymers is still ambiguous due to a wide range of paracrystallinity (disorder). Here, we advance this understanding by presenting the relationship between transport, electronic density of states and scattering parameter in conducting polymers. We show that the tail of the density of states possesses a Gaussian form confirmed by two-dimensional tight-binding model supported by Density Functional Theory and Molecular Dynamics simulations. Furthermore, by using the Boltzmann Transport Equation, we find that transport can be understood by the scattering parameter and the effective density of states. Our model aligns well with the experimental transport properties of a variety of conducting polymers; the scattering parameter affects electrical conductivity, carrier mobility, and Seebeck coefficient, while the effective density of states only affects the electrical conductivity. We hope our results advance the fundamental understanding of charge transport in conducting polymers to further enhance their performance in electronic applications.

Improved Alignment of PEDOT:PSS Induced by in-situ Crystallization of "Green" Dimethylsulfone Molecules to Enhance the Polymer Thermoelectric Performance.

Dimethylsulfone (DMSO2), a small organic molecule, was observed to induce the alignment of poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) via in-situ crystallization in PEDOT:PSS mixture, which was verified by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and atomic force microscopy (AFM). A chemically stable dopant, DMSO2, remarkably raised the electrical conductivity of the PEDOT:PSS film, which was fabricated from pre-mixed solution of PEDOT:PSS and DMSO2, up to 1080 S/cm, and more importantly, such a PEDOT:PSS film showed a long-term humidity stability and it retained near 90% electric conductivity after 60 days, suggesting DMSO2 is promising for an eco-friendly alternative to replace dimethyl sulfoxide (DMSO), ethylene glycol (EG) and various acids dopants that have been widely employed to dope and post-treat PEDOT:PSS. Pairwise interaction energies and free energy of solvation between PEDOT:PSS and DMSO2 were calculated by first-principles and molecular mechanics, respectively, revealing the mechanism of DMSO2 in enhancing the electrical conductivity.

Unprecedented Enhancement of Thermoelectric Power Factor Induced by Pressure in Small-Molecule Organic Semiconductors.

Establishing the relationship between pressure and heat-electricity interconversion in van der Waals bonded small-molecule organic semiconductors is critical not only in designing flexible thermoelectric materials, but also in developing organic electronics. Here, based on first-principles calculations and using naphthalene as a case study, an unprecedented elevation of p-type thermoelectric power factor induced by pressure is demonstrated; and the power factor increases by 267% from 159.5 µW m-1 K-2 under ambient conditions to 585.8 µW m-1 K-2 at 2.1 GPa. The underlying mechanism is attributed to the dramatic inhibition of lattice-vibration-caused electronic scattering. Furthermore, it is revealed that both restraining low-frequency intermolecular vibrational modes and increasing intermolecular electronic coupling are two essential factors that effectively suppress the electron-phonon scattering. From the standpoint of molecular design, these two conditions can be achieved by extending the π-conjugated backbones, introducing long alkyl sidechains to the π-cores, and substituting heteroatoms in the π-cores.

Polymer morphology and interfacial charge transfer dominate over energy-dependent scattering in organic-inorganic thermoelectrics.

Hybrid (organic-inorganic) materials have emerged as a promising class of thermoelectric materials, achieving power factors (S2σ) exceeding those of either constituent. The mechanism of this enhancement is still under debate, and pinpointing the underlying physics has proven difficult. In this work, we combine transport measurements with theoretical simulations and first principles calculations on a prototypical PEDOT:PSS-Te(Cux) nanowire hybrid material system to understand the effect of templating and charge redistribution on the thermoelectric performance. Further, we apply the recently developed Kang-Snyder charge transport model to show that scattering of holes in the hybrid system, defined by the energy-dependent scattering parameter, remains the same as in the host polymer matrix; performance is instead dictated by polymer morphology manifested in an energy-independent transport coefficient. We build upon this language to explain thermoelectric behavior in a variety of PEDOT and P3HT based hybrids acting as a guide for future work in multiphase materials.

Optimizing special quasirandom structure (SQS) models for accurate functional property prediction in disordered 2D alloys.

2D materials such as MXenes have garnered attention in a wide field of applications ranging from energy to environment to medical. Properties of 2D materials can be tailored via alloying and in some cases, solid-solutions (disordered alloys) are formed. To predict the disordered alloy properties via first-principles, the model structure needs to imitate the random arrangements of alloyants and yet remains computationally tractable. Using density functional theory and the cluster expansion method, we investigate the accuracy of using of special quasirandom structures (SQSs) for predicting disordered 2D alloy properties, evaluating the effect of SQS supercell size on the prediction quality of formation energies, elastic properties, and structural parameters. We illustrate the findings with 5 different disordered binary [Formula: see text] MXene alloy systems (where M = Ti and M' = Zr, Hf, V, Nb, or Ta), demonstrating that SQSs around 6-8 times the primitive cell (N = 6-8) are sufficient to attain convergence in the property predictions versus supercell size. For formation energies, SQSs with N > 4 are found to reproduce the formation energies of the fully disordered phase within ~2.5 meV. For the simulation of the experimentally-synthesized TiNbCO2, we find convergence in structural parameters and elastic tensors at N ~ 6. We traced the convergence of the predictions to the convergence in the band structure-related properties via analysis of the electronic densities-of-states and the projected crystal overlap Hamilton population. Our findings suggest that modest sized SQSs would reproduce the properties of disordered MXene alloys. The results should help guide the investigations of structure-property relationships in other disordered 2D materials as well.

Poly(nickel-ethylenetetrathiolate) and Its Analogs: Theoretical Prediction of High-Performance Doping-Free Thermoelectric Polymers.

It is generally deemed that doping is a must for polymeric materials to achieve their high thermoelectric performance. We herein present the first report that intrinsically metallic behaviors and high-performance thermoelectric power factors can coexist within doping-free linear-backbone conducting polymers, poly(nickel-ethylenetetrathiolate) and its analogs. On the basis of density functional calculations, we have corroborated that four crystalline π- d conjugated transition-metal coordination polymers, including poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4) exhibit intrinsically metallic behavior arising from the formation of dense intermolecular interaction networks between sulfur/selenium atoms. They show moderate carrier concentrations (1019-1021 cm-3) and decent conductivities (103-104 S cm-1), among which, poly(Ni-C2S4), poly(Ni-C2Se4) and poly(Pd-C2S4) possess high power factors (∼103 µW m-1 K-2).