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.

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.

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.

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.

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).

Theoretical investigation on carbon nucleation on nickel carbides at initial stages of single-walled carbon nanotube formation.

It is a long-standing controversy whether metal carbide clusters do exist during the nucleation and growth process of single-walled carbon nanotubes (SWCNTs). In the current work, we are trying to elucidate the carbon nucleation on nickel carbides during the initial stages based on density functional theory calculated formation energy and chemical potential for a series of Ni55Cn carbides (n is the number of carbon atoms dissolved in the Ni55 cluster). It is found that the formation energies of the Ni55Cn carbides decrease gradually with an increase of dissolved carbon atomic numbers, meaning the Ni55Cn carbides are thermodynamically stable. Meanwhile, the calculated chemical potentials indicate that not only nickel carbides are preferentially formed during the initial stage of the SWCNT nucleation, but also saturated nickel carbides may be able to exist during the nucleation and growth process of SWCNTs. In addition, the nickel carbides have a high selectivity for the formation of the carbon pentagon and carbon structures with pentagon-incorporated end-edge according to the adsorption energies. All of these findings provide opportunities in controlling the growth of the SWCNTs.

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.

Origin of long-range ferromagnetic ordering in metal-organic frameworks with antiferromagnetic dimeric-Cu(II) building units.

Even though metal-organic frameworks (MOFs) derived from antiferromagnetic dimeric-Cu(II) building units and nonmagnetic molecular linkers are known to exhibit unexpected ferromagnetic behavior, a comprehensive understanding of the underlying mechanism remains elusive. Using a combined theoretical and experimental approach, here we reveal the origin of the long-range ferromagnetic coupling in a series of MOFs, constructed from antiferromagnetic dimeric-Cu(II) building blocks. Our studies show that the strong localization of copper vacancy states favors spontaneous spin polarization and formation of local moment. These copper vacancy-induced moments are coupled via the itinerant electrons in the conjugated aromatic linkers to establish a long-range ferromagnetic ordering. The proposed mechanism is supported by direct experimental evidence of copper vacancies and the magnetic hysteresis (M-H) loops.

Stress induced half-metallicity in surface defected germanium nanowires.

Germanium nanowires (GeNWs) with single, double, quadruple and octuple surface dangling bonds (SDBs) are investigated using density-functional-theory calculations. We show that single SDB defected GeNWs remain semiconducting as their non-defected form while double or multiple SDB defects result in either semiconducting or metallic GeNWs, depending on the defect's locations on the surface. More importantly, we show that the electronic properties of surface defected GeNWs can also be fine-tuned by applying tensile and compressive strains. Upon the right loading, the surface defected GeNWs become half-metallic. In addition, we determine that the surface defected GeNWs can be classified into three classes: (1) GeNWs with zero magnetic moment, which are either metallic or semiconducting; (2) GeNWs with net magnetic moments equal to the number of SDBs, which are semiconducting with distinct spin-up and spin-down configurations; and (3) GeNWs with net magnetic moments significantly lower than the number of SDBs. We also find that only the defected GeNWs that fall under (3) are potentially half-metallic. Our results predict that half-metallic GeNWs can be obtained via engineering of the surface defects and the structures without the presence of impurity dopants.

Reactive sites for chiral selective growth of single-walled carbon nanotubes: a DFT study of Ni55-C(n) complexes.

The physical and electronic properties of single-walled carbon nanotubes (SWCNTs) are determined by their chirality. The chirality selection mechanism in SWCNT growth is not fully understood. In this study, the interaction between near-armchair (n,5), where n = 6, 7, 8, and 9, zigzag (9,0), and armchair (5,5) nanotubes and a fully relaxed Ni(55) metal cluster during the early stage of growth is studied by density functional theory calculations. We found that kink sites at the end edge of (n,5) nanotubes are more reactive than other sites based on the charge transfer analysis at the Ni-C interface. The frontier orbitals of the (6,5) and (7,5) caps are localized on their kink-step sites, which stretch outward from the carbon cap surface, having typical 2p(z) orbital feature of carbon atom with high reactivity. Such favorable frontier orbital spatial orientation and location is ideal to incorporate more carbon species. These reactive sites may lead to the faster growth rate, resulting in the chirality selectivity toward the (6,5) and (7,5) nanotubes. In contrast, the frontier orbitals of (8,5) and (9,5) caps spread over the entire carbon cap surface. Adding carbon species at these sites may lead to the chirality change or formation of other carbon structures. Our results showed that the spatial distribution and orientation of frontier orbitals is useful in explaining the chiral selectivity. Engineering catalyst clusters to control these reactive sites has high potential to further improve chirality control in SWCNT synthesis.

Structures and magnetism of multinuclear vanadium-pentacene sandwich clusters and their 1D molecular wires.

Two types of multinuclear sandwich clusters, (V(3))(n)Pen(n+1), (V(4))(n)Pen(n+1) (Pen = Pentacene; n = 1, 2), and their corresponding infinite one-dimensional (1D) molecular wires ([V(3)Pen](∞), [V(4)Pen](∞)) are investigated theoretically, especially on their magnetic coupling mechanism. These sandwich clusters and molecular wires are found to be of high stability and exhibit intriguing magnetic properties. The intra-layered V atoms in (V(3))(n)Pen(n+1) clusters prefer antiferromagnetic (AFM) coupling, while they can be either ferromagnetic (FM) or AFM coupling in (V(4))(n)Pen(n+1) depending on the intra-layered V-V distances via direct exchange or superexchange mechanism. The inter-layered V atoms favor FM coupling in (V(3))(2)Pen(3), whereas they are AFM coupled in (V(4))(2)Pen(3). Such magnetic behaviors are the consequence of the competition between direct exchange and superexchange interactions among inter-layered V atoms. In contrast, the 1D molecular wires, [V(3)Pen](∞) and [V(4)Pen](∞), appear to be FM metallic with ultra high magnetic moments of 6.8 and 4.0 µ(B) per unit cell respectively, suggesting that they can be served as good candidates for molecular magnets.

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.

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.

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.

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.

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.

Water induced electrical hysteresis in germanium nanowires: a theoretical study.

We apply DFT calculations to evaluate the electronic properties of germanium nanowires (GeNWs) upon adsorption of water molecules and reveal the possible causes of the experimentally observed electrical hysteresis in GeNWs based electronic devices. We show that the absorption of water molecules on the GeNW surface would lead to the formation of hydroxyl passivated GeNWs (OH-GeNWs). The first step of the formation mechanism is physisorption of water molecules toward a Ge atom then followed by dissociation of water molecules to form OH-GeNWs, consistent with experimental observation of reversible and irreversible electrical hystereses. More importantly, we also predict that the effective masses of OH-GeNWs depend strongly on their growth orientation and depend nonlinearly on the OH coverage percentage. We propose that the electrical hysteresis phenomenon observed in experiment can be attributed to the formation of OH-GeNWs with different OH coverage percentages, along with different alignments of the OH groups on the GeNW surface, and also the presence of surface trap state defects, during the different stages of I-V measurement.

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.

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.