Self-powered,high response and fast response speed metal–insulator–semiconductor structured photodetector based on 2D MoS2

Here, we firstly fabricated a metal–insulator–semiconductor (MIS) (Pd/Al₂O₃/MoS₂) self-powered photodetector based on MoS₂, which is sensitive to the illumination of light without any external bias, exhibiting a high responsivity of 308 mA W⁻¹. Under bias, it shows a ratio of photocurrent to dark current exceeding 3705, a high photoresponsivity of 5.04 A W⁻¹, and a fast response/recovery time of 468 ms/543 ms. The optoelectronic performances of the photodetector are closely related to the insulating layer, which can suppress the dark current of the photodetectors, and prevent strong current drifting and degradation by environmental effects, playing a key role in carrier tunneling. Furthermore, we used a thin HfO₂ film as the insulating layer to improve the optoelectronics performance of the MIS structured self-powered photodetector, which presented a high responsivity of 538 mA W⁻¹ at 0 bias. With an applied bias, it exhibits an on/off ratio up to 6653, a photoresponsivity of 25.46 A W⁻¹, and a response/recovery time of 7.53 ms/159 ms. Our results lead to a new way for future application of high performance MIS structured photodetectors based on 2D MoS₂.

A MIS structured self-powered photodetector of Pd/HfO₂/MoS₂ was fabricated by inserting a thin insulator, which has a fast response/recovery speed.     

Unusual semiconductor–metal–semiconductor transitions in magnetite Fe3O4 nanoparticles

Magnetite (Fe₃O₄) nanoparticles were successfully prepared by a co-precipitation method. Rietveld refinement on the X-ray diffraction pattern confirmed the development of a single-phase cubic spinel structure with space group Fd$\bar{3}$m. However, ⁵⁷Fe Mössbauer spectroscopy suggested the presence of Fe³⁺ and Fe^2.5+ (mixed Fe³⁺ and Fe²⁺) ions at the tetrahedral and octahedral sites of the inverse spinel structure, respectively. Impedance spectroscopy measurements showed a discontinues variation in the temperature dependence of the sample''s resistive behavior, indicating the appearance of semiconductor–metal–semiconductor like transitions between the temperature range of 293 and 373 K. A similar dual transition was also observed from the dielectric and conductivity measurements around the same temperature regions. The observed unusual transition is explained in term of the competitive effects among the hopping of localized/delocalized and short-range/long-range charge carriers present in the sample. Moreover, the prepared sample exhibits colossal dielectric permittivity (∼10⁶), reduced tangent loss (∼0.2) and moderate conductivity (>10⁻⁶ S cm⁻¹) values, making Fe₃O₄ nanoparticles a potential candidate for electromagnetic absorbing materials.

Herein, we report the existence of a novel semiconductor–metal–semiconductor type transition in Fe₃O₄ nanoparticles by employing impedance spectroscopy techniques.     

The spin–orbit–phonon coupling and crystalline elasticity of LaCoO3 perovskite

Based on an integrated study of magnetic susceptibility, specific heat, and thermal expansion of single-crystal LaCoO₃ free from cobalt and oxygen vacancies, two narrow spin gaps are identified before and after the phonon softening of gap size ΔE ∼ 0.5 meV in a CoO₆-octahedral crystal electric field (CEF) and the thermally activated spin gap Q ∼ 25 meV, respectively. Significant excitation of Co³⁺ spins from a low-spin (LS) to a high-spin (HS) state is confirmed by the thermal activation behavior of spin susceptibility χ^S of energy gap Q ∼ 25 meV, which follows a two-level Boltzmann distribution to saturate at a level of 50% LS/50% HS statistically above ∼200 K, without the inclusion of a postulated intermediate spin (IS) state. A threefold increase in the thermal expansion; coefficient (α) across the same temperature range as that of thermally activated HS population growth is identified, which implies the non-trivial spin–orbit–phonon coupling caused the bond length of Co^3+(LS↔HS)–O fluctuation and the local lattice distortion. The unusually narrow gap of ΔE ∼ 0.5 meV for the CoO₆ octahedral CEF between e_g–t_2g indicates a more isotropic negative charge distribution within the octahedral CEF environment, which is verified by the Electron Energy Loss Spectroscopy (EELS) study to show nontrivial La–O covalency.

Considering the before and after phonon softening, the gap in a CoO₆-octahedral crystal electric fields (CEF) and the thermally activated spin gap, were observed of ∼0.5 meV and Q ∼ 25 meV in defect-free LaCoO₃ single crystal, respectively.     

Free and self-trapped exciton emission in perovskite CsPbBr3 microcrystals

The all-inorganic perovskite CsPbBr₃ has been capturing extensive attention due to its high quantum yield in luminescence devices and relatively high stability. Its luminescence is dominated by free exciton (FE) recombination but additional emission peaks were also commonly observed. In this work, a CsPbBr₃ microcrystal sample in the orthorhombic phase was prepared by the chemical vapor deposition method. In addition to the FE peak, a broad emission peak was found in this sample and it was attributed to self-trapped excitons (STEs) based on its photophysical properties. The STE emission can only be observed below 70 K. The derived Huang–Rhys factor is ∼12 and the corresponding phonon energy is 15.3 meV. Its lifetime is 123 ns at 10 K, much longer than that of FE emission. The STE emission is thought to be an intrinsic property of CsPbBr₃.

A broad STE emission band together with a FE emission was found at low temperature in a CsPbBr₃ microcrystal sample prepared by CVD method.     

Coupled magnetic–elastic and metal–insulator transition in epitaxially strained SrMnO3/BaMnO3 superlattices

The spin–phonon coupling and the effects of strain on the ground-state phases of artificial SrMnO₃/BaMnO₃ superlattices were systematically investigated using first-principles calculations. The results confirm that this system has antiferromagnetic order and an intrinsic ferroelectric polarisation with the P4mm space group. A tensile epitaxial strain can drive the ground state to another antiferromagnetic–ferroelectric phase and then to a ferromagnetic–ferroelectric phase with the Amm2 space group, accompanied by a change in the ferroelectric polarisation from an out-of-plane direction to an in-plane direction. In contrast, a compressive strain could induce a transition from the antiferromagnetic insulator phase to the ferromagnetic metal phase.

The epitaxial strain can induce interesting physical phase transitions in SrMnO₃/BaMnO₃ superlattices.     

Synthesis,structure, and fluorescence properties of a calcium-based metal–organic framework

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O. This MOF possesses a new structure composed of calcium clusters and H₂NDC linker anions and shows a unique fluorescence property; it exhibits a fluorescence peak at 395 nm (λ_ex = 350 nm) at room temperature, which is blue-shifted compared with that exhibited by the free H₂NDC ligand. One of the possible mechanisms for this fluorescence is likely attributable to a ligand-to-metal charge transfer (LMCT) transition and is the first example of a calcium-based MOF exhibiting blue-shifted fluorescence due to LMCT.

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O.     

Double-exponential refractive index sensitivity of metal–semiconductor core–shell nanoparticles: the effects of dual-plasmon resonances and red-shift

In order to improve the refractive index sensitivity of a localized surface plasmon resonance (LSPR) sensor, we present a new type of LSPR sensor whose refractive index sensitivity can be improved by greatly increasing the plasmon wavelength red-shift of metal–semiconductor core–shell nanoparticles (CSNs). Using extended Mie theory and Au@Cu_2−xS CSNs, we theoretically investigate the optical properties of metal–semiconductor CSNs in the entire near-infrared band. Compared with dielectric–metal and metal–metal CSNs under the same conditions, the metal–semiconductor CSNs have a higher double-exponential sensitivity curve because their core and shell respectively support two LSPRs that greatly increase the LSPR red-shift to the entire near-infrared range. It is worth noting that the sensitivity can be improved effectively by increasing the ratio of the shell-thickness to core-radius, instead of decreasing it in the case of the dielectric–metal CSNs. The underlying reason for the enhancement of sensitivity is the increase of repulsive force with the enlargement of shell thickness, which is different from the dielectric–metal CSNs. This design method not only paves the way for utilizing metal–semiconductor CSNs in biology and chemistry, but also proposes new ideas for the design of sensors with high sensitivity.

We present a new type of localized surface plasmon resonance (LSPR) sensor whose refractive index sensitivity can be improved by greatly increasing the plasmon wavelength red-shift of metal–semiconductor core–shell nanoparticles (CSNs).     

Tunnable rectifying performance of in-plane metal–semiconductor junctions based on passivated zigzag phosphorene nanoribbons

Using first principles density functional theory, we perform a systematic study of the band structures of passivated zigzag phosphorene nanoribbons (ZPNRs) and the transport properties of in-plane metal–semiconductor junctions. It is found that the ZPNR passivated by H, Cl or F atoms is a semiconductor, and the ZPNR passivated by C, O or S atoms is a metal. Therefore, ZPNRs with different passivated atoms can be fabricated into an in-plane metal–semiconductor junction. The calculated current–voltage characteristics indicate that these in-plane metal–semiconductor junctions can exhibit excellent rectification behavior. More importantly, we find that the type of passivated atom plays a very important role in the rectification ratio of this in-plane metal–semiconductor junction. The findings are very useful for the further design of functional nanodevices based on ZPNRs.

Using first principles density functional theory, we perform a systematic study of the band structures of passivated zigzag phosphorene nanoribbons (ZPNRs) and the transport properties of in-plane metal–semiconductor junctions.     

Energy-adaptive resistive switching with controllable thresholds in insulator–metal transition

Resistive switching has provided a significant avenue for electronic neural networks and neuromorphic systems. Inspired by the active regulation of neurotransmitter secretion, realizing electronic elements with self-adaptive characteristics is vital for matching Joule heating or sophisticated thermal environments in energy-efficient integrated circuits. Here we present energy-adaptive resistive switching via a controllable insulator–metal transition. Memory-related switching is designed and implemented by manipulating conductance transitions in vanadium dioxide. The switching power decreases dynamically by about 58% during the heating process. Furthermore, the thresholds can be controlled by adjusting the insulator–metal transition processes in such nanowire-based resistive switching, and then preformed in a wide range of operating temperatures. We believe that such power-adaptive switching is of benefit for intelligent memory devices and neuromorphic electronics with low energy consumption.

Adaptive energy-scaling resistive switching with active response and self-regulation via controllable insulator–metal transition shows promise in energy-efficient devices.     

A review on development of metal–organic framework-derived bifunctional electrocatalysts for oxygen electrodes in metal–air batteries

Worldwide demand for oil, coal, and natural gas has increased recently because of odd weather patterns and economies recovering from the pandemic. By using these fuels at an astonishing rate, their reserves are running low with each passing decade. Increased reliance on these sources is contributing significantly to both global warming and power shortage problems. It is vital to highlight and focus on using renewable energy sources for power production and storage. This review aims to discuss one of the cutting-edge technologies, metal–air batteries, which are currently being researched for energy storage applications. A battery that employs an external cathode of ambient air and an anode constructed of pure metal in which an electrolyte can be aqueous or aprotic electrolyte is termed as a metal–air battery (MAB). Due to their reportedly higher energy density, MABs are frequently hailed as the electrochemical energy storage of the future for applications like grid storage or electric car energy storage. The demand of the upcoming energy storage technologies can be satisfied by these MABs. The usage of metal–organic frameworks (MOFs) in metal–air batteries as a bi-functional electrocatalyst has been widely studied in the last decade. Metal ions or arrays bound to organic ligands to create one, two, or three-dimensional structures make up the family of molecules known as MOFs. They are a subclass of coordination polymers; metal nodes and organic linkers form different classes of these porous materials. Because of their modular design, they offer excellent synthetic tunability, enabling precise chemical and structural control that is highly desirable in electrode materials of MABs.

This review paper is based on importance of metal–organic framework-derived bifunctional electrocatalysts for oxygen electrodes in metal–air batteries (MABs), related to the Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER).     

Confinement of a Au–N-heterocyclic carbene in a Pd6L12 metal–organic cage

A Au(i)–N-heterocyclic-carbene (NHC)-edged Pd₆L₁₂ molecular metal–organic cage is assembled from a Au(i)–NHC-based bipyridyl bent ligand and Pd²⁺. The octahedral cage structure is unambiguously established by NMR, electrospray ionization-mass spectrometry and single crystal X-ray crystallography. The electrochemical behaviour was analyzed by cyclic voltammetry. The octahedral cage has a central cavity for guest binding, and is capable of encapsulating PF₆⁻ and BF₄⁻ anions within the cavity.

A Au(i)–NHC-edged Pd₆L₁₂ molecular cage is assembled from a Au(i)–NHC-based bipyridyl bent ligand and Pd²⁺.     

Exciton–phonon coupling in two-dimensional layered (BA)2PbI4 perovskite microplates

Two-dimensional layered (BA)₂PbI₄ (BA = C₄H₉NH₃) perovskites are emerging as a new class of layered materials and show great potential in optoelectronic applications. Elucidating how exciton–phonon interaction affects the excitonic emission is of great importance for a better knowledge of their optoelectronic properties. In this letter, we synthesized high-quality (BA)₂PbI₄ microplates via solution methods, and dual-excitonic emission peaks (surface-emission and interior-emission) were detected from the as-grown samples at low temperatures. Furthermore, we determine the energies for the longitudinal optical phonon modes to be ∼27 and ∼18 meV, and the exciton–phonon coupling strengths to be ∼177 and ∼21 meV for the surface-emission and interior-emission bands, respectively. Compared to the interior-emission band, the stronger exciton–phonon interaction results in a considerable degree of spectral broadening and red-shift for the surface-emission with increasing temperature. In contrast, the (OA)₂PbI₄ (OA = C₈H₁₇NH₂) microplates with longer alkyl chains between Pb–I layers, exhibit only one excitonic emission peak, as well as a large exciton–phonon coupling strength. Our work clarifies the influence of exciton–phonon coupling on the excitonic emission of (BA)₂PbI₄ microplates, and also suggests the intrinsic relationship between the exciton–phonon coupling and the length of organic carbon chain ligands.

The effect of exciton–phonon coupling on the excitonic emission of two-dimensional layered (BA)₂PbI₄ (BA = C₄H₉NH₃) perovskites.     

The pioneering role of metal–organic framework-5 in ever-growing contemporary applications – a review

MOF-5 with a Zn(ii) cluster and terephthalic acid is a distinctive porous material among the metal–organic frameworks (MOFs), with unique physical, chemical and mechanical properties. MOF-5 based composites possess ample applications in modern chemistry. Huge surface area, suitable pore dimensions and scope of tunability make MOF-5 noteworthy in advanced materials. The extensive features of MOF-5 provided an opportunity for researchers to explore atomic/molecular scale materials. Various MOF-5 based composites have been designed with revamped properties appropriate to the application by altering and fabricating MOF-5 in situ or using a post-synthetic approach. Surface modification via the dispersion and impregnation of active substances into the pores of MOF-5 enhances its applicability. The boundless topologies and morphologies of MOF-5 combined with other chemical entities has provided opportunities in various fields, including catalysis, gas storage and sensors. The present review illuminates the leading role of MOF-5 and its composites in contemporary applications based on the current literature in heterogeneous catalysis, H₂ and CO₂ storage and sensors.

MOF-5 with a Zn(ii) cluster and terephthalic acid is a distinctive porous material among the metal–organic frameworks (MOFs), with unique physical, chemical and mechanical properties.     

Surface-tension-confined assembly of a metal–organic framework in femtoliter droplet arrays

Metal–organic frameworks (MOFs), produced by metal ions coordinated to organic linkers, have attracted increasing attention in recent years. For the utilization in MOFs in numerous applications, achieving positioned MOF growth on surfaces is essential to fabricate multiple-functional devices. We develop a novel miniaturized method to realize surface-tension-confined assembly of HKUST-1 in femtoliter droplet arrays. HKUST-1 crystal arrays grown by evaporation-induced crystallization are observed, and five typical crystal morphologies (i.e., hexagonal, irregular hexagonal, triangular, arc-like and ribbon-like crystals) are found in the large area on the patterned substrate during crystallization. Our research provides a better understanding of the formation mechanism of MOF crystals in confined sessile droplets. The key factors determining HKUST-1 single-crystal growth are the internal flows in an evaporating droplet and consequently aggregation induced by the combination of metallic Cu(ii) and BTC ions. Understanding the formation of different morphologies of HKUST-1 crystals is useful to guide the production of crystals with desired shapes for various applications.

The key factors determining HKUST-1 single-crystal growth are the internal flows in an evaporating droplet and consequently aggregation induced by the combination of metallic Cu(ii) and BTC ions.     

Tuning the structure and properties of a multiferroic metal–organic-framework via growing under high magnetic fields

High magnetic field-induced synthesis has been demonstrated to tune the structure and properties of the multiferroic metal–organic framework [(CH₃)₂NH₂][Mn(HCOO)₃]. The crystals obtained under 90 kOe exhibit a lower ferroelectric polarization value and reduced magnetic susceptibility, compared to crystals obtained without a field, which is attributed to structural changes induced by the high magnetic field.

High magnetic field-induced synthesis has been demonstrated to tune the structure and properties of the multiferroic metal–organic framework [(CH₃)₂NH₂][Mn(HCOO)₃].

In 2009, Jain et al. reported multiferroic behavior in a family of perovskite metal–organic frameworks (MOFs), which stimulated considerable experimental and theoretical efforts to search for new multiferroic materials among perovskite MOFs.¹ In particular, MOFs with the ABX₃ perovskite-like structure are of great interest because the variable A and B components provide plenty of room for adjusting the physical and chemical properties in a simple crystalline structure.^2–25 One such example is formate-based perovskites, [A–M–(HCOO)₃]. They contain a divalent transition metal ion (M²⁺) which occupies the center of a BO₆ octahedron, linked by a formate (HCOO⁻) bridging ligand to form a cavity which is occupied by an organic cation. The ordering of hydrogen bonding (N–H⋯O) of the alkylammonium cation triggers the ferroelectic ordering at low temperature, while the weak ferromagnetism arises from metal ions.It is well known that volatilization and hydrothermal/solvothermal synthesis are the main methods to grow formate-based perovskites, which involve nucleation and growth processes that are affected by physical and chemical factors. Modifying or controlling the physical environment is a simple means to affect the processes of crystallization in order to fine-tune the crystal structure and properties. On the other hand, the effect of pressure on the structure modification of multiferroic MOFs has recently been demonstrated, which showed that ferroelectric polarization could be enhanced by more than three times by applying a compressive strain to the multiferroic frameworks.¹⁴ Also, density functional theory calculations indicated that changing the magnitude or the canting of the organic molecular dipole can tune the ferroelectric polarization in the new class of multiferroic metal–organic frameworks.^4,15 These experimental results further highlight the potential of these flexible MOF perovskites to undergo large structural changes in response to an external stimulus.Crystal growth under a magnetic field is an interesting research topic because a magnetic field may provide a special environment. From the latter half of the 1990s, many studies of magnetic field effects (MFEs) on protein crystallization have been carried out.^26–28 It has been found that magnetic fields can induce molecular ordering in most organic polymer and biological macromolecules due to the magnetic susceptibility anisotropy of the individual C–C, C–O, C–H, and O–H bonds and their relative orientation in the crystal, or the formation of an interlinked network of biological macromolecules under a high magnetic field.^29–31 Also, the MFEs on the growth of materials have been extensively studied by our group, which found that both the structure and the properties of materials can be regulated by magnetic fields.^32–38 For example, it is believed that during Co₃O₄ growth, the alignment of the spins and thus the magnetic and crystal lattices of Co₃O₄ are influenced by the external magnetic field.³⁵ In this paper, we extend the study to grow single crystals of formate-based perovskites [(CH₃)₂NH₂][Mn(HCOO)₃] (DMMnF) under a high magnetic field. DMMnF is the first prototypical multiferroic formate-based perovskite to be discovered¹ which exhibits an order-disorder ferroelectric transition below 185 K. It is also a weakly canted antiferromagnet (T_c = 8.5 K) with a 0.08° canting angle. Moreover, DMMnF is suggested to be thermodynamically more stable than other formate-based perovskites and is easily obtained by a solvothermal process. The main purpose of this study is to show the possibility that the structure and properties of multiferroic MOFs could be tuned by synthesis under high magnetic field, and to try to understand the mechanism. In this work, the growth of DMMnF crystals was carried out with and without an applied magnetic field. The resulting crystals were labeled as AF-DMMnF (applied field) and ZF-DMMnF (zero field), with AF-DMMnF indicating the product obtained under a high magnetic field, and ZF-DMMnF indicating the case without a magnetic field.The AF-DMMnF and ZF-DMMnF micro-crystal powders (obtained from grinding the as-grown single crystals) and single crystals were characterized by X-ray diffraction (XRD). As shown in Fig. S2a,^† the clear sharp peaks of both samples are in good agreement with the theoretical patterns calculated from the single-crystal data.¹⁵ Moreover, the peaks of the powder samples could be attributed to the diffraction planes of the trigonal phase DMMnF at room temperature, confirming the phase purity of both samples (Fig. S2b^†).Dielectric constant measurements were carried out on single crystals of AF-DMMnF and ZF-DMMnF. The dielectric constants of both samples show a clear anomaly close to 185 K on cooling (Fig. 1a). The shape of the dielectric plot indicates that the samples are undergoing a paraelectric to ferroelectric phase transition, which is consistent with the literature.^1,16 It should be noted that the dielectric constant of AF-DMMnF is lower than that of ZF-DMMnF above 185 K. Fig. 1b shows the temperature dependence of the electric polarization. Below the phase transition temperature, the electric polarization values of AF-DMMnF are much lower than those of ZF-DMMnF, suggesting that the electric polarization of DMMnF in the ferroelectric state is reduced by growing under a high magnetic field. The highest experimental electric polarizations of AF-DMMnF and ZF-DMMnF within the measured temperature range are ∼0.3 μC cm⁻² and ∼1 μC cm⁻², respectively, as shown in Fig. 1b. It is well known that the ferroelectrics of DMMnF arise from the hydrogen bonds between the DMA⁺ cations and the formate framework. The reduction in electric polarization indicates that the hydrogen bond-related interactions may have changed. As is known, the Raman spectrum is sensitive to changes in chemical bonding. The Raman spectra of the samples at different temperatures are presented in Fig. 2, and the main vibration modes assigned to HCOO⁻ and DMA⁺ have been marked. All bands observed below 300 cm⁻¹ can be attributed to the lattice modes (Fig. 2a). It is worth noting that the intensities of ν₁(HCOO⁻), ν₂(HCOO⁻) and ν₃(HCOO⁻) are weakened in AF-DMMnF compared to those in ZF-DMMnF. In contrast, the intensity of ν_s(CNC) at 897 cm⁻¹ in AF-DMMnF is increased compared to that in ZF-DMMnF. The obvious weakening in the vibrational modes of the formate ions in AF-DMMnF could presumably be influenced by the strength of the N–H⋯O hydrogen bonds between the DMA⁺ cations and the formate framework.¹⁵ It is suggested that the band at 2788 cm⁻¹ can be unambiguously assigned to the stretching modes of the NH₂ group which directly involve hydrogen-bonds.¹² Most importantly, at 140 K, the intensity of ν(NH₂) in AF-DMMnF is much lower than that in ZF-DMMnF, as shown in Fig. 2d. This result, combined with the weakened vibration modes of HCOO⁻, indicates the reduced strength of the hydrogen-bonds in AF-DMMnF, which is responsible for the decrease in polarization values. Several claims have been made that magnetic fields change the physicochemical properties of water or prepared laboratory solutions, by influencing nucleation and growth, chemical equilibria, and so on.^39–41 An enhancement in the hydrogen-bonded strength under a magnetic field of 100 kOe was observed, which was caused by increased electron delocalization in the hydrogen-bonded molecules.^42,43 Our case can be considered as an example of MFEs on a hydrogen-bonded structure, although the mechanism has as yet not been adequately explained. Fig. S3^† is the optical image of the AF-DMMnF and ZF-DMMnF surfaces. The AF-DMMnF exhibits uniform growth steps, which may be attributed to slower nucleation. However, irregular growth layers are observed on the surfaces of the ZF-DMMnF because of rapid nucleation.¹⁵Open in a separate windowFig. 1(a) Dielectric constants of AF-DMMnF and ZF-DMMnF as a function of temperature at 1 kHz; (b) the electric polarization of AF-DMMnF and ZF-DMMnF as a function of temperature. Electric polarization was measured after cooling the sample at a voltage of 750V cm⁻¹.Open in a separate windowFig. 2Detail of the Raman spectra corresponding to the spectral ranges 25–300, 700–1200, 1300–1600 and 2650–3200 cm⁻¹ at different temperatures for the samples. Black lines: 140 K for ZF-DMMnF; red line: 170 K for ZF-DMMnF; blue line: 140 K for AF-DMMnF; pink line: 170 K for AF-DMMnF.Wang et al. have shown that DMMnF is a canted weak ferromagnet with a T_c value of 8.5 K.⁴⁴ It is suggested that the spin canting may originate from the noncentrosymmetric character of the three-atom formate bridge, CHOO⁻. Fig. 3a and b show the magnetization as a function of temperature measured at 200 Oe. Both samples show a clear magnetic phase transition at ca. 8.5 K. Furthermore, the isothermal magnetization M(T, H) at 1.8 K is shown in Fig. 3c. The magnetization increases almost linearly from 0 to 10 kOe. It is worth mentioning that the magnetic susceptibilities of AF-DMMnF are lower than those of ZF-DMMnF. In the low-field range at 1.8 K, a hysteresis loop can be observed with remnant magnetizations M_R = 0.0069 μB and 0.0045 μB for ZF-DMMnF and AF-DMMnF, respectively. The canting angle α is related to M_R and M_S through sin(α) = M_R/M_S (M_S = 5 μB for a spin-only Mn^II ion). In ZF-DMMnF, the canting angle α is estimated to be about 0.08°, which is consistent with the reported result.⁴⁴ However, the canting angle α in AF-DMMnF is reduced to 0.05° according to the above formula. In a canted antiferromagnetic material, neighboring spins do not align in a strictly parallel manner, but cant each other at a certain angle. During the growth process, the applied magnetic field may disturb this manner and change the angles between the directions of neighboring spins. The decrease in canting angle in AF-DMMnF will generate fewer uncompensated spins, leading to reduced magnetization. This result proves that growth under a magnetic field may be used to control the degree of spin-canting, and then influence the magnetic properties.Open in a separate windowFig. 3Temperature dependence of χ_M of AF-DMMnF (a) and ZF-DMMnF (b) measured at 200 Oe from 2 to 20 K (ZFC and FC). (c) Field-dependent isothermal magnetization M(T, H) for AF-DMMnF and ZF-DMMnF at 1.8 K from 0 to 10 kOe. (d) The hysteresis loop measured at 1.8 K.To further investigate the magnetic states in ZF-DMMnF and AF-DMMnF below T_c, a low temperature electron spin resonance (ESR) technique is employed. ESR embodies the internal environment (crystal field and internal magnetic field) of magnetic ions. Both samples possess a temperature-independent peak and another peak which moves to a low magnetic field when the temperature decreases (Fig. 4). The temperature-independent peak comes from the transition between the Zeeman energy levels of S = ±1/2, and the low-field peaks are derived from the transition between the Zeeman energy level of S = 1/2 and that of S = 3/2 (zero splitting energy, D < 0) or S = −1/2 and S = −3/2 (D > 0).⁴⁵ Interestingly, the separation distances between the two peaks of AF-DMMnF are smaller than those of ZF-DMMnF (Fig. 4), indicating the increased zero splitting energies in AF-DMMnF. Moreover, additional weak peaks (circled) appear in the ESR spectrum of ZF-DMMnF, which are probably caused by a small number of magnetic domains in different directions. In other words, the magnetic field makes the magnetic domains of AF-DMMnF more consistent, which will contribute to the dielectric constants and electric polarization values. However, the slight contribution of the domains cannot compensate for the decreases in dielectric constant and electric polarization values induced by the reduction in strength of the hydrogen-bonds. Therefore, AF-DMMnF exhibits a reduced dielectric constant and electric polarization. The increased zero splitting energies, combined with more consistent magnetic domains, further proves that the structure of DMMnF can be regulated by growth under high magnetic fields.Open in a separate windowFig. 4The ESR spectra of AF-DMMnF (a) and ZF-DMMnF (b) at selected temperatures.In summary, for the first time, it has been demonstrated that the structure and properties of a multiferroic MOF can be tuned by growth under a high magnetic field. The crystals obtained under a high magnetic field exhibit a much lower electric polarization value, which is attributed to the reduced strength of the hydrogen bonds. Moreover, a decrease in magnetic susceptibilities and remnant magnetization was observed due to the reduction in the spin canting angle. With the emergence of superconducting technology, a high magnetic field could be obtained easily, which would open a possible way to tailor the structures and properties of multiferroic MOFs by high magnetic field-induced synthesis.     

A photosensitive metal–organic framework having a flower-like structure for effective visible light-driven photodegradation of rhodamine B

Porphyrin-based metal–organic frameworks (MOFs) have great photocatalytic potential due to their good photosensitivity. Their photocatalytic performance is not only determined by molecular structure but also by morphology. Flower-like MOFs are considered to be good materials for catalysis due to their larger specific surface area, more exposed active sites, and good stability. Here, we first proposed a method to synthesize flower-like porphyrin-based MOFs using trifluoroacetic acid as a morphology control agent. These MOFs had a large BET surface area (605.04 m² g⁻¹), a stable structure and a complete morphology. Meanwhile, we discussed their self-assembly process and mechanism in detail. In addition, we studied the photocatalytic performance of flower-like porphyrin-based MOFs and found that the flower-like Cu-TCPP (TCPP = tetrakis(4-carboxyphenyl)porphyrin) has excellent photocatalytic activity. Its photodegradation efficiency toward the cationic dye rhodamine B reached 88% within 100 min and the sample still maintained its stable catalytic activity and complete flower-like morphological structure after five repeated uses. Furthermore, this synthetic strategy can be extended to control the morphology of other MOFs.

Using the method of adding morphological control agent-trifluoroacetic acid (TFA), a flower-like porphyrin-based metal organic framework can be prepared, which shows a good effect on the photocatalytic degradation of rhodamine B.     

Facile synthesis of a dual-phase CsPbBr3–CsPb2Br5 single crystal and its photoelectric performance

The emerging metal-halide perovskites are promising for next generation optoelectronic devices. Recently, all-inorganic halide perovskites have been developed and show significantly improved stability compared with organic–inorganic hybrid halide perovskites. Here, we report a facile method based on the coffee ring effect of solvents to synthesize dual-phase CsPbBr₃–CsPb₂Br₅ single crystal microsheets for the first time. The prepared dual-phase CsPbBr₃–CsPb₂Br₅ single crystal is composed of a tetragonal crystalline phase of CsPb₂Br₅ and a monoclinic phase of CsPbBr₃ according to X-ray diffraction (XRD) patterns. The sharp XRD peaks indicate the high crystallinity of the as-synthesized dual-phase CsPbBr₃–CsPb₂Br₅ microsheets. CsPbBr₃ is mainly distributed on the edge of the microsheets based on photoluminescence (PL) mapping images. Besides, a photodetector based on the dual-phase CsPbBr₃–CsPb₂Br₅ microsheets exhibits good performance with a high on/off photocurrent ratio of 300 and a photoresponsivity of 2.68 mA W⁻¹. The rise and decay times of the CsPbBr₃–CsPb₂Br₅ microsheet photodetector are around 25.3 ms and 29.6 ms, respectively. The experimental results indicate that the dual-phase CsPbBr₃–CsPb₂Br₅ microsheet could be a good candidate for the fabrication of high-performance micro photodetectors compatible with practical applications.

Dual-phase CsPbBr₃–CsPb₂Br₅ single crystal microsheets were synthesized by a simple method based on the coffee ring effect.     

A three-dimensional metal–organic framework for a guest-free ultra-low dielectric material

A three-dimensional metal–organic framework compound [NH₂(CH₃)₂]₂[Zn₃(bpdc)₄]·3DMF (1) shows two step dielectric relaxation and its guest-free framework (1′) possesses an ultra-low κ value of 1.80 (at 100 kHz, it is the lowest value for MOFs reported to date) over a wide temperature range and high thermal stability.

A MOFs compound [NH₂(CH₃)₂]₂[Zn₃(bpdc)₄]·3DMF (1) shows two step dielectric relaxation and its guest-free framework (1′) possesses an ultra-low κ value of 1.80 (at 100 kHz) over a wide temperature range and high thermal stability.

The design and synthesis of low dielectric constant (low-κ) materials has been a subject of interest in terms of their potential for use in high performance electronic devices. Materials with extremely low-dielectric constants have been targeted as interlayer dielectrics (ILD) because they decrease the cross-talk noise, propagation delay, and power dissipation in most electronic components. ^1–5 Indeed, the search for new low-κ materials replacing silicon dioxide (SiO₂) as an ILD has always been dictated by industrial needs, resulting in a strong connection between fundamental research and technology.⁶ Many materials have been proposed and studied as potential candidates; two major classes are dense organic polymers and porous inorganic-based materials. Some dense organic polymers could have κ below 2.2, but they suffer from concerns of low thermal stability and thermal conductivity. For porous inorganic-based low-κ materials, sol–gel silica, doped oxides and mesoporous silica have been extensively studied.⁷ but its low mechanical strength, wide pore size distribution, and hydrophilicity have been cited as concerns.As air or vacuums have the lowest dielectric constant (κ = 1.01), the partial replacement of solid network with air or a vacuum appears to be the more intuitive and direct option to the development of new low-κ ILD materials. Thus, as per the International Technology Roadmap for Semiconductors (ITRS), robust porous materials and air gap structures will become target low-κ materials in the near future.⁸ Metal–organic frameworks (MOFs) with a well-defined monodisperse porosity, large surface area, ultra-low densities, high stability and easy tunability of the surface and structural properties have potential for meet the demands for use as stable low-κ materials.⁹ MOFs have been extensively studied over the past decade for their applications in gas storage, sensors, chemical separation, catalysis, drug delivery and biomedical imaging.^10–12 However, their electrical properties and applications in microelectronics remain under researched.¹³ MOFs should be stiffer and harder than other low-density amorphous inorganic or organic polymers because of their ordered framework and rigid organic linkers. With tunable structural properties, high porosity, and thermal/mechanical stability, MOFs represent an ideal replacement as an ILD material. Hermann and coworkers presented a brief theoretical model for using MOFs as low-κ materials in microelectronics applications.¹⁴ However, these theoretical calculations did not take into account the orientational and ionic contributions to the molecular polarizability, which drastically contribute to the dielectric constant. These theoretical results encouraged us to search for new MOFs materials with experimentally ultra-low κ values.In this work, we report on the preparation of a MOFs, [NH₂(CH₃)₂]₂[Zn₃(bpdc)₄]·3DMF (1) (H₂bpdc = 4,4′-biphenyldicarboxylic acid), which have 3D frameworks with high thermal stability (Fig. S1^†) and a ultra-low κ values of its guest-free sample 1′. The 1′ possesses a very low κ values of 1.80 (at 100 kHz) and high thermal stability at temperatures up to 360 °C (Fig. S2^†), making it a potential candidate for use as an ILD. To the best of our knowledge, to date, the κ values of 1′ is the lowest value for MOFs reported. Furthermore, compound 1 shows dielectric relaxation and anomalies in the temperature range of 35–140 °C. Dielectric relaxation and anomalies of 1 is related to reorientation of the dipole moment of surface absorbed water and guest DMF molecules, respectively.The compound 1 was obtained from the solvothermal reaction of Zn(NO₃)₂·6H₂O, H₂bpdc and [NH₂(CH₃)₂]Cl in DMF. X-ray crystallographic analysis reveals that it crystallizes in the space group Pna2₁.^§ The asymmetric unit contains of three crystallographically distinct Zn²⁺ ions, four deprotonated bpdc²⁻ ligands, two [NH₂(CH₃)₂]⁺ ions and three free DMF molecules. Three crystallographically independent Zn²⁺ ions have two coordination modes (Fig. 1a). The Zn(1) and Zn(3) adopts a four-coordinated and formed slightly distorted tetrahedral geometry, and the Zn(2) adopts a six-coordinated geometry. O13 and O16 atoms originated from monodentate coordination of the bpdc²⁻ ligands and the other oxygen atoms are coordinated by μ2-modes bpdc²⁻ ligands to Zn²⁺ ions. The shortest and longest Zn–O distance is 1.881(10) and 2.088(9) Å, respectively. Each Zn(2) atom is connected Zn(1) atom and Zn(3) atom by three bridged bidentate bpdc²⁻ ligands to form trinuclear building blocks. As shown in Fig. 1b, trinuclear building blocks are further linked together by bpdc²⁻ ligands make up the 3D anionic framework with two different channels, and the channels is occupied by [NH₂(CH₃)₂]⁺ ions and disordered DMF guest molecules. Parallel to the ac plane, the monodentate bpdc²⁻ ligands bridge trinuclear building blocks to afford layers stacking, and the layers are pillared by bidentate bpdc²⁻ ligands to give rise to a regular 3D network (Fig. 1c), and channel dimensions is about 13 × 18 Å along the b-axis direction. The triangle cage was formed along the c-axis direction with small channel (Fig. 1d). Overall, two individual triangle cage are independent interpenetrated to form the entire framework of 1 (Fig. 1e). It should be noted that although the framework of 1 is interpenetration networks, it is still highly porous. After the removal of solvent molecules in the channels, the accessible volume of 1 is 40.2%.Open in a separate windowFig. 1Structure of 1 (a) trinuclear metal cluster building blocks; (b) 3D anionic framework with two different channels; (c) regular channel along the b-axis direction; (d) the triangle cage along the c-axis direction; (e) two independent interpenetrated triangle cage.The temperature dependent dielectric properties were investigated in the temperature rang of 30–135 °C, and two-step dielectric relaxation were observed. As show in Fig. 2a, it is clear that compound 1 shows the first step dielectric relaxation in the temperature range of 30–80 °C. When 1 was heated from 30 to 37 °C, the dielectric constant progressive increased and reaches a maximum of 174.4 at 10³ Hz. Further increase in temperature results in the dielectric constant of 1 slowly decreasing and dielectric peak disappears. The first step dielectric relaxation is due to the relaxation of absorbed water molecules in the sample surface. The dielectric relaxation signal was not observed in the cooling process (from 95 °C to 30 °C) for losing surface water (Fig. 2b). The κ value is directly related to the polarization phenomena. The higher the polarization, the greater the increase in κ value will be. Usually, the MOF materials with low κ value feature the reorientational motions of polar guest molecules being restricted at low temperature or frameworks solvent-free. However, for 1, the thermally assisted dynamical dipole motion due to polar DMF molecules is appeared. The guest molecules get enough excitation thermal energy to be able to obey the change under the external electric field more easily in the high temperature regime, and the reorientational dynamics of guest molecules is activated above 105 °C. This in return enhances their contribution to the polarization leading to an sharp increase of dielectric permittivity value. At f = 10⁵ Hz, the dielectric constant reaches a maximum of 237, and then sharply decreased when the temperature increased. In the following cooling process, a very low κ value was observed and no dielectric relaxation was occurred (Fig. 2c). The second step dielectric relaxation at different frequency are shown in Fig. 2d, which can be ascribed to the guest polar DMF reorientational motions. In addition, the dielectric loss values shows similar features in the selected frequency range (Fig. S3^†).Open in a separate windowFig. 2(a) Temperature-dependent real part dielectric constant (ε′) in the temperature range of 30–95 °C at selected frequency of 1; (b) plots of ε′ vs. T in the 30–95 °C range at 5 × 10³ Hz with the heating (black dot) and cooling models (red dot); (c) plots of ε′ vs. T in the 100–135 °C range at 10⁵ Hz with the heating (black line) and cooling models (red line; (d) temperature-dependent ε′ in the temperature range of 30–135 °C.Removing polar guest molecules from the framework may be decreases the polarization and the possibility of any type of hydrogen bonding or ionic interactions between the framework and guest molecular, hence, the κ value will also decreases. The guest-free sample 1′ were obtained by simply heating method. The PXRD patterns of the 1 match well with the 1′ (Fig. S4^†), thus demonstrating the phase were unaltered. The dielectric properties of guest-free sample 1′ were investigated. As shown in Fig. 3, after removing the polar DMF molecules, a very low κ value of 1.78 at 100 kHz at 40 °C with a low dielectric loss (0.005) was observed (Fig. S5^†). It is very interesting as the temperature increase from 40 to 130 °C, κ value increases very slowly. κ value is 1.99 at 130 °C. With the ac electric field frequency increasing, the κ value slightly decrease (Fig. 3a). It is noteworthy that, to date, dielectric investigations of MOFs have received relatively little attention, although a few exciting examples have been reported. Only a limited number of MOFs have been reported to possess ultra-low κ value.^15,16 The ultra-low κ values for a few MOFs are shown in Table S2.^†17 To the best of our knowledge, the κ values of 1′ is the lowest value for MOFs reported. From the published paper and our results, to obtained the ultra-low κ MOFs, ligands should have high symmetry and small polarity and polar guest molecular should be avoid. Furthermore, some small counter ions could be decrease κ values. As the vacuum has the lowest dielectric constant, thus, κ values can be reduced significantly by increasing porosity of MOFs. High thermal stability and MOFs thin-film growth are required for the practical applications of ultra-low κ MOFs in microelectronics. ZIF-8 films with κ value of 2.4 were deposited on silicon wafers and characterized in order to assess their potential as future insulators (low-κ dielectrics) in microelectronics.¹⁸ We recently reported a hydrogen bonding MOFs [Zn(H₂EIDA)₂(H₂O)]·2DMF, which exhibited low-κ behaviour, but its thermal stability was not perfect.Open in a separate windowFig. 3Temperature-dependent real part dielectric constant (ε′) at selected frequency of 1′.     

Hollow terbium metal–organic-framework spheres: preparation and their performance in Fe3+ detection

Hollow metal–organic framework (MOF) micro/nanostructures have been attracting a great amount of research interest in recent years. However, the synthesis of hollow metal–organic frameworks (MOFs) is a great challenge. In this paper, by using 1,3,5-benzenetricarboxylic acid (H₃BTC) as the organic ligand and 2,5-thiophenedicarboxylic acid (H₂TDC) as the competitive ligand and protective agent, hollow terbium MOFs (Tb-MOFs) spheres were synthesized by a one-pot solvothermal method. By comparing the morphology of Tb-MOFs in the presence and absence of H₂TDC, it is found that H₂TDC plays a key role in the formation of the hollow spherical structure. Single crystal analyses and element analysis confirm that H₂TDC is not involved in the coordination with Tb³⁺. Interestingly, Tb-MOFs can be used as the luminescent probes for Fe³⁺ recognition in aqueous and N,N-dimethylformamide (DMF) solutions. In aqueous solution, the quenching constant (K_SV) is 5.8 × 10⁻⁴ M⁻¹, and the limit of detection (LOD) is 2.05 μM. In DMF, the K_SV and LOD are 9.5 × 10⁻⁴ M⁻¹ and 0.80 μM, respectively. The sensing mechanism is that the excitation energy absorption of Fe³⁺ ions reduces the energy transfer efficiency from the ligand to Tb³⁺ ions.

(a) Pictures of Tb-MOFs suspension (left) and Fe³⁺ (right) under 365 nm illumination. (b) Pictures of Fe³⁺ with (left) and without (right) Tb-MOFs. (c) Pictures of Tb-MOFs powder before (left) and after (right) Fe³⁺ adsorption.     

Active control of broadband plasmon-induced transparency in a terahertz hybrid metal–graphene metamaterial

A hybrid metal–graphene metamaterial (MM) is reported to achieve active control of broadband plasmon-induced transparency (PIT) in the THz region. The unit cell consists of one cut wire (CW), four U-shaped resonators (USRs) and monolayer graphene sheets under the USRs. Via near-field coupling, broadband PIT can be produced through the interference between different modes. Based on different arrangements of graphene positions, not only can we achieve electrical switching of the amplitude of broadband PIT, but can also realize modulation of the bandwidth of the transparent window. Simultaneously, both the capability and region of slow light can be dynamically tunable. This work provides schemes to manipulate PIT with more degrees of freedom, which will find significant applications in multifunctional THz modulation.

A hybrid metal–graphene metamaterial (MM) is reported to achieve active control of broadband plasmon-induced transparency (PIT) in the THz region.