Metallic Phase Research Articles

UV-random lasing of ZnO films decorated with Au nanoparticles and modification of lasing threshold by surface plasmon effect

Threshold energy values for random lasing action are determined from spectral and temporal coherence studies in systems with incoherent feedback. Zinc oxide films with and without their surface having been decorated with gold nanoparticles stand simultaneously for the active and the scattering medium, whereas pumping is carried out by a conventional laser system operating at the picosecond pulse regime. The influence of the gold surface on both the threshold and the intensity of the optical response is investigated. Whenever localized surface plasmon resonances are supported, electron mobility at the gold boundaries facilitates reinsertion of the trapped electrons into shallow defect levels to the conduction band. In consequence the characteristic visible emission from ZnO (green) due to transitions from such levels is suppressed, thus lowering the pumping threshold prerequisite for random lasing emission. The tunning of the random lasing threshold is achievable by means of control on the morphology and size effects of the gold nanoparticles. This contribution shows that specific post-thermal treatments could be exploited to adjust the lasing properties of novel metal decorated systems, which already feature random lasing before decoration. In principle, the extension of the decoration should be rather small to minimize response losses due to screening side effects of the metal phase, which obtrudes direct contact of the pumping photons with the semiconductor system.

Pyrometallurgical Reduction of Copper Slag with Biochar for Metal Recovery

AbstractLarge amounts of slag are generated during pyrometallurgical processing in copper production. Due to the presence of valuable elements, the improper disposal of huge quantities of copper slag produced, results in significant loss of resources as well as environmental issues. Analyses of the copper slag show that it contains valuable metals, particularly copper and nickel. In this work, four biochars were employed as fossil-free reducing agents to recover valuable metals from the slag. Reduction experiments were performed in a vertical furnace at temperatures 1250, 1300 and 1350 °C for 60 min in order to investigate the effect of temperature. Moreover, the effect of time on reduction progress was studied at 1250 °C and the concentrations of CO and CO2 in the off-gas were measured with a gas analyzer. Copper slag was reacted with metallurgical coke for comparison and the products were analyzed with EPMA and LA-ICPMS. The results revealed that reduction rapidly progresses to the formation of metal alloy within 10 min. Valuable metals like copper, nickel and arsenic were the first to be reduced to the metal phase. As reduction time increased, iron was also reduced and combined with the metal droplet. The use of biochar as reductant was shown to be more effective than coke especially at lower temperatures. In addition, thermodynamic modelling was performed with FactSage and HSC and compared with the experimental results. The simulations with HSC showed the sequence of reactions taking place and the calculations by FactSage were in agreement with the experiments. Graphical Abstract

Inverted optical functionality of phase change material surfaces through Mie resonances of VO2@Si core–shell structures

Vanadium dioxide (VO2) has attracted extensive attention due to its reversible transition from the insulator to metal phase at a critical temperature of 68°C. Below the critical temperature VO2 transmits the infrared radiation in the insulator phase, whereas above the critical temperature VO2 reflects the infrared portion of the incident radiation. However, smart surface interfaces for high-temperature emitter surfaces require the opposite functionality within the 1–3 µm spectral range. Here, we demonstrate that a core–shell structure, composed of VO2@Si, which is deposited on a thin layer of Ag, achieves the inverted optical functionality within the 1–3 µm spectral range, making it ideal as smart interfaces for radiative heat applications as high-temperature emitters. The proposed material architecture also increases the thermal stability of VO2 in addition to enhancing its optical properties in near-infrared region. The results were obtained using numerical simulations. Our results indicate that in its metallic state, the core–shell structure with metallic underlayer promotes efficient absorption in the near-infrared spectrum. On the other hand, in its insulating state dielectric resonances within the core–shell structure along with the metallic underlayer, resulting in increased reflection, offer inverse optical functionalities. Our findings present a significant step toward designing dynamic filters that can efficiently capture and respond to changing conditions in the near-infrared spectrum.

Development of Self-Doped Monolayered 2D MoS2 for Enhanced Photoresponsivity.

Transition metal dichalcogenides (TMDs) exist in two distinct phases: the thermodynamically stable trigonal prismatic (2H) and the metastable octahedral (1T) phase. Phase engineering has emerged as a potent technique for enhancing the performance of TMDs in optoelectronics applications. Nevertheless, understanding the mechanism of phase transition in TMDs and achieving large-area synthesis of phase-controlled TMDs continue to pose significant challenges. This study presents the synthesis of large-area monolayered 2H-MoS2 and mixed-phase 1T/2H-MoS2 by controlling the growth temperature in the chemical vapor deposition (CVD) method without use of a catalyst. The field-effect transistors (FETs) devices fabricated with 1T/2H-MoS2 mixed-phase show an on/off ratio of 107. Photo response devices fabricated with 1T/2H-MoS2 mixed-phase show ≈55 times enhancement in responsivity (from 0.32 to 17.4AW-1) and 102 times increase in the detectivity (from 4.1×1010 to 2.48×1012cmHzW-1) compare to 2H-MoS2. Introducing the metallic 1T phase within the 2H phase contributes additional carriers to the material, which prevents the electron-hole recombination and thereby increases the carrier density in the 1T/2H-MoS2 mixed-phase in comparison to 2H-MoS2. This work provides insights into the self-doping effects of 1T phase in 2H MoS2, enabling the tuning of 2D TMDs properties for optoelectronic applications.

A bidirectional broadband multifunctional terahertz device based on vanadium dioxide

We propose a switchable and bidirectional metamaterial terahertz absorber/reflector based on vanadium dioxide. Simulation results show that the device can achieve broadband absorption and reflection by adjusting the phase state of vanadium dioxide. When the vanadium dioxide is in the insulating phase, the device can be a perfect bidirectional broadband absorber with more than 90% absorptivity under normal forward/backward TE/TM-polarized incident wave in the frequency range from 1.9 THz to 10 THz. This excellent absorption characteristic is insensitive to the polarization angle of the incident wave. When the vanadium dioxide is in the metal phase, the device achieves more than 90% reflectivity in the low-frequency range under forward/backward TE-polarized incident wave. In the high-frequency range, the device has both reflection and absorption effects on the TE-polarized wave. When vanadium dioxide is in two different phase states, the performance of device is not sensitive to the electromagnetic wave incidence angle. The change in vanadium dioxide conductivity does not affect the stable performance of the device at high (low) frequencies when used as an absorber (reflector). The proposed device has potential applications in terahertz energy harvesting, detection, sensing, optical switching, and shielding.

Novel emergent phases in a two-dimensional superconductor

In this letter, we report our observation of an extraordinarily rich phase diagram of a LaScO3/SrTiO3 heterostructure. Close to the superconducting transition temperature, the system hosts a superconducting critical point of the Infinite-randomness type characterized by an effective dynamical exponent νz that diverges logarithmically. At lower temperatures, we find the emergence of a magnetic field-tuned metallic phase that co-exists with a quantum Griffiths phase (QGP). Our study reveals a previously unobserved phenomenon in 2D superconductors—an unanticipated suppression of the QGP below a crossover temperature in this system. This concealment is accompanied by the destruction of the superconducting quantum critical point (QCP) signaled by a power-law divergence (in temperature) of the effective dynamical exponent. These observations are entirely at odds with the predictions of the infinite-randomness scenario and challenge the very concept of a vanishing energy scale associated with a QCP. We develop and discuss possible scenarios like smearing of the phase transition that could plausibly explain our observations. Our findings challenge the notion that QGP is the ultimate ground state in two-dimensional superconductors.

Insight into phase stability and thermoelectric properties of semiconducting iron silicides with manganese substitution: β-Fe1−xMnxSi2 (0≤x≤0.05)

Metal-doped iron silicides (β-FeSi2) are promising thermoelectric materials for high-temperature applications. However, the addition of metal dopants usually causes the formation of secondary metallic phases, resulting in the degradation of thermopower. Here we report the stability of semiconducting β-phase (higher than 95 %) of Mn-doped β-FeSi2 obtained at Mn concentration from 1 % to 5 %, where the samples were prepared through direct arc melting and heat treatment process. The carrier density remarkably increases with Mn content, but the mobility decreases. The electrical resistivity significantly decreases with Mn content. The Seebeck coefficient of Mn-doped samples is improved and stable at high-temperature regions because of the reduction of the bipolar effect and β-phase stability. The thermal conductivity slightly increases with Mn. As a result, the maximum thermoelectric power factor PF = 970 μWm−1K−2 and dimensionless figure of merit ZT = 0.12 at 800 K are obtained in a 3 % Mn-doped sample. The stability of β-phase and the improved carrier density are the origins of enhanced thermoelectric properties, where the Seebeck coefficient is improved, and the electrical resistivity is reduced. Our study provides insight into the importance of phase stability for enhancing thermoelectric transport in Mn-doped β-FeSi2.

2H-SnS2 assembled with petaloid 1T@2H-MoS2 nanosheet heterostructures for room temperature NO2 gas sensing.

In this study, we explored the gas-sensing capabilities of MoS2 petaloid nanosheets in the metallic 1T phase with the commonly investigated semiconducting 2H phase. By synthesizing SnS2 nanoparticles and MoS2 petaloid nanosheets through a hydrothermal method, we achieve notable sensing performance for NO2 gas at room temperature (27 °C). This investigation represents a novel study, and to the best of our knowledge no, prior similar investigations have been reported in the literature for 1T@2HMoS2/SnS2 heterostructures for room temperature NO2 gas sensing. The formed heterostructure between SnS2 nanoparticles and petaloid MoS2 nanosheets exhibits synergistic effects, offering highly active sites for NO2 gas adsorption, consequently enhancing sensor response. Our sensor demonstrated a remarkable sensing response (R a/R g = 7.49) towards 1 ppm of NO2, rapid response time of 54 s, baseline recovery in 345 s, good selectivity and long-term stability, underscoring its potential for practical gas-sensing applications.

Unveiling the overlooked role of structural heterogeneity within Fe-N-C single atom catalysts for Fenton-like reactions: Efficient decontamination of pharmaceuticals from wastewater and source-separated urine

Recognizing the structural heterogeneity of Metal-N-C single atom catalysts (M-N-C SACs) and further revealing the impact of other metal phases that may be generated during the preparation process on the catalytic activity of SACs are essential for the rational design of M-N-C SACs. Through the post-Fe(III)-N coordination strategy, an extremely reactive heterogeneous Fe-N-C SACs including Fe-Nx coordinating sites and carbon-iron hybrids into the N-doped graphene-containing carbon nanoflakes (Fe3C/Fe-Nx@GCN) is designed at the atomic level. As a stabilized Fenton-like activator, the catalyst exhibits a high mass activity of 2.08 × 105 min−1 mol−1 for the catalytic oxidation of recalcitrant organic. The experiments and density functional theory (DFT) calculations reveal that specific type of N-coordinated Fe sites with a lower d-band center position is prone to Fe-oxo species production. However, the incorporation of carbon-iron hybrids induces charge redistribution in the active metal centers, further enhancing catalyst’s adsorption energy on peroxymonosulfate (PMS) and effectively modulating the reaction energy barriers. Additionally, the electron transfer pathway dominated by potential differences was confirmed by appropriate reactive substrate orbital energies. This study preferentially provides perspectives on the synergistic relationship between carbon-iron hybrids and Fe-Nx in PMS-AOPs and proposes the multiple-reaction pathways for eliminating pharmaceuticals from the source-separated urine.

Gallium distribution between slag and metal phases during the carbothermal reduction of bauxite

Gallium distribution between slag and metal phases during the carbothermal reduction of bauxite

Polarization-insensitive terahertz ultra-wideband absorber with an actively tunable bandwidth

This study presents a novel, to our knowledge, polarization-insensitive terahertz absorber with an actively adjustable bandwidth. The absorber consists of a vertically stacked three-layer structure with target-shaped resonant layers. Its absorption bandwidth is dynamically controlled, exhibiting ultra-broadband characteristics in the metallic phase. The absorber achieves over 90% absorption from 1.8 to 31.7 THz, with a relative bandwidth of 178%. In the insulating phase, the device transforms into a narrowband absorber, showing a precise absorption peak within the 0 to 20 THz frequency range. Notably, this absorber demonstrates polarization-insensitive and wide-angle absorption properties, making it suitable for different application scenarios. Compared to traditional broadband absorbers, the absorber proposed in this study features an ultra-wide absorption bandwidth and switchable operating modes, enhancing its flexibility and adaptability. The excellent performance of the absorber indicates its suitability for various applications, such as sixth-generation (6G) wireless communication, security detection, imaging, military stealth technology, and energy harvesting.

Green synthesis of nanostructured 1T/2H-MoS2 hybrid phase with polyol solvents and microwave heating

Green synthesis approaches have attracted greatly of attention in recent years since they address the issues associated with sustainability than conventional synthesis methods. New research fields in green nanoscience are being developed as a result of the incorporation of green chemistry principles into nanoscience. In this paper, the successful microwave-assisted green synthesis of MoS2 nanoparticles in a single pot using polyol solvents such as ethylene glycol and glycerol is demonstrated. The coexistence of 1T and 2H phases in MoS2 nanomaterials was determined using advanced techniques such as XRD, Raman, XPS, and TEM images. The highest 1T proportion obtained was 84.5% when compared to the 2H phase. The reaction mechanism and the phase transition between 1T and 2H were described and illustrated. The role of polyol solvents in the practical synthesis of nano MoS2 under microwave heating is also evaluated and explained. Due to the ability of the metallic 1T phase to enhance electrical conductivity, it is believed that hybrid nanostructures exhibit superior electrochemical performance for energy storage and conversion applications.

Constrained patterning of orientated metal chalcogenide nanowires and their growth mechanism

One-dimensional metallic transition-metal chalcogenide nanowires (TMC-NWs) hold promise for interconnecting devices built on two-dimensional (2D) transition-metal dichalcogenides, but only isotropic growth has so far been demonstrated. Here we show the direct patterning of highly oriented Mo6Te6 NWs in 2D molybdenum ditelluride (MoTe2) using graphite as confined encapsulation layers under external stimuli. The atomic structural transition is studied through in-situ electrical biasing the fabricated heterostructure in a scanning transmission electron microscope. Atomic resolution high-angle annular dark-field STEM images reveal that the conversion of Mo6Te6 NWs from MoTe2 occurs only along specific directions. Combined with first-principles calculations, we attribute the oriented growth to the local Joule-heating induced by electrical bias near the interface of the graphite-MoTe2 heterostructure and the confinement effect generated by graphite. Using the same strategy, we fabricate oriented NWs confined in graphite as lateral contact electrodes in the 2H-MoTe2 FET, achieving a low Schottky barrier of 11.5 meV, and low contact resistance of 43.7 Ω µm at the metal-NW interface. Our work introduces possible approaches to fabricate oriented NWs for interconnections in flexible 2D nanoelectronics through direct metal phase patterning.

Toward Scalable Electrochemical Exfoliation of Molybdenum Disulfide Powder through an Accessible Electrode Design.

Cathodic electrochemical intercalation/exfoliation of transition metal dichalcogenides (TMDs) with bulky tetraalkylammonium-based cations is gaining popularity as it avoids the semiconducting (2H) to metallic (1T) phase transformation in TMDs like molybdenum disulfide (MoS2) and, generally, produces sheets with a larger aspect ratio - important for achieving conformal sheet-to-sheet contact in optoelectronic devices. Large single crystals are typically used as the precursor, but these are expensive, often inaccessible, and result in limited quantities of material. In this paper, a 3D-printable electrochemical cell capable of intercalating gram-scale quantities of commercially available TMD powders is presented. By incorporating a reference electrode in the cell and physically restraining the powder with a spring-loaded mechanism, the system can probe the intercalation electrochemistry, for example, determining the onset of intercalation to be near -2.5V versus the ferrocene redox couple. While the extent of intercalation depends on precursor quantity and reaction time, a high yield of exfoliated product can be obtained exhibiting average aspect ratios as high as 49 ± 44 similar to values obtained by crystal intercalation. The intercalation and exfoliation of a wide variety of pelletized commercial powders including molybdenum diselenide (MoSe2), tungsten diselenide (WSe2), tungsten disulfide (WS2), and graphitic carbon nitride (gCN) are also demonstrated.

Quantum phase transitions in one-dimensional nanostructures: a comparison between DFT and DMRG methodologies.

In the realm of quantum chemistry, the accurate prediction of electronic structure and properties of nanostructures remains a formidable challenge. Density functional theory (DFT) and density matrix renormalization group (DMRG) have emerged as two powerful computational methods for addressing electronic correlation effects in diverse molecular systems. We compare ground-state energies ( ), density profiles ( ), and average entanglement entropies ( ) in metals, insulators and at the transition from metal to insulator, in homogeneous, superlattices, and harmonically confined chains described by the fermionic one-dimensional Hubbard model. While for the homogeneous systems, there is a clear hierarchy between the deviations, , and all the deviations decrease with the chain size; for superlattices and harmonic confinement, the relation among the deviations is less trivial and strongly dependent on the superlattice structure and the confinement strength considered. For the superlattices, in general, increasing the number of impurities in the unit cell represents lower precision in the DFT calculations. For the confined chains, DFT performs better for metallic phases, while the highest deviations appear for the Mott and band-insulator phases. This work provides a comprehensive comparative analysis of these methodologies, shedding light on their respective strengths, limitations, and applications. The DFT calculations were performed using the standard Kohn-Sham scheme within the BALDA approach. It integrated the numerical Bethe-Ansatz (BA) solution of the Hubbard model as the homogeneous density functional within a local-density approximation (LDA) for the exchange-correlation energy. The DMRG algorithms were implemented using the ITensor library, which is based on the matrix product states (MPS) ansatz. The calculations were performed until the energy reaches convergence of at least .

Graphene and Vanadium Dioxide-Based Terahertz Absorber with Switchable Multifunctionality for Band Selection Applications.

This study proposes a multifunctional absorber in the terahertz (THz) regime based on vanadium dioxide (VO2) and graphene with either-or band selector applications, which can be realized by electrically and thermally controlling the Fermi energy level of graphene and vanadium dioxide, respectively. The broadband absorption can be achieved with absorptance exceeding 90%, when the VO2 film is in the metallic phase and the Fermi energy levels of the upper and lower graphene layers are simultaneously set to 0.6 and 0 eV, respectively. The double narrowband can be realized when the VO2 film is in the insulating phase and the Fermi energy levels in upper and lower graphene layers are set as 0 and 0.8 eV, respectively. By flexibly shifting between the broadband and the double narrowband, the proposed absorber can be used as an either-or band selector, corresponding optional bandwidth from 2.05 to 2.35 THz, and 3.25 to 3.6 THz. Furthermore, single narrowband absorption can be achieved by setting the conductivity of the VO2 film to appropriate values. The proposed absorber can be used in the THz regime in applications such as multifunctional devices, switches, cloaking objects, and band selectors.

Manipulating the metal–insulator transitions and thermoelectric bi-functionality for correlated vanadium dioxide pellets

The electron correlations as triggered by on-site Coulomb repulsion within correlated vanadium dioxide (VO2) open up an emerging paradigm to explore new electronic phases and promising device applications. Apart from correlated electrical transport, the thermoelectric thermopower (S) of VO2 also undergoes abrupt variation across the critical temperature (TMIT) that sheds lights on the potential thermoelectric functionality. Nevertheless, bridging bi-functionality of VO2 associated with the thermistor and thermoelectric properties is still restricted by the limitation in improving the absolute magnitude of S and temperature coefficient of resistance (TCR). Herein, we demonstrate the widely adjustable metal–insulator transition (MIT) behavior and overlooked thermoelectric performance for Ti-substituted VO2 pellets that enable the bi-functionality strategy as combined with thermistor and thermoelectric properties. As-achieved tunable TMIT while maintaining large |TCR| is herein achieved for Ti-substituted VO2 pellets that enables practical device applications near room temperature. Apart from the well-known MIT functionality, we reveal the overlooked thermoelectric properties for VO2 via coherently co-sintering with TiO2 that introduces a new freedom (thermopower). Specifically, the largely enhanced thermopower observed for insulating V1−xTixO2 exceeds the one for pristine VO2 by five times, while the thermoelectric power factor for its metallic phase is comparable to typical organic or oxide thermoelectric materials. The presently achieved thermistor and thermoelectric bi-functionality (e.g., |S| > 100 μV/K and |TCR| > 1 K−1) for Ti-substituted VO2 pellets extends the horizons in material designs that combines such a bi-functionality to achieve both the passive and active sensing for improving the accuracy in thermal perturbations of VO2 bulk as infrared detectors near room temperature.

Construction of an Ohmic Contact Cathode by Two Metal Sulfides for efficient Capture and Catalysis of Polysulfide.

The slow reaction kinetics and severe shuttle effect of lithium polysulfide make Li-S battery electrochemical performance difficult to meet the demands of large electronic devices such as electric vehicles. Based on this, an electrocatalyst constructed by metal phase material (MoS2) and semiconductor phase material (SnS2) with ohmic contact is designed for inhibiting the dissolution of lithium polysulfide with improving the reaction kinetics. According to the density-functional theory calculations, it is found that the heterostructured samples with ohmic contacts can effectively reduce the reaction-free energy of lithium polysulfide to accelerate the sulfur redox reaction, in addition to the excellent electron conduction to reduce the overall activation energy. The metallic sulfide can add more sulfophilic sites to promote the capture of polysulfide. Thanks to the ohmic contact design, the carbon nanotube-MoS2-SnS2 achieved a specific capacity of 1437.2 mAhg-1 at 0.1 C current density and 805.5 mAhg-1 after 500 cycles at 1 C current density and is also tested as a pouch cell, which proves to be valuable for practical applications. This work provides a new idea for designing an advanced and efficient polysulfide catalyst based on ohmic contact.

Bottom-up Synthesis of Highly Chiral 1T Molybdenum Disulfide Nanosheets.

Chirality in inorganic nanostructures has recently stimulated the attention of many researchers, both to unravel fundamental questions on the origin of chirality in inorganic and hybrid materials, as well as to introduce novel promising properties that are originated by the symmetry breaking. MoS2 is one of the most investigated among the large family of layered transition metal dichalcogenides. In particular, the metastable metallic 1T-MoS2 phase is of large interest for potential applications. However, due to thermodynamic reasons, the synthesis of 1T-MoS2 phase is quite challenging. Herein, we present the first synthesis of chiral 1T-MoS2 phase which shows remarkably high chiroptical activity with a g-factor up to 0.01. Chiral 1T-MoS2 was produced using tartaric acid as a chiral ligand to induce symmetry breaking during the material's growth under hydrothermal conditions, leading to the formation of distorted hierarchical nanosheet assemblies exhibiting chiral morphology. Thorough optimization of the synthetic conditions was carried out to maximize chiroptical activity, which is strongly related to the nanostructures' morphology. Finally, the formation mechanism of the chiral 1T-MoS2 nanosheet assemblies was investigated, focusing on the role of molecular intermediates in the growth of the nanosheets and the transfer of chirality.

Absorption and Reflection of Switchable Multifunctional Metamaterial Absorber Based on Vanadium Dioxide

A dynamically tunable terahertz broadband absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The absorber consists of two patterned VO2 layers and a metal bottom layer separated by two polytetrafluoroethylene (PTFE) dielectric layers. Simulation results show that the absorption exceeds 90% in the frequency range of 2.4–11 THz with a relative bandwidth of 128.4% under normal incidence. When VO2 is in the metal phase, the designed absorber functions as an ideal absorber. The absorption rate can be flexibly adjusted from 2% to 99% as vanadium dioxide transitions from the insulator phase to the metal phase. Therefore, the newly developed broad structure has the capability to seamlessly transition between functioning as an absorber or reflector through modifications in the conductivity of VO2 from the insulator phase to the metal phase. Moreover, further insight into the underlying physical processes can be gained by studying the insensitivity of the proposed absorber to the polarization of incident light and its ability to achieve high absorption across a wide range of incident angles. Impedance matching theory and electric field distribution of the absorber are investigated. The THz absorber has many potential applications in fields such as THz sensors, modulation, and switches.