Momentum matching induced giant magnetoresistance in two-dimensional magnetic tunnel junctions.

Giant magnetoresistance was first experimentally discovered in three-dimensional magnetic tunnel junctions (MTJs) in the late 1980s and is of great importance in nonvolatile memory applications. How to achieve a magnetoresistance as large as possible is always a central task in the study of MTJs. However, it is normally only of the order of magnitude of tens of percent in traditional MTJs. The ideal situation is the metal-insulator transition together with the magnetization reversal of one magnetic lead. In this work, we will show that this can be achieved using a two-dimensional ferromagnetic zigzag SiC nanoribbon junction based on quantum transport calculations performed with a combination of density functional theory and non-equilibrium Green's function. Specifically, with the magnetization configuration switching of the two leads from parallel to anti-parallel, the junction will change abruptly from a conducting state to an insulating state, although the two leads are always metallic, with both spin up and spin down channels crossing the Fermi level simultaneously. Extensive analysis indicates that the insulating state in the anti-parallel magnetic configuration originates not from any present mechanisms that cause full suppression of electron transmission but from momentum direction mismatching. This finding suggests a fantastic mechanism for achieving magnetoresistance or electrical switching in nanoscale devices by manipulating band dispersion.

MgF2:Mn2+: novel material with mechanically-induced luminescence.

Mechanoluminescent (ML) materials can directly convert external mechanical stimulation into light without the need for excitation from other forms of energy, such as light or electricity. This alluring characteristic makes ML materials potentially applicable in a wide range of areas, including dynamic imaging of force, advanced displays, information code, storage, and anti-counterfeiting encryption. However, current reproducible ML materials are restricted to sulfide- and oxide-based materials. In addition, most of the reported ML materials require pre-irradiation with ultraviolet (UV) lamps or other light sources, which seriously hinders their practical applications. Here, we report a novel ML material, MgF2:Mn2+, which emits bright red light under an external dynamic force without the need for pre-charging with UV light. The luminescence properties were systematically studied, and the piezophotonic application was demonstrated. More interestingly, unlike the well-known zinc sulfide ML complexes reported previously, a highly transparent ML film was successfully fabricated by incorporating MgF2:Mn2+ into polydimethylsiloxane (PDMS) matrices. This film is expected to find applications in advanced flexible optoelectronics such as integrated piezophotonics, artificial skin, athletic analytics in sports science, among others.

Near-Infrared Broadband ZnTa2O6:Cr3+ Phosphor for pc-LEDs and Its Application to Nondestructive Testing.

Broadband near-infrared (NIR) phosphors are necessary materials for developing portable NIR light sources. Moreover, exploiting an NIR phosphor with a main peak located beyond a wavelength of 900 nm remains a challenge because this spectral range has great potential in biological nondestructive testing and solution testing. In this study, a range of Cr3+-doped ZnTa2O6 (ZTO) phosphors were completely synthesized by a solid-state method, which show broadband Cr3+ emission centered at 935 nm with a large full width at half maximum (FWHM) of 185 nm due to two distorted octahedral sites. A packaged phosphor-converted light-emitting diode (pc-LED) device is used to penetrate a 5-cm-thick chicken breast and identify diverse solutions based on differences in the measured transmission spectra. The results indicate broad application prospects in the field of biological tissue penetration and solution analysis.

On-Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition.

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction.

Interlayer Quasi-Bonding Interactions in 2D Layered Materials: A Classification According to the Occupancy of Involved Energy Bands.

Recent studies have revealed that the interlayer interaction in two-dimensional (2D) layered materials is not simply of van der Waals character but could coexist with quasi-bonding character. Herein, we classify the interlayer quasi-bonding interactions into two main categories (I: homo-occupancy interaction; II: hetero-occupancy interaction) according to the occupancy of the involved energy bands near the Fermi level. We then investigate the quasi-bonding-interaction-induced band structure evolution of several representative 2D materials based on density functional theory calculations. Further calculations confirm that this classification is applicable to generic 2D layered materials and provide a unified understanding of the total strength of interlayer interaction, which is a synergetic effect of the van der Waals attraction and the quasi-bonding interaction. The latter is stabilizing in main category II and destabilizing in main category I. Thus, the total interlayer interaction strength is relatively stronger in category II and weaker in category I.

Ladder Phenylenes Synthesized on Au(111) Surface via Selective [2+2] Cycloaddition.

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.

Electrical contacts to few-layer MoS2 with phase-engineering and metal intercalation for tuning the contact performance.

Due to Fermi-level pinning in metal-two-dimensional MoS2 junctions, improving the performance of MoS2-based electrical devices is still under extensive study. The device performance of few-layer MoS2 depends strongly on the number of layers. In this work, via density-functional theory calculations, a comprehensive understanding from the atomistic view was reached for the interlayer interaction between metal and few-layer MoS2 with phase-engineering and intercalation doping, which are helpful for improving the contact performance. These two methods are probed to tune the performance of few-layer MoS2-based field-effect transistors, and both of them can tune the Schottky barrier height. Phase-engineering, which means that the MoS2 layer in contact with metal is converted to the T phase, can transform the Schottky barrier from n- to p-type. Intercalation doping, which takes advantage of annealing and results in metal atom interaction in between MoS2 layers, makes the MoS2 layers become quasi-freestanding and converts the indirect bandgap into direct bandgap. Our atomistic insights help improve the performance of few-layer MoS2-based electronic devices.

CNSi/MXene/CNSi: Unique Structure with Specific Electronic Properties for Nanodevices.

2D materials have been interesting for applications into nanodevices due to their intriguing physical properties. In this work, four types of unique structures are designed that are composed of MXenes and C/N-Si layers (CNSi), where MXene is sandwiched by the CNSi layers with different thicknesses, for their practical applications into integrated devices. The systematic calculations on their elastic constants, phonon dispersions, and thermodynamic properties show that these structures are stable, depending on the composition of MXene. It is found: 1) different from MXene or N-functionalized MXene (M2 CN2 ), SiN2 /M2 X/SiN2 possess new electronic properties with free carriers only in the middle, leading to 2D free electron gas; 2) CNSi/MXene/CNSi shows an intrinsic Ohmic semiconductor-metal-semiconductor (S-M-S) contact, which is potential for applications into nanodevices; and 3) O/M2 C/SiN2 and N/M2 C/OSiN are also stable and show different electronic properties, which can be semiconductor or metal as a whole depending on the interface. A method is further proposed to fabricate the 2D structures based on the industrial availability. The findings may provide a novel strategy to design and fabricate the 2D structures for their application into nanodevices and integrated circuits.

Dynamic fingerprint of fractionalized excitations in single-crystalline Cu3Zn(OH)6FBr.

Beyond the absence of long-range magnetic orders, the most prominent feature of the elusive quantum spin liquid (QSL) state is the existence of fractionalized spin excitations, i.e., spinons. When the system orders, the spin-wave excitation appears as the bound state of the spinon-antispinon pair. Although scarcely reported, a direct comparison between similar compounds illustrates the evolution from spinon to magnon. Here, we perform the Raman scattering on single crystals of two quantum kagome antiferromagnets, of which one is the kagome QSL candidate Cu3Zn(OH)6FBr, and another is an antiferromagnetically ordered compound EuCu3(OH)6Cl3. In Cu3Zn(OH)6FBr, we identify a unique one spinon-antispinon pair component in the E2g magnetic Raman continuum, providing strong evidence for deconfined spinon excitations. In contrast, a sharp magnon peak emerges from the one-pair spinon continuum in the Eg magnetic Raman response once EuCu3(OH)6Cl3 undergoes the antiferromagnetic order transition. From the comparative Raman studies, we can regard the magnon mode as the spinon-antispinon bound state, and the spinon confinement drives the magnetic ordering.

Strain-driven phase transition and spin polarization of Re-doped transition-metal dichalcogenides.

Two-dimensional transition metal dichalcogenides (TMDCs) are promising in spintronics due to their spin-orbit coupling, but their intrinsic non-magnetic properties limit their further development. Here, we focus on the energy landscapes of TMDC (MX2, M = Mo, W and X = S, Se, Te) monolayers by rhenium (Re) substitution doping under axial strains, which controllably drive 1H ↔ 1Td structural transformations. For both 1H and 1Td phases without strain, Re-doped TMDCs have an n-type character and are non-magnetic, but the tensile strain could effectively induce and modulate the magnetism. Specifically, 1H-Re0.5Mo0.5S2 gets a maximum magnetic moment of 0.69 µB at a 6% uniaxial tensile strain along the armchair direction; along the zigzag direction it exhibits a significant magnetic moment (0.49 µB) at a 2.04% uniaxial tensile strain but then exhibits no magnetism in the range of [5.10%, 7.14%]. By contrast, for 1Td-Re0.5Mo0.5S2 a critical uniaxial tensile strain along the zigzag direction reaches up to ∼9.18%, and a smaller uniaxial tensile strain (∼5.10%) along the zigzag direction is needed to induce the magnetism in 1Td-Re0.5M0.5Te2. The results reveal that the magnetism of Re-doped TMDCs could be effectively induced and modulated by the tensile strain, suggesting that strain engineering could have significant applications in doped TMDCs.

Flexoelectricity Driven Fano Resonance in Slotted Carbon Nanotubes for Decoupled Multifunctional Sensing.

Multifunctionality, interference-free signal readout, and quantum effect are important considerations for flexible sensors equipped within a single unit towards further miniaturization. To address these criteria, we present the slotted carbon nanotube (CNT) junction features tunable Fano resonance driven by flexoelectricity, which could serve as an ideal multimodal sensory receptor. Based on extensive ab initio calculations, we find that the effective Fano factor can be used as a temperature-insensitive extrinsic variable for sensing the bending strain, and the Seebeck coefficient can be used as a strain-insensitive intrinsic variable for detecting temperature. Thus, this dual-parameter permits simultaneous sensing of temperature and strain without signal interference. We further demonstrate the applicability of this slotted junction to ultrasensitive chemical sensing which enables precise determination of donor-type, acceptor-type, and inert molecules. This is due to the enhancement or counterbalance between flexoelectric and chemical gating. Flexoelectric gating would preserve the electron-hole symmetry of the slotted junction whereas chemical gating would break it. As a proof-of-concept demonstration, the slotted CNT junction provides an excellent quantum platform for the development of multistimuli sensation in artificial intelligence at the molecular scale.

Mechanism of charge redistribution at the metal-semiconductor and semiconductor-semiconductor interfaces of metal-bilayer MoS2 junctions.

Layer-number-dependent performance of metal-semiconductor junctions (MSJs) with multilayered two-dimensional (2D) semiconductors has attracted increasing attention for their potential in ultrathin electronics and optoelectronics. However, the mechanism of the interaction and the resulting charge transfer/redistribution at the two kinds of interfaces in MSJ with multilayered 2D semiconductors, namely, the metal-semiconductor (M-S) and the semiconductor-semiconductor (S-S) interfaces, have not been well understood until now, although that is important for the overall Schottky barrier height and the energy-band-offset between different layers of the 2D semiconductors. Here, based on state-of-the-art density functional theory calculations, the mechanisms of bonding and asymmetric electron redistribution at the M-S and S-S interfaces of metal-bilayer MoS2 junctions are revealed. Multiple mechanisms collectively contribute to the electron redistribution at the two kinds of interfaces, and the dominant mechanism depends on both the dimensionality (2D vs 3D) and the work function of metal electrodes. For the M-S interface, the pushback effect and metal-induced gap states play a dominant role for MSJs with 3D metal, while the covalent-like quasi-bonding feature appears for MSJs with medium-work-function 2D metals, and charge transfer plays a main role for MSJs with 2D metals that have very large or small work functions. For the S-S interface, it inherits the electron-redistribution behavior of the M-S interface for MSJs with 2D metal, while opposite electron-redistribution appears in MSJs with 3D metal. These mechanisms provide general insights and new concepts to better understand and use MSJs with multilayered 2D semiconductors.

Atomistic insights into the screening and role of oxygen in enhancing the Li+ conductivity of Li7P3S11-xOx solid-state electrolytes.

Herein, we implement first-principles calculations to design Li7P3S11-xOx at an atomic scale, aiming to obtain stable Li7P3S11-xOx-type solid electrolyte materials with good Li+ conductivity. After searching for chemical potentials, Li2O2 is expected to be the potential raw material, and it can afford the most favorable growth environment for the synthesis of Li7P3S11-xOx (x = 0.25, 0.50, 0.75 and 1). Among these compounds, it is found that Li7P3S10.25O0.75 exhibits the most desirable Li+ conductivity of 109 mS cm-1 at 300 K, which is far higher than that of Li7P3S11 (50 mS cm-1 at 300 K). By structural analysis, it is demonstrated that the Li diffusion pathway in Li7P3S10.25O0.75 is significantly broadened relative to that in Li7P3S11 (71.38 Å3vs. 69.48 Å3), which breaks the bottleneck during Li diffusion. Moreover, the resistance of Li ion diffusion in Li7P3S10.25O0.75 decreases due to the balance of interactions between Li and its neighbouring atoms at the transition state, which induces a much lesser energy barrier of Li7P3S10.25O0.75 than that of Li7P3S11 (0.20 eV vs. 0.31 eV). Moreover, introducing Li vacancies is unlikely to alter the essence of the inherent superionic conductivity of Li7P3S10.25O0.75. Furthermore, Li7P3S10.25O0.75 can maintain good thermal stability and similar electrochemical stability to Li7P3S11. This study successfully clarifies the role of oxygen in enhancing the Li+ conductivity of Li7P3S11-xOx. Moreover, it affords a new strategy to design other solid-state electrolytes with good Li+ conductivity.

Polarity and Spin-Orbit Coupling Induced Strong Interfacial Exchange Coupling: An Asymmetric Charge Transfer in Iridate-Manganite Heterostructure.

Charge transfer is of particular importance in manipulating the interface physics in transition-metal oxide heterostructures. In this work, we have fabricated epitaxial bilayers composed of polar 3d LaMnO3 and nonpolar 5d SrIrO3. Systematic magnetic measurements reveal an unexpectedly large exchange bias effect in the bilayer, together with a dramatic enhancement of the coercivity of LaMnO3. Based on first-principle calculations and X-ray absorption spectroscopy measurements, such a strong interfacial magnetic coupling is found closely associated with the polar nature of LaMnO3 and the strong spin-orbit interaction in SrIrO3, which collectively drive an asymmetric interfacial charge transfer and lead to the emergence of an interfacial reentrant spin/superspin glass state. Our study provides a new insight into the charge transfer in transition-metal oxide heterostructures and offers a novel means to tune the interfacial exchange coupling for a variety of device applications.

Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations.

Computational tools based on density-functional theory (DFT) enable the exploration of the qualitatively new, experimentally attainable nanoscale compounds for a targeted application. Theoretical simulations provide a profound understanding of the intrinsic electronic properties of functional materials. The goal of this protocol is to search for photocatalyst candidates by computational dissection. Photocatalytic applications require suitable band gaps, appropriate band edge positions relative to the redox potentials. Hybrid functionals can provide accurate values of these properties but are computationally expensive, whereas the results at the Perdew-Burke-Ernzerhof (PBE) functional level could be effective for suggesting strategies for band structure engineering via electric field and tensile strain aiming to enhance the photocatalytic performance. To illustrate this, in the present manuscript, the DFT based simulation tool VASP is used to investigate the band alignment of nanocomposites in combinations of nanotubes and nanoribbons in the ground state. To address the lifetime of photogenerated holes and electrons in the excited state, nonadiabatic dynamics calculations are needed.

Highly in-plane anisotropic 2D semiconductors ß-AuSe with multiple superior properties: a first-principles investigation.

Discovering highly in-plane anisotropic two-dimensional (2D) semiconductors with multiple superior properties (good stability, widely tunable bandgap and high mobility) are of great interest for fundamental studies and for developments of novel (opto)electronic devices. By means of state-of-the-art first-principles calculations, herein we present a thorough investigation on the stability, electronic properties and promising applications of previously unexplored 2D semiconductors-gold-selenium (ß-AuSe) with strong in-plane anisotropy, whose layered bulk counterpart was synthesized fifty years ago. We show that they have stable structures, widely tunable bandgap varying from 1.66 eV in monolayer to 0.70 eV in five-layer, strong light absorption coefficient (~105 cm-1) within the whole visible light range, and high/ultrahigh carrier mobility (103-105 cm2 V -1 s -1). More importantly, they show highly in-pane anisotropic behaviors in absorption coefficients, photoconductance and carrier mobility. Especially, the anisotropic ratio of carrier mobility is much higher than the literature reported ones. The above findings show that the in-plane anisotropic 2D ß-AuSe are promising candidates for developing polarization-sensitive photodetectors, synaptic devices and micro digital inverters based on multiple superior properties and highly anisotropic behaviors. Besides, few-layer ß-AuSe systems can serve as channel materials in field-effect transistors with high mobility or be applied in solar cells with strong light absorption. Our findings demonstrate that few-layer 2D ß-AuSe have great potential for multifunctional applications and thus stimulate immediately experimental interests.

Atomic-Scale Dynamics and Storage Performance of Na/K on FeF3 Nanosheet.

Developing highly efficient FeF3-based cathode materials for Na/K-ion batteries is greatly needed, which needs long cycling life and rate performance besides large voltage and capacity. Accordingly, we designed a two-dimensional (2D) FeF3 nanosheet to obtain highly efficient Na/K-ion batteries. Moreover, first-principles calculations were implemented to discuss systematically the Na and K storage mechanism on the FeF3(012) nanosheet. The adsorption energies of Na and K are -3.55 and -3.98 eV, respectively, which can guarantee the Na/K loading process. Interestingly, Na and K adatoms on FeF3(012) prefer to get together in the form of the Na dimer and K tetramer, respectively. Energy barrier of the K tetramer is lower than that of the Na dimer (0.43 eV vs 0.45 eV). As a result, the K tetramer possesses a larger diffusion coefficient than the Na dimer (4.22 × 10-10 cm2·s-1 vs 3.32 × 10-10 cm2·s-1). That is to say, good Na/K-ion mobility can be achieved. Also, the FeF3(012) nanosheet exhibits high initial discharge voltage (4.10 V for K and 3.74 V for Na). Moreover, it has a stable discharge voltage curve in Na/K-ion batteries. Besides, the FeF3(012) nanosheet is more favorable to be fabricated as a flexible cathode material for potassium batteries. Therefore, the 2D FeF3 nanosheet belongs to a promising cathode material in Na/K-ion batteries.

The shielding effects of a C60 cage on the magnetic moments of transition metal atoms inside the corner holes of Si(111)-(7 × 7).

The strong interaction between transition metal (TM) atoms and semiconductor surface atoms may diminish the magnetic moments of the TM atoms and prevent them from being used as single atom spin-based devices. A carbon cage that can encapsulate TM atoms and isolate them from interacting with surface atoms is considered to protect the magnetic moments of the TM atoms. We have studied the magnetic moments of Fe, Co, and Ni atoms adsorbed inside the corner hole of Si(111)-(7 × 7) by using first-principles calculations based on the density functional theory. The results show that when Co and Ni atoms are directly adsorbed inside the corner hole, the magnetic moments are 1.353µB and 0, respectively. However when a C60 cage is used to encapsulate the atoms, the magnetic moments increase to 1.849µB and 0.884µB, respectively. The results show a clear protecting effect of a carbon cage. For Fe with and without C60, the magnetic moments are 2.909µB and 2.825µB, respectively. The presence of a C60 cage can also maintain their magnetic moments. Further analysis shows that the TM atoms possess magnetic moments when the conduction electrons are localized around them. All the results can be well understood in the framework of the Anderson impurity model. Our results demonstrate that a carbon cage may effectively protect the magnetic moments of TM atoms. This provides a new strategy for developing single atom spin-based devices on semiconductors.

Novel Molecular Doping Mechanism for n-Doping of SnO2 via Triphenylphosphine Oxide and Its Effect on Perovskite Solar Cells.

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n-type doping of oxide semiconductors. Herein, successful n-type doping of SnO2 is demonstrated by a simple, air-robust, and cost-effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P+ O- σ-bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi-photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO2 , which enlarges the built-in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO2 /perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

Multifunctional two-dimensional semiconductors SnP3: universal mechanism of layer-dependent electronic phase transition.

Two-dimensional (2D) semiconductors SnP3 are predicted, from first-principles calculations, to host moderate band gaps (0.72 eV for monolayer and 1.07 eV for bilayer), ultrahigh carrier mobility (∼104 cm2 V-1 s-1 for bilayer), strong absorption coefficients (∼105 cm-1) and good stability. Moreover, the band gap can be modulated from an indirect character into a direct one via strain engineering. For experimental accessibility, the calculated exfoliation energies of monolayer and bilayer SnP3 are smaller than those of the common arsenic-type honeycomb structures GeP3 and InP3. More importantly, a semiconductor-to-metal transition is discovered with the layer number N > 2. We demonstrate, in remarkable contrast to the previous understandings, that such phase transition is largely driven by the correlation between lone-pair electrons of interlayer Sn and P atoms. This mechanism is universal for analogues phase transitions in arsenic-type honeycomb structures (GeP3, InP3 and SnP3).