Phase-Dependent Magnetic Proximity Modulations on Valley Polarization and Splitting.

Proximate-induced magnetic interactions present a promising strategy for precise manipulation of valley degrees of freedom. Taking advantage of the splendid valleytronic platform of transition metal dichalcogenides, magnetic two-dimensional VSe2 with different phases are introduced to intervene in the spin of electrons and modulate their valleytronic properties. When constructing the heterostructures, 1T-VSe2/WX2 (X = S and Se) showcases significant improvement in the valley polarizations at room temperature, while 2H-VSe2/WX2 exhibits superior performance at low temperatures and demonstrates heightened sensitivity to the external magnetic field. Simultaneously, considerable valley splitting with a large geff factor up to -29.0 is observed in 2H-VSe2/WS2, while it is negligible in 1T-VSe2/WX2. First-principles calculations reveal a phase-dependent magnetic proximity mechanism on the valleytronic modulations, which is dominated by interfacial charge transfer in 1T-VSe2/WX2 and the proximity exchange field in 2H-VSe2/WX2 heterostructures. The effective control over valley degrees of freedom will bridge the valleytronic physics and devices, rendering enormous potential in the field of valley quantum applications.

Ferromagnetic Insulating Ground-State Resolved in Mixed Protons and Oxygen Vacancies-Doped La0.67Sr0.33CoO3 Thin Films via Ionic Liquid Gating.

The realization of ferromagnetic insulating ground state is a critical prerequisite for spintronic applications. By applying electric field-controlled ionic liquid gating (ILG) to stoichiometry La0.67Sr0.33CoO3 thin films, the doping of protons (H+) has been achieved for the first time. Furthermore, a hitherto-unreported ferromagnetic insulating phase with a remarkably high Tc up to 180 K has been observed which can be attributed to the doping of H+ and the formation of oxygen vacancies (VO). The chemical formula of the dual-ion migrated film has been identified as La2/3Sr1/3CoO8/3H2/3 based on combined Co L23-edge absorption spectra and configuration interaction cluster calculations, from which we are able to explain the ferromagnetic ground state in terms of the distinct magnetic moment contributions from Co ions with octahedral (Oh) and tetrahedral (Td) symmetries following antiparallel spin alignments. Further density functional theory calculations have been performed to verify the functionality of H+ as the transfer ion and the origin of the novel ferromagnetic insulating ground state. Our results provide a fundamental understanding of the ILG regulation mechanism and shed light on the manipulating of more functionalities in other correlated compounds through dual-ion manipulation.

Material Design of Ultra-Thin InN/GaN Superlattices for a Long-Wavelength Light Emission.

GaN heterostructure is a promising material for next-generation optoelectronic devices, and Indium gallium nitride (InGaN) has been widely used in ultraviolet and blue light emission. However, its applied potential for longer wavelengths still requires exploration. In this work, the ultra-thin InN/GaN superlattices (SL) were designed for long-wavelength light emission and investigated by first-principles simulations. The crystallographic and electronic properties of SL were comprehensively studied, especially the strain state of InN well layers in SL. Different strain states of InN layers were applied to modulate the bandgap of the SL, and the designed InN/GaN heterostructure could theoretically achieve photon emission of at least 650 nm. Additionally, we found the SL had different quantum confinement effects on electrons and holes, but an efficient capture of electron-hole pairs could be realized. Meanwhile, external forces were also considered. The orbital compositions of the valence band maximum (VBM) were changed with the increase in tensile stress. The transverse electric (TE) mode was found to play a leading role in light emission in normal working conditions, and it was advantageous for light extraction. The capacity of ultra-thin InN/GaN SL on long-wavelength light emission was theoretically investigated.

Simultaneously Regulated Highly Polarized and Long-Lived Valley Excitons in WSe2/GaN Heterostructures.

Interlayer excitons, with prolonged lifetimes and tunability, hold potential for advanced optoelectronics. Previous research on the interlayer excitons has been dominated by two-dimensional heterostructures. Here, we construct WSe2/GaN composite heterostructures, in which the doping concentration of GaN and the twist angle of bilayer WSe2 are employed as two ingredients for the manipulation of exciton behaviors and polarizations. The exciton energies in monolayer WSe2/GaN can be regulated continuously by the doping levels of the GaN substrate, and a remarkable increase in the valley polarizations is achieved. Especially in a heterostructure with 4°-twisted bilayer WSe2, a maximum polarization of 38.9% with a long lifetime is achieved for the interlayer exciton. Theoretical calculations reveal that the large polarization and long lifetime are attributed to the high exciton binding energy and large spin flipping energy during depolarization in bilayer WSe2/GaN. This work introduces a distinctive member of the interlayer exciton with a high degree of polarization and a long lifetime.

Manipulations of Electronic and Spin States in Co-Quantum Dot/WS2 Heterostructure on a Metal-Dielectric Composite Substrate by Controlling Interfacial Carriers.

Charge and spin are two intrinsic attributes of carriers governing almost all of the physical processes and operation principles in materials. Here, we demonstrate the manipulation of electronic and spin states in designed Co-quantum dot/WS2 (Co-QDs/WS2) heterostructures by employing a metal-dielectric composite substrate and via scanning tunneling microscope. By repeatedly scanning under a unipolar bias, switching the bias polarity, or applying a pulse through nonmagnetic or magnetic tips, the Co-QDs morphologies exhibit a regular and reproducible transformation between bright and dark dots. First-principles calculations reveal that these tunable characters are attributed to the variation of density of states and the transition of magnetic anisotropy energy induced by carrier accumulation. It also suggests that the metal-dielectric composite substrate is successful in creating the interfacial potential for carrier accumulation and realizes the electrically controllable modulations. These results will promote the exploration of electron-matter interactions in quantum systems and provide an innovative way to facilitate the development of spintronics.

Enhanced light extraction efficiency of GaN-based green micro-LED modulating by a thickness-tunable SiO2 passivation structure.

Green micro-light emitting diodes (micro-LEDs) is one of the three primary color light sources as full-color display, which serves as a key research object in the field of micro-LED display. As the micro-LED size decreases, the surface-area-to-volume ratio of the device increases, leading to more serious damage on the sidewall by inductively coupled plasma (ICP) etching. The passivation process of SiO2 provides an effective method to reduce sidewall damage caused by ICP etching. In this work, green rectangular micro-LEDs with passivation layer thickness of 0∼600 nm was designed using the finite-difference time-domain (FDTD) simulation. In order to verify the simulation results, the micro-LED array was fabricated by parallel laser micro-lens array (MLA) lithography in high speed and large area. The effect of the SiO2 passivation layer thickness on the performance of the green micro-LED was analyzed, which shows that the passivation layer thickness-light extraction efficiency curve fluctuates periodically. For the sample with 90 nm thickness of SiO2 passivation layer, there exists a small leakage current and higher operating current density, and the maximum external quantum efficiency (EQE) is 2.8 times higher than micro-LED without SiO2 passivation layer.

Controllable Resistive Switching in ReS2 /WS2 Heterostructure for Nonvolatile Memory and Synaptic Simulation.

Memristors with nonvolatile storage performance and simulated synaptic functions are regarded as one of the critical devices to overcome the bottleneck in traditional von Neumann computer architecture. 2D van der Waals heterostructures have paved a new way for the development of advanced memristors by integrating the intriguing features of different materials and offering additional controllability over their optoelectronic properties. Herein, planar memristors with both electrical and optical tunability based on ReS2 /WS2 van der Waals heterostructure are demonstrated. The devices show unique unipolar nonvolatile behavior with high Roff /Ron ratio of up to 106 , desirable endurance, and retention, which are superior to pure ReS2 and WS2 devices. When decreasing the channel length, the set voltage can be notably reduced while the high Roff /Ron ratios are retained. By introducing electrostatic doping through the gate control, the set voltage can be tailored in a wide range from 4.50 to 0.40 V. Furthermore, biological synaptic functions and plasticity, including spike rate-dependent plasticity and paired-pulse facilitation, are successfully realized. By employing optical illumination, resistive switching can also be modulated, which is dependent on the illumination energy and power. A mechanism related to the interlayer charge transfer controlled by optical excitation is revealed.

In-depth insights into polarization-dependent light extraction mechanisms of AlGaN-based deep ultraviolet light-emitting diodes.

The AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) dominated by transverse-magnetic (TM) polarized emission suffer from extremely poor light extraction efficiency (LEE) from their top surface, which severely limits the device performance. In this study, the underlying physics of polarization-dependent light extraction mechanisms of AlGaN-based DUV LEDs has been explored in depth via simple Monte Carlo ray-tracing simulations with Snell's law. It is especially worth noting that the structures of the p-type electron blocking layer (p-EBL) and multi-quantum wells (MQWs) have a significant impact on light extraction behavior, especially for TM-polarized emission. Thus, an artificial vertical escape channel (named GLRV) has been constructed to efficiently extract the TM-polarized light through the top surface, by adjusting the structures of the p-EBL, MQWs, sidewalls, and using the adverse total internal reflection in a positive manner. The results show that the enhancement times of the top-surface LEE is up to 18 for TM-polarized emission in the 300 × 300 µm2 chip comprising a single GLRV structure, and further increases to 25 by dividing this single GLRV structure into a 4 × 4 micro-GLRV array structure. This study provides a new perspective for understanding and modulating the extraction mechanisms of polarized light to overcome the inherently poor LEE for the TM-polarized light.

P-type nitrogen-doped ß-Ga2O3: the role of stable shallow acceptor NO-VGa complexes.

In-depth understanding of the acceptor states and origins of p-type conductivity is essential and critical to overcome the great challenge for the p-type doping of ultrawide-bandgap oxide semiconductors. In this study we find that stable NO-VGa complexes can be formed with ε(0/-) transition levels significantly smaller than those of the isolated NO and VGa defects using N2 as the dopant source. Due to the defect-induced crystal-field splitting of the p orbitals of Ga, O and N atoms, and the Coulomb binding between NO(II) and VGa(I), an a' doublet state at 1.43 eV and an a'' singlet state at 0.22 eV above the valence band maximum (VBM) are formed for the ß-Ga2O3:NO(II)-VGa(I) complexes with an activated hole concentration of 8.5 × 1017 cm-3 at the VBM, indicating the formation of a shallow acceptor level and the feasibility to obtain p-type conductivity in ß-Ga2O3 even when using N2 as the dopant source. Considering the transition from NO(II)-V0Ga(I) + e to NO(II)-V-Ga(I), an emission peak at 385 nm with a Franck-Condon shift of 1.08 eV is predicted. These findings are of general scientific significance as well as technological application significance for p-type doping of ultrawide-bandgap oxide semiconductors.

Rhodium-embedded UV photodetectors based on localized surface plasmon resonance on AlN/GaN.

We report a remarkably enhanced responsivity of metal-semiconductor-metal photodetectors embedded with a large-scale periodicity and highly uniform rhodium nanoparticle array based on localized surface plasmon resonance. In this study, we used theoretical simulations of the absorption, scattering, and extinction behaviors, as well as the near electromagnetic field distributions to predict the plasmon resonance wavelength of quasi-triangular-shaped rhodium nanoparticles. More specifically, we successfully implemented a hexagonal close-packed structure with the individual quasi-triangular-shaped rhodium nanoparticle on the AlN/GaN structure by self-assembly nanosphere technology. The characterization results showed that the device embedded with rhodium nanoparticles had a reduced dark current of 7 × 10-14 A, and the maximum responsivity was shifted to a longer wavelength of approximately 310 nm compared to the device without rhodium nanoparticles. Moreover, at a wavelength of 324 nm, the enhancement ratio of the responsivity was as high as 56. Our study makes a significant contribution to the literature with a highly uniform, large-scale distributed rhodium nanoparticle array for enhancing the performance of AlGaN-based photodetectors in the UV region.

Reply to the 'Comment on "Regulating the valence level arrangement of high-Al-content AlGaN quantum wells using additional potentials with Mg doping"' by W. Lambrecht, Phys. Chem. Chem. Phys., 2023, 25, DOI: 10.1039/D2CP01080A.

We are happy to receive the attention of Prof. Lambrecht regarding our paper and we appreciate his comments. However, it is hard to agree with his judgement about the "incorrect application", "incorrect interpretation", and the work being "misleading". Therefore, we would like to provide a defense and further discussion in this reply.

Modulation of ZnO Nanostructure for Efficient Photocatalytic Performance.

Structure has been considered to play an important role in photocatalytic performance of the semiconductors, but the intrinsic factors were rarely revealed. Herein, ZnO nanomaterials in the structures of thin film, nanowire array and nanosheet array were synthesized, and their structural characteristics, optical properties, photocurrent response and photocatalytic efficiency were compared with each other for illustrating the issue. The photoluminescence intensity decreased in the order of nanosheets, thin film and nanowires for improved lifetime of the photoexcited charges. The absorption of the nanosheets and nanowires improved obviously in the visible range with a redshift of the absorption edge than that of the thin film. The nanowires possessed the highest response current of 82.65 µA at a response time of 2.0 ms in a sensitivity of 87.93 at the light frequency of 1 Hz, and gained the largest catalytic efficiency of 2.45 µg/cm2 h for the methylene blue degradation in UV light. Nevertheless, the improvement of catalytic efficiency of the nanosheets (up to 42.4%) was much larger than that of nanowires (5.7%) and thin film (2.6%) for the Au coating. The analysis revealed that the photocatalytic efficiency of the ZnO nanomaterials was modulated by the structure as it contained different surface area, roughness, defect and doping states, vacancies, polar and non-polar crystalline faces, which would provide structural design of semiconductor nanomaterials for the photoelectric and photocatalytic applications.

Nonvolatile Electrical Valley Manipulation in WS2 by Ferroelectric Gating.

Valleytronics in transition metal dichalcogenides has been intensively investigated for potential applications in next-generation information storage, data processing, and signal transmission devices. Here a ferroelectric gating approach is engaged in achieving nonvolatile electrical tuning of the valley-excitonic properties of monolayer and bilayer WS2. The gating effects include carrier doping and ferroelectric coupling, which are further distinguished by comparing two geometries where the gate electrodes are in direct contact with or insulated from the WS2 crystal. The results show that the carrier doping from gate electrodes acts on WS2 through carrier screening, which only moderately alters the valley polarization. In contrast, the ferroelectric gating promotes electron-phonon interaction, introduces a strong surface polarization field, and controls the interfacial charge trapping/detrapping, causing a Stark shift in exciton energy and strongly enhancing room-temperature valley polarization. In bilayer WS2, the intralayer-interlayer exciton transition is further induced, contributing to even higher valley polarization. The ferroelectric coupling effect can still be maintained after the removal of gate voltage, showing its nonvolatile nature. The role of ferroelectricity is further verified by the anomalous temperature dependence in valley polarization. This work has revealed effective electrical control over valley excitons in semiconductors through interaction with ferroelectric materials. The reported high room-temperature valley polarization in WS2 will boost the development of valleytronics devices.

Band alignment of ZnO-based nanorod arrays for enhanced visible light photocatalytic performance.

A ternary semiconductor ZnO/MoS2/Ag2S nanorod array in an intimate core-shell structure was synthesized on glass substrates. The physicochemical properties and photocatalytical performance of the specimen were characterized and compared with single ZnO and binary ZnO/Ag2S and ZnO/MoS2 nanorod arrays. It is found that the coating layers depressed the band edge emission of the ZnO core, improved light absorption in the visible range, reduced charge transfer resistance, and increased photocatalytic activity. The ternary heterojunction nanorod array possessed full solar absorption with an efficiency of 52.88% for the degradation of methylene blue under visible light in 30 min. The efficiency was higher than other arrays and was 7.6 times that of the ZnO array. Theory analysis revealed that the coating layer brought different band alignment in the heterojunctions for efficient charge separation and conduction, which was beneficial for the photocatalytic performance.

Simplified numerical modeling for Fano interference-induced asymmetric light reflectance effect using equivalent medium theory.

Localized surface plasmons exhibit promising capabilities in optoelectronic devices. In most cases, the metal nanoparticle arrays are located on interfaces or inside optical cavities. Fano interferences have been observed and explained via the interference between the waves generated by the localized surface plasmon and dielectric interfaces. Conventionally, these Fano interferences are modeled using the modified Fresnel equation. However, certain issues persist in the fundamental physics or in the numerical calculation process. Here, we adopt the equivalent medium theory (Maxwell-Garnett theory, MGT) to calculate and elucidate Fano interferences in different structures, in the region comprising nanoparticle arrays and dielectrics equivalent to a homogeneous layer of media via the mean field theory. Using this method, the Fano interference can be modeled by mixing different materials, i.e., metals and dielectrics in these cases. Furthermore, a multiple-layered equivalent medium theory is proposed to significantly improve the scalability of this simplified numerical method. In other words, this method can be easily extended to nanoparticles with different shapes, sizes, and materials; in addition, it exhibits robust practicability. Compared with the modified Fresnel equation and finite-difference time-domain methods, this MGT-based method can effectively minimize the calculation process, which is beneficial to the prospective application of plasmon photonics.

High-Efficient Spin Injection in GaN at Room Temperature Through A Van der Waals Tunnelling Barrier.

Achieving high-efficient spin injection in semiconductors is critical for developing spintronic devices. Although a tunnel spin injector is typically used, the construction of a high-quality tunnel barrier remains a significant challenge due to the large lattice mismatch between oxides and semiconductors. In this work, van der Waals h-BN films with the atomically flat interface were engaged as the tunnel barrier to achieve high spin polarization in GaN, and the spin injection and transport in GaN were investigated systematically. Based on the Hanle precession and magnetic resistance measurements, CoFeB was determined as an optimal spin polarizer, bilayer h-BN tunnelling barrier was proven to yield a much higher spin polarization than the case of monolayer, and appropriate carrier concentration as well as higher crystal equality of n-GaN could effectively reduce the defect-induced spin scattering to improve the spin transport. The systematic understanding and the high efficiency of spin injection in this work may pave the way to the development of physical connotations and the applications of semiconductor spintronics.

Colorful Conductive Threads for Wearable Electronics: Transparent Cu-Ag Nanonets.

Electronic textiles have been regarded as the basic building blocks for constructing a new generation of wearable electronics. However, the electronization of textiles often changes their original properties such as color, softness, glossiness, or flexibility. Here a rapid room-temperature fabrication method toward conductive colorful threads and fabrics with Ag-coated Cu (Cu-Ag) nanonets is demonstrated. Cu-Ag core-shell nanowires are produced through a one-pot synthesis followed by electroless deposition. According to the balance of draining and entraining forces, a fast dip-withdraw process in a volatile solution is developed to tightly wrap Cu-Ag nanonets onto the fibers of thread. The modified threads are not only conductive, but they also retain their original features with enhanced mechanical stability and dry-wash durability. Furthermore, various e-textile devices are fabricated such as a fabric heater, touch screen gloves, a wearable real-time temperature sensor, and warm fabrics against infrared thermal dissipation. These high quality and colorful conductive textiles will provide powerful materials for promoting next-generation applications in wearable electronics.

Towards n-type conductivity in hexagonal boron nitride.

Asymmetric transport characteristic in n- and p-type conductivity has long been a fundamental difficulty in wide bandgap semiconductors. Hexagonal boron nitride (h-BN) can achieve p-type conduction, however, the n-type conductivity still remains unavailable. Here, we demonstrate a concept of orbital split induced level engineering through sacrificial impurity coupling and the realization of efficient n-type transport in 2D h-BN monolayer. We find that the O 2pz orbital has both symmetry and energy matching to the Ge 4pz orbital, which promises a strong coupling. The introduction of side-by-side O to Ge donor can effectively push up the donor level by the formation of another sacrificial deep level. We discover that a Ge-O2 trimer brings the extremely shallow donor level and very low ionization energy. By low-pressure chemical vapor deposition method, we obtain the in-situ Ge-O doping in h-BN monolayer and successfully achieve both through-plane (~100 nA) and in-plane (~20 nA) n-type conduction. We fabricate a vertically-stacked n-hBN/p-GaN heterojunction and show distinct rectification characteristics. The sacrificial impurity coupling method provides a highly viable route to overcome the n-type limitation of h-BN and paves the way for the future 2D optoelectronic devices.

Enhanced Faraday effects of magneto-plasmonic crystals with plasmonic hexagonal hole arrays.

Magneto-optical (MO) properties of the bilayed Au/BIG and trilayered Au/BIG/Au magneto-plasmonic crystals (MPCs) were analyzed by the finite-difference time-domain method. In contrast to the low deflection angle and transmission of the smooth thin film, all the heterostructures with perforated holes in the top Au film displayed a similar trend with two strong resonant bands in Faraday rotation and transmittance in the near infrared wavelength range. The bands and electric distribution relative to the component and hole structure were revealed. The MPC with plasmonic hexagonal holes was found to own superior Faraday effects with distinctive anisotropy. The evolution of the resonant bands with the size and period of hexagonal holes, the thickness of different layers, and the incident light polarization was illustrated. The Faraday rotation of the optimized bilayed and trilayered hexagonal MPCs was improved 15.3 and 17.5 times, and the transmittance was enhanced 12.1 and 11.1 folds respectively at the resonant wavelength in comparison to the continuous Au/BIG film, indicating that the systems might find potential application in MO devices.

Upconversion under Photon Trapping in ZnO/BN Nanoarray: An Ultrahigh Responsivity Solar-Blind Photodetecting Paper.

Solar-blind photodetectors (PDs) are widely applicable in special, military, medical, environmental, and commercial fields. However, high performance and flexible PD for deep ultraviolet (UV) range is still a challenge. Here, it is demonstrated that an upconversion of photon absorption beyond the energy bandgap is achieved in the ZnO nanoarray/h-BN heterostructure, which enables the ultrahigh responsivity of a solar-blind photodetecting paper. The direct growth of ultralong ZnO nanoarray on polycrystalline copper paper induced by h-BN 2D interlayer is obtained. Meanwhile, strong photon trapping takes place within the ZnO nanoarray forest through the cyclic state transition of surface oxygen ions, resulting in an extremely high absorption efficiency (> 99.5%). A flexible photodetecting paper is fabricated for switchable detections between near UV and deep UV signals by critical external bias. The device shows robust reliability, ultrahigh responsivity up to 700 A W-1 @ 265-276 nm, and high photoconductive gain of ≈2 × 103 . A negative differential resistance effect is revealed for driving the rapid transfer of up-converted electrons between adjacent energy valleys (Γ to A) above the critical bias (3.9 V). The discovered rationale and device structure are expected to bring high-efficiency deep UV detecting and future wearable applications.