Large-Scale Direct Growth of Monolayer MoS2 on Patterned Graphene for van der Waals Ultrafast Photoactive Circuits.

Two-dimensional (2D) van der Waals heterostructures combine the distinct properties of individual 2D materials, resulting in metamaterials, ideal for emergent electronic, optoelectronic, and spintronic phenomena. A significant challenge in harnessing these properties for future hybrid circuits is their large-scale realization and integration into graphene interconnects. In this work, we demonstrate the direct growth of molybdenum disulfide (MoS2) crystals on patterned graphene channels. By enhancing control over vapor transport through a confined space chemical vapor deposition growth technique, we achieve the preferential deposition of monolayer MoS2 crystals on monolayer graphene. Atomic resolution scanning transmission electron microscopy reveals the high structural integrity of the heterostructures. Through in-depth spectroscopic characterization, we unveil charge transfer in Graphene/MoS2, with MoS2 introducing p-type doping to graphene, as confirmed by our electrical measurements. Photoconductivity characterization shows that photoactive regions can be locally created in graphene channels covered by MoS2 layers. Time-resolved ultrafast transient absorption (TA) spectroscopy reveals accelerated charge decay kinetics in Graphene/MoS2 heterostructures compared to standalone MoS2 and upconversion for below band gap excitation conditions. Our proof-of-concept results pave the way for the direct growth of van der Waals heterostructure circuits with significant implications for ultrafast photoactive nanoelectronics and optospintronic applications.

An all phosphorene lattice nanometric spin valve.

Phosphorene is a unique semiconducting two-dimensional platform for enabling spintronic devices integrated with phosphorene nanoelectronics. Here, we have designed an all phosphorene lattice lateral spin valve device, conceived via patterned magnetic substituted atoms of 3d-block elements at both ends of a phosphorene nanoribbon acting as ferromagnetic electrodes in the spin valve. Through First-principles based calculations, we have extensively studied the spin-dependent transport characteristics of the new spin valve structures. Systematic exploration of the magnetoresistance (MR) of the spin valve for various substitutional atoms and bias voltage resulted in a phase diagram offering a colossal MR for V and Cr-substitutional atoms. Such MR can be directly attributed to their specific electronic structure, which can be further tuned by a gate voltage, for electric field controlled spin valves. The spin-dependent transport characteristics here reveal new features such as negative conductance oscillation and switching of the sign of MR due to change in the majority spin carrier type. Our study creates possibilities for the design of nanometric spin valves, which could enable integration of memory and logic elements for all phosphorene 2D processors.

Vacancy-Engineered Nickel Ferrite Forming-Free Low-Voltage Resistive Switches for Neuromorphic Circuits.

Innovations in resistive switching devices constitute a core objective for the development of ultralow-power computing devices. Forming-free resistive switching is a type of resistive switching that eliminates the need for an initial high voltage for the formation of conductive filaments and offers promising opportunities to overcome the limitations of traditional resistive switching devices. Here, we demonstrate mixed charge state oxygen vacancy-engineered electroforming-free resistive switching in NiFe2O4 (NFO) thin films, fabricated as asymmetric Ti/NFO/Pt heterostructures, for the first time. Using pulsed laser deposition in a controlled oxygen atmosphere, we tune the oxygen vacancies together with the cationic valence state in the nickel ferrite phase, with the latter directly affecting the charge state of the oxygen vacancies. The structural integrity and chemical composition of the films are confirmed by X-ray diffraction and hard X-ray photoelectron spectroscopy, respectively. Electrical transport studies reveal that resistive switching characteristics in the films can be significantly altered by tuning the amount and charge state of the oxygen vacancy concentration during the deposition of the films. The resistive switching mechanism is seen to depend upon the migration of both singly and doubly charged oxygen vacancies formed as a result of changes in the nickel valence state and the consequent formation/rupture of conducting filaments in the switching layer. This is supported by the existence of an optimum oxygen vacancy concentration for efficient low-voltage resistive switching, below or above which the switching process is inhibited. Along with the filamentary switching mechanism, the Ti top electrode also enhances the resistive switching performance due to interfacial effects. Time-resolved measurements on the devices display both long- and short-term potentiation in the optimized vacancy-engineered NFO resistive switches, ideal for solid-state synapses achieved in a single system. Our work on correlated oxide forming-free resistive switches holds significant potential for CMOS-compatible low-power, nonvolatile resistive memory and neuromorphic circuits.

High current treated-passivated graphene (CTPG) towards stable nanoelectronic and spintronic circuits.

Achieving enhanced and stable electrical quality of scalable graphene is crucial for practical graphene device applications. Accordingly, encapsulation has emerged as an approach for improving electrical transport in graphene. In this study, we demonstrate high-current treatment of graphene passivated by AlOx nanofilms as a new means to enhance the electrical quality of graphene for its scalable utilization. Our experiments and electrical measurements on large-scale chemical vapor-deposited (CVD) graphene devices reveal that high-current treatment causes persistent and irreversible de-trapping density in both bare graphene and graphene covered by AlOx. Strikingly, despite possible interfacial defects in graphene covered with AlOx, the high-current treatment enhances its carrier mobility by up to 200% in contrast to bare graphene samples, where mobility decreases. Spatially resolved Raman spectroscopy mapping confirms that surface passivation by AlOx, followed by the current treatment, reduces the number of sp3 defects in graphene. These results suggest that for current treated-passivated graphene (CTPG), the high-current treatment considerably reduces charged impurity and trapped charge densities, thereby reducing Coulomb scattering while mitigating any electromigration of carbon atoms. Our study unveils CTPG as an innovative system for practical utilization in graphene nanoelectronic and spintronic integrated circuits.

Strain-controlled spin transport in a two-dimensional (2D) nanomagnet.

Semiconductors with controllable electronic transport coupled with magnetic behaviour, offering programmable spin arrangements present enticing potential for next generation intelligent technologies. Integrating and linking these two properties has been a long standing challenge for material researchers. Recent discoveries in two-dimensional (2D) magnet shows an ability to tune and control the electronic and magnetic phases at ambient temperature. Here, we illustrate controlled spin transport within the magnetic phase of the 2D semiconductor CrOBr and reveal a substantial connection between its magnetic order and charge carriers. First, we systematically analyse the strain-induced electronic behaviour of 2D CrOBr using density functional theory calculations. Our study demonstrates the phase transition from a magnetic semiconductor → half metal → magnetic metal in the material under strain application, creating intriguing spin-resolved conductance with 100% spin polarisation and spin-injection efficiency. Additionally, the spin-polarised current-voltage (I-V) trend displayed conductance variations with high strain-assisted tunability and a peak-to-valley ratio as well as switching efficiency. Our study reveals that CrOBr can exhibit highly anisotropic behaviour with perfect spin filtering, offering new implications for strain engineered magneto-electronic devices.

Twist-assisted optoelectronic phase control in two-dimensional (2D) Janus heterostructures.

Atomically thin two-dimensional (2D) Janus materials and their Van der Waals heterostructures (vdWHs) have emerged as a new class of intriguing semiconductor materials due to their versatile application in electronic and optoelectronic devices. Herein, We have invstigated most probable arrangements of different inhomogeneous heterostructures employing one layer of transition metal dichalcogenide, TMD (MoS2, WS2, MoSe2, and WSe2) piled on the top of Janus TMD (MoSeTe or WSeTe) and investigated their structural, electronic as well as optical properties through first-principles based calculations. After that, we applied twist engineering between the monolayers from 0[Formula: see text] 60[Formula: see text] twist angle, which delivers lattice reconstruction and improves the performance of the vdWHs due to interlayer coupling. The result reveals that all the proposed vdWHs are dynamically and thermodynamically stable. Some vdWHs such as MoS2/MoSeTe, WS2/WSeTe, MoS2/WSeTe, MoSe2/MoSeTe, and WS2/MoSeTe exhibit direct bandgap with type-II band alignment at some specific twist angle, which shows potential for future photovoltaic devices. Moreover, the electronic property and carrier mobility can be effectively tuned in the vdWHs compared to the respective monolayers. Furthermore, the visible optical absorption of all the Janus vdWHs at [Formula: see text] = 0[Formula: see text] can be significantly enhanced due to the weak inter-layer coupling and redistribution of the charges. Therefore, the interlayer twisting not only provides an opportunity to observe new exciting properties but also gives a novel route to modulate the electronic and optoelectronic properties of the heterostructure for practical applications.

Insights and Implications of Intricate Surface Charge Transfer and sp3-Defects in Graphene/Metal Oxide Interfaces.

Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiOx, the AlOx/graphene interface shows the presence of appreciable sp3 defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp3 bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp3 hybridization at the AlOx/graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlOx barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.

Reply to the 'Comment on "Ultralow magnetostrictive flexible ferromagnetic nanowires"' by D. Faurie, N. Challab, M. Haboussi, and F. Zighem, Nanoscale, 2022, 14, DOI: 10.1039/D1NR01773J.

In the comment to our paper, D. Faurie et al. have carried out simulations on Co-nanowires subjected to tensile stress perpendicular to the length of the nanowires. According to their simulation, the low effective magnetostriction constant of the Co nanowires results from a very low transfer of stress. They suggest that a higher transfer of stress would be obtained if the wires are bent along the length of the nanowires. Here we compare the result of magneto-optical experiments conducted by bending the nanowires both along and perpendicular to their long axis. The obtained effective magnetostriction of the Co-nanowires is, within the experimental resolution, independent of the bending direction.

Combined Bottom-Up and Top-Down Approach for Highly Ordered One-Dimensional Composite Nanostructures for Spin Insulatronics.

Engineering magnetic proximity effects-based devices requires developing efficient magnetic insulators. In particular, insulators, where magnetic phases show dramatic changes in texture on the nanometric level, could allow us to tune the proximity-induced exchange splitting at such distances. In this paper, we report the fabrication and characterization of highly ordered two-dimensional arrays of LaFeO3 (LFO)-CoFe2O4 (CFO) biphasic magnetic nanowires, grown on silicon substrates using a unique combination of bottom-up and top-down synthesis approaches. The regularity of the patterns was confirmed using atomic force microscopy and scanning electron microscopy techniques, whereas magnetic force microscopy images established the magnetic homogeneity of the patterned nanowires and absence of any magnetic debris between the wires. Transmission electron microscopy shows a close spatial correlation between the LFO and CFO phases, indicating strong grain-to-grain interfacial coupling, intrinsically different from the usual core-shell structures. Magnetic hysteresis loops reveal the ferrimagnetic nature of the composites up to room temperature and the presence of a strong magnetic coupling between the two phases, and electrical transport measurements demonstrate the strong insulating behavior of the LFO-CFO composite, which is found to be governed by Mott-variable range hopping conduction mechanisms. A shift in the Raman modes in the composite sample compared to those of pure CFO suggests the existence of strain-mediated elastic coupling between the two phases in the composite sample. Our work offers ordered composite nanowires with strong interfacial coupling between the two phases that can be directly integrated for developing multiphase spin insulatronic devices and emergent magnetic interfaces.

Ultralow magnetostrictive flexible ferromagnetic nanowires.

The integration of magneto-electric and spintronic sensors to flexible electronics presents a huge potential for advancing flexible and wearable technologies. Magnetic nanowires are core components for building such devices. Therefore, realizing flexible magnetic nanowires with engineered magneto-elastic properties is key to flexible spintronic circuits, as well as creating unique pathways to explore complex flexible spintronic, magnonic, and magneto-plasmonic devices. Here, we demonstrate highly resilient flexible ferromagnetic nanowires on transparent flexible substrates for the first time. Through extensive magneto-optical Kerr experiments, exploring the Villari effect, we reveal an ultralow magnetostrictive constant in nanowires, a two-order reduced value compared to bulk values. In addition, the flexible magnetic nanowires exhibit remarkable resilience sustaining bending radii ∼5 mm, high endurance, and enhanced elastic limit compared to thin films of similar thickness and composition. The observed performance is corroborated by our micro-magnetic simulations and can be attributed to the reduced size and strong nanostructure-interfacial effects. Such stable magnetic nanowires with ultralow magnetostriction open up new opportunities for stable surface mountable and wearable spintronic sensors, advanced nanospintronic circuits, and for exploring novel strain-induced quantum effects in hybrid devices.

Highly-efficient growth of cobalt nanostructures using focused ion beam induced deposition under cryogenic conditions: application to electrical contacts on graphene, magnetism and hard masking.

Emergent technologies are required in the field of nanoelectronics for improved contacts and interconnects at nano and micro-scale. In this work, we report a highly-efficient nanolithography process for the growth of cobalt nanostructures requiring an ultra-low charge dose (15 µC cm-2, unprecedented in single-step charge-based nanopatterning). This resist-free process consists in the condensation of a ∼28 nm-thick Co2(CO)8 layer on a substrate held at -100 °C, its irradiation with a Ga+ focused ion beam, and substrate heating up to room temperature. The resulting cobalt-based deposits exhibit sub-100 nm lateral resolution, display metallic behaviour (room-temperature resistivity of 200 µΩ cm), present ferromagnetic properties (magnetization at room temperature of 400 emu cm-3) and can be grown in large areas. To put these results in perspective, similar properties can be achieved by room-temperature focused ion beam induced deposition and the same precursor only if a 2 × 103 times higher charge dose is used. We demonstrate the application of such an ultra-fast growth process to directly create electrical contacts onto graphene ribbons, opening the route for a broad application of this technology to any 2D material. In addition, the application of these cryo-deposits for hard masking is demonstrated, confirming its structural functionality.

Ultimate Spin Currents in Commercial Chemical Vapor Deposited Graphene.

Establishing ultimate spin current efficiency in graphene over industry-standard substrates can facilitate research and development exploration of spin current functions and spin sensing. At the same time, it can resolve core issues in spin relaxation physics while addressing the skepticism of graphene's practicality for planar spintronic applications. In this work, we reveal an exceptionally long spin communication capability of 45 µm and highest to date spin diffusion length of 13.6 µm in graphene on SiO2/Si at room temperature. Employing commercial chemical vapor deposited (CVD) graphene, we show how contact-induced surface charge transfer doping and device doping contributions, as well as spin relaxation, can be quenched in extremely long spin channels and thereby enable unexpectedly long spin diffusion lengths in polycrystalline CVD graphene. Extensive experiments show enhanced spin transport and precession in multiple longest channels (36 and 45 µm) that reveal the highest spin lifetime of ∼2.5-3.5 ns in graphene over SiO2/Si, even under ambient conditions. Such performance, made possible due to our devices approaching the intrinsic spin-orbit coupling of ∼20 µeV in graphene, reveals the role of the D'yakonov-Perel' spin relaxation mechanism in graphene channels as well as contact regions. Our record demonstration, fresh device engineering, and spin relaxation insights unlock the ultimate spin current capabilities of graphene on SiO2/Si, while the robust high performance of commercial CVD graphene can proliferate research and development of innovative spin sensors and spin computing circuits.

Bi-stimuli assisted engineering and control of magnetic phase in monolayer CrOCl.

Magnetic phase control and room temperature magnetic stability in two-dimensional (2D) materials are indispensable for realising advanced spintronic and magneto-electronic functions. Our current work employs first-principles calculations to comprehensively study the magnetic behaviour of 2D CrOCl, uncovering the impact of strain and electric field on the material. Our studies have revealed that uniaxial strain leads to the feasibility of room temperature ferromagnetism in the layer and also detected the occurrence of a ferromagnetic → antiferromagnetic phase transition in the system, which is anisotropic along the armchair and zigzag directions. Beyond such a strain effect, the coupling of strain and electric field leads to a remarkable enhancement of the Curie temperature (Tc) ∼ 450 K in CrOCl. These predictions based on our detailed simulations show the prospect of multi-stimuli magnetic phase control, which could have great significance for realizing magneto-mechanical sensors.

High efficiency spin filtering in magnetic phosphorene.

Phosphorene has a unique set of characteristics such as a semiconducting nature, good carrier mobility and low-spin orbit coupling aspects which makes it a highly prospective two dimensional material for cross-hybrid architectures in nanoelectronics, spintronics, and optoelectronics. In the spintronic context, the creation of a stable magnetic order in phosphorene can be immensely beneficial for designing phosphorene spin circuits. In this work, we present high efficiency spin filtering behaviour in magnetically rendered phosphorene. First, we calculate the effect of doping various 3d block elements in phosphorene to introduce a stable magnetic order. Next, by varying doping concentrations in distinct doping configurations, an extensive phase diagram has been obtained depicting the presence of various electronic and magnetic states. This allows us to achieve a high magnetisation in the presence of various transition metal atoms, with a spin polarisation of ∼100% in half-metallic regimes. The transport behaviour reveals a map of the spin injection efficiency showing enhancement with doping concentration and reaching a perfect spin filtering capacity of ∼100% in the presence of Ti, Cr, Mn, Co, and Fe atoms. The present results offer new insights into engineered designs of multi-functional phosphorene spintronic circuits.

Flexible transparent graphene laminates via direct lamination of graphene onto polyethylene naphthalate substrates.

Graphene, with its excellent electrical, mechanical, and optical properties, has emerged as an exceptional material for flexible and transparent nanoelectronics. Such versatility makes it compelling to find new pathways to lay graphene sheets onto smooth, flexible substrates to create large-scale flexible transparent graphene conductors. Here, we report the realization of flexible transparent graphene laminates by direct adhesion of chemical vapor deposition (CVD) graphene on a polyethylene naphthalate (PEN) substrate, which is an emerging standard for flexible electronics. By systematically optimizing the conditions of a hot-press technique, we have identified that applying optimum temperature and pressure can make graphene directly adhere to flexible PEN substrates without any intermediate layer. The resultant flexible graphene films are transparent, have a standard sheet resistance of 1 kΩ with high bending resilience, and high optical transmittance of 85%. Our direct hot-press method is achieved below the glass transition temperature of the PEN substrate. Furthermore, we demonstrate press-assisted embossing for patterned transfer of graphene, and hence it can serve as a reliable new means for creating universal, transparent conducting patterned films for designing flexible nanoelectronic and optoelectronic components.

Dramatic magnetic phase designing in phosphorene.

Phosphorene is a unique two-dimensional semiconductor that exhibits huge potential for nanoelectronic, optoelectronic and spintronic applications and their cross-hybrid electronics. In particular, creation of magnetic phases in phosphorene selectively can provide a multitude of opportunities for developments in 2D spintronic circuits. Doping phosphorene with transition metal atoms can induce sustainable magnetic ordering, making it a diluted magnetic system, however, the viability of high temperature magnetic phases and potential control remain unanswered. In this work, using first-principles calculations, we uncover the impact of doping phosphorene with various 3d block elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) in increasing order of atomic number at various levels of doping. Such an extensive study helps us to find the doping conditions that lead to remarkable feasibility of ferromagnetism and antiferromagnetism up to a strikingly large temperature ∼1150 K, evaluated by mean field theory. The doping concentration and atom type can be used to systematically tune the phases from ferromagnetic and antiferromagnetic to non-magnetic ground states. Our work provides new guidelines for engineering multi-functional spintronic components using phosphorene as a base material for all-phosphorene spintronics.

Two-Dimensional Flexible High Diffusive Spin Circuits.

Owing to their unprecedented electronic properties, graphene and two-dimensional (2D) crystals have brought fresh opportunities for advances in planar spintronic devices. Graphene is an ideal medium for spin transport while being an exceptionally resilient material for flexible nanoelectronics. However, these extraordinary traits have never been combined to create flexible graphene spin circuits. Realizing such circuits could lead to bendable strain-spin sensors, as well as a unique platform to explore pure spin current based operations and low-power 2D flexible nanoelectronics. Here, we demonstrate graphene spin circuits on flexible substrates for the first time. Despite the rough topography of the flexible substrates, these circuits prepared with chemical vapor deposited monolayer graphene reveal an efficient room temperature spin transport with distinctively large spin diffusion coefficients ∼0.2 m2 s-1. Compared to earlier graphene devices on Si/SiO2 substrates, such values are up to 20 times larger, leading to one order higher spin signals and an enhanced spin diffusion length ∼10 µm in graphene-based nonlocal spin valves fabricated using industry standard systems. This high performance arising out of a characteristic substrate terrain shows promise of a scalable and flexible platform towards flexible 2D spintronics. Our innovation is a key step for the exploration of strain-dependent 2D spin phenomena and paves the way for flexible graphene spin memory-logic units and planar spin sensors.

Unconventional strain-dependent conductance oscillations in pristine phosphorene.

Phosphorene is a single elemental, two-dimensional semiconductor that has quickly emerged as a high mobility material for transistors and optoelectronic devices. In addition, being a 2D material it can sustain high levels of strain, enabling sensitive modification of its electronic properties. In this paper, we investigate the strain dependent electronic properties of phosphorene nanocrystals. By performing extensive calculations we determine the electrical conductance as a function of uniaxial, as well as biaxial strain stimuli and uncover a unique zone phase diagram. This enables us to uncover conductance oscillations in pristine phosphorene for the first time, by the simple application of strain. We show that such unconventional current-voltage behaviour is tuneable by the nature of strain, and that an additional gate voltage can modulate the amplitude (peak to valley ratio) of the observed phenomena and its switching efficiency. Furthermore, we show that the switching is highly robust against doping and defects. Our detailed results present new leads for innovation in strain based gauging and high-frequency nanoelectronic switches of phosphorene.

Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide.

The two-dimensional (2D) semiconductor molybdenum disulfide (MoS2) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS2 (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5-2% has been observed, corresponding to spin polarization of 5-10% in the measured temperature range of 300-75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS2 spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance.

Inversion of Spin Signal and Spin Filtering in Ferromagnet|Hexagonal Boron Nitride-Graphene van der Waals Heterostructures.

Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.