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Two-dimensional (2D) layered materials have an atomically thin and flat nature which makes it an ultimate candidate for spintronic devices. The spin-valve junctions (SVJs), composed of 2D materials, have been recognized as unique features of spin transport polarization. However, the magnetotransport properties of SVJs are highly influenced by the type of intervening layer (spacer) inserted between the ferromagnetic materials (FMs). In this situation, the spin filtering effect at the interfaces plays a critical role in the observation of the magnetoresistance (MR) of such magnetic structures, which can be improved by using promising hybrid structure. Here, we report MR of bilayer graphene (BLG), single-layer MoSe2 (SL-MoSe2), and BLG/SL-MoSe2 heterostack SVJs. However, before annealing, BLG and SL-MoSe2 SVJs demonstrate positive MR, but after annealing, BLG reverses its polarity while the SL-MoSe2 maintains its polarity and demonstrated stable positive spin polarizations at both interfaces due to meager doping effect of ferromagnetic (FM) contacts. Further, Co/BLG/SL-MoSe2/NiFe determines positive MR, i.e., ~ 1.71% and ~ 1.86% at T = 4 K before and after annealing, respectively. On the contrary, NiFe/BLG/SL-MoSe2/Co SVJs showed positive MR before annealing and subsequently reversed its MR sign after annealing due to the proximity-induced effect of metals doping with graphene. The obtained results can be useful to comprehend the origin of polarity and the selection of non-magnetic material (spacer) for magnetotransport properties. Thus, this study established a new paragon for novel spintronic applications.

Vertical integration of two dimensional (2D) layered materials is indispensable in making van der Waals (vdWs) heterostructures for promising electronic and optoelectronic devices. Herein, we report excellent electrical and photoelectrical measurements where the current ON & OFF ratio of FET is increased by decreasing the temperature in the graphene/ReSe2/graphene heterojunction. We investigated the photoresponsivity in broad spectral range (UV-Vis-NIR) and achieved high photoresponsivity of 1.5 × 107 A W−1 and external quantum efficiency of ∼64% at λ = 220 nm. Further, the photovoltaic effect was examined, which significantly modulated the short circuit current (Isc) from 4.2 × 10−8 A to 2.6 × 10−7 A and open-circuit voltage (Voc) from 0.21 V to 0.44 V at different wavelengths (1064, 840, 514 and 220 nm), attributed to the photo-generation and recombination rate of the carriers. Moreover, photoresponsivity was observed near 1.2 × 106, 8.6 × 106 and 1.5 × 107 A W−1 by applying different gate biases (0, 20 and 40 V), respectively. Further, we have explored the photocurrent and photoresponsivity at different intensities of incident light (200, 260, 400, 620 and 850 μW cm−2). In addition, we calculated the rise and decay response times of photodetectors at different wavelengths and power densities, which depend upon the trap sites in the energy band of ReSe2. These devices opened up new ways to improve the performance of photodetectors from the UV to the NIR region.

openaire: EC/H2020/834742/EU//ATOP Two-dimensional (2D) transition metal dichalcogenides have attracted vibrant interest for future solid-state device applications due to their unique properties. However, it is challenging to realize 2D material based high performance complementary devices due to the stubborn Fermi level pinning effect and the lack of facile doping techniques. In this paper, we reported a hybrid Gr/Ni contact to WS2, which can switch carrier types from n-type to p-type in WS2. The unorthodox polarity transition is attributed to the natural p-doping of graphene with Ni adsorption and the alleviation of Fermi level pinning in WS2. Furthermore, we realized asymmetric Ni and Gr/Ni hybrid contacts to a multilayer WS2 device, and we observed synergistic p-n diode characteristics with excellent current rectification exceeding 104, and a near unity ideality factor of 1.1 (1.6) at a temperature of 4.5 K (300 K). Lastly, our WS2 p-n device exhibits high performance photovoltaic ability with a maximum photoresponsivity of 4 × 104 A W-1 at 532 nm wavelength, that is 108 times higher than that of graphene and 50 times better than that of the monolayer MoS2 photodetector. This doping-free carrier type modulation technique will pave the way to realize high performance complementary electronics and optoelectronic devices based on 2D materials.

NiMnSn ferromagnetic shape memory alloys exhibit martensitic transformation at low temperatures, restricting their applications. Therefore, this is a key factor in improving the martensitic transformation temperature, which is effectively carried out by proper element doping. In this research, we investigated the martensitic transformation and magnetic properties of Ni43Mn46-x SmxSn11 (x = 0, 1, 2, 3) alloys on the basis of structural and magnetic measurements. X-ray diffraction showed that the crystal structure transforms from the cubic L21 to the orthorhombic martensite and gamma (γ) phases. The reverse martensitic and martensitic transformations were indicated by exothermic and endothermic peaks in differential scanning calorimetry. The martensitic transformation temperature increased considerably with Sm doping and exceeded room temperature for Sm = 3 at. %. The Ni43Mn45SmSn11 alloy exhibited magnetostructural transformation, leading to a large magnetocaloric effect near room temperature. The existence of thermal hysteresis and the metamagnetic behavior of Ni43Mn45SmSn11 confirm the first-order magnetostructural transition. The magnetic entropy change reached 20 J·kg−1·K−1 at 266 K, and the refrigeration capacity reached ~162 J·Kg−1, for Ni43Mn45SmSn11 under a magnetic field variation of 0–5 T.

The main challenge encountered by most 2D materials for their use in non-volatile memory technology is their low Ron/off ratio. Recently, black phosphorus (BP), an emerging 2D layered material has replaced transition metal dichalcogenides (TMDS) due to its unique electronic properties. In this paper, we have investigated the resistive switching behavior of BP Conductive Bridge Random Access Memory (CBRAM), in which resistive switching is driven by the formation of a metallic (Cu) filament and the active layer is solely composed of BP thin films. The on/off ratio is controlled by using different thicknesses of BP. It is notably found that the Ron/off ratio has a strong dependence on the thickness of BP. Thicker BP devices showed stable bipolar switching with a low operating voltage of 0.6 eV ± 0.1 V and an Ron/off of 104. The main hindrance to commercial applications of BP devices is that the BP films are susceptible to degradation on exposure to ambient conditions. To make these BP devices environmentally stable, we have utilized the strategy of growing native oxide on thicker BP devices as a protective layer by utilizing deep ultra violet light and elucidated an improved Ron/off up to 105. To the best of our knowledge, here, we report for the first time the resistive switching characteristics of BP CBRAM.

The diversity of brain functions depend on the release of neurotransmitters in chemical synapses. The back gated three terminal field effect transistors (FETs) are auspicious candidates for the emulation of biological functions to recognize the proficient neuromorphic computing systems. In order to encourage the hysteresis loops, we treated the bottom side of MoTe2 flake with deep ultraviolet light in ambient conditions. Here, we modulate the short-term and long-term memory effects due to the trapping and de-trapping of electron events in few layers of a MoTe2 transistor. However, MoTe2 FETs are investigated to reveal the time constants of electron trapping/de-trapping while applying the gate-voltage pulses. Our devices exploit the hysteresis effect in the transfer curves of MoTe2 FETs to explore the excitatory/inhibitory post-synaptic currents (EPSC/IPSC), long-term potentiation (LTP), long-term depression (LTD), spike timing/amplitude-dependent plasticity (STDP/SADP), and paired pulse facilitation (PPF). Further, the time constants for potentiation and depression is found to be 0.6 and 0.9 s, respectively which seems plausible for biological synapses. In addition, the change of synaptic weight in MoTe2 conductance is found to be 41% at negative gate pulse and 38% for positive gate pulse, respectively. Our findings can provide an essential role in the advancement of smart neuromorphic electronics.

In this study, the dominant role of the top electrode is presented for Nb2O5-based devices to demonstrate either the resistive switching or threshold characteristics. These Nb2O5-based devices may exhibit different characteristics depending on the selection of electrode. The use of the inert electrode (Au) initiates resistive switching characteristics in the Au/Nb2O5/Pt device. Alternatively, threshold characteristics are induced by using reactive electrodes (W and Nb). The X-ray photoelectron spectroscopy analysis confirms the presence of oxide layers of WOy and NbOx at interfaces for W and Nb as top electrodes. However, no interface layer between the top electrode and active layer is detected in X-ray photoelectron spectroscopy for Au as the top electrode. Moreover, the dominant phase is Nb2O5 for Au and NbO2 for W and Nb. The threshold characteristics are attributed to the reduction of Nb2O5 phase to NbO2 due to the interfacial oxide layer formation between the reactive top electrode and Nb2O5. Additionally, reliability tests for both resistive switching and threshold characteristics are also performed to confirm switching stabilities.

Two-dimensional (2D) transition metal dichalcogenides have attracted vibrant interest for future solid-state device applications due to their unique properties. However, it is challenging to realize 2D material based high performance complementary devices due to the stubborn Fermi level pinning effect and the lack of facile doping techniques. In this paper, we reported a hybrid Gr/Ni contact to WS2, which can switch carrier types from n-type to p-type in WS2. The unorthodox polarity transition is attributed to the natural p-doping of graphene with Ni adsorption and the alleviation of Fermi level pinning in WS2. Furthermore, we realized asymmetric Ni and Gr/Ni hybrid contacts to a multilayer WS2 device, and we observed synergistic p-n diode characteristics with excellent current rectification exceeding 104, and a near unity ideality factor of 1.1 (1.6) at a temperature of 4.5 K (300 K). Lastly, our WS2 p-n device exhibits high performance photovoltaic ability with a maximum photoresponsivity of 4 × 104 A W-1 at 532 nm wavelength, that is 108 times higher than that of graphene and 50 times better than that of the monolayer MoS2 photodetector. This doping-free carrier type modulation technique will pave the way to realize high performance complementary electronics and optoelectronic devices based on 2D materials.

Vertical integration of two dimensional (2D) layered materials is indispensable in making van der Waals (vdWs) heterostructures for promising electronic and optoelectronic devices. Herein, we report excellent electrical and photoelectrical measurements where the current ONOFF ratio of FET is increased by decreasing the temperature in the graphene/ReSe

Correlation between the resistive switching characteristics of Au/Zn-doped CeO2/Au devices and ionic mobility of CeO2 altered by the dopant concentration were explored. It was found that the ionic mobility of CeO2 has a profound effect on the operating voltages of the devices. The magnitude of operating voltage was observed to decrease when the doping concentration of Zn was increased up to 14%. After further increasing the doping level to 24%, the device hardly exhibits any resistive switching. At a low doping concentration, only isolated Vo existed in the CeO2 lattice. At an intermediate doping concentration, the association between dopant and Vo formed (Zn, Vo)× defect clusters. Low number density of these defect clusters initially favored the formation of Vo filament and led to a reduction in operating voltage. As the size and number density of (Zn, Vo)× defect clusters increased at a higher doping concentration, the ionic conductivity was limited with the trapping of isolated Vo by these defect clusters, which resulted in the diminishing of resistive switching. This research work provides a strategy for tuning the mobility of Vo to modulate resistive switching characteristics for non-volatile memory applications.

The control of resistive switching, in low-cost solution-processed CuxO thin films, was demonstrated on the basis of intentional manipulation of intrinsic point defects. Cu interstitials offered a unique way to create metallic Cu filament even in the absence of electrochemically active Cu top electrode. The concentration of these Cu interstitials was controlled by annealing the CuO films at low temperature (300 °C) in Ar environment with different oxygen contents. By varying the oxygen content, profound effect was observed on the resistivity of CuxO thin films, which in turn controlled the memory windows in different devices. Annealing at 0% O2 atmosphere created abundant cationic defects, which resulted in poor switching behavior. With the addition of 20% oxygen and increased annealing time, transition of CuO to Cu2O, determined by X-ray diffraction and Raman spectroscopy, resulted in deterministic increase in on/off ratio by 3 orders of magnitude and improved endurance. On increasing the oxygen content ...

In this study, we report the Poole-Frenkel induced threshold switching characteristics of niobium dioxide (NbO2) films by tuning oxygen stoichiometry. Similar to correlated oxides, NbO2 is also found sensitive to oxygen related defects. The oxygen stoichiometry of NbO2 films was varied by changing the oxygen flux during its synthesis in reactive RF-sputter. Nb2O5 dominant phase of as-grown films was detected during Raman spectroscopy. For the reduction of Nb2O5 films, we chose tungsten as a top electrode due to its considerable oxygen affinity. After the required forming process, stable threshold switching was observed which is attributed to the reduction of Nb2O5 phase to NbO2 due to interfacial WOx layer formation between top electrode and Nb2O5. Increase of O2 flux during deposition leads to an increase in the selectivity, threshold voltage and off-state resistance. These devices showed good thermal stability up to 160 °C making them useful for RRAM and neuromorphic computing applications.

This study elucidates a comparison between zinc oxide (ZnO) and zinc oxide:Polyethylenimine ethoxylated (ZnO:PEIE) nanocomposite as an electron transport layer (ETL). The impact of ZnO:PEIE annealing temperature to increase the fill factor (FF) and current density (Jsc) of the devices was first studied after optimizing the doping concentration of PEIE. It was found that 0.4 wt% concentration and 150 °C annealing temperature of ZnO:PEIE ETL showed a superior performance for the devices. The introduction of PEIE promoted the charge transport by decreasing the work function of ZnO, which caused an improvement in the conductivity and current density of devices. For ZnO, the efficiency was 1.25%, which was enhanced to 1.46% with 0.4 wt% PEIE doping and further enhanced to 1.53% at 150 °C annealing temperature. The crystallinity of the ETL can be enhanced by choosing an effective annealing temperature that assists in increasing the current density and fill factor. This research work offers a desirable approach for the synthesis of high-performance organic solar cells.

Controlled resistive switching behavior has shown potential for brain-inspired functionalities and feasibility for multi-functional photelectric sensors. For the first time, bias-polarity dependent switching characteristic is observed in forming-free W/Nb2O5/(Pt, ITO) sandwiched structures. When voltage bias was applied to the W electrode, a multi-level cell (MLC) with more than two switching states was observed while a single-level (SLC) with binary switching states was observed with Pt or ITO biasing electrode. Moreover, devices with Pt electrodes exhibit higher SET voltages (>1) while devices with ITO electrodes exhibit lower SET voltages (

Correlation between the resistive switching characteristics of Au/Zn-doped CeO2/Au devices and ionic mobility of CeO2 altered by the dopant concentration were explored. It was found that the ionic mobility of CeO2 has a profound effect on the operating voltages of the devices. The magnitude of operating voltage was observed to decrease when the doping concentration of Zn was increased up to 14%. After further increasing the doping level to 24%, the device hardly exhibits any resistive switching. At a low doping concentration, only isolated Vo existed in the CeO2 lattice. At an intermediate doping concentration, the association between dopant and Vo formed (Zn, Vo)× defect clusters. Low number density of these defect clusters initially favored the formation of Vo filament and led to a reduction in operating voltage. As the size and number density of (Zn, Vo)× defect clusters increased at a higher doping concentration, the ionic conductivity was limited with the trapping of isolated Vo by these defect clusters, which resulted in the diminishing of resistive switching. This research work provides a strategy for tuning the mobility of Vo to modulate resistive switching characteristics for non-volatile memory applications.

The development of the novel three-terminal hybrid lateral memristor and transistor device called memtransistor, has successfully provided additional functionalities in memory devices. However, their high operating voltage is critical. In this report, we have utilized the vertical heterojunction of Copper/Rhenium diselenide/Graphene to obtain low power gate tunable memristor. In such devices, resistive switching is governed by barrier height at the Rhenium diselenide/Graphene) interface, which controls the flow of electrons to neutralize Cu ions for the formation of the Cu filament. Therefore, barrier height is manipulated from ∼60 meV at Vg = 90 V to ∼828 meV at Vg = - 90 V by tuning the work function of the mono layer graphene with back-gate voltages. Subsequently, the tuning of barrier height ultimately modulates the operating voltage from 0.53 V to 3.67 V and Ron/Roff ratio from 102 to 105 by changing the Vg from 90 V to −90 V respectively. Moreover, the deep ultraviolet light assisted resistance switching effect is also investigated in gate-controlled Copper/Rhenium diselenide/Graphene devices. Thus, gate modulation and deep ultraviolet light irradiation make it compatible for future application in integrated optoelectronic systems.

Two-dimensional (2D) layered materials and their heterostructures have opened a new avenue for next-generation spintronic applications, benefited by their unique electronic properties and high crystallinity with an atomically flat surface. Here, we report magnetoresistance of vertical magnetic spin-valve devices with multi-layer (ML) MoSe2 and its heterostructures with few-layer graphene (FLG). We employed a micro-fabrication procedure to form ultraclean ferromagnetic–non-magnetic–ferromagnetic interfaces to elucidate the intrinsic spin-transferring mechanism through both an individual material and combinations of 2D layered materials. However, it is revealed that the polarity of tunneling magnetoresistance (TMR) is independent of non-magnetic spacers whether the spin valve is composed of a single material or a hybrid structure, but it strongly depends on the interfaces between ferromagnetics (FMs) and 2D materials. We observed positive spin polarizations in ML-MoSe2 and FLG/ML-MoSe2/FLG tunnel junctions, whereas spin-valve devices comprised of FLG/ML-MoSe2 showed a reversed spin polarization and demonstrated a negative TMR. Importantly, in Co/FLG/ML-MoSe2/FLG/NiFe devices, the polarization of spin carriers in the FM/FLG interface remained conserved during tunneling through MoSe2 flakes in spin-transferring events, which is understandable by Julliere’s model. In addition, large TMR values are investigated at low temperatures, whereas at high temperatures, the TMR ratios are deteriorated. Furthermore, the large values of driving ac-current also quenched the amplitude of TMR signals. Therefore, our observations suggest that the microscopic spin-transferring mechanism between ferromagnetic metals and 2D materials played a momentous role in spin-transferring phenomena in vertical magnetic spin-valve junctions.

The phase transformation induced resistive switching behavior of an Al/Cu2Se/Pt device was studied. While the device did not demonstrate any resistive switching behavior at room temperature, it exhibited resistive switching behavior at 125 °C, near the transition temperature of copper(I) selenide (Cu2Se) (137 °C), where Cu2Se is known to transform from the monoclinic to superionic phase. The increase in ionic conductivity and structural variations (from ordered to disordered structure) associated with phase transformation were observed to be responsible for the origin of the switching behavior and increase in the on/off resistance ratio near the transition temperature. Thermodynamic calculations showed that a reduction in Gibb's free energy of nucleation and an increase in the migration speed of the Cu ion associated with the ionic conductivity and order to disorder the transition of the Cu2Se at the transition temperature were the important factors responsible for the reduction in the SET voltages at 155 °C.

In this paper, we present the effect of different catalysts (Mn, Au and Sn) on the room temperature photoluminescence and Raman spectra of VLS grown ZnS nanostructures. The catalysts found to self-dope the ZnS nanostructures during its growth were confirmed by XRD and XPS results. The extent of self-doping, which depended on the type and size of catalysts, was observed strongly to have affected the optical properties of ZnS nanostructures, particularly intrinsic defects like S and Zn vacancies. Intense broad bands in the visible due to intrinsic defects, namely Zn and S vacancies were observed, which were quite different for each catalyst as well as for the size of the catalyst. Au and Mn catalyzed ZnS nanostructures also showed creation of catalyst related defects, which were absent in the case of Sn. From the PL spectra, an estimation of the Zn and S vacancies was made for each type of catalyzed ZnS nanostructures. Surface optic (SO) phonon modes of the ZnS nanostructures were also observed to behave differently for the three catalysts. The dielectric continuum model was applied to determine the correlation length and variation in the surface potential modulations in these nanostructures. It was deduced from optical studies that the location of dopant in self-doped ZnS nanostructures strongly affected the luminescence properties. Finally it was concluded that self-doping using a suitable catalyst can be a simple and controllable way to dope ZnS nanostructures with tailored optical characteristics.

We report the effect of Mn self-doping in Mn catalyzed ZnS nanostructures grown via vapor liquid solid mechanism, which also resulted in the formation of additional impurity minority phases like ZnO and MnO2. The synthesized ZnS nanostructures were subsequently annealed in the range of 500 °C – 700 °C in an inert environment to remove impurity phases and enhance the incorporation of dopant. Room temperature photoluminescence showed strong defect assisted luminescence. It was observed that green emission due to intrinsic defects of ZnS nanostructures was reduced in magnitude and Mn related orange/red luminescence increased in magnitude in nanostructures annealed at high temperature. The presence of impurity phases led to the observation of surface optical and interface phonon modes as observed in the Raman spectroscopy. Dielectric continuum and phonon confinement models were employed to determine the correlation lengths of the optical phonon modes.

In this paper, we demonstrate that vapor-liquid-solid (VLS) mode of growth can be employed to obtain three-dimensional (3D cubes or 2D platelets) mesoscopic structures with unusual morphologies and sizes using buffer layer assisted (BLA) grown catalysts. The unusual structures could not be grown by using normally deposited catalyst. It is demonstrated that the shape and the contact angle of the catalyst are the key parameters to realize such an unusual growth. AFM images of the BLA grown Mn nanoparticles showed small contact area and large contact angle with the substrate. The VLS growth was performed in the ambient and in the presence of a carrier gas. The growth done in the presence of a carrier gas led to creation of oxygen vacancies, which affected the ultimate morphology of the grown structure. It was observed that oxygen deficient environment led to the growth of hierarchical micropellets. On the contrary, the presence of oxygen resulted in the quasi symmetrical growth of micro/nano cubes. Finally, the growth mechanism was explained using thermodynamic approach, considering the interface energies and change in the Gibb's free energy. XRD, Raman and PL characterization techniques were employed and found in good agreement with proposed growth mechanism. Self doping of the Mn catalyst was also observed in cubic structures.

The use of dopant as a catalyst in the vapor-liquid-solid mode of growth offers a unique way to dope simultaneously the growing nanostructure. This paper reports the use of Mn as a catalyst to grow ZnS nanostructures and simultaneously doping it. It is also shown here that the diffusion rate of Mn in ZnS can be varied to compete with the growth rate of the nanostructures. The diffusion of Mn is found to depend strongly on the Mn layer thickness. The composition of nanowires is determined from X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) measurements. XPS revealed that Mn diffuses only on the surface forming MnS2 compound and its concentration on the surface of nanowires shows a strong dependence on its droplet size. The thermodynamic models are used to examine the interplay of the growth of ZnS nanostructures and Mn diffusion in them. The results show that uniform doping during the growth can be achieved by carefully tuning the growth temperature and the catalyst layer thickness or c...