Low Power Volatile and Nonvolatile Memristive Devices from 1D MoO2-MoS2 Core-Shell Heterostructures for Future Bio-Inspired Computing.

Memristors-based integrated circuits for emerging bio-inspired computing paradigms require an integrated approach utilizing both volatile and nonvolatile memristive devices. Here, an innovative architecture comprising of 1D CVD-grown core-shell heterostructures (CSHSs) of MoO2-MoS2 is employed as memristors manifesting both volatile switching (with high selectivity of 107 and steep slope of 0.6 mV decade-1) and nonvolatile switching phenomena (with Ion/Ioff ≈103 and switching speed of 60 ns). In these CSHSs, the metallic core MoO2 with high current carrying capacity provides a conformal and immaculate interface with semiconducting MoS2 shells and therefore it acts as a bottom electrode for the memristors. The power consumption in volatile devices is as low as 50 pW per set transition and 0.1 fW in standby mode. Voltage-driven current spikes are observed for volatile devices while with nonvolatile memristors, key features of a biological synapse such as short/long-term plasticity and paired pulse facilitation are emulated suggesting their potential for the development of neuromorphic circuits. These CSHSs offer an unprecedented solution for the interfacial issues between metallic electrodes and the layered materials-based switching element with the prospects of developing smaller footprint memristive devices for future integrated circuits.

Estimation of spectroscopic parameters and TL glow curve analysis of Eu3+-activated CaY2O4 phosphor.

The solid-state reaction method was utilised to create a down-conversion phosphor in an air environment in CaY2O4:Eu3+ nanocrystalline material. The calcination temperature was set at 1000 °C, and the sintering temperature was set at 1300 °C. Following annealing, confirmation of the crystallinity quality of the phosphor was accomplished by the use of X-ray diffraction analysis. The particle size was predicted to be 43.113 nm using Scherrer's formula. To produce down-conversion luminescence spectra, an excitation wavelength of 247 nm was applied with a fluorescence spectrophotometer. The PL got increasingly intense as the concentration of the dopant increased. The maximum intensity was measured at 2.0 mol% of Eu3+ ion, which gradually decreased as the concentration increased because of concentration quenching. To analyse spectrophotometric peak determinations, the approach developed by the Commission Internationale de l'Éclairage (CIE) was used. Thermoluminescence (TL) glow curve analysis of the CaY2O4:Eu3+-doped phosphor manufactured here revealed a wide TL centred at 225 °C, which comprised of so many peaks that may be extracted by the computerised glow curve deconvolution (CGCD) approach using glow-fit software. The associated kinetic parameters were then determined. The prepared phosphor may be useful for application in various display devices upon excitation by 247 nm; the prominent 613 nm peak of the Eu3+ ion (5D0 → 7F2) electric dipole transition features a red component. CaY2O4:Eu3+ phosphors show promise as materials for potential use in phosphor-converted white LEDs in the field of solid-state lighting technology. The linear connection that the TL glow curve has with UV dose provides evidence for its possible use in dosimetry.

Optimizing the luminescence efficiency of an europium (Eu3+) doped SrY2O4 phosphor for flexible display and lighting applications.

This research paper reports the synthesis and luminescence study of an Eu3+ activated SrY2O4 phosphor prepared by a modified solid-state reaction method with varying concentrations of Eu3+ ions (0.1-2.5 mol%). X-ray diffraction (XRD) revealed the orthorhombic structure and Fourier transform infrared spectroscopy (FTIR) methods were used to analyse the produced phosphors. Photoluminescence emission and excitation spectra were recorded for varying concentrations of Eu3+ ions, and an optimum concentration of 2.0 mol% was found to produce the highest intensity. Under 254 nm excitation the emission peaks were found to be at 580 nm, 590 nm, 611 nm and 619 nm, corresponding to transitions at 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2 respectively. Because of Eu3+ inherent luminosity, these emission peaks indicate radiative transitions between excited states of ions, making them useful for developing white light-emitting phosphors for optoelectronic and flexible display applications. The 1931 CIE (x, y) chromaticity coordinates were calculated from the photoluminescence emission spectra and found to be near white light emission, indicating the potential application of the prepared phosphor for light emitting diodes (white component). TL glow curve analysis was also performed for various concentrations of doping ions and UV exposure times, and a single broad peak was observed at 187 °C. Using the computerised glow curve deconvolution (CGCD) method, kinetic parameters were computed.

Intense green light emission and thermoluminescence glow curve analysis of Tb3+ -activated CaY2 O4 phosphor.

Here, the synthesis and luminescence analysis of the Tb3+ -activated phosphor were reported. The CaY2 O4 phosphors were synthesized using a modified solid-state reaction method with a variable doping concentration of Tb3+ ion (0.1-2.5 mol%). As synthesized, the phosphor was characterized using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analysis techniques for the optimized concentration of doping ions. The prepared phosphor showed a cubic structure, and FTIR analysis confirmed functional group analysis. It was discovered that the intensity of 1.5 mol% was higher than at other concentrations after the photoluminescence (PL) excitation and emission spectra were recorded for different concentrations of doping ions. The excitation was monitored at 542 nm, and the emission was monitored at 237 nm. At 237 nm excitation, the emission peaks were found at 620 nm (5 D4 →7 F3 ), 582 nm (5 D4 →7 F4 ), 542 nm (5 D4 →7 F5 ), and 484 nm (5 D4 →7 F6 ). The 1931 CIE (x, y) chromaticity coordinates showed the distribution of the spectral region calculated from the PL emission spectra. The values of (x = 0.34 and y = 0.60) were very close to dark green emission. Therefore, the produced phosphor would be very useful for light-emitting diode (green component) applications. Thermoluminescence glow curve analysis for various concentrations of doping ions and various ultraviolet (UV) exposure times was carried out, and a single broad peak was found at 252°C. The computerized glow curve deconvolution method was used to obtain the related kinetic parameters. The prepared phosphor exhibited an excellent response to UV dose and could be useful for UV ray dosimetry.

Pulsed Carrier Gas Assisted High-Quality Synthetic 3R-Phase Sword-like MoS2: A Versatile Optoelectronic Material.

Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3R)-phase MoS2 in a well-defined sword-like geometry having lengths up to 120 µm, uniform width of 2-3 µm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3R-phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS2 swords signifies the potential of MoS2 to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm2/V-s and Ion/Ioff ratio of ∼106 further confirm the electronic-grade standard of this 3R-phase MoS2. These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.

A υ-Constrained Matrix Adaptation Evolution Strategy With Broyden-Based Mutation for Constrained Optimization.

To solve the nonconvex constrained optimization problems (COPs) over continuous search spaces by using a population-based optimization algorithm, balancing between the feasible and infeasible solutions in the population plays an important role over different stages of the optimization process. To keep this balance, we propose a constraint handling technique, called the υ -level penalty function, which works by transforming a COP into an unconstrained one. Also, to improve the ability of the algorithm in handling several complex constraints, especially nonlinear inequality and equality constraints, we suggest a Broyden-based mutation that finds a feasible solution to replace an infeasible solution. By incorporating these techniques with the matrix adaptation evolution strategy (MA-ES), we develop a new constrained optimization algorithm. An extensive comparative analysis undertaken using a broad range of benchmark problems indicates that the proposed algorithm can outperform several state-of-the-art constrained evolutionary optimizers.

Author Correction: Planar and van der Waals heterostructures for vertical tunnelling single electron transistors.

The original version of this Article contained an error in the spelling of the author Matthew Holwill, which was incorrectly given as Mathew Holwill. This has now been corrected in both the PDF and HTML versions of the Article.

Planar and van der Waals heterostructures for vertical tunnelling single electron transistors.

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.

Work function modulation and thermal stability of reduced graphene oxide gate electrodes in MOS devices.

Work function (WF) tuning of the contact electrodes is a key requirement in several device technologies, including organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and complementary metal oxide semiconductor (CMOS) transistors. Here, we demonstrate that the WF of the gate electrode in an MOS structure can be modulated from 4.35 eV (n-type metal) to 5.28 eV (p-type metal) by sandwiching different thicknesses of reduced graphene oxide (rGO) layers between top contact metals and gate dielectric SiO2. The WF of the gate electrode shows strong dependence on the rGO thickness and is seen to be nearly independent of the contact metals used. The observed WF modulation is attributed to the different amounts of oxygen concentrations in different thicknesses of rGO layers. Importantly, this oxygen concentration can also be varied by the reduction extent of the graphene oxide as experimentally demonstrated. The results are verified by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses. The obtained WF values are thermally stable up to 800 °C. At further high temperatures, diffusion of metal through the rGO sheets is the main cause for WF instability, as confirmed by cross-sectional high-resolution transmission electron microscopy analysis. These findings are not limited to MOS devices, and the WF modulation technique has the potential for applications in other technologies such as OLEDs and OPVs involving graphene as conducting electrodes.

Electrical transport and electromigration studies on nickel encapsulated carbon nanotubes: possible future interconnects.

We nominate the nickel filled multiwalled carbon nanotubes (MWNTs) as potential candidates to cope with challenges in persistent scaling for future interconnect technology. The insights into electrical transport through nickel filled carbon nanotubes provide an effective solution for major performance and reliability issues such as the increasing resistivity of metals at reduced scales, electromigration at high current densities and the problem of diffusion and corrosion faced by the existing copper interconnect technology. Furthermore, the nickel filled MWNTs outperform their hollow counterparts, the unfilled MWNTs, carrying at least one order higher current density, with increased time to failure. The results suggest that metal filled carbon nanotubes can provide a twofold benefit: (1) the metal filling provides an increased density of states for the system leading to a higher current density compared to hollow MWNTs, (2) metal out-diffusion and corrosion is prevented by the surrounding graphitic walls.

Electrical transport and breakdown in graphene multilayers loaded with electron beam induced deposited platinum.

We demonstrate here the effect of electron beam induced deposited platinum on the electrical transport through multilayer graphene sheets. Platinum metal is deposited at different positions on the graphene multilayers, i.e., including as well as excluding the bottom contact sites and the change in electrical conductance of the same multilayer graphene sheets before and after platinum deposition is segregated. An improvement in electrical conductance is observed even if the metal is deposited at the part of the graphene sheets that does not touch the bottom gold electrodes, and hence this experimental approach directly demonstrates that the contact improvement is not the sole reason for the improved electrical conduction. The improvement in electrical performance of the graphene sheets is explained in terms of the doping of graphene sheets caused by the charge transfer between the deposited metal and the graphene and thereby modified density of states for electrical conduction. Metal deposition also leads to the increased interlayer interaction of the graphene sheets as revealed by the transmission electron microscopy analysis. Further, two types of breakdown behaviors viz. sharp and stepped breakdowns observed for these graphene devices are explained in terms of the effective graphene-metal contact area. These studies reveal the implications of top metal contact fabrication on graphene for electronic devices.

Healing of broken multiwalled carbon nanotubes using very low energy electrons in SEM: a route toward complete recovery.

We report the healing of electrically broken multiwalled carbon nanotubes (MWNTs) using very low energy electrons (3-10 keV) in scanning electron microscopy (SEM). Current-induced breakdown caused by Joule heating has been achieved by applying suitably high voltages. The broken tubes were examined and exposed to electrons of 3-10 keV in situ in SEM with careful maneuvering of the electron beam at the broken site, which results in the mechanical joining of the tube. Electrical recovery of the same tube has been confirmed by performing the current-voltage measurements after joining. This easy approach is directly applicable for the repairing of carbon nanotubes incorporated in ready devices, such as in on-chip horizontal interconnects or on-tip probing applications, such as in scanning tunneling microscopy.