A facile chemical method of acid treatment has been followed for the cutting of graphene oxides sheet to extract graphene oxide quantum dots (GOQDs) in aqueous medium at different pH. Strong blue emission and excitation dependent photoluminescence (PL) spectrum are observed in GOQDs. Relative PL quantum yield is measured as high as ∼30% for the GOQDs synthesized at pH11. Presence of defect related states and oxygen containing functional groups in GOQDs are confirmed through Raman and Fourier transform infrared analysis, respectively. High resolution transmission electron microscopy analysis suggests that the sizes of GOQDs are distributed in the range ∼2.0-8.0 nm with d-spacing of 0.245 nm. AFM measurement confirms the topography height of GOQDs in the range ∼6.0-9.0 nm. Synthesized GOQDs at different pH are dispersed in poly (vinyl alcohol) (PVA) matrix and spin coated to fabricate polymer nanocomposites (PNCs) devices. Electrical studies have been performed which display writeonce-read-many characteristics and the set voltage (V SET ) increases with the increase in pH of the GOQDs. Very low V SET ∼−0.9 V and / I I ON OFF ∼10 4 have been obtained for the device containing GOQDs synthesized at pH1. Retention tests up to 10 4 s are performed for the PNCs devices in order to confirm the stability. A suitable energy band diagram is proposed to discuss the carrier transport through the composites devices.
Hybrid composites comprised of layered two-dimensional (2D) materials have attracted growing attention, especially in large-area electronics. Herein, we exploit the synergistic interfacial effect of molybdenum disulfide (MoS2) grafted reduced graphene oxide (rGO) and study the memristive characteristics of the layered hybrid rGO-MoS2 nanocomposites (NCs). The synthesized materials are analyzed using optical, structural, elemental, and morphological techniques. The average number of layers (~ 6 to 10) is estimated from the intense X-ray diffraction peak of the polycrystalline materials. The intercalation of rGO in the rGO-MoS2 NCs results in an increased energy bandgap compared to pristine MoS2. The photoluminescence study exhibits dominant blue emission due to the restoration of the sp2-hybridized carbon domain in the reduced sheets. The surface topography of the NCs shows a 3D flower-like structure with multiple nano-petals interconnected in the form of nanosheets. The average diameter of the flower-shaped particles is calculated to be ~258 and ~301 nm for MoS2 and rGO-MoS2, respectively. The crossbar devices (ITO/rGO-MoS2/Cu) in a sandwich configuration (thickness ~700 nm) are fabricated, displaying stable and repeatable bipolar resistive switching characteristics. The trapping and de-trapping of the charge carriers at the rich sulfur vacancies are responsible for bipolar memory behavior.
Heterostructures of two-dimensional (2D) layered materials, integrating two or more building blocks with complementing counterparts, can regulate the confinement and transportation of charge carriers via vacancy-induced defect and interfacial states. Herein, reduced graphene oxide-molybdenum disulfide (rGO-MoS2) nanohybrid were fabricated and reinforced with various polymers [PMMA, PVDF, and PMMA-PVDF (20:80) blend] to study the resistive memory properties in a metal-insulator-metal configuration. The SEM analysis presents a hierarchical 3D flower-like MoS2 intercalated with rGO nanosheets. TEM image exhibits MoS2 nanoflakes well interspersed and grafted on layered rGO sheets, forming sandwich heterostructures. Raman analysis shows a higher ID/IG ratio for rGO-MoS2 than rGO, demonstrating numerous defect states in rGO. The XRD analysis of the polymer blend containing rGO-MoS2 exhibits β-crystal phases with a polarity-dependent internal electric field (E-field). The J-V characteristics of pure MoS2-polymer films display a write-once-read-many (WORM) behavior with a current ION/IOFF ratio of ~102-103, in contrast to pristine polymer films exhibiting repeatable electrical hysteresis. Instead, the rGO-MoS2 -based devices display bipolar characteristics (ION/IOFF ratio of ~103-104) due to charge transfer interaction with the conductive carbon substrates. The ferroelectric polarization-induced E-field coupled with the external bias is responsible for the improved memristive performances. A plausible conduction mechanism is proposed to discuss the carrier transport through the devices.
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