Accepted Manuscript Reduced graphene oxide preparation and its applications in solution-processed writeonce-read-many-times graphene-based memory device Poh Choon Ooi, Muhammad Aniq Shazni Mohammad Haniff, M.F. Mohd Razip Wee, Chang Fu Dee, Boon Tong Goh, Mohd Ambri Mohamed, Burhanuddin Yeop Majlis PII:
S0008-6223(17)30880-1
DOI:
10.1016/j.carbon.2017.09.004
Reference:
CARBON 12334
To appear in:
Carbon
Received Date: 17 April 2017 Revised Date:
29 August 2017
Accepted Date: 2 September 2017
Please cite this article as: P.C. Ooi, M.A.S.M. Haniff, M.F.M.R. Wee, C.F. Dee, B.T. Goh, M.A. Mohamed, B.Y. Majlis, Reduced graphene oxide preparation and its applications in solutionprocessed write-once-read-many-times graphene-based memory device, Carbon (2017), doi: 10.1016/ j.carbon.2017.09.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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AgNWs
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Multi-stacked layer
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Quartz
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Reduced Graphene Oxide Preparation and Its Applications in Solution-Processed Write-Once-Read-Many-Times Graphene-Based Memory Device
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Institute of Microengineering and Nanoelectronic, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia 2 Advanced Devices Lab, MIMOS Berhad, Technology Park Malaysia, 57000 Kuala Lumpur, Malaysia 3 Low Dimensional Materials Research Centre (LDMRC), Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
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Poh Choon Ooi1,*, ɫ ·Muhammad Aniq Shazni Mohammad Haniff 2, ɫ· M. F. Mohd. Razip Wee 1·Chang Fu Dee1,*·Boon Tong Goh3· Mohd Ambri Mohamed1·Burhanuddin Yeop Majlis1
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ABSTRACT:
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this work. Thermally reduced graphene oxide (rGO) on quartz substrate prepared in the
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ambient of C2H2/H2 plasma treatment was used as bottom conductive electrode to replace the
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commonly-used bottom conductive indium-tin-oxide layer. The morphology of the rGO film
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was characterized and being used for device fabrication. The device was fabricated in the
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simple structure of silver nanowires/nanocomposite/rGO/quartz and the nanocomposite was
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prepared by mixing the graphene quantum dots and graphene oxide in ethanol. Current-
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voltage (I-V) measurement of the fabricated device shows current bistablity with the similar
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behavior as write-once-read-many-times (WORM) memory device. The ON/OFF ratio of the
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current bistability for the devices was as large as 1 x 103 with retention stability up to 1 x 104
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s. The direct tunnelling, trapped-charge limited-current, and Ohmic conduction were
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proposed as dominant conduction mechanisms through the fabricated NVM devices based on
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the obtained I-V characteristics.
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We fabricated graphene-based non-volatile memory device by solution-processed route in
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Keywords: Graphene, graphene quantum dots, graphene oxide, reduced graphene oxide, graphene-based, non-volatile memory, conduction mechanisms
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These authors contributed equally to this work. *Corresponding author: E-mail address:
[email protected] (P. C. Ooi);
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1. Introduction
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The two-dimensional carbon nanomaterials, such as graphene have been widely explored
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recently to demonstrate its potential applications in biomedical study and by fabricating
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graphene-based nano-scaled electronic, optoelectronic, photovoltaic, and sensing devices [1-
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4]. Graphene has received significant research attention due to its unique properties of planar
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structure, high electrical conductivity, high transparency and flexibility, large surface area,
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and high mechanical stability [5-7]. Moreover, its advantages of high density of states, high
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mobility, high work function, and low dimensionality compared with the conventional charge
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trap materials [5, 8] suggested that graphene could be used as the potential material in non-
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volatile memory (NVM) devices as the charge trap medium [4, 5, 9, 10].
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For enhancing memory data retention in real applications, the nanometer-sized graphene
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pieces such as graphene quantum dots (GQDs) would be preferable to be integrated for
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device fabrication owing to the discrete charge trap materials offers the advantage of
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constricting lateral charge movement as a result of the nanocrystals are separated from each
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other [5, 11].
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work. Even though some investigations of the NVM devices fabricated with GQDs by using
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a solution process route have been conducted [8, 10], studies on the device performances by
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incorporating GQDs in the graphene oxide (GO) matrix with reduced graphene oxide (rGO)
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prepared by C2H2/H2 plasma treatment as a bottom conductive layer to realize fully graphene-
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based materials NVM devices have not yet been performed.
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Consequently, GQDs will be used as a discrete charge trap medium in this
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GO the non-conducting material will be used to replace the solution polymer matrix as
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it does not require high temperature treatment after the deposition process and the
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nanocomposite mixture offering the potential of cost-effective, large-scale production, Page 2 of 16
ACCEPTED MANUSCRIPT environmental friendliness and simple method to develop graphene-based electronic devices
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[12-14]. Meanwhile, the commonly used bottom conductive indium-tin-oxide (ITO) layer
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will be replaced by conductive rGO thin film owing to the costly ITO, limited rare element
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indium, mechanically brittle, chemically unstable and poor adhesion to organic materials [15-
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17]. Alternatively, rGO also provided an excellent conductivity and transparency but more
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importantly better chemical stability and flexibility than ITO [18]. However, the quality of
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typical rGO lattice is much lower compared to that of a CVD grown graphene due to high
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structural defects, such as vacancies and topological defects after reduction [19]. Thus far,
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only little progress has been made to improve the graphitic structure by healing of the lattice
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defects to produce high-quality rGO with excellent electrical performance [20, 21].
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Therefore, in this study, the rarely reported thermal reduction method with C2H2/H2
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plasma treatment will be used to produce high-quality and conductive rGO thin films from
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the synthesized GO solution on the quartz substrate. The advantage of C2H2/H2 plasma
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treatment is to heal the lattice defects of rGO by the addition of carbon atoms in clustering of
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the sp2 domains. Then, the two-terminal, fully graphene-based NVM device, in the simple
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metal-insulator-metal (MIM) structure of silver nanowires/nanocomposite/rGO/quartz will be
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fabricated using the fully solution-processed route. Solution-processing deposition techniques
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have been particularly attractive because of interest in realizing transparent and flexible
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electronic devices without the need of complicated fabrication processes that require
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expensive vacuum equipment, high-temperature fabrication steps and high manufacturing
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cost [10, 22, 23]. The rGO and nanocomposite mixture will be deposited using the spin-
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coating method while the top metal electrode contact could be formed by the spray-coating
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method to avoid the expensive and time-consuming metal vacuum evaporation process [22].
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2. Experimental
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In this study, the bottom conductive rGO thin film was derived from the synthesized GO
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flakes. GO was synthesized from graphite powder using a typical modified Hummer’s
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method [24]. The GO flakes suspended in ethanol with a concentration of 1.0 mg/mL were
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ultrasonicated for 30 min to obtain a homogeneous GO solution. The GO flakes were then
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deposited on the 1 cm x 1 cm quartz substrate by the spin-coating method at a rate of 3000
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rpm for 40 seconds. At this rate, the solvent was rapidly evaporated, thus leaving the 5 nm
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GO flakes on the substrate in the form of thin film due to the strong van der Waals forces
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interaction. The reduction of GO film was begun by thermal annealing at 700 ˚C under
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vacuum atmosphere for 6 hours to effectively diminish a substantial amount of oxygen
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functional groups in the carbon network. By maintaining the temperature of 700 ˚C, the rGO
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film was carefully plasma-treated using a mixture of acetylene/hydrogen (C2H2/H2) gas at a
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flow rate of 40 sccm/10 sccm and a plasma power of 20 W for 2 min. This process is inferred
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to improve the carbon crystalline structure from the substitution of carbon atoms in the
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aromatic hexagonal lattice of graphene. The morphologies of the rGO film on the substrate
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were characterized prior to device fabrication.
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The nanocomposite mixture was prepared by mixing the GQDs in ethanol (1mg/ml)
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purchased from ACS Material and the synthesized GO solution with the volume ratio of 2:1 and
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agitated for 15 minutes to ensure the homogenous distribution of the GQDs in the GO solution
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matrix [Refer to Fig. S1 in Supplementary Material for the nanocomposite characterization].
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Then, a 7 nm mixture layer was then spin-coated at 2500 rpm for 40 s on top of the rGO film and
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a reference device without GQDs also fabricated with the same spin-coating condition. Finally,
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the top silver nanowires (Ag NWs) electrodes were spray-coated with the aid of 0.5 mm diameter
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circular shadow mask at 0.1 MPa on a 100 oC hot plate to complete the graphene-based devices
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structure of (a) AgNW/GO/rGO/quartz and (b) AgNW/nanocomposite/rGO/quartz, denoted as
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ACCEPTED MANUSCRIPT reference device and NVM device. Ag was used as the top metal contact in this work due to it is a
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renowned low-cost metal yet from the same noble metal category as compared with Au [25].
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Additionally, Al was not used as it yielded a higher turn-on voltage in our earlier reported work
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[26]. Fig. 1 shows the schematic diagram of the fabricated devices whereas the inset in Fig. 1(b)
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shows the transmission electron microscopy (TEM) cross-sectional image for the fabricated
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NVM device.
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The morphologies of rGO film on the substrate were characterized by a field emission
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scanning electron microscopy (FESEM) system (JEOL, JSM 7500 F) operated at an
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accelerating voltage of 2.0 kV, a high-resolution transmission electron microscopy (HRTEM)
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system (JEOL, JEM-2010) operated at an accelerating voltage of 200 kV, and an atomic force
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microscopy (AFM) system in a semi-contact mode (NTEGRA Spectra, MT-MDT). For the
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HRTEM studies of rGO film, the sample was prepared by dipping a carbon-coated TEM grid
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into the GO solution and allowing it to dry at room temperature. The reduction of the sample
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was then carried out by the same reduction method to the as-prepared rGO/quartz substrate.
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Meanwhile, the FEI Tecnai TF20, a 200 kV FEG HRTEM was used to obtain the cross-
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sectional structure of the fabricated device. The samples of GO, before and after reduction
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with C2H2/H2 plasma treatment have been characterized by X-ray photoemission
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spectroscopy (XPS) system (Scanning XPS Microprobe, PHI Quantera II), ultraviolet-visible
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(UV-Vis) spectroscopy system (Agilent Cary 7000), and Raman spectroscopy system
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(NTEGRA Spectra, MT-MDT). For the electrical measurements, the rGO/quartz sample was
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cut into a square shape with size of 1 × 1 cm2. The electrical properties of rGO films were
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measured with a Hall-effect measurement system (Ecopia, HMS-5300) based on the van der
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Pauw method. Four contacts were made by soldering indium dots of about 0.5 mm on each of
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the edge of sample to create ohmic contact with rGO films. The current-voltage (I-V)
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measurements of the fabricated NVM devices were performed at room temperature by using
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Keithley 4200-SCS semiconductor characterization system. Bias voltages were applied to the
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bottom rGO electrode with respect to the top metal electrode for all measurements.
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3. Results and Discussion
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Fig. 2(a) shows the FESEM image of rGO films prepared on quartz substrate with flake size
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up to 50 µm. It was observed that the rGO flakes are of unsymmetrical shape in random
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multi-stacked order due to the van der Waals interactions. The visibility color contrast of rGO
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films indicates that it consists of few layers. The TEM image in Fig. 2(b) clearly shows the
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presence of mostly few-layered and folding-structured rGO films, which is almost electron
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transparent from its low magnification image. Interestingly, the HRTEM images in Fig. 2(c-
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d) reveal the periodic bright spots in the rGO films, suggesting the significant restoration of
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sp2 carbon domains in the carbon crystalline lattice after plasma treatment in the C2H2/H2
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ambient [27]. It should be noted that few portion of the rGO films (indicated by blue box)
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show clear Moiré patterns in the HRTEM image arising from rotational stacking faults (see
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Fig. 2(c)), indicating the van der Waals nature of such rGO films [28]. However, the
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characteristic of hexagonal atomic lattices with an estimated carbon–carbon bond length (acc)
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of ~1.4 Å and a lattice spacing (a) of ~2.4 Å are clearly observable in the HRTEM image (see
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Fig. 2(d)), which in good agreement with theoretical model [29]. In addition, the typical
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AFM image shows that the rGO film consists of continuous flake structures as shown in Fig.
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2(d) and the height profile analysis confirms that the rGO is primarily composed of thin film
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(≤ 5 layers) with an average thickness of approximately 5.12 nm. Next, UV-Vis spectra
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studies were conducted on the both samples prepared on quartz substrates to evaluate the
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optical properties of GO and rGO. Fig. 2(e) shows the comparison of the optical
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transmittance for the GO and rGO samples in the visible range. At λ = 550 nm, it was
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observed that the GO and rGO films are transparent with a light transmittance of 96.4% and
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ACCEPTED MANUSCRIPT 89.2%, respectively. This evidence can be clearly seen from the photograph images for the
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both films prepared on a quartz substrate as inserted in Fig. 2(e), where the light brown GO
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was entirely changed to a light grey after the reduction process. The observation suggests that
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the reduction of GO enhances its reflection of incident light, thus decreasing the
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transmittance. Moreover, the optical band gap (Eg) of GO and rGO can be also obtained from
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Tauc plots using the following equation [30]: (αhv)2 ∝ (hv – Eg), where α is the absorption
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coefficient and hv is the proton energy. The values of Eg have been estimated by taking the
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intercept from the fitting of the (αhv)2 ~ hv plots at (αhv)2 = 0, as shown in Fig. 2(f), giving
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an indirect band gap of GO and rGO to be ~4.0 eV and ~2.75 eV, respectively.
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In order to study the quantitative elemental compositions and chemical states of GO
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and rGO, XPS studies were carefully performed on the sample before and after plasma
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treatment on the same probed spot. The XPS spectra survey scan for both GO and rGO is
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shown in Fig. 3(a). A slight shift in the XPS bands of C 1s towards higher binding energy
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was clearly seen for the GO sample, reflecting surface charging effect due to its nature of
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insulating behavior. It was observed that the typical GO sample has a prominent intensity of
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O 1s arising from the high content of oxygen functional groups. Meanwhile, the O 1s peak is
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seen to have a weak intensity with about 7.65 at.% of oxygen in rGO after the reduction
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process, indicating the substantial removal of oxygen functional groups. Based on the atomic
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concentration data, the C/O atomic ratio of GO and rGO was determined to be 1.81 and 10.9,
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respectively. The increased C/O atomic ratio in the rGO indicates the effective removal of
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oxygen functional groups and the enhanced sp2 carbon restoration during the plasma
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treatment. Further analysis to support this finding can be also proven by the deconvoluted C
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1s spectra of GO and rGO as shown in Fig. 3(b) and (c). The prominent peak at ~284.5 eV
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associated to the C=C sp2 was evidently for the rGO, while the other peaks of oxygen
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functional groups such as C-OH, C=O and COOH bonds became much weaker in
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comparison to that of GO, demonstrating the reduced nature of rGO films.
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The Raman spectra of both GO and rGO films were examined with a Raman
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excitation wavelength of 473.0 nm. The typical spectra obtained with four intense features:
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the G band at ~1600 cm-1, D band at 1356 cm-1, 2D band at 2679 cm-1 and D+G band at
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~2950 cm-1, are shown in Fig. 3(d). For the GO sample, the D band is seen to have a
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comparable intensity to the G band, indicating the presence of mixed sp2 and sp3
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hybridizations. Meanwhile, a higher intensity of the G band arising from the stretching of C-
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C bonds is apparently observed for the rGO sample, suggesting the significant reduction and
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lattice restoration in the sp2 hybridized carbon domains. In this case, the intensity ratio of G
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and D peaks (IG/ID) of rGO was about ~1.33, much higher than that of GO (~0.97), which
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reflects to the higher relative amount of sp2 hybridization on the graphene surface. Based on
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the Tuinstra-Koenig relation [31], a rough estimation of the average crystallite size (La) can
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be determined for both the GO and rGO films using the following equation: La = (2.4 × 10-
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)·λ4·(ID/IG)-1, where λ is the Raman excitation wavelength. Assuming that the C2H2/H2
plasma treatment was uniform on the film surface, the La value of rGO films was estimated to
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be about ~16.0 nm, which is slightly higher compared to that of GO films (~11.7 nm). This
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observed higher La in rGO suggests the effective healing of lattice defects by the addition of
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carbon atoms in clustering of the sp2 domains during the C2H2/H2 plasma treatment.
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Furthermore, the lattice restoration of sp2 domains in the aromatic carbon structure is further
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examined by analyzing the intensity ratio of 2D and D+G peaks (I2D/ID+G). The enhancement
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of I2D/ID+G ratio of rGO can be clearly observed (~1.08) compared to that of GO (~0.68),
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which suggests that the significant recovery and the larger-area π-conjugated structure.
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According to the IG/ID and I2D/ID+G ratios, it is confirmed that the sp2 hybridized carbon atoms
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in the rGO films were enhanced after the C2H2/H2 plasma treatment. On the other hand, an analysis using the temperature dependence of the conductance
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was carried out to identify the possible carrier transport mechanism for the rGO films. Fig. 4
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shows the conductance versus temperature plot of the rGO films, where the measurements
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were investigated in a temperature range from 100 to 350 K. It was observed that the
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conductance of rGO films increases with increasing temperature, hence exhibiting a typical
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semiconducting behavior.
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behavior in the lightly-reduced GO films can be explained by variable-range hopping (VRH)
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between the localized states due to the electron confinement effect in the large disordered
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regions composed of point defects and sp3 carbon domains [32-34]. The VRH model in a
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two-dimensional (2D) system gives the relation between conductance G and temperature T as
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G(T) α exp(-B/T1/3), where B is the hopping parameter [35]. In our case, however, the
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signature of VRH behavior in the rGO film can be only observed from a linear plot of ln (G)
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versus T-1/3 at low temperature regime up to T = 210 K. Above this threshold, the contribution
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of thermally activated (TA) carriers is expected to dominate the electrical conduction in the
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rGO films, which can be fitted well by the Arrhenius model: G(T) α exp(-Ea/kBT), where G is
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the conductance, Ea is the activation energy, and kb is the Boltzmann constant. The Ea derived
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from the fitted line in the inset of Fig. 4 is about 41.0 meV and comparable to the value
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obtained by the previous works [36, 37]. At room temperature (T = 299 K), the observation
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indicates that the band-like transport is more dominant than VRH due to the delocalization of
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the charge carriers in the enhanced sp2 carbon domains. As a result, the excellent electrical
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properties of rGO with high carrier mobility ~ 90 ± 4.1 cm2 V-1 s-1 and low sheet resistance
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~510 ± 10.3 Ω/sq. at T = 299 K were obtained through the Hall-effect measurement system.
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It is worthy to mention that our approach to prepare rGO have been significantly improved
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the carrier mobility and the sheet resistance as compared to the others as reported elsewhere
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using different reduction approach [38-40]. The obtained rGO film is commonly p-type because of the interaction with the water
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molecules in the ambient attributed to the carbon atoms tend to share the electrons with the
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oxygen atoms, thus leaving hole in the carbon network [41]. The work function for the
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prepared rGO film in this study was 4.7 eV determined by the ultraviolet photoelectron
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spectroscopy (UPS) measurement, relatively higher than the work function of AgNWs, ~ 4.5
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eV [42]. Thus, the electrons emission will be preferable from AgNWs electrode when
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performing the electrical characterization in this work. Fig. 5 shows the I–V measurement for
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the fabricated NVM device with the applied voltage was swept from – 3 V to 3 V and vice
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versa. When the voltage was swept from 0 to 3 V during the 1st–Sweep, there was an abrupt
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increase in current at 0.35 V (VON), which switched the device from low-current state to high-
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current state. The device remained at high-current state when voltage was swept from 3 to 0
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V. When the bias polarity was changed from 0 to – 3 V, the device remained at high-current
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state even when the voltage was continued to sweep from – 3 V to 0. The presence of I–V
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hysteresis window in Fig. 5 suggesting the charge storage capability could be strongly
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associated to presence of GQDs in the fabricated NVM device as there is negligible
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hysteresis window for the reference device without GQDs as shown in the left inset [5]. The
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right inset in Fig. 5 shows the fabricated transparent NVM device on quartz substrate.
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As aforementioned, the device remained at high-current state even though the
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opposite polarity was applied and indicating the behavior of write-once-read-many-times
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(WORM) memory device. The observed non-erasing WORM memory device behavior was
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further proven by the additional voltage sweeping indicated by 2nd–Sweep and 3rd–Sweep as
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shown in Fig. 5(a) and this observation could be explained by the formation of irreversible
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ACCEPTED MANUSCRIPT conductive filament in the device. Three distinct regions were marked in the plotted I–V
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curve as shown in Fig. 5(a) to understand the possible dominant conduction mechanisms
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occurred in the WORM memory device using a curve fitting method by double log plot the
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obtained I–V data. In region I, the low voltage region, the occurrence of thermionic emission
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current conduction is a commonly observed mechanism related to thermally-generated
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electrons [43]. Besides, due to the thin 7 nm nanocomposite mixture layer with embedded
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GQDs, there is the probability of electrons injected from AgNWs electrode via direct
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tunneling into the GO dielectric as the obtained experimental data fits well to follow
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ln(I)∝ln(V/d), where d is the thickness of dielectric [44]. In region II, when the voltage is
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increased to VON, the curve-fitted slope is approximately 4.8, and the transport mechanism
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switches to follow a typical space-charge-limited current model with traps, which is known as
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trapped-charge limited current (TCLC) mechanism as the slope is much greater than 2. In this
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region, the increase of applied voltage will increase the density of free carriers from injection
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to a certain value that the Fermi level moves up above the various electron trapping levels
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occurred and the electrons tunnel from the GO layer are trapped at the various energy levels
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as illustrated by the plausible electronic band structure in Fig. 5(b) [45, 46]. When the trap
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sites due to the presence of GQDs start to fill an abrupt increase in current is observed as can
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be seen from the I-V 1st-Sweep as shown in Fig.5(a) [45].
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Once all the traps are filled, the transport mechanism switches to obey Ohmic model
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in region III with slope value of 1.2 as shown in Fig. 5(a). The Ohmic transportation might be
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associated to the irreversible electro-migration process of oxygen ions from the rGO bottom
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conductive electrode [47, 48]. After the application of higher forward voltage, the oxygen
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ions from the rGO electrode could migrate to the AgNWs/GO interface and permanently
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annihilate the positively charged oxygen vacancies which form the conductive filament and
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ACCEPTED MANUSCRIPT the annihilation of oxygen vacancies could result in the permanent rupture of the filament.
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When the filament is oxidized or broken off, it is unlikely to be formed again as there are no
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sufficient oxygen vacancies electrically driven to construct the conductive filament, yielded
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the irreversible of high-current state to low-current state [47, 48]. Another probable
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explanation for this ohmic transportation could be also deduced to the diffusion of metallic
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electrode into the nanocomposite layer due to the high local heat generation caused by the
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passing current switching the conduction from direct tunneling to ohmic. Therefore, it is
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believed that the n≈4.8 in region II might not be attributed TCLC mechanism but merely a
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transition from direct tunneling (region I) to irreversible Ag filament conduction in region III
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[26, 49, 50]. Subsequently, a retention test was conducted to examine the stability of the
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fabricated WORM memory devices. The electrical characterization demonstrated that there is
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without significant deterioration of the both current states up to 1 x 104 s with the ON/OFF
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current ratio of 103 even though slightly current fluctuation in the low-current state was
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observed as shown in Fig. 6. The stable and distinguishable current states are essential feature
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to avoid misreading in the Boolean logic system [9].
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4. Conclusion
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Graphene-based NVM devices were fabricated using fully solution-processed route. The
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bottom conductive layer on quart substrate was formed by rGO thin films as a result of a
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thermal reduction of GO and the plasma treatment of rGO in the C2H2/H2 ambient on a quartz
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substrate. The C/O atomic ratio in rGO was estimated to be 10.9 with high carrier mobility ~
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90 ± 4.1 cm2 V-1 s-1 and low sheet resistance ~ 510 ± 10.3 Ω/sq. at T = 299 K were measured.
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The nanocomposite mixture and AgNWs were then deposited on the rGO film to construct
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the MIM device. The fabricated NVM device demonstrated similar behavior as non-erasing
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WORM memory device after electrical characterization. The direct tunnelling, trapped-
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mechanisms occurred in the fabricated NVM devices based on the I-V curve fitting analysis.
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The Ohmic conduction might be associated to the irreversible electro-migration process of
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oxygen ions from rGO bottom conductive electrode and yielded permanent storage of the
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high-current or irreversible Ag filament conduction in this study. Retention test were also
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conducted and the both current states of the device was stable up to 1 x 104 s with the
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ON/OFF current ratio of 103 was achieved. The permanent data storage and high retention
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stability with distinguishable current states show promising applications in the next-
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generation electronic devices such as in transparent embedded system with see-through
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memory and disposable/single-use medical devices.
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Acknowledgments
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This research was supported in-part by the Research University Grant from Universiti
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Kebangsaan
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UKM/NANOMITE/04/01, the Higher Institutions Centre of Excellence (HICOE) (AKU95)
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from Ministry of Education, Malaysia and Fundamental Research Grant Scheme (FRGS) of
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FP011-2015A from the Ministry of Higher Education, Malaysia.
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Figure captions
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Fig. 1: Schematic diagram of the cross-sectional structure for the fabricated MIM devices (a)
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reference device and (b) NVM device. The inset shows the TEM image of the cross-sectional
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Fig. 6: Retention stability test of the fabricated NVM device.
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