MoS2-Based Piezotronic

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Ultra low-cost, large area Graphene-MoS based piezotronic memristor on paper: A Systematic study for both DC and AC inputs Bhavaniprasad Yalagala, Parikshit Sahatiya, Venkat Mattela, and Sushmee Badhulika ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00086 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Electronic Materials

Ultra low-cost, large area Graphene-MoS2 based piezotronic memristor on paper: A Systematic study for both DC and AC inputs Bhavaniprasad Yalagala#, Parikshit Sahatiya#, Venkat Mattela and Sushmee Badhulika* Department of Electrical Engineering, Indian Institute of Technology Hyderabad Hyderabad, 502285, India *Corresponding author: E-mail: [email protected]; Telephone: 040-23018443 Fax 04023016032 # Equal Contributed First Authors ABSTRACT: This report is the first demonstration of the fabrication of 2-dimensional (2D) nanohybrid i.e., Graphene (Gr)/ MoS2 based resistive random access memory (RRAM) on paper substrate. Combining Graphene with MoS2 on paper helps in improving the overall mobility while still maintaining the dielectric property needed for resistive switching (RS) and ensures a durable, reliable and repeatable performance due to their high Young moduli. The Gr/MoS2 based paper memory exhibited excellent stable RS behavior, endurance up to 5x102 cycles, ON/OFF ratio of 104 with data retention capacity tested for 104 seconds. The impedance spectroscopy analysis on the fabricated memristor revealed capacitive behavior which upon application of continuous time signals interestingly exhibited programmability i.e. resistance variation in the low frequency regime which disappears at high frequencies. Further, upon bending, due to the piezoelectric nature of the odd layered MoS2, the memristor exhibited lower set and reset values of 3.3 V and -3.3V respectively which could be attributed to the piezotronic effect at the junction of Gr/MoS2. Detailed switching behavior is explained in terms of conduction bridge mechanism wherein the schottky barrier created at the Gr/MoS2 interface helps in the effective movement of the ions. The strategy presented here is a major step ahead in the development of next generation Papertronics based ultra-low-cost, disposable memory devices with ease of handling data for security, medical and Internet of Things (IoT) applications. KEYWORDS: Memristor, papertronics, piezotronic, disposable memory, resistive switching.

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Introduction Memristors are considered to be the most potential candidates for future non-volatile memories and artificial intelligence, because of their simple sandwich-like device structure, extremely low operating voltages, nanoscale device dimensions, faster switching speeds, multi-threshold switching, high switching endurance and good CMOS compatibility [1-2]. Since their inception, research towards the development of memristors has been focused on fabricating them on conventional silicon substrate utilizing various functional nanomaterials which is achieved by the use of highly sophisticated technique which are not only expensive but also energy inefficient [35]. In addition, focus of this research has been primarily towards achieving high performance based RRAMs in optimal conditions while the reliability of memristors under external stress induced bend and on flexible substrates remains unexplored. Furthermore, since memory is ubiquitous in semiconductor industry and its large scale production leads to increase in the E-waste [6], efforts are being made to fabricate the same on flexible and biodegradable substrates rather than on conventionally utilized Si or polymeric substrates without compromising on the performance of the fabricated device. In addition, since the programmability of the memristor depends on the ionic movements in the dielectric material, the instability caused due to the strain induced in the flexible substrate and functional nanomaterial significantly affects the reliability [7]. Traditionally used materials for the fabrication of memristors such as metal oxides exhibit very low young’s modulus of elasticity and hence there is an active search for novel materials with superior mechanical properties for memristor applications [8]. Two Dimensional materials are excellent for flexible electronics because of their outstanding mechanical and electrical property combined with the structural stability [9-11]. Molybdenum disulfide (MoS2) is a layered material possessing variable bandgap and dielectric property making it suitable for memristor application [12-14]. Graphene, on the other hand possesses excellent electronic mobility and has been utilized as a transport material in variety of electronic applications [15-17]. Combining Graphene with MoS2 on a flexible platform will not only help in improving the overall mobility while still maintaining the dielectric property needed for the resistive switching of the device but also ensures a durable, reliable and repeatable performance due to their high Young modulus [18]. Odd layers of MoS2 are known to be piezoelectric in nature due to the cento symmetric nature of MoS2 [19]. Cellulose paper is an excellent alternative to polymeric


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substrates because of its excellent biodegradability, low cost, recyclability, light weight and mechanical flexibility [20]. Also, the conduction mechanism of the memristor relies on the transport of the metal contacts ions from one electrode to another. Cellulose paper being porous in nature helps in the diffusion of the metal ions in localized defect sites i.e. traps present in MoS2 which is not possible in most of the traditional polymeric flexible substrates. Further, cellulose paper not only acts as a substrate but also as an active dielectric which simplifies the system and reduces the overall cost of the fabricated device [21]. Despite, the excellent properties offered by cellulose paper, the fabrication of paper based memristor still remains unexplored. The challenge for the development of the 2D materials based flexible memristors is intensified because of the lack of suitable fabrication techniques. The conventional approach is the use of chemical vapor deposition (CVD) technique for the direct deposition of MoS2 on rigid and flexible substrates [22]. However, the substrates that can be utilized for CVD are limited and often come with multiple, erroneous transfer processes thereby leading to variation in device to device performance. Further, CVD process is also accompanied by defining contacts utilizing Electron beam lithography (EBL) making the fabrication process complicated and expensive. Thus the direct growth of 2D materials on the substrate of choice still remains a challenge. Recently our group demonstrated the direct growth of MoS2 on different flexible substrates using solution processed hydrothermal method and the performance was found to be comparable to the CVD grown MoS2 [14]. This report is the first demonstration of a paper based flexible memristor device utilizing a nanohybrid comprising of multilayer Gr/MoS2 (MGM) with silver, copper as top and bottom electrodes respectively exhibiting excellent repeatable resistive switching (RS) behavior with an endurance value up to 5x102 cycles, ON to OFF switching ratio of ≈104 along with outstanding flexibility. Further, a complete investigation on the role of Gr during RS mechanism suggests that Gr plays a very crucial role in achieving better performance primarily in lower switching voltages along with flexibility. Also, the capacitance effect in a typical metal insulator metal (MIM) device is experimentally demonstrated using impedance spectroscopy analysis wherein the fabricated Gr/MoS2 exhibited programmability i.e. resistance variation with frequency in the low frequency signals and vanishes at high frequencies. The piezoelectric nature of the odd layers of MoS2 added to the reduced SET voltages due to the piezotronic effect at the schottky barrier between Gr and



MoS2 interface. Finally, a hybrid composite based novel MGM device exhibited excellent mechanical robustness against various bending cycles of ≈103, along with its impressive features like biodegradability and low cost. As per the authors’ knowledge, this report is a first demonstration of Gr/MoS2 on cellulose paper based resistive memory. Experimental section: Growth of Gr-MoS2 on Cellulose Paper: 0.1 wt% of Gr was dispersed in N-methyl-2-pyrrolidone using ultra-sonication. Cellulose filter paper was dipped in Gr dispersion for 2 h followed by drying at 70 °C for 2 h. The same process was repeated for 3 times. The Gr coated cellulose paper was further utilized as the substrate for the direct growth of MoS2 wherein it was transferred to the seed solution containing sodium molybdate (10mM) and thiourea (20 mM) in distilled water for 1 h followed by drying of Gr/ cellulose paper at 70 °C for 30 min. Followed by the seeding process, the seed coated Gr/cellulose paper was transferred to Teflon lined autoclave containing the nutrient solution of sodium molybdate (50 mM) and thiourea (100 mM) in 40 mL of DI water. The hydrothermal process was carried out at 200°C for 20 hours followed by the natural cooling to the room temperature. The sample obtained after natural cooling was dried at 70 °C for 30 min resulting in growth of MoS2 on Gr/cellulose paper. Device fabrication: The as-synthesized Gr/MoS2 cellulose paper was cut in desired dimensions, length (L  8 mm) and width (W  5 mm). The top metal electrode was defined using Ag paste followed by heating in oven for 30 minutes. The copper tape was utilized as the bottom electrode of the similar dimensions which forms MIM device structure as Cu/Gr-MoS2/Ag. Similar methodology was adopted for the fabrication of Cu/MoS2/Ag device. Electrical Characterization: The electrical measurements like current v/s voltage measurements were carried out using a Keithley 4200 and 2450 SCS instrument. Impedance characterization was performed using CHI Electrochemical Workstation 660E. Analysis of continuous AC signals was performed using function generator and Digital Storage CRO from Textronics.


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Materials and Characterization: Graphene was procured from Graphene Supermarket, USA with 12 nm ﬂake size. Sodium molybdate and thiourea were purchased from Sigma-Aldrich and were used as received. The structural characteristics of the prepared hybrids were investigated using X’pert PRO XRD with Cu Kα radiation. Raman spectra were obtained from Raman spectrometer (Senterra inVia opus, Bruker) having an excitation wavelength of 532 nm. FESEM analysis was performed by ZEISS Ultra-55 SEM to study morphology. UV–visible–NIR spectra were obtained using LAMBDA UV/Vis/NIR spectrophotometers (PerkinElmer).

Results and discussion: Fabrication of a MGM device on a porous and rough surfaced flexible cellulose paper is a quite challenging task. However, one major advantage of cellulose paper is its porous nature helps in the easy diffusion of silver atoms in localized defect sites i.e. traps present in solution MoS2 which is not possible in most of the traditional polymeric flexible substrates [23]. Further, the cellulose paper not only acts as a substrate but also acts as a dielectric (Gr/MoS2) which further helps in the reduction in the cost of the overall device. Herein, solution processed hydrothermal process was utilized for the direct growth of MoS2 on Gr/cellulose paper which assist in the diffusion of silver (Ag) which also serves as the TE and BE of the fabricated device. The growth of MoS2 on Gr/cellulose paper was well characterized using sophisticated X-ray photoelectron spectroscopy (XPS),

X-ray

diffraction

spectroscopy

(XRD)

and

Raman

analysis.

Morphological

characterizations were performed using Field Emission Scanning Electron Microscopy (FESEM). XPS characterizations revealed the presence of Mo 3d3/2 and Mo 3d5/2 peaks at 228.7, 232.1, 229.6 and 232.8 eV in deconvoluted spectra suggesting the formation of both 2H and 1T phases of MoS2 with 2H phase to be dominant. The deconvoluted S2p peaks are observed at 161.7 and 163.2 eV further confirming the growth of MoS2 on Gr/cellulose paper. Deconvoluted peaks at 285.1 and 288.8 eV suggest the presence of carbon attributing to hydroxyl and carboxyl group due to the exposure of Gr to ambient atmosphere and responsible for making it p type with a small bandgap of meVs which is clearly shown in supporting information figure S1 a, b & c. XRD analysis suggests the growth of rhombohedral and hexagonal phase of MoS2 wherein the peaks match well with the JCPDS card no - 37-1942 and 00-017-0744 which is clearly shown in supporting



information figure S2. Raman studies were performed on Gr-MoS2 hybrid composite which is clearly shown in supporting information figure S3 a & b. FESEM results confirmed the uniform growth of MoS2 on cellulose paper substrate and higher magnification FESEM images suggests the micro-flower morphology wherein individual MoS2 nanoflakes are observed which are clearly shown in supporting information figure S4 a, b & c. The further detailed explanation on chemical and morphological characterization details of the same can be found in a recent report from our lab [14]. A brief overview on complete fabrication process is shown in figure 1.

Figure 1: Fabrication procedure of Graphene-MoS2 RRAM Figure 2a depicts the schematic drawing of as fabricated novel hybrid Gr/MoS2 based bipolar RRAM with silver (Ag) and copper (Cu) as its top and bottom electrodes respectively on disposable paper substrate. The current v/s voltage measurements of as fabricated MGM device have been performed using Keithley 2450 source meter by applying positive voltage on top electrode (Ag) while bottom electrode (Cu) being connected to ground. Figure 2b shows I-V characteristics of the Gr/MoS2 based memristor device clearly exhibiting excellent resistance switching for an applied voltage range of -4V to +4V with switching occurring at ~3.6 V. The reason for the high switching voltage can be attributed to the thickness of the cellulose paper (~ 180 μm). Figure 2c shows resistance variation with an applied voltage which clearly illustrates that a perfect switching from OFF to ON state occurs at a particular set voltage (Vset) ≈3.6V and reverse switching occurs at reset voltage (Vreset) ≈ -3.6V indicating formation and rupture of


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localized silver conducting paths at set and reset voltages inside MGM device. Owing to another important parameter which is RS window margin, the fabricated MGM device exhibited a very good ROFF/RON ratio of ≈104 which can clearly distinguish two states namely high resistance state (HRS) and low resistance state (LRS). Figure 2d clearly explains the complete RS mechanism wherein initially the MGM device is in its pristine state thereby exhibiting a very high resistance ( ≈ 1.5MΩ). As the applied voltage increases in positive direction the fabricated device exhibited Ohmic behavior as depicted as (stage I) in figure 2d until it reaches to a threshold voltage i.e. Vset ≈ 3.6V. As the voltage is further increased, a sudden abrupt change in the current value from few orders of µA to mA was observed suggesting a clear switching of its state to LRS i.e. from MΩ to KΩ which is referred as programming in memory (stage II). During reverse sweep, i.e. voltage range from +4V to -3.6V, device maintains its same LRS passing through origin, which is shown as stage III and IV in figure 2d and at a particular voltage (Vreset) ≈ -3.6V reverse switching occurs from LRS to HRS (Stage V) which then repeat continuously for multiple cycles. To demonstrate the repeatability of the fabricated Gr/MoS2 memristor, 5x102 cycles of continuous tests were performed to check its retaining switching capability. As shown in figure 2e, negligible change in the performance of the fabricated Gr/MoS2 memristor was observed suggesting an excellent repeatable nature of the device over 5x102 cycles. Excellent stable repeatability with good switching ratio is clearly shown in figure 2g by a plot of double logarithmic scale of I-V. Figure 2h clearly depicts as fabricated device stability and also shows a very slight variation in HRS and LRS. Data retention is one of the important performance metric of a memory device which mainly signifies the data holding capability. Data retention characteristics of as fabricated MGM device is plotted in figure 2h for a longer duration of 104 seconds which clearly signifies the excellent data storage capability. To further explore the understanding of the complete conduction mechanism of the fabricated MGM device, variations in the slopes (logarithmic scales) of the fabricated memristor I-V characteristics were studied which gives information regarding the resistance variation of the fabricated memristor. Figure 2f displays the variation in the slopes of the I-V characteristics in logarithmic scale of the fabricated device clearly suggesting the switching behavior. This can be attributed mainly due to localized silver conductive path and trap sites in Graphene-MoS2. Due to porous nature of paper, even after uniform growth of MoS2 on Gr/cellulose paper, there exist defects in forms of structural disorder due to the sulfur vacancies, which acts as trap sites and has



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a major effect on carrier mobility and charge transport of the ions. The conduction mechanism of as fabricated Gr/MoS2 memristor can be explained by considering three regions, wherein region-I is basically a low voltage region from 0V to1.8V, region-II is from 1.8V to Vset (3.5V) and region –III is a reverse sweep voltage region for a voltage range from 4V to 0V. In Region-I the calculated slope value obtained is ≈1 as shown in figure 2f, which is mainly due to lower applied voltage creating a very low electric field thereby exhibiting a linear characteristic or nearly ohmic behavior due to the thermal emission behavior inside the MoS2 domination. As the applied voltage increases and approaches ≈2V, a slope value of ≈2.03 is observed as shown in figure 2f wherein all the porous sites of the Gr/MoS2 are occupied creating a strong electric field thereby allowing the trap assisted space charge current to flow inside MGM active layer. This is explained by considering equation proposed by Mark-Helfrich describing Space charge limited current (SCLC) charge transport with traps given as [24]:

1―𝑚

𝐽=𝑞

{

𝜇𝑛𝑁0

𝜀𝑟𝜀0𝑚 𝑁𝑡(𝑚 + 1)

𝑚 2𝑚 + 1 𝑚 + 1 𝑉𝑚 + 1

}{

𝑚+1

}

………(1)

𝐿2𝑚 + 1

Where 𝜇𝑛 is mobility of MGM, 𝑁0 is effective density of states in conduction band, Nt is trap concentration, V is applied voltage, 𝑚 + 1 is slope of double logarithm J-V plot and L is the distance from TE to BE. As the external applied voltage increases, porous sites located internally inside the paper start occupying gradually and injected charges are free to move through localized graphene-MoS2 interfacial sites. Moreover, these can form a metallic filament bridge based conduction path thereby increasing the density of trapped charge carriers leading to a space charge limited conduction current (SCLC) conduction mechanism and hence an abrupt switching happens at Vset. During the reverse voltage sweeping region i.e. in region- III, as fabricated device maintains its LRS with the calculated slope value as ≈1.03 clearly indicating a ohmic behavior which is clearly shown in figure 2f. Here the current varies linearly with the applied voltage which is mainly attributed due to the already formed silver based metallic bridging between TE and BE and the device maintains its LRS state until a large reverse voltage namely Vreset of approximately -3.6V is applied across the device.


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Figure 2: Electrical characteristics of as fabricated cellulose paper memory. a) schematics of the multilayer Gr/MoS2 (MGM) device b) DC I-V characteristics of MGM device mechanism c) Resistance vs voltage showing set @ 3.6V and reset @-3.6V voltages d) semi-logarithmic I-V curves showing resistive switching e) plot of absolute current vs voltage indicating excellent stable switching for different number of cycles f) Typical plot of Log I vs Log V showing different slopes indicated with different colors for both HRS and LRS g) double logarithmic plot of I-V showing its excellent repeatability for several cycles h) Typical plot of absolute resistance vs number of cycles showing excellent resistive switching endurance over 500 cycles i) Data retention plot of absolute resistance vs time.



A complete microscopic view of resistive switching (RS) mechanism of the as fabricated MGM device is shown in figure 3. RS mechanism is mainly due to electroforming and electro breaking of conductive Ag filament which is formed due to oxidation and reduction of silver at both the top and bottom electrodes. Hydrothermal growth of MoS2 on Gr/cellulose paper, which is performed at a temperature of 200°C, creates traps on the top surface of as fabricated Graphene-MoS2. Micro flowered structure of grown MoS2 along with porous sites of paper can eventually acts as the host locations for silver atoms to partially penetrate on to the top surface portion of as fabricated MGM device clearly shown in figure 3a. As the device is in its pristine state, i.e. HRS some of the silver atoms gets trapped only on the top surface especially nearer to the TE. As soon as external electric field increases, all traps start to get occupied with charged carriers which transport towards BE through Graphene-MoS2 micro flowers. There is the formation of space charged regions at two interfaces namely Graphene-MoS2 interface and metal-MoS2 interface which assist the silver ions to drifts towards BE via MGM active layer shown in figure 3b. In addition, charge transport takes place via hopping of these metallic silver filaments in MGM structure thereby forming a localized conduction paths from TE to BE due to the reduction of the silver ions near the BE resulting in abrupt change of state from HRS to LRS i.e. from OFF to ON as clearly shown in the figure 3c. The formation of conduction path is due to unique layered micro flowered crystal structure of Graphene-MoS2 and the porous nature of the cellulose paper which is lacking in traditional metal oxides like TiO2, HfO2 etc. As the polarity reverses, stored charges get de-trapped from the space charge regions resulting in the empty trap sites in space charge region and hopped metallic silver filaments gets ruptured i.e. electro breaking which is consistent with previous reports [25]. Hence the device instantly changes its state from LRS to HRS shown in above figure 3d. In order to investigate the role of Gr in the performance of MGM device, a controlled set of similar experiments were performed with pristine MoS2 as an active layer with varying bottom electrodes like Cu, Ag, ITO shown in supporting information S5. After a thorough analysis of the fabricated pristine MoS2 based device, it was observed that the device exhibits a proper RS behavior but at a much larger set and reset voltages given as Vset ≈ 8.6V and Vreset ≈ -9V respectively and with almost 50% reduced window margin of ≈102 in comparison with MGM device as clearly shown in (supporting information) figure S5a. Further, to understand the role of metal, different metal contacts were utilized for the fabrication of MGM structure such as Ag/MoS2/Ag, Ag/MoS2/ITO, Ag/MoS2/Cu which interestingly exhibited different responses shown in supporting information


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S5. With Ag/MoS2/Ag, the device demonstrated linear I-V characteristics with no switching, suggesting the diffusion of silver atoms in to porous sites and forming a complete short from TE to BE and its I-V characteristics are shown in supporting information figure S5b. It is important to note that here, the Ag metal contacts were fabricated utilizing Ag paste and hence application of Ag on both top and bottom electrode results in the diffusion of Ag from top and bottom and forming a conduction path. Similarly, with Ag/MoS2/ITO the fabricated device also exhibited linear characteristics as shown in figure S5c. It should be noted that ITO contact was fabricated by placing ITO coated PET on MoS2/cellulose paper which results in a small air gap which does not allow the Ag ions to reach the bottom electrode. With Ag/MoS2/Cu, the device exhibited switching but a higher set and reset voltage and also the ON/OFF ratio was found to be 100. Recently zaho et.al used controlled defect induced graphene as an additional layer to enhance the data retention capability of the memristor based non-volatile memory [26]. In order to explain the role of Gr in MGM device, a complete understanding of interfacial studies at Graphene-MoS2 interface was done with the help of band diagram of Gr/MoS2 as clearly shown in figure 3e. Graphene which is a semimetal upon exposure to ambient atmosphere gets easily oxidized and behaves like a p-type semiconductor with bandgap of few meVs [27]. Few layers MoS2 behaves as an n-type semiconductor which has a band gap of ≈ 1.53 eV. When the junction of MoS2 and Gr is formed, the electrons from the MoS2 diffuse to Gr and at equilibrium there is an alignment of Fermi level which creates a small band bending at the interface which in turn creates local internal high electric field. Due to this high electric field, the ionic drift velocity of metallic Ag+ ions increases which assists in transport mechanism of the fabricated device. In case of pristine few layer MoS2, conductive filament bridge formation happens only due to external fields and effect of internal effect is totally nullified which results in smaller drift velocity with much lesser diffusion coefficient of silver ions. In addition, at high electric fields ionic drift velocity is given as [28]

𝐷

𝑣𝑑(𝐴𝑔) = 𝑘𝐵𝑇𝐸0 exp (E│𝐸0)…….(2) Where 𝑣𝑑(𝐴𝑔) is the drift velocity of silver ions, 𝑘𝐵 is Boltzman’s constant, E is Electric field, D is diffusion coeeficient of the silver aoms and T is the Temperature. At room temperature, under equilibrium conditions, drift velocity (vd) is proportional to diffusion coefficient of silver which is obtained from the above equation (2). As discussed, upon inclusion of Gr there is a formation of



schottky barrier height which produces an internal electrical field thereby increasing the drift velocity. This increase in the drift velocity also increases the diffusion constant of Ag ions through Gr/MoS2 thereby obtaining higher switching ratio at lesser set and reset voltage when compared to pristine MoS2.

Figure 3: Schematic diagram showing complete resistive switching mechanism a) During pristine state, silver atoms partially penetrating on to the top surface portion of as fabricated MGM device b) migration of penetrated metallic silver (Ag+) ions due to positive external electrical field. c) formation of localized metallic filament based conduction bridge via hopping mechanism.


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d) rupture of hopped metallic silver filaments due to reverse external electrical field. e) Energy band diagram of Gr/MoS2 (where ϕB, WB are schottky barrier height and width respectively) To further explore the memristor for its analog applications, continuous time signal with frequency and amplitude as controlling parameters was applied. It was observed that the fabricated device exhibited programmability and interestingly also showed a basic phenomenon of charging and discharging at very low and medium frequencies. To understand the exact cause for such unique exponential behavior, impedance spectroscopy and frequency response analysis on the fabricated device was performed. Figure 4a shows impedance spectroscopy of as fabricated device wherein it clearly depicts the existence of capacitive behavior which is coined up as inherent capacitance that is present in the MGM structure. Similar behavior was observed by Qingjiang et al., where they proved that co-existence of capacitance along with memristors is possible and its switching is independent on frequency [29]. While most of the current research on memristors is focused on achieving better switching performance, the current work presents a comprehensive analysis of the switching mechanism along with a detailed study of the internal capacitance of the fabricated memristor, for which a very few reports are available till date. In this work, capacitance value has been calculated from impedance spectroscopy technique by applying a square wave of amplitude 2V with DC biasing voltage as zero for a wide frequency range of 0.1Hz to 1MHz. The amplitude of 2V was chosen by measuring the open circuit potential before performing the impedance spectroscopy. Experimental results from impedance spectroscopy clearly confirmed that a negative reactance exists which corresponds to capacitance (CM) value of 10pF is clearly depicted in equivalent circuit in inset of figure 4a. This is attributed mainly due to its unique MIM structure which naturally exhibits capacitance property. In order to further confirm the effect of capacitance, frequency response analysis has been done on the fabricated device for a frequency sweep ranging from 0.1Hz to 1MHz and plotted real part of total impedance i.e. resistance Vs frequency as shown in supporting information figure S6a. At very low and medium frequencies, memristor exhibits an inverse relationship between resistance and frequency. Similarly, at very high frequencies the fabricated memristor is independent of frequency with a constant value of memristor, i.e. the MGM was observed to lose its storing capacity and behaves like a resistor with an almost fixed value of resistance. Moreover a phase – frequency response plot shows a negative phase value which clearly confirms existence of capacitance dominant behavior supported by a basic phase equation of θ = tan ―1 (𝑋/𝑅) by considering its overall impedance as 𝑍 = 𝑅 + 𝑗𝑋. At very high frequencies



both phase and capacitance also exhibit almost a constant value which is shown supporting information figure S6b. To have further understanding on the unique capacitive behavior of as fabricated memristor, an experimental setup consisting of a memristor connected in series with a standard test resistor(R) of R≈1kΩ was applied with an input which is a continuous time varying signal. The main goal here was to observe programmability and the capacitance behavior for different frequencies at a constant voltage for the fabricated MGM device. Figure 4b shows the standard experimental setup where an input test signal i.e. symmetrical square wave with fixed amplitude of 4V was applied from a function generator for various frequencies (ranging from low to high). Figure 4c shows the output waveforms for an input square wave of 2V (peak) at high frequency of 50 KHz with a slight reduction in amplitude to ≈3.5V which is mainly due to voltage drop across the memristor. This result is consistent with the impedance spectroscopy results wherein at higher frequencies it was observed that memristor behaves like a resistor and does not have switching behavior. As the frequency gradually decreases and at a frequency of 0.5 KHz a clear variation in resistance approximately from 140kΩ to 90kΩ i.e. programmability was observed which is depicted in figure 4d. In addition, as fabricated memristor shows capacitive dominant behavior with observable charging and discharging of the input signal. Hence, the voltage variation in the input signal is attributed to the memristor behavior wherein the programmability of the resistance is observed and the charging and discharging behavior is due to the inherent capacitance that is present in the memristor structure. More interestingly at very low frequency of approximately at 50Hz, a unique behavior of exponential rise and fall was observed along with an increased variation in resistance from 140kΩ to 90kΩ as shown in figure 4e. In order to have a precise view of the mechanism of frequency response behavior at high frequencies, it is speculated that silver atoms which are located in defect sites nearer to the TE gets reduced, but due to its continuous nature of an applied input, at very high frequencies reduced silver ions will be in an oscillatory phase and are not able to form a complete conducting path from TE to BE leading to a very slight variation in resistance or no change in resistance i.e. memristor loses its non-volatility.


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Figure 4: a) Impedance spectrum of as fabricated MGM device for a frequency range of (0.1Hz1MHz) accomplished via an AC signal at a peak value of 1V with an inset showing its approximated equivalent circuit experimental results illustrating variation of output voltage (resistance) for as fabricated device for an applied continuous AC (square) input of amplitude 4Vpp at various frequencies b) Experimental set up for measuring output voltage across memristor c) variation in resistance for a very high frequency of 50KHz clearly showing minor variation in output voltage d) variation in resistance for an frequency of 0.5KHz clearly showing variation in output voltage e) variation in resistance at a very low frequency of 50Hz for a standard test input of symmetrical square wave of 4Vp-p along with its exponential charging and discharging behavior. Mechanical Robustness/Flexibility and Piezotronics: To check the durability and the mechanical robustness of the fabricated flexible paper based memristor, bending studies were performed wherein the device was bent and bought back to its initial position and memristor characteristics were examined. Figure 5a shows the IV characteristic of the fabricated memristor in the sweeping range of -4V to 4V for 1000 bending cycles wherein very minor change in the performance of the device was observed suggesting the robust nature of the Gr/MoS2 device. This can be attributed to the fact that both MoS2 and Graphene possess excellent mechanical strength and are very well suitable for potential applications in military and aerospace. Moreover, growth of this hybrid nano-composite on a flexible paper substrate provides excellent flexibility. Furthermore, in order to study the effect of bending on the piezoelectric nature of MoS2, the device was bent and then the measurements were recorded. Figure 5b shows the IV curves of the fabricated memristor device in the sweep range of -4V to 4V with and without strain wherein it was clearly observed that under bend conditions the device exhibits higher ON/OFF ratio of ≈5 x 104 and also lower set and reset values of 3.3V and -3.3 V respectively. Moreover, a plot of absolute current vs voltage under strains is clearly shown in supporting information figure S7. This can be attributed to the piezoelectric nature of the odd layer of MoS2. In a recent report from our lab, it was observed that the growth of MoS2 under optimized conditions on Gr/cellulose paper yields trilayer MoS2 which exhibits piezoelectric property [19]. Under bending conditions, there is a lowering of the conduction band energy of MoS2 which results in the increase in the


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schottky barrier height (ϕBP) and width (WBP). This increase in the schottky barrier height increases the electric field at the MoS2/Gr interface thereby increasing the drift velocity of the Ag ions. The increase in the drift velocity of the Ag ions allows them to reach the bottom electrode at a lower voltage and hence the set and reset values under bend conditions are found to be low when compared to relaxed state. This phenomenon wherein the external bending causes an enhanced behavior is termed as piezotronics which is depicted with the help of energy band diagram shown in figure 5c. Figure 5d clearly shows optical image of as fabricated MGM device clearly exhibiting its mechanical robustness.

Figure 5: a) Plot of I-V curve of as fabricated MGM device for repeated bending up to 1000 cycles exhibiting excellent mechanical robustness b) I-V curves under different strains c) Energy band



diagram of Gr-MoS2 with piezotronic effect d) optical image of as fabricated MGM device demonstrating flexibility. There are reports on RRAM utilizing few-layer MoS2 as an active material for various applications [30-32]. Wang et al., reported MoS2 with Graphene as a contact on a polymeric substrate as a memristor exhibiting excellent switching behavior [33]. MoS2 was deposited on silicon substrate using exfoliation process which has a lot of post-processing and utilizes E-beam lithography for metal contacts which not only increases the complexity but also is energy inefficient. Further, the demonstration is performed on Silicon substrate which does not serve to the benefits of flexible electronics. Wu et al., demonstrated N doped MoS2-PVP deposited on ITO substrate as a dielectric material for memristor wherein MoS2 was synthesized using exfoliation process [34]. Zhou et al., fabricated memristor based on MoS2 microspheres on ITO substrate which acts a bottom electrode which demonstrates the formation and rupture of Ag filament using in-situ EDAX [35]. Lien et al., developed all printed paper memory using conventional TiO2 material. However, the fabrication process was performed using printing technique which uses solvents to disperse the active material which degrades the pristine properties of the material. In this work, we demonstrate low cost biodegradable paper based memristor using 2D nanocomposite Graphene and MoS2 as active materials. MoS2 was grown on Graphene coated cellulose paper using one step hydrothermal method. The use of Graphene created electric field at the MoS2/Gr junction which not only increases the drift velocity of the Ag ions but also increases the diffusion of the Ag ions in MoS2. Further, by applying different frequency AC signals programmability was observed which opens new avenues for analog applications using memristor. The present novel approach of flexible, disposable, low cost paper based memristor hold tremendous potential in the field of digital and analog applications. Here in this current work i.e. fabrication of memristor device is on a cellulose paper substrate which is best suited for low cost, flexible and biodegradable applications. Moreover, synthesis of MGM device is done using a two-step direct growth, facile, hydrothermal method which is entirely scalable and can is thus suited for large area production. However, there are still a few challenges like higher surface roughness, non-uniformity etc. which can be improved by adding some of the polymers at the interfaces which not only will acts as a buffer layer but also will reduce the surface roughness without much effect on the device performance. A comparison table with different performance metrics of non-volatile memory of various RRAM’s using MoS2 as an active layer is listed in table 1 which further signifies the


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excellent performance of the as fabricated MGM device. Here in this current work i.e. fabrication of memristor device is on a cellulose paper substrate which is best suited for low cost, flexible and biodegradable applications. Moreover, synthesis of MGM device is done using a two-step direct growth, facile, hydrothermal method which is entirely scalable and can is thus suited for large area production. However, there are still a few challenges like higher surface roughness, non-uniformity etc. which can be improved by adding some of the polymers at the interfaces which not only will acts as a buffer layer but also will reduce the surface roughness without much effect on the device performance. Table 1: Comparison of as fabricated MGM device with previous reports with MoS2 as active material. Material (Active Layer)

Voltage Sweep range(V)

ROFF/RON ratio

Switching Endurance

Mechanical Robustness

Fabrication cost

Ref

MoS2PVP

-6 to +6

≈102

---

NO

HIGH

[36]

MoS2P123

-4 to +4

≈5X102

---

NO

HIGH

[37]

MoS2ZIF

-4 to +4

≈104

---

NO

HIGH

[38]

MoS2

-2 to +2

≈102

200

NO

HIGH

[39]

MoS2Gr

-4 to +4

≈104

500

YES

LOW

[This work]

Conclusion: This work is the first demonstration of a cellulose paper based flexible memristor device utilizing a nanohybrid composite of multilayer Gr/MoS2 (MGM) with silver, copper as top and bottom metal electrodes respectively exhibiting excellent repeatable resistive switching (RS) behavior with an endurance value up to 5x102 cycles, ON to OFF switching ratio of ≈104. A detailed investigation of the role of Gr during RS mechanism suggests that it helps in the formation of a schottky barrier with MoS2 thereby achieving better performance primarily in lower switching voltages. Also, the capacitance effect in the fabricated MIM device was experimentally demonstrated using impedance spectroscopy analysis wherein the fabricated Gr/MoS2 exhibited ACS Paragon Plus Environment


programmability i.e. resistance variation with frequency which is predominant at low frequencies and very less at high frequencies. The piezoelectric nature of the odd layers of MoS2 added to the enhanced ON/OFF ratio due to the piezotronic effect at the schottky barrier between Gr and MoS2 interface. Finally, a hybrid composite based novel MGM device exhibited excellent mechanical robustness against various bending cycles of >103, along with its impressive features like biodegradability and low cost. The successful demonstration of the Gr/MoS2 on cellulose paper based resisitive memory opens up new avenues towards the development of low-cost, disposable and temporary memories which find potential applications in consumer electronics, Internet of Things (IoT), security etc. Acknowledgement A part of the reported work (characterization) was carried out at the IITBNF, IITB under INUP which is sponsored by DeitY, MCIT, Government of India. SB acknowledges financial assistance from Scientific and Engineering Research Board (SERB) grant SB/WEA-03/2017. Supporting information: The complete characterization data like X-Ray Photoelectron spectroscopy (XPS), X-Ray diffraction (XRD), Raman spectroscopy, related to Graphene, MoS2, Graphene-MoS2 nano-hybrid, Field emission scanning electron microscopy (FESEM), current vs voltage characteristics for various device configurations like Ag/MoS2/Cu, Ag/MoS2/Ag, Ag/MoS2/ITO, Plot corresponding to resistance variation with phase and frequency, a plot of absolute current vs voltage for as fabricated Ag/Gr-MoS2/Cu device under different strains.

References 1. Hu, S. G., Wu, S. Y., Jia, W. W., Yu, Q., Deng, L. J., Fu, Y. Q., & Chen, T. P. Review of nanostructured resistive switching memristor and its applications. Nanoscience and Nanotechnology Letters, 2014, 6(9), 729-757. 2. Strukov, D. B., Snider, G. S., Stewart, D. R., & Williams, R. S. The missing memristor found. Nature, 2008, 453(7191), 80. 3. Sun, Y., Zhao, X., Song, C., Xu, K., Xi, Y., Yin, J., & Lv, H. Performance‐Enhancing Selector via Symmetrical Multilayer Design. Advanced Functional Materials, 2019 1808376.


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Strachan, J. P., Pickett, M. D., Yang, J. J., Aloni, S., David Kilcoyne, A. L., Medeiros‐Ribeiro, G., & Stanley Williams, R. Direct identification of the conducting channels in a functioning memristive device. Advanced materials, 2010, 22(32), 35733577. 5. Cavallini, M., Hemmatian, Z., Riminucci, A., Prezioso, M., Morandi, V., & Murgia, M. Regenerable resistive switching in silicon oxide based nanojunctions. Advanced Materials, 2012, 24(9), 1197-1201. 6. Cui, J., & Forssberg, E. Mechanical recycling of waste electric and electronic equipment: a review. Journal of hazardous materials, 2003, 99(3), 243-263. 7. Gergel-Hackett, N., Hamadani, B., Dunlap, B., Suehle, J., Richter, C., Hacker, C., & Gundlach, D. A flexible solution-processed memristor. IEEE Electron Device Letters, 2009, 30(7), 706-708. 8. Choi, S., Lee, H., Ghaffari, R., Hyeon, T., & Kim, D. H. Recent advances in flexible and stretchable bio‐electronic devices integrated with nanomaterials. Advanced Materials, 2016, 28(22), 4203-4218. 9. Akinwande, D., Brennan, C. J., Bunch, J. S., Egberts, P., Felts, J. R., Gao, H., & Liechti, K. M. A review on mechanics and mechanical properties of 2D materials—Graphene and beyond. Extreme Mechanics Letters, 2017, 13, 42-77. 10. Gomathi, P. T., Sahatiya, P., & Badhulika, S. Large‐Area, Flexible Broadband Photodetector Based on ZnS–MoS2 Hybrid on Paper Substrate. Advanced Functional Materials, 2017, 27(31), 1701611. 11. Late, D. J., Huang, Y. K., Liu, B., Acharya, J., Shirodkar, S. N., Luo, J., & Rao, C. N. R. Sensing behavior of atomically thin-layered MoS2 transistors. ACS Nano, 2013, 7(6), 4879-4891. 12. Sahatiya, P., & Badhulika, S. Fabrication of a solution-processed, highly flexible few layer MoS2 (n)–CuO (p) piezotronic diode on a paper substrate for an active analog frequency modulator and enhanced broadband photodetector. Journal of Materials Chemistry C, 2017, 5(44), 11436-11447. 13. Sahatiya, P., Jones, S. S., & Badhulika, S. 2D MoS2–carbon quantum dot hybrid based large area, flexible UV–vis–NIR photodetector on paper substrate. Applied Materials Today, 2018, 10, 106-114.



14. Sahatiya, P., Jones, S. S., & Badhulika, S. Direct, large area growth of few-layered MoS2 nanostructures on various flexible substrates: growth kinetics and its effect on photodetection studies. Flexible and Printed Electronics, 2018, 3(1), 015002. 15. Allen, M. J., Tung, V. C., & Kaner, R. B. Honeycomb carbon: a review of graphene. Chemical reviews, 2009, 110(1), 132-145. 16. Choi, W., Lahiri, I., Seelaboyina, R., & Kang, Y. S. Synthesis of graphene and its applications: a review. Critical Reviews in Solid State and Materials Sciences, 2010, 35(1), 52-71. 17. Geim, A. K. Graphene: status and prospects. Science, 2009, 324(5934), 1530-1534. 18. Frank, I. W., Tanenbaum, D. M., van der Zande, A. M., & McEuen, P. L. Mechanical properties of suspended graphene sheets. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 2007, 25(6), 2558-2561. 19. Sahatiya, P., Kannan, S., & Badhulika, S. Few layer MoS2 and in situ poled PVDF nanofibers on low cost paper substrate as high performance piezo-triboelectric hybrid nanogenerator: Energy harvesting from handwriting and human touch. Applied Materials Today, 2018, 13, 91-99. 20. Yalagala, B. P., Sahatiya, P., Kolli, C.S.R., Khandelwal, S., Mattela, V., & Badhulika, S. V2O5 Nanosheets for Flexible Memristors and Broadband Photodetectors. ACS Appl. Nano Mater, 2019, 2 (2), 937-947 21. Sahatiya, P., & Badhulika, S. Wireless, Smart, Human Motion Monitoring Using Solution Processed Fabrication of Graphene–MoS2 Transistors on Paper. Advanced Electronic Materials, 2018, 4(6), 1700388. 22. Tongay, S., Fan, W., Kang, J., Park, J., Koldemir, U., Suh, J., & Sinclair, R. Tuning interlayer coupling in large-area heterostructures with CVD-grown MoS2 and WS2 monolayers. Nano Letters, 2014, 14(6), 3185-3190. 23. Henriksson, M., Berglund, L. A., Isaksson, P., Lindström, T., & Nishino, T. Cellulose nanopaper structures of high toughness. Biomacromolecules, 2008, 9(6), 1579-1585. 24. Koehler, M., & Biaggio, I. Influence of diffusion, trapping, and state filling on charge injection and transport in organic insulators. Physical Review B, 2003, 68(7), 075205.


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25. Liu, S., Lu, N., Zhao, X., Xu, H., Banerjee, W., Lv, H., & Liu, M.

Eliminating

Negative‐SET Behavior by Suppressing Nanofilament Overgrowth in Cation‐Based Memory. Advanced Materials, 2016, 28(48), 10623-10629. 26. Zhao, X., Ma, J., Xiao, X., Liu, Q., Shao, L., Chen, D., & Cao, R. Breaking the Current‐Retention Dilemma in Cation‐Based Resistive Switching Devices Utilizing Graphene with Controlled Defects. Advanced Materials, 2018, 30(14), 1705193. 27. Sahatiya, P., & Badhulika, S. UV/ozone assisted local graphene (p)/ZnO (n) heterojunctions as a nanodiode rectifier. Journal of Physics D: Applied Physics, 2016, 49(26), 265101. 28. Chen, W., Tappertzhofen, S., Barnaby, H. J., & Kozicki, M. N. SiO2 based conductive bridging random access memory. Journal of Electroceramics, 2017, 39(1-4), 109-131. 29. Qingjiang, L., Khiat, A., Salaoru, I., Papavassiliou, C., Hui, X., & Prodromakis, T. Memory impedance in TiO2 based metal-insulator-metal devices. Scientific Reports, 2014, 4, 4522. 30. Cheng, P., Sun, K., & Hu, Y. H. Memristive behavior and ideal memristor of 1T phase MoS2 nanosheets. Nano Letters, 2015, 16(1), 572-576. 31. Sangwan, V. K., Lee, H. S., & Hersam, M. C. (2017, December). Gate-tunable memristors from monolayer MoS2. IEEE International Electron Devices Meeting (IEDM) 2017 (pp. 5-1). 32. Li, D., Wu, B., Zhu, X., Wang, J., Ryu, B., Lu, W. D., & Liang, X. MoS2 Memristors Exhibiting Variable Switching Characteristics toward Biorealistic Synaptic Emulation. ACS Nano, 2018, 12(9), 9240-9252. 33. Wang, M., Cai, S., Pan, C., Wang, C., Lian, X., Zhuo, Y.,& Liang, S. J. Robust memristors based on layered two-dimensional materials. Nature Electronics, 2018, 1(2), 130. 34. Wu, Z., Wang, T., Sun, C., Liu, P., Xia, B., Zhang, J., & Gao, D. Resistive switching effect of N-doped MoS2-PVP nanocomposites films for nonvolatile memory devices. AIP Advances, 2017, 7(12), 125. 35. Zhou, G. D., Lu, Z. S., Yao, Y. Q., Wang, G., Yang, X. D., Zhou, A. K., & Song, Q. L. Mechanism for bipolar resistive switching memory behaviors of a self-assembled threedimensional MoS2 microsphere composed active layer. Journal of Applied Physics, 2017, 121(15), 155302.



36. Liu, J., Zeng, Z., Cao, X., Lu, G., Wang, L. H., Fan, Q. L., & Zhang, H. Preparation of MoS2‐Polyvinylpyrrolidone Nanocomposites for Flexible Nonvolatile Rewritable Memory Devices with Reduced Graphene Oxide Electrodes. Small, 2012 8(22), 35173522. 37. Tan, C., Qi, X., Liu, Z., Zhao, F., Li, H., Huang, X., & Tang, Z. Self-assembled chiral nanofibers from ultrathin low-dimensional nanomaterials. Journal of the American Chemical Society, 2015 137(4), 1565-1571. 38. Huang, X., Zheng, B., Liu, Z., Tan, C., Liu, J., Chen, B., & Zhang, W. Coating twodimensional nanomaterials with metal–organic frameworks. ACS Nano, 2014 8(8), 86958701. 39. Zhou, G. D., Lu, Z. S., Yao, Y. Q., Wang, G., Yang, X. D., Zhou, A. K., & Song, Q. L. Mechanism for bipolar resistive switching memory behaviors of a self-assembled threedimensional MoS2 microsphere composed active layer. Journal of Applied Physics, 2017, 121(15), 155302.


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