V2O5 Nanosheets for Flexible Memristors and Broadband

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V2O5 Nanosheets for Flexible Memristors and Broadband Photodetectors Bhavaniprasad Yalagala, Parikshit Sahatiya, Chandra Sekhar Reddy Kolli, Shivam Khandelwal, Venkat Mattela, and Sushmee Badhulika ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02233 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 1, 2019

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ACS Applied Nano Materials

V2O5 Nanosheets for Flexible Memristors and Broadband Photodetectors BhavaniPrasad Yalagala#, Parikshit Sahatiya#, Chandra sekhar Reddy Kolli, Shivam Khandelwal, 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 Synthesis of vanadium oxide family compounds is challenging owing to their affinity to exist in different oxidation states and is further intensified by the lack of suitable techniques for their direct growth on flexible substrates thus limiting their applications in flexible electronics. In this report, we demonstrate one-step fabrication of two dimensional(2D) V2O5 based versatile papertronics (paper electronics) platform on low cost cellulose paper substrate and its application towards broadband photo detection and resistive memories. The porous nature of cellulose paper helps in the uniform growth of 2D V2O5 not only on the surface but also in bulk thereby assisting in the easy diffusion of silver ions(Ag+) in the defect sites of V2O5 unlike in conventional flexible polymeric substrates thereby assisting in the resistive switching mechanism. 2D V2O5 on cellulose paper based memory exhibited ON/OFF ratio to be 3.5 x 102 and the Vset and Vreset voltage to be ~ +1V and -1V respectively with excellent endurance and the retention capacity up to 500 cycles. The synthesized 2D V2O5 nanosheets exhibited broadband absorption ranging from ultraviolet (UV) to visible region with an optical bandgap calculated to be 2.4 eV making it suitable for broadband photodetection. Responsivity under UV and visible light illumination were found to be 31.5 mA/W and 20.2 mA/W respectively which are better than V2O5 based photodetectors fabricated using sophisticated methods. The fabricated broadband photodetector exhibited excellent mechanical stability with excellent retention in responsivity values over 500 cycles. The strategy outlined here presents a novel, low cost and one step approach for fabricating devices on paper that find wide applications in flexible electronics. Keywords: 2D V2O5, hydrothermal synthesis, Papertronics, memristor, broadband photodetector

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Introduction Today, for low cost electronic devices and circuits with reduced complexity, cellulose paper has already proven to be the substrate which can host variety of applications including sensors [1], supercapacitors [2], solar cells [3], antennas etc. [4]. Further, it has also been utilized as a disposable and “one time use” sensor/device to mitigate the E-waste issue caused by the conventional silicon technology [5]. This is due to its excellent properties of eco-friendliness, low cost, flexible, lightweight and recyclable over other flexible polymeric substrates. Further, the state of the art production technology for cellulose paper allows for the rapid production which can clearly meet the demand for electronics in near future [6]. The issue is the inability of the cellulose paper to be microfabrication compatible wherein most of the solvent based approaches for the fabrication of the devices on cellulose paper fail. Most of the fabrication approaches on cellulose paper includes screen printing, inkjet printing, direct writing etc. [7-8] which involve complex preprocessing of the inks. The synthesis procedure of such inks engages the pristine materials to be incorporated into suitable solvents using physical mixing or sonochemistry which further degrades the electronic and the mechanical properties of the pristine material. In addition, purification methods involved are sophisticated wherein the size of the nanomaterials should be less than the size of the nozzle of the inkjet printer. Hence it would be interesting to explore a process wherein the functional material can be directly grown on cellulose paper. Flexible electronic devices are the basis of next generation electronics and have evolved as a parallel research to conventional silicon technology. In this approach, there has been a surge of emerging materials exhibiting flexibility, transparency, low dimensionality and broadband absorption [9]. Transition metal oxides have gained significant attention for the fabrication of highly efficient electronic devices which find wide applications in sensors, memories, solar cells, security, surveillance and healthcare due to their excellent electronic and optical properties [1011] and reversible phase transitions from metals and semiconductors. Among the members of the vanadium oxides family, Vanadium Pentoxide (V2O5) possesses excellent electronic and chemical properties which make it a potential candidate for electronic applications compared to other oxides of vanadium [12]. There are reports on V2O5 being utilized for the fabrication of field effect transistor’s (FET), chemical sensors, energy harvesters etc. [13-14]. V2O5 possesses direct bandgap of 2.2 eV to 2.7 eV which makes it an ideal choice for optoelectronic applications such as photodetectors [15]. There are recent reports on synthesizing different structures of V2O5 such


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as nanowires, nanotubes, nanospheres for its utilization photodetectors fabricated using different techniques [16-17]. Although there are reports on one dimensional (1D) structures of V2O5, its low surface area limits the transport of charges and becomes a challenge in defining contacts on 1D structures. 2D structures have inherent advantage of high exposed area and establishing electrical contacts on 2D structures are relatively easy. The deposition or growth of vanadium oxides family is a challenging task because of its affinity to exist in different oxidation states. Control in the synthesis parameters is required for growth of VO2 or V2O5. To synthesize V2O5, numerous methods are utilized like sol-gel, hydrothermal, electrospinning and sputtering [18-19]. Other deposition techniques include physical vapor deposition which are not only time consuming but also energy inefficient [20]. Compared to these methods, hydrothermal synthesis offers the advantages of controlling the morphology and oxidation states by varying parameters such as time and temperature. Despite the excellent properties offered by V2O5, direct growth of V2O5 on cellulose paper and its utilization for various electronic applications remains unexplored. Flexible resistive memories have gained a lot of momentum because of their numerous potential applications in the field of artificial intelligence, analog electronics and neuromorphic computing etc. [21]. The external applied bias control the movement of the silver ions and the oxygen vacancies thereby changing the resistance of the device due to the formation and rupture of the conductive filament under opposite polarity. 2D V2O5 is a wide bandgap semiconductor and possesses dielectric property thereby making it suitable for resistive memory applications. The integration of 2D V2O5 layered structure on porous cellulose paper utilizing hydrothermal process allows for the uniform growth not only on surface but also in bulk thereby allowing for the easy diffusion of silver atoms in the defect sites of V2O5 unlike in conventional flexible polymeric substrates thereby assisting in the resistive switching mechanism. Despite the advantages offered by 2D V2O5 and its ease of integration on cellulose paper, studies along using the combination for flexible, disposable resistive memories still remains unreported. For fabrication of memristor, the metal contacts need to be fabricated on top and bottom sides of V2O5. The metal contacts on top and bottom side of V2O5 restricts its usage in other applications wherein the sensing material should be exposed to the stimuli. In such cases, if the positions of the metal contacts are changed, it can lead to different varied applications. To explore this, instead of



top and bottom electrode, planar electrodes are fabricated for its application in broadband photodetection ranging from ultraviolet (UV) to visible illumination. In this report, we demonstrate for the first time, direct growth of V2O5 nanosheets on cellulose paper using simple and cost effective, solution processed hydrothermal method and its utilization as resistive memory and broadband photodetector ranging from UV to visible spectrum. Optimization in terms of hydrothermal process was performed to obtain 2D V2O5 nanosheets on cellulose paper. 2D V2O5 on cellulose paper based resistive memory exhibited the ON/OFF ratio and Vset voltage with excellent endurance and retention capacity up to 500 cycles. Further detailed switching mechanism is discussed with respect to the formation and rupture of the conductive filament. Also, the absorbance of 2D V2O5 nanosheets from UV to visible region ensured the successful performance of broadband photodetector. Responsivity under UV and visible light illumination were found to be 31.5 mA/W and 20.2 mA/W respectively. To the best of the authors’ knowledge, this report is the first demonstration of 2D V2O5 nanosheets directly grown on cellulose paper via simple hydrothermal technique and its application as resistive memory and broadband photodetector. Experimental Section Solution processed hydrothermal route was followed for fabrication of V2O5 on cellulose paper substrate. Ammonium Metavanadate (NH4VO3) as a main precursor and reducing agent Hydroxylamine Hydrochloride (NH2OH.HCl) were used and no other additives were used for uniform growth of V2O5 nanosheets. Cellulose paper was dipped into 3mmol NH4VO3 seed solution for 1 hr followed by drying at 70°C for 15 minutes. The seed coated substrate was then transferred to autoclave containing 30mmol of NH4VO3 and 20mmol of NH2OH.HCl nutrient solution and kept at 200°C for 20 hrs. Post hydrothermal, the V2O5 grown cellulose paper substrate was dried at 70ºC for 15 minutes. Silver paste was utilized as contacts on both end of the as grown V2O5 on cellulose paper. Schematic of the device fabrication is as shown in figure 1.


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Figure 1: Schematic of the fabrication of 2D V2O5 nanosheets for resistive memory and broadband photodetector

Results and discussions Flexible electronics has been in the forefront of electronics research due to its wide potential range of applications in sensors, transistors, energy harvesters etc. But the fabrication of most of such devices require highly sophisticated cleanroom techniques which are not only expensive but also energy inefficient. There are flexible substrates whose properties can be utilized for sensing applications but are not cleanroom compatible. Hence there is an urgent need to develop processes to directly grow/deposit nanomaterials on the substrate of choice. Chemical vapor deposition is a viable technique to grow single layer 2D materials but is plagued by complexities of the transfer technique which introduces variability in device performance. Moreover, all substrates are not CVD compatible. Solution processed synthesis method such as hydrothermal are alternative for direct growth of nanomaterials on the substrate of choice. Further, the morphology of the nanomaterial can be tuned by varying the hydrothermal parameters. Herein, direct growth of 2D V2O5 nanosheets on cellulose paper was employed by the use of hydrothermal method.



Temperature of the hydrothermal method was optimized to 200°C wherein no formation of V2O5 was observed below 200°C and increasing the temperature beyond 200°C degrades the substrate properties. To confirm the chemical composition of the synthesized 2D V2O5, Raman analysis of V2O5 was performed and is as shown in Figure 2a. The vibrations at 145 and 195cm-1 are due to the VO5 VO5 modes [15]. Peak at 994 cm-1 corresponds to the doubly coordinated oxygen bonds (V=O) and 694 cm-1 peak can be attributed to triply coordinated oxygen bonds due to the stretching modes at atomic level. The Raman peak at 490 cm-1 corresponds to the stretching mode of V-O3-V and the bending vibration of the V=O bonds are observed at 284 and 407 cm-1 modes [22]. Here, Energy-dispersive X-ray (EDX) wasn’t performed, since XPS provides the elemental composition. Authors tried performing AFM studies for the surface roughness analysis, but due to the rough surface of cellulose paper and the microfiber like morphology the tip of the AFM was damaged. To further confirm the elemental composition and the oxidation state of as synthesized V2O5, Xray photoelectron spectroscopy (XPS) analysis was performed. Figure 2b shows the deconvoluted spectra of V2O5 grown on cellulose paper wherein a peak at 517.9 eV confirms the V+5 oxidation state of V2O5 and can be assigned to V 2p3/2. Also, figure 2c shows the peak at 530.6 eV which is due to the presence of oxygen and corresponds to O 1s. Combining the results from Raman, XRay Diffraction (XRD) (supporting information SI, Figure S1) and XPS study, it is concluded that the as grown film on cellulose paper is V2O5. Here we still tried to perform TEM analysis on the V2O5 grown cellulose paper but due to the large thickness of the cellulose paper (180 microns), it was very difficult to prepare sample for TEM analysis. Further, performing TEM studies on the V2O5 powder sample would not be beneficial and also would be misleading as the substrate play an important role in the morphology and the crystal orientation of the nanomaterials synthesized utilizing hydrothermal method.

To study the morphology of as synthesized V2O5, Field emission scanning electron microscopy (FESEM) imaging was performed wherein nanosheet structure like morphology was observed as shown in figure 2d. High yield of stacked V2O5 nano sheets in the range of below 200 nm were observed. Residual nanorods structures arising from the hydrothermal reaction are also visible which can be attributed to mechanical brittleness of V2O5 during hydrothermal reaction


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[23]. The nanosheet morphology was found to be consistent with the in-plane vibrations observed in Raman spectra of V2O5 where the peaks corresponding to y-x (out-of-plane) were missing. Figure 2e shows the high magnification image of V2O5 wherein individual nanosheet appear to transparent demonstrating the growth of few layer V2O5 nanosheet.

Figure 2: a) Raman spectra of 2D V2O5 nanosheets grown on cellulose paper b) Deconvoluted XPS spectra for V 2p3/2 and c) for O 1s d) FESEM of V2O5 demonstrating 2D nanosheet like morphology e) high magnification FESEM image showing few layer V2O5 nanosheet The mechanism for the formation of 2D V2O5 nanosheet can be described by the steps followed during the hydrothermal reaction. For direct growth of V2O5 on cellulose paper two step hydrothermal method was employed where the cellulose paper substrate was first subjected to seeding process followed by the hydrothermal growth of 2D V2O5 nanosheets. Seeding layers



forms nucleation sites which helps in the growth of V2O5 during hydrothermal process. Seeding layer consists of dip coating the cellulose paper substrate in the precursor which forms nanoparticles acting as a nucleation sites for growth. During the hydrothermal process, NH4V4O10 first form in the reaction which possess same crystal structure as that of V2O5 and results in the formation of nanorods like structures. Then, nanorods in the solution grow together along the specific facets to form V2O5 nanosheets by the process of Ostwald ripening [24]. As the hydrothermal reaction progresses, small nanosheets agglomerate and high nanosheets breaks. With further reaction going on, all the nanorods are converted into well-defined V2O5 nanosheets where several nanosheets stack epitaxially so as to minimize the energy. It should be noted that several nanorods are still seen in the FESEM image (figure 2) which is due to the mechanical brittleness of redox V2O5 caused by the increased disorder and decreased coherence length. Authors indeed tried taking cross section imaging but because of the cellulose paper whose thickness is ~ 180 µm the imaging was not proper. Further, due to the stacking of the individual V2O5 the thickness measurement for individual V2O5 was not possible. Also, the individual stacking of V2O5 is possible by the Van der Waals forces and hence the nanosheet morphology that is observed from the FESEM images can be considered as 2D material.

2D V2O5 on cellulose paper as resistive memory Figure 3a shows the schematic for experimental set up of as fabricated device with vanadium pentoxide (V2O5) grown on cellulose paper as active switching layer, silver (Ag), copper as top electrode (TE) and bottom electrode (BE) respectively. Figure 3b represents I-V curve of as fabricated device, with all its electrical measurements performed upon application of external bias on TE and connecting BE to ground, as can be clearly seen, it exhibits an excellent bipolar RS mechanism. Initially as the device is its pristine state, it will be in its high resistance state (HRS) i.e. the OFF state and then exhibits a stable switching behavior for a DC voltage sweep ranging from -1V to 1V. In order to have a clear understanding of the RS mechanism, a semi logarithm plot of absolute current vs voltage was plotted as is shown in figure 3c. As voltage starts increasing in forward direction at one particular voltage which is known to be as its set voltage of VSET ≈ 1V an abrupt change in current from 10nA to 3.3µA occurs, which clearly indicates change of device


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state from high resistance state (HRS) to low resistance state (LRS) for a compliance current set to 4µA and reverse switching of LRS to HRS at a reset voltage of VRESET ≈ -1V. Figure 3d depicts semi logarithmic plot of current vs voltage for number of cycles exhibiting almost stable switching mechanism at the same set and reset voltages. Moreover, the fabricated device exhibited excellent switching repeatability i.e. switching endurance for almost 500 cycles, with a decent resistance window margin of RON/ROFF ≈ 3.5 x 102 clearly suggesting that it can function as an excellent memory device. Data retention capacity is one of the important characteristic feature which determine the stability of a memory device. In case of the as fabricated Ag/V2O5/Cu device, it maintains its stable HRS and LRS states for a retention time of 1500 secs as clearly shown in figure 3e. In order to further examine the current conduction mechanism of the as fabricated Ag/V2O5/Cu device, a plot of double logarithm of I-V was drawn as shown in figure 3f. In region 1, i.e. in a small positive voltage region i.e. from (0V - 0.5V), current in HRS slowly starts increasing linearly with a slope of almost ≈1.69 (indicated with a pink line arrow). This clearly suggests a near nonohmic behavior, which is mainly due to lower electric fields created internally between two metallic electrodes. Moreover, at this lower electric field, a very less number of charge carriers get injected inside the active region of device from TE, as compared to localized internal thermal carriers. Thus the current density of the device is mainly due to this higher concentration of localized thermal carriers. In region 2, i.e. for applied bias voltages greater than 0.5V, electric field starts increasing leading to an increase in the number of external injected charge carriers which slowly start occupying all the traps present in active layer of the device. Moreover, current in this region starts increasing in an exponential manner clearly indicated by a calculated slope value of ≈1.9 i.e. I ~V2 and the traps most commonly refers to the defects or vacant sites, where in this case corresponds to both the defect sites in V2O5 as well as porous sites present inside the cellulose paper [25]. At one particular voltage known as the set voltage, all the traps get completely filled and the device current values change abruptly, clearly suggesting device has switched its state from HRS to LRS and calculated slope is very high at that instance (indicated with a yellow line arrow) and clearly shown in figure 3f. This typical behavior of current conduction mechanism which is mainly due to traps is considered to be as trap-assisted space charge limited current (SCLC) or Mark-Helfrich law with its current density (J) is given as [26]



{

J = q1 ― mμnN0

m 2m + 1 m + 1 Vm + 1 ……. Nt(m + 1) m+1 L2m + 1 εrε0m

}{

}

(1)

Where q is the charge, µn is the mobility of charge carrier, Nt is the trap density, m+1 is the slope of I-V plot, V is the applied voltage, L is the width of the active layer etc. Subsequently, the device in its LRS exhibits a linear I-V characteristics i.e. ohmic behavior with its calculated slope value as ≈1 which is clearly shown in figure 3f (indicated with a blue line arrow) suggesting that there might be a formation of localized conductive paths between TE and BE. Here in our case, a plausible explanation to this is the penetration of Ag filaments inside the porous sites of cellulose paper during fabrication process.


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Figure 3: Electrical characteristics of as fabricated Ag/V2O5/Cu device a) schematic showing layer stack along with measurement set up for electrical characterization with TE (Ag) always connected to positive terminal and BE(Cu) is grounded. b) plot of I-V curve illustrating complete DC switching characteristics of as fabricated Ag/V2O5/Cu device c) Semi logarithmic plot of absolute current Vs voltage clearly indicating switching behavior with set and reset values as 1V and -1V respectively d) Semi logarithmic plot of absolute current Vs voltage clearly indicating excellent switching endurance for number of cycles e) Retention characteristics of Ag/V2O5/Cu device showing very good window margin along with retention of states f) Double logarithmic plot of IV for positive region along with slopes in different regions depicting different mechanisms responsible for charge transport. A complete RS mechanism of as fabricated Ag/V2O5/Cu device is explained in figure 4. RS mechanism in the device can be mainly attributed to metal cation based conduction bridge RS mechanism i.e. due to formation and rupture of localized metallic conductive filament (Ag) between TE and BE. However, possibility of occurrence of both conduction bridging mechanism(CBRAM) and redox based valence switching mechanism(VCM) is possible in as fabricated device. There is still lot of open questions in the exact description of the switching mechanism and switching kinetics. Moreover, to clearly identify the movement of metallic cations a controlled analysis using the Conductive Atomic Force Microscopy(CAFM) for as fabricated device is needed, but due to high surface roughness of cellulose paper and the microfiber like morphology, the tip of the CAFM will easily get damaged. Even though both mechanisms are possible in V2O5 based RRAM, as per the authors knowledge, due to more surface roughness and porosity of paper substrate there is greater possibility of silver ions gets trapped in the porous sites along with defect sites located on surface. Mobility of silver ions is higher compared to oxygen anions, in which movement of silver ions is much more quicker compared to oxygen vacancies which further leads to the CBRAM mechanism. Moreover, from authors point of view, formation of oxygen vacancies is less which is confirmed using XRD, XPS and hence no movement of vacancies is possible. More importantly an abrupt rise and fall of current at set and reset voltages is one possible reason, which mostly happens in conduction bridging mechanism and one more conformation from slopes calculation that it is varying linearly(ohmic) during reverse sweeping which is less frequently occurs in the VCM. Moreover, compared to VCM, CBRAM is a very matured theory and a lot of experimental proofs have also stated the existence of movement of



metal cations like Ag+, Cu+2 etc. compared to oxygen vacancies. However, from the previous reports, most of the low and moderate bandgap materials exhibits CBRAM mechanism whereas high bandgap materials like HfO2 exhibits VCM. As fabricated device is in its pristine state, externally applied electric field created is zero and most of the silver atoms gets trapped inside the porous sites of cellulose paper and the device will be in its HRS, mainly due to non-existence of localized conductive metallic bridge between TE and BE. V2O5 with a band gap of 2.4 eV behaves as a good dielectric material with its measured resistance value in terms of few MΩ. Initially, during the deposition of silver as a TE, some of the silver atoms penetrate in to the porous sites i.e. traps which are present at boundaries of nanosheets in the active layer of V2O5 deposited on cellulose paper, which further act as the localized metallic filaments. Moreover, there is very less possibility of formation and movement of vanadium or oxygen due to their high valences under external bias. However, it is equally important to consider, that only some of the silver atoms which are uniformly distributed only on the top surface of the active layer participated in formation of metallic bridge which is clearly depicted in figure 4a. As the externally applied bias slowly increases i.e. in low voltage region, Ag atoms which are accumulated near TE, gets oxidized and form Ag+ ions inside nanosheets of 2D structured V2O5 as illustrated in figure 4b. Upon application of larger voltages, much stronger electric field gets created which in turn lowers the barriers for the Ag+ ions, which then slowly start drifting towards BE with field directed thermally generated charge transport mechanism. At these higher electric fields, there will be soft breakdown of dielectric strength of V2O5 leading to formation of some defect sites producing V+5 and O2- and silver ions quickly start hopping from TE to BE. Silver ions after reaching the BE gets reduced and finally form a localized conductive path resulting in flow of higher currents producing lower resistance values [27]. Moreover, at the same instant, device abruptly changes its state from HRS to LRS considered to be as SET state of memory device which is clearly shown in figure 4c. As the polarity of applied voltage reverses, at one particular voltage namely VRESET, a very high electric field is generated in opposite direction to previous state leading to breakdown of the formed metallic filament bridge i.e. Ag gets redistributed at the near boundaries of BE, then instantly device switches its state from LRS to HRS which is illustrated in figure 4d. Moreover, all the previously trapped charges get detrapped from porous sites disturbing the localized variable hopped metallic filament based bridges. On continuous application of set and reset voltages for number of times makes the device to change its state from HRS to LRS and vice versa. Platinum,


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gold are much costlier as compared to copper. Since our main objective is to fabricate a low cost memory device authors have chosen cellulose paper as substrate, copper as bottom electrode and still its performance doesn’t get effected. Moreover, standard electrode potential of copper is almost comparable to the gold, platinum.

Figure 4: Schematic diagram explaining resistive switching mechanism of V2O5 based RRAM a) Device in its pristine state with partially penetrated silver atoms during fabrication b) As applied positive bias slowly starts increasing leading to oxidation of silver and migration of silver ions towards bottom electrode c) completely migrated silver ions and forming a metallic conduction bridge indicating a SET process d) complete disrupt of metallic conduction bridge due to applied positive bias clearly indicating a RESET process. A generalized memristor model which can exactly fit the experimental results provides a lot of scope for number of circuit simulations which can then be explored for a multiple parameter optimization by considering a wide range of inputs [28]. Model fitting studies for as fabricated Ag/V2O5/Cu device was performed by considering the memristor working phenomenon with the available basic electronic components such as comparators, voltage sources, logic gates etc. In this case, the memristor was considered as a two terminal resistance switching device with two states i.e. a HRS and LRS in which the state resistance values correspond to ROFF, RON respectively, which are extracted from our experimental work. Memristor behavior was considered to be as a voltage controlled current source with a gain value of unity. A complete schematic describing



memristor modelling and the schematic set up for modelled memristor I-V plot can be found in (supporting information SI, Figure S2a). All simulations were performed using LTSPICE simulator. Interestingly, the simulated and experimental values were exactly in agreement with very slight variation in set and reset voltages as shown in figure 5a. All parameters considered for modelling are listed in (supporting information table S1). In order to further explore various electronic applications related to digital logic gates, memories etc. such modelled memristors are very helpful from design perspective. A plot of output voltage, current v/s time for modeled memristor can be found in (supporting information Figure S2b, c). A testbench setup for memristor model simulation in LTSPICE is shown in (supporting information Figure S2d).

Mechanical robustness and flexibility Most of the research in the field of memories focuses on achieving better switching performance in terms of higher resistance window margin, retention and endurance [29]. However, efforts to develop flexible memories which have wide applications in flexible electronics has received scant attention. Interestingly, a very few studies have been reported with V2O5 as an active material for RRAM applications. Most recently Putrolainen et.al reported Si/SiO2/V2O5/Au device exhibiting a bipolar resistive switching [30]. However, the RON/ROFF was very low as compared to that of the as fabricated device in this work. In addition, it was fabricated on a traditional Silicon substrate using conventional cleanroom based techniques such as sputtering which is not only expensive but time consuming. Cellulose paper, on the other hand, offers numerous advantages in terms of low cost, flexible, simple and facile fabrication. In order to have a thorough understanding of the mechanical robustness of the fabricated device, the paper based memristor was subjected to different bends i.e. 500 cycles and I-V plot after repeated bends is shown in figure 5b. In order to further examine the flexibility of the as fabricated V2O5 based RRAM device, it was subjected to bending for 500 cycles of arbitrary radii. It should be noted that the bending test was performed by bending the device to arbitrary radii and brought to its initial position followed by which the measurements were performed. Interestingly, the device exhibited negligible change in the performance even after 500 cycles suggesting the mechanical robustness of the device. Interestingly, the device didn’t lose its flexibility and exhibited very good switching mechanism without any deviation in switching performance. Moreover, device showed a very slight variation


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in switching voltages as well as retention, which confirms its stable behavior under bending conditions.

Figure 5: a) Hysteresis I-V plot of both fabricated and simulated devices which exactly match with each other and also switch at same set and reset voltages b) Hysteresis I-V plot of as fabricated device under various bending tests showing its excellent stability. 2D V2O5 as broadband photodetector To further utilize it as a broadband photodetector, absorbance spectra was studied using UVVisible-Near-Infrared(NIR) spectroscopy. Figure 6 shows the UV-vis-NIR spectrum of V2O5 where its exhibits strong absorbance in UV range (220-360 nm) where as weak and constant absorption curve at visible region. There is a peak at starting of the visible spectrum at around ~420 to 500 nm which suggest that synthesized V2O5 can also be utilized as broadband photodetector covering UV and visible range as shown in figure 6. Optical bandgap of synthesized V2O5 was calculated to be 2.4 eV using tauc’s plot shown in inset of figure 6.



Figure 6: UV-vis spectroscopy of synthesized 2D V2O5 nanosheets Inset is the Tauc’s plot showing the optical bandgap of 2.4 eV. To study the behavior of fabricated V2O5 device under UV illumination, IV studies were performed using Ag paste as contacts which exhibits ohmic contact as shown in fig 7. Prior to measuring the UV response, devices were kept in dark for 12 hours so as to stabilize them. UV lamp (365 nm) and visible light (554 nm) were used for illumination. Figure 7a shows IV characteristics with different UV light intensities wherein increment in current is observed with intensity. This can be attributed to the increased number of photo generated electron-hole pair under high intensity illumination. Same experiments were repeated for visible light illumination and similar results were observed wherein increasing the visible light illumination intensities increases the photocurrent as shown in figure 7c. Figure 7b shows the temporal response of V2O5 photodetector upon increasing UV illumination for 3 cycles where the illumination was switched “ON” and “OFF” at repeated intervals. 28 % increment in the normalized resistance was observed at the highest intensity of UV illumination (13.8 mW/cm2). Similar measurements were repeated for visible light and 18 % increment in the normalized resistance was observed at the highest intensity (7.2 mW/cm2) suggesting that the fabricated device was more responsive to UV illumination when compared to visible light illumination as shown in figure 7d. This can also be attributed to the more absorbance observed for V2O5 in UV region when compared in visible region. It should be


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noted that even though the intensities used for the measurements are not same, responsivity and external quantum efficiency were calculated at the same intensities of UV and visible light. Responsivity and external quantum efficiency (EQE) are important parameters for evaluating the performance of photodetector. Responsivity can be defined as the photocurrent generated per unit power of incident light and EQE is the number of electrons generated per incident photon and are given by the following equations [31]. Rλ =

Iλ Pλ × A

EQE = hc X Rλ /eλ



Figure 7: a) I-V characteristic of fabricated 2D V2O5 device under UV illumination b) temporal response of fabricated 2D V2O5 device under UV illumination c) IV characteristic of fabricated 2D V2O5 device under visible illumination d) temporal response of fabricated 2D V2O5 device under visible illumination Figure 8a shows the responsivity graph of fabricated device with different intensities of UV and visible light. As intensity of the illumination increased, increased in the responsivity was observed for both UV and visible light illumination with device exhibiting more responsivity towards UV when compared to visible light illumination which can be attributed to the fact that the absorbance of V2O5 is more in UV region when compared to visible region thereby generating more photo generated carriers under UV illumination. It should be noted that even though pristine 2D V2O5 was utilized and no transport material was deposited or grown for enhancing the responsivity, observed responsivity was higher than previously reported papers on V2O5 photodetectors. This can be attributed to the 2D nanosheet morphology wherein the ballistic transport of the photo generated carriers becomes prominent. Further to the study the reliability of the fabricated photodetector, the device was subjected to 500 bending cycles wherein the device was bent and bought to its initial position and responsivity measurements were performed. As can be seen from figure 8b, no notable difference in the responsivity values were observed even after 500 bending cycles demonstrating the robust nature of the fabricated device. Graph showing the detectivity of the fabricated photodetector with varying intensities is as shown in (Supporting information S3). The ON/OFF repeatability is poor and the possible reason for this is the defects introduced in V2O5 during the hydrothermal growth on cellulose paper. Further, the surface roughness of the cellulose paper is very high which hinders the mobility of the photogenerated carriers thereby decreasing the responsivity and also the ON/OFF ratio of the fabricated photodetector. Here, it could be further optimize by initially treating the cellulose paper to reduce the surface roughness and optimize the hydrothermal parameters for the reducing the defects introduced during the hydrothermal.

The transport mechanism of 2D V2O5 broadband photodetector can be best explained by energy band diagram as shown in figure 8c. Electron affinity of V2O5 is 4.3 eV and the optical bandgap calculated from the absorption spectra was 2.4 eV. Under UV illumination, when the illumination


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energy is greater than or equal to the bandgap of V2O5, electrons-hole pairs are generated. Due to the applied electric field and the electric field created at the metal-V2O5 interface, the electronhole pair gets separated which leads to the increase in photocurrent. It should be noted that the electric field is only present at the metal-V2O5 interface and not in the areas far away from the metals which leads to lower responsivity values. It can be increased by incorporating transport material with V2O5 whose conduction band lies below the conduction band of V2O5. Incorporation of the transport material not only increases the complexity in the fabrication process but also sometimes leads to the recombination of the photo generated carriers if the placements of the metal contacts are not studied properly. Responsivity of the photodetector can also be increased if the morphology of the grown material can be optimized and suited to behave as a ballistic transport which increases the mobility due to the reduced collision thereby avoiding the recombination. The photo generated electron-hole pair gets collected at Ag contact whose conduction band lies at a lower energy level than V2O5 conduction band.



Figure 8: a) Responsivity calculations of the fabricated 2D V2O5 device under different illumination intensities b) Responsivity v/s bending cycles demonstrating the reliability of the fabricated device c) Band diagram of fabricated 2D V2O5 device with Ag paste as contacts. There are few reports on V2O5 based photodetectors using various synthesis procedures. Fu et al reported V2O5 nanowire synthesis using hydrothermal and utilized it for UV photodetector [32]. Song et al., demonstrated the synthesis of V2O5 by hydrothermal method followed by calcination process [33]. Pawar et al., synthesized V2O5 using hydrothermal method and demonstrated multifunctional sensors [34]. But these reports involved drop casting method for fabrication of sensors on glass and Si/SiO2 substrates which often leads to device to device variation. Kim et al., reported the deposition of V2O5 on Si/SiO2 substrate by sputtering method for broadband photodetector [35]. Moreover, the substrates used were rigid which restrict their use in flexible electronics applications. Here, we demonstrate one step direct growth of V2O5 nanosheets on flexible cellulose paper substrates using simple solution processed hydrothermal method. The use of hydrothermal method avoids the complexity of transfer process used in Chemical Vapor deposition (CVD) method and reduces the overall cost. The method presented here is scalable and can be extended to grow any functional material of choice on wide area. There are few reports which utilize V2O5 as an active material for the fabrication of memristor. Ying et.al deposited V2O5 on a traditional non-flexible silicon substrate which is used for resistive random access memory (RRAM) application using ion beam sputtering which is not only an expensive technique but also is time consuming compared to inkjet printing, spin coating, electrodeposition etc. Lin et.al demonstrated deposition of V2O5 on silicon substrate for fabrication of RRAM device using RF sputtering which is not suitable for most flexible substrates. Moreover, sputtering targets are very costly and need high vacuum and sophisticated controlled environment for deposition. Current state of the art which uses V2O5 as an active layer for RRAM application are listed in below table 1. In comparison to them, this work reports the successful fabrication of V2O5 based flexible RRAM and broad band photodetector on rough surfaced cellulose paper using a single step hydrothermal process. The overall performance of both photodetector and memristor are exceptionally good. Model fitting studies for memristor which exactly matches with the as fabricated memristor have been studied. The as fabricated memristor due to its excellent features will be further explored for various analog and digital applications in the near future.


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Table 1: State of the art comparisons with previously published reports which utilizes V2O5 as an active material for RRAM application. S.NO

Active Material

RON/ROFF

1

V2O5

≈10

2 3

V2O5/Sm2 ≈100 O3 V2O5 ≈10

4

V2O5

3.5x102

Operating voltage

Flexibility

Switching Substr Endurance ate

-6V to +6V

NO

100

Silicon [36]

-3V to +3V

NO

Not reported Not reported 500

Silicon [37]

-12V to +12V NO -1V to +1V

YES

Ref

Silicon [38] Paper

This work

Conclusion In this report, we demonstrate the growth of V2O5 nanosheets on flexible cellulose paper substrate using simple solution processed hydrothermal method and subsequently utilize it as a memristor and broadband photodetector. FESEM and Raman confirmed the formation of well stacked thin nanosheets of V2O5. 2D V2O5 on cellulose paper as resistive memory exhibited an excellent ON/OFF ratio of 3.5 x 102 and the Vset and Vreset voltage as ~ -1V and 1V respectively with endurance and retention up to 500 cycles. Further, UV-visible spectroscopy analysis revealed broadband absorption of the synthesized 2D V2O5 from UV to visible region. By altering the placement of the metal contacts to planar electrodes, the 2D V2O5 on cellulose paper was further explored for broadband photodetector wherein the responsivity values calculated under UV and visible light illumination were 31.5 mA/W and 20.2 mA/W respectively. Successful fabrication of V2O5 based flexible papertronics device is a major step ahead in multifunctional electronic applications which hold tremendous potential in the field of flexible electronics and optoelectronics. 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.



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Supporting information: X-Ray Diffraction(XRD) analysis for as fabricated V2O5, to support the experimentally obtained data, a simulation model for memristor was developed. A complete set of constant values for memristor model in tabular format, Equivalent circuit diagram for memristor model, working of memristor model, voltage across memristor vs time, current across memristor vs time, testbench setup for memristor model simulation, Graph showing the detectivity of the fabricated photodetector with varying intensities. References 1. Kanaparthi, S., & Badhulika, S. Low cost, flexible and biodegradable touch sensor fabricated by solvent-free processing of graphite on cellulose paper. Sensors and Actuators B: Chemical, 2017 242, 857-864. 2. Feng, J. X., Ye, S. H., Wang, A. L., Lu, X. F., Tong, Y. X., & Li, G. R. Flexible Cellulose Paper‐based Asymmetrical Thin Film Supercapacitors with High‐Performance for Electrochemical Energy Storage. Advanced Functional Materials, 24, 7093-7101, 2014. 3. Gao, C., Yuan, S., Cui, K., Qiu, Z., Ge, S., Cao, B., & Yu, J. Flexible and Biocompatibility Power Source for Electronics: A Cellulose Paper Based Hole‐Transport‐Materials‐Free Perovskite Solar Cell. Solar RRL, 2, 1800175, 2018. 4. Kanaparthi, S., Sekhar, V. R., & Badhulika, S. Flexible, eco-friendly and highly sensitive paper antenna based electromechanical sensor for wireless human motion detection and structural health monitoring. Extreme Mechanics Letters, 2016, 9, 324-330. 5. Kumar, A., Holuszko, M., & Espinosa, D. C. R. recycling p E-waste: an overview on generation,

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192x132mm (96 x 96 DPI)


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V2O5 Nanosheets for Flexible Memristors and Broadband

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