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Organic molecule stabilized bismuth sulfide nanoparticles: A potential system for bistable resistive memory application
Physica E 116 (2020) 113787
Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Organic molecule stabilized bismuth sulfide nanoparticles: A potential system for bistable resistive memory application Venkata K. Perla, Sarit K. Ghosh, Tarasankar Pal, Kaushik Mallick * Department of Chemical Sciences, University of Johannesburg, P.O. Box: 524, Auckland Park, 2006, South Africa
A B S T R A C T
The performance of the microelectronic devices depends on the material used for the fabrication of the device and also the device architecture. In this paper, we demonstrate the synthesis of ‘organic molecule stabilized bismuth sulfide nanoparticles’ (BSN) by applying a one-pot, wet-chemical route. The as-synthesized material was characterized using microscopic and optical techniques. The organic-inorganic composite system displayed electronic memory effect based on the unipolar resistive switching phenomenon. The current-voltage characteristic of the device is fitted with the thermionic emission and space charge limited current model for the OFF-state, whereas, the ON-state is fitted with the combination of trapped-space charge limited current and trap-free space charge limited current model. The study of endurance and the nonvolatile behaviour of the device was performed for 103 cycles and for 2 � 103 s, respectively. The system demonstrated that the power drops do not have any impact on either ‘ON’ or ‘OFF’ state.
1. Introduction Nonvolatile electrically bistable memory devices, based on inorganic-organic hybrid system, have emerged as the most projecting candidates due to unique advantages of simple fabrication process, lower power consumption, large memory density and fast operating speed [1,2]. A wide variety of organic materials, including organic polymers and monomers in combination with and inorganic crystals, such as metal nanoparticles, various metal oxides and sulfides have been applied to obtain better device performance [2–6]. The bistable memory devices are simple to fabricate without addi tional components, such as, sources and drains. Several mechanisms have been proposed to explain the electron transport phenomenon of the resistive switching memory devices made with organic-inorganic com bination as an active material. The memory device, composed with dodecanethiol protected gold nanoparticles along with hydroxyquinoline-polystyrene, sandwiched between two aluminium electrodes showed the electrical transition from the low-conductivity state to high-conductivity state and vice-versa at 2.8 and 1.8 V, respectively [2]. The current-voltage (I–V) properties of the device for the high-conductivity state followed the direct tunnelling mechanism at the low voltage region, whereas, Fowler–Nordheim tunnelling mecha nism was the dominant conduction mechanism at the high voltage range. The polyaniline-gold nanoparticle based device exhibited very interesting bistable electrical behaviour, where at the potential value of þ3 V the device transform from a low conductive state (OFF) to a
high-conductive state (ON) due to the electric-field-induced charge transfer, from the imine nitrogen of the polyaniline to the gold nano particles [7]. Electrically bistable device fabricated based on semi conducting polymer and thiol-capped copper sulfide nanocrystals displayed electrical bistability and negative differential resistance characteristics in current-voltage measurement with the ON/OFF cur rent ratio of 104 [6]. The current-voltage curve for the OFF-state was followed the space-charge limited current (SCLC) model, whereas, the ON state exhibited the Ohmic conduction mechanism. The current-voltage characteristics of an electrically bistable device, based on poly-(N-vinylcarbazole)-silver sulfide nanocomposite [8], has been reported where the thermionic emission model (charge injection from the electrodes) and the trap-controlled space charge limited current (SCLC) model, within the range of 0–7 V and 7–10 V, respectively, was proposed for the OFF-state, whereas, the ON state was described by an ohmic model. The bistable effects of poly-(methyl methacrylate) stabi lized zinc oxide nanoparticles embedded on flexible polyethylene tere phthalate substrate was investigated [9]. At the low voltage range (below 1.0 V), for ON-state, the current-voltage characteristics was related to the thermionic emission (TE) conduction model, whereas, within the range of 1.0–3.6 V, the transport phenomenon fitted with the SCLC model (current value proportional to the square of the voltage). In this study, we fabricated the organic-inorganic composite system by exploiting the complexation behaviour of aniline (as a ligand) with metal cation [10,11] with the subsequent formation of organic molecule protected metal sulfide nanoparticles by the addition of sulfide anion.
* Corresponding author. E-mail address: [email protected] (K. Mallick). https://doi.org/10.1016/j.physe.2019.113787 Received 19 July 2019; Received in revised form 18 September 2019; Accepted 16 October 2019 Available online 19 October 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
V.K. Perla et al.
Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113787
Scheme 1. Synthesis route of bismuth sulfide nanoparticles and charge transfer mechanism from electrode to active material.
The electrical property of the synthesized composite system (aniline protected bismuth sulfide nanoparticles) was investigated for the ap plications in next-generation nonvolatile flash memory devices with low-power and ultrahigh-density elements. 2. Experimental details 2.1. Materials All the chemicals, bismuth (III) nitrate pentahydrate [Bi (NO3)3⋅5H2O], aniline (PhNH2) and sodium sulfide (Na2S) were purchased from Sigma Aldrich and used without further purification. 2.2. Synthesis For a particular synthesis method, Bi (NO3)3⋅5H2O (0.485 g) was solubilized in HNO3 followed by the addition of certain required amount of distilled water. The Bi (NO3)3⋅5H2O solution was added to PhNH2 and a white precipitation was appeared. The aqueous solution of sodium sulfide (Na2S) was added to the white precipitation and left for 6 h. The solid material was collected through filtration and dried under vacuum. The dried material was keep in an oven for 3 h at 120 � C. The synthesis method has been schematically displayed in Scheme 1. The synthesized materials were characterized using different tech niques to extract the surface, microscopic and optical properties. The material was also used to fabricate a device for the current-voltage measurement to study the nonvolatile memory effect.
Fig. 1. (A) Room temperature x-ray diffraction pattern of Bi2S3 material indexed according to the orthorhombic crystal structure (JCPDS-170320). The In-set figure shows the SED pattern and the unit cell representation of Bi2S3 projected along b-axis. The red, green and yellow spheres correspond to bis muth (Bi1 and Bi2) cations and sulphur (S) anion respectively. (B) TEM image of the as-synthesized bismuth sulfide nanoparticles and the size distribution (histogram). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
source meter with the scan rate of 0.17 V s 1 and a step voltage of 0.1 V. The write-read-erase-read (WRER) parameter was measured with Keithley 2612 B dual channel source meters.
2.3. Material characterization The synthesized samples were characterized by X-ray diffractometer (XRD) (Shimadzu XD-3), and studied under the operating voltage of 40 kV within the diffraction angle range (2θ) from 20� to 60� . Micro scopy study was performed using JEOL (JEM-2100) transmission elec tron microscope (TEM). The sample for TEM analysis was prepared by depositing the material onto a carbon coated copper grid. Raman spectrum was acquired using the 514.5 nm of an argon ion laser as the excitation source. Light dispersion was undertaken via the single spec trograph stage of a Jobin-Yvon T64000 Raman spectrometer. Fourier transform infrared spectroscopy (FTIR) spectrum was collected using Shimadzu IRAffinity-1 with the spectral resolution of 0.5 cm 1. The current-voltage (I–V) characteristic were measured using a Keithley 2401
2.4. Device fabrication At first, the gold bottom electrode was printed on a flexible paper substrate and the synthesized material (BS) was deposited on the top of the printed electrode using a spin-coating technique with the thickness of approximately 5 μm. Finally, the top electrode was fabricated with the gold. The material was sandwiched between top and bottom electrodes, with an active area of 0.5 � 0.5 mm2. The fabricated device, Au||BSN|| Au, was applied for the current-voltage measurement. The bottom and top electrode of the device was connected to the positive terminal and ground terminal of the instrument, respectively. Gold electrodes were fabricated with PVD (physical vapour deposition) cathode sputter coater 2
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113787
Fig. 3. The semi-log current-voltage (I–V) characteristics of the device from 0 to þ5 V (∎), from þ5 to 0 V (●), from 0 to -5 V (▴) and from 5 to 0 V (▾).
anion coordinated by two bismuth cations (Bi1 and Bi2) in an asym metrical arrangement due to large Bi–S bonds length [12]. The crys talline nature of the nanoparticles was also confirmed through selected area (electron) diffraction (SAD) pattern (in-set, Fig. 1A) analysis. The TEM image (Fig. 1B) shows the nanosized Bi2S3 particles (dark spots) are highly dispersed within the organic matrix. The histogram exhibited the particle size distribution, which is about 2–10 nm (in-set, Fig. 1B). The spectral behaviour of the bismuth nitrate and aniline complex is revealed through infra-red spectroscopy technique (IR spectra of the (a) aniline and (b) bismuth-aniline complex). In the IR spectra (Fig. 2A), both symmetric and asymmetric N–H stretching bands have been observed for both aniline and bismuth-aniline complex in the range of 3500-3300 cm 1. The nature of the band indicates that the complexa tion has an influence on the N–H stretching vibration. The NH2 out-ofplane bending involves the hydrogen wagging both above and below the plane, defined by C–N bond, appear at 870 and 750 cm 1 in aniline spectra, which are disappeared in the IR spectrum of bismuth-aniline complex. The aromatic C–H in-plane bending vibration at 1050 and 1080 cm 1 and the C–N stretching vibration of the aromatic amine at 1268 cm 1 is visible in aniline but absent in bismuth-aniline complex. In the spectrum (a), the peak at 1611 cm 1 represents the N–H bending vibration, whereas, for bismuth-aniline complex the shifting of the peak has been recorded at 1635 cm 1, spectrum (b). From the spectrum it is evident that the incorporation of bismuth nitrate in aniline leads to the formation of bismuth (Bi3þ)-aniline complex. Fig. 2B shows the Raman shift of Bi2S3 nano particle in the range of 50–350 cm 1 when excited with a wavelength of 514.5 nm. The overall Raman shift was deconvo luted into 8-Lorentzian peaks. The lower frequency peaks positioned at 62 cm 1 and 74.5 cm 1 is due to the an-harmonic vibration of optical phonon. Other peaks positioned at 110, 150, 189, 240 and 274 cm 1 are assigned as Ag and B1g modes, associated with transverse optical (TO) and longitudinal optical (LO) phonon vibration respectively [13,14]. Towards higher frequency (>150 cm 1) peaks become weaker in in tensity and asymmetrical broadened effect was observed. Earlier, this behaviour was recognized as phonon dispersion effect from band localization in a quantum confinement system [15]. The semi-log representation of the current-voltage (I–V) character istic of the device Au||BSN||Au, is displayed in Fig. 3. A sweeping of voltage, from 0 to þ5 V, Fig. 3 (b), displayed an abrupt improvement of current value at 3.6 V, known as SET process (shifting the device from the low conductance state to high conductance state). During reverse sweep (from þ5 to 0 V), at 1.0 V the device started dropping its con ductivity and eventually shifted to the low conductance state at 0.25 V, known as RESET process (shifting the device from the high conductance state to low conductance state). The left hand side of Fig. 3 (a), exhibited two SET processes at 1.4 V and 3.8 V, during the voltage sweep from
Fig. 2. Raman shift of Bi2S3 nanoparticles in the range from 50 to 350 cm 1. The recorded spectrum (black curve) was fitted (green curve) according to the deconvolution method into eight Lorentzian peaks (red curves). (For interpre tation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(EMSCOPE SC 500) at the rate of 1.5 mA. 3. Result and discussion The formation of bismuth sulfide nanoparticles can be proposed as follows. The first step is a complexation phenomenon between bismuth nitrate and aniline whereas the second step lead to the formation of bismuth sulfide due the addition of sodium sulfide. The bismuth sulfide nanoparticles are stabilized and functionalized by the nitrogen (N) lonepair in the amine group of the aniline moiety. The x-ray diffraction pattern of Bi2S3, material was recorded in the range (2θ) from 200 to 600 is shown in Fig. 1A. The experimentally observed pattern was indexed according to the (JCPDS: 170320) orthorhombic crystal structure that belong to the space group of Pbmn. The lattice parameter values, a ¼ 11.317(6) Å, b ¼ 3.987(1) Å, and c ¼ 11.167(6) Å, were confirmed from the JCPDS card number. The sharp and well defined diffraction peaks indicated the crystalline nature of the material. No secondary peak was observed and the synthesized material retained the stoichiometric ratio within the limit of x-ray measurement. The unit cell representation of the Bi2S3, material is shown in Fig. 1A (inset), projected along b-axis that formed a layered structure of bismuth-sulfide polyhedra. Within the polyhedra the S 3
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113787
Fig. 4. The log (I) – log (V) characteristic of the device.
Scheme 2. The conduction mechanism of the device both for OFF state and ON state for the (a) side of Fig. 3.
0 to -5 V, whereas, the RESET process started at 0.9 V, during the voltage sweep from 5 to 0 V. The overall I–V characteristic of the de vices represented as unipolar switching behaviour. The experimental current-voltage curve of the devices was fitted by several theoretical models to analyze the carrier transport mechanism and is revealed in Fig. 4. We found a quasi-symmetric unipolar currentvoltage performance of the aniline molecule stabilized bismuth sulfide based device (Au||BSN||Au), where both sides follow the identical mechanistic behaviour. For the purpose of simplification we only consider the left-hand side of the curve for discussion. The log (I) – log (V) characteristic of the device is presented in Fig. 4. Fig. 4 (a), OFFstate, in the low voltage region ( 1.4 V) the conductivity model fits with the thermionic emission (TE) behaviour, whereas the voltage re gion from 1.5 V to 3.8 V, the conductivity characteristic followed the space charge limited current (SCLC) model, with the slope of 2.3. After that, the DUT showed a sharp transition from OFF-state to ON-state. In the ON-state, the device displayed a trapped-space charge limited cur rent (TCLC) model, with the slope of 8.3 and later transform to trap free SCLC model (α ¼ 1.9), followed by a thermionic emission behaviour. A similar conductivity behaviour was found in Fig. 4 (b), the right-hand side of the current-voltage curve. In the present system, bismuth sul fide played the role as carrier traps (Scheme 2). At low voltage region (below the 1.4 V) the transport property of the device is fitted with the
Fig. 5. Endurance study of the device, for both the sides of the semi-log I–V curve, for 103 cycles. (a) The average ON-OFF ratio is 103, (a) side of Fig. 3. (b) The average ON-OFF ratio is 3 � 102, (b) side of Fig. 3.
thermionic emission, which indicates limited number of thermally induced charge carriers cross the potential barrier at the interface be tween electrode and active material. Within the voltage range between 1.5 V and 3.8 V, the log (I) – log (V) characteristic fitted with SCLC with the slope value of 2.3, which indicates that the trapped electrons within the interface act as space charges [16,17]. Above the threshold voltage (> 3.8 V), the trapped electrons by the defect sites of bismuth sulfide dispersed exponentially within the forbidden gap and conse quently the device reach to the ON-state [18]. During the voltage sweep from 5 to 0 V, within the voltage region between -5 V and -1 V, the log (I) – log (V) exhibits a linear slope with the α value of 1.9 comparable to SCLC indicate no empty traps available to further trap the charge carrier 4
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113787
Fig. 6. The nonvolatile memory study of the device, for both the sides of the semi-log I–V curve, for the total time duration of 2 � 103 s. (a) The ON-OFF ratio is 103, (a) side of Fig. 3. (b) The ON-OFF ratio is 3 � 102, (b) side of Fig. 3.
Fig. 7. The Write-Read (1)-Erase-Read (0), W-R (1)-E-R (0), study of the device, for both the sides of the semi-log I–V curve (Fig. 3). (a) The W-R (1)-E-R (0) voltages are 5, 1.3, 0 and 1.3 V, respectively, with the corresponding current responses of the (a) side of Fig. 3. (b) The W-R (1)-E-R (0) voltages are þ5, þ3, 0 and þ3 V, respectively, with the corresponding current responses of the (b) side of Fig. 3.
species [19]. At the low voltage, < 1 V, the current-voltage curve behave thermionic emission mechanism [9,20]. A similar type of mechanism can be proposed for the right-hand side of the current-voltage curve. The endurance study of the Au||BSN||Au based device was per formed applying 50% duty cycle of the pulse for the time period of 0.1 s, in-set, Fig. 5 (a) and (b). Both sides of the I–V curve was considered for the endurance study of the DUT. The device was subjected to -5 V and þ5 V for few seconds to imprint the ON-state for LHS and RHS, respectively. For the I–V curve of the LHS and the RHS, the read-voltage of 1.3 V and 3 V were applied, respectively, for 103 cycles. Endurance study consists of write once in to the device and read for 103 cycles, similar to Write once and Read many times (WROM), or simply read only memory (ROM) type of memory device [21]. The extracted ON-OFF ratio (average) from the endurance study were found 1 � 103 and 3 � 102 for LHS and RHS, respectively. Nonvolatile study for the resistive switching memory device is critically important in the information technology industry for evaluating the persistence of the data. The de vice was subjected to -5 V and þ5 V for few seconds to imprint the ON-state for LHS and RHS, respectively. For the I–V curve of the LHS and the RHS, the read-voltage of 1.3 V and 3 V were applied, respectively, for 2 � 103 s. The nonvolatile study of the Au||BSN||Au based device was per formed applying 0.16% duty cycle of the pulse for the time period of 60.1 s, in-set, Fig. 6 (a) and (b). Both sides of the I–V curve was considered for the nonvolatile study of the DUT. Both sides exhibited stable ON-state and OFF-state with the ON-OFF ratio of 1 � 103 (LHS) and 3 � 102 (RHS) for 2 � 103 s. The study also revealed that both ONand OFF-states are not fluctuating irrespective of interrupted power supply.
The device was repeatedly demonstrated ‘write (W)’, ‘read (R)’ and ‘erase (E)’ under ambient condition for both sides of the current-voltage curve (extracted from the DUT) and revealed in Fig. 7. W-R(1)-E-R(0) cycle consists of one write pulse, one erase pulse and two read pulse, R (1) and R(0), with equal magnitude. The write pulse established the high conductance state of the device. The R (1) pulse was used to confirm the conductive state of the device. The erase pulse make the device to the low conductance state and again the R(0) pulse probe and confirms the conductive state of the device. Low voltage for erase is used to ground all the electrons instantly, created by TCLC. For LHS (Fig. 3), the applied W-R (1)-E-R (0) voltage pulse were 5, 1.3, 0 and 1.3 V, respectively, with the duration of 0.8 s for one cycle, and the corresponding current responses is displayed in Fig. 7 (a). A clear distinction between R (1) and R (0) has been observed with the current difference of 1 μA. In a similar way, for RHS (Fig. 3) the applied W-R (1)-E-R (0) voltage pulse were þ5, þ3, 0 and þ3 V, respectively, for the same duration as above, and the cor responding current responses is displayed in Fig. 7 (b) with the current difference of 0.1 μA between R (1) and R (0). The difference of current response between R (1) and R (0) indicate that the device is capable to distinguish bit (0) from bit (1). It is also evident from the literature that the current difference or the ratio for two Read states can be increased by increasing the time period for the Write and Erase voltages for WriteRead (1)-Erase-Read (0) cycle [4]. 5
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113787
Appendix A. Supplementary data
Several reasons are associated with the asymmetric nature and the presence of multiple SET processes in a current-voltage behaviour (Fig. 3). For a metal-semiconductor-metal type of device (a) at the region of low voltage, thermionic emission (TE) play the role for electron transfer. For TE, the electrons are generated at the interface of the electrode and the active material [20]. Due to the inhomogeneity of the interface, an asymmetric nature within I–V curve can be formed, (b) the sweep rate, voltage window and compliance current can influence the I–V symmetry [22,23]. Low sweep rate with a large voltage window and appropriate compliance current may eliminate the asymmetric nature of the I–V curve, (c) the top electrode is in the contact with atmospheric moisture whereas the bottom electrode is in-between the substrate and the active material, which may have an impact on asymmetric nature of the I–V curve [24] and (d) for a multicomponent system (organ ic-inorganic), in this present study, two SET process could be due to the contribution from the pure organic component (at 1.4 V) and BSN system (at 3.8 V). A similar phenomenon has also been observed at the right-hand side of Fig. 3, at 0.7 and 3.6 V.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.physe.2019.113787. Conflicts of interest The authors declare no conflict of interest. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
4. Conclusion
[10] [11] [12]
The fabrication of a nonvolatile bistable memory device has been reported in this manuscript based on organic molecule stabilized bis muth sulfide nanoparticles. The X-ray diffraction pattern indicated that the nanoparticles are belong to the orthorhombic crystal structure with the space group of Pbmn. The bismuth sulfide nanoparticles are stabi lized and functionalized by the lone-pair of nitrogen in the amine group of the aniline moiety and which is considered to have a fundamental role in resistive switching behaviour. The device exhibited unipolar switch ing phenomenon and the bismuth sulfide played the role as carrier traps. In both sides of the current-voltage curve, the transport mechanism showed the thermionic emission and space charge limited current model for the OFF state, whereas for the ON-state, the device displayed a trapped-space charge limited current model, which later transform to trap-free space charge limited current model followed by a thermionic emission behaviour.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Acknowledgments
[23]
This study was financially supported by the Faculty of Science and the Global Excellence and Stature programme, University of Johannesburg.
[24]
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Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Organic molecule stabilized bismuth sulfide nanoparticles: A potential system for bistable resistive memory application Venkata K. Perla, Sarit K. Ghosh, Tarasankar Pal, Kaushik Mallick * Department of Chemical Sciences, University of Johannesburg, P.O. Box: 524, Auckland Park, 2006, South Africa
A B S T R A C T
The performance of the microelectronic devices depends on the material used for the fabrication of the device and also the device architecture. In this paper, we demonstrate the synthesis of ‘organic molecule stabilized bismuth sulfide nanoparticles’ (BSN) by applying a one-pot, wet-chemical route. The as-synthesized material was characterized using microscopic and optical techniques. The organic-inorganic composite system displayed electronic memory effect based on the unipolar resistive switching phenomenon. The current-voltage characteristic of the device is fitted with the thermionic emission and space charge limited current model for the OFF-state, whereas, the ON-state is fitted with the combination of trapped-space charge limited current and trap-free space charge limited current model. The study of endurance and the nonvolatile behaviour of the device was performed for 103 cycles and for 2 � 103 s, respectively. The system demonstrated that the power drops do not have any impact on either ‘ON’ or ‘OFF’ state.
1. Introduction Nonvolatile electrically bistable memory devices, based on inorganic-organic hybrid system, have emerged as the most projecting candidates due to unique advantages of simple fabrication process, lower power consumption, large memory density and fast operating speed [1,2]. A wide variety of organic materials, including organic polymers and monomers in combination with and inorganic crystals, such as metal nanoparticles, various metal oxides and sulfides have been applied to obtain better device performance [2–6]. The bistable memory devices are simple to fabricate without addi tional components, such as, sources and drains. Several mechanisms have been proposed to explain the electron transport phenomenon of the resistive switching memory devices made with organic-inorganic com bination as an active material. The memory device, composed with dodecanethiol protected gold nanoparticles along with hydroxyquinoline-polystyrene, sandwiched between two aluminium electrodes showed the electrical transition from the low-conductivity state to high-conductivity state and vice-versa at 2.8 and 1.8 V, respectively [2]. The current-voltage (I–V) properties of the device for the high-conductivity state followed the direct tunnelling mechanism at the low voltage region, whereas, Fowler–Nordheim tunnelling mecha nism was the dominant conduction mechanism at the high voltage range. The polyaniline-gold nanoparticle based device exhibited very interesting bistable electrical behaviour, where at the potential value of þ3 V the device transform from a low conductive state (OFF) to a
high-conductive state (ON) due to the electric-field-induced charge transfer, from the imine nitrogen of the polyaniline to the gold nano particles [7]. Electrically bistable device fabricated based on semi conducting polymer and thiol-capped copper sulfide nanocrystals displayed electrical bistability and negative differential resistance characteristics in current-voltage measurement with the ON/OFF cur rent ratio of 104 [6]. The current-voltage curve for the OFF-state was followed the space-charge limited current (SCLC) model, whereas, the ON state exhibited the Ohmic conduction mechanism. The current-voltage characteristics of an electrically bistable device, based on poly-(N-vinylcarbazole)-silver sulfide nanocomposite [8], has been reported where the thermionic emission model (charge injection from the electrodes) and the trap-controlled space charge limited current (SCLC) model, within the range of 0–7 V and 7–10 V, respectively, was proposed for the OFF-state, whereas, the ON state was described by an ohmic model. The bistable effects of poly-(methyl methacrylate) stabi lized zinc oxide nanoparticles embedded on flexible polyethylene tere phthalate substrate was investigated [9]. At the low voltage range (below 1.0 V), for ON-state, the current-voltage characteristics was related to the thermionic emission (TE) conduction model, whereas, within the range of 1.0–3.6 V, the transport phenomenon fitted with the SCLC model (current value proportional to the square of the voltage). In this study, we fabricated the organic-inorganic composite system by exploiting the complexation behaviour of aniline (as a ligand) with metal cation [10,11] with the subsequent formation of organic molecule protected metal sulfide nanoparticles by the addition of sulfide anion.
* Corresponding author. E-mail address: [email protected] (K. Mallick). https://doi.org/10.1016/j.physe.2019.113787 Received 19 July 2019; Received in revised form 18 September 2019; Accepted 16 October 2019 Available online 19 October 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis route of bismuth sulfide nanoparticles and charge transfer mechanism from electrode to active material.
The electrical property of the synthesized composite system (aniline protected bismuth sulfide nanoparticles) was investigated for the ap plications in next-generation nonvolatile flash memory devices with low-power and ultrahigh-density elements. 2. Experimental details 2.1. Materials All the chemicals, bismuth (III) nitrate pentahydrate [Bi (NO3)3⋅5H2O], aniline (PhNH2) and sodium sulfide (Na2S) were purchased from Sigma Aldrich and used without further purification. 2.2. Synthesis For a particular synthesis method, Bi (NO3)3⋅5H2O (0.485 g) was solubilized in HNO3 followed by the addition of certain required amount of distilled water. The Bi (NO3)3⋅5H2O solution was added to PhNH2 and a white precipitation was appeared. The aqueous solution of sodium sulfide (Na2S) was added to the white precipitation and left for 6 h. The solid material was collected through filtration and dried under vacuum. The dried material was keep in an oven for 3 h at 120 � C. The synthesis method has been schematically displayed in Scheme 1. The synthesized materials were characterized using different tech niques to extract the surface, microscopic and optical properties. The material was also used to fabricate a device for the current-voltage measurement to study the nonvolatile memory effect.
Fig. 1. (A) Room temperature x-ray diffraction pattern of Bi2S3 material indexed according to the orthorhombic crystal structure (JCPDS-170320). The In-set figure shows the SED pattern and the unit cell representation of Bi2S3 projected along b-axis. The red, green and yellow spheres correspond to bis muth (Bi1 and Bi2) cations and sulphur (S) anion respectively. (B) TEM image of the as-synthesized bismuth sulfide nanoparticles and the size distribution (histogram). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
source meter with the scan rate of 0.17 V s 1 and a step voltage of 0.1 V. The write-read-erase-read (WRER) parameter was measured with Keithley 2612 B dual channel source meters.
2.3. Material characterization The synthesized samples were characterized by X-ray diffractometer (XRD) (Shimadzu XD-3), and studied under the operating voltage of 40 kV within the diffraction angle range (2θ) from 20� to 60� . Micro scopy study was performed using JEOL (JEM-2100) transmission elec tron microscope (TEM). The sample for TEM analysis was prepared by depositing the material onto a carbon coated copper grid. Raman spectrum was acquired using the 514.5 nm of an argon ion laser as the excitation source. Light dispersion was undertaken via the single spec trograph stage of a Jobin-Yvon T64000 Raman spectrometer. Fourier transform infrared spectroscopy (FTIR) spectrum was collected using Shimadzu IRAffinity-1 with the spectral resolution of 0.5 cm 1. The current-voltage (I–V) characteristic were measured using a Keithley 2401
2.4. Device fabrication At first, the gold bottom electrode was printed on a flexible paper substrate and the synthesized material (BS) was deposited on the top of the printed electrode using a spin-coating technique with the thickness of approximately 5 μm. Finally, the top electrode was fabricated with the gold. The material was sandwiched between top and bottom electrodes, with an active area of 0.5 � 0.5 mm2. The fabricated device, Au||BSN|| Au, was applied for the current-voltage measurement. The bottom and top electrode of the device was connected to the positive terminal and ground terminal of the instrument, respectively. Gold electrodes were fabricated with PVD (physical vapour deposition) cathode sputter coater 2
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Fig. 3. The semi-log current-voltage (I–V) characteristics of the device from 0 to þ5 V (∎), from þ5 to 0 V (●), from 0 to -5 V (▴) and from 5 to 0 V (▾).
anion coordinated by two bismuth cations (Bi1 and Bi2) in an asym metrical arrangement due to large Bi–S bonds length [12]. The crys talline nature of the nanoparticles was also confirmed through selected area (electron) diffraction (SAD) pattern (in-set, Fig. 1A) analysis. The TEM image (Fig. 1B) shows the nanosized Bi2S3 particles (dark spots) are highly dispersed within the organic matrix. The histogram exhibited the particle size distribution, which is about 2–10 nm (in-set, Fig. 1B). The spectral behaviour of the bismuth nitrate and aniline complex is revealed through infra-red spectroscopy technique (IR spectra of the (a) aniline and (b) bismuth-aniline complex). In the IR spectra (Fig. 2A), both symmetric and asymmetric N–H stretching bands have been observed for both aniline and bismuth-aniline complex in the range of 3500-3300 cm 1. The nature of the band indicates that the complexa tion has an influence on the N–H stretching vibration. The NH2 out-ofplane bending involves the hydrogen wagging both above and below the plane, defined by C–N bond, appear at 870 and 750 cm 1 in aniline spectra, which are disappeared in the IR spectrum of bismuth-aniline complex. The aromatic C–H in-plane bending vibration at 1050 and 1080 cm 1 and the C–N stretching vibration of the aromatic amine at 1268 cm 1 is visible in aniline but absent in bismuth-aniline complex. In the spectrum (a), the peak at 1611 cm 1 represents the N–H bending vibration, whereas, for bismuth-aniline complex the shifting of the peak has been recorded at 1635 cm 1, spectrum (b). From the spectrum it is evident that the incorporation of bismuth nitrate in aniline leads to the formation of bismuth (Bi3þ)-aniline complex. Fig. 2B shows the Raman shift of Bi2S3 nano particle in the range of 50–350 cm 1 when excited with a wavelength of 514.5 nm. The overall Raman shift was deconvo luted into 8-Lorentzian peaks. The lower frequency peaks positioned at 62 cm 1 and 74.5 cm 1 is due to the an-harmonic vibration of optical phonon. Other peaks positioned at 110, 150, 189, 240 and 274 cm 1 are assigned as Ag and B1g modes, associated with transverse optical (TO) and longitudinal optical (LO) phonon vibration respectively [13,14]. Towards higher frequency (>150 cm 1) peaks become weaker in in tensity and asymmetrical broadened effect was observed. Earlier, this behaviour was recognized as phonon dispersion effect from band localization in a quantum confinement system [15]. The semi-log representation of the current-voltage (I–V) character istic of the device Au||BSN||Au, is displayed in Fig. 3. A sweeping of voltage, from 0 to þ5 V, Fig. 3 (b), displayed an abrupt improvement of current value at 3.6 V, known as SET process (shifting the device from the low conductance state to high conductance state). During reverse sweep (from þ5 to 0 V), at 1.0 V the device started dropping its con ductivity and eventually shifted to the low conductance state at 0.25 V, known as RESET process (shifting the device from the high conductance state to low conductance state). The left hand side of Fig. 3 (a), exhibited two SET processes at 1.4 V and 3.8 V, during the voltage sweep from
Fig. 2. Raman shift of Bi2S3 nanoparticles in the range from 50 to 350 cm 1. The recorded spectrum (black curve) was fitted (green curve) according to the deconvolution method into eight Lorentzian peaks (red curves). (For interpre tation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(EMSCOPE SC 500) at the rate of 1.5 mA. 3. Result and discussion The formation of bismuth sulfide nanoparticles can be proposed as follows. The first step is a complexation phenomenon between bismuth nitrate and aniline whereas the second step lead to the formation of bismuth sulfide due the addition of sodium sulfide. The bismuth sulfide nanoparticles are stabilized and functionalized by the nitrogen (N) lonepair in the amine group of the aniline moiety. The x-ray diffraction pattern of Bi2S3, material was recorded in the range (2θ) from 200 to 600 is shown in Fig. 1A. The experimentally observed pattern was indexed according to the (JCPDS: 170320) orthorhombic crystal structure that belong to the space group of Pbmn. The lattice parameter values, a ¼ 11.317(6) Å, b ¼ 3.987(1) Å, and c ¼ 11.167(6) Å, were confirmed from the JCPDS card number. The sharp and well defined diffraction peaks indicated the crystalline nature of the material. No secondary peak was observed and the synthesized material retained the stoichiometric ratio within the limit of x-ray measurement. The unit cell representation of the Bi2S3, material is shown in Fig. 1A (inset), projected along b-axis that formed a layered structure of bismuth-sulfide polyhedra. Within the polyhedra the S 3
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Fig. 4. The log (I) – log (V) characteristic of the device.
Scheme 2. The conduction mechanism of the device both for OFF state and ON state for the (a) side of Fig. 3.
0 to -5 V, whereas, the RESET process started at 0.9 V, during the voltage sweep from 5 to 0 V. The overall I–V characteristic of the de vices represented as unipolar switching behaviour. The experimental current-voltage curve of the devices was fitted by several theoretical models to analyze the carrier transport mechanism and is revealed in Fig. 4. We found a quasi-symmetric unipolar currentvoltage performance of the aniline molecule stabilized bismuth sulfide based device (Au||BSN||Au), where both sides follow the identical mechanistic behaviour. For the purpose of simplification we only consider the left-hand side of the curve for discussion. The log (I) – log (V) characteristic of the device is presented in Fig. 4. Fig. 4 (a), OFFstate, in the low voltage region ( 1.4 V) the conductivity model fits with the thermionic emission (TE) behaviour, whereas the voltage re gion from 1.5 V to 3.8 V, the conductivity characteristic followed the space charge limited current (SCLC) model, with the slope of 2.3. After that, the DUT showed a sharp transition from OFF-state to ON-state. In the ON-state, the device displayed a trapped-space charge limited cur rent (TCLC) model, with the slope of 8.3 and later transform to trap free SCLC model (α ¼ 1.9), followed by a thermionic emission behaviour. A similar conductivity behaviour was found in Fig. 4 (b), the right-hand side of the current-voltage curve. In the present system, bismuth sul fide played the role as carrier traps (Scheme 2). At low voltage region (below the 1.4 V) the transport property of the device is fitted with the
Fig. 5. Endurance study of the device, for both the sides of the semi-log I–V curve, for 103 cycles. (a) The average ON-OFF ratio is 103, (a) side of Fig. 3. (b) The average ON-OFF ratio is 3 � 102, (b) side of Fig. 3.
thermionic emission, which indicates limited number of thermally induced charge carriers cross the potential barrier at the interface be tween electrode and active material. Within the voltage range between 1.5 V and 3.8 V, the log (I) – log (V) characteristic fitted with SCLC with the slope value of 2.3, which indicates that the trapped electrons within the interface act as space charges [16,17]. Above the threshold voltage (> 3.8 V), the trapped electrons by the defect sites of bismuth sulfide dispersed exponentially within the forbidden gap and conse quently the device reach to the ON-state [18]. During the voltage sweep from 5 to 0 V, within the voltage region between -5 V and -1 V, the log (I) – log (V) exhibits a linear slope with the α value of 1.9 comparable to SCLC indicate no empty traps available to further trap the charge carrier 4
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Fig. 6. The nonvolatile memory study of the device, for both the sides of the semi-log I–V curve, for the total time duration of 2 � 103 s. (a) The ON-OFF ratio is 103, (a) side of Fig. 3. (b) The ON-OFF ratio is 3 � 102, (b) side of Fig. 3.
Fig. 7. The Write-Read (1)-Erase-Read (0), W-R (1)-E-R (0), study of the device, for both the sides of the semi-log I–V curve (Fig. 3). (a) The W-R (1)-E-R (0) voltages are 5, 1.3, 0 and 1.3 V, respectively, with the corresponding current responses of the (a) side of Fig. 3. (b) The W-R (1)-E-R (0) voltages are þ5, þ3, 0 and þ3 V, respectively, with the corresponding current responses of the (b) side of Fig. 3.
species [19]. At the low voltage, < 1 V, the current-voltage curve behave thermionic emission mechanism [9,20]. A similar type of mechanism can be proposed for the right-hand side of the current-voltage curve. The endurance study of the Au||BSN||Au based device was per formed applying 50% duty cycle of the pulse for the time period of 0.1 s, in-set, Fig. 5 (a) and (b). Both sides of the I–V curve was considered for the endurance study of the DUT. The device was subjected to -5 V and þ5 V for few seconds to imprint the ON-state for LHS and RHS, respectively. For the I–V curve of the LHS and the RHS, the read-voltage of 1.3 V and 3 V were applied, respectively, for 103 cycles. Endurance study consists of write once in to the device and read for 103 cycles, similar to Write once and Read many times (WROM), or simply read only memory (ROM) type of memory device [21]. The extracted ON-OFF ratio (average) from the endurance study were found 1 � 103 and 3 � 102 for LHS and RHS, respectively. Nonvolatile study for the resistive switching memory device is critically important in the information technology industry for evaluating the persistence of the data. The de vice was subjected to -5 V and þ5 V for few seconds to imprint the ON-state for LHS and RHS, respectively. For the I–V curve of the LHS and the RHS, the read-voltage of 1.3 V and 3 V were applied, respectively, for 2 � 103 s. The nonvolatile study of the Au||BSN||Au based device was per formed applying 0.16% duty cycle of the pulse for the time period of 60.1 s, in-set, Fig. 6 (a) and (b). Both sides of the I–V curve was considered for the nonvolatile study of the DUT. Both sides exhibited stable ON-state and OFF-state with the ON-OFF ratio of 1 � 103 (LHS) and 3 � 102 (RHS) for 2 � 103 s. The study also revealed that both ONand OFF-states are not fluctuating irrespective of interrupted power supply.
The device was repeatedly demonstrated ‘write (W)’, ‘read (R)’ and ‘erase (E)’ under ambient condition for both sides of the current-voltage curve (extracted from the DUT) and revealed in Fig. 7. W-R(1)-E-R(0) cycle consists of one write pulse, one erase pulse and two read pulse, R (1) and R(0), with equal magnitude. The write pulse established the high conductance state of the device. The R (1) pulse was used to confirm the conductive state of the device. The erase pulse make the device to the low conductance state and again the R(0) pulse probe and confirms the conductive state of the device. Low voltage for erase is used to ground all the electrons instantly, created by TCLC. For LHS (Fig. 3), the applied W-R (1)-E-R (0) voltage pulse were 5, 1.3, 0 and 1.3 V, respectively, with the duration of 0.8 s for one cycle, and the corresponding current responses is displayed in Fig. 7 (a). A clear distinction between R (1) and R (0) has been observed with the current difference of 1 μA. In a similar way, for RHS (Fig. 3) the applied W-R (1)-E-R (0) voltage pulse were þ5, þ3, 0 and þ3 V, respectively, for the same duration as above, and the cor responding current responses is displayed in Fig. 7 (b) with the current difference of 0.1 μA between R (1) and R (0). The difference of current response between R (1) and R (0) indicate that the device is capable to distinguish bit (0) from bit (1). It is also evident from the literature that the current difference or the ratio for two Read states can be increased by increasing the time period for the Write and Erase voltages for WriteRead (1)-Erase-Read (0) cycle [4]. 5
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Appendix A. Supplementary data
Several reasons are associated with the asymmetric nature and the presence of multiple SET processes in a current-voltage behaviour (Fig. 3). For a metal-semiconductor-metal type of device (a) at the region of low voltage, thermionic emission (TE) play the role for electron transfer. For TE, the electrons are generated at the interface of the electrode and the active material [20]. Due to the inhomogeneity of the interface, an asymmetric nature within I–V curve can be formed, (b) the sweep rate, voltage window and compliance current can influence the I–V symmetry [22,23]. Low sweep rate with a large voltage window and appropriate compliance current may eliminate the asymmetric nature of the I–V curve, (c) the top electrode is in the contact with atmospheric moisture whereas the bottom electrode is in-between the substrate and the active material, which may have an impact on asymmetric nature of the I–V curve [24] and (d) for a multicomponent system (organ ic-inorganic), in this present study, two SET process could be due to the contribution from the pure organic component (at 1.4 V) and BSN system (at 3.8 V). A similar phenomenon has also been observed at the right-hand side of Fig. 3, at 0.7 and 3.6 V.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.physe.2019.113787. Conflicts of interest The authors declare no conflict of interest. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
4. Conclusion
[10] [11] [12]
The fabrication of a nonvolatile bistable memory device has been reported in this manuscript based on organic molecule stabilized bis muth sulfide nanoparticles. The X-ray diffraction pattern indicated that the nanoparticles are belong to the orthorhombic crystal structure with the space group of Pbmn. The bismuth sulfide nanoparticles are stabi lized and functionalized by the lone-pair of nitrogen in the amine group of the aniline moiety and which is considered to have a fundamental role in resistive switching behaviour. The device exhibited unipolar switch ing phenomenon and the bismuth sulfide played the role as carrier traps. In both sides of the current-voltage curve, the transport mechanism showed the thermionic emission and space charge limited current model for the OFF state, whereas for the ON-state, the device displayed a trapped-space charge limited current model, which later transform to trap-free space charge limited current model followed by a thermionic emission behaviour.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Acknowledgments
[23]
This study was financially supported by the Faculty of Science and the Global Excellence and Stature programme, University of Johannesburg.
[24]
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