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Ag heterostructure
Materials Letters 188 (2017) 351–354
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Structure and properties of nanoscale Ag2S/Ag heterostructure A.I. Gusev n, S.I. Sadovnikov Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2016 Received in revised form 8 November 2016 Accepted 30 November 2016 Available online 30 November 2016
Ag2S/Ag heteronanostructure has been produced by a simple one-stage chemical deposition from aqueous solutions of silver nitrate, sodium sulfide, and sodium citrate with the use of monochromatic light irradiation. The formation of Ag2S/Ag nanocomposite structures is confirmed by X-ray analysis, highresolution electron microscopy, and energy dispersion analysis. It is established that in the contact layer between silver sulfide and silver, nonconductingα-Ag2S acanthite transforms into superionic β-Ag2S argentite under the action of external electric field. The scheme of the operation of a resistive switch based on an Ag2S/Ag heteronanostructure is proposed. & 2016 Elsevier B.V. All rights reserved.
Keywords: Acanthite - argentite phase transformation Silver sulfide Silver Heteronanostructure Resistive switch
1. Introduction Unique chemical, structural, optical and conductive properties make silver sulfide an excellent substance for preparation of heteronanostructures. Among composite heterostructures of silver sulfide, the semiconductor/metal heteronanostructure Ag2S/Ag attracts special attention. It can be used in resistive switches and nonvolatile memory devices [1–4]. The action of the switch is based on the phase transformation between nonconducting α-Ag2S acanthite and superionic β-Ag2S argentite. According to the phase diagram of the system Ag – S [5], low-temperature semiconducting phase α-Ag2S (acanthite) with monoclinic crystal structure exists at temperatures below 450 K. Under equilibrium conditions, cubic phase β-Ag2S (argentite) exists in the temperature range 452– 859 K and has a superionic conductivity. Known methods for the preparation of heteronanostructures of Ag2S or Ag mainly deal with the synthesis of nanoparticles of one species with the subsequent growth of other species nanoparticles [6–10]. These methods are rather expensive and time-consuming. Chemical deposition is a promising route for preparing semiconducting nanoparticles [11–16], thin-film structures and heteronanostructures [10,17]. Earlier, possibility of creation an atomic switch based on thin films of silver sulfide, fabricated on the surface of the silver electrode was discussed in [18]. Present paper is devoted to synthesis of Ag2S/Ag heteronanostructure by hydrochemical bath deposition n
Corresponding author. E-mail address: [email protected] (A.I. Gusev).
http://dx.doi.org/10.1016/j.matlet.2016.11.111 0167-577X/& 2016 Elsevier B.V. All rights reserved.
using for this purpose separate Ag and Ag2S nanoparticles, which are in direct contact, and the possibility of application of this Ag2S/ Ag heteronanostructure as resistive switch. Present work is a continuation of our studies of nanostructured silver sulfide [14,15,19–25].
2. Experimental Ag2S/Ag heteronanostructure has been synthesized by chemical deposition from aqueous solutions of AgNO3, Na3C6H5O7 ≡ Na3Cit, and Na2S with concentrations 50, 25, and (25-δ) mmol l 1, where δ E 0.5 mmol l 1, respectively. Small deficiency of Na2S is necessary for the deposition of Ag nanoparticles along with Ag2S and the formation of Ag2S/Ag heteronanostructure. Synthesis was carried out under illumination by LED with a 450 nm wavelength and the irradiation intensity of 15 mW cm 2. Synthesis was carried out in the following sequence: a complexing agent was added to silver nitrate in the dark; then a solution of Na2S was poured into the prepared solution. As a result, deposition of silver sulfide occurred by the following reaction
2AgNO3 + Na2S
Na3C6H5O7 → Ag2S ↓ + 2NaNO3 .
(1)
Further the solution was irradiated with monochromatic light. In accordance with photochemical reaction,
λ C6H5O73 − + 2Ag+ → C5H4 O52 − + CO2 + H+ + 2Ag ↓ , C6H5O73 −
þ
(2)
the citrate ions reduce the Ag ions to Ag nanoparticles in aqueous solutions and transform into acetone-1,3-dicarboxylate
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ions C5H4O52 −. The reduction of silver at the surface of Ag2S nanoparticles leads to the formation of the Ag2S/Ag heteronanostructures. Size distributions of Ag and Ag2S nanoparticles in synthesized colloidal solutions with Ag2S/Ag heteronanostructures are shown in Fig. S1 (see “Supplementary Material”). Produced heteronanostructure was examined by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX) analysis, and Brunauer-Emmett-Teller methods. The average particle size D (to be more precise, the average size of coherent scattering regions (CSR)) in deposited powders was estimated by XRD method from the diffraction reflection broadening [13] (see experimental details in “Supplementary Material” also).
3. Results and discussion The XRD pattern of Ag2S/Ag heteronanostructure is shown in Fig. 1a. The heteronanostructure contain monoclinic silver sulfide with α-Ag2S acanthite structure and metallic cubic silver Ag. Detailed XRD studies of the crystal structure of α-Ag2S acanthite phase were performed earlier [14,20–22]. The crystal structure of bcc β-Ag2S argentite was determined in study [21] and described in detail in Crystallographic information file (CCDC reference number 1062400) of Electronic Supplementary information attached to the article [21]. The quantitative analysis of the XRD pattern and comparison with data [19] have shown that the observed set of diffraction reflections corresponds to nanocrystalline nonstoichiometric monoclinic (space group P21/c) acanthite αAg1.93S and cubic (space group Fm3m ) silver Ag. The diffraction reflection broadening is indicative of the nanosized state of the both phases. The content of Ag and Ag2S in the deposited nanopowder is equal to 7.5 and 92.5 wt%. The HRTEM image of Ag2S/Ag heteronanostructure is shown in Fig. 1b. It is seen that the Ag2S and Ag nanoparticles are in direct contact and form the heteronanostructures. In present study the indices (hkl) of the diffraction reflections obtained by Fast Fourier Transformation (FFT) have been determined with taking into account interplanar distances dhkl and angles φrefl between observed reflections, i. e. the angle between the straight lines passing through each reflection and the central spot (000). The diffraction pattern (Fig. 1c) obtained by FFT of HRTEM image of the whole this composite heteronanostructure contains
two set of diffraction reflections corresponding to monoclinic silver sulfide and cubic silver. The diffraction patterns (d) and (e) are obtained by FFT from areas (1) and (2) isolated by green and orange quadrates. The observed set (d) of spots (111), (200), and (11-1) corresponds to the [01–1] plane of the reciprocal lattice of cubic Ag. The interplanar distances for area (2) and the set (e) of spots (01-3), (12-2), and (111) correspond to monoclinic α-Ag2S acanthite. The produced Ag2S/Ag heteronanostructure combines ionic and electronic conductors. The heterostructures of this type containing Ag and Ag2S nanofibers or a silver film with Ag2S nanoclusters are considered as a potential basis for creating resistive switches and nonvolatile memory devices [2–4,18,26]. The resistive switches consist of a superionic conductor located between two metal electrodes. In the case of Ag2S/Ag heterostructures, one of the electrodes is silver, and the second electrode can be such metals as Pt, Au, and W. In this work, Ag2S/Ag heterostructure formed by Ag2S and Ag nanoparticles has been produced by a simple method of hydrochemical bath deposition. Deposition of Ag2S/Ag heterostructures on a substrate coated with a thin conducting metallic layer will make it possible to form a structure, which can work as a resistive switch. The action of the switch is based on the phase transformation of nonconducting α-Ag2S acanthite into β-Ag2S argentite exhibiting superionic conduction. The transition into a high-conduction state is due to abrupt disordering of the cationic sublattice. In works [27,28] it was shown that a high-conduction state of a crystal can be achieved by external electric field induced “melting” of the cationic sublattice taking place without heating of the crystal. Such transformation occurring as a result of applied external electric field was confirmed by the authors [2–4] with respect to nanocrystalline silver sulfide. The effect of external electric field induced abrupt disordering allows the realization of the superionic state of silver sulfide at room temperature. We have studied the switching processes in Ag2S/Ag heteronanostructure. For this purpose, a metallic Pt microcontact was supplied to Ag2S/Ag heteronanostructure and bias voltage was impressed so that Ag electrode was charged positively. When positive bias voltage increases to 500 mV, the conduction of the heteronanostructure grows and the nanodevice transforms into the conducting state (i. e., the on-state). The bias back to negative values decreases the conduction and the nanodevice transforms into the off-state.
Fig. 1. (a) XRD pattern and (b) HRTEM image of Ag2S/Ag heteronanostructure. (c), (d), and (e) diffraction patterns obtained by FFT of HRTEM image of the whole composite heteronanostructure and its areas (1) and (2). The long and short ticks on XRD pattern correspond to reflections of cubic metallic Ag and monoclinic Ag2S silver sulfide, respectively.
A.I. Gusev, S.I. Sadovnikov / Materials Letters 188 (2017) 351–354
353
Fig. 2. HRTEM images of region of transition between Ag and Ag2S for off-state (a) and on-state (b) of Ag2S/Ag heteronanostructure. The Pt electrode is located on the top part of the image, and Ag electrode is in the bottom part. Diffraction patterns (c) and (d) are obtained by FFT of HRTEM images (a) and (b), respectively. The on-state is induced by the bias voltage applying. When Ag2S/Ag heteronanostructure is transformed from the off-state into the on-state, along with Ag spots, the (011) and (112) spots of β-Ag2S argentite appear on the diffraction pattern (d) instead of acanthite spots.
Fig. 2 displays a region of Ag2S/Ag heteronanostructure where change of crystal structure at the transition from the off-state (Fig. 2a) to the on-state (Fig. 2b) can be observed. Using FFT of HRTEM images, we obtained the diffraction patterns (Figs. 2c, 2d). The diffraction pattern of Ag2S/Ag heteronanostructure in the off-state is shown in Fig. 2c. This diffraction pattern contains (111), (11-1) spots and twinning reflection (111)* corresponding to cubic (space group Fm3¯ m ) silver, as well as (2-12) and (030) spots corresponding to monoclinic (space group P21/c) α-Ag2S acanthite. The observed angle of 100.3° between (2-12) and (030) spots of monoclinic acanthite coincides within measurement error with the theoretical value 100.7°. Then, a positive bias was applied to the Ag2S/Ag heteronanostructure in order to turn it on. HRTEM image of Ag2S/Ag heteronanostructure in the on-state and its diffraction pattern are presented in Figs. 2b and 2d, respectively. The diffraction pattern (Fig. 2d) contains two sets of spots corresponding to two cubic phases. The (111), (200), (1-1-1) spots and the twinning spot (00-
2)* correspond to cubic (space group Fm3¯ m ) silver, and the (011) and (112) spots correspond to cubic (space group Im3¯ m ) β-Ag2S argentite. The observed angle of 30.1° between the (011) and (112) spots of cubic β-Ag2S argentite coincides with the theoretical value 30°. Experimental angles between diffraction spots of cubic silver (Figs. 2c, 2d) coincide with theoretical values. Thus, the applied bias really leads to the appearance of conducting β-Ag2S argentite instead of nonconducting α-Ag2S acanthite and the formation of conductive channel from argentite βAg2S and silver Ag. A current voltage I(U) characteristic (a) of the switching and the schematic operation of a switch based on Ag2S/Ag heteronanostructure are shown in Fig. 3. The initial Ag2S phase is a nonconducting acanthite α-Ag2S (Fig. 3(1)). When a positive bias is applied, Ag þ cations start to move toward the negatively charged cathode M and are reduced to Ag atoms during their transport. At the same time, the α-Ag2S phase transforms into superionic βAg2S argentite (Fig. 3(2)), and a continuous conductive channel is
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Fig. 3. Generalized scheme of the operation of an Ag2S/Ag-based switch: (a) typical current voltage characteristic of the switching; (1) initial nonconducting state; (2) the appearance of a conductive channel upon the application of an external electric field that induces the transformation of acanthite α-Ag2S into argentite β-Ag2S; (3) work onstate with a continuous conductive channel; (4,5) off-state and break down of the conductive channel upon the application of negative bias and the transformation of argentite into initial acanthite; (6) initial nonconducting state after disappearance of the conductive channel and turning-off of the switch. The numbers from 1 to 6 for different states of switching correspond to the order of the events on current voltage characteristic (a).
formed (Fig. 3(3)). The continuous conductive channel which is formed from argentite β-Ag2S and silver Ag is retained, when the external field is turned off. This phenomenon can be considered as a memory effect (Fig. 3(3)). If a negative (reverse) bias is applied to the switch, the Ag nanocrystals start dissolving in argentite, the Ag þ cations move to the anode, argentite transforms into the initial acanthite again, the conductive channel breaks down, and offstate is realized (Fig. 3(4,5)). Because of the formation of nonconducting acanthite, the conductive channel disappears, the switch transforms into the initial state and is turned off (Fig. 3(6)). If positive bias is applied once again, the destroyed conductive channel is restored due to the appearance of argentite and the formation of silver.
4. Conclusion The Ag2S/Ag heteronanostructure is formed in aqueous solutions of AgNO3, Na2S, and Na3Cit with decreased concentration of sodium sulfide during synthesis in the light. The appearance of Ag nanoparticles is due to photochemical reduction of some Ag þ ions by citrate ions. The produced Ag2S/Ag heteronanostructure combines ionic and electronic conductors. A high-conducting state of this heteronanostructure can be induced by external electric field owing to phase transformation of nonconducting acanthite into argentite exhibiting superionic conduction. The argentite β-Ag2S which appears as a result of the phase transformation and metallic silver Ag together form the conducting channel. The scheme of the operation of a resistive switch based on an Ag2S/Ag heteronanostructure is proposed. The main potential application of Ag2S/Ag heteronanostructure is a creation of resistive switches and nonvolatile memory devices.
Acknowledgements This work is financially supported by the Russian Science Foundation (Grant 14-23-00025) through the Institute of Solid State Chemistry of the Ural Branch of the RAS.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.11.111.
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Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structure and properties of nanoscale Ag2S/Ag heterostructure A.I. Gusev n, S.I. Sadovnikov Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2016 Received in revised form 8 November 2016 Accepted 30 November 2016 Available online 30 November 2016
Ag2S/Ag heteronanostructure has been produced by a simple one-stage chemical deposition from aqueous solutions of silver nitrate, sodium sulfide, and sodium citrate with the use of monochromatic light irradiation. The formation of Ag2S/Ag nanocomposite structures is confirmed by X-ray analysis, highresolution electron microscopy, and energy dispersion analysis. It is established that in the contact layer between silver sulfide and silver, nonconductingα-Ag2S acanthite transforms into superionic β-Ag2S argentite under the action of external electric field. The scheme of the operation of a resistive switch based on an Ag2S/Ag heteronanostructure is proposed. & 2016 Elsevier B.V. All rights reserved.
Keywords: Acanthite - argentite phase transformation Silver sulfide Silver Heteronanostructure Resistive switch
1. Introduction Unique chemical, structural, optical and conductive properties make silver sulfide an excellent substance for preparation of heteronanostructures. Among composite heterostructures of silver sulfide, the semiconductor/metal heteronanostructure Ag2S/Ag attracts special attention. It can be used in resistive switches and nonvolatile memory devices [1–4]. The action of the switch is based on the phase transformation between nonconducting α-Ag2S acanthite and superionic β-Ag2S argentite. According to the phase diagram of the system Ag – S [5], low-temperature semiconducting phase α-Ag2S (acanthite) with monoclinic crystal structure exists at temperatures below 450 K. Under equilibrium conditions, cubic phase β-Ag2S (argentite) exists in the temperature range 452– 859 K and has a superionic conductivity. Known methods for the preparation of heteronanostructures of Ag2S or Ag mainly deal with the synthesis of nanoparticles of one species with the subsequent growth of other species nanoparticles [6–10]. These methods are rather expensive and time-consuming. Chemical deposition is a promising route for preparing semiconducting nanoparticles [11–16], thin-film structures and heteronanostructures [10,17]. Earlier, possibility of creation an atomic switch based on thin films of silver sulfide, fabricated on the surface of the silver electrode was discussed in [18]. Present paper is devoted to synthesis of Ag2S/Ag heteronanostructure by hydrochemical bath deposition n
Corresponding author. E-mail address: [email protected] (A.I. Gusev).
http://dx.doi.org/10.1016/j.matlet.2016.11.111 0167-577X/& 2016 Elsevier B.V. All rights reserved.
using for this purpose separate Ag and Ag2S nanoparticles, which are in direct contact, and the possibility of application of this Ag2S/ Ag heteronanostructure as resistive switch. Present work is a continuation of our studies of nanostructured silver sulfide [14,15,19–25].
2. Experimental Ag2S/Ag heteronanostructure has been synthesized by chemical deposition from aqueous solutions of AgNO3, Na3C6H5O7 ≡ Na3Cit, and Na2S with concentrations 50, 25, and (25-δ) mmol l 1, where δ E 0.5 mmol l 1, respectively. Small deficiency of Na2S is necessary for the deposition of Ag nanoparticles along with Ag2S and the formation of Ag2S/Ag heteronanostructure. Synthesis was carried out under illumination by LED with a 450 nm wavelength and the irradiation intensity of 15 mW cm 2. Synthesis was carried out in the following sequence: a complexing agent was added to silver nitrate in the dark; then a solution of Na2S was poured into the prepared solution. As a result, deposition of silver sulfide occurred by the following reaction
2AgNO3 + Na2S
Na3C6H5O7 → Ag2S ↓ + 2NaNO3 .
(1)
Further the solution was irradiated with monochromatic light. In accordance with photochemical reaction,
λ C6H5O73 − + 2Ag+ → C5H4 O52 − + CO2 + H+ + 2Ag ↓ , C6H5O73 −
þ
(2)
the citrate ions reduce the Ag ions to Ag nanoparticles in aqueous solutions and transform into acetone-1,3-dicarboxylate
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A.I. Gusev, S.I. Sadovnikov / Materials Letters 188 (2017) 351–354
ions C5H4O52 −. The reduction of silver at the surface of Ag2S nanoparticles leads to the formation of the Ag2S/Ag heteronanostructures. Size distributions of Ag and Ag2S nanoparticles in synthesized colloidal solutions with Ag2S/Ag heteronanostructures are shown in Fig. S1 (see “Supplementary Material”). Produced heteronanostructure was examined by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX) analysis, and Brunauer-Emmett-Teller methods. The average particle size D (to be more precise, the average size of coherent scattering regions (CSR)) in deposited powders was estimated by XRD method from the diffraction reflection broadening [13] (see experimental details in “Supplementary Material” also).
3. Results and discussion The XRD pattern of Ag2S/Ag heteronanostructure is shown in Fig. 1a. The heteronanostructure contain monoclinic silver sulfide with α-Ag2S acanthite structure and metallic cubic silver Ag. Detailed XRD studies of the crystal structure of α-Ag2S acanthite phase were performed earlier [14,20–22]. The crystal structure of bcc β-Ag2S argentite was determined in study [21] and described in detail in Crystallographic information file (CCDC reference number 1062400) of Electronic Supplementary information attached to the article [21]. The quantitative analysis of the XRD pattern and comparison with data [19] have shown that the observed set of diffraction reflections corresponds to nanocrystalline nonstoichiometric monoclinic (space group P21/c) acanthite αAg1.93S and cubic (space group Fm3m ) silver Ag. The diffraction reflection broadening is indicative of the nanosized state of the both phases. The content of Ag and Ag2S in the deposited nanopowder is equal to 7.5 and 92.5 wt%. The HRTEM image of Ag2S/Ag heteronanostructure is shown in Fig. 1b. It is seen that the Ag2S and Ag nanoparticles are in direct contact and form the heteronanostructures. In present study the indices (hkl) of the diffraction reflections obtained by Fast Fourier Transformation (FFT) have been determined with taking into account interplanar distances dhkl and angles φrefl between observed reflections, i. e. the angle between the straight lines passing through each reflection and the central spot (000). The diffraction pattern (Fig. 1c) obtained by FFT of HRTEM image of the whole this composite heteronanostructure contains
two set of diffraction reflections corresponding to monoclinic silver sulfide and cubic silver. The diffraction patterns (d) and (e) are obtained by FFT from areas (1) and (2) isolated by green and orange quadrates. The observed set (d) of spots (111), (200), and (11-1) corresponds to the [01–1] plane of the reciprocal lattice of cubic Ag. The interplanar distances for area (2) and the set (e) of spots (01-3), (12-2), and (111) correspond to monoclinic α-Ag2S acanthite. The produced Ag2S/Ag heteronanostructure combines ionic and electronic conductors. The heterostructures of this type containing Ag and Ag2S nanofibers or a silver film with Ag2S nanoclusters are considered as a potential basis for creating resistive switches and nonvolatile memory devices [2–4,18,26]. The resistive switches consist of a superionic conductor located between two metal electrodes. In the case of Ag2S/Ag heterostructures, one of the electrodes is silver, and the second electrode can be such metals as Pt, Au, and W. In this work, Ag2S/Ag heterostructure formed by Ag2S and Ag nanoparticles has been produced by a simple method of hydrochemical bath deposition. Deposition of Ag2S/Ag heterostructures on a substrate coated with a thin conducting metallic layer will make it possible to form a structure, which can work as a resistive switch. The action of the switch is based on the phase transformation of nonconducting α-Ag2S acanthite into β-Ag2S argentite exhibiting superionic conduction. The transition into a high-conduction state is due to abrupt disordering of the cationic sublattice. In works [27,28] it was shown that a high-conduction state of a crystal can be achieved by external electric field induced “melting” of the cationic sublattice taking place without heating of the crystal. Such transformation occurring as a result of applied external electric field was confirmed by the authors [2–4] with respect to nanocrystalline silver sulfide. The effect of external electric field induced abrupt disordering allows the realization of the superionic state of silver sulfide at room temperature. We have studied the switching processes in Ag2S/Ag heteronanostructure. For this purpose, a metallic Pt microcontact was supplied to Ag2S/Ag heteronanostructure and bias voltage was impressed so that Ag electrode was charged positively. When positive bias voltage increases to 500 mV, the conduction of the heteronanostructure grows and the nanodevice transforms into the conducting state (i. e., the on-state). The bias back to negative values decreases the conduction and the nanodevice transforms into the off-state.
Fig. 1. (a) XRD pattern and (b) HRTEM image of Ag2S/Ag heteronanostructure. (c), (d), and (e) diffraction patterns obtained by FFT of HRTEM image of the whole composite heteronanostructure and its areas (1) and (2). The long and short ticks on XRD pattern correspond to reflections of cubic metallic Ag and monoclinic Ag2S silver sulfide, respectively.
A.I. Gusev, S.I. Sadovnikov / Materials Letters 188 (2017) 351–354
353
Fig. 2. HRTEM images of region of transition between Ag and Ag2S for off-state (a) and on-state (b) of Ag2S/Ag heteronanostructure. The Pt electrode is located on the top part of the image, and Ag electrode is in the bottom part. Diffraction patterns (c) and (d) are obtained by FFT of HRTEM images (a) and (b), respectively. The on-state is induced by the bias voltage applying. When Ag2S/Ag heteronanostructure is transformed from the off-state into the on-state, along with Ag spots, the (011) and (112) spots of β-Ag2S argentite appear on the diffraction pattern (d) instead of acanthite spots.
Fig. 2 displays a region of Ag2S/Ag heteronanostructure where change of crystal structure at the transition from the off-state (Fig. 2a) to the on-state (Fig. 2b) can be observed. Using FFT of HRTEM images, we obtained the diffraction patterns (Figs. 2c, 2d). The diffraction pattern of Ag2S/Ag heteronanostructure in the off-state is shown in Fig. 2c. This diffraction pattern contains (111), (11-1) spots and twinning reflection (111)* corresponding to cubic (space group Fm3¯ m ) silver, as well as (2-12) and (030) spots corresponding to monoclinic (space group P21/c) α-Ag2S acanthite. The observed angle of 100.3° between (2-12) and (030) spots of monoclinic acanthite coincides within measurement error with the theoretical value 100.7°. Then, a positive bias was applied to the Ag2S/Ag heteronanostructure in order to turn it on. HRTEM image of Ag2S/Ag heteronanostructure in the on-state and its diffraction pattern are presented in Figs. 2b and 2d, respectively. The diffraction pattern (Fig. 2d) contains two sets of spots corresponding to two cubic phases. The (111), (200), (1-1-1) spots and the twinning spot (00-
2)* correspond to cubic (space group Fm3¯ m ) silver, and the (011) and (112) spots correspond to cubic (space group Im3¯ m ) β-Ag2S argentite. The observed angle of 30.1° between the (011) and (112) spots of cubic β-Ag2S argentite coincides with the theoretical value 30°. Experimental angles between diffraction spots of cubic silver (Figs. 2c, 2d) coincide with theoretical values. Thus, the applied bias really leads to the appearance of conducting β-Ag2S argentite instead of nonconducting α-Ag2S acanthite and the formation of conductive channel from argentite βAg2S and silver Ag. A current voltage I(U) characteristic (a) of the switching and the schematic operation of a switch based on Ag2S/Ag heteronanostructure are shown in Fig. 3. The initial Ag2S phase is a nonconducting acanthite α-Ag2S (Fig. 3(1)). When a positive bias is applied, Ag þ cations start to move toward the negatively charged cathode M and are reduced to Ag atoms during their transport. At the same time, the α-Ag2S phase transforms into superionic βAg2S argentite (Fig. 3(2)), and a continuous conductive channel is
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Fig. 3. Generalized scheme of the operation of an Ag2S/Ag-based switch: (a) typical current voltage characteristic of the switching; (1) initial nonconducting state; (2) the appearance of a conductive channel upon the application of an external electric field that induces the transformation of acanthite α-Ag2S into argentite β-Ag2S; (3) work onstate with a continuous conductive channel; (4,5) off-state and break down of the conductive channel upon the application of negative bias and the transformation of argentite into initial acanthite; (6) initial nonconducting state after disappearance of the conductive channel and turning-off of the switch. The numbers from 1 to 6 for different states of switching correspond to the order of the events on current voltage characteristic (a).
formed (Fig. 3(3)). The continuous conductive channel which is formed from argentite β-Ag2S and silver Ag is retained, when the external field is turned off. This phenomenon can be considered as a memory effect (Fig. 3(3)). If a negative (reverse) bias is applied to the switch, the Ag nanocrystals start dissolving in argentite, the Ag þ cations move to the anode, argentite transforms into the initial acanthite again, the conductive channel breaks down, and offstate is realized (Fig. 3(4,5)). Because of the formation of nonconducting acanthite, the conductive channel disappears, the switch transforms into the initial state and is turned off (Fig. 3(6)). If positive bias is applied once again, the destroyed conductive channel is restored due to the appearance of argentite and the formation of silver.
4. Conclusion The Ag2S/Ag heteronanostructure is formed in aqueous solutions of AgNO3, Na2S, and Na3Cit with decreased concentration of sodium sulfide during synthesis in the light. The appearance of Ag nanoparticles is due to photochemical reduction of some Ag þ ions by citrate ions. The produced Ag2S/Ag heteronanostructure combines ionic and electronic conductors. A high-conducting state of this heteronanostructure can be induced by external electric field owing to phase transformation of nonconducting acanthite into argentite exhibiting superionic conduction. The argentite β-Ag2S which appears as a result of the phase transformation and metallic silver Ag together form the conducting channel. The scheme of the operation of a resistive switch based on an Ag2S/Ag heteronanostructure is proposed. The main potential application of Ag2S/Ag heteronanostructure is a creation of resistive switches and nonvolatile memory devices.
Acknowledgements This work is financially supported by the Russian Science Foundation (Grant 14-23-00025) through the Institute of Solid State Chemistry of the Ural Branch of the RAS.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.11.111.
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