Reversible Negative Resistive Switching in an Individual Fe@Al2O3

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Functional Nanostructured Materials (including low-D carbon)

Reversible negative resistive switching in an individual Fe@Al2O3 hybrid nanotube for nonvolatile memory Yalong Ye, Jie Zhao, Li Xiao, Baochang Cheng, Yanhe Xiao, and Shuijin Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01153 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

Reversible negative resistive switching in an individual Fe@Al2O3 hybrid nanotube for nonvolatile memory Yalong Ye,† Jie Zhao,† Li Xiao,† Baochang Cheng,†,‡,* Yanhe Xiao,† and Shuijin Lei,† †

School of Materials Science and Engineering, Nanchang University, Jiangxi 330031, P. R. China, and

‡

Nanoscale Science and Technology Laboratory, Institute for Advanced Study, Nanchang University, Jiangxi 330031, P. R. China

ABSTRACT: Hybrid nanostructures can show enormous potential in different areas due to their unique structural configurations. Herein, Fe@Al2O3 hybrid nanotubes are constructed via a homogeneous coprecipitation method followed by subsequent annealing at reducing atmosphere. The introduction of zero bandgap Fe nanocrystals in the wall of ultrawide bandgap Al2O3 insulator nanotubes results in the formation of charge trap centers, and correspondingly a single hybrid nanotube-based two-terminal device can show reversible negative resistive switching (RS) characteristics with symmetrical negative differential resistance (NDR) at relatively high operation bias voltages. At a large bias voltage, holes and electrons can be injected into traps at two ends from electrodes, respectively, and then captured. The bias voltage dependence of asymmetrical filling of charges can lead to a reversible variation of built-in electromotive force, and therefore the symmetrical negative RS with NDR arises from two reversible back-to-back series bipolar RS. At a low readout voltage, the single Fe@Al2O3 hybrid nanotube can show an excellent nonvolatile memory feature with a relatively large switching ratio of ~30. The bias-governed reversible negative RS with superior stability, reversibility, nondestructive readout as well as remarkable cycle performance makes it a potential candidate in next-generation erasable nonvolatile resistive random access memories.

*

Address correspondence to [email protected]. ACS Paragon Plus Environment

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KEYWORDS: hybrid nanostructure ·charge traps ·negative resistive switching ·reversible charge injection ·nonvolatile memory.

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1. INTRODUCTION Among all of the information storage technologies, the resistive random access memory (RRAM) has been developing as one of the most promising memory devices for next generation, including the resistance-based memory and the neuromorphic device applications, owing to its superior performance, such as simple structure, high storage density, fast response, low power dissipation, excellent scalability, non-destructive readout and so on.1-8 Commonly, the memory device based on RRAM is made into typical metal-insulator-metal (MIM) structure. The device can display high resistance state (HRS) and low resistance state (LRS) by imposing external voltage, which correspond to the “1” and “0” logic states, combining with write and erase processes. At present, the RRAMs are mainly focused on two types: unipolar and bipolar resistive switching (RS), however, the nonvolatile memories based on a negative differential resistance (NDR) effect have also drawn considerable attention due to the potential applications in next-generation RRAM.9-12 The controlling mechanisms of RS behavior, including oxygen vacancy migration,13,14 conducting filament,15,16 Mott transition,17,18 space-charge-limited current,19,20 trapping/detrapping of charges,21,22 and Poole–Frenkel emission23,24 have gotten a great deal of discussion, although the RS performance itself has been found in a variety of materials included phosphides,25 chalcogenides,26 nitrides,27,28 and binary/complex oxides.29,30 Among these, the utilization of Al2O3 in RRAM has been mentioned rarely, and moreover they are all based on thin film structure.21-37 Although a negative differential resistance (NDR) behavior was observed in Al2O3 combining with the influence of water, its real origin is still not very clear.12 In addition, the sandwich structure RRAM, which based on Ag, CoPtx, SiGe, and Ru nanocrystals embedded in Al2O3 , was studied as well.38-42 The introduction of nanocrystals in oxide films can enhance the local electric field, and then improve the dispersion of the RRAM conversion parameters significantly. However, the feasible fabrication processing of RRAM devices with embedded nanocrystals is still lacking. For Fe nanocrystals, additionally, they have unique magnetic, catalytic, optical, and other properties making them useful in many advanced nanotechnological applications, and they have been


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regarded as the most promising candidate for ultrahigh-density magnetic recording and informationstorage media.43 In this work, individual Fe@Al2O3 hybrid nanotubes, synthesized via a homogenization precipitation method followed by heat treatment under reducing atmosphere, were connected by semi-dried silver paste to fabricate a typical MIM structure two-terminal nanodevice. For single zero band gap metal Fe and ultra-wide bandgap insulator Al2O3, it is very difficult to appear RS behavior. However, the introduction of Fe nanocrystals into Al2O3 nanotube wall results in the formation of hybrid nanostructures. It is very easy for charge traps to form in the nanohybrids, which make it feasible to rewritable non-volatile memory effects. Therefore, the behavior of the typical symmetrical negative RS in company with NDR appears in the two-terminal nanodevices based on individual Fe@Al2O3 hybrid nanotubes, and correspondingly a built-in electromotive force induced by reversible charge injection and extraction at two ends of onedimension nanohybrids is proposed.

2. EXPERIMENTAL METHODS 2.1 Synthesis of Fe@Al2O3 hybrid nanotubes. In this work, Fe@Al2O3 hybrid nanotubes, in which ultrafine metal Fe nanocrystals are homogeneously distributed in the wall of Al2O3 nanotubes, are synthesized via a homogenization coprecipitation method followed by heat treatment under weak reducing atmosphere composed of 90%N2+10%H2. First of all, 0.0025mol FeSO4·7H2O, 0.005mol Al(NO3)·9H2O, 0.6mol CO(NH2)2, and 0.003mol [(C16H33)N+(CH3)Br-, CTAB] were dissolved in 50 ml deionized water and stirred to obtain a solution. The solution was put into a stainless steel autoclave. Then, it was heated at 125 oC for 15 h and subsequently heated at 180 oC for 9 h. After cooling to room temperature naturally, the resulting product was collected, washed with deionized water and alcohol, and dried in oven at 60 oC for about 10 h. Thereafter, the powders were annealed at 600 oC for 3 h under N2 contained 10%H2 at a constant 25 ml/min. After the reaction was over, Fe@Al2O3 hybrid nanotubes were obtained. 2.2 Fabrication of devices based on individual Fe@Al2O3 hybrid nanotubes. For the fabrication of devices, Fe@Al2O3 hybrid nanotubes were transferred onto insulated Al2O3 ceramic substrates which ACS Paragon Plus Environment

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were washed by ultrasound with deionized water and alcohol, and Cu lead wires were connected to the both ends of a relatively long nanotube with semi-dried silver. Then, they were covered by PDMS to isolate air and vapor. An integral MIM two-terminal device could be obtained after heating at 70 oC for 10 min to make PDMS solidified. 2.3 Characterization of materials and measurement of devices. The composition and microstructure of the Fe@Al2O3 hybrid nanotubes were analyzed by the way of X-ray diffraction (XRD; Bruker D8 Advance with Cu Kα radiation), field emission scanning electron microscopy (FESEM; FEI Quanta 200F), high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100, operated at 200 kV), and X-ray photoelectron spectroscope (XPS; ESCALAB 250). The measurement of electric performance was performed by a combination of synthesized function generator (Stanford Research System Model DS345) and low-noise current preamplifier (Stanford Research System Model SR570).

3. RESULTS AND DISCUSSION The XRD pattern for as-synthesized product is exhibited in Figure 1a. Peaks at 44.6°and 82.3°, which respectively correspond to (110) and (221) Miller indices of a body-centered cubic (bcc) structure Fe (JCPDS No. 06-0696), reveal the existence of elemental Fe. Peaks at 35.5°and 64.9°can be indexed to (111) and (020) Miller indices of face-centered cubic (fcc) structure γ-Al2O3 (JCPDS No. 35-0121, indicating the presence of Al2O3 in as-synthesized product as well. The product is further determined by FESEM and TEM, as shown in Figure 1b-f. As seen from FESEM image in Figure 1b, the nanostructures display rod-like morphology. The TEM image in Figure 1c further verifies that the rods are composed of nanotubes, and moreover the diameter of outer wall is around 300 nm. Some small black granules with a size of about 10 nm can be found uniformly in the wall of the nanotubes. Then, the HRTEM lattice fringe image (Figure 1e) of the small granules is obtained and its corresponding fast Fourier transformation (FFT) analysis (Figure 1f) further confirms that the Fe nanocrystal has a bcc structure stretching along the [001] direction. Composition analysis of the nanotubes is tested by energy-dispersive X-ray spectroscopy (EDS) equipped in TEM, as showed in Figure 1d. Besides Cu peaks from the carbon-coated TEM grid, Al, O and Fe peaks are present in the spectrum. This demonstrates that the product is consisted of Al, O and Fe ACS Paragon Plus Environment

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elements. XPS is additionally applied to explore the valence of Fe, as shown in Figure 2. As seen from the XPS spectrum of Fe@Al2O3, two obvious peaks at 724.3 eV and 711 eV, which correspond to 2p1/2 and 2p/2 of Fe, respectively, indicate the presence of Fe2+ and Fe3+. The peak at 720 eV is related to Fe0. Compared with the XPS data, the XRD pattern does not show any other Fe oxides, indicating that the

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precipitation of Fe exists as Fe0 in the nanohybrids, and trace Fe2+ and Fe3+ exist in Al2O3 lattice.44,45

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Figure 1. Morphology and structure characterization of as-synthesized product. (a) XRD pattern. (b) FESEM image, showing rod-like morphology. (c) TEM bright-field image, displaying tube-like structure. (d) EDS spectrum of a single nanotube, demonstrating that the product was composed of Al, O and Fe elements. (e) High-magnification TEM image of tube wall, revealing the existence of a large amount of crystalline particles in wall. (f) A HRTEM image of a single particle in tube wall, verifying that crystalline particle is Fe with a cubic structure. The inset is its corresponding FFT pattern, indexed to the [001] zone axis of bcc Fe.


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Figure 2. XPS spectra of as-synthesized Fe@Al2O3 nanohybrids. (a) Full spectra; (b), (c), and (d) high resolution XPS spectrum of O 1s, Fe 2P, and Al 2p core-level, respectively. To explore the electrical performance of Fe@Al2O3 hybrid nanotubes in different conditions, the I–V characteristics of a single Fe@Al2O3 hybrid nanotube-based nanodevice were measured at 293 K, as illustrated in Figure 3. Figure 3a shows the I-V cyclic sweep curves at a bias voltage of 10 V. It can visibly be seen that RS behavior, accompanied by NDR effect, is symmetrically displayed in the positive and negative bias part of cyclic I-V curves. With the voltage increases from 0 to +10 V, the corresponding current first rises and then falls. Through careful observation, it reaches a maximum at the voltage of around +4 V, and then decreases rapidly, showing a NDR behavior at the rise stage. The resistance changes evidently from LRS to HRS. Looking at this segment that the voltage ranges from +10 to -10 V, the resistance has been kept HRS until the voltage changes into zero. Equally, I-V curves show the same variation law under the voltage sweep with reverse negative bias. The current increases first, and then decreases sharply when the reverse voltage exceeds around -4 V, showing a NDR behavior as well.


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Figure 4 Memory performance of an individual Fe@Al2O3 hybrid nanotube-based two-terminal device. The blue and red curves represent the applied voltage and the response current or resistance, respectively. (a) Applying a consecutive pulse voltage of +10~+0.6 V, showing that the resistance of the device is about 3.2 MΩ at readout voltage of 0.6 V. (b) Applying a consecutive plus voltage of -10~+0.6 V, showing that the resistance of the device is about 0.38 MΩ at the same readout voltage of 0.6 V. (c) A programmable pulse voltage for writing-reading access. Writing and erasing voltages are set as -10 V and +10 V, respectively, and readout voltages are set as +0.2, +0.6, +0.8, +1.0, +1.5 and +2.0 V, respectively. (d) Multiple circular memory processes at readout voltage of +0.6 V, showing regularity and repetitiveness of the device. (e) An enlargement of reading–writing–reading–erasing–reading process, and measurement ACS Paragon Plus Environment

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voltages of -10, +0.6, and +10 V represent writing, reading, and erasing voltages, respectively. (f) The variation curves of LRS and HRS currents with time, showing an endurance performance. In order to better understand and clarify the phenomena associated with negative RS and NDR, I–V characteristics were also measured by cyclic sweep voltages of 1, 2, 4, 6, 8 and 10 V, respectively, as shown in Figure 3b. Their I–V curves can show noticeable differences when applying different cyclic sweep voltages. When the applied bias voltage is lower than about 2 V, I-V curve only shows a monotonous variation, as shown in Figure 3e. Moreover, a linear relation of ln(I/V) versus V1/2 can be found at the rise stage of positive bias, as illustrated in Figure 3f, indicating that the device can conduct by an electric-field-enhanced thermal emission from a bulk trap state into a continuum of electronic state, namely Poole−Frenkel (P−F) emission, and furthermore the Ag electrode and hybrid nanostructure interface is Ohmic contact.47 When the applied bias voltage exceeds about 3 V, NDR phenomena appear symmetrically at the rise stage of forward and reverse bias voltage in the cyclic I-V curves. With increasing cyclic sweep voltage, moreover, the transition point (reset voltage) of NDR, that is the voltage that the resistance changes from LRS to HRS, becomes larger. The finding indicates that the reset voltage is intensively dependent on the highest imposed voltage. In addition, I-V characteristics were also measured by only applying positive cyclic sweep voltages within the range from 0 to 1, 2, 4, 6, 8 and 10 V, respectively, as illustrated in Figure 3c. It can be seen that all the currents vary uniformly with voltage, and moreover the NDR behavior is not found, indicating that NDR phenomena appear only after applying a relatively large reverse bias voltage. Near zero point, a magnified view is shown in Figure 3d. It can distinctly be seen that all I-V curves deviate downward from zero point, indicating that a presence of nanobattery-like electromotive force between two end electrodes after being subjected to a bias voltage,46 and furthermore the potential is positive at the end connected with positive electrode. This indicates that the holes are injected into Fe-related traps at the end connected with positive electrode, resulting in an oxidation of Fe0 to a high valence. In addition, the charge injection-induced potential increases with the


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increase of applied bias voltage, and contrarily the charge injection-induced current decreases, indicating that the electromotive force is more distinct with the increase of externally applied bias voltage. (a)

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Figure 5. Multicyclic I-V characteristics measured at different bias voltages, showing a good stability and repeatability. (a) 4 V; (b) 6 V; (c) 7 V; (d)10 V. As mentioned above, two relatively large hysteresis loops exist symmetrically in the cyclic I-V curves of an individual Fe@Al2O3 hybrid nanotube-based two-terminal device at a relatively large operation bias voltage, indicating a good nonvolatile data storage capability. In order to explore further the RS phenomena and the information storage performance, the measurement of consecutive pulse voltage between +10~+0.6 V and -10~+0.6 V were performed, respectively, as shown in Figure 4a and b. As seen from them, there is almost no difference in aspect of the resistance under applying relatively large voltages of +10 and -10 V. At the same readout voltage of +0.6 V, however, the resistance was much large after applying a pulse voltage of -10 V compared with +10 V, showing a negative RS nature. The resistance of the device reaches about 3.2 MΩ at a readout voltage of 0.6 V after applying a voltage of +10 V. But it changes into around 0.38 MΩ at the same readout voltage after applying a voltage of -10 V. Comparing the formation condition of LRS and HRS, the imposition of high voltage is a decisive factor, and furthermore the polarity of RS is reversible. At a low readout voltage, in other words, the device can turn into LRS after loading a relatively large reverse bias voltage, and then it can recovery into HRS after loading a relatively large forward bias voltage. The memory measurement was conducted by a pulse ACS Paragon Plus Environment

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voltage and the corresponding results were shown in Figure 4c and d. The writing and erasing is reversible. In order to fix writing and erasing voltages, here, they are set as -10 V and +10 V, respectively. Then, the readout voltages are set as 0.2, 0.6, 0.8, 1.0, 1.5 and 2.0 V. For a better observation, part curves are enlarged or shrunk properly. As seen from Figure 4c, the response currents are different at different readout voltages, and moreover they can all show relatively good memory performance. When the readout voltages are higher or lower than 0.6 V, however, the stabilization of LRS and HRS reduces greatly and the switching ratio of HRS to LRS might be small, and furthermore the response sensitivity is low. Given all that, the value of 0.6 V, which not only has the mentioned advantages but also achieves low power consumption of 15 nW at LRS, is selected as optimal readout voltage, and the multiple circular memory measuring processes are illustrated in Figure 4d. the regularity and repetitiveness are observed in the multiple circular memory processes, demonstrating an excellent stability of device. As seen from a part enlargement of the reading–writing–reading–erasing–reading process exhibited in Figure 4e, the information can be written at the readout voltage of 0.6 V after applying a relatively large negative bias of -10 V, namely LRS, and it is erased after applying a relatively high positive bias of +10 V, namely HRS. In addition, the ratio of LRS to HRS, namely memory window, can achieve ~30. Owing to good symmetry of their I–V characteristics, it can be concluded that the device can show similar rewritable memory property by applying relatively low opposite readout voltage. The current curves show a completely reversible, stable, and nondestructive property. As seen from Figure 4f, it exhibits clearly the relational curves about time and readout current when nanodevice is in the condition of HRS and LRS, respectively. At a reading voltage of 0.6 V, it can be found that there is no significant variation at both HRS and LRS after 1.2×105 seconds, showing that the nonvolatile nature of the devices is stable. Figure 5 show further cyclic I-V curves with consecutive sweeping at different bias voltages. It can be seen that the I-V curves all show NDR behavior at the measurement bias voltage ranging from 4 to 10 V. After many cycles, moreover, I-V curves can still remain the same shape, demonstrating an excellent stability.


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Figure 7. After applying a relatively large fixed bias voltage in two opposite directions, respectively, I-V characteristics at 0.6 V bias voltage, showing that I-V curves deviate from zero point symmetrically. (a) After applying -10 V; (b) after applying +10 V. For a deeper insight into the origin of negative RS and the influence of temperature on electric transport, the I–V characteristics of the single-Fe@Al2O3 hybrid nanotube were also measured at 1 V bias voltage at 343 and 293 K, respectively, as illustrated in Figure 6. The nanodevice is conductive at 293 K, and furthermore the current almost exhibits a monotonous variation with voltage. However, it becomes into a nonconductive state at 343 K. This finding indicates the electric properties is strongly determined by traps. They can be filled up at 293 K, and therefore the devices can conduct by an electric-field-enhanced thermal emission of trapped charges. However, the trapped charges can be excited thermally at 343 K,


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resulting in the emptying of traps. Therefore, the device becomes into a nonconductive state and the conductivity descends remarkably. (a)

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Figure 8. The schematic diagrams of charge distribution and the corresponding energy band structure in different conditions. The purple and orange arrows correspond to the direction of external electric field (E) and built-in polarization field (Ep), the white and sky-blue balls represent holes and electrons, Φh and Φe are the depth of hole and electron traps, K is Boltzmann constant, and T is adiabatic temperature, respectively. (a) The emptying of traps resulting from the thermal excitation of captured charges when the external temperature is higher than 343 K, showing a nonconductive state. (b) A certain amount of charges are captured by traps, and an electric-field-enhanced thermal emission (P-F emission) from trap states into a continuum of electronic state governs the electric transport. (c) At a relatively large forward bias voltage, holes and electrons can respectively be injected into the traps at two ends from electrodes, resulting in an asymmetrical filling of charges. (d) After applying a relatively large forward bias voltage, ACS Paragon Plus Environment

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part injected charges can permanently be stored at zero or relatively small bias voltage, resulting in the formation of built-in electromotive force or Fermi level difference. (e) At a low reverse bias voltage, the direction of external electric field is opposite to the built-in electromotive force, and therefore the builtin electromotive force is weaken and the device is LRS. (f) At a moderate reverse bias voltage, the reverse polarization makes the built-in electromotive force disappear, and correspondingly the charges are uniformly distributed in the traps and the device current reaches a maximum (reset point). After applying fixed bias voltages of +10 and -10 V, respectively, I-V characteristics are measured at 0.6 V bias voltage as well, as shown in Figure 7. It can be seen that I-V curve deviates downward from zero point after applying +10 V fixed bias voltage, and contrarily it deviates upward from zero point after applying -10 V fixed bias voltage. The results further verify nanobattery-like electromotive force exists reversibly between two ends at zero bias voltage after applying a relatively large fixed bias voltage in two opposite directions, namely the presence of charge injection-induced redox after completely withdrawing externally applied large electric field. Moreover, the polarity of RS is reversible, determined by the direction of loaded bias voltage. The schematic diagrams are given in Figure 8 to explain the above electric transport phenomena. Fe nanocrystals are uniformly embedded in the wall of Al2O3 nanotubes, resulting in the formation of hybrid nanostructures. Therefore, quantities of traps are formed, and furthermore they can capture and store charges. In addition, trace Fe2+ and Fe3+ exist in the nanohybrids as well, confirmed by XPS analysis and moreover their valence can vary between +2 and +3. Therefore, the nanohybrid can well capture charges and then store. When the device is heated to 343 K, the trapped charges can be excited since the energy of thermal excitation is higher than that of trap ionization, resulting in an emptying of traps. Therefore, the device is nonconductive at relatively high temperature, as illustrated in Figure 8a. At a relatively low temperature, such as 293 K, some charges can be captured due to the decrease of thermal excitation energy, resulting in a part filling of traps. At a low bias voltage, therefore, the device can conduct by an electricfield-enhanced thermal emission from a bulk trap state into a continuum of electronic state, that is P−F ACS Paragon Plus Environment

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emission, as shown in Figure 8b. At a relatively large voltage which is higher than that of trap ionization potential, as seen from Figure 8c, the energy band tilts, holes and electrons can be injected into traps at two ends of a single hybrid nanotube from positive and negative electrodes, respectively, resulting in an asymmetrical filling of charges. After applying a relatively large forward voltage, the injected charges can well be captured by traps, and moreover the asymmetrical filling of charges can be kept. The asymmetrical distribution of charges leads to the difference of Fermi level position at two ends, that is, nanobattery-like electromotive force is formed in the single nanostructure. Therefore, the built-in potential difference makes I-V curve deviate downward from zero point, that is, the device can show a nanobatterylike electromotive force effect after applying a relatively large voltage, as shown in Figure 8d. If a low forward voltage is applied to the device, its direction of electric field is the same as that of electromotive force, resulting in an enhancement of electromotive force, and hence the device conductivity is relatively poor, namely HRS. If a low reverse voltage is applied to the device, its electric field direction is opposite to that of the electromotive force, resulting in a weakness of electromotive force, and accordingly the device conductivity is relatively good, namely LRS, as shown in Figure 8e. As a consequence, the device can show a negative RS behavior with a large switching ratio of LRS to HRS at a relatively low readout voltage of 0.6 V, and more the properties are intensively dependent on the applied voltage. At a relatively low readout voltage and temperature, the injection and extraction of charges cannot occur, and accordingly the memory can retain for a long time. If a relatively large reverse voltage is applied to the device, the reverse injection of charges can occur. When the charges are uniformly redistributed in the traps, the reverse injection makes the electromotive force disappear, and correspondingly the current reaches a maximum, namely reset point, as illustrated in Figure 8f. With further increasing reverse voltage, the NDR phenomenon appears due to a stronger reverse injection of changes in two ends from electrodes, that is, the built-in electromotive force is again formed in opposite direction. Therefore, the symmetrical negative RS with NDR arises from two reversible back-to-back series bipolar RS

4. CONCLUSION


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Fe@Al2O3 hybrid nanotubes were synthesized via a homogenization coprecipitation method followed by heat treatment under reducing atmosphere. The introduction of Fe nanocrystals in the wall of Al2O3 nanotubes results in the formation of hybrid nanostructures, and correspondingly the storage centers of charges can be constructed. At relatively large external electric fields, electrons and holes can be injected into traps at two ends of an individual Fe@Al2O3 nanotube-based two-terminal nanodevice, respectively, and then effectively captured and stored by Fe nanocrystal-related trap centers, resulting in the formation of a voltage dependence of reversible built-in electromotive force between two ends of individual hybrid nanotubes. Two reversible back-to-back series bipolar RS results in the symmetrical negative RS with NDR. Moreover, ~30 switching ratio can be obtained with a non-destructive readout effect. The nanostructure devices based on one-dimensional nanohybrids will contribute to the development of new types of memory components.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86-791-8396-9329.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (51571107, 51462023), and the Major Program of the Natural Science Foundation of Jiangxi Province (20152ACB20010). REFERENCES


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Reversible Negative Resistive Switching in an Individual Fe@Al2O3

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