Plasma-Induced Nonvolatile Resistive Switching with Extremely Low

Nov 16, 2016 - Low power consumption is crucial for the application of resistive random access memory. In this work, we present the bipolar resistive ...
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Plasma-Induced Nonvolatile Resistive Switching with Extremely Low SET Voltage in TiOxFy with AgF Nanoparticles Xiangyu Sun,† Chuangui Wu,*,† Yao Shuai,*,† Xinqiang Pan,† Wenbo Luo,† Tiangui You,‡ Agnieszka Bogusz,‡ Nan Du,‡ Yanrong Li,† and Heidemarie Schmidt‡ †

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China ‡ Material Systems for Nanoelectronics, Technische Universität Chemnitz, Chemnitz 09126, Germany ABSTRACT: Low power consumption is crucial for the application of resistive random access memory. In this work, we present the bipolar resistive switching in an Ag/TiOxFy/Ti/Pt stack with extremely low switch-on voltage of 0.07 V. Operating current as low as 10 nA was also obtained by conductive atomic force microscopy. The highly defective TiOxFy layer was fabricated by plasma treatment using helium, oxygen, and carbon tetrafluoride orderly. During the electroforming process, AgF nanoparticles were formed due to the diffusion of Ag+ which reacted with the adsorbed F− in the TiOxFy layer. These nanoparticles are of great importance to resistive switching performance because they are believed to be conductive phases and become part of the conducting path when the sample is switched to a low-resistance state. KEYWORDS: plasma treatment, bipolar resistive switching, low operating voltage, low power consumption, nanoparticle

1. INTRODUCTION It is getting difficult to get Si-based flash memory devices to meet our needs because of the high energy consumption and scaling limit.1,2 As one of the most promising candidates for the next-generation nonvolatile memory devices, resistive random access memory (RRAM) has attracted considerable attention thanks to its simple structure, low power consumption, good reliability, and excellent scalability.3−5 Generally, resistive switching (RS) between two different resistance states, namely, high-resistance state (HRS) and low-resistance state (LRS), can be generated in a simple layered metal−insulator−metal structure.6 Resistive switching has been observed in a large variety of insulating materials among binary metal oxides,7 ternary metal oxides,8 organic materials,9,10 and graphene-based thin films.11 Most of the insulators are fabricated by techniques such as pulsed laser deposition,8 plasma-enhanced chemical vapor deposition,12 and thermal annealing.13 However, nonstandard integrated circuit procedures have been an obstacle to industrial RRAM production for many years. Along with different metal electrodes, diverse switching mechanisms such as electrochemical metallization (ECM) effect, valence change memory effect, and interface effect have been proposed and elaborated upon, while other mechanisms are still under discussion.14 ECM is a switching mechanism with typical pronounced bipolar switching behavior.14,15 In ECM switching systems, a chemically active electrode (such as Ag or Cu), a solid electrolyte insulator, and an inert electrode (such as Au or Pt) are normally required © 2016 American Chemical Society

for metallic ionization, cation migration, and electrocrystallization during the switching operation, respectively.14 It is reported that low writing current of 160 μA,16 high ON/ OFF ratio (i.e., ratio of device resistance in HRS (RHRS) to device resistance in LRS (RLRS)) exceeding 5 orders of magnitude17 and small RS device area less than 25 × 25 nm2 have been achieved during the past few years.18 As a result, tremendous development of RRAMs with small scale and low power consumption has been made. However, most of the RS devices have to endure high switch-on voltages (VSET) during SET operation, that is, switching from HRS to LRS, and high switch-off voltage (VRESET) during RESET operation, that is, switching from LRS to HRS. Generally, in ECM switching systems, VSET of over 1 V is required.19 As it is known that the VSET is closely related to the film thickness, length of conductive path, ion migration distance, ionic mobility, and chemical reaction energy,14 many efforts have been made to reduce the operating voltage. However, until now, VSET for ultrathin devices with 20 nm thick dielectric layers is still as large as 0.5 V.20 It is worth noting that, in the Ag/Ge−Se/Pt system, the silver ions diffuse into the Ge−Se layer and form AgSe nanocrystal particles during the electroforming. The nanoparticles act as conductive phases, which largely shorten the length of conductive filaments as well as the ion migration distance. As a result, VSET was reduced to around 0.2 V.21 Received: September 1, 2016 Accepted: November 16, 2016 Published: November 16, 2016 32956

DOI: 10.1021/acsami.6b11049 ACS Appl. Mater. Interfaces 2016, 8, 32956−32962

Research Article

ACS Applied Materials & Interfaces In this paper, a fluorine-doped highly defective TiOx (TiOxFy) layer was prepared by plasma treatment. An extremely low operating voltage of 70 mV was obtained in an Ag/TiOxFy/Ti/Pt stack. The device was prepared at room temperature (RT) using standard semiconductor processes, equipment, and materials. As one of the complementary metal oxide semiconductor compatible metals, silver was used as an ideal chemically active electrode with high mobility for the redox reaction and conductive filament (CF) formation in TiOxFy layer. Stable ON/OFF ratio of over 104 was achieved for at least 7000 s. According to the results obtained from conductive atomic force microscopy (cAFM) measurements, operating current as low as 10 nA was achieved. The switching mechanism was further investigated by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and voltage step experiment. Obtained results provide insight into thoroughly understanding the transport characteristics in the Ag/TiOxFy/Ti/Pt stacks.

Electrical measurements were carried out with a Keithley 2400 SourceMeter, with grounded Ti/Pt bottom electrode (BE). Voltage pulse measurements were done using an AFC 3031c voltage source. cAFM (Agilent Technologies 5420 scanning probe microscope) was used to observe the switching property under a current compliance level of 10 nA. The chemical states of titanium, oxygen, and fluoride in the TiOxFy layer were characterized by XPS (XSAM800). The microstructures of the samples in as-prepared state and electroformed state were investigated by TEM (JEM-3010).

3. RESULTS AND DISCUSSIONS 3.1. Electronic Performance. The Ag/TiOxFy/Ti/Pt stack with testing area of 0.07 mm2 was measured at RT. The current−voltage (I−V) characteristics are presented in Figure 2a. Current compliance was set to 1 mA to prevent electric breakdown. The pristine sample was in the OFF state with resistance of about 10 MΩ. BE was grounded and the voltage sweep sequence applied to TE was conducted as 0 V → negative voltage → positive voltage →0 V, as illustrated by the red arrows in Figure 2a. During the electroforming, the sample switched from initial HRS to LRS when the positive bias reached +0.5 V. Typical bipolar resistive switching behavior was observed in the subsequent switching cycles with an even lower VSET (in comparison to that of the electroforming step) of +0.10 V and VRESET of −0.11 V. I−V curves follow a counterclockwise direction and the ON/OFF ratio was around 2 × 104. Polarity of the switching operations is consistent with most of the RS behaviors with ECM mechanism,15 in which the SET process is induced by positive bias while the RESET process occurs at negative bias. This indicates that the resistive switching of investigated sample is possibly dominated by the migration and reaction of Ag+ ions. To explore its RS performance at elevated temperature, retention test at 85 °C was measured. As indicated by Figure 2b, both HRS and LRS of the sample remain stable for over 600 min of continuous testing. Current at low-resistance state is narrowly distributed between 55 and 75 μA, while current at high-resistance state is in the range from 60 to 70 nA, obtained by the read voltage of +0.025 V. This proves that the sample is capable of high-temperature applications as RRAM. Endurance property of the device was also measured. Positive triangular sweeps from 0 to 0.5 V and negative triangular sweeps from 0 to −0.5 V were applied to the device alternately to ensure thorough SET and RESET processes. After each sweep, a read voltage of +0.025 V was applied to detect the resistance state of the device. Results in Figure 2c indicate stable ON and OFF resistances with narrow distributions for over 300 testing cycles. Good repeatability has been obtained. The transport properties were further investigated by cAFM measurements using the contact mode of AFM with a platinum-coated conductive tip. This experiment was conducted to demonstrate good RS property under low operating current, which was systematically confined to 10 nA. Figure 2d presents the scheme of the cAFM measurement and the obtained I−V curve. Voltage sweep started from −0.5 V, as indicated by the red arrow. Similar to the I−V characteristic in Figure 2a, the sample was abruptly switched from as-prepared HRS to LRS at +0.45 V, and was switched back to HRS at −0.22 V, with the current limited to 10 nA. This result indicates that the sample can be switched with a very low operating current and is expected to enable the applications with low power consumption. Voltage step measurements were conducted at RT to understand and explain the kinetics of the SET process. Pulse

2. EXPERIMENTS Fabrication process of Ag/TiOxFy/Ti/Pt/Si stacks was carried out at RT. First, a Ti (100 nm) layer and a Pt (120 nm) layer were subsequently deposited on a Si substrate by magnetron sputtering. After that, another Ti film of 200 nm was sputtered on the Pt layer. The 200 nm thick Ti film was half-masked by Parafilm (from BEMIS Company). TiOxFy layer was prepared by plasma treatment with helium, oxygen, and carbon tetrafluoride (CF4) orderly using a deep reactive ion etching machine (MNL/D III, Trion). First, helium plasma treatment was conducted at a pressure of 40 mTorr and gas flow rate of 40 SCCM (cubic centimeter per minute at standard pressure and temperature), to remove the native oxide layer. During the 7 s reaction, the inductively coupled plasma (ICP) power and reactive ion etching (RIE) power, which determine the plasma density and etching energy, were set as 2500 and 300 W, respectively. The high processing power enabled a remarkable bombardment to the sample surface, which could easily remove the native oxide layer on top of the Ti film. After that, helium was replaced by oxygen with the same gas flow rate and the same pressure in the reaction chamber to produce TiOx layer. The ICP power and RIE power were adjusted to 1000 and 30 W, respectively. After an oxidation process for 120 s, a titanium oxide layer has been formed. In the last stage of the process, oxygen was substituted by CF4 for fluorine implantation. The gas flow rate was kept at 40 SCCM and the inner pressure was tuned to 20 mTorr. ICP and RIE power were kept at 1000 and 30 W for another 60 s reaction for finally fabricating the ca. 150−180 nm thick TiOxFy layer. Samples not treated with CF4 were also prepared for comparison. Circular Ag (200 nm) top electrodes (TE) with diameters ranging from 200 to 500 μm were produced by magnetron sputtering, assisted by a copper shadow mask. The sketch diagram of the Ag/TiOxFy/Ti/ Pt/Ti/Si stacked device is shown in Figure 1.

Figure 1. Sketch diagram of the layered structure with measurement setup. 32957

DOI: 10.1021/acsami.6b11049 ACS Appl. Mater. Interfaces 2016, 8, 32956−32962

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Figure 2. (a) Typical I−V curves of the forming cycle and subsequent cycle with counterclockwise voltage direction labeled with red arrows. (b) The stable resistances in both HRS and LRS at 85 °C for over 600 min retention test; (c) endurance for SET and RESET for over 300 testing cycles. (d) I−V curve obtained by cAFM on an Ag/TiOxFy/Ti/Pt stack with circular Ag electrode and sketch of cAFM testing setup; (e) read currents obtained at a reading bias of 0.01 V after writing pulses with different constant pulse widths (100, 10, 1, 0.5, 0.4, 0.3, 0.2, and 0.1 ms) and different write voltage amplitudes applied to an Ag/TiOxFy/Ti/Pt stack in HRS; (f) VSET varies with eight different pulse widths (100, 10, 1, 0.5, 0.4, 0.3, 0.2, and 0.1 ms) and linear fitting was applied to VSET at small pulse width.

According to the slope in the fitting line shown in Figure 2f, which is −0.54, the calculated coefficient α is obtained as 0.12. This value is comparable to that of metallization processes in other solid electrolytes, for example, α amounts to 0.18 in the Ag/GeSe/Pt electrolyte system.23 This indicates that the ECM mechanism is responsible for the RS behavior and the switching speed is limited by the growth of silver filaments. According to the results presented in Figure 2e, VSET is less than 0.1 V for pulse width of 100 ms. The lowest VSET value of 0.07 V was acquired during voltage sweep with the same pulse width. Compared to the other switching system with low VSET, the value between 0.2 and 1.0 V was frequently reported for the ultrathin atomic switching oxide films.24−26 A small VSET of 0.075 V was obtained by Tamura et al., but a pulse length of 4 s was necessary to switch the sample from 1 MΩ to 10 kΩ.27 In this work, VSET as small as 0.070 V with pulse width of 100 ms is sufficient to switch the sample from 1 MΩ to 300 Ω. Taking into account the operating current as low as 10 nA, the Ag/ TiOxFy/Ti/Pt stack reveals high potential for future RRAM production with high performance and low power consumption. In addition, the investigations on the RS unit with thinner TiOxFy layer are ongoing, to achieve extensive applications. 3.2. Microstructures Measured by TEM. To understand the performance and the low power consumption of SET operation in Ag/TiOxFy/Ti/Pt stack, TEM was employed to characterize the microstructures of the as-prepared and of the electroformed samples. Figure 3a presents the layered structure in the as-prepared sample revealed by TEM. Three layers with different contrasts can be distinguished, known as Pt, Ti, and TiOxFy as indicated in Figure 3a. The 50 nm thick Ti layer between Pt and TiOxFy film remains due to the depth limit of plasma treatment. Highresolution TEM image of TiOxFy layer marked with a red circle in Figure 3a is shown in Figure 3b. TiOxFy layer shows highly amorphous structure, which is often observed in plasma-treated oxide layers.20 Physical bombardment by the active atom species, including O2−, O−, F−, and others during plasma treatment is believed to be responsible for formation of the

voltages with different pulse widths were applied to switch the sample from HRS to LRS. After each writing pulse, a read voltage of +0.01 V was applied to detect the resistance state of the Ag/TiOxFy/Ti/Pt stack. Stable and reproducible read currents were obtained, as shown in Figure 2e; different VSET were obtained for each pulse width where the read current abruptly increased. Figure 2f shows the relationship between VSET and logarithm of pulse width, which is derived from the voltage step measurements. A linear fitting, expressing an exponential relationship between VSET and pulse width, can be applied when the pulse width is smaller than 1 ms. However, VSET changes slightly and is likely to approach a critical threshold voltage when the pulse width is larger than 1 ms. It has to be mentioned that the data shown here did not reach the limit of the switching speed of the sample. Higher switching speed of 1 μs was achieved with SET voltage of 5 V. The obtained exponential relationship is commonly observed in ECM systems, which is determined by the growth speed of metallic filament. The threshold of VSET is mainly related to the critical overpotential for silver nucleation on the cathode.22 Further attention was paid to the exponential relationship between switching voltage VSET and pulse width to understand the conducting mechanism. If one assumes that the switching speed is limited by the growth speed of silver filaments, the dependence of VSET on pulse width can be expressed as follows,14 VSET =

Q αze kT kT ln γ + ln SET2 αze αze i0πrf kT

(1)

where e is the elementary charge, k is the Boltzmann constant, T is the temperature, z is the metal valence (here z = 1 for silver ions Ag+), γ is the sweep rate which can be converted into effective pulse width, QSET is the charge needed for the SET process, i0 is the exchange current density, and α is the cathodic charge-transfer coefficient expressing the kinetics of electrochemical reactions, which can be determined by the experiment result. 32958

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relates to the formation of AgF. Therefore, forming free RS unit could be obtained by introducing Ag+ ions into TiOxFy layer during the fabrication process. Further analysis by energydispersive spectrometry (EDS) was applied to the area in Figure 3d to confirm this point. The spectrum shown in Figure 3f reveals the presence of silver, titanium, oxygen, and fluorine, which demonstrates that the Ag has diffused into TiOxFy layer.29 However, since the sensitivity of fluorine is much lower than other elements, the relative intensities of fluorine peak is much weaker than expected.30 3.3. X-ray Photoelectron Spectroscopy. To understand how the AgF nanoparticles were formed during electroforming, XPS analyses of titanium, oxygen, and fluorine were conducted at RT on the as-prepared samples directly after plasma treatment (without TE). The sample treated without CF4 was also examined for comparison. Carbon detected in both samples was judged as adventitious because of the following reason. During CF4 plasma treatment, carbon was always presented in the positively charged groups such as CF3+, CF22+, and CF3+. Under the positive RIE bias, these groups would never reach the sample surface. The spectrum of three main contributions of titanium, oxygen, and fluorine were analyzed and are shown separately below. The Ti 2p spectra of both samples are shown in Figure 4a. Two strong peaks centered at 458.9 and 464.7 eV corresponding to Ti4+ are observed in both samples. In comparison, two weak peaks centered at 455.5 and 461.6 eV were observed in sample treated without CF4, revealing the existence of Ti3+ and Ti2+.31,32 This indicates that a deep oxidation reaction occurs during the CF4-assisted plasma treatment, which is most likely due to the high chemical activity of the F− ions. In Figure 4b, fitting of O 1s spectra is presented with the three main peaks centered at 528.8, 529.9, and 531.2 eV, representing lattice oxygen, defect oxygen, and adsorption oxygen, respectively.31,33 According to the effective areas of each peak in the sample treated without CF4, TiO2 lattice oxygen is the major component while defect oxygen and adsorption oxygen are the minor components. However, after CF 4 plasma treatment, the oxygen state has changed dramatically. The ratio between defect oxygen and adsorption oxygen largely increased and became the major components, which was probably induced by F− ions. To clarify the effect of F− on the change of oxygen states, F 1s spectra are displayed in Figure 4c. Two main peaks centered at 685.6 and 688.2 eV are observed. The larger contribution at 688.2 eV corresponds to F− ions that substitute lattice oxygen in TiOx, forming TiO2−xFx groups, which was positively charged due to the lack of oxygen. According to O 1s spectra, the replaced oxygen is very likely to be adsorbed on the lattice. This phenomenon can be ascribed to the electrostatic attraction by the positively charged TiO2−xFx groups. The other contribution of F 1s spectra at 685.6 eV is assigned to F− ions that were adsorbed on the lattice surface.34,35 Consequently, the chemical environment among the TiOx layer after CF4 plasma treatment contains TiO2, TiO2−xFx, defect oxygen, physically adsorbed F− ions, and O2− ions. TiO2−xFx groups act as positive centers with the absence of oxygen (defect oxygen) while the adsorption ions are recognized as mobile ions which can move from one lattice surface to another. Furthermore, the activation energy for drift of Ag+ ions in this highly defective layer is much lower than the pure TiOx layer and correspondingly mobility for Ag+ ions is

Figure 3. (a) TEM image of the layered structure of the as-prepared sample and (b) high-resolution TEM image of the area marked with a red circle within the TiOxFy layer in Figure 3a; (c) TEM image of the layered structure of the electroformed sample and (d) high-resolution TEM image of the area marked with a red circle within TiOxFy layer in Figure 3c; (e) the crystal structures of blue ellipses areas in Figure 3d, analyzed by SAED; (f) EDS result reveals the presence of Ag and F in the electroformed TiOxFy layer.

amorphous structure. Such amorphous structure is favored for ECM switching because the long-range disordered structure allows Ag+ for high mobility and low activation energy.28 In comparison, the microstructure of electroformed sample is shown in Figure 3c,d. A small void is observed at the interface between the TiOxFy layer and TE, which is indicated in Figure 3c. This can be caused by the dissipation of silver when Ag+ ions diffuse into TiOxFy layer during the SET process. Figure 3d presents the magnified area of TiOxFy film marked by a red circle in Figure 3c. No obvious conducting path is found. However, it can be clearly seen that uniformly distributed nanoparticles with 2−5 nm in diameter are dispersed in the TiOxFy layer of the electroformed sample. Selected area electron diffraction (SAED) was applied to investigate the crystal structure of observed nanoparticles. According to the electron diffraction patterns shown in Figure 3e and based on the diameters of the diffraction rings, the crystal spaces are obtained as 0.189, 0.134, and 0.11 nm, and from the ratio of the crystal spaces, the crystal faces of [110], [200], and [211] can be determined, respectively. On the basis of the standard crystalline parameter, the obtained space values match the primitive cubic AgF lattice parameters with over 90% similarity. Therefore, it is reasonable to conclude that the nanoparticles observed in Figure 3d are AgF nanocrystals which were formed during electroforming and distributed in the TiOxFy layer with distances of about 2 nm between each other. Learning from above, the electroforming process of the sample most possibly 32959

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According to Yang’s theory, filament would first bridge the TE and newly appeared nanoparticles.38,39 Silver filaments gradually grow and bridge the nanoparticles with the top electrode. These nanoparticles can now be recognized as a part of the top electrode and the same process happens as above until the silver filament finally connects TE and BE via the AgF nanoparticles (Figure 5c,d). This model is in agreement with observed positive bias for electroforming and for SET process; that is, the diffusion-induced crystallization starts from the TE where Ag+ ions start to diffuse in the electric field toward BE. Thus, the growth of networks of Ag filament from TE to BE can be deduced. The work presented by Kozicki et al. for the Ag/Ge−Se/Ni RS system provides an explanation of the behavior of charged species during SET process.40 A similar model can be developed to explain the bipolar resistive switching behavior of the discussed Ag/TiOxFy/Ti/Pt system. As an ionic crystal, the AgF nanoparticles act as Ag+ reservoirs which store and release Ag+ ions. The Ag+ ions diffuse from AgF nanoparticles toward BE and metallic filament forms during the SET process. These dissipative Ag+ cations are constantly supplied by the Ag+ ions from TE when positive bias is applied. Therefore, the AgF nanoparticles are conductive nodes which deliver Ag+ ions from one to another through the TiOxFy layer and finally become part of the conducting path. Thus, the silver filaments bridge the AgF nanoparticles instead of the complete TiOxFy layer during SET process and the length of the filament is significantly shortened. Consequently, less Ag+ ions are needed to be transported during SET for filament growth. Meanwhile, the diffusion distance of Ag+ ions is also reduced to the distance between neighbored AgF nanoparticles, that is, approximately 2−5 nm. In other words, lower activation energies are needed for electrochemical reaction and structural deformation in electrolyte.28 Thus, a resistive switching system with extremely low VSET can be realized. When negative bias applies, the conductive filament dissolves and a dielectric gap forms, which is known as the RESET process. It has to be mentioned that the existence of AgF nanoparticles largely reduces the effective thickness of the dielectric layer (TiOxFy). The local electric field is enhanced especially at the space between neighbored nanoparticles. In this case, the filament prefers to bridge the nearest nanoparticles instead of directly growing toward electrode. Therefore, a meshlike conducting path is proposed as shown in Figure 5d.

Figure 4. XPS results showing (a) binding energy of Ti 2p spectra in samples treated with CF4 and without CF4; (b) binding energy of O 1s spectra in samples treated with CF4 and without CF4; (c) binding energy of F 1s spectra in samples treated with CF4.

4. CONCLUSIONS Highly defective fluorine-doped TiOx layer was fabricated by plasma treatment. Silver was used as an active top electrode for resistive switching with ECM mechanism. AgF nanoparticles were formed in oxide layer during the electroforming. The AgF nanoparticles played a crucial role for the resistive switching performance. Bipolar switching behavior with extremely low VSET of 0.07 V was achieved. Stable ON/OFF ratio above 2 × 104 was obtained for over 7000 s. The sample was able to work under low operating current within 10 nA. These properties are competitive compared with resistive switching performance reported for other ECM systems. Above all, the extremely low VSET is expected to be a promising characteristic for future applications such as artificial synapses for neuromorphic computing which require low power consumption.

expected to be higher. Consequently, the F−-doped TiOxFy layer becomes a good solid electrolyte for Ag+.36 When positive bias is applied to the Ag/TiOxFy/Ti/Pt stack and enough Ag+ ions diffuse into TiOxFy layer, crystallization can easily take place and a new phase appears.37 The model shown in Figure 5 is proposed to discuss the change within the TiOxFy layer during the electroforming process. First, silver top electrode is partially oxidized under the positive bias, followed by the diffusion of Ag+ ions into TiOxFy layer (Figure 5a), which serves as solid electrolyte. Second, the diffused Ag+ ions react with active F− ions and AgF nanoparticles are formed. After that, some Ag+ ions continue diffusing toward the bottom electrode while others start to reduce at the surface of the nanoparticles (Figure 5b). 32960

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Figure 5. Model explaining (a) Ag+ ions for diffusion and pre-existent F− ions in TiOxFy layer; (b) formation of AgF nanoparticles; (c) bridging AgF nanoparticles by networks of conducting filaments; (d) formation of meshlike conducting path between TE and BE by Ag filament and AgF nanoparticles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.G.W.). *E-mail: [email protected] (Y.S.). ORCID

Chuangui Wu: 0000-0001-6491-7422 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51102037) and the National Natural Science Foundation of China (No. 51402044).

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DOI: 10.1021/acsami.6b11049 ACS Appl. Mater. Interfaces 2016, 8, 32956−32962

Research Article

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DOI: 10.1021/acsami.6b11049 ACS Appl. Mater. Interfaces 2016, 8, 32956−32962