Chemical Vapor-Deposited Vanadium Pentoxide Nanosheets with

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Chemical Vapor Deposited Vanadium Pentoxide Nanosheets with Highly Stable and Low Switching Voltages for Effective Selector Devices Sunghun Lee, Jinsu Kim, Jae Ho Jeon, Minho Song, Seonyeong Kim, Young Gyu You, Sung Ho Jhang, Sunae Seo, and Seung-Hyun Chun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15686 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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ACS Applied Materials & Interfaces

Chemical Vapor Deposited Vanadium Pentoxide Nanosheets with Highly Stable and Low Switching Voltages for Effective Selector Devices

Sunghun Lee†, Jinsu Kim†, Jae Ho Jeon†, Minho Song†, Seonyeong Kim†, Young Gyu You‡, Sung Ho Jhang‡, Sun-ae Seo†*, and Seung-Hyun Chun†*

†

Department of Physics and Astronomy, Sejong University, Seoul 05006, Republic of Korea ‡

School of Physics, Konkuk University, Seoul 05029, Republic of Korea

*

E-mail: [email protected] (S.S.) and [email protected] (S.H.C.)

KEYWORDS: vanadium pentoxide, nanosheets, selector devices, threshold switching, thermal annealing treatment

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ABSTRACT Recently, attempts to overcome the physical limits of memory devices have led to the development of promising materials and architectures for next-generation memory technology. The selector device is one of the essential ingredients of high-density stacked memory systems. However, complicated constituent deposition conditions and thermal degradation are problematic, even with effective selector device materials. Herein, we demonstrate the highly stable and low threshold voltages of vanadium pentoxide (V2O5) nanosheets synthesized by facile chemical vapor deposition, which have not been previously reported on the threshold switching properties. The electrons occupying trap sites in polycrystalline V2O5 nanosheet contribute to the perfectly symmetric threshold switching feature at the bias polarity and low threshold voltages in V2O5, confirmed by high-resolution transmission electron microscopy measurements. Furthermore, we find an additional PdO interlayer in V2O5 nanodevices connected with a Pd/Au electrode after thermal annealing treatment. The PdO interlayer decreases the threshold voltages, and the Ion/Ioff ratio increases because of the increased trap density of V2O5. These studies provide insight into V2O5 switching characteristics, which can support low power consumption in non-volatile memory devices.

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1. INTRODUCTION Charge-based memory technology are facing physical limitations, despite the development of lithography and exploration of various materials.1 To overcome the scaling limit and to exploit fast switching kinetics and low power consumption, various types of non-volatile memory have attracted attention as promising next-generation memory devices.2-7 Among them, resistive random access memory (ReRAM) has received intensive interest due to its low power consumption, multilevel switching, long retention time, outstanding cycling endurance, and high integration capacity.8-11 To realize a high-density ReRAM system with terabyte data storage, such as a 3-dimensional crossbar stacked structure, selector devices are required to suppress the sneak current.12 Recent progressive research on high-performance selector devices has highlighted suitable control of both positive and negative polarities for effective memory performance, particularly a stable and low threshold switching (TS) voltage, which is independent of the bias polarity.13,14 Moreover, since the subsequent memory switching behavior after TS requires increasing set and/or reset voltages in serially combined one switch-one resistor (1S-1R) devices compared to those of individual devices, lower threshold and holding voltages are beneficial from a power consumption perspective.12,13 To create high-performance selector devices that allow accurate information storage and retrieval, various TS materials have been investigated. Chalcogenide materials, such as Si and/or Ge-based As-Te thin films, show excellent TS features, but these materials require the fabrication of complex materials and experience thermal degradation after repeated cycling.12,13,15 Additionally, volatile TS characteristics have been observed in thin films of metal-oxide materials, such as NbO2, VO2, and NiO2, which possess Mott-type metal-toinsulator transitions due to strongly correlated d-electrons.16-21 V2O5 has garnered attention due to its splendid potential for applications, such as electrochromic devices, optical switching devices, smart windows, and chemical gas sensors.22-27 The chemically stable van 3



der Waals layered structure of V2O5 permits intercalation with active metal complexes, leading to research on reversible cathode materials for batteries.28-31 Although the electrical properties of V2O5 have received attention in several theoretical and experimental papers,32-34 reports on its use in memory applications are rare.35 Herein, we report the highly stable and low TS behavior of V2O5 nanosheets synthesized by chemical vapor deposition (CVD). The as-synthesized V2O5 nanosheets have well-defined shapes, such as hexagons, triangles, and truncated triangles, with widths of several μm. A Pd electrode-connected V2O5 nanosheet device with a lateral device configuration showed low threshold and holding voltages and perfectly symmetric switching features at bias polarities. Such low switching voltages greatly benefit from low power consumption. Moreover, we observed a large hysteresis window after thermal annealing treatment, implying higher on-off current ratio and larger threshold voltages differentials. The cross-sectional transmission electron microscopy (TEM) images were used to demonstrate the mechanism of TS behavior in V2O5 nanosheets.

2. EXPERIMENTAL DETAILS 2.1. Synthesis of V2O5 nanosheets. V2O5 nanosheets were synthesized by a chemical vapor transport method using a thermal furnace equipped with a 2-inch diameter quartz tube. An alumina boat containing 40 mg of VCl3 powder (Sigma-Aldrich, ≥ 97%) and c-cut sapphire substrates were placed at the middle and edge (10 cm away from the alumina boat with a 1 cm distance gap) of the heating zone, respectively (Figure 1a). Prior to the heating process, Ar gas (500 sccm) was used to purge the furnace tube for 20 min, and then a mixed gas of Ar (200 sccm) and H2 (10 sccm) flowed until the internal chamber pressure reached 1 atm. Next, the bubble-lock valve was opened to maintain the 1 atm condition. Afterwards, the heating zone was heated to 620 °C within 20 min. The temperature was held at the set 4


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temperature for 20 min, and then, the furnace was naturally cooled to room temperature with Ar gas (200 sccm). 2.2. Fabrication and thermal annealing treatment of the V2O5 nanosheet device. The V2O5 nanosheets were immersed in isopropanol and dispersed onto a SiO2/Si substrate with 9 Au point contacts that was prepared in advanced by a photolithography. Pd/Au (50/250 nm) or Au (300 nm) electrodes connected with a single V2O5 nanosheet were defined by a conventional e-beam lithography technique, followed by deposition by using an e-beam evaporator. A thermal annealing treatment was performed in a high-vacuum annealing system. The temperature was ramped up to 200 °C in 5 min and sustained under ~10-6 Torr. After 2 h, the furnace was cooled to room temperature naturally under the same high pressure. 2.3. Characterization and measurement of transport properties. The morphology of asgrown V2O5 nanosheets were systemically characterized with optical microscopy (Leica DM 2700M), scanning electron microscopy (SEM, Tescan, VEGA-3), and transmission electron microscopy (TEM, JEOL, JEM-2100F; acceleration voltage of 200 kV). X-ray diffraction (XRD) patterns were collected using a PANalytical Empyrean diffractometer with X-ray source of Mo and Ag at 60 kV and analysed based on Joint Committee on Powder Diffraction Standards (ICSD) data to confirm the crystal structure. Raman measurements were performed with a Raman spectrometer (Renishaw inVia system) by focusing the laser radiation with 1 mW power, centered at 514 nm. All electrical measurements were carried out at room temperature using a Keithley 4200-SCS instrument.

3. RESULTS AND DISCUSSION V2O5 nanosheets were synthesized on c-cut sapphire substrates in a horizontal furnace equipped with a 2-inch quartz tube by a facile CVD method without any catalysts as shown in Figure 1a. Anhydrous VCl3 powder was used as the vanadium precursor. In our setup, the 5



temperature, which determined the vaporization and the growth, was constant in the central area and sharply decreased moving toward the edges (temperature profile graph in Figure 1a). No vacuum pump was used and the pressure was maintained at 1 atm using a bubble-lock exhaust system. The detailed experimental setup is described in the experimental section. The formation of the V2O5 nanosheets can be attributed to the reaction of VCl3 vapor with oxygen present in the quartz tube or supplied by partial oxygen leakage into the furnace.36 Various types of V2O5 nanosheets with three distinguished shapes, i.e., hexagons, triangles, and truncated triangles, were synthesized on sapphire substrates. Figures 1b and c show representative optical images and their corresponding SEM images of each nanosheet. The edge lengths of the as-synthesized V2O5 nanosheets (in the case of the truncated triangle, the longest edge) were in the range of 2 to 5 μm, and the surfaces of the nanosheets partially included defects such as holes and wrinkles due to the very reactive halide gas precursor (see more SEM images, Figure S1).37 To examine the crystal structure, we carried out an XRD analysis of the as-grown V2O5 nanostructure ensemble on a sapphire substrate as shown in Figure 2a. The observed XRD patterns well matched the orthorhombic V2O5 phase with lattice parameters of a = 1.1516 nm, b = 0.3566 nm, and c = 0.4374 nm (space group Pmmn59, JCPDS card no. 41-1426), and did not show a secondary phase or impurities. Moreover, the predominant peaks of the (00l) series indicate the van der Waals layered structure of V2O5 nanosheets along the c-axis. The peaks marked with asterisks in Figure 2a originated from the holder and the sapphire substrate. Figure 2b shows the Raman spectrum of the individual V2O5 nanosheets, which have peaks in the range from 50 to 1100 cm-1. The observed 10 Raman peaks are quite similar to those of previously reported V2O5 structures.38-40 The dominant peak at 140 cm-1 is related to the vibration mode of the V-O-V chain, implying the existence of layered orthorhombic V2O5 structures.38 The observed peak at 991 cm-1 corresponds to the terminal oxygen 6


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(V5+=O) stretching mode, clearly reflecting the stoichiometry of the V2O5 nanosheets.39 To characterize the crystal structure of the as-grown V2O5 nanosheets, we performed TEM and high-resolution TEM (HRTEM) analyses. A well-defined V2O5 nanosheet with a hexagon shape is shown in Figure 3a. The two enlarged regions show crystal domains with different growth directions, indicating the poly-crystalline nature of the as-grown V2O5 nanosheets. For instance, the magnified HRTEM images marked with red and blue squares (Figures 3b and C) show domains with 0.274 nm and 0.358 nm lattice spacings, corresponding to (011) and (010) planes of the orthorhombic V2O5 phase, respectively. The insets of Figures 3b and c show the two-dimensional fast Fourier transform (FFT) from the HRTEM images. The growth mechanism of the V2O5 nanosheets has two possible reaction paths: i) The VCl3 gas phase vaporized in the heating zone of the furnace is transferred onto the substrate, where the temperature is lower than that in the vaporized region (Figure 1a), followed by a reaction with oxygen in the quartz tube; ii) since metal halide precursors can be easily hydrolyzed due to their high sensitivity to moisture, VOCl3 may be formed,41 and this VOCl3 vapor participates in the reaction, leading to the creation of additional V2O5 nanosheets on the substrate. The device fabrication for the electrical transport studies was conducted by a standard lithographic technique with an individual V2O5 nanosheet, wherein the nanosheet thickness of approximately 200 nm was measured by cross-sectional TEM. After dropping a V2O5 nanosheets solution onto a rinsed SiO2/Si substrate, contact electrodes were made using photolithography and electron-beam lithography, followed by the deposition of 50 nm thick Pd and 250 nm thick Au layers. The optical image of the as-fabricated V2O5 nanodevice is displayed in the left inset of Figure 4. Figure 4 shows the I-V characteristics of the V2O5 nanosheet device. We observed volatile TS behavior between -1 V and 1 V, which is independent of the bias polarities. When the 7



applied voltage is higher than 0.54 V, the current abruptly increases; i.e., the V2O5 nanosheet changes from an off-state to an on-state at the threshold voltage (Vth). When the voltage is applied in the reverse direction, a sudden decrease in the current is observed at 0.46 V, i.e., the holding voltage (Vhold), followed by retracing the off-state current path. The Vth and Vhold values in the negative voltage regime are almost equal to those in the positive voltage regime. These symmetric TS characteristics have been reported for selector-switching devices with materials showing a metal-insulator transition (MIT)42,43 or a phase-change transition (PCT).15,44 To understand the underlying conduction mechanism of the TS characteristics of the V2O5 nanosheets, the I-V curve in Figure 4 was plotted on a double-logarithmic scale (the right inset of Figure 4). The rescaled I-V curve exhibits a linear relationship with voltage (I ∝ V) in the low-voltage region, indicating Ohmic contact. In the high-voltage region, however, the current quadratically varies with the voltage, i.e., I ∝ V2. This I-V relationship is characteristic of trap-controlled space charge limited conduction (SCLC), following Ohm’s law and then Mott-Gurney square law when assuming the presence of shallow traps.45 The traps are likely to be oxygen vacancies, and trap sites are created just below the conduction band edge in transition metal oxides.46 In fact, the I-V curve in Figure 4 was collected after several sweep cycles. After voltage sweep cycles in a low-resistance state, a transition to a high-resistance state was observed with a slightly high off-current (Figure S2). The large defect density in the poly-crystalline V2O5 nanosheets could lead to an increase in the background leakage current. As shown in Figure 5a, then, the TS characteristics were stable during repeated cycles. For several tens of cycles, Vth and Vhold values of 0.54 V and 0.46 V, respectively, were consistently obtained, and the same values were measured in the negative-voltage regime, indicating the independence of the bias polarities in volatile V2O5 switching. 8


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Since the MIT temperature of V2O5 has been considered to be approximately 280 °C,47 the observed threshold voltages at room temperature might be insufficiently low to cause MIT due to relatively large specific heat capacity.48 We performed high temperature XRD measurement to verify whether the TS characteristics are driven by structural or electrical relaxation. There was no significant difference in predominant peaks among various temperatures ranged from room temperature to 350 °C as shown in Figure S3, implying that our V2O5 nanosheets undergo a MIT without structural relaxation. Additionally, in contrast to other MIT materials,49 the small change in electrical conductivity was observed as only oneorder of magnitude. Although some papers have demonstrated that the threshold switching in NbO2 occur at lower temperature than its MIT temperature, based on thermal runaway model,50 the mechanism of MIT is still controversial. As demonstrated above, the conduction mechanism of V2O5 nanosheet devices follows the trap-filled-limited behavior. The trap sites attributed to structural feature, i.e., poly-crystalline, become energetically favorable, when the voltage is applied. After the electrons are fully occupied at trap sites, the abruptly surged current is observed at Vth. When the bias is applied in reverse, the trapped electrons in conduction band edge revert to the pristine state at Vhold. We investigated the effect of an annealing treatment on the TS behavior of a V2O5 nanosheet device, and the temperature was set to 200°C in a high-vacuum chamber (~10-6 Torr). The compliance current was 500 μA to avoid breakdown of the switching device. As seen in Figure 5b, interestingly, we observed similar TS characteristics with lower threshold voltages (0.34 V for Vth and 0.24 V for Vhold) and a larger Ion/Ioff ratio than those obtained before the annealing treatment. Figure 5c illustrates the Vth and Vhold values before/after the annealing treatment. Highly stable threshold voltages were measured, and the Vth and Vhold values decrease by 37% and 49%, respectively, compared to the values obtained before the annealing treatment. These results, i.e., the perfectly symmetric features of the polarity and 9



the high low threshold voltages observed in the V2O5 nanosheet device, indicate V2O5 is a candidate for high-performance selector devices with low power consumption, which are an essential part of high-density 3-dimensional crossbar stacked memory devices. The Ion/Ioff ratio increased at least 5 times compared to that before the annealing treatment, which was estimated under a 500 μA compliance current. In comparison to other materials used in selector devices, the performances of our V2O5 nanosheets after annealing are truly outstanding as shown in Table 1. Since V2O5 consists of a layered structure, studies on physical phenomena via the intercalation of various materials between interlayers have been attractive in battery and energy research fields.23,28,40,53-56 When such materials with layered structures are treated by thermal annealing, the oxygen or impurities bound in the interlayer space can be extracted, leading to changes in the electrical properties.57,58 To clearly elucidate the lower threshold voltages in V2O5 nanosheets, cross-sectional TEM measurements using a V2O5 nanosheet device that experienced an electric field stress were carried out. As seen in Figure 6a, a V2O5 nanosheet with a 200 nm thickness was surrounded by a Pd contact metal. We observed V2O5 nanosheet with both poly-crystalline (c-V2O5) and amorphous (a-V2O5) phases, which was confirmed by the contrast and FFT from the HRTEM images (Figure S4). Additional a-V2O5 layer formed between c-V2O5 nanosheet and Pd contact metal gives rise to the lower offcurrent than before the annealing treatment. We noted that a thin PdO layer formed in the interlayer between the Pd contact electrode and the V2O5 nanosheet after the thermal annealing treatment. Figure 6b shows an enlarged image of the top area (red colored square) of the V2O5 nanosheet device. PdO also penetrates the space between the V2O5 nanosheet and substrate (Figure 6a). To clarify the composition of the interlayer, energy dispersive spectroscopy was performed and showed the presence of V, O and Pd elements, as shown in Figures 6c and d. Moreover, the regular spot configuration 10


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in the FFT confirmed the tetragonal PdO phase (JCPDS card no. 41-1107, Figure S5). As mentioned above, in the conduction mechanism of the V2O5 nanosheet device, the conduction electrons are occupied at the shallow trap sites close to the conduction band of V2O5. We confirmed the presence of a thin PdO layer partially surrounding the V2O5 nanosheet after the thermal annealing treatment. This PdO layer causes the trap density of V2O5 to increase, resulting in decreasing average inter-trap distance.12,59 By the trapcontrolled SCLC model, we can conjecture the increment of trap density from I-V curve (Figure S6). In the case of increased trap density, since the energy difference or barrier of neighboring two traps becomes lower, the relatively small energy to trap and/or release is required, leading to decreasing switching voltage.60 As a result, we observed a large hysteresis window, higher Ion/Ioff ratio and larger ΔV (=Vth–Vhold) after the thermal annealing treatment. A high on-state drive current density is estimated to be 3 MA/cm2, which is comparable to the performance of other excellent selector devices.61 To understand the influence of the contact metal electrode, we investigated a V2O5 nanosheet device with a deposited Au electrode. We did not find evidence of TS characteristics, and a lower off-current was revealed (Figure S7) even at high voltages after thermal annealing treatment. In the cross-sectional TEM measurements, no additional oxide interlayer was found (Figure S8). Moreover, a somewhat thick a-V2O5 phase might hinder overcoming the energy barrier required for a conducting filamentary forming, but further experiments are needed.

4. Conclusion In summary, we demonstrated the synthesis of V2O5 nanosheets with well-defined shapes by a facile thermal CVD method and the stable and low threshold voltages of a V 2O5 nanosheet device with a Pd/Au contact electrode. Based on the detailed TEM measurements, the TS 11



characteristics of the devices were ascribed to trap-controlled space charge conduction behavior of V2O5. Moreover, a PdO interlayer formed at the interface between the contact electrode and the V2O5 nanosheet after a thermal annealing treatment, which resulted in more stable and lower threshold voltages than those obtained before the annealing treatment due to increasing defect density. To expand the use of materials that have potential in high-density, crossbar stacked memory applications, V2O5 could provide an effective platform for developing novel next-generation memory devices.

Supporting Information The supporting Information is available free of charge on the ACS Publications website at DOI: xxx/xxx. Additional SEM images of V2O5 nanosheets, I-V characteristics evolution until threshold switching, high temperature XRD patterns, poly-crystal and amorphous phase of V2O5 nanosheet, TEM analyses of PdO interlayer, Scaling of trap-controlled SCLC I-V curve after vacuum annealing treatment, Cross-sectional TEM images of V2O5 nanosheet device contacted with Au metal electrode.

ACKNOWLEDGMENTS We thank Dr. Young Haeng Lee (Korea Institute of Science and Technology) for the HRXRD measurements. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT and MOE) (Nos. 2010-0020207, 2016R1E1A1A01942649, 2018R1A5A6075964 and 2018R1D1A1B07048109).

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Table 1. Performance of representative materials for selector devices Materials NiO VOx/AlOy HfO2 As-Te-Ge-Si-N VO2 V2O5 V2O5 (annealed)

Vth 5.4 V 1.08~1.82 V 5V 1.2 V 0.35 V 0.54 V 0.34 V

Vh 3.8 V 0.12~0.54 V 2.5 V 1.1 V 0.2 V 0.46 V 0.24 V

Ion/Ioff 50~300 104 80 50 50 20 100

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Endurance 120 cycles 100 cycles 107 cycles 108 cycles 100 cycles 100 cycles

Refs. [21] [51] [52] [13] [42] This work This work

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Figure 1. Synthesis and morphological characterization of the V2O5 nanosheets. (a) Schematic of V2O5 nanosheet growth along a temperature gradient. The distances between the VCl3 precursor and growth substrate (D1) and the VCl3 precursor position from the furnace edge (D2) are 10 cm and 20 cm, respectively. The bubble-lock system allows the inert pressure to remain at 1 atm with flowing Ar carrier gas. (b) Representative optical and (c) SEM images of the as-synthesized V2O5 nanosheets with well-defined shapes, such as hexagon, triangle, and truncated triangle. All scale bars are 3 μm.

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Figure 2. Spectroscopic analyses of the as-synthesized V2O5 nanosheets. (a) XRD pattern of a V2O5 nanosheet ensemble synthesized on a sapphire substrate. All the peaks are indexed to orthorhombic V2O5 phase (JCPDS card no. 41-1426). The peaks marked with asterisks originate from the holder and sapphire substrate. Inset: enlarged XRD patterns around 42°, clearly indicating separated peaks. (b) Raman spectrum of the V2O5 nanosheets.

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Figure 3. TEM characterization of an individual V2O5 nanosheet. (a) Low-magnified TEM image of hexagonal V2O5 nanosheets. Different contrast indicates varying thickness and crystal domain. (b) and (c) Enlarged HRTEM images taken from the areas marked with red and blue squares, respectively. The 0.274 nm and 0.358 nm lattice spacings correspond to the (011) and (010) growth directions, respectively, of the orthorhombic V2O5 phase. Inset: FFT from the HRTEM of (b) and (c)

Figure 4. I-V characteristics of the V2O5 nanosheet device operated in the TS mode. The ±Vth (Vhold) values that cause the switch to turn on or off are clearly observed and are independent of the bias polarities. Left inset: optical image of the V2O5 nanosheet device in contact with the Pd/Au electrode. Right inset: I-V curve fitted on a log-log scale using the positive voltage regime of the main figure, which agreed with the trap-controlled SCLC mechanism.

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Figure 5. Electrical properties influenced by a thermal annealing treatment. I-V characteristics (a) before and (b) after a thermal annealing treatment under several tens of positive and negative voltage sweeps. Compliance currents before and after a thermal annealing treatment are 1 mA and 500 μA, respectively. (c) Vth (square) and Vhold (circle) of the V2O5 nanosheet device in the positive (black) and negative (red) voltage regimes. After a thermal annealing treatment, both Vth and Vhold values are clearly more stable than the values before a thermal annealing treatment. 24


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Figure 6. TEM characterization of the V2O5 nanosheet device experiencing an electrical field. (a) The cross-sectional TEM image of the V2O5 nanosheet device in contact with the Pd/Au metal electrode. A phase-change transition and V2O5 surrounded by a PdO interlayer are observed. All materials and phases are denoted in the figure. (b) Enlarged HRTEM image taken from the area marked by a red square. Detailed analysis of PdO is described in the Supplementary Information. (c) Enlarged scanning TEM image taken from the area marked by a blue square for EDS compositional mapping, as shown in (d)

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Chemical Vapor-Deposited Vanadium Pentoxide Nanosheets with

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