Interface Thermodynamic State-Induced High-Performance

With the applied positive potential, the migration of oxygen vacancies within a single crystal generates oxygen vacancies at their vacant places, wher...
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Interface Thermodynamic State-Induced High-Performance Memristors Adnan Younis,† Dewei Chu,*,† Chang Ming Li,*,‡ Theerthankar Das,§ Shama Sehar,§ Mike Manefield,§ and Sean Li†,‡ †

School of Materials Science and Engineering and §Centre for Marine Bio-Innovation, School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney 2052, NSW, Australia ‡ Institute for Clean Energy & Advanced Materials (ICEAM), Southwest University, Beibei, Chongqing 400715, P. R. China. S Supporting Information *

ABSTRACT: A new class of memristors based on long-range-ordered CeO2 nanocubes with a controlled degree of selfassembly is presented, in which the regularity and range of the nanocubes can be greatly improved with a highly concentrated dispersed surfactant. The magnitudes of the hydrophobicity and surface energy components as functions of surfactant concentration were also investigated. The self-assembled nanostructure was found to demonstrate excellent degradation in device threshold voltage with excellent uniformity in resistive switching parameters, particularly a set voltage distribution of ∼0.2 V over 30 successive cycles and a fast response time for writing (0.2 μs) and erasing (1 μs) operations, thus offering great potential for nonvolatile memory applications with high performance at low cost.

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INTRODUCTION Memristors work on the principle of storing information by changing their resistance states in a reversible manner, a resistive switching process, thus resulting in a new class of memory devices, which promise low power consumption than flash memory and have the opportunity to provide even faster responses. Resistive switching (RS) phenomena have been explored in a variety of materials, from binary and complex metal oxides1−8 to solid electrolytes9,10 and organic-based materials.11 Despite significant improvements to date,12−14 the practical implementation of high-performance memristors still poses great challenges in terms of obtaining favorable performance characteristics such as switching speed, power consumption, signal-to-noise (OFF/ON) ratio, cycling endurance, and suppression of sneak currents in the same device system.15 As the span of resistive switching phenomena spreads in a very thin layer of a few nanometers, the reduction of the size of an individual memory cell to a few nanometers is extremely desirable. To fabricate nanoscale memory devices, top-down and bottom-up approaches have been utilized to prepare building blocks such as nanodots, nanorods, nanocubes, and nanowires.16−18 The bottom-up approach, as a cost-effective © 2014 American Chemical Society

method that mainly relies on the self-assembly mechanism, is considered more promising for the fabrication of sophisticated components than the top-down method.16 Also, the selfassembly technique had been found to be quite useful for improved device performances. For example, improvements in RS characteristics and enhancements in photoluminescence properties using self-assembled gold−peptide nanofiber nanostructures19 and spherical bolaamphiphiles20 were recently reported. Cerium oxide (CeO2) nanomaterials have attracted great attention because of their wide application in catalysts, solar cells, fuel cells, gate materials for metal-oxide semiconductor devices and phosphors, and so on.21−24 There have been many reports on the size- and shape-controlled synthesis of CeO2 nanocrystals,25−28 for example, by tuning the concentration of reactants and stabilizing agents, varying the water/toluene ratio in the reaction system,26 and using a liquid−liquid interface.28 To date, only a few reports have been published on the Received: November 14, 2013 Revised: January 8, 2014 Published: January 9, 2014 1183

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Characterization. For structural analysis (X-ray diffraction), scanning electron microscopy (SEM), contact-angle measurements, and resistive switching characterization, gold-coated silicon was used as a substrate, and the previously prepared suspensions were drop-coated three times to complete thin film preparations. Henceforth, the samples are denoted as follows: D-1, original suspension; D-2, 2 vol % OLA; D-3, 3 vol % OLA; D-4, 4 vol % OLA; and D-5, 5 vol % OLA. Optical images of all suspensions are shown in the Supporting Information (Figure S1). Structural analysis of the as-synthesized CeO2 films was carried out using a Philips X’pert Multipurpose X-ray Diffraction System (MPD) with Cu Kα radiation, whereas scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out using SEM Nova 230 and Philips CM200 microscopes with accelerating voltages up to 200 kV. The contact angles of all prepared films (with varying OLA concentrations from 0 to 7 vol %) were measured with standard polar and nonpolar probe liquids (water, formamide, and diiodomethane, respectively) using a goniometer (KSV model 200, KSV instrumentation Pvt. Ltd., Helsinki, Finland) according to the sessile drop technique.32 To conduct surface thermodynamic analysis, the measured contact angles of samples D-1−D-7 (where D-7 is an additional suspension solution containing 7 vol % OLA) were converted to Lifshitz−van der Waals (γLW) and acid−base (γAB) surface free energy components using the LW−AB approach.32 Subsequently, the acid−base component was separated into electrondonating (γ−) and electron-accepting (γ+) parameters.30,31 Different components of the surface energies were then further used to calculate the total interfacial free energy of CeO2 nanocube self-assembly to the bare substrate (total ΔG value) at close contact and separated into Lifshitz−van der Waals (LW ΔG value) and acid−base (LW ΔG value) parts.30,31 The resistive switching characteristics were measured using an Autolab 302N electrochemical workstation controlled with Nova software. Before measurements, a small-area electrode (Au) with square patterning and sized about 250 μm in diameter was sputtered through masks onto the devices.

controlled synthesis of CeO2 nanocrystals for memristor applications.29 For the bottom-up method, not only is the morphologycontrolled synthesis of the building blocks (nanocrystals) important, but the way in which nanocrystals interact with each other (interfacial effect) is also crucial for their diverse applications, especially RS characteristics. However, hardly any reports have demonstrated the effects of self-assembly on either the resistive switching characteristics or the physicochemical interactions that drive the self-assembly of nanocubes on a substrate. It is expected that the degree of self-assembly will affect the interface between adjacent nanocrystals and, thus, can play an important role in the electrical properties of the assembly. In a previous study, we induced the self-assembly of a CeO2 quantum dot (QD) array to tune the resistive switching characteristics of ZnO memristors.25 Physicochemical interactions, including Lifshitz−van der Waals, hydrophobic, hydrogen-bonding/acid−base, and ionic bonding interactions, in colloids and surface science play an important role in dictating the attachment of nanocrystals to a substrate, self-aggregation, or agglomeration. Herein, the physicochemical interactions that drive the self-assembly of CeO2 nanocubes on a substrate are demonstrated through surface thermodynamics. Surface thermodynamics that consider only nonspecific interactions depend on their component interaction forces: attractive long-range Lifshitz−van der Waals interactions based on dipole−dipole interactions arising from entire nanocubes and attractive or repulsive short-range acid− base interactions.30 The acid−base interactions form the basis for the hydrophobicity of the particle surface as a reflection of their chemical composition.31 Herein, we report a systematic study on tuning the degree of self-assembly of CeO2 nanocubes by varying the surfactant concentration. Furthermore, the surface thermodynamics was investigated by calculating surface energy components, and a tremendous uniformity in resistive switching characteristics was obtained, along with a considerable reduction in device set, rest voltage, and power consumption as a consequence of improved self-assembly in CeO2 nanocubes. To the best of our knowledge, the present report is the first attempt to relate the self-assembly of memristor devices to their surface thermodynamics and resistive switching characteristics.

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RESULTS AND DISCUSSION The purity and crystallinity of the as-prepared CeO2 nanocube thin films (D-1−D-5) were examined by X-ray diffraction (XRD). All diffracted peaks (Figure 1) can be indexed as the

METHODS

Synthesis. All chemicals used in this work were obtained from Sigma and used without further purification. In the first step, CeO2 nanocubes were prepared by a hydrothermal process.25,27 In a typical synthesis, 15 mL of 16.7 mmol/L cerium(III) nitrate solution was added to a 50 mL autoclave, and then 15 mL of a mixture of toluene, oleic acid (OLA; 0.56 mL, 8:1 OLA/Ce), and tert-butylamine (0.15 mL) was added to the autoclave in open air without stirring. The sealed autoclave was heated at 200 °C for 30 h and then immediately cooled to room temperature. The upper organic crude solution was washed with absolute ethanol and centrifuged at 16000 rpm for 5 min to separate the CeO2 nanocubes. The separated CeO2 nanocubes were redispersed in 3 mL of toluene to form an original suspension solution composed of CeO2 nanocubes. The concentration of OLA in the already-prepared original suspension was increased systematically (0, 2, 3, 4, and 5 vol %) to form five different suspensions. To prepare thin films using these suspensions, a gold-coated silicon wafer was used as the substrate, and the drop-coating method was employed to form thin films. After that, the as-prepared films were treated with UV radiation for 2 h to remove extra oleic acid and other organics.

Figure 1. X-ray diffraction patterns of all prepared devices (D-1−D-5).

face-centered-cubic pure phase [space group Fm3̅m (No. 225)] of ceria (JCPDS no. 00-034-0394). The strongest peak, (200), among all CeO2 samples indicates that the nanocubes were all oriented vertically to the substrate. The increase in (200) peak intensity for D-3 can be attributed to the high self-assembly and vertical orientation of CeO2 nanocubes on the substrate. Figure 2 shows TEM images of CeO2 nanocrystals obtained with various OLA concentrations. A monolayer of CeO2 nanocubes can be visualized but with many empty/vacant spaces (Figure 2a). The number and area of these empty spaces 1184

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Figure 2. TEM images of CeO2 nanocubes having OLA concentrations of (a) 0, (b) 2, (c) 3, (e) 4, and (f) 5 vol %. Image d shows the boxed area in panel c at high magnification. The inset in panel f is a high-magnification HRTEM image of the boxed region showing overlapping nanocubes.

Table 1. Contact Angle and Surface Free Energy Components of Bare Substrate (Au−Si) and Samples D-1−D-5 and D-7 surface energy (mJ/m2) contact angle (θ, deg) sample

water

bare substrate (Au−Si) D-1 D-2 D-3 D-4 D-5 D-7

95 ± 1 90 ± 0.5 96 ± 1 116 ± 1 109 ± 1 106 ± 1 103 ± 1

formamide 82 73 76 82 79 77 76

± ± ± ± ± ± ±

1 1 1 1 1 1 1

AB (acid−base)

diiodomethane 48 48 55 59 58 58 56

± ± ± ± ± ± ±

γLW (Lifshitz−van der Waals)

γ− (electrondonating)

± ± ± ± ± ± ±

5±1 5 ± 0.5 2 ± 0.4 0±0 0±0 0±0 0.1 ± 0.08

2 1 1 1 1 0.5 1

35 36 31 29 30 30 31

1 0.5 0.5 0.4 0.6 0.8 0.6

γ+ (electronaccepting) 0±0 0±0 0±0 0.02 ± 0.02 ± 0.05 ± 0.02 ±

0.05 0.05 0.05 0.05

γAB 0±0 0±0 0±0 0±0 0±0 0.02 ± 0.05 0.1 ± 0

γtotal (γLW + γAB) 35 36 31 29 30 30 31

± ± ± ± ± ± ±

1 0.5 0.5 0.4 0.6 0.8 0.6

Figure 3. Interfacial free energies of CeO2 nanocube self-assembly on a substrate: (a) total, (b) acid−base, and (c) Lifshitz−van der Waals energies.

acid-stabilized CeO2 nanocrystals are drawn to the rapidly enlarged liquid−gas interface and immediately assembled into monodisperse ordered nanostructures, whereas the substrate and postdeposition UV treatment help to decompose and evaporate extra suspension. To validate this assumption, Raman spectroscopy was conducted for two D-3 samples (one treated with UV radiation and the other untreated), as shown in Figure S2 (Supporting Information). The OLA peaks in untreated sample were obvious, while for the UV treated sample, no obvious peaks correspond to OLA were detected. For the formation of a nearly perfect, monodisperse self-assembled nanostructure layer, the suspension should have a proper oleic acid concentration, and in our case, the appropriate concentration of oleic acid was ∼3 vol %.

decreased with a slight improvement in the order of selfassembly when the OLA concentration was increased to 2% (Figure 2b). With a further increase in the OLA concentration to 3%, a nearly perfect monolayer and self-assembled nanostructure was formed (Figure 2c); a high-magnification image of the boxed area in Figure 2c is presented in Figure 2d. It is known that surfactant molecules can play a key role in forming ordered assemblies.27 In this study, the surfactant, oleic acid, provides a necessary driving force for CeO2 nanocrystals to form an ordered nanostructure. The driving force is suggested to be the traction force originating from the surfactant absorbed on the nanocrystals when surfactant molecules are concentrated on a rapidly newly forming liquid−gas interface to decrease the surface tension. The oleic 1185

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Figure 4. (a) Current−voltage relationship on a semilogarithmic scale. (b−d) Statistical distributions of (b) set voltages, (c) HRSs, and (d) LRSs for 30 cycles for devices D-1−D-5.

above. Thus, the hydrophobicities and surface energies mainly rely on the self-assembly of the CeO2 nanocubes regardless of the nature of the surface. It is very clear from our results that the coated CeO2 nanocube surface by itself is hydrophobic (90° water contact angle) and has a very low surface energy and attractive Gibbs free energy (ΔG). It is obvious that hydrophobic surfaces are well-known to have low surface energies.33 Further use of OLA increased the hydrophobicity without much affecting the surface energy (which is already very low regardless of the presence of OLA, e.g., D-1) or total Gibbs free energy (ΔG). Therefore, variations in the hydrophobicity and interfacial surface energy suggest that the selfassembly of CeO2 nanocubes on the Au−Si surface is driven by the oleic acid-mediated surface hydrophobicity of nanocubes. However, the decrease in the hydrophobicity and multilayer formation of CeO2 nanocubes for samples D-4−D-7 might be due to the adsorption of excess oleic acid on the CeO2 nanocubes to promote the cross-linking of the nanocubes (D4−D-7) and, thus, to affect CeO2 nanocube self-aggregation. The presence of biomolecules/polymers such as amino acids, fatty acids, lipids, proteins, and nucleic acids extending up to tens of nanometers from the adsorbed particle surface has the potential to promote their self-aggregation by influencing their surface properties such as cross-linking and acid−base interactions.34−36 The current−voltage (I−V) curves of all prepared memristor devices (D-1−D-5) presented in Figure 4a exhibit resistive switching (RS) effects. Repetitive dc cycling characteristics were measured for the samples to examine the reliability of the devices. Panels b−d of Figure 4 show the distributions of set voltages and high-resistance and low-resistance states (HRSs and LRSs), respectively, over 30 successive cycles. For D-3, the distribution of the set voltage was restricted to 0.2 V, which was much lower than the voltage distributions of 0.77, 0.2953, 0.342, and 0.674 V for D-1, D-2, D-4, and D-5, respectively. This reflects the better uniformity in setting the threshold voltage for the device D-3 ON state. Moreover, D-3 also demonstrates a higher degree of uniformity in both

Samples D-1 and D-2 were not able to yield ordered structures (Figure 2a,b), whereas sample D-3 led to the formation of an ordered monolayer structure (Figure 2c,d). It is worth noting that a multilayered nanostructure can also be prepared from the redispersed nanocrystals by increasing the oleic acid concentration as depicted from Figure 2e,f. In the inset of Figure 2f, the selected highlighted area is magnified, and the HRTEM image confirms the overlapping of adjacent nanocubes. Table 1 reports the surface hydrophobicity and surface energies of a gold-coated silicon substrate and CeO2 nanocubebased films (D-1−D-5 and D-7). The water contact angle of CeO2 nanocubes increased from 90° for D-1 to 116° for D-3; with a further increase in oleic acid (to 7 vol %, D-7), the water contact angle decreased to ∼103°. However, no prominent difference in surface energy components was observed. The observed Lifshitz−van der Waals and electron-donating/accepting parameters of the acid−base component indicate that the concentration of oleic acid mediates hydrophobic forces to dictate both the self-assembly and self-aggregation of the nanocubes. The interfacial energies of the CeO2 nanocube assemblies and bare substrate (Au−Si) presented in Figure 3 indicate that both the Lifshitz−van der Waals and acid−base energies are attractive. However, the self-assembly of CeO2 is more favorable or attractive for D-3, D-4, D-5, and D-7 than for D1 and D-2. This is due to the increase in surface hydrophobicity of CeO2 nanocubes with increasing oleic acid concentration. Oleic acid, which is a hydrophobic, nonpolar, water-insoluble fatty acid, promotes hydrophobic interactions between CeO2 nanocubes and the substrate. In conjunction with the results presented in Figure 2, sample D-3 makes a uniform selfassembly on the Au−Si substrate. To ensure consistency with our TEM results, highly polished carbon-coated copper strips were also used as bare substrates for contact angles measurements. The similar pattern for films formation was adopted as mentioned before. Interestingly, the contact angles of all three solutions were consistent (±2−3%) with the trend described 1186

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Figure 5. (a) Program and (b) erase characteristics for devices D-1−D-5 composed of CeO2 nanocubes.

resistance states (HRSs and LRSs) over D-1, D-2, D-4, and D5, as depicted in Figure 4c,d. The pulse characteristics of all of the devices (D-1−D-5) were investigated to define the program and erase (P/E) conditions, in which the typical programming and erasing characteristics under ac pulse biases were measured over pulse widths ranging from 1 ns to 100 ms with pulse heights of +3 V and −3 V. Figure 5a,b shows the current at 0.5 V as a function of the P and E times. In the program test, all devices were switched from a high- to a low-resistance state at +3 V/0.1 ms, +3 V/20 μs, +3 V/0.2 μs, +3 V/2 μs, and +3 V/10 μs for D-1− D-5, respectively. In addition, the devices completely swapped their states from low to high resistance at −3 V/0.1 ms, −3 V/ 20 μs, −3 V/1 μs, −3 V/2 μs, and −3 V/10 μs, respectively. This indicates that the degree of self-assembly significantly affects or/and improves the device response time. To explain resistive switching behavior in the CeO2 nanocube-based memristor device, the I−V curves for D-3 were replotted on a log−log scale as shown in Figure 6. The

It is well-known that defect formation energies at surfaces differ from those in the bulk. This leads to an electric gradient near the surface to maintain thermodynamic equilibrium. Volume changes related to point defects in space charge layers can produce strains, significantly changing the thermodynamic equilibrium near surfaces in ionic solids.40 With the applied positive potential, the migration of oxygen vacancies within a single crystal generates oxygen vacancies at their vacant places, where they localized on/near surface. Because of the closely packed structure and comparatively weakly bonded surface atoms as compared to their counterparts in the bulk, the intergrain paths are much shorter than the grain size. (The UV treatment is more likely to form a tightly packed matrix by reducing the space among cubes, which is much less than the size of a single cube.) The high potential further induces electric stress on localized vacancies, and as a consequence, electrons can tunnel through the interfacial barrier. This argument is quite reasonable in a way, suggesting that, for films with thicknesses of a few hundred nanometers, the effective electric field is on the order of ∼100 kV/cm, which is strong enough to overcome the interfacial barriers among grains. In other words, the defects on the crystal surface, such as cation or anion dangling bonds, make broad electronic states near the band edge. They serve as hole or electron traps, which can act as acceptors for carriers. Also, the nanocube assembly has a large interfacial volume that acts as trap sites, the current would flow through the filaments by the SCLC conduction as filling nearby trap sites.39 For low potential, the injected charge carriers can be captured in the traps, when the concentration of the injected charge carriers is lower than the thermally generated carrier density, so that ohmic conduction is dominant. With increasing voltage, the injected current become higher; therefore, SCLC would predominate during filling of the trap sites. As a result, a conducting filament is formed, resulting in an abrupt current increase to 10−3−10−2 A and a state transition to the ON state. When in the ON state (highly conductive state) at positive and negative voltages, the characteristics of the ON-state currents completely show an ohmic current. The multilayer assembly of the nanocubes on each other in a nearly perfect self-assembled structure (D-3) can provide better contact area/platforms (the high probability of layer-by-layer assembly with each nanocube and its interface with another nanocube reside over the underlying nanocube and its interface) to generate conducting filaments more easily and efficiently as compared to imperfect self-assembled structure, as shown schematically in Figure 7. As a result, the D-3 device

Figure 6. Plot of log I vs log V under positive bias, fitted for the ohmic conduction and trap-controlled space-charge-limited-current (SCLC) mechanism for the Au/CeO2/Au/Si (D-3) device.

observed linear relation indicates that the electrical conduction follows either ohmic or strong space-charge-limited current (SCLC), which is known as a trap-charge-limited current (TCLC) with trap models (slope (α ≫ 2).37 For the low-voltage range (