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Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals Joonseok Yoon,† Howon Kim,† Xian Chen,‡ Nobumichi Tamura,§ Bongjin Simon Mun,*,∥,⊥ Changwoo Park,#,▽ and Honglyoul Ju*,† †

Department of Physics, Yonsei University, Seoul 03722, Republic of Korea Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong 999077, China § Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Department of Physics and Photon Science, School of Physics and Chemistry and ⊥Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea # Division of Applied Chemistry and Biotechnology, Hanbat National University, Daejeon 34158, Republic of Korea ▽ Advanced Nano Products, Sejong, 30077, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We investigated the control of two important parameters of vanadium dioxide (VO2) microcrystals, the phase transition temperature and speed, by varying microcrystal width. By using the reflectivity change between insulating and metallic phases, phase transition temperature is measured by optical microscopy. As the width of square cylinder-shaped microcrystals decreases from ∼70 to ∼1 μm, the phase transition temperature (67 °C for bulk) varied as much as 26.1 °C (19.7 °C) during heating (cooling). In addition, the propagation speed of phase boundary in the microcrystal, i.e., phase transition speed, is monitored at the onset of phase transition by using the high-speed resistance measurement. The phase transition speed increases from 4.6 × 102 to 1.7 × 104 μm/s as the width decreases from ∼50 to ∼2 μm. While the statistical description for a heterogeneous nucleation process explains the size dependence on phase transition temperature of VO2, the increase of effective thermal exchange process is responsible for the enhancement of phase transition speed of small VO2 microcrystals. Our findings not only enhance the understanding of VO2 intrinsic properties but also contribute to the development of innovative electronic devices. KEYWORDS: vanadium dioxide, metal−insulator transition, phase transition temperature, phase transition speed, size effect, microcrystal, high-speed resistance measurement sistors.14 Recently, fundamental understandings such as discovery of triple point in VO2 nanobeam and the role of anharmonic lattice dynamics significantly enhance our view on the underlying physics of the MIT.15,16 Also, recent advances of field-effect transistors based on VO2, which is the one of the most promising applications, have revealed critically important results such as oxygen migration to ionic liquid and strong localization of field-induced excess carriers.17,18 Despite these tremendous recent progresses, many challenging tasks still remain before VO2 can be utilized as a reliable material for commercial products. Specifically, among the many critical tasks, control of the phase transition temperature and speed is one of the highest priority tasks because the functionality of

1. INTRODUCTION VO2 has received much attention because of its intriguing physical properties as a strongly correlated electron system and its enormous potential for future technological applications.1−11 At just above room temperature (∼67 °C), VO2 undergoes a first-order metal−insulator transition (MIT) with a dramatic change in electrical resistance of 4−5 orders of magnitude.2,3 Also, at the moment of the MIT, VO2 undergoes a first-order structural phase transition from a monoclinic to a rutile structure. One important aspect of the phase transition of VO2 is that it can be induced by diverse methods other than thermal activation, such as the application of pressure,4 light,5,6 current,7,8 and voltage.9 Moreover, in the case of an optically induced MIT, the transition occurs on an ultrafast time scale.5,6 These unique physical properties make VO2 one of the best candidates for future advanced materials in a variety of applications, including ultrafast sensors,10 optical switching devices,11 memories,12 tunable resonators,13 and Mott tran© 2015 American Chemical Society

Received: November 17, 2015 Accepted: December 29, 2015 Published: December 29, 2015 2280

DOI: 10.1021/acsami.5b11144 ACS Appl. Mater. Interfaces 2016, 8, 2280−2286

Research Article

ACS Applied Materials & Interfaces

of VO2. Sample inspection and the method for control of the sample temperature are given in the Supporting Information. Crystal structures before and after MIT onset were confirmed by synchrotron Laue microdiffraction from Beamline 12.3.2 of the Advanced Light Source (Supporting Information). Because the onset of phase transition in our microcrystals was characterized by the propagation of the phase boundary from one end of the crystal to the other (Supporting Information), we were able to estimate the speed of the phase boundary during MIT, i.e., the phase transition speed, from measurements of crystal length and the propagation time of the phase boundary. To monitor the propagation time and the phase transition speed, the high-speed resistance of VO2 crystals was measured using a dc two-contact four-probe method. High-speed resistance measurements with a data collection rate of 5000 events per second were carried out using a source meter (Keithley 2611A). Details of the photolithography process used for resistance measurement of VO2 microcrystals are provided in Supporting Information.

VO2 and its potential scope of application depend strongly on these two parameters. Over the years, many research groups have made intense efforts not only to identify the origin of the phase transition in VO2, but also to control the occurrence of the phase transition onset by varying many physical parameters, such as doping,19,20 strain,21,22 pulse voltage,23,24 and size.25−27 For example, phase transition temperature has been reported to decrease in response to doping with high valence (≥4+) ions, e.g., W6+, Nb5+, and Mo6+. However, doping with trivalent ions, e.g., Cr3+, Fe3+, Ga3+, and Al3+, resulted in opposite results, i.e., the increase of phase transition temperature.19,20 Recently, Whittaker et al. reported that the phase transition temperature could be decreased by oxygen vacancy doping.26 Varying crystal dimensions is another approach that has been used to control the onset of the phase transition in VO2. Lopez et al. reported the heterogeneous nature of the nucleation process and the possibility of increasing the VO2 phase transition temperature by reducing the dimensions of VO2 by embedding VO2 nanoparticles in SiO2 matrices.25,28 These authors postulated that the defects in VO2 nanoparticles acted as nucleation sites for the phase transition. In contrast, in solid-state physics, the spatial confinement of ordinary materials on ultrasmall scales often leads to new physical phenomena. Various physical properties of solid states, e.g., band gaps,29−31 structural strength,32 magnetism,33 ferroelectricity,34 and thermoelectric power,35 change dramatically as the dimensions of the material are reduced. In the case of VO2, the probability of phase transition is expected to be strongly dependent on the size of the sample.25 As sample size decreases, the probability of phase transition decreases. Consequently, the onset temperature of phase transition of smaller samples is expected to increase (decrease) during heating (cooling) compared to that of larger samples. Moreover, the transition speed of VO2 is greatly limited by thermal dissipation conditions because of the exchange of latent heat with the surrounding environment, indicating that the effective surface area for thermal interaction may be an important parameter determining the transition speed of VO2. In this report, we describe how to tune the temperature and speed of the phase transition of VO2 microcrystals by systematically controlling microcrystal size. By varying the dimensions of the VO2 microcrystals, we demonstrate that the onset temperature for phase transition can be varied from 67.4 °C up to 93.1 °C, while the transition speed can be enhanced from 4.6 × 102 up to 1.7 × 104 μm/s. Using statistical behavior analysis of the first-order phase transition with a size-dependent thermal dissipation effect, we investigate the correlation among phase transition temperature, speed, and the size of VO2 microcrystals. The ability to control the temperature and speed of the phase transition will contribute to the future use of VO2 in advanced devices.

3. RESULTS AND DISCUSSION For the present study of size dependence of MIT in VO2, highquality square-cylinder-shaped VO2 microcrystals were fabricated, as shown in Figure 1. Width range (w) of VO2

Figure 1. Scanning electron microscopy image of square-cylindershaped VO2 microcrystals with widths of (a) 67 μm, (b) 4.3 μm, and (c) 1.3 μm. Optical microscope images of a small VO2 microcrystal with a width of 3.1 μm and length of 42 μm, (d) just before and (e) after the MIT.

microcrystals was from ∼1 to ∼70 μm. A few selected SEM images of VO2 microcrystals are shown in Figure 1a−c; the cross section of the microcrystals was almost square. In a previous report, we showed that high-quality VO2 single crystals show a heterogeneous first-order phase transition propagating from one end of the crystal surface to the other (Figure S2 and Video S1).37 Figure 1d,e shows optical microscopy images of a VO2 microcrystal with a length of 42 μm and width of 3.1 μm just before and after the phase transition during heating. This crystal exhibited single-domain characteristics of phase transition without any grain boundaries or domain structures. That is, there was either a pure metallic or insulating single domain across the crystals, and very few defects were present in these high-quality single crystals. Hence, the probability of phase transition initiating from a defect inside the crystals was assumed to be negligible. The optical images shown in Figure 1d,e are different from those of VO2 nanowires or thin films, where the metallic and insulating phases coexist during phase transition.21,22,38 In addition, the heterogeneous phase transition in these crystals occurs from the end surfaces and proceeds with phase boundary motion.37 This differs from the results reported by Lopez et al.;28 they found that MIT was initiated by heterogeneous nucleation at extrinsic defect sites in the crystals. Previous synchrotron X-ray microdiffraction results of identical microcrystals revealed that the growth direction

2. EXPERIMENTAL DETAILS Free-standing high-quality VO2 microcrystals with a square cross section were prepared on an Al2O3 boat by a self-flux evaporation method. Details of the crystal elaboration process have been provided previously.36 The microstructure and composition of VO2 crystals were characterized by scanning electron microscopy (SEM; JSM7001F, JEOL). Once the microcrystals were prepared, we employed optical microscopy to monitor the onset of the phase transition in these VO2 microcrystals.37 Because of different optical reflectivity between the metal and insulator phases of VO2, optical microscopy can provide direct real-time images of phase transition on the surface 2281


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Figure 2. (a) Schematic drawing of the crystal size dependence of the MIT temperature of VO2 microcrystals. Smaller crystals tended to have higher transition temperatures because of a lower probability of phase transition. (b) Dependence of the MIT temperature on crystal size (width) during heating (empty red square) and cooling (empty blue circle). The inverse of the end surface area (1/S) versus the difference in the MIT temperature (|T − TC|) curve of VO2 microcrystals after (c) heating and (d) cooling.

on the surface area of both ends, S (= 2w2). If σ is the density of suitable heterogeneous nucleating sites per unit surface, then the probability of finding one nucleation site in a small area (dS) at the end surface is defined by σdS. Because this probability is very small, we assume that the probability of occurrence of more than two sites is negligible. Thus, the probability, F, that the surface contains at least one nucleation site is F = 1 − exp[−σS].43−45 To consider the contribution of temperature to the probability, σ can be written as BΔgnex, where B is a constant, n is an exponent, and Δgex is the driving force of phase transition, i.e., the Gibbs free energy difference per unit volume between metal and insulator phases.28,45 If we further assume that phase transition occurs when F had the same (or most probable) probability regardless of size in Figure 2b, then σS becomes a constant. Note that Δgex is directly proportional to |T − TC| and σS = BΔgnexS = C|T − TC|nS = D where both C and D are constants. Finally, by taking the logarithm of both sides of the equation, C 1 we get log S = n log|T − TC| + log D . To test this derivation,

(length direction) of VO2 microcrystals is along the c direction of the rutile phase (cR) [001].36,37 Next, the onset temperature of phase transition in VO2 microcrystals was monitored as the size (crystal width (w)) of the crystals was varied from 1.9 to 67 μm. Figure 2a shows schematics of the experimental setup for thermal optical microscopy of free-standing VO2 microcrystals. As discussed in Figure S2, the phase transition of VO2 occurs with phase boundary motion along the cR axis, and propagation of the MIT starts from one end of the microcrystals, suggesting that phase transition onset may be linked to the end surface areas (2w2). This is clearly different behavior compared to that of stressinduced multiple domains in VO2 nanobeam.39−41 To test this hypothesis, we plotted the onset temperature of the MIT phase transition as a function of the width of the microcrystals, w, as shown in Figure 2b. Interestingly, the phase transition temperature rapidly increased (decreased) as the width of the crystals decreased in both the heating and cooling processes. In the case of the 3.6 μm thick microcrystals, the onset temperature of the phase transition was as high (low) as 93.1 °C (49.2 °C) during heating (cooling). This change in onset temperature of phase transition is larger than those reported for doped VO2 crystals (e.g., Cr, Fe, Ga, and Al) by ∼10 °C at most.19,20 Also, because the measurements of MIT are carried out on free-standing microcrystals, our results are free from any strain effect that can be induced by substrate.42 Furthermore, hysteresis, the difference in phase transition onset temperature between heating and cooling processes, increased as w decreased, e.g., the hysteresis of the 3.6 μm thick crystal was as large as 43.9 °C. The MIT temperature and hysteresis width of each measured sample are shown in Table S1. To analyze the results shown in Figure 2, we applied the classical nucleation theory of phase transition.43 In general, heterogeneous nucleation, i.e., the onset of phase transition, occurs preferentially at sites such as grain boundaries, surfaces, or intrinsic and extrinsic defects. In the case of high-quality single microcrystals, the preferential sites of nucleation are the two end surfaces, and nucleation starts at either one or both ends of the surfaces. Because the effective surface energy at both end surfaces is believed to be lower than that on the long side surfaces, the nucleation probability is expected to depend

1

the values of log S and log|T − TC| were plotted, and a linear relationship was found, as expected (shown in Figure 2c,d). The average value (66.6 °C) of the phase transition temperature during heating (67.6 °C) and that during cooling (65.5 °C) for the largest crystal (w = 67 μm) was used as the C TC value. By fitting the data shown in Figure 2c,d, n and log D values during heating (cooling) of 1.6 ± 0.4 (1.9 ± 0.3) and −3.9 ± 0.4 (−3.9 ± 0.2), respectively, were obtained. The exponent (∼1.9) for the cooling side was practically the same as that (∼1.6) of the heating side within experimental error. These exponents may be essential for theoretical models of nucleating defect creation. Based on the above discussion, the MIT temperature of microcrystals during heating and cooling can be

(

calculated as T+ = TC(66.6 °C) + 1.7 × 102 °C

(

T− = TC(66.6 °C) − 8.5 × 101 °C

1 w (μm)

1 w (μm)

1.2

)

and

1.1

)

, where “+” and

“−” signs refer to heating and cooling cycles, respectively. Hysteresis can be expressed as 2282


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Figure 3. (a) Schematic of the patterned crystal sample circuit geometry with Au (1000 nm)/Cr(10 nm) contacts used for high-speed and temperature-dependent resistance measurements. (b) Temperature dependence of the resistance of VO2 crystals with widths of 1.9 μm. Inset shows an optical microscope image of the VO2 crystal used for the resistance measurements. (c) High-speed resistance measurements of VO2 crystal in b. Insets show schematics of the phase boundary motion during MIT. (d) Dependence of the phase boundary speed (v) on crystal width (w) during heating (empty red squares) and cooling (empty blue circles).

of the resistance curve in the temperature range of 50−60 °C in Figure 3b, the band gap Eg of VO2 was calculated to be 0.6−0.7 eV when VO2 was assumed to be an intrinsic semiconductor.46 These calculated band gaps are in good agreement with the previously reported value of 0.7 eV.47,48 Furthermore, the crystal structures before and after MIT onset, shown in Figure S5, were confirmed by synchrotron Laue microdiffraction to correspond to the M2 and R phases, respectively. Figure 3c displays high-speed resistance measurements of VO2 crystals with widths of 1.9 μm during MIT under heating. (See also Figure S6.) As seen in Figure 3c, resistance decreased at an almost constant rate during the MIT, which is in agreement with phase boundary motion. The phase transition time was defined as the time required for the phase boundary to travel from one contact to the other (= l/v), where l and v are the length between electrical contacts and phase boundary speed, respectively. Phase transition time dropped rapidly from ∼1 s to ∼3 ms as the width (w) of the microcrystals decreased from 40 to 1.9 μm. Figure 3d shows the dependence of the phase boundary speed during heating and cooling on crystal width. Although the speed of the MIT for large crystals (w = 40 μm) was 4.6 × 102 μm/s, the speed of the MIT for smaller crystals (w = 1.9 μm) was as high as 1.7 × 104 μm/s, which is almost 37 times faster than that of the large crystals. The width dependence of the phase boundary speed could be well-fitted as an inverse function of the width, v (μm/s) = a , with a =

ΔT = T+ − T − ⎛ 1 ⎞1.2 = 1.7 × 102 °C⎜ ⎟ ⎝ w (μm) ⎠ ⎛ 1 ⎞1.1 + 8.5 × 101 °C⎜ ⎟ ⎝ w (μm) ⎠

On the basis of our analyses, hysteresis is expected to increase with a decrease in the crystal width. T− may therefore be lower than room temperature when the width is below 1 μm. In this case, hysteresis is expected to be large at ∼100 °C. Large hysteresis is an important property of VO2-based optical switches, memory, and related fields because it is related to the ability to maintain a stable phase under a large external impulse. Because the size of the crystals appeared to be closely related to the phase transition temperature, we next investigated the effect of crystal size on the phase transition speed. A dc twocontact four-probe method are used for high-speed resistance measurement, shown in Figure 3a. Though the onset of MIT and its propagation along long (> ∼200 μm) samples can be observed under an optical microscope, the same measurements are difficult to perform in short samples with a length < ∼200 μm because of the speed of the MIT and the limited shutter speed of a CCD camera for a given sample size. Indeed, the CCD camera attached to the optical microscope was only able to take up to 60 frames per second or images with ∼17 ms between images. Figure 3b shows one of the measurements of the temperature dependence of the resistance of a VO2 crystal with a width of 1.9 μm. The inset in Figure 3b shows an optical image of the crystal with Au (1000 nm)/Cr (10 nm) electrical contacts for a width of 1.9 μm taken during the resistance measurements. The VO2 crystal showed a sharp phase transition with a rectangular hysteresis loop. The temperature dependencies of resistance curves according to width were measured (Figure S4 and Table S2). The MIT onset temperature based on high-speed resistance measurements during heating was in the range of ∼62 to ∼67 °C. On the basis

w (μm)

(3.3 ± 0.1) × 104 μm2/s (a = (3.8 ± 0.1) × 104 μm2/s) during heating (cooling). Assuming that the fitting results were valid for widths down to 0.01 μm, i.e., the limit of the linear plot in Figure 3d, then the speed during heating was expected to be 3.3 × 106 μm/s for that width. That is, an on/off-switch-based on phase transition of VO2 can be as fast as ∼10 ns for a 10 nm wide, 30 nm long crystal, and this ultrafast phase boundary speed can be used to regulate the switching time of future VO2 devices. It is important to understand why the transition speed showed a 1/w dependence in Figure 3d. There are several 2283


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Actually, microcrystals used in this experiment show MIT from insulating monoclinic M2 to metallic R phases, which is different than the well-known MIT between M1 and R. From previous papers, the size effect of M1 phase shows suppression of MIT to room temperature.26,27 But in this experiment, totally different behavior was observed; phase transition temperature increases as the microcrystal size decreases for heating. This implies that unknown properties associated with M2 phases without any defects can be related to new information on M2 in VO2.

parameters that may play a role in transition speed, such as Gibbs free energy, Joule heating, electric field strength, and thermal dissipation. First, let us consider the Gibbs free energy. Transition temperatures from high-speed resistance measurements were in the range of ∼62 to 67 °C (∼50 to 57 °C) during heating (cooling). If Gibbs free energy plays a major role, then the phase boundary speed is expected to increase as the Gibbs free energy increases with increasing |T − TC|. Thus, during heating, crystals with a higher phase transition temperature should have undergone a faster MIT. As shown in Table S2, crystals with w = 40 μm had a phase transition temperature of 66.3 °C, which is higher than either of the phase transition temperatures (∼62 °C) of smaller crystals (w = 2.5 and 4.1 μm). Thus, crystals with w = 40 μm were expected to have a much faster MIT speed than crystals with w = 2.5 and 4.1 μm. However, the crystals with w = 40 μm had the lowest MIT speed among those of the crystals described above. Because the transition speed did not show significant dependence on the magnitude of |T − TC|, we concluded that the effect of the driving force or the Gibbs free energy on MIT speed was small. Next, we examined the effect of Joule heating (current) and the magnitude of electric field on MIT speed. We measured the MIT speed with currents of 0.1 and 1 μA; thus, current and the magnitude of the electric field were varied simultaneously by a factor of 10 (Figure S8). Phase boundary speeds of 0.1 and 1 μA were found to have practically the same value of 1.5 × 104 μm/s. If either Joule heating or the magnitude of the electric field plays a critical role in the transition speed, then the transition speed should vary considerably because Joule heating and the magnitude of the electric field in the crystal differed significantly. However, the speeds in both cases were almost identical, which indicated that the dependence of MIT speed on the magnitude of the current or the magnitude of the electric field was small. Finally, we examined the possibility that the size dependence of the phase transition speed originated from size-dependent thermal dissipation. First, we assumed that the thermal time constant of the crystal was roughly the same as that of a uniformly heated square cylinder (section 12 in the Supporting Information). The thermal time constant, τ, can be viewed as the build-up time necessary for the onset of a heat flux after a temperature gradient is imposed on a given sample. The τ of uniformly heated square cylinder is given by τ = ρVc , where ρ,

4. CONCLUSIONS We investigated the control of the phase transition temperature and speed in VO2 microcrystals. With optical microscopy and high-speed electrical transport measurement, we show that the crystal width has a significant role in metal−insulator transition of VO2. As the width of microcrystals decreases, the phase transition temperature increases (decreases) for heating (cooling) leading to large hysteresis. Using a statistical description of a heterogeneous nucleation process, we show that the phase transition probability of VO2 depends on the end surface width. Also, the effective thermal exchange model was applied to explain the variation of phase transition speeds with different sample size. In this report, we demonstrate that the onset and speed of the MIT can be finely and effectively tuned by controlling microcrystal size, which will promote understanding of the intrinsic properties of VO2 and its application to advanced electronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11144. Schematics of heating stage and position of microcrystals during thermal triggering. Optical microscope images of phase boundary motion of VO2 crystal during MIT. MIT temperature and hysteresis width for microcrystals used in Figure 2b. Schematic diagrams of photolithography processes for temperature-dependent resistance measurement of VO2 microcrystal. Temperature-dependent resistance measurement of VO2 microcrystal with different widths. Micro Laue diffraction images. Highspeed resistance measurement for different microcrystals. Temperature-dependent resistance measurement showing training effect. Current and electric field dependence of the high-speed resistance measurement. Time constant for the cooling of a square cylinder. (PDF) Optical microscope video of phase boundary motion of VO2 crystal during MIT.(AVI)

κA

V, c, κ, and A, density, total volume, heat capacity of the unit mass of the cylinder, proportional constant and total bottom surface area, respectively (section 12 in Supporting Information). In the case of a square cylinder, A = lw and V = lw2. The ρwc time constant becomes τ = κ ∝ w , where ρ is the density of VO2. That is, the thermal constant (τ) is linearly proportional to the width (τ ∝ w), whose dependence originates from the increasing surface to volume ratio (∼(1/w) for square cylinders) with decreasing size. Therefore, smaller crystals have shorter time constants than larger crystals. In rough approximation, the time constant τ is the same as the phase transition time. Thus, the phase transition speed is roughly equal to (or proportional to) l/τ, and the speed depends on 1/ w, as observed. Because the thermal constant and transition speed showed the same dependence, we believe that these two parameters have the same physical origin from a thermal dissipation mechanism.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.J. thanks the Basic Science Research Program for support through grants from the National Research Foundation of Korea (NRF) funded by the Korean Government (MOE), NRF-2012R1A1A2006948. B.S.M. acknowledges support from 2284


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NRF-2015R1A5A1009962(C-AXS, SRC), NRF2015R1A2A2A01004084 and GIST College’s 2015 TBP Research Fund. H.J. thanks Yonsei University for financial support during sabbatical leave in 2015. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Science Division, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231 at Lawrence Berkeley National Laboratory.

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