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Article

Controlling the temperature and speed of the phase transition of VO microcrystals 2

Joonseok Yoon, Howon Kim, Xian Chen, Nobumichi Tamura, Bongjin Simon Mun, Changwoo Park, and Honglyoul Ju ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11144 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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ACS Applied Materials & Interfaces

Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals

Joonseok Yoon,† Howon Kim,† Xian Chen,‡ Nobumichi Tamura,¶ Bongjin Simon Mun,*,§,ǁ 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 ¶

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California

94720, USA §

Department of Physics and Photon Science, School of Physics and Chemistry,

Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea ǁ

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

*Corresponding authors: [email protected] (B. S. Mun); [email protected] (H. L. Ju)

KEYWORDS: vanadium dioxide, metal-insulator transition, phase transition temperature, phase transition speed, size effect, microcrystal, high-speed resistance 1

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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 cylindershaped 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 dependent of 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 development of innovative electronic devices.

1.

INTRODUCTION VO2 has received much attention because of its intriguing physical properties as

strongly correlated electron system and enormous potential for future technological applications.1-11 At just above room temperature (~67°C), VO2 undergoes a first order metalinsulator transition (MIT) with a dramatic change in electrical resistance of 4 to 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 2


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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 transistors.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 of these tremendous recent progresses, many challenging tasks still remain before VO2 can be utilized as a reliable material for commercial products. Especially, among the many critical tasks, control of the phase transition temperature and speed is one of the highest priority tasks, because the functionality of 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+, Nb+5, Mo6+. However, the doping with trivalent ions, e.g. Cr3+, Fe3+, Ga3+, 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 nano-particles in SiO2 matrices.25,28 These authors postulated that the 3



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defects in VO2 nano-particles acted as nucleation sites for the phase transition. On the other hand, in solid-state physics, the spatial confinement of ordinary materials on ultra-small 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 due to 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 systematic 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 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.

2.

EXPERIMENTAL DETAILS Free standing high quality VO2 microcrystals with a square cross section were 4


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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) (JSM-7001F, JEOL). Once the microcrystals were prepared, we employed optical microscopy to monitor the onset of the phase transition in these VO2 microcrystals.37 Due to 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 of VO2. Sample inspection and the method for control of the sample temperature are given in 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 (See 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 (see 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 highspeed resistance of VO2 crystals was measured using a dc two-contact four-probe method. High-speed resistance measurements with a data collection rate of 5,000 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.

3.

RESULTS AND DISCUSSION

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Figure 1. Scanning electron microscope image of square cylinder-shaped 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, just before (d) and after (e) the MIT.

For the present study of size dependence of MIT in VO2, high-quality square cylinder-shaped VO2 microcrystals were fabricated, as shown in Figure 1. Width range (w) of VO2 microcrystals was from ~1 to ~70 μm. A few selected SEM images of VO2 microcrystals are shown in Figures 1a-1c; 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 (also see Figure S2 and Video S1 in the Supporting Information).37 Figures 1d-1e show optical microscope 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 6


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optical images shown in Figures 1d-1e 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 micro-diffraction results of identical microcrystals revealed that the growth direction (length direction) of VO2 microcrystals is along the c-direction of the rutile phase (cR), [001].36,37

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 due to 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

1 end surface area ( ) versus the difference in the MIT temperature ( | T  TC | ) curve of VO2 S microcrystals after (c) heating and (d) cooling.

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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 m. Figure 2a shows schematics of the experimental set-up 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 stress-induced 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 mthick 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, Al) by ~10°C at most.19,20 Also, considering the measurements of MIT are carried out on free-standing microcrystals, our results are free from any strain effect which 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 m-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 8


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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 on the surface area of both ends, S (=2w2). If σ is the density of suitable heterogeneous nucleating sites per unit surface, 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. Then, the probability, F, that the surface contains at least one nucleation site is F  1  exp[  S ] .43-45 To consider the contribution of temperature to the probability, σ can be written as

Bg ex n , where B is a constant, n is an exponent, and g ex 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, σS becomes a constant. Note that g ex is directly proportional to |T TC |,  S  Bgex n S  C | T  TC |n S  D , where both C and D

are constants. Finally, by taking the logarithm of both sides of the equation, we get

log

1 C 1  n log | T  TC |  log . To test this derivation, the values of log and log | T TC | S D S

were plotted, and a linear relationship was found, as expected (shown in Figures 2c-2d). 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 TC value. By fitting the data shown in Figures 2c-2d, n and log

C values during heating (cooling) of D

1.6  0.4 ( 1.9  0.3 ) and 3.9  0.4 ( 3.9  0.2 ), respectively, were obtained. The 9



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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 1.2

 1  T  TC (66.6 C)  1.7 10 C    w(  m)  2

cooling

can

be

calculated

as 1.1

 1  and T  TC (66.6 C)  8.5 10 C    w(  m)  1

,

where “+” and “  ” signs refer to heating and cooling cycles, respectively. Hysteresis can be 1.2

1.1

 1   1  1 expressed as T  T  T  1.7 10 C    8.5 10 C   . Based on our  w(  m)   w(  m)  2

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 two contact 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 m) samples can be observed under an optical microscope, the same measurements are difficult to perform in short samples with a length < ~200 m due to 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

10


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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 square) and cooling (empty blue square). 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 m 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 in Supporting Information). The MIT onset temperature based on high-speed resistance measurements during heating was in the range of ~62 to ~67°C. Based on the resistance curve in the 11



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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 (also see Figure S6 in the Supporting Information). 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 ), where l and v 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 m. Figure 3d shows the dependence of the phase boundary speed during heating and cooling on crystal width. While 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  (3.3  0.1) 104 μm2 s ( a  (3.8  0.1) 104 μm 2 s ) w(μm)

during heating (cooling). Assuming the fitting results were valid for widths down to 0.01 μm, 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 12


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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 dependence in w

Figure 3d. There are several 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, 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 peed among those of the crystals described above. Since 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 in Supporting Information]. 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, the transition speed should vary considerably, as 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 13



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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 Supporting Information]. 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  

A , Vc

where A, ρ, V, and c are total bottom surface area, density, total volume, and heat capacity of the unit mass of the cylinder, respectively [Section 12 in Supporting Information]. In the case of a square cylinder, A = lw and V = lw2. The time constant becomes  

 wc  w , where ρ 

is the density of VO2. That is, the thermal constant (τ) is linearly proportional to the width (   w ), whose dependence originates from the increasing surface to volume ratio ( ~

1 for w

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)

and the speed depends on

l



,

1 , as observed. Because the thermal constant and transition speed w

showed the same dependence, we believe that these two parameters have the same physical origin from a thermal dissipation mechanism. Actually, microcrystals used in this experiment shows MIT from insulating monoclinic M2 to metallic R phases, which is different to well-known MIT between M1 and 14


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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 implicates that unknown properties associated to M2 phases without any defects can be related to new information on M2 in VO2.

4.

CONCLUSION

In summary, 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 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 to understand the intrinsic properties of VO2 and its application to advanced electronic devices.

ASSOCIATED CONTENT Supporting Information

Schematics of heating stage and position of microcrystals during thermal triggering (Figure S1). Optical microscope images and video of phase boundary motion of VO2 crystal during MIT (Figure S2 and Video S1). MIT temperature and hysteresis width for microcrystals used in Figure 2b (Table S1). Schematic diagrams of photolithography processes for temperature15



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dependent resistance measurement of VO2 microcrystal (Figure S3). Temperature-dependent resistance measurement of VO2 microcrystal with different widths (Figure S4 and Table S2). Micro Laue diffraction images (Figure S5). High speed resistance measurement for different microcrystals (Figure S6). Temperature-dependent resistance measurement showing training effect (Figure S7). Current and electric field dependence of the high speed resistance measurement (Figure S8). Time constant for the cooling of a square cylinder. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (B. S. Mun) *E-mail: [email protected] (H. L. Ju) Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS

H. L. Ju and B. S. Mun would like to thank the Basic Science Research Program for support through grants from the National Research Foundation of Korea (NRF) funded by the Korean Government (MOE) (2012R1A1A2006948 & 2012R1A1A2001745). B. S. Mun would like to acknowledge support from SRC (C-AXS, NRF-2015R1A5A1009962) and GIST College's 2015 TBP Research Fund. H. L. Ju thanks Yonsei University for financial support during sabbatical leave in 2015. The Advanced Light Source is supported by the Director, Office of 16


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Science, Office of Basic Energy Sciences, Materials Science Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 at Lawrence Berkeley National Laboratory.

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REFERENCES

(1) Yang, Z.; Ko, C.; Ramanathan, S. Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions. Annu. Rev. Mater. Res. 2011, 41, 337-367. (2) Morin, F. J. Oxides which Show a Metal-to-Insulator Transition at the Neel Temperature. Phys. Rev. Lett. 1959, 3, 34-36. (3) Kim, D. H.; Kwok, H. S. Pulsed Laser Deposition of VO2 Thin Films. Appl. Phys. Lett. 1994, 65, 3188-3190.

(4) Arcangeletti, E.; Baldassarre, L.; Di Castro, D.; Lupi, S.; Malavasi, L.; Marini, C.; Perucchi, A.; Postorino, P. Evidence of a Pressure-Induced Metallization Process in Monoclinic VO2. Phys. Rev. Lett. 2007, 98, 196406. (5) Cavalleri, A.; Toth, C.; Siders, C. W.; Squier, J. A.; Raksi, F.; Forget, P.; Kieffer, J. C. Femtosecond Structural Dynamics in VO2 during an Ultrafast Solid-Solid Phase Transition. Phys. Rev. Lett. 2001, 87, 237401. (6) Kübler, C.; Ehrke, H.; Huber, R.; Lopez, R.; Halabica, A.; Haglund, R. F.; Leitenstorfer, A. Coherent Structural Dynamics and Electronic Correlations during an Ultrafast Insulator-toMetal Phase Transition in VO2. Phys. Rev. Lett. 2007, 99, 116401. (7) Kim, H.-T.; Chae, B.-G.; Youn, D.-H.; Maeng, S.-L.; Kim, G.; Kang, K.-Y.; Lim, Y.-S. Mechanism and Observation of Mott Transition in VO2-Based Two- and Three-Terminal Devices. New J. Phys. 2004, 6, 52. (8) Tselev, A.; Budai, J. D.; Strelcov, E.; Tischler, J. Z.; Kolmakov, A.; Kalinin, S. V. Electromechanical Actuation and Current-Induced Metastable States in Suspended SingleCrystalline VO2 Nanoplatelets. Nano Lett. 2011, 11, 3065-3073. (9) Stefanovich, G.; Pergament, A.; Stefanovich, D. Electrical Switching and Mott Transition in VO2. J. Phys.: Condens. Matter 2000, 12, 8837-8845. (10) Strelcov, E.; Lilach, Y.; Kolmakov, A. Gas Sensor Based on Metal-Insulator Transition in 18


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


VO2 Nanowire Thermistor. Nano Lett. 2009, 9, 2322-2326. (11) Gao, Y.; Luo, H.; Zhang, Z.; Kang, L.; Chen, Z.; Du, J.; Kanehira, M.; Cao, C. Nanoceramic VO2 Thermochromic Smart Glass: A Review on Progress in Solution Processing. Nano Energy 2012, 1, 221-246. (12) Lee, M. J.; Park, Y.; Suh, D. S.; Lee, E. H.; Seo, S.; Kim, D. C.; Jung, R.; Kang, B. S.; Ahn, S. E.; Lee, C. B.; Seo, D. H.; Cha, Y. K.; Yoo, I. K.; Kim, J. S.; Park, B. H. Two Series Oxide Resistors Applicable to High Speed and High Density Nonvolatile Memory. Adv. Mater. 2007, 19, 3919-3923. (13) Liu, M.; Hwang, H. Y.; Tao, H.; Strikwerda, A. C.; Fan, K.; Keiser, G. R.; Sternbach, A. J.; West, K. G.; Kittiwatanakul, S.; Lu, J.; Wolf, S. A.; Omenetto, F. G.; Zhang, X.; Nelson, K. A.; Averitt, R. D. Terahertz-Field-Induced Insulator-to-Metal Transition in Vanadium Dioxide Metamaterial. Nature 2012, 487, 345-348. (14) Hormoz, S.; Ramanathan, S. Limits on Vanadium Oxide Mott Metal-Insulator Transition Field-Effect Transistors. Solid-State Electron. 2010, 54, 654-659. (15) Park, J. H.; Coy, J. M.; Kasirga, T. S.; Huang, C.; Fei, Z.; Hunter, S.; Cobden, D. H. Measurement of a solid-state triple point at the metal-insulator transition in VO2. Nature 2013, 500, 431-434. (16) Budai, J. D.; Hong, J.; Manley, M. E.; Specht, E. D.; Li, C. W.; Tischler, J. Z.; Abernathy, D. L.; Said, A. H.; Leu, B. M.; Boatner, L. A.; McQueeney, R. J.; Delaire, O. Metallization of vanadium dioxide driven by large phonon entropy. Nature 2014, 515, 535-539. (17) Jeong, J.; Aetukuri, N.; Graf, T.; Schladt, T. D.; Samant, M. G.; Parkin, S. S. P. Suppression of Metal-Insulator Transition in VO2 by Electric Field–Induced Oxygen Vacancy Formation. Science 2013, 339, 1402-1405. (18) Martens, K.; Jeong, J. W.; Aetukuri, N.; Rettner, C.; Shukla, N.; Freeman, E.; Esfahani, D. N.; Peeters, F. M.; Topuria, T.; Rice, P. M.; Volodin, A.; Douhard, B.; Vandervorst, W.; 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Samant, M. G.; Datta, S.; Parkin, S. S. P. Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO2. Phys. Rev. Lett. 2015, 115, 196401. (19) Zhang, S.; Kim, I. S.; Lauhon, L. J. Stoichiometry Engineering of Monoclinic to Rutile Phase Transition in Suspended Single Crystalline Vanadium Dioxide Nanobeams. Nano Lett. 2011, 11, 1443-1447.

(20) Lee, S.; Cheng, C.; Guo, H.; Hippalgaonkar, K.; Wang, K.; Suh, J.; Liu, K.; Wu, J. Axially Engineered Metal–Insulator Phase Transition by Graded Doping VO2 Nanowires. J. Am. Chem. Soc. 2013, 135, 4850-4855. (21) Wei, J.; Wang, Z. H.; Chen, W.; Cobden, D. H. New Aspects of the Metal-Insulator Transition in Single-Domain Vanadium Dioxide Nanobeams. Nat. Nanotechnol. 2009, 4, 420424. (22) Cao, J.; Ertekin, E.; Srinivasan, V.; Fan, W.; Huang, S.; Zheng, H.; Yim, J. W. L.; Khanal, D. R.; Ogletree, D. F.; Grossmanan, J. C.; Wu, J. Strain Engineering and One-Dimensional Organization of Metal-Insulator Domains in Single-Crystal Vanadium Dioxide Beams. Nat. Nanotechnol. 2009, 4, 732-737. (23) Driscoll, T.; Kim, H.-T.; Chae, B.-G.; Kim, B.-J.; Lee, Y.-W.; Jokerst, N. M.; Palit, S.; Smith, D. R.; Di Ventra, M.; Basov, D. N. Memory Metamaterials. Science 2009, 325, 15181521. (24) Bae, S.-H.; Lee, S.; Koo, H.; Lin, L.; Jo, B. H.; Park, C.; Wang, Z. L. The Memristive Properties of a Single VO2 Nanowire with Switching Controlled by Self-Heating. Adv. Mater. 2013, 25, 5098-5103.

(25) Lopez, R.; Feldman, L. C.; Haglund, R. F. Size-Dependent Optical Properties of VO2 Nanoparticle Arrays. Phys. Rev. Lett. 2004, 93, 177403. (26) Whittaker, L.; Jaye, C.; Fu, Z.; Fischer, D. A.; Banerjee, S. Depressed Phase Transition in Solution-Grown VO2 Nanostructures. J. Am. Chem. Soc. 2009, 131, 8884-8894. 20


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Hongwei, L.; Junpeng, L.; Minrui, Z.; Hai, T. S.; Haur, S. C.; Xinhai, Z.; Lin, K. Size Effects on Metal-Insulator Phase Transition in Individual Vanadium Dioxide Nanowires. Opt. Express 2014, 22, 30748-30755. (28) Lopez, R.; Haynes, T. E.; Boatner, L. A.; Feldman, L. C.; Haglund, R. F. Size Effects in the Structural Phase Transition of VO2 Nanoparticles. Phys. Rev. B 2002, 65, 224113. (29) Hu, Y.; Churchill, H. O. H.; Reilly, D. J.; Xiang, J.; Lieber, C. M.; Marcus, C. M. A Ge/Si Heterostructure Nanowire-Based Double Quantum Dot with Integrated Charge Sensor. Nat. Nanotechnol. 2007, 2, 622-625. (30) Wolkin, M. V.; Jorne, J.; Fauchet, P. M.; Allan, G.; Delerue, C. Electronic States and Luminescence in Porous Silicon Quantum Dots: The Role of Oxygen. Phys. Rev. Lett. 1999, 82, 197-200. (31) Kagan, C. R.; Murray, C. B.; Nirmal, M.; Bawendi, M. G. Electronic Energy Transfer in CdSe Quantum Dot Solids. Phys. Rev. Lett. 1996, 76, 1517-1520. (32) Bažant, Z. P.; Le, J.-L.; Bazant, M. Z. Scaling of Strength and Lifetime Probability Distributions of Quasibrittle Structures Based on Atomistic Fracture Mechanics. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 11484-11489. (33) Jun, Y.-W.; Huh, Y.-M.; Choi, J.-s.; Lee, J.-H.; Song, H.-T.; Kim, S.; Yoon, S.; Kim, K.S.; Shin, J.-S.; Suh, J.-S.; Cheon, J. Nanoscale Size Effect of Magnetic Nanocrystals and Their Utilization for Cancer Diagnosis via Magnetic Resonance Imaging. J. Am. Chem. Soc. 2005, 127, 5732-5733.

(34) Ishikawa, K.; Yoshikawa, K.; Okada, N. Size Effect on the Ferroelectric Phase Transition in PbTiO3 Ultrafine Particles. Phys. Rev. B 1988, 37, 5852-5855. (35) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043-1053. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(36) Mun, B. S.; Chen, K.; Leem, Y.; Dejoie, C.; Tamura, N.; Kunz, M.; Liu, Z.; Grass, M. E.; Park, C.; Yoon, J.; Lee, Y. Y.; Ju, H. Observation of Insulating–Insulating Monoclinic Structural Transition in Macro-Sized VO2 Single Crystals. Phys. Status Solidi RRL 2011, 5, 107-109. (37) Mun, B. S.; Chen, K.; Yoon, J.; Dejoie, C.; Tamura, N.; Kunz, M.; Liu, Z.; Grass, M. E.; Mo, S.-K.; Park, C.; Lee, Y. Y.; Ju, H. Nonpercolative Metal-Insulator Transition in VO2 Single Crystals. Phys. Rev. B 2011, 84, 113109. (38) Qazilbash, M. M.; Brehm, M.; Chae, B. G.; Ho, P. C.; Andreev, G. O.; Kim, B. J.; Yun, S. J.; Balatsky, A. V.; Maple, M. B.; Keilmann, F.; Kim, H. T.; Basov, D. N. Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging. Science 2007, 318, 1750-1753. (39) Wu, J. Q.; Gu, Q.; Guiton, B. S.; de Leon, N. P.; Lian, O. Y.; Park, H. Strain-induced self organization of metal-insulator domains in single-crystalline VO2 nanobeams. Nano Lett. 2006, 6, 2313-2317.

(40) Cao, J.; Gu, Y.; Fan, W.; Chen, L. Q.; Ogletree, D. F.; Chen, K.; Tamura, N.; Kunz, M.; Barrett, C.; Seidel, J.; Wu, J. Extended Mapping and Exploration of the Vanadium Dioxide Stress-Temperature Phase Diagram. Nano Lett. 2010, 10, 2667-2673. (41) Jones, A. C.; Berweger, S.; Wei, J.; Cobden, D.; Raschke, M. B. Nano-optical Investigations of the Metal-Insulator Phase Behavior of Individual VO2 Microcrystals. Nano Lett. 2010, 10, 1574-1581. (42) O’Callahan, B. T.; Jones, A. C.; Park, J. H.; Cobden, D. H.; Atkin, J. M.; Raschke, M. B. Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2. Nature commun. 2015, 6, 6849. (43) Porter, D. A.; Easterling, K. E.; Sherif, M. Phase Transformation in Metals and Alloys. 3rd ed; CRC Press: 2009. (44) Freund, J. E.; Walpole, R. E. Mathematical Statistics, 3rd ed; Prentice-Hall, Englewoods 22


Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cliffs: NJ, 1987. (45) Chen, I.-W.; Chiao, Y.-H.; Tsuzaki, K. Statistics of Martensitic Nucleation. Acta Metall. 1985, 33, 1847-1859.

(46) Blackmore, J. S. Semiconductor Statistics, Pergamon Press, Oxford, 1962. (47) Brassard, D.; Fourmaux, S.; Jean-Jacques, M.; Kieffer, J. C.; El Khakani, M. A. Grain Size Effect on the Semiconductor-Metal Phase Transition Characteristics of MagnetronSputtered VO2 Thin Films. Appl. Phys. Lett. 2005, 87, 051910. (48) Liu, W.-T.; Cao, J.; Fan, W.; Hao, Z.; Martin, M. C.; Shen, Y. R.; Wu, J.; Wang, F. Intrinsic Optical Properties of Vanadium Dioxide near the Insulator−Metal Transition. Nano Lett. 2011, 11, 466-470.

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TABLE OF CONTENTS GRAPHIC AND SYNOPSIS

The control of the temperature and speed of phase transition of VO2 was investigated by varying the crystal size. By decreasing the width of square cylinder-shaped microcrystals from ~70 to ~1 μm, the phase transition temperature varied as much as 26.1°C (19.7°C) during heating (cooling), while the phase transition speed increased from 4.6 × 102 to 1.7 × 104 μm/s.

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Controlling the Temperature and Speed of the ... - ACS Publications

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