Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in

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Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in Doped VO2 Nanowires Hasti Asayesh-Ardakani,†,‡ Anmin Nie,†,‡ Peter M. Marley,§ Yihan Zhu,∥ Patrick J. Phillips,‡ Sujay Singh,⊥ Farzad Mashayek,∇ Ganapathy Sambandamurthy,⊥ Ke-bin Low,¶ Robert F. Klie,‡ Sarbajit Banerjee,§ Gregory M. Odegard,* and Reza Shahbazian-Yassar*,†,‡,∇

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†

Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan 49933-1295, United States ‡ Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607-7059, United States § Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14260-3000, United States ∥ Advanced Membranes and Porous Materials Center, King Abdullah University of Science & Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ⊥ Department of Physics, University at Buffalo, State University of New York, Buffalo, New York 14260-3000, United States ∇ Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607-7059, United States ¶ Research Resource Center, University of Illinois at Chicago, Chicago, Illinois 60607-7059, United States S Supporting Information *

ABSTRACT: There has been long-standing interest in tuning the metal−insulator phase transition in vanadium dioxide (VO2) via the addition of chemical dopants. However, the underlying mechanisms by which doping elements regulate the phase transition in VO2 are poorly understood. Taking advantage of aberration-corrected scanning transmission electron microscopy, we reveal the atomistic origins by which tungsten (W) dopants influence the phase transition in single crystalline WxV1−xO2 nanowires. Our atomically resolved strain maps clearly show the localized strain normal to the (122)̅ lattice planes of the low W-doped monoclinic structure (insulator). These strain maps demonstrate how anisotropic localized stress created by dopants in the monoclinic structure accelerates the phase transition and lead to relaxation of structure in tetragonal form. In contrast, the strain distribution in the high W-doped VO2 structure is relatively uniform as a result of transition to tetragonal (metallic) phase. The directional strain gradients are furthermore corroborated by density functional theory calculations that show the energetic consequences of distortions to the local structure. These findings pave the roadmap for lattice-stress engineering of the MIT behavior in strongly correlated materials for specific applications such as ultrafast electronic switches and electro-optical sensors. KEYWORDS: VO2, nanowires, doping, phase transition, scanning transmission electron microscopy, strain mapping

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usage of VO2 in many applications such as thermo/electrochromics,11−13 Mott transistors,14,15 memristors,16 thermal actuators,4,17 gas sensors,18 strain sensors,19 and temperature sensors.20 On the other hand, it is necessary to adjust the narrow temperature window around the transition temperature (TMIT) of VO2 to suit different needs. Doping of different transition metals into the VO2 structure can modify TMIT, preferentially stabilize the R phase, or alter the phase diagram to favor stabilization of the metastable M2 phase. Generally, dopants

anadium dioxide (VO2), an iconic example of a correlated electron material, has received significant attention as a result of its abruptly discontinuous metal−insulator transition (MIT) at ∼340 K, close to room temperature.1,2 The MIT in VO2 is associated with a structural phase transition from a monoclinic (M1), insulating phase, to a tetragonal rutile (R), metallic phase. The M1 phase is the stable and thermodynamically favored phase for undoped and strain free VO2 samples at room temperature. However, additional factors such as doping and sample/substrate strain can result in stabilization of an insulating, metastable monoclinic phase (M2).2−8 The MIT in VO2 can be observed by noticeable resistivity and optical transparency changes spanning several orders of magnitude.2,5,9,10 These unique properties have inspired the proposed © 2015 American Chemical Society

Received: February 24, 2015 Revised: September 29, 2015 Published: October 12, 2015 7179

DOI: 10.1021/acs.nanolett.5b03219 Nano Lett. 2015, 15, 7179−7188

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Figure 1. STEM analysis on single WxV1−xO2 (x = 0.2 atom %) nanowire. (a) Atomic-resolution HAADF image and (b) FFT of (a). (a−b) indicate that (a) is acquired along the [4̅11̅] zone axis of monoclinic structure. (c) Solid sphere model of the monoclinic structure corresponding to [4̅11̅] zone axies and (d) simulated diffraction pattern based on model (c), both confirm the crystal structure orientation of (a). (e−f) Intensity profiles from Area 1 and Area 2 revealing the atomic columns with higher intensity compared to the rest of columns. (g−i) Simulated STEM-HAADF images corresponding to different depth position of W atoms (ZW) and intensity profiles from the red dotted box. The high intensity columns verify the existence of W in these columns.

with a higher oxidation state such as W6+, Mo5+, and Nb5+ are found to decrease TMIT,21−27 whereas Cr3+, Al3+, Fe3+, and Ga3+ characterized by a lower oxidation state increase the TMIT.28−34 The most effective dopant among these transition metals in terms of enabling a wide range of TMIT is tungsten (W), which reduces TMIT for nanowires with a nearly linear rate of (45−56) K/atom % W.24 This enables precise control over TMIT over an expansive temperature range. Much research thus far has been focused on analysis of MIT and mapping the structural phase progression using X-ray diffraction,34−40 X-ray absorption spectroscopy,34−36,41−44 Raman spectroscopy,36,40,45,46 atomic force microscopy,47−50 and optical microscopy3,7,39,49,51 techniques. However, these methods lack the spatial resolution needed to identify and monitor the changes in local atomic structure during the MIT. Transmission electron microscopy (TEM) has the ability to

achieve resolutions better than 1 Å and, thus, is an ideal technique for such studies at atomic resolution. Sohn et al.37 studied the growth characteristics and phase transition of VO2 using electron diffraction analysis at low and high temperature by in situ heating in a TEM setup. The transition from monoclinic to tetragonal structure initiated by a near-infrared excitation reported by using four-dimensional (4D) femtosecond electron diffraction.52 Electron diffraction was also used to demonstrate inhomogeneous doping of VO2 nanowires with W. Nanowires prepared by their chemical vapor transport method were more extensively doped at the tip and less so in the middle, which gave rise to distinctive selected area electron diffraction (SAED) patterns in these two regions for the same nanowire.34 In addition, TEM diffraction analysis was used to confirm the difference between transition temperatures in heating and cooling cycles of nonporous/nanotubular VO2. 7180

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Figure 2. STEM analysis on single WxV1−xO2 (x = 0.8 atom % W) nanowire. (a) HAADF atomic-resolution image and (b) depicts the FFT of (a). (c) Solid sphere model of the tetragonal crystal structure along [011] zone axis, which matches the experimentally obtained HAADF image. (d) Simulated diffraction pattern constructed from (c). (e−f) The line intensity profiles from Area 3 and Area 4. (g−i) Simulated STEM-HAADF images corresponding to different depth position of W atoms (ZW) along the [011] zone axis of tetragonal and intensity profiles from the red dotted box.

The phase transitions were observed at 70 and 45 °C for heating and cooling cycles, respectively.53 Cheng et al.54 reported high-resolution transmission electron microscopy imaging of a phase transition in a VO2 rod, activated in situ by a high intensity electron beam. Although recent TEM studies have revealed some aspects of structural details of the phase transition, the atomic scale mechanisms of the phase transition in doped VO2 remains unclear. In order to exploit the celebrated electronic switching property of VO2, the ability to control domain structures and phase transitions is of critical significance. In this work, we present a comprehensive aberration corrected scanning transition electron microscopy (STEM) investigation of the phase transition in individual single-crystalline WxV1−xO2 nanowires with different doping concentration of W (0.2, 0.5, and 0.8 atom %). The transition temperatures are expected to be around 330, 314, and 295 K for samples with 0.2, 0.5, and 0.8 atom % W, respectively, as reported by Wu et al.24 Initially,

W atoms were detected in the monoclinic and tetragonal structures by atomic resolution Z-contrast imaging. Subsequently, atomic resolution strain analysis revealed that strain distribution to be quite distinct between the two crystal structures. Interestingly, the anisotropic strain normal to the (122̅) lattice planes of monoclinic is observed as main component to accommodate the phase transition. In addition, the electron energy loss spectroscopy (EELS) was performed around W dopants to demonstrate the local lattice conformation under the influence of tendency for charge neutralization. In this study, nanowires were chosen with different amount of W (x = 0.2, 0.5, and 0.8 atom %). Hereafter, the nanowires with 0.2 and 0.5 atom % W are referred as low-doped and 0.8 atom % W is referred as high-doped nanowires, respectively. Room temperature electrical characterization of the low-doped nanowires inside TEM (Supporting Information (SI) Figure S1.a−b) indicated that monoclinic structure is in insulator state 7181

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other spots at the specific areas are 1.94 and 1.76, respectively. To verify the above observations, the Kirkland multislice electron-scattering simulation59 was used (for more details see SI Note S3). The simulations were conducted 25 nm [4̅11̅] supercell by replacing V with W atoms on the top, middle, and bottom levels (Figure 1g−i). One can conclude that the intensity variations in HAADF images are sufficient for identification of W atoms depending on the position of W along the atomic columns. Subsequently, atomic-resolution HAADF imaging and EELS spectrum analysis were carried out for the high W-doped nanowires. The thickness of imaging area in the 0.8 atom % W nanowire was measured to be 28 nm, respectively (SI Note S2). The HAADF image is shown in SI Figure S5, where the image intensity variation is mainly due to the sample thickness difference. A trend subtract filter was applied to remove the background noise from the original HAADF.60 The trend subtraction filter improves the intensity of images but does not affect the image intensity caused by the W dopants as it is obvious by comparing the raw and processed images (Figure 2a and SI Figure S5.a−b). The atomic-resolution HAADF image of nanowire with 0.8 atom % W (Figure 2a) and FFT image of HAADF (Figure 2b) are concordant with the [011] zone axis of the tetragonal structure. The excellent overlap of the tetragonal solid sphere model acquired along the [011] zone axis (Figure 2c) with the experimental image and matching of the FFT with the simulated diffraction pattern (Figure 2d) further corroborates the tetragonal structure. Analogous to the previously discussed HAADF images, some spots are observed with higher intensity, and they correspond to the locations of incorporated W atoms in the structure (some of the spots indicated by arrows in Figure 2a). The brighter spots in the high-doped nanowires are much more abundant than low-doped nanowires as illustrated using arrows. The line intensity profiles of Area 3 and Area 4 are shown in Figure 2e and f, respectively. The intensity ratio of atomic column including W atoms over the averaged intensity of other atomic column in areas 3 and 4 were 1.69 and 1.62, respectively. The simulation results for 28 nm [011] supercell and line profiles from the simulated STEM images are displayed in Figure 2g−i. The simulated images confirm the detection of the W in different depth of atomic column. As previously mentioned, high image intensity variation is expected in HAADF images due to the existence of W atoms in the matrix of VO2. However, electron probe dechanneling should be considered particularly in high symmetry zone axis orientation of crystalline materials. In this phenomenon, the electron probe and the intensity scattered by atoms are focused and drawn onto arranged atomic columns and that can alter the intensity of the image.61 The dechanneling effect could vary depends on material, orientation, and thickness of the sample.57,62,63 The results of Kirkland multislice electronscattering simulation can also be used to study the electron dechanneling effect. Figure 1g−i and Figure 2g−i corresponding to 0.2 and 0.8 atom % W nanowires, respectively, show that W atoms can be detected at the expected position after 25 and 28 nm and the intensity change of atoms around W atoms are minimal. The comparison between different setting locations of W atoms along the atomic column shows that the intensity variation is low in the thickness ranges used to obtain the experimental HAADF images.

up to 9 V (SI Figure S1.c) and high-doped nanowires exhibited Ohmic behavior (SI Figure S1.d). When the monoclinic structure of the low-doped nanowire and tetragonal structure of the high-doped nanowire at room temperature (∼300 K) is considered, increasing the doping concentration of W from 0.2 to 0.8 atom % W results in phase transition from monoclinic to tetragonal phase at room temperature. Dispersions of V, O, and W elements were investigated in WxV1−xO2 nanowires by energy-dispersive X-ray spectroscopy (EDS). SI Figure S2.a−c displays the selected area of a nanowire that was used for EDS mapping, elemental distributions of V, O, and W, and the overlaid distribution of each element. EDS mapping reveals that dispersion of W in the nanowires is relatively homogeneous. Due to low concentration of W in the 0.2 and 0.5 atom % W nanowires, the EDS signals from W atoms are very weak. As reasonably expected, EDS mapping does not sufficiently differentiate the atomic-scale perturbations induced by doping. As such, further characterization was carried out by analysis of atomic-resolution high angle annular dark field (HAADF) images. HAADF imaging is also known as Z (atomic number) contrast imaging, where the contrast is related to the averaged Z1.7 of a given atomic column.55,56 Therefore, according to the theoretical calculation, with the substitution of W with V atoms in the crystal structure, a distinct variation of intensity is expected to be seen due to the fact that W atoms (Z = 74) should scatter electrons approximately seven times more than V atoms (Z = 23). However, electron probe dechanneling and the variation in W atom position within the atomic columns may overshadow the theoretically calculated intensity difference in the HAADF images of W−VO2 matrix.57,58 The thickness of 0.2 and 0.5 atom % W nanowires were estimated to be 25 and 34 nm, respectively, based on the lowloss EELS from the imaging area (see SI Note S2 for more information). Atomic-resolution images of 0.2 atom % W nanowire are shown along two different zone axes in Figure 1a and SI Figure S3.a. The atomic-resolution HAADF image (Figure 1a) and fast Fourier transform (FFT) of the HAADF image (Figure 1b) at the first zone axis represent the [4̅11̅] zone axis of the VO2 monoclinic structure. This was confirmed by the solid sphere model of the monoclinic structure along the [4̅11̅] zone axis (Figure 1c, which is partially overlapped on the HAADF image) and the simulated diffraction pattern (Figure 1d) constructed based on the model. SI Figure S3.a depicts the atomic-resolution HAADF image acquired along the second zone axis. The solid sphere model along the [001] zone axis of the monoclinic structure (SI Figure S3.c) is matched with the HAADF atomic-resolution image (SI Figure S3.a). In addition, the FFT of HAADF (SI Figure S3.b) and simulated diffraction pattern (SI Figure S3.d) are validated by the atomic-resolution observation. The same analysis were performed on the atomicresolution HAADF images of the nanowire with 0.5 atom % W (SI Figure S4). The results confirm the monoclinic structure for 0.5 atom % W nanowire along the [4̅11̅] zone axis. The high-resolution HAADF images (Figure 1a, SI Figures S3.a and S4) acquired along the [001] and [4̅11̅] clearly show the V atoms as bright spots. Despite the relatively low-dopant concentration of 0.2 and 0.5 atom % W, the intensity of selected spots stand out in marked contrast to the remaining spots as delineated in Figure 1a and SI Figure S3.a and S4. Quantitative image contrast analyses have been performed with line profiles of intensity (Figure 1e−f) in Area 1 and Area 2 (from four spots before and after the high-contrast atoms). The intensity ratios of the brighter spot over the average intensity of 7182

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Figure 3. Strain maps calculated by Peak Pairs Algorithm corresponding of HAADF images of 0.2 and 0.8 atom % W nanowires. (a) and (b) Strain maps in perpendicular direction to (122̅) and along to (122̅) lattice plane for monoclinic structure (0.2 atom % W), respectively. (c) and (d) Strain maps in normal direction to (01̅1) and (200) lattice plane of tetragonal structure (0.8% at.% W), respectively. The selected areas in (a) and (b) numbered as 1−6 show the positions of W atoms in HAADF image (Figure 1a). This directly displays the relation between the localized distortions to the position of W atoms in the monoclinic structure. In contrast, the strain maps of the tetragonal structure (0.8% atom % W) are almost uniform and do not present any localized distortions.

More quantitative analysis of the atomic-resolution images reveals additional important details such as the presence of localized strain and lattice distortions. This quantitative analysis was performed using peak pairs analysis (PPA).64,65 In brief, PPA is a real space method that estimates the strain by investigating the position of intensity maxima in the image and then comparing them to predicted values (see SI Note S4 for more details). For the first set of analyses, the atomic-resolution images of low W-doped nanowires (Figure 1a and SI Figure S4) were analyzed by peak pairs algorithm and the resulting strain maps are shown in Figure 3a−b and SI Figure S6.a−b. The black area around the strain maps corresponds to the edge of the imaging area and is not used for our strain analysis. Figure 3a displays the strain map perpendicular to the (122̅) lattice plane of monoclinic structure, and Figure 3b shows the strain map along the (122̅) lattice plane. Figure 3a shows more localized strain than Figure 3b. By locating the position of W atoms in the strain maps, it is obvious that W dopants induce a greater localized strain normal to the (122̅) lattice plane than along it. The same analysis was performed on the 0.5 atom % W nanowire as it can be seen in the SI Figure S.6a−b. The

results show that as the number of W dopants increases, the induced-localized strains start to merge together and the strain maps capture the collective behavior rather than local variations. Next, the same analyses were performed on the atomic-resolution images of high-doped nanowires Figure 2a. The strain maps in normal directions of (011̅) and (200) lattice planes are shown in Figure 3c and d, respectively. The strain maps in high W-doped nanowires (tetragonal structure) do not represent any localized strain and they are mostly uniform. To verify this contention, density functional theory (DFT) simulations were conducted for VO 2 monoclinic, VO 2 tetragonal, WxV1−xO2 monoclinic, and WxV1−xO2 tetragonal systems x = 1.0 atom % W. Further details of these simulations are described in the SI Note S5. The absolute value of the maximum shear stresses in each of the four systems was calculated to determine the relative amount of internal distortional stress. Figure 4 shows the maximum shear stresses in each system normalized with respect to the VO2 monoclinic system. The data in the figure indicates that the addition of W dopants induces a greater internal distortional stress in the lattice, with a 34% increase in the monoclinic system and a 7183

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In this equation E is the electronic energy of the material in consideration; EV, EO, and EW are the electronic energies of pure V, O, and W materials, respectively; and x, y, and z are the numbers of V, O, and W atoms in the simulation unit cell, respectively. The values of electronic energies, P, and V were determined from the DFT simulations. The phase transformation Gibbs free energy results are ΔGVO2 = −155.5 eV and ΔGWVO2 = −761.2 eV. The negative values of these quantities indicate the drive of these materials to transform to the tetragonal phase. The more negative value for the WxV1−xO2 material indicates that the presence of W dopants creates a dramatically stronger thermodynamic drive to transform to the tetragonal phase. The origins of strain induced by the addition of W dopants can be attributed to the physical mismatch between W and V atoms and also the tendency for charge neutrality due to electronic mismatch between W and V atoms. The lattice distortion due to radial mismatch between W and V atoms has been observed as changes and variations in local distances of W and V bonds by X-ray absorption fine structure.35,41,67,68 To investigate the change of V valence because of W addition in VO2 system, EELS was also used in this study. The V oxidation state can be quantified based on the VL3, VL2 and OK fingerprint edges of EELS.69 Here, ΔE (L3-OK) was used to identify the valence of V. The EELS map was recorded in locations with and without W dopants. Two different series of EELS were acquired in the site with W dopants, which are displayed as V− W−1 and V−W−2 in Figure 5. A set of EELS was recorded for

Figure 4. Maximum shear stress in different VO2 structures calculated by DFT. Maximum shear stresses in each of the four simulated systems, normalized with respect to the VO2 monoclinic material. Also shown in the figure are the DFT models for each system.

0.14% in the tetragonal system. The stress vectors associated with the (122̅) lattice plane were calculated with Cauchy’s law, t = Tn, where T is the stress tensor determined in the DFT simulations and n is the unit normal to the lattice plane. The component of the stress vector normal to the lattice plane was determined as tn = t·n, where the dot denotes a scalar product of two vectors. The principal stresses were calculated by finding the eigenvalues of the stress tensor. The maximum values of the shear stresses were determined using methods described elsewhere.66 The magnitude of the component of the stress vector normal to the (122̅) plane in the WxV1−xO2 monoclinic system was 6.7% higher than in the VO2 monoclinic system, whereas the magnitude of the corresponding stress vector along the (122̅) plane in the WxV1−xO2 monoclinic system was only 0.9% higher than in the VO2 monoclinic system. This indicates that the addition of W atoms corresponds to larger increases in stress normal to (122)̅ than along it. The maximum shear stress in the WxV1−xO2 monoclinic system was 12% larger than that in the WxV1−xO2 tetragonal system, indicating the relaxation of the internal distortional stresses during the monoclinic to tetragonal transition with W doping. It is also interesting that the addition of W dopants in tetragonal structure only increases the internal distortional stress by 0.14%. Because there is a oneto-one relationship between stress and strain in this material system, it follows that these predictions are in agreement with the above observations associated with Figure 3. To emphasize the influence of these distortional stresses on the thermodynamic drive for transforming the VO2 and WxV1−xO2 materials from the monoclinic to tetragonal phases, the Gibbs free energy associated with the phase transformations was determined using

Figure 5. EELS data in areas with W and without W atoms. V−W−1 (blue curve) and V−W−2 (green curve) EELS spectra were recorded in a site where W atoms are present, and VO2 spectrum (red curve) is recorded in a site without W atoms.

locations without W (VO2 spectrum in Figure 5). The energy shifts of VO2 and V−W−2 spectra are almost the same (ΔE1 = 10.2 and 10.3, respectively). As reported,69 ΔE1 corresponds to V4+ and the energy shift of V−W−1, ΔE2 = 10.8, matches with V3+. Because only two V4+ ions are reduced to V3+ ions as results of one W6+ ion substitution,32 such variation between energy shift in the EELS signals around the W atoms (e.g., V− W−1 and V−W−2) are expected. This excitation state change corroborates with observation of two sets of EELS in the W dopants site corresponding to existence of both V3+ and V4+ ions. In the case of W-doped VO2, V4+ is substituted by

ΔG VO2 = (Uf + PV )VO2 rutile − (Uf + PV )VO2 monoclinic ΔG WVO2 = (Uf + PV )Wx V1−xO2rutile − (Uf + PV )Wx V1−xO2monoclinic

where P and V are the pressure and volume of the corresponding materials, respectively, and Uf is the internal energy of formation given by Uf = E − xE V − yEO − zE W 7184

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Figure 6. Probability of having 0−2 W atoms in one atomic column. The predicted probabilities are based on the binomial distribution calculation, and the measured values are based on the HAADF images (see more details in SI Note S6 and Table S1). The predicated and measured probabilities values are in good agreements.

W6+,33,35,68 which is shown to induce a reduction in the intensity of the pre-edge feature in the VK‑edge X-ray absorption near edge structure accompanied by a shift to lower energies. These spectral features arise from the local reduction of vanadium sites to V3+ and are a result of the softening of V−O bonds adjacent to the dopant sites and electron localization.35 Notably, analogous changes in the intensity and peak position of the VK‑edge pre-edge feature has also been observed upon Mo doping.26 The observation of homogeneous strain distribution in highdoped nanowire can be discussed from two viewpoints: (i) the effect of localized electron density after the transition and (ii) the difference between monoclinic and tetragonal structures. A characteristic behavior of Mott insulators is that below a critical carrier density, added electrons remain localized at dopant sites.70,71 The origin of this electron localization continues to be debated,72,73 but it is thought to be due to formation of small polarons and is facilitated by local lattice distortion around the dopant sites. Such distortion is indeed verified by our atomically resolved measurements presented here. However, above a critical threshold density corresponding to the Mott criterion 3

structure (SI Figure S7.c) and the corresponding [100] zone axis (SI Figure S7.d) of the tetragonal structure. The phase transition in VO2 is associated with a subtle distortion of vanadium atom positions.76,77 In the monoclinic crystal structure, V4+−V4+ bonds have two different distances of 2.65 and 3.12 Å, which constitute V4+ zigzag type chains (SI Figure S7.c). In contrast, V atoms in the tetragonal structure align in linear chains along the c axis with a uniform V4+−V4+ distance of 2.85 Å (SI Figure S7.d).73 In other words, V atoms are distorted from the zigzag configuration in the low-symmetry monoclinic structure to linear chains in the high-symmetry tetragonal structure, which is also accompanied by a change in the overlap of the dxy orbitals and closing of the bandgap.78 In the case of W-doped VO2, W dopants substitute vanadium atoms, and locally manifest stress on the V−V homopolar band.79 Because of the lower symmetry and different V−V bond lengths in the monoclinic structure, the local stress will appear anisotropic. Such stress heterogeneity will be manifested as nonuniform strain islands around the W dopants in the lowdoped monoclinic structure (Figure 3a−b). Further increase of W dopants in the monoclinic matrix induces more distortions until the internal stress reaches a threshold where the crystal structure becomes thermodynamically unstable and transforms to tetragonal. On the other hand, considering the symmetric of the tetragonal phase and the uniform lengths of V−V bonds, the stress distribution due to addition of dopants will appear to be homogeneous around V atoms. This homogeneous stress results in uniform strain as observed in Figure 3c−d. Although the influence of external stress or strain in metal− insulator phase transition of VO2 has been extensively studied,7,45 the effect of internal stresses are less known. It is interesting to study and compare how internal strains caused by different defects like doping and twining play role in metal− insulator phase transition of VO2. Li et al.80 reported that highly strained places in the thin monoclinic films, such as twin boundaries, resemble tetragonal structures. These imply that twin boundaries act as tetragonal nucleation sites during the

(nc) ah* = 0.25

where nc is the critical carrier concentration and ah* is the Bohr radius; an insulator to metal transition is induced, and as a result, the electron density is no longer localized on adjacent vanadium sites. The relaxation of these sites with elimination of the small polaron is reflected in the homogeneous strain map observed for the tetragonal phase.74,75 Furthermore, the differences in strain distribution between monoclinic and tetragonal phases can be discussed from structural perspective. The V and O atoms in monoclinic and tetragonal crystal structures are located in general positions of P21/c and P42/mnm space group, respectively (SI Figure S7.a− b). The structural phase transition between the two phases is best viewed along the [010] zone axis of the monoclinic 7185

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the same extent. The lattice parameters of the monoclinic crystal are clearly modified far more than that of the tetragonal phase, or in other words, the strain built up within this phase is considerably greater than in the tetragonal phase. This leads to the substantial supercooling of the tetragonal phase upon cooling. In other words, the tetragonal phase is stabilized far below the expected metalinsulator transition temperature while cooling owing to the greater anisotropic strains exhibited in the monoclinic phase. This picture is consistent with our observations35 that increased hysteresis upon W doping is primarily a result of the metal (tetragonal) → insulator (monoclinic) transition being depressed in temperature substantially more than the insulator (monoclinic) → metal (tetragonal) transition. In summary, atomic resolution studies were carried out on individual single crystalline WxV1−xO2 nanowires. Atomic resolution imaging reveals that W dopants introduce localized strain to the (122̅) lattice planes of the monoclinic structure. These localized lattice distortion accelerate the phase transition from monoclinic to tetragonal structure. This new understanding can be extended to other VO2 nanomaterials that are heterogeneously doped with heavier elements such as Mo, Nb, and Ga. This is because extensive research has shown that such elements can also influence the MIT transition points.26,27,34 Future work will focus on investigation of the strain evolution in low-doped nanowires during heating above MIT temperatures. This allows for direct analysis of strain evolution during monoclinic to tetragonal crystal transition.

MIT transition and change the transition temperature. Balakrishnan et al.81 also reported structural relaxation as film cracking during the phase transition centered around twinned platelets. The distribution of W atoms in the form of clusters or distribution of individual atoms can also influence the distribution of internal strains within the crystal structures and eventually modifying the metalinsulator transition of VO2. As reported,82 W clusters reduced the W-induced distortion in the structure and concentrated structural distortion in the W cluster area through tetragonal features. Therefore, further analysis was performed to reveal the possibility of W clustering in the VO2 structure. As expected, intensity of HAADF images are directly corresponded to Z of atoms in each column and the probability of containing w number of W dopants in each column can be measured by the statistical analysis of the HAADF images (SI Note S6 and Figure S8).83 On the other hand, the same probability can be calculated by the binomial distribution theory: Pn,c(w) = (n!/(w!(n−w)!))cw (1−c)n−w, where n is the number of atoms (total of V and W) in each column, w is the number of W in each column, and c is the W concentration in the sample. The probability of finding 0 to 2 atoms of W in one column for all of the nanowires is represented in Figure 6 (for more details see SI Table S1). The values of measured probabilities based on HAADF images are consistent with theoretical probabilities based on the binomial distribution as shown in Figure 6. For instance, the measured probability of having two tungsten atoms in one column of Figure 2a is 6.4%, and this probability is 5.7% based on the binomial distribution calculation for the 0.8 atom % W sample. These results clearly verify that the chance of W clustering is negligible and W atoms do not form a percolative pathway. At these concentrations, W atoms appear to be homogeneously dispersed as individual atoms (Figures 1 and 2). Additionally, our past X-ray absorption fine structure studies do not indicate any W−W bonds within the doped nanowires.35 The strain analysis discussed in this paper can shed light into hysteresis behavior previously reported by our group in Wdoped VO2 nanowires.24,25,35,73 Undoped VO2 nanowires prepared by hydrothermal methods exhibit sharply discontinuous phase transitions approaching 4 orders of magnitude within 1 °C temperature ranges.84 In contrast, W doping reduces the magnitude of the phase transition by 1−2 orders of magnitude.24 In fact, we observed that the resistance of insulating phase decreased from 106 Ohms for undoped VO284 to 104 Ohms for 0.6 atom % W doping, and to 103.5 Ohms for 1.14 atom % W doping.24 Interestingly, the resistance of metallic phase is always on the order of 102 Ohms and does not dramatically change as a function of W doping. These data demonstrate that the diminished magnitude of the transition in W-doped VO2 results from the decreased resistivity (increased conductance) of the insulating phase upon W doping. This observation supports the idea that the induced local strain due to W dopants (as well as the increased carrier density) to a great extent affects the resistance of monoclinic structure in comparison to the tetragonal phase. Cao et al.2 also reported the decreasing trend in resistance of insulator phase for VO2 beam under different axial compression. This trend results in significant decrease of MIT threshold voltage and current. The role of W doping in affecting the hysteresis can be related to the structural strain observed in the sense that it modifies the lattice parameters of the monoclinic and tetragonal crystals but not to

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03219. Material, methods, specimen preparations, and detailed calculation and simulations. (PDF)

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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 R.S.Y. acknowledges financial support from the National Science Foundation (Award No. CMMI-1200383). The acquisition of the UIC JEOL JEM-ARM200CF is supported by an MRI-R2 grant from the National Science Foundation (Grant No. DMR-0959470). Support from the UIC Research Resources Center is also acknowledged. G.M.O. would like to acknowledge the use of SUPERIOR, a high-performance computing cluster at Michigan Technological University. P.M. and S.B. acknowledge support from the National Science Foundation under IIP 1311837 and from the Research Corporation for Science Advancement through a Cottrell Scholar Award. S.S. and G.S. are supported by National Science Foundation (DMR 0847324).

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