Direct evidence of M₂ phase during the monoclinic-tetragonal (rutile) phase transition of W-doped VO₂ nanowires

The metal-insulator transition (MIT) of VO₂ has attracted the attention of many theorists and experimentalists for more than fifty years since the first discovery of this phenomenon by Morin.¹ However, the underlying mechanism of the MIT is still debated as a result of the simultaneously occurrence of both Peierls and Mott signatures during the transition.^2,3 The unique aspects of this phenomenon include a structural phase transition, sharp modulation of resistivity, and optical transparency by several order of magnitudes at ∼68 °C.³ The transition in VO₂ can be triggered by thermal, electrical, optical, and mechanical excitation. These unique properties have inspired many potential applications such as smart windows,⁴ thermo/electrochromics,⁵ Mott transistors,⁶ nanoactuators,^7,8 and sensors.⁹ 

The structural phase transition of VO₂ occurs from a low-temperature monoclinic M₁ phase to a high–temperature rutile R phase. Several other insulating polymorphs of VO₂ are known including the monoclinic (M₂) and triclinic (T) phases. The atomic arrangements of different phases can be explained by considering two interpenetrating sets of parallel chains of vanadium atoms, which are surrounded by six oxygen atoms (distorted octahedron). In the R phase, the chains of vanadium atoms are linearly arrayed and periodic, whereas in the M₁ phase all the chains are dimerized. In the M₂ phase, only half of the chains are dimerized, whereas the T phase is intermediate between M₁ and M₂. These insulating phases and their different properties add interesting aspects and complexity to the phase diagram of VO₂. The insulating phases can be stabilized in VO₂ with doping, introduction of defects, and as a result of anisotropic strain.^3,10,11 However, scarce little attention has been focused on the phase evolution and phase coexistence of doped phases.

On the other hand, the coexistence of multiple domains and phases was reported under conditions, where a pure phase might be expected. Cao et al. reported that the resistivity of the M₂ phase is three times higher than of the M₁ phase.¹² The pronounced differences in electrical, mechanical, and magnetic properties of insulting phases have been observed,^13–15 which play a critical role in device applications. Therefore, it is essential to map and control the appearance of different phases during the phase transition.

Much research thus far has been focused on analysis and distinction of different phases progression during the phase transition using X-ray diffraction,^12,13,16,17 X-ray absorption spectroscopy,^18,19 Raman spectroscopy,^17,20,21 atomic force microscopy,^22,23 and optical microscopy^8,15,22 techniques. Although the recent studies^24,25 using the infrared scattering-scanning near-field optical microscopy (s-SNOM) could achieve nanometer-scale spatial resolution,²⁶ most of the other methods lack the spatial resolution needed to identify and monitor nanoscale changes during the phase transition, nanoscale phase formation, and the relative spatial connectivity of M₁, M₂, and R phases remains to be fully understood. Transmission electron microscopy (TEM) has the ability to achieve a spatial resolution better than 1 Å and, thus, is an ideal technique for such studies at atomic resolution. In a related work, electron beam diffraction was only used to distinguish between the insulating (M₁ and M₂) phases of VO₂ in strain-induced nanowires.¹³ An in situ push–to-pull TEM method was used to estimate the Young moduli of M₁ and M₂ phases to be 128 ± 10 and 156 ± 10 GPa, respectively.

Although the recent TEM studies have revealed some details regarding the structural 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 the domain structures and phase transitions is of critical significance. The present work investigates the in situ phase transition of highly W-doped VO₂ in the individual single-crystalline nanowires of VO₂. The experiments were performed inside the chamber of aberration corrected scanning transmission electron microscopy (STEM) using an in-situ cooling holder. The structural phase transition was investigated through a complete cooling and heating cycle. Atomic resolution images and phase reconstruction at high resolution clearly show the appearance of the M₂ phase and demonstrate the remarkable coexistence of the R, M₁, and M₂ phases. The stabilization of the M₂ phase is attributed to the localized stress induced by W atoms doped substitutionally within the structure.

In this study, we have investigated the in situ phase transition of high-doped VO₂ with 0.8 at. % W concentration, which has a transition temperature around 20 °C.²⁷ The W_xV_1–xO₂ nanowires were synthesized by the hydrothermal reduction of bulk V₂O₅ in the presence of tungstic acid as a dopant precursor. The detailed synthetic procedures are described elsewhere.¹⁸ Figures 1(a) and 1(b) show an SEM image and powder XRD pattern of the W-doped VO₂ nanowires, respectively. The XRD reflections can be indexed to the rutile phase (P4₂/mnm space group) and can indicate a phase-pure sample. The previous studies by the Raman microprobe analysis have further allowed for unequivocal phase identification.²¹ The nanowires were dispersed on a lacy carbon coated copper grid for TEM characterization. A Gatan double-tilt liquid nitrogen cooling stage (model 636) was used for in situ cooling experiments, and the sample was kept at each temperature for more than 30 min for the stabilization of holder and phases. General characterization and atomic resolution high angle annular dark field (HAADF) imaging were performed by a probe-corrected JEOL JEM-ARM200CF equipped with a cold field emission gun, operated at 200 kV with a convergence angle of 22 mrad, and the inner angle of HAADF detector was 90 mrad. The SingleCrystal software²⁸ was used for TEM diffraction modeling, and crystal structural information of VO₂ different phases was obtained from the Pearson's Crystal data.²⁹ The electron beam energy was set to 200 keV, and the unit cells were oriented to the specific orientation of specimen for TEM diffraction modeling.

Figure 2(a) shows an atomic-resolution HAADF image acquired from a nanowire. The atomic arrangement and FFT of Figure 2(a) are concordant with the $[100]$ zone axis of the rutile structure. Next, the nanowire was cooled to a temperature of −180 °C inside the microscope. The HAADF image of the nanowire at −180 °C and FFT of the image (Figure 2(b) and inset) represent the $[01¯0]$ zone axis of the M₁ phase. The comparison between the FFT panels in Figures 2(a) and 2(b) demonstrates the phase transformation of the W-doped nanowire from rutile to monoclinic upon cooling. Next, the sample was brought back to 25 °C; the atomic-resolution image at room temperature (Figure 2(c)) and selected area FFTs from top right corner and bottom left corner of the image (the insets of Figure 2(c)) confirm the coexistence of different phases of VO₂. Finally, the sample was heated back up to 70 °C and then allowed to cool back to 25 °C. Atomic resolution images of the nanowire only show the existence of the rutile structure (Figure 2(d)). Mapping the atomic structure across a complete thermal cycle thus illustrates the reversibility of the phase transition in W-doped VO₂.

To identify and verify the observed phases of VO₂, solid sphere atomic arrangement of V and O atoms and corresponding simulated diffraction pattern of related phases were considered. In general, the symmetry of rutile structure belongs to P4₂/mnm space group, whereas the monoclinic structures (M₁ and M₂, respectively) belong to P2₁/c and C2/m. The solid sphere model of rutile structure along the $[100]$ zone axis is shown in Figure 3(a). All lattice planes corresponding to different M₁, M₂ and R phases can be related as per the following transition matrices¹⁰ 

Equivalent zone axes can be calculated based on the lattice planes. The monoclinic counterpart zone axes of $[100]$R are along $[01¯0]$ for M₁ and $[001¯]$ for M₂ structures. The solid sphere model of related monoclinic zone axes is shown in Figures 3(b) and 3(c). The left panels of Figures 3(a)–3(c) demonstrate the arrangement of only V atoms (blue spheres), which are observable in the HAADF images; the right panels illustrate the arrangement of V and O atoms (red spheres) for each phase and zone axis. The differences between the atomic arrangements of different phases are quite subtle, as can be seen in Figures 3(a)–3(c). However, the simulated diffraction patterns constructed from solid sphere models along $[100]$ R, $[01¯0]$ M₁, and $[001¯]$ M₂ (Figures 3(d)–3(f), respectively) demonstrate the distinctive characteristic features of each phase. The lattice reflections marked with blue circles are common between rutile and the two monoclinic phases. Several new reflections appear in the monoclinic phases, which are marked by green circles. In addition to the green-labeled features common to both monoclinic phases, the M₂ phase has several additional lattice reflections; the highest intensity pair is denoted with red circles. Such an analysis facilitates the mapping of three phases across the imaged sections. If an area only shows reflections marked with blue, it is assigned to be entirely in the rutile phase. The emergence of green and red-labeled lattice reflections in addition to the blue reflections denotes the appearance of the M₁ and M₂ phases, respectively. All lattice planes shown in Figures 3(d)–3(f) and related lattice spacings are tabulated in Figure 3(g).

Based on the analysis noted above, the top right corner of Figure 2(c) has a rutile structure, whereas the bottom left corner of the image has an M₁ structure. Additional atomic-resolution images at room temperature, which were acquired from other areas of the nanowire, are shown in Figures 4(b) and 4(c). The FFTs of atomic resolution images (insets of Figures 4(a)–4(c)) show the reflections of all the phases. Distinct lattice planes of each phases are mapped using different colors, as noted and marked in Figures 3(d)–3(f). High-resolution images have been reconstructed based on the unique reflections of each phase (Figures 4(d)–4(f)). The areas colored in blue represent the rutile structure. Overlapping blue and green domains suggest the existence of the M₁ phase. Concurrent blue, green, and red reflections correspond to the M₂ phase. Interestingly, the M₂ phase is stabilized as nanoscale grains, spanning only a few unit cells, in M₁ and R phases.

Even though the intermediacy role of M₂ phase in phase transition is not clear, the observation of this phase close to the phase transition is believed to be a consequence of inhomogeneous strain.^12,22,25,30 Indeed, the stabilization of this phase has been observed upon uniaxial tensile strain or Cr and Al doping using the Raman microprobe or micro-X-ray diffraction analyses^11,17,31,33 The M₂ phase was discernible in the previous Raman microprobe studies on W-doped samples; this is not entirely surprising given the nanoscale dimensions of the domains that span only a few unit cells.²¹ The observations here thus provide the first clear evidence of stabilization of the M₂ phase along with M₁ and R phases in W-doped VO₂ nanowires in proximity of the transition temperature. The appearance of the M₂ phase is observed across different domains through multiple experiments although the specific dimensions of the domains are dependent on the thermal history, reflecting the inherent stochasticity of nucleation processes.

The Ginzburg-Landau analysis of the structural phase transformation of VO₂ by Tselev et al.³⁰ noted that M₁ and M₂ phases are equivalent solutions for resolving the instability of rutile phase. The transformation from rutile to M₁ and M₂ phases happens at the same special point in the Brillouin zone, and the energetically favorable transformation is determined by slight perturbations such as local strain gradients. The possibility of M₂ phase as an intermediate phase between R and M₁ was discounted by these researchers. However, the existence of M₂ phase in response to existence of strain is generally proven. In the previous work, we reported³² that anisotropic localized strain caused by adding W dopants to the VO₂ matrix can strongly influence the phase transition. The origin of this localized strain can be due to physical mismatch between the W and V atoms or due to the tendency of the W⁶⁺ dopants to add electron density to the lattice. However, this localized strain can renormalize the coefficients of Ginzburg-Landau free energy function and stabilize the M₂ phase. The coexistence of M₁, M₂ and R phases as observed in HAADF images is thus reasonably attributed to the local anisotropic strain gradients resulting from W doping.

The hysteresis behavior of W-doped VO₂ nanowires, which was previously reported by our group,^18,21,27,33 can clarify the induced localized strain and consequently the existence of M₂ nanograins in the sample. W doping reduces the magnitude of the phase transition by 1−2 orders of magnitude, as compared to undoped nanowires, where the transitions span four orders of magnitude.³⁰ The tensile strain generated within the crystal structure favors stabilization of the M₂ phase with respect to the M₁ phase. The role of W doping in modulating the hysteresis can be related to modification of the lattice parameters that results in the structural strain and formation of M₂ phase. Due to the appearance of M₂ phase, the phase transition after cooling happens above the room temperature. However, the nanoscale dimensions of the M₂ grains (spanning only a few unit cells) and homogenous distribution of W atoms in VO₂ yield relatively drastic modulations of the conductivity upon thermal cycling, as compared to other dopants with larger M₂ grains. On the other hand, the formation and stabilization of M₂ phase is noteworthy especially for device application due to the different electrical, mechanical and magnetic properties. The existence of the M₂ and the controlling of this phase in relationship to M₁ and R phases are important.

In summary, the in situ atomic resolution studies were carried out on individual single crystalline W_xV_1–xO₂ nanowires (x = 0.8 at. %) during the cooling and heating experiments. Atomic resolution imaging reveals the stabilization of M₂ phase in the presentence M₁ and R phases just before the phase transformation. This is directly linked to strain induced by W dopants and renormalization of coefficients in favor of M₂ instead of M₁ phase. The homogenously distributed W in the VO₂ matrix results in evenly spreading the strain and the consequent formation of M₂ nanograins.

R.S.Y. acknowledges the financial support from the National Science Foundation (Award No. CMMI-1619743). The acquisition of the UIC JEOL JEM-ARM200CF is supported by the MRI-R2 grant from the National Science Foundation (Grant No. DMR-0959470). Support from the UIC Research Resources Center is also acknowledged. P.M., E.B., and S.B. acknowledge the support of this work from the National Science Foundation (Award No. IIP 1311837).