Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Since Morin reported observing the metal-to-insulator transition (MIT) of VO 2 in 1959 1 , VO 2 has been widely studied to understand the origin of its MIT [2][3][4][5][6][7] and to use it in practical applications including smart windows, batteries, transistors, ultrafast switches, and gas sensors [8][9][10][11][12][13][14][15] . The MIT of VO 2 can be induced by different factors such as heat, an electric field, doping, oxygen vacancy, photons, and a magnetic field 1,[5][6][7][16][17][18][19][20] . A typical critical temperature (T c ) of the MIT of VO 2 is approximately 68 °C 5 . However, previous studies showed that the T c is very sensitive to structural strain [21][22][23] . Cao and coworkers observed that VO 2 beams with multiple domains have different T c values, resulted in a dull transition 22 . Since the MIT of VO 2 is accompanied by a first-order structural phase transition (SPT) from a monoclinic phase (M1) to a rutile phase (R) via a M2 phase, which is a mixture of the M1 and R phases, the structural changes could be related to the MIT. For the last half decade, arguments have continued as to whether this structural transition directly induces the MIT of VO 2 2-8,24-26 . The electrical resistivity change of VO 2 between insulator and metallic phases is approximately four orders of magnitude and the MIT is quite abrupt 5,7 . Many efforts have been made to understand the mechanism of VO 2 MIT both theoretically and experimentally [2][3][4][5][6][7][8][24][25][26][27][28][29] . Many researchers attributed the abrupt MIT of VO 2 to the SPT 2,3,24,25,30 , while others argued that the abrupt MIT can be induced by the change of carriers from holes (insulator) to electrons (metal), supporting the Mott transition of VO 2 8,18 . The abruptness of the MIT of VO 2 has become a further issue, in addition to its origin.
A single crystal VO 2 has a distinct MIT temperature 22,31 while the MIT of grained VO 2 is dull, occurring over a wide range of temperature 7,21 . The T c and the MIT curve of VO 2 are very sensitive to structural disorder and strain 19,[32][33][34] . When a VO 2 film consists of grains, their structural disorder and distortion can take various forms, resulting in each grain having an individual T c . Furthermore, Qazilbash demonstrated that the MIT of VO 2 could occur at slightly different temperatures for even the same grain by using infrared (IR) mapping measurements 5 . Structural disorder and defects can prevent the movement of conduction electrons and also demolish a bandgap, creating bands of impurity near the Fermi level 35 . As a result, the T c of MIT can shift towards a higher or lower temperature [21][22][23] . Structural disorder, strain, and defects can be created by different conditions, including

Results
The temperature-dependent electrical properties of Cr-and Co-VO 2 . Implanted Cr and Co ions can act as dopants and induce structural defects in VO 2 . Previous studies showed that the lattice constants of Crdoped VO 2 increased 37,39 while the T c shifted towards a higher temperature 38,39 . Figure 1 shows the temperaturedependent resistance from the Cr-VO 2 films before and after Cr-ions implantation. The typical T c value of single crystal VO 2 is approximately 68 °C 5,31 . However, the T c value of a VO 2 film is substantially affected by structural strain and disorder and varies according to the substrate 21,32,32 . The T c of ~78 °C for the pristine VO 2 films in Fig. 1 before the ion implantation is ascribed to structural strain due to a lattice mismatch between the VO 2 films and Al 2 O 3 substrates 21,33,34 . The T c values of pristine VO 2 films in Fig. 2, particularly in Fig. 2(c), are somewhat lower than that in Fig. 1. The T c values suggest that the growth conditions of VO 2 films in Fig. 1 were somewhat different from those in Fig. 2, although the difference was not conscious during growth. A small deviation of the characteristics of the pristine VO 2 films does not seriously affect the main conclusions of this study because the changes of MIT features before and after ion implantation are directly compared from the same specimen. The Figure 1. Temperature-dependent electrical resistance for Cr-VO 2 films with a Cr ion energy and a flux of (a) 30 keV and 10 12 ions/cm 2 , (b) 50 keV and 10 12 ions/cm 2 , (c) 50 keV and 10 13 ions/cm 2 , and (d) 50 keV and 5 × 10 13 ions/cm 2 , respectively. Solid lines and circles are the resistances for the same Cr-VO 2 film before and after Cr-ion implantation, respectively. XAFS was simultaneously measured at the temperatures of the circles. Red and blue colors indicate the resistance for heating and cooling processes, respectively. The dotted lines are a guide for the eye. www.nature.com/scientificreports/ resistance curves of the Cr-VO 2 films with a flux of 10 12 ions/cm 2 show that the T c values during heating and cooling shift towards lower and higher temperatures relative to those before Cr ion implantation, respectively. As a result, the width of a hysteresis loop from the Cr-VO 2 films, particularly with a low energy of Cr ions, is significantly narrower than that of the pristine VO 2 . For Cr ion fluxes of 10 13 ions/cm 2 and 5 × 10 13 ions/cm 2 , the resistance curves become similar to that of the pristine VO 2 . At a flux of 5 × 10 13 ions/cm 2 , the T c value is ~ 2 degrees higher than that before ion implantation, as shown in Fig. 1(d). This is consistent with the previous studies of V 1−x Cr x O in which the T c was shifted towards a higher temperature 38,39 . The T c increase of 2 degrees of Cr-VO 2 roughly corresponds to the Cr concentration of ~ 3%, compared to a previous study of V 1−x Cr x O 2 39 . The T c shift and the resistance changes of Cr-VO 2 can be understood in terms of structural disorder and doping effects due to implanted Cr ions. The structural damage due to implanted ions is discussed in detail in the supplementary materials.
Zou and coworkers reported that the width of the hysteresis loop of temperature-dependent resistance for VO 2 was reduced by Cr doping and that the MIT features of VO 2 disappeared at a Cr concentration ratio of 14% 39 . The resistance measurements of the Cr-and Co-VO 2 films show that the MIT characteristics of VO 2 mostly disappear when the flux of Cr and Co ions exceeds 10 15 ions/cm 2 , which corresponds to ~ 50% of the VO 2 cells being hit by implanted ions. The probability of a conventional cell of a film being hit by implanted ions can be estimated as is the distribution function of ions in a film and A is the normalization factor. The details of the probability are described in the supplementary materials. The ion tracks and the local disorder due to implanted ions likely destroy the MIT characteristics. The T c shift of the Cr-VO 2 films with an ion flux of 10 12 ions/cm 2 suggests that the structural disorder due to ion implantation dominantly contributes to the electrical properties because of the negligible doping effect at a concentration ratio of 0.00023%. The concentration ratio is discussed in the supplementary materials in detail. When the ion flux increases over 10 13 ions/cm 2 , the doping effects due to the implanted ions may influence the MIT of VO 2 because of the extremely low charge carrier density of insulating VO 2 , although structural disorder still dominantly affects the MIT. Figure 2 shows the temperature-dependent electrical resistance of the Co-VO 2 films with different fluxes and energies of Co ions. The distribution of implanted Co ions in VO 2 is quite similar to that of Cr ions, as shown in the supplementary materials. When Co ions with a flux of 10 13 ions/cm 2 and an energy of ≤ 70 keV are implanted on VO 2 films, the T c values for both heating and cooling processes shift towards lower temperatures relative to those before the ion implantation. This is consistent with Co-doped VO 2 42 . The width reduction of the resistance hysteresis loop is similar to that of the Cr-VO 2 films with low ion fluxes. The resistance curves of the Co-VO 2 films with an energy of 30-50 keV and a flux of 10 14 ions/cm 2 show very weak MIT features near the T c of 65 °C (data Figure 2. Temperature-dependent electrical resistance for Co-VO 2 films with a Co ion energy and a flux of (a) 30 keV and 10 13 ions/cm 2 , (b) 50 keV and 10 13 ions/cm 2 , (c) 70 keV and 10 14 ions/cm 2 , and (d) 100 keV and 10 14 ions/cm 2 , respectively. Solid lines and circles are the resistances for the same Co-VO 2 film before and after Co-ion implantation, respectively. XAFS was simultaneously measured at the temperatures of the circles. Red and blue colors indicate resistance for heating and cooling processes, respectively. The dotted lines are a guide for the eye.  36 , implanted ions with the energy of 30-50 keV can affect the entire film through the grain boundaries and the lateral surfaces of the grains 46 . A lack of MIT features in Co-VO 2 with a Co ion energy of 30 keV and a flux of 10 14 ions/cm 2 (data not shown here) can be ascribed to a substantial structural disorder and distortion existing in the entire film due to the ion implantation. Structural disorder and distortion in Cr-and Co-VO 2 films which are created due to the implanted ions may not be uniformly distributed and can be more concentrated on near the surface than the bottom because the ion energy of several tens keV is insufficient to create a uniform defect in VO 2 films with a mean thickness of ~ 130 nm. At a Co ion energy of 100 keV, the resistance curves of the Co-VO 2 films show sharp MIT features and the T c values become similar to those before implantation during both heating and cooling. This is substantially different from those for a low energy of Co ions and sharply contrasts to previous works of Co-added VO 2 42 . The ion-flux-dependent behavior of T c values of Co-VO 2 is similar to that of Cr-VO 2 , as shown in Fig. 1. As the flux of both Co and Cr ions exceeds a certain value, T c shifts towards a higher and lower temperature relative to that for a low flux during heating and cooling, respectively, while the sharpness of MIT is not greatly affected, as shown in Figs. 1 and 2. A similar behavior of T c was also observed from Ti-added VO 2 44 . When the flux of Cr and Co ions with the energy of 30-50 keV is larger than 10 14 ions/cm 2 , the MIT features are significantly diminished. The critical flux of the ions increases when ion energy increases, as shown in Fig. 2. This is an evidence that ions with a lower energy more effectively create structural disorder, particularly near the surface, than the ions with a higher energy.
The temperature-dependent XANES and the pre-edge peaks of Cr-VO 2 and Co-VO 2 . Implanted Cr 3+ and Co 2+ ions can affect the charge currier density of conduction bands and the local density of states around the V atoms in VO 2 . X-ray absorption near edge structure (XANES) detects the local density of empty states around a probing atom 7 . Figure 3 shows XANES from the Cr-VO 2 films at the V K edge. The main absorption edge energy near 5478 eV from the Cr-VO 2 films is nearly identical to that of a pristine VO 2 film, which indicates that the chemical valance state and the 4p states of the V atoms in VO 2 are little affected by implanted Cr ions at a flux ≤ 5 × 10 13 ions/cm 2 . The intensity of the pre-edge peak near 5470 eV from the Cr-VO 2 films increases dramatically for an ion flux ≥ 10 13 ions/cm 2 , but only increases slightly for a flux of 10 12 ions/cm 2 relative to that of the pristine VO 2 . The pre-edge peaks consist of two peaks corresponding to the t 2g and e g states of V 3d orbitals of VO 2 , which are separated by approximately 2.0 eV 7 . The intensity increase of the pre-edge peak at the V K edge was also observed from Ti-added VO 2 44 . Since a lack of doping effects is expected when Ti 4+ s are replaced at V 4+ sites in VO 2 , the pre-edge peak changes are mainly attributed to structural changes around the V atoms. The position of the pre-edge peak is shifted by approximately 0.5 eV for a Cr ion flux ≥ 10 13 ions/cm 2 relative to that of the pristine VO 2 , as shown in Fig. 3 (b). These intensity increases and the position shift of the pre-edge peak might be due to local structural distortion around the V atoms. Figure 4(a, b) show the temperature-dependent XANES and pre-edge peaks, respectively, from a pristine VO 2 film in the temperature range of 30-100 °C. The main absorption edge is almost unaffected by the increasing temperature while the pre-edge shows a temperature-dependent behavior. In a pristine VO 2 film, the dull pre-edge peak corresponds to the t 2g and e g bands which have the energy difference of ~ 2.0 eV 7 . The direct band gap of VO 2 is ~ 0.65 eV at room temperature 47,48 and the Fermi level lies in the lower t 2g band 2,3,7 . Figure 3(d) shows that the intensity of the pre-edge peak decreases and the position shifts towards a higher energy when VO 2 is heated from 30 to 100 °C. The separation of the two pre-edge peaks does not change greatly but the peak www.nature.com/scientificreports/ positions shift by ~ 0.5 eV towards a higher energy when the structural symmetry of VO 2 changes from M1 to a rutile (or M2) phase 7 . The pre-edge peak of the pristine VO 2 shows a shift at ~ 68 °C, as shown in Fig. 4(b). This is prior to the T c of ~ 75 °C. The temperature-dependent behavior of the pre-edge peak is directly related to the local structural changes around the V atoms 7 . Electrical resistance measurements from Cr-VO 2 films with even a low Cr ion flux show substantial changes in the T c value of the MIT relative to that before ion implantation, as shown in Fig. 1. XANES and the pre-edge peak reveal that a Cr-VO 2 film with a small flux of Cr ions shows similar behavior as that of pristine VO 2 , as shown in Fig. 4(c, d). The pre-edge peak from the Cr-VO 2 film shifts at ~ 68 °C and ~ 65 °C while the T c values of MIT are ~ 71 °C and ~ 65 °C during heating and cooling, respectively, as shown in Fig. 1(a). The pre-edge peak transitions roughly agree with the MITs of the film, although they do not occur simultaneously at the same temperature. This is consistent with pristine VO 2 7 . For a Cr ion flux ≥ 10 13 ions/cm 2 , the intensity of the pre-edge dramatically increases relative to that of pristine VO 2 while the resistance curves are comparable to that before ion implantation. Figure 5(a, c) show the temperature-dependent XANES of the Cr-VO 2 films with a flux ≥ 10 13 ions/cm 2 . The pre-edge peaks of the films show nearly no temperature dependence in the temperature range of 40-100 °C. This sharply contrasts to that of pristine VO 2 and Cr-VO 2 with a low flux of the Cr ion beam. The resistance curves of the Cr-VO 2 films show clear MIT features, as shown in Fig. 1(c, d). This temperature independence of the pre-edge peaks of the Cr-VO 2 films is obvious evidence confirming that the pre-edge peak at the V K edge is irrelevant to the MIT of VO 2 . The pre-edge peak of a single crystal VO 2 accidently changes with temperature due to the transition of local structural properties and the local density of states around the V atoms. This study suggests that a structural disorder can remove the correlation between the pre-edge peak and the electrical properties of VO 2 . Figure 6(a) shows XANES at the V K edge from Co-VO 2 films with different energies and Co ion fluxes. The main absorption edges of the Co-VO 2 films are nearly identical to that of pristine VO 2 . This confirms the permanence of both the chemical valance states and the local density of states around the V atoms in the Co-VO 2 films, relative to those of the pristine VO 2 film. The intensity of the pre-edge peaks slightly increases at a Co ion flux of 10 14 ions/cm 2 . The XANES of Co-VO 2 films is quite different from that of Cr-VO 2 films, as shown in Fig. 3. Figure 6(c) shows the temperature-dependent XANES of Co-VO 2 with a Co ion energy of 70 keV and a flux of 10 14 ions/cm 2 . No significant changes in the main absorption edge energy of the Co-VO 2 film are observed in the temperature range of 30-100 °C. The pre-edge peak behavior of Co-VO 2 is similar to that of the pristine VO 2 but it is completely different from that of Cr-VO 2 with a Cr flux ≥ 10 13 ions/cm 2 . The pre-edge peak shifts at 62 °C towards a higher energy, as shown in Fig. 6(d), which is slightly prior to the T c of 65 °C, as shown in Fig. 2(c). The different behaviors of the pre-edge peaks from the Cr-VO 2 and Co-VO 2 films are mainly attributed to the different local structural properties around the V atoms.
Local structural properties around vanadium atoms of Cr-VO 2 and Co-VO 2 . Extended XAFS (EXAFS), which is small oscillations above the main absorption edge, as partially shown in Figs. 3, 4, 5 and 6, can detect the local structural properties around a probing atom [50][51][52] . After the atomic background was determined using the IFEFFIT software package 53 , EXAFS was obtained and analyzed using the standard procedure 54 . www.nature.com/scientificreports/ Raw EXAFS in k-space is presented in the supplementary materials. Figure 7 shows Fourier transformed EXAFS from the pristine VO 2 and Cr-VO 2 films in r-space. The peak positions of EXAFS correspond to the mean atomic distances from a V atom. They are approximately 0.3 Å shorter than the true atomic positions because the phase shift of back-scattered photoelectrons is not accounted for. Figure 7(a, b) show temperature-dependent EXAFS for the pristine VO 2 film for heating and cooling, respectively. The two peaks near 1.5 Å correspond to the six  www.nature.com/scientificreports/ V-O pairs of a VO 6 octahedron in VO 2 at lower temperatures; these become one sharp peak at higher temperatures because VO 2 has monoclinic and rutile (or M2) phases at low and high temperatures, respectively. A peak near 3.0 Å, mainly corresponding to eight vertex V atoms, slightly moves towards a longer distance in the rutile (or M2) phase, compared to that in M1. The EXAFS data of a pristine VO 2 were quantitatively fitted to the EXAFS theory 51 and the fit results are described elsewhere in literatures 7,36 . The SPTs of the pristine VO 2 film are observed at ~ 70 °C and ~ 62 °C for heating and cooling, respectively. The SPT temperatures do not match with either the T c of MIT or the pre-edge peak transitions. This agrees with previous reports 7, 33 . From the Cr-VO 2 film with a Cr ion energy of 50 keV and a flux of 10 12 ions/cm 2 , the SPTs are observed at 68 °C and 63 °C for heating and cooling, respectively, as shown in Fig. 7(c, d). The temperature difference between the SPTs for heating and cooling is approximately 5 °C, which is comparable to the resistance curve after ion-implantation, as shown in Fig. 1(a). This strongly suggests that the electrical property changes and the T c shifts of Cr-VO 2 films are highly related to the structural changes due to ion implantation. In addition, this indicates that the implanted Cr ions with the energy of 50 keV cause structural disorder and distortion around V atoms in the entire Cr-VO 2 film. An obvious MIT is observed from the Cr-VO 2 films with a flux of Cr ions ≥ 10 13 ions/cm 2 with no changes of the pre-edge peaks in the temperature range of 30-100 °C. Figure 8 shows EXAFS from the Cr-VO 2 films with a Cr ion flux ≥ 10 13 ions/cm 2 . At this Cr ion flux, there are clear SPTs near 72 °C and 64 °C for heating and cooling, respectively, as shown in Fig. 8(a, b). The SPT temperatures are comparable to the MIT T c values of 75 °C and 65 °C for heating and cooling, respectively, as shown in Fig. 1(c). This result is substantially different from that of a pristine VO 2 , which shows very different the T c values of MIT and SPT during both heating and cooling. The similar T c values of the MIT and the SPT in Cr-VO 2 and Co-VO 2 may suggest that Cr-and Co-VO 2 films structurally soften relative to a pristine VO 2 film due to ion implantation. Figure 8(c, d) show Fouriertransformed EXAFS of Cr-VO 2 with a flux of 5 × 10 13 ions/cm 2 . EXAFS shows no SPT of the first two peaks in 1.0-2.0 Å in the temperature range of 40-100 °C, indicating no SPT of the VO 6 octahedron in Cr-VO 2 . There are slight changes in the position and the shape of the EXAFS peaks in the r-range of 2.2-3.3 Å, as shown in Fig. 8(c, d), which correspond to ten V atoms: two V atoms are located above and below along the b-axis and the other eight are located at the vertexes of the rutile VO 2 . The position and the shape of the V peak near 3.0 Å are evidently changed at the T c of the SPT of a pristine VO 2 . A small shift of the V peak of the Cr-VO 2 film with a flux of 5 × 10 13 ions/cm 2 occurs at 73 °C and 61 °C for heating and cooling, respectively. The small shift of the V peak is reproducible and consistent during heating and cooling processes, although it is quite weak. The T c values of sharp MITs from the Cr-VO 2 film are observed at 80 °C and 65 °C for heating and cooling, respectively, as shown in Fig. 1(d). No transitions in either the pre-edge peak or in the V-O distance of the Cr-VO 2 film with a flux of 5 × 10 13 ions/cm 2 are observed, whereas a small shift occurs in the V-V distance. This indicates that www.nature.com/scientificreports/ both the pre-edge peak and the VO 6 octahedron, which are directly related to the crystal field effects, are nearly irrelevant to the MIT of VO 2 . The structural change of the V sites could drive the MIT of VO 2 , although the V sites cannot maintain even a correct rutile symmetry above the T c due to structural disorder. The transitions of the pre-edge peaks and the VO 6 octahedrons occur accidentally with the SPT of VO 2 crystals because the pre-edge peak is very sensitive to the nearest neighboring atoms around a probing atom and the V-O distance changes with the SPT. The EXAFS of the Cr-VO 2 films suggests that a structural change of the V sites is related to the MIT, although the temperatures of the structural changes are not identical to the T c values of the MIT during both heating and cooling. Figure 9(a, b) show temperature-dependent EXAFS from Co-VO 2 with a Co ion flux of 10 14 ions/cm 2 . EXAFS shows the SPTs of the films occurring at ~ 62 °C and ~ 56 °C for heating and cooling, respectively, which are comparable to the T c values of 65 °C and 55 °C, as shown in Fig. 2(c). The SPT of Co-VO 2 simultaneously appears at both O and V atomic sites at the same temperature. The SPT of Co-VO 2 is similar to that of pristine VO 2 but it is quite different from that of Cr-VO 2 with a Cr ion flux of 5 × 10 13 ions/cm 2 . The distortion of atomic pairs in Cr-VO 2 and Co-VO 2 is more obviously seen when the EXAFS data are directly compared to those of pristine VO 2 , as shown in Fig. 9(c, d). The EXAFS of pristine VO 2 shows two obvious peaks in the r-space of 1.0-2.0 Å, which correspond to six V-O pairs. When the ion flux increases, the EXAFS peak intensity is decreased, the shape is deformed, and the positions are shifted relative to those of the pristine VO 2 . The significant deformation of the first two peaks of the Cr-and Co-VO 2 films indicates that the VO 6 octahedrons are seriously distorted due to the implanted ions. When the flux of Cr ions is larger than 5 × 10 13 ions/cm 2 , VO 2 cannot maintain standard VO 6 octahedrons. EXAFS reveals that for an ion energy of 50 keV and a flux of 10 13 ions/cm 2 , the first peaks of Co-VO 2 are more distorted than that of Cr-VO 2 . The second peak of EXAFS at ~ 3.0 Å, which mainly corresponds to eight V atoms at the vertexes of a rutile phase VO 2 , is also affected by the implanted ions but the distortion of the V-V pairs is less significant than that of the V-O pairs, as shown in Fig. 9(c, d). For a Cr ion flux of 10 12 ions/ cm 2 , the intensity and the shape of the second peak from Cr-VO 2 with an energy of both 30 keV and 50 keV are similar to those of the pristine VO 2 , implying that the V sites are slightly affected by the implanted Cr ions. As the ion flux increases, the structural distortion of the V sites also increases in both Cr-VO 2 and Co-VO 2 . When the energy of Co ions becomes 100 keV, structural distortion, particularly at V sites, is somewhat reduced, compared to that at low ion energy, as shown in Fig. 9(d). This is not observed in Cr-VO 2 . The positions and shapes of the EXAFS peaks from Cr-VO 2 with a Cr ion flux of 5 × 10 13 ions/cm 2 are significantly different from those of other specimens, implying serious distortion existing in all atomic sites. Interestingly, the position and shape of the third two peaks near 4.0 Å in Fig. 9(c, d) are similar to those of the pristine VO 2 , although the intensity www.nature.com/scientificreports/ is quite weak. The third peaks mainly correspond to further V atomic shells beyond a conventional cell of a rutile-phased VO 2 . Those peaks of Cr-VO 2 with a flux of 5 × 10 13 ions/cm 2 show a slight temperature-dependent behavior, as shown Fig. 8(c, d). This is a further evidence that the V sites of Cr-VO 3 with a flux of 5 × 10 13 ions/ cm 2 still experience a weak SPT during both heating and cooling processes.

Discussion
Many researchers have observed that the MIT of a single crystal VO 2 occurs simultaneously with its SPT at T c ≈ 68 °C 5,29 . Since the MIT of VO 2 is accompanied by an SPT, the contribution of each structural change, such as VO 6 octahedrons, V-V dimers, and vertex V arrays, on the MIT is indistinguishable because the changes occur simultaneously in a single crystal VO 2 . EXAFS from Cr-VO 2 and Co-VO 2 films shows independent changes of structural properties of atomic shells during MIT. The direct comparison of electrical resistance and EXAFS measurements suggests that the contribution of the VO 6 octahedrons on the MIT is negligible. The energy states of V 3d orbitals are split in the e g and t 2g bands due to the crystal field effects of a VO 6 octahedron in VO 2 .
In Cr-VO 2 with a flux ≥ 10 13 ions/cm 2 the V-O pairs are significantly disordered, so that the VO 6 octahedrons cannot have regular splitting of the e g and t 2g bands. This prevents any regular alignment of V 3d orbitals in the specimen and can exclude the possibility of conduction electrons jumping from a lower energy band of the d xy and d xz orbitals to a higher energy band of the d (d x 2 −y 2 ) to trigger the metallic phase VO 2 . A lack of temperaturedependent features of the pre-edge peak from Cr-VO 2 with a flux ≥ 10 13 ions/cm 2 is further evidence of no regular splitting of V 3d states because the pre-edge peak corresponds to the e g and t 2g bands 7 . A dramatic increase of the pre-edge peak intensity of Cr-VO 2 indicates an increase of local density of states in the V 3d orbitals due to the structural distortion of VO 6 octahedrons. The pre-edge peak intensity of the V K edge increased and decreased for V 2 O 5 and V 2 O 3 , respectively, relative to that of VO 2 , because there are more empty states in the V 3d orbitals of V 2 O 5 than of V 2 O 3 41,55 . The dull pre-edge peak of VO 2 at the V K edge consists of two peaks which correspond to the V 3d t 2g and e g states, respectively, with an energy gap of ~ 2.0 eV 7 . XANES cannot detect the direct band gap because of its resolution limit. XANES from pristine VO 2 shows that the intensities of the t 2g (lower peak) and e g (upper peak) states decreases and increases, respectively, with no change of the band gap between the two states during heating 7 . As a result, the pre-edge peak seems to be shifted towards a higher energy for heating, as shown in Fig. 4(b, d). The pre-edge peaks of Cr-VO 2 with a flux ≥ 10 13 ions/cm 2 show that the t 2g band at ~ 5467.5 eV nearly disappears while the peak intensity of the e g states at ~ 5469.5 eV is very strong with no temperature dependence, as shown in Fig. 5(b, d). For the Cr-VO 2 films with a flux ≥ 10 13 ions/cm 2 , the lack temperature dependence of the pre-edge peak strongly implies no changes in the local density of states of the V 3d orbitals in the temperature www.nature.com/scientificreports/ range of 30-100 °C, although the films experience MIT and SPT. This is evidence indicating that the t 2g and e g bands split by the crystal field effects are irrelevant to the MIT of VO 2 and that an SPT of short-range orderings around the V atoms in VO 2 does not directly contribute to the MIT. EXAFS shows an SPT and no SPT at the O sites of Cr-VO 2 with a flux of 10 13 and 5 × 10 13 ions/cm 2 , respectively, while an SPT is observed at the V sites for both fluxes. This implies that the pre-edge peak of VO 2 is mainly contributed by the nearest neighboring O atoms, rather than by the second neighboring V atoms. This contrasts to the previous studies of the pre-edge peak of transition metals, in which the authors discussed the contribution of the second neighboring atoms on the pre-edge peaks 41,44,56 .
On the other hand, traditional band theory cannot predict the bandgap of ~ 0.65 eV for M1-phased VO 2 at room temperature 6 . Based on a structural-driven Peierls transition mechanism, two different distances of V-V pairs in M1 VO 2 were introduced to understand the insulating phase of VO 2 6,27,57-60 . The dimerization model also cannot explain the measured bandgap of VO 2 at room temperature 6 . Our EXAFS measurements and calculations [Supplementary Materials] on Cr-VO 2 with a Cr flux of 5 × 10 13 ions/cm 2 reveal a substantial amount of structural disorder at both oxygen and vanadium sites. The linear defects created due to the implanted ions are placed perpendicular to the current direction in the DC electrical resistance measurements of the films. Cr-VO 2 barely maintains a crystalline structure without regular V-V dimers and does not show an SPT of V-V dimers due to the enormous amount of structural disorder and distortion in the V sites, although there are obvious MITs during both heating and cooling. The distances of the V-V dimers are approximately ~ 2.5 Å and ~ 3.2 Å in the M1 phase and become ~ 2.8 Å in the rutile phase 27,36 . Our result is further evidence that a V-V dimerization model cannot explain the MIT mechanism of VO 2 . However, a structural-driven Peierls transition may not be excluded as an explanation for the MIT of VO 2 because the EXAFS peaks which mainly correspond to the V sites show a weak SPT at T c , as shown in Fig. 8(c, d). The sharp MIT features of the resistance curve from Cr-VO 2 with a Cr ion flux of 5 × 10 13 ions/cm 2 strongly suggest a transition of interaction between conduction electrons at T c . The EXAFS and resistance measurements of the Cr-VO 2 film support that the MIT is highly related to the interaction of conduction electrons and is triggered by the alignment of the V atomic arrays near T c .
Previous studies reported that the T c values of Cr-added and Co-added VO 2 shifted towards higher and lower temperatures, respectively [37][38][39][40]42 . The T c values of both Cr-and Co-VO 2 films with a flux ≥ 5 × 10 13 ions/ cm 2 shift towards a higher temperature. Since the concentration ratio of the implanted ions is only ~ 0.023% for an ion flux of 10 14 ions/cm 2 , the doping effects of the ions could be negligible. V 1−x Ti x O 2 also showed that the T c decreased and increased for low and high concentrations of Ti 4+ , respectively 44 . Ti 4+ ions which are mostly replaced at the V 4+ sites of VO 2 can cause the disorder and distortion of the V sites without doping effects. Previous studies of heavy ion irradiation with high energy on VO 2 showed that the resistivity and the T c value of VO 2 were considerably modified due to an extra structural disorder 61,62 . Hofsäss and coworkers showed that 1 GeV 238 U swift heavy ions substantially decreased the T c value of VO 2 , although no surface hillocks were observed 61 . When 200 meV Ag 9+ -ions with a high flux bombarded VO 2 , the surface and the crystal symmetry of VO 2 were seriously damaged. Both T c value and resistivity jump size of the MIT of VO 2 continuously decreased when the fluence of Ag 9+ ions increased 62 . This result is somewhat different from that of Cr-VO 2 , Co-VO 2 , and V 1−x Ti x O 2 , as discussed above. Since most of implanted Cr and Co ions remain in VO 2 films, they play as impurities in addition to the ion tracks. Impurities in VO 2 can modify the band structure, contribute the charge carrier density of the conduction band, disturb the SPT, and interrupt the propagation of electrons. When Cr concentration increased in VO 2 , both lattice constants and structural disorder of V 1−x Cr x O 2 increased, while the T c value moved towards a higher temperature during both heating and cooling 38,39 . This is comparable to the T c behavior of Cr-VO 2 with the ion flux of 5 × 10 13 ions/cm 2 .
The resistance and EXAFS measurements of the Cr-and Co-VO 2 films with different ion energies and fluxes show that the SPT always occurs before and after the MIT during heating and cooling, respectively. This indicates that a percolation effect is negligible in the systems and that an SPT, particularly the V atomic arrays, is an essential prerequisite for the MIT of VO 2 . This corresponds to that of the pristine VO 2 . A few defects in VO 2 assist SPTs during heating and cooling whereas many defects interrupt SPTs. As a result, △T c (T c heating − T c cooling ) becomes small and large, as shown in Figs. 1 and 2, respectively. When the concentration of defects is larger than a critical value, the MIT of VO 2 can be totally destoryed, as reported in previous studies 61,62 . The total amount of defects in a film due to implanted ions increases with increase in the flux and the penetration depth of the ions because the ions create linear tracks. When the energy of the implanted ions increases, the penetration depth is expanded, leading to the creation of more defects in the film. This scenario is consistent with the results of the Crand Co-VO 2 films with different energies and the same flux. For an ion flux of 10 14 ions/cm 2 , MIT features from Cr-VO 2 are significantly reduced while an obvious MIT is observed from Co-VO 2 . EXAFS reveals that Cr ions more seriously affect the O sites of VO 2 than those of Co ions, as shown in Figs. 8 and 9. The penetration depths of the two ions on VO 2 are roughly the same, as shown in the supplementary materials. Researchers observed that the T c values of V 1−x Cr x O 2 and V 1−x Co x VO 2 increased and decreased, respectively, relative to that of pristine VO 2 38,39,42 , This implies that the contributions of Cr and Co ions on the MIT of VO 2 are not the same, as they interrupt and assist the SPT, respectively. EXAFS measurements reveal that Cr ions more effectively destroy the crystalline structure of VO 2 than Co ions do. The different effects of Cr and Co ions to VO 2 could be attributed to the different radii and the different oxidation states of Cr 3+ and Co 2+ ions. This study indicates that the T c of VO 2 can be increased or decreased by careful selection of a proper species of ions with different energies and fluxes.

Conclusions
For the Cr and Co ion fluxes ≤ 10 14 ions/cm 2 , both Cr-and Co-VO 2 show sharp MIT features near T c . The T c of both the Cr-and Co-VO 2 films with a low ion flux is lower than that before ion implantation, while it shifts toward a higher temperature for a high ion flux. This indicates that the T c of VO 2 can be engineered by properly www.nature.com/scientificreports/ selecting the flux, energy, and species of ion beam. Both Cr and Co ions create a substantial amount of structural disorder and distortion in VO 2 . Based on resistance and EXAFS measurements, model calculations suggest that a sharp and abrupt MIT and SPT can occur in VO 2 unless more than 5% of the V sites are disturbed by impurities. Temperature-dependent XANES from Cr-VO 2 at the V K edge showed that the pre-edge peak alone cannot fully describe either the MIT or the SPT. Temperature-dependent resistance and EXAFS measurements reveal that crystal field splitting in the VO 6 octahedron of VO 2 does not play a critical role in the MIT. These study results suggest that an SPT of the V atomic arrays and the interaction of V 3d 1 electrons are the necessary conditions for the MIT of VO 2 , supporting both the structural-driven-Peierls and Mott-Hubbard models. This study also shows that ion-implantation techniques can be widely used to engineer the T c and the MIT of VO 2 , and particularly of VO 2 nanostructures, without degrading the sharpness of the MIT features 23 .

Methods
Synthesis of VO 2 films. The b-oriented VO 2 films were fabricated on α-Al 2 O 3 (0001) substrates using direct current (DC)-sputtering deposition from a vanadium target with a purity of 99.95%. The base vacuum of the growth chamber was 10 -6 Torr and the pressure was kept at 10 -3 Torr during the deposition. Ar gas was used as the plasma and the substrate temperature was maintained at ~ 500 °C. After deposition, the films were annealed at 500 °C for 30 min with a mixture gas flow of Ar: O 2 = 300:1. More details of VO 2 film fabrication can be found elsewhere in the literatures 7,36 .
Cr and Co ion implantation on VO 2 films. Cr and Co ions with an energy of 30-100 keV and a flux of In-situ XAFS measurements. Temperature-dependent XAFS measurements were conducted from Cr-VO 2 and Co-VO 2 films and a pristine VO 2 film as a counterpart at the V K edge (5465 eV). XAFS measurements were performed with a fluorescence mode using a Si(111) double-crystal monochromator at beamline 8C of the Pohang Light Source (PLS) and beamline 20-BM of the Advanced Photon Source (APS). The XAFS data were taken with an unpolarized geometry where the angle between the film surface and the incident X-ray beam was fixed at 45 degrees. During in-situ temperature-dependent XAFS measurements, the DC electrical resistance was simultaneously measured from the same specimens 7,33 . A thermocouple was directly contacted to the surface of a VO 2 film to accurately measure the true temperature of the film in real time. The resistance and the temperature were recorded after the temperature at each set temperature was stabilized. The temperature was monitored and controlled within ± 0.1 degree during the XAFS scans and the resistance measurements. Each XAFS scan took approximately 15 min.
DC resistance measurements. Two-probe DC-resistance measurements were performed from pristine, Cr-VO 2 , and Co-VO 2 films before ion-implanted at the applied voltage of 0.5 V using a Keithley 2400 SourceMeter 7,36 . After the ions were implanted, the resistance measurements were simultaneously performed with in-situ XAFS measurements.