Large Scale Synthesis of Nanopyramidal-Like VO2 Films by an Oxygen-Assisted Etching Growth Method with Significantly Enhanced Field Emission Properties

The present investigation reported on a novel oxygen-assisted etching growth method that can directly transform wafer-scale plain VO2 thin films into pyramidal-like VO2 nanostructures with highly improved field-emission properties. The oxygen applied during annealing played a key role in the formation of the special pyramidal-like structures by introducing thin oxygen-rich transition layers on the top surfaces of the VO2 crystals. An etching related growth and transformation mechanism for the synthesis of nanopyramidal films was proposed. Structural characterizations confirmed the formation of a composite VO2 structure of monoclinic M1 (P21/c) and Mott insulating M2 (C2/m) phases for the films at room temperature. Moreover, by varying the oxygen concentration, the nanocrystal morphology of the VO2 films could be tuned, ranging over pyramidal, dot, and/or twin structures. These nanopyramidal VO2 films showed potential benefits for application such as temperature−regulated field emission devices. For one typical sample deposited on a 3-inch silicon substrate, its emission current (measured at 6 V/μm) increased by about 1000 times after the oxygen-etching treatment, and the field enhancement factor β reached as high as 3810 and 1620 for the M and R states, respectively. The simple method reported in the present study may provide a protocol for building a variety of large interesting surfaces for VO2-based device applications.


Introduction
Structural characteristics including dimensionality, morphology, and crystal structure can have significant influences on the properties and applications of functional materials. The development of rational processes for synthesizing materials with tunable micro-/nanostructures is essential for both fundamental research and industrial applications. Multivalent transition metal oxides, e.g., TiO x , VO x , WO x , and NiO x , usually exhibit a wealth of interesting and useful properties that are beyond that of conventional semiconductors [1,2], due to their abounding types of order in the spin, charge, or orbital degree of freedom, which are associated with the convertible valence states of metal cations [3]. Among these oxides, vanadium dioxide (VO 2 ), a prototypical strongly correlated electron oxide, has drawn growing attention because of its famous reversible first-order metal-to-insulator phase transition (MIPT), which occurs at an accessible temperature of 68 • C [4][5][6]. Upon the MIPT, the crystalline structure changes from a high-symmetry tetragonal rutile phase (P4 2 /mnm, R phase) to a lower symmetry monoclinic phase (P2 1 /c, M 1 phase) with dimerized V-V pairs exhibiting alternating zig-zag like chains [4,5]. Noticeable physical property changes such as optical transmittance [7][8][9], electrical conductivity [10,11], permittivity [12,13], and magnetic susceptibility [14,15] can be observed, making these kind of materials potentially suitable for a great many stimuli-responsive devices, e.g., ultrafast optical switches [16,17], sensing devices [18,19], next-generation terahertz transistors [20], non-Boolean computing materials [21], uncooled infrared bolometer [22,23], and other novel concepts of switching devices [24][25][26][27][28].
For device-based applications of VO 2 materials [29], integration methods for film techniques are quite imperative, having the advantages of excellent purity, uniform thickness, feasibility in layer stacking, and are also compatible with large industrial level production. Many constructive methods for film deposition, ranging over both vapor and solution based techniques including sputtering [30][31][32], physical vapor deposition (PVD) [33], chemical vapor deposition (CVD) [34,35], pulsed laser deposition (PLD), sol-gel [36,37], and polymer-assisted deposition (PAD) [38,39] have been developed for VO 2 films, which have shown excellent controls over the crystalline phases and chemical compositions. Recently, wafer-scale processing with superior thickness uniformity and valence state control have been demonstrated by rf magnetron sputtering [40] and a combinatorial approach [1], giving rise to constructive bridges to further fill the gaps between fundamental research and commercial devices. However, among the numerous reports on the synthesis of VO 2 films, only a few distinct surface structures, e.g., rough pores [41], subwavelength nanoholes [42], nanotetrapods [43], nanobeams [19], have been developed. This may derive from the complexity of the vanadium oxide system [44] which has multiple oxidation states (from +2, as in VO, to +5, as in V 2 O 5 ), different elemental compositions (such as V n O 2n−1 and V n O 2n+1 ) [45,46], and various coordination polyhedra (including the tetrahedron, trigonal bipyramid, square pyramid, regular octahedron, and distorted octahedron) [47]; however, these severely restrict their applications as high performance devices. Thus, the development of an effective strategy for the synthesis of VO 2 films with desirable surface structures is still urgently required.
In this article, we reported on a novel wafer-scale oxygen-etching strategy that could successfully transform plain planar VO 2 thin films into a highly textured nanopyramidal surface structure. The oxygen atmosphere applied in the experiments provides a mild environment (within the tolerance and stability) for VO 2 chemical composition, which prevents further oxidizing into high valence vanadium oxides (e.g., V 2 O 5 , V 6 O 13 ), although their surface structures change significantly from relatively smooth morphologies into nanopyramidal structures. Moreover, aside from the monoclinic M1 phase VO 2 , another metastable M2 phase, which is most frequently observed at high temperature during the metal-to-insulator phase transition, was also found in this investigation, even at room temperature. A temperature responsive field emission device was developed to broaden the applications of this special nanostructure. For a 3-inch wafer-scale VO 2 film sample directly grown on silicon wafer, an increase of about three orders in the emission currency was observed after the oxygen-etching treatment, and the field enhancement factor β reached 3810 and 1620 for the M and R states, respectively, which are comparable with the reported ZnO nanobullets [48] and CdS nanobelts [49]. The simple and large-area available processing reported in this work may provide a practical strategy for the production of VO 2 -based integration devices.

Materials
Vanadium dioxide thin films were synthesized via a solution-based process developed previously in our laboratory [7,38]. Vanadium pentoxide (V 2 O 5 , analytically pure), polyvinylpyrrolidone (PVP, K90, average molecular weight 10,300,000) and diamide hydrochloride (N 2 H 4 ·HCl, analytically pure) were used as the starting materials to form a 0.2 M vanadium precursor. Silicon pre-treated sequentially with ethyl alcohol, HCl, and NH 3 ·H 2 O were used as the deposition substrates. The precursor films were prepared by spin-coating at 1000 rpm for 30 s and dried at 60 • C for 10 min, and then were annealed in a N 2 atmosphere at 520 • C for 1 h to obtain pure VO 2 films (M1/R).
The oxygen-etching processing was performed in a quartz tube furnace (120 cm in length with a heating zone of around 20 cm in the middle). A pressure gauge was used to monitor the pressure inside the quartz tube, and two valves were installed at the gas inlet and outlet ends. A scheme for the furnace tube is shown in Scheme 1. For the oxygen-etching growth process, the above VO 2 samples were loaded and rapidly heated to 540 • C in vacuum at a heating rate of 50 • C/min. The valve at the gas outlet end was then closed, and different amounts of the N 2 /O 2 mixture (with an oxygen volume ratio of 2%) were introduced through the gas inlet. The content of oxygen introduced during annealing was monitored by the pressure changes recorded on the pressure gauge. The etching process was sustained for different spans of time, and textured vanadium oxide films with various morphologies were obtained. A schematic showing the evolution of the film morphology is given in Scheme 2. The oxygen-etching processing was performed in a quartz tube furnace (120 cm in length with a heating zone of around 20 cm in the middle). A pressure gauge was used to monitor the pressure inside the quartz tube, and two valves were installed at the gas inlet and outlet ends. A scheme for the furnace tube is shown in Scheme 1. For the oxygen-etching growth process, the above VO2 samples were loaded and rapidly heated to 540 °C in vacuum at a heating rate of 50 °C/min. The valve at the gas outlet end was then closed, and different amounts of the N2/O2 mixture (with an oxygen volume ratio of 2%) were introduced through the gas inlet. The content of oxygen introduced during annealing was monitored by the pressure changes recorded on the pressure gauge. The etching process was sustained for different spans of time, and textured vanadium oxide films with various morphologies were obtained. A schematic showing the evolution of the film morphology is given in Scheme 2. Scheme 2. Schematic of VO2 nanopyramid synthesis from solution processed VO2 nanofilms.

Characterization
The surface morphologies of the films were determined by field emission scanning electron microscopy (SEM, JSM 6700F, JEOL, Tokyo, Japan). TEM images were acquired by transmission electron microscopy (TEM, JEM2010, JEOL, Tokyo, Japan). The samples for TEM were prepared by scraping the films from the substrates using a stainless steel blade; the resulting material was then dispersed in ethanol. X-ray diffraction was performed on a D/max 2550V x-ray diffractometer (Rigaku, Tokyo, Japan, Cu Kα, λ = 0.15406 nm). The film thickness was determined using a Taylor-Hobson surface profile measuring system by measuring at least four different points per sample. The structures and compositions of the samples were characterized by Raman spectroscopy and performed on a Raman microscope spectrometer (Raman, inVia Reflex, Renishaw, Gloucestershire, England) using a 514.5 nm laser.
To measure the field emission (FE) performance (as a cathode), the as-obtained nanofilms were placed before phosphor/indium tin oxide (ITO)/glass (anode), separated by two Teflon spacers with a thickness of 100 μm. The FE properties were measured under a high vacuum level of approximately 5  Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials deposition substrates. The precursor films were prepared by spin-coating at 1000 rpm for 30 s and dried at 60 °C for 10 min, and then were annealed in a N2 atmosphere at 520 °C for 1 h to obtain pure VO2 films (M1/R). The oxygen-etching processing was performed in a quartz tube furnace (120 cm in length with a heating zone of around 20 cm in the middle). A pressure gauge was used to monitor the pressure inside the quartz tube, and two valves were installed at the gas inlet and outlet ends. A scheme for the furnace tube is shown in Scheme 1. For the oxygen-etching growth process, the above VO2 samples were loaded and rapidly heated to 540 °C in vacuum at a heating rate of 50 °C/min. The valve at the gas outlet end was then closed, and different amounts of the N2/O2 mixture (with an oxygen volume ratio of 2%) were introduced through the gas inlet. The content of oxygen introduced during annealing was monitored by the pressure changes recorded on the pressure gauge. The etching process was sustained for different spans of time, and textured vanadium oxide films with various morphologies were obtained. A schematic showing the evolution of the film morphology is given in Scheme 2. Scheme 2. Schematic of VO2 nanopyramid synthesis from solution processed VO2 nanofilms.

Characterization
The surface morphologies of the films were determined by field emission scanning electron microscopy (SEM, JSM 6700F, JEOL, Tokyo, Japan). TEM images were acquired by transmission electron microscopy (TEM, JEM2010, JEOL, Tokyo, Japan). The samples for TEM were prepared by scraping the films from the substrates using a stainless steel blade; the resulting material was then dispersed in ethanol. X-ray diffraction was performed on a D/max 2550V x-ray diffractometer (Rigaku, Tokyo, Japan, Cu Kα, λ = 0.15406 nm). The film thickness was determined using a Taylor-Hobson surface profile measuring system by measuring at least four different points per sample. The structures and compositions of the samples were characterized by Raman spectroscopy and performed on a Raman microscope spectrometer (Raman, inVia Reflex, Renishaw, Gloucestershire, England) using a 514.5 nm laser.
To measure the field emission (FE) performance (as a cathode), the as-obtained nanofilms were placed before phosphor/indium tin oxide (ITO)/glass (anode), separated by two Teflon spacers with a thickness of 100 μm. The FE properties were measured under a high vacuum level of approximately 5 × 10 −5 Pa at room temperature and 100 °C, respectively. The measured emission area was 1 × 1 cm 2 .

Results and Discussion
The as-prepared VO2 thin films before and after oxygen-etching treatment were characterized by room temperature XRD measurement ( Figure 1). All of the recorded diffraction peaks for the Scheme 2. Schematic of VO 2 nanopyramid synthesis from solution processed VO 2 nanofilms.

Characterization
The surface morphologies of the films were determined by field emission scanning electron microscopy (SEM, JSM 6700F, JEOL, Tokyo, Japan). TEM images were acquired by transmission electron microscopy (TEM, JEM2010, JEOL, Tokyo, Japan). The samples for TEM were prepared by scraping the films from the substrates using a stainless steel blade; the resulting material was then dispersed in ethanol. X-ray diffraction was performed on a D/max 2550V X-ray diffractometer (Rigaku, Tokyo, Japan, Cu Kα, λ = 0.15406 nm). The film thickness was determined using a Taylor-Hobson surface profile measuring system by measuring at least four different points per sample. The structures and compositions of the samples were characterized by Raman spectroscopy and performed on a Raman microscope spectrometer (Raman, inVia Reflex, Renishaw, Gloucestershire, England) using a 514.5 nm laser.
To measure the field emission (FE) performance (as a cathode), the as-obtained nanofilms were placed before phosphor/indium tin oxide (ITO)/glass (anode), separated by two Teflon spacers with a thickness of 100 µm. The FE properties were measured under a high vacuum level of approximately 5 × 10 −5 Pa at room temperature and 100 • C, respectively. The measured emission area was 1 × 1 cm 2 .

Results and Discussion
The as-prepared VO 2 thin films before and after oxygen-etching treatment were characterized by room temperature XRD measurement ( Figure 1). All of the recorded diffraction peaks for the original films obtained at 540 • C in a N 2 atmosphere can be assigned to the pure monoclinic phase of VO 2 (indexed as VO 2 (M1), JCPDS card no. 72-0514, P2 1 /c, a = 0.57 nm, b = 0.45 nm, c = 0.54 nm, and β = 122.61 • ), in agreement with previous investigations [50,51]. However, after the introduction of oxygen with a controllable amount (0.12 kPa, sample labeled as S-0.12k in the following discussion) during the etching process, a slightly shifted diffraction peak of M1 VO 2 (011) (from 27.86 • to about 28.01 • ) and an extra peak at 28.53 • were observed. The diffraction pattern at approximately 28.53 • could not be assigned directly to any of the vanadium oxide phases, although it was close to the main diffraction peak of the (201) crystalline plane for M2 phase VO 2 (JCPDS card no. 33-1441, space group C2/m, a = 0.908, b = 0.576, c = 0.453, and β = 91.3 • ). The crystallinity increased after the oxygen treatment, as deduced from the decreased full widths at half maximum (FWHM), which were 0.43 • and 0.20 • for the major diffraction peaks of the plain nanofilm and nanopyramid film, respectively. original films obtained at 540 °C in a N2 atmosphere can be assigned to the pure monoclinic phase of VO2 (indexed as VO2 (M1), JCPDS card no. 72-0514, P21/c, a = 0.57 nm, b = 0.45 nm, c = 0.54 nm, and β = 122.61°), in agreement with previous investigations [50,51]. However, after the introduction of oxygen with a controllable amount (0.12 kPa, sample labeled as S-0.12k in the following discussion) during the etching process, a slightly shifted diffraction peak of M1 VO2 (011) (from 27.86° to about 28.01°) and an extra peak at 28.53° were observed. The diffraction pattern at approximately 28.53° could not be assigned directly to any of the vanadium oxide phases, although it was close to the main diffraction peak of the (201) crystalline plane for M2 phase VO2 (JCPDS card no. 33-1441, space group C2/m, a = 0.908, b = 0.576, c = 0.453, and β = 91.3°). The crystallinity increased after the oxygen treatment, as deduced from the decreased full widths at half maximum (FWHM), which were 0.43° and 0.20° for the major diffraction peaks of the plain nanofilm and nanopyramid film, respectively.

Figure 1.
Room temperature x-ray diffraction (XRD) pattern for the VO2 nanofilms deposited on the silicon substrate before and after oxygen injection. The inset shows the enlarged results of the XRD data obtained between 27° and 30° for these films.
The structures and compositions of the films before and after oxygen injection were further confirmed by room-temperature Raman spectra ( Figure 2). For the original VO2 films, Raman modes corresponding to M1 phase VO2 were found, with peaks centered at 192, 223, 307, 338, 392, 440, 494, 612, and 816 cm −1 [51,52], respectively. However, after oxygen injection during post-annealing at 540 °C, other modes centered at approximately 271, 432, and 646 cm −1 appeared aside from the modes of the M1 phase of VO2. These modes can be assigned to the metastable M2 phase of VO2, whose Raman modes centered at around 226, 272, 293, 338, 432, 495, and 645 cm −1 [53]. The existence of these Raman modes, especially the characteristic V-V vibration mode at 646 cm −1 , indicates that treating the VO2 films with a controllable amount of oxygen produced the VO2 M1/M2 composite structures, agreeing with the XRD results. The structures and compositions of the films before and after oxygen injection were further confirmed by room-temperature Raman spectra ( Figure 2). For the original VO 2 films, Raman modes corresponding to M1 phase VO 2 were found, with peaks centered at 192, 223, 307, 338, 392, 440, 494, 612, and 816 cm −1 [51,52], respectively. However, after oxygen injection during post-annealing at 540 • C, other modes centered at approximately 271, 432, and 646 cm −1 appeared aside from the modes of the M1 phase of VO 2 . These modes can be assigned to the metastable M2 phase of VO 2 , whose Raman modes centered at around 226, 272, 293, 338, 432, 495, and 645 cm −1 [53]. The existence of these Raman modes, especially the characteristic V-V vibration mode at 646 cm −1 , indicates that treating the VO 2 films with a controllable amount of oxygen produced the VO 2 M 1 /M 2 composite structures, agreeing with the XRD results. XPS spectra ( Figure 3) were further conducted to investigate the compositions and valence states of the vanadium in the films. Surface contamination was removed before the measurements by an Ar ion (1 keV) etching treatment. As shown in a wide-range survey XPS spectrum, a typical XPS spectra ( Figure 3) were further conducted to investigate the compositions and valence states of the vanadium in the films. Surface contamination was removed before the measurements by an Ar ion (1 keV) etching treatment. As shown in a wide-range survey XPS spectrum, a typical prepared sample mainly consisted of vanadium and oxygen, besides the Si signals from the silicon substrate and carbon from the oil residues during pumping by a diffusion pump. The high-resolution XPS (HRXPS) profile of V2p shows the following two major valence states for the vanadium in the pyramidal VO 2 films: V 4+ with a binding energy of 516.0 eV and V 5+ with a binding energy of 517.2 eV [52,54]. The quantitative analysis by peak fitting indicates a composition of VO 2.10 for the pyramidal films, which is evidence of the oxidization of the surface of VO 2 during the oxygen etching treatment. Room temperature Raman spectra of VO2 nanofilms deposited on the silicon substrate before and after oxygen injection. The excitation power for the Raman spectra was fixed at 1 mW.
XPS spectra ( Figure 3) were further conducted to investigate the compositions and valence states of the vanadium in the films. Surface contamination was removed before the measurements by an Ar ion (1 keV) etching treatment. As shown in a wide-range survey XPS spectrum, a typical prepared sample mainly consisted of vanadium and oxygen, besides the Si signals from the silicon substrate and carbon from the oil residues during pumping by a diffusion pump. The high-resolution XPS (HRXPS) profile of V2p shows the following two major valence states for the vanadium in the pyramidal VO2 films: V 4+ with a binding energy of 516.0 eV and V 5+ with a binding energy of 517.2 eV [52,54]. The quantitative analysis by peak fitting indicates a composition of VO2.10 for the pyramidal films, which is evidence of the oxidization of the surface of VO2 during the oxygen etching treatment. To investigate the influence of oxygen on the morphology evolution of the pyramidal-like VO2 films, FE-SEM observations ( Figure 4) were undertaken for the samples treated with oxygen for different times. From the SEM images, one can see that the growth of VO2 nanopyramids in the O2/N2 atmosphere unexpectedly followed the routes of particle top-surface sharpening, size enlarging, and then film structure loosening. Moreover, the growth rate for the oxygen treated sample was much higher than that of the sample annealed in a N2 atmosphere (see the dotted lines in Figure 4d). The particle sizes measured were 71 nm, 110 nm, and 162 nm for the samples treated in oxygen-containing environments for 5 min, 15 min, and 60 min, respectively; while the particle sizes of the samples obtained by annealing in a pure N2 atmosphere were only tens of nanometers. In fact, our further experiments confirmed that for samples treated without oxygen injection (in a N2 atmosphere), significant particle growth could only be observed when annealing at higher To investigate the influence of oxygen on the morphology evolution of the pyramidal-like VO 2 films, FE-SEM observations ( Figure 4) were undertaken for the samples treated with oxygen for different times. From the SEM images, one can see that the growth of VO 2 nanopyramids in the O 2 /N 2 atmosphere unexpectedly followed the routes of particle top-surface sharpening, size enlarging, and then film structure loosening. Moreover, the growth rate for the oxygen treated sample was much higher than that of the sample annealed in a N 2 atmosphere (see the dotted lines in Figure 4d). The particle sizes measured were 71 nm, 110 nm, and 162 nm for the samples treated in oxygen-containing environments for 5 min, 15 min, and 60 min, respectively; while the particle sizes of the samples obtained by annealing in a pure N 2 atmosphere were only tens of nanometers. In fact, our further experiments confirmed that for samples treated without oxygen injection (in a N 2 atmosphere), significant particle growth could only be observed when annealing at higher temperatures (e.g., above 700 • C), indicating that a new growth pattern was involved in the growth of nanopyramidal VO 2 films.
The structure and composition of the pyramid films were further characterized by TEM, high resolution TEM (HRTEM), and fast Fourier transformation (FFT), and the typical results are given in Figure 5. As shown in the low magnification TEM image (Figure 5a), the pyramidal-like morphology for one individual particle can be deduced from the contour stripes. The FFT pattern (Figure 5c) for the total area of Figure 5b indicates that the major structure of the particle is M1 phase VO 2 (in the direction of [010]). However, when the FFT patterns were performed in selected small regions, from the outer side to the inner one of the particle (as shown in Figure 5e-h), patterns belonging to M2 phase VO 2 (in the direction of [200]) gradually appeared and became stronger and stronger. These results agreed well with the XRD and Raman results, further confirming the formation of composite phases in the pyramidal films. temperatures (e.g., above 700 °C), indicating that a new growth pattern was involved in the growth of nanopyramidal VO2 films. . The relationship between the particle size and annealing time for N2 annealing and oxygen assisted annealing is shown in Figure 4d. The particle size for each film was obtained by randomly counting 100 particles in the SEM images. The insets in Figures 4a,b show the high magnification surface images for the corresponding sample, and the inset in Figure 4c shows the enlarged cross-section image of the sample.
The structure and composition of the pyramid films were further characterized by TEM, high resolution TEM (HRTEM), and fast Fourier transformation (FFT), and the typical results are given in Figure 5. As shown in the low magnification TEM image (Figure 5a), the pyramidal-like morphology for one individual particle can be deduced from the contour stripes. The FFT pattern (Figure 5c) for the total area of Figure 5b indicates that the major structure of the particle is M1 phase VO2 (in the direction of [010]). However, when the FFT patterns were performed in selected small regions, from the outer side to the inner one of the particle (as shown in Figures 5e-h), patterns belonging to M2 phase VO2 (in the direction of [200]) gradually appeared and became stronger and stronger. These results agreed well with the XRD and Raman results, further confirming the formation of composite phases in the pyramidal films. , and 60 min (c). The relationship between the particle size and annealing time for N 2 annealing and oxygen assisted annealing is shown in (d). The particle size for each film was obtained by randomly counting 100 particles in the SEM images. The insets in (a,b) show the high magnification surface images for the corresponding sample, and the inset in (c) shows the enlarged cross-section image of the sample. As reported by others [53,55,56], the M2 phase is another monoclinic phase of VO2 with a free energy close to that of M1, which can be stabilized by doping [55] or compressive uniaxial stress in the R-[110] direction [56] (equivalent to a tensile strain along the c axis in R phase or a axis in the M1 phase). Considering the XRD results (Figure 1) where the diffraction patterns of M1 phase VO2 (011) (or R phase VO2 (110)) shifted toward high angles, a compressing stress along the R-[110] (or As reported by others [53,55,56], the M2 phase is another monoclinic phase of VO 2 with a free energy close to that of M1, which can be stabilized by doping [55] or compressive uniaxial stress in the R-[110] direction [56] (equivalent to a tensile strain along the c axis in R phase or a axis in the M1 phase). Considering the XRD results (Figure 1) where the diffraction patterns of M1 phase VO 2 (011) (or R phase VO 2 (110)) shifted toward high angles, a compressing stress along the R-[110] (or M1-[011]) direction can be expected, which will be favorable for stabilizing the metastable M2 phase. Moreover, the M2 phase has also been reported to be related with the incorporation of V 5+ ions in the crystal lattices (by lower valence Cr 3+ doping [55], or non-stoichiometric engineering caused by annealing in an oxygen-rich condition [57]). From the high-resolution TEM image in Figure 5b, we could see a distinct thin transition layer on the surface of the pyramidical VO 2 nanoparticle, which had a lower contrast and less rigid lattice fringes when compared with the inside VO 2 crystals, and the FFT pattern for this layer indicated the structure of M1 phase VO 2 . Considering the special oxygen treatment and the XPS results, this thin layer, especially for the outer parts, should be composed of slightly oxidized vanadium-oxide species, which works as an intermediate layer between the oxygen environment and the inside VO 2 crystals in the annealing process.
When oxygen is injected into the vacuum tube, the top surfaces of the VO 2 nanoparticles are expected initially to react with the excess O 2 to form oxygen-rich vanadium oxides. The most far-gone form of these oxides is expected to be the highest valence state oxide of V 2 O 5 (T m , V 2 O 5 = 690 • C), which is highly volatile and can easily evaporate from the surface of the nanoparticles at the annealing temperature of 540 • C (the reaction could be expressed as VO 2 In fact, evaporated V 2 O 5 residues with a characteristic orange color can even be observed at the cold end of the quartz tube. The samples obtained with a higher oxygen injection content in our further experiments also confirmed the existence of V 2 O 5 crystals surrounding the VO 2 particles (to be discussed in the following section). An oxygen-related selective etching growth model can be described for the formation of a pyramidal film morphology.
As shown in Scheme 3, when oxygen is injected at a high temperature, the top surfaces of the VO 2 nanoparticles react immediately with the excess oxygen to form a thin layer of oxygen-rich vanadium oxides, and the ultimate oxide of V 2 O 5 in the thin layers evaporates partly into the gas phase, which causes the etching of the films. More importantly, due to the diversity in the active energies for different facets of VO 2 , when the oxygen is injected in a controlled low amount (as discussed for the S-0.12k sample), the reaction speed for different facets will vary. Specifically, because the monoclinic facet (011) is the lowest energy facet in VO 2 , the other reactive facets may react (or etch) preferentially with O 2 to form volatile V 2 O 5 and leave the particle surface. This can cause the preservation of VO 2 (011) after etching, which agrees well with the observation that the (011) M1 orientation is highly preferred after oxygen etching. (to be discussed in the following section). An oxygen-related selective etching growth model can be described for the formation of a pyramidal film morphology. As shown in Scheme 3, when oxygen is injected at a high temperature, the top surfaces of the VO2 nanoparticles react immediately with the excess oxygen to form a thin layer of oxygen-rich vanadium oxides, and the ultimate oxide of V2O5 in the thin layers evaporates partly into the gas phase, which causes the etching of the films. More importantly, due to the diversity in the active energies for different facets of VO2, when the oxygen is injected in a controlled low amount (as discussed for the S-0.12k sample), the reaction speed for different facets will vary. Specifically, because the monoclinic facet (011) is the lowest energy facet in VO2, the other reactive facets may react (or etch) preferentially with O2 to form volatile V2O5 and leave the particle surface. This can cause the preservation of VO2 (011) after etching, which agrees well with the observation that the (011) M1 orientation is highly preferred after oxygen etching. However, for samples with higher oxygen injection contents, the above selective etching patterns were gradually eliminated. Figure 6 shows the SEM and TEM images for the samples treated with higher oxygen injection amounts of 0.20 kPa and 0.24 kPa (labeled as S-0.20k and S-0.24k, respectively). We can see from the SEM images that the top surfaces became much smoother for the S-0.20k film, and even evolved into a spherical (or semi-spherical) morphology for the S-0.24k film, indicating a significant suppression over the facet selective etching growth pattern. Furthermore, for the S-0.24k film, the spherical particles showed a distinct melted droplet-like morphology at the bottom of the etched particles, and large amounts of these particles showed a neck-connected structure (in the inset of Figure 6b). However, for samples with higher oxygen injection contents, the above selective etching patterns were gradually eliminated. Figure 6 shows the SEM and TEM images for the samples treated with higher oxygen injection amounts of 0.20 kPa and 0.24 kPa (labeled as S-0.20k and S-0.24k, respectively). We can see from the SEM images that the top surfaces became much smoother for the S-0.20k film, and even evolved into a spherical (or semi-spherical) morphology for the S-0.24k film, indicating a significant suppression over the facet selective etching growth pattern. Furthermore, for the S-0.24k film, the spherical particles showed a distinct melted droplet-like morphology at the bottom of the etched particles, and large amounts of these particles showed a neck-connected structure (in the inset of Figure 6b). TEM and HRTEM images were further taken to investigate the detailed structures of these films, and the results are given in Figure 6. As shown in Figures 6c,d, the thin transition layers that implied a surface oxidization process could be clearly found on the top surface of the particles for the S-0.20k film, although their lattice fringes were distorted more severely, indicating a much deeper oxidization level and more severe lattice defects with a higher oxygen injection content. For the S-0.24k film, the two distinct structures (in Figure 6b), e.g., the upside nanoparticles and the bottom-laying molten droplets, specifically, were also distinguishable in the TEM images ( Figure  6e), and were assigned to the M1 phase VO2 and orthorhombic V2O5 (JCPDS Card File No. 41-1426) by their corresponding selected area electron diffraction patterns (the insets of Figure 6e). Furthermore, the top surface of the connected particles in Figure 6b and its inset were also confirmed as VO2 twin crystals with a M1 phase. Further investigation by XRD characterization indicated that a strongly orientated characteristic along the [011] crystallographic orientation of M1 phase VO2 could be observed for both films, which agreed well with the proposed selective etching mechanism of preserving the stable (011) crystal facets while consuming others during the oxygen treatment. Moreover, when compared with the nanopyramidal films, both films exhibited a smaller FWHM for the (011) diffraction peak, recorded with 0.11° and 0.17° for S-0.20k and S-0.24k, respectively, indicating an increased crystallinity or prompted growth process in this orientation during annealing. However, for the S-0.24k sample, the oxygen concentration applied at 540 °C was TEM and HRTEM images were further taken to investigate the detailed structures of these films, and the results are given in Figure 6. As shown in Figure 6c,d, the thin transition layers that implied a surface oxidization process could be clearly found on the top surface of the particles for the S-0.20k film, although their lattice fringes were distorted more severely, indicating a much deeper oxidization level and more severe lattice defects with a higher oxygen injection content. For the S-0.24k film, the two distinct structures (in Figure 6b), e.g., the upside nanoparticles and the bottom-laying molten droplets, specifically, were also distinguishable in the TEM images (Figure 6e), and were assigned to the M1 phase VO 2 and orthorhombic V 2 O 5 (JCPDS Card File No. 41-1426) by their corresponding selected area electron diffraction patterns (the insets of Figure 6e). Furthermore, the top surface of the connected particles in Figure 6b and its inset were also confirmed as VO 2 twin crystals with a M1 phase. Further investigation by XRD characterization indicated that a strongly orientated characteristic along the [011] crystallographic orientation of M1 phase VO 2 could be observed for both films, which agreed well with the proposed selective etching mechanism of preserving the stable (011) crystal facets while consuming others during the oxygen treatment. Moreover, when compared with the nanopyramidal films, both films exhibited a smaller FWHM for the (011) diffraction peak, recorded with 0.11 • and 0.17 • for S-0.20k and S-0.24k, respectively, indicating an increased crystallinity or prompted growth process in this orientation during annealing. However, for the S-0.24k sample, the oxygen concentration applied at 540 • C was too high to stabilize the VO 2 crystals, and large amounts were oxidized to be orthorhombic V 2 O 5 , which can be seen from the XRD results in Figure 7, in accordance with the TEM observations of the bottom melted droplet-like compounds in Figure 6e. All of the above results indicate that the formation of VO2 nanopyramidal structures represents the interplay of the thermodynamic stability of different vanadium oxide phases and the kinetics of vanadium oxidation. With a low oxygen injection content (as discussed for the S-0.12k sample), the reactive facets located on the surfaces of pristine VO2 crystals react immediately with O2 to form thin oxygen rich transition layers, although some of these layers are gradually oxidized into highly volatile oxides of V2O5, resulting in a selective etching growth pattern for the nanopyramidal structures. When a higher content of oxygen was injected (as discussed for the S-0.20k sample), the selective etching process may have been suppressed to a certain extent because of the rapid reaction with a large amount of surrounding oxygen, which could be deduced from the gradually smoothened particle morphology. Furthermore, when the applied oxygen content was too high (as discussed for the S-0.24k sample), large amounts of V2O5, in the vapor phase and/or melted droplet states at the temperature of 540 °C, could be formed aside from the VO2 crystals. The growth patterns for this kind of nanostructure were followed, partly with the routes of oxygen related etching growth as we proposed, although a liquid involved growth mechanism [58,59] should also be considered. After all, by simply controlling the oxygen injection content as discussed above, highly textured VO2 films, especially the nanopyramidal-like nanostructures that have rarely been reported, can be formed, indicating that the current method by oxygen relation etching growth is of competitive potential for the synthesis of nanostructured VO2 films.
Moreover, to explore potential applications of the oxygen etching treated nanostructured VO2 films, field emission (FE) devices were assembled and investigated. The FE device is one of the key features by which nanostructured materials can eject electrons from their surfaces into the vacuum energy level under high electric fields [60,61], and useful applications such as high performance x-ray sources [62,63] can be produced based on the FE properties. According to reports from Yin and Yu [64], VO2 nanobundles exhibit a high field enhancement factor (β) of 1020-1400, which is better than that of Si [65] and Ge nanowires [66], and is comparable to ZnO nanobullets [48] and CdS nanobelts [49]. Furthermore, the Fermi level of VO2 increases distinctly due to the metal-to-insulator phase transition, resulting in a rapid decrease in the work function [64], which makes VO2 materials quite suitable in applications such as temperature-regulated field emission devices. Comparatively, nanopyramidal-like VO2 nanostructures present promising sharp tips that can facilitate electron emission, so a higher field emission performance can be expected.
The relationship of the field emission current density J with the applied electric field E (J-E curves) for the device based on the VO2 thin films before and after the oxygen-etching treatment (corresponding to sample S-0.12k) is shown in Figure 8. The FE measurements were performed at both 25 and 100 °C, respectively. A schematic of the field emission measurement is given in Figure   Figure 7. XRD results for VO 2 films that were annealed at 540 • C for 1 h with 0.20 kPa (a) and 0.24 kPa (b) oxygen injection. The insets in the XRD patterns indicate the growth mode of the corresponding films.
All of the above results indicate that the formation of VO 2 nanopyramidal structures represents the interplay of the thermodynamic stability of different vanadium oxide phases and the kinetics of vanadium oxidation. With a low oxygen injection content (as discussed for the S-0.12k sample), the reactive facets located on the surfaces of pristine VO 2 crystals react immediately with O 2 to form thin oxygen rich transition layers, although some of these layers are gradually oxidized into highly volatile oxides of V 2 O 5 , resulting in a selective etching growth pattern for the nanopyramidal structures. When a higher content of oxygen was injected (as discussed for the S-0.20k sample), the selective etching process may have been suppressed to a certain extent because of the rapid reaction with a large amount of surrounding oxygen, which could be deduced from the gradually smoothened particle morphology. Furthermore, when the applied oxygen content was too high (as discussed for the S-0.24k sample), large amounts of V 2 O 5 , in the vapor phase and/or melted droplet states at the temperature of 540 • C, could be formed aside from the VO 2 crystals. The growth patterns for this kind of nanostructure were followed, partly with the routes of oxygen related etching growth as we proposed, although a liquid involved growth mechanism [58,59] should also be considered. After all, by simply controlling the oxygen injection content as discussed above, highly textured VO 2 films, especially the nanopyramidal-like nanostructures that have rarely been reported, can be formed, indicating that the current method by oxygen relation etching growth is of competitive potential for the synthesis of nanostructured VO 2 films.
Moreover, to explore potential applications of the oxygen etching treated nanostructured VO 2 films, field emission (FE) devices were assembled and investigated. The FE device is one of the key features by which nanostructured materials can eject electrons from their surfaces into the vacuum energy level under high electric fields [60,61], and useful applications such as high performance X-ray sources [62,63] can be produced based on the FE properties. According to reports from Yin and Yu [64], VO 2 nanobundles exhibit a high field enhancement factor (β) of 1020-1400, which is better than that of Si [65] and Ge nanowires [66], and is comparable to ZnO nanobullets [48] and CdS nanobelts [49]. Furthermore, the Fermi level of VO 2 increases distinctly due to the metal-to-insulator phase transition, resulting in a rapid decrease in the work function [64], which makes VO 2 materials quite suitable in applications such as temperature-regulated field emission devices. Comparatively, nanopyramidal-like VO 2 nanostructures present promising sharp tips that can facilitate electron emission, so a higher field emission performance can be expected.
The relationship of the field emission current density J with the applied electric field E (J-E curves) for the device based on the VO 2 thin films before and after the oxygen-etching treatment (corresponding to sample S-0.12k) is shown in Figure 8. The FE measurements were performed at both 25 and 100 • C, respectively. A schematic of the field emission measurement is given in Figure 8a, and a photograph of a large area of VO 2 nanopyramidal film (diameter of~7.6 cm) obtained by this method is given in Figure 8b. As shown in the J-E curves (Figure 8c,d), the device based on the original VO 2 film (the dark yellow lines) showed no or undetectable FE properties at either low or high temperature, while the device made with the nanopyramidal VO 2 film after oxygen-etching treatment showed a highly enhanced emission currency at both low and high temperatures, showing an exponential profile versus the applied electric fields. The emission currencies measured at a fixed electric field of 6 V/µm were 1.6 × 10 −6 A/cm 2 (or 7.2 × 10 −7 A/cm 2 ) and 1.6 × 10 −3 A/cm 2 (or 1.9 × 10 −5 A/cm 2 ) at the R (or M) state before and after the oxygen-etching treatment, indicating an improvement of nearly 1000 times (or 26 times) in emission currency at the R (or M) state. The increase in emission currency can be expected to originate from the significant changes in film particle morphologies, which can pare away the difficulties of electron field emission for sharp-tipped pyramidal surfaces. Furthermore, the work function of the M1 phase was 0.07 eV, higher than that of the R phase [64], so its improvement in the field emission property was much more significant in the R state after transforming from plain thin films with no FE properties. Nanomaterials 2019, 9, x FOR PEER REVIEW 11 of 15 treatment showed a highly enhanced emission currency at both low and high temperatures, showing an exponential profile versus the applied electric fields. The emission currencies measured at a fixed electric field of 6 V/μm were 1.6 × 10 −6 A/cm 2 (or 7.2 × 10 −7 A/cm 2 ) and 1.6 × 10 −3 A/cm 2 (or 1.9 × 10 −5 A/cm 2 ) at the R (or M) state before and after the oxygen-etching treatment, indicating an improvement of nearly 1000 times (or 26 times) in emission currency at the R (or M) state. The increase in emission currency can be expected to originate from the significant changes in film particle morphologies, which can pare away the difficulties of electron field emission for sharp-tipped pyramidal surfaces. Furthermore, the work function of the M1 phase was 0.07 eV, higher than that of the R phase [64], so its improvement in the field emission property was much more significant in the R state after transforming from plain thin films with no FE properties. The Fowler-Nordheim (F-N) plots of the ln(J/E 2 )-1/E relationship [61] are also shown in the insets of Figures 8c,d. The straight lines fitted in the F-N plots confirmed the electron field emission characters for the pyramidal VO2 film, which could not be observed for the original plain VO2 thin film. By determining the slop of the F-N plot, one can obtain the data of the field enhancement factor (β), estimated to be about 3810 and 1620 for the M and R states, respectively, which were much higher than those of the reported VO2 nanobundles [64], and comparable with high quality ZnO nanoneedles [67], making the pyramidal VO2 films prepared by the present method attractive for FE applications. Moreover, the feasible and large-area available synthetic strategy reported here can also provide an efficient tool to build a variety of interesting nanoscale VO2 surfaces, which is beneficial for VO2 and other transition metal oxide based device applications. The Fowler-Nordheim (F-N) plots of the ln(J/E 2 )-1/E relationship [61] are also shown in the insets of Figure 8c,d. The straight lines fitted in the F-N plots confirmed the electron field emission characters for the pyramidal VO 2 film, which could not be observed for the original plain VO 2 thin film. By determining the slop of the F-N plot, one can obtain the data of the field enhancement factor (β), estimated to be about 3810 and 1620 for the M and R states, respectively, which were much higher than those of the reported VO 2 nanobundles [64], and comparable with high quality ZnO nanoneedles [67], making the pyramidal VO 2 films prepared by the present method attractive for FE applications. Moreover, the feasible and large-area available synthetic strategy reported here can also provide an efficient tool to build a variety of interesting nanoscale VO 2 surfaces, which is beneficial for VO 2 and other transition metal oxide based device applications.

Conclusions
In this paper, we report on an effective wafer-scale oxygen-assisted etching growth strategy for transforming plain VO 2 thin films into highly textured nanopyramidal thin films. The controllable amount of oxygen incorporated during the post annealing process reacted rapidly with the reactive VO 2 planar surface to form volatile V 2 O 5 , which resulted in the effective etching and formation of pyramidal-like VO 2 films. The structural investigation indicated that the VO 2 films had a composite M1/M2 structure at room temperature. Field emission measurements further showed that the etching process could successfully transform planar VO 2 films with no field emission performance into nanopyramidal films with significant field emission performance. The emission currencies at a fixed electric field of 6 V/µm could be improved by 1000 times at the R state before and after the oxygen etching treatment, and the field enhancement factor β reached 3810 and 1620 for the M and R states, respectively, which was quite high amongst all reports. All these results indicate that the technique developed here is an efficient surface building and reconstruction strategy for VO 2 -based thin films with potential applications in high-performance FE devices.