Advancements, Challenges and Prospects of Chemical Vapour Pressure at Atmospheric Pressure on Vanadium Dioxide Structures

Vanadium (IV) oxide (VO2) layers have received extensive interest for applications in smart windows to batteries and gas sensors due to the multi-phases of the oxide. Among the methods utilized for their growth, chemical vapour deposition is a technology that is proven to be industrially competitive because of its simplicity when performed at atmospheric pressure (APCVD). APCVD’s success has shown that it is possible to create tough and stable materials in which their stoichiometry may be precisely controlled. Initially, we give a brief overview of the basic processes taking place during this procedure. Then, we present recent progress on experimental procedures for isolating different polymorphs of VO2. We outline emerging techniques and processes that yield in optimum characteristics for potentially useful layers. Finally, we discuss the possibility to grow 2D VO2 by APCVD.


General Information
CVD is a practical method of atomistic or near atomistic deposition having the ability to synthesize well-controlled dimensions and structures at reasonably low temperatures, high purity and in multiple formats such as single layer, multi-layer, composite and finally functional coatings. In its simplest incarnation, CVD encompasses a single precursor gas flowing into a chamber containing the substrate to be coated. Although, there are exceptions, the vapour of the reactive compound, usually an easily volatilized liquid or in some cases a solid, is sublimed directly and transported to the reaction zone by a carrier gas. A thin film is then deposited by chemical reaction or decomposition of the gas mixture on the substrate surface or in its vicinity at a defined temperature.
The precursors used within a variety of CVD techniques can be single source or dual source in origin. Single source precursors contain all the groups/elements required for successive thin film production. On the other hand, dual source precursors involve the interaction between multiple precursors for the synthesis of thin films. In each case, it is vital for production of thin films to deliver the gas phase precursors with a carrier gas. The most common carrier gases are N 2 , He or Ar, especially when highly reactive or pyrophoric reactants are used and in some cases, reactions entail an energy input from the carrier gas, e.g., H 2 or O 2 enrichment.
Reactor systems in CVD processes must allow controlled transport of the reactant and diluent gases to the reaction zone, maintain a defined substrate temperature and safely remove the gaseous Reactor systems in CVD processes must allow controlled transport of the reactant and diluent gases to the reaction zone, maintain a defined substrate temperature and safely remove the gaseous by-products. These functions should be fulfilled with sufficient control and maximal effectiveness, which requires optimum engineering design and automation. The reactor in which the thin film deposition actually takes place is the essential part of the system and must be designed according to the specific chemical process parameters. To coat layers using Chemical Vapour Deposition at Atmospheric Pressure (APCVD), four basic types can be classified according to their gas flow and operation principles: 1. Horizontal tube displacement flow type. 2. Rotary vertical batch type. 3. Continuous-deposition type using premixed gas flow. 4. Continuous-deposition type employing separate gas streams.

CVD Processes
Any CVD process including APCVD involves the subsequent operations. First, the reacting gas is directed into the reactor. The gas moves towards its thermal equilibrium temperature and composition through gas-phase collisions and reactions. Near-equilibrated species are then transported to the reaction surface, the surface chemical reactions commence and the thin film is formed. The processes are summarized below (Figure 1) [1]: 1. Creation of active gaseous reactants. 2. Transport of the precursor to the CVD reactor. 3. Decomposition of gas phase precursor to remove gaseous by-products and grow reactive intermediates. 4. Gaseous reactants transportation onto substrate area. 5. Surface diffusion for nucleation and thin film growth. 6. Desorption of by-products and mass transport away from active reactive zone.

APCVD
A schematic presentation of the APCVD system is shown in Figure 2. It is designed with no joints in all outlet lines to avoid blocking. A flow of inert gas, usually nitrogen, is passed through the apparatus during all operations. The amount of the precursor delivered into the reactor is calculated from the Equation (1) a VP F 760 VP 24.4 (1)

APCVD
A schematic presentation of the APCVD system is shown in Figure 2. It is designed with no joints in all outlet lines to avoid blocking. A flow of inert gas, usually nitrogen, is passed through the apparatus during all operations. The amount of the precursor delivered into the reactor is calculated from the Equation (1) where a, is the amount of precursor (mol min −1 ), VP, is the vapour pressure of precursor at the bubbler's temperature (mm Hg), F, is the nitrogen flow rate through the bubbler (L min −1 ) and 24.4, is a constant for the molar volume of an ideal gas at standard temperature and pressure (L mol −1 ). In a typical APCVD experiment, once all temperatures are stabilized over time, the N 2 is passed through the bubblers and then the precursor gas flow rate is directed into the mixing chamber where the mixture begins in order to be utilized before entering the reaction chamber for the deposition to take place. Once the allotted time is complete, the precursor bubbler is closed. The reactor heater is turned off and the substrate is allowed to cool down under an atmosphere of N 2 . Ideally, the carrier gas inlet flows should be fully saturated with precursor vapour; this can be achieved with knowledge of the precursor volatility and vapour pressure and then controlled by the carrier gas flow and bubbler temperature using flow meters and heating jackets. where a, is the amount of precursor (mol min −1 ), VP, is the vapour pressure of precursor at the bubbler's temperature (mm Hg), F, is the nitrogen flow rate through the bubbler (L min −1 ) and 24.4, is a constant for the molar volume of an ideal gas at standard temperature and pressure (L mol −1 ). In a typical APCVD experiment, once all temperatures are stabilized over time, the N2 is passed through the bubblers and then the precursor gas flow rate is directed into the mixing chamber where the mixture begins in order to be utilized before entering the reaction chamber for the deposition to take place. Once the allotted time is complete, the precursor bubbler is closed. The reactor heater is turned off and the substrate is allowed to cool down under an atmosphere of N2. Ideally, the carrier gas inlet flows should be fully saturated with precursor vapour; this can be achieved with knowledge of the precursor volatility and vapour pressure and then controlled by the carrier gas flow and bubbler temperature using flow meters and heating jackets.

Vanadium Oxides
The binary Vanadium-Oxygen phase diagram consists of a large number of phases between V2O3 and VO2 of the form VnO2n−1 commonly known as the Magneli phases [2] that exhibit distinctive electrical and optical properties. The variety of Vanadium-Oxygen stoichiometries emerges from the ability of vanadium atoms to adopt multiple oxidation states, which consequently results in synthetic challenges to control the structure of the materials [3].
More than ten kinds of crystalline phases of VO2 have been reported elsewhere, whereas some examples are monoclinic VO2 (M), tetragonal VO2 (R) and several metastable forms of VO2 (A), VO2 (B) and VO2 (C) [4]. Among these phases, only the rutile VO2 (R/M) phase undergoes a fully reversible metal insulator transition at a critical temperature (Tc) [1], where an abrupt alteration in optical and electronic properties is observed making it ideal for optoelectronic switches [5], memristors [6], artificial neuron networks [7,8] and intelligent window coatings [9,10].
The high temperature phase (T > Tc), has a tetragonal type structure characterized by chains of edge sharing [VO6] octahedral along the c-axis with equidistant vanadium atoms (V-V = 2.88 Å) [11]. While, the low temperature structure involves V 4+ -V 4+ pairing with alternate shorter (0.265 nm) and longer (0.312 nm) V 4+ -V 4+ distances along the a-axis and tilting with respect to the rutile c-axis [11]. At 25 °C, the lattice has unit cell parameters; a = 5.75 Å, b = 4.52 Å, c = 5.38 Å and β = 122.60° [12]. The lattice is the result of the distortion occurring at the high temperature metallic tetragonal phase.
The mechanism of metal insulator transition in VO2 has been investigated through computational, experimental and theoretical studies [13][14][15]. Nevertheless, the mechanism of the transition remains unresolved, since the VO2 phases exhibit diverse lattice structures but have analogous electronic properties.

Vanadium Oxides
The binary Vanadium-Oxygen phase diagram consists of a large number of phases between V 2 O 3 and VO 2 of the form V n O 2n−1 commonly known as the Magneli phases [2] that exhibit distinctive electrical and optical properties. The variety of Vanadium-Oxygen stoichiometries emerges from the ability of vanadium atoms to adopt multiple oxidation states, which consequently results in synthetic challenges to control the structure of the materials [3].
More than ten kinds of crystalline phases of VO 2 have been reported elsewhere, whereas some examples are monoclinic VO 2 (M), tetragonal VO 2 (R) and several metastable forms of VO 2 (A), VO 2 (B) and VO 2 (C) [4]. Among these phases, only the rutile VO 2 (R/M) phase undergoes a fully reversible metal insulator transition at a critical temperature (T c ) [1], where an abrupt alteration in optical and electronic properties is observed making it ideal for optoelectronic switches [5], memristors [6], artificial neuron networks [7,8] and intelligent window coatings [9,10].
The high temperature phase (T > T c ), has a tetragonal type structure characterized by chains of edge sharing [VO 6 ] octahedral along the c-axis with equidistant vanadium atoms (V-V = 2.88 Å) [11]. While, the low temperature structure involves V 4+ -V 4+ pairing with alternate shorter (0.265 nm) and longer (0.312 nm) V 4+ -V 4+ distances along the a-axis and tilting with respect to the rutile c-axis [11]. At 25 • C, the lattice has unit cell parameters; a = 5.75 Å, b = 4.52 Å, c = 5.38 Å and β = 122.60 • [12]. The lattice is the result of the distortion occurring at the high temperature metallic tetragonal phase.
The mechanism of metal insulator transition in VO 2 has been investigated through computational, experimental and theoretical studies [13][14][15]. Nevertheless, the mechanism of the transition remains unresolved, since the VO 2 phases exhibit diverse lattice structures but have analogous electronic properties.

Advancements
There have been numerous studies on the VO 2 grown by APCVD since Maruyama and Ikuta utilized vanadium (III) acetylacetonate (V(acac) 3 ) as a single-precursor to deposit polycrystalline pure VO 2 films on fused quartz and sapphire single crystals [16]. In this review article, we will focus on the progress taking place during the last four years regarding the control of the processing parameters to isolate the VO 2 phases strengthening the functional properties of APCVD VO 2 layers.
The growth of amorphous pure and tungsten doped VO 2 coatings is possible on SnO 2 -precoated glass substrates using vanadyl (V) triisopropoxide (VO(OC 3 H 7 ) 3 ) as single-precursor [9,10,17,18]. It is interesting to note that the presence of tungsten in the lattice of VO 2 changed the surface morphology to worm-like ( Figure 3) from granular structure [9]. This approach has several advantages including the high vapour pressure of the precursor (i.e., decomposition over time and transport of unknown species are prevented). Additionally, the operations are simplified by removing the commonly necessary oxygen source, which is usually provided either in the form of pure gas or from an extra bubbler through H 2 O or alcohol. Vanadyl (IV) acetylacetonate (VO(acac) 2 ) along with propanol, ethanol and O 2 gas as oxygen sources is accomplished to grow VO 2 of different crystalline orientations [19,20]. The a-axis textured monoclinic is enhanced with propanol and ethanol, while the 022-oriented single phase V 2 is obtained with O 2 gas possessing grains (a-axis coatings) and agglomeration of grains forming rod-like structures (002-oriented phases). On controlling the oxygen gas flow rate (Figure 4), isolated monoclinic and metastable VO 2 phases can also be achieved using VO(acac) 2 as vanadium precursor on flexible [21] and SnO 2 -precoated glass substrates [22].

Advancements
There have been numerous studies on the VO2 grown by APCVD since Maruyama and Ikuta utilized vanadium (III) acetylacetonate (V(acac)3) as a single-precursor to deposit polycrystalline pure VO2 films on fused quartz and sapphire single crystals [16]. In this review article, we will focus on the progress taking place during the last four years regarding the control of the processing parameters to isolate the VO2 phases strengthening the functional properties of APCVD VO2 layers.
The growth of amorphous pure and tungsten doped VO2 coatings is possible on SnO2-precoated glass substrates using vanadyl (V) triisopropoxide (VO(OC3H7)3) as single-precursor [9,10,17,18]. It is interesting to note that the presence of tungsten in the lattice of VO2 changed the surface morphology to worm-like ( Figure 3) from granular structure [9]. This approach has several advantages including the high vapour pressure of the precursor (i.e., decomposition over time and transport of unknown species are prevented). Additionally, the operations are simplified by removing the commonly necessary oxygen source, which is usually provided either in the form of pure gas or from an extra bubbler through H2O or alcohol. Vanadyl (IV) acetylacetonate (VO(acac)2) along with propanol, ethanol and O2 gas as oxygen sources is accomplished to grow VO2 of different crystalline orientations [19,20]. The a-axis textured monoclinic is enhanced with propanol and ethanol, while the 022-oriented single phase VO2 is obtained with O2 gas possessing grains (a-axis coatings) and agglomeration of grains forming rod-like structures (002-oriented phases). On controlling the oxygen gas flow rate (Figure 4), isolated monoclinic and metastable VO2 phases can also be achieved using VO(acac)2 as vanadium precursor on flexible [21] and SnO2-precoated glass substrates [22].
SnO2 was chosen as a substrate due to the similar crystalline structure with VO2, which can act as a template for the growth of rutile VO2 and promote the crystallinity of the oxide [23].

Advancements
There have been numerous studies on the VO2 grown by APCVD since Maruyama and Ikuta utilized vanadium (III) acetylacetonate (V(acac)3) as a single-precursor to deposit polycrystalline pure VO2 films on fused quartz and sapphire single crystals [16]. In this review article, we will focus on the progress taking place during the last four years regarding the control of the processing parameters to isolate the VO2 phases strengthening the functional properties of APCVD VO2 layers.
The growth of amorphous pure and tungsten doped VO2 coatings is possible on SnO2-precoated glass substrates using vanadyl (V) triisopropoxide (VO(OC3H7)3) as single-precursor [9,10,17,18]. It is interesting to note that the presence of tungsten in the lattice of VO2 changed the surface morphology to worm-like ( Figure 3) from granular structure [9]. This approach has several advantages including the high vapour pressure of the precursor (i.e., decomposition over time and transport of unknown species are prevented). Additionally, the operations are simplified by removing the commonly necessary oxygen source, which is usually provided either in the form of pure gas or from an extra bubbler through H2O or alcohol. Vanadyl (IV) acetylacetonate (VO(acac)2) along with propanol, ethanol and O2 gas as oxygen sources is accomplished to grow VO2 of different crystalline orientations [19,20]. The a-axis textured monoclinic is enhanced with propanol and ethanol, while the 022-oriented single phase VO2 is obtained with O2 gas possessing grains (a-axis coatings) and agglomeration of grains forming rod-like structures (002-oriented phases). On controlling the oxygen gas flow rate (Figure 4), isolated monoclinic and metastable VO2 phases can also be achieved using VO(acac)2 as vanadium precursor on flexible [21] and SnO2-precoated glass substrates [22].
SnO2 was chosen as a substrate due to the similar crystalline structure with VO2, which can act as a template for the growth of rutile VO2 and promote the crystallinity of the oxide [23].   SnO 2 was chosen as a substrate due to the similar crystalline structure with VO 2 , which can act as a template for the growth of rutile VO 2 and promote the crystallinity of the oxide [23].

Challenges
A comparative study among VO(acac) 2 and VCl 4 , the most utilized vanadium precursors for APCVD VO 2 , indicated that the transport rate of VO(acac) 2 is lower than VCl 4 [24]. This can be handled by increasing the temperature and the N 2 flow rate in the bubbler. However, this is not anticipated because the precursor may decompose over time leading to irreproducible delivery rates and the transport of unknown species. On the other hand, VCl 4 is highly reactive with H 2 O resulting in inhomogeneous films [25]. A new approach uses the ethyl acetate (EtAc) as an excellent oxygen precursor resulting in the precise control of the growth rate and porosity of the films after the optimization of VCl 4 /EtAc system [26]. A route to improve this system involves the combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) to determine the effect of the substrate choice on the VO 2 formation for functional properties such as thermochromism [27]. It is then possible to grow VO 2 ( Figure 5) onto substrates that induce lattice matching (SnO 2 ) or others (F-doped SnO 2 ) that promote a destabilization of V 4+ ions and a further increase in V 5+ deteriorating the functional properties ( Figure 6).

Challenges
A comparative study among VO(acac)2 and VCl4, the most utilized vanadium precursors for APCVD VO2, indicated that the transport rate of VO(acac)2 is lower than VCl4 [24]. This can be handled by increasing the temperature and the N2 flow rate in the bubbler. However, this is not anticipated because the precursor may decompose over time leading to irreproducible delivery rates and the transport of unknown species. On the other hand, VCl4 is highly reactive with H2O resulting in inhomogeneous films [25]. A new approach uses the ethyl acetate (EtAc) as an excellent oxygen precursor resulting in the precise control of the growth rate and porosity of the films after the optimization of VCl4/EtAc system [26]. A route to improve this system involves the combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) to determine the effect of the substrate choice on the VO2 formation for functional properties such as thermochromism [27]. It is then possible to grow VO2 ( Figure 5) onto substrates that induce lattice matching (SnO2) or others (F-doped SnO2) that promote a destabilization of V 4+ ions and a further increase in V 5+ deteriorating the functional properties ( Figure 6).
Furthermore, monoclinic VO2 exhibits poor adhesion and is chemically susceptible to attack, restricting the use as solar control coating. In that respect, multi-functional, robust APCVD VO2/SiO2/TiO2 films on glass substrates demonstrates excellent solar modulation properties, high transparency and resistance to abrasion compared to single VO2 films of the same thickness [28].

Challenges
A comparative study among VO(acac)2 and VCl4, the most utilized vanadium precursors for APCVD VO2, indicated that the transport rate of VO(acac)2 is lower than VCl4 [24]. This can be handled by increasing the temperature and the N2 flow rate in the bubbler. However, this is not anticipated because the precursor may decompose over time leading to irreproducible delivery rates and the transport of unknown species. On the other hand, VCl4 is highly reactive with H2O resulting in inhomogeneous films [25]. A new approach uses the ethyl acetate (EtAc) as an excellent oxygen precursor resulting in the precise control of the growth rate and porosity of the films after the optimization of VCl4/EtAc system [26]. A route to improve this system involves the combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) to determine the effect of the substrate choice on the VO2 formation for functional properties such as thermochromism [27]. It is then possible to grow VO2 ( Figure 5) onto substrates that induce lattice matching (SnO2) or others (F-doped SnO2) that promote a destabilization of V 4+ ions and a further increase in V 5+ deteriorating the functional properties ( Figure 6).
Furthermore, monoclinic VO2 exhibits poor adhesion and is chemically susceptible to attack, restricting the use as solar control coating. In that respect, multi-functional, robust APCVD VO2/SiO2/TiO2 films on glass substrates demonstrates excellent solar modulation properties, high transparency and resistance to abrasion compared to single VO2 films of the same thickness [28].  Furthermore, monoclinic VO 2 exhibits poor adhesion and is chemically susceptible to attack, restricting the use as solar control coating. In that respect, multi-functional, robust APCVD VO 2 /SiO 2 /TiO 2 films on glass substrates demonstrates excellent solar modulation properties, high transparency and resistance to abrasion compared to single VO 2 films of the same thickness [28].

Prospects and Outlook
In the field of APCVD VO 2 , the altering of the processing parameters and the manipulation of the substrate surface is just starting to be understood. New evolvements in experimental procedures such as the utilization of single vanadium precursor and the oxygen source have addressed APCVD routes in isolating the intrinsic material properties. There are numerous exciting challenges in developing VO 2 with functional properties, which expand our understanding of the underlying chemistry and potentially lead to anticipated applications.
Two-dimensional (2D) VO 2 can also be possible by APCVD through Computational Fluid Dynamics (CFD) simulations. CFD simulations are performed to evaluate and define the whole experimental process, before, while and after the experimental procedure isolating the intrinsic material properties (Figure 7). CFD results of exhaust and quartz tube presented the simulation procedure regarding the flow rates and the temperature distribution along the boundaries of the metallic parts. The flow rate of N 2 was set at 0.1 L min −1 and the temperature in the inner boundaries was at 300 • C. Every aspect of the APCVD process is simulated to approach the optimal characteristics of the oxide in tandem to the surface to be deposited. Prospects in developing the growth of high-quality large-area materials with well-defined sizes, high dispersion and excellent control on layer thickness will then appear. The potential impact is illustrated by considering the exploitation possibilities of the high-performance materials by APCVD to create advanced devices for practical applications.

Prospects and Outlook
In the field of APCVD VO2, the altering of the processing parameters and the manipulation of the substrate surface is just starting to be understood. New evolvements in experimental procedures such as the utilization of single vanadium precursor and the oxygen source have addressed APCVD routes in isolating the intrinsic material properties. There are numerous exciting challenges in developing VO2 with functional properties, which expand our understanding of the underlying chemistry and potentially lead to anticipated applications.
Two-dimensional (2D) VO2 can also be possible by APCVD through Computational Fluid Dynamics (CFD) simulations. CFD simulations are performed to evaluate and define the whole experimental process, before, while and after the experimental procedure isolating the intrinsic material properties (Figure 7). CFD results of exhaust and quartz tube presented the simulation procedure regarding the flow rates and the temperature distribution along the boundaries of the metallic parts. The flow rate of N2 was set at 0.1 L min −1 and the temperature in the inner boundaries was at 300 °C. Every aspect of the APCVD process is simulated to approach the optimal characteristics of the oxide in tandem to the surface to be deposited. Prospects in developing the growth of high-quality large-area materials with well-defined sizes, high dispersion and excellent control on layer thickness will then appear. The potential impact is illustrated by considering the exploitation possibilities of the high-performance materials by APCVD to create advanced devices for practical applications.