Interface-Driven Thermo-Electric Switching Performance of VO$^+$ Diffused Soda-Lime Glass

Strongly confined NaVO$^+$ segregation and its thermo-responsive functionality at the interface between simple sputter-deposited amorphous vanadium oxide thin films and soda-lime glass was substantiated in the present study by in-situ temperature-controlled Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The obtained ToF-SIMS depth profiles provided unambiguous evidence for a reversible transformation that caused systematic switching of the NaVO$^+$/ Na$^+$ and Na$^+$/ VO$^+$ intensities upon cycling the temperature between 25 $^\circ$C and 340 $^\circ$C. Subsequently, NaVO complexes were found to be reversibly formed (at 300 $^\circ$C) in vanadium oxide diffused glass, leading to thermo-responsive electrical behaviour of the thin film glass system. This new segregation -- and diffusion-dependent multifunctionality of NaVO$^+$ -- points towards applications as an advanced material for thermo-optical switches, in smart windows or in thermal sensors.


Introduction
Vanadium oxide is one of the most promising and deeply studied materials in smart and energy materials applications. Recently, the thermoelectric behaviour of vanadium oxides has been recognized as highly attractive concerning meeting energy efficiency requirements.
Recent reports on the Seebeck coefficient of a vanadium pentoxide (V2O5) thin film on silica glass (−680 µV K −1 ) 1 and annealed bulk V2O5 (−618 µV K −1 ) 2 are highly promising for the development of a V2O5 based thermoelectric generator. Additionally, there has been an enormous focus on, and wide studies of V2O5 thin films for micro and thin-film battery applications due to their promising higher specific capacity 3 . In the attempt to replace Li-ion batteries, current research is focused mainly on Na ion-based solutions due to the abundant availability and low-cost of sodium. Thus, in addition to the studies on the Li-ion insertion and extraction mechanisms 4,5 , the Na-ion insertion and extraction mechanisms for vanadium oxidebased systems is also being investigated 6,7 . The intercalation reaction of Na into the NaVOx product layer under a chemical gradient is being intensely studied for vanadium oxide-based materials in Na ion battery applications [8][9][10] .
Although vanadium pentoxide is being explored for many functional applications, its metal-insulator transition behaviour is highly debated. Various investigations have reported significantly different temperatures at which this transition is observed, viz. ~ 127 °C 11 , 260 °C 12 or 338 °C 13 . Vanadium pentoxide (V2O5) is observed to exhibit distinguishable and unique functional properties when it is deposited on different type of substrates [13][14][15] . Moreover, vanadium containing amorphous semiconducting glasses are also being extensively studied, for example as smart functioning glass 16 . However, there is no fundamental understanding of the mechanisms responsible for these exciting temperature dependent properties of V2O5 or their temperature dependence. For a final break-through commercialization, cost-effective and simple processing techniques must be developed along with an in-depth understanding of the material system for the entire temperature interval under consideration.
The present study focuses on the functional behaviour of cost-effectively sputter deposited vanadium oxide thin films on a commercial glass substrate such as soda-lime glass and a detailed investigation of the temperature-dependent reversible interactions of native elements in the amorphous vanadium pentoxide/soda-lime glass system by in-situ temperaturecontrolled Time-of-Flight Secondary-Ion Mass Spectrometry (ToF-SIMS). Additionally, temperature-dependent conductivity measurements were carried out to understand the thermoelectric behaviour of the V2O5/soda-lime glass.

Materials and Methods
Thin films of V2O5 were deposited on glass substrates (Thermo Fisher Scientific Silicabased microscope slides) using a vanadium metal target (99.95% purity) with a room temperature reactive sputtering process. The DC Magnetron sputtering instrument (BesTech) was operated at a power of 40 W for the current study. The vacuum recipient was pre-evacuated to a pressure < 7.5× 10-8 mbar and the working pressure was adjusted to 5× 10-3 mbar with an O to Ar ratio of (2:3) in terms of partial pressures. The thicknesses of the V2O5 thin films were measured by a surface profilometer (Bruker DektakXT). The thicknesses were averaged over a minimum of 5 measurements on at least 2 similarly processed samples to approx. 80 nm.
The ToF-SIMS depth profiles were measured using a custom instrument which is largely equivalent to the IONTOF M6. The spectrometer was run at an operating pressure of <10 −8 mbar. The depth profiles were measured in a dual-beam mode, using a 30 keV Bi 3+ primary ion source with a pulsed current of 0.02 pA for analysis and a 1 keV Ar source with a current of 200 nA for sputtering. Analysis was performed over a 100×100 µm 2 area and sputtering was performed over a 500×500 µm 2 area. The sputtering depth was calibrated via the known film thickness and assuming a constant sputtering rate. Since we are not quantifying any diffusion properties of the elements, this approximation is suitable for the present purposes.
The depth profiles are shown as dead-time corrected absolute intensities without normalization and averaged over the analysed area, assuming a lateral homogeneity of the samples over the entire area of the thin film on the glass substrate.
An Axis-Ultra spectrometer (Kratos, Manchester, UK) with charge neutralizer was used to obtain X-ray photoelectron spectra with monochromatic Al Kα radiation (hν = 1486.6 eV).
Acceleration voltage and emission were set as 12 kV and 10 mA, respectively. An Impedance spectrometer (Agilent 4192A LF impedance analyser) were used to analyse the ac conductivity changes of the thin film in the temperature range 25 ˚C to 340 ˚C. For this reason, spring loaded Au contacts were configured and tested. The frequency range of 5 Hz to 2 MHz was utilized with multiple intervals. The AC voltage was set to 1.0 V for all measurements. Figure 1a show the depth profile and mass spectrum (See Extended Data Figure 1) of soda-lime glass, respectively, substantiating the presence of Na + as a major constituent of the glass along with minor K + and Ca + additives. The intensities of Na + are found to be almost constant through the sputtered glass substrate.

Results and Discussion
The vanadium oxide thin film was sputter deposited on soda-lime glass as explained in the Materials and Methods section. Figure 1b shows the ToF-SIMS depth profile through V2O5 analysed at 25 °C. The signals for VO + , Na + , NaVO + and Si + are shown. The ToF-SIMS spectrum also contains signals from V + and a wide range of VxOy + cluster ions in the V2O5 layer as well as traces of K + , Ca + and Al + in the glass substrate. These signals are not included in Figure 1b for the sake of clarity.
The ToF-SIMS depth profile shows the presence of Na + through the entire thickness (~80 nm) of the thin film already in the as-deposited state. This is likely due to diffusion of Na into the amorphous vanadium oxide thin film during the sputter deposition with the soda-lime glass substrate acting as a source for Na + . The surface is found to be enriched with oxygen, suggesting the presence of higher oxide states of vanadium along with minute trace amounts of Na + . Further, the intensity of Na + is negligible at the surface, which is also supported by XPS analysis. Extended clearly reveals the presence of V and O species in the deposited thin films along with low levels of C and N, typical of organic contaminants commonly observed on samples that have been exposed to ambient air 17 . No traces of Na were revealed on the XPS survey spectra as shown in Fig. S2 a. The interface between the thin film and the glass substrate is characterized by enrichment/segregation of Na + and the formation of NaVO + (see Figure 1). The NaVO + profile is roughly Gaussian shaped and an inverse Gaussian-like peak is observed for V + at the film/substrate interface. Na + diffusion is not expected to alter the oxidation state of V by the negative charge-feedback mechanism for the transition metal oxides as explained by Raebiger et al. 18 Extended Figure 2b shows the core level XPS spectrum of V and O. The binding energies of 517.3 and 530.2 eV were detected for V2p3/2 and O1s, respectively, in the asprepared thin film V2O5 / glass. The binding energy splitting between the V2p3/2 and V2p1/2 states was found to be 7.2 eV. The peak values are in good agreement with the previously reported data for stoichiometric V2O5 at room temperature within an experimental uncertainty of about ±0.1eV 13 . Since the core level peaks of O1s and V2p3/2 are narrow and represent a single oxidation state, the peaks were not deconvoluted further to quantify minor oxidation states.
Interestingly, the ToF-SIMS depth profile showed unambiguous evidence for the occurrence of (sputtering-induced) diffusion of V + and its oxides into the glass substrate. The ToF-SIMS mass spectrum of soda-lime glass shown in Extended Figure 1 substantiates the absence of V in the substrate prior to the film deposition. Hence, diffusion of V + into the glass and the formation of NaVO + near/at the interface occurs either during or after the sputter deposition of the V2O5 thin film.
Diffusion of Na + from silica-based glass into a vanadium oxide thin film has been reported previously 19 . On the other hand, soda-lime glass as a substrate for VO2 coating was very rarely utilized 20 . It was found to be highly challenging to optimize the coating procedure for thermochromic applications 21   In Figure 2, the ToF-SIMS depth profiles recorded for VO + , Na + and NaVO + during thermal cycling are shown for two limiting temperatures of 25 °C and 340 ˚C. An enrichment of Na + ions is prominent at the V2O5 / glass interface at temperatures above 300 ˚C (See Extended Data Figure 4). During heating, the intensity of V and its oxides is seen to increase, their distribution is flattened out and the sputter-induced "gap" at the interface disappears upon heating to 340 ˚C. The intensity of the NaVO + signal is also reduced at the interface at 340 ˚C as shown in Fig. 2 (c) and it appears again when the temperature is decreased to 25 °C.
Simultaneously, an interface-related gap in the VO + distribution is formed when the temperature is lowered below 300 °C.
Extended Data Figure 4 substantiates a systematic and reversible evolution of the Na + , NaVO + and VO + signals. Remarkably, the initial (as-sputtered) intensity levels are retained upon cooling to 25 ˚C. Simultaneously, the NaVO + interface and the substrate-related peaks disappear. The sample was cycled three times between 25 ˚C and 340 ˚C to ensure the reversibility of the aforementioned reactions presented in Fig. 3 (a-c).

Figure 2.
In-situ ToF-SIMS depth profiles of V2O5 on glass, comparing three subsequent temperature cycles between 25 ˚C and 340 ˚C and analysing the distribution of a) VO + , b) Na + and c) NaVO + Figure 3 shows the 3D maps corresponding to the distributions of VO + , Na + and NaVO + at 25 ˚C and 340 ˚C, respectively. It is evident that the expected reaction Na + VO  NaVO occurs uniformly in planes parallel to the film/substrate interface for the whole ToF-SIMS sputtered area of 100×100 µm 2 .

Advanced Functionalities
Three unique temperature-dependent reaction mechanisms are found in the V2O5/sodalime glass system.
(i) Change of electrical conductivity of the V2O5 / glass system In the present study, the temperature dependent conductivity increments (Figure 5 a, b) and the ToF-SIMS results elucidating reversible temperature-dependent solid-state reactions in the V2O5 thin film /soda-lime glass provide evidence of the possibility for the fabrication of a thermoelectric generator 16,28 or smart window applications 29 , as have been preliminarily attempted by others for vanadium oxides and its composites [30][31][32][33][34][35] . Further, here it is important to emphasise that the investigated V2O5/soda-lime glass system displays at least 80% optical transparency in the visible spectral region with an optical band gap of 2.3eV. This property combination is highly beneficial for the fabrication of transparent thin-film based thermoelectric generators and thermal sensor devices.
(ii) Formation of NaVO + in the glass matrix Vanadium, with its stabilized oxide and ionic states, diffused into the glass matrix in the presence of Na + . During heating, Na + and VO + combined and formed NaVO + (Supplementary details Figure S2). Ultimately, the reaction of Na + VO = NaVO is expected to produce excess electrons. Upon cooling, the NaVO + separated into ionic states of Na + and VO + , which is already demonstrated as stored charge concentration in the dielectric glass medium. The current study verifies charging and discharging mechanisms by solid state reactions upon thermal cycling. Hence the current results indicate that the produced electricity from the thermal behaviour of vanadium oxides can be stored directly into the soda-lime glass.
(iii) Thermal response of the confined ionic interface layer The systematic alteration of ionic states (VO and NaVO complex) by segregation shown by in-situ ToF-SIMS (Supplementary details Figure S2) were well correlated with the concurrently recorded higher frequency response of the electrical conductivity (Fig. 5). Since we expect the NaVO + nano-interface to outperform with conventional thin film-based devices regarding the electrical conductivity and optical response, we are proposing a conceptual design of a 'thermodynamically confined ionic interface/segregation layer' to serve as a thermo-electrical/thermo-optical switch or sensor.

Concluding Remarks
The present work primarily focused on the meticulous characterization of a vanadium oxide thin film coated on commercial glass to reveal its smart functional behaviour. The observed results substantiate the formation of a thermodynamically confined ionic interface.
The diffused vanadium in the glass was introduced by a simple sputter deposition process.
Furthermore, the thermal behaviour of the thin film/ glass system is highly reversible and stable. Recent in-situ TEM work 36  In conclusion, the present study demonstrates the potential of V2O5 thin films for a selfchargeable transparent thin film thermo-electric generator on a commercial window glass and many more applications which can be realised in the near future. The obtained results of vanadium diffusion in the glass show new perspectives concerning the basic understanding of the thermal response of vanadium oxide-based semiconducting glass bulk systems.

Extended Data Figures
. Figure 1. Mass spectrum of uncoated soda-lime glass which shows the absence of vanadium and its oxides   In-situ temperature based ToF-SIMS depth profiles cycles of V2O5/glass. Time sequence showed by arrows starting from 300 ˚C to 340 ˚C and subsequent cooling cycle followed by cycles 2 and 3.