Growth and optical properties of Ca x CoO 2 thin ﬁlms

(cid:1) Phase-pure Ca x CoO 2 thin ﬁlms were obtained by annealing Ca(OH) 2 /Co 3 O 4 multilayers. (cid:1) The Ca x CoO 2 thin ﬁlms showed good transmittance properties. (cid:1) The electrical conductivity of Ca x CoO 2 was improved by incorporating Ca 3 Co 4 O 9 .


Introduction
The layered cobaltates A x CoO 2 (A: Li, Na, Ca, Ba, Sr; 0 < x < 1) have complex crystal structures composed of CdI 2 -type CoO 2 layers and A x layers [1][2][3][4][5] alternately stacked along the c-axis.They show anisotropic transport properties due to this inherently laminated structure and have been investigated for applications in Li-and Na-ion batteries (Li x CoO 2 and Na x CoO 2 ) [6,7], as thermoelectrics (Na x CoO 2 , and Ca x CoO 2 ) [2,8] and for their optical transparency as thin films [9].Among these layered cobaltates, Ca x CoO 2 (0.15 x 0.5) shows higher thermal stability in air [10][11][12] than Li x CoO 2 , and Na x CoO 2 .The latter two have lower long-term stability because of the reactive alkaline Li and Na elements.Preparation methods for A x CoO 2 include solid-state reactions [1,7] and physical vapor deposition (PVD) methods such as pulsed laser deposition (PLD) [6,13] and magnetron sputtering [3,8].Furthermore, Nafree cobaltates (Ca x CoO 2 , Sr x CoO 2 , Ba x CoO 2 ) can be synthesized by ion-exchange using Na x CoO 2 as precursor [4,[14][15][16].In previous work, Ca x CoO 2 thin films were obtained through a two-step sputtering/annealing approach [17] which can be used to synthesize also Ca 3 Co 4 O 9 , another layered cobaltate in the same materials system, and tailor its nanoporosity and mechanical flexibility with retained thermoelectrics properties [18][19][20].Ca x CoO 2 can occur as an intermediate step in the synthesis of Ca 3 Co 4 O 9 [11,21].However, for the synthesis of Ca x CoO 2 it has been challenging to obtain

⇑ Corresponding authors.
E-mail addresses: binbin.xin@liu.se(B.Xin), per.eklund@liu.se(P.Eklund).[17].For materials design and property tailoring in these materials, it is therefore important to understand and exploit the parameter space with respect to phase control and its implications for properties.
In the present paper, pure Ca x CoO 2 thin films are synthesized by annealing of Ca(OH) 2 /Co 3 O 4 multilayer films, where the Ca(OH) 2 was formed by moisture treatment of CaO/Co 3 O 4 multilayer films, as described elsewhere [20].The use of Ca(OH) 2 /Co 3 O 4 starting multilayers enables the synthesis of pure Ca x CoO 2 thin films which exhibit an average optical transmittance of approximately 36% in the visible region and above 70% in the near-infrared (NIR) region.In addition, the electrical conductivity can be increased by incorporating a secondary Ca 3 Co 4 O 9 phase into the Ca x CoO 2 thin films.

Materials and methods
CaO/Co 3 O 4 multilayers were deposited on muscovite mica and sapphire (00 l) (1 cm Â 1 cm) at (300 °C and 600 °C) by rfmagnetron reactive sputtering from Ca and Co targets with a 0.27 Pa (2 mTorr) gas mixture of a 0.5% O 2 / 99.5% Ar (flow percent- age).The deposition system is described in detail elsewhere [22].The as-deposited CaO/Co 3 O 4 multilayer films consisted of eight layers of CaO (top layer) and eight layers of Co 3 O 4 .The asdeposited CaO/Co 3 O 4 multilayer films were exposed to humid environments with water activity a H 2 o (=relative humidity at constant temperature) of 0.88 at room temperature for two days.Moisture exposure led to the conversion of CaO into Ca(OH) 2 as described earlier [20].The Ca(OH) 2/ Co 3 O 4 multilayer films were annealed at 650 °C in air for 2 h.The temperature was increased from room temperature to the annealing temperature with a ramping rate of 20 °C/ min.In addition, the Ca(OH) 2/ Co 3 O 4 multilayer films sputter-deposited at 300 °C were annealed at different temperatures (630 °C, 650 °C, and 670 °C (ramping rate 10 °C/ min)) in order to evaluate the effect of annealing temperature on phase formation of Ca x CoO 2 which is relative to when Ca 3 Co 4 O 9 is formed [21].
X-ray diffractometry (XRD) was carried out using a PANalytical X'Pert PRO instrument with Cu Ka radiation (k = 0.15406 nm) and a Ni filter.Scanning electron microscopy (SEM) was carried out using a LEO Gemini 1550 Zeiss instrument operated at 10 keV.The Ca/Co ratio was determined using energy-dispersive X-ray spectroscopy (EDS) by measuring at several positions on the surface and crosssection of each sample.The electrical resistivity was determined from the sheet resistance measured with a four-point probe Jandel RM3000 station.The resistivity was calculated by multiplying sheet resistance and the film thickness determined from crosssection SEM images.The error mainly originates from the thickness estimation (up to 5%).A Perkin Elmer Lambda 900 UV-Vis-NIR absorption spectrometer was used for transmittance measurements.The Seebeck coefficient a was measured by a home-built thermoelectric measurement setup equipped with; 1) two Peltier heat sources for creating a temperature gradient in the sample on each side of the sample; 2) Two Cu metal electrodes (1 mm Â 9 mm) contact for the hot and cold side for the voltage measurement; 3) two K-type thermocouples (electrically insulated) placed at the same position as the electrical contact.The temperature gradient is controlled using the Peltier modules and the voltage is measured with a Keithley 2001 multimeter.The Seebeck coefficient a defined as DV=DT was calculated from the slope of V = f(DT) (0 K < DT < 10 K).Five different temperature differences of approximately 2 K, 4, K, 6 K, 8 K, and 10 K were applied, with the cold side in all cases set at 300 K, and the Seebeck coefficient is determined as the average of these measurements.The estimated error margins are ~10% with the main contributions being the (separate) thermocouple and voltage readings.

Results and discussion
Fig. 1 a, b, and c show XRD patterns of the films on sapphire and mica, as-deposited, exposed to moisture, and annealed at 650 °C, respectively.For the as-deposited films on sapphire at 600 °C (Fig. 1a), the XRD pattern shows peaks at 2h = 17.9°, 32.37°, and 44.32°, which can be identified as Ca(OH) 2 001, CaO 111, and Co 3 O 4 400, respectively.The presence of Ca(OH) 2 001 is due to a reaction between CaO with water vapor, as explained earlier [20].The low intensity and broad peak at 2h = 36.3°canbe attributed to either Co 3 O 4 311 or CoO 111.After moisture treatment, the CaO 111 peak disappears while the peak intensity of Ca(OH) 2 001 increases and the peaks of Co 3 O 4 remain essentially the same.After annealing at 650 °C, only two peaks from the film are visible, situated at 2h = 16.48°and33.21°These peaks can be identified as Ca x -CoO 2 001 and 002.The XRD patterns of the films deposited on mica at 600 °C (Fig. 1b) and 300 °C (Fig. 1c) show only one peak at 2h = 32.37°,identified as CaO 111.The peaks of Ca(OH) 2 and Co 3 O 4 are overlapped by the peaks from mica [20] and cannot be observed in this XRD pattern.After moisture treatment, the CaO 111 peak disappears.Similar, the Ca x CoO 2 001 and 002 can be confirmed in the XRD pattern of the film growing on mica after annealing at 650 °C.Fig. 1 d, e, and f show top-view and cross-sectional SEM images of the annealed films deposited on sapphire and mica, corresponding to the films in the XRD patterns in Fig. 1 a, b, and c, respectively.The annealed film deposited on sapphire (Fig. 1d) is relatively smooth.In contrast, the annealed film deposited on mica shows irregularly distributed platelets on the surface.The film thicknesses of the annealed film deposited at 600 °C are around 160 nm (Fig. 1d, e).For the film deposited on mica at 300 °C, the microstructural features of the annealed films become smaller and the thickness is around 600 nm (Fig. 1f).
Fig. 2a shows XRD patterns of the films deposited at 300 °C on mica after annealing at the different temperatures of 630 °C, 650 °C, and 670 °C.For the films annealed at 630 °C and 650 °C, the XRD patterns are very similar to those in Fig. 1, with the peaks at 2h = 16.48°and33.21°identified as Ca x CoO 2 001 and 002.The lattice parameter c of Ca x CoO 2 films, obtained from the XRD patterns, is approximately 5.37 Å.In contrast, for the higher annealing temperature of 670 °C, the low-angle peak is instead located at 16.48°to 16.57°and two peaks appear at 24.96°and 42.16°, indicating the presence of a new phase in the film.This XRD pattern exhibits peaks at 2h = 16.57°,24.96°, 33.42°and 42.16°, which can be identified as Ca 3 Co 4 O 9 002, 003, 004, and 005, respectively.The lattice parameter c of the Ca 3 Co 4 O 9 film was calculated to be 10.69 Å, which is slightly smaller than twice that of Ca x CoO 2 [12,21,23].As an intermediate phase, Ca x CoO 2 can convert into in Ca 3 Co 4 O 9 when the annealing temperature is above the formation temperature of Ca 3 Co 4 O 9 as a consequence of the similarities in the layered structure [21].to 650 °C (Fig. 2b and c).For an annealing temperature at 670 °C, nanopores can be observed in the film, indicating that under these conditions, pore-formation accompanies Ca 3 Co 4 O 9 formation (Fig. 2d), as we have previously reported [19,20].With increasing annealing temperature, the thickness of films decreases slightly from 166 nm to 149 nm (Fig. 2b-d).The annealed samples were analyzed by EDS and the elemental ratio Â (Ca:Co) was 0.5:1.Fig. 3a shows the transmission properties of annealed films with different deposition temperature.The average optical transmittance is approximately 36% in the visible region for the Ca x CoO 2 thin film annealed at 650 °C with deposition temperature (300 °C) and is higher than the film at same annealed temperature with higher deposition temperature (600 °C).This transmission is typical of layered cobaltates, similar to reported values for Na x CoO 2 [9] and Ca 3 Co 4 O 9 [24], but much lower than p-type transparent conducting oxides (TCOs) [25,26].In the near-infrared (NIR) region, the average optical transmittance is above 70% for the Ca x CoO 2 film deposited at 300 °C and 60% for the film deposited at 600 °C.Their optical band gaps can be estimated by the equation [27] ahv where a is the optical absorption coefficient, hm is the photon energy, A is a constant, and m is 1/2 for a direct band transition and 2 for an indirect band transition.The optical absorption coefficient a is determined by [28] a ¼ where d is the film thickness, T is the transmittance and R is reflectance.R has been measured and a typical value is about 7%, meaning that the correction due to R will be small and can for the present Fig. 3b shows the transmission properties of films deposited at 300 °C after annealing at 630 °C, 650 °C, and 670 °C(with ramping rate 10 °C/ min).The average optical transmittance is ~26% in the visible region for the Ca x CoO 2 films (annealed at 630 °C and 650 °C).The optical band gap of films annealed at 630 °C and 650 °C is 2.55 eV (Fig. 4d).The Ca x CoO 2 film annealed at 650 °C with the ramping rate 10 °C/ min shows different transmisson properties and larger optical band gap compared with the film annealed at 650 °C with higher ramping rate 20 °C/ min (Fig. 3a  and c).In addition, the optical transmittance decreases significantly when the annealing temperature is increased to 670 °C.The optical band gap of the Ca 3 Co 4 O 9 film annealed at 670 °C is 2.37 eV (Fig. 3d), which is similar to values obtained from Ca 3 Co 4 -O 9 [29].
The Ca x CoO 2 thin films have high resistivity, 260-275 mX cm for the films deposited at 300 and 600 °C and annealed at 650 °C, much higher values than earlier reports [15,30].The resistivity decreases to 228 mX cm, 96 mX cm, and 9 mX cm for the films annealed at 630 °C, 650 °C, and 670 °C (with ramping rate 10 °C/ min), respectively (Fig. 4).The decrease in resistivity is correlated with the amount of Ca 3 Co 4 O 9 phase formed.In additional, the resistivity of the annealed films at 650 °C was reduced by 50% when the temperature ramp rate was decreased from 20 °C/min to 10 °C/min.These samples differ somewhat in that they have different surface morphology, but similar XRD patterns.The surface morphology became smoother, with only a minor amount of Ca 3 -Co 4 O 9 obtained when the film was annealed with the lower ramp rate for annealing temperature of 650 °C.These results indicate that the ramp rate has effect on the amount of Ca 3 Co 4 O 9 formed, which in turn can be beneficial to improve the electrical conductivity.The Seebeck coefficients were measured at room temperature (Fig. 4) and are in the range 130-140 mV/K, without significant differences between samples, which is somewhat higher than the reported value of in-plane Seebeck coefficient for Ca x CoO 2 epitaxial thin films [3,10] and comparable to the value of bulk polycrystalline Ca x CoO 2 [31].

Conclusion
Phase-pure Ca x CoO 2 thin films were synthesized by annealing of multilayer Ca(OH) 2 /Co 3 O 4 films.Pure Ca x CoO 2 thin films exhibit an average optical transmittance of approximately 36% in the visible region and greater than 70% in the near-infrared (NIR) region.In addition, the electrical conductivity can be increased by incorporating a secondary Ca 3 Co 4 O 9 phase into the Ca x CoO 2 ; thus, electrical conductivity can be improved without undesired large changes in optical properties and Seebeck coefficient.Therefore, the electrical conductivity may be improved with retained high transparency and Seebeck coefficient by the ratio of Ca:Co and phase composition in Ca x CoO 2 thin films.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Materials & Design 210 (2021) 110033 Contents lists available at ScienceDirect Materials & Design j o u r n a l h o m e p a g e : w w w .e l s e v i e r .c o m / l o c a t e / m a t d e s phase-pure films, as films tended to contain a substantial amount of Co 3 O 4 distributed in the Ca x CoO 2 thin film

Fig. 1 .
Fig. 1.X-ray diffractograms from CaO/Co 3 O 4 multilayer films, exposed to moisture (a H2O = 0.88), and annealed at 650 °C: as-deposited on (a) sapphire at 600 °C and (b) mica at 600°and on (c) mica at 300 °C; the insets show the cross-sections of the films.

Fig. 2 b
Fig.1 d, e, and f show top-view and cross-sectional SEM images of the annealed films deposited on sapphire and mica, corresponding to the films in the XRD patterns in Fig.1a, b, and c, respectively.The annealed film deposited on sapphire (Fig.1d) is relatively smooth.In contrast, the annealed film deposited on mica shows irregularly distributed platelets on the surface.The film thicknesses of the annealed film deposited at 600 °C are around 160 nm (Fig.1d, e).For the film deposited on mica at 300 °C, the microstructural features of the annealed films become smaller and the thickness is around 600 nm (Fig.1f).Fig.2ashowsXRD patterns of the films deposited at 300 °C on mica after annealing at the different temperatures of 630 °C, 650 °C, and 670 °C.For the films annealed at 630 °C and 650 °C, the XRD patterns are very similar to those in Fig.1, with the peaks at 2h = 16.48°and33.21°identified as Ca x CoO 2 001 and 002.The lattice parameter c of Ca x CoO 2 films, obtained from the XRD patterns, is approximately 5.37 Å.In contrast, for the higher annealing temperature of 670 °C, the low-angle peak is instead located at 16.48°to 16.57°and two peaks appear at 24.96°and 42.16°, indicating the presence of a new phase in the film.This XRD pattern exhibits peaks at 2h = 16.57°,24.96°, 33.42°and 42.16°, which can be identified as Ca 3 Co 4 O 9 002, 003, 004, and 005, respectively.The lattice parameter c of the Ca 3 Co 4 O 9 film was calculated to be 10.69 Å, which is slightly smaller than twice that of Ca x CoO 2[12,21,23].As an intermediate phase, Ca x CoO 2 can convert into in Ca 3 Co 4 O 9 when the annealing temperature is above the formation temperature of Ca 3 Co 4 O 9 as a consequence of the similarities in the layered structure[21].Fig. 2 b, c, and d exhibit top-view and cross-sectional SEM images of these annealed films.The surface morphology becomes smoother when the annealed temperature increases from 630 °C

Fig. 3 .
Fig. 3.The transmittance spectrum after subtraction of the mica contribution of (a) the Ca x CoO 2 thin film annealed at 650 °C with different deposition temperature (300 °C and 600 °C) and (b) films deposited at 300 °C with different annealed temperature: 630 °C, 650 °C and 670 °C; (c) and (d) corresponding plot of ((ahm) 2 versus hv for their respective transmittance measurements.