# Fileset

[coatings---Enhancing Thermoelectric Performance The Impact of Carbon Incorporation in Spin-Coated Al-Doped ZnO Thin Films.pdf](https://mdr.nims.go.jp/filesets/78619dce-0fc6-4341-ac11-0bdc7002dd3e/download)

## Creator

Alberto Giribaldi, Cristiano Giordani, Giovanna Latronico, [Cédric Bourgès](https://orcid.org/0000-0001-9056-0420), Takahiro Baba, Cecilia Piscino, Maya Marinova, [Takao Mori](https://orcid.org/0000-0003-2682-1846), Cristina Artini, Hannes Rijckaert, Paolo Mele

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Enhancing Thermoelectric Performance: The Impact of Carbon Incorporation in Spin-Coated Al-Doped ZnO Thin Films](https://mdr.nims.go.jp/datasets/d499a3f8-0ff6-4dcd-8996-d404822064e6)

## Fulltext

Enhancing Thermoelectric Performance: The Impact of Carbon Incorporation in Spin-Coated Al-Doped ZnO Thin FilmsAcademic Editor: Carlos JoseMacedo TavaresReceived: 19 December 2024Revised: 15 January 2025Accepted: 16 January 2025Published: 19 January 2025Citation: Giribaldi, A.; Giordani, C.;Latronico, G.; Bourgès, C.; Baba, T.;Piscino, C.; Marinova, M.; Mori, T.;Artini, C.; Rijckaert, H.; et al.Enhancing ThermoelectricPerformance: The Impact of CarbonIncorporation in Spin-CoatedAl-Doped ZnO Thin Films. Coatings2025, 15, 107.https://doi.org/10.3390/coatings15010107Copyright: © 2025 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).ArticleEnhancing Thermoelectric Performance: The Impact of CarbonIncorporation in Spin-Coated Al-Doped ZnO Thin FilmsAlberto Giribaldi 1,*, Cristiano Giordani 2,3 , Giovanna Latronico 4 , Cédric Bourgès 5,6 , Takahiro Baba 7,8,Cecilia Piscino 1, Maya Marinova 9, Takao Mori 7,8 , Cristina Artini 1,10 , Hannes Rijckaert 11and Paolo Mele 12,*1 Department of Chemistry and Industrial Chemistry, University of Genoa, Via Dodecaneso 31,16146 Genoa, Italy; cecilia.piscino@edu.unige.it (C.P.); artini@chimica.unige.it (C.A.)2 Institute of Physics, Faculty of Exact and Natural Sciences, Universidad de Antioquia—UdeA, Calle 70No. 52-21, Medellín 050010, Colombia; cristiano.giordani@udea.edu.co3 Grupo Productos Naturales Marinos, Faculty of Pharmaceutical and Food Sciences, Universidad deAntioquia—UdeA, Calle 70 No. 52-21, Medellín 050010, Colombia4 National Research Council, Institute of Condensed Matter Chemistry and Technologies forEnergy (CNR-ICMATE), Via G. Previati, 1/E, 23900 Lecco, Italy; giovannalatronico@cnr.it5 International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), 1-1 Namiki,Tsukuba 305-0044, Japan; cedric.bourges@unilim.fr6 National Scientific Research Center, Institut de Recherche Sur Les Céramiques (CNRS-IRCER), UMR 7315,Centre Européen de la Céramique, 12 Rue Atlantis, 87068 Limoges, France7 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),1-1 Namiki, Tsukuba 305-0044, Japan; baba.takahiro@nims.go.jp (T.B.); mori.takao@nims.go.jp (T.M.)8 Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai,Tsukuba 305-8577, Japan9 National Scientific Research Center, National Agronomy Research Institute (CNRS-INRA), Centrale Lille,Université Artois, FR 2638—IMEC—Institut Michel-Eugène Chevreul, Université de Lille, 59000 Lille, France;maya.marinova@univ-lille.fr10 National Research Council, Institute of Condensed Matter Chemistry and Technologies forEnergy (CNR-ICMATE), Via De Marini 6, 16149 Genova, Italy11 Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium;hannes.rijckaert@ugent.be12 College of Engineering, Shibaura Institute of Technology, 307 Fukasaku, Minuma-ku, Saitama 337-8570, Japan* Correspondence: s4839837@studenti.unige.it (A.G.); pmele@shibaura-it.ac.jp (P.M.)Abstract: In the present study, for the first time, aluminum-doped zinc oxide (AZO) thinfilms with nanoinclusions of amorphous carbon have been synthesized via spin coating,and the thermoelectric performances were investigated varying the aging period of thesolution, the procedure of carbon nanoparticles’ addition, and the annealing atmosphere.The addition of nanoparticles has been pursued to introduce phonon scattering centersto reduce thermal conductivity. All the samples showed a strong orientation along the[002] crystallographic direction, even though the substrate is amorphous silica, with anintensity of the diffraction peaks reaching its maximum in samples annealed in the presenceof hydrogen, and generally decreasing by the addition of carbon nanoparticles. Absolutevalues of the Seebeck coefficient improve when nanoparticles are added. At the sametime, electric conductivity is higher for the sample with 1 wt.% of carbon and annealedin Ar with 1% of H2, both increasing in absolute value with the temperature rise. Amongall the samples, the lowest thermal conductivity value of 1.25 W/(m·K) was found atroom temperature, and the highest power factor was 111 µW/(m·K2) at 325 ◦C. Thus, theintroduction of carbon effectively reduced thermal conductivity, while also increasing thepower factor, giving promising results for the further development of AZO-based materialsfor thermoelectric applications.Coatings 2025, 15, 107 https://doi.org/10.3390/coatings15010107https://doi.org/10.3390/coatings15010107https://doi.org/10.3390/coatings15010107https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/coatingshttps://www.mdpi.comhttps://orcid.org/0000-0001-5432-8085https://orcid.org/0000-0002-9959-9302https://orcid.org/0000-0001-9056-0420https://orcid.org/0000-0003-2682-1846https://orcid.org/0000-0001-9699-7973https://orcid.org/0000-0002-6078-2919https://orcid.org/0000-0002-9291-7646https://doi.org/10.3390/coatings15010107https://www.mdpi.com/article/10.3390/coatings15010107?type=check_update&version=2Coatings 2025, 15, 107 2 of 12Keywords: aluminum-doped zinc oxide; spin coating; carbon nanoparticles;thermoelectrics; thin films1. IntroductionThermoelectric devices for energy harvesting have attracted interest in recent yearsdue to the need for more eco-friendly fuels to pursue the “energy transition” from fossilto renewable energy sources [1–5]. Thermoelectrics are not the final solution to this envi-ronmental issue, but they may constitute one of the many features of a new low-impactenergetical landscape. Thermoelectric devices do not have moving parts and just rely onthe thermoelectric properties of the materials they are made of, and on the temperaturegradient they experience. To evaluate the performance of a thermoelectric material, andhence of a thermoelectric device, it is necessary to refer to the figure of merit ZT, introducedby Abraham F. Ioffe in his works on thermoelectricity [6–8]. As shown in Equation (1),this value depends on the Seebeck coefficient (S), the electric conductivity (σ), and thethermal conductivity (k), resulting from the sum of the electron (kel) and the phonon (kph)contribution [8]:ZT =σ · S2kel + kph·T (1)Semiconductor materials, such as metal oxides for high-temperature applications [9],clathrates [10], skutterudites [11], half-Heusler intermetallics [12], some Zintl com-pounds [13], and chalcogenide compounds for low thermal conductivity and dopingpossibilities [14], are the most promising thermoelectric materials due to their potentialvarious applications in this field [2,3,15].Although most metal oxides usually show a low ZT due to their low σ and high k val-ues, Al-doped zinc oxide (AZO) is among the compounds having moderate thermoelectricproperties, which makes it suitable for high-temperature applications, i.e., in the temper-ature range where other thermoelectric materials with better performances are unstable.Moreover, AZO is nontoxic, cheap, and easy to synthesize, thus favoring its large-scaleapplications [16]. AZO has a ZnO hexagonal structure; namely, it is a semiconductor witha wide band gap of 3.4 eV and has proper doping with 2% of aluminum, which increasesits electrical conductivity and therefore its power factor [17]. However, the high thermalconductivity remains an issue, keeping the ZT value low. Nanostructuring, dimensionalityreduction, and the addition of nanoinclusions are approaches aimed at decreasing the valueof kph [18,19], as all of them introduce scattering centers for phonons, which are responsible,along with conduction electrons, for thermal conductivity in crystalline solids. AZO thinfilms have already been investigated to determine the influence of the reduced dimension-ality and the corresponding microstructure effect on its electrical properties. In particular, ithas been found that the density of carriers and their scattering is mainly influenced by thepresence of grain boundaries at the studied composition [20], and some interesting resultsare a resistance of 320 mΩ in a film with a thickness of 780 nm [21], a power factor of 550mW/(m·K2), and a thermal conductivity lower than 2 W/(m·K) [22,23], as well as a sheetresistance of 2.08 Ω/sq for a 1 µm thick film [24]. These promising results call for tuning ofthe thermoelectric performances of AZO through the control of morphology.The subject of this work is the synthesis and the study of the thermoelectric propertiesof thin films of AZO with nanoinclusions of carbon with an average size of the primaryparticles being 12 nm, which can act as scattering centers for phonons reducing the valueof kph without heavily affecting the other transport properties of the material. The carbonnanoparticles have been chosen because of their low price and use in large-scale applica-Coatings 2025, 15, 107 3 of 12tions. Since the approach consisting of the addition of nanoparticles to reduce the thermalconductivity has already been successfully applied to other systems, the thermoelectricperformances of AZO thin films with nominal composition Al0.02Zn0.98O are expected tobe further enhanced. In particular, the figure of merit ZT of In0.53Ga0.47As was increasedby a factor of 2 with ErAs nanoparticles [25] and a thermal conductivity of 1 W/(m·K)in Si has been achieved with the introduction of Ge nanoparticles with a tailored sizedistribution [26]. Thin films were prepared by spin coating; the technique has been chosenfor its operational simplicity and tunability of the different parameters, such as the agingperiod of the solution [27–31].2. Materials and MethodsThe solutions for the spin coating have been prepared dissolving Zn(CH3COO)2·2H2O(Kanto Chemical Co., Inc., Tokyo, Japan, 99.0% min) and AlCl3·6H2O (Kanto ChemicalCo., Inc., Tokyo, Japan, 98.0%) in 15 mL of isopropanol and 5 mL of ethanol in dueamounts to obtain a solution of the precursor with a 98:2 ratio (0.15 mol/L for Zn and0.003 mol/L for Al), corresponding to the chosen nominal composition. Then about 250 µLof 2-aminoethanol (Kanto Chemical Co., Inc., Tokyo, Japan, 99.0% min.) was added understirring as a complexing agent, keeping the temperature at 60 ◦C until the volume of thesolution was halved. The ratio between Al and Zn was the same for all the samples. Anaging process, consisting of keeping the solution at room temperature for a certain time,as a step for a modified sol–gel method, was carried out on all the solutions, prior to thespin-coating process [24,27,28,30].The series aged for different periods were prepared; they are referred to as “AZO0 day” to “AZO 3 day”, thus containing the information about the days of aging undergoneby the spin-coating solution. Increasing the aging time results in an increasing opalescenceand viscosity of the sol. Two drops of these solutions were deposited on a silica substrate(Crystal Base Co., Ltd., Osaka, Japan) of 10 mm × 10 mm with one smooth face, spun at3000 rpm for 30 s with a K-35951 Kyowariken system, and fired at 350 ◦C for 10 min ina tubular furnace (TMF-500N, AS-ONE Co., Osaka, Japan) in ambient atmosphere. Afterrepeating the process five times, films were annealed at 500 ◦C for 6 h under differentatmospheres: low vacuum, O2, Ar, and Ar + 1% of H2. In Figure 1, the whole syntheticprocedure is schematized. Amorphous carbon nanoparticles (BLACK PEARLS®2000,CABOT Co., Boston, MA, USA) were added in fractions of 0.5 wt.%, 1 wt.%, and 2 wt.%related to the quantity of the resulting AZO. The addition was performed before an agingprocess of 1 day in all the series and after the aging solely in the series annealed in Ar with1% of H2, using ultrasounds to homogenize the suspension. Samples added with carbonnanoparticles before and after aging are named “AZO-C” and “AZO-C after”, respectively,with the name followed by the weight fraction of particles and the annealing atmosphere.Coatings 2025, 15, 107 3 of 13   nanoparticles have been chosen because of their low price and use in large-scale applications. Since the approach consisting of the addition of nanoparticles to reduce the thermal conductivity has already been successfully applied to other systems, the thermoelectric performances of AZO thin films with nominal composition Al0.02Zn0.98O are expected to be further enhanced. In particular, the figure of merit ZT of In0.53Ga0.47As was increased by a factor of 2 with ErAs nanoparticles [25] and a thermal conductivity of 1 W/(m·K) in Si has been achieved with the introduction of Ge nanoparticles with a tailored size distribution [26]. Thin films were prepared by spin coating; the technique has been chosen for its operational simplicity and tunability of the different parameters, such as the aging period of the solution [27–31]. 2. Materials and Methods The solutions for the spin coating have been prepared dissolving Zn(CH3COO)2·2H2O (Kanto Chemical Co., Inc., Tokyo, Japan, 99.0% min) and AlCl3·6H2O (Kanto Chemical Co., Inc., Tokyo, Japan, 98.0%) in 15 mL of isopropanol and 5 mL of ethanol in due amounts to obtain a solution of the precursor with a 98:2 ratio (0.15 mol/L for Zn and 0.003 mol/L for Al), corresponding to the chosen nominal composition. Then about 250 µL of 2-aminoethanol (Kanto Chemical Co., Inc., Tokyo, Japan, 99.0% min.) was added under stirring as a complexing agent, keeping the temperature at 60 °C until the volume of the solution was halved. The ratio between Al and Zn was the same for all the samples. An aging process, consisting of keeping the solution at room temperature for a certain time, as a step for a modified sol–gel method, was carried out on all the solutions, prior to the spin-coating process [24,27,28,30]. The series aged for different periods were prepared; they are referred to as “AZO 0 day” to “AZO 3 day”, thus containing the information about the days of aging undergone by the spin-coating solution. Increasing the aging time results in an increasing opalescence and viscosity of the sol. Two drops of these solutions were deposited on a silica substrate (Crystal Base Co., Ltd., Osaka, Japan) of 10 mm × 10 mm with one smooth face, spun at 3000 rpm for 30 s with a K-35951 Kyowariken system, and fired at 350 °C for 10 min in a tubular furnace (TMF-500N, AS-ONE Co., Osaka, Japan) in ambient atmosphere. After repeating the process five times, films were annealed at 500 °C for 6 h under different atmospheres: low vacuum, O2, Ar, and Ar + 1% of H2. In Figure 1, the whole synthetic procedure is schematized. Amorphous carbon nanoparticles (BLACK PEARLS®2000, CABOT Co., Boston, MA, USA) were added in fractions of 0.5 wt.%, 1 wt.%, and 2 wt.% related to the quantity of the resulting AZO. The addition was performed before an aging process of 1 day in all the series and after the aging solely in the series annealed in Ar with 1% of H2, using ultrasounds to homogenize the suspension. Samples added with carbon nanoparticles before and after aging are named “AZO-C” and “AZO-C after”, respectively, with the name followed by the weight fraction of particles and the annealing atmosphere.  Figure 1. Synthesis protocol of the spin-coating film deposition of the samples. Figure 1. Synthesis protocol of the spin-coating film deposition of the samples.Films were characterized through X-ray diffraction (XRD) by a SmartLab (Rigaku Co.,Tokyo, Japan) using the wavelength Kα of Cu (voltage 40 kV, current 50 mA) in the rangeCoatings 2025, 15, 107 4 of 1210–100◦ with a step of 0.02◦, a speed of 2◦/min, and a slit of 5 mm. The SEM imagingwas made with a JSM-7100F (JEOL Ltd., Tokyo, Japan) Field Emission Scanning ElectronMicroscope using the secondary electrons signal and an operating voltage of 10 kV. Thethicknesses of the films were determined with an MI4050 Focused Ion Beam System (FIB)(Hitachi, Tokyo, Japan) to etch the sample and then measure it via imaging mode. Themicrostructure of the AZO thin films was analyzed by transmission electron microscopy(TEM). For TEM investigations, a cross-sectional lamella was prepared by FIB, employingan FEI Nova 600 Dual Beam system (Nanolab, Waltham, MA, USA), based on the proceduredescribed in this technical note [32]. The experiment was performed on the TITAN Themisat 300 kV in Scanning TEM (STEM) mode with high-angle annular dark-field (HAADF)and energy-dispersive X-ray spectroscopy (EDX). The STEM has been performed using asemi-convergence of 20 mrad and a spot size of 0.5 nm. Preliminary room temperaturevalues of the Seebeck coefficient and sheet resistance (RS) have been pursued via a PortableThermoelectric Meter–3 (JouleYacht, Wuhan, China) and a Loresta-FX MCP-T380 (NHInstruments, Willich, Germany) using the RMH504 four-point probe (NH Instruments,Willich, Germany). S and σ were simultaneously measured by the four-probe methodusing a ZEM-3 (ULVAC Advance-Riko, Yokohama, Japan) device in temperature underpartial helium pressure, from 25 ◦C to 325 ◦C with a step of 50 ◦C. The cross-plane thermalconductivity (kcp) has been determined by a picosecond thermoreflectance method using afront-heating front-detection setup [33].3. Results and DiscussionThe XRD patterns of all the samples exhibit a peak at 2θ = 34.4◦ corresponding tothe [002] crystallographic direction of the P63mc ZnO wurtzite phase; just one other peakappears at 2θ~73.6◦ related to the [004] direction. This evidence denotes a strongly orientedgrowth of the diffraction domains of thin films, even if the deposition has been carriedon amorphous silica, also observed when depositing thin films of the same material bypulsed laser deposition (PLD) [22,34,35]. The intensity of the [002] peak decreases with theaddition of carbon nanoparticles, and it depends on the annealing atmosphere, being thehighest for samples treated in Ar + 1%H2, followed by samples treated in Ar (Figure 2).This result is in accordance with the observation of another study, that stated the suitabilityof an annealing in the presence of H2 with respect to one with air for AZO [36]. An intensitydecrease (Figure 3) and a peak area decrease (Figure 4) were observed when nanoparticleswere added to the spin-coating solution after the aging period. Using the relation betweendhkl and lattice parameters in a hexagonal structure, the value of c was determined to be0.5080(2) nm for all the samples with the exception of the ones with nanoparticles annealedin Ar + 1% H2, which show a slightly higher c value of 0.5085 nm. This value is significantlylower than the one found in the literature for both ZnO and Fe-doped ZnO (c of 0.5205 nmfor the former and c of 0.5202 nm for the latter with 10% Fe) [37] and from the JCPDS89-7102, as a consequence of the presence of the smaller Al instead of Zn.The microstructure observed by SEM of the films without carbon consists of a homo-geneous texture with surface ripples (Figure 5a,b), which seem to be reduced by longeraging. The addition of carbon nanoparticles completely removes these deformations andleaves a wave-like pattern of more rough regions with a radial distribution from the centerof the substrate, visible at lower magnification (Figure 5c). This is probably due to thedeposition method, which involves the spinning of the substrate around the normal axisthrough its center. A peculiar morphology is present solely in the sample annealed in thepresence of hydrogen: some grains of the order of 100 nm seem to stand out from thesurface of the plain regions as well as of the rough ones (Figure 5d). These samples arealso the only ones whose color remains dark and resembles the respective spin-coatingCoatings 2025, 15, 107 5 of 12solutions after the thermal treatment, suggesting the preservation of the nanoparticles afterthe thermal treatment.Coatings 2025, 15, 107 5 of 13    Figure 2. XRD patterns of samples without carbon nanoparticles and annealed under different atmospheres, namely, Ar + H2, Ar, O2, and vacuum. The calculated pattern for the wurtzite type ZnO phase is shown as a reference [37]. The broad peak between 15° and 25° is due to the amor-phous phase of the silica substrate.  Figure 3. XRD patterns of all the samples annealed in Ar + H2.  Figure 2. XRD patterns of samples without carbon nanoparticles and annealed under differentatmospheres, namely, Ar + H2, Ar, O2, and vacuum. The calculated pattern for the wurtzite type ZnOphase is shown as a reference [37]. The broad peak between 15◦ and 25◦ is due to the amorphousphase of the silica substrate.Coatings 2025, 15, 107 5 of 13    Figure 2. XRD patterns of samples without carbon nanoparticles and annealed under different atmospheres, namely, Ar + H2, Ar, O2, and vacuum. The calculated pattern for the wurtzite type ZnO phase is shown as a reference [37]. The broad peak between 15° and 25° is due to the amor-phous phase of the silica substrate.  Figure 3. XRD patterns of all the samples annealed in Ar + H2.  Figure 3. XRD patterns of all the samples annealed in Ar + H2.Coatings 2025, 15, 107 5 of 13    Figure 2. XRD patterns of samples without carbon nanoparticles and annealed under different atmospheres, namely, Ar + H2, Ar, O2, and vacuum. The calculated pattern for the wurtzite type ZnO phase is shown as a reference [37]. The broad peak between 15° and 25° is due to the amor-phous phase of the silica substrate.  Figure 3. XRD patterns of all the samples annealed in Ar + H2.  Figure 4. Areas of the [002] diffraction peaks in the series with the addition of nanoparticles beforethe aging (AZO-C) and the series with the addition afterward (AZO-C after).Coatings 2025, 15, 107 6 of 12Coatings 2025, 15, 107 6 of 13   Figure 4. Areas of the [002] diffraction peaks in the series with the addition of nanoparticles before the aging (AZO-C) and the series with the addition afterward (AZO-C after). The microstructure observed by SEM of the films without carbon consists of a homo-geneous texture with surface ripples (Figure 5a,b), which seem to be reduced by longer aging. The addition of carbon nanoparticles completely removes these deformations and leaves a wave-like pattern of more rough regions with a radial distribution from the center of the substrate, visible at lower magnification (Figure 5c). This is probably due to the deposition method, which involves the spinning of the substrate around the normal axis through its center. A peculiar morphology is present solely in the sample annealed in the presence of hydrogen: some grains of the order of 100 nm seem to stand out from the surface of the plain regions as well as of the rough ones (Figure 5d). These samples are also the only ones whose color remains dark and resembles the respective spin-coating solutions after the thermal treatment, suggesting the preservation of the nanoparticles af-ter the thermal treatment.  Figure 5. SEM secondary electrons images of (a) AZO 1 day; (b) detail of the highlight in (a); (c) AZO-C 0.5% Ar + H2 with the limit of the center of the film and the direction of the wave-like pat-tern marked; (d) detail of (c). The observed thickness of the films from the cross-sectional HAADF images acquired with the STEM is 161 ± 3 nm for “AZO 1 day Ar + H2” and 123 ± 3 nm for “AZO-C 1% Ar + H2”. The single-layer thickness is, on average, higher in “AZO 1 day” and has a lower variability than in “AZO-C 1% Ar + H2“. Moreover, in “AZO-C 1% Ar + H2“, the thickness decreases from the first to the last layer in order of deposition; the grains are generally smaller and their packing is less dense than in “AZO 1 day Ar + H2”. Amorphous regions, which may be in part the carbon nanoparticles, since some of them show dimensions and shapes coherent with the nanoparticles employed, are found in this sample (Figure 6). The resulting morphology, with more abrupt discontinuities, such as grain boundaries, dis-continuous interfaces, and amorphous portions of materials, can explain the further sup-pression of the thermal conductivity observed in “AZO-C 1% Ar + H2“. The presence of aggregates or nanoparticles of C, however, is not fully confirmed. Figure 5. SEM secondary electrons images of (a) AZO 1 day; (b) detail of the highlight in (a); (c) AZO-C 0.5% Ar + H2 with the limit of the center of the film and the direction of the wave-like patternmarked; (d) detail of (c).The observed thickness of the films from the cross-sectional HAADF images acquiredwith the STEM is 161 ± 3 nm for “AZO 1 day Ar + H2” and 123 ± 3 nm for “AZO-C 1%Ar + H2”. The single-layer thickness is, on average, higher in “AZO 1 day” and has a lowervariability than in “AZO-C 1% Ar + H2”. Moreover, in “AZO-C 1% Ar + H2”, the thicknessdecreases from the first to the last layer in order of deposition; the grains are generallysmaller and their packing is less dense than in “AZO 1 day Ar + H2”. Amorphous regions,which may be in part the carbon nanoparticles, since some of them show dimensions andshapes coherent with the nanoparticles employed, are found in this sample (Figure 6).The resulting morphology, with more abrupt discontinuities, such as grain boundaries,discontinuous interfaces, and amorphous portions of materials, can explain the furthersuppression of the thermal conductivity observed in “AZO-C 1% Ar + H2”. The presenceof aggregates or nanoparticles of C, however, is not fully confirmed.The results of the measurements of room temperature S and RS have been used asscreening tests to determine the most promising samples in terms of thermoelectric per-formance. All the samples show negative S, with AZO being an n-type semiconductor,so the discussion below is based on the module of this quantity. The trends of S and RSare very similar through the same series. The aging period does not heavily influencethe value of S for up to two days, keeping a stable value of about −180 µV/K at roomtemperature for the first three samples of the series (from AZO 0 day Ar + H2 to AZO2 day Ar + H2). On the contrary, when the aging reaches three days (AZO 3 dayAr + H2), S and RS abruptly decrease (Figure 7). These films have a rather low RS atroom temperature, in the order of the tens of kΩ/sq. Overall, taking into account the XRDpatterns, the thermoelectric features, and the complexity of the method, the best choiceseems to be 1-day aging. Therefore, the following discussions refer to samples synthesizedfrom a solution aged for 1 day.Coatings 2025, 15, 107 7 of 12Coatings 2025, 15, 107 7 of 13    Figure 6. Cross-sectional TEM images of (a) AZO 1 day Ar + H2 and (b) AZO-C 1% Ar + H2, high-lighting the amorphous areas more likely to be carbon nanoparticles; and HAADF-STEM images of (c) AZO 1 day Ar + H2 and (d) AZO-C 1% Ar + H2, with the layer structure highlighted. The results of the measurements of room temperature S and RS have been used as screening tests to determine the most promising samples in terms of thermoelectric per-formance. All the samples show negative S, with AZO being an n-type semiconductor, so the discussion below is based on the module of this quantity. The trends of S and RS are very similar through the same series. The aging period does not heavily influence the value of S for up to two days, keeping a stable value of about −180 µV/K at room temper-ature for the first three samples of the series (from AZO 0 day Ar + H2 to AZO 2 day Ar + H2). On the contrary, when the aging reaches three days (AZO 3 day Ar + H2), S and RS abruptly decrease (Figure 7). These films have a rather low RS at room temperature, in the order of the tens of kΩ/sq. Overall, taking into account the XRD patterns, the thermoelec-tric features, and the complexity of the method, the best choice seems to be 1-day aging. Therefore, the following discussions refer to samples synthesized from a solution aged for 1 day. Figure 6. Cross-sectional TEM images of (a) AZO 1 day Ar + H2 and (b) AZO-C 1% Ar + H2,highlighting the amorphous areas more likely to be carbon nanoparticles; and HAADF-STEM imagesof (c) AZO 1 day Ar + H2 and (d) AZO-C 1% Ar + H2, with the layer structure highlighted.Coatings 2025, 15, 107 8 of 13    Figure 7. Preliminary room temperature values of S and RS of the samples AZO without carbon and annealed in Ar + H2, showing the dependence on the aging period of the solution. The addition of carbon nanoparticles, carried out to reduce thermal conductivity by the introduction of scattering centers, has a different influence on the room temperature values of S and RS depending on the atmospheric conditions adopted for the annealing process, as depicted in Figure 8. In samples “AZO-C” annealed in Ar and Ar + H2, an overall reduction in S and RS can be observed with increasing the fraction of carbon nano-particles. The trend is inverted in the samples “AZO-C after” annealed in Ar + H2, while no clear dependence appears in samples annealed in O2 and vacuum, whose higher sheet resistance and higher Seebeck coefficient are a direct consequence of the far lower conduc-tivity. The samples annealed in Ar + H2 are the only two series that seem to preserve the carbon nanoparticles after the thermal treatment, thanks to the mild reducing conditions. Out of these samples, the ones synthesized with the carbon introduced before aging also show a RS lower by an order of magnitude with respect to the others. The other annealing atmospheres, namely, Ar (blue curve), O2 (green curve), and vacuum (yellow curve), cause an abrupt increase in RS and consequently in S at room temperature compared to the ones annealed in Ar + H2 (red curve), as shown in Figure 8. In particular, S ranges from −200 µV/K to −650 µV/K in these samples against the values between −100 µV/K and −200 µV/K of “AZO-C Ar + H2”, while RS increases by almost two orders of magnitude, from around 0.04 MΩ/sq to values ranging from 0.2 MΩ/sq to 1.5 MΩ/sq. Therefore, the best compromise between S and RS stands in the samples annealed in Ar + H2 with the carbon nanoparticles added before the aging of 1 day. Figure 7. Preliminary room temperature values of S and RS of the samples AZO without carbon andannealed in Ar + H2, showing the dependence on the aging period of the solution.The addition of carbon nanoparticles, carried out to reduce thermal conductivity by theintroduction of scattering centers, has a different influence on the room temperature valuesof S and RS depending on the atmospheric conditions adopted for the annealing process, asdepicted in Figure 8. In samples “AZO-C” annealed in Ar and Ar + H2, an overall reductionin S and RS can be observed with increasing the fraction of carbon nanoparticles. The trendis inverted in the samples “AZO-C after” annealed in Ar + H2, while no clear dependenceappears in samples annealed in O2 and vacuum, whose higher sheet resistance and higherSeebeck coefficient are a direct consequence of the far lower conductivity. The samplesannealed in Ar + H2 are the only two series that seem to preserve the carbon nanoparticlesafter the thermal treatment, thanks to the mild reducing conditions. Out of these samples,Coatings 2025, 15, 107 8 of 12the ones synthesized with the carbon introduced before aging also show a RS lower by anorder of magnitude with respect to the others. The other annealing atmospheres, namely,Ar (blue curve), O2 (green curve), and vacuum (yellow curve), cause an abrupt increase inRS and consequently in S at room temperature compared to the ones annealed in Ar + H2(red curve), as shown in Figure 8. In particular, S ranges from −200 µV/K to −650 µV/Kin these samples against the values between −100 µV/K and −200 µV/K of “AZO-C Ar +H2”, while RS increases by almost two orders of magnitude, from around 0.04 MΩ/sq tovalues ranging from 0.2 MΩ/sq to 1.5 MΩ/sq. Therefore, the best compromise between Sand RS stands in the samples annealed in Ar + H2 with the carbon nanoparticles addedbefore the aging of 1 day.Coatings 2025, 15, 107 9 of 13   (a) (b) Figure 8. Preliminary room temperature values of S (a) and RS (b) of samples AZO, as a function of C addition in the 1-day aging samples, annealed at 500 °C in various atmospheres. From the room temperature thermoelectric performances and the considerations about the retention of the nanoparticles, the samples that show the best trade-off between low resistance and high Seebeck coefficient, while seemingly preserving the carbon nano-particles, are the samples “AZO-C Ar + H2”. Out of these, “AZO-C 1% Ar + H2” has been chosen as a representative sample of the series. Therefore, “AZO-C 1% Ar + H2” has been measured, along with “AZO 1 day Ar + H2” as a reference without carbon, with the ZEM-3 system under partial helium pressure and σ was calculated using the thickness obtained via TEM observations. Both S and σ increase with temperature and show a hysteresis be-tween heating and cooling, as evident from the data depicted in Figure 9. These show anomalous behavior with abrupt modifications of their properties when heated for the first time, without changes in the appearance of the thin film, probably because the meas-urement itself acted as an annealing, as observed in other studies [38,39]. S seems to in-crease when the nanoparticles are added, in contrast with what was observed at room temperature, even though the values are comparable between the samples with different fractions of C. Instead, no meaningful conclusions can be drawn for the dependence of σ, which is higher for “AZO-C 1% Ar + H2” even in comparison with “AZO 1 day Ar + H2”. The power factor reaches its maximum of about 111 µW/(m·K2) at 325 °C in “AZO-C 1% Ar + H2” (Figure 9c), which also shows the lowest value of k. The values of k at room temperature of the thin films have been calculated from the thermoreflectance data and assuming the heat capacity and density of AZO being 480 J/(kg·K) and 5.195 g/cm3, re-spectively. The sample “AZO 1 day Ar + H2” shows a kcp of 2.55 ± 0.28 W/(m·K), against 1.25 ± 0.15 W/(m·K) of “AZO-C 1% Ar + H2”. This means that the carbon addition caused a lowering of k, although the details of the mechanism that causes this suppression and the presence of carbon are yet to be confirmed. Furthermore, these values are both signif-icantly lower than 10 W/(m·K), the value typically encountered in films of the same mate-rial obtained by pulsed laser deposition [40], suggesting the influence of the deposition method. In particular, the layer-based morphology resulting from the spin-coating method introduces interfaces and discontinuities that act as scattering centers for pho-nons, at least for the cross-plane direction of the thin film. Figure 8. Preliminary room temperature values of S (a) and RS (b) of samples AZO, as a function ofC addition in the 1-day aging samples, annealed at 500 ◦C in various atmospheres.From the room temperature thermoelectric performances and the considerations aboutthe retention of the nanoparticles, the samples that show the best trade-off between lowresistance and high Seebeck coefficient, while seemingly preserving the carbon nanopar-ticles, are the samples “AZO-C Ar + H2”. Out of these, “AZO-C 1% Ar + H2” has beenchosen as a representative sample of the series. Therefore, “AZO-C 1% Ar + H2” has beenmeasured, along with “AZO 1 day Ar + H2” as a reference without carbon, with the ZEM-3system under partial helium pressure and σ was calculated using the thickness obtained viaTEM observations. Both S and σ increase with temperature and show a hysteresis betweenheating and cooling, as evident from the data depicted in Figure 9. These show anomalousbehavior with abrupt modifications of their properties when heated for the first time,without changes in the appearance of the thin film, probably because the measurementitself acted as an annealing, as observed in other studies [38,39]. S seems to increase whenthe nanoparticles are added, in contrast with what was observed at room temperature,even though the values are comparable between the samples with different fractions of C.Instead, no meaningful conclusions can be drawn for the dependence of σ, which is higherfor “AZO-C 1% Ar + H2” even in comparison with “AZO 1 day Ar + H2”. The powerfactor reaches its maximum of about 111 µW/(m·K2) at 325 ◦C in “AZO-C 1% Ar + H2”(Figure 9c), which also shows the lowest value of k. The values of k at room temperature ofthe thin films have been calculated from the thermoreflectance data and assuming the heatcapacity and density of AZO being 480 J/(kg·K) and 5.195 g/cm3, respectively. The sampleCoatings 2025, 15, 107 9 of 12“AZO 1 day Ar + H2” shows a kcp of 2.55 ± 0.28 W/(m·K), against 1.25 ± 0.15 W/(m·K)of “AZO-C 1% Ar + H2”. This means that the carbon addition caused a lowering of k,although the details of the mechanism that causes this suppression and the presence ofcarbon are yet to be confirmed. Furthermore, these values are both significantly lowerthan 10 W/(m·K), the value typically encountered in films of the same material obtainedby pulsed laser deposition [40], suggesting the influence of the deposition method. Inparticular, the layer-based morphology resulting from the spin-coating method introducesinterfaces and discontinuities that act as scattering centers for phonons, at least for thecross-plane direction of the thin film.Coatings 2025, 15, 107 10 of 13    (a) (b)  (c) Figure 9. S (a) and σ (b) of samples “AZO 1 day Ar + H2” and “AZO-C Ar + H2”, as a function of temperature during heating and cooling; (c) calculated power factor of the samples “AZO 1 day Ar + H2” and AZO-C 1% Ar + H2” showing the dependence with respect to the temperature dur-ing heating and cooling. 4. Conclusions The [00l] oriented AZO thin film samples, synthesized via spin coating, showed en-hanced thermoelectric performance, thanks to both the chemical deposition method and the introduction of nanoparticles. The lowest thermal conductivity value was 1.25 W/(m·K) at room temperature, and the highest power factor was 111 µW/(m·K2) at 325 °C. The crystallinity degree increases with the aging period of the spin-coating solution and decreases if the carbon nanoparticles are added. The amorphous carbon nanoparticles, which seem to be retained only after annealing in Ar + H2, cause inhomogeneity in the morphology of the thin films and do not heavily influence the values of the Seebeck coef-ficient, though electrical conductivity slightly increases, and the cross-plane thermal con-ductivity is efficiently suppressed. In this respect, carbon seems to induce a slight strain in the lattice, as suggested by the values of the c-axis, and mass fluctuations without dampening the electrical conductivity, giving better results when the addition is per-formed before the aging process. Further studies on spin-coated AZO thin films are to be pursued, for instance, to evaluate the layers’ thickness dependence from the synthetic pro-tocol and its drawbacks on the thermoelectric properties, and to optimize the method. Moreover, further investigation about the retention (through Raman Spectroscopy or Figure 9. S (a) and σ (b) of samples “AZO 1 day Ar + H2” and “AZO-C Ar + H2”, as a function oftemperature during heating and cooling; (c) calculated power factor of the samples “AZO 1 day Ar+ H2” and AZO-C 1% Ar + H2” showing the dependence with respect to the temperature duringheating and cooling.4. ConclusionsThe [00l] oriented AZO thin film samples, synthesized via spin coating, showed en-hanced thermoelectric performance, thanks to both the chemical deposition method andthe introduction of nanoparticles. The lowest thermal conductivity value was 1.25 W/(m·K)at room temperature, and the highest power factor was 111 µW/(m·K2) at 325 ◦C. The crys-tallinity degree increases with the aging period of the spin-coating solution and decreasesif the carbon nanoparticles are added. The amorphous carbon nanoparticles, which seemCoatings 2025, 15, 107 10 of 12to be retained only after annealing in Ar + H2, cause inhomogeneity in the morphology ofthe thin films and do not heavily influence the values of the Seebeck coefficient, thoughelectrical conductivity slightly increases, and the cross-plane thermal conductivity is ef-ficiently suppressed. In this respect, carbon seems to induce a slight strain in the lattice,as suggested by the values of the c-axis, and mass fluctuations without dampening theelectrical conductivity, giving better results when the addition is performed before the agingprocess. Further studies on spin-coated AZO thin films are to be pursued, for instance, toevaluate the layers’ thickness dependence from the synthetic protocol and its drawbackson the thermoelectric properties, and to optimize the method. Moreover, further investi-gation about the retention (through Raman Spectroscopy or Induction-Coupled PlasmaOptical Emission Spectroscopy, for instance) and the exact role of the carbon nanoparti-cles in the kcp suppression are necessary to better apply this solution to this and otherthermoelectric materials.Author Contributions: Conceptualization, A.G., P.M. and G.L.; methodology, A.G., P.M., C.G., H.R.,M.M. and G.L.; formal analysis, P.M. and A.G.; investigation, A.G., P.M., G.L., C.B., T.B., M.M.and C.G.; resources, P.M., C.A. and T.M.; data curation, A.G.; writing—original draft preparation,A.G.; writing—review and editing, P.M., A.G., G.L, C.G., C.B., T.B., C.A., T.M., H.R., C.P. and M.M.;visualization, A.G.; supervision, P.M. and C.A.; project administration, P.M.; funding acquisition, T.M.All authors have read and agreed to the published version of the manuscript.Funding: H.R. acknowledges support and funding as a postdoctoral fellow in fundamental researchof the Research Foundation—Flanders (FWO) under grant number 1273621N. T.M. and T.B. arethankful for the support from JST Mirai JPMJMI19A1.Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: The data presented in this research study are available in this article.Acknowledgments: P.M. and C.G. are thankful to the Shibaura Institute of Technology (Japan) andUniversidad de Antioquia (Colombia) for their cooperation framework agreement and support for theproject n. 2022-48030 titled: “Development of oxide thin films embedded with polymer nanoparticlesfor the realization of environmentally friendly thermoelectric harvesters”. P.M, A.G., and G.L. aregrateful to Alicja Klimkowicz (Shibaura Institute of Technology, Japan) for the assistance in themicroscopical observations of nanoparticles. P.M. and C.G. are also grateful to Antonino Tamburello(UER, Rome, Italy) for insightful inspiration.Conflicts of Interest: The authors declare no conflicts of interest.References1. Riffat, S.B.; Ma, X. Thermoelectrics: A review of present and potential applications. Appl. Therm. Eng. 2003, 23, 913–935.[CrossRef]2. Sootsman, J.R.; Chung, D.Y.; Kanatzidis, M.G. New and old Concepts in Thermoelectric Materials. Angew. Chem. 2009, 48,8616–8639. [CrossRef] [PubMed]3. He, J.; Tritt, T.M. Advances in Thermoelectric Materials Research: Looking Back and Moving Forward. Science 2017, 357, 1369.[CrossRef] [PubMed]4. Russ, B.; Glaudell, A.; Urban, J.; Chabinyc, M.L.; Segalman, R.A. Organic thermoelectric materials for energy harvesting andtemperature control. Nat. Rev. Mater. 2016, 1, 16050. [CrossRef]5. D’Angelo, M.; Galassi, C.; Lecis, N. Thermoelectric Materials and Applications: A Review. Energies 2023, 16, 6409. [CrossRef]6. Ioffe, A.F. Semiconductor Thermoelements and Thermoelectric Cooling; Infosearch Ltd.: London, UK, 1957.7. Ioffe, A.F. The Revival of Thermoelectricity. Sci. Am. 1958, 199, 31–37.8. Vedernikov, M.V.; Iordanishvili, E.K. AF Ioffe and Origin of Modern Semiconductor Thermoelectric Energy Conversion. InProceedings of the Seventeenth International Conference on Thermoelectrics. Proceedings ICT98 (Cat. No. 98TH8365), Nagoya,Japan, 28–28 May 1998; pp. 37–42.https://doi.org/10.1016/S1359-4311(03)00012-7https://doi.org/10.1002/anie.200900598https://www.ncbi.nlm.nih.gov/pubmed/19866458https://doi.org/10.1126/science.aak9997https://www.ncbi.nlm.nih.gov/pubmed/28963228https://doi.org/10.1038/natrevmats.2016.50https://doi.org/10.3390/en16176409Coatings 2025, 15, 107 11 of 129. Walia, S.; Balendhran, S.; Nili, H.; Zhuiykov, S.; Rosengarten, G.; Wang, Q.H.; Bhaskaran, M.; Sriram, S.; Strano, M.S.; Kalantar-zadeh, K. Transition metal oxides—Thermoelectric properties. Progr. Mater. Sci. 2013, 58, 1443–1489. [CrossRef]10. Dolyniuk, J.-A.; Owens-Baird, B.; Wang, J.; Zaikina, J.V.; Kovnir, K. Clathrate thermoelectrics. Mater. Sci. Eng. R Rep. 2016, 108,1–46. [CrossRef]11. Zhao, C.; Wang, M.; Liu, Z. Research progress on preparation methods of skutterudite. Inorganics 2022, 10, 106. [CrossRef]12. Aversano, F.; Palumbo, M.; Ferrario, A.; Boldrini, S.; Fanciulli, C.; Baricco, M.; Castellero, A. Role of secondary phases and thermalcycling on thermoelectric properties of TiNiSn half-Heusler alloy prepared by different processing routes. Intermetallics 2020, 127,106988. [CrossRef]13. Bano, S.; Chetty, R.; Jayachandran, B.; Mori, T. Mg3(Sb,Bi)2-based materials and devices rivalling bismuth telluride for thermo-electric power generation and cooling. Device 2024, 2, 100408. [CrossRef]14. Wang, Y.; Lin, P.; Lou, Q.; Zhang, Z.; Huang, S.; Lu, Y.; He, J. Design guidelines for chalcogenide-based flexible thermoelectricmaterials. Mater. Adv. 2021, 2, 2584–2593. [CrossRef]15. Artini, C.; Pennelli, G.; Graziosi, P.; Li, Z.; Neophytou, N.; Melis, C.; Colombo, L.; Isotta, E.; Lohani, K.; Scardi, P.; et al. Roadmapon Thermoelectricity. Nanotechnology 2023, 34, 292001. [CrossRef] [PubMed]16. Wang, Y.; Zhou, C.; Elquist, A.M.; Ghods, A.; Saravade, V.G.; Lu, N.; Ferguson, I. A review of earth abundant ZnO-based materialsfor thermoelectric and photovoltaic applications. In Proceedings of the Proceedings Volume 10533, Oxide-based Materials andDevices IX, San Francisco, CA, USA, 27 January–1 February 2018; p. 105331R.17. Zhou, B.; Chen, L.; Li, C.; Qi, N.; Chen, Z.; Su, X.; Thang, X.F. Significant Enhancement in the Thermoelectric Performance ofAluminum-doped ZnO Tuned by Pore Structure. ACS Appl. Mater. Interfaces 2020, 12, 51669–51678. [CrossRef]18. Vineis, C.J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M.G. Nanostructured Thermoelectrics: Big Efficiency Gains from SmallFeatures. Adv. Mater. 2010, 22, 3970–3980. [CrossRef]19. Noori, A.; Masoumi, S.; Hashemi, N. Temperature Dependence of the Seebeck Coefficient in Zinc Oxide Thin Films. J. Phys. Conf.Ser. 2017, 939, 012013. [CrossRef]20. Jia, J.; Oka, N.; Kusayanagi, M.; Nakatomi, S.; Shigesato, Y. Origin of Carrier Scattering in Polycrystalline Al-doped ZnO Films.Appl. Phys. Express 2014, 7, 105802. [CrossRef]21. Zhou, H.M.; Yi, D.; Yu, Z.; Xiao, L.; Li, J. Preparation of Aluminum Doped Zinc Oxide Films and the Study of their Microstructure,Electrical and Optical Properties. Thin Solid. Films 2007, 15, 6909–6914. [CrossRef]22. Mele, P.; Saini, S.; Honda, H.; Matsumoto, K.; Miyazaki, K. Effect on Substrate on Thermoelectric Properties of Al-doped ZnOThin Films. Appl. Phys. Lett. 2013, 102, 253903. [CrossRef]23. Nam, W.H.; Lim, Y.S.; Choi, S.M.; Seo, W.S.; Lee, J.Y. High-Temperature Charge Transport and Thermoelectric Properties of aDegenerately Al-doped ZnO Nanocomposite. J. Mater. Chem. 2012, 22, 14633–14638. [CrossRef]24. Tonny, K.N.; Rafique, R.; Sharmin, A.; Bashar, M.S.; Mahmood, Z.H. Electrical, Optical and Structural Properties of TransparentConducting Al doped ZnO (AZO) Deposited by Sol-Gel Spin Coating. AIP Adv. 2018, 8, 065307. [CrossRef]25. Kim, W.; Zide, J.; Gossard, A.; Klenov, D.; Stemmer, S.; Shakouri, A.; Majumdar, A. Thermal Conductivity Reduction andThermoelectric Figure of Merit Increase by Embedding Nanoparticles in Crystalline Semiconductors. PRL 2006, 96, 045901.[CrossRef] [PubMed]26. Zhang, H.; Minnich, A.J. The Best Nanoparticle Size Distribution for Minimum Thermal Conductivity. Sci. Rep. 2015, 5, 8995.[CrossRef] [PubMed]27. Wang, Y.; Xu, G.; Yang, J.; Mao, W.; Wang, J.; Liu, Z.; Dong, Y.; Yang, S.; Li, J. Fabrication of AZO and FAZO Films Using Low-costSpin-Coating Method. Opt. Mater. 2022, 126, 112204. [CrossRef]28. Lee, W.; Leem, J.Y. Enhancement of the Ultraviolet Photoresponsivity of Al-doped ZnO Thin Films Prepared by using the Sol-gelSpin-coating Method. J. Korean Phys. Soc. 2018, 72, 610–614. [CrossRef]29. Nga, Z.N.; Chana, K.Y.; Tohsophonb, T. Effects of Annealing Temperature on ZnO and AZO Films Prepared by Sol–Gel Technique.Appl. Surf. Sci. 2012, 258, 9604–9609. [CrossRef]30. Kumar, K.D.A.; Valanarasu, S.; Kathalingam, A.; Ganesh, V.; Shkir, M.; AlFaify, S. Effect of solvents on sol–gel spin-coatednanostructured Al-doped ZnO thin films: A film for key optoelectronic applications. Appl. Phys. A 2017, 123, 801. [CrossRef]31. Sengupta, J.; Sahoo, R.K.; Mukherjee, C.D. Effect of annealing on the structural, topographical and optical properties of sol–gelderived ZnO and AZO thin films. Mater. Lett. 2012, 83, 84–87. [CrossRef]32. Rijckaert, H. Preparation of the S/TEM Lamella of Commercial REBa2Cu3O7-x Coated Conductors by FIB. IEEE Trans. Appl.Supercond. 2024, 34, 7500707. [CrossRef]33. Baba, T.; Baba, T.; Mori, T. Fourier Transform Thermoreflectance method Under Front-Heat Front-Detect Configuration. Int. J.Thermophys. 2021, 45, 61. [CrossRef]34. Chen, X.; Guan, W.; Fang, G.; Zhao, X.Z. Influence of substrate temperature and port-treatment on the properties of AnO:Al thinfilms prepared by pulsed laser deposition. Appl. Surf. Sci. 2005, 252, 1561–1567. [CrossRef]https://doi.org/10.1016/j.pmatsci.2013.06.003https://doi.org/10.1016/j.mser.2016.08.001https://doi.org/10.3390/inorganics10080106https://doi.org/10.1016/j.intermet.2020.106988https://doi.org/10.1016/j.device.2024.100408https://doi.org/10.1039/D0MA01018Ahttps://doi.org/10.1088/1361-6528/acca88https://www.ncbi.nlm.nih.gov/pubmed/37019100https://doi.org/10.1021/acsami.0c16506https://doi.org/10.1002/adma.201000839https://doi.org/10.1088/1742-6596/939/1/012013https://doi.org/10.7567/APEX.7.105802https://doi.org/10.1016/j.tsf.2007.01.041https://doi.org/10.1063/1.4812401https://doi.org/10.1039/c2jm31763jhttps://doi.org/10.1063/1.5023020https://doi.org/10.1103/PhysRevLett.96.045901https://www.ncbi.nlm.nih.gov/pubmed/16486849https://doi.org/10.1038/srep08995https://www.ncbi.nlm.nih.gov/pubmed/25757414https://doi.org/10.1016/j.optmat.2022.112204https://doi.org/10.3938/jkps.72.610https://doi.org/10.1016/j.apsusc.2012.05.156https://doi.org/10.1007/s00339-017-1426-zhttps://doi.org/10.1016/j.matlet.2012.05.130https://doi.org/10.1109/TASC.2024.3438690https://doi.org/10.1007/s10765-024-03351-1https://doi.org/10.1016/j.apsusc.2005.02.137Coatings 2025, 15, 107 12 of 1235. Alyamani, A.; Sayari, A.; Albadri, A.; Albrithen, H.; El Mir, L. Structural, morphological and optical characterizations of ZnO:Althin films grown on silicon substrates by pulsed laser deposition. Eur. Phys. J. Plus 2016, 131, 328. [CrossRef]36. Wang, W.P.; Chen, Y.Y.; Hsu, J.C.; Wang, C. Structural, optical and electrical properties of aluminum doped ZnO films annealed inair and hydrogen atmosphere. J. Non-Cryst. Solids 2014, 383, 131–136. [CrossRef]37. Saadi, H.; Rhouma, F.I.H.; Benzarti, Z.; Bougrioua, Z.; Guemazi, S.; Khirouni, K. Electrical Conductivity Improvement of Fedoped ZnO Nanopowders. Mater. Res. Bull. 2020, 129, 110884. [CrossRef]38. Abdullin, K.A.; Gabdullin, M.T.; Zhumagulov, S.K.; Ismailova, G.A.; Gritsenko, L.V.; Kedruk, Y.Y.; Mirzaeian, M. Stabilization ofthe Surface of ZnO Films and Elimination of the Aging Effect. Materials 2021, 14, 6535. [CrossRef]39. Zhang, W.; Chen, X.; Ma, Y.; Xu, Z.; Wu, L.; Yang, Y.; Tsang, S.W.; Chen, S. The Positive Aging Effect of ZnO NanoparticlesInduced by Surface Stabilization. J. Phys. Chem. Lett. 2020, 11, 5863–5870. [CrossRef]40. Darwish, A.M.; Muhammad, A.; Sarkisov, S.S.; Mele, P.; Saini, S.; Liu, J.; Shiomi, J. Thermoelectric Properties of Al-doped ZnOComposite Films with Polymer Nanoparticles Prepared by Pulsed Laser Deposition. Composites Part. B 2019, 167, 406–410.[CrossRef]Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1140/epjp/i2016-16328-7https://doi.org/10.1016/j.jnoncrysol.2013.03.001https://doi.org/10.1016/j.materresbull.2020.110884https://doi.org/10.3390/ma14216535https://doi.org/10.1021/acs.jpclett.0c01640https://doi.org/10.1016/j.compositesb.2019.02.043 Introduction  Materials and Methods  Results and Discussion  Conclusions  References