Sonia Grimbuhler
email: sonia.grimbuhler@irstea.fr

Jean-François Viel

Development and psychometric evaluation of a safety climate scale for vineyards

Keywords: Safety climate, Summed score, conceptual framework, management commitment, communication and feedback, rules and practices, knowledge, structural equation modeling Abbreviation: SEM, structural equation model(ing)

Background: This study aimed to develop a questionnaire-based tool measuring the safety climate in vineyards and to assess its psychometric properties.

Methods: A literature search was conducted to identify the dimensions and items that constitute the safety culture construct in various occupational sectors and to draft a conceptual framework.

Content validity appraisal was performed by 16 farm managers or pesticide operators. The resulting preliminary conceptual framework consisted of 9 dimensions and 42 questions. Then, a telephone survey was conducted in the French Aquitaine (Bordeaux) region with 312 vineyard workers. Itemtotal correlation tests, Cronbach's alpha analysis and a principal component analysis were performed to confirm the unidimensionality of the scale under construction. Structural equation modelling (SEM) techniques were used to verify the model hypothesized from the exploratory analyses and to determine how well it fits the data.

Results: Exploratory analyses resulted initially in a 9-dimension, 20-item safety climate questionnaire.

Internal consistency proved good with a Cronbach's alpha equal to 0.81. The SEM approach suggested two dimension groupings for a better fit of the data (7 dimensions operationalized through the same 20 items). Internal model parameters showed that the more influential dimensions of safety climate were Management commitment, Communication and feedback, Rules and practices, and Knowledge (all standardized path coefficients ≥ 0.7).

Conclusions: Owing to its good psychometric properties, we hope this score will help in drawing up relevant interventions aimed at improving safety culture, raising pesticide risk awareness, and hopefully inducing more sustainable practices in the medium-term future.

Introduction

Grapevines are perennial crops that suffer from a number of diseases, such as downy mildew, powdery mildew, botrytis brunch rot or black rot, which reduce grape yields. Despite efforts to select resistant grape varieties, pesticides remain the main solution used by farmers to reduce the extent of diseases. In France, winegrowing represents 4% of utilized agricultural area within the country but accounts for 14% of chemical inputs expenditure [START_REF] Aubert | Between the approved and the actual dose. A diagnosis of pesticide overdosing in French vineyards[END_REF].

Reducing the consumption of chemical inputs, fertilizers and pesticides has become a primary objective in France. The ambitious "Ecophyto 2018" scheme (launched in 2008) aimed at cutting pesticide use by 50% over the next decade while maintaining agricultural performance.

Unfortunately, this national action plan proved over-optimistic and, instead of declining, pesticide use rose. In vineyards, the treatment frequency index (TFI -built with the normalized doses of pesticides actually applied by the wine producers) has regularly increased over the years 2006[START_REF] Zohar | Thirty years of safety climate research: reflections and future directions[END_REF], and 2013 in all French vineyards (for example, 14.2, 14.1, 16.9 and 14.2, 15.0, 19.8 in all French vineyards (for example, 14.2, 14.1, 16.9 and 14.2, 15.0, 19.8, in the Bordeaux and Bourgogne vineyards, respectively) [START_REF] Pujol | Phytosanitary product use in vineyards and climate: results from surveys on agricultural practices[END_REF]. This trend could be explained by interannual variability in climatic conditions or parasite pressure, but also by changes in farming practices of the wine producers. A recent study showed that the effect of weather condition was moderate although significant in half of the vineyards, leaving most of the TFI variations unexplained [START_REF] Pujol | Evolution of farming practices in the French vineyard -The use of pesticides 2006-2013[END_REF]. A recent survey conducted in 105 French vineyards revealed that 24% of winegrowers systematically apply excessive doses of pesticides [START_REF] Aubert | Between the approved and the actual dose. A diagnosis of pesticide overdosing in French vineyards[END_REF]. Therefore, winegrowing represents a major challenge for reducing phytosanitary treatments.

Although health problems first affect workers who handle pesticides, many vineyard farmers have been resistant to change for more sustainable practices (one must bear in mind that the spraying culture in vineyards spans a century or more), unless this change results in clear economic benefits, or they understand the importance of non-obvious long-term benefits. Changes in pest management are driven by individual (education level) and farm (size, solvency level, indebtedness, subscription of crop insurance policies) characteristics [START_REF] Aubert | Between the approved and the actual dose. A diagnosis of pesticide overdosing in French vineyards[END_REF]. Above these change drivers, farmers placing more value on their overall health also place more value on the benefits of implementing farm safety practices [START_REF] Hodne | Development of the Farm Safety and Health Beliefs Scale[END_REF]. Regarding pesticide safety, farmers who report experiencing adverse health problems have a higher level of pesticide risk awareness and are more likely to use alternative pest management practices than farmers who do not report having such problems [START_REF] Lichtenberg | Adverse health experiences, environmental attitudes, and pesticide usage behavior of farm operators[END_REF]. Based on these observations, it can be assumed that farmers who anticipate long-term positive effects from virtuous practices have also more heightened concerns about pesticide pollution and are more familiar with safety culture principles.

The safety culture concept is easily traced back to the 1986 Chernobyl nuclear disaster in the former Soviet Union. In its aftermath, a UK nuclear safety panel developed one of the first and most commonly used definitions of safety culture: "the product of individual and group values, attitudes, perceptions, competencies and patterns of behavior that can determine the commitment to, and the style and proficiency of an organization's health and safety management system" (Health and safety commission, 1993).

The term "safety climate" appeared earlier in the academic literature when [START_REF] Zohar | Safety climate in industrial organizations: theoretical and applied implications[END_REF] measured workers' perceptions of work safety in various industries. He defined safety climate as a "summary molar perceptions that employees share about their work environment" and assessed it through a questionnaire whose items (questions) measured a set of 8 factors (dimensions) revealing shared perceptions. Compared to safety culture, safety climate is more applied to employee psychological characteristics, corresponding to values, attitudes and perceptions that employees have about safety in their organization [START_REF] Do Nascimento | Psychometric model for safety culture assessment in nuclear research facilities[END_REF]. Providing a "snapshot" of culture at a given moment in time, it can be influenced by recent events. In short, safety climate is considered the measurable aspect of safety culture [START_REF] Huang | Development and validation of safety climate scale for lone workers using truck drivers as exemplar[END_REF].

The academic and scientific interest in safety climate measurement methods has resulted in a proliferation of assessment instruments, most of them based on self-assessment questionnaires, applied in different sectors, mainly in health and production areas. Specific attention to the agriculture industry (among which grape growing) has not been given and it remains unclear how these principles can be applied to this highly segmented industrial sector where similar regulations, organization structures and lines of command do not exist. In particular, long-term vineyard employees work regularly or occasionally on their own, without access to immediate support from work colleagues or managers.

Assessment of safety culture in vineyards could help understand the resistance to change and, subsequently, introduce positive attitudes and behavior towards occupational safety and health and facilitate the adoption of best practices. Therefore, this work aimed to develop a questionnairebased tool measuring safety climate in vineyards and to assess its psychometric properties.

Methods

Development of the safety climate scale

Conceptual definition of the construct

As there is no consensus about the dimensions that make up the concept of safety culture, a literature search was conducted to identify the dimensions and items that constitute the safety culture construct in the industrial segment, armies, transport organizations or the health sector. The literature search consisted of two stages. Firstly, the 20 most cited articles published between 1980 and 2006 with the terms "safety climate", safety culture", "safety behavior" or "safety performance" in the title, abstract or keywords were extracted from the Scopus database. Among them, 8 articles were screened out because of the absence of questionnaire. A similar search strategy retrieved 11 non-redundant articles for the time period 2007-2012. As a result, a total of 23 papers were identified as being of sufficient quality and relevance to merit data extraction (Cooper and Phillips, 2004;[START_REF] Cox | Assessing safety culture in offshore environment[END_REF][START_REF] Dodek | Organizational and safety culture in Canadian intensive care units: relationship to size of intensive care unit and physician management model[END_REF][START_REF] Fogarty | Safety climate and the theory of planned behavior: towards the prediction of unsafe[END_REF]Gershon et al., 2000;[START_REF] Griffin | Perception of safety at work: a framework for linking safety climate to safety performance, knowledge, and motivation[END_REF]Jliang et al., 2010;[START_REF] Mearns | Safety climate, safety management practice and safety performance in offshore environments[END_REF][START_REF] Morrow | Relationships between psychological safety climate facets and safety behavior in the rail industry: a dominance analysis[END_REF][START_REF] Neal | The impact of organizational climate on safety climate and individual behavior[END_REF][START_REF] Neal | A study of the lagged relationships among safety climate, safety motivation, safety behavior, and accidents at the individual and group levels[END_REF][START_REF] Probst | Accident under-reporting among employees: testing the moderating influence of psychological safety climate and supervisor enforcement of safety practices[END_REF][START_REF] Raftopoulos | Safety culture in the maternity units: a census survey using the safety attitudes questionnaire[END_REF][START_REF] Sexton | The safety attitudes questionnaire: psychometric properties, benchmarking data and emerging research[END_REF][START_REF] Singer | Workforce perceptions of hospital safety culture: development and validation of the patient safety climate in healthcare organizations survey[END_REF][START_REF] Singer | Relationship of safety climate and safety performance in hospital[END_REF][START_REF] Sorra | Multilevel psychometric properties of the AHRQ hospital survey on patient safety culture[END_REF][START_REF] Turnberg | Evaluation of healthcare safety climate measurement tool[END_REF][START_REF] Varonen | The safety climate and its relationship to safety practices, safety of the work environment and occupational accidents in eight wood-processing companies[END_REF][START_REF] Vinodkumar | Safety management practices and safety behaviour: assessing the mediating role of safety knowledge and motivation[END_REF][START_REF] Williamson | The development of a measure of safety climate: the role of safety perceptions and attitudes[END_REF][START_REF] Zohar | Safety climate in industrial organizations: theoretical and applied implications[END_REF][START_REF] Zohar | A multilevel model of safety climate: cross level relationships between organization and group level climates[END_REF]. We have, moreover, considered a Health and Safety Executive report [START_REF] Davies | Summary guide to safety climate tools[END_REF], as it provides information on a number of questionnaire-based tools measuring the safety climate of organizations and offers general guidance to compile a customized survey questionnaire.

After a careful comparison of individual questionnaire dimensions, we have discarded the dimensions cited in less than 20% of questionnaires (e.g., management style, perceived supervisor competence) and those embedded in more general ones. As a result, the preliminary conceptual framework of safety climate construct was defined through the following 9 dimensions: Management commitment (managers' demonstrated value of and commitment to workers' safety), Training (process providing employees with knowledge and skills to perform their work in a safe and healthy way), Communication and feedback (interactions between employees and supervisors), Rules and best practices (specific to the activities conducted), Knowledge (specific to the activities conducted), Safety compliance (state of being in accordance with established safety standards and regulations), Safety participation (proactive orientation toward safety management and risk prevention), Teamwork climate (quality of collaboration between employees from the same work unit) and Safety system (systematic approach to managing safety, including organizational structures, accountabilities, policies and procedures).

Then, each questionnaire from the 24 documents was taken and examined item by item. Similar and identical items were grouped together. As the groupings developed, suitable labels were selected. This was not restricted to the original set of item titles where alternative labels were considered to be more appropriate. Some items were reworded or rephrased to suit vineyard working practice. In particular, to adapt the questionnaire to the agriculture sector, "company" was replaced by "farm", "employee" by "farm worker", "rules and procedures" by "rules and best practices", etc. Moreover, some focus was given to pesticide exposure. As a result, the preliminary conceptual framework of the safety climate construct was based on 9 dimensions operationalized through 49 items.

Content validity

To assess the degree of agreement of the items in relation to the construct, content validity appraisal was performed by a team of 16 farm managers or pesticide operators working in vineyards of various acreage (from 3 to 145 hectares). They were asked to assess the degree of relevance of each item intended to measure the related construct and to check general syntax and semantic problems.

Survey administration

A telephone survey was conducted in the Aquitaine region (southwestern France) in January-February 2015 by a unique professional investigator. We used stratified random sampling to control for heterogeneity in vineyard characteristics and grape growing practices. The Bordeaux region was divided into 11 winegrowing sub-regions (strata). A sample of vineyard farms from each sub-region was randomly selected from the electronic yellow pages. Its size was proportionate to the total number of vineyard farms of the stratum. Farm managers or pesticide operators were contacted by phone during their working hours. A total of 343 eligible participants were successfully reached, of whom 312 accepted to complete the safety climate questionnaire (31 refusals).

Statistical analysis

The response to each of the items was a 5-point Likert scale (1 = disagree strongly, 2 = disagree slightly, 3 = neutral, 4 = agree slightly, 5 = agree strongly). For five negatively worded questions, reverse scores were calculated. Item scores were then summed.

Item-total correlation tests were performed to check if any item was consistent with the averaged behavior of the others. Following [START_REF] Devellis | Scale Development: Theory and Applications[END_REF], an item-total correlation < 0.30 indicated some factor outside of the whole scale and the corresponding item was discarded. This process was iterated until all correlations were ≥0.30. Internal consistency of the resulting set of items was measured by Cronbach's alpha coefficient. Reliability was considered good if alpha was greater than 0.70.

A principal component analysis (PCA) was used to confirm the unidimensionality of the questionnaire by checking the items' loadings on the first component.

Structural equation modelling (SEM) techniques were used to verify the model hypothesized from the exploratory analyses and to determine how well it fits the data. The SEM consisted in an external model representing the relationships between the latent (unmeasured) variables (the dimensions or constructs) and manifest (measured) variables (their items) as well as an internal model representing the relationships between the dimensions and another latent variable, the safety climate.

Standardized path coefficients were calculated, indicating the relative strength and sign of the relationships between variables while controlling for prior variables. The best fit model was assessed with the comparative fit index (CFI), the Tucker-Lewis index (TLI), the root mean square error approximation (RMSEA), and the standardized root mean square residual (SRMR). The following cutoff values were applied to assess the quality of the fit: > 0.95, > 0.95, < 0.05, and < 0.07, respectively.

All statistical analyses were performed using the Commander and lavaan packages from R software (R Development Core Team, 2018).

Results

Pre-testing process

The main actions taken as a result of the 16 farm managers or pesticide operators' judgment were as follows: seven questions with an insufficient degree of relevance or redundancy were removed and those presenting writing inconsistencies were accordingly changed. This content validation process resulted in the preliminary conceptual framework definition of the safety climate, consisting of 9 dimensions and 42 questions (Table 1).

Survey participants

The demographic characteristics and occupational factors for the 312 survey participants are reported in Table 2. Most respondents were male, aged over 40, had over 10 years of experience in agriculture, and were certified for pesticide application.

Reliability analysis

Because of a high rate of missing values (168 of 312), one item ("Actions taken by the agricultural cooperative aim at reducing exposure to pesticides") was removed from analyses. First-round item correlations are reported in Table 1. At this stage, the 19 items (of 41) with a corresponding coefficient <0.30 were discarded. Two further items ("Safety training given to me is adapted to the tasks I perform when handling pesticides" and "Safety regulation and good practices are adapted to work situations") were further removed in the second run of the analysis (both correlation coefficients equal to 0.28). This procedure resulted in a 9-dimension, 20-item safety climate questionnaire (grey shaded items in Table 1). Thus the total score of the scale could range from 20 to 100, a higher score reflecting a better safety climate. In this survey, the safety climate score ranked from 50 to 100 (mean: 82.9, standard deviation: 9.1).

Internal consistency of this 20-item version proved good with a Cronbach's alpha equal to 0.81. In the PCA analysis, the first component accounted for 23.4% of the total variance (while the second component explained 8.4% of the total variance), all factor loadings being positive and ranging from 0.18 to 0.29.

Confirmatory analysis

Using the SEM approach, the original structure in 9 dimensions and 20 items showed an acceptable fit to the data (CFI = 0.91, TLI = 0.90, RMSEA = 0.05, SRMR = 0.06). Path coefficients between items, dimensions and safety climate were all positive and highly significant (p values ≤ 0.0001).

However, modification indices showed that correlated error terms between Management commitment and Training dimensions, and also between Safety compliance and Safety system dimensions, were high (modification indices > 17), showing that these pairs of dimensions had something in common. As the items within these couples of latent variables were close semantically, we have slightly modified the conceptual framework by incorporating the single item from the Training dimension ("I am encouraged to attend safety training programs regularly") into the Management commitment dimension, and the single item from the Safety system dimension ("Personal protective equipment is adapted to my work activities") into the Safety compliance dimension. Safety participation remained as a one-item separate dimension. The SEM converged after 67 iterations. As a result, the final version of the questionnaire measuring the safety climate in vineyards is composed of 7 dimensions and 20 items (Table 3).

Figure 1 depicts the internal and external models and their standardized path coefficients. This SEM modeling fitted the data well (CFI = 0.97, TLI = 0.96, RMSEA = 0.03, SRMR = 0.05). All path coefficients remained positive and highly significant (p values ≤ 0.0001). External model parameters showed that the effects of the items on their dimension differed. For example, they varied from 0.3 to 0.6 and from 0.4 to 0.8 for the Management commitment and Safety compliance dimensions, respectively. Item effects were more homogeneous for the remaining dimensions. Internal model parameters revealed some heterogeneity regarding the impact of the 7 dimensions on the safety climate, the more influential being the Management commitment, Communication and feedback, Rules and practices, and Knowledge (all standardized path coefficients ≥ 0.7).

Discussion

The agriculture industry is a latecomer to the safety climate discussion. This significant gap in the field prompted us to develop and test the reliability and validity of a new instrument (7 dimensions operationalized through 20 items) designed for measuring safety climate in vineyards.

This study has several strengths. First, our approach to developing an adequate psychometric model to evaluate the safety climate in vineyards complies with the methodological principles that are recommended for research instrument modeling [START_REF] Do Nascimento | Psychometric model for safety culture assessment in nuclear research facilities[END_REF]. Second, using stratified random sampling made the study population as representative as possible. Evidence of construct validity provides confidence that the item measurements taken from the sample represent the true score in the vineyard-working population from the Aquitaine region. Third, the sample size and objective performance metrics used in this study strengthen the psychometric property and utility of this newly developed safety climate scale. This research has the limitations that are inherent in studies using perceptions and self-reported response data in a cross-sectional approach. While safety culture is more stable, safety climate is subject to fluctuations in response to local variable changes and can, therefore, be seen as an organization's temporal state of safety at a discrete point in time [START_REF] Huang | Development and validation of safety climate scale for lone workers using truck drivers as exemplar[END_REF]. Moreover, different vineyards are likely to have different managerial styles, attitudes towards safety issues, and levels of quality of the supervisor-employee relationship, inducing some heterogeneity. Criterion validity of the score could not be established because of the lack of safety behavior data that would have required ethnographic approaches based on observation and interview. Finally, we were also not able to set score limits to distinguish between performance levels and identify specific vineyards with low levels of safety climate to guide the implementation of strategies to improve safety culture.

A safety climate measurement tool suited for the vine-growing industry had not been introduced before the current study to the best of our knowledge. As Zohar suggested (2010), development of industry-specific climate scales should be encouraged as it is likely to identify new, contextdependent targets of climate perception in respective industries. We built our conceptual framework without any assumption on the number and composition of dimensions and their underlying causal structure. As all items predicted their corresponding dimensions (p values ≤ 0.0001) at the outset, no model-trimming was required. Only some fine-tuning was performed, based on the modification indexes to achieve better fit, as it made sense semantically. This new safety climate scale has been validated as a whole. Item groupings (or dimensions) were only indicative although helpful in the identification of a core safety climate item set and the confirmatory analysis. For these reasons, we discourage any dimension sub-scoring, all the more as the number of items per dimension is small, ranging from one to six.

Not surprisingly, the safety management practice dimensions (Management commitment, Communication and feedback, Rules and best practices), as well as the Knowledge dimension (a determinant of safety performance), turned out to have the most impact [START_REF] Flin | Measuring safety climate in health care[END_REF]. The components of safety performance (Safety compliance and Safety participation) rank lower, suggesting that organization governance was considered more influential than individual behaviors on safety climate. Finally, the lowest impact of the teamwork climate dimension may be related to the lone working that characterizes a number of vineyard tasks. Therefore, expectations of workrelated social support could be moderate and, at least, not considered to heavily influence the safety climate in these mainly individual practices. Further research across vineyards located in various countries would be useful in interpreting more accurately the core constructs of this new safety climate instrument.

Conclusion

The practical and theoretical significance of safety climate as a construct stems from its ability to predict safety behavior and safety-related outcome (here in the sense of lower pesticide use rather than fewer worker injuries) [START_REF] Huang | Development and validation of safety climate scale for lone workers using truck drivers as exemplar[END_REF]. It is hoped that the safety climate tool assessed in this study will help researchers, occupational health experts and vineyard managers in drawing up relevant interventions aimed at improving safety culture, raising pesticide risk awareness, and hopefully inducing more sustainable practices in the medium-term future.

Table 1

Conceptual framework of the safety climate and first-round item-total correlations (N=312).

Dimensions (construct) Indicators Item-total correlation

Management commitment MC1 Pesticide safety is given a high priority by farm management 0.48 MC2 Actions are undertaken to reduce pesticide exposure in the farm 0.38 MC3 Farm management strives to reduce pesticide exposure in farmworkers 0.28 MC4 Actions taken by the agricultural cooperative aim at reducing exposure to pesticides a NA MC5 Actions taken by health promoters aim at reducing exposure to pesticides 0.33 MC6 I have been consulted about safety issues when using pesticides 0.39 MC7 I am encouraged to become involved in pesticide safety matters 0.36 MC8 I trust the expertise of the people in charge of health risk reductions of pesticide use 0.

Dimensions Items

Management commitment Pesticide safety is given a high priority by farm management Actions are undertaken to reduce pesticide exposure in the farm Actions taken by health promoters aim at reducing exposure to pesticides I have been consulted about safety issues when using pesticides

IFig. 1 .

 1 Fig. 1. Final structural equation model with standardized path coefficients (all p values ≤ 0.0001, covariances have been omitted).

Table 2

 2 Characteristics of the participants in the safety climate survey (N=312).

	Gender
	Female	20 (6.4)
	Male	292 (93.6)
	Age (years)
	< 40	60 (19.3)
	40 -49	96 (30.8)
	50 -59	97 (31.0)
	≥ 60	59 (18.9)
	Experience in agriculture (years)
	< 10	15 (4.8)
	10 -19	64 (20.5)
	20 -29	95 (30.5)
	30 -39	86 (27.5)
	≥ 40	52 (6.7)
	Position
	Farm worker	13 (4.2)
	Farmer / farm manager	299 (95.8)
	Pesticide applicator certificate
	No	32 (10.3)
	Yes	280 (89.7)
	Pesticide-related activities
	Preparation
	No	46 (14.7)
	Yes	266 (85.3)
	Application
	No	48 (15.4)
	Yes	264 (84.6)
	Equipment cleaning
	No	48 (15.4)
	Yes	264 (84.6)

Table 3

 3 Final version of the questionnaire measuring safety climate for vineyards.

Acknowledgements

We are grateful to the pesticide operators and vineyard managers who made this study possible.

Moreover, we thank Mandy Lambert and Mireille Grandjean for their technical assistance.

The authors declare no conflicts of interest.

Funding sources

The French Crop Protection Industry Association (UIPP) funded the National Research Institute of Science and Technology for Environment and Agriculture for the field study within the framework of the Safe Use Initiative project. The funder had no role in the design or conduct of this study, the analysis or interpretation of the data, or the preparation of this manuscript.

Authors' contributions

SG generated the idea for the paper and supervised the fieldwork and data collection. JFV conducted the statistical analyses. SG and JFV wrote and approved the final manuscript.


Xavier Portier

Christian Hébert

Emrick Briand

Jacques Perriere

Eric Millon

Christophe Cachoncinlle

Magdalena Nistor

Nathalie Jedrecy

C Hebert

J Perrière

Microstructure of nanocomposite wurtzite-spinel (Fe:ZnO)-(Zn:Fe3O4) epitaxial films

Keywords: Nanocomposite, phase separation, zinc oxide, orientation relationships, transmission electron microscopy

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Since several decades, oxide-based nanocomposite materials may be obtained in bulk form by crystallization or precipitation of a phase (metal or oxide) inside a ceramic (i.e. oxide) matrix [START_REF] Wu | Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries[END_REF][START_REF] Miller | Nanoscale metal oxide-based heterojunctions for gas sensing: A review[END_REF]. In thin film form, this approach has been explored via the phase separation [START_REF] Zheng | Multiferroic BaTiO3-CoFe2O4 nanostructures[END_REF][START_REF] Millon | Phase separation in nanocomposite indium tin oxide thin films grown at room temperature: on the role of oxygen deficiency[END_REF], and such a combination of simple materials revealed much better properties than each component alone [START_REF] Li | High performance gas sensors based on in-situ fabricated ZnO-polyaniline nanocomposite: The effect of morphology on the sensing properties[END_REF]. Depending on the constituents and the growth conditions, nanocomposite thin films may recover multiple organization levels, ranging from small nanocrystallites embedded in a matrix [START_REF] Jedrecy | High magnetoresistance at low magnetic fields in self-assembled ZnO-Co nanocomposite films[END_REF][START_REF] Corral-Flores | Synthesis and characterization of novel CoFe2O4-BaTiO3 multiferroic core-shell-type nanostructures[END_REF], to horizontally stacked layers or vertically aligned pillars [START_REF] Zheng | Multiferroic BaTiO3-CoFe2O4 nanostructures[END_REF][START_REF] Chen | Microstructure, vertical strain control and tunable functionalities in self-assembled, vertically aligned nanocomposite thin films[END_REF]. Particular emphasis has been devoted to oxide-based nanocomposite thin films consisting in self-organized vertical hetero-structures and which arise from immiscible phases, such as BaTiO3-CoFe2O4 [START_REF] Macmanus-Driscoll | Self-Assembled Heteroepitaxial Oxide Nanocomposite Thin Film Structures: Designing Interface-Induced Functionality in Electronic Materials[END_REF][START_REF] Zhang | Phase-field model for epitaxial ferroelectric and magnetic nanocomposite thin films[END_REF]. The assembly mechanisms of these epitaxial oxide nanocomposite films have been presented and discussed [START_REF] Macmanus-Driscoll | Self-Assembled Heteroepitaxial Oxide Nanocomposite Thin Film Structures: Designing Interface-Induced Functionality in Electronic Materials[END_REF].

Due to its semiconducting and piezoelectric properties, wurtzite ZnO has been used in different oxide nanocomposites [START_REF] Jedrecy | High magnetoresistance at low magnetic fields in self-assembled ZnO-Co nanocomposite films[END_REF][START_REF] Fix | Electric-Field Control of Ferromagnetism in a Nanocomposite via a ZnO Phase[END_REF]. On the other way, spinel Fe3O4 or ZnFe2O4 phases have largely been studied as bulk or thin film for their magnetic properties. The ZnO-ZnFe2O4 nanocomposite in bulk or thin film form, is promising as negative electrode for lithium ion battery [START_REF] Woo | Synthesis and lithium electrode application of ZnO-ZnFe2O4 nanocomposites and porously assembled ZnFe2O4 nanoparticles[END_REF], or electrode materials for supercapacitor applications [START_REF] Agyemang | Electrospun ZnFe2O4-based nanofiber composites with enhanced supercapacitive properties[END_REF], for gas sensing [START_REF] Liu | Light irradiation enhanced triethylamine gas sensing materials based on ZnO/ZnFe2O4 composites[END_REF],

or solar photoelectrochemical water oxidation [START_REF] Cho | Self-Assembled Heteroepitaxial Oxide Nanocomposite for Photoelectrochemical Solar Water Oxidation[END_REF] and visible light photocatalytic activity [START_REF] Li | Facile synthesis of highly efficient ZnO/ZnFe2O4 photocatalyst using earth-abundant sphalerite and its visible light photocatalytic activity[END_REF]. Good thermoelectric properties have additionally been reported [START_REF] Liang | Thermoelectric Transport Properties of Fe-Enriched ZnO with High-Temperature Nanostructure Refinement[END_REF]. All these works evidence the importance in controlling the nature of the phases, their precise composition, structure and microstructure, for leading to the expected physical and chemical properties. In particular, a great attention has to be paid on the cationic distribution in the different separated phases, especially concerning the spinel one which may range from Fe3O4 to ZnFe2O4. In that sense, the wurtzite-spinel oxide composite deserves to be studied in detail.

Starting from the wurtzite ZnO in which all Zn 2+ are in tetrahedral position, the substitution of Zn 2+ by Fe 2+ leading to the Zn1-xFexO composition may occur for x<0.02 (Zn0.98Fe0.02O) in air, at 800°C for bulk material [START_REF] Degterov | Experimental study of phase equilibria and thermodynamic optimization of the Fe-Zn-O system[END_REF]. In the same conditions, the insertion of Zn in the Fe3O4 network is more effective, as the structure is preserved with 33% of Zn 2+ (x = 0.66) that corresponds to the ZnFe2O4 spinel phase. For x ranging from 0.66 to 1, a solid solution between normal and inverse spinel is formed, for which the lattice parameter follows the Vegard's law with a slight positive deviation indicating a normal structure preferably [START_REF] Schaefer | Eectrochemical determination of thermodynamic properties and X-ray diffraction investigation of the Fe3O4-ZnFe2O4 system[END_REF]. In the intermediary composition range (0.02 < x < 0.66), a bi-phased mixture of ZnO and Zn1-yFe2+yO4 (0 ≤ y ≤ 1) is obtained.

It is worth noticing that the composition ranges presented above are notably modified in thin film form. In a previous publication [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF], we reported a study of the phase diagram of Zn1-xFexO films grown by pulsed-laser deposition (PLD), by determining the nature of the phases present in the films as a function of the Fe concentration. The films were grown using a Zn1-xFexO target, at moderate substrate temperature (500°C) and under residual vacuum (10 -6 mbar pressure). ZnO wurtzite-based films may be epitaxially grown onto sapphire substrate down to x = 0.35 while pure epitaxial spinel films are formed for x > 0.8 [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF]. The optical, electrical and magnetic properties of such wurtzite-spinel films were discussed in relation with their structural characteristics [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF][START_REF] Jedrecy | Magnetic and magnetotransport properties of ZnxFe3-xO4-y thin films[END_REF][START_REF] Brachwitz | Evolution of magnetization in epitaxial Zn1-xFexOz thin films (0< x < 0.66) grown by pulsed laser deposition[END_REF]. In the intermediate region (0.35 < x < 0.8), the phase separation occurs leading to both wurtzite and spinel phases. As the wurtzite lattice may contain a certain fraction of Fe, while the spinel lattice may contain a certain amount of Zn, we will use in the following the notations Fe:ZnO and Zn:Fe3O4 to refer to the wurtzite and spinel phases, respectively. These nanocomposite films formed in the (0.35 < x < 0.8) domain were found to present electrical resistivity (from 1 to 5 10 -2 Ω.cm) and optical properties (large absorption in a wide wavelength range) well suited to photocatalytic applications. In addition, the presence of multiple boundaries between the two phases is particularly interesting in thermoelectrics.

In the present paper, we have studied in details the texture, microstructure and epitaxial relationships of the wurtzite and spinel phases in the nanocomposite films formed in the intermediate region (0.35 < x < 0.8), with emphasis on the nature of the interfaces between the two oxide phases. Using X-ray diffraction and transmission electron microscopy, we have precisely determined the epitaxial relationships between the wurtzite (or spinel) phase and the c-cut sapphire substrate, and between the wurtzite and spinel crystallites inside the nanocomposite film. These relationships are discussed as a function of the variation in Zn content in the different phases and various interfaces.

Experimental

The nanocomposite (Fe:ZnO)-(Zn:Fe3O4) films were grown by PLD using a frequency quadrupled Nd:YAG laser ((λ = 266 nm, τ= 7 ns, 10 Hz repetition rate) in the experimental set up already described [23]. Different Zn-Fe-oxide ceramic targets with a Zn/Fe ratio equal to 0.5/0.5 or 0.6/0.4, were irradiated by laser pulses (100 mJ/cm 2 ). The film growth was carried out on c-cut sapphire substrates, at 500°C under 5 10 -7 mbar, i.e. the residual vacuum.

After the growth, the nanocomposite films were cooled down to room temperature under the same pressure. As previously reported [START_REF] Brachwitz | Evolution of magnetization in epitaxial Zn1-xFexOz thin films (0< x < 0.66) grown by pulsed laser deposition[END_REF], the nanocomposite films present excess of iron with respect to the target composition.

The film thickness and chemical composition of the films were quantitatively determined by Rutherford backscattering spectrometry (RBS), using the 2 MeV Van de Graaff accelerator of the INSP laboratory. Such measurements gave the amount of Fe and Zn cations, with a good accuracy (1%), while the oxygen content was only determined with a 4% precision, because of the low RBS yield on light elements like oxygen. The nature of the crystalline phases present in the films was determined by X-ray diffraction analyses (XRD) using an Xpert Panalytical 4 circles diffractometer (Cu Kα radiation). The nature of the phases was investigated using the θ-2θ Bragg geometry, while the epitaxial relationships between the different phases and the c-cut sapphire substrate were determined using asymmetric diffraction, i.e. pole figure measurements.

Transmission electron microscopy (TEM) was used to characterize the local structure and composition of the films: the grain size and morphology, the orientation relationships between the substrate and the grains as well as those between the stacked grains, and the local chemical compositions of the grains and interfaces. The microscope used was a double corrected JEOL ARM 200F cold FEG operated at 200 kV, equipped with an energy dispersive X-ray (EDX) spectrometer (CENTURIO set up from JEOL) and an electron energy loss spectrometer (EELS, GIF QUANTUM 965ER setup from GATAN). The microscope was also implemented with a scanning system (STEM) combined with HAADF JEOL and GATAN detectors. The spatial resolution was 1 Å in TEM and 0.8 Å in STEM-HAADF mode. All the images were recorded and analyzed with the commercial software from GATAN called DIGITAL MICROGRAPH (DM2 version). The TEM foils were prepared by Focused Ion Beam (Helios Dual Beam nanolab660 FIB system). To protect the film surface from the gallium ion beam induced damage, a protective carbon layer and two platinum layers (one deposited electronically and the other one with the Ga beam) were deposited on the sample's surface prior to the thinning process.

Results

Chemical composition and microstructure of the nanocomposite films

Different nanocomposite films with wurtzite and spinel phases were obtained for a Fe/(Fe+Zn) ratio x between 0.50 and 0.75. We recall here that while the wurtzite phase is always present for x lower than 0.80 [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF], its presence is negligible for higher Fe concentrations (the Zn:Fe3O4 spinel phase is only formed). The phase separation leading to the presence of both wurtzite and spinel also depends upon the oxygen pressure during the growth, as observed in other cases [START_REF] Millon | Phase separation in nanocomposite indium tin oxide thin films grown at room temperature: on the role of oxygen deficiency[END_REF]. Indeed, in Fe3O4 the O/Fe ratio is equal to 1.33, and this means that an O/Fe ratio close to this value must be reached for the formation of the spinel phase. A too high ratio would lead to the formation of Fe2O3. In this work, the phase separation was obtained in the 5 10 -7 to 5 10 -6 mbar oxygen pressure range.

A typical RBS spectrum of a (Fe:ZnO)-(Zn:Fe3O4) nanocomposite film is shown in Fig. 1.

The RUMP simulation program was used to obtain the concentration profile of the elements.

The experimental spectrum cannot be fitted using an homogeneous composition. The simulated curve (continuous line) on the spectrum is obtained considering a Zn enrichment (or Fe depletion) towards the film-substrate interface, i.e from Zn0.23Fe0.77 composition at the top surface of the film, towards Zn0.26Fe0.74 at the interface with the substrate. The origin of this difference may be either an inward diffusion of Zn towards the interface or an outward Fe diffusion towards the surface during the film growth. This last possibility has been observed in the case of annealing at 700°C of Fe implanted in ZnO crystal with formation of the spinel phase [START_REF] Zhou | Crystallographically oriented magnetic ZnFe2O4 nanoparticles synthesized by Fe implantation into ZnO[END_REF]. Various contrasts are clearly visible from one grain to another. They correspond mainly to orientation changes (Bragg contrast) and/or composition variations (Z contrast). Another explanation could be the variation in the foil thickness due to the FIB system using a Ga beam for the thinning process but such a phenomenon would have led to lines and trenches in the image from the top to the bottom since the thinning process is unidirectional. Contrast variations are also visible in the grains. They are more likely explained by local strains modifying the Bragg conditions in these regions and may be by some local concentration fluctuations. Most of the grain boundaries are not sharp despite the good texture of the film with respect to the substrate. Figure 2(b) is a STEM HAADF image of a different region of the film than that presented in Fig. 2(a), and showing a clear variation in the chemical contrast from grain to grain. The whiter regions are richer in the heaviest element (Zn). Figures 2(c),(d) and (e) are STEM EDX images (chemical maps) for Zn, O and Fe respectively using their K edges. The oxygen map has a uniform contrast whereas the Zn map reveals some brighter regions indicating an enrichment of Zn especially close to the substrate and at junctions between large grains. The Fe map shows a Fe deficiency where a higher Zn content is observed. This result suggests that most of the grains are Zn:Fe3O4 with only a few small elongated Fe:ZnO ones located at the film/substrate interface and between large spinel crystallites.

Typical X-ray diffraction diagrams of nanocomposite films with x values of about 0.60 and 0.75 are presented in Fig. 3. In each case, two characteristic peaks related to the two phases, i.e. the 0002 reflection from the (hexagonal) wurtzite phase and the 222 reflection from the (cubic) spinel phase are observed, attesting the preferential texture for these two phases. Except these two peaks, only the 111 and 333 reflections from the spinel phase (and the 0003 and 0006 reflections from the substrate) are present when scanning on a larger 2θ angular range (10-60°, not shown). For the wurtzite phase, the 0002 peak is broad and shifted towards lower 2θ values than that of bulk ZnO, indicating an increase of the c-axis parameter. This increase has been already observed [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF][START_REF] Yamashita | The relationships between microstructure and crystal structure in zincite solid solutions[END_REF], and it can be related to the presence of Fe 3+ ions in substitution of Zn in the wurtzite matrix [START_REF] Yamashita | The relationships between microstructure and crystal structure in zincite solid solutions[END_REF]. The broadening of this peak can be due to both the large variation in the Fe concentration in the wurtzite phase, and the small size of the wurtzite crystallites. On the contrary, the position of the 222 spinel peak is well defined and its FWHM does not change with the film composition. In fact, the Zn doped ferrite is thermodynamically stable up to high Zn concentration (33%), and the variation of the spinel parameter with the Zn content is rather low. At last, the sharpness of the 222 peak of the spinel phase is consistent with the larger spinel grain size.

Epitaxial relationships of the wurtzite and spinel phases on sapphire

The wurtzite and spinel phases are both textured, and their in-plane epitaxial relationships have been determined by pole figure measurements. This relationship corresponds to a 30° rotation of the hexagons of the (0001) Fe:ZnO wurtzite lattice plane with respect to the hexagons of the (0001) Al2O3 lattice plane, and it will be noted by hereafter W-30°. This is the classical epitaxial relationship observed for pure ZnO films grown on c-cut sapphire substrate at T ≥ 500°C [START_REF] Ohkubo | Investigation of ZnO/sapphire interface and formation of ZnO nanocrystalline by laser MBE[END_REF][START_REF] Craciun | Epitaxial ZnO films grown on sapphire (001) by ultraviolet-assisted pulsed laser deposition[END_REF]. This epitaxial relationship was always observed, whatever the Fe concentration in the wurtzite phase, when this phase was present. The comparison of the respective in-plane lattice spacing for the (or [210])

sapphire and [2-1-10] (or [100]) wurtzite directions shows a very large lattice mismatch [START_REF] Narayan | Defects and interfaces in epitaxial ZnO/alpha-Al2O3 and AlN/ZnO/alpha-Al2O3 heterostructures[END_REF].

In the frame of the domain matching epitaxy [START_REF] Narayan | Defects and interfaces in epitaxial ZnO/alpha-Al2O3 and AlN/ZnO/alpha-Al2O3 heterostructures[END_REF][START_REF] Narayan | Recent progress in thin film epitaxy across the misfit scale[END_REF] such a W-30° in plane orientation is explained by the coincidence of m lattice units of the substrate with p lattice units of the film. This relationship also corresponds to a 30° rotation of the hexagons of the (111) spinel plane with respect to the hexagons of Al2O3, and it will be noted by S-30°. Notice that in the (111) lattice plane of the spinel, the lattice parameter is equal to ac/√2 where ac is the parameter of the cubic cell. The S-30° orientation has already been observed in the case of a single-phase Zn:Fe3O4 film on sapphire substrate, with high Fe concentrations (> 0.80) and for which, the wurtzite phase is not present [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF]. For this S-30° epitaxial relationship, as reported in Table I, the lattice mismatch corresponds to m = 7 and p = 5, leading to a lattice mismatch δ = 0.9% and a minimum size of epitaxial domain equal to 42 Å.

The 6 other poles in Fig. 4(b), correspond to the following epitaxial relationship:

Zn:Fe3O4 [10-1] // Al2O3 [2-1-10] (or [100]) S-0°
In this case, the hexagons of the (111) oriented zinc ferrite spinel crystallites have the orientation of the hexagons in the (0001) plane of Al2O3, i.e. the "hexagon on hexagon" orientation, which will be noted by S-0°. For this epitaxial relationship, 4 zinc ferrite units and 5 sapphire units are matching with a very low value of lattice mismatch δ equal to 0.15% (Table I).

The presence of two series of poles in Fig. 4(b), means that two kinds of (111) spinel crystallites are present in the nanocomposite film. Looking more precisely at the 6 poles of the S-0° epitaxy, it appears that 3 of these 6 poles are more intense than the others. Actually, the azimuthal positions of these 3 poles are exactly the same as those of the 10-11 poles of the Al2O3 substrate (not shown in the figure). These 3 poles correspond thus to spinel crystallites with (111) planes presenting a three-fold symmetry, as it is the case for the (0001) planes of Al2O3. The other poles have presumably the six-fold symmetry. Two kinds of S-0° spinel crystallites would thus be present: (i) with a three-fold symmetry, and (ii) with an apparent six-fold symmetry.

In addition, a detailed analysis of the six S-0° poles in Fig. 4 This in-plane orientation which is noted S-11°, corresponds to the coincidence of 5 Al2O3 lattice unit with 6 zinc ferrite lattice units, leading to a -1.9 % lattice mismatch and D = 62 Å for the minimum epitaxial domain (Table I). Although this lattice mismatch is largely higher than that for S-0° (-0.15%), zinc ferrite crystallites can be grown following this orientation.

Relative orientations and compositions of the wurtzite and spinel crystallites

The above analysis has allowed the determination of the specific orientations of the various crystallites with respect to the sapphire substrate. However, the nanocomposite nature of the films has to be taken into account. Actually, TEM images of the films show that in addition to wurtzite and spinel crystallites directly grown on Al2O3, crystallites inside the film present specific relative in plane orientations. A typical example is presented in Fig. 5 -a S-0° spinel crystallite with a 30° rotation with respect to the wurtzite, as previously observed.

The matching relationships, lattice mismatch and the minimum domain size corresponding to these epitaxial relationships are given in Table I. Regarding the domain sizes, it is not obvious to compare directly the data provided by the XRD data and those observed by electron microscopy. However, one can notice that the dimensions of the grains perpendicular to the film shown in the HRTEM image in Fig. 5 are of the same order of magnitude than those reported in Table 1.

The poles figures (Fig. 4) and TEM image (Fig. 5), agree with the fact that the wurtzite phase shows a sole in-plane orientation (W-30°). Actually, these W-30° wurtzite crystallites are grown directly on Al2O3, or inside the film, on spinel crystallites presenting the S-30° orientation. The (W-30° on S-30°) epitaxy, which corresponds to a hexagon on hexagon scheme, is indeed favored by a low lattice mismatch (δ =0.3%) together with a low domain size (D =36 Å), while the (W-0° on S-30°) epitaxy would lead for equivalent δ to a domain size of at least 100 Å.

On the contrary, the spinel phase may present different in-plane orientations. The S-30° orientation is observed for spinel crystallites grown on spinel crystallites (S-0°) (Figure 5), and also on the c-cut sapphire substrate (Figure 6). Both epitaxies may be explained (Table I) by a low lattice mismatch associated to a low domain size; on spinel, two possibilities are given: a relatively high lattice mismatch (1%) with a low domain size (42 Å), and secondly a low mismatch (0.3%) with a high domain size (114 Å).

The in-plane orientation S-0° is observed for spinel crystallites grown either on Al2O3 (Figure 6), or on W-30° wurtzite crystallites inside the film (Figure 5). The corresponding very low lattice mismatch (δ = -0.05%) and low domain size (D = 62 Å) of this epitaxial orientation explain its presence. The epitaxial growth of (111) oriented ferrite or Zn-doped ferrite on (0001) ZnO has already been reported [START_REF] Zhou | Crystallographically oriented magnetic ZnFe2O4 nanoparticles synthesized by Fe implantation into ZnO[END_REF], and a 30° in-plane rotation has been observed, in agreement with our results. Moreover, since the W-30° wurtzite presents a sixfold symmetry, it comes that the spinel crystallites growing on them with the S-0° orientation will present the same symmetry in the pole figure, i.e. a six-fold symmetry. It can thus be concluded that the S-0° crystallites presenting the three-fold symmetry in the pole figure would correspond to crystallites directly grown on the Al2O3 substrate.

The S-11° orientation relationship is more complicated to interpret. A possible explanation could be the result of stress between well oriented grains during the film growth but deeper investigation is needed to elaborate a possible formation mechanism.

Concerning the origin of the S-0° and S-30° orientations of the spinel crystallites directly grown on the Al2O3 substrate, the comparison of the associated lattice mismatch and domain size (Table I) indicates that the S-0° would be the more favorable case. Our TEM results show that the most frequently observed is the 30° rotation for the spinel phase, the S-0° usually appearing when the wurtzite phase is present, i.e. for sufficient Zn concentrations.

Additional insights have been obtained through the analysis of the Zn and Fe composition of the spinel crystallites. Figure 6 shows HRTEM images and EDX atomic profiles corresponding to S-30° and S-0° crystallites directly grown on Al2O3. For the S-30° orientation (Figures 6(a) and (b)), the EDX profiles indicate a constant concentration of Fe and Zn, through the whole spinel crystallite. The cationic distribution Zn0.21Fe0.79 as deduced from these EDX profiles must be compared to the overall distribution obtained by RBS, that is Zn0.25Fe0.75. A slight deficit in Zn exists in the S-30° crystallites, which can be related to the presence of the wurtzite phase in the nanocomposite film.

Differently, for the S-0° spinel crystallite, a Zn enrichment is observed at the interface with the substrate, accompanied by a corresponding decrease of the Fe content (Figures 6(c) and (d)). The cationic distribution deduced from EDX in the region close to the interface with the substrate is Zn0.39Fe0.61 (to be compared to Zn021Fe0.79 for S-30°). This corresponds to a spinel highly rich in Zn, with the composition Zn1.17Fe1.83O4. This behavior was systematically observed in the case of spinel crystallites directly grown on the sapphire substrate with the S-0° orientation, and would mean that above a certain increase in the Zn concentration in the zinc ferrite, a change in the in-plane epitaxial relationship with the c-cut sapphire substrate will occur (from S-30° to S-0°). The driving force for the S-0° orientation would thus be the Zn enrichment at the interface with the substrate.

Similar phenomena can be observed when the wurtzite phase is present. Indeed, Fig. 7(a) shows an HRTEM image of a region with a rather thin wurtzite layer (about 8 nm thick)

between the substrate and a S-0° oriented spinel. The EDX analysis (Fig. 7(b)) leads to a Zn0.5Fe0.5 cationic distribution in the wurtzite crystallite, followed by a decrease of the Zn concentration in the spinel phase. The S-0° oriented spinel phase is related to the high Zn concentration at the interface with the wurtzite crystallite, since in the "bulk" of the S-0° spinel crystallite, a Zn0.2Fe0.8 cationic distribution is present. In the case of two spinel crystallites separated by a wurtzite crystallite (Figures 7(c) and (d)), the presence of the wurtzite crystallite leads also to a change in the spinel orientation. In the middle part of the image in Fig. 7(c), the wurtzite presents the "hexagon on hexagon" orientation with respect to the S-30° spinel crystallite underneath. In the upper part, the spinel (S-0°) presents a 30° rotation with the wurtzite W-30°. The EDX scan in the middle part of the image (Fig. 7(d))

shows a general increase of the Zn concentration (on average Zn0.5Fe0.5) as expected for the wurtzite phase formation (W-30°) on the S-30° spinel phase. A steep decrease of the Zn concentration is noticed at the interface with the S-0° spinel, together with a sharp interface, compared to the case of the interface with the S-30° spinel.

Discussion

The above results show well-defined epitaxial relationships between the wurtzite and spinel phases and the Al2O3 substrate. These specific in-plane orientations are directly related to changes in the Zn concentration in the spinel crystallites. Magnetite Fe3O4 is an inverse spinel, where the tetrahedral sites are occupied by Fe 3+ while the octahedral sites contain Fe 2+

and Fe 3+ ions. On the contrary, ZnFe2O4 is a normal spinel, the octahedral sites being occupied by only Fe 3+ ions. The occupancy of Zn 2+ , Fe 2+ and Fe 3+ cations in octahedral or tetrahedral sites is governed by ionic size effect and preferred environment related to crystal field. It has been calculated that the site preference of Zn 2+ is in tetrahedral geometry [START_REF] Fitzner | Thermodynamic properties and cation distribution of the ZnFe2O4-Fe3O4 spinel solid-solutions at 900°C[END_REF].

The following reaction:

Zn 2+ octa + Fe 3+ tetra Zn 2+ tetra + Fe 3+ octa is exothermal (-5.02 kJ at 900°C) [START_REF] Fitzner | Thermodynamic properties and cation distribution of the ZnFe2O4-Fe3O4 spinel solid-solutions at 900°C[END_REF]. This reaction may occur when iron atoms in pure magnetite are substituted by Zn 2+ . It comes that a cationic exchange from Fe 2+ octa and Fe 3+ tetra in magnetite allows to insert Zn 2+ in tetrahedral sites in magnetite leading to switch to the normal spinel structure of zinc ferrite. The S-0° orientation would thus correspond to the epitaxial growth of normal spinel (Zn rich) on the c-cut Al2O3, while the S-30° orientation would correspond to the epitaxial growth of inverse spinel on the sapphire.

The S-0° oriented crystallites present a three-fold symmetry in the pole figure of Fig. the existence of crystallites presenting different symmetries in the pole figure has been previously reported in the case of the growth of In2O3 bixbyite films on c-cut sapphire substrate [START_REF] Seiler | Epitaxial undoped indium oxide thin films: Structural and physical properties[END_REF]. This was interpreted assuming that "ordered" and "disordered" bixbyite crystallites were present in the films, the "disordered" crystallites having a random distribution of oxygen vacancies in their (111) planes leading to a six-fold symmetry.

Differently, a fixed distribution of oxygen vacancy in this plane led to the three-fold symmetry of the ordered bixbyite crystallites. A similar approach can be proposed here, based on the fact that Zn is randomly substituted to Fe in tetrahedral sites of the spinel lattice [START_REF] Harris | Cation distribution in NiZn-ferrite films determined using x-ray absorption fine structure[END_REF]. If these Zn ions occupy fixed lattice positions in the (111) planes, an ordered crystallite with a three-fold symmetry would be obtained. On the contrary, if the Zn ions are randomly distributed in the tetrahedral sites of the (111) planes, a disordered crystallite with a six-fold symmetry will be obtained.

In the epitaxial growth of a film on a substrate, the nature of the first plane on the film grown on the substrate is of prime importance. Indeed, the precise atomic repartition in this plane will define the interface energy which will play a major role in the epitaxial relationships between film and substrate. The (000l) plane of the sapphire substrate is a polar plane, and in the growth conditions the last sapphire plane would be an oxygen plane [START_REF] Wang | Effect of hydrogen on Al2O3/Cu interfacial structure and adhesion[END_REF][START_REF] Yahiro | Growth mode of (111) oriented spinel type ZnFe2O4 thin film by laser-molecular beam epitaxy technique[END_REF].

Accordingly, the first plane of the growing zinc ferrite would be a cationic plane. In the spinel structure, two kinds of cationic (111) planes are present: one is named the Kagomé plane (K plane) with a specific site distribution [START_REF] Yahiro | Growth mode of (111) oriented spinel type ZnFe2O4 thin film by laser-molecular beam epitaxy technique[END_REF][START_REF] Liu | Epitaxial growth of (111)-oriented spinel CoCr2O4/Al2O3 heterostructures[END_REF], and the other one presents a triangular network (T planes). Moreover, the K plane only contains octahedral sites, while two different T planes are identified containing either only tetrahedral sites (Tt planes), or only octahedral sites (To planes). In the spinel structure, the (111) planes distribution is the following: K-Tt-To-Tt-K [START_REF] Liu | Epitaxial growth of (111)-oriented spinel CoCr2O4/Al2O3 heterostructures[END_REF]. In the case of a low Zn concentration in the crystallites (corresponding to the S-30° epitaxy), both Zn and Fe will be present in the Tt planes (inverse spinel). Assuming that the Zn and Fe are randomly distributed in such Tt planes, this will lead to a six-fold symmetry for such crystallites. Differently, for a high Zn concentration (corresponding to the S-0° epitaxy), Zn will occupy the sites of the Tt planes (normal spinel), and a three-fold symmetry is possible for such high Zn concentration crystallites. It follows that a Tt plane would be the first plane on the sapphire for the S-0° epitaxy.

Then, it seems reasonable to assume that the S-30° epitaxy would correspond to a Kagome plane as the first plane on the sapphire substrate for the crystallite with a low concentration in Zn. Indeed, the triangular planes Tt and To present the same site distribution, even if these sites are occupied be different cations (Zn or Fe), and they would correspond to the S-0° epitaxy.

These nanocomposite films formed during this work, look like the random stacking of wurtzite and spinel platelets with variable thicknesses, i.e. 5 to 15 nm for the wurtzite and 5 to 30 nm for the spinel phase. The formation of the wurtzite platelets is a priori not expected.

Indeed, the pure ZnO phase classically presents a c-axis texture with a columnar microstructure, owing to the high growth rate following the c-axis. However, the incorporation of Fe 3+ in the ZnO network not only leads to an increase of the c-axis parameter, but in addition a transition of the ZnO grain morphology towards plate-like zincite crystal has been already reported [START_REF] Yamashita | The relationships between microstructure and crystal structure in zincite solid solutions[END_REF].

Such a microstructure for the wurtzite -spinel nanocomposite films is quite different from the regular stacking observed in some nanocomposite films like HfO2-SiO2 [START_REF] Liu | Surface-directed spinodal decomposition in hafnium silicate thin films[END_REF] or YBCO-BZO [START_REF] Yang | Self-assembled multilayers and enhanced superconductivity in (YBa2Cu3O7-x)[END_REF] for example. The formation of such regular multilayer nanocomposite films was assumed to be due to the surface-directed spinodal decomposition (SDSD), based on the preferential attraction of the surface to one of the two components [START_REF] Puri | Surface effects on spinodal decomposition in binary-mixtures and the interplay with wetting phenomena[END_REF]. SDSD simulations show the formation perpendicular to the surface of a composition wave propagating in the film. This leads to the formation of a regular multilayered heteroepitaxial phase assemblage [START_REF] Puri | Surface effects on spinodal decomposition in binary-mixtures and the interplay with wetting phenomena[END_REF]. It must be noticed that the high temperature annealing (1350°C) of a bulk ZnO-Fe3O4 material [START_REF] Liang | Thermoelectric Transport Properties of Fe-Enriched ZnO with High-Temperature Nanostructure Refinement[END_REF], leads via the spinodal decomposition, to a regular nanostructure with alternate layers of wurtzite (Fe rich ZnO) and spinel (Zn rich Fe3O4) with an average grain boundary spacing value of about 200 nm [START_REF] Liang | Thermoelectric Transport Properties of Fe-Enriched ZnO with High-Temperature Nanostructure Refinement[END_REF]. In this case, owing to the high temperature treatment, the diffusion of Fe and Zn is sufficient to form the regular nanostructure.

Even if we have observed a slight enrichment in Fe towards the surface of the nanocomposite films formed at 500°C, it is not possible to conclude that the surface-directed spinodal decomposition process can be at the origin of the microstructure observed in this work. Taking into account the fact that a Zn enrichment appears at the interface between wurtzite and spinel phase (Figures 6 and7], and that the Zn concentration plays an important role in the in-plane orientation of the spinel phase, a possible explanation of the microstructure observed in this work can be proposed according to the specificity of the PLD growth.

As noted above, the main phase observed in the nanocomposite films is the spinel phase, with the presence of thinner wurtzite platelets. The overall cationic composition of the film determined by RBS is Zn0.27Fe0.73, but this is only a mean value. In fact, the PLD can be considered as a random process, in the sense that the flux of the Zn and Fe species coming from the target will not be the same at the same time in all the points of the substrate.

Moreover, for a given point of the substrate, the flux of Zn and Fe species will not be the same as a function of time. This means that locally, the Zn and Fe concentrations measurable by EDX technique can be different from the mean values determined by RBS, and will be different as a function of time. As a matter of fact, considering the first step of the film deposition on sapphire substrate, locally a Zn-rich region can exist in some points. If this Zn enrichment is not too large (< 33%) it can be at the origin of the S-30° orientation for the spinel phase (as observed in Figures 6(c) and (d)), by the formation of a Zn-rich spinel phase.

On the contrary, if this Zn enrichment is important (about 50%), the formation of the wurtzite phase at the interface with the Al2O3 substrate can occur (as it is shown in Figures 6(a) and (b)). However, this Zn local enrichment cannot occur during all the film growth, and when the Zn concentration decreases, the spinel phase will grow on top of the W-30° crystallite with the S-30° orientation (as shown in Fig. 6(a)).

Similar phenomena can occur inside the film, and the Zn enrichment on top of a spinel S-30° crystallite, will lead to the formation wurtzite W-30° (Fig. 7). Then a decrease of the Zn concentration will lead to the formation of the spinel phase with the S-30° orientation (Fig. 7).

If the Zn enrichment locally inside the film is not sufficient to induce the formation of the wurtzite phase, it could lead to a change from S-0° towards S-30° orientation (as shown in Fig. 5). The driving force for the wurtzite-spinel nanocomposite film would thus be the change of composition in the film as a function of the position and of time.

As previously mentioned [START_REF] Perrière | Zn1-xFexO films: from transparent Fe-diluted ZnO wurtzite to magnetic Zn-diluted Fe3O4 spinel[END_REF], the nanocomposite epitaxial wurtzite-spinel films based on earth-abundant and low cost materials, presenting both a good optical absorption in a large domain of the solar spectrum, and a noticeable electronic conductivity, could be an interesting approach for photocatalytic applications. The nanocomposite wurtzite-spinel films could also be of interest in thermoelectric applications. In fact, the microstructure observed in this work with a large number of boundaries between the two phases is particularly interesting in view to obtain a low thermal conductivity value [START_REF] Liang | Relation between thermoelectric properties and phase equilibria in the ZnO-In2O3 binary system[END_REF]. The thermal conductivity of the bulk ZnO-ZnFe2O4 material presenting alternate layers of wurtzite and spinel with a 200 nm spacing was found around 10 W/mK, i.e. a value 10 times lower than the thermal conductivity of bulk ZnO [START_REF] Liang | Thermoelectric Transport Properties of Fe-Enriched ZnO with High-Temperature Nanostructure Refinement[END_REF]. Taking into account that the density of boundaries is ten times higher in our nanocomposite wurtzite -spinel film (spacing about 20 nm), we could expect a further decrease of the thermal conductivity down to 1-2W/mK.

In summary, nanocomposite films formed with both wurtzite (Fe:ZnO) and spinel (Zn:Fe3O4) phases have been grown by PLD on c-cut sapphire substrate. Both phases present preferential axis growth according to the (0001) wurtzite and (111) spinel planes parallel to the basal (000l) sapphire plane. The wurtzite crystallites are epitaxially grown following to a 30° rotation of the hexagons of the (0001) Fe:ZnO plane with respect to the hexagons of the (0001) Al2O3 plane. On the contrary, the spinel crystallites show two main in-plane epitaxial relationships corresponding to the coincidence of spinel and sapphire hexagons without rotation (S-0°), or a 30° rotation between the hexagonal cells (S-30°). In addition, these specific W-30°, S-0° and S-30° in-plane relationships have been also observed for the wurtzite and spinel crystallites inside the film. The presence of S-0° or S-30° orientations for spinel crystallites on the (000l) sapphire planes has been related to the Zn concentration in the (111) family planes of spinel crystallites. The S-30° and S-0° orientations correspond to low and high Zn concentrations in the spinel phase, respectively. This result is tentatively explained by the variation of Zn 2+ ion concentration in the tetrahedral sites in the 111 triangular planes (Tt plane).

Table captions

Figure captions

Figure 2 (

 2 Figure 2(a) is a bright field TEM image of a nanocomposite film taken along the [01-10] (or [120]) direction of the sapphire substrate. Large elongated grains are lying parallel to the film/substrate interface, i.e. the nanocomposite looks like a random stacking of crystallites.

Figure 4

 4 figure. The azimuthal position of the six 10-11 Fe:ZnO poles with respect to the sapphire substrate leads to the following in plane epitaxial relationship :

  The m and p values are defined as the minimum integers satisfying the relation: mds = pdf, where ds and df are the respective atomic distances in the substrate and film parallel directions[START_REF] Narayan | Recent progress in thin film epitaxy across the misfit scale[END_REF][START_REF] Sbai | Epitaxial growth of titanium oxide thin films on c-cut and alpha-cut sapphire substrates[END_REF]. In the present case, along the (or [210]) axis of sapphire it comes that (m = 7, p = 6) leads to a lattice mismatch δ = 1.41 % and a minimum size of epitaxial domain D equal to 34 Å, as indicated in Table I which presents the matching relationships, lattice mismatch δ and size of epitaxial domain D, for the various epitaxial relationships evidenced in this work. In Fig. 4(b), the pole figure was recorded for the 220 spinel reflection at 2θ = 29.80°. Twelve poles are observed at Ψ = 35.60°, and they correspond to two series of 6 poles which characterize different epitaxial relationships. The first one (indicated by #) corresponds to the following in-plane epitaxial relationship with the sapphire substrate: Zn:Fe3O4 [10-1] // Al2O3 [10-10] or [210] S-30°

  (b) shows that these poles present symmetric shoulders in their azimuthal positions, i.e. at about 10/11° with respect to the central pole position. The following in-plane epitaxial relationships can explain the presence of these shoulders: Zn:Fe3O4 [11-2] // Al2O3 [41-50] (or [320]) S-11°

  which shows an HRTEM image where Fe:ZnO and Zn:Fe3O4 successive crystallites following the growth direction are observed. The fast Fourier transforms (FFT) are also shown for each crystallite. Starting from the c-cut sapphire substrate, the following epitaxial crystallites are found: -a wurtzite crystallite with the as-expected W-30° orientation, Fe:ZnO [2-1-10] (or [100]) // Al2O3 [10-10] (or [210]) -a spinel with a S-0° orientation (with respect to the sapphire substrate) meaning that this crystallite is 30° rotated with respect to the underneath W-30° wurtzite crystallite Zn:Fe3O4 [11-2] // Fe:ZnO [2-1-10] (or [100]) -a spinel with a S-30° orientation, meaning a 30° rotation with respect to the underlying S-0° spinel crystallite Zn:Fe3O4 [10-1] // Zn:Fe3O4 [11-2]-a wurtzite crystallite with a W-30° orientation on a S-30°, i.e. hexagon on hexagon with respect to the underlying spinel crystallite Fe:ZnO [2-1-10] (or [100]) // Zn:Fe3O4

4

  (b), while the S-30° crystallites lead to a six-fold symmetry in this pole figure. Such a, i.e.

Fig. 1 :

 1 Fig. 1: Typical RBS spectrum for a nanocomposite film grown on c-cut sapphire substrate at 500°C and 5 10 -7 mbar residual pressure. The solid line is the simulated spectrum obtained by the use of the RUMP program.

Fig. 2 :

 2 Fig. 2: (a) Bright field TEM image of the Zn-Fe-O film. Contrasts in the image are due to Bragg contrast and/or composition of the grains. (b) STEM HAADF dark field image with the corresponding Zn (c), O (d) and Fe (e) chemical maps obtained by STEM EDX.

Fig. 3 :

 3 Fig. 3: θ-2θ diffraction patterns recorded on nanocomposite (wurtzite and spinel phases) films grown at 500°C under residual vacuum (10 -7 mbar). The vertical dashed lines figure the bulk positions.

Fig. 4 :

 4 Fig. 4: Pole figures recorded on a nanocomposite (Zn0.27Fe0.73O1.33) film for (a) the wurtzite phase and the (10-11) reflection (2θ = 36.25°); (b) the spinel phase and the (220) reflection (2θ = 29.8°).

Fig. 5 :

 5 Fig. 5: HRTEM image of a region showing the different orientations of wurtzite and spinel grains with respect to the substrate.

Fig. 6 :

 6 Fig. 6: (a) HRTEM image and (b) EDX profile of S-30 oriented spinel at the bottom of the film. (c) HRTEM image and (d) EDX profile of S-0 oriented spinel at the bottom of the film. The EDX profiles have been recorded along a 60 nm line from the film towards the substrate in both cases. The interfacial region rich in Zn of the S-0° spinel is delimited by dashed lines in (d).

Fig. 7 :

 7 Fig. 7: (a) HRTEM image and (b) EDX profile of S-0 oriented spinel and W-30 oriented wurtzite at the bottom of the film (recorded from the film towards the substrate as indicated by the arrow in (a)). (c) STEM HAADF dark field image of two S-30 and S-0 oriented spinel grains separated by a W-30 oriented wurtzite crystallite, (d) EDX profile across the three grains recorded from the top to the bottom of the region as indicated by the arrow in (b). The interfacial regions rich in Zn are delimited by dashed lines in (b) and (d).

Fig. 1 :

 1 Fig. 1: Typical RBS spectrum for a nanocomposite film grown on c-cut sapphire substrate at 500°C and 5 10 -7 mbar residual pressure. The solid line is the simulated spectrum obtained by the use of the RUMP program.

Fig. 2 :

 2 Fig. 2: (a) Bright field TEM image of the Zn-Fe-O film. Contrasts in the image are due to Bragg contrast and/or composition of the grains. (b) STEM HAADF dark field image with the corresponding Zn (c), O (d) and Fe (e) chemical maps obtained by STEM EDX.

Fig. 3 :Fig. 4 :

 34 Fig. 3: θ-2θ diffraction patterns recorded on nanocomposite (wurtzite and spinel phases) films grown at 500°C under residual vacuum (10 -7 mbar). The vertical dashed lines figure the bulk positions.

Fig. 5 :

 5 Fig. 5: HRTEM image of a region showing the different orientations of wurtzite and spinel grains with respect to the substrate.

Fig. 6 :

 6 Fig. 6: (a) HRTEM image and (b) EDX profile of S-30 oriented spinel at the bottom of the film. (c) HRTEM image and (d) EDX profile of S-0 oriented spinel at the bottom of the film. The EDX profiles have been recorded along a 60 nm line from the film towards the substrate in both cases. The interfacial region rich in Zn of the S-0° spinel is delimited by dashed lines in (d).

Fig. 7 :

 7 Fig. 7: (a) HRTEM image and (b) EDX profile of S-0 oriented spinel and W-30 oriented wurtzite at the bottom of the film (recorded from the film towards the substrate as indicated by the arrow in (a)). (c) STEM HAADF dark field image of two S-30 and S-0 oriented spinel grains separated by a W-30 oriented wurtzite crystallite, (d) EDX profile across the three grains recorded from the top to the bottom of the region as indicated by the arrow in (c). The interfacial regions rich in Zn are delimited by dashed lines in (b) and (d).

Table I :

 I Matching orientation relationships, lattice mismatch δ and size of epitaxial domain D, for the various epitaxial configurations evidenced in this work. The m, p and D values are given with respect to the mentioned directions and the lattice mismatch is calculated with respect to the substrate or the template layer.

Acknowledgments

X. Portier would like to thank the French ANR (Agence Nationale de la Recherche) institution for the EQUIPEX "GENESIS" grant "ANR-11-EQPX-0020" in the frame of the "Investissements d'avenir" as well as FEDER and the Normandie Region funding that allowed the acquisition and the use of the FIB system (HELIOS Nanolab 600 from FEI) and the EELS spectrometer (QUANTUM ER965 from GATAN). N. Jedrecy thanks the "ANR-15-CE09-0005-01" for their funding and warmly acknowledges D. Demaille for the preliminary TEM analyses conducted on the nanocomposite films. C. Cachoncinlle, E. Millon and J. Perrière would like to thank T. Orlowski for her help during specific manipulations.

S-0° on sapphire Zn:Fe3O4 [10-1] // Al2O3 [2-1-10]

S-11° on sapphire Zn:Fe3O4 [11-2] // Al2O3 [41-50] S-0° on wurtzite W-30° Zn:Fe3O4 [11-2] // Fe:ZnO [2-1-10] W-30° on spinel S-30° Fe:ZnO [2-1-10] // Zn:Fe3O4 [10-1]

S-30° on spinel S-0°