Valérie Potin
email: valerie.potin@u-bourgogne.fr

Houssem Boukhalfa

Nicolas Martin

Oblique angle co-deposition of nanocolumnar tungsten thin films with two W sources: Effect of pressure and target current

Keywords: Writingoriginal Tungsten, co-sputtering, GLAD, nanocolumns, microscopy

Two series of tungsten thin films are sputtered on silicon and glass substrates by oblique angle co-deposition technique with an original configuration. Two opposite distinct tungsten targets are simultaneously used, both tilted with an oblique angle of 80°. The growth is performed at low (0.33 Pa) and high (1.5 Pa) argon sputtering pressure and the current intensity applied to the targets varies between 50 and 250 mA. The effect of these deposition parameters on the films microstructure and electrical properties is investigated by scanning and transmission electron microscopy, X-ray diffraction and pole figures, and van der Pauw method. Due to self-shadowing effect, all tungsten sputtered thin films are porous and columnar. At low pressure, the columnar tilt angle β can be tuned with the target current intensity until obtaining vertical columns. X-ray diffraction and pole figure analyses point out a A15 crystal structure (β-W) and a uniaxial fiber texture with a <100> growth direction. In contrast, tungsten thin films deposited at high pressure present a cauliflower structure and are poorly crystallized. Both deposition parameters also affect the films electrical resistivity and anisotropy. These behaviors are discussed and linked to the microstructure and crystallography.

Introduction

The modification of thin films properties due to oblique incidence of particles has been first reported by Knorr and Hoffman [START_REF] Knorr | Dependence of Geometric Magnetic Anisotropy in Thin Iron Films[END_REF] and by Smith [START_REF] Smith | Anisotropy in Permalloy Films[END_REF]. The oblique angle deposition (OAD) is mainly performed using physical vapor deposition (PVD) and is characterized by an oblique incidence of the incoming particle flux, as the source makes an angle α with the direction normal to the substrate [START_REF] Van Kranenburg | Tailoring growth and local composition by oblique-incidence deposition: a review and new experimental data[END_REF]. As certain areas of the growing film are shadowed for direct impingement, a selective competition occurs between the shadowed and non-shadowed areas and the so-called "self-shadowing effect" results in anisotropic characteristics in thin films, eventually lessened by adatom surface diffusion [START_REF] Dirks | Columnar Microstructure in Vapor-Deposited Thin Films[END_REF][START_REF] Abelmann | Oblique evaporation and surface diffusion[END_REF]. Generally, OAD thin films are characterized by a higher porosity and tilted columnar structures in comparison to thin films elaborated with a normal incidence. This anisotropic microstructure generates anisotropic behavior in optical [START_REF] Schmidt | [END_REF], magnetic [START_REF] Keitoku | Grain Structure and Magnetic Anisotropy of Fe Film Evaporated Obliquely from Two Sources[END_REF] and electrical [START_REF] Beainou | Resistivity anisotropy of tilted columnar W and W-Cu thin films[END_REF] properties. The porous character of the OAD thin films leads to an increased surface area that is a key parameter for many applications such as sensors, electrochemistry and catalysis [START_REF] Matolin | [END_REF][10][START_REF] Chundak | Tailoring of highly porous SnO2 and SnO2-Pd thin films[END_REF].

The GLancing Angle Deposition (GLAD) is a technique derived from OAD that allows also to grow three-dimensional (3-D) nanostructures by altering the substrate motion and/or rotation [START_REF] Robbie | Fabrication of thin films with highly porous microstructures[END_REF][START_REF] Robbie | Sculptured thin films and glancing angle deposition: Growth mechanics and applications[END_REF][START_REF] Robbie | Advanced techniques for glancing angle deposition[END_REF]. Nanostructures including tilted or vertical columns, zigzags, S-shapes, C-shapes, spirals and helices are sculptured by modifying the polar (α) and azimuthal () angles between the normal to the substrate and the incident deposition flux [10,[START_REF] Hawkeye | Glancing Angle Deposition of Thin Films: Engineering the Nanoscale[END_REF]. The nanoarchitectured structures fabricated by GLAD technique have demonstrated enhanced capabilities in many applications including chemical and biological sensors, optical and electronic devices, magnetic storage as well as innovative coatings and applications [START_REF] Hawkeye | Glancing Angle Deposition of Thin Films: Engineering the Nanoscale[END_REF][START_REF] He | Advanced multi-component nanostructures designed by dynamic shadowing growth[END_REF].

To form multi-composition or multi-component structures, simultaneous or sequential oblique deposition of two materials from two different sources positioned at equal or different J o u r n a l P r e -p r o o f angles has also been proposed [START_REF] Keitoku | Inhomogeneous Concentration Distribution in Obliquely Evaporated Fe-Cu Films[END_REF]. Several systems were grown by OAD co-deposition technique with two different sources such as Fe-Cu [START_REF] Keitoku | Inhomogeneous Concentration Distribution in Obliquely Evaporated Fe-Cu Films[END_REF], Co-M (M=Cr, Ag and Ta) [START_REF] Van Kranenburg | Tailoring growth and local composition by oblique-incidence deposition: a review and new experimental data[END_REF] and Pt-M (M=Al, Si, Ti, Zr, Sn and Ce) [START_REF] Watanabe | [END_REF] thin films. GLAD nanostructures were also obtained by co-deposition technique for Si-M (M=Cr, Pt, Ag, Cu and Ta) [START_REF] Kesapragada | Two-component nanopillar arrays grown by Glancing Angle Deposition[END_REF][START_REF] He | Designing Catalytic Nanomotors by Dynamic Shadowing Growth[END_REF] as well as numerous metals, semiconductors and compounds [START_REF] He | Advanced multi-component nanostructures designed by dynamic shadowing growth[END_REF]. By using two sources with oblique incidence, the morphology and/or elemental composition become also anisotropic. The 3-D growth is controlled by key parameters such as the angles of the incident fluxes, the deposition rates, the eventual substrate rotation/motion (and its speed) as well as usual deposition parameters [10,[START_REF] Hawkeye | Glancing Angle Deposition of Thin Films: Engineering the Nanoscale[END_REF]. By modifying the deposition sequence (simultaneous or sequential) and the substrate rotation (stationary, continuous or intermittent), it is also possible to control the distribution of the components both laterally and vertically within the 3-D nanostructures [START_REF] Kesapragada | Two-component nanopillar arrays grown by Glancing Angle Deposition[END_REF][START_REF] Zhou | Multi-component nanostructure design by atomic shadowing[END_REF].

OAD and GLAD co-depositions are generally performed with two different materials obliquely deposited from two different targets. Only a few studies were performed on thin films prepared with two sources of a similar material. In 1981, Keitoku and Nishioka studied the grain structure and magnetic anisotropy of Fe films evaporated obliquely from two iron sources [START_REF] Keitoku | Inhomogeneous Concentration Distribution in Obliquely Evaporated Fe-Cu Films[END_REF].

They reported that the iron films were formed by vertical columns, with an inclination of the columns with respect to the substrate surface normal almost equal to zero. This was in contrast with the columnar tilt angle equal to 45° measured for Fe films elaborated with a similar angle of incidence but with only one iron source [START_REF] Hara | Origin of Anomalous Magnetic Anisotropy of Iron Films Evaporated at Oblique Incidence[END_REF]. In 1995, Robbie et al. [START_REF] Robbie | Fabrication of thin films with highly porous microstructures[END_REF] evaporated MgF2 films by using two similar and opposite sources and obtained very porous thin films with isolated columns separated by large voids. In 2006, Kesapragada and Gall [START_REF] Kesapragada | Anisotropic broadening of Cu nanorods during glancing angle deposition[END_REF] deposited Cu nanorods on a patterned Si substrate using two opposite Cu sources. A two steps GLAD process was performed with continuous substrate rotation followed by deposition on stationary substrate. The first step J o u r n a l P r e -p r o o f led to vertical columns and the second one to anisotropic broadened columns with an increase of the nanorod width in the direction parallel to the fluxes.

In this study, we report on the oblique angle deposition of tungsten thin films by magnetron sputtering using two opposite targets of tungsten. Former studies have been performed on tungsten (with one target) [START_REF] Beainou | Resistivity anisotropy of tilted columnar W and W-Cu thin films[END_REF][START_REF] Chargui | Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films[END_REF], W-WO3 [START_REF] Martin | Architectured columns with a metal-dielectric periodic nanostructure[END_REF], W-Cu [START_REF] Beainou | Correlation between structure and electrical resistivity of W-Cu thin films prepared by GLAD co-sputtering[END_REF][START_REF] Beainou | Contrasted morphologies in nanostructured Janus W-Cu columns[END_REF] and W-Ag [START_REF] Boukhalfa | Microstructural analysis and electrical behaviours of co-sputtered W-Ag thin films with a tilted columnar architecture[END_REF] deposited by magnetron sputtering in OAD configuration. Structure and electrical properties of such thin films are clearly dependent on the deposition parameters, mainly the sputtering pressure and the current intensity applied to the targets. Typically, if one the fluxes is dominant, it can largely influence the microstructure and the elemental composition of the thin films. Thus, by varying the current intensity applied to the targets, W-to-Cu and W-to-Ag atomic concentration ratio is tuned from 0.2 to 10 [START_REF] Beainou | Correlation between structure and electrical resistivity of W-Cu thin films prepared by GLAD co-sputtering[END_REF][START_REF] Boukhalfa | Microstructural analysis and electrical behaviours of co-sputtered W-Ag thin films with a tilted columnar architecture[END_REF]. In order to better understand the influence of these deposition parameters on the morphology and the electrical resistivity, OAD thin films have been deposited using two similar W targets. Several deposition parameters such as sputtering pressure, deposition temperature, oblique incidence angle have been reported to influence the growth and the final microstructure of the OAD and GLAD thin films [10]. So, for this study, many parameters such as α angles, deposition temperature, distances between the substrate and the targets are fixed during the deposition. Only two deposition parameters have been modified: the argon pressure and the intensity of the current applied to the targets. To investigate the influence of the sputtering pressure on the microstructure of the films, two argon sputtering pressures are chosen: 0.33 Pa and 1.5 Pa.

Moreover, the current applied to one of the tungsten targets is set to 140 mA whereas the current to the second one is varied from 50 to 250 mA. The influence of these parameters on the morphology, crystallography and electrical resistivity of the tungsten thin films is studied and discussed.

J o u r n a l P r e -p r o o f

Material and methods

Thin films are deposited by DC magnetron sputtering from two opposite tungsten metallic targets (both are 51 mm diameter and 99.9 at.% purity) on silicon and glass substrates (size of 25  12 mm 2 ) (Fig. 1). and to 95 mm for W2. The substrate can be fixed or rotated following the ϕ angle (not used in this study). W atoms are deposited on substrate defining a x-y plane whereas the z-axis is defined by the direction perpendicular to the substrate.

The deposition is performed at room temperature in a homemade 40 L vacuum chamber at a base pressure below 10 -5 Pa. The distances between the center of the substrate and the center of W1 and W2 targets are equal to 65 mm and 95 mm, respectively. W1 and W2 targets are inclined according to the same incident angle equal to 80° defined by the normal to the substrate and the normal to the targets (αW1 for W1 target and αW2 for W2 target). The sputtering is performed simultaneously J o u r n a l P r e -p r o o f at room temperature in a pure argon atmosphere. Two samples series are produced with two argon sputtering pressures of 0.33 and 1.5 Pa. The current of W2 target is varied from 50 to 250 mA, IW1 being kept unchanged at 140 mA. More details about the deposition facilities can be found in [START_REF] Beainou | Correlation between structure and electrical resistivity of W-Cu thin films prepared by GLAD co-sputtering[END_REF].

The films thickness measured by means of a Tencor Alpha Step IQ profilometer varies between 270 and 460 nm. The morphology of thin films deposited on silicon substrate is observed with a scanning electron microscope (SEM) Hitachi SU8230 from cross-section and surface views.

The crystallographic structure of the W films is characterized by X-ray diffraction (XRD).

Measurements are carried out using a Bruker D8 diffractometer with a copper line focus X-ray tube (Cu λKα1 = 0.154060 nm) with a θ/θ configuration. Scans are performed with a step of 0.0204° per 1 s and a 2θ angle ranging from 20 to 120°. The pole figures are performed using a Bruker D8 Discover GADDS diffractometer with a point focus cobalt X-ray tube. A 500 µm collimator is added to better focus the incident X-ray beam on the sample surface and the step size is 3° azimuthally (φ = 0°-358°). The transmission electron microscopy (TEM) analyses are performed using a JEOL 2100 FEG microscope with a 200 kV acceleration voltage. For TEM observations, scratched pieces of W thin films are dispersed on a lacey carbon film supported by a copper grid.

TEM images are analyzed using the GATAN Digital Micrograph software and simulated with the Java Electron Microscopy Software (JEMS). The resistivity measurements are carried out at room temperature using the four probe van der Pauw method. The resistivity anisotropy is defined and measured [START_REF] Bierwagen | Mobility and carrier density in materials[END_REF] as the ratio between electrical resistivity following perpendicular and parallel directions to the sputtered particle fluxes [START_REF] Kasper | Measuring anisotropic resistivity of single crystals using the van der Pauw technique[END_REF].

Results and discussion

J o u r n a l P r e -p r o o f Top-view and cross-section scanning electron microscopy (SEM) images of the W thin films deposited at low (0.33 Pa) and high (1.5 Pa) argon sputtering pressures and with various W2 target current intensities (IW2 varying from 50 to 250 mA) are visible in Fig. 2 and3. All SEM observations have been performed at the center of the samples, but are representative of the whole thin films' morphology, except at extremities. For all sputtering conditions, thin films appear as porous and columnar with separated and more or less inclined columns. It is well admitted that the morphology of OAD and GLAD thin films is mainly controlled by the self-shadowing effect [START_REF] Smith | Oblique-Incidence Anisotropy in Evaporated Permalloy Films[END_REF], enhanced for high oblique incidence angle of the fluxes ( > 70°). It can also be seen that the exhibited morphology strongly depends on the argon sputtering pressure. Indeed, surface SEM observations exhibit two kinds of morphology at both pressures (Fig. 2). At low pressure, the morphology appears to be asymmetric as some columns are connected forming elliptical structures perpendicular to the W particle fluxes. The bundled columns are also separated by elongated voids. This anisotropic microstructure has already been observed for tungsten thin films deposited by OAD with one source [START_REF] Beainou | Resistivity anisotropy of tilted columnar W and W-Cu thin films[END_REF][START_REF] Chargui | Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films[END_REF] as well as for W-Cu and W-Ag thin films co-deposited by OAD [START_REF] Beainou | Resistivity anisotropy of tilted columnar W and W-Cu thin films[END_REF][START_REF] Boukhalfa | Microstructural analysis and electrical behaviours of co-sputtered W-Ag thin films with a tilted columnar architecture[END_REF]. At high pressure, the morphology appears to be more isotropic with a cauliflower structure. Some voids separate the agglomerated columns and are also slightly perpendicular to the W particle fluxes. The agglomerated columns do not show the elliptical shape reported for thin films produced at low pressure, but rather a circular one. Therefore, the comparison of top-view SEM images indicates that W thin films are columnar, with agglomerated columns separated by voids.

The growth is also more anisotropic for thin films produced at low pressure.

J o u r n a l P r e -p r o o f

The phenomenon of asymmetric morphology and more specifically the coalescence of columns in a specific direction named "bundling" have been reported in many studies and are related to the one-dimensional nature of the shadowing effect [START_REF] Van Kranenburg | Tailoring growth and local composition by oblique-incidence deposition: a review and new experimental data[END_REF]10]. The bundling phenomenon consists in associating nanocolumns, laterally connected in a direction perpendicular to the particle flux. The formation of the bundled columns has been linked to a preferential trapping mechanism leading to a faster lateral growth of the nanocolumns, in the direction perpendicular to the particle fluxes until their coalescence [10]. In our study, as the two targets are opposite, shadowing effect still occurs in a one-dimensional way (incidence takes place in the x-z plane). As the growth keeps going on, nanocolumns are still merging in the direction perpendicular to the particle fluxes (ydirection) and the shadowing effect prevents the columns to be connected in the parallel direction (x-direction).

The difference of morphology at low and high pressures is linked to the more or less ballistic character of W sputtered atoms. At low pressure (0.33 Pa), ballistic sputtered particles are mainly produced and the particle fluxes are highly directional [10]. In contrast, a pressure increase corresponds to an increase of the density of argon atoms available for ionization. Therefore, at higher pressure, W sputtered particles undergo more scattering and the mean free path strongly decreases. The mean free path for sputtered W atoms in argon atmosphere is inversely proportional to the argon sputtering pressure following this equation (Eq. 1) [START_REF] Vüllers | Alphavs. beta-W nanocrystalline thin films: A comprehensive study of sputter parameters and resulting materials[END_REF]:

 = k B T √2 π (r Ar +r W ) 2 p (1)
with kB is the Boltzmann's constant (1.3810 -23 J.K -1 ), T the temperature (300 K), p the pressure (Pa) and rAr = 97 pm and rW = 130 pm the covalent radii of argon and tungsten, respectively. It is calculated equal to 55 mm for p = 0.33 Pa and equal to 12 mm for p = 1.5 Pa. Moreover, the target-J o u r n a l P r e -p r o o f to-substrate distances being equal to 65 mm and 95 mm for W1 and W2 targets, respectively, the sputtered W particles are even more scattered at high pressure. Consequently, their kinetic energy decreases and the fluxes are also less directional. Thus, it can be considered that the W sputtered atoms evolve from a mostly ballistic behavior to a mostly thermalized one when the pressure changes from 0.33 to 1.5 Pa. The same behavior has been reported for W thin films grown by GLAD with one source (α = 85°, constant rotation of the substrate) [START_REF] Khedir | Morphology control of tungsten nanorods grown by glancing angle RF magnetron sputtering under variable argon pressure and flow rate[END_REF]. So, at higher pressure, thermalized particles experience numerous collisions and accordingly they are less directional than ballistic ones. Thus, the one-directional nature of the shadowing effect is altered and the anisotropic bundling phenomenon does not occur at high pressure. Finally, we note that both observed morphologies well correspond to the main features of  and  generic microstructures developed by theoretical model to describe the different growth regimes of gold thin films as a function of the oblique incidence angle α and the sputtering pressure [10]. -type microstructure described as vertical and coalescent columns with a high density of micropores is obtained for a high degree of thermalization whereas -type microstructure described as tilted and isolated columns is obtained for a low thermalization degree. In our deposition conditions, when the argon sputtering pressure increases from 0.33 to 1.5 Pa, the thermalization degree also increases from 0.5 to 3.2 and the microstructure evolves from a -type to a δ-type structures.

Cross-section SEM images reveal a columnar microstructure with columns inclined towards W1 target (Fig. 3). For all deposited films, thin columns are formed at the interface with the silicon substrate.

J o u r n a l P r e -p r o o f

Afterwards, during the growth, the thinnest columns disappear in favor of larger columns that widen. Consequently, the column's width increases with thickness. This column broadening is J o u r n a l P r e -p r o o f linked to the extinction of small columns being shadowed by bigger ones. It has been reported that in the case of W thin films deposited by GLAD technique, columns width and length are related by a power law, with an exponent p = 0.34 [START_REF] Karabacak | Scaling during shadowing growth of isolated nanocolumns[END_REF]. The main evolution concerns the columnar tilt angle β defined as the angle between the normal to the substrate surface and the inclined columns direction (Fig. 3). Since W1 target is closer to the substrate holder than W2 target (65 mm and 95 mm, respectively), columns are oriented towards the W1 particle flux, whatever the IW2 current intensity applied to W2 target.

However, the increase of the IW2 current leads to a decrease of the columnar tilt angle β. It is worth noticing that the influence of the W2 particle flux becomes more visible as higher IW2 currents are applied, especially at lower pressure. For these low pressure conditions, β presents a strong variation from 39° for IW2 = 50 mA to 2° for IW2 = 250 mA. In contrast, at high pressure, β is quasi constant with angles varying from 11° to 8° (Table 1). A comparison can be made with W thin films prepared with only one W target and with quite similar conditions (p = 0.35 Pa and p = 1.5 Pa, target-to-substrate distance = 65 mm, α = 80° and IW = 140 mA) in the same homemade PVD chamber. At low and high pressures, W thin films exhibit a columnar tilt angle β equal to 39°

J o u r n a l P r e -p r o o f and 18°, respectively [START_REF] Chargui | Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films[END_REF]. The value obtained for β at high pressure with one target is higher than all column angles obtained with two W targets, whatever the IW2 current. At low pressure and with one target, β angle is similar to the maximum value obtained with two opposite W targets (for IW2 = 50 mA). As the target-to-substrate distance is also shorter for W1 than for W2 target (65 mm versus 95 mm), one can even consider that the particle flux coming from W2 target is negligible for IW2 = 50 mA.

OAD thin films show a columnar morphology and are characterized by an inclination of the columns towards the particle flux direction. The columnar tilt angle  is systematically lower than the oblique incidence angle . This value depends on many parameters such as the substrate temperature, the deposition rate and the partial pressure. Several models have been proposed to connect the columnar tilt angle  and the oblique incidence angle α [START_REF] Van Kranenburg | Tailoring growth and local composition by oblique-incidence deposition: a review and new experimental data[END_REF][START_REF] Robbie | Advanced techniques for glancing angle deposition[END_REF][START_REF] García-Martín | [END_REF][36]. Among the different empirical models, the tangent and the cosine rules have attracted much attention. The empirical "tangent rule" [START_REF] Nieuwenhuizen | Microfractography of thin films[END_REF] gives better results for smaller α while the "cosine rule" suggested by Tait et al. [START_REF] Tait | Modelling and characterization of columnar growth in evaporated films[END_REF] is proposed for larger α. For α = 80°, this is verified as the first equation predicts β = 71° and the second one β = 56°. However, these rules suppose that the relations between the angles are purely geometric and are not dependent on both materials and elaboration parameters [START_REF] García-Martín | [END_REF]. This explains why some discrepancies are observed when one target is used and even more when the deposition takes place with two opposite targets [36].

Higher argon sputtering pressures lead to a strong decrease of W particle energy and directionality. Therefore, thermalization effect induces at high pressure a lesser influence of target currents on β angle [START_REF] García-Martín | [END_REF]. In contrast, in the ballistic approach, the particles impinge on the film surface along straight and oblique trajectories. They remain at the landing location giving rise to tilted nanocolumns [10]. Due to the shadowing effect, deposited particles accumulate within J o u r n a l P r e -p r o o f certain areas corresponding to the first location of growth. These grains lead to shadowing areas, preventing other particles to deposit within. This selective growth between shadowed and nonshadowed areas induces the formation of tilted columns towards the particle flux direction. In the case of two targets instead of one, an effective source direction must be considered [START_REF] Leamy | Microstructure and magnetism in amorphous rare-earth-transition-metal thin films[END_REF]. But the growth mechanism with a strong influence of the self-shadowing effect still applies. As the two opposite W targets have a common plane (x-z plane), the self-shadowing effect occurs perpendicularly to this plane of incidence. This leads to a preferential aggregation of the grains perpendicular to the incident plane rather than parallel to it. Already deposited grains shadow adjacent regions from direct impingement. As the thickness increases, the self-shadowing effect is even more pronounced and a columnar structure with a specific orientation is obtained. The columnar grains aggregate along a row perpendicular to the plane of incidence. Since the two sources are symmetrically installed relative to the substrate axis, the particle fluxes would have the same contribution to the film growth (i.e., incoming fluxes and atom energies would be the same when impinging on the growing columns) leading to the deposition of vertical columns, as previously reported for Fe thin films by Keitoku et al. [START_REF] Keitoku | Inhomogeneous Concentration Distribution in Obliquely Evaporated Fe-Cu Films[END_REF]. Otherwise, they will be inclined towards the most powered target in terms of flux and energy [START_REF] He | Advanced multi-component nanostructures designed by dynamic shadowing growth[END_REF].

In our study, even if identical  angles are imposed, as the substrate-to-target distances are not equal for both sources (65 mm and 95 mm), the fluxes are not diametrically opposed. Thus, our configuration of the deposition system is not symmetric and the two incoming particle fluxes do not contribute equally to the columnar growth. Consequently, the angle  is not equal to zero important target-to-substrate distance for W2 target compared to W1 one is then compensated by the higher applied IW2 current intensity, leading to balanced particle fluxes coming from the two opposite sides.

Fe, MgF2 and Cu vertical columns were also previously obtained using two opposite sources of materials [START_REF] Keitoku | Grain Structure and Magnetic Anisotropy of Fe Film Evaporated Obliquely from Two Sources[END_REF][START_REF] Robbie | Fabrication of thin films with highly porous microstructures[END_REF][START_REF] Kesapragada | Two-component nanopillar arrays grown by Glancing Angle Deposition[END_REF]. Instead of using two MgF2 sources, Robbie et al. [START_REF] Robbie | Fabrication of thin films with highly porous microstructures[END_REF] proposed to use only one source and to rotate the substrate rapidly with 180° steps. However, zigzag structure was obtained instead of vertical columns. To control the columnar tilt angle independently of the film porosity, more complicated GLAD configurations have been proposed with specific parameters. For example, if α angle is kept constant whereas the substrate is rapidly rotated with a continuous or intermittent rotation, vertical columns are obtained [START_REF] Robbie | Advanced techniques for glancing angle deposition[END_REF][START_REF] Karabacak | β-phase tungsten nanorod formation by oblique-angle sputter deposition[END_REF][START_REF] Krishnan | [END_REF]. GLAD is a very useful technique to obtain 3-D nanostructures but it implies the control of polar (α) and azimuthal (ϕ) angles. OAD configuration with two opposite sources appears to be another method to easily obtain inclined or vertical columns, by simply playing on the target current intensity.

X-ray diffraction analyses point out differences for W thin films prepared at low and high argon sputtering pressure (p = 0.33 and 1.5 Pa). W thin films deposited at 0.33 Pa exhibit peaks largely more intense and clearly defined (Fig. 4-a). Thus, the decrease of the argon sputtering pressure is linked to a better crystallization. The most intense peak is detected at 2 = 35. Previous studies have shown that tungsten is mainly formed in the thermodynamically stable body-centered-cubic (bcc) α-W phase, but can occur as a metastable cubic A-15 structure, also referred as -W [START_REF] O'keefe | Phase transformation of sputter deposited tungsten thin films with A15 structure[END_REF][START_REF] Shen | Influences of oxygen on the formation and stability of A15 β-W thin films[END_REF][START_REF] Rossnagel | The Phase Transformation of Thin Sputter-deposited Tungsten Films at Room Temperature[END_REF][START_REF] Chen | [END_REF]. The unit cell of A15 structure is described as an ordered phase where the body centered lattice sites are occupied by O atoms whereas two lattice on each of the faces are occupied by W atoms. The lattices parameters are equal to a = 0.316 nm and a = 0.504 nm for α and β phases, respectively. The β-W phase is stabilized by the presence of oxygen acting as impurities, but without forming a tungsten oxide compound [START_REF] O'keefe | Phase transformation of sputter deposited tungsten thin films with A15 structure[END_REF][START_REF] Shen | Influences of oxygen on the formation and stability of A15 β-W thin films[END_REF][START_REF] Weerasekera | Structure and stability of sputter deposited beta-tungsten thin films[END_REF][START_REF] Chattaraj | [END_REF]. Even if -W is often described as an ordered A-15 unit cell with a W3O stoichiometry, the tungsten thin films designated as -W do not present an ordered structure and the oxygen concentration in the films is often below 15 at.% but does not reach the 25 at.%, corresponding to a W3O stoichiometry [START_REF] O'keefe | Phase transformation of sputter deposited tungsten thin films with A15 structure[END_REF][START_REF] Shen | Influences of oxygen on the formation and stability of A15 β-W thin films[END_REF][START_REF] Chattaraj | [END_REF]. The stabilization of β-W has also been attributed to interstitial incorporation of argon or nitrogen impurities as well as to the stacking fault W3W in the A15 structure [START_REF] Shen | Structure and properties of stacking faulted A15 tungsten thin films[END_REF][START_REF] Barmak | [END_REF]. The occurrence of -W, -W or a mixture of both is strongly sensitive to a variety of deposition parameters such as presence of underlayers and type of substrates, films thickness, pressure of inert or reactive gas, deposition temperature, substrate bias, deposition power as well as deposition rates [START_REF] Vüllers | Alphavs. beta-W nanocrystalline thin films: A comprehensive study of sputter parameters and resulting materials[END_REF][START_REF] Rossnagel | The Phase Transformation of Thin Sputter-deposited Tungsten Films at Room Temperature[END_REF][START_REF] Barmak | [END_REF].

J o u r n a l P r e -p r o o f

J o u r n a l P r e -p r o o f

A better crystallinity has been reported for W thin films deposited at lower pressure by conventional sputtering [START_REF] Vüllers | Alphavs. beta-W nanocrystalline thin films: A comprehensive study of sputter parameters and resulting materials[END_REF] as well as for W thin films deposited by OAD or GLAD [START_REF] Chargui | Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films[END_REF][START_REF] Khedir | Morphology control of tungsten nanorods grown by glancing angle RF magnetron sputtering under variable argon pressure and flow rate[END_REF]. It is due to the ballistic regime inducing lesser scattering and higher kinetic energy of W sputtered particles. This would also explain the higher intensity of (200) -W peak with the increase of IW2.

A low sputtering pressure favors energetic W atoms and thus their diffusion on the growing film.

In contrast, at high pressure, the lower energy, mobility and surface diffusion disrupt the crystallization process reducing the thin films' crystallinity. Khedir et al. [START_REF] Khedir | Morphology control of tungsten nanorods grown by glancing angle RF magnetron sputtering under variable argon pressure and flow rate[END_REF] confirmed the preponderance of the metastable β-W phase at low pressure and of the stable α-W phase at high pressure. They proposed that oxygen atoms could more easily incorporate at interstitial positions at low pressure whereas Weerasekera et al. [START_REF] Weerasekera | Structure and stability of sputter deposited beta-tungsten thin films[END_REF] have also linked the stability of the -W phase to the amount of incorporated oxygen, more abundant at low deposition rate. In our case, the average deposition rate is equal to 10.5 nm.min -1 at low pressure (p = 0.33 Pa) and to 19.7 nm.min -1 at high pressure (p = 1.5 Pa) (Table 1). So, as the deposition rate increases at high pressure, the incorporation of residual oxygen would be more difficult and the β-W phase would be less stabilized at high pressure.

Using GLAD technique with continuous substrate rotation around the normal to the substrate, the -W phase is reported to be favored compared to α-W one at low pressure [START_REF] Karabacak | β-phase tungsten nanorod formation by oblique-angle sputter deposition[END_REF]. Due to different adatom mobilities, the α-W and β-W phases are supposed to grow faster laterally and vertically, respectively. The shadowing effect favors the β-W phase that predominates over the α-W phase during OAD and GLAD growth. Using the two-step procedure proposed by Robbie et al.

[14] with a fast rotation of 180° of the substrate followed by pauses for deposition, Krishnan et al.

[41] also obtained vertical columns with -W phase. With other GLAD parameters characterized by a "flipping rotation" mode, Chen et al. [START_REF] Chen | [END_REF] reported the growth of vertical nanorods with the J o u r n a l P r e -p r o o f α-W phase, the β-W phase being obtained under conditions similar to Karabacak et al. [START_REF] Karabacak | β-phase tungsten nanorod formation by oblique-angle sputter deposition[END_REF] (continuous substrate rotation). The occurrence of α-W phase was linked to a modification of the shadowing effect that is not enough efficient to promote the β-phase. Therefore, besides the aforementioned classical deposition parameters, OAD and GLAD parameters such as  angle, eventual motion and rotation of the substrate as well as in our case the two confocal W targets are additional deposition parameters that must be considered. This explains why it is not trivial to predict the occurrence of α and/or β phases as it is impacted by so many deposition parameters.

J o u r n a l P r e -p r o o f X-ray pole figure measurements are performed on the sample deposited at low pressure with IW2 = 200 mA to gain information about main texture component (Fig. 5). As the X-rays reach the silicon substrate, texture information is obtained from the whole deposited thin film. The reconstruction of pole figures (RPF) is performed (using Labotex software) for {200}, {210} and {211} planes of β-W, the first two ones corresponding to the two most intense XRD peaks (Fig. 3-a). The {200} reconstructed pole figure exhibits a very intense central spot, typical of {100} fiber pole figure, indicating that {200} are planes parallel to the surface. We note that a slight tilt could be applied with a shift towards {10 1 1} explained as the columns are not vertical for this sample but are inclined towards W1 target with β = 14°. So, W columnar thin films are characterized by a strong {200} texture. Fig. 5-a also shows a quasi-continuous ring corresponding to {211} planes, pointing out the contribution of the fiber structure. In Fig. 5-b, the {210} RPF is characterized by the absence of a central spot and the presence of two quasi-continuous rings corresponding to {210} planes. Both features confirm the {200} texture and the fiber texture. A weak additional {211} fiber texture is present in Fig. 5-c as the {211} RPF exhibits a weak central spot and also in the {200} RPF (Fig. 5-d) with a weak secondary ring, suggesting an eventual secondary growth on the columns. Some fishbone-like shape secondary structures have previously been reported on the W-face of W-Cu thin films deposited by OAD [START_REF] Beainou | Contrasted morphologies in nanostructured Janus W-Cu columns[END_REF]. The {200} texture combined to the fiber texture pointed out by RPF measurements indicates a random rotation of the J o u r n a l P r e -p r o o f columns around preferential <100> growth direction. RPF method has not been applied to a sample deposited at high argon pressure as the XRD data indicate a poor crystallization. Moreover, the proximity of {210} β-W and {110} α-W would induce misinterpretations of the pole figures. We note that the surface energy of the different facets of α-W and β-W crystals has been calculated by density functional theory (DFT) energy calculations and reported in a database by Tran et al. [50].

For α-W, {110} planes exhibit the lowest surface energy with E = 3.23 J.m -2 whereas {100} and {210} planes of β-W have the lowest and second-lowest surface energy with E = 3.29 J.m -2 and E = 3.40 J.m -2 , respectively. Therefore, the exhibited {200} texture corresponds to the thermodynamically favored crystal planes of β-W. J o u r n a l P r e -p r o o f W thin films are also analyzed by high resolution transmission electron microscopy (HRTEM). Low-magnification TEM images are visible in Fig. 6-a and 6-b for the W thin films deposited at 1.5 Pa and 0.33 Pa (both with IW2 = 200 mA). As the TEM samples have been prepared by scratching off, the silicon substrate is not visible. Figure 6-a confirms the columnar cauliflower structure already observed by SEM for samples deposited at high pressure. For sample deposited at low pressure, the columns are agglomerated and the coalescence effect is clearly visible as the width of the columns is smaller at their bottom part (Fig. 6-b). As the TEM samples have not been thinned down and as tungsten is a heavy element, only a few columns can be imaged for HRTEM. The W thin film deposited at high pressure displays a highly defective polycrystalline structure with crystallite size below 3 nm (not shown here), confirming the poor crystallization pointed out by XRD. In contrast, the sample deposited at low pressure displays some columns well orientated in zone axis (even if defective) and some HRTEM images have been recorded (Fig. 6c). The analysis of the HRTEM images combined with calculated fast Fourier transform (FFT) patterns confirms that the columns are rather monocrystalline with a well-defined orientation (Fig. 6-d). They all correspond to a [001] zone axis with crystallographic {200} planes (d = 0.252 nm) of the β-W structure. These results agree with and confirm XRD and RPF ones. As the {200} planes are perpendicular to the growth direction of the columns, the growth direction corresponds to <100> direction and therefore, to the preferential growth direction of bcc crystals [50].

DC electrical resistivity and resistivity anisotropy are determined at room temperature for OAD W thin films deposited on glass substrate by the van der Pauw method. For samples deposited at low pressure, the resistivity gradually increases as a function of IW2 current intensity from 1.1510 -5 to 4.8510 -5 Ω m (Fig. 7-a).

J o u r n a l P r e -p r o o f These values are two or three orders of magnitude higher than the ones reported for bulk W (ρW = 5.4410 -8 Ω m) [START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF]. This higher resistivity is attributed to the enhancement of the electron scattering by surfaces and grain boundaries, more present in porous thin films. SEM observations have pointed out the presence of large voids between the tilted columns for W films deposited at low pressure. Moreover, it has been shown that the electrical resistivity depends on the crystallographic structure. Indeed, the electrical resistivity of α-W is one order of magnitude lower than β-W one [START_REF] Vüllers | Alphavs. beta-W nanocrystalline thin films: A comprehensive study of sputter parameters and resulting materials[END_REF][START_REF] Karabacak | β-phase tungsten nanorod formation by oblique-angle sputter deposition[END_REF][START_REF] O'keefe | Phase transformation of sputter deposited tungsten thin films with A15 structure[END_REF]. This explains the gradual increase of the electrical resistivity for J o u r n a l P r e -p r o o f samples grown at low pressure as it can be linked to the higher amount of β-W phase pointed out by XRD.

For samples deposited at high pressure, the resistivity also varies as a function of the IW2 current intensity. It gradually decreases from 7.0010 -5 to 4.6710 -5 Ω m before rising to 6.8010 - 5 Ω m (Fig. 7-b). Such resistivity values are higher than those previously measured for W films prepared at low pressure. At first, it can be assigned to the disordered structure recorded from XRD analyses (Fig. 4). Assuming that a small grain size, the presence of stacking faults and the occurrence of the β-W phase favor a high film resistivity, the second W target is not only a source of matter but also supply energy from the incoming particle flux. For the lowest IW2, W atoms diffusing on the columns side opposite to the W1 target migrate more easily to stable crystalline α-W growth sites, which reduces the films resistivity. A further increase of IW2 and so, of the particle flux coming for the target W2 enhances the defect incorporation due to a growth competition between two particle fluxes. This competition favors the formation of the β-W phase (more intense β (200) diffracted signal at 2 = 35.52° from XRD in Fig. 4) and thus, an increase of the resistivity is produced.

J o u r n a l P r e -p r o o f

Using the Bierwagen method [START_REF] Bierwagen | Mobility and carrier density in materials[END_REF], the electrical resistivity has been measured in directions parallel (x-axis) and perpendicular (y-axis) to the particle fluxes. Then, the anisotropic resistivity is defined as the resistivity ratio following the parallel and perpendicular directions of the particle fluxes. At low pressure, the ratio is comprised between 1.59 and 1.89 whereas at high pressure, the ratio is nearly constant and close to 1.35. This anisotropic resistivity is linked to the elliptical shape already reported by SEM observations. In particular, the higher values obtained at low pressure correspond to an emphasized anisotropic morphology due to the bundling effect. The resistivity is favored in the direction parallel to the particle fluxes as the columns are separated by voids. Simultaneously, the conductivity is enhanced in the direction perpendicular to the particle fluxes as the columns are agglomerated along this direction because of the bundling effect. At high pressure, the constant electrical anisotropy is assigned to the less anisotropic and rather homogeneous cauliflower microstructure whatever IW2 current intensity. Scattering of electrons is especially significant in this kind of microstructure since the bundled columns exhibit a small cross section as shown from SEM observations in figures 2 and 3. These nanocolumns are more or less connected to each other's and are separated by voids acting as barriers of potential for charge carriers.

Conclusion

In this study, a specific configuration is proposed to deposit tungsten thin films by cosputtering on silicon and glass substrates using oblique angle deposition. The deposition is simultaneously performed from two distinct and opposite tungsten sources. Both targets are focused on the substrate with the same incident angle α = 80°. Due to the shadowing effect, typical of OAD deposition, we observe that all samples are porous and columnar. The morphology, J o u r n a l P r e -p r o o f and to 95 mm for W2. The substrate can be fixed or rotated following the ϕ angle (not used in this study). W atoms are deposited on substrate defining a x-y plane whereas the z-axis is defined by the direction perpendicular to the substrate. J o u r n a l P r e -p r o o f -The column tilt angle is tunable at low sputtering pressure changing one current target.

List of figures

-A cauliflower morphology is systematically produced at high sputtering pressure.

-Pressure and target current both influence electrical conductivity of nanocolumnar films.

J o u r n a l P r e -p r o o f

Fig. 1 :

 1 Fig. 1: Schematic representation of the OAD setup. Two tungsten targets W1 and W2 are tilted

Fig. 2 :

 2 Fig. 2: Top-view scanning electron microscopy observations of W thin films deposited at a) low

Fig. 3 :

 3 Fig. 3: Cross-section scanning electron microscopy observations of W thin films deposited at a)

  for the same target current intensities and columns tilt towards the closest W1 target. However, modifying the IW2 current intensity allows to change the columnar tilt angle . For IW1 = 140 mA and IW2 = 250 mA, quasi-vertical columns are obtained with  = 2°. The consequence of the more J o u r n a l P r e -p r o o f

  55° and is assigned to the (200) planes (2 = 35.52°) of the body-centered cubic (bcc) β-W phase (ICDDpdf # 65-6453). Its intensity strongly increases with the IW2 current intensity. An asymmetric, broad and weak peak is recorded at a diffraction angle 2 around 39.5°-40.5° (Fig. 4-b). It is attributed to the sample deposited with IW2 = 50 mA to a mixture of the (210) planes of the β-W phase and of the (110) planes of the α-W phase, respectively. For other IW2 current intensities, the peak attributed to (110) planes of -W phase is hardly distinguishable. For W thin films deposited J o u r n a l P r e -p r o o f at high pressure, weaker diffracted signals with broader peaks are reported (Fig. 4-c). Both α and β phases of tungsten are reported with peaks recorded at 2 = 35.44°, 39.88° and 40.25°, corresponding to β (200), β (210) and α (110) planes (2 = 35.52°, 39.89° and 40.26°), respectively. If the intensities of the two last peaks are rather constant, the intensity of the first one also increases with IW2 current intensity.

Fig. 4 :

 4 Fig. 4: X-ray diffraction patterns recorded in θ/θ configuration of W thin films prepared at low

Fig. 5 :

 5 Fig. 5: a, d) {200}, b, e) {210} and c, f) {211} X-ray reconstructed pole figures (RPF) for the W

Fig. 6 :

 6 Fig. 6: TEM images of columnar W thin films deposited at a) high pressure and b) low pressure

Fig. 7 :

 7 Fig. 7: DC electrical resistivity and resistivity anisotropy measured at room temperature as a

  Broad and low diffracted signals are systematically observed for W films deposited at high pressure, which corresponds to a low crystal size (a few nanometers) and thus, a reduced mean free path of electrons. The surprising minimum of resistivity observed for IW2 = 140 mA can be explained neither from the evolution of the films' morphology (SEM pictures show similar top and cross-section features), nor by signification variations of the crystal structure (mixture of β-W and α-W phases poorly crystallized from XRD patterns). Although this minimum of resistivity at IW2 = 140 mA remains an open question, one may suggest some correlations between the current IW2, growing defects and proportion of β-W and α-W phases in the films.

J o u r

  n a l P r e -p r o o f crystallography and the electrical resistivity of the W thin films are tailored by the variation of argon sputtering pressure and current intensity applied to the second W target (IW2). At low pressure, XRD analyses reveal the predominance of the metastable phase β-W and a <100> uniaxial fiber texture. Deposition at high argon pressure results in a cauliflower microstructure and a poor crystallization with a mixture of α-W and β-W phases. At low argon pressure (0.33 Pa), the columnar tilt angle β varies with IW2 until obtaining vertical columns whereas samples deposited at high pressure (1.5 Pa) do not show any dependence. This work shows the meaningful effects of deposition conditions on the structure and morphology of tungsten films. Their modification as a function of the argon pressure is mainly linked to the occurrence of ballistic and diffusive regimes, inducing loss of energy and directionality of the particle fluxes. CRediT authorship contribution statement Valerie Potin: Writing -Review & editing. Houssem Boukhalfa: Data curation. Nicolas Martin: Writing -Review & editing.

Figure 1 :

 1 Figure 1: Schematic representation of the OAD setup. Two tungsten targets W1 and W2 are tilted

Figure 2 :

 2 Figure 2: Top-view scanning electron microscopy observations of W thin films deposited at a) low

Figure 3 :

 3 Figure 3: Cross-section scanning electron microscopy observations of W thin films deposited at a)

Figure 4 :

 4 Figure 4: X-ray diffraction patterns recorded in θ/θ configuration of W thin films prepared at low

Figure 5 :

 5 Figure 5: a, d) {200}, b, e) {210} and c, f) {211} x-ray reconstructed pole figures (RPF) for the W

Figure 6 :

 6 Figure 6: TEM images of columnar W thin films deposited at a) high pressure and b) low pressure

Figure 7 :

 7 Figure 7: DC electrical resistivity and resistivity anisotropy measured at room temperature as a

Table 1 :

 1 Deposition rate and columnar tilt angle (β angle) for W thin films prepared at low (LP) and high (HP) pressures with different IW2 target currents.

		0.33 Pa (LP)		1.5 Pa (HP)
	IW2 target	Deposition rate	β angle	Deposition rate	β angle
	current (mA)	(nm.min -1 )	(°)	(nm.min -1 )	(°)
	50	8.2	39	17.7	11
	100	11	22	13.2	18
	140	10.4	16	19.7	10
	200	10.5	15	22.3	3
	250	12.5	3	25.5	8

Table 1 :

 1 Deposition rate and columnar tilt angle (β angle) for W thin films prepared at low (LP) and high (HP) pressures with different IW2 target currents.

		0.33 Pa (LP)		1.5 Pa (HP)
	IW2 target	Deposition rate	β angle	Deposition rate	β angle
	current (mA)	(nm.min -1 )	(°)	(nm.min -1 )	(°)
	50	8.2	39	17.7	11
	100	11	22	13.2	18
	140	10.4	16	19.7	10
	200	10.5	15	22.3	3
	250	12.5	3	25.5	8

J o u r n a l P r e -p r o o f

Acknowledgments

This work has been supported by the Region Bourgogne Franche-Comté and by EIPHI Graduate School (Contract 'ANR-17-EURE-0002'). The authors are grateful to Loïc Chossemier, Nicolas J o u r n a l P r e -p r o o f

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Geoffroy, Rémi Chassagnon and Cédric Thomas for helping in samples characterization and for fruitful discussions.

J o u r n a l P r e -p r o o f

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:


Elie Geagea

Judicael Jeannoutot

Frank Palmino

Frédéric Chérioux
email: frederic.cherioux@femto-st.fr

On-surface reactivity of disubstituted-bianthryl molecules on Cu(111) and Au(111) surfaces

On-surface p-conjugated 1D polymers, like graphene nanoribbons, have emerged as a class of promising materials. On-surface chemical properties of 9,9'-bianthryl molecules are widely developed as they can be used as starting building blocks to provide graphene nanoribbons. Here, we propose to investigate the chemical behavior of 10,10'-disubstituted-9,9'-bianthryl molecules on Cu(111) and Au(111) surfaces by using scanning tunneling microscopy under ultra-high vacuum. We demonstrated that the balance between molecule-molecule interaction, molecule-substrate interaction, and molecular rearrangement, drastically alter the chemical properties of the adsorbed molecule by thermal annealing.

Introduction

During the three two decades, molecular self-assembly has been widely investigated on different kinds of substrates [1][2][3][4][5]. More recently, on-surface chemistry has emerged to elaborate covalent nanostructures [6][7][8][9][10][11][12][13][14][15][16][17][18]. All these nanostructures have been obtained by bottom-up approaches and their structures are atomically precise, as proven by scanning probe microscopies, that can be used for great potential applications in molecular electronics, spintronics, energy, catalysis, and other fields. Graphene nanoribbons (GNRs) are 1D graphene stripes that are widely investigated due to their remarkable electronic properties compared to 2D graphene [19][20]. To achieve GNRs with a well-defined edge configuration and width, and accordingly control these electronic properties, growth from wisely-chosen molecular building-blocks was optimized thanks to atomic-scale characterization using scanning probe microscopies under ultra-high vacuum (UHV) [21][START_REF] Sakaguchi | Homochiral Polymerization-Driven Selective Growth of Graphene Nanoribbons[END_REF][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. The two most commonly used precursors for the on-surface synthesis of GNRs are 10,10'-dihalogenated-9,9'-bianthryl derivatives [21][START_REF] Sakaguchi | Homochiral Polymerization-Driven Selective Growth of Graphene Nanoribbons[END_REF][23][24][25][26][27][28][29] and 9,9'-bianthryl [30][31][32][33][34][35][36][37][38]. 10,10'-dihalogenated-9,9'-bianthryl derivatives lead to the formation of a large variety of GNRs on different kinds of coinage metal surfaces in a two-steps reaction: (i) a thermally induced Ullmann cross-coupling followed by (ii) a cyclodehydrogenation [21][START_REF] Sakaguchi | Homochiral Polymerization-Driven Selective Growth of Graphene Nanoribbons[END_REF][23][24][25][26][27][28][29]. Non-halogenated 9,9'-bianthryl derivatives lead to chiral (3,1)-GNRs, but only on a copper surface. [30][31][32][33][34][35][36][37][38] After few controversies, [36][37][38] the currently accepted reactional mechanism for the growth of chiral (3,1)-GNRs on a Cu(111) surface is based on a first thermal annealing leading to a protopolymer thanks to on surfaceassisted generation of a 2,2'-diradical species. Then, further heating gives the targeted chiral (3,1)-GNR as result of ring-closure driven by a cyclodehydrogenation. Most of the reaction intermediates involved in this mechanism have been observed. [30][31][32][33][34][35][36][37][38] Here, we investigate the on-surface chemistry of two 10,10'-disubstituted-9,9'-bianthryl derivatives induced by thermal annealing on Cu(111) and Au(111). Experiments were performed in ultra-high vacuum (UHV) and all adsorbates are fully characterized by scanning tunneling microscopy that provides images with submolecular resolution. We demonstrate that the molecule-surface couple plays a major role in the chemical transformation. In the case of Cu(111), 10,10'-disubstituted-9,9'-bianthryl are not transformed into (3,1)-GNRs due to the lack of reactivity of the starting building blocks, while on Au(111), surface only the formation of coordination polymers is observed.

Experimental

Molecules

All reagents were purchased from Aldrich, except Pd(PPh3)4 which was purchased from Strem chemical, and used as received. The silica gel used for column chromatography was purchased from Merck. The deuterated NMR solvents were purchased from Euriso-top. The NMR spectra were recorded using a Bruker AC-300 MHz spectrometer. The two molecules, 10,10'-di-(4''-cyanophenyl)-9,9'-bianthryl (CPBA) and 10,10'-di-(4''-pyridyl)-9,9'-bianthryl (PBA) were synthesized by a procedure adapted from the literature [39][40]. Basically, 10,10'dibromo-9,9'-bianthryl, 4'-cyanophenyl boronic acid pinacol ester or 4'-pyridyl boronic acid pinacol ester, cesium carbonate and Pd(PPh3)4 as a catalyst were dissolved in tetrahydrofuran. The resulting solid, obtained after removing of volatiles, was purified by column chromatography (silica gel, cyclohexane/dichloromethane 1:1) to give a pure white solid [41].

Surfaces and STM experiments

The first step consists of the preparation of the Cu(111) and Au(111) surfaces by argon ions sputtering cycles at 1.2 kV followed by thermal annealing at 723 K and 673 K for Cu111) and Au(111), respectively. This takes place inside a preparation chamber maintained under UHV conditions with a base pressure lower than 2•10 -10 mbar and coupled to an Omicron variable temperature Scanning Tunneling Microscope (Omicron VT-STM XA). Once the cleanness of Cu(111) and Au(111) surface is confirmed by analysis of STM images, we proceed to the deposition of molecules by thermal sublimation from a quartz crucible with a corresponding temperature of 458 K for PBA and 468 K for CPBA. During deposition the substrate was held at room temperature then transferred and cooled to 110 K on the STM stage for acquiring images. Each image process was carried out using SPIP software.

Results

We synthesized 10,10'-di(4"-pyridyl)-9,9'-bianthryl (PBA) and 10,10'-di(4"cyanophenyl)-9,9'-bianthryl (CPBA) respectively, that are based on bianthryl core surrounded by two lateral aryl groups covalently bound to the 10 and 10' positions (Figure 1). We chose cyanophenyl and pyridyl groups because they promote the formation of supramolecular selfassemblies on different types of surfaces [3][4][5]. The length is 1.67 nm and 2.08 nm for PBA and CPBA, respectively, while the distance between the extremities of the two anthracenyl rings is 0.79 nm and (Figure 1). PBA and CPBA molecules were deposited at the monolayer threshold by thermal sublimation under UHV on a Cu(111) and an Au(111) substrate kept at room temperature. As revealed in STM images recorded at 110 K, no isolated molecule was observed but only large extended 2D islands on the two chosen surfaces, each building-up of highly organized periodic protrusions (Figure 2). In the case of PBA/Cu(111) and CPBA/Cu(111) interfaces, large extended 2D islands are constituted by the periodic arrangement of several bright protrusions. The repeat unit pattern of each STM image is in accordance with the distance (0.8 ± 0.02 nm) between the edges of the topmost carbon ring of each of the two anthracenyl subunits of molecules (Figure 1). For PBA/Cu(111) interface, the repeat unit pattern is constituted by three pairs of disjoined protrusions rotated by 120° with different brightness (Figure 2a). In the case of CPBA/Cu(111) interface, 2D islands are made of bright lines separated by darker stripes. The periodicity between the bright lines is 1.35 ± 0.02 nm. Each line exhibits paired bright protrusions separated by 0.8 ± 0.02 nm. In the case of PBA/Au(111) interface, the repetitive unit consists of three pairs of disjoined protrusions (Figure 2a) rotated by 120°. In each domain, all repetitive units are rotated only clockwise or only anti-clockwise, leading to homochiral domains. The distance measured between the disjoined protrusions is 0.8 ± 0.02 nm. For the CPBA molecules deposited on Au(111), a compact periodic network constituted by bright lines, which were separated by darker strips can be observed. The periodicity between the bright lines is 1.28 ± 0.02 nm. Each line is made up of paired bright protrusions separated by 0.8 ± 0.02 nm.

Based on our STM observations and measurements, we propose the models of the four investigated interfaces. On the basis of the features of PBA and CPBA molecules (Figure 1), each bright-paired protrusion observed in Figure 2 The four investigated supramolecular networks are based on T-Shape p-p molecule-molecule interactions located between the lateral functional group (e.g. Pyridyl and cyano for PBA and CPBA, respectively, Figure 3) and the centre of the anthracenyl ring [42].

We investigated the chemical transformation of the four supramolecular self-assemblies induced by thermal annealing. Each supramolecular network was exposed to subsequent thermal annealing up to 673 K on both Cu(111) and Au(111) surfaces, respectively, from 1 hour to 4 hours. Below these two temperatures, all the starting self-assemblies are kept without any visible modification of the corresponding STM images. On a Au(111) surface, after thermal annealing, the two starting periodic supramolecular networks (Figure 2c-d) are transformed into disordered nanostructures (Figure 5). In addition, the resulting nanostructures are constituted by cross-shaped bright protrusions instead of bright-paired protrusions, as initially depicted in Figure 2c-d. The length of these new protrusions is 1.73 ± 0.05 nm and 2.23 ± 0.05 nm for PBA and CPBA, respectively while their width is 1.05 ± 0.05 nm. The bright protrusions lead to the formation of straight or reticulated nanolines including three-of four connecting nodes. The distance separating bright protrusions is longer in the case of CPBA than in the case of PBA (Figure 5).

Discussion

We made different thermal annealing, with temperatures from 400 K to 773 K. However, we never observed linear nanostructures corresponding to functionalized (3,1) GNRs, but only disordered features. This result is very surprising because, according to the reported reaction mechanism, the presence of aryl substituents at the 10 and 10' positions of a bianthryl building blocks should be in agreement with the formation of corresponding (3,1) GNRs [21][START_REF] Sakaguchi | Homochiral Polymerization-Driven Selective Growth of Graphene Nanoribbons[END_REF][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. However, as no (3,1) GNR was observed by using PBA and CPBA as building blocks on Au(111) and Cu(111) surfaces, we conclude that the surface-induced cleavage of C-H bond of 2 and 2' positions is not possible by annealing on a Cu(111). This claim is supported by the canonical forms of bianthryl molecule that govern the reactivity of fused six-membered aromatic rings. We assume that the thermally-induced cleavage of C-H bonds is only possible on 10 and 10' positions on Cu(111) and Au(111) surfaces, to give biradical species that undergo to (3,1) GNRs [41]. In the case of PBA and CPBA molecules, as the 10 and 10' positions are substituted by pyridyl or cyanophenyl groups, respectively, the formation of (3,1) GNRs are impossible on the two investigated surfaces.

In the case of Cu(111) surface, which is a more reactive surface than Au(111) [43], the thermal annealing is strong enough to induce at least one C-H bond cleavage per PBA or CPBA molecules, but without any regiospecificity. Therefore, the resulting radicals, due to surfaceassisted C-H bond cleavages, lead to the formation of disordered nanostructures (Figure 6), as observed on STM images (Figure 4). In the case of Au(111) surface, the cleavage of C-H bonds to lead to radicals is not possible. However, thermal annealing can induce intramolecular cyclodehydrogenation reactions. This type of reaction is well-known on Au(111) surface [21][START_REF] Sakaguchi | Homochiral Polymerization-Driven Selective Growth of Graphene Nanoribbons[END_REF][23][24][25][26][27][28]. The driven force of cyclodehydrogenation is the formation of more conjugated compounds. One consequence of this increased conjugation is the flattening of involved molecules. By the way, the average distance between Au(111) surface and flat-PBA or flat-CPBA molecules is shorter than those between surface and respectively, PBA and CPBA (Figure 6). Therefore, the interaction of nitrogen atoms of pyridyl or cyanophenyl moieties with the gold atoms of the surface is reinforced, which promotes the formation of coordination polymers [44][45][46]. The pyridyl or cyano moieties are ligand for several kind of metal atoms, due to the presence of the lone pair of electrons on the nitrogen atom. Due to the strength and the directionality of these metalligand bonds, the corresponding coordination polymers exhibit a high degree of reticulation on metal substrates like on an Au(111) surface, even if it is still rare compared to other coinage surfaces [44][45][46]. These coordination properties support the observation of straight and reticulated nanolines in STM images (Figure 5).

Conclusions

We have investigated the on-surface chemistry of two 10,10'-disubstituted-9,9'-bianthryl derivatives induced by a thermal annealing on Cu(111) and Au(111) surfaces. We have demonstrated that the choice of building blocks strongly alters the on-surface chemical behavior The presence of aryl substituents on the 10,10'-poisitions avoid the formation of mandatory radicals which are implied in the formation of (3,1)-GNRs. Depending of the reactivity of the surface, we have shown that 2D-extended periodic supramolecular networks are converted into coordination polymers on Au(111) surface while they led to highly disordered nanostructures on the Cu(111) surface. This strategy paves the way to new possibilities for multi-functional nanostructures by using on-surface assisted synthesis.

Figure 1 .

 1 Figure 1. CPK models of 10,10'-di-(4''-pyridyl)-9,9'-bianthryl (PBA) and 10,10'-di-(4''cyanophenyl)-9,9'-bianthryl (CPBA) molecules respectively.

Figure 2 .

 2 Figure 2. STM images of self-assembled molecules. a) PBA/Cu(111) (Vs = 0.9 V, It = 10 pA, 15 nm x 15 nm, inset 7 nm x 4 nm), b) CPBA/Cu(111) (Vs = -2.2 V, It = 10 pA, 20 nm x 20 nm, inset 8 nm x 4 nm), c) PBA/Au(111) (Vs = -1.5 V, It = 10 pA, 30 nm x 30 nm, inset 4 nm x 4 nm), d) CPBA/Au(111) (Vs = 1.5 V, It = 10 pA, 20 nm x 20 nm, inset 4 nm x 3 nm).

Figure 3 .

 3 Figure 3. a) STM image of PBA supramolecular network on a Cu(111) surface with corresponding superimposed model and unit cell vectors: UPBA = 1.98 nm, VPBA = 2.20 nm, ϴPBA = 77°, b) Representation of T-Shape p-p molecule-molecule interactions between adjacent pyridyl rings and the centre of the anthracenyl ring, separated by a distance of 0.42 nm, c) STM image of CPBA supramolecular network on a Cu(111) surface with corresponding superimposed model and unit cell vectors: UPBA = 0.85 nm, VPBA = 1.63 nm, ϴPBA = 82°, d) Representation of T-Shape p-p molecule-molecule interactions between cyano-phenyl rings, pointing towards the centre of the anthracenyl ring (0.52 nm), e) STM image of PBA supramolecular network on an Au(111) surface with corresponding superimposed model and unit cell vectors: UPBA = 2.25 nm, VPBA = 2.25 nm, ϴPBA = 60°, f) Representation of T-Shape p-p molecule-molecule interactions between two adjacent pyridyl rings, separated by a distance of 0.42 nm, and two nitrogen atoms point towards the centre of the anthracenyl ring of a third PBA molecule (0.41 nm), g) STM image of CPBA supramolecular network on an Au(111) surface with corresponding superimposed model and unit cell vectors: UCPBA = 2.58 nm, VCPBA = 1.45 nm, ϴCPBA = 94°, h) Representation of T-Shape p-p molecule-molecule interactions between cyano moieties and the centre of the anthracenyl ring (0.45 nm).

Figure 4 .Figure 5 .

 45 Figure 4. a) STM images of disordered features attained following subsequent thermal annealing up to 673 K on Cu(111) surface of : a) PBA (Vs = 1.8 V, It = 10 pA, 60 nm x 60 nm) an b) CPBA (Vs = -1.2 V, It = 10 pA, 60 nm x 60 nm).

Figure 6 .

 6 Figure 6. Thermal-induced transformation of PBA and CPBA molecules on a Cu(111) and an Au(111) surface. On Cu(111), the thermal annealing leads to disordered nanostructures, while on Au(111) the starting molecules are flattened due to intramolecular cyclodehydrogenation reactions.

Cu(111) On-surface coordination X X Au X X X X X X X X X X Disordered nanostructuresCoordination polymers

Acknowledgments

The authors acknowledge the financial support from the French National Research Agency through contract OVATION (ANR-19-CE09-0020) and from the Pays de Montbéliard Agglomération. The authors thank Pr. Dr. Alain Rochefort (Polytechnique Montréal, Canada) for fruitful discussions.


Moustafa Rodolphe Boudot

Abdel Hafiz

Michael Petersen

Claudio Calosso

All-optical microwave feedback oscillator with atomic cell resonator

All-optical microwave feedback oscillator with atomic cell resonator R. Boudot, 1 M. Abdel Hafiz, 1 M.Petersen, 1 C. E. Calosso, 2 and E. Rubiola 1, 2 1) FEMTO-ST, CNRS, Université Bourgogne Franche-Comté, Besançon, France 2) INRIM, Strada delle Cacce 91, Torino, Italy (*Electronic mail: rodolphe.boudot@femto-st.fr)

We report the demonstration of an all-optical self-sustained cell-based microwave feedback oscillator. In this scheme, a microwave resonance, optically-induced in a buffer-gas filled vapor cell resonator through coherent population trapping (CPT), is detected by a fast photodiode, amplified and used to drive back, through a frequency divider, a Mach-Zehnder electro-optic modulator in a sustaining loop configuration. The total gain and phase of the system was measured in open-loop configuration with a network analyzer. In good agreement with values predicted by the Leeson effect, the CPT feedback oscillator demonstrates an absolute phase noise of -24 dBrad 2 /Hz at 1 Hz offset frequency, compatible with a fractional frequency stability of 8 × 10 -12 at 1 s, and a phase noise floor of -112 dBrad 2 /Hz, limited by the low microwave power available at the photodiode output. The amplitude noise of the oscillator shows a comparable noise floor and, for offset frequencies lower than 200 Hz, a 1/ f dependence, due to the presence of the frequency divider in the loop.

Coherent population trapping (CPT) 1,2 is a quantum interference process in which atoms are optically pumped into a dark state by interacting with a dual-frequency laser field that connects two atomic ground states to a common excited state. The fulfillment of the null Raman detuning condition keeps the atoms in a coherent superposition of the two ground states with a typical relaxation time T 2 . In buffer-gas vapor cell experiments, the microwave hyperfine coherence lifetime T 2 is usually in the 0.1 -10 ms range, yielding ultimate atomic resonance linewidths ∆ν = 1/(πT 2 ) in the 30 -3000 Hz range (neglecting power broadening effects) and then Q-factors, if using Cs or Rb atoms, of 10 6 -10 8 . The exquisite properties of CPT resonances make them attractive candidates for a plethora of applications including laser spectroscopy [START_REF] Wynands | Precision spectroscopy with coherent dark states[END_REF] , atomic frequency standards [START_REF] Vanier | Atomic clocks based on coherent population trapping: a review[END_REF][START_REF] Yun | High-performance coherent population trapping clock with polarization modulation[END_REF][START_REF] Liu | Low-drift coherent population trapping clock based on laser-cooled atoms and highcoherence excitation fields[END_REF][START_REF] Hafiz | Symmetric autobalanced Ramsey interrogation for high-performance coherent population-trapping vapor-cell atomic clock[END_REF][START_REF] Yun | High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser[END_REF] , sensors [START_REF] Schwindt | Chipscale atomic magnetometer[END_REF][START_REF] Breschi | A high-sensitivity pushpull magnetometer[END_REF] , slow-light experiments [START_REF] Bajcsy | Stationary pulses of light in an atomic medium[END_REF] or laser cooling [START_REF] Aspect | Laser cooling below the one-photon recoil energy by velocityselective coherent population trapping: theoretical analysis[END_REF] . Most CPT-based vapor cell atomic clocks operate as a passive atomic frequency standard [START_REF] Vanier | The quantum physics of atomic frequency standards[END_REF] . In this case, the atomic resonance is probed by scanning the frequency of an opticallycarried microwave signal delivered by a local oscillator (LO) and is detected with a slow photodiode as an increase of the light power transmitted through the vapor cell. A derivative error signal, crossing zero at the resonance extremum, is then obtained by probing both sides of the resonance and can be processed to stabilize the LO frequency onto the atomic frequency. An alternative approach, known as the CPT maser [START_REF] Godone | Coherent-populationtrapping maser: Noise spectrum and frequency stability[END_REF] , consists to place the cell in an electromagnetic cavity in order to produce an oscillating magnetization responsible for the emission of a coherent signal emission at the atomic ground state hyperfine transition frequency. Other studies have demonstrated the possibility to directly detect, at the cell output with a fast photodiode, microwaves generated through stimulated Raman scattering (SRS) [START_REF] Vukicevic | Compact diode-laser based rubidium frequency reference[END_REF][START_REF] Balet | VCSEL-based Raman frequency reference on Rubidium atoms[END_REF] , electromagneticallyinduced transparency (EIT) [START_REF] Vukicevic | Compact diode-laser based rubidium frequency reference[END_REF] or CPT [START_REF] Strekalov | Stabilizing an optoelectronic microwave oscillator with photonic filters[END_REF][START_REF] Matsko | Magnetometer based on the optoelectronic oscillator[END_REF][START_REF] Strekalov | Application of vertical cavity surface emitting lasers in self-oscillating atomic clocks[END_REF][START_REF] Strekalov | All-Optical Atomic Clock on a Chip: Progress Report[END_REF][START_REF] Yim | Oscillator-free atomic clock using a multimode laser[END_REF][START_REF] Yim | Optelectronic oscillator stabilized to an intra-loop atomic filter with a coherent population trapping resonance[END_REF] . These approaches have been used to phase-lock, to a microwave atomic cell frequency reference, neighboring modes of multi-mode [START_REF] Yim | Oscillator-free atomic clock using a multimode laser[END_REF] or current-modulated lasers [START_REF] Yim | Optelectronic oscillator stabilized to an intra-loop atomic filter with a coherent population trapping resonance[END_REF] , or to construct self-oscillating atomic cell oscillators [START_REF] Strekalov | Stabilizing an optoelectronic microwave oscillator with photonic filters[END_REF][START_REF] Matsko | Magnetometer based on the optoelectronic oscillator[END_REF][START_REF] Strekalov | Application of vertical cavity surface emitting lasers in self-oscillating atomic clocks[END_REF][START_REF] Yim | Optelectronic oscillator stabilized to an intra-loop atomic filter with a coherent population trapping resonance[END_REF] . In this paper, we investigate the unusual concept of a CPTbased vapor cell atomic feedback oscillator [START_REF] Strekalov | Stabilizing an optoelectronic microwave oscillator with photonic filters[END_REF][START_REF] Matsko | Magnetometer based on the optoelectronic oscillator[END_REF][START_REF] Strekalov | Application of vertical cavity surface emitting lasers in self-oscillating atomic clocks[END_REF] , comparable in its architecture to those of opto-electronic oscillators (OEOs) [START_REF] Yao | Optoelectronic microwave oscillator[END_REF][START_REF] Li | Tutorial on optoelectronic oscillators[END_REF] , with the fiber delay line being replaced by the alkali vapor cell. In our setup, a high-Q atomic resonance, initiated through CPT in a buffer-gas filled Cs vapor cell, is detected with a fast photodiode, amplified and sustained in a loop oscillator configuration, by driving back through a frequency divider, a diode laser modulated with a fibered Mach-Zehnder electro-optic modulator (EOM). The latter is driven at half of the clock transition frequency and biased at its dark point, such that a proper CPT-resonant dual-frequency optical field, free from excess sidebands, is generated by both first-order optical sidebands. For better understanding of actual limitations of the cell-based feedback oscillator, a simple and straightforward phase noise metrology analysis is used. The phase noise of the Cs cell atomic oscillator is -24 dBrad 2 /Hz at f = 1 Hz, compatible with a frequency stability of 8 × 10 -12 at 1 s, and -112 dBrad 2 /Hz at f = 10 kHz. These results, at least one order of magnitude better than those previously-reported by atomic cell-based OEOs, are in good agreement with the Leeson model [START_REF] Leeson | A simple model of feedback oscillator noise spectrum[END_REF][START_REF] Rubiola | Phase noise and frequency stability in oscillators[END_REF] . The amplitude modulation (AM) noise of the oscillator was also measured, showing a comparable floor level and flicker of amplitude dependence for f < 200 Hz.

Figure 1 presents the experimental setup. The optical setup used for this study was mainly extracted from the existing one described in Ref. [START_REF] Hafiz | Symmetric autobalanced Ramsey interrogation for high-performance coherent population-trapping vapor-cell atomic clock[END_REF] . The heart of the experiment is a 2-cm diameter and 5-cm long buffer-gas filled (N 2 -Ar mixture) Cs vapor cell. The cell is maintained in a temperaturestabilized physics package that also integrates a solenoid, for optional application of a static magnetic field parallel used to raise the Zeeman degeneracy. This ensemble is surrounded by a double-layer mu-metal magnetic shield. In the present study, most of the experiments were performed at null magnetic field. These conditions were found to be favorable for increasing the atomic signal and subsequently improve the oscillator phase noise performances. A CPT atomic resonance can be created in the cell by making the atoms interact with a dual-frequency optical field. For this purpose, we use a Distributed Feedback (DFB) diode laser, tuned on the Cs D 1 line at 894.6 nm, externally-modulated by a fibered Mach-Zehnder electro-optic modulator (EOM), ultimately driven by the feed- back loop at 4.596 GHz, such that both first-order optical sidebands produce the CPT state. The EOM is temperature stabilized and the EOM is biased at the dark point such that the optical carrier at the EOM output is strongly reduced. At the output of the EOM, a fraction of the light is extracted and directed to an evacuated Cs vapor cell (not shown on Fig. 1 for clarity) that can be used for laser frequency stabilization using dualfrequency sub-Doppler spectroscopy [START_REF] Hafiz | Doppler-free spectroscopy on the Cs D 1 line with a dual-frequency laser[END_REF][START_REF] Brazhnikov | Dual-frequency sub-Doppler spectroscopy: Extended theoretical model and microcell-based experiments[END_REF] . The light propagates then into an acousto-optical modulator (AOM), driven with a radiofrequency signal at 122 MHz (-1-order). The AOM is used to compensate for the optical frequency shift induced by buffer gas in the CPT cell and for control of the laser power. The 1-cm diameter laser beam is then sent into the CPT cell, filled with 15 Torr of a N 2 -Ar buffer gas mixture (P Ar /P N 2 = 0.6). For tests where a static magnetic field was applied, the CPT interaction was produced with the pushpull optical pumping (PPOP) scheme [START_REF] Jau | Push-pull optical pumping of pure superposition states[END_REF][START_REF] Liu | Coherent-population-trapping resonances in buffer-gas-filled Csvapor cells with push-pull optical pumping[END_REF] . The CPT-induced microwave resonance at the cell output is detected by a fast photodiode (FPD). The microwave resonance (A) is then first amplified by three serial amplifiers with a total gain of about 70 dB and band-pass filtered (bandwidth < 50 MHz). A phase shifter is then placed to ensure the Barkhausen phase condition of the oscillator loop. At the phase shifter output, the 9.192 GHz signal is attenuated by 6 dB, frequency-divided by 2, band-pass filtered and again amplified in order to drive the EOM at 4.596 GHz. We note that the frequency divider (Hittite HMC361) acts as an amplitude limiter, yielding for a large input power range (E) to a power of 22 dBm to drive the EOM. A 6-dB microwave coupler is placed right after the amplification stage (B) to extract the output microwave signal from the oscillator for analysis (S).

In a first step, we have measured some characteristics of the oscillator system in open-loop configuration using a net-work analyzer. For this, the loop is opened between points E and D. The signal from the port 1 of the analyzer is injected in the frequency divider (E), with a power of about 0.5 dBm and the output of the 6-dB attenuator (D) is connected to the port 2 of the analyzer. Figure 2 shows an example of the total gain and phase (S 21 parameter) of the system, for a total laser power entering the cell of 1.3 mW and no static mag- netic field applied. The total gain G of the system is at resonance 1.3 dB, slightly above the required oscillation condition G = 0 dB. The loaded Q-factor Q L of the atomic resonator, defined as Q L = 1 2 dϕ dν 0 ν 0 , with dϕ dν 0 , the resonance frequency discriminator-based phase-frequency slope and ν 0 the resonance frequency (∼ 9.192640 GHz), is here 1.6 × 10 6 . Following these tests, we have performed some closed-loop tests and started the self-sustained oscillation by satisfying the Barkhausen conditions. Figure 3 shows the microwave signal obtained at the 6-dB coupler input (B), extracted from a measurement performed with a spectrum analyzer at the point S.

The microwave power at (B) is about 11 dBm.

For further analysis, we have measured the absolute phase noise performances of the Cs-cell feedback oscillator. Results, shown on Fig. 4 for a laser power of 1.3 mW at the cell input and null static magnetic field, are completed by the measurement of the residual phase noise of the total system. The latter is the phase noise of all the components of the loop, except the Cs cell, measured separately, in the same conditions as in the oscillator, using a standard mixer-based phase detection scheme. The white phase noise is of -112 dBrad 2 /Hz, in agreement with the expected value S ϕ 0 = FkT P A , where F (5 dB) and P A (-57 dBm) are the noise figure and input microwave power of the amplification stage respectively, and kT is the thermal energy [START_REF] Boudot | Phase noise in RF and microwave amplifiers[END_REF] . For offset frequencies f < f c , with f c 250 Hz here, the spectrum exhibits a phase flicker noise f -1 slope, yielding -90 dBrad 2 /Hz at f = 1 Hz. We suspect that the main contribution of the system residual phase noise is the EOM stage, that includes the by-2 divider, the high-power 4.6 GHz amplifier and the EOM. We have measured the residual noise of the arm composed of the by-2 divider and high-power 4.6 GHz amplifier, at the level of about -105 dBrad 2 /Hz at f = 1 Hz, well below the detection limit of the setup. However, its conjunction with the EOM always led, in a reproducible way, to about -90 dBrad 2 /Hz at f = 1 Hz. This degradation is not fully understood yet.

f 3 f 1 f 0 f 2 f L f c
oscillator noise residual noise FIG. 4. Absolute phase noise at 9.192 GHz of the Cs cell atomic feedback oscillator (red line) and residual phase noise of the sustaining loop (black line). The measured Leeson frequency f L is about 2.1 kHz, slightly lower than the calculated one f L = ν 0 /2Q L = 2.8 kHz. The transition from the f -3 to the f -2 slope is obtained around the corner frequency f c of the residual noise.

We found in the literature measurements of EOMs with lower residual phase noise [START_REF] Volyansky | Applications of the optical fiber to the generation and measurement of low-phase-noise microwave signals[END_REF] but also some with worse performances [START_REF] Barke | EOM sideband phase characteristics for the spaceborne gravitational wave detector LISA[END_REF] . The contribution of the photodetection stage (photodiode + 3 serial microwave amplifiers at 9.192 GHz) was measured well below the total detection limit, at the level of -108 dBrad 2 /Hz at f = 1 Hz.

The absolute phase noise of the Cs cell oscillator is measured in good agreement with the Leeson effect [START_REF] Leeson | A simple model of feedback oscillator noise spectrum[END_REF][START_REF] Rubiola | Phase noise and frequency stability in oscillators[END_REF] . The phase noise spectrum is well-described by the power-law performances represent an important advance for CPT oscillators, since they are at least one order of magnitude better than those described in Refs [START_REF] Vukicevic | Compact diode-laser based rubidium frequency reference[END_REF][START_REF] Matsko | Magnetometer based on the optoelectronic oscillator[END_REF][START_REF] Strekalov | Application of vertical cavity surface emitting lasers in self-oscillating atomic clocks[END_REF] . However, at the present state, they are not yet at the level of state-of-the-art sapphire oscillators [START_REF] Ivanov | Low phase-noise sapphire crystal microwave oscillators: current status[END_REF] , delay-line [START_REF] Li | Tutorial on optoelectronic oscillators[END_REF] and micro-resonator-based OEOs [START_REF] Maleki | The optoelectronic oscillator[END_REF] or standard CPT clocks [START_REF] Yun | High-performance coherent population trapping clock with polarization modulation[END_REF][START_REF] Hafiz | Symmetric autobalanced Ramsey interrogation for high-performance coherent population-trapping vapor-cell atomic clock[END_REF][START_REF] Yun | High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser[END_REF] . The main limitation of the present approach is represented by the phase fluctuations induced by the sustaining loop, in particular the EOM stage, that are directly converted into frequency noise through the CPT oscillator loop. Also, the floor is limited by the low microwave power available at the input of the first amplifier following the photodetection stage. Below the Leeson frequency, this floor is converted into white frequency noise (WFN) whose level is b -2 = -45 dBrad 2 Hz, corresponding to a fractional frequency stability of 4 × 10 -13 at 1 s, a level comparable to those demonstrated by stateof-the-art CPT clocks [START_REF] Yun | High-performance coherent population trapping clock with polarization modulation[END_REF][START_REF] Hafiz | Symmetric autobalanced Ramsey interrogation for high-performance coherent population-trapping vapor-cell atomic clock[END_REF][START_REF] Yun | High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser[END_REF] . Notice that the WFN level is due solely to fundamental quantities, like signal-to-noise ratio of the resonance and the Leeson frequency, i.e. atomic quality factor, and sets the ultimate limit of the CPT oscillator once other technical noises, like the one from the EOM stage, are made negligible. Several approaches might be explored to improve the current oscillator performances, including the use of high-power photodiodes [START_REF] Fortier | Photonic microwave generation with high-power photodiodes[END_REF] to increase the level of microwave power and then reduce the phase noise floor, the reduction of phase fluctuations of the loop or, at the condition of detecting a powerful-enough signal, the application of a static magnetic field to increase the resonance Q-factor and then reduce the Leeson frequency.

S ϕ ( f ) = ∑ 0 i=-3 b i f i ,
We have also started the oscillation onto the Cs atom 0-0 transition, with application of a static magnetic field. In this case, a degradation of the oscillator phase noise by about 5 dB for 5 < f < 1000 Hz and up to 10 dB for the noise floor was observed, explained by the reduction of the number of atoms involved in the magnetic-insensitive 0-0 transition and therefore by a reduction of the resonance signal. Degradation of CPT-based oscillator performance was also observed in this case in previous studies [START_REF] Matsko | Magnetometer based on the optoelectronic oscillator[END_REF][START_REF] Strekalov | Application of vertical cavity surface emitting lasers in self-oscillating atomic clocks[END_REF] . We also did some tests at null magnetic field using circularly-polarized light. With similar experimental parameters, phase noise results comparable with those obtained with PPOP were obtained.

In a last part, we have performed the measurement of the AM noise of the cell-based oscillator. Corresponding results, extracted from a second measurement campaign with comparable conditions to those described in Fig. 1, are shown in Fig. 5. AM and PM noise spectra were simultaneously acquired with a phase noise analyzer (Rohde-Schwarz FSWP). We note first that the oscillator phase noise is similar to the one shown in Fig. 4. The AM noise spectrum shows a noise floor comparable to the phase noise floor, as expected since noise floors result from an additive noise mechanism, and a flicker of amplitude with S α = 2 × 10 -9 / f . The absence of a visible Leeson effect on the AM noise spectrum is well explained by the formalism developed in Ref. [START_REF] Rubiola | A generalization of the Leeson effect[END_REF] , and is due to the frequency divider, acting as a hard amplitude limiter. Because of the divider, the output amplitude of the chain that drives the EOM is a constant, independent of the input amplitude. Thus, the gain of the amplifier chain is inversely proportional to the input amplitude, and the slope of the normalized gain -γ, defined in Ref. [START_REF] Rubiola | A generalization of the Leeson effect[END_REF] , is -1. With γ = 1, the amplitude-noise transfer function, described in Eq. 68 of Ref. [START_REF] Rubiola | A generalization of the Leeson effect[END_REF] , equals 1. The physical meaning of the Leeson effect is that the oscillator integrates the phase fluctuations circulating in the feedback loop. The same happens with gain fluctuations, with the obvious difference that saturation prevents the amplitude from diverging. In our case, unlike "regular" oscillators and lasers, the behavior of the oscillation amplitude depends mainly on the saturation induced by the divider, which breaks the integration process mentioned above. The Leeson effect is then not visible on the AM noise and the AM noise is determined by the divider, the EOM and the microwave amplifier that follows. The AM noise of the oscillator then contains only white and 1/ f processes. Not surprisingly [START_REF] Boudot | Phase noise in RF and microwave amplifiers[END_REF] , the AM flicker corner of the amplification chain is quite similar to that of phase noise.

In conclusion, we have reported the characterization of a microwave feedback oscillator that uses an optically-excited buffer gas-filled vapor cell as a resonator. In this approach, the microwave atomic resonance induced by CPT is detected by a fast photodiode at the cell output and maintained by a sustaining amplifier stage that feeds back an electro-optic modulated laser through a frequency divider. The phase noise of the Cs cell atomic feedback oscillator is -24, -82 and -112 dBrad 2 /Hz, for offset frequencies of 1 Hz, 100 Hz and 10 kHz, respectively. These results are in good agreement with the Leeson model, with a noise floor limited by the low detected microwave power and a stability level at 1 s currently degraded by the flicker phase noise of the EOM stage converted into frequency fluctuations of the oscillator loop. In addition, we have measured the AM noise of the cell-based oscillator. The latter exhibits for f < 200 Hz a 1/ f slope and a noise floor comparable to the phase noise floor. The absence of a visible Leeson effect onto the AM noise spectrum was explained by the presence of a divider in the loop acting as a hard amplitude limiter.

  FIG. 1. (Color online) Simplified schematic of the CPT-based Cs cell feedback oscillator. The reference of the oscillator loop consists of the atomic resonance produced in the vapor cell through CPT spectroscopy. At the cell output, the resonance is detected by a fast photodiode, amplified, filtered with a band-pass filter (BPF), frequency-divided by 2 and again amplified to drive the EOM that modulates the DFB laser.Microwave isolators were used in the setup but are not shown here for clarity. The AOM is used to shift the laser frequency and control the laser power. An inset shows the CPT diagram. A linear polarizer (P) is inserted right after the cell to align respective involved in the PPOP scheme along a single axis and make possible the detection of the microwave beatnote. The typical power values at different points of the system are given for information.

FIG. 2 .FIG. 3 .

 23 FIG.2. Total gain and phase (S 21 parameter, magnitude and phase) of the system, measured with a network analyzer in open loop configuration (input: E, output: D). The center of the frequency axis corresponds to an atomic resonance frequency ν 0 ∼ 9.192640 GHz, shifted from the exact Cs clock transition frequency due to the presence of buffer gas in the cell. A 100-pt averaging was applied onto the data.

2 0= 8 ×

 28 with coefficients b 0 = -112 dBrad 2 /Hz, b -2 = -45 dBrad 2 Hz and b -3 = -24 dBrad 2 Hz 2 , respectively. The associated fractional frequency stability is a flicker frequency noise of 2 ln 2 b -3 ν 10 -12 . These

FIG. 5 .

 5 FIG.5. AM noise and phase noise of the Cs cell feedback oscillator, measured simultaneously under identical conditions. Spurious removal was performed by the noise analyzer.

G. Alzetta, A. Gozzini, L. Moi and G. Orriols, An experimental method for the observation of r.f. transitions and laser beat resonances in oriented Na vapour, Nuovo CimentoB 36, 5-20 (1976).

ACKNOWLEDGMENTS

This work has been partly funded by Conseil Régional Bourgogne Franche-Comté with the HACES project (grant 2018-04768), and in part by Agence Nationale de la Recherche (ANR) in the frame of the LabeX FIRST-TF (Grant ANR 10-LABX-48-01) and EquipX Oscillator-IMP (Grant ANR 11-EQPX-0033) projects. This work has been supported by the EIPHI Graduate school (Grant ANR-17-EURE-0002).

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.