Iann C Gerber
email: igerber@insa-toulouse.fr

R Poteau

Critical assessment of charge transfer estimates in non-covalent graphene doping

Critical assessment of charge transfer estimates in non-covalent graphene doping

I.C. Gerber,

I. INTRODUCTION

Graphene, a single exfoliated layer of graphite, was first obtained by the group of Novoselov [START_REF] K S Novoselov | Electric Field Effect in Atomically Thin Carbon Films[END_REF]. Due to its special honeycomb structure, implying only sp 2 carbon atoms, the valence band (VB) and the conduction band (CB) intersect at the Dirac point. Interestingly the energy-momentum dispersion is linear around the Dirac point [START_REF] Geim | The rise of graphene[END_REF]. As a result ballistic charge transport has been observed [START_REF] Das Sarma | Electronic transport in twodimensional graphene[END_REF], and ideally this property could be useful in wide range of applications. Therefore, graphene-based devices have been considered as a promising alternative to conventional silicon-based ones. By shifting the Fermi level relative to the Dirac point, the carrier type and concentration in graphene can be controlled through electrical doping either with an external electric field [START_REF] K S Novoselov | Electric Field Effect in Atomically Thin Carbon Films[END_REF][START_REF] Zhang | Experimental observation of the quantum Hall effect and Berry's phase in graphene[END_REF][START_REF] K S Novoselov | Two-dimensional gas of massless Dirac fermions in graphene[END_REF] or chemical doping with different atoms and/or molecules, see Ref. [START_REF] Liu | Chemical doping of graphene[END_REF][START_REF] Georgakilas | Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications[END_REF][START_REF] Dirian | Low dimensional nanocarbons -chemistry and energy/electron transfer reactions[END_REF][START_REF] Georgakilas | Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications[END_REF] and references therein. Among all the possibilities offered by a chemical functionalization of a graphene, the physisorption of some electron donor/acceptor molecules is particularly simple, non-destructive and effective [START_REF] Chen | Surface Transfer p-Type Doping of Epitaxial Graphene[END_REF][START_REF] Voggu | Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques[END_REF]. The mechanism of charge transfer (CT) between electron donor/acceptor (EDA) and a graphene monolayer has been elucidated by one of the author [START_REF] Hu | Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene[END_REF], completed by a later study [START_REF] Chen | Energy Level Alignment and Charge Carrier Mobility in Noncovalently Functionalized Graphene[END_REF]. CT occurs as soon as a difference in electronic chemical potentials is present at the interface. This difference is thus simply determined by the relative positions of the graphene Fermi level and the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the adsorbate. If the LUMO of the adsorbate lies lower in energy than the Fermi level of graphene, electrons will flow from graphene to the adsorbate making graphene doped by holes, i.e p-type. On contrary adsorbates with the HOMO lying above the graphene Fermi level will act as donors, thus n-type doping graphene.

Theoretically speaking it means that to be predictive and quantitative, a DFT study of the interaction between any electron donor/acceptor molecule and graphene, needs to include (i) a correct description of the electronic structure of the molecule, more specifically to put the HOMO/LUMO energy at the right position and (ii) since the adsorbate is only physisorbed, a correct description of the van der Waals forces is mandatory, to put the molecule at the right distance above graphene. Indeed in the work of Hu et al it has been shown that CT is crucially dependent of the distance between the EDA molecule and graphene [START_REF] Hu | Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene[END_REF]. The main concern of all the previous studies in this field were devoted to theoretically determine the existing CT, mainly thanks to the "Quantum Theory of Atoms in Molecules" (QTAIM), that is able to partition the electronic density based on topological arguments [START_REF] Bader | Atoms in Molecules: A Quantum Theory[END_REF]. Mind that vdW interactions were not systematically included in previous works dealing with the interaction between organic EDA molecule with graphene [START_REF] Arun | Tuning the Electronic Structure of Graphene by Molecular Charge Transfer: A Computational Study[END_REF][START_REF] Lu | Tuning the Electronic Structure of Graphene by an Organic Molecule[END_REF][START_REF] Zhang | Tuning the electronic structure and transport properties of graphene by noncovalent functionalization: effects of organic donor, acceptor and metal atoms[END_REF][START_REF] Sun | Linear tuning of charge carriers in graphene by organic molecules and charge-transfer complexes[END_REF][START_REF] Chi | First principle study of the interaction and charge transfer between graphene and organic molecules[END_REF][START_REF] Denis | Chemical Reactivity of Electron-Doped and Hole-Doped Graphene[END_REF][START_REF] Kong | Molecular adsorption on graphene[END_REF][START_REF] Pablo | Strong N-Doped Graphene: The Case of 4-(1,3-Dimethyl-2,3-dihydro-1 H-benzoimidazol-2-yl)phenyl)dimethylamine ( N-DMBI)[END_REF][START_REF] Nishidate | Tuning the work function of graphene with the adsorbed organic molecules: firstprinciples calculations[END_REF][START_REF] Yang | Many-body dispersion effects on the binding of TCNQ and F4-TCNQ with graphene[END_REF]. Thus it is challenging to correctly describe CT complexes usually since (i) the local density approximation (LDA) or generalized gradient approximations (GGA) of the exchange-correlation functionals tend to over or under-bind respectively, (ii) the position of the HOMO or LUMO adsorbate with respect to the Fermi level of graphene may be erroneous. We propose here, to investigate the use of hybrid functionals, which provide electronic structures, especially HOMO-LUMO gap, in better agreement with experimental data, to estimates CT for two prototypical EDA molecules, the electron-acceptor TCNE (tetracyano-ethylene) and the electron donor TTF (tetrafulvalene) in interaction with graphene. We also compare different methods for the projections/partitioning of the electronic densities, to quantify the CT.

II. COMPUTATIONAL DETAILS

The atomic structures have been obtained from DFT calculations using the VASP package. [START_REF] Kresse | Ab initio molecular dynamics for liquid metals[END_REF][START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF] It uses the projector-augmented wave scheme [START_REF] Blöchl | Projector augmented-wave method[END_REF][START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented-wave method[END_REF] to treat core electrons.

The 2s and 2p states of carbon and nitrogen, 1s state of hydrogen and 3s and 3p states of sulfur have been treated explicitly in the valence. The graphene model used is made of (7×7) primitive cell, with a relaxed C-C bond-length of 1.42 Å. As usual in this type of calculations, we have applied a supercell approach, with a vacuum region of more than 16 Å to avoid interaction between periodic images in the perpendicular direction. On top of this substrate one single molecule (TCNE or TTF) has been absorbed, in various sites of high symmetry using the notations of a previous work [START_REF] Hu | Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene[END_REF]. All atoms were allowed to relax with a force convergence criterion below 0.005 eV/ Å, and to ensure a reliable distance between the adsorbate and graphene, we have accounted for van der Waals interactions through the use of opt-B86b-DF scheme [START_REF] Klimeš | Chemical accuracy for the van der Waals density functional[END_REF][START_REF] Klimeš | Van der Waals density functionals applied to solids[END_REF] for every geometry optimization runs. The plane-wave basis set cutoff energy was set to 400 eV with a Gaussian smearing method of 0.1 eV width, in order to assure well converged total energy and force values, in conjunction with a (3×3×1) Γ-centered k-point grid. Choice of the exchange-correlation functional has been tested to estimate CT, by using spin polarized version of the Generalized Gradient Approximation (GGA) PBE [START_REF] Perdew | Generalized gradient approximation made simple[END_REF], two hybrid functionals PBE0 [START_REF] John P Perdew | Rationale for Mixing Exact Exchange with Density Functional Approximations[END_REF][START_REF] Adamo | Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model[END_REF] and HSE [START_REF] Heyd | Assessment and Validation of a Screened Coulomb Hybrid Density Functional[END_REF][START_REF] Heyd | Energy Band Gaps and Lattice Parameters Evaluated with the Heyd-Scuseria-Ernzerhof Screened Hybrid Functional[END_REF][START_REF] Paier | Screened Hybrid Density Functionals Applied to Solids[END_REF], as well as RSHXLDA scheme [START_REF] Gerber | Hybrid functional with separated range[END_REF][START_REF] Iann | Range separated hybrid density functional with long-range Hartree-Fock exchange applied to solids[END_REF] known to produce correct long-range exchange behavior. Bader charge analysis, based on Atoms in Molecule theory [START_REF] Bader | Atoms in Molecules: A Quantum Theory[END_REF], were carried out using Henkelman's Group program [START_REF] Henkelman | A fast and robust algorithm for Bader decomposition of charge density[END_REF][START_REF] Sanville | Improved grid-based algorithm for Bader charge allocation[END_REF][START_REF] Tang | A grid-based Bader analysis algorithm without lattice bias[END_REF], after a careful checking of the integration grid dimension controlled by the Fast Fourier Grid parameters. Those charges have been compared with the ones from a Mulliken-like population analysis (MPA) within VASP. Indeed it is well known that absolute partial charge values can vary signicantly between different partitioning schemes. [START_REF] Kenneth | Comparison of atomic charges derived via different procedures[END_REF][START_REF] Leenaerts | Paramagnetic adsorbates on graphene: A charge transfer analysis[END_REF] MPA from VASP wavefunction was performed by integrating up to the Fermi energy the Density of States projected onto an atomic Slater basis set (pDOS). These charges are hereafter mentioned as pMPA charges. The projection of the PAW wavefunction was achieved with the Lobster software, using the pbeVASPfit basis set [START_REF] Volker | Crystal orbital hamilton population (COHP) analysis as projected from plane-wave basis sets[END_REF][START_REF] Maintz | Analytic projection from plane-wave and PAW wavefunctions and application to chemical-bonding analysis in solids[END_REF][START_REF] Maintz | LOB-STER: A tool to extract chemical bonding from plane-wave based DFT[END_REF]. The charge spilling, a criterion that assesses the quality of the projection, was systematically lower than 1.0%. Various electronic density decomposition schemes (MPA, AIM-Bader, Natural Population Analyzis [START_REF] Reed | Natural population analysis[END_REF], CM5 [START_REF] Aleksandr | Charge model 5: An extension of hirshfeld population analysis for the accurate description of molecular interactions in gaseous and condensed phases[END_REF]) were also applied to TCNE and TTF molecules alone, using the 6-311++G(d,p) Gaussian basis sets within the G09 software [START_REF] Frisch | Gaussian 09 Revision D.01[END_REF]. These wavefunction analysis were carried out with the Multiwfn software [START_REF] Lu | Multiwfn: A multifunctional wavefunction analyzer[END_REF].

III. RESULTS

A. Free compounds

Several atomic charges calculation methods are first going to be evaluated on the gasphase TCNE and TTF molecules. Two procedures based on molecular orbitals were examined -i.e. in the framework of Gaussian atomic basis sets -the Mulliken population analysis (MPA) and the Natural population analysis (NPA). They have been compared to two methods based on the density distribution: the Bader's Atom In Molecule (AIM), also known as Bader charges, and the Charge Model 5 (CM5), which is an extension of Hirshfeld population analysis. AIM charges were also calculated from the density functional electronic charge distributions calculated in a pseudopotential -plane waves basis set, under periodic boundary conditions, i.e. with a supercell approach. Molecular orbitals based methods cannot be directly used with such methods, both because a density matrix in a localized atom-centered basis set is required, and because the electronic wave function for periodic systems is given in reciprocal space, thus yielding electronic states delocalized over all space. A way to circumvent these fundamental challenges is to employ a plane waves to atomic orbitals projection scheme. This is the strategy used in the present study, taking advantage on a projected density of states, from which an MPA can be easily achieved. It must be underlined that since the resulting charges, referred to as pMPA charges, are obtained within an orthonormal minimal Slater basis set, they are not expected to be similar to the Gaussian atomic basis sets MPA charges. On top of that, the PBE GGA functional will be compared to the PBE0 hybrid functional. The comparison of the AIM or pMPA charges in these free compounds with the complexes will then show where the electron density depletion or increase does occur upon an intermolecular CT phenomenon.

TCNE molecule.

The first general comment is that PBE0 and PBE atomic charges are very close, whatever the method ( As shown previously [START_REF] Hu | Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene[END_REF], if several adsorption sites presenting high symmetry can be tested, indeed they are almost degenerate, with energy differences smaller than 0.2 kcal/mol.

In this respect we propose here to discuss only the most stable state of adsorption for TTF and TCNE, starting for Bridge and Hollow geometries respectively, since the effect of the orientation of the molecule with respect to the graphene layer is really weak with CT differing by less than 0.02e at maximum between different adsorption sites. The selected optimized stacking geometries are depicted in Fig. 1. We recall the importance of including van der Waals forces in the xc-functional term to yield correct adsorption height: with opt-B86b-DF TCNE lies 3.2 Å above the graphene, while with PBE it is 3.38 Å. the same change is observed for the TTF molecule. The corresponding adsorption energies are in line with previous studies with values around -30 kcal/mol when dispersion forces are included in the electronic correlation term. On top of these geometries we have performed AIM-Bader analysis, with various xc-functionals and also compared for two density decomposition schemes, namely AIM-Bader and pMPA.

xc-functional

In figure 2, TCNE and TTF CTs estimated by AIM-Bader scheme are shown for four selected xc-functionals on the same set of geometries. Surprisingly, for the acceptor molecule, the effect of using hybrid-type functional is weak, of the order of the integration's precision.

Only RSHXLDA functional provides a slightly smaller CT probably due to its long-range exchange corrected HF term, compared to GGA estimates. So for this particular molecule, the estimates of CT by Bader analysis is more influenced by the distance between the molecule and graphene [START_REF] Hu | Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene[END_REF], and not primarily by the difference in the electronic structure of the molecule due to the use of hybrid functional, which usually enhances HOMO-LUMO gap. Indeed these values are 2.7, 3.7, 4.4 and 8.0 eV for PBE, HSE, PBE0 and RSHXLDA respectively. In other words, space CT seems to be mainly governs by density overlaps between the two subsystems and globally the use of HF exchange term does not affect this overlap for the TCNE case. For the donor case, the situation is different, the PBE values is more than three times larger than with hybrid functionals, for the same molecule-graphene distance. When comparing the fundamental gaps for the freestanding TTF molecule which are 2.1, 3.1, 3.9 and 7.0 eV for PBE, HSE, PBE0 and RSHXLDA respectively, one cannot find the origin of the discrepancy, the differences between GGA and hybrid values being very similar to the TCNE case. Here we propose that GGA tendency to over-delocalize electronic density already presented as a manifestation of self-interaction error (SIE), [START_REF] Zheng | Delocalization error of density-functional approximations: A distinct manifestation in hydrogen molecular chains[END_REF] is thus responsible for the CT differences. Indeed if one looks at figure 3a In Table III, CT evaluations with pMPA and AIM for both molecules and two different xc-functionals are given. We also provides the final magnetic moment of the calculation cell, since it is a relevant physical property [START_REF] Leenaerts | Paramagnetic adsorbates on graphene: A charge transfer analysis[END_REF] and a direct manifestation of the CT. Indeed since a pristine graphene is diamagnetic, any charge changes (being positive or negative) on graphene do not affect the total magnetic moment, contrarily to the adsorbate. overestimates the CT for TTF-Graphene system, since a zero magnetic moment is obtained for this system. Slightly more reliable results are obtained with the use of a hybrid functional, the PBE0 CT being -0.04e, making the CT much smaller for this donor molecule than for the acceptor TCNE molecule. Experimentally, CTs are observed in G-band shifts in Raman spectra for some graphene flakes in interaction with TTF solution [START_REF] Voggu | Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques[END_REF], the shifts being smaller for TTF than for TCNE at the same concentration. Thus n-doping of graphene is much less efficient than p-doping, as observed experimentally. Moreover, in order to explain the discrepancy between the AIM CT value and the zero magnetic moment for the TTF molecule, one may suspect that some weak hybridization starts to occur between the adsorbate and the graphenic substrate, making the entire system diamagnetic.

Let us now compare the pMPA and AIM atomic charges within PBE0 calculations (Fig. 1). As already found in freestanding TCNE, ethylenic carbons are found to be negative with pMPA and positive with AIM. Whereas the CT is found to be fairly the same with both methods, they do not agree in the charge redistribution from graphene to TCNE. According to AIM the CT occurs mainly toward nitrogen atoms (-1.14e vs. -1.05e in the freestanding TCNE), whereas according to pMPA it is both nitrogen and ethylenic carbon atoms that benefit from the CT (C/N: -0.29e/-0.39e vs. -0.17e/-0.27e). Regarding now TTF, both methods find that the CT toward the graphene is weak, and the comparison between the charges given in Figure 3 and those reported in Table II shows so that there is not significant charge redistribution upon the adsorption of TTF on graphene.

IV. CONCLUSION

To sum up, as already mentioned in a previous work, the primary interaction between an EDA molecule and a pristine graphene is of physisorption type.

  ) C(≡N) N C(=C) C(≡N) N MPA a 2.29 -1.03 -0.11 2.46 -1.10 -0.13 pMPA b -0.18 0.37 -0.27 -0.17 0.37 -0.27 NPA -0.15 0.25 -0.17 -0.14 0.25 -0.18 AIM a 0.16 0.83 -0.91 0.18 0.91 -1.00 AIM b 0.25 0.87 -0.99 0.31 0.89 -1.05 CM5 0.08 0.20 -0.24 0.09 0.21 -0.25TABLE I: Partial atomic charges in TCNE (in units of e). a : Gaussian basis set (G09); b : PW basis set (VASP).

FIG. 1 :

 1 FIG. 1: Comparison of the AIM-Bader (bottom) and pMPA (top) partial atomic charges on EDAs (left: TCNE, right TTF). Atom colors are set up according to the charged-based color coded scheme plotted on the left.

FIG. 2 :FIG. 3 :

 23 FIG. 2: AIM-Bader charge transfer with respect to xc-functional choice, for TCNE (black) and TTF (red) molecules.

  It appears thus naturalto describe those systems with the help of an exchange-correlation functional that treats explicitly van der Waals forces. This imposes a correct distance between the adsorbate and the carbon-based substrate and at the same time authorize CT through space since electronic densities overlap. To estimate this CT we have considered two different schemes: one based on the AIM-Bader theory on one side and Mulliken-like scheme based on an atomic Slater basis set projection. AIM Bader charge transfer appear reliable for both donor and acceptor molecules, pMPA scheme seems to overestimate those CTs. However the intra-molecular partitioning are clearly different, for TCNE molecule for instance even some changes of sign are observed between the two schemes for the charge of two central C atoms. This makes the discussion of the extra-charge localization over the constituents of the molecule difficult. For the calculations considering the TTF molecule, the estimated CT is much weaker. When a portion of exact-exchange in included, this CT is even more decreased, making GGA calculations not reliable for this system. Globally this weak CT values is in line with experimental data, graphene is indeed less efficiently n-doped than p-doped, at the same concentration of dopants. More systematic studies, especially with others donor molecules, are thus further required. Out of these results, we can only advice to use both partitioning schemes, firstly to double-check the charge transfer values and secondly since they appear complementary in terms of extra possible analysis, one based of the topological aspects of the electronic density, the other scheme being based on a more natural molecular-orbitals like analysis.

TABLE II :

 II tableI). The explicit exchange contribution in the functional does not significantly affect this property. Although basis sets and pseudopotentials differ between VASP and G09, AIM charges are found to be almost the same, thus showing a weak basis set dependency of this density partitioning scheme. Not surprisingly, MPA and pMPA charges strongly differ from each other. It is a good point for pMPA, since MPA atomic charges are questionable, with a 2.29e (PBE) or 2.46e (PBE0) very strong charge on the ethylenic carbon atoms and a strongly negative charge on the cyano groups carbon atoms (PBE: -1.03e, PBE0: -1.10e). Interestingly, it can be seen from table I that pMPA and NPA charges are very similar, whereas CM5 charges are to some extent found to be closer to pMPA and NPA than to AIM. All methods agree that nitrogen atoms are Partial atomic charges in TTF (in units of e).

			PBE			PBE0
		C(central) C(lateral) H	S C(central) C(lateral) H	S
	MPA a	0.63	-0.12 0.26 -0.45	0.66	-0.12 0.27 -0.48
	pMPA b	-0.51	-0.57 0.37 0.46	-0.49	-0.56 0.37 0.45
	NPA	-0.45	-0.40 0.24 0.38	-0.44	-0.39 0.24 0.37
	AIM a	-0.22	-0.15 0.09 0.17	-0.24	-0.16 0.09 0.18
	AIM b	-0.24	-0.20 0.14 0.18	-0.24	-0.20 0.15 0.17
	CM5	-0.05	-0.11 0.12 0.02	-0.04	-0.11 0.12 0.01

a : Gaussian basis set (G09); b : PW basis set (VASP).

negatively charged, with a ca. -1e charge with AIM and a -1 /4 e or -1 /5 e charge with the other methods. Whereas the global charge of cyano groups is found to be slightly negative with AIM and CM5 (between -0.04e and -0.1e), it is slightly positive with pMPA and NPA (∼ 0.1e). As a consequence ethylenic carbon atoms exhibit a negative charge according to pMPA and NPA and a positive charge with AIM and CM5, which is apparently more in line with the electron-withdrawing character of cyano groups. However, what is more important is the relative value of atomic charges upon the replacement of a functional group, a trend well accounted for by pMPA and NPA when comparing to ethylene (-0.50e and -0.38e respectively). TTF molecule. As observed previously with TCNE, PBE0 and PBE charges are very similar (table

II

). MPA again provides very different charges from the other methods, and pMPA and NPA charges are still very close. Whereas in TCNE the AIM charges were close to unity on the C and N atoms of the cyano groups, no large intermolecular charge transfer is found here with this method. On the contrary, it is now the pMPA and NPA methods that provide the highest charge values. It is worth mentioning that CM5 charges are close to zero, in contrast with the other procedures. pMPA, NPA and AIM actually qualitatively agree, with positive sulfur atoms, negative central carbon atoms and a slightly positive C 2 H 2 moiety.

  This indirect estimate of the CT is accurate if the adsorbed molecules do not interact strongly with the π and π * orbitals of the graphene. Since TCNE and TTF molecules are physisorbed on graphene, one can be confident in this values. By comparing electron density partitioning schemes, the main comment is that pMPA approach always provides larger CT than AIM, and suffers from xc-functional's choice too. Regarding TTF, larger absolute values are yielded with PBE than with PBE0. Interestingly AIM CT values for the TCNE molecule are very close to the magnetic moment value for both functionals, when one can observe larger differences for the TTF molecule. From this table, one can indeed conclude that PBE

Acknowledgments

I. C. Gerber and R. Poteau thanks the CALMIP initiative for the generous allocation of computational times, through the project p0812, as well as the GENCI-CINES, GENCI-IDRIS and GENCI-CCRT for the A004096649 grant.


domain: chim phys

text:
J Dugay

R P Tan

A Loubat

L.-M Lacroix

J Carrey

Pier F Fazzini

B Chaudret

M Respaud

Tuning Deposition of Magnetic Metallic Nanoparticles from Periodic Pattern to Thin Film Entrainment by Dip Coating Method

In this paper, we report the self-assembly of chemically synthesized metallic CoFe nanoparticles (NPs) from periodic pattern to thin film. NPs depositions were carried out inside a coupled glovebox-sputtering system to prevent NP oxidation and to tune the surface energy of the substrate through oxygen plasma treatment. A home-made dip coating setup allowing the vertical withdrawal of substrate from the colloidal suspension was used. We have systematically investigated the effects of NP concentration, withdrawal speed, nature (amino and acid groups) and amount of surfactants in suspension on the quality of the deposit on hydrophilic substrates (SiO 2 or Au). The stripe thickness, width and spacing, formed by the deposit, can be controlled by the NP concentration and the withdrawal speed. The surfactant concentration generally tends to decrease the NP coverage, except for amine which induced an improved coverage on silica substrate. We report here for the first time that dip coating is a suitable and reproducible technique to deposit magnetic metallic NPs over large areas from stripes pattern to thin films entrainment.

Introduction

Self-assembly is a physical concept present in Nature from atomic scale to large-scale structures of the universe. Such phenomenon can be defined as the self-assembly of similar entities in patterns or structures, without any human intervention, but in a controlled environment. [START_REF] Whitesides | Self-Assembly at All Scales[END_REF] Nowadays, the recent developments in colloidal synthesis allows the growth and the self-assembly of artificial nanometer-scale metallic, semiconductor, and/or magnetic building blocks thanks to a fine tuning of NP morphological characteristic such as size, size-distribution and aspect ratio, and their physico-chemical properties governed by capping surfactants. [START_REF] Talapin | Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications[END_REF] Thanks to these unprecedented properties, magnetic NPs (MNPs) may be used for spintronics devices applications. [START_REF] Reiss | Magnetic nanoparticles: Applications beyond data storage[END_REF][START_REF] Chappert | The emergence of spin electronics in data storage[END_REF] For instance a new type of magnetoresistance [START_REF] Tan | Transport in Superlattices of Magnetic Nanoparticles: Coulomb Blockade, Hysteresis, and Switching Induced by a Magnetic Field[END_REF] and tunneling magnetoresistance (TMR) up to room temperature [START_REF] Dugay | Room-Temperature Tunnel Magnetoresistance in Self-Assembled Chemically Synthesized Metallic Iron Nanoparticles[END_REF] have been recently observed in MNPs surrounded by organic ligands. However, a crucial issue for such applications is the perfect control of MNPs deposit directly onto prefabricated electrodes in form of chains, arrays, and networks. [START_REF] Koh | Strategies for Controlled Placement of Nanoscale Building Blocks[END_REF] Such a control would allow on one hand, to tune the strength of the dipolar interactions between MNPs and thus increase the amplitude of the TMR. [START_REF] Tan | Numerical simulations of collective magnetic properties and magnetoresistance in 2D ferromagnetic nanoparticle arrays[END_REF] We emphasize that to observe TMR in MNPs assemblies, drastic precautions against oxidation need to be taken. [START_REF] Dugay | Room-Temperature Tunnel Magnetoresistance in Self-Assembled Chemically Synthesized Metallic Iron Nanoparticles[END_REF] On the other hand, the control could also permits to study the influence of dimensionality and structural disorder on transport mechanism and therefore on the resulting magneto-transport properties. [START_REF] Tran | Sequential tunneling and inelastic cotunneling in nanoparticle arrays[END_REF][START_REF] Pauly | Co-tunneling Enhancement of the Electrical Response of Nanoparticle Networks[END_REF] One way to integrate the NPs into patterned devices is based on physical and/or chemical templates, obtained by lithography, [START_REF] Cui | Integration of Colloidal Nanocrystals into Lithographically Patterned Devices[END_REF][START_REF] Dai | Programmable Nanoparticle Ensembles via High-Throughput Directed Self-Assembly[END_REF] microcontact printing, [13][START_REF] Park | Langmuir Monolayers of Co Nanoparticles and Their Patterning by Microcontact Printing[END_REF][START_REF] Kraus | Nanoparticle printing with single-particle resolution[END_REF][START_REF] Jie | Laterally patterned magnetic nanoparticles[END_REF] micro-injection molding in capillaries, [START_REF] Cavallini | Multiple-length-scale patterning of magnetic nanoparticles by stamp assisted deposition[END_REF] dip pen lithography, [START_REF] Basnar | Dip-Pen-Nanolithographic Patterning of Metallic, Semiconductor, and Metal Oxide Nanostructures on Surfaces[END_REF][START_REF] Bellido | Controlled Positioning of Nanoparticles on Graphene by Noninvasive AFM Lithography[END_REF] elimination lithography on self-assembled monolayers [START_REF] Krämer | Scanning Probe Lithography Using Self-Assembled Monolayers[END_REF] or a combination of such approaches. [START_REF] Fustin | Parameters Influencing the Templated Growth of Colloidal Crystals on Chemically Patterned Surfaces[END_REF][START_REF] Ma | Electrostatic Funneling for Precise Nanoparticle Placement: A Route to Wafer-Scale Integration[END_REF] Nevertheless, the one-step self-assembly of NPs on solid substrates directly from colloidal solution is highly desirable. To date, several strategies have been developed: drop-casting of NP suspension on solid substrates [START_REF] Bigioni | Kinetically driven self assembly of highly ordered nanoparticle monolayers[END_REF][START_REF] Adachi | Stripe Patterns Formed on a Glass Surface during Droplet Evaporation[END_REF] or at the surface of an immiscible polar organic subphase, [START_REF] Dong | Two-Dimensional Binary and Ternary Nanocrystal Superlattices: The Case of Monolayers and Bilayers[END_REF][START_REF] Dong | Multiscale Periodic Assembly of Striped Nanocrystal Superlattice Films on a Liquid Surface[END_REF] langmuir-blodgett method, [START_REF] Aleksandrovic | Preparation and Electrical Properties of Cobalt-Platinum Nanoparticle Monolayers Deposited by the Langmuir-Blodgett Technique[END_REF][START_REF] Huang | Spontaneous formation of nanoparticle stripe patterns through dewetting[END_REF] convective-self assembly [START_REF] Bodnarchuk | Energetic and Entropic Contributions to Self-Assembly of Bi-nary Nanocrystal Superlattices: Temperature as the Structure-Directing Factor[END_REF][START_REF] Farcau | Tunable Conductive Nanoparticle Wire Arrays Fabricated by Convective Self-Assembly on Nonpatterned Substrates[END_REF] or spin coating. [START_REF] Johnston-Peck | Formation and Grain Analysis of Spin-Cast Magnetic Nanoparticle Monolayers[END_REF] In spite of extensive efforts, a direct deposition of NPs on solid surfaces with controlled arrangements -from stripes pattern to thin film entrainment -with tunable spacing, thickness and organization remains a challenge, especially when metallic magnetic nano-objects are involved. Indeed, to prevent NP from oxidation, the deposition based on polar organic subphase should be avoided while the atmosphere should be strictly controlled. Glovebox could enable such a control, however, space limitation and handling difficulty arisen. To our knowledge, no report on a controlled deposit method compatible with the metallic character of magnetic NPs assembly has been reported so far. MNPs can be synthesized following various method, [START_REF] Lu | Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application[END_REF] one of which being the organometallic approach, which consists in the decomposition of a metal complex thanks to a reducing agent under mild conditions in presence of organic surfactants. [START_REF] Chaudret | Organometallic approach to nanoparticles synthesis and selforganization[END_REF] The final MNP is surrounded by surfactants which prevent their aggregation and ensure their stabilization in organic solvents with fairly low polarity. [START_REF] Yin | Colloidal nanocrystal synthesis and the organic-inorganic interface[END_REF] An excess amount of surfactants is usually needed to insure the stabilization of the colloidal solution, thus, the uncoordinated surfactants can interact with the substrate and modify the morphology of the deposits. [START_REF] Kwon | The effect of excess surfactants on the adsorption of iron oxide nanoparticles during a dip-coating process[END_REF] The surfactants also affect the suspension properties such as evaporation rate, surface tension and colloidal/selfassembly behavior. The exact composition of the solution (amount and nature of surfactants) varying slightly with the synthesis, the detailed study of the influence of deposit parameter must be performed on the very same MNP batch.

Here we present the self-assembly of chemically synthesized metallic CoFe MNPs carried out with a home-made dip coating setup placed directly inside a coupled glovebox-sputtering system. Such a configuration preserves the good properties of the MNPs during the deposition while permitting the modulation of the interfacial energy of the substrates by oxygen plasma treatment. CoFe MNPs were synthesized by an organometallic approach in presence of amine and acid as surfactants. High yield and reproducibility were obtained. Such features permit to perform several experiments with the same batch in order to figure out the impact of the different key parameters. MNP concentration, withdrawal speed, nature and amount of surfactants in suspension and substrate surface state have a strong influence on the self-assembly process and allow controlling the nature of the deposit from stripes to thin film entrainment.

Experimental Section

Materials and Methods

Chemicals and materials. Chemicals and materials. All syntheses were prepared and purified under argon or dihydrogen using Fischer-Porter bottles techniques, a glove box and argon/vacuum lines. Mesithylene (99 %), tetrahydrofurane (THF, 99 %), were purchased from VWR Prolabo and distilled and degassed through three freeze-pump-thaw cycles. Hexadecylamine (HDA, 99 %), oleic acid (OA, 99 %) and stearic acid (SA, 99 %), were purchased from Sigma-Aldrich, iron(0)pentacarbonyl (Fe(CO) 5 , 99,5%) from Acros organics and cyclooctadienecyclooctenyl (Co(η 3 -C 8 H 13 )(η 4 -C 8 H 12 ), 99 %) from Nanomeps and used as received.

Fabrication and preparation of the substrates. Standard photolithography was used to make gold electrodes (5 nm Ti/30 nm Au) on 0.25 cm 2 squares silicon wafers covered by a 300 nm layer of thermally grown silica (SiO 2 ). Resina layer used to protect the substrates was removed with acetone, ethanol and deionized water and dried under nitrogen flow. UV ozone treatment of 15 min render them hydrophilic prior to their introduction in the glove box. The surface wettability was characterized by contact angle measurements performed at room temperature with a GBX Digidrop. Each contact angle represents an average value of three measurements on different sample areas. Oxygen plasma could be used to modulate the surface energy of substrates stocked at least one day in advance in the glovebox, using a MP450s Plassys system coupled to the glove box. The working pressure was maintained at 8 mTorr, and a flow rate of 4 and 20 sccm respectively for argon and oxygen has been used with under 10W RF power.

Synthesis and characterization of CoFe MNPs.

Core/shell carbide CoFe MNPs were prepared according to a modified published procedure. [START_REF] Desvaux | FeCo nanoparticles from an organometallic approach: synthesis, organisation and physical properties[END_REF] In brief, Co(η 3 -C 8 H 13 )(η 4 -C 8 H 12 ) (1 mmole, 278 mg) was mixed with amine (HDA, 1 mmole, 242 mg) and acids (SA, 1 mmole, and OA, 1 mmole) in 10mL of Mesitylene. The reaction was stirred magnetically for 10 min. Fe(CO) 5 (2 mmole, 280µL) was then injected and 40 mL of Mesitylene added. The reaction is pressurized under 3 bars of dihydrogen and let to react at 150 °C for 48h. To remove excess surfactants, three washing process and magnetic separation using mesitylene were used, the NP solution was then evaporated under vacuum to get a dried powder.

A droplet of a colloidal suspension of CoFe MNPs was deposited inside a glovebox on covered holey copper grids. The latters were then characterized by TEM (transmission electron microscopy), HRTEM (high-resolution TEM), STEM-HAADF (scanning transmission electron microscopy using a high angle annular dark field detector) and EELS (electron energy loss spectroscopy). XRD (X-ray diffraction) and Mössbauer spectroscopy were carried out on a powder of CoFe MNPs that were prepared and sealed under an argon atmosphere. TEM image (see Figure S1) of the MNPs was recorded with a JEOL-JEM 1011F, operating at 100 kV. The polydispersity in size was calculated from the size measurements of more than 2000 NPs using ImageJ software. 37 STEM-HAADF observations (see Figure S2 (ab) and EELS analysis (c)) were carried out at the LMA laboratory in Zaragoza, using a Cs corrected Titan Microscope equipped with an XFEG source. XRD measurements (see Figure S2 (d)) were performed on a PANalytical Empyrean diffractometer using Co-Kα radiation at 45 KV and 40 mA. The iron state and its environment (see Figure S3) were analyzed by Mössbauer spectroscopy (WIS-SEL, 57Co source).

Deposit of CoFe MNPs. Every steps were performed in the coupled glovebox-sputtering system under inert gas (Ar) to prevent NPs from oxidation. The MNPs were redispersed in Tetrahydrofurane (THF), solvent suitable for dip-coating deposit thanks to its low boiling point (66°C 38 ) [START_REF] Jang | Highly Crystalline Soluble Acene Crystal Arrays for Organic Transistors: Mechanism of Crystal Growth During Dip-Coating[END_REF] and surface tension (28.10 -3 N.m 38 ). [START_REF] Grosso | How to exploit the full potential of the dip-coating process to better control film formation[END_REF] Colloidal solution with desired concentration were obtained from MNP powder diluted in the appropriate volume of solvent under ultrasonic bath (15 min, 40°C). Dip-coating experiments were performed using the home-made setup shown in Figure 1. The substrate is maintained with a small amount of vacuum grease on a cover slip. Such configuration enables an easy handling and prevents any deposit on the substrate backside, altering magnetic measurements performed on the final device. The cover slip is maintained vertically using a self grip tweezer. The first step consists in completely immersing the substrate in a 1 mL teflon beaker filled beforehand with the suspension of MNPs. The substrate is then withdrawn vertically at a constant speed by action of a stepper motor at a speed ranging from 13 µm.s -1 to 660 µm.s -1 .

Characterization of assemblies. Deposit patterns formed by the MNPs were imaged by optical microscope (Olympus BXFM) equipped with a CCD camera (Olympus DP20) integrated in the glove-box. Field emission gun scanning electron microscopy (SEM-FEG Hitachi S-4800) was performed at 30 KV. Topographical imaging of films was performed in tapping mode at room temperature with a Digital Instruments/Veeco Dimension 3100 Atomic Force Microscope using Si tips (frequency range: 240-380 kHz, Force: 42 N.m -1 ). AFM image processing and rendering was analyzed with WsXM data analysis software. [START_REF] Horcas | A software for scanning probe microscopy and a tool for nanotechnology[END_REF] The magnetic properties of assemblies were investigated by using a superconducting quantum interference device (SQUID) magnetometer MPMS XL SQUID (7 T). Magnetization versus applied magnetic field curves were recorded at 2K without applying any magnetic field referred as ZFC (Zero field cooling). Then, the field cooling curves (FC) were recorded after the temperature was set down 2 K under 5 T. The substrates containing the assemblies of MNPs were placed in a parallel direction to the one of the applied magnetic field.

Results and discussions

Results

Core/shell carbide CoFe MNPs used hereafter exhibit a polydisperse size distribution with a diameter centered on 25.4 ± 9.7 nm and 14 ± 4 nm separated by organic ligands with a thickness distribution centered on 2.3 ± 1 nm (see Figure S1). STEM-HAADF images highlighted the polycrystalline morphology of the CoFe MNPs (see Figure S2 (a-b)) while STEM-EELS analysis evidenced a core-shell structure (respectively Co/Fe). It will be noted that the shell (see orange color in Figure S2 (c)) is very thin compared to the core. XRD mea-surements, used to characterize the phases present in a powder of CoFe MNPs, revealed that the crystallization of the core is a ε cobalt phase while the signature of Fe could be masked by the small size of crystallites and the strong signal of cobalt. Such unexpected phase suggests the presence of carbon formed during the synthesis, and its diffusion inside the MNPs structure confirmed by Mossbauer spectroscopy showing a carbide phase following a stoichiometry of Fe 1-x Co x C y (see figure S3). A powder sample exhibited a weak ferromagnetism character at 200 K as determined by SQUID magnetometry (M S = 140 A.m 2 .kg -1 , and H C = 6 mT). The fact that the mean magnetization at saturation is below the expected value (M S bulk = 174 A.m 2 .kg -1 ) could be related to the presence of carbon inside the MNPs structure and/or carbon monoxide on the surfaces of the latters. The absence of a shift of the coercive field at 2 K after ZFC and FC (see Figure S4) confirms that the MNPs are not oxidized after synthesis.

MNPs concentration

We investigated the effect of the concentration of the CoFe MNP suspension on the deposit pattern. The concentration was varied from [5 mmol.L -1 ] to [50 mmol.L -1 ]. The minimum value was limited by the accuracy of the microbalance giving an error around 15 %, while the highest concentration was the threshold above which the MNPs were not effectively stabilized in the solvent. Except for the highest concentration, colloidal solution were stable for several hours, much longer than dip coating experiments (≈ 6 min per deposit).

Figure 2 (a) depicts optical images of four deposits obtained respectively for 5, 10, 20 and 50 mmol.L -1 at a fixed withdrawal speed (13 µm.s -1 ). Corresponding scanning electron microscopy (SEM) images taken in random areas are shown in Figure S5. Formation of stripe patterned layer, perpendicular to the withdrawal direction (horizontal on the images) was observed for each sample. The width and spacing of the stripes could be monotonically tuned by the concentration of the CoFe MNP suspensions as evidenced in Figure 2 (b). We observed an increase of the averaged width as well as the stripes spacing when the MNP concentration increases, similarly to what can be found in the literature. [START_REF] Watanabe | Mechanism for stripe pattern formation on hydrophilic surfaces by using convective self-assembly[END_REF] Thickness of the deposit, quantified in number of layers, was evaluated from AFM images both on stripes or in between, depicted respectively in black or red in Figure S6.

For the lowest MNP concentration (see the Figure S6 (a)), thickness of a quarter of a layer was detected, corresponding to the substrate roughness between stripes, thus, no MNPs deposit. This roughness must be removed from further quantitative analysis. To further quantify the results obtained, two parameters were introduced i) the mean number of layers NL (Figure 2 c)) which can be quantified by:

NL = N L × Ptot(N L ) P tot (N L ) dN L (1)
and ii) the MNP coverage Γ (Figure 2 d)) corresponding to:

Γ = P MNP (N L ) P tot (N L ) dN L × 100% (2)
where N L , P MNP (N L ) and P tot (N L ) are respectively the distribution of the number of layers, the probability corresponding to the distribution of the number of layers without the peak corresponding to the substrate roughness (i.e the absence of MNPs) and the probability of the entire distribution. The mean number of layers increases with the MNP concentration both within or in between stripes, respectively black and red symbols. A plateau seems to appear for stripes of 3.5 layers above [20 mmol.L -1 ]. In agreement, the MNP coverage evolves toward a dense deposit (100% coverage) at high concentration. Above [50 mmol.L -1 ], dense layer could be deposited but stripes were composed of multilayers (3 to 4). The almost linear increase of the mean number of layers (for the three lowest concentration) as well as a high and almost constant MNP coverage are in perfect agreement with the literature. [START_REF] Fustin | Parameters Influencing the Templated Growth of Colloidal Crystals on Chemically Patterned Surfaces[END_REF][START_REF] Kim | Rapid Fabrication of Two-and Three-Dimensional Colloidal Crystal Films via Confined Convective Assembly[END_REF] The deviation observed at the highest concentration, could be understood by the unstable character of the colloidal suspension, the actual concentration of suspended MNPs being lower than the calculated one. Another hypothesis would be that, since the static contact angle formed by a droplet of THF on our substrates is very low (≈ 7 °), the thickness of the stripes is limited at high concentration by the dynamic contact angle. This could clear up why the thickness saturates while the width of the stripes still growths. In the seeking of an homogeneous monolayer deposition, consisting of a mean thickness of one, both on and between stripes with a high MNP coverage, the best compromise was obtained for a [10 mmol.L -1 ] concentration. This concentration was kept constant for the rest of the study.

Withdrawal speed

We have studied the impact of withdrawal speed on the deposit properties. Experiments have been performed for [10 mmol.L -1 ] concentration at respectively 0, corresponding to the natural evaporation of the MNP solution,13, 15, 19, 29 and 180 µm.s -1 . Except for the highest withdrawal speed, which exhibited clusters of a few isolated MNPs induced by the strong stretching of the meniscus, [START_REF] Ghosh | Spontaneous Pattern Formation by Dip Coating of Colloidal Suspensions on Homogeneous Surfaces[END_REF] stripe patterned layer perpendicular to the withdrawal direction were detected from optical and SEM images (Figure 3 (a)), similarly to the MNP concentration study.

Figure 3 (b) summarizes the evolution of the width and the spacing of the stripes (respectively black squares and red triangles). The stripe width decreased exponentially with withdrawal speed, as previously reported. [START_REF] Ghosh | Spontaneous Pattern Formation by Dip Coating of Colloidal Suspensions on Homogeneous Surfaces[END_REF] Concerning the spacing of the stripes, previous works have shown two behaviors. On one hand, an almost constant spacing with the withdrawal speed (measured for a speed below ≈ 3 µm.s -1 ) [START_REF] Watanabe | Mechanism for stripe pattern formation on hydrophilic surfaces by using convective self-assembly[END_REF][START_REF] Ghosh | Spontaneous Pattern Formation by Dip Coating of Colloidal Suspensions on Homogeneous Surfaces[END_REF] have been recently reported. On the other hand, a non-monotonous behaviour against the withdrawal speed was exhibited on a broader range (below ≈ 150 µm.s -1 ). [START_REF] Yabu | Preparation of Self-Organized Mesoscale Polymer Patterns on a Solid Substrate: Continuous Pattern Formation from a Receding Meniscus[END_REF] The fast withdrawal induced a shorter accumulation time of MNPs at the contact line, resulting in thinner stripes. As expected, mean number of layers de-creased also drastically both in and between stripes (Figure 3 (c)) as well as the density of the deposit, evaluated by the MNP coverage parameter (Figure 4 and Supplementary Figure S7). Stripe width and thickness and MNP coverage evolved similarly towards a diminution for higher withdrawal speed. However, transition could be observed from an "evaporation" regime at low speed, with large and thick stripes, to an "intermediate regime", with a weak dependence of the stripe width with withdrawal speed associated to a low thickness. [START_REF] Grosso | How to exploit the full potential of the dip-coating process to better control film formation[END_REF][START_REF] Le Berre | From Convective Assembly to Landau-Levich Deposition of Multilayered Phospholipid Films of Controlled Thickness[END_REF][START_REF] Jing | Drying of Colloidal Suspensions and Polymer Solutions near the Contact Line: Deposit Thickness at Low Capillary Number[END_REF] Withdrawal speed (µm.s -1 ) MNPs coverage (%) Figure 4 shows the evolution of the MNP coverage on and between stripes (respectively black and red curves) deduced from AFM measurements against the withdrawal speed (see Figure S7) using Equ.(2) and the methodology described in the previous part. The MNP coverage presents both on and between stripes a monotonous decrease with increasing withdrawal speed. 13 µm.s -1 speed appeared as the best compromise to control the deposit thickness, close to a monolayer in and between stripes, while providing the best MNP coverage (65 %). This withdrawal speed will be kept constant for the rest of the study.

Surfactants concentration

The impact of surfactants was systematically studied through the controlled addition of amine (hexadecylamine -HDA) or acid (oleic acid -OA). The chemical affinities of surfactants with silica substrate or gold electrodes was investigated by immersing substrates separately into [10 mmol.L -1 ] solution of OA and HDA in THF (Figure S8). While a reference sample in THF reveals an homogeneous contrast (Figure S8 (c)), the presence of OA (liquid at room temperature) leaded to multiple droplets randomly distributed on both silica and gold (Figure S8 (a)) while HDA (solid) was solely observed on and at the vicinity of gold electrodes (see Figure S8 (b)). While both amine and carboxylic groups of acid can form self-assembled monolayers on gold surfaces in low-polar solvent, [START_REF] Paik | Adsorption of Carboxylic Acids on Gold by Anodic Reaction[END_REF][START_REF] Xu | Molecular interactions between organized, surface-confined monolayers and vapor-phase probe molecules. 6. Insitu FT-IR external reflectance spectroscopy of monolayer adsorption and reaction chemistry[END_REF] the affinity of surfactants with silica strongly depends of the substrate pending group. Silanol (Si-OH) could form hydrogen bonds with both surfactants, while only acid could interact with siloxane bridges (Si-O-Si), thus amine exhibit a lower reactivity towards silica compared to acid.

We then studied the influence of the ligand concentration on the MNP deposit at optimized speed (13 µms -1 ) and concentration [10 mmol.L -1 ]. Controlled concentration of surfactants (varying from 0 to [10 mmol.L -1 ], referred to as 1 equivalent per mole of metal, eq.) were added to the colloidal solution and sonicated for 15 min. Optical images revealed a clear evolution of the color of the deposits, while SEM images addressed the deposit morphology (Figure 5). In presence of OA, MNP coverage decreased, 3D isolated islands and regions evidenced by a black contrast in SEM images, probably filled with organic molecules, arisen both on Au and SiO 2 surface (Figure S8 (a)). While the same behaviour was observed in presence of HDA on Au electrodes, the MNP coverage on SiO 2 increases with HDA (Figure 5 and Figure S9). Ligand physisorption, enhanced at the three-phase contact line, Acid oleic (OA) concentration Hexadecylamine (HDA) concentration 500 µm 500 µm 500 µm 20 µm 15 µm 10 µm 0 eq 0.1 eq 1 eq 0 eq 0.1 eq 1 eq 0.05 eq 0.1 eq 1 eq 0.05 eq 0.1 eq 1 eq 20 µm 10 µm 10 µm 500 µm 500 µm 500 µm ). The concentration of the surfactants has been varied by adding the latters in a powder form directly inside the colloidal suspensions of CoFe MNPs beforehand prepared at optimized concentrations ([X ] = [10 mmol.L -1 ]).

Excess amount of (a) 0, 0.1 and 1 equivalent of OA and (b) 0.05, 0.1 and 1 equivalent of HDA has been added. All SEM images are located on silica and endowed, in inset, with magnified view.

could form a shear barrier to prevent MNPs interaction with the surface, decreasing the MNP coverage. [START_REF] Kwon | The effect of excess surfactants on the adsorption of iron oxide nanoparticles during a dip-coating process[END_REF][START_REF] Yoon | Single and Multiple-Step Dip-Coating of Colloidal Maghemite (γ -Fe 2 O 3 ) Nanoparticles onto Si, Si 3 N 4 , and SiO 2 Substrates[END_REF][START_REF] Wang | Deposition and characterization of large-scale FePt nanoparticle monolayers on SiO2/Si surface[END_REF] In the case of low affinity between the surfactants and the surface, no steric hindrance were encountered, as evidenced for HDA on SiO 2 . Surfactants could enhance the MNPs-MNPs interactions as well as the depletion force, increasing the MNP coverage as previously reported. [START_REF] Bigioni | Kinetically driven self assembly of highly ordered nanoparticle monolayers[END_REF][START_REF] Lau | Enhanced Ordering in Gold Nanoparticles Self-Assembly through Excess Free Ligands[END_REF] Surface energy of the substrates

We studied so far the deposition on silica and gold surfaces beforehand cleaned by ultraviolet (UV)/ozone and then stored inside the glove box. However, the surface state evolved as evidenced by the drastic increase of the contact angle of deionized water from 10 • and reach 60 • after 6 hours (Figure S10). Such evolution is probably due to the physisorption of organic molecules or contaminants. To address the effect of surface energy and solvent wettability issue we performed oxygen plasma treatment of different time. Dip coating of treated substrate were performed under optimized conditions (v = 13 µm.s -1 and [X] = [10 mmol.L -1 ]).

For long plasma treatment stripes were less visible, suggesting that the deposition mechanism is close to a thin film entrainment (Figure 6). SEM images confirmed that MNP coverage significantly increased compared to a deposit on bare substrate (Figure S5 (b)) for comparison). Such enhancement of the MNP coverage could be explained by i) the removing of physisorbed molecules which, as previously discussed, decrease the interaction energy between the MNPs and the substrate, [START_REF] Kwon | The effect of excess surfactants on the adsorption of iron oxide nanoparticles during a dip-coating process[END_REF] and/or ii) a lower contact angle, which permits a higher evaporation rate and a homogeneous thickness of the deposit. [START_REF] Monteux | Packing and Sorting Colloids at the Contact Line of a Drying Drop[END_REF] Concerning this last point, the average spacing between two successive stripes as well as the width of the stripes increases nonlinearly with the oxygen plasma treatment duration (Figure 7). Such experiments demonstrates indirectly the decrease of the contact angle acting on the triple line. Indeed, a smaller contact angle (for a fixed withdrawal speed) is known to avoid the MNPs to diffuse toward to the triple contact line and entails that the next stripe starts its formation farther (i.e the average spacing increases). [START_REF] Ray | Dynamic Self-Assembly of Polymer Colloids To Form Linear Patterns[END_REF] Moreover, it has been shown that a low contact angle provide a convective self-assembly with wider stripes, while increasing contact angle progressively forms incompletely layers with defects until prevent MNPs from being deposited on the substrates when the contact angle reaches around 20 °. 55 To conclude on that point, the increase of the average spacing between two consecutive stripes combined to an increase of the widths constitute a fingerprint of the decrease of the contact angle when the oxygen plasma treatment duration is longer.

The intrinsic magnetic character of the CoFe MNPs used in this work could also be an important factor guiding the self-assembly through magnetostatic dipole-dipole interactions (U). In particular, it has been shown that, for U > 8 k B T , characteristic and noticeable self-organizations of MNPs are observed (rings, chains, ... [START_REF] Bishop | Nanoscale Forces and Their Uses in Self-Assembly[END_REF] ). In our case, one can estimate the dipolar energy between two MNPs i and j using U ≈ (µ i µ j )/σ 3 i, j where µ i = M s V i is the magnetization carried by a MNP i of volume V i and σ i, j the center-to-center distance between the MNPs i and j. Due to the bidisperse character of the MNPs (displayed on their size distribution in Figure S1), we obtain U ≈ 124 and 316 k B T for the small and the larges MNPs respectively. Despite these large values of (U >> k B T ), we never clearly observed the characteristic structures induced by a self-assembly governed by dipolar interactions. [START_REF] Bishop | Nanoscale Forces and Their Uses in Self-Assembly[END_REF] In fact, many theoretical works demonstrated that the probability of forming MNPs chains are strongly reduced in bimodal systems compared to monodisperse ones. [START_REF] Minina | Microstructure of bidisperse ferrofluids in a thin layer[END_REF][START_REF] Kruse | Monte Carlo simulations of polydisperse ferrofluids: Cluster formation and field-dependent microstructure[END_REF][START_REF] Wang | Structure and magnetic properties of polydisperse ferrofluids: A molecular dynamics study[END_REF] This can be qualitatively explained by the presence of the small MNPs which i/ lowered the effective dipolar interactions seen by the larges MNPs, and ii/ increase the breakup probability of long chains through their Brownian motion; both phenomena leading to only 2-3 MNPs aggregation.

Finally, magnetic measurements have been performed on a deposit of CoFe MNPs using the optimized conditions described above (speed 13 µm.s -1 , concentration 10 mmol.L -1 on a silicon substrate with native thin oxide cleaned by plasma oxygen during 20 min). To preserve the magnetic properties of the MNPs, but also their geometrical arrangement after dip-coating, a thin resin layer (≈ 40 nm thick) has been deposited followed by four annealing steps (1 min at 90°, 110°, 170°, 250°), still inside the glovebox (see Figure 8). The magnetic measurements have been performed with the magnetic field applied in the plane of the substrate. Interestingly, the hysteresis loops measured at 2 K after field cooling under 5 T did not exhibit any exchange bias features, meaning that no oxidation occurred during the whole process, i.e the magnetic properties of the MNPs are preserved. It will be noted that this deposition method was successfully extended to MNPs displaying monodisperse size distribution for different materials, ligands and to various type of substrates. These latter works will be reported elsewhere.

Outline

In conclusion, we have systematically investigated the influence of MNP concentration, withdrawal speed, nature/concentration of the surfactants and the surface state of the substrates on the deposit obtained by dip-coating technique. We successfully performed deposition of metallic magnetic MNPs on hydrophilic surfaces under inert atmosphere thanks to an home-made set-up designed in a couple glovebox-sputtering system, to prevent any oxidation of the deposit. The film thickness can be controlled by either the MNP concentration or the withdrawal speed. Deposition of single monolayers could be reached for a concentration of [X] = [10 mmol.L -1 ] and a withdrawal speed v = 13 µm.s -1 . On contrary, very thick periodic stripes can be formed either at high MNP concentration or without any withdrawal force, corresponding to the natural evaporation of the MNP suspension. By adding surfactants containing amino and acid groups in two distinct freshly synthesized suspensions, we have demonstrated how drastically the nature and the amount of such surfactants modifies the morphology of the deposit. For surfactants strongly interacting with the surface, lower MNP coverage were obtained, physisorbed surfactants acting as shearing barrier towards MNP adhesion. When surfactants do not exhibit preferential interaction, for instance for HDA on SiO 2 , improved coverage were obtained, probably due to the enhancement of MNPs-MNPs interactions and depletion force in suspension. A 20 min plasma treatment of the samples prior to dipcoating yield the highest MNP coverage. Interestingly, those preliminary experiments permits to achieve dense monolayer of monodisperse MNPs (Fe, Co, CoFe) on silica surfaces functionalized by APTES molecules, gold surfaces as well as thin resina layer while preserving the magnetic properties of the MNPs which will be published elsewhere.

Figure 1 :

 1 Figure 1: (a) Sketch of the home-made dip coating setup used in this work (b). Schematic drawing of the situation at the beginning of the dip-coating process. (c) Schematic representation of MNPs accumulation and the flow pattern when the substrate is removed from the suspension.

Figure 2 :

 2 Figure 2: (a) Optical images of the stripe patterns formed perpendicularly to the withdrawal direction on the silicon oxide and gold electrodes. All deposits were performed at constant withdrawal speed (13 µm.s -1 ) for different MNP concentration: 5, 10, 20 and [50 mmol.L -1 ]. (b) Summary of the averaged width of the stripes (red triangles) and the average spacing between the consecutive stripes (black circles). The error bars correspond to statistical dispersion of the measurements performed on the entire surface of the substrates. (c)-(d) Summary of respectively the mean number of layers and the MNP coverage (deduced from AFM measurements) as a function of the concentration.

Figure 4 :

 4 Figure4: Evolution against the withdrawal speed of the MNP coverage deduced from AFM measurements (see FigureS7). Black (red) curves represent MNP coverage on (between) stripes.

Figure 3 :

 3 Figure 3: (a) Representative images of the stripes obtained by the dip coating method at different withdrawal speeds (vertically downwards, at zero withdrawal speed (corresponding to the natural evaporation of the MNP suspension), 13 µm.s -1 and 180 µm.s -1 ). MNP concentration was ([10 mmol.L -1 ]). The left column corresponds to optical images. The right column shows SEM images of a zone located on the silica layer between two consecutive stripes. (b) Evolution of the spacing (black squares) and the width (red triangles) of consecutive stripes with the withdrawal speed. Error bars correspond to the statistical dispersion of measurements performed on all of the substrate (5 mm). (c) Evolution of the mean number of layers of the stripes against the withdrawal speed. Black (red) curves represent the mean MNP coverage on (between) stripes.

Figure 5 :

 5 Figure 5: Optical and SEM images of the deposits obtained by the dip coating method at the optimized speed (v = 13 µm.s -1 ). The concentration of the surfactants has been varied by adding the latters in a powder form directly inside the colloidal suspensions of CoFe MNPs beforehand prepared at optimized concentrations ([X ] = [10 mmol.L -1 ]).

Figure 7 :

 7 Figure7: Evolution of the spacing (black circles) and the width (red triangles) of consecutive stripes against the oxygen plasma treatment for different exposition durations. The first point (t = 0) is the reference for this study, where a substrate of the same batch was stored in the glovebox at least for one day without oxygen plasma treatment. Error bars correspond to the statistical dispersion of measurements performed on all the substrate (5 mm).

Figure 6 :

 6 Figure 6: Optical images of samples deposited after different oxygen plasma treatment durations : 2 min, 5 min and 20 min respectively from the left to the right. Other conditions are v = 13 µm.s -1 and [X] = [ 10mmol.L -1 ]). On the bottom, high resolution SEM images located on silica for the longest plasma treatment are shown.

Figure 8 :

 8 Figure8: At the top, a cross section TEM picture of the sample used for the magnetic measurements. On the bottom: in-plane hysteresis loops of the sample measured at 2K after zero-field cooling (ZFC) (black squares) and after field-cooling (FC) under 5T (red circles).

Acknowledgement

This work was partly supported by the CNRS LAAS member of french RENATECH network. The authors gratefully acknowledge C.Nayral and F.Delpech for fruitful discussions.

Supporting Information Available

 8) Optical images taken after the substrates were immersed in suspensions of OA and HDA dissolved in THF as well as pure THF. ( 9) A high resolution SEM image located between two consecutive stripes when a large excess of HDA (1 eq) were added to the suspension. (10) Evolution against the time of the contact angle of cleaned substrates and then stored in glovebox.

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