domain: chim phys

text:
Busheng Wang

Isabelle Braems
email: isabelle.braems@cnrs-imn.fr

Shunsuke Sasaki

Laurent Cario

Stéphane Jobic

Prediction of a New Layered Polymorph of FeS 2 with Fe 3+ S 2-(S 2 ) 1/2 2-Structure

Keywords: Two-dimensional material, DFT, evolutionary algorithm, iron sulfides, polymorph

The never-elucidated crystal structure of metastable iron disulfide FeS 2 resulting from the full deintercalation of Li in Li 2 FeS 2 has been cracked thanks to crystal structure prediction searches based on an evolutionary algorithm combined with first-principles calculations accounting for experimental observations. Besides the newly layered C2/m polymorph of iron disulfide, two-dimensional dynamically stable FeS 2 phases are proposed containing sulfides and/or persulfide S 2 motifs.

Lamellar transition-metal disulfides MS 2 (M = transition element) have been intensively investigated as host lattices for chemical and electrochemical intercalation of alkali metals (e.g. Li, Na and K). [START_REF] Zak | Alkali Metal Intercalated Fullerene-like MS 2 (M= W, Mo) Nanoparticles and Their Properties[END_REF][START_REF] Hu | MoS 2 Nanoflowers with Expanded Interlayers as High-Performance Anodes for Sodium-Ion Batteries[END_REF][START_REF] Stephenson | Lithium Ion Battery Applications of Molybdenum Disulfide (MoS 2 ) Nanocomposites[END_REF][START_REF] Ren | MoS 2 as a Long-Life Host Material for Potassium Ion Intercalation[END_REF] Indeed, these materials provided key information in the understanding of redox mechanisms in solids, and served for a while as models for secondary batteries. Simultaneously, the deintercalation process, which falls into the category of soft chemistry routes, [START_REF] Zhou | Metastable Layered Cobalt Chalcogenides from Topochemical Deintercalation[END_REF][START_REF] Wan | Tuning Two-Dimensional Nanomaterials by Intercalation: Materials, Properties and Applications[END_REF] was used to stabilize metastable MS 2 edifices by topotactical deinsertion of the A cation from an AMS 2 pristine compound. The resulting metastable structure retains the original host features with empty A sites. For instance, metastable CdI 2 -type VS 2 and cubic TiS 2 could be obtained from LiVS 2 and CuTi 2 S 4 in presence of iodine and bromine as oxidizing agent, respectively. [START_REF] Murphy | Preparation and Properties of LixVS 2 (0 < x < 1)[END_REF][START_REF] Sinha | Lithium Intercalation in Cubic TiS 2[END_REF] Whatever the intercalation/deintercalation regimes, only the cationic redox center was at work in the reductive/oxidative reactions, yet one can wonder what happens to the host structure when deintercalation involves not only a cationic but also an anionic species. Such a situation was first observed for the Li 2 FeS 2 ternary sulfide, which was evidenced to allow a full deintercalation of Li, [START_REF] Rouxel | Les Composites Intercalaires Na x TiS 2 . Etude Gènérale Des Phases Na x TiS 2 et K x TiS 2[END_REF][START_REF] Leblanc-Soreau | Les Intercalaires A x TiS 2 et A x ZrS 2 . Structure et Liaisons[END_REF][START_REF] Cousseau | Behavior of Zirconium Disulfide in Presence of Solutions of Alkali-Metals in Liquid-Ammonia-Intercalates Na x ZrS 2 and K x ZrS 2 and K x ZrS 2 and Phase (NH 3 )ZrS 2[END_REF][START_REF] Arnaud | Etude Structurale Des Composés M x TiSe 2 (M= Fe, Co, Ni)[END_REF] foreshadowing to some extent the forthcoming investigations on anionic redox chemistry in oxides for application in high-energy Li-ion batteries. [START_REF] Tarascon | In Situ Structural and Electrochemical Study of Ni 1-x Co x O 2 Metastable Oxides Prepared by Soft Chemistry[END_REF][START_REF] Sathiya | Reversible Anionic Redox Chemistry in High-Capacity Layered-Oxide Electrodes[END_REF][START_REF] Assat | Practical Assessment of Anionic Redox in Li-Rich Layered Oxide Cathodes: A Mixed Blessing for High Energy Li-Ion Batteries[END_REF][START_REF] Mccalla | Visualization of OO Peroxo-like Dimers in High-Capacity Layered Oxides for Li-Ion Batteries[END_REF][START_REF] Subban | Search for Li-Electrochemical Activity and Li-Ion Conductivity among Lithium Bismuth Oxides[END_REF][START_REF] Saha | Exploring the Bottlenecks of Anionic Redox in Li-Rich Layered Sulfides[END_REF][START_REF] Yahia | Unified Picture of Anionic Redox in Li/Na-Ion Batteries[END_REF] Nevertheless, the metastable iron disulfide phase obtained from totally delithiated Li 2 FeS 2 precursor has not been crystallographically characterized, and remains unknown.

Unraveling this long-sought crystal structure is needed. Therefore, in this study we investigated thermodynamically metastable two-dimensional (2D) novel iron disfulfides FeS 2 using ab initio crystal structure prediction (CSP) investigations.

The Li 2 FeS 2 precursor, prepared via a ceramic route from Li 2 S and FeS, is a layered material (see Scheme 1). [START_REF] Batchelor | Crystal Structure of Li 2 FeS 2[END_REF] Li 2 FeS 2 may be regarded as consisting of 2 /  [LiFeS 2 ] slabs with mixed Li + /Fe 2+ edge-sharing tetrahedral sites separated by monolayers of Li + cations in octahedral coordination. The overall charge balance can be written (Li + ) 2 (Fe 2+ )(S 2-) 2 . While the detailed mechanism of delithiation is still under study, [START_REF] Hansen | Cation and Anion Redox in Lithium-Rich Iron Sulfide Cathodes[END_REF] some features have been confirmed experimentally and are ascertained: the first deintercalation of half of the Li leads to the oxidation of Fe 2+ (d [START_REF] Wan | Tuning Two-Dimensional Nanomaterials by Intercalation: Materials, Properties and Applications[END_REF] HS configuration, HS indicates high spin) into Fe 3+ (d 5 HS configuration) as evidenced by infrared (IR) and Mössbauer spectroscopies. [START_REF] Brec | Redox Processes in the Li x FeS 2 /Li Electrochemical System Studied through Crystal, Mössbauer, and EXAFS Analyses[END_REF][START_REF] Blandeau | Transition-Metal Dichalcogenides from Disintercalation Processes. Crystal Structure Determination and Mossbauer Study of Li 2 FeS 2 and Its Disintercalates Li x FeS 2 (0.2 ⩽ X ⩽ 2)[END_REF] Beyond this composition, the complete departure of lithium results in the oxidation of the anions S ), as a study by IR spectroscopy of the delithiated phase [START_REF] Gard | Infrared Study of Lithium Intercalated Phases in the Li x FeS 2 System (0⩽ X⩽ 2). Characterization of a New Iron Disulfide[END_REF] showed the existence of an absorption band around 469 cm -1 assigned unambiguously to the stretching mode of a singlybonded (S-S) ) ratio of (2:1), so that the charge balance of this metastable FeS 2 material can be unambiguously be written as Fe 3+ S 2-(S 2 ) 2- 1/2 . Note that the existence of Fe 3+ HS ion in tetrahedral sites is rather rare but has already been encountered in Fe-S clusters used as biological condensers within Fe-S proteins. [START_REF] Broderick | Iron-Sulfur Clusters in Enzyme Catalysis[END_REF] Finally, due to the crystal arrangement of the precursor (stacking of LiFeS 2 and Li layers), a lamellar feature is expected for FeS 2 . While novel experiments comforted both the cation and anion redox chemistry encountered in layered Li 2-

x FeS 2 as well as the structural features, i.e. tetrahedral Fe 3+ cations and S 2 2-persulfides, [START_REF] Hansen | Cation and Anion Redox in Lithium-Rich Iron Sulfide Cathodes[END_REF] to date, the complete crystal structure of the investigated lamellar FeS 2 compound remains unknown, mainly due to a low crystallinity. Unravelling a crystal structure of a given composition is a classical crystal structure prediction (CSP) problem that can be solved in silico by locating the lowest energetic phase on the potential energy surface (PES). Here, our search differs in two ways of the classical CSP problem: (i) the aforementioned lamellar FeS 2 compound is a metastable phase, i.e. its structural arrangement is expected to lie in energy significantly above the thermodynamically stable pyrite and marcasite structures, [26][START_REF] Grønvold | Heat Capacities of Iron Disulfides Thermodynamics of Marcasite from 5 to 700 K, Pyrite from 300 to 780 K, and the Transformation of Marcasite to Pyrite[END_REF][START_REF] Sun | First-Principles Electronic Structure and Relative Stability of Pyrite and Marcasite: Implications for Photovoltaic Performance[END_REF][START_REF] Zhang | Relative Stability of FeS 2 Polymorphs with the Random Phase Approximation Approach[END_REF][START_REF] Spagnoli | Density Functional Theory Study of the Relative Stability of the Iron Disulfide Polymorphs Pyrite and Marcasite[END_REF] (ii) the structure must meet the available experimental data, i.e. Fe 3+ cations in tetrahedral coordination of sulfur atoms, and the coexistence of (S 2 ) 2-dimers and sulfide anions with a 1:2 S 2 2-:S 2-ratio.

Therefore, to find out the thermodynamically metastable FeS 2 phase(s), we employed a constrained fixed-composition evolutionary algorithm (EA) specifically developed in Poitiers (constrained EA) [START_REF] Huang | Pressure-Induced Polymerization of CO 2 in Lithium-Carbon Dioxide Phases[END_REF][START_REF] Huang | Computational Materials Discovery: Prediction of Carbon Dioxide and Nitrogen-Based Compounds under Pressure Using Density Functional Theory and Evolutionary Algorithm[END_REF] that only keeps generated structures satisfying the aforementioned criteria, and discards the others. Constrained EA is based on the evolutionary algorithm method implemented in the USPEX code [START_REF] Oganov | Crystal Structure Prediction Using Ab Initio Evolutionary Techniques: Principles and Applications[END_REF][START_REF] Oganov | How Evolutionary Crystal Structure Prediction Works and Why[END_REF][START_REF] Lyakhov | New Developments in Evolutionary Structure Prediction Algorithm USPEX[END_REF][START_REF] Zhou | Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions[END_REF] that has been successfully applied to a large number of different systems. All structure relaxations (shape, volume, and atomic positions) and energy calculations were performed with the VASP 5.4.4 code. [START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF] As the whole first-principles procedure involves several levels of theory to fully explore the PESs, optimize the selected low-energy FeS 2 candidates, and calculate their electronic and bonding properties, please refer to the Supporting Information for the methodology details. In the following, all structural parameters and enthalpies are given at the SCAN+U+rVV10 level of theory, [START_REF] Sun | Strongly Constrained and Appropriately Normed Semilocal Density Functional[END_REF][START_REF] Cococcioni | Linear Response Approach to the Calculation of the Effective Interaction Parameters in the LDA+ U Method[END_REF][START_REF] Sabatini | Nonlocal van Der Waals Density Functional Made Simple and Efficient[END_REF][START_REF] Peng | Versatile van Der Waals Density Functional Based on a Meta-Generalized Gradient Approximation[END_REF] while the energy gap is computed at the hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional [START_REF] Heyd | Hybrid Functionals Based on a Screened Coulomb Potential[END_REF] level of theory (see section S1 in the Supporting Information).

Relying on the lamellar structure of the Li 2 FeS 2 host, we first investigated twodimensional (2D) FeS 2 monolayers, then their tridimensional (3D) stacking. Using constrained EA on isolated 2D FeS 2 single layers, we located one admissible phase that crystallizes in the C2/m space group (SG n°12, Z=4, hereafter named C2/m-2D). Its structure type strongly differs from the MS 2 slabs commonly met in lamellar dichalcogenides where metals are six-fold coordinated by anions (octahedron or trigonal prism). [START_REF] Ataca | Single-Layer MX 2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-like Structure[END_REF] The C2/m-2D structure (Figure 1a) can be described as built upon one- ) 1/2 charge balance. This structure type is dynamically stable as its phonon dispersion curve does not present any imaginary frequencies in the entire Brillouin zone (see Figure 1b). We also evaluated its kinetic and thermal stabilities by running ab initio molecular dynamics simulations at temperatures in the range of 300~1000 K (see the Supporting Information). All of the atoms in the 2D layer vibrate only slightly around their equilibrium positions during annealing at 300 K for 10 ps, indicating the stability of the structure at room temperature. Despite the fluctuation of atoms, the whole structure is maintained at 600 and 900 K, while the system shows reconstruction at 1000 K. This indicates that the dynamically stable structure C2/m-2D is definitively viable up to ca.

dimensional 1 /  [Fe 2 S
900 K: no structural transformations are observed in the iron and sulfur network, indicating the existence of a substantial kinetic barrier which leads to a trapping of this metastable 2D phase, in line with its experimental availability.

Finally, this layer is predicted to be a semi-conductor (HSE06 gap of 0.90 eV, see Figure S3) with antiferromagnetically coupled edge-sharing (FeS 4 ) tetrahedra within 1/[Fe 2 S 4 ] ribbons along the a axis, these ribbons been connected via S 2 dumbbells along the b axis (see the studied magnetic configurations in Figure S2). As indicated on the electronic density of states curve (Figure 1 d-e), the Fe 3d-spin up and spin down blocks are well separated from each other by about 6 eV while the vacant antibonding σ* (S-S) lies much higher than these two blocks, in full agreement with the expected Fe 3+ high spin and S 2 2-electronic configuration. It is remarkable that this structure is more stable in energy than hypothetical 2D-FeS

2 f 1H 1T 1T b c 1T' c 1H-MoS 2 ,
1T-MoS 2 and 1T'-MoS 2 , respectively. [START_REF] Ataca | Single-Layer MX 2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-like Structure[END_REF][START_REF] Chou | Understanding Catalysis in a Multiphasic Two-Dimensional Transition Metal Dichalcogenide[END_REF] Both 1H-D 1T'-2D FeS 2 are local minima on the PES, while 1T-2D is dynamically unstable (see the Supporting Information). The enthalpy of C2/m-2D is shown to be 92 meV/f.u. lower than the 1T'-2D (with Fe in a distorted octahedron), and 633 meV/f.u. lower than the 1H-2D (with Fe in a trigonal prism) structures that are proved to be stable for other transition-metal disulfides. [START_REF] Ataca | Single-Layer MX 2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-like Structure[END_REF] This clearly asserts that Fe 4+ cations are instable in sulfide environment, and that Fe 3+ cations in tetrahedral sites are not oxidizing enough towards S 2-species to trigger the full transformation of regular S 2-anions into (S 2 ) 2-dimers as observed in pyrite or marcasite.

To some extent, C2/m-2D FeS 2 exhibits an intermediate charge balance between 2D (M 4+ (S 2-) 2 ) and 3D (M 2+ (S 2 2-)) transition element dichalcogenides. Nevertheless, let us notice that an unconstrained search among possible 2D layers of FeS 2 led us to unravel another dynamically metastable structure that would crystallize in the P-1 space group.

This structure is more stable than C2/m-2D, however it is built upon FeS 5 motifs (vide infra).

We applied our constrained EA algorithm to the search of the bulk layered structure based on the assembly of C2/m-2D monolayers. The resulting lowest crystal structure, denoted C2/m-3D hereafter, consists of the unchanged aforementioned monolayers, well

se f c b - f
). Its structure is displayed in Figure 2(a). We checked that C2/m-3D is a local minimum on the PES (all frequencies are positive over the Brillouin zone in the phonon dispersion curve in Figure 2b). Bulk C2/m-3D FeS 2 is an antiferromagnetic semiconductor with an HSE06 energy gap of 0.84 eV (see the Supporting Information). One can see clearly on its phonon density of states the S 2 dimer signature, i.e. a S-S stretching mode computed at 505 cm -1 within the harmonic approximation that is in good agreement with the experimental value of 469 cm -1 obtained by IR spectroscopy (4 meV difference between experimental and theoretical values). [START_REF] Gard | Infrared Study of Lithium Intercalated Phases in the Li x FeS 2 System (0⩽ X⩽ 2). Characterization of a New Iron Disulfide[END_REF] We evaluated the exfoliation energy of

C2/m-3D FeS 2 at 0.26 J/m 2 c σ) 1 G the Supporting
Information), which are lower than that of graphite (0.37 J/m 2 45 and 2.10 GPa [START_REF] Zhao | Obtaining Two-Dimensional Electron Gas in Free Space without Resorting to Electron Doping: An Electride Based Design[END_REF] ).

Considering that graphene has been experimentally exfoliated, it is highly probable that a two-dimensional FeS 2 material should be obtained. ) 26 for marcasite structures. The relative errors of predicted equilibrium volume of the unit cell with respect to experimental results are as low as 0.5% (159.6 3 1 3 1 3 1 3 for pyrite and marcasite, respectively). Both the structural and energetic properties are in very good agreement with the experimental and theoretical data, which validates our methodological approach. Finally, note that the third structure in energy ranking (therefore before the C2/m-3D that is ranked at the fourth position), to be denoted P-1-3D, displays 5-coordinated iron atoms and contains both sulfides and persulfides moieties (see Figure 3 c). In this layered material, each S 2 dimer links two [Fe 2 S 2 ] infinite chains as found in C2/m-3D phase, but here each sulfur atom of a S 2 unit bridges two iron sites within a single chain resulting in 5-coordinated square pyramidal iron atoms. This structural filiation between C2/m and P-1 layers is depicted in the Supporting Information. This new material is predicted to be antiferromagnetic and lies 282 meV/f.u. above the pyrite and 143meV/f.u. below C2/m-3D. The synthesis of this newly layered five-coordinated P-1-3D FeS 2 will be a true challenge. structure that meets all experimental criteria is framed in red, and the specific FeS 4 tetrahedral environment of the iron atom is displayed.

To conclude, we performed evolutionary crystal structure prediction searches with prior structural constraints derived from several spectroscopies to investigate the crystal structure of the twice delithiated Li 2 FeS 2 compound. We propose a dynamically stable phase C2/m-2D and its tridimensional derivative C2/m-3D built upon C2/m-2D layers as metastable phases with a crystal structure that fully matches the Fe 3+ S is to be related to its coordination geometry, since Fe in the tetrahedral sites found in C2/m-2D

FeS 2 are stabilized with d 5 high-spin configuration (Fe 3+ ), then being a milder oxidizing agent than Fe in octahedral sites of Fe in pyrite or marcasite that favours d 6 low-spin configuration (Fe 2+ ) to fill its t 2g levels. Along our exploration of the energy landscape, we discover the possible existence of another FeS 2 polymorph with five-fold coordinated Fe that would be a challenge to form. Moreover, further work will explore the complexity of the anionic/cationic redox competition in chalcogenides to generate new phases assisted by calculations.

Scheme 1 .

 1 Scheme 1. Crystal structure of Li 2 FeS 2 . The sulfur atoms are in yellow, the lithium and

  4 ] ribbons consisting of edge-shared FeS 4 tetrahedra (mean d Fe-S = 2.27 ) running along the [010] direction with zig-zag iron rows (d Fe-Fe = 2.82 ). These blocks are linked to each other along the [100] axis via S-S bonds (d S-S = ) to form a corrugated 2 /  [FeS 2 ] layer with characteristics in full agreement with a (Fe 3+ )(S 2-)(S 2 2-

Figure 1 .

 1 Figure 1. (a) Top view and side view of the FeS 2 monolayer C2/m-2D found in this work

Figure 2 .

 2 Figure 2. Layered metastable C2/m-3D FeS 2 : (a) Crystal structure and enlightened FeS 4 tetrahedra and S 2 2-dumbbells. (b) Brillouin zone for C2/m-3D, and selected k-path is

Figure 3 .

 3 Figure 3. All dynamically stable 3D FeS 2 structures investigated in this paper as a function of

  2-(S 2 ) 2- 1/2 formulation issued from Mössbauer, IR and EXAFS spectroscopic measurements. Such an intermediate oxidation level of C2/m-2D FeS 2

ACKNOWLEDGMENT

We thank the ANR PRCI Predict_2D_Nanomat (P.I. G.F.), the FEDER and Nouvelle Aquitaine Region (France). We also acknowledge the High-Performance Computing Centers of Poitiers University Mésocentre SPIN, France) and the Irène/TGCC, and Jean Zay/IDRIS GENCI (France) under projects no. X2016087539 and Jean Zay Challenge 2019 for allocation of computing time.

ASSOCIATED CONTENT

Supporting information

Methodological details; calculated structural parameters and energies for all 2D and 3D FeS 2

Notes

The authors declare no competing financial interests.


Special Issue on Monolithic 3D: Technology, Design and Computing Systems Applications Perspectives

Guest Editors:

Sébastien THURIES, sebastien.thuries@cea.fr, CEA-LIST Aida TODRI-SANIAL, aida.todri@lirmm.fr, CNRS/LIRMM

Scope and Purpose:

Over the last 50 years, Computing Systems have successfully surfed the Moore's law wave. Currently, there is much effort spent on M3D technology development and system-level design. Yet, numerous challenges remain. For example, the development of "cold process" to monolithically fabricate a reliable device on top of another one. Another challenge is on the understanding of the physical issues (electro-thermal) and design of energy efficient and reliable M3D systems. Ultimately, CAD tools for planar technology must be adapted for M3D to enable new computing architectures for large on-chip memory bandwidth. This special issue aims at addressing these problems to speed up the emergence of High Energy Efficient Computing Systems.

Topics of Interest:

This special issue is looking for innovative research on Monolithic 3D computing systems over multiple scales of circuit design, microarchitecture, system applications and software development. The areas of interests include but not limited to:

• Systems and applications • Hardware architectures and micro architectures • Hardware -Software co-design • Model and simulation • Design tools and framework, covering any of architecture -circuit -technology levels.

• Reliability and Testability

Interested authors should submit their papers to mc.manuscriptcentral.com/jetc, and select the paper type "Special Issue on Monolithic 3D." The author guidelines can be found at jetc.acm.org/authors.cfm.

Important Dates

• Manuscript submissions due: September 30, 2020 • First round of reviews completed: November 11, 2020 • Revised manuscripts due: December 15, 2020 • Second round of reviews completed: January 1, 2021

• Final manuscripts due: January 3, 2021

For questions or further information, please contact jetc.monolithic3d-editors@acm.org.

  Today, we observe a clear stagnation in this miniaturization race while a new generation of applications (such as Artificial Intelligence) are producing unprecedented amounts of raw data. To overcome this major challenge, new technologies aiming at revising classical Computing Architecture that are continuously moving data from the processing unit to memory are essential. Monolithic-3D technology (M3D) has the potential to improve the energy efficiency of new computing architectures. Indeed, thanks to its nano-scale Monolithic Inter Tier Via that is 100x smaller than state of the art Through Silicon Vias, M3D opens the door for new computing systems immersed in memory.


Mohamed Baccour

Alexandra Lamotte

Kento Sakai

Eric Dubreucq

Ahmad Mehdi

Kenji Kano

Anne Galarneau

Jullien Drone

Nicolas Brun

Production of formate from CO 2 gas under ambient conditions: towards flow-through enzyme reactors

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


D V Beznasyuk

P Stepanov

Jean-Luc L Rouvière

F Glas

M Verheijen

J Claudon

Moïra Hocevar

Full characterization and modeling of graded interfaces in a high lattice-mismatch axial nanowire heterostructure

teaching and research institutions in France or abroad, or from public or private research centers.

I. INTRODUCTION

Semiconductor nanowires offer the unique opportunity to realize coherent axial heterostructures which associate materials having vastly different lattice parameters [START_REF] Jegenyes | High piezoelectric conversion properties of axial InGaN/GaN nanowires[END_REF][START_REF] Caroff | InSb heterostructure nanowires: MOVPE growth under extreme lattice mismatch[END_REF][START_REF] Wen | Formation of compositionally abrupt axial heterojunctions in silicon-germanium nanowires[END_REF] or crystalline structures [START_REF] Hocevar | Growth and optical properties of axial hybrid III-V/silicon nanowires[END_REF][START_REF] Dheeraj | Growth and characterization of wurtzite GaAs nanowires with defect-free zinc blende GaAsSb inserts[END_REF]. In addition, the nanowire geometry can be adjusted to finely engineer its photonic and electronic properties [START_REF] Zhang | III-V nanowires and nanowire optoelectronic devices[END_REF][START_REF] Krogstrup | Singlenanowire solar cells beyond the Shockley-Queisser limit[END_REF][START_REF] Wen | Theoretical analysis and modeling of light trapping in high efficicency GaAs nanowire array solar cells[END_REF][START_REF] Hu | Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications[END_REF][START_REF] Quan | Nanowires for photonics[END_REF]. Brought together, these appealing features promise a wealth of applications in optoelectronics [START_REF] Zhang | III-V nanowires and nanowire optoelectronic devices[END_REF][START_REF] Quan | Nanowires for photonics[END_REF].

Prototypes of laser diodes [START_REF] Quan | Nanowires for photonics[END_REF][START_REF] Eaton | Semiconductor nanowire lasers[END_REF] and quantum light sources [START_REF] Reimer | Bright single-photon sources in bottom-up tailored nanowires[END_REF], white light emitting diodes [START_REF] Sadaf | Alternating-current InGaN/GaN tunnel junction nanowire white-light emitting diodes[END_REF][START_REF] Nguyen | Engineering the carrier dynamics of InGaN nanowire white light-emitting diodes by distributed p-AlGaN electron blocking layers[END_REF], solar cells [START_REF] Krogstrup | Singlenanowire solar cells beyond the Shockley-Queisser limit[END_REF][START_REF] Wen | Theoretical analysis and modeling of light trapping in high efficicency GaAs nanowire array solar cells[END_REF][START_REF] Hu | Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications[END_REF], and high efficiency photodetectors [START_REF] Gibson | Tapered InP nanowire arrays for efficient broadband high-speed single-photon detection[END_REF][START_REF] Lapierre | A review of III-V nanowire infrared photodetectors and sensors[END_REF] were recently developed in nanowire heterostructures. Today, numerous material combinations have been explored to realize axial nanowire heterostructures [START_REF] Hocevar | Growth and optical properties of axial hybrid III-V/silicon nanowires[END_REF][START_REF] Holm | Surface-passivated GaAsP single-nanowire solar cells exceeding 10% efficiency grown on silicon[END_REF][START_REF] Bakkers | Epitaxial growth of III-V nanowires on group IV substrates[END_REF][START_REF] Barrigón | Synthesis and applications of III-V nanowires[END_REF]. In all cases, the control of the strain level around the interface is critical, because above a certain threshold, elastic energy is plastically released via the formation of dislocations [START_REF] Glas | Critical dimensions for the plastic relaxation of strained axial heterostructures in free-standing nanowires[END_REF][START_REF] Kavanagh | Misfit dislocations in nanowire heterostructures[END_REF].

Dislocations act as recombination centers for photons and electrons and degrade the materials properties by reducing the light emission or detection efficiency, and the carrier density.

One solution to realize defect-free interfaces is to reduce the nanowire lateral dimensions. Indeed, for a given couple of materials A and B, and thus a given lattice mismatch, there exists a critical nanowire radius below which coherent growth of B on top of A is possible regardless of the height of B.

* Corresponding author: moira.hocevar@neel.cnrs.fr

The mismatch strain is then partially and elastically relaxed 52 at the nanowire sidewalls. This critical radius, which separates 53 the domains of elastic and plastic strain relaxation, is well 54 understood in the case of a sharp interface [START_REF] Glas | Critical dimensions for the plastic relaxation of strained axial heterostructures in free-standing nanowires[END_REF][START_REF] Larsson | Strain mapping in free-standing heterostructured wurtzite InAs/InP nanowires[END_REF].

55

However, sharp interfaces still present a large resid-56 ual strain, which may be detrimental for applications. 57 For example, the electron-hole wave-function overlap de-58 creases in InAs/GaAs quantum dot nanowires, resulting in 59 longer exciton lifetimes and nonradiative recombination [START_REF] Niquet | Electronic properties of InAs/GaAs nanowire superlattices[END_REF]. 60 High interface strains also lead to potential barriers for 61 charge carriers, which limits their transport in quantum dot 62 nanowire devices [START_REF] Niquet | Quantum dots and tunnel barriers in InAs/InP nanowire heterostructures: Electronic and optical properties[END_REF]. Moreover, a high interfacial strain 63 can enhance piezoelectric effects which degrade performances 64 of nanowire based solar cells [START_REF] Golam Sarwar | Exploiting piezoelectric charge for high performance graded InGaN nanowire solar cells[END_REF]. Finally, in the case of 65 large lattice mismatch, coherent growth is only possible for 66 a very limited range of radii, severely limiting the accessible 67 geometries. As an example, for a lattice mismatch of 7%, 68 the critical radius is as low as 10 nm. Implementing graded 69 interfaces, with a smooth chemical profile, offers a solution to 70 overcome these limitations. Despite a few works [START_REF] Golam Sarwar | Exploiting piezoelectric charge for high performance graded InGaN nanowire solar cells[END_REF][START_REF] Erhard | Optical control of internal electric fields in band gap-graded InGaN nanowires[END_REF][START_REF] Zhang | Impeded thermal transport in composition graded SiGe Nanowires[END_REF], this 71 strategy remains to be thoroughly explored.

72

In this work we investigate both theoretically and experi-73 mentally graded interfaces in axial nanowire heterostructures. 74 A theoretical model specifies the wire radii compatible with 75 coherent growth for various interface lengths and lattice mis-76 matches. While we specifically consider the representative 77 case of interfaces whose chemical profile is described by an 78 error function, these calculations could be easily extended to 79 other profiles. We compare these predictions to experiments realized with the highly mismatched InAs/GaAs material system. The nanowire heterostructures are grown by molecular beam epitaxy (MBE), using a gold droplet as a catalyst.

We perform a complete characterization of the interface: the chemical profile is obtained by energy dispersive x-ray spectroscopy (EDX) analysis, while the structural characterization is conducted through high-resolution transmission electron microscopy (TEM). In the case of coherent growth, the maps of the mismatch strain obtained by geometrical phase analysis (GPA) are in excellent agreement with finite element simulations. This analysis confirms in particular that mechanical strain is largely reduced by interface grading. More generally, interface grading constitutes a novel tuning knob to adjust the physical properties of nanowire heterostructures.

II. RESULTS AND DISCUSSION

As schematized in Fig. 1(a), we consider an infinitely long nanowire oriented along the z direction, with a circular section of radius R. The nanowire features a graded interface between two materials A and B. They have different lattice parameters a, which leads to the lattice mismatch ε m = (a Ba A )/a A .

The interface is centered at z = 0, and the fractions n A and n B = 1n A of the two species follow a smooth profile.

For small-scale compositional gradients (on the order of the nanowire diameter), the interface chemical profile is usually well represented by an error function [START_REF] Schlesinger | Determination of the Interdiffusion of Al and Ga in Undoped (Al,Ga)As/GaAs Quantum wells[END_REF][START_REF] Goren | Barrier formation at graded HgTe/CdTe heterojunctions[END_REF][START_REF] Han | Quantitative atomic resolution at interfaces: Subtraction of the background in STEM images with the example of (Ga, In) P/GaAs structures[END_REF]. Specifically, we assume:

n B = 1 2 1 + erf z L with erf z L = 2 √ π z L 0 e -u 2 du, (1)
where L measures the interface length [Fig. 1(b)]. For a given couple of materials, and in the framework of linear elasticity, the amplitude of the strain generated around the interface is controlled by the normalized interface length α = L/R.

Intuitively, one thus expects that the critical radius R c below which coherent growth is possible increases with α. To determine R c for a given lattice mismatch and interface profile, we compare the energies of the system in two states, namely state (1) with a purely elastic relaxation of the mechanical strain, and state (2) with a single dislocation segment lying perpendicular to the nanowire axis. The critical radius is then defined as the radius above which state (2) has an energy lower than state (1). In state (1), the system energy reduces to elastic energy. This quantity is evaluated with finite element software, assuming mechanical isotropy for the materials. The energy in state (2) is obtained with the method of Spencer and Tersoff considering an edge dislocation [START_REF] Spencer | Stresses and first-order dislocation energetics in equilibrium Stranski-Krastanow islands[END_REF][START_REF] Gatti | Modeling the plastic relaxation onset in realistic SiGe islands on Si(001)[END_REF]. More details on these calculations are given in Sec. S1 of the Supplemental Material [33]. Composition-graded interfaces thus considerably extend the 134 domain where coherent growth is possible: an interface length 135 over tens of nanometers is sufficient to completely suppress 136 the constraint on the nanowire dimensions. This is in contrast 137 to thin film epitaxy [START_REF] Zou | Misfit dislocations and critical thickness in InGaAs/GaAs heterostructure systems[END_REF] where composition-graded buffer 138 layers need to be larger than hundreds of nanometers and to 139 selective area growth of planar nanowires where composition-140 graded interfaces of tens of nanometers are not sufficient 141 to release the mechanical strain and suppress misfit disloca-142 tions at the interface [START_REF] Friedl | Template-assisted scalable nanowire networks[END_REF]. These results, obtained with the 143 parameters of the InAs/GaAs material system, capture the 144 general benefits of interface grading. Furthermore, they can 145 be used to estimate R c for graded interfaces involving other 146 material systems. When precise values are required, one can 147 employ the same method to determine R c using the specific 148 In(Ga)As/GaAs material system thanks to the possibility of 156 creating ternary alloys. We focus here on high-mismatch het-157 erostructures with ε m 5.7%. We grow our In(Ga)As/GaAs 158 nanowire heterostructures by MBE using the gold assisted 159 vapor-liquid-solid mechanism [START_REF] Beznasyuk | Dislocation-free axial InAs-on-GaAs nanowires on silicon[END_REF]. The nanowire radius is 160 controlled by the catalyst dimensions, while the interface 161 length can be controlled by adjusting the growth conditions. 162 Indeed, interface grading occurs in particle-seeded nanowire 163 systems and is attributed to the solubility of the growth species 164 in the liquid droplet which constitutes a reservoir [START_REF] Li | Transition region width of nanowire hetero-and pn-junctions grown using vapor-liquidsolid processes[END_REF]. This 165 "reservoir effect" can be tuned or suppressed by carefully 166 adjusting the growth parameters and the droplet dimensions 167 to form either sharp or controlled graded interfaces [START_REF] Dick | Controlling the abruptness of axial heterojunctions in III-V nanowires: Beyond the reservoir effect[END_REF][START_REF] Priante | Sharpening the interfaces of axial heterostructures in self-catalyzed AlGaAs nanowires: Experiment and theory[END_REF][START_REF] Dubrovskii | Understanding the vapor-liquid-solid growth and composition of ternary III-V nanowires and nanowire heterostructures[END_REF]. STEM image yields a nanowire radius of 10.5 nm and suggests that the position of the interface stands right after the bottleneck visible in Fig. 2(a). This is confirmed by the energy dispersive x-ray spectroscopy (EDX) line profile measured along the nanowire axis [Fig. 2(b)]. The bottom segment is composed of pure GaAs and the upper one is made of an InGaAs ternary alloy with an average indium composition of 0.8. The corresponding lattice mismatch is ε m = 5.7% (wurtzite a lattice parameter). Note that EDX radial profiles across both the InGaAs segment and the top of the GaAs segment do not reveal any radial shell. As shown in Fig. 2(c), the chemical profile of the interface is very well reproduced by an error function. The fit of the data to Eq. ( 1) leads to an interface length L of 5.9 nm [Fig. 2(c)] which corresponds to a reduced interface length α = 0.56. The nanowire radius lies well below the critical value R c = 56 nm, calculated from the experimental value of α and ε m [Fig. 1(c)].

To investigate the crystalline quality of the nanowire heterostructure, we image different areas of NW 1 by high resolution HAADF STEM followed by fast Fourier transform. Both GaAs and InGaAs segments have the wurtzite (WZ) crystal structure except for a small zinc blende (ZB) insertion in the interface region (Supplemental Material S3). Figures 3(a) and 3(b) are additional HR STEM images in two different orientations and do not reveal any misfit dislocation in the crystal at the InGaAs/GaAs interface. As predicted by our calculations, the crystalline integrity of our nanowire is preserved and the mismatch strain at the interface is relaxed elastically.

Across the interface, the lattice parameters a and c are modified both by compositional changes and by mechanical strain. We employ geometric phase analysis (GPA) to image the c-and a-lattice strain, i.e., the c-and a-lattice deformations with respect to a reference chosen here as unstrained c-GaAs and a-GaAs: c c = c-c GaAs c GaAs and a a = a-a GaAs a GaAs , respectively [START_REF] Cooper | Strain mapping of semiconductor specimens with nm-scale resolution in a transmission electron microscope[END_REF]. To map the c-lattice (a-lattice) strain around the interface, we use the high resolution S3). We choose a medium-size mask of ∼0.15| g| (with g the reciprocal lattice vector) in order to preserve a balance between a good spatial resolution and a high signal-to-noise ratio [START_REF] Vallet | Highly strained AlAs-type interfaces in InAs/AlSb heterostructures[END_REF][START_REF] Li | Full-field strain mapping at a Ge/Si heterostructure interface[END_REF].

Figure 3(c) [Fig. 3(d)] shows the resulting color-coded map of c/c ( a/a) in the a-c plane. The bottom part of the wire corresponds to unstrained GaAs ( c/c = a/a = 0%). The top part of the nanowire features a maximum deformation of the c and a planes with respect to GaAs which is consistent with unstrained wurtzite In 0.8 Ga 0.2 As (composition found by EDX), which indicates full relaxation far from the interface. We observe a transition region around the InGaAs/GaAs interface indicating that the lattice is gradually stretched. Importantly, there is no discontinuity (or defects) in the transition regions for c/c and a/a, confirming the absence of misfit dislocations at the interface [START_REF] Frigeri | Interface abruptness and strain relaxation in selfassisted grown InAs/GaAs nanowires[END_REF][START_REF] De La Mata | Atomic scale strain relaxation in axial semiconductor III-V nanowire heterostructures[END_REF]. The transition region is thicker in the center than on the nanowire edges, showing that the a-and c-lattice parameters recover faster their unstrained characteristic value near the nanowire sidewalls than at the nanowire center. It is indeed more difficult to release strain in the core of the nanowire than on the free sidewalls. To get a complementary insight on strain relaxation, we vi-

249

We now quantitatively compare the experimental GPA data 250 to numerical simulations. We first calculate the mechanical 251 strain tensor ε around the nanowire interface using finite 252 element software (COMSOL Multiphysics), the values of 253 lattice constants [START_REF] Kriegner | Unit cell structure of crystal polytypes in InAs and InSb nanowires[END_REF][START_REF] Jacobsson | Phase transformation in radially merged wurtzite GaAs nanowires[END_REF] and stiffness coefficients [START_REF] Wang | First-principles study on elastic properties and phase stability of III-V compounds[END_REF] for WZ 60 nm, respectively) correspond to the dimensions of NW 1 . Since these lengths are already both much larger than R, the results will also apply to nanowires featuring longer segments. We also include the interface chemical profile as determined from the fit to the EDX measurement [Fig. 2(b)]. Finally, we take into account the mechanical anisotropy associated with the wurtzite nanowire crystal (see the Supplemental Material S4 Methods). The a-lattice strain is then deduced using the relation a/a = [a loc (ε xx + 1)a GaAs ]/a GaAs . Here a loc is the local unstrained lattice parameter, determined from the measured chemical profile in Fig. 2(c) and using a linear interpolation between GaAs and InAs, a GaAs is the unstrained lattice parameter of GaAs and ε xx = a-a loc a loc is the mechanical strain along the x axis. Similarly, we have c/c = [c loc (ε zz + 1)c GaAs ]/c GaAs . In addition, in order to account for the depth of focus of STEM imaging (around 10 nm), the theoretical data are averaged along the nanowire depth (details in the Supplemental Material S4).

Figure 4(a) and 4(b) compare the experimental and simulated a-and c-lattice strain along the nanowire axis. Without any free parameter, we obtain for c/c an excellent agreement between the simulated and experimental profiles (the discrepancy is lower than 0.5%). For a/a, the agreement is good, but the theory predicts a slightly lower In composition in the In 0.8 Ga 0.2 As segment than observed in the experimental data. We attribute this to the noise in the experimental GPA data [Fig. 3(d)], which increases the uncertainty on the unstrained reference region. Both c/c and a/a increase gradually from 0% (GaAs reference) to about 5.9% and 5.7% (In 0.8 Ga 0.2 As segment) respectively, which is consistent with The maximum hydrostatic strain / as well as the maxima of the mechanical strain components ε xx , ε yy , and ε zz are plotted versus the interface length L.

the calculated lattice mismatch with the WZ phase. Furthermore, we also compute theoretical 2D maps of c/c and a/a (Supplemental Material S4). They both reproduce the features observed in the experimental maps. In particular, the different radial strain profiles extracted from the GPA maps at different z show that the InGaAs segment experiences an increase in the c-lattice strain on the side walls with respect to the center of the segment, while the GaAs segment sees a decrease in the c-lattice strain. These data follow the simulated radial strain profiles (Supplemental Material S4). Overall, this demonstrates that we have a quantitative understanding of the structural properties of the interface.

We build on this understanding to discuss the distribution of mechanical strain around the InGaAs/GaAs interface. exceeds the interface length (L = 5.9 nm), and is in fact roughly set by the nanowire diameter, in agreement with the Saint Venant's principle. ε zz and ε xx show a maximum around 0.5%, indicating that the mismatch strain is largely decreased but not fully released. Finally, both ε zz and ε xx feature large spatial inhomogeneities. In particular, ε xx presents several longitudinal oscillations between tensile and compressive deformation. Importantly, these marked strain inhomogeneities will introduce a spatial modulation of the band structure [START_REF] Niquet | Electronic properties of InAs/GaAs nanowire superlattices[END_REF][START_REF] Niquet | Quantum dots and tunnel barriers in InAs/InP nanowire heterostructures: Electronic and optical properties[END_REF] which should be taken into account in the design of nanowire devices.

Figure 5 illustrates the dramatic influence of interface grading on the strain fields. We consider an InGaAs/GaAs nanowire with the same dimensions and composition as NW 1 and plot the maximal values of ε xx , ε yy , and ε zz as a function of the interface length L. In the case of a sharp interface (L = 0) ε zz and ε xx reach 1.7% and 2.6%, respectively. A graded interface with L = 5.9 nm (NW 1 ) is already sufficient to decrease ε zz by a factor of 3, and ε xx by a factor close to 6. We note here that interface grading has a stronger influence on the transverse strain components. Of course, 326 increasing L leads to a further decrease of the strain but for 327 the investigated interface lengths, the spatial extension of the 328 strained region is roughly the same (Supplemental Material 329 S4). We next consider the hydrostatic strain / = ε xx + 330 ε yy + ε zz , which has an important impact on the band gap and 331 the conduction band offsets [START_REF] Niquet | Electronic properties of InAs/GaAs nanowire superlattices[END_REF]. Its maximum value is also 332 plotted in Fig. 5: it is reduced from 4.2% down to 0.8% as the 333 interface length increases from 0 to 8.4 nm. Modest interface 334 grading thus already results in a strong reduction of the strain 335 level.

336

Next, we consider additional nanowire samples to further 337 support the theoretical predictions of the coherent growth do-338 mains. The results are summarized in Fig. 6, which confronts 339 the theory to experimental results obtained with two families 340 of samples. The first set of nanowires (NW 2 to NW 4 ) features 341 ε m of 5.7%, R around 10 nm, and α ranging from 0.25 to 342 0.56. For all these nanowires, the mismatch strain is always 343 elastically released by the sidewalls, as shown on HRTEM 344 images by the absence of dislocations at the interface and on 345 GPA color-coded maps by the curvature of the a and c planes 346 (Supplemental Material S5). As shown in Fig. 6, this nanowire 347 family falls in the predicted coherent domain. The second set 348 of InAs/GaAs nanowire samples with α being 0.48 and 0.67 349 for a unique ε m of 7.1% (NW 5 and NW 6 ). We observe by 350 HRTEM that the nanowires present defects at the InAs/GaAs 351 interface. GPA color-coded maps confirm the presence of 352 misfit dislocations and reveal plane bending (Supplemental 353 Material S6). In those nanowires the mismatch strain is re-354 leased both via plastic and elastic relaxation. We finally plot 355 the experimental data in Fig. 6: these thick nanowires fall 356 in the plastic relaxation region, confirming here as well the 357 predictions.

358

Our study is of particular significance when it comes 359 to realize optoelectronic devices using semiconductor het-360 erostructures. Material combinations such as, for exam-361 ple, InP/InSb (ε m = 10%) and GaN/InN (ε m = 11%) are 362 important for photovoltaic and optoelectronic applications but 363 their structural quality and therefore their physical functions 364 suffer from an extremely high lattice mismatch. As seen in 365 previous works, reducing the diameter is not always possible or sufficient to prevent plastic relaxation [START_REF] Frigeri | Interface abruptness and strain relaxation in selfassisted grown InAs/GaAs nanowires[END_REF][START_REF] Pitanti | InAs/InP/InSb Nanowires as Low Capacitance n-n Heterojunction Diodes[END_REF]. Thus, Theoretical predictions confirmed by our experimental data show that the domains for coherent growth can be extended using compositional gradients of few nanometers. Beyond the realization of coherent heterointerfaces, interface grading offers an additional tuning knob to control residual strain in the nanowire, and thus to fine-tune its optoelectronic properties.

Figure 1 (FIG. 1 .

 11 FIG. 1. Axial nanowire heterostructure with graded interfaces. (a) Schematics of a nanowire heterostructure composed of two materials A and B. The nanowire features a circular section (radius R), its longitudinal axis coincides with the z direction. (b) Composition profile along the nanowire axis described by an error function [Eq. (1)]. Over the interface length L, the composition varies by 52% of the total composition jump. (c) Calculated critical radius R c as a function of the mismatch strain ε m between A and B for different values of α = L/R (α = 0 corresponds to an abrupt interface).

FIG. 2 .

 2 FIG. 2. InGaAs/GaAs axial nanowire heterostructure: chemical characterization. (a) Dark-field TEM image of NW 1 taken along the [2-1-10] zone axis. The position of the interface is indicated by the white arrow. Moiré fringes are visible in the nanowire and are due to the coincidence periods between the scanning step of the electron beam and the interatomic potential. (b) EDX composition profile measured along the nanowire axis [blue arrow in (a)]. (c) Zoom on the interface profile. The fit to an error function yields L = 5.9 nm.

168

  Figure 2(a) shows an image of NW 1 , obtained by scanning

  [2 11 0] [01 10] HAADF-STEM image shown in Fig. 3(a) [Fig. 3(b)]. GPA is then performed to the image by applying a mask around the (000 2) (2 11 0) Bragg peak in the Fourier transform visible in the inset of Fig. 3(a) [Fig. 3(b)] (Supplemental Material

FIG. 3 .

 3 FIG. 3. InGaAs/GaAs axial nanowire heterostructure: high resolution structural characterization. HAADF STEM image taken along the [2-1-10] viewing direction (a) and the [01-10] viewing direction (b). The insets show the corresponding fast Fourier transform (FFT). Map of the mismatch strain c/c (c) and a/a (d) obtained by applying GPA on (a) and (b), respectively. Corresponding numerical moiré patterns (e) and (f). The scale bars are identical for (a), (c), and (e). Similarly, the scale bars are identical for (b), (d), and (f).

237

  sualize the arrangement of the crystal planes with a numerical 238 moiré technique [46] [Figs. 3(e) and 3(f)]. We obtain a moiré 239 pattern from the geometric phase images of Figs. 3(a) and 3(b) 240 using Fourier filtering of the (000 2) and (2 11 0) Bragg peaks, 241 respectively. We observe that the distance between planes is 242 larger in the upper segment than in the bottom segment. Far 243 from the interface, the planes are parallel to each other and are 244 strain-free. Near the InGaAs/GaAs interface, at the sidewalls, 245 the planes bend dramatically. This large deformation is due to 246 elastic relaxation of the mismatch strain at the nanowire free 247 surfaces. Note that plane bending is also evidenced in lattice 248 rotations maps obtained by GPA (not shown).

254FIG. 4 .

 4 FIG. 4. Experimental and simulated strain profiles along the z axis. Evolution of c/c (a) and a/a (b) across the interface. The experimental GPA data profiles (green, with standard deviation as error bars) were extracted from the strain maps and averaged over a lateral sampling of 80 pixels. The simulated profiles were extracted in the central part of the nanowire. They correspond to calculated data along the nanowire axis only (blue) and to calculated data averaged along the nanowire depth (purple). Evolution of the calculated mechanical strain components ε zz (c) and ε xx (d) across the interface.

FIG. 5 .

 5 FIG. 5. Influence of the interface length on the mechanical strain.The maximum hydrostatic strain / as well as the maxima of the mechanical strain components ε xx , ε yy , and ε zz are plotted versus the interface length L.

  Figures 4(c) and 4(d) show the calculated strain components ε zz and ε xx along the nanowire axis (z). Both components are zero far from the interface, and feature significant amplitude over a domain which is 30-40 nm long. Its size significantly

FIG. 6 .

 6 FIG.6. Elastic and plastic relaxation in axial nanowire heterostructures. The elastic and plastic domains are separated by a line corresponding to the calculated R c for In 0.8 Ga 0.2 As/GaAs (dashed blue) and InAs/GaAs (solid red). Above the line, theory predicts plastic relaxation. The circles indicate the experimental data from dislocation-free (open) and plastically relaxed (solid) structures.

367374

  the design of nanowire devices with compositionally graded interfaces has the potential to reduce materials constraint on 369 the device dimensions. Importantly, a compositional grading 370 over few nanometers at nanowire interfaces is sufficient to 371 reduce most of the strain without altering the required physical In conclusion, we fully characterized high lattice-mismatch 375 axial In(Ga)As/GaAs heterostructure nanowires featuring 376 graded interfaces. The heterostructure shows a preserved crys-377 talline quality with a mismatch strain released elastically, via 378 plane bending. Full elastic relaxation occurs at the nanowire 379 sidewalls while the remaining strain is localized in the cen-380 tral area of the nanowire, larger than the interface length.

ACKNOWLEDGMENTS

Assistance with the MBE growth by Yann Genuist is gratefully acknowledged. Martien den Hertog, Eric Robin, and Joël Cibert are acknowledged for fruitful discussions. The authors acknowledge financial support by the LABEX LANEF (ANR-10-LABX-51-01), Université Grenoble Alpes (program AGIR), the Thomas Jefferson Fund of the Embassy of France in the United States and the FACE Foundation, as well as the ANR HYBRID (ANR-17-PIRE-0001) and QDOT (ANR-16-CE09-0010-01).

The authors declare no conflict of interest.


Alexander Winterl
email: alexander.winterl@fau.de

Sebastian Richter

Aymeric Houstin

Anna P Nesterova

Francesco Bonadonna

Werner Schneider

Ben Fabry

Céline Le Bohec

Daniel P Zitterbart

micrObs -A customizable time-lapse camera for ecological studies

Keywords: automated camera system, image processing, remote sensing, ecology, collective behavior, wildlife monitoring Specifications table Hardware name micrObs Subject area, Biological Science, Environmental Sciences

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

flocks of birds [START_REF] Ballerini | Interaction ruling animal collective behavior depends on topological rather than metric distance: Evidence from a field study[END_REF], mating of mosquitoes [START_REF] Butail | Reconstructing the flight kinematics of swarming and mating in wild mosquitoes[END_REF], hunting strategies in bats [START_REF] Hristov | Applications of thermal infrared imaging for research in aeroecology[END_REF], grouping of midges [START_REF] Attanasi | Collective Behaviour without Collective Order in Wild Swarms of Midges[END_REF], or the huddling of emperor penguins (Aptenodytes forsteri) (Richter et al., 2018b). The recording hardware used in such studies can vary from camera traps [START_REF] Jones | Data Descriptor: Time-lapse imagery and volunteer classifications from the Zooniverse Penguin Watch project[END_REF], semi-automatic camera systems [START_REF] Lynch | A high-resolution panorama camera system for monitoring colony-wide seabird nesting behaviour[END_REF], DSLR cameras with custom trigger electronics [START_REF] Newbery | An automated camera system for remote monitoring in polar environments[END_REF], to autonomous multicamera arrays (Richter et al., 2018a). Also, requirements regarding frame rate and image resolution can vary greatly. For studies that focus on slowly changing parameters such as population size and breeding site occupancy, long image intervals (1 image per hour to 1 image per day) are sufficient, and accordingly the total number of acquired images remains small, with typically less than a few thousand images per camera and season. By contrast, behavioral studies require higher frame rates to identify individual motion and interaction between individuals, ranging from 1 frame per minute for studying breeding penguins [START_REF] Jones | Data Descriptor: Time-lapse imagery and volunteer classifications from the Zooniverse Penguin Watch project[END_REF][START_REF] Zitterbart | Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle[END_REF] to video frame rates [START_REF] Butail | Reconstructing the flight kinematics of swarming and mating in wild mosquitoes[END_REF] or even high-speed cameras [START_REF] Attanasi | Collective Behaviour without Collective Order in Wild Swarms of Midges[END_REF] for studying collective order in swarms of midges.

Unfortunately, frame rates between one frame per second to one frame per minute are often not available in commercial camera traps and time-lapse cameras, or if they are, longer recording periods of several days or weeks cannot be achieved for various reasons such as mechanical camera failure after 100,000 images (Improve Photography, 2020). This lack of long-term recording hardware is especially critical for studies in which the behavior of a species needs to be observed over a significant part of its annual cycle, for example emperor penguins that remain continuously at the breeding colony for over 130 days (Richter et al., 2018b). Another disadvantage especially of low-cost commercial systems is their poor image quality as regards image resolution and noise performance in low-light conditions.

Here, we present a low-cost, high-resolution, sturdy and water-resistant time-lapse camera system (micr0bs) (Fig. 1) based on customized consumer components. The camera system was designed to record time-lapse images continuously and autonomously at frame rates between 0.5 frames per second and 1 frame per day with an image storage capacity of 1024 GByte (~200,000 jpeg-compressed 16 MPix images). Furthermore, the system was optimized for minimum power consumption and offers auxiliary data logging capabilities, for example to record local weather conditions. micr0bs systems have been deployed since 2013 under the harsh climatic conditions of Antarctica and the Sub-Antarctic Islands, with some of the cameras having recorded more than 1 million images (Table 6).

2.

Hardware description

The camera system was designed to handle a large range of time-lapse applications. This foremost includes a wide range of image recording rates (2s -24h), low power consumption and large image storage capacity to maximize runtime. The camera system offers auxiliary data logging capabilities, for example to record local weather conditions. We foresee the main applications to fall into one of the following categories:

• Continuous observation of individual behavior, e.g. tracking predation events in a seabird colony at high temporal resolution of 1 image every 5 seconds for 12 days

• Investigation of collective behavior events, e.g. observing penguin huddle formation at intermediate temporal resolution of 1 image every minute for 20 weeks • Long-term monitoring of landscapes, aggregations, or population sizes of animals, e.g. observing nesting seabirds at low temporal resolution of 1 image per hour for 4 years

Camera choice

Existing solutions for automated image capture can be divided into four categories: camera traps, action cameras, consumer digital single lens reflex (DSLR) cameras, and computer-assisted image acquisition system based on industrial cameras. In Table 1, we compare representative examples of such systems to our approach (micrObs).

Camera traps such as the Bushnell impulse trail (Bushnell Outdoor Products, 2020) are designed for long deployment times, low energy uptake, and harsh environmental conditions. Typically, they are used to record a picture/video on demand after being triggered by motion or a thermal signal from a passing animal. These systems often support time-lapse imaging but cannot record more than several thousand images during a deployment period due to their limited memory capacity and battery lifespan. Furthermore, their image quality is limited by low-resolution lenses, small apertures, lack of automatic aperture and focus settings as well as small sensor sizes and low image resolution.

Action cameras such as the goPro Hero 8 (goPro, 2020) are primarily used for frame rates between 30 fps and 120 fps, but they also support time-lapse modes [START_REF] Gopro | HERO8 Black HDR Stabilizing Camera | GoPro[END_REF]. Similar to camera traps, they are limited by battery capacity and image storage capacity. There are solutions bypassing this issue with additional hardware [START_REF] Blinkx | Performance Binoculars, Riflescopes, Rangefinders & Night Vision Equipment[END_REF][START_REF] Greene | CoralCam: A flexible, low-cost ecological monitoring platform[END_REF]. However, further drawbacks of these cameras are their wide-angle lenses and small image sensors that perform poorly under low-light conditions.

The use of consumer DSLR cameras for time-lapse recordings is widespread in construction progress documentation (Harbortronics Inc., 2020). In addition, there is a noncommercial system already in use to study penguin population dynamics [START_REF] Newbery | An automated camera system for remote monitoring in polar environments[END_REF][START_REF] Southwell | First comprehensive abundance survey of a newly discovered Adélie penguin breeding metapopulation in the Robinson Group of islands, Mac.Robertson Land, East Antarctica[END_REF]. These systems are closest to our requirement in image quality and range of time-lapse intervals. However, DSLR cameras suffer from mechanical wear of the mirror and shutter, which are moved during each image capture. This limits the lifetime of a DSLR camera to about 50,000 to 100,000 images. Furthermore, in extreme cold, DSLR fail mechanically: shutter and mirror move slowly, become stuck or damaged [START_REF] Zitterbart | Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle[END_REF]. The DSLR in both systems can be replaced with a mirrorless interchangeable-lens camera with an electronic shutter, however, the relatively high cost of the commercial system and the absence of open soft-and hardware documentation for the noncommercial system remain an issue.

Custom image acquisition systems based on industrial cameras and controllers (Richter et al., 2018a) are expensive, bulky, have a high energy consumption, and require the user to develop customized algorithms for the automatic control of aperture, gain, exposure time, and focus distance. Software and hardware for compression and management of the recorded images further increase the complexity.

Our "micrObs" imaging system is based on the Panasonic Lumix DMC G5, a commercially available digital mirrorless camera that offers an electronic shutter, external trigger, optional external power supply, and high-quality interchangeable fixed or adjustable focal length lenses with large aperture. We use a wide-to-medium angle telephoto lens (Panasonic H-FS014042E) with a focal length of 14-42 mm (28-48 mm in 35 mm equivalent). We use the manual focus option to set the focus point (usually to infinity). This prevents blurred images due to an unsuccessful autofocus search in scenes with poor light or poor visibility, and further reduces the number of moving parts.

Camera control

The recording system is controlled by an Arduino Nano microcontroller [START_REF] Arduino | Arduino Nano | Arduino Official Store[END_REF] mounted onto a custom printed circuit board. The microcontroller monitors battery voltage and ambient light conditions. It powers and boots the camera, and triggers the image acquisition (see Fig. 2 and Fig. 3).

The microcontroller also logs the data from a GPS module, a weather station (AWS), or any other RS232-capable device, to an interchangeable memory card (microSD). We choose the Arduino over alternative controllers with lower power requirements because of the large user community providing existing hardware and software solutions such as the SD card board.

We control the camera's boot and shut down commands by replacing the camera's manual power switch with an electronic switch. This is necessary to ensure a proper shut-down sequence, as simply disconnecting the external power supply leads to an erroneous numbering of the recorded images.

A major limitation of a consumer camera is the inaccuracy of their image timestamps. Even if the camera's clock is set properly, it will drift noticeable during long-term deployments. Therefore, in addition to the image timestamp provided by the camera, we log the microcontroller time of each image trigger event in a file. To prevent time drift, we synchronize the microcontroller's clock once per hour (configurable) to UTC time provided by the GPS module. To reference the camera-provided image timestamp to the microcontroller-provided trigger time, we acquire two images with no (minimal) time delay each time the microprocessor's clock is synchronized.

For most applications, ambient light conditions will occasionally be insufficient for image acquisition, e.g. due to day-night cycles or weather changes. To save energy and storage space, micrObs measures the ambient light intensity with a photoresistor and interrupts recording if it falls below a user-defined threshold.

To simplify the camera setup procedure under poor weather conditions, the user can adjust the frame rate and illumination threshold via the front panel of the control box (see Fig. 1A). A display informs the user of image interval, current ambient illumination, illumination threshold, connected RS232 devices such as a weather station, battery voltage, availability and functionality of GPS, and memory card.

The control box can in principle control any camera that offers external power supply and shutter release. The voltage levels of the camera power supply and the shutter release are adapted to our camera model but can be changed by adjusting resistors R6 and R7 and the voltage converter TSR-2490 (see design file MainPCB.sch).

Other components

The housing of the recording system needs to withstand severe weather conditions like heavy rain, drifting snow or ice buildup as well long-term UV exposure. The housing is chosen as small as possible to reduce wind-induced vibrations. It holds the camera, the control unit, and the GPS receiver (NL-604P, Navilock). We chose a plastic housing with rubber sealing (Protector 1150, Peli). Cables to the battery box and external weather station are connected to the controller via waterproof cable-glands.

The cutout for the camera's viewport through the enclosure is covered with a standard 52 mm UV filter used for photography and sealed with a custom-made rubber gasket that is compressed to the enclosure by a bracket. The housing is water resistant (comparable to IP66 rating) but not submersible.

It is attached to a tripod via a custom pan-tilt mount that is easily operable and fits most tripods. The tripod (berlan BST285A) used in our application is a standard surveillance equipment tripod and was chosen based on its low cost and light weight (aluminum construction).

If a rechargeable battery with solar panels is to be used, the solar panel is mounted on a separate aluminum box (Zarges) that also houses the charge controller and battery. For transportation, the camera case is designed to fit into the battery box as well. Depending on battery weight and size, transport and setup of the system is possible with one or two persons.

For the battery, we chose a deep cycle, non spillable, lead gel battery (Lifeline GPL-U1T), which, for our application, was the best compromise between weight (10.9kg), price (170€) and capacity (33Ah, 12V). A 30 W solar module (Offgridtec 3-01-001530) that fits on the aluminum box charges the battery with a 6A 12V charge regulator (Steca Solsum 0606). syncRenameFiles.py: This file contains a self-explanatory (help function) python (version>3.4) script used to synchronize the recorded images from the camera memory card with the timestamps and meta information from the control box' memory card. 4 in Supplementary File 2.

Design files

Design files summary

Bill of materials

We included a detailed bill of materials with the other design files (10.5281/zenodo.3756718) and in Supplementary File 1.

The control box housing can be ordered from a laser cutting service or manufactured at a local fab lab using the provided drawings. The control box housing (parts C1-C8, see Fig. 4 in Supplementary File 2) is made out of 3 mm black cast acrylic glass (PMMA). The window parts W1 and W2 are made out of 3 mm acrylic glass, and W3 is made out of 1 mm thick chlorine free rubber.

The electronic parts and most of the smaller mechanical parts can be purchased through online retailers such as conrad.de or mouser.com.

Aluminum for larger mechanical components (e.g. for the tripod mount) can be purchased from a local hardware store. We manufactured the tripod mount at our institute's mechanical workshop.

The hardware costs for a minimum configuration of the system consisting of the enclosure box, camera, control electronics and GPS are 440 €. This does not include the costs for an external power supply (289 €) and tripod mounting (228 €). All prices include German VAT of 19 %.

Building Instructions

We provide a detailed step-by-step building instruction in Supplementary File 2. The instruction contains descriptions and pictures of the assembly of the control box, the customization of the camera, the construction of the front panel, a drilling plan for the outer housing, and a building instruction for the window mount. It does not contain instructions on how to cut acrylic glass, how to manufacture the pan and tilt mount, how to make a PCB from the design files or how to solder components. If an in-house workshop for these tasks is not available, they can be outsourced (e.g. Online PCB Services, Laser Cutting Services). In the following, we give a brief summary of the instructions:

Pry open the camera body and replace the hardware switch by two cables connected to the former hardware switch pads. Drill a hole in the camera body to feed-through the cables. Re-assemble the camera body. Replace the camera battery with the DC-adapter. Flash the Arduino with the C++ software (software.ino) using the Arduino IDE software and a mini USB connector. Put the Arduino in the socket on the main board. Close the control box housing. Assemble the trigger cable from the core of the audio jack (P6) with two 0.5 mm² wires. Assemble the light sensor from the photoresistor (R0), the plastic mantle of the audio jack (P6) and two 0.5 mm² wires. Drill holes for the camera mount, window, light sensor, GPS, and power cable gland (T1, T2) in the housing. Assemble the window mount from the laser cut window mount parts (W1-W4) and secure it to the housing. Mount the camera and GPS inside the housing. Assemble the GPS connector plug.

Connect the camera to the control box via the trigger cable, customized camera power switch cable, and dc adapter cable. Double check (!) the polarity of the camera power connection. (See chapter 5 in Supplementary File 2.)

Connect the light sensor and GPS. Put a SD card in the camera and a micro SD card in the control box. Push the debug button and connect the control box to power. The system should boot, start up the camera, and display the system parameters. (See chapter 6 in Supplementary File 2)

We use the transport box as a housing for the battery and charge regulator and as a support for the solar panel. Detailed instruction for the customization of the transport box, wiring, mounting the solar panel and mechanically securing the battery are given in chapter 7 in Supplementary File 2.

Operation instructions

We include a user's manual in Supplementary File 3, which explains the setup in the field, front panel setup options and synchronization software. In the following, we describe the typical use case.

The battery box with the camera and tripod can be easily carried by two people. Remove the camera housing and solar panels from the transport box and connect cables to the outer sockets. For most applications, the camera should be positioned as high as possible above the imaged area to achieve a high observation elevation angle. This is usually associated with a high wind load. We reduce windinduced shaking and tilting by attaching a heavy weight to the camera mount/tripod. For example, a tarpaulin or net can be attached to the tripod, which is then loaded with stones or other heavy material available on site. Alternatively, tent pegs can be used to secure the tripod.

The camera mount allows for +/-15 degrees of tilt and 360° of panning. For better access while setting pan, tilt, and recording parameters, the camera housing can be opened and closed without moving or touching the camera.

To adjust recording settings, first switch to setup mode by pressing the red setup button (red LED will turn on). The button will click and stay in the latched position. While in setup mode, no images are acquired. To resume time-lapse operation, click the setup button again (red LED will turn off).

While in setup mode, the control box' display shows the current parameters of the system: Image interval, ambient light threshold, GPS activity, weather station activity, µSD card presence and power supply voltage. To change those parameters, set the image interval, the GPS and weather station usage and the light sensor threshold using their respective control/button. The settings will be saved when the setup mode is exited.

After exiting the setup mode, recording will start immediately, and the control box will display the cumulative number of recorded images after each trigger event. The "Power" LED indicates the supply of the camera with power, the "Boot" LED indicates if the camera is switched on, the "Trigger" LED lights up shortly when an image is triggered.

If micrObs is disconnected from power intentionally or unintentionally (e.g. to exchange the battery), it is not necessary to reset any parameters. The control box saves the last setup on its µSD card and continues recording when re-powered. Likewise, the system resumes recording after a power loss, e.g. when connected to a variable power source like a photovoltaic panel. Control box memory card and camera memory card should always be exchanged together, to keep the timestamp and meta information synchronous with the recorded images. We provide a software tool for reordering and renaming of image files based on the timestamps from the control box' µSD card.

Validation and characterization

micrObs supports 9 different time lapse intervals between 2 s (fastest) and 1 h. The use of the light sensor, GPS, and a second RS232 device can be enabled or disabled. If needed, these default time lapse intervals can be changed in the Software.ino file, but 2 s is the fastest interval due to camera limitations. The performance characteristics and limitations with our configuration (12 V 30 Ah battery (3,600 Wh), 1 TByte memory card for the images) are:

• Capture more than 200,000 images (this requires recharging of the battery)

• Capture up to 64,000 images on a single battery charge • Record for up to of 98 days on a single battery charge • Relative deviation of the timer interval from the target value is less than 0.1 %

Timing and power drain

We minimize the energy consumption of our recording system by optimizing the component uptime, keeping components only powered when necessary. Therefore, we switch-off the camera between two images if the image interval is larger than 10 s or when there is not sufficient ambient light (e.g. night, storm). Power consumption is 152.4 mW during the power-down phases (Fig. 4B). A single charge of a 3,600 Wh (12V 30 Ah) battery lasts for up to 64,000 images recorded at the highest frame rate, or up 98 days when images are recorded at the lowest frame rate (if no additional RS232 device other than the GPS device is powered, see Table 5). The light sensor (to automatically turn-off recording during low-light conditions), GPS, and a second RS232 device can be switched on or off. The power consumption during the power-down phases can be further lowered to 122 mW by removing the microcontroller's status LEDs and removing the built-in power converter of the Arduino-controller, which are not necessary for operation. If solar power (with a small buffer battery) is sufficient to power micr0bs continuously (e.g. 3W average consumption, 30W solar panel), the system's maintenance interval is only limited by the camera's image memory card size. With a 1TB memory card, the camera can capture more than 200,000 16 MPix 98% quality jpeg images. micrObs supports 9 different image intervals between 2s and 1h. Each image is provided with a GPS time stamp. We verified the image interval by measuring the timing accuracy of the Arduino's internal clock. In particular, we measured four different micrObs devices at four settings for the sleep timing (the time the system spends in deep sleep without interruption) and image intervals (the actual time between images): 0.5 s sleep at 1 h image interval, 0.5 s sleep at 10 s interval, 1 s sleep at 30 s interval, and 0.5 s sleep at 30 s interval. We then compared the Arduino's internal timestamp to a network time protocol-synchronized pc timestamp and found that the relative deviation from the target interval for each of the devices was below 0.1% (Fig. 4A).

Field test

With five separate micrObs system, we recorded time-lapse images of penguin colonies of three species (Emperor penguins (Aptenodytes forsteri), Adélie penguins (Pygoscelis adeliae), and King penguins (A. patagonicus)) at three different locations: Pointe Géologie archipelago (Adélie Land, Antarctica), Kerguelen (Ratmanoff Cape), and Crozet (Baie du Marin on the Possession Island). During 6 years of operation, we recorded over 6 million images (see Table 6). [START_REF] Barbraud | Population trends of penguins in the French Southern Territories[END_REF]. During breeding, adult penguins alternate between breeding and foraging, forming a continuous stream of thousands of penguins arriving and leaving their colony. Therefore, we persistently encounter trafficking individuals between the open sea and the breeding sites. We continuously observed an area of high traffic over 21 days. To achieve reliable tracking of animals (which are indistinguishable from each other), the distance between two individuals must not be greater than the distance one individual covers between two images. Therefore, the image interval of 2 seconds was selected to track the penguins, yielding 398,149 images in total.

We applied an object detection and tracking algorithm to the image series, which generated 7,413,146 detections of individuals, and concatenated them to 373,290 penguin tracks. Individual tracks of 1628 penguins during a 17 min recording period are shown in Fig. 5A. Penguin tracks separated into penguins arriving and leaving their colony site are shown in Fig. 5B.

We find that penguins use preferential pathways to commute over the sea ice on short time scales (Fig. 5C), creating "penguin roads". These penguin roads, however, are not stable over time but meander across the sea ice area during the 21 day recording period so that over time a substantial part of the sea ice area was utilized by the Adélie penguins (Fig. 5F). The high-density areas (yellow) in Fig. 5F represent points along the road where penguins frequently stop and rest, resulting in a high area usage. We also find that animals leaving and returning to the colony prefer separate trails (Fig. 1D) to avoid collisions.

Ecological study on emperor penguins

micrObs was previously used in a published study that analyzed the behavioral response of emperor penguins (Aptenodytes Forsteri) to weather changes (Richter et al., 2018b).

While breeding and fasting under the harsh conditions of the Antarctic winter, emperor penguins (Aptenodytes Forsteri) form tight groups to conserve energy [START_REF] Ancel | Energy saving in huddling penguins[END_REF]. This strategy, called huddling, is the key to their reproductive success. From micrObs time lapse recordings at 30 s time intervals, we measured the area covered by the animals to estimate the colony density. We then interpreted this density as an order parameter for studying colony phase transitions (from a dilute phase to a dense huddling phase) as function of an apparent temperature (Richter et al., 2018b). This apparent temperature is computed as a linear combination of four meteorological parameters (ambient temperature, wind speed, relative humidity, solar radiation) and serves as the thermodynamic variable in a phase transition model. The model predicts the colony's state depending on the environmental conditions. For this study, we analyzed data from 8 days of recordings (2767 images) between 6 th of May and 11 th of May 2014 at DDU (see Table 4). The system operated in cold (average temperature -16.8°C) and low illumination conditions (average solar radiation 1878 W/m²). The 20 Ah battery was replaced every 3 days.

Declaration of interest

None.

Human and animal rights

This study was approved by the French ethics committee (APAFIS#4897-2015110911016428 and APAFIS#9496-201707131540776) and the French Polar Environmental Committee of the Terres Australes et Antarctiques Françaises (TAAF project implementation and access permits # 2012-117 & 2012-126, 2013-74 & 2013-82, 2014-116 & 2014-132, 2015-52 & 2015-105, 2016-76 & 2016-82, 2017-92 & 2017-102, 2018-116 & 2018-129, 2019-107 & 2019-115) and conducted in accordance with its guidelines.

Fig. 1 :

 1 Fig. 1: micrObs recording system in the field. (A) The camera housing contains the control box and camera. (B) System deployed in the field at Pointe Géologie archipelago, Adélie Land, Antarctica (66°42′01″S 139°49′41″E, May 2019). The camera housing is mounted on a tripod with a pan/tilt ball mount. The system is powered by a 12 V car battery with a solar panel and charge regulator. (C) Image recorded with micrObs. (D) Enlarged detail: Individual animals are clearly distinguishable.

Fig. 2 :Fig. 3 :

 23 Fig. 2: Schematic of the camera control setup.

Fig. 4 :

 4 Fig. 4: Validation of time interval precision and power drain. (A) Deviation of the image interval relative to the target interval for different settings. Each point corresponds to an independent measurement, colors indicate the performance of different microcontrollers. The boxplot (median, 25 and 75 percentiles) shows the data pooled over all measurements from all microcontrollers for the same setting. Whiskers show the 1.5 interquartile range. (B) Power drain during image capture without GPS or other RS232 device. The pink area corresponds to the energy consumed for the capture of one image (20.3 Ws). The blue shaded area indicates the power consumption in standby mode (152 mW). Vertical lines indicate the phases of image capture initiated by the microcontroller.

Fig. 5 :

 5 Fig. 5: (A) Image recorded with micrObs. The dashed curves show circles with distances of 50 m, 100 m, and 200 m around the camera. The solid red lines mark a line of 10 m length at the respective distances from the camera. Insets show examples of penguins at the respective distances. (B) and (C) are images recorded with micrObs overlaid with penguin tracks during a 17 min observation period. Each individual track in (B) is marked by a separate color. Colors in (C) indicate the track

Table 1

 1

	: Comparison of four time-lapse recording systems for frame rates between 1/s and 1/day.
	*Images per battery charge are calculated based on a 3Ah 12V battery for comparability.
	References: 1 (Bushnell Outdoor Products, 2020), 2 (Harbortronics Inc., 2020), 3 (Newbery and
	Southwell, 2009), 4 (BlinkX, 2020), 5 (goPro, 2020), 6 (Canon, 2016), 7 (Canon, 2018), 8 (Bushnell
	Outdoor Products, 2017) 9 (Panasonic Corporation, 2012).
	System Type	Camera Trap	Action	DSLR camera		Mirrorless
			Camera			camera
	Name	Bushnell	GoPro HERO8	Cyclapse	Newbery &	micrObs
		Impulse Trail	+ CamDo		Southwell
			BlinkX		2009
	Price	265 € 1	692 € 4,5	3372 € 2	N . A .	957 €
					(~1000 €) 3
	Electrical	Yes 8	Y e s 5	N o	N o 3	Y e s 9
	Shutter
	Mirror	No 8	N o 5	Y e s 7	Y e s 3	N o 9
	Camera	integrated 8	GoPro Hero 8	Canon EOS-	Canon EOS-	Panasonic
			5	1500D / Rebel	1300D / Rebel	Lumix G5
				T7 2	T6 3,6
	Other	No 8	GoPro Hero 5	Yes 2	Y e s 3	Y e s
	Cameras		to 8 4
	Possible
	Image	8 MPix 8	8 M Pi x 5	24 MPix 7	6.3 MPix 6	15.9 MPix 9
	Resolution
	FOV (typical)	38° 8	68°-122° 5	23-63° 7	23-63° 3	23°-63°
	Other Lenses	No 8	N o 5	Y e s 7	Y e s 6	Y e s
	Possible
	Images per	5400 8	2890 4	2100-3500 2	~3700 3	6400
	Battery
	Charge*

Table 2 :

 2 Hardware design files.MainPCB.sch: Documentation file containing the circuit schematic for the custom electronic board.MainPCB.brd: The print circuit board layout file of the main board of the control unit. Most manufacturers accept this filetype, otherwise it can be opened with the free version of AutoCAD Eagle, to derive other file types (e.g. Gerber files). The print circuit board layout file. See MainPCb.sch. Most manufacturers accept this filetype, otherwise it can be opened with the free version of AutoCAD Eagle, to derive other file types (e.g. Gerber files).PanTiltMount.stp: A step file containing the drawing of the pan and tilt mount. It can be opened with many CAD programs like SolidEdge, which we used for creating the drawing.

	Design file name	File type	Open source license	Location of the file
	MainPCB.sch	EAGLE Circuit file	CC-BY 4.0	10.5281/zenodo.3756
				718
	MainPCB.brd	EAGLE Board file	CC-BY 4.0	10.5281/zenodo.3756
				718
	FrontPCB.sch	EAGLE Circuit file	CC-BY 4.0	10.5281/zenodo.3756
				718
	FrontPCB.brid	EAGLE Board file	CC-BY 4.0	10.5281/zenodo.3756
				718
	PanTiltMount.stp	step file with CAD	CC-BY 4.0	10.5281/zenodo.3756
		drawing		718

Table 3 :

 3 Software files. .cpp: Get and set micrObs inputs and outputs. Handling of analog averaging and multiplexer. microbs_IO.h: Settings and definitions of inputs and outputs. microbs_com_v0.cpp: Communication with GPS, weather station and control box memory card. microbs_com_v0.h: Settings and definitions of communication modules. microbs_display_v0.cpp: Front panel routines for display and LEDs. Debug strings for display. Set/get methods for system settings (interval, light sensitivity, GPS/weather station activity). microbs_displa_v0.h: Definitions for front panel routines. microbs_time.cpp: Timing routines: interval timer, syncing with GPS, time display, reset functions.

Software.ino: Arduino Project including main routine for micrObs. Project management. microbs_IOmicrobs_time.h: Definitions of timing routines.

Software Libraries: Contains Open Source Libraries like Adafruit_LED_Backpack, Low-Power, SdFat.

Table 4 :

 4 Short bill of materials.

				Cost per unit	Total cost		Materia
	Designator	Component	Number	-currency	-currency	Source of Material	l type
	Camera	Panasonic	1	99 €	99 €	mpb.com	other
		Lumix G5
	Lens	Panasonic	1	64 €	64 €	mpb.com	other
		Lumix G Vario
		14-42mm
		Vario f/3.5-5.8
		ASPH Mega OIS
	housing	Pelicase 1150	1	42 €	42 €	pelishop.com	other
	Battery	Lifeline GPL-	1	170 €	170 €	akkuangebote.de other
		U1T
	Charge	Steca Solsum	1	20.99 €	20.99 €	conrad.de	other
	Regulator	0606

  Acryfix, Evonic). Drill the M2 threads in the sides and the 6 mm countersunk holes in the top and bottom piece of the control box housing. (See chapter 4.1 in Supplementary File 2.)

	acrylic glass (Produce the Main PCB and Frontpanel PCB (printed circuit board) from the design files MainPCB.brd
	and FrontPCB.brd. Solder the electronic components (D1, IC1, Q1-Q9, R1-R23, REED, TSR-1-2450, TSR-
	1-2490, U1, X1, X2, Ca1, Ca2) to the main board. (See chapter 3 in Supplementary File 2.)
	Mount the main board into the control box housing. Assemble the connectors for the weather station
	and GPS, mount them to the control box, and connect them to the main PCB. Assemble the front panel
	from the control box housing top acrylic glass piece (C7, C8 x2), the front panel circuit board, buttons
	(B1, B2, B3) , hexadecimal encoder (H), potentiometer (M), knobs (K1,K2), display (D) and LEDs (L1-
	L7). Assemble the connector ribbon cable (MM) with the connectors (P4, P5). Connect front panel and
	main PCB. Put the SD card reader in the socket on the main board. (See chapter 4.2 in Supplementary
	File 2.)
	(See chapter 2 in Supplementary File
	2.)

Table 5 :

 5 Runtime and image memory usage for different recording intervals.

	Image	# Images	Runtime	Camera memory usage
	interval		(30Ah 12V battery)
	2s	64k	35h	319 GB
	5s	64k	88h	319 GB
	10s	64k	7 days	319 GB
	30s	56k	19 days	277 GB
	1min	46k	32 days	232 GB
	5min	20k	69 days	100 GB
	10min	12k	81 days	59 GB
	30min	4421	92 days	22 GB
	1h	2284	95 days	11 GB
	6h	391	97 days	2 GB
	24h	98	98 days	0.5 GB

Table 6 :

 6 Operation statistics since 2013.

	Location	Species	# Images	Year
	Crozet	King penguin	126 k	2013
	Pointe Géologie	Emperor penguin	1,957 k	2014
	Kerguelen	King penguin	130 k	2014
	Pointe Géologie	Emperor penguin	968 k	2015
	Pointe Géologie	Adélie penguin	830 k	2015
	Pointe Géologie	Emperor penguin	519 k	2016
	Pointe Géologie	Adélie penguin	671 k	2016
	Crozet	King penguin	181 k	2016
	Pointe Géologie	Emperor penguin	402 k	2017
	Crozet	King penguin	269 k	2018
	Total		6,053 k	6 years
	7.2

.1 Path finding in foraging Adélie penguins During

  the austral summer, approximately 47,000 Adélie penguin pairs (Pygoscelis adeliae) breed at Pointe Géologie close to the French Antarctic research station of Dumont d'Urville (Adélie Land, Antarctica)

Acknowledgements

This study was funded by the Deutsche Forschungsgemeinschaft (DFG) grants FA336/5-1 and ZI1525/3-1 in the framework of the priority program "Antarctic research with comparative investigations in Arctic ice areas", by The Penzance Endowed Fund and The Grayce B. Kerr Fund in Support of Assistant Scientists. This study was also supported by the Institut Polaire Francais Paul-Emile Victor (IPEV) within the framework of the Programmes 137-ANTAVIA (data collection at Crozet and Terre Adélie) and 354-ETHOTAAF (Kerguelen data collection), by the Centre Scientifique de Monaco (CSM) with additional support from the LIA-647 and RTPI-NUTRESS (CSM/CNRS-UNISTRA), and by the Centre National de la Recherche Scientifique (CNRS) through the Programme Zone Atelier de Recherches sur l'Environnement Antarctique et Subantarctique (ZATA). We thank the IPEV logistics team, the overwinterers at Dumont d'Urville, at Crozet Island, and at Kerguelen Islands for their support. We thank Michael Hoffman for building micrObs control units and reviewing our camera customization instruction.