J Neukampf
email: julia.neukampf@erdw.ethz.ch

B S Ellis

O Laurent

L K Steinmann

T Ubide

M Oeser

T Magna

S Weyer

O Bachmann

Time scales of syneruptive volatile loss in silicic magmas quantified by Li isotopes

Most explosive, silicic volcanoes spend thousands of years in repose between eruptive events. The timing of the switch from repose to eruption is key to interpreting monitoring signals and improving the safety of people living close to active volcanoes.

We addressed this question using a novel technique based on lithium isotopic (δ 7 Li) and elemental concentration profiles within plagioclase crystals from the Mesa Falls Tuff of the Yellowstone volcanic system (California, USA), constraining volatile degassing to occur on minimum time scales of tens of minutes prior to eruption. During this ephemeral time, Li abundances drop by a factor of four to 10 from crystal cores to rims, accompanied by an increase in δ 7 Li of as much as 10‰, reflecting diffusion-driven

INTRODUCTION

A key aim of volcanology is to improve the forecasting of volcanic eruptions in order to safeguard millions of people who live in volcanically prolific regions.

Understanding magma dynamics, particularly the time scales of bubble nucleation and magma degassing, which control eruptive explosivity (e.g., [START_REF] Sparks | Physical aspects of magmatic degassing: I. Experimental and theoretical constraints on vesiculation[END_REF][START_REF] Mangan | Delayed, disequilibrium degassing in rhyolite magma: Decompression experiments and implications for explosive volcanism[END_REF], can help minimize this hazard. The study of volcanic deposits remains fundamental to understanding and quantifying the processes occurring immediately prior to eruption [START_REF] Humphreys | Magma evolution and opensystem processes at Shiveluch Volcano: Insights from phenocryst zoning[END_REF][START_REF] Cashman | Vertically extensive and unstable magmatic systems: A unified view of igneous processes: Science[END_REF]. This requires the ability to define and characterize the factors that lead to changes in chemical (major and trace elements, volatile contents) [START_REF] Shea | Cracking the olivine zoning code: Distinguishing between crystal growth and diffusion[END_REF] and isotopic compositions in melt and associated crystals [START_REF] Oeser | Processes and time scales of magmatic evolution as revealed by Fe-Mg chemical and isotopic zoning in natural olivines[END_REF]. In particular, crystals in rapidly quenched, nonwelded deposits can record magmatic histories from storage in the magma chamber to eruption and provide information about changes in volatile concentration, magma composition, temperature, and pressure before and during eruptions (e.g., [START_REF] Sparks | The dynamics of bubble formation and growth in magmas[END_REF][START_REF] Ellis | Post-eruptive mobility of lithium in volcanic rocks[END_REF]. The Yellowstone volcanic system (California, USA) represents the archetypal supervolcano. Silicic volcanism at Yellowstone has predominantly produced rhyolitic lavas punctuated by three voluminous caldera-forming eruptions (Huckleberry Ridge Tuff, Mesa Falls Tuff, and Lava Creek Tuff) [START_REF] Christiansen | The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana[END_REF]. The 1.3 Ma Mesa Falls Tuff (MFT; Fig. 1) represents a large-volume explosive event that produced a Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 3 of 18 variably welded ignimbrite sheet overlying fallout deposits [START_REF] Rivera | Zircon petrochronology and 40 Ar/ 39 Ar sanidine dates for the Mesa Falls Tuff: Crystal-scale records of magmatic evolution and the short lifespan of a large Yellowstone magma chamber[END_REF]. While the MFT is well characterized in many respects, the final processes that tipped the system from stability to eruption remain unknown.

The groundmass glass from the MFT is depleted in lithium (Li) (36-52 ppm) relative to quartz-hosted melt inclusions (70-397 ppm) [START_REF] Neukampf | Partitioning and isotopic fractionation of lithium in mineral phases of hot, dry rhyolites: The case of the Mesa Falls Tuff[END_REF]. We propose that Li was lost with a H 2 O-rich fluid upon syneruptive degassing. Although the fluid-melt partitioning behavior of Li is debated, natural fluid-melt inclusion pairs consistently show that Li tends to partition in favor of a H 2 O-and Cl-rich fluid phase, thereby depleting the melt in Li [START_REF] Zajacz | Determination of fluid/melt partition coefficients by LA-ICPMS analysis of co-existing fluid and silicate melt inclusions: Controls on element partitioning[END_REF][START_REF] Fiedrich | Melt and fluid evolution in an upper-crustal magma reservoir, preserved by inclusions in juvenile clasts from the Kos Plateau Tuff, Aegean Arc, Greece[END_REF], as supported by some experimental results [START_REF] Webster | Partitioning of lithophile trace elements between H 2 O and H 2 O + CO 2 fluids and topaz rhyolite melts[END_REF].

During bubble formation, k Kinetic isotopic fractionation has been proposed to occur between the faster-diffusing 6 Li and the slower-diffusing 7 Li during bubble formation, enriching the vapor in 6 Li and, conversely, the melt in 7 Li [START_REF] Vlastélic | Lithium isotope fractionation during magma degassing: Constraints from silicic differentiates and natural gas condensates from Piton de la Fournaise volcano Publisher[END_REF]. These processes, as shown experimentally [START_REF] Holycross | Diffusive fractionation of Li isotopes in wet, silicic melts[END_REF] and theoretically [START_REF] Watson | Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas[END_REF], can produce significant Li elemental and isotopic variations between groundmass glass and mineral phases as the system strives to reestablish equilibrium. The extent to which diffusion can homogenize these records reflects the time between onset and termination of diffusion upon quenching. Here, we use a novel technique combining Li concentration and Li stable-isotope ratios [START_REF] Steinmann | Multi-stage magma evolution in intra-plate volcanoes: Insights from combined in situ Li and Mg-Fe chemical and isotopic diffusion profiles in olivine[END_REF] in plagioclase phenocrysts from the MFT and applying diffusion modeling to constrain time scales of Li loss from the melt.

LITHIUM CONCENTRATION MAPS AND PROFILES

Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 4 of 18 Seven plagioclase crystals from the nonwelded portion of the MFT were geochemically characterized by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Only plagioclase crystals rimmed by groundmass glass were used, to ensure that crystal rim measurements truly reflect plagioclase in contact with the surrounding melt. Lithium concentration maps obtained by LA-ICPMS (Figs. 2A and2B) reveal homogeneous crystal interiors with high Li concentrations mantled by a narrow zone (≤200 µm) where Li contents continuously decrease toward crystal edges (Figs. 2C and2D).

Concentration profiles through crystals confirm high-Li cores, from 12 to 33 ppm depending on the crystal (mean 22 ppm; n = 1440), decreasing to 19 to ~5 ppm at the very edge of the crystals (mean 12 ppm; n = 208). The anorthite content (An) varies between crystals (An 17-31 ) but is homogenous within grains, and all other major and trace elements lack any strong, systematic core-to-rim variation (Fig. 2C and2D). Some crystals exhibit similar Li depletion as in rims along cracks within the crystal interior similar to that in rims. These cracks likely formed due to depressurization during eruptive ascent allowing melt inclusions to expand and fracture their host plagioclase (Fig. 2B). Interestingly, concentration profiles perpendicular to magmatic cracks return steeper depletion profiles (Fig. 2D) compared to measurements at crystal rims. This supports the interpretation that the cracks developed at a later stage during ascent, leading to shorter Li diffusion times.

IN SITU δ 7 Li MEASUREMENTS

Lithium isotopic composition (δ 7 Li) was measured in situ using a femtosecond The observed Li loss toward crystal edges and magmatic cracks requires charge balancing by other elements. Hydrogen is unsuitable, as it would behave similarly to Li.

The charge balance can occur by an exchange reaction of Li + + Al 3+ → Si 4+ and/or oxidation of Fe 2+ to Fe 3+ during eruption. Although none of these elements correlates with the observed edgeward decrease in Li, their higher abundance relative to the small concentration change (~20 ppm) observed in Li challenges the identification of the charge-balance mechanism. We favor the oxidation of Fe 2+ to Fe 3+ because it is independent from elemental diffusion.

The variation in Li abundance cannot result from simple crystal-liquid equilibrium partitioning during crystal growth because Li is moderately incompatible in plagioclase (K d [[Define]] = 0.1-0.7; [START_REF] Neukampf | Partitioning and isotopic fractionation of lithium in mineral phases of hot, dry rhyolites: The case of the Mesa Falls Tuff[END_REF] and other mineral phases present in the MFT [START_REF] Tomascak | Advances in Lithium Isotope Geochemistry[END_REF][START_REF] Neukampf | Partitioning and isotopic fractionation of lithium in mineral phases of hot, dry rhyolites: The case of the Mesa Falls Tuff[END_REF]. Hence, continued crystal growth would enrich Li in the melt, thereby increasing the Li concentration toward the plagioclase rim assuming that Li is not lost into the volatile phase. This process would result in isotopically lighter rims because the resulting concentration Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 6 of 18 gradient would foster 6 Li diffusion into the crystal as observed for magmatic olivine and clinopyroxene [START_REF] Jeffcoate | Li isotope fractionation in peridotites and mafic melts[END_REF]. Magma recharge with a Li-poor, 7 Li-enriched melt can be excluded because no other element correlates with the decrease in Li toward plagioclase rims.

The decreased Li concentration and elevated δ 7 Li toward crystal edges can be explained by scavenging of Li from the melt into the vapor phase during magma ascent and degassing (e.g., [START_REF] Genareau | New insight into explosive volcanic eruptions: Connecting crystal-scale chemical changes with conduit-scale dynamics[END_REF][START_REF] Giuffrida | Ultrafast syn-eruptive degassing and ascent trigger high-energy basic eruptions[END_REF][START_REF] Steinmann | Multi-stage magma evolution in intra-plate volcanoes: Insights from combined in situ Li and Mg-Fe chemical and isotopic diffusion profiles in olivine[END_REF]. 6 Li preferentially moves into the vapor phase during vesiculation [START_REF] Watson | Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas[END_REF], resulting in the depletion of 6 Li along with Li concentrations in the melt, inducing a disequilibrium between plagioclase and melt (Fig. 3). This disequilibrium causes diffusion of Li-preferentially 6 Li because it diffuses faster than 7 Li-from the rim of the plagioclase into the melt. This in turn leads to a disequilibrium within the plagioclase between the Li-depleted rim and its interior, driving Li diffusion toward the rims. Such a scenario results in the observed depletion of Li and enrichment of 7 Li in the plagioclase rims.

DIFFUSION MODELING OF LITHIUM CONCENTRATIONS AND δ 7 Li

Lithium diffusion profiles in plagioclase can be used to determine time spans of this disequilibrium process (Fig. 4). Estimating such time scales depends on the applied diffusion coefficient (D), requiring an accurate temperature determination. Here, we investigated a temperature range based on different mineral thermometers for the MFT [START_REF] Tollan | Assessing magmatic volatile equilibria through FTIR spectroscopy of unexposed melt inclusions and their host quartz: A new technique and application to the Mesa Falls Tuff[END_REF]. Diffusion coefficients are calculated using the Arrhenius parameters for Li that were experimentally determined for an anorthitic plagioclase [START_REF] Giletti | Alkali diffusion in plagioclase feldspar[END_REF]. The published 6 Li diffusion coefficient can be applied because Li Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 7 of 18 diffusivity in feldspar is compositionally independent [START_REF] Giletti | Alkali diffusion in plagioclase feldspar[END_REF]. The minimum applied temperature is 789 °C based on a sanidine-melt thermometer [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF] with a corresponding 𝐷𝐷 6 Li = 1.05 × 10 -11 m 2 /s. Lower temperatures are implausible because the melt was rapidly quenched upon eruption as supported by the absence of microlites. Moreover, the pre-eruptive water content of the MFT melt was ~3 wt% [START_REF] Tollan | Assessing magmatic volatile equilibria through FTIR spectroscopy of unexposed melt inclusions and their host quartz: A new technique and application to the Mesa Falls Tuff[END_REF]. At the pre-eruptive storage pressure of the MFT (<1.5 kbar; [START_REF] Befus | Magma storage and evolution of the most recent effusive and explosive eruptions from Yellowstone Caldera[END_REF], this implies minimum temperatures of 770-780 °C [START_REF] Holtz | Maximum and minimum water contents of granitic melts generated in the crust: A reevaluation and implications[END_REF]. The maximum temperature is 856 °C, which is determined by a magnetiteilmenite thermometer [START_REF] Sauerzapf | The titanomagnetiteilmenite equilibrium: New experimental data and thermo-oxybarometric application to the crystallization of basic to intermediate rocks[END_REF], with a corresponding 𝐷𝐷 6 Li = 2.79 × 10 -11 m 2 /s. Both calculated diffusion coefficients are applied to the concentration profiles.

To calculate time scales for the isotope profiles, 6 Li and 7 Li are considered as two independent species with different diffusion coefficients. Because no published diffusion coefficient is available for 7 Li in plagioclase [START_REF] Giletti | Alkali diffusion in plagioclase feldspar[END_REF], we recalculated them[[the diffusion coefficients? Or the time scales?]] based on those used for 6 Li and considering that 6 Li moves 3.5% faster than 7 Li in melt [START_REF] Holycross | Diffusive fractionation of Li isotopes in wet, silicic melts[END_REF]. This calculation yields 𝐷𝐷 7 Li = 1.01 × 10 -11 m 2 /s for 789 °C and 𝐷𝐷 7 Li = 2.69 × 10 -11 m 2 /s for 856 °C. We recognize that the relative diffusivity of the two isotopes in plagioclase could differ from that in melt and may furthermore be composition dependent, as observed for olivine (in which 6 Li moves 3% faster; [START_REF] Dohmen | Diffusion of Li in olivine; Part I: Experimental observations and a multi species diffusion model[END_REF] and clinopyroxene (in which 6 Li moves 4% faster; [START_REF] Richter | Lithium isotope fractionation by diffusion in minerals; Part 1: Pyroxenes[END_REF]. However, these differences in relative isotope diffusivities between phases are much smaller than that resulting from our inferred temperature range. The δ 7 Li fit is calculated as the ratio of individual Li isotope paths (Equation S1 in the Supplemental Material 1 ). Estimated time Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 8 of 18 scales are between 1-13 min at 789 °C and 0.3-4.5 min at 856 °C for concentration profiles, and 0.5-15 min at 789 °C and 0.2-6 min at 856 °C for δ 7 Li profiles. The results for the diffusion modeling of concentration and isotopic profiles are thus mutually consistent.

Figure 4C shows that for some profiles, a slight mismatch exists between time scales derived from concentration profiles and isotope profiles. This can be a result of different errors that are introduced into the model, e.g., different spatial resolution and analytical errors on the measurements and/or inaccurate diffusion coefficients. To better estimate errors on the time scales, the model boundary conditions (Tables S1-S3 in the Supplemental Material) were set to match the highest and the lowest 7 Li/ 6 Li ratio within analytical uncertainty (Fig. 4B) and modeled for the best fit. The time difference between the best fit and the model using the highest and the lowest 7 Li/ 6 Li ratio is used as a more realistic uncertainty (Fig. 4C) on the time scales derived from the diffusion models.

Crystal size has no influence on the estimated time scales, but crystal geometry and magmatic cracks can have a non-negligible impact. There might be a diffusional anisotropy of Li isotopes along different crystallographic directions, as reported for olivine [START_REF] Dohmen | Diffusion of Li in olivine; Part I: Experimental observations and a multi species diffusion model[END_REF]. [START_REF] Audétat | Copper and Li diffusion in plagioclase, pyroxenes, olivine and apatite, and consequences for the composition of melt Publisher[END_REF] recognized that Li has two, slowversus fast-diffusing pathways in plagioclase. If the used diffusion coefficients represent the latter, the resulting time scales are minima. However, even considering that Li diffusion could be two orders of magnitude slower, the maximum time scales would increase to only a few hours. Our results agree well with decompression time scales from a study of the Huckleberry Ridge Tuff [START_REF] Myers | Prolonged ascent and episodic venting of discrete magma batches at the onset of the Huckleberry Ridge supereruption[END_REF], which yields durations of <12 h to 5 days. A recent study of Li concentration profiles in plagioclase from Mount Etna Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 9 of 18 (Italy; [START_REF] Giuffrida | Ultrafast syn-eruptive degassing and ascent trigger high-energy basic eruptions[END_REF] predicts time scales of ascent and degassing within minutes.

Other studies have also recognized Li gradients in plagioclase (e.g., [START_REF] Kent | Vapor transfer prior to the October 2004 eruption of[END_REF][START_REF] Charlier | Lithium concentration gradients in feldspar and quartz record the final minutes of magma ascent in an explosive supereruption[END_REF], but these investigations have suggested magmatic processes other than degassing to be responsible for formation of Li gradients.

With particular attention to the suitability of individual crystals, e.g., identifying the rims that were in contact with melt, crystal geometry, and three-dimensional effects, the new combination of Li concentrations and in situ δ 7 Li measurements in phenocrysts represents a powerful tool and affords new opportunities for understanding time scales of late magmatic processes. Our results show that at minimum a few tens of minutes elapse between the onset of the eruptive volatile degassing of the magma and quenching. The possibility of such rapid shifts from quiescence to explosivity underscores the need for the improved understanding of the dynamics of magmatic systems and the causes for the formation of a magmatic volatile phase. This study strengthens the role of petrology in assisting in the key observations in monitoring potentially explosive volcanoes in real time. [[In the figure, insert a space between "Li" and "(‰)" at the upper left corner; replace hyphen with a space before each instance of "fit". In panel B, insert a space between "Li" and "(‰)". In panel C, reword axis descriptions to place units of measure (min) in parentheses-e.g., "Time from δ 7 Li (min)". In panels D and E, italicize instances of "n"; make instances of "prob." lowercase; insert a space between each temperature value and "°C"]]

FIGURE CAPTIONS

  laser ablation system coupled to a multicollector ICPMS (fs-LA-MC-ICPMS) along Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G47764.1 Page 5 of 18 transects following crystal zoning patterns. The δ 7 Li profiles exhibit large but systematic differences (of as much as 10‰; Fig.2E), with Li-poor plagioclase rims isotopically heavier (-3.8‰ to +5.9‰; mean +1.0‰ ± 2.5‰; two standard errors [2 SE], n = 19) than the Li-rich cores (-11‰ to +2‰; mean -4.2‰ ± 2‰; 2 SE, n = 85). Measurements associated with magmatic cracks mimic those from crystal rims (-4.9‰ to +5.4‰; mean +1.2‰ ± 2.6‰; [[2 SE? Alternatively, state at the definition of "2 SE" above that all error values throughout the paper represent 2 SE]] n = 13), resulting in a broad negative correlation between Li concentrations and δ 7 Li across all crystals (Fig.2F).

Figure 1 .Figure 2 .Figure 3 .Figure 4 .

 1234 Figure 1. Map showing sample location (44°7′8.49″N, 111°26′39.49″W; Ellis et al.,

Supplemental Material. Supplemental material and analytical data. Please visit https://doi.org/10.1130/XXXXX to access the supplemental material, and contact editing@geosociety.org with any questions.

ACKNOWLEDGMENTS

We thank R. Dohmen for help with, and discussions on, the diffusion modeling. This work was supported by the Swiss National Science Foundation (grant 200021_166281 to Ellis).[[Would you like to acknowledge this paper's reviewers, perhaps by name?]]


Guillaume Schweicher

Guangfeng Liu
email: gliu@ulb.ac.be

Pierre Fastré

Roland Resel

Mamatimin Abbas

Guillaume Wantz

Yves Henri Geerts

Directional crystallization of C8-BTBT-C8 thin films in a temperature gradient

come   L'archive ouverte pluridisciplinaire

Introduction

In the last three decades, thin-film of organic semiconductors (OSCs) have attracted considerable attention, because they are key components for constructing the next-generation lightweight, stretchable and wearable optoelectronic organic devices. [1][START_REF] Bao | Organic Field-Effect Transistors[END_REF][START_REF] Uoyama | Highly efficient organic light-emitting diodes from delayed fluorescence[END_REF][START_REF] Schweicher | Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?[END_REF][START_REF] Zaumseil | Electron and Ambipolar Transport in Organic Field-Effect Transistors[END_REF][START_REF] Skabara | Close Encounters of the 3D Kind -Exploiting High Dimensionality in Molecular Semiconductors[END_REF][START_REF] Sergeyev | Discotic liquid crystals: a new generation of organic semiconductors[END_REF] The control of thickness, roughness, and crystalline morphology at different length-scales and over large area are crucial for the performances of OSCs. [START_REF] Rivnay | Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale[END_REF][START_REF] Salleo | Microstructural Characterization and Charge Transport in Thin Films of Conjugated Polymers[END_REF][START_REF] Delongchamp | Molecular Characterization of Organic Electronic Films[END_REF] Crystalline morphology is defined here as comprising crystal shape, crystal size, crystal orientation, crystal alignment, less ordered regions, defects, polymorphs, and grain boundaries. The fabrication of high-quality, large-scale (> 1 cm 2 ) single crystalline thin-film of OSCs remains challenging. [START_REF] Diao | Morphology control strategies for solution-processed organic semiconductor thin films[END_REF][START_REF] Li | Toward high-mobility organic field-effect transistors: Control of molecular packing and large-area fabrication of single-crystal-based devices[END_REF][START_REF] Virkar | Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors[END_REF][START_REF] Liu | Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications[END_REF] Thin films of OSCs are fabricated either by vapor deposition, solution processing, or from melt, each method bearing its own advantages and disadvantages. Vapor deposition, which is only applicable to small molecules, allows the reproducible obtaining of thin films with controlled thickness and roughness. The orientation of the molecules versus a substrate, which is governed by the anisotropy of surface tension of the different crystallographic faces, is generally spontaneously achieved. Grain size and grain boundary width are controllable by pressure, deposition rate and temperature conditions. However, the alignment of crystals along a specific in plane direction generally requires preliminary substrate patterning. [START_REF] Virkar | Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors[END_REF][START_REF] Wen | Experimental Techniques for the Fabrication and Characterization of Organic Thin Films for Field-Effect Transistors[END_REF][START_REF] Park | Crystallization-Induced Properties from Morphology-Controlled Organic Crystals[END_REF] A plethora of solution deposition techniques, such as for example drop-casting, spin-coating, inkjet printing, dip coating, blade coating, zone casting, and edge casting, have been developed for both small molecules and polymers. [START_REF] Diao | Morphology control strategies for solution-processed organic semiconductor thin films[END_REF][START_REF] Park | Crystallization-Induced Properties from Morphology-Controlled Organic Crystals[END_REF] They evidently require a sufficient solubility in solvents. Drop-casting allows nearly no control of either thickness, roughness, or morphology. Spin-coating is a widely applicable method for obtaining films of tailored thickness and roughness, but with poor control of crystalline morphology. Inkjet printing has the advantage of easy patterning of thin films on the micron scale. [START_REF] Minemawari | Inkjet printing of singlecrystal films[END_REF] Dip coating, blade coating, zone casting, and edge casting are directional methods with good control of thickness, roughness, and crystalline morphology, including some alignment. [START_REF] Liu | Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications[END_REF][START_REF] Wen | Experimental Techniques for the Fabrication and Characterization of Organic Thin Films for Field-Effect Transistors[END_REF][START_REF] Pisula | A Zone-Casting Technique for Device Fabrication of Field-Effect Transistors Based on Discotic Hexa-peri-hexabenzocoronene[END_REF] Over the years, shearing has emerged as one of the best technique for thin film fabrication. [START_REF] Giri | Tuning charge transport in solutionsheared organic semiconductors using lattice strain[END_REF][START_REF] James | Controlling Microstructure of Pentacene Derivatives by Solution Processing: Impact of Structural Anisotropy on Optoelectronic Properties[END_REF][START_REF] Diao | Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains[END_REF] Thin rather uniform aligned films are achievable and even some new polymorphs have been obtained. [START_REF] Giri | Tuning charge transport in solutionsheared organic semiconductors using lattice strain[END_REF][START_REF] Diao | Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains[END_REF] Through optimization of solvent evaporation conditions, wafer scale single crystals as thin as one molecular layer have been obtained by the shearing technique. [START_REF] Yamamura | Wafer-scale, layer-controlled organic single crystals for highspeed circuit operation[END_REF] A less visited approach is the production of crystalline thin films from the melt. [START_REF] Maunoury | Melt-Processing of Conjugated Liquid Crystals: A Simple Route to Fabricate OFETs[END_REF][START_REF] Ribierre | A solvent-free and vacuum-free melt-processing method to fabricate organic semiconducting layers with large crystal size for organic electronic applications[END_REF][START_REF] Kim | Solvent-Free Directed Patterning of a Highly Ordered Liquid Crystalline Organic Semiconductor via Template-Assisted Self-Assembly for Organic Transistors[END_REF][START_REF] Fujieda | Directional solidification of C8-BTBT films induced by temperature gradients and its application for transistors[END_REF] In the past few years, our group has investigated the directional crystallization of organic solids starting from their melt by using a temperature gradient (∇T) (Figure 1). [START_REF] Schweicher | Toward Single Crystal Thin Films of Terthiophene by Directional Crystallization Using a Thermal Gradient[END_REF][START_REF] Chattopadhyay | Accessing Phase-Pure and Stable Acetaminophen Polymorphs by Thermal Gradient Crystallization[END_REF][START_REF] Resel | Polymorphism of terthiophene with surface confinement[END_REF][START_REF] Panini | Crystal Growth Alignment of β-Polymorph of Resorcinol in Thermal Gradient[END_REF] This technique, also known as directional crystallization or solidification, is essentially the adaptation of the method of Percy Bridgman to thin films of molecular compounds using the set-up invented by Kenneth Jackson. [START_REF] Jackson | Kinetic Processes: Crystal Growth, Diffusion, and Phase Transitions in Materials[END_REF] Although this technique has commonly been used for the study of organic compounds such as for example n-alkanes and mnitroaniline, [START_REF] Asano | Melt-crystallization of n-alkanes and polyethylene in a temperature gradient[END_REF][START_REF] Asano | Melt-crystallization of n-alkanes and polyethylene in a temperature gradient[END_REF][START_REF] Asano | Crystallization of a low molecular weight polyethylene and paraffins under a temperature gradient[END_REF][START_REF] Stevenson | Growth and characterisation of single crystal optical fibre waveguides -meta-nitroaniline[END_REF][START_REF] Murayama | Control of crystal orientation in an organic-crystal-cored fibre[END_REF] it has not widely been studied to control the crystalline morphology of OSC thin films. This technique presents several advantages: i) nucleation and growth processes are decoupled, ii) growth occurs preferentially through some selected crystallographic faces, iii) uniaxial alignment takes place, iv) the number of molecules entering the crystal phase per unit time and through some crystallographic faces is externally controllable. [START_REF] Schweicher | Toward Single Crystal Thin Films of Terthiophene by Directional Crystallization Using a Thermal Gradient[END_REF][START_REF] Chattopadhyay | Accessing Phase-Pure and Stable Acetaminophen Polymorphs by Thermal Gradient Crystallization[END_REF][START_REF] Resel | Polymorphism of terthiophene with surface confinement[END_REF][START_REF] Panini | Crystal Growth Alignment of β-Polymorph of Resorcinol in Thermal Gradient[END_REF] Up to now, the temperature gradient method has been applied to rather thick films (µm thick) sandwiched between two glass slides to avoid the unwanted dewetting of the melt state. [START_REF] Tsui | Polymer Thin Films[END_REF] As a consequence, film thickness could hardly be controlled unless for OSCs exhibiting a smectic phase before the isotropization temperature (Tiso). Indeed, smectic phases are less prone to dewetting and tend to spread on substrates, allowing direct processing in a thermal gradient from a non-sandwiched thin film as we will describe further now. [START_REF] Dohr | Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films[END_REF][START_REF] Iino | Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films[END_REF] Firstly, a thin film of arbitrary thickness is conventionally fabricated by spin-coating. Then, the gradient method exploiting the crystal to smectic transition is used as a post-deposition processing method to tailor crystalline morphology. 2,7-dioctyl [1]benzothieno[3,2b] [1]benzothiophene, abbreviated C8-BTBT-C8, which is a largely studied OSC, forms a smectic A (SmA) phase from 110 to 126°C and qualifies as adequate molecular system for such study. [START_REF] Fujieda | Directional solidification of C8-BTBT films induced by temperature gradients and its application for transistors[END_REF][START_REF] Ebata | Highly Soluble [1]Benzothieno[3,2b]benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors[END_REF][START_REF] Izawa | Molecular Ordering of High-Performance Soluble Molecular Semiconductors and Re-evaluation of Their Field-Effect Transistor Characteristics[END_REF][START_REF] Grigoriadis | Viscoelastic, and Dielectric Properties of Symmetric and Asymmetric Alkyl[END_REF] The goal of this work is to investigate the crystalline morphology of thin films of C8-BTBT-C8 as a function of the directional crystallization conditions. Materials. C8-BTBT-C8 and 2,7didodecyl [1]benzothieno [3,2-b][1]benzothiophene, abbreviated C12-BTBT-C12, have been synthesized according to literature. [START_REF] Ebata | Highly Soluble [1]Benzothieno[3,2b]benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors[END_REF] Before using, these two compounds were purified by sublimation in a tube furnace. HPLC-grade (>99.8%, Sigma-Aldrich) toluene solvent was used without further purification. 15 mm × 10 mm × 0.7 mm sized glass substrates (D 263 Borosilicate thin glass from Präzisions Glas & Optik GmbH, Germany) were used for the experiments. They were sonicated for 10 minutes in deionized water and acetone, then blown dry with nitrogen and exposed 30 minutes to an ultraviolet-ozone (Procleaner plus; Bioforce Nanosciences) treatment before the experiments.

2.

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC was performed at 10 ℃.min -1 under a dry nitrogen gas atmosphere in a Perkin Elmer Diamond 6 instrument (temperature range 30-150 ℃). TGA measurement was performed in a Perkin Elmer Pyris 6 TGA. Powder sample underwent a linear ramp of increasing temperature up to 600 ℃ at a rate of 10 ℃.min -1 in an inert atmosphere of nitrogen.

Polarized Optical Microscopy (POM).

The ∇T hot stage is mounted on a POM (Nikon Eclipse 80i) allowing in situ observations. The pictures of the thin films were recorded before and after the thermal gradient treatment by a Carl Zeiss AXiO Imager. A1 POM. 4. Temperature gradient apparatus. The set-up shown in Figure 1, consists of a Linkam GS350 system presenting two individual heating stages separated by a gap. The distance (gap) between the two stages where the thermal gradient will be generated is of 2 mm. During the ∇T experiments, the hot stage is set at a temperature Th, situated between the isotropization temperature of the material, C8-BTBT-C8 (Tiso), and the LC to crystal transformation temperature (TLC-Cr); while the cold stage is maintained at a temperature Tc, below TLC-Cr of C8-BTBT-C8. Tiso and TLC-Cr temperature values were evaluated by DSC. The system is covered by a hermetic lid so that the system is thermally independent from the laboratory environment. 5. C8-BTBT-C8 thin films fabrication by the ∇T method. C8-BTBT-C8 thin films were initially prepared by spin-coating (Specialty Coating Systems Model P6700 Series spin-coater) with different solution concentrations (2.3 mg.ml -1 , 5.4 mg.ml -1 , 10.5 mg.ml -1 , 13.5 mg.ml -1 , and 20 mg.ml -1 ; solvent: toluene), on pre-cleaned 15 mm × 10 mm × 0.7 mm thin glass substrates. The spin-coating parameters are as following: 150 μL solution, 150 RPM (5s, the first step), 1500 RPM (5s, the second step), and 3000 RPM (30s, final step). The thickness of thin films was measured by X-ray reflectivity (XRR). Thin films of C8-BTBT-C8 were then processed in the ∇T. The ∇T treatments for ten spin-coated samples, produced using a 20 mg.ml -1 concentration, were carried out using two Th -Tc couples: 1) 120 ℃ -90 ℃ and 2) 120 ℃ -60 ℃, and five different pulling rates (vp: 1 μm.s -1 , 5 μm.s -1 , 10 μm.s -1 , 25 μm.s -1 and 50 μm.s -1 ) respectively, see Table 1 for full conditions.

X-ray measurements. X-ray reflectometry (XRR):

XRR curves of the C8-BTBT-C8 thin films were collected on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å) at room temperature. 2θ ranges between 0.4° and 3.8° with an angular resolution of 0.005° per step and a typical counting time

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Please do not adjust margins of 5s, using a θ/θ (specular) reflection geometry. XRR curves are presented as the scattering intensity versus the scatter vector, qz (Å -1 ), using Equation 1: q = 4π × sinθ/λ, where λ = 1.5418 Å. The thickness of the thin films can be evaluated by the angular separation between two maxima (constructive interference) or minima (destructive interference) of consecutive fringes using Equation 2: n = 2π/(q1-q2), where n is the thickness of thin film; qi is the momentum transfer of an extremum. The XRR curves show a total external reflection plateau up to qz = 0.34 nm -1 (on glass substrates) providing information on the critical angle. The value of the total reflection critical angle is similar to the one achieved in our previous XRR studies of spin-coated thin films of C8-BTBT-C8 on Si-wafers covered with a 150 nm thermally grown oxide layer (qz = 0.31 nm -1 ). [START_REF] Dohr | Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films[END_REF] The FWHM (full width at half the maximum of a diffraction peak) of the (002) Bragg peak (qz around 4.34 nm -1 ) was determined. The coherence length, LC (perpendicular to the substrate surface in this case, that is, the vertical crystallite size), was estimated using the Scherrer equation [START_REF] Warren | X-Ray Diffraction[END_REF] (Equation 3: LC = 2π/Δq, Δq is the FWHM). Specular X-ray diffraction (sXRD): sXRD pattern of C8-BTBT-C8 powder sample was collected on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å) at room temperature, in the 2θ range between 2° and 40° with an increment of 0.02° per step and a counting time of 0.5 s per step at room temperature. sXRD pattern of the LC phase was collected on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å) and equipped with a MRI (Material Research Instruments) heating stage for high temperature measurements, using similar parameters except a 2θ range between 2° and 30°. sXRD patterns for thin films were collected on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.5418 Å), in the 2θ range between 2° and 10° with an increment of 0.02° per step and a counting time of 0.15 s per step at room temperature. In-plane GIXD: in-plane grazing incidence XRD measurements were performed on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.5418 Å). An in-plane 2θχ/ϕ scan is a technique for measuring lattice planes normal to the surface of a thin film. The direction of the scatter vector is kept constant. A grazing-incident X-ray beam is employed in a 2θχ/ϕ scan inplane XRD measurement so that the incident X-ray beam travels a long distance inside the thin film, and thus high diffraction intensities from the film could be obtained. Meanwhile, the angle of the incident X-ray beam can be controlled independently on the sample rotation angle (ϕ) and the position of the detector (2θ χ ) in the geometry of a 2θχ/ϕ scan. 2θχ/ϕ scans for thin films were carried out in the 2θχ range between 5° and 50° with a sampling of 0.02° per step and a speed of 0.08° min -1 at room temperature. The beam incident angle αi has been kept at 0.18°. XRD pole figures: pole-figure measurement is an XRD technique where the diffraction angle (2θ) is fixed and the diffracted intensity is collected by varying the tilt angle from the sample surface normal direction (Ψ) and the rotation angle around the sample surface normal (φ). The obtained diffracted intensity data can be plotted as a function of the tilt angle and rotation angle to quantitatively evaluate the orientation texture of the sample. Pole figures for the different C8-BTBT-C8 thin film samples were measured on a PHILIPS X'Pert system equipped with an ATC3 cradle using Cr Kα radiation (λ = 2.291 Å) and a graphite secondary monochromator. The schematic representation of the pole-figure measurement geometry is shown in Figure S13. Pole figures were plotted using the software STEREOPOLE. [START_REF] Salzmann | software for the analysis of X-ray diffraction pole figures with IDL[END_REF] 7. Optical profilometer. The surface morphology study of the C8-BTBT-C8 thin films before and after the ∇T treatment was carried out using an optical profilometer (Veeco NT9080) by a phase shifting interferometry (PSI) mode. Each image was analysed by the software Vision 4.3 to get the 3D surface morphology and surface roughness statistics (Ra). Ra is the arithmetical mean deviation of the assessed profile.

Equation 4:

( , )

A

Ra z x y dxdy A = 

, where A is the area of the defined range, |z(x,y)| is the distance between a point (x,y) in the plane and the center plane.

Crystal morphology calculation.

The crystal morphology of C8-BTBT-C8 was predicted using the software Mercury 3.10.3 based on the Bravais-Friedel-Donnay-Harker (BFDH) method. [START_REF] Bravais | Etudes Cristallographiques[END_REF][START_REF] Friede | Crystal Morphologies[END_REF][START_REF] Donnay | A new law of crystal morphology extending the law of Bravais[END_REF] 9. Calibration of the magnitude of the temperature gradient setup (Gexp). Previous work has shown that the effective magnitude of temperature gradient (Gexp) that takes place between the hot and cold zones is less than the magnitude simply calculated by the equation as G = (Th -Tc)/x, where x = 2.0 mm is the gap between the hot and cold stages. [START_REF] Schweicher | Toward Single Crystal Thin Films of Terthiophene by Directional Crystallization Using a Thermal Gradient[END_REF][START_REF] Chattopadhyay | Accessing Phase-Pure and Stable Acetaminophen Polymorphs by Thermal Gradient Crystallization[END_REF][START_REF] Resel | Polymorphism of terthiophene with surface confinement[END_REF][START_REF] Panini | Crystal Growth Alignment of β-Polymorph of Resorcinol in Thermal Gradient[END_REF] The calibration of the ∇T setup has been carried out as previously reported [START_REF] Panini | Crystal Growth Alignment of β-Polymorph of Resorcinol in Thermal Gradient[END_REF] and is detailed in supporting information (Table S1 and Figure S1). G1exp ≈ 6 and G2exp ≈ 11°C.mm -1 for Th -Tc couples 120 ℃ -90 ℃ and 120 ℃ -60 ℃, respectively.

Results and discussion

Crystal structure and thermotropic behaviour of C8-BTBT-C8.

The crystal structure of C8-BTBT-C8, first reported by Takimiya et. al. from single crystal X-ray diffraction measurements performed at 293 K, belongs to a monoclinic system, with a space group P21/a and unit cell parameters a = 5.907 Å, b = 7.890 Å, c = 29.086 Å, and β = 91.94°. [START_REF] Ebata | Highly Soluble [1]Benzothieno[3,2b]benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors[END_REF] The crystal morphology of C8-BTBT-C8, shown in Figure 2a, presents a lamellar hexagonal shape with edge-on upstanding molecules on the main face (001). The thermal stability of C8-BTBT-C8 is higher than 300 ℃ (onset Temperature 334 ℃, shown in the TGA curve in Figure S2). The Cr-to-LC and LC-to-Cr phase transitions are reproducibly observed in the DSC curves (Figure 2b), showing a liquid crystal phase from 108.9 ℃ to 125 ℃ upon heating. The fan shaped features visible in the POM image at 124 ℃ (Figure 2c) combined to the only two diffraction peaks visible in the sXRD pattern recorded at 120 ℃ (Figure 2d) confirms the high temperature SmA phase of C8-BTBT-C8, in excellent agreement with previous data. [START_REF] Dohr | Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films[END_REF][START_REF] Ebata | Highly Soluble [1]Benzothieno[3,2b]benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors[END_REF] 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins 2. Crystallization of C8-BTBT-C8 thin films in ∇T using the LC to Cr transition.

Crystallization conditions.

First, thin films of C8-BTBT-C8 were prepared on borosilicate glass substrates by spin-coating from toluene solutions, using five different concentrations: 2.3 mg.ml -1 , 5.4 mg.m -1 , 10.5 mg.ml -1 , 13.5 mg.ml -1 and 20 mg.ml -1 . These films, numbered as samples 1-5, serve as comparison point with samples 6-19 processed in thermal gradient, see Table 1. Note that borosilicate glass exhibits a higher thermal conductivity, λ = 0.926 Wm -1 K -1 than the one of C8-BTBT-C8 along the a and b directions, λ ≈ 0.18 Wm -1 K -1 . 48 Moreover, C8-BTBT-C8 films are much thinner than glass slides. These two conditions ensure a rapid local thermalization of C8-BTBT-C8 films with their environment. Crystallization of C8-BTBT-C8 occurred in films 1-5 upon solvent evaporation during spin-coating with not much control (vide infra). Second, films of increasing thickness have been subjected to crystallization in the same ∇T conditions (films 6 to 10). Third, the thickest films, spin-coated at 20 mg.ml -1 , have been crystallized in various ∇T conditions, i.e. with 1 ≤ Vp ≤ 50 μm.s -1 and with using Gexp ≈ 6 and 11 °C.mm -1 , to explore a large set of experimental conditions (films 10 to 19). The cooling rate at the growth front has been varied by nearly two order of magnitudes, i.e. ranging from 0.36 to 33 °C.min -1 (Table 1).

Films of different thicknesses and subjected to the same thermal gradient process.

Optical profilometry, POM, XRR, and sXRD have been combined to assess thickness, roughness and crystalline morphology of the thin films prepared using different concentrations. The spin-coated thin film 1 at a very low concentration was pretty homogeneous; but the thin films became more and more inhomogeneous with aggregation of small crystals for samples 2-5, as shown in Figure 3a, 3c and Figure S3 (3D views). Likely, increasing inhomogeneities result from heterogeneous type dewetting which may occur during spin-coating. [START_REF] Tsui | Polymer Thin Films[END_REF][START_REF] Konnur | Instability and Morphology of Thin Liquid Films on Chemically Heterogeneous Substrates[END_REF] It is known that dust particles can serve as nucleation points in heterogeneous dewetting by weakening mechanically thin films, locally. The DSC curves shown in Figure 2b suggest that some dust particles are presented in the C8-BTBT-C8 powder sample, result of a ratio of TLC-Cr / TCr-LC = 0.96 (367.15 K / 382.05 K). A decrease of this ratio close or below 0.8 usually indicates that contamination is significantly reduced. [START_REF] Jackson | Nucleation from the Melt[END_REF] Moreover, the number of dust particles scales with volume. [START_REF] Jackson | Nucleation from the Melt[END_REF] Consequently, for a given sample area, there should be less dust particles in thinner films than in thicker ones, making thicker films more prone to heterogeneous dewetting. Figure 3e shows that the surface roughness values (Ra in nm) of films 1-5 increases non-linearly with concentrations. Only films 1 and 2 showed Kiessig fringes in their XRR curves (Figure 4a), allowing to determine a thickness of 13.3 and 26.3 nm, respectively.

We now turn our attention to thin films 6-10, subjected to ∇T treatments, which differ by their thickness, see Table 1. They were analysed by POM, optical profilometer, XRR and sXRD. Figure 3b, 3d and Figure S3 show that films 6-8 exhibit a dendritic growth pattern and a partial coverage of glass substrates. But for the film 9, the dendritic growth is avoided to some degree. However, the continuity of this thin film is still poor. A rather uniform surface morphology has been obtained for film 10. The dendritic growth was avoided, and the thin film covered the whole area of the substrate, although there were still some small pin-hole-like defects in the sample. As shown in Figure 3e and Table S2, the ∇T treatment leads to a little increase of Ra values,

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Please do not adjust margins in the 6.5 to 15.6 nm range, for films 6-9. But, for film 10, the surface roughness decreases, i.e. Ra 23.07 nm (before ∇T) versus Ra 5.44 nm (after ∇T). Films 6-9, give rise to distinctive XRR curves with Kiessig fringes, as displayed in Figure 4b, allowing the determination of their thicknesses. The thickness scales linearly with concentration as can be seen in Figure 4c and Table S2. However, no prominent Kiessing fringes can be observed for film 10, because the thickness of the film is too large, resulting in the period of the oscillation is too small for the resolution of our X-ray optics. A noticeable observation in Figure 4c is that the thickness of the film 1 vs 6 and of the film 2 vs 7 apparently increases. This peculiarity likely results from the discontinuity of thin films 6-7 after ∇T processing. Actually, the current results corroborate our previous reports on the annealing of spin-coated C8-BTBT-C8 thin films, without ∇T processing, but for which a disordered-layers-to-ordered-islands transition was observed. [START_REF] Dohr | Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films[END_REF][START_REF] Dohr | Dynamics of Monolayer-Island Transitions in 2,7-Dioctylbenzothienobenzthiophene Thin Films[END_REF] An intense diffraction peak is present at qz around 2.2 nm -1 in all films (Figure 4a and4b). The peak is identified as the 001 Bragg peak of the crystal structure of C8-BTBT-C8 (Figure S6). This, combined to the solely observation of 00l family within the whole specular scans of samples (see Figure S7), suggests a preferential orientation of the (00l) planes parallel to the glass substrate for all films, in agreement with previous reports. [START_REF] Gbabode | X-ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1]Benzothieno[3,2b]benzothiophene Derivatives in Bulk and Thin Films[END_REF] The sXRD data also highlight a decrease of the FWHM (Figure S8, Figure 4d, and Table S2) of the 002 Bragg peak, which is attributed to the increasing of the vertical crystallite size (Figure 4e, Table S2) after the ∇T treatment, for all concentrations. The FWHM of the (002) reflection has been used for the analysis because the impact of the Fresnel reflectivity for (002) is less pronounced than that for (001). Altogether, these results suggest a better out-of-plane crystalline order of the thin films after the ∇T treatment. The results of the in-plane GIXD of films 5 and 10 are displayed in Figure S9. Random in-plane alignment of crystallites was revealed for film 5, as shown by the presence of all observable hk0 reflections in the 5-50° 2θχ range. The inplane 2θχ/ϕ scan of film 10 presents intense {020} reflections until the second order, highlighting an in-plane alignment. However, a 200 reflection is still observed, although with very small intensity. This can be attributed to the multiple in-plane alignments of the sample. The observation demonstrates that the in-plane crystalline alignment of the thin film is dramatically improved by the ∇T approach. All results presented above indicate that the optimal concentration of the C8-BTBT-C8 solution is 20 mg.ml -1 to produce continuous, uniform and low roughness thin films on glass substrates by the ∇T approach. S2.

Please do not adjust margins Please do not adjust margins S2.

Thin films of C8-BTBT-C8 with comparable thickness but treated by different ∇T conditions.

Thin films 10-19, obtained at 20 mg.ml -1 , have been selected to investigate the influence of the ∇T conditions on thickness, roughness, and crystalline morphology of the thin films. The investigated ∇T conditions are listed in Table 1. After the ∇T treatments, roughness and surface morphology were analysed by POM and optical profilometer. The dendritic growth patterns and the partial covering of the substrate, previously observed for thin films produced using lower solution concentrations, appear for the films 11-16, treated by lower pulling rates/cooling rates (Vp ≤ 10 μm.s -1 , Cexp ≤ 6.41°C.min -1 ). However, films 10, 17, 18, and 19 cover almost completely the substrates and present large apparently uniform domains. The surface morphology of films 14 and 19 is depicted in Figure 5a and 5b, highlighting the features of partial or complete covering of the glass substrate. The complete set of POM and optical profilometer pictures of films 10-19 is presented in the supporting information, see Figure S4 and S5. As shown in Figure 5c and Table S3, Ra values of films 10-19 decrease by a factor of five to reach a minimum value of about 5 nm. Many small cracks are found in films 10-19, which were generated upon cooling from Tc to room temperature. They are believed to originate from the difference in the thermal expansion coefficients of C8-BTBT-C8 and the rigid substrate. Figure S11a displays the XRR curves of films 10-19. Although the ∇T treatments flatten the film surface and make the films better covering of the substrate, the thicknesses of the films prepared at 20 mg.ml -1 are too large, causing the oscillation period to approach the limit of our X-ray instrument resolution. Only films 11 -14 and 16 show observable Kiessig fringes before qz = 1.7 nm -1 , allowing to evaluate the thicknesses roughly are > 130 nm (Figure S11b, see also in Table S3). S3. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins S3.

Similarly to the samples prepared from different concentrations, the observation of a strong diffraction peak at qz around 2.2 nm - 1 combined to the solely observation of 00l family through the whole specular diffraction pattern for films 10-19 (see Figure S10) highlights a preferential orientation of the (00l) planes parallel to the glass substrate. Besides, compared with the just coated film 5, the FWHM of the (002) reflection of films 11 -19 is reduced (Figure 6a and Figure S12), resulting from the increasing of the vertical crystallite size after the ∇T treatments (Figure 6b). The results highlight again the ∇T approach can improve the out-of-plane crystalline order of the thin films. But there is no remarkable difference of the FWHM among films 11 -19 (Table S3) dues to the limit of the resolution of the Xray optics. To assess the in-plane alignment of crystallites in the samples, pole figure measurements were carried out for films 10-19 and for film 5, which has undergone no ∇T processing (Figure 7 and Figure S14). The particular {113} reflection was chosen because of its high peak intensity (Figure S6). As expected, a totally random in-plane alignment of crystallites was observed for the coated film 5, as evidenced by a wide intensity distribution of the {113} reflection along with azimuthal and meridional rotations of the sample (Figure 7a). This result is consistent with the presence of 110, 020, 120, 200, 130, and 040 peaks in-plane 2θχ/ϕ scan of film 5, displayed in Figure S9. Importantly, the ∇T processing on films 10-19 causes a much better in-plane alignment of crystallites, as shown in Figure 7b-7d and Figure S13. Among films 10, 17, 18, and 19 with good continuity and low surface roughness, number 10 exhibits the best in-plane alignment of crystallites with the narrowest discrete spots, suggesting that highly aligned crystalline domains were produced using Vp = 25 µm.s -1 , Gexp = 6 °C.mm -1 , and Cexp = 9 °C.min -1 . However, the uniaxial alignment that takes place exhibit various angles regarding the ∇T direction. Films 10-19 are definitively polycrystalline even if large domains are observed. This observation agrees with the POM images in Figures 5 andFigure S4. The lack of univocal uniaxial alignment contrasts with the one observed for terthiophene and resorcinol processed in comparable ∇T conditions. [START_REF] Schweicher | Toward Single Crystal Thin Films of Terthiophene by Directional Crystallization Using a Thermal Gradient[END_REF][START_REF] Panini | Crystal Growth Alignment of β-Polymorph of Resorcinol in Thermal Gradient[END_REF] The reason is likely related to the a and b parameters that differ only slightly and to the plate-like crystal morphology, shown in Figure 2a, which results from rather comparable crystal growth rate of planes hk0. Noteworthy is also that the thermal conductivity coefficients along a and b are the same.

Conclusions

In this contribution, we have explored the post-deposition processing of thin films of C8-BTBT-C8 compound by directional crystallization using a temperature gradient. The smectic phase of C8-BTBT-C8 prevents dewetting and allows to decouple film formation by spin-coating from the control of the crystalline morphology. Results obtained by POM, optical profilometer, XRR, sXRD, GIXD and pole figures put forward a dramatic effect of the temperature gradient processing. Roughness drops and films flatten significantly. They cover much better the substrate. Considerably larger and more Please do not adjust margins Please do not adjust margins ordered crystal domains are obtained. However, no univocal uniaxial alignment is observed. This study demonstrates also that directional crystallization is achievable using a liquid crystal to crystal phase transition. This journal is © The Royal Society of Chemistry 20xx

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Fig. 1

 1 Fig.1 Schematic drawing of the ∇T set-up. Tiso and TLC-Cr are the isotropization and the liquid-crystal to crystal phase transition temperatures, respectively. Th and Tc are the temperatures of the hot and cold stage. ΔT is the temperature difference between the hot and cold stage. Vp and Vc are the lateral pulling rate and the crystal growth rate of the sample. The sample is constituted of a glass substrate covered by a thin film of C8-BTBT-C8. Inset: the molecular structure of C8-BTBT-C8.

Fig. 2

 2 Fig.2 (a) Predicted crystal morphology of C8-BTBT-C8 based on the BFDH method. (b) DSC trace of C8-BTBT-C8. (c) POM picture of the fan shaped texture of the SmA liquid crystal phase of C8-BTBT-C8 recorded at 124 ℃. (d) sXRD pattern of the SmA phase recorded at 120 ℃.

Fig. 3

 3 Fig.3 Surface morphology of the C8-BTBT-C8 spin-coated thin films produced using different solution concentrations before: a) optical profilometry images and c) POM images; and after the ∇T treatment; b) optical profilometry images and d) POM images, recorded at room temperature. The gradient direction is highlighted (bottom). e) Evolution of the film surface roughness (Ra) as a function of the solution concentration. See also TableS2.

Fig. 4

 4 Fig.4 XRR curves of the C8-BTBT-C8 thin films (1-10) before (a) and after (b) the ∇T treatment, recorded at room temperature. Evolution of the film thicknesses (c), the FWHM of the (002) Bragg peaks (d), and the vertical crystallite size (e) as a function of the solution concentrations before and after the ∇T treatments. See also TableS2.

Fig. 5

 5 Fig. 5 The surface morphology (optical profilometry and POM images) of the C8-BTBT-C8 thin films 14 (a) and 19 (b) presents the features of partial or complete covering of the glass substrate. Pictures were recorded at room temperature. The gradient direction is highlighted (bottom). (c) Evolution of the film surface roughness (Ra) as a function of the pulling rate (Vp) for different Th -Tc couples. See also TableS3.

Fig. 6

 6 Fig. 6 Evolution of the FWHM of the (002) Bragg peaks of the samples(a) and the vertical crystallite size of films 10 -19 as a function of the pulling rate (Vp) for two Th -Tc couples (b). The data of just coated film 5 are shown for comparison. See also in TableS3.

  48

Fig. 7

 7 Fig.7 (a) Pole figure of {113} reflections for the spin-coated film 5 before the ∇T treatment. Pole-figure measurements of {113} reflection for films 10 (b), 14 (c) and 19 (d), after the ∇T treatments. The black solid dot highlights the temperature gradient direction.

Table 1 .

 1 ∇T crystallization conditions for the C8-BTBT-C8 thin films.

	Films Concentration	Th	Tc	Vp	Gexp	Cexp*
	No.	mg.ml -1	℃	℃ μm.s -1	℃.mm -1	℃.min -1
	6	2.3	120 90	25	6	9
	7	5.4	120 90	25	6	9
	8	10.5	120 90	25	6	9
	9	13.5	120 90	25	6	9
	10	20	120 90	25	6	9
	11	20	120 90	1	6	0.36
	12	20	120 60	1	11	0.66
	13	20	120 90	5	6	1.8
	14	20	120 60	5	11	3.3
	15	20	120 90	10	6	3.6
	16	20	120 60	10	11	6.6
	17	20	120 60	25	11	16.5
	18	20	120 90	50	6	18
	19	20	120 60	50	11	33

* The cooling rate at the growth front Cexp (°C.min -1 ) is simply given by Cexp = Gexp × Vp

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements G.L. thanks the European Union's Horizon 2020 Research and Innovation Program under the Marie Sklodowska-Curie grant agreement n°. 791207 (PARADA) and postdoctoral fellowship support from the FNRS for the POLYMOL project n° 1.B.253.20F. YG is thankful to the Belgian National Fund for Scientific Research (FNRS) for financial support through research projects BTBT n° 2.4565.11, Phasetrans n° T.0058.14, Pi-Fast n° T.0072.18, and 2D to 3D No. 30489208. Financial supports from the French Community of Belgian (ARC n° 20061) and by the Walloon Region (WCS No. 1117306, SOLIDYE n° 1510602) are also acknowledged. G.S. acknowledges postdoctoral fellowship support from the FNRS.

Conflicts of interest

There are no conflicts to declare.