Frank David
email: frank.david@live.fr

Cédric Hubas

Helène Laguerre

Aïcha Badou

Gwen Herault

Théo Bordelet

Nadia Améziane

Aicha Badou

Herault

Nadia Ameziane Bd

Food sources, digestive efficiency and resources allocation in the sea cucumber 2 Holothuria forskali (Echinodermata: Holothuroidea): insights from pigments and fatty

Keywords: sea cucumbers, echinoderms, Holothuria forskali, gut contents, pigments, fatty 51 acids 52

    Food sources, digestive efficiency and resource allocation in the sea cucumber Holothuria forskali (Echinodermata: Holothuroidea): Insights from pigments and fatty acids

 (Bouland et al. 87 1982)and then demonstrated on natural populations that favour the ingestion of 40-60 µm 88 sand grain size (Mezali and Soualili 2013) and organic-rich particles (Belbachir et al. 2014).

89

The digestive tract is mostly filled with sand grains although the species is most generally 90 observed on rocky substrates (Massin andJangoux 1976, Tuwo andConand 1992, pers. 91 observations). Out of the sand, the gut content is dominated by sponge spicules, diatoms, 92 cyanophyceae, macrophytes, foraminifera, crustaceans and seagrass fragments, with large site 93 variability (Belbachir and Mezali 2018, Badou pers. observations). Detritus and fungal 94 hyphae were also recorded [START_REF] Roberts | Feeding and digestive strategies in 711 deposit-feeding holothurians[END_REF].

95

Yet, the ingestion of coarse fragments of macrophytes has been suggested to be the 96 result of a not so efficient feeding selectivity. The concentration of certain items in the gut of 97 holothurians may thus be the result of particles physical properties (such as specific gravity, 98 density or microtopography of the surface) rather than selection of particles that are organic-99 rich or within a specific size range [START_REF] Roberts | Feeding and digestive strategies in 711 deposit-feeding holothurians[END_REF]. More generally, deposit-feeding 100 holothurians process large quantities of nutrient poor sediments and ingested food items may 101 not all be assimilated. Other studies than those previously conducted are thus required to 102 understand nutritional requirements of H. forskali for a rearing purpose. Feeding requirements 103 in marine species have generally been assessed using feeding trials and calculating optimal 104 growth rate (e. g. Kanazawa et al. 1979[START_REF] Domínguez-Godino | 582 Seagrass debris as potential food source to enhance Holothuria arguinensis' growth in 583 aquaculture[END_REF]) and/or maximal 105 hydrolytic activity (Van Wormhoudt et al. 1980) related to the inclusion of a given amount of 106 a specific compound in the food. In this study, we propose a "natural populations" oriented 107 approach to characterise food sources, digestive efficiency and resources allocation based on 108 differences in the composition of pigments and fatty acids (FA), that are reliable biomarkers 109 in ecological studies (Madji et al. 2018), between gut fractions, faeces and tissues of H.

110 forskali. This work aims at improving the basic knowledge of H. forskali nutrition in the wild 111 to facilitate the development of a commercial food for its rearing in captivity. We 112 hypothesised that (i) some food items are preferentially digested during gut transit and others 129 elimination (hindgut from the second descending segment of the intestine to the rectum). The 130 midgut was carefully emptied to avoid remains of food and its first half was isolated from the 131 rete mirabile and conserved for further analysis. Gonads and muscles isolated from muscular 132 bands were also sampled. Histological examination of the gonads indicated that they were at 133 the end of the gametogenesis and that collected individuals (six females and four males) were 134 about to spawn (Tuwo andConand 1992, Santos et al. 2015). Blades of brown (Laminaria 135 sp.) and red (Cryptopleura ramosa) macroalgae were collected by hand and sediment was 136 sampled in the depressions using a 50 mL syringe connected to a 5 mm inner diameter plastic 137 tube. Surface water was also collected and vacuum-filtered through pre-combusted (5 h at 357 Féral (1985) measured higher lipid levels in the anterior intestine wall compared to the 358 oesophagus of Leptosynapta galliennei (51.9 ± 3.4 μg mg -1 vs. 7.6 ± 3.5 μg mg -1 , 359 respectively). Assuming similar differences in H. forskali, cell wall lysis would be expected to 360 cause higher lipid contents in the midgut compared to the foregut content. The opposite was 361 measured in our study, confirming that cell wall lysis caused by the sampling stress was 362 minor. In an aquaculture perspective, further investigations would be required to determine 363 whether the nutritional upgrading of detrital organic matter takes place in the sediment and/or 364 in the foregut and which organisms mediate the biosynthesis pathways of HUFA. Actually, if 377 (1983) where one-third to one-half of the organic carbon was lost during gut passage in three 378 species of holothurians and two species of echinoids from shallow tropical waters (Discovery 379 Bay, Jamaica). Yet, not all compounds were similarly digested as total concentration of 380 pigments did not significantly vary during gut transit, while total FA experienced more than 381 three-fold reduction between foregut and hindgut (2.6 ± 1.3 mg g -1 in the foregut vs. 0.7 ± 0.5 382 mg g -1 in the hindgut), which confirms our hypothesis (i.e. that some food items are 383 preferentially digested during gut transit and others remain untransformed).

384

The absence of significant changes in relative proportions and total concentrations of Féral (1985) where the author measured especially high levels of 429 lipids in the anterior intestine walls of Leptosynapta galliennii (i.e. lipid droplets). Féral and 430 Massin (1982) suggested that the digestive tract may serve as a storage organ and Allen 431 (1968) measured highest incorporation rates of radioacetate in lipids of the gut isolated from 432 the rete mirabile of H. forskali. All these results suggest that the fatty acid composition of 433 holothuroids digestive tract deserves particular attention. The energy storage role of this organ 434 has however been dismissed by Féral (1985) who did not measure any decay in the anterior 435 intestine lipid content of adult L. galliennii before and after two weeks of starvation.

385
436 Similarly, no seasonal difference was measured in the digestive tract lipid content of 437 Molpadia violacea by [START_REF] Féral | Level, content and energetic equivalent of the main 595 biochemical constituents of the subantarctic molpadid holothurian Eumolpadia violacea 596 (Echinodermata) at two seasons of the year[END_REF] Male gonad (n = 4) Saturated 12:0 0.3 ± 0.1 a 0.1 ± 0.1 b 0.1 ± 0.0 b 0.1 ± 0.1 b 14:0 2.5 ± 0.4 c 3.9 ± 0.8 b 5.3 ± 0.5 a 2.2 ± 1.5 c 15:0 0.7 ± 0.2 b 0.7 ± 0.2 b 1.3 ± 0.2 a 0.5 ± 0.3 b 16:0 5.3 ± 0.9 b 6.0 ± 1.1 b 8.9 ± 0.9 a 5.0 ± 1.7 b 17:0 0.7 ± 0.1 c 0.9 ± 0.1 b 1.1 ± 0.1 a 1.4 ± 0. 0.8 ± 0.2 c 0.9 ± 0.1 b 1.3 ± 0.2 a 0.7 ± 0.1 c 20:4ω6

19.5 ± 0.9 a 11.0 ± 1.9 b 7.4 ± 1.1 c 12.8 ± 2.5 b 20:5ω3 11.9 ± 1.5 b 15.9 ± 1.8 a 11.5 ± 1.8 b 18.5 ± 4.0 a 20:2ω6 0.8 ± 0.2 b 1.3 ± 0.3 a 1.0 ± 0.1 a 1.4 ± 0.6 a Unidentified HUFA 1.3 ± 0.2 1.0 ± 0.2 0.8 ± 0.1 0.9 ± 0.1 22:5ω6

1.1 ± 0.1 a 0.7 ± 0.2 b 0.5 ± 0.1 c 0.8 ± 0.2 b 22:6ω3 2.0 ± 0.4 b 2.7 ± 0.5 a 2.0 ± 0.6 b 2.1 ± 0.6 ab ∑C16PUFA 0.9 ± 0.2 b 1.4 ± 0.4 a 1.9 ± 0.6 a 0.8 ± 0. Branched 14:0iso 0.5 ± 0.1 bc 0.6 ± 0.1 b 1.0 ± 0.2 a 0.3 ± 0.2 c 15:0iso 1.7 ± 0.4 bc 2.1 ± 0.4 b 3.6 ± 0.6 a 1.2 ± 0.8 c 15:0anteiso 0.7 ± 0.2 bc 0.8 ± 0.2 b 1.4 ± 0.3 a 0.5 ± 0.3 c 17:0iso 0.8 ± 0.1 c 1.1 ± 0.1 b 1.6 ± 0.2 a 0.9 ± 0.

  sediment has first been suggested through histological analysis

113

  remain untransformed; (ii) specific elements are preferentially retained and/or converted for 114 physiological functions such as growth, tissues regeneration or reproduction. of the species Holothuria (Panningothuria) forskali Delle Chiaje, 119 1823 were sampled on May 2, 2019 east of the Glenan Islands (Brittany -France; 47.710°N, 120 3.948°E) at 8-11 m deep in the early afternoon (bottom water temperature ≈ 16°C). The 121 sampling area was characterised by a rocky substrate where Laminaria sp. (Phaeophyceae -122 Laminariales) forests develop along with various species of red algae. Patches of sediment 123 dominated by sand and shell fragments accumulated in the depressions. Sea cucumbers were 124 immediately dissected in the field and gut content was divided in three parts (Fig. 1) 125 corresponding to specific digestive functions as proposed by Massin and Jangoux (1976): 126 storage (foregut from the upper part of the digestive tract to 2 cm above the attachment point 127 of the rete mirabile), digestion (midgut corresponding to the first descending and ascending 128 segments of the intestine and sometimes referred in the literature as anterior intestine) and

Fig. 3 : 4 :

 34 Boxplot of the organic matter content (%) in wild H. forskali samples and nearby 780 sediment. Broad lines indicate median, box edges refer to 1 st and 3 rd quartiles and circles 781 indicate outliers. Letters indicate significant differences between samples (Van der Waerden 782 test, α = 5%) Total pigment concentration in aquaculture and wild samples (μg mg dw -1 ). Broad 785 lines indicate median and box edges refer to 1 st and 3 rd quartiles. Eicosanoid ratios in gut content and tissues of wild H. forskali samples. Letters 789 indicate significant differences between samples (Van der Waerden test, α = 5%). Broad lines 790 indicate median, box edges refer to 1 st and 3 rd quartiles and circles indicate outliers. 791 Page 32 of 73 Aquaculture Nutrition

  that diatoms and/or brown macroalgae are important food sources for H.308 forskali in our study(Schmid et al. 2005), while neoxanthin may originate from decomposing 309 pelagic green microalgae as this carotenoid is present in high amounts in suspended 310 particulate matter (16.7 ± 0.8 %) and found almost exclusively in Prasinophyceae and 311 Chlorophyceae(Mackey et al. 1996, Brotas and Plante-Cuny 2003).312Conversely, low proportions of lutein in gut contents of wild H. forskali indicate that 313 the species does not feed on the red algae Cryptopleura ramosa found in close proximity and 314 exhibiting high levels of this carotenoid (37.0 ± 0.4 %). More largely, all groups of red algae 315 exhibit high contents of antheraxanthin, lutein or zeaxanthin[START_REF] Schubert | Carotenoid composition of marine 725 red algae[END_REF]). These 316 carotenoids were not detected or below 1 % in H. forskali gut contents. Lutein is also an protists, also named "non-pigmented" protists in the old scientific literature.341 Other authors mentioned elevated levels of HUFA in the gut content of holothurians. Mfilinge 342 and Tsuchiya (2016) reported 21.8 % of HUFA in the foregut content of H. leucospilota and 343 31.4 % in H. atra. Both species where assumed to feed on the nearby sediment that exhibited 344 only 9.5 and 16.8 % of HUFA, respectively. High levels of HUFA in the foregut were 345 attributed to the specific selection of algae and bacteria rich detrital particles (Mfilinge and 346 Tsuchiya 2016). Although microalgae have long been considered the major de novo producer 347 of PUFA in marine food webs (Nichols 2003), diatoms, that are the dominant microalgae 348 consumed by H. forskali in our study, are usually poor in FA 20:4ω6 (Dalsgaard et al. 2003, 349 Lang et al. 2011), suggesting that the HUFA measured in the foregut content of H.

	F o r 147 station of Agrocampus-Ouest maintains 2-4 cm juveniles in 25 L plastic tanks cleaned daily, 148 and feeds them with a mixture of microalgae produced on site (the two Bacillariophyceae 149 Chatoceros sp. and Skeletonema sp. and the Prymnesiophyceae Isochrysis sp.). Finally, the 150 AquaB Company in Plobannalec-Lesconil maintains 2-4 cm juveniles in 50 cm diameter 151 polyamide sieves (20 μm mesh size) placed in cylindroconical tanks with a circulating filtered 152 seawater system, and feeds them with decomposed green macroalgae (Ulva sp. or 153 Enteromorpha sp.). Green seaweeds are gathered on nearby beaches, left to decompose 154 during a few weeks in aerated cylindroconical tanks and the mixture is filtered before feeding 155 events to collect particles in the size 50-300 μm. All experimental stations constantly renew 156 the water in culture tanks with filtered seawater pumped nearby, maintaining water 157 parameters similar to that of the ocean. The list of collected samples and major associated 159 months after recollection. o r o r o r o r o r r o 158 weaknesses is presented in Table 1. All samples were kept at -25°C and analysed within 2 F F F F F 307 individuals indicate 317 F 340 by heterotrophic
	160	P P P P P P P
	161 162	e e r Organic matter of wild H. forskali gut contents and nearby sediment was quantified 3. 2. Sample processing e e r e e r e e r e e r e e r e e r
	R e v i e w 163 gravimetrically after combustion of freeze-dried material in a muffle furnace (4 h at 450°C). 164 Pigments were analysed by high performance liquid chromatography (HPLC) according to 165 Brotas and Plante-Cuny (2003). Sub-samples of freeze-dried material (10-15 mg for sediment, R e v R e v R e v R e v R e v R e v 166 gut contents and faeces and 5-10 mg for potential food sources) were incubated with 2 mL of 167 methanol (buffered with 2% ammonium acetate) during 15 min, at -25°C in the dark. Extracts 168 were then filtered with 0.2 μm PTFE syringe filters and analysed within 16 h using an Agilent i e w i e w i e w i e w i e w i e w
	169 1260 Infinity HPLC composed of a quaternary pump (VL 400 bar), a UV-VIS photodiode
	170 array detector (DAD 1260 VL, 250-900 nm) and a 100 μl automatic sample injector
	139 171 refrigerated at 4°C in the dark. Chromatographic separation was carried out using a C18
	172 column for reverse phase chromatography (Supelcosil, 25 cm long, 4.6 mm inner diameter).
	140	3. 2. Collected of feed and faeces of farmed individuals
	141	We collected food sources and corresponding faeces of holothurians opportunistically
	142 in May-June 2019 at the experimental sites where the different partners of the HOLOFARM
	143 research project maintain livestock. The Concarneau marine station maintains broodstock in
	144 500 L tanks with 5 cm of coarse sand on the bottom to allow microbial development, and
	145 feeds them with a combination of Shellfish Diet 1800 ® (Reed Mariculture) with a paste
	146 mixture of Grower Fertil ® (Le Gouessant) shrimp pellets and fresh mussels. The Beg Meil
		6 11 12

138 450°C) and pre-weighted glass fibre filters (Whatman GF/F 0.7 μm).

173 The solvents used were A: 0.5 M ammonium acetate in methanol and water

(85:15, v:v)

, B:

174 acetonitrile and water

(90:10, v:v)

, and C: 100% ethyl acetate. The solvent gradient was set 175 according to

Brotas and Plante-Cuny (2003)

, with a 0.5 mL min -1 flow rate. Identification and 176 calibration of the HPLC peaks were performed with antheraxanthin, ββ-carotene, 177 canthaxanthin, chlorophyll a, chlorophyll b, chlorophyll c2, diatoxanthin, diadinoxanthin, 178 fucoxanthin and pheophytin a standards. We identified all detected peaks by their absorption 179 spectra and relative retention times using the Agilent OpenLab software. Quantification was important component of green macroalgae such as Ulva sp. that may reach the sea floor in 318 this area once in decomposition but that is not ingested by H. forskali. High levels of lutein in 319 the faeces of sea cucumber juveniles fed with decomposed green algae confirm that lutein can 320 be used as a reliable tracer of green algae in the diet of H. forskali, and that pigment 321 composition of farmed animal faeces roughly resembled that of their diet. Finally, low 322 proportions of violaxanthin and FA 18:4ω3 in gut contents reveal that brown macroalgae are 323 barely ingested, as they usually exhibit high levels of these compounds

(Fleurence et al. 1994, 324 Schmid et al. 2005)

. Thus, the vegetal food sources ingested by H. forskali in our study are 325 mainly diatoms.

326

In contrast to pigments, fatty acids were not only more concentrated in foregut 327 contents compared to the sediment but also differed in composition, suggesting that other 328 food sources not containing pigments or exhibiting similar pigment compositions than 329 unsorted sediment enter the diet of H. forskali. Higher proportions of the FA 20:4ω6, the sum 330 of highly unsaturated FA (HUFA ≥ 20 carbon chain-length and 2 double bounds), and lower 331 proportions of the FA 16:0 in the foregut content compared to the sediment indicate that the 332 sea cucumbers ingest particles with especially high nutritional value. Actually, HUFA are 333 precious compounds with roles in membrane properties and immune response (Arts and 334 Kohler 2008, Twining et al. 2016). They are usually rapidly degraded in decaying organic 335 matter (Wakeham 1995, Vivier et al. 2019). Such elevated proportions of HUFA in the 336 foregut content of H. forskali suggest that ingested particles are upgraded regarding their 337 nutritional value before transiting in the gut (i.e. in the nearby sediment or in the oesophagus). 338 Trophic upgrading and enhancement of PUFA relative proportions throughout the first levels 339 of the food web is usual in the marine realm (Desvilettes and Bec 2009), and can be mediated forskali 350 may rather originate from bacteria (Nichols 2003), heterotrophic protists (Desvilettes and Bec 351 2009) and/or their symbiotic relationship (Gast et al. 2009). Ginger et al. (2000) also 352 measured high amounts of HUFA in the gut contents of holothurians, but they attributed these 353 elevated levels to the lysis of gut walls, caused by stress during animals recovery. Yet, Ginger 354 et al. (2000) worked on abyssal holothurians and change in barometric pressure during the 355 ascent phase may have caused a sufficient stress to cause cell wall lysis, which may not have

356 occurred in our study as sea cucumbers were gathered only at a few meters deep. In addition,

  365 possible, feeding the microbiote of H. forskali instead of providing feeds of high nutritional 366 value may be of interest in a sustainable aquaculture perspective. Algerian Posidonia meadows (10.9 ± 2.6 % in our study, 8.6 ± 6.1 % measured by Mezali and 374 Soualili 2013 in the Sidi-fredj peninsula and ~ 8.3 ± 2.7 % found byBelbachir et al. 2014 in 375 the Stidia region). The one-third reduction in OM content during gut transit (14.1 ± 4.5 % OM 376 in foregut vs. 9.1 ± 2.5 % OM in faeces) is similar to previous observations by Hammond

	373
		F
		o r
		P
		e e r
		R e v
		i e w
	367
	368	5. 2. Digestive efficiency
	369	The significant but weak differences in OM between foregut contents and faeces
		13

370 suggest that H. forskali have to process large quantities of sediment to cover its nutritional 371 needs. In the midgut (the largest compartment) of H. forskali found on rocky substrate in the 372 Glenan Islands, the OM content was slightly above that of the complete gut measured in

  Yet, inefficient digestion of living diatoms raises the question of reason for their 398 ingestion, that may be due to inefficient feeding selectivity, or to the feeding on compounds 399 that do not require cell lysis. The most abundant compounds that are released by microalgaeMühlenbruch et al. 2018) especially in terms of carbohydrates 407 when they are exposed for instance to changing environmental conditions such as pH and 408 surface roughness[START_REF] Riera | Biofilm monitoring as a tool to assess 708 the efficiency of artificial reefs as substrates: Toward 3D printed reefs[END_REF]. Ingest and force diatoms to release nutritive compounds 409 may thus be a very efficient way to obtain energy for a deposit-feeding organism such as H. with a high nutritional value) are those that are preferentially assimilated by the 414 sea cucumbers, while increase in proportions of the FA 16:0 and the sum of SFA indicates 415 that remaining compounds are in a more advanced state of degradation with lower nutritional 416 value. The almost absence of significant differences between the FA profiles of nearby 417 sediment and sea cucumber faeces indicates that H. forskali have little influence on sediment 418 composition, except concerning the proportion of branched FA, indicative of bacteria

	406 increase their exudation rate (410 forskali.
	411	In contrast to diatom biomarkers, HUFA proportions significantly decreased during
	412 gut transit (31.8 ± 9.2 % in the foregut vs. 19.8 ± 4.2 % in the hindgut), confirming that these
	400 in the adjacent environment are exopolymer secretions, mostly on the form of polysaccharides 401 (Decho 1990, Mühlenbruch et al. 2018). Such compounds may provide a source of carbon for o r P e e r R e v i e w 413 compounds (425 426 5. 3. Resources allocation 427 High contents of FA in the digestive tract of H. forskali are in accordance with 402 deposit feeders (Roberts et al. 2000), as previously evidenced in various echinoderms (e.g. F 428 histological observations of
		14

pigments during gut transit indicates that H. forskali poorly utilises such resources, and 386 confirms that pigments are good biomarkers of diet. In addition, these results suggest that 387 living diatoms, that are the main pigmented food sources consumed by H. forskali in our 388 study, are not digested during gut transit. Fresh pelagic microalgae were administrated to 389 farmed juveniles. Given that H. forskali is a deposit feeding species, most of ingested cells 390 were probably dead algae, which induced lower relative proportions of chlorophyll a,

391

chlorophyll c and fucoxanthin in juveniles faeces compared to their living food sources, along 392 with the almost exclusive presence of pheophorbide a in faeces, a degradation product of 393 chlorophylls (Roy et al. 2011), that was absent from the fresh cultured microalgae. Inefficient 394 digestion of living diatoms was also confirmed by higher relative proportions of C 16 PUFA 395 and the FA 16:1ω7, both biomarkers of diatoms (Dalsgaard et al. 2003, Brett et al. 2009, 396 Kelly and Schiebling 2012) in the hindgut compared to the foregut contents of wild H. 397 forskali. in 403 the holothurian Isostichopus badionotus studied by Baird and Thistle 1986 and in the 404 brittlestar Amphipholis gracillima studied by Hoskins et al. 2003). In stressing conditions (as 405 is most probably the digestive tract of holothurians for diatoms), algal cells are known to 419 (Kaneda 1991, Kelly and Schiebling 2012). Actually, lower BrFA proportions in faeces (2.0 ± 420 0.3 %) compared to nearby sediment (4.6 ± 1.2 %), along with similar total FA 421 concentrations, suggest that H. forskali has the ability to reduce bacterial loads on the sea 422 floor. Thus, as other species of sea cucumbers, H. forskali may be an interesting candidate for 423 integrated multitrophic aquaculture (Zamora et al. 2016), as already proposed by MacDonald 424 et al. (2003).

Table 1 :

 1 whereas a storage role of the digestive tract List of collected samples and major weaknesses associated

	F
	o r
	P
	e e r
	R e v
	i e w

acids in the testis and ovary of cultured and wild sweet smelt Plecoglossus altivelis. Kelly, J., & Scheibling, R. (2012). Fatty acids as dietary tracers in benthic food webs. Marine Ecology 648 Progress Series, 446, 1-22. https://doi.org/10.3354/meps09559 649 Laguerre, H., Raymond, G., Plan, P., Améziane, N., Bailly, X., Le Chevalier, P. (2020). First 650 description of embryonic and larval development, juvenile growth of the black sea-cucumber 651 Holothuria forskali (Echinodermata: Holothuroidea), a new species for aquaculture in the 652

Table 4 :

 4 Fatty acid proportions of wild H. forskali tissues (mean ± SD). Letters in exponent 769 indicate significant differences between groups (Van der Waerden test, α = 5%). Note that 770 traces of 18:3ω3 may have co-eluted with 18:1ω9 and traces of 16:0iso may have co-eluted 771 with 16:4ω3. LC-MUFA Long-Chain MUFA ; HUFA Highly Unsaturated FA 772

	Fatty acids (%)	Muscle	Digestive tract	Female gonad
		(n = 10)	(n = 10)	(n = 6)

Aquaculture Nutrition

Mostaganem (Algérie). 6.

Belbachir, N., & Mezali, K. (2014). Comportement alimentaire sélectif de certaines espèces

https://doi.org/10.1007/BF00397516

Féral, J.-P. (1989). Activity of the principal digestive enzymes in the detritivorous apodous

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Jeong, B.-Y., Jeong, W.-G., Moon, S.-K., & Ohshima, T. (2002). Preferential accumulation of fatty

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R Core Team (2019). R: A language and environment for statistical computing. R Foundation for

Stott, F. C. (1957). Observations on the food canal and associated structures in the holothurian

Holothuria forskali Delle Chiaje. Proceedings of the Zoological Society of London, 129(1),

129-136. https://doi.org/10.1111/j.1096-3642.1957.tb00283.x

Tuwo, A., & Conand, C. (1992). Reproductive biology of the holothurian Holothuria forskali

(Echinodermata). Journal of the Marine Biological Association of the United Kingdom, 72(4),

in nature: What we know and what we need to learn. Oikos, 125(6), 749-760.

https://doi.org/10.1111/oik.02910

Van Wormhoudt A. & Bellon C. (1991). Digestion et reproduction chez les crustacés décapodes. In :

Barnabé, G. (Eds.), Bases biologiques et écologiques de l'aquaculture. Lavoisier press, pp. 213-

Vivier, B., David, F., Marchand, C., Thanh-Nho, N., & Meziane, T. (2019). Fatty acids, C and N

This study is part of the HOLOFARM research project funded by the European 21 Maritime and Fisheries Fund (in measure 47: Aquaculture Innovations) and coordinated by 22 Agrocampus-Ouest. The authors are especially grateful to Grégory Raymond, from 23 Agrocampus-Ouest, who coordinates the project and provided juvenile faeces and fresh 24 microalgae, to Julien Grys and Patrick Plan, from the AquaB company, who provided juvenile 25 faeces and green algae, and to all other partners of the Holofarm research project, the 26 Biotechnology and Marine Chemistry Laboratory of Quimper, the Roscoff Biological Station 27 and the Regional Shellfish Farming Committee of South Brittany.

The data that support the findings of this study are available from 31 the corresponding author upon reasonable request.

Food sources

Laminaria sp. (brown macroalgae) 2 0.0 ± 0.0 6.6 ± 0.6 0.0 ± 0.0 8.4 ± 0.1 38.5 ± 0.5 0.0 ± 0.0 0.0 ± 0.0 15.1 ± 0.2 0.0 ± 0.0 26.9 ± 0.2 4.5 ± 0.1 2.3 ± 0.2

Cryptopleura ramosa (red macroalgae) 2 0.0 ± 0.0 15.3 ± 0.6 0.0 ± 0.0 0.0 ± 0.0 0.4 ± 0.0 37.0 ± 0.4 0.0 ± 0.0 1.5 ± 0.0 0.0 ± 0.0 39.4 ± 0.7 6.4 ± 1.0 7.0 ± 0.4

Ulva sp. (green macroalgae) 2 1.7 ± 0.1 10.5 ± 0.1 5.9 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 53.4 ± 0.3 6.8 ± 0.5 18.0 ± 0.2 0.0 ± 0.0 0.0 ± 0.0 3.6 ± 0.1 8. Cultured microalgae 2 0.0 ± 0.0 10.7 ± 0.4 0.0 ± 0.0 29.7 ± 4.0 52.6 ± 5.8 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 6.9 ± 2.2 n.d.

Shelfish Diet 1800 2 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 13.1 ± 0.0 7.8 ± 0.3 0.9 ± 0.1 4.7 ± 0.3 0.4 ± 0.0 71.6 ± 0.7 0.2 ± 0.0 1.3 ± 0.0 256.8 ± 4.0

Grower Fertil + fresh mussels 2 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 29.0 ± 1.7 0.0 ± 0.0 2.2 ± 0.6 0.0 ± 0.0 64.5 ± 2.0 3.7 ± 0.7 0.6 ± 0.1 4.9 ± 1.7

Suspended particulate matter 2 0.0 ± 0.0 7.5 ± 0.3 0.2 ± 0.0 44.0 ± 0.3 23.0 ± 0.1 0.0 ± 0.0 16.7 ± 0.8 0.0 ± 0.0 0.0 ± 0.0 2.9 ± 0.0 5.8 ± 0.2 3.6 ± 1.1

Faeces of aquacultured individuals

Juveniles fed decomposed Ulva sp. 3 0.6 ± 0.1 0.2 ± 0.0 0.3 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 11.4 ± 1.2 1.2 ± 0.2 3.7 ± 0.4 74.0 ± 2.8 7.8 ± 0.9 0.8 ± 0.2 24.8 ± 3.2

Juveniles fed decomposed Enteromorpha sp. Adults fed Shellfish Diet 1800 + Grower Fertil + fresh mussels 3 0.0 ± 0.0 0.1 ± 0.0 0.0 ± 0.0 9.1 ± 1.3 6.9 ± 2.5 0.4 ± 0.1 3.1 ± 0.4 0.5 ± 0.3 77.7 ± 5.3 1.6 ± 0.6 0.5 ± 0.3 25.5 ± 21.2

Wild H. forskali samples

Nearby sediment 4 0.0 ± 0.0 0.7 ± 0.4 0.2 ± 0.1 12.2 ± 3.1 5.9 ± 2.6 1.0 ± 0.2 2.5 ± 0.8 0.5 ± 0.4 71.4 ± 4.7 3.9 ± 1.1 1.6 ± 0.2 1.4 ± 0.9

Foregut content 10 0.0 ± 0.0 0.4 ± 0.4 0.2 ± 0.1 14.9 ± 5.2 4.9 ± 2.3 0.7 ± 0.3 2.8 ± 0.6 0.7 ± 0.1 66.3 ± 9.0 6.9 ± 4.5 2.2 ± 0.7 5.9 ± 2.2

Midgut content 10 0.0 ± 0.0 0.2 ± 0.1 0.2 ± 0.1 16. 2 18.7 ± 2.1 34.1 ± 3.1 10.7 ± 0.9 0.6 ± 0.1 0.1 ± 0.0 0.2 ± 0.1 2.4 ± 0.2 38.8 ± 2.9 Cultured diatoms (brown microalgae) 2 27.4 ± 3.2 64.2 ± 3.4 12.4 ± 4.9 20.2 ± 1.8 Shelfish Diet 1800 2 14.9 ± 0.0 23.7 ± 0.1 9.5 ± 0.2 0.4 ± 0.0 0.9 ± 0.0 1.5 ± 0.0 31.4 ± 0.0 Grower Fertil + fresh mussels 2 19.6 ± 1.4 29.8 ± 1.6 10.0 ± 2.0 3.2 ± 0.1 0.3 ± 0.0 0.2 ± 0.0 4.9 ± 0.0 32.3 ± 0.1 Suspended particulate matter 2 33.7 ± 1.5 66.9 ± 4.2 7.0 ± 2.2 0.4 ± 0.1 0.6 ± 0.4 1.4 ± 0.5 22.8 ± 3.2

Faeces of farmed individuals

Juveniles fed decomposed Ulva sp. 3 22.7 ± 6.0 47.7 ± 9.6 13.1 ± 2.2 0.3 ± 0.3 0.9 ± 0.5 34.5 ± 6.8 Juveniles fed decomposed Enteromorpha sp.

3 29.4 ± 6.4 53.9 ± 11.5 5.8 ± 2.2 0.2 ± 0.4 0.5 ± 0.8 30.0 ± 5.8 Juveniles fed fresh microalgae 3 31.8 ± 17.8 60.9 ± 28.1 9.6 ± 7.6 0.3 ± 0.2 0.1 ± 0.1 0.0 ± 0.1 0.0 ± 0.1 0.7 ± 0.7 22.2 ± 16.9 Adults fed Shelfish Diet 1800 + Grower Fertil + fresh mussels 3 29.1 ± 2.5 45.6 ± 1.1 9.4 ± 1.1 5.6 ± 1.7 0.3 ± 0.1 0.8 ± 1.3 0.2 ± 0.0 8.1 ± 0.6 33.4 ± 0.8

Wild H. forskali samples

Nearby sediment 4 23.0 ± 2.9 a 42.0 ± 3.1 a 11.3 ± 4.7 bc 3.3 ± 2.3 0.5 ± 0.4 0.1 ± 0.2 c 5.0 ± 3.0 b 30.9 ± 2.7 Foregut content 10 12.0 ± 6.0 b 30.7 ± 7.4 b 9.9 ± 4.0 c 2.3 ± 1.1 0.4 ± 0.3 2.2 ± 0.6 a 1.7 ± 1.1 a 9.4 ± 3.2 a 29.1 ± 3.2 Midgut content 10 14.1 ± 3.7 b 33.5 ± 3.6 b 14.0 ± 3.7 ab 1.0 ± 0.5 0.7 ± 0.4 1.3 ± 0.6 b 1.2 ± 0.5 a 6.2 ± 1.6 b 30.7 ± 2.1 Hindgut content 10 21.3 ± 5.2 a 39.7 ± 6.0 a 17.1 ± 3.7 a 1.3 ± 0.5 0.3 ± 0.4 1.0 ± 0.7 b 0.5 ± 0.4 bc 4.2 ± 2.1 b 30.6 ± 3.2 Freshly released faeces 4 23.7 ± 0.9 a 40.6 ± 1.1 a 15.0 ± 1.8 ab 2.1 ± 0.6 0.2 ± 0.1 2.2 ± 1.0 a 0.9 ± 0. 2 0.2 ± 0.0 16.4 ± 1.1 9.9 ± 0.4 10.8 ± 0.9 0.0 ± 0.0 21.0 ± 1.3 41.3 ± 2.3 0.1 ± 0.1 10.5 ± 2.4 Cryptopleura ramosa (red macroalgae) 2 0.6 ± 0.0 3.5 ± 0.1 1.8 ± 0.3 0.7 ± 0.3 5.9 ± 0.7 7.7 ± 0.7 0.4 ± 0.0 3.6 ± 0.0 Ulva sp. (green macroalgae) 2 7.6 ± 0.5 0.4 ± 0.0 0.4 ± 0.0 14.1 ± 0.8 1.0 ± 0.1 4.1 ± 0.7 Decomposed Ulva sp.

2 1.3 ± 0.3 0.5 ± 0.2 1.6 ± 0.4 2.5 ± 0.7 0.8 ± 0.1 5.3 ± 1.1 11.4 ± 1.8 11.6 ± 2.2 5.5 ± 0.0 Enteromorpha sp. (green macroalgae) 2 1.6 ± 0.1 5.9 ± 0.4 0.8 ± 0.0 2.0 ± 0.0 3.0 ± 0.0 29.6 ± 0.8 0.6 ± 0.1 9.6 ± 0.9 Decomposed Enteromorpha sp. 2 0.9 ± 0.1 0. 3 0.5 ± 0.4 0.6 ± 0.5 0.2 ± 0.4 0.5 ± 0.9 0.3 ± 0.5 1.0 ± 1.8 5.5 ± 2.9 10.6 ± 3.2 1.0 ± 0.5 Juveniles fed fresh microalgae 3 1.9 ± 1.7 0.5 ± 0.5 1.0 ± 0.9 2.7 ± 2. 3 3.5 ± 3.3 7.8 ± 7.0 11.8 ± 10.4 4.7 ± 0.5 15.8 ± 21.8 Adults fed Shelfish Diet 1800 + Grower Fertil + fresh mussels 3 1.3 ± 0.2 1.2 ± 0.2 0.6 ± 0.3 3.3 ± 1.2 2.4 ± 0.7 9.4 ± 0.6 18.8 ± 0.4 1.7 ± 0.8 8.1 ± 8.5

Wild H. forskali samples Nearby sediment 4 1.8 ± 1.0 b 1.1 ± 0.6 3.7 ± 1.2 c 9.2 ± 3.9 2.6 ± 1.0 17.1 ± 6.9 c 21.0 ± 6.5 c 4.5 ± 1.2 a 0.3 ± 0.1 d Foregut content 10 1.7 ± 0.8 b 1.3 ± 0.3 8.6 ± 2.7 a 15.0 ± 5.3 5.1 ± 4.9 31.8 ± 9.2 a 34.9 ± 8.7 a 3.1 ± 1.0 b 2.6 ± 1.3 a Midgut content 10 2.5 ± 0.7 ab 1.6 ± 0.2 6.3 ± 1.7 b 13.2 ± 2.6 3.2 ± 0.9 24.7 ± 5.0 ab 29.5 ± 4.7 ab 4.1 ± 1.3 a 1.1 ± 0.4 b Hindgut content 10 2.9 ± 0.5 a 1.5 ± 0.2 4.6 ± 1.4 c 11.1 ± 2.2 2.7 ± 0.9 19.8 ± 4.2 bc 24.9 ± 4.2 bc 2.7 ± 0.9 bc 0.7 ± 0.5 c Freshly released faeces 4 2.7 ± 0.5 ab 1.6 ± 0.4 5.0 ± 1.1 bc 10.1 ± 1.7 2.0 ± 0.7 18.3 ± 2.4 bc 23.5 ± 2.4 bc 2.0 ± 0.


Magatte Gueye

Alexandre Vercouter

Rémy Jouclas

David Guérin

Vincent Lemaur

Guillaume Schweicher

Stéphane Lenfant

Aleandro Antidormi

Yves Geerts

Claudio Melis

N Gueye

Aleandro Anhdormi

Jérôme Cornil

Dominique Vuillaume

Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

Keywords: thermal conducHvity, scanning thermal microscopy (SThM), approach to equilibrium molecular dynamics (AEMD), organic semiconductor, organic thermoelectricity

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

INTRODUCTION.

Organic materials have recently aWracted a great interest for poten'al thermoelectric applica'on because commercial modules are built with materials such as bismuth telluride (Bi 2 Te 3 )-based alloys 1 which are toxic, expensive, and energy-consuming for processing. The candidates range from polymers (like PEDOT:PSS, PEDOT:OTf, and other deriva'ves) with high electrical conduc'vity and Seebeck coefficient (up to ∼5,000-6,000 S cm -1 and ∼1 mV K -1 ), [2][3][4][5] down to self-assembled monolayers and single molecule junc'ons (based mainly on alkyl chains, oligo(phenylene ethynylene)s, benzene, C 60 ,…). [6][7][8][9][10][11][12][13] In the laWer case, quantum interference effects can be exploited to tailor and op'mize the thermoelectric proper'es of the molecules. [14][15][16][17] In contrast to the charge transport proper'es, the thermal conduc'vity (κ) of thin films of small πconjugated organic semiconductors (OSCs) appears less studied at both the experimental and theore'cal levels. The in-plane κ of various molecular thin films (pentacene, TPD, Alq3, C 60 , PCBM, rubrene, DNTT, ….) 4,[18][19][20][21][22][23][24][25][26] has been measured in the range 0.1 -0.8 W m -1 K -1 at the macroscale (ac-calorimetry, 3ω Joule hea'ng, 'me domain thermo-reflectance….). Only a very few reports have been published on the out-of-plane κ of these organic materials at the nanoscale, e.g. using the scanning thermal microscope (SThM). 27,28 On the theore'cal side, the predic'on of κ for OSCs and the defini'on of structureproperty rela'onships is also scarcely addressed. The κ values can be es'mated by models 29 based on: (i) collec've excita'ons of phonons via the resolu'on of the Boltzmann Transport Equa'on (BTE); (ii) atomic displacements via Molecular Dynamics (MD) approaches such as the Green-Kubo formalism or the Non Equilibrium Molecular Dynamics method (see below). The BTE approach is known to be far much prohibi've in terms of computa'onal cost than MD-based techniques. Thus, κ of a few molecules (pentacene, C 60 , PCBM, H 2 Pc, TPD,..) [30][31][32][33][34][35][36][37][38] have been essen'ally calculated by various methods belonging to the "MD-class" in order to es'mate the anisotropy of κ along the long axis of the molecules versus in the perpendicular intralayer direc'ons.

In this work, we measure by SThM the out-of-plane κ at the nanoscale of thin films (40-400 nm thick) of [1]benzothieno [3,2-b][1]benzothiophene (BTBT) and 2,7-dioctyl [1]benzothieno [3,2-b][1]benzothiophene (C8-BTBT-C8). 39,40 These molecules have promising performances for organic electronics with reproducible transistor hole mobili'es in excess of 10 cm 2 V -1 s -1 and up to ∼ 200 cm 2 V -1 s -1 at the local scale. 40,41 Since their thermoelectric proper'es were not inves'gated experimentally up to now, this has s'mulated the present experimental and theore'cal works to determine the structure-property rela'onships of BTBT deriva'ves, especially the role of alkyl chains on the thermal transport. We demonstrate (SThM) that κ BTBT = 0.63 ± 0.12 W m -1 K -1 is larger than κ C8-BTBT-C8 = 0.25 ± 0.13 W m -1 K -1. The nanoscale thermal resistance is weakly dependent on the film thickness, as predicted by a simple analy'cal model of the constric'on thermal resistance for a surface coated by a thin film. The experimental results are supported by the theore'cal es'mates of κ obtained by the AEMD (Approach to the Equilibrium Molecular Dynamics) method. 42 We find κ along the long caxis of the molecules larger for BTBT (1.04 W m -1 K -1 ) than for C8-BTBT-C8 (0.72 W m -1 K -1 ). The results also point to a decrease of κ in the a-b plane upon alkyla'on (from 0.6-1.3 W m -1 K -1 for BTBT to 0.26-0.33 W m -1 K -1 for C8-BTBT-C8).

RESULTS.

Scanning thermal microscopy results.

Figure 1 shows the typical topographic and thermal voltage images of a C8-BTBT-C8 film prepared by spin-coa'ng (see Methods). The organic thin film has a staircase topography (Fig. 1-a, height profile Fig. 1-c, red curve) with an incomplete surface coverage leaving apparent several zones of the underlying Si/ SiO 2 substrate. The same type of "staircase" topography is observed for all samples (Figs. S1 and S2 in the ESI), in agreement with previous results. 43 In contrast, the SThM thermal voltage (Fig. 1-b) shows a featureless structure (Fig. 1-c, blue curve) for all samples (Figs. S1 and S2 in the ESI). The thermal voltage V SThM is related to the thermal resistance of the sample by: (1) where T sample is the temperature of the surface sample, T amb the ambient temperature, R th the thermal resistance of the sample and the thermal flux.

We analyzed the SThM image to determine the thermal resistance of the films at various loca'ons with various thicknesses t, taking the thermal resistance of the Si/SiO 2 substrate as a reference (Fig. 1-d). (t) the thermal resistance of the organic film of thickness t and R org-ox the thermal resistance of the organic-oxide interface (Fig. 1-d).

V SThM ∝ T sample -T amb ( ) ∝ R th ! Q ! Q spin-coated
We determined κ using the null-point method, NP-SThM. 44 This differen'al method is suitable to remove the parasi'c contribu'ons (air conduc'on, etc…).

When the 'p contacts the sample surface (C), both the sample and parasi'c thermal contribu'ons are involved, whereas, just before physical 'p contact (non contact, NC), only the parasi'c thermal contribu'ons are involved. We measured the thermal voltage V SThM -z traces at several places on the organic films and on the substrate zones. The 'p temperature is determined from V SThM (see the ESI). The thermal conduc'vity is determined using the following rela'on: 44 (2)

where α and β are calibrated parameters related to the SThM equipment and 'p (Fig. S3 in the ESI) and T amb the room temperature (α = 25.6 W/m.K, β = 21.6 K/K and T amb = 22.5 °C). From a linear fit on these data, we get: κ C8-BTBT-C8 = 0.25 ± 0.13 W m -1 K -1 , κ BTBT = 0.63 ± 0.12 W m -1 K -1 and κ sub = 1.57 ± 0.43 W m -1 K -1 . A key finding is that the thermal conduc'vity of BTBT is larger than that of C8-BTBT-C8, in full consistency with our AEMD simula'ons (see below). For the Si/SiO 2 substrate, we found a value close to that of bulk SiO 2 , in agreement with previous measurements by NP-SThM 44 showing that the effec've κ is that of bulk SiO 2 (see Fig. 6-b in Ref. 44 and Fig. S4) if the SiO 2 thickness is larger than ∼ 100 nm (here 200-500 nm).

The ra'o of the thermal voltage measured on the organic domain over that on the substrate (Fig. 1), V SThM-org /V SThM-sub is related to the ra'o of the corresponding thermal resistance of each zone. For the Si/SiO 2 substrate, the constric'on thermal resistance [START_REF] Carslaw | ConducHon of Heat in Solids[END_REF] is R sub =1/4rκ ox = 9x10 6 K W -1 with κ ox the "bulk" SiO 2 value (1.4 W m -1 K -1 ) and r is the radius of the SThM 'p thermal contact (es'mated to be ≈ 20 nm, see the ESI). In order to determine the R org (t) from this SThM voltage ra'o, we need to es'mate the various interfacial resistances (Kapitza resistance) [START_REF] Kapitza | [END_REF] which cannot be directly measured here.

Reported values for a large variety of interfaces 27,[47][48][49][50][51][52][53][54] range typically between 10 -9 and 10 -6 m 2 K W -1 . For simplicity, we consider the lower limit value for all interfaces. In that case, the interface thermal resistances are negligible (8x10 5 K

W -1 ) compared to R sub and R org (R sub =1/4rκ ox = 9x10 6 K W -1 , R org =1/4rκ org = T C -T amb = α 1 κ + β ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ T NC -T C ( ) 2-5x10 7 K W -1
, considering the values of κ org determined above). In this oversimplified case, the ra'o of the thermal voltage measured on the organic domain versus over the substrate is given by , assuming the same thermal flux on the oxide and the organic domain (see the ESI), where is the effec've constric'on thermal resistance measured by the sharp SThM 'p at the surface of the organic thin film. We consider a simple analy'cal model derived by Dryden 55 for film with t/r>2 (here t > 40 nm):

(3)

The first term on the right-hand side stands for the constric'on thermal resistance of the bulk organic film (t → ∞) and the second term represents the effect of the underlying substrate covered by the film of thickness t. Figure 3 shows the measured obtained from V SThM-org /V SThM-sub ra'os picked up at various loca'ons on the organic films with various thicknesses (Fig. 1, Figs. S1 and S2, in the ESI) and on the underlying substrate, taking R sub =1/4rκ ox = 9x10 6 K W -1 . For t → ∞, tends to saturate at a larger value for C8-BTBT-C8 than for BTBT although the difference is weak. This trend is in agreement with the NP-SThM determina'on that κ BTBT > κ C8-BTBT-C8 . From the fits of Eq. 3 (solid lines in Fig. 4), we obtain a mean value (see the ESI) κ BTBT = 1.37 ± 0.01 W m -1 K -1 and κ C8- BTBT-C8 = 1.35 ± 0.01 W m -1 K -1 which are not significantly different. This implies that, even though the same trends are observed as with the NP-SThM method, this image analysis approach is perturbed by the interfacial thermal resistance, which more or less masks the actual values of the organic film thermal resistance.

V SThM-org V SThM-sub = R org * (t ) R sub ! Q R org * (t ) R org * (t ) = 1 4rκ org - 1 2πrκ org r t ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ln 2 1+κ org κ sub ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ R org * (t )
Previous works 27,28,49,50,54 also reported SThM 'p-organic materials and organic-SiO 2 interfacial thermal resistances larger than 10 7 K W -1 (or > ∼ 10 -8 m 2 K W -1 ),

i.e., larger than R org of our C8-BTBT-C8 and BTBT films.

R org * (t ) R org * (t )
Theory.

We use the Approach to the Equilibrium Molecular Dynamics (AEMD) method 42 to compute the la~ce κ. We ignored the contribu'on of the electronic κ at this stage because the thermal transport is barely controlled by electrons in most neutral and weakly doped organic semiconductors (OSCs). 56 This argument is supported by the newly developed molecular Wiedemann-Franz model 57 which predicts an electronic contribu'on to the heat transport smaller by several orders of magnitude than the corresponding la~ce contribu'on. In brief, the key steps of the AEMD methodology are the following: (i) applying a perfectly monitored thermal pulse on a simula'on box; (ii) recovering the ini'al thermodynamic equilibrium during a fast transient regime; (iii) fi~ng the 'me-decaying temperature difference between the right and le• parts of the system from a reliable solu'on of the one-dimensional heat equa'on in order to evaluate the thermal diffusivity, D. 58 This alterna've scheme has the benefit to be far less 'me-consuming than the previously cited methods due to the rapid dissipa'on of the thermal gradient. Then κ=DρC P is finally obtained, provided the density, ρ, and the specific heat, C P , of the system are known (see details in the ESI). It is also of prime importance to account for the size-dependence of the κ values deduced from this approach since phonons having a mean free path larger than the cell dimension do not effec'vely contribute to κ. An extrapola'on procedure is thus needed to get rid of these size effects 59 and to extract a quasilength-free la~ce κ from the linear regression of 1/κ versus 1/L (L the length of the box size along the direc'on of heat propaga'on). 1).

There is a fairly good quan'ta've agreement between experimental measurements and theore'cal es'mates. Interes'ngly, the calcula'ons indicate that the overall calculated conduc'vity (along the 3 crystal axes) of BTBT is higher than for C8-BTBT-C8 (Table 1), in full consistency with the experimental measurements probing essen'ally heat transport along the c axis.

Table 1. Calculated and measured thermal conducHvity values (W m

-1 K -1 )
DISCUSSION.

Since no theore'cal and experimental data of the out-of-plane κ was previously reported for these BTBT deriva'ves, we compare our values with other results obtained for various OSCs. The measured nanoscale out-of-plane κ C8-BTBT-C8 = 0.25 ± 0.13 W m -1 K -1 , is on par with values reported for other small molecule thin films by SThM: 0.15-0.20 W m -1 K -1 for methylstyryl-benzene, 28 0.15-0.4 W m -1 K -1

for CuPc (Cu-phthalocyanine) and 0.15-0.25 W m -1 K -1 for PbCu. 27 They are also consistent with the corresponding values obtained by characteriza'on methods at the macroscale on rubrene (0.07 W m -1 K -1 ) 23 and DNTT (0.45 W m -1 K -1 ). 24 The experimental out-of-plane κ BTBT = 0.63 ± 0.12 W m -1 K -1 lies at the highest limit of reported values: 0.45 W m -1 K -1 for DNTT 24 (to the best of our knowledge and excluding the highly unusual value of 21 W m -1 K -1 reported for crystal of TIPSpentacene 61,62 ).

The calcula'ons of the la~ce κ were performed on single crystals while the SThM measurement was carried out on polycrystalline samples containing a certain amount of disorder. It is also known (X-ray diffrac'on) that a disordered layer of less than ca. 10 nm exists at the SiO 2 /BTBT interfaces. 39 To cope with this "experimental" disorder we compare the measured values with an effec've isotropic κ eff defined as 63 where the in-plane κ is . The calculated values are given in Table 1. A ra'o of 2-2.5 between the thermal conduc'vi'es of BTBT and C8-BTBT-C8 is observed at both the experimental and theore'cal level. For both materials, the agreement between the evaluated κ eff and the measurements (Table 1) is excellent, suppor'ng the existence of a thin disordered layer at the SiO 2 /BTBT interface with a lower thermal conduc'vity (i.e.

ab-plane).

We do find a rather isotropic (3D) behavior of the heat propaga'on in the two deriva'ves in deep contrast with the charge transport proper'es featuring a two-dimensional character in presence of a herringbone arrangement; 60,64,65 note that a significant thermal conduc'vity along the three crystallines axes has also been reported theore'cally for the DNTT single crystal 36 . The κ ra'os for a/ b, c/a and c/b are respec'vely 2.22, 0.79 and 1.76 for BTBT and 1.27, 2.18 and 2.77 for C8-BTBT-C8. Morever, we evidence a no'ceable drop in the ab-plane κ when octyl chains are added on each side of the aroma'c cores, with anisotropic factors of ∼ 3.97 and ∼ 2.27. This complements the theore'cal

κ eff = κ c κ ab κ ab = κ a κ b κ a C 0 /κ a C 8 κ b C 0 /κ b C 8
study of Shi et al. 32 showing that κ is marginally affected by the presence of terminal saturated chains in Cn-BTBT-Cn (with n = 8;10;12). It is worth stressing that the drop in thermal conduc'vity upon addi'on of saturated chains has been also clearly observed in recent κ measurements performed on DNTT and C8-DNTT-C8 thin films in the a-b plane, and fully supported again by our AEMD calcula'ons. 66 To gain a deeper insight into the underlying physical mechanisms, we explore the spa'al character of the vibra'onal modes, by es'ma'ng their par'cipa'on ra'o (PR). [START_REF] Bell | Methods in Computa'onal Physics: Advances in Research and Applica'ons[END_REF] This parameter is a quan'ta've measure of the spa'al extension of vibra'ons, allowing to classify them into extended (large PR) and localized modes (PR ~ 0). It has been shown [START_REF] Cappai | [END_REF] that extended modes are generally more effec've in transpor'ng heat across the material than localized modes. The details of the calcula'ons are given in the ESI. The par'cipa'on ra'o of vibra'ons in BTBT and C8-BTBT-C8 are shown in Fig. 5 as a func'on of their frequency. We observe that PR in BTBT takes larger values than C8-BTBT-C8 over the whole frequency spectrum. This shows that BTBT hosts vibra'onal modes are spa'ally more extended than those in C8-BTBT-C8. Hence, BTBT alkyla'on leads to a strong localiza'on of the vibra'onal modes and consequently to a strong reduc'on of the overall thermal conduc'vity.

CONCLUSION.

In summary, we demonstrate that the thermal conduc'vity of BTBT, κ BTBT = 0.63 ± 0.12 W m -1 K -1 is larger than for C8-BTBT-C8, κ C8-BTBT-C8 = 0.25 ± 0.13 W m -1 K -1 . The nanoscale thermal resistance is weakly dependent on the film thickness, as predicted by a simple analy'cal model of the constric'on thermal resistance for a surface coated by a thin film. The experimental results are further supported by the theore'cal es'mates of the thermal conduc'vity obtained by the AEMD (Approach to the Equilibrium Molecular Dynamics) method. Moreover, we have not only demonstrated the drop of the thermal conduc'vity upon introduc'on of saturated chains but we have also provided a clear ra'onale based on the degree of delocaliza'on of the intermolecular vibra'onal modes. Finally, our calcula'ons point to significant thermal conduc'vity (3D-like) values along the 3 crystalline direc'ons, with anisotropy factors between the crystalline direc'ons below 1.8 for BTBT and below 2.8 for C8-BTBT-C8, in deep contrast with the charge transport proper'es featuring a two-dimensional character for these materials.

METHODS.

Synthesis and sample fabricaAon. BTBT and C8-BTBT-C8 were synthesized according to the reported procedures. 69,70 The thin films were deposited by spin-coa'ng on thermal SiO 2 /Si-n + according to previously reported methods. 39,43 Nine samples (5 C8-BTBT-C8, 4 BTBT) were prepared using various spin-coa'ng parameters to vary the film thickness (see the ESI).

Scanning Thermal Microscope (SThM). SThM 71,72 were carried out with a Bruker ICON machine equipped with the Anasys SThM module. All measurements were done at room temperature in an air-condi'oned laboratory (T amb = 22.5 °C, rela've humidity of 35-40 %). In the scanning mode, the topography and thermal voltage were recorded simultaneously. We used both the DC method or the 3ω-SThM. 73 The thermal conduc'vity is determined by using the null-point SThM method (see the ESI). 44 CalculaAons of the laDce thermal conducAvity. All MD simula'ons have been performed in the LAMMPS so•ware package 74 by means of the Op'mized Poten'als for Liquid Simula'ons All-Atoms (OPLS-AA) force-field (see the ESI). In addi'on, we also es'mated the par'cipa'on ra'o to characterize the vibra'onal proper'es of both BTBT and C8-BTBT-C8. This parameter is a quan'ta've measure of the spa'al extension of vibra'ons (see the ESI). calcula'ons of the vibra'onal proper'es. D.V. conducted the project, analyzed the results and wrote the paper with the help of all the authors. All authors have given approval to the final version of the manuscript. # These authors (M.N.G. and A.V.) contributed equally to this work.

versus distance curve (V SThM -z). At the transi'on from a non-contact (NC, 'p very near the surface) to a contact (C, 'p on the surface) situa'on, we observe a temperature jump, T NC -T C , which is used to determine the sample thermal conduc'vity according to the protocol described in Ref. 3. The temperature jump is measured from the approach trace only (to avoid any ar'fact due to wellknown adhesion hysteresis of the retract curve) and averaged over the 25 recorded V SThM -z traces. This differen'al method is suitable to remove the parasi'c contribu'ons (air conduc'on, etc…): at the contact (C) both the sample and parasi'c thermal contribu'ons govern the V SThM signal, whereas, just before physical 'p contact (NC), only the parasi'c thermal contribu'ons are involved.

The plot of the temperature jump, T NC -T C , versus the sample temperature at contact T C is linear and its slope is inversely propor'onal to the thermal conduc'vity. The 'p-sample temperature T C increases with the supply voltage of the Wheatstone bridge V DC (typically from 0.4 to 1 V).

Thermal flux. The determina'on of the thermal resistance from the ra'o of the thermal voltage measured on the organic domain versus over the substrate is valid under the hypothesis that all the electrical power delivered to the 'p is dissipated into the sample and the thermal flux is constant. 4,5 Albeit not strictly verified, this is jus'fied, in first approxima'on, since the constric'on resistances on the two zones are quite similar around 10 7 K W -1 (R sub =1/4rκ ox = 9x10 6 K W -1 , R org =1/4rκ org = 2-5x10 7 K W -1 , considering the values of κ org determined by the NP-SThM). However, in addi'on to taking into account the interface resistances (see text), a more precise analysis of data in Fig. 3 implies that a correc'on factor should be applied to determine the ra'o from the ra'o of the SThM voltages, . Since the thermal conduc'vity is lower for C8-BTBT-C8 than for BTBT, the correc'on factor should be higher for C8-BTBT-C8.

R org * R sub R org * (t ) R sub = V SThm-org V SThM-sub ( ) ! Q sub ! Q org ( ) ! Q sub ! Q org

Theore.cal methods

Molecular Dynamics. A wide range of Molecular Dynamics (MD) simula'ons has been developed to assess the laqce thermal conduc'vity of a plethora of materials and complex nanostructures. 6,7 Among them, the Green-Kubo (GK) formalism 8 based on the fluctua'on-dissipa'on theorem provides a direct access to the (off-)diagonal elements of the two-dimensional thermal conduc'vity tensor. Nevertheless, ensuring the proper convergence of the auto-and crosscorrela'on func'ons of the heat current may be a non-trivial task, especially for sos molecular systems. A second class of techniques built on a Non-Equilibrium Molecular Dynamics (NEMD) scheme 9 requires important computa'onal resources to ar'ficially maintain the system out of equilibrium while es'ma'ng its thermal conduc'vity in a sta'onary state. The key steps of the AEMD methodology are the following: (i) applying a perfectly monitored thermal pulse on a simula'on box; (ii) recovering the ini'al thermodynamic equilibrium during a fast transient regime; (iii) fiqng the 'me-decaying temperature difference between the right and les parts of the system from a reliable solu'on of the onedimensional heat equa'on in order to evaluate the thermal diffusivity, D. 10 This alterna've scheme has the benefit to be far less 'me-consuming than the previously cited methods due to the rapid dissipa'on of the thermal gradient. The AEMD approach also contrasts with both the Equilibrium Molecular Dynamics (EMD) and the Non Equilibrium Molecular Dynamics (NEMD)

techniques by es'ma'ng the phononic contribu'on to the thermal transport with no need for any instantaneous heat flux; moreover, the defini'on of the heat flux in the LAMMPS sosware 11 has been recently iden'fied as physically inconsistent for the study of molecular systems. 12,13 With the AEMD approach, the thermal conduc'vity κ=DρC P is finally obtained, provided the density, ρ, and the specific heat, C P , of the system are known. 10 The parameter C P is typically

∂T ∂t = D ∂ 2 T ∂x 2
es'mated to be 3R (with R the ideal gas constant), as predicted by the Dulong-Pe't model. 14 Note that no quantum correc'ons are taken into account because MD simula'ons are performed at 300K while many OSCs are characterized by a very low Debye temperature θ D 15,16 so that the classical approxima'on used in the Dulong-Pe't rela'onship is s'll valid. It is also of prime importance to account for the size-dependence of the laqce thermal conduc'vity deduced from this approach since phonons having a mean free path larger than the cell dimension do not effec'vely contribute to κ. An extrapola'on procedure is thus needed to get rid of these size effects 17 and to extract a quasi-length-free laqce thermal conduc'vity from the linear regression of 1/κ versus 1/L (L the length of the box size along the direc'on of heat propaga'on). We have carefully reparameterized the torsional poten'al terms against sophis'cated quantumchemical calcula'ons, with atomic charges calculated at the 1.14*CM1A-LBCC level. 18 The implementa'on of the force field was insured by the LigParGen free server. 19 is the best compromise between reasonable computa'onal 'me and sufficient accuracy for the es'ma'on of the laqce thermal conduc'vity.

Es?ma?on of the par?cipa?on ra?o. In order to characterize the vibra'onal proper'es of both BTBT and C8-BTBT-C8, we performed the diagonaliza'on of the corresponding dynamical matrix given by:

(s1)
where m i is the mass of the i th atom, F iα is the force on the i th atom along the α direc'on due to a displacement of atom j along the β direc'on. La'n indices are used for labelling atoms while Greek leWers indicate the (x,y,z) Cartesian components. The diagonaliza'on of the dynamical matrix was obtained by using the SLEPc library 20 in order to obtain the eigenvectors e s and eigenvalues ω 2 s where s = 1, . . . ,3N counts the system eigenmodes. The calcula'on of the force first deriva'ves have been performed using a numerical finite difference procedure by considering an atomic displacement as small as 5 x 10 -4 Å. Once the eigenvectors have been calculated, we es'mated the par'cipa'on ra'o (PR) as: 21 (s2)

PR yields an es'ma'on of the contribu'on of a subgroup of atoms in a specific vibra'onal mode. The actual localized or extended character of that mode is

ΔT = T 1 -T 2 C n e -α n 2 Dt n=1 5 ∑ D iα , jβ = -1 m i m j ∂F iα ∂r jβ PR = 1 N e i ,s 2 i=1 N ∑ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 e i ,s 4

NP-SThM calibra.on.

The sample temperature is calculated from the measured thermal voltage by (Eq.S1)

V SThM = GR tip T sample ( ) = G R amb + λ T sample -T amb ( )

⎡ ⎣ ⎤ ⎦
where G is the known transfer func'on of the Wheatstone bridge and the amplifier gain (1.08 x V DC V/Ω) for the DC-SThM methods, 22 8.2x10 -2 x V AC V/Ω for the 3ω-SThM. The 'p resistance R 'p varies linearly with sample temperature, and has been calibrated given R amb = 290.2 Ω and λ = 0.27 Ω/°C.

To determine the thermal conduc'vity from data in Fig. 3 (here 200-500 nm). 3 This behavior is in agreement with a simple analy'cal model derived by Dryden 23 for film with t/r > 2 (r being the radius of the SThM 'p with r 'p = 100 nm (data from Bruker), the rela've humidity φ = 0.35-0.4 (aircondi'oned laboratory, values checked during the measurements) and the contact angle of the concave meniscus between the 'p and the surface θ ≈ 30° as measured for π-conjugated molecular crystals in Ref. 26. We get r ≈ 20 nm. The water meniscus contact angle depends on the surface energy of the sample, and thus should, in principle, not be the same on the organic domains and the uncovered SiO 2 surface, the later being more hydrophilic than the organic materials. However, we cannot perform standard water contact angle measurements inside the micrometer size uncovered SiO 2 (see Fig. S1 andS2),

and we consider the same value of 30° in both cases. Moreover, it is likely that the SiO 2 surface is covered by organic contaminants from the spin-coa'ng deposi'on technique.

FiNed parameters of Eq. 3.

Table S2 gives the values of the fiWed parameters (κ org ) for the data shown in Fig. 4 (main text).

Table S2. Fit parameters (Fig. 4, Eq. 3)

We get the average values κ BTBT = 1.37 ± 0.01 W m -1 K -1 and κ C8-BTBT-C8 = 1.35 ± 0.01 W m -1 K -1 . We note that a values of r = 80 nm has been used to these fits.

Using r = 20 nm gave poor fits. The value r = 80 nm should indicate a more

Figure 1 .

 1 Figure 1. (a) Topographic (10 μm x 10 μm) and (b) SThM thermal voltage (at the output of the Wheatstone bridge, 3ω-SThM method) images of a C8-BTBT-C8 film

Figure 2

 2 Figure 2 show 25 typical 'p temperature versus distance (z-trace) curves measured on the C8-BTBT-C8 domain and on the nearby apparent Si/SiO 2substrate (circled bullets on the SThM images in the insets). When approaching the heated 'p to the surface, the 'p temperature decreases gradually because the heat transfer through the air gap is increased. At contact, we observe a sudden decrease from T NC to T C , due to the addi'onal heat flux through the 'psample contact. Figure2-cshows the T C versus T NC -T C curves for the C8-BTBT-C8 and BTBT samples and on the apparent substrate zone, where T C and T NC are the measured temperature at contact and non-contact condi'ons.

Figure 2 .

 2 Figure 2. 25 Hp temperature versus Hp verHcal displacement curves (z-trace, approach, 0 corresponds to the Hp retracted) measured at V DC = 0.6 V on (a) a C8-BTBT-C8 domain (sample #5), (b) on the Si/SiO 2 substrate (as indicated by the circled bullets). (c) Temperature jump T NC -T C versus temperature at Hp contact T C for C8-BTBT-C8 (◆), BTBT (•) and the Si/SiO 2 substrate (◼). The increasing T C corresponds to a supply voltage V DC from 0.6 -0.9 V (by step of 0.1 V) of the Wheatstone bridge (0.6 -1 V, step of 0.1 V on the substrate). Solid lines are the linear fits. Each data point (C8-BTBT-C8 and BTBT) is the average of 3 measurements at 3 different locaHons (25 V SThM -z traces at each locaHon). The data points for the substrate are averaged from the data acquired on the 2 samples.

Figure 3 .

 3 Figure 3. EffecHve constricHon thermal resistance

∂x 2 !Figure 4 .Figure 4

 244 Figure 4. Inverse of the lance thermal conducHvity as a funcHon of the inverse of the simulaHon box length along direcHons a, b and c for (a) BTBT and (b) C8-BTBT-C8. Each inset represents the molecular packing of BTBT and C8-BTBT-C8 in the ab plane. For sake of clarity, we did not represent the alkyl chains in (b). The two compounds crystallize into a monoclinic structure (a = 5.854 Å, b = 7.960 Å, c = 11.809 Å, α = 105.990° for BTBT and a = 5.927 Å, b = 7.880 Å, c = 29.180 Å, β = 92.443° for C8-BTBT-C8; respecHvely); both of them exhibit a very similar layered herringbone packing, 60 as shown in the insets.

Figure 5 .

 5 Figure 5. EsHmated ParHcipaHon RaHo (PR) as a funcHon of the vibraHonal frequency for BTBT (blue) and C8-BTBT-C8 (red).

  Supercells ranging from 10 to 52 units cell have been generated along each direc'on of interest for heat transport and periodic boundary condi'ons were applied along the main three axes. First, atomic posi'ons were op'mized at 0 K while keeping fixed the cell parameters before relaxing the whole simula'on box during a second energy minimiza'on. Aserwards, systems have been aged in the canonical NVT ensemble (mole (N), volume (V) and temperature (T)) and in the isothermal-isobaric NPT ensemble (mole (N), pressure (P) and temperature (T)) during 1 ns in ambient condi'ons, using a Nose-Hoover thermostat and barostat. Then, a step-like temperature profile was created by simultaneously "freezing" atomic mo'ons in one half of the simula'on box while hea'ng up [cooling down, respec'vely] the second part of the system alterna'vely at < T1 > = 362.5 K (< T2 > = 237.5 K, respec'vely) via NVT simula'ons (∼ 250 ps). Finally, an NVE (mole (N), volume (V) and energy (E)) simula'on las'ng up to ∼ 3 ns was achieved to dissipate the ini'al 125 K thermal gradient. Throughout a post-processing step, the monitored temperature offset was fiWed by a sum of exponen'als expressed as , where C n and α n are coefficients depending on ini'al condi'ons. A fiqng func'on based on five terms

∑

  Figures S1. Typical topographic and thermal voltage images of the five C8-BTBT-C8 measured by 3ω-SThM (V AC = 0.2 V, 1 kHz), samples #1-4, and by DC-SThM (V DC = 0.5V), sample #5.

Figure S2 .

 S2 Figure S2. Typical topographic and thermal voltage images of the BTBT samples measured by 3ω-SThM (V AC = 0.2 V, 1 kHz) for samples #1 and 2 and samples #3,4 by DC-SThM (V DC = 0.6V).

  and using Eq. (2), we calibrated the null-point SThM according to the protocol in Ref. 3. The same T C vs. T NC -T C measurements were done on two materials with well-known thermal conduc'vity (below) : a glass slide (1.3 W/m.K) and a low doped silicon wafer with its na've oxide (150 W/m.K). From a linear fit on the data, we get α = 25.6 W m -1 K -1 and β = 21.6 K/K.

Figure S3 .

 S3 Figure S3. T C vs. T NC -T C plot for the two reference materials. Solid lines are the linear fits.

  SchemaHc of the nanoscale SThM measurement. On the substrate, the thermal resistance is R th-ox =R t-ox +R sub , with R t-ox the thermal resistance of the Hp-SiO 2 interface and R sub the thermal resistance of the substrate. Over the organic domains, we have R th-org =R t-org +R org (t)+R org-ox +R sub, with R t-org the thermal resistance of the Hp-organic interface, R org

on Si/SiO 2 substrate, (c) height (red) and thermal voltage profiles (blue). (d)

hydrophilic surface (Eq. S2, lower θ, but Eq. (S2) has a limit r = 21.7 nm for θ=0°, thus this simple model is inappropriate in the case of highly hydrophilic surface).

Nevertheless, this uncertainty in the determina'on of r does not change the conclusion of the NP-SThM measurements that the thermal conductance of BTBT is higher than for C8-BTBT-C8 (r is included in the calibra'on parameters).

ASSOCIATED CONTENT

Electronic Supplementary Informa'on (ESI) is available: Sample fabrica'on, SThM methods, theore'cal methods, addi'onal SThM images, NP-SThM calibra'on, es'ma'on of the constric'on thermal resistance of the Si/SiO 2 substrate, es'ma'on of the radius of the thermal contact between 'p and surface, fiWed parameters of Eq. 3.

AUTHOR CONTRIBUTIONS

CONFLICTS OF INTEREST.

The authors declare no compe'ng financial interest.

ACKNOWLEDGEMENTS.

M.G., D.G., S.L. and D.V. thank the ANR for financial support (ANR-16-CE05-0029). 8. FiWed parameters of Eq. 3.

Y.G. is thankful to the

Sample fabrica.on

The Si-n + (highly doped ∼10 -3 Ω.cm)/SiO 2 substrate (200 -500 nm of thermal SiO 2 ) were first cleaned in CH 2 Cl 2 (3mn, sonicated), in IPA (isopropyl alcohol) solu'on (3mn, sonicated) and then submiWed during 10 mn to UV-ozone cleaning. They were used immediately. The C8-BTBT-C8 and BTBT (scheme 1) solu'ons (5mg/mL in toluene for BTBT and 5 mg/mL in tetrahydrofuran for C8-BTBT-C8) were spin coated at 1000 rpm for 9s followed by 30 s at 1500 rpm, or directly at 4500 rpm for 30 s. We used two methods to deposit a drop of the solu'on onto the substrate: we dropped the solu'on on the substrate and started the spin-coater (method A) or we dropped the solu'on while the spin coater was already running (method B). Table S1 summarizes the spin-coa'ng condi'ons for the 9 samples. The films are not uniform and we have indicated the maximum thickness of the films as measured from AFM images (see Figs. S1 andS2).

SThM measurements

Topographic and thermal images. In the scanning mode, the topography and thermal voltage were recorded simultaneously. We used both the DC method or the 3ω-SThM. The SThM 'p is a lithographed Pd wire, i.e. resistance, inserted in a Wheatstone bridge, which is supplied by a DC voltage, V DC . The 'p is used to heat the sample (the higher the DC voltage, the higher the heat flux and the 'p temperature) and to measure the 'p/surface temperature, in the so-called ac've mode. The 'p resistance measured by the Wheatstone bridge connected to a voltage amplifier. From this output SThM voltage, we deduced the 'p resistance, which is calibrated versus temperature. Thus, the SThM voltage recorded at the amplifier output can be converted to 'p/sample temperature. To minimize parasi'c contribu'ons (air conduc'on, radia'on) and thus increase the sensi'vity of the 'p/sample contribu'on, we also use the 3ω-SThM method, 1,2 in which the Wheatstone bridge is supplied by an AC voltage (0.2 V at 1 kHz) and we measure the third harmonic signal at the Wheatstone bridge output with a lock-in.

Null-point SThM method. The null-point SThM 3 was used at selected points on the organic films (organic domains) and/or the uncovered Si/SiO 2 substrate. From a previously recorded topographic/thermal image a zone of interest is selected where we define a 5x5 grid, each point spaced by 10 nm. At each point of the grid, in the z-trace mode (approach and retract) we recorded the SThM voltage R sub =1/4rκ ox = 9x10 6 K W -1 with κ ox the "bulk" SiO 2 thermal conduc'vity (1.4 W m -1 K -1 ). Copyright (2011) American Chemical Society).

Es.ma.on of the radius of the thermal contact between .p and surface.

The thermal contact radius is calculated following the approach reported in Ref.

24 taken into account the mechanical 'p radius r 'p and the size of the water meniscus at the 'p/surface interface. The thermal radius of the thermal contact is given by 25 (Eq. S2) r th = 2.08 -r tip cosθ lnϕ