Erwann Legrand
email: erwann.legrand@sb-roscoff.fr

Pascal Riera

Loïc Pouliquen

Olivier Bohner

Thierry Cariou

Sophie Martin

Ecological characterization of intertidal rockpools: Seasonal and diurnal monitoring of physico-chemical parameters

Keywords:

Diurnal and seasonal variations in temperature, salinity, oxygen concentration, carbonate chemistry parameters and nutrient concentration were monitored within mid-tidal rockpools on the north-western coast of Brittany, France. We also examined the importance of rockpool physical characteristics (e.g. shore height, surface, volume and depth) on the physico-chemical variability. Rockpool shore height and depth appeared as the main physical factors driving physico-chemical variations. Shallow upper shore rockpools exhibited the largest diurnal and seasonal fluctuations of temperature, oxygen concentration and pH T . Conversely, deeper and lower shore rockpools experienced weaker variations, excepted for salinity and nutrient concentrations. The physico-chemical environment was likely to be strongly influenced by rockpool community composition, especially in summer when rockpools were colonized by dense canopy-forming algae. During the day-time emersion, photosynthesis increased oxygen concentration and pH T but decreased nutrient concentrations in upper shore rockpools. The increase in rockpool pH T provided an alkaline environment conducive to coralline algal calcification. Conversely, at night, upper shore rockpools presented a strong decrease in oxygen concentration and pH T due to the high community respiration, increasing the dissolution in coralline algae. Therefore, rockpools are characterized by complex interactions between physical and chemical parameters and biological processes. Rockpools appear as suitable environments to examine the physiological processes set up by species to face natural variability, especially in a context of climate change.

Introduction

Intertidal communities are exposed to a dynamic environment influenced by tidal flooding [START_REF] Widdows | Impact of biotic and abiotic processes on sediment dynamics and the consequences to the structure and functioning of the intertidal zone[END_REF]. Organisms living at the interface between land and sea are exposed to consecutive periods of emersion and immersion. During low tides, organisms undergo harsh environmental conditions characterized by high temperature variations and solar radiation, low humidity and windy conditions [START_REF] Martínez | Physical factors driving intertidal macroalgae distribution: physiological stress of a dominant fucoid at its southern limit[END_REF][START_REF] Tomanek | Physiological ecology of rocky intertidal organisms: A synergy of concepts[END_REF]. These physical conditions take species close to their physiological thresholds [START_REF] Fernández | Additive effects of emersion stressors on the ecophysiological performance of two intertidal seaweeds[END_REF], through a rapid desiccation and damages to photosynthetic and cellular components [START_REF] Collén | Reactive oxygen metabolism in intertidal Fucus spp. (Phaeophyceae)[END_REF][START_REF] Hoffman | Interactions between UV radiation and temperature limit inferences from single-factor experiments[END_REF].

On the sea shore, rockpools are patchy depressions which retain seawater during low tide. This intertidal marine environment acts as refuge for many intertidal species [START_REF] Underwood | The effects of tidal height, wave-exposure, seasonality and rock-pools on grazing and the distribution of intertidal macroalgae in New-South-Wales[END_REF][START_REF] Zander | Vertical Distribution Patterns, Intertidal Fishes[END_REF], due to the lower amplitude of physico-chemical fluctuations compared with emerging substrate [START_REF] Metaxas | Community structure and organization of tidepools[END_REF]. However, physico-chemical variations within rockpools remain much greater than those of nearshore seawater [START_REF] Morris | Diurnal and seasonal variation in physicochemical conditions within intertidal rock pools[END_REF]. Strong diurnal and seasonal variations in rockpools have been recorded for temperature, oxygen concentration, pH and salinity [START_REF] Daniel | Diurnal variations in physico-chemical conditions within intertidal rockpools[END_REF][START_REF] Morris | Diurnal and seasonal variation in physicochemical conditions within intertidal rock pools[END_REF], but never in terms of whole carbonate chemistry and nutrients. In addition to temporal variations, rockpool physico-chemical parameters are highly variable in space, depending mainly on the wave exposure and the shore height [START_REF] Firth | The importance of water-retaining features for biodiversity on artificial intertidal coastal defence structures[END_REF][START_REF] Martins | Community structure and functioning in intertidal rock pools: effects of pool size and shore height at different successional stages[END_REF].

Rockpools are also characterized by strong interactions between physico-chemical parameters and biological processes [START_REF] Benedetti-Cecchi | The interplay of physical and biological factors in maintaining mid-shore and low-shore assemblages on rocky coasts in the north-west Mediterranean[END_REF][START_REF] Underwood | The effects of tidal height, wave-exposure, seasonality and rock-pools on grazing and the distribution of intertidal macroalgae in New-South-Wales[END_REF]. In particular, species distribution and abundance are controlled by both abiotic factors [START_REF] Huggett | Some relationships between elevation, physicochemical variables and biota of intertidal rock pools[END_REF][START_REF] Metaxas | Community structure and organization of tidepools[END_REF][START_REF] Truchot | Oxygen and carbon-dioxide in the marine intertidal environment -Diurnal and tidal changes in rockpools[END_REF] and species interactions, such as herbivory, predation and competition [START_REF] Metaxas | Community structure and organization of tidepools[END_REF]. The high productivity observed in these habitats is mostly explained by the diversity of seaweeds [START_REF] Araujo | Macroalgal communities of intertidal rock pools in the northwest coast of Portugal[END_REF] and invertebrates [START_REF] Ganning | Studies on chemical, physical and biological conditions in Swedish rockpool ecosystems[END_REF] which develop in rockpools [START_REF] Firth | Biodiversity in intertidal rock pools: Informing engineering criteria for artificial habitat enhancement in the built environment[END_REF].

In this context, the present study aims to (1) quantify diurnal and seasonal variations in multiple physico-chemical parameters (e.g. temperature, salinity, oxygen concentration, carbonate chemistry and nutrient concentration), which naturally occur within intertidal rockpools and (2) assess the importance of rockpool physical characteristics (e.g. shore height, surface, volume and depth) on the physico-chemical variability. Seasonal variations within rockpools were examined over a period of 14 months, during which two diurnal monitoring of physicochemical parameters were carried out, one in the winter and one in the summer. Finally, the composition of the community present in each rockpool was examined in the winter and the summer to better understand the influence exerted by biotic factors on the physico-chemical variability.

Materials and methods

Seasonal monitoring

From January 2015 to March 2016, 5 rockpools were monitored every 2 weeks during spring tides under fair weather conditions. Rockpools were situated in a rocky shore at the Bloscon site in Roscoff, Brittany, France (48 • 43'28N 03 • 58'08W). Rockpools monitored were distributed at different heights above chart datum (C.D., Table 1) in the mid-tidal zone and were colonized by coralline algae, non-calcareous algae and associated fauna (Fig. 1). All selected rockpools were filled with seawater at each high tide and were isolated from the sea at each low tide. Selected rockpools presented different volumes and depth to assess relationships between physical structure, physico-chemical variability and assemblage composition.

Measurements of temperature, dissolved oxygen concentration, pH T (pH expressed on the total hydrogen ion concentration scale) and salinity were performed at different locations across of each rockpool, using a multimeter (HQ40D, Hach Lange, Ltd portable LDO). All measurements and samples were made at the end of the emersion of rockpools, just before they were covered by the tide. Throughout the year, measurements were made at the same positions and depth in each rockpool and seawater was homogenized before each measurement. Seawater pH and temperature measurements were realized using a probe associated with a temperature sensor with an accuracy of 0.3 • C and 0.02 pH units, respectively (PHC101, Hach Lange, IntelliCAL). The pH probe was calibrated using Tris/HCl and 2-aminopyridine/HCl buffers [START_REF] Dickson | Guide to Best Practices for Ocean CO 2 Measurements[END_REF]. Dissolved oxygen concentrations (mg L -1 ) were measured with a luminescent dissolved oxygen probe (LDO101, Hach Lange, IntelliCAL) with an accuracy of 0.02 mg L -1 and converted to oxygen concentrations in µmol L -1 . Salinity was determined using a conductivity probe (CDC401, Hach Lange, IntelliCAL, accuracy of 0.1). Seawater samples were also taken in the rockpools for total alkalinity (A T ) measurements. 60 mL seawater samples for A T analyses were filtered through 0.7 µm Whatman GF/F filters and poisoned with a mercuric chloride solution to prevent biological activity [START_REF] Dickson | Guide to Best Practices for Ocean CO 2 Measurements[END_REF]. A T was determined by an opencell titration on an automatic titrator (Titroline alpha, Schott SI Analytics, Mainz, Germany) according to the method developed by [START_REF] Dickson | Guide to Best Practices for Ocean CO 2 Measurements[END_REF]. A T was calculated using a Gran function applied to pH values ranging from 3.5 to 3.0 [START_REF] Dickson | Guide to Best Practices for Ocean CO 2 Measurements[END_REF] and corrected using standard reference material provided by Andrew G. Dickson (CRM Batch 111). Saturation state of seawater with respect to aragonite (Ω Ar ) and saturation state of seawater with respect to calcite (Ω Ca ) were calculated from pH T , A T , temperature and salinity using the CO2SYS software [START_REF] Lewis | Program developed for CO 2 system calculations[END_REF], using constants from [START_REF] Mehrbach | Measurement of apparent dissociation-constants of carbonic-acid in seawater at atmospheric-pressure[END_REF] refitted by [START_REF] Dickson | A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media[END_REF]. Nitrite (NO -

2 , µmol L -1 ), nitrate (NO - 3 , µmol L -1 ), phosphate (PO 3- 4 , µmol L -1
) and silicate (Si(OH) 4 , µmol L -1 ) concentrations were monitored once a month. For NO - 2 , NO - 3 and PO 3- 4 measurements, 60 mL of seawater was sampled and filtered through 0.7 µm Whatman GF/F filters in 100 mL plastic containers. Then, samples were frozen (-20 • C) pending analyses. For Si(OH) 4 , 60 mL seawater samples were filtered through 0.8 µm Millex cellulose ester filters in 100 mL plastic containers and were kept refrigerated pending analyses. Nutrient samples were measured using a AA3 auto-analyzer (Seal Analytical) according to the method of [START_REF] Aminot | Dosage automatique des nutriments dans les eaux marines: méthodes en flux continu[END_REF], with an accuracy of 1 nmol L -1 for NO - 2 , 0.02 µmol L -1 for NO - 3 , 1 nmol L -1 for PO 3- 4 , and 0.01 µmol L -1 for Si(OH) 4 . Seasonal physico-chemical parameters were compared with those of near shore seawater (when available), obtained from SOMLIT (Service d'Observation en Milieu LITtoral, INSU-CNRS) station, located in the Estacade site in Roscoff.

Diurnal monitoring

Among the five rockpools chosen for the seasonal monitoring, three of them were selected for diurnal measurements (rockpools 2, 4 and 5, Table 1). Two 24-h monitoring were realized during spring tides, in summer (September 3rd, 2015) and in winter (March 10th, 2016). Measurements of temperature, pH T , oxygen concentration, salinity and A T were realized every hour during the emersion period, and Ω Ar and Ω Ca were calculated as described above. Near shore seawater was also monitored and sampled just before and after the emersion period. All samples were analyzed as described above.

Community composition

After the one-year monitoring, biomass sampling was realized in the 5 rockpools in summer and in winter. 10 × 10 cm quadrats were randomly placed in rockpools and all the seaweeds and animals in the quadrats were sampled. The number of quadrats per rockpool depended on the size of the rockpools (4 quadrats for rockpools 2 and 4, 5 quadrats for rockpool 3 and 8 quadrats for rockpools 1 and 5). In the laboratory, macrofauna and seaweed species were sorted and the main species were identified. Species dry weight (DW) was determined after drying fresh samples at 60 • C for 48 h. Species were then burned for 6 h at 500 • C in a furnace to obtain ash-free dry weight (AFDW; [START_REF] Crisp | Energy flow measurements[END_REF]. For the CCA L. incrustans, the biomass could not be determined precisely without deteriorating rockpools. Therefore, the cover percent was estimated for each quadrat and AFDW was obtained from previously established relationships between the surface covered by the alga and the AFDW.

Data analysis

Differences in physico-chemical parameters among rockpools were examined from seasonal data using a three-way permutational multivariate analysis of variance (PERMANOVA), based on Euclidian distance [START_REF] Anderson | A new method for non-parametric multivariate analysis of variance[END_REF]. PERMANOVAs were run with 4999 permutations [START_REF] Anderson | A new method for non-parametric multivariate analysis of variance[END_REF]. These statistical analyses were performed with the PRIMER 7 & PERMANOVA+ software package. The Principal component analysis (PCA) was completed using R 3.2.2 version from seasonal data. The PCA was performed using the ''factoextra'' package to test the correlation between environmental and physical variables and determine which variables explain most of the space-time variability observed between rockpools.

Results

Seasonal variations

The physico-chemical parameters exhibited significant differences between rockpools (PERMANOVA, F = 44.9, p <0.001, n = 29) except between rockpools 3 and 4 (Table 2, Figs. 2 and3). The rockpools 1 and 2 showed the largest amplitude for all physico-chemical parameters, except for salinity and nutrient concentrations. Conversely, the rockpool 5 presented weak seasonal variations for all parameters except for salinity and nutrient concentrations, which exhibited the largest variations. Physicochemical parameters showed strong differences according to seasons. At the end of the emersion period, the seawater temperature within rockpools varied from 7.9 • C in February 2015 in rockpool 3 to 26.9 • C in July in rockpool 1. Minimum oxygen concentrations were recorded in the rockpool 5 in January 2016 (242 µmol O 2 L -1 ), while maximum concentrations were obtained in the rockpool 1 in November, reaching 1040 µmol O 2 L -1 . The rockpool 5 showed the lowest salinity in January (33.4), while the highest values were recorded in rockpool 2 in May and August (36.6). The minimum pH T was 7.79 and was measured in rockpool 5 in January 2016. The maximum pH T value was 10.01, recorded in rockpool 1 in April. A T varied from 1553 µmol L -1 in early September to 2588 µmol L -1 in late January 2016 in rockpools 2 and 5, respectively. Carbonate saturation states of seawater with respect to aragonite (Ω Ar ) and calcite (Ω Ca ) are the lowest in rockpool 5 in January 2016 (1.3 and 2.1, respectively) and the highest in rockpool 1 in May (11.5 and 17.9,respectively).

NO - 2 concentrations varied from 0.00 µmol L -1 in August in rockpool 1 to 0.65 µmol L -1 in early September in rockpool 5 (Fig. 3). The rockpool 1 also presented the lowest NO - The first principal component (PC1) of the PCA was strongly positively correlated with depth, nutrients and A T and negatively correlated with increasing shore height, pH T , oxygen concentration and salinity (Fig. 4a). The second principal component (PC2) indicated an opposition between temperature and oxygen concentration. The different nutrients were clearly positively correlated between them and A T , but negatively correlated with oxygen concentration, pH T , temperature and salinity.

From the PCA, the within-rockpool seasonal variability appeared mainly driven by temperature (Fig. 4b). The rockpool 5 differed from the others through much higher nutrient concentrations and A T , and lower pH T , salinity and oxygen concentrations. Conversely, the rockpools 1 and 2 were characterized by high pH T , salinity and oxygen concentrations, but low nutrient concentration and A T .

Diurnal variations

Diurnal variations exhibited differences among the 3 rockpools monitored both in the summer 2015 (Fig. 5) and the winter 2016 (Fig. 6). While rockpool 1 presented high variations for all physicochemical parameters, variations in rockpool 5 were close to those of nearshore seawater. Furthermore, higher diurnal amplitudes were observed in the summer for all parameters, except for salinity for which variations are greater in winter. In the winter, diurnal temperature in rockpool 2 varied from 7.2 • C to 12.0 • C at the end of night-and day-time emersion, respectively. In the summer, temperature ranged from 13.4 • C at night to 20.9 • C at the middle of day-time emersion.

The salinity in rockpools was higher in the summer than in the winter. In the summer, the rockpools 2 and 3 exhibited low salinity variations during night-and day-time emersion. Salinity fluctuations in these rockpools were close to the values of nearshore seawater. Conversely, the rockpool 5 presented higher variations, with a decrease in salinity until 35.0 at the end of the emersion period. Oxygen concentrations during night-time emersion reached minimum concentrations in rockpool 2, at 53 and 10 µmol O 2 L -1 in the winter and the summer, respectively. During the day-time emersion, maximum oxygen concentrations were measured in rockpool 3 in winter (913 µmol O 2 L -1 after 6 h of emersion) and rockpool 2 in summer (955 µmol O 2 L -1 after 2 h of emersion). In rockpool 5, variations in oxygen concentration were lower than in other rockpools. A reduction in oxygen concentration was observed during the day-time emersion in the summer. As for oxygen concentration, pH T was constantly reduced at night down to a minimum value was reached in rockpool 2 at the end of the emersion period (7.57 and 7.49 in the winter and the summer, respectively). During the day, maximal pH T values of 9.14 in the winter and 9.90 in the summer were also reached in rockpool 2. Conversely, the rockpool 5 presented low diurnal pH T variations both in the winter and the summer. A pH T reduction was also observed during the day-time emersion in the summer. In the winter, A T increased at night in rockpool 2 only (maximum of 2566 µmol L -1 ), while in the summer the rise in A T was observed within the 3 rockpools monitored. During the day-time emersion, A T was strongly reduced in rockpools 2 and 3 for each season, with extremely low values reached in rockpool 2 in summer (1553 µmol L -1 ). As for other parameters, A T presented weak variations in rockpool 5. The carbonate saturation state with respect to aragonite (Ω Ar ) and calcite (Ω Ca ) decreased at night in the 3 rockpools monitored, for each season. During the night-time emersion, Ω Ar declined below 1 within rockpools 2 and 3, both in the winter and the summer. Conversely, during the day-time emersion, Ω Ar and Ω Ca increased significantly in rockpools 2 and 3. Maximum values of 11 and 17 were reached in rockpool 2 in the summer, for Ω Ar and Ω Ca , respectively. Except in the rockpool 5, in which Ω variations were close to those of near-shore seawater.

Community composition

Regarding the floral composition, strong differences were observed between rockpools and according to the season (Table 3). L. incrustans attained the highest biomasses in rockpools 1 and 3 in winter and summer, respectively. Conversely, higher biomass of E. elongata was recorded within rockpool 5 both in summer and winter. Fleshy algae biomass ranged from 204.5 g AFDW m -2 to 712.3 g AFDW m -2 in summer (rockpools 2 and 4, respectively) and from 3.7 g AFDW m -2 to 228.0 g AFDW m -2 in winter (rockpools 2 and 5, respectively). In the summer, rockpool 1 was dominated by the green algae Ulva intestinalis and the brown seaweed S. muticum while the rockpool 2 was mainly composed by Ulva sp. In winter, fleshy algae were nearly absent in these two rockpools. S. muticum was the dominant fleshy alga within the rockpool 3 both in the summer and the winter, although winter biomass was only half that of summer biomass. In the rockpool 4, the fleshy algal biomass was lower in winter compared with summer being mainly composed by the brown alga F. serratus. The fleshy algal biomass of rockpool 5 consisted of more diverse and sparse seaweeds. Although Ulva sp. and S. muticum were still abundant in this rockpool, the greater part of summer biomass corresponded to a few small feet of Laminaria digitata. In winter, fleshy algae community was dominated by undetermined filamentous red algae which grew on E. elongata.

Table 3

Winter and summer biomasses (in grams of ash-free dry weight) of the dominant coralline and fleshy algae present in the five rockpools monitored for seasonal variations at the Bloscon site in Roscoff (Brittany, France). The total biomass of fauna ranged from 4.2 g AFDW m -2 (rockpool 5) to 11.8 g AFDW m -2 (rockpool 4) in summer and from 0.7 g AFDW m -2 (rockpool 1) to 12.4 g AFDW m -2 (rockpool 2) in winter. The gastropod Gibbula umbilicalis dominated the faunal species in all the rockpools both in winter and summer. The biomass of G. umbilicalis extended from 0.75 g AFDW m -2 to 4.14 g AFDW m -2 in summer (rockpools 5 and 3, respectively) and from 0 g AFDW m -2 to 6.1 g AFDW m -2 in winter (rockpools 4 and 2, respectively).

Discussion

Rockpool environment is governed by complex interactions between physical, chemical and biological parameters [START_REF] Martins | Community structure and functioning in intertidal rock pools: effects of pool size and shore height at different successional stages[END_REF]. Rockpools can be described by evaluating their shore height, surface, depth and volume. Among these characteristics, shore height appeared as one of the main physical factors controlling the physico-chemical variability within rockpools, as evidenced by [START_REF] Morris | Diurnal and seasonal variation in physicochemical conditions within intertidal rock pools[END_REF]. Consistently with their findings, rockpools 1 and 2 located on the upper shore exhibited the largest diurnal and seasonal fluctuations of temperature, oxygen and pH T . In contrast, the rockpool 5 was the lowest on the shore and the deepest and presented weaker variations in temperature, oxygen and pH T but higher nutrient concentrations and total alkalinity. In accordance with the present results, [START_REF] Martins | Community structure and functioning in intertidal rock pools: effects of pool size and shore height at different successional stages[END_REF] suggested that rockpools depth was more important in controlling physico-chemical variability than surface and volume.

During the day, upper shore rockpools were directly exposed to incident light, increasing the seawater temperature. Moreover, rockpools higher on the shore zone are exposed to external conditions for longer than lower ones, increasing the duration of thermic exchange with the atmosphere [START_REF] Martins | Community structure and functioning in intertidal rock pools: effects of pool size and shore height at different successional stages[END_REF]. This results in larger diurnal and seasonal variations in temperature than in lower shore rockpools. Similarly at night, the temperature within rockpools was mainly driven by the atmospheric temperature and wind exposure. In both the winter and the summer, atmospheric temperature at night was lower than near-shore seawater. As a consequence, during the emersion period, the temperature decrease at night was more important in high shore rockpools. In contrast to the upper shore rockpools, lower shore rockpools presented lower seasonal and diurnal variations for all parameters, except for salinity and nutrients. The salinity showed seasonal variability, with higher values in the summer, when evaporation was maximal. However, freshwater seepage may occur within the rockpool 5, as the salinity in this rockpool was lower than near-shore seawater both for seasonal and diurnal measurements. This statement is in accordance with the higher nutrient values measured in this rockpool.

As for intertidal emerging substrate, light intensity and temperature exert a strong influence on biological processes [START_REF] Fernández | Additive effects of emersion stressors on the ecophysiological performance of two intertidal seaweeds[END_REF][START_REF] Martínez | Physical factors driving intertidal macroalgae distribution: physiological stress of a dominant fucoid at its southern limit[END_REF]. Indeed, rockpool communities undergo a wide range of irradiance, determined by the seasonal, daily and tide cycles, and the cloud cover (Figs. 7 and 8;[START_REF] Häder | Effects of solar radiation on the patagonian rhodophyte Corallina officinatis (L.)[END_REF]. In the present study, the CCA L. incrustans colonized all rockpools, but reached the highest biomass in shallow high shore rockpools, both in the winter and the summer. This suggested that this species was stress-tolerant and was able to withstand high temperatures and light intensities [START_REF] Dethier | The ecology of intertidal algal crusts: Variation within a functional group[END_REF][START_REF] Kooistra | Zonation patterns in intertidal pools and their possible causes: a multivariate approach[END_REF]. Conversely, the articulated coralline algae E. elongata attained higher biomass within deeper and shaded rockpools. Egilsdottir et al. ( 2016) related a strong inhibition of photosynthesis and calcification in E. elongata under irradiances beyond 200 µmol photons m -2 s -1 in the summer. In the present study, E. elongata was almost absent from upper shore rockpools, where incident light intensity reached up to 800 and 1600 µmol photons m -2 s -1 in the winter and the summer, respectively (Fig. 8). Hence, conversely to L. incrustans, E. elongata may tolerate lower light intensities [START_REF] Egilsdottir | Photosynthesis and calcification in the articulated coralline alga Ellisolandia elongata (Corallinales, Rhodophyta) from intertidal rock pools[END_REF][START_REF] Korbee | A novel in situ system to evaluate the effect of high CO 2 on photosynthesis and biochemistry of seaweeds[END_REF] and may benefit from the shaded environment provided by deeper and sheltered rockpools.

In the summer, shallow upper shore rockpools were dominated by Ulva intestinalis and Ulva sp., respectively, although Sargassum muticum was co-dominant within the rockpool 1. Ulva is a cosmopolitan genus in intertidal environments, especially rockpools, and is characterized by a strong ability to cope with changing light conditions and tolerate high light intensities [START_REF] Altamirano | Long-term effects of natural sunlight under various ultraviolet radiation conditions on growth and photosynthesis of intertidal Ulva rigida (Chlorophyceae) cultivated in situ[END_REF][START_REF] Gómez | Patterns of photosynthesis in 18 species of intertidal macroalgae from southern Chile[END_REF][START_REF] Ruangchuay | Effects of culture conditions on the growth and reproduction of Gut Weed, Ulva intestinalis Linnaeus (Ulvales, Chlorophyta)[END_REF]. S. muticum was the dominant canopy-forming species in the rockpool 3. As for Ulva sp., [START_REF] Norton | The growth and development of Sargassum muticum (Yendo) Fensholt[END_REF] demonstrated that S. muticum have a broad tolerance for irradiance, salinity and temperature.

Within the rockpool 4, a dense canopy of the brown algae Fucus serratus was observed both in the winter and the summer. Thus, rockpools 1 to 4 are covered by a dense canopy of U. intestinalis, S. muticum and/or F. serratus from spring to late summer, when light intensities are high. [START_REF] Olabarria | Response of macroalgal assemblages from rockpools to climate change: effects of persistent increase in temperature and CO 2[END_REF] highlighted the importance of canopy composition for understory species. For most intertidal seaweeds, the high light intensities in the summer often greatly exceed their needs for photosynthesis [START_REF] Franklin | The changing irradiance environment: consequences for marine macrophyte physiology, productivity and ecology[END_REF]. Although photoinhibition may affect these canopyforming species, they are also likely to act as a protective layer for understory species, which may benefit from reduced light stress for their metabolic activity [START_REF] Bordeyne | Metabolic activity of intertidal Fucus spp. communities: evidence for high aerial carbon fluxes displaying seasonal variability[END_REF][START_REF] Tait | Dynamics of productivity in naturally structured macroalgal assemblages: importance of canopy structure on light-use efficiency[END_REF]. In the summer, L. incrustans and E. elongata would thus be protected from photoinhibition by the dense canopy formed by U. intestinalis, S. muticum and F. serratus. This statement may be consistent with the bleaching observed in E. elongata when colonizing rockpool edges without canopy (personal observation). The bleaching in coralline algae under high irradiances has already been reported by [START_REF] Häder | Effects of solar radiation on the patagonian rhodophyte Corallina officinatis (L.)[END_REF].

Communities inhabiting rockpools also exerted a strong influence on the physico-chemical variations. During the day-time emersion, a part of incident light was used by seaweeds for their photosynthesis, reducing inorganic carbon and nutrients in the seawater during the emersion period and increasing oxygen concentrations. Ulva spp. are known to have a high plasticity in utilizing different forms of inorganic carbon through the use of powerful carbon concentration mechanisms (CCM; [START_REF] Axelsson | Affinity, capacity and oxygen sensitivity of two different mechanisms for bicarbonate utilization in Ulva lactuca L. (Chlorophyta)[END_REF][START_REF] Axelsson | Two modes of bicarbonate utilization in the marine green macroalga Ulva lactuca[END_REF][START_REF] Rautenberger | Saturating light and not increased carbon dioxide under ocean acidification drives photosynthesis and growth in Ulva rigida (Chlorophyta)[END_REF].

This process may confer a competitive advantage to Ulva spp., especially when CO 2 is limited [START_REF] Axelsson | Two modes of bicarbonate utilization in the marine green macroalga Ulva lactuca[END_REF][START_REF] Larsson | Photosynthetic carbon utilization by Enteromorpha intestinalis (Chlorophyta) from a Swedish rockpool[END_REF]. Similarly, the ability of S. muticum to use bicarbonates (HCO - 3 ) through CCM makes it unlikely to be limited in CO 2 for its photosynthesis [START_REF] Thomas | Bicarbonate ion assimilation in photosynthesis by Sargassum muticum[END_REF]. Interestingly, no clear seasonal trend was measured in rockpools in terms of oxygen concentrations. Within upper shore rockpools, oxygen concentration varied greatly throughout the year with a peak detected in November, although light intensities were reduced compared with the summer. Several explanations could be provided:

(1) the high light intensities in summer induce photoinhibition in seaweeds colonizing shallow and high shore rockpools, limiting oxygen concentration increase [START_REF] Egilsdottir | Photosynthesis and calcification in the articulated coralline alga Ellisolandia elongata (Corallinales, Rhodophyta) from intertidal rock pools[END_REF][START_REF] Häder | Effects of solar radiation on the patagonian rhodophyte Corallina officinatis (L.)[END_REF][START_REF] Raven | Ecophysiology of photosynthesis in macroalgae[END_REF]. ( 2) The high photosynthetic rates combined with the important canopy biomass in summer severely reduced the nutrients which rapidly became limiting for seaweed primary production [START_REF] Henley | Photoacclimation and photoinhibition in Ulva rotundata as influenced by nitrogen availability[END_REF][START_REF] Metaxas | Spatial and temporal variability of macrobenthic communities in tidepools on a rocky shore in Nova-Scotia, Canada[END_REF]. In the autumn, the highest nutrient availability and the lowest light intensities may increase photosynthesis and oxygen release in rockpools. Oxygen concentration within the lower shore rockpool was more stable during the year than in other rockpools, with concentrations close to those of near-shore seawater. This rockpool was more shaded than other rockpools due to the presence of high surrounding rocks. Although nutrient concentrations were higher, the lower incident light may limit the community production.

During the night-time emersion, the community respiration decreased the oxygen concentration in the three rockpools monitored, both in the winter and the summer. However, the higher seaweed biomass in the summer reduced drastically oxygen concentrations, especially within rockpools 2 and 3. The higher temperature in the summer at night may increase the metabolic rates of rockpool communities [START_REF] Tagliarolo | Carbon emission associated with respiration and calcification of nine gastropod species from the intertidal rocky shore of Western Europe[END_REF][START_REF] Zou | Responses of dark respiration in the light to desiccation and temperature in the intertidal macroalga, Ulva lactuca (Chlorophyta) during emersion[END_REF] reducing oxygen concentration more severely. A plateau was reached from 4 am in these two rockpools, which became almost anoxic at the end of the night-time emersion. Conversely, in the winter, the decrease in oxygen concentration at night was more linear until the end of the night-time emersion. Temperature also influences gas solubility, and [START_REF] Truchot | Oxygen and carbon-dioxide in the marine intertidal environment -Diurnal and tidal changes in rockpools[END_REF] suggested that the increase in oxygen concentration during the day and the decrease at night may be limited by diffusion process across the air-water interface.

By their photosynthesis, seaweeds also reduced the carbon dioxide (CO 2 ) concentration in the rockpools during the emersion period. As consequence, the pH T of seawater increased, and reached 10 in spring in the rockpool 1, during the bloom of Ulva intestinalis, consistent with the results of [START_REF] Björk | Why is Ulva intestinalis the only macroalga inhabiting isolated rockpools along the Swedish Atlantic coast?[END_REF] for isolated rockpools in Sweden. A peak of pH T was also observed in November in the rockpool 2, which corresponded to the maximum oxygen concentration in this rockpool. Conversely, at night, the community respiration during emersion increased the CO 2 concentrations and reduced the pH T down to 7.5. The important seaweed biomass in the summer and the high respiration at night severely decreased the pH T until a plateau was reached when oxygen has been consumed.

The variations in pH T have a strong influence on the carbonate saturation state with respect to aragonite (Ω Ar ) and calcite (Ω Ca ). In marine systems, the equilibrium between the different inorganic carbon species is a major factor influencing the calcification process [START_REF] Borowitzka | Calcification in algae: Mechanisms and the role of metabolism[END_REF]. Ω defines the ability of calcareous species to produce their skeleton, through the precipitation of calcium carbonate (CaCO 3 ) [START_REF] Kleypas | Impacts of ocean acidification on coral reefs and other marine calcifiers: a guide for future research[END_REF][START_REF] Raven | Effects on marine algae of changed seawater chemistry with increasing atmospheric CO 2[END_REF]. The relationship between saturation state values and coralline calcification and growth is well known [START_REF] Kuffner | Decreased abundance of crustose coralline algae due to ocean acidification[END_REF]. Despite the presence of seasonal variations, Ω was much greater than 1 in upper shore rockpools during the day-time emersion, making the environment more conducive to calcification. In Several studies underlined the interaction between photosynthesis and calcification processes in coralline algae [START_REF] Comeau | Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate[END_REF][START_REF] Mccoy | Diurnal and tidal patterns of carbon uptake and calcification in geniculate inter-tidal coralline algae[END_REF]. The ''trans calcification'' process described by [START_REF] Mcconnaughey | Calcification generates protons for nutrient and bicarbonate uptake[END_REF] suggests that calcareous algae may uptake seawater HCO - 3 and convert it to CO 2 thanks to proton transport through the organism. A part of this CO 2 would be used for algal photosynthesis while another part would be converted to produce CO 2- 3 used for calcification. Although this process was evidenced in the freshwater alga Chara corallina [START_REF] Mcconnaughey | Calcification generates protons for nutrient and bicarbonate uptake[END_REF], ''trans calcification'' may also occur in geniculate coralline algae [START_REF] Mccoy | Diurnal and tidal patterns of carbon uptake and calcification in geniculate inter-tidal coralline algae[END_REF] and L. incrustans. Within rockpools, where CO 2 concentrations may be low during the day, this mechanism may provide sufficient inorganic carbon substrate to increase both photosynthetic and calcification rates. This statement would be consistent with the decrease in A T observed during the day-time emersion in the summer within upper shore rockpools. Temperature increase in the summer may also have a positive effect on calcification and growth rates of coralline algae [START_REF] Martin | Primary production, respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides[END_REF][START_REF] Steller | Effect of temperature on photosynthesis, growth and calcification rates of the freeliving coralline alga Lithophyllum margaritae[END_REF]. Finally, a part of the calcification can also be attributed to calcareous fauna, since [START_REF] Tagliarolo | Carbon emission associated with respiration and calcification of nine gastropod species from the intertidal rocky shore of Western Europe[END_REF] highlighted an increase in calcification rates in summer in several intertidal gastropods, including G. umbilicalis.

E. elongata is known to have high calcification rates, with a light to dark ratio of 3.6 [START_REF] El Haikali | Estimation of photosynthesis and calcification rates of Corallina elongata Ellis and Solander, 1786, by measurements of dissolved oxygen, pH and total alkalinity[END_REF]. In a recent study, [START_REF] Egilsdottir | Photosynthesis and calcification in the articulated coralline alga Ellisolandia elongata (Corallinales, Rhodophyta) from intertidal rock pools[END_REF] estimated that calcification in E. elongata on the north coast of Brittany was 3.3 µmol CaCO 3 g DW -1 h -1 in the summer and 1.1 µmol CaCO 3 g DW -1 h -1 in the winter. During night-time emersion, the increase in A T reflected the dissolution of coralline algae, explained by the decline of pH T and Ω. This suggests that the increase in dissolution rates during night-time emersion would be offset by high calcification rates during the daytime emersion. In a Mediterranean shallow infralittoral ecosystem dominated by E. elongata, [START_REF] Bensoussan | Community primary production and calcification in a NW Mediterranean ecosystem dominated by calcareous macroalgae[END_REF] also evidenced strong diurnal variations, with higher calcification rates when light intensities were maximal. In the present study, biomass of E. elongata reached 982 g DW m -2 in rockpool 5. This value is lower than those reported for infralittoral communities on the Costa Brava (1110 to 4015 g DW m -2 ; [START_REF] Ballesteros | Composición y estructura de la comunidad infralitoral de Corallina elongata Ellis & Solander, 1786, de la Costa Brava (Mediterráneo occidental)[END_REF], but is close to the lower values reported by El [START_REF] El Haikali | Estimation of photosynthesis and calcification rates of Corallina elongata Ellis and Solander, 1786, by measurements of dissolved oxygen, pH and total alkalinity[END_REF] on the Gulf of Marseilles (820 to 2544 g DW m -2 ). Moreover, in south-west Wales, [START_REF] Edyvean | Growth rates of Lithophyllum incrustans (Corallinales, Rhodophyta) from South West Wales[END_REF] evidenced that CaCO 3 deposition rate for L. incrustans was 378.96 g m -2 plant surface area year -1 . Within rockpools, many factors may influence the calcification rates in coralline algae, such as rockpools exposition to incident light, the seawater physico-chemical parameters, but also the interaction with other species. However, the present results evidence that coralline algae colonizing rockpools can be considered as a key element of carbonate cycles in the shallow coastal waters.

This study provides evidence that although seaweeds were likely to strongly influence physico-chemical variations in rockpools, these variations were also governed by more complex interactions between physical parameters of rockpools (e.g. shore height, depth, light exposure, temperature. . . ), chemical processes (gas exchange between the seawater and the atmosphere, interaction between physico-chemical parameters) and biotic factors (metabolic processes of species, nutrient/light limitation). Rockpools can thus be considered as complex dynamic environments, unique in their functioning due to the large number of interactions between physical, chemical and biological factors.

Within rockpools, physico-chemical variations mainly depend on their depth and the duration of their isolation from the sea during low tide. Seaweed species colonizing rockpools can thus survive harsh environmental conditions, such as low oxygen concentration and pH T at night or high temperature and low nutrient concentrations during the day. However, species require a regular exchange of seawater for regulating physico-chemical parameters and recover from the emersion period. In coralline algae, low pH T and carbonate saturation state at night may have deleterious consequences on their calcareous skeleton by increasing dissolution [START_REF] Comeau | Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate[END_REF][START_REF] Egilsdottir | Photosynthesis and calcification in the articulated coralline alga Ellisolandia elongata (Corallinales, Rhodophyta) from intertidal rock pools[END_REF].

The present study evidenced that rockpool organisms undergo physico-chemical variations that may greatly exceed those predicted with climate change for the end of the century [START_REF] Gattuso | Contrasting futures for ocean and society from different anthropogenic CO 2 emissions scenarios[END_REF]. However, few studies took into account both emersion variability and inter-specific interactions [START_REF] Olabarria | Response of macroalgal assemblages from rockpools to climate change: effects of persistent increase in temperature and CO 2[END_REF]. Regarding climate change experiments, particular attention should be paid to the natural variability that tested species undergo. Therefore, rockpools appear as suitable environments to examine the physiological processes set up by species to face natural variability.

Fig. 1 .

 1 Fig. 1. Pictures of rockpools 1 to 5 in summer (A to E) and winter (F to J). All rockpools were located in the Estacade site in Roscoff (Brittany, France).

  3 (0.06 µmol L -1 ) and PO 3- 4 (0.04 µmol L -1 ) concentrations in May and June, respectively. The highest NO - 3 and PO 3- 4 concentrations were measured in rockpool 5 in late January 2016 (19.65 µmol NO - 3 L -1 ) and early March 2015 (1.07 µmol PO 3- 4 L -1 ), respectively. Si(OH) 4 concentrations reached minimum concentrations of 0.51 µmol L -1 in rockpool 3 in April and maximum concentrations of 7.41 µmol L -1 in rockpool 5 in September.

Fig. 2 .

 2 Fig. 2. Seasonal variations in physico-chemical parameters within 5 rockpools monitored every two weeks during spring tides from January 2015 to March 2016. Ω Ar and Ω Ca were calculated using the CO2SYS software. The near shore seawater parameters (dot line) were obtained from the SOMLIT station located in the Estacade site in Roscoff (Brittany, France).

Fig. 3 .

 3 Fig. 3. Seasonal variations in nutrient concentrations within 5 rockpools monitored once a month during spring tides from January 2015 to March 2016. The near shore seawater parameters (dot line) were obtained from the SOMLIT station located in the Estacade site in Roscoff (Brittany, France).

Fig. 4 .

 4 Fig. 4. Representation of (a) the correlation circle between physico-chemical variables measured within each rockpool (black arrows), the physical characteristics of rockpools (gray arrows; SH: shore height) and the two principal components and (b) contribution of the space-time measurements to the two principal components.

Fig. 5 .

 5 Fig. 5. Summer diurnal variations in physico-chemical parameters within 3 rockpools monitored every hours during emersion in September 2015. The near shore seawater parameters were measured at the beginning and at the end of the emersion. Ω Ar and Ω Ca were calculated using the CO2SYS software. The gray section represents the period during which all the rockpools were covered by the tide.

Fig. 6 .

 6 Fig. 6. Winter diurnal variations in physico-chemical parameters within 3 rockpools monitored every hours during emersion in March 2016. The near shore seawater parameters were measured at the beginning and at the end of the emersion. Ω Ar and Ω Ca were calculated using the CO2SYS software. The gray section represents the period during which all the rockpools were covered by the tide.

Fig. 7 .

 7 Fig. 7. Seasonal variations in light intensity measured at noon in air at the Station Biologique de Roscoff from January 2016 to April 2016 (dot line). Black dots represent the mean irradiance measured in air during day-time emersion for each seasonal measurement.

Fig. 8 .

 8 Fig. 8. Winter and summer diurnal variations in light measured in air in the Bloscon site in March 2016 and September 2015 respectively. The gray section represents the period during which all the rockpools were covered by the tide. Irradiance during immersion was estimated using extinction coefficient of light in seawater.

Table 1

 1 Physical characterization of the five rockpools monitored for seasonal variations at the Bloscon site in Roscoff (Brittany, France). Mean emersion duration was calculated from the water height obtained from the ''Service Hydrographique et Océanique de la Marine (SHOM)''. Surface area is the surface of water measured at low tide from pictures. Min and max emersion duration were determined over January 2015 to March 2016.

	Rockpool	Shore	Mean emersion	Min emersion	Max emersion	Surface area (m 2 )	Volume (m 3 )	Maximum	Surface/volume ratio
		height (m)	duration (h)	duration (h)	duration (h)			depth (cm)
	1	6.4	7 h 40	7 h 01	10 h 13	2.58	0.294	23	8.77
	2	6.2	7 h 20	6 h 51	9 h 22	0.54	0.063	25	8.56
	3	5.8	6 h 40	6 h 33	8 h 02	0.79	0.175	31	4.50
	4	4.9	5 h 30	5 h 25	5 h 47	0.57	0.087	32	6.51
	5	4.4	4 h 40	3 h 55	5 h 23	1.16	0.234	46	4.95

Table 2

 2 Results of PERMANOVA's post-hoc pairwise tests performed on the physico-chemical parameters obtained from the seasonal monitoring of the five rockpools (n = 29). Significant results are in bold (p<0.05).

		Rockpool 1		Rockpool 2		Rockpool 3		Rockpool 4
		t	p	t	p	t	p	t	p
	Rockpool 2	2.5	0.004	-	-	-	-	-	-
	Rockpool 3	3.0	<0.001	3.7	<0.001	-	-	-	-
	Rockpool 4	3.3	<0.001	2.9	<0.001	1.5	0.14	-	-
	Rockpool 5	11.1	<0.001	12.3	<0.001	11.0	<0.001	13.1	<0.001

Acknowledgments

The authors thank Jérôme Coudret, Stéphane Loisel, Marine Macé, Robin Marsac, Doriane Stagnol, Aline Migné and Louis Girard for their field assistance. We also thank the SOMLIT (Service d'Observation en Milieu LITtoral, INSU-CNRS) program for the temperature data sets provided. This work was supported by the Brittany Regional Council and the French National Research Agency via the ''Investment for the Future'' program IDEALG (n • ANR-10-BTBR-04).


Wasim J Mir

Clément Livache

Nicolas Goubet

Bertille J Martinez

Amardeep Jagtap

Audrey Chu

Nathan Coutard

Hervé Cruguel

Thierry Barisien

Sandrine Ithurria

Angshuman Nag

Benoit Dubertret

Abdelkarim Ouerghi

Mathieu G Silly

Emmanuel Lhuillier

Strategy to overcome recombination limited photocurrent generation in CsPbX 3 nanocrystal arrays

Keywords: CsPbX3, Perovskite nanocrystals, Transport, Photoconduction, Electronic Properties

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.

Lead halide-based perovskite materials have raised significant scientific and technological interests in the field of solar cells because of their defect-tolerant band structure. 1 This structure leads to large open-circuit voltages (>1 V) and thus to high power conversion efficiencies. 2 Improved efficiencies of around 20 % for hybrid perovskites 3 and above 13% for perovskite quantum dot-based devices 4 , 5 have been reported so far. The idea of using these defect-tolerant lead halide-based perovskite nanocrystals for designing high-efficiency light-emitting devices was later proposed by the Kovalenko group. 6 A paradigm shift was proposed: instead of preventing the wave function from encountering surface traps by building a shell around the nanocrystal, a defect-tolerant material ensures that no surface traps are generated in the mid-gap region. As a result, CsPbX3 (with X a halide anion) nanocrystals are able to generate high photoluminescence (PL) efficiency6 (≈ 90 % based on core only objects) and are obtained through straightforward and reliable synthesis. Their exceptional light emitting properties have been used to demonstrate low thresholds for lasing, 7 single photon emission 8,9 and light-emitting diodes with narrow emission linewidth. 10,11,12, As a result, both high light-emitting efficiency and defect-tolerant nature make them promising candidates for a new generation of nanocrystal-based optoelectronic devices. [START_REF] Konstantatos | Colloidal Quantum Dot Optoelectronics and Photovoltaics[END_REF][START_REF] Talapin | [END_REF] As a new class of colloidal nanomaterials, lead halide-based perovskite nanocrystals (PNCs) raise strong interest because of their unique properties, yet reveal some new challenges. In particular, these materials present some stability issues: PNCs degrade under an electron beam 15 and undergo structural phase changes or ligand-induced stoichiometry change. 16 As a result, PNCs need to be handled with great care and the conventional method to build conductive nanocrystal-based films need to be revised. For example, a phase transfer ligand exchange toward S 2- capping, although quite successful for metal chalcogenide nanocrystals, 17,18 cannot be applied to PNCs. Indeed, such a ligand exchange performed on PNCs leads to the formation of a conductive and photoresponsive film of…PbS nanocrystals (see Supporting Information (SI) and Figure S6). Similarly, ion gel gating 19,20 cannot be straight forwardly applied because the ion gel dissolves the PNCs. As a consequence, the whole surface chemistry toolbox used to tune the tunnel barrier associated with the capping ligands and used to make film photoconductive needs to be revisited.

The recent report of PNC-based solar cell4 with relatively high photon conversion efficiency (13 %) has demanded for a deeper investigation of the charge transport properties of PNC arrays. However current transport experiments 21,22,23 from PNC-based devices have been performed on diode structures. 24,25 In such device geometries, the transport of the PNC layer is blurred with effects coming from the electron and hole extraction layers. This makes the basic understanding of the transport more complex.

In this paper, we use a combination of optical and photoemission spectroscopy and transport measurements to highlight the current limitations of the photocurrent generation in PNC arrays. We provide evidence that the fast bimolecular recombination of PNC is the main limitation and is responsible for the short photocarrier lifetime. We thus propose to use a nanotrench device geometry to boost the carrier injection at a given high field and enhance photocurrent generation by a factor of 1,000.

We conducted transport measurements on a series of CsPbBrxI3-x samples. This manuscript will focus on one particular PNC composition CsPb(Br0.65,I0.35)3. Energy dispersive X-ray spectroscopy (see Figure S1) reveals that the actual composition is slightly more Br-rich than expected and that the actual iodine content is close to 25%. The choice of CsPb(Br0.65,I0.35)3 was initially driven by a trade-off between the material stability (higher for Br-rich samples) and the optimal value of the band gap for solar cell application (better suited with iodine). These CsPb(Br0.65,I0.35)3 PNCs were grown using previously described methods 5,7 and present a band edge energy of 2.29 eV (see Figure 1(a)), which is close to the bulk band gap of the material having same halide composition and also consistent with their weak confinement. Transmission electron microscopy (TEM) imaging (see Figure 1(b)) indeed reveals a particle size in the range of 12 ± 2 nm, which is larger than the Bohr diameter of this material (7 nm).6 Their time-resolved PL decay dynamics (see Figure 1(c)) is fitted with a biexponential decay function with two time constants, 5 ns and 25 ns, which will be further discussed in the results and discussion section. This study of the spectroscopic properties of CsPb(Br0.65,I0.35)3 PNCs is complemented by X-ray photoemission spectroscopy, intended to probe the electronic spectrum on the absolute energy scale. 26 This work is of utmost importance to probe and quantify the Schottky barrier at the metal-PNC interface. Measurements were conducted at the TEMPO beamline of the SOLEIL synchrotron on PNC films prepared by following the same protocol used for transport studies at the device scale. For more details about the processing of the photoemission data as well as the analysis of the core level see Figure S2-S4 given in the SI. The cut-off of the secondary electrons reveals the work function (WF) to be ≈ 4.1 eV, see Figure S5(a) and is used to scale the spectrum on the absolute energy scale. The low binding energies part of the XPS spectrum, unveils the energy of the valence band with respect to the Fermi level (≈1.45 eV) (see Figure S5(b)). The PL band gap 2.29 eV (see Figure 1(a)) is used to set the energy of the conduction band with respect to the valence band. This allows us to propose the electronic energy spectrum for the CsPb(Br0.65,I0.35)3 PNC ensemble (see Figure 1(d)). The Fermi level is quite deep in the band gap of the material but lies closer to the conduction band, suggesting a slight n-type character of the PNCs, which is consistent with recent magneto-optical spectroscopic measurements. After synthesis of the desired PNC composition, colloidal dispersion of PNCs was drop-casted over predesigned electrodes (see the methods section for details about fabrication and see the inset of Figure 2(b) for the schematic of the device used). The insulating long chain oleylammonium capping ligands were then removed by following a previously reported procedure.4 Briefly, thin film of the PNCs is dipped in a solution of lead acetate in methyl acetate. Using Fourier-transform infrared spectroscopy (FTIR), we find that the absorption relative to the C-H bond, which is signature of long chain organic capping ligands, is strongly suppressed after the ligand exchange step (see Figure S7(a)).

Finally, the device was mildly annealed at 100 °C for two minutes to dry the film and improve the inter-nanocrystal coupling. A higher annealing temperature (200 °C) or longer annealing time leads to strong inter-PNC sintering, resulting in a loss of their nano-crystalline aspect as revealed by scanning electron microscopy (SEM) imaging Figure S7(d)-S7(g), X-ray diffraction Figure S7(b) and conduction measurements Figure S8(a)-S8(b). On the other hand, ligand exchange followed by a short mild annealing at 100 °C preserves the PNC aspect, and only a limited change, from 10 to 15 nm, of the Scherrer size extracted from the diffraction pattern is observed (see Figure S7(c)).

Thin films of PNCs reaching a thickness of 200-400 nm were prepared using a multistep procedure in order to obtain an efficient ligand exchanged film. Multistep deposition is also used to fill up the pin holes or cracks generated by the volume reduction resulting from the removal of long chain organic ligands and annealing steps. Once deposited on metallic electrodes, the thin film of CsPb(Br0.65,I0.35)3 PNCs presents a conductive ohmic behavior (see Figure 2(a)), in spite of the somewhat large band offset (≈ 0.5 eV for holes) between the bands of CsPb(Br0.65,I0.35)3 and the gold electrodes (see Figure 1(d)). Under 532 nm irradiation (2 W.cm -2 ), a significant rise in the conductance is observed, typically by four orders of magnitude (see Figure 2(a)). We further cross-checked that the method used to make the thin film conductive and photoconductive is not limited to this particular halide content. In particular, we demonstrated that the method can be applied to other shapes, including nanocubes (0D objects) and nanoplatelets 28 (2D objects, NPL) (see Figure S9), and to other Br:I ratios (see Figure S10). The typical responsivity of the film spans from 200 µA.W -1 to 1 mA.W -1 depending on the incident power. The scaling of the photocurrent with the incident light power follows a power law with a 0.64 exponent 29 (see Figure 2(b)). This value is closer to 0.5 expected for bimolecular recombination (electron-hole recombination through radiative and non-radiative paths) than to the value of 1 that results from a monomolecular process (trapping) usually observed for metal chalcogenide nanocrystals. 30 This supports the hypothesis of the lower role of surface trapping in PNCs, as expected for a defect-tolerant material in which trap states are away from the band gap. We also investigated the temperature dependence of the current (see Figure 2(c)). Close to room temperature, the current follows an Arrhenius law, which suggests that transport occurs through a hopping process to the nearest neighbour. The associated activation energy (240 meV) is smaller than half the band gap expected for an intrinsic semiconductor and is qualitatively consistent with the deviation of the Fermi level with respect to the middle of the band gap observed from X-ray photoemission and PL spectroscopy (see Figure 1(d)). We then investigated time-resolved carrier dynamics at the device scale. This is especially challenging because of the insulating character of the film. Under dark conditions, film resistance is typically in the 10 GΩ range and this is a crucial difference with respect to narrower band gap nanocrystals (PbSe) previously probed using a high bandwidth transient photocurrent setup. 31,32 Measuring fast dynamics in the GHz range requires a special design where both sample stimulation (pulsed ns laser) and acquisition stage have to be compatible with the targeted GHz bandwidth of the setup (see Figure 3(a) for the schematic of the setup). The experiments are conducted on transparent electrodes made of ITO so as to (i) reduce the large Schottky barrier experienced by the electron, and (ii) to avoid any heating effects of the substrate which may lead to an artificial bolometric photoresponse of the device. We further cross-check that the phototransport properties of the films are effectively preserved (see Figure S11). The transient photocurrent in PNCs is extremely fast with a main decay time of 1.2 ns. Typically, two time constants can be extracted from the decay curve (Figure 3(b) and S12): a fast component of ≈1.2 ns and a slower contribution around 7.3 ns. No long-lived tail is observed, which is consistent with a limited role of deep surface trap states, contrary to what can be observed on metal chalcogenide nanocrystals. 33 These transient photocurrent dynamics have to be correlated with the PL dynamics (Figure 1(c)). In such PL measurements, electron-hole pairs are generated by the laser pulse and the intra-PNC decay of the exciton population is followed. This PL decay is a good probe for the excited state carrier density. The fact that similar dynamics are observed using photocurrent experiments suggests that the current magnitude is actually driven by the photogenerated carrier population of PNCs. 31 This observation is also consistent with the flux dependence of the photocurrent Figure 2(b) which implies that the photocurrent generation is limited by interband gap recombination. In the current device, the diffusion length can be estimated using an Einstein diffusion equation: we found

nm e T µk D L b D 58     
, assuming a mobility of 0.26 cm 2 V -1 s -1 as recently determined from field effect transistor measurements 34 and τ = 5 ns from our photocurrent decay measurements. This is much smaller than the electrode spacing (20 µm). As a result, only the carriers in the closest vicinity of the electrodes can be collected as all the other carriers undergo recombination before reaching the electrodes. On the other hand, the obtained diffusion length value of ≈60 nm is very close to the length of a PNC layer used for solar cells (≈200 nm) which explain the recently obtained high power conversion efficiency. We finally investigated the effect of electric field on transport, in particular in the high field regime. To do so, we built nanotrench electrodes see the inset of Figure 3(c). Along this nanotrench, the two electrodes lie 60 nm apart and have a high aspect ratio (length of 15 µm). Moreover, they present the key advantage of being built with a high fabrication success ratio (>80%) using only two optical lithography steps 30,35 in spite of the sub-wavelength size of the trench. To follow the electric field dependence of the conductance over almost four orders of magnitude, we used µm spaced interdigitated electrodes for the low field range and nanotrench electrodes for the high field range. The fact that the conductivity curves resulting from the two types of electrodes overlap (see Figure 3(c)) and suggests that the transport mechanism is unchanged. The latter remains dominated by the nearest neighbour hopping mechanism, which is consistent with the fact that 4-5 PNCs can fit within the nanotrench gap (60 nm). In the low field range, the conductance of the PNC film is field-independent. On the other hand, above 50 kV.cm -1 , we observe a significant rise in the conductance with the applied bias. This threshold corresponds to an energy drop per PNC of ≈50 meV, which likely results from the charging energy of the PNC. By applying a large electric field, we are able to inject charges in PNCs more efficiently and the effect of the electrode structure on photocurrent is quite dramatic. The responsivity of the film is enhanced by more than a factor 1,000 as we switch from µm spaced electrodes to nanoscale devices (see Figure 3(d)). This increase arises from a combination of (i) an easier charge injection into PNC and (ii) an increase of gain due to the short inter-electrode spacing. In a photoconductor, the gain is the ratio of the photocarrier lifetime (≈ 5 ns) over the transit time (the time to go from one electrode to the other). In this nanotrench geometry, the transit time is reduced by a factor of 300 as compared to µm spaced electrodes. 30,36 Because of the n-type nature of these PNCs, the electron will recirculate several times during the hole carrier lifetime, leading to photogating 37 and to the observed enhanced photocurrent. 38 In conclusion, using a combination of DC and time resolved transport measurements, we provide evidence that the photocurrent of PNC arrays is limited by the fast bimolecular recombination of the material. This leads to fast recombination of the photogenerated electron-hole pair which allows for high bandwidth photodetection with time response as short as 1.2 ns but limits the absolute photoresponse of the film. We then propose to use nanotrench electrodes to achieve better charge injection at high electric fields and maintained the best correlation between the device size and the material diffusion length. Thanks to this strategy, we have enhanced the photoresponse by a factor of 1,000.

"Supplementary Material" include informations about material synthesis, material characterization, photoemission data acquisition and processing, informations on film preparation for transport and additional transport measurements.

27

 27

Figure 1 .

 1 Figure 1. (a) Absorption and PL spectra of CsPb(Br0.65,I0.35)3. (b) TEM image of the CsPb(Br0.65,I0.35)3 PNCs. (c) Room temperature time-resolved PL signal for a film of CsPb(Br0.65,I0.35)3 PNCs. Both photoemission and PL measurements were carried out on thin films treated with saturated lead acetate in methylacetate. (d) Electronic structure in absolute energy scale of CsPb(Br0.65,I0.35)3 PNCs. The WF of gold (5.1 eV) and ITO (4.7 eV) are also provided.

Figure 2 .

 2 Figure 2. (a) IV curves of a film of CsPb(Br0.65,I0.35)3 PNCs under dark conditions and under illumination at different light irradiance (λ=532 nm), under ambient atmosphere and at room temperature over 20 µm spaced interdigitated Au electrodes. (b) Current as a function of the light irradiance (λ=532 nm) for the same film shown in part (a). The inset shows scheme of the device. (c) Current under a voltage of 20 V as a function of temperature for a film of CsPb(Br0.65,I0.35)3 PNCs measured under vacuum and in dark conditions.

Figure 3 .

 3 Figure 3. (a) Schematics of the high bandwidth transient photocurrent setup. (b) Transient photocurrent signal obtained in a film of CsPb(Br0.65,I0.35)3 PNCs at room temperature. The film is deposited on interdigitated electrodes made of ITO/PET. (c) Conductivity as a function of the applied electric field under dark condition in the case of CsPb(Br0.65,I0.35)3 PNCs. The data have been obtained from interdigitated electrodes at low electric field and from the nanotrench electrodes for the high field regime. The inset is a SEM image of uncovered nanotrench electrodes (i.e., the blackline is the trench). (d) Responsivity of a film of CsPb(Br0.65,I0.35)3 PNCs over µm spaced interdigitated electrodes and over nanotrench electrodes.

Acknowledgements

We acknowledge the use of clean room facilities from the consortium "Salles Blanches Paris Centre -SBPC". This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K via the grant dopQD. This work was supported by French state funds managed by the ANR within the Investissements d'Avenir program under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE. Wasim J. Mir acknowledges CEFIPRA for the Raman-Charpak fellowship and CSIR India for the PhD fellowship. EL is thankful for the support of the ERC starting grant (project: blackQD -grant 756225).


Nicolas Goubet

Amardeep Jagtap

Clément Livache

Bertille Martinez

Hervé Portalès

Zhen Xiang

Ricardo Xu

Benoit Lobo

Emmanuel Dubertret

Lhuillier

Terahertz HgTe Nanocrystals: Beyond Confinement

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.

TeraHertz HgTe nanocrystals: beyond confinement

Nicolas Goubet 1,2,3 , Amardeep Jagtap 1 , Clément Livache 1,2,3 , Bertille Martinez 1,2,3 , Hervé Portalès 4 , Xiang Zhen Xu 2,3 , Ricardo P.S.M. Lobo 2,3 , Benoit Dubertret 2,3 , Emmanuel Lhuillier

ABSTRACT:

We report the synthesis of nanocrystals with optical feature in the THz range. To do so, we develop a new synthetic procedure for the growth of HgTe, HgSe and HgS nanocrystals, with strong size tunability from 5 nm to 200 nm. This is used to tune the absorption of the nanocrystal all over the infrared range up to the teraHertz (from 2 µm to 65 µm for absorption peak and even 200 µm for cut-off wavelength). The interest for this procedure is not limited to large size since for small objects we demonstrate low aggregation, good shape control (i.e. spherical object) while using non-expansive and simple mercury halogenide precursors. By integrating these nanocrystals into an electrolyte-gated transistor, we evidence a change of carrier density from p-doped to n-doped as the confinement is vanishing.

Colloidal quantum dots (CQD) have garnered significant scientific and technological interests because of their tunable optical features from ultraviolet (UV) to infrared (IR). While their photoluminescence appears as the first mass market application, CQD are actually as interesting for the design of low cost optoelectronic devices that combine the ease of processing of solution-processable material with the robustness of inorganic systems. This is especially true in the IR range where current technologies remain pricy. CQD offer an interesting platform, however reaching mid (λ>3 µm) and far infrared wavelength is challenging on a material perspective. [1][2][3] Only few materials can address absorption tunability up to the terahertz range: graphene 4 , doped semiconductors (Si, InSb) 5,6 and topological insulators such as Bi2Se3 7 . However, the strong field localization of terahertz plasmon in a subwavelength volume makes that materials need to be designed in gratings. The wet chemical synthesis can be a potential alternative and easy way to produce terahertz materials. To reach this goal, the current synthetic procedures need to be pushed toward CQD-based terahertz technology. Mercury chalcogenides (HgX: HgS, 8,9 HgSe 10-12 and HgTe 13 ) have reached the highest maturity to address this part of the electromagnetic spectrum because they combine mid-IR tunable optical features with photoconduction. Lots of effort have been made for the integration of HgX CQD, including the design of photoconductive 14,15 , photovoltaic, 16 phototransistor, devices 17 , plasmon assisted photodetection, [18][19][20] multicolor detection 21,22 and even hybrid with graphene 23,24 . A recent review on the field is given in ref 25 .

Among HgX, HgTe is the compound that leads to the best performances, with the lowest dark current and the fastest time response. 25 However, the current state of the art synthesis for mid-IR HgTe CQD relies on Keuleyan et al. synthesis and keep suffering from several limitations, which are discussed in more details in the supporting information. 26,27 In particular the material grows with an anisotropic shape (tetrapod or tetrahedron, see figure S1-S4) which tends to self-assemble into pairs (see figure S4) and leads to a loss of colloidal stability even for moderate size CQD. This is extremely detrimental to the design of pinhole free layer for vertical geometry device. Moreover, the final objects are strongly faceted and present the least reactive facet, which is limiting for growth of heterostructures. This perspective is even more challenging due to the high reactivity of mercury with chalcogenides. Consequently, nucleation and growth steps are difficult to be time-separated and low temperature growth is used in current synthesis (60-120°C), making the obtained material incompatible with usual high temperature (200-300°C) growth process of wide band gap colloidal material. 8,28 In term of shape control HgTe has not yet reached the maturity of CdSe. 2D HgTe platelets have recently been proposed by Izquierdo et al. 29 but they remain limited to the near-IR range. Shen et al 30 have also proposed the use of a more reactive precursor such as bis-(trimethylsilyl)telluride (TMSTe) as a way to improve sphericity and aggregation. However, TMSTe is expensive and has a very limited stability, which prevents such synthesis to be viable at least at the industrial scale. Last, the current range of size spans from 3 to 20 nm, which makes that the obtained material is still strongly confined since HgTe presents a large exciton Bohr radius of 40 nm. 31 This prevents current material to address wavelengths above 12 µm 27,32 and also limits the emergence of topologically protected states which are expected for poorly confined objects. 33,34 In this paper, we report a new and simple synthetic procedure for the growth of HgTe CQD. We demonstrate size control from 5 nm to above 200 nm, which allows for the first time to cover the whole infrared range from 2 µm up to the THz range (65 µm) with a single material. In particular, we focus on long wavelengths (10-100 µm range), which are non-addressable based on previously reported synthesis. We also integrate these materials into electrolyte-gated transistors and demonstrate that HgTe CQD switch from p-type for the smallest size to n-type for the largest CQD. To achieve a higher degree of control on HgTe CQD synthesis, we investigate for less reactive precursors. Our approach is based on the simultaneous injection of HgX2 (with X=Cl, Br or I) and Te complexed with trioctylphosphine (TOP) in a hot solution of oleylamine. It is worth noting that the procedure can be extended to the growth of HgSe and HgS (see figure S9 and S10), while using NaBH4 reduced Se and elemental sulfur in oleylamine, respectively. The size of the CQD is driven by three key parameters, which are temperature, the reaction time and the chosen mercury halide precursor. By tuning the reaction temperature from 120 °C up to 340 °C, the mean size of the CQD can be tuned from 5 nm up to 200 nm, see Figure 1c, 2 and Figure S5. Thus, it is possible to obtain absorption all over the infrared range from 2 µm up to 65 µm, see Figure 1. This approach presents the key advantages to both be able to synthesize small CQD sizes with sharp excitonic transition, but is also well suited to obtain large CQD, which are able to address long wavelength (>12 µm). Figure 1b shows how the Long-Wavelength Infrared range (LWIR, 8-12 µm range), Very Long-Wavelength Infrared range (VLWIR, 12-20 µm range) and THz (λ>30 µm) can be finely addressed using the developed protocol. The optical features have been pushed to an energy typically twice smaller than the previous record. 10 In the case of large CQD, the spectrum is made of two components: an interband absorption at high energy and an intraband/plasmonic peak at low energy, see Figure 1a andb. The exact nature of the long wavelength feature actually depends on particle size and doping level. 8 The energy of this transition can be made low enough, so that the peak overlaps with the absorption resulting from the LO phonon line (at 123 cm -1 ). The Raman spectrum in figure S11 confirms the phonon character of this line. Another striking behavior is the exceptional narrowness of the intraband/plasmonic peak (< 200 cm -1 ) compared to what it can be for doped nanocrystals of Si 35 or oxides [36][37][38] (with a FWHM >1000 cm -1 ). While the role of reaction time is rather trivial with an increase of the final size with time, the role of halide is more complex. We observe that chloride leads to larger objects compare to the other halides (>Br>I), see Figure 2. This trend is confirmed by following the absorption of the solution at 415 nm, which is the signature of the amount of crystalline HgTe, see figure S5. 32 Even at low temperature (120 °C), the reaction is almost complete after 3 min in the case of HgCl2, while it is still unfinished after 3 h while using HgI2. As shown in Figure 2a, d ande, iodine precursor leads to more faceted objects. This may be attributed to the soft base character of iodine that allow a better bonding and poisoning surface made of soft acid mercury. 39 As recently proposed by Shen et al, 30 it is possible to tune the magnitude of the long wavelength peak of the absorption spectra while tuning the reaction stoichiometry. For moderate size CQD (12-40 nm), only the interband contribution is observed in the case of stoichiometric reaction (Hg:Te=1:1), while the long wavelength feature is obtained in Hg rich condition, see figure S8. Hg rich material tends to be n-type doped, which favor the observation of the intraband/plasmonic feature as observed for HgS and HgSe which are intrinsically n-doped. 40 Transmission electron microscopy (TEM) images (Figure 2) clearly show a limited aggregation. Moreover, it is possible at low temperature to obtain almost nearly spherical objects, see Figure 2 f and g. As shown in Figure 2 d, e andh, the size dispersion is large for large objects. However, this is not critical since the plasmon linewidth of CQD is driven by the doping level rather than the confinement. 8 The crystalline nature of the obtained material is well revealed by high resolution TEM (Figure 2 g) as well as by X-ray diffraction (Figure 2i). The diffraction pattern is consistent with the zinc-blende structure of HgTe. To better highlight the role of doping on the optical properties, we integrate the film of HgTe CQD into electrolyte-gated transistor (see Figure 3a for a scheme of the device). 41 Such gating method is convenient because it combines low bias operation thanks to the large gate capacitance with air operability. As the nanoparticle size is increased, the material switches from p-type (conduction under hole injection) (Figure 3b) to ambipolar (Figure 3c) and finally to n-type only (conduction under electron injection) for the largest sizes (Figure 3d). This change of behavior is further quantized by the increase of the ratio of electron mobility over hole mobility while the band gap is reduced, see Figure 3e. This emergence of n-type conduction is similar to what has been observed for HgS 8 and HgSe 10 nanocrystals. Due to the combination of narrow band gap and high work function in HgX CQD, the material can be reduced by the environment. 42 This reduction however occurs for much larger size (ie lower confinement) because of the lower work function of HgTe material compared with HgS and HgSe. 40 This is interesting on an applied point of view because it preserves the gate effect even for large particles, while for other chalcogenides gate control is lost due to their too metallic character. 43 This means that even for the largest particle the number of carrier per particle remains in the range of what can be tuned by electrolyte gating (0-6 electrons typically). As a result, the long wavelength feature observed in the absorption spectra is probably not fully plasmonic and preserves an intraband character. In summary, we report a new synthetic procedure for HgTe nanocrystal, which allows an unprecedented tuning of nanocrystal sizes from small scale to far beyond the exciton Bohr radius. This leads to the first demonstration of colloidal nanocrystals with optical feature in the THz range. The synthesis is also promising for small size because of the low aggregation and good shape control (ie spherical objects) of the synthetized nanocrystals. Moreover, the synthesis relies on low reactivity precursor of mercury with low cost. The transport properties of the nanocrystals show a change of majority carrier density from p-type for small CQD to n-type for the largest object.

CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Additional information includes experimental details on HgTe, HgSe and HgS nanocrystal synthesis, TEM pictures of HgTe CQD shape evolution with time and self-assembly in dimer, reactivity of different HgX precursors, absorption spectra with different stoichiometry of HgTe CQD, size distribution, experimental details on electron microscopies, UV-Vis, FTIR and Raman spectroscopies (PDF)

Table of Content

1 * 1

 11 Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005 Paris, France 2 LPEM, ESPCI Paris, PSL University, CNRS, F-75005 Paris, France 3 Sorbonne Université, CNRS, LPEM, F-75005 Paris, France 4 Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8233, MONARIS, 4 Place Jussieu, 75005 Paris, France Supporting Information Placeholder

Figure 1 .

 1 Figure 1. (a). absorption spectra of HgTe CQD with various sizes. (b) absorption spectra of HgTe CQD with various sizes, presenting a plasmonic feature in the LWIR, VLWIR or THz range. (c) Cut off wavelength of the interband transition (for material without plasmonic feature) and plasmonic peak wavelength (for doped material) as a function of the CQD size (diameter for spherical object). SWIR corresponds to Short-Wavelength Infrared range (1.5 to 3 µm), MWIR to Mid-Wavelength Infrared range (3 to 8 µm), LWIR to Long-Wavelength Infrared range (8 to 12 µm), and VLWIR to Very Long-Wavelength Infrared range (12 to 30 µm). Inset: full width at half maximum (FWHM) of intraband peak as function CQD size.

Figure 2

 2 Figure 2 (a), (b) and (c) are Transmission Electron Microscopy (TEM) images of HgTe CQD synthetized at 180°C with HgI2, HgBr2 and HgCl2 as Hg precursor, respectively. (d) and (e) are TEM images of HgTe CQD synthetized using HgI2 as precursor at 250°C and 300°C, respectively. (f) and (g) are low and high resolution images of spherical HgTe CQD with limited aggregation. (h) is an Scanning Electron Microscopy (SEM) image of HgTe CQD synthetized at 300°C using HgCl2 and (i) the corresponding X-ray diffractogram.

Figure 3

 3 Figure 3 (a) Scheme of an electrolyte gated transistor which channel is made of S 2-capped HgTe CQD. Transfer curves (current as a function of gate bias) for HgTe CQD with an excitonic feature at 4000cm -1 (b), a cut off at 2000cm (c) and a plasmonic feature at 450cm -1 (d). (e) Variation of the ratio of the electron mobility over the hole mobility as a function of the interband gap.

ACKNOWLEDGMENT

EL acknowledges the financial support of the European Research Council (ERC) starting grant (blackQD -n°756225). We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre". We thank Agence Nationale de la Recherche for funding through grant Nanodose and H2DH. This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K. This work was supported by French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE. This work is also supported by the NextIR grant funded by Nexdot.

AUTHOR INFORMATION Corresponding Author

* el@insp.upmc.fr

Notes

The authors declare no competing financial interests.