MD, PhD Nicolas Bertheuil
email: nbertheuil@gmail.com

MD, PhD Franck-Marie Leclere

MD Farid Bekara

MD, PhD Eric Watier

Erwan Flecher

Jerome Duisit

MD, PhD Erwan Flécher

DDS, MD, PhD Jérôme Duisit

Superior epigastric artery perforator flap for reconstruction of deep sternal wound infection

Keywords: American Society of Anesthesiologists CABG, coronary artery bypass grafting IMAP, internal artery mammary perforator SEAP, superior epigastric artery perforator

Introduction

Patients who undergo to cardiac surgery have most often several comorbidities, often leading to multiple post-operative complications. Mediastinitis is a common complication after cardiac surgery, occurring approximately up to 3.4% of patients [START_REF] Goh | Post-sternotomy mediastinitis in the modern era[END_REF] . It's a life-threatening complication with a mortality rating up to 16% [START_REF] Aliu | Mortality trends and the effects of debridement timing in the management of mediastinitis in the United States, 1998 to 2010[END_REF] . In few cases, after multiple reoperations with aggressive surgical debridement, a deep sternal wound dehiscence required to be covered by a plastic surgeon. Classical local or locoregional flaps such as pectoralis major muscle, rectus abdominis muscle, latissimus dorsi muscle or omentum can be used. [START_REF] Jurkiewicz | Infected median sternotomy wound. Successful treatment by muscle flaps[END_REF] These flaps are quickly harvested, but are associated with subsequent donor site morbidities. In this context, perforator propeller flaps represent the ultimate progress in the history of reconstructive surgery: indeed, they provide a highly reliable reconstructive method, with two options regarding the anterior thoracic wall reconstruction: the internal mammary artery perforator (IMAP) [START_REF] Kannan | The internal mammary artery perforator flap and its subtypes in the reconstruction of median sternotomy wounds[END_REF] or the superior epigastric artery perforator (SEAP) flaps [START_REF] Eburdery | Management of Large Sternal Wound Infections With the Superior Epigastric Artery Perforator Flap[END_REF] . Their main interest is to be harvested quickly, as compared with free flaps (procedure duration), along with low donor site morbidity as compared with the muscular flaps (functional sequelae). Moreover, the IMAP flap is not feasible after coronary artery bypass graft (CABG), when the entire length of both internal mammary vessels have been harvested: the SEAP flap remains feasible, due to the retrograde superior epigastric vessels flow.

Herein, we report one of the largest series of SEAP flaps for deep sternal wound coverage. We describe the flap harvesting, the post-operative outcomes and their management.

Methods

We report our experience with seven patients who underwent prior reconstruction with SEAP flaps, between March 2015 and June 2017 in our department of Plastic and Reconstructive Surgery. During this period, we prospectively recorded the following features: age, sex, body mass index, type of cardiac surgery and comorbidities. The following peroperative data were recorded: American Society of Anesthesiologists (ASA) score, sternal defect size, dimension of the skin paddle of the flap, operative time, as well as the post-operative complication and postoperative survival. This study was performed in accordance with the principles of the Declaration of Helsinki (1964) and in agreement with the french Bioethics Laws of July 7 th , 2011.

Surgical techniques

The procedure is performed under general anesthesia, and in a supine position. Perforator vessels are identified vertically with an 8-MHz acoustic handheld Doppler (Hadeco ES 100 VX) [START_REF] Chaput | Perforator detection with a hand-held Doppler device: importance of the learning curve[END_REF] , on each side above the rectus muscle near the defect to choose the best perforating vessels. Preoperative design is based on the location of these perforators (as a propeller perforator mapping) and adapted perioperatively. We create an elliptical skin flap to allow primary closure of the donor site oriented in the direction of the perforasome 7 , in the inframammary fold. The surgery is divided in three steps: firstly, flap harvesting; secondly, deep sternal wound debridement; and finally, flap rotation and defect closure.

Flap harvesting

Firstly, the SEAP flap dissection starts: we perform a medial vertical small incision to search the previously detected perforators, and dissect under loupe magnification (x3.5). If the vessels are not found, or their caliber is too small to support the flap, a common feature in this population with polyarterial diseases, the dissection is pursued on the other side, through the same incision. When the SEAP vessels are found and selected, perforators are dissected under the fascia and skeletonized. Thereafter, the distance between the main vessel and the end of the loss of substance is measured, as well as the width of the defect. Finally, the drawing is readjusted and the skin ellipse incised and the flap elevated. The perforator vessels dissection is pursued intra-muscularly, to achieve optimal flap mobility without tension on the pedicle.

Sternal wound debridement

This step is always achieved once we have made sure that the flap could cover the defect. We then perform an aggressive debridement, with multiple bone sampling in order to provide an adequate antibiogram and adapt the infectious treatment.

Defect closure

The flap is then carefully rotated 90 degrees to cover the defect, and attached with weak tighten [START_REF] Chaput | Reply: The Stitches Could Be the Main Risk for Failure in Perforator-Pedicled Flaps[END_REF] Skoog interrupted sutures (3/0 Ethilon, Ethicon, Inc., Somerville, N.J), over suction drains.

The donor site is closed primarily in three planes, by interrupted absorbable sutures (Polysorb 2 ; Covidien, Dublin, Ireland ; and Monocryl 3/0 ; Ethicon, Inc., Somerville, N.J.) and by a running suture (Monocryl 4/0) or staples.

Post-operative care

Patients were monitored hourly on the first two days then 3 times a day until the end of the first week. The skin flap was checked on the following elements: color, heat, skin flexibility. Drains were removed if done less than 20 mL/24 h. Empirical antibiotic treatment was adapted to the result of the samples. All patients wore compression stockings and received subcutaneous lowmolecular weight heparin to prevent phlebitis for 15 days after surgery.

Results

The mean age was 72.1 ± 9.5 years with a mean body mass index at 27.5 ± 6.1 kg/m 2 . There were four male and three female patients. The initial cardiac procedure was a coronary artery CABG (n=2), aortic valve replacement (n=3) and a combined aortic valve replacement-CABG (n=2). The mean preoperative ASA score was 3 (n=5) and 4 (n=2) (Table 1). The defect size ranged between 32 to 170 cm 2 (Table 2).

The overall mean operative time was 156.9 ± 30.1 min for the three steps procedure. The mean size of the skin paddle was 155.7 ± 52.8 cm 2 (ranging 6-9 x 14-27 cm). The mean hospitalization time after flap coverage was 31.6 ± 16 days. All SEAP flaps achieved a successful entire coverage of the defect, but only two patients were free from any flap-related complications.

The figure 1 and2 show a deep wound sternal infection cover with a SEAP flap.

Five patients had venous congestion of the flap, treated by leech therapy [START_REF] Herlin | Leech therapy in flap salvage: Systematic review and practical recommendations[END_REF] : among them, the two first patients presented complete venous congestion resolution, one have distal partial necrosis treated by with V-Y wound closure. The last two patients presented a necrosis of the entire superficial flap layer, but the tissue underneath the superficialis fascia remained viable: the complication originated from a problem of venous return as well as arterial inflow (Table 2). For these last two patients wound closure was achieved by secondary healing using Algostéril (Brothier, Nanterre, France) dressing. One patient had a recurrent infection with cutaneous fistula at 9 postoperative months. He was re-operated with re-elevation of the lateral haft of the flap for perform a total sternum bone ablation to allow disease-control and wound healing. One patient, discharged at postoperative day 18 died of a cardiac arrest at postoperative day 27.

Discussion

Deep sternal wound infections and mediastinitis are serious complications following cardiac surgery and represent a real surgical challenge. In rare cases and despite initial surgical debridement and antibiotherapy, reconstructive surgery is needed for deep sternal wound coverage. Before the advent of perforator flaps, local muscular flaps such as pectoralis major and rectus abdominis flaps, or locoregional flaps like latissimus dorsi flap, were routinely used and associated with significant donor site morbidity. Omentum flaps were also used, associated with poor cosmetic results and major risk of eventration.

Perforator flaps offer new tools in the reconstructive armada, in which the propeller flaps are a refinement [START_REF] Pignatti | The "Tokyo" consensus on propeller flaps[END_REF] . These flaps achieve a robust reconstruction with well-vascularized tissues and can address large tissue defects, offering a great flexibility and plasticity near to the defect, as well as a direct closure of donor site without any muscular sequalae.

Our study shows that this flap can achieve mediastinal coverage, regardless of cardiac surgery performed (even after CABG) unlike IMAP propeller flaps that are no longer feasible when both mammary arteries have been removed (4 of our 7 patients).

We achieved 100% success coverage, making SEAP flaps a good first choice option for these patients in our experience. However, the postoperative management remains difficult, with a significant complication rate (71.4%). We do recommend close monitoring of the flap by a well-trained specialized plastic surgery team with dedicated nurse care. Post-operative complications can be explained by the fact that the caliber of the perforating vessels is often small, associated with a short length of the pedicle. These anatomical particularities may explain a great sensitivity of this flap to the twist and thus to the venous suffering. In a previous teams works [START_REF] Florczak | The Use of Pedicled Perforator Flaps in Chest Reconstruction: A Systematic Review of Outcomes and Reliability[END_REF] on propeller perforator flaps for chest wall defect, the SEAP flap had already been associated with a higher complication rate. As previously said, this can rely on intrinsic characteristic of the flap (caliber vessels, length of the pedicle, retrograde vessels flow after CABG) but also on specific characteristics of theses patients (arterial disease, smoking status, ASA high score, etc...). In addition, experienced that the cover of the entire length of the sternum with SEAP flap, could favor the problem of venous return. Indeed, we observed that when the length to cover is greater than 20 cm we had a postoperative complication in venous return. Further studies will explore if this parameter may represent the limit of the venous perforasome [START_REF] Chaput | Delayed procedure in propeller perforator flap: Defining the venous perforasome[END_REF] . Our results are comparable to those from Wettstein et al [START_REF] Wettstein | Superior epigastric artery perforator flap for sternal osteomyelitis defect reconstruction[END_REF] using the same flap on a series of patients similar to ours. Indeed, they also reported 100% coverage success rate, but with an overall complication rate of 55.6% and among them three (33.3%) partial flap necrosis. Eburdery et al 5 reported a venous congestion in one of four patients treated with SEAP flap, resolving after leech therapy. As in our experience, it is interesting to note that the venous congestion appeared on a flap more than 20 cm-long. Moreover, in 1 patient a pectoralis major flap was needed to cover the upper third of the sternum to limit the SEAP length. Besides, Wettstein et al did not specify the length of theirs flaps in the manuscript.

The other perforator propeller flap being an alternative to cover sternal defect is the IMAP flap.

Kannan et al 4 reported 4 patients covered by IMAP propeller flap with 2 patients presenting partial necrosis. The IMAP propeller flaps were used to treat superficial defect whereas they preferred musculo-cutaneous IMAP flap to cover deeper mediastinal defects. It should be noted that the authors selected their cases to perform IMAP flaps, according to the initial cardiac surgery, even if in one case after CABG the perforating vessels were still present: maybe because the internal mammary artery was harvested being skeletonized. The IMAP flap is equivalent to the SEAP flap in term of cover capacity, as well as regarding the morbidity, but is not feasible in all patients especially after those after CABG. It has now been prooved that muscle flaps do not provide any additional benefit in the treatment of underlying infections, as compared with perforator flap [START_REF] Buono | Muscular Versus Non-Muscular Free Flaps for Soft Tissue Coverage of Chronic Tibial Osteomyelitis[END_REF][START_REF] Hong | The Efficacy of Perforator Flaps in the Treatment of Chronic Osteomyelitis[END_REF][START_REF] Thiessen | Flap surgery for pressure sores: should the underlying muscle be transferred or not? Journal of plastic, reconstructive & aesthetic surgery[END_REF] , and create more donor site morbidity. In our opinion, these latest flaps should be used as a second intention, after perforator flap failure.

Even if demonstrative and representing one the largest series, this monocenter study remains limited by the small number of patients included. One should keep in mind that such population of patients remains rare even in high volume cardiac surgery departments. It is currently difficult to compare extensively SEAP and IMAP flaps, considering the lack of randomized studies and in view of the few cases published. Future meta-analytic studies are required to resolve this issue. However, since SEAP can be realized in all patients unlike IMAP, it became our preference as first line of treatment in this population. It would also be interesting in the future to compare the results of these flaps with those using the older classical muscle flap technique.

Conclusion

The SEAP Perforator propeller flap is a reliable and safe surgical technique to achieve deep sternal wound infections treatment. This technique requires well-trained surgeons and postoperative team. Propeller perforator flaps change the management of these complex patients and may be currently considered as a first line option. These flaps open new possibilities in cardiac and thoracic reconstruction.

Authors contributions statement

FIGURE 2 (

 2 FIGURE 2(a) Preoperative SEAP flap markings with mapped perforating vessels. (b) Intraoperative view of flap harvesting (c) Intraoperativeview of the perforating vessels. (d) Postoperative view at 12 months

Figure 1 .

 1 Figure 1. A A 74-year old man with history of aortic valvular replacement, complicated with

Figure 2 .

 2 Figure 2. A A 79 old year man with history of aortic valvular replacement, complicated by

Table 1 -

 1 General patient data

	Patient	Age	Gender	BMI	Tobacco status	Original procedure	comorbidities	ASA
	1	70	M	27.0	weaned tobacco	ARV	HBP, IHD,COB	3
	2	76	M	25.7	weaned tobacco	CABG	HBP, IHD, obliterative arterial disease	3
	3	78	F	40.1	active consumption	ARV	HBP, circulatory arrest, arrhythmias, smoking	4
	4	74	M	28.7	non smoker	ARV	HBP, IHD	3
	5	83	F	23.6	non smoker	CABG	IHD,obliterative arterial disease, immunosupression	4
	6	53	F	21.1	non smoker	CABG+ARV	HBP, IDH, obliterative arterial disease	3
	7	71	M	26.1	non smoker	CABG+ARV	obliterative arterial disease	3

CABG: coronary artery bypass graft; ARV: Aortic Valve Replacement; HBP: hight blood pressure; DM: Diabetes mellitus; IHD:Ischemic Heart Disease; COB: Chronic Obstructive Bronchopathy

Table 2

 2 Flap Characteristics and outcomes

	Patient Age Gender Cause of defect	Defect size (in cm)	Flap skin paddle size (in cm)	Operative time	Flap related complications	Outcomes	Follow up (months)
	1	70	M	Mediastinitis	20x6	27x8	190	Venous congestion, leech therapy	Healed wound, Alive	66
	2	76	M	Mediastinitis	17x7	20x9	190	Venous congestion, leech therapy	Healed wound, death (D25)	1
			F	Mediastinitis	17x10	24x9	152	No	Healed wound, Alive	56
	4	74	M	Mediastinitis	16x10	18x8	180	No	Recurrent infection at 9 months, healed wound after reoperation, Alive	51
	5	83	F	Mediastinitis	16x6	22x7	116	Venous congestion, leech therapy, partial distal necrosis	Healed wound, Alive	45
	6	53	F	Mediastinitis	8x4	14x6	140	Venous congestion, leech therapy, superficial skin layer necrosis	Healed wound, Alive	42
	7	71	M	Mediastinitis	20x6	16x6	130	Venous congestion, leech therapy, superficial skin layer necrosis	Healed wound, Alive

Conflict of interest statement and source of funding

The authors received no funding support for the research of this article and declared no potential conflicts of interest with the respect to the research, authorship and/or publication. interpretation of the data and drafting of the manuscript, final approval of the version to be published. Jérome Duisit: drafting of the manuscript, final approval of the version to be published.

Franck Marie

Funding statement

The authors received no funding support for the research of this article

Conflict of interest statement

The authors declared no potential conflicts of interest with the respect to the research, authorship and/or publication.


Killian Denoue

David Le Coq
email: david.lecoq@univ-rennes1.fr

Christophe Calers

Antoine Gautier

Louisiane Verger

Laurent Calvez

New synthesis route for glasses and glassceramics in the Ga 2 S 3 -Na 2 S binary system

New synthesis route for glasses and glass-ceramics in the

Ga2S3-Na2S binary system

Introduction

Lithium-ion batteries dominate the current energy storage market. They are mainly used for mobile and automotive applications. However, the demand for energy storage devices is constantly growing not only in the automotive sector with electric vehicles, but also for domestic stationary power supplies and large wind and solar farms 1 2 3 4 . This increasing demand requires new batteries with higher energy and power density and good reliability. Studies have shown that all-solid-state sulfurbased lithium batteries can have more interesting power and energy density than liquid systems 5 6 . The advantage of all-solid-state batteries lies in the use of an inorganic solid electrolyte. It is nonflammable, which means that these batteries do not suffer from leakage or volatilization, which increases their safety, unlike those using liquid electrolyte solutions 7 8 . Nevertheless, resistance at the different interfaces between solid materials usually limits all-solid-state batteries performances. In addition, during cycling, interphases can form at the interfaces and degrade the performance of these batteries [START_REF] Banerjee | Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes[END_REF] .

Despite a major presence of lithium in secondary batteries, it is not an abundant material in the earth's crust and its price has therefore increased significantly due to geopolitical reason 2 10 . On the opposite, sodium has almost unlimited resources, a low environmental impact, and is economically more advantageous. Thus, all-solid-state sodium batteries could be a judicious alternative for the next generation of low-cost batteries 11 12 , especially for stationary applications.

Different solid electrolytes have already been developed for sodium batteries, such as the oxide βalumina Na2O-11Al2O3 and NASICON-type Na3+xScxZr2-x(SiO4)2(PO4) with room temperature conductivities of 1.2 x 10 -3 S/cm 13 and 4.0 × 10 -3 S/cm [START_REF] Ma | Scandium-Substituted Na3Zr2(SiO4)2(PO4) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors[END_REF] , respectively. Compared to oxide-based electrolytes, solid sulfide electrolytes have mechanical properties that facilitate electrolyte-electrode contact by uniaxial pressing at near-ambient temperatures 15 16 . Indeed, sulfur electrolytes have a lower Young's modulus than their oxide counterparts and their increased elasticity participates in maintaining solid-solid contact at the interfaces during variations in electrode volume during chargedischarge cycles [START_REF] Sakuda | Sulfide Solid Electrolyte with Favorable Mechanical Property for All-Solid-State Lithium Battery[END_REF] . In addition, inorganic vitreous materials, and more specifically sulfur-based glasses, have several advantages such as non-flammability, single ion conduction, and an ionic conductivity generally higher than the one of the corresponding crystal [START_REF] Viallet | Glasses and Glass-Ceramics for Solid-State Battery Applications[END_REF] .

Numerous studies have therefore been carried out on sulfur compounds, in particular in Na2S-X binary and Na2S-X-Y ternary systems, with X = P2S5 15 , GeS2 [START_REF] Souquet | Glass Formation and Ionic Conduction in the M2S-GeS2 (M = Li, Na, Ag) Systems[END_REF] , or SiS2 20 and Y = Ga2S3 21 , NaCl [START_REF] Chu | Room-Temperature All-Solid-State Rechargeable Sodium-Ion Batteries with a Cl-Doped Na3PS4 Superionic Conductor[END_REF] , or SnS2 23 . As a result, room temperature conductivities ranging from 10 -8 S/cm up to more than 10 -4 S/cm have been achieved. However, no electrical conducting data are available on the binary Ga2S3-Na2S. To the best of our knowledge, the only referenced work published by Palazzi et al. was essentially devoted to the determination of the vitreous domain obtained by melt quenching, ranging from 76[Ga2S3]-24[Na2S] to 59[Ga2S3]-41[Na2S] [START_REF] Palazzi | Etude Du Système Ga2S3-Na2S. Mise En Évidence d'une Zone Formatrice de Verre[END_REF] . Sodium content in glasses obtained by melt quenching method is limited by the problem of sodium diffusion in silica. Therefore, our study focuses on the expansion of the vitreous domain in the pseudo binary Ga2S3-Na2S using high-energy mechanical milling. This technique is a simple, rapid, no-waste, and energy-efficient pathway compared to the melt quenching technique. Moreover, the ball-milling technique offers possibilities to synthesize materials without solvent or gas, metastable phases or materials that usually require high temperatures. Indeed, chalcogenide glasses have already been produced using this synthesis method 25 26 27 . Then, composition dependence of ionic conductivities of (100-x)[Ga2S3]-x[Na2S] glasses is examined. Next, a glass-ceramic processing is carried out in order to investigate the impact of the crystallization on the electronic properties that prevents the sodium dendrite formation under current. Finally, the presented method allows an electronical stabilization of the samples meaning that this process is promising to produce high sodium content electrolytes for all-solid-state batteries.

Experimental

Synthesis

Compositions belonging to the binary (100-x)[Ga2S3]-x[Na2S] (with x= 10, 20, 30, 40, 50, 60, 70, 80 and 90) were synthesized by mechanical milling using a planetary mill (Fritsch, Pulverisette 7). 5 g of a mixture of Na2S (Alfa Aesar, 95% purity) and Ga2S3 are weighed in stochiometric proportions. Ga2S3 was synthesized by mechanical milling following an experimental process given in a previous report [START_REF] Dénoue | Mechanochemical Synthesis and Structural Characterization of Gallium Sulfide Ga2S3[END_REF] . The starting elements are then hand-ground and the mixture is placed in a zirconia bowl (volume of 45 mL) with ZrO2 balls (4 mm diameter), respecting a ball/powder weight ratio equivalent to 20/1. Mechanochemical synthesis was performed at a speed of 600 rpm, until total amorphization. The bowls were opened every hour to scrub and hand grind the material in order to obtain a uniform powder. Finally, 10 mm diameter pellets with a thickness between 1 and 2 mm were obtained by vacuum pressing under 1.5 T the previous synthesized powders. The whole process is carried out under a controlled atmosphere of N2. To study the impact of crystallization on conductivity properties, the previously obtained amorphous pellets were placed in a silica tube sealed under vacuum and annealed at Tx + 10 (310 °C) for 24 hours.

Characterization techniques

X-ray diffraction (XRD) measurements were performed at room temperature on samples protected from air by a Kapton (polyimide) window to avoid moisture corrosion of fine submicron particles. They were recorded in the 10-90° 2θ range with a 0.0261° step size with each data point having an effective total counting time of 40 s/step, using a PANalytical X'Pert Pro diffractometer (Cu Kα1, Kα2 radiations, λKα1 = 1.54056 Å, λKα2 = 1.54439 Å, 40 kV, 40 mA, PIXcel 1D detector). The counting time was multiplied by twelve when it was necessary to reduce the background noise. Data collector and HighScore Plus software were used for recording and analysis of the patterns, respectively.

A differential scanning calorimeter (DSC Q20Thermal Analysis) was used to characterize the thermal properties of the synthesized materials. Measurements were performed from room temperature (RT) up to 550 °C with a heating rate of 10 °C/min under nitrogen atmosphere. Characteristic temperatures including glass transition temperature (Tg) and crystallization onset temperature (Tx) were determined.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analysis were collected on JEOL JSM-IT 300 using an acceleration voltage of 15 kV and a magnification x 500 to x 3500 to check the surface, the cross section of pellets and the distribution of elements. To compare the amorphous samples in the pseudo binary system Ga2S3-Na2S with the precursors used, pellets of Ga2S3 and Na2S were also vacuum pressed under 1.5 T.

In order to perform electrical measurements, pellets were polished. A thin layer of gold was deposited to ensure good electrical contact with electrodes of the measurement cell. A pressure of approximately 80 kPa was also applied. All measurements were performed with Autolab workstation (PGSTAT302N with FRA32M and ECD module) from 293 K up to 353 K with a frequency range from 1 MHz down to 5 mHz and a signal amplitude of 200 mV. A potentiostatic chronoamperometric measurement was used to determine the electronic conductivity. This technique consists of applying a constant potential difference (100 mV in our case) to a steel/electrolyte/sodium cell. The resulting current measured once the stabilization is obtained is due to the electron motion.

Results and discussion

Compositions ranging from (100-x)[Ga2S3]-x[Na2S] with 10 ≤ x ≤ 90 were prepared. The amorphization process has been investigated by X-ray diffraction analyses on fresh powder every 5 h of milling. As an example, Figure 1-a presents the evolution of amorphization of the 20Ga2S3-80Na2S composition. Na2S and Ga2S3 precursors are detected up to 15 h of grinding. After 20 h of milling, their characteristic peaks are not detected anymore, and a supplementary XRD acquisition was then performed with a longer counting time (twelve times more) in order to confirm the absence of any residual peaks. In this case, the compound is therefore considered amorphous. For (100-x)[Ga2S3]-x[Na2S] with 20 ≤ x ≤ 80, total amorphization has been achieved after 20 h to 38 h of milling, showing no dependence with the Na2S amount. However, for compounds with x = 10 and x = 90, amorphization by mechanochemistry synthesis was not possible. Indeed, Ga2S3 and Na2S are still detected as a majority phase after 15 h and 25 h of milling time, respectively. More importantly, no evolution of the residual peaks of Ga2S3 and Na2S is observed after an extending milling time up to 20 h and 30 h, respectively. Consequently, these compounds are considered outside the glassy zone accessible by high-energy milling.

In this study, we found that the glass-forming range of the (100-x)[Ga2S3]-x[Na2S] binary system is 20 ≤ x ≤ 80, meaning that our experimental conditions show a real benefit compared to the work of Palazzi, in which a glassy domain of 24 ≤ x ≤ 41 was evidenced [START_REF] Palazzi | Etude Du Système Ga2S3-Na2S. Mise En Évidence d'une Zone Formatrice de Verre[END_REF] . Palazzi used the melt quenching technique in a vitreous carbon nacelle with GaOOH and Na2CO3 as starting materials, under a H2S flow [START_REF] Palazzi | Etude Du Système Ga2S3-Na2S. Mise En Évidence d'une Zone Formatrice de Verre[END_REF] . The incorporation of twice as much Na + within the amorphous phase can be explained by the choice of mechanosynthesis: the mechanical energy of the impact of balls against the powder creates a local melting/quenching area in which higher quenching rates occur, possibly higher than 100 °C/s [START_REF] Xue | Amorphization by Mechanical Milling for Making IR Transparent Glass-Ceramics[END_REF] . An extended amorphous domain can therefore be potentially reached using mechanical milling. The amorphous powders were pressed under vacuum (10 -2 Pa) in a 10 mm die at room temperature. The obtained pellets were studied by impedance spectroscopy. Insets in Figure 2 . Two parts differ from this diagram: (i) in the high and medium frequency regions, the semicircle represents the impedance of the sample; and (ii) in the low frequency region, the polarization tail, is typical of a polarization at the electrode. The latter results from the accumulation of charge carriers (Na + ) at pellet-electrode interface and is characteristic of ion conductors. It probably means that the conductivity can be assumed to be mainly ionic. From these diagrams collected at different temperatures, a sample resistance R (Ohm), corresponding to the diameter of the semi-circle, is extracted and equation (1) allows to determine the conductivity σ (S.cm -1 ):

𝜎 = 𝑒/(𝑅 × 𝑆) (1)
With e and S corresponding to the thickness (cm) and the area (cm²) of the pellet, respectively.

Figure 2 shows that ionic conductivity increases with the Na2S content measured at 25°C, following a nonlinear monotonous trend. As expected, the 20[Ga2S3]-80[Na2S] glass composition, corresponding to the highest Na content, reaches the highest conductivity σ = 1.9 x 10 -6 S.cm -1 at 25°C, while the lowest Na content sample 80[Ga2S3]-20[Na2S] possesses a conductivity of σ = 1.6 x 10 -10 S.cm -1 .

Comparison with the pseudo-binary system GeS2/Na2S shows that the conductivities at room temperature of 20[Ga2S3]-80[Na2S] and 50[GeS2]-50[Na2S] (σ = 1 x 10 -6 S.cm -1 ) are roughly the same [START_REF] Souquet | Glass Formation and Ionic Conduction in the M2S-GeS2 (M = Li, Na, Ag) Systems[END_REF] .

Figure 3 shows the evolution of the ionic conductivity of each sample as a function of the reverse temperature. As in figure 2, the ionic conductivity increases with the Na2S content, whatever the temperature. For all compounds the conductivity is thermally activated and follows an Arrhenius law:

𝜎 = ! ! " × exp -- # " $ # " / (2)
With σ0 the pre-exponential factor (S.cm -1 .K), T the temperature (K), Ea the activation energy (eV), and kb the Boltzmann constant.

Nevertheless, two points at 70°C and 80°C for the 20[Ga2S3]-80[Na2S] sample do not obey the

Arrhenius law (open squares in figure 3). This behavior let suppose that an instability of this composition with temperature occurs, which could be characterized by a structural network modification. Indeed, the highest conductivity of all investigated samples, σ = 1.6 x 10 -4 S.cm -1 , is obtained at 80°C for this composition. The activation energies, which were calculated from equation 2, are summarized in Table 1. No influence of the Na2S content is observed and their values are around 0.56 ±0.04 eV. We focused our investigation on the 20[Ga2S3]-80[Na2S] compound, which offers the best ionic conductivity. A solid electrolyte must have both a high ionic conductivity and a low electronic conductivity to promote the circulation of electrons outside the circuit and power a device connected to the battery. The electronic conductivity of our compounds has been measured using the potentiostatic chronoamperometric technique. The residual current I∞ is related to the electronic conductivity σe following equation 3:

𝜎 % = & $ ×% (×) (3)
With I∞ the residual current (A), e the thickness of the sample (cm), U the voltage (V), and S the surface area of the sample (cm²).

Potentiostatic chronoamperometry measurements show that the 20[Ga2S3]-80[Na2S] compound is not electronically stable: in figure 5a, a continuous increase of the current is observed, instead of a drop followed by a plateau. SEM-EDS mapping operated after measurement shows the formation of Na-rich area on the side of the 20[Ga2S3]-80[Na2S] pellet in contact with the blocking electrode. This clearly shows the formation of metallic sodium dendrites during measurement, which is likely to happen with this Na-rich composition. Consequently, even if the ionic conductivity of this sample is promising, the growth of metallic sodium dendrites during potentiostatic chronoamperometry experiments requires a new or adapted strategy. Literature reveals the existence of superconducting phases by crystallizing amorphous compounds, such as Na3PS4 [START_REF] Hayashi | Superionic Glass-Ceramic Electrolytes for Room-Temperature Rechargeable Sodium Batteries[END_REF] . Crystallization experiments were therefore carried out using the amorphous 20[Ga2S3]-80[Na2S] sample. Pellets of amorphous 20[Ga2S3]-80[Na2S] were placed in a vacuum-sealed tube and put in an annealing furnace at Tx + 10 (310 °C) for 24 hours. These annealing parameters were fixed after many experimental attempts from which the aim was the obtention of a thin and homogeneous crystalline powder. The diffractogram obtained (figure 6), reveals the presence of two polymorphs of GaNa3S3: I-GaNa3S3 (JCPDS no. 01-084-2770) and II-GaNa3S3 (JCPDS no. 01-084-2771). Two peaks between 16 and 17° are not indexed by these phases. GaNa3S3 corresponds to a starting stoichiometry of 25[Ga2S3]-75[Na2S]. Thus, there is an excess of Na2S that might react with the silica tube and form NaSi (JCPDS no. 00-019-1231) and S (JCPDS no. 01-078-0793). Nevertheless, EDS analyses do not reveal the presence of Si in the material and consequently the peaks not indexed by GaNa3S3 are attributed to sulfur. It can also be underlined that the synthesis of I-and II-GaNa3S3 has already been reported in the literature by heating stoichiometric amounts of Ga, Na, and S in a graphitized quartz glass tube sealed under vacuum at about 650 and 800 °C, respectively 34 35 . We prove here that GaNa3S3 can be synthesized at lower temperature (310 °C) by crystallization of an amorphous precursor. Supplementary investigations are in progress on the 25[Ga2S3]-75[Na2S] composition to more specifically evaluate the impact of milling on the polymorph obtained. Complex impedance and potentiostatic chronoamperometric measurements were performed in order to evaluate the ionic and electronic contribution in the conductivity of the glass-ceramic compound (figures 5b and 5c). First, the ionic conductivity at 25°C of the glass-ceramic compound appears to be slightly lower than for the amorphous sample (σglass-ceramic = 7.1 x 10 -7 S.cm -1 and σglass = 1.9 x 10 -6 S.cm - 1 ). Compared to other glass compounds of the binary system, the value of σglass-ceramic is similar to the conductivity of the 30[Ga2S3]-70[Na2S] glass (σ = 7.5 x 10 -7 S.cm -1 ). We also noticed that the deviation to the Arrhenius law observed for the two highest temperatures in the glass sample is deeply reduced in the glass-ceramic. These data clearly support the structural stabilization of the composition by ceramization. Secondly, the potentiostatic chronoamperometric measurement shows that the compound is electronically stable (Figure 5c). Indeed, a current drop followed by a plateau is observed, the latter corresponding to the residual current within the material. According to equation (3), an electronic conductivity σe = 5.8 x 10 -8 S.cm -1 is calculated, which is approximately 12 times lower than the ionic conductivity. Moreover, EDS mapping of this compound (figure 5c) shows a homogeneous distribution of the elements (Ga, S, and Na) after measurement and does not show dendrites. These results are consistent with the potentiostatic chronoamperometry measurement and confirm its electronic stability. In summary, the strategy implemented to stabilize compounds of the chosen binary system by a glass-ceramic processing turned out to be promising, with a potential as solid-state electrolyte for sodium batteries. Nevertheless, from a solid electrolyte point of view, since the difference between σi and σe is still low, some adjustments in the composition have to be done to increase it. Consequently, future investigations must be devoted to reducing the electronic conductivity. One judicious way could consist in the addition of an electronegative element in the composition, such as chlorine, to prevent the electron motion in the compounds. Thus, a substitution of a part of Na2S by NaCl have to be considered. Another way could be the addition of a low content of a network former -modifier as for example (Si, Sn, In, Ge, …). Moreover, the conductivity is not the only key parameter for a good solid electrolyte, and further investigations should be carried out to determine its chemical stability with regard to the electrodes and to determine its electrochemical behavior during cycling tests, especially at the electrolyte-electrode interfaces.

Conclusion

This work shows that the glass-forming region of the (100-x)[Ga2S3]-x[Na2S] pseudo-binary system can be extended to 20 ≤ x ≤ 80 by mechanical-milling synthesis. Conductivities of pressed pellets of amorphous powders were measured for the first time. This study shows that the higher the Na2S content is, the lower the Tg is and the higher the conductivities are. The best ionic conductivity is obtained for the glass composition 20[Ga2S3]-[80Na2S] with σ = 1.9 x 10 -6 S.cm -1 at 25°C and σ = 1.6 x 10 -4 S.cm -1 at 80°C. Unfortunately, this compound is unstable since the formation of sodium dendrites is observed once it is under current. In order to thwart the dendritic growing, a glass-ceramic processing was operated. Its crystallization mainly reveals the formation of GaNa3S3 polymorphs. The conductivity of the glass-ceramic is lightly lower than for the glass counterpart but the crystallized sample has the advantage to be electronically stable. Even if the ratio between σi and σe is still too low to positively consider application as solid electrolyte in all-solid-state sodium batteries, the presented results are very promising from a methodological aspect.

Figure 1 :

 1 Figure 1: (a) XRD diagrams of 20[Ga2S3]-80[Na2S] samples prepared by ball milling for different durations. A longer counting was used for the powder after 20 h of milling in order to improve the statistic. (b) DSC trace of the 20[Ga2S3]-80[Na2S] sample after 20h of milling.

  show the Nyquist diagrams of compounds 70[Ga2S3]-30[Na2S], 50[Ga2S3]-50[Na2S], and 20[Ga2S3]-80[Na2S]

Figure 2 :

 2 Figure 2: Evolution of the ionic conductivity as a function of the molar percentage of Na2S for the vacuum pressed pellets, at room temperature. The solid line is a guide for the eye. Insets highlight the Nyquist's diagrams for amorphous samples (100-x)[Ga2S3]-x[Na2S], with x = 30, 50, and 80 at 25°C.

Figure 3 :

 3 Figure 3: Evolution of the ionic conductivity of vacuum pressed pellets as a function of the reverse temperature. The two open black squares correspond to data for 20[Ga2S3]-80[Na2S] that do not obey the Arrhenius law.

Figure 4 :

 4 Figure 4: SEM images of the surface (a) and cross section (b) of 20[Ga2S3]-80[Na2S]; the surface (c) and cross section (d) of 50[Ga2S3]-50[Na2S]; the surface (e) and cross section (f) of 80[Ga2S3]-20[Na2S]; the cross sections of Na2S (g) and Ga2S3 (h).

Figure 5 :

 5 Figure 5: (a) Potentiostatic chronoamperometric measurement of amorphous compound 20[Ga2S3]-80[Na2S]. (b) Comparison of the ionic conductivity between amorphous compound 20[Ga2S3]-80[Na2S] and annealed 20[Ga2S3]-80[Na2S] at 310°C for 24h as a function of the reverse temperature. (c) Potentiostatic chronoamperometric measurement of the annealed 20[Ga2S3]-80[Na2S] sample at 310°C for 24h. The insets are EDS mappings of a fragment of the respective pellets after measurement, with Ga in red, S in green, and Na in blue.

Figure 6 :

 6 Figure 6: XRD diagram of 20[Ga2S3]-80[Na2S] samples annealed at 310°C for 24h. The two lowermost diagrams represent the peaks position of the two monoclinic GaNa3S3 polymorphs (JCPDS no. 01-084-2770 and JCPDS no. 01-084-2771).

Table 1 :

 1 Characteristics of amorphous compounds belonging to the pseudo binary Ga2S3-Na2S system: onset glass transition temperature Tg, onset crystallization temperature Tx, and activation energy Ea.

	Compound	Tg °(C) (± 5°C)	Tx (°C) (± 5°C)	Ea (eV)
	20[Ga2S3]-80[Na2S]	263		298		0.574(18)*
	30[Ga2S3]-70[Na2S]	290		340	419	0.592(15)
	40[Ga2S3]-60[Na2S]	285		324		0.551(16)
	50[Ga2S3]-50[Na2S]	304		405		0.540(13)
	60[Ga2S3]-40[Na2S]	300	356	394		0.524(13)
	70[Ga2S3]-30[Na2S]	449		533		0.576(12)
	80[Ga2S3]-20[Na2S]	328		437	>550	0.604(19)
	*calculation carried out up to 1000/T = 3.003.

Acknowledgment

Authors would like to acknowledge the IUF (Institut Universitaire de France). This publication is supported by the European Union through the European Regional Development Fund (ERDF), the Ministry of Higher Education and Research, the French region of Brittany and Rennes Métropole.


Rodolphe Valleix

Dr Qian Zhang

Prof Damien G Boyer
email: damien.boyer@sigma-clermont.fr

Philippe Boutinaud

Geneviève Chadeyron

Yongjun Feng

Dr Hanako Okuno

Dr François Réveret

Horst Hintze-Bruening

Fabrice Leroux
email: fabrice.leroux@uca.fr

Prof P Chadeyron

Dr F Boutinaud

Leroux

A first wide-open LDH structure hosting InP/ZnS QDs: a new route toward efficient and photostable red-emitting phosphor

Keywords: Indium phosphide, Quantum dots, Bola-amphiphiles, Layered double hydroxide, Silicone composite film, Red-emitting phosphor

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

Semiconductor nanocrystals (NCs) form a well-known class of emerging materials which find applications in many forefront fields such as biological imaging/labelling, [1] lasers, [2] displays [3] and solid-state lighting. [4] Colloidal quantum dots (QDs) are among the most promising materials to integrate this technology. These NCs exhibit a three-dimensional quantum confinement which endows them with outstanding optical properties, such as size-tunable emission, in addition to presenting a broad absorption range. Environmental concerns have forced the scientific community to move from the well-established CdSe QDs to less toxic Cd-free compositions. Today, indium phosphide (InP) is one of the most-studied substitutes. Despite the fact that the synthesis of InP QDs was first described 25 years ago by Mićić et al., [5] it remained scarcely used because of the pyrophoric and toxic nature of the phosphorous precursor, tris-(trimethylsilyl)-phosphine (P(TMS) 3 ). In 2013, Song et al. reported on the synthesis of InP QDs through a cost-effective and safer way using aminophosphines (e.g. P(N(CH 3 ) 2 ) 3 ). [6] This method presents an acceptable size distribution despite a photoluminescence quantum yield (PLQY) lower than that attained with P(TMS) 3 . Moreover, the main drawback of InP-based QDs is their loss of stability under continuous photonic excitation compared to Cd-based QDs. This issue can however be overcome by increasing the thickness of the shell using a core-shell structure such as InP/ZnSe. [7] Thus a multi-shelled InP core like InP/ZnSeS/ZnS is reported to reach a near-unity PLQY with high stability under continuous UV light irradiation. [8] So far, other structures like InP/ZnSe/ZnS [9] and InP/GaP/ZnS [10] exhibit good photonic stability, but their synthesis requires almost twelve hours to complete and their scale-up is highly unlikely. Another enticing approach to stabilizing InP-based QDs is to employ a protective inorganic host structure. Evidently, such a host structure must show high flexibility to accommodate the size of QDs. Hitherto, metal organic frameworks (MOFs) or layered (2D) structures have appeared as relevant host structures to embed QDs. MOFs presenting large open structures associated with high surface areas have been extensively studied, and their association with QDs, whether in an "onto" or "into" mode, find applications, as reviewed for photocatalysis, [11] and energy [12] as well as light-harvesting. [13] As far as the topic of QDs armored by 2D structures is concerned, a heterogeneous hybrid structure is formed using the layer-by-layer (LbL) method. Based on electrostatic interaction, the LbL process requires monolayers or highlydispersed sheets to fabricate films to be implemented into optical devices. In combination with a 2D structure and exemplified by layered double hydroxide (LDH), the LbL approach involves positively-charged self-assembled rigid platelets alternating with soft organic segments, usually a polymer, where QDs are encapsulated, as exemplified by CdSe/CdS/ZnS in poly(maleic acid-altoctadecene), [14] CdTe and CdSe/ZnS in polyvinyl alcohol (PVA), [15] CdTe QDs directly in the LDH interlamellar space [16,17] or, as for dihydrolipoic acid-decorated PbS QDs and exfoliated LDH nanosheets, by drop-casting. [18] As for the multi-shelled QD approach, LbL technology is time-consuming, and probably limited to the fabrication of thin films; moreover, the exfoliation process is also limited to small LDH concentrations in large quantities of solvent, hindering any scale-up. An alternative is to accommodate QDs directly into the LDH interlamellar space by a straightforward and scalable method. LDH-type materials are built from edge-sharing octahedral units forming positive layers by cation substitution from brucite M II (1-x) M III x with the presence of the interleaved species, anions and water molecules. Usually, and as for mesoporous MOFs using large-size templating agents to entrap QDs such as perovskite QDs, [19] surfactant molecules are selected. Such long-chain fatty acid anions, such as arachidate, are found to prop apart the layered structure. The process is of Langmuir-Blodgett (LB) [20] type or through the delamination of dodecyl sulfate (DS) LDH exfoliated into 1-butanol and restacked onto (dodecanethiol-capped 4-nm) CdSe. [21] Bearing in mind that QDs should be fully embedded to be protected by the mineral host, the "into" approach is preferred to immobilisation « onto » the platelets; the layered structure should then match the QDs size without being disrupted (delaminated). Usual surfactant molecules are here discarded, as they are tethered at one end-molecule only, making it possible for the sheets to delaminate and, as the size needed for the alkyl chain will be probably too long (over 5 to 6 nm length), thus rendering the surfactant too waxy to mix intimately with LDH platelets. As far as the size of the basal spacing is concerned, polymeric bola-amphiphiles, [22a,22b,22c] in particular ,-carboxylate-terminated hydrophobic backbones (Figure S1) are reported to generate an interlayer space as large as 5 to 6

nm, [23a,23b] close here to the size of the QDs. It is found that LDH phases are highly compatible with such bi-terminal molecules, which is reminiscent of terephthalate intercalation pioneered by Drezdon. [24] Relying upon previous studies, [23a] the first QD-LDH hybrid materials are synthesized via an original method where organo-modified bola-LDH hybrid structures are placed in contact with oleylamine-capped QDs in suspension in chloroform (Figure 1). The interlayer spacing is adjusted by selecting the suitable bola-amphiphile size to load InP/ZnS QDs. The resulting composites are straightforward to produce since they are free of any QD ligand exchange procedure. This last point is in most cases a prerequisite to integrate a host structure, but it generally leads to a significant drop in the optical properties. [25] Figure 1. Schematic representation of the synthesis of a QD-LDH composite structure.

InP-core QDs were produced by a previously published hot-injection method by first mixing zinc and indium halides in oleylamine (OLAm). [26] Then the phosphorous precursor was injected at high temperature, thereby producing tetrahedral zinc-blende type InP cores (ESIsection 1 and Figure S2a andb). The ZnS shelling was processed by bringing the sulfur precursor to the mixture, followed by the zinc carboxylate. The temperature was then increased to 260°C to ensure the formation of a ZnS layer on the InP core (ESI section 1). Both the tetrahedral shape and the desired zinc-blende crystalline structure were preserved after shell deposition (Figure 2a, 2b and 3a).

The obtained InP/ZnS QDs were characterized by STEM-HAADF (Figure 2a andb) and powder small angle X-ray scattering (SAXS, Figure 2c) to estimate their size and size distribution. The scattering vector q QDs of the QDs obtained by SAXS has a value of 1.3 nm -1 , which gives a realspace distance d QDs = 4.9 nm after conversion using Equation 1.

(

This calculated dimension agrees well with the edge length of the QDs measured at 4.4 nm by STEM-HAADF.

Uncoated InP cores do not show any fluorescence due to the presence of under-coordinated surface bonds acting as charge carrier traps. [27] Fluorescence appears with the ZnS shelling, which passivates dangling bonds present at the InP surface. Correspondingly, the excitonic absorption of the InP/ZnS QDs presents a half width at half maximum (HWHM [START_REF] Hwhm | Half-Width at Half-Maximum, used instead of FHWM since one can only experimentally deduce half of the absorption peak[END_REF] ) of 34 nm (110 meV), in line with earlier values reported for these QDs, [START_REF] Williams | [END_REF] and appears to be red-shifted by ~10 nm with respect to the InP-core excitonic transition (Figure S2c). This red shift is ascribed to exciton delocalization throughout the shell. [26,30] The 360 nm-excited emission of well-dispersed InP/ZnS QDs suspended in chloroform at a concentration of 1.5 g.L -1 consists of a single peak at 624 nm (1.99 eV), with a full width at half maximum (FWHM) of 59 nm (187 meV), see Figure 2d. This result is consistent with the literature. [31] Bola-LDHs were synthesized using the polyol method from zinc and aluminum salts and bolaamphiphile polymer. This suspension was mixed with a suspension of InP/ZnS QDs to obtain the desired QD-bola-LDH hybrids by diffusion of the QDs into the interlamellar space.

The successful insertion of the QDs was confirmed by combining XRD and SAXS to obtain the entire sequence of the characteristic (00l) reflections of the lamellar architecture, see Figure 3. The peak positions as deduced by SAXS were collected by means of the first derivative for better precision (Figure S3). For bola-LDH, the characteristic Bragg peaks of Zn 2 Al-LDH exhibit a series of basal reflections (00l) at low angles and the (110) reflection at a higher angle, indicating that a layered structure and an intralayer structural ordering are formed, respectively. The SAXS and XRD data (Table 1) indicate a layered structure with a large number of harmonic peaks resulting in a basal spacing of 6.47 ± 0.06 nm and 6.92 ± 0.02 nm for bola-LDH and QD-bola-LDH, respectively. These values agree with the distances measured by TEM: 6.4 nm for bola-LDH and 7.3 nm for QD-bola-LDH (Figure 3c and3d). Interestingly, QD diffraction peaks are still observed in the hybrid structure even with a loading as low as 3 wt. %, due to the high electronic contrast (Figure 3a right). Note that the (003) reflection is hidden in the SAXS pattern (Figure 3b) because it overlaps with the scattering of larger particles (aggregates). For QD-bola-LDH, the Bragg reflections for both XRD and SAXS patterns are slightly broadened compared to the QD-free pristine assembly (Figure 3a andb). At first glance, this may indicate a decrease in the structural coherence length resulting from a limited number of stacked layers in the hybrid LDH. The expansion of the basal spacing from 6.47 to 6.92 nm is subsequent to the rearrangement of the interleaved bola anions to accommodate InP/ZnS QDs. This occurs through a tilting or kinking of the bi-terminal bola groups attached to the LDH lamellar network. This interlamellar length of 6.92 nm determined by SAXS supports the fact that QDs (and ligands after slight rearrangements) enter the interlamellar space as isolated, non-aggregated guests. The evaporating solvent method appears therefore to be a relevant route for the intercalation of QDs within the interlayer space of a highly expandable bola-LDH host. [32a] LDH is pushed here to its limit through a stable pillar-structure to fit QDs size. The innovative SMIS approach, different from classical methods using simple carbonate [14,32b,32c] or hybrid [21,32d] LDH phases or LbL successive depositions, [15,17] may be considered as a breakthrough in both intercalation chemistry and polymer filler aspect for advanced materials. Dispersing bola-LDHs in chloroform has no effect on the characteristic lamellar structure (Figure S4). Figure S5 depicts the FT-IR spectrum of bola-LDH and QD-bola-LDH compounds;

no modification is noted. Since the vibrations associated with the bola backbone remain the same, this eliminates stretching as an explanation of the basal expansion and supports the fact that this expansion has a tilting origin. Considering a layer thickness and a bola length of 7.3 nm, [33] the tilt angle between bola and LDH mineral network is about 39°. This angle drops to 26° after the addition of the QDs, and confirms the success of the incorporation. Two other types of LDHs were investigated as hosts for QDs, where carbonate and dodecyl sulfate spacers were used to fabricate CO 3 -LDH and DS-LDH, respectively. The corresponding XRD and SAXS patterns indicate no expansion of the interlamellar distance in these cases, i.e. no incorporation of the QDs within the HDL gallery (Figure S6). The resulting powders should be seen as a physical mixture in which QDs are localized in the vicinity of the platelets in an « onto » mode rather than « into » the interlayer space. Other bi-tethered spacers, such as terephthalic acid (TA) and ,-dicarboxylic acid polyethylene glycol (PEG 1000 ), are employed to accommodate QDs. For TA-LDH, no changes are noticed neither on XRD nor on SAXS data after fabricating the QD-TA-LDH indicating that the QDs fail to be incorporated into the interlamellar space (Figure S7a andb). In the case of the PEG 1000 , home-made synthesized (ESIsection 1), presenting both the length (8.3 nm) [34] and the /  functionalization comparable to ones of the bola (Figure S8a), the syntheses by coprecipitation or by the polyol method fail to produce the desired hybrids. As presented on the XRD patterns (Figure S8b andc), the spacers are counter-anion of the starting salts, nitrates and acetates for co-precipitation and polyol route, respectively. However, several vibrations bands are attributed to PEG molecules on the FT-IR spectrum (Figure S8d). Indeed, PEG 1000 molecules are not rigid enough to allow them to act as spacers, their high degree of flexibility forces them instead to be adsorbed on the LDH layers, this reminiscent of superplasticizers adsorption in cement chemistry. [35] relevant optical synergistic effect between QDs and Bola-LDH is evidenced. The role played by the layered host is limited to the solid state dilution effect with no energetic transfer from the host matrix to the QDs. Fluorescence decays are investigated at room temperature to get insight of the energy transfer operating among QDs in the hybrid LDH materials. The decay profiles are displayed in the Inokuti-Hirayama (I.-H.) model in the dipole-dipole approximation. [37a] This model derives from the Förster-Dexter theory of energy transfer. [37b,37c] The corresponding data are shown

where I(t) is the luminescence intensity at time t, is the lifetime,  is the energy transfer probability and n = 6 in the dipole-dipole approximation: [37a] ,  and the pre-exponential factor A were considered as adjustable parameters. Their values are given in Table 2 and spectra appear on Figure S12. Inspection of these data reveals that the lifetime of pristine InP/ZnS QDs, either as a powder or as a suspension in CHCl 3 , is the same ( ~ 50 ns), with a value consistent with the literature. [38a,38b] The  rate of the powder, QD-DS-LDH and QD-CO 3 -LDH is roughly two orders of magnitude larger than in the suspension, which accounts for effective energy transfer among QDs in these close-packed architectures. Conversely, the spectroscopic parameters ,  of QD-Bola-LDH hybrid compare well those of pristine QDs free in solvent and further demonstrates inefficient energy transfer in this system. This confirms the observations made previously by XRD, SAXS and absorbance spectrum concluding that the QDs are well dispersed and isolated in QD-Bola-LDH. The QD-spacer-LDH hybrids were encapsulated in silicone films of relevance for solid-state lighting applications to evaluate their performance (Figure S13). As illustrated by optical microscopy images, a homogeneous film is obtained with QD-bola-LDH fillers throughout the Luminescence intensity, evaluated as the integrated area below the emission spectrum, is about 6 times higher for the InP/ZnS (0.27 wt. %) QD-bola-LDH @ silicone composite. A ≈ 20% rise in the initial emission intensity is observed in this sample. Similar behavior was observed in several other structures based on PbS [39] and CdSe [40] QDs, and is explained by the adsorption of water molecules onto the QD surface or by the rearrangement of the surface ligands. The latter seems unlikely in the present case, however, because of the presence of silicone immobilizing the QDs in the LDH interlayer space. A focus on the time-dependence of emission spectra in the early stages of illumination (i.e. < 4 h, see Figure 6) reveals a blue shift of ≈ 8 nm of the emission band (Figure S20), which is associated with a shrinkage of the InP core. This phenomenon is ascribed to the diffusion of water and oxygen molecules due to the mismatched interface (8 % in terms of cell parameter) between the InP core and the ZnS shell, followed by local photo-oxidation processes. [40] Despite the faster rate of photo-induced quenching of the luminescence in the InP/ZnS (0.27 wt. %) QD-bola-LDH @ silicone composite, the emitted intensity still remains about 3 times that of InP/ZnS (0.27 wt. %) QD @ silicone composite. This demonstrates the efficient shielding character of the bola-LDH architecture vs. the deleterious action of external irradiation on the InP/ZnS QDs. Moving to W-LED solid-state lighting applications, composite films of silicone-containing commercial YAG-Ce 3+ and InP/ZnS (1 wt. %) QD-bola-LDH phosphors combined with a 465 nm commercial LED in remote mode with the objective of lowering the colour temperature (CT) and increasing the colour rendering index (CRI) of the device with respect to the cold-white emitting blue LED + YAG-Ce 3+ system (Figure 7). The photometric parameters of the different configurations shown in Figure 7 were compiled. For configuration 1, the emission is centered on the greenish-yellow spectral range and the overall produced color corresponds to cool white with parameters CT = 6144 K and CRI 76. To recapitulate, the SMIS method using a bi-tethered spacer such as bola-type molecule is demonstrated to be original and straightforward to accommodate controllable amounts of core-shell InP/ZnS QDs within the interlamellar space of a LDH host structure. Focusing on the size-matching of the bi-tethered spacer and its QD guest nanoentities is preferred to ensure the incorporation of QDs into the interlamellar space of the LDH. The resulting highly promising red-emitting InP/ZnS QD-bola-LDH hybrid powder phosphor shows an emission peak at the same wavelength and as narrow as the QDs dispersed in a solution. The controlled entrapping of the InP/ZnS QDs within the hybrid bola-LDH host results in optimized solid-state dilution, significant enhancement of the ssPLQY without QD-QD energy transfer and improved photo-stability. Furthermore, the SMIS method paves the way towards the production of synergistic QD-LDH systems which are easy to disperse in polymers and show efficient and sustainable optical properties; this is barely feasible using LB or LbL strategies. As a proof of concept, a LED-based structure consisting of a blue LED chip combined in remote mode with silicone films loaded with InP/ZnS-bola-LDH hybrids and commercial YAG:Ce 3+ phosphors was qualified with a color temperature of 3576 K and a color rendering index of 93.8, thus opening very promising perspectives in terms of lighting applications.

Figure 2 .

 2 Figure 2. Representative STEM-HAADF images of the synthesized oleylamine-capped tetrahedral

Figure 3 .

 3 Figure 3. XRD patterns of QD-bola-LDH (red), bola-LDH (blue) powders and QD powder (black)

Figure 4 .

 4 Figure 4. Emission spectra of synthesized QDs and QD-spacer-LDH powders (a), comparison

  Figure S12. The (I.-H.) model accounts for energy transfer in absence of migration following Equation 2,

Figure 6 .

 6 Figure 6. Variation in the integrated areas of the emission spectra as a function of irradiation time

7 .

 7 When the red-emitting QD-bola-LDH composite film is added, as shown in configuration 2, the photometric parameters change to CT = 5410 K and CRI = 82.2, which gives warmer white light. Configuration 3 yields CT = 4648 K and CRI = 84.9, which is significantly warmer. The difference is due to the fact that the emission produced by the YAG:Ce 3+ film is reabsorbed by the QD-bola-LDH composite film in this configuration, which thus reinforces the contribution of the red component to the global spectrum. Photometric parameters CT = 3576 K and CRI = 93.8 are finally reached when adding a second 120 µm-thick QD-bola-LDH film as in configuration 4. This fulfils expectations of a further breakthrough in the W-LED field.

Figure 7 .

 7 Figure 7. Emission spectra, photometric parameters and designed white LED prototypes

Table 1 .

 1 Data collection reporting X-ray diffraction angles, SAXS q-values and the corresponding d-value for QD-bola-LDH and bola-LDH hybrid structures. Distance d was calculated using Bragg equation (i.e. 2.d.sin() = ) and Equation1from XRD data and SAXS data, respectively

			QD-bola-LDH			bola-LDH
	(hkl)	XRD (°)	SAXS (nm -1 )	d a) (nm)	XRD (°)	SAXS (nm -1 )	d a) (nm)
	(003)	-	0.89	7.06	1.36	0.98	6.50 / 6.41
	(006)	-	1.78	7.06	2.68	1.94	6.59 / 6.48
	(009)	3.81	2.72	6.96 / 6.93	4.05	2.89	6.54 / 6.52
	(0012)	5.08	-	6.96	5.45	3.87	6.49 / 6.49
	(0015)	6.39	-	6.92	6.83	-	6.46
	(0018)	7.66	-	6.92	8.27	-	6.41
	(0021)	9.01	-	6.87	9.64	-	6.42
	a)

Table 2 .

 2 Fitting parameters obtained for the studied compounds.

	Compounds	 (ns -1 	R²	 (ns)
	QD-Bola-LDH	0.28	0.9993	51
	QD-DS-LDH	7.79	0.9956	86
	QD-CO 3 -LDH	3.80	0.9938	73
	QDs powder	4.63	0.9989	54
	QDs in CHCl 3 (1.5 g.L -1 )	0.08	0.9993	53

Acknowledgements R.V. and Q.Z. contributed equally to the present work. The authors would like to acknowledge the Centre Imagerie Cellulaire Sante (CICS) (UCA PARTNER -Clermont-Ferrand, FRANCE) and 2MAtech, for their technical support and expertise on TEM and FEG/SEM analyses. Thomas Lohmeier, former working student at BASF Coatings Division, for the supply of bola-amphiphiles. Q.Z. gratefully acknowledges the National Natural Science Foundation of China (No. U1707603) and the financial support from China Scholarship Council (No. 201906880038).

The photoluminescence of QD-CO 3 -LDH and QD-DS-LDH hybrid powders excited at 360 nm remains weak and comparable to the luminescence of pristine InP/ZnS QDs powder (Figure 4a). In particular, the emission is much weaker with respect to the same QDs dispersed in CHCl 3 (Figure 2d), due to an enhancement of the reabsorption processes caused by multiple light scattering. The red-shift from 624 nm to 684 nm of the QDs emission is due to a dipole-dipole energy transfer induced by the aggregation state of the QDs in their powder state. [36] Same conclusions are made on the QD-TA-LDH compound; a red-shifted ( em = 673 nm) and broader spectral signature (FWHM = 82 nm) than in the case of InP/ZnS QDs in CHCl 3 albeit with a higher intensity than the DS and CO 3 compounds (Figure S7c). In contrast, QD-Bola-LDH hybrid powders show bright emission with a maximum (636 nm) close to the emission of InP/ZnS QDs suspended in chloroform (Figure 4b). The FWHM of the InP/ZnS QD-bola-LDH emission (184 meV) also compares well with that of well-dispersed QDs (187 meV), which confirms the efficient solid-state dilution effect provided by the bola-LDH host and the correlated narrow size distribution of the QDs (Figure S9).

In a concomitant manner, the solid-state internal photoluminescent quantum yield (ssPLQY) of the QD-bola-LDH hybrid is the highest over the excitation range 300 -600 nm (Figure 4c): the ssPLQY, amounts to 18 % for 3 wt. % of InP/ZnS QDs loaded in the bola-LDH, compared to 6.8 % for pristine InP/ZnS QDs powders. The ssPLQY of the other two QD-spacer-LDH hybrid powders remains very low whatever the excitation wavelength. This clearly highlights the crucial role of guest-to-host size matching. Figure 4d accounts for the QD-loading dependence of the ssPLQY in the QD-bola-LDH system upon excitation using two wavelengths that correspond to common commercial excitation sources (see Figure S10 for ssPLQY variation over the full excitation range i.e. 300 -600 nm). A maximum of 18 % is obtained for a loading rate of 3 wt. %, a value that provides the most suitable dilution of the QDs within the architecture of the hybrid powder.

Moreover, as it could be seen on the absorbance spectra (Figure S11) Bola-LDH compound does not absorb light in the range of relevance (i.e. > 365 nm). Since most of incident light is absorbed by the incorporated QDs and that the Bola-LDH structure does not show any fluorescence signal, no silicone film; QD aggregates are clearly observed and identified by EDX in the other cases (Figures S14 to S18). The homogeneity trend is QD-bola-LDH > QD-DS-LDH > QD-CO 3 -LDH > QDs, with QD-bola-LDH film being the most homogeneous, which was also evidenced by SEM/EDX analyses. This confirms the rather poor compatibility between InP/ZnS QDs and silicone.

Representative 120 µm-thick composite films are exposed to the 365 nm excitation of a lowpressure mercury lamp in Figure 5 and S7d. The InP/ZnS (0.27 wt. %) QD-bola-LDH @ silicone composite clearly shows brighter and more homogeneously-distributed luminescence, as the direct consequence of the above-described microstructure. The CIE color coordinates (Figure S19) associated with the luminescence of each film further confirm the more saturated red color exhibited by InP/ZnS (0.27 wt. %) QD-bola-LDH @ silicone film. The photostability behavior of InP/ZnS (0.27 wt. %) QD-bola-LDH @ silicone and InP/ZnS (0.27 wt. %) QDs @ silicone films are compared in Figure 6 upon continuous 375 nm irradiation provided by a UVA LED at room temperature. The incident luminous power experienced by the samples amounted 61.2 W/m 2 . This is a key experiment in the perspective of a further technological application. Note that the other two composite films were not tested due to a lack of photoluminescence in these experimental conditions.

Experimental Section

All synthetic details (QDs, LDH matrices and QD-LDH composites), along with all the film formation procedures and the devices used for each characterization are described in the Supporting Information.

Supporting Information