Walid Younes
email: walid.younes@irit.fr

Sylvie Trouilhet
email: sylvie.trouilhet@irit.fr

Franc ¸oise Adreit
email: francoise.adreit@irit.fr

Jean-Paul Arcangeli
email: jean-paul.arcangeli@irit.fr

Agent-mediated application emergence through reinforcement learning from user feedback

Keywords: Agent, multi-agent system, online learning, reinforcement learning, user feedback, service discovery, selection and composition, ambient intelligence

Cyber-physical and ambient systems surround the human user with applications that should be tailored as possible to her/his preferences and the current situation. We propose to build them automatically and on the fly by composition of software components present at the time in the environment, but without prior expression of the user's needs or process specification or composition model. In order to produce knowledge useful for automatic composition in the absence of an initial guideline, we have developed a generic solution based on lifelong online reinforcement learning. It is decentralized within a multi-agent system where agents learn incrementally from user feedback to satisfy her/him. Different use cases have been experimented in which applications, adapted to the user and the situation, are composed and emerge automatically and continuously.

I. INTRODUCTION

Cyber-physical and ambient systems consist of fix or mobile devices connected by one or several communication networks. These devices host software components that provide services and may require other services to operate. Components are software building blocks that can be assembled by connecting required services to provided ones to compose applications [START_REF] Sommerville | Component-based software engineering[END_REF]. For example, assembling a standard interaction component present in a smartphone (e.g. a slider or a speech recognition component), a software adapter and a connected lamp realizes an application allowing a user to control the level of ambient lighting.

Hardware and software components are generally multitenant and independently managed: they are developed, installed and activated, independently of each other. Due to the mobility of devices and users, they may appear or disappear with unpredictable dynamics, giving to cyber-physical and ambient systems an open and unstable nature. Besides, the This work is partially supported by the region Occitanie, the program FEDER-FSE Midi-Pyrénées et Garonne, and the University of Toulouse III. number of components may be large. In such a context, component assemblies are hard to design, maintain and adapt.

At the core of these systems, the human user can use applications at her/his disposal. Ambient intelligence [START_REF] Sadri | Ambient intelligence: A survey[END_REF] aims to offer a personalized environment adapted to the situation, i.e. to provide the right service at the right time, by anticipating user's needs, which may change over time.

Our project aims to design and build a solution that automatically and dynamically assembles software components in order to build "composite" applications adapted to the ambient environment and the user, i.e. operational, useful and usable. Our approach breaks with the classic top-down mode for application development: building an assembly is not driven by explicit user's needs or goals, nor by a predefined process or model; on the contrary, composite applications are built on the fly in bottom-up mode from the components available at the time in the ambient environment. Thus, applications emerge from the environment, taking advantage of opportunities [START_REF] Younes | Towards an intelligent user-oriented middleware for opportunistic composition of services in ambient spaces[END_REF]. In this context, the user does not request for a service or an application: on the contrary, emerging applications are provided in push mode.

The solution relies on a middleware, called Opportunistic Composition Engine (OCE), that periodically detects the components and their services present in the ambient environment, designs assemblies of components by connecting services in an opportunistic way, and proposes them to the user. In the absence of prior explicit guidelines, OCE automatically learns the user's preferences according to the situation in order to later maximize her/his satisfaction. Learning is achieved online by reinforcement. It is decentralized within a multi-agent system (MAS) in which agents interact via a protocol that supports dynamic service discovery and selection. To learn from the user and for the user, the latter is put in the loop.

This paper focuses on agent learning and decision making within the MAS. Opportunistic composition raises other questions (related to heterogeneity, security, reliability, resource limitation, etc.) that are out of the scope of this paper.

The paper is organized as follows. Section II presents the main architectural principles. Agents' behavior and cooperation features are described in Section III. Section IV focuses on the online reinforcement learning solution. Experimental validation results are reported in Section V. Section VI analyzes the related work and positions our solution. Last, Section VII summarizes the contribution and discusses open issues.

II. ARCHITECTURE OF THE COMPOSITION SYSTEM

The software architecture, which a simplified view is given in Fig. 1, has been defined in order to meet the automation requirements of opportunistic composition. This section presents the main features of this architecture.

A. Overall architecture

The Opportunistic Composition Engine (OCE) is responsible for designing applications for the user by assembling available business and interaction components. To do this, it senses the components and their services that are present in the ambient environment, then manages the connections and disconnections between required and provided services without relying on prior explicit guidelines, thereby generating on the fly composite applications.

As there is no guideline, unknown and surprising applications may emerge. Those should be presented to the user. Indeed, we consider their deployment should remain under human control whatever the engine's choices are. This point is particularly important in the field of human-machine interaction, for which the control by the user of her/his interaction environment is of the highest importance [START_REF] Bach | Adaptation of ergonomic criteria to humanvirtual environments interactions[END_REF]. As a consequence, the user must be put "in the loop".

Therefore, OCE proposes the emergent applications to the user and she/he decides at the end on their deployment: she/he can accept or reject a proposed application, but also modify it. After acceptance, the emergent application is automatically deployed and so usable.

Nevertheless, even if the user shares responsibilities with the engine, she/he should be involved as little as possible [START_REF] Bach | Adaptation of ergonomic criteria to humanvirtual environments interactions[END_REF]. OCE should therefore be as autonomous as possible. So, it complies with the autonomic computing principles and the MAPE-K model [START_REF] Kephart | The vision of autonomic computing[END_REF]: in a cyclic way, OCE monitors (M) the surrounding environment including the user's reaction to the presented application (accept, reject, modify), analyzes (A) it, and plans (P) assemblies based on its knowledge (K); execution (E) is the presentation of the planned assembly to the user.

B. MAS-based decentralization

We have designed OCE as a multi-agent system (MAS) since agents and MAS address major challenges raised by ambient and IoT systems [START_REF] Savaglio | Agent-based Internet of Things: State-of-the-art and research challenges[END_REF]: decentralization, distribution, scalability, dynamics and adaptiveness. Agents are autonomous entities that cooperate to achieve a common mission. Here, each provided or required service is separately managed by a dedicated service agent, that is aimed at connecting (or disconnecting) the service.

Decentralizing OCE architecture as a MAS leads to an additional requirement concerning inter-agent cooperation: to find an adequate connection, a service agent has to communicate by message with other agents to reach an agreement 1 . Thus, we have designed ARSA [START_REF] Younes | Towards an intelligent user-oriented middleware for opportunistic composition of services in ambient spaces[END_REF], an advertisement-based interaction protocol close to the Contrat Net Protocol [START_REF] Smith | The contract net protocol: High-level communication and control in a distributed problem solver[END_REF]. ARSA relies on asynchronous (non-blocking) messages, and supports cooperation between agents in a context of dynamics and openness: inter-agent cooperation goes on even if messages are lost or services disappear with their agents; besides, agents associated to appearing services are automatically integrated through advertizing or receiving advertisements from the others.

Under ARSA, service agents may send 4 types of message: (1) Advertize -the sender declares its service is available for binding (this is close to publication of services in SOA but concerns the required services in the same way) (2) Replythe sender answers positively to an advertizer agent (negative answers are dropped) (3) Select -when receiving a Reply message (it may receive none, one or several replies), an agent can drop, memorize, or select it (it can also select a previously memorized reply) then send a Select message to the replier (4) Agree -the agent which has received a Select message finally agrees (if so, the connection can be effectively made).

III. AGENT'S BEHAVIOR AND COOPERATION

Each OCE agent manages a software service and interacts with other agents via the ARSA protocol in order to connect its service 2 . Periodically, an agent perceives a set of ARSA messages. Then it decides which one it is going to answer to. For that, it first builds its own representation of the current situation from the messages it received. Next, it compares this current situation with the situations it has already encountered, and scores it from similar ones. Lastly, it chooses the action to be performed (i.e. the agent to answer to). This is repeated along the message exchanges until the end of the current OCE cycle (an OCE cycle consists in sensing the environment, planning an assembly, presenting it, and getting user feedback).

A. Construction of the current situation

A current situation is a local (individual) view of the ambient environment, i.e. a set of agents with whom it is possible to connect. The current situation S i t of an agent A i lists the service agents sensed by A i (i.e. the ones from which A i has received a message) during the OCE cycle t, which services are compatible3 with the one of A i . It is a set of pairs (A j , ARSA T ype) where A j identifies the message sender and ARSA T ype is Advertize, Reply, Select or Agree. S i t is incrementally built and updated throughout the OCE cycle t4 .

B. Comparison with the reference situations

Over time, depending on the hardware and software components in the varying ambient environment, an agent may encounter various situations. A reference situation Ref i k is a situation, numbered k, that an agent A i has encountered in the past (in a previous OCE cycle). Close to the current situation, it is composed of a set of agents, which services are compatible with the one of A i , sensed by A i in the environment at a given time:

Ref i k is a set of pairs (A j , Score i j )
, where A j is the identifier of a message sender, and Score i j is a numerical value that represents A i 's interest in connecting its service to the one of A j in this situation (Sec. IV-B explains how this knowledge is built and maintained through learning).

A i memorizes a set of reference situations that constitutes its knowledge base, noted Ref i . Comparison step aims to select from Ref i the reference situations that are similar or identical to S i t . The idea is to be able to repeat a decision made in the past in identical or close situations.

Comparison between the current situation S i t and a reference situation Ref i k is based on the identifiers of the agents present in the situations, regardless of message types and scores. The Similar S function returns a set of reference situations with a real number for each of them, that measures the degree of similarity

d k = |S i t ∩Ref i k | |S i t ∪Ref i k | (i.
e. the proportion of agents in common between S i t and Ref i k -known as the Jaccard coefficient). In addition, the returned set contains only reference situations with a d k greater than or equal to a threshold ξ. If the current situation S i t already exists identically in Ref i , only the latter is selected (the other similar situations are overlooked).

Let Sit be the set of all possible current situations, and Ref the set of all possible reference situations; thus, P(Ref ) is the set of all possible agents' knowledge base. Similar S is defined as follows:

Similar S : Sit × P(Ref ) → P(Ref × [0, 1]) (S i t , Ref i ) → {(Ref i k , d k )} Ref i k ∈Ref i , ξ≤d k ≤1 (1)

C. Scoring the current situation

The function Score Situation assigns a numerical value to each agent A j of S i t , in order to choose one of them later. If S i t has been recognized as a reference situation, the scores are replicated identically. Otherwise, the score Score i j of A j is calculated from the scores of A j in the selected reference situations: it is the average score of A j weighted by the respective degrees of similarity.

An agent that A i hasn't met in the past may appear. It is scored depending on a novelty sensitivity coefficient ν (0 ≤ ν ≤ 1), which reflects the user's degree of acceptability of novelty. With a probability equal to ν, it is assigned a higher score than the better one in the situation; with a probability (1 -ν), it is assigned a medium score.

Last, if none reference situation similar to S i t exists, the score of each agent of the current situation is set to 1 n , n being the number of agents in S i t .

D. Choosing the agent to connect to At this point, A i chooses one agent from the scored current situation. To do this, several strategies are possible depending on the scores and the message types. Here, priority is given to the agent with the best score. In case of equality, priority depends on the received message type (choice is random if the type is the same), ordered from highest to lowest as follows: Agree, Select, Reply, Advertize5 . When the decision is made, A i sends a message to the chosen agent A j according to the ARSA protocol.

Algorithm 1 summarizes the agent's decision process.

Algorithm Inter-agent cooperation and connection agreements contribute to the consistency of OCE global decision and the emergence of cohesive composite applications. Section IV-B points out the complementary contribution of the user to this issue through her/his feedback.

IV. LEARNING PRINCIPLES

OCE cannot base its decision on guidelines specified in advance: indeed, the user is not able to explicit a priori and exhaustively her/his needs and preferences in any situation, nor to translate them into assembly plans, because of the dynamics and unpredictability of the surrounding environment, the variability of her/his needs and the combinatorics generated by the number of components. Thus, OCE has to learn what the user prefers at a given moment in order to make decisions in same or similar future situations.

A. How to learn?

The lack of initial data and solutions known in advance makes supervised and unsupervised learning impossible. In addition, environment dynamics, with services appearing and disappearing unpredictably, makes very difficult or even impossible to build a static model of prediction or classification. Hence, learning must be done online: OCE iteratively learns by progressive adaptation of the agents' knowledge. Agents increment and update their knowledge as the experience goes along, according to the interactions with the user and the feedback she/he provides.

According to the online learning model, OCE makes a "prediction" (the assembly), and the "environment" (the user) provides an answer about its correction depending on its preferences and actual needs. However, the feedback given in our case by the user does not have the accuracy of the answer given by the environment in standard online learning: in particular, it can evolve over time as the situations and the user's needs or preferences change. For this reason, we hybridize the principles of online learning with those of reinforcement learning [START_REF] Sutton | Reinforcement Learning: An Introduction[END_REF]. Reinforcement learning aims at learning what to do (mapping situations to actions) so as to maximize a numerical reward. It allows the learner to adapt over the long term by interacting with its environment. Here, the user's response helps to reinforce the agents' knowledge. Then, agent's decisions at the OCE cycle t + 1 rely on the knowledge that it has accumulated up to the cycle t.

Note that this approach does not exclude the use of a priori knowledge (for example, general rules or patterns for assembling business components, ergonomic rules for assembling interaction components), that could be provided initially.

Finally, due to the dynamics of both the surrounding environment and the user's needs, learning must also be lifelong, which does not exclude phases of knowledge stabilization.

B. Agent-level learning based on user feedback

Agents' knowledge is created and updated at the end of the OCE cycle, after the interaction with the user. At this point, the user feedback (concerning the entire assembly) is sent back to OCE and propagated to every agent A i involved in the assembly (connected to an agent A j ). Each A i has to build a new reference situation from both its scored current situation and a reward derived from the feedback about its connection choice. β being the reinforcement factor (β > 0), this reward r i j is computed as follows: 1) The user has entirely accepted the presented assembly.

A i decision to connect to A j is rewarded positively:

r i j = β 2)
The user has entirely rejected the presented assembly.

A i decision to connect to A j is penalized:

r i j = -β 3)
The user has modified the presented assembly -If the user has replaced the connection between A i and A j by one between A i and A h , the first connection is penalized and the new one is positively rewarded:

r i j = -β r i h = |Score i j -Score i h | + β -If the
user has kept (respectively removed) a connection: the reward is computed as in the case of an acceptance (respectively rejection) of the whole assembly.

-If the connection between A i and A j didn't exist in the proposed assembly, and the user has added this connection, r i j must be such that, in the future, in the same situation, A i will prefer A j to all the other agents. Hence (S i t being the current situation of A i ):

r i j = max A k ∈S i t Score i k + β
The new reference situation is derived from the current situation of A i (a set of agents A j with their scores Score i j on which A i has made its decision, see Sec. III): in case of reward, Score i j is updated according to the formula (2) derived from the one of the "bandit algorithm" [START_REF] Sutton | Reinforcement Learning: An Introduction[END_REF], where α is the learning factor (0 ≤ α ≤ 1). There, (1 -α)Score i j represents the part of knowledge that A i keeps from the past and α • r i j the part it learns in the current OCE cycle.

Score i j = Score i j + α(r i j -Score i j ) (2)
Last, the reference situation is normalized so as the sum of the scores equals 1, and stored in Ref i .

C. Exploitation vs exploration

When deciding of an action, reinforcement learning usually supposes a balance between exploitation of learned data and some exploration in order to build new data. This balance is determined by a value , 0 ≤ ≤ 1: along the ARSA exchanges, the chosen agent (see Sec. III-D) may be the "best" with a probability (1 -) that promotes knowledge exploitation, or another agent with a probability , promoting exploration.

V. PROTOTYPE AND EXPERIMENTATION

This section presents the current state of implementation, and several experiments which show that construction of current situations, creation and evolution of knowledge, and its exploitation by the agents are carried out correctly.

A. Implementation

A prototype version of OCE has been implemented in Java according to the principles presented above. It works in conjunction with a user-dedicated Interactive Control Environment (ICE) [START_REF] Koussaifi | Ambient intelligence users in the loop: Towards a model-driven approach[END_REF], and can be connected to real component platforms. To experiment, we have developed an interface that simulates the interaction between OCE and the ambient environment: it allows to enter components by hand and run OCE (with ICE) cycle by cycle and trace agents' behaviors and knowledge.

B. Experimentation

Here is the use case. Mary is at work. The ambient space holds components supplied by her company: a room Planner providing the Book service, a booking Desk providing the Order service and requiring both Book and Notify, and a Tactile input device that requires Order. There also are Mary's personal components: Text and Voice input interfaces both requiring Order, and Mary's Calendar that provides Notify.

In what follows, we demonstrate OCE behavior with α = 0.4, β = 1, = 0.2, ν = 0.33 and ξ = 0.5. We focus on the agent B that manages the provided Order service of Desk. We show how it works and learns about the agents A 1 , A 2 , and A 3 that respectively manage the Order services required by Text, Voice and Tactile. To experiment, we assume the same environment (described above) in the first three cases and activity recognition. Managing user attention and disturbance is a major requirement. For us, it remains an open issue.

Service selection mainly aims at meeting quality of service (QoS) requirements [START_REF] Sheng | Web services composition: A decade's overview[END_REF]. A composition algorithm based on the clustering of services in relation to QoS is proposed in [START_REF] Khanouche | Clustering-based and QoS-aware services composition algorithm for ambient intelligence[END_REF]. In order to continuously adapt component-based software systems and build "emergent software", authors of [START_REF] Filho | Defining emergent software using continuous self-assembly, perception, and learning[END_REF] propose a learning system based on reinforcement learning that monitors at runtime a set of known possible configuration for a given goal, and choose the one that maximize extrafunctional criteria. Here, the configuration emerge, not the functionality. In [START_REF] Wang | Adaptive service composition based on reinforcement learning[END_REF], self-adaptive composition of Web services in dynamic environments maximizes the global QoS of the composition: service composition is modelled as a Markovian decision process with several alternative processes, the best one being chosen using a Q-learning algorithm (a sort of reinforcement learning algorithm). Our solution doesn't rely on specified QoS attributes nor precisely optimize a particular QoS criterion: in a way, the quality of a proposed assembly is set by the user depending on her/his preferences, then provided to the engine as feedback data that drive OCE future decisions.

Wang et al. propose a distributed algorithm to optimize dynamically Web service compositions in a varying environment: within a MAS, agents learn by reinforcement using a Q-learning algorithm and share their experience to improve efficiency and speed up the learning rate [START_REF] Wang | A multi-agent reinforcement learning approach to dynamic service composition[END_REF]. In [START_REF] Charif | Dynamic service composition enabled by introspective agent coordination[END_REF], agent coordination supports choreography of services and relies on dialogue and the history of conversations. These cooperative approaches seem promising and could improve our solution by adding coordination between agents.

VII. CONCLUSION

This paper presented the principles of a new solution for user-oriented automated service composition in ambient spaces. Based on online reinforcement learning from user feedback, it makes new applications emerge in bottom-up mode without prior expression of needs, goals, or composition models. By not embedding any predefined QoS criteria nor user-specific preferences, our solution is generic (regardless of the user and the application area) and evolutive (the user's preferences may change over time).

Each service is managed by an agent which learns from the user and makes local decisions about connection to maximize user satisfaction while limiting her/his involvement. For that, the agents observe partially the environment through the messages they receive, which avoids having to predefine then identify global situation. However, having only a local view might not be enough for efficient learning and consistent decision making. Here, the user "in the loop" has a major role in term of decision consistency by giving a global feedback which is dispatched to the agents and transformed into knowledge. Driven by this common knowledge, the aggregation of the agents' local decisions makes sense from a global perspective. However, for greater consistency, our solution should evolve towards multi-agent learning [START_REF] Albrecht | Autonomous Agents Modelling Other Agents: A Comprehensive Survey and Open Problems[END_REF] based on knowledge sharing between agents and increased coordination.

Fig. 1 .

 1 Fig. 1. Simplified architectural view

In this paper, we disregard service description and matchmaking issues, as well as efficiency of the routing of messages.

To simplify, we improperly talk about connection between agents.

Two services are compatible if one of them, P , is provided and the other, R, is required and if P includes R. If so, R and P may be connected.

When updating the current situation with A j , if A j already exists in the current situation with a different message type, the most recent one is retained.

In relation to the learning method, Sec. IV-C explains why and how the chosen agent is not automatically the best scored.

we make agents' knowledge vary. The experiences described below can be viewed in a video 6 .

1) Composition without knowledge: OCE is started and runs until an assembly emerges. Since B has no knowledge yet, the same score of 1 3 is assigned to A1, A2 and A3, that are the candidates for connection in the current situation of B. Fig. 2 shows a screenshot of the ICE interface with the resulting assembly (here, the random choice of B has been A 1 ): an application that allows Mary to book a room for a meeting. Agents' IDs have been added on the screenshot. As Mary prefers the Voice component, she modifies the assembly using ICE functionalities. Fig. 3 shows the result. 3) Composition with approximate knowledge: OCE has previously run and, C 1 , C 2 , C 3 being agents managing services previously encountered, Ref B contains 6 https://www.irit.fr/ ∼ Sylvie.Trouilhet/demo/wetice2020.mp4 Experimentation on more complex real use cases is underway, as well as quantitative performance evaluation depending on the number of services and the possible connections.

VI. RELATED WORK

Sheng et al. [START_REF] Sheng | Web services composition: A decade's overview[END_REF] survey standards and research work on Web service composition. They identify automation of service selection, composition adaptability, scalability, and customization as major requirements, and state that adaptability and autonomy of service composition and ubiquitous service composition are issues that require significant research efforts.

The automatic composition problem differs on whether a model of the composition is known in advance or not. In the first case, the problem is to find the different services that make possible the instantiation of the model while adapting it to the context. For example, MUSIC [START_REF] Rouvoy | MUSIC: middleware support for self-adaptation in ubiquitous and service-oriented environments[END_REF] supports plan selection at runtime and their implementation and adaptation to the context to maximize application utility. In [START_REF] Karchoud | Longlife application[END_REF], a rule-based engine builds applications at runtime and pushes them to the user when particular contextual situations are detected. In the second case, goals or pre-and/or post-conditions are specified, and services are built to satisfy them. For example, MUSA [START_REF] Cossentino | MUSA: a Middleware for User-driven Service Adaptation[END_REF] supports service composition and adaptation in dynamic and unpredictable environments based on user goals, which may change dynamically. In [START_REF] Mayer | Smart Configuration of Smart Environments[END_REF], the surrounding environment is automatically and dynamically configured based on goals and the services present at the time. Solutions for service composition in ambient systems, based on goals expressed in different ways, are studied in [START_REF] Stavropoulos | A survey of service composition in ambient intelligence environments[END_REF]. Hence, in any slution, service composition is made up top-down from a pre-declared specification contrary to our bottom-up approach, which somehow learns these specifications at runtime. Ambient intelligence systems aim at minimizing user involvement. In [START_REF] Evers | The user in the loop: Enabling user participation for self-adaptive applications[END_REF], the user can select, accept, reject, or adjust applications, change her/his preference, even put off automatic adaptation. In [START_REF] Karami | User in the Loop: Adaptive Smart Homes Exploiting User Feedback-State of the Art and Future Directions[END_REF], the emphasis is put on feedback: authors argue that user preferences and profile can be learned (by semi-supervised reinforcement learning), and associated to


Aurore Didier

David Sarmiento Castillo

David Sarmiento

Castillo Excavations

Jean-Marie Casal

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

AMRI EXCAVATIONS

(1959-1962) Th e site of Amri is located on the west bank of the Indus River, about 110 north of Hyderabad in Sindh. Situated near foothills of the Kirthar Range, the mound of 8 hectares, was excavated, providing the remains of a Chalcolithic/Early Bronze Age occupation (period I, ca. 3500-2500 BC) associated with architecture in mudbrick and stone. Th e period I material assemblage also evidenced close cultural and technological relationships with the craft productions of Balochistan. From 2500 BC, the site shows an occupation (period III A,B,C/D, ca. 2500-1900 BC) characteristic of the three periods of the Indus civilization. Amri remains, with Kot-Diji, a reference site for the periodization of the Lower Indus valley and for studying the transition between the pre-Indus and the Indus periods.

PIRAK EXCAVATIONS

(1968-1974) Th e site of Pirak (12 hectares) is located 20 km at the East of Mehrgarh, in the Kachi-Bolan region (Balochistan). Th e excavation has shown the existence of an occupation dated between 1700 BC and 700 BC. Th e oldest levels (ca. 1700 BC) evidenced the setting of a new rural economy, involving the cultivation of rice, which requires a well-controlled irrigation system. At this time, substantial mudbrick architectures were developed in a rather standardized plan with symmetrical niches on several levels inside the rooms, a type of plan which was still in use in the Kachi region some 60 years ago. Pirak also provided the remains of hand-made painted pottery and the fi rst occurrence of horse, horse rider, camels and donkeys fi gurines. In the following periods (1500-1200 BC), wheel-made pottery and iron objects start to be produced.

  Th e site of Nindowari is located in the Ornach valley in Kalat District (Balochistan). Th e fi eld work, conducted by J.-M. Casal, is the only large-scale excavation ever carried out at a site of the 3 rd millennium BC Kulli Culture, which was partly contemporary with the Indus Civilization. Th e excavation records, recently published by J.-F. Jarrige, G. Quivron and C. Jarrige (2011), provide unique insights into the origins and development of the Kulli Culture (2600-1900 BC) characterized, among other features, by an original pottery style.

  Period ID pottery (ca.2600-2500 BC).Northern view of the mound of Amri.Plan of the excavated remains inAreas A and B. Nindowari (2011, fi g. 42). Period II (2600-2300 BC) and period III (2300-1900 BC) pottery from Nindowari. Indus Civilization steatite stamp seals from Nindowari period III (2300-1900 BC) Anthropomorphic fi gurines in terracotta Camel and horse-rider terracotta fi gurines Hand-made painted pottery Plan of level 9 at Pirak. Fouilles de Pirak (1979, Vol II, fi g 15)

curator of the Musée Guimet and seconded to the French


Alexandra Cantarano

Denis Testemale

Sònia Sousa Nobre

Audrey Potdevin

Rémy Bruyère

Aude Barbara

Jean-Louis F Hazemann

Alain Ibanez

Géraldine Dantelle

Sonia Sousa Nobre

Twofold advantage of gas bubbling for the advanced solvothermal preparation of efficient YAG:Ce nanophosphors

Introduction

With the advent of the semiconductor-based nanotechnologies, miniaturization has become more and more accessible, leading to tremendous innovations in electronics, photonics, sensors, etc. Over the last decade, solid-state lighting, through Light-Emitting Diodes (LEDs) and more recently with Organic LED technologies, has emerged, offering in particular drastic energy savings. However, many challenges remain in this field, such as the improvement of light extraction from white LEDs or displays, for greater efficiency. Recent developments include elaboration of micro-LEDs and semiconductor nanostructuration. [START_REF] Ley | Strain relaxation of InGaN/GaN multi-quantum well light emitters via nanopatterning[END_REF][START_REF] Jiao | The effects of nanocavity and photonic crystal in InGaN/GaN nanorod LED arrays[END_REF] Phosphor elaboration is also a key factor in optimizing the efficiency and stability of phosphor-converted LED (pc-LED) lighting devices. [START_REF] Smet | Selecting conversion phosphors for white light-emitting diodes[END_REF][START_REF] George | Phosphors for Solid-State White Lighting[END_REF][START_REF] Xia | Ce3+-Doped garnet phosphors: composition modification, luminescence properties and applications[END_REF][START_REF] Li | Recent progress in luminescence tuning of Ce3+ and Eu2+-activated phosphors for pc-WLEDs[END_REF] In this context and for the development of new applications, particularly for miniaturization involving solution-based deposition techniques (e.g. ink-jet printing [START_REF] Zhan | Inkjet-printed optoelectronics[END_REF] ), it is required to couple the semiconducting heterostructures with phosphors of reduced size. [START_REF] Jiang | Light-emitting diodes of colloidal quantum dots and nanorod heterostructures for future emissive displays[END_REF] Some pioneer works have been performed using submicronic Ce 3+ -doped Y3Al5O12 (YAG:Ce) phosphors, the currently used phosphor in white light LEDs. [START_REF] Schimpke | Phosphor-converted white light from blue-emitting InGaN microrod LEDs[END_REF][START_REF] Guan | Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors[END_REF] To meet the phosphor size demand (< 100 nm), there is a great interest in mastering the chemical synthesis of YAG:Ce nanocrystals (NCs), including the control of their size and crystal quality to ensure high photoluminescence efficiency, quantified by the internal quantum yield (iQY). Among the different methods proposed to synthesize YAG:Ce NCs, the prominent ones comprise combustion, [START_REF] Costa | Synthesis of Nd-YAG Material by Citrate-Nitrate Sol-Gel Combustion Route[END_REF][START_REF] Haranath | Enhanced luminescence of Y3Al5O12:Ce3+ nanophosphor for white light-emitting diodes[END_REF] coprecipitation, [START_REF] Caponetti | Luminescence properties of neodymium-doped yttrium aluminium garnet obtained by the co-precipitation method combined with the mechanical process[END_REF][START_REF] Wang | Preparation and photoluminescence properties of YAG:Ce3+ phosphors by a series of amines assisted co-precipitation method[END_REF] sol-gel, [START_REF] Murai | Enhanced absorption and emission of Y3Al5O12:Ce3+ thin layers prepared by epoxide-catalyzed solgel method[END_REF][START_REF] Veith | Low temperature synthesis of nanocrystalline Y3Al5O12 (YAG) and Ce-doped Y3Al5O12 via different sol-gel methods[END_REF][START_REF] Potdevin | Sol-gel based YAG:Tb3+ or Eu3+ phosphors for application in lighting sources[END_REF] and solvothermal processes. [START_REF] Inoue | Glycothermal synthesis of metal oxides[END_REF][START_REF] Kasuya | Glycothermal synthesis and photoluminescence of YAG:Ce3+ nanophosphors[END_REF][START_REF] Byun | Solvothermally grown Ce3+-doped Y3Al5O12 colloidal nanocrystals: spectral variations and white LED characteristics[END_REF] In most of these techniques, an annealing step is required to crystallize YAG phase, inducing NC agglomeration if no care is taken during the thermal treatment.

A recent paper reports the precipitation and crystallization of NCs previously embedded inside a LiCl matrix, preventing their aggregation, resulting in YAG:Ce NCs with a size smaller than 100 nm and a iQY of 50 %. However, the presence of CeO2 parasitic phase is also detected. [START_REF] Gaiser | Y3Al5O12:Ce nanoparticles made by ionic-liquid assisted particle formation and LiCl-matrix-treated crystallization[END_REF] Finally, the solvothermal method, often employed to produce nanocrystals with good crystallinity, 22 enables to produce non-aggregated nanometersized YAG:Ce NCs that can be then be dispersed in ethanol thanks to the presence of organic groups resulting from the synthesis solvent and forming a stabilizing corona around the NCs that can thus be used individually. [START_REF] Asakura | Effects of citric acid additive on photoluminescence properties of YAG:Ce3+ nanoparticles synthesized by glycothermal reaction[END_REF][START_REF] Dong | Non-photobleaching YAG:Ce nanoparticles for optical imaging with blue excitation[END_REF][START_REF] Odziomek | Highly luminescent YAG:Ce ultra-small nanocrystals, from stable dispersions to thin films[END_REF] However, two challenges remain for the solvothermal method: (1) to control the nanoparticle crystal quality and (2) to master the cerium oxidation state.

Regarding the first point, the control of crystal quality implies to master nucleation and growth mechanisms of YAG:Ce NCs. The nucleation step is particularly difficult to control in autoclave, which can be considered as a "black box". Indeed, the pressure is generally autogenous and thus increases gradually as well as temperature, leading to non-stabilized and inappropriate conditions as nucleation must be confined in time, or eventually in space. [START_REF] Lamer | Theory, Production and Mechanism of Formation of Monodispersed Hydrosols[END_REF] This generally leads, for YAG NC syntheses, to a "cauliflower" morphology, with nanoparticles being formed of many small NCs (a few nm), which is certainly the consequence of an uncontrolled nucleation step. [START_REF] Ramanujam | Solvothermal nanoYAG synthesis: Mechanism and particle growth kinetics[END_REF][START_REF] Vorsthove | Efficiency issues in Ce3+ doped YAG nanocrystals[END_REF] As a result, due to numerous surface defects and a lack of good crystallinity of these particles, their photoluminescence efficiency is limited with weak iQY = ca. 20-30 %. [START_REF] Dantelle | Luminescent coatings prepared from optimized YAG:Ce nanoparticles[END_REF][START_REF] Kasuya | Photoluminescence Enhancement of PEG-Modified YAG:Ce3+ Nanocrystal Phosphor Prepared by Glycothermal Method[END_REF][START_REF] Aboulaich | Rapid synthesis of Ce3+doped YAG nanoparticles by a solvothermal method using metal carbonates as precursors[END_REF] Strategies to increase the iQY, while preserving the nanometer size of particles, include surface modification (PEG coating, [START_REF] Isobe | Low-temperature wet chemical syntheses of nanocrystal phosphors with surface modification and their characterization[END_REF] growth of a protective layer, 33 surface passivation [START_REF] Nyman | Nano-YAG:Ce Mechanisms of Growth and Epoxy-Encapsulation[END_REF] ) and protective thermal annealing. [START_REF] Revaux | Improvement of luminescent properties and photostability of YAG nanoparticle-based films[END_REF] The surface-modified NCs present iQY around 35 %, while the latter method leads to NCs with iQY of 60 %, but this multi-step process cannot be scaled up due to a timeconsuming and hazardous process involving hydrofluoric acid. [START_REF] Revaux | Improvement of luminescent properties and photostability of YAG nanoparticle-based films[END_REF] The second challenge concerns the Ce oxidation state control. We recently showed, through in situ X-Ray absorption spectroscopy (X-ray Absorption Near Edge Spectroscopy, XANES), that the solvothermal method induces the partial oxidation of Ce 3+ ions into Ce 4+ ions upon synthesis. [START_REF] Dantelle | A new solvothermal method for the synthesis of size-controlled YAG:Ce single-nanocrystals[END_REF] Not only Ce 4+ ions are known to be optically silent (i.e. do not participate to photoluminescence), [START_REF] Herrmann | Spectroscopic properties of cerium-doped aluminosilicate glasses[END_REF][START_REF] Wang | Semiquantitative estimation of Ce3+/Ce4+ ratio in YAG:Ce3+ phosphor under different sintering atmosphere[END_REF] but they also may have a role as luminescent quenchers. [START_REF] Sontakke | Effect of synthesis conditions on Ce3+ luminescence in borate glasses[END_REF] One hypothesis is that they would act as traps for Ce 3+ luminescence, thus reducing the iQY. [START_REF] Orera | Ce3+↔Ce4+ conversion in ceria-doped zirconia single crystals induced by oxidoreduction treatments[END_REF] In this study, we initially tackled the second issue, with the aim of preserving cerium ions in their 3+ oxidation state by bubbling a reducing gas into the precursor solutions. The idea was to mimic the solid-state reaction for the formation of YAG:Ce 3+ phosphors, for which the annealing treatment is performed under Ar/H2 gas to maintain cerium ions in their 3+ oxidation state. [START_REF] George | The local environment of the activator ions in the solid state lighting Y3-xCexAl5O12[END_REF] The relative proportion Ce 3+ :Ce 4+ was monitored by XANES spectroscopy. In addition, we evidenced that gas bubbling has a significant effect on the nucleation and growth steps of NCs, allowing the production of high quality, nonaggregated YAG:Ce NCs through a one-step process exhibiting one of the highest iQYs ever reported in the literature.

Results and Discussion

In order to lower the Ce 3+ oxidation during the synthesis of YAG:Ce nanocrystals, a reducing gas (Ar/H2) was bubbled into the initial solution of 1,4-butanediol and precursors for two hours at low flow rate, prior to the solvothermal reaction. For comparison, another YAG:Ce sample was synthesized after bubbling an oxidizing gas (O2), into the initial solution during 2 h. Finally, a third sample was prepared without gas bubbling. These samples were characterized by XANES and photoluminescence spectroscopy to examine their Ce 3+ :Ce 4+ ratio and respective iQY. Following these results, the bubbling process was optimized and its impact on the NC structure and morphology was investigated.

Controlling the Ce 3+ :Ce 4+ ratio by gas bubbling. XANES spectra of both nanophosphor samples involving gas bubbling are shown in Figure 1, along with XANES spectra of reference samples. Note that the staring cerium precursor is Ce(acetate)3 and contains only Ce 3+ ions (Figure S1). [START_REF] Chaurand | Multiscale X-ray computed tomography to detect and localize metal-based nanomaterials in lung tissues of in vivo exposed mice[END_REF] Both nano-YAG powders contain Ce 4+ ions, as evidenced by the presence of the peak at 5739 eV. [START_REF] Tella | Chronic dosing of a simulated pond ecosystem in indoor aquatic mesocosms: fate and transport of CeO2 nanoparticles[END_REF][START_REF] Cafun | Absence of Ce3+ Sites in Chemically Active Colloidal Ceria Nanoparticles[END_REF] However, the proportion of Ce 4+ is slightly higher in the sample resulting from the O2 bubbling (Ce 3+ :Ce 4+ = 64:36 ± 2) than in the sample resulting from the Ar/H2 bubbling (Ce 3+ :Ce 4+ = 72:28 ± 2). This trend is consistent with the reducing character of the Ar/H2 gas and the oxidizing character of O2 gas. However, on the contrary to the solid-gas reaction occurring in the solid-state process, the presence of H2 dissolved in the solvent does not enable to prevent significantly the Ce 3+ →Ce 4+ oxidation reaction. Interestingly, both samples arising from Ar/H2 and O2 bubbling, although they have slightly different Ce 3+ :Ce 4+ ratios, present the same high iQY value (around 45%, Table 1). This would suggest that showing that Ce 4+ ions do not act as traps for Ce 3+ luminescence. On the other hand, the nanophosphor obtained without gas bubbling exhibits a significantly lower iQY (38%), while the Ce 3+ :Ce 4+ ratio is in between the values of the two bubbled samples (Ce 3+ :Ce 4+ = 68:32 ± 2) (Table 1). Hence, the nature of the gas bubbled inside the precursor solution positively influences the proportion of Ce 3+ :Ce 4+ in YAG:Ce NCs but does not allow to prevent completely Ce 3+ oxidation into Ce 4+ , which is mainly driven by the solvothermal process itself. Indeed, the cerium oxidation most likely comes from the partial decomposition of 1,4-butanediol into tetrahydrofuran and water starting from 250°C and 180 bar during synthesis, [START_REF] Odziomek | Highly luminescent YAG:Ce ultra-small nanocrystals, from stable dispersions to thin films[END_REF][START_REF] Hunter | Kinetics and Mechanism of Tetrahydrofuran Synthesis via 1,4-Butanediol Dehydration in High-Temperature Water[END_REF] and from the partial decomposition of the organics precursors when working at high pressure, just above 300°C. [START_REF] Bourns | The vapour-phase dehydration of butanediols[END_REF][START_REF] Inoue | Reactions of rare earth acetate hydrates in glycols at high temperatures[END_REF][START_REF] Inoue | Thermal Reaction of Aluminum Alkoxide in Glycols[END_REF] Interestingly, the presence of gas bubble influences the iQY of YAG:Ce NCs as an iQY of 38 % is obtained when no bubbling is performed, whereas an iQY value of 45 % is obtained when a gas is bubbled in the precursor solution.

Influence of gas bubbling on YAG:Ce NC morphology

Hereafter, we present in detail how the bubbling treatment impacts the YAG:Ce NC morphology and optical properties. Different YAG:Ce NC samples were obtained, resulting from a synthesis in 1,4-butanediol or in 1,5-pentanediol. The latter has been chosen for its higher viscosity (128 mPa.s for 1,5pentanediol [START_REF] Werle | Alcohols, Polyhedral[END_REF] vs 84.9 mPa.s for 1,4-butanediol, 50 at 20°C), allowing a more efficient bubbling in the precursor solutions, and potentially a more efficient bubble trapping.

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Please do not adjust margins Different Ar/H2 bubbling durations (0, 2, 4 and 24 h) were studied, with identical solvothermal conditions (320°C, 200 bar). The optimal bubbling duration has been adjusted to 4 h based on NC coherence length evolution (Figure S2). As a reference, their morphological and optical properties will be compared to a sample obtained without bubbling. Pure garnet-type YAG phase was successfully synthesized for all samples, as shown in Figure 2a from powder X-ray Diffraction (PXRD) pattern analysis. Similar diffraction patterns were obtained for the samples synthesized in 1,5-pentanediol also confirming the absence of crystallized impurity. The unit cell parameter was determined to be a = 12.066 ± 0.013 Å for all samples synthesized in 1,4-butanediol and a = 12.090 ± 0.025 Å for the samples synthesized in 1,5pentanediol. On the other hand, the different bubbling treatments have no impact on the lattice parameter, whatever the nature of gas and bubbling time. These unit cell values, 12.066 Å and 12.090 Å, are larger than for the bulk YAG crystal, which is of 12.012 Å. [START_REF] George | The local environment of the activator ions in the solid state lighting Y3-xCexAl5O12[END_REF] These higher unit cell parameters for nano-YAG are reported in the literature and decrease with thermal treatments, [START_REF] Revaux | Improvement of luminescent properties and photostability of YAG nanoparticle-based films[END_REF][START_REF] Hosokawa | Defect structure of rare-earth aluminum garnets obtained by glycothermal method[END_REF][START_REF] Xu | Solvothermal Synthesis and Luminescence Properties of Yttrium Aluminum Garnet Monodispersed Crystallites with Well-Developed Faces[END_REF] which suggests that it comes from the local disorder of the nanometer sized NCs and not because of the synthesis process itself. It might arise from internal strain inside NCs. [START_REF] Dantelle | A new solvothermal method for the synthesis of size-controlled YAG:Ce single-nanocrystals[END_REF] Indeed, maximal internal strains, ɛ, inside the samples were deduced from the Le Bail fits: ɛ = 12 ± 5 ‰ for all NCs synthesized in 1,4-butanediol and ɛ = 26 ± 6 ‰ for all NCs synthesized in 1,5-pentanediol. The nature of the gas does not seem to impact the strains inside the NCs but the nature of the solvent did. Moreover, a higher maximal strain for the samples synthesized in 1,5-pentanediol is in good agreement with a higher unit cell parameter. The coherence length, Lc, of YAG:Ce NCs calculated from PXRD and their diameter obtained from TEM image analysis are compared in Figure 3 for samples synthesized in 1,4-butanediol and in 1,5-pentanediol, respectively. In both series, Lc as well as the average size increase when gas bubbling is performed in the initial solutions whatever the nature of the gas. Syntheses made in 1,5-pentanediol result in also higher NC diameter and Lc than in 1,4-butanediol. The Lc increase observed in both solvents when bubbling gas can be associated with the parallel increase of the NC diameter, but also with an improved crystallinity in the samples. Hence, bubbling gas in the precursor solutions allows the preparation of YAG:Ce NCs with bigger size and improved crystallinity, whatever the nature of the gas. This journal is © The Royal Society of Chemistry 20xx

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Please do not adjust margins TEM images (Figure 4) show that the NCs obtained in this study present a good crystallinity, even for samples prepared with no bubbling, compared to cauliflower-shape NCs, commonly seen in the literature, which arise from a massive agglomeration of primary YAG:Ce nanoparticles. [START_REF] Kasuya | Glycothermal synthesis and photoluminescence of YAG:Ce3+ nanophosphors[END_REF][START_REF] Vorsthove | Efficiency issues in Ce3+ doped YAG nanocrystals[END_REF][START_REF] Isobe | Low-temperature wet chemical syntheses of nanocrystal phosphors with surface modification and their characterization[END_REF] This is due to the synthesis conditions of the modified solvothermal process, where an external pressure of 200 bar is ab initio applied. [START_REF] Dantelle | A new solvothermal method for the synthesis of size-controlled YAG:Ce single-nanocrystals[END_REF] The synthesis at 320°C and around 200 bar increases the crystallinity of the particles, with the atomic planes running over the single crystals as observed in TEM images (Figure 4 (a) and (b)) and the crystalline facets clearly visible. Moreover, when using the bubbling protocol, TEM pictures show an increase of the NCs size (Figure 4 (c) and (d) and Figure S3 for images taken at lower magnification) as well as their crystallinity (more faceted crystals) in perfect agreement with the coherence lengths extracted from PXRD patterns. A consequence of these morphological changes of NCs is the enhancement of their iQY (Figure 5). Indeed, size increase induces a reduction of NC specific surface, leading to fewer surface traps responsible for non-radiative de-excitations. Consistently, as NCs obtained in 1,5-pentanediol are bigger than those obtained in 1,4butanediol, higher iQYs are observed: for the former 50% with no bubbling and 57% to 66% when bubbling, while for the latter we measured an iQY of 38% with no bubbling and an iQY between 42% and 52% when bubbling, Figure 5. Thus, a record iQY of 66% has been obtained for 200-nm NCs synthesized in 1,5-pentanediol with an Ar-bubbling. In addition, one can note the high iQY (52 %) obtained for the 60-nm NCs, whose size is more appropriate for the targeted applications. Undoubtedly, the bubbling process notably impacts the nanocrystallisation mechanisms during the solvothermal process. The most likely hypothesis is that the presence of bubbles in the precursor solution promotes an earlier nucleation step. This then allows more time to the nuclei to grow and coalesce through the self-oriented coalescence mechanism, previously observed by several works. [START_REF] Ramanujam | Solvothermal nanoYAG synthesis: Mechanism and particle growth kinetics[END_REF][START_REF] Guo | Bright YAG:Ce Nanorod Phosphors Prepared via a Partial Wet Chemical Route and Biolabeling Applications[END_REF] Thus, a significant improvement in crystallinity is observed by PXRD and TEM: increase of the coherence length, Figure 3, and appearance of NC facets, respectively. This earlier nucleation step compared to no bubbling experiments is certainly favoured by a heterogeneous nucleation of YAG NCs at the bubblesolution interface. [START_REF] Thanh | Mechanisms of Nucleation and Growth of Nanoparticles in Solution[END_REF] To confirm this hypothesis and, as the presence of gas bubbles under pressure is a delicate and rather undocumented matter, we carried out in situ Dynamic Light scattering (DLS) measurements using a home-made autoclave with optical windows, described in [55] and illustrated in Figure 6a. The experiment was performed with the solvent only after gas (Ar/H2) bubbling in order to avoid any parasitic diffusion that could result from the addition of metal precursors. Gas bubbles were either directly visualized using a 2 MPixel Basler acA1600-20uc camera, based on a Sony CCD sensor with 4x4 µm 2 pixel size, or, for smaller bubbles, detected through in situ DLS experiments using a portable Cordouan particle size analyser (VASCO Kin). The pressure was increased progressively up to 100 bar. Under atmospheric pressure, bubbles with a size of 40 µm were observed by the camera. As the pressure increases, their diameter decreases (10 µm at 5 bar) due to the compression and the increased gas solubility. [START_REF] Battino | The solubility of gases in liquids[END_REF] At 100 bar, the camera was unable to observe any microscopic bubbles, indicating either the disappearance of bubbles at such pressure, or the presence of nanometer-sized bubbles.

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Please do not adjust margins At such sizes, the buoyancy of the bubbles is very weak and their motion is consequently dominated by the thermal Brownian movement. Therefore, in situ DLS experiments can be performed to highlight their potential existence. Indeed, as for DLS measurements on colloidal solutions were the particles are driven by a Brownian motion, the movement of the bubbles will induce temporal fluctuations in the intensities of the light they scatter. These fluctuations may be analysed by deriving the normalized autocorrelation functions of the measured intensities (IACF). In the absence of bubbles, the IACF is expected to be flat and equal to 1 during the whole acquisition time. On the contrary, in the presence of diffusing bubbles, the IACF is expected to present an exponential decay as a function of time with a characteristic time related to the size of the nanobubbles. 57, [START_REF] Kobayashi | Measurements of ultrafine bubbles using different types of particle size measuring instruments[END_REF] The experiments were realized by illuminating the solution through the optical window and collecting the light scattered at 170° from the incident beam, through the same window. The recorded IACF is presented in Figure 6b and clearly shows the expected exponential time-decay that signs the presence of bubbles. However, because of the weakness of the scattered intensities, due to the small size of the bubbles, the amplitude of the ACF is weak and a large background signal is observed. A fitting of the IACF was performed following the well-known cumulants method [START_REF] Koppel | Analysis of Macromolecular Polydispersity in Intensity Correlation Spectroscopy: The Method of Cumulants[END_REF] to access the values of the mean time-decay τc and that of the polydispersity index (PDI), which is related to the width of the diameter distribution. The bubble diameter is then calculated knowing that tc = 1/q 2 D, where |q|=4pnsin(q/2)/l is the scattering vector, n being the solvent refractive index, q the scattering angle and l the laser wavelength; and D = (kBT/h)/3pd is the translational diffusion coefficient of the bubbles, kB being the Boltzmann constant, T This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins and h the temperature and the viscosity of the solvent, respectively, and d the hydrodynamic diameter of the bubbles. The best fit of the IACF, also presented in Figure 6b, was found for tc = 2.2 ± 0.3 ms, which leads to a mean diameter d = 6 ±1 nm, as T = 20 °C, n = 1.44, λ = 635 nm, and η = 128 mPa.s. The inset of Figure 6b represents the diameter distribution obtained considering a normal distribution and a standard deviation of 45 % providing from the PDI value. It is to note that in situ DLS experiments on small bubbles may lead to a partial heterodyning of the measurements arising from stray light and scattering from the solvent. [START_REF] Flammer | Dynamic light scattering with singlemode receivers: partial heterodyning regime[END_REF] The occurrence of such phenomena may not be completely ruled out here, however, a partial heterodyning would lead to an underestimate of the bubble size of several tens of percent, but does not question their existence. Bubble-assisted nucleation process has already been demonstrated for coprecipitation reactions, occurring at mild temperature and ambient pressure, [START_REF] Lynch | Gas-Bubble Effects on the Formation of Colloidal Iron Oxide Nanocrystals[END_REF][START_REF] Fatemi | Micro-bubbles as heterogeneous nucleation sites for crystallization in continuous microfluidic devices[END_REF] but also for sonocrystallisation processes. [START_REF] Kordylla | Modeling ultrasound-induced nucleation during cooling crystallization[END_REF][START_REF] Guo | Effect of ultrasound on the heterogeneous nucleation of BaSO4 during reactive crystallization[END_REF] Here, we demonstrated that under solvothermal conditions, bubbles also modify the nanocrystallisation mechanism, through an heterogeneous nucleation with a significant reduction of the energy barrier for nucleation, compared to homogeneous nucleation. This favoured the occurrence of a strong confinement in time of the nucleation step at the early stage of the solvothermal process (Figure 6c), giving then the time to YAG primary grains to grow and aggregate to form larger NCs by self-oriented coalescence, [START_REF] Ramanujam | Solvothermal nanoYAG synthesis: Mechanism and particle growth kinetics[END_REF] leading to larger YAG:Ce NCs with enhanced crystal quality (Figure 4) and associated iQY.

Experimental

Synthesis of YAG:Ce nanocrystals. A modified-solvothermal process was developed to synthesize 1 mol% Ce 3+ -doped YAG NCs. Compared to conventional solvothermal process where the pressure inside the autoclave is autogenous, an external pressure is here applied, as detailed hereafter. Yttrium acetate tetrahydrate (Alfa Aesar, 99.9%), cerium acetate dihydrate (STREM chemicals, 99.9%) and aluminum isopropoxide (Acros organics, 98%) with a concentration of 0.18 mol.L -1 were mixed in stoichiometric proportions and dispersed either in 1,4butanediol or in 1,5-pentanediol. The resulting precursor solutions were stirred at room temperature for 72 h. Then, different gases (oxidizing, O2; neutral; Ar and reducing; Ar/H2 gas mixture (95-5 vol.%)) were bubbled inside the precursor solutions with a volumetric flow rate assessed at 2.5 cm 3 /min ± 0.5 cm 3 /min. Different bubbling duration were studied, from 0 to 24 h. After the bubbling treatment, the precursor solution was poured into a 500 mL home-made autoclave. Note that one solution without bubbling treatment was synthesized as a reference to study the effect of these treatments on the morphology and optical properties of YAG:Ce NCs. The autoclave was purged and then filled with Argon gas up to 60 bar. The autoclave was heated to 320°C for 2h30 and then cooled down to room temperature. The pressure inside the autoclave increased from 60 bar at the beginning of the reaction to around 200 bar at the end of the heating step due to the precursor degradation and the liquid to gas transformation of the solvent. The products of the reaction were then washed three times in ethanol through successive centrifugations. For powder characterizations, the products were dried in oven at 100°C for 1 h under air.

Structural characterizations. Powder X-ray diffraction (PXRD) diagrams were recorded on a Siemens D8 Advance diffractometer (λCu = 1.54056 Å). Scans were collected in the 2Ɵ range 16-90°, with a step of 0.005° and 1s of acquisition time.

The nanopowders were homogeneously deposited onto a silicon wafer. The unit cell parameter and the coherence length were determined using the Le Bail method on the XRD patterns. [START_REF] Bail | Whole pattern decomposition methods and applications: a retrospection[END_REF] NC morphology and size were investigated by Transmission Electron Microscopy (TEM, Philips CM300). NC average size was obtained from the TEM image analysis. Photoluminescence iQY was measured on nanophosphor powder, using an integrating sphere (Quantaurus-QY Hamamatsu). More specifically, a small amount of powder was deposited in a quartz cuvette, placed at the bottom of an integrating sphere. The powder was excited at 460 nm and the emission was collected from 500 to 700 nm. The iQY was obtained after measuring a blank sample, consisting in the same empty cuvette. The iQY measurements were repeated twice for each sample to ensure good repeatability leading to iQY values with an uncertainty of ± 5%. XANES experiments, performed in the high energy resolution fluorescence-detected (HERFD) mode at the L3 edge of cerium, were achieved at the FAME-UHD beamline at the European Synchrotron Radiation Facility (ESRF Grenoble). [START_REF] Proux | High-Energy Resolution Fluorescence Detected X-Ray Absorption Spectroscopy: A Powerful New Structural Tool in Environmental Biogeochemistry Sciences[END_REF] The photon energy was scanned from 5.68 keV to 5.85 keV using a Si(220) double-crystal monochromator. A helium chamber was mounted between the autoclave, the crystal analyzer spectrometer and the detector to avoid partial beam absorption by air. The X-ray fluorescence signal was recorded with a five Ge (331) crystal analyzer and a Vortex-Ex detector. The beam size was 300 x 100 µm 2 (horizontal x vertical FWHM). The energy calibration was done using the CeO2 spectrum. The nanophosphor powder was compacted between two layers of Kapton tape. The goal of these ex situ experiments was to determine the oxidation state of cerium in nanophosphors synthesized after bubbling of various gases in the precursor solution. The Ce 3+ : Ce 4+ ratio was determined by fitting the experimental data with a linear combination of XANES spectra of standards, i.e. CeO2 nanopowder as a reference for Ce 4+ and Ce 3+ -doped YAG single crystal as a reference for Ce 3+ , using the Demeter software. 67 The ratio is given with an uncertainty of 2%.

Conclusions

Please do not adjust margins Please do not adjust margins Nanocrystals of YAG:Ce were synthesized by a modified solvothermal process involving the bubbling of a gas in the precursor solutions. The set of gas bubbled investigated in this study was selected to contain: one reductive gas, one oxidant and one neutral. First, our study shows that gas bubbling positively influences the Ce 3+ :Ce 4+ proportion in YAG nanocrystals. Second, the bubbling of any of these gasses favoured the nanocrystallisation through a heterogeneous nucleation mechanism at the early stage of the solvothermal process thanks to the formation of nanometer-sized bubbles acting as nucleation centers. This favoured in a second step the growth of NCs increasing their size and crystallinity. The enhancement in the crystallinity provided a breakthrough in YAG:Ce nanocrystals by enabling us to reach iQY of 52% for 60 nm crystals and 66% for 200 nm crystals, which is among the highest iQY ever obtained for YAG:Ce without any annealing treatment. We believe that this method, involving heterogeneous nucleation, a generic process at the bubblesolution interface, can be applied to other types of nanocrystals and would benefit to the entire community working on the nanocrystal synthesis in solution.

Figure 1 .

 1 Figure 1. (a) XANES spectra of the standards used for the linear combination (b) XANES spectra of the YAG:Ce nanophosphors obtained by solvothermal synthesis performed after Ar/H2 or O2 gas bubbling for two hours in the precursor solution. (c) Zoom on the peaks characteristics of Ce 3+ absorption (at 5725 eV) and of Ce 4+ absorption (at 5738 eV). The Ce 3+ :Ce 4+ ratio is indicated for each sample.

Figure 2 :

 2 Figure 2: (a) PXRD patterns of YAG:Ce nanophosphors resulting from the bubbling of different gas for 4 hours in 1,4-butanediol. For comparison, the PXRD pattern of the sample obtained without bubbling is reported (b) PXRD pattern of YAG:Ce nanophosphors obtained without bubbling and its associated Le Bail fit.

  Figure 2b presents a typical PXRD pattern recorded for the sample prepared without bubbling and its associated Le Bail fit. From Le Bail fits, the unit cell parameter, a, was determined, along with the coherence length. Only PXRD patterns of the samples synthesized in 1,4butanediol are given as typical examples in Figure 2.

Figure 3 :

 3 Figure 3: Diameter (filled) and Lc (patterned) of YAG:Ce NCs synthesized in 1,4butanediol (orange) and 1,5-pentanediol (blue) with no bubbling and with 4 hr bubbling of O2, Ar and Ar/H2 gas.

Figure 4 :

 4 Figure 4: TEM images of YAG:Ce NCs as a function of bubbling treatment and solvent: (a) No bubbling and (b) Ar/H2 4 h -bubbling in 1,4-butanediol, (c) no bubbling and (d) Ar/H2 4 h -bubbling in 1,5-pentanediol. In the inset of images (a) and (b) the Fourier transforms corresponding to the areas circled in red are presented, proving the single crystal character of the NCs.

Figure 5 :

 5 Figure 5: iQY for the different samples synthesized in (a) 1,4-butanediol (circles) and in (b) 1,5-pentanediol (triangles).

Figure 6 :

 6 Figure 6: (a) Scheme of the experimental set-up used to assess the presence of gas bubbles under pressure. (b) Experimental (diamonds) and calculated (line) IACF measured in the solvent after gas bubbling, under 100 bar. Inset: diameter distribution of the bubbles obtained from the parameters of the calculated IACF. (c) Scheme of the proposed explanation for increased growth and crystallinity when bubbling a gas inside the precursor solution (bottom) compared to with no bubbling (top) as a function of the synthesis time.

Table 1 .

 1 Ce

	O2 bubbling	64:36	46%
	Ar/H2 bubbling	72:28	45%
	No bubbling	68:32	38%

Sample

Ce 3+ :Ce 4+ ratio (± 2) iQY (± 5%) 3+ :Ce 4+ ratios for YAG:Ce NCs obtained after bubbling O2 or Ar/H2 gas for two hours in the precursor solution, or without any bubbling. Their associated iQY values are also reported

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

Acknowledgements

The authors thank the French ANR for funding this research through the NanophosforLED project (ANR-17-CE09-0035-01).

Conflicts of interest

There are no conflicts to declare.


Dr Jean-François Jarrige

A Didier

D Sarmiento

Castillo / Photos

: Mai -Mafm

General Curator of Patrimoine. He was director of the Musée Guimet from 1986 to 2008. Research Supervisor at CNRS. Member of the Mission Archéologique de l'Indus (MAI) in Pakistan under Jean-Marie Casal's supervision (excavations at Nindowari and Pirak from 1964 to 1975). Th en, from 1975, MAI director and excavation director at Mehrgarh and Nausharo. He has recently published, with Catherine Jarrige and Gonzague Quivron, the site of Nindowari (2011) and the site of Mehrgarh (Neolithic Period)

Th e site of Mehrgarh, located in the north-western part of the Kachi-Bolan region (Balochistan), is a vast archaeological area of about 300 ha covered with remains left by a continuous occupation from the 8 th to the 3 rd millennium BC. Th e excavation of the Neolithic levels (period I-II, ca. 8 th -6 th millennium BC) has provided the so far earliest evidence of farming economy in South Asia involving (among the domesticated species) goats, sheep, cattle, barley and wheat. For period I, 7 meters of aceramic deposits (1700 m 2 ) were unearthed and 77 multi-roomed buildings built in mudbrick were exposed. Alternating with nine building levels, nine levels of graveyards dug in the debris of the abandoned houses were documented (318 graves). Th e majority of the burials provided a rich funerary material assemblage which included shell headbands, necklaces in shell, in unfi red and fi red steatite, clay fi gurines, stone objects, lithics and the fi rst evidences of the use of copper and cotton in South Asia. Th e study of several human skeletons also provided the earliest evidence of dentistry in the archaeological records. From around 6000 BC (period II), pottery occurred in the Neolithic levels at Mehrgarh and the architectural structures now include large compartmented storehouses used as granaries.

MEHRGARH CHALCOLITHIC (5000-2600 BC) AND MEHRGARH VIII (2100-1900 BC)

Period III at Mehrgarh (5 th millennium-beginning of the 4 th millennium BC) is characterized by the development of various specialized craft techniques, particularly in the pottery, fi nely manufactured with coil-building and wheel-throwing techniques, and decorated with geometric and (later) naturalistic motifs. During this period, the fi rst metal objects using the lost-wax casting technique and copper alloy with lead were produced while the funerary goods include sometimes necklaces with glazed steatite beads. Periods IV-V (3600-3100 BC) are distinguishable by a profusion of new painted ceramic styles. Periods VI and VII (3100-2600 BC) see the development of architectural complexes, of new ceramic techniques such as the painted grey ware and of very fi ne and elaborated terracotta human fi gurines linked to a long tradition starting in the Neolithic. Aft er a gap, the site is reoccupied as a cementery at the end of the 3 rd millennium BC (Mehrgarh VIII, ca. 2100-1900 BC). Evidences of material stylistically linked with the Oxus Civilization in Central Asia were discovered in the Mehrgarh VIII burials. Period VII building with numerous pottery in situ.

Granaries of period IIA (6500-5500 BC).

Pottery and seals from periods IV-VII (3600-2600 BC).

Evolution of the terracotta fi gurines at Merhgarh from Period I (8 th -7 th millennium BC) to Period VII (2900-2600 BC)

A grave from the Mehrgarh VIII cemetery (2100-1900 BC) with material similar to the Oxus Civilization kind.


Sylvain Regny

Kévin Bredillet

Jérémy Riporto

Isabelle Gautier-Luneau

Yannick Mugnier

Ronan Le Dantec
email: ronan.le-dantec@univ-smb.fr

Géraldine Dantelle
email: geraldine.dantelle@neel.cnrs.fr

Dual light-emitting Yb 3+ ,Er 3+ -doped La(IO 3 ) 3 iodate nanocrystals: up-conversion and second harmonic generation

Keywords: nanocrystals, iodate compounds, second harmonic generation (SHG), photoluminescence, up-conversion

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

Introduction

With the advent of nanotechnologies and nanomedicine, numerous fluorescent optical nanoprobes have been designed for in vitro, ex vivo and in vivo bio-imaging, such as dye-doped silica nanoparticles, 1 rare-earth doped nanocrystals, 2 and nanodiamonds 3 as recently reviewed. [START_REF] Wolfbeis | An overview of nanoparticles commonly used in fluorescent bioimaging[END_REF] For in vivo experiments, penetration depth is a crucial parameter [START_REF] Gigan | Optical microscopy aims deep[END_REF] and the use of emitters with excitation properties in the near-infrared (NIR) region and the so-called biological transparency windows (BWs) is required. Based on down-conversion fluorescence, Nd 3+ -doped fluorescence probes [START_REF] Dantelle | Nd 3+ -doped Gd3Sc2Al3O12 nanocrystals: towards efficient nanoprobes for temperature sensing[END_REF][START_REF] Del Rosal | García Sol: Nd 3+ ions in nanomedicine: Perspectives and applications[END_REF] or more complex multi-doped architectures [START_REF] Wang | Epitaxial Seeded Growth of Rare-Earth Nanocrystals with Efficient 800 nm Near-Infrared to 1525 nm Short-Wavelength Infrared Downconversion Photoluminescence for In Vivo Bioimaging[END_REF][START_REF] Tan | Rare-earth-doped fluoride nanoparticles with engineered long luminescence lifetime for time-gated in vivo optical imaging in the second biological window[END_REF] have raised high interest with their excitation at 808 nm and their emission in the first (750 -950 nm), second (1000 -1350 nm) and third (1500 -1800 nm) BWs, ensuring deep tissue imaging while minimizing tissue autofluorescence and photon scattering. Similarly, PbS or InP Quantum Dots appear as good candidates for biological applications thanks to their tunable NIR emission. [START_REF] Chinnathambi | Recent advances on fluorescent biomarkers of near-infrared quantum dots for in vitro and in vivo imaging[END_REF] Among other recent developments, the use of nanoprobes excited through a two-photon process has also been studied, allowing an excitation in the BWs and resulting in a higher spatial selectivity than the one-photon down-conversion fluorescence, as the emission only occurs at the focal point of the excitation laser. Up-conversion photoluminescence (UC-PL), two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG) are the different envisioned processes. The latter, characterized by transitions from virtual levels, offers the advantage of a tunable excitation in the material transparency range and in the different BWs.

To benefit from the advantages of the different fluorescence-based and nonlinear optical (NLO) imaging techniques, bimodal nanoprobes were recently introduced. [START_REF] Mayer | Dual light emitting nanoparticles: second harmonic generation combined with rare-earth photoluminescence[END_REF] For instance, UC-PL and SHG-active nanoprobes would allow to combine the advantages of both processes: photoluminescence can be easily detected using conventional microscopy platforms but is a slow process (typical excited state lifetimes for rare-earth emitters are of the order of tens of µs to ms), whereas NLO imaging from Harmonic Nanoparticles (HNPs) [START_REF] Pantazis | Second harmonic generating (SHG) nanoprobes for in vivo imaging[END_REF][START_REF] Staedler | Harmonic Nanocrystals for Biolabeling: A Survey of Optical Properties and Biocompatibility[END_REF] is less implemented on commercial microscopes. SHG and Third Harmonic Generation (THG) signals are polarization-sensitive, instantaneous processes and their simultaneous acquisition results in rapid 2D scans and a better imaging selectivity in demanding applications. [START_REF] Dubreil | Multi-harmonic Imaging in the Second Near-Infrared Window of Nanoparticle-Labeled Stem Cells as a Monitoring Tool in Tissue Depth[END_REF] In addition, SHGand THG-active nanoprobes do not evidence any blinking, photobleaching or saturation process, allowing prolonged studies. [START_REF] Slenders | Image Correlation Spectroscopy with Second Harmonic Generating Nanoparticles in Suspension and in Cells[END_REF][START_REF] Le | Photostable second-harmonic generation from a single KTiOPO4 nanocrystal for nonlinear microscopy[END_REF] However, only a very limited number of nanosources producing both SHG and UC-PL have been reported in the literature, [START_REF] Mayer | Dual light emitting nanoparticles: second harmonic generation combined with rare-earth photoluminescence[END_REF] due to the difficulty to find a single-phase nanomaterial that is both non-centrosymmetric and a good host for luminescent species. Acentric α-La(IO3)3 is known for its SHG activity as initially demonstrated from comparative micron-sized powder experiments. [START_REF] Ok | New Metal Iodates: Syntheses, Structures, and Characterizations of Noncentrosymmetric La(IO3)3 and NaYI4O12 and Centrosymmetric β-Cs2I4O11 and Rb2I6O15(OH)2•H2O[END_REF][START_REF] Taouti | Structures, Thermal Behaviors, and Luminescent Properties of Anhydrous Lanthanum Iodate Polymorphs[END_REF] Recently, we reported the synthesis of α-La(IO3)3 nanocrystals and the quantitative assessment of its orientation-averaged SHG efficiency [START_REF] Regny | Dantelle: Microwave Synthesis and Up-Conversion Properties of SHG-Active α-(La,Er)(IO3)3 Nanocrystals[END_REF] that was found comparable to the other HNPs of interest (KTP, BaTiO3, LiNbO3, etc). [START_REF] Joulaud | Characterization of the Nonlinear Optical Properties of Nanocrystals by Hyper Rayleigh Scattering[END_REF][START_REF] Riporto | Second harmonic spectroscopy of ZnO, BiFeO3 and LiNbO3 nanocrystals[END_REF] We also evidenced the possibility of doping α-La(IO3)3 nanocrystals with Er 3+ ions by La 3+ substitution and showed that Er 3+ -doped α-La(IO3)3 nanocrystals emit both UC-PL and SHG signals under 800-nm and 980-nm excitations. Here we report on multifunctional Yb 3+ ,Er 3+ -doped α-La(IO3)3 nanocrystals demonstrating optimized UC-PL properties and a strong SHG activity. In addition, because of the resonant conditions of UC-PL and the intrinsic non-resonant behaviour of SHG in the material transparency range, special attention has been paid to a possible interplay between the two signals when the excitation wavelength is scanned around the Yb 3+ and Er 3+ absorption bands.

Results and discussion

Synthesis of α-La1-y-xYbyErx(IO3)3 nanocrystals

α-La1-y-xYbyErx(IO3)3 nanocrystals, with y = 0.1 and x = 0.005, 0.01 and 0.02, were synthesized using a microwave-assisted hydrothermal method, as reported in [19]. In the following, the nanocrystals will be referred to as α-La0.9-xYb0.1Erx(IO3)3. Briefly, LaCl3, YbCl3, ErCl3 and HIO3 precursors (all purchased from Sigma-Aldrich, purity > 99.9 %) were mixed in water and introduced into a quartz vessel, tightly sealed. The mixture was then heated through microwave radiations (power: 800 W) at 250°C for 10 minutes. After cooling down (cooling rate: 10°C.min -1 ), the obtained powders were washed with pure water and dried at room temperature. Nanocrystals were then mixed in ethylene glycol and sonicated for 15 minutes at a typical concentration of 0.1 mg.mL -1 so as to obtain dispersions for the subsequent optical characterizations. Note that the quantity of inserted rare-earth ions inside the α-La(IO3)3 matrix is relatively large (>10 mol.%) and that the term "doping" may not be strictly speaking the more appropriate. However, for sake of consistency with this research field, it will be used throughout this paper.

Characterization of nanocrystal structure and morphology

Obtained α-La0.9-xYb0.1Erx(IO3)3 powders were characterized by powder X-ray diffraction (PXRD) using a Siemens D8 Advance diffractometer (Cu Kα1 radiation, 40 mA, 40 kV). Figure 1a shows the PXRD diagrams of La0.9-xYb0.1Erx(IO3)3 with x = 0.005, 0.01 and 0.02, along with their associated Le Bail refinements, based on already published results as input parameters (ICSD card #171250). [START_REF] Ok | New Metal Iodates: Syntheses, Structures, and Characterizations of Noncentrosymmetric La(IO3)3 and NaYI4O12 and Centrosymmetric β-Cs2I4O11 and Rb2I6O15(OH)2•H2O[END_REF] No phase impurity was detected. This is consistent with the results published by M.B. Taouti et al. on α-La(IO3)3 and by Y. Suffren et al. on β-La(IO3)3, as both polymorphs can incorporate more than 10 mol.% of Yb 3+ . [START_REF] Taouti | Structures, Thermal Behaviors, and Luminescent Properties of Anhydrous Lanthanum Iodate Polymorphs[END_REF][START_REF] Suffren | Differences and Similarities between Lanthanum and Rare-Earth Iodate Anhydrous Polymorphs: Structures, Thermal Behaviors, and Luminescent Properties[END_REF] The unit cell parameters of La0.9-xYb0.1Erx(IO3)3 were obtained from the Le Bail fits and are shown in Figure 1b. As the concentration of Er 3+ is raised the unit cell parameters a, b, and c become smaller according to the Vegard's law. It is consistent with the decrease of the ionic radius from La 3+ (rLa3+=1.216 Å in a 9-fold coordinated environment) to Er 3+ (rEr3+=1.062 Å in a 9-fold coordinated environment) and evidences the substitution of La 3+ by Er 3+ upon doping.

In addition, the coherence length, Lc, of the crystalline domains obtained from the Le Bail fits assuming an isotropic broadening is about 60 ± 10 nm for each composition. 4) [START_REF] Ok | New Metal Iodates: Syntheses, Structures, and Characterizations of Noncentrosymmetric La(IO3)3 and NaYI4O12 and Centrosymmetric β-Cs2I4O11 and Rb2I6O15(OH)2•H2O[END_REF] ) again in good agreement with the reduction of ionic radius from La 3+ to Yb 3+ (rYb3+=1.042 Å) and with the substitution of La 3+ by Yb 3+ .

The obtained α-La0.9-xYb0.1Erx(IO3)3 powders were dispersed in ethylene glycol using ultrasound bath (for 15 minutes at room temperature). Dilute solutions were obtained by successive dilutions and drop-casted onto Transmission Electron Microscopy (TEM) grids (Cu, 300 mesh, Electron Microscopy Sciences). After a drying of 4 days at 80°C to remove any trace of solvent, the carbon grids were inserted into a Transmission Electron Microscope (TEM, Philips CM300) operating at 300 kV and 77 K. Figure 2 shows a typical TEM image of α-La0.88Yb0.1Er0.02(IO3)3, representative of all α-La0.9-xYb0.1Erx(IO3)3 compounds. One can observe that the nanocrystals are homogeneous in size, with a typical average size of 50 ± 10 nm. This value is consistent with the coherence length (Lc = 60 ± 10 nm) previously determined and attests the single-crystal character of the obtained α-La0.9-xYb0.1Erx(IO3)3 nanocrystals. It is worth mentioning that, as previously reported in [19], doped and un-doped α-La(IO3)3 nanocrystals present a strong affinity for the TEM carbon-coated Cu grids, resulting in a high coverage density.

In addition, Dynamic Light Scattering (DLS) measurements performed from the ethylene glycol dispersions of α-La0.9-xYb0.1Erx(IO3)3 nanocrystals confirm the above size estimations (TEM and PXRD) whatever the Er 3+ concentration (Insert in Figure 2).

Evidence of multiple emission from α-La0.9-xYb0.1Erx(IO3)3 dispersions

The orientation-averaged optical properties of α-La0.9-xYb0.1Erx(IO3)3 nanocrystals dispersed in ethylene glycol were studied using the Hyper Rayleigh Scattering (HRS) configuration with a tunable (700-1300 nm) vertically polarized femtosecond Laser (Insight X3 Spectra-Physics, 80 MHz, 100-120 fs) as the excitation source. [START_REF] Regny | Dantelle: Microwave Synthesis and Up-Conversion Properties of SHG-Active α-(La,Er)(IO3)3 Nanocrystals[END_REF][START_REF] Riporto | Second harmonic spectroscopy of ZnO, BiFeO3 and LiNbO3 nanocrystals[END_REF] Emitted signals were collected perpendicularly to the laser beam through a spectrometer (Andor Shamrock 193) coupled to a CCD camera (Andor iDus 401). The power-law dependences were obtained by adjusting the excitation power from 250 to 800 mW thanks to a half-wave plate associated with a polarizer cube. The peak intensity within the sample ranges from 0.5 to 1.6 GW cm -2 and the average intensity from 4.5 to 13 kW cm -2 . Note that the peak intensity range is to be considered for the Second Harmonic Scattering (SHS) process, whereas, the average intensity is assumed for the UC-PL signals since the typical excited state lifetime of Er 3+ is of the order of several tens of µs, i.e. much longer than the excitation fs laser pulse.

All dispersions of α-La0.9-xYb0.1Erx(IO3)3 nanocrystals in ethylene glycol emit several characteristic optical signals corresponding to SHG and UC-PL, as illustrated in Figure 3a for two resonant excitation wavelengths at 800 nm (red curve) and 980 nm (dark red curve).

Arrows indicate the SHS signal whose position varies with the excitation wavelength, namely λSHS = 400 nm for λexc = 800 nm and λSHS = 490 nm for λexc = 980 nm. In addition, the power-law dependence of the SHS intensity scales quadratically with the peak intensity as expected for SHS (Figure S1). The other characteristic peaks centred at 525 nm, 550 nm and 660 nm are independent of the excitation wavelength and correspond to the UC-PL process through the well-known Er 3+ electronic transitions. As reminded in Figure S2, the peaks at 525 nm, 550 nm and 660 nm are associated with the 2 H11/2 → 4 I15/2, 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions, respectively. [START_REF] Dieke | Spectra and Energy Levels of Rare Earth Ions in Crystals[END_REF][START_REF] Peijzela | A complete 4f n energy level diagram for all trivalent lanthanide ions[END_REF] When plotted in logarithmic scales, the power dependence of the UC-PL intensity of each emission wavelength (Figure 3b) scales as 𝐼 ! ".$ where 𝐼 ! is the average excitation intensity. For the UC-PL emission, the sequential absorption of n photons can lead to different values of the exponent according to the competition between the linear decay upconversion processes [START_REF] Pollnau | Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems[END_REF] . The value here obtained at n = 1,5 is typical of the up-conversion processes in co-doped Yb 3+ ,Er 3+ systems under such excitation intensities. [START_REF] Pollnau | Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems[END_REF][START_REF] Suyver | Anomalous power dependence of sensitized upconversion luminescence[END_REF] In terms of relative intensities between the SHS and UC-PL signals and after a proper calibration of the wavelength-dependent response of the experimental setup [START_REF] Riporto | Second harmonic spectroscopy of ZnO, BiFeO3 and LiNbO3 nanocrystals[END_REF] , the signals at 400 nm, 525 nm and 550 nm are very similar under an excitation wavelength set at 800 nm and whatever the Er content (x = 0.005, 0.01 and 0.02). However, when the resonant excitation is shifted to 980 nm, a much more efficient UC process can be noticed comparatively to the almost constant SHS signal. Note that the overall spectral dependence of the SHS and UC-PL signals according to the excitation wavelength is further studied in the last section.

Spectral response of the SHS and UC-PL signals under resonant conditions

To understand better the evolution of SHS and UC-PL emissions with the excitation wavelength, the emission spectra of the α-La0.88Yb0.1Er0.02(IO3)3 nanocrystals dispersed in ethylene glycol were recorded by scanning both in the 780-820 nm and 900-1020 nm ranges.

After fitting of each characteristic peak with Gaussian functions, the UC-PL at 550 nm and SHS intensities are reported in Figure 4. For both excitation ranges, the SHS signal appears rather constant with the excitation wavelength (Figure 4, black curves). On the contrary, the UC-PL signal strongly depends on the excitation wavelength (Figure 4, green curves). Such evolution of the UC-PL signal is directly correlated to the real absorption levels of Er 3+ and Yb 3+ whereas SHS only involves virtual states. In Figure 4a, the green curve corresponds to the typical absorption of Er 3+ around 800 nm, namely to the 4 I15/2 → 4 I9/2 electronic transition. [START_REF] Blasse | Luminescent Materials[END_REF] In Figure 4b, the green curve is well correlated to the 2 F7/2 → 2 F5/2 transition of the Yb 3+ excitation spectrum. Note also that the absorption cross-section of Er 3+ around 980 nm is 10 times smaller than that of Yb 3+ and thus Er 3+ ions do not contribute to the excitation spectrum. [START_REF] Huang | Origin of near to middle infrared luminescence and energy transfer process of Er 3+ /Yb 3+ co-doped fluorotellurite glasses under different excitations[END_REF] Hence, according to the excitation wavelength, two UC-PL mechanisms can be suggested:

• For an excitation at 980 nm, the UC-PL process first involves Yb 3+ ions. After absorption of the incident photons to the 2 F5/2 excited state, energy is transferred to the neighbouring Er 3+ ions in their vicinity. Because of the long lifetime of the 4 I11/2 level of Er 3+ , further energy transfer from Yb 3+ is repeated (before de-excitation of Er 3+ ) thus allowing population of higher excited states. A two-photon process thus results in the Er 3+ PL emission at 525nm and 550 nm from the 2 H11/2 and 4 S3/2 levels, respectively. In addition, a three-photon process from the 2 H9/2 is also observed as evidenced in Figure 3a by the weak emission at ~400 nm.

• For an excitation at 800 nm, the UC-PL process occurs exclusively through the Er 3+ ions, as Yb 3+ shows no absorption in this range. Er 3+ ions are first excited to the 4 I9/2 level before absorption of a second photon populating higher excited states of Er 3+ and leading to a PL emission from the 2 H11/2 and 4 S3/2 levels.

The highest conversion efficiency of the UC-PL process, illustrated in Figure 3a for an excitation wavelength at 980 nm, is thus consistent with the absorption cross-section of Yb 3+

which is known to be much larger than that of Er 3+ at 800 nm. [START_REF] Dong | Energy Transfer in Lanthanide Upconverting Studies for Extended Optical Applications[END_REF] Similarly, when Yb 3+ is used as a sensitizer, a three-photon process with an emission at ~ 400 nm is only detectable when pumping at 980 nm.

Regarding the SHG conversion efficiency, the SHS signal is measured constant within the experimental uncertainties (evaluated at 15 % 21 ) (Figure 4) while scanning in both excitation ranges (780-820 nm and 900-1020 nm). Hence, the SHG signal is not modulated in both excitation ranges, on the contrary to UC-PL process which is strongly dependent on the excitation wavelength. Under such resonant excitation conditions for the Er 3+ and Yb 3+ ions, the absence of interplay between the SHS and UC-PL processes can be clearly noticed, indicating that the real transition levels of the lanthanide ions are not involved in the SHS emission. The absence of interplay here observed for the UC-PL and SHG processes might be attributed to the intrinsic properties of lanthanum iodate compounds themselves. Indeed, the SHG efficiency in iodate materials is mainly attributed to the stereoactive lone-pair of I 5+ , [START_REF] Chang | Alignment of Lone Pairs in a New Polar Material: Synthesis, Characterization, and Functional Properties of Li2Ti(IO3)6[END_REF][START_REF] Sun | PbPt(IO3)6(H2O): a new polar material with two types of stereoactive lone-pairs and a very large SHG response[END_REF] whereas the photoluminescence comes from the lanthanide ions, substituting La 3+ as evidenced by the PXRD results. Such a behaviour would deserve further comparison with other non-centrosymmetric structures like LiNbO3 for which co-doping with Er 3+ and Yb 3+ ions has already been achieved.

Conclusions

Microwave-assisted hydrothermal reaction allows to successfully produce α-La0.9-xYb0.1Erx(IO3)3 nanocrystals with x = 0.005, 0.01 and 0.02. Under near-infrared resonant excitations at around 800 nm and 980 nm, corresponding to the Er 3+ and Yb 3+ excited levels, a simultaneous emission of UC-PL and SHS signals can be observed. Very notably, Yb 3+ act as a strong sensitizer for the UC-PL at 980 nm and no competition between the two processes (UC-PL and SHS) could be evidenced in the co-doped lanthanide iodate nanocrystals.

For

  La0.9Yb0.1(IO3)3, the values of the cell parameters can be derived through the extrapolation of the linear regression at x = 0 from Figure 1b. It results in a = 12.5110(5) Å, b = 7.0860(5) Å and c = 27.7965(5) Å. All these values are smaller than that of pure α-La(IO3)3 (a = 12.526(2) Å, b = 7.0939(9) Å and c = 27.823(

Figure 1 :

 1 Figure 1: (a) PXRD diagrams of α-La0.9-xYb0.1Erx(IO3)3 for x = 0.005, 0.01 and 0.02 and their associated Le Bail fits. α-La(IO3)3 is a monoclinic phase, crystallizing in the Cc space group. (b) Evolution of the unit cell parameters a, b and c as a function of x in α-La0.9-xYb0.1Erx(IO3)3.

Figure 2 :

 2 Figure 2: Typical TEM image of α-La0.88Yb0.1Er0.02(IO3)3 nanocrystals recorded at 77 K. In the inset is shown the size distribution by intensity measured by DLS from La0.88Yb0.1Er0.02(IO3)3 nanocrystals dispersed in ethylene glycol.

Figure 3 .

 3 Figure 3. (a) Emission spectra of α-La0.88Yb0.1Er0.02(IO3)3 nanocrystal dispersions in ethylene glycol under a 800-nm excitation (red curve) and a 980-nm excitation (dark red curve). For sake of clarity, the spectra have been stacked. The arrows point out the SHG signals, whereas the up-conversion emission is evidenced at 525 nm, 550 nm and 660 nm. ê is discussed in the text. (b) Power-law dependence of the UC-PL intensity at 525 nm, 550 nm and 660 nm, in logarithmic scale under a 980-nm excitation. Data correspond to the symbols and dashed lines to the linear fit giving a slope of n= 1.5.

Figure 4 .

 4 Figure 4. Spectral response of the emission intensity of the UC-PL at 550 nm (green curves) and SHG signals (black curves) in the 780-820 nm (a) and 900-1030 nm (b) ranges for α-La0.88Yb0.1Er0.02(IO3)3 nanocrystals dispersed in ethylene glycol. These spectra were recordedfor a peak intensity of 1 GW.cm -2 . Note that for sake of comparison of SHS and UC-PL intensity, both y-axis scales were kept identical in graph (a). On graph (b), as the UC-PL intensity is much more intense than SHS, the y-axis scales were chosen differently. It is possible to compare the absolute intensity values of the two graphs (a) and (b) as a thorough calibration of the set-up has been performed as the function of the excitation wavelength.[START_REF] Riporto | Second harmonic spectroscopy of ZnO, BiFeO3 and LiNbO3 nanocrystals[END_REF]

Acknowledgements

The authors thank the French-Swiss Interreg V and French ANR programs for financial supports (Projects OncoNanoscreen and Racine) and the 2015-2020 French Contrat Plan État Région (project E-TIME, SYMME).

Supplementary Material Available: Power dependence of SHS signal and schematic of the energy diagram of Yb 3+ and Er 3+ .