Roger Bostelman
email: roger.bostelman@nist.gov

Sebti Foufou

Steve Legowik

Hong Tsai

Hong

Mobile Manipulator Performance Measurement Towards Manufacturing Assembly Tasks

Keywords: mobile manipulator, performance measurement, ASTM F45, artifacts, ground truth

Mobile manipulator performance measurement research is relatively minimal as compared to that of robot arms. Measurement methods, such as optical tracking systems, are useful for measuring the performance of mobile manipulators, although at a much higher relative cost as compared to artifacts. The concept of using test artifacts demonstrates to potential manufacturers and users of mobile manipulator systems that relatively low cost performance measurement methods exist. This paper discusses the concept of reconfigurable mobile manipulator artifacts that were designed and built. An artifact was then used through experimentation to measure the performance of a mobile manipulator to demonstrate the feasibility of the test method. Experimental results show a promising test method to measure the performance of mobile manipulators that are to be used for manufacturing assembly tasks, where at least the mobile manipulator tested has the capability to perform assembly to 1 mm positional accuracy or greater.

Introduction

Mobile robots and mobile manipulators have been popular research topics [START_REF] Shneier | Literature Review of Mobile Robots for Manufacturing[END_REF][START_REF] Katz | The UMass mobile manipulator UMan: An experimental platform for autonomous mobile manipulation[END_REF][START_REF] Hamner | An autonomous mobile manipulator for assembly tasks[END_REF][START_REF] Djebrani | Modeling and Control of an Omnidirectional Mobile Manipulator[END_REF][START_REF] Vannoy | Real-time Adaptive Motion Planning (RAMP) of mobile manipulators in dynamic environments with unforeseen changes[END_REF]. But, in general mobile manipulators have been further investigated recently and are now becoming commercial tools for industrial use [START_REF]Yaskawa Motoman MH80 robot unloading trucks -from Wynright Corporation[END_REF][START_REF] Guizzo | Meka Robotics, Announces Mobile Manipulator With Kinect and ROS[END_REF][START_REF] Green | KUKA Falls First, Buys Swisslog for $335M. Who's Next?[END_REF]. In research, considerations have focused on the coordination of movements of the robot and the base since redundant degrees-of-freedom (DoF) exist by adding the moving base. An example mobile manipulator consists of a six DoF robot arm (manipulator) mounted onboard a wheeled base (e.g., automatic guided vehicle (AGV) or mobile robot) with two translational and one rotational DoF in the horizontal plane for a total of nine DoF. [START_REF] Miksch | Performance-Functional Based Controller Design for a Mobile Manipulator[END_REF]. Some mobile manipulators have more or fewer DoF and may also be equipped with vertical axis motion control of the robot arm base.

As with robot arm or AGV performance, it is important for manufacturers and users of mobile manipulators to implement performance measurements to understand their system capabilities for appropriate application. For example, a user may wish to apply a mobile manipulator to assemble an engine having relatively high tolerances and associated costs as compared to inserting relatively low tolerance and cost rivets into sheet metal covers.

Measurements of the performances of mobile manipulators performing standard tasks (poses and motions) are non-existent except for simply ensuring that the task has been more or less completed. Robot performance measurements may include path comparison and path drawing, Cartesian and polar coordinate measuring, triangulation, optical tracking, inertial measuring, as well as the difference in position and orientation, or pose, of mainly the end of arm tooling from the commanded robot pose. Ground truth measurement using motion tracking systems of various techniques provides relatively accurate robot joint, segment, or tool point position information such that comparisons can be made to the commanded pose. Summarizing review of robot, mobile robot, and mobile manipulator performance measurement research shows this as being relatively new to the research community [START_REF] Bostelman | Survey of Research for Performance Measurement of Mobile Manipulators[END_REF].

A survey of research on performance measurement of mobile manipulators [START_REF] Bostelman | Survey of Research for Performance Measurement of Mobile Manipulators[END_REF] was published by the National Institute of Standards and Technology (NIST) as basis for research in the Robotic Systems for Smart Manufacturing (RSSM) Program [START_REF]Robotic Systems for Smart Manufacturing Program[END_REF]. The Program develops and deploys advances in measurement science that enhance U. S. innovation and industrial competitiveness by improving robotic system performance and other aspects to achieve dynamic production for assembly-centric manufacturing. Recently, NIST has been measuring performance of mobile manipulators using both a motion tracking system and artifacts designed at NIST. The artifacts can provide an inexpensive, yet low uncertainty method for manufacturers and users to measure the performance of mobile manipulators.

Performance standards, such as ASTM F45 [START_REF]ASTM F45 Committee on Driverless Automatic Industrial Vehicles[END_REF], can also benefit from the use of low cost, high accuracy artifacts to develop generic test methods so that manufacturers and users perform similar and comparative tests. Specifically, ASTM F45.02 Docking and Navigation subcommittee is developing work item WK50379 [START_REF][END_REF] on docking unmanned ground vehicles and their onboard equipment, such as manipulators. In this work, the use of artifacts is being considered as ground truth for mobile manipulator performance measurement.

This paper discusses the design and use of the NIST artifacts, called reconfigurable mobile manipulator artifacts (RMMAs), in measuring mobile manipulator performance. The concept of using artifacts and programmed algorithms to control the manipulator are discussed. An experiment demonstrating feasibility and experimental results is then discussed followed by conclusions that suggest follow-on measurements.

Performance Measurement using Artifacts

The concept includes positioning a mobile manipulator next to an artifact as well as positioning and orienting the end of arm tool (EOAT) attached to the manipulator at specific locations above an artifact to digitally detect fiducials with known uncertainty. The performance evaluation criteria include the:  Time to register the mobile manipulator to the artifact  Time to move from the registration points to the assembly points  Repeatability after registration  Number of search steps equating to the initial distance from registration/assembly points  Detection of reflectors with varying diameters Artifacts, called reconfigurable mobile manipulator artifacts (RMMAs), were designed at NIST to include square, circle, triangle, straight and curved lines, and sinusoidal geometric patterns of tapped holes drilled into machined plates with tolerance of +/-0.025 mm. The static and index artifact, RMMA-1, is shown in Figure 1(a) beside a mobile manipulator. A dynamic RMMA, RMMA-2, is shown in Figure 1(b). Static means a stopped vehicle with the only onboard manipulator moving to detect fiducials. Index means the mobile vehicle moves from one static location to another where the manipulator cannot physically reach all patterns from one stopped location. Dynamic means both a continuously moving vehicle and onboard manipulator to detect fiducials. Figure 1(c) shows a 457.2 mm square pattern of four reflector fiducials located at the square corners of RMMA-1 and Figure 1(d) shows a close-up of the reflector inside the square tube reflector housing (which also supports a collimator to be explained later). Both RMMAs can be in a horizontal (as tested in this research), vertical, or other orientation, at short-to-tall heights, and even configured overhead as would be typical of assembly in manufacturing facilities for an unlimited set of performance measurement possibilities. The EOAT was a retroreflective laser sensor (RLS) that emits light to a reflector and is detected by the RLS. A camera, with a light source, could instead be used as the detection sensor, especially with a larger diameter reflector or other spot. For the RLS/reflector concept, no camera software algorithm was required as the RLS connected directly into one of the manipulator digital inputs. The reflectors can have specific diameters depending upon the required uncertainty for their location. The manipulator1 used has manufacturer's specified repeatability of 0.1 mm [START_REF][END_REF] and the AGV navigation sensor has manufacturer's specified resolution of 1 mm [START_REF]SICK 2D Laser Scanners[END_REF]. No information is provided by the AGV manufacturers to specify the vehicle performance such as position accuracy. As published in [START_REF] Bostelman | Performance Measurement of Mobile Manipulators[END_REF], 6.3 mm diameter reflectors were used to test mobile manipulator uncertainty as an initial concept feasibility test.

For our tests, detector-to-reflector distance parallel to the laser axis was approximately 127 mm where the minimum and maximum detection distances are 100 mm and 10 m respectively. The distance would be representative of a programmed waypoint above and in-line with the next manipulator task point aligned to grip or insert a part or perform another task. The desired uncertainty may be, for example, a part insertion alignment tolerance required for a manufacturing assembly process. Moving along this grip/insertion line, parallel to the laser, at the aligned pose to the task point, also provides some knowledge of insertion performance (i.e., if the task point is continuously detected along the grip/insertion line).

Each adapter, to be screwed into the various patterns of holes, supports a background target, a circular reflector, a square tube reflector housing, and a cylinder used as a light collimator. Circular collimators are inserted into three dimensional (3D) printed (blue) square tubes that house a micro reflector, a reflector cover with a specific diameter hole, through which exposes the reflector, on top of the reflector and the collimator on top of the cover. Flat background targets, measuring 7.6 cm diameter with 6.4 mm incremental rings are perpendicular to each collimator and sometimes used as a simple visual cue for the test director when the manipulator does not align with the reflector. The adapter, reflector, housing, target, and collimator can be perpendicular to the flat surface or rotated to pitch angles between ± 90° and yaw angles between 0° and 360°. The reflector and the collimator inside can be any diameter, dependent upon the sensor specification and the desired measurement uncertainty. Experiments for this paper utilized 1 mm through 6.3 mm diameter reflector cover holes where the 1 mm diameter hole was used for registering the manipulator to the reflector center.

Without the collimator, as shown in Figure 1(c), the reflector can be detected at approximately ± 20° to the vertical axis. For the collimators used, the reflector can be detected at a maximum 3.2 mm radius from the reflector center. It can be detected at approximately ± 7° to the vertical axis when using a 12.7 mm inside-diameter collimator. Collimators could be made with even smaller inside-diameters to force more perpendicular manipulator tool point axis pose to the reflector.

The RLS, shown in Figure 1(a), shows the sensor mounted in-line and perpendicular to the manipulator tool point. Initial alignment to the reflector can occur using one of several methods briefed in [START_REF] Bostelman | Performance Measurement of Mobile Manipulators[END_REF]. For our experiments, we aligned the RLS using the manipulator jog mode from the teach pendant until the laser detected the two registration reflectors for both the circle and square reflector patterns. Therefore, we could read directly from the teach pendant the end-effector coordinates to return to during our experiment.

An optical tracking measurement system was initially used as ground truth [START_REF] Bostelman | Dynamic Metrology Performance Measurement of a Six Degree-Of-Freedom Tracking System used in Smart Manufacturing[END_REF] for comparison to the use of artifacts and to measure all system components simultaneously for test method development. The ground truth system has static positional accuracy of 0.02 mm, however costs approximately 20 times the cost of the artifact concept. Figure 2 shows a snapshot of the optical tracking system markers used as fiducials and positioned on the square pattern of collimators. The tracking system measured the performance of the vehicle, manipulator, and artifact within the same system of reference. In this paper, the focus is to discuss the comparison between only the manipulator and artifact as a low cost, relatively high accuracy measurement method. The potential for artifacts being made using three dimensional (3D) printing could lower the cost further by an order of magnitude, or 200 times, as demonstrated through machining costs of the RMMAs and the 3D printing of parts used with the artifact.

Experiments

The experiment consisted of moving the AGV from a home position away from the RMMA-1. The AGV control program moved the AGV to the first location where its position and orientation or pose was pre-determined by the AGV control program. AGV orientation angles were programmed to be at 45° with respect to the RMMA-1. Upon completion of the pattern detection for a location, the AGV moved to the second location and pose, and so forth until six locations were completed. The AGV completed the test by returning to the home position.

A modified registration method for registering the mobile manipulator to the RMMA was recently developed that uses the components shown in Figure 3, including a 3D printed aperture housing, collimator, micro-reflector, and a camera iris aperture. The aperture allows the opening to the reflector to not only be any size, but to also center the opening. This ensures that the two registration fiducials are centered on the reflector even when using a much larger reflector for all other fiducials. The smallest aperture opening used was 1 mm diameter while all other fiducials were 3.2 mm or 6.3 mm diameter. The RLS did not return a 'detect' at a smaller diameter than 1 mm diameter aperture setting.

A circular search pattern was used in previous tests [START_REF] Bostelman | Performance Measurement of Mobile Manipulators[END_REF] to register the manipulator to the first reflector. Once the first fiducial was acquired, it was possible for a registration skew to occur as only the edge of the pattern may have been detected and the opposite side of the second fiducial used for registration could cause an incorrect performance measurement of the mobile manipulator. The circular search began with a step increment chosen to be approximately half of the diameter of the fiducial being tested. For example, an initial step size of 3.1 mm was chosen for a 6.3 mm fiducial to be detected. However, after the initial circle of steps was completed, the step moved to the next larger 3.1 mm circle step radius (e.g., from 3.1 mm to 6.2 mm radius from the start location) and at the same step arc angle (e.g., 15°) causing much larger steps to occur as the circle pattern grew. Instead, a square pattern was tested for the research described in this paper that kept the same step size throughout the entire search. An example of the square step pattern is shown in Figure 4 where the search begins away from the reflector at the chosen start point (yellow arrow dotted end) and each step moves along the small white or gray arrows until the RLS detects the reflector with the red arrow step. The manipulator was first moved from a stowed location over the body of the AGV to a staging location directly in front of the AGV. The manipulator was then moved from the staging location to the first of the two registration reflectors. The staging location was chosen so that the manipulator could make a straight line motion from the staging location to a registration reflector located in front of, or to either side of, the AGV without colliding with its shoulder joint. After moving to the first registration reflector, the manipulator performed a square spiral search to determine the exact location of the reflector. When it determined the location of the first registration reflector, the program repeated the process with the second registration reflector. When the locations of the two registration reflectors were determined, the program had sufficient information to compute the locations of the other fiducials in the square or circular patterns. The initial search was not counted as a performance criteria since the mobile manipulator could use various types of registration techniques, such as: physical contact using a touch probe [START_REF] Mastercal | [END_REF], cameras detecting fiducials [START_REF] Atcheson | CALTag: High Precision Fiducial Markers for Camera Calibration[END_REF], or laser interferometry, theodolites, and coordinate measuring arms [START_REF] Mauricio | Robot Calibration: Modeling Measurement and Applications[END_REF]. However, for comparison to repeatability, the initial registration number of iterations count was logged and included in results.

Once the locations of all reflectors in the pattern were computed, the manipulator cycled through them a set number of times -32 times for each pattern in this experiment. At each reflector, the RLS checked to see if the manipulator was aligned with the reflector.

When the test was completed, the manipulator was moved to the staging location and then the stow location. When the manipulator was back in the stow location, the Mobile Manipulator program signaled to the AGV control program that it was clear to move.

The positions of the index fiducials for the targets were recorded prior to performing the repeatability tests. The AGV was first moved to a location where it could reach both of the index fiducials. The current location and orientation of the AGV was recorded. The arm was repositioned manually until the sensor detected alignment with each of the index fiducials, and the manipulator position was recorded. This information, along with the manipulator base position relative to the vehicle's coordinate system, allowed the correct manipulator coordinates for the index fiducials to be calculated for an arbitrary AGV location. This allowed the AGV to approach the target/work area from any direction and to compensate for variation in the AGV's stopping pose.

The calibration of the manipulator base location involved recording the position of one or more fiducials from a variety of locations. Both the AGV location and the manipulator coordinates of the fiducials were recorded. This data was processed using an iterative, non-linear model to find the best value of the base position and orientation.

Results

The mobile manipulator performance measurement results using the RMMA-1 included only the detection of reflectors for each pattern and after initial registration. By comparison, the initial number of search steps used to register the manipulator at the first reflector was recorded. Results are shown in Table 1. The repeatability performance measurement process began once the mobile manipulator was registered to the artifact after initial registration and moving through the square or circle pattern one time and with the AGV statically positioned at a pattern. Measurements of 'detect' or '1' were logged for each RMMA-1 reflector location. If a search was required to find the fiducial after registration, the measurement at that reflector was counted as a 'no detect' and the number of search steps was recorded.

Table 1 shows: the consecutive position number and programmed AGV position, the AGV pose angle (heading), the circle or square pattern being detected, the total number of reflectors to detect for 32 pattern iterations after the registration pattern, the reflector diameters for each pattern (rounded to whole numbers), the number of reflectors detected and detection percentage, and the initial number of search steps needed to register to the first reflector after the AGV stopped. The AGV stop points programmed are shown in Figure 4. The lines leading to the stop points indicate the AGV orientation.

The table shows very high repeatability results at 97% or above as shown in the "% detected" column of the table. The results demonstrate a good test procedure for determining repeatability of a mobile manipulator to register to and access assembly points within the reflector diameters chosen. Further tests are required to understand direct connections between mobile manipulator performance and system pose, for example, suggesting that AGV pose at 0° provides higher performance than at other angles. Results here do not show this since position 6 included the AGV being at 90° and yet, was repeatable to 100%. Several additional tests are envisioned as well, such as: repeatability of the same pattern followed by the other pattern, both from different AGV poses; using the 1 mm registration reflectors for all patterns followed by the same size reflectors for all, such as 3 mm; and using 1 mm diameter reflectors for all points within each pattern to provide a possible detection limit.

Conclusion and Future Work

As discovered in a NIST survey of mobile manipulator research [START_REF] Bostelman | Survey of Research for Performance Measurement of Mobile Manipulators[END_REF], performance measurement of these systems is minimal as compared to robot arms. Measurement methods, such as using optical tracking systems, are useful methods for measuring mobile manipulator performance, although at a much higher cost. The use of known artifacts, called reconfigurable mobile manipulator artifacts, to measure the performance of mobile manipulators is being researched at NIST to demonstrate the feasibility of the test method. The concept of using artifacts demonstrates to potential manufacturers and users of mobile manipulator systems that relatively low cost performance measurement methods exist. Artifacts, such as the RMMA-1 and in the future, RMMA-2, allow an unlimited number of performance measurement configurations. The measurement of mobile manipulator repeatability and accuracy for very low resolution tasks (e.g., positioning bags of product) through very high resolution tasks (e.g., assembly of parts for manufacturing) is achievable through the use of RMMAs. Static and index tests have been completed using this method and have proven feasible. Experimental results show a promising test method to measure performance of mobile manipulators that are to be used for manufacturing assembly tasks, where at least the mobile manipulator tested has the capability to perform assembly to 1 mm positional accuracy or greater. Future test method developments should not only include dynamic mobile manipulator performance measurements, but also include the suggested tests in the results section. Additionally, rapid registration techniques, finer retroreflective laser sensors allowing smaller diameters, and in turn, physically providing peg-in-hole measurements with variable peg and hole chamfers, are expected to provide even higher performance measurements towards assembly applications of mobile manipulators.

Figure 1 :

 1 Figure 1: Reconfigurable mobile manipulator artifacts (RMMAs) showing (a) static and index version (RMMA-1) and (b) dynamic version (RMMA-2). (c) Static and index version square pattern of reflectors and (d) close-up, top view of an illuminated reflector inside the square tube. The inset in (a) shows the retroreflective laser sensor used to detect reflectors.

Figure 2 :

 2 Figure 2: RMMA with optical tracking system markers on each collimator. Markers are also shown in Figure 1(a) attached to the retroreflective laser sensor held by the manipulator.

Figure 3 :

 3 Figure 3: (a) Components used for registration and (b) registration fiducial mechanism attached to the RMMA. The Mobile Manipulator program controlled the manipulator during the tests. It interfaced with the AGV directly to obtain the current AGV position and orientation, and it interfaced with the AGV control program (Transport Structure) running on the Order Manager application to coordinate the motion of the arm with the motion of the AGV. The AGV control program signaled the Mobile Manipulator program when it arrived at one of the stop or test locations. The AGV control program also sent the identification number of the test location. The Mobile Manipulator program read the current AGV pose and used it to compute the initial search location of the two registration reflectors in the target pattern (circle or square). Additional patterns could also have been used in the Mobile Manipulator program.

Figure 4 :

 4 Figure 4: Example square step search pattern drawing. The pattern begins with the yellow arrow dotted end and ends when the reflector is detected with the red arrow search step.

Figure 2 :

 2 Figure 2: Stop points

Disclaimer: Commercial equipment and materials are identified in order to adequately specify certain procedures. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.


Clément Livache

Bertille Martinez

Eva Izquierdo

Marion Dufour

Hervé Cruguel

Sébastien Royer

Xiangzhen Zhen Xu

Sandrine Ithurria

Emmanuel Lhuillier

Herve Cruguel

Sebastien Royer

Shape and confinement control in mid and far infrared nanocrystals

Keywords: nanocrystals, narrow band gap, mid and far-infrared, self-doping

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

INTRODUCTION

Colloidal quantum dots (CQD) have been attracting a growing interest over the past few years. Their integration as the next generation of phosphors for display has pushed their use up to the industrial scale. However, in this application, the CQD remain electrically passive and only their photoluminescence (PL) is used. The use of CQD for optoelectronics [START_REF] Talapin | Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications[END_REF] remains far more challenging because both optical and transport properties need to be addressed.

On the material side, most of the effort has been focused on cadmium chalcogenides because of their visible tunable optical features. Nevertheless, in the visible range, current optoelectronic devices (source and detector) already combine high efficiency and cost effectiveness. This is not the case in the IR where low cost devices are generally associated with lower performances. Since organic electronics are ineffective in the mid IR and that current technologies based on epitaxially grown semiconductors are unlikely to bring a cost disruption, alternative materials still have to be identified. CQD can become the building block of a new generation of infrared devices as long as they can demonstrate competitive device performances [START_REF] Rogalski | Third-generation infrared photodetector arrays[END_REF].

Here, we introduce some of the recently obtained progress on the synthesis of HgTe and HgSe CQD [START_REF] Kershaw | Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties[END_REF][START_REF] Lhuillier | Mid-IR Colloidal Nanocrystals[END_REF]. We present early demonstration of mid infrared optical features thanks to the growth of large and poorly confined HgTe CQD. Recently, a next step has been reached on the shape control of HgTe CQD, with the growth of HgTe 2D nanoplatelets [START_REF] Izquierdo | Strongly confined HgTe 2D nanoplatelets as narrow near infrared emitter[END_REF]. Their anisotropic shape brings enhanced optical features with narrower and faster emission. Finally, we discuss the recent progress obtained with self-doped CQD of HgSe. They present additional optical features resulting from intraband absorption, which is tunable from 3 to 20 µm [START_REF] Lhuillier | Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz[END_REF].

DISCUSSION

In order to use CQD in mid IR applications and in particular for photodetection, it is essential to demonstrate optical absorption above 3 µm. This value of 3 µm is typically the limit above which the self-emitting blackbody radiation prevails over the reflection of light coming from a warm source. It also corresponds to the beginning of an atmospheric transparency window, which ranges up to 5 µm and allows long distance thermal imaging.

Among the potential CQD materials to reach 3 µm, mercury chalcogenides (HgTe [START_REF] Kovalenko | Colloidal HgTe nanocrystals with widely tunable narrow band gap energies: from telecommunications to molecular vibrations[END_REF][START_REF] Lhuillier | Optical properties of HgTe colloidal quantum dots[END_REF] and HgSe) with their semimetal bulk nature appear quite promising [START_REF] Allan | Tight-binding calculations of the optical properties of HgTe nanocrystals[END_REF]. In addition, their proximity with cadmium chalcogenides make them benefit from all the developed experience on CQD synthesis over the past 25 years. [START_REF] Keuleyan | Mid-infrared HgTe colloidal quantum dot photodetectors[END_REF] b is the associated absorption spectrum with a broad absorption edge resulting from the limited size control. c TEMimage of HgTe CQD obtained from ref [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF]. The associated optical spectra for different sizes of CQD are shown on part d. e. is a TEM image of te HgTe CQD obtained from ref [START_REF] Keuleyan | Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 μm[END_REF]. The associated optical features for different sizes of HgTe CQD are given in part f.

Figure 1 a TEM image of early HgTe nanocrystals with absorption edge reaching the 3-5 µm range, adapted from ref

The first CQD addressing the 3-5 µm window have been obtained by Keuleyan et al [START_REF] Keuleyan | Mid-infrared HgTe colloidal quantum dot photodetectors[END_REF]who revised the synthesis of HgTe CQD in organic medium. This early material was poorly controlled in term of size, and only the absorption edge was actually reaching this infrared window, see Figure 1a-b. The high level of the material aggregation was quite detrimental to the material cut off wavelength. On the other hand it leads to a strong level of inter dot coupling [START_REF] Lhuillier | Thermal properties of mid infrared Colloidal quantum dot detectors[END_REF] and leads to relatively high carrier mobility in the range of 1 cm 2 V -1 s -1 . In 2012, the synthetic procedure was improved to reduce the aggregation [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF], see Figure 1c. As the nanoparticle diameter gets more defined, the optical features becomes sharper, see Figure 1d. In 2014, the same group proposed another improvement of the HgTe CQD synthesis [START_REF] Keuleyan | Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 μm[END_REF] and demonstrate an improved monodispersity. The absorption spectrum becomes more structured and reaches a similar level of maturity as what can be obtained with CdSe, see Figure 1f. In term of cut off wavelength the current record for HgTe CQD is at 12 µm [START_REF] Keuleyan | Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 μm[END_REF].

In spite of this huge progress, the optical features of HgTe remain limited by the inhomogeneous broadening (ie the CQD size distribution). It was demonstrated by Ithurria et al [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF] with cadmium chalcogenides that 2D nanoplatelets (NPL) can lead to narrower optical features [START_REF] Nasilowski | 2D Colloidal nanocrystals[END_REF][START_REF] Lhuillier | 2D Colloidal Metal Chalcogenides Semiconductors: Synthesis, Spectroscopy and Applications[END_REF] . This improvement actually results from the atomic scale control of this object thickness. 2D NPL present only one confined direction (i.e. the thickness) and the lateral extension span from the Bohr radius up to 1 µm [START_REF] Pedetti | Optimized Synthesis of CdTe Nanoplatelets and Photoresponse of CdTe Nanoplatelets Films[END_REF] and is consequently not confined. The growth mechanism associated with these 2D NPL, make that no roughness is obtained in the confined direction. As a result the linewidth associated with the PL is only limited by homogeneous broadening. So far, the direct synthesis of HgTe NPL has not been reported. Izquierdo et al [START_REF] Izquierdo | Strongly confined HgTe 2D nanoplatelets as narrow near infrared emitter[END_REF] used an indirect path to obtain these NPL. They first synthetized CdTe NPL and in a second step used a cation exchange [START_REF] De Trizio | Forging Colloidal Nanostructures via Cation Exchange Reactions[END_REF][START_REF] Son | Cation exchange reactions in ionic nanocrystals[END_REF][START_REF] Beberwyck | Cation Exchange: A Versatile Tool for Nanomaterials Synthesis[END_REF] procedure to remove the Cd 2+ cations from the NPL and replace them by mercury. The challenge comes from the fact that the 2D shape has to be preserved to avoid either the CQD dissolution or shape reconstruction, see Figure 2b. To do so, a bulky precursor of Hg is prepared by complexing the Hg 2+ ions with long alkane chains. After the cation exchange, the optical features of the CdTe NPL initially in the green shift toward near infrared, see Figure 2a. The obtained materials present a narrow PL of 55 meV for the entire measurement. For the sake of comparison, the other nanocrystal materials (PbS and CuGaInS(e) ) emitting in the same range of wavelength present a linewidth of 100 meV, even for single particle measurements. Another striking optical feature resulting from the 2D shape is the fast PL emission of these NPL. The PL decay has been determined to be around 60 ns. Due to the higher binding energy resulting from the 2D shape. The PL decay is typically 50 times faster than what can be obtained with PbS and CIGS, Transfer curve for a film of HgTe NPL. Adapted from ref [START_REF] Izquierdo | Strongly confined HgTe 2D nanoplatelets as narrow near infrared emitter[END_REF] We further investigate the transport properties of these 2D NPL. To do so, we incorporate the NPL into an electrolytic transistor, where the gate is obtained by ion gel gating [START_REF] Lhuillier | Electrolyte Gated Field Effect Transistor to Probe the Surface Defects and Morphology in Films of Thick CdSe Colloidal Nanoplatelets[END_REF][START_REF] Lhuillier | Electrolyte gated colloidal nanoplatelets based phototransistor and its use for bicolor detection[END_REF]. This geometry allows us to explore large carrier density injection thanks to gate capacitance in the range of some µF.cm -2 and in volume gating volume [START_REF] Lhuillier | Investigating the n and p type electrolytic charging of colloidal nanoplatelets[END_REF]. To build the device, we first exchange the NPL ligands. The initial long capping ligands (oleylamine and oleic acid) are replaced by short S 2-ions [START_REF] Nag | Metal-free Inorganic Ligands for Colloidal Nanocrystals: S 2-, HS -, Se 2-, HSe -, Te 2-, HTe -, TeS3 2-, OH -, and NH2as Surface Ligands[END_REF]. In a second step, the ion gel made of LiClO4 in PEG is deposited on the top of the film. The initial CdTe NPL present a p-type behavior, see Figure 2c whereas after the cation exchange, the HgTe NPL become n-type, see Figure 2d.

To further explore the infrared properties of CQD and push their absorption toward longer wavelength, an interesting approach is to switch from interband transition in narrow band gap material to intraband transition. To observe intraband transition in CQD, some carrier(s) have to be injected within the conduction band. This can be obtained by optical pumping [START_REF] Jeong | Mid-Infrared Photoluminescence of CdS and CdSe Colloidal Quantum Dots[END_REF] or electrochemical injection [START_REF] Shim | N-type colloidal semiconductor nanocrystals[END_REF]. However, these approaches remain poorly convenient for the design of device. Recently, self-doped CQD presenting mid infrared intraband absorption have been obtained with HgS [START_REF] Jeong | Air-Stable n-Doped Colloidal HgS Quantum Dots[END_REF] and HgSe [START_REF] Lhuillier | Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz[END_REF][START_REF] Deng | Colloidal Quantum Dots Intraband Photodetectors[END_REF]. We proposed an improved synthesis for HgSe with a wider range of size [START_REF] Lhuillier | Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz[END_REF]. The typical absorption spectrum of the HgSe CQD presents two main contributions. In the near IR, we observe a broad absorption relative to interband absorption and in the mid and far infrared, we observe a peak relative to the intraband transition. Transport measurements, again conducted in an ion gel transistor configuration, have confirmed this doping and demonstrate its n-type nature (ie electron doping) [START_REF] Lhuillier | Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz[END_REF][START_REF] Robin | Surface Control of Doping in self-doped Nanocrystals[END_REF]. In this case we rather use As2S3 as capping ligand because of its low absorption in the mid infrared and also because of the previously demonstrated capability of this ligand to generate high mobility and photoresponsivity [START_REF] Lhuillier | Mid-Infrared HgTe/As2S3 FETs and photodetectors[END_REF][START_REF] Yakunin | High Infrared Photoconductivity in Films of Arsenic-Sulfide-Encapsulated Lead-Sulfide Nanocrystals[END_REF]. We demonstrate the possibility to tune the HgSe CQD size from 4 to 50 nm while tuning the Se precursor (Se complexed with trioctylphosphine or SeS2), reaction duration and temperature (70°C-120°C). As a result, the intraband peak energy can be tuned from 3 to 20 µm, see Figure 3b andd. The obtained material presents no aggregation, as shown on the TEM image, see Figure 3c. The origin of the self-doping was attributed to a reduction of the HgSe by its environment because of the large work function of the bulk [START_REF] Nethercot | Prediction of Fermi Energies and Photoelectric Thresholds Based on Electronegativity Concepts[END_REF] and limited interband band edge energy. This brings the conduction band below the O2/H2O redox couple and the negatively charged CQD becomes the stable form. We demonstrate that the doping magnitude can be tuned by charging the capping ligands [START_REF] Robin | Surface Control of Doping in self-doped Nanocrystals[END_REF], we typically observe that the carrier density can be tuned by a factor 10 from 2 carriers per dot with dodecanetiol capping down to 0.2 carrier per dot with S 2-capping ions.

CONCLUSION

Significant progress has been obtained on the synthesis of mercury chalcogenides CQD with infrared optical absorption. Since the early nanocrystals absorbing above 3 µm in 2011, 2D materials have been proposed and synthetized with a strategy to achieve narrower optical features. Compared to previously existing nanocrystals, the 2D NPL of HgTe have enhanced PL performances with PL a narrower, faster and greater efficiency (about 10%).

Concerning the mid infrared absorption, the synthesis of self-doped nanocrystals made of HgS and HgSe, have for the first time allows the observation of intraband features in the mid IR. It addresses the atmospheric transparency window, without the constrain of optical or electrochemical pumping. This interband absorption is tunable up to the THz range. The possibility to use intraband transition opens new possibilities for the design of infrared CQD based device.

Figure 2 a

 2 Figure 2 a. Absorption spectra of CdTe and HgTe NPL. b. TEM image of HgTe NPL. c. Transfer curve (drain and gate current as a function of gate bias) for CdTe NPL film measured in an electrolytic transistor configuration at room temperature. d.Transfer curve for a film of HgTe NPL. Adapted from ref[START_REF] Izquierdo | Strongly confined HgTe 2D nanoplatelets as narrow near infrared emitter[END_REF]

Figure 3 a

 3 Figure 3 a. Absorption spectrum for two sizes of HgSe CQD. b. Zoom on the intraband contribution of the absorption spectra of HgSe CQD for four sized of HgSe CQD. c. TEM image of HgSe CQD. d. Energy dependence of the intraband transition as a function of the CQD size. Adapted from ref [6].

ACKNOWLEDGEMENTS

This work was supported by French state funds managed by the ANR within the Investissements d'Avenir program under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE led by Sorbonne Universités. SI thanks the ANR for funding through the Nanodose grant.