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Optoelectronic of narrow band gap nanocrystals

Periodic Reporting for period 2 - blackQD (Optoelectronic of narrow band gap nanocrystals)

Reporting period: 2019-08-01 to 2021-01-31

Colloidal nanocrystals are promissing building blocks for optoelectronic devices. They have encouter a large sucess as downconverter for display and now offer great promisses for the design of low cost infrared sensors.
Current technologies dedicated to infrared and more particularly for infrared sensing present a prohibitive cost which prevent the mass market use of such technologies. As a result infrared remains to date limited to defense and astronomy, while if lower cost devices were available, other applications such as self driven vehicles and industrial vision will also be enable.
The blackQD aims to push this technology. To ensure the sucess of the project, all aspects from material science to final device are included in the workframe. The three pilars of the project are (i) better material, (ii) deeper material understanding and (iii) better device.
Regarding material, the project will mostly focus on narrow band gap materials such as PbS and HgTe. Identified material chalenges are the following : improvement of the material surface chemistry to improve stability and transport properties, screening for alternatives to heavy metal and finally control of doping.
Because the design of better device require a deep knowledge of the material. blackQD dedicates a significant effort to the basic investigation of the electronic properties (work function, doping level, band alignement and band bending) of the targeted active material. Systematic characterization of the material as a function of composition size, composition and surface chemistry will be conducted. The electronic transport properties will also be investigated and in particular under the perspective of doping control thanks to gate.
The last part of the project is dedicated to expand the design toolbox of infrared nanocrystal based sensor. Specific geometries will be targeted to adress question such as dark current reduction, achieve fast time response or enhance charge dissociation.
The final goal of the project is to achieve a device operated in the short wave and mid wave infrared comptible with room temperature operation and which performance overcome the one of current bolometer.
Project is organized along three workpackages : (i) better material, (ii) deeper material understanding and (iii) better device. in the following i have selected some key results for each work packages

*Development of the first THz absorbing nanocrystals : Even though the project is mostly focused on the short (1.5-3 µm) and mid wave infrared (3-5µm), it is of utmost interst to test the potential of the material for even longer wavelength. Over the pas decade there was a chalenge to push the absorption of nanocrystal toward longer wavelength as it enable new type of application (thermal imaging for mid infrared). We have develloped a new synthetic procedure to grow large (weakly confined) HgTe nanocrystals. Thanks to this procedure absorption of HgTe nanocystal film reaches up to 60 µm peak and up to 200 µm for the cut-off wavelength. This is by far the reddest values reported. This result has lead to a patent application (Far infrared and THz nanocrystals and uses of thereof, FR 17 59276 (2017), E. Lhuillier, N. Goubet, Y.-P. Lin, FR 17 59276) and to a publication Terahertz HgTe nanocrystals: beyond confinement, N. Goubet, A. Jagtap, C. Livache, B. Martinez, H. Portales, X. Zhen Xu, R.P.S.M. Lobo, B. Dubertret, E. Lhuillier, J. Am. Chem. Soc. 140, 5053 (2018).
*Development of new synthetic process compatible with mass scale production : Current best performing nanocrystals for IR sensing are based on lead and mercury chalcogenides which raises some obvious toxicity issue. The synthetic procedure needs to minimize work foce exposure to heavy metal. Current synthesis were actually very dilute which generate cost (solvent and facilities) and increase tne number of synthesis to conduct to obtain a given amount of material. We have work on a new procedure, where the precursor of Hg is liquid Hg. This synthesis is 10-100 times more concentrated than any previous procedure. This result has led to a patent application Method of preparation of nanoparticles using mercury thiolate compounds, E. Lhuillier, N. Goubet, EP 19 306 130.6. and to a publication Near to Long Wave Infrared Mercury Chalcogenide Nanocrystals from Liquid Mercury, N. Goubet, M. Thomas, C. Gréboval, A. Chu, J. Qu, P. Rastogi, S.-S. Chee, M; Goyal, Y. Zhang, X. Z. Xu, G. Cabailh, S. Ithurria, E. Lhuillier, J Phys Chem C 124, 8423 (2020)
*Development of new surface chemistry to enable the formation of thick strongly photoconductive film. The conventional way to prepare film of nanocrystal relies on the deposition of a film which is then followed a liogand exchange step. Her ewe have develloped an ink which ease the formation of thick film (up to 600 nm) while being compatioble with large thin film mobilty . This result has been publised in HgTe Nanocrystal Inks for Extended Short Wave Infrared Detection, B. Martinez, J. Ramade, C. Livache, N. Goubet, A. Chu, C. Gréboval, J. Qu, W. L. Watkins L. Becerra, E. Dandeu, J.-L. Fave, C. Méthivier, E. Lacaze, E. Lhuillier, Adv Opt Mat 7, 1900348 (2019)

*Regarding the electronic structure of the material, we have systematically characterize the band alignment of HgTe and HgSe as a function of size and surface chemistry. Most of these measurements have been conducted using photoemission beamline on Soleil synchrotron. These results have led to several publications Nano Lett 17, 4067 (2017), ACS Appl. Mater. Interfaces 9, 36173 (2017), J. Phys. Chem. C 122, 859–865 (2018), ACS Phot. 5, 4569 (2018), ACS Appl. Mater. Interfaces 11, 33116 (2019), Nanoscale 11, 3905 (2019)

*Regarding the transport properties I has identified the control of carrier density as a key parameter. As a result, i wanted to develop new strategies to gate nanocrystal film. We have investigated in particular the potential of ionic glass to obtain high capacitance gating. These results have led to several publications Nano Lett 19, 3981 (2019), Adv Func Mat 29, 1902723 (2019), ACS Nano 14, 4567 (2020)

*Finally on the device side, we have explored new device geometries by proposing new layers for the design of photovoltaic devise based on interband (ACS Phot. 5, 4569 (2018)) or intraband transition (Nature Comm 10, 2125 (2019)). We have also tested the potential of the coupling of the HgTe nanocrystal to 2D material such as graphene used as infrared transparent electrodes leading to enhanced charge dissociation once coupled with an ionic glass gate (ACS Nano 14, 4567 (2020))
On the material side, we have developed the reddest nanocrystals reported and are currently the only group to have reported nanomaterial with absorption above 10 µm

We have reported the first SWIR focal plane array based on HgTe with room temperature operation and we will work in optimizing the deposition process and go toward full characterization of the component

During the first 30months of the project, we have pushed the understanding of the material and now aim to use this knowledge to design new device architecture. In particular we are working on a new device scheme at the nanoscale in order for the device size to match the diffusion length, this direction is also promising to reduce dark current since the latter is proportional to the electrically active volume
Infrared image obtained from HgTe nanocrystal based focal plane array