Infrared photodetection based on colloidal nanoparticles is a promising path toward low cost devices. However, mid-infrared absorption relies on interband transitions in heavy metal-based materials, which is a major flaw for the development toward mass market. In the quest of mercury-free infrared active colloidal materials, we here investigate Ag2Se nanoparticles presenting intraband transition between 3 and 15 µm. With photoemission and infrared spectroscopy, we are able to propose an

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... ic spectrum of the material in absolute energy scale. We also investigate the origin of doping and demonstrate that it results from a cation excess under Ag + form. We demonstrate photoconduction into this material under resonant excitation of the intraband transition. However, performances are currently quite weak with (i) a slow photoresponse (several seconds), and (ii) some electrochemical instabilities at room temperature. To whom correspondence should be sent: el@insp.upmc.fr