3 research outputs found
Plasmon Spectroscopy and Chemical Structure of Small Bimetallic Cu<sub>(1ā<i>x</i>)</sub>Ag<sub><i>x</i></sub> Clusters
The
optical properties of small CuāAg bimetallic clusters
have been experimentally and theoretically investigated in relation
to their chemical structure analyzed by high resolution transmission
electron microscopy (HRTEM). Cu <sub>(1ā<i>x</i>)</sub>Ag<sub><i>x</i></sub> clusters of about 5 nm in diameter
are produced in a laser vaporization source with a well-defined stoichiometry
(<i>x</i> = 0, 25, 50, 75, and 100%) and dispersed in an
alumina matrix. Absorption spectra are dominated by a broad and strong
surface plasmon resonance whose shape and location are dependent on
both cluster composition and sample aging. Detailed modeling and systematic
calculations of the optical response of pure and oxidized mixed clusters
of various chemical structures have been carried out in the framework
of classical and semiquantal formalisms. Optical and HRTEM measurements
combined with theoretical predictions lead to the conclusion that
these bimetallic clusters are not alloyed at the atomic scale but
rather present a segregation of chemical phases. Most likely, they
adopt a Cu@Ag coreāshell configuration. Moreover, the nanoparticle
oxidation process is consistent with the formation of a copper oxide
layer by dragging out inner copper atoms to the cluster surface
Intraband Mid-Infrared Transitions in Ag<sub>2</sub>Se Nanocrystals: Potential and Limitations for Hg-Free Low-Cost Photodetection
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
Ag<sub>2</sub>Se nanoparticles presenting intraband transition between
3 and 15 Ī¼m. With photoemission and infrared spectroscopy, we
are able to propose an electronic spectrum of the material in the
absolute energy scale. We also investigate the origin of doping and
demonstrate that it results from a cation excess under the Ag<sup>+</sup> 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
Material Perspective on HgTe Nanocrystal-Based Short-Wave Infrared Focal Plane Arrays
After the use of nanocrystals as light downconverters,
infrared
sensing appears to be one of the first market applications where they
can be used while being both electrically and optically active. Over
recent years, tremendous progress has been achieved, leading to an
apparent rise in the technological-readiness level (TRL). So far,
the efforts have been focused on PbS nanocrystals for operation in
the near-infrared. Here, we focus on HgTe since its narrower band
gap offers more flexibility to explore the extended short-wave and
midwave infrared. We report a photoconductive strategy for the design
of short-wave infrared focal plane arrays with enhanced image quality.
An important aspect often swept under the rug at an early stage is
the material stability. It appears that HgTe remains mostly unaffected
by oxidation under air operation. The evaporation of Hg, a potentially
dramatic aging process, only occurs at temperatures far beyond the
focal plane arrayās standard working temperature. The main
bottleneck appears to be the particle sintering resulting from joule
heating of focal plane arrays. This suggests that a cooling system
is required, whose first role is to prevent the material from sintering
even before targeting dark current reduction