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In the early 2000s, the commercialization of CMOS image sensors led to smaller and smaller—and cheaper and cheaper—digital cameras. Now the thinnest of mobile phones contains at least two camera modules, and all except the most dedicated photographers have stopped carrying a separate camera, concluding that the camera sensors in their phones take pictures that are good enough.

Colloidal semiconductor nanocrystals are important building blocks for low-cost, solution-processed electronic devices with tunable functionalities. Considerable progress is made in improving charge transport through nanocrystal films by exchanging long insulating ligands with shorter passivating ligands. To take full advantage of this strategy, it is equally important to fabricate close-packed structures that reduce the average interparticle spacing. Yet it remains a challenge to retain long-range, close-packed order after ligand exchange. Here, a novel one-step in situ ligand-exchange method is demonstrated that enables rapid (5 min) ligand exchange of nanocrystal films, which are more than 50 layers thick. Using this simple and efficient method, it is shown that the face-centered cubic ordering of 500 nm thick PbSe nanocrystal films is retained after ligand exchange from oleic acid to benzoic acid. Moreover, it is demonstrated that PbSe nanocrystal photodetectors with a well-ordered structure have superior optoelectronic properties compared to disordered films; ordered films have a 16× higher responsivity of ≈0.25 A W-1 at 1 V and a 2× faster response time. As far as it is known, this is the first report to realize a rapid one-step ligand exchange through a thick superlattice film with retention of long-range order.

International audience; Two-dimensional colloidal nanoplatelets (NPLs), owing to the atomic-level control of their confined direction (i.e., no inhomogeneous broadening), have demonstrated improved photoluminescence (PL) line widths for cadmium chalcogenide-based nanocrystals. Here we use cation exchange to synthesize mercury chalcogenide NPLs. Appropriate control of reaction kinetics enables the 2D morphology of the NPLs to be maintained during the cation exchange. HgTe and HgSe NPLs have significantly improved optical features compared to existing materials with similar band gaps. The PL line width of HgTe NPLs (40 nm full width at half-maximum, centered at 880 nm) is a factor of 2 smaller than typical PbS nanocrystals (NCs) emitting at the same wavelength. The PL has a lifetime of 50 ns, almost 2 orders of magnitude shorter than small PbS colloidal quantum dots (CQDs), and a quantum yield of ∼10%, almost 2 orders of magnitude shorter than small PbS colloidal quantum dots (CQDs). These materials are promising for a large variety of applications spanning from telecommunications to the design of colloidal topological insulators.

We report the synthesis of high-quality single-crystal two-dimensional, layered nanostructures of silicon telluride, Si2Te3, in multiple morphologies controlled by substrate temperature and Te seeding. Morphologies include nanoribbons formed by VLS growth from Te droplets, vertical hexagonal nanoplates through vapor–solid crystallographically oriented growth on amorphous oxide substrates, and flat hexagonal nanoplates formed through large-area VLS growth in liquid Te pools. We show the potential for doping through the choice of substrate and growth conditions. Vertical nanoplates grown on sapphire substrates, for example, can incorporate a uniform density of Al atoms from the substrate. We also show that the material may be modified after synthesis, including both mechanical exfoliation (reducing the thickness to as few as five layers) and intercalation of metal ions including Li+ and Mg2+, which suggests applications in energy storage materials. The material exhibits an intense red color corresponding to...

International audience; We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material,. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping of the HgTe nanoplatelets by S 2-ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 µs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100 ns to 500 ns range, which suggests that the current performances are rather limited by geometrical factors.

The photoluminescence quantum yield of HgTe colloidal quantum dots is measured from 1800 to 6500 cm–1. There is a steep drop to low energy reminiscent of the generic gap law. However, direct evidence of energy transfer to the C–H stretch and overtone vibrations is apparent when temperature tunes the PL wavelength of a given sample through the vibrational resonances. Calculations based on the radiative rate and resonant energy transfer to the ligand vibrations appear to account for much of the quantum yield drop. Power-dependent photoluminescence lifetime measurements on 3.7 nm particles show fast, ∼50 ps, biexciton lifetime similar to other colloidal quantum dot systems of similar sizes.

International audience; Colloidal quantum dots presenting optoelectronic properties into the mid-infrared are reviewed with an emphasis on HgTe. Interband transitions with narrow band gap material and intraband transitions with wide band gap semiconductors can both address the infrared range of wavelengths. Semimetals are particularly promising since, by controlling the particle size, a gap can in principle be opened and tuned through the full infrared spectrum. HgTe has been quite successful recently, and colloidal synthesis has allowed the tuning of the absorption edge from 1 and 5 µm at room temperature. The cutoff wavelength and absorption coefficient of these materials have been discussed. Electrochemistry experiments have demonstrated that both n and p nanoparticles are stable. Films of the HgTe colloidal quantum dots have shown room temperature photoconduction when excited in the mid-IR. With novel processing leading to improved detectivity, these materials are becoming of significant interest as an alternative to the much more expensive epitaxial detector technologies.

Les dispositifs optoelectroniques bases sur les semi-conducteurs III-V et II-VI sont actuellement realises selon une approche top-down. En depit de leurs performances, certaines de ces structures sont proches de leur limite quantique, ce qui rend l'optimisation de la couche active difficile. Une alternative interessante a ces mate-riaux vient de l'approche colloidale qui permet a la fois une reduction drastique des couts tout en conservant un niveau de performance raisonnable. Ces materiaux sont actuellement utilises du proche ultraviolet jusqu'au proche infrarouge en tant que marqueurs biologiques, diodes electroluminescentes ou pour le photo-voltaique. L'utilisation de materiaux semi-metalliques tels le HgTe avec des longueurs d'onde de coupure dans la gamme 3 a 5 μm ouvre la voie a de nouvelles applications pour ces nanoparticules, en parti-culier en imagerie infrarouge. Dans cet article nous nous attachons a montrer que les principales objections a l'encontre des materiaux colloidaux peuvent etre refu-tees grâce a un controle precis de la chimie de surface.

Films of colloidal quantum dots of HgTe can be charged n- and p-type by electrochemistry. Infrared spectroscopy of the charged dots reveals interband bleach and intraband absorption. Similar mobilities, up to 0.1 cm2/(V s), are observed for both carriers, but p-type films show larger photocurrent while n-type films show larger magnetoresistance.

A basic and affordable experimental apparatus is described that measures the static contact angle of a liquid drop in contact with a solid. The image of the drop is made with a simple digital camera by taking a picture that is magnified by an optical lens. The profile of the drop is then processed with ImageJ free software. The ImageJ contact angle plugin detects the edge of the drop and fits its profile to a circle or an ellipse. The tangent to the triple line contact is calculated and drawn by the ImageJ software, thus, returning the value of the contact angle with acute precision on the measurement.

HgS nanocrystals show a strong mid-infrared absorption and a bleach of the near-infrared band edge, both tunable in energy and reversibly controlled by exposure to solution ions under ambient conditions. The same effects are obtained by applying a reducing electrochemical potential, confirming that the mid-infrared absorption is the intraband transition of the quantum dot. This is the first time that stable carriers are present in the quantum state of strongly confined quantum dot in ambient conditions. The mechanism by which doping is achieved is attributed to the rigid shifts of the valence and conduction band with respect to the environment, similar to the sensitivity of the work function of surfaces to adsorbates.

We report the synthesis of high-quality single-crystal two-dimensional, layered nanostructures of silicon telluride, Si2Te3, in multiple morphologies controlled by substrate temperature and Te seeding. Morphologies include nanoribbons formed by VLS growth from Te droplets, vertical hexagonal nanoplates through vapor-solid crystallographically oriented growth on amorphous oxide substrates, and flat hexagonal nanoplates formed through large-area VLS growth in liquid Te pools. We show the potential for doping through the choice of substrate and growth conditions. Vertical nanoplates grown on sapphire substrates, for example, can incorporate a uniform density of Al atoms from the substrate. We also show that the material may be modified after synthesis, including both mechanical exfoliation (reducing the thickness to as few as five layers) and intercalation of metal ions including Li(+) and Mg(2+), which suggests applications in energy storage materials. The material exhibits an intense red color corresponding to its strong and broad interband absorption extending from the red into the infrared. Si2Te3 enjoys chemical and processing compatibility with other silicon-based material including amorphous SiO2 but is very chemically sensitive to its environment, which suggests applications in silicon-based devices ranging from fully integrated thermoelectrics to optoelectronics to chemical sensors.

HgTe colloidal quantum dots are synthesized with high monodispersivity with sizes up to ∼15 nm corresponding to a room temperature absorption edge at ∼5 μm. The shape is tetrahedral for larger sizes and up to five peaks are seen in the absorption spectra with a clear size dependence. The size range of the HgTe quantum dots is extended to ∼20 nm using regrowth. The corresponding room temperature photoluminescence and absorption edge reach into the long-wave infrared, past 8 μm. Upon cooling to liquid nitrogen temperature, a photoconductive response is obtained in the long-wave infrared region up to 12 μm. Configuration-interaction tight-binding calculations successfully explain the spectra and the size dependence. The five optical features can be assigned to sets of single hole to single electron transitions whose strengths are strongly influenced by the multiband/multiorbital character of the quantum-dot electronic states.

International audience; Colloidal quantum dots (CQDs) with mid-infrared (IR) tunable bandgaps present a new paradigm for mid-IR materials, but advances in charge transport and collection are needed for practical use in electronic applications. By replacing the organic ligands in films of HgTe CQDs with As 2 S 3 , a problem of oxidation in air is avoided and charge carrier mobilities are improved 100 fold compared to the standard organic ligand exchange. The composite inorganic HgTe/As 2 S 3 material allows for ambipolar field effect transistors with on-off ratios up to 10 7 , and photodetectors with high sensitivity, reaching 3x10 10 Jones at 230 K with a 3.5 micron cutoff wavelength.

HgTe colloidal quantum dot have been demonstrated for mid-infrared photoconduction. The potential and challenges associated to the materials are discussed.

International audience; Les dispositifs optoélectroniques basés sur les semi-conducteurs III-V et II-VI sont actuellement réalisés selon une approche top-down. En dépit de leurs performances, certaines de ces structures sont proches de leur limite quantique, ce qui rend l'optimisation de la couche active difficile. Une alternative intéressante à ces maté-riaux vient de l'approche colloïdale qui permet à la fois une réduction drastique des coûts tout en conservant un niveau de performance raisonnable. Ces matériaux sont actuellement utilisés du proche ultraviolet jusqu'au proche infrarouge en tant que marqueurs biologiques, diodes électroluminescentes ou pour le photo-voltaïque. L'utilisation de matériaux semi-métalliques tels le HgTe avec des longueurs d'onde de coupure dans la gamme 3 à 5 μm ouvre la voie à de nouvelles applications pour ces nanoparticules, en parti-culier en imagerie infrarouge. Dans cet article nous nous attachons à montrer que les principales objections à l'encontre des matériaux colloïdaux peuvent être réfu-tées grâce à un contrôle précis de la chimie de surface.

International audience; The use of colloidal material offers an interesting alternative to top down approaches for the realization of low cost infrared detectors. We demonstrate photoconduction in thin films of a colloidal material in the mid-infrared (up to 7 μm), using HgTe colloidal quantum dots. Thin films of the colloidal quantum dots have a large absorption coefficient (>104 cm−1), and the photoconductive response is dramatically improved by encapsulating the nanoparticle into an inorganic matrix of As2S3. Such devices show fast response and large detectivity (>1010 jones) at temperatures above 200 K.

The transport and thermal properties of HgTe colloidal quantum dot films with cut-off wavelengths in the mid-IR are investigated. The cut-off wavelength of this material can be tuned over the 3-5 \mu m range, which makes it a promising alternative to existing high cost detectors. Post deposition processes such as ligand exchange and atomic layer deposition are investigated as a way to increase the carrier mobility., Comment: 7 pages, 7 figures

HgTe colloidal quantum dots are prepared via a simple two-step injection method. Absorption and photodetection with sharp edges, as well as narrow photoluminescence, are tunable across the near and mid-IR between 1.3 and 5 μm.

International audience; We report on HgTe colloidal-based quantum-dot mid-infrared (IR) photode-tectors. We demonstrate tuning the cutoff wavelength of this detector from the near IR up to 7 lm. Responsivity values of films of HgTe nanoparticles a few hundred nanometers thick are in the 100 mA W À1 range even at room temperature. The detectivity of this film is limited by large 1/f noise. Time responses as short as 200 ns are achieved.

Researchers show that thin films containing HgTe quantum dots with diameters of around 10 nm exhibit a photoresponse in the mid-infrared that extends to wavelengths as long as 5 µm. Such films could become the basis of a new form of low-cost mid-infrared photodetector.

International audience; Room temperature photodetection with HgTe colloidal quantum films is reported between 2 and 5 µm for particles of sizes between ~5 and ~12 nm diameter, and photodetection extends to 7 µm at 80 K. The size-tuning of the absorption of HgTe colloidal quantum dots, their optical cross section and the infrared absorption depth of films are measured. The tuning with radius is empirically given by ${\lambda }_{\mathrm{BE}}^{\mathrm{QD}}=\frac{30.5}{\sqrt{1+(43/{R}_{\mathrm{QD}})^{2}}-1}$, where R is in nm. The optical cross section of the colloidal dots at 415 nm is approximately proportional to their volume and given by ${\sigma }_{\mathrm{Hg}}^{415}=2.6\pm 0.4\times 1{0}^{-17}~{\mathrm{cm}}^{2}/\mathrm{mercury}$ atom. The size-dependent optical cross section at the band edge ~1.5 × 10−15 cm2 is consistent with the expected oscillator strength of the quantum dots. The absorption depth of HgTe colloidal dot films is short, about 1–2 µm, which is an advantage for thin film devices. These properties agree rather well with the expectation from the k ⋅ p model. HgTe colloidal quantum dot thin films show a strong tuning with temperature with a large positive thermal shift between 0.4 and 0.2 meV K−1, decreasing with decreasing size within the size range studied and this is attributed primarily to electron–phonon effects.

International audience; HgTe colloidal quantum dot films are studied for photodetection over the 3–5 lm atmospherictransparency window. The temperature dependence of the conductivity indicates that the materialbehaves approximately as an intrinsic semiconductor. In photoconduction, the responsivity can beas high as several hundred mA W1 at room temperature. The dark current presents 1/f noise whichis larger than that for homogeneous conductors, and this noise decreases with temperature.A specific detectivity of 2109 Jones is obtained for a sample with a 6 lm cut-off wavelength at130 K. These values are obtained for the thickest films studied (400 nm) and whose thicknessesare still much less than the optical absorption length. The time response can be faster than 100 ns.

International audience; Self-doped colloidal quantum dots (CQDs) attract a strong interest for the design of a new generation of low-cost infrared (IR) optoelectronic devices because of their tunable intraband absorption feature in the mid-IR region. However, very little remains known about their electronic structure which combines confinement and an inverted band structure, complicating the design of optimized devices. We use a combination of IR spectroscopy and photoemission to determine the absolute energy levels of HgSe CQDs with various sizes and surface chemistries. We demonstrate that the filling of the CQD states ranges from 2 electrons per CQD at small sizes (