Confined Acoustic Phonons in Colloidal Nanorod Heterostructures Investigated by Nonresonant Raman Spectroscopy and Finite Elements Simulations

Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials. A prominent material for light emitting applications are CdSe/CdS core–shell dot-in-rods. Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by nonresonant Raman spectroscopy with laser excitation energy lower than their bandgap. With high signal-to-noise ratio in the frequency range from 5–50 cm–1, we reveal distinct Raman bands that can be related to confined extensional and radial-breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature of the acoustic vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an oscillation of a CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architectu..

Broadband amplified spontaneous emission and random lasing from wurtzite CdSe/CdS 'giant-shell' nanocrystals

Colloidal nanocrystals (NCs) are attractive materials for light-emitting applications thanks to their flexible synthesis, size-dependent properties, and bright emission. Yet, colloidal NCs still present a narrow gain band (full-width half maximum around 10 nm), which limits their application to single-color lasers. Widening of the gain band by specifically engineered NCs can further improve the prospect of this class of materials toward the fabrication of solution-processed white-emitting or color-tunable lasers. Here, we report broadband amplified spontaneous emission (ASE) from wurtzite CdSe/CdS "giant-shell" nanocrystals (g-NCs) with an unprecedented large core up to 7.5 nm in diameter that were synthesized through a continuous injection route. The combination of large core and shell enables ASE from different CdSe optical transitions as well as from the CdS. Importantly, thin films of g-NCs with a large CdSe core (7.5 and 5.1 nm in diameter) show ASE at different colors with a similar threshold, indicating that light emission amplification can be achieved from different optical transitions simultaneously. Tuning of the core diameter allows obtaining ASE in a wide spectral range, and blending of g-NCs with different core sizes gives rise to a continuous amplified spontaneous emission band from green to red (510 to 650 nm). Drop-cast films of CdSe/CdS g-NCs demonstrate simultaneous dual-color random lasing under nanosecond-pulsed excitation

Directional Fluorescence Spectral Narrowing in All-Polymer Microcavities Doped with CdSe/CdS Dot-in-Rod Nanocrystals

We report on the fluorescence properties of high optical quality all-polymer planar microcavities embedding core 12shell dot-in-rod CdSe/CdS nanocrystals. Properly tuned microcavities allow a 10-fold sharpening of the nanocrystals fluorescence spectrum, resulting in a reduction of the bandwidth from 24 to 2.4 nm, which corresponds to a quality factor larger than 250. A 5-fold peak photoluminescence intensity enhancement is measured, while the overall number of emitted photons is reduced. Time-resolved photoluminescence and quantum yield for microcavities and suitable references show the presence of two decays related to differences in nanocrystal size distribution. The slower decay rate, which becomes faster when the nanocrystals are embedded into the microcavity, is assigned to longer nanorods with emission spectrally overlapped to the cavity mode. Conversely, the short-living component, which is assigned to an impurity of shorter nanorods, remains unaffected by the microcavity

A novel approach to noisy gates for simulating quantum computers

We present a novel method for simulating the noisy behaviour of quantum computers, which allows to efficiently incorporate environmental effects in the driven evolution implementing the gates acting on the qubits. We show how to modify the noiseless gate executed by the computer to include any Markovian noise, hence resulting in what we will call a noisy gate. We compare our method with the IBM Qiskit simulator, and show that it follows more closely both the analytical solution of the Lindblad equation as well as the behaviour of a real quantum computer, where we ran algorithms involving up to 18 qubits; as such, our protocol offers a more accurate simulator for NISQ devices. The method is flexible enough to potentially describe any noise, including non-Markovian ones. The noise simulator based on this work is available as a python package at this link: https://pypi.org/project/quantum-gates

Properties of gravitoturbulent accretion disks

We explore the properties of cold gravitoturbulent accretion disks - non-fragmenting disks hovering on the verge of gravitational instability - using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular momentum transport in a thin accretion disk and the disk cooling in a steady state. Assuming that opacity is dominated by dust we are able to self-consistently derive disk properties for a given $\dot M$ assuming marginal gravitational stability. We also allow external irradiation of the disk and account for a non-zero background viscosity which can be due to the MRI. Spatial transitions between different co-existing disk states (e.g. between irradiated and self-luminous or between gravitoturbulent and viscous) are described and the location of the boundary at which disk must fragment is determined in a variety of situations. We demonstrate in particular that at low enough $\dot M$ external irradiation stabilizes gravitoturbulent disk against fragmentation all the way to infinity thus providing means of steady mass transport to the central object. Implications of our results for the possibility of planet formation by gravitational instability in protoplanetary disks and star formation in the Galactic Center and for the problem of feeding supermassive black holes in galactic nuclei are discussed.Comment: 12 pages, 3 figures, submitted to Ap

fully solution processed conductive films based on colloidal copper selenide nanosheets for flexible electronics

A novel colloidal synthesis of copper selenide nanosheets (NSs) with lateral dimensions of up to 3 μm is developed. This material is used for the fabrication of flexible conductive films prepared via simple drop-casting of the NS dispersions without any additional treatment. The electrical performance of these coatings is benchmarked against copper selenide spherical nanocrystals (SNCs) in order to demonstrate the advantage of 2D morphology of the NSs for flexible electronics. In this contest, Cu2−xSe SNC films exhibit higher conductivity but lower reproducibility due to the formation of cracks leading to discontinuous films. Furthermore, the electrical properties of the films deposited on different flexible substrates following their bending, stretching and folding are studied. A comparison of Cu2−xSe SNC and CuSe NS films reveals an increased stability of the CuSe NS films under mechanical stress applied to the samples and their improved long-term stability in air

Two Massive, Low-Luminosity Cores Toward Infrared Dark Clouds

This article presents high-resolution interferometric mosaics in the 850 micron waveband of two massive, quiescent infrared dark clouds. The two clouds were chosen based on their likelihood to represent environments preceding the formation of massive stars. The brightest compact sources detected in each cloud have masses of approximately 110 and 60 solar masses with radii < 0.1 pc, implying mean volume densities of approximately 1 million particles per cubic centimeter and mean column densities of about 1 gram per square centimeter. Supplementary data show these cores to be cold and inactive. Low upper limits to their bolometric luminosities and temperatures place them at a very early stage of evolution while current models of massive star formation suggest they have the potential to form massive stars.Comment: 6 pages, 5 figures. Accepted for publication by the Astrophysical Journa

Motile bacteria leverage bioconvection for eco-physiological benefits in a natural aquatic environment

IntroductionBioconvection, a phenomenon characterized by the collective upward swimming of motile microorganisms, has mainly been investigated within controlled laboratory settings, leaving a knowledge gap regarding its ecological implications in natural aquatic environments. This study aims to address this question by investigating the influence of bioconvection on the eco-physiology of the anoxygenic phototrophic sulfur bacteria community of meromictic Lake Cadagno.MethodsHere we comprehensively explore its effects by comparing the physicochemical profiles of the water column and the physiological traits of the main populations of the bacterial layer (BL). The search for eco-physiological effects of bioconvection involved a comparative analysis between two time points during the warm season, one featuring bioconvection (July) and the other without it (September).ResultsA prominent distinction in the physicochemical profiles of the water column centers on light availability, which is significantly higher in July. This minimum threshold of light intensity is essential for sustaining the physiological CO2 fixation activity of Chromatium okenii, the microorganism responsible for bioconvection. Furthermore, the turbulence generated by bioconvection redistributes sulfides to the upper region of the BL and displaces other microorganisms from their optimal ecological niches.ConclusionThe findings underscore the influence of bioconvection on the physiology of C. okenii and demonstrate its functional role in improving its metabolic advantage over coexisting phototrophic sulfur bacteria. However, additional research is necessary to confirm these results and to unravel the multiscale processes activated by C. okenii’s motility mechanisms

Photonic Crystals for Plasmonics: From Fundamentals to Superhydrophobic Devices

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