• Energy Research

We introduce an optical parametric amplifier, pumped by an amplified femtosecond Yb:KGW laser, which directly generates broadly tunable mid-infrared (MIR) pulses, covering the whole vibrational spectrum from 3 to 10 µm. The avoidance of the traditional difference-frequency generation stage to access the MIR range simplifies the setup while enabling high conversion efficiencies. The two-stage design employs in the second stage either periodically poled lithium niobate, optimized for the CH/OH stretching region (3-5 µm) or LiGaS2, which allows extending the tunability to the fingerprint region (up to 10 µm). We anticipate applications of this versatile source to ultrafast vibrational spectroscopy and infrared microscopy.

Dibenzo[a,m]dinaphtho[ef,hi]coronene with zigzag and fjord edges was synthesized and characterized, demonstrating a nonplanar structure with near-infrared stimulated emission with a relatively long lifetime and dual-amplified spontaneous emission.

AbstractVertical heterostructures of transition metal dichalcogenides (TMDs) host interlayer excitons with electrons and holes residing in different layers. With respect to their intralayer counterparts, interlayer excitons feature longer lifetimes and diffusion lengths, paving the way for room temperature excitonic optoelectronic devices. The interlayer exciton formation process and its underlying physical mechanisms are largely unexplored. Here we use ultrafast transient absorption spectroscopy with a broadband white-light probe to simultaneously resolve interlayer charge transfer and interlayer exciton formation dynamics in a MoSe2/WSe2 heterostructure. We observe an interlayer exciton formation timescale nearly an order of magnitude (~1 ps) longer than the interlayer charge transfer time (~100 fs). Microscopic calculations attribute this relative delay to an interplay of a phonon-assisted interlayer exciton cascade and thermalization, and excitonic wave-function overlap. Our results may explain the efficient photocurrent generation observed in optoelectronic devices based on TMD heterostructures, as the interlayer excitons are able to dissociate during thermalization.

AbstractCopper chalcogenides are materials characterized by intrinsic doping properties, allowing them to display high carrier concentrations due to their defect‐heavy structures, independent of the preparation method. Such high doping enables these materials to display plasmonic resonances, tunable by varying their stoichiometry. Here, plasmonic dynamics is studied in drop‐cast Cu9S5 (digenite) nanocrystals (NCs) film using ultrafast pump–probe spectroscopy. The NCs are synthesized by thermal annealing of copper foil using chemical vapor deposition (CVD), followed by sonication and drop‐casting of the isolated few‐layered flakes on different substrates. The samples display a broad localized surface plasmon resonance (LSPR) in the near‐infrared (NIR), peaking at 2100 nm. The free carrier response is further confirmed by fitting the linear absorption with a Drude–Lorentz effective medium approximation model. The high temporal resolution allows to measure the relaxation dynamics of the photo‐excited holes, which are dominated by a fast decay (τ1 = 360 ± 20 fs) and correspond to hole–phonon scattering processes, followed by a long‐lived (τ2 > 1 ns) signal associated with phonon–phonon scattering relaxation. These results confirm the possibility of fabricating Cu9S5 films retaining the plasmonic properties of individual NCs, anticipating integrating these films into heterojunctions with suitable hole acceptor materials to build hot‐hole‐transfer‐based optoelectronic devices.

In this work, we observe plasmon induced hot electron extraction in a heterojunction between indium tin oxide nanocrystals and monolayer molybdenum disulphide. We study the sample with ultrafast differential transmission exciting the sample at 1750 nm where the intense localized plasmon surface resonance of the indium tin oxide nanocrystals is and where the monolayer molybdenum disulphide does not absorb light. With the excitation at 1750 nm we observe the excitonic features of molybdenum disulphide in the visible range, close to the exciton of molybdenum disulphide. Such phenomenon can be ascribed to a charge transfer between indium tin oxide nanocrystals and monolayer molybdenum disulphide upon plasmon excitation. These results are a first step towards the implementation of near infrared plasmonic materials for photoconversion. 12 pages, 3 figures

The endeavor of the scientific community to maximize the possibility to harvest Sun irradiation for energy production is mainly devoted to the improvement of the power conversion efficiency of devices and to the extension of the spectral range in which solar devices operate. Considering that a significant portion of the Sun irradiation at the ground level is in the infrared, the research on materials and systems that operate in such region is gaining increasing attention. In this review, we will report recent advancements in multijunction solar cells, inorganic-organic perovskite solar cells, organic solar cells, colloidal quantum dot solar cells focusing on the absorption of such devices in the infrared. In addition, the use of upconverting nanostructures will be introduced as a way to indirectly exploit infrared radiation to increase power conversion efficiency of photovoltaic devices. Moreover, we will describe plasmon induced hot electron extraction based solar cells, that are particularly promising in absorbing the infrared portion of the Sun irradiation when the active materials are doped semiconductors, which show intense plasmonic resonances in the infrared. The review includes the optical spectroscopy tools to study the hot electron extraction from doped semiconductor-based heterojunctions.