arXiv subjects

Iwan Moreels

Publications and source records attributed to Iwan Moreels.

At least 19 recordsLinked to original sources

arXiv2026

Controlling Electron-Beam-Induced Charging in Colloidal Quantum Dots

Colloidal quantum dots (QDs) are attractive nanoscale emitters, yet their cathodoluminescence (CL) response remains poorly understood and often unstable under electron-beam excitation, limiting CL spectroscopy and electron-beam-based device processing. Here, we investigate the CL mechanism and strategies to improve its stability using highly photostable, structurally homogeneous giant-shell CdSe/CdS QDs combined with in situ CL and photoluminescence (PL) measurements. By identifying distinct signatures of excited states in both lifetime and spectral measurements, we demonstrate that the CL response is governed by electron-beam-induced charging. Charge accumulation drives multiexciton generation even at relatively low currents, leading to a pronounced blueshift, shorter average lifetimes, and rapid cathodobleaching. To test this further, we employ indirect excitation to reach sub-pA currents beyond the limits of typical electron beams, showing that neutral-exciton emission can be partially recovered and cathodobleaching mitigated, although charging cannot be fully suppressed. Furthermore, by replacing long insulating ligands with shorter ones, we improve charge drainage and strongly suppress biexciton formation. Together, these results show that biexciton formation can be controlled by limiting charge accumulation, providing a practical route toward stable CL for spectroscopy, imaging, and electron-beam-compatible photonic devices.

cond-mat.mes-hall↗

arXiv2026

Graphene-based technologies for energy applications, challenges and perspectives

Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

cond-mat.mtrl-sci↗

arXiv2026

Efficient charge transfer in solution-processed PbS Quantum Dot-reduced graphene oxide hybrid materials

Quantum dot - graphene hybrid materials have raised significant interest due to the unique synergy of the optical properties of colloidal quantum dots (QDs) and the transport properties of graphene. This stimulated the development of low-cost and up-scalable solution-processed strategies for hybrid materials with potential application in light harvesting and opto-electronic devices. Here we report a versatile covalent-linking based approach for the functionalization of reduced graphene oxide (rGO), to prepare a variety of QD-rGO hybrid dispersions with QDs of different size and composition (PbS, PbS/CdS and CdSe QDs), and shape (CdSe/CdS dot-in-rods). We achieved a well-controlled QD coverage of the rGO sheets by functionalizing the rGO surface with mercapto-silane linkers. A further spectroscopic investigation of near-infrared PbS QD-rGO materials demonstrates efficient electronic coupling between both materials. The QD photoluminescence emission quenching and exciton lifetime shortening up to 95%, together with subtle graphene Raman G-band shifts upon QD linking, supports electron transfer as the dominant relaxation pathway from the QD to the rGO. The use of core/shell PbS/CdS QDs allows tuning of the transfer efficiency from 94% for a 0.2 nm thin CdS shell, down to 30% for a 1.1 nm thick shell.

cond-mat.mes-hall↗

arXiv2022

Purifying single photon emission from giant shell CdSe/CdS quantum dots at room temperature

Giant shell CdSe/CdS quantum dots are bright and flexible emitters, with near-unity quantum yield and suppressed blinking, but their single photon purity is reduced by efficient multiexcitonic emission. We report the observation, at the single dot level, of a large blueshift of the photoluminescence biexciton spectrum ($24\pm5$ nm over a sample of 32 dots) for pure-phase wurtzite quantum dots. By spectral filtering, we demonstrate a 2.3 times reduction of the biexciton quantum yield relative to the exciton emission, while preserving as much as 60% of the exciton single photon emission, thus improving the purity from $g_2(0)=0.07\pm0.01$ to $g_2(0)=0.03\pm0.01$. At larger pump fluency the spectral purification is even more effective with up to a 6.6 times reduction in $g_2(0)$, which is due to the suppression of higher order excitons and shell states experiencing even larger blueshift. Our results indicate the potential for synthesis engineered giant shell quantum dots, with further increased biexciton blueshift, for quantum optical applications requiring both high purity and brightness.

cond-mat.mes-hall↗

arXiv2020

Electrical control of single-photon emission in highly-charged individual colloidal quantum dots

Electron transfer to an individual quantum dot promotes the formation of charged excitons with enhanced recombination pathways and reduced lifetimes. Excitons with only one or two extra charges have been observed and exploited for very efficient lasing or single quantum dot LEDs. Here, by room-temperature time-resolved experiments on individual giant-shell CdSe/CdS quantum dots, we show the electrochemical formation of highly charged excitons containing more than twelve electrons and one hole. We report the control over intensity blinking, along with a deterministic manipulation of quantum dot photodynamics, with an observed 210-fold increase of the decay rate, accompanied by 12-fold decrease of the emission intensity, while preserving single-photon emission characteristics. These results pave the way for deterministic control over the charge state, and room-temperature decay-rate engineering for colloidal quantum dot-based classical and quantum communication technologies.

physics.optics↗

arXiv2020

Objective-free excitation of quantum emitters with a laser-written micro parabolic mirror

The efficient excitation of quantum sources such as quantum dots or single molecules requires high NA optics which is often a challenge in cryogenics, or in ultrafast optics. Here we propose a 3.2 um wide parabolic mirror, with a 0.8 um focal length, fabricated by direct laser writing on CdSe/CdS colloidal quantum dots, capable of focusing the excitation light to a sub-wavelength spot and to extract the generated emission by collimating it into a narrow beam. This mirror is fabricated via in-situ volumetric optical lithography, which can be aligned to individual emitters, and it can be easily adapted to other geometries beyond the paraboloid. This compact solid-state transducer from far-field to the emitter has important applications in objective-free quantum technologies.

physics.optics↗

arXiv2020

Giant-Shell CdSe/CdS Nanocrystals: Exciton Coupling to Shell Phonons Investigated by Resonant Raman Spectroscopy

The interaction between excitons and phonons in semiconductor nanocrystals plays a crucial role in the exciton energy spectrum and dynamics, and thus in their optical properties. We investigate the exciton2 phonon coupling in giant-shell CdSe/CdS core-shell nanocrystals via resonant Raman spectroscopy. The Huang-Rhys parameter is evaluated by the intensity ratio of the longitudinal-optical (LO) phonon of CdS with its first multiscattering (2LO) replica. We used four different excitation wavelengths in the range from the onset of the CdS shell absorption to well above the CdS shell band edge to get insight into resonance effects of the CdS LO phonon with high energy excitonic transitions. The isotropic spherical giant-shell nanocrystals show consistently stronger exciton-phonon coupling as compared to the anisotropic rod-shaped dot-in-rod (DiR) architecture, and the 2LO/LO intensity ratio decreases for excitation wavelengths approaching the CdS band edge. The strong exciton-phonon coupling in the spherical giant-shell nanocrystals can be related to the delocalization of the electronic wave functions. Furthermore, we observe the radial breathing modes of the GS nanocrystals and their overtones by ultralow frequency Raman spectroscopy with nonresonant excitation, using laser energies well below the band gap of the heteronanocrystals, and highlight the differences between higher order

physics.chem-ph↗

arXiv2020

Solution-processed silver sulphide nanocrystal film for resistive switching memories

Resistive switching memories allow electrical control of the conductivity of a material, by inducing a high resistance (OFF) or a low resistance (ON) state, using electrochemical and ion transport processes. As alternative to high temperature and vacuum-based physical sulphurization methods of silver (Ag), here we propose, as resistive switching medium, a layer built from colloidal Ag$_{2-x}$S nanocrystals -compatible with solution-processed approaches. The effect of the electrode size (from macro- to micro-scale), composition (Ag, Ti and Pt) and geometry on the device performance together with the electrochemical mechanisms involved are evaluated. We achieved an optimized Ag/Ti bowtie proof-of-concept configuration by e-beam lithography, which fulfils the general requirements for ReRAM devices in terms of low power consumption and reliable $I_{ON}/I_{OFF}$ ratio. This configuration demonstrates reproducible switching between ON and OFF states with data endurance of at least 20 cycles; and an $I_{ON}/I_{OFF}$ ratio up to 10$^3$ at low power consumption (0.1V readout), which outperforms previous results in literature for devices with resistive layers fabricated from silver chalcogenide nanoparticles.

physics.app-ph↗

arXiv2020

Core/Shell CdSe/CdS Bone-Shaped Nanocrystals with a Thick and Anisotropic Shell as Optical Emitters

Colloidal core/shell nanocrystals are key materials for optoelectronics, enabling control over essential properties via precise engineering of the shape, thickness, and crystal lattice structure of their shell. Here, we apply the growth protocol for CdS branched nanocrystals on CdSe nanoplatelet seeds and obtain bone-shaped heterostructures with a highly anisotropic shell. Surprisingly, the nanoplatelets withstand the high growth temperature of 350 °C and we obtain structures with a CdSe nanoplatelet core that is overcoated by a shell of cubic CdS, on top of which tetrahedral CdS structures with hexagonal lattice are formed. These complex core/shell nanocrystals show a bandedge emission around 657 nm with a photoluminescence quantum yield of ca. 42 % in solution, which is also retained in thin films. Interestingly, the nanocrystals manifest simultaneous red and green emission, and the relatively long wavelength of the green emission indicates charge recombination at the cubic/hexagonal interface of the CdS shell. The nanocrystal films show amplified spontaneous emission, random lasing, and distributed feedback lasing when the material is deposited on suitable gratings. Our work stimulates the design and fabrication of more exotic core/shell heterostructures where charge carrier delocalization, dipole moment, and other optical and electrical properties can be engineered.

physics.chem-ph↗

arXiv2020

Composition, Size, and Surface Functionalization dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on the surface passivation. We present a study on the temperature dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by the surface passivation, and a post-synthesis ligand exchange with a quaternary ammonium bromide (QAB) results in a more stable passivation over a larger temperature range. The PL intensity is highest from 50K-250K, which indicates that the ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), the surface passivation still needs improvement toward increased emission intensity in nanocrystal films.

physics.chem-ph↗

arXiv2020

Extending the colloidal transition metal dichalcogenide library to ReS2 nanosheets for application in gas sensing and electrocatalysis

Among the large family of transition metal dichalcogenides (TMDCs), recently ReS2 has stood out due to its nearly layer-independent optoelectronic and physicochemical properties. These are related to its 1T distorted octahedral structure, which leads to strong in-plane anisotropy and the presence of active sites at its surface, which makes ReS2 interesting for applications such as gas sensors and catalysts for H2 production. However, the current fabrication methods for ReS2 use chemical or physical vapor deposition (CVD or PVD) processes that are costly and involve complex and lengthy fabrication procedures, therefore limiting their large-scale production and exploitation. To address this issue, we developed a colloidal synthesis approach, which allows the production of ReS2 to be attained at temperatures below 360 Celsius degrees and with reaction times < 2 h, resulting in a more cost-efficient strategy than the CVD and PVD methods. By combining the solution-based synthesis with surface functionalization strategies, we demonstrate the feasibility of colloidal ReS2 nanosheet films for gas sensing of different toxic gases, moisture and other volatile compounds with highly competitive performance in comparison with devices built with CVD-grown ReS2 and MoS2. In addition, the integration of the ReS2 nanosheet films in assemblies, in which they are deposited on top of networks of carbon nanotubes, allowed us to fabricate electrodes for electrocatalysis for H2 production in both acid and alkaline conditions. Results from proof-of-principle devices show an electrocatalytic overpotential that is competitive with devices based on ReS2 produced by CVD, and even with MoS2, WS2 and MoSe2 electrocatalysts.

physics.app-ph↗

arXiv2019

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetism owing to the uncompensated spins of dangling bonds (DBSs). We reveal that by optical spectroscopy in high magnetic fields up to 15 Tesla using the exciton spin as probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the DBS and exciton spin polarization as well as by the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction can be adjusted by the nanoplatelet growth conditions.

cond-mat.mes-hall↗

arXiv2019

Dye-Sensitized Ternary Copper Chalcogenide Nanocrystals: Optoelectronic Properties, Air Stability and Photosensitivity

We report on the effect of ligand exchange of copper sulphide selenide as well as copper selenide nanocrystals with the organic pi-system Cobalt beta-tetraaminophthalocyanine and analyse changes in the structural, optical as well as electric properties of thin films of these hybrid materials. A strong ligand interaction with the surface of the nanocrystals is revealed by UV-vis absorption and Raman spectroscopy. Grazing-incidence small-angle X-ray scattering studies show a significant contraction in the interparticle distance upon ligand exchange. For copper-deficient copper selenide, this contraction has a negligible effect on electric transport, while for copper-deficient copper sulphide selenide, the conductivity increases by eight orders of magnitude and results in metal-like temperature-dependent transport. We discuss these differences in the light of varying contributions of electronic vs. ionic transport in the two materials and highlight their effect on the stability of the transport properties under ambient conditions. With photocurrent measurements, we demonstrate high optical responsivities of 200-400 A W-1 for the phthalocyanine-capped copper sulphide selenide and emphasize the beneficial role of the organic pi-system in this respect, which acts as an electronic linker and an optical sensitizer at the same time.

cond-mat.mtrl-sci↗

arXiv2018

Solution-processed hybrid graphene flake/2H-MoS2 quantum dot heterostructures for efficient electrochemical hydrogen evolution

We designed solution-processed, flexible hybrid graphene flake/2H-MoS2 quantum dot (QD) heterostructures, showing enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) with respect to their native individual components. The 2H-MoS2 QDs are produced through a scalable, environmentally friendly, one-step solvothermal approach from two-dimensional (2D) 2H-MoS2 flakes obtained by liquid phase exfoliation (LPE) of their bulk counterpart in 2-Propanol. This QDs synthesis avoids the use of high boiling point and/or toxic solvents. Graphene flakes are produced by LPE of graphite in N-Methyl-2-pyrrolidone. The electrochemical properties of 2H-MoS2 QDs and their HER-favorable chemical and electronic coupling with graphene enable to reach a current density of 10 mA/cm^2 at an overpotential of 136 mV, surpassing the performances of graphene flake/2H-MoS2 (1T-MoS2) flake heterostructures. Our approach provides a shortcut, viable and cost-effective method for enhancing the 2D materials electrocatalytic properties.

cond-mat.mtrl-sci↗

arXiv2018

Graphene-Based Hole Selective Layers for High-Efficiency, Solution-Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes

Regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the work-horse of organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ) configuration with phenyl-C61-butyric acid methyl ester (PCBM) for solution-processed hydrogen-evolving photocathodes, reaching cathodic photocurrents at 0 V vs. RHE (J0V vs RHE) of up to 8 mA cm-2. The photoelectrochemical performance of these photocathodes strongly depends on the presence of the electron (ESL) and hole (HSL) selective layer. While TiO2 and its sub-stoichiometric phases are consolidated ESL materials, the currently used HSLs (e.g., MoO3, CuI, PEDOTT:PSS, WO3) suffer electrochemical degradation under hydrogen evolution reaction (HER)-working conditions. In this work, we use solution-processed graphene derivatives as HSL to boost efficiency and durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high performance. In fact, our devices show cathodic J0V vs RHE of 6.01 mA cm-2, onset potential (Vo) of 0.6 V vs. RHE, ratiometric power-saved efficiency (ϕsaved) of 1.11% and operational activity of 20 hours in 0.5 M H2SO4 solution. Moreover, the designed photocathodes are effectively working in different pH environments ranging from acid to basic. This is pivotal for their exploitation in tandem configurations, where photoanodes operate only in restricted electrochemical conditions. Furthermore, we demonstrate the scalability of our all-solution processed approach by fabricating a large-area (~9 cm2) photocathode on flexible substrate, achieving remarkable cathodic J0V vs RHE of 2.8 mA cm-2, Vo of 0.45 V vs. RHE and ϕsaved of 0.31%. This is the first demonstration of high-efficient rr-P3HT:PCBM flexible photocathodes based on low-cost and solution-processed manufacturing techniques.

physics.app-ph↗

arXiv2018

Single-mode lasing from colloidal water-soluble CdSe/CdS quantum dot-in-rods

Core-shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that makes them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties, poses a major challenge. This work demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 microJ/cm2) as the pristine material (115 microJ/cm2). To demonstrate feasibility for lasing applications, self-assembled micro lasers are fabricated via the coffee-ring effect that display single-mode operation and a very low threshold of around 10 microJ/cm2. The performance of these micro lasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods.

physics.app-ph↗

arXiv2016

Strong exciton-photon coupling with colloidal nanoplatelets in an open microcavity

Colloidal semiconductor nanoplatelets exhibit quantum size effects due to their thickness of only few monolayers, together with strong optical band-edge transitions facilitated by large lateral extensions. In this article we demonstrate room temperature strong coupling of the light and heavy hole exciton transitions of CdSe nanoplatelets with the photonic modes of an open planar microcavity. Vacuum Rabi splittings of $66 \pm 1$ meV and $58 \pm 1$ meV are observed for the heavy and light hole excitons respectively, together with a polariton-mediated hybridisation of both transitions. By measuring the concentration of platelets in the film we compute the transition dipole moment of a nanoplatelet exciton to be $μ= (575 \pm 110)$ D. The large oscillator strength and fluorescence quantum yield of semiconductor nanoplatelets provide a perspective towards novel photonic devices, combining polaritonic and spinoptronic effects.

cond-mat.mes-hall↗

arXiv2015

p-State Luminescence in CdSe Nanoplatelets: The Role of Lateral Confinement and an LO Phonon Bottleneck

We report excited state emission from p-states at excitation fluences well below ground state saturation in CdSe nanoplatelets. Size dependent exciton ground state-excited state energies and dynamics are determined by three independent methods, time-resolved photoluminescence (PL), time-integrated PL and Hartree renormalized k$\cdot$p calculations -- all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Our results suggest that the PL decay of CdSe platelets is governed by an LO-phonon bottleneck, related to the reported low exciton phonon coupling in CdSe platelets and only observable due to the very large oscillator strength and energy spacing of both states.

cond-mat.mes-hall↗