Showing 1–5 of 5 results for author: Penfold, T

Unveiling Zn incorporation in CuInS$_2$ quantum dots: X-ray and optical analysis of doping effects, structural modifications and surface passivation

Authors: Andrés Burgos-Caminal, Brener R. C. Vale, André F. V. Fonseca, Juan F. Hidalgo, Elisa P. P. Collet, Lázaro García, Víctor Vega-Mayoral, Saül Garcia-Orrit, Iciar Arnay, Juan Cabanillas-González, Laura Simonelli, Ana Flávia Nogueira, Marco Antônio Schiavon, Thomas J. Penfold, Lazaro A. Padilha, Wojciech Gawelda

Abstract: Quantum dots (QDs) exhibit unique properties arising from their reduced size and quantum confinement effects, including exceptionally bright and tunable photoluminescence. Among these, CuInS$_{2}$ QDs have gained significant attention owing to their remarkable broadband emission, making them highly desirable for various optoelectronic applications requiring efficient luminescent nanomaterials. How… ▽ More Quantum dots (QDs) exhibit unique properties arising from their reduced size and quantum confinement effects, including exceptionally bright and tunable photoluminescence. Among these, CuInS$_{2}$ QDs have gained significant attention owing to their remarkable broadband emission, making them highly desirable for various optoelectronic applications requiring efficient luminescent nanomaterials. However, maximizing radiative recombination in CuInS$_{2}$ QDs often necessitates minimizing intragap trap states. A common approach involves the introduction of Zn during the synthesis, which typically promotes the formation of a ZnS shell that passivates the QD surface. Despite its importance, the characterization and quantification of Zn incorporation using conventional techniques, such as optical spectroscopy or electron microscopy, remains challenging. In this study, we utilized X-ray absorption spectroscopy (XAS), in both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectral ranges, to investigate Zn incorporation into CuInS$_{2}$ QDs with element-specific precision. This approach allowed us to detect the formation of a ZnS surface shell and to resolve the spatial distribution of Zn atoms within the QD lattice, distinguishing between Zn as a substituent, or as an interstitial defect. Additionally, we explored the optical and dynamical properties of CuInS$_{2}$ QDs using time-resolved optical spectroscopies, particularly in the presence of electron and hole acceptors. These results provide deeper insights into the role and effectiveness of the Zn-induced passivating layer, paving the way for optimizing QD performance in photoluminescence applications. △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: arXiv admin note: text overlap with arXiv:2412.15418

Selective tracking of charge carrier dynamics in CuInS2 quantum dots

Authors: Andrés Burgos-Caminal, Brener R. C. Vale, André F. V. Fonseca, Elisa P. P. Collet, Juan F. Hidalgo, Lázaro García, Luke Watson, Olivia Borrell-Grueiro, María E. Corrales, Tae-Kyu Choi, Tetsuo Katayama, Dongxiao Fan, Víctor Vega-Mayoral, Saül García-Orrit, Shunsuke Nozawa, Thomas J. Penfold, Juan Cabanillas-Gonzalez, Shin-Ichi Adachi, Luis Bañares, Ana F. Nogueira, Lázaro A. Padilha, Marco A. Schiavon, Wojciech Gawelda

Abstract: CuInS2 quantum dots have been studied in a broad range of applications, but despite this, the fine details of their charge carrier dynamics remain a subject of intense debate. Two of the most relevant points of discussion are the hole dynamics and the influence of Cu:In synthesis stoichiometry on them. It has been proposed that Cu-deficiency leads to the formation of Cu2+, affecting the localizati… ▽ More CuInS2 quantum dots have been studied in a broad range of applications, but despite this, the fine details of their charge carrier dynamics remain a subject of intense debate. Two of the most relevant points of discussion are the hole dynamics and the influence of Cu:In synthesis stoichiometry on them. It has been proposed that Cu-deficiency leads to the formation of Cu2+, affecting the localization of holes into Cu defects. Importantly, it is precisely these confined hole states which are used to explain the interesting photoluminescence properties of CuInS2 quantum dots. We use static X-ray spectroscopy to reveal no evidence for a measurable amount of native Cu2+ states in Cu-deficient samples. Instead, the improved properties of these samples are explained by an increase of crystallinity, reducing the concentration of mid gap states. Furthermore, to understand the charge carrier dynamics, herein we employ ultrafast optical transient absorption, and fluorescence up-conversion spectroscopies in combination with ultrafast X-ray absorption spectroscopy using a hard X-ray free electron laser. We demonstrate that in non-passivated samples, holes are transferred from Cu atoms in sub-picosecond timescales. We assign this transfer to occur towards the thiol-based ligands. Finally, we observe that Cu-deficient samples are more robust against the photothermal heating effects of using higher laser fluences. This is not the case for the stoichiometric sample, where heating effects on the structure are directly observed. △ Less

Submitted 19 December, 2024; originally announced December 2024.

The Photochemistry of Rydberg Excited Cyclobutanone: Photoinduced Processes and Ground State Dynamics

Authors: Julien Eng, Conor Rankine, Thomas Penfold

Abstract: Owing to ring-strain, cyclic ketones exhibit complex excited-state dynamics with multiple competing photochemical channels active on the ultrafast timescale. While the excited-state dynamics of cyclobutanone after $π^{\ast}\leftarrow n$ excitation into the lowest-energy excited singlet state (S$_1$) has been extensively studied, the dynamics following 3$s\leftarrow n$ excitation into the higher-ly… ▽ More Owing to ring-strain, cyclic ketones exhibit complex excited-state dynamics with multiple competing photochemical channels active on the ultrafast timescale. While the excited-state dynamics of cyclobutanone after $π^{\ast}\leftarrow n$ excitation into the lowest-energy excited singlet state (S$_1$) has been extensively studied, the dynamics following 3$s\leftarrow n$ excitation into the higher-lying singlet Rydberg (S$_2$) state are less well understood. Herein, we couple quantum and excited-state trajectory surface-hopping molecular dynamics simulations to study the relaxation of cyclobutanone following 3s$\leftarrow n$ excitation and to predict the ultrafast electron diffraction scattering signal that we anticipate to arise from the relaxation dynamics that we observe. Our simulations indicate that relaxation from the initially-populated singlet Rydberg state occurs on the hundreds-of-femtosecond to picosecond timescale consistent with the symmetry-forbidden nature of the state-to-state transition involved. Once cyclobutanone has relaxed non-radiatively to the electronic ground state (S$_0$), the vibrationally hot molecules have sufficient energy to form multiple fragmentory products on the electronic ground-state surface including C$_2$H$_4$ + CH$_2$CO (C2; 20%), and C$_3$H$_6$ + CO (C3; 2.5%). We discuss the limitations of our simulations, how these may influence the outcome of the excited-state dynamics we observe, and -- ultimately -- the predictive power of the simulated experimental observable. △ Less

Submitted 14 February, 2024; originally announced February 2024.

Disentangling the Evolution of Electrons and Holes in photoexcited ZnO nanoparticles

Authors: Christopher J. Milne, Natalia Nagornova, Thomas Pope, Hui-Yuan Chen, Thomas Rossi, Jakub Szlachetko, Wojciech Gawelda, Alexander Britz, Tim B. van Drie, Leonardo Sala, Simon Ebner, Tetsuo Katayama, Stephen H. Southworth, Gilles Doumy, Anne Marie March, C. Stefan Lehmann, Melanie Mucke, Denys Iablonskyi, Yoshiaki Kumagai, Gregor Knopp, Koji Motomura, Tadashi Togashi, Shigeki Owada, Makina Yabashi, Martin M. Nielsen , et al. (5 additional authors not shown)

Abstract: The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy and ab-initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exc… ▽ More The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy and ab-initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The X-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the X-ray absorption signal is found to rise in ~1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly-charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ~100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice. △ Less

Submitted 6 October, 2023; originally announced October 2023.

Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins

Authors: Dominik Kinschel, Camila Bacellar, Oliviero Cannelli, Boris Sorokin, Tetsuo Katayama, Giulia F. Mancini, Jeremy R. Rouxel, Yuki Obara, Junichi Nishitani, Hironori Ito, Terumasa Ito, Naoya Kurahashi, Chika Higashimura, Shotaro Kudo, Theo Keane, Frederico A. Lima, Wojciech Gawelda, Peter Zalden, Sebastian Schulz, James Budarz, Dmitry Khakhulin, Andreas Galler, Christian Bressler, Christopher J. Milne, Thomas Penfold , et al. (4 additional authors not shown)

Abstract: In haemoglobin (consisting of four globular myoglobin-like subunits), the change from the low-spin (LS) hexacoordinated haem to the high spin (HS) pentacoordinated domed form upon ligand detachment and the reverse process upon ligand binding, represent the transition states that ultimately drive the respiratory function. Visible-ultraviolet light has long been used to mimic the ligand release from… ▽ More In haemoglobin (consisting of four globular myoglobin-like subunits), the change from the low-spin (LS) hexacoordinated haem to the high spin (HS) pentacoordinated domed form upon ligand detachment and the reverse process upon ligand binding, represent the transition states that ultimately drive the respiratory function. Visible-ultraviolet light has long been used to mimic the ligand release from the haem by photodissociation, while its recombination was monitored using time-resolved infrared to ultraviolet spectroscopic tools. However, these are neither element- nor spin-sensitive. Here we investigate the transition state in the case of Myoglobin-NO (MbNO) using femtosecond Fe Kalpha and Kbeta non-resonant X-ray emission spectroscopy (XES) at an X-ray free-electron laser upon photolysis of the Fe-NO bond. We find that the photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2) deoxy-myoglobin (deoxyMb) haem occurs in ca. 800 fs, and that it proceeds via an intermediate (S = 1) spin state. The XES observables also show that upon NO recombination to deoxyMb, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ca. 30 ps. Thus, the entire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process. △ Less

Submitted 12 May, 2020; originally announced May 2020.