Time-resolved spectroscopy | EPFL Graph Search

In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds. All time-resolved spectra are suitable to be analyzed using the two-dimensional correlation method for a correlation map between the peaks. Flash photolysis Ultrafast laser spectroscopy Transient-absorption spectroscopy (TAS), also known as flash photolysis, is an extension of absorption spectroscopy. Ultrafast transient absorption spectroscopy, an example of non-linear spectroscopy, measures changes in the absorbance/transmittance in the sample. Here, the absorbance at a particular wavelength or range of wavelengths of a sample is measured as a function of time after excitation by a flash of light. In a typical experiment, both the light for excitation ('pump') and the light for measuring the absorbance ('probe') are generated by a pulsed laser. If the process under study is slow, then the time resolution can be obtained with a continuous (i.e., not pulsed) probe beam and repeated conventional spectrophotometric techniques. Time-resolved absorption spectroscopy relies on our ability to resolve two physical actions in real time. The shorter the detection time, the better the resolution. This leads to the idea that femtosecond laser based spectroscopy offers better resolution than nano-second laser based spectroscopy. In a typical experimental set up, a pump pulse excites the sample and later, a delayed probe pulse strikes the sample. In order to maintain the maximum spectral distribution, two pulses are derived from the same source. The impact of the probe pulse on the sample is recorded and analyzed with wavelength/ time to study the dynamics of the excited state.

Multiple Effects Induced by Mo6+ Doping in BiVO4 Photoanodes

Kevin Sivula, Charles Roger Jean Lhermitte, Nukorn Plainpan, Annalisa Polo, Ivan Grigioni

Mo6+ doping increases the photoelectrochemical performance of BiVO4 photoanodes in water oxidation. Herein, the underlying mechanisms is elucidated through a systematic structural, morphological, and photoelectrochemical investigation on photoelectrodes of pure and Mo6+ doped BiVO4 prepared by a novel multistep spin-coating deposition approach, leading to multilayer flat films with high optical transparency. Transient absorption spectroscopy in the nano- to microsecond time scale reveals a longer lifetime of photogenerated holes in the doped films. Besides confirming that Mo6+ ions improve the electron transport in the material bulk, impedance spectroscopy also reveals the crucial role of the dopant on the surface properties of BiVO4 photoanodes. The presence of intra-bandgap states, acting as traps of photogenerated charge carriers in pure BiVO4, is detected through the build-up of the interfacial surface state capacitance. The limited activity of pure BiVO4 in water oxidation is largely improved upon 3 at% Mo6+ incorporation, ensuring a more efficient charge carrier transport with respect to pure BiVO4, together with the beneficial passivation of its trap surface states.

WILEY-V C H VERLAG GMBH2022

Ultrafast carrier and quasiparticle dynamics in strongly confined perovskite nanoplatelets

Etienne Christophe Socie

Over the past decade, lead halide perovskites (LHPs) have received considerable attention thanks to their impressive optoelectronic properties. Today, LHP-based devices are one of the most efficient single-junction solar cells, with power-conversion efficiencies reaching 25.7%. Likewise, nanostructures of the same material have emerged with tremendous potential for light-emitting and lasing applications. The perovskite complex and dynamic structures also make them fascinating for fundamental photophysical studies, not only to apply in the context of the device improvements but also for a much broader class of semiconductors.Typically, charge carrier dynamics on the ultrafast timescale, from a few nanoseconds down to tens of femtoseconds, are monitored by time-resolved spectroscopic techniques using ultrashort laser pulses. The aim of this work is to associate transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy for investigating early photophysical mechanisms in low-dimensional perovskite nanocrystals (PNCs). While most available TRPL experimental techniques are limited by a temporal resolution of a few picoseconds, broadband fluorescence upconversion spectroscopy (FLUPS) is employed to obtain two-dimensional spectro-temporal data with a sub-picosecond temporal resolution.Chapters 1 and 2 provide the scientific background relevant to the thesis. Chapter 1 establishes the fundamental photophysical concepts for bulk and confined semiconductors and provides a general overview of the PNC optoelectronic properties, reviewing the relevant literature. The second chapter focuses on TRPL spectroscopy and details the theoretical and experimental approaches for FLUPS, the primary technique employed in this work.Chapter 3 describes the synthetic procedure to obtain colloidal blue-emitting CsPbBr3 nanoplatelets (NPls). The product is then characterized by standard methods, including transmission electron microscopy, cyclic voltammetry and linear UV-Vis absorption and emission spectroscopy. The consequences of strong quantum and dielectric confinements are highlighted and induce unique optoelectronic properties. The importance of capping ligands engineering for passivating the perovskite surface states is also stressed. Capping ligands can also increase the NPl emission efficiency and long-term stability.Chapters 4 and 5 discuss the charge carrier dynamics under weak and strong excitation regimes. By combining FLUPS and TA data, complex photophysical behaviors within the first ten picoseconds are revealed. Under low excitation intensities, the NPl dynamics are dominated by an ensemble of independent excitons. Besides, at high pump fluences, enhanced carrier-carrier interactions induce surprisingly stable and emissive exciton-exciton pairs, also called biexcitons. The properties and recombination mechanisms of the biexcitons display typical quantum well signatures.Finally, chapter 6 investigates the charge separation and subsequent charge recombination mechanisms in CsPbBr3 NPls combining FLUPS, TA and nanosecond flash photolysis measurements. NPl-acceptor complexes are synthesized by attaching phenothiazine and p-benzoquinone to the NPl surface, acting as hole and electron acceptors, respectively. Excitation energy-dependent FLUPS measurements are carried out to unveil fast interfacial charge transfer from the perovskite band edge and ultrafast hot carrier transfer from higher energy levels.

EPFL2022

Ultrafast carrier and quasiparticle dynamics in strongly confined perovskite nanoplatelets

Etienne Christophe Socie

EPFL2022