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Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.


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TL;DR: In this article, the authors calculate the initial thermalization rate of a perovskite from the scattering time in the Frohlich polaron model to be 78 meV ps-1 for CH3NH3PbI3.
Abstract: Halide perovskites show unusual thermalization kinetics for above-bandgap photoexcitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The crossover between low-fluence and high-fluence “phonon bottleneck” cooling is due to a Mott transition where the polarons overlap (n ≥ 1018 cm–3) and the phonon subpopulations are shared. We calculate the initial rate of cooling (thermalization) from the scattering time in the Frohlich polaron model to be 78 meV ps–1 for CH3NH3PbI3. This rapid initial thermalization involves heat transfer into optical phonon modes coupled by a polar dielectric interaction. Further cooling to equilibrium over hundreds of picoseconds is limited by the ultralow thermal conductivity of the perovskite lattice.

128 citations

Journal ArticleDOI
TL;DR: The electrostatic charge and photoionization characteristics of 5-nm CdSe nanocrystals were directly observed with electrostatic force microscopy (EFM) in dry air at room temperature as mentioned in this paper.
Abstract: The electrostatic charge and photoionization characteristics of 5-nm CdSe nanocrystals were directly observed with electrostatic force microscopy (EFM) in dry air at room temperature. Measurements were made on individual nanocrystals, as well as on those in self-assembled rafts. Nanocrystals are initially charge neutral if protected from sources of light. However, over a few weeks some nanocrystals develop a single positive charge if exposed to ambient light. The determination of the charge magnitude per nanocrystal within the framework of EFM theory is described. EFM measurements with simultaneous above band gap laser photoexcitation provide direct evidence of nanocrystal photoionization. A small percentage of photoionized nanocrystals exhibit a blinking behavior in their charge. The linear dependence of nanocrystal photoionization rates on excitation intensity indicates that the ionization process occurs via a single photon. EFM measurements of core/shell CdSe nanocrystals show that photoionization is slower in the presence of an electron barrier at the nanocrystal surface. Photoionization and subsequent neutralization are quantitatively modeled with a twolevel system.

128 citations

Journal ArticleDOI
TL;DR: It is inferred from the data that photoexcitation initially leads to formation of bound electron-hole pairs in the form of neutral excitons, which can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts.
Abstract: The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA), and terahertz (THz) conductivity measurements. The PL, TA, and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron–hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site, the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a ...

128 citations

Journal ArticleDOI
TL;DR: It is noticed that while temperature sensitivity and luminescence intensity are optically stable, increased excitation intensity to generate heat above room temperature may saturate the sensing capacity of temperature feedback, so a dual beam photoexcitation scheme is proposed as a solution for possible light-induced hyperthermia treatment.
Abstract: The current frontier in nanomaterials engineering is to intentionally design and fabricate heterogeneous nanoparticles with desirable morphology and composition, and to integrate multiple functionalities through highly controlled epitaxial growth Here we show that heterogeneous doping of Nd3+ ions following a core–shell design already allows three optical functions, namely efficient (η > 72%) light-to-heat conversion, bright NIR emission, and sensitive (SR > 01% K−1) localized temperature quantification, to be built within a single ca 25 nm nanoparticle Importantly, all these optical functions operate within the transparent biological window of the NIR spectral region (λexc ∼ 800 nm, λemi ∼ 860 nm), in which light scattering and absorption by tissues and water are minimal We find NaNdF4 as a core is efficient in absorbing and converting 808 nm light to heat, while NaYF4:1%Nd3+ as a shell is a temperature sensor based on the ratio-metric luminescence reading but an intermediate inert spacer shell, eg NaYF4, is necessary to insulate the heat convertor and thermometer by preventing the possible Nd–Nd energy relaxation Moreover, we notice that while temperature sensitivity and luminescence intensity are optically stable, increased excitation intensity to generate heat above room temperature may saturate the sensing capacity of temperature feedback We therefore propose a dual beam photoexcitation scheme as a solution for possible light-induced hyperthermia treatment

127 citations

Journal ArticleDOI
TL;DR: In this paper, a collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B(1A1) excited state at 121.6 nm was investigated.
Abstract: The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three‐body fragmentation yielding ground state atoms. The observed energy disposal has been rationalized by considering the motion of a wavepacket launched on the B state surface at a geometry corresponding to the ground state equilibrium configuration. Electronically excited OH(OD) fragments result from that fraction of the photoexcited molecules that dissociate on the B state surface; their rotational excitation results from the marked angular anisotropy of the B state surface. Ground state OH(OD) fragments can arise as a result of radiationless transfer to the lower A(1B1) or X(1A1) surfaces. The wavepacket calculations show that B■X transfer via the conical intersection linking these two surfaces leads to the most highly rotationally excited OH(OD) fragments. These calculations also show that the contribution made by B■A radiationless transfer to the overall rotational distribution in the ground state OH(OD) fragments scales with the amount of a‐axis rotational excitation in the photoexcited molecules: The detailed form of the OH(OD) product state population distribution is thus predicted to be temperature dependent.

127 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023249
2022529
2021221
2020204
2019183
2018256