We study electron mobilities in nanoporous and single-crystal titanium dioxide with terahertz time domain spectroscopy. This ultrafast technique allows the determination of the electron mobility after carrier thermalization with the lattice but before equilibration with defect trapping states. The mobilities reported here for single-crystal rutile (1 cm2/(V s)) and porous TiO2 (10(-2) cm2/(V s)) therefore represent upper limits for electron transport at room temperature for defect-free materials. The large difference in mobility between bulk and porous samples is explained using Maxwell-Garnett effective medium theory. These results demonstrate that electron mobility is strongly dependent on the material morphology in nanostructured polar materials due to local field effects and cannot be used as a direct measure of the diffusion coefficient.
This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (Eox = +0.74 to 0.80 V, Ered = -2.66 to 2.80 eV vs Fc/Fc+), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (EgCV = 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400-420 degrees C) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from Tg = 63 degrees C for T1 to 116 degrees C for T4) and show very efficient blue photoluminescence (lambdaPL = 398-422 nm) in both solution (PhiPL = 70-86%) and solid state (PhiPL = 43-60%). Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (approximately 340-400 nm) to colored (from red to purple color; approximately 500-600 nm) in the oxidized state.
We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots. Time-resolved luminescence and terahertz spectroscopy reveal that the rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes the first direct, quantitative measurement of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 0:15 ps time scale. DOI: 10.1103/PhysRevLett.96.057408 PACS numbers: 78.67.Hc, 65.80.+n, 73.21.La, 73.22.Dj Semiconductor quantum dots (QDs) exhibiting strong quantum confinement of electrons provide a rare opportunity to study the fundamental properties of electronic excitations in a size regime between the atomic and bulk limits. Electron confinement gives rise to discrete energy levels [with (1S e ; 1P e ; . . . ) electron and (1S 3=2 ; 1P 3=2 ; . . . ) hole states] [1], rather than the continuum of states present in bulk semiconductors. In both bulk and QD semiconductors, absorption of a photon generates ''hot'' charges with energies in excess of the band edge. While in bulk materials cooling occurs readily through sequential one-phonon emission, the electron energy level spacing in QDs (typically hundreds of meV) is large compared to typical longitudinal optical phonon frequencies (25 meV), and electron cooling via coupling to phonons is expected to be slow. Accordingly, it has been proposed that cooling in QDs is hindered by a so-called ''phonon bottleneck'' [2], though cooling rates comparable to those in bulk have been observed [3][4][5][6], suggesting that other effects may prevail. The measurement and understanding of the decay dynamics of the different electron states is also of technological importance: QDs are increasingly finding applications as the active component in single-photon emitters, lightemitting diodes [7], photovoltaic cells [8,9], lasers [10], and photon up-converters; for all these applications, knowledge and manipulation of the decay dynamics of the hot and cold electron-hole (exciton) states is a prerequisite.In the prototypical case of CdSe QDs, the radiative lifetime of the lowest exciton 1S 3=2 1S e cold state has been studied extensively by time-resolved luminescence measurements [11]. However, when the hot 1P 3=2 1P e exciton is generated through optical excitation, this state is not observed in the emission spectrum [1,11]. This means that nonradiative processes, i.e., electron cooling (1P e ! 1S e ) and hole cooling (1P 3=2 ! 1S 3=2 ), compete effectively with radiative decay of the hot exciton. Thus, fast electron and/or hole cooling processes must occur, in contradiction with the predicted phonon bottleneck. To explain the apparent fast cooling, an Auger-like mechanism has been proposed [5,6,12 -14], in which efficient transfer of energy from electrons to holes occurs followed by relatively fast hole relaxation through the more closely spaced valence levels [15]. Other explanations...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.