95 research outputs found
Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots
We examine the multiple exciton population dynamics in PbS quantum dots by
ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We
simultaneously probe the first three excitonic transitions over a broad
spectral range. Transient spectra show the presence of first order bleach of
absorption for the 1S_h-1S_e transition and second order bleach along with
photoinduced absorption band for 1P_h-1P_e transition. We also report evidence
of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures
of carrier multiplication (multiexcitons for the single absorbed photon) from
analysis of the first and second order bleaches, in the limit of low absorbed
photon numbers (~ 10^-2), at pump energies from two to four times the
semiconductor band gap. The multiexciton generation efficiency is discussed
both in terms of a broadband global fit and the ratio between early- to
long-time transient absorption signals.. Analysis of population dynamics shows
that the bleach peak due to the biexciton population is red-shifted respect the
single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure
Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films
Colloidal semiconductor nanocrystals have attracted significant interest for applications in solution-processable devices such as light-emitting diodes and solar cells. However, a poor understanding of charge transport in nanocrystal assemblies, specifically the relation between electrical conductance in dark and under light illumination, hinders their technological applicability. Here we simultaneously address the issues of 'dark' transport and photoconductivity in films of PbS nanocrystals, by incorporating them into optical field-effect transistors in which the channel conductance is controlled by both gate voltage and incident radiation. Spectrally resolved photoresponses of these devices reveal a weakly conductive mid-gap band that is responsible for charge transport in dark. The mechanism for conductance, however, changes under illumination when it becomes dominated by band-edge quantized states. In this case, the mid-gap band still has an important role as its occupancy (tuned by the gate voltage) controls the dynamics of band-edge charges
A nanocomposite ultraviolet photodetector based on interfacial trap-controlled charge injection
Ultraviolet photodetectors have applications in fields such as medicine, communications and defence1, and are typically made from single-crystalline silicon, silicon carbide or gallium nitride p–n junction photodiodes. However, such inorganic photodetectors are unsuitable for certain applications because of their high cost and low responsivity (<0.2 A W−1)2. Solution-processed photodetectors based on organic materials and/or nanomaterials could be significantly cheaper to manufacture, but their performance so far has been limited2,3,4,5,6,7. Here, we show that a solution-processed ultraviolet photodetector with a nanocomposite active layer composed of ZnO nanoparticles blended with semiconducting polymers can significantly outperform inorganic photodetectors. As a result of interfacial trap-controlled charge injection, the photodetector transitions from a photodiode with a rectifying Schottky contact in the dark, to a photoconductor with an ohmic contact under illumination, and therefore combines the low dark current of a photodiode and the high responsivity of a photoconductor (∼721–1,001 A W−1). Under a bias of <10 V, our device provides a detectivity of 3.4 × 1015 Jones at 360 nm at room temperature, which is two to three orders of magnitude higher than that of existing inorganic semiconductor ultraviolet photodetectors
Effect of correlation and dielectric confinement on 1S1/2(e)nS3/2(h)Excitons in CdTe/CdSe and CdSe/CdTe Type-II quantum dots
Controlled reduction of graphene oxide is an alternative and promising method to tune the electronic and optically active energy gap of this two-dimensional material in the energy range of the visible light spectrum. By means of ab initio calculations, based on hybrid density functional theory, that combine the Hartree–Fock method with the generalized gradient approximation (GGA), we investigated the electronic, optical, and radiative recombination properties of partially reduced graphene oxide, modelled as small islands of pristine graphene formed in an infinite sheet of graphene oxide. We predict that tuning of optically active gaps, in the wide range from ∼6.5 eV to ∼0.25 eV, followed by the electron radiative transition times in the range from ns to μs, can be effected by controlling the level of oxidization
Solution-processed semiconductors for next-generation photodetectors
Efficient light detection is central to modern science and technology.Current photodetectors mainly use photodiodes based on crystalline inorganic elementalsemiconductors, such as silicon, or compounds such as III–V semiconductors. Photodetectorsmade of solution-processed semiconductors — which include organic materials, metal-halideperovskites and quantum dots — have recently emerged as candidates for next-generation lightsensing. They combine ease of processing, tailorable optoelectronic properties, facile integrationwith complementary metal–oxide–semiconductors, compatibility with flexible substrates andgood performance. Here, we review the recent advances and the open challenges in the field ofsolution-processed photodetectors, examining the topic from both the materials and the deviceperspective and highlighting the potential of the synergistic combination of materials and deviceengineering. We explore hybrid phototransistorsand their potential to overcome trade-offsin noise, gain and speed, as well as the rapid advances in metal-halide perovskite photodiodesand their recent application in narrowband filterless photodetection
Light emission efficiency and dynamics in silicon-rich silicon nitride films
Light-emitting Si-rich silicon nitride (SRN) films were fabricated by plasma enhanced chemical vapor deposition followed by thermal annealing and the SRN external quantum efficiency was measured. The SRN light emission temperature dependence and recombination dynamics were also studied. Small emission thermal quenching from 4to330K with wavelength dependent, nanosecond recombination lifetime was observed. Light emission from SRN systems can provide alternative routes towards the fabrication of efficient Si-based optical devices
Quantum coherence of multiple excitons governs absorption cross-sections of PbS/CdS core/shell nanocrystals
半導体ナノ粒子の光吸収効率の増加メカニズムを解明 --高効率な太陽電池や光検出器へ期待--. 京都大学プレスリリース. 2018-08-22.Multiple excitons in semiconductor nanocrystals have been extensively studied with respect to unique carrier dynamics including quantized Auger recombination and implementation in optoelectronic devices such as solar cells and photodetectors. However, the generation mechanism of multiple excitons still remains unclear. Here, we study instantaneous and delayed multiple exciton generation processes in PbS/CdS core/shell nanocrystals. The absorption cross-sections of biexcitons and triexcitons are identical to that of single excitons under instantaneous excitation with a single pulse. In contrast, the delayed excitation using double pulses shows a reduction of the biexciton and triexciton absorption cross-sections. Our theoretical analysis reveals that the excitonic coherence assists the generation of multiple excitons and that the reduction of multiple exciton absorption cross-sections is caused by the reduction of coherent excitation pathways. We clarify that exciton coherences play a key role in multiple exciton generation processes and seamlessly connect the identical and reduced multiple exciton absorption cross-sections
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