14 research outputs found
Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon
We have shown theoretically that efficient multiple exciton generation (MEG)
by a single photon can be observed in small nanocrystals (NCs). Our quantum
simulations that include hundreds of thousands of exciton and multi-exciton
states demonstrate that the complex time-dependent dynamics of these states in
a closed electronic system yields a saturated MEG effect on a picosecond
timescale. Including phonon relaxation confirms that efficient MEG requires the
exciton--biexciton coupling time to be faster than exciton relaxation time
Bright triplet excitons in lead halide perovskites
Nanostructured semiconductors emit light from electronic states known as
excitons[1]. According to Hund's rules[2], the lowest energy exciton in organic
materials should be a poorly emitting triplet state. Analogously, the lowest
exciton level in all known inorganic semiconductors is believed to be optically
inactive. These 'dark' excitons (into which the system can relax) hinder
light-emitting devices based on semiconductor nanostructures. While strategies
to diminish their influence have been developed[3-5], no materials have been
identified in which the lowest exciton is bright. Here we show that the lowest
exciton in quasi-cubic lead halide perovskites is optically active. We first
use the effective-mass model and group theory to explore this possibility,
which can occur when the strong spin-orbit coupling in the perovskite
conduction band is combined with the Rashba effect [6-10]. We then apply our
model to CsPbX3 (X=Cl,Br,I) nanocrystals[11], for which we measure size- and
composition-dependent fluorescence at the single-nanocrystal level. The bright
character of the lowest exciton immediately explains the anomalous
photon-emission rates of these materials, which emit 20 and 1,000 times
faster[12] than any other semiconductor nanocrystal at room[13-16] and
cryogenic[17] temperatures, respectively. The bright exciton is further
confirmed by detailed analysis of the fine structure in low-temperature
fluorescence spectra. For semiconductor nanocrystals[18], which are already
used in lighting[19,20], lasers[21,22], and displays[23], these optically
active excitons can lead to materials with brighter emission and enhanced
absorption. More generally, our results provide criteria for identifying other
semiconductors exhibiting bright excitons with potentially broad implications
for optoelectronic devices.Comment: 14 pages and 3 figures in the main text, Methods and extended data 16
pages which include 11 figures, and supporting information 28 page
Bright triplet excitons in caesium lead halide perovskites
Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund’s rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the ‘dark exciton’. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX_3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX_3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices
Symmetry Breaking Induced Activation of Nanocrystal Optical Transitions
We have analysed the effect of symmetry breaking on the optical properties of semiconductor nanocrystals due to doping by charged impurities. Using doped CdSe nanocrystals as an example, we show the effects of a Coulomb center on the exciton fine-structure and optical selection rules using symmetry theory and then quantify the effect of symmetry breaking on the exciton fine structure, modelling the charged center using a multipole expansion. The model shows that the presence of a Coulomb center breaks the nanocrystal symmetry and affects its optical properties through mixing and shifting of the hole spin and parity sublevels. This symmetry breaking, particularly for positively charged centers, shortens the radiative lifetime of CdSe nanocrystals even at room temperature, in qualitative agreement with the increase in PL efficiency observed in CdSe nanocrystals doped with positive Ag charge centers [A. Sahu et.al., Nano Lett. 12, 2587, (2012)]. The effect of the charged center on the photoluminescence and the absorption spectra is shown, with and without the presence of compensating charges on the nanocrystal surface. While spectra of individual nanocrystals are expected to shift and broaden with the introduction of a charged center, configuration averaging and inhomogeneous broadening are shown to wash out these effects. The presence of compensating charges at the NC surface also serves to stabilize the band edge transition energies relative to NCs with no charge centers
Symmetry Breaking Induced Activation of Nanocrystal Optical Transitions
We have analysed the effect of symmetry breaking on the optical properties of semiconductor nanocrystals due to doping by charged impurities. Using doped CdSe nanocrystals as an example, we show the effects of a Coulomb center on the exciton fine-structure and optical selection rules using symmetry theory and then quantify the effect of symmetry breaking on the exciton fine structure, modelling the charged center using a multipole expansion. The model shows that the presence of a Coulomb center breaks the nanocrystal symmetry and affects its optical properties through mixing and shifting of the hole spin and parity sublevels. This symmetry breaking, particularly for positively charged centers, shortens the radiative lifetime of CdSe nanocrystals even at room temperature, in qualitative agreement with the increase in PL efficiency observed in CdSe nanocrystals doped with positive Ag charge centers [A. Sahu et.al., Nano Lett. 12, 2587, (2012)]. The effect of the charged center on the photoluminescence and the absorption spectra is shown, with and without the presence of compensating charges on the nanocrystal surface. While spectra of individual nanocrystals are expected to shift and broaden with the introduction of a charged center, configuration averaging and inhomogeneous broadening are shown to wash out these effects. The presence of compensating charges at the NC surface also serves to stabilize the band edge transition energies relative to NCs with no charge centers
Direct Observation of Photoexcited Hole Localization in CdSe Nanorods
Quantum-confined
1D semiconductor nanostructures are being investigated
for hydrogen generation photocatalysts. In the photoreaction, after
fast electron transfer, holes that remain in the nanostructure play
an important role in the total quantum yield of hydrogen production.
Unfortunately, knowledge of hole dynamics is limited due to lack of
convenient spectroscopic signatures. Here, we directly probe hole
localization dynamics within CdSe nanorods (NRs) by combining transient
absorption (TA) and time-resolved terahertz (TRTS) spectroscopy. We
show that when methylene blue is used as an electron acceptor, the
resulting electron transfer occurs with a time constant of 3.5 ±
0.1 ps and leaves behind a delocalized hole. However, the hole quickly
localizes in the Coulomb potential well generated by the reduced electron
acceptor near the NR surface with time constant of 11.7 ± 0.2
ps. Our theoretical investigation suggests that the hole becomes confined
to a ∼±0.8 nm region near the reduced electron acceptor
and the activation energy to detrap the hole from the potential well
can be as large as 235 meV