14 research outputs found
Suppressed Carrier Scattering In Cds-encapsulated Pbs Nanocrystal Films
One of the key challenges facing the realization of functional nanocrystal devices concerns the development of techniques for depositing colloidal nanocrystals into electrically coupled nanoparticle solids. This work compares several alternative strategies for the assembly of such films using an all-optical approach to the characterization of electron transport phenomena. By measuring excited carrier lifetimes in either ligand-linked or matrix-encapsulated PbS nanocrystal films containing a tunable fraction of insulating ZnS domains, we uniquely distinguish the dynamics of charge scattering on defects from other processes of exciton dissociation. The measured times are subsequently used to estimate the diffusion length and the carrier mobility for each film type within the hopping transport regime. It is demonstrated that nanocrystal films encapsulated into semiconductor matrices exhibit a lower probability of charge scattering than that of nanocrystal solids cross-linked with either 3-mercaptopropionic acid or 1,2-ethanedithiol molecular linkers. The suppression of carrier scattering in matrix-encapsulated nanocrystal films is attributed to a relatively low density of surface defects at nanocrystal/matrix interfaces
Exciton Diffusion in Nanocrystal Solids
This dissertation work is focused on exploring the unique optical and electronic properties of several semiconductor nanostructures and devices based on them. The present research demonstrates the near-field confinement of light achieved through the use of small-diameter Au nanoparticles embedded into a PbS nanocrystal solid. Using this strategy, we developed plasmonic solar cells that can harness the emission of Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The contribution of Au near-field emission toward the charge carrier generation was successfully proved through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, compare to PbS-only devices.
Moreover, unique behavior of semiconductor nanocrystals makes them to be promising candidates not only for photovoltaics but for light-emitting applications as well. In light-emitting devices NC solids are designed to have large interparticle gaps that minimize exciton diffusion to dissociative sites. This strategy reduces electrical coupling between nanoparticles in a film, making the injection of charges inefficient. We demonstrated that bright luminescence from nanocrystal solids can be achieved without compromising their electrical conductivity. Our research showed that solids featuring low absorption-emission spectral overlap exhibit slower exciton diffusion to recombination centers, promoting longer exciton lifetimes. As a result, enhanced emission is achieved despite a strong electronic coupling. The inverse correlation between film luminescence and absorption-emission spectral overlap was verified by the comparison of CdSe/CdS and ZnSe/CdS solids and further confirmed in two control systems (ZnTe/CdSe and Mn2+-doped ZnCdSe/ZnS).
Another challenging task in the development of quantum dot based solids lies in the studying the motion of neutral excitons. The nature of the exciton dissociation mechanism as well as exciton diffusion trajectories in nanocrystal solids remain poorly understood. We developed an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length and the exciton diffusivity
Prospects and applications of plasmon-exciton interactions in the near-field regime
Plasmonics is a rapidly developing field at the boundary of fundamental sciences and device engineering, which exploits the ability of metal nanostructures to concentrate electromagnetic radiation. The principal challenge lies in achieving an efficient conversion of the plasmon-concentrated field into some form of useful energy. To date, a substantial progress has been made within the scientific community in identifying the major pathways of the plasmon energy conversion. Strategies based on the hot electron injection and the near-field energy transfer have already shown promise in a number of proof-of-principle plasmonic architectures. Nevertheless, there are several fundamental questions that need to be addressed in the future to facilitate the transition of plasmonics to a variety of applications in both light amplification and optical detection. Of particular interest is a plasmon-induced resonance energy transfer (PIRET) process that couples the plasmon evanescent field to a semiconductor absorber via dipole-dipole interaction. This relatively unexplored mechanism has emerged as a promising light conversion strategy in the areas of photovoltaics and photocatalysis and represents the main focus of the present minireview. Along these lines, we highlight the key advances in this area and review some of the challenges associated with applications of the PIRET mechanism in nanostructured systems
Double-Well Colloidal Nanocrystals Featuring Two-Color Photoluminescence
Ratiometric sensing
strategy relies on the ratio of the two photoluminescence
(PL) signals originating from the same nano-object for detecting the
changes in the surrounding media. Recently, such dual-color emission
has been demonstrated in semiconductor colloids, where the PL signal
from a quantum-confined domain was complemented with the secondary
emission from transition metal ions or a bulk-like structure. Here,
we report on the development of dual-color nanocrystal colloids featuring
a combination of two quantum-confined emitters within the same nano-object.
The reported morphology relies on double-well core/barrier/shell arrangement,
where zero-dimensional excitons of the core component (PbS) can coexist
with two-dimensional excitons of the shell domain (CdSe). As a result,
the core and shell emission bands can be independently tuned across
880–1500 nm and 600–650 nm spectral windows, respectively.
A CdS potential barrier at the PbS/CdSe interface was designed to
suppress the energy and charge diffusion between the two domains allowing
both emission bands to exhibit quantum yields over 10%. Fabricated
colloids were demonstrated as dual-color probes for sensing the redox
environment, where both the energetics and the timing of photoinduced
charge transfer to an add-on analyte could be inferred from the ratiometric
measurements
Enhanced Emission of Nanocrystal Solids Featuring Slowly Diffusive Excitons
Solution
processing of semiconductor nanocrystal (NC) solids represents
an attractive platform for the development of next-generation optoelectronic
devices. In search of enhanced light-emitting performance, NC solids
are typically designed to have large interparticle gaps that minimize
exciton diffusion to dissociative sites. This strategy, however, reduces
electrical coupling between nanoparticles in a film, making the injection
of charges inefficient. Here, we demonstrate that bright emission
from nanocrystal solids can be achieved without compromising their
electrical conductivity. Our study shows that solids featuring a low
absorption-emission spectral overlap (<i>J</i>) exhibit
an intrinsically slower exciton diffusion to recombination centers,
promoting longer exciton lifetimes. As a result, enhanced emission
is achieved despite a strong electronic coupling. The observed phenomenon
was found consistent with a decreased resonant energy transfer in
films exhibiting a reduced <i>J</i> value. The inverse correlation
between film luminescence and <i>J</i> was revealed through
a comparative analysis of CdSe/CdS and ZnSe/CdS solids and further
confirmed in two control systems (ZnTe/CdSe and Mn<sup>2+</sup>-doped
ZnCdSe/ZnS). Exceptionally slow exciton diffusion (∼0.3 ms)
and high brightness were observed for Mn<sup>2+</sup>-doped Zn<sub>1–<i>x</i></sub>Cd<sub><i>x</i></sub>Se/ZnS
NC films exhibiting a nearly vanishing <i>J</i> parameter.
We expect that the demonstrated combination of electrical coupling
and bright emission in nanocrystal solids featuring low <i>J</i> can benefit the development of nanocrystal light-emitting technologies
Mapping the Exciton Diffusion in Semiconductor Nanocrystal Solids
Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal–metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling
Plasmonic Nanocrystal Solar Cells Utilizing Strongly Confined Radiation
The ability of metal nanoparticles to concentrate light <i>via</i> the plasmon resonance represents a unique opportunity for funneling the solar energy in photovoltaic devices. The absorption enhancement in plasmonic solar cells is predicted to be particularly prominent when the size of metal features falls below 20 nm, causing the strong confinement of radiation modes. Unfortunately, the ultrashort lifetime of such near-field radiation makes harvesting the plasmon energy in small-diameter nanoparticles a challenging task. Here, we develop plasmonic solar cells that harness the near-field emission of 5 nm Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The interfaces of Au and PbS domains were designed to support a rapid energy transfer at rates that outpace the thermal dephasing of plasmon modes. We demonstrate that central to the device operation is the inorganic passivation of Au nanoparticles with a wide gap semiconductor, which reduces carrier scattering and simultaneously improves the stability of heat-prone plasmonic films. The contribution of the Au near-field emission toward the charge carrier generation was manifested through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, as measured relative to PbS-only devices
Lifting the Spectral Crosstalk in Multifluorophore Assemblies
A general
strategy for measuring the energy-transfer efficiencies
in multifluorophore assemblies is demonstrated. The present method
is based on spectral shaping of the excitation light with molecular
solutions representing donor and acceptor fluorophores, which causes
a suppressed excitation of the respective donor and acceptor molecules
in the sample. The changes in the acceptor emission resulting from
spectral shaping of the excitation light are then used to determine
the energy-transfer efficiencies (<i>E</i><sub><i>x</i></sub>) associated with all participating donor–acceptor pairs.
Here, the technique is demonstrated through energy-transfer (ET) measurements
in a 4-fluorophore construct featuring a DNA supported assembly of
three donor/donor-relay (Cy3, Cy3.5, and Cy5) and one acceptor (Cy5.5)
molecules. The resulting <i>E</i><sub><i>x</i></sub> were validated using the standard photoluminescence (PL) quenching
approach as well as measurements of partial 2- and 3-dye assemblies.
The present work highlights general benefits of the spectrally shaped
excitation approach to measuring donor–acceptor energetics,
including the ability to resolve the spectral cross talk between multiple
fluorophores and to exclude charge transfer contributions into donor
PL quenching
Plasmon-Induced Energy Transfer: When the Game Is Worth the Candle
The
superior optical extinction characteristics of noble metal nanoparticles
have long been considered for enhancing the solar energy absorption
in light-harvesting devices. The energy captured through a plasmon
resonance mechanism can potentially be transferred to a surrounding
semiconductor matrix in the form of excitons or charge carriers, offering
a promising light-sensitization strategy. Of particular interest is
the plasmon near-field energy conversion, which is predicted to yield
substantial gains in the photocarrier generation. Such a short-range
interaction, however, is often inhibited by processes of backward
electron and energy transfer, which obscure its net benefit. Here,
we employ sample-transmitted excitation photoluminescence spectroscopy
to determine the quantum efficiency for the plasmon-induced energy
transfer (ET) in assemblies of Au nanoparticles and CdSe nanocrystals.
The present technique distinguishes the Au-to-CdSe ET contribution
from metal-induced quenching processes, thus enabling accurate estimates
of the photon-to-exciton conversion efficiency. We show that in the
case of 9.1 nm Au nanoparticles only 1–2% of the Au absorbed
radiation is converted to excitons in the surrounding CdSe nanocrystal
matrix. For larger, 21.0 nm Au, the photon-to-exciton conversion efficiency
increases to 29.5%. The results of the present measurements were used
to develop an empirical model for estimating the maximum gain in plasmon-induced
carriers versus the mass fraction of Au in a film
Suppressed Carrier Scattering in CdS-Encapsulated PbS Nanocrystal Films
One of the key challenges facing the realization of functional nanocrystal devices concerns the development of techniques for depositing colloidal nanocrystals into electrically coupled nanoparticle solids. This work compares several alternative strategies for the assembly of such films using an all-optical approach to the characterization of electron transport phenomena. By measuring excited carrier lifetimes in either ligand-linked or matrix-encapsulated PbS nanocrystal films containing a tunable fraction of insulating ZnS domains, we uniquely distinguish the dynamics of charge scattering on defects from other processes of exciton dissociation. The measured times are subsequently used to estimate the diffusion length and the carrier mobility for each film type within the hopping transport regime. It is demonstrated that nanocrystal films encapsulated into semiconductor matrices exhibit a lower probability of charge scattering than that of nanocrystal solids cross-linked with either 3-mercaptopropionic acid or 1,2-ethanedithiol molecular linkers. The suppression of carrier scattering in matrix-encapsulated nanocrystal films is attributed to a relatively low density of surface defects at nanocrystal/matrix interfaces