Exciton Engineering Research Articles

Lifetime enhancement of exciplex based organic light-emitting diodes by triplet exciton engineering

A device approach to improve the device lifetime of exciplex organic light-emitting diodes was studied by managing the intermolecular interaction and concentration quenching of exciplexes. The exciplexes were made up of a p-type material and a bipolar n-type material, in which the ratio of the p-type material was less than 50% for dispersion in the n-type material to copy the conventional host-dopant type emitting layer. The exciplexes generated between the p-type material and n-type material were dispersed in the n-type material. The device lifetime of the exciplex organic light-emitting diodes was extended by almost four times through control of the p-type material content by mimicking the host-dopant system. The analysis of the lifetime improvement mechanism revealed that suppression of triplet exciton loss by strong intermolecular interaction and concentration quenching was the major pathway to stabilize the exciplex devices by dispersion. Additionally, the device lifetime of the fluorescence emitter doped exciplex devices was extended by more than three times. Therefore, the exciplexes with a p-type material dispersed in the bipolar n-type material were effective to elongate the device lifetime of the exciplex based organic light-emitting diodes.

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs.

We investigate the excitonic peak associated with defects and disorder in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs). To uncover the intrinsic origin of defect-related (D) excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer MoS2. Our results suggest that D excitons are neutral excitons bound to ionized donor levels, likely related to sulfur vacancies, with a density of 7 × 1011 cm-2. To study the extrinsic contribution to D excitons, we controllably deposit oxygen molecules in situ onto the surface of MoS2 kept at cryogenic temperature. We find that, in addition to trivial p-doping of 3 × 1012 cm-2, oxygen affects the D excitons, likely by functionalizing the defect sites. Combined, our results uncover the origin of D excitons, suggest an approach to track the functionalization of TMDCs, to benchmark device quality, and pave the way toward exciton engineering in hybrid organic-inorganic TMDC devices.

Self-trapped exciton engineering for white-light emission in colloidal lead-free double perovskite nanocrystals

Intrinsic broadband photoluminescence (PL) of self-trapped excitons (STEs) are systematically studied in lead-free double perovskite nanocrystals (NCs). It is clarified that bandgap (direct/indirect) has important influence on the PL properties of STEs: indirect bandgap NCs exhibit strong exciton-phonon coupling which results in non-radiative STEs, while direct bandgap NCs exhibit moderate exciton-phonon coupling, inducing bright STE PL. Furthermore, by alloying K+ and Li+ ions in Cs2AgInCl6 NCs, the NCs exhibit broadband white-light emission. Charge-carrier dynamics study indicates that the efficient white-light emission originates from the further suppressed non-radiative processes of the STEs in the direct bandgap structure. This work may deepen the understanding of STEs and guide the design of high-performance lead-free perovskites.

Perfectly imperfect: a review of chemical tools for exciton engineering in single-walled carbon nanotubes

This review investigates the concept of introducing disorder to single-walled carbon nanotubes for enhancement of their optical properties.

Carbon nitride with electron storage property: Enhanced exciton dissociation for high-efficient photocatalysis

Excitonic effect, originated from the strong Coulomb attraction between electron and hole, plays an important role in the photocatalytic process of polymeric materials but has been long ignored. In view point of hot-carriers generation, the dissociation of Frenkel excitons is proposed as an effective way to improve the photocatalytic performance of polymeric photocatalysts. Herein, by taking graphitic carbon nitride (g-C3N4) as an example, we verify that endowing g-C3N4 with electron storage ability can facilitate exciton dissociation by extracting electrons from bound electron-hole couples around the electron stotage sites, therefore enhancing the hot-carriers harvest and suppressing the charge recombination. Benefiting from these advantages, the as-prepared material demonstrates excellent photocatalytic performance for both H2 evolution and H2O2 generation. As a result, the apparent quantum yield (AQY) for H2 evolution at 420 nm reaches 55%, which is much higher than most of the reported polymeric materials. The study described here offers a new way for designing advanced polymeric photocatalysts toward high performance solar energy conversion via excitonic engineering.

(Invited) Imaging Dopamine Neuromodulation with Single Wall Carbon Nanotube Sensors

Neurons communicate through chemical neurotransmitter signals that either terminate at the postsynaptic process (“wired transmission”) or diffuse beyond the synaptic cleft to modulate the activity of larger neuronal networks (“volume transmission”). Molecules such as dopamine belong to the latter class of neurotransmitters, for which real-time imaging of the signal’s spatial propagation would constitute a major advance in neurochemical imaging. To this end, we present a nanoscale near-infrared fluorescent sensor for dopamine and demonstrate its efficacy for imaging dopamine volume transmission in the extracellular space of the brain striatum and prefrontal cortex. The sensor is developed from polymers pinned to the surface of single wall carbon nanotubes (SWNT) in which the surface-adsorbed polymer is the recognition moiety and the carbon nanotube the fluorescence transduction element. Using this paradigm, we describe the design, characterization, and implementation of polymer-functionalized SWNT nanosensors and present recent progress in imaging the dynamics of evoked and spontaneous dopamine release in mouse acute brain slices. Fluorescence imaging of dopamine volume transmission with our probes provides much needed spatial information and high resolution of dopamine dynamics in contrast with traditional electrochemical dopamine measurements obtained with fast scan cyclic voltammetry (FSCV). Whereas FSCV measures dopamine concentration at a single point arising from the activity of a large number of dopamine terminals, our probes capture local release arising from single and multiple terminals within the imaging field of view. From imaging data, we can describe localized release and clearance kinetics of dopamine neuromodulation. In the brain striatum, the nanosensors record spatially and temporally resolved dynamic behavior of dopamine on length scales that encompass single terminals and inter-terminal distances, and enable investigation of the effect of antidepressant drugs such as Nomifensine on dopamine dynamics. In the prefrontal cortex (PFC), an area of sparse dopaminergic innervation, dopamine nanosensors capture dynamics elicited by single terminals. In both the striatum and PFC, optogenetics enables selective stimulation of dopaminergic terminals to evoke dopamine release. We further describe how nanoparticle exciton engineering can be used to tune sensor performance and design sensors best suited for in vivo experimentation. Molecular dynamics modeling elucidates the physicochemical phenomena underlying the dopamine sensing process and stochastic simulations reveal that optical nanosensors can capture millisecond-scale changes during phasic and tonic firing of dopaminergic neurons. Combining our experimental and computational work, we outline the functional range of this technology for imaging dopamine neuromodulation in biologically complex and optically dense live brain. Our experimental and theoretical results show that near-infrared (nIR) neurotransmitter nanosensor constructs can relay information about neuronal signaling in the tissue-compatible nIR optical window, with spatiotemporal scales that capture both single synaptic release events and ensemble terminal behavior suitable for in vivo behavioral experimentation.

Emergence of new red-shifted carbon nanotube photoluminescence based on proximal doped-site design.

Single-walled carbon nanotubes (SWNTs) show unique photoluminescence (PL) in the near-infrared (NIR) region. Here we propose a concept based on the proximal modification in local covalent functionalization of SWNTs. Quantum mechanical simulations reveal that the SWNT band gap changes specifically based on the proximal doped-site design. Thus, we synthesize newly-designed bisdiazonium molecules and conduct local fucntionalisation of SWNTs. Consequently, new red-shifted PL (E112*) from the bisdiazonium-modified SWNTs with (6, 5) chirality is recognized around 1250 nm with over ~270 nm Stokes shift from the PL of the pristine SWNTs and the PL wavelengths are shifted depending on the methylene spacer lengths of the modifiers. The present study revealed that SWNT PL modulation is enable by close-proximity-local covalent modification, which is highly important for fundamental understanding of intrinsic SWNT PL properties as well as exciton engineering–based applications including photonic devices and (bio)imaging/sensing.

First-Principles Method of Propagation of Tightly Bound Excitons: Verifying the Exciton Band Structure of LiF with Inelastic x-Ray Scattering

We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures.