Carrier multiplication in semiconductor nanocrystals detected by energy transfer to organic dye molecules

Single-Photon Emission from Organic Dye Molecules Adsorbed on a Quantum Dot via Energy Transfer

Single-photon emissions from individual emitters are crucial in fundamental science and quantum information technologies. Multichromophoric systems, comprising multiple dyes, can exhibit single-photon emissions through efficient annihilation between the excited states; however, exploring this phenomenon in complex systems remains a challenge. In this study, we investigated the photon statistics of emissions from multiple perylene bisimide (PBI) dyes adsorbed onto the surface of CdSe/ZnS quantum dots (QDs). When multiple PBIs were simultaneously excited by both direct excitation and energy transfer from the QD, multiphoton emissions from the PBIs were observed. Conversely, when the QDs were selectively excited, multiple PBIs exhibiting single-photon emission through energy transfer from the QDs to the PBIs were found. These results highlight the intriguing interplay between multichromophoric systems and QDs, offering valuable insights into the development of efficient single-photon sources in quantum information technologies.

Photocatalysis in Two-Dimensional Black Phosphorus: The Roles of Many-Body Effects

Two-dimensional (2D) black phosphorus (BP) has drawn tremendous attention in solar-light-driven catalytic processes for its intriguing chemical and physical properties. Benefiting from the highly anisotropic electronic structure induced by its puckered crystal geometry, 2D BP tends to have greater confinement with respect to traditional inorganic nanomaterials, thereby leading to robust many-body effects. Such Coulomb-interaction-mediated effects dominate the electronic and optical properties of 2D BP-based nanosystems, where exotic correlations between photoinduced species give rise to unique photoexcitation processes that are closely associated with the involved photocatalytic behavior. In this Perspective, we highlight the critical role of many-body effects in 2D BP-based photocatalysis and exemplify the relationships between the correlated photoinduced species-dominated photoexcitation processes and photocatalytic behavior involved therein. The relevant challenges and opportunities in pursuing efficient 2D BP-based solar energy utilization are also discussed.

A feasibility study of unconventional planar ligand spacers in chalcogenide nanocrystals

The solar cell efficiency of chalcogenide nanocrystals (quantum dots) has been limited in the past by the insulation between neighboring quantum dots caused by intervening, often long-chain, aliphatic ligands. We have conducted a computationally based feasibility study to investigate the use of ultra-thin, planar, charge-conducting ligands as an alternative to traditional long passive ligands. Not only might these radically unconventional ligands decrease the mean distance between adjacent quantum dots, but, since they are charge-conducting, they have the potential to actively enhance charge migration. Our ab initio studies compare the binding energies, electronic energy gaps, and absorption characteristics for both conventional and unconventional ligands, such as phthalocyanines, porphyrins and coronene. This comparison identified these unconventional ligands with the exception of titanyl phthalocyanine, that bind to themselves more strongly than to the surface of the quantum dot, which is likely to be less desirable for enhancing charge transport. The distribution of finite energy levels of the bound system is sensitive to the ligand's binding site and the levels correspond to delocalized states. We also observed a trap state localized on a single Pb atom when a sulfur-containing phenyldithiocarbamate (PTC) ligand is attached to a slightly off-stoichiometric dot in a manner that the sulfur of the ligand completes stoichiometry of the bound system. Hence, this is indicative of the source of trap state when thio-based ligands are bound to chalcogenide nanocrystals. We also predict that titanyl phthalocyanine in a mix with chalcogenide dots of diameter ∼1.5 Å can form a donor-acceptor system.

Energy Transfer of Biexcitons in a Single Semiconductor Nanocrystal

Photoluminescence (PL) decay dynamics of multiexcitons in semiconductor nanocrystals (NCs) are dominated by the nonradiative Auger effect, making it difficult to explore their basic optical processes such as radiative recombination and energy transfer (ET). Here we constructed a single-particle ET system by attaching several acceptor dyes to the surface of a donor NC to study the ET of biexcitons at a single-NC level. By comparing the single-exciton and biexciton PL lifetimes of the same donor NC before and after the acceptor dyes were bleached, their respective ET lifetimes could be reliably extracted without the Auger influence. From statistical measurements on a large number of single ET particles, the average ET rate ratio between biexcitons and single excitons was estimated to be larger than four, and the same scaling rule could be naturally extended to their radiative rates.

Carrier Multiplication in a Single Semiconductor Nanocrystal

To confirm the existence of the carrier multiplication (CM) effect and estimate its generation efficiency of multiple excitons in semiconductor nanocrystals (NCs), it is imperative to completely exclude the false contribution of charged excitons from the measured CM signal. Here we place single CdSe NCs above an aluminum film and successfully resolve their UV-excited photoluminescence (PL) time trajectories where the true and false CM signals are contained in the blinking "on" and "off" levels, respectively. By analyzing the PL dynamics of the on-level photons, an average CM efficiency of ∼20.2% can be reliably estimated when the UV photon energy is ∼2.46 times the NC energy gap.

Optofluidic FRET lasers using aqueous quantum dots as donors

An optofluidic FRET (fluorescence resonance energy transfer) laser is formed by putting FRET pairs inside a microcavity acting as a gain medium. This integration of an optofluidic laser and the FRET mechanism provides novel research frontiers, including sensitive biochemical analysis and novel photonic devices, such as on-chip coherent light sources and bio-tunable lasers. Here, we investigated an optofluidic FRET laser using quantum dots (QDs) as FRET donors. We achieved lasing from Cy5 as the acceptor in a QD-Cy5 pair upon excitation at 450 nm, where Cy5 has negligible absorption by itself. The threshold was approximately 14 μJ mm(-2). The demonstrated capability of QDs as donors in the FRET laser greatly improves the versatility of optofluidic laser operation due to the broad and large absorption cross section of the QDs in the blue and UV spectral regions. The excitation efficiency of the acceptor molecules through a FRET channel was also analyzed, showing that the energy transfer rate and the non-radiative Auger recombination rate of QDs play a significant role in FRET laser performance.

A near IR photosensitizer based on self-assembled CdSe quantum dot-aza-BODIPY conjugate coated with poly(ethylene glycol) and folic acid for concurrent fluorescence imaging and photodynamic therapy

Nanocomposite comprised of CdSe quantum dot-thiophene-substituted aza-BODIPY conjugate coated with FA and PEG has been developed for simultaneous fluorescence imaging and photodynamic therapy of HeLa cells via Förster resonance energy transfer.

Prospects of nanoscience with nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Enhanced carrier multiplication in engineered quasi-type-II quantum dots

One process limiting the performance of solar cells is rapid cooling (thermalization) of hot carriers generated by higher-energy solar photons. In principle, the thermalization losses can be reduced by converting the kinetic energy of energetic carriers into additional electron-hole pairs via carrier multiplication (CM). While being inefficient in bulk semiconductors this process is enhanced in quantum dots, although not sufficiently high to considerably boost the power output of practical devices. Here we demonstrate that thick-shell PbSe/CdSe nanostructures can show almost a fourfold increase in the CM yield over conventional PbSe quantum dots, accompanied by a considerable reduction of the CM threshold. These structures enhance a valence-band CM channel due to effective capture of energetic holes into long-lived shell-localized states. The attainment of the regime of slowed cooling responsible for CM enhancement is indicated by the development of shell-related emission in the visible observed simultaneously with infrared emission from the core.

Carrier multiplication can improve the performance of solar cells, but its efficiency is still not high enough to considerably increase the power output of practical devices. Cirloganu et al. show that appropriately designed core-shell quantum dots can enhance the carrier multiplication yield four-fold.

Reducing the efficiency droop by lateral carrier confinement in InGaN/GaN quantum-well nanorods

Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes (LEDs). In this paper, we report the suppression of efficiency droop induced by the process of density-activated defect recombination in nanorod structures of a-plane InGaN/GaN QWs. In the high carrier density regime, the retained emission efficiency in a dry-etched nanorod sample is observed to be over two times higher than that in its parent QW sample. We further argue that such improvement is a net effect that the lateral carrier confinement overcomes the increased surface trapping introduced during fabrication.

Broadband optical non-linearity induced by charge-transfer excitons in type-II CdSe/ZnTe nanocrystals

Benefiting from excitonic charge-transfer states, an efficient non-linear optical response is observed in type-II nanocrystals by four-wave mixing when the incident photon energy lies below the bandgaps of constituent semiconductors. The non-linear optical properties in nanocrystals can be manipulated by the band alignment of the core-shell components, which serves as a promising platform to design broadband non-linear optical devices.