Triplet excitons are pervasive in both organic and inorganic semiconductors but generally remain confined to the material in which they originate. We demonstrated by transient absorption spectroscopy ...that cadmium selenide semiconductor nanoparticles, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surfacr-anchored polyaromatic carboxylic acid acceptors, extending the excited-state lifetime by six orders of magnitude. Net triplet energy transfer also occurs from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in an aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to the bulk solution implies that such materials are generally effective surrogates for molecular triplets. The nanoparticles could thereby potentially sensitize a range of chemical transformations that are relevant for fields as diverse as optoelectronics, solar energy conversion, and photobiology.
The generation and transfer of triplet excitons across semiconductor nanomaterial-molecular interfaces will play an important role in emerging photonic and optoelectronic technologies, and ...understanding the rules that govern such phenomena is essential. The ability to cooperatively merge the photophysical properties of semiconductor quantum dots with those of well-understood and inexpensive molecular chromophores is therefore paramount. Here we show that 1-pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. This phenomenon results from a near quantitative triplet-triplet energy transfer from the nanocrystals to 1-pyrenecarboxylic acid, producing a molecular triplet-state 'reservoir' that thermally repopulates the photoluminescent state of CdSe through endothermic reverse triplet-triplet energy transfer. The photoluminescence properties are systematically and predictably tuned through variation of the quantum dot-molecule energy gap, temperature and the triplet-excited-state lifetime of the molecular adsorbate. The concepts developed are likely to be applicable to semiconductor nanocrystals interfaced with molecular chromophores, enabling potential applications of their combined excited states.
The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor–ligand interface, we demonstrate ...that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the “degraded” nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.
Recent progress in the colloidal synthesis of inorganic nanocrystals has led to the realization of complex, multidomain nanoparticle morphologies that give rise to advanced optoelectronic properties. ...Such nanocomposites are particularly appealing for photocatalytic applications where tunable absorption, extensive charge separation, and large surface-to-volume ratios are important. To date, heterostructured nanocrystals featuring a metal catalyst and a semiconductor “chromophore” component have shown compelling efficiencies in photoreduction reactions, including sacrificial hydrogen production. Time-resolved optical studies have attributed their success to a near-complete separation of photoinduced charges across dissimilar nanoparticle domains. The spectroscopy approach has also identified the key performance-limiting factors of nanocrystal catalysts that arise from inefficient extraction of photoinduced charges to catalytic sites. Along these lines, the main scope of present-day efforts targets the improvement of interstitial charge transfer pathways across the chromophore-catalyst assembly through the design of high-quality stoichiometric interfaces.
Photocatalytic systems based on colloidal semiconductor nanocrystals have gained considerable attention owing to potential benefits that include a visible-range light extinction and a low spatial ...overlap of photoinduced charges. When coupled to metal catalysts, nanocrystal sensitizers have demonstrated a compelling performance in homogenous photoreduction reactions, including the degradation of organic dyes and hydrogen generation. Going beyond half-cycle reactions, however, the progress in the field of nanocrystal photocatalysis has been rather limited. Here, we review some of the challenges associated with photocatalytic applications of colloidal semiconductor nanocrystals and highlight possible directions aimed toward their resolution. A particular emphasis was made on new paradigms in this field, including the possibility of harvesting triplet excitons and utilizing nanocrystal assemblies to accumulate multiple charges at the reaction site.
Efficient, fast, and robust scintillators for ionizing radiation detection are crucial in various fields, including medical diagnostics, defense, and particle physics. However, traditional ...scintillator technologies face challenges in simultaneously achieving optimal performance and high-speed operation. Herein we introduce colloidal quantum shell heterostructures as X-ray and electron scintillators, combining efficiency, speed, and durability. Quantum shells exhibit light yields up to 70,000 photons MeV
at room temperature, enabled by their high multiexciton radiative efficiency thanks to long Auger-Meitner lifetimes (>10 ns). Radioluminescence is fast, with lifetimes of 2.5 ns and sub-100 ps rise times. Additionally, quantum shells do not exhibit afterglow and maintain stable scintillation even under high X-ray doses (>10
Gy). Furthermore, we showcase quantum shells for X-ray imaging achieving a spatial resolution as high as 28 line pairs per millimeter. Overall, efficient, fast, and durable scintillation make quantum shells appealing in applications ranging from ultrafast radiation detection to high-resolution imaging.
The nature of exciton−plasmon interactions in Au-tipped CdS nanorods has been investigated using femtosecond transient absorption spectroscopy. The study demonstrates that the key optoelectronic ...properties of composite heterostructures comprising electrically coupled metal and semiconductor domains are substantially different from those observed in systems with weak interdomain coupling. In particular, strongly coupled nanocomposites promote mixing of electronic states at semiconductor−metal domain interfaces, which causes a significant suppression of both plasmon and exciton excitations of carriers.
Ultrafast transient absorption spectroscopy was used to investigate the nature of photoinduced charge transfer processes taking place in ZnSe/CdS/Pt colloidal heteronanocrystals. These nanoparticles ...consist of a dot-in-a-rod semiconductor domain (ZnSe/CdS) coupled to a Pt tip. Together the three components are designed to dissociate an electron–hole pair by pinning the hole in the ZnSe domain while allowing the electron to transfer into the Pt tip. Separated charges can then induce a catalytic reaction, such as the light-driven hydrogen production. Present measurements demonstrate that the internal electron–hole separation is fast and results in the localization of both charges in nonadjacent parts of the nanoparticle. In particular, we show that photoinduced holes become confined within the ZnSe domain in less than 2 ps, while electrons take approximately 15 ps to transition into a Pt tip. More importantly, we demonstrate that the presence of the ZnSe dot within the CdS nanorods plays a key role both in enabling photoinduced separation of charges and in suppressing their backward recombination. The implications of the observed exciton dynamics to photocatalytic function of ZnSe/CdS/Pt heteronanocrystals are discussed.
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