Engineering Efficient Photon Upconversion in Semiconductor Heterostructures

Abstract

Photon upconversion is a photophysical process in which two lowenergy photons are converted into one high-energy photon. Photon upconversion has broad appeal for a range of applications from biomedical imaging and targeted drug release to solar energy harvesting. Current upconversion nanosystems, including lanthanide-doped nanocrystals and triplet−triplet annihilation molecules, have achieved upconversion quantum yields on the order of 10−30%. However, the performance of these materials is hampered by inherently narrow absorption cross sections and fixed energy levels originating in atomic, ionic, or molecular states. Semiconductors, on the other hand, have inherently wide absorption cross sections. Moreover, recent advances enable the synthesis of colloidal semiconductor nanoparticles with complex heterostructures that can control band alignments and tune optical properties. We synthesize and characterize a three-component heterostructure that successfully upconverts photons under continuous-wave illumination and solarrelevant photon fluxes. The heterostructure is composed of two cadmium selenide quantum dots (QDs), an absorber and emitter, spatially separated by a cadmium sulfide nanorod (NR). We demonstrate that the principles of semiconductor heterostructure engineering can be applied to engineer improved upconversion efficiency. We first eliminate electron trap states near the surface of the absorbing QD and then tailor the band gap of the NR such that charge carriers are funneled to the emitting QD. When combined, these two changes result in a 100-fold improvement in photon upconversion performance.

NIR-to-visible upconversion in quantum dots via a ligand induced charge transfer state

“…This can be accomplished by careful synthesis of the QD heterostructure in specic architectures, such that the two quantum emitters are spatially separated by a potential barrier. This was previously demonstrated for UC both in the vis-to-vis 11,12 and the NIR-to-vis 13,14 regime. In both of these realizations, a low band-gap dot is rst excited, followed by further excitation of the hole either by intraband absorption or by an Auger mechanism.…”

Ratiometric upconversion nanothermometry with dual emission at the same wavelength decoded via a time-resolved technique

“…Taking all these factors into consideration, it is still difficult for different wavelength dual emission ratiometric nanothermometers to eliminate errors originating from tissue absorption and scattering in vivo. Recently, there has been renewed interest in UCL emission of quantum dots (QDs), which can be excited by a broad wavelength light under a low power density [38][39][40][41] . Due to the participation of phonons in this UCL process, the UCL emission shows excellent temperature-dependent properties, which make it a suitable candidate for fabricating a nanothermometer.…”

Band Gap Engineering Improves the Efficiency of Double Quantum Dot Upconversion Nanocrystals

“…The above reported estimates for UC efficiency are quite significantly higher than those observed in other double QD UC experiments, which were done either under higher repetition rate pulsed pumping (1 KHz) or under CW illumination. In particular, it is notable that the saturation power for CW operation as measured by Milleville and coworkers (albeit on a different system) is 5 W cm −2 , comparable (in terms of average power) to what we get using 5 ns pulsed excitation at 10 Hz, which is very surprising considering the fact that the lifetime of the intermediate excited state is on the scale of 1 µs. We therfore set out to measure the effect of the regime of illumination on the UC efficiency.…”

“…The two emitting nanocrystals are electronically coupled due to close spatial proximity; hence, carriers excited in the low energy state by one low‐energy photon can leave a confined hole and a relatively delocalized electron. Following a second photon absorption, the hole can be further excited and relax to the high‐energy state, leading to upconversion by radiative recombination with the delocalized electron . Similar to other upconversion systems, upconversion QDs are characterized by two important figures of merit: the maximal upconversion quantum yield (UCQY) and the saturation intensity.…”

“…It was concluded that for these QDs, the main mechanism via which upconversion proceeds is by interband absorption followed by Auger recombination . CW pumped upconversion was also observed in a seeded rod system, where the upconversion efficiency was shown to crucially depend on the details of the heterostructure, such as alloying and composition grading . However, despite promising properties of the this family of upconverting nanocrystals, their use in any practical applications has been hampered by the fairly low UCQY and relatively high saturation threshold.…”