Using light to control matter has captured the imagination of scientists for generations, as there is an abundance of photons at our disposal. Yet delivering photons beyond the surface to many photoresponsive systems has proven challenging, particularly at scale, due to light attenuation via absorption and scattering losses. Triplet−triplet annihilation upconversion (TTA-UC), a process which allows for low energy photons to be converted to high energy photons, is poised to overcome these challenges by allowing for precise spatial generation of high energy photons due to its nonlinear nature. With a wide range of sensitizer and annihilator motifs available for TTA-UC, many researchers seek to integrate these materials in solution or solid-state applications. In this Review, we discuss nanoengineering deployment strategies and highlight their uses in recent state-of-the-art examples of TTA-UC integrated in both solution and solid-state applications. Considering both implementation tactics and application-specific requirements, we identify critical needs to push TTA-UC-based applications from an academic curiosity to a scalable technology.

PbS nanocrystals are critical materials for infrared optoelectronics, but the persistent challenge in synthesizing small nanocrystals with narrow line widths demands improved mechanistic understanding. Here, we show that the conventional hot-injection synthesis of PbS nanocrystals per Hines exhibits two-step kinetics involving an intermediate species. The intermediate is small, lead-rich, and has characteristic, reproducible, visible-wavelength emissionall consistent. with a PbS prenucleation cluster (PNC). We then demonstrate that high-pK a amines disrupt the PNC, accelerating nanocrystal nucleation and enabling the synthesis of PbS nanocrystals with diameters as small as ⌀ ∼ 1.7 nm and distinct ensemble absorption peaks (hν = 2.2 eV, λ = 560 nm) in reactions allowed to run to completion. We show that the basicity of the amine additive controls the average size of nanocrystals at reaction completion, which we understand by incorporating metastable PNCs into reaction models that partition monomers between nanocrystal nucleation and nanocrystal growth. This conceptual advance permits the routine synthesis of ultrasmall PbS NCs with excitonic absorption line widths that are up to 25% narrower than previously reported for comparable sizes (⌀: 1.7–3 nm, λpeak,abs: 560–885 nm, hνpeak,abs: 2.2–1.4 eV). This reduced electronic dispersity will enhance device performance, and the underlying insight is further evidence of the exquisite ability of metal-complexing additives to direct the bottom-up syntheses of nanostructured materials.

For gold nanoparticles stabilized with cetyl trimethylammonium bromide (CTAB) and polymer ligands, increase in solvent polarity leads to stabilization–aggregation–stabilization–aggregation transitions.

Metal-halide perovskite nanocrystals have demonstrated excellent optoelectronic properties for light-emitting applications. Isovalent doping with various metals (M 2+ ) can be used to tailor and enhance their light emission. Although crucial to maximize performance, an understanding of the universal working mechanism for such doping is still missing. Here, we directly compare the optical properties of nanocrystals containing the most commonly employed dopants, fabricated under identical synthesis conditions. We show for the first time unambiguously, and supported by first-principles calculations and molecular orbital theory, that element-unspecific symmetry-breaking rather than element-specific electronic effects dominate these properties under device-relevant conditions. The impact of most dopants on the perovskite electronic structure is predominantly based on local lattice periodicity breaking and resulting charge carrier localization, leading to enhanced radiative recombination, while dopant-specific hybridization effects play a secondary role. Our results suggest specific guidelines for selecting a dopant to maximize the performance of perovskite emitters in the desired optoelectronic devices.

Ultraviolet (UV) light can trigger a plethora of useful photochemical reactions for diverse applications, including photocatalysis, photopolymerization, and drug delivery. These applications typically require penetration of high energy photons deep into materials, yet delivering these photons beyond the surface is extremely challenging due to absorption and scattering effects. Triplet-triplet annihilation upconversion (TTA-UC) shows great promise to circumvent this issue by generating high energy photons from incident lower energy photons. However, molecules that facilitate TTA-UC usually have poor water solubility, limiting their deployment in aqueous environments. To address this challenge, we leverage a nanoencapsulation method to fabricate water-compatible UC micelles, enabling on-demand UV photon generation deep into materials. We present two iridium-based complexes for use as TTA-UC sensitizers with increased solubilities that facilitate the formation of highly emissive UV-upconverting micelles. Furthermore, we show this encapsulation method is generalizable to nineteen UV-emitting UC systems, accessing a range of upconverted UV emission profiles with wavelengths as low as 350 nm. As a proof-of-principle demonstration of precision photochemistry at depth, we use UV-emitting UC micelles to photolyze a fluorophore at a focal point nearly a centimeter beyond the surface, revealing opportunities for spatially controlled manipulation deep into UV-responsive materials.

Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has significant implications for producing high energy light beyond a material's surface. However, the deployment of UC materials has been stymied due to poor material solubility, high concentration requirements, and oxygen sensitivity, ultimately resulting in reduced light output. Toward this end, nanoencapsulation has been a popular motif to circumvent these challenges, but durability has remained elusive in organic solvents.Recently, a nanoencapsulation technique was engineered to tackle each of these challenges, whereupon an oleic acid nanodroplet containing upconversion materials was encapsulated with a silica shell. Ultimately, these nanocapsules (NCs) were durable enough to enable triplet fusion upconversion-facilitated volumetric threedimensional (3D) printing. By encapsulating upconversion materials with silica and dispersing them in a 3D printing resin, photopatterning beyond the surface of the printing vat was made possible. Here, video protocols for the synthesis of upconversion NCs are presented for both small-scale and large-scale batches. The outlined protocols serve as a starting point for adapting this encapsulation scheme to multiple upconversion schemes for use in volumetric 3D printing applications.

Over the past few years, high external quantum efficiencies (EQE) have been achieved for blue, green, red, and near infrared perovskite light-emitting diodes (PeLEDs), and their energy efficiencies are approaching the efficiencies of III-V based LEDs. Beyond the visible regime, ultraviolet light offers great promise for many applications such as disinfection, which has become increasingly important since the COVID-19 pandemic. However, PeLEDs demonstrate poor performance in the violet/ultraviolet region, with reports of violet PeLED performance hindered by poor thin film quality. In this work, we improve the uniformity of perovskite film by adding water into the precursor solution to engineer the crystallization process of spin-coated 2D perovskite. The PeLEDs deliver bright violet emission at 408 nm, with a maximum external quantum efficiency of 0.41%, a fivefold increase over control devices. This work demonstrates viable steps towards cost-effective, efficient ultraviolet PeLEDs.

Nanoparticles (NPs) functionalized with polymers have a broad range of applications; however, a key challenge is to achieve high-yield and time-efficient isolation of the polymer-capped NPs from free polymers. Here, we introduce a highly effective thermodynamically driven approach to isolate polymer-grafted NPs. By controllably reducing the solvent quality for the polymer tethers, cellulose nanocrystals end-grafted with poly(N-isopropyl acrylamide) (pNIPAm) were isolated from free pNIPAm. More specifically, under poor solvency conditions, polymer-tethered NPs aggregated and precipitated, while the free polymer remained in the supernatant. The method was validated for the NPs end-grafted with pNIPAm with different molecular weights. The approach was theoretically rationalized using a scaling approach. Based on the NP isolation time and separation efficiency, the solvent-mediated method favorably compared with conventional dialysis and membrane filtration separation techniques and thus can be used for a variety of polymer-functionalized NPs.