Abstract: Metal halide perovskites possess desirable optical, material, and electrical properties which have had substantial impact on next-generation optoelectronics. However, given the toxicity of lead, alternative lead-free perovskite semiconductors are needed. By fully replacing lead with rare-earth elements, one can simultaneously address toxicity concerns and achieve comparable optoelectronic performance. Here, we demonstrate the synthesis of two-dimensional europium halide perovskite nanoplatelets governed by the formula L2EuX4 where L is an organic ligand and X is a halide anion. The structure, morphology, and composition of the nanoplatelets are confirmed by XRD, AFM, and XPS. Deep blue-emitting PEA2EuBr4 perovskite nanoplatelets are synthesized in both the solution- and solid-states with photoluminescence emission centered at 446 nm and CIE color coordinates of (0.1515, 0.0327) and (0.1515, 0.0342), respectively. Additionally, near-ultraviolet PEA2EuCl4 perovskite nanoplatelets are synthesized in both the solution- and solid-states with photoluminescence emission centered at 400 nm and 401 nm, respectively. Overall, europium halide perovskite nanoplatelets offer a lead-free alternative for deep blue, violet, and near-ultraviolet light emission – charting new pathways for optoelectronics in this energy regime.

Abstract: Emissive metal halide perovskites (MHPs) have emerged as excellent candidates for next-generation optoelectronics, due to their sharp color purity, inexpensive processing, and bandgap tunability. However, the development of violet and ultraviolet light-emitting MHPs has lagged behind, due to challenges related to material and device stability, charge carrier transport, tunability into the ultraviolet spectrum, toxicity, and scalability. Here, we review the progress of both violet and ultraviolet MHP nanomaterials and light-emitting diodes, including materials synthesis and device fabrication across various crystal structures and dimensions (e.g., bulk thin films, 2D thin films, nanoplatelets, nanocrystals, quantum dots, and more) as well as lead-free platforms (e.g., rare-earth halide perovskites). By highlighting several pathways to continue the development of violet and ultraviolet light-emitting MHPs while also proposing tactics to overcome their immediate challenges, state-of-the-art violet and ultraviolet MHP materials and devices can be achieved for important applications in public health, 3D printing, nanofabrication, optical communications, and more.

Recently, the incorporation of chiral ligands within halide perovskites has yielded both spin-polarized currents and chiral light emission—unlocking new possibilities for opto-spintronic devices based on this semiconductor class [1]. However, the spatial variability at the nanoscale, as well as its consequences on spin transport, in these chiral halide perovskite (CHP) materials remains an open question. In a recent study published in National Science Review, Li and co-workers employ Kelvin probe force microscopy (KPFM) to map the chiral-induced spin selectivity (CISS) effect with nanoscale resolution within CHP thin films [2]. This method offers a nanoscale viewpoint into the CISS effect within CHPs, which could lead to new observations regarding the mechanisms that drive the CISS effect.

Abstract: Applying to graduate school can be difficult for various reasons, such as garnering enough research experience to submit compelling applications, understanding unspoken expectations related to graduate education from trusted mentors, and crafting the various components needed for an application. Further, these difficulties are amplified for underserved students who may lack support in navigating these obstacles given a shortage of representation in engineering academia. Here, we showcase the efforts of the Stanford Engineering Research Introductions Organization (SERIO) to address these challenges through an initial multi-day program for early-stage undergraduate students, a long-term mentorship program between current Stanford graduate students and U.S. undergraduates, and tailored graduate application preparatory resources. We also analyze the impact of SERIO’s efforts on the undergraduate students, including those that have successfully applied to graduate school. Altogether, we emphasize the benefit of creating SERIO-like organizations across the U.S. to support future generations of graduate students from all backgrounds.

Abstract: Micro- and nanoscale fabrication, which enables precise construction of intricate three-dimensional structures, is of foundational importance for advancing innovation in plasmonics, nanophotonics, and biomedical applications. However, scaling fabrication to industrially relevant levels remains a significant challenge. We demonstrate that triplet-triplet annihilation upconversion (TTA-UC) offers a unique opportunity to increase fabrication speeds and scalability of micro- and nanoscale 3D structures. Due to its nonlinearity and low power requirements, TTA-UC enables localized polymerization with nanoscale resolutions while simultaneously printing millions of voxels per second through optical parallelization using off-the-shelf light-emitting diodes and digital micromirror devices. Our system design and component integration empower fabrication with a minimum lateral feature size down to 230 nm and speeds up to 112 million voxels per second at a power of 7.0 nW per voxel. This combination of high resolution and fast print speed demonstrates that TTA-UC is a significant advancement in nanofabrication technique, evidenced by the fabrication of hydrophobic nanostructures on a square-centimeter scale, paving the way for industrial nanomanufacturing.

Abstract: Upconversion (UC) of low-energy photons to higher-energy photons enables exciting advances in 3D printing, bioimaging, and more. Particularly, the UC of near-infrared (NIR) photons is identified as a process that can enhance photovoltaic, night vision, and anti-counterfeiting technologies. Triplet–triplet annihilation UC is especially attractive due to its low UC thresholds and broadband, tunable absorption. However, state-of-the-art NIR-to-visible solid-state UC thin films made of PbS quantum dots and rubrene are limited by 1) low absorption of NIR photons, 2) inefficient energy transfer, and 3) parasitic back transfer processes, leading to low efficiencies unsuitable for wide application. Here, a multi-layer architecture that allows for strongly absorbing UC films with improved efficiencies is proposed. 5-tetracene carboxylic acid is used as an interlayer to improve energy transfer to rubrene and alleviate parasitic back transfer leading to a factor of five improvement in efficiency. Finally, UC anti-counterfeiting is demonstrated, highlighting the film's potential for UC-facilitated technologies.

Abstract: Advances in perovskite light-emitting diodes (PeLEDs) have established them as viable candidates for next-generation displays and lighting across the entire visible spectrum, with recent investigations extending their emissive properties into the deep blue and violet regions. However, achieving shorter emission wavelengths presents a significant challenge due to the larger band gaps required of both the perovskite and charge transport materials, compounding the difficulty in managing electron-hole pair recombination dynamics necessary for efficient electroluminescence. In this work, we precisely tune the halide composition in two-dimensional perovskites, successfully extending the band gap to 3.1 eV. By introducing an optimized dual-electron transport layer architecture, we improve electron injection and hole confinement within the perovskite matrix, culminating in a high-purity electroluminescent emission at 399 nm with a maximum external quantum efficiency of 0.16%, a benchmark for PeLEDs operating in this spectral domain. These findings highlight the potential of large-band-gap perovskite materials for next-generation light-emitting applications.

Standfirst: Photoresponsive perovskite light-emitting diodes can be used to build multifunctional displays that can function as touch screens, light sensors and image sensors.

Abstract: To foster equity, inclusion, and diversity within engineering departments' graduate student populations, active support of traditionally underrepresented undergraduates early in their academic journey is essential. Here, we outline the efforts of the Stanford Engineering Research Introductions Organization (SERIO) to address systemic barriers, encourage early engagement with research opportunities, and prepare early-stage underrepresented students for admission to engineering graduate programs. We highlight new pathways for institutions to foster their underrepresented undergraduate student population and facilitate their pursuit of advanced engineering education. 

Abstract: Although perovskite light-emitting diodes (PeLEDs) have demonstrated external quantum efficiencies (EQEs) well over 20%, their instability limits their commercial viability. Incorporating transition-metal dopants has previously improved the brightness, stability, and efficiency of PeLEDs. Here, we dope Mn2+ ions into a quasi-bulk 3D perovskite and introduce tris(4-fluorophenyl)phosphine oxide (TFPPO) to achieve a 14.0% peak EQE and 128,000 cd/m2 peak luminance. Whereas incorporating TFPPO into PeLEDs dramatically increases their EQE, it also severely compromises their stability. At a 5 mA/cm2 electrical current bias, PeLEDs fabricated without TFPPO (2.97% EQE) and with TFPPO (14.0% EQE) decay to half their maximum luminance in 37.0 and 2.54 min, respectively. In order to investigate this trade-off in EQE and stability, we study both photophysical and optoelectronic characteristics before and after PeLED electrical operation. Although Mn2+-doped PeLEDs hold the potential to enable bright and efficient lighting, device stability degradation mechanisms require further investigation.

Abstract: High external quantum efficiencies (EQEs) 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. 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 films by adding water into the precursor solution to engineer the crystallization process of spin-coated 2D perovskites. The resulting improved film uniformity, coupled with the reduction in nanoplate size, reduces leakage current and promotes faster recombination rates. The fabricated PeLEDs deliver bright violet emission at 408 nm with a maximum external quantum efficiency of 0.41%, a 5-fold increase over control devices. This work demonstrates viable steps toward cost-effective, efficient ultraviolet PeLEDs.

Abstract: Metal-halide perovskite nanocrystals have demonstrated excellent optoelectronic properties for light-emitting applications. Isovalent doping with various metals (M2+) 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.