Pournima Narayanan

All Publications

• Alleviating Parasitic Back Energy Transfer Enhances Thin Film Upconversion ADVANCED OPTICAL MATERIALS Narayanan, P., Hu, M., Pucurimay, L., Gallegos, A. O., Zhou, Q., Belliveau, E., Ahmed, G. H., Fernandez, S., Michaels, W., Murrietta, N., Mutatu, V. E., Feng, D., Hamid, R., Yap, K. K., Schloemer, T. H., Jaramillo, T. F., Kats, M. A., Congreve, D. N. 2025

  View details for DOI 10.1002/adom.202500252

  View details for Web of Science ID 001495471700001

• Understanding the Formation Dynamics and Physical Properties of Nanocapsules Using Charge Detection Mass Spectrometry. ACS nano Harper, C. C., Schloemer, T. H., Jordan, J. S., Heflin, N., Narayanan, P., Zhou, Q., Congreve, D. N., Williams, E. R. 2024

  Abstract

  Characterizing the size, structure, and composition of nanoparticles is vital in predicting and understanding their macroscopic properties. In this work, charge detection mass spectrometry (CDMS) was used to analyze nanocapsules (∼10-200 MDa) consisting of a liquid oleic acid core surrounded by a dense silica outer shell. CDMS is an emerging method for nanoparticle analysis that can rapidly measure the mass and charge of thousands of individual nanoparticles. We find that increasing the feed volume of the tetraethylorthosilicate (TEOS) precursor added to form the silica shell of the nanocapsules yielded both higher and broader nanocapsule mass distributions with differentiable densities. A two-dimensional mass versus charge analysis also revealed the formation of two distinct populations of nanocapsules. These two nanocapsule morphologies were also present in transmission electron microscopy (TEM) images and exhibited low-density spherical cores and crescent-shaped cores where the remainder of the core volume was "filled in" by more dense silica. Nanocapsule shell growth kinetics over a ∼48-h synthesis period were also monitored by sampling the reaction mixture at various times, quenching the sampled aliquots, and then characterizing these time-resolved samples by CDMS. The CDMS data reveal three distinct growth phases in nanocapsule formation; rapid initial nucleation, an "inverted" distribution of silica growth, and a final slow growth phase where the rate of mass increase and final nanocapsule masses are dictated by the initial TEOS feed volumes. CDMS-enabled understanding of the diverse nanocapsule sizes, morphologies, and growth dynamics will allow us to better predict nanocapsule properties while reducing the experimental burden in optimizing nanocapsules for real-world applications.

  View details for DOI 10.1021/acsnano.4c12461

  View details for PubMedID 39723934

• 2D mixed halide perovskites for ultraviolet light-emitting diodes DEVICE Hu, M., Lyu, J., Murrietta, N., Fernandez, S., Michaels, W., Zhou, Q., Narayanan, P., Congreve, D. N. 2024; 2 (11)

  View details for DOI 10.1016/j.device.2024.100511

  View details for Web of Science ID 001359154500001

• Bulk Heterojunction Upconversion Thin Films Fabricated via One-Step Solution Deposition. ACS nano Hu, M., Belliveau, E., Wu, Y., Narayanan, P., Feng, D., Hamid, R., Murrietta, N., Ahmed, G. H., Kats, M. A., Congreve, D. N. 2023

  Abstract

  Upconversion of near-infrared light into the visible has achieved limited success in applications due to the difficulty of creating solid-state films with high external quantum efficiency (EQE). Recent developments have expanded the range of relevant materials for solid-state triplet-triplet annihilation upconversion through the use of a charge-transfer state sensitization process. Here, we report the single-step solution-processed deposition of a bulk heterojunction upconversion film using organic semiconductors. The use of a bulk heterojunction thin film enables a high contact area between sensitizer and annihilator materials in this interface-triplet-generation mechanism and allows for a facile single-step deposition process. Demonstrations of multiple deposition and patterning methods on glass and flexible substrates show the promise of this materials system for solid-state upconversion applications.

  View details for DOI 10.1021/acsnano.3c06955

  View details for PubMedID 37963265

• Promoting multiexciton interactions in singlet fission and triplet fusion upconversion dendrimers. Nature communications He, G., Churchill, E. M., Parenti, K. R., Zhang, J., Narayanan, P., Namata, F., Malkoch, M., Congreve, D. N., Cacciuto, A., Sfeir, M. Y., Campos, L. M. 2023; 14 (1): 6080

  Abstract

  Singlet fission and triplet-triplet annihilation upconversion are two multiexciton processes intimately related to the dynamic interaction between one high-lying energy singlet and two low-lying energy triplet excitons. Here, we introduce a series of dendritic macromolecules that serve as platform to study the effect of interchromophore interactions on the dynamics of multiexciton generation and decay as a function of dendrimer generation. The dendrimers (generations 1-4) consist of trimethylolpropane core and 2,2-bis(methylol)propionic acid (bis-MPA) dendrons that provide exponential growth of the branches, leading to a corona decorated with pentacenes for SF or anthracenes for TTA-UC. The findings reveal a trend where a few highly ordered sites emerge as the dendrimer generation grows, dominating the multiexciton dynamics, as deduced from optical spectra, and transient absorption spectroscopy. While the dendritic structures enhance TTA-UC at low annihilator concentrations in the largest dendrimers, the paired chromophore interactions induce a broadened and red-shifted excimer emission. In SF dendrimers of higher generations, the triplet dynamics become increasingly dominated by pairwise sites exhibiting strong coupling (Type II), which can be readily distinguished from sites with weaker coupling (Type I) by their spectral dynamics and decay kinetics.

  View details for DOI 10.1038/s41467-023-41818-1

  View details for PubMedID 37770472

  View details for PubMedCentralID 6338432

• Spatially Controlled Uv Light Generation at Depth Using Upconversion Micelles. Advanced materials (Deerfield Beach, Fla.) Zhou, Q., Wirtz, B. M., Schloemer, T. H., Burroughs, M. C., Hu, M., Narayanan, P., Lyu, J., Gallegos, A. O., Layton, C., Mai, D. J., Congreve, D. N. 2023: e2301563

  Abstract

  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, a nanoencapsulation method is leveraged to fabricate water-compatible UC micelles, enabling on-demand UV photon generation deep into materials. Two iridium-based complexes are presented for use as TTA-UC sensitizers with increased solubilities that facilitate the formation of highly emissive UV-upconverting micelles. Furthermore, this encapsulation method is shown to be 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, UV-emitting UC micelles are used 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. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/adma.202301563

  View details for PubMedID 37548335

• Water additives improve the efficiency of violet perovskite light-emitting diodes MATTER Hu, M., Fernandez, S., Zhou, Q., Narayanan, P., Saini, B., Schloemer, T. H., Lyu, J., Gallegos, A. O., Ahmed, G. H., Congreve, D. N. 2023; 6 (7): 2356-2367

  View details for DOI 10.1016/j.matt.2023.05.018

  View details for Web of Science ID 001058067600001

• Controlling the durability and optical properties of triplet-triplet annihilation upconversion nanocapsules. Nanoscale Schloemer, T. H., Sanders, S. N., Narayanan, P., Zhou, Q., Hu, M., Congreve, D. N. 2023

  Abstract

  Deep penetration of high energy photons by direct irradiation is often not feasible due to absorption and scattering losses, which are generally exacerbated as photon energy increases. Precise generation of high energy photons beneath a surface can circumvent these losses and significantly transform optically controlled processes like photocatalysis or 3D printing. Using triplet-triplet annihilation upconversion (TTA-UC), a nonlinear process, we can locally convert two transmissive low energy photons into one high energy photon. We recently demonstrated the use of nanocapsules for high energy photon generation at depth, with durability within a variety of chemical environments due to the formation of a dense, protective silica shell that prevents content leakage and nanocapsule aggregation. Here, we show the importance of the feed concentrations of the tetraethylorthosilicate (TEOS) monomer and the methoxy poly(ethyleneglycol) silane (PEG-silane) ligand used to synthesize these nanocapsules using spectroscopic and microscopy characterizations. At optimal TEOS and PEG-silane concentrations, minimal nanocapsule leakage can be obtained which maximizes UC photoluminescence. We also spectroscopically study the origin of inefficient upconversion from UCNCs made using sub-optimal conditions to probe how TEOS and PEG-silane concentrations impact the equilibrium between productive shell growth and side product formation, like amorphous silica. Furthermore, this optimized fabrication protocol can be applied to encapsulate multiple TTA-UC systems and other emissive dyes to generate anti-Stokes or Stokes shifted emission, respectively. These results show that simple synthetic controls can be tuned to obtain robust, well-dispersed, bright upconverting nanoparticles for subsequent integration in optically controlled technologies.

  View details for DOI 10.1039/d3nr00067b

  View details for PubMedID 37000152

• Nanoengineering Triplet-Triplet Annihilation Upconversion: From Materials to Real-World Applications. ACS nano Schloemer, T., Narayanan, P., Zhou, Q., Belliveau, E., Seitz, M., Congreve, D. N. 2023

  Abstract

  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.

  View details for DOI 10.1021/acsnano.3c00543

  View details for PubMedID 36800310

• Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals JOURNAL OF PHYSICAL CHEMISTRY LETTERS Wilson, M. B., Hasham, M., Narayanan, P., Villanueva, F., Green, P. B., Imperiale, C. J. 2023: 1899-1909

  Abstract

  Nanocrystal (NC)-sensitized triplet-fusion upconversion is a rising strategy to convert long-wavelength, incoherent light into higher-energy output photons. Here, we chart the photophysics of tailor-functionalized CdSe NCs to understand energy transfer to surface-anchored transmitter ligands, which can proceed via correlated exciton transfer or sequential carrier hops. Varying NC size, we observe a pronounced acceleration of energy transfer (from kquench = 0.0096 ns-1 ligand-1 to 0.064 ns-1 ligand-1) when the barrier to hole-first sequential transfer is lowered from 100 ± 25 meV to 50 ± 25 meV. This acceleration is 5.1× the expected effect of increased carrier wave function leakage, so we conclude that sequential transfer becomes kinetically dominant under the latter conditions. Last, transient photoluminescence shows that NC band-edge and trap states are comparably quenched by functionalization (up to ∼98% for sequential transfer) and exhibit matched dynamics for t > 300 ns, consistent with a dynamic quasi-equilibrium where photoexcitations can ultimately be extracted even when a carrier is initially trapped.

  View details for DOI 10.1021/acs.jpclett.2c03443

  View details for Web of Science ID 000933297400001

  View details for PubMedID 36780580

• Triplet Fusion Upconversion Nanocapsule Synthesis. Journal of visualized experiments : JoVE Schloemer, T. H., Sanders, S. N., Zhou, Q., Narayanan, P., Hu, M., Gangishetty, M. K., Anderson, D., Seitz, M., Gallegos, A. O., Stokes, R. C., Congreve, D. N. 2022

  Abstract

  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 three-dimensional (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.

  View details for DOI 10.3791/64374

  View details for PubMedID 36155426

• Luminescence Enhancement Due to Symmetry Breaking in Doped Halide Perovskite Nanocrystals. Journal of the American Chemical Society Ahmed, G. H., Liu, Y., Bravic, I., Ng, X., Heckelmann, I., Narayanan, P., Fernandez, M. S., Monserrat, B., Congreve, D. N., Feldmann, S. 2022

  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.

  View details for DOI 10.1021/jacs.2c07111

  View details for PubMedID 35977424