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University of Pennsylvania · Electrical Engineering
Active 1982–2026
Cherie Kagan is the Stephen J. Angello Professor and holds professorships in Electrical and Systems Engineering as well as Materials Science and Engineering at the University of Pennsylvania. Her primary focus area is Materials Chemistry. She can be contacted via email at kagan@seas.upenn.edu and is located in Room 230 of the VLEST Building. The information provided highlights her interdisciplinary roles bridging engineering and materials science, emphasizing her expertise in materials chemistry.
Chitosan Infiltrated TiO2 Nanocrystal Composite Optical Metasurfaces for Colorimetric Leaf Sensing
ChemRxiv · 2026-01-06
We report high figure-of-merit optical leaf sensors based on dielectric metasurfaces and stimuli-responsive polymers. The metasurfaces have narrowband resonances and are broadly transparent across the visible spectrum to allow photosynthesis for unobstructive monitoring of crop health. These sensors are fabricated at scale using direct nanoimprint lithography into UV curable-TiO2 nanocrystal (NC) inks to structure optical metasurfaces, followed by a room-temperature ligand exchange process to create a nanoporous TiO2 scaffold. The nanoporous TiO2 metasurfaces are then infiltrated by a stimuli-responsive polymer while retaining their geometry and optical quality. As a proof of concept, we demonstrate optical humidity sensors by incorporating the moisture-responsive chitosan biopolymer into the nanoporous TiO2 metasurfaces, resulting in a 430 % improvement in sensitivity compared to conventional RI sensors where the polymer surrounds the metasurface. The sensors are mounted on leaves and the highly reflective resonances, which are readily distinguishable from the leaf background, respond dynamically to leaf water stress, enabling passive, battery-free monitoring of leaf surface humidity.
arXiv (Cornell University) · 2026-01-16
Colloidal semiconductor nanocrystals are promising building blocks for optoelectronics due to their solution processability, spectral tunability, and ability to self-assemble into complex architectures. However, their use in lasing application remains limited by high working thresholds, rapid nonradiative losses from Auger recombination, and sensitivity to environmental conditions. Here, we report hybrid microscale supraparticles composed of core/shell CdSe/ZnS quantum dots (QDs) and CdSe/CdxZn1-xS nanoplatelets (NPLs), which overcome these limitations through efficient, cavity-mediated energy funneling and coupling. Broadband absorbing QDs rapidly transfer excitation to narrow emitting NPLs, enabling stable whispering gallery mode lasing with a low threshold of 0.35 mJ/cm2. These supraparticles retain optical performance after prolonged exposure to air, water, and continuous irradiation, offering practical advantages for optoelectronic devices and advanced pigment technologies. Ultimately, our approach provides a versatile, programmable platform for optical amplification and tunable emission control within colloidal photonic architectures. Keywords
ACS Nano · 2026-03-16
We report all-inorganic, bicontinuous, bandgap-engineered epitaxially fused PbSe QD/CdS matrix heterostructures achieved through postdeposition sequential colloidal atomic layer deposition (c-ALD). The CdS matrix grows epitaxially on the PbSe QDs to ultimately fully infill the interstitial space between fused PbSe QD arrays in an interpenetrating fashion, resulting in bicontinuous semiconductor heterostructures. The low-energy excitonic absorbance of the epitaxially fused PbSe QD assembly is maintained, and the absorbance at energies above the CdS matrix bandgap increases. The c-ALD grown CdS matrix enhances the oxidative and thermal stability of the QD assemblies, allowing us to preserve the QD/matrix heterostructure upon annealing at 150 °C. By controlling the number of c-ALD cycles and by thermal annealing, we tailor stoichiometry and modulate carrier type, concentration, and mobility probed in the platform of field-effect transistors and the dark current density and responsivity of infrared-absorbing PbSe QD/CdS matrix heterostructure photoconductors. Photoconductors treated with c-ALD and annealed showed an increase in photocurrent modulation, enhancing infrared photoresponsivity. The bicontinuous, bandgap-engineered semiconductor QD/matrix heterostructures provide an architecture that promises the high mobility charge transport and long carrier lifetimes needed to achieve high speed and high quantum efficiency electronic and optoelectronic devices.
ACS Nano · 2026-01-02
Colloidal semiconductor nanocrystals are promising building blocks for optoelectronics due to their solution processability, spectral tunability, and ability to self-assemble into complex architectures. However, their use in lasing application remains limited by high working thresholds, rapid nonradiative losses from Auger recombination, and sensitivity to environmental conditions. Here, we report hybrid microscale supraparticles composed of core/shell CdSe/ZnS quantum dots (QDs) and CdSe/CdxZn1–xS nanoplatelets (NPLs), which overcome these limitations through efficient, cavity-mediated energy funneling and coupling. Broadband absorbing QDs rapidly transfer excitation to narrow emitting NPLs, enabling stable whispering gallery mode lasing with a low threshold of 0.35 mJ/cm2. These supraparticles retain optical performance after prolonged exposure to air, water, and continuous irradiation, offering practical advantages for optoelectronic devices and advanced pigment technologies. Ultimately, our approach provides a versatile, programmable platform for optical amplification and tunable emission control within colloidal photonic architectures.
ACS Applied Nano Materials · 2025-08-14 · 4 citations
We report the fabrication and characterization of responsive dielectric metasurfaces composed of titanium dioxide (TiO2) nanocrystals (NCs) and chitosan hydrogel biopolymer composites. Chitosan is controllably introduced into aqueous TiO2 NC dispersions to formulate adaptive inks. The inks are then used in a nanoimprinting process to produce moisture-responsive metasurfaces with high quality factor (Q), quasi-guided mode (QGM) resonances. The metasurfaces function as environmental relative humidity (RH) sensors, exhibiting a 250% enhancement in sensitivity with the addition of chitosan─to a maximum of 0.29 nm/%RH. This work presents a scalable, single-step, room-temperature strategy for producing optically tunable metasurfaces in the visible, offering high sensitivity and quality factor resonances, and enabling enhanced optical humidity sensor performance.
Shape effects on the 2D self-assembly of lithographically fabricated nanoparticles
Nanoscale · 2025-01-01 · 1 citations
Lithographically-defined polygonal Au nanoplates assemble at liquid-air interface. Shape governs 2D order: hexagonal>pentagonal>square, correlated with coordination/tiling. Binary mixtures with compatible six-fold coordination show enhanced ordering.
Prospects of Nanoscience with Nanocrystals: 2025 Edition
ACS Nano · 2025-09-03 · 27 citations
Nanocrystals (NCs) of various compositions have made important contributions to science and technology, with their impact recognized by the 2023 Nobel Prize in Chemistry for the discovery and synthesis of semiconductor quantum dots (QDs). Over four decades of research into NCs has led to numerous advancements in diverse fields, such as optoelectronics, catalysis, energy, medicine, and recently, quantum information and computing. The last 10 years since the predecessor perspective "Prospect of Nanoscience with Nanocrystals" was published in ACS Nano have seen NC research continuously evolve, yielding critical advances in fundamental understanding and practical applications. Mechanistic insights into NC formation have translated into precision control over NC size, shape, and composition. Emerging synthesis techniques have broadened the landscape of compounds obtainable in colloidal NC form. Sophistication in surface chemistry, jointly bolstered by theoretical models and experimental findings, has facilitated refined control over NC properties and represents a trusted gateway to enhanced NC stability and processability. The assembly of NCs into superlattices, along with two-dimensional (2D) photolithography and three-dimensional (3D) printing, has expanded their utility in creating materials with tailored properties. Applications of NCs are also flourishing, consolidating progress in fields targeted early on, such as optoelectronics and catalysis, and extending into areas ranging from quantum technology to phase-change memories. In this perspective, we review the extensive progress in research on NCs over the past decade and highlight key areas where future research may bring further breakthroughs.
ACS Nano · 2025-06-05 · 3 citations
We report room-temperature observations of CuZn–VS quantum emitters in individual ZnS:Cu nanocrystals (NCs). Using time-gated imaging, we isolate the distinct ∼3-μs-long, red photoluminescence (PL) emission of CuZn–VS defects, enabling their precise identification and statistical characterization. The emitters exhibit distinct blinking and photon antibunching, consistent with individual NCs containing two to four CuZn–VS defects. The quantum emitters’ PL spectra show a pronounced blue shift compared to NC dispersions, likely due to photochemical and charging effects. Emission polarization measurements of quantum emitters are consistent with a σ-character optical dipole transition and the symmetry of the CuZn–VS defect. These observations motivate further investigation of CuZn–VS defects in ZnS NCs for use in quantum technologies.
ACS Applied Nano Materials · 2025-02-20 · 4 citations
Plasmonic metasurfaces composed of nanodimer repeat units with ultranarrow gaps have strong fields confined to the subwavelength gaps, which can enable resonance wavelength tuning and polarization control. We report a scalable fabrication process that takes advantage of nanoimprint lithography and the solution processability and surface conformability of colloidal nanocrystal dispersions to realize large-area, geometrically engineered nanodimer metasurfaces from a single master template. Geometrical control of the nanodimer gap is achieved through a combination of controlled wet etching of bilayer imprint resists and solution-based nanocrystal deposition and ligand exchange. Using a master template with 50 nm gaps and tailoring the wet etch, nanocrystal concentration, and ligand exchange time, we achieve large-area (1 cm2) metasurfaces with nanogaps tailorable from 72 nm to as narrow as 21 nm, even to the point of fusing the nanorods. We characterize the gap-size-dependent spectral response in the near-infrared, which shows increased electric field confinement and polarization dependence when the gap narrows. This process is promising in its potential for scalable manufacturing and nanogap engineering, using a single master template, of nanodimer metasurfaces, which are of particular interest for applications in refractive index sensing.
Three-Dimensional Colloidal Nanocrystal Metamaterials
2025-07-21
NSF Engineering Research Center for the Internet of Things for Precision Agriculture (IoT4Ag)
NSF · $31.5M · 2020–2026
Designing the Electronic Properties of PbSe Nanowires for Optoelectronic Devices
NSF · $375k · 2013–2017
Engineering Organic-Inorganic Hybrid Materials for the Conversion of Solar Energy
NSF · $300k · 2009–2013
NSF · $300k · 2012–2015
Shape-Dependent Electronic and Optical Properties in Semiconductor Nanowires
NSF · $386k · 2008–2012
Christopher B. Murray
University of Pennsylvania
Stephen J. Angello
A.S. Watson (Netherlands)
Timothy Watson
Tan Tock Seng Hospital
Claudia Felser
Harvard University Press
Carolyn Duran
Johns Hopkins University
Ph.D., Electrical Engineering
University of Pennsylvania
M.S., Electrical Engineering
University of Pennsylvania
B.S., Electrical Engineering
University of Pennsylvania
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Todd Osman
A.S. Watson (Netherlands)
Kammrath Weiss
O January
National Student Clearinghouse Research Center