Cherie R Kagan Research Group

Electrical and Systems Engineering - Chemistry - Materials Science and Engineering

Group Photo 2018


The Kagan Group explores the chemical and physical properties of nanostructured and organic materials and integrates these materials in electronic, optoelectronic, optical, thermoelectric and bioelectronic devices. We combine the flexibility of chemistry and bottom-up assembly with top-down fabrication techniques to design novel materials and devices. We explore the structure and function of these materials and devices using spatially- and temporally-resolved optical spectroscopies, AC and DC electrical techniques, electrochemistry, scanning probe and electron microscopies and analytical measurements.

Research Highlights 4/15/2018:
3D Nanofabrication via Chemo-Mechanical Transformation of Nanocrystal/Bulk Heterostructures

High yield, functional 3D cell-sized machines are achieved by ligand-chemistry-induced folding of planar nanocrystal/bulk hetero-structures. Design rules have been established to fabricate various 3D architectures and unique physical functions can be incorporated through careful selection of nanocrystals. This technique promises to find applications in a variety of fields including artificial motors, nano-robots, and chiral optical metasurfaces.

Angle-Independent Optical Moisture Sensors Based on Hydrogel-Coated Plasmonic Lattice Arrays

We construct an angle-independent optical sensor to measure soil moisture by coating hydrogel on top of an ultrathin, plasmonic Au nanorod lattice array. Refractive index changes of the hydrogel upon exposure to moisture are transduced into spectra shifts on the resonances of the array.

Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties

2D hybrid perovskites are widely tailorable in composition, structure, and dimensionality and provide an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In this Perspective, we correlate the unique structural and optoelectronic properties in semiconducting 2D perovskites and discuss current and future applications for this diverse class of materials. We describe the role of the heavy atoms in the inorganic framework; the geometry and chemistry of organic cations; and the "softness" of the organic-inorganic lattice on the electronic structure and dynamics of electrons, excitons, and phonons that govern the physical properties of these materials.

The Effect of Dielectric Environment on Doping Efficiency in Colloidal PbSe Nanostructures

We show dielectric confinement effects limit the doping efficiency of lead chalcogenide nanostructures in low dielectric constant media. As the dielectric mismatch between the nanostructure and its surrounding environment decreases, the doping efficiency is enhanced, consistent with our theoretical model.

Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites

In collaboration with the Roy and Owen groups at Columbia, we measure the lifetime and diffusion length of single crystals of lead bromide perovskites APbBr3 (A=methylammonium, formamidinium, Cs) using scanning photocurrent microscopy and time resolved microwave conductivity spectroscopy. We find that the hole diffusion length is on average an order of magnitude longer than the electron diffusion length, irrespective of the A cation. This limitation is an important factor to consider when designing and optimizing perovskite solar cells.

Plasmon Resonances in Self-Assembled Two-Dimensional Au Nanocrystal Metamolecules

We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer �metamolecules� as the number of nanocrystals is systematically varied. Darkfield scattering spectroscopy of individual self-assembled oligomers in combination with theoretical simulations is used to map the wavelength and character of plasmonic modes as a function of metamolecule size and degree of disorder in NC packing.

Hierarchical Materials Design by Pattern Transfer Printing of Self-Assembled Binary Nanocrystal Superlattices

We demonstrate the fabrication of hierarchical materials, structured on multiple length scales, by combining self-assembly and transfer printing of binary nanocrystal superlattices.

Engineering the Surface Chemistry of Lead Chalcogenide Nanocrystal Solids to Enhance Carrier Mobility and Lifetime in Optoelectronic Devices

We introduce a stepwise, hybrid ligand-exchange method for lead chalcogenide nanocrystal (NC) thin films using the compact inorganic ligand thiocyanate and the short organic ligand benzenediothiolate. We show that hybrid exchange enhances both carrier mobility and lifetime in NC thin films and allows the demonstration of optoelectronic devices with enhanced power conversion and quantum efficiency.

High-Strength Magnetically Switchable Plasmonic Nanorods Assembled from a Binary Nanocrystal Mixture

We report the fabrication of multifunctional 'smart' nanoparticle systems by combining top-down fabrication and bottom-up self-assembly methods. Particularly, we template nanorods from a mixture of superparamagnetic Zn0.2Fe2.8O4 and plasmonic Au nanocrystals. Nanoscrytal-derived superparamagnetism prevents these nanorods from spontaneous magnetic-dipole-induced aggregation, while their magnetic anisotropy makes them responsive to an external field. Ligand exchange drives Au nanocrystal fusion and forms a porous network, imparting the nanorods with high mechanical strength and polarization-dependent, infrared surface plasmon resonances. The combined superparamagnetic and plasmonic functions enable switching of the infrared transmission of a hybrid nanorod suspension using an external magnetic field.

Direct Observation of Electron-Phonon Coupling and Slow Vibrational Relaxation in Organic-Inorganic Hybrid Perovskites

We harness the large exciton binding energy in 2D perovskites to directly observe electron-phonon coupling and slow vibrational relaxation using steady-state and time-resolved optical spectroscopy.

Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells

Correlated scanning confocal photoluminescence and photocurrent microscopies are used to show that contrary to general expectation, higher photoluminesence quenching is not always indicative of higher photocurrent in bulk-heterojunction organic solar cells. This work suggests that photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for solar cells.

Advanced Architecture for Colloidal PbS Quantum Dot Solar Cells Exploiting a CdSe Quantum Dot Buffer Layer

We introduce a CdI2-treated CdSe quantum dot buffer layer at the junction between ZnO nanoparticles and PbS quantum dots to improve the performance of the solar cell devices. After optimizing the carrier concentration and energy band alignment of the CdSe QD buffer layer in the heterojunction, we find suppressed interface recombination and additional photogenerated carriers attributed to the buffer layer, leading to a 25% increase in solar power conversion efficiency.

Building Devices from Colloidal Quantum Dots

Kagan, Lifshitz, Sargent, and Talapin review recent progress in tailoring colloidal quantum dots and their assemblies to build electronic and optoelectronic devices.


Congratulations Nuri!

Congratulations and best wishes to Nuri Oh, now Professor of Materials Science and Engineering, Hanyang University.

Congratulations Scott!

Scott has successfully defended his thesis, and shall now be known only as Dr. Stinner. Congrats!

Congratulations Eric!

Eric Wong has been selected for the ESE Grad Award for Best Undergraduate Research Mentor. Congrats, Eric!

Congratulations Wenxiang!

Wenxiang has successfully defended his thesis, and shall now be known only as Dr. Chen. Congrats!

Congratulations Nick!

Nick has successfully defended his thesis, and shall now be known only as Dr. Greybush. Congrats!

Congratulations Wenxiang!

Congratulations to Wenxiang on receiving the 2017 S. J. Stein Prize "for superior achievement in the field of new or unique materials or applications for materials in electronics."

Congratulations Lizzy!

Congratulations to Elizabeth Dresselhaus on her selection for a Fulbright Award to study next year in the Netherlands!

Congratulations Derrick and Guannan!

Derrick Butler and Guannan Liu have been selected to receive the Masters Scholar Award from the MSE Department. Congratulations, and good luck on your research!

Congratulations Lizzy and Ana!

Undergraduate group members Elizabeth Dresselhaus and Ana Cohen have been recognized by Penn Current as being part of the founding leadership of Penn's Women in Physics Club, a group which supports undergraduate women at Penn pursuiting Physics degrees.

Congratulations Jingsong!

Jingsong Meng has been selected to receive the Masters Scholar Award from the MSE Department. Congratulations, and good luck on your research!

Congratulations Nick!

Nick Greybush was awarded 2nd place in the Student Poster Competition at the Singh Nano Week Conference for his poster on Plasmonic Oligomers. Congrats!

Congratulations TJ!

TJ has been appointed Asst. Professor of the School of Integrative Engineering at Chung-Ang University. Congratulations, and good luck!

Congratulations to Ed Goodwin!

Ed has successfully defended his thesis, and shall now be known only as Dr. Goodwin. Congrats!

Congratulations to our Graduating Students!

Our graduating students will be continuing their education:
  • Emily - PhD program in MSE at Univ. of Michigan
  • Devika - PhD program in MSE at UT Austin
  • Matt - PhD program in EE at UC Berkeley
  • Prashanth - PhD program in EE at Univ. of Utah
  • Chawit - PhD program in Integrated in Photonic and Electronic Systems at Cambridge University

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