Cherie R Kagan Research Group

Electrical and Systems Engineering - Chemistry - Materials Science and Engineering

Group Photo 2018

Welcome

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

http://kagan.seas.upenn.edu/index.php 5/28/2019:
Designing Strong Optical Absorbers via Continuous Tuning of Interparticle Interaction in Colloidal Gold Nanocrystal Assemblies

We program the optical properties of colloidal Au nanocrystal (NC) assemblies via an unconventional ligand hybridization (LH) strategy to precisely engineer interparticle interactions and design materials with optical properties difficult or impossible to acheive in bulk form. Long-chain hydrocarbon ligands used in NC synthesis are partially exchanged, from 0% to 100%, with compact thiocyanate ligands by controlling the reaction time for exchange. Our LH strategy may be applied to the design of materials constructed from NCs of different size, shape, and composition for specific applications.

2/26/2019:
Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites

2D hybrid perovskites are broadly tuneable in structural and optoelectronic properties in part because of the variety of organic cations that can be incorporated into the structure. Even though charge carriers and excitons are confined to the metal-halide layers in Type I 2D hybrid perovskites, we demonstrate that longer organic cations create a more disordered energy landscape in the ground state, which we observe through broader, blue-shifted excitonic absorption spectra. In contrast, polaron formation in the excited state stabilizes a structural and electronic minimum seen by photoluminescence spectra that are largely invariant with cation length.

2/1/2019:
Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange

Most high-performance Nanocrystal (NC) devices and circuits have been achieved using NCs containing toxic elements, which may limit their further device development. We fabricate Al2O3-encapsulated, air-stable CuInSe2 NC field-effect transistors (FETs) using a solution-based, post-deposition, sequential cation exchange process with linear (saturation) electron mobilities of 8.2±1.8 cm2/Vs (10.5±2.4 cm2/Vs) and with current modulation of 105, comparable to that for high-performance Cd-, Pb-, and As-based NC FETs. The CuInSe2 NC FETs are used as building blocks of integrated inverters to demonstrate their promise for low-cost, low-toxicity NC circuits.

1/10/2019:
Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers

We use template-assisted self-assembly to direct colloidal Au nanorods to form "open" triangular nanorod assemblies. The triangular nanorod arrangement supports magnetic plasmonic modes where electric fields are directed along the triangle perimeter and magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods impart either a left- or right-handed sense of rotation to the structures, which generates a chiroptical response under unidirectional oblique illumination.

9/26/2018:
Nanoimprinted chiral plasmonic substrates with three-dimensional nanostructures

Chiral plasmonic metamaterials can have giant optical chirality and be used in bio-sensing and light manipulation. We report a large-area fabrication method to prepare chiral substrates patterned with arrays of multilayer, three-dimensional nanostructures using a combination of nanoimprint lithography and glancing angle deposition. As one typical example, arrays of L-shaped chiral plasmonic nanostructures exhibit giant optical chirality in the infrared region with an experimentally achieved g-factor as high as 0.38. To demonstrate scalability, we fabricated a 1 cm2 chiral substrate with uniform chiral optical property.

9/24/2018:
Ultrasensitive, Mechanically Responsive Optical Metasurfaces via Strain Amplification

We report a structurally-reconfigurable, optical metasurface constructed by integrating a plasmonic lattice array in the gap between a pair of symmetric microrods that serve to locally amplify the strain created on an elastomeric substrate by an external mechanical stimulus. The strain on the metasurface is amplified by a factor of 1.5 to 15.9 relative to the external strain by tailoring the microrod geometry. The demonstrated mechano-sensitivity of the optical responses of the plasmonic lattice array is a factor of 10 greater than that of state-of-the-art stretchable plasmonic resonator arrays

9/12/2018:
Flexible colloidal nanocrystal electronics

Kagan reviews the physics and chemistry of colloidal semiconductor nanocrystals (NCs) that have lead to recent leaps forward in the performance of their field-effect transistors and the device design and fabrication that have allowed the demonstration of flexible colloidal NC integrated circuits. Taking stock of the advances made in the science and engineering of NC systems, challenges and opportunities are presented to develop next-generation, colloidal NC electronic materials and devices, important to their potential in future computational and in Internet of Things applications.

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.

Announcements

5/17/19:
Congratulations, Hak-Jong!

Best wishes to Hak-Jong Choi as he returns to South Korea to begin his career at the Korean Institute of Machinery & Materials!

5/1/19:
Congratulations, Daniel!

Congratulations to Daniel Straus as he receives the 2019 John G Miller Award for the Most Outstanding Doctoral Thesis in the Department of Chemistry.

2/26/19:
Congratulations Leo!

Congratulations to Leo on defending his PhD thesis! We wish him the best as he transitions to a Postdoc at Yale.

11/30/18:
Congratulations Martin!

Congratulations to Martin Sarott on his Masters graduation from ETH and on being awarded the Willi-Studer prize for the best overall grade. We wish him the best of luck as he starts his PhD!

10/31/18:
Congratulations Daniel!

Congratulations to Daniel on defending his PhD thesis! We wish him the best as he starts a Postdoc at Princeton.


9/30/18:
Congratulations Martin!

Congratulations to Martin Sarott on completing an outstanding Masters thesis during his 6 month visiting research term from ETHZ. Best wishes, Martin!

9/10/18:
Congratulations Scott!

Congratulations and best wishes to Scott Stinner as he starts his new job at Seren IPS.

9/4/18:
Congratulations Eric!

Eric Wong has successfully defended his thesis, and is happily on his way to a fellowship in Seattle. Conrats, Eric!

9/1/18:
Congratulations Qinghua!

Qinghua Zhao has been selected as a VEIST Graduate Fellow. Congrats, Qinghua!

2/5/18:
Congratulations Nuri!

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

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