Cherie R Kagan Research Group

Electrical and Systems Engineering - Chemistry - Materials Science and Engineering

Group Photo 2016

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 2/27/2017:
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.


2/12/2017:
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.


2/1/2017:
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.


12/12/2016:
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.


11/7/2016:
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.


10/5/2016:
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.


10/3/2016:
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.






9/20/2016:
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.




8/26/2016:
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.




8/9/2016:
Roadmap on Optical Metamaterials

Kagan and Murray describe using nanoparticles as building blocks for the design of optical metamaterials.




4/8/2016:
Exploiting the Colloidal Nanocrystal Library to Construct Electronic Devices

We exploit metallic, semiconducting, and insulating colloidal nanocrystals and design the materials, interfaces, and processes to construct all-nanocrystal electronic devices on flexible plastics using solution-based processes.




3/22/2016:
At the Nexus of Food Security and Safety: Opportunities for Nanoscience and Nanotechnology

Kagan describes the opportunities for nanoscience and nanotechnology to address the grand challenge at the nexus of food security and safety in an ACS Nano editorial.


11/9/2015:
Increased Carrier Mobility and Lifetime in CdSe Quantum Dot Thin Films through Surface Trap Passivation and Doping

We investigate the effect of introducing indium in CdSe QD thin films on the dark mobility and the photogenerated carrier mobility and lifetime using field-effect transistor (FET) and time-resolved microwave conductivity (TRMC) measurements. As the amount of indium increases, the FET and TRMC mobilities and the TRMC lifetime increase. The increase in mobility and lifetime is consistent with increased indium passivating midgap and band-tail trap states and doping the films, shifting the Fermi energy closer to and into the conduction band.




Charge transport in strongly coupled quantum dot solids

Kagan and Murray review the chemical and physical origins of high mobility charge transport in quantum dot (QD) solids that is being exploited in devices. They also present future prospects for QD materials and device design.

9/25/2015:
Flexible, High-Speed CdSe Nanocrystal Integrated Circuits

We report large area, flexible, photopatterned integrated circuits based on CdSe nanocrystal semiconductors. Our results show high performance devices with transistor mobilities exceeding 10 cm2/Vs, amplifiers with ~7 kHz bandwidth, and oscillators with sub-10 µs stage delays as well as NAND and NOR logic gates.

Announcements

3/19/17:
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."

3/19/17:
Congratulations Lizzy!

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

3/4/2017:
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!

12/8/2016:
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.

10/31/2016:
Congratulations Jingsong!

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

10/24/2016:
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!

9/17/2015:
Congratulations TJ!

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

9/10/2015:
Congratulations to Ed Goodwin!

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

5/01/2015:
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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Electrical & Systems Engineering, Moore Building
200 South 33rd Street, Philadelphia, PA 19104