Christopher B. Murray - US grants

The grand challenge in efficiently harvesting and converting solar radiation into electricity lies in engineering materials on multiple length scales with architectures that direct the flow of energy and the transfer and transport of charge, as in naturally occurring light harvesting systems. Organic-inorganic hybrids, prepared from functional, electro-active organic and nanostructured inorganic materials, combine desirable and tunable chemical and physical properties of the constituent organic and inorganic building blocks in a single composite, making them promising systems for solar technologies. Hybrid materials incorporate the low-cost, large-area processing and high absorbance and quantum efficiencies of organic materials with the adjustable optical properties, high carrier conductivities, and good photostability of inorganic nanostructures. Solar photovoltaic and luminescent solar concentrator technologies will be dramatically advanced if the organic and inorganic building blocks of hybrid structures can be positioned and oriented on the nanometer scale to regulate the competitive processes of charge transfer and transport, emission, and energy transfer.

Hybrid organic-inorganic materials promise one of the best architectures for ultra-low-cost photovoltaic devices. Currently, the efficiency of hybrid photovoltaic devices is limited by the availability of red-absorbing, high-mobility organic and inorganic components (to match the solar spectrum and efficiently collect charge) and of composites with structures that achieve high surface area junctions, yet form well-connected organic and inorganic pathways. This project aims to produce significantly improved hybrid structures for photovoltaics. Improved hybrid materials may also enable creation of high-efficiency luminescent solar concentrators, which currently are limited in performance by materials challenges; organic and inorganic materials alone have not been found to satisfy the broad-spectrum collection, near-unity photoluminescence efficiency, low re-absorption, and good photostability required.

This project brings together advances in chemical synthesis, mathematical modeling, and self-organization to control the position and orientation of organic and inorganic building blocks, exploiting advances at the frontier of chemistry, materials science, and mathematics. We will combine precisely controlled 1) molecular and supramolecular dendrimeric systems tailored to assemble with different structural motifs and 2) nanocrystals of tunable size, shape, and composition that self-assemble into single and multi-component superlattices. Structural, optical, and electrical probes will be combined with mathematical modeling of the effects of interface geometry to optimize charge transfer and transport, emission, and energy transfer. The results will enable engineering of organic-inorganic materials that will be integrated in photovoltaic devices and luminescent solar concentrators.

More broadly, the research program will develop new synthetic methods and mathematical formalisms for the self-assembly of hybrid materials with tailored architectures that is important to provide materials with superior structural, electronic, and optical properties. These materials have applications in imaging, therapeutics, and information technology, in addition to energy harvesting. The project's emphasis on mathematical techniques for engineered self-assembling systems offers the potential for impact in robotics and biological systems. The project will also electronically and optically probe and establish mathematical models of the behavior of organic-inorganic heterojunctions key to their application in a range of electronic and optical devices.

The objective of this research is to develop innovative methods to assemble and manufacture nanoscale materials and devices for applications in photonics, phononics, electronics, and chemical and biological sensing. The approach is to use the advanced lithographic methods of the proposed nanoimprintor to define and probe structures with desired complexity and with nanometer scale features that will transform our fundamental understanding of physical phenomena and biological processes and open doors to new devices for a wide-range of applications.
The intellectual merit of the research, utilizing the proposed nanoimprintor, is to pattern molecular, carbon-based, and inorganic nanocrystalline materials into functional architectures, to explore fundamental phenomena, and to exploit these materials in optical, electrical, energy, mechanical, and sensing devices. Mechanical systems show unique properties at the nanometer scale that can be probed in fabricated structures and harnessed in ultrafast, low-power nanoelectromechanical devices. Surfaces patterned at nanometer and micron scales mimic the extraordinary chemical and mechanical properties of biological systems and are being engineered to influence cell function and tissue regeneration and enabling model organs to be constructed.
The broader impact of the proposed tool is to the research and training of a diverse body of students, teachers, and researchers. The tool will be open to current and future users at Penn and regional academic and industrial institutions. It will be used by undergraduate and graduate students and postdocs from a wide-range of academic disciplines in new course curricula and research, as well being used in demonstrations to K-12 students and teachers.

PI: Baxter, Jason / Murray, Christopher
Proposal Number: 1333649 / 1335821
Institution: Drexel University / University of Pennsylvania
Title: Collaborative Research: Ultrafast Carrier Dynamics in Semiconductor Nanocrystal Solar Cells

Close-packed arrays of semiconductor nanocrystals (NCs), or quantum dots, are ideal systems for fundamental investigations of photo-induced charge and energy transfer in interacting quantum-confined materials. The materials, diameters, and arrangement of the NCs can be used to tune the inter-NC coupling to exploit both the properties of the individual NCs and the long-range effects of the solid. The emergent optical, electronic, and thermal properties of NC superlattices may lead to transformational improvements in applications including photovoltaics, photonics, and thermoelectrics.

The broad objectives of this proposal are (1) to understand ultrafast charge carrier generation, separation, recombination, and transport phenomena in semiconductor nanocrystal superlattices, and (2) to control these fundamental photophysical processes to improve solar cell performance. Specifically, we will investigate films of CdSe, CdTe, and Cu2ZnSnS4 (CZTS) NCs. CdSe and CdTe NCs are excellent model systems because their synthesis and optical properties are well-understood, enabling fundamental ultrafast studies of carrier dynamics in glassy arrays and ordered superlattices of a single monodisperse NC species, as well as binary NC superlattices. CZTS NCs provide an exciting new direction for high efficiency photovoltaics made from non-toxic, earth-abundant elements. The PIs will refine the synthesis of monodisperse CZTS NCs to enable meaningful ultrafast spectroscopic characterization.

This approach centers on time-resolved terahertz spectroscopy (TRTS) and femtosecond visible/infrared transient absorption (TA) to probe intraband and interband transitions, respectively. THz spectroscopy is an ideal, non-contact probe of electronic materials because the THz frequency regime (0.1 - 3 THz) brackets typical carrier scattering rates in semiconductors. THz spectroscopy is unique in its abilities to distinguish between excitons and free carriers and to measure their dynamics on sub-picosecond to nanosecond time scales, providing an excellent complement to our steady-state field effect transistor (FET) measurements. Pump-probe TRTS and TA are ideal techniques to investigate the dynamics of interfacial charge transfer, recombination, and inter-NC transport of photoexcited carriers on their natural time and energy scales.

This work will advance our understanding of the physical phenomena that govern ultrafast exciton and free carrier dynamics in NCs and NC superlattices. Specific studies will include: (1) Determining mechanisms of charge transport in NC superlattices, e.g. by extended states or by activated hopping; (2) Measuring dynamics of inter-NC coupling, interfacial charge transfer, and long-range charge transport in superlattices of a single monodisperse NC species; (3) Determining the dependence of dynamics and transport mechanisms on NC size, capping ligand, inter-NC spacing, and long range order; (4) Understanding charge separation and transport in binary NC superlattices; and (5) Incorporating good candidate materials into solar cells to demonstrate improvements in efficiency that result from carefully designed NC architectures. This work will address the challenge of maintaining quantum-confined NC photophysics while also enabling long range charge transport necessary for devices. PI Baxter?s expertise in ultrafast spectroscopy and solar cells and PI Murray?s expertise in synthesis of NCs and superlattices make the team well-equipped to carry out this work.

The understanding of fundamental photophysical processes such as interfacial charge transfer, recombination, and inter-NC transport in NC superlattices developed here can be applied to create high-efficiency NC solar cells. Availability of efficient, low-cost, clean, and sustainable solar cells made from earth-abundant, non-toxic materials would transform the US energy portfolio. This project will result in the education and training of two Ph.D. students and multiple undergraduates. Additionally, PI Baxter is developing new courses on "Fundamentals of Solar Cells" and lab-based "Nanomanufacturing for Energy Applications" for students from both universities. Outreach will extend to K-12 students by the PIs? continued participation in NanoDay@Penn, Philly Materials Day at Drexel, and mentoring local high school teachers through NSF RET and university programs. These programs are particularly beneficial for underrepresented groups since they target students and teachers from the School District of Philadelphia, whose student body is over 80% minorities.

In this collaborative project funded by the Macromolecular, Supramolecular and Nanochemistry program of the Chemistry Division, Professor Jason Baxter of Drexel University and Professor Christopher Murray of the University of Pennsylvania are developing new particles comprised of two components. The core is a nanoparticle made from a semiconductor, and attached to the core are branched organic molecules called dendrimers. These nanocrystal-dendrimer (NCD) hybrid particles offer promising and superior optical and electronic properties that can be tailored through chemical design of the constituent organic and semiconductor components to manipulate the flow of charge and energy on nanometer length scales. The NCD platform has potential for broad impact across many fields and applications, including solar cells, solid state lighting and display technology, biological imaging, and photocatalytic fuel production and chemical synthesis.

In this project, inorganic NC donors are functionalized with tailored dendrimer ligands that incorporate molecular acceptors, precisely controlling the acceptor location and orientation to enable fundamental investigations of charge and energy transfer. Studies of these NCD hybrids are greatly advancing understanding of photophysical processes at organic / inorganic interfaces and are establishing new design rules to optimize the coupling between the quantum states of the NCs and the molecular energy levels of the functionalized dendrons. The NCD hybrids harness advances in both organic and inorganic synthesis to offer a new class of solution-processable building blocks to engineer the flow of energy and the separation and transport of charge. Objectives of this project include synthesizing and characterizing a library of NCD hybrid nanostructures as well as measuring dynamics and understanding mechanisms of ultrafast charge and energy transfer in NCDs. The broader technical impacts of this work include potential societal benefits from the design and discovery of next-generation particles that offer superior performance in optoelectronic and photocatalytic devices. Broader non-technical impacts include education and training of graduate and undergraduate students, as well as outreach to underrepresented groups within the West Philadelphia Promise Zone.

Non-Technical Description
This Major Research Instrumentation grant allows acquisition of an advanced microscope at the Singh Center for Nanotechnology at the University of Pennsylvania. This microscope uses two different types of particles - electrons and ions - to form high magnification images and give quantitative information about the chemical composition of materials. The microscope is outfitted with a number of special tools that make it particularly capable of characterizing soft matter: materials such as plastics, liquid crystal polymers and biological cells. These materials have been very difficult to characterize in the past as they are very fragile when exposed to high-energy electrons and ions. This instrument includes the ability to freeze samples to very low temperatures to mitigate this problem. It plays a vital role in the education of undergraduate and graduate students, visiting scientists, post-doctoral research associates and local high school students and teachers through existing NSF-funded Center efforts on the campus. The instrument is also used in advanced undergraduate/graduate laboratory classes across multiple departments. Additionally, its location in a fully staffed, open-access NSF National Nanotechnology Coordinated Infrastructure facility allows scientists and engineers from multiple nearby companies and universities to utilize the tool for their own work.

Technical Description
The focused ion beam (FIB) / scanning electron microscope (SEM) enables five major research activities: (1) serial sectioning (cryo)tomography, (2) cryo-sample preparation for subsequent scanning transmission electron microscopy (S/TEM) characterization, (3) time-of-flight secondary ion mass spectroscopy (ToF-SIMS) characterization, (4) nanofabrication, and (5) S/TEM sample preparation. Serial sectioning allows in-depth understanding of the three-dimensional structure of liquid crystal polymers, colloidal nanoparticle superlattices, polymer/nanoparticle composites and other hierarchical materials. Cryo-transfer and FIB-based sample preparation is used to develop methods for characterizing semi-crystalline polymers, colloidal nanoparticle superlattices and polymer/nanoparticle composites. ToF-SIMS allows researchers to understand diffusion in polymeric materials with unprecedented resolution. The instrument supports multiple nanofabrication efforts at the Singh Center's Quattrone Nanofabrication Facility and enables new fabrication approaches for two-dimensional materials. Finally, the low-voltage/low-current capabilities enables ultrathin sample preparation for use in the Singh's S/TEM instruments across a range of research projects which require exquisite atomic-scale characterization.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.