Stephanie Lee - US grants

The industrialized world and the United States in particular faces a critical Helium shortage, which threatens a multitude of industrial and medical applications requiring cryogenic cooling. For example, Magnetic Resonance Imagery relies on liquid Helium to cool the high-power magnets needed to create interior body images for clinical analysis and medical intervention. Helium is also indispensable to the U.S. space exploration program and other civilian and military uses. This proposal requests funding to purchase an instrument called attoDRY1100 that allows for carrying out sophisticated optical characterization of nanomaterials and devices at extremely low temperatures and high magnetic fields without the need for liquid Helium. This cryogen-free system completely eliminates the current annual liquid Helium operation costs of about $50,000 at Stevens Institute of Technology, and more importantly, avoids the annual consumption of about 5000 liters of a precious noble gas, thereby contributing to a "greener campus". The equipment will be incorporated into the shared-user Nanophotonics Lab at Stevens where it will serve the needs of a growing user base of over 30 students and staff representing 11 research groups from 5 departments at Stevens. The cryogen-free system will advance several NSF-funded projects that address fundamental questions of the light-matter interaction of nanomaterials at the forefront of current research. These projects also target a broad range of photonic devices that have potential transformative applications, such as in national security and sustainable energy. The broader impacts will come from the fact that the requested instrument will be housed in a central user facility which has a history of successful multidisciplinary training of a broad range of students from high school to postdoctoral researchers addressing forefront research in nanotechnology. The PI's will further expand their outreach activities to currently several hundred high schools students in the framework of the SEED program of the American Chemical Society and the ECOES program at Stevens, that will specifically include hands-on education on various nanotechnology tools, including the attoDRY1100.

This proposal requests funding to purchase the attoDRY1100 cryogen-free cryostat with an integrated CFM-I scanning-probe confocal microscope insert. The cryogen-free system will benefit several NSF-funded projects that address fundamental questions of the light-matter interaction in nanomaterials, which include heterostructures of graphene and transition-metal dichalcogenides, ultra-clean carbon nanotubes, lithium-niobate microcavities, catalytic surfaces for biomass conversion, and solution-processed hybrid inorganic-organic semiconductor thin films. These projects target a broad range of photonic devices, including on-chip photodetectors, all-optical modulators, quantum light sources, thin-film organic solar cells, and Zeno-effect-based switches, which imply transformative applications in national security, sustainable energy, and beyond. The attoDRY1100 instrument offers a wealth of new functionalities and measurement modes highly desirable to the ongoing research projects. These include hyperspectral 2D-mapping of photoluminescence and photocurrent signals, cryogenic photolithography with 10 nm resolution, magneto-luminescence studies of confined excitons, in-situ Raman mapping of surface catalysis, and long-term drift-free quantum-optics studies of single quantum emitters.

Every hour the sun provides more than enough energy to satisfy the annual energy requirements of the human population. Full exploitation of this abundant sustainable resource will require efficient means for its economical harvesting. Organic solar cells, which are composed of polymers with various carbon-based additives, are promising vehicles to convert solar energy into electricity on the basis of their flexibility, lightweight nature, and potential for large-area coverage. The conversion efficiencies of current organic solar cells, however, are relatively low and their costs are prohibitively high. The use of high-throughput continuous manufacturing methods, such as inkjet printing and roll-to-roll processing has the potential to reduce the cost of manufacturing. Furthermore, if the organic cell microstructures are favorably controlled during their continuous fabrication, their conversion efficiencies can be increased. This project aims to develop a fundamental understanding of the dynamics of the shearing processes during continuous mixing and deposition of the polymer/additive mixtures so that solar cell structures with greater light conversion efficiencies can be obtained while reducing the manufacturing expense. This multidisciplinary project will serve as a fertile training ground for graduate students and will be integrated into outreach activities for underrepresented groups in science and engineering.

Photoactive layers of organic solar cells are comprised of polymer-small molecule nanocomposites, and the crystal size and crystallinity of the small molecule component are critical microstructural factors for light conversion efficiency and long-term stability. This research will investigate how the deformation history applied to polymer-small molecule nanosuspensions prior to and during film deposition affects crystal sizes and nucleation densities of small molecules to impact the efficiency and stability of organic solar cells. This objective will be accomplished by: (1) mapping processing-structure relationships between nanocomposite composition, solution shearing conditions, and resultant small molecule crystallization outcomes; (2) executing a preshearing and coating process that is compatible with industrially-relevant rates to impose target shear histories prior to and during film deposition; and (3) evaluating solar cell performance to determine the effects of small molecule crystallization on light conversion efficiency and stability. By systematically exploring the effects of polymer rheology and processing conditions on the shear induced crystallization of small molecules, mathematical modeling-based design rules will be established to guide the development of continuous processing methods capable of evoking desired crystallization outcomes.

Non-technical abstract

Crystals are straight by definition. They have sharp edges and flat faces. They are polyhedra. However, molecular crystals that twist as they grow are remarkably common, albeit little known. More than one third of simple molecular crystals are capable of forming twisted morphologies. As a largely unexplored phenomenon, crystal twisting introduces a new dimension to materials design. Plastic electronic devices, e.g. foldable LCD screens, smart phones, computers, and solar panels, depend on the shapes of tiny crystals that carry electricity. This collaborative project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, uncovers at a fundamental science level the effect of twisted morphologies on the propagation of electrical current and light through crystals comprising organic semiconductors in order to usher in the next age of personal consumer electronic devices as well as critical technologies associated with renewable energy. Early results show that twisting boosts conductivity. Outreach activities embracing literature, art, history, and education reflect the themes of crystals and chirality (items that can be distinguished from their mirror image are chiral, for example a hand) that undergird these scientific efforts, with a focus on using intriguing aspects of twisted crystals as a platform to engage K-12 students in STEM-related activities in the NY metro area.

Technical Abstract

Helicoidal crystals with pitches from 1-500 microns can carry charge when grown from molecules that form traditional organic semiconductors. At the level of devices, twisting on these length scales can have critical consequences on light propagation and charge injection, extraction and hopping. To elucidate the general effect of twisting on such processes, a series of semiconducting compounds are induced to twist as they crystallize from the melt into thin films as part of this collaborative research, which is supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF. Conductive and photoconductive atomic force microscopy and charge mobility measurements using a field-effect transistor platform are performed on these helicoidal crystals as a function of pitch to determine the modulation of electric field- and photo-induced charge transport locally along and perpendicular to the twisting axes. As optoelectronic devices typically require specific crystal orientations within active layers for optimal performance, electrocrystallization is applied to molten organic conductors to collimate twisted crystals on electrode surfaces. Electrical magnetochiral anisotropy measurements, in conjunction with complete imaging polarimetry unique to the PIs' laboratories, are actualized in the search for chiral defects introduced via twisting. In doing so, this research uncovers fundamental mechanisms of crystal growth while addressing inherent limitations in the field of organic electronics, including large charge transport anisotropies along less accessible crystallographic directions and difficulties in tuning molecular interactions independent of molecular structure.

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.