Office: KINSC E304A
Nanoporous 2D materials for Gas Separation
Graphene, a one-atom-thick planar allotrope of carbon, has extraordinary thermal and electrical conductivity and mechanical strength. Moreover, experiment and theory indicate that a single graphene sheet is impermeable to gases even as small as helium; pores are required for transmission of atoms or molecules. Interestingly, these types of pores can be synthesized in a bottom-up fashion using the tools of organic chemistry, or produced as natural defects in bilayer silica. We have been investigating how these nanoporous two-dimensional materials can be used for chemical and isotopic separations. Because quantum tunneling plays a role in the transmission of atoms through these pores, even at room temperature, this leads to new types of effects which have not previously been utilized in separations.
- "Helium Isotope Enrichment by Resonant Tunneling through Nanoporous Graphene Bilayers" J. Phys. Chem. A 118, 6457-6465 (2014)
- "Entropy-driven Molecular Separations in 2D-Nanoporous Materials, with Application to High-performance Paraffin/Olefin Membrane Separations" J. Phys. Chem. C 117 17050-17057 (2013)
- "Noble Gas Separation using PG-ESX (X=1,2,3) Nanoporous Two-dimensional Polymers" J. Phys. Chem. C, 117, 393-402 (2013)
- "Carbon Dioxide Separation with a Two-Dimensional Polymer Membrane" ACS Appl. Mater. Interfaces 4, 3745-3752 (2012)
- "Helium Tunneling through Nitrogen-Functionalized Graphene Pores: Pressure- and Temperature-Driven Approaches to Isotope Separation" J. Phys. Chem. C 116, 10819-10827 (2012).
- "Thermally-driven Isotope Separation Across Nanoporous Graphene" Chem. Phys. Lett.
521, 118-124 (2012).
- "Fluorinated and nanoporous graphene materials as sorbents for gas separations" ACS Appl. Mater. Interfaces 3, 4451-4458 (2011).
- "Helium Separation Using Porous Graphene Membranes" J. Phys. Chem. Lett. 1, 2284 (2010)
Organically-templated Inorganic Materials
This project centers on non-centrosymmetric organically-templated inorganic solids, in collaboration with experimental work by my Haverford colleague Prof. Alexander Norquist. Non-centrosymmetry (i.e., absence of an inversion symmetry) of these crystals gives them important technological properties, such as piezoelectric, pyroelectric, non-linear optical, and ferroelectric effects. Since only a small fraction of crystals are non-centrosymmetric, it is important to develop new ways to make these types of materials, and understanding the factors that lead to non-centrosymmetry is necessary so we can rationally design new materials. By combining experiment and computation, we have worked to quantify the “charge density matching” model for understanding the molecular factors giving rise to different layer morphologies and crystal symmetries of these types of materials, and to apply new ways of assigning local atomic charges from planewave pseudopotential density functional calculations using the iterative-Hirshfeld method. More recently, we have been exploring the use of the Non-Covalent Interaction (NCI) index to identify/visualize/quantify non-covalent interactions in these materials, and applications of data-mining/machine learning to accelerate the discovery of new solid-state materials.
- "Machine learning assisted materials discovery using failed experiments" Nature 533, 73-76 (2016)
- "Probing structural adaptability in templated vanadium selenites" Polyhedron 114, 184-193 (2016)
- "The role of inorganic acidity on templated vanadate composition and dimensionality" J. Solid State Chem. 236, 215-221 (2016)
- "EQeq+C: An Empirical Bond-Order Corrected Extended Charge Equilibration Method", J. Chem. Theory Comput. 11, 3364-3374 (2015)
- "Role of Noncovalent Interactions in Vanadium Tellurite Chain Connectivities" Inorg. Chem. 54, 694-703 (2015)
- "Formation principles for vanadium selenites: the role of pH on product composition" Inorg. Chem. 53, 12027-12035 (2014)
- "Formation principles for templated vanadium selenite oxalates" Cryst. Growth Des. 13 4504-4511 (2013)
- “Steric-Induced Layer Flection in Templated Vanadium Tellurites” Cryst. Growth Des. 13, 2190-2197 (2013)
- “Role of Hydrogen-Bonding in the Formation of Polar Achiral and Nonpolar Chiral Vanadium Selenite Frameworks” Inorg. Chem. 51, 11040-11048 (2012).
- "Inducing polarity in [VO3]nn- chain compounds using asymmetric hydrogen-bonding networks", J. Solid State Chem. 86-93 (2012).
- "Beyond Charge Density Matching: The Role of C–H···O Interactions in the Formation of Templated Vanadium Tellurites" Cryst. Growth Des. 11, 4213-4219 (2011).
- "Understanding an order-disorder phase transition in ionothermally synthesized gallium phosphates" Cryst. Growth Des. 11, 3065-3071 (2011)
- "[R-C7H16N2][V2Te2O10] and [S-C7H16N2][V2Te2O10]; new polar templated vanadium tellurite enantiomers" J. Solid State Chem. 184, 1445-1450 (2011).
- "The Role of Stereoactive Lone Pairs in Templated Vanadium Tellurite Charge Density Matching" Inorg. Chem. 49, 5167 (2010)
- "Bio-Inspired Electroactive Organic Molecules for Aqueous Redox Flow Batteries. 1. Thiophenoquinones" J. Phys. Chem. C 119, 21800-21809 (2015)
- "From computational discovery to experimental characterization of a high hole mobility organic crystal" Nature Commun. 2, 437 (2011).
- "Predicting organic thin-film transistor carrier type from single molecule calculations" Comput. Theoret. Chem. 966, 70 (2011)
- "Multiple Exciton Generation in Graphene Nanostructures" J. Phys. Chem. C 114, 14332 (2010)
- "Theoretical Characterization of the Air-Stable, High-Mobility Dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene Organic Semiconductor" J. Phys. Chem. C. 114, 2334 (2010)
Organic Aqueous Redox Flow Battery Materials
Low-cost electrical energy storage is crucial for widespread adoption of renewable energy, but current electrochemical (battery) technologies are too expensive. Redox Flow Batteries utilizing water- soluble organic redox couples are a new strategy for low-cost, eco-friendly, and durable stationary electrical energy storage. While prototypes have been demonstrated in the laboratory, two physicochemical properties must be improved to increase the energy density: (1) Increasing the range of half-cell voltages; (2) Increasing the solubility in water. Using quantum chemical calculations, we can predict the voltages and using cheminformatics models (and a little statistical thermodynamics) we can also estimate the solubility. Moreover, by developing cheminformatics models of these quantum chemical results, we can dramatically decrease the amount of computational time needed to find the best candidates. Currently we are working on exploring molecular frameworks modeled on those used by bacteria for electron transfer processes. In collaboration with Prof. Jason Schmink (Bryn Mawr College) we are also working on designing new, easily-synthesized molecules with high performance characteristics.
High-performance organic semiconductors are important for the development of low-cost printable electronics and large-area (e.g., photovoltaic) applications. However, compared to traditional inorganic semiconductors, organic semiconductors typically have low charge mobility, a measure of how rapidly and easily charges can move through the material. Charge mobility determines both the switching speed (important for printable electronics and radio-frequency identification (RFID) tag applications), and is also related to efficiency losses in organic photovoltaics. In collaboration with the groups of Prof. Zhenan Bao (Stanford), Prof. Alan Aspuru-Guzik (Harvard), and Prof. Sergio Granados-Focil (Clark) we have recently demonstrated how theoretical calculations can guide the development of new high-performance organic semiconductor materials. Current work in this area includes development of new n-channel organic semiconductors with high performance and developing computationally efficient schemes for identifying the best candidate molecules.
One spin-off of this work involves polyaromatic hydrocarbon (PAH) (“nanographene” or “graphene nanoparticles”) molecules. Some of the earliest organic semiconductors, such as tetracene and pentacene, belong to this class of molecules. New synthetic methods make it possible to construct larger, more stable PAHs with a variety of shapes, providing new ways to tune the material properties. Based on theoretical calculations we have been able to show that a large class of graphene nanoparticles exhibit efficient multiple exciton generation, a process where the excess energy contained in high-energy photons is captured and converted into an additional charge excitation in the material rather than being dissipated as heat. MEG can be used to create solar cells that exceed the Shockley-Queisser thermodynamic efficiency limit, and thus have the potential to improve the performance and reduce the cost of solar cells.