Research

Nanoporous graphene

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. We have been investigating how these nanoporous forms of graphene 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. Alternatively, fluorinating these structures can lead to selective adsorption of molecules on the surface, which can be used for gas separations related to pollution control and renewable energy.

Recent publications:

• "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)

Organic semiconductors

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.

Recent publications:

• "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)

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 been working 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.

Recent publications:

• “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]_n^n- 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)