Person:

Forsythe, Martin Blood Zwirner

The virtual high-throughput screening framework of the Harvard Clean Energy Project allows for the computational assessment of candidate structures for organic electronic materials – in particular photovoltaics – at an unprecedented scale. We report the most promising compounds that have emerged after studying 2.3 million molecular motifs by means of 150 million density functional theory calculations. Our top candidates are analyzed with respect to their structural makeup in order to identify important building blocks and extract design rules for efficient materials. An online database of the results is made available to the community.

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni2+ in aqueous NH3 solution under aerobic conditions produces Ni_3(HITP)_2(HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metal−organic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 and 40 S·cm−1, respectively, both records for MOFs and among the best for any coordination polymer.

Diketopyrrolopyrroles (DPPs) have recently gained attention as building-blocks for organic semiconducting polymers and small molecules, however the semiconducting properties of their hydrogen-bonded (H-bonded) pigment forms have not been explored. Herein we report on the performance of three archetypical H-bonded DPP pigments, which show ambipolar carrier mobilities in the range 0.01–0.06 cm2/V s in organic field-effect transistors. Their semiconducting properties are correlated with crystal structure, where an Hbonded crystal lattice supports close and relatively cofacial p–p stacking. To better understand transport in these systems, density functional theory calculations were carried out, indicating theoretical maximum ambipolar mobility values of 0.3 cm2/V s. Based on these experimental and theoretical results, H-bonded DPPs represent a viable alternative to more established DPP-containing polymers and small molecules where H-bonding is blocked by N-alkylation.

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni2+ in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metalorganic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 Scm -1 and 40 Scm -1, respectively, both records for MOFs and among the best for any coordination polymer.

An accurate treatment of the long-range electron correlation energy, including van der Waals (vdW) or dispersion interactions, is essential for describing the structure, dynamics, and function of a wide variety of systems. Among the most accurate models for including dispersion into density functional theory (DFT) is the range-separated many-body dispersion (MBD) method [A. Ambrosetti et al., J. Chem. Phys., 2014, 140, 18A508], in which the correlation energy is modeled at short-range by a semi-local density functional and at long-range by a model system of coupled quantum harmonic oscillators. In this work, we develop analytical gradients of the MBD energy with respect to nuclear coordinates, including all implicit coordinate dependencies arising from the partitioning of the charge density into Hirshfeld effective volumes. To demonstrate the efficiency and accuracy of these MBD gradients for geometry optimizations of systems with intermolecular and intramolecular interactions, we optimized conformers of the benzene dimer and isolated small peptides with aromatic side-chains. We find excellent agreement with the wavefunction theory reference geometries of these systems (at a fraction of the computational cost) and find that MBD consistently outperforms the popular TS and D3(BJ) dispersion corrections. To demonstrate the performance of the MBD model on a larger system with supramolecular interactions, we optimized the C60@C60H28 buckyball catcher host–guest complex. In our analysis, we also find that neglecting the implicit nuclear coordinate dependence arising from the charge density partitioning, as has been done in prior numerical treatments, leads to an unacceptable error in the MBD forces, with relative errors of ∼20% (on average) that can extend well beyond 100%.

Accurate treatment of the long-range electron correlation energy, including dispersion interactions, is essential for describing the structure, dynamics, and function of a wide variety of systems. Among the most accurate models for including dispersion into density functional theory (DFT) is the range-separated many-body dispersion (MBD) method [A. Ambrosetti et al., J. Chem. Phys. 2014, 140, 18A508], in which the long-range correlation energy is computed from a model system of coupled quantum harmonic oscillators. In this work, we seek to extend the applicability of the MBD model by developing the analytical gradients necessary to compute MBD corrections to ionic forces, unit-cell stresses, phonon modes, and self-consistent updates to the Kohn-Sham potential. We include all implicit coordinate dependencies arising from charge density partitioning, as we find that neglecting these terms leads to unacceptably large relative errors in the MBD forces. Such errors would impact the predictive nature of ab initio molecular dynamics simulations employing MBD. We develop a new efficient implementation of the MBD correlation energy and forces within the Quantum ESPRESSO software package and rigorously test its numerical stability and convergence properties for condensed phase simulations. Additionally, we re-parameterize the MBD model for use with a wide variety of generalized gradient approximation exchange-correlation functionals. We demonstrate the efficiency and accuracy of these MBD gradient corrections for optimizations of isolated dispersively bound molecular systems, as well as representative condensed phase systems including adsorbed hydrocarbons, layered materials, and hydrogen-bonded crystals. Where highly accurate reference geometries are available, we find the DFT+MBD method significantly improves the predicted structures of these systems and consistently outperforms popular pairwise-additive DFT-D dispersion corrections. Though significant work remains in the benchmarking and testing of these contributions to the MBD model, we are optimistic that these methodological developments will enable many exciting discoveries of beyond-pairwise dispersive effects in organic materials.

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ~ 10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO(_{2}) thin film in high vacuum.