SCIENTIFIC ACHIEVEMENTS

Solvent-type-dependent polymorphism and charge transport in a long fused-ring organic semiconductor.

Chen JShao MXiao KRondinone AJLoo YLKent PRSumpter BGLi DKeum JKDiemer PJAnthony JEJurchescu ODHuang J.

Abstract
Crystalline polymorphism of organic semiconductors is among the critical factors in determining the structure and properties of the resultant organic electronic devices. Herein we report for the first time a solvent-type-dependent polymorphism of a long fused-ring organic semiconductor and its crucial effects on charge transport. A new polymorph of 5,11-bis(triethylsilylethynyl)anthradithiophene (TES ADT) is obtained using solvent-assisted crystallization, and the crystalline polymorphism of TES ADT thin films is correlated with their measured hole mobilities. The best-performing organic thin film transistors of the two TES ADT polymorphs show subthreshold slopes close to 1 V dec(-1), and threshold voltages close to zero, indicating that the density of traps at the semiconductor-dielectric interface is negligible in these devices and the observed up to 10-fold differences in hole mobilities of devices fabricated with different solvents are largely resultant from the presence of two TES ADT polymorphs. Moreover, our results suggest that the best-performing TES ADT devices reported in the literature correspond to the new polymorph identified in this study, which involves crystallization from a weakly polar solvent (such as toluene and chloroform).

Highlight Narrative
Polymorphism is widely encountered in organic solids due to the relatively weak intermolecular interactions (van der Waals type). In this work, we demonstrate that the specific intermolecular interactions between the solvent and aggregated clusters in the initial stage of crystallization can cause a remarkable polymorphism during the solution crystallization of TES ADT. This control over polymorphism is important because crystalline packings/polymorphs of TES ADT when processed at room temperature critically affect device performance, changing the maximum hole mobility by up to 10 times under the same testing conditions.

Title
Solvent-type-dependent polymorphism and charge transport in a long fused-ring organic semiconductor

Scientific Achievement
We report a solvent-type-dependent polymorphism of a long fused-ring organic semiconductor and its crucial effects on charge transport.

Above - Selected area electron diffraction patterns of TES ADT thin films slowly crystallized from THF and toluene solutions in [001] zone.

Significance and Impact
A newly discovered polymorph of 5,11-bis(triethylsilyl-ethynyl) anthradithiophene (TES ADT) obtained using toluene solvent-assisted crystallization exhibits up to 10-fold higher hole mobility in organic thin film transistor devices than devices fabricated with other different solvents. These results underscore the importance of crystalline polymorphism of organic semiconductors as a critical factor in determining the structure and properties of the resultant organic electronic devices.

Research Details

  1. A method for obtaining the new polymorph of TES ADT and its structure.
  2. Mechanism for charge transport enhancement based on intrinsic packing of the resultant TES ADT polymorphs.
  3. Density functional theory UV-Vis absorption and simulation.

Title
Using Solvents to Improve Charge Transport

Scientific Achievement
We show how solvent polarity and intermolecular interactions between the solvent and aggregated clusters in the initial stage of crystallization of  a long fused-ring organic semiconductor cause a remarkable polymorphism during the solution crystallization.

Above - Optical micrographs of 5,11-bis(triethylsilylethynyl) anthradithiophene films prepared in toluene (Left) and THF (right) solvents.

Significance and Impact
By using appropriate solvents, a solvent-assisted crystallization is promoted that yields materials that can exhibit up to 10-fold differences in the maximum hole mobilities in organic thin film transistor devices.

Research Details

  1. Using a combination of quantum calculations, experimental crystallization and characterization, we developed a method for a highly important class of fused-ring organic materials in order to yield films with significantly improved charge mobilities.

Acknowledgement of Support
This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy. The computations were performed using the resources of the National Energy Research Scientific Computing Center, which are supported by the Office of Science of the U.S. Department of Energy under Contract no. DE-AC02-05CH11231.

Citation
J. Chen, M. Shao, K. Xiao, A. J. Rondinone, Y-L. Loo, P. R. C. Kent, B. G. Sumpter, D. Li, J. K. Keum, P. J. Diemer, J. E. Anthony, O. D. Jurchescu, J. Huang, "Solvent-type-dependent polymorphism and charge transport in a long fused-ring organic semiconductor", Nanoscale,DOI: 10.1039/c3nr04341j.

 

Understanding the origin of high-rate intercalation pseudocapacitance in Nb2O5 crystals

Andrew A. Lubimtsev, Paul R. C. Kent, Bobby G. Sumpter and P. Ganesh  

Abstract
Pseudocapacitors aim to maintain the high power density of supercapacitors while increasing the energy density towards those of energy dense storage systems such as lithium ion batteries. Recently discovered intercalation pseudocapacitors (e.g. Nb2O5) are particularly interesting because their performance is seemingly not limited by surface reactions or structures, but instead determined by the bulk crystalline structure of the material. We study ordered polymorphs of Nb2O5 and detail the mechanism for the intrinsic high rates and energy density observed for this class of materials. We find that the intercalating atom (lithium) forms a solid solution adsorbing at specific sites in a network of quasi-2D NbOx faces with x = {1.3, 1.67, or 2}, donating electrons locally to its neighboring atoms, reducing niobium. Open channels in the structure have low diffusion barriers for ions to migrate between these sites (EDescription: similar] 0.28–0.44 eV) comparable to high-performance solid electrolytes. Using a combination of complementary theoretical methods we rationalize this effect in LixNb2O5 for a wide range of compositions (x) and at finite temperatures. Multiple adsorption sites per unit-cell with similar adsorption energies and local charge transfer result in high capacity and energy density, while the interconnected open channels lead to low cost diffusion pathways between these sites, resulting in high power density. The nano-porous structure exhibiting local chemistry in a crystalline framework is the origin of high-rate pseudocapacitance in this new class of intercalation pseudocapacitor materials. This new insight provides guidance for improving the performance of this family of materials.

Highlight Narrative
Electrochemical energy storage is an essential means of storing/retrieving energy with ubiquitous use in applications from powering portable electronics to vehicular transportation. Our current energy ecosystem demands that we overcome fundamental bottlenecks in materials and design that limit simultaneous high energy density and power-density with long cycle life. The present work highlights a detailed mechanism explaining the origins of high-capacity pseudocapacitance in niobium pentoxide, where local oxidation/reduction reactions occur along with electrostatic charge storage coupled to efficient mass mobility. This new mechanism provides a means for future design/discovery of new energy storage materials with greatly improved energy capacity and very long cycle life.

Title
Understanding the origin of high-rate intercalation pseudocapacitance in Nb2O5 crystals

Scientific Achievement
Using first principles calculations, we unravel a detailed mechanism explaining the high-rate pseudocapacitance in niobium pentoxide.

Above - Computed CV profile shows a pseudocapacitative slope over the entire range of Li-ion intercalation in Nb2O5. 

Above - Overlapped lithium (blue) trajectories from ab initio molecular dynamics show diffusion pathways through the Nb2O5 crystal.

Significance and Impact
Our modern energy ecosystem demands that we overcome fundamental bottlenecks in materials and design that limit simultaneous high energy density and power-density with long cycle life. The detailed mechanism for high-rate pseudocapacitance in niobium pentoxide materials provides a blueprint for efficient design/discovery of greatly improved energy storage materials.

Research Details

  1. First principles simulation of pseudocapacitance
  2. Improved computational workflow to reliably predict the stability and dynamics of ions in solids at finite temperatures
  3. Insights and approach enable opportunities for designing new materials exhibiting improved energy storage capacities

Title
Rapid Electrical Energy Storage even at High Capacities

Scientific Achievement
Using a combination of complementary methods based on the theory of quantum mechanics, we unravel the principles behind a new mechanism to rapidly increase the amount of energy stored even at very high capacities

Above - Electron transfer is seen to occur from lithium reducing niobium atoms in the Nb2O5 solid explaining its high energy density. The orange lines indicate the two dimensional open pathways for the lithium ions to diffuse with very low barrier. The combination of a large electron transfer and availability of open pathways lead to both a high energy and a high power-density in this system.

Significance and Impact
Our modern energy ecosystem demands that we overcome fundamental bottlenecks in materials and design that limit simultaneous high energy density and power-density with long cycle life. Our fundamental insights detailing the inner workings of a high capacity electrical energy storage material provides a blueprint for efficient design/discovery of greatly improved materials.

Research Details

  1. Quantum mechanics based understanding of how to rapidly store energy at high capacities in an electrical storage material
  2. Improved computational workflow to reliably predict such phenomenon using supercomputers
  3. Insights enabling opportunities for designing materials that can store more energy and operate at high power.

Acknowledgement of Support:
This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy. The computations were performed using the resources of the CNMS and the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory.

Citation
Andrew Lubimtsev, Paul R. C. Kent, Bobby G. Sumpter, P. Ganesh, "Understanding the Origin of High-Rate Intercalation Pseudocapacitance in Nb2O5 Crystals", (2013) J. Mater. Chem. ADOI: 10.1039/C3TA13316H

 

A Quantum Trajectory-Electronic Structure Approach for Exploring Nuclear Effects in the Dynamics of Nanomaterials

Sophya Garashchuk, Jacek Jakowski, Lei Wang, and Bobby G. Sumpter

Abstract
A massively parallel, direct quantum molecular dynamics method is described. The method combines a quantum trajectory (QT) representation of the nuclear wave function discretized into an ensemble of trajectories with an electronic structure (ES) description of electrons, namely using the density functional tight binding (DFTB) theory. Quantum nuclear effects are included into the dynamics of the nuclei via quantum corrections to the classical forces. To reduce computational cost and increase numerical accuracy, the quantum corrections to dynamics resulting from localization of the nuclear wave function are computed approximately and included into selected degrees of freedom representing light particles where the quantum effects are expected to be the most pronounced. A massively parallel implementation, based on the message-passing interface allows for efficient simulations of ensembles of thousands of trajectories at once. The QTES-DFTB dynamics approach is employed to study the role of quantum nuclear effects on the interaction of hydrogen with a model graphene sheet, revealing that neglect of nuclear effects can lead to an overestimation of adsorption.

Highlight Narrative
Quantum behavior of nuclei can play an important role for chemical reactivity and optoelectric properties. We have developed a massively parallel, direct quantum molecular dynamics method and used it to study adsorption of hydrogen/deuterium on a graphene "flake". The colliding atoms are treated as quantum particles whose wavefunction is represented by an ensemble of quantum trajectories (shown as a swarm of particles), moving according to the quantum potential. The electronic potential energy is computed using Self-Consistent Density Functional Theory Tight Binding. Localization of the colliding atomic wavefunction significantly influences the dynamics of the graphene carbons that subsequently alters the adsorption probabilities.

Title
A Quantum Trajectory-Electronic Structure Approach for Exploring Nuclear Effects in the Dynamics of Nanomaterials

Scientific Achievement
We describe a massively parallel, direct quantum molecular dynamics method that combines a quantum trajectory representation of the nuclear wavefunction discretized into an ensemble of trajectories with an electronic structure description of the electrons.

Above Left: A snapshot of a graphene flake under bombardment by an ensemble of 1000 H atoms. Above Right:Time-dependent adsorption probability of hydrogen on the graphene flake. The results are obtained from classical dynamics, the developed method, and from exact quantum evolution.

Significance and Impact
The hybrid nuclear dynamics method allows treatment of nuclear effects that are important in the interaction of hydrogen with a model graphene sheet, revealing that neglect of these effects can lead to an overestimation of adsorption.

Research Details

  1. A scalable first principles quantum dynamics simulation method that combines electronic structure with quantum treatment for selected nuclei.
  2. Approach that enables opportunities for designing new materials exhibiting improved properties for energy science applications.
  3. Quantum nuclear effects are responsible for increased adsorption selectivity of deuterium over hydrogen on a graphene flake.

Title
Are there Nuclear Effects in the Dynamics of Nanomaterials?

Scientific Achievement
Using a combination of complementary methods based on the theory of quantum mechanics, we have developed a new computational capability to study the dynamics of materials under diverse environmental situations.

Above - A snapshot of a flake of carbon (graphene) under bombardment by 1000 H atoms.

Significance and Impact
Our modern energy ecosystem demands that we overcome fundamental bottlenecks in materials and design that limit our progress towards securing a bright energy future.  To this end, advanced computational simulations can provide a means to study and understand possible materials for use in energy storage or conversion devices.  The new advances for modeling dynamics, with the inclusion of nuclear effects, provides improved capabilities to study energy materials properties and processes.

Research Details

  1. A method based on quantum mechanics to understand how nuclear effects modify the dynamics of nanoscale materials
  2. Quantum nuclear effects are responsible for increased adsorption selectivity of deuterium over hydrogen on a graphene flake

Acknowledgement of Support
This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy.

Citation
Sophya Garashchuk, Jacek Jakowski, Lei Wang, and Bobby G. Sumpter, "A Quantum Trajectory-Electronic Structure Approach for Exploring Nuclear Effects in the Dynamics of Nanomaterials", (2013) J. Chem. Theory Comput.,9, 5221–5235 (2013). DOI: 10.1021/ct4006147

 

Spin-resolved self-doping tunes the intrinsic half-metallicity of AlN nanoribbons

Alejandro Lopez-Bezanilla, P. Ganesh,  Paul R. C. Kent, Bobby G. Sumpter

Abstract

We present a first-principles theoretical study of electric field-and straincontrolled intrinsic half-metallic properties of zigzagged aluminium nitride (AlN) nanoribbons. We show that the half-metallic property of AlN ribbons can undergo a transition into fully-metallic or semiconducting behavior with application of an electric field or uniaxial strain. An external transverse electric field induces a full charge screening that renders the material semiconducting. In contrast, as uniaxial strain varies from compressive to tensile, a spin-resolved selective self-doping increases the half-metallic character of the ribbons. The relevant strain-induced changes in electronic properties arise from band structure modifications at the Fermi level as a consequence of a spin-polarized charge transfer between p-orbitals of the N and Al edge atoms in a spin-resolved self-doping process. This band structure tunability indicates the possibility of designing magnetic nanoribbons with tunable electronic structure by deriving edge states from elements with sufficiently different localization properties. Finite temperature molecular dynamics reveal a thermally stable half-metallic nanoribbon up to room temperature.

Highlight Narrative

A first-principles theoretical study of electric field- and strain-controlled intrinsic half-metallic properties of zigzagged aluminium nitride (AlN) nanoribbons reveal that the half-metallic property of AlN ribbons can undergo a metallic or semiconducting transition with application of an electric field or uniaxial strain. An external transverse electric field induces full charge screening rendering the material semiconducting, while an uniaxial strain varying from compressive to tensile causes the spin-resolved selective self-doping to increase the half-metallic character of the ribbons. The relevant strain-induced changes in electronic properties arise from band structure modifications at the Fermi level as a consequence of a spin-polarized charge transfer between p-orbitals of the N and Al edge atoms in a spin-resolved self-doping process. The band structure tunability indicates the possibility of rationally designing magnetic nanoribbons with tunable electronic structure by deriving edge states from elements with sufficiently different localization properties.

Title

Spin-Resolved Self-Doping Tunes The Intrinsic Half-Metallicity Of AlN Nanoribbons

Scientific Achievement

Demonstrated how and why the half-metallic property of aluminum nitride (AlN) nanoribbons can undergo a transition to fully-metallic or semiconducting behavior with application of an electric field or uniaxial strain.

Above - Top, the real space distribution of the net-spin density corresponding to the electronic states of an AlN nanoribbon ([AlN]6(2)). The red and green isosurfaces correspond to net spin-↑ and spin-↓ electron densities respectively. Bottom, the spin-resolved band structure, within a narrow energy window around the Fermi level for different types/amount of strain.

Significance and Impact

The band structure tunability of AlN indicates the possibility of rationally designing magnetic nanoribbons with "on-demand" electronic structure. An external transverse electric field induces a full charge screening that renders the material semiconducting, while an uniaxial strain varying from compressive to tensile causes the spin-resolved selective self-doping to increase the half-metallic character of the ribbons.

Research Details

Title

Tuning Electronic Properties on Demand

Scientific Achievement

We demonstrated that aluminum nitride (AlN) nanoribbons have a rich variety of "on demand" tunable electronic properties due to a spin-resolved self-doping.

Above Left, the real space distribution of the net-spin density corresponding to the electronic states of an AlN nanoribbon ([AlN]6(2)). The red and green isosurfaces correspond to net spin-↑ and spin-↓ electron densities respectively. Above Right, electronic band structure for the configuration. The spin orientations for Al- and -N edge atom are given by arrows at the bottom of the band structure.

Significance and Impact

Materials exhibiting metallic behavior for electron spins with one orientation and insulating for the spins with the opposite orientation are critical for modern applications of  spintronics. The results of this study provide a "materials recipe" for designing systems for exhibiting these properties by deriving edge states  from elements with sufficiently different localization properties.

Research Details

This work enabled the development of an Improved computational capability and understanding of the electronic structure of complex layered materials.  The approach provides enhanced opportunities for designing new materials exhibiting tunable electronic properties, e.g., multifunctional materials by design.

Acknowledgement of Support

This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy. The computations were performed using the resources of the CNMS and the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory.

Citation

Alejandro Lopez-Bezanilla, P. Ganesh,  Paul R. C. Kent, Bobby G. Sumpter, Spin-resolved self-doping tunes the intrinsic half-metallicity of nanoribbons, Nano Res. DOI 10.1007/s12274-013-0371-1