A Magnetic Analogue to Thermoelectric Effect: Coupled Phonon-Magnon Diffusion and the Magnon Cooling Effect
Dec 1, 2014 8:15am ‐ Dec 1, 2014 8:30am
Identification: CC1.01
We study the coupled transport of phonons and magnons in a single-domain ferromagnet under concurrent temperature and external magnetic field gradients, and show it is a close analogue to the conventional coupled electron-phonon transport that leads to the thermoelectric effect. Working within the framework of Boltzmann transport equation, we derive the constitutive equations for the coupled phonon-magnon transport, and the corresponding conservation laws. Especially we show both the similarity and the difference between the roles played by the electrochemical potential for electrons and the external magnetic field for magnons. Our equations reduce to the original Sanders-Walton two-temperature model under a uniform external field, but predict a new magnon cooling effect driven by a non-uniform magnetic field, which resembles the conventional Peltier effect. We estimate the magnitude of the cooling effect in yttrium iron garnet, and show it is within current experimental reach. With properly optimized materials, the predicted cooling effect can potentially supplement the conventional magnetocaloric effect in cryogenic applications in the future. This work is supported partially by S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, and partially by the Air Force Office of Scientific Research Multidisciplinary Research Program of the University Research Initiative (AFOSR MURI) via Ohio State University.
First-Principles Calculations of the Thermoelectric Properties of Silicon
Dec 1, 2014 8:30am ‐ Dec 1, 2014 9:00am
Identification: CC1.02
This talk will discuss our recent work on using first-principles simulations to investigate both electron and phonon thermoelectric transport in silicon. Electron-phonon and electron-impurity scatterings are computed from first principles calculations to obtain electron relaxation times due to both phonon and impurity scattering. The energy dependent relaxation times are then used in the Boltzmann transport theory to obtain the electrical conductivity, Seebeck coefficient and electronic thermal conductivity. The anharmonic force constants are derived from first-principles and used to compute phonon relaxation times based on Fermi’s golden rule. The energy dependent mean free paths are computed for both electrons and phonons. The electrical and thermal conductivity accumulation functions in silicon with respect to electron and phonon mean free paths are compared. The electron-filtering concept is examined using our energy dependent transport data. It demonstrates the capability to quantitatively investigate various electron engineering approaches. By combining all electron and phonon transport properties from first-principles, we predict the full thermoelectric properties of the bulk and nanostructure of silicon over a range of temperatures from 100 to 400 K and doping levels from to cm-3, showing good agreement with experiments. This work is supported by S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.
Performance Enhancement of Pentacene Based Organic Field-Effect Transistor through DNA Interlayer
Dec 1, 2014 8:30am ‐ Dec 1, 2014 8:45am
Identification: A1.01
Organic field-effect transistors (OFETs) with a top-contact structure were fabricated by applying deoxyribonucleic acid (DNA) as a hole injection layer between organic semiconductor and electrousing a simple spray-coating process. Compared with that of the bare OFET, the OFET incorporated with a DNA hole injection layer exhibited a significant enhancement of field-effect mobility from 0.02 to 0.104 cm2 V−1 s−1. By analyzing the electrical characteristics of these OFETs and the surface morphology of the pentacene film, the results showed that the dipole formation effect brought by the DNA interlayer effectively reduced the contact resistance between the gold electrodes and the pentacene film. Consequently, improved hole injection was obtained along with the enhanced electrical characteristics. Moreover, DNA is soluble in aqueous solvent only and compared to spin-coating process, spray-coating process is beneficial to film formation of DNA aqueous solution with high viscosity and thus make DNA interlayer well incorporated in OFET device. In order to confirm the performance enhancement effect brought by DNA interlayer, OFETs based on another p-type organic semiconductor of α-hexathiophene and the OFETs based on silver electrodes were also investigated. The results showed that the α-hexathiophene based OFETs also exhibited significant performance improvement and the large injection barrier between silver electrodes and the pentacene organic semiconductor was significantly reduced by DNA interlayer. In addition, among the four kinds of base-pairs in DNA molecule, the guanine base-pair exhibits the lowest oxidation potential owing to its peculiar sequence of H-bond donor or acceptor groups. Therefore, the guanine base-pair was selected to be further analyzed and the results confirmed the existence of the dipole formation effect in DNA molecule. Moreover, in the case of organic electronics, environmental stability is another crucial feature. The main discrepancy of pentacene film after exposure to air is the generation of the deep trap states. When the OFET is exposed to air, deep trap states which result from the absorption of H2O and other molecules lead to a degradation in performance. When a DNA hole injection layer was introduced, the surface of pentacene film was protected from direct contact with ambient conditions and the degradation of the device was thus slowed down.
A Theory and Challenges for Coarsening in Microstructure
Dec 1, 2014 8:30am ‐ Dec 1, 2014 9:00am
Identification: NN1.01
Cellular networks are ubiquitous in nature. Most engineered materials are polycrystalline microstructures composed of a myriad of small grains separated by grain boundaries, thus comprising cellular networks. The grain boundary character distribution (GBCD) is an empirical distribution of the relative length (in 2D) or area (in 3D) of interface with a given lattice misorientation and normal. During the coarsening, or growth, process, an initially random grain boundary arrangement reaches a steady state that is strongly correlated to the interfacial energy density. In simulation, if the given energy density depends only on lattice misorientation, then the steady state GBCD and the energy are related by a Boltzmann distribution. This is among the simplest non-random distributions, corresponding to independent trials with respect to the energy. Why does such simplicity emerge from such complexity?
Here we describe an entropy based theory which suggests that the evolution of the GBCD satisfies a Fokker-Planck Equation, an equation whose stationary state is a Boltzmann distribution. The properties of the evolving network that characterize the GBCD must be identified and appropriately upscaled or 'coarse-grained'. This entails identifying the evolution of the statistic in terms of the recently discovered Monge-Kantorovich-Wasserstein implicit scheme. The undetermined diffusion coefficient or temperature parameter is found by means of a convex optimization problem reminiscent of large deviation theory.
Joint work with K. Barmak (Columbia), M. Emelianenko (George Mason). Y. Epshteyn (Utah), R. Sharp (Globys), and S. Ta'asan (CMU)
Fundamentals of Hybrid Halide Perovskites: Semiconductors with a Twist
Dec 1, 2014 8:30am ‐ Dec 1, 2014 9:00am
Identification: W1.01
Perovskites are the wonder compounds of materials science, with examples of ferromagnets, ferroelectrics, multiferroics, superconductors, semiconductors, ion conductors, insulators and, most recently, highly efficient photovoltaics. This talk will address the chemical and physical factors that make these materials, and in particular hybrid organic-inorganic halide perovskites, unique.
Recently, we have been addressing the success of methylammonium lead iodide in solar cells from atomistic and electronic structure modelling [1-3]. The hybrid material satisfies the basic optoelectronic criteria essential for an active photovoltaic layer (spectral response with light electron and hole effective masses). Relativistic and many-body corrections are shown to be essential to describing the electronic band structure. In addition, the system is structurally and compositionally flexible with large dielectric constants, and the ability to alloy on each of the three perovskite lattice sites.
One anomalous behaviour of hybrid perovskite solar cells is hysteresis in the photovoltaic current-voltage response, which we demonstrate has a contribution from the orientational disorder of the methylammonium cations. The rotation-libration of the molecular dipoles results in a rich domain structure that is sensitive to both temperature and the external electric field.
1. F. Brivio, A. B. Walker and A. Walsh, APL Materials 1, 042111 (2013)
2. J. M. Frost, K. T. Butler, F. Brivio, C. H. Hendon, M. van Schilfgaarde and A. Walsh, Nano Letters, 14, 2584 (2014)
3. F. Brivio, K. T. Butler, A. Walsh and M. van Schilfgaarde, Physical Review B 89, 115204 (2014)
A Quantum Informed Homogenized Energy Model for Ferroelectric Materials
Dec 1, 2014 8:30am ‐ Dec 1, 2014 8:45am
Identification: XX1.03
Density function theory (DFT) calculations are integrated into a continuum mechanics formulation for simulating macroscale ferroelectric constitutive behavior. Theoretical relations associated with the Hellman-Feynman stress theory are used to guide the energy and electromechanical constitutive relations in the homogenized, continuum approximation of the stored energy function. Challenges associated with addressing uncertainty in length scales between the atomic lattice scale and the continuum mesoscale are treated using stochastic homogenization and Bayesian statistics. This is achieved by quantifying DFT energy, stress, and polarization over a range of constrained lattice dimensions and internal atomic configurations. The DFT energy calculations are fit to a continuum scale stored energy function using Markov chain Monte Carlo algorithms and Bayesian statistics. Probabilities distributions of the material parameters are used to guide the development of stochastic based homogenization of the stored energy. Ferroelectric hysteresis curves are numerically simulated on lead titanate and compared to data in the literature.
Semiconducting Polymer-Dipeptide Nanostructures by Ultrasonically-Assisted Self-Assembling
Dec 1, 2014 8:45am ‐ Dec 1, 2014 9:00am
Identification: A1.02
Mostly of the organic electronic devices are built from synthetic small molecules, conjugated polymers, oligomeric or blended semiconducting materials. Other straightforward approaches include self-organized columnar stacks of aromatic compounds like discotic liquid crystals (DLCs), bent-core molecules and polyaromatic dendrons. Besides the organic π-conjugated classes cited above a quite different alternative has been emerged in the recent years for device fabrication. Vladu, Sariciftci and Bauer called this new class as “exotic” materials and it comprehends biological or bioinspired materials like paper, leather, silk, gelatine, DNA and peptides. The motivation behind the use of such biodegradable materials as substrate, dielectrics or semiconductors is to generate more sustainable and eco-friendly electronics.
In this contribution, we report for the first time the preparation of a hybrid material having (L)-diphenylalanine (Phe-Phe or FF) as biological component and semiconducting polymers (SP) (e.g. P3HT, PFO or PCDTBT) as organic semiconductor. Conventionally, FF nanostructures have been obtained by simple mixing a small amount of highly concentrate (ca. 100 mg/mL) solution of FF in fluorinated solvent (e.g. hexafluoro isopropanol or hexafluoro ethanol) with deionized water (DI). By doing so, FF molecules self-assembly mostly into bundles of nanotubes. Our attempts using the conventional synthesis protocol were fruitless to produce FF nanostructures with SP. To enhance the incorporation of semiconducting polymer and also to get control over the shape and size distribution of FF structures we developed a procedure using ultrasound energy. In a brief, to a solution of FF in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was added a stoichiometric amount of SP in 1,2-dichlorobenzene (DCB) and 100 mL of DI water. The process of self-organization initiated spontaneously by ultrasound tip giving rise to FF:P3HT hybrid material as a light-violet precipitated. The temperature was kept at 0-4 °C with an ice-water bath during all the process. After ceased the ultrasound energy application the material was allowed to stand for 8h. The crude product was washed and harvested with centrifugation-redispersion cycles (2265 g) using DCB as solvent. Then, the resulting solid was finally dried in vacuum oven at 60 °C for 24h. The hybrid materials as prepared were fully characterized by SEM, optical microscopy, dynamic light scattering (DLC), circular dichroism (CD), cyclic voltammetry (CV), FT-IR, XRD and thermal gravimetric analysis (TGA).
Those new bio-organic materials have the semiconducting properties of the conjugate polymers while keep the inherent self-organization of biological systems. Benefiting from such synergy high-performance organic electronic devices like OFETs can be envisioned.
In this contribution, we report for the first time the preparation of a hybrid material having (L)-diphenylalanine (Phe-Phe or FF) as biological component and semiconducting polymers (SP) (e.g. P3HT, PFO or PCDTBT) as organic semiconductor. Conventionally, FF nanostructures have been obtained by simple mixing a small amount of highly concentrate (ca. 100 mg/mL) solution of FF in fluorinated solvent (e.g. hexafluoro isopropanol or hexafluoro ethanol) with deionized water (DI). By doing so, FF molecules self-assembly mostly into bundles of nanotubes. Our attempts using the conventional synthesis protocol were fruitless to produce FF nanostructures with SP. To enhance the incorporation of semiconducting polymer and also to get control over the shape and size distribution of FF structures we developed a procedure using ultrasound energy. In a brief, to a solution of FF in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was added a stoichiometric amount of SP in 1,2-dichlorobenzene (DCB) and 100 mL of DI water. The process of self-organization initiated spontaneously by ultrasound tip giving rise to FF:P3HT hybrid material as a light-violet precipitated. The temperature was kept at 0-4 °C with an ice-water bath during all the process. After ceased the ultrasound energy application the material was allowed to stand for 8h. The crude product was washed and harvested with centrifugation-redispersion cycles (2265 g) using DCB as solvent. Then, the resulting solid was finally dried in vacuum oven at 60 °C for 24h. The hybrid materials as prepared were fully characterized by SEM, optical microscopy, dynamic light scattering (DLC), circular dichroism (CD), cyclic voltammetry (CV), FT-IR, XRD and thermal gravimetric analysis (TGA).
Those new bio-organic materials have the semiconducting properties of the conjugate polymers while keep the inherent self-organization of biological systems. Benefiting from such synergy high-performance organic electronic devices like OFETs can be envisioned.
The Impact of Broadening Participation in Undergraduate Research
Dec 1, 2014 8:45am ‐ Dec 1, 2014 9:15am
Identification: AAA1.01
This talk will discuss the role undergraduate research can play in student retention, academic and social integration and engagement, and the pursuit of graduate education with a focus on historically underrepresented students and community college transfer students. Drawing from the relevant literature, the presenter will discuss the rationale for using undergraduate research to retain and engage students, the role of undergraduate research in encouraging students to pursue graduate education, and the presenter will also share a model for early engagement of students in research including a description of lessons learned from a 20 year program to provide early engagement at the University of Michigan including key program components, peer advising, research seminars, and skill-building workshops), administrative structures, student and faculty recruitment, and data from a carefully designed, longitudinal assessment of the impact of undergraduate research on student retention and academic success through to graduate school. The presenter will also share other models from different types of institutions. Participants will have the opportunity to conduct an institutional audit of the feasibility of adapting the program models to engage and retain diverse students on their own campuses.
Vapor-Condensed Nanolenses for Label-Free Nanoparticle and Virus Imaging Using Lensfree Holographic On-Chip Microscopy
Dec 1, 2014 9:00am ‐ Dec 1, 2014 9:15am
Identification: D1.02
For the label-free detection, enumeration, imaging, and sizing of nanoparticles and viruses, electron microscopy constitutes the gold standard despite its high capital cost, limited field of view, and specialized sample preparation requirements. Alternative nanoparticle detection and sizing methods, such as dynamic light scattering, do not provide individual particle sizes as well as the location and spatial distribution information that is offered through imaging techniques. On the other hand, standard optical imaging techniques, such as brightfield microscopy, typically do not provide strong enough signal-to-noise (SNR) and signal-to-background ratios to observe individual nanoparticles. To overcome these various limitations in label-free nanoparticle and virus detection and quantification, we use an alternative imaging approach based on lensfree holographic on-chip microscopy. This approach provides the advantages of ultra-large fields of view >20 mm2 (>100 fold larger than conventional microscopes with comparable resolution), as well as compatibility with field-portable and cost-effective implementations. However, lensfree on-chip holographic microscopy has traditionally been limited in SNR, hampering its ability to detect individual nanoparticles.
To boost the sensitivity of this computational microscopy modality, we have created a vapor-deposition approach to form tunable liquid polyethylene glycol nanolenses that self-assemble around the target nanoparticles [1]. These nanolenses greatly enhance the holographic optical signatures, enabling the detection of spheroids with diameters smaller than 40 nm and rod-shaped particles with diameters below 20 nm. The signal enhancement provided by these nanolenses agrees well with theoretical predictions based on the simultaneous numerical modeling of the liquid lens interface shape and the optical diffraction through the system. We demonstrate that this procedure is compatible with specific component detection in heterogeneous mixtures through the use of functionalized surface capture. One important application is virus detection, where we have used self-assembled nanolenses to enable the imaging of single viral particles in lensfree holographic optical microscopy [2]. In order to facilitate easy application of this approach in field settings, we have combined the nanolens self-assembly and imaging into a single portable device prototype device that monitors the nanolens formation in situ. Such a device is particularly well-suited for point-of-care or limited-resource settings where nanoparticle or virus detection and sizing are required, e.g. in medical diagnostics (viral load measurements) or environmental monitoring.
[1] E. McLeod, C. Nguyen, P. Huang, W. Luo, M. Veli, and A. Ozcan, “Tunable vapor-condensed nanolenses,” ACS Nano, under review.
[2] O. Mudanyali, E. McLeod, W. Luo, A. Greenbaum, A. F. Coskun, Y. Hennequin, C. P. Allier, and A. Ozcan, Nature Photonics, 7 247-254 (2013).
To boost the sensitivity of this computational microscopy modality, we have created a vapor-deposition approach to form tunable liquid polyethylene glycol nanolenses that self-assemble around the target nanoparticles [1]. These nanolenses greatly enhance the holographic optical signatures, enabling the detection of spheroids with diameters smaller than 40 nm and rod-shaped particles with diameters below 20 nm. The signal enhancement provided by these nanolenses agrees well with theoretical predictions based on the simultaneous numerical modeling of the liquid lens interface shape and the optical diffraction through the system. We demonstrate that this procedure is compatible with specific component detection in heterogeneous mixtures through the use of functionalized surface capture. One important application is virus detection, where we have used self-assembled nanolenses to enable the imaging of single viral particles in lensfree holographic optical microscopy [2]. In order to facilitate easy application of this approach in field settings, we have combined the nanolens self-assembly and imaging into a single portable device prototype device that monitors the nanolens formation in situ. Such a device is particularly well-suited for point-of-care or limited-resource settings where nanoparticle or virus detection and sizing are required, e.g. in medical diagnostics (viral load measurements) or environmental monitoring.
[1] E. McLeod, C. Nguyen, P. Huang, W. Luo, M. Veli, and A. Ozcan, “Tunable vapor-condensed nanolenses,” ACS Nano, under review.
[2] O. Mudanyali, E. McLeod, W. Luo, A. Greenbaum, A. F. Coskun, Y. Hennequin, C. P. Allier, and A. Ozcan, Nature Photonics, 7 247-254 (2013).
Engaging the Public in Constructive Conversations about the Future of Technology
Dec 1, 2014 9:00am ‐ Dec 1, 2014 9:30am
Identification: FF1.03
Public outreach is often framed as a process of developing lesson plans with an aim of convincing the public to believe a series of facts and/or accept a specific viewpoint. These one-way patterns of communication, however, frequently fall short in at least two important ways. First, the public has many sources of information so getting the attention of a number of people is difficult. And second, the organizations that put together such programs are missing a major source of information that could help them to achieve their goals. Over the past several years museum professionals with the Nanoscale Informal Science Education Network (NISE Net) and scholars at Arizona State University’s Center for Nanotechnology in Society have been developing techniques that go beyond this “linear model.” These techniques enable two way conversations in which members of the public learn about scientific achievements in a specific area and then build their own ideas of what they think a better future would like with them. What we have found is that the visitors to science museums in the United States are not only able to participate in constructive conversations, but that they are hungry for them. This talk will outline some of the lessons learned through this partnership and offer suggestions for how they can be implemented in other spheres.