John Kieffer

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Name: Kieffer, John

Organization: University of Michigan , USA

Department: Department of Materials Science and Engineering

Title: Professor(PhD)

TOPICS

Material Chemistry

Chemicals
References

Elastic properties and short-range structural order in mixed network former glasses

Co-reporter:Weimin Wang;Randilynn Christensen;Brittany Curtis;David Hynek;Sydney Keizer;James Wang;Steve Feller;Steve W. Martin

Physical Chemistry Chemical Physics 2017 vol. 19(Issue 24) pp:15942-15952

Publication Date(Web):2017/06/21

DOI:10.1039/C6CP08939A

Elastic properties of alkali containing glasses are of great interest not only because they provide information about overall structural integrity but also they are related to other properties such as thermal conductivity and ion mobility. In this study, we investigate two mixed-network former glass systems, sodium borosilicate 0.2Na2O + 0.8[xBO1.5 + (1 − x)SiO2] and sodium borogermanate 0.2Na2O + 0.8[xBO1.5 + (1 − x)GeO2] glasses. By mixing network formers, the network topology can be changed while keeping the network modifier concentration constant, which allows for the effect of network structure on elastic properties to be analyzed over a wide parametric range. In addition to non-linear, non-additive mixed-glass former effects, maxima are observed in longitudinal, shear and Young's moduli with increasing atomic number density. By combining results from NMR spectroscopy and Brillouin light scattering with a newly developed statistical thermodynamic reaction equilibrium model, it is possible to determine the relative proportions of all network structural units. This new analysis reveals that the structural characteristic predominantly responsible for effective mechanical load transmission in these glasses is a high density of network cations coordinated by four or more bridging oxygens, as it provides for establishing a network of covalent bonds among these cations with connectivity in three dimensions.

Lithium Ion Conducting Poly(ethylene oxide)-Based Solid Electrolytes Containing Active or Passive Ceramic Nanoparticles

Co-reporter:Weimin Wang, Eongyu Yi, Anthony J. Fici, Richard M. Laine, and John Kieffer

The Journal of Physical Chemistry C 2017 Volume 121(Issue 5) pp:

Publication Date(Web):January 2, 2017

DOI:10.1021/acs.jpcc.6b11136

Polymer-based solid electrolytes containing ceramic nanoparticles are attractive alternatives to liquid electrolytes for high-energy density Li batteries. In this study, three different types of fillers have been dispersed in poly(ethylene) oxide (PEO) polymer matrices, and the effects on the resulting ionic conductivity of the nanocomposites have been examined. In this respect, the efficacy of one active, liquid-feed flame spray pyrolysis synthesized amorphous Li1.3Al0.3Ti1.7(PO4)3 (LATP), and two passive filler materials, TiO2 and fumed silica nanoparticles, are compared. Nanocomposite electrolytes are prepared with up to 20 wt % particle loadings. PEO/LiClO4 with 10 wt % LATP nanoparticles exhibits an ionic conductivity of 1.70 × 10–4 S·cm–1 at 20 °C, the highest among the surveyed systems, despite exhibiting comparable or higher degrees of crystallinity and glass transition temperatures than the systems containing passive fillers. The ionic conductivity of the composites with LATP nanoparticles exceed that of the polymer matrix by 1 to 2 orders of magnitude. We attribute this remarkable enhancement to cation transport within the interphase region surrounding the particles, which achieves percolation at low nanoparticle loading. The development of this interphase structure is influenced by the active nature of the LATP filler, and we estimate the inherent conductivity of the interphase to be 3 to 4 times higher than the maximum measured value.

Alkane–Metal Interfacial Structure and Elastic Properties by Molecular Dynamics Simulation

Co-reporter:Katherine Sebeck, Chen Shao, and John Kieffer

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 26) pp:16885-16896

Publication Date(Web):June 10, 2016

DOI:10.1021/acsami.6b01665

The structure of amorphous materials near the interface with an ordered substrate can be affected by various characteristics of the adjoining phases, such as the lattice spacing of the adherent surface, polymer chain length, and adhesive strength. To discern the influence of each of these factors, four FCC metal lattices are examined for three chain lengths of n-alkane and van der Waals interfacial interactions are controlled by adjusting the Lennard–Jones 12-6 potential parameters. The role of interaction strength is investigated for a single chain length and substrate combination. Four nanoconfined systems are also analyzed in terms of their mechanical strength. A strong layering effect is observed near the interface for all systems. The distinctiveness of polymer layering, i.e., the maximum density and spatial extent, exhibits a logarithmic dependence on the interaction strength between polymer and substrate. Congruency with the substrate lattice parameter further enhances this effect. Moreover, the elastic modulus of the alkane phase as a function of layer thickness indicates that the effects of ordering within the structure extend beyond the immediately obvious interfacial region.

The effects of extended conjugation length of purely organic phosphors on their phosphorescence emission properties

Co-reporter:Dongwook Lee, Xiao Ma, Jaehun Jung, Eun Jeong Jeong, Hossein Hashemi, Avi Bregman, John Kieffer and Jinsang Kim

Physical Chemistry Chemical Physics 2015 vol. 17(Issue 29) pp:19096-19103

Publication Date(Web):16 Jun 2015

DOI:10.1039/C5CP01003A

We synthesized a series of purely organic phosphors, bromobenzaldehyde derivatives, with varying conjugation length to investigate the effects of conjugation length on their phosphorescence emission properties. As the conjugation length increases phosphorescence efficiency decreases with a redshift in the emission color at 77 K. Our computational results imply that this correlation is related to the intersystem crossing rate and that the rate is determined by spin–orbit coupling strength rather than by simply the energy difference between the lowest lying singlet and triplet states. TD-DFT calculations show that the S1 → T1 transition occurs more dominantly than the S1 → T2 transition for all cases. Moreover, singlet excited states are localized on the aldehyde functional group, regardless of the conjugation length, while triplet excited states are evenly distributed over the conjugated backbone. Consequently, as the conjugation length increases, the larger spatial separation between singlet and triplet states diminishes the spin–orbit coupling efficiency, resulting in reduced phosphorescence.

Spatially resolved, in situ elastic modulus of thermoset polymer amidst carbon fibers in a polymer matrix composite

Co-reporter:Michael Aldridge, Anthony Waas, John Kieffer

Composites Science and Technology 2014 Volume 98() pp:22-27

Publication Date(Web):27 June 2014

DOI:10.1016/j.compscitech.2014.03.002

The mechanical properties of thermoset resins used in carbon fiber composites depend on their processing conditions. The inclusion of fibers into the resin locally affects the thermal balance, creates chemical inhomogeneities, and templates structural developments in the polymer near the fiber surface. In this study we use Raman and Brillouin light scattering to investigate the effect of carbon fibers on the mechanical properties of an epoxy matrix. Our results show that the longitudinal modulus of epoxy within a fiber tow is about 3.75% lower than the modulus of the epoxy outside of the tow. Furthermore, within a fiber tow, the modulus depends on the local packing density of the carbon fibers. Comparison between our Brillouin and Raman measurements suggest that the observed spatial inhomogeneity in elastic properties of the matrix is not a result of residual stresses within the matrix but more likely due to structural reorganization in the interfacial region.

In Situ Analysis of the Relationship between Cure Kinetics and the Mechanical Modulus of an Epoxy Resin

Co-reporter:Michael Aldridge, Alan Wineman, Anthony Waas, and John Kieffer

Macromolecules 2014 Volume 47(Issue 23) pp:8368-8376

Publication Date(Web):November 25, 2014

DOI:10.1021/ma501441c

The cure kinetics of epoxy cured with an amine hardener is investigated using a combination of light scattering techniques. Concurrent Raman and Brillouin scattering are used to monitor the formation of epoxy networks over time in terms of both the structural connectivity of the network and the evolution of chemical bonding configurations. The relationship between the elastic properties of the network and the degree of cure was found to depend on the cure temperature and the resin–hardener stoichiometry. A numerical model was created to determine the degree of cure and elastic modulus as a function of time and cure conditions. Accordingly, the modulus of the epoxy comprises two contributions: one directly related to the concentration of covalent bonds that form and another one due to nonbonding interactions that arise as the network relaxes into an optimally packed configuration.

Interfacial Adhesive Properties between a Rigid-Rod Pyromellitimide Molecular Layer and a Covalent Semiconductor via Atomistic Simulations

Co-reporter:Maxim A. Makeev, Philippe H. Geubelle, Nancy R. Sottos, and John Kieffer

ACS Applied Materials & Interfaces 2013 Volume 5(Issue 11) pp:4702

Publication Date(Web):April 22, 2013

DOI:10.1021/am3031163

We conducted a comprehensive atomistic simulation study of the adhesive properties of aromatic rigid-rod poly-[(4,4′diphenylene) pyromellitimide] on a dimer-reconstructed silicon surface. We describe the structural developments within the adherent’s interfacial region at the atomistic scale, and evaluate the energetics of the adhesive interactions between bimaterial constituents. In particular, we observe a transition between noncontact and contact adhesion regimes as a function of the interfacial bonding strength between the polyimide repeat units and the silicon substrate. This transition is manifest by a three- to four-fold increase in adhesive energy, which is entirely attributable to structural relaxation in the organic layer near the interface, revealing the importance of accurately describing structural details at interfaces for reliable interfacial strength predictions. The underlying molecular reconfigurations in the pyromellitimide layer include preferred orientation of the rigid-rod molecules, molecular stacking, ordering, and the local densification. The role of each of these factors in the adhesive behavior is analyzed and conclusively described. Where possible, simulation results are compared with theoretical model predictions or experimental data.Keywords: adhesion; glass transition; interface; molecular dynamics; polyimide; structure−property relations;

Organic Dye Design Tools for Efficient Photocurrent Generation in Dye-Sensitized Solar Cells: Exciton Binding Energy and Electron Acceptors

Co-reporter:Bong-Gi Kim;Chang-Gua Zhen;Eun Jeong Jeong;Jinsang Kim

Advanced Functional Materials 2012 Volume 22( Issue 8) pp:1606-1612

Publication Date(Web):

DOI:10.1002/adfm.201101961

Abstract

The relationship between the exciton binding energies of several pure organic dyes and their chemical structures is explored using density functional theory calculations in order to optimize the molecular design in terms of the light-to-electric energy-conversion efficiency in dye-sensitized solar cell devices. Comparing calculations with measurements reveals that the exciton binding energy and quantum yield are inversely correlated, implying that dyes with lower exciton binding energy produce electric current from the absorbed photons more efficiently. When a strong electron-accepting moiety is inserted in the middle of the dye framework, the light-to-electric energy-conversion behavior significantly deteriorates. As verified by electronic-structure calculations, this is likely due to electron localization near the electron-deficient group. The combined computational and experimental design approach provides insight into the functioning of organic photosensitizing dyes for solar-cell applications. This is exemplified by the development of a novel, all-organic dye (EB-01) exhibiting a power conversion efficiency of over 9%.

Spectral mode assignment for binary silicate glasses using molecular dynamics simulations

Co-reporter:A.K. Upadhyay, K. Sebeck, J. Kieffer

Journal of Non-Crystalline Solids 2012 Volume 358(Issue 23) pp:3348-3354

Publication Date(Web):1 December 2012

DOI:10.1016/j.jnoncrysol.2012.08.019

The vibrational behavior of binary silicate glasses is modeled using molecular dynamics (MD) simulations. The prevailing procedure for assigning vibrational modes to structural features is through comparison with spectra of known substances. The explicit knowledge of atomic trajectories from MD simulations allows for a direct observation of vibrational motion in specific frequency bands and decomposition into the predominant vibrational modes. We demonstrate a Fourier transform method for determining the vibrational motion associated with a particular frequency. We compare literature assignments for α-cristobalite, vitreous silica, β Na2O2SiO2 and vitreous Na2O2SiO2 to this method. Our analysis shows that while it is necessary for the network to be disrupted and non-bridging oxygen atoms to form for vibrational motion to occur at 1205 cm− 1, the mode is in fact not specific to that structural moiety.Highlights► Present Fourier filtering of atomic trajectories FFAT method for IR spectral analysis. ► For crystalline and amorphous SiO2 FFAT assignments agree well with literature data. ► Network modification introduces specific IR spectral features.. ► For Na2O2SiO2, both BO and NBO are active in the 1100 cm− 1 and 1200 cm− 1 modes.

Achieving Highly Efficient Fluorescent Blue Organic Light-Emitting Diodes Through Optimizing Molecular Structures and Device Configuration

Co-reporter:Chang-Gua Zhen;Yan-Feng Dai;Wen-Jin Zeng;Zhun Ma;Zhi-Kuan Chen

Advanced Functional Materials 2011 Volume 21( Issue 4) pp:699-707

Publication Date(Web):

DOI:10.1002/adfm.201002165

Abstract

Based on the results of first-principles calculations of the electronic properties of blue light-emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron-withdrawing groups into the molecules to balance hole and electron injection and transport for organic light-emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky-blue device. Finally, by doping the emitter in a suitable host material, 4,4’-bis(carbazol-9-yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.147, 0.139) is achieved.

Brillouin Light Scattering Investigation of the Mechanical Properties of Layer-by-Layer Assembled Cellulose Nanocrystal Films

Co-reporter:L. Sui, L. Huang, P. Podsiadlo, N. A. Kotov, and J. Kieffer

Macromolecules 2010 Volume 43(Issue 22) pp:9541-9548

Publication Date(Web):October 22, 2010

DOI:10.1021/ma1016488

Composite thin films containing cellulose nanocrystal (cellN) polyanions embedded between either poly(diallyldimethylammonium chloride) (PDDA) or chitosan were fabricated using the layer-by-layer (LBL) deposition technique. The in-plane and out-of-plane elastic constants of the composites were measured using Brillouin light scattering as a function of film thickness and cellulose content. Compared to the pure cast polymer films, the addition of cellN raises the elastic constants within the growth plane by a factor of 2 and 3 for [chitosan/cellN] and [PDDA/cellN] films, respectively, while in the growth direction the elastic constant increases by 50% for [PDDA/cellN] and not at all for [chitosan/cellN]. With increasing amounts of cellN in the films, the stiffness increases in the growth plane at a higher rate than in the growth direction. These trends reflect the contribution of the cellulose nanocrystals within and cross layers to load transmission. The results are interpreted in terms of processes that occur during film deposition and the resulting spatial arrangements of the nanocrystals.

Tuning Electronic Properties of Functionalized Polyhedral Oligomeric Silsesquioxanes: A DFT and TDDFT Study

Co-reporter:Chang-Gua Zhen, Udo Becker and John Kieffer

The Journal of Physical Chemistry A 2009 Volume 113(Issue 35) pp:9707-9714

Publication Date(Web):August 11, 2009

DOI:10.1021/jp903796m

The structure and electronic properties of polyhedral oligomeric silsesquioxane (POSS) cages functionalized with different organic groups have been studied using density functional theory and time-dependent density functional theory calculations. Accordingly, the POSS-T8 cage is quite rigid upon functionalization and thus provides a means for spatially separating conjugated organic fragments, which is useful for the realization of novel organic molecular architectures for light-emitting diodes. Moreover, electronic properties can be tuned through the choice of functional groups and their positioning on or within the POSS cage. Attaching an electron-donating group, such as 4-carbazolephenyl, to the silicon atom at the corner of the cage raises the HOMO level, while attaching an electron-withdrawing group, such as 4-cyanophenyl, or inserting an inert molecule, such as N2, into the POSS cage lowers the LUMO level. Frontier orbital analysis indicates that the POSS cage is partially conjugated and serves a role as electron acceptor. Carrier transport rates are discussed in the frame of Marcus’ electron hopping theory. On the basis of the calculated reorganization energies, these POSS compounds can be used as carrier transporting or blocking materials, depending on the functionalization. Exciton binding energies strongly depend on the spatial arrangement of frontier orbitals rather than on molecular sizes.

The effects of extended conjugation length of purely organic phosphors on their phosphorescence emission properties

Co-reporter:Dongwook Lee, Xiao Ma, Jaehun Jung, Eun Jeong Jeong, Hossein Hashemi, Avi Bregman, John Kieffer and Jinsang Kim

Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 29) pp:NaN19103-19103

Publication Date(Web):2015/06/16

DOI:10.1039/C5CP01003A

Elastic properties and short-range structural order in mixed network former glasses

Co-reporter:Weimin Wang, Randilynn Christensen, Brittany Curtis, David Hynek, Sydney Keizer, James Wang, Steve Feller, Steve W. Martin and John Kieffer

Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 24) pp:NaN15952-15952

Publication Date(Web):2017/05/15

DOI:10.1039/C6CP08939A