Stress Driven Growth of CNT Microstructures with Multi-Directional Trajectories iconStress Driven Growth of CNT Microstructures with Multi-Directional Trajectories

Stress Driven Growth of CNT Microstructures with Multi-Directional Trajectories

Dec 1, 2014 11:30am ‐ Dec 1, 2014 11:45am

Identification: MM1.07

Scalable fabrication of microstructures with freeform geometries, in particular curved and re-entrant shapes, is limited by the constraints of fabrication methods that present a tradeoff between structural complexity and throughput. We present a scalable fabrication technique to produce freeform Carbon Nanotube (CNT) microstructures with exceptional uniformity by controlling the growth rate of aligned CNTs via engineered catalyst film stack. The growth rate of CNTs can be controlled by introducing a growth retardant layer underneath the regular catalyst film stack. Offset patterning of the CNT growth catalyst and growth retardant layer with varying thicknesses is used to locally modulate the CNT growth rate at inter- and intra-structural level which enables multi-height scale structures and curved structures whose trajectories depend on the strain mismatch within the structures. The final shape of the curved CNT microstructures can be designed via finite element modeling, and compound catalyst shapes produce microstructures with multi-directional curvature and unusual self-organized patterns. Conformal coating of the CNTs enables tuning of the mechanical properties independently from the microstructure geometry, representing a versatile principle for design and manufacturing of complex microstructured surfaces. This principle can be applied toward applications demanding uniform arrays of complex structures over large areas, such as micro-architectured composite materials for maximized stiffness and damping properties, and nature-inspired superhydrophobic surfaces with intricate micro- and nanoscale features for directional wetting properties.
Mechanical Behavior of Nanotwinned Al and Ag/Al Nanolayers iconMechanical Behavior of Nanotwinned Al and Ag/Al Nanolayers

Mechanical Behavior of Nanotwinned Al and Ag/Al Nanolayers

Dec 1, 2014 11:30am ‐ Dec 1, 2014 11:45am

Identification: XX2.03

Nanotwins readily form in numerous fcc metals with low stacking fault energy (SFE). However, growth twins rarely form in Al due to its high SFE. Here, by using thin inter- or buffer layers of a low SFE fcc metal (Ag), we overcome the SFE barrier and successfully grow high-density coherent and incoherent twin boundaries into Al [D. Bufford et al, Materials Research Letters, 1 (2013) 51-60; http://dx.doi.org/10.1080/21663831.2012.761654.]. We identify three mechanisms that enable growth twin formation in Al, and demonstrate enhanced mechanical strength in twinned Al.

Furthermore we will show that epitaxial Ag/Al multilayer films have high hardness (up to 5.5 GPa) in comparison to monolithic Ag and Al films (2 and 1 GPa). High-density nanotwins and stacking faults appear in both Ag and Al layers, and stacking fault density in Al increases sharply with decreasing individual layer thickness, h. Hardness increases monotonically with decreasing h, and no softening occurs. In comparison, epitaxial Cu/Ni multilayers reach similar peak hardness when h ≈ 5 nm, but soften at smaller h. High strength in Ag/Al films is primarily a result of layer interfaces, nanotwins, and stacking faults, which are strong barriers to transmission of dislocations. This research is funded by DOE-Office of Basic Energy Sciences.

Ab Initio Study of Nano-Structured Half-Heusler Alloys iconAb Initio Study of Nano-Structured Half-Heusler Alloys

Ab Initio Study of Nano-Structured Half-Heusler Alloys

Dec 1, 2014 11:45am ‐ Dec 1, 2014 12:00pm

Identification: CC1.09

Recent improvements in the performance of thermoelectric materials have resulted from adding nano-structures in order to scatter heat carrying phonons. While this approach effectively reduces the lattice thermal conductivity, it is often accompanied by large drops in the electrical conductivity caused by mobility reductions. In this work we show that bulk forms of Half-Heusler (HH) alloys can be combined with nano-scale Full-Heusler (FH) inclusions to simultaneously improve the power factor and reduce thermal conductivity. HH structures are of the form MNiSn and MCoSb (M= Ti, Zr, or Hf) and the FH counterparts are created by filling the vacancies on the Ni or Co planes respectively, resulting in MNi2Sn and MCo2Sb. Previous experimental results have shown the FH nano-inclusions being coherently integrated into the matrix HH material resulting in enhanced ZT which has been attributed to an energy filtering mechanism that occurs at the HH-FH semi-coherent boundaries as well as moderate reductions in thermal conductivity by nano-inclusion phonon scattering. Ab Initio calculations, in combination with a cluster expansion, are used to test the stability of FH structures in the HH matrix and create a thermodynamic pseudo-binary phase diagram for MNiSn-MNi2Sn compositions. In addition, electronic structure and lattice dynamics are investigated in order to elucidate possibilities for future approaches to enhance ZT. This research is supported by the Department of Energy, Office of Basic Energy Sciences under Award # DE-SC-0008574.
Semiconductor Nanoplatelets: A New Colloidal System for Low-Threshold, High-Gain Stimulated Emission iconSemiconductor Nanoplatelets: A New Colloidal System for Low-Threshold, High-Gain Stimulated Emission

Semiconductor Nanoplatelets: A New Colloidal System for Low-Threshold, High-Gain Stimulated Emission

Dec 1, 2014 11:45am ‐ Dec 1, 2014 12:00pm

Identification: II1.09

Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. QWs have several advantages as gain media in semiconductor lasers, including tunable emission wavelengths and low threshold currents. So far, however, QWs have been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically in large volumes and at low cost. In these quantum-dot (QD) systems, however, carriers are confined in all three dimensions, and only a small number of exciton states exist at the optical bandgap energy. Since QDs cannot be packed together closer than their diameters, the maximum optical gain in a layer of QDs is limited. Moreover, initial demonstrations of optical gain from colloidal QDs involved high thresholds, which were attributed to rapid Auger processes. Attempts to reduce thresholds based on designing QDs to reduce the effects of Auger recombination have culminated in thresholds as low as ~26 µJ/cm2. In this case, however, the lower threshold comes at the expense of the maximum obtainable gain, because the QDs have think shells that reduce their packing density.

Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). The faces of these platelets are capped with organic ligands, and the platelets are typically surrounded by solvents or by air. This means that carrier confinement and exciton binding energies are much stronger in colloidal NPLs than in epitaxial QWs. The stronger confinement and binding energies, in turn, are likely to result in significantly different carrier dynamics.

We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials. 

We therefore investigated amplified spontaneous emission (ASE) using close-packed films of NPLs. We observed thresholds as low as 6 µJ/cm2, more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is as high as 600 cm-1, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.
"From Nanostructured to Thin-Film Perovskite Solar Cells" - the 2014 MRS Outstanding Young Investigator Award talk icon"From Nanostructured to Thin-Film Perovskite Solar Cells" - the 2014 MRS Outstanding Young Investigator Award talk

"From Nanostructured to Thin-Film Perovskite Solar Cells" - the 2014 MRS Outstanding Young Investigator Award talk

Dec 1, 2014 12:05pm ‐ Dec 1, 2014 1:15pm

Second generation solar cells based on thin films of polycrystalline semiconductors promise to reduce the cost of sunlight-to-electricity conversion compared to first generation crystalline silicon. Efficient thin-film absorber materials can fulfil the multiple roles of light-absorption, charge separation, and transport of both holes and electrons out of the device. A third generation of materials, which can be processed with solution-based techniques at low-temperature, such as printing, should ultimately lead to the least expensive solar cell technology. However, most of the materials processable with the lowest cost methods usually require complex architectures of distributed heterojunctions to ionise tightly bound electron-hole pairs. This inherently introduces losses at the high density of internal material interfaces. Recently organic-inorganic metal halide perovskite absorbers have rocketed to the forefront of PV research as efficient solar cell materials, which seem to be both simple to process and promise to reach the highest efficiencies. This paradigm shift arguably represents a 4th generation of photovoltaics.

Here I will present our developments in perovskite solar cells, highlighting how the technology has mutated and evolved from a nanostructured solar cell to a thin film device. I will present our recent results on improving and understanding perovskite solar cells, with both device based and spectroscopic investigations and highlight some of the reasons why these materials work so well and the future prospects.

Hierarchical 3-D Nano-Architectures for Biomimetics, Batteries, and Lightweight Structural Materials iconHierarchical 3-D Nano-Architectures for Biomimetics, Batteries, and Lightweight Structural Materials

Hierarchical 3-D Nano-Architectures for Biomimetics, Batteries, and Lightweight Structural Materials

Dec 1, 2014 1:30pm ‐ Dec 1, 2014 2:00pm

Identification: XX3.01

Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hi­e­r­a­­r­chical design of 3-dimensional nano-architectures. Such structural meta-materials exhibit superior thermo­mechanical pro­­­per­­ties at ex­tre­me­ly low mass densities (lighter than aerogels), making these solid foams ideal for many scientific and tech­no­lo­gi­cal applications. The dominant de­for­mation mechanisms in such “meta-materials”, where individual constituent size (nanometers to microns) is compa­rable to the characteristic microstructural length scale of the constituent solid, are essentially un­known. To harness the lucrative properties of 3-dimensional hierarchical nanostructures, it is critical to assess mechanical properties at each relevant scale while capturing the over­all structural complexity.

We present the fabrication of 3-dimensional nano-lattices whose constituents vary in size from several nanometers to tens of microns to millimeters. We discuss the deformation and mechanical properties of a range of nano-sized solids with different microstructures deformed in an in-situ nanomechanical instrument. Attention is focused on the interplay between the internal critical microstructural length scale of materials and their external limitations in revealing the physical mecha­nisms which govern the mechanical deformation, wherecompeting material- and structure-induced size effects drive overall properties.

We focus on the deformation and failure in metallic, ceramic, and glassy nano structures and discuss size effects in nanomaterials in the framework of mechanics and physics of defects. Specific discussion topics include: fabrication and characterization of hierarchical 3-dimensional architected meta-materials for applications in biomedical devices, ultra lightweight batteries, and damage-tolerant cellular solids, nano-mechanical experiments, flaw sensitivity in fracture of nano structures.

Artificial Physical and Chemical Awareness (proprioception) from Polymeric Motors iconArtificial Physical and Chemical Awareness (proprioception) from Polymeric Motors

Artificial Physical and Chemical Awareness (proprioception) from Polymeric Motors

Dec 1, 2014 1:30pm ‐ Dec 1, 2014 2:00pm

Identification: A2.01

Designers and engineers have been dreaming for decades with motors sensing, by themselves, working and surrounding conditions, as biological muscles do originating proprioception. The evolution of the working potential, or that of the consumed electrical energy, of electrochemical artificial muscles based on electroactive materials (intrinsically conducting polymers, redox polymers, carbon nanotubes, fullerene derivatives, grapheme derivatives, porphyrines, phtalocyanines, among others) and driven by constant currents senses, while working, any variation of the mechanical (trailed mass, obstacles, pressure, strain or stress) thermal or chemical conditions of work. They are linear faradaic polymeric motors: currents control movement rates and charges control displacements. One physically uniform artificial muscle includes one chemically based polymeric motor and several sensors working simultaneously under the same driving reaction. Actuating (current and charge) and sensing (potential and energy) magnitudes are present, simultaneously, in the only two connecting wires and can be read by the computer at any time. From basic polymeric, mechanical and electrochemical principles a basic equation is attained. It includes and describes, simultaneously, the polymeric motor characteristics (rate of the muscle movement and muscle position) and the working variables (temperature, electrolyte concentration and mechanical conditions). By changing working conditions experimental results overlap theoretical predictions. The ensemble computer-generator-muscle-theoretical equation constitutes and describes artificial mechanical, thermal and chemical proprioception of the system. Proprioceptive tools and zoomorphic or anthropomorphic soft robots can be envisaged.

Acknowledgments: Authors acknowledge financial support from Spanish Government (MCI) Project MAT2011-24973, Jose G. Martinez acknowledges to the Spanish Education Ministry for a FPU grant (AP2010-3460).

References

[1] T. F. Otero and J. G. Martinez, Biomimetic intracellular matrix (ICM) materials, properties and functions. Full integration of actuators and sensors. J. Mater. Chem. B, 1, 26-38 (2013)
[1] T. F. Otero, J. J. Sanchez & J. G. Martinez, Biomimetic Dual Sensing-Actuators Based on Conducting Polymers. Galvanostatic Theoretical Model for Actuators Sensing Temperature. J. Phys. Chem. B 116, 5279-5290 (2012).
[2] J. G. Martinez & T. F. Otero, Biomimetic Dual Sensing-Actuators: Theoretical Description. Sensing Electrolyte Concentration and Driving Current. J. Phys. Chem. B, 116, 9223-9230 (2012).
[3] J. G. Martinez & T. F. Otero, Mechanical awareness from sensing artificial muscles: experiments and modeling. Sens. Actuators B-Chem. 195, 365-372 (2014)
Mesoscale to Molecular Model for Particle Mediated Crystal Growth iconMesoscale to Molecular Model for Particle Mediated Crystal Growth

Mesoscale to Molecular Model for Particle Mediated Crystal Growth

Dec 1, 2014 1:30pm ‐ Dec 1, 2014 1:45pm

Identification: NN2.01

Understanding the mechanism of crystal growth through oriented attachment of nanoparticles, such as in the self-assembly of metal oxide minerals in aqueous solution, poses many challenges, but also opens vast opportunities for materials design. We present a theoretical approach for modeling solvent controlled interactions between nanoparticles that reaches into the mesoscale, while retaining molecular details of the interacting particle surfaces and intervening solvent. The total Hamiltonian of the system includes contributions from long-range particle-particle dispersion interactions across solvent, that accounts for the influence of solvent structuring on the high frequency dielectric response and ion screening of the static response, and contributions from ion-mediated interactions. The latter include direct Coulomb interactions between ions and mineral surfaces with discrete facet-dependent distribution of charges, image interactions, interactions arising from density (excluded volume) and charge density (ion correlation) fluctuations, ion-mineral and ion-water dispersion interactions. The, ion-mineral dispersion contribution depends on dynamic excess polarizabilities of ions in water and on the dynamic dielectric function of the mineral surfaces providing the link between macroscopic and microscopic dispersion terms.

The model was validated against its ability to reproduce ion activity in 1:1, 2:1 and 3:1 electrolyte solutions in the 0-2M concentration range, and its ability to capture the qualitative ion-specific effect in 1:1 electrolytes at the air-water interface. We apply the approach to understand the influence of pH on facet-dependent interactions between anatase TiO2 nanoparticles.

An REU in Glass Science - Lessons and Legacy for Undergraduate Materials Education iconAn REU in Glass Science - Lessons and Legacy for Undergraduate Materials Education

An REU in Glass Science - Lessons and Legacy for Undergraduate Materials Education

Dec 1, 2014 1:45pm ‐ Dec 1, 2014 2:00pm

Identification: AAA3.02

The International Materials Institute for New Functionality in Glass (IMI-NFG - www.lehigh.edu/imi) was established in 2004 [1]. Two important goals of this institute have been to facilitate international collaboration in glass research and to support the training of a professional workforce. The REU program has been an important component of our mission since 2005. Glass science provides the central theme of our program, which has both an international and domestic component. In this paper we provide a summary of our accomplishments and experiences with this unique and innovative, materials focused, multidisciplinary REU program. While glass provides the unifying theme of our program, the research topics and project advisers come from various departments including biology, physics, environmental and civil engineering, electrical engineering, etc. This mode of REU operation has provided an effective means to introduce faculty from other departments to new and innovative applications of glass as an enabling material for their own work.

For the domestic REU, we partner with the Physics REU Program at Lehigh, sharing many crucial administrative aspects as well as housing, seminars and social events. The complementary approach of the two REU programs has been very beneficial to both of them. We will discuss details of our approaches to recruitment, selection and creating an environment conducive to cohesive and motivated community of young researchers. While the majority of student experiences have been through individual projects, we have also experimented with small teams headed by a single adviser. These teams emphasize a cooperative education model and the group can thus accommodate a greater diversity of individual student skills and experiences. Such projects tend to provide opportunity for open-ended exploration within the context of a general goal. Some of the projects are specifically focused on developing low-cost, hands-on methods for teaching glass science to others through a more experiential and intuitive approach. All of these REU projects have been incorporated in an on-line collection of activities for the science education community at large as reported previously [2,3].

For the international component, our role has been to support glass faculty in the country, who wish to engage their own undergraduate students in a glass based summer research in the lab of an international colleague. These experiences require considerable preparation and individual attention. We will discuss the challenges as well as some of our successful strategies to deal with both the international and domestic programs.

1. IMI-NFG is supported by National Science Foundation (DMR-0844014).
2. W. Heffner and H. Jain, Mater. Res. Soc. Symp. Proc., Vol. 1233, 2010.
3. W. Heffner and H. Jain, Mater. Res. Soc. Symp. Proc., Vol. 1657, 2014.
Physics of Local Crystallography: Phases, Symmetries, and Defects from the Bottom Up iconPhysics of Local Crystallography: Phases, Symmetries, and Defects from the Bottom Up

Physics of Local Crystallography: Phases, Symmetries, and Defects from the Bottom Up

Dec 1, 2014 1:45pm ‐ Dec 1, 2014 2:00pm

Identification: NN2.02

Progress in high-resolution electron and probe based, real space imaging techniques like (Scanning) Transmission Electron Microscopy (STEM) and Scanning Probe Microscopy (SPM) has consistently delivered imaging of atomic columns and surface atomic structures with ever growing precision. As the instruments evolve, the basic data processing principle - analysis of structure factor, or essentially a two point correlation function averaged over probing volume � remains invariant since the days of Bragg. We propose a multivariate statistics based approach to analyze the coordination spheres of individual atoms to reveal preferential structures and symmetries.

The underlying mechanism is that for each atom, i, on the lattice site with indices (l, m), we construct a near coordination sphere vector , where is the radius-vector to j/2-th nearest neighbor. Once the set of Ni vectors is assembled, it is analyzed though cluster analysis and other multivariate methods to reveal and extract regions of symmetry, distortions, different phases, boundaries, defects, etc., that can be back projected on the atomically mapped surface. Results are presented on various model and real material systems including La0.7Sr0.3MnO3, BiFeO3, LaCoO3 and discussed in light of physical parameter extraction.

Acknowledgement:

Research for (AB, QH, AB, SJ, SVK) was supported by the US Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Research was conducted at the Institute for Functional Imaging of Materials and Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy.