2014 MRS Spring Meeting

Apr 21, 2014 ‐ Apr 25, 2014


The 2014 MRS Spring Meeting & Exhibit featured 57 technical sessions and was chaired by Jose A. Garrido, Sergei V. Kalinin, Edson R. Leite, David Parrillo and Molly Stevens.


Sessions

Theory Assisted Design of 3D Networked Binary Metal Nanoparticle -Triblock Terpolymer Superstructures

Apr 22, 2014 9:00am ‐ Apr 22, 2014 9:15am

Identification: QQ1.02

Controlling superstructure of binary nanoparticle (NP) mixtures in three dimensions (3D) from self-assembly opens enormous opportunities for the design of novel materials with unique properties for a variety of applications, e.g. energy related, photonic, and phononic, applications. Here, we present a synthetic approach toward such materials from bottom-up type block copolymer (BCP)—metal nanoparticle (NP) co-assembly.

A BCP was used as a structure-directing agent for controlling spatial arrangement of metal NPs. Structure control of functional NPs at the nanoscale, mediated by BCP micro-phase separation, provides facile routes to nanostructured materials. In order to efficiently predict nanostructures of such materials, a novel theoretical approach was developed, allowing a level of complexity usually reserved to more costly molecular simulation treatments. The theory exhibits quantitative agreement with the experiment of a highly ordered 3-dimensional (3D) chiral metal nanoparticle (NP) network, synthesized via triblock terpolymer / ligand-stabilized NP self-assembly, and provides design criteria for controlling a range of NP based nanomaterial structures.

The intimate coupling of synthesis, in-depth electron tomographic characterization, and a recently developed field theory enables exquisite control of superstructure in highly ordered porous 3D continuous networks from single and binary mixtures of metal NPs. Quantitative analysis provided insights into short- and long-range NP-NP correlations, and local and global contributions to structural chirality in the networks. Results provide design rules for next generation mesoporous network superstructures from binary NP mixtures for potential applications in areas including catalysis and plasmonics

Speaker(s):
  • Kahyun Hur, Cornell University, Korea Institute of Science and Technology

Similarity Functions for Datamining Compositional and Structural Relations Between Materials

Apr 22, 2014 9:00am ‐ Apr 22, 2014 9:30am

Identification: WW1.03

The availability of large amounts of data generated by high-throughput computing or experimenting has generated interested in the application of machine learning techniques to materials science [1]. Machine learning of materials behavior requires the use of feature vectors or descriptors that capture the essential compositional or structural information that is most likely to influence a property. We will present a new method for assessing the similarity of material compositions. A similarity measure is important for the classification and clustering of compositions. The similarity of the material compositions is calculated utilizing a data-mined ionic substitutional similarity based upon the probability with which two ions will substitute for each other within the same structure prototype. The method is validated via the prediction of crystal structure prototypes for oxides from the Inorganic Crystal Structure Database. It performs particularly well on the quaternary oxides, predicting the correct prototype within 5 guesses 90% of the time. We expect that this compositional similarity measure can be used to classify other properties as well.

[1] C. Fischer et al, Predicting Crystal Structure: Merging Data Mining with Quantum Mechanics, Nature Materials, 5 (8), pp. 641-6 (2006). G. Hautier et al, Data Mined Ionic Substitutions for the Discovery of New Compounds, Inorganic Chemistry, 50 (2), 656-663 (2011).

Speaker(s):

High Voc a-Si:H Solar Cells with Silicon Oxide Doped Layers for Applications in Multijunction Thin-Film Silicon Solar Cells

Apr 22, 2014 9:15am ‐ Apr 22, 2014 9:30am

Identification: A1.03

The a-Si:H solar cells having high open-circuit voltage (Voc>950 mV) are highly desirable for the top junction in thin-film silicon based tandem and triple-junction solar cells. Except for the high Voc, the top cell should also have high spectral response (between 350-600 nm wavelength range) to allow thinner absorber layer in order to reduce the light-induced degradation. Therefore, doped layers with high transparency are required. Commonly, the a-Si:H solar cells deploy a-SiC:H as p-layer and nc-Si:H or a-Si:H as n-layer. Those doped layers will inevitably lead to high parasitic absorption losses, and thus make it difficult to achieve sufficient photocurrent with a thin absorber layer.

In this contribution, we will first discuss how to achieve high Voc by processing a-Si:H at high-pressure and high-power regime. Then deployment of highly transparent silicon oxide (SiOx) doped layers will be discussed to obtain better spectral response (or external quantum efficiency) and higher Voc than the conventional doped layers. Specifically, following key points will be presented in the conference.

1. The a-Si:H is deposited at high-pressure (>5 mbar ) and high-power (>0.1 W/cm2) regime, which results in larger bandgap than materials commonly processed at low-pressure and low-power regime. The bandgap of a-Si:H can be tuned by H2/SiH4 dilution, power and pressure. High performance device-grade a-Si:H can be obtained over wide deposition window, in contrast to the narrow window at low-pressure regime.

2. Highly transparent p-SiOx:H with sufficient conduction is investigated. Firsly, it should have good ohmic contact with front ZnO TCO. Second, the control of crystallinity of p-SiOx:H is a critical point to obtain high Voc for a particular absorber layer. Finally, a very thin layer of i-SiOx:H inserted in the p/i interface can significantly reduce the boron diffusion during the deposition of i-layer, and thus considerably improve the blue spectral response. Consequently, EQE higher than 70% at λ=400 nm is achievable.

3. Low absorption n-SiOx:H to replace absorptive n-aSi:H is necessary to achieve high spectral response over 500-700 nm wavelength range. Furthermore, the low refractive-index n-SiOx:H layer can also function as intermediate reflection layer in multijunction devices. Control of the i/n interface is crucial to achieve high FF. The insertion of an ultra-thin (<3 nm) n-aSi:H or n-aSiOx layer can significantly increase the FF of solar cells and result in FF comparable to cells with a-Si:H n-layer, without reduction of spectral response compared to single n-SiOx layer. After optimization of the p-SiOx:H and n-SiOx:H doped layers, a-Si:H solar cells with high Voc, high FF and excellent spectral response is obtained (Voc>960 mV, FF>74%, and efficiency>10%). The light-induced degradation of solar cells with SiOx:H doped layers are investigated, and will be compared to the solar cells with conventional doped layers.

Speaker(s):

Nanoporous Materials Chemistry for CO2 Capture and Separation

Apr 22, 2014 9:15am ‐ Apr 22, 2014 9:45am

Identification: I1.03

Control of carbon dioxide emissions without significant penalties requires effective CO scrubbing from point sources, such as fossil fuel burning power plants, cement factories and steel making. Capturing process is the most costly; hence the research is directed to finding solutions to it.

Solids with slight chemisorptive nature are identified as most likely candidates for a sustainable solution. Nanoporous (pore size < 100 nm) materials show considerable CO uptakes and are likely to replace monoethanol amine (MEA) solutions for industrial CO capture. We have developed nanoporous covalent organic polymers (COPs), which show significant capacities and selectivities for CO.

To name a few, COP-1 shows 5.6 g/g CO uptake at 200 bar and 45°C, COP-2 shows a CO/H selectivity of over 10k:1, COP-79 has a CO/N selectivity of 308 at 50°C, COP-83 has CO2 uptake capacity of 5 mmol/g at 298 K and 1 bar and COP-97 showed an uptake of 8 % (w/w) CO in 2 minutes from a simulated flue gas mixture (CO2 15%, HO 3.8%, He 81.2%, 40 C, flow rate : 80 mL/min). Our results point to an ideal nanoporous structure to be made from a highly porous, inexpensive, physisorptive solid, which is chemically modified with chemisorptive functionalities such as amines.

References:

1. H. A. Patel, et al., Nature Commun., 4:1357, (2013)

2. H. A. Patel, et al., Adv. Funct. Mater., 23, 2270-2276 (2013).3. H. A. Patel, et al., J. Mater. Chem., 22, 8431-8437 (2012).

4. Image adapted from The Economist, Mar 5, 2009.

Speaker(s):

Nucleation and Growth of Secondary Phases on Electrochemically-Active, Perovskite-Oxide Surfaces

Apr 22, 2014 9:15am ‐ Apr 22, 2014 9:30am

Identification: L1.03

The oxygen evolution (OER) and reduction reactions (ORR) are ubiquitous in many energy conversions. Of particular interest are reactions that take place over the solid/gas interface, for example, in elevated-temperature solid oxide fuel cells and electrolyzers operating at 500-800°C. Despite that OER and ORR generally dominate the overpotential in these electrochemical cells, an in-depth understanding of what controls reaction rate has not been achieved yet. In particular, the formation of secondary phases at the surface of electrocatalysts due to cation segregation and phase separation are believed to influence OER and ORR rates strongly.

Here, we combine in-situ and ex-situ microscopy and spectroscopy techniques to follow the formation and growth of such secondary phases at the surface of perovskite (ABO) electrocatalysts. A compositional range of dense, epitaxial thin film electrodes were grown by pulsed-laser deposition. The nucleation and growth of secondary phases were observed by in-situ environmental SEM as a function of temperature and oxygen partial pressure. Auger microscopy was also used to link surface morphology to the nanoscale chemical composition. These results were combined with current-voltage measurements to assess the formation pathway of secondary phases and their impact on OER and ORR rates.

Speaker(s):

Unraveling Micro-Mechanisms of Grain Boundary Migration Using Molecular Dynamics Simulation and Reaction Path Techniques

Apr 22, 2014 9:15am ‐ Apr 22, 2014 9:30am

Identification: QQ1.03

Understanding grain boundary (GB) migration mechanisms plays a key role in understanding the deformation mechanics of nano-crystalline materials. Despite the many theories have been proposed, there still exists widespread disagreement in the research community. For instance, the normal direction diffusion model is often assumed in conventional grain growth models, but recent studies have suggested that shear-coupled grain boundary migration is the dominate mechanism for low or high angle structures during stress driven dynamics. This study addresses the competition between the two mechanisms in nano-crystalline copper by using molecular dynamics simulations to characterize symmetric tilt grain boundary migration in response to an external driving force. The fundamental idea is to first determine an order parameters using principal coordinate analysis and then find the reaction pathways under different simulation conditions by minimum free energy path (MFEP) search techniques. Once the MFEP is found, the free energy profile for GB migration can be computed from thermodynamic integration. Our preliminary results show that migration behavior of a symmetric tilt grain boundary with various misorientation angles can be well represented by two order parameters, and surprisingly the MFEP for most misorientation GBs has a zigzag shape instead of the commonly observed a smoothed interface. This work was supported by a grant from the Office of Naval Research.

Speaker(s):

Observing Nickel Germanide Formation in 1-Dimension

Apr 22, 2014 9:15am ‐ Apr 22, 2014 9:30am

Identification: UU1.05

One of the major roadblocks in developing Ge nanowire transistors is in finding low resistance ohmic contacts. This is due to the fact that surface states between a metal and a Ge nanowire contribute to large Fermi level pinning and hence detrimental large Schottky barriers for the devices. Metal alloys with Ge, i.e. germanides, have been suggested as a possible solution. Nickel germanide has shown promise as the germanide of choice due to its thermal stability and low resistivity [1].

One problem which has been identified with Ni-germanides for bulk systems is the agglomeration of the germanide and hence an increase in resistivity [2]. The finite dimensions of the nanorods should eliminate this due to a limited Ni/Ge interface. In-situ TEM annealing at 500°C of Ni germanides supplied first insights into the kinetics of the nanowire Ni germanide formation suggesting a linear growth rate and a sharp interface with the Ge nanowire [3].

Our studies further expand this knowledge by looking at the formation of germanides restricted to 1-dimensional Ge nanorods in contact to a finite source of Ni i.e. Ni cap. The nanorod diameters were scaled down to 20 nm using electron beam lithography (EBL) and reactive ion etching (RIE) using (100), (110) and (111) Ge substrates.

Hence we establish the effect of the Ni-germanide formation with a decreasing interface area, deterministically controlled crystal orientation and dopant level. Ni-capped Ge nanorods with varying size and crystal orientation have been annealed in-situ in the TEM to observe the Ni-germanide formation. The consumption of the Ni as it migrates into the Ge is observed and corresponding germanide growth rates were determined. Other methods of annealing such as microwave and rapid thermal annealing (RTA) will also be presented in this study as a comparison to the in-situ annealing.

1. Brunco, D. P. et al. Germanium MOSFET devices: Advances in materials understanding, process development, and electrical performance. Journal of the Electrochemical Society 155, H552-H561, doi:10.1149/1.2919115 (2008).

2. Lee, J.-W. et al. Enhanced Morphological and Thermal Stabilities of Nickel Germanide with an Ultrathin Tantalum Layer Studied by Ex Situ and In Situ Transmission Electron Microscopy. Microscopy and Microanalysis 19, 114-118, doi:doi:10.1017/S1431927613012452 (2013).

3. Tang, J. et al. Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors. Nanotechnology 21, doi:10.1088/0957-4484/21/50/505704 (2010).

Speaker(s):
  • Roisin Kelly, Tyndall National Institute, University College Cork

Hydrogenated Amorphous Silicon Germanium by Hot Wire CVD as an Alternative for Microcrystalline Silicon in Tandem and Triple Junction Solar Cells

Apr 22, 2014 9:30am ‐ Apr 22, 2014 9:45am

Identification: A1.04

The manufacturing cost of thin film Si-based tandem and triple junction cells and modules is at present too high to meet current module market prices. Conventionally, microcrystalline silicon is used as the low-bandgap absorber in micromorph solar cells (a-Si/µc-Si tandem cells). However, due to the considerable thickness needed for the µc-Si:H absorber, it takes three to four times as many deposition reactors compared to single junction cells to produce tandem cells, leading to high cost of ownership.

One of the approaches to reduce processing time of the low-bandgap layer(s) in multijunction silicon-based solar cells is the use of hydrogenated amorphous silicon germanium (a-SiGe:H). In general, however, a-SiGe:H has not been considered a viable option because of (i) the high defect density for PECVD a-SiGe:H, at band gaps < 1.4 eV, and (ii) the cost of GeH4. On the other hand, due to its direct gap nature, the thickness of an a-SiGe:H absorber layer can be kept 10 times smaller than that of µc Si:H.

We are investigating whether a-SiGe:H can be reconsidered for inexpensive production of multijunction thin film Si based solar cells if HWCVD is used as the deposition method. HWCVD is a simple and low-cost deposition technique allowing high deposition rates while maintaining good defect passivation. Early results reported by NREL include the achievement of material with a band gap close to that of µc Si:H (1.2 eV) with an equivalent photoresponse (in excess of two orders of magnitude). Their work has led to 8.64% single junction cells without any band-gap profiling in the absorber layer.

We now continued this development to provide a novel thin film alloy for the struggling micromorph technology. We have produced a-SiGe:H materials with Tauc band gaps ranging from 1.6 eV down to 1.2 eV. Due to the efficient dissociation of silane and germane gases at the hot filament, a high deposition rate is achieved. Moreover, the dissociation rate of germane is three times faster than that of silane. The deposition rate for the lowest-gap material is 0.7 nm/s and is always higher than 0.5 nm/s. With this deposition rate, an active absorber layer (i-layer) of 150 nm is readily deposited within 5 minutes. This should be compared to the roughly one-hour long deposition time needed to deposit a 2-µm thick µc Si:H film at the commonly used deposition rate of 0.5 nm/s.

Using a GeH4/SiH4 ratio of 1, we deduce from Raman spectroscopy that the films already contain 60-70% Ge, showing that Ge is preferentially incorporated in the film. We will report an extensive microstructure analysis and will present our first cells.

Speaker(s):
  • Ruud Schropp, ECN - Solliance, Eindhoven University of Technology

Engineering Synaptic Electrodes to Drive Self-Assembly of Neural Interfaces

Apr 22, 2014 9:30am ‐ Apr 22, 2014 9:45am

Identification: Z1.03/AA1.03

Bioelectronic medicine has numerous promising applications for the treatment of diseases and disorders of the nervous system, but also many challenges. Two of the key limiting factors to the development of next-generation neural interfaces are the low charge transfer area and poor neural tissue integration of conventional metallic electrodes [1, 2]. The “synaptic electrode" concept draws upon knowledge from both implantable neurostimulator and bioelectrode research, and combines it with the principles of tissue engineering.

The basis of this technology is a conductive hydrogel (CH) which provides a new approach to tailoring the neural interface by decreasing the strain mismatch while providing a conductive path within a soft, deformable hydrogel matrix [3]. A typical CH consists of a biosynthetic hydrogel integrated with a CP, such as poly(ethylene dioxythiophene) (PEDOT). The hydrogel is a co-polymer of poly(vinyl alcohol) (PVA) and a modified biological molecule. Varying the type of biomolecule has allowed the properties of the CH to be tailored such that specific cells will interact with the electrode coating.

CH efficacy and safety has been demonstrated in vivo, with reduced scar tissue compared to conventional platinum interfaces. The “synaptic electrode” construct is produced by the addition of a degradable PVA layer overlying the CH, in which cells can be encapsulated. This hydrogel layer provides both the ability to encapsulate cells within the electrode and simultaneously deliver therapeutic agents to promote regeneration or directed growth of neurons. Specifically, co-cultures of neurons and supporting glia have been embedded in the electrode coating, and found to survive and differentiate to produce active neural processes. The neural networks grown within the hydrogel matrix are excitable at lower thresholds than typical neural tissue at the implant interface.

It is expected that integration of this bioelectrode into neural tissue will create a “synaptic electrode” to directly interact with excitable target tissue. These studies provide evidence that next-generation electrode arrays can be developed to safely deliver stimulus and accurately record from high density electrode arrays using natural synaptic processes. These bioactive neural interfaces create a technology platform which can be tailored for bioelectronic applications such as functional electrical stimulation (FES), nerve guides, bionic ear and eye devices and deep brain stimulators.

References:

1. Ludwig, K.A., et al., Chronic neural recordings using silicon microelectrode arrays electrochemically deposited with poly(3,4-ethylenedioxythiophene) (PEDOT) film. J Neural Eng, 2006. 3: p. 59-70.2. Green, R.A., et al., Conducting polymers for neural interfaces: Challenges in developing an effective long-term implant. Biomaterials, 2008. 29: p. 3393-9.3. Green, R.A., et al., Conductive hydrogels: Mechanically robust hybrids for use as biomaterials. Macromol Biosci, 2012. 12.

Speaker(s):

Efficient Solution-Processed Small Molecule Solar Cell by Incorporating High Molecular Weight Polystyrene

Apr 22, 2014 9:30am ‐ Apr 22, 2014 9:45am

Identification: C3.04

Bulk-heterojuction (BHJ) solar cells with bicontinuous interpenetrating network of organic donor (D) and acceptor (A) offer promising merits such as low cost, lightweight and flexibility and have been considered as potential contender for novel energy market. The power conversion efficiencies (PCEs) of BHJ solar cells based on conjugated polymer donors have been exceeding to 10 %. Recently, molecular donors with well-defined dimensions are being studied intensively, leading to small molecule BHJ solar cells which exhibit comparable PCEs ~7 %.

Although there are much attractive properties along with molecular donors, it is generally pretty difficult to obtain film with good wettability from molecular donors when compared to analogous polymer systems. Moreover, the molecular films are so thin that it is not possible to achieve sufficiently high optical densities upon solar spectrum. To circumvent these problems, a novel method of active layer control by incorporating a small addition of an inert high molecular weight polymer to molecular D/A system was investigated. It is found that one can obtain increase in film thickness without sacrificing desirable morphology of phase separation and structural order. Specifically, high molecular weight polystyrene (PS, Mn=20,000,000) is incorporated to a p-DTS(FBTTh2)2:PC70BM blend on an order of 1-5 % by weight relative to the BHJ components. It shows that the device performance is extremely beneficial with a PCE increasing from 7.0 % to 8.2 %.

Furthermore, different techniques such as grazing incidence wide-angle X-ray scattering (GIWAXS), dynamic secondary ion mass spectroscopy (DSIMS), cross-sectional transmission electron microscopy (TEM) and bright-field TEM were employed to illustrate the film composition and morphology. It reveals that the critical accumulation of insulating PS within the interior of film and away from the charge-collecting electrodes.

Speaker(s):
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