2015 MRS Spring Meeting

Apr 6, 2015 ‐ Apr 10, 2015


The 2015 MRS Spring Meeting and Exhibit was held in San Francisco, California. Technical symposia were held within the following topical clusters: Energy, Nanomaterials, Electronics and Photonics, Soft and Biomaterials, and General – Fabrication and Characterization. Access to all of the content below is free – simply login with your MRS username and password for complete access. Don’t have a username and password? Create a free account.


Sessions

Full Description of the Optical Behavior of Perovskite Solar Cells

Apr 9, 2015 4:30pm ‐ Apr 9, 2015 4:45pm

Identification: C9.04

Perovskite solar cells are driven a paradigm shift in photovoltaics (PV) since they are breaking with the existing tradeoff between efficiency and fabrication cost.1,2 A little over two years this solar cell technology is at the top of the emerging PV efficiency race, reaching certified values that surpass all the ones attained in cells based on solution-processed materials. Herein we provide a full description of the optical behavior of perovskite solar cell devices. We performed an in-depth experimental and theoretical analysis of the optical effects occurring in state-of-the-art solution-processed perovskite cells, i.e. organic-inorganic metal halide perovskite, prepared using a one-step deposition method, sandwiched between an n-type and p-type charge collection layer. We developed a theoretical model using a method based on the transfer matrix formalism,3 which allows us to calculate the electric field intensity within the layered structure and thus to visualize the effect of each layer comprising the cell on the spatial distribution of the radiation in the PV device. To check the suitability of the proposed theoretical model, we fitted the experimental reflectance, transmittance and absorptance spectra of devices measured at normal incidence. The layout of the device determines the field intensity distribution along the cell and hence the spatial and spectral distribution of optical absorption in the device. This allows us to estimate the amount of light that is captured by each absorbing layer in the perovskite cell, and hence to discriminate between productive (absorbed in the perovskite material) and parasitic (absorbed in other components such as FTO, or the metallic contact) absorption. Our work also provides a theoretical framework in which the optical response of solar cells integrating photonic enhancing components can be rationalized. Also, as demonstrated in other PV technologies, photonic designs may lead to resonant photocurrent generation4 to improve the efficiency of the device. References: 1. Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T. N., & Snaith, H. J. Science 338, 643-647 (2012). 2. Liu, M., Johnston, M. B. & Snaith, H. J. Nature 501, 395-398 (2013). 3. Lozano, G., Colodrero, S., Caulier, O., Calvo, M. E., & M�guez, H. J. Phys. Chem. C 114, 3681-3687 (2010). 4. Anaya, M., Calvo, M. E., Luque-Raig�n, J. M. & M�guez. J. Am. Chem. Soc. 135, 7803-7806 (2013).

Speaker(s):
  • Miguel Anaya, Institute of Materials Science of Seville (CSIC-US)

Atomic Structure and Monolayer Stability in 2D Elemental Layered Materials: Silicene and Germanene

Apr 9, 2015 4:30pm ‐ Apr 9, 2015 4:45pm

Identification: O8.07

Two-dimensional materials are one of the most active areas of nanomaterials research. Here we report the structural stability, electronic and vibrational properties of different monolayer configurations of the group IV elemental materials silicene and germanene. The structure of the stable configurations is calculated and in the case of planar and a low (

Speaker(s):

Fermi Level Pinning in Metal/Al2O3/InGaAs Gate Stack

Apr 9, 2015 4:45pm ‐ Apr 9, 2015 5:00pm

Identification: AA10.11

In0.53Ga0.47As is considered as one of the most attractive semiconductors for high electron mobility channels complementary metal oxide semiconductor transistors. Fixed charges and traps (e.g. border traps) within the oxide layer, and trap states produced by defects at the Al2O3/InGaAs interface were found in many studies of Al2O3/InGaAs gate stacks. Post oxide deposition annealing and post metal deposition annealing help in improving the quality of the Al2O3/InGaAs system by reducing the above mentioned traps and charges. In this study the effect of post metal deposition annealing on the effective work function (EWF) in metal/Al2O3/InGaAs gate stacks was investigated. Al2O3 was deposited by thermal atomic layer deposition (ALD) using a standard trimethylaluminum/H2O process on n-type InGaAs(100) substrates. The samples were then annealed at 400�C for 30 min in vacuum (P

Speaker(s):
  • Roy Winter, Technion - Israel Institute of Technology

A Novel Synthetic Strategy for Environmentally Benign Processing of Polymer Semiconductors

Apr 9, 2015 4:45pm ‐ Apr 9, 2015 5:00pm

Identification: CC11.12

Recent intensive research into the development of high mobility polymer semiconductors has achieved field effect mobilities higher than 10 cm2/Vs, when fabricated as a polymer thin film transistor. However, most of this work has been aimed at achieving a high mobility/reliability in polymer semiconductors, with very few reports regarding the use of non-chlorinated solvents in the processing of polymer semiconductors. From an industrial point of view, the chlorinated solvent that is currently the most widely used in the processing of polymer semiconductors should be excluded owing to its high environmental cost. Here, we show that controlling the random copolymerization between two different diketopyrrolopyrole-based conducting units by varying the relative quantity of each represents a suitable synthetic strategy to increase the solubility of polymer semiconductors in a non-chlorinated solvent, without compromising high charge carrier mobility. Highly performing and reliable polymer thin film transistors processed from environmentally benign solvents such as tetralin are demonstrated for the first time, resulting in a mobility of greater than 5 cm2/Vs.

Speaker(s):

Tribotronics: A New Field Coupled Semiconductor and Triboelectricity

Apr 9, 2015 4:45pm ‐ Apr 9, 2015 5:00pm

Identification: P8.08

Recently, the invention of a triboelectric nanogenerator (TENG) has provided an effective approach to convert ambient mechanical energy into electricity. The working principle of the TENG is based on the coupling of contact electrification and electrostatic induction. Contact-induced charge transfer between two materials with opposite tribopolarity results in a potential difference when they are separated. This potential difference is an inner electrical signal created by the external mechanical force, which could be used as a gate signal to tune/control the carrier transport characteristics in FET as the same effect as applying a gate voltage. Utilizing the coupled metal oxide semiconductor field-effect transistor and triboelectric nanogenerator, we demonstrate an external force triggered/controlled contact electrification field-effect transistor (CE-FET), in which an electrostatic potential across the gate and source is created by a vertical contact electrification between the gate material and a �foreign� object, and the carrier transport between drain and source can be tuned/controlled by the contact-induced electrostatic potential instead of the traditional gate voltage. With the two contacted frictional layers vertically separated by 80 ?m, the drain current is decreased from 13.4 to 1.9 ?A in depletion mode and increased from 2.4 to 12.1 ?A in enhancement mode at a drain voltage of 5 V. Compared with the piezotronic devices that are controlled by the strain-induced piezoelectric polarization charged at an interface/junction, the CE-FET has greatly expanded the sensing range and choices of materials in conjunction with semiconductors. The CE-FET is likely to have important applications in sensors, human silicon technology interfacing, MEMS, nanorobotics, and active flexible electronics. The CE-FET is based on the MOSFET and contact electrification effect, which can be used as fundamental components in novel triboelectronic devices and systems. We call this tribotronics, a new field of research and applications in flexible electronics. Tribotronics is about the devices fabricated using the electrostatic potential created by triboelectrification as a �gate� voltage to tune/control charge carrier transport in the semiconductor. Tribotronics is a field by coupling the semiconductor with triboelectricity, which is an extension of piezotronics first proposed in 2007, and also a profound application of TENG first proposed in 2012. By the three-way coupling among triboelectricity, semiconductor, and photoexcitation, plenty of potentially important research fields are expected to be explored in the near future.

Speaker(s):

Intrinsic and Induced Chirality of CdSe Nanocrystals

Apr 10, 2015 8:15am ‐ Apr 10, 2015 8:30am

Identification: GG8.00

Colloidal semiconductor nanocrystals are under intense development for a range of applications, including light-emitting devices for energy-efficient displays and light sources, light-harvesting elements for exploitable and low-cost photovoltaics, logical elements for quantum computing, as well as for many biomedical applications such as imaging, drug delivery, and sensors. In particular, the key advantage of using of artificial nanocrystals in biology, medicine and pharmacology is an ability to manipulate the units of matter at the scale close to molecular one. At this scale many biological processes involve mechanism of molecular recognition, and it is well known that chirality plays a key role in this mechanism. Potentially any nanocrystal can be intrinsically chiral due to their low symmetry and presence of bulk and surface defects. Chirality of the semiconductor nanocrystals makes possible their �lock and key� interaction with biological objects since nanocrystals� sizes are comparable to the sizes of biomolecules and the pores of cell membranes. Here we investigate intrinsic chirality of CdSe nanocrystals using a technique of circular dichroism (CD). We develop an experiment under assumption that as-prepared ensemble is a mixture of achiral nanocrystals and equal amounts of nanocrystals with opposite chirality, and possesses no optical activity. Consequently, to obtain an experimental evidence of intrinsic chirality of the semiconductor nanocrystals, it is necessary to separate their enantiomers. We show that optically active the quantum dots and quantum rods samples can be obtained by preferentially extracting either L- or D-enantiomers of nanocrystals from optically inactive sample. We use an original method of the enantioselective phase transfer of the nanocrystals from organic solvent to water assisted by the chiral molecules of ligand. This method allows us to isolate L- and D-enantiomers of the nanocrystals into different organic and aqueous phases and investigate their CD behaviour. The aqueous phase is enriched with nanocrystals with the same chirality as the ligand molecules used for the phase transfer, for example, L-nanocrystals in the case of using L-cysteine. In the contrast, the organic phase is enriched with the nanocrystals, whose interaction with chiral ligands was ineffective. These nanocrystals are still capped by achiral ligands. The aqueous and organic fractions possess almost mirror image CD signals, but the value of intrinsic CD signal in organic solvent is less. The reason for this difference may be in induced increasing of CD after adsorption of chiral ligands. The experiments with other types of quantum-confined objects such as nanoplatelets, nanotetrapods, and dot-in-rods also show increasing of CD signal after capping with chiral ligands. Importantly, CD increasing differs for different types of nanocrystals. In particular, CD increasing up to 30 times is observed for the nanoplatelets.

Speaker(s):

Nanostructured Charge Collecting Materials for Highly Efficient Perovskite Solar Cells

Apr 10, 2015 9:00am ‐ Apr 10, 2015 9:30am

Identification: C11.02

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 15% and further improvements are expected up to 20%. These novel organometal halide perovskite absorbers which possess exceptionally strong and broad light absorption enable to approach the performances of the best thin film technologies. Especially, the cost-effective solution process for perovskite solar cells at a low temperature makes them viable to realize flexible thin film solar cells. Efficient charge collection is one of critical issues in perovskite solar cells [1-3]. The charge collection efficiency for perovskite solar cells can be enhanced by controlling nanostructure or exploiting new materials. In this presentation, we demonstrate that nanostructured materials such as SnO2@TiO2 core-shell nanowires and 3D-ITO nanowire/TiO2 nanoparticle materials can facilitate charge transport in perovskite solar cell. Also, MgO nanolayer coated on TiO2 nanoparticles can efficiently retard charge recombination. Finally, we introduce highly bendable 12 % perovskite solar cells based on ITO/PEN substrates [4]. The energy conversion efficiency did not change after 1000 cycle of bending test with 10mm bending radius which demonstrates a feasibility of highly bendable perovskite solar cells without efficiency degradation. References (1) K. Mahmood, B. S. Swain and H. S. Jung, Nanoscale, 2014, 6, 9127 (2014). (2) G. S. Han, S. Lee, J. H. Noh, H. S. Chung, J. H. Park, B. S. Swain, J.-H. Im, N.-G. Park and H. S. Jung, Nanoscale, 6, 6127 (2014). (3) G. S. Han, H. S. Chung, B. J. Kim, D. H. Kim, J. W. Lee, B. S. Swain, K. Mahmood, J. S. Yu, N.-G. Park, J. H. Lee and H. S. Jung,, J. Mater. Chem. A DOI: 10.1039/C4TA03684K (2014). (4) B.-J. Kim, D. H. Kim, Y.-Y. Lee, H.-W. Shin, G. S. Han, J. S. Hong, K. Mahmood, T. Ahn, Y.-C. Joo, K. S. Hong, N.-G. Park, S. Lee and H. S. Jung, Energy & Environmental Science, DOI: 10.1039/C4EE02441A (2014).

Speaker(s):

Study on the Contact between MoS2 and Metals

Apr 10, 2015 9:00am ‐ Apr 10, 2015 9:15am

Identification: O10.02

Field effect transistors based on single layer or few layer MoS2 have been demonstrated to have impressive characteristics, such as high ON/OFF current ratio, reasonable electron mobility and a subthreshold swing approaching the theoretical limit at room temperature. However, the metal-MoS2 contact resistance is still much higher than the metal-Si contact and metal-graphene contact. Here, we study the resistance distribution at the metal-MoS2 contact through a multi-electrode measurement with specially designed thin electrodes. We find that the sheet resistance of the 2D MoS2 increases obviously after contacting the metal. The parameters related to the contact resistance, such as transfer length, sheet resistance of the MoS2 and contact resistivity between the 2D materials and the metal electrode, are all changed by the gate voltage. Furthermore, we study the ways to reduce the contact resistance and interesting results are obtained.

Speaker(s):

First-Principles Investigation of H2O Adsorption on SnO2-TiO2 Gas Sensors

Apr 10, 2015 9:15am ‐ Apr 10, 2015 9:30am

Identification: TT15.05

Metal oxides are highly interesting materials for numerous industrially and economically important applications as a result of their semiconducting properties. Wide band gap metal oxides such as SnO2 and TiO2 are used, for example, in solar cells [1] or photocatalysis [2]. Moreover, their ability to change conductivity when gaseous molecules are reacting with the surface makes them particularly applicable for chemoresistive portable gas sensors. A main drawback of metal oxide materials both in photocatalysis and as gas sensors, however, is the reaction of their surfaces with water vapor. In other words, changes in the relative humidity of the environment can significantly influence the performance of the metal oxide. This is one of the major shortcomings of SnO2-based gas sensors used, for example, in breath analysis. Experimentally, it has been shown that this can be overcome by doping of SnO2 with other metal atoms, such as Ti [3]. In this project, density functional theory calculations have been utilized to simulate the formation of SnO2-TiO2 solid solutions demonstrating favorable distribution of Ti on the SnO2 surface, in particular on six-fold coordinated sites. Changes in the electronic structure of such SnO2-TiO2 surfaces leads to a destabilization of dissociatively adsorbed H2O species. A minimum in the H2O stability at low coverage has been found at a surface Ti-content of 25%. At high coverage, H2O is drastically destabilized when increasing the surface Ti-content from 0 to 30%. The overall minimum in the H2O stability can thus be assigned to a surface Ti-content of 25-30%. This gives a possible explanation for the minimum in cross-sensitivity to humidity found experimentally for Ti-doped particles [3]. [1] J. F. Wager, Science 300 (2003) 1245. [2] A. Fujishima and K. Honda, Nature 238 (1997) 37. [3] A. Tricoli, M. Righettoni and S. E. Pratsinis, Nanotechnology 20 (2009) 31552.

Speaker(s):

Brush-Hydrogels with Graded and Gradient-Like Properties: From Organic to Hybrid Biomimetic Coatings

Apr 10, 2015 10:15am ‐ Apr 10, 2015 10:30am

Identification: GG8.05

The fabrication of organic coatings with well-defined structure and physico-chemical properties is of particular interest in several disciplines. Bio-organic layers on different length-scales, featuring variable composition, structure and modulus have been observed in several biological systems, such as human cartilage, mammalian skin and the nacre of oyster shells [1-3]. These complex materials consist of mechanically graded structures that resist and respond to the external normal and shear forces, protecting underlying tissues from incurring damage. Among the many �natural� coating systems, human epidermis is constituted by different layers of cells which present diverse properties to confer resistance against abrasion. In order to mimic these natural structures, materials scientists have been made numerous efforts to fabricate coatings with graded and gradient-like mechanical properties within a single film. Despite this, the fabrication of a full-organic, polymer-based coating architecture featuring nano-scale variations of properties still represents a challenging task. In order to fabricate polymer films presenting discontinuous and continuous variations of mechanical and chemico-physical properties we applied sequential surface-initiated polymerization (SIP) of different monomer compositions to create polymer brush/hydrogel films with vertically defined structure. Specifically, poly(hydroxyethyl methacrylate) (PHEMA) brush and brush-hydrogel layered films were synthesized by surface-initiated atom transfer polymerization (SI-ATRP) in the presence of different concentrations of diethylene and tetraethylene glycol dimethacrylate (DEG/TEGDMA). Sequential SI-ATRP of monomer mixtures was employed to fabricate brush-hydrogel films presenting vertically graded properties. Alternatively, continuous variation of monomers during the SI-ATRP resulted in brush-hydrogels featuring gradient-like variations of polymer architecture through the film thickness. The chemical, mechanical and tribological properties of the films were characterized by ellipsometry, FT-IR and colloidal probe atomic force microscopy (CP-AFM). All these measures confirmed the influence of polymer architecture on the properties of the films at specific depths. Additionally, graded and gradient-like brush-hydrogels were used as reactors for the synthesis of polymer-inorganic hybrids presenting metal nanoparticles (NPs) embedded within the films. Depending on the vertical crosslinker content, different NPs morphologies were obtained at determined depths. Through this multi-step fabrication, polymer-inorganic hybrids with vertically defined structures and variable NPs loading were obtained. In conclusion we believe these methods will represent an easy and effective mean to form coatings with defined mechanical properties and tunable optical characteristics. 1. R. A. Stockwell et al., Nature 1967 2. J. C. Mackenzie, Nature 1969 3. H. D. Espinosa et al., Nat. Commun. 2011

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