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.
Piezotronics based on piezoelectric fine wires have show diverse of potential applications [1-3]. A piezotronic device is normally characterized by its capable of output pulsed piezoelectrical signals upon a load of strain. Strain sensor or tactile sensor could be a promising application for piezotronic devices [1, 2]. This field is pioneered by Zhong Lin Wang’s group. Zhong Lin Wang’s group has just demonstrated a tactile imaging device based on an array of piezotronic transistors . This work opened a broad range of potential applications such as human-electronic interfacing, smart skin and micro- and nanoelectromechanical systems. The unique thing of this type of tactile sensor is that its function mechanism is similar with that of tactile sensing of human beings, which is essentially a bio-piezo effect. In this report, we demonstrate a horizontally aligned piezotronic component based on a ZnO piezoelectric fine wire bridging between two copper electrodes. We integrate a fiber across the ZnO piezoelectric fine wire and so it forms a ridge across the piezotronic component after PDMS packaging. The ridge over the piezotronic component serves as a strain introducing component mimic human being finger prints. The devices are used to touch (press and slide on) a series of objects with period textures on the surface. The output piezoelectric pulses are recorded and correlated to the period textures of the objects been touched.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61172041, 91123018, 61172040), and the Fundamental Research Funds for the Central Universities. The authors also thank Prof. Yong Qin for his valuable comments on the device fabrications.
 Wenzhuo Wu, Xiaonan Wen, Zhong Lin Wang, Taxel-Addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging, Science 340, 952 (2013).
 Rusen Yang, Yong Qin, Liming Dai and Zhong Lin Wang, Power generation with laterally packaged piezoelectric fine wires, Nature Nanotechnology, 4, 34-39 (2009).
 Jun Zhou, Yudong Gu, Peng Fei, Wenjie Mai, et al., Flexible Piezotronic Strain Sensor, Nano Lett. 8, 3035-3040 (2008).Speaker(s):
The scaling of logic CMOS devices requires the use of high mobility semiconductor channel materials such as InGaAs. However, the interfaces of III-V and high K oxides such as Al2O3 and HfO2 can still possess a high interface state density, D_it. Various methods have been used to lower this density, such as choice of oxide, and recently the use of nitrogen treatment of the interface. We have calculated that the replacement of the last layer of the III-V by an Al-N layer greatly reduces the interface defect state density, because N-N dimers do not form, N dangling bond states are deep in the valence band and Al dangling bond states lies higher in the conduction band . This result was recently confirmed experimentally [2,3]. Here, we extend these calculations to other III-Vs such as InGaSb and GaSb where a similar behavior is found.
1. Y Guo, L Lin, J Robertson, App Phys Lett 102 091 606 (2013)
2. T Aoki et al, Tech Digest SSDM (2013) J-8-33. V Chobpattana... S Stemmer, App Phys Lett 102 022907 (2013)Speaker(s):
I will present methods to fabricate, test and characterize ZnO nanorod-based energy harvesters and the impact of surface passivation of the ZnO on power output for a vibrational harvesting system and a photovoltaic. Previous studies have focused on improving the performance of these devices. In this regard, suppression of semiconductor carrier screening and improvement in device architecture had been reported. However, the impact of ZnO nanorod passivation on p-n junction devices has not been studied.
The first section of the presentation will discuss the passivation of ZnO nanorods using CuSCN in ZnO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) p-n junction-type piezoelectric energy harvesting devices. The maximum power density across its optimal load is determined by testing the device across variable resistors showing higher output from passivated devices. The increase in voltage and voltage-driven current is discussed with relation to changes in the carrier lifetime.A comparison between passivated and non-passivated ZnO devices shows that the passivated device generated an open-circuit voltage of 212 mV and short circuit current of 1 mA/cm2 which was around twice the value of non-passivated device (90 mV and 0.66 mA/cm2). When tested across a range of load resistances, the maximum power density was also found to have almost doubled from 36.00 ÂµW/cm2 across a 1.38 kΩ load for the non-passivated device to 64.40 µW/cm2 (1.67 kΩ) for the passivated device. We deduce that the voltage and voltage-driven current density of the passivated ZnO device improved due to three-fold increase in carrier life time.
The second part of the presentation will focus on the photovoltaic (PV) effect of perovskite materials and methods of passivating the surface of ZnO to enable thin film growth. I will report on the design, fabrication and testing of a solid-state perovskite enhanced ZnO solar cell. The p-type perovskite material used was bismuth ferrite (BFO), so far not implemented in a ZnO solar cells. The cell uses FTO and Au electrodes sandwiching semiconductor layers and a CuSCN hole conductor, the ZnO was sensitized with N719. XRD confirmed the presence of BFO with the presence of a single  BFO peak. The results show a significant increase in the performance with efficiencies increased by almost 3 times with the BFO coating with an efficiency of 0.272%. The BFO acts as a buffer from electron recombination. However, the relative increase in performance of the solar cell when BFO was added shows promise of BFO as a PV material.In summary I will show that there are a range of energy harvesting devices that can be produced using a surface passivation layer on a ZnO nanorod architecture and that by careful control of the process parameters enhanced energy harvesting is possible.Speaker(s):
The growth of Sn seeded Si and Ge nanowires in high density directly from an evaporated catalyst layer on stainless steel is reported. Nanowire growth was achieved in low cost glassware apparatus using the vapor phase of a high boiling point organic solvent as the growth medium. The nanowires are single crystalline with predominant <111> growth directions. High resolution transmission electron microscopy, dark field scanning transmission electron microscopy and energy dispersive X-ray analysis are used to elucidate the interface between the seed and the nanowire. The growth method was then extended to the formation of Si/Ge axial heterostructure nanowires using the versatile wet chemical approach. The growth method exploits the low solubility of Si and Ge within the Sn catalyst material to produce abrupt heterojunctions characterized using aberration corrected scanning transmission electron microscopy and atomic level electron energy loss spectroscopy. Additional analysis focussed on the role of crystallographic defects in determining interfacial abruptness and the preferential incorporation of metal catalyst atoms near twin defects in nanowires.
1. Emma Mullane, Tadhg Kennedy, Hugh Geaney, Calum Dickinson, and Kevin M. Ryan, 25 (9), 1816-1822
2. Hugh Geaney, Emma Mullane, Quentin M. Ramasse, and Kevin M. Ryan, 13 (4), 1675-1680Speaker(s):
Light-induced degradation (LID) of amorphous silicon (a-Si:H) solar cells due to the Staebler-Wronski effect has been widely discussed in literature. However, it has been most often discussed with respect to the degradation of the intrinsic absorber layer.
In the present study, LID of a-Si:H solar cells is studied with respect to the amorphous silicon carbide (p-(a-SiC:H)) layer that is part of the window layer of high efficiency solar cells. We have deposited solar cell series varying the p-(a-SiC:H) thickness and the substrate roughness of single junction solar cells in superstrate configuration. The solar cell design is state-of-the-art using low-pressure chemical vapor deposition zinc-oxide for front and back contacts that are in contact with p- and n-doped silicon oxide layers. Plasma-enhanced chemical vapor deposition (PECVD, 40 and 13 MHz) has been used for all silicon layers, using a cluster tool with dedicated chambers for p-doped, intrinsic, and n-doped layers.
During light soaking, a systematic open-circuit voltage (Voc) increase could be observed for thin p-layers, while Voc decreases for thick p-layers. This effect is more pronounced for rough than for smooth substrates: The critical p-(a-SiC:H) thickness, at which light soaking has no effect on the Voc, increases with increasing substrate roughness. These Voc changes have a strong impact on the conversion efficiency of the solar cells. First, the optimum p-(a-SiC:H) thickness depends on the substrate roughness. Second, highest stabilized cell efficiencies are obtained using thinner p-(a-SiC:H) layers than what is optimum in initial state. Different contributions of short-circuit current, fill factor, and Voc to LID of the conversion efficiency are discussed. All trends could be reproduced using different cell designs in three different PECVD systems.
To discriminate the effect of effective p-layer thickness on rough substrates, the nominal thicknesses are corrected by the effective surface as determined from AFM measurements.
Different mechanisms could lead to the observed Voc changes. These are investigated by bias light and bias voltage dependent EQE measurements and by analyzing the degradation /annealing kinetics of the solar cells. The changes are related to layer properties as measured by ellipsometry, photothermal deflection spectroscopy, and conductivity. Finally, we will briefly discuss our latest tandem and triple junction solar cells where we incorporated these a-Si:H cells as top cells.Speaker(s):
A great deal of interest is being focused on environmentally safe earth abundant materials including iron sulfide (FeS) in the pyrite phase. While promising, iron sulfide is presenting serious material science challenges. In addition the band gap of pyrite is much less than the optimal value of 1.36 eV. Another excellent candidate for an earth abundant absorber material is WSe which can be directly grown as a p-type semiconductor with a band gap near 1.4 eV. In this work we present the structural, optical, and electrical properties of thin film WSe2 grown via the selenization of sputter deposited tungsten films. We will show that highly textured films with an optical band gap in range of 1.45 eV, and absorption coefficients greater than 105/cm across the visible spectrum can be easily achieved. In addition we will present Hall Effect and carrier density measurements as well, where will show densities in to 10-10 cm range and p-type Hall mobilities in the 10 cm/V-s range can be obtained. We employ these results to numerically simulate heterojunction solar cells based on this material, where we will show efficiencies greater than 20% are possible.Speaker(s):
The pressure of academic life often hinders our ability as faculty to constructively support students who may face increased challenges, may not have had equal opportunities to learn, or may not be perceived as competent within the “mainstream” scientific culture. Yet, experience shows that such students bring a fresh and innovative outlook to research projects that often lead to superior results and ultimately high level positions.
The role that faculty play for such students, from providing research experience for high school and undergraduate students to mentoring graduate students and postdocs, is critical. This talk focuses on practical ways to develop an inclusive environment within one’s research group where different approaches are valued, to establish a critical mass for students of all backgrounds, and to openly discuss differences as a means to strengthen the team. It also illustrates the value of mentoring external students and having our own students mentored to become better coaches of our own groups. Finally, it re-emphasizes the need for all educators to engage the broadest segments of the population in STEM fields if the US is to remain competitive.Speaker(s):
The mechanisms and efficiency of charge transport in lithium peroxide (Li2O2) are key factors in understanding the performance of non-aqueous Li-air batteries. Towards revealing these mechanisms, here we use first-principles calculations to predict the concentrations, mobilities, and conductivities of various charge carriers and intrinsic defects in Li2O2. We compare transport rates within the baseline case of (pristine) bulk Li2O2, at selected low-energy surfaces, and within amorphous phases. While transport within the bulk is predicted to be low, higher concentrations of charge carriers at both surfaces and within amorphous regions are shown to enhance conductivity in the vicinity of these features. Our calculations reveal that changes in the charge state of O2 dimers controls the defect chemistry and conductivity of Li2O2. More generally, we describe how the presence of a species that can change charge state - e.g., O2 dimers in alkaline metal-based peroxides - may impact rechargeability in metal-air batteries.Speaker(s):
Graphene, the two-dimensional material with honeycomb structure, possesses superior physical properties and thus is a promising candidate for a variety of electronic devices. Actually, there are some issues which should be studied to accomplish the fabrication of devices with high performance. The area of graphene films obtained from mechanically exfoliation is localized while the graphene films synthesized by chemical vapor deposition need to be transferred in spite of the much more complete area. In our investigation, the plasma treatment followed by annealing process is utilized in order to obtain large-scale graphene films from bulk SiC via Nitridation-Induced Carbon Condensation. After exposure of N2 plasma, the N2 annealing process, which promotes nitrogen ions to react with Si and simultaneously condense C around the surface of the SiC, is implemented. A thin silicon nitride layer formed during nitridation may squeeze carbon atoms. Eventually, a uniform large-scale graphene film on SiC wafer will be achieved. The Raman analysis shows typical spectra of graphene and the XPS results indicate that the formation of silicon nitride layer. Furthermore, the TEM images verify that there are several layers of graphene on the top of SiC. In this research, we create a simple method for synthesizing large-scale graphene.Speaker(s):
Nanodiamond powder produced by detonation synthesis is the most promising nanofiller for composites . It is made of diamond particles of ~5 nm in diameter, combining fully accessible surface with a rich and tailorable surface chemistry. Nanodiamond has unique optical, electrical, thermal, and mechanical properties, and is biocompatible and non-toxic. In order to fully benefit from the potential of nanodiamond in nanocomposites, several important issues must be addressed, such as uniformity of nanodiamond dispersion in the matrix, nanodiamond-matrix interface, and the properties of the polymer interphase formed in the vicinity of nanoparticles. These issues can be addressed by different purification, dispersion, and surface modification strategies. Covalent linking of hydrophobic molecules , improves dispersions of nanodiamond in hydrophobic polymers. Reactions of nanodiamond functional groups with the polymer matrix can be used to design a nanofiller-matrix interface and produce a significant volume of interphase in the composite . The interphase formation depends on how nanodiamond changes the structure of polymer in the vicinity of the nanoparticle . Incorporation of nanodiamond into polymers may improve their mechanical properties, thermal conductivity, UV-absorption, and other properties in many practical applications.
1. Mochalin, V. N.; Shenderova, O.; Ho, D.; Gogotsi, Y., The properties and applications of nanodiamonds. Nature Nanotechnology 2012, 7, 11-23.
2. Mochalin, V. N.; Gogotsi, Y., Wet Chemistry Route to Hydrophobic Blue Fluorescent Nanodiamond. Journal of the American Chemical Society 2009, 131, 4594-4595.
3. Mochalin, V. N.; Neitzel, I.; Etzold, B. J. M.; Peterson, A.; Palmese, G.; Gogotsi, Y., Covalent Incorporation of Aminated Nanodiamond into an Epoxy Polymer Network. ACS Nano 2011, 5, 7494-7502.
4. Guo, S.; Solares, S. D.; Mochalin, V.; Neitzel, I.; Gogotsi, Y.; Kalinin, S. V.; Jesse, S., Multifrequency imaging in the intermittent contact mode of atomic force microscopy: beyond phase imaging. Small 2012, 8, 1264-9.Speaker(s):