bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Metal Organic Frameworks for Thermoelectric Energy Conversion Applications
Authors: Talin Albert Alec
Published: February 2020

Motivated by low cost, low toxicity, mechanical flexibility, and conformability over complex shapes, organic semiconductors are attractive as thermoelectric (TE) materials to replace the costly, brittle, and non-eco-friendly inorganic TEs for near ambient temperature applications. Metal organic frameworks (MOFs) are extended, crystalline compounds consisting of metal ions interconnected by organic ligands forming a crystalline, nanoporous structure, share many of the attractive features of all-organic polymers, including solution processability and low thermal conductivity. A potential advantage of MOFs and Guest@MOF materials for TE applications is their synthetic and structural versatility, which allows both the electronic and geometric structure to be tuned through the choice of metal, ligand, and guest molecules. These unique features could solve the long-standing challenge of finding stable, high ZT n-type organic semiconductors, as well as promote high charge mobility as a result of the long-range crystalline order inherent in MOFs. In addition, the low atomic density and long, bridge-like bonding characteristic implies that MOFs should exhibit low thermal conductivity, which is an attractive feature for thermoelectric energy conversion. In my presentation I will review the recent advances in the synthesis of MOF and Guest@MOF TEs and discuss how the Seebeck coefficient, electrical conductivity and the thermal conductivity could be tuned to further optimize their TE performance.

This work was supported by the Sandia Laboratory Directed Research and Development (LDRD) Program. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
(Invited) Soft Mode Phase Transition and the Lattice Thermal Conductivity of PbTe-Based Materials
Authors: Savic Ivana
Published: February 2020

Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [1,2]. The underlying idea is that soft phonons lead to intrinsically low thermal conductivity, while possibly preserving electronic transport properties. Using first principles simulations, we show that driving PbTe near the transition from the rocksalt to a rhombohedral structure will significantly reduce its lattice thermal conductivity. We illustrate this concept by applying biaxial tensile (001) strain to both PbTe and its alloy with rocksalt PbSe, and also by alloying PbTe with rhombohedral GeTe [3]. Moreover, we investigate in detail how tuning the proximity to the soft optical mode phase transition via chemical composition affects the lattice thermal conductivity of (Pb,Ge)Te alloys [4]. We show that the anharmonic contribution to the lattice thermal conductivity is minimized at the phase transition due to the maximized acoustic-optical interaction. Furthermore, mass disorder shifts the conductivity minimum towards the composition at which the scattering due to mass disorder is maximized. The total lattice thermal conductivity and its anharmonic contribution vary continuously between the rocksalt and rhombohedral phases as expected for the second-order phase transition. Our results show that alloys with soft optical mode transitions are promising materials for achieving low thermal conductivity and possibly high thermoelectric efficiency.

[1] D. T. Morelli, V. Jovovic, and J. P. Heremans, Phys. Rev. Lett. 101, 035901 (2008).

[2] L.-D. Zhao et al, Nature 508, 373 (2014).

[3] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 93, 104304 (2016).

[4] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 95, 144302 (2017).
(Invited) Is Ion Migration Needed for Resistance Memory and Memristor?
Authors: Chen I-Wei
Published: February 2020

This presentation will focus on the electronic behavior of resistance switching memory, and contrast it with the expectation of ionic and Joule heating mechanisms that have received much attention in this field. Several prototypical memories, ranging from the filamentary type to the nanometallic type, will be examined. Filamentary memory is believed to undergo resistance switching along a nanoscale filament between the electrodes, and there is extreme concentration of ion and heat flow along the filament. It usually requires dielectric breakdown (called “forming”) prior to operation, or else the device remains resistive and cannot be switched. Nanometallic memory is believed to undergo resistance switching only when the film is thinner than a certain thickness, and it switches uniformly by way of voltage-trapping/detrapping of electrons, which alter the localization length of electron wave function. Although their switching and conduction mechanisms have been thought to be entirely different, one relying on ions, the other electrons, there are surprising similarities between the two, summarized below.

*Their high resistance states are insulating, and their low resistance states are metallic, having opposite signs of temperature sensitivity of resistance, as evidenced by resistivity data down to 18 mK.

*Their high resistance states are unstable under a mechanical pressure, which triggers them to switch to the low resistance states in less than a picosecond without any voltage assistance.

*Their low resistance states are stable under a mechanical pressure.

*Their on/off switching is triggered when their switching elements experience a critical voltage, which because of load sharing can be substantially less than the device voltage.

Since as-fabricated nanometallic memory is already in the low resistance state, it is also stable under a mechanical pressure. However, a virgin, highly resistive filamentary memory can be alternatively “formed” to the low resistance state by a pressure alone, without going through dielectric breakdown. This, along with pressure-triggered switching from the high resistance state to the low resistance state, provides unequivocal evidence that forming and switching can proceed without any ion migration, even in a filamentary memory.

In addition to the above, voltage-controlled switching is difficult to rationalize by the existing switching models for filamentary memory. These models all invoke a critical amount of electric current and joule heating to break and reconnect filaments, which are not likely to be voltage-controlled processes.

Lastly, filamentary memory has been used to emulate synapses. But all synaptic ion channels (Na or Ca) are voltage-gated. Therefore, the finding that both nanometallic and filamentary memories are voltage-controlled is fundamentally relevant to using memristors for neuromorphic computing.
Mercury Selective GaN HEMT Sensor for Dynamic Water Quality Monitoring
Authors: Sukesan Revathi , Chen Yi-Ting , Wang Yu-Lin
Published: February 2020

Mercury is an extremely toxic metal that can cause irreversible damage to human, animal and plant life. Even a very low level exposure to mercury over a long period of time is detrimental. Monitoring mercury levels in drinking water, water bodies and other drinks are thus necessary.

A method which can detect mercury level instantly from the sample would be ideal, but traditional methods are really complex and time consuming, they also require bulky and costly apparatus and in some cases requires really high temperature up to 10000K.

Recently ion-selective electrodes have been quite popular due to their portability and selectivity also they are economical than the traditional methods. In our research, we utilize an AlGaN/GaN high electron mobility transistor by functionalizing the gate area with a polyvinyl chloride (PVC) based ion selective membrane. The functionalized HEMT device is applied with pulsed gate bias at reference gate electrode positioned at fixed distance from the membrane functionalized channel region. The gate bias drops across the test solution, influencing the membrane on the channel where specific ionophore captures the target heavy metal ion. Thus the liquid gated HEMT’s drain current response is a function of the concentration of the target heavy metal ion. The unique sensing structure design enables direct detection of target ions in a background matrix of high ionic concentration solution. Ionophores are added to the membrane which increases the affinity to the particular ion and promises high specificity and low affinity for other ions.

The sensing technique enables direct detection of target ions in the medium using GaN HEMT and innovative biosensor packaging methodology, a portable biosensor system has been designed and fabricated which can provide quantitative detection in 10 minutes with a 10 µl sample.

The sensor showed a rapid and stable response when introduced to solutions of varying Hg2+ concentrations. An interesting thing about the technique is that that the pH of the sample is between 6-7, which helps in real time testing and have a fast response time. ISFET sensors are portable and robust, they also show better selectivity and sensitivity when compared to traditional electrochemical ion selective electrodes and for AlGaN/GaN devices with other forms of functionalization. These devices have much greater possibilities for development and integration as they don't require a reference electrode which reduces the manufacturing costs of ISFET. The development of this sensor will help in providing real time monitoring of water quality in rivers or water bodies, tap water and even beverages which will be a great boon to humankind.

Figure 1
(Invited) Low-Resistance Ohmic Contacts to Al0.45Ga0.55n/ Al0.3Ga0.7n HEMTS
Authors: Klein Brianna , Baca Albert G. , Armstrong Andrew M. , Allerman Andrew A. , Douglas Erica A. , Sanchez Carlos , Kotula Paul , Miller Mary , Reza Shahed
Published: February 2020

RF amplifiers and power electronics transistors in III-N semiconductors are increasingly important in emerging applications, as are photo-transistors for the purpose of UV light detection. Increasing the Al fraction in AlGaN-based HEMT devices enables transistors with higher breakdown voltages and a potentially favorable breakdown - specific on-resistance tradeoff. However, the advantages of higher aluminum content comes with a major challenge: Ohmic contacts with acceptable contact resistance become increasingly problematic with increasing Al content in the channel and barrier layer. We report sub 10-4 W-cm2 specific contact resistivity (rc) for a 30% channel, improving on the best prior result of approximately 2x10-4 W-cm2for AlGaN channels of at least 30% Al.

We investigated five different Ohmic contact metallizations consisting of X/Al/Y/Au where X = Ti, Nb/Ti, Zr, or V, and Y = Ni, Mo, or V (Table 1) as function of alloying conditions. The MOCVD-grown structure (Figure 1) is on sapphire substrates and consists of a 1.6 µm AlN buffer layer, a 4.15 µm thick Al0.3Ga0.7N channel layer and a 50 nm thick Al0.45Ga0.55N barrier with Rsheet of 3500 Ω/□ and ns= 5x1012/cm2. Representative current-voltage plots measured on circular TLM structures are shown in Figure 2. Ohmic contacts with average specific contact resistance (pc) of 7x10-5 Ω-cm2 (3.3x10-5 Ω-cm2) extracted from device (TLM) data were achieved with a conventional Ti/Al/Ni/Au metal stack and a multi-temperature stepped-anneal process. Structural characterization was carried out on the Ti/Al/Ni/Au contacts, to compare Ohmic and Schottky conditions. TEM was used to characterize the interaction between the contact metals and the AlGaN, revealing an interface consisting primarily of Ti-Au with limited indication of spiking behavior. Using the optimized Ohmic contact process, HEMTs with 10 µm source-to-drain spacing and 2 µm gate lengths were fabricated; current densities of 78 mA/mm were achieved, and parasitic loading of RON was reduced.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Figure 1
(Invited) GaN-Based Device Reliability Investigations through Advanced TEM Techniques
Authors: Specht Petra , Anderson Travis J , Koehler Andrew D. , Dubon Oscar D , Weatherford Todd R
Published: February 2020

Device reliability is a diverse topic, and influenced by a variety of factors. Device reproducibility which relies heavily on the specific device process, but also depends on local material quality is commonly focused on by industry. Device performance changes under operation and/or under specific environments, such as elevated temperatures, radiation or chemistry (often water-related) may have to be taken into account as well. The investigation of device performance changes can profit from accompanying TEM analysis, but only if the necessary TEM sample preparation and the imaging / spectroscopy technique do not produce their own artifacts. Examples of a device structure characterization include inter-diffusion phenomena at interfaces, diffusion of metal atoms from the device contacts, oxidation / corrosion effects and the study of extended defects as well as their impact on diffusion. AlGaN/GaN:Si HEMTs and GaN-based vertical power device structures are currently studied. This presentation will demonstrate how advanced TEM characterization can significantly improve device performance and reliability, which is enabled through advanced TEM sample preparation and electron dose control during imaging, which is particularly important at locally defective device areas.
An Investigation of Smart Aqueous Binder for Novel Solid Electrolyte Interphase Formation on Silicon in Lithium Ion Battery
Authors: Lin Pin-Ling , Wang Fu-Ming
Published: February 2020

Polysiloxane is an environmental friendly chemical that is used in adhesives, medicine, and insulations, respectively. Currently, the solid electrolyte interphase (SEI) formation on cathode and anode in lithium ion battery is important due to the ionic diffusivity enhancement and lowers the irreversibility of electrochemical reaction. Silicon is regarded as one of the most promising anodes for lithium ion battery owing to its large theoretical energy density (about 4200 mAh g−1). However, its practical application is limited by structure degradation.

In this study, a new compound has been developed, which is combined with the functions of SEI formation and particle adhesion in electrode. This new technology is used to eliminate the huge irreversible reaction on first cycle and effectively extend the cycle life. The new compound was composed by the bismaleimide (BMI) and the polymethylhydrosiloxane (PMHS) through hydrosilylation reaction (shown in Figure 1). The functionalized aqueous product provides three performances including excellent adhesion property with silicon particles on backbone structure, high ionic conductive SEI with high reduction potential on imide structure, and ionic hopping site on acrylate ethyl oxide side chain.

It is the first time that combines binder, SEI, and ionic transfer functions in one material in order to solve the uneven distribution of electrochemical reaction on silicon particles. In addition, we anticipate that the volume expansion and irreversible reaction on silicon can be effectively eliminated by this new technology. We also expect that this functionalized smart aqueous binder delivers outstanding electrochemical behavior and are looking forward to eradicate the disadvantages of silicon.

Figure 1
(Invited) 2-Dimensional Layered Materials for Si Technology
Authors: Shin Hyeon-Jin
Published: February 2020

Two dimensional (2D) layered materials are crystalline materials with layered structures, including Graphene, h-BN, and Transition Metal Di-chalcogenides (TMD’s). Each of their layers is consisting of one or a few atomic layers and they form van der Waals interactions with neighboring layers. Recently, they have been studied intensively due to their extraordinary properties, such as, flexibility and transparency. In addition, they have exceptional electronic, optoelectronic, chemical and mechanical properties. For example, Graphene has high electron mobility, chemical inertness, and thermal conductivity, while TMD has high photo responsivity. Based on their properties, many potential applications were proposed and demonstrated.

We have been investigated 2D layered materials for Si technology, but not for the active materials. We have focused 2D layered materials as interface materials due to the chemical inertness and their atomically thin nature. Especially, Graphene has been suggested as a promising material for future interconnects because of its unique electrical and chemical properties. For instance, they are good candidates for diffusion barrier.[1]

Also, they are good candidates for interface materials between metal and Si to reduce the Schottky barrier heights and contact resistance in source and drain, which is one of the most critical issues for scaling down.[2]

In this talk, we will cover and discuss the possibility of Graphene and other 2D layered materials for interconnects and contact resistance reducer in Si technology. In addition, we will also cover other potential applications based on 2D materials’ unique properties, such as, chemical inertness and atomic thick nature.

[1] L.Li et al., “Verticle and lateral copper transport through graphene layer”, ACS Nano, 9 (8), pp. 8361-8367 (2015)

[2] K.-E. Byun et al., “Graphene for true ohmic contact at metal-semiconductor junctions”, Nano Letters, 13 (9), pp. 4001-4005 (2013)
(Invited) Orientation Dependent GaAs Nanowire Schottky Solar Cells with 16% Efficiency
Authors: Han Ning , Ho Johnny C
Published: February 2020

Nowadays, high efficiency and low cost solar cell is still technological challenging. In this study, crystalline GaAs NWs with the uniform, pure <110>, <111> orientations and other different mixture ratios can be successfully prepared by tuning the catalyst thickness, nucleation and growth temperatures in the two-step chemical vapor deposition. The single GaAs NWs are fabricated into Au-Al asymmetric Schottky contact solar cells 1 by a simple ultraviolet photolithography process. The low-cost NW solar cell shows a record high efficiency of ~16% under air mass 1.5 global (AM 1.5G) illumination, which produces another record in addition to the vertically aligned p-i-n GaAs NW solar cells previously reported (40%), where the superior photovoltaic performance is attributed to the light concentrating effect of NWs with diameter in the order of light wavelength 2. Employing lift-off resists, three-layer NW parallel arrays can be easily attained for X-ray diffraction in order to evaluate their growth orientation along with the fabrication of NW parallel arrays based Schottky photovoltaic devices for the subsequent performance assessment3. Notably, the open circuit voltage of purely <111>-oriented NW arrayed cells is far higher than that of <110>-oriented NW arrayed counterparts, which can be interpreted by the different surface Fermi level pinning existed on various NW crystal surface planes due to the different As dangling bond densities.

Fig1. (a) Output IV curve of a single GaAs NW solar cell and (b) the orientation dependent PV property of printed GaAs NW array solar cells

Ref.

1. Han, N.; Yang, Z.; Wang, F.; Dong, G.; Yip, S.; Liang, X.; Hung, T. F.; Chen, Y.; Ho, J. C., High performance GaAs nanowire solar cells for flexible and transparent photovoltaics. ACS Appl. Mater. Interfaces
2015,
7, 20454-20459.

2. Krogstrup, P.; Jorgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Morral, A. F. I., Single-nanowire solar cells beyond the Shockley-Queisser limit. Nat. Photonics
2013,
7 (4), 306-310.

3. Han, N.; Yang, Z. X.; Wang, F. Y.; Yip, S.; Li, D. P.; Hung, T. F.; Chen, Y. F.; Ho, J. C., Crystal Orientation Controlled Photovoltaic Properties of Multilayer GaAs Nanowire Arrays. ACS Nano
2016,
10 (6), 6283-6290.

Figure 1
Improving Triboelectric Nanogenerator Performance Using AAO and Nanopatterned PDMS
Authors: Huynh Nghia Dinh , Choi Dukhyun
Published: February 2020

The huge environmental crises nowadays from energy consumption pushes the use of green energy grow rapidly, such as the harvest of solar, waves, wind, geothermal, or mechanical energy sources. Among these techniques, triboelectric nanogenerators (TENG) have a promising potential based on the triboelectrification and electrostatic induction. Even though, TENG have high power conversion efficiency in many previous study but low output power. Here, we focus on the improvement of the output of TENG based on anodic aluminum oxide (AAO) and nanopatterned polydimethylsiloxane (NP-PDMS) soft-lithography. The TENGs were fabricated by using PDMS with a nanoporous AAO template as a replica mold. The AAO nanopore arrays can be formed by a low cost, light weight, and simple manufacturing process. For bottom electrode, after PDMS was laminated on the nanoporous AAO arrays (PDMS/AAO), it was stuck to Aluminum foil. In order to peel off the nanostructure PDMS from AAO arrays easily, the AAO was treated by HDFS (heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane) before laminating. By changing the period and diameter of NP-PDMS, we find the optimum TENG output. For the top electrode, we test with AAO and Al foil. With external input frequency of 5 Hz and force of 5N to the TENG device, we can obtain maximum output of 520V open-circuit voltage and 24uA short-circuit current for Al foil as top electrode and 125nm period, 60nm diameter NP-PDMS as bottom electrode. This TENG device after rectification was able to drive 180 LEDs connected in series.
(Invited) Controllable Growth of High-Performance Gasb Thin Nanowires By a Surfactant Assisted CVD Process
Authors: Yang Zaixing
Published: February 2020

Although various device structures based on GaSb nanowires have been realized, further performance enhancement suffers from uncontrolled radial growth during the nanowire synthesis, resulting in non-uniform and tapered nanowires with diameters larger than few tens of nanometers. However, high-quality and uniform GaSb NWs with diameters down to 20 nm have been prepared by using a sulfur surfactant technique in our previous works, which efficiently stabilizes the high-energy sidewalls for homogenous NW formation during the chemical vapor deposition (CVD) process.1 At the same time, the growth direction and corresponding carrier mobilities are still found to be heavily depended on the NW diameters, in which the <111>-orientated GaSb NWs exhibit the highest hole mobility because of the optimized surface passivation and roughness in preference to the minimized surface traps, indicating the potential applications of these phase pure NWs for high-performance electronics.2

Using CMOS-compatible Pd catalysts and a complementary experimental and theoretical approach, we demonstrate successfully the formation of high-performance, <111>-oriented GaSb nanowires (NWs) via vapor-solid-solid (VSS) growth mode in surfactant-assisted chemical vapor deposition. When back-gated field-effect transistors (FETs) are fabricated using these NWs, the Pd-catalyzed GaSb NW exhibits a superior peak hole mobility of ~330 cm2V-1s-1, and this value is close to the mobility limit for a NW channel diameter of ~30 nm with a free carrier concentration of ~1018 cm-3, indicating excellent homogeneity of the NW phase purity, the growth orientation, the surface morphology and the electrical characteristics. Contact printing process is also used to fabricate large-scale assembly of Pd-catalyzed GaSb NW parallel arrays, and a calculated peak hole mobility of ∼65cm2V-1s-1 is achieved, confirming the potential construction and application of these high-performance electronic devices. 3

Acknowledgment

We acknowledge “Qilu young scholar” program of Shandong University.

References

1. Yang, Z.-x.; Han, N.; Fang, M.; Lin, H.; Cheung, H.-Y.; Yip, S.; Wang, E.-J.; Hung, T.; Wong, C.-Y.; Ho, J. C., Surfactant-assisted chemical vapour deposition of high-performance small-diameter GaSb nanowires. Nature Communications
2014,
5, 5249.

2. Yang, Z.-x.; Yip, S.; Li, D.; Han, N.; Dong, G.; Liang, X.; Shu, L.; Hung, T. F.; Mo, X.; Ho, J. C., Approaching the Hole Mobility Limit of GaSb Nanowires. Acs Nano
2015,
9 (9), 9268-9275.

3. Yang, Z.-x.; Liu, L.; Yip, S.; Li, D.; Shen, L.; Zhou, Z.; Han, N.; Hung, T. F.; Pun, E. Y.-B.; Wu, X.; Song, A.; Ho, J. C., Complementary Metal Oxide Semiconductor-Compatible, High-Mobility,< 111>-Oriented GaSb Nanowires Enabled by Vapor-Solid-Solid Chemical Vapor Deposition. ACS nano
2017, DOI:
 10.1021/acsnano.7b01217.
(Invited) Investigation of Single WS2 Nanotubes Leads to New Observations and Potential Applications
Authors: Tenne Reshef
Published: February 2020

Multiwall nanotubes and fullerene-like nanoparticles of WS2 are produced in large amounts, offering numerous opportunities for their investigation and future applications. While many studies focus on the preparation and studying the characteristics of bulk matrices incorporating these nanotubes, the investigation of individual nanotubes permits addressing intriguing issues, encompassing their unique optical, electrical and mechanical properties.

In this report, the author will dwell on several new studies. FET of WS2 nanotubes with liquid ionic gating demonstrated that the nanotubes become superconducting at low temperatures (5.8 K).1 The magnetoresisitance exhibited strong Little-Parks oscillations under magnetic field parallel to the tube axis, which originates from the interference of the supercurrent along the circumference of the nanotube. The coupling of the supercurrent with the magnetic field in the chiral tubes produces nonreciprocal second harmonic signal.

In another study, the electromechanical characteristics of a torsion device based on single nanotube was studied.2 A high frequency resonator with high quality factor was established. The high quality factor of the WS2-based device was attributed to the interlocking of the layers, which minimize the dissipative losses due to reciprocating motion of the nanotubes walls during torsion.

The wetting of single WS2 nanotube retracted from a liquid surface was studied experimentally and by detailed computational anlaysis.3  Nanotubes with different diameter and having open or closed tips were studied. It was found that the water does not wet the outer wall of the nanotubes. However, nanotubes with open end of diameter below say 40 nm exhibited strong interaction with the water surface. Careful analyses indicated that strong capillary action leads to the filling of the hollow nanotube core which leads to large dissipated energy during withdrawal of the nanotube.

At the end of the talk, the author will briefly deliberate on the commercial applications of fullerene-like WS2 nanoparticles in advanced lubrication and the industrial prospects of nanotubes thereof.

References

1. F. Qin, W. Shi, T. Ideue, M. Yoshida, A. Zak, R. Tenne, T. Kikitsu, D. Inoue, D. Hashizume, and Y. Iwasa, Nature Comm., DOI: 10.1038/ncomms14465 (2017).

2. Y. Divon, R. Levi, J. Garel, D. Golberg, R. Tenne, A. Ya’akobovitz and E. Joselevich, Nano Lett.
17, 28-35 (2017).

3. O. Goldbart, S.R. Cohen, I. Kaplan-Ashiri, P. Glazyrina, H.D. Wagner, A. Enyashin and R. Tenne, Proc. US Natl. Acad. Sci. (PNAS)
113, 13624-13629 (2016).
(Invited) Performance Improvement of Silver-Nanowire-Based Flexible Transparent Electrodes By Capillary-Force-Induced Cold Welding
Authors: Liu Yuan , Ren Zhifeng , Guo Chuan-Fei
Published: February 2020

Currently, silver-nanowire-based (AgNW-based) flexible transparent electrodes exhibit the most promising performance. The electrical performance of a AgNW network depends strongly on the wire-wire junction contact. Several techniques of nanowelding or soldering have been reported to reduce the wire-wire junction resistance, but these methods may either have adverse effects on the nanowires or the substrate, or require either specific facilities or additional materials as the “solder”.

We would like to report on a new method based on capillary-force-induced cold welding. We show at nanoscale, capillary force can be used as a powerful driving force to effectively induce the self-limited cold-welding of wire-wire junctions for AgNWs. The capillary-force-induced welding can be performed by a moisture application process which is simple, fast and does not require additional materials or expensive facilities. We are able to achieve a significant improvement in electrical conductivity and flexibility, but negligible changes in transparency.

We have also demonstrated that this method is effective to heal damaged AgNW films of wearable electronics. Our capillary-force-based method may also be useful in the welding of other metal NWs and the fabrication of nanostructures or smart assemblies for versatile flexible optoelectronic applications.
(Invited) Overview of ALD Synthesized MoS2 Thin Films
Authors: Baumgart Helmut , Zeng Min , Zhang Kai , Chen Xin , Lin Pengtao , Li Qiliang
Published: February 2020

Two-dimensional (2-D) layered transition metal dichalcogenides, such as MoS2, WSe2, WS2 and MoSe2 have received a lot of attention recently due to their significant thermal, mechanical, electrical and optical properties. Molybdenum Disulfide (MoS2), one of the transition metal dichalcogenides of interest for 2-D electronic systems, exhibits not only good thermal stability and high melting point, but also excellent light absorption, fast photo-response and sizeable band gap properties. Therefore, these unique properties make MoS2 a promising material for various electronic devices, photodetectors, sensors and catalysis applications.

Many techniques have been developed for the synthesis of MoS2 films on different substrates in recent years, such as pulsed laser deposition (PLD), chemical vapor deposition (CVD). However, very little is known about atomic layer deposition (ALD) synthesis of MoS2 films there are very only few references. ALD technique exhibits self-limiting atomic layer reactions in each introduced ALD cycle. It can accurately control film layer thickness stoichiometry, composition, uniformity, and sharp interface. Furthermore, ALD also can be used to deposit conformal film onto very complex structures.

Here we report on a large-area synthesis of MoS2
films on native oxide covered and hydroxyl terminated Si substrates by using the Savannah 100 ALD system from Ultratech/Cambridge Nanotech. Molybdenum hexacarbonyl (Mo(CO)6) and dimethyldisulfide (CH3SSCH3, DMDS) are employed as the chemical ALD precursors for Molybdenum and Sulfur, respectively. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 200, 230 and 250 oC. The crystal structure and chemical composition of WSe2 films were identified by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The film morphology was characterized by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The optical properties of ALD MoS2 films were analyzed by Raman spectroscopy and femtosecond transient absorption spectroscopy.
(Invited) Semiconducting Polymer-Semiconducting Single-Walled Carbon Nanotubes Hybrid Nanocomposites
Authors: Lee Dennis , Park Geonhee , Shin Hyeon-Jin , Kim Woo-Jae
Published: February 2020

Semiconducting single-walled carbon nanotubes (SWNTs) show promise as core materials for next-generation solar cells and nanoelectronic devices. However, most commercial SWNT production methods generate mixtures of metallic SWNTs (m-SWNTs) and semiconducting SWNT (sc-SWNTs). Therefore, sc-SWNTs must be separated from their original mixtures before use. In this study, we investigated a polymer-based, noncovalent sc-SWNT separation approach, which is simple to perform and does not disrupt the electrical properties of the SWNTs, thus improving the performance of the corresponding sc-SWNT-based applications. By systematically investigating the effect that different structural features of the semiconductor polymers have on the separation of sc-SWNTs, we discovered that the length and configuration of the alkyl side chains and the rigidity of the backbone structure exert significant effects on the efficiency of sc-SWNT separation. We also found that electron transfer between the semiconducting polymers and sc-SWNTs is strongly affected by their energy-level alignment, which can be tailored by controlling the donor-acceptor configuration in the polymer backbone structures. Among the polymers investigated, the highly planar P8T2Z-C12 semiconducting polymer showed the best sc-SWNT separation efficiency and unprecedentedly strong electronic interaction with the sc-SWNTs, which is important for improving their performance in applications.
(Invited) Novel Nanophotonic Devices By Engineering Light-Matter Interactions in Low-Dimensional Systems
Authors: Agarwal Ritesh
Published: February 2020

Strongly confined electrical, optical and thermal excitations drastically modify material’s properties and break local symmetries that can enable precisely tunable novel responses and new functionalities. With an emphasis on low-dimensional materials such as nanowires and monolayer MoS2, we will discuss how extreme confinement of fields interacting with materials produces new and unexpected materials response. For example, we will discuss how the strong plasmonic fields can lead to a new paradigm of nanoscale Si photonics such as optical emission in the visible region and nonlinear optical devices. Furthermore, by utilizing the fundamental symmetry breaking properties of fields, new quantum phenomena such as chirality-dependent optical and electronic properties will be discussed in non-chiral materials and utilized to enable new functionalities that are only possible in strong spin-orbit coupled materials. The role of geometry in low-dimensional systems to produce new properties in the presence of symmetry breaking fields will be discussed. Finally, effect of plasmons on light matter interactions in 2D excitonic crystals will be discussed, which can be engineered to produce novel responses such as enhanced and tunable emission, Fano resonances and strong exciton-plasmon polaritons, which can be precisely controlled by geometry and applied fields to produce novel device concepts.
(Invited) Incommensurate Epitaxy of Low-Dimensional Metal Halides
Authors: Shi Jian
Published: February 2020

In conventional chemical/ionic epitaxy, both symmetry and wavelength of lattice potentials of substrate largely determine epilayer’s structure and physical property. Thus, to grow high quality thin film, great efforts have been taken to search for proper substrate. In this talk, I will discuss our recent results on the growth of low dimensional halide perovskite via incommensurate epitaxy. It is found that lattice mismatch and crystal symmetry become insignificant in such epitaxy. The growth results seem to be connected with a static distortion wave prediction in 1977.
(Invited) Guided Nanowire Optoelectronics
Authors: Joselevich Ernesto
Published: February 2020

The large-scale assembly of NWs with controlled orientation on surfaces remains one challenge toward their integration into practical devices. During the last few years we reported the growth of perfectly aligned, millimeter-long, horizontal NWs of GaN [1], ZnO [2], ZnSe [3], ZnTe [4], CdSe [5] and other materials, with controlled crystallographic orientations on different planes of sapphire [1-5], SiC [6], quartz [7], and spinel [8]. The growth directions and crystallographic orientation of the NWs are controlled by their epitaxial relationship with the

substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. As a proof of concept for future applications, we demonstrated the massively parallel “self-integration” of NWs into circuits via guided growth [9]. Here we will show how guided nanowires with complex morphologies and heterostructures can be used for the bottom-up fabrication of nano-optoelectronic devices, including photodetectors, photodiodes and photovoltaic cells [10]
(Invited) UVPL Imaging of 4H-SiC Wafers Along the Device Processing Chain
Authors: Kallinger Birgit , Kaminzky Daniel , Edokam Francis , Berwian Patrick , Friedrich Jochen , Schöck Johannes , Bauer Anton
Published: February 2020

The market for power electronic devices is highly competitive among devices made of silicon (Si), silicon carbide (4H-SiC) and gallium nitride (GaN) – devices with best performance and reliability at lowest cost will win the race. The price of SiC devices depends on material costs and device production yield, the latter one is limited by technological and structural defects. The device performance is a matter of device design and process technology, and the device reliability is often limited by structural defects. Hence, a characterization method for structural defects in substrates, epiwafers and partially processed wafers is a keystone for development of competitive SiC power electronic devices. Photoluminescence imaging with excitation in the ultraviolet range (UVPL imaging) is an established and powerful technique for such process-accompanying measurements of SiC wafers and we will review its current status in our talk.

In this paper we will compare different UVPL imaging systems available on the market in terms of UV excitation parameters and their PL detectors and discuss their defect detection capabilities. We will present the visibility of structural defects in substrates and epiwafers with regard to their defect-specific PL wavelength as well as excitation wavelength and intensity. SiC epiwafers can be optically stressed by using high excitation intensity, i.e. the conversion of basal plane dislocations to stacking faults can be provoked in unprocessed material as shown in figure 1. This “optical stress test” can be used for material quality testing prior to device processing as well as for scientific questions such as the velocities of dislocations. Furthermore, we will present UVPL images of partially processed SiC wafers revealing technologically induced structural defects. In case of 6.5 kV PIN diodes, the UVPL measurements allow even for prediction of bipolar device degradation.

UVPL imaging provides fast, non-destructive defect characterization on full-wafer area and allows for targeted process optimization in epitaxial growth and device processing as well as for process control measurements. Defects can be tracked from substrate/epiwafer to different stages of device production. Hence, UVPL imaging is of great importance for cost reduction and reliability improvement of SiC power electronic devices.

Figure 1
Evaporation of Water from Gas Diffusion Layers
Authors: Büchi Felix N , Lal Sreeyuth , Lamibrac Adrien
Published: February 2020

Evaporation of water from gas diffusion layers (GDL) is an important process for water management in polymer electrolyte fuel cells. At temperatures around 80 °C and locally under-saturated conditions, evaporation can have a significant impact on the GDL saturation.

 However, little is known about the evaporation rates of water in gas diffusion layers. First ex-situ results at room temperature and for low saturations have been reported in [1]. But for a better understanding of the evaporation rates under real operating temperatures, generic data is required at different temperatures and saturation levels.

 Using a special setup, which is applicable also for X-ray imaging of water saturation, water surface and GDL structure, evaporation rates are determined for well-controlled gas speeds (over the surface of the GDL),, temperatures of up to 80 °C and relevant liquid saturations. Figure 1 shows data (including reproducibility) for Toray TGPH 060 material as function of temperature up to 80 °C, at a capillary pressure of 10 mbar and gas velocity of 6 m/s.

[1] I. V. Zenyuk, A. Lamibrac, J. Eller, D. Y. Parkinson, F. Marone, F. N. Büchi, and A. Z. Weber, J. Phys. Chem. C, 2016, 120 (50), pp 28701–28711

Figure 1: Evaporation rates from Toray TGPH 060 gas diffusion layer material a) as function of temperature at a gas velocity of 6.0 m/s; b) as function of gas velocity on the GDL surface at 60 °C; both for a dry gas stream and capillary pressure of 10 mbar; data shown based on preliminary set-up.

Figure 1
A Novel Oems Method to Monitor Direct Ethanol Fuel Cells in Operando
Authors: Hong Yu-Hao , Zhou Zhi-You , Zhan Mei , Wang Yu-Cheng , Sun Shi-Gang
Published: February 2020

On-line electrochemical mass spectrometry (OEMS) is a powerful analytic tool for electrochemical reactions. To prevent water into ultra-high vacuum, the inlet of mass spectrometer needs to be covered by a hydrophobic film, which makes it to detect only gaseous and volatile species. Herein, we developed a new OEMS technique based on differential vacuum liquid introduction system through a capillary, so that low-volatile species can be detected effectively. Due to small size of the capillary, the MS technique can in-operando 
monitor the working status of different components of fuel cells by changing the positions of the capillary. We detected ethanol, acetaldehyde, and acetic acid from a direct ethanol fuel cell, and found significant difference in their concentrations when the sampling inlet capillary was inserted into catalyst layer or flow field.
The result on catalyst layer shows that about 0.3M ethanol in catalyst layer was used, half was converted into acetic acid, the other half was acetaldehyde. However, when we positioned the sampling capillary into graphite flow field outlet. The generated acetic acid was detected at concentration ~0.86 mM at 45 mA cm-2
. This great difference in concentration in catalyst layer or into flow field indicates the mass transfer is not good of the MEA and the serpentine flow field. In addition, we observed considerable O2 crossover from cathode to anode to chemically oxidize ethanol into acetic acid, thus lead to ethanol waste and formation of mixed potential. 

Figure 1
(Invited) Methods for Understanding and Mitigating High Current Density Performance Losses in Low Loaded Pt-Based PEMFCs
Authors: Neyerlin Kenneth Charles , Anderson Lawrence , Chuang Abel , More Karren L. , Ahluwalia Rajesh , Mauger Scott A , Bender Guido , Pivovar Bryan S , Gu Wenbin , Kumaraguru Swami , Kongkanand Anusorn , Kocha Shyam S.
Published: February 2020

Significant advances in the development of electrocatalysts that exceed the DOE target of 440 mA/mgPt (H2/O2, 0.90V, 80oC, 100% RH, ptotal=150 kPa) for ORR activity have placed proton exchange membrane fuel cells on a promising path towards achieving the DOE target of a 0.1 mgPt/cm2
elec cathode loading by 2020. However, unanticipated voltage losses that manifest at high current density and low Pt loading have prevented the attainment of the 0.125 gPGM/kWrated 2020 target.1 While some of the observed voltage losses for low-loaded Pt electrodes (< 0.1 mgPt/cm2
elec) has been attributed to oxide dependent kinetics that manifest at the lower iR-free potentials (< 0.75V),2 it is believed that a significant portion of this unanticipated loss stems from an oxygen transport resistance local to or associated with electrochemically accessible Pt surface area.2-4 While the exact cause of this phenomenon remains unknown, studies have demonstrated that this loss both: 1) scales with total Pt surface area4 and 2) can be associated with the incorporation of ionomer into the cathode electrode.3 As such, this loss has been termed the local Pt resistance (RO2
Pt).

In this work we have applied a variety of in-situ electrochemical diagnostics across a range of material sets (e.g. electrocatalysts, carbon supports, ionomers, and membranes) in order to understand their impact on high current density operation in low-Pt loaded electrodes. Values derived for RO2
Pt will be compared to those determined from ex-situ measurements in an effort to elucidate the fundamental reasons for the observed performance loss.

Additionally, parallel approaches involving novel and state-of-the-art, electrocatalysts, electrodes and MEA designs aimed at mitigating performance loss at high current density and low Pt loading will be presented.

Acknowledgements

This work was funded through the DOE FC-PAD Consortium and by the U.S. Department of Energy under CRADA #CRD-14-539.

References

1. https://energy.gov/eere/fuelcells /doe-technical-targets polymer-electrolyte-membrane-fuel-cell-components

2. T. A. Greszler, D. Caulk, and P. Sinha, Journal of the Electrochemical Society,
159 (12), F831-F840 (2012).

3. H. Iden, S. Takaichi, Y. Furuya, T. Mashio, Y. Ono, and A. Ohma, Journal of Electroanalytical Chemistry,
694 37-44 (2013).

4. A. Kongkanand and M. F. Mathias, Journal of Physical Chemistry Letters,
7 (7), 1127-1137 (2016).
Quantitative Measurement of Local Fuel Cell Electrode Potentials
Authors: Spendelow Jacob S , Dumont Joseph H , Mukundan Rangachary , Castanheira Luis , Hinds Gareth , Borup Rod L.
Published: February 2020

Measurement of local electrode potentials in operating fuel cells can provide important information about spatial variations in performance and durability. These measurements are useful for improved understanding of localized effects associated with contaminants, startup/shutdown, electrode irregularities, membrane cracks/holes, and fuel starvation. Accurate measurement of local electrode potentials requires incorporation of multiple reference electrodes into the fuel cell hardware. By connecting external reference electrodes to the outside of the MEA using ionomer salt bridges, local potential measurements can be obtained without disturbing cell operation [1].

Contaminants such as CO can cause strong localized electrode poisoning in operating fuel cells. Incorporation of multiple reference electrodes has enabled measurement of the distribution of anode losses in a fuel cell exposed to CO, as depicted in Fig. 1. Similar techniques can be used to measure and deconvolute losses associated with the anode and cathode. However, robust ionic connectivity between the MEA and the external reference electrodes is required to enable quantitative potential measurements. Excessive impedance in this ion-conducting pathway can result in slow response times, reference potential drift, and non-quantitative measurements. This work describes improved techniques for integrating external reference electrodes, allowing more accurate quantification of local electrode potentials.

Fig. 1. Change in cell voltage and local anode potential (vs SHE) during exposure to CO gas (~400 s) and removal of CO (~1,800 s).

References

E. Brightman, G. Hinds, Journal of Power Sources 267 (2014) 160-170.

Acknowledgements

This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and the U.K. National Measurement System.

Figure 1