bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Fundamental Modeling of Radiation Effects in AlGaN/GaN HEMTs
Authors: Patrick Erin , Law Mark E , Li Shun , Hwang Ya-Hsi , Ren Fan , Pearton Stephen J.
Published: February 2020

AlGaN/GaN HEMTs have high radiation tolerance compared to Si and GaAs transistors. They are also well suited for high power and high frequency applications and thus may be advantageous for satellite-based communication systems. Radiation studies have shown the following DC performance changes: threshold voltage shift (positive), transconductance reduction, and saturation drain current reduction.

The basic phenomenon leading to degradation of DC performance is the creation of point defects such as Ga or N vacancies and subsequent ionization of the traps. We have successfully modeled DC degradation due to radiation effects with FLOODS, a TCAD finite-element simulator. The simulation of trapped negative charges in the GaN buffer layer explained experimental results. As Figure 1 shows, there is a positive shift in threshold voltage that increases with the concentration of negatively charged traps.

Another phenomenon that occurs in irradiated AlGaN/GaN HEMTs is an increase in breakdown voltage and apparent enhanced reliability. The fundamental mechanism for this phenomenon is unclear. We have shown that the negatively-trapped charges in the GaN buffer are not the primary mechanism and that charging at the AlGaN surface is a plausible reason. This work explores the fundamental mechanisms for charging of the AlGaN surface. We look at the transient effects of electron and hole creation during radiation and subsequent interfacial trapping mechanisms.

Figure 1. Positive shift in threshold voltage with respect to trap concentration in GaN buffer layer.
(Invited) AlGaN/GaN HEMT Reliability and Trap Detection Using Optical Pumping
Authors: Cheney David
Published: February 2020

Although the AlGaN/GaN HEMT continues to draw attention because of its high-thermal stability, extended frequency response, and high break-down voltage, it has not gained the full confidence of designers due to its inconsistent reliability. One of the prominent reliability issues that plague AlGaN/GaN HEMTs is the creation of traps that not only degrade performance, but can lead to device failure. By optically pumping the device, with different wavelengths of light in the visible spectrum (red, green, blue, and violet) and measuring the drain current response, trap energies, trap densities, and time constants are determined.

The light sources used in the optical pumping technique consist of four low power lasers: red, green, blue, and violet. The response of the drain current to the illumination is measured using high-speed data acquisition equipment with sampling rates up to 100k samples/s. In the initial experiments a device was exposed to each laser independently from longest to shortest wavelength, i.e., red then green then blue then violet.

AlGaN/GaN HEMTs with gates ranging from 0.125x150 to 0.170x150 microns were pumped before and after high temperature dc stress. The dc stress was at the bias point of VG=-2 and the channel power was maintained at one watt by adjusting the drain voltage at constant temperature of 150⁰C. Devices that degraded gradually have significant increases in the blue energy range which suggests an increase oxygen on nitrogen substitutional defects [M.A. Reshchikov and H. Morkoc; JAP 97, 061301 (2005)]. Other devices showed degradation with noisy fluctuations in drain current (up to 20 mApp), but when optically pumped, showed very little change from the pre-stress response. Trap creation is not suspected in the degradation in these parts, but may be a result of contact or final metal failure.

Trap energies can be determined by analyzing the decay of the drain current after the part has been illuminated. Curve fitting techniques determine the time constants of the decaying curve and when measured at multiple temperatures, activation energies are determined with Arrhenius analysis. An example using this technique with the blue laser (450nm) at 10 temperatures showed a trap of 0.14eV which the literature suggests a carbon on gallium substitutional [Levinshtein, et al., Properties of Advanced Semiconductor Materials : Gan, Aln, InN, BN, SiC, SiGe, 1st ed. New York: Wiley-Interscience, 2001].

The author gratefully acknowleges the support of a DOD MURI monitored by the AFOSR and colleagues who contributed to this work: Rick Deist, Dr. Brent Gila, and Prof. Steve Pearton.
Current Confinement Effect of InGaN Devices by Forming Photoelectrochemical-Oxidized GaN Nanoporous Structures
Authors: Lin Chia-Feng , Lee Wen-Che , Chen Yen-Lun , Tseng Yi-Hui , Dai Jing-Jie , Han Jung
Published: February 2020

Aperture of InGaN light emitting diode (LED) structure was demonstrated by forming oxidized GaN nanoporous structure bellow the InGaN active layer. The fabricated processes consisted of a laser drilling hole process, a photoelectrochemical (PEC) lateral etching process, and a oxidation process. The LED structures consisted of a 30nm-thick GaN buffer layer, a 2μm-thick undoped GaN layer, a 2μm-thick n-type GaN layer, 0.7μm-thick current blocking layers, 10 pairs of the InGaN/GaN multiple quantum wells (MQWs) active layers, and a 0.2μm-thick magnesium-doped p-type GaN layer. 0.1μm u-GaN/0.5μm n-GaN:Si/0.1μm u-GaN layers were designed as current blocking layers for PEC processes. The current block layer The active layers consisted of a 30Å-thick InGaN-well layer and a 70Å-thick GaN-barrier layer. High resistance oxidized GaN nanoporous structure was formed embedded the LED structure that the injection current can confine in an aperture region. The aperture regions of the InGaN devices were observed clearly in optical and scanning electron microscopy (SEM) images. The peak photoluminescence (PL) spectra of the InGaN active layers were measured at 439.5nm and 435.6nm without and with embedded oxidized GaN nanoporous structure, respectively. The oxidized GaN nanoporous structure acted a current confinement structure that has potential for vertical cavity surface emitting laser applications.
Field Effect Transistor with Electrodeposited Nanowire Channels
Authors: Matei Elena , Florica Camelia , Costas Andrea , Enculescu Monica , Evanghelidis Alexandru , Enculescu Ionut
Published: February 2020

Zinc oxide is one of the most intense studied materials these days and the applications sought range from energy production and photocathalysis to light emitting devices, sensors or logic circuits. A wide band gap (3.3 eV) semiconductor, with an excitonic binding energy of 60 meV zinc oxide is attractive besides other properties to its high occurrence/low cost and non-toxicity. The material is also remarkable from the point of view of polymorphism of its nanostructures, from belts and rods to platelets and flowers, tunable as a function of preparation method and specific conditions. Different electronic devices were fabricated using zinc oxide, among them of utmost importance being field effect transistors. ZnO nanowires were prepared by electrochemical deposition in polycarbonate ion track templates. After the deposition process the polymer templates were dissolved in dichloromethane and the nanowires were harvested by ultrasonication in isopropyl. A droplet of nanowire suspension was placed on a Si/SiO2substrate patterned with interdigitated electrodes. By means of electron beam lithography single nanowires were selected and provided with electrical contacts. We found that in order to obtain reliable electrical contacts and typical field effect characteristics the electrode deposition process needs to be adapted to the 3 D shape of the wires and that annealing and passivation treatments are necessary. Our results show an excellent behaviour of the nanowire based devices, including here typical field effect transistor behaviour, with saturated source – drain characteristics and Ion to Ioff ratios of about 104. The transport properties show carrier mobility in excess of 100 cm2/Vs.
(Invited) Crystal Defects in Wide Bandgap Semiconductors
Authors: Shenai Krishna , Christou Aristos , Dudley Michael , Ragothamachar Balaji , Singh Rajendra
Published: February 2020

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects [1]. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role crystal defects paly on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme environments. This paper will review the current state-of-the-art of material defects in SIC and GaN power semiconductors, and the potential role these defects may have on high-density power switching electronics. A review of silicon technology development and manufacturing evolution will be made to draw a parallel between the state of silicon and wide bandgap power electronics.

[1] K. Shenai et al, “Current status and emerging trends in wide bandgap (WBG) semiconductor power devices,” ECS J. Solid State Sci. and Tech.
2(8), N3055-N3063, Jul 2013
(Invited) Hydrogen Sensing Characteristics of Gallium Nitrides with Various Crystal Planes
Authors: Kim Hyonwoong , Baik Kwang Hyeon , Ren Fan , Pearton Stephen J. , Jang Soohwan
Published: February 2020

Hydrogen has driven great attention as one of the most promising alternative energies. It does not produce any harmful byproducts such as carbon dioxide and nitrogen oxide during the conversion. However, energy conversion using hydrogen is still at research stage due to the possibility of explosion in the air even though it is almost matured in the respect of scientific technology. It is very important to detect low concentration of hydrogen promptly without false alarm in that it can promote the use of hydrogen energy to daily life applications.

Recently, we demonstrated ultra-sensitive hydrogen sensors based on GaN using platinum nano-networks grown by facile solution method. Extremely large surface to volume ratio of nano-networks dramatically improved the sensitivities of the hydrogen sensors. Size, density and distribution of nano-networks can be controlled and optimized by the synthesis conditions and coating methods, and the selective area deposition of platinum nano-networks on the GaN surface was achieved for the fabrication of sensors.

Nonpolar and semipolar GaN is of great current interest due to its promise for eliminating internal electric fields, which exist in conventional c-plane III-nitrides. In the polar crystal orientations, both spontaneous and piezoelectric polarization cause the quantum-confined Stark Effect (QCSE), resulting in the reduction of radiative recombination rate in quantum wells used as active layers in LEDs due to the separation of wavefunctions of electron and hole. There have been recent many reports on growth and fabrication of LEDs with nonpolar and semipolar GaN on SiC, LiAlO2, Al2O3 and bulk GaN substrates.

In this study, we investigated dependence of hydrogen sensing characteristics on various crystal planes of GaN including conventional gallium polar c-plane (0001), nitrogen polar c-plane (000-1), a-plane (11-20), m-plane (1-100), and semipolar plane (11-22). Density functional theory indicates much higher affinity of nitrogen in GaN to hydrogen and stronger bonding. Each plane has its own surface configuration and shows different hydrogen responsivity. In case of (11-22) plane GaN, its current response to hydrogen is very large even though its surface is terminated with gallium atoms, since nitrogens position right beneath the gallium atoms.
Air-Hybrid Dbr Structure for the Improvement of Light Output Power in Algainp-Based LED
Authors: Oh Hwa Sub
Published: February 2020

In this paper, we report the air gap induced hybrid DBR-based LED (AH-DBR LED) structures for the efficient light extraction efficiency. For verifying the influence of the AH-DBR structures in 630 nm emitting LED devices, we compare two LED structures with conventional and air gap induced hybrid DBRs.

The conceptual light paths reflected from conventional DBR and AH-DBR structures are depicted in Fig. 1. The conventional DBRs mainly reflect the light within the yellow region propagating vertically. However, the reflectivity becomes negligible when the incident angle is larger than the critical angle (θc) and the only small portion of light can be extracted out of chip through side facet. For supplementing this drawback of the conventional DBR structures, air gap is inserted in the side of the DBR structures. In the AH-DBR structures, the incident light larger than the θc can be effectively reflected by the air gap to enhance the light extraction through the side facet of the chip.

To verify the performance of our proposed LED structures, AlGaInP-based LEDs with the conventional DBRs and AH-DBR structures are prepared. Fig. 2 shows the cross-sectional schematics and actual scanning electron microscope (SEM) images of the conventional DBR LED structures and the AH-DBR LED structures. The AlxGa1-xAs DBRs are well etched away to 38 mm in depth and formed relatively well-defined AH-DBR structures as shown in Fig. 2(b). Also, phosphide LED structures are not etched seriously and well maintained.
(Invited) Surface Modification of Wide Band-Gaped Anode of Dye-Sensitized Solar Cells by Ionic Liquid Molecules
Authors: Wu Wei-Lin , Chen Yijia J , Lin Ivan J. B. , Lin Ken , Huang Sheng-Fu
Published: February 2020

Dye-sensitized solar cells have attracted worldwide attention due to its unique properties such as cost effective, temperature insensitive, and with high quantum efficiency under low illuminations. Among all studies, the topic of replacement of commonly used liquid electrolyte solution to reduce solution leakage becomes one of the emerging research focuses. The replacement of solvent of electrolyte solution by ionic liquid is one of the approaches. The reduced leakage by these approaches is expected to realize yet at the expense of reduced solar cell efficiency, due to lowered charge transfer rates in the electrolyte solutions. However, we found that the partial replacement of commonly used electrolyte solution by imidazolium salt solution enhances the solar cell efficiency instead.

In this study, we report that the surface treatment of the wide band-gaped anode of titanium oxide by ionic liquid molecules accounts for at least part of the solar cell efficiency enhancement. We found that the imidazole-derived molecules act as barrier for electron-hole premature recombination between photoelectrons transported on the conduction band of anode and the holes in the electrolyte on the anode/electrolyte interface. The significant efficiency enhancement is not necessarily due to increased dye adsorption on anodes, because we can manage a reduced dye adsorption condition while the solar cell still provides enhanced efficiency.

The ligand of imidazolium salts plays a very crucial role on the effectiveness of imidazole-derived molecules on the enhancement of solar cell efficiency. The presence of sulfite ligand secures the molecule adhesion on titanium oxide through the sulfite ligand itself.

We would like to present the parameters of treatment of such sequence of imidazole and dye adsorption, the use of solvent, dip duration time, and solution concentration. From the interrogation of the experiment results, we can confirm the exact role of imidazole-derived molecules on the enhancement of the solar cell efficiency.
Resistive Memory Switching in Pulsed Laser Deposited YCrO3 Thin Films
Authors: Sharma Yogesh , Misra Pankaj , Katiyar Ram S.
Published: February 2020

The pulsed laser deposited polycrystalline YCrO3 (YCO) thin films sandwiched between two Pt-electrodes have been investigated to explore unipolar resistive switching phenomenon. As fabricated Pt/YCO/Pt structure showed a stable resistance ratio (Rratio) of four orders of magnitude (at 0.2 V readout bias) between low resistance state (LRS) and high resistance state (HRS) and non-overlapping switching voltages in the range of 0.75- 1 V and 3.4- 4.5 V, respectively. Moreover, cyclic endurance up to 50 set/reset cycles and data retention up to 103 seconds were presented to confirm the reliability of the Pt/YCO/Pt structure. The current conduction mechanisms of the device for low and high resistance states are dominated by Ohmic behavior and trap controlled space–charge limited current, respectively. The resistance switching between HRS and LRS was ascribed to formation/rupture of conductive filaments. This work provides the promising multifunctional oxide material for non-volatile memory applications.
(Invited) The Interplay of Particle Size and Doping Concentration on the Electronic Structure of Doped Quantum Dots
Authors: Wright Joshua , Lawson Stuart , Meulenberg Robert W
Published: February 2020

Understanding the role that chemical dopants play in modifying the properties of quantum dots (QDs) has been an active field of research for the last decade. For instance, certain chemical impurities can induce unique magnetic properties in otherwise non-magnetic materials, or even present a wide range of new optical properties. In this presentation, we will discuss our efforts towards investigating the effects of copper doping in CdSe QDs. Extended x-ray absorption fine structure (EXAFS) spectroscopy measurements provide conclusive evidence for substitutional doping of Cu in the CdSe lattice. EXAFS suggests the local coordination environment is reduced, likely due to surface doping. Both x-ray absorption near edge structure spectroscopy (XANES) and theoretical modeling are used to examine effects of hybridization on the conduction band minimum (CBM) in doped CdSe quantum dots (QDs). Experimentally, Cd M3-edge XANES provides evidence for a lowering of the CB minimum for Cu doped CdSe QDs that is dependent on Cu concentration. Theoretical modeling suggests the effects of hybridization between Cu and Cd atoms in the QD can explain our experimental results. The hybridization effect leads to active emissive states below the CBM resulting in tunable near-infrared photoluminescence. Our work shows that a simple chemical model can provide a predictive tool towards probing the effects of hybridization on the CB levels in QDs.
(Invited) Rare Earth Luminescence in Nanostructured Amorphous Silicon Alloys
Authors: Tessler Leandro R.
Published: February 2020

Amorphous silicon and its alloys are widely used in semiconductor industry. Due to short range order, doping can be achieved by substituting phosphorus or boron for silicon. Moreover, the optical bandgap can be increased by alloying with C, N or O or decreased by alloying with Ge. Due to its amorphous nature the network can accommodate a variety of elements up to relatively high concentrations at the expense of an increased density of defects in the bandgap. Rare earth elements (RE) have been introduced in the form of trivalent ions in amorphous silicon or its wider bandgap alloys. They present characteristic intra-4f luminescence if their chemical neighborhood is non-centrosymetric and the electric dipole forbidden transitions become partially allowed. In the amorphous alloys excitation through the network can be efficiently achieved when a RE transition corresponds to roughly half the bandgap. The RE act as nucleation centers to induce formation of Si nanocrystals under high temperature annealing.

Samples were prepared by reactive RF sputtering from a Si target partially covered with metallic RE platelets using appropriate reactive atmospheres. Annealing was used either to optimize the photoluminescence or to induce the formation of Si nanocrystals.

We will present characterization by photoluminescence and EXAFS of Er, Nd, Eu and Tb in a-Si:H, a-SiOx:H and a-SiNx:H.

The RE ions tend to occupy low coordination low symmetry sites in the network in the amorphous samples. This favors relatively strong wide photoluminescence lines. The RE present two luminescence lifetimes, the fast component determined by the host and the slow associated to the local symmetry of the RE ions.

Models for the excitation of luminescence and recombination processes and their relation with the local nanostructure around the RE ions will be presented.
(Invited) Lanthanides Fluorides Doped Nanocrystals for Biomedical Applications
Authors: Podhorodecki Artur , Noculak Agnieszka , Banski Mateusz , Sojka Bartlomiej , Zelazo Anna , Misiewicz Jan , Cichos Jakub , Karbowiak Miroslaw , Zasonska Beata , Horak Daniel , Sikora Bozena , Elbaum Danek , Dumych T , Bilyy Rostyslav , Szewczyk M
Published: February 2020

Introducing to medicine and biology concept of optical markers in tremendous way has changed the recent status of these two important disciplines. This was mainly due to strong development in imaging techniques which recently allow us to investigate both static as well dynamic properties of living cells, their components and their interactions with external factors. Recently used molecular markers including organic dyes, fluorescent proteins or chelates containing lanthanide ions have several significant limitations. One of the alternatives for molecular markers are inorganic quantum dots (ie. CdSe, CdS) which are recently commonly used in many academic works. However, even if they are much better from physico-chemical point of view, from the application point of view at this moment they use is still questionable mainly because of their high risk of toxicity and difficulties in obtaining infrared excitation/emission. One of the solutions, combining advantages of both concepts, is to make nontoxic inorganic nanocrystals doped with lanthanide ions (RENCs).

In this work, we will present optical (photoluminescence (PL), PL excitation, PL decay, FTIR, Raman) and structural (XRD, HR-TEM) results obtained for NaYF4 and NaGdF4 nanocrystals doped with Eu, Er, Tm and Yb ions. Based on obtained results, we will discuss examples of ion-ion interaction like: cross-relaxation, energy migration or up-conversion [1, 2] and influence of these interactions on resultant emission properties of RENCs. Additionally, we will discuss the effect of the nanocrystals surface and their core-shell geometry on the excitation and emission properties of lanthanides in RENCs [1, 3]. Finally, we will discuss a few examples of potential application of RENCs in bio-medicine.

Acknowledgements

The authors would like to thank the Polish National Centre for Research and Development for their financial support under the LIDER project No. 014/L-2/10.

References

[1] A. Podhorodecki, M.Banski, A. Noculak, B. Sojka, G. Pawlik, J. Misiewicz, On the nature of carriers relaxation and ion-ion interactions in ultrasmall
b
-NaYF4 nanocrystals-effect of the surface, Nanoscale 5 (2013) 429.

[2] A. Podhorodecki, M.Banski, J.Misiewicz, M. Afzaal, P. O Brien, D. Cha, X. Wang, The Multicolor light emitters based on energy exchange between Tb and Eu ions co-doped into ultrasmall
b
-NaYF4 nanocrystals, J.Materials Chemistry 22 (2012) 5356.

[3] M. Banski, M. Afzaal, A. Podhorodecki, Jan Misiewicz, Paul O'Brien, Passivation of lanthanide surface sites in ultrasmall NaYF4:Eu3+ nanocrystals, J. Nanoparticles Research 14 (2012) 1228.
(Invited) Inhomogeneous Strain in Silicon Photonics
Authors: Wehrspohn Ralf , Schriever Clemens , Schilling Jörg
Published: February 2020

A review of recent progress in the field of strained silicon photonics is presented. The application of strain to waveguide and photonic crystal structures can be used to alter the linear and nonlinear optical properties of these devices. Here, methods for the fabrication of strained devices are summarized and recent examples of linear and nonlinear optical devices are discussed. Furthermore, the relation between strain and the enhancement of the second order nonlinear susceptibility is investigated, which may enable the construction of optically active photonic devices made of silicon.
(Invited) Rare Earth Doped Metal-Oxide-Semiconductor Structures: A Promising Material System or a Dead End of Optoelectronic Evolution?
Authors: Rebohle Lars , Berencén Yonder , Braun M. , Garrido Blas , Hiller Daniel , Liu B. , Ramírez Joan Manel , Sun Jiaming , Wutzler Rene , Helm Manfred , Skorupa Wolfgang
Published: February 2020

Integrated photonics is a key technology of the 21st century, and the electrically driven, integrated light emitter is an important building block, but difficult to realize. Thus, an enormous variety of different materials and material systems have been investigated in the past, ranging from the various approaches to integrate III-V semiconductors to the different types of Si-based light emission. Within the latter group rare earth (RE) implanted MOS structures feature a high conformity with standard CMOS processes combined with the excellent optical properties of RE elements.

After more than 20 years of intense research in the field of Si-based light emission the results are mixed. With respect to important key parameters, most of all power efficiency, Si-based light emitters are not yet able to compete with their counterparts based on compound or organic semiconductors. The present contribution discusses the problems of Si-based light emitters at the example of RE-doped MOS structures, and compares various light emitter designs and their potential to overcome these problems.

In detail, the power efficiency, the operation lifetime and the operation voltage of Tb- and Er-doped MOS structures are investigated. The main electroluminescence excitation mechanism is impact excitation of hot electron which plays an ambivalent role: efficient excitation is often related with efficient defect creation. In addition, a dark zone close to the injecting interface limits the scalability towards low voltages. The excitation mechanism und thus the performance of the light emitter is affected by the structure and composition of the dielectric stack of the MOS structure. Within this study, several host materials for the RE ions, namely stoichiometric and Si-rich silicon oxide or silicon nitride; different fabrication methods, namely plasma enhanced chemical vapour deposition, ion implantation and atomic layer deposition; and the use of additional buffer or injection layers are investigated. Finally, a short perspective to potential applications is given.
A Chemically Modified Sensor for the Electroanalytical Quantification of Clomiphene Citrate in Pharmaceuticals Formulation
Authors: Jain Rajeev , Kumar Dhanjai , LAL Vikas
Published: February 2020

A sensitive electroanalytical method for quantification of clomiphene citrate in pharmaceutical formulation has been developed on the basis of the enhanced electrochemical response at glassy carbon electrode modified with nano-composite film of bismuth (III) oxide -multiwalled carbon nanotubes (Bi2O3-MWCNTs).A well developedanodic peak for the oxidation of clomiphene was obtained in acidic medium using BR buffer at modified and unmodified electrodes. The influence of optimisation parameters such as pH and optimal loading of composite mixture on the analytical performance of the sensor was evaluated. The morphological characteristics, effective electrode surface area and charge transfer properties of the developed Bi2O3-MWCNTs/GCE sensor were studied by scanning electron microscopy (SEM), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) respectively. The fabricated sensor based on the Bi2O3-MWCNTs modified electrode showed electrocatalytic activity in terms of significant improvement of the anodic current of clomephene and lowering of the detection limit. Under optimized conditions, the squarewave oxidation peak current of clomiphene was linear over the concentration range of 50 ng mL-1 to10 µg mL-1. The obtained results suggest that the developed sensor can be successfully used for the determination of clomiphene in its commercial pharmaceutical formulations.
Revealing Morphotropic Phase Boundary and Phase Transition Behavior in Strained BiFeO3 Thin Films
Authors: Chiu Chung-Hua , Liang Wen-I , Huang Chun-Wei , Chu Ying-Hao , Wu Wen-Wei
Published: February 2020

As the advance of technology, devices with multi-function have dictated the material design. Complex oxide materials, having the spatial variation of electron charge and lattice degree of freedom at interface, have attracted wide attention because of their various properties. Recently, strained bismuth ferrite (BiFeO3, BFO) thin film is the most promising multifunctional material due to its spontaneous magnetic, ferroelectric and unique electromechanical properties at room temperature. In this study, we have investigated the temperature-dependent structural and strain-driven phase transition in epitaxial BFO thin films deposited on LaAlO3 substrates. Atomic force microscopy (AFM) and X-ray diffraction spectroscopy (XRD) demonstrated that the tetragonal-like phase (T-phase) and rhombohedral-like phase (R-phase) were involved in the strain BFO thin films. High-resolution transmission electron microscopy (HRTEM) images show that the T-phase BFO has a large c-axis parameter of ~4.65 Å, whereas R-phase BFO has a relatively small c-axis parameter of ~4.05 Å. We further find that the tilt angle between R/T phases is about 4.65° along the a-axis. Using in-situ TEM, we observed that the BFO thin film exhibited R→T phase transition with increasing temperature. Upon increasing the temperature from room temperature to 200℃, we found a significant increase in the width of T-phase region with the broader morphology phase boundary (MPB). Interestingly, upon further increasing the temperature to 250~300℃, the MPB disappears gradually with the change of tilt angle. Eventually, only T-phase BFO would be preserved at high temperature. Our study directly demonstrates the detail of structural changes with temperature effect, which may provide important information for material design and promising multifunctional applications in future technology.
Influence of Bound Metals on the Electrical Properties of Single Molecule Junction Porphyrin-Imides Linked to SWNTs
Authors: Handayani Murni , Gohda Shun , Tanaka Hirofumi , Tanaka Daisuke , Ogawa Takuji
Published: February 2020

Since Aviram and Ratner proposed that a single molecule could act as a molecular rectifier, containing a donor and an acceptor separated by a sigma bond, many such molecules have been synthesized for use in devices. Among these, porphyrins are widely used as molecular electronic devices owing to their structural flexibility as well as their well-developed synthetic chemistry, which allows their physical and chemical properties to be adjusted, making them ideally suited for binding to metals. In this research, porphyrin-imide dyads with perpendicular arrangements containing different metals (zinc and rhodium) bound to porphyrin molecules have been synthesized (Figure 1) and the effect of these “center metals” on the electrical properties of single molecule junction porphyrin-imides connected to SWNTs have been investigated.

DFT calculations for porphyrin-imide dyads revealed that the HOMO and LUMO energy levels of the dyads were completely separated, indicating the absence of any intramolecular interactions between the donor and acceptor. Furthermore, these calculations also revealed that the molecular orbital of rhodium porphyrin-imide for HOMO-2, HOMO-3, and LUMO-1 were entirely localized on the rhodium center, whereas no localization on the zinc center of the molecular orbital for zinc porphyrin-imide was observed. Based on these DFT results, it was predicted that employing zinc and rhodium as center metals would result in the corresponding dyads having different electrical properties.

The connection of single molecule junctions for porphyrin-imides linked covalently to SWNTs was evidenced by the coordination of the rhodium porphyrin-imide dyad with pyridine-tert-dodec-AuNPs (~3 nm in size) used as a marker. AFM topography was employed in order to confirm that the AuNPs were attached to the molecule. Moreover, the topography revealed the presence of particles with a height of 2.6 nm, which subsequent TEM observations revealed to be part of a single-molecule junction (Figure 2c). The electrical properties of these dyads were measured by point-contact current imaging atomic force microscopy (PCI-AFM). The results for the rectification behavior of single molecule junction zinc porphyrin-imide revealed larger values, with an average rectification ratio of 26, compared to the rhodium porphyrin-imide that had an average rectification ratio of 6. This differential conductance indicated that the HOMO-LUMO gap can be controlled by changing the center metal.

References:

L. Hong, H. Tanaka, T. Ogawa, J. Mater. Chem. C, 1137 (2013)

H. Tanaka, L. Hong, M. Fukumori, R. Negishi, Y. Kobayashi, D. Tanaka, and T. Ogawa

Nanotechnology, 23, 215701 (2012).

H. Ozawa, M. Kawao, H. Tanaka, Langmuir 2007, 23, 6365-6371
Designing Hybrid Inorganic-Organic Materials By Utilizing Sol-Gel Chemistry for Batteries and Energy Storage
Authors: Huang Fei , Cornelius Chris J
Published: February 2020

The focus of this work is to investigate the morphology, physical properties, and ion transport of tetraethyl orthosilicate (TEOS)-titanium isopropoxide (TIP)-polymer hybrid membranes depending on different composition. TEOS and TIP are two precursors used in sol-gel process to prepare Si-Ti mixed materials as the inorganic component. Penta block copolymer (PBC) is used as the organic component, which is poly(t-butyl styrene-b-ethylene-co-propylene-b-styrene-co-styrenesulfonate-b-ethylene-co-propylene-b-t-butyl styrene). Unlike the previous work done by soaking membrane in inorganic sol or mixing polymer with inorganic particles, TEOS-TIP-PBC hybrid membranes were prepared from homogenous inorganic-organic solutions by controlling different TEOS: TIP molar ratio and different concentration of inorganic components in hybrid membranes. Contact angle data by Kruss DSA 100 surface analyser, water uptake, proton conductivity by Autolab potentiostat, mechanical properties by dynamic mechanical analysis (DMA), and thermal stability by thermal gravity analysis (TGA) were investigated and highly affected by the composition. The most interesting results we found is that the proton conductivity of hybrid membranes with both acid and salt form was significantly increased compared with the pure PBC membranes, which opens the potential that these novel hybrid materials can be used in fuel cells, lithium-ion battery, or vanadium redox flow battery. In the future work, other polymers with sulfonate group, such as Nafion, will be included to prepare hybrid membranes. Moreover, other characterizations and applications, such as transmission electron microscopy (TEM), small angle x-ray scattering (SAXS), methanol permeability, and single cell performance of these hybrid membranes will be further investigated.
High Density ZnO Nanowire Arrays for Dye-Sensitized Solar Cells Fabrication
Authors: Zhao Yang , Xu Cheng , Zou Yongjie , Ziegler Kirk J
Published: February 2020

Dye-sensitized solar cells (DSSCs) have been extensively studied for the past few years because of their low-cost and high-efficiency. The surface area of the photoanode, which determines the amount of photons absorbed, is one of the most critical factors to improve the efficiency of DSSCs. Although conventional TiO2 nanoparticle-based DSSCs have shown promising results, their performance has been limited to ~11% because of diffusion and back reaction problems. Due to its excellent band gap (3.37 eV) and high exciton binding energy (60 meV) properties, one-dimensional ZnO nanowire arrays are capable of overcoming the limitation of nanoparticle-based DSSCs by improving electron injection and axial electron diffusion. Based on models proposed by our group, the efficiency of DSSCs based on TiO2 core-shell ZnO nanowire arrays (40 µm in lengths, 40 nm in diameter and ~1011 /cm2 of density) can significantly surpass current efficiencies. To reach our objective, we focused on pH value control and using two-step anodized aluminum oxide (AAO) as a template to control the dimension of ZnO nanowire arrays. The synthesis was carried out using both hydrothermal and CVD methods. SEM, TEM, and XRD were applied to characterize these ZnO nanowire arrays. This technique provides a new approach to obtain vertically-aligned ZnO nanowire arrays with both high density and aspect-ratio through a large-scale process.

Keywords:

Dye sensitized solar cell (DSSC), ZnO Nanowire Arrays, AAO Template, Hydrothermal, CVD
In Situ Surface Enhanced Raman Spectroscopic Studies on Manganese Oxide
Authors: Pavlovic Zoran Miroslav , Ranjan Chinmoy
Published: February 2020

Non noble metal "earth abundant" electrocatalysts will significantly reduce the costs and make it possible to use electrochemical water splitting on a very large scale. Non noble metal catalysts can be an effective replacement even if their electrochemical activities are a fraction of their noble metal counterparts. If these catalysts can be developed to an extent where their operational lifetime would become comparable to their noble metal counterparts, a large part of the problem would be solved.

Whether in acidic, alkaline or neutral media, catalyst corrosion remains a significant challenge to oxygen evolution. MnO2 is earth abundant and well known for catalyzing oxygen evolution. A form of MnOxcluster is known to catalyze water oxidation in photosynthesis. The problem is that the catalyst comes with a significant corrosion problem.

MnO2 + 2H2O --> MnO4
- + 4H+ + 3e- E0= 1.69V vs. NHE

Besides that MnO2-xnaturally exists in various polymorphs, with various amounts of defects each of which is known to have a different electrocatalytic performance. Poor ballistic electronic conduction (important for good electrode materials) is also a prevalent issue with these oxides. These catalysts although with immense potential remain a long way from successful implementation within electrolysers. In order to understand the limitations of this catalyst under the operational conditions, it is important to study this material in situ.

Setup for in situ Surface Enhanced Raman Spectroscopy is used for tracking the structural changes of hydrous MnOx while changing the potential in the range of 0 - 2,0 V. Electrodeposition of MnOx on the surface of Au was achieved by passing an anodic current of 50 mC/cm2 at 1,6V. The resulting MnO2 phase could be termed as the alpha/MnO2 phase. The structural integrity of the MnOx was studied by following the Mn-O-Mn stretching various potential ranges in various electrolytes. Onset of phase changes and corrosion could be clearly established.
Effect of Pressure and Temperature on Nucleation and Growth Kinetics of Titanium Oxide Nanoparticles
Authors: Gupta Ankur , Das Soumen , Seal Sudipta
Published: February 2020

With the development of advanced processing and characterization tools, nanotechnology is emerging as a potential area which can deal with energy and environmental challenges. Titanium oxide nanoparticles are redox active nanoparticles and used in various applications as catalyst. The catalytic property of these nanoparticles varies with the phase, size, shape and environment. Phase and size of TiO2 nanoparticles varies significantly with the processing temperature and pressure. In this present work, TiO2 nanoparticles are synthesized at different temperature from sub-zero to 200°C with varying pressure (1atm-500psi) and time (1-144hrs). TiO2 nanoparticles are obtained by the hydrolysis of titanium isopropoxide precursor. The objective of this study is to understand the effect of pressure and temperature on nucleation and growth kinetics of synthesized TiO2 nanoparticles. These nanoparticles are characterized using x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), fourier transform infrared (FR-IR) spectroscopy, visible and ultra violet (UV-Vis) spectroscopy, and photo luminance (PL).
Trends in Adsorption Characteristics of Organic Molecules on Transition Metal Surfaces: Role of Surface Chemistry and Van Der Waals Interactions
Authors: Yildirim Handan , Kara Abdelkader
Published: February 2020

The accurate description of interface characteristics between organic molecules and metal surfaces has long been debated in theoretical studies. A well-founded description of interface geometry and adsorption energy is highly desirable for these systems. Using first principles calculations with the inclusion of van der Waals interactions, we examine adsorption characteristics of few organic molecules on several transition metal surfaces. Our aim is to obtain insights into the role of vdW interactions in the adsorption characteristics as well as to build understanding how these functional treat the adsorption in varying surface chemistries. Furthermore, the comparisons made between the results obtained using different vdW functionals for each organic molecule type provide the means to assess their performance.

This work is funded by the U.S. Department of Energy Basic Energy Science under Contract No DE-FG02-11ER16243
The Effect of Different Gel Electrolytes on Graphene Based Solid-State Supercapacitors （oral)
Authors: Chen Qiao
Published: February 2020

Graphene is a good candidate for the electrode materials for supercapacitors, as it’s excellent electrical, mechanical and optical properties. A all solid-state supercapacitors based on graphene was designed to study the effect of different electrically conductive substance (H3PO4, H2SO4, KOH, NaOH, KCl, NaCl) in polymer polyethylene terephthalate (PVA) gel electrolytes on the electrochemical performance of this supercapacitor,

Large multilayer graphene films were grown on copper foils by chemical vapor deposition (CVD) method. The few-layer graphene film grown by chemical vapor deposition was cut into squares with sides of 7~10 centimeters and was transferred onto the PET substrate directly. This supercapacitor is easily assembled through binding the two graphene electrodes together by gel electrolyte. Thus the device simply contains two film electrodes separated by solid electrolyte (gel electrolyte solidifies by water loss) and two flexible substrates, so it is ultrathin and high flexible.

The experiment demonstrated the specific capacitance of this graphene based solid-state supercapacitors varies with different electrically conductive substance in gel electrolytes with tremendous dissimilar cyclic voltammetry curves. The supercapacitors used PVA-H2SO4 electrolyte show higher device specific capacitance of 0.82mF/cm2 at a scanning rate of 100mV/s than other electrolyte. By optimization the concentration of H3PO4 from 0.01mol to 0.09mol in equal amount of PVA gel, the device specific capacitance of this supercapacitors was elevated from 0.07mF/cm2 to 2.64mF/cm2. At the same time the internal resistance decreased with the adding of H3PO4. The graphene-based supercapacitors provided imponderable changes of electrochemical properties during bend to 180-degree. This supercapacitors also possess great stability after 5000 circulations with more than 92% specific capacitance retained.
Graphene Oxide and Metalloid Polymer Hybrids Composites based Au Electrodes for Durable Glucose Sensing
Authors: Lee Min-Ho , Kim Hye Youn
Published: February 2020

A new class of functionalized graphene oxide (FGO) has been fabricated and coated on Au electrodes. FGO based amperometric glucose sensor platformhas been fabricated and tested for basic characterization. Spectroscopic characterization indicated that metalloid polymer hybrids (MPHs) are well distributed, with a strong binding affinity between the GO. The synergic features of the metalloid polymer and the GO resulted in a unique three dimensional nano-architecture on a Au based PCB. In addition to FGO, multiple layers were coated on top of FGO providing efficient blocking of interents in urine samples. The amperometric response against glucose concentration in solution shows a correlation coefficient of 0.998 and wide linear range of up to 55.5mM. The stability test were performed based on over period of 25 days and their amperometric results were compared with commercially available sensors. The applicability tests were also performed with urine samples collected from the patients with hyperglycemia.
Systems Level Approaches for on-Body Sensing Devices
Authors: Jur Jesse S , Misra Veena , Muth John F.
Published: February 2020

The NSF Nanosystems Engineering Research Center (NERC) for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) is aimed at develop and employ nano-enabled energy harvesting and storage, nanodevices and sensors to create innovative battery-free, body-powered, and wearable health monitoring systems. These sensor systems will result in a paradigm shift in health informatics enabled by wearable nanotechnologies that monitor individual health parameters and environmental exposures. Long-term sensing will enable patients, doctors, and scientists to make direct correlations between health and environmental toxins leading to chronic disease prediction, management and treatment. ASSIST advances will accelerate environmental health research and clinical trials as well as inform environmental policy. Effort will be described related to the challenges in the integration of fundamental nanotechnology research in ASSIST into practical, wearable devices. Particular focus will be placed on integration challenges as it relates to energy harvesting from the body, system level low-power sensing strategies, and improving the device-skin interface for long term wearability.
The Influence of the High Temperature Oxidation Regime on Residual Stresses in Oxide Scales on Fecral-Type Alloys
Authors: Jedlinski Jerzy , Grosseau Poussard Jean Luc , Dubiel Marcin , Zurek Zbigniew , Dąbek Jarosław
Published: February 2020

The alumina-forming FeCrAl-type commercial alloys manufactured in different ways, conventional melting (Kanthal AF), mechanical alloying (MA956) and powder metallurgy (Kanthal APM), were oxidized at 1100⁰C, in air for 5 hours under isothermal and thermal cycling conditions. Subsequently, after final cooling down the samples to the room temperature, the residual stresses in oxide scales as well at their phase composition and morphology were determined using Photoluminescence Spectroscopy and/or Scanning Electron Microscopy.

Different residual stresses were found in scales growing on various materials, under different regimes. Moreover, they depended on the region of the scale. These differences are discussed in terms of the response of the scale-substrate system to growth, structural and thermal stresses.