bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon Nanotubes As Electronic Connections Between Molecules and Substrates
Authors: Shapter Joe George , Moore Katherine , Yu Jingxian , Flavel Ben , Abell Andrew , Shearer Cameron , Ellis Amanda
Published: February 2020

Carbon nanotube (CNT) electrodes for electrochemistry were fabricated by chemical anchoring directly CNTs to various substrates to produce vertically-aligned shortened architectures. The electrodes were subsequently covalently loaded with various molecules including ferrocenes, porphyrins and peptides. The electron transfer (ET) capabilities were probed with cyclic voltammetry. Generally CNT electrodes comprised of double-walled CNTs (DWCNTs) demonstrated significantly higher peak current compared to their single-walled counterparts (SWCNTs) suggesting that DWCNTs may offer a useful alternative to SWCNTs in future electrochemical sensors and biosensors.
(Invited) Fabrication and Dielectric Properties of BaTiO3 Thin Films on Polycrystalline Ni Foils
Authors: Du Hui , Liang Weizheng , Gao Min , Zhang Y. , Chen C. L. , Lin Yuan
Published: February 2020

The BaTiO3 (BTO) thin films were deposited on polycrystalline Ni foils using a chemical solution approach. We show that the interface between the BTO film and the Ni foil plays a critical role in determining the dielectric and magnetoelectric (ME) properties of this flexible assembly. A pretreatment process of nickel foils with hydrogen peroxide (H2O2) solution has been conducted to form thin nickel oxide layers on the surfaces for the growth of BTO films. Both the concentration of H2O2 solution and the pretreated time were found to strongly affect the dielectric constant of BTO films, which is likely to be related to the evolution of nickel oxide at the interface during the growth of BTO films. The growth dynamics was systematically studied to optimize the single phase BTO films with good dielectric properties. Our results have demonstrated that not only a well-controlled interface between the BTO film and the Ni substrate can be achieved, but also an good dielectric and ME performance can be accomplished.
An Etch Stop and Sacrificial Materials Study for 3D NEMS-CMOS Co-Integration
Authors: Philippe Julien , Ouerghi Issam , Pollet Olivier , Hentz Sébastien , Arcamone Julien , Ernst Thomas
Published: February 2020

In this paper, we compare several materials that can be used as sacrificial and etch stop layer for 3D NEMS-CMOS co-integration.

During these last 10 years Very Large Scale Integration techniques, well developed for transistors, have been used for the M/NEMS world (Micro/Nano Electro-Mechanical Systems). This allowed the construction of various kinds of miniaturized nano-sensors, especially resonators that can be used for gas sensing, mass spectrometry and molecules recognition for example [1,2]. Nevertheless, the use of such nano-sensors requires a CMOS electronic circuit for the readout part and for the oscillating-loop creation with the nano-resonator. The CMOS part can be on a separated die linked to the NEMS resonator using wire bonding connections (Fig.1-a).

However, in order to reduce signal transmission loss and allows the fabrication of a sensor with a high density of resonator, the best approach consists in processing both NEMS and CMOS on the same die (Fig.1-b and c) [3,4], in particular following a 3D scheme (Fig.1-c) [5,6]. The NEMS release process, commonly performed by a HF (Hydrogen Fluoride) vapor etching operation, is the final step of the integration. It implies the selection of two kinds of materials. The first one is a sacrificial layer in order to achieve a well-controlled and clean release of the mechanical structure. The second one is an etch stop layer located between the NEMS and CMOS for protecting all the area under the nano-resonator (Fig.2). In such 3D approach, etch stop and sacrificial layers can be deposited at high temperature before the molecular bonding of the mechanical part with the electronic part.

Two parameters are critical for the selection of those materials; the etch rate and the presence and dimensions of the residues (see Fig.3, 4, 5 and 6) which may result from a silicon-fluorine complex precipitation during the etching process [7]. These residues must not disturb the mechanical resonator during its oscillations.

This paper will present results about etch-rates and residual dimensions on HF vapor etching of materials used for the NEMS realization. It is shown in Fig.3 and Fig.6 that USG (Undoped Silicon Glass) and SiH4, usually used in back-end processes, are not suitable for the vapor HF release contrary to TEOS (tetraethyl-orthosilicate) Low Rate (LR) and boron-nitride (BN). Our study shows that the latter constitutes a good etch stop layer for HF vapor processing within a 3D integration scheme.

[1] J. Arcamone et al, IEDM 2011 IEEE International, pp.29.3.1, 29.3.4, 5-7 Dec. 2011

[2] Y.T. Yang et al, Nano lett., Vol. 6, No. 4, Apr. 2006

[3] E.Ollier et al, MEMS 2012

[4] J. Arcamone et al, Tech. Digest. IEDM 2012, pp. 15.4.1-15.4.4

[5] P.Batude et al, J.Emerging Sel. Top.Circuits Syst, Vol.2, No.4, Dec. 2012

[6] J-C. Souriau, IEEE Trans. Compon., Packag. Manuf. Technol. Vol. 1, No.6, June 2011

[7] H. Ritala et al, ECS Trans. 2009 25(5): 355-358
Comparison between Equilibrium Voltage Step and Charge Pumping Techniques for Characterizing near Si-SiO2 Interface Traps
Authors: Guenifi Naima , Bauza Daniel
Published: February 2020

In conventional MOS structures, the interface region between silicon and its oxide contains defects which play an important role in the performance and reliability of the devices by acting as charge traps. Traps having response times greater than that of conventional (and so-called “fast”) interface traps are known as slow traps. These traps are supposed to exchange charges with the semiconductor by tunnelling, their response time depending on their energy level and distance from the interface. Due to their wide range of response times and depending on the measurement technique employed, they are often indistinguishable from fast ones or from fixed oxide charges.

Several techniques have been used to characterize electrically these different kinds of traps in MOS capacitors or MOSFET structures. Recently, the study on MOS capacitors of current transients that follow the application of a small gate voltage step due to the change in the charge state of the traps has been proposed by Tanner & al. [1]. The technique was used to characterize traps generated in the oxide by constant current stressing [1]. A few years later, Spillane et al. [2] introduced the McWhorter tunnelling model [3] to evaluate oxide depth from the traps time constant and study the slow states density versus oxide depth.

In this paper, after recalling the principle of the Equilibrium Voltage Step (EVS) technique, this technique is used to extract the profile in three dimensions (Nt, x, E) of the trap density near the Si-SiO2interface of a lightly stressed MOS structure (Fig. 1). This has been achieved using the models depicted in Refs. [2] and [4] using a device with a rather low interface states density.

These results are summarized in two dimensions (Nt,x) in Fig. 2 where they are compared with those obtained with charge pumping (CP) [4], [5]. A good coherence between the results recorded using the two techniques is obtained. The traps densities measured using EVS are in the same order of magnitude as those obtained with CP. Both trap profiles show the same tendencies even though their shapes are somewhat different. From that point of view, CP may be more efficient in distinguishing between interface and near oxide traps. However, the shape of trap profiles extracted previously using current-DLTS [6] is very close to that measured here using EVS method. In this paper, these results will be compared and discussed.

[1] P. Tanner, S. Dimitrijev, and H.B. Harrison, Electronics Letters, 31, 1880 (1995).

[2] M. P. Spillane, S. Taylor and M. J. Uren, Microelectronic Engineering, 48, 155 (1999).

[3] A. L. McWhorter, Semiconductor Surface Physics, Univ. Pennsylvania Press, 1957, pp. 207-228.

[4] D. Bauza and Y. Maneglia, IEEE Trans. Electron Devices, 44, 2262 (1997).

[5] Y. Maneglia, F. Rahmoune and D. Bauza, J. Appl. Phys., 97, 145021 (2005).

[6] D. Bauza, J. Appl. Phys., 84, 6178 (1998).
(Invited) Monolithic Three-Dimensional Integrated Circuits: Process and Design Implications
Authors: Geng Hui , Maresca Luke , Cronquist Brian , Or-Bach Zvi , Shi Yiyu
Published: February 2020

Conventional three-dimensional integrated circuits (3D ICs) stack multiple dies vertically for higher integration density, shorter wirelength, smaller footprint, faster speed and lower power consumption. However, the through-silicon-vias (TSVs) in die-stacking based 3D ICs are large in size (>1um) and reduce the benefits that can be attained by the technology. In this paper, we will introduce a new fabrication process that facilitates monolithic 3D die integration and yields much smaller TSVs (~50 nm). We will also discuss various design benefits brought by the monolithic 3D IC technology.
(Invited) Effects of Annealing Process on Morphology, Crystal Structure, and Photoelectrical Properties of CZTS Thin Films
Authors: Cao Lei , Sui Jing , Zhang Qian , Zhu Qianqian , Dong Hongzhou , Dong Lifeng
Published: February 2020

In this study, Cu2ZnSnS4 (CZTS) thin films were directly formed on FTO glass substrates by a simple ethanol-thermal method, and the films were annealed at 823K under different atmosphere environments. Field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), and photoelectrical measurements were used to investigate effects of annealing parameters in surface morphology, chemical compositions, and crystal structures of the thin films as well as photoelectrical properties, respectively. All the thin films were smooth and compact, but their color and micro-topography were different through different annealing atmosphere. The addition of sulfur during the annealing process is necessary to tailor sulfur concentration in the films and the crystallization of CZTS phase. Furthermore, the films exhibited p-type semiconductor characteristics, which is desirable for their applications in thin films solar cells.
A Cahn Hilliard Modeling of Metal Oxide Thin Films for Advanced CMP Applications
Authors: Basim G. Bahar , Sengul Yasemin , Karagoz Ayse
Published: February 2020

Chemical mechanical planarization (CMP) process enables topographic selectivity through formation of a protective oxide thin film on the recessed locations of the deposited metal layer while a continuous chemical oxidation reaction is followed by mechanical abrasion takes place on the protruding locations [1]. This paper demonstrates a new approach to CMP process optimization in terms of slurry formulations by analyzing the nature of the protective metal oxide nano films and modeling their growth through a Cahn Hilliard Equation (CHE) approach [2]. The commonly utilized models on the material removal rate calculations are also revisited to address the differences from the typical approaches to the dynamic removal models at the elevated layers.

In previous work we have demonstrated the formation and mechanical properties of the chemically modified metal oxide thin films in CMP and discussed the stresses develop at the interfaces delineating the stability and protective nature of the chemically altered films [3]. The preliminary model is developed on the very well established tungsten films by assuming a constant temperature and metal oxide thin film volume to apply the CHE approach. As the CMP nano metal oxide films of tungsten have been shown to be self-protective, these assumptions are valid. Thin film analyses were conducted through XRD, XRR and FTIR characterization and compared to the theoretical calculations for the modeling simulations. The atomic force microscopy surface structures observed on the tungsten wafers are shown to be predicted by the CHE approach as spinodal decomposition structures. This new modeling approach is going to help in formulating optimal slurries for the new generation CMP materials by considering not only the active material removal concept but also the very critical planarization requirement through enabling protective oxide layers for planarization.

References:

Kaufman, F.B., Thomson, D.B. Broadie R.E., Jaso, M.A., Guthrie, W.L., Pearson, M.B., Small, M.B., Journal of the Electrochemical Society, 138,(1991) 3460.

Gamerio, M., Mischaikow, K., Wanner, T., Acta Materialia 53 (2005) 693-704.

Basim, G.B., Karagoz, A., Ozdemir, Z. ECS Trans. “Metal Oxide Nano Film Characterization for CMP Optimization” 50 (39) (2013) 3-7.
UV VIS Studies of Ce(III)/Ce(IV) Redox Reactions to Understand Abrasive and Work Surface Interactions during STI CMP
Authors: T.D. Umashankar , Ramanathan S
Published: February 2020

In the current fabrication processes for integrated chips (IC), shallow trench isolation (STI) scheme is used to isolate and insulate transistors. Chemical mechanical planarization (CMP) is a key step in STI integration scheme. High selectivity slurries (HSS) which yield high silicon dioxide to silicon nitride polish rate are often used, and this is achieved by controlling the solid loading and the chemical concentration of additives [1]. Ceria based slurries with suitable additives are known to provide high selectivity [2, 3, 5]. Ceria is known to interact chemically with the silicon dioxide surface during CMP [2,3] and it is proposed that Ce(III) might play a role in the chemical interaction [4], but the exact nature of the species interacting with oxide are not known.

Addition of hydrogen peroxide (H2O2) to ceria based slurries is reported to suppress the oxide and nitride removal rates [4]. However, when other abrasives are employed, hydrogen peroxide does not modify the removal rate significantly [4]. Ce (III) and Ce (IV) ions are known to exhibit UV VIS absorption peak at different wavelength regions.

In this work a study on the kinetics of Ce (III) and Ce (IV) ions in the presence of H2O2is undertaken using UV-Visible Spectroscopy. Ce (III) chloride and Ce (IV) sulfate are employed for the initial study and the effect of hydrogen peroxide on changing the equilibrium composition is analyzed. The absorption is also monitored as a function of pH and time, to understand the dynamics of the system. Further, ceria abrasives are employed in the study and the nature of the species dissolving into the aqueous medium in presence of and absence of hydrogen peroxide, at different pH values are analyzed. The results show that the equilibrium between Ce (III) and Ce (IV) is shifted by the hydrogen peroxide which may help explain the suppression of oxide and nitride CMP rate by the addition of peroxide to ceria based slurries.

References:

1. J.M. Steigerwald, S.P. Murarka, R.J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials, John Wiley and Sons, New York, 1997.

2. L.M. Cook, Chemical process in glass polishing, J. Non-Cryst. Solids 120 (1990) 152–171.

3. L.Y. Wang, K.L. Zhang, Z.T. Song, et al., Ceria concentration effect on chemical mechanical polishing of optical glass, Appl. Surf. Sci. 253 (2007) 4951–4954.

4. R.Manivannan, S.Ramanathan, Appl Surf Sci.,Volume 255 (2009), 3764–3768

5. Naresh K. Penta, B.C. Peethala, S.V.Babu et al, Role of Hydrogen Bonding on the adsorption of several amino acids on SiO2 and Si3N4 and selective polishing of these materials using ceria dispersions, Colloids and Surfaces A : Physiochem. Eng. Apects 429(2013)67-73
(Invited) Significant Enhancement of High-Ns
Electron Mobility in Ge n-MOSFETs with Atomically Flat Ge/GeO2 Interface
Authors: Lee Choonghyun , Nishimura Tomonori , Lu Cimang , Zhang Wenfeng , Nagashio Kosuke , Toriumi Akira
Published: February 2020

The demonstration of high mobility Ge n-MOSFETs has been a great challenge for high-performance Ge CMOS, because electron mobility was limited by a large amount of interface states density (Dit
) near the conduction band edge, which acts as Coulomb scattering centers. We significantly reduced Dit
(<1011 eV-1cm-2) at the Ge/GeO2 interface with high-pressure oxidation (HPO), and very high electron mobility in Ge n-MOSFETs was demonstrated thanks to the dramatic reduction of Coulomb scattering. However, the rapid degradation of high-Ns
electron mobility in Ge n-MOSFETs is still one of the greatest concerns in Ge CMOS technology. Although there are many possible origins so far considered, the degradation mechanism is still unclear. In this work, we present (i) the atomically flat Ge(111) surface, followed by (ii) the layer-by-layer oxidation for achieving the atomically flat Ge/GeO2 interface, and examine the effect of surface roughness (SR) scattering on high-Ns
electron mobility in Ge n-MOSFETs. The layer-by-layer oxidation on atomically flat Ge(111) surface was investigated. It was found that surface roughness was formed by conventional oxidation process, while low-temperature high-pressure oxidation provided the atomically flat Ge/GeO2 interface. On the basis of this understanding, we have examined the effect of surface roughness scattering on high-Ns
electron mobility in Ge n-MOSFETs. The highest electron mobility of 582 cm2/Vs at Ns
=1x1013 cm-2 was achieved by the formation of atomically flat Ge/GeO2 interface thanks to the layer-by-layer oxidation.
(Invited) Reducing EOT and Interface Trap Densities of High-k/III-V Gate Stacks
Authors: Chobpattana Varistha , Mates Thomas , Mitchell William , Zhang Jack , Stemmer Susanne
Published: February 2020

In0.53Ga0.47Asis currently being investigated as a channel material for III-V metal-oxide-semiconductor field effect transistors (MOSFETs) for post-Si complementary CMOS applications. A major obstacle facing the development of MOSFETs with In0.53Ga0.47As channels is the high density of traps (Dit
) at the gate dielectric/III-V semiconductor interface. Interface trap densities are relatively straightforward to detect for In0.53Ga0.47As channels, because of its narrow band gap (0.75 eV); at room temperature they cause, for example, a frequency-dependent hump in the depletion region of capacitance-voltage (CV) characteristics, and peaks in normalized conductance maps. To reduce the large trap densities at interfaces between In0.53Ga0.47As and the gate dielectric (typically Al2O3 or HfO2) various surface preparation and passivation methods are currently being investigated in the literature.

In this presentation, we will discuss the development of novel, in-situ surface preparation methods prior to atomic layer deposition (ALD) of gate dielectrics on In0.53Ga0.47As, and their influence on EOT scaling and Dit
. The surface treatments employed here consist of in-situ cleaning involving alternating cycles of nitrogen plasma and trimethylaluminum (TMA) pulses prior to ALD of different dielectrics. We demonstrate metal-oxide-semiconductor capacitors (MOSCAPs) with HfO2 and HfO2/Al2O3 dielectrics with very low Dit
(in the low 1012 cm-2 eV-1 range) that can be scaled to sub-nm EOT (accumulation capacitances exceeding 3 µF/cm2 at 1 MHz for n-In0.53Ga0.47As). To elucidate the mechanisms by which these novel surface preparation methods allow for reducing both Dit
and EOT, we present complementary studies of the MOSCAP electrical properties (GV and CV), interface chemistry and surface morphology using XPS, TEM, SIMS and atom probe tomography, as a function of surface cleaning parameters. Because the surface preparation methods employed here allow for both low Dit
and low EOT, well behaved gate stacks can also be obtained on p-In0.53Ga0.47As channels. Comparisons of the CV results for p- and n-channels with calculated CV allow for analysis of the limitations of currently employed methods of Dit
analysis, such as the Terman method.
(Invited) 2.5D Advanced System-in-Package: Processes, Materials & Integration Aspects
Authors: Shenoy Ravindra V , Lai Kwan-Yu , Gusev Evgeni
Published: February 2020

Over the next several years, the semiconductor roadmap presents several disruptive challenges:

the cost structure in terms of normalized cost per gate is changing dramatically. The conventional cost reduction from one node to the next is diminishing. Some of the advanced nodes are expected to end up costing more than the previous nodes due to increased process complexity and lithography steps

increasing number of features and new “More than Moore” functionalities need to be integrated

the end of CMOS scaling is going to lead to new devices & technologies for continuation of Moore’s law

While the above presents challenges for the conventional system on chip (SOC) integration, it presents an opportunity for advanced “2.5D system in package (SIP)” integration where multiple chips could be integrated on an interposer.

Silicon interposer technology with through silicon vias has been extensively explored and evaluated for 2.5D SIP integration. Successful integration of memory and logic chips on silicon interposer has been demonstrated. However, the commercialization of this technology into high volume mobile/consumer applications has been limited by cost. A number of alternative technologies are being actively explored for interposers. The talk will review new materials, processes and integration aspects of interposer based 2.5D advanced system in package.
Stabilizing Redox Active Molecules in Aqueous Electrolytes: A Path through Molecular Spectroscopy
Authors: Murugesan Vijayakumar , Li Bin , Nie Zimin , Wang Wei , Hu Jianzhi , Sprenkle Vincent
Published: February 2020

Redox flow batteries (RFB) are promising Large scale energy storage device (>kWh) which are compatible with intermittent energy conversion devices such as solar and wind energy. The efficiency of RFB critically depends on the chemical and thermal stability of redox active species (such as vanadium–bromide, iron–chromium, zinc–bromine, and all-vanadium) dissolved in aqueous electrolyte medium. Evidently, research efforts are fueled towards designing better electrolytes which can host stable redox active species through wide temperature and electrochemical window. In this aspect, our research group recently reported mixed acid based electrolyte system which offers better energy density through stabilizing vanadium (V) species in the aqueous solution [1]. This novel electrolyte design mainly arises from the synergetic effort between electrolyte synthesis efforts and fundamental studies about the molecular structure and stability of redox active species. A comprehensive analysis of chemistry behind redox active species in aqueous electrolyte system is done through combining experimental molecular spectroscopic studies (optical, magnetic resonance and X-ray spectroscopy) with density functional theory (DFT)-based simulations [2-4]. In this presentation, a detailed molecular level insight about the structure and dynamics of redox species in aqueous electrolytes gained through molecular spectroscopy will be presented. This fundamental insight will enhance the ability to predict the chemical stability of redox active molecules within aqueous electrolyte which is crucial in designing better aqueous electrolyte for RFB systems.

Acknowledgements
: The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery and Energy Reliability (OE) (under Contract No. 57558). We also are grateful for insightful discussions with Dr. Imre Gyuk of the DOE-OE Grid Storage Program. The spectroscopic work was conducted in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at PNNL. Pacific Northwest National Laboratory is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

References
:

Li, L., Kim, S., Wang, W., Vijayakumar, M., Nie, Z., Chen, B., Zhang, J., Xia, G., Hu, J., Graff, G., Liu, J. and Yang, Z., Adv. Energy Mater., 1(2011), 394–400

M. Vijayakumar, L. Li, G. Graff, J. Liu, H. Zhang, Z. Yang, J.Z. Hu, J. Power Sources, 196 (2011) 3669-3672.

M. Vijayakumar, L. Li, Z. Nie, Z. Yang, J. Hu, Phys. Chem. Chem. Phys, 14 (2012) 10233-10242.

M. Vijayakumar, W. Wang, Z. Nie, V. Sprenkle, J. Hu, J. of Power Sources, 241 (2013) 173-177.
Predicting the Solubility of Organic Redox Molecules in Non-Aqueous Flow Battery Electrolytes
Authors: Murugesan Vijayakumar , Wang Wei , Wei Xiaoliang , Cosimbescu Lelia , Xu Wu , Sprenkle Vincent , Liu Jun
Published: February 2020

Large scale energy storage systems are essential to sustain an electrical supply grid which are powered by multiple energy conversion technologies including intermittent sources such as solar and wind energies. Redox Flow Batteries (RFB) is emerging as potential candidate for such a grid scale energy storage systems. Typically, the efficiency and energy density of the RFB depends on the operating voltage and solubility of redox active species in electrolyte system. The restriction in operating voltage associated with aqueous electrolytes fueled research towards non-aqueous organic electrolyte system which have wider electrochemical window. However, the solubility of redox active species in non-aqueous electrolyte system still remains as a major hurdle in realizing the commercial non-aqueous RFB technology. Apparently, the search for optimal electrolyte system and redox active species is actively pursued. In this regard, our research group developed a novel ferrocenium based ionic liquid as redox active species. The solubility of these ferrocenium species in traditional carbonate and/or ether based solvents depend on the solvation structure and dynamics of electrolyte system. To unravel the solvation structure and possible dissolution mechanism we carried out density functional theory (DFT) based calculations and verified the results with X-ray and Nuclear Magnetic Resonance (NMR) spectroscopic measurements. This powerful combination of theory and experimental measurements helped us to derive the solubility mechanism of redox active species in non-aqueous electrolyte system. Our efforts are focused towards developing a predictive level model which can aid in rational designing of optimal redox molecule and solvent systems for non-aqueous RFB technology.
Development of Non-Aqueous Redox Flow Batteries at Pacific Northwest National Laboratory
Authors: Wei Xiaoliang , Wang Wei , Cosimbescu Lelia , Xu Wu , Vijayakumar Murugesan , Liu Tianbiao , Shao Yuyan , Liu Jun , Sprenkle Vincent
Published: February 2020

Redox flow battery (RFB) is considered as one of the most promising large-scale stationary energy storage technologies and has attracted much attention both academically and industrially. The unique cell architecture renders RFB with attractive advantages including separation of energy and power, exceptional design flexibility, excellent scalability and modularity, long service life, high efficiency, etc.Therefore, RFB is a well-suitable option to stabilize the power grid and tackle the intermittency of the renewable energies. Conventional aqueous RFBs have achieved significant progress such as vanadium-vanadium, iron-vanadium, zinc-bromine, polysulfide-bromine systems. However, aqueous RFBs also are confined by two major intrinsic limitations: narrow voltage window to avoid water electrolysis and relatively low concentrations of the electroactive materials, making them generally low energy density systems.

Non-aqueous electrolytes provide wider electrochemically stable voltage window and examples having cell potential of >2V have been demonstrated recently. The expansion of the operational cell potential has a direct impact on the energy density of the non-aqueous RFB system. Meanwhile, the wider voltage windows make more redox couples available as electroactive material choices and increase the possibility of using multiple electron transfer redox reactions. Therefore, a number of non-aqueous RFB systems have been proposed and investigated [1]. Most of these systems are based on organic active materials that have the advantages of convenient structural tunability and availability of different redox mechanisms.

Here we report the development of non-aqueous RFBs at PNNL with the focus on several lithium-organic RFB systems, each adopting a different organic cathode material [2]. Chemical functionalization to the organic active materials was carried out to increase its solubility targeting high energy density energy storage systems. The flow cells of these systems demonstrated excellent electrochemical performance with impressive cell efficiencies, high operational current density, and remarkable cycling stability that exceeded those of other reported non-aqueous RFBs. Effective protection of the lithium anode employed in these lithium-organic RFB systems will be discussed.

References

1. W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater., 23, 970 (2013).

2. W. Wang, W. Xu, L. Cosimbescu, D. Choi, L. Li, Z. Yang, Chem. Commun.
48, 6669 (2012).

Figure 1. A lithium – anthraquinone non-aqueous RFB: (a) CV curves of 0.25M structurally modified anthraquinone in 1.0M LiPF6/PC electrolyte with lithium foil as the reference electrode; (b) Cycling energy efficiency and discharge energy density. [2]
(Invited) Planar Field Effect Transistor Biosensors: Toward Single Molecular Detection and Clinical Applications
Authors: Wang Yuji , Sondergaard Patricia Casal , Theiss Andy , Lee Stephen C. , Lu Wu
Published: February 2020

Field effect transistor (FET) based biosensors that can translate directly specific bindings between receptors and target analytes to electrical signals have attracted considerable interests for label free, rapid, low-cost detection of biomolecules. However, it is challenging to detect biomarkers at low concentrations in physiological buffers with high ionic strengths due to the small Debye length. FET biosensors on a planar configuration are highly preferable because of the process compatibility with microelectronic manufacturing. III-nitride semiconductor materials have been considered as a promising candidate for biosensing applications due to their unique properties, such as chemical inertness, non-toxicity, and excellent thermal stability. In particular, III-nitrides FETs are ion-impermeable and highly stable in electrolytes, making them ideal for detection at ultralow concentrations in fluids with high ionic strengths. Here we present a recessed gate planar AlGaN/GaN heterojunction FET (HFET) for biodetection. The gate area of the AlGaN/GaN HFET was recessed by inductively coupled plasma (ICP) etching to shift the threshold voltage to near zero volt regime for sensitivity enhancement. As such, the device works at the subthreshold regime. The biosensing measurement was performed on a non-bias gate setting to eliminate the noise and electrochemical activities from the gate electrode. The signal-to-noise ratio of these recessed AlGaN/GaN FET biosensors is two orders of magnitude higher than that of the Si nanowire FET biosensors previously reported. Detection of streptavidin at concentration of 16 aM in PBS physiological buffer has been demonstrated. To the best of our knowledge, this is the highest sensitivity ever reported on a field effect-based biosensor in physiological buffer. These devices have demonstrated their potential in biochemical and clinical applications. We have successfully detected MIG (monokine induced by interferon gamma, an indicator of transplant rejection in patients when detected at a concentration level above nano Molar) in clinical samples. Utilizing of ion impermeable semiconductors like GaN opens up opportunities of FET-based biosensors for direct detection of biomarkers in physiological fluids at low concentrations for early disease diagnosis with excellent manufacturing compatibility.
Effect of Proton Irradiation on DC Performance and Reliability of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors
Authors: Xi Y. Y. , Hwang Ya-Hsi , Hsieh Yueh-Ling , Li Shun , Ren Fan , Pearton Stephen J. , Patrick Erin , Law Mark E , Yang Gwangseok , Kim Hong-Yeol , Kim Jihyun , Baca Albert G. , Allerman Andrew A. , Sanchez Carlos
Published: February 2020

AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for applications such as military radar and satellite-based communications systems. Circular devices, with smaller junction areas and higher effective gate width/gate length, are better suited for high power, high frequency application. Since the circular devices have been shown to improve the device performance, the effect of radiation on their reliability is an important topic. There are many studies characterizing the performance under proton, neutron, and electron irradiations for rectangular AlGaN/GaN HEMTs. However, the studies for circular-shaped devices are limited. In this work, we report the effect of proton irradiation on circular-based AlGaN/GaN HEMT. DC performance, breakdown voltage, and gate lag measurement were done to characterize the devices.

AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by molecular beam epitaxy (MBE). The layer structure included a thin AlGaN nucleation layer followed with a 2 µm thick undoped GaN buffer topped by a 25 nm thick unintentionally doped Al0.28GaN layer. Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2-based discharges. The Ohmic contacts were formed by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). Gates with the dimension of 1.5 µm × 314 µm were defined by lift-off of e-beam deposited Ni/Au metallization. The contacts were annealed at 850°C for 45 s under a flowing N2 ambient in Heatpulse 610T system. The final step was deposition of e-beam evaporated Ti/Au (300 Å/1200 Å) interconnection contacts. Devices were exposed to protons with 5 MeV and different doses of 2×1014 proton/cm2. The device DC performance was characterized using an HP 4156 parameter analyzer. The breakdown voltage was conducted with a Tektronix 370A curve tracer. FLOODS TCAD finite-element solver was used to simulate the electrical field around the gate edge to confirm the experimental result.

As shown in Figure 1, drain current IVs, IDSS, decreased from 454 to 398 mA/mm after irradiation with a proton dose of 2×1014 cm-2. Moreover, resistance between source and drain increased and extrinsic transconductance decreased after irradiation. However, the drain breakdown voltage increased from 150 to 230 V. The improvement of the breakdown voltage was due to the reduction of electric field at the edge of the gate, caused by the defects generated by proton irradiation at AlGaN/GaN interface, which was confirmed with simulation, as shown in Figure 2.

Both gate and drain pulse measurements were also performed to verify the introductions of implanted defects into the HMET structure, which will be discussed in the talk.
Impact of Annealing on Contact Formation and Stability of IGZO TFTs
Authors: Mudgal Tarun , Walsh Nathaniel , Manley Robert G. , Hirschman Karl D
Published: February 2020

Oxide semiconductors are candidate materials to replace amorphous silicon in thin-film transistors (TFTs) for LCD and OLED backplanes where the demand on performance is increasing. Indium-Gallium-Zinc-Oxide (IGZO) has shown promise because it exhibits high electron mobility and can utilize the existing fabrication infrastructure for flat-panel display manufacturing. However the acceptance of IGZO requires exceptional uniformity and stability in device performance. The necessity to anneal sputtered IGZO for improved device operation has been widely established; however the temperature, gas ambient, and process integration details vary.

In this work, bottom-gate top-contact TFTs are fabricated using sputtered IGZO. Molybdenum and aluminum were investigated as contact metals which defined the working source/drain electrodes. Annealing processes applied either before or after metal deposition were investigated. Annealing was done at 400 °C in various gas ambients including air, oxygen, nitrogen, forming gas (5% H2 in N2) and vacuum. Channel passivation materials including SiO2 and Al2O3 were also investigated. The annealing ambient and arrangement of process steps were found to have a significant influence on the contact behavior and the uniformity and stability of electrical characteristics. These details have a significant impact on circuit operation, which will be discussed along with results of test circuits including inverters and ring oscillators.
A Molecular Dynamics Simulation Study of Electrical Double Layers Formed By an Organic Solvent/Ionic Liquid Mixture at Charged Surfaces
Authors: HU Zongzhi , Vatamanu Jenel , Bedrov Dmitry
Published: February 2020

Tetraethylammonium tetrafluoroborate in acetonitrile solvent is commonly used as electrolyte in supercapacitors. For example, this electrolyte was utilized to probe an unusual capacity increase in subnanometer C-based nanopores. Understanding the electrolyte structuring near electrode is important in elucidating the mechanism of electric double layer formation and the increase of energy densities stored by capacitors. In this work we study the electric double layer structure formed at the interface between this electrolyte and an atomically flat conductive electrode surface as a function of the applied electrode potential. Both the electrode and the electrolyte were modeled as fully polarizable. The uncharged surfaces have stronger affinity for the cation than for BF4 anion. The solvent is adsorbed at the positive surface and repelled from the negative one. Based on electrolyte rearrangement at electrode surfaces we explain the unusual shape of differential capacitance which is continuously increasing in the electrode potential window of -3V to +3V.
Electrochemical Performance of Lithium-Sulfur Cells in Various Organic Electrolyte Solutions
Authors: Koh Jeong Yoon , Kim Eun Hee , Park Seong Soon , Jung Yongju
Published: February 2020

Lithium-ion batteries (LIB) have been widely used as the power source of portable electronic devices, and their application area have been recently expanded to electric vehicles and large-scale storage system. However, current LIB technologies are faced with severe limitations of cost and energy density. For this reason, a number of researchers are trying to find alternative systems to replace lithium-ion batteries. Lithium-sulfur (Li-S) batteries are considered as one of strong candidates due to high theoretical capacity (1675 mAh/g-sulfur) and low cost of active materials. It has been known that the electrochemical reactions of the sulfur cathode are fairly complicated, which consist of several charge transfer reactions and complex chemical reactions. It is commonly accepted that the physicochemical and electrochemical properties of polysulfides and lithium anodes are critically dependent on the type of organic solvent. In result, electrochemical performance of Li-S cells is strongly affected by electrolyte systems. Up to now, however, a few studies on electrolyte systems for Li-S cells have been presented. In this study, comprehensive studies to find a suitable electrolyte solution are presented. Chemical reactivity and stability of polysulfides and lithium metal in a number of organic solvents are systematically investigated, and finally optimal electrolyte systems for Li-S cells are suggested.
Effect of Waveforms of Applied Voltage on CO2 Reforming of CH4 in an Atmospheric Plasma Reaction
Authors: Nguyen Duc Ba , Lee Won Gyu
Published: February 2020

CO2 reforming of CH4 to syngas has been the subject of great interest because two greenhouse gases are consumed and several valuable products can be produced. The products, such as ammonia, methanol, acetic acid, dimethyl ether, synthetic gasoline, and so on, can be used as the basic materials for other chemical reactions. Recently and traditionally, a catalytic method, a plasma method and a combined catalytic and plasma method have been employed for the reaction of CO2 reforming of CH4.

Reaction with a catalyst is still considered an attractive method. However, the high temperature operation required for this method increases its cost. Also, the continuous formation and deposition of carbon black on the catalyst surface leads to the deactivation of the catalyst, which is the main disadvantage of this method. As an effective approach, a plasma method was suggested as a dry method of reforming methane without the concern about the formation of carbon black. Several plasma systems have been employed for CO2 reforming of CH4, such as thermal plasma, corona discharge, arc discharge and dielectric barrier discharge (DBD). Among the plasma systems, reaction by DBD has produced a relatively low degree of conversion of feed gases and selectivity of products. Therefore, many research groups have researched ways to improve the efficiency of reaction by DBD. Several research groups presented that CO2-CH4 reforming could be improved by a pulse type power supply. It was reported that the reaction by a DBD reactor was enhanced using a pulsed plasma, while the pulse width was not effective to enhance the conversion of reactants.

In this study, CO2 reforming of CH4 to syngas was performed by an atmospheric dielectric barrier discharge reactor immersed in an electrically insulating oil bath. For comparison of the effects of applied voltage waveforms on the reaction, two kinds of waveforms, namely, sinusoidal and pulse, were supplied to the plasma reactor for CO2 reforming of CH4. Several parameters such as reaction time and applied voltage were evaluated for the conversion of reactants, the selectivity of products, and energy efficiency for both waveforms of applied voltage.
Growth and Transfer of Nanowires with High Density and Aspect-Ratio Onto Flexible Substrates
Authors: Xu Cheng , Zhao Yang , Chen Shikai , Liu Jie
Published: February 2020

Nanowire-based devices have gained tremendous interest because of their potential in fabricating devices with ultra-high energy conversion and storage, such as photovoltaics, thermoelectrics, capacitors, batteries, spintronics, and piezoelectrics. Since the device performance is often directly proportional to the active surface area, fabricating vertically aligned nanowire arrays with high density and aspect ratio is important to maximize their efficiencies beyond the current limit. More importantly, many of these devices require substrates with specific properties of flexibility, transparency, and light weight. Thus, the key challenge lying ahead is not only to improve methods to maximize surface area of nanowires but also fabricate them onto a desired substrate. Currently, researchers have not been able to produce nanowire devices with maximum density. Many methods struggle with difficulties in obtaining catalytic seeds with small diameter and maintaining nanowires with small diameter during growth due to the high surface energy associated with substrates. While templated approaches provide an excellent way of demonstrating nanowire density control, it is difficult to form and remove the back electrode and integrate these nanowires into useful devices. In this study, we focused on using the anodized alumina oxide (AAO) as a template for nanowire deposition and then transferring the vertically aligned nanostructure onto a transparent flexible substrate. AAO provides maximum density with its closely-packed hexagonal pore arrangement and, by carefully tuning the experimental conditions, the pore diameter and interpore distance can be precisely controlled. To demonstrate the transfer process, nanowires with various dimensions and materials, including Au, Ag, Cu, Pt, and ZnO, were fabricated using electrodeposition. These embedded nanowires were then lifted off and transferred onto a polydimethylsiloxane (PDMS) substrate. During the AAO etching and drying process, an electrostatic repulsion (ESR) technique was applied to prevent aggregation and keep the nanowires vertically aligned. The current-voltage (IV) response before and after nanowire transfer were measured and compared to demonstrate the efficiency of the transfer process. As a result, nanowires were fabricated on a flexible substrate with densities and aspect-ratios of 1011/cm3 and 1000:1, respectively. This technique provides a new route of fabricating large-scale, cost-effective flexible optoelectronic devices.

Keywords: Dye sensitized solar cell (DSSC), ZnO nanowires, solar cell, alumina template, electrodeposition, flexible electronics
Photoelectrochemical Analysis of Nanostructured Titanium Oxide Conjugated to Organic Molecules for Hydrogen Evolution
Authors: Giraldo Manuel Louis , Shepard Cathrine , Sakthivel Tamil Selvan , Saraf Shashank , Gupta Ankur , Leuenberger Michael , Seal Sudipta
Published: February 2020

Photocatalytic water splitting using TiO2 for hydrogen applications has been limited by its inability to absorb wavelengths within the visible light range. TiO2 covalently linked with folic acid (FA) by the linker molecule 3-aminopropyl)-diethoxy-methylsilane (APTMS) has shown to exhibit photoluminescence under visible light [1]. In this study, functionalized TiO2 was used as a photocatalyst for water oxidation which was measured using a photoelectrode setup. In order to measure the photoelectric current generated by the catalyst chronoamperometry techniques were employed. In addition, the Butler-Volmer equation was used to model the relationship between the quasi-fermi energy of electron photocarriers, current density, and the ultimately the hydrogen evolution rate.

Experimentation focused on comparing the volume of hydrogen evolution induced by three different photocatalysts: commercially available TiO2 (P25), sol-gel prepared TiO2, and functionalized sol-gel prepared TiO2. P25 was used for a comparison with TiO2 and TiO2 functionalized layers. The photochemical cell configuration allowed for the efficient reduction of electron-hole pair recombination and separated hydrogen and oxygen evolution.

Platinum was placed in the acid solution as a cathode (counter electrode) and the nanostructured coatings on stainless steel were placed in the basic solution as an anode (working electrode). The two cells were separated by a nafion 211 cation exchange membrane film and a xenon lamp was directed at the anode through a quartz window in the plexiglass cell. The anode was connected to the cathode using a stainless steel wire which allowed various external bias voltages to be applied to the system. The voltages applied to the system were critical in calculating the hydrogen evolution rates.

Figure 1: Chronoamperometric comparison of P25, TiO2 , TiO2-APTMS-FA at .55 V

1. Turkowski, V., et al., Linker-Induced Anomalous Emission of Organic-Molecule Conjugated Metal-Oxide Nanoparticles. ACS Nano, 2012. 6(6): p. 4854-4863.
Effect of Varying Polyethylene Glycol Chain Length on Enzyme Activity
Authors: Neal Craig J. , Das Soumen , Saraf Shashank , Seal Sudipta
Published: February 2020

It is well-established in research literature that ceria nanoparticles (CNPs) are able to undergo redox reaction with several oxygen-containing species in single or double electron transfers. The oxidase-mimetic ability of these particles, specifically, has found use in several biosensor devices. In this study we sought to characterize and optimize the use of ceria nanoparticles coated in polyethylene glycol (PEG) for use in biosensors. Specifically, molecular weight of adsorbed polymer was varied, particles were allowed to age in solution, and each solution was assayed for catalase activity. Interestingly, this study revealed that different polymer chain lengths of PEG influence self-assembly of particles in solution, thus varying oxidase activity.