bluuebunny/crossref_metadata_2025 huggingface dataset:
Graphene-Coated Silicon Alloy with High Capacity and Good Cycling Stability for Lithium-Ion Batteries
Authors: Kim Sang-Hyung , Yang Hwi Soo , Yook Seung Hyun , G Kannan Aravindaraj , Kim Seon-Kyong , Park Cheol-Ho , Kim Dong-Won
Published: February 2020

The growing demand for high-performance energy storage devices for applications in portable electronic devices, electric vehicles and energy storage systems requires dramatic improvement in the energy density and cycle life. Silicon-based anode materials hold promise for these applications due to their high theoretical capacity and abundant availability for large-scale applications.  However, silicon anodes are known to have problems such as electrode pulverization due to its high volume expansion during repeated cycling and poor cycle life due to unstable solid-electrolyte interphase (SEI) layer. To solve these problems, many studies have been carried out in terms of thin film, nanostructure, alloy and composites. In this work, we synthesized Si-based alloy materials with high specific capacity. To improve their cycling stability, the silicon alloy materials were surface modified with reduced graphene oxide (rGO).  The rGO layer formed on the Si alloy played as a flexible confinement for accommodating volume changes during cycling.  And also, its high mechanical strength prevented the pulverization of the electrode.  In addition, its 2-dimensional structure enabled to electronically connect the individual Si alloy particles to the current collector. Their interfacial studies and cycling performances are investigated by electrochemical impedance spectroscopy, XPS, FE-SEM, HR-TEM and cycling test. Detailed characterization of the rGO-coated silicon alloy materials along with their electrochemical performance will be presented.
25 Mg NMR Studies of Mg-Ion Battery Materials
Authors: Wang Hao , Sa Niya , Proffit Danielle L. , Senguttuvan Premkumar , Kim Chunjoong , Cabana Jordi , Poeppelmeier Kenneth , Kinnibrugh Tiffany L. , Chapman Karena W , Burrell Anthony K. , Vaughey John T. , Key Baris
Published: February 2020

Multivalent-ion chemistries such as Mg-ion are emerging as alternative battery systems to Li-ion. Current Mg-ion chemistries are limited to relatively low voltages and relatively low reversible specific capacities (1-2). Recent research on potential high voltage Mg-ion cathode materials and alternative anode materials such as transition metal oxides and metal alloys have highlighted the urgent need to understand structure activity relationships and insertion/intercalation phenomenon for development of such systems (3). Solid state NMR is a powerful tool to investigate local structure and insertion/intercalation phenomena, particularly for batteries as shown for Li-ion chemistries with 6Li and 7Li NMR (4, 5). However, the low natural abundance (10%) of the NMR active Mg isotope (25Mg), highly quadrupolar nuclear spin of 25Mg (spin 5/2) and very low gyromagnetic ratio (i.e 30.6 MHz Larmor frequency relative to 1H = 500 MHz) limits the effective use of 25Mg NMR for solid Mg-ion battery materials (6). In this work, despite the challenges of 25Mg NMR, our recent efforts to characterize Mg environments in cathode materials such as MgMn2O4 spinels, MgV2O5, in anode materials such as h-TiO2 and Mg alloys will be presented. Chemical magnesiation using dibutylmagnesium and preliminary electrochemical (de)magnesiation and the structural changes induced will be discussed. The results will summarize the effectiveness of the method in distinguishing side reactions or undesirable conversion reactions, including amorphous phases, from intercalation phenomenon.

1. D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, and E. Levi, Nature,
407 (6805), 724-727 (2000).

2. H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour, and D. Aurbach, Energ Environ Sci,
6 (8), 2265-2279 (2013).

3. Magnesium Batteries 1 and 2, 224th Electrochemical Society Meeting, San Francisco CA, 2013

4. C. P. Grey and N. Dupre, Chem. Rev. (Washington, DC, U. S.), 104, 4493 (2004).

5. B. Key, R. Bhattacharyya, M. Morcrette, V. Seznec, J. M. Tarascon and C. P. Grey, Journal of the American Chemical Society, 131, 9239 (2009).

6. R. Dupree and M. E. Smith, Journal of the Chemical Society-Chemical Communications, 1483 (1988).
Electrochemical Reaction Mechanism in a High Capacity Zinc-Ion Battery System
Authors: Alfaruqi Muhammad Hilmy , Gim Jihyeon , Song Jinju , Kim Sungjin , Mathew Vinod , Jo Jeonggeun , Lee Seulgi , Islam Saiful , Kim Jaekook
Published: February 2020

Earlier efforts to develop an aqueous zinc-ion battery (ZIB), which facilitates energy storage/conversion via Zn-ion intercalation/de-intercalation, has paved the way not only to realizing safe and environmentally benign energy devices, but also to reducing the processing costs of next generation batteries. [1,2] From the viewpoint of developing low-cost and environmentally safe rechargeable batteries that can be easily scaled up for large-scale production, the present study reports on a high surface area mesoporous tunnel-type γ-MnO2 cathode obtained by a simple template-free ambient temperature strategy and subsequent annealing at low temperatures for ZIB applications. An in-depth study on the structural transformation in a mesoporous γ-MnO2 cathode during electrochemical reaction in a zinc-ion battery (ZIB) has been conducted.

A combination analyses of in-situ synchrotron XANES and XRD reveal that the tunnel-type parent γ-MnO2 experiences a structural transformation to spinel-type Mn(III) phase (ZnMn2O4) and two new intermediary Mn(II) phases namely, tunnel-type γ-ZnxMnO2 and layered-type L-ZnyMnO2 and that these phases with multioxidation states co-exist after complete electrochemical Zn-intercalation. On sequential Zn-de-intercalation, a majority of these phases with multi-oxidation states was observed to revert back to the parent γ-MnO2 phase. The mesoporous γ-MnO2 cathode exhibits an initial discharge capacity of 285 mAh g-1 at 0.05 mA cm-2 with a defined plateau at around 1.25 V vs. Zn/Zn2+. Further, ex-situ HR-TEM studies of the discharged electrode aided to identify the lattice fringe widths corresponding to the Mn(III) and Mn(II) phases and the stoichiometric composition estimated by ICP appear to be in agreement with the in-situ findings.

References

C. Xu, B. Li, H. Du, F. Kang, Angew. Chem. Int. Ed., 51, 933, (2012).

C. Yuan, Y. Zhang, Y. Pan, X. Liu, G. Wang, D. Cao, Electrochim. Acta, 116, 404 (2014).
Enhanced Cycling Stability of Hybrid Li-Air Batteries Enabled By Ordered Pd3fe Intermetallic Electrocatalyst
Authors: Cui Zhiming , Li Longjun , Manthiram Arumugam , Goodenough John B.
Published: February 2020

Hybrid Li-air batteries, in which a solid electrolyte separates the lithium metal anode in an aprotic electrolyte from the air electrode in an aqueous electrolyte, have attracted considerable attention because they possess high theoretical energy density and potentially overcome the problems of conventional aprotic Li-air batteries. However, current hybrid systems still suffer from poor cycling stability. The problem mainly lies in the sluggish kinetics of oxygen reduction reaction (ORR) and poor durability of the cathode catalysts, which actually is also the major limiting factor for other energy conversion and storage technologies such as fuel cells and metal-air batteries. Therefore, it is highly desirable to develop highly active and stable catalysts for the ORR in aqueous media. Herein, we report an ordered Pd3Fe intermetallic catalyst synthesized with a nanoparticle-KCl matrix method. Such structurally ordered phase can provide predictable control over structure, geometric, and electronic effect, not afforded by disordered alloys.  Further, as the order in intermetallic phases arises from the high enthalpy of mixing, a higher chemical and structural stability than found in its corresponding disordered alloy can be expected. Our results show ordered Pd3Fe/C catalysts exhibit much higher activity and durability for the ORR in alkaline media than disordered Pd3Fe/C, Pd/C, and Pt/C. The ordered Pd3Fe/C catalyst enables a long-term cycling performance of hybrid Li-air batteries over 880 h with only 0.08 V increase in round-trip overpotential. The extraordinarily high performance of ordered Pd3Fe/C catalyst provides a very promising alternative to the conventional Pt/C catalyst for air cathode in alkaline electrolyte.
Increasing the Capacity Performance in Lithium Metal Secondary Battery By Using Nafion-Coated Cathodes
Authors: Bae Ki Yoon , Kim Byung Hyuk , Yoon Woo Young
Published: February 2020

Lithium metal has a high theoretical energy density (3860 mAh g-1), which makes it a good anode material for use in secondary lithium batteries. Instead of a lithium-foil electrode, a lithium-powder electrode is used as anode material to prohibit dendrite growth and to improve on the electrochemical behavior[1], [2], When lithium-powder is used directly as a anode electrode, the non-lithiated cathode must have the ability of intercalation and deintercalation of lithium ions.

Lithium trivanadate (LiV3O8, LVO) is a promising non-lithiated cathode material in which lithium ions can inserted and removed from the LVO’s layered structure. LVO is relatively high theoretical capacity (280 mAh g-1) and good cycle stability. However, because of the low conductivities of LVO, the cell exhibits adverse effects in rate retention and degradation during cycle is progressed. Nafion is a well-known cation exchange polymer with high ionic conductivity and excellent stability, which can be coated onto the surface of cathode to solve the LVO’s drawback.

Lithium-powder particles and lithium trivanadate are synthesized by the droplet emulsion technique (DET) and solid-state method, respectively. And Nafion-coated cathode is easily synthesized by using a dip-coating technique. Dip-coating is a facile and fast process, and a simple way to generate a thin uniform film on the surface of LVO[3].

The structure, morphology, and electrochemical properties of lithium metal secondary battery’s electrode were investigated by the implementation of X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and energy dispersive spectroscopy (EDS)[4]. All cells were cycled 50 times at a C-rate of 0.2 and various C-rate with the cut-off voltage ranging from 1.8 to 4.0V (versus Li/Li+). The result was analyzed by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV)[5].

As the dip-coating the nafion is performed, layered cathode electrode has higher ionic conductivity. Thanks to effect of coating, lithium-powder/LVO cell appears better electrochemical performance, rate retention in various c-rate and mitigated degradation.

[1] J. S. Kim, W. Y. Yoon, Electrochim. Acta 2004, 50, 531–534

[2] J. H. Chung, W. S. Kim, W. Y. Yoon, S. W. Min, B. W. Cho, J. Power Sources 2006, 163, 191-195

[3] S. J. Oh, J. K. Lee, W. Y. Yoon, ChemSusChem 2014, 7, 2562-2566

[4] D. Wang, L. Cao, J. Huang, J. Wu, Cera. Inte, 38, 2012, 2647-2652

[5] Y.Q. Qiao, J.P. Tu, X.L. Wang, J. Zhang, Y.X. Yu, and C.D. Gu, J. Phys. Chem, 115, 2011 25508-25518
Discharge and Recharge Characteristics in an Mg/O2 Battery with a Mixed-Phase Discharge Product
Authors: Vardar Gulin , Sleightholme Alice , Siegel Donald J , Monroe Charles W
Published: February 2020

By combining a multivalent cation with the high capacity of a gas-breathing positive electrode, the energy density of a non-aqueous Mg/O2 battery is projected to surpass that of other ‘beyond-Li-ion’ chemistries. Nevertheless, research into Mg/O2 systems has been limited. A rechargeable, non-aqueous Mg/O2 cell that operates at room temperature will be discussed and used to clarify fundamental electrochemical characteristics, including the nature of the product phase(s) and the reaction mechanism for discharge. The discharge product differs from alkali-metal-based chemistries in that it is a mixed phase, primarily comprising crystalline MgO, with a substantial minority of MgO2. The open-circuit cell voltage is 2.0 V, lower than the theoretically expected ~2.9 V. Both the low voltage and the two-phase discharge product are consistent with a multi-step discharge reaction in which a superoxide (O2
–) intermediate forms at ~2 V vs. Mg/Mg2+. Chemical disproportionation reactions subsequently yield MgO and MgO2, but do not contribute to the cell’s electrical energy output. During the charging step, MgO2 is preferentially decomposed. Bypassing this multi-step mechanism in favor of direct electrochemical MgO
x
formation would allow for higher discharge potential and consequently higher energy density.
Enabling Rechargeable Magnesium Batteries through Bottom up Design Strategies
Authors: Mohtadi Rana , Tutusaus Oscar , Arthur Timothy S. , Mizuno Fuminori
Published: February 2020

Recently, rechargeable magnesium battery has been attracting attention as a candidate for a post Li-ion battery owing to a high volumetric capacity of 3832 mAh/ cm3, and the absence of dendrites formation which eliminates a major safety concern hampering the utilization of Li metal anodes [1].  In addition, the abundance and low cost of Mg metal makes this battery very attractive for future commercialization. Nonetheless, current Mg battery technology suffers from several drawbacks which hamper its utilization. These are primarily related to the absence of practical high voltage cathodes and the occurence of severe corrosion of the current collectors caused by the electrolytes.  In this presentation, we will explain the current challenges associated with these batteries and offer potential approaches which could be used to overcome these challenges. In addition, we will discuss our electrolyte’s development research strategy using a new class of electrolytes based on magnesium borohydride and its derivatives which resulted in the first simple type, all around and non-corrosive salt that could be used in magnesium batteries [2,3,4,5].  The electrolyte prepared was used in the first demonstration of high voltage Mg battery coin cell making these batteries a step closer to becoming practical [5].

[1] a) H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour, D. Aurbach, Energy Environ. Sci. 2013, 6, 2265-2279; b) J. Muldoon, C. B. Bucur, A. G. Oliver, T. Sugimoto, M. Matsui, H. S. Kim, G. D. Allred, J. Zajicek, Y. Kotani, Energy Environ. Sci. 2012, 5, 5941-5950; c) R. Mohtadi, F. Mizuno  Beilstein J. Nanotechnol. 2014, 5, 1291–1311.

[2] R. Mohtadi, M. Matsui, T. S. Arthur, S.-J. Hwang, Angew. Chem. Int. Ed. 2012, 51,9780 –9783.

[3] T. J. Carter, R. Mohtadi, T. S. Arthur, F. Mizuno, R. Zhang, S. Shirai, J. W. Kampf,Angew. Chem. Int. Ed. 2014, 53, 3173-3177.

[4] O. Tutusaus, R. Mohtadi, ChemElectroChem. 2014 DOI: 10.1002/celc.201402207.

[5] O. Tutusaus, R. Mohtadi et al. Angew. Chem. Int. Ed. 2015, Accepted.
Materials Design Rules for Multi-Valent Ion Mobility in Intercalation Structures
Authors: Rong Ziqin , Malik Rahul , Canepa Pieremanuele , Gautam Gopalakrishnan Sai , Liu Miao , Jain Anubhav , Persson Kristin A , Ceder Gerbrand
Published: February 2020

The diffusion of ions in solid materials plays an important role in many aspects of materials science, such as the geological evolution of minerals, materials synthesis, and in device performance across several technologies. For example, the realization of multi-valent (MV) batteries, which offer a realistic route to superceding the electrochemical performance of Li-ion batteries, hinges on the discovery of host materials that possess adequate mobility of the multi-valent intercalant to support reasonable charge and discharge times. This has proven especially challenging, motivating the current investigation of ion mobility (Li+, Mg2+, Zn2+, Ca2+, and Al3+) in spinel Mn2O4, olivine FePO4, layered NiO2, and orthorhombic δ-V2O5. In this study, we not only quantitatively assess these structures as candidate cathode materials, but also isolate the chemical and structural descriptors that govern MV diffusion.  Our finding that matching the intercalant site preference to the diffusion path topology of the host structure controls mobility more than any other factor, leads to practical and implementable guidelines to find fast-diffusing MV ion conductors.
Tailored Carbon Nanotubes for High-Performance Lithium-Sulfur Batteries
Authors: Jeong Yo Chan , Yang Seung Jae , Lee Kunsil , Kim Taehoon , Kim Jae Ho , Jung Haesol , Cho Young Shik , Park Chong Rae
Published: February 2020

Significant effort has been invested into addressing critical problems of lithium-sulfur batteries since they are considered as the next generation energy storage material due to their high specific energy density and low cost.1 Critical problems still remain including poor cycle life and rate capability originated from the insulating nature of sulfur and polysulfide intermediate dissolution into the electrolyte.2 Despite the considerable success in alleviating the problems such as drastic capacity fading and poor rate capability, commercial application still has a long way to go, especially at high current rate conditions.

 Here, we report a noble carbon matrix for lithium-sulfur batteries based on a partially unzipped multi-walled carbon nanotube(MWCNT) with favorable properties for the improved lithium-sulfur batteries.  Partially unzipped MWCNT has additional benefit in that it was prepared by facile and scalable way without use of toxic or expensive materials. In contrast to MWCNT and fully unzipped nanoribbon, partially unzipped MWCNT clearly exhibited a unique structure of coexistence of carbon nanotube and nanoribbon in one tube as illustrated in Figure 1. The experimental optimization results were fully discussed with TEM, SAED patterns, EDS mapping, RAMAN, XPS, BET isotherms and EIS characterizations.

 Partially unzipped MWCNT showed increased surface area and pore volume with preserved electron conductive pathway owing to opened inner pores of MWCNT. Newly developed the pores of partially unzipped MWCNT were decorated with oxygen containing functional groups and acted as a stable polysulfide reservoir as illustrated in Figure 2.  The synergistic effect of unique pore structure and oxygen containing functional groups led to the improved electrochemical performance at a high current rate of 8375 mA g-1 (5 C). Partially unzipped MWCNT-sulfur composite delivered 688.5 mAh g-1 at the initial discharge and retained 544.2 mAh g-1 after 100 cycles at the high current rate of 5 C as given in Figure 3.

 Furthermore, to understand the mechanism of the improved electrochemical performance of partially unzipped MWCNT, Partially unzipped MWCNT-sulfur was studied utilizing the Monte Carlo (MC) simulations. This modelling explains why the polysulfide dissolution into the bulk electrolyte was alleviated thus led to the improved electrochemical performance.

 We believe that our work can provide insight to many researchers in that partially unzipped MWCNT opens up a new concept of exploiting MWCNT inner pores.

Reference

1. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A. et al. The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat. Chem. 2013, 5, 518-524.

2. Manthiram, A.; Fu, Y.;Su, Y.-S. Challenges and Prospects of Lithium-Sulfur Batteries. Acc. Chem. Res. 2012, 46, 1125-1134.

Figure 1
Understanding Li-S Chemistry with First-Principles Analysis
Authors: Balbuena Perla B , Burgos Juan Carlos , Ma Yuguang , Camacho-Forero Luis Eduardo , Smith Taylor , Bertolini Da Silva Oliveira Samuel
Published: February 2020

Fulfilling the promise of Li-S batteries depends on the implementation of strict controls on the degradation processes that are involved in a strongly correlated system. A highly reactive Li anode requires of a special electrolyte formulation able to control dendrite formation and its subsequent effects on the cell performance. In addition, the presences of soluble polysulfides (PS) that travel from cathode to anode substantially change the anode interfacial chemistry. On the other hand, intimately mixed sulfur-carbon cathode formulations are usually employed to enhance the cathode electronic conductivity. As a consequence, sulfur reduction reactions at the cathode interface are affected by the presence of porous carbon structures.  Thus, a complex chemistry requires of numerous tools to identify processes and mechanisms with the goal of guiding improvements in materials aimed to extended battery life. Among these tools, first-principles density functional theory and ab initio molecular dynamics simulations yield information regarding molecular mechanisms that is not readily available or difficult to obtain from experimentation. Here we focus on two aspects: interfacial electrolyte decomposition reactions on the Li anode surface leading to the initial stages of the solid-electrolyte interphase (SEI), and sulfur reduction at the cathode.  We study the reduction mechanisms of a series of electrolyte solutions including  1M solution of Lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) in pure solvents such as Dioxolan (DOL), 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (D2), dimethoxyethane (DME), and ethylene carbonate (EC) and mixtures  of DOL/DME, and DOL/D2. The reduction mechanisms of soluble PS species such as S8Li2, S6Li2 and S4Li2 at the anode surface are also investigated, along with their effect on the electrolyte decomposition reactions.  At the cathode side we characterize discharge reactions and their voltage profiles for sulfur reduction in contact with the electrolyte, versus sulfur reduction in contact with carbon and the electrolyte. Also we analyze effects of carbon structure on diffusion and dissolution of sulfur anions and PS species as well as on Li2S precipitation.
(Invited) Epitaxial III-Nitride Film Growth in a Single Wafer Rotating Disk MOCVD Reactor
Authors: Papasouliotis George D , Su Jie , Krishnan Balakrishnan , Arif Ronald
Published: February 2020

Epitaxy of GaN on large-size Si wafers can enable the adoption of AlGaN-based High Electron Mobility Transistors (HEMTs) in a broad range of applications ranging from consumer electronics, solar and wind power, power supplies, and automotive, to the potential integration with Si-based CMOS technologies. Device performance dictates stringent requirements for epitaxial film quality and uniformity, while wafer bow management during growth facilitates the use of larger diameter substrates. Uniform temperature profile and gas distribution in the MOCVD reactor become critical elements for film performance and, ultimately, technology adoption. In this work, we are reporting on the epitaxy of Al/Ga/N containing films on 200 mm Si wafers. The TurbodiscÒ, vertical, rotating disk reactor used for epitaxy encompasses the fundamental reaction and high velocity laminar flow characteristics of its predecessor batch systems, thus enabling stable, repeatable operation, long PM cycles, and direct, model-based process scale up. Moreover, it incorporates advanced features, which will be discussed, for alkyl/hydride flow distribution and temperature uniformity, which translate into excellent uniformity in epitaxial layer thickness, alloy composition, and doping profile across the wafer. Results on AlGaN/GaN heterostructures, thickness uniformity of GaN 0.48%, 1σ, and AlN 0.86%, 1σ, Al composition uniformity in AlGaN of 0.289%, 1σ, and 2DEG performance are presented. Uniform incorporation of dopant species (Mg and C) across the substrates is reported. The average Hall mobility is > 1700 cm2/V.s, with sheet carrier concentration at 9.5x1012/cm2, and sheet resistance 380 Ohm/sq for the 5 points across the wafer. Results on the repeatability of film quality, FWHM of GaN (002) and (102) showing < 1.5% 1σ over 20  runs, and average Al composition of AlGaN barrier, showing variation of 0.37 % 1σ over 16  runs, are presented. Wafer bow can be precisely engineered and controlled through the thickness and composition of the stress compliance structure.
(Invited) Semiconductor Nanowires and Nanosheets for Extremely Widely Tunable Lasing
Authors: Ning Cun-Zheng
Published: February 2020

One of the greatest advantages of semiconductor nanomaterials is the ability to grow very dissimilar materials together or on a given substrate, due to the larger tolerance to lattice mismatching than thin film growth. This advantage can be explored for a wide array of applications such as full spectrum solar cells, full-color emission, and widely tunable lasing. In this talk, we will review the most recent progress in the growth and applications of semiconductor nanowires and nanosheets in producing extremely widely tunable lasing from a given substrate, or from a given nanostructure. Both material growth and device characterization will be discussed.
(Invited) Lead Halide Perovskite Nanowire and Nanoplate Lasers with Low Lasing Thresholds and High Quality Factors
Authors: Jin Song
Published: February 2020

The remarkable performance of lead halide perovskites in solar cells can be attributed to the long carrier lifetimes and low non-radiative recombination rates, the same physical properties that are ideal for semiconductor lasers. Here we first report new insights on the crystal growth of the perovskite materials and the solution growth of single crystal nanowires, nanorods, and nanoplates of methylammonium lead halide and other perovskites via a dissolution-recrystallization pathway from lead iodide or lead acetate films coated on substrates. We further show room temperature and wavelength tunable lasing from single crystal lead halide perovskite nanowires with the lowest lasing thresholds (220 nJ/cm2) and highest quality factors (Q  ~ 3600) reported to date for semiconductor nanowire lasers. The lasing threshold corresponds to a charge carrier density as low as 1.5×1016/cm3. Kinetic analysis based on time-resolved fluorescence reveals little charge carrier trapping in these single crystal nanowires and gives estimated lasing quantum yields approaching 100%. Such lasing performance, coupled with facile solution phase growth of single crystal NWs and broad tunability of emission color from stoichiometry, makes lead halide perovskites ideal materials for the development of nano-photonics, in parallel with the rapid development in photovoltaic technology from the same materials.
(Invited) Electronics Based on Monolayers
Authors: Li Lain-Jong
Published: February 2020

Transition metal dichalcogenides (TMDs) are attractive for energy and electronic applications. Our recent success in vapor phase growth of TMD monolayer [1] has stimulated the research in growth and applications [2].  In this presentation,  I would discuss the synthesis and characterizations of crystalline MoS2 and WSe2 monolayers. These layer materials can be transferred to desired substrates, making them suitable building blocks for constructing multilayer stacks for various applications [3].

Heterostructures of 2d materials formed by vertical stacking have been realized recently via transfer of their exfoliated flakes, where their properties are dominated by the stacking orientation and strength of interlayer coupling. The method to determine valence band and conduction band alignment for various TMD materials is proposed [4].

Another very attractive structure is lateral heterostructure in which the junction is atomically sharp and the active region can be as narrow as few strings of atoms at the junction areas. This structure offers much easier band offset tuning since materials are spatially separated. The direct growth of such lateral heterostructures will be presented. Preliminary results and perspectives on monolayer electronics shall be discussed.

[1] Adv. Mater. 24, 2320 (2012); ACS Nano 8, 923 (2014).   [2] Nat. Chem. 5, 263-275 (2013); Chem. Soc. Rev. (2015) DOI: 10.1039/C4CS00256C.    [3] ACS Nano 8, 2951 (2014); ACS Nano 8, 8317 (2014). [4] arXiv:1406.5137 (2015)
(Invited) Functional Nanostructures for Highly Efficient Photoelectrochemical Water Splitting
Authors: Zhang Yuegang
Published: February 2020

Photoelectrochemical (PEC) water splitting is one of the efficient ways to convert sunlight to the cleanest fuel – hydrogen. The development of photoelectrode materials with a suitable bandgap for efficient light harvesting and fast charge transfer is one of the key challenges for PEC water splitting. In this talk, I will show our recent results using functional nanostructure design to improve the broadband solar absorption and charge transfer process. By growing omnidirectional, densely packed branches on TiO2 nanowires, we demonstrated a 3D hyperbranched hierarchical TiO2 nanowire (HHNW) architecture that could significantly enhance the performance of PEC water splitting. Another approach to increase visible light absorption is to use the surface plasmon resonance (SPR) effect. We demonstrated an approach to decorate TiO2 nanorods (TNRs) with Au nanoparticles (AuNPs) and graphene quantum dots (GQDs). The incident photon-to-current conversion efﬁciency measurement showed that the PEC activity of the AuNPs and GQDs-loaded TNRs was enhanced in the whole UV-visible region. The experimental analysis indicated that the AuNPs contributed to the surface plasmon resonance (SPR) phenomenon in the UV region and SPR-mediated hot electron injection in the visible region. The GQDs contributed to the improved electron injection in the UV region, multiphoton excitations and hot electron injection in the visible region. The multi-level functionalization of nanostructures have proved to be a synergistic way to realize highly efficient PEC Water Splitting
(Invited) Two-Dimensional Layered Materials/Silicon Heterojunctions for Energy and Optoelectronic Applications
Authors: Jie Jiansheng
Published: February 2020

Two-dimensional (2D) layered materials such as graphene, metal dichalcogenides, and topological insulators have attracted much attention due to their extraordinary properties. However, their applications in optoelectronic devices are hindered by the relatively low light absorption of monolayer materials as well as difficulties in controllable doping and p-n junction fabrications. In comparison to the formation of homojunctions or heterojunctions, respectively, from one or two different 2D layered materials, the formation of heterojunctions by combining a 2D layered material with a semiconductor offers a feasible and easier way to construct high-performance optoelectronic devices by harnessing the advantages of both materials. Herein, we report a systematic investigation on 2D layered material/silicon heterojunction based photovoltaic and optoelectronic devices. It was found that interface charge recombination and graphene conductivity play important roles in determining solar cell performance. By suppressing surface recombination and optimizing graphene layers and doping level, we demonstrate power conversion efficiencies (PCEs) over 14% for graphene/silicon heterojunctions. A new type of solar cell based on the graphene quantum dots (GQDs)/silicon heterojunctions was also developed. Owing to the extraordinary optical and electrical properties of GQDs, the GQDs/Si heterojunctions show a promising efficiency over 12%. Further, ultra-fast photodetectors were constructed from the MoS2/Si, MoSe2/Si, and Bi2SeTe2/Si heterojunctions. Our work reveals the great potential of two-dimensional layered materials/silicon heterojunctions for high-performance photovoltaic and optoelectronic devices.
(Invited) Enhanced Energy Harvesting Scheme Utilizing Hierarchical Micro/Nanostructures
Authors: Chueh Yu-Lun
Published: February 2020

In this talk, I will introduce the development of low dimensional nanomaterials in my group and its applications on solar cell and solar energy storage. In the first part of my talk, I will introduce the development of hierarchical nanostructures used to enhance the energy harvesting based on photo-solar and thermal-solar in my group.  First, I will report fabrication of a large area Cu(In,Ga)Se2 nanotip arrays (CIGS NTRs) by using one step Ar+ milling process without template. By controlling milling time and incident angles, the length of CIGS NTRs with adjustable tilting orientations can be precisely controlled. The CIGS NTRs have very low reflectance < 0.1 % at incident wavelengths between 300 nm to 1200 nm. Open circuit voltage and short circuit current of CIGS NTRs solar cell were measured to be ~390 mV and ~22.56 mA/cm2, yielding the filling factor and the efficiency of 59 % and 5.2 %, respectively. In contrast to CIGS thin film solar cell with efficiency of 3.2 %, the nanostructured CIGS NTRs can have efficiency enhancement of ~160 % due to the higher light absorption ability because of the nanostructure.  In the 2nd part of my talk, an enhanced heat capacity of modified HITEC molten salt by doping of metal NPs will be reported. We encapsulate the Tin alloy into Sn/SiOx core-shell NPs to prevent mixture with salt when operation temperature is higher than the melting point of Tin. The SiOx shell layer can protect the Sn NPs to be oxidized and maintains a melted status during the heat cycles, providing a stable leant heat value of 28.36 J/g. Besides, effective heat capacity of HITEC salt with Sn/SiOx core-shell NPs can be enhanced to be 30 % a mixture percentage of 5 wt%. The merits of this approach show a low-cost and easy way to enhance the thermal properties of current molten salt without complicated manufacturing.
(Invited) Wearable Electronics Using Low-Dimensional Nanomaterials
Authors: Park Jang-Ung
Published: February 2020

Transparent conductive electrodes (TCEs), which have transparency and conductivity, are of increasing significance and demand for diverse application areas such as information (displays, touch screen), energy (solar cell, architectural), and environment (sensors). Nowadays, indium tin oxide (ITO) deposited by sputtering process is used as the main TCE material. Although the ITO presents low sheet resistance (~ 30 Ohm/sq) and high transparency (~ 90 %), its fragility limits many potential applications in flexible and stretchable electronics. For these reason, many types of alternative materials such as conducting polymers, carbon nanotubes, graphene, and metal nanowires, have been studied as candidates of the flexible and stretchable TCEs. Although many of these candidates exhibit good mechanical flexibility and stretchability, none shows significantly higher conductivity and transparency together, compared to ITO. Here, we present a simple fabrication process of high-performance, stretchable TCEs based on the metal nanofibers which have long and continuous web geometries, hybrid with 2D material graphene. These TCEs show superb electric conductivity (~ 1 Ohm/sq) with high transparency (~ 91 %) as well as substantial flexibility and stretchability. We believe these TCEs based on the nanostructures present a promising strategy toward flexible and wearable electronics, and indicate the promise of future electronics beyond the limits of conventional ITO.
(Invited) Laser Cooling in Semiconductors
Authors: Zhang Jun
Published: February 2020

Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, in a phenomenon known as laser cooling. In gaseous matter, an extremely low temperature can be obtained in diluted atomic gases by Doppler cooling; in solid-state materials, laser cooling is achieved by the annihilation of phonons, which are quanta of lattice vibrations, during anti-Stokes luminescence.

Here we will show my studies how to use laser to cool whole semiconductor by using unresolved sideband anti-Stokes luminescence upconversion and how to use laser to cool specific optical phonon vibration by using resolved sideband Raman cooling technique. We demonstrate a net cooling by about 40 kelvin in a semiconductor using group-II–VI cadmium sulphide nanoribbons, or nanobelts, starting from 290 kelvin. We use a pump laser with a wavelength of 514 nanometres, and obtain an estimated cooling efficiency of about 1.3 per cent and an estimated cooling power of 180 microwatts. At 100 kelvin, 532-nm pumping leads to a net cooling of about 15 kelvin with a cooling efficiency of about 2.0 per cent. In resolved sideband regime, we experimentally demonstrate spontaneous Raman cooling and heating of longitudinal optical phonon with a 6.23 THz frequency in polar semiconductor zinc telluride nanobelts. We use the exciton to resonate and assist photo-elastic Raman scattering from LOPs due to the large exciton-LOP coupling. The cooling (heating) is mediated by detuning the laser pump to lower (higher) energy sideband, and spontaneous scattering photon resonates with exciton at anti-Stokes (Stokes) side, that beat and photo-elastically attenuate (enhance) the dipole oscillation of the optical phonon. 

References:

J. Zhang, D. H. Li, R. J. Chen, Q. H. Xiong, Laser cooling of a semiconductor by 40 Kelvin, Nature, 493, 504-508 (2013)

J. Zhang, L. C. Kwek, Q. H. Xiong, Resolved Sideband Raman cooling of optical phonons in semiconductor materiaals, Nature Photonics, under review （2015）
(Invited) Operando Measurement of Energy Band Alignment and Built-in Potential in Thin-Film Photovoltaic Devices
Authors: Chen Liwei
Published: February 2020

The energy band alignment in thin-film or low-dimensional devices is extremely important for charge separation, carrier transport and collection, as well as undesirable recombination. However, experimental determination of the energy level alignment and the quantitative measurement of related parameters, such as the built-in potential (Vbi
), have been difficult to realise, especially under typical operating conditions for the devices. Indirect methods, such as Mott-Schottky analysis, electroabsorption spectroscopy, and dark current density-voltage (J-V) curve fitting, have been applied to measure the Vbi
in solar cells, but the quantitative information has been inaccurate or inconsistent.

Recently, SKPM has been used to image cross sections of solar cells to probe the SP depth profile across multiple layers. The vacuum level (VL) in an energy band structure is defined as the energy of an electron resting right at the surface of a material; thus, the VL alignment within a device can be obtained by multiplying the SP depth profile by a constant, i.e., the electron charge. In principle, this approach offers the advantage of being able to directly visualise the energy band alignment across the device. However, a major obstacle has limited the application of this approach in solar cell research: quantitatively accurate measurements and interpretations have yet to be realised in cross-sectional SKPM studies. Critical parameters for the operating mechanism of solar cells, such as the open-circuit voltage (Voc
) and Vbi
, are often found to be much smaller than expected from J-V characterisation. Overall, the results obtained from cross-sectional SKPM measurements are often uncorrelated with the device performance and J-Vcharacteristics and are thus not applicable for understanding practical devices in their actual operating states.

In this presentation, we address the challenges in quantitative SKPM potentiometry in low-dimensional devices using organic photovoltaic (OPV) cells as model systems. Cross sections of OPV devices are exposed by using ion beam milling; and the SP depth profiles of the OPV device under operating conditions are directly visualised by SKPM imaging on the cross section. The tip/cantilever convolution effect due to finite tip size and cantilever beam cross-talk is identified as the source of the systematic underestimate of SP differences in cross-sectional SKPM measurements. A bias compensation method is developed to obtain quantitative measurements of internal potential difference parameters, such as Voc
, Vtd
and Vbi
, in OPV devices. This method is generally applicable to other thin-film and low-dimensional devices.
Nanopatterned Fiber Based Textile Triboelectiric Nanogenerator
Authors: Seung Wanchul , Kim Sang-Woo
Published: February 2020

There has been increasing demand for highly efficient portable energy harvesters due to the development and mass consumption of portable electronic devices. Photovoltaic, thermoelectric, piezoelectric and triboelectric energy scavengers have become strong candidates for portable energy harvesters in future self-powered portable electronic device applications.Among these, Triboelectric energy harvesters that can convert mechanical energy into electrical energy have recently attracted attention. Compared to time-constrained photovoltaic, piezoelectric energy harvesters are not limited in time and space because mechanical energy sources such as body movement, heartbeat, blood ﬂow, wind, tide, low-frequency seismic vibrations, and sound from the surrounding environment are prevalent.

In this study, we report the textile based as a substrate for foldable and wearable triboelectric power generators. Nano-size pattened PDMS on the conductive woven fabric substrate by dip-coated method. For practical applications, nano patterned conductive woven fabric based power generator showed the continuous driving of the liquid crystal display screen and light emitting diode successfully. Furthermore, textile based power generator showed high sensitivity acts low applying for of under 1kgf.

The results presented here may create new possibilities for energy harvesting wearable devices, particularly in applications that require wearability and flexibility.
Highly Flexible and Transferable Supercapacitors with Ordered Three-Dimensional MnO2/Au/MnO2 Nanospikes Arrays
Authors: Gao Yuan , Lin Qingfeng , Tavakoli Mohammad Mahdi , Leung Siu-Fung , Fan Zhiyong
Published: February 2020

Ordered three-dimensional nanostructures such as arrays of nanospikes, nanocones, and nanotubes naturally have large interface/surface area, which are highly attractive for energy storage application, particularly for high-performance flexible supercapacitors. Here, we report a unique MnO2/Au/MnO2 Nanospikes (MAMNSPs) supercapacitor structure based on ultra-thin, free-standing 3-D gold (Au) NSPs film. The NSPs films are highly flexible and transferable to arbitrary type of flexible substrate to enable applications that require high flexibility. The large surface area of this unique structure leads remarkable enhancement in electrochemical performance, 1.9 and 4.26 times capacitance as compared with MnO2/Au NSPs (MANSPs) and MnO2/Planar (MAPL) electrodes, respectively. The all-solid-state symmetric supercapacitors based on MAMNSPs electrodes have been fabricated and systematic performance characterizations showed that the devices have high volumetric capacitance of 20.35 F cm-3 and specific energy of 1.75 × 10-3 Wh cm-3. In addition, bendability measurement showed that the supercapacitor devices are highly flexible and reliable. With virtue of simple fabrication procedures and enhanced electrochemical performance, such 3-D structures have highly promising potency for portable and flexible energy storage systems for a wide range of practical applications.

Figure 1
(Invited) Interface and Dielectric Engineering for High-Performance Top-Gated MoS2 Field Effect Transistors
Authors: Zou Xuming , Liao Lei
Published: February 2020

In recent years, due to the intriguing electrical and optical characteristics, two dimensional layered materials such as MoS2 have attracted tremendous research attention. However, until now, majority of the efforts have been focused on the integration of MoS2 devices in the back- or dual-gated geometry due to the difficulty of compact and conformal top-gated dielectric deposition directly onto the 2-D channel for the realization of high-performance top-gated FETs. In order to integration in TFT circuit for practical application, top-gated FETs with high-k dielectric is necessary. In this regard, interface or dielectric engineering is an important step towards the practical implementation of MoS2 devices with the optimized performance.

Here, we explore the case of interface engineering by utilizing an ultrathin metallic oxide (MgO, Al2O3 and Y2O3) buffer layer inserted between the ALD-HfO2 and MoS2 channel in order to achieve conformal HfO2/MoS2 interfaces with the minimal interface defect density. Exploiting these enhanced gate stack dielectrics, we attain the highest saturation current (526 μA/μm) of any MoS2 transistor reported to date, which is comparable to the same scaled state-of-the-art Si MOSFETs. At the same time, these devices also exhibit the impressive room-temperature mobility (63.7 cm2/V·s), on/off current ratio (>108) and near-ideal sub-threshold slope (SS = 65 mV/decade).

Although Y2O3/HfO2/MoS2 structure improves the device performance greatly, the degradation in mobility is unavoidable. We then further utilize BN as dielectric layer to improve the mobility of top-gated MoS2 device. The mobility value is close to 100 cm2/V·s,

Demonstration of all these suggests that the performance of few-layer MoS2 FETs can reach near intrinsic limits at room temperature along with the proper interface engineering and propose future directions to improve electrical characteristics in layered semiconductors.
(Invited) A Metal Oxide Antifuse-Diode Device
Authors: Kuo Yue
Published: February 2020

Crystalline metal oxides have been used in various micro and nano electronics and optoelectronics, such as conductors, superconductors, and light emitting devices (LEDs). The amorphous metal oxide dielectric is usually a high-k material suitable for the insulation purpose. The doped metal oxide, such as the Zr-doped HfO2 (ZrHfO), high-k thin film has many advantageous material and electrical properties, such as the high crystallization temperature, low interface layer thickness in contact with Si, and low interface density of states (1). When nanocrystals are embedded into the high-k metal oxide dielectric thin film, electrons and charges can be stored for a long period of time, which is suitable for the nonvolatile memory application (2). Recently, it was proved by the author’s group that the broad band white light could be emitted from the metal oxide dielectric thin film after the dielectric breakdown due to the thermal excitation of the nano size conductive paths (3). In this talk, a new high-k dielectric based device that shows both antifuse and diode functions is reported. Figure 1 shows a large “programmed” to “unprogrammed” leakage current ratio of 105 is obtained from an original MOS capacitor made of a high-kgate dielectric on a p-type wafer, which fits the antifuse requirement. Figure 2 shows that for a same device in the programmed state, the leakage current increases with the increase of the magnitude of the gate voltage in the negative voltage range. However, the leakage current remains low and changes little over a large range of gate voltage in the positive voltage range, which is similar to the diode behavior. On the other hand, the leakage current-voltage curve of the new device is almost linear while that of the conventional diode follows the exponential relationship. The operation physics, structure, and material properties of the new device will be discussed in detail.   

Y. Kuo, J. Lu, S. Chatterjee, J. Yan, H. C. Kim, T. Yuan, W. Luo, J. Peterson, and M. Gardner, “Sub 2 nm Thick Zirconium Doped Hafnium Oxide High-K Gate Dielectrics,” ECS Trans., 1(5), 447-454 (2006).

Y. Kuo, invited, “Nanocrystals Embedded High-k Nonvolatile Memories – bulk film and nanocrystal material effects,” ECS Trans., 53(4), 121-128 (2013).

Y. Kuo, invited, “A Solid State Thin Film Incandescent Light Emitting Device,” Proc. IDEM, 104-107 (2014).  

Figure 1