Browsing by Author "Ajayan, Pulickel"

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    A Preliminary Characterization of the Interfacial Properties of Solution Spun Carbon Nanotube Fibers
    (2023-11-27) Curtis, Nicholas Alexander; Rahman, Muhammad; Ajayan, Pulickel
    This thesis presents new data regarding the characterization of interfacial properties of highly aligned and densely packed solution-spun carbon nanotube fibers (CNTF) as a first step in a more sophisticated study on CNTF composites and their response to impact. Through characterization of the interphase region of a matrix-filler composite system, commentary on the efficiency of stress transfer between the bulk and reinforcement, and thus the quality of the adhesive bond, is provided. In order to acquire the necessary data for proper analysis, the single fiber pull-out test is employed using a high-precision rheometer. The data collected and presented herein are preliminary data for initial and first-ever insight into the newly developed CNT fibers, but larger batch sample sizes are required for increased statistical confidence in the results. Given the large impact of processing parameters on the mechanical characteristics of heterogeneous composite materials, minimal quantitative comparison to external studies is made. Instead, processes, characterization schemes, data reduction and analysis methods, and assessments are presented for simplified implementation of future data points in order to increase the significance of the results.
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    A thermally-invariant, additively manufactured, high-power graphene resistor for flexible electronics
    (IOP, 2017) Michel, Monica; Biswas, Chandan; Tiwary, Chandra S.; Saenz, Gustavo A.; Hossain, Ridwan F.; Ajayan, Pulickel; Kaul, Anupama B.
    Solution processed two-dimensional (2D) layered materials and their integration with additive manufacturing techniques, such as ink-jet printing, is a facile approach for incorporating these exotic materials into device platforms for flexible electronics. In this work, graphene ink formulations are successfully utilized toward the design and fabrication of high-power resistive structures that are printed on both rigid and flexible substrates and have the potential to deliver close to 10 W of power. A near-flat, negative temperature coefficient of resistivity (TCR) is measured with an activation energy E a ~ 2.4 meV for electron hopping, which is 100×  lower compared to E a values for high TCR materials. The TCR and E a values are amongst the lowest reported for 2D layered material systems. The thermal-invariance of resistivity for such high-power graphene printed resistors is attractive for applications, for example to provide a stable heating source for flexible electronics over extreme thermal environments. The transport characteristics of the ink-jet printed features is modeled as a composite structure in order to explain the thermal response which appears to be mediated via defects in the sonicated graphite, and correlates well to inferences made from Raman spectroscopy and transmission electron microscopy analysis conducted on the printed graphene structures. In order to fabricate such functional structures with ink-jet printing, the active nozzle number, printing passes, and annealing conditions are shown to play an important role to determine line resolution, and also dictate the morphological and electronic transport characteristics of the printed graphene features.
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    Additive manufacture-assisted method for making structural elements having controlled failure characteristics
    (2023-08-15) Sajadi, Seyed Mohammad; Meiyazhagan, Ashokkumar; Boul, Peter; Rahman, Muhammad; Thaemlitz, Carl; Ajayan, Pulickel; Rice University; Saudi Arabian Oil Company; William Marsh Rice University; United States Patent and Trademark Office
    A process for making a layered multi-material structural element having controlled mechanical failure characteristics. The process includes the steps of: supplying a cementitious layer and forming a polymer layer on the cementitious layer by additive manufacture such that the polymer layer has a first thickness and the cementitious layer has a second thickness, wherein the polymer layer comprises a polymer and the cementitious layer comprises a cementitious material; and allowing the polymer from the polymer layer to suffuse into the cementitious layer for a period of time to obtain a suffused zone in the cementitious layer such that the suffused zone has a third thickness that is less than half the second thickness.
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    Additive manufacture-assisted method for making structural elements having controlled failure characteristics
    (2024-06-18) Sajadi, Seyed Mohammad; Meiyazhagan, Ashok Kumar; Boul, Peter; Rahman, Muhammad; Thaemlitz, Carl; Ajayan, Pulickel; Rice University; Saudi Arabian Oil Company; United States Patent and Trademark Office
    A process for making a layered multi-material structural element having controlled mechanical failure characteristics. The process includes the steps of: supplying a cementitious layer and forming a polymer layer on the cementitious layer by additive manufacture such that the polymer layer has a first thickness and the cementitious layer has a second thickness, wherein the polymer layer comprises a polymer and the cementitious layer comprises a cementitious material; and allowing the polymer from the polymer layer to suffuse into the cementitious layer for a period of time to obtain a suffused zone in the cementitious layer such that the suffused zone has a third thickness that is less than half the second thickness.
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    Additive manufacture-assisted method for making structural elements having controlled failure characteristics
    (2024-05-28) Sajadi, Seyed Mohammad; Meiyazhagan, Ashok Kumar; Boul, Peter; Rahman, Muhammad; Thaemlitz, Carl; Ajayan, Pulickel; Rice University; Saudi Arabian Oil Company; United States Patent and Trademark Office
    A process for making a layered multi-material structural element having controlled mechanical failure characteristics. The process includes the steps of: supplying a cementitious layer and forming a polymer layer on the cementitious layer by additive manufacture such that the polymer layer has a first thickness and the cementitious layer has a second thickness, wherein the polymer layer comprises a polymer and the cementitious layer comprises a cementitious material; and allowing the polymer from the polymer layer to suffuse into the cementitious layer for a period of time to obtain a suffused zone in the cementitious layer such that the suffused zone has a third thickness that is less than half the second thickness.
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    Additive manufacture-assisted method for making structural elements having controlled failure characteristics
    (2024-05-28) Sajadi, Seyed Mohammad; Meiyazhagan, Ashok Kumar; Boul, Peter; Rahman, Muhammad; Thaemlitz, Carl; Ajayan, Pulickel; Rice University; Saudi Arabian Oil Company; United States Patent and Trademark Office
    A process for making a layered multi-material structural element having controlled mechanical failure characteristics. The process includes the steps of: supplying a cementitious layer and forming a polymer layer on the cementitious layer by additive manufacture such that the polymer layer has a first thickness and the cementitious layer has a second thickness, wherein the polymer layer comprises a polymer and the cementitious layer comprises a cementitious material; and allowing the polymer from the polymer layer to suffuse into the cementitious layer for a period of time to obtain a suffused zone in the cementitious layer such that the suffused zone has a third thickness that is less than half the second thickness.
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    Ambient mechanochemical solid-state reactions of carbon nanotubes and their reactions via covalent coordinate bond in solution.
    (2016-03-29) Kabbani, Mohamad; Ajayan, Pulickel
    In its first part, this thesis deals with ambient mechanochemical solid-state reactions of differently functionalized multiple walled carbon nanotubes (MWCNTs) while in its second part it investigates the cross-linking reactions of CNTs in solution via covalent coordinate bonds with transitions metals and carboxylate groups decorating their surfaces. In the first part a series of mechanochemical reactions involving different reactive functionalities on the CNTs such as COOH/OH, COOH/NH2 and COCl/OH were performed. The solid-state unzipping of CNTs leading to graphene formation was confirmed using spectroscopic, thermal and electron microscopy techniques. The non-grapheme products were established using in-situ quadruple mass spectroscopy. The experimental results were confirmed by theoretical simulation calculations using the ‘hot spots’ protocol. The kinetics of the reaction between MWCNT-COOH and MWCNT-OH was monitored using variable temperature Raman spectroscopy. The low activation energy was discussed in terms of hydrogen bond mediated proton transfer mechanism. The second part involves the reaction of MWCNTII COOH with Zn (II) and Cu (II) to form CNT metal-organic frame (MOFs) products that were tested for their effective use as counter-electrodes in dyes sensitized solar cells (DSSC). The thesis concludes by the study of the room temperature reaction between the functionalized graphenes, GOH and G'-COOH followed by the application of compressive loads. The 3D solid graphene pellet product (~0.6gm/cc) is conductive and reflective with a 35MPa ultimate strength as compared to 10MPa strength of graphite electrode (~2.2gm/cc).
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    Carbon Based Nanomaterials for Electrochemical Energy Storage Applications
    (2015-04-22) Li, Lei; Tour, James M.; Ajayan, Pulickel; Martí, Angel A.
    Ever-growing energy needs, limited energy resources, and the need to decrease soaring greenhouse gas emissions have brought about an urgent demand on the pursuit of energy alternatives, includ¬ing both renewable energy sources and sustainable storage technologies. Electrochemical capacitors (ECs) and reversible lithium ion batteries (LIBs) are two promising energy storage technologies that are well positioned to satisfy this need in a green energy future. However, their large-scale deployment has been significantly hindered by several major technological barriers, such as high cost, intrinsically poor safety characteristic, limited life, and low energy density and/or power density. One promising solution is to develop advanced electrodes materials for these devices. In this thesis, various nanomaterials and nanostructures have been developed to improve the electrochemical performance of ECs and LIBs. My thesis begins with the introduction of energy storage systems of ECs and LIBs in Chapter 1. Chapter 2 to 4 discuss the synthesis of nitrogen-doped carbonized cotton, brush-like structured nanocomposites of polyaniline nanorods-graphene nanoribbons, laser induced graphene-MnO2, and laser induced graphene-polyaniline and their applications in ECs. All of them demonstrated excellent performance in energy storage, showing high potential applications as electrode materials in ECs. Chapter 5 to 8 discuss a graphene wrapping strategy designed to synthesize graphene-metal oxide/sulfide-graphene nanoribbons, including graphene-MnO2-GNRs, graphene-NiO-rGONRs, graphene-Fe3O4-GNRs, and graphene-FeS-GNRs. This sandwich structure mitigated the pulverization of these anode materials from their conversion reactions during extended cycling, leading to a large improvement in the cycling stability of anodes in LIBs. To address the volume change of SnO2-based anode materials, a facile and cost-effective approach was developed to prepare a thin layer SnO2 on reduced graphene oxide nanoribbons. Chapter 9 discusses how this nanocomposite demonstrated excellent cycling stability with high capacity. For LIBs cathode materials, a hierarchical polyaniline matrix was designed to reduce the dissolution of the intermediate lithium polysulfide into the electrolyte as shown in Chapter 10. This material showed great improvement in cycling stability with high capacity.
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    Carrier transfer dynamics between atomic layers in Van Der-Waals heterostructures probed by ultrafast spectroscopies
    (2017-10-30) Wen, Xiewen; Lou, Jun; Ajayan, Pulickel
    In the past decade, inspired by the discovery and of graphene, two dimensional (2D) materials which only consist of a single layer of atoms have attracted attention of scientists. A series of 2D materials including semiconducting transition metal dichalcogenides, black Phosphors, Silicene, insulating h-BN, and a number of metallic and semi-metallic materials have been synthesized and studied. Many novel physical phenomena and unique applications have been explored. Based on the rich 2D materials library, in recent years, a new sort of materials – Van der Waals (VDW) heterostructure has been created and investigated, unlike conventional semiconductor heterostructures, VDW Heterostructures do not require lattice matching and complicated growth, and they rely on VDW forces between 2D materials so they can be fabricated via artificial stacking, however they can exhibit great deal of phenomena and applications which conventional heterostructures can or cannot realize. Now people tend to believe band structures of individual layers experienced none to slight change in the VDW Heterostructures, so the charge transfers between layers become the crucial factors determining the properties of the VDW Heterostructures. Microscopies, electronics and spectroscopies are main means of studying interlayer interactions, in this thesis, we creatively utilized microscopic optical pump, mid-IR probe ultrafast spectroscopies to reveal the charge transfer dynamics of several VDW Heterostructure systems with different band alignments, include MoS2/WS2, MoS2/MoSe2, MoSe2/Graphene, MoS2/MoSe2/Graphene with different stacking orders, and we observed the charge transfer dynamics and the formation and extinction of interlayer excitons in VDW Heterostructures, a series of novel physical phenomena have been discovered experimentally. And by studying plasmonics-2D material and VDW Heterostructures systems, hot electron injection pathways were clearly revealed for the first time. The work in this thesis not only clarify some controversial opinions of the newly emerging VDW Heterostructures fields but also provide significant experimental evidence for future research.
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    Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser
    (AIP Publishing, 2021) Britz, Alexander; Attar, Andrew R.; Zhang, Xiang; Chang, Hung-Tzu; Nyby, Clara; Krishnamoorthy, Aravind; Park, Sang Han; Kwon, Soonnam; Kim, Minseok; Nordlund, Dennis; Sainio, Sami; Heinz, Tony F.; Leone, Stephen R.; Lindenberg, Aaron M.; Nakano, Aiichiro; Ajayan, Pulickel; Vashishta, Priya; Fritz, David; Lin, Ming-Fu; Bergmann, Uwe
    Femtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d5/2 core level (M5-edge, 572–577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400 ± 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1–2 ps timescale, which is interpreted as electron–hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.
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    Chemical Vapor Deposition of Two-Dimensional Materials and the Investigation of Applications and Properties
    (2017-04-12) Keyshar, Kunttal; Ajayan, Pulickel
    Two-Dimensional Transition metal dichalcogenides (TMDs) have attracted significant research attention due to their appealing electronic, optical and catalytic properties. In order to probe and utilize some of the unique properties of TMDs there must be a scalable approach to growing large area monolayers. The research presented will be in regards to the development of chemical vapor deposition techniques for TMDs, specifically ReS2, and the development of a metrology tool for experimentally determining band alignments for layered materials. We intensively characterize the first monolayer grown ReS2 crystals using various spectroscopy and microscopy techniques and investigate the electronic and catalytic properties. Furthermore, we experimentally determine band alignments for various TMDs using a novel photoemission electron microscopy technique with deep ultraviolet light. Band alignments experimentally determined using this technique provide essential information to the scientific community for building complex electronic and optoelectronic devices with TMDs.
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    Cobalt-free cathode materials for Li-ion batteries: synthesis, processing, characterization and evaluation of the electrochemical performances
    (2023-04-20) Xu, Jianan; Ajayan, Pulickel
    Li-ion batteries cathode materials play an important role in limiting the overall capacity and energy density of the cell, due to their relatively lower specific capacity (~200 mAh/g) compared to that of the anode counterparts (Graphite 372 mAh/g, Si 4212 mAh/g). The chronological development of the classical cathode materials is reviewed. Li1.2Mn0.4Ti0.4O2 (LMTO) belonging to the emerged cation-disordered rock-salt cathodes family, is then selected due to its high specific capacity larger than 250 mAh/g and the Co-free inexpensive feature. While the typical cathode material loading is only 70.0 wt.% with additional components, a chemical vapor deposition method is applied to increase the electrical conductivity by coating a carbon layer on the LMTO particle surface. On the other hand, carbon nanotube is selected to be the ball milling additive and a 78.7 wt.% cathode loading is reached, which increases the cathode gravimetric capacity from 44 mAh/g to 121 mAh/g after fifty cycles. Li1.079Ni0.400Ti0.476O2 is then selected and ball-milled, and the PITT measurement is applied to extract the Li+ diffusivity value. The obtained DLi+ on the order of 10-15 cm2/s successfully explains the kinetic requirement for the particle size less than 200 nm, giving a 10 mA/g current density. Furthermore, the effects of ball milling on the electrochemical capacity and interfacial stability of Li2MnO3 cathode material is studied, which reveals the inducing of Li2CO3 contaminant species even though the capacity is largely enhanced, its further decomposition into CO2 during first charge and the reduction of CO2 to carbonate species during the following discharge. Next, an all-fluorinated electrolyte is coupled with the LMTO material which increases the coulombic efficiency to 90.0 % and 99.0 % for the first cycle and subsequent cycles, respectively, with a F-rich cathode-electrolyte-interphase characterized by XPS. Finally, several types of hydrothermal methods and a flash joule heating method are adopted to directly synthesize the LMTO material with nano-sized primary particles. As a result, spinel structure LiMnTiO4 is found to be the stable phase and LMTO can only be synthesized via traditional solid-state method.
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    Designing Anisotropic Functional Polymers: From Shape-Shifting Elastomers to 2-Dimensional Films
    (2021-08-06) Barnes, Morgan G.; Ajayan, Pulickel; Verduzco, Rafael
    Polymers are ubiquitous in our everyday lives largely due to their low cost, processibility and broad range of mechanical properties. As technology advances researchers aim to develop new multi-functional polymers with unique mechanical, electrical, and optical properties such as shape-shifting soft robots and high-strength high-toughness films for aerospace applications. However, traditional isotropic polymers, with random polymer chain conformations, are not effective for many of these next-generation materials. There is increased interest in developing anisotropic polymers where researchers can precisely tune both the chemical makeup and physical alignment of polymer chains to obtain desired functionality. In this thesis, I develop two distinct intrinsically anisotropic polymers - liquid crystal elastomers (LCEs) and covalent organic frameworks (COFs) - which exhibit unique physical properties. LCEs are soft materials that directly couple the anisotropy of liquid crystals to a loosely crosslinked polymer network. When exposed to a stimulus, such as heat, a reversible phase transition occurs which alters the optical and mechanical properties of the material and is promising in applications ranging from high damping materials to soft-robotics. Here, I develop a new method to program reversible complex actuations into shape shifting LCEs including a self-curling flower and a film that morphs in the topographical features of a human face. Next, I translate the synthesis procedure to a new reactive 3D printing method to enable more sophisticated shape changes. Finally, I can readily tune the actuation temperature of these LCEs so that they can actuate from body heat which is useful for biomedical applications. COFs are highly porous and crystalline polymers with tunable pore sizes and functionality with applications in catalysis, filtration, energy storage, and high-strength films. However, COFs are traditionally very difficult to process and current methods to obtain free-standing films are not scalable and yield modest crystallinities. Here, I obtain high surface area and crystalline free-standing films directly from monomers. This procedure is quick and inexpensive compared to previous methods and is industrially scalable. Therefore, this is an important advancement in COF processing that has the potential to greatly expand research focused on the application of these multifunctional materials.
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    Energy Storage Capacity and Superconductivity of Nanosized Titanium Diboride, and Multifunctionality of Carbon-based Nanostructures: Development of Nano-engineered Solutions
    (2019-09-30) Zhou, Zhou; Ajayan, Pulickel; Kono, Junichiro; Bayazitoglu, Yildiz
    Nanotechnology has risen into prominence since the discovery of the “buckyball” in 1985,2 due to the enhanced tunability and performance of nanomaterials.3 Keenly awaited, scalable and facile application of nanotechnology, however, remains challenging. In petroleum industry, for instance, implementation barriers in scalability, controllability, and profitability have been hindering the advancement of nanotechnology innovations. The potential of fine tuning material properties and creating novel solutions is yet to be realized. Industrial friendly, scalable synthesis of nanosized titanium diboride and multifunctional nanostructures are exploited in this thesis, to include chemical vapor deposition, liquid exfoliation and electrochemical deposition. State of the art characterization techniques reveal atomic level properties in physical structure and chemical composition. After iterative material development cycles, the performance of prototypes are evaluated experimentally and theoretically. Lithium ion storage capacity and type II superconductivity are first time reported for nanosized titanium diboride. Remarkable theoretical capacity of 385.7 mAh/g and superconductive critical temperature of 5.8 K are attributed to the dimensional confinement of the nanoscale. Titanium diboride nanoparticles exhibit remarkable charge storage capacity, demonstrating great potential for applications as lithium ion battery anode and supercapacitor material. Their high energy storage capacity together with their newly discovered superconductivity manifest the distinctive material characteristics induced by dimensional confinement. Looking beyond the enhancement of material properties offered by the nanoscale, the multifunctionality of nanostructures are explored. Impelled by the virtues of carbon nanotubes and Fe@C core-shell nanoparticles, multifunctional, nano-engineered prototypes are designed and fabricated, combining hydrophobicity, mechanical and chemical resistance, and superparamagnetic, florescent and photocatalytic properties. The multifunctionality of infiltrated carbon nanotubes and Fe@C-CNx nanostructures appeal to various applications such as protective composite and reusable photocatalyst. Bridging the gap between academic research and industrial application, nano-engineering and design thinking approaches in this thesis develop nanostructures to solve explicit problems. Size confinement induced properties and innovative designs of nano-engineered structures are vital to convey the value of nanotechnology. The developed prototypes provide innovative solutions to various existing problems, including low durability of drilling tools, high friction in mechanical operations, critical environment energy storage and hazardous water waste.
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    Exfoliation, Characterizations, and Applications of 2D Gallium Sheets
    (2016-11-30) Zhang, Yuan; Ajayan, Pulickel
    The isolation of graphene from graphite have boomed the two dimensional materials research for many years. The metallic layers have been studied rarely, so I decided to explore the two-dimensional gallium layer. Unlike the traditional top-down techniques of exfoliation, I here develop a novel solid-melt interface exfoliation technique, which take advantages of the interface between the solid and molten phases of the gallium. Some characterization figures will be reported to study the thickness, the morphology, the crystal nature, and the composition details. I also used the standard electron beam lithography to fabricate the gallium device, which helps to investigate the electronic properties of the atomic-thick gallium layer. At last, to prove the scalability and reproducibility of the solid-melt exfoliation technique, I used a new stamping technique to create gallium layers showing a successful rate between 80%-90%, which could be established to a unique and innovative metal stamping technique in future industry. Also, I provided several possible improvements and insights about my own project, which may worth a try in future study. At last, the progress and challenges of the two dimensional materials area will be covered according to my own understanding.
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    Hierarchical Design of Two-Dimensional Conjugated Porous Organic Polymers: Synthesis, Properties, and Tailored Applications
    (2022-11-30) Gayle, Jessica Mae; Ajayan, Pulickel; Vajtai, Robert
    Over recent years, there has been a growing interest in utilizing two-dimensional conjugated porous organic polymers (2D-C-POPs) for a variety of applications including robust sensing, membrane separation, and controlled drug delivery. 2D-C-POPs are composed of carbon-rich aromatic skeletons and can be predesigned using a modular templated synthesis approach to promote polymerization in 2D. The most widely reported strategy to tailor the skeletal structures of 2D-C-POPs, involves the selection and tuning of building blocks to control the pore size, linkage chemistry, π-electron conjugation, and heteroatom functionalities. The scope for 2D-C-POPs functional applications is limited by challenges associated with fabricating them into scalable and adaptable form factors. In this thesis, I have explored the hierarchical design, synthesis, and properties of amorphous and crystalline 2D-C-POP materials tailored for environmental and biomedical applications. Anisotropic 2D-C-POP films were readily tuned with different heteroatoms functionalities and processed using a facile drop-cast approach which produced uniform free-standing amorphous films of adaptable surface areas and thicknesses. Even though the films lacked long-range order, they exhibited inherent microporosity, out-of-plane anisotropy and were found composed of disorderly stacked 2D polymeric sheets that could be easily exfoliated into atomic-thin layers. The films displayed high sensitivity and fast response in the robust colorimetric sensing of acids and could be adapted into different device architectures for real-time sensing applications. These imine-linked films were then compared to analogs assembled from fluorine enriched building blocks. The addition of fluorine promoted tailorable surface properties, enhanced order in sheet stacking, and related functional properties. Covalent Organic Frameworks (COFs) have long-range order and offer high surface areas and uniform pores architectures that make them an attractive platform for controlled-drug release. In this work, the novel synthesis of drug COF aerogels using one-pot synthesis approach to yield crystalline gels in a matter of minutes is reported. The hierarchical pore structure of the aerogels offers higher drug loading potential and improved and more controlled drug adsorption and release. Together these directions entail two orthogonal approaches of hierarchical design of conjugated porous organic polymer architectures for enhancing the adaptability of these rich structures and their excellent properties in diverse application platforms.
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    K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2
    (Springer Nature, 2022) Bae, Soungmin; Matsumoto, Kana; Raebiger, Hannes; Shudo, Ken-ichi; Kim, Yong-Hoon; Handegård, Ørjan Sele; Nagao, Tadaaki; Kitajima, Masahiro; Sakai, Yuji; Zhang, Xiang; Vajtai, Robert; Ajayan, Pulickel; Kono, Junichiro; Takeda, Jun; Katayama, Ikufumi
    In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.
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    Materials and Electromechanical Engineering of a Haptic System
    (2022-04-22) Ojha, Ved; Ajayan, Pulickel; O'Malley, Marcia
    Haptic technologies are set to play a key role in the design of next generation human-computer interfaces. While haptic feedback is being leveraged in areas such as consumer electronics, there is much to be done to create haptic systems that add value to our daily life. With the coming AR/VR revolution, wearables that add a sense of touch will enable truly immersive experiences. Haptic feedback is already augmenting professional training simulators used by astronauts, surgeons, soldiers to optimize learning routines. In other applications, haptic interfaces have already been shown to enable the deaf to "hear" again and the blind to navigate through environments with greater accuracy. Haptic feedback, therefore, is poised to redefine communications technology as a whole. I have explored actuation technologies, actuators with advanced polymers for generating crisp haptic feedback, data processing techniques and software design for triggering complex haptic patterns using sound signals, and low cost electronic design methodologies for driving arrays of electromagnetic actuators. I have demonstrated the above by designing a portable haptic vest using 112 eccentric rotating mass actuators, a haptic sleeve with 8 linear resonant actuators, and demonstrated a PVDF-TrFE polymer based piezoelectric actuator as a flexible and biocompatible alternative to traditional actuation technologies. A drawback that plagues most haptic systems is high power consumption, especially when operating at peak loads. When constructing energy efficient portable haptic systems, one must use an energy storage device capable of delivering high power while maintaining a stable energy density. Current lithium-ion batteries used in consumer electronics are energy dense but have low rate capability. I have explored a system known as a hybrid supercapacitor using a novel \ch{rGO/Nb2O5} nanocomposite anode, capable of delivering high energy at high current rates. As an intermediate energy storage device between batteries and supercapacitors it is ideal for analyzing the limitations of current energy storage systems and designing future energy solutions.
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    Mechanics of Platelet-Matrix Composites across Scales: Theory, Multiscale Modeling, and 3D Fabrication
    (2015-04-24) Sakhavand, Navid; Shahsavari, Rouzbeh; Nagarajaiah, Satish; Ajayan, Pulickel
    Many natural and biomimetic composites - such as nacre, silk and clay-polymer - exhibit a remarkable balance of strength, toughness, and/or stiffness, which call for a universal measure to quantify this outstanding feature given the platelet-matrix structure and material characteristics of the constituents. Analogously, there is an urgent need to quantify the mechanics of emerging electronic and photonic systems such as stacked heterostructures, which are composed of strong in-plane bonding networks but weak interplanar bonding matrices. In this regard, development of a universal composition-structure-property map for natural platelet-matrix composites, and stacked heterostructures opens up new doors for designing materials with superior mechanical performance. In this dissertation, a multiscale bottom-up approach is adopted to analyze and predict the mechanical properties of platelet-matrix composites. Design guidelines are provided by developing universally valid (across different length scales) diagrams for science-based engineering of numerous natural and synthetic platelet-matrix composites and stacked heterostructures while significantly broadening the spectrum of strategies for fabricating new composites with specific and optimized mechanical properties. First, molecular dynamics simulations are utilized to unravel the fundamental underlying physics and chemistry of the binding nature at the atomic-level interface of organic-inorganic composites. Polymer-cementitious composites are considered as case studies to understand bonding mechanism at the nanoscale and open up new venues for potential mechanical enhancement at the macro-scale. Next, sophisticated mathematical derivations based on elasticity and plasticity theories are presented to describe pre-crack (intrinsic) mechanical performance of platelet-matrix composites at the microscale. These derivations lead to developing a unified framework to construct series of universal composition-structure-property maps that decode the interplay between various geometries and inherent material features, encapsulated in a few dimensionless parameters. Finally, after crack mechanical properties (extrinsic) of platelet-matrix composites until ultimate failure of the material at the macroscale is investigated via combinatorial finite element simulations. The effect of different composition-structure-property parameters on mechanical properties synergies are depicted via 2D and 3D maps. 3D-printed specimens are fabricated and tested against the theoretical prediction. The combination of the presented diagrams and guidelines paves the path toward platelet-matrix composites and stacked-heterostructures with superior and optimized mechanical properties.
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    Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites
    (JoVE, 2016) Kim, Hojin; Zhu, Bohan; Chen, Huiying; Adetiba, Oluwatomiyin; Agrawal, Aditya; Ajayan, Pulickel; Jacot, Jeffrey G.; Verduzco, Rafael
    LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and as soft robots. Here, we demonstrate the preparation of shape-responsive liquid crystal elastomers (LCEs) and LCE nanocomposites along with characterization of their shape-responsiveness, mechanical properties, and microstructure. Two types of LCEs — polysiloxane-based and epoxy-based — are synthesized, aligned, and characterized. Polysiloxane-based LCEs are prepared through two crosslinking steps, the second under an applied load, resulting in monodomain LCEs. Polysiloxane LCE nanocomposites are prepared through the addition of conductive carbon black nanoparticles, both throughout the bulk of the LCE and to the LCE surface. Epoxy-based LCEs are prepared through a reversible esterification reaction. Epoxy-based LCEs are aligned through the application of a uniaxial load at elevated (160 °C) temperatures. Aligned LCEs and LCE nanocomposites are characterized with respect to reversible strain, mechanical stiffness, and liquid crystal ordering using a combination of imaging, two-dimensional X-ray diffraction measurements, differential scanning calorimetry, and dynamic mechanical analysis. LCEs and LCE nanocomposites can be stimulated with heat and/or electrical potential to controllably generate strains in cell culture media, and we demonstrate the application of LCEs as shape-responsive substrates for cell culture using a custom-made apparatus.