Browsing by Author "Ajayan, Pulickel M"

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    Growth of 2D materials and application in electrochemical energy conversion
    (2016-11-09) Ye, Gonglan; Ajayan, Pulickel M; Vajtai, Robert
    The discovery of graphene in 2004 has generated numerous interests among scientists for graphene’s versatile potentials. The enthusiasm for graphene has recently been extended to other members of two-dimensional(2D) materials for applications in electronics, optoelectronics, and catalysis. Different from graphene, atomically-thin transition metal dichalcogenides (TMDs) have varied band gaps and would benefit for applications in the semiconductor industry. One of the promising applications of 2D TMDs is for 2D integrated circuits to replace current Si based electronics. In addition to electronic applications, 2D materials are also good candidates for electrochemical energy storage and conversion due to their large surface area and atomic thickness. This thesis mainly focuses on the synthesis of 2D materials and their application in energy conversion. Firstly, we focus on the synthesis of two-dimensional Tin Disulfide(SnS2). SnS2 is considered to be a novel material in 2D family. 2D SnS2 has a large band gap (~ 2.8 eV) and high carrier mobility, which makes it a potential applicant for electronics. Monolayer SnS2 with large scale and high crystal quality was successfully synthesized by chemical vapor deposition (CVD), and its performance as a photodetector was examined. The next chapter demonstrated a generic method for growing millimeter-scale single crystals as well as wafer-scale thin films of TMDs. This generic method was obtained by studying the precursors’ behavior and the flow dynamics during the CVD process of growing MoSe2, and was extended to other TMD layers such as millimeter-scale WSe2 single crystals. Understanding the growth processes of high quality large area monolayers of TMDs is crucial for further fundamental research as well as future development for scalable complex electronics. Besides the synthesis of 2D materials with high qualities, we further explored the relationship between defects and electrochemical properties. By directly observing and correlating the microscale structural changes of TMD monolayers such as MoS2 to the catalytic properties, we were able to provide insight on the fundamental catalytic mechanism for hydrogen evolution reaction. Finally, we used the 2D materials to build up 3D architectures, showing excellent performance in energy storage and conversion. For example, we used graphene as a conductive scaffold to support vanadium oxide (V2O5) on nanoscale, and achieved high performances for supercapacitors. Also, we applied the Pt anchored N-doped graphene nanoribbons as the catalyst for methanol electro oxidation, and reported the best performance among Pt/Carbon-based catalysts.
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    Materials Compositions for Lithium Ion Batteries with Extended Thermal Stability
    (2016-11-30) Kalaga, Kaushik; Ajayan, Pulickel M
    Advancements in portable electronics have generated a pronounced demand for rechargeable energy storage devices with superior capacity and reliability. Lithium ion batteries (LIBs) have evolved as the primary choice of portable power for several such applications. While multiple variations have been developed, safety concerns of commercial technologies limit them to atmospheric temperature operability. With several niche markets such as aerospace, defense and oil & gas demanding energy storage at elevated temperatures, there is a renewed interest in developing rechargeable batteries that could survive temperatures beyond 100oC. Instability of critical battery components towards extreme thermal and electrochemical conditions limit their usability at high temperatures. This study deals with developing material configurations for LIB components to stabilize them at such temperatures. Flammable organic solvent based electrolytes and low melting polymer based separators have been identified as the primary bottleneck for LIBs to survive increasing temperature. Furthermore, thermally activated degradation processes in oxide based electrodes have been identified as the reason for their limited lifetime. A quasi-solid composite comprising of room temperature ionic liquids (RTILs) and Clay was developed as an electrolyte/separator hybrid and tested to be stable up to 120oC. These composites facilitate complete reversible Li intercalation in lithium titanate (LTO) with a stable capacity of 120 mAh g-1 for several cycles of charge and discharge while simultaneously resisting severe thermal conditions. Modified phosphate based electrodes were introduced as a reliable alternative for operability at high temperatures in this study. These systems were shown to deliver stable reversible capacity for numerous charge/discharge cycles at elevated temperatures. Higher lithium intercalation potential of the developed cathode materials makes them interesting candidates for high voltage lithium batteries, which may be dubbed as the next generation devices. Architectural engineering of battery components to amplify the device performance is also discussed. 3D electrode structures developed using CVD and electrodeposition techniques demonstrated significant enhancement in performance when compared to their 2D analogues. The study has established the prospects of LIBs at high temperatures through material tuning and engineering approaches and envisage a scope for viable devices.
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    Multicomponent Chalcogenides
    (2019-08-27) Susarla, Sandhya; Ajayan, Pulickel M
    Transition metal dichalcogenides (TMDs), a class of two-dimensional (2D) materials, are proposed to be the next generation materials for optoelectronic, spintronic, and valleytronic devices due to their direct semiconducting bandgap, strong spin-orbit coupling and non-equivalent K points in momentum space. However, pristine TMDs fall short for these purposes due to their fixed band gap and low life times of intrinsic excitons. From a materials design perspective, alloying and heterostructure formation with TMDs are some of viable solutions. The first part of this thesis discusses TMDs design for optoelectronics and valleytronics. For optoelectronic applications, multicomponent alloying is used: different strategies like binary, non-isomorphous quaternary, and isomorphous quaternary alloying have been adopted. For valleytronics, the focus is on tuning the long-lifetime interlayer (IL) excitons present in vertical 2D heterostructures by straining and twisting. The second part of this thesis details the synthesis and emergent properties of a bulk binary chalcogen alloy (S-Se). Combining insulating S and Se results in the formation of a flexible alloy with very high dielectric constant and strength. It is believed that this S-Se alloy could perfectly bridge the gap between conventional brittle ceramics and flexible polymers.
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    Synthesis and electrochemical evaluation of active materials for energy storage applications
    (2022-08-10) Castro, Samuel; Ajayan, Pulickel M
    Since its successful commercialization in the early 1990s, Li-ion batteries (LIBs) have been one of the key factors in the technological growth of portable electronics, the aerospace industry, biomedical applications, and means of transportation. Research in the battery field has focused on synthesizing insertion cathode materials that rely on transition metals like Co, Mn, and Ni to improve figures of merit like the energy density and life cycle while reducing the cost per energy unit. Recently, there's been an increasing interest in employing LIBs for operation in harsh thermal environments such as outer space, military conditions, the oil and gas industry, and applications with a demand for high-power applications that causes a rise in the internal temperature to higher than 80 C. Unfortunately, the state-of-the-art materials currently used in commercial LIBs (LCO, NMC, NCA) suffer from structural instability upon heating that causes cathode degradation and decreases the electrode/cell performance. There is an area of opportunity in the development of materials that are structurally and electrochemically stable at high temperatures (>55 ºC). In the quest for cathode materials for high-temperature LIBs, phase-changing materials that modulate their electrical properties upon heating appear as a possible solution. Furthermore, there is a focus on replacing components like Cobalt that are scarce and toxic, and their extraction involves an unethical workforce. Herein, Sulfur based cathodes gain attention due to their high theoretical capacity and low cost. This thesis concentrates the work done on the synthesis, structural characterization, and electrochemical evaluation of materials with potential applications as insertion and conversion cathodes for LIBs and Li-metal batteries. A Vanadium dioxide cathode material, synthesized by a hydrothermal reduction reaction, shows promising specific capacities, high-capacity retention, and better rate capability upon a reversible phase transition. A Sulfur-selenium cathode composite was synthesized on carbon matrixes and used as a cathode material in a next-generation battery.
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    Two-dimensional molybdenum ditelluride (MoTe2): synthesis, characterization, and application
    (2018-04-19) Zhang, Xiang; Ajayan, Pulickel M
    Recent research efforts in two-dimensional (2D) materials have shown an increasing focus on molybdenum ditelluride (MoTe2). Unlike other TMDs, MoTe2 is distinguished by the existence of two stable phases, hexagonal 2H phase and monoclinic 1T’ phase, both of which can be synthesized directly. 2H MoTe2 is a semiconductor with a band gap of ~ 1 eV, while 1T’ MoTe2 is metallic which can be transformed to a type-II Weyl semimetal at low temperature. The semiconductor-metal junction between 2H and 1T’ MoTe2 shows the potential to resolve the issue of the existence of high Schottky barrier in traditional devices. MoTe2 does not only possesses a myriad of physical properties to further unravel but also shows great potential towards various industrial applications such as analog circuits and spintronics. Here in this thesis, chapter 1 gave a brief introduction to 2D materials. Several common synthesis methods, characterization methods, and applications were discussed. Chapter 2 focused on the phase-controlled synthesis of large-area MoTe2 films and 2H/1T’ MoTe2 heterostructures by chemical vapor deposition. A series of techniques have been used to systematically characterize the synthesized MoTe2 films. In chapter 3, several types of MoTe2-based devices were fabricated and measured. We demonstrated an electrical device across the one-step-synthesized 2H/1T’ MoTe2 in-plane heterostructure, where 1T’ phase serves as the contact electrodes for the 2H phase channel. An improved current density was observed compared with deposited metal electrodes on top. In chapter 4, we studied the Raman enhancement on MoTe2 films. MoTe2-based 2D heterostructures exhibit the potential as novel platforms for surface-enhanced Raman scattering (SERS) application. Our recent efforts in the spatial phase-targeted synthesis of 2H and 1T’ MoTe2 were presented in chapter 5. This strategy was suitable not only for large-scale patterns but also for small features. In chapter 6, we used the ultrafast electron diffraction (UED) to investigate the nonradiative process in 2H MoTe2 at SLAC National Accelerator Laboratory. This thesis doesn’t only study the fundamental properties of MoTe2, but also paves the way towards the large-scale application of MoTe2 in electronic and optoelectronic devices.