Browsing by Author "Lou, Jun"

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    2D Optoelectronics: Challenges and Opportunities
    (2015-12-17) Lei, Sidong; Ajayan, Pulickel M; Lou, Jun; Kono, Junichiro
    Indium Selenide (InSe) is one of atomically layered 2D materials attracting broad interests recently, because of its good optoelectronic properties. Based on the challenges of 2D optoelectronics, several topics will be covered in this defense, such as trap states and low absorption rate. InSe is selected as a platform to study these topics. The localized states and trap states in InSe system was characterized through low temperature photocurrent measurement to reveal the evolution of band structure and origin of the localized states in few layered InSe. It is found the surface electron orbitals contribute to the localized states. By modifying the surface electron via metallic ions, the Fermi level can be tuned significantly and the inertia surface of the pristine 2D surface can be sensitized for functionalization. Via this method, the InSe photodetector can be improved by organic photosensitive molecules. On the other hand, local gating can induce trap states in 2D materials, helping to improve the photoresponse, but slowing down the response speed. By utilizing this effect, 2D charge coupled device can be fabricated to serve as flexible image sensor which can help correct the optical aberration. The discussion is based on InSe, however, the principle is very universal that can be easily apply to other 2D system. The research can help to promote the research and device development in 2D optoelectronics.
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    A cohesive law for interfaces in graphene/hexagonal boron nitride heterostructure
    (AIP Publishing LLC., 2014) Zhang, Chenxi; Lou, Jun; Song, Jizhou
    Graphene/hexagonal boron nitride (h-BN) heterostructure has showed great potential to improve the performance of graphene device. We have established the cohesive law for interfaces between graphene and monolayer or multi-layer h-BN based on the van der Waals force. The cohesive energy and cohesive strength are given in terms of area density of atoms on corresponding layers, number of layers, and parameters in the van der Waals force. It is found that the cohesive law in the graphene/multi-layer h-BN is dominated by the three h-BN layers which are closest to the graphene. The approximate solution is also obtained to simplify the expression of cohesive law. These results are very useful to study the deformation of graphene/h-BN heterostructure, which may have significant impacts on the performance and reliability of the graphene devices especially in the areas of emerging applications such as stretchable electronics.
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    Additive manufacturing and mechanical properties of 2D h-BN reinforced nanocomposite
    (2022-12-02) Zhang, Boyu; Lou, Jun
    Ceramic and inorganic materials have extraordinary physical properties, such as high modulus, high strength, high hardness and heat resistant. However, the low toughness hinders the wide application of ceramic. Although ceramic can withstand high load in the elastic stage, once the crack emerged inside the bulk ceramic, it will propagate fast and cause the dramatic failure of the whole structure. To solve this problem, several toughening mechanisms has been developed, such as bridging mechanism and crack deflection. The main idea is to hinder the crack propagation or increase the energy needed for the crack propagation. One effective way is the add reinforcements in the ceramic to create reinforced ceramic composites. Graphene has been verified to be a promising candidate for reinforcing ceramic matrix composites due to its extremely high elastic modulus (~1 TPa) and intrinsic strength (~130 GPa). However, similar to ceramics, graphene also has brittle fracture behavior, and its fracture toughness is only about 4 MPa·m1/2. h-BN is a kind of 2D material which has a similar lattice structure to graphene, with B and N atoms adjacent to each other. It is well known for it dielectric properties and widely used as dielectric substrate and protective layer. People has found its robustness since it can dramatically increase the sample survival rate if used as protective layer. However, its toughness mechanical property has not been systematically studied since recent years. In this thesis, the mechanical properties of h-BN, interfacial mechanical properties of h-BN/ceramic nanocomposites and additive manufacturing h-BN/silica nanocomposites are shown. The intrinsic toughening mechanism, h-BN/ceramic nanocomposites toughening mechanism are systematically discussed. In chapter 2, the intrinsic toughening fracture behavior of single layer h-BN was firstly introduced and a dual fracture mode (asynchronous and synchronous fracture) of multilayer h-BN caused by interlayer mechanical coupling effect was shown. In chapter 3, by using nanoindentation-assisted micro-mechanical devices integrated with scanning electron microscopy (SEM), the interfacial sliding and failure behaviors between h-BN and PDC were systematically studied. In chapter 4 and 5, 2PP 3D printing technique was developed to create high quality silica and h-BN/silica nanostructures with sub-200 nm. High Q microtoroid resonators and strong photoluminescence of rare earth doped silica nanostructures were demonstrated and mechanical properties of h-BN/silica nanocomposites was studied. Overall, this thesis contributes to the knowledge of toughening mechanism of h-BN and h-BN nanocomposites and shows the advanced way to fabricate inorganic nanocomposites with nanoscale resolution.
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    Application of Compact, Geometrically Complex Shape Memory Alloy Devices for Seismic Enhancement of Highway Bridge Expansion Joints
    (2014-04-25) McCarthy, Emily Ruth; Padgett, Jamie E.; Stanciulescu, Ilinca; Lou, Jun; DesRoches, Reginald
    Highway bridges are an important part of transportation networks. They provide connectivity across waterways, ravines and other roadways, reducing commuting times and facilitating social community. The disruption of their effective operation caused by earthquake damage has lasting effects based on repair costs, road closure times, traffic rerouting causing extended commute times and additional CO2 emissions, and the potential prevention of emergency responders being able to reach affected regions. Bridge expansion joints have historically been recognized as the most vulnerable component in the bridge system during these seismic events, causing dramatic disruption to bridge functionality because of their location in bridges (points of discontinuity in deck systems). Expansion joint systems are placed in these locations of discontinuity and accommodate bridge movements from thermal effects while facilitating safe driving surfaces across large gaps in the roadway. Commonly installed systems are not designed to survive seismic events, instead failure is assumed and replacement necessary to return the bridge to its functional state. When damaged, the large gaps they span can be un-crossable without external intervention, resulting in non-functioning bridges even when the structural system remains sound. Expensive and complex expansion systems exist, which prevent seismic damage, however they are used mostly in highly seismic regions and limitedly elsewhere. This dissertation provides an expansion joint design that is economical and superior in seismic performance to the commonly installed service level expansion joints so that more bridges in moderate seismic regions can be equipped with expansion systems able to accommodate large longitudinal displacement demands from earthquakes. The use of innovative shape memory alloy (SMA) springs enables a single support bar modular bridge expansion joint (one type of large capacity expansion joint) to accommodate seismic level longitudinal displacements while maintaining existing performance behavior for service level thermal expansion demands. Through limited alteration of the existing configuration, costs are minimized. The resulting design is experimentally and analytically shown to be superior in performance and able to prevent expansion joint system failure during dynamic loading. The use of fragility curves, which are probabilistic statements of demand exceeding capacity, offers a means of measuring performance over a range of earthquake intensities. Convolution with seismic hazard curves for some moderate seismic zones in the US over a range of time intervals provide information on lifetime seismic risk, valuable information for a cost benefit analysis that concludes investment in SMA springs for enhancement of modular bridge expansion joints is worthwhile for the cost reduction they offer over the life of the bridge.
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    Area-selective atomic layer deposition on 2D monolayer lateral superlattices
    (Springer Nature, 2024) Park, Jeongwon; Kwak, Seung Jae; Kang, Sumin; Oh, Saeyoung; Shin, Bongki; Noh, Gichang; Kim, Tae Soo; Kim, Changhwan; Park, Hyeonbin; Oh, Seung Hoon; Kang, Woojin; Hur, Namwook; Chai, Hyun-Jun; Kang, Minsoo; Kwon, Seongdae; Lee, Jaehyun; Lee, Yongjoon; Moon, Eoram; Shi, Chuqiao; Lou, Jun; Lee, Won Bo; Kwak, Joon Young; Yang, Heejun; Chung, Taek-Mo; Eom, Taeyong; Suh, Joonki; Han, Yimo; Jeong, Hu Young; Kim, YongJoo; Kang, Kibum
    The advanced patterning process is the basis of integration technology to realize the development of next-generation high-speed, low-power consumption devices. Recently, area-selective atomic layer deposition (AS-ALD), which allows the direct deposition of target materials on the desired area using a deposition barrier, has emerged as an alternative patterning process. However, the AS-ALD process remains challenging to use for the improvement of patterning resolution and selectivity. In this study, we report a superlattice-based AS-ALD (SAS-ALD) process using a two-dimensional (2D) MoS2-MoSe2 lateral superlattice as a pre-defining template. We achieved a minimum half pitch size of a sub-10 nm scale for the resulting AS-ALD on the 2D superlattice template by controlling the duration time of chemical vapor deposition (CVD) precursors. SAS-ALD introduces a mechanism that enables selectivity through the adsorption and diffusion processes of ALD precursors, distinctly different from conventional AS-ALD method. This technique facilitates selective deposition even on small pattern sizes and is compatible with the use of highly reactive precursors like trimethyl aluminum. Moreover, it allows for the selective deposition of a variety of materials, including Al2O3, HfO2, Ru, Te, and Sb2Se3.
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    Blueshift of the A-exciton peak in folded monolayer 1H-MoS2
    (American Physical Society, 2013) Crowne, Frank J.; Amani, Matin; Birdwell, A. Glen; Chin, Matthew L.; O'Regan, Terrance P.; Najmaei, Sina; Liu, Zheng; Ajayan, Pulickel M.; Lou, Jun; Dubey, Madan
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    Carbon Nanotube Doping Procedures for Three-Dimensional Macro-Structures and Gallium-Nitride Functionalization
    (2014-05-05) Hashim, Daniel Paul; Ajayan, Pulickel M.; Lou, Jun; Rau, Carl
    Carbon nanotubes (CNTs) in all of their forms are considered “gamechanger” materials that will revolutionize the modern world through many diverse applications. Over 20 years of research has gone into CNT materials, yet we still see their limited use in feasible real-world applications. Part of the reason is because it still remains a challenge for materials scientists to engineer these extraordinary nano-scale building blocks into covalently interconnected three-dimensional (3-D) structures, and to realize macro-scaled sizes via a bulk synthesis process. Another challenge is being able to create CNT-semiconductor hybrid materials by covalently joining other useful semiconductor compounds with CNTs in order to harness their value for electronics applications. The experimental research compiled in the first part of this thesis pioneers an innovative approach to synthesize 3-D macro-structured forms of CNTs by utilizing a heteroatom doping strategy via chemical vapor deposition (CVD). The importance of substitutional doping effects of boron on CNT structural morphology is characterized experimentally and theoretically for the first time so as to create a robust, solid, 3-D networked, CNT “sponge” form. The CNT “sponge” was characterized to exhibit an exotic combination of multifunctional properties including high porosity, high surface area, low density, superhydorphicity, oleophilicity, ferromagnetism, and good elastic mechanical performance. It was also demonstrated that 3-D porous CNT “sponges” could be used for environmental needs as reusable oil spill sorbent materials in seawater. In an effort to combine group III–V semiconductors with CNTs, the second part of this thesis involve a simple solution-based technique for gallium functionalization of nitrogen-doped multi-wall carbon nanotubes. With an aqueous solution of a gallium salt (GaI3), it was possible to form covalent bonds between the Ga3C ion and the nitrogen atoms of the doped carbon nanotubes to form a gallium nitride–carbon nanotube hybrid at room temperature. This functionalization was evaluated by x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy.
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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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    Compositional and Heterostructure Engineering in Low Dimensional Metal Halide Perovskites
    (2024-01-22) Liu, Yifeng; Lou, Jun
    Metal halide perovskites (MHPs) have become the preeminent semiconducting materials serving for photovoltaics, light-emitting diodes, photodetectors, lasers and photocatalysis, attributed to their superior optoelectronic properties. Low-dimensional MHPs with unique properties originating from quantum confinement and dimension dependence have garnered considerable attention in photonic and electronic studies. To enrich the function versatility and performance optimization to address the practical challenges, chemical engineering strategies have been applied in MHPs including compositional modification, heterostructure fabrication, and composite synthesis. This thesis will center on the compositional and heterostructure engineering in low-dimensional MHPs, specifically lead-free bismuth halide perovskites, for the unprecedented MHP research on photonics, spin physics and energy conversion. First, chemical vapor deposition and anion exchange protocols to synthesize bismuth halide perovskite nanoflakes with controlled dimensions and variable compositions are established. In particular, the gradient bromide distribution by controlling the anion exchange and diffusion is spatially resolved by the time-of-flight secondary ion mass spectrometry. Moreover, the optical waveguiding properties of bismuth halide perovskites can be modulated by the flake thickness and anion composition. Next, a novel metal ion doping protocol through the vapor phase metal halide insertion reaction to the CVD-grown ultrathin Cs3BiBr6 perovskites is presented. The Fe-doped Cs3BiBr6 (Fe: Cs3BiBr6) II perovskites demonstrate that the iron spins are successfully incorporated into the lattice, as revealed by the spin-phonon coupling below the critical temperature Tc around 50 K observed through the temperature-dependent Raman spectroscopy. Furthermore, the phonons exhibit significant softening under the applied magnetic field, possibly originating from magnetostriction and spin exchange interaction. In addition to the compositional engineering in ultrathin bismuth halide perovskite crystals, the direct CVD growth of the Cs3Bi2I9 based heterostructures is introduced. Cs3Bi2I9-MoSe2 and Cs3Bi2I9-MoS2 heterostructures are synthesized to investigate the strong interlayer couplings within the heterostructures. Cs3Bi2I9-graphene heterostructures are developed for future applications in X-ray sensing transistors. Finally, the synthesis of lead-free bismuth halide perovskite nanocrystals encapsulated by the covalent organic frameworks (PNCs- COFs) via an in-situ growth approach is reported. The PNCs-COFs were further applied as photocatalysts for free-radical polymerizations, photo-activating different co-initiators via both hole and electron transfer mechanisms in aqueous and organic phases. The excellent photocatalytic performance of PNCs-COFs was confirmed by high monomer conversion (up to 97.5%), diverse functional group tolerance and recyclability. These protocols provide facile, universal, well-adaptable and efficient avenues for exploring the versatilities of Bi-based perovskite materials in low dimensions. The endeavors devote to shedding light on the potential of low-dimensional MHPs (bismuth halide perovskites) as promising semiconductors for optoelectronics, spin physics and solar energy harvesting.
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    Computational framework for the analysis of hybrid masonry systems using an improved non-local technique
    (2014-12-05) Gao, Zhenjia; Stanciulescu, Ilinca; Padgett, Jamie; Lou, Jun; Willam, Kaspar
    Hybrid masonry structures combine the ductility of steel components with the shear strength of reinforced masonry panels. The goal of this research is to provide a sound basis for the design of an optimal type of hybrid structure that can be implemented as a new lateral-force-resisting system in high seismic regions. The most challenging part in the hybrid structure simulation is to capture the behaviour of concrete under different loading scenarios. This thesis sets up a computational framework for the analysis of hybrid masonry systems using an improved non-local technique, including the contributions such as: adopting the consistent linearisation technique to improve the computational efficiency of the non-local one-scalar damage model; presenting a new way to calibrate parameters in the tension damage law in the two-scalar damage model by correlating them to the ones in the one-scalar damage model; designing a data structure to save the domain information for each material point in order to apply the non-local technique; proposing an automatic parameter calibration procedure based on the Nelder-Mead simplex method for the two-scalar damage model utilizing the global system testing data; proposing and identifying the internal variable to be non-localized to enhance a new damage model to obtain the mesh regularization solution. Finally, this thesis performs a system-level numerical study of the energy dissipation mechanisms of hybrid masonry structures under cyclic loading. The numerical studies extrapolate test data to a wider range of structural configurations in terms of various connector strengths and different masonry panels to maximize seismic energy dissipation. This work also investigates the influence of the load transfer mechanism on the lateral strength, stiffness, energy dissipation capacity and deformation pattern of the hybrid system. Findings from the numerical studies performed in this work confirm the feasibility of using hybrid structures in high seismic areas.
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    Computational study of synthesis, structure, property and application of low-dimensional materials
    (2014-07-29) Liu, Yuanyue; Yakobson, Boris I.; Lou, Jun; Tour, James M.
    Low-dimensional materials have attracted intense interest due to their unconventional properties and promising potential for applications. In this thesis, state-of-art computational methods are employed to study the syntheses, structures, properties, and applications of low-dimensional materials, including carbon nanotube, graphene, boron nitride and transition metal dichalcogenides. Special focus is given to the atomistic mechanism of chemical vapor deposition growth, defect structure and properties, Li-ion battery, and catalytic hydrogen production. Design of novel materials based on Materials Genome approach will also be presented.
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    Computer simulations on mechanical and electrical properties of nanoscale materials
    (2013-12-06) Hua, Ming; Yakobson, Boris I.; Lou, Jun; Kono, Junichiro
    Nanoscale materials have highly regular atomistic structures with very few defects due to their small sizes. The small size and near-perfect structure give such materials unique properties compared with materials at a larger scale. This work investigates the structures and properties of several nanoscale materials using various computer simulation methods. The great strength of carbon nanotubes comes from the strong covalent bonding between carbon atoms, and has been of great interest in research, however both the theoretical and experimental results obtained are in a wide range. In this work, different atomic mechanisms about the nucleation of structural failure are proposed and analyzed, revealing the competition of two routes of forming defects--brittle bond breaking and plastic yield. The relevance of these two routes are shown to be dependent on nanotube symmetry, test time, and temperature. The nanotube strength is decided by the dominant route chosen under these parameters. Helical symmetry exists in many nanoscale structures, but it's far less utilized in computer simulations compared with translational and rotational symmetry. In this work a model for helical symmetry in tight-binding computational method is developed, then the implemented code are used to calculate the structure of thin silicon nanowires, as well as the properties of twisted armchair graphene nanoribbons, such as their deformation energy, band gap, and electrical conductance. Inspired by carbon nanotube, this work also investigates very thin silicon nanotubes. They are shown to have stable structures when filled with various metal atoms along the axis. They can also go through significant structural changes from one stable atomistic configuration to another. Such thin metal-endohedral silicon nanotubes can then combine to form thicker silicide wires that are morphologically identical to experimental disilicide wires synthesized from epitaxial growth.
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    CVD Grown Graphene-Based Materials: Synthesis, Characterization and Applications
    (2015-04-22) Ma, Lulu; Ajayan, Pulickel M; Lou, Jun; Zheng, Junrong; Vajtai, Robert
    Graphene draws a lot of attention due to its exceptional electrical, mechanical, thermal, optical and chemical properties. However, its zero bandgap is a limitation for electronics applications and its two-dimensional (2D) nature is a limitation for large scale, volumetric and macroscopic applications. Doping graphene with heteroatoms and creating graphene hetero-structures are two approaches herewith suggested to sidestep the above limitations. The illustration of thus approaches begins with the chemical vapor deposition (CVD) growth of graphene in the form of either atomically thin films or 3D porous structures; which involves the synthesis of several structures such as nitrogen-doped graphene, graphene-carbon nanotube hybrids, in-plane graphene-boron nitride heterostructures, and graphene-molybdenum carbide hybrids. The analysis of impurities in CVD grown graphene at the atomic scale and the measurement of fracture toughness of graphene will then follow. Furthermore, the potential applications of as-synthesized materials like field emitters, supercapacitors, and catalysts for water splitting are discussed.
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    CVD-grown monolayered MoS2 as an effective photosensor operating at low-voltage
    (IOP Publishing, 2014) Perea-López, Néstor; Lin, Zhong; Pradhan, Nihar R.; Iñiguez-Rábago, Agustín; Elías, Ana Laura; McCreary, Amber; Lou, Jun; Ajayan, Pulickel M.; Terrones, Humberto; Balicas, Luis; Terrones, Mauricio
    We report the fabrication of a photosensor based on as-grown single crystal monolayers of MoS2synthesized by chemical vapor deposition (CVD). The measurements were performed using Au/Ti leads in a two terminal configuration on CVD-grown MoS2 on a SiO2/Si substrate. The device was operated in air at room temperature at low bias voltages ranging from −2 V to 2 V and its sensing capabilities were tested for two different excitation wavelengths (514.5 nm and 488 nm). The responsivity reached 1.1 mA W−1 when excited with a 514.5 nm laser at a bias of 1.5 V. This responsivity is one order of magnitude larger than that reported from photo devices fabricated using CVD-grown multilayered WS2. A rectifying-effect was observed for the optically excited current, which was four times larger in the direct polarization bias when compared to the reverse bias photocurrent. Such rectifying behavior can be attributed to the asymmetric electrode placement on the triangular MoS2 monocrystal. It is envisioned that these components could eventually be used as efficient and low cost photosensors based on CVD-grown transition metal dichalcogenide monolayers.
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    Defect-mediated transport and electronic irradiation effect in individual domains of CVD-grown monolayer MoS2
    (AIP Publishing, 2015) Durand, Corentin; Zhang, Xiaoguang; Fowlkes, Jason; Najmaei, Sina; Lou, Jun; Li, An-Ping
    The authors study the electrical transport properties of atomically thin individual crystalline grains of MoS2ﾠwith four-probeﾠscanning tunneling microscopy.ﾠTheﾠmonolayerﾠMoS2ﾠdomains are synthesized byﾠchemical vapor depositionﾠon SiO2/Si substrate. Temperature dependent measurements on conductance andﾠmobilityﾠshow that transport is dominated by an electron charge trapping and thermal release process with very lowﾠcarrier densityﾠandﾠmobility.ﾠThe effects of electronicﾠirradiationﾠare examined by exposing the film toﾠelectron beamﾠin theﾠscanning electron microscopeﾠin an ultrahigh vacuum environment. Theﾠirradiationﾠprocess is found to significantly affect theﾠmobilityﾠand theﾠcarrier densityﾠof the material, with the conductance showing a peculiar time-dependent relaxation behavior. It is suggested that the presence of defects in active MoS2ﾠlayer and dielectric layer create charge trapping sites, and a multiple trapping and thermal release process dictates the transport andﾠmobilityﾠcharacteristics. Theﾠelectron beamﾠirradiationﾠpromotes the formation of defects and impact the electrical properties of MoS2. Our study reveals the important roles of defects and theﾠelectron beamﾠirradiationﾠeffects in the electronic properties of atomic layers of MoS2.
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    Efficient hydrogen evolution in transition metal dichalcogenides via a simple one-step hydrazine reaction
    (Springer Nature, 2016) Cummins, Dustin R.; Martinez, Ulises; Sherehiy, Andriy; Kappera, Rajesh; Martinez-Garcia, Alejandro; Schulze, Roland K.; Jasinski, Jacek; Zhang, Jing; Gupta, Ram K.; Lou, Jun; Chhowalla, Manish; Sumanasekera, Gamini; Mohite, Aditya D.; Sunkara, Mahendra K.; Gupta, Gautam
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    Electrical and Optoelectronic Devices from Two-Dimensional Materials for Advanced and Integrated Functionalities
    (2018-11-30) Jin, Zehua; Ajayan, Pulickel M.; Lou, Jun
    The past decade saw rapid development of two-dimensional (2D) layered materials. Being thin and flexible, 2D materials show unique properties as well as high-performance for a variety of electrical and optoelectronic applications. Despite ever-growing progresses on developing 2D materials based devices, there is great amount of room for improving device performance as well as seeking additional functionalities. For example, electrical contact serves as the bottleneck for electrical or optoelectronic devices based on 2D materials. A systematic solution, preferably in large scale, would greatly boost the applicability of 2D materials in various device structures. Talking about semiconductor devices, how to reduce dark current as well as boost device speed has been a major challenge facing photodetectors based on 2D transition metal dichalcogenides. In addition, in real-world applications, many photodetectors work in a more complicated physical principle. For example, image sensors work in a charge integration manner. Can those 2D high-performance photodetectors demonstrate this compatibility? Furthermore, can we further combine the high-performance of 2D optoelectronics with its integrability, to realize its sensitivity on various surfaces? In this thesis, I illustrate my efforts in solving or partially solving the above few questions, aiming to achieve advanced and integrated functionalities of 2D materials. Specifically, I have tried tackling the contact resistance issue by direct synthesis of mixed phase in-planar junctions in large scale. In addition, photodetectors based on III-VI InSe materials as well as its junction structures have been explored for improved photodetection performance. Furthermore, a novel device transfer technique has been developed, enabling the transfer of 2D sensors to a variety of surfaces for near-field sensing applications.
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    Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition
    (American Institute of Physics, 2013) Amani, Matin; Chin, Matthew L.; Birdwell, A. Glen; O'Regan, Terrance P.; Najmaei, Sina; Liu, Zheng; Ajayan, Pulickel M.; Lou, Jun; Dubey, Madan
    Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition. The layer qualities and physical properties were characterized using high-resolution Raman and photoluminescence spectroscopy, scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET devices was measured using field effect mobility measurements as a function of temperature. The back-gated devices had mobilities of 6.0 cm2/V s at 300K without a high-j dielectric overcoat and increased to 16.1 cm2/V s with a high-j dielectric overcoat. In addition the devices show on/off ratios ranging from 105 to 109.
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    Electrochemical Behavior of Two-Dimensional Atomic Layers
    (2016-11-22) Zhang, Jing; Lou, Jun
    In this thesis several aspects of the electrochemical behaviors of two-dimensional layered materials are discussed. First, large-area continuous few-layer molybdenum disulfide film is prepared by simple solid-gas elemental reaction and transferred onto fluorine doped tin oxide glass substrate as the counter electrode for dye-sensitized solar cells. The catalytic activity of the MoS2 atomic layers are dramatically improved by craving the MoS2 film and creating artificial edges on it. Electrochemical analysis shows that the edges contribute to the improve catalytic activity of MoS2. Second, large-area continuous hexagonal boron nitride film is grown by chemical vapor deposition method. The film is transferred onto copper substrate and tested as the corrosion-inhibiting coating in sodium sulphate aqueous solution. The sample with 30nm h-BN coating shows significantly suppressed copper dissolution peak and only one fourth of the corrosion rate of the uncoated sample. Electrochemical impedance spectroscopy analysis reveals that the charge transfer resistance is much higher when h-BN film is present. Third, we invent a local probe electrochemical measurement method and successfully applied it to the electrolysis catalytic activity measurement of various kinds of transition metal dichalcogenides. The catalytic activity and turnover frequencies of the 2H-MoS2 basal plane versus edge as well as the 1T’-MoS2 basal plane are identified by this measurement. At the same time, the basal plane activity and turnover frequencies of transition metal dichalcogenides from different element groups has been obtained. We have shown that the general trend of the transition metal dichalcogenides in the form of volcano plot follows the trend of metals. VB-VIA dichalcogenides have been identified as the preferential selection for hydrogen evolution reaction catalysts. Last, we discussed the measurement of layered materials in photoelectrolysis using the local probe electrochemical method. Gallium selenide as a good photoconductor, is examined as the photoelectrolysis catalyst and shows promising photoelectrochemical hydrogen evolution performance. The turnover frequency and photon-to-electron conversion efficiency are obtained from our measurements.
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    Electromechanical Investigation of Low Dimensional Nanomaterials for NEMS Applications
    (2011) Lu, Hao; Lou, Jun
    Successful operation of Nano-ElectroMechanical Systems (NEMS) critically depends on their working environment and component materials' electromechanical properties. It is equally important that ambient or liquid environment to be seriously considered for NEMS to work as high sensitivity sensors with commercial viabilities. Firstly, to understand interaction between NEMS oscillator and fluid, transfer function of suspended gold nanowire NEMS devices in fluid was calculated. It was found that NEMS's resonance frequency decreased and energy dissipation increased, which constrained its sensitivity. Sensitivity limit of NEMS oscillators was also considered in a statistical framework. Subsequently, suspended gold nanowire NEMS devices were magnetomotively actuated in vacuum and liquid. Secondly, electromechanical properties of gold nanowires were carefully studied and the observed size effect was found to agree with theory, which predicted small changes of electromechanical property compared with bulk gold materials. Finally, it is well recognized that continuous development of new NEMS devices demands novel materials. Mechanical properties of new two-dimensional hexagonal Boron Nitride films with a few atomic layers were studied. Outlook of utilizing ultrathm BN films in next generation NEMS devices was discussed.