Browsing by Author "Ma, Lulu"

Now showing 1 - 4 of 4
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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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    Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides
    (The Optical Society, 2016) Razanoelina, Manjakavahoaka; Bagsican, Filchito Renee; Kawayama, Iwao; Zhang, Xiang; Ma, Lulu; Murakami, Hironaru; Vajtai, Robert; Ajayan, Pulickel M.; Kono, Junichiro; Tonouchi, Masayoshi
    As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 1011 cm−2, which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.
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    Structural determination of Enzyme-Graphene Nanocomposite Sensor Material
    (Springer Nature, 2019) Rai, Durgesh K.; Gurusaran, Manickam; Urban, Volker; Aran, Kiana; Ma, Lulu; Qian, Shuo; Narayanan, Tharangattu N.; Ajayan, Pulickel M.; Liepmann, Dorian; Sekar, Kanagaraj; Álvarez-Cao, María-Efigenia; Escuder-Rodríguez, Juan-José; Cerdán, María-Esperanza; González-Siso, María-Isabel; Viswanathan, Sowmya; Paulmurugan, Ramasamy; Renugopalakrishnan, Venkatesan; Li, Pingzuo
    State-of-the-art ultra-sensitive blood glucose-monitoring biosensors, based on glucose oxidase (GOx) covalently linked to a single layer graphene (SLG), will be a valuable next generation diagnostic tool for personal glycemic level management. We report here our observations of sensor matrix structure obtained using a multi-physics approach towards analysis of small-angle neutron scattering (SANS) on graphene-based biosensor functionalized with GOx under different pH conditions for various hierarchical GOx assemblies within SLG. We developed a methodology to separately extract the average shape of GOx molecules within the hierarchical assemblies. The modeling is able to resolve differences in the average GOx dimer structure and shows that treatment under different pH conditions lead to differences within the GOx at the dimer contact region with SLG. The coupling of different analysis methods and modeling approaches we developed in this study provides a universal approach to obtain detailed structural quantifications, for establishing robust structure-property relationships. This is an essential step to obtain an insight into the structure and function of the GOx-SLG interface for optimizing sensor performance.
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    Using the Plasmon Linewidth To Calculate the Time and Efficiency of Electron Transfer between Gold Nanorods and Graphene
    (American Chemical Society, 2013) Hoggard, Anneli; Wang, Lin-Yung; Ma, Lulu; Fang, Ying; You, Ge; Olson, Jana; Liu, Zheng; Chang, Wei-Shun; Ajayan, Pulickel M.; Link, Stephan; Laboratory for Nanophotonics
    We present a quantitative analysis of the electron transfer between single gold nanorods and monolayer graphene under no electrical bias. Using single-particle dark-field scattering and photoluminescence spectroscopy to access the homogeneous linewidth, we observe broadening of the surface plasmon resonance for gold nanorods on graphene compared to nanorods on a quartz substrate. Because of the absence of spectral plasmon shifts, dielectric interactions between the gold nanorods and graphene are not important and we instead assign the plasmon damping to charge transfer between plasmon-generated hot electrons and the graphene that acts as an efficient acceptor. Analysis of the plasmon linewidth yields an average electron transfer time of 160 ± 30 fs, which is otherwise difficult to measure directly in the time domain with single-particle sensitivity. In comparison to intrinsic hot electron decay and radiative relaxation, we furthermore calculate from the plasmon linewidth that charge transfer between the gold nanorods and the graphene support occurs with an efficiency of ∼10%. Our results are important for future applications of light harvesting with metal nanoparticle plasmons and efficient hot electron acceptors as well as for understanding hot electron transfer in plasmon-assisted chemical reactions.