Propagative Exfoliation of High Quality Graphene

通讯作者地址: Deng, SL (通讯作者) Xiamen Univ, Dept Chem, Coll Chem & Chem Engn, Xiamen 361005, Peoples R China. [email protected]; [email protected] quality graphene materials that readily disperse in water or organic solvents are needed to achieve some of the most ambitious applications. However, current synthetic approaches are typically limited by irreversible structural damages, little solubility, or low scalability. Here, we describe a fundamental study of graphene chemistry and covalent functionalization patterns on sp(2) carbon lattices, from which a facile, scalable synthesis of high quality graphene sheets was developed. Graphite materials were efficiently exfoliated by reductive, propagative alkylation. The exfoliated, propagatively alkylated graphene sheets (PAGenes) not only exhibited high solubility in common solvents such as chloroform, water, and N-methyl-pyrrolidone, but also showed electrical conductivity as high as 4.1 X 10(3) S/m, which is 5 orders of magnitude greater than those of graphene oxides. Bright blue photoluminescence, unattainable in graphene, was also observed. We attribute the rise of blue photoluminescence in PAGenes to small on-graphene sp(2) domains created by the propagative covalent chemistry, which may expand from graphene edges or existing defect sites leaving sp(2)-hybridized patches interlaced with sp(3)-hybridized regions. The intact sp(2) domains enable effective electrical percolation among different graphene layers affording the observed high electrical conductivity in PAGene films.National Key Basic Research Program of China 2013CB933901 National Natural Science Foundation of China 21171140 21021061 21031004 U1205111 Natural Science Foundation of Fujian Province of China 2013J01056 Fundamental Research Funds for the Central Universities University of Maryland U.S. National Science Foundation CAREER CHE-105551

Development of grafting strategies for the polymer functionalisation of graphene

Graphene is well-known for its exceptional mechanical, electrical, and thermal properties, but its potential is yet to be fully realised in bulk applications due to difficulties in obtaining a large yield of high-quality individually-dispersed graphene sheets. In this thesis, reductive exfoliation of bulk graphite is demonstrated as a promising and versatile method which allows the isolation of single- and few-layer graphenes; the production of reduced graphene, or ‘graphenide’, solutions shows varying efficiency in different solvents. Subsequent functionalisation of graphenide dispersions with various electrophiles, including 1-bromododecane, anionic monomers such as methyl methacrylate, and bromine, results in increased solubility in organic solvents, without damage to the graphene basal plane. Exact characterisation and quantification of grafting is complicated by the presence of solvent remaining between graphene layers, a phenomenon which is not significant in other carbon nanomaterials. Reductive alkylation was carried out on five different graphitic starting materials including two types of natural flake graphite, shear-exfoliated graphite platelets, graphite nanofibres, and few-layer graphene. The study reveals pronounced differences in the obtained grafted species with respect to the degree of functionalisation and residual solvent, exfoliation efficiency and product homogeneity. These results are shown to be dependent on the size and nature of the starting material, with few-layer graphene showing the highest grafting ratios. Few-layer graphene was also functionalised with various molecular weight poly(methyl methacrylate) (PMMA) polymers by grafting-to and grafting-from approaches; the grafting ratios were higher for the grafting-from approach and the products showed a far greater dispersibility in acetone (up to 920 µg/mL). In parallel with these direct polymer-grafting strategies on few-layer graphene, the reduction method was used to dissolve and brominate few-layer graphene sheets, achieving direct covalent attachment of bromine to the graphene framework. The brominated few-layer graphenes provide a convenient, stable, liquid-phase precursor, suitable for the synthesis of a variety of directly functionalised graphenes. As an example, the brominated species was used to initiate atom transfer radical polymerisation, to obtain PMMA-grafted graphene, which was six times more dispersible in acetone than controls. In addition, brominated graphene is active for nucleophilic substitution reactions, as illustrated by the preparation of methoxypolyethylene glycol- and hydroxyl-substituted derivatives. Grafting ratios for these polymer-functionalised materials varied between 6 and 25% and all graphene derivatives showed increased solubility in organic solvents, highlighting the potential of this route for preparing large quantities of dispersed graphene with minimal damage to the carbon framework.Open Acces

Double Tips for In-Plane Polarized Near-Field Microscopy and Spectroscopy

Near-field optical microscopy and spectroscopy provide high-resolution imaging below the diffraction limit, crucial in physics, chemistry, and biology for studying molecules, nanoparticles, and viruses. These techniques use a sharp metallic tip of an atomic force microscope (AFM) to enhance incoming and scattered light by excited near-fields at the tip apex, leading to high sensitivity and a spatial resolution of a few nanometers. However, this restricts the near-field orientation to out-of-plane polarization, limiting optical polarization choices. We introduce double tips that offer in-plane polarization for enhanced imaging and spectroscopy. These double tips provide superior enhancement over single tips, although with a slightly lower spatial resolution (∼30 nm). They enable advanced studies of nanotubes, graphene defects, and transition metal dichalcogenides, benefiting from polarization control. The double tips allow varied polarization in tip-enhanced Raman scattering and selective excitation of transverse-electric and -magnetic polaritons, expanding the range of nanoscale samples that can be studied

Direct Nano-Imaging of Light-Matter Interactions in Nanoscale Excitonic Emitters

Strong light-matter interactions in localized nano-emitters when placed near metallic mirrors have been widely reported via spectroscopic studies in the optical far-field. Here, we report a near-field nano-spectroscopic study of the localized nanoscale emitters on a flat Au substrate. We observe strong-coupling of the excitonic dipoles in quasi 2-dimensional CdSe/CdxZnS1-xS nanoplatelets with gap mode plasmons formed between the Au tip and substrate. We also observe directional propagation on the Au substrate of surface plasmon polaritons launched from the excitons of the nanoplatelets as wave-like fringe patterns in the near-field photoluminescence maps. These fringe patterns were confirmed via extensive electromagnetic wave simulations to be standing-waves formed between the tip and the emitter on the substrate plane. We further report that both light confinement and the in-plane emission can be engineered by tuning the surrounding dielectric environment of the nanoplatelets. Our results lead to renewed understanding of in-plane, near-field electromagnetic signal transduction from the localized nano-emitters with profound implications in nano and quantum photonics as well as resonant optoelectronics.Comment: manuscript + supporting informatio

Synthesis, Characterization and Properties Studies of Alkylated Graphene and Graphene Quantum Dots

石墨烯自被发现并报道以来，因其独特的结构和优异的物理化学性质成为物理、化学、材料科学等领域研究的热点。然而已有的制备方法中石墨烯的产率和产量都不尽如人意，合成得到的石墨烯也很难分散在一般的溶剂中，极大地限制了石墨烯在实际领域的应用。因此，发展一种可分散、高质量石墨烯的制备方法成为相关领域中的一个重点和难点课题。此外，纯的石墨烯是零带隙半导体，因此不会有光致发光现象，大大限制了它在光电器件中的应用。最近研究发现，当石墨烯的宽度小于10nm后，由于量子限域效应和边界效应的影响，得到的石墨烯纳米带或石墨烯量子点能够表现出半导体特性。这一发现突破了二维石墨烯材料本身的限制，激起了科学家对石墨烯纳米带和...Graphene has attracted enormous attention over the past few years due, in part, to the remarkable electrical, mechanical, and thermal properties of this new material. However, low yield and throughput remain limiting factors in producing large quantities of materials needed for many practical applications, such as capacitors, fuel cells, catalysts, and sensors. In addition, graphene materials prod...学位：理学硕士院系专业：化学化工学院_无机化学学号：2052011115154

Towards compact phase-matched and waveguided nonlinear optics in atomically layered semiconductors

Nonlinear frequency conversion provides essential tools for light generation, photon entanglement, and manipulation. Transition metal dichalcogenides (TMDs) possess huge nonlinear susceptibilities and 3R-stacked TMD crystals further combine broken inversion symmetry and aligned layering, representing ideal candidates to boost the nonlinear optical gain with minimal footprint. Here, we report on the efficient frequency conversion of 3R-MoS2, revealing the evolution of its exceptional second-order nonlinear processes along the ordinary (in-plane) and extraordinary (out-of-plane) directions. By measuring second harmonic generation (SHG) of 3R-MoS2 with various thickness - from monolayer (~0.65 nm) to bulk (~1 {\mu}m) - we present the first measurement of the in-plane SHG coherence length (~530 nm) at 1520 nm and achieve record nonlinear optical enhancement from a van der Waals material, >10^4 stronger than a monolayer. It is found that 3R-MoS2 slabs exhibit similar conversion efficiencies of lithium niobate, but within propagation lengths >100-fold shorter at telecom wavelengths. Furthermore, along the extraordinary axis, we achieve broadly tunable SHG from 3R-MoS2 in a waveguide geometry, revealing the coherence length in such structure for the first time. We characterize the full refractive index spectrum and quantify both birefringence components in anisotropic 3R-MoS2 crystals with near-field nano-imaging. Empowered with these data we assess the intrinsic limits of the conversion efficiency and nonlinear optical processes in 3R-MoS2 attainable in waveguide geometries. Our analysis highlights the potential of 3R-stacked TMDs for integrated photonics, providing critical parameters for designing highly efficient on-chip nonlinear optical devices including periodically poled structures, resonators, compact optical parametric oscillators and amplifiers, and optical quantum circuits

Perfect Absorption and Strong Coupling in Supported MoS2 Multilayers

Perfect absorption and strong coupling are two highly sought-after regimes of light-matter interactions. Both regimes have been studied as separate phenomena in excitonic 2D materials, particularly in MoS2. However, the structures used to reach these regimes often require intricate nanofabrication. Here, we demonstrate the occurrence of perfect absorption and strong coupling in thin MoS2 multilayers supported by a glass substrate. We measure reflection spectra of mechanically exfoliated MoS2 flakes at various angles beyond the light-line via Fourier plane imaging and spectroscopy and find that absorption in MoS2 monolayers increases up to 74% at the C-exciton by illuminating at the critical angle. Perfect absorption is achieved for ultrathin MoS2 flakes (4-8 layers) with a notable angle and frequency sensitivity to the exact number of layers. By calculating zeros and poles of the scattering matrix in the complex frequency plane, we identify perfect absorption (zeros) and strong coupling (poles) conditions for thin (<10 layers) and thick (>10 layers) limits. Our findings reveal rich physics of light-matter interactions in bare MoS2 flakes, which could be useful for nanophotonic and light harvesting applications