[Show abstract][Hide abstract] ABSTRACT: We investigated the strong coupling between the excitons of ZnO nanowires (NWs) and the localized surface plasmons (LSPs) of individual Ag nanoparticles (NPs) by monochromated electron energy loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscopy (STEM) instrument. The EELS results confirmed that the hybridization of the ZnO exciton with the LSPs of the Ag NP created two plexcitons: the lower branch plexcitons (LPs) with a symmetrical dipole distribution and the upper branch plexcitons (UPs) with an anti-symmetrical dipole distribution. The spatial maps of the LP and UP excitations reveal the nature of the LSP-exciton interactions. With decreasing size of the Ag NP the peak energies of the LPs and UPs showed a blue-shift and an anti-crossing behavior at the ZnO exciton energy was observed. The coupled oscillator model explains the dispersion curve of the plexcitons and a Rabi splitting energy of ~170 meV was deduced. The high spatial and energy resolution STEM-EELS approach demonstrated in this work is general and can be extended to study the various coupling interactions of a plethora of metal-semiconductor nanocomposite systems.
[Show abstract][Hide abstract] ABSTRACT: We fabricate carbon nanotube (CNT)-field effect transistors (FETs) with a changeable channel length and investigate the electron transport properties of single-walled, double-walled and triple-walled CNTs under uniaxial strain. In particular, we characterize the atomic structure of the same CNTs in the devices by transmission electron microscopy and correlate the strain-induced electronic property change to the chirality of the CNTs. Both the off-state resistance and on-state resistance are observed to change with the axial strain following an exponential function. The strain-induced band gap change obtained from the maximum resistance change in the transfer curve of the ambipolar FETs is quantitatively compared with the previous theoretical prediction and our DFTB calculation from the chirality of the CNTs. Although following the same trend, the experimentally obtained strain-induced band gap change is obviously larger (57%-170% larger) than the theoretical results for all the six CNTs, indicating that more work is needed to fully understand the strain-induced electronic property change of CNTs.
[Show abstract][Hide abstract] ABSTRACT: The dynamic structural behavior in DNA due to interaction with cisplatin is essential for the functionality of platinum-based anti-cancer drugs. Here we report a novel method to monitor the interaction progress in DNA-cisplatin reaction in real time with a solid-state nanopore. The interaction processes are found to be well elucidated by the evolution of the capture rate of DNA-cisplatin complex, which is defined as the number of their translocation events through the nanopore in unit time. In the first stage, the capture rate decreases rapidly due to DNA discharging as the positive-charged hydrated cisplatin molecules initially bond to the negative-charged DNA and form mono-adducts. In the second stage, by forming di-adducts, the capture rate increases as DNA molecules are softened, appears as the reduced persistence length of the DNA-cisplatin adducts. In the third stage, the capture rate decreases again as a result of DNA aggregation. Our study demonstrates a new single-molecule tool in exploring dynamic behaviors during drug-DNA reactions and may have future application in fast drug screening.
[Show abstract][Hide abstract] ABSTRACT: The growth of zigzag single-walled carbon nanotubes (SWNTs) is most challenging among all types of SWNTs, with the highest reported selectivity of ~7%. Here we realized the dominant growth of (16,0) tubes at the abundance near ~80% by using intermetallic W6Co7 catalysts containing plenty amount of (1 1 6) planes together with optimizing the growth conditions. These (1 1 6) planes may act as the structure templates for (16,0) SWNTs due to the geometrical match between the open end of (16,0) tube and the atomic arrangements of the (1 1 6) planes in W6Co7. Using catalysts with designed structure as solid state template at suitable kinetic conditions offers a strategy for selective growth of zigzag SWNTs.
Journal of the American Chemical Society 06/2015; 137(27). DOI:10.1021/jacs.5b04403 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A lateral piezopotential-gated field-effect transistor is realized by operation of scaning tunneling microscope (STM) tip onto an individual ZnO nanowires inside high-resolution transmission electron microscope (TEM). The electric transport behavior of ZnO nanowires under bending by point contact of STM tip at the cross section of nanowire end shows that, the nanowire conductance decreases up to 2 orders of magnitude when a 2.63% bending strain is applied. Experimental results and their detailed analysis reveal that the regarded change in conductance is not due to Schottky barrier at the contact interface but originates from the carrier depletion caused by the lateral piezoelectric potential within the nanowire. The bending strain-gated transistor is one of the new type piezotronics devices.
Nano Energy 04/2015; 13. DOI:10.1016/j.nanoen.2015.02.030 · 10.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Few-layer graphene was successfully tailored with smooth edges along crystallographic directions by Joule heating-driven tungsten nanoparticles inside a transmission electron microscope. The dynamic process was monitored in real time at the atomic resolution level. These high-resolution in-situ observations show that the neighboring graphene layers joined together to form closed edges, which is in contrast to the supposed open edges formed with hydrogen passivation. The tungsten nanoparticles transformed to W2C in the intermediate stage of etching and to WC after etching, suggesting that carbon dissolution helped the continuous action of the metal nanoparticles in the catalytic anisotropic etching reaction.
[Show abstract][Hide abstract] ABSTRACT: Using selected-area low-energy electron diffraction analysis, we showed strict orientational alignment of monolayer hexagonal boron nitride (h-BN) crystallites with Cu(100) surface lattices of Cu foil substrates during atmospheric pressure chemical vapor deposition. In sharp contrast, the graphene-Cu(100) system is well-known to assume a wide range of rotations despite graphene's crystallographic similarity to h-BN. Our density functional theory calculations uncovered the origin of this surprising difference: The crystallite orientation is determined during nucleation by interactions between the cluster's edges and the substrate. Unlike the weaker B- and N-Cu interactions, strong C-Cu interactions rearrange surface Cu atoms, resulting in the aligned geometry not being a distinct minimum in total energy. The discovery made in this specific case runs counter to the conventional wisdom that strong epilayer-substrate interactions enhance orientational alignment in epitaxy and sheds light on the factors that determine orientational relation in van der Waals epitaxy of 2D materials.
Proceedings of the National Academy of Sciences 11/2014; 111(47). DOI:10.1073/pnas.1405613111 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The possibility to induce magnetism in light-element materials that contain only s and p electrons is of fundamental and practical importance. Here, weak high-temperature ferromagnetism is observed in carbon-doped boron nitride (B-C-N) nanosheets. The bulk-quantities of B-C-N nanosheets that are free of metallic impurities are prepared through a multi-step process. These B-C-N samples exhibit ferromagnetic hysteresis stable at room temperature and above, with saturation magnetization and coercivity comparable to the previously reported results of defective graphite samples. The ferromagnetic response disappears upon the removal of carbon dopants from the BN lattice, indicating that the observed magnetism originates from substitutional carbon-doping rather than from extrinsic magnetic impurities. On the basis of first-principle calculations it is shown that not only substitutional carbon doping in a honeycomb BN lattice favors spontaneous spin polarization and local moment formation, but also that the spin moments can exhibit long-range magnetic ordering.
[Show abstract][Hide abstract] ABSTRACT: The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.
[Show abstract][Hide abstract] ABSTRACT: High-energy lithium battery materials based on conversion/alloying reactions have tremendous potential applications in new generation energy storage devices. However, these applications are limited by inherent large volume variations and sluggish kinetics. Here we report a self-adaptive strain-relaxed electrode through crumpling of graphene to serve as high-stretchy protective shells on metal framework, to overcome these limitations. The graphene sheets are self-assembled and deeply crumpled into pinecone-like structure through a contraction-strain-driven crumpling method. The as-prepared electrode exhibits high specific capacity (2,165 mAh g(-1)), fast charge-discharge rate (20 A g(-1)) with no capacity fading in 1,000 cycles. This kind of crumpled graphene has self-adaptive behaviour of spontaneous unfolding-folding synchronized with cyclic expansion-contraction volumetric variation of core materials, which can release strain and maintain good electric contact simultaneously. It is expected that such findings will facilitate the applications of crumpled graphene and the self-adaptive materials.
[Show abstract][Hide abstract] ABSTRACT: Nanostructured silicon anodes, which possess extremely high energy density and accommodate large strain without pulverization, have been developed rapidly for high-power lithium ion batteries. Here, using in situ transmission electron microscopy, the lithiation behavior of silicon nanowires with diameters smaller than 60 nm was investigated. The study demonstrated a direct dependence of the self-limiting lithiation on the pristine diameter. A "punch-through" lithiation process at the core of nanowires with pristine diameters slightly larger than the self-limiting threshold is suggested to occur with the consequent formation of a stage structure. Our work demonstrates the crucial role of mechanical stress and local defects in determining the migration and geometry of the reaction front at the mesoscopic scale. This intriguing finding holds critical significance for the application of silicon nanostructures in high-power lithium ion batteries.
[Show abstract][Hide abstract] ABSTRACT: Solid electrolyte based-resistive memories have been considered to be a potential candidate for future information technology with applications in non-volatile memory, logic circuits and neuromorphic computing. A conductive filament model has been generally accepted to be the underlying mechanism for the resistive switching. However, the growth dynamics of such conductive filaments is still not fully understood. Here, we explore the controllability of filament growth by correlating observations of the filament growth with the electric field distribution and several other factors. The filament growth behavior has been recorded using in situ transmission electron microscopy. By studying the real-time recorded filament growth behavior and morphologies, we have been able to simulate the electric field distribution in accordance with our observations. Other factors have also been shown to affect the filament growth, such as Joule heating and electrolyte infrastructure. This work provides insight into the controllable growth of conductive filaments and will help guide research into further functionalities of nanoionic resistive memories.
Nano Research 07/2014; 7(7-7):1065-1072. DOI:10.1007/s12274-014-0469-0 · 7.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotubes have many material properties that make them attractive for applications. In the context of nanoelectronics, interest has focused on single-walled carbon nanotubes (SWNTs) because slight changes in tube diameter and wrapping angle, defined by the chirality indices (n, m), will shift their electrical conductivity from one characteristic of a metallic state to one characteristic of a semiconducting state, and will also change the bandgap. However, this structure-function relationship can be fully exploited only with structurally pure SWNTs. Solution-based separation methods yield tubes within a narrow structure range, but the ultimate goal of producing just one type of SWNT by controlling its structure during growth has proved to be a considerable challenge over the last two decades. Such efforts aim to optimize the composition or shape of the catalyst particles that are used in the chemical vapour deposition synthesis process to decompose the carbon feedstock and influence SWNT nucleation and growth. This approach resulted in the highest reported proportion, 55 per cent, of single-chirality SWNTs in an as-grown sample. Here we show that SWNTs of a single chirality, (12, 6), can be produced directly with an abundance higher than 92 per cent when using tungsten-based bimetallic alloy nanocrystals as catalysts. These, unlike other catalysts used so far, have such high melting points that they maintain their crystalline structure during the chemical vapour deposition process. This feature seems crucial because experiment and simulation both suggest that the highly selective growth of (12, 6) SWNTs is the result of a good structural match between the carbon atom arrangement around the nanotube circumference and the arrangement of the catalytically active atoms in one of the planes of the nanocrystal catalyst. We anticipate that using high-melting-point alloy nanocrystals with optimized structures as catalysts paves the way for total chirality control in SWNT growth and will thus promote the development of SWNT applications.
[Show abstract][Hide abstract] ABSTRACT: Getting a grip on the switching mechanism in nanoionic resistive memories: the bipolar electrochemical mechanism for mass transfer of Ag in nanoscale SiO2 is disclosed. The in-situ atomic-level experiments provide detailed evidence of the mass-transfer process under external electric fields. The mass transfer of Ag directly leads to conductive filament formation and disruption, which is responsible for the switching mechanism in nanoionic resistive memories.
[Show abstract][Hide abstract] ABSTRACT: Defect engineering in graphene is important for tailoring graphene's properties thus applicable in various applications such as porous membranes and ultra-capacitors. In this paper, we report a general route towards defect- and pore- engineering in graphene through remote plasma treatments. Oxygen plasma irradiation was employed to create homogenous defects in graphene with controllable density from a few to ≈10(3) (μm(-2) ). The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well-defined pore size of a few nm or above. The achieved smallest nanopores are ≈2 nm in size, showing the potential for ultra-small graphene nanopores fabrication.
Small 06/2014; 10(11). DOI:10.1002/smll.201303671 · 8.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Optical absorption is the most fundamental optical property characterizing light-matter interactions in materials and can be most readily compared with theoretical predictions. However, determination of optical absorption cross-section of individual nanostructures is experimentally challenging due to the small extinction signal using conventional transmission measurements. Recently, dramatic increase of optical contrast from individual carbon nanotubes has been successfully achieved with a polarization-based homodyne microscope, where the scattered light wave from the nanostructure interferes with the optimized reference signal (the reflected/transmitted light). Here we demonstrate high-sensitivity absorption spectroscopy for individual single-walled carbon nanotubes by combining the polarization-based homodyne technique with broadband supercontinuum excitation in transmission configuration. To our knowledge, this is the first time that high-throughput and quantitative determination of nanotube absorption cross-section over broad spectral range at the single-tube level was performed for more than 50 individual chirality-defined single-walled nanotubes. Our data reveal chirality-dependent behaviors of exciton resonances in carbon nanotubes, where the exciton oscillator strength exhibits a universal scaling law with the nanotube diameter and the transition order. The exciton linewidth (characterizing the exciton lifetime) varies strongly in different nanotubes, and on average it increases linearly with the transition energy. In addition, we establish an empirical formula by extrapolating our data to predict the absorption cross-section spectrum for any given nanotube. The quantitative information of absorption cross-section in a broad spectral range and all nanotube species not only provides new insight into the unique photophysics in one-dimensional carbon nanotubes, but also enables absolute determination of optical quantum efficiencies in important photoluminescence and photovoltaic processes.
Proceedings of the National Academy of Sciences 05/2014; 111(21). DOI:10.1073/pnas.1318851111 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report a scalable growth of monolayer MoS2 films on SiO2 substrates by chemical vapor deposition. As-grown polycrystalline MoS2 films are continuous over the entire substrate surface with a tunable domain size from ~20nm up to ~1μm. An obvious blue-shift (up to 80meV) of photoluminescence peaks was observed from a series samples with different domain sizes. Back-gated field effect transistors based on polycrystalline MoS2 film with a typical domain size of ~600nm shows field mobility of ~7cm2/Vs and on/off ratio of ~106, comparable to those achieved from exfoliated MoS2. Our work provides a route towards scaled-up synthesis of high-quality monolayer MoS2 for electronic and optoelectronic devices.