Huaichao Yang

Peking University, Peping, Beijing, China

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Publications (21)65.64 Total impact

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    ABSTRACT: Three-dimensional graphene foam (GF) is synthesized by CVD. When water flows through GF, electricity is induced. The direction of the induced current is dominated by the flow direction of water; the value of induced current is related to the flow velocity but has no relationship with the flow direction and external bias voltage.
    Nanoscale 10/2013; · 6.23 Impact Factor
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    ABSTRACT: In this work, the doping of n-layer graphenes with C60 is investigated via Raman spectroscopy. The results indicate that C60 can induce hole doping in graphenes, and that the doping level is closely related to the layer number of the graphenes. Moreover, the level of doping in the hybrid of C60 on graphene (C60/G) is more significant than that in the hybrid of graphene on C60 (G/C60).
    Nanoscale 07/2013; · 6.23 Impact Factor
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    ABSTRACT: Intramolecular junctions can be formed in single-walled carbon nanotubes (SWNTs) by introducing a pentagon and/or heptagon into the hexagonal carbon lattice. The realization of these carbon-based molecular electronics is still quite challenging. Here, we report that nickel or cobalt catalyzed etching can be applied to partially unzip an SWNT into an intermolecular junction of SWNT/graphene nanoribbon, directly confirmed by atomic force microscopy and Raman.
    Small 05/2013; · 7.82 Impact Factor
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    ABSTRACT: In this work, we present studies of the effects of electron-beam irradiation on the modification of single-layer graphene. Micro-Raman spectra show that the D, D′, and D + G Raman bands, which are invisible for pristine graphene, appear after the graphenes are irradiated by low-energy electron-beam irradiation (10 keV), and the intensities of these peaks increase with increasing irradiation time, indicating disorder in graphene. The characteristics of G and 2D bands of graphene are also studied before and after irradiation. In the meantime, the height of graphene is studied by atomic force microscopy and found to increase for increasing irradiation time due to the contaminant deposition on graphene. The effects introduced by irradiation can be recovered partly by vacuum annealing. These results provide important information about the modification of graphene under electron-beam irradiation.
    The Journal of Physical Chemistry C. 05/2013; 117(19):10079–10085.
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    ABSTRACT: In this work, we report the fluorination of edges and central areas of monolayer graphene by SF6 and CHF3 plasma treatments. After fluorination by SF6 plasma, G and 2D peaks of Raman spectroscopy for the edges have upshifts, which are much bigger than the upshifts for central areas of monolayer graphene. For the intensity ratio of I(2D)/I(G), it becomes smaller after SF6 plasma treatments and magnitude of change is similar for the edges and that of the central areas. These observations indicate that the fluorination by SF6 plasma treatments can induce p-doping to graphene, which is more significant for the edges comparing to the central areas. Moreover, the ratio of I(D)/I(G) becomes larger both for the edges and the central areas. For CHF3 plasma treatments, although similar results can be obtained, the p-doping to graphene is less and more defects are introduced comparing to SF6 plasma treatment. Therefore, for fluorination of monolayer graphene, SF6 plasma is better than CHF3 plasma.
    Journal of Nanoscience and Nanotechnology 02/2013; 13(2):1331-4. · 1.15 Impact Factor
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    ABSTRACT: Graphenes were prepared via micromechanical cleavage of natural graphite flakes and transferred onto a silicon wafers with a 285 nm SiO2 thick film on the surface. Nickel was used as the electrodes to study the magnetoresistance of graphenes devices at room temperature. We find that the magnetoresistances of graphenes shows positive sign while the magnetoresistances of pure nickel have negative sign at the same external magnetic field when a Z-direction magnetic field (0.24 T, graphenes on the X-Y plane) is applied. The magnitude of magnetoresistance of graphenes is about +0.20% and 5 times larger than that of the same thickenss pure nickel film. When the magnetic field is along the X-direction (-0.26 T, perpendicular to Y-direction current), the magnetoresistance of graphenes and nickel film both have negative sign and value of -0.04% and -0.03%, respectively. The magnetoresistance of graphenes and nickel film both have positive sign and value of +0.09% and +0.04%, when the magnetic field is along the Y-direction (-0.24 T, parallel to Y-direction current), respectively. The mechanism of these observations is attributed to the edge ferromagnetism of graphenes. Our work shows that grapheses may play an important role in spin device operated at room temperature.
    Journal of Nanoscience and Nanotechnology 02/2013; 13(2):1125-8. · 1.15 Impact Factor
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    ABSTRACT: In this work, a rope of single-walled carbon nanotubes is prepared by using a diamond wire drawing die. At atmospheric condition, the electrical conductance and the thermoelectric voltage of single-walled carbon nanotubes rope have been investigated with the hot-side temperature ranging from 292 to 380 K, and cold-side temperature at 292 K. For different temperatures in the range of 292 to 380 K at hot-side, the current-voltage curves are almost parallel to each other, indicating that the electrical conductance does not change. The dynamic characteristics of voltage at positive, zero and negative current bias demonstrate that a thermoelectric voltage is induced with a direction from hot- to cold-side. The induced thermoelectric voltage shows linear dependence on the temperature difference between hot- and cold-side. The thermoelectric power of single-walled carbon nanotubes rope is found to be positive and has a value about 17.8 +/- 1.0 microV/K. This result suggests the hole-like carriers in single-walled carbon nanotubes rope. This study will pave the way for single-walled carbon nanotubes based thermoelectric devices.
    Journal of Nanoscience and Nanotechnology 02/2013; 13(2):1335-8. · 1.15 Impact Factor
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    ABSTRACT: Using Raman spectroscopic studies, we firstly report that Pd film deposition can induce a tensile strain at the interface between Pd and n-layer graphenes, which results in the splitting of the G peak and a red Raman shift of the 2D peak in monolayer graphene, and red Raman shifts of G and 2D peaks for other n-layer graphenes. In particular, this kind of tensile strain can be used as an effective way for edge modification or strain engineering in monolayer graphene.
    Nanoscale 11/2012; · 6.23 Impact Factor
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    ABSTRACT: In this work, the fluorination of n-layer graphene is systematically investigated using CHF₃ and CF₄ plasma treatments. The G and 2D Raman peaks of graphene show upshifts after each of the two kinds of plasma treatment, indicating p-doping to the graphene. Meanwhile, D, D' and D + G peaks can be clearly observed for monolayer graphene, whereas these peaks are weaker for thicker n-layer graphene (n ≥ 2) at the same experimental conditions. The upshifts of the G and 2D peaks and the ratio of I(2D)/I(G) for CF₄ plasma treated graphene are larger than those of CHF₃ plasma treated graphene. The ratio of I(D)/I(G) of the Raman spectra is notably small in CF₄ plasma treated graphene. These facts indicate that CF₄ plasma treatment introduces more p-doping and fewer defects for graphene. Moreover, the fluorination of monolayer graphene by CF₄ plasma treatment is reversible through thermal annealing while that by CHF₃ plasma treatment is irreversible. These studies explore the information on the surface properties of graphene and provide an optimal method of fluorinating graphene through plasma techniques.
    Nanotechnology 03/2012; 23(11):115706. · 3.84 Impact Factor
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    ABSTRACT: We report that single-walled carbon nanotubes (SWNTs) can be etched into graphene nanoribbons (GNRs) by iron etching, which is confirmed by Raman spectroscopy and transmission electron microscopy. Compared with SWNTs, there are some unique features in Raman spectra of GNRs: symmetric G peak with no splitting, larger Raman intensity of 2D peak than G peak, and lower frequency and narrower full width at half maximum for 2D peak. Similar to radial breathing modes in SWNTs, theoretically predicted radial breathing-like mode of GNRs is also observed: a clear and prominent peak around 223 cm−1 in the low frequency regions. This work paves the way for future studies of nanodevices based on SWNT-GNR heterojunction.
    Applied Physics Letters 03/2012; 100(10). · 3.79 Impact Factor
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    ABSTRACT: The effects of confinement in channels between single-walled carbon nanotubes (SWNTs) on the water to ice transition temperature are studied in this work. The channels are provided in the SWNT rope, which are formed between SWNTs when SWNT films are treated with diamond wire drawing dies. Gold wires and aluminum tapes are used as electrodes to connect SWNT rope. When we carried out the measurement, a part of SWNT rope was dipped into water, and the electrical property of the SWNT rope was monitored with decreasing temperature. A sharp jump in voltage is found when the temperature is 274.1 K, which represents the transition temperature of confined water in the channels between SWNTs. These results indicate that the temperature of water–ice transition increases due to its confinement in nanometer size channels compared to that of bulk water. The mechanisms of our observation are discussed.
    Journal of Adhesion Science and Technology 01/2012; · 0.90 Impact Factor
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    ABSTRACT: We report that iron particles-assisted etching can be applied to selectively remove single-walled carbon nanotubes (SWNTs) with diameters smaller than 1.2 nm via the catalytic hydrogenation of carbon in a film of SWNTs, which is proven by a Raman spectroscopy-based technique.
    Chemical Communications 12/2011; 48(7):1042-4. · 6.38 Impact Factor
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    ABSTRACT: We report thickness-dependent morphologies of a Pd film on n-layer graphenes. Via Raman spectroscopy-based technique, obvious charge transfer has been observed among Pd and graphenes, which is also dependent on the layer number. With the increase of the layer number, the Pd film becomes coarser, and the electron transfer becomes lower.
    Chemical Communications 09/2011; 47(33):9408-10. · 6.38 Impact Factor
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    ABSTRACT: In this work, we report the layer-dependent fluorination of n-layer graphenes by SF6 plasma treatment. For monolayer graphene, D, D′, and D+G peaks of Raman spectroscopy can be clearly observed, whereas these peaks are absent for thicker n-layer graphenes (n ≥ 2) at the same experimental conditions. This indicates that monolayer graphene is much more susceptible to being fluorinated than thicker graphenes. These results can be well explained by larger corrugations of monolayer graphene than those of thicker graphenes. Meanwhile, the fluorination of n-layer graphenes is reversible after vacuum annealing. These studies provide useful information for exploiting the useful surface information of the fluorination of n-layer graphenes through plasma techniques.
    The Journal of Physical Chemistry C. 08/2011; 115(34).
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    ABSTRACT: Ferromagnetism in graphite/graphenes is attractive for fundamental science and potential applications in carbon-based magnetism and spintronics. In this work, we show that magnetic particle inspection can be miniaturized to detect local magnetic moments with a high spatial resolution of 1.0 nm using scanning electron microscopy. A metal nanowire and adjacent nanogap can be found at the edges of graphenes and graphite for atoms with magnetic moments (Fe, Co, Ni, Mn, Pd, Al), whereas no similar characteristics are found for diamagnetic metals (Au, Ag). By investigating these features under an external magnetic field and at different temperatures, we discuss possible mechanisms and propose that intrinsic ferromagnets exist and form a one-dimensional array at the edges of graphenes and graphite. Meanwhile, the size of individual magnets (<4.8 Å), orientation, magnitude (0.45 μB per carbon edge atom) of magnetic moments, and their Curie temperature (>95 °C) are obtained, which are novel and interesting.
    The Journal of Physical Chemistry C. 07/2011; 115(32).
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    ABSTRACT: After thermal deposition of silver films onto n-layer graphenes, the following results have been obtained. First, the dependence of silver morphologies on the layer number is studied via controlling the sample temperature at 298, 333, and 373 K. This can be attributed to the changes in surface properties and/or surface diffusion coefficient of n-layer graphenes at different temperatures. Second, Raman scattering of n-layer graphenes is greatly enhanced after Ag deposition and the enhancement factors depend on the layer number of n-layer graphenes. Monolayer graphene has the largest enhancement factors, and the enhancement factors decrease with layer number increasing. For graphite, almost no enhancement effect has been detected. Third, the dependences of the enhancement factors on laser wavelength, thickness, and morphologies (nanoparticle size and spacing) of silver film are also studied. The Raman enhancement observed here is mainly attributed to the coupled surface plasmon resonance (SPR) absorption of silver nanoparticles.
    The Journal of Physical Chemistry C. 05/2011; 115(23).
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    ABSTRACT: In this work, Raman signals from crystal violet (CV) molecules on n-layer graphenes are clearly observed while these signals are invisible on SiO2/Si substrate under the same deposition condition with the same concentration of CV solution. This indicates that n-layer graphenes can be used as substrates for enhancing Raman signals of adsorbed CV molecules. The enhanced efficiency is found to be closely related to the layer number n. These conclusions can be further confirmed by Raman spectra of CV molecules on gold-decorated n-layer graphenes. The Raman enhancement effect of n-layer graphenes is attributed to chemical mechanism (CM) while electromagnetic mechanism (EM) dominates the enhancement effect of gold. Thus, graphene provides a convenient way to study CM exclusively. The morphology and density of gold nanostructures on n-layer graphenes play a significant role in the EM related Raman enhancement effect.
    The Journal of Physical Chemistry C. 05/2011; 115(20).
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    ABSTRACT: Raman spectra of monolayer graphene at various temperatures (303–473 K) are measured. In Raman scattering with wave numbers ranging from 1200 to 3400 cm−1, the four main Raman peaks (G, 2D, T + D and 2D') show temperature-dependent behaviour, but have different frequency shifts with increase in temperature. We propose that the peak frequency shift is related mainly to the elongation of C–C bond due to thermal expansion or anharmonic coupling of phonon modes, and oxygen-induced strong hole doping on the graphene surface. The doping effect can be confirmed from the frequency shifts, full-width at half-maximum as well as the area and intensity ratios of G and 2D peaks in temperature-dependent Raman scattering of graphene, room-temperature Raman spectra of pristine graphene and graphene cooled down after Raman measurement at 473 K in air. Therefore, the oxygen doping effect and temperature effect coexist in temperature-dependent Raman scattering of monolayer graphene.
    Journal of Physics D Applied Physics 04/2011; 44(18):185404. · 2.53 Impact Factor
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    ABSTRACT: We report that gold thermally deposited onto n-layer graphenes interacts differently with these substrates depending on the number layer, indicating the different surface properties of graphenes. This results in thickness-dependent morphologies of gold on n-layer graphenes, which can be used to identify and distinguish graphenes with high throughput and spatial resolution. This technique may play an important role in checking if n-layer graphenes are mixed with different layer numbers of graphene with a smaller size, which cannot be found by Raman spectra. The possible mechanisms for these observations are discussed.
    Journal of the American Chemical Society 01/2010; 132(3):944-6. · 10.68 Impact Factor
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    ABSTRACT: Graphene, a new two-dimensional carbon material, is a rising star of physics, chemistry and materials science. In this work, we report the recent experimental researches on the Raman spectra and the temperature-dependent features of graphenes and carbon nanoscrolls, which are evolved from graphene and have an open tubular structure. The layer-dependent Raman enhancing characteristics of n-layer graphenes for crystal violet, and the thickness-dependent morphologies of gold on n-layer graphenes are also systematically investigated. Meanwhile, the aggregations of ferromagnetic and paramagnetic atoms at edges of graphenes and graphite are observed and the mechanisms are discussed. KeywordsN-layer graphene–Raman spectroscopy–edge ferromagnetism
    Science China: Physics, Mechanics and Astronomy 54(10):1729-1738. · 1.17 Impact Factor