Controlled fabrication of intermolecular junctions of single-walled carbon nanotube/graphene nanoribbon.

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, it is reported 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 spectroscopy.

Fluorination of edges and central areas of monolayer graphene by SF6 and CHF3 plasma treatments.

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.

Negative and positive magnetoresistance of graphenes at room-temperature.

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.

Thermoelectric power of a single-walled carbon nanotubes rope.

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.

Layer-dependent fluorination and doping of graphene via plasma treatment.

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.

Thickness-dependent morphologies of gold on n-layer graphenes.

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.

Power generation from water flowing through three-dimensional graphene foam.

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.

Tuning the layer-dependent doping effect of graphenes by C60.

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).

Large physisorption strain and edge modification of Pd on monolayer graphene.

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.

Preferential elimination of thin single-walled carbon nanotubes by iron etching.

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.

Layer-dependent morphologies and charge transfer of Pd on n-layer graphenes.

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.