Baoqing Zeng

The effect of ambient humidity on the electrical properties of graphene oxide films

We investigate the effect of water adsorption on the electrical properties of graphene oxide (GO) films using the direct current (DC) measurement and alternating current (AC) complex impedance spectroscopy. GO suspension synthesized by a modified Hummers method is deposited on Au interdigitated electrodes. The strong electrical interaction of water molecules with GO films was observed through electrical characterizations. The DC measurement results show that the electrical properties of GO films are humidity- and applied voltage amplitude-dependent. The AC complex impedance spectroscopy method is used to analyze the mechanism of electrical interaction between water molecules and GO films in detail. At low humidity, GO films exhibit poor conductivity and can be seen as an insulator. However, at high humidity, the conductivity of GO films increases due to the enhancement of ion conduction. Our systematic research on this effect provides the fundamental supports for the development of graphene devices originating from solution-processed graphene oxide.

Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres

We present a plasmonic nanostructure design by embedding a layer of hexagonal periodic metallic nanospheres between the active layer and transparent anode for bulk heterojunction organic solar cells. The hybrid structure shows broadband optical absorption enhancement from localized surface plasmon resonance with a weak dependence on polarization of incident light. We also theoretically study the optimization of the design to enhance the absorption up to 1.90 times for a typical hybrid active layer based on a low band gap material.

A Double-Layer Chiral Metamaterial with Negative Index

In this paper, a double-layer metamaterial is proposed composing of twisted metallic foil pairs and dielectric spacer. Negative refractive index can be realized originating from chiral configuration of this metamaterial. With precious tuning of rotated angle in each metallic foil pair and dielectric thickness h, different chirality parameters and negative refraction indices can be obtained. These results help to give a more profound understanding on chirality and to design new chiral metamaterial.

Tunable positive temperature coefficient of resistivity in an electrically conducting polymer/graphene composite

The graphene nanosheet (GNS)/ultrahigh molecular weight polyethylene composite with a two-dimensional conductive network of GNSs exhibits an increasing positive temperature coefficient (PTC) of resistivity while thermally treated at a certain temperature. This anomalous phenomenon is originated from the reduced viscosity of polymer matrix, crystallization induced local flow and weak interactions among the overlapping joints of GNSs, which allow GNSs to migrate to the polymer matrix, thus weakening the conductive paths and increasing the PTC intensity. A facile approach is accordingly developed to prepare a conductive polymer composite with a tunable PTC intensity.

High throughput exfoliation of graphene oxide from expanded graphite with assistance of strong oxidant in modified Hummers method

The large-quantity (Grams) of graphene oxide (GO) was prepared by modified Hummers method, which the purifying process of GO suspension was much faster by filtering dried GO agglomeration dispersed into deionized water than usually used directed filter. The GO could be easily reduced to graphene.

Field emission of silicon nanowires grown on carbon cloth

A low operating electric field has been achieved on silicon nanowires grown on carbon cloth. The silicon nanowires were grown on carbon cloth via the vapor-liquid-solid reaction using silane gas as the silicon source and gold as catalyst from the decomposition of hydrogen gold tetrachloride. An emission current density of 1mA∕cm2 was obtained at an operating electric field of 0.7V∕μm. Such low field is resulted from a high field enhancement factor of 6.1×104 due to the combined effects of the high intrinsic aspect ratio of silicon nanowires and the woven geometry of carbon cloth. Such results may lead silicon nanowire field emitters to practical applications in vacuum microelectronic devices including microwave devices.

Field emission of silicon nanowires

Field emission of single crystal silicon nanowires of 100nm in diameter grown at 480°C from silane using Au as catalyst has been investigated. An emission current density of 1mA∕cm2 over a 0.2cm2 area was obtained at an electric field of 3.4V∕μm with a turn-on field of 2V∕μm at 0.01mA∕cm2. The annealing of the as-grown samples at 550°C in vacuum has drastically improved the field emission performance. The low growth and annealing temperatures make the process applicable to glass substrates.

Metallic nanomesh electrodes with controllable optical properties for organic solar cells

We have fabricated a metallic nanomesh using nanosphere lithography and metal evaporation. The metallic nanomesh has a precisely controlled nanostructure showing excellent uniformity with hexagonally arrayed periodic circular holes. A P3HT:PCBM organic solar cell, with the gold nanomesh electrode, demonstrates a high fill factor of 61% and a considerable power conversion efficiency of 3.12%. Electromagnetic simulation indicates that the optical properties of the metallic nanomesh can be optimized for organic photovoltaic devices by tuning the film thickness, hole diameter, and periodicity. These results show the promising potential of using a metallic nanomesh as the transparent electrode in organic solar cells.

Enhancement of field emission of aligned carbon nanotubes by thermal oxidation

To improve the field emission current density of aligned carbon nanotubes grown by thermal chemical vapor deposition, postgrowth thermal annealing was conducted at 850°C for 1h in vacuum plus at 465°C for 2h in air. It was found that the highest field emission current density significantly improved by a factor of 4 (from 19to79mA∕cm2), which is due to the substantial increase of the emitting area of the carbon nanotubes after annealing. This result is important for applications of using carbon nanotubes as high current electron sources, microwave devices, flat panel displays, etc.

Enhanced field electron emission of graphene sheets by CsI coating after electrophoretic deposition.

Because of the large quantities of edges, graphene can serve as an efficient edge emitter for field emission (FE). Cesium iodide (CsI) coating was promising to enhance the electron emission and utilized in FE applications. In this work, FE of graphene sheets after electrophoretic deposition (ED) was studied. Electron emission property of GS was obviously improved by coating with CsI. The turn-on field of GS decreased from 4.4 to 2.5 V/ μm; and threshold field decreased from 9 to 5.8 V/μm, respectively. This FE improvement must due to a higher effective density of emission site generated around the GS surface after coating. Scanning electron microscopy (SEM) and computation were taken to reveal the influence after coating. Investigations of CsI coated MWCNTs were also compared in order to better understand the origin of the low turn-on electric field obtained by GS.