Ultrathin Metal Film on Graphene for Percolation-Threshold-Limited Thermal Emissivity Control.

Translucent Au/graphene hybrid films are shown to be effective in reducing thermal emission from the underlying surfaces when the deposition thickness of Au is close to the percolation threshold. The critical Au deposition thickness for an abrupt change in emissivity is reduced from 15 nm (Si substrate) to a percolation-threshold-limited thickness of 8.5 nm (graphene/Si substrate) because of the chemical inertness of graphene leading to the deposited Au atoms forming a thin, crystalline layer. The effect of the graphene layer on the optical properties of the hybrid film is highlighted by a drastic increase in infrared absorptivity, whereas the visible absorptivity is marginally affected by the presence of a graphene layer. The level of thermal emission from the Au/graphene hybrid films with the percolation-threshold-limited Au thickness is stable even with high background temperatures of up to 300 °C and mechanical strains of ≈4%. As an example of a thermal management application, an anti-counterfeiting device is demonstrated; thermal-camouflage-masked text fabricated with an Au/graphene hybrid film is discernible only using a thermographic camera. Ultrathin metal film assisted by a graphene layer will provide a facile platform for thermal management with semi-transparency, flexibility, and transferability to arbitrary surfaces.

Tunneling Spectroscopy for Electronic Bands in Multi-Walled Carbon Nanotubes with Van Der Waals Gap.

Various intriguing quantum transport measurements for carbon nanotubes (CNTs) based on their unique electronic band structures have been performed adopting a field-effect transistor (FET), where the contact resistance represents the interaction between the one-dimensional and three-dimensional systems. Recently, van der Waals (vdW) gap tunneling spectroscopy for single-walled CNTs with indium-metal contacts was performed adopting an FET device, providing the direct assignment of the subband location in terms of the current-voltage characteristic. Here, we extend the vdW gap tunneling spectroscopy to multi-walled CNTs, which provides transport spectroscopy in a tunneling regime of ~1 eV, directly reflecting the electronic density of states. This new quantum transport regime may allow the development of novel quantum devices by selective electron (or hole) injection to specific subbands.

Enhanced Neurite Outgrowth by Intracellular Stimulation.

Electrical stimulation through direct electrical activation has been widely used to recover the function of neurons, primarily through the extracellular application of thin film electrodes. However, studies using extracellular methods show limited ability to reveal correlations between the cells and the electrical stimulation due to interference from external sources such as membrane capacitance and culture medium. Here, we demonstrate long-term intracellular electrical stimulation of undamaged pheochromocytoma (PC-12) cells by utilizing a vertical nanowire electrode array (VNEA). The VNEA was prepared by synthesizing silicon nanowires on a Si substrate through a vapor-liquid-solid (VLS) mechanism and then fabricating them into electrodes with semiconductor nanodevice processing. PC-12 cells were cultured on the VNEA for 4 days with intracellular electrical stimulation and then a 2-day stabilization period. Periodic scanning via two-photon microscopy confirmed that the electrodes pierced the cells without inducing damage. Electrical stimulation through the VNEA enhances cellular differentiation and neurite outgrowth by about 50% relative to extracellular stimulation under the same conditions. VNEA-mediated stimulation also revealed that cellular differentiation and growth in the cultures were dependent on the potential used to stimulate them. Intracellular stimulation using nanowires could pave the way for controlled cellular differentiation and outgrowth studies in living cells.

Vertical nanowire probes for intracellular signaling of living cells.

The single living cell action potential was measured in an intracellular mode by using a vertical nanoelectrode. For intracellular interfacing, Si nanowires were vertically grown in a controlled manner, and optimum conditions, such as diameter, length, and nanowire density, were determined by culturing cells on the nanowires. Vertical nanowire probes were then fabricated with a complimentary metal-oxide-semiconductor (CMOS) process including sequential deposition of the passivation and electrode layers on the nanowires, and a subsequent partial etching process. The fabricated nanowire probes had an approximately 60-nm diameter and were intracellular. These probes interfaced with a GH3 cell and measured the spontaneous action potential. It successfully measured the action potential, which rapidly reached a steady state with average peak amplitude of approximately 10 mV, duration of approximately 140 ms, and period of 0.9 Hz.

Large current difference in Au-coated vertical silicon nanowire electrode array with functionalization of peptides.

Au-coated vertical silicon nanowire electrode array (VSNEA) was fabricated using a combination of bottom-up and top-down approaches by chemical vapor deposition and complementary metal-oxide-semiconductor process for biomolecule sensing. To verify the feasibility for the detection of biomolecules, Au-coated VSNEA was functionalized using peptides having a fluorescent probe. Cyclic voltammograms of the peptide-functionalized Au-coated VSNEA show a steady-state electrochemical current behavior. Because of the critically small dimension and vertically aligned nature of VSNEA, the current density of Au-coated VSNEA was dramatically higher than that of Au film electrodes. Au-coated VSNEA further showed a large current difference with and without peptides that was nine times more than that of Au film electrodes. These results indicate that Au-coated VSENA is highly effective device to detect peptides compared to conventional thin-film electrodes. Au-coated VSNEA can also be used as a divergent biosensor platform in many applications.