Session V30: Nanotechnology Applications: NEMS, CNTs, Graphene, and Nanoscale Devices

Ultrathin single crystal diamond nanomechanical resonators

We report the fabrication and operation of ultrathin ($<$ 100 nm) high-frequency single crystal diamond nanomechanical resonators. Suspended diamond dome structures were created in a microwave hydrogen plasma which undercuts a sacrificial damage layer formed by medium energy (150-180 keV) ion implantation at a high fluence ($>$1$\times$10$^{16}$ ions/cm$^{2}$ ). Under high vacuum conditions we measure resonant frequencies between 70 MHz and 550 MHz with quality factors ranging from $\sim$ 750 to over 1000 using an optical technique. Mechanical, thermal and optical properties in combination with chemical inertness and bio-compatibility of single crystal diamond make these high-frequency, low-mass dome resonators suitable for sensing applications.   

MEMS based directional sound sensor with electronic readout

Despite a spatial separation of only 0.5 mm between its two ears, the \textit{Ormia ochracea} fly is able to accurately determine the direction of a sound source due to a flexible structure which mechanically couples the fly's two tympana. The coupled system responds to acoustic excitation with a combination of fundamental modes that amplify the interaural pressure differences in both amplitude and phase. Theoretical background, finite-element modeling, and experimental results are presented from a MEMS device which was designed and produced to mimic the fly's hearing system, providing directionally dependent response with nanometer scale amplitudes in the 3-5 kHz range. The device, which is approximately 1 mm x 2 mm of 10$\mu m$ silicon-on-insulator, uses out-of-plane interdigitated comb-finger capacitors for transduction of the response signal instead of the complicated optical schemes that are sometimes used.   

Parametric resonance based mass sensing under ambient and liquid conditions

The parametric excitation of a cantilever for AFM applications using a closed-loop electronic feedback has been previously discussed [1, 2]. The parametrically excited cantilever enables the detection of small frequency shifts due to its sharp, controllable and non-Lorentzian resonance peak, making it a suitable detector for mass sensing applications. By attaching a small particle of hygroscopic material at the free end of a cantilever, the mass can be controllably changed as the humidity is varied. The increase in mass due to the adsorbed water causes a small downshift in the resonance frequency of the cantilever that can easily be detected because of the narrow resonance peak under parametric excitation. Using a commercial cantilever, the smallest mass change observed is of the order 1 x 10$^{-12}$ grams under ambient conditions. Efforts to extend this work to improve dynamic sensing under liquids will also be described. \\[4pt] [1] M. Moreno-Moreno \textit{et al.}, Appl. Phys. Lett. \textbf{88}, 193108 (2006). \\[0pt] [2] G. Prakash \textit{et al.}, Phys. Rev. B, \textbf{79}, 094304 (2009).   

Self-assembly behavior of poly(fluorenyl styrene)-\textit{block}-poly(2-vinyl pyridine) and their blends with single wall carbon nanotubes \textit{(SWCNT)}

We describe a supramolecular strategy to disperse carbon nanotubes in block copolymer matrices. To achieve the desired functions and morphologies, comb-type architectures in which one and two fluorene units attached on the styrene ring of polystyrene-\textit{block}-poly(2-vinyl pyridine) were studied. Depending on the pendant fluorene units, the block ratio, the casting solvent and thermal annealing history, multiple morphologies were found. The phase diagram, compared to PS-$b$-P2VP, was interpreted in terms of the conformational asymmetry arising from grafting of fluorene units of variable lengths. Hydrogen bonds between COOH-SWCNT and P2VP favor miscibility of SWCNT within P2VP domains and the blending of these two components is reflected both on the final morphologies and on the electron conductivity of the blends.   

High spatial resolution thermal imaging of metallic nanowires using a scanning fluorescent nanoprobe.

We have studied the temperature increase of metallic nanowires excited by an electrical current. The measurements have been performed using a recently developed scanning thermal microscope that uses a small fluorescent nanocrystal glued at the end of an atomic force microscope tip as a nanoscale sensor. With this system, we have been able to observe the heating of 100nm wide titanium nanowires deposited on an oxidized silicon substrate. By studying wires of different geometries, we have found that the thickness of the interfacial silicon dioxide layer has a strong influence on the heating and the heat dissipation phenomena in the wires. In addition, we have also been able to observe the heating of microwires on which defects have been created by electron beam lithography. A strong temperature increase is visible in the vicinity of the defects where the electrical current density is the highest.   

Electrical properties and memory effects of field-effect transistors from networks of single and double-walled carbon-nanotubes

We study field-effect transistors made of single and double-walled carbon nanotube networks for applications as memory devices. The transfer characteristics of the transistors exhibit a reproducible hysteresis which enables their use as nano-sized memory cells with operations faster than 10 ms, endurance longer than 10$^{+4}$ cycles and charge retention of few hours in air. We propose water enhanced charge trapping at the SiO$_{2}$/air interface as the dominant mechanism for charge storage. We show that charge storage can be improved by limiting device exposure to air.   

Telescopic hot double wall carbon nanotube for nanolithography

Two main challenges in improving the use of an atomic force microscope tip for nanolithography have been identified for all types of methods for surface modification. One challenge is achieving high spatial resolutions, which is directly related to the sharpness of the tip; the other one is the accurate control of the tip-surface distance, which affects the quality of the surface modification. A telescopic hot double wall carbon nanotube for nanolithography that improves the spatial resolution and successfully solves the problem of maintaining a constant tip-surface distance is proposed. The system consists of a finite length outer tube attached to an atomic force microscope cantilever, while the inner tube with length larger than the outer one is free. By studying the heat transfer in the double wall carbon nanotube/surface, it is found that the size of the thermal spot on the surface is mainly determined by the inner tube diameter indicating that high spatial resolution can be achieved if small diameter nanotubes are used. The interaction forces in the system are of van der Waals type and we show that the inner tube is located always at the same energetically favorable distance from the surface. Since the inner tube can move telescopically along the double wall carbon nanotube axis, the tip/surface distance is maintained constant due to the van der Waals interaction, which in turn eliminates the need of an active feedback loop.   

Experimental Demonstration and Characterization of on-chip high speed graphene interconnects

Graphene has been considered as one of the most promising candidates for future interconnect technology. In spite of the promising theoretical predictions and DC characterization results about the excellent current-carrying capability of graphene nanoribbons, experimental demonstration and characterization of high speed signaling performance of graphene interconnects is still very limited. Here we present the first monolithic integration of graphene with commercial CMOS technology and the first experimental demonstration of on-chip graphene interconnects that operates above 1 GHz. We also studied the the dependence of high frequency performance on graphene interconnect physical dimensions. Important physical parameters like mean free path of graphene are extracted from experimental data. We compared our experimental results with previous theoretical predictions and gave experimental performance projection of on-chip graphene nanoribbon interconnects with linewidth smaller than 100nm.   

Memory Effects in Capacitive Devices

We suggest a possible realization of a solid-state memory capacitive (memcapacitive) device [1]. Our approach relies on a slow polarization rate of a medium between capacitor plates. We consider a multilayer sandwich structure with a non-linear electronic transport (tunneling) between the layers and show memory phenomena in such a system. Our results indicate a possibility of information storage in memcapacitive devices as well as an interesting behavior of such devices in electronic circuits. \\[4pt] [1] M. Di Ventra, Y.V. Pershin and L.O. Chua, Proc. IEEE 97, 1717 (2009).   

Conductance of the capacitively coupled single-electron transistor with a Tomonaga-Luttinger liquid island in the Coulomb blockade regime

We consider transport properties of the capacitively coupled single-electron transistor with a one-dimensional island. We describe the island using the Tomonaga-Luttinger liquid model, and obtain the analytic expressions for the conductance in the limits of low and high voltages and temperatures, and for arbitrary electromagnetic environment. The results show potential for applications of such nanoscale devices in electrical metrology.   

Nuclear Spin-Lattice Relaxation in Silicon Nanoparticles

Measurements of $^{29}$Si nuclear spin-lattice relaxation times ($T_{1}$) of silicon nanoparticles as a function of size, fabrication method, doping, magnetic field, and optical illumination are presented. The decay of nuclear polarization is found to have components both dependent and independent of magnetic field. Optical illumination is found to reduce $T_{1}$ in small nanoparticles, but has no effect in larger particles. These results indicate the competing roles of paramagnetic impurities and nuclear spin diffusion as relaxation mechanisms in silicon nanoparticles. This work is motivated by the application of silicon nanoparticles to hyperpolarized magnetic resonance imaging [1]. We conclude that silicon nanoparticles should be able to hold large nuclear polarizations for long time periods under varying magnetic environments. \\ \\[0pt] [1] J. W. Aptekar et al., ACS Nano, (in press, 2009), arXiv:0902.0269.   

Analysis of the Polarity of the Threshold Voltage in Au/Monolayer/Ag Cross-wire Tunnel Junctions

The development of a nanoscale switch element is an important step in producing the next generation of low-power, low-cost, high density electronic devices. The cross-wire tunnel junction has been shown to provide a reliable platform for analyzing the switching characteristics of Au/monolayer/Ag junctions. Our previous results suggest that changing the moiety of thiolates in the monolayer used in the junction could be responsible for changing the polarity of the threshold voltage necessary to induce the switching behavior of the junction. Here we use molecules with different surface functional groups of CH$_{3}$, OH, and Br with high, medium, and low wettabilities to investigate whether the surface energy of the monolayer directly affects the polarity of the threshold voltage.   

A new route to large area graphene by selective pulsed laser ablation

Because of its remarkable electronic properties graphene has a very bright technological future. One of the current challenges is the fabrication of defect free graphene over large areas, a must for device technology. We present a new route for producing a uniform, large area single layer graphene by selectively ablating multilayer graphene by pulsed laser irradiation. An energy density window is found within which all the layers except a single layer graphene are successfully ablated at room temperature either in Ar atmosphere or vacuum. The residual defects after laser ablation observed by Raman spectroscopy has little influence on the intrinsic electron mobility. These defects are removed by a post ablation high temperature annealing. In a bottoms-up approach, this process is being coupled with a CVD graphene deposition system which can potentially produce high purity single layer graphene on a large scale.   

Lattice-resolved frictional pattern probed by tailored carbon nanotubes

In this study, we demonstrate a high-resolution friction profiling technique using the synchronous atomic/lateral force microscopy (AFM/LFM). The atomic resolution is achieved by our special carbon nanotube (CNT) probes made via in situ tailoring and manipulation inside an ultra-high vacuum transmission electron microscope (UHV TEM). The frictional pattern mapped on graphite displays a periodic distribution similar to the atomic (0001)-oriented graphite lattice structure. Furthermore, the electro-thermal process in the UHV TEM renders a graphite-caped CNT tip, which delivers the nanotribology study within two graphite layers by the LFM measurement on graphite. The synchronous AFM and LFM images can discern a spatial shift between the atomic points and local friction maxima. We further interpret this shift as caused by the lattice distortion, which in turn induces irreversible energy dissipation. We believe this is the origin of atomic friction in the sub-nanonewton scale.