Jason Simmons

Optically modulated conduction in chromophore-functionalized single-wall carbon nanotubes

We demonstrate an optically active nanotube-hybrid material by functionalizing single-wall nanotubes with an azo-based chromophore. Upon UV illumination, the conjugated chromophore undergoes a cis-trans isomerization leading to a charge redistribution near the nanotube. This charge redistribution changes the local electrostatic environment, shifting the threshold voltage and increasing the conductivity of the nanotube transistor. For a approximately 1%-2% coverage, we measure a shift in the threshold voltage of up to 1.2 V. Further, the conductance change is reversible and repeatable over long periods of time, indicating that the chromophore-functionalized nanotubes are useful for integrated nanophotodetectors.

Photogating carbon nanotube transistors

Optoelectronic measurements of carbon nanotube transistors have shown a wide variety of sensitivites to the incident light. Direct photocurrent processes compete with a number of extrinsic mechanisms. Here we show that visible light absorption in the silicon substrate generates a photovoltage that can electrically gate the nanotube device. The photocurrent induced by the changing gate voltage can be significantly larger than that due to direct electron-hole pair generation in the nanotube. The dominance of photogating in these devices is confirmed by the power and position dependence of the resulting photocurrent. The power dependence is strongly nonlinear and photocurrents are measured through the device even when the laser illuminates up to 1mm from the nanotube.

Electrically directed assembly and detection of nanowire bridges in aqueous media

Although dielectrophoresis has been used previously to manipulate a variety of nanoscale materials, manipulation in ionic solutions is more difficult due to the high dielectric constant of water and the formation of electrical double-layers. Here, we report experiments aimed at the manipulation of nanowires in aqueous media and real-time detection of nanowire bridging events. Real-time video images demonstrate the ability to manipulate individual nanowires in aqueous media by capturing them along the edges of electrodes, and using a slow fluid flow to transport them until they bridge across micron-sized electrode gaps. By using special cancellation schemes, we demonstrate that it is possible to eliminate the effects of background currents through the electrolyte, and to electrically detect the bridging of electrodes by individual nanowires and nanowire bundles. These results have been obtained using gold nanowires with diameters ranging from ~50 to 250?nm, ~50?nm diameter silicon nanowires, and ~70?nm diameter carbon nanofibres.

Template-directed carbon nanotube network using self-organized Si nanocrystals

We demonstrate a way to direct carbon nanotube growth using Si nanocrystals that are self-ordered via the thermal decomposition of thin silicon-on-insulator substrates. The Si nanocrystals are about 90nm wide and 100–150nm tall, with 200nm spacing. Nanotubes connect the silicon nanocrystals to form a network. Nanotubes selectively appear between tops of the Si nanocrystals. We show that the flow pattern of the carbon feedstock in the chemical vapor deposition growth process is disturbed by the geometric effect of the Si nanocrystals, providing a mechanism for growth between the tops of the Si nanocrystals.

Predicting the results of chemical vapor deposition growth of suspended carbon nanotubes.

The successful growth of suspended carbon nanotubes is normally based on purely empirical results. Here we demonstrate the ability to predict the successful suspension of nanotubes across a range of trench widths by combining experimental growth data with a theoretical description of nanotube mechanics at the growth temperature. We show that rare thermal oscillations much larger than the rms amplitude combined with the large nanotube-substrate adhesion energy together are responsible for unsuccessful nanotube suspensions. We derive an upper limit on the number of deleterious nanotube-substrate interactions that can be tolerated before successful growth becomes impossible, and we are able to accurately explain literature reports of suspended nanotube growth. The methodology developed here should enable improved growth yields of suspended nanotubes, and it provides a framework in which to analyze the role of nanotube-substrate interactions during nanotube growth by chemical vapor deposition.

Photodetector based on networks of carbon nanotubes on decomposed SOI

We report a novel method of fabricating self-assembled carbon nanotube (CNT) on Si nanocrystals and the photocurrent from this network. Silicon-on-insulator (SOI) substrate with 10nm thin top silicon layer is annealed at elevate temperature in an ultra-high vacuum environment. The Si layer dewets and aggregates into Si nanocrystal islands with dimensions about 90 nm high, 100-150 nm wide, and 200nm apart. 1nm thin Fe film is deposited on the decomposed SOI as catalyst for CNT growth. The growth is done by chemical vapor deposition (CVD) at 900 °C with a flow of CH4 at 400sccm and H2 at 20sccm. The CVD grown CNTs show strong preferential growth on the top portion of the Si nanocrystals and form a suspended network connecting the nanocrystals. No photolithographic process is needed to create this self-assembled CNT network. We find that the reason that few CNT are found on the oxide surface is because of the influence of the island topography on the CH4 gas flow pattern, with feedstock unable to reach the oxide surface when the islands are close to each other. We demonstrate that, by shining a low power 650nm wavelength commercial red laser pointer on this network, it generates photocurrent on the level of 20nA photocurrent under 1 volt bias condition. Since a 100 mW 1.175 μm wavelength IR laser does not generate any distinguishable photocurrent in our measurement setup, we believe the photocurrent generated by 650 nm red laser mainly comes from the Si nanocrystals instead of the CNTs. We demonstrate that a dense, self-assembled CNT network can be formed on the decomposed Si nanocrystals and can be used as conducting media for electric measurement.

Pattern Formation on Silicon-on-Insulator

The strain driven self-assembly of faceted Ge nanocrystals during epitaxy on Si(001) to form quantum dots (QDs) is by now well known. We have also recently provided an understanding of the thermodynamic driving force for directed assembly of QDs on bulk Si (extendable to other QD systems) based on local chemical potential and curvature of the surface. Silicon-on-insulator (SOI) produces unique new phenomena. The essential thermodynamic instability of the very thin crystalline layer (called the template layer) resting on an oxide can cause this layer, under appropriate conditions, to dewet, agglomerate, and self-organize into an array of Si nanocrystals. Using low-energy electron microscopy (LEEM), we observe this process and, with the help of first-principles total-energy calculations, we provide a quantitative understanding of this pattern formation. The Si nanocrystal pattern formation can be controlled by lithographic patterning of the SOI prior to the dewetting process. The resulting patterns of electrically isolated Si nanocrystals can in turn be used as a template for growth of nanostructures, such as carbon nanotubes (CNTs). Finally we show that this growth may be controlled by the flow dynamics of the feed gas across the substrate.