Ruth Zhang

Structure determination of individual single-wall carbon nanotubes by nanoarea electron diffraction

In this letter, we report an electron diffraction determination of chiral vectors (n,m) of individual single-wall carbon nanotubes (SWNTs). Electron diffraction patterns from individual SWNTs were recorded on imaging plates using a parallel electron beam over a section of tube of ∼50 nm long. Using two tubes of 1.39 and 3.77 nm in diameter, we show that the details of electron diffuse scattering can be detected for both the small and large tubes. The quality of diffraction patterns allows the accurate measurement of both the diameters and chiral angles of SWNTs for a direct determination of chiral vectors. The electron diffraction technique is general and applicable to other forms of individual nanostructures.

In situ detection of cytochrome c adsorption with single walled carbon nanotube device

We report on the in situ detection of cytochrome c adsorption onto individual SWNT transistors via the changes in the electron transport properties of the transistors.

First Demonstration of AC Gain From a Single-walled Carbon Nanotube Common-Source Amplifier

First demonstration of AC gain from a single-walled carbon nanotube transistor is presented. A top-gated carbon nanotube field-effect transistor (CNFET) is configured as a common-source amplifier and frequency response function of the amplifier is measured. Evidence of unambiguous signal amplification is observed in time domain as well as frequency domain up to a unity gain frequency of approximately 560 kHz. The observed roll-off in frequency is solely due to the RC time constant of the measurement apparatus. A specifically designed circuit compatible SPICE model for the CNFET is used to model both DC and AC characteristic with the same set of physical parameters for the first time. Good agreement between measurement and simulation is obtained. For a device without the parasitic load capacitance, the predicted intrinsic unity voltage gain frequency is 29 GHz and the cut-off frequency is ~ 50 GHz.

Selective formation and alignment of InAs quantum dots over mesa stripes along the [011] and [001] directions on GaAs (100) substrates

We have studied the selective formation of InAs self-organized quantum dots on top of [001]- and [011]-oriented mesa stripes on patterned GaAs (100) substrates. The GaAs stripes are also grown by selective area epitaxy. The dot density and spatial distribution depend on both the stripe orientation and the width of the (100) top facet of the stripe. The density is higher for stripes aligned in the [001] direction, and lower for those aligned in the [011] direction, respectively, when compared to that obtained on a planar substrate under the same growth conditions. In addition, the dot uniformity is improved by reducing the top facet width below 200 nm in the growth of the mesa stripes, and well-aligned rows of dots are obtained for sub-100-nm widths.We have studied the selective formation of InAs self-organized quantum dots on top of [001]- and [011]-oriented mesa stripes on patterned GaAs (100) substrates. The GaAs stripes are also grown by selective area epitaxy. The dot density and spatial distribution depend on both the stripe orientation and the width of the (100) top facet of the stripe. The density is higher for stripes aligned in the [001] direction, and lower for those aligned in the [011] direction, respectively, when compared to that obtained on a planar substrate under the same growth conditions. In addition, the dot uniformity is improved by reducing the top facet width below 200 nm in the growth of the mesa stripes, and well-aligned rows of dots are obtained for sub-100-nm widths.

SELECTIVE POSITIONING OF INAS SELF-ORGANIZED QUANTUM DOTS ON SUB-250 NM GAAS FACETS

We have selectively grown GaAs mesas with (100) top facets that range from several μm to less than 250 nm in width on an oxide-patterned GaAs substrate, and InAs self-organized quantum dots (SOQDs) on top of the facets. For a given amount of InAs deposited, the dot density varies with the facet width, and is higher than the density on a planar, nonpatterned substrate. The SOQDs also tend to form at the facet edges. These observations are indicative of a strong surface diffusion effect of In-containing species adsorbed on the various crystalline facets of the GaAs mesas.

The contrast mechanism in low voltage scanning electron microscopy of single-walled carbon nanotubes

The contrast mechanism for scanning electron microscopy images of single-walled carbon nanotubes (SWNTs) is studied for samples on various substrates using low voltage scanning electron microscopy (LVSEM). We show that the contrast of SWNT images is greatly affected by a charging effect, where the brightness strongly depends on the supply of charges available to the SWNTs and the charging characteristics of the substrate. We demonstrate that the charging effect in LVSEM can be utilized to obtain images of various SWNT device structures to provide rapid feedback for the device fabrication process.

Field-effect and single-electron transistors based on single-walled carbon nanotubes catalyzed by Al/Ni thin films

Growth of single-walled carbon nanotubes (SWNTs) via a chemical vapor deposition technique catalyzed by a thin Al/Ni stack is presented. Catalyst islands are defined by lithography, thin film evaporation and lift off technique. The process is fully compatible with conventional micro- and nanolithography techniques. Micrometer size semiconducting SWNT devices have been used to explore field-effect properties of the nanotubes at room temperature while smaller length semiconducting SWNT islands are used to study single-electron charging effects at low temperatures.

Spatially Selective Formation of InAs Self-organized Quantum Dots on Patterned GaAs (100) Substrates

Self-organized InAs quantum dots are selectively formed on patterned GaAs (100) substrate by chemical beam epitaxy. Dot formation on top of sub-µm sized mesa stripes is a function of the stripe top width, the stripe orientation, as well as the growth conditions. The dot density is higher for stripes aligned in [001] direction, and lower for stripes aligned in the [011] direction, respectively, when compared to that obtained on a non-patterned substrate under the same growth conditions. We attribute these effects to dissimilar surface migration behavior of In adatoms on different side facets of the mesa stripes. The spatial distribution of the self-organized quantum dots (SOQDs) on top of [001] stripes becomes more uniform as the top facet width decreases, and self-alignment of the dots into well-defined rows is achieved by manipulating the growth conditions and mesa size.

Control of single-wall-nanotube field-effect transistors via indirect long-range optically induced processes

We observe significant changes in the response of single-wall-carbon-nanotube-based field-effect transistors upon photoexcitation in the presence of optically active conjugated polymer network. The primary features observed are in the form of an increase in the current in the depletion mode upon photoexcitation. Pulsed measurements indicate that the transistor enters the depleted state prior to the rise in current brought about by the transfer of the photogenerated carriers from the semiconducting polymer to the nanotube under depletion bias.

Controlled growth of quantum dots on mesa top

For ridge-shaped GaAs mesas grown by selective area epitaxy in the oxide opening of the patterned substrate, the width of the mesa top can be controlled by growing a calculated thickness of the GaAs. InAs can then be selectively grown on the mesa top to obtain well defined rows of self-organized quantum dots. The shape of the oxide opening is tailored in this study to achieve the desired dot distributions along the length of a ridge. The results show individual row control of the dots which makes possible the design of novel device structures.