Barry Chin Li Cheung Publications

Energy Gaps in “Metallic” Single-Walled Carbon Nanotubes

Min Ouyang et al. — Fri, 03 Apr 2009 12:31:25 PDT

Metallic single-walled carbon nanotubes have been proposed to be good one-dimensional conductors. However, the finite curvature of the graphene sheet that forms the nanotubes and the broken symmetry due to the local environment may modify their electronic properties. We used low-temperature atomically resolved scanning tunneling microscopy to investigate zigzag and armchair nanotubes, both thought to be metallic. “Metallic” zigzag nanotubes were found to have energy gaps with magnitudes that depend inversely on the square of the tube radius, whereas isolated armchair tubes do not have energy gaps. Additionally, armchair nanotubes packed in bundles have pseudogaps, which exhibit an inverse dependence on tube radius. These observed energy gaps suggest that most “metallic” single-walled nanotubes are not true metals, and they have implications for our understanding of the electronic properties and potential applications of carbon nanotubes.

6:1 aspect ratio silicon pillar based thermal neutron detector filled with ¹⁰B

R. J. Nikolic et al. — Tue, 04 Nov 2008 07:06:47 PST

Current helium-3 tube based thermal neutron detectors have shortcomings in achieving simultaneously high efficiency and low voltage while maintaining adequate fieldability performance. By using a three-dimensional silicon p-i-n diode pillar array filled with boron-10 these constraints can be overcome. The fabricated pillar structured detector reported here is composed of 2 μm diameter silicon pillars with a 4 μm pitch and height of 12 μm. A thermal neutron detection efficiency of 7.3+/−0.6% and a neutron-to-gamma discrimination of 10⁵ at 2 V reverse bias were measured for this detector. When scaled to larger aspect ratio, a high efficiency device is possible.

The creation of organic and biological nanostructures at surfaces using scanning probe nanolithography

B. L. Weeks et al. — Wed, 12 Mar 2008 07:18:43 PDT

As the size of microelectronic devices continues to shrink and the desire to build in hierarchical structures of organic and biological materials grows, control of the chemistry and structure of materials at the molecular level will become increasingly important. Conventional lithographic techniques to pattern polymeric thin films are beginning to reach their resolution limit and several alternative "bottom-up" strategies have emerged that use the scanning probe microscope to manipulate matter at the atomic or molecular scale. Of these new scanning probe nanolithography (SPN) techniques, dip-pen nanolithography (DPN) [I] and scanning probe nanografting (SNG) [2] are particularly promising.

The DPN methodology utilizes the tip of an atomic force microscope (AFM) as a "nanopen" to transport an "ink containing organic molecules onto a solid support as illustrated in Figure 8. l. Using the same tip to "write" and subsequently "read patterns, it is possible to create nanoscale patterns of alkyl thiols with remarkable resolution (~10 nm) and simultaneously control the chemical functionality of the written areas. Examples of patterns created using a variety of inks are shown in Figure 8.2.

Growth of nanotubes for probe microscopy tips

Jason H. Hafner et al. — Wed, 05 Mar 2008 08:08:33 PST

Carbon nanotubes, which have intrinsically small diameters and high aspect ratios and which buckle reversibly, make potentially ideal structures for use as tips in scanning probe microscopies, such as atomic force microscopy (AFM)^{1, 2, 3, 4}. However, the present method of mechanically attaching nanotube bundles for tip fabrication is time consuming and selects against the smallest nanotubes, limiting the quality of tips. We have developed a technique for growing individual carbon nanotube probe tips directly, with control over the orientation, by chemical vapor deposition (CVD) from the ends of silicon tips. Tips grown in this way may become widely used in high-resolution probe microscopy imaging.

Covalently Functionalized Nanotubes as Nanometer-Sized Probes in Chemistry and Biology

Stanislaus S. Wong et al. — Wed, 05 Mar 2008 08:04:43 PST

Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM)^{1, 2, 3}. Their high aspect ratio, for example, opens up the possibility of probing the deep crevices⁴ that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips⁵. Another characteristic feature of nanotubes is their ability to buckle elastically^{4, 6}, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level⁷. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein–ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions⁸, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology.

Roadmap for High Efficiency Solid-State Neutron Detectors

R. J. Nikolic et al. — Wed, 27 Feb 2008 09:52:37 PST

Solid-state thermal neutron detectors are generally fabricated in a planar configuration by coating a layer of neutron-to-alpha converter material onto a semiconductor. The as-created alpha particles in the material are expected to impinge the semiconductor and create electron-hole pairs which provide the electrical signal. These devices are limited in efficiency to a range near (2-5%)/cm² due to the conflicting thickness requirements of the converter layer. In this case, the layer is required to be thick enough to capture the incoming neutron flux while at the same time adequately thin to allow the alpha particles to reach the semiconductor. A three dimensional matrix structure has great potential to satisfy these two requirements in one device. Such structures can be realized by using PIN diode pillar elements to extend in the third dimension with the converter material filling the rest of the matrix. Our strategy to fabricate this structure is based on both “top-down” and “bottom-up” approaches. The “top down” approach employs high-density plasma etching techniques, while the “bottom up” approach draws on the growth of nanowires by chemical vapor deposition. From our simulations for structures with pillar diameters from 2 μm down to 100 nm, the detector efficiency is expected to increase with a decrease in pillar size. Moreover, in the optimized configuration, the detector efficiency could be higher than 75%/cm². Finally, the road map for the relationship between detector diameter and efficiency will be outlined.

Future of Semiconductor Based Thermal Neutron Detectors

R. J. Nikolić et al. — Wed, 27 Feb 2008 09:47:34 PST

Thermal neutron detectors have seen only incremental improvements over the last decades. In this paper we overview the current technology of choice for thermal neutron detection – ³He tubes, which suffer from, moderate to poor fieldability, and low absolute efficiency. The need for improved neutron detection is evident due to this technology gap and the fact that neutrons are a highly specific indicator of fissile material. Recognizing this need, we propose to exploit recent advances in microfabrication technology for building the next generation of semiconductor thermal neutron detectors for national security requirements, for applications requiring excellent fieldability of small devices. We have developed an innovative pathway taking advantage of advanced processing and fabrication technology to produce the proposed device. The crucial advantage of our Pillar Detector is that it can simultaneously meet the requirements of high efficiency and fieldability in the optimized configuration, the detector efficiency could be higher than 70%.

Searching for Smart Durable Coatings to Promote Bone Marrow Stromal Cell Growth While Preventing Biofilm Formation

Fereydoon Namavar et al. — Wed, 27 Feb 2008 09:41:40 PST

There is a great need to develop methods to regulate cellular growth in order to enhance or prevent cell proliferation, as needed, to either improve health or prevent disease. In this work we evaluated the adhesion, survival and growth of bone marrow stromal cells on the surface of several new ion beam engineered nano-crystals of ceramic hard coatings such as zirconium, titanium, tantalum and cerium oxides. Cell adhesion and growth on the ceramic coatings were compared to adhesion and growth on a nano-crystalline silver coating which is known to possess antibacterial properties. The initial results of a study to determine the effect of nanocrystalline titanium and silver coating on staphylococcus aureus biofilm growth is also discussed.

Fabrication of Pillar-Structured Thermal Neutron Detectors

Rebecca J. Nikolić et al. — Wed, 27 Feb 2008 08:16:53 PST

Pillar detector is an innovative solid state device structure that leverages advanced semiconductor fabrication technology to produce a device for thermal neutron detection. State-of-the-art thermal neutron detectors have shortcomings in achieving simultaneously high efficiency, low operating voltage while maintaining adequate fieldability performance. By using a 3-dimensional silicon PIN diode pillar array filled with isotopic boron 10, (¹⁰B) a high efficiency device is theoretically possible. The fabricated pillar structures reported in this work are composed of 2 μm diameter silicon pillars with a 4 μm pitch and pillar heights of 6 and 12 μm. The pillar detector with a 12 μm height achieved a thermal neutron detection efficiency of 7.3% at 2V.

Atomically Resolved Single-Walled Carbon Nanotube Intramolecular Junctions

Min Ouyang et al. — Fri, 22 Feb 2008 07:43:14 PST

Intramolecular junctions in single-walled carbon nanotubes are potentially ideal structures for building robust, molecular-scale electronics but have only been studied theoretically at the atomic level. Scanning tunneling microscopy was used to determine the atomic structure and electronic properties of such junctions in single-walled nanotube samples. Metal-semiconductor junctions are found to exhibit an electronically sharp interface without localized junction states, whereas a more diffuse interface and low-energy states are found in metal-metal junctions. Tight-binding calculations for models based on observed atomic structures show good agreement with spectroscopy and provide insight into the topological defects forming intramolecular junctions. These studies have important implications for applications of present materials and provide a means for assessing efforts designed to tailor intramolecular junctions for nanoelectronics.

Magnetic Clusters on Single-Walled Carbon Nanotubes: The Kondo Effect in a One-Dimensional Host

Teri W. Odom et al. — Fri, 22 Feb 2008 07:39:36 PST

Single-walled carbon nanotubes are ideal systems for investigating fundamental properties and applications of one-dimensional electronic systems. The interaction of magnetic impurities with electrons confined in one dimension has been studied by spatially resolving the local electronic density of states of small cobalt clusters on metallic single-walled nanotubes with a low-temperature scanning tunneling microscope. Spectroscopic measurements performed on and near these clusters exhibit a narrow peak near the Fermi level that has been identified as a Kondo resonance. Using the scanning tunneling microscope to fabricate ultra-small magnetic nanostructures consisting of small cobalt clusters on short nanotube pieces, spectroscopic studies of this quantum box structure exhibited features characteristic of the bulk Kondo resonance, but also new features due to finite size.

Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing

Thomas Rueckes et al. — Fri, 22 Feb 2008 07:35:33 PST

A concept for molecular electronics exploiting carbon nanotubes as both molecular device elements and molecular wires for reading and writing information was developed. Each device element is based on a suspended, crossed nanotube geometry that leads to bistable, electrostatically switchable ON/OFF states. The device elements are naturally addressable in large arrays by the carbon nanotube molecular wires making up the devices. These reversible, bistable device elements could be used to construct nonvolatile random access memory and logic function tables at an integration level approaching 1012 elements per square centimeter and an element operation frequency in excess of 100 gigahertz. The viability of this concept is demonstrated by detailed calculations and by the experimental realization of a reversible, bistable nanotube-based bit.

Carbon nanotube atomic force microscopy tips: Direct growth by chemical vapor deposition and application to high-resolution imaging

Chin Li Cheung et al. — Fri, 22 Feb 2008 07:31:49 PST

Carbon nanotubes are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. This communication describes methods for the direct growth of carbon nanotube tips by chemical vapor deposition (CVD) using ethylene and iron catalysts deposited on commercial silicon-cantilever-tip assemblies. Scanning electron microscopy and transmission electron microscopy measurements demonstrate that multiwalled nanotube and single-walled nanotube tips can be grown by predictable variations in the CVD growth conditions. Force-displacement measurements made on the tips show that they buckle elastically and have very small (≤100 pN) nonspecific adhesion on mica surfaces in air. Analysis of images recorded on gold nanoparticle standards shows that these multi- and single-walled carbon nanotube tips have radii of curvature of 3–6 and 2–4 nm, respectively. Moreover, the nanotube tip radii determined from the nanoparticle images are consistent with those determined directly by transmission electron microscopy imaging of the nanotube ends. These molecular-scale CVD nanotube probes have been used to image isolated IgG and GroES proteins at high-resolution.

Direct haplotyping of kilobase-size DNA using carbon nanotube probes

Adam T. Woolley et al. — Fri, 22 Feb 2008 07:27:40 PST

We have implemented a method for multiplexed detection of polymorphic sites and direct determination of haplotypes in 10-kilobasesize DNA fragments using single-walled carbon nanotube (SWNT) atomic force microscopy (AFM) probes. Labeled oligonucleotides are hybridized specifically to complementary target sequences in template DNA, and the positions of the tagged sequences are detected by direct SWNT tip imaging. We demonstrated this concept by detecting streptavidin and IRD800 labels at two different sequences in M13mp18. Our approach also permits haplotype determination from simple visual inspection of AFM images of individual DNA molecules, which we have done on UGT1A7, a gene under study as a cancer risk factor. The haplotypes of individuals heterozygous at two critical loci, which together influence cancer risk, can be easily and directly distinguished from AFM images. The application of this technique to haplotyping in population-based genetic disease studies and other genomic screening problems is discussed.

Structural biology with carbon nanotube AFM probes

Adam T. Woolley et al. — Fri, 22 Feb 2008 07:23:30 PST

Carbon nanotubes represent ideal probes for high-resolution structural and chemical imaging of biomolecules with atomic force microscopy. Recent advances in fabrication of carbon nanotube probes with sub-nanometer radii promise to yield unique insights into the structure, dynamics and function of biological macromolecules and complexes.

Atomic force microscopy investigation of virus aggregation and assembly at chemical templates formed by scanned probe nanolithography

Chin Li Cheung et al. — Thu, 07 Feb 2008 08:11:47 PST

Aggregation and assembly of macromolecules are important processes in a number of scientific fields including structural biology, medicine, and materials science. For example, growth of well-ordered two-dimensional (2-D) arrays and bulk crystals remains the rate-limiting step in macromolecular structure determination. Uncontrolled aggregation of proteins is the source of a number of devastating pathologies such as Creutzfeldt-Jakob syndrome. Moreover, the demonstrated ability of engineered viruses and proteins to act as templates for growth of inorganic nanostructures is driving a need for methods to deterministically pattern their assembly at surfaces in order to fabricate hierarchical materials and devices.

Fabrication of nanopillars by nanosphere lithography

Chin Li Cheung et al. — Thu, 07 Feb 2008 08:06:07 PST

A low cost nanosphere lithography method for patterning and generation of semiconductor nanostructures provides a potential alternative to the conventional top-down fabrication techniques. Forests of silicon pillars of sub-500 nm diameter and with an aspect ratio up to 10 were fabricated using a combination of the nanosphere lithography and deep reactive ion etching techniques. The nanosphere etch mask coated silicon substrates were etched using oxygen plasma and a timemultiplexed “Bosch” process to produce nanopillars of different length, diameter and separation. Scanning electron microscopy data indicate that the silicon etch rates with the nanoscale etch masks decrease linearly with increasing aspect ratio of the resulting etch structures.

Structural and functional imaging with carbon nanotube AFM probes

J. H. Hafner et al. — Wed, 06 Feb 2008 11:30:10 PST

Atomic force microscopy (AFM) has great potential as a tool for structural biology, a field in which there is increasing demand to characterize larger and more complex biomolecular systems. However, the poorly characterized silicon and silicon nitride probe tips currently employed in AFM limit its biological applications. Carbon nanotubes represent ideal AFM tip materials due to their small diameter, high aspect ratio, large Young’s modulus, mechanical robustness, well-defined structure, and unique chemical properties. Nanotube probes were first fabricated by manual assembly, but more recent methods based on chemical vapor deposition provide higher resolution probes and are geared towards mass production, including recent developments that enable quantitative preparation of individual single-walled carbon nanotube tips [J. Phys. Chem. B 105 (2001) 743]. The high-resolution imaging capabilities of these nanotube AFM probes have been demonstrated on gold nanoparticles and well-characterized biomolecules such as IgG and GroES. Using the nanotube probes, new biological structures have been investigated in the areas of amyloid-beta protein aggregation and chromatin remodeling, and new biotechnologies have been developed such as AFM-based haplotyping. In addition to measuring topography, chemically functionalized AFM probes can measure the spatial arrangement of chemical functional groups in a sample. However, standard silicon and silicon nitride tips, once functionalized, do not yield sufficient resolution to allow combined structural and functional imaging of biomolecules. The unique end-group chemistry of carbon nanotubes, which can be arbitrarily modified by established chemical methods, has been exploited for chemical force microscopy, allowing single-molecule measurements with well-defined functionalized tips.

Direct Imaging of Human SWI/SNF-Remodeled Mono- and Polynucleosomes by Atomic Force Microscopy Employing Carbon Nanotube Tips

Gavin R. Schnitzler et al. — Wed, 06 Feb 2008 11:25:16 PST

Chromatin-remodeling complexes alter chromatin structure to facilitate, or in some cases repress, gene expression. Recent studies have suggested two potential pathways by which such regulation might occur. In the first, the remodeling complex repositions nucleosomes along DNA to open or occlude regulatory sites. In the second, the remodeling complex creates an altered dimeric form of the nucleosome that has altered accessibility to transcription factors. The extent of translational repositioning, the structure of the remodeled dimer, and the presence of dimers on remodeled polynucleosomes have been difficult to gauge by biochemical assays. To address these questions, ultrahigh-resolution carbon nanotube tip atomic force microscopy was used to examine the products of remodeling reactions carried out by the human SWI/SNF (hSWI/SNF) complex. We found that mononucleosome remodeling by hSWI/SNF resulted in a dimer of mononucleosomes in which ~60 bp of DNA is more weakly bound than in control nucleosomes. Arrays of evenly spaced nucleosomes that were positioned by 5S rRNA gene sequences were disorganized by hSWI/SNF, and this resulted in long stretches of bare DNA, as well as clusters of nucleosomes. The formation of structurally altered nucleosomes on the array is suggested by a significant increase in the fraction of closely abutting nucleosome pairs and by a general destabilization of nucleosomes on the array. These results suggest that both the repositioning and structural alteration of nucleosomes are important aspects of hSWI/SNF action on polynucleosomes.

Growth and fabrication with single-walled carbon nanotube probe microscopy tips

Chin Li Cheung et al. — Wed, 06 Feb 2008 11:18:16 PST

Single-walled carbon nanotube (SWNT) probe microscopy tips were grown by a surface growth chemical vapor deposition method. Tips consisting of individual SWNTs (1.5–4 nm in diameter) and SWNT bundles (4–12 nm in diameter) have been prepared by design through variations in the catalyst and growth conditions. In addition to high-resolution imaging, these tips have been used to fabricate SWNT nanostructures by spatially controlled deposition of specific length segments of the nanotube tips.