Category: Carbon

Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas

Renato Pinto Reveggino

PECVD_CNTs_APL2005

Carbon nanotubes CNTs are among the important materials for the advancement of future nanoscience and nano-technology. Recently, vertically aligned multiwalled carbon nanotubes VA-MWCNTs have gained additional attention for innovative applications, such as nanotubes antennas,1vertical transistors,2,3 and vertical biosensors.4 Ideally, these applications require VA-MWCNTs to be grown at desired patterns. Plasma-enhanced chemical vapor deposition PECVD is the only technique for growing individual VA-MWCNTs at desired locations. However, the graphitic order of these VA-MWCNTs is inferior to the multiwall CNTs grown by arc discharge5 and thermal chemical vapordeposition.6–8 This has been a long-standing issue for realis-tic uses of VA-MWCNTs in applications, such as electron field emission devices.9–13

 

Nanotubes and Related Nanostructures–2014

Renato Pinto Reveggino

opl1700fm

Carbon nanotubes and related nanostructures, including nanosheets have attracted tremendous attention for their unique structures and intriguing properties. These nanomaterials have been widely investigated—from theory, synthesis, and characterization to applications in electronic devices, energy generation and storage, biological and chemical sensors, etc. In addition, non-carbon nanostructures such as nanotubes and nanosheets of boron nitride (BN) have gained increasing interest. To facilitate scientific interaction among students and researchers on the latest advancements in this area, Symposium MM – Nanotubes and Related Nanostructures, was organized and held on Apr. 21–25 at the 2014 MRS Spring Meeting in San Francisco, California. The symposium organizers were Don Futaba (National Institute of Advanced Industrial Science and Technology), Annick Loiseau (Laboratoire d’Etude des Microstructures), Yoke Khin Yap (Michigan Technological University), and Ming Zheng (National Institute of Standards and Technology). This proceedings volume consists of peer-reviewed papers presented in the symposium, including invited and contributed presentations. These papers represent a snapshot of topics discussed in both theoretical and experimental aspects. We hope this publication will contribute toward productive research in the area of nanotubes and related nanostructures.

A simple scheme of molecular electronic devices with multiwalled carbon nanotubes as the top electrodes

Renato Pinto Reveggino

Molectronics-Carbon-2012

A simple fabrication scheme for molecular electronic junctions is presented with multi-walled carbon nanotubes (MWCNTs) as the top electrodes. Results indicate that our approach retains the molecular character of the chosen molecules [a self-assembled mono-layer of octadecanethiol on gold bottom electrodes] and opens the door for studying a wide variety of organothiol candidates for molecular electronics. The fabrication scheme is designed in a way that it can be modified into all-carbon devices in the future by using graphitic carbon bottom electrodes functionalized with nitrozoabenzene, for example, and MWCNTs or graphene as the top electrodes. Alternatively, the scheme is applicable for all-gold devices with gold bottom electrodes and gold nanowire top electrodes.

Switching Behaviors of Graphene- Boron Nitride Nanotube Heterojunctions

Renato Pinto Reveggino

Graphene-BNNT junctions-2015

High electron mobility of graphene has enabled their application in high-frequency analogue devices but their gapless nature has hindered their use in digital switches. In contrast, the structural analogous, h-BN sheets and BN nanotubes (BNNTs) are wide band gap insulators. Here we show that the growth of electrically insulating BNNTs on graphene can enable the use of graphene as effective digital switches. These graphene-BNNT heterojunctions were characterized at room temperature by four-probe scanning tunneling microscopy (4-probe STM) under real-time monitoring of scanning electron microscopy (SEM). A switching ratio as high as 105 at a turn-on voltage as low as 0.5V were recorded. Simulation by density functional theory (DFT) suggests that mismatch of the density of states (DOS) is responsible for these novel switching behaviors.

Glucose Biosensors Based on Vertically-Aligned Multi-walled Carbon Nanotubes

Renato Pinto Reveggino

Glucose-MRS2010

Vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs) were grown using plasma

enhanced chemical vapor deposition (PECVD) technique. These VA-MWCNTs were then dip

coated by Poly methyl methacrylate (PMMA) followed by annealing. Samples were then

polished to expose the tips of CNTs. Biological molecules Glucose Oxidase (GOx) were then

immobilized on the exposed tips of these nanoelectrode ensembles. Here we present further

characterization of these devices, with results on the detection limits and measurement stability.

We found that these sensors can be reused for longer than six months when kept in proper

storage conditions.

Effect of graphitic order on field emission stability of carbon nanotubes

Renato Pinto Reveggino

FE-nanotech-2007

We observed current density (J ) dependent degradation in field emission current from multiwalled carbon nanotubes (MWCNTs). These degradations are recoverable and can be explained by emission current-induced dislocations along the MWCNTs. MWCNTs grown by thermal chemical vapour deposition (CVD) can emit stable current continuously for at least 1200 min with upper current density limits of ∼0.5 mA cm−2. In contrast, this upper limit is <40 μA cm−2 for nanotubes grown by plasma-enhanced CVD (PECVD), although higher J is possible with relatively shorter stability duration. High-resolution transmission electron microscopy and Raman spectroscopy indicate higher graphitic order of the thermal CVD grown MWCNTs as compared to PECVD grown MWCNTs. Our study suggests that graphitic order affects their upper performance limits of long-term emission stability, although the effects from adsorbates cannot be completely ignored. These results indicate that field emission cannot be considered as an ideal quantum tunnelling process. The effect of electron transport along CNTs before electron tunnelling must be considered.

Comparing Field Emission Stability of Lithography-free, Modified Multi-Walled Carbon Nanotubes

Renato Pinto Reveggino

FE-MRS2010

Field emission from carbon nanotubes (CNTs) has been known for more than a decade but there is no commercialized product available in the market. Apparently, we need to improve our basics understanding on stable field emission from CNTs. Here we compared the field emission properties of as grown vertically-aligned multi-walled carbon nanotubes (MWCNTs) to two types of modified MWCNTs: 1) Conical bundles of opened-tip MWCNTs, and 2) Opened-
tip MWCNTs embedded in poly-methyl methacrylate (PMMA). We found that both types of modified MWCNTs have lower emission thresholds and better emission stability than the as grown samples. Among these modified samples, MCNTs embedded in PMMA has lower emission thresholds and better emission stability. We attributed these improvements to the filling of spacing between MWCNTs with PMMA that has higher dielectric constant than vacuum.

Very Stable Electron Field Emission from Strontium Titanate Coated Carbon Nanotube Matrices with Low Emission Thresholds

Renato Pinto Reveggino

FE-matrices-ACS-Nano2013

Novel PMMASTOCNT matrices were created by opened-tip vertically aligned multiwalled carbon nanotubes (VAMWCNTs) with conformal coatings of strontium titanate (STO) and poly(methyl methacrylate) (PMMA). Emission threshold of 0.8 V/μm was demonstrated, about 5-fold lower than that of the as-grown VA- MWCNTs. This was obtained after considering the related band structures under the perspective of work functions and tunneling width as a function of the STO thickness. We showed that there is an optimum thickness of STO coatings to effectively reduce the work function of CNTs and yet minimize the tunneling width for electron emissions. Furthermore, simulation and modeling suggest that PMMASTOCNT matrices have suppressed screening effects and Coulombs’ repulsion forces between electrons in adjacent CNTs, leading to low emission threshold, high emission density, and prolonged emission stability. These findings are important for practical application of VA-MWCNTs in field emission devices, X-ray generation, and wave amplification.

Stability of field emission current from various types of carbon nanotube films

Renato Pinto Reveggino

FE-DMR2006

A series of emission current measurements were taken from various types of multiwalled carbon nanotube (MWCNT) films in order to examine their stability for electron field emission. We found that the MWCNTs films grown by the catalytic thermal chemical vapor deposition (CVD) method exhibited much improved emission stability as compared to MWCNT films grown by the plasma-enhanced CVD (PECVD) method. We explain this difference of performance by referring to the graphitic order of these MWCNTs as detected by transmission electron microscopy and Raman spectroscopy. Results indicate that MWCNTs with high-order tubular structures demonstrate stable electron field emission. The best performing sample exhibits a constant current degradation of ̈3% per hour at an emission current density of ̈1 mJ/cm2.

Enhanced field emission stability and density produced by conical bundles of catalyst-free carbon nanotubes

Renato Pinto Reveggino

FE-carbon2010

Self-assembled bundling and catalyst removal can enhance the field emission stability and density of vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs). These catalyst-free, opened tip, VA-MWCNTs offered better emission stability than the as grown samples. Both the emission stability and density were further enhanced as the opened-tip MWCNTs self-assembled into arrays of conical bundles. Theoretical simulation suggests that higher emission density was due to the reduced screening effects. The simulated local fields at the tips of the bundles suggest for a two-order of magnitude lower electric field loading on MWCNTs and contribute to prolong emission stability needed for practical applications.