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.
Author: Renato Pinto Reveggino
First-principles study of strain-induced modulation of energy gaps of graphene/BN and BN bilayers
First-principles calculations based on density functional theory are performed on graphene/BN and BN bilayers to investigate the effect of the strain on their energy gaps. For the graphene/BN bilayer, the bands have characteristic graphenelike features with a small band gap at K. Application of strain modulates the band gap, whose magnitude depends on the strength of interaction between constituent monolayers. For the BN bilayer, on the other hand, a large band gap is predicted, which remains nearly the same for small strains. The increased inhomogeneity in charge density of different carbon sublattices due to a stronger interplanar interaction is the cause of the predicted variation in the band gap with strains applied along the perpendicular direction in the graphene/BN bilayer.
Glucose Biosensors Based on Vertically-Aligned Multi-walled Carbon Nanotubes
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
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
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
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
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
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.
Field emission and strain engineering of electronic properties in boron nitride nanotubes
FE-and-Electronic-strains-Nanotech2012
The electrical properties of boron nitride (BN) nanostructures, particularly BN nanotubes (NTs), have been studied less in comparison to the counterpart carbon nanotubes. The present work investigates the field emission (FE) behavior of BNNTs under multiple cycles of FE experiments and demonstrates a strain-engineering pathway to tune the electronic properties of BNNTs. The electrical probing of individual BNNTs were conducted inside a transmission electron microscope (TEM) using an in situ electrical holder capable of applying a bias voltage of up to 110 V. Our results indicate that in the first cycle a single BNNT can exhibit the current density of ∼1 mA cm−2 at 110 V and the turn-on voltage of 325 V μm−1. However, field emission properties reduced considerably in subsequent cycles. Real-time imaging revealed the structural degradation of individual BNNTs during FE experiments. The electromechanical measurements show that the conductivity of BNNTs can be tuned by means of mechanical straining. The resistance of individual BNNTs reduced from 2000 to 769 M and the carrier concentration increased from 0.35 × 1017 to 1.1 × 1017 cm−3 by straining the samples up to 2.5%.
Stable Electron Field Emission from PMMACNT Matrices
We have created PMMACNT matrices by embedding opened-tip vertically aligned multiwalled
carbon nanotubes (VA-MWCNTs) with poly(methyl methacrylate) (PMMA). These PMMACNT matrices are
excellent electron field emitters with an emission threshold field of 1.675 V/m, more than 2-fold lower that
that of the as-grown sample. In addition, the emission site density from these matrices is high, merely filling up
the entire sample surface. Emission stability test at 1.35 mA/cm2 was performed continuously for 40 h with no
significant degradation. On the basis of our theoretical simulation and hypothetical modeling, we attribute these
performances to the reduced screening effect and fewer Joule heatings due to the shorter effective transport
distance of the electrons in MWCNTs.