Quantum-mechanical simulations of field emission from carbon nanotubes

We present simulations of field emission from 2-nm long open (5,5), closed (5,5) and open (10,0) carbon nanotubes. Besides usual effects associated with the field-emission process, the total-energy distributions of the field-emitted electrons present peaks that are shifted by the electric field. Their sharpness and the evolution of their amplitude when changing the electric field depend on the semiconducting or metallic character of the nanotube.     

Interaction between carbon nanotubes and substrate and its implication on field emission mechanism

The interaction between carbon nanotubes (CNTs) and substrate plays an important role in the process of field emission. A double-barrier model is adopted to analyze the difference of field emission for various CNT films. Result shows that the width of interface barrier determines the emission performance. For CNTs on titanium, the best emission performance is attributed to the removal of interlayer barrier by the formation of conductive titanium carbide. These facts might shed new light on the field emission mechanism for CNTs.     

Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

Electronic and optical properties of double-walled armchair carbon nanotubes

Magnetoelectronic structures of double-walled armchair carbon nanotubes are calculated according to the tight-binding model. Their features are dominated by the intertube interactions, the symmetric configurations, the magnetic flux, and the Zeeman splitting. The drastic changes of the low energy states, such as energy dispersion, wave function, and Fermi level, which also rely on the different symmetries, are caused by the intertube interactions. The magnetic flux could change linear bands into parabolic bands, destroy state degeneracy, open an energy gap, and shift Fermi level. The magnetic flux and the intertube interactions, however, compete with each other in the metallic or semiconducting behavior. The Zeeman splitting would suppress the metal–semiconductor transition while the opposite is true of the magnetic flux. The main characteristics of energy bands are directly reflected in the magneto-optical absorption spectra. The different symmetric configurations can be distinguished by the absorption peaks, and the threshold absorption frequency is not identical with the energy gap.     

In-situ studies of electron field emission of single carbon nanotubes inside the TEM

Electron field emission characteristics of individual multi-walled carbon nanotubes (MWCNTs) were investigated in situ inside the transmission electron microscope (TEM). For a single MWCNT it was found that while field-emission can hardly occur from the side of the nanotube, a curved nanotube may result in finite side emission and the best emission geometry is the top emission geometry. Current-voltage (I-V) measurements made at different vacuum conditions and voltage sweeps emphasize the importance of the adsorbates on the electron field emission of MWCNTs. For a contaminated MWCNT, although the field emission current was reduced, the stability of its emission was improved. A current of up to several tens of μA was observed for a single MWCNT, but it was found that long time emission usually results in drastic structure damage that may lead to sudden emission failure.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

Flame-synthesis of carbon nanotubes on silicon substrates and their field emission properties

We reported the flame-synthesis of patterned multiwalled carbon nanotubes (CNTs) on silicon substrate by a shadow mask and their field emission properties. It was found that CNTs with tangled and curved morphology were preferentially grown around the cracked edges of Ni dot pattern. A crack-induced catalyst-activation growth mechanism was proposed. The patterned CNTs fabricated by such a simple flame-synthesis method exhibited good field emission characteristics with uniform emission patterns and reproducible and stable emission behaviors, although the CNTs possessed many defective graphite layers and showed relatively higher turn-on and threshold field than other reported CNTs grown by chemical vapor deposition. Our results demonstrated that such a low-cost and scaleable CNT pattern fabrication process can be expected to have favorable applications in field emission devices.     

Optical excitations of finite carbon nanotubes

Optical excitations of finite carbon nanotubes by the cross polarized light are studied within the gradient approximation. They are dominated by the quantum size effects. The absorption spectra exhibit rich absorption peaks, mainly owing to many zero-dimensional discrete states. They strongly depend on the length, the radius, the chiral angles, and the magnetic flux. The absorption peaks gradually group together as the length increases. The threshold excitation energy (ω_th) decreases in the increasing of radius. The dependence of ω_th on length is monotonous for zigzag nanotubes, while it is oscillatory for armchair nanotubes. The threshold excitation energy is constant for the sufficiently long carbon nanotubes. Chiral carbon nanotubes exhibit very complicated optical spectra, compared with achiral carbon nanotubes. The magnetic flux reduces the threshold excitation energy and increases the number of absorption peaks.     

Persistent currents in finite zigzag carbon nanotubes

Magnetic properties of finite zigzag carbon nanotubes are studied within the tight-binding model. The spin-B interaction (Zeeman splitting) causes the metal-semiconductor transition and thus produces a large persistent current (J) with special jump structures. This effect makes all zigzag carbon nanotubes exhibit a gigantic paramagnetism. It also destroys the periodicity of magnetic properties. The dependence on the magnetic flux, the length (w), the radius (r), the temperature (T), and the chirality (zigzag or armchair) is strong. The amplitude of J quickly decreases with increasing of (w, r, T). Zigzag carbon nanotubes differ significantly from armchair carbon nanotubes (or infinite zigzag carbon nanotubes) in features such as magnetic susceptibility and in special structures in J.     

Non-reactive rf treatment of multiwall carbon nanotube with inert argon plasma for enhanced field emission

External rf plasma irradiation of well-aligned multiwall carbon nanotubes (MWCTs) was demonstrated as a simple purification process. The plasma exposure to argon (Ar) gas was performed in situ in order to modify any structural defects and to vaporize contamination of the MWCTs grown at the extremely low temperature of 400 °C. Scanning electron microscopy and transmission electron microscopy confirmed the catalytic particle removal from the MWCT edges. In addition, Fourier transformer infrared spectroscopy and energy dispersive spectrometry showed that Ar plasma atoms were not chemically terminated by carbon atoms because of the inertness of the Ar atoms. The structural enhancement in the MWCTs resulted in a reduction of the turn-on field from 1.65 V/μm to 0.93 V/μm and an increase in the field emission current.     

Multi-walled carbon nanotubes on amorphous carbon films

Nanotubular structures composed of layered graphite sheets or other layered materials have been studied intensely by both scanning and transmission electron microscopy. In this paper, we will show how graphite structures, that are inherent to the production process of the amorphous carbon support films, used for both SEM and TEM studies can be easily mistaken for the actual sample structures. We will further report that these artifacts appear in both commercial as well as homemade holey carbon support films on copper grids, and suggest that to successfully study the “real” nanotubular structures only support films made from materials other than carbon should be used.     

Electronic and vibrational excitations in carbon nanotubes

We investigated the electronic and vibrational properties of single wall carbon nanotubes by reflection electron energy loss and X-ray absorption spectroscopies. We report on single particle excitations measured at the C-1s edge and on collective excitations of the valence band region. The comparison between the two techniques allows us to locate empty electronic states of the carbonaceous sample. Loss spectra taken in the infrared region reveal two loss features at 90 and 170 meV assigned to the excitation of optical phonon modes.     

Transfer-matrix simulations of electronic transport in single-wall and multi-wall carbon nanotubes

We present simulations of electronic transport in single-wall and multi-wall carbon nanotubes, which are placed between two metallic contacts. We consider situations where the electrons first encounter a singe-wall nanotube (corresponding to either the inner or the outer shell of the (10, 10)@(15, 15)@(20, 20) and (10, 10)@(20, 10)@(20, 20) nanotubes), before encountering the multi-wall structures. The role of this two-step procedure is to enforce the electrons to enter a single shell of the multi-wall nanotubes, and we study how from that point they get redistributed amongst the other tubes. Because of reflections at the metallic contacts, the conductance of finite armchair nanotubes is found to depend on the length of the tubes, with values that alternate between three separate functions. Regarding the transport in multi-wall nanotubes, it is found that the electrons keep essentially propagating in the shell in which they are initially injected, with transfers to the other tubes hardly exceeding one percent of the whole current. In the case where the three tubes are conducting, these transfers are already completed after four nanometers. The conductance and repartition of the current present then oscillations, which are traced to the band structure of the nanotube. The transfers between the shells and the amplitude of these oscillations are significantly reduced when the intermediate tube is semiconducting.     

Nucleation stability of diamond nanowires inside carbon nanotubes: A thermodynamic approach

Aiming at synthesis diamond nanowires, a simple thermodynamic approach was performed with respect to the effect of nanosize-induced additional pressure on the Gibbs free energy of critical nuclei to elucidate diamond nucleation inside carbon nanotubes upon chemical vapor deposition, based on the carbon thermodynamic equilibrium phase diagram. Notably, these analysis showed that the diamond nucleation would be preferable inside a carbon nanotube due to the effect of surface tension induced by the nanosize curvature of the carbon nanotube and diamond critical nuclei, compared with diamond nucleation on the flat surface of a silicon substrate. Meanwhile, the metastable phase region of diamond nucleation would be driven into a new stable phase region in the carbon thermodynamic equilibrium phase diagram by the effect of nanosize-induced additional pressure. Eventually, we predicted that carbon nanotubes would be an effective path to grow diamond nanowires by chemical vapor deposition.     

Carbon nanotubes field emission integrated triode amplifier array

Because the high frequency operation of a field emission triode amplifier is dictated by the cutoff frequency and not the electron transit time, a high ratio of transconductance, g_m to the overlapping interelectrode capacitance, C_g is the desired outcome. Consequently, to achieve high frequency performance of the CNT amplifier array in this study, C_g was reduced by performing a dual-mask photolithography process to minimize the overlapping gate area, and, the insulating layer's thickness was increased. Moreover, wedge-shaped CNT emitter arrays are employed to increase emission sites, resulting in return higher g_m. Both dc and ac performance of the amplifier were characterized. The triode amplifier array exhibited a high current of  0.32 mA (74 mA/cm²), g_m of  63 μS and voltage gain of  18 dB. Frequency response of the triode amplifier up to 20 kHz was also investigated. A theoretical cutoff frequency of > 70 MHz could be achieved with proper shielding of the test setup.     

Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method

Thin multiwalled carbon nanotube (t-MWCNTs)-based field emitters are made by use of a spray method. The number of tube walls is between 2 and 6, with the corresponding outer diameters between 3 and 6 nm. They were dispersed in dichloroethane and sprayed onto metal-deposited indium tin oxide glass. After heat treatment, they were found to be tightly adhered to metal electrode. Excellent field emission characteristics were exhibited, with a large field enhancement factor and low turn-on voltage, comparable to those of singlewalled CNTs. However, the t-MWCNTs demonstrated a significantly lower degradation rate than SWCNTs in the emission current. This high emission stability was attributed to their stable edge structures, similar to conventional large-diameter MWCNTs. Therefore, t-MWCNTs could be utilized as an alternative material for field emitters.     

Metal-semiconductor heterojunctions in T-shaped carbon nanotubes

We report a possible metal-semiconductor carbon nanotube heterojunction which can be done fusing perpendicularly a metallic nanotube to a semiconducting one. Octagonal defects help to make the 90° bend necessary for T-junctions. These molecules could be used as building blocks of nanoelectronic devices.     

Mechanical properties of carbon nanotubes: a fiber digest for beginners

A condensed review of mechanical properties of carbon nanotubes is given. Theory as well as experiments is examined with a view to extracting the fundamental elements that should allow the reader to build his own perspective of the subject.     

Field emission enhancement of amorphous carbon films by nitrogen-implantation

Effect of nitrogen-implantation on electron field emission properties of amorphous carbon films has been examined. Raman and X-ray photoelectron spectroscopy measurements reveal different types of C-N bonds formed upon nitrogen-implantation. The threshold field is lowered from 14 to 4 V/μm with increasing the dose of implantation from 0 to 5 × 10¹⁷ cm⁻² and the corresponding effective work function is estimated to be in the range of 0.01-0.1 eV. From the perspective of tetrahedron bond formation, a mechanism for the nitrogen-lowered work function is proposed, suggesting that both the nitrogen nonbonding (lone pair) and the lone-pair-induced carbon antiboding (dipole) states are responsible for lowering the work function and hence the threshold field.