Effect of temperature on the electron field emission from aligned carbon nanofibers and multiwalled carbon nanotubes

Effect of temperature and aspect ratio on the field emission properties of vertically aligned carbon nanofiber and multiwalled carbon nanotube thin films were studied in detail. Carbon nanofibers and multiwalled carbon nanotube have been synthesized on Si substrates via direct current plasma enhanced chemical vapor deposition technique. Surface morphologies of the films have been studied by a scanning electron microscope, transmission electron microscope and an atomic force microscope. It is found that the threshold field and the emission current density are dependent on the ambient temperature as well as on the aspect ratio of the carbon nanostructure. The threshold field for carbon nanofibers was found to decrease from 5.1 to 2.6 V/μm when the temperature was raised from 300 to 650 K, whereas for MWCNTs it was found to decrease from 4.0 to 1.4 V/μm. This dependence was due to the change in work function of the nanofibers and nanotubes with temperature. The field enhancement factor, current density and the dependence of the effective work function with temperature and with aspect ratio were calculated and we have tried to explain the emission mechanism.     

Efficient field emission from coiled carbon nano/microfiber on copper substrate by dc-PECVD

Crystalline coiled carbon nano/micro fibers in thin film form have been synthesized via direct current plasma enhanced chemical vapor deposition (PECVD) on copper substrates with acetylene as a carbon precursor at 10 mbar pressure and 750 °C substrate temperature. The as-prepared samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD pattern as well as selected area electron diffraction (SAED) pattern showed that the samples were crystalline in nature. SEM and HRTEM studies showed that as synthesized coiled carbon fibers are having average diameter ∼100 nm and are several micrometers in length. The as-prepared samples showed moderately good electron field emission properties with a turn-on field as low as 1.96 V/μm for an inter-electrode distance 220 μm. The variation of field emission properties with inter-electrode distance has been studied in detail. The field emission properties of the coiled carbon fibrous thin films are compared with that of crystalline multiwalled carbon nanotubes and other carbon nanostructures.     

Optimization of cuprous oxide nanocrystals deposition on multiwalled carbon nanotubes

Cu (I) phenyl acetylide was used as a source of copper to achieve a homogeneous distribution of Cu₂O nanocrystals (10–80 nm) decorated on multiwalled carbon nanotubes (MWCNTs) having an average diameter of 10 nm. Pristine MWCNTs were first oxygen-functionalized by treating them with a mixture of concentrated (H₂SO₄/HNO₃ : 3/1) acids and the products were characterized by X-ray powder diffraction, transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and thermogravimetric analysis. An easy, efficient and one-step impregnation method was followed to produce copper-containing nanoparticles on the MWCNTs. The copper-treated MWCNTs dried at room temperature were seen to be well decorated by copper-containing nanoparticles on their outer surface. The MWCNTs were then heat-treated at 400 °C in a nitrogen atmosphere to produce a homogeneous distribution of cuprous oxide nanocrystals on their surface. By varying the ratio of copper to oxygen-functionalized MWCNTs, Cu₂O nanocrystals decorated on MWCNTs with different copper content can be obtained.     

Laser backwriting process on glass via ablation of metal targets

Ablation of metal targets onto pyrex glass substrates, using a Q-switched Nd:YAG laser working at 355 nm, was used to study the potential of a laser backwriting process for the fabrication of optical waveguides via an index of refraction change. Metal foils of stainless steel, aluminum, copper, brass and gold have been used as blanks and irradiated by focusing the laser beam through a cylindrical lens under continuous movement in a direction perpendicular to the irradiation. An horizontal setup was found suitable to improve the effect of the plume in the sample. Results were obtained for two different configurations. Transversal profiles were analysed using a contact profilometer, comparing results obtained for the different configurations, traverse speeds and metal targets used. Two ablation regimes were identified, which are related to a critical laser fluence value of 2.7 J/cm². Surface micrographs obtained by scanning electron microscopy are discussed, together with the characteristics of the structures attained, taking into account the optical and thermal properties of the ablated metal blanks.     

Temperature and substrate dependence of structure and growth mechanism of carbon nanofiber

The carbon nanofibers were grown on Ni/Si and Ni/Ti/Si substrates in 1 atm CH₄ atmosphere at 640 °C and 700 °C by thermal chemical vapor deposition method. The carbon nanofibers were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectrometry for morphology, microstructure, and crystallinity. The electron emission property of carbon nanofibers was also investigated by current-voltage (I-V) measurement. The results showed that the solid amorphous carbon nanofibers could be grown on Ni/Si substrate at 640 °C through tip growth mechanism, the carbon nanotubes could be grown on Ni/Si substrate at 700 °C through tip growth mechanism, and the carbon nanotubes could be grown on Ni/Ti/Si substrate at 700 °C through root growth mechanism.     

Novel room temperature ionic liquids of hexaalkyl substituted guanidinium salts for dye-sensitized solar cells

Nanoparticles are attracting increasing interest because of their high potential for a great number of practical applications, such as optical and electronic devices, nanoscale storage, and delivery systems. Using Cu-phthalocyanine as precursor material, we have synthesized multi-shell graphitic carbon nanospheres without and with metal encapsulation, depending on the pyrolysis conditions. The encapsulated elemental copper nanocrystals achieved using that route were of the order of 50 nm in size. The particles were characterized in detail by high-resolution transmission electron microscopy (HRTEM) and by energy filtering microscopy (EFTEM). The concentric graphitic carbon shells of the as-grown particles were clearly discernable. After in situ high-temperature annealing, an increase in the degree of order was observed. Under high-voltage electron irradiation and heating, a melting point reduction of the enclosed nanosized copper of more than 200 K could be detected, as compared to the melting point 1083 °C of bulk copper. Time-resolved imaging revealed the displacement of the melting copper by migration through the carbon shells, leaving intact carbon cages with a central hole. At intermediate stages of this process the transformation into a hexagonal morphology of the copper nanocrystals was observed. PACS 61.46.+w; 61.48.+c; 68.37.Lp     

Formation of metallic NbSe2 nanotubes and nanofibers

We succeed in synthesizing NbSe₂ nanotubes along with nanofibers by chemical vapor transportation. They are stable crystalline systems and can be synthesized reproducibly in a nearly equilibrium reacting process. We have investigated these nanosize structures of NbSe₂ by transmission electron microscopy and electron diffraction. Both of the structures have a similar size of 100–200 nm in diameter. While nanotubes consist of rolled-up NbSe₂ layers, nanofibers are a pile of thin flat layers. We propose a mechanism of the formation of NbSe₂ nanotubes and nanofibers on the basis of deseleniditive transition from a NbSe₃ fiber-shaped crystal. We also measured electrical resistance of the nanofibers with conductive atomic force microscopy and demonstrated that the material show metallic behavior at room temperature.     

Controllable one-step synthesis of magnetite/carbon nanotubes composite and its electrochemical properties

Magnetite nanocrystals are deposited on carbon nanotubes by a reflux method in diethylene glycol. The morphological characterization proves that magnetite nanocrystals are decorated on the external surfaces of carbon nanotubes. The crystal size of magnetite nanocrystals can be readily tuned by adjusting the content of sodium acetate, but the content of sodium acetate has little effect on the amount of magnetite. The magnetite/carbon nanotubes composites exhibit an initial capacity as high as 840 mAh g⁻¹ and an excellent cycling performance for lithium storage. The reversible capacity, as high as 390 mAh g⁻¹, can be maintained after 75 charge/discharge cycles. The research has potential implications for the application of magnetite/carbon nanotubes composites as anode materials of lithium ion batteries.     

Opposed flow oxy-flame synthesis of carbon and oxide nanostructures on molybdenum probes

The formation of carbon and metal-oxide nanostructures on molybdenum probes inserted in a counter-flow oxy-fuel flame is studied experimentally. Flame position and probe diameter were varied to achieve a controlled growth of carbon and metal-oxide nanostructures at fuel and oxygen-rich flame zones. Mo probes of 1-mm diameter were introduced in the flame at various heights, starting from the upper hydrocarbon-rich zone on the fuel side of the flame to the oxygen-rich zone on the oxidizer side. High density layers of carbon nanocoils (CNCs) and filamentous structures containing ribbon shapes and straight nanofibers were formed in the upper hydrocarbon-rich flame zone. The formation of carbon micro-trees was observed on the fuel side closer to the flame front. The structures formed in the oxidizer part of the flame were composed of molybdenum-oxides. MoO₂ micron-sized channel structures were formed on the oxidizer side in the vicinity of the flame front. The micro-channels had rectangular and square-framed shapes; they were completely hollow, closed, and semi-open with a small circular cavity at their tips. The application of probes with diameters of 0.75 and 0.25 mm resulted in the formation of spectacular 3-D structures with unique and distinct morphologies.     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

Ab initio modeling of the interaction of electron beams and single-walled carbon nanotubes

Single-walled carbon nanotubes are readily observable in a scanning electron microscope, which traditional models fail to explain. We present an ab initio model to explain how the electron beam can interact with these structures despite the very small, nanoscale, interaction volume. In particular, we show how the electron beam can generate very strong secondary electron emission from the tip of a nanotube under external electric field. The approach may also be used in modeling the interaction of charged particles with nanostructures in other applications such as electron detection.     

SEM Investigation of the electrical properties of silicon ribbons

The structure and electrical properties of silicon ribbons grown on a substrate by the Ribbon Growth on Substrate (RGS) method method for solar cell applications have been investigated in secondary electron and electron beam induced current modes of scanning electron microscopy. The growth method and growth conditions have provided the formation of the coarse-grained structure of silicon, in which the majority of grains are separated by twin boundaries and the dislocation density does not exceed 10⁶ cm⁻². According to the electron beam induced current investigations, the recombination contrast from twin boundaries is extremely low at 300 K, only a small amount of twin boundaries show an increase in the contrast upon cooling, and the contrast from dislocations is almost absent in the temperature range from 100 to 300 K.     

Preparation and structure of carbon encapsulated copper nanoparticles

Carbon-encapsulated copper nanoparticles were synthesized by a modified arc plasma method using methane as carbon source. The particles were characterized in detail by transmission electron microscope, high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray diffraction, thermogravimetric and differential scanning calorimetry. The encapsulated copper nanoparticles were about 30 nm in diameter with 3–5 nm graphitic carbon shells. The outside graphitic carbon layers effectively prevented unwanted oxidation of the copper inside. The effect of the ratio of He/CH₄ on the morphologies and the formation of the carbon shell were investigated.     

Synthesis and characterization of titanium nitride, niobium nitride, and tantalum nitride nanocrystals via the RAPET (reaction under autogenic pressure at elevated temperature) technique

TiN, NbN, and TaN nanocrystals have been selectively prepared through a simple, solvent-free, and convenient reaction under autogenic pressure at moderate temperature (RAPET) process at 350 °C for 12 h, reacting transition metal chlorides and sodium azide. The nanostructures obtained are characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). A reaction mechanism is suggested based on the experimental results. These rapid reactions produce nanocrystals of TiN, NbN, and TaN with average sizes of approximately 30, 28, and 27 nm, respectively (as calculated from X-ray line broadening). An octahedral inorganic fullerene was detected among the various structures of the TiN.     

Carbon nanofibers and multiwalled carbon nanotubes from camphor and their field electron emission

Vertically aligned carbon nanofibers (CNF) and multiwalled carbon nanotubes (MWCN) have been synthesized from camphor by catalytic thermal CVD method on Co and Co/Fe thin films (for CNF) and on silicon substrates using a mixture of camphor and ferrocene (for MWCN). CNF and MWCN are studied by field emission scanning electron microscopy, high-resolution transmission electron microscopy, visible Raman spectroscopy, X-ray diffraction in order to get insight into the microstructure and morphology of these materials. Field electron emission study indicates turn-on field of about 2.56, 3.0 and 6.5 V/μm for MWCN, Co/CNF and Co/Fe/CNF films, respectively. The best performance of MWCN in field electron emission among the materials studied can be due to the highest aspect ratio, good graphitization and good density.     

Modified laser ablation process for nanostructured thermoelectric nanomaterial fabrication

A modified pulsed laser deposition process was used to enhance the nanostructure generation inside Bi₂Te₃ nanocrystals. In this process, an additional femotosecond laser beam was used to add an energy shock on the ablated flume, which can result in rich nanostructures embedded inside Bi₂Te₃ nanocrystals. A large Si wafer was used to ‘freeze’ such nanostructures and to effectively collect such nanostructured nanocrystals for further processing. The generated nanocrystals were studied by X-ray diffraction and scanning transmission electron microscopy, and the results prove the existence of such embedded nanostructures. Such nanocrystals were also characterized electrically and thermally for the conductivity measurements.     

In situ synthesis of cobalt nanocrystal hierarchies in a transmission electron microscope

We report a versatile electron beam (e-beam) synthesis method for the local fabrication of ferromagnetic nanocrystals “on demand”. A localized irradiation in a transmission electron microscope (TEM) is used to convert a raw cobalt fluoride material into ferromagnetic metal by means of formation of a short-range ordered distribution of well-defined faceted three-dimensional (3D) cobalt nanocrystals on the carbon substrate. A range of sizes and morphologies can be obtained, depending on the size, intensity, and acceleration voltage of the e-beam and on the initial size/thickness of the 3D raw fluoride materials, with 300 kV acceleration voltage and thermionic LaB₆ emission found most favorable. The nanofabrication of locally quasi-monodispersed, small sized, and well-distributed 3D nanocrystals opens up the possibility to generate particle arrays on demand with desirable magnetic properties.     

Synthesis of starfish-like Cu2O nanocrystals through ��-irradiation and their application in lithium-ion batteries

In this study, single-crystalline starfish-like cuprous oxide (Cu₂O) nanocrystals with the backbones lengths in the range of 660 nm~16 ??m are successfully prepared through ??-irradiation, the cetyltrimethylammonium bromide (CTAB) is used as a capping material or soft colloidal templates. Without the addition of CTAB in the reaction system, irregular Cu₂O nanoclusters were obtained and their diameter is about 200 nm~1 ??m. Controlling the concentration ratio of CTAB to the copper ions, starfish-like morphology of Cu₂O can be obtained in high yield. Their structures are characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The possible growth mechanism of the starfish structure is discussed in the text. For potential application in lithium-ion batteries, an electrode made of the starfish-like Cu₂O shows excellent electrochemical cycling performance and high-rate capability. Compared with the Cu₂O nanoclusters, the starfish-like Cu₂O exhibits an improved electrochemical cycling stability. The capacity of the starfish-like Cu₂O can maintain 340 and 215 mAh g^?1 after 50 cycles at the rate of 0.1 C and 5 C, respectively. The reversible capacity holds 60% as the discharge?Ccharge rate even increases by 50 times.     

Atom beam sputtered Mo2C films as a diffusion barrier for copper metallization

The saddle field fast atom beam sputtered (ABS) 50 nm thick molybdenum carbide (Mo₂C) films as a diffusion barrier for copper metallization were investigated. To study the diffusion barrier properties of Mo₂C films, the as-deposited and annealed samples were characterized using four probes, X-ray diffraction, field enhanced scanning electron microscopy, energy dispersive X-ray analysis, atomic force microscopy and Rutherford back scattering techniques. The amorphous structure of the barrier films along with presence of carbon atoms at the molybdenum carbide-silicon interface is understood to reduce effective grain boundaries and responsible for increased thermal stability of Cu/Mo₂C/Si structure. The lowest resistivity of the as-deposited molybdenum carbide barrier films was ∼29 μΩ cm. The low carbon containing molybdenum carbide was found thermally stable up to 700 °C, therefore can potentially be used as a diffusion barrier for copper metallization.     

Formation of high aspect ratio polyamide-6 nanofibers via electrically induced double layer during electrospinning

In the present study, the formation of high aspect ratio nanofibers in polyamide-6 was investigated as a function of applied voltage ranging from 15 to 25 kV using electrospinning technique. All other experimental parameters were kept constant. The electrospun polyamide-6 nanofibers were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF). FE-SEM images of polyamide-6 nanofibers showed that the diameter of the electrospun fiber was decreased with increasing applied voltage. At the critical applied voltage, the polymer solution was completely ionized to form the dense high aspect ratio nanofibers in between the main nanofibers. The diameter of the polyamide-6 nanofibers was observed to be in the range of 75-110 nm, whereas the high aspect ratio structures consisted of regularly distributed very fine nanofibers with diameters of about 9-28 nm. Trends in fiber diameter and diameter distribution were discussed for the high aspect ratio nanofibers. TEM results revealed that the formation of double layers in polyamide-6 nanofibers and then split-up into ultrafine fibers. The electrically induced double layer in combination with the polyelectrolytic nature of solution is proposed as the suitable mechanisms for the formation of high aspect ratio nanofibers in polyamide-6.