Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm² and 7.0 mA/cm², respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.     

Effective electron emitters by molybdenum oxide-coated carbon nanotubes core–shell nanostructures

In this article, we showed that simple metal oxide coatings such as MoO₃ can be an effective enhancer for carbon nanotubes (CNTs) in field emission (FE) performance. For comparison, the FE properties of the pristine vertically aligned multi-walled CNTs with the metal oxide-coated CNTs were investigated. The metal oxide coating of the pristine CNTs was carried out by metal–organic chemical vapor deposition (MOCVD) method at 400 °C using Mo(CO)₆ as the precursor. The core–shell structure of the nanocomposite was studied by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) results showed that the surface of the coating material was mainly MoO₃. FE test indicated that the MoO₃-coated CNTs film exhibited an enhanced performance than the pristine CNTs with a turn-on field of 1.33 V μm⁻¹ and a field enhancement factor β estimated to be ~7000. Ultraviolet photoelectron spectroscopy (UPS) results confirmed a lower electron emission barrier height for MoO₃-coated CNTs than for the pristine CNTs. The mechanism of the enhanced FE performance is discussed based on Schottky barrier effect.     

Electron field emission properties of carbon nanoflakes prepared by RF sputtering

Carbon nanoflakes (CNFs) with corrugated geometry were synthesized using RF sputtering process with Ar/CH₄ gas mixture. Transmission electron microscopic examination reveals that the introduction of H₂ in sputtering chamber leads to the preferential etching of amorphous carbons, while maintaining integrity for the nano-crystalline phases. The proportion of nano-sized crystalline clusters is thus increased, which improved the electron field emission (EFE) properties of the materials, viz. with turn-on field of E ₀ = 6.22 V/μm and FEE current density of J _e = 90.1 μA/cm² at 11.0 V/μm. The cathodes made of screen printing of CNFs-Ag paste exhibit even better EFE properties than the as-deposited CNFs. The EFE of the CNFs cathodes can be turned on at E ₀ = 5.71 V/μm, achieving J ₀ = 340.1 μA/cm² at 11.0 V/μm applied field. These results showed that the CNFs are inheritantly more robust in device fabrication process than the other carbon materials and thus possess better potential for electron field emitter applications.     

Magnetic field enhances the graphitized structure and field emission effect of carbon nanotubes

This study uses a low temperature thermal chemical vapor deposition with an applied external magnetic field to grow carbon nanotubes (CNTs) on Ni/Ag-printed glass substrates. A mixture of C₂H₂ and H₂ gas was used for the growth of the CNTs. A Ni catalyst layer was deposited on the Ag-printed glass substrate by pulse electroplating. Scanning electron micrographs as well as the presence of two sharp peaks at 1320 cm⁻¹ (D band) and 1590 cm⁻¹ (G band) in the Raman spectra indicate that the graphitized structure of CNTs synthesized under a magnetic field has higher quality (i.e., a D-band to G-band intensity ratio of 0.303) than CNTs synthesized without a magnetic field. Transmission electron micrographs show a fine Ni catalyst at the tip of the tube for CNTs synthesized under a magnetic field, exhibiting a CNT “tip-growth” model. The synthesis of CNTs in the presence of a magnetic field also generates better field emission properties and better lighting morphology than without a magnetic field.     

Enhancing electron field emission of carbon nanoflakes by hydrogen post-annealing process

In this study, the carbon nanoflakes (CNFs) fabricated by sputtering were chosen as the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The as-deposited CNFs were annealed in the furnace under hydrogen atmosphere. The results showed that the optimum field emission properties with smaller turn-on field and larger current density were obtained at annealing temperature of 600 °C for 10 min. The hydrogen thermal annealing has chemical etching on the surface of the CNFs and produces appropriate emission site density to increase the emission current density. The turn-on field was reduced from 6.7 to 5.8 V/μm and electric current density was increased from 22 to 187 μA/cm² under 8 V/μm after hydrogen thermal annealing.     

Synthesis of orthorhombic and cubic PbF<Subscript>2</Subscript> by hydrothermal method

Orthorhombic (α-) and cubic (β-) PbF₂ have been successfully synthesized via a simple hydrothermal process at 200 °C for 8 h using Pb(C₂H₃O)₂ and NH₄F as the raw reaction materials. The crystal structure, morphology, and optical properties of the as-synthesized samples were characterized by X-ray powder diffraction, scanning electron microscopy, and photoluminescence spectroscopy. XRD and SEM results show that the uniform β-PbF₂ microspheres and porous-microspheres with the average diameter about 3 and 5 μm, respectively, were synthesized assisting with citric acid and α-PbF₂ microrods and microtubes with a diameter about 15 and 10 μm, respectively, were synthesized assisting with acetic acid. The reaction conditions influencing the synthesis of these PbF₂ microstructures, such as reaction time, the amount of CTAB, and the molar ratio of F/Pb were investigated. On the basis of a series of observations, phenomenological elucidation of a mechanism for the growth of the β-PbF₂ microspheres and multispheres has been presented. Room-temperature photoluminescence measurements indicated that the as-prepared α-PbF₂ microrods exhibit a strong green emission.     

Characterization and enhanced field emission properties of carbon nanotube bundle arrays coated with N-doped nanocrystalline anatase TiO2

Anatase TiO₂ nanocrystals (NCs) were deposited onto patterned carbon nanotube (CNT) bundle arrays to form a TiO₂/CNT composite using metal organic chemical vapor deposition (MOCVD) using titanium-tetraisopropoxide (Ti(OC₃H₇)₄) as a source reagent. The N-doped TiO₂/CNT composite was then fabricated using nitrogen plasma treatment. The structural and spectroscopic properties of TiO₂/CNT composites were characterized by field-emission scanning electron microscopy, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. The combined geometrical structure and low electron affinity effects of N-doped TiO₂ led to a low turn-on field of 1.0 V μm⁻¹ at a current density of 10 μA cm⁻², a low threshold field of 1.9 V μm⁻¹ at a current density of 1 mA cm⁻², a high field enhancement factor of 3.0 × 10³, and long-term stability for the N-doped TiO₂/CNT composite. The results revealed that the N-doped TiO₂/CNT composite can be a potential candidate for field emission devices.     

Field emission characteristics of carbon nanofibers grown on copper micro-tips at low temperature

Carbon nanofibers (CNFs) were grown on copper micro-tips formed by electroplating. The nickel layer electroplated over the copper micro-tips was used as a catalyst. The CNFs were synthesized by using plasma-enhanced chemical vapor deposition (PECVD) of C₂H₂ and NH₃ at 480 °C. The copper micro-tips were formed by high current pulse electroplating, which played a significant role in characterizing our CNFs. The CNFs grown on the copper micro-tips showed outstanding field emission performance and stability, whose turn-on field, defined as one at the current density of 10 μA/cm², was 1.30 V/μm and the maximum current density reached 5.39 mA/cm² at an electric field of 4.9 V/μm.     

Plasma treatment effects on surface morphology and field emission characteristics of carbon nanotubes

In this study, electron field-emission properties of carbon nanotube films (CNTs) grown on silicon substrate before and after tetrafluoromethane (CF₄), hydrogen (H₂) and argon (Ar) plasma etchings were investigated. The CNTs were synthesized by thermal decomposition of methane in the presence of nickel catalyst. Our research results reveal that plasma treatment can modify the surface morphology and enhance the field-emission characteristics of CNTs regardless of the plasma used. The CNTs treated by both non-reactive and reactive plasmas have a higher density of defect and a smaller average diameter reflecting the etching effects of plasma treatments. In addition, higher emission currents and lower turn-on electric fields are also obtained for the CNTs after plasma treatment. As expected, reactive plasma treatment has a more pronounced effect on the surface morphology and field-emission characteristics of the synthesized CNTs than non-reactive plasma treatment. In particular, a huge increase in emission current (more than three orders of magnitude at high electric fields) and a substantial lower turn-on electric field are found for the CNTs after H₂ plasma treatment. This huge increase in the emitted current is primarily caused by the increase in the density of field-emission sites resulting from the change of surface morphology and the –CH _x nanoparticles redeposited on the CNTs.     

Aligned graphene nanoribbons and crossbars from unzipped carbon nanotubes

Aligned graphene nanoribbon (GNR) arrays have been made by unzipping of aligned single-walled and few-walled carbon nanotube (CNT) arrays. Nanotube unzipping was achieved by a polymer-protected Ar plasma etching method, and the resulting nanoribbon array can be transferred onto any chosen substrate. Atomic force microscope (AFM) imaging and Raman mapping on the same CNTs before and after unzipping confirmed that ˜80% of CNTs were opened up to form single layer sub-10 nm GNRs. Electrical devices made from the GNRs (after annealing in H₂ at high temperature) showed on/off current (I _on/I _off) ratios up to 10³ at room temperature, suggesting the semiconducting nature of the narrow GNRs. Novel GNR-GNR and GNR-CNT crossbars were fabricated by transferring GNR arrays across GNR and CNT arrays, respectively. The production of such ordered graphene nanoribbon architectures may allow for large scale integration of GNRs into nanoelectronics or optoelectronics.     

Characterization of IrO2/CNT nanocomposites

IrO₂ nanocrystals (NCs) were grown on vertically aligned carbon nanotube (CNT) templates, forming IrO₂/CNT nanocomposites, by metal organic chemical vapour deposition using (C₆H₇)(C₈H₁₂)Ir as a source reagent. The surface morphology, structural and spectroscopic properties of the nanocomposites were characterized using field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman scattering. IrO₂ varied from particle- to tube-like NCs as the deposition time increased from 5 to 60 min. The particle-like IrO₂ NCs may be used as a protective layer on CNTs, providing stable and uniform field emission application. The tube-like structure may increase the surface-to-volume ratio which makes the IrO₂/CNT nanocomposites as an attractive candidate for the supercapacitor application.     

Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Growth and microstructures of carbon nanotube films prepared by microwave plasma enhanced chemical vapor deposition process

Well-aligned carbon nanotubes (CNTs) were grown on iron coated silicon substrates by microwave plasma enhanced chemical vapor deposition. Effect of plasma composition on the growth and microstructures of CNTs were investigated by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and optical emission spectroscopy. Morphology and microstructure of nanotubes were found to be strongly dependent on the plasma composition. Aligned bamboo-shaped nanotubes consisting of regular cone shaped compartments were observed for C₂H₂/NH₃/N₂ and C₂H₂/NH₃/H₂ gas mixtures. Randomly oriented or no nanotubes growth were observed in C₂H₂/H₂ and C₂H₂/N₂ gas mixtures respectively. CNTs grown in nitrogen rich plasma had more frequent short compartments while compartment length increased with decreasing nitrogen concentration in the plasma. Raman spectroscopy of CNTs samples revealed that CNTs prepared in nitrogen rich plasma had higher degree of disorder than those in low nitrogen or nitrogen free plasma. In-situ optical emission spectroscopy investigations showed that CN and H radicals play very important role in both the growth and microstructure of CNTs. Microstructure of CNTs has been correlated as a function of CN radical concentration in the plasma. It is suggested that presence of nitrogen in the plasma enhances the bulk diffusion of carbon through the iron catalyst particles which causes compartment formation. Based on our experimental observations, growth model of nanotubes under different plasma composition has been suggested using base growth mechanism.     

Dispersion assessment and studies on AC percolative conductivity in polymer-derived Si–C–N/CNT ceramic nanocomposites

Nanocomposites comprise polysilazane-derived SiCN ceramic charged with carbon nanotubes (CNTs) have been prepared by dispersion of multi-walled CNTs with a diameter of 80 nm in a cross-linked polysilazane (HTT 1800, Clariant) using a simple roll-mixer method. Subsequently, the composites were warm pressed and pyrolyzed in argon atmosphere. Scanning electron microscopy (SEM) and 3D Raman imaging techniques were used as major tools to assess the dispersion of CNTs throughout the ceramic matrix. Furthermore, studies on the effect of the volume fraction of CNTs in the nanocomposites on their electrical properties have been performed. The specific bulk conductivities of the materials were analyzed by AC impedance spectroscopy, revealing percolation thresholds (ρ_c) at CNT loadings lower than 1 vol%. Maximum conductivity amounted to 7.6 × 10⁻² S/cm was observed at 5 vol% CNT. The conductivity exponent in the SiCN/CNT composites was found equal to 1.71, indicating transport in three dimensions.     

Influence of Fe nanoparticles diameters on the structure and electron emission studies of carbon nanotubes and multilayer graphene

In this paper we report the effect of Fe film thickness on the growth, structure and electron emission characteristics of carbon nanotubes (CNTs) and multilayer graphene deposited on Si substrate. It is observed that the number of graphitic shells in carbon nanostructures (CNs) varies with the thickness of the catalyst depending on the average size of nanoparticles. Further, the Fe nanoparticles do not catalyze beyond a particular size of nanoclusters leading to the formation of multilayer graphene structure, instead of carbon nanotubes (CNTs). It is observed that the crystallinity of CNs enhances upon increasing the catalyst thickness. Multilayer graphene structures show improved crystallinity in comparison to CNTs as graphitic to defect mode intensity ratio (I_D/I_G) decreases from 1.2 to 0.8. However, I_2D/I_G value for multilayer graphene is found to be 1.1 confirming the presence of at least 10 layers of graphene in these samples. CNTs with smaller diameter show better electron emission properties with enhancement factor (γ_C = 2.8 × 10³) in comparison to multilayer graphene structure (γ_C = 1.5 × 10³). The better emission characteristics in CNTs are explained due to combination of electrons from edges as well as centers in comparison to the multilayer graphene.     

～2 μm Tm3+/Yb3+-doped tellurite fibre laser

We present, for the first time, laser emission in the range 1.910–1.994 μm in Tm³⁺/Yb³⁺-doped tellurite fibre when pumped using an Yb³⁺-doped double-clad silica fibre laser operating at 1.088 μm. With this pump scheme there was strong pump excited state absorption (ESA) which caused upconversion emission at 800 nm and 480 nm due to the Tm³⁺: ³H₄ → ³H₆ and Tm³⁺: ¹G₄ → ³H₆ transitions, respectively. This strong ESA limited the maximum slope efficiency to 10% with respect to absorbed pump power, and the maximum output power to 67 mW. This is however the highest output power which has been achieved in a tellurite fibre laser so far. The lowest observed threshold was 114 mW for a 22 cm long fibre and a 90% reflective output coupler. Further power scaling was limited due to thermal damage at the pump end of the fibre. The optimum fibre length for this arrangement was around 16 cm but lasing was achieved in lengths ranging from <9 to 30 cm. Tellurite glass offers significant advantages over silicate and fluoride glasses which make it a very promising material for compact, medium power, near and mid-IR fibre lasers for range-finding, medical and atmospheric monitoring and sensing applications.     

Experimental Reconstruction of Thermal Parameters in CNT Array Multilayer Structure

The thermal conductivities, thermal diffusivity, thermal anisotropy ratio, and thermal boundary resistance for the multilayered microstructure of a carbon nanotube (CNT) array are reconstructed experimentally using the 3ω method with two different width metal heaters. The thermal impedance in the frequency domain and sensitivity coefficients are introduced to simultaneously determine the multiple thermal parameters. The thermal conductivity at 295 K is 38 W · m⁻¹ · K⁻¹ along the nanotube growth direction, and two orders of magnitude lower in the direction perpendicular to the tubes with the anisotropy ratio as large as 86. Separation of the contact and CNT array resistances is realized through circuit modeling. The measured thermal boundary resistances of the CNT array/Si substrate and insulating diamond film interfaces are 3.1 m² · K · MW⁻¹ and 18.4 m² · K · MW⁻¹, respectively. The measured thermal boundary resistance between the heater and diamond film is 0.085 m² · K · MW⁻¹ using a reference sample without a CNT array. The thermal conductivity for a CNT array already exceeds those of phase-changing thermal interface materials used in microelectronics.     

Growth of carbon nanotubes using a thermal insulator investigated by in-situ mass spectroscopy

The low-temperature synthesis (500-560 °C) of carbon nanotubes (CNTs) on a triple-layered catalyst, Al/Fe/Mo was performed using complex hydrocarbon radicals which were produced from pyrolysis of C₂H₂. These radicals were produced using a high-temperature heater (~ 830 °C), but the substrate where the CNTs were grown was placed on a thermal insulator. This then enabled the substrate to be at a much lower temperature (500-560 °C). A simulated temperature distribution inside the chamber was also used to describe this low-temperature configuration. The synthesis of CNTs relies on the thermal dissociation and dissociative recombination of C₂H₂ for the formation of complex high-order radicals (i.e. C₆H₉, C₅H₉, and C₆H₁₃), and their presence was confirmed by in-situ mass spectroscopy. To explain this, a simple gas-phase radical chain process and a growth model are presented.     

Sol–gel preparation and luminescence of RE<Subscript>3</Subscript>BO<Subscript>6</Subscript>: Dy<Superscript>3+</Superscript> (RE = La,Y, Gd) microparticles with hybrid precursors

Dysprosium ion activated RE₃BO₆ (RE = La, Y, Gd) polycrystalline phosphors have been prepared via a modified sol–gel process with urea as fuel and rare earth nitrates as precursors. The crystal phase and the morphology of the phosphors are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The average particle size after heat treatment at 1,000 °C is around 5 μm by particle size determination technique. The characteristic transitions of Dy³⁺ due to ⁴F_9/2 → ⁶H_15/2 (blue) and ⁴F_9/2. → ⁶H_13/2 (yellow) can be observed in the emission spectra and the yellow-to-blue intensity ratio (Y/B) can be changed with the variety of the doped concentration of Dy³⁺ ion. Zeta potential shows the relationship between the surface charges to the pH value in suspension.     

Field emission characteristics of CNTs synthesized by bias-assisted ICPHFCVD and ICPCVD techniques

Carbon nanotubes (CNTs) were vertically well-grown on Ni/Cr-deposited glass substrates at 580 °C by ICPCVD and bias-assisted ICPHFCVD techniques. The vertically well-aligned CNTs showed multi-walled type with hollow structure. The measured critical current density on CNTs grown by the ICPCVD technique was 1.0×10^–6 A cm^–2 at 5 V m^–1 of turn-on field and 7.7×10^–5 A cm^–2 at 7.8 V m^–1 of the critical field. On the other hand, the critical current density on CNTs grown by the bias-assisted ICPHFCVD technique was 3.7×10^–7 A cm^–2 at 3 V m of turn-on field and 3.3×10^–4 A cm^–2 at 6.8 V m^–1 of the critical field, respectively. On comparing the two processes, it can be concluded that CNTs grown by bias-assisted ICPHFCVD are more suitable than those grown by ICPCVD for the possible application of field emission displays (FEDs).