Umesh Palnitkar

Self-Assembled Growth, Microstructure, and Field-Emission High-Performance of Ultrathin Diamond Nanorods

We report the growth of ultrathin diamond nanorods (DNRs) by a microwave plasma assisted chemical vapor deposition method using a mixture gas of nitrogen and methane. DNRs have a diameter as thin as 2.1 nm, which is not only smaller than reported one-dimensional diamond nanostructures (4-300 nm) but also smaller than the theoretical value for energetically stable DNRs. The ultrathin DNR is encapsulated in tapered carbon nanotubes (CNTs) with an orientation relation of (111)diamond//(0002)graphite. Together with diamond nanoclusters and multilayer graphene nanowires/nano-onions, DNRs are self-assembled into isolated electron-emitting spherules and exhibit a low-threshold, high current-density (flat panel display threshold: 10 mA/cm2 at 2.9 V/microm) field emission performance, better than that of all other conventional (Mo and Si tips, etc.) and popular nanostructural (ZnO nanostructure and nanodiamond, etc.) field emitters except for oriented CNTs. The forming mechanism of DNRs is suggested based on a heterogeneous self-catalytic vapor-solid process. This novel DNRs-based integrated nanostructure has not only a theoretical significance but also has a potential for use as low-power cold cathodes.

Field emission effects of nitrogenated carbon nanotubes on chlorination and oxidation

With reference to our recent reports [Appl. Phys. Lett. 90, 192107 (2007); Appl. Phys. Lett. 91, 202102 (2007)] about the electronic structure of chlorine treated and oxygen-plasma treated nitrogenatcd carbon nanotubes (N-CNTs), here we studied the electron field emission effects on chlorination (N-CNT:Cl) and oxidation (N-CNT:O) of N-CNT. A high current density (J) of 15.0 mA/cm(2) has been achieved on chlorination, whereas low J of 0.0052 mA/cm(2) is observed on oxidation compared to J=1.3 mA/cm(2) for untreated N-CNT at an applied electric field E-A of similar to 1.9 V/mu m. The turn-on electric field (E-TO) was similar to 0.875. The 1.25 V/mu m was achieved for N-CNT:C1 and N-CNT:O, respectively, with respect to E-TO= 1.0 V/mu n for untreated one. These findings are due to the formation of different bonds with carbon and nitrogen in the N-CNT during the process of chlorine (oxygen)-plasma treatment by the charge transfer, or else that changes the density of free charge carriers and hence enhances (reduces) the field emission properties of N-CNTs:C1 (N-CNTs:O)

Enhancement in electron field emission in ultrananocrystalline and microcrystalline diamond films upon 100 MeV silver ion irradiation

Enhanced electron field emission (EFE) behavior was observed in ultrananocrystalline diamond (UNCD) and microcrystalline diamond (MCD) films upon irradiation with 100 MeV Ag9+-ions in a fluence of 5×1011 ions/cm2. Transmission electron microscopy indicated that while the overall crystallinity of these films remained essentially unaffected, the local microstructure of the materials was tremendously altered due to heavy ion irradiation, which implied that the melting and recrystallization process have occurred along the trajectory of the heavy ions. Such a process induced the formation of interconnected nanocluster networks, facilitating the electron conduction and enhancing the EFE properties for the materials. The enhancement in the EFE is more prominent for MCD films than that for UNCD films, reaching a low turn-on field of E0=3.2 V/μm and large EFE current density of Je=3.04 mA/cm2 for 5×1011 ions/cm2 heavy ion irradiated samples.

Electron Field Emission of Silicon-Doped Diamond-Like Carbon Thin Films

In this work we demonstrate that the field emission characteristics of disordered Si-doped diamond-like carbon (DLC) thin films depend not only on properties of the conductive clustered sp2 phase and the insulating sp3 matrix (or sp2/sp3 ratio) but also on the presence of Si–Hn and C–Hn species in the film. The presence of such species reduces the hardness of the film and simultaneously enhances the field emission performance. A turn on electric field (ETOF) of 6.76 V/µm produced a field emission current density of ~0.2 mA/cm2, when an electric field of ~20 V/µm was applied. The Fowler–Nordheim (FN) tunneling model is appropriate to explain the field emission mechanism only within limited range of the current density. However, it is found that there is an apparent crossover between space charge limited current (SCLC) and the Frenkel effect due to impurities incorporated during the fabrication of Si-DLC films. This combined effect (SCLC + Frenkel) allows for the emission of electrons from the top of the reduced barriers due to the formation of comparatively soft DLC:Si films. The emission also occurs through tunneling from one conductive cluster (sp2 C=C) to another separated by an insulating matrix (sp3 C–C) after reducing the effective depth of a trap on application of high electric field.

Field emission enhancement in ultrananocrystalline diamond films by in situ heating during single or multienergy ion implantation processes

The single or multienergy nitrogen (N) ion implantation (MENII) processes with a dose (4×1014 ions/cm2) just below the critical dose (1×1015 ions/cm2) for the structural transformation of ultrananocrystalline diamond (UNCD) films were observed to significantly improve the electron field emission (EFE) properties. The single energy N ion implantation at 300 °C has shown better field emission properties with turn-on field (E0) of 7.1 V/μm, as compared to room temperature implanted sample at similar conditions (E0=8.0 V/μm) or the pristine UNCD film (E0=13.9 V/μm). On the other hand, the MENII with a specific sequence of implantation pronouncedly showed different effect on altering the EFE properties for UNCD films, and the implantation at 300 °C further enhanced the EFE behavior. The best EFE characteristics achieved for the UNCD film treated with the implantation process are E0=4.5 V/μm and current density of (Je)=2.0 mA/cm2 (at 24.5 V/μm). The prime factors for improving the EFE properties are presumed to...

Interesting trends in direct current electrical conductivity of chemical vapor deposited diamond sheets

Self-supported diamond sheets of the thickness ranging from 15 to 30 μm were prepared using hot filament chemical vapor deposition technique. The controlled variation of the deposition parameters resulted in the sheets with varying amount of nondiamond impurities. Routine characterization of the sheets was carried out using scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, Fourier transform infrared spectroscopy, and Positron annihilation spectroscopy techniques. Detailed measurements of room temperature electrical conductivity (σ300), current–voltage (I–V) characteristics, and annealing studies on the sheets deposited with various structural disorder have yielded useful information about the electrical conduction in this interesting material. σ300 and I–V characteristic measurements were done in sandwiched configuration taking care off the surface effects. The diamond sheets deposited at low deposition pressure (Pd<60 Torr) contain negligible nondiamond impurities and show σ300≅10−6...