Ikerionwu A. Akwani

Effects of O2, Ar, and H2 gases on the field-emission properties of single-walled and multiwalled carbon nanotubes

We compare the effects of O2 , Ar, and H2 gases on the field-emission ~FE! properties of single-walled carbon nanotubes ~SWNTs! and multiwalled carbon nanotubes ~MWNTs!. We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O 2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after operation in UHV. The higher operating voltages in an O 2 environment cause a permanent decrease of the FE current and an increase in the turn-on field of MWNTs. The ratios of the slopes before and after O 2 exposure are approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively.

Optical loss mechanisms in GeSiON planar waveguides

A double frequency overtone of the fundamental N–H bond related vibration was found to dominate the optical loss in the 1.5–1.6 μm range in high index GeSiON-based planar waveguides deposited by plasma-enhanced chemical vapor deposition using hydrogen-based precursors. The fundamental N–H bond-related absorption and its double frequency overtone were measured to be ∼560 cm−1 and 1.8 cm−1, respectively, resulting in their ratio of 310±10. The optical loss, extracted from analysis of the N–H-related absorption is consistent with the directly measured optical loss in the waveguide structures. It was shown that using the deuterated precursors results in incorporation of N–D bonds in the GeSiON films. The N–H bond absorption band was eliminated in the deuterated films, leading to low optical losses in the communication band.

Effects of O2, H2, and N2 gases on the field emission properties of diamond-coated microtips

We report the effects of O2, H2, and N2 residual gases on the field emission properties of uncoated and diamond-coated individual Mo microtips. The microtips are made using electrochemical etching techniques and positioned 5 μm from the anode using a scanning tunneling microscopy system. We observe that the field emission (FE) current and turn-on voltage of diamond-coated microtips are significantly less degraded by O2 exposure than those of uncoated Mo microtips. H2 exposure enhances the FE properties of both uncoated and diamond-coated microtips, while N2 exposure does not have any significant effect.

Atomic structure of the diamond (100) surface studied using scanning tunneling microscopy

Atomic resolution ultrahigh‐vacuum scanning tunneling microscopy studies of chemical‐vapor‐deposition‐grown epitaxial diamond (100) films are reported. After growth, the surface of the epitaxial films is amorphous at the atomic scale. After 2 min of exposure to atomic hydrogen at 30 Torr, the surface is observed to consist of amorphous regions and (2×1) dimer reconstructed regions. After 5 min of exposure to atomic hydrogen, the surface is observed to consist mostly of (2×1) dimer reconstructed regions. These observations are compared with a recent model for chemical‐vapor‐deposition diamond growth. Tunneling current versus voltage spectroscopy of undoped and boron‐doped epitaxial diamond (100) films is also reported.

Effect of SP 3 /( SP 2 + SP 3 ) Carbon Fraction on the Photoelectric Threshold and Electron Affinity of Diamond Films

We report a significant decrease in the photoelectric threshold of chemical vapor deposition grown diamond films as the fraction of sp 3 carbon to sp 2 plus sp 3 carbon in the films decreases. Raman spectroscopy and x-ray photoelectron spectroscopy are used to characterize the different forms of carbon in the films and the sp 3 /( sp 2 + sp 3 ) carbon fraction at the surface. We observe a decrease in the photoelectric threshold from 4.5 eV to 3.9 eV as the sp 3 /( sp 2 + sp 3 ) carbon fraction at the surface decreases from 71% to 55%. Ultraviolet photoelectron spectroscopy of the films shows that they have a negative electron affinity surface. Therefore, the work function of the films decreases from 4.5 eV to 3.9 eV. We propose that the decrease in photoelectric threshold is due to a decrease in the band gap of sp 2 - sp 3 carbon networks at the grain boundaries. The observed decrease in photoelectric threshold can be used to tailor the electronic properties of diamond films for specific applications.

Structural and Electronic Properties of Negative Electron Affinity Epitaxial Diamond (110) Films Studied Using Atomic Resolution UHV STM

We report ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) studies of the structural and electronic properties of epitaxial diamond (110) films. We observe that epitaxial diamond (110) films grow very rough due to striations. The striations are found to be due to the appearance of(111) faces and contain (100) steps. UHV STM atomic resolution images of the diamond (110) films show a (lxi) zigzag structure that measures 1.5 ± 0.1 A × 1.5 ± 0.1 A, in agreement with theoretical predictions for the hydrogen terminated diamond (110) surface. Ultraviolet spectroscopy shows that the epitaxial films have a photoelectric threshold of 5.3 ± 0.1 eV, providing evidence that the films have a negative electron affinity surface.

Variation of the Photoelectric Threshold of Boron Doped Diamond Films

Boron doped polycrystalline diamond films grown on p-type single-crystal Si substrates using chemical vapor deposition with a gas mixture of hydrogen, methane and diborane were characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy and photoelectric current measurements. The energy distributions are not sensitive to boron doping for diborane concentrations from 0 to 4.75 ppm, although the boron doping modifies the surface morphology and the photoemission intensity. The photoemission intensity is high where the microcrystalline content is highest (at diborane concentrations of 2.91 and 4.75 ppm. The photoelectric threshold is found to be at 4.38 eV, in agreement with earlier measurements. The present results are characteristic of valence band emission at 4.38, 4.63, 4.92, 5.12 and 5.30 eV for incident photons between 4.87 and 5.63 eV.