C. M. Yeh

Liquid-crystal alignment on a-C:H films by nitrogen plasma beam scanning

A plasma beam scanning treatment has been developed to modify the surface of the hydrogenated amorphous carbon a-C:H film on the indium tin oxide glass. The plasma beam scanning treatment makes the a-C:H film an excellent layer for liquid-crystal alignment. The qualities of a-C:H films were characterized by using atomic force microscope, micro-Raman spectroscopy, and field-emission scanning electron microscope. The ultrathin a-C:H films were deposited at 50% CH4/H2+CH4 gas ratio, 100 W radio-frequency power, and a gas pressure of 10 mtorr for 15 min by capacitive-coupled plasma chemical-vapor deposition method. The twist nematic cells were filled with liquid crystal ZLI-2293 on the a-C:H film treated with different nitrogen plasma beam scanning time. The grooving mechanism is considered not responsible for the liquid-crystal LC alignment. Raman spectra suggest that a bond-breaking process of aromatic rings occurs in the aC:H film. The O1s ,C 1s, and N1s core-level spectra support that the nitrogen plasma beam scanning treatment induces a bond-breaking process of aromatic rings to create available carbon dangling bonds for the formation of C‐O bonds. The newly formed C‐O bonds are “directional,” which favor the LC alignment on the a-C:H film.

Field emission from a composite structure consisting of vertically aligned single-walled carbon nanotubes and carbon nanocones

Vertically aligned single-walled carbon nanotubes (VA-SWCNTs) have been fabricated on carbon nanocones (CNCs) in a gravity-assisted chemical vapour deposition (CVD) process. The CNCs with nanoscale Co particles at the top were first grown on the Co/Si(100) substrate biased at 350?V in a plasma enhanced chemical vapour deposition process. The CNCs typically are ~200?nm in height, and their diameters are ~100?nm near the bottom and ~10?nm at the top. The nanoscale Co particles ~10?nm in diameter act as catalysts which favour the growth of VA-SWCNTs out of CNCs at 850??C in the gravity-assisted CVD process. The average length and the growth time of VA-SWCNTs are ~150?nm and 1.5?min, equivalent to a growth rate of ~6??m?h?1. The diameters of VA-SWCNTs are estimated to be 1.2?2.1?nm. When VA-SWCNTs are fabricated on CNCs, the turn-on voltage is reduced from 3.9 to 0.7?V??m?1 and the emission current density at the electric field of 5?V??m?1 is enhanced by a factor of more than 200. The composite VA-SWCNT/CNC structure is potentially an excellent field emitter. The emission stability of the VA-SWCNT/CNC field emitter is discussed.

Stress relaxation in the GaN∕AlN multilayers grown on a mesh-patterned Si(111) substrate

300×300μm2 crack-free GaN∕AlN multilayers of 2μm in thickness have been successfully grown on the Si(111) substrate patterned with the SixNy mesh by metal-organic chemical-vapor deposition. The in-plane stress exhibits a U-shape distribution across the “window” region, supported by the Raman shift of the GaN E2(TO) mode. This indicates a stress relaxation abruptly occurring near the edge of the window region due to the freestanding surface (11¯01) or (112¯2). The in-plane stress is almost relaxed at the corner of the window region due to three freestanding surfaces (11¯01), (112¯2), and (101¯1). The maximum in-plane stress is located near the surface of the multilayers at the center of the window region, supported by the Raman measurements and the failure observations. The role of the SixNy mesh in the stress relaxation is discussed.

Field Emission from a Carbon Nanofiber/Carbon Nanocone Composite Structure Fabricated by a Two-Step Growth Process

A composite structure of carbon nanofiber (CNF) and carbon nanocone (CNC) has been successfully fabricated on the Co/Al/Si(100) substrates by using microwave plasma-enhanced chemical vapor deposition. A hydrogen pretreatment is required to form Co-Al nanoparticles on the substrate at 450°C, which is essential for the subsequent CNF/CNC growth. The CNF/CNC composite structure is fabricated by a two-step process in which bundles of CNFs are formed on the as-pretreated substrate at 450°C and a CNC structure is subsequently formed at 650°C by a coalescence mechanism. Raman spectra indicate that sp 2 bonds in C=C chains play a role in connecting CNFs in each bundle during the formation of CNCs. An upward growth of CNFs on the top of CNCs occurs during coalescing CNFs. The CNF/CNC composite structure exhibits very good field emission characteristics, better than CNFs. The optimum turn-on field for the CNF/CNC structure is 2.5 V/μm, which is defined as the electric field at a current density of 10 μA/cm 2 . The current density is 4.5 mA/cm 2 at 6 V/μm.

Band gap shift in the GaN∕AlN multilayers on the mesh-patterned Si(111)

The band gap shift in the 80×80μm2 crack-free GaN∕AlN multilayers on the mesh-patterned Si(111) has been characterized by cathodoluminescence (CL) and Raman techniques. The GaN band gap derived from CL spectra depends on the spatial point inside a mesh, which changes from 3.413eV (at center) to 3.418eV (at edge) and to 3.426eV (at corner). The band gap shift is attributed to the variation of tensile stress inside the mesh, confirmed by Raman mapping. The shift of GaN band gap per unit stress is determined to be 0.03eV∕GPa.

Field Emission from Hydrogenated Amorphous Carbon Nanotips Grown on Cu ∕ Ti ∕ Si ( 100 )

Hydrogenated amorphous carbon (a-C:H) nanotips have been successfully grown on Cu/Ti/Si(100) by microwave plasma-enhanced chemical vapor deposition. A Cu etching process occurs simultaneously during the growth of the a-C:H nanotips. Both the Cu etching and a-C:H nanotips growth rates continuously increase with time. In the beginning, the Cu etching rate is approximately 3 nm/min and the upward growth rate of the a-C:H nanotips is approximately 1.2 nm/min. At the end of the growth, the Cu etching rate reaches 9 nm/min and the upward growth rate of the a-C:H nanotips reaches 5 nm/min. An etching-growth mechanism has been proposed to explain the formation of a-C:H nanotips on Cu/Ti/Si( 100). The structure of the a-C:H nanotips exhibits very good field-emission characteristics where a low turn-on field of 3.2 V/μm at 10 μA/cm 2 is achieved.

Effect of gravity on the growth of vertical single-walled carbon nanotubes in a chemical vapor deposition process

Vertically aligned single-walled carbon nanotubes (SWCNTs) can be fabricated on the Co∕Si(100) substrate in a gravity assisted chemical vapor deposition process. The Co∕Si(100) substrate is tilted such that its surface normal points downwards in the chemical vapor deposition process. All the SWCNTs are lined up with the direction of gravity. The length of SWCNTs increases with growth time. A modified vapor-liquid-solid mechanism is proposed to explain the role of gravity in the precipitation of SWCNTs and in the lineup behavior of SWCNTs.

Effects of time on the quality of vertically oriented single-walled carbon nanotubes by gravity-assisted chemical vapour deposition

The qualities of freestanding single-walled carbon nanotubes (SWCNTs) grown on Co/Si(100) in the gravity-assisted chemical vapour deposition (CVD) process have been investigated. Vertically oriented SWCNTs of high quality appear at a growth time of less than 3 min, verified by the clear radial breathing mode (RBM) in the Raman spectra and by the high resolution images taken by a transmission electron microscope (TEM). At a growth time of 3 min, vertical and looped SWCNTs co-appear on the substrate. A SWCNT longer than ~1 µm tends to bend into a semicircular loop. At a longer growth time such as 20 min, the coating of amorphous carbon (a-C) on the SWCNT becomes dominant, which is attributed to catalytic poisoning. A relatively long SWCNT (~10–40 µm) surrounded with a thick a-C layer is the final structure. The bending stiffness of the a-C tube is estimated ~15 times larger than that of the SWCNT, which helps to keep the a-C/SWCNT composite structure in a nearly vertical shape. A mechanism has been proposed to explain the coating of a-C on SWCNTs.

Gravity-assisted chemical vapor deposition of vertically aligned single-walled carbon nanotubes

Temperature and CH 4 /H 2 ratio of gas-flow rates are the two factors that strongly affect the qualities of vertically aligned single-walled carbon nanotubes (SWCNTs) in gravity-assisted chemical vapor deposition (CVD). The qualities of SWCNTs and other carbon products grown by gravity-assisted CVD were characterized by scanning electron microscopy and Raman spectroscopy. At temperatures between 850 and 900°C, SWCNTs of very good quality stand alone on the substrate. At other temperatures, nanofibers or irregular islands of carbon are present on the substrate. The CH 4 /H 2 ratio influences the quality of SWCNTs more abruptly than temperature. At low ratio, no carbon nanotube is formed. The window of CH 4 /H 2 ratio for the growth of vertically aligned SWCNTs ranges from 160:80 to 160:40. At a ratio higher than 160:40, multiwalled CNTs replace SWCNTs and become the dominant product.