J. J. Li

Dielectric properties of Ba0.6Sr0.4TiO3∕Mg2SiO4∕MgO composite ceramics

The dielectric properties of BSTO∕Mg2SiO4∕MgO composite ceramics have been investigated systematically. The dielectric properties of BSTO composites under a dc bias field can be interpreted by the “soft-mode” theory near the phase transition and Johnson’s phenomenological equation far above the transition temperature. Compared with most of the other BSTO composite ceramics, BSTO∕Mg2SiO4∕MgO not only can be sintered at a lower temperature but also keep a higher dielectric tunability versus a lower dielectric constant. For example, BSTO∕35wt.%Mg2SiO4∕15wt.%MgO has a dielectric tunability ∼13.18% (under 2kV∕mm biasing) versus a dielectric constant er(0)∼118.40 at 25°C. It suggests that the dielectric properties are influenced to a great extent by the microstructure, and the dielectric tunability is enhanced when more continuous BSTO phase is obtained.

Field electron emission from individual diamond cone formed by plasma etching

Field electron emission properties of individual diamond cone were investigated using a customized double-probe scanning electron microscope system. The diamond cone was formed by maskless ion sputtering process in bias-assisted hot filament chemical vapor deposition system. The as-formed sharp diamond cone coated with high-sp2-content amorphous carbon exhibited high emission current of about 80μA at an applied voltage of 100V. The field emission was stable and well in consistent with the conventional Fowler-Nordheim emission mechanism, due to a stabilization process in surface work function. It has demonstrated the possibility of using individual diamond cone as a point electron emission source, because of its high field electron emission ability and stable surface state after the process of work function stabilization.

Field emission from high aspect ratio tubular carbon cones grown on gold wire

Tubular carbon cones (TCCs) with nanometer-sized tips and micrometer-sized roots, having a herringbone hollow interior surrounded by helical sheets of graphite coiling around, were grown on Au wires by hot filament chemical vapor deposition (HFCVD). These TCCs exhibit excellent field emission properties with a very low threshold field of 0.27V∕μm and a corresponding current density of about 1μA∕cm2; and a stable emitting current density of 1.9mA∕cm2 can be obtained at only 0.6V∕μm. Their low effective work function of ∼0.0056eV and their conical bases—which effectively reduce the screening effect due to sufficient distance between adjacent tubular cones—are both favorable to field emission enhancement.

Chemical gases sensing properties of diamond nanocone arrays formed by plasma etching

A uniform diamond nanocone array was formed by plasma etching of diamond film in a hot filament chemical vapor deposition (HFCVD) system. A surface amorphous carbon coating layer, which is formed during CH4/H2 plasma-etching process, was removed by Ar plasma in a reactive ion etching system. The hydrogenation of diamond nanocones was performed in H2 ambience by using the same HFCVD system. The air-diluted NH3 and NO2 gases sensing properties of the diamond cone arrays had been studied by using electric current versus measurement time characteristics at room temperature. The repeatable chemical sensing properties of the hydrogenated diamond cone array sensor are enhanced, in comparison with as-formed diamond film. Surface two-dimensional hole gas structure and greatly increased surface-to-volume ratio both play a key role for the excellent detection performance. As-formed diamond nanocone arrays show a promising prospect for applications as chemical sensor for both reducing (NH3) and oxidizing (NO2) gases.

The field emission properties of high aspect ratio diamond nanocone arrays fabricated by focused ion beam milling

Abstract High aspect ratio diamond nanocone arrays are formed on freestanding diamond film by means of focused ion beam (FIB) milling technology and hot-filament chemical vapor deposition (HFCVD) method. The structure and phase purity of an individual diamond nanocone are characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The result indicatesthat the diamond cones with high aspect ratio and small tip apex radius can be obtained by optimizing the parameters ofFIB milling and diamond growth. The diamond nanocone arrays were also used to study the electron field emission propertiesand electric field shielding effect, finding high emission current density, low threshold and weak shielding effect, all attributable to the high field enhancement factor and suitable cone density of the diamond nanocone emitter.

Effect of grain size and pores on the dielectric constant of nanocrystalline diamond films

The nanocrystalline diamond films with different morphologies and roughness were synthesized by a bias-assisted hot filament chemical vapor deposition method. It was found that the nanocrystalline diamond film exhibited low-k dielectric properties with the increase of CH4 concentration during diamond deposition. The low-k nanocrystalline diamond film with grain size of around 40nm and dielectric constant of 2.4 was obtained at the CH4 concentration of 16% and the bias of −140V. The low dielectric constant can be mainly attributed to the decrease of diamond grain sizes and the formation of more nanopores in as-grown nanocrystalline diamond film, both of which were discussed in details based on the grain size determined band gap expansion effect and the two-phase dielectric mixing model, respectively.

Diamond cone arrays with controlled morphologies formed by self-organized selective ions sputtering

Controlled preparation of nanoscale materials and the underlying mechanisms are essential issues nowadays. Here, we report a significant subtractive formation process of large-area diamond conical nanostructure arrays using a hot filament chemical vapor deposition (HFCVD) system with negative biasing of the substrates, and the etching effect of energetic ions on the formation of diamond cone arrays with controlled morphology has been studied in detail. It shows that methylic ions dominantly contribute to diamond cone formation based on a neutral-ion charge exchange collision model. The self-organized selective sputtering process of as-formed hillock bottoms on a roughened surface by low energetic ions plays a key role for the formation and development of diamond cones. The cone morphologies under various experimental parameters are systematically studied, and they nicely confirm and supplement the as-established cone formation mechanism.

Enhanced photoluminescence from porous silicon by hydrogen-plasma etching

Porous silicon (PS) was etched by hydrogen plasma. On the surface a large number of silicon nanocone arrays and nanocrystallites were formed. It is found that the photoluminescence of the H-etched porous silicon is highly enhanced. Correspondingly, three emission centers including red, green, and blue emissions are shown to contribute to the enhanced photoluminescence of the H-etched PS, which originate from the recombination of trapped electrons with free holes due to SiO bonding at the surface of the silicon nanocrystallites, the quantum size confinement effect, and oxygen vacancy in the surface SiO2 layer, respectively. In particular, the increase of SiOx(x<2) formed on the surface of the H-etched porous silicon plays a very important role in enhancing the photoluminescence properties.

Space-charge-limited leakage current characteristics influenced by field-dependent permittivity in high dielectric constant and ferroelectric thin films

The current-voltage relationship dominated by space-charge-limited conduction for high dielectric constant and ferroelectric thin films, including (Ba,Sr)TiO3 and Pb(Zr,Ti)O3, is derived taking into account the electric field dependence of permittivity. Typical current-voltage characteristics are given by the derived relationship with empirical values for its parameters. The obtained current characteristics might account for the high-field quasi-Ohmic region observed experimentally in the leakage current characteristics of a very thin BaTiO3 single crystal in the previous study [Morrison et al., Appl. Phys. Lett. 86, 152903 (2005)].

Enhanced electron field emission from oriented AlN films

(002) and (100) oriented AlN films were deposited on silicon substrates and tungsten tips by radio frequency magnetron reactive sputtering. The electron field emission (FE) properties of (002) and (100) oriented AlN films were investigated and found to be significantly different in emission threshold and current density. The threshold electric field was only 0.14V∕μm for (002) oriented AlN film—far lower than the threshold of 1.13V∕μm for (100) oriented AlN film on tungsten tips. Maximum FE currents of 183 and 27μA were obtained for (002) and (100) oriented AlN films on tungsten tips, respectively. Comparative analysis showed that the absence of linear relation in Fowler-Nordheim plots could be attributed to the high current density integrated over the emitting areas. The excellent FE property of (002) oriented AlN film can be attributed to its vertically oriented grains and the broad distribution of defect-related subbands within its band gap as analyzed by photoluminescence spectra from (002) and (100) ...