Chiu-Yee Chan

Structuring nanodiamond cone arrays for improved field emission

A structuring method capable of producing uniform, large-area cone arrays of diamond films was developed. The technique employs bias-assisted reactive ion etching and is applicable to any structure of diamond films ranging from microcrystalline to nanocrystalline. Variation of the etching conditions enables control of the cone density, geometry, and height. Surface nanostructuring of cone arrays significantly improves the field emission properties of diamond films of all kinds. The turn on field is reduced to 6 and 10 V/μm for nanodiamond and microdiamond films, respectively, (compared to >25 V/μm for as deposited surfaces). Lower cone density yields better field electron emission (lower turn-on electrical field) due to the screening in high-density cone arrays. The field emission properties are determined by both the enhancement factor of the cone array and the emitting properties of the material. The field electron emission properties of nanodiamond arrays are better than cone arrays of single crystalli...

Formation chemistry of perovskites with mixed iodide/chloride content and the implications on charge transport properties

Although Cl-doping is a common technique for achieving high photovoltaic (PV) performance, the Cl content is negligibly small and cannot easily be tuned. Therefore, we herein study the formation chemistry of Cl-doped perovskites by examining the chemical interactions between thermally evaporated MAI and PbCl2 through X-ray photoemission spectroscopy (XPS). We show that PbCl2 is not stable at the MAI/PbCl2 contact surface. The Cl atom readily detaches from the PbCl2, which subsequently initiates electron transition from Pb to MAI. Via thermal-evaporation, a perovskite with a high PbCl2 content can be prepared and examined. We found that the presence of metallic Pb, associated with increased Cl content, can quench the photogenerated exciton in PV devices. By optimizing the ratio of MAI : PbCl2, a perovskite solar cell with ∼6% efficiency was obtained.

Oriented single-crystal diamond cones and their arrays

One of the major problems in material science has been the difficulty in modification of the most endurable material, diamond, due to its extreme hardness and chemical inertness. Here, we report the development of a conical structure of diamond by performing bias-assisted reactive ion etching in hydrogen plasma. The diamond cones produced by this method are uniformly distributed over large areas on silicon substrates. Each cone was identified to be a single crystal with an apical angle as small as 28° and a very sharp tip (tip radii ∼2 nm). Their [001] axes are perpendicular to the substrate surface and parallel to each other. Such striking structures of individual single-crystal diamond cones and their arrays, in addition to their scientific value, may lead to a breakthrough in the design of high-performance mechanical and electronic devices.

Thick and adherent cubic boron nitride films grown on diamond interlayers by fluorine-assisted chemical vapor deposition

Using plasma-enhanced chemical vapor deposition (PECVD) based on fluorine chemistry, the limitations hindering the practical use of cubic boron nitride (cBN) films in mechanical applications have been overcome. The CVD method presented is characteristic with (a) the direct cBN growth on diamond without soft, noncubic BN interface layers, (b) the synthesis of cBN films with extraordinary adhesion to the substrates and high mechanical properties, and (c) the scalable process providing thick, large-area cBN films at high deposition rate even on rough and untreated surfaces. These prime technological properties open the route to the mechanical exploitation of cBN films, particularly in tribological and tool applications. The reduction of the bias voltage in the PECVD process presented to a value of −20V not only provides high-quality films, but also gives physical insight into the cBN growth mechanism.

Mechanical and tribological properties of diamond-like carbon films prepared on steel by ECR-CVD process

Abstract Diamond-like carbon (DLC) films were prepared on AISI 440C steel substrates at room temperature by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C 2 H 2 /Ar plasma under different conditions. In order to prevent the inter-diffusion of carbon and improve the adhesion strength of DLC films, functionally gradient Ti/TiN/TiCN/TiC supporting underlayers were deposited on the steel substrates in advance. Using the designed interfacial transition layers, relatively thick DLC films (1–2 μm) were successfully prepared on the steel substrates without delamination. By optimizing the deposition parameters, DLC films with hardness up to 28 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball were obtained. In addition, the specific wear rates of the films were found to be extremely low (∼10 −17 m 3 /Nm). The friction-induced graphitization mechanism of DLC was confirmed by micro-Raman analysis.

Wear-resistant multilayered diamond-like carbon coating prepared by pulse biased arc ion plating

Diamond-like carbon coatings have been deposited by a pulse biased arc ion plating. In order to improve their adherence to metal substrate, two systems of graded transition layers, namely Ti/TiN/TiC and Ti/TiCN/TIC, have been applied. The structure and composition of the diamond-like carbon/transition composite coatings were studied by scanning electron microscopy, Raman spectroscopy and energy dispersive X-ray spectroscopy. The total thickness of the coatings was within a range of 1.0-2.0 mum. Such multi-layer coatings showed excellent properties including high hardness, low friction coefficient and long wear-resistant lifetime. Diamond like carbon coatings as well as their wear tracks developed by sliding steel balls have been investigated by scanning electron microscopy. The results of the analysis, particularly that of the tribological study showed that the wear resistance and film-to-substrate adherence of diamond like carbon coatings with stainless steel surface were dramatically improved by using a graded transition layer and pulse biased arc ion plating

Mechanical properties of DLC films prepared in acetylene and methane plasmas using electron cyclotron resonance microwave plasma chemical vapor deposition

Abstract Diamond-like carbon (DLC) films were deposited on silicon using methane and acetylene plasma induced by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD). The mechanical properties of DLC films were characterized by micro-Raman system, atomic force microscope, tribometer, nano-indenter used for both hardness and nano-scratch test measurements. The mechanical properties of both DLC films, prepared in methane and acetylene plasmas, respectively, strongly depended on the kinetic energy of impinging particles. The deposition at −120 V substrate bias gave rise to DLC films with the best mechanical properties for both methane and acetylene plasmas. The hardness measurements with variable indentation depth showed the characteristic changes in hardness values implying elastic deformations of supporting substrates. The maximum hardness value of DLC M films was 20 GPa while that of DLC A films was 28 GPa. However, the hardness dropped when DLC films were prepared at substrate biases more negative than −120 V due to the thermal graphitization. The improvement in DLC properties usually provided the films with smaller hydrogen content and higher density of sp 3 bondings. These parameters were engineered through controlling the deposition parameters. Particularly, the bombardment of growing DLC films by energetic ions showed to be extremely important to yield films with lower internal stress.

Film thickness effects on mechanical and tribological properties of nitrogenated diamond-like carbon films

Nanoindentation, nanoscratch and ball-on-disk tests were used to determine the film thickness effect on the mechanical and tribological properties of nitrogenated diamond-like carbon (CNx) films, which were deposited on Si (100) substrates by an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MP-CVD) system. Except for the film with a thickness of 20 nm, there existed peak values of hardness for all films thicker than 44 nm. When the thickness ranged from 44 to 235 nm, the peak values of hardness and the corresponding indentation depth increased along with increasing film thickness. The results of scratch resistance tests showed that the critical loads of the fracture were independent of thickness for thinner films, however, they increased rapidly with increasing thickness for thicker films. Ball-on-disk sliding tests indicated that the friction coefficient decreased with increasing thickness for thinner films, while there was no obvious thickness dependence of the friction coefficient for relatively thicker films. The formation of transferred layer and graphitization of the films during sliding resulted in the decrease of friction coefficients at early stages of sliding.

Mechanical properties and corrosion studies of amorphous carbon on magnetic disks prepared by ECR plasma technique

Abstract Diamond-like carbon (DLC) films were prepared on magnetic disk surfaces using an electron cyclotron resonance assisted microwave plasma chemical vapor deposition (ECR-MPCVD) system with variable radio-frequency (r.f.) substrate bias. Surface roughness of DLC deposited on hard disks was investigated by atomic force microscopy (AFM), which revealed that the DLC coated surfaces are smoother than those of the uncoated disks. Nitrogen incorporation into the films (a-C:N) further reduced the root-mean-square (RMS) roughness to 2.5 A for films prepared at a substrate bias of −120 V. Scratch resistance was improved when the DLC coatings were deposited at bias voltages greater than −90 V. However, the nitrogen introduction into the films deteriorated their scratch resistance. When the DLC films were subjected to an accelerated corrosive environment, pinhole density remarkably varied with the deposition conditions. The DLC films deposited with lower substrate biases and nitrogen incorporation resulted in poorer corrosion resistance. Internal stress, hydrogen content and graphitic cluster size of these films were correlated with data acquired by Raman analysis.

The growth of thick cBN films employing fluorine chemistry and ECR deposition

Abstract Energetic ion bombardment in N2 and Ar plasmas conventionally applied to cubic boron nitride (cBN) deposition restricts the size of cBN crystallites and induces high internal stress levels, limiting the non-delaminating film thickness obtainable (to ∼100 nm). The introduction of fluorine chemistry via a complex He–Ar–N2–BF3–H2 plasma produced in an electron cyclotron resonance (ECR) system overcomes such limitations and enables the preparation of thick cBN films (>1 μm) with low stress. The films were deposited using a low substrate bias (−40 V) and high substrate temperature (900 °C). Fourier-transform infrared (FTIR) spectroscopy shows that the films are composed of >80% cBN. Detailed analysis of BN film structures employing high-resolution transmission electron microscopy (HRTEM) and transmission electron energy loss spectroscopy (EELS) shows that a cBN layer with high phase purity is formed on top of an initial turbostratic BN (tBN) layer, which accounts for the small hexagonal BN (hBN) signal in FTIR spectra. The appearance of characteristic transverse optical (TO) phonon and longitudinal phonon (LO) modes of cBN in visible Raman spectra demonstrates the larger crystalline size (∼100 nm, also confirmed by HRTEM) in contrast to the films reported previously. The ion bombardment, gas composition, substrate temperature and growth time significantly affect the phase purity and crystallinity of the cBN films formed, as further elucidated in this paper.