Crystal structures and phase transformation in In2Se3 compound
semiconductor have been studied by electron
diffraction, high resolution electron microscopy and X-ray diffraction (XRD) together
with optical absorption measurements.
The time-temperature-transformation (TTT) diagram reveals that there
exist only two phases in In2Se3 and the transformation
temperature is 853 K. The transformation from the high temperature phase to the low
requires a long incubation time for crystal nucleation and
a relatively high temperature for crystal growth.
The low and high temperature phases are the vacancy ordered in screw form (VOSF)
phase and the layer structure phase, respectively. Both phases possess semiconducting
optical properties and are constructed on the basis of
a tetrahedral bonding structure.
The VOSF phase is of a defect wurtzite structure, in which vacancies
on 1/3 of the cation sites are ordered in screw form along the c-axis.
The space group is P61 or P65 with
a=7.14 Å, c=19.38 Å, Z=6.
The layer structure is constructed of five-layer Se–In–Se–In–Se sets and the sets
are linked by weak van der Waals' force with stacking sequence of ABC.
The space group is R3m with a = 4.00 Å,
c = 28.80 Å, Z=3 (indexed in hexagonal system).
In the layer structure, structure vacancies on 1/3 of the cation sites aggregate
to form a vacancy plane for every three In-planes.
The structural difference between the two phases is most clearly characterized
by the difference in coordination numbers of the Se atoms.
Nanostructures and mechanical properties on the surface of two kinds of tribofilm formed from zinc dialkyl-dithiophosphate ͑ZDDP͒ and molybdenum dithiocarbamate ͑MoDTC͒ additives, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using an atomic force microscopy ͑AFM͒ phase imaging technique. The level of interactive force between the tip and sample was modulated for distinguishing well-defined structures and mechanical properties of individual components not only on the uppermost surface but also in the underlying area near the surface in the AFM tapping mode. It was found that the MoDTC/ZDDP tribofilm possessed a lower surface modulus than the ZDDP film in the elastic deformation range. Most importantly, nanostrips oriented in the sliding direction were found in the MoDTC/ZDDP tribofilm at a depth of around 10 nm from the surface. These nanostrips possessed lower shearing stress than the surface matrix and formed the inner skin layer, which exhibited lower friction behavior than that of the ZDDP tribofilm. These results agreed with our recent nanoindentation and nanoscratch measurements for estimating the mechanical and frictional properties of MoDTC/ZDDP and ZDDP tribofilms. These findings and previous surface analytical results suggest that the nanostrips act as a type of solid lubricant, such as MoS 2 single sheets, to lower the boundary friction coefficient.
Surface physical properties and chemical states on a nanometer scale were investigated to provide a direct insight into the mechanism of near-frictionless performance displayed by diamond-like carbon (DLC) coatings in engine oil lubricants. A mechanism was revealed by combining nanoprobe methods with surface chemical analysis. The near-frictionless behavior observed in the lubricants was found to stem from strong repulsive force between hydrogen-terminated carbon chains, originating from the bonding of oil additive molecules on the DLC coating surface producing sliding contact between H-terminated alkyl chain layers.In the last few years, an impressive finding was made that low-cost, low-wear, chemically stable diamond-like carbon (DLC) films can exhibit a superlow friction performance, not only observed in the sliding of DLC film containing a large amount of hydrogen in a dry nitrogen atmosphere [1], but also seen in the sliding of hydrogen-free DLC film under an oil lubricant condition [2-4] for a wide range of industrial applications. The friction reduction mechanism has attracted widespread interest with regard to scientific understanding for putting this finding to practical use. However, several researches for the case exhibited only in a dry nitrogen atmosphere were reported [5][6][7][8]. Dickrell et al. proposed a closed-form time-and position-dependent model for fractional coverage, based on the adsorption of environmental contaminants and their subsequent removal through sliding contact [5,6]. Erdemir suggested a dipole configuration that gives rise to repulsion rather than attraction between the hydrogen-terminated sliding surfaces of the DLC films [7]. Dag et al. modeled the sliding friction between hydrogenated diamond surfaces for explaining the superlow friction originating from the steady repulsive interaction between the sliding surfaces [8]. In any case, there is no direct analytical evidence showing the adsorbed contaminants or the dipole configuration formed on sliding surfaces to support these friction reduction models.Our recent studies have demonstrated that nanometerscale properties, referred to here as analytical properties, such as the nanostructure, nanoproperties, and nanofunctions of the surface materials act as extremely important factors in controlling macro-scale tribological performance [9][10][11][12]. For the purpose of elucidating these controlling factors and their correlations, it is indispensable to obtain direct surface information, including various structural, physical, and chemical properties or characteristics on a nanometer scale without any influence from surface roughness. Probing techniques such as atomic force microscopy (AFM) and the nanoindentation method are powerful tools for ascertaining the real nature of sliding surface [9][10][11][12].
Nanometer-scale differences in mechanical and structural properties between the molybdenum- dithiocarbamate/zinc-dialkylsithiophosphate (MoDTC/ZDDP) tribofilm and ZDDP tribofilm were successfully evaluated by using atomic force microscopic phase-image techniques, Auger electron spectroscopy and X-ray photo spectroscopy. It is well known that the MoDTC/ZDDP tribofilm exhibits markedly lower friction behavior than the ZDDP tribofilm. To elucidate the mechanism of friction reduction originating from the MoDTC additive, attention was focused on property differences in the surface area in particular, from the uppermost surface to an underlying region of less than 10 nm in depth. It was found that the friction reduction due to the MoDTC/ZDDP additives originates from an inner skin layer formed by MoS2 nanostrips just below the surface. The MoS2 nanostrips were oriented in the sliding direction, had low yield strength and acted as a solid lubricant in lowering the friction coefficient of the MoDTC/ZDDP tribofilm.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.