MoS₂ solid-lubricating film fabricated by atomic layer deposition on Si substrate

Friction induces a great deal of energy waste in numerous sectors of the society, and the efforts of human to reduce it have never been stopped in the past thousands of years. Recently, layered materials such as graphite^1–3 and transition metal dichalcogenides (TMDCs)^4–6 have attracted wide attentions for their ability to reduce friction effectively as the surface lubricants. MoS₂, as one number of TMDCs, interaction of the adjacent layers is van der Waals forces. So, the interlayer-interfaces-sliding possesses nominal frictional coefficient owing to low interfacial shear strengths from the weak interlayer interactions within MoS₂.^1,7,8 Meanwhile, due to the hexagonal networks formed by Mo and S atoms with stable covalent bonds in layer, MoS₂ can withstand heavy load with high compressive strength in the out-of-layer direction.⁹ These excellent characters ensure MoS₂ as solid-lubricating film and protective surface coating.^2,10–13

Recently, MoS₂ film grown by atomic layer deposition (ALD) has aroused wide concerns. Chemisorption and self-limiting chemical reactions of ALD can provide MoS₂ film with strong interaction to the substrate and controllable number of layers (NL).^14–20 This strong interaction can bind MoS₂ film strongly to the substrate, which can ensure it does not fall off in the lubrication process.²¹ ALD of MoS₂ contains four steps: pulse and purge molybdenum precursor, pulse and purge sulfur precursor, which mean the MoS₂ film with desired thickness can be got by repeating these steps. As a simple and stable method to grow conformal and uniform film, ALD provides a possibility for the excellent material in large scale applications.

In this work, we report MoS₂ solid-lubricating film directly grown by ALD on Si substrate using MoCl₅ and H₂S as precursors. Different methods were used to observe the grown MoS₂ films. Moreover, nanotribological properties of the grown MoS₂ film were observed by an atomic force microscope (AFM).

A single side polished bare Si wafer was chosen as the growth substrate. After acetone and ethanol cleaning in an ultrasonic bath, the wafer was put into a commercial ALD setup (SUNALETMR-100, Picosun) to grow the MoS₂ films. The growth temperature was kept at 460 °C under the pressure of 5.3 hPa. The schematic illustrations of ALD MoS₂ film were shown in Fig. 1. As precursors, MoCl₅ (99.6%) and H₂S (99.6%) were alternately vaporized into ALD reaction chamber by N₂ (99.999%) flow at a rate of 50 sccm. Before the growth, in order to the sufficient vapor pressure, MoCl₅ was heated to the preset temperatures and kept for half an hour. Consequently, an ALD cycle contains four steps: pulsing the MoCl₅ (0.5 s), purging the reaction chamber (30 s), pulsing the H₂S (0.5 s), purging the reaction chamber again (30 s). The MoS₂ film with desired thickness can be got by repeating these steps. Tribological properties of the grown MoS₂ film were observed by an AFM tip.

Uniformity, species composition and content analysis of the grown MoS₂ film were observed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy was used to observe the crystal structure of the MoS₂ film, using an excitation light of 532 nm laser with 0.8 mW power and 1 µm spot. The crystal structure of the MoS₂ film was also characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD was carried out with Cu Kα radiation (λ = 1.54 Å) at 35 mA and 50 kV. HRTEM was taken using an accelerating voltage of 200 kV. The tribological properties of the grown MoS₂ films were studied by AFM in contact mode. A Si probe with a normal spring constant of 2.7 N/m and a tip radius of about 10 nm was used for scanning the surface of the grown MoS₂ film. During the friction measurement, the tip was scanned along the direction perpendicular to the cantilever, in which the signal of torsion corresponded to the friction force was extracted. The applied load range and the scan velocity are 10-50 nN and 10 µm/s. Moreover, adhesion force experiments were carried out in the force-displacement mode. During the measurements, the temperature and the relative humidity were kept at 27 ± 1 °C and 50–60 % respectively. Only relative friction forces (units, mV) were measured owing to the lack of a calibration standard.

Thicknesses of the grown MoS₂ films were observed by AFM. According to AFM images and height profiles (Figs. 2(a) and 2(b)), thicknesses of the MoS₂ films obtained by 100-ALD cycles and 10-ALD cycles are ∼43.2 nm and ∼2.51 nm respectively. Temperature window of ALD MoS₂ is shown in Fig. 2(c). If the temperature is lower than 440 °C, the surface reactions will be incomplete. Elevating the temperature could increase the film thickness by improving the chemical reactivity and occupations of MoCl₅ and H₂S. However, if the temperature is higher than 470 °C, thermal decomposition will occur, which leads to the decrease of the film thickness. According to Fig. 2(d), growth-per-cycle (GPC) value of the MoS₂ film obtained at 460 C was calculated to be 0.42 nm/cycle.

MoS₂ film obtained by 100-ALD-cycle on the Si substrate was observed by SEM (Fig. 3(a)), which indicates the MoS₂ film surface is uniform and compact. Raman spectra are shown in inset. A_1g mode and E_2g¹ mode are sharply shown at 408 cm^-1 and 383cm^-1, which indicates the grown film with high crystallinity is bulk 2H-MoS₂. EDS spectra were used to observe the composition of the MoS₂ film in Fig. 3(b). Atomic percentages of atoms S (2.61%) and Mo (1.31%) confirm the grown film is MoS₂. The crystal structure of the grown MoS₂ film was also characterized by HRTEM. Typical layered structure of MoS₂ can be observed in Fig. 3(c). The cross-sectional HRTEM image of the MoS₂ film is shown in Fig. 3(d), which indicates the spacing is 0.65nm for (002) lattice planes paralleled to the substrate. A planar HRTEM image of the MoS₂ film and the corresponding fast Fourier transformation (FFT) pattern are shown in Figs. 3(e) and 3(f). The typical atomic spacing of MoS₂ can be observed from the clear hexagonal sets of dots in the reciprocal space image, which indicates the inner hexagon is belong to (100) planes with spacing of 0.26nm, and the outer is belong to (110) planes with spacing of 0.16nm.

HRTEM was used to measure grain size of the grown MoS₂. There are two grains A (size of ∼9.1 nm) and B (size of ∼15.1 nm) in the planar view (Fig. 4(a)), indicating crystal size of c-axis (002) orientation may be within 9-15 nm. XPS spectra for the grown MoS₂ are shown in Figs. 4(b) and 4(c). The stoichiometric ratio was determined by the ratio of the S2p_3/2 and Mo3d_5/2 peaks. S/Mo ratio of 1.85 indicates a slight lack of sulfur in the grown MoS₂ film.

The friction measurement was carried out by an AFM tip (Fig. 5(a)). Inset is SEM image of tip. AFM images of bare Si substrate and 10-ALD-cycle MoS₂ film are shown in Figs. 5(b) and 5(c), in which RMS roughness values are 0.48 and 0.72 nm respectively. According to Fig. 5(c), the smooth and compact film is made up of nano-crystalline MoS₂. Inset shows high resolution XRD of the grown MoS₂ film. A diffraction peak related to the (002) plane of MoS₂ at 14.2 degree is sharply shown. Presence of the unique (002) peak indicates the hexagonal MoS₂ film is highly oriented, suggesting the (002) basal planes of as-grown MoS₂ film can be parallel to the Si substrate. The crystal size was calculated by the Debye-Scherrer equation:

where D is the size of crystal, λ is the wavelength of X-ray (λ=1.54Å), β is the full width at half-maximum (FWHM) value, and θ is the Bragg angle. The crystal size of c-axis (002) orientation was obtained as 11.4 nm. This agrees with the HRTEM data well. XRD results indicate that the AFM tip is slid on the (002) plane of nano-crystalline MoS₂ in friction measurement process. Friction forces F_f(Si) of Si and F_f(MoS2) of MoS₂ film under different loads are shown in Fig. 5(d). Obviously, MoS₂ film can effectively reduce the friction force by about 30-45% ((F_f(Si) - F_f(MoS2))/F_f(Si)) under different loads. The friction force of Si increases linearly with load. However, the friction force of MoS₂ is nonlinear with load, which may be attributed to the interlayer-interfaces-sliding. The interlayer-interfaces-sliding endows MoS₂ with nominal friction force, and the sliding processes are different owing to various strains caused by different loads.

During the sliding of AFM tip, friction force can be divided into two parts, one is related to the applied load of tip, the other is related to the adhesion force between the tip and the sample surface. Adhesion forces of Si and 10-ALD-cycle MoS₂ film are shown in Fig. 6. Their water contact angles are shown in Inset. Adhesion forces both of Si and MoS₂ almost keep unchanged under different loads. It can be observed that the adhesion force of MoS₂ film is significantly lower than that of Si, which indicates the friction force of MoS₂ induced by adhesion force should be lower than that of Si. Adhesion force is composed of various forces, such as electrostatic force, van der Waals force and capillary, in which the capillary has the strongest influence. For the spherical tip and the flat surface, the capillary force can be calculated as

where R is the tip radius, γ_L is the surface tension of water, θ is the contact angle of water. According to Fig. 6, the contact angle of MoS₂ film is 88.7°, which is bigger than 69.2° of Si, indicating the capillary force of MoS₂ film is smaller than that of Si. So, the lower friction force of MoS₂ related to the adhesion force is mainly owing to the smaller capillary force.

In summary, MoS₂ solid-lubricating film has been directly fabricated on Si substrates by ALD using MoCl₅ and H₂S as precursors. The grown MoS₂ film can effectively reduce the friction force by about 30-45% under different loads. Besides the interlayer-interfaces-sliding, the smaller capillary is another reason why the grown MoS₂ film has smaller friction force than that of Si. These results indicate the huge application value of the grown MoS₂ film as a solid lubricant.

This work is financially supported by the Natural Science Funds for Distinguished Young Scholar of Jiangsu Province (BK20170023), the National Natural Science Foundation of China (51675360, 51675502, 51775105, 51775001, 51775530, 51775051), the Fundamental Research Funds for the Central Universities (3202006301, 3202006403), the Qing Lan Project of Jiangsu Province, the International Foundation for Science, Stockholm, Sweden, the Organization for the Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu (F/4736-2), the grants from Top 6 High-Level Talents Program of Jiangsu Province(2017-GDZB-006, Class A), the Natural Science Foundation of Jiangsu Province (BK20150505), the Tribo1ogy Science Fund of State Key Laboratory of Tribology (SKLTKF15A11), Open Research Fund of State Key Laboratory of High Performance Complex Manufacturing, Central South University (Kfkt2016-11), the Scientific Research Foundation of Graduate School of Southeast University (YBPY1703), and Open Research Fund of State Key Laboratory of solid lubrication.