Structure and optical properties of 2D layered MoS₂ crystals implemented with novel friction induced crystal growth Open Access

Molybdenum disulfide (MoS₂) is a two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductor crystal that exhibits optoelectronic properties suitable for low electrical power, high-speed devices.¹ Recently, our group has studied properties of 2D materials.^2–4 Bulk MoS₂ is an indirect bandgap semiconductor with a bandgap of 1.29 eV, while monolayer MoS₂ is a direct bandgap semiconductor with a bandgap of 1.8 eV.^5–7 The bandgap decreases as the number of layers is increased. In 2011, monolayer MoS₂ based field effect transistors (FETs) were realized by Kis’s group,⁸ which inspired us to develop MoS₂ devices. Also, MoS₂ logic circuits and phototransistors have been demonstrated,^9–11 and extensive MoS₂ device research has been focused on spintronics¹² as well as valleytronics.¹³ The applications for MoS₂ are expected to be in a number of areas, including flexible solar cells,^14,15 superconductors¹⁶ and highly active catalysts.¹⁷ 

For these studies, the MoS₂ layers had been prepared by mechanical exfoliation, sometimes called the “Scotch tape method”¹⁸ or synthesized via chemical vapor deposition (CVD).¹⁹ MoS₂ is also familiar as a solid lubricant. MoS₂ lubrication was developed and used to reduce friction in machines including car engines. To realize lower friction, MoS₂ formed of well-stacked layers was grown on the surface, and a layered structure of MoS₂ at the mechanical interface has been observed using transmission electron microscopy (TEM).²⁰ However the semiconductor properties of friction induced MoS₂ are unknown. Tribological studies have been used to determine the friction coefficient. Friction phenomena have mainly been analyzed by examining the surface morphology, the chemical composition and the molecular structure using laser microscopy, X-ray photoelectron spectroscopy (XPS) and infrared/Raman spectroscopy, respectively. In this study we used X-ray diffraction (XRD), and Raman and photoluminescence spectroscopies to examine the structure and optical properties of friction induced MoS₂ crystals grown at the interface between two materials. Stainless steel and Ge(100) were used as substrates. Stainless steel is commonly used for tribological studies and the well-known tribochemical properties of MoDTC confirm that the in-house experimental setup used for the friction experiments in this study is suitable. Germanium is one of the more attractive substrates for spintronic applications because a well-interfaced heterostructure with stable spin current can be formed with MoS₂. In addition, selective growth of MoS₂ films at specified positions using this friction-induced method enables savings to be made, since a dry vacuum pump is not needed and a photolithography process can be omitted from the fabrication process, making it similar to a rubbing process. In this study, friction induced crystal growth has been realized for the first time, and the indications are that the optical quality of the MoS₂ obtained by this method under optimum conditions is comparable to exfoliated MoS₂ and CVD grown MoS₂.

The friction experiments were performed using a ball on plate method on a rotating stage, as shown in Fig. 1. Two different substrates were examined. One was SUS430 with the surface mechanically polished by #400 paper, giving it an RMS surface roughness of 0.02. The other was an undoped Ge(100) wafer with a resistance of over 30 Ω-cm. The ball was made of high carbon chromium bearing steel SUJ2 and had a diameter of 8 mm and a roughness of 0.008. These roughness values were measured using a 3D laser microscope (Shimadzu OLS4100). The experiments were conducted under a load of 10 N with a sliding speed of 2.5 mm/s. The solution used was poly-α-olefin (PAO) at temperatures of 80∼150 °C. This was blended with molybdenum dithiocarbamate (MoDTC) and calcium sulfonate. The MoS₂ was synthesized at the surface using this lubricant. The morphology of the MoS₂ on the substrates was examined using a laser microscope (Keyence Corp), and the structure was analyzed using X-ray diffraction measurements (Bruker D8 Advance) with Cu Kα radiation (λ = 0.154 nm).

After rinsing the substrate in alcohol, room-temperature μ-Raman and photoluminescence (PL) spectra were measured in the back-scattering configuration using a JASCO NRS-5100 spectrometer. Excitation was by a SHG YVO₄ laser at 532 nm and 0.8 mW. The laser was focused by a 100× objective lens. The beam spot size was about 1 μm diameter, which is close to the diffraction limit defined by the laser wavelength. The spectral resolution was 0.4 cm^-1.

The behavior of the friction coefficient as a function of sliding time for SUS430 and Ge(100) at temperatures of 150 °C and 80 °C are shown in Figs. 2(a) and 2(b), respectively. One oscillation period of the friction coefficient corresponds to one rotation of the stage. The friction coefficients at the beginning of the experiments are 0.6 and 0.13, which decrease gradually with sliding time. Sliding for 600 and 300 seconds decreases the friction coefficients to 0.3 and 0.03 for SUS430 and Ge(100), respectively. The tribological properties for SUS430 are similar to those for steel.²¹ The decrement indicates that crystalline MoS₂ is formed at the surface. The morphology of the MoS₂ crystals are shown in the insets in Figs. 2(a) and 2(b). The shape of the MoS₂ crystals is approximately triangular with cracked corners. The side lengths of the triangular domains are 50 μm and 20 μm for the SUS430 and Ge(100) substrates, respectively.

The XRD patterns of the surfaces of the SUS430 and Ge(100) substrates are shown in Figs. 3(a) and 3(b), respectively. The diffraction peak at 2θ=14.6° corresponds to the (002) plane of hexagonal MoS₂ (JCPDS card No: 75-1539). The prominence of the (002) peak indicates the presence of well-stacked layered structures along the (002) direction.

Figures 4 and 5 show Raman and PL spectra, respectively, of friction induced MoS₂ crystals grown on the SUS430 and Ge(100) substrates at room temperature. The excitation wavelength was 532 nm (2.33 eV). In the Raman spectrum, two characteristic vibration modes, E¹_2g and A_1g are seen, which are attributed to the in-plane vibration of molybdenum and sulfur atoms and the out-of-plane vibration of sulfur atoms, respectively. The number of MoS₂ layers can be estimated from the frequency difference between the E¹_2g and A_1g peaks.^22–24 For MoS₂ grown on SUS430 these two modes are centered at 383.8 cm^-1 and 408.8 cm^-1, respectively. The frequency difference of 25 cm^-1 is indicative of N-layers (N > 6) of MoS₂. For the MoS₂ grown on Ge(100) the frequencies of E¹_2g and A_1g are 384 cm^-1 and 408.4 cm^-1 respectively, a difference of 24.4 cm^-1, indicating the formation of N-layers (N > 6) of MoS₂. For exfoliated MoS₂ with 5 or 6 layers, these peaks are observed around 382 cm^-1 and 407 cm^-1, a frequency interval of 25 cm^-1.²² This blue shift in the spectrum indicates that the friction induced MoS₂ crystal is compressed.²⁵ The full width at half maximum (FWHM) of the E¹_2g peak is used as an indicator of the crystalline quality.²⁶ The FWHMs of the E¹_2g peaks for MoS₂ grown on SUS430 and Ge(100) substrates are 2.6 cm^-1 and 3.4 cm^-1, respectively, and that of 5 layers of CVD and atomic layer deposition (ALD) grown MoS₂ is 10 cm^-1.^27,28 This suggests that the crystalline quality of the friction induced MoS₂ is good.

The PL spectra of the friction induced MoS₂ crystal grown on SUS430 and Ge(100) at room temperature are shown in Figs. 5(a) and 5(b), respectively. For SUS430 a broad peak is observed around 657 nm, and a shoulder peak appears on the lower energy side. These two peaks are found to be at 1.89 eV (656 nm) and 1.79 eV (694 nm) by peak fitting. The PL peak at 1.89 eV can be attributed to observation of the A peak which is assigned to neutral exciton emission.^29,30 The blueshift of the A peak is caused by tensile stress, which may be due to the friction.³¹ The peak at 1.79 eV is not emission from the charged exciton recombination channels (trions, A^-). The energy difference between A and A^- is around 30 meV.²⁹ In addition to the relatively broad photoluminescence, a sharp Raman mode can be observed at 628 nm (1.97 eV/2873 cm^-1), which corresponds to Raman-active CH₃ stretching vibrations. The Raman peak at 628 nm is also observed in the MoS₂ on Ge(100), as shown in Fig. 5(b). The oil on the surface was rinsed with alcohol. However the rinsing time was only 10 seconds in order to avoid removal of the MoS₂ crystal from the surface. It is known that, for monolayers and multi-layered MoS₂, two PL peaks located at around 1.85 eV (A peak) and 1.98 eV (B peak) can be observed.^27,30,32 The B peak arises from exciton emission from another direct transition between the conduction band and the lower lying valence band. In this study, the B peak is not clear because it is close to the Raman peak of the CH₃ stretching vibrations.

Very little to no PL intensity is observed for thicker films or bulk MoS₂ due to the decrease in the intraband relaxation rate from the excitonic states with increasing film thickness, a consequence of bulk MoS₂ being an indirect bandgap semiconductor like silicon. As shown in Fig. 5(b), PL signal is not appeared for the MoS₂ grown on Ge(100). We find PL peaks in the spectra of the few layers of MoS₂ grown on SUS430, as shown in Fig. 5(a), which is caused by the structural discontinuity at the crystal edges.³³ 

While more studies are necessary to understand this friction induced crystal growth mechanism, these results suggest that a comparable optical quality can be obtained for friction induced MoS₂ grown under optimum conditions to MoS₂ grown by exfoliation or CVD. The friction load used in this study, 10 N, is lower than the breaking strength of MoS₂, 23 GPa, and can be modified to improve the growth conditions.³⁴ The number of layered MoS₂ is expected to be controlled by friction load.

We have used XRD, and Raman and photoluminescence spectroscopies to characterize friction induced crystalline MoS₂ grown on solid surfaces. Under rotational sliding with a 10 N load at temperatures of 80∼150 °C, layered structures of MoS₂ crystal along the (002) direction were grown on SUS430 and Ge(100) substrates. The MoS₂ crystal shape appears triangular with side lengths up to 20∼50 μm. The distance between the Raman frequencies of the E¹_2g and A_1g peaks indicates that the thickness is N-layers (N > 6). MoS₂ grown on SUS430 shows photoluminescence from the A peak (exciton) at room temperature.

This research was supported by Tohoku University: Advanced Research and Education Center for Steel (ARECS).