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Encapsulation of transition metal dichalcogenides crystals with room temperature plasma deposited carbonaceous films

Encapsulation and its effect on two-dimensional (2D) dichalcogenides layered materials are of significant importance for electronic device fabrication. Here, we developed a dry and rapid encapsulation method for the 2D dichalcogenides by coating a carbonaceous film at room temperature using a plasma technique. Thin encapsulation film is coated on dichalcogenide crystals by microwave assisted surface wave plasma (MW-SWP) chemical vapor deposition (CVD) within a very short duration. Photoluminescence (PL) analysis shows the influence on excitation as well as the emission light with the coating of a carbonaceous film. The encapsulated dichalcogenide samples remain intact after heating at 200 °C as observed by PL and microscopic analysis. We also demonstrate obtaining stable photoconductivity for the dichalcogenide layers with encapsulation of a carbonaceous film. The rapid and dry coating technique at room temperature can be extended to various other 2D materials as the MW-SWP process allows damage-free encapsulation.


Introduction
In recent years, transition metal dichalcogenides (TMDCs) and other two-dimensional (2D) layered materials have attracted signicant interest for nanoelectronic and optoelectronic device applications. [1][2][3][4][5] The TMDCs are a large family of semiconductors (e.g. MoS 2 , WS 2 etc.), metals (e.g. TiS 2 , VSe 2 ), superconductors (e.g. TaS 2 , NbSe 2 ), semimetals (e.g. WTe 2 and TiSe 2 ) and insulators (e.g. HfS 2 ). [6][7][8][9][10][11][12][13][14][15][16] The thickness dependent electronic and optoelectronic properties of group VI dichalcogenides (MX 2 , M ¼ Mo, W; X ¼ S, Se, etc.) semiconductors are quite promising for novel optoelectronic devices. 17,18 Transition from indirect band gap to direct band gap for the semiconducting trigonal prismatic MX 2 has been revealed as the thickness reduces to monolayer. [19][20][21][22] The MX 2 semiconductor with excellent optical absorption in visible range and large exciton binding energy has attracted wide attention for highperformance photodetectors, efficient solar cells, and multifunctional nanoelectronics. [23][24][25][26] TMDCs layered materials have been derived by exfoliation process, hydrothermal synthesis and chemical vapor deposition (CVD) techniques. [27][28][29][30] The CVD technique has a signicant advantage for the synthesis of largearea polycrystalline lms, single crystal domains, vertical or lateral heterostructures of two different dichalcogenides. [29][30][31][32] Recent studies show structural transformation and degradation of MX 2 layered materials with oxygen exposure, UV irradiation, ozone exposure and heating under ambient conditions. [33][34][35][36] Signicant structural degradation of MX 2 layers has been also observed with long-term exposure to ambient conditions. 36 The other 2D layered semiconductors, such as black phosphorus are more unstable and degradable under ambient conditions. 37 In this context, encapsulation and device engineering are the key aspects that need to be addressed in the coming years for practical application of these layered semiconductors. Gao et al., has demonstrated coating of poly(methyl methacrylate) (PMMA) and Parylene C on MoS 2 and WS 2 layers to protect from ageing. 36 Ahn et al. has demonstrated the transferring process of PMMA/h-BN layer on WSe 2 layer as degradation protective layer. 38 Recently, Woodhead et al. have developed an optical solution to encapsulate monolayer MX 2 from physical damage and degradation. 39 It has been also reported that depending on the layered materials a compressive or other lattice strain can be observed with the encapsulation layer. 39,40 In contrast to the previous reports, we developed a rapid and dry approach to fully encapsulate MX 2 layered materials preventing degradation in ambient conditions. As follows, we demonstrate the encapsulation and PL emission properties with the plasma CVD deposited carbonaceous lm on MX 2 layers.

Synthesis of MoS 2 and WS 2 crystals
MoS 2 and WS 2 crystals were grown on SiO 2 /Si substrate by thermal chemical vapor deposition (CVD) method by controlling the growth parameters. The synthesis was performed in a quartz tube CVD system with a mixture of argon (Ar) and H 2 gas ow at atmospheric pressure. Prior to the thermal CVD growth, the substrate was cleaned using acetone and following isopropyl alcohol (IPA) for 15 min each by sonication. Sulfur powder (1 g) in a ceramic boat was inserted into the quartz tube of length 90 cm and diameter of 4.5 cm. It was placed in a low-temperature furnace (LTF) heated to 180 C. The substrate and MoO 3 and WO 3 powder (10 mg) were kept on a ceramic boat for the growth of MoS 2 and WS 2 crystals, respectively in two separate experiments. MoS 2 and WS 2 crystals were synthesized at 750 and 850 C, respectively in the high-temperature furnace (HTF). Both the experiments were carried out using 100 standard cubic centimeters per minute (sccm) of argon and H 2 gas mixture (97 : 3).

Deposition of a carbonaceous lm by plasma CVD
Carbonaceous lm was deposited by microwave assisted surface wave plasma (MW-SWP) chemical vapor deposition (CVD) technique on the MoS 2 and WS 2 crystals synthesized on SiO 2 /Si substrates. A metal grid was placed above the MoS 2 and WS 2 crystal samples (Fig. S1 †) to avoid plasma induced deformation of the TMDCs layers. The MW-SWP CVD chamber was evacuated to 5 Â 10 À4 Pa prior to the deposition. The thin lm deposition was performed at 10-20 Pa gas composition pressure with a ow rate of 100-200 sccm of Ar and 10-50 sccm of CH 4 . The microwave power was xed as 800 W with negligible amount of uctuation. Deposition of the thin lms was carried out for a duration of 60-240 s at room temperature ($24 C).

Characterization of prepared samples
Prepared samples were characterized by optical microscopy, Raman spectroscopy, eld-emission scanning electron microscopy (FE-SEM), photoluminescence (PL) spectroscopy and X-ray photoelectron spectroscopy (XPS). Optical microscopy analysis was performed using the digital optical microscope VHX-500 in reectance mode with a Moticam 2000 2.0 M pixel camera. Raman analysis was carried out using the NRS 3300 laser Raman spectrometer with a laser excitation wavelength of 532.08 nm. FE-SEM study was performed with JEOL JSM-7800F with an accelerating voltage of 5 kV. Infrared (IR) spectroscopy analysis was carried out with grazing incident using JASCO IRT-5000 FT-IR microscope. Absorbance and transmittance of the carbonaceous lm was measured by JASCO V-670K spectrophotometer. XPS analysis was performed by VersaProbe using monochromated Al Ka excitation source (1486.6 eV). PL analysis was carried with a confocal laser scanning microscope (NX-3DFLIM-N03, Tokyo Instruments, Japan) equipped with a Nd:YVO 4 diode laser. For the device fabrication, Au electrodes were deposited by the thermal evaporation technique using ULVAC VPC-260F. The current-voltage (I-V) characteristics were measured using a two probe system and a Keithley 2401 SourceMeter. Fig. 1a shows optical microscope image of MoS 2 crystal synthesized on the SiO 2 /Si substrate by thermal CVD technique. Triangular shaped MoS 2 crystals with curved edges and size of around 20 mm were obtained in the particular growth condition. Fig. 1b shows an AFM image of the as-synthesized MoS 2 crystal. The line prole shows a thickness of nearly $0.9 nm for the individual MoS 2 crystals. The synthesized samples were transferred to MW-SWP CVD chambers for coating without any other treatment. Microwave excited plasma sources have been used for effective deposition of an insulating encapsulation layer for the conventional semiconductors. 41,42 We explore the possibility of 2D layered materials encapsulation at room temperature by SW-MWP technique. The MW-SWP can produce an over-dense plasma even at a low gas pressure, with a density higher than the cut-off density (7.4 Â 10 16 m À3 for 2.45 GHz). The deposition process was rapid and substrates were place at a longer distance from plasma shower which allowed to maintain the substrate temperature at $24 C. Fig. 1c shows an optical microscope image of the coated MoS 2 crystals. The triangular MoS 2 crystal remain intact without any visible structural damage. Fig. 1d shows a higher resolution optical microscopy image of the MoS 2 triangular crystals, where the morphology is unaffected by deposition of the encapsulation lm. Unlike other plasma technique, the MW-SWP is a promising tool for large-area carbonaceous lm deposition and useful to avoid plasma induced damages of the substrate surface. 43 The details of the deposition process and photograph of the deposited lm are included in ESI (Fig. S1 and S2 †). Fig. 2a shows an IR absorption spectra of the carbonaceous lm coated on the MoS 2 /SiO 2 /Si sample ($15 nm). Few weak peaks are observed in the frequency range from 1300 to 1600 cm À1 , where the peak centered at $1530 cm À1 can be assigned to C]C stretching vibrations. At a higher frequency range, three sharper peaks at 2870, 2925 and 2961 cm À1 were observed corresponding to the C-H vibrations. These absorption peaks can be assigned to sp 3 CH x symmetric and asymmetric bands. Fig. 2b shows a typical Raman spectra of the carbonaceous coating. There was no signicant Raman features for the plasma deposited carbonaceous lm, other than a broad hump and reduction in intensity of Si Raman peak. A carbon lm coated around $150 C with similar plasma parameters shows broad D and G peaks, clearly indicating the difference in carbon structures for room temperature deposited lm (Fig. S3 †). The presence of sp 3 CH x bonds and no clear Raman peaks (D and G bands) signies formation of insulating carbonaceous lm with polymeric nature. 43,44 The room temperature deposited carbon lm is signicantly different than that of normal crystalline and amorphous carbon forms. Fig. 2c shows transmittance and absorbance of the carbonaceous lm deposited on glass substrate. The coated lm shows excellent transparency in the wavelength range of 0.3-2 mm. More than 95% transparency is obtained in the visible light region, which can be tuned with appropriate thickness of the deposited lm. The coated lm on SiO 2 /Si substrate was characterized by Raman spectroscopy with and without the TMDCs crystals. Fig. 2d and e shows Raman spectra of the MoS 2 crystals with and without encapsulated lm. The Raman vibration modes E 1 2g and A 1g for MoS 2 are observed at 383.2 and 403.3 cm À1 , (frequency difference of $20 cm À1 ) corresponding to inplane vibration of molybdenum and sulfur and out-of-plane vibration of sulfur atoms, respectively. We observed a reduction in Raman peak intensity for the encapsulated sample, while no signicant shi in peak position. Inuence of the coated layer was further analyzed by PL study to investigate the possible effect on PL excitation and light emission. Fig. 3a shows FE-SEM image of the as-synthesized MoS 2 crystals on SiO 2 /Si substrate by the CVD technique. The triangular shaped MoS 2 crystals can be conrmed by the FE-SEM analysis. Fig. 3b shows the FE-SEM image of the plasma deposited carbonaceous lm on MoS 2 crystals. The surface morphology of the encapsulated MoS 2 sample was observed by FE-SEM analysis using the upper electron detector (UED) and 2 kV accelerating voltage. The triangular MoS 2 crystals can be conrmed below the encapsulation layer, although the nonconducting encapsulation lm signicantly affects the micrograph. Previously, plasma functionalization of TMDCs layer has been also demonstrated for modication of electronic properties. PL properties of the encapsulated MoS 2 layer were investigated considering the surface functionalization by the plasma process and possible effect on PL emission. 45 Fig. 3c shows the PL spectra of coated and non-coated MoS 2 crystal. Quenching and red shi of the PL peak for the encapsulated MoS 2 sample was observed (Fig. 3c). The PL quenching and shi conrm inuence of PL excitation and emission of light for the MoS 2 crystal with the encapsulation. Woodhead et al. have also reported signicant suppression of the PL peak of MoS 2 monolayer by solution coated epoxy due to the compressive strain. 39 Compressive strain can modulate the direct to indirect band gap of MoS 2 crystals, which reduces the emission intensity. The required values of strain to induce a shi from direct to indirect bandgaps for MoS 2 and WS 2 are 0.5% and 1% respectively. 22 However, the reduction and red shi of the PL peak with the carbonaceous encapsulated layer cannot solely conrm as compressive strain, which was further investigated for the triangular WS 2 crystals. Fig. 4a shows optical microscope image of CVD synthesized triangular WS 2 crystals on SiO 2 /Si substrate. Fig. 4b shows optical microscope image of WS 2 crystals on the SiO 2 /Si substrate aer coating a thin carbonaceous lm ($15 nm). Similarly, Fig. 4c shows an optical microscope image of WS 2 sample with a thicker coated layer ($30 nm). The difference in coating thickness can be also identied with variation in contrast of the optical images. The WS 2 crystals remain unaffected aer the room temperature deposited encapsulation layer as observed from the optical microscopy images. Further, Raman and X-ray photoelectron spectroscopy (XPS) analysis were carried out to investigate the structural properties. XPS analysis of the as growth dichalcogenide layers and encapsulated samples are included in ESI (Fig. S4 and S5 †). Fig. 4d shows the Raman spectra of the WS 2 crystals before and aer coating the encapsulation layer. The as-synthesized WS 2 crystals on SiO 2 /Si substrate shows characteristic Raman shi at $356 and $423 cm À1 corresponding to E 2g mode for in-plane vibration of tungsten and sulfur atoms and A 1g mode for out-of-plane vibration of sulfur atoms, respectively. Although, the Raman shi does not show any considerable change from the original sample, the intensity of Raman peak show reduction with increasing thickness of encapsulation layer. The structure and the chemical nature of the WS 2 crystals is virtually unaffected by carbonaceous coating as the Raman vibration modes are at the same frequency for as synthesized as well as coated samples. Raman analysis also does not show substantial evidence of strain, while signicant reduction and shi of PL peak were evident. 36,39,46,47 Fangfei Li et al. has reported only small shi in Raman peaks with applying a signicantly high pressure (0.84 GPa) to induce a compressive strain. 46 The encapsulation behavior of WS 2 crystals with the plasma deposited carbonaceous lm was investigated by accelerated heating test in atmospheric condition. Fig. 5a shows FE-SEM image of a pristine triangular WS 2 crystals aer heating in atmosphere at 200 C for 1 hour. White traces and their branched structures appear thought-out the crystals, signifying the heat-induced degradation of the MX 2 layer under atmospheric conditions. Fig. 5b shows FE-SEM image of the encapsulated WS 2 crystals aer the same accelerated heating test. The triangular WS 2 crystals remain intact, however, it is difficult to identify any kind of traces as observed for the uncoated sample due to the presence of the encapsulation layer. PL analysis was carried out to investigate the encapsulation effect of the WS 2 crystals before and aer the heating of the samples. Fig. 5c shows PL spectra for the uncoated and encapsulated WS 2 samples. The intensities were normalized for maximum value to obtain a clear idea of the relative changes in PL. A sharp PL peak at 655 nm (1.89 eV) was obtained for the pristine WS 2 crystals. Signicant difference in PL peak was observed with coating a carbonaceous layer on the WS 2 crystals. Coating a thin encapsulation layer ($15 nm) on the WS 2 crystals, decrease in PL intensity and red shi (shied to 662 nm corresponding to 1.87 eV) was observed (Fig. 5c). By increasing the thickness of the encapsulation layer ($30 nm), we observed a much reduced PL peak with the similar red shi. It conrms the inuence of the carbonaceous lm thickness on the PL excitation as well as the emitted light. Then, PL analysis was performed for the coated and uncoated WS 2 samples aer heating at 200 C in atmospheric condition. Fig. 5d shows the normalized PL spectra for the heating tested samples. Aer the heating process, both the encapsulated WS 2 samples show similar intensity PL peaks with a peak position at 650 nm (1.90 eV). The PL peak of the uncoated WS 2 crystals was reduced due to the degradation under atmospheric heating as observed from the SEM analysis. Interestingly, the PL peak position of the coated sample slightly blue shied ($5 nm) and signicantly differed prior to the heating ($12 nm). The effect is complicated to understand, as different process can occurs, such as reduction in internal absorption of emitted light and change in induced strain. 48 Then, the coated samples were further tested by heating at a higher temperature (250 C) in atmospheric condition.  sample aer the heating experiment. We observed crack formation in some of the WS 2 crystals aer the heating process. Fig. 6d shows a higher magnied view of the coated WS 2 crystals with clearly visible cracks. The cracks appeared in the WS 2 crystals, which propagate in hexagonal patterns, with nearly 120 angles. This signies localized heat induced defect enhancement in the individual encapsulated WS 2 crystals. 36 Signicantly, we observed the appearance of the cracks only in few WS 2 crystals, where all other crystals remain unaffected. The reason behind this anomaly can be attributed to the difference in defect level of individual crystals and variation in induced strain with thermal expansion differences. It can be expected that better thermal conductivity of the TMDCs crystals will lead to relatively higher local temperature of WS 2 than the other areas of carbonaceous encapsulated sample. Previously, Yan et al. has reported the local heat induced thermal expansion properties of the TMDCs layer. 40 The effect of encapsulation layer on electrical and optical properties of MoS 2 was studied by Schottky junction fabrication. Fig. 6a shows the schematic diagram of the Au/MoS 2 /Au device with the carbonaceous layer. The distance between two Au electrodes was approximately 200 mm covering MoS 2 layer in between as shown in the optical microscope image (Fig. 6b). I-V characteristics of the device was measured with (white light, $100 mW cm À2 ) and without light illumination conditions. Fig. 6c shows I-V characteristic under dark and light conditions of the as-fabricated device with plasma deposited encapsulated layer. Nonlinear Schottky junction behavior was obtained for the semiconducting MoS 2 layer and Au metal electrode, where the encapsulated layer did not show much inuence on junction behavior. With illumination of light, we observed signicant photo-response for Schottky junction with photo-exciton in the MoS 2 layer. Then, the device was annealed at 200 C under atmospheric condition for 1 h as discussed above. Fig. 6d shows I-V characteristic of the same device aer annealing. The Schottky junction behavior and photo-responsivity was unchanged and no hysteresis was observed with the annealing in presence of the encapsulating layer. Thus, the effectiveness of carbonaceous encapsulating layer on electrical and photoresponsive properties of MoS 2 layer was conrmed.

Conclusion
In conclusion, we have demonstrated deposition of a carbonaceous lm at room temperature on MoS 2 and WS 2 crystals by the SW-MWP CVD technique as an encapsulation layer. The encapsulation process is dry, rapid and nondestructive, which can be carried out at room temperature for large area substrate. PL analysis showed the inuence on excitation as well as the emission light with the coating of a carbonaceous lm. The encapsulated samples remained intact as observed by the optical microscopy, SEM and PL analysis in an accelerated degradation test by heating in ambient condition (humidity $65%). We also observed change in PL emission for the encapsulated WS 2 layer aer heating process, attributing to reduction in internal absorption of emitted light and change in induced strain. We have also demonstrated obtaining stable photoconductivity for the TMDCs dichalcogenide layers with encapsulation of a carbonaceous lm. The rapid and dry coating technique at room temperature can be extended to various other 2D materials as MW-SWP process allows damage free encapsulation.

Conflicts of interest
There are no conicts to declare.