Impact of crystallinity on thermal conductivity of RF magnetron sputtered MoS2 thin films

This study investigates the effects of sulfur atomic defects and crystallinity on the thermal conductivity of MoS2 thin films. Utilizing scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and Raman spectroscopy, we examined MoS2 films, several nanometers thick, deposited on Si/SiO2 substrates. These films were prepared via a combination of RF magnetron sputtering and sulfur vapor annealing (SVA) treatment. Structural analyses, including cross-sectional STEM and in-plane and out-of-plane XRD measurements, revealed an increase in the S/Mo ratio and grain size of the MoS2 films following SVA treatment. Notably, the in-plane thermal conductivity of MoS2 films treated with SVA was found to be at least an order of magnitude higher than that of films without SVA treatment. This research suggests that the in-plane thermal conductivity of MoS2 thin films can be significantly enhanced through crystallinity improvement via SVA treatment.


Introduction
Transition metal dichalcogenides (TMDCs) are a class of two-dimensional (2D) semiconductor materials characterized by a layered structure, 1) high mobility, 2) mechanical flexibility, 3,4) and permeability. 4)They exhibit superior thermoelectric conversion characteristics compared to conventional materials such as Bi 2 Te 3 . 5)Molybdenum disulfide (MoS 2 ), a prominent TMDC, has been extensively researched for applications in various electronic and optical devices, including transistors 6,7) and photodetectors. 8,9)Recent studies have focused on developing thermoelectric conversion devices utilizing MoS 2 's exceptional characteristics. 10,11)For high-performance thermoelectric devices using MoS 2 , enhancing the area and uniformity of MoS 2 thin films is crucial.RF magnetron sputtering and CVD are established methods for fabricating large-area, nanometer-scale MoS 2 thin films.][14] Mortazavi et al. studied the impact of grain boundaries and crystallite size on the thermal properties of CVD-derived MoS 2 using nonequilibrium molecular dynamics (NEMD) simulations. 12)Wang and Tabarraei theoretically demonstrated that molybdenum and sulfur vacancies in MoS 2 nanoribbons reduce thermal conductivity, as determined through reverse NEMD simulations. 14)However, to our knowledge, the effects of grain boundaries, crystallite sizes, and atomic defects on the thermal conductivity of MoS 2 thin films have not yet been investigated experimentally.Given the recent trend toward miniaturization and integration of thermoelectric devices, experimental insights into the thermal conductivity of MoS 2 thin films are essential.
Sulfur vapor annealing (SVA), a process involving the encapsulation of MoS 2 thin films with sulfur powder in a quartz tube under argon flow and subsequent heating, has been recognized as a method to enhance sulfur atom integration and crystallinity in MoS 2 thin films. 2,15)To evaluate the thermal conductivity of MoS 2 thin films with varying structures, we employed a combination of RF magnetron sputtering and SVA, preparing MoS 2 thin film samples both with and without SVA treatment.This study comprehensively investigates the effects of grain boundaries, crystallite sizes, and atomic defects on the thermal conductivity of MoS 2 thin films through structural analysis using cross-sectional scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD), alongside thermal conductivity analysis via the optothermal Raman (OTR) method.

Experimental method
2.1.Method for preparing MoS 2 thin films MoS 2 thin films were deposited on Si/SiO 2 substrates using RF magnetron sputtering.Prior to sputtering, the Si/SiO 2 substrates were cleaned using a sulfuric acid and hydrogen peroxide mixture to eliminate surface contamination.This cleaning process, known as SPM cleaning, prepared the substrates for deposition.Subsequently, the cleaned Si/SiO 2 substrates were placed in the deposition chamber.The MoS 2 thin films were then deposited with a target-to-substrate distance of 150 mm and an argon flow rate of 7 sccm, maintaining a partial pressure of 0.55 Pa.This study aimed to produce MoS 2 thin films with varying degrees of crystallinity.To achieve this, the sputtering power was alternated between 20 and 80 W. The substrate temperature was maintained at room temperature for films produced at 20 W sputtering power.For films sputtered at 80 W, the substrate temperature was maintained at 300 °C.Additionally, SVA-treated samples were also prepared.For the SVA process, an as-deposited (As-depo) MoS 2 thin film sample was placed in a quartz tube with sulfur powder under an argon flow of 100 sccm at a pressure of 100 Pa.The sample was then annealed at 700 °C for 60 min.

Measurement conditions
STEM observation and electron energy loss spectroscopy (EELS) measurements were conducted using an aberrationcorrected scanning transmission electron microscope (JEM-ARM300F, JEOL).To minimize sulfur atom sputtering due to electron beam exposure, 16) the acceleration voltage was set to 80 kV.The electron probe's convergence semiangle and probe current were set at 24 mrad and ∼50 pA, respectively.EELS spectra were collected using a GIF Quantum ER imaging filter system, with the energy dispersion set at 0.4 eV/ch.Cross-sectional STEM samples were prepared using a focused ion beam system, while plan-view STEM samples were prepared through mechanical polishing and Ar ion milling.
2D XRD, in-plane XRD, and X-ray reflectivity (XRR) measurements were performed using a Rigaku SmartLab with a 9 kW Cu source and a HyPix-3000 2D detector.For 2D XRD, the beam was collimated to 0.5 mmj.A 2D diffraction image centered in the out-of-plane direction was obtained through a 2D time-delay integration measurement, with the detector positioned approximately 150 mm from the sample.In-plane XRD measurements utilized an X-ray beam shaped to 0.1 mm × 10 mm, irradiated at an angle of 0.2°, and the measurements were performed in zero-dimensional mode with a 0.5°solar slit.XRR measurements aimed to determine film thickness via symmetrical scanning of a 0.05 mm × 10 mm wide beam with a 0.25 mm wide receiving slit.
Raman spectroscopy was conducted using a Nanophoton Raman-11 system.All spectra were acquired with a 20x objective lens (numerical aperture of 0.40) and a 488 nm laser wavelength under atmospheric conditions.A 2400 line/mm grating was used to capture the A , 1g E , 2g 1 and Si (LO) peaks of MoS 2 in the same wavenumber region.
[19][20][21][22] OTR calculates these parameters by analyzing the dependence of a sample's Raman peak on sample temperature and laser spot radius.Gertych et al. utilized xy-mapping measurements of OTR on polycrystalline MoS 2 thin films prepared by liquid exfoliation to calculate the thermal conductivity and interfacial thermal conductance. 17)Because of its potential to evaluate thermal conductivity and interfacial thermal conductance with high accuracy by considering sample inhomogeneity, we adopted this method to investigate the thermal conductivity of MoS 2 thin films.For xy-mapping scanning, measuring points were set at 5 μm intervals across a 50 μm × 50 μm area on the sample surface, collecting Raman spectra from 100 points.The laser exposure time was set to 1 s for each condition and measuring point, where the sample temperature and the laser spot radius were varied.The sample temperature was controlled using a DSC600 temperature variable stage (Linkam Corporation).Sample temperatures varied from 298 to 407 K, corresponding to stage temperatures between 303 and 423 K, adjusted in 30 K intervals and verified using a type K thermocouple.Measurements were conducted at least 15 min after temperature adjustments to ensure stable sample temperatures.The laser spot radius on the sample surface was varied by adjusting the distance between the sample surface and the laser focus position.The laser spot radius was measured using the knife-edge method and corresponded to the distance from the laser focus position to the sample surface. 23)The laser intensity for these measurements was measured using an Ophir StarLite with a 3 A thermopile sensor.The shift in the A 1g peak position caused by laser irradiation could affect the accuracy of thermal conductivity and interfacial thermal conductance measurements.Minimizing sample damage due to laser irradiation was essential to maintain measurement accuracy.Following Gertych et al.'s approach, 17) we monitored the A 1g peak's stability under continuous laser exposure to determine an optimal laser power condition.A variation within 0.1 cm −1 over 60 s of continuous irradiation (considered negligible for thermal conductivity and interfacial thermal conductance values) was deemed acceptable.Given the total irradiation time of 40 s at the same measurement position, we selected a laser power condition that maintained peak position variation within this range.

Results and discussion
Cross-sectional bright-field STEM (BF-STEM) images were used to examine the nano-structures of MoS 2 sputtered films [Fig.1(a)].In samples prepared at 20 W with SVA and those prepared at 80 W, both As-depo and with SVA, a distinct layered structure spanning several tens of nanometers was observed horizontally relative to the Si/SiO 2 substrate.This structure comprised nanometer-sized polycrystalline grains with the c-axis of each grain oriented perpendicular to the substrate.The 20 W As-depo sample did not exhibit a layered structure, suggesting its amorphous nature.The increased crystallinity in the 80 W As-depo sample compared to the 20 W As-depo sample might be attributed to the higher substrate temperature (300 °C versus room temperature), which facilitated crystal growth and enhanced the crystallinity of the MoS 2 thin film. 15)In contrast to the lowcrystallinity 20 W As-depo film, the 20 W SVA film exhibited a pronounced layered structure, attributed to the SVA treatment's role in remedying sulfur defects and improving crystallinity. 15)While both 80 W As-depo and 80 W SVA films exhibited clear layered structures, the 80 W As-depo sample showed noncrystalline areas around the MoS 2 film, indicating the presence of amorphous components.Similar to the comparison between the 20 W As-depo and 20 W SVA films, the 80 W SVA sample demonstrated sulfur defect rectification and enhanced crystallinity following SVA treatment.
Figure 1(b) presents a plan-view STEM-ADF (annular dark-field) image of an MoS 2 sputtered film following SVA treatment, providing insights into the in-plane structure.Given that ADF imaging facilitates the layer count in 2D materials, 24) the image revealed thickness inhomogeneity in the film.The grain size varied from several to 10 nm, suggesting abundant grain boundaries.The presence of different moiré patterns in the two-layer regions indicated random stacking patterns.
STEM-EELS elemental mappings were conducted to assess the impact of SVA treatment on the composition of MoS 2 sputtered films.I).The S/Mo ratio improvement following SVA treatment, regardless of sputter power, reaffirms that SVA treatment effectively remedies sulfur defects in MoS 2 sputtered films. 15)n-plane and out-of-plane XRD measurements were conducted to evaluate the crystallinity changes in MoS 2 sputtered films before and after SVA treatment [Figs.2(a     The spectra were calibrated, and the intensity was normalized using the Si Raman peak at 520 cm −1 .The inset of Fig. 2(c) magnifies the Raman spectra of the 20 W As-depo sample by a factor of 100.Peaks at ∼380 cm −1 and ∼410 cm −1 correspond to the E 2g 1 and A 1g vibrational modes of MoS 2 , respectively.These peaks were clearly identifiable in the 20 W SVA, 80 W As-depo, and 80 W SVA samples.Despite the amorphous structure indicated by the STEM-BF image in Fig. 1(a), the E 2g 1 and A 1g peaks were not clearly observable in the 20 W As-depo sample (see inset).The detection of MoS 2 (100) and (110) peaks in the 20 W As-depo sample's in-plane XRD spectra suggests the presence of some MoS 2 polycrystalline grains.The broadness of the E 2g 1 and A 1g peaks in the 80 W As-depo is attributed to sulfur defects and low crystallinity. 15)The intensities of these peaks increased after SVA treatment in both 20 W and 80 W sputtered films, indicating sulfur defect rectification and crystallinity enhancement.Peak positions and FWHMs of the E 2g 1 and A 1g peaks were determined through peak fitting of the Raman spectra presented in Fig. 2(c) and are detailed in Table I.Notable red-and blue-shifts of the E 2g 1 and A 1g peaks were observed after SVA treatment, alongside decreases in their FWHMs.These changes in the E 2g 1 and A 1g peaks suggest an enhancement in the crystallinity of the MoS 2 sputtered films due to SVA treatment. 15)By analyzing the peak intensities  055508-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd and FWHM of these peaks, the crystallinity of the MoS 2 sputtered films appears to follow the order 20 W SVA > 80 W SVA > 80 W As-depo > 20 W As-depo.This order is in alignment with the XRD results.It is hypothesized that the 80 W As-depo film exhibits higher crystallinity than the 20 W As-depo film because the sputter source imparts higher energy to the substrate at a higher sputtering power.Interestingly, despite both the 20 W SVA and 80 W As-depo films showing an S/Mo ratio of ∼1.5 (as indicated in Table I), the 20 W SVA film demonstrated superior crystallinity.
Subsequently, the study focused on the OTR measurements to investigate the effects of changes in crystallinity induced by SVA treatment on the thermal properties of the MoS 2 sputtered films.The OTR measurements were primarily centered on the A 1g peak due to its higher intensity and more symmetrical nature compared to the E 2g 1 peak.For brevity, the results for 80 W SVA, which exhibited a pronounced intensity of the A 1g peak, are presented.The thermal properties of the 20 W As-depo film were not evaluated due to the low intensity of its Raman peak.Further details on 20 W SVA and 80 W As-depo are provided in Supplementary Data S.1.
Figure 3(a) illustrates the spatial distribution of the A 1g peak position obtained through mapping measurements, varying sample temperature and laser intensity.The sample temperature refers to the temperature measured in the absence of laser irradiation.Figure 3(b) shows the average Raman spectra from 100 pixels of mapping, acquired under a laser intensity of ∼0.12 mW in Fig. 3(a).The A 1g peak position is observed to continuously red-shift with increasing sample temperature, attributable to lattice vibration relaxation. 27)igure 3(a) shows the red-shift of the A 1g peak with increasing sample stage temperature and laser intensity.Figure 3(c) presents the average value of the A 1g peak position from 100 pixels obtained by the mapping measurement in Fig. 3(a) (the error bar represents the standard deviation).The shift in the A 1g peak position was approximately the same under all temperature conditions.We extrapolated the fitted line and determined the intercept, which corresponds to the A 1g peak position without laser heating, as shown in Fig. 3(d).This plot illustrates the sample temperature dependence of the A 1g peak position in the absence of laser heating, with the slope ( T c ) of the plot calculated as 0.0148 0.0002 cm K .
T T shows the spatial distribution of the A 1g peak position, determined through mapping measurements that varied the laser spot radius and intensity.The color scale represents the A 1g peak position.Figure 4(b) illustrates the average Raman spectra from these measurements, conducted at a laser intensity of ∼0.24 mW.As indicated in Fig. 4(b), the A 1g peak position blue-shifted with an increasing laser spot radius and intensity.A smaller laser spot radius correlates to a higher laser power density on the sample, thereby facilitating a more pronounced local temperature rise in response to laser intensity changes.Figure 4(a) clearly demonstrates the red-shift of the A 1g peak with a decreasing laser spot radius and increasing laser intensity.Figure 4(c) plots the average A 1g peak position from 100 pixels, as determined in the mapping measurement in Fig. 4(a), with error bars indicating standard deviation.Extrapolating the straight lines for each laser spot radius r 0 in Fig. 4(c) yielded the slope of the fitted line.The results were 6.58 0.09 cm mW ), and 5.1 ). Supplementary Data S.1 includes results for 20 W SVA and 80 W As-depo.The value P c and T c in this study align closely with those in previous research, 27) supporting the validity of these results.
Figure 4(d) presents the derivative of the sample temperature (T ) with respect to the laser power absorbed by the MoS 2 sputtered film (P abs ), calculated using Eq. ( 1) for three different spot radii r .0 where a and w represent the absorbance and the A 1g peak position of the MoS 2 sputtered film, respectively.An a value of 28% for 488 nm was used, based on previous research on MoS 2 thin films sputtered by the RF magnetron sputtering method. 26)The thermal conductivity were determined by ensuring that the theoretical thermal resistance (R m ) from the heat conduction equation (see Supplementary Data S.2) matched the value for each spot radius condition.
The calculated values of s k and g for 20 W SVA and 80 W Asdepo are summarized in Table I.Laser irradiation increases the temperature of both the MoS 2 thin film and the underlying Si/SiO 2 substrate.Here, the temperature of the MoS 2 thin film (T ) and the Si/SiO 2 substrate (T a ) are expressed as variables through the temperature difference T T a q = -( )to solve the heat conduction equation (see Supplementary Data S.2).Consequently, the temperature increase in the Si/SiO 2 substrate is implicitly accounted for in the analytical results for the thermal conductivity ( s k ) and the interfacial thermal conductance (g).In a previous study [Ref.19], the finite element method was employed to compare scenarios: one where the temperature variation of the Si/SiO 2 substrate beneath the MoS 2 thin film was explicitly considered and another where it was not.The findings indicated that in regions where the interfacial thermal conductance (g) is low, there is no significant difference in the thermal conductivity ( s k ) of MoS 2 .The values of g derived in this study range from 0.04 to 0.29 MW m K , 2 /( • ) which is very low, suggesting that the derived values of MoS 2 's thermal conductivity are highly reliable.
The 20 W SVA film exhibited a thermal conductivity approximately an order of magnitude higher than that of the 80 W As-depo film despite both having similar S/Mo ratios of around 1.5.This significant difference in thermal conductivity, despite nearly identical S/Mo ratios, is attributed to the marginally larger crystallite size in the SVA-treated samples and their enhanced crystallinity.Thermal conductivity is significantly influenced by the mean free path (MFP) of phonons.In MoS 2 , the MFP of low-frequency phonons is approximately 100 nm, while that of HF phonons is reported to be less than 10 nm. 28)Typically, low-frequency phonons are the major contributors to thermal conductivity.The crystallite size in both films is approximately 10 nm, and the presence of grain boundaries considerably impacts their thermal conductivity. 29)Additionally, point defects lead to interactions between low-and HF phonons, resulting in reduced thermal conductivity. 30)Therefore, in the lowercrystallinity 80 W As-depo film, the decrease in thermal conductivity is likely due to a combination of point defects and other factors.The 80 W SVA film, with a similar in-plane crystallite size as the 20 W SVA film, demonstrated the highest thermal conductivity, at 44 ± 14 W/(m•K).This value significantly exceeds the previously reported range for polycrystalline MoS 2 films [0.27 to 2.0 W/(m•K)] 13,17) and is comparable to that of high-crystallinity MoS 2 flakes prepared on Si/SiO 2 substrates via mechanical exfoliation [52-67 W/(m•K)]. 19,21,31)This enhanced thermal conductivity can be attributed to the substantially lower sulfur defect density in the MoS 2 film, which has been shown to induce phonon scattering in theoretical studies. 32)Comparing the properties of the 20 W SVA film and the 80 W SVA film, sulfur defects are though to play a more pivotal role in influencing phonon scattering than grain boundaries do.Therefore, addressing sulfur defects might be more crucial than increasing crystalline size in MoS 2 sputtered films to enhance thermal conductivity.The MoS 2 samples studied here comprise several monolayers, with the out-of-plane lattice spacing in the 80 W SVA film reduced by approximately 10%.This reduction likely strengthens van der Waals interactions, leading to enhanced out-of-plane thermal conductivity and, consequently, overall improved thermal conductivity.It has been reported that the out-of-plane thermal conductivity of MoS 2 decreases with an increase in the twist angle of in-plane grains. 33,34)However, as shown in Fig. 1(b), the grains and layers in our samples appear randomly oriented vertically, suggesting a percolative connection between each grain and layer, which may also contribute to the observed thermal conductivity.][37][38][39] A wide range is attributable to several factors, including the number of layers in the film, the degree of crystallinity in both the film and the underlying substrate, and the characteristics of the interface.Even in cases where the substrate exhibits low crystallinity, such as amorphous, high interfacial thermal conductance has been reported. 40,41)This phenomenon is often attributed to a considerable overlap in the density of states of the phonon vibrational modes between the thin film and the substrate, which are crucial for interfacial thermal conduction.Interestingly, 80 W SVA films display a higher degree of crystallinity compared to both 20 W SVA and 80 W As-depo films.Despite this enhanced crystallinity, these films exhibit lower interfacial thermal conductivity when interfaced with an amorphous SiO 2 substrate.This reduction in conductivity might be due to a smaller overlap of the relevant vibrational modes that facilitate interfacial thermal conduction.However, further investigation is essential to elucidate the details of the interfacial states.

Conclusions
In conclusion, our study successfully evaluated the impact of crystallinity on the thermal conductivity of MoS 2 sputtered films through comprehensive structural analysis methods, including XRD, STEM, and OTR spectroscopy.SVA treatment was found to effectively remedy sulfur defects in the films, leading to a significant improvement in macroscopic crystallinity by increasing the size of polycrystalline grains.This improvement in crystallinity directly correlates with a substantial increase-of at least an order of magnitude-in the in-plane thermal conductivity of the sputtered films.Notably, the rectification of sulfur defects emerges as a key factor in enhancing in-plane thermal conductivity.Our results highlight the critical role of improved crystallinity of MoS 2 thin films in achieving high in-plane thermal conductivity.Our measurements and data analysis were technically Figure 1(c) compares molybdenum and sulfur maps, clearly showing an increased sulfur concentration in the 20 W SVA sample, while sulfur was nearly absent in the 20 W As-depo sample.The contrast in the ADF image primarily reflects the distribution of Mo atoms, as the ADF signal is approximately proportional to the square of the atomic number. 25)Figure 1(d) shows the S/Mo ratios derived from the cross-sectional EELS mappings in Fig. 1(c).The S/Mo ratio for the 20 W As-depo sample was 0.0, indicating a lack of sulfur in the sputtered film.However, the S/Mo ratio for the 20 W SVA sample improved to ∼1.5.While the S/Mo ratio for the 80 W As-depo sample was 1.5, it increased to 1.8 in the 80 W SVA sample, nearing the stoichiometric ratio of pristine MoS 2 (S/Mo ratios for each sample are listed in Table ) and 2(b)].In-plane XRD measurements distinctly revealed MoS 2 (100), (110), and (200) peaks in the 20 W SVA, 80 W As-depo, and 80 W SVA samples.However, for the 20 W As-depo sample, while MoS 2 (100) and (110) peaks were marginally observed,

Fig. 1 .
Fig. 1.(a) Cross-sectional STEM-BF images.(b) Plan-view STEM-ADF image of a MoS 2 sputtered film after SVA treatment.(c) Cross-sectional STEM-ADF image and EELS elemental maps of molybdenum and sulfur.(d) S/Mo ratios.

the MoS 2 (
200) peaks were scarcely detectable.In-plane XRD peak intensity increased and full width at half maximum (FWHM) decreased following the SVA treatment for both 20 W and 80 W sputtering powers.Out-of-plane XRD measurements exhibited a trend similar to that of the in-plane XRD results.The MoS 2 (002) peak was prominently detected in the 20 W SVA, 80 W As-depo, and 80 W SVA samples, while it was not clearly discernible in the 20 W As-depo sample.The distinct MoS 2 (002) peaks indicate that the c-axis of MoS 2 polycrystalline grains is oriented perpendicular to the substrate plane, aligning with the STEM-BF image observations in Fig.1(a).2,26)Following SVA treatment, an increase in out-of-plane XRD peak intensity and a decrease in FWHM were observed for both sputtering powers.The in-plane and out-of-plane crystallite sizes derived from XRD measurements are summarized in TableI(the out-ofplane crystallite size of the 20 W As-depo sample could not be calculated due to weak XRD peak intensity).Notably, the in-plane crystallite size of the MoS 2 film sputtered at 20 W increased 2.3 times after SVA treatment.For the film sputtered at 80 W, the in-plane and out-of-plane crystallite sizes increased by 1.4 and 1.5 times, respectively, following the SVA treatment.This enlargement in crystallite size corresponds to the observed changes in film structure following SVA treatment, where a distinct layered structure became evident.Based on in-plane crystallite size, the crystallinity trend of the sputtered films is estimated as 20 W SVA > 80 W SVA > 80 W As-depo > 20 W Asdepo.The out-of-plane crystallite size comparison was not considered due to potential influences from the thickness of the sputtered film.

Figure 2 (
c) presents the Raman spectra of MoS 2 sputtered films (20 W As-depo, 20 W SVA, 80 W As-depo, and 80 W SVA), acquired at room temperature on the sample surface.

Fig. 2 .
Fig. 2. (a) In-plane and (b) out-of-plane XRD spectra of 20 W As-depo/SVA and 80 W As-depo/SVA. (c) Raman spectra averaged from 100 pixel of data acquired in a 10 × 10 μm 2 area on the sample surface using xy-mapping mode.

Fig. 3 .
Fig. 3. OTR mapping measurement exploring the sample temperature effect in 80 W SVA films.(a) Mapping of the A 1g peak position for 100 pixels in a 10 × 10 μm 2 area.(b) Average Raman spectra for 100 pixels.For clarity, the vertical axis is adjusted with offsets.(c) Average A 1g peak position with linear fits.Error bars represent standard deviations.(d) Sample temperature dependence of the A 1g peak position in the absence of laser heating.

Fig. 4 .
Fig. 4. OTR mapping measurement exploring the effect of laser spot radius in 80 W SVA films.(a) Mapping of the A 1g peak position for 100 pixels in a 10 × 10 μm 2 area.(b) Average Raman spectrum for 100 pixels.For clarity, the vertical axis is adjusted with offsets for each laser spot radius.(c) Average A 1g peak position with linear fits.Error bars represent standard deviations.(d) Derivative of the sample temperature T against laser intensity absorbed by the sample P , abs plotted for different laser spot radii.The red solid line represents the fitted theoretical curve.The red dotted lines correspond to the theoretical curves with g values of 0.03 MW m K

Table I .
Results of structural analysis of MoS 2 sputtered films.
©2024The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd