Work Function Modulation of Molybdenum Disulfide Nanosheets by Introducing Systematic Lattice Strain

Tuning the surface electronic properties of 2D transition metal dichalcogenides such as Molebdenum disulfide (MoS2) nanosheets is worth exploring for their potential applications in strain sensitive flexible electronic devices. Here in, the correlation between tensile strain developed in MoS2 nanosheets during swift heavy ion irradiation and corresponding modifications in their surface electronic properties is investigated. With prior structural characterization by transmission electron microscopy, chemically exfoliated MoS2 nanosheets were exposed to 100 MeV Ag ion irradiation at varying fluence for creation of controlled defects. The presence of defect induced systematic tensile strain was verified by Raman spectroscopy and X-ray Diffraction analysis. The effect of ion irradiation on in–plane mode is observed to be significantly higher than that on out-of-plane mode. The contribution of irradiation induced in-plane strain on modification of the surface electronic properties of nanosheets was analyzed by work function measurement using scanning Kelvin probe microscopy. The work function value is observed to be linearly proportional to tensile strain along the basal plane indicating a systematic shifting of Fermi surface with fluence towards the valence band.

the system which on the other hand can affect the surface electronic properties. However, there is no experimental result reported till date regarding the tuning of work function of MoS 2 nanosheets with lattice strain.
We, here, report for the first time the use of SHI irradiation in introducing controlled tensile strain in MoS 2 nanosheets to modify its work function in a systematic manner. Tuning the WF of MoS 2 has great potential for achieving optimized performance in flexible electronic devices. In this work MoS 2 nanosheets have been synthesized using sono-chemical exfoliation method, which involves ultrasonication of MoS 2 flakes in organic solvent (DMF), followed by centrifugation. These nanosheets were characterized initially by different spectroscopy techniques such as Transmission electron microscopy (TEM) and Field Emission Scanning Electron Microscopy (FESEM) for the analysis of surface morphology. To investigate the optical properties of the nanosheets Raman spectroscopy has been employed. Scanning Kelvin probe microscopy (SKPM) provided the WF measurement. SHI beams are expected to change the surface potential of the system. This change in the surface potential of the system significantly affects the surface electronic properties of the material. A distinct relation has been observed between lattice strain and electronic disorder in MoS 2 . This work will also be of immense importance for further theoretical study of strain related electronic properties of few layer TMDs where Density Functional Theory (DFT) calculation gets complicated as inter-layer van-der-Waal force has to be taken into account.

Results and Discussion
Crystallinity of suspended MoS 2 nanosheets (placed on TEM grid) was confirmed from TEM studies. TEM image of MoS 2 nanosheets are presented in Fig. 1(a). Electron diffraction of nanosheets shows the atomic structure of planes present in MoS 2 as shown in the inset of Fig. 1(a). High resolution TEM image showing the atomic structure of MoS 2 is presented where a clear view of lattice fringes is shown in Fig. 1(b). The lattice spacing is of the order of 0.283 nm, which corresponds to (100) plane of MoS 2 21 .
Layer morphology of pristine MoS 2 is also confirmed from its FESEM image as shown in Fig. 1(c). Figure 1(d) exhibits surface morphology of MoS 2 sheets irradiated at 5 × 10 13 ions.cm −2 . Formation of extended defects in MoS 2 nanosheets in the form of latent tracks is clearly visible from FESEM image. Effect of irradiation induced disorders can be well reflected from modification in vibrational spectra of MoS 2 nanosheets using Raman spectroscopy.
Raman spectra of pristine as well as irradiated MoS 2 nanosheets were recorded at five different spots covering almost full area using 532 nm laser excitation source with average power ~100 μW. Figure 2(a) shows Raman spectra of pristine and irradiated MoS 2 nanosheets. The peak intensities have been obtained by fitting the peaks assuming Lorentzian distribution. Two prominent characteristic modes (E 1 2g and A 1g ) are observed around 400 cm −1 (Fig. 2a). The in-plane E 1 2g mode corresponds to opposite vibration of two S atoms with respect to Mo atom. Whereas A 1g mode arises due to the out of plane vibration of only S atoms in opposite direction 22 . The other modes like (E 1g and E 2 2g ) of MoS 2 could not be detected due to selection rule for scattering geometry (E 1g ) 1 of the spectrometer or because of the limited rejection of the Rayleigh scattered radiation (E 2 2g ) 2 23 However, in irradiated nanosheets, in addition to E 1 2g and A 1g peaks, another Raman (LA) peak at 235 cm −1 corresponding to defects is also detected (Fig. 2b). The presence of this mode in irradiated MoS 2 nanosheets corresponds to defects generated by SHI irradiation 24 . The sharp increase in peak intensity indicates the increase in number of defects with fluence. We have calculated the average inter defect distance where, σ is the density of ions impinging on the surface given by σ = it/Ae, here i is the ion current, 't' is the exposure time to the ion beam, ' A' is the rastered area and 'e' is the elementary charge 24 . L D decreases from 14 nm to 1.4 nm for fluence from 5 × 10 11 to 5 × 10 13 ions.cm −2 .
The vacancies created per ion calculated Using SRIM 2008 Monte Carlo simulation for 100 MeV Ag ion on MoS 2 target is found to be 106. Position of the E 1 2g and A 1g peaks are plotted against the fluence in Fig. 3(a). In both the cases as compared to pristine MoS 2 nanosheets, a systematic red shift for both E g 2 1 and A 1g modes were observed with increasing fluence and the magnitude of peak shift is more pronounced at higher fluences. This happens due to stretching of bond length which reduces the effective force between the atoms and hence the reduction of force constant between them leading to reduced frequency of vibration. This nature of peak shift rules out the possibility of charge transfer as the primary cause and point out the presence of tensile strain 24,25 leading to the increase in the bond length with fluence. The evolution of Raman spectra of MoS 2 with strain is investigated from the variation of full width at half maximum (FWHM) with fluence as shown in Fig. 3(b). Lorentzian fitting of the peaks observed in Raman spectra provides information about the structural disorder via FWHM, thus indicating strain related variations. The broadening of two peaks was observed with increasing fluence which may be attributed to the lattice defects.
From the strain-induced shift of Raman-active phonon mode, we can measure strain using Raman Spectroscopy. Tensile strain can be expressed by the Gruneisen parameters (γ) which uniquely depends on the material. Gruneisen parameter is a remarkable physical quantity, which is related to the Raman shifts of atoms in a material in the microscopic field 21 . The strain can be calculated using the following formula 21,26 .
where, ω 0 and ω are the initial and shifted wave number respectively. We have taken γ E ( ) g 2 1 = 0.21 and γ (A 1g ) = 0.42 27,28 . As previously discussed, both E g 2 1 and A 1g modes in MoS 2 are very sensitive to fluence of ion beam irradiation. In our case, we observed maximum E g 2 1 peak shift of 5.1 cm −1 and A 1g peak shift of 3.06 cm −1 at a fluence of 5 × 10 13 ions.cm −2 as compared to that of the pristine sample. Figure 3c,d shows the variations in strain which is related to the change in the position of E g 2 1 and A 1g modes at different fluences in SHI irradiated MoS 2 nanosheets. The position of both the peaks shifts linearly towards the lower wavenumber indicating the variation of tensile strain along a-as well as c-axis 29 . The in-plane mode under tensile strain gets stretched along a-axis and brings compression mostly along b-axis in the same plane which compensates the compression in the out-of-plane mode in the vertical plane because van der Waals force resists the compression, in the same way while it was under tensile strain 29 . The variation of strain in both A 1g and E g 2 1 mode as a function of fluence is plotted in Fig. 3(e). It is observed that, for A 1g mode the strain is increasing as a function of ion fluence until a maximum possible strain was attained and subsequently, it gets partially relaxed. It can be seen that the effect of ion irradiation on in-plane (E g 2 1 mode) strain is significantly higher than that on out-of-plane (A 1g mode) strain. The difference in their magnitude increases with fluence. This difference can be attributed to the fact that in-plane (E 1 2g ) mode involves vibration of both Mo as well as S atoms in the basal plane while in the out-of plane (A 1g ) mode only S atoms vibrate along c-axis and Mo atoms remains fixed, thus making A 1g mode stiffer as compared to in-plane mode. Our samples being few layers of stacked MoS 2 nanosheets, one can expect the van der Waals interactions to produce more restoring force constant along c-axis causing gradual stiffening of out-of-plane mode and thus it (A 1g mode) gets less affected due to irradiation 30,31 . This bond nature will affect the kind of defects induced in this system due to ion irradiation. It is reported that electron irradiation produces a number of defects (mostly vacancies) like V s , V s2 , V MoS3 , V MoS6 and S2 Mo etc. as maximum formation energy for them is 15 eV per atom in the basal plane of MoS 2 nanosheets 32 . Therefore, it is quite obvious that 100 MeV Ag ion beam having high electron energy loss (S e ) of the order of 17.26 keV/nm will create these kind of vacancies as well as extended defects in MoS 2 in-plane mode resulting in expansion of in-plane Mo-S bond. In addition, normal incidence of ions on MoS 2 nanosheets is expected to result in more number of defects in basal plane rather than along the out-of-plane direction of MoS 2 nanosheets. Even then, high S e value can generate a few number of defects along the out of plane mode which are reflected in increasing variation of A 1g mode with fluence which in due course gets relaxed beyond a fluence of 5 × 10 12 ions.cm −2 , where, the defects start overlapping (Fig. 3e).
To reinforce the observed results in variation of strain with ion fluence, quantification of strain is also done using X-ray diffraction studies. XRD spectra of pristine and 100 MeV Ag ion irradiated MoS 2 thin films at different fluences are shown in Fig. 4. The peak at 14.9° of (002) plane is the most prominent XRD peak of MoS 2 nanosheets 33 . The changes observed in the X-ray diffraction spectra of SHI irradiated samples, are due to the disordering of the original crystalline structure of MoS 2 . This (002) peak (at 14.9°) gets significantly diminished with increase in fluence showing the occurrence of complete amorphization beyond a fluence of 5 × 10 13 ions.cm −2 .
The peak of (002) plane (c-axis) shifts to lower angle, indicating that there is an out of plane expansion, however small, in the irradiated MoS 2 nanosheets. The atomic plane spacing in the MoS 2 nanosheets can be calculated using Bragg law 34 : Here 'd' is the atomic plane spacing, θ is the Bragg angle, λ is the wavelength of incident X-ray and n is the diffraction order. The calculated 'd' value (0.598 nm) for (002) is larger for irradiated sample as compared to the pristine MoS 2 (0.594 nm). The larger d value for (002) plane represents the presence of tensile strain along [001] direction in the irradiated sample 34 . By recording 2θ shifts of MoS 2 (002) peak as a function of fluence and calculating the corresponding d-spacing, the total elastic strain was calculated from the following relation between FWHM (B) and lattice strain (ε) 35 , where, θ is Bragg's diffraction angle. It is not possible to find the in-plane strain in MoS 2 nanosheets, as there exists no peak corresponding to this plane in XRD except (002) plane (Fig. 4). Therefore, we have compared the results obtained from XRD and Raman measurements only for out of plane strain in MoS 2 nanosheets induced due to SHI irradiation. The results from these two methods are strikingly very close to each other as presented in Table 1.

Mechanism of SHI interaction.
The mechanism of defect creation on MoS 2 nanosheets by SHI irradiation needs discussion. It is well reported that an energetic ion passing through matter loses its energy via nuclear energy loss (S n ) and electronic energy loss (S e ) 19 . Values of S e , S n and range (R) of 100 MeV Ag ion irradiation on MoS 2 nanosheets calculated using Monte Carlo simulations (SRIM 2008) were found to be 17.26 keV/nm, 0.101 keV/nm and 9.60 μm respectively. The ion interaction can result in creation of point defects, extended defects and even latent tracks in the target depending on above mentioned ion parameters. Variation of S e and S n with fluence is given in Supplementary information (Fig. S1). At keV energy range, predominant nuclear energy loss results in only point defects on the target. But in case of ions incident at MeV energy range, most part of the energy is lost through electronic excitation process. Normally two basic models such as Coulomb explosion and thermal spike model respectively are used to explain the early and later part of the ion interaction with matter 36 .
On the basis of Coulomb explosion model relaxation of excess energy via radial impulses of atoms lying in the surroundings of ion path is expected to generate outward spreading of +ve ions away from the ion trajectory. These atomic movements give rise to radially expandable stress and strain around the ion track. Thermal spike model explains the relaxation of excess energy via energy transfer from excited electronic system to target atoms in a span of 10 −16 s, thus leading to a local temperature increase followed by local thermalization via heat transfer from electron to lattice between 10 −14 S to 10 −12 s. Dense electronic excitation and efficient electron-phonon coupling can create a defected zone surrounding a ion trajectory termed as latent track. In the present situation, high value of S e (17.26 keV/nm) is expected to create extended defects and even latent tracks in MoS 2 nanosheets. From observed XRD results (Fig. 4) we could expect a complete amorphization of MoS 2 nanosheets along the trajectory at a higher fluence beyond 5 × 10 13 ions.cm −2 . The amorphized region surrounding the ion trajectory is normally termed as latent track. Radius of these latent tracks can be estimated from the fluence dependent variation of any physical property of the nanosheets ( Supplementary Information, Fig. S2). The schematic diagram of the effect of ion interaction on MoS 2 is shown in Fig. 5. For convenience, only top view of the ion interaction with nanosheets   Table 1. Comparison of out-of-plane strain in pristine and irradiated MoS 2 nanosheets obtained from Raman and XRD Techniques.
is illustrated. SHI beams incident at normal angle on nanosheets are expected to generate defects, like vacancies, which are marked as circles in irradiated sample. It is reported that SHI induced defects affect the work function (WF) of the oxide nanomaterials in a proportionate manner 19 . Similar effect can also be expected in SHI irradiated MoS 2 nanosheets. The modification in WF of MoS 2 nanosheets can be correlated to SHI induced strain in it. To further illustrate the WF modulation by strain, we investigated the variation of both tensile strain and WF with fluence. The variation of defect induced strain with fluence normally causes the Fermi level of the material to shift either towards valence band or conduction band 37 . The magnitude of shifting of Fermi level is estimated from the contact surface potential difference (CPD) measurement using SKPM.

CPD t ip sample
where ɸ tip is the known WF of reference Au tip and ɸ sample is the WF of the sample. 'e' is the electric charge. The CPD mappings at different fluences are shown in the Fig. 6. Contour mapping is given in the supplementary information (Fig. S3). With increase in fluence, the CPD value decreases and corresponding WF increases. The measured CPD value for pristine MoS 2 was found to be 561 mV. The CPD value for 100 MeV Ag ion irradiated thin film decreases from 313 mV to −381 mV for fluence varying from 5 × 10 11 to 5 × 10 13 ions.cm −2 . Using equation 4, we find that the WF increases with increasing ion fluence for all irradiated MoS 2 nanosheets. For pristine MoS 2 it is calculated to be 4.53 eV. For increasing fluence in orders of magnitude (from 5 × 10 11 ions.cm −2 onwards), the WF values are 4.78 eV, 5.21 eV and 5.48 eV respectively. To observe the correlation between lattice strain (in-plane and out-of-plane) and induced WF, all of them are plotted against the fluence. Figure 7a shows the variation of WF and out-of-plane strain with fluence. At low fluence up to 5 × 10 12 ions.cm −2 , both of them increases and beyond it there occurs a deviation in variation of out-of-plane strain with fluence having an indication of relaxation. The difference in variation could not establish any proportionate relation between WF and out-of-plane strain. The variation of work function and in-plain tensile strain with respect to the fluence is shown in Fig. 7b where both of them increase with fluence. Shifting of Fermi level towards the valance band is expected from the variation of WF with fluence. Figure 7c gives the conclusive correlation between WF and in-plane strain of MoS 2 where a linear relationship is found to be exist between them. This may be correlated to the roles played by defects generated in the wake of SHI irradiation. Due to the defect induced tensile strains, the separation between atoms gets increased and it causes the orbital overlap to lower down. When the atoms are moved farther apart from one another, the work function of the valence band maximum attains a more symmetrical shape and the location of the valance band maximum changes from K-point to the Г-point as the crystal is stretched 37 . This leads to decrease in valance bandwidth as well as Fermi energy level, which in turn increases the work function of MoS 2 nanosheets.
In conclusion, introduction of controlled tensile strain using 100 MeV Ag ion beam for tailoring the surface electronic property of MoS 2 nanosheets has been demonstrated. Raman and XRD results support the fact. The difference in variation of tensile strain along the in-plane and out-of-plane may be dependent on the nature of Mo-S bonds along a-b plane and c-axis. XRD results indicate the complete amorphization of the latent tracks in MoS 2 nanosheets at higher fluence. The change in work functions of MoS 2 as a result of induced tensile strain is estimated from contact potential difference measurements using scanning Kelvin probe microscopy. The work function of MoS 2 nanosheets is found to be linearly dependent on the tensile strain along the basal plane. Our  results suggest that the modification of the lattice strain by SHI irradiation provides an efficient way to engineer the band structure of MoS 2 . The results reported in this paper are relevant for further development of electronic applications of MoS 2 as well as for device modeling. This work will also open up the way for DFT calculation for strain induced modification of work function of MoS 2 nanosheets.

Methods
Bulk MoS 2 powder (Sigma-Aldrich) was dissolved in DMF (Sigma-Aldrich) solvent at a concentration of 0.5 mg/ ml. The solution was sonicated for 6 h in ultrasonic bath. During the bath sonication process, there occur increase in pressure and temperature due to solvo-dynamic forces. These forces are capable of overcoming the van der Waals force between 2D layers, thus resulting in gradual exfoliation of layers from bulk MoS 2 . After sonication, the suspension was centrifuged at 1000 rpm for 15 min to obtain supernatant for further processing. The solution was drop casted on the silicon substrates. Subsequently, the samples were annealed at 153 °C for 1 hour to evaporate the solvent. The samples were then irradiated with 100 MeV Ag ions at different fluences (5 × 10 11 , 5 × 10 12 , 5 × 10 13 ions.cm −2 ) using the 16 MV Pelletron accelerator at IUAC, New Delhi. Ion irradiations were carried out at normal incidence and room temperature. The pressure in the irradiation chamber was of the order of 10 −6 torr. The average ion current was maintained at 2 particle nano ampere (1pnA = 6.25 × 10 9 ions/s). The ion beam was magnetically scanned for uniform irradiation covering the complete sample surface. The range of the ion beam was ~9.6 micron which is much larger than the thickness of the nanosheets, thus avoiding the possibility of implantation of silver ions on nanosheets.
Materials characterization. The pristine as well as SHI irradiated nanosheets were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Raman Spectroscopy and scanning Kelvin probe microscopy (SKPM) for structural, morphological, optical and surface electronic studies respectively. XRD measurement was done in a PANalytical X'pert pro set up with CuKα radiation (1.54 Å). FESEM images were taken using Tescan Model LYRA 3 XMU. Raman spectroscopic measurements were carried out by using WiTec Model alpha 300 having 532 nm excitation. The work function of pristine and SHI irradiated MoS 2 were obtained using SKPM from KP Technology, United Kingdom. In addition, Transmission Electron Microscope (TEM, JEOL 2100 F) studies were carried out on suspended MoS 2 nanosheets (on TEM grids) before they were deposited on the Si-substrate.