**2. Experimental methods and results**

#### **2.1 RF sputtering**

The Molybdenum di-Sulfide (MoS2) films were fabricated with a high vacuum Kurt J. Lesker© PVD 75 machine; applying RF-sputtering at a rate of 2.26 Å/sec at 275 W of plasma power over 4″-diameter silicon oxide (SiO2) wafers. The films were deposit from commercial MoS2 99.9% targets (Kurt J. Lesker). By using dwell time of 300 seconds a film thickness value of ~100 nm was achieved as indicated by profilometry measurements, **Figure 1E**.

#### **2.2 Scanning electron microscopy**

The film morphology and crystallographic structure were investigated using scanning and high-resolution transmission electron microscopy (SEM, TEM). SEM

**65**

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

with Lince & Fleischauer [20].

*thickness ~ 100 nm for 300 seconds a rate of 2.26 Å/sec.*

**Figure 1.**

was performed in a Hitachi® SU5500 unit, equipped with Energy-dispersive X-ray spectroscopy (EDS) unit and operated at 30 kV with 8A of current to avoid surface damage on the film. Observations indicate a high-degree of porosity and vertically aligned MoS2 film matrix, as presented in **Figure 2A–C** which is in agreement with Kong et al*.* [18]. EDS analysis reveals the two main signals that correspond to Sulfur-Kα and Molybdenum-Lα at 2.4 keV, as presented in **Figure 2E**, in agreement

*Collage of photographic images taken at the Center for Integration of nanotechnologies-Albuquerque, NM.* **(A)** *High-vacuum sputtering device, overview.* **(B)** *RF-magnetrons with MoS2 target mounted.* **(C)** *Table showing the sample mounting.* **(D)** *Optical inspection of the films.* **(E)** *Profilometer measurements indicating the film* 

The microstructure of MoS2 thin matrix was also studied using *Scanning Electron Transmission Microscopy* (STEM) using a Cs-corrected 2200-JEOL, with STEM unit, equipped with a high-angle annular dark-field (HAADF) detector, X-Twin lenses and CCD camera. A lamella was prepared using Focus-Ion Beam model JEOL JEM 9320 at 30 kV and 25 mA, MoS2 film surface coated with gold and gallium. *Atom Probe Tomography* (APT) was performed on Cameca® LEAP 4000X high-resolution system in laser pulse mode (wavelength ~355 nm), measurements were taken at 60 K with evaporation rate of 0.5 and laser frequency of 100 kHz, laser beam was set to 70pJ/V, all data was reconstructed using IVAS© 3.6.10a package. The samples were prepared using focus ion beam FEI Strata dual-beam instrument coupled with micromanipulator Oxford® Omniprobe® 200 by lift-out method as described by Szász et al*.* [21]. MoS2 film surface was protected using platinum layer, and cuts were done at 30 kV at 260pA gun power. Using both techniques, it was possible to determine chemical composition, and spatial resolution of S-Mo-S distribution along film matrix, stacking and orientation, as presented in **Figure 3**. In the image the top part corresponds to MoS2 and uppermost bright layer is due to gold gallium coating.

**2.3 Transmission electron microscopy and atom probe tomography**

#### **Figure 1.**

*2D Materials*

100 *e*

<sup>−</sup>/Å2

transport on MoS2 films.

**2.1 RF sputtering**

**2. Experimental methods and results**

profilometry measurements, **Figure 1E**.

**2.2 Scanning electron microscopy**

with a band gap ranging between 0.9 and 1.8 eV as calculated theoretically by first principles methods and as measured experimentally by Kam & Parkinson using photo-spectroscopy as a function of crystal orientation [5, 6]. The crystal structure of MoS2 is hexagonal with space group R3m (*a = b =* 3.16 Å and *c =* 18.41 Å), having *d*-bonded layers of S-Mo-S along a-b plane which are stacked along c-axis by weak Van der Waals forces with 6.2 Å of separation within layers [7]. The crystal structure was studied using electron microscopy techniques as described by Chianelli et al*.* who were able to observe its layered structure [8]. However, electron beam dosage during electron microscopy studies plays an important role to avoid any structural damage as described by Ponce et al*.* when using TEM technique who concluded high-resolution imaging at operational voltages near ~80 kV [9] to be possible. By *in-situ* TEM, Helveg et al*.* were able to synthesize small clusters of MoS2 from molybdenum oxide and hydrogen sulfide gases at beam radiation dosage of

s [10]. The mechanical properties were studied by Casillas et al*.* achieving

an atomistic observation of a resilient nature on MoS2 laminates at 8GPa of external applied pressure and its mechanical recovery during *in-situ* AFM on TEM sample holder [11]. Applying atomic force microscopy (AFM), Bertolazzi et al*.* determined a Young modulus values of 270GPa ± 100GPa and fracture strength of 16~30GPa in MoS2 layers as suspended in patterned silicon wafers [12, 14], and Castellanos-Gomez et al*.* estimated an average Young modulus *E* = 330GPa in suspended MoS2 sheets over patterned silicon wafer [13]. The mechanical properties were studied by density functional theory and molecular dynamics, Jiang et al*.* calculated a theoretical Poisson's ratio value of *v* = 0.29 applying Stillinger-Weber potential [15]. The reactive empirical bond-order (REBO) potential was used by Li et al*.* to understand structural effects at chemical bonding within S-Mo-S layers, their findings indicate induced vacancies on the basal plane can influence Poisson's ratio values [16]. The atom probe tomography enables the chemical understanding with three-dimensional spatial resolution and was applied to determine dopants, contamination and ionic distribution within semiconducting matrix [17], Singh et al*.* used APT technique to determine distribution of Ti over MoS2 matrix [18]. Regarding electrical transport, Lia et al*.* [4] and Samuel et al*.* [38] performed transport electrical measurements encountering a linear ohmic behavior in MoS2. This chapter covers mechanical, electrical and microstructure characterization by electron microscopy, low angle x-ray, atom probe tomography and CSM-nanoindentation to obtain information about crystal growth, elastic modulus (*E*), hardness (*H*) and electrical

The Molybdenum di-Sulfide (MoS2) films were fabricated with a high vacuum Kurt J. Lesker© PVD 75 machine; applying RF-sputtering at a rate of 2.26 Å/sec at 275 W of plasma power over 4″-diameter silicon oxide (SiO2) wafers. The films were deposit from commercial MoS2 99.9% targets (Kurt J. Lesker). By using dwell time of 300 seconds a film thickness value of ~100 nm was achieved as indicated by

The film morphology and crystallographic structure were investigated using scanning and high-resolution transmission electron microscopy (SEM, TEM). SEM

**64**

*Collage of photographic images taken at the Center for Integration of nanotechnologies-Albuquerque, NM.* **(A)** *High-vacuum sputtering device, overview.* **(B)** *RF-magnetrons with MoS2 target mounted.* **(C)** *Table showing the sample mounting.* **(D)** *Optical inspection of the films.* **(E)** *Profilometer measurements indicating the film thickness ~ 100 nm for 300 seconds a rate of 2.26 Å/sec.*

was performed in a Hitachi® SU5500 unit, equipped with Energy-dispersive X-ray spectroscopy (EDS) unit and operated at 30 kV with 8A of current to avoid surface damage on the film. Observations indicate a high-degree of porosity and vertically aligned MoS2 film matrix, as presented in **Figure 2A–C** which is in agreement with Kong et al*.* [18]. EDS analysis reveals the two main signals that correspond to Sulfur-Kα and Molybdenum-Lα at 2.4 keV, as presented in **Figure 2E**, in agreement with Lince & Fleischauer [20].

#### **2.3 Transmission electron microscopy and atom probe tomography**

The microstructure of MoS2 thin matrix was also studied using *Scanning Electron Transmission Microscopy* (STEM) using a Cs-corrected 2200-JEOL, with STEM unit, equipped with a high-angle annular dark-field (HAADF) detector, X-Twin lenses and CCD camera. A lamella was prepared using Focus-Ion Beam model JEOL JEM 9320 at 30 kV and 25 mA, MoS2 film surface coated with gold and gallium. *Atom Probe Tomography* (APT) was performed on Cameca® LEAP 4000X high-resolution system in laser pulse mode (wavelength ~355 nm), measurements were taken at 60 K with evaporation rate of 0.5 and laser frequency of 100 kHz, laser beam was set to 70pJ/V, all data was reconstructed using IVAS© 3.6.10a package. The samples were prepared using focus ion beam FEI Strata dual-beam instrument coupled with micromanipulator Oxford® Omniprobe® 200 by lift-out method as described by Szász et al*.* [21]. MoS2 film surface was protected using platinum layer, and cuts were done at 30 kV at 260pA gun power. Using both techniques, it was possible to determine chemical composition, and spatial resolution of S-Mo-S distribution along film matrix, stacking and orientation, as presented in **Figure 3**. In the image the top part corresponds to MoS2 and uppermost bright layer is due to gold gallium coating.

**Figure 2.**

**(A, B)** *Scanning electron micrograph of MoS2 film matrix at magnification of 6000x.* **(C)** *SEM image able to observe laminates vertically aligned at magnification of 200,000x.* **(E)** *Energy disperse spectrum from surface to MoS2 film matrix to determine chemical composition.*

The atom probe tomography (APT) is a technique used to understand in a threedimensional reconstruction with high-spatial resolution the chemical distribution and composition as indicated by Kelly & Miller [17]. A sample is placed in the main APT chamber to undergo an ionizing evaporation process at a high electric field triggered by a laser pulse; the potential energy of an atom at the sample surface, as caused by the applied voltage on the sample *neV,* is converted into kinetic energy ~1/2*mv*<sup>2</sup> in the vicinity of the tip. This relationship, in order to understand the mass-to charge-state ratio *m/n* of evaporated ions, is given by Eq. (1); with *n* as number of electrons removed from the ion, *e* electron charge *(−1.62 × 10−19*C), *V* total applied voltage, *m* is atomic mass and speed of atoms are given by conventional *v* = *d*/*t,* which is with good approximation constant, distance *d* and lastly *t* is the time of flight, as described by the schematic drawing taken from Kelly and Larson [23]. Short laser pulses (<1 ns) are used for APT and can field evaporated for almost any material regardless of its electrical conductivity as described by Kellogg et al*.* [24].

**67**

**Figure 3.**

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

samples using a FIB (**Figures 5–9**).

**2.4 Raman spectroscopy**

Modes of vibration at E<sup>1</sup>

20o

< θ < 80o

in **Figure 11**.

rate of 0.05 s<sup>−</sup><sup>1</sup>

schematically **Figure 10** (insets).

Nowadays, usage of APT to survey spatial distribution of atomistic species in semiconducting devices like n-doped metal-oxide field effect transistors [25] and Singh et al. applied with high success to titanium-MoS2 and strontium oxide-MoS2 films [18]. In this case, APT measurements were performed to understand the spatial distribution of MoS2 film matrix. **Figure 4** illustrates the preparation of APT

The Raman spectroscopy was obtained using Alpha 300RA system equipped with a 532 nm Nd-YAG laser and a 100X 0.9 NA objective. The laser power was varied to avoid surface damage; with no additional sample preparation during study.

and A1

X-ray diffraction was collected using a Panalytical X-Pert system with source

described by Liu et al*.* while characterizing same layers of MoS2 [26] and presented

The nanoscale mechanical properties were evaluated to obtain Elastic modulus (*E*) and Hardness (*H*) of MoS2 thin films; this was possible using an Agilent nanoindenter model G200 coupled with a DCM II head instrument and Berkovich diamond indenter tip radius of 20 ± 5 nm, penetration depth limit of 400 nm, strain

Poisson's coefficient of *ν* = 0.22. The equipment was calibrated using a standard

**Left:** *Cross-sectional view of MoS2 film on TEM.* **Right:** *Chemical composition at locations as indicated in the* 

, and harmonic displacement and frequency of 1 nm and 75 Hz,

of CuK<sup>α</sup> *λ =* 1.41 Å radiation. The grazing incidence angle was fixed at 0.5<sup>o</sup>

vibrations caused by dangling bongs on S-Mo-S chemical structure as indicated

g = 407 cm<sup>−</sup><sup>1</sup>

with a graphite flat crystal monochromator,

are indicators of sulfur

with

2g = 378 cm<sup>−</sup><sup>1</sup>

**2.5 Grazing incidence X-ray diffraction (GIXD)**

and step size of 0.02o

**2.6 Nanoscale mechanical properties**

*image (violet squares), obtained during TEM observations.*

(1)

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

$$\frac{m}{n} = \frac{2e}{d^2} (V\_{\rm dc}) t^2 \tag{1}$$

Nowadays, usage of APT to survey spatial distribution of atomistic species in semiconducting devices like n-doped metal-oxide field effect transistors [25] and Singh et al. applied with high success to titanium-MoS2 and strontium oxide-MoS2 films [18]. In this case, APT measurements were performed to understand the spatial distribution of MoS2 film matrix. **Figure 4** illustrates the preparation of APT samples using a FIB (**Figures 5–9**).

#### **2.4 Raman spectroscopy**

*2D Materials*

**66**

~1/2*mv*<sup>2</sup>

**Figure 2.**

*to MoS2 film matrix to determine chemical composition.*

The atom probe tomography (APT) is a technique used to understand in a threedimensional reconstruction with high-spatial resolution the chemical distribution and composition as indicated by Kelly & Miller [17]. A sample is placed in the main APT chamber to undergo an ionizing evaporation process at a high electric field triggered by a laser pulse; the potential energy of an atom at the sample surface, as caused by the applied voltage on the sample *neV,* is converted into kinetic energy

**(A, B)** *Scanning electron micrograph of MoS2 film matrix at magnification of 6000x.* **(C)** *SEM image able to observe laminates vertically aligned at magnification of 200,000x.* **(E)** *Energy disperse spectrum from surface* 

 in the vicinity of the tip. This relationship, in order to understand the mass-to charge-state ratio *m/n* of evaporated ions, is given by Eq. (1); with *n* as number of electrons removed from the ion, *e* electron charge *(−1.62 × 10−19*C), *V* total applied voltage, *m* is atomic mass and speed of atoms are given by conventional *v* = *d*/*t,* which is with good approximation constant, distance *d* and lastly *t* is the time of flight, as described by the schematic drawing taken from Kelly and Larson [23]. Short laser pulses (<1 ns) are used for APT and can field evaporated for almost any material regardless of its electrical conductivity as described by Kellogg et al*.* [24].

The Raman spectroscopy was obtained using Alpha 300RA system equipped with a 532 nm Nd-YAG laser and a 100X 0.9 NA objective. The laser power was varied to avoid surface damage; with no additional sample preparation during study. Modes of vibration at E<sup>1</sup> 2g = 378 cm<sup>−</sup><sup>1</sup> and A1 g = 407 cm<sup>−</sup><sup>1</sup> are indicators of sulfur vibrations caused by dangling bongs on S-Mo-S chemical structure as indicated schematically **Figure 10** (insets).

#### **2.5 Grazing incidence X-ray diffraction (GIXD)**

X-ray diffraction was collected using a Panalytical X-Pert system with source of CuK<sup>α</sup> *λ =* 1.41 Å radiation. The grazing incidence angle was fixed at 0.5<sup>o</sup> with 20o < θ < 80o and step size of 0.02o with a graphite flat crystal monochromator, described by Liu et al*.* while characterizing same layers of MoS2 [26] and presented in **Figure 11**.

#### **2.6 Nanoscale mechanical properties**

The nanoscale mechanical properties were evaluated to obtain Elastic modulus (*E*) and Hardness (*H*) of MoS2 thin films; this was possible using an Agilent nanoindenter model G200 coupled with a DCM II head instrument and Berkovich diamond indenter tip radius of 20 ± 5 nm, penetration depth limit of 400 nm, strain rate of 0.05 s<sup>−</sup><sup>1</sup> , and harmonic displacement and frequency of 1 nm and 75 Hz, Poisson's coefficient of *ν* = 0.22. The equipment was calibrated using a standard

#### **Figure 3.**

**Left:** *Cross-sectional view of MoS2 film on TEM.* **Right:** *Chemical composition at locations as indicated in the image (violet squares), obtained during TEM observations.*

#### **Figure 4.**

**Left:** *Scanning transmission electron micrograph of transversal section of MoS2 film.* **Right:** *Selected area diffraction patterns for three different sites as indicated by red circles, the top part corresponds to textural MoS2 matrix. Table indicates principal with (101) and (112) for MoS2 in agreement with obtained by GDRX.*

#### **Figure 5.**

*High-resolution STEM image showing a vertical growth of MoS2 crystallites as confirmed by 0.62 nm interlayer distance in (002) basal plane, in agreement. Image taken with rights and permissions from IOP-surf. Topogr.: Metrol. Prop.© Ramos et al. [22].*

fused silica sample, under test parameters of *C0 =* 24.06, *C1 =* −184.31, *C2 =* 6532.04, *C3 =* −25482.45, and *C5 =* 19015.30 as constant area of contact for continuous stiffness method (CSM) as described in detail by Li et al*.* [27]. All data was recorded by AFM Nano Vision© system attached to the nanoindenter system. The estimated values for hardness (H) and elastic modulus (E) were calculated using Eq. (2) to

**69**

et al*.* [28].

**Figure 7.**

**Figure 6.**

*S* =

*micromanipulators), as discussed by Szász et al. [21].*

*Schematic drawing of APT (taken from [23]).*

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

determine stiffness *S*, when comparing to silicon substrates (001) surface termination and applying a continuous stiffness method as described extensively by Pharr

*Scanning electron images taken during lift-out procedure to prepare a needle for atom probe tomography in MoS2 thin film (green square and circles are the areas of interest and cut using Ga ions and Omiprobe®* 

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ 1 \_\_\_

cosφ − (*Ks* − *m*ω<sup>2</sup>

In Eq. (2), **ω** is the excitation frequency, (**Zo**) displacement amplitude, (**φ**) phase angle, and (**F0**) is the excitation amplitude, all those values can be obtained if the machine parameters load-frame stiffness **Kf** and stiffness of springs (**Ks**) as well the mass *m* are known input values during nanoindentation test. The coating hardness of film Hf can be estimated using a work indentation model described by

) − \_\_1 *Kf*| −1

(2)


*F*0 *Z*0 *MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

*2D Materials*

**Figure 4.**

**68**

**Figure 5.**

*Metrol. Prop.© Ramos et al. [22].*

fused silica sample, under test parameters of *C0 =* 24.06, *C1 =* −184.31, *C2 =* 6532.04, *C3 =* −25482.45, and *C5 =* 19015.30 as constant area of contact for continuous stiffness method (CSM) as described in detail by Li et al*.* [27]. All data was recorded by AFM Nano Vision© system attached to the nanoindenter system. The estimated values for hardness (H) and elastic modulus (E) were calculated using Eq. (2) to

*High-resolution STEM image showing a vertical growth of MoS2 crystallites as confirmed by 0.62 nm interlayer distance in (002) basal plane, in agreement. Image taken with rights and permissions from IOP-surf. Topogr.:* 

**Left:** *Scanning transmission electron micrograph of transversal section of MoS2 film.* **Right:** *Selected area diffraction patterns for three different sites as indicated by red circles, the top part corresponds to textural MoS2 matrix. Table indicates principal with (101) and (112) for MoS2 in agreement with obtained by GDRX.*

#### **Figure 6.** *Schematic drawing of APT (taken from [23]).*

#### **Figure 7.**

*Scanning electron images taken during lift-out procedure to prepare a needle for atom probe tomography in MoS2 thin film (green square and circles are the areas of interest and cut using Ga ions and Omiprobe® micromanipulators), as discussed by Szász et al. [21].*

determine stiffness *S*, when comparing to silicon substrates (001) surface termination and applying a continuous stiffness method as described extensively by Pharr et al*.* [28].

## [28].

$$S = \left| \frac{1}{\frac{F\_0}{Z\_0} \cos \alpha - \left(K\_s - m \,\alpha^2\right)} - \frac{1}{K\_f} \right|^{-1} \tag{2}$$

In Eq. (2), **ω** is the excitation frequency, (**Zo**) displacement amplitude, (**φ**) phase angle, and (**F0**) is the excitation amplitude, all those values can be obtained if the machine parameters load-frame stiffness **Kf** and stiffness of springs (**Ks**) as well the mass *m* are known input values during nanoindentation test. The coating hardness of film Hf can be estimated using a work indentation model described by

#### **Figure 8.**

*Scanning electron images and line scan EDS to map chemical composition on the needle; molybdenum and sulfur atoms were detected over MoS2 section (~ 110 nm).*

#### **Figure 9.**

**Top:** *Atomic concentration profile from atom probe tomography for the MoS2 thin film (~110 nm).* **Bottom:** *Portion of APT needle where corresponding to MoS2 highest concentration as shown on ions table on left (yellow and blue arrows), in agreement with Singh et al. [18].*

Eq. (3), having **Hc** as composed film/substrate hardness; **Hs** and **Hf** as substrate and film hardness, the constant *k* represents a fitting parameter determined experimentally from the variation of **Hc** with relative indentation depth (*β* = **Hc**/*t*).

$$H\_{\text{c}} = H\_{\text{s}} + \frac{H\_{\text{f}} - H\_{\text{s}}}{1 + k\beta^2} \tag{3}$$

**71**

**Figure 11.**

**Figure 10.**

*agreement with Kong et al. [19].*

*The Raman spectra with two characteristic modes of vibrations at E1*

*agreement with Liu et al. [26] for vertical aligned layers.*

*Grazing incidence x-ray diffraction it was possible to observe a dominant (101) reflection at 2θ ~30°, in* 

*2g at 378 cm<sup>−</sup><sup>1</sup>*

 *and A1*

*g at 407 cm<sup>−</sup><sup>1</sup>*

*, in* 

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

[29] and Hurtado-Macias et al*.* [30].

methyltrimethoxysilane.

geometry, in our case a pyramidal shape, as described by Domínguez-Rios et al.

By using CSM method, it is was possible to estimate elastic modulus and hardness values as follows: Three regions of test are observed in the **Figures 12** and **13**, where region I is hardness values for MoS2 crystallites with penetration depth of 0–90 nm, having no influence from silicon oxide substrate and a hardness value of *H* = 6.0 ± 0.1 GPa and elastic modulus of *E* = 136 ± 2 GPa. The region II, which has a penetration deep of 90–120 nm both values of elastic modulus and hardness are increased, meaning a clear influence by silicon oxide substrate, as confirmed by profilometry a thin film thickness of ~105 nm (both insets of **Figure 1**). The region III with penetration depth of 120–150 nm represents a hardness and elastic modulus of silicon oxide substrate, which are in partial agreement with Malzbender & With [31] to whom performed similar experiment on SiO2 spin coated with

(4)

The elastic modulus *E* can be estimated using Eq. (4); having **E***eff* as effective reduced elastic modulus of the system in array film/substrate, contact area is determinate by **A** as function of the penetration depth, *ν* is the Poisson ratio, *t* represents film thickness, and *α* is a parameter which depends on the material and the indenter *MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

*2D Materials*

**Figure 8.**

*sulfur atoms were detected over MoS2 section (~ 110 nm).*

*(yellow and blue arrows), in agreement with Singh et al. [18].*

**70**

**Figure 9.**

Eq. (3), having **Hc** as composed film/substrate hardness; **Hs** and **Hf** as substrate and film hardness, the constant *k* represents a fitting parameter determined experimentally from the variation of **Hc** with relative indentation depth (*β* = **Hc**/*t*).

**Top:** *Atomic concentration profile from atom probe tomography for the MoS2 thin film (~110 nm).* **Bottom:** *Portion of APT needle where corresponding to MoS2 highest concentration as shown on ions table on left* 

*Scanning electron images and line scan EDS to map chemical composition on the needle; molybdenum and* 

The elastic modulus *E* can be estimated using Eq. (4); having **E***eff* as effective reduced elastic modulus of the system in array film/substrate, contact area is determinate by **A** as function of the penetration depth, *ν* is the Poisson ratio, *t* represents film thickness, and *α* is a parameter which depends on the material and the indenter

(3)

geometry, in our case a pyramidal shape, as described by Domínguez-Rios et al. [29] and Hurtado-Macias et al*.* [30].

$$\frac{1}{E\_{\text{eff}}} = \frac{(1 - \nu\_{\text{f}}^2)}{E\_{\text{f}}} (1 - \text{e}^{-\text{at}/\sqrt{\lambda}}) + \frac{(1 - \nu\_{\text{s}}^2)}{E\_{\text{s}}} (\text{e}^{-\text{at}/\sqrt{\lambda}}) + \frac{(1 - \nu\_{\text{i}}^2)}{e\_{\text{i}}} \tag{4}$$

By using CSM method, it is was possible to estimate elastic modulus and hardness values as follows: Three regions of test are observed in the **Figures 12** and **13**, where region I is hardness values for MoS2 crystallites with penetration depth of 0–90 nm, having no influence from silicon oxide substrate and a hardness value of *H* = 6.0 ± 0.1 GPa and elastic modulus of *E* = 136 ± 2 GPa. The region II, which has a penetration deep of 90–120 nm both values of elastic modulus and hardness are increased, meaning a clear influence by silicon oxide substrate, as confirmed by profilometry a thin film thickness of ~105 nm (both insets of **Figure 1**). The region III with penetration depth of 120–150 nm represents a hardness and elastic modulus of silicon oxide substrate, which are in partial agreement with Malzbender & With [31] to whom performed similar experiment on SiO2 spin coated with methyltrimethoxysilane.

#### **Figure 10.**

*The Raman spectra with two characteristic modes of vibrations at E1 2g at 378 cm<sup>−</sup><sup>1</sup> and A1 g at 407 cm<sup>−</sup><sup>1</sup> , in agreement with Kong et al. [19].*

#### **Figure 11.**

*Grazing incidence x-ray diffraction it was possible to observe a dominant (101) reflection at 2θ ~30°, in agreement with Liu et al. [26] for vertical aligned layers.*

#### **Figure 12.**

*Nanoindentation curves estimated experimentally using continuous stiffness method (CSM), the curve corresponds to regions I, II, III. In region I the estimated elastic modulus is E = 136 ± 2 GPa corresponds to 0–90 nm of penetration depth, which is indicated to be only for MoS2 film, in agreement with [28].*

**Figure 13.**

*The region I corresponds to hardness values of H = 10.5 ± 0.1 GPa at 0–90 nm of penetration depth corresponding to MoS2 layer. The regions II and III on both curves corresponds to mixed stage MoS2/SiO2 and SiO2 substrate reason of an increase on both values are observed, in agreement with [28].*

The obtained values for hardness and elastic modulus are smaller estimations when comparing with results as presented by Bertolazzi et al*.* [12, 14] for single layers of MoS2; we believe this occurs because of low dimension laminates can be stronger than stacking of MoS2 crystallites. The applied force was done over (001)-basal plane as suspended on patterned silicon holes [12, 14], and in this case indenter tip can sweep MoS2 crystallites over surface area. For that reason, our research team proceed to estimate film adherence by using AFM scratching technique in

**73**

**Figure 14.**

**Figure 15.**

encountering a deformation 0.85 μm2

**2.7 Electrical transport and resistivity**

with a residual groove width 1 μm (a total

groove height 125 nm and pile up height 40 nm), as presented in **Figure 14**, along with indentation sites completed to obtain elastic modulus and hardness values.

*Graphical description of the four-point method implemented for electrical transport in MoS2 film.*

**Left:** *Scanning electron micrograph indicating the nine zones of nanoindentation made with diamond indenter tip to estimate the elastic modulus and hardness values on MoS2 film.* **Center:** *Scanning electron micrograph showing cracks over triangle shape indentation as indicated by red arrows.* **Right:** *Atomic force microscope* 

*starching zone to estimate a stiffness values of 4.27kN/m over the MoS2 film.*

The electrical transport of the MoS2 film matrix was investigated using fourpoint probe method as indicated in **Figure 15**, equipped with Keithley 4200-SCS in applied voltage range from −3.5 to 3.5 V. The transport measurements were done at room temperature and by direct contact to the MoS2 film surface, no especial solder or metallic glue was used. Also, they were completed in the presence of light and dark conditions, the results indicate a linear Ohmic behavior, as presented in

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512* *MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

#### **Figure 14.**

*2D Materials*

**Figure 12.**

**72**

**Figure 13.**

The obtained values for hardness and elastic modulus are smaller estimations when comparing with results as presented by Bertolazzi et al*.* [12, 14] for single layers of MoS2; we believe this occurs because of low dimension laminates can be stronger than stacking of MoS2 crystallites. The applied force was done over (001)-basal plane as suspended on patterned silicon holes [12, 14], and in this case indenter tip can sweep MoS2 crystallites over surface area. For that reason, our research team proceed to estimate film adherence by using AFM scratching technique in

*The region I corresponds to hardness values of H = 10.5 ± 0.1 GPa at 0–90 nm of penetration depth corresponding to MoS2 layer. The regions II and III on both curves corresponds to mixed stage MoS2/SiO2 and* 

*SiO2 substrate reason of an increase on both values are observed, in agreement with [28].*

*Nanoindentation curves estimated experimentally using continuous stiffness method (CSM), the curve corresponds to regions I, II, III. In region I the estimated elastic modulus is E = 136 ± 2 GPa corresponds to 0–90 nm of penetration depth, which is indicated to be only for MoS2 film, in agreement with [28].*

**Left:** *Scanning electron micrograph indicating the nine zones of nanoindentation made with diamond indenter tip to estimate the elastic modulus and hardness values on MoS2 film.* **Center:** *Scanning electron micrograph showing cracks over triangle shape indentation as indicated by red arrows.* **Right:** *Atomic force microscope starching zone to estimate a stiffness values of 4.27kN/m over the MoS2 film.*

#### **Figure 15.**

*Graphical description of the four-point method implemented for electrical transport in MoS2 film.*

encountering a deformation 0.85 μm2 with a residual groove width 1 μm (a total groove height 125 nm and pile up height 40 nm), as presented in **Figure 14**, along with indentation sites completed to obtain elastic modulus and hardness values.

#### **2.7 Electrical transport and resistivity**

The electrical transport of the MoS2 film matrix was investigated using fourpoint probe method as indicated in **Figure 15**, equipped with Keithley 4200-SCS in applied voltage range from −3.5 to 3.5 V. The transport measurements were done at room temperature and by direct contact to the MoS2 film surface, no especial solder or metallic glue was used. Also, they were completed in the presence of light and dark conditions, the results indicate a linear Ohmic behavior, as presented in

**Figure 16.**

*Top: I-V curves measured in the presence of white light and using four-point method MoS2 film. Bottom: I-V curves measured film in dark-room under otherwise conditions. When comparing both measurements it is possible to observe a change on slope, which is related to resistive values as presented in* **Figure 17***.*

#### **Figure 17.**

*Resistivity ρ values, calculated from I-V curves measured in the presence of white light and dark environment by using four-point method over MoS2 film's surface. Red curve corresponds to presence of light shining on surface and blue curve to dark-room conditions.*

**Figure 16** and resistivity values in **Figure 17**, from **Tables 1** and **2** it was possible to determine values differences when white light is present, some authors refer this as photo-voltaic effect due to its intrinsic semiconductor nature of MoS2 [2, 32, 33].

**75**

**Acknowledgements**

A1

**Table 1.**

**Table 2.**

g = 407 cm<sup>−</sup><sup>1</sup>

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*

**3. Discussion and conclusion**

By using radio frequency sputtering techniques at high-vacuum it was possible to fabricate MoS2 films with thickness of ~100 nm over pristine silicon oxide (SiO2) wafers. The film surface analysis was carried out using electron microscopy and spectroscopy techniques and results indicate molybdenum di-sulfide had a vertical crystallite growth as shown in **Figures 2C** and **5**. Energy disperse confirms Sulfur-Kα (60%) and Molybdenum-Lα (40%) at 2.4 keV signal; and Raman

**Size (m) Impedance Resistance Power Resistivity (***Ω/m)* **5 × 10<sup>−</sup><sup>3</sup>** 0.4064 1.41 × 106 0.0381 26.507 **10 × 10<sup>−</sup><sup>3</sup>** 0.508 1.83 × 106 0.0381 27.473 **15 × 10<sup>−</sup><sup>3</sup>** 0.6096 2.89 × 106 0.0254 24.106

**Size (m) Impedance Resistance Power Resistivity (***Ω/m)* **5 × 10<sup>−</sup><sup>3</sup>** 0.4064 1.86 × 106 0.0381 35.047 **10 × 10<sup>−</sup><sup>3</sup>** 0.508 2.46 × 106 0.0381 36.605 **15 × 10<sup>−</sup><sup>3</sup>** 0.6096 3.92 × 106 0.0254 33.256

*Values of impedance, resistance, power and resistivity measured without presence of white light.*

*Values of impedance, resistance, power and resistivity measured in presence of white light.*

of stacking between 7 layers along (002)-basal plane and to confirm vertical growth in agreement with Kong et al*.* [19], and APT preliminary measurements indicate a large quantity of sulfur and molybdenum with no grain boundaries or high impurities within film matrix for specific thin film growth using RF-sputtering conditions. From electrical transport measurements, it was possible to determine a linear Ohmic behavior and excitation when external visible light was on and off during four-point probe measurements as indicated by **Figure 16**, the resistivity values 26.5 *Ω/m* versus 35.0 *Ω/m* for off and on in external visible light, as possible caused by intrinsic semiconductor nature of MoS2 in agreement with [2, 32, 33]. The mechanical properties were also investigated as previously reported by Ramos et al*.* [22] for indenter penetrating film surface 0-90 nm it was possible to estimate hardness of *H* = 10.5 ± 0.1 GPa and elastic modulus *E* = 136 ± 2 GPa by the continuous stiffness method [28]. It was concluded that MoS2 films are a promising semiconducting material candidate for large scale photovoltaic applications, due to low cost and reliable and straight forward approach for homogenous low dimension films and its possible chemical combination with other group VI semiconducting materials as indicated by Najmaei et al*.* [34, 35] and Matis et al*.* [36] and Paranjape et al*.* [37–42].

The principal author thanks Universidad Autónoma de Ciudad Juárez for financial support by PIVA-2017 project titled: *"Fabricación de plantillas metálicoorgánicas: Una ruta hacia celdas solares más eficientes"* and PROFOCIE 2015. The

. From high-resolution STEM it was possible to determine a degree

2g = 378 cm<sup>−</sup><sup>1</sup>

and

spectroscopy modes of vibration at surface corresponding to E1

*MoS2 Thin Films for Photo-Voltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.83512*


**Table 1.**

*2D Materials*

**74**

**Figure 17.**

**Figure 16.**

*surface and blue curve to dark-room conditions.*

**Figure 16** and resistivity values in **Figure 17**, from **Tables 1** and **2** it was possible to determine values differences when white light is present, some authors refer this as photo-voltaic effect due to its intrinsic semiconductor nature of MoS2 [2, 32, 33].

*Resistivity ρ values, calculated from I-V curves measured in the presence of white light and dark environment by using four-point method over MoS2 film's surface. Red curve corresponds to presence of light shining on* 

*Top: I-V curves measured in the presence of white light and using four-point method MoS2 film. Bottom: I-V curves measured film in dark-room under otherwise conditions. When comparing both measurements it is possible to observe a change on slope, which is related to resistive values as presented in* **Figure 17***.*

*Values of impedance, resistance, power and resistivity measured without presence of white light.*


**Table 2.**

*Values of impedance, resistance, power and resistivity measured in presence of white light.*

## **3. Discussion and conclusion**

By using radio frequency sputtering techniques at high-vacuum it was possible to fabricate MoS2 films with thickness of ~100 nm over pristine silicon oxide (SiO2) wafers. The film surface analysis was carried out using electron microscopy and spectroscopy techniques and results indicate molybdenum di-sulfide had a vertical crystallite growth as shown in **Figures 2C** and **5**. Energy disperse confirms Sulfur-Kα (60%) and Molybdenum-Lα (40%) at 2.4 keV signal; and Raman spectroscopy modes of vibration at surface corresponding to E1 2g = 378 cm<sup>−</sup><sup>1</sup> and A1 g = 407 cm<sup>−</sup><sup>1</sup> . From high-resolution STEM it was possible to determine a degree of stacking between 7 layers along (002)-basal plane and to confirm vertical growth in agreement with Kong et al*.* [19], and APT preliminary measurements indicate a large quantity of sulfur and molybdenum with no grain boundaries or high impurities within film matrix for specific thin film growth using RF-sputtering conditions. From electrical transport measurements, it was possible to determine a linear Ohmic behavior and excitation when external visible light was on and off during four-point probe measurements as indicated by **Figure 16**, the resistivity values 26.5 *Ω/m* versus 35.0 *Ω/m* for off and on in external visible light, as possible caused by intrinsic semiconductor nature of MoS2 in agreement with [2, 32, 33]. The mechanical properties were also investigated as previously reported by Ramos et al*.* [22] for indenter penetrating film surface 0-90 nm it was possible to estimate hardness of *H* = 10.5 ± 0.1 GPa and elastic modulus *E* = 136 ± 2 GPa by the continuous stiffness method [28]. It was concluded that MoS2 films are a promising semiconducting material candidate for large scale photovoltaic applications, due to low cost and reliable and straight forward approach for homogenous low dimension films and its possible chemical combination with other group VI semiconducting materials as indicated by Najmaei et al*.* [34, 35] and Matis et al*.* [36] and Paranjape et al*.* [37–42].

#### **Acknowledgements**

The principal author thanks Universidad Autónoma de Ciudad Juárez for financial support by PIVA-2017 project titled: *"Fabricación de plantillas metálicoorgánicas: Una ruta hacia celdas solares más eficientes"* and PROFOCIE 2015. The

Laboratorio Nacional de Nanotecnología of Centro de Investigación en Materiales Avanzados (CIMAV-Chihuahua) and ICNAM program of Kleberg Advanced Laboratory Center of University of Texas at San Antonio both for the usage of electron microscopy and characterization equipment and facilities. Part of this work was performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. We are grateful to the Karlsruhe Micro and Nano Facility (KNMF) of Karlsruhe Institute of Technology for usage of Atom Probe Tomography and FIB instruments, under proposal 2018– 020-022609. To facultad de ciencias de la electronica of Benemerita Universidad Autónoma de Puebla for usage of four-point probe station and laboratory facilities.
