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Abstract
Nickel-doped tungsten telluride films (Ni/WTe2) were prepared by direct current-radio frequency (DC-RF) co-sputtering technique, and the doping content of Ni elements in the films was varied by changing the DC target power. We investigated the effect of metal doping and different doping concentrations on the nonlinear absorption and nonlinear refraction of the doped films. The nonlinear absorption coefficients and nonlinear refraction coefficients of the doped films were measured at the 532 nm using the ps Z-scan technique with significant enhancement over the undoped WTe2 films. It is shown that the saturation absorption effect and self-dispersion effect of WTe2 film can be effectively tuned by doping transition metal.
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1. Introduction
Since the birth of the first ruby laser, people gradually discovered many novel phenomena, such as frequency doubling, stimulated Raman scattering, two-photon absorption, saturation absorption, self-scattering [1,2]. These unusual phenomena led to the birth of the field of nonlinear optics. With the further development of this discipline, researchers found that the mechanism based on nonlinear optics can realize key technologies such as frequency conversion, modulation, Q-switching and mode-locking of lasers [3]. Thus, the quest for nonlinear optical materials with large nonlinear optical coefficients, ultra-fast response times and high stability was crucial. While two-dimensional transition metal dihalides (TMDs) benefit from quantum-limited effects and the elimination of interlayer coupling, making these materials a more significant improvement in optoelectronics than bulk materials, we believe that TMDs have broad application prospects within the field of nonlinear optics [4].
In recent years, Tungsten telluride (WTe2) has been extensively studied in the field of third-order nonlinear optics. WTe2 is a Group VI tellurides narrow bandgap semiconductor with a bulk bandgap of 0.7 eV and a monolayer bandgap of 1.18 eV [5]. It has been reported that this narrow bandgap semiconductor has wider saturation absorption properties and higher carrier mobility. The crystal structure of WTe2 is shown in Fig. 1(a), which belongs to the cubic crystal system, space group Pnm21 [6]. As in the structure of other transition metal sulfides, the sulfur group elements sandwich the metal elements in between. It has been shown that the surface of WTe2 is easily oxidized under natural environmental conditions, but the effect of this oxidation decreases with increasing thickness [7]. Therefore, two-dimensional nanofilms of WTe2 prepared by magnetron sputtering, in terms of oxidation resistance, will outperform nanosheets and nanowires prepared by liquid phase exfoliation and chemical vapor deposition techniques. Currently, some papers have reported that WTe2 films have NLO responses. Firstly, it has been reported that WTe2 films act as saturable absorbers in thulium-doped fiber (TDF) lasers to produce stable ultrafast pulses at 2 $\mu m$ [8]. In addition, the WTe2 saturable absorber with a nonlinear absorption coefficient of −3.78 × 10−5 cm/W was prepared by magnetron sputtering in 2020, and a mode-locked laser with a pulse width of 164 fs at 1557.71 nm was obtained using this saturated absorbent material [5]. In recent years, some studies have shown that doping of transition metals (Ni, Sn, Al, Co, etc.) within the films can enhance the nonlinear electromagnetic and optical properties [9].
Fig. 1. (a) WTe2 crystal structure (b) WTe2 thin film prepared by magnetron sputtering (c) Ni/WTe2 thin film prepared by magnetron sputtering.
In this paper, dense WTe2 and Ni/WTe2 films were prepared using DC-RF target sputtering. The size and content of Ni particles in the films can be controlled by varying the DC target power, which characterized by emission electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive spectroscopy (EDS) techniques. The results of Raman spectroscopy and X-ray photoelectron spectroscopy indicate the existence of charge transfer between Ni and WTe2 and the formation of Ni-Te chemical bonds. The linear optical properties of the WTe2 and Ni/WTe2 films were analysed by UV-Vis absorption spectroscopy and the optical band gap was estimated. The nonlinear absorption and refraction properties of the films were measured using open-aperture Z-scan (OA-Z-scan) and closed-aperture Z-scan (CA-Z-scan), which showed that the doping of Ni resulted in an increase of the nonlinear absorption coefficient by two orders of magnitude and the nonlinear refraction coefficient by one order of magnitude for the WTe2 films.
2. Experiment
Using a 99.9% pure WTe2 target (diameter 60 mm, thickness 3 mm) fixed on the RF target, the quartz substrate was placed in an ultrasonic cleaner to remove surface contaminants, then was placed in a tray and fed into the deposition chamber. When the deposition chamber was evacuated to 6.0 × 10−4 pa, the pre-sputtering was started for 10 min, the RF target power was 60 W, the substrate heating temperature was 100 °C, and the argon gas flow rate was 20 sccm. After pre-sputtering, the baffle is opened for formal sputtering, and the deposition time is 30 min to obtain WTe2 films. The Ni target (diameter 60 mm, thickness 3 mm) and WTe2 target were mounted on the DC target and RF target respectively to prepare Ni/WTe2 thin film, keeping the experimental operation steps the same as before, modulating the DC target power of 2W, 4W and 6W to deposit for 30 min to obtain (2W, 4W,6W) Ni/WTe2 films.
In this work, the morphology of the samples was characterized by SEM and AFM.The results of EDS were used to analyze the elemental distribution of the films and to quantify the elemental content. The vibrational modes and chemical valence states of the WTe2 and Ni/WTe2 films were analyzed by Raman (Raman) and X-ray photoelectron spectroscopy (XPS). The samples were characterized by UV-Vis spectrophotometer. The nonlinear absorption coefficients and nonlinear refraction coefficients of the films were measured by OA-Z-scan and CA-Z-scan devices (wavelength:532 nm, pulse width:15 ps).
3. Results and discussion
SEM and AFM in Fig. 2 show the surface morphology of WTe2 films and 2w, 4w and 6w Ni/WTe2 films respectively, and it can be seen that WTe2 molecules and Ni atoms are uniformly distributed on the substrate in the form of particles. As the sputtering power of the Ni target increases, the particle size of the Ni nanoparticles in the films increases, so that the smooth films become denser with Ni doping. The average roughness of the films analyzed by the NanoScope Analysis program was 1.48 nm, 2.31 nm, 4 nm and 7.48 nm, respectively, proportional to the DC target power.
Fig. 2. (a-d) SEM images of WTe2 and 2w, 4w and 6w Ni/WTe2 films (ai-di) AFM images of WTe2 and 2w, 4w and 6w Ni/WTe2 films.
The EDS energy spectra (Fig. 3) quantitatively demonstrates the content of elemental Ni in the doped films as 12.78% (2 W), 43.84% (4 W) and 52.09% (6 W), indicating that the metal doping concentration increases with increasing target sputtering power. The similar situation has been previously reported in the literature [10,11].
Figure 4 shows the Raman spectra of WTe2 and Ni/WTe2 films. The WTe2 films show four vibrational peaks $\textrm{A}_2^5$, $\textrm{A}_2^\textrm{4}$, $\textrm{A}_\textrm{1}^\textrm{8}$ and $\textrm{A}_\textrm{1}^\textrm{5}$ located at 86.7 cm−1, 104 cm−1, 130 cm−1 and 163 cm−1, respectively, which are in agreement with the previous work [12]. The presence of the vibrational peaks at $\textrm{A}_2^\textrm{4}$ implies that the film has several layers of WTe2, so that the film can resist oxidation to some extent and keep the main components of the film stable at room temperature. The Raman images of Ni/WTe2 films do not show any new phonon vibrational peaks, but the peaks of the vibrational peaks of the Raman patterns all appear to be enhanced. This phenomenon is due to the surface-enhanced Raman scattering (SERS) effect in the metal-semiconductor doped system, the addition of metallic Ni enhances the charge transfer in the film and improves the sensitivity of SERS; on the other hand, the presence of a large number of Ni particles on the doped thin surface generates SPR effect, which enhances the electromagnetic field and thus the Raman scattering signal [13]. It can be seen from Fig. 4(a) that with the increase of Ni target power, Raman vibration peak also increases, which is related to the increase of Ni element content in the film, which is consistent with the EDS results.
Fig. 4. (a) Raman spectra of WTe2 and (2w,4w,6w) Ni/WTe2 films (b-e) XPS spectra of O, Te, Ni and W in Ni/WTe2 films.
In order to investigate the chemical state of Ni in doped WTe2 films, the XPS technique was used to exhibit the characteristic peaks of W, Te, Ni,as shown in Fig. 4(b-d).There are two very distinct peaks in the XPS spectrum of Ni, located at 853 eV and 855.6 eV at the binding energy. Both peaks belong to the Ni 2P3/2 orbital, the peak at 853 eV being attributed to the presence of metallic Ni and the peak at 855.6 eV due to the presence of Ni2+ in NiTe [14]. The two peaks at 572.6 eV and 583 eV in the XPS spectra of Te are attributed to the Te 3d orbital, which belong to WTe2 and NiTe. The two weak peaks at 572.2 eV and 586.8 eV binding energies are attributed to the formation of a chemical bond between Te and O, indicating a slight oxidation of the sample. The two peaks at 247 eV and 260 eV are attributed to the W 4d orbital [7]. Analysis of the combined XPS results confirms that part of the element Ni in the doped films is present in the NiTe compound as Ni2+ ions and the other part as singlet Ni in the films.
We measured the absorption spectra of WTe2 thin films and (2W, 4W, 6W) Ni/WTe2 thin films by UV spectrophotometer (Fig. 5). We found that the absorption edges of the samples were red-shifted when Ni was doped into the WTe2 films, and the distance shifted to the infrared band was larger with increasing doping concentration, which indicated that the doping metal caused a change in the band gap of the WTe2 films, so we used Tauc's equation [15] to calculate the optical band gap of the WTe2 films and (2W, 4W, 6W) Ni/WTe2 films as follows:
where α is the linear optical coefficient, A is a constant, Eg is the optical band gap, hν is the energy possessed by the incident light photon, and the index n is 1, 2, 3. The optical band gaps of WTe2 film and (2W, 4W, 6W) Ni/WTe2 film are 2.2, 2.0, 1.9, 1.76 eV.The doping of the metal leads to a red shift in the absorption spectrum and a reduction in the band gap because Ni replaces W in the WTe2 lattice, as the radius of the Ni ion is 0.69 Å, which is larger than the radius of the W ion 0.62 Å [16]. Therefore, this substitution doping leads to a shortening of the bond lengths resulting in a change in the intrinsic frequency and thus the red shift phenomenon is observed in the absorption spectrum.
As the DC target power is increased, the optical band gap continues to decrease, which may be related to the particle size in the film, Norifusa Satoh proposed a relationship between the band gap Eg and the particle size [17]
Figure 6 shows the schematic diagram of the optical path for the Z-scan experiment.The incident laser is divided into two beams after passing through the beam splitter (BS). One beam is received by the detector (PD1), which is used to detect the scattering of the incident energy; the other beam is converged by the lens to excite the sample to be measured, and the laser passes through the sample and is again received by another detector. The aperture is placed in front of PD2, and the OA-Z-scan device measures the nonlinear absorption, while the CA-Z-scan device measures the nonlinear refraction. The WTe2 films and (2W, 4W, 6W) Ni/WTe2 films are subjected to Z-scan tests under the excitation of a laser with the wavelength of 532 nm, the pulse width of 15 ps, the beam waist radius of 22 µm, and the energy of 1 µJ, and the following OA-Z-scan plots and CA-Z-scan curves are obtained (Fig. 7). The analyzed OA-Z-scan curve rises and then falls in a peak-like pattern, showing the saturation absorption effect (SA). The CA-Z-scan curve showed peak and then valley, indicating that the nonlinear refractive properties of the films were self-defocusing effect.
Fig. 7. OA-Z-scan (a) and CA-Z-scan (b) curves of WTe2 film and (2w, 4w, 6w) Ni/WTe2 films (c) Variation of Ni power effects on the Ni content in the films as well as the bandgap of the films (d) Impact of the variation of Ni power on the nonlinear absorption coefficient β and nonlinear refraction coefficient n2 of the film.
The sample was tested by Z-scan to calculate the third-order NLO coefficients as follows [18–20].
The nonlinear absorption is calculated as
Leff in the formula is the effective thickness of the sample, which can be calculated using the following equation. The laser intensity at the focus of the formula ${I_{0(t)}}$ is calculated as follows where c is the ratio of laser incident energy to pulse width and r is the beam waist radius.The nonlinear refractive index equation is
To better understand the internal charge transfer path of the doped film, we use the nonlinear absorption results to analyze and construct an energy level diagram as shown in Fig. 8. When the metal is in contact with the semiconductor, the electrons are transferred from the inside of the metal to the semiconductor because the work function of the metal Ni (4.33 eV) is smaller than that of WTe2 (4.56 eV) [21,22]. Finally, an internal electric field is formed at the metal-semiconductor interface with the direction pointing from the metal to the semiconductor. When the laser excites the doped film, the electrons on the valence band of WTe2 absorb photons and migrate to the conduction band, and these photons flow to the metal Ni by the effect of the built-in electric field. These electrons continue to absorb photons in the metal to leap to the high energy level and relax from the high energy level to the nearest conduction band of WTe2, thus achieving a dynamic circulation of electrons at the metal-semiconductor interface, which hinders the compounding of electrons and holes inside WTe2 making the saturation absorption effect of WTe2 enhanced, and also enhances the saturation absorption effect of metallic Ni.
Fig. 8. (a) Schematic diagram of Ni in contact with WTe2 to form the built-in electric field (b) Energy level diagram of Ni/WTe2 film.
In addition, as more Ni is doped into the WTe2 film, the Ni-Ni monopolar bond in the film changes to a Ni-Te heteropolar bond because the electronegativity of Ni is less than that of Te. According to the Mott-Davis model, there is no strict valence band top and conduction band bottom in amorphous semiconductors but rather band tails at the edge of the energy band, and the formation of more heteropolar bonds reduces the width of the band tails, decreasing the trap for trapping photogenerated carriers, thus enhancing the probability of photon leap [23,24].
The result of the CA-Z-scan image shows a peak followed by a valley curve indicating that the Ni/WTe2 and WTe2 films have a self-scattering property, and the nonlinear refractive index of the undoped WTe2 film of −5.22 × 10 −14 m2/w is enhanced by an order of magnitude after metallic Ni doping. We calculated the Z-coordinate difference between peak and trough and found the relationship of △Zp-v > 1.7Z0, indicating that the nonlinear refraction effect is due to the thermo-optical effect [25,26,27]. Under laser irradiation at high repetition rates, the nanostructures are locally heated to generate high temperatures resulting in a thermal lensing effect on the sample. It has been shown that the doping of metallic Ni into semiconductors such as two-dimensional transition metal sulfides decrease the overall thermal conductivity of the material, which contributes more to the enhancement of the thermal lensing effect of the film, thus increasing the nonlinear refractive index of the film [28,29,30]. We have reviewed the NLO parameters of some typical 2D materials, and after comparing them in Table 1, it is concluded that the NLO coefficients of our prepared Ni-doped WTe2 films are more than three orders of magnitude higher than the reference data, thus indicating that metal doping has a tremendous improvement in the performance of WTe2 as a saturable absorber for Q-switching and mode-locked lasers.
Table 1. NLO coefficients of thin films.
4. Summary
In this work, the morphology, structure and optical properties of Ni-doped WTe2 thin films have been investigated. The samples were subjected to OA-Z-scan and CA-Z-scan technologies under visible light excitation at 532 nm. The OA-Z-scan showed that the saturation absorption effect of the WTe2 films was enhanced with the addition of Ni, which was attributed to the formation of Ni-Te and the charge transfer between the Ni and WTe2 contact surfaces The CA-Z-scan curves show that the addition of the metal improved self-dispersion characteristics of the samples, which is due to the enhancement of the thermal lensing effect of the films. In summary, this paper confirms that Ni doping can play a modulating role in the NLO response of WTe2 films, which provides an important reference value for the subsequent search for more suitable WTe2 dopants.
Funding
Natural Science Foundation of Heilongjiang Province (LH2020F032).
Acknowledgement
JiaXiang Mu: Conceptualization, Data curation, Methodology, Software, Writing - original draft. Qi Zhang: Project administration, Resources. XiYi Yuan: Investigation. Mukhtiar Ali: Validation.Hong Qi, Fei Wang: Funding acquisition. Wen-Jun Sun: Formal analysis, Funding acquisition, Supervision, Visualization, Writing - review & editing. Ming Li: Supervision, Visualization
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Ref. [31,32]. In addition, data on the results of research results derived from the article may be obtained from the corresponding authors upon reasonable request.
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