2D materials coated on etched optical fibers as humidity sensor

In this investigation, etched-fibers are coated by 2D layers such as Molybdenum disulfide (MoS2), Molybdenum diselenide (MoSe2) and composition of graphene and graphene oxide (G/GO) to modify humidity sensing. The relative differentiation of attenuations (RDA) in presence of relative humidity (RH) is measured by Optical Loss Test Set at two standard-wavelengths-telecommunication (1310 nm and 1550 nm). Results show that the etched single-mode fiber (ESMF) coated with G/GO has relatively high and one by one function for RDA versus RH (more than 30%). Also, its sensitivity and variance are reasonable. The MoSe2 based sensor is applicable at humidity below 30% because of higher RDA. However, it is not useful at humidity more than 30% due to the absence of one by one function for RDA versus RH. Besides, ESMF coated with MoS2 has indistinctive behavior and is not useful as a humidity sensor.


Scientific Reports
| (2021) 11:1771 | https://doi.org/10.1038/s41598-020-79563-w www.nature.com/scientificreports/ atoms respectively. The humidity sensor based on the side-polished optical fiber coated with these nanosheets have been investigated at 1550 nm wavelength [33][34][35] . The use of mechanical polishing method results in insertion loss of the side-polished fiber and smoothness of edges is difficult. However, chemical polishing methods have preserved the cylindrical shape of the optical fiber and is much simpler and more practical than other methods. As far as we have researched, so far the optical fibers that have been chemically thinned and layered with 2D materials have not been applied as humidity sensor.
In this study, for the first time, chemical etching the surface of single-mode optical fiber (SMF) by hydrofluoric acid (HF) as a simple polish method and coating 2D materials such as the composition of G and GO (G/ GO) to use the advantage of both them, MoS 2 and MoSe 2 on the etched-SMF (ESMF) are used to investigate the humidity sensing properties of optical fiber. So, at first G/GO, MoS 2 and MoSe 2 are synthesized and qualified by Scanning Electron microscopy (SEM), Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray spectroscopy (EDX), Raman, FTIR (Fourier-transform infrared spectroscopy) and XRD (X-ray Diffraction) analysis. Then, etched-SMFs are coated by synthesized 2D materials. In the end, the relative differentiation of attenuation (RDA), repeatability, sensitivity, and variance versus relative humidity (RH) at two standard-wavelengths-telecommunication (1310 nm and 1550 nm) are measured.
Also, the behavior of the introduced humidity sensor has been analyzed based on the surface functional groups and the semiconductor type (n-type or p-type) of the coated 2D materials on the ESMF. We have not found this type of analysis in the field of humidity sensing by fiber optic as far as we have studied.

Materials and methods
In this research, SMF is used as the main part of the optical-humidity sensor. To modify the sensing, the diameter of SMF reduced by corroding it in HF acid and layered with 2D materials (G/GO, MoSe 2 , and MoS 2 ) to achieve desirable evanescence field. The Optical Loss Test Set (OLTS) as an analyzer (Optical power source (OPS) and Optical power meter (OPM)) was used to measure RDA versus RH.

Synthesis.
Graphene. The G/GO layer is synthesized by Liquid Phase Exfoliation of Graphite Oxide. Usually, the composition of G and GO produces in this method. After synthesis of Graphite Oxide powder by Hummer's method 36,37 , 5 mg of it disperses in 10 cc distilled water and sonicates to separate individual Graphite Oxide layers. The transparent liquid above sediments that contain few/mono-layer of G/GO is achieved after centrifugation at 6000 rpm for 3 h. The quality of G/GO layers is investigated by SEM, TEM, EDX, Raman, FTIR and XRD analysis.
The SEM and TEM images display the microstructure of synthesized G/GO (Fig. 1a,b). The corresponding EDX analysis (Fig. 1c) indicates the presence of some impurity in the results and high O/C ratio.
The Raman spectra of G/GO layer show two important peaks called D and G bands (D ~ 1197 and G ~ 1634 cm −1 ). The G and D bands correspond to the presence of sp 2 carbon-type structures within the sample, and defects in the hexagonal structure of graphite respectively (Fig. 1d) 38,39 . The presence of C-O, C=O and C-H bands at 1020, 1624 and 2,923 cm −1 respectively are evident from the FTIR spectra (Fig. 1e.). Also, the spread peak around 3430 cm −1 is due to the tensile vibration of phenol (C-OH) groups 40,41 .
The synthesis of MoSe 2 was down by Solvothermal method 24 in normal ambient environmental conditions. In this method Se powder, Na 2 MoO 4 , distilled water, and Hydrazine Hydrate are mixed in a container. The materials are stirred constantly and heated with rate 2 • C/min for 45 min. Then, the temperature of the homogenize solution is maintained at 120 ℃ for an hour. Afterward, to increase the pH of the solution up to 12, Hydrazine Hydrate is gradually added and the color of the solution turned to dark brown.
After water evaporation by kept the solution in oven at 60 ℃ for 45 min, the residual powder was washed with water and ethanol 3 times and again was kept in oven at 60 ℃ for 10 h until all the water was evaporated and the dark powder was left.
To easy distribution of MoSe 2 powder in water, the obtained powder is stirred in alcohol for 12 h, dried in the environmental conditions and finally, 5mgr of it dispersed in 10 cc distilled water and sonicates to separate individual layers.
The transparent liquid above sediments that contain few/mono-layer of MoSe 2 is achieved after centrifugation at 6000 rpm for 3 h. The quality of MoSe 2 layers is investigated by TEM, SEM, EDX, Raman, FTIR and XRD analysis. Figure 2a,b show the morphology of MoSe 2 by SEM and TEM images. The few-layer structure of MoSe 2 is seen in these pictures. Figure 2c implies the presence of Mo and Se in synthesized MoSe 2 by EDX.
MoS 2 . Two-dimensional MoS 2 sheets synthesis by chemical exfoliation method. In this method, MoS 2 was dispersed in a mixture of ethanol and water with 45% and 55% volume ratio. After stirring the composition for 10 min, it was sonicated for 12 h in 40 Hz frequency to separate MoS 2 sheets from each other. The quality of MoS 2 layers is investigated by TEM, SEM, EDX, Raman, FTIR, and XRD analysis (Fig. 3).  Fig. 3a and Fig. 3b) and indicate to broad sheets of MoS 2 . The purity of MoS 2 sheets is shown in the EDX spectrum (Fig. 3c). The Raman spectra of MoS 2 layer (Fig. 3d) exhibit two important peaks, the in-plane E 2g (~ 381 cm -1 ), and the out-of-plane A 1g (~ 405 cm -1 ). E 2g mode corresponds to the Mo and S atoms vibrating in one direction, while A 1g mode is due to the Sulfur atoms vibrating 14,25 . The difference between these two modes (~ 24 cm -1 ) can be used as a reliable identification for few-layers MoS 2 46 . The FTIR spectrum (Fig. 3e) shows the presence of Mo-S, C-H, and Mo-O bands at 600, 1396, and 1639 cm −1 respectively. The appearance of a spread peak in the region of 3287 is related to the tensile vibration of OH groups 47 .
The XRD of MoS 2 sheets in Fig. 3f illustrates Fabrication of humidity sensor. In this research, SMF is used as the main part of the optical-humidity sensor. To modify the sensing, the diameter of SMF reduces from 34.45 µm by corrosion. For this purpose, 3 cm of the protective layer of SMF was stripped and wiped up with alcohol, then put it in HF acid for 60 min. The process etched some of the clad-layer on the core of SMF to achieve the desirable evanescence field. Figure 4a-c shows the SEM images of ESMF.
In order to make more changes in the refractive index by water absorption, 2D materials have deposited on the surface of ESMF by dip coating in transparent solution contain one or few layers of G/GO, MoS 2 and MoSe 2 . These solutions are prepared by adding 5 mg of synthesized powders to 10 cc Distilled water separately, sonicating for 2 h, and centrifuging. The SEM images of ESMF coated with G/GO, MoSe 2 and MoS 2 illustrate in Fig. 4d-f respectively. Also, the preparation of optical fiber for humidity sensing schematically is shown in Fig. 5.
The OPS and OPM are used as OLTS to analyze RDA, repeatability, sensitivity, and variance versus RH. To create the RH in the range of 20-90% in 25 ∘C, we used a cloud chamber consist of humidifier, humidity control box, temperature controller, and standard humidity sensor as shown in Fig. 5b. Due to the presence of temperature and humidity control system in cloud chamber, these quantities are uniform and constant throughout chamber.

Result
RDAs as a function of RH for ESMF with different coatings are illustrated in Fig. 6a and b for 1310 nm and 1550 nm wavelengths. As shown in these figures, ESMF coated with MoS 2 has zero RDA at RH below 30% and at higher RH has lower RDA relative to ESMFs coated with MoSe 2 and G/GO. So it cannot use as an efficient humidity sensor. Unlike it, the ESMF coated with MoSe 2 has very good humidity sensing at low RH up to 30%. However, the humidity sensor based on MoSe 2 is not applicable at medium and high RH due to the lake of the one-to-one function of RH. The ESMF coated with G/GO has high RDA, especially at RH higher than 40%, and also its RDA has the one-to-one function of RH. The optical humidity sensing mechanism of coated ESMF is related to variation of refractive index by bonding between water molecules and the surface of 2D materials. This physical interaction changes the density of carriers in 2D materials coated on the ESMF and results in the variation of electronic gap energy (E g ). According to the Penn model, E g are inversely correlated to refractive index 49 .  The presence of functional groups and defects determine the type of synthesized semiconductor powder. For example, impurities such as oxygen, water, carbon dioxide, etc. cause to p-type, and others such as hydroxyl, ether, and epoxide groups convert the intrinsic semiconductor to n-type. So, with respect to the FTIR spectrum (Fig. 1e), G/GO has hydroxyl groups and behaves as an n-type semiconductor. On the other hand, MoSe 2 has not hydroxyl group and the presence of Mo bonds (Fig. 2e)  Taking out the electrons from a p-type surface like MoSe 2 , by water adsorption, decreases the refractive index because of the increase in E g . This mechanism in G/GO as an n-type semiconductor is vice versa. However, MoSe 2 coated ESMF at low humidity adsorb H 2 O and show high RDA relatively, and saturate in RH about 30%. At higher humidity, MoSe 2 saturate and will not adsorb more H 2 O. As regards, the functional groups at G/GO can absorb H 2 O up to 90% RH and the its refractive index increase by adsorption of water molecules on G/GO coated ESMF because of the increment of carrier densities.
The MoS 2 coated ESMF has low RDA and indeterminate humidity sensing behavior because of competition between donor and acceptor of electrons impurities on its surface.
As shown in Fig. 6, RDA of G/GO in the telecommunication's standard wavelengths, 1310 nm and 1550 nm, is higher than MoSe 2 , and MoS 2 . In addition to the changes in the density of carriers with respect to changes in relative humidity, the adsorption of water molecules between the graphene oxide sheets cause to change in refractive index and improve its humidity sensing 50 . So other quantities just measure for G/GO-based ESMF.
The repeatability curve shows a reduction of RDA during consecutive tests (Fig. 7a,c), due to saturation hydrogen bonding in G/GO structures. It seems thermal treatment of G/GO coated ESMF at 90 °C can reactive the sensor because of the separate the oxygen and water molecules from the G/GO surface 16,51 . To calculate the formulation for measuring RH from Attenuation, reversing curves are illustrated and fitted by MATLAB, which are shown in Fig. 7b,d. Sensitivity has defined as a ratio between the differential of the output signal and measured properties (humidity) 52 . Sensitivity for the humidity sensors coated with G/GO, MoSe 2 and MoS 2 are illustrated in Fig. 8. It is obvious from this figure, sensitivity of G/GO (MoS 2 ) coted ESMF at RH more than 30% (60%) increases and has more magnitude in 1550 nm wavelength relative to 1330 nm wavelength. While, MoSe 2 coated ESMF has very low sensitivity at both wavelengths.
Variances of any test from the average value are plotted in Fig. 9. As shown, variance for the investigated sensor is small in the RHs less than 40%. In addition, variance is so little in 1310 nm wavelength than 1550 nm. So, results obtained from the humidity sensor based on etched-fiber coated with G/GO have better accuracy at low humidity and 1310 nm wavelength.     License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.