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Abstract
One important use of molybdenum disulfide (MoS2) could be in making sensing and detection devices with optical chip or fiber. Here, MoS2 nanosheets coated on side-polished optical fiber (SPF) is proposed, which can enhance the localized interaction between evanescent light of fiber core and MoS2 nanosheets, this can motivate greatly sensing and detection performance. Moreover, the MoS2 nanosheet possesses exceedingly high surface/volume ratio. By combining the MoS2 nanosheets and the side-polished fiber, humidity sensing characteristics has been demonstrated. The optical transmitted power (OTP) of the MoS2-based SPF changes with a negative correlation to the variation of relative humidity (RH) in experiments. The OTP changes of the MoS2-based SPF as an exponential function can reach ~13.5dB (~54 fold enhancement) when the RH ranges from 40%RH to 85%RH. Furthermore, experiments on the monitoring of human breath have also been conducted to evaluate the response time (0.85 s) and the recovery time (0.85 s). The performance comparison between this proposed device and the other recent-developed fiber-optic humidity sensing devices in literature illustrates the superiority of the MoS2-based SPF in humidity sensing and monitoring of human breath, which paves a path for the MoS2 nanosheets to integrate in lab-on-fiber sensing and detection devices.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since graphene was first isolated from graphite, two-dimensional (2D) nanomaterials have been studied extensively and applied in various fields due to its distinct characteristics associated with their 2D morphology [1–4]. Layered transition metal dichalcogenides (TMDs) is a kind of 2D materials. The d-electrons’ interactions of layered TMDs can give rise to new physic phenomena [5]. As a typically layered TMD material, layers of molybdenum disulfide (MoS2) has two hexagonal planes of S atoms coordinated through ionic-covalent interactions with the Mo atoms in a trigonal prismatic arrangement [6]. The quantum confinement effects on electronic structure and optical properties of MoS2 have been investigated in MoS2 thin films as well as in MoS2 nanoplates and nanotubes [7]. This material has been considered as a promising candidate for next-generation flexible electronic [8–12], optoelectronic [13–17] and optical devices [6,18,19].
Some humidity sensors are fabricated with nanocomposite film deposited on prism [21,22], others use laser beam to transmit through the sensing film [23]. Burman et al. reported MoS2/GO (graphene oxide) nanocomposite based sensing layers for resistive humidity sensor on a Si/SiO2 substrate containing aluminum electrodes [24]. Ravindra et al. demonstrated the response of resistive sensor with tungsten disulphide (WS2) nanosheets varied ~25.6 times from 40% RH to 80% RH on interdigitated electrodes realized over Si/SiO2 substrate [25]. On-chip MoS2-based electrochemical sensing devices might not be suitable for remote detection, flammable explosive environment or environment with strong electromagnetic interference. Compared to conventional electronic or on-chip sensors, fiber-optic devices are low-cost, simply-structured and resistant to corrosive, strong electromagnetic interference, high temperature and radiation effects. Moreover, they are compatible with current fiber-optic communication network. Therefore, combining optical fiber and MoS2 nanosheet is considered to take advantage of the superior properties of these two. To date, the fiber-optic humidity sensing devices using MoS2 as sensitive material with relatively large dynamic response and fast response (<1 s) has few been reported, neither did the elucidation of the sensing mechanism.
Side-polished fiber (SPF) is made by polishing a standard single mode optical fiber, which offers a more robust platform than that of micro and nanostructured optical fiber [26]. By removing a part of fiber cladding, its evanescent light can interact with the deposited material. Consequently, various devices using SPF with this feature were reported, such as temperature sensor [27], fiber-optic violet light sensing device [28], ultraviolet (UV) detection device [29,30] etc.
In this paper, we fabricated an all-optically fiber-optic humidity sensing device by combining the SPF and the MoS2 nanosheets. The MoS2 nanosheets were deposited onto the polished surface of the SPF through evaporating MoS2 solution. SPF not only has high compatibility with fiber-optic system, but also have flat polished surface for containing sensitive materials, which makes it an ideal and robust platform to combine with MoS2 nanosheets to form a lab-on-fiber device. When the environmental humidity changes (40%RH- 85%RH), the optical transmitted power (OTP) of the proposed MoS2-based SPF responses with a largely dynamic range of ~13.5dB owning to the enhanced interaction between evanescent light and MoS2 nanosheets (large surface/volume ratio). The experimental results demonstrate the properties and reveal its humidity sensing characteristics and the monitoring performance of human breath.
2. Device fabrication
The device was fabricated with SPF as shown in the structure diagram in Fig. 1(a). The SPF is made by polishing a standard single mode optical fiber (SMF-28, with its diameter of 125μm and the diameter of fiber core of ~8μm) via wheel polishing method [27]. The polished length of the SPF was ~15mm and the depth ~57.5μm as shown in Fig. 1(b), which meant the residual thickness of cladding was ~1μm. The SPF was then fixed on a glass slide by ultraviolet curing adhesive to enhance the mechanical strength of the device. The polished surface was kept upward and surrounded by a basin (40mm × 8mm × 2 mm) fabricated by the ultraviolet curing adhesive, as displayed in Fig. 1(a).
Few layers MoS2 solution (concentration: 1mg/ml, XFNANO) was prepared and used in our experiments. To avoid agglomeration, the MoS2 solution was first treated by ultrasonication for ~30 minutes. Then 0.5ml solution of few-layer MoS2 was dropped into the basin. The concentration and the volume of this solution will determine the average thickness of deposition, which can be a trade-off between the insertion loss and the dynamically response range of the fabricated device. After 4 hours of natural evaporation at room temperature, the device for humidity sensing was obtained. The SPF wrapped with MoS2 nanosheets was observed by scanning electron microscope (SEM) as shown in Fig. 2(a) and 2(b).
Fig. 2 (a) SEM images of the polished surface with MoS2 nanosheets. (b) Enlarged view of the region marked by white square in (a).
Atomic force microscopy (AFM) was used to measure the thickness (15-180nm) and the morphology of MoS2 nanosheets as shown in Fig. 3, which indicated the deposited MoS2 nanosheets with stacked layers were not homogenously distributed on the polished surface, but with randomly layered structures and wrinkles. Raman spectra (illuminated by a 514.5nm laser) of the MoS2 nanosheets were measured with a Raman Microscope (RENISHAW, UK) as shown in Fig. 4. The E12g mode is a characteristic peak caused by in-plane motion between Mo and S atoms, while the A1g mode is generated by an out-of-plane vibration of two S atoms [34]. We observe that the in-plane E12g mode is at 381.24 cm-1 and the out-of-plane A1g mode at 404 cm-1. Thus confirm the MoS2 nanosheets having been successfully coated on the SPF.
Fig. 3 Atomic force microscopy (AFM) image of MoS2 on the SPF and the cross-sectional profile along the sampled line.
3. Experiments and analysis
3.1 Experimental setup
The experimental setup was constituted of a highly steady 1550 nm DFB laser source (SOF-155-D DFB LASER), a coupler, a temperature-humidity chamber (BPS-100CL), two optical power meters and a personal computer, As shown in Fig. 5. Two fiber-optic samples (bare SPF and SPF deposited with MoS2 nanosheets) were well-placed in the chamber and connected with the coupler using the DFB laser as input and the optical power meters as output. A commercial humidity sensor (Testo 175H1) was utilized to monitor the RH of the chamber in real time. During the humidity sensing experiments, the temperature in the chamber was set at 27°C, while the RH in the chamber increased from ~40%RH to ~85%RH and then decreased from ~85%RH to ~40%RH with steps displayed in Fig. 6(a). Each humidity step lasted ~10 minutes to establish a stable humidity. Both the OTP and the RH were recorded by the computer during the whole experimental process.
Fig. 6 (a) Variation of the RH in the chamber. (b) Variation of relative power (RP) of bare SPF. (c) Variation of RP for SPF device with MoS2. (d) Relation between RH and RP of MoS2-based device.
3.2 Experimental results and discussion
The RH variation in the chamber was depicted in Fig. 6(a). The fluctuations in the duration of stable humidity step were caused by the feedback tuning of the chamber. The OTP variation of the bare SPF in Fig. 6(b) was ~0.25dB without showing any dependence on the RH variation. The OTP variation of the MoS2-based SPF, as depicted in Fig. 6(c), changed with laddering characteristics and corresponding tendency with highest variation of ~13.5dB, which is ~54 times as large as that of the bare SPF sample. Thus, the MoS2 nanosheet as sensitive nanomaterial which can enhance the localized interaction in the interface between the SPF and the MoS2 nanosheets play a crucial role in the enhancement of humidity sensing.
The relation between the RH and the variations of relative power (RP) can be obtained from Fig. 6(a) and 6(c) as depicted in Fig. 6(d). The abscissa and ordinate values of each point in Fig. 6(d) are the average values of each step in Fig. 6(a) and 6(c) respectively. The RP decreases from about −2.70dB to −15.27dB when the RH increases from ~40%RH to ~85%RH, and it increases from −15.27dB to about −2.53dB when the RH decreases from ~85%RH to ~40%RH. The OTP changes much larger when the humidity in relatively high RH (>70%). A nonlinear curve fitting to the relationship between the RH (RH ascending and RH descending respectively) and the relative power (RP) was made and shown as the blue line (with circle marker) and red line (with cross marker) in Fig. 6(d). Equation (1) below presents the fitting curve with a correlation coefficient of 99.9% (RH ascending) and the following Eq. (2) presents the fitting curve with a correlation coefficient of 99.3% (RH descending) respectively:
To confirm the repeatability and stability of the sensing properties, we adjusted the RH from ~47% directly up to ~73% and tested back and forth for several times as displayed in Fig. 7(a). As the humidity varied, the OTP changed accordingly as shown in Fig. 7(b), which demonstrated the repeatability and reversibility and thus verified that the device can be functioned for humidity sensing. The data in the black square (2000th s to 2600th s) and that in the red square (3100th s to 3700th s) in Fig. 7 were enlarged to show in Fig. 8(a) and 8(b) respectively. The RH fluctuations (feedback tuning of servo system) inside the chamber could be tracked and followed by the OTP variation of the MoS2-based fiber-optic device at ~73%RH and ~47%RH.
Fig. 7 (a) Variation of RH in the chamber. (b)Variation of relative power through the MoS2-base SPF device.
Fig. 8 (a) Enlarged view of the first marked rectangle (black) in Fig. 7(a) from 2000th s to 2600th s. (b) Enlarged view of second marked rectangle (red) in Fig. 7(b) from 3100th s to 3700th s.
The use of MoS2 nanosheets deposited on SPF for humidity sensing can be explained principally as follows: While the humidity in the chamber increases, the concentration of H2O molecules increases, and the H2O molecules will be adsorbed by the MoS2 nanosheets, which induces the changing of concentration of the charge carriers [20,31,32]. And an amount of charges will be transferred from H2O to MoS2 [33]. This will induce the change for the effective refractive index (RI) of the deposited MoS2 nanosheets. Therefore, following a RH increase, the imaginary part of the RI of the MoS2 nanosheets will be enhanced, which increases the light absorption [34], while the OTP of the MoS2-based SPF decreases. Hence, the enhancement of humidity sensing can be achieved.
3.3 Human breath monitoring
In order to further explore the sensing characteristics (e.g. response time and recovery time) and application of the MoS2-based SPF, experiments on monitoring human breath were conducted. The MoS2-based SPF at room temperature was tested for human breath with experimental setup as Fig. 9(a). When the deep breath (exhale/inhale) repeated for ~100 seconds, the OTP varied in accordance with the evolution of the exhale/inhale cycles as depicted in Fig. 9(b). Each cycle (include rising time and recovery time) was completed in ~8-10 seconds with a maximum extinction ratio of ~7dB.
Fig. 9 (a) Experimental setup for monitoring human breath. (b) Response of the SPF with MoS2 in deep breathing process.
To further investigate the monitoring performance, experiments on different breathing patterns such as short and slight breathing were also performed. Once the breath starts, the fast repetition of ~1.6 s-2.5 s (one exhalation and inhalation cycle) can be tracked by the OTP variation as illustrated in Fig. 10(a). The relative variation of OTP is of ~1.1-1.9dB, which is smaller comparing to the deep breath. When the breathing was off, the OTP could be recovered relatively quickly to its equilibrium level. The OTP response from the 47th second to 52th second in Fig. 10(a) (dashed red rectangle) was enlarged as Fig. 10(b), which revealed the best response time of ~0.85 s and the best recovery time of ~0.85 s for monitoring the short and slight human breathing.
Fig. 10 (a) Response characteristic of fast human breath process. (b) Enlarged view of the response from 47th s-52th s to evaluate the response time and recovery time.
The comparison between our proposed device and other types of recent-developed fiber-optic humidity sensors in literature is given in Table 1. The MoS2 nanosheets based SPF possesses quicker response and recovery speed with relatively large dynamic response (13.5dB/(40%RH-85%RH)). These demonstrated performances reveal the MoS2 nanosheets as enhanced interaction and sensitive nanomaterial could be a highly potential candidate for lab-on-fiber sensing and detection devices.
Table 1. Comparison of the main performance between the proposed MoS2 nanosheets based SPF sensing device and other recent-developed fiber-optic sensing devices in literature
4. Conclusions
In conclusion, MoS2 nanosheets wrapped on the SPF is proposed for fiber-optic compatible humidity sensing device. Due to the enhanced interaction between the evanescent light of SPF and MoS2 nanosheets (with large surface/volume ratio), the OTP variation of the MoS2-based device can achieved a largely dynamic response of ~13.5dB for the RH ranging from ~40% to ~85%. It reveals that the applied MoS2 nanosheets enhance significantly the humidity sensing performance. Experiments on monitoring human breathing have also been performed which demonstrate the monitoring performance for different breath patterns and evaluate its response time (0.85 s) and the recovery time (0.85 s). The experimental results and performance analysis validate that the MoS2 nanosheets are highly potential for integrating in lab-on-fiber sensing and detection devices.
Funding
National Natural Science Foundation of China (NSFC) (61775084, 61405075, 61475066, 61675092, 61177075, 61275046); Guangdong Natural Science Funds for Distinguish Young Scholar (2015A030306046); National Major Project of China (22104001, 22117001); Natural Science Foundation of Guangdong Province (2016TQ03X962); China Scholarship Council (201606785015).
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