Photocurrent Response in Multiwalled Carbon Nanotube Core

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Photocurrent Response in Multiwalled Carbon Nanotube Core-Molybdenum Disulfide Shell Heterostructures Lili Gong, Liangjun Wang, Junpeng Lu, Cheng Han, Wei Chen, and Chorng Haur Sow J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06609 • Publication Date (Web): 02 Oct 2015 Downloaded from http://pubs.acs.org on October 8, 2015

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The Journal of Physical Chemistry

Photocurrent Response in Multiwalled Carbon Nanotube Core-Molybdenum Disulfide Shell Heterostructures Lili Gong,* Liangjun Wang,* Junpeng Lu,* Cheng Han,† Wei Chen*,†,‡ and Chorng Haur Sow*,‡ *

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542,

†

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Center for Advanced 2D Materials and Graphene Research Center, National University of Singapore, 6 Science Drive 2, Singapore 117546 ‡

Abstract: In this report, few layered molybdenum disulfide (MoS2) shell was coated on core multiwalled carbon nanotube (CNT) by a facile solvothermal method. The morphology and high crystallinity of this structure were demonstrated and verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). After integrated into planar device, CNT-MoS2 core-shell structure exhibits clear photoresponse and a wide response range upon laser illumination. In addition, the device shows a bias dependent and position sensitive photocurrent effect. Further experiments show that larger photocurrent was obtained under laser illumination with longer wavelength. Both the photocurrent and response speed are enhanced when the device is placed under vacuum condition. The simple material synthesis and device fabrication method used in this work may provide a practical strategy for low cost and large scale optical applications. Introduction Following the graphene revolution, great attention has been paid to layered transition metal dichalcogenides (LTMDs) that offer unique properties such as sizeable band gaps and versatile chemistry. Among all, molybdenum disulfide (MoS2), is regarded as one of the most promising LTMD due to its stability, abundance and flexibility. In its bulk form, MoS2 has an indirect band gap of around 1.29eV corresponding to infrared wavelength; while single layer MoS2 is a direct band gap semiconductor with a band gap of 1.9eV, which lies in visible wavelength region.1 This indirect-to-direct transition as well as the band gap change makes MoS2 an extremely interesting candidate for optical applications such as photodetectors,2-4 light emitter,5-6 photovoltaics7 and optoelectronic memory devices.8-9 However, optoelectronic applications based on MoS2 and other LTMDs are still hampered by the low responsivity stemming from their low carrier mobility.10-11 To enhance the photoresponse, several strategies have been implemented including applying optical microcavity to trap light,5 using optical waveguide,12 placing plasmonic nanostructures to enhance surface plasmons13-14 and focused laser healing.15 Moreover, for optoelectronics, the combination of LTMDs and conducting materials such as graphene can also be an effective approach.9, 16-17 For these hybrid materials, electron-hole pairs produced by incident light in LTMDs can be easily separated at the interface and one type of charge carriers are transferred to graphene, resulting in long recombination time, thus high photoresponse is achieved. In this report, we make use of the highly conducting nature of multiwalled carbon nanotube (CNT) to construct CNT-MoS2 core-shell structure by a simple solvothermal method and investigate its optoelectronic properties. Previously, CNT and MoS2 hybrid structures with various morphologies have been successfully synthesized18-21 and studied for lithium ion batteries,22-25 supercapacitors26-27 and hydrogen evolution reactions.28 These researches mainly take advantage of the large surface area, efficient edge sites and high conductance of the hybrid material. However, little research utilizes the photosensitivity of CNT and MoS2 hybrid material where MoS2 can work as light sensitizer and CNT help to conduct light induced charge carriers. Here, we constructed CNT-MoS2 core-shell structure into planer device and tested its ACS Paragon Plus 1 Environment


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optoelectronic properties for the first time. Under illumination, the device shows clear photoresponse and the photocurrent depends on both power density and wavelength. When illuminated by laser with a wavelength of 532nm, the photoresponsivity is calculated to be 24mA/W. Further experiment shows that a ~70% enhancement in photocurrent can be achieved in high vacuum condition. It is also worth mentioning that the capability to be fabricated into large scale and uniform thin network makes our CNT-MoS2 core-shell structure a viable candidate for low-cost applications.

Experimental section Synthesis of CNT-MoS2 core-shell structure 20 mg of acid-treated multiwalled CNT powder was dispersed in 12 ml dimethylformamide (DMF) by sonication and violent stirring until homogenous suspension was achieved. Then 40 mg (NH4)2MoS4 powder was added to the suspension and stirred for another 10 minutes. The well-dispersed suspension was then transferred to a Teflon-lined stainless steel autoclave and heated at 200 for 15 hours. After naturally cooled down to room temperature, the black precipitate was collected by centrifugation, washed thoroughly with DI water and absolute ethanol and dried at 60 overnight. The as-obtained product was further annealed at 800 ℃ in the atmosphere of 5% H2 balanced by argon for 2h. Materials characterization The morphologies of the core-shell structure were characterized by field-emission scanning electron microscope (FESEM, JEOL JSM-6700F, 5kV) and transmission electron microscope (TEM, JEOL JEM3010 LaB6). Crystallographic information was collected by X-ray diffraction (XRD: Philips X'PERT MPD, Cu-KR radiation, =1.5418 Å).The Raman spectra were obtained by using Renishaw inVia 2000 with a laser wavelength of 532nm. The X-ray photoelectron spectroscopy (XPS) was performed on VG ESCALAB Mk II system. Optical images were taken by Olympus DP73 microscope. Thin network and device fabrication 3.5mg of CNT-MoS2 core-shell hybrid was dissolved in 1%wt triton 114 solution, sonicated and stirred for 1h violently. Then the suspension was filtered and washed through mixed cellulose ester (MCE) membrane with an average pore size of 220nm by DI water. After that, the thin film was dried in vacuum chamber for 1 day. Then gold electrode (thickness around 100nm) was sputtered on the sample by using a mask with a gap of around 90 m. Separate devices of bare CNT or MoS2 were fabricated under the same condition. Characterization The morphology of CNT-MoS2 core-shell structure was demonstrated by TEM. As can be clearly seen in Figure 1a, CNT backbone with diameter of around 40nm was tightly sheathed with MoS2 layered structures, with layer number varying from 2 to 12. In the high resolution TEM image (Figure 1b), the interplanar distance for MoS2 and CNT layers can be identified to be 0.64nm and 0.37nm respectively. These values are in good agreement with the values for d(002) spacing of MoS2 and the lattice spacing between CNT basal planes.29-30

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Figure 1. (a) Low and (b) High magnification TEM image of CNT-MoS2 core-shell structure.

To further examine the crystallinity of CNT-MoS2 core-shell structure synthesized by the solvothermal method, X-ray diffraction (XRD) and Raman spectroscopy were employed. Figure 2a compares the XRD pattern for CNT-MoS2 and bare CNT. As is demonstrated, the main peak for bare CNT locates at 2 =26 , which should be assigned to its (002) plane.31 For CNT-MoS2 hybrid, the peak from the contribution of CNT locates at 26.1 , which is very close to that of bare CNT, suggesting that MoS2 outer shells do not introduce much distortion to CNT surface. Other peaks centered at 14.0 , 33.2 , 39.3 , 58.3 can be attributed to (002), (100), (103) and (110) planes of MoS2.20 The high quality of CNT-MoS2 hybrid is also verified by Raman spectroscopy (Figure 2b). Signals from MoS2 appear in low Raman shift region (Figure 2c). Sharp peaks from in-plane E and out-of-plane A1g vibration mode of MoS2 are located at 383.6cm-1 and 408.3cm-1, which verifies its high crystallinity. The interval for these two peaks is measured to be 24.7cm-1, indicating 6 layers on average.32 Figure 2d compares the Raman spectra for both CNT-MoS2 hybrid and bare CNT in high Raman shift region by normalizing them according to the intensity of D band. The D and G peaks, which respectively characterize the disordering in graphitic layers and in-plane stretching, are located at ACS Paragon Plus 3 Environment


1352.8 cm-1 and 1585.0 cm-1 for both CNT-MoS2 hybrid and bare CNT. The similarity in peak position and shape in these two spectra may indicate a negligible change in electronic structure of CNT upon coating of MoS2 layers. However, contrary to Koroteev’s study, CNT-MoS2 core-shell structure shows a higher ratio of D and G peak intensities (ID/IG ratio) compared with bare CNT, which may suggest the introduction of defects to CNT surface by MoS2 outer shells in CNT-MoS2 core-shell structure.19

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Figure 2. (a) XRD pattern and (b) Raman spectra for CNT-MoS2 core-shell structure. (c) Raman spectra of low Raman shift region for CNT-MoS2 core-shell structure. (d) Raman spectra of high Raman shift region for both CNT-MoS2 core-shell structure and bare CNT. The two spectra are normalized according to the intensity of D band. The chemical states of Mo, S and C in CNT-MoS2 core-shell structure were investigated by XPS. In Figure 3a, the two peaks located at 232.65eV and 229.45eV can be indexed to Mo 3d3/2 and 3d5/2 doublet,24 indicating a +4 oxidation state. The small peak centered at 226.55eV corresponds to the binding energy of S 2s orbital. For the spectrum of S element (Figure 3b), the two peaks located at 163.45eV and 162.35 eV arise from the binding energy of S 2p1/2 and 2p3/2 orbitals.33 As for C (Figure 3c), the spectrum for CNTMoS2 (red curve) displays a curve shape similar to that of bare CNT (black curve), which suggests that negligible C-O bond, C=O bond or functional groups such as -COOH, -OH were introduced to CNT surface via acid treatment. This result implies that the chemical states of carbon atoms did not vary significantly upon the coating of MoS2. Moreover, instead of covalent bonds, most MoS2 layers may connect to CNT surface by weak van der Waals force.


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Figure 3. XPS spectra for CNT-MoS2 core-shell structure and bare CNT: (a) Mo spectrum and (b) S spectrum for CNT-MoS2 core-shell structure. (c) C spectrum for both CNT-MoS2 core-shell structure and bare CNT.

Device fabrication CNT-MoS2 thin network was fabricated with the aid of mixed cellulose ester (MCE) membrane by vacuum filtration (Figure 4a). Unlike other wet-chemistry film fabrication methods such as spin-coating, airbrushing and dip coating, vacuum filtration can provide a cleaner and more uniform film surface for nanotube-like structure. Besides, the fabricated network can be cut into arbitrary size and shape and transferred to any substrate by dissolving the MCE membrane in acetone and washing for several times. A photograph of CNT-MoS2 thin network transferred to a glass slide is shown in Figure 4b. For a better knowledge of the thickness and surface condition of the network, cross sectional and surface morphologies were characterized by SEM. As demonstrated in Figure 4c, d and e, the thin network presents a thickness of around 8 m on average and the nanotubes on the surface are dense and highly entangled, which provides sufficient current pathways, thus enhancing the contact and reducing electrical resistance.34 The network was made into devices by sputtering gold on the surface as electrode using a shadow mask and a two-probe configuration was adopted. Figure 4f shows the optical microscopy picture for the electrodes. The width of the channel is around 90 m and the thickness of gold is around 100nm. The whole device fabrication process is highly controllable and repeatable, which promotes the reliability of our photocurrent measurement.



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Figure 4. Characterization of CNT-MoS2 network and device. (a) Photograph of CNT-MoS2 network covering an area of around 12 cm2 on top of the MCE membrane. (b) CNT- MoS2 network transferred to a glass slide. (c) Cross sectional image of CNT-MoS2 network with an average thickness of around 8 μm. (d) Magnified cross sectional image. (e) Surface of CNT-MoS2 network. (f) Optical image of gold electrodes. The scale bars are (a) 5mm (b) 1cm (c) 20 μm (d) 2 m (e) 1 m and (f) 50 m.

Results and Discussions After device fabrication, the optoelectronic properties of the hybrid material were tested. A focused laser beam with wavelength of 532nm and spot size of around 3 μm2 was employed as the light source and shined at the center of the electrodes. The power was maintained at 2mW. Figure 5a shows the I-V characteristic. The current is linearly dependent on voltage both in dark and under illumination. The slightly steeper slope under illumination suggests a positive enhancement upon photon injection. The extracted photocurrent, shown as red curve in Figure 5b, roughly exhibits a linear behavior at low voltage (-2 to 2V). It should be emphasized that the magnitude of the dark current is about 2 orders of magnitude larger than the photocurrent. Hence the observed photocurrent represents a small increment over the dark current. The mechanism for the positive photocurrent could be attributed to the transfer of photo-excited charge carriers to CNT surface. Upon laser illumination, electron-hole pairs are generated inside MoS2 layers. Then electrons can be transferred to CNT surface. These transferred electrons increase the charge carrier number in CNT which results in the positive photoresponse.


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Figure 6. (a) Photocurrent response under different power intensities. Inset shows the linear dependence of photocurrent on power. (b) Cycling stability. To understand the relationship between photocurrent and laser power, current versus time (I-t) characteristic under irradiation of different power densities was measured. For the convenience of adjusting and measuring power, a broad beam laser with adjustable power densities was used as light source. The wavelength is 532nm and the spot size is around 2 mm2. During the measurement, the bias voltage is maintained at 2V. As demonstrated in Figure 6a, CNT- MoS2 gives clear photoresponse as the power density varies from 60mW/cm2 to 7.7mW/cm2. This indicates its potential use towards photodetecting under widerange of intensities. Moreover, an apparent drop in photocurrent is observed when power density decreases. A linear relationship between photocurrent and incident power can be easily found, as shown in the inset. It is worth noticing that, laser power was adjusted and measured between laser is turned off and turned on, which guarantees the reliability of our experimental results. The photoresponsivity is calculated from above results to be 24mA/W according to the formula R=Iph/(SeffP). Here, R is photoresponsivity; Iph is the photocurrent; Seff is the effective working area defined by the area between two electrodes; P is the power density of incident laser. This photoresponsivity value is better than that of single wall carbon nanotube and single layer MoS2 p-n diode at 532nm wavelength (around 10mA/W).35 An increase in dark current can be noticed in Figure 6a. This could be originated from the heating effect on CNT, which corresponds to the result that multiwalled carbon nanotube demonstrates a negative dρ/dT value at room temperature (ρ is resistivity and ACS Paragon Plus 7 Environment

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T is temperature).36 In our system, both the laser irradiation and the Joule heat could be the heat source. According to Figure 6a, the dark current appears to increase at a constant rate regardless of the change in laser density. Moreover, if the bias voltage changes while the laser density remains the same (see supplementary information Figure S4), the dark current increases more under higher voltages. Thus, the most possible cause of the dark current rise could be the Joule heat. Next, a focused laser beam with a power of 1mW and spot size of around 3 m2 was applied to examine the cycling stability of the hybrid material. The bias voltage was maintained at 2V. Figure 6b shows the I-t characteristics, where the laser was periodically irradiated onto the center of the device. After irradiation for 10 cycles, only 3% of photocurrent shrinkage was observed. This persistent photoresponse indicates a good cycling stability, which is a significant aspect for photodetecting applications.


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Figure 7. Photocurrent dynamics of CNT-MoS2 core-shell structure and bare CNT under focused laser beam (wavelength: 532nm, power: 1mW) with a spot size of around 5 m2. CNT-MoS2 core-shell structure: (a) Typical time-resolved photoresponse (b) Enlarged growth and decay process (c) Fitted exponential functions ACS Paragon Plus 9 Environment


for growth and (d) Decay. Bare CNT: (e) Typical time-resolved photoresponse (f) Enlarged growth and decay process (g) Fitted exponential functions for growth and (h) Decay process. To gain insight to the generation mechanism of the photocurrent, a more detailed study was carried out on the time-characteristic of the photoresponse for both CNT-MoS2 core-shell structure and bare CNT. The typical I- t characteristics with the bias voltage maintaining at 2V for CNT-MoS2 was shown in Figure 7a. Upon turning on laser, the current exhibits a rapid rise, followed by a slower increase until saturated. When laser is removed, the current drops fast initially, then the decay process slows down and a persistent photoconductivity appears. Similar current dropping behavior also shows up in MoS2-graphene hybrid9 and CVD-MoS2.37 The mechanism will be discussed later. From the enlarged growth and decay curve in Figure 7b, the growth and decay time for CNT-MoS2 is measured to be 1.8s and 2.9s (The growth time is defined as the time interval for photocurrent to change from 10% to 90% of its total increase. The decay time is the time interval to change from 90% to 10% of the total drop). To further understand the growth and decay mechanism, exponential functions were applied to fit the two processes. The fitting equations are shown below: I(t)rise=I0r+ A{exp[(t-t0)/τ ]} +B{exp[(t-t0)/ τ ]}, (1) I(t)fall=I0f+ A{exp[-(t-t0)/τ ]}+B{exp[-(t-t0)/ τ ]}, (2) I0r and I0f are the current before laser is turned on and turned off. τ , τ and τ , τ are time constants for growth process and decay process, respectively. Figure 7c and d show the fitting curves. The time constant for the photocurrent rise is found to be τ =0.072s, τ =2.95s and τ =0.14s, τ =2.11s for decay process. These fitting results clearly indicate that both the growth and decay process consist of two parts with one faster and the other one slower. I-t characteristics were also conducted for bare CNT. Since the resistance of bare CNT is much lower than CNT-MoS2 hybrid material, lower voltage of 0.13V is applied to guarantee a similar dark current to that of CNT-MoS2. As shown in Figure 7e and f, the response of bare CNT towards laser is much slower. For a clear comparison, all the parameters for the time resolved response of both CNTMoS2 and bare CNT are listed in Table 1. Due to the much slower response of CNT, it is reasonable to attribute the slower part in growth and decay process for CNT-MoS2 to the contribution of CNT. Whereas, the fast rise and fall response should stem from MoS2, since MoS2 is able to give an extremely fast on-off speed within the time scale of tens of microseconds.38 Table 1. Parameters for growth and decay process for CNT-MoS2 core shell structure and bare CNT. Growth Decay τ (s) τ (s) τ (s) τ (s) Time(s) Time(s) CNT-MoS2 core shell 1.8 2.7 0.072 2.95 0.14 2.11 structure Bare CNT 22.4 14.6 1.97 16.0 2.2 11.7

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Figure 8. Photocurrent of different spots across the two gold electrodes under external bias of (a) 2V and (b) 1 mV. (c) Typical positive and negative photoresponse at positions of -50 m and 50 m under bias of 1 mV. As our device comprises of different active components, it would be worthwhile to investigate which is the main contributor to the photocurrent. We take advantage of our focused laser beam (wavelength: 532nm, power: 1mw) to measure the photocurrent of different spots along a line across the two electrodes. Firstly, the external voltage is set to be 2V and the positive bias is applied to the left electrode. As can be seen in Figure 8a, the photocurrent varies according to position with the center reaching the maximum. Due to the thinness of gold (around 100nm) and the roughness of the network surface, photocurrent is also obtained in the electrode region as the laser beam is able to penetrate through the electrode. Then the experiment was repeated under a much smaller external bias of 1mV (Figure 8b). Expectedly, a much smaller photocurrent (of the order of nA) was observed but the highest photoresponse appears at the two inner interfaces between gold electrodes and CNT-MoS2 network with reversed sign. Much less photoresponse shows up in the middle. Figure 8c depicts the typical positive and negative transient photocurrent at the interfaces (position: -50 m and 50 m, respectively). Similar position effect was also observed in CNT networks.39-41 In our case, firstly we consider a simplified case that only one CNT-MoS2 nanotube connects the two electrodes. A simplified band diagrams for the circuit are shown in Figure 9. First consider the case when the bias is low (Figure 9c) and the focused laser beam is illuminated at the center portion of the CNT-MoS2 sample (at position 0 m). Photon energy is absorbed by the hybrid material, resulting in the generation of electronhole pairs. These carriers would diffuse randomly in the network of the hybrid since there is only a very small field. Thus high scattering lost and high recombination result in very negligible photocurrent. This is consistent with the observation shown in center part of Figure 8(b). As the laser beam approaches the goldCNT-MoS2 hybrid interface (at position of -50 m), the focused laser beam should generate similar amount of electron-hole pairs since the laser power is the same. High recombination of the electron-hole pairs should still occur but some of the hot electrons can overcome the energy barrier via tunneling or thermal emission and cross over to the gold electrode. This gives rise to a small amount of photocurrent when the laser beam is present near the interface. When the laser beam approaches the other gold-CNT-MoS2 hybrid interface (at position of 50 m), similar phenomenon occurs but the direction of motion of the electron is reversed. This gives rise to a reversal in sign of the photocurrent. In fact whenever the laser beam is illuminated at an interface with gold on the left and hybrid on the right such as positions of -50 m and 200 m, we observed positive photocurrent. On the other hand, whenever the laser beam is illuminated at an interface with gold on the right and hybrid on the left such as positions of -200 m and 50 m, we observed negative photocurrent. When a large external bias is applied, a potential drop is formed between the electrodes (Figure 9b). The photogenerated electron-hole pairs can be separated by the strong local electric field. Naturally, the magnitude of the photocurrent detected will be much higher than the photocurrent detected at low bias. This is evident when we compare Figure 8(a) and Figure 8(b). Other notable feature from Figure 8(a) and (b) is that whenever the laser beam is focused onto the gold electrode, we detected photocurrent as well. This is attributed to the fact that we only deposited a thin layer (100nm) of gold onto the nanotubes network. Laser light is able to penetrate the electrode and exert an influence on the hybrid sample below the gold electrode. Naturally, the intensity of the laser beam will be reduced and thus results in a lower photocurrent. Another interesting feature shown in Figure 8(a) is that the photocurrent reaches its maximum when the laser beam is focused at the center of the hybrid network. The photocurrent at the center is about 15% higher than the photocurrent detected when the laser beam is illuminated at the interface. We attribute this observation to interplay of multiple factors. Since our sample comprises of physically entangled network of nanotubes, the motion of the charge carriers across the sample will be subjected to the influence of multiple scattering. For ACS Paragon Plus 11 Environment


example, when the laser beam is focused near the left interface, electrons and holes will be generated locally. The electrons generated will be readily collected by the gold electrode nearby. However, the holes will need to travel across the full length of the network to the other electrode in order to contribute to the measured current. Travelling those mazes of interconnected networks implies a high chance of scattering lost. Likewise, when the laser beam is focused near the right interface, the holes generated will be readily collected but the electrons will need to travel across the network with high chance of scattering lost. On the other hand, when the laser beam is focused at the center of the hybrid, the electrons and holes generated will travel in the opposite direction and travel for a shorter distance before reaching the respective electrodes on both sides. This may result in a lesser degree of net loss and thus the photocurrent reaches a maximum when the laser beam is illuminated at the center of the sample.

Figure 9. Simplified band diagrams for conditions of (a) equilibrium (b) under illumination with large external bias and (c) under illumination with small external bias.

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Time (s) Time (s) Figure 10. (a) Photoresponse towards different wavelengths in ambient. (b) Photoresponse towards different wavelengths in high vacuum of 3x10-6 mbar. The photoresponse of the device towards different wavelengths under both ambient and vacuum conditions were also investigated. During the test, laser with a certain wavelength was irradiated onto the device for two cycles before the wavelength was changed. All the lasers come from one optical fiber and have the same spot size (around 2mm2) and power density (40mW/cm2). Figure 10a shows the I-t characteristic towards four wavelengths obtained in ambient (Bias voltage: 2V). The device appears to provide higher photocurrent towards longer wavelength. To testify this trend observed, we changed the sequence of laser irradiation and repeated the experiments for four times to avoid random error and to exclude the influence of sample damage induced by laser. As a result, the positive dependence of photocurrent on wavelength was also spotted and the average photocurrent with error bars towards different wavelengths was shown in the inset. This is consistent with the wavelength dependent study for few-layer ACS Paragon Plus 12 Environment

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MoS2 in ref 38. Similar experiments were also conducted in high vacuum with a pressure of 3x10-6 mbar. As shown in Figure 9b, the photocurrent enhancement towards wavelength increase still appears. Besides, compared with ambient condition, a ~70% increase in photocurrent and a large enhancement in the slow response speed (both growth time and decay time less than 1s) can be observed. In addition, the persistent photoconductivity behavior is largely suppressed. It is clear that the adsorbates (charge impurities or moistures) in ambient strongly affect the optoelectronic properties of the hybrid material. The change in photocurrent and response speed should be attributed to the outer MoS2 shell which is exposed to ambient. The adsorbates in ambient may act as electron trap centers in MoS2, which can increase carrier scattering and extend recombination process.37, 42 Therefore, lower photocurrent and slower response speed is obtained in ambient.

Conclusion In summary, the molybdenum disulfide (MoS2) and carbon nanotube (CNT) core-shell structure has been synthesized and fabricated into planer devices successfully. The high crystallinity is verified by TEM, XRD, Raman spectroscopy and XPS. Upon laser illumination in ambient, the hybrid material gives clear photoresponse and shows the capability for light sensing under various power intensities and wavelengths, which makes it a promising candidate in photodetecting. For a better understanding of the photocurrent, experiment was also conducted in vacuum, and four laser wavelengths were included. Compared with the photocurrent obtained in ambient, a large photocurrent increase and a faster response speed were observed in vacuum. This may be due to the trapping effect induced by adsorbed charge impurities from air. The wavelength-dependent study shows that larger photocurrent is obtained when illumination light has longer wavelength. In addition, bias dependent and position sensitive photocurrent effects were found, which indicates the existence of schottky barriers. Both the synthesis for CNT-MoS2 core-shell structure and device fabrication in our work is convenient and easy to control, which makes CNT-MoS2 core-shell structure a suitable candidate for low-cost optical applications.

Acknowledgements The authors acknowledge the financial support from Singapore MOE grant R143-000-559-112, under which the high vacuum photocurrent measurement system is set up.

Author information Corresponding author: Sow Chorng Haur E-mail: [email protected]

Associated content The supporting Information is available free of charge via the internet at http://pubs.acs.org. Raman spectroscopy of the hydrothermal product before and after H2 annealing; EDS mapping of CNTMoS2 core-shell structure; thermogravimetric analysis for CNT-MoS2 hybrid; cycling stability for CNTMoS2 under different bias voltages; I-V characterization and cycling stability for bare CNT; I-V characterization for bare MoS2.



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