Facile synthesis of large-area and highly crystalline WS2 film on dielectric surfaces for SERS

Accepted Manuscript Facile synthesis of large-area and highly crystalline WS2 film on dielectric surfaces for SERS Zhen Li, Shouzhen Jiang, Shicai Xu, Chao Zhang, Hengwei Qiu, Peixi Chen, Saisai Gao, Baoyuan Man, Cheng Yang, Mei Liu PII:

S0925-8388(16)30127-X

DOI:

10.1016/j.jallcom.2016.01.126

Reference:

JALCOM 36477

To appear in:

Journal of Alloys and Compounds

Received Date: 27 November 2015 Revised Date:

7 January 2016

Accepted Date: 17 January 2016

Please cite this article as: Z. Li, S. Jiang, S. Xu, C. Zhang, H. Qiu, P. Chen, S. Gao, B. Man, C. Yang, M. Liu, Facile synthesis of large-area and highly crystalline WS2 film on dielectric surfaces for SERS, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.126. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile synthesis of large-area and highly crystalline WS2 film on dielectric surfaces for SERS Zhen Lia, Shouzhen Jiang a, Shicai Xu b, Chao Zhang a, Hengwei Qiu a, Peixi Chen a,

a

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Saisai Gaoa, Baoyuan Man a,*, Cheng Yang a and Mei Liua

School of Physics and Electronics, Shandong Normal University, Jinan250014,

b

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People’s Republic of China

College of Physics and Electronic Information, Shandong Provincial Key Laboratory

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of Functional Macromolecular Biophysics, Institute of Biophysics Dezhou University,

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Dezhou 253023, People’s Republic of China

E-mail:[email protected]. cn (Baoyuan Man).

ACCEPTED MANUSCRIPT Abstract A facile fabrication of high-quality and large-area tungsten disulfide (WS2) layers is demonstrated using a thermal decomposition of tetrathiotungstates

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((NH4)2WS4) with two annealing process. During synthesis, the first annealing step is utilized to achieve lateral epitaxial growth of the WS2 and create seamless and large-area WS2 film. The second annealing step can offer an S-rich and high

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temperature condition, which is beneficial for the high quality of the WS2 film.

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Scanning electron microscopy, Raman spectroscopy and atomic force microscopy confirm the presence of large-area and high-quality WS2 film. The crucial role of the S, H2 and the effect of the temperature during the experiment are also investigated. Furthermore, the potential application of the prepared WS2 as a substrate for Raman

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enhancement is first discussed using R6G molecules as probe molecule. Keywords: Transition metal dichalcogenides, tungsten disulfide, two-dimensional

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materials, SERS, layered materials

ACCEPTED MANUSCRIPT Introduction Recently, transition metal dichalcogenides (TMDCs) with two-dimensional (2-D) layer structure analogous to graphene has been attracting extensive scientific interests

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[1-13]. The TMDCs can be divided into metal, semimetal, or semiconductor due to the coordination and oxidation of the metal [8-10]. Among them, the semiconducting phases (MoS2, WS2, etc) have exhibited many unique properties. The optical and

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electronic properties of the materials are quite different such as valleytronics [14-16],

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when the indirect band gap in the bulk-layer form transformed into direct in the single-layer form. Molybdenum disulfide (MoS2) has showed great potential applications in 2-D semiconductors with extraordinary electronic and optical properties in such materials [1], and there have been many studies about it [4-7].

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Analogous to MoS2, WS2 layers are composed by a slab S-W-S sandwich crystallize in a Vander Waals layered structure. It possesses many remarkable characteristics such as the coupled spin and valley physics [17], the indirect-to-direct band-gap transition,

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high photoluminescence emission efficiency of monolayer WS2 [14] and band

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structure tunability with strain [18]. Furthermore, the WS2 layer can combine with other 2-D materials to form a large category of layered composites for further applications [19-21].

There have been various applications on WS2 such as field-effect transistors

(FET) [20-23], hydrogen evolution [24], and saturable absorber of mode-locked laser [25] etc. Most of applications are based on the electrical measurement and the Raman enhancement effect on the WS2 film has not been investigated up to now. The Raman

ACCEPTED MANUSCRIPT enhancement effect is a practical phenomenon which is used for light-matter interaction and matter-matter interaction studies and for microanalytical applications [26]. At this time, MoS2 film as a substrate for surface enhanced Raman scattering

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(SERS) has been proved and note that a flat surface has promising importance for further application of SERS [27]. Similar to MoS2, the WS2 film is a very flat and uniform substrate which may provide a good choice for SERS. We will explore

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potential application of the WS2 as a substrate for Raman enhancement.

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To cater to the needs of various applications, significant attempts have been made currently to prepare WS2 thin films by different routes such as chemical exfoliation [24, 28], mechanical exfoliation [14, 17, 29], and sulfurization of tungsten oxide films [30, 31]. Nevertheless the method to synthesize large-area and

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high-quality WS2 layers is still rare. Here, an approach for the large-area and high-quality deposition of the WS2 thin film is offered. It has been reported that the thermolysis of (NH4)2WS4 in N2 atmosphere for the

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growth of WS2 nanotubes [32]. The (NH4)2WS4 is firstly transformed into WS3 at the as shown in aq (1). Then, the WS3 will further decompose

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temperature 180~280

into WS2 (aq2), which needs a higher temperature of ~600

. In this work, the

(NH4)2WS4 directly transforms to WS2 under a mixture of Ar and H2 carrier gas as described in eq (3) with the temperature lowered to 500 280 ( NH 4 ) 2WS 4 180 ~ → 2 NH 3 + H 2 S + WS 3

WS 3 600  →WS 2 + S

. (1)

(2)

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( NH 4 ) 2WS 4 + H 2 500  → 2 NH 3 + 2 H 2 S + WS 2

(3)

In order to improve the crystallization quality of WS2 films, it is reasonable to

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increase the thermolysis temperature. However, we find that the WS2 easily decomposes in H2 atmosphere when the temperature is higher than 500 . Meanwhile, the direct annealing of the (NH4)2WS4 at a higher temperature may be affected by the

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presence of oxygen if there is no protection of H2. Thus, two annealing process are

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conducted in this study.

Experimental

Figure 1(a) schematically shows the first step for the synthesis of WS2 films on SiO2 substrate. The 1ml dimethylsulfoxide (DMSO) was added into the high purity

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(NH4)2WS4 (Alfa Aesar purity of 99.99%; 0.01g) powder to form a 1wt% solution. The (NH4)2WS4 solution was treated by sonication in ultrasonic cleaner for 20 min with the power of 80 watt to break down the undissolved particles. Then we made a

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thin and uniform (NH4)2WS4 film by spinning (NH4)2WS4 solution onto SiO2

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substrates with a spinner at a rotating speed of 2000 rmp for one minute. The sample was put in the horizontal constant temperature zone after 20 min baking treatment at 120

. When the pressure was pumped to 10-3 Pa by a molecular pump, the quarz tube

was heated to 500

at 10

/min for the first annealing with gas mixture (Ar/H2 =

80/20 sccm) to efficiently remove the byproducts separated from the precursors. The (NH4)2WS4 precursors were thermally decomposed into WS2 after 60 min reaction. The samples were cooled to room temperature naturally to obtain lateral epitaxial

ACCEPTED MANUSCRIPT structure. Then, the quartz tube was heated again until the center of the furnace reached 800

for the second annealing in the atmosphere of Ar/S as shown in Fig.

1(b). S powder (99.5%, Alfa Aesar; 500mg) was placed in the low-temperature zone which sublimates the S powders into sulfur vapors. Ar is used to avoid

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of about 200

the system oxidation and carry sulphur vapors to the surface of the WS2. After 30 min treatment, the samples were cooled to room temperature naturally.

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Apparatus and characterization

micros-copy (SEM,

Zeiss

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Surface morphologies of the WS2 were observed using scanning electron Gemini Ultra-55) with energy-dispersive X-ray

spectroscopy (EDX) for chemical analysis. The Raman spectroscopy was performed using a Raman spectrometer (Horiba HR Evolution 800) with laser excitation at

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532nm. The morphological changes of WS2 layers were characterized by atomic force microscopy (AFM, Park XE100). The crystalline quality and the single-crystalline structure of WS2 thin films were characterized by XRD (Bruker D8). The transmission

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electron microscopy (TEM) and selected area electron diffraction (SAED) were

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carried out by a transmission electron microscopy system (Hitachi H-800).

Results and discussion Figure 1a schematically shows the first step for the synthesis of WS2 films on

SiO2 substrate. The first annealing with gas mixture (Ar/H2) make the (NH4)2WS4 precursors thermally decompose into seamless and large-area WS2 film. Then, in order to get the high crystallization quality of WS2 films, the quartz tube was heated again for the second annealing in the atmosphere of Ar/S as shown in Figure 1(b).

ACCEPTED MANUSCRIPT Figure 2(a) exhibits SEM image of the WS2 thin layers after the second annealing at 800

. The edge region of the WS2 film is clearly observed which can visually

identify the presence of the large-area, uniform and continuous WS2 films. In order to

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observe the surface morphology clearly, SEM image of WS2 film at a higher magnification is shown in Figure 2(b). A very flat and uniform WS2 film is clearly observed. The uniform and thin WS2 films can be attributed to the dilute (NH4)2WS4

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solution and fast spin-coating. Furthermore, slow heating and natural cooling process

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can also facilitate the lateral growth of WS2. Figure 2(c) provides strong and direct evidence that the WS2 films are free of impurities at the detectable resolution of the EDX. The WS2 film possesses a reasonable chemical composition (atomic ratio W: S approximate 1:2) as shown in the inset in Figure 2(c). We use XRD to characterize the

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crystal structure of the obtained WS2 layers. Figure 2(d) shows the four noteworthy peaks at 2θ = 14.3° , 28.9° , 44.1° and 60.1° assigned as the (002), (004), (006) and (008) respectively (a=0.315nm, c=1.23nm, Powder diffraction file (PDF) no.

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84-1398). Here, only the (002) family of diffraction peaks are observed, indicating the

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hexagonal structure of the WS2 sheets without any other impurities and a high crystallinity. We can also identify that the periodicity of WS2 film plane is in c-axis as a strong (002) peak is observed. For the as-grown WS2 film just after the first annealing treatment, almost no peaks can be detected, which suggests the low quality of the WS2 film. To further reveal the importance of the second annealing treatment, the Raman spectra were used to characterize the obtained WS2 film. The in-plane vibrational

ACCEPTED MANUSCRIPT mode [E12g(M)], the second-order mode of longitudinal acoustic mode [2LA(M)] and the in-plane optical mode [E12g(Γ)] for WS2 should appear at approximate 343, 350 and 355cm-1 respectively according to the previous experimental studies [16, 30, 32,

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33]. However, note that the high-intensity 2LA(M) mode located at 350 cm-1 overshadow the E12g(M) at 343 cm-1 and E12g(Γ) at 355 cm-1. While other peaks of the WS2 such as the combination modes of 2LA-2E22g, 2LA-E22g and the out-of-plane

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vibrations of the sulfur atoms [A1g] separately appear at 296, 320 and 417cm-1 as

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shown in the Figure 2(e).

Figure 2(e) shows the Raman spectra of the WS2 films, where the red and the black lines represent separately that of WS2 films obtained without and with the second annealing treatment. The full-with at half maximum (FWHM) and intensity of

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the characteristic peaks of the WS2 film can be used to identify the quality of the WS2 layers. The large FWHM and weak intensity of 2LA(M) and A1g peaks can be observed, which demonstrates the poor crystal structure of the WS2 just after the first

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annealing treatment. In contrast, the high intensity of the 2LA(M) and A1g peaks and

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their narrow FWHMs suggest that the crystallization of the WS2 films after the second annealing treatment is greatly improved, which is in good agreement with the XRD results.

In addition, the number of WS2 layers can be estimated by the frequency

difference ∇ between 2LA(M) and A1g. The ∇ value obtained in Figure 2(e) is around 69 cm-1 corresponding to the multilayer [34]. To make it clear, we used AFM to identify the thickness of the obtained WS2. Figure 2(f) shows the AFM image WS2

ACCEPTED MANUSCRIPT layers which reveals the film is uniformly flat with the thickness around 3 nm, indicating that the layers of the prepared WS2 is 4 L, which is in agreement with the Raman results. The more diluted (NH4)2WS4 solution and faster spin-coating may

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contribute to the thinner WS2 films and we will explore these effects in the following work. In order to further demonstrate the uniformity of the WS2 film, the 10 ×10 µm2 Raman mappings are obtained as shown in Figure 2(g) and 2(h). The small color

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variations of both the 2LA(M) and A1g mode indicate the well uniformity of the WS2

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film.

Transmission electron microscopy (TEM) was also used to evaluate the microstructure and crystallinity of the WS2 film. The TEM image in Figure 2a shows the edge of WS2 layers where the contrast is relatively flat with a folded region

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(marked orange). The high-resolution tunneling electron microscopy (HRTEM) image in Figure 2b reveals the periodic atom arrangement of the WS2 film at a selected location, which demonstrates that the WS2 film is highly crystalline. The folded edge

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of the WS2 film is showed in Figure 3(b) inset, where the four layers of WS2 are

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clearly identified. Thus, the number of layers of the WS2 film can be also resolved under top-view TEM. Additionally, the distinct and bright sixfold symmetric symmetry spot pattern shown in the Figure 3(c) also indicates the high crystallization and the single crystalline nature of the WS2 thin film, which is in well accordance with the XRD and Raman analysis. Additionally, we have also performed the other comparative measurements. Figure 3(d) and inset respectively show the TEM image and SAED pattern of the WS2 film simply after the first annealing treatment. Note that

ACCEPTED MANUSCRIPT the WS2 film shows nearly noncystalline structures. It is noteworthy pointing out that the WS2 film will become polycrystal if the (NH4)2WS4 is decomposed without H2 in the first annealing process as shown in

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Figure 4(a). In order to investigate this phenomenon, XRD is conducted on the WS2 sheets in the first annealing respectively with H2 (black line) and without H2 (red line). A weak and broad peak between 20° and 25° is clearly observed in the sample

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obtained without H2 as shown in Figure 4(b) where the WO3 shows many peaks in

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general. This broad peak implies that this may be the result from the mixing of weak WO3 peaks, and the WS2 films contain extremely small crystallites or nuclei of WO3. This is may be the reason why the SAED pattern of the WS2 shows the ring which stands for polycrystal. Thus, the transformation process of (NH4)2WS4 to WS2 will be

process.

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affected by the presence of oxygen if there is no protection of H2 in the first annealing

To investigate the vital role of the S powders, we conducted a contrast

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experiment without S powder during the second annealing. The SAED, Raman

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spectra and XRD were used. The blurry and dim sixfold symmetric symmetry spot in the SAED pattern (shown in the inset of Figure 5(a)) indicates the low crystallinity of the WS2 film obtained without S powder. For the XRD pattern, the (002), (004), (006) and (008) diffraction peaks are clearly observed for the WS2 film annealed with S during the second annealing, but only (002), (004) peaks are detected for that annealed without S as shown in Figure 5 (a). The XRD results also demonstrate the crystallinity of the WS2 film annealed with S is better than that without S. The broad

ACCEPTED MANUSCRIPT FWHM and the lower intensity of 2LA(M) and A1g in the Raman spectra (red line in Figure 5(b)) proved once more that the quality of WS2 layers without S is poor. The possible factor may explain this: the WS2 will further lose the S2- in the high

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temperature above 500 , which may reduce the crystallization quality of the WS2 film. Consequently, to obtain highly crystalline WS2 film, it is crucial to create an S-rich condition in the second annealing process, which can effectively restrain the

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separation of S2- and further improve the crystallinity of the WS2 layers.

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Additionally, we have also studied the temperature effect for the quality of WS2 film during the second annealing. The crystal quality of the WS2 films were prepared in 900

differ very minor when compared with that obtained in 800

(Figure 5(c)).

However, the WS2 films are no longer continuous and transform into many hexagonal

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or quasi-hexagonal sheets as shown in Figure 5(d) and inset. These hexagonal sheets uniformly spread on the SiO2 substrates covering a large area. What’ more, the WS2 film curls up due to the high temperature and long thermostatic time during the

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second annealing to form the nano-tube analogues marked with orange arrows shown

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in the inset of Figure 5(d), which should be the reason why the WS2 film is not continuous.

To investigate the potential application of the prepared WS2 as a substrate for

Raman enhancement, R6G molecules were deposited on the WS2/SiO2 and the SiO2 substrates respectively to compare the Raman signals. We collect the Raman spectra from both WS2/SiO2 and the SiO2 substrate under the same condition (532nm excitation, 48mw laser power, etc). The Raman spectra of R6G on the WS2/SiO2 with

ACCEPTED MANUSCRIPT the concentration from 10-4 to 10-7 are shown in the Figure 6(a). The Raman peaks at 611, 774, 1186, 1360, 1504, 1575 and 1650 cm-1 are assigned to the R6G molecules and the 351 and 419 cm-1 are assigned to the WS2 film, which are in agreement well However, for the signal obtained directly from the

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with the previous works [17,35].

SiO2 substrate, strong fluorescence background was observed, the fluorescence background overshadow the Raman signal of R6G except the 611cm-1 peaks as shown

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in the Figure 6c. Thus, the intensity of the 611 cm-1 peak with different concentrations

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was collected to investigate the enhancement factor of WS2/SiO2 as shown in Table 1. The peaks of R6G at 611 cm-1 on the WS2/SiO2 are about 1.5 times stronger than those on the SiO2 substrate. In addition, the minimum detectable concentration of R6G in aqueous solution is one order of magnitude lower than those on the SiO2

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substrate. Thus, the WS2 film exhibits a clear SERS ability.

To further investigate the dynamic variation of Raman intensity versus the concentration of R6G on the WS2/SiO2, the peak at 611 cm-1 was selected. The

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average value of the intensity based on six spectra randomly collected from the

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WS2/SiO2 is chosen to guarantee the dependability of the data. A good linear SERS response from 10−7 to 10 −4M of R6G is obtained with the concentration plotted in log scale as shown in the Figure 6(c). The coefficient of determination (R 2) of the linear fit calibration curve for the peaks of 611 cm

−1

is reached 0.9885. These results

indicate WS2/SiO2 can provide reliable and clear SERS signals. Considering the principle of the Raman enhancement, there are possible three factors: Firstly, Similarly to MoS2, the chemical mechanism is possible for the Raman

ACCEPTED MANUSCRIPT enhancement on WS2. The WS2 film is relatively smooth making a small distance between the WS2 and the R6G molecules which is convenient for charge transfer between the molecule and the substrate. Secondly, the HOMO (-5.7eV) and LUMO

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(-3.4eV) of the R6G molecules are located on both sides of the Fermi level (-3.2eV) of WS2, which makes the charge transfer easily between WS2 and the R6G molecules. Additionally, the high crystallinity of the WS2 film we obtained results in less lattice

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defects, which contributes to the transmission of charge carriers and further lead to

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large SERS activity.

We also investigate the use of WS2 film in compositing with the metal nanopaticles. Compare the Raman signals from a WS2/Cu nanopaticles (CuNPs) SERS substrate (red line) and pure CuNPs (blue line) SERS substrate (without

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molecules) as shown in Figure 6(d). Raman spectra obtained from the CuNPs substrate shows two obvious peaks (1370 and 1560 cm-1) with a strong background which imply the presence of surface carbons, and is kown as photocarbonization. It is

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also noted that the WS2/CuNPs substrate only shows two clear characteristic peaks of

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WS2 with a clean baseline in the Raman spectra for the photocarbonization background is nearly not observed. There was no Raman probe molecules introduced here, thus this carbonization background is normally considered to be caused by surface carbon-based adsorbates from the atmosphere. By encapsulating the surface of CuNPs, the WS2 film can suppresses the catalytic activity of CuNPs and prevents the photo-induced carbonization. This finding paves the way for the combination of WS2 film and metal nanopaticles for better SERS substrates and further study are now in

ACCEPTED MANUSCRIPT progress in our group.

Conclusions In conclusion, we have proposed a facile way to obtain high-quality and

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large-area WS2 layers using a thermally decomposition of (NH4)2WS4 with two annealing process. The high-quality and large-area WS2 thin layers are evidenced by various spectroscopic and microscopic characterizations including Raman, SEM,

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AFM, XRD, TEM and SEAD. With a series of contrast experiments, we also

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demonstrated the crucial role of the S, H2 and the temperature during the experiment for the deposition of high-quality and large-area WS2 thin layers. Using R6G as probe molecule, we also verify that the WS2 film has obvious Raman enhancement and fluorescence quenching effect. Besides, the WS2 film can suppresses the catalytic

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activity of metal nanopaticles and prevents the photo-induced carbonization when composite with metal nanopaticles. This method provides a promising technique for the practical applications of WS2, such as in electronic and optoelectronic fields.

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Acknowledgments

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The authors are grateful for financial support from the National Natural Science Foundation of China (11474187, 61205174 and 11404193), Excellent Young Scholars Research Fund of Shandong Normal University and Shandong Province Natural Science Foundation ((ZR2014FQ032 and ZR2013EMM009).

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Li, H. Qiu, ACS Appl. Mater. Inter. 7 (2015) 10977−10987.

Figure 1. (a) Schematic illustration of the first process for the synthesis of WS2 films on SiO2 substrate. (b) the second annealing in the (Ar+S) gas environment with the

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zone.

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WS2 in the high-temperature zone and a boat of S powders in the low-temperature

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Figure 2. (a) SEM image of WS2 thin layers on SiO2 substrates performed on the edge region at a low magnification. (b) SEM image of WS2 film at a higher magnification

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(c) EDX data of the same sample. Inset: quantitative atomic analysis of the W and S

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elements. (d), (e) XRD pattern and Raman spectra for the WS2 films, where the red and the black represent WS2 films separately obtained after the first annealing and the second annealing. (f) AFM image of WS2 layers selected from the red box area in Fig. 2(a) and its corresponding height profile; (g) (h) the scanning Raman 2LA(M) and A1g band mappings of the WS2 film.

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Figure 3. (a) and (b) TEM images of the WS2 thin film after the second annealing. Inset in (b) is the edge of WS2 film, where four layers are identified. (c) The SAED pattern of the WS2 sample discussed in (b). TEM image (d) and the SAED pattern

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(inset) of the WS2 layers simply after the first annealing treatment.

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Figure 4. (a) the SAED pattern for WS2 layers annealed with pure Ar in the first

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annealing process. (b) XRD patterns of the WS2 sheets obtained in the (Ar + H2)

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(black line) and in pure Ar (red line) during the first annealing.

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Figure 5. (a) XRD patterns for the WS2 sheets. Inset: SAED pattern of the WS2 layers annealed without S in the second annealing process. (b) Raman spectra for the WS2

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sheets. (c) XRD patterns for the WS2 sheets obtained in 800

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(black line) respectively during the second annealing. (d) SEM image of WS2 layers after the second annealing at the temperature of 900

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higher magnification (Inset).

at a low magnification and a

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Figure 6. (a) Raman signals of R6G deposited on the WS2/SiO2 with different

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concentrations from 10-4 to 10-7 M at 532nm laser excitation. (b) Raman signals of R6G on SiO2 substrate with different concentrations from 10-4 to 10-6 M. (c) Raman

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intensity of R6G peaks at 611 cm-1 on WS2/ SiO2 as a function of the molecular concentration. (d) Raman spectrum of CuNPs (the blue line) and WS2 / CuNPs (the red line).

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Table 1. Intensity of the 613cm-1 Raman peaks from the WS2/SiO2 and SiO2

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substrates.

ACCEPTED MANUSCRIPT Highlights: 1. A method to synthesize large-area and high-quality WS2 thin film is offered. 2. The crucial role of the sulphur and H2 during the experiments is investigated.

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3. Obvious SERS and fluorescence quenching effect of the WS2 film is exhibit.