Centimeter-scale polycrystalline monolayer (1L) WS2 films with the presence of high-surface-density grain boundaries (GBs) were successfully synthesized on SiO2 (300 nm)/Si substrates in a home-made CVD system as reported in our previous works61,62, which are confirmed by measurements of optical microscopy (OM), fluorescence (FL), atomic force microscopy (AFM), Raman and photoluminescence (PL) (see Supplementary Figs. S1, 2, and details about CVD growth and characterization of OM, FL, AFM, Raman and PL). Because of the preferential adsorption of contaminants on GBs in 1L WS2 film under exposure to air, the GB profiles can be observable even in the OM images60. Figures 1a and b show the low and high magnification AFM images of the as-grown polycrystalline 1L WS2 film on Si substrate after one-week exposure to air for sufficient adsorption on GBs. The GB profiles can be clearly visualized between irregular-shaped WS2 grains with the lateral dimension of less than 100 nm in average. The nano-scale grain sizes are the direct evidence of high-surface-density GBs in the CVD-grown polycrystalline 1L WS2 film. The atomic structure of GBs was investigated in detail by using scanning transmission electron microscopy (STEM). The low magnification high angle annular dark field STEM (HAADF-STEM) image of Fig. 1c clearly shows the GBs and the nano-scale grains with lateral sizes of less than 100 nm, confirming the AFM observations. In the corresponding selected area electron diffraction (SAED) of Fig. 1d, the observed diffraction rings instead of individual spots suggest that the nano-scale grains in polycrystalline 1L WS2 film were grown in nano-scale lateral sizes and without a preferential orientation. Statistical analysis of the grains indicates that the lateral size dominantly lies in the range from 20 to 55 nm, as shown by the inset of Fig. 1c, demonstrating the high grain density (1010-1011 cm− 2) and high GB surface density. Atomic structure of the GBs on the as-synthesized WS2 film is distinguished using atomic-resolution HAADF-STEM imaging. After systematic examination of multiple boundary locations (Supplementary Fig. S4), we confirmed that the WS2 grains in the as-synthesized film are indeed stitched together via GBs. It is established in previous research that the atomic make-up of GBs is diverse. In our sample, GBs are primarily consisted of 5–7 members of rings with a sulfur-rich chemical composition, as illustrated in Fig. 1e.
To demonstrate the application potential of GB-rich 2D layered materials as biochemical sensor materials, we fabricated the SPR sensors based on GB-rich 1L WS2 film for ultra-sensitive detection of Hg2+. The basic setup of our sensor device (fabrication details in Methods) and the SPR imaging system are schematically illustrated in Supplementary Fig. S3. The detection principle of SPR sensors lies in the ultra-sensitivity of SPR signals (i.e. resonance angle of θ in the present work) to minute changes in refractive index of the sensing surface. In this case, upon Hg2+ adsorption onto GB-rich 1L WS2 film (sensor material), its refractive index changes, thus SPR curve shifts, providing quantitative information of Hg2+ on the 1L WS2 film. By using Snell’s Law and the N-layer transfer matrix method, we performed the computational simulation to profile the vertical electric field distribution of Au film before and after the transferring of a 1L WS2 film (Supplementary Fig. S5). Compared with bare Au, the electric field is significantly enhanced upon the incorporation of GB-rich 1L WS2, and it is further increased to a maximum at the interface between 1L WS2 film and sensing medium. The 1L WS2 integration induced electric field enhancement between Au film and sensing medium signifies that the proposed SPR sensor is sensitive to slight changes in sensing medium, thus strongly suggests the suitability of 1L WS2 film for SPR sensor. Generally, SPR signal optimization can be induced via incorporation of many other 2D layered materials onto Au film,65,66 which is favorable for detection sensitivity. However, this effect itself is insufficient for high sensor performance, and the fundamentally indispensable key factor is the high analyte adsorption ability of the sensor material.
To demonstrate the significant role of GBs in physi and/or chemi-adsorption of Hg2+, the CVD-grown 1L WS2 single crystals (see Supplementary Fig. S6) based SPR sensor for Hg2+ detection was also evaluated. Compared with the rich structural defects along GBs in polycrystalline 1L WS2 film, the 1L WS2 single crystal has much fewer structural defects on the surface, but possesses edge defects. As revealed in the previous studies,14 for the biochemical sensors based on 2D layered crystals, preferential adsorption occurs along material edges. Thus, the Hg2+ sensing abilities of SPR sensors based on GB-rich 1L WS2 film and 1L WS2 single crystal were investigated in detail for comparison. Figure 2a shows the angle-resolved SPR spectra of GB-rich 1L WS2 film and 1L WS2 single crystal in ultrapure water and at increasing concentrations of Hg2+ aqueous solution (10− 18 − 10− 11 M). In contrast to those of the 1L WS2 single crystal based SPR sensor, the SPR spectra based on GB-rich 1L WS2 film display much more prominent right-shift with increasing Hg2+ concentration, indicating adsorption of more Hg2+ ions. Figure 2b shows the determined resonance angle shift (∆θ) as a function of Hg2+ concentration for the SPR sensors based on GB-rich 1L WS2 film, in which the extracted one of 1L WS2 crystal is also included for comparison. Distinctly, at the same Hg2+ concentration, the GB-rich 1L WS2 sensor exhibits a much larger angle shift (∆θ) than that of the 1L WS2 crystal one. The degree of angle shift is proportional to the amount of adsorbed Hg2+. Even at attomolar-level concentration (13 milli-degree for 10− 18 M), the adsorption of Hg2+ by the GB-rich 1L WS2 film brings about discernible change in SPR resonance angle. Notably, the GB-rich 1L WS2 sensor displays the wide detectable dynamic range from 10− 11 to 10− 18 M, covering 7 orders of magnitude. Compared with that of the 1L WS2 single crystal sensor, the observed much larger ∆θ of GB-rich 1L WS2 sensor indicates the substantially larger amount of Hg2+ ions adsorbed onto GB-rich 1L WS2. Thus, this drastic sensor performance difference not only demonstrates the outstanding sensitivity of GB-rich WS2 sensor device, but, more importantly, reveals the significant role of high-surface-density GBs in sensor performance.
Adsorption or binding sites are energetically active positions, which are widely accepted to exist in structural defects of 2D layered materials, such as vacancy, antisite, substitution, edge, and GB.14,26,27,48–58,67−71 Structurally, although 1L WS2 single crystals contain a certain amount of defects on surface and along edges, the surprisingly high surface-density of GBs in polycrystalline 1L WS2 film immensely increase the amount of defects per unit area, providing the primary analyte binding sites. Thus, for the SPR sensor based on polycrystalline 1L WS2 film, the observed superior sensitivity of Hg2+ detection is considered to result from the rich GBs.
For fundamental understanding on the role of GBs in Hg2+ adsorption of polycrystalline 1L WS2 film, we performed DFT calculations by considering the chemisorption of Hg2+ ions on GBs and GB-free area for comparison, as shown in Figs. 3a, b and supplementary Fig. S7 (also see supplementary for calculation details).The adsorption energies Eads of Hg2+ ions on different positions around GBs are listed between Figs. 3a and b. It is evident to find that Hg prefer to adsorb on the pentagon hollow site along GBs. Moreover, the adsorption energies of Hg around the GBs are also lower than that on pristine 1L WS2 crystal (see Supplementary Fig. S7). This indicates the GBs behave like the Hg traps. From the charge density difference plot shown in Fig. 3b, we can find the apparent charge transfer between Hg and GBs. There are 0.1 negative charge transfer from Hg to neighboring S atoms, indicating the formation of covalent bonds between Hg and S atoms. The formation of Hg-S bond is substantiated by X-ray photoelectron spectroscopy (XPS) measurements of the as-synthesized GB-rich 1L WS2 film before and after Hg2+ detection. Figures 3c-e show the XPS spectra of W4f, S2p and Hg4f core levels before and after Hg2+ detection. For the as-synthesized GB-rich 1L WS2 film, the two deconvoluted W4f peaks at 32.63 and 34.79 eV (Fig. 3c top) and the two deconvoluted S2p peaks at 162.55 and 163.71 eV (Fig. 3d top) are ascribed to WS2,72 while no trace of Hg is observed (Fig. 3e top). For the GB-rich 1L WS2 film after Hg2+ detection in 10− 9 M Hg2+ solution and then rinse with ultra-pure water for three times, obvious sign of S-Hg bonds is observable in the XPS spectrum of Hg4f core level (Fig. 3e bottom), and the main deconvoluted peaks of W4f and S2p are slightly shifted toward higher binding energies in contrast to those of the as-synthesized GB-rich 1L WS2 film. The two deconvoluted W 4f peaks at 33.05 and 35.22 eV can be still related to WS2. On the XPS spectra of S2p core level, however, in addition to the two strong deconvoluted peaks at 162.55 and 163.71 eV from WS2, two new weak deconvoluted peaks at 161.67 and 163.11 eV are recognized to be produced by the formation of S-Hg bonds during Hg2+ detection. The formation of S-Hg bonds after Hg2+ detection, which can induce changes in chemical environment for W and S atoms in the WS2 film, can also be the underlying origin for the observed slight shift of the XPS W4f and S2p peaks from WS2 toward higher binding energies. The combined theoretical calculations and XPS measurements provide concrete evidence for the preferential adsorption of Hg2+ on the GBs via the formation of S-Hg bonds. Therefore, the rich chemically active sites on GBs in polycrystalline 1L WS2 film serve as efficient probes for ultra-sensitive detection of Hg2+ ions.
As one of the crucial performance criteria, the selectivity of Hg2+ detection for the SPR sensor based on GB-rich polycrystalline 1L WS2 film was also assessed. The evaluation was performed by comparing the sensor responses to 10− 12 M of Hg2+ and common interfering ions such as Pb2+, Mn2+, Cu2+, Fe3+, Zn2+, Co2+, Cr3+ and Mg2+. The determined resonance angle shifts ∆θ in Fig. 4a show that the value of ∆θ induced by Hg2+ adsorption is at least ~ 3X higher than those produced by other ions, indicating that the GB-rich WS2 sensor is highly selective towards Hg2+ detection.
To further validate the superior sensing capacity of the GB-rich WS2 film, we compared its LOD for Hg2+ with those of previously reported high-performance Hg2+ sensing materials.17,73−135 As presented in Fig. 4b. the LOD of GB-rich WS2 film for Hg2+can reach down to 1aM (~ 600 ions·cm− 3), according to the IUPAC guideline of 3:1 signal to noise ratio, which clearly outperforms the previous ion sensors based on conventional sensor materials.
The superior performance of GB-rich 1L WS2 based SPR sensor in Hg2+ detection relies on the presence of sufficient active sites along the rich GBs for the preferential and efficient adsorption of Hg2+. Therefore, not only the polycrystalline 1L WS2 film, the GB-rich polycrystalline 1L and few-layer films of any other layered metal sulphides would also be expected to serve as sensor materials for the similar ultra-sensitive detection of Hg2+ ions. As a demonstration, the CVD-grown GB-rich polycrystalline 1L MoS2 film62 was used to fabricate the SPR sensor for detection of Hg2+ ions. As shown in the SPR spectra and the determined resonance angle shifts of Δθ (Supplementary Fig. S8), the superior sensor sensitivity down to trace attomolar-level quantification (LOD of 1 aM) is also observed for the GB-rich 1L MoS2 film, comparable to those of the GB-rich 1L WS2 film, which further demonstrates that the concept of GB-rich 2D layered materials for high-performance sensors is universal and comprehensive.