1. Introduction

2. Materials and Methods

Figure 1. Growth and characterization of MoN nanosheets. (a) Schematic illustration of the two-step CVD growth of the MoN nanosheets. (b) Optical microscopy image of the MoO₂ nanosheets. (c) Optical microscopy image of the MoN nanosheets. (d) AFM characterization of a MoO₂ nanosheet corresponding to the hexagonal dashed box in b. the white curve is the height profile along the white dotted line, indicating the thickness of the nanosheet is 6.6 nm. (e) AFM characterization of the MoN nanosheet after nitridation. The white curve is the height profile along the white dotted line, indicating the thickness of the nanosheet is 6.6 nm. (f) The Raman spectra of MoO₂ nanosheets. (g) Raman spectra evolution of the nanosheets with the increase of the nitridation time, the insert image demonstrates the characteristic Raman peak of MoN at near 160 cm^-1.

Figure 1. Growth and characterization of MoN nanosheets. (a) Schematic illustration of the two-step CVD growth of the MoN nanosheets. (b) Optical microscopy image of the MoO₂ nanosheets. (c) Optical microscopy image of the MoN nanosheets. (d) AFM characterization of a MoO₂ nanosheet corresponding to the hexagonal dashed box in b. the white curve is the height profile along the white dotted line, indicating the thickness of the nanosheet is 6.6 nm. (e) AFM characterization of the MoN nanosheet after nitridation. The white curve is the height profile along the white dotted line, indicating the thickness of the nanosheet is 6.6 nm. (f) The Raman spectra of MoO₂ nanosheets. (g) Raman spectra evolution of the nanosheets with the increase of the nitridation time, the insert image demonstrates the characteristic Raman peak of MoN at near 160 cm^-1.

3. Results and Discussion

3.1. Preparation and Characterization of MoN Nanosheets

A two-step CVD process was used to prepare the MoN nanosheets on SiO₂/Si substrates. Firstly, MoO₂ nanosheets were synthesized by atmospheric pressure CVD using MoO₃ powder as precursor in a high-temperature tube furnace (Figure 1a). Then the as-grown MoO₂ nanosheets was replaced in the high-temperature tube furnace with two temperature zones for nitridation to prepare the MoN nanosheets, where urea was used as a nitrogen source. Details of the synthesis process can be found in the Experimental Section and Figure S1. Figure 1b illustrates the optical image of the MoO₂ nanosheets, most of which are in the shape of parallelograms and hexagons. The standard geometric configuration of the nanosheets indicates that the nanosheets are single crystals. As shown in Figure 1c, the nanosheets maintain their original shape after nitridation, while the color of the MoN nanosheets slightly deepens compare to the MoO₂ nanosheets. As the thickness of the nanosheets almost remains unchanged (~6.6 nm) after nitridation (Figure 1d-e), the color variation of the nanosheets indicates that the refractive index of the nanosheets have changed after nitridation³⁷, which can regard as an earlier evidence of the successful synthesis of the MoN nanosheets. To further confirm that, Raman spectroscopy characterization of the nanosheets was carried out before and after the nitridation process. As illustrated in Figure 1f, typical Raman peaks of MoO₂ nanosheets were observed at 124, 208, 230, 347, 363, 498, 570 and 746 cm^-1.⁷ With the increasing of the nitridation time, those typical peaks of MoO₂ nanosheets gradually vanished (Figure 1g), with a new Raman peak emerging at near 160 cm^-1. The new Raman peak belongs to MoN, which is consistent with previous report.³⁸ The results confirmed the successful preparation of the MoN nanosheets.

The as-grown MoN nanosheets are metallic material with good conductivity. As illustrate in Figure 2a, the band structure of MoN nanosheet was calculated by density functional theory, no bandgap was observed at the Fermi level. The high density of states (DOS) found at the Fermi level (Figure 2b) indicates the intrinsic metallic property of the MoN nanosheets. To further confirm the metallic characteristic, the current-voltage (I-V) curves of the MoN nanosheets under different temperature were measured (Figure S2). The resistivity of the MoN nanosheets are between 6-10×10^-8 Ω/m, which is comparable to that of metals. Meanwhile, the resistivity increases with the increasing of the temperature. The linear temperature dependence of the resistivity is highly consistent with metals. The high carrier density of MoN nanosheets enable it to support surface plasmon resonance at visible range³⁹. As indicated in the UV-vis spectroscopy of the nanosheet in Figure 4d, a distinct absorption band around 500 nm was observed, which can be attributed to the surface plasmon resonance of the MoN nanosheets³⁵. The absorption band slightly red shifts with an increase in intensity upon the increasing of the nitridation time. As the absorption band is near 532 nm, one of the mostly used commercial laser wavelength in Raman spectroscopy, the MoN nanosheets are hopefully to be used as surface enhanced Raman scattering substrate.

Figure 2. Physical properties of the MoN nanosheets. (a) Band structure of MoN nanosheets obtained by first principle calculation. (b) Density of states at the Fermi level. (c) I-V curves of MoN nanosheets under different temperature, the insert is the optical image of the device, the scale bar is 10 m. (d) UV-vis absorption spectroscopy of MoN nanosheets with different nitridation time.

Figure 2. Physical properties of the MoN nanosheets. (a) Band structure of MoN nanosheets obtained by first principle calculation. (b) Density of states at the Fermi level. (c) I-V curves of MoN nanosheets under different temperature, the insert is the optical image of the device, the scale bar is 10 m. (d) UV-vis absorption spectroscopy of MoN nanosheets with different nitridation time.

3.2. SERS Property of the MoN Nanosheets

To investigate the SERS performance of MoN nanosheets, a commonly used Rhodamine 6G (R6G) molecules were chosen as the Raman probe. Before the Raman spectroscopy collection, the SiO₂/Si substrates with MoN nanosheets were immersed in R6G solutions with concentrations ranging from 10^-3 to 10^-8 M for 20 minutes to adsorb R6G molecules, after which the samples were dried by high-speed nitrogen flow. The Raman spectroscopy was collected using 532 nm laser, with the laser spot located in the middle of the nanosheets. As shown in Figure 3a, no characteristics Raman peaks were detected on the SiO₂/Si substrate. However, distinct Raman peaks of R6G molecules locate at 612 (R1), 772 (R2), 1363 (R3), and 1651 (R4) cm^-1 were observed on MoN nanosheets, indicating the excellent SERS activity of MoN nanosheets. The Raman peaks R1 and R2 are assigned to the in-plane and out-of-plane bending modes of the carbon and hydrogen atoms in the xanthenes skeleton, Raman peaks R3 and R4 are assigned to the aromatic C-C stretching vibration modes.⁴⁰

Figure 3. SERS property of MoN nanosheets. (a) Raman spectra of R6G molecules adsorbed on SiO₂/Si and MoN after soaking in the R6G solution with a concentration of 10^-6 M. (b) Raman spectrum of R6G molecules adsorbed on MoN nanosheets by soaking in the R6G solution with various concentration. (c) The intensity of four typical Raman peaks of R6G molecules extracted from (b). (d) The enhancement factor of four typical Raman peaks of R6G at different concentration levels.

Figure 3. SERS property of MoN nanosheets. (a) Raman spectra of R6G molecules adsorbed on SiO₂/Si and MoN after soaking in the R6G solution with a concentration of 10^-6 M. (b) Raman spectrum of R6G molecules adsorbed on MoN nanosheets by soaking in the R6G solution with various concentration. (c) The intensity of four typical Raman peaks of R6G molecules extracted from (b). (d) The enhancement factor of four typical Raman peaks of R6G at different concentration levels.

In order to further reveal the SERS effect of the MoN substrate, the Raman spectra of R6G molecules at different concentrations on the MoN substrate were studied. As shown in Figure 3b, the Raman signal intensity of the R6G decreases with the decreasing of concentration. When the concentration is as low as 4×10^-8 M, all the characteristic peaks of R6G molecules can be identified. Therefore, the MoN nanosheets SERS substrate can achieve a minimum concentration of detection of 4×10^-8 M. In addition, the characteristic peak R1 can be identified even when the concentration further decreases to 1×10^-8 M. The correlation between the intensity of the four characteristic peaks of R6G molecules and the concentration of the R6G solution was extracted, as shown in Figure 3c. It can be observed that the intensity of the Raman peaks increases as the concentration increases and eventually saturated at higher concentration, which can be explained by the Brunauer–Emmett–Teller theory. Furthermore, the enhancement factors of the MoN substrate were calculated by taking the ratio of the Raman intensity of the four characteristic peaks of R6G on MoN substrate and that of bulk R6G (Details in Experimental Section)²³. The calculation results are shown in Figure 3d. The enhancement factors of the four characteristic peaks decrease with the increase of the concentration of R6G solution. The enhancement factor can reach 3×10⁵ when the concentration is 10^-7 M, which is comparable to noble metal SERS substrate without hot spots⁴⁰. Under the same concentration, the enhancement factors of Raman peaks R1, R2, R3, and R4 decrease in sequence. The difference in the enhancement factors of various characteristic Raman peaks can be ascribed to the binding and geometry of R6G molecules absorbed on the MoN nanosheet surface.⁴¹

3.3. Raman Signal Uniformity of the MoN Nanosheets

In addition to the sensitivity, the Raman signal uniformity is another important factor for SERS substrate. Unlike those SERS substrates with nanoparticles or rough surface, the 2D configuration of the MoN nanosheets enable the uniform absorption of the probe molecules.⁴² Not only the number of the absorbed probe molecules on the surface of MoN nanosheets can be more uniform. Moreover, the geometry of the molecular absorbed on a flat surface can also be more coincident, resulting a cleaner SERS signals. To illustrate the signal uniformity of the MoN nanosheets, methyl violet was used as the probe molecules to avoid the resonant Raman scattering. A large MoN nanosheet was selected as the SERS substrate to perform Raman mapping test. As shown in Figure 4a, the Raman mappings based on three typical Raman peaks of the methyl violet located at 914, 1182 and 1622 cm^-1 show excellent uniformity within the MoN nanosheets. To illustrate it more clearly, the Raman spectra along the green dotted line in Figure 4a are plotted in Figure 4b. The characteristic Raman peaks of methyl violet are invisible on SiO₂ substrate as shown by the front clarets line. Moreover, the relative standard deviations (RSD) of the three typical Raman peaks are calculated by randomly selected 50 points within the MoN nanosheets, as shown in Figure 4(c-e), where the RSD for three typical Raman peaks are 4.4 %, 5.4% and 5.7%, respectively. The results indicate that the MoN nanosheets can be used as SERS substrate with high uniform SERS signals.

Figure 4. SERS signal uniformity on MoN nanosheets. (a) Optical image of a MoN nanosheet and the Raman mapping based on three typical Raman peaks of methyl violet molecules, the mapping area is marked by yellow dotted square. The scale bar is 10 m. (b) Raman spectra of methyl violet molecules along the green dashed line in panel (a). (c-e) The Raman intensity of three typical peaks 914, 1182 and 1622 cm^-1 collected from 50 randomly selected points on MoN nanosheets, the calculated RSD are 4.4%, 5.4% and 5.7%, respectively.

Figure 4. SERS signal uniformity on MoN nanosheets. (a) Optical image of a MoN nanosheet and the Raman mapping based on three typical Raman peaks of methyl violet molecules, the mapping area is marked by yellow dotted square. The scale bar is 10 m. (b) Raman spectra of methyl violet molecules along the green dashed line in panel (a). (c-e) The Raman intensity of three typical peaks 914, 1182 and 1622 cm^-1 collected from 50 randomly selected points on MoN nanosheets, the calculated RSD are 4.4%, 5.4% and 5.7%, respectively.

3.4. Thickness Dependent SERS Effect of the MoN Nanosheets Substrate

Distinct thickness dependent SERS effect was observed in the MoN nanosheets substrate. MoN nanosheets with different thickness were chosen to collect the Raman spectra after soaking in R6G solution with a concentration of 10^-7 M. The optical images of the MoN nanosheets are shown in Figure 5a. The thickness of those samples was determined by AFM as shown in Figure S4, which are 3.9, 4.5, 5, 6.8, 10.2 and 13.6 nm, respectively. The Raman signal intensity decrease with the increasing of the thickness of the nanosheets, as illustrated in Figure 5b. To illustrate it more clearly, the four characteristic Raman peaks of the R6G on different substrate was collected in Figure 5c. The intensity of the four Raman peaks decreases with the increasing of the thickness of the MoN nanosheets substrate. To reveal the mechanism of the thickness dependent SERS effect of the MoN substrate, the UV-vis absorption spectroscopy of the MoN substrate was taken as shown in Figure 5d. A red-shift of the absorption band with the increasing of the thickness was observed, meanwhile, the absorption band intensity decreases with the increasing of the thickness. As the absorption band of the MoN substrate was ascribed to the surface plasmon resonance, the red-shift and the decrease of intensity can be well explained by the energy loss of the surface plasmon resonance with the increased thickness⁴³. The thickness dependent SERS effect of the MoN nanosheets implies that better SERS effect can be achieved with thinner MoN, further effort is worth to take to synthesis thinner MoN nanosheets with larger scale.

Figure 5. Thickness dependent SERS effect of the MoN nanosheets. (a) Optical microscopy of MoN nanosheets with different thickness. (b) Raman spectra of R6G (10^-7 M) on MoN substrates with different thickness. (c) Raman intensity of four typical Raman peaks with different thickness. (d) Thickness dependent of UV-vis absorption of MoN nanosheets.

Figure 5. Thickness dependent SERS effect of the MoN nanosheets. (a) Optical microscopy of MoN nanosheets with different thickness. (b) Raman spectra of R6G (10^-7 M) on MoN substrates with different thickness. (c) Raman intensity of four typical Raman peaks with different thickness. (d) Thickness dependent of UV-vis absorption of MoN nanosheets.

3.5. Thermal Stability of the MoN Nanosheets and Its Reusability for SERS

In addition to high sensitivity, the stability is another important factor for SERS substrate. As shown in Figure 6a, the MoN nanosheets show high thermal stability, which can stand for more than 300℃ annealing in air. The maximum tolerable temperature of MoN nanosheets is far larger than the precursor MoO₂ nanosheets.⁴⁴ When the annealing temperature increased to 350℃, the color of the nanosheets changes from orange to purple, indicating the oxidation of the nanosheets. The obtained oxide is mainly MoO₃, as indicated by the Raman spectra of the purple nanosheets⁴⁵. When the annealing temperature was below 300℃, the SERS effect preserved well as indicated in Figure 6b. Distinct Raman signals of 10^-6 M R6G can be observed after annealing at 300℃. The high thermal stability of the MoN nanosheets making it possible to be reused as SERS substrate, because heating was found to be an efficient way to remove molecules from the substrate. Meanwhile, the planar configuration of MoN nanosheets also makes it easier for the absorbed molecule detaching from the surface than those substrate with complex nanostructures. To verify the reusability of the MoN nanosheets SERS substrate, cycle tests of the MoN SERS substrate was performed. As shown in Figure 6c, the Raman signal of R6G can be identified after 20 reusing cycles. To illustrate it more clearly, the typical Raman peaks of R6G are shown in Figure 6d, no obvious decay of the intensity was observed in both 4 typical Raman peaks.

Figure 6. Thermal stability of MoN nanosheets and its reusability as SERS substrate. (a) Optical microscopy images of MoN nanosheets after heating at 300℃ and 350℃ in air for 20 min. The Raman spectra of the purple nanosheets shown characteristic Raman peaks at 156, 300 and 816 cm^-1, which are the characteristic Raman peaks of -MoO₃⁴⁶. (b) Raman spectra of R6G (10^-6 M) on MoN substrates after annealing at different temperature. (c) The Raman spectrum of R6G (10^-6 M) on the MoN substrates reusing for 20 cycles. (d) Intensity of 4 typical Raman peaks extract from c.

Figure 6. Thermal stability of MoN nanosheets and its reusability as SERS substrate. (a) Optical microscopy images of MoN nanosheets after heating at 300℃ and 350℃ in air for 20 min. The Raman spectra of the purple nanosheets shown characteristic Raman peaks at 156, 300 and 816 cm^-1, which are the characteristic Raman peaks of -MoO₃⁴⁶. (b) Raman spectra of R6G (10^-6 M) on MoN substrates after annealing at different temperature. (c) The Raman spectrum of R6G (10^-6 M) on the MoN substrates reusing for 20 cycles. (d) Intensity of 4 typical Raman peaks extract from c.

4. Conclusions

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