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Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDCs), can exhibit novel phenomena distinct from their bulk counterparts. In this work, we report the growth of 2D MoS2 with the configuration of a multilayer MoS2 nanodot stacked on the center of the monolayer MoS2 flake by a single-step and catalyst-free chemical vapor deposition method. The local photoluminescence of the multilayer MoS2 nanodot is significantly enhanced in comparison to that of the basal monolayer MoS2. The reason of this novel phenomenon is owing to the existence of oxygen-related defects, which resulted from the tapered morphology of MoS2 nanodots with many edge states and adsorbed O molecules. The results presented here may open a pathway to modulate the optical properties of future optoelectronic devices using MoS2 flakes with special configuration.
© 2017 Optical Society of America
1. Introduction
Two-dimensional (2D) transitional metal dichalcogenides (TMDCs) are widely investigated due to their unique properties and potential for various electronic and optoelectronic applications [1, 2]. Molybdenum disulfide (MoS2), one of the most promising members of 2D TMDCs, has drawn increasing attention due to its thickness-dependent electrical and optical properties [2, 3]. In particular, the bandgap of MoS2 can be tuned from an indirect bandgap of 1.2 eV for bulk to a direct bandgap of 1.9 eV for monolayer [4, 5]. Thus, MoS2 has gained much attention for its potential applications in diverse optoelectronic devices [6]. It is well known that the performance of mono- or few-layer MoS2 based devices is determined by their fundamental characteristics, including layer number, morphology, and quality. In this regard, it is essential to fabricate uniform high-quality MoS2 with a desired morphology and physical properties. A variety of methods, including exfoliation [7], hydrothermal synthesis [8], and chemical vapor deposition (CVD) [9], has been developed to fabricate the 2D materials. In particular, CVD is considered as one of the most effective and feasible approaches to synthesize high quality MoS2 monolayers with a desired configuration.
Recently, significant effort has been devoted to develop monolayer MoS2-based photonic devices with outstanding PL performance. However, the PL property of as-prepared MoS2 monolayer has lagged behind expectation for a high quality direct band gap semiconductor [10, 11]. The reason for the weak PL is ascribed to defects [12] and formation of negatively charged excitons (negative trions) in the n-doped MoS2 [13]. Several alternative strategies are being adopted to control and tune the PL emission, such as chemical treatment [14], surface plasmonic excitation [15], quantum dots deposition [16], and the use of certain substrates (BN or metals) [17, 18]. However, most of these approaches are relatively complicated. Naturally, it is necessary to find a pathway to obtain peculiar MoS2 flakes with excellent PL performance by a one-step growth method.
In this work, we demonstrated the growth of a small multilayer MoS2 nanodot stacked on large size MoS2 monolayer by a one-step CVD method. Surprisingly, the MoS2 nanodot shows extraordinary local PL emission, whose PL intensity is several times enhanced in comparison to the outside region of monolayer MoS2 flake. Power-dependent PL demonstrated the existence of defects or impurities in the outside region of the MoS2 flakes, whereas EDS and post-annealing measurements indicated an increase in the adsorbed oxygen content at the center of the nanodot. These results indicate that the observed PL enhancement might be ascribed to the synergistic effect of the adsorbed O and more edge states of the special tapered edge structure at the center nanodot and the existence of defects in outside region of the triangular monolayer.
2. Experimental
2.1 Sample preparation
In this work, single- and few-layer MoS2 were grown on 285 nm SiO2/n-Si substrate by a one-step CVD in a two-temperature-zone horizontal tubular furnace equipped with a 4 cm diameter quartz tube. Prior to growth, substrates were cleaned with acetone, 2-propanol, and deionized water. In brief, MoO3 (Alfa Aesar, 99.9%) and S powder (Alfa Aesar, 99.5%) were chosen as Mo and S sources, respectively. Typically, S (~200 mg) and MoO3 (~6 mg) powders were contained in two quartz boats and placed in two separated temperature zones along the gas flow direction. One piece of SiO2/Si substrate (1 cm × 2 cm) was placed face down to quartz boat containing MoO3 powder. The distance between S powder and MoO3 was around 25 cm. The furnace was evacuated and purged with Ar (purity, 99.999%) several times to completely remove oxygen and moisture from the chamber. Argon was used as carrier gas to deliver Mo and S sources to SiO2/Si substrate with a flow rate of 50-100 cm3/min. Two temperature zones were heated to target values at a rate of ~10 °C /min, and then maintained for 15-30 min. The S was heated to 270 °C with Ar flow, and the substrate zone was heated to 780 °C. After the furnace was cooled to room temperature under Ar flow, the as-grown samples were removed for structural, morphological and optical characterization.
2.2 Characterizations
The as-prepared MoS2 flakes were systematically characterized through optical microscopy (Nikon OPTIPHOT-100), transmission electron microscope (TEM, JEOL-2100F), PL and Raman spectroscopy (RENISHAW invia, ∼532 nm excitation wavelength and 50 × objective), and atomic force microscopy (AFM, Bruker Multimode 8).
3. Results and discussion
Figure 1(a) shows a typical optical microscopy image of the MoS2 sample grown on SiO2/Si substrate. The substrate is covered with isolated triangular MoS2 flakes, which is a manifestation of three-fold rotational symmetry of MoS2 lattice. The edge lengths of isolated triangular MoS2 flakes are in the range of 10 to 50 μm. The optical contrast of the center of the triangular flakes is higher than that of its outside area, indicating the thickness variation from center to edge. Figure 1(b) is a representative triangular MoS2 flake with the edge length of ~22.5 μm. A dot formed at the center of the triangular flake can be clearly observed, which is the original nucleation site for the crystal [19].
Fig. 1 (a) Optical image of MoS2 flakes deposited on SiO2/Si substrate; (b) a representative optical image of the triangular MoS2 flake.
Figure 2(a) is an AFM topography image of an individual MoS2 flake. Obviously, there is a bright MoS2 nanodot at the center of the triangular MoS2 flake. Figure 2(b) depicts the height profile scan along the green line b shown in Fig. 2(a). The thickness of the edge region in the flake is ~0.7 nm, which is in agreement with the thickness of reported CVD-grown MoS2 monolayers [20]. Figure 2(c) presents the height profile scan along the green line c shown in Fig. 2(a). The height and lateral dimensions of the MoS2 nanodot are ~185 nm and ~1.1 μm, respectively, suggesting that the MoS2 nanodot is multilayer. Notably, the diameter of the nanodot gradually decreases along the axial direction from bottom to top, leading to the formation of tapered structures with a sharp top. Figures 2(d) and 2(e) represent a high resolution AFM image and a height profile of the MoS2 nanodot along the green dotted line in Fig. 2(d). The lateral dimension decreases with the height, showing shrinkage in layer-by-layer MoS2 nanodots. There is an abrupt contrast change at the center of the nanodot, indicating the existence of a flat high terrace (bright area) with a height higher than 20 nm. The step height of ~0.7 nm shown in Fig. 2(e) is consistent with the thickness of MoS2 monolayer. Therefore, the center nanodots are approximately stacked 30 layers. This step-like feature results in numerous edges of the center nanodots. The crystalline nature of the MoS2 flake is also confirmed by high-resolution TEM characterization with a spacing of ∼0.27 nm of the (100) plane (Fig. 2(f)).
Fig. 2 (a) the AFM image of the triangular MoS2 flake; (b)-(c) the corresponding height profile along the green lines b and c in (a); (d) high-resolution TEM image of the edge of a MoS2 flake; (e) The high resolution AFM image of a MoS2 nanodot at the center of a triangular flake; (f) step height profile of the corresponding region of the MoS2 nanodot.
PL spectra from the MoS2 nanodot and outside region of the triangular MoS2 flake are depicted in Fig. 3(a). Two prominent peaks located at around 672 nm (~1.84 eV) and 623 nm (~1.99 eV) (the left inset of Fig. 3(a)), attributed to A1 and B1 excitons [21], are observed in both PL spectra. Remarkably, peak B1 intensity of the nanodot (center region) is approximately 10 times enhanced in comparison to that of the edge region. Figure 3(b) is a typical PL mapping image. The difference in the contrast between the MoS2 nanodot at the center and the outside region indicates that the difference of PL performance. In order to further access the layer-number of the different regions in triangular MoS2 flake, Raman characterization is performed with the same laser excitation source as PL testing. As the Raman spectra provided in Fig. 3(c), two typical Raman peaks can be observed clearly from the nanodot and edge of the MoS2 flake, which correspond to out-plane A1g and in-plane E12g vibration modes at the Γ-point of the hexagonal Brillouin zone of 2D-MoS2. The peak frequency differences between A1g and E12g modes of edge region and nanodot are 19.4 cm−1 and 24.9 cm−1, respectively, indicating that edge region is monolayer and center region (nanodot) is multilayer [22]. The insets of Fig. 3(c) show the Raman intensity distribution mappings of A1g and E12g peaks of a whole triangular MoS2 flake. The integrated peak intensity of A1g and E12g from the center nanodot, corresponding to the brighter color region, is significant higher than that of outside part, further confirming the MoS2 nanodot is multilayer.
Fig. 3 (a) The PL spectra from the center and outside region of a single triangular MoS2 flake, left inset showing the enlarged PL spectra of B1 peak; (b) the corresponding PL mapping of 672 nm; (c) the corresponding Raman spectra, insets showing the Raman mappings of the E2g and A1g peaks.
To further study the enhanced PL properties of CVD-grown MoS2 nanodot stacked on monolayer MoS2, various MoS2 flakes, including truncated triangle, four-point star, and six-point star, are prepared for comparison. Figure 4 depicts the PL spectra of the center and outside parts of the MoS2 flakes with different configurations. The insets are the corresponding optical images of truncated triangle, four-point star, and six-point star. The PL peak intensities at the center regions of the MoS2 flakes are distinctly higher than that of the outside region, and the B1 peak intensity ratio of center/edge is in the range of 1.16~3.57. In addition, no obvious peak shifting of A1 and B1 is observed in different regions. Interestingly, the PL result obtained differs from the thickness-dependent PL characteristic of reported MoS2 [4].
Fig. 4 The PL spectra of the center and edge regions of the MoS2 flakes with different shapes: (a) truncated triangular flake, (b) four-point star, (c) six-point star.
Raman spectra of MoS2 flakes with different morphologies are shown in Fig. 5. All samples exhibit two prominent Raman peaks corresponding to the first-order A1g and E12g modes. For the outside regions of the three samples, the peak frequency differences between A1g and E12g modes are 19.6 cm−1, 20.8 cm−1, and 20.1 cm−1, respectively, indicating the outside regions are monolayer. Meanwhile, peak frequency differences of the center regions are 24.1 cm−1, 24.7 cm−1, and 23.8 cm−1, respectively, confirming the center regions are multilayer.
Fig. 5 The Raman spectra from the center and outside region of the MoS2 flakes with different domains: (a) truncated triangular flake, (b) four-point star, (c) six-point star.
One of the characteristics of MoS2 is the tunable band gap 1.2 eV for bulk (indirect) and 1.9 eV for monolayer (direct) [10]. And it was reported that the PL peak position and intensity are dependent on the thickness (layer number) of MoS2 [4]. However, in this study, the PL of multilayer MoS2 nanodot (center region) of the MoS2 flakes is markedly stronger than that of the monolayer (outside region). Furthermore, two PL peak positions are approximately the same. The result seems to be different from the thickness-dependent PL performance. In order to figure out the origination of the extraordinary PL phenomenon, statistical A1 peak ratio of multilayer/monolayer from 32 reported papers and measured A1 peak ratio of multilayer/monolayer from 15 samples in our work are presented in Fig. 6. The A1 peak intensity ratio of multilayer/monolayer from reported literatures is lower than 1, which is attributed to the direct-indirect band gap transition as the thickness increasing [10]. However, the A1 peak intensity ratio of multilayer/monolayer in this work is in the range of 0.8~10, which is distinct higher than the reported value, meaning the PL of the MoS2 nanodot stacked on the MoS2 flakes is enhanced.
Fig. 6 Statistical survey of A1 peak intensity ratio of multilayer/monolayer from the reported references and our work.
PL properties of MoS2 crystals were reported to be affected by defects, environmental oxidation, doping, and strain [19, 23, 24]. Generally, defects within the semiconductors usually cause PL quenching or act as nonradiative recombination centers [25]. As discussed above, the MoS2 nanodot is clearly a layer-by-layer structure with dimensions gradually reducing along the axial direction, leading to the formation of a tapered nanodot. Compared to micrometer-scale CVD-MoS2 flake, the multilayered MoS2 nanodot has a larger amount of edge states than that in the outside region of the triangular flake, which might contribute to the observed PL enhancement. Power-dependent PL spectra of the monolayer MoS2 flake at outside-region are shown in Fig. 7(a). The A and B excitonic transitions are clearly observed with A being dominant. With increasing excitation power from 0.1%P0 to 10%P0, the intensity of both A and B emission increase accordingly. Using a Gaussian fitting, the raw data of PL spectrum obtained under 10%P0 can be fitted into A0, A-, and B, as presented in the inset of Fig. 7(b). The as-prepared MoS2 is usually n-doped due to the presence of defects or substrate unintentional doping [13]. Subsequently, the photoexcited electron-hole pairs would bind with excess electrons to form negative trions, labeled as A- with lower energy, and A0 is the neutral exciton with higher energy. All the power-dependent PL spectra are deconvoluted into A0, A-, and B, as displayed in Fig. 7(b) on a log-log scale. The slopes indicate the relation between peak intensity (I) and excitation power (L), I∝Ln. For A0, A-, and B, n is 0.98, 0.91, and 0.97, respectively. It is expected that n is in between 1 and 2 for exciton-like transitions in the above band gap excitation. The slightly smaller n (n<1) for A- and A0 implies a defects/impurities-related recombination path [25]. Therefore, the power-dependent PL analysis of MoS2 monolayer confirms the existence of defects, which quench the intrinsic PL or act as nonradiative recombination sites.
Fig. 7 (a) Power-dependent PL spectra of the outside region of the MoS2 sample in Fig. 1(a) at RT under the excitation wavelength of ~532 nm; (b) integrated PL intensity (I) of features A0, A-, and B as a function of the laser intensity (L) plotted on a log-log scale, inset showing the peak fitting and decomposition of the sample under 10% P0 laser power and the origin of features A0, A-, and B.
The improved PL of MoS2 nanodot stacked on MoS2 monolayer flakes might be owing to the physical or chemical adsorption of O2 and physical adsorption of H2O molecules on MoS2. By the adsorption of O2/H2O on MoS2, the excess electrons are depleted, leading to switch the dominant PL process from trion recombination (A-) to exciton recombination (A0), the PL intensity therefore is improved. Meanwhile, the O ions are more reactive and easily interact with MoS2 at the defect sites [26]. In order to clarify the effect of absorbed O2/H2O on PL performance of MoS2, we annealed the sample at 500 °C for 15 min in vacuum. PL spectra of multilayered MoS2 nanodot and basal MoS2 monolayer before and after annealing are presented in Fig. 8. As a result, the PL of the multilayered nanodot was enhanced significantly in comparison to the PL of the basal monolayer. We ascribe this phenomenon to a desorption of absorbed O2 and H2O species during the annealing process. The results suggest the absorbed O by MoS2 nanodot might contribute to the enhanced PL. Therefore, the improved PL intensity of the MoS2 nanodot at the center of triangular MoS2 monolayer might be ascribed to the synergistic effect of the adsorbed O and more edge states of the special tapered edge structure at the center nanodot and the existence of defects in outside region of the triangular monolayer.
Fig. 8 The PL spectra from center and edge region of the MoS2 crystal (a) before and (b) after annealing.
4. Conclusion
In conclusion, we have synthesized and characterized multilayered MoS2 nanodots stacked on monolayered MoS2 flakes obtained by CVD method. Extraordinary high local PL emission has been observed from the MoS2 nanodots. Based on the experimental analysis, the enhanced PL of multilayered MoS2 nanodots is owing to the tapered morphology of MoS2 nanodots with many edge states and adsorbed O molecules. The findings presented here supply a new approach to modify the PL properties of MoS2 by the formation of pyramid-like morphology under appropriate growth conditions.
Funding
National Natural Science Foundation of China (51572092).
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