Nanoparticle@MoS2 Core-Shell Architecture: Role of the

Article Cite This: Chem. Mater. 2018, 30, 4675−4682

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Nanoparticle@MoS2 Core−Shell Architecture: Role of the Core Material Jennifer G. DiStefano,† Yuan Li,†,‡ Hee Joon Jung,†,‡ Shiqiang Hao,† Akshay A. Murthy,† Xiaomi Zhang,† Chris Wolverton,† and Vinayak P. Dravid*,†,‡,§ Department of Materials Science and Engineering, ‡Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, and §International Institute for Nanotechnology (IIN), Northwestern University, Evanston, Illinois 60208, United States

Chem. Mater. 2018.30:4675-4682. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/05/18. For personal use only.

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ABSTRACT: Core@shell architectures provide a rich platform for designing new geometries composed of various functional nanomaterials. Recent work has shown that Au@MoS2 core@shell structures exhibit strong light−matter interactions and promising optoelectronic device performance. However, the role of the core on Au@MoS2 growth dynamics is not well understood, leaving the question of if this unusual structure is extendable to other materials systems unanswered. Herein, we present unambiguous evidence of MoS2 encapsulation of new crystalline and even noncrystalline core materials, including Ag and silica. High-resolution transmission electron microscopy shows intimate contact between each core material and their highly crystalline, conformal MoS2 shells. We propose a generalized growth mechanism for these structures, which is supported by density functional theory energy calculations and implies wider applicability of transition metal dichalcogenide encapsulation to other functional nanoparticles. Further, we demonstrate that altering the core material is a useful methodology to achieve distinct optical responses, as reflected in the photoluminescence measurements and corroborated by discrete dipole approximation calculations. By exploring the role of the core material on synthesis and properties in this architectural platform, we introduce a multiplexed nanoparticle@MoS2 paradigm with numerous viable avenues for future structural and property investigation.

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encapsulation of plasmonic nanoparticles (NP), though only complete encapsulation of Au has been definitively demonstrated through advanced microscopy.23−29 These Au@MoS2 structures have demonstrated attractive performance in a variety of applications including photodetectors,27 surface enhanced Raman spectroscopy (SERS),23 and bioimaging.25 By varying the nature of the nanoparticle core material as well as the type and characteristics of the shell using this geometry, there are abundant opportunities to explore and engineer potentially useful properties. However, some reports have hypothesized that the well-established affinity between gold and sulfur30 may be the key enabler for the formation of Au@ MoS2 structures.24,26 Before a library of this unique core@ TMD architecture can be realized, a deeper understanding of the feasibility and extent of this geometry is necessary. Herein, we demonstrate that a natural affinity between TMD and core is not a necessary condition for construction of these core@shell architectures. Namely, we report successful encapsulation of diverse nanoparticle cores with conformal MoS2, including both crystalline, plasmonic Ag and noncrystal-

raditional composite materials produce unique material characteristics and have enabled significant progress in many engineering fields by combining components with vastly different properties.1−3 When applied to nanoscale geometries, the ability of composites to derive new functionality is particularly powerful. Core@shell architectural nanocomposites have an extensive history of producing remarkable and synergistic effects, including protection of reactive core materials,4,5 increased chemical functionalization,6 and a multitude of enhanced or entirely new optical and catalytic properties.7−10 The highly multifunctional nature of this architecture has been utilized across materials systems ranging from quantum dots11−14 to bimetallic nanoparticles15,16 to graphite17,18 and shown promise in numerous technological applications.19−22 Such unique nanocomposite architectures have recently been realized with two-dimensional (2D) transition metal dichalcogenides (TMDs). In core@TMD shell structures, a vast parameter space including size, shape, and composition is available for modulating properties. When a conformal coating is achieved, this architecture also has the unique advantage of maximizing the surface area of the nanocomposite interface, thereby increasing interaction between a functional core and TMD shell.23 Recently, multiple studies have examined MoS2 © 2018 American Chemical Society

Received: March 30, 2018 Revised: June 16, 2018 Published: June 18, 2018 4675

DOI: 10.1021/acs.chemmater.8b01333 Chem. Mater. 2018, 30, 4675−4682

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Chemistry of Materials

Figure 1. Structural demonstration of NP@MoS2 core@shells. (a−c) Bright field TEM; (d−f) HRTEM; (g−i) FFT of core@shell nanoparticles for Ag@MoS2 (left column), Au@MoS2 (middle column), and SiO2@MoS2 (right column). HRTEM images exhibit the interface between MoS2 shell and core. Examples of defects in the MoS2 shell are visible in panel e. FFT patterns confirm the structures of MoS2 and cores. Note that Ag and Au are imaged along the FCC [211] zone axis while silica is amorphous. Extra, theoretically forbidden FFT spots are generated due to irregular but repeating variation in intensity of (111) planes caused by thickness variation (visible in panels d and e) and are indicated as 01̅1 in panels g and h, for example. MoS2 {004} spots are indicated as yellow circles with connecting dotted lines in panels g and h.

nanoparticle cores, commercially available 50 nm diameter particles (NanoComposix) in water were dispersed on an SiO2/Si substrate by spin-coating. MoS2 shells were then directly grown on these nanoparticles using the conventional CVD process involving molybdenum trioxide (MoO3) and sulfur powder precursors,31,32 with consistent growth conditions for each core material. Structural and chemical analyses of the core@shells via transmission electron microscopy (TEM) demonstrate similar encapsulation of each type of core material. Figure 1a−c shows high-resolution TEM (HRTEM) images of the three types of core@shells. Cross sections of MoS2 shells are distinctly visible extending from each core surface, regardless of whether the nanoparticle is slightly faceted in nature. These shells are approximately two to five layers thick. High-resolution images in Figure 1d−f indicate that the interplanar d-spacing of MoS2 is ∼6.4 Å, consistent with previous reports of interlayer spacing.24,27 The shells appear to be highly crystalline with defects largely localized to areas of maximum strain, such as extreme curvature at faceted corners, as is evident in Figure 1e. Despite inevitable changes in orientation following transfer from the growth substrate to a lacey carbon film on Cu grid, conformal shells are consistently observed in the 2D projection of TEM for all three types of core@shell architectures. The structural characteristics of the different core and shell materials were further analyzed through fast Fourier transform (FFT) of the HRTEM images (Figure 1g−i) and diffraction

line, dielectric silica, expanding the NP@TMD portfolio. We show that formation of this nanoscale architecture and the intimate interface between core and shell is invariable between the two core materials and similar to that of Au@MoS2. In contrast, the interaction between these NP@MoS2 structures and incident light is strongly core-dependent. We present evidence that this NP@MoS2 architecture can be extended to many other core materials for potentially diverse properties and phenomena. Additionally, the MoS2 shells supported by plasmonic Ag and Au cores exhibit significantly enhanced photoluminescence (PL) emission, which we attribute to localized surface plasmon resonance (LSPR), in contrast to the poor PL from encapsulated silica cores. By investigating MoS2 encapsulation of various nanoparticles, this work leverages the complex core@TMD architecture to further probe and design TMD nanocomposites.

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RESULTS AND DISCUSSION Structural Analysis. MoS2 encapsulation of Au, Ag, and silica nanoparticles was achieved using a modified chemical vapor deposition (CVD) process. To produce Au and Ag nanoparticle cores, thin films of each metal were first evaporated onto separate Si substrates with a 285 nm SiO2 layer and subsequently annealed at 800 and 450 °C, respectively, to form nanoparticles which were then encapsulated with MoS2 layers. The resultant size distributions of Ag and Au were 53 ± 15 and 49 ± 17 nm, respectively. For silica 4676


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Figure 2. HAADF STEM images and corresponding EDS overlay color-map of each core@shell. Top row: HAADF STEM images; Bottom row: EDS overlays, with green representing sulfur and red representing the respective core element (Ag, Au, Si). (a, d) Ag@MoS2; (b, e) Au@MoS2; (c, f) SiO2@MoS2. The MoS2 shell is evident in each HAADF image and also apparent in the EDS overlay as the green sulfur signal extending slightly beyond the red core signals.

corresponding to the Mo 3d3/2 and 3d5/2 peaks, respectively (Figure S5b). Additionally, the S 2s peak is observed near 226.5 eV. These spectra indicate the expected 4+ and 2− oxidation states of Mo and S, respectively, in MoS2, and are consistent with previous reports, suggesting no chemical bonding between core and shell materials.34,35 The XPS spectra of each bare core material demonstrate the expected chemical states (Figure S5c−e) as well. Proposed Encapsulation Mechanism. Interestingly, we observe similar conformal encapsulation with an intimate core@shell interface for each core material and further demonstrate this at various size scales (Figure S6). While Au and Ag are known to have strong bonding affinity to sulfur, this is not true of silica, and yet, we observe no apparent difference in encapsulation quality.36,37 To aid the investigation of the fundamental formation mechanism of these core@shell structures, we utilized density functional theory (DFT) to calculate the interfacial energies between the basal plane of MoS2 and each core material, including various faces for each metal (Table S1). MoS2 on the (111) face of Ag exhibits the lowest interface energy and, therefore, is the most favorable of the studied cores. Given that direct MoS2 growth on Au and SiO2 substrates has been experimentally demonstrated,38,39 we expect lower energy interfaces (e.g., Ag/MoS2), to also successfully form, thereby supporting our experimental core@shell results. Next, we consider the impact of nanoparticle curvature. We approximated the strain energy due to curvature, defined as K2 ,

(Figure S3). The Ag and Au cores in Figure 1g,h are aligned approximately along a [211] zone axis of the expected facecentered cubic (FCC) crystal structure, corresponding to the HRTEM images in Figure 1a,b, respectively. In contrast, the FFT for SiO2@MoS2 (Figure 1i) shows a cloudy broad intensity distribution without distinct spots due to the amorphous nature of the silica core. The {004} MoS2 spots can be observed in each FFT pattern. Diffraction patterns (Figure S3) were taken from a large number of particles for statistical analysis and confirm the expected FCC structure of Au and Ag, as well as the structure of MoS2. The half circles highlight the positions of the ring-type polycrystalline diffraction patterns from MoS2 and the corresponding core materials. The positions of these rings match well with reported d-spacings. The elemental makeup of these structures is revealed by scanning transmission electron microscopy−X-ray energy dispersive spectroscopy (STEM-EDS) (Figure 2). The shells are clearly visible based on contrast differences in each STEM image taken in high angular annular dark-field (HAADF) mode (Figure 2a−c), wherein the contrast is derived from atomic number and mass thickness.33 As a result, the higher atomic number metal cores appear brighter and the amorphous silica core appears darker in relation to MoS2. EDS color-map overlays clearly show the spatial distribution of each element (Figure 2d−f). Ag, Au, and Si, each colored red, make up the respective core materials, with the green sulfur signal extending slightly beyond the core in each case. Individual EDS maps can be found in Figure S4. We further demonstrate the expected chemical nature of each element is present in the core@shell structures using Xray photoelectron spectroscopy (XPS) (Figure S5). Overall, the Mo and S peak positions are consistent among the three types of cores. The broad S peak for each core@shell can be deconvolved into two smaller peaks corresponding to the 2p1/2 and 2p3/2 orbitals with binding energies of 163.5 and 162.4 eV, respectively (Figure S5a). Mo peaks from each of the three core@shell types are observed near 232.7 and 229.5 eV,

r

where the MoS2 bending stiffness (K) is 9.61 eV and r is the radius of curvature of the nanoparticle (25 nm).40,41 The approximate curvature energy in each of our core@shells is J 2.46 × 10−3 2 , which is orders of magnitude smaller than the m calculated interface energies. This is consistent with other reports suggesting that strain energy in MoS2 only becomes significant for much smaller radii.42−44 In fact, we demonstrate conformal encapsulation of a sub-10 nm diameter Au core (Figure S6a), with diameters as small as 2 nm expected to be 4677


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Figure 3. Mechanism of core@shell CVD growth. Step 1: MoS2 nucleation sites form on the arbitrary nanoparticle surface. Step 2: Interface between core and shell forms, and eventually, full encapsulation is achieved by an initial shell layer. Step 3a: Additional shells are grown using the initial shell as a template in slow deposition (i.e., normal growth) conditions. Step 3b: Under fast deposition rate conditions, MoS2 “wing” formation occurs.

Figure 4. Core-dependent optical properties. (a) Raman spectroscopy of each core@shell demonstrating the characteristic MoS2 phonon modes and similar mode separations. (b) Photoluminescence spectra of each core@shell. The plasmonic core@shell structures demonstrate significantly higher intensities compared to the dielectric core@shell structure. (c) Electric field intensity maps at 535 nm calculated using DDA. (d) DDA calculations of absorbance indicating the C exciton of MoS2 and LSPR peaks of Ag and Au.

possible according to reported strain energy calculations.42 Additionally, the presence of defects in the shell would result in strain relaxation and further reduce this curvature energy. Accordingly, the growth dynamics on surfaces of a 25 nm

radius can be considered comparable to that of planar structures. This energy analysis is further supported by our experimental observations, where conformal growth of the first MoS2 layer is observed in each of our nanoparticle systems. 4678


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multilayer MoS2, we expect relatively low PL intensities in these indirect bandgap materials. As such, the silica core@shell demonstrates a low PL intensity consistent with these expectations (Figure 4b). However, the PL intensity corresponding to the MoS2 A exciton for the Au and Ag cores is significantly higher than that of the silica core. Au and Ag are both plasmonic materials and therefore produce strong local electromagnetic fields when irradiated, causing increased exciton formation in the surrounding MoS2 shell. Strong interaction with plasmonic nanoparticles and enhanced PL intensity from TMDs has previously been demonstrated in TMD/plasmonic systems,50−52 so it follows that the two plasmonic cores would demonstrate notably higher PL intensities than the dielectric silica core. Utilization of this structure therefore provides a method to enhance the traditionally low PL intensity of multilayer TMDs and serves as an example of the property engineering possible with these core@shells. Simulated electric field distribution maps provide further insight into our PL observations (Figure 4c). Using discrete dipole approximation (DDA), we simulated isolated particles with 50 nm diameters of each core material and three layers of MoS2 shell, which serves as an appropriate model for our system. A hollow MoS2 shell was additionally considered for reference. The four near-field maps are compared at a wavelength of 535 nm to closely match the PL measurement taken at an incident wavelength of 532 nm. In all cases, the field is primarily localized in the MoS2 shell, but the intensity varies greatly. The two plasmonic cores of Ag and Au exhibit the greatest field localization, and both have |E/E0|max2 values of 10.7. In contrast, the hollow MoS2 shell and SiO2@MoS2 exhibit much lower field intensity, with |E/E0|max2 values of 4.83 and 4.16, respectively. This calculation is consistent with our PL findings, indicating that the local field enhancement caused by the LSPR of the plasmonic cores can help explain the PL enhancement in MoS2. Beyond the impact on the emissive properties in MoS2, the optical tunability provided by these core structures is readily apparent in the optical absorbance of each core@shell structure. As evident in Figure 4d, the simulated absorbance peaks and their locations depend highly on the core material utilized. Three distinct spectral features are apparent, corresponding to the LSPR peak of each plasmonic material and an excitonic transition in MoS2. The Au and Ag LSPR peaks are observed at 538 and 463 nm, respectively, showing a red-shift compared to the resonance of the bare cores, which are at wavelengths of 513 and 375 nm, respectively (Figure S7a). This can be attributed to the modified dielectric environment induced by MoS2 encapsulation. The position of such plasmon peaks is sensitive to not only the refractive index but also the thickness of the surrounding environment, according to Mie theory.53 Therefore, changes in the layer number of the MoS2 shell can noticeably shift the LSPR of a plasmonic core, exhibiting another opportunity for property engineering through these structures (Figure S7b).54,55 The expected C exciton of MoS2 can be clearly observed around 400 nm in the cases of the silica and Ag cores and the hollow MoS2 shell.56 A slight shoulder in the Au core spectrum can also be attributed to the C exciton. We attribute the higher intensity in the case of Ag@MoS2 to the proximity of the MoS2 C exciton to the Ag LSPR, where there is sufficient peak overlap to cause emission enhancement. In the case of the MoS2 shell, we attribute the intensity of the C

Therefore, we believe that conformal encapsulation is possible with other core materials where a planar interface is achievable with MoS2. It follows that this architecture is extendable to larger cores (Figure S6c), where the strain energy due to curvature is further reduced. According to the above analysis, the growth mechanism of MoS2 encapsulation can be divided into three general steps: (1) nucleation of MoS2 on the nanoparticle surfaces; (2) growth of the first conformal shell; (3) subsequent growth of additional shells (Figure 3). On the basis of the low interfacial energies explored through DFT, local nucleation centers are predicted to first form on the surface of the nanoparticle. Beyond initial nucleation, it is critical to control the deposition rate of MoS2 molecular clusters to achieve the conformal shells discussed here. The growth process utilized to create conformal shells occurs under relatively slow deposition conditions (Figure 3, Step 3a). However, when the growth process is altered to induce a higher deposition rate, a variation on this pristine encapsulation occurs in the form of MoS2 “wings”, where flakes extend laterally from the core@shell (Figure 3, Step 3b). These structures are observed when the sulfur powder is placed in a hotter region of the furnace (i.e., closer to the growth substrate) than the normal growth setup, which increases the sulfur vapor pressure. In turn, this produces a higher partial pressure of MoO3−x, resulting in an increased deposition rate of MoS2 molecular clusters.31,45,46 Therefore, we believe this rapid impingement results in growth of both additional shells on the nanoparticle surface and MoS2 planar sheets (or “wings”). Though these structures are outside the scope of this study, their high number of edge sites could be leveraged for applications such as hydrogen evolution reaction.47 The growth mechanism proposed above is corroborated by an earlier proposed graphite encapsulation model.48 According to the literature on curved graphite, subsequent layers after the first shell tend to mimic the morphology of the layer beneath (i.e., homoepitaxy), meaning the core has little role in the growth of subsequent layers. This is consistent with our proposed slow deposition pathway resulting in typical pristine encapsulation (Figure 3, Step 3a). Further, at a faster deposition rate, planar graphitic sheets grow away from the nanoparticle core, as observed for MoS2 in our winged growth pathway (Figure 3, Step 3b). Core-Dependent Optical Properties. When keeping shell parameters (layer number, strain, etc.) constant, the core material can greatly impact the optical emission of MoS2. Raman spectroscopy demonstrates the signature MoS2 E2g1 (in-plane) and A1g (out-of-plane) Raman modes for each core@shell, further confirming the crystallinity of the MoS2 shells (Figure 4a). The Ag@MoS2 structures are observed to exhibit the highest peak intensities, over twice that of SiO2@ MoS2 structures without a plasmonic core, which is not surprising as Ag nanoparticles are a common enhancement enabler in SERS.49 Layer thickness and strain have been demonstrated to affect MoS2 Raman peak positions, and both factors are expected to play a role in our structures.39 Because consistent core size results in similar MoS2 strain for all three types of core@shells, Raman data can then identify regions of consistent layer thickness for PL analysis. PL spectra (Figure 4b) are collected from the same point as their respective Raman spectra in Figure 4a, which have similar peak separations and are representative of spectra collected for each core@shell. Because the shells are comprised of 4679


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Chemistry of Materials Notes

exciton peak to a waveguiding effect. Waveguides typically require a higher index of refraction shell surrounded by lower index medium, and hollow-core waveguides are one such structure known to effectively direct light, suggesting hollow MoS2 shells may cause a similar effect.57,58 The intensity of the C exciton is noticeably lower in the cases of the Au and silica cores because the spectral position of the Au LSPR does not overlap with the C exciton to cause notable enhancement, and silica has no LSPR. We believe such property diversity based on the core material paves the way for exploration of additional properties via other core materials in this unusual architecture.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation (NSF) under Grant No. DMR-1507810. This work made use of the EPIC and Keck-II facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; the State of Illinois, through the IIN. Access to facilities of high-performance computational resources at Northwestern University is acknowledged. J.G.D. gratefully acknowledges support from the National Science Foundation Graduate Research Fellowship Program (NSFGRFP). The authors thank Dr. Sungkyu Kim, Dr. Qianqian Li, and Dr. Fengyuan Shi for providing TEM expertise and Dr. Eve Hanson for insightful discussions.

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SUMMARY AND CONCLUSIONS We report conformal and crystalline encapsulation of diverse nanoparticles by MoS2. HRTEM confirms the role of the core material to be minimal in determining the structure, where an intimate interface between the MoS2 shell and core forms regardless of core material. The DFT energy analysis suggests that conformal encapsulation can be achieved using core materials on which planar MoS2 growth occurs, offering extendibility to a wide variety of core materials beyond Au, Ag, and silica. Further, core-dependent optical property engineering of MoS2 was experimentally demonstrated through significant changes in PL intensity and corresponding DDA calculations, with potential for future application in optoelectronic devices such as photodetectors. A large parameter space is available for design of future nanocomposites, where one can imagine replacing core materials with other classes of functional materials, while tuning properties by varying size, shape, and type of TMD. We believe this work provides the foundation for an array of future nanoparticle@TMD architectures utilizing this intimate interface, greatly expanding the engineering opportunities available through nanocomposites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01333.

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REFERENCES

Experimental methods, DDA and DFT calculation details, electron diffraction analysis, supplementary figures of SEM, STEM-EDS maps, various core sizes, XPS, and theoretical effect of shell thickness on plasmonic resonance, and DFT interfacial energy values (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jennifer G. DiStefano: 0000-0002-8829-0692 Yuan Li: 0000-0001-7452-1149 Akshay A. Murthy: 0000-0001-7677-6866 Chris Wolverton: 0000-0003-2248-474X Vinayak P. Dravid: 0000-0002-6007-3063 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 4680


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