Optically Active 1D MoS2 Nanobelts - ACS Applied Materials

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Optically-active 1-D MoS nano-belts Akshay A. Murthy, Yuan Li, Edgar Palacios, Qianqian Li, Shiqiang Hao, Jennifer G. DiStefano, Chris Wolverton, Koray Aydin, Xinqi Chen, and Vinayak P. Dravid ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16892 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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ACS Applied Materials & Interfaces

Article Type: Letter

Optically-active 1-D MoS2 nano-belts Akshay A. Murthy#,† Yuan Li#,†,‡ Edgar Palacios,

∥

Qianqian Li,†,‡ Shiqiang Hao,† Jennifer G.

DiStefano, †,§ Chris Wolverton,† Koray Aydin,∥ Xinqi Chen,†,‡ Vinayak P. Dravid *,†,‡,§

#

A. M. and Y. L. Contributed equally to this work

†

Department of Materials Science and Engineering, ‡Northwestern University Atomic and

Nanoscale Characterization Experimental (NUANCE) Center,

§

International Institute for

Nanotechnology (IIN), and ∥Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA

*Corresponding author Vinayak P. Dravid: [email protected] KEYWORDS: Transition metal dichalcogenides, MoS2, chemical vapor deposition, photodetectors, synthesis

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ABSTRACT: Transition metal dichalcogenides can be synthesized in a wide range of structures. 1-D geometries, including nanotubes and nanowires, are especially intriguing due to enhanced light-matter interactions stemming from both the thickness and width possessing subwavelength dimensions. In this letter, we demonstrate the synthesis of 1-D MoS2 nano-belts through chemical vapor deposition and examine the mechanism driving the formation of this material. We also report enhanced light scattering within these structures. Finally, we investigate the phototransistor behavior of MoS2 nano-belts and observed a photoresponsivity around 1.5 A/W, an order of magnitude greater than analogous multilayer 2-D MoS2 sheets reported previously.


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Transition metal dichalcogenides (TMDs) have garnered a great deal of research interest due to their naturally layered structures and property changes as a function of number of layers, especially down to the monolayer limit. These van der Waals solids can be synthesized into various structural architectures by overcoming the weak forces holding together the layered units1, 2. In the case of 2-D confinement, TMDs, such as MoS2 and WS2, exhibit fundamentally divergent optical properties from their bulk structures, including enhanced photoemission3, second harmonic generation4, and valley polarization5. When the dimensionality is lowered further to produce 1-D structures, TMDs are predicted to demonstrate magnetism and electronic phase transitions, as well6. Furthermore, as is apparent from the emergent properties seen in Si and III-V semiconducting nanowires, the dimensional confinement in 1-D structures can modify optical properties through substantial light-matter coupling and lead to scattering cross-sections an order of magnitude greater than their physical cross-sections7. This coupling is a result of differences in the dielectric function between the semiconducting media and the external environment that allow 1-D structures to act as dielectric waveguides that can be modeled using the same theory developed for Fabry-Perot cavities8. This resonance between 1-D structures and light has allowed for the design of 1-D optical devices, such as photodetectors9, LEDs10, and photovoltaic cells11.While there have been a few reports12-14 on MoS2 nano-belts with similarly intriguing 1-D geometries, the focus has mainly been on the synthesis protocol and structural characterization. This has caused fundamental insights on optical properties in 1-D geometries of MoS2 to significantly lag those of 2-D geometries3,

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, while leaving potentially valuable material

properties of MoS2 nano-belts, such as photoconductivity, largely concealed.


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Herein we provide a thorough characterization of these lesser-explored, 1-D MoS2 nanobelt structures through a suite of characterization methods. Following synthesis of these structures via a modified chemical vapor deposition (CVD) method, we use spectroscopy and microscopy techniques to understand the chemical and structural morphology of these materials. Armed with these findings, we are able to propose a growth mechanism governing the formation of these structures. Scattering peaks resulting from enhanced interaction between incident light and the subwavelength width of the structure were detected via optical measurements. In light of these peaks, which can induce electromagnetic field enhancement within and around the structures, the nano-belts were characterized as a photoconductor and exhibited significant photoresponsivities of 1.5 A/W. These notable properties combined with the simple synthesis protocol for these structures provides an avenue to interface them with traditional 2-D building blocks moving forward. The synthesis of MoS2 nano-belts was accomplished via CVD (see experimental section in SI for details) on Si and 300 nm SiO2/Si substrates. An optical image of the resultant structures is provided in Fig. 1a. As apparent from AFM (Figs. 1c-d), these structures tend to have thicknesses around 30-40 nm with widths around 150 nm. X-ray photoelectron spectroscopy further confirmed (Figs. 2a-b) the expected chemical states of Mo4+ and S2- for the case of Mo-S bonding. We observed an additional weak peak indicating the presence of oxide states of molybdenum, which is likely due to the inevitable deposition of MoOx on the substrate during the CVD synthesis process. Raman spectroscopy complements this data as both the inplane (E12g) and out-of-plane (A1g) MoS2 Raman-active modes are evident (Fig. 2c). In this case, the spacing between the two modes is 25.7 cm-1, and this spacing between modes corresponds to


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an MoS2 nanosheet with greater than 4 layers, which matches the case for a 30-40 nm thick structure4, 16. TEM was used to better understand the chemical homogeneity and structural morphology of these materials. The energy dispersive spectroscopy (EDS) maps of randomly selected individual MoS2 nano-belts, presented in Fig. 1h, indicate that both Mo and S are present uniformly throughout on the nano-belt. According to previous literature, the 3R stacking phase is present in these nano-belts, based on convergent beam electron diffraction (CBED) and aberration-corrected TEM imaging12,

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. To supplement these articles, we conducted high-

resolution TEM imaging (Fig. 1f), which suggests an atomic spacing of 0.27 nm in the (100) plane in the 3R-MoS2 phase. It is noteworthy that there is some variance in this lattice spacing as a function of direction. This strain likely manifests itself in the form of diffraction contrast in the low magnification TEM image in Fig. 1b. The belt-like, as opposed to wire-like or cylindrical, geometry of these materials leads to some variance in the atomic structure between the top surface (Fig. 1e) and the edge surface (Fig. 1f), as well. Additionally, the selected area electron diffraction (SAED) patterns (Fig. 1g) in the [11ത1ത] direction confirms the single crystalline nature of these nano-belts, as diffraction spots from the ሼ110ሽ family of planes are evident. Furthermore, the six-fold symmetry present in this diffraction pattern corroborates with the sixfold symmetry atomic structure present in 3R stacking. Based on our previous TEM findings regarding the formation of a MoS2 fullerene nuclei seeding the growth of MoS2 flakes, it is apparent that the ratio between the vapor pressures of the sulfur and transition metal oxide precursors initiate the growth of different MoS2 structures18. To better understand the growth mechanism governing these nano-belts, which we have found to be lacking in the field, we investigated the difference in product morphology by introducing the


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sulfur powders into location 1 (Fig. S1) of the reaction chamber at different stages of the ramping process from room temperature to 700°C. Since a single zone furnace was used and the sulfur powder was moved into the same location each time, the temperature of location 1 and thus, the sulfur vapor pressure was dependent on when the sulfur was introduced. As apparent from the optical microscopy images seen in Fig. S1, the sulfur introduction condition is intrinsically linked to the structural moiety formed. If the sulfur is introduced either slightly sooner (ܶெ௢ைయ = 550°C) or slightly later (ܶெ௢ைయ = 650°C) than in the standard growth process, MoS2 flakes are formed without any nano-belts present (Fig. S1a and S1c). Based on these results, we propose the following explanations for each of the three conditions studied: (1) early sulfur introduction (ܶெ௢ைయ = 550°C and Fig. S1a), (2) optimal sulfur introduction (ܶெ௢ைయ = 600°C and Fig. S1b), and (3) late sulfur introduction (ܶெ௢ைయ = 650°C and Fig. S1c). In the first condition, we believe the situation most closely follows the mechanism detailed by Cain et al18 as the sulfur is initially moved into a temperature region that remains below its melting temperature (ܶௌ௨௟௙௨௥ ≈ 100°C). In this mechanism, as the temperature in both regions of the furnace subsequently rises, a sulfur deficient atmosphere initially develops leading to the condensation of partially sulfurized MoO3-xS cores on the surface. These cores subsequently seed the 2-D flakes seen in our study as the sulfur vapor pressure continues to rise. In the third condition, at the moment the sulfur is introduced, both the MoO3 and the sulfur are significantly more volatile than in the first condition (ܶௌ௨௟௙௨௥ ≈ 150°C), as each is placed in temperature regions that are closer or above its melting temperature19. Since, however, flake formation is once again evident, we argue that the high MoO3 vapor pressure leads once more to a sulfur deficient atmosphere and the same growth mechanism governs material formation .18


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The condition of sulfur introduction at 600°C is unique, though, as once again the sulfur is moved into a temperature region that exceeds its melting temperature (ܶௌ௨௟௙௨௥ ≈ 125°C), but in this condition the initial vapor pressure of the oxide species is not as high as it is initially in condition 3. As a result, the environment quickly reaches a sulfur rich regime, and this environment has been known to produce fullerene-like structures20. Based on this information, we propose a new mechanism where MoO3-x nano-belts are formed first, and then are instantaneously sulfurized through a process similar to fullerene formation, thereby forming MoS2 nano-belts. We propose this since MoO3-x has been previously synthesized into nano-belt form21-23. A scheme representing the types of nanostructures that can form is presented in Fig. S2 based on the experimental conditions used in previous reports18, 20, 24-27. Following the examination of these MoS2 nano-belts, we investigated their optical properties to determine the nature of light-matter interactions of the 1-D structures seen in Fig. 1a due to the dimensional confinement. The nano-belts in this region have the following approximate dimensions from AFM: length = 2.0 um, width = 150 nm +/- 25 nm, and thickness = 40 nm +/- 10 nm and lie on a Si substrate with a native oxide layer of approximately 2 nm. Though this material exhibits an indirect bandgap due to its multilayered nature as has been shown previously17 and in Fig. S3, photoluminescence spectrometry was conducted and a peak corresponding to the higher energy, “A” exciton direct band gap transition is present at 1.81 eV in Fig. 2d, as has been shown for 2-D multilayered MoS2. 3, 12, 17 Figure 2e shows localized reflectance spectra from the nanobelts using polarized reflectance measurements. The spectra were obtained using incident light polarized perpendicular to the nano-belt axis to couple with dielectric resonances and were taken from a


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250 nm x 200 nm region limited by the spatial resolution of the system. These spectra are representative of the nano-belts, as indicated by the reflectance map (Fig. S4). As is apparent, there are three peaks in the differential reflectance spectra, occurring at 525 nm, 658 nm and 721 nm. To quantitatively explain the presence of these peaks, we conducted FDTD calculations on nano-belt structures with similar geometries, which predicted extinction peaks at 626 nm and 686 nm (Figure 2f). It is important to note, however, that the extinction spectrum is calculated as the sum of scattering and absorption, and if we compare the two, we see that the scattering appears to dominate due to the large scattering cross sections originating from strong magnetic resonances within the dielectric nano-belt media, which has been seen previously in semiconducting nanowires28. To further elucidate this, we plot the magnetic field maxima within the nano-belt and peaks at 545 nm, 635 nm and 700 nm are apparent, which correspond well with peaks seen in the scattering spectrum (Fig. S5). Additionally, the peak around 625 nm results from a combination of absorption through exciton formation and scattering phenomena as seen in the FDTD simulations in Figure 2f. The peaks seen are generally supported by the experimental optical measurements, and differences in the refractive index and substrate effects can likely explain the slightly red-shifted FDTD simulation spectra from experiments. In order to leverage the appreciable light-matter interactions within these structures, twoterminal measurements were conducted on these nano-belts both with and without a white light source (360-600 nm), as seen in the Fig. 3a schematic. We studied two device geometries in this study: (1) a channel composed of networks of MoS2 nano-belts that were allowed to freely intersect (Fig. 3b) and (2) a channel composed of individual nano-belts (Fig. 3c). The transfer and output curves of the former device with and without illumination are shown in Figs. 3d and


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e, respectively. Fig. 3d is indicative of standard n-type MoS2 transistor behavior29 and consistent with the semiconducting band structure seen in Fig. S3. The photoconductive effect leads to a significant photoresponse throughout the entire gate voltage range and is most significant when the gate voltage was less than the threshold voltage (-10V) due to the minimal dark currents in this regime. The output curves measured under VG=0V and VG=50V both with and without illumination (Fig. 3e) are fairly linear, and the photoresponse is generally consistent with the observations from the transfer curves. The electrical characteristics of the second geometry are shown in Figs. 3f and g. Similar n-type behavior is observed, however, the photoresponse exceeds the first device geometry. The lesser photoconductivity in the former device likely results from scattering at the interfaces between nano-belts, which reduces the mean free path of photogenerated carriers. Additionally, the larger channel length (20 µm vs. 2 µm) of the former device likely reduces the collection efficiency of photogenerated carriers at the electrical contacts. Photoresponsivity, defined as the photocurrent generated by unit incident power illuminated on the device (R = I / P), is considered to be a critical indicator of photocarrier generation ability in a photo-sensing device. The variation of photoresponsivity as a function of illumination power for various nano-belt devices was generated from I-t plots of the second transistor geometry examining the generation and decay of photocurrent upon periodic illumination (VD = 1 V; VG = 10 V) (Fig. 3h). We observed a general decreasing trend in photoresponsivity with increasing incident power likely due to saturation of photocarriers (Fig. 3i), which is consistent with previous literature29. The photoresponsivity values of 2-µm nanobelt transistors were found to be between 1 A/W and 1.5 A/W, which is about 10x larger than those reported for similar multilayer MoS2 nanosheets29. We believe this results from particular


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wavelengths of light coupling with the dielectric media, as explored through reflection measurements. In light of this enhanced photoresponsivity, it is apparent that MoS2 1-D nanobelts can serve as promising low-dimensional platforms for optoelectronic applications. We report the synthesis of MoS2 nanostructures exhibiting unique 1-D geometries comprising of rectangular cross-sections (of tens of nanometers) with extended lengths (tens of micrometers). We propose these structures form under sulfur-rich CVD conditions when MoO3-x nano-belts are sulfurized. We further probed their optical properties and detected scattering peaks not seen in planar MoS2, which can induce electromagnetic field enhancement within and around the structures. Finally, stemming from these attractive optical properties, the nano-belts were characterized as a photoconductor, with responsivities around 1.5 A/W making them competitive with analogous 2-D multilayer TMDs. This significant responsivity for a photoconductor, motivates the use of these distinctive low-dimensional materials in future 1-D/2D architectures, which leverage the direct band gap nature of single layer MoS2 with the light confining properties present in one-dimensional geometries, for photocatalytic or optoelectronic applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Experimental details of growth, reflection measurements, and device fabrication; Schematics outlining products depending on CVD growth conditions; DFT calculations


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of 3R MoS2 band structure; Reflection maps of nanobelt region; FDTD simulations of magnetic resonances in nano-belts and for nanobelts with varying widths;

Author Information Corresponding author *Vinayak P. Dravid: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. #A. M. and Y. L. Contributed equally to this work Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This material is based upon work supported by the National Science Foundation under Grant No. DMR-1507810. This work made use of the EPIC, Keck-II, and/or SPID facility(ies) 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-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This work also utilized Northwestern University Micro/Nano Fabrication Facility (NUFAB), which is partially supported by Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the Materials Research Science and Engineering Center (DMR-1720139), the State of Illinois, and Northwestern University.


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FIGURES

Fig. 1. CVD synthesis of 1-D MoS2 nano-belts. (a) Optical image of the MoS2 nano-belts on Si substrate. (b) Low magnification TEM image of the nano-belt. (c) AFM image of the nano-belts. (d) Height profile of nanobelts derived from AFM. (e-f) TEM images showing the lattice structure at the edge (e) and center (f) of the nano-belts. (g) Diffraction pattern collected from nanobelt. (h) STEM-based EDS mapping of a single MoS2 nano-belt.


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Fig. 2. Spectroscopic characterization and optical properties of MoS2 nano-belts. (a,b) XPS indicating the chemical states of the Mo 3d (a) and S 2p (b). (c) Raman spectra obtained the MoS2 nano-belts. (d) Photoluminescence spectra for nano-belts. (e) Experimental reflectance spectrum from nano-belts. (f) FDTD simulation of extinction, scattering, and absorption cross section spectra for nano-belts.


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Fig. 3. MoS2 nano-belt-based optoelectronic device. (a) Typical structure of a field-effect phototransistor based on the MoS2 nano-belts. (b) Optical image of one device composed of randomly-dispersed network of MoS2 nano-belts. (c) Another device design ensures transport across individual nano-belts between two interdigitated electrodes. (d) Transfer curves and (e) output curves of device (b) upon illumination. (f) Transfer curves and (g) output curves of device (c) upon illumination. (h) Photocurrent variation upon periodic ON/OFF illumination with increasing power. (i) Photocurrent and photoresponsivity as a function of illumination power extracted from (h).


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Optically Active 1D MoS2 Nanobelts - ACS Applied Materials

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