Scalable Patterning of MoS2 Nanoribbons by Micromolding in

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Scalable Patterning of MoS2 Nanoribbons by Micromolding in Capillaries Yu-Han Hung, Ang-Yu Lu, Yung-Huang Chang, Jing-Kai Huang, Jeng-Kuei Chang, Lain-Jong Li, and Ching-Yuan Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05827 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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Scalable Patterning of MoS2 Nanoribbons by Micromolding in Capillaries Yu-Han Hung†#, Ang-Yu Lu^, Yung-Huang Chang^, Jing-Kai Huang^, Jeng-Kuei Chang%, Lain-Jong Li^, and Ching-Yuan Su †#%* #

Graduate Institute of Energy Engineering, National Central University, Tao-Yuan

32001 , Taiwan ^ Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Kingdom of Saudi Arabia. †

Dep. of Mechanical Engineering, National Central University, Tao-Yuan 32001 ,

Taiwan %

Graduate Institute of Material Science and Engineering, National Central University, Tao-Yuan 32001 , Taiwan

To whom correspondence should be addressed: (C. Y. Su) : [email protected]

Abstract In this study, we report a facile approach to prepare dense arrays of MoS2 nanoribbons by combining procedures of micromolding in capillaries(MIMIC) and thermolysis of thiosalts ((NH4)2MoS4) as the printing ink. The obtained MoS2 nanoribbons revealed a thin thickness reaching as low as 3.9 nm, the width ranged from 157 nm to 465 nm, and the length was up to 2 centimeters. MoS2 nanoribbons with an extremely high aspect ratio (length/width) of ~7.4x108 were achieved. The MoS2 pattern can be printed on versatile substrates, such as SiO2/Si, sapphire, Au film, FTO/glass, and graphene-coated glass. The degree of crystallinity of the as-prepared MoS2 was discovered to be adjustable by varying the temperature through post annealing. The high-temperature thermolysis (1000°C) results in high high qualitative and conductive samples, and field-

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effect transistors based on the patterned MoS2 nanoribbons were demonstrated and characterized, where the carrier mobility was comparable to that of thin film MoS2. In contrast, the low-temperature treated samples (170°C) result in a unique nano-crystalline MoSx structure (x~2.5), where the abundant and exposed edge sites were obtained from highly dense arrays of nanoribbon structures by this MIMIC patterning method. The patterned MoSx was discovered to reveal superior electrocatalytic efficiency (an overpotential of ~211 mV at 10 mA/cm2 and a Tafel slope of 43 mV/dec) in the hydrogen evolution reaction (HER) when compared to the thin-film MoS2. The report introduces a new concept for rapidly fabricating cost-effective and high-density MoS2/MoSx nanostructures on versatile substrates, which may pave the way for potential applications in nano-electronics/optoelectronics and frontier energy materials. KEYWORDS: MoS2, Nano imprint, Patterning, hydrogen evolution reaction (HER), Field effect transistors(FET)

Introduction Transition metal dichalcogenides (TMDs), such as MoS2, have recently attracted considerable attention due to the their multifunctional material properties such as the presence of an appropriate band gap, high carrier mobility, and catalytic activity, leading to various potential applications in nanoelectronics,1-3 optical electronics, 5

3-

energy generation/storage,6-10 and highly active catalyst.11-15 Meanwhile, MoS2 is non-

poisonous, abundant and inexpensive, which makes it suitable for worldwide use and product development. To make this material more practical for use in industry, scalable and cost-effective synthesis methods are required. The current methods to synthesize MoS2 include: (1) thermolysis of thiosalts, such as the frequently used precursor (NH4)2MoS4.16(2) sulfurization of molybdenum17 or molybdenum oxide films18(3) the vapor phase transport method,

19-20

and (4) the vapor phase chemical reaction of


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molybdenum oxide and sulfur vapor.

21-22

Among these approaches, the method of

thermolysis from thiosalts shows various advantages such as easy processing and the controllable crystallinity of MoS2, allowing it to be useful for versatile applications. For example, thermolysis temperatures lower than 200 ° C result in a defective MoSx structure, which is beneficial for electrochemical reactions due to its abundant active sites with high electroactivities.12, 23 On the other hand, the degree of MoS2 crystallinity and domain size could be controlled by tuning the post-annealing temperature gradually to 1000°C, which leads to high-quality MoS2 that shows promising applications for semiconducting 2D-electronics and optical electronics due to its band gap and superior carrier transport properties.16 For practical usage, especially for electronics and optoelectronics, these applications require technologies to precisely pattern the MoS2 structure. Currently, most patterned 2D materials are based on the top-down approach of removing regions of unwanted material, where the fabrication procedure still requires many processing steps. For example, in MoS2-based electronics, prior to device integration, MoS2 conducting channels are normally patterned by E-beam/photolithography followed by oxygen/Ar plasma etching on MoS2 thin-film. Additionally, the performance of etched MoS2 structures degrade due to their abundant edge defects.24-25 Another scalable and low-cost strategy using laser patterning on MoS2 thin-films could achieve a fine electrode pattern with a length of 4.5 mm and a width of 820 µm.26 However, this approach was limited to micro-scaled patterns. In addition, Espinosa et al. reported a more direct fabrication of nano-scaled MoS2 field-effect transistors by scanning probe lithography, which can achieve a MoS2 channel 200 nm in width, but the method was time-consuming and thus lacked scalability. Clearly, in spite of several



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efforts thus far, the nano-scaled patterning of MoS2 with a facile, scalable and costeffective approach remains an obstacle, thus new technologies are necessary. In 1995, the patterning technology called micromolding in capillaries(MIMIC) have been developed and pioneered by G. M. Whitesides, where the micrometer- and submicrometer scale structured patterns was performed with an easy and scalable way.27-28 When carried out this process, the pre-defined elastomeric stamp was placed on the target substrate. A drop of a fluid containing precursor to be patterned was placed at the end of the mold, the channels was then filled by the fluid by capillary action. The precursor was dried and post-treated by thermal, solidify or cross-linking ways, depending on the precursors that used, to finalized the required patterning structures. Via this MIMIC approach, various microstructures of polymers, ceramics, inorganic salts, and crystalline particles were fabricated. 28-29

Here, we report a patterning technology for MoS2/MoSx based on MIMIC using (NH4)2MoS4 as ink. The various widths of MoS2/MoSx nanoribbons were controlled by designing the as-prepared polydimethylsiloxane (PDMS) stamp. Characterizations such as Raman spectrum, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electrical measurements were employed to characterize the quality of the asprepared MoS2 nanoribbons. To evaluate the potential applications of the proposed approach, the field-effect transistor (FET) and electrocatalytic performance for the hydrogen evolution reaction (HER) were carried out. This is the first time that a patterning approach has been developed to provide a reliable, facile, and cost-effective process without sacrificing the MoS2 performance in applications.


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Results and Discussion In this work, the patterning for MoS2 nanoribbons was based on the concept of nano-imprint technology, where a stamp mold with a pre-designed pattern and ink (precursor) were used. Figure 1a shows an illustration of the printing procedures. (Further details can be found in the experimental section). In brief, the elastic stamps were prepared by casting polydimethylsiloxane (PDMS) on the mold, which was obtained from commercial digital optical discs including a compact disc (CD), a digital versatile disc (DVD), and a Blue-Ray disc (BD), where the dimensions of the track width (W)/pit (P) are 600 nm/1.6 µm (CD), 320 nm/740 nm (DVD) and 130 nm/320 nm (BD). When printing the MoS2 pattern, the substrate was contact with the designed stamp, and then, a drop of (NH4)2MoS4 was cast on the end of stamp, followed by a free-drying step to remove the solvent. After de-lamination of the stamp from substrate, a thermolysis step was carried out at 170°C or 1000°C for the formation of crystallized MoS2. The photograph images in Figure 1a demonstrate that MoS2 nanoribbon arrays were uniformly printed both on fluorine-doped tin oxide (FTO) glass and SiO2/Si substrates over 2.6 cm in length. Note that the printed regions displayed iridescence phenomena. To the best of our knowledge, the iridescence phenomena of nanopatterned MoS2 have not yet been reported. The iridescence is caused by multiple light reflections from the periodic grating on the MoS2/substrate interface, where the interference of reflections modulate the incident light by selectively amplifying specific frequencies. The results show that highly uniform and controllable patterns were achieved through this approach, which can potentially offer optoelectronic applications such as full bandgaps in photonic crystals, broadband transmission of light, omnidirectional light trapping in solar cell, and colorful displays films. Figure 1b shows an optical micrograph of a typical top view for an imprinted pattern with arrays of MoS2 nanoribbons using a 320-nm-width stamp, indicating large-area uniformity and a regular pattern with high reliability can be obtained by this method. To evaluate the



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detail features of the as-printed structure, AFM was performed on the printed MoS2 on the SiO2/Si substrate.

Figure 1. (a) The schematic illustration for the patterning of MoS2 nanoribbon arrays on FTO glass and SiO2/Si substrates by a stamp printing method. The labels P and W indicate the pit and the ribbon width of the PDMS stamp, respectively. (b) A typical optical micrograph for a printed MoS2 sample (with the 320 nm stamp) on the SiO2/Si substrate.

Figure 2 shows the typical AFM images and the corresponding step profiles for the MoS2 pattern fabricated by stamps of various widths (i.e., 600 nm, 320 nm and 130 nm) followed by thermolysis at 1000°C. The thickness for these three cases is less than 9 nm, and it reached as low as ~3.9 nm by selecting the 600-nm-width stamp, indicating than an extremely thin film of MoS2 was achieved. Figure S1 shows the AFM analysis of patterned MoS2 (600nm) by altering concentration of the precursor solution. The result indicate the thickness could be adjusted from 14 nm to 9 nm when lower the concentration from 0.05g/mL to 0.01g/5mL, suggesting the layer thickness was adjustable using this method by variation of the precursor concentration. Moreover, the


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width of the MoS2 nanoribbons printed with the 600-, 320- and 130-nm-width stamps were 465, 272 and 157 nm, respectively. The AFM analysis of disk molds and replicated PDMS molds surface were shown in Figure S2 and S3. The summarized detail dimensions of disk molds, PDMS stamps, and the patterned MoS2 was compiled in Table S1. Compared with the scale of the designed stamp pattern, the width of the printed nanoribbons from 600 nm and 320 nm stamps showed a ~15% decrease, presumably due to the volume shrinking of the precursor when it was subjected to solvent drying and the subsequent thermal decomposition after the thermolysis procedure. The printed MoS2 nanoribbons showed a regular and continuous shape over a large area, especially for nanoribbons with widths of 465 nm and 272 nm; meanwhile, the fractured edge and disconnected nanoribbons were clearly seen for the sample from the 130-nm-width stamp, as indicated by white arrows in Figure 2b, which can be attributed to the severe volume shrinking that induced Rayleigh-Plateau instability. In spite of this, the length of the printed nanoribbons can be up to 2 centimeters. Therefore, the extremely high aspect ratio (length/width) of 7.4x108 was achieved on the MoS2 nanoribbons for the case of the 272-nm-width nanoribbon.



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Figure 2. The typical AFM images and the corresponding step profiles for the MoS2 pattern on the SiO2/Si substrate by variously scaled stamps of (a) 600 nm, (b) 320 nm, and (c) 130 nm. Note that the enlarged AFM image for the case of MoS2 (130 nm)/SiO2 reveals large amount of damaged and disconnected nanoribbons, as indicated by white arrows.

The thermal decomposition of (NH4)2MoS4 in an atmosphere of H2/Ar has been reported to result in the conversion from (NH4)2MoS4 to MoS3 at 120-360°C, and it was further converted from MoS3 to MoS2 above 800°C.16 These results indicate that the degree of MoS2 crystallinity can be controlled by the thermolysis temperature. In this work, two thermolysis temperatures, 1000°C and 170°C, were applied to the printed MoS2 samples. Here, we focus on high- (1000 ° C) and low-temperature (170 ° C)


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crystallized MoS2 for applications in electronics and electrocatalysts for the HER, respectively. Figure 3a, b shows the XRD and Raman spectra for MoS2 samples, where the feature peaks indicate the higher crystallinity of MoS2 at 1000oC than that of 170oC annealing. Moreover, the Raman spectra acquired on various points (P1~P6 as Figure 3b) in the same samples, suggesting the continuity and homogeneity of these samples.



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Figure 3. The (a)XRD and (b)Raman spectra for the MoS2 and MoSx samples. Raman spectrum for the MoS2 samples shows distinct feature peaks at about 378 cm-1 and 402 and A1g mode of a typical layered MoS2 nanostructure, cm-1 correspond to the E

respectively.The labelled P1-P6 indicate various data points acquired along the patterned MoS2 ribbons.


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Figure 4a is a representative Raman spectrum for the patterned MoS2 sample with the 1000°C thermolysis, where the distinct feature peaks at 377.6 cm-1 and 402.4 cm-1 ) and the out-of-plane mode (A1g) of correspond to the in-plane Mo-S phonon mode (E

a typical layered MoS2 nanostructure, respectively.30-31 Moreover, the peak intensity ratio of A1g/E was measured to be 1.4, which suggests the strengthened vibration of

the A1g mode. This result indicates the ordered and edge-terminated structure of the printed MoS2 nanoribbons,32-33 which is consistent with the AFM observation in Figure 2. In addition to Raman characterization, photoluminescence (PL) is frequently employed to evaluate the crystallinity of MoS2 structures.16,

34

Figure 4b shows a

typical PL spectrum for the printed MoS2 samples (on the sapphire substrate) after it was subjected to the 1000°C annealing, where the feature peak at ~672 nm indicates that highly crystalline MoS2 was obtained.

To investigate the electrical properties of the printed arrays of MoS2 nanoribbons, a bottom-gated field-effect transistor was fabricated by depositing a Au/Ti electrode on the printed MoS2 samples on SiO2(300 nm)/Si. Figure 4c shows the representative output characteristics (drain current vs. drain voltage, Ids - Vds) for the device obtained from a printed MoS2 sample (320 nm width) after the 1000°C thermolysis. The inset in Figure 4c shows the top-view optical micrograph of such device. Figure 4d shows the typical transfer curve (drain current Ids vs. drain voltage Vgs) for the MoS2 device, where it exhibits n-type semiconducting characteristics, which is consistent with other reported works.25 In addition, the field-effect carrier mobility was extracted according to the fitted linear region of the obtained transfer curve using the equation µ = (L/WCoxVd)(∆Id/∆Vg), where L and W are the channel length and width, respectively, and Cox is the gate capacitance(11.5 nF/cm2).

16

The average field-effect electron

mobility of the MoS2 devices was 1.2x10-3 cm2/vs. This average was compiled from six effective data points ranging from 7.2x10-4 to 3.1x10-3 cm2/vs, which was close to the previously reported data (2x10-2 cm2/vs) on a MoS2 thin-film made by a similar



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thermolysis approach.16 The mobility was still low for practical application (>30 cm2 V−1 s−1), the value may be further improved by choosing proper dielectric material to suppress the charge impurity scattering from the gate oxide, and increasing the longrange ordering of MoS2 lattice.

Figure 4. (a) The representative Raman and (b) PL spectra for the printed MoS2 samples on SiO2/Si substrate. (c) The output characteristics (Ids-Vds) with various gate voltages for a field-effect transistor fabricated on the printed MoS2 samples (320-width). The inset shows the optical micrograph for such device, where the scale bar is ~200 µm. (d) The corresponding typical transfer curve (Ids-Vgs) for the device fabricated using printed MoS2.


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In addition to the high degree of MoS2 crystallinity obtained by the high thermolysis temperature of 1000°C discussed above, the low temperature thermolysis at 170°C was also studied. In this case, the MoS2 nanostructured pattern was found to imprint on versatile substrates such as glass, SiO2/Si, sapphire, Au film, FTO glass, and graphene-coated glass (see Figure S4), indicating that a wide range of applications can be achieved with this approach. Figure 5 shows the optical micrograph and AFM images for the printed MoS2 samples on both the FTO- and graphene-coated glass after thermolysis at 170°C. The detailed AFM features show that the MoS2 nanoparticles preferentially formed at the low thermolysis temperature (170°C). Figure S5 shows a statistical particle size analysis for thin-film and printed MoS2 on both FTO and graphene substrates. The average particle size of the nanoribbon samples was clearly ~50% smaller than that of thin film MoS2 samples, which could be attributed to size effects during the nucleation that was strongly correlated to the annealing temperature and the volume confinement. The significant reduction in the particle size shown by this printing approach is beneficial for electrocatalytic activity, which will be discussed in detail in the followed discussion.



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Figure 5. The typical optical micrographs and their corresponding AFM images on detailed features of the printed MoSx (320 nm and 600 nm width) on both of FTO- and graphene-coated glass substrates after thermolysis at 170°C.

The nano-structures of MoS2 from the low thermolysis temperature (

Scalable Patterning of MoS2 Nanoribbons by Micromolding in

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