Nano-patterned High-frequency Supporting Structures Stably

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Nano-patterned High-frequency Supporting Structures Stably Eliminate Substrate Effects Imposed on Two-dimensional Semiconductors Soonmin Yim, Hyeuk Jin Han, Jaebeom Jeon, Kiung Jeon, Dong Min Sim, and Yeon Sik Jung Nano Lett., Just Accepted Manuscript • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Nano-patterned High-frequency Supporting Structures Stably Eliminate Substrate Effects Imposed on Two-dimensional Semiconductors Soonmin Yim†, Hyeuk Jin Han†, Jaebeom Jeon†, Kiung Jeon†, Dong Min Sim†, and Yeon Sik Jung*, † †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea

*Corresponding author’s e-mail: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT

Despite the outstanding physical and chemical properties of two-dimensional (2D) materials, due to their extremely thin nature, eliminating detrimental substrate effects such as serious degradation of charge-carrier mobility or light-emission yield remains a major challenge. However, previous approaches have suffered from limitations such as structural instability or the need of costly and high-temperature deposition processes. Herein, we propose a new strategy based on the insertion of high-density topographic nanopatterns as a nano-gap-containing supporter between 2D materials and substrate to minimize their contact and to block the substrate-induced undesirable effects. We show that well-controlled high-frequency SiOx nanopillar structures derived from the self-assembly of Si-containing block copolymer securely prevents the collapse or deformation of transferred MoS2 and guarantee excellent mechanical stability. The nano-gap supporters formed below monolayer MoS2 leads to dramatic enhancement of the photoluminescence emission intensity (8.7-fold), field-effect mobility (2.0fold, with a maximum of 4.3 fold), and photoresponsivity (12.1-fold) compared to the sample on flat SiO2. Similar favorable effects observed for graphene strongly suggest that this simple but powerful nano-gap-supporting method can be extensively applicable to a variety of lowdimensional materials and contribute to improved device performance.

KEYWORDS: Suspended architecture, Nano-gap supporting structures, Two-dimensional materials, Substrate effect, Block copolymer


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Recent studies on two-dimensional (2D) materials have shown great promise in the areas of both fundamental physics and potential applications based on their widely tunable properties from insulator to conductor1-4 and unconventional properties induced via multi-layer stacking.5-7 Among these studies, some have targeted the possibility of potentially replacing Si with 2D semiconducting materials due to their inherent flexibility, high transparency, and excellent carrier transport properties, which are important characteristics for flexible electronics.1,

2

In

particular, the gigantic gate controllability based on the atomic thickness of 2D material can eliminate short-channel effects (SCEs),8, 9 which are one of the bottlenecks in next-generation integrated devices.10,

11

However, the extremely thin nature of 2D semiconductors inevitably

accompanies oversensitivity to the surrounding environment – for example, geometric and electrical randomness of the substrate resulting from buried charge, surface roughness, defects, and impurities,12-15 which may be huge obstacles to achieve reliable device performance. Therefore, the elimination of substrate effects is necessary to employ 2D materials in various types of high-performance applications. One of the conventional approaches to avoid substrate effects is to etch the underlying layer below 2D materials. The 2D materials are thereby spatially separated from the underlying substrate and therefore their intrinsic material properties can be recovered. This suspended geometry of 2D materials has shown usefulness for fundamental studies on their intrinsic mechanical, optical, and electrical properties.16-19 In addition, improved device characteristics such as low electrical noise and high performance have been achieved using this method.20-25 However, this suspended structure suffers from poor mechanical stability, low yield for device fabrication, limited choice of contact materials, and difficulty of changing the device architecture.26-29 Also, there are a number of processing issues, mainly due to the wet chemical


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processes that must be used for removing the underlying layer. The capillary force between the suspended materials and the substrate during evaporation of the wet solution often collapses the suspended 2D materials (Figure S1), resulting in a low fabrication yield. In order to circumvent these problems, a critical-point-drying technique was employed, although the high pressure involved in the process could present another issue.27 Another critical issue is limited variability of device architectures, such as the stacking of multilayer 2D materials in three dimensions or the formation of hetero-structures with other 2D materials.5-7 All of these issues are closely related to the instability of the suspended geometry and incompatibility with multiple lithography processes due to the absence of a supporting material underneath the 2D material. To resolve the low stability issues of fully-suspended architecture, insertion of atomically flat materials (i.e. hexagonal boron nitride (h-BN)) between MoS2 and the substrate was demonstrated, and successfully blocked the influences of the substrate.4, 12, 30, 31 The advantages of thin h-BN such as a low defect density, high flexibility, and smooth surface are compatible with 2D material systems and devices. Because the h-BN can support the entire 2D channel region while blocking the undesirable effects from the underlying layer, outstanding mechanical stability is achieved compared to the above-mentioned suspended structure. However, large-area growth of h-BN using chemical vapor deposition (CVD) or atomic layer deposition (ALD) accompanies issues such as imperfect crystal quality, insufficient uniformity, and high growth temperature.4, 30 Herein, we report that “nano-patterned gap” geometries based on self-assembled nanostructures can reliably restore the inherent physical properties of 2D semiconductors by minimizing contact with the underlying substrate while maintaining structural stability. The nano-patterned gap-supporting strategy enables the maintenance of a constant gap distance


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between 2D materials and the substrate, while providing a stable mechanical supporting effect. This apparent suspending effect is induced by high-frequency topographic nanostructures derived from the self-assembled Si-containing block copolymers (BCPs), and effectively prevents contact of the 2D materials with the underlying substrate surface areas.32-35 Influences of nano-patterned gaps on the physical properties of MoS2 were systematically investigated by Raman

spectroscopy,

photoluminescence

(PL)

measurements,

and

carrier

transport

characteristics using field effect transistors (FETs). We report an 8.7-fold increase of photoluminescence 2.0-fold enhancement of field effect mobility (maximum 4.3-fold) compared to the same MoS2 flakes on a flat SiO2 substrate. Furthermore, a top-gate field-effect transistor device can be successfully fabricated, which is highly challenging to fabricate with the previously demonstrated fully-suspended architecture, confirming the excellent stability of the nano-patterned gap supported MoS2.

Modulation of the properties of 2D materials by the BCP-generated topographic structures was investigated based on a comparison of the properties of MoS2 flakes supported by flat SiO2 (flat silica supported MoS2; FSS-MoS2) and the same MoS2 on a nano-patterned SiO2 substrate (nanopatterned high-frequently supported MoS2; NHS-MoS2), as shown in Figures 1a, c. Monolayer MoS2 was mechanically exfoliated from a bulk sample and was first transferred on to a flat SiO2 substrate for various characterizations such as Raman and PL analyses.36, 37 To prepare a surfacemodulated substrate, we used self-assembly of Si-containing BCPs. Nanostructure formation based on BCP self-assembly provides multiple advantages such as excellent resolution down to sub-10 nm, large-area coverage, scalability, uniformity, high-throughput, and geometric controllability.38-41 Moreover, Si-containing BCPs offer robust silica nanostructures without the


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need of an additional pattern transfer step or the incorporation of inorganic elements, and thus offer superior mechanical and chemical stability of transferred 2D materials.42 We used poly(styrene-b-dimethlysilioxane) (PS-b-PDMS) BCP that has a total molecular weight of 28 kg mol-1 to self-assemble into nanoscale spheres with an average diameter of 10 nm and a periodicity of 25 nm, as shown in Figure 1b, d.43-51 Plasma oxidation was used to convert the self-assembled PS-b-PDMS BCP film into rigid nanoscale silica supporters.43-51 We chose the BCP considering its high-frequency pattern formation capability, which is needed for our strategy, as supported by Figures 2c-g and Figure S2.32, 34 The MoS2 flakes on flat SiO2 were transferred onto various nano-patterned surfaces including a BCP-treated substrate via a polymer carrier film, as shown in Figure 1a. The geometry of NHS-MoS2 was observed by cross-sectional transmission electron microscopy (TEM), as presented in Figure 1e. It was confirmed that MoS2 was successfully separated from the substrate while maintaining a constant gap distance of approximately 7.5 nm. The BCP-generated nanostructures support MoS2 with a minimal contact area (< 8% of the entire surface, estimated from top-down SEM and cross-section TEM images). The small contact area is due to the rounded shape of the nanostructures derived from the spherical domains of the BCP morphology. As shown in Figure 1f, contact of the NHS-MoS2 sheet with the bottom substrate requires extremely high bending energy cost due to the short period of the supporting nanostructures, as will be discussed later. Therefore, the suspended structure is favorable rather than a corrugated geometry following the surface morphology of the underlying substrate.32, 34, 5254

The optical characteristics of FSS- and NHS-MoS2 were investigated by Raman and PL measurements with an excitation wavelength of 514 nm (Figure 2). Raman spectroscopy is a


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widely used analysis tool for identification of thickness, internal stress, and carrier concentration of 2D materials.36, 37, 55, 56 The black line of Figure 2a shows the Raman spectra of monolayer FSS-MoS2, where the separated two peaks originated from in-plane (E12g) and out-of-plane (A1g) vibrational modes of layered atoms. The frequency difference of both peaks is less than 19 cm-1 (E12g ~ 386.7 cm-1 and A1g ~ 403.8 cm-1), indicating that the samples are monolayer MoS2.36, 37 The colored plots of Figure 2a demonstrate the Raman spectra of FSS-MoS2 (black), NHS-MoS2 (red), and NHS-MoS2 FET (blue), respectively. Compared to the spectrum of FSS-MoS2, the peak intensity ratio of A1g/E12g is increased from 1.00 to 1.25 using the BCP-treated substrate, suggesting a decrease of the n-type carrier concentration by physically separating the MoS2 from the substrate.56 Meanwhile, the Raman peak position was also monitored to identify the doping effect as a result of incorporating the nano-gap supporter. The peak of the E12g phonon mode shows a clear red-shift (from 386.7 cm-1 to 384.7 cm-1), and this may indicate elimination of the n-doping effect imposed by the SiO2 substrate.55-58 To explore the origin of the modified optical responses of MoS2 by incorporating NHA, a more systematic analysis was performed and is discussed in detail at the later part of this paper. The incorporation of the NHS structure also caused a significant change of the PL spectra, as shown in Figure 2b. The emission from the direct optical transition of carriers is composed of A and B excitons located at ~ 620 nm (2.00 eV) and ~ 680 nm (1.82 eV), respectively, which originated from valance-band splitting due to strong spin-orbit coupling.59, 60 The emission from A exciton can be deconvoluted with A0 (exciton) and X- (negatively charged exciton, trion) peaks, and the change of their relative ratio implies alteration of the charged environment.59-62 By transferring MoS2 onto NHS, drastic changes of the PL intensity and peak position were observed, as presented in Figure 2b. The A exciton peak position is clearly blue-shifted from


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680.7 nm (black) to 654.2 nm (red), while its intensity is magnified by up to 8.7 times compared to the same sample on flat SiO2. These results can be attributed to the elimination of random scattering centers caused by the substrate, significantly reducing the probability of radiative recombination of electron-hole pairs.4,

14

To obtain a more in-depth understanding of this

phenomenon, emission spectra of the FSS- and NHS-MoS2 were further deconvoluted into A0 and X- peaks, as shown in Figure S3. Based on the considerable enhancement of A0 for the NHSMoS2, it can be concluded that the dominant mechanism of PL emission is driven by more effective recombination of neutral excitons, which is less probable in the FSS-MoS2 sample due to undesirable n-doping effect imposed by the substrate. As shown in Figure 2a, b, excellent mechanical and chemical stability of our architecture is also supported by the observation that Raman and PL spectra were not notably changed after the FET fabrication process consisting of several photolithography, metallization, and lift-off steps. Because the previously reported fully-suspended architecture is not suitable for applying those steps, the confirmed stability of our architecture would be beneficial for practical, highperformance device applications that require multiple stacking of material or device layers.5-7, 2629

Various supporting structures such as high-frequency (hexagonal nanodots and nanogrooves) and low-frequency (nanogrooves) geometries were explored for the NHS-architecture. As shown in Figures 2c-g, high-frequency geometries with a periodicity of 100 nm or smaller are more suitable to support MoS2 by maintaining a nano-gap, as confirmed by the blue shifted PL spectra compared to the initial PL spectra from the same MoS2 flakes, as shown in Figure 2c and Figure S4. All the peak positions of the PL spectra were shifted to approximately 650 nm, and this result is consistent with previous studies on fully suspended MoS2.4, 12, 14 In contrast, the MoS2 flakes


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followed the topography of the low-frequency patterns with a periodicity of 400 nm. These results suggests that, for the low-frequency supporting structures, the reduction of substrate surface energy through the contact of MoS2 on the substrate surface dominates over increased strain energy. Structural suitability of our high-frequency supporting geometry is also supported by the 2D materials on a micron-scale structure regime.52-54 As reported in previous papers, the major factor to determine the geometry of 2D materials on supporting structures is competition between (i) strain energy of 2D flakes to fit on the surface structures and (ii) attractive energy between the 2D material and the surface of the nanostructures. The relationship and competition of energetic terms are theoretically discussed in ref. 32. In our approach, we designed the substrate geometry to maximize the strain energy penalty. The bending energy (Eb) of MoS2 per unit area is described by the following equation:32, 54, 63 4πସ DAଶ Eୠ = λସ where A, λ, and D are bended amplitude, period of structures, and bending rigidity of the material, respectively. Additionally, the bending rigidity of material (D) can be estimated using the following equation: Ehଷ D= 12(1 − νଶ ) where E, h, and ν are Young’s modulus, thickness, and Poisson’s ratio of the material. As shown in the equations, the most effective geometrical factor of the substrate that influences the bending energy (Eb) is the period (λ) of the underlying structures (inversely proportional to the fourthpower of λ). These modeling results motivate the use of smaller λ for the supporting structure.53 Although we used extremely thin monolayer forms of 2D materials (both of MoS2, and graphene),52-54 successful suspension of 2D materials was achieved as shown above, suggesting


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the excellent supporting characteristic of our high-frequency nanostructures with an extremely small λ. To demonstrate wider applicability of our nano-gap-supporting architecture, we also tested graphene on a BCP-treated substrate, as shown in Figure S5. It was reported that the optical and electrical properties of graphene can be strongly affected by the surrounding environment, especially the substrate conditions.15 We transferred mechanically exfoliated graphene from a flat substrate to a NHS substrate for comparison of properties. Raman spectra were measured and plotted as black (FSS-Graphene) and red (NHS-Graphene) colored lines. Both spectra were measured from the same graphene sample displayed in the upper right inset of the figure. Before and after the transfer process, a D-band peak was not detected, indicating the high quality of graphene and its preservation during the transfer process. In contrast, a G-band shift (from 1594 cm-1 to 1583 cm-1) and a ratio change of G/2D intensity (from 0.32 to 0.99) were observed, implying a reduction of substrate effects, which is consistent with the results of MoS2 on NHS. Therefore, based on the following discussion, we conclude that our strategy may be extensively applicable to various 2D materials by effectively removing substrate effects. As aforementioned, the origins of the modified optical property are most likely the doping environment imposed by substrate removal. Applied strain33, 64, 65 and dielectric environment66 are other possible causes that can also influence the optical properties. As mentioned and shown in Figure 2, by floating monolayer MoS2 on the nano-patterned supporters, we observed a shift and intensity ratio change for both Raman and PL spectra. (i) Blue-shift and intensity enhancement of PL spectra and (ii) change of Raman peak intensity ratio are the two main phenomena.


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MoS2 is locally and maximally strained at the supported peak region, and relaxes with greater distance away from this point, while the remaining region is purely suspended. With strainmajority assumption, the applied biaxial tensile strain on MoS2 is estimated to be about 0.4%,33 which would be accompanied with a red-shifted (~ 50 meV) PL peak position and a decrease of intensity.33 This is, however, opposite to the phenomenon observed in this study. On the other hand, a dielectric screening effect can also influence the PL spectra of MoS2, resulting in a small red-shift of the PL peak position with a decrease of the dielectric constant.66 Although our architecture leads to a decrease of the dielectric constant below MoS2, the amount of the decrease is relatively small compared to that in a previous report,66 and the direction of the PL shift is opposite to our result. In addition, the dielectric environmental changes do not influence the Raman spectra.66 Therefore, the effect of dielectric screening appears to be only a minor factor in terms of explaining our observation. In contrast, the doping-majority assumption accommodates both observed phenomena. Changes of the Raman peak ratio (A1g/E12g) are an important indicator for changes of the doping condition in MoS2, as investigated by the electrochemical method.67 A decrease of the electron concentration in MoS2, which is expected for the suspended structure, leads to an increase of the A1g/E12g value,

67

which is consistent with our result. In addition, two aspects of PL spectra

changes - blue-shift (~ 74 meV) and increased intensity (by 8.7 times) – are noted. The obtained optical phenomena are explainable by the change from a trion-dominant to exciton-dominant carrier recombination mechanism.4,

12, 14

Thus, carrier population changes induced by the

markedly reduced contact area with the substrate provide a more convincing explanation of our observations and are fully consistent with previous results. This is further supported by the optical characteristics of graphene. The intensity ratio of Raman modes (G/2D) was reported to


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be an important indicator for the doping condition of graphene.68 As graphene becomes neutral, the Raman intensity ratio of G/2D is decreased, which is consistent with our result. Further high resolution analyses such as scanning tunneling spectroscopy may give additional insight to confirm the doping-induced effects.33 Because a shift of the PL peak position reflects the degree of the substrate effects, the structural robustness of the sample can be assessed by tracking the change of PL characteristics with the air storage time. As presented in Figure 3a, the position of the PL peak was blue-shifted from 677 nm to 652 nm by floating MoS2 on NHS, and the shift was slowly recovered by storage in air. It is thus necessary to confirm whether this change is caused by a gradual increase of the contact area between MoS2 and the NHS structure or by other factors such as gas adsorption. It was reported that adsorption of oxygen and moisture in the atmosphere can affect the electrical and optical properties of MoS2 and can degrade its properties.69, 70 To verify the origin of the relaxation, after keeping the sample for 130 days, we performed thermal treatment at 200°C for 20 min and measured the PL spectra for comparison. It was confirmed that the improved optical properties by NHS were fully recovered, as shown in Figure 3b, which can be attributed to the desorption of adsorbed oxidizing molecules.69 Therefore, it is clear that the NHS architecture is stable for a long period of air-storage time without structural collapse or any other permanent degradation. The secure NHS structure even allows the fabrication of a top-gate FET device while maintaining the suspending effect of the architecture, which was not possible with the previous fully suspended structure. The mechanical stability of the suspended architecture should be tested because it is important that it will be able to withstand various wet/dry processes for fabricating sophisticated device architecture5-7 and encapsulate (passivate) devices by dielectric


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material deposition.71 Additional lithography processes on top of the NHS-MoS2 were performed to fabricate top-gate FETs. The schematic and corresponding photographic images of the entire device are shown in Figure 3c. Even under the harsh processing conditions of repeated lithography, metallization, and lift-off process cycles, normal device operation results (Figure S6), indicating no noticeable damage on the MoS2 layer suspended on the NHS structures. This is also supported by the cross-sectional TEM image (Figure 3d) showing that the continuous nanogap between NHS-MoS2 and the bottom substrate was maintained uniformly. These results show that NHS-MoS2 has excellent mechanical stability and is promising for a variety of applications with more complex device architectures. To shed light on the electrical properties of the NHS-MoS2, we fabricated a number of bottomgate FET devices via conventional photolithography. Practical FET devices require high mobility to enhance the device operation speed and a low subthreshold swing (SS) value to reduce power consumption and to improve switching performance simultaneously.20,

72

Electrical

characteristics of the devices were measured under dark and vacuum conditions (< 100 mTorr), as shown in Figure 4a. Both NHS-MoS2 and FSS-MoS2 devices exhibited an acceptable Ion/Ioff ratio (> 105) at a drain-to-source voltage (Vds) of 10 mV, and n-type FET characteristics indicated by the increase of current level (Ids) with an increase of gate voltage (Vgs) from - 40 V to + 40V with a 0.5 V step. As a consequence of eliminating unintended doping and scattering effects imposed by the substrate, the NHS-MoS2 FET exhibits a higher current level than the FSS-MoS2 FET at the same Vgs, and shows a negative shift of the threshold voltage (from - 14 V to - 24 V, calculated by extrapolation of linear transfer curves), suggesting an n-type shift of electrical properties.55, 57, 58 For a more quantitative analysis, we also calculated the field effect mobility (µ), Ion/Ioff ratio, and SS of both devices, as shown in Figures 4b-d.55, 58 The average


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field-effect mobility of the device was enhanced from 14.64 cm2 V-1 s-1 to 30.62 cm2 V-1 s-1 for the NHS-MoS2 FET, while the SS factor decreased by 29.71%. Although a slight decrease of Ion/Ioff (from 1.24 × 105 to 1.16 × 105) was observed (Figure 4c), this amount of change (6%) may be within the device-to-device variation range, considering the similar values (Ion/Ioff ~ 2 × 105) obtained for devices based on CVD-grown MoS2 shown in Figure S7. On the other hand, the drastic decrease of SS (from 4.71 V decade-1 to 2.84 V decade-1) may have originated from the improvement of the MoS2/substrate interface.72,

73

The enhancement of µ is statistically

confirmed, as presented in Figure 4e. The NHS-MoS2 FETs (µ = 34.9 ± 10.1 cm2 V-1 s-1) generally showed higher mobility values than those (17.6 ± 3.5 cm2 V-1 s-1) of the FSS-MoS2 FETs. We also prepared FETs using single-batch CVD-grown MoS2 to reduce material-to-material variation. The device performance of the suspended CVD-grown MoS2 FETs is displayed in Figure S7, showing consistent improvements of device characteristics by the supporting nanogap approach. For example, the field-effect mobility increased by ~ 430% using the NHS substrate. From the transfer curves of multiple devices, it is obvious that the electrical property is highly correlated with the support architecture of the devices (Figure S7). Therefore, we conclude that NHS-MoS2 on NHS shows consistent results with previous studies in terms of the carrier transport property,4, 14 while guaranteeing improved mechanical stability and processing simplicity. One notable strength of suspended MoS2 FETs is high photocurrent generation and rapid heat dissipation capabilities due to the absence of a substrate and the extension of surface area.21, 22, 74, 75

Exploiting these advantages, we demonstrate a photodetector device based on bottom-gate

FETs. Figures 5a, b show configurations of the NHS-MoS2 FET and the working principle of the 14 ACS Paragon Plus Environment

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device under illumination, respectively. In addition, Figures 5c-f provide photoelectric characteristics of the FSS and NHS devices, which were measured at Vds of 1 V under illumination of a 625 nm laser with varying power (1.5 µW, 2.2 µW, 10.7 µW, 24.1 µW, and 114.0 µW) of the incident laser source. The photoresponse characteristics of the FET devices in Figures 5c, d clearly show that current generation in NHS-MoS2 (Figure 5d) by light illumination is much greater than that of FSS-MoS2 (Figure 5c). Based on the device area, the responsivity of the FSS device was calculated to be 0.10 A W-1 (Figure 5g) under 1.5 µW incident light power. In contrast, the NHS device showed substantially higher current density and responsivity (1.21 A W-1 under the same illumination), which is a clear indicated of improved properties caused by the nano-gap-supporting topographic nanostructures.

In summary, we demonstrated a practical nano-gap-supporting strategy to stably and reliably suspend ultrathin 2D materials to avoid the detrimental effects caused by contact with the substrate. Based on the advantages of excellent regularity, robustness, and large-area uniformity of self-assembled BCP nano-patterns, our approach enabled creation of a highly stable suspending architecture without sacrificing mechanical stability. As a result of floating MoS2 using the high-frequency nanostructure, the photoluminescence spectrum showed 8.7 times enhanced intensity compared to a reference sample placed on a flat SiO2 surface. Moreover, the electrical characterization on an FET device based on the nano-gap-supported MoS2 reveals 2.0fold enhanced field-effect mobility. The fabrication of a top-gate FET device and successful device operation results confirmed its outstanding mechanical stability. Furthermore, the photoresponsivity of the nano-gap-supported MoS2 was improved by more than 12.1 times. These improved optical and electrical properties of 2D materials can be attributed to the


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markedly more balanced charge neutrality than that of substrate-contacted materials. Importantly, we showed that these desirable effects are also effective for other 2D materials such as graphene. Therefore, we believe that our nano-patterned nano-gap approach can contribute to performance enhancements of future optoelectronic applications based on low-dimensional materials while ensuring processing simplicity, large-area processing, mechanical stability, and excellent yield.

Experimental Section. Preparation of nano-gap supporting nanostructures. Poly(styrene-b-dimethylsiloxane) (PS-bPDMS) block copolymers (BCPs) were purchased from Polymer Source Inc. (Canada). Sphereforming PS-b-PDMS BCPs with a molecular weight of 28 kg mol-1 and 56 kg mol-1 were coated on a Si substrate and were thermally annealed at 150°C in a vacuum chamber for 30 min and 60 min. For pattern development, CF4 plasma (source power = 50 W, 21 sec) followed by O2 plasma (source power = 60 W, 50 sec) treatment was used to etch the top PDMS layer and PS matrix while converting PDMS microdomains to SiOx nanostructures. The working pressure and gas flow rate during the plasma treatment were kept at 15 mTorr and 30 sccm, respectively. Preparation of 2D materials. Monolayer MoS2 flakes were mechanically exfoliated from bulk crystals (2d Semiconductors, Inc.) onto thermally grown 300 nm-thick-SiO2 on a highly doped Si wafer using a conventional Scotch tape method.3 The thickness of MoS2 was firs defined by optical contrast of the flakes with a bare substrate, and confirmed by Raman spectroscopy. Similarly, monolayer graphene was also mechanically exfoliated from bulk crystals (Graphene Supermarket, Inc.). MoS2 grown via chemical vapor deposition (CVD) was purchased from


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6Carbon, Inc. Flakes of the 2D materials were transferred onto a 300-nm-thick SiO2/p++ Si wafer for measurement of pristine properties. Transfer method for 2D materials. Referring to the reported water-assisted transfer method, we employed a similar procedure.76 Polystyrene (PS) with a total molecular weight of 280 kg mol-1 as a transfer and handling medium was first dissolved in toluene with a specific weight fraction (wt%, 0.5 – 1.0 wt%). After mechanical exfoliation of monolayer MoS2 (Figure 1a (i)), the PS film was spun-cast on the MoS2/SiO2 (~ 50 nm thickness, measured from cross-sectional SEM image). The PS/MoS2 film was then delaminated and floated on DI water by injection of DI water between PS/MoS2 and the hydrophilic SiO2 surface.76 The floated PS/MoS2 film was then scooped out by a prefabricated nanostructure substrate, and PS was washed out by toluene. To confirm the reliability, we performed a comparison of PL spectra by transferring MoS2 to a flat SiO2 substrate again. (Figure S8) As confirmed by the absence of noticeable changes of the PL spectra, the process employed here resulted in negligible effects on the transferred 2D material. This is again confirmed by the absence of a D-band of the transferred graphene on an NHS substrate, as shown in Figure S5. Fabrication of various support nanostructures. For fabrication of the structure in Figure 2d, we used a higher-molecular-weight BCP (PS-b-PDMS, total molecular weight of 56 kg mol-1) than we mainly used. The BCP solution dissolved in a mixed solvent (toluene, heptane, and acetone) with 1 wt% is spin-coated on a SiO2/Si substrate, and thermally annealed at 150°C in a vacuum chamber for 60 min. For pattern development, CF4 plasma (source power = 50 W, 21 sec) followed by O2 plasma (source power = 60 W, 50 sec) treatment was used to etch the top PDMS layer and PS matrix while converting PDMS microdomains to SiOx nanostructures. The working pressure and gas flow rate during the plasma treatment were kept at 15 mTorr and 30


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sccm, respectively. For fabrication of structure (e), we employed solvent-assisted nanotransfer printing (S-nTP), which was previously reported by our group.77 The early step for master template fabrication is described in Note 1 in the Supporting Information and ref. 77. Using SnTP, we fabricated aligned Cr nanowires (NWs) with periodicity of ~ 50 nm on a flat SiO2 (300 nm)/Si substrate. As a protective mask pattern, Cr NWs effectively block the incident plasma treatment. The Bosch process (repeated SF6 and C4F8 plasma treatments) consisted of SF6 plasma (source power = 80 W, 10 s) followed by C4F8 plasma (source power = 80 W, 7 s) treatment and was repeated six times. Cr NWs were then removed by application of Cr-5 wet etchant (Cyantek, USA) for 3 min. Finally, we fabricated high-frequency nanogroove structures. (Figure 2e). For the fabrication of nanogroove structures (f, g), we employed capillary force lithography (CFL).78 Master templates with a varied period (50 nm – 400 nm) were fabricated by double patterning photolithography. To replicate the master templates, polyurethane acrylate (PUA, MINS311RM, Changsung Sheet, Korea) was poured on the master substrate and cured by exposure of UV for 5 min (RX-H400D, Raynics, Korea). PS with a total molecular weight of 28 kg mol-1 dissolved in toluene (2 wt%) was spin-coated on a SiO2 substrate. The fabricated PUA replica was gently covered on the surface of the PS/SiO2 substrate to form conformal contact (hot plate with heating at 100oC). During 8 min of heating, PS chains filled patterned spaces on the PUA surface. After removal of the PUA replica, PS grooves were formed on the SiO2 substrate. For pattern development, O2 plasma (source power = 60 W, 25 sec) treatment was used to etch the residual PS film between grooves. The underlying SiO2 substrate was then etched by CF4 plasma (source power = 100 W, 100 sec), and residual PS was thermally (550oC) decomposed in an air environment for 2 hours. Finally, we fabricated low frequency nanogroove structures (Figure 2f, g).


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Fabrication of FETs. Bottom-gate MoS2 FETs were fabricated using conventional photolithography.58 Photoresist (AZ5214E) was spin-casted on a MoS2/SiO2/Si substrate, and electrode regions with a ~ 3 µm channel are defined by alignment and exposure equipment (MDA-8000B). After the development step, an 80 nm Au layer was deposited by thermal evaporation. A Si wafer was highly doped with boron (resistivity < 0.005 Ω cm) to be used as a bottom gate of the FET. To fabricate top-gate FETs, 150-nm-thick SiO2 was deposited by ebeam evaporation. Gold top gate electrodes were also fabricated via photolithography. Prior to the electrical property measurement, annealing (200°C, Ar, 1 hour) was conducted to improve the contact property of the device. Characterization. The self-assembled nanostructures were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). A dispersive Raman spectroscopy system (HORIBA, ARAMS) equipped with an Ar ion continuous wave (CW) laser (λ = 514.5 nm) was used to characterize 2D materials on flat silica and nano-patterned nano-gap structures. Electrical characterization of the FETs was carried out using a Keithley 4200-SCS parameter analyzer under a low vacuum (< 100 mTorr) condition. The transfer characteristics were measured with sweeping Vgs from - 40 V to 40 V with a step of 0.5 volts (bottom-gate FETs) and sweeping Vgs from - 5 V to 20 V with a step of 0.5 volts (top-gate FETs). To measure the photoresponse of the devices, a laser beam with a wavelength of 625 nm was vertically illuminated under vacuum conditions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 19 ACS Paragon Plus Environment

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Fabrication procedure of the NHS supporting geometry. Structural stability test result of fully suspended MoS2. Additional characterization data (PL spectra and electrical characteristics) of suspended MoS2 on NHS. Electrical characterization results for CVDgrown monolayer MoS2 on NHS. Raman spectra for suspended graphene on NHS. Optical measurement data showing the negligible effect of the transfer process. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. (NRF-2016M3D1A1900035)


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FIGURES

Figure 1. Illustration of supporting MoS2 by nano-patterned high-frequency supporting structures for enhancement of photoluminescence (PL) efficiency. (a) Procedure for preparation of monolayer MoS2 and transfer process, and (b) NHS fabrication on SiO2 using selfassembly of Si-containing PS-b-PDMS block copolymers (BCP), (c) Transfer of monolayer MoS2 onto NHS, showing considerably enhanced optical response. (d) SEM image of fabricated SiO2 nanostructures (in birds-eye view). The inset SEM image was obtained from a 55o tilt view of the same sample, and the scale bar of the inset image represents a length of 100 nm. (e) Cross-


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sectional TEM image of suspended bi-layer MoS2. The yellow arrows indicate the locations of nanostructures fabricated by self-assembly of PS-b-PDMS BCP. (f) Illustration of suspended MoS2 on NHS, showing a cross-sectional view of suspended MoS2. The magnified image at the upper panel presents an enlargement of the surface area by NHS and the creation of a nanogap between MoS2 and the basal plane.


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Figure 2. Optical property modulation of monolayer MoS2 using various NHS. (a) Raman spectra of the same monolayer MoS2 supported on a flat SiO2 substrate (black) and suspended on NHS (red and blue), respectively. (b) Photoluminescence (PL) spectra of the same monolayer MoS2 supported on a flat SiO2 substrate (black) and suspended on NHS (red and blue), respectively. (excitation wavelength of laser = 514 nm). Blue plots in (a) and (b) were measured after fabrication of the bottom-gate FET, showing robust mechanical and chemical stability of the NHS architecture. (c) Emission centers of A excitons plotted as a function of the geometry of 29 ACS Paragon Plus Environment

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the supporting nanostructures. The samples were observed by tilt-view of SEM images, as shown in Figures 2d-g. (d) – (f) High-frequency and (g) low-frequency nanopatterns. (d) nanodots: diameter = 25 nm, period = 50 nm, (e) nanogrooves: width = 20 nm, periodicity = 50 nm, (f) nanogrooves: width = 50 nm, periodicity = 100 nm, (g) nanogrooves: width = 50 nm, periodicity = 400 nm.


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Figure 3. Stability evaluation of NHS architecture. (a) PL spectra of the same NHS-MoS2 over storage time in air. (b) Positions of PL maxima (A exciton peak) shown as a function of air storage time of the samples. Heat treatment in a vacuum furnace at 200oC for 20 min was employed to remove physically adsorbed molecules. (c) Schematic and optical image of top-gate FET. The top-gate fabrication process was carried out with the bottom-gate FETs based on NHSMoS2. (d) Cross-sectional TEM image of top-gate FET with NHS-MoS2. The right panel shows a magnified image of the yellow rectangle in the left panel.


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Figure 4. Electrical characterization results on NHS-MoS2 FETs. (a) Transport property of NHS-MoS2 FET (red triangle) and FSS-MoS2 FET (black plot) at a source-drain voltage (Vds) of 10 mV. (b) Field-effect mobility, (c) on/off current ratio, and (d) subthreshold swing values calculated (a) with and without NHS. (e) Distribution of field-effect mobility collected from eight devices. 32 ACS Paragon Plus Environment

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Figure 5. Photoresponses of MoS2 FETs under illumination (λ = 625 nm). (a) Schematic of the device used for the measurement. (b) Band structure change of the device upon light illumination. Change of current density – drain voltage characteristics of (c) FSS-MoS2 FET and (d) NHS-MoS2 FET at various power levels. Black, red, orange, green, blue, and purple colored plots indicate dark condition and light illumination with a source power of 1.5 µW, 2.2 µW, 10.7 33 ACS Paragon Plus Environment

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µW, 24.1 µW, and 114.0 µW, respectively. (e) - (f) Current density change depending on gate voltage and illumination laser power for (e) FSS-MoS2 FET and (f) NHS-MoS2 FET. (g) Generated photocurrent as a function of incident illumination power.


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TABLE OF CONTENTS GRAPHIC


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Table of Contents Graphic 174x151mm (150 x 150 DPI)

ACS Paragon Plus Environment

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Figure 1. Illustration of supporting MoS2 by nano-patterned high-frequency supporting structures for enhancement of photoluminescence (PL) efficiency. (a) Procedure for preparation of monolayer MoS2 and transfer process, and (b) NHS fabrication on SiO2 using self-assembly of Si-containing PS-b-PDMS block copolymers (BCP), (c) Transfer of monolayer MoS2 onto NHS, showing considerably enhanced optical response. (d) SEM image of fabricated SiO2 nanostructures (in birds-eye view). The inset SEM image was obtained from a 55o tilt view of the same sample, and the scale bar of the inset image represents a length of 100 nm. (e) Cross-sectional TEM image of suspended bi-layer MoS2. The yellow arrows indicate the locations of nanostructures fabricated by self-assembly of PS-b-PDMS BCP. (f) Illustration of suspended MoS2 on NHS, showing a cross-sectional view of suspended MoS2. The magnified image at the upper panel presents an enlargement of the surface area by NHS and the creation of a nanogap between MoS2 and the basal plane. 253x190mm (150 x 150 DPI)


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Figure 2. Optical property modulation of monolayer MoS2 using various NHS. (a) Raman spectra of the same monolayer MoS2 supported on a flat SiO2 substrate (black) and suspended on NHS (red and blue), respectively. (b) Photoluminescence (PL) spectra of the same monolayer MoS2 supported on a flat SiO2 substrate (black) and suspended on NHS (red and blue), respectively. (excitation wavelength of laser = 514 nm). Blue plots in (a) and (b) were measured after fabrication of the bottom-gate FET, showing robust mechanical and chemical stability of the NHS architecture. (c) Emission centers of A excitons plotted as a function of the geometry of the supporting nanostructures. The samples were observed by tilt-view of SEM images, as shown in Figures 2d-g. (d) – (f) High-frequency and (g) low-frequency nanopatterns. (d) nanodots: diameter = 25 nm, period = 50 nm, (e) nanogrooves: width = 20 nm, periodicity = 50 nm, (f) nanogrooves: width = 50 nm, periodicity = 100 nm, (g) nanogrooves: width = 50 nm, periodicity = 400 nm. 315x256mm (150 x 150 DPI)


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Figure 3. Stability evaluation of NHS architecture. (a) PL spectra of the same NHS-MoS2 over storage time in air. (b) Positions of PL maxima (A exciton peak) shown as a function of air storage time of the samples. Heat treatment in a vacuum furnace at 200oC for 20 min was employed to remove physically adsorbed molecules. (c) Schematic and optical image of top-gate FET. The top-gate fabrication process was carried out with the bottom-gate FETs based on NHS-MoS2. (d) Cross-sectional TEM image of top-gate FET with NHS-MoS2. The right panel shows a magnified image of the yellow rectangle in the left panel. 254x174mm (150 x 145 DPI)


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Figure 4. Electrical characterization results on NHS-MoS2 FETs. (a) Transport property of NHS-MoS2 FET (red triangle) and FSS-MoS2 FET (black plot) at a source-drain voltage (Vds) of 10 mV. (b) Field-effect mobility, (c) on/off current ratio, and (d) subthreshold swing values calculated (a) with and without NHS. (e) Distribution of field-effect mobility collected from eight devices. 156x284mm (150 x 150 DPI)


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Figure 5. Photoresponses of MoS2 FETs under illumination (λ = 625 nm). (a) Schematic of the device used for the measurement. (b) Band structure change of the device upon light illumination. Change of current density – drain voltage characteristics of (c) FSS-MoS2 FET and (d) NHS-MoS2 FET at various power levels. Black, red, orange, green, blue, and purple colored plots indicate dark condition and light illumination with a source power of 1.5 µW, 2.2 µW, 10.7 µW, 24.1 µW, and 114.0 µW, respectively. (e) - (f) Current density change depending on gate voltage and illumination laser power for (e) FSS-MoS2 FET and (f) NHS-MoS2 FET. (g) Generated photocurrent as a function of incident illumination power. 275x287mm (150 x 128 DPI)


Nano-patterned High-frequency Supporting Structures Stably

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