Electronic Structure Mosaicity of Monolayer Transition Metal

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Functional Nanostructured Materials (including low-D carbon)

Electronic Structure Mosaicity of Monolayer Transition Metal Dichalcogenides by Spontaneous Pattern Formation of Donor Molecules Hisashi Ichimiya, Masahiro Takinoue, Akito Fukui, Takeshi Yoshimura, Atsushi Ashida, Norifumi Fujimura, and Daisuke Kiriya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03367 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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ACS Applied Materials & Interfaces

Electronic Structure Mosaicity of Monolayer Transition Metal Dichalcogenides by Spontaneous Pattern Formation of Donor Molecules Hisashi Ichimiya1, Masahiro Takinoue2, Akito Fukui1, Takeshi Yoshimura1, Atsushi Ashida1, Norifumi Fujimura1 and Daisuke Kiriya1,3*

1

Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuen-cho,

Naka-ku, Sakai-shi, Osaka 599-8531, Japan

2

Department of Computer Science, Tokyo Institute of Technology, Yokohama, Japan

3

PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,

Saitama 332-0012, Saitama, Japan

KEYWORDS

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Transition metal dichalcogenides, Benzyl viologen, Molecular doping, Spontaneous pattern formation, Photoluminescence, Electronic mosaicity

ABSTRACT

Modulating the electronic structure of semiconducting materials is critical to developing high-performance electronic and optical devices. Transition metal dichalcogenides (TMDCs) are atomically thin semiconducting materials. However, before they can be used successfully in electronic and optical devices, the modulation of their carrier concentration on the nanometer scale must be achieved. Molecular doping has been successful in modulating the carrier concentration; however, the scientific approach for selective and local carrier doping at the nanometer scale is still missing. Here, we demonstrate a proofof-concept of modulating the carrier concentration of TMDCs laterally on a scale of around 100 nm using spontaneous pattern formation of an ultrathin film consisting of molecular electron dopants. When the water made contact with the molecular film (~10 nm), a


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spontaneous pattern formation was observed on both monolayer and bulk TMDCs. We revealed that the pattern-formation dynamics and nanoscopic flow rate of the molecules were highly dependent on the thickness of the TMDCs, since the band gap varies based on the number of layers. Analysis of topographic images of the molecular patterns and photoluminescence spectra of the TMDCs indicated that the spontaneously patterned molecular films induced a local carrier doping; the estimated depletion width is on the nanometer scale. Our results demonstrate a spontaneous formation of a mosaic electronic structure. This work is useful for making tiny-scale electronic junctions on TMDCs and semiconducting materials to make numerous p/n junctions simultaneously for optoelectronic devices.


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INTRODUCTION

Transition metal dichalcogenides (TMDCs) have received much attention as materials for next-generation devices.1-2 TMDCs are constructed by stacking three atomic (~0.7 nm) layers, giving them the potential to be used for making atomically thin transistors, lightemitting diodes, photovoltaics and topological devices.1-4 To accomplish this, modulating the carrier concentrations of the TMDCs must be achieved.2,

5-6

Since high-energy

processing such as ion implantation is not appropriate for the thin layers of TMDCs, molecular surface-charge transfer doping has been developed as a possible solution.5-6 With this method, molecules barely making contact with the TMDCs can modulate the carrier concentrations of TMDCs at a wide range of concentration levels. Benzyl viologen (BV) is one of the strongest electron donor molecules for use in such a technique, and because of its strong doping ability showed modulation from a semiconducting to a metallic state.7

So far, the majority of research on molecular doping has focused on modulation of entire TMDC flakes, resulting in the whole surface area of the material or device being uniformly


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doped.5 However, to harness materials’ performance from external modules or to be able to use TMDCs in the development of future optoelectronic devices, modulating carrier concentrations in local regions is required.8 The conventional technique is to use photoresists to make tiny patterns, but the donor molecules, including BV molecules, are normally dissolved in hydrophobic solvents that dissolve photoresists, making local carrier doping with this method difficult. We recently developed a technique for causing spontaneous pattern formation of an ultrathin (~10 nm) BV molecular film on bulk TMDCs and demonstrated modulation of the bulk carrier concentrations.9 Here, we report the spontaneous pattern formation of BV molecular film on different thicknesses of TMDCs, including monolayer, and examine the local carrier modulation of TMDCs in terms of the relationship among optoelectronic properties and topographic analysis (Figure 1). This work leads to a method for combining natural and engineering systems to generate many p/n junctions simultaneously and spontaneously on semiconducting materials.

RESULTS AND DISCUSSION


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We first examined the doping ability of the BV molecules for monolayer TMDCs (MoS2 and WSe2) by using both Raman and photoluminescence (PL) spectroscopy. TMDC flakes were mechanically exfoliated on a Si substrate covered with thermally grown SiO2.

The original monolayer MoS2 (as exfoliated, no BV molecular film) exhibited two main

Raman characteristic peaks at 386 and 404 cm-1, corresponding to the E’ and A’ modes, respectively (Figure 2a).10 By spin casting the BV molecular solution on the monolayer MoS2, a softening of the A’ Raman mode was observed, which is consistent with the results of a previous study.7 The PL intensity decreased, and the peak shifted to a lower energy in the monolayer MoS2, which also indicated electron transfer doping from the BV molecules.11-12 In the case of monolayer WSe2, both original and BV-treated monolayers showed Raman peaks around 250 and 265 cm-1; these peaks could be assigned to the combination of E’ and A’ modes, and a double-resonance effect involving a longitudinal acoustic phonon.13 The full width at half maximum at 250 cm-1 was 3.1 and 4.1 cm-1 for the original and BV-treated monolayer WSe2 samples, respectively. This broadening would have been a result of the shifting of the Raman signals as shown in the case of the monolayer MoS2. Figure 2d shows the PL spectra before and after the BV molecules were


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treated on the monolayer WSe2. The BV-treated showed a significant decrease in PL intensity and in the peak shift, indicating electron transfer doping from the BV molecules, as shown in the case of the monolayer MoS2.14 According to the Raman and PL spectroscopies, electron transfer between the BV molecules and the monolayer TMDCs was confirmed.

We examined the spontaneous pattern formation of the BV molecules on bulk and monolayer TMDCs. Figure 3 shows atomic force microscope (AFM) images of the spontaneously formed BV molecule pattern on TMDCs of various thicknesses. The BV film (~10 nm) exhibited a spontaneous pattern formation by interfacing with water, as shown in the previous report (Figures 3a-c and S1).9 Interestingly, the patterns were highly dependent on the thickness of the MoS2 (Figure 3b); patterns of tiny holes (on 2L, 10L and bulk) and isolated dots (on monolayer) were observed. The thickness-dependent pattern formation was also observed on WSe2 (Figure 3c); a network pattern and patterns of tiny holes were observed on the monolayer and bulk WSe2, respectively. This layerdependent pattern formation originated from the different flow rates of BV molecules in


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the molecular film on the TMDCs. Since MoS2 and WSe2 show layer-dependent band gaps, and the monolayer is larger than the bulk,15 the extent of the electron transfer between the BV molecules and the TMDCs should depend highly on the number of layers, even though the surface elemental structure between each layer of the TMDCs is the same (Figure 3d). This topographic difference was also revealed by the delta interface area of the BV film (the amount of increase in the surface area) calculated using AFM images (Figure 3e). In the case of the monolayer MoS2, a large increase in the interface area was observed compared to the bulk MoS2, which would be related to the high flow rate of the BV molecules on the monolayer. Qualitative analysis of the flow rates of the BV molecules showed 21 × 103 nm3/min and 5 × 103 nm3/min on monolayer and bulk MoS2, respectively (Figure S2). The results show that patterns can be exhibited even on the monolayer TMDCs and that the flow rate of the BV molecules is highly dependent on the electronic structure of the substrate TMDCs.

To analyze how the patterned BV molecules affect the electronic structure of TMDCs, we examined the relationship between the spontaneously formed patterns and the TMDCs’


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optoelectronic properties. Figures 4a and S3 show various PL spectra of monolayer WSe2, a p-type semiconductor, covered with a patterned BV (n-type dopant) film. Covering the monolayer WSe2 with BV molecules drastically reduced the PL intensity due to a large amount of electron doping.14 Since the Debye length in the sample can be estimated to be ~1 nm, the monolayer WSe2 would be uniformly affected by the BV film. Contact with water dramatically influenced the PL intensity, according to the spontaneous pattern formation, as shown in Figures 4a and b. Figures 4b and S4 portray the topographic occupancy of the BV film using the AFM images. From the PL spectra in Figure 4a, the amount of BV area from the optical point of view was identified as being determined by the following equation:

Itotal = IBV ×  + Iori × (1-)

(1)

where Itotal, IBV and Iori are the PL intensity of the entire area (measured value), of the BVcovered area, and of the original monolayer WSe2, respectively, and  is the estimated area ratio of the BV-covered area. As shown in Figure 4c, the calculated occupancy of the BV-covered area from the AFM images and the PL spectra exhibited a linear


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relationship, which indicates a low PL intensity for the BV-covered area, while the exposed area, representing the original WSe2, shows a high PL intensity. These data can be explained by the local carrier injection through the spontaneous pattern formation of the dopant BV molecules. For further analysis, we calculated the expected depletion area using the following equation for a depletion model for the heterostructures of a conventional p-n junction (Figure 4d),16

Wd = xp +xn

xp =

(2)

2𝑁𝑑𝜀0𝜀𝑟𝑉𝑏𝑖 𝑞𝑁𝑎(𝑁𝑎 + 𝑁𝑑)

, xn =

2𝑁𝑎𝜀0𝜀𝑟𝑉𝑏𝑖 𝑞𝑁𝑑(𝑁𝑎 + 𝑁𝑑)

(3)

where Wd. xp, xn, Nd, Na, 0, r, Vbi and q are the depletion width, the depletion width on the p-side, the depletion width on the BV-doped side, the carrier concentration of the BVdoped WSe2, the carrier concentration of the original WSe2, the vacuum dielectric constant, the relative dielectric constant of WSe2, the built-in potential and the elementary charge, respectively. The analysis indicates a depletion width in the range of 1 to 25 nm (Figure S5). We further confirmed the hypothesis of the above local carrier doping by using a thinner BV film on WSe2 via a 1/10- or 1/100-diluted BV solution, which showed


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a tiny PL intensity, as shown in the BV-covered WSe2, illustrated in Figures 2d and 4a (Figure S6). In addition, we examined the reprocessing of BV molecules onto the BVpatterned WSe2 to fill the exposed area in order to confirm the re-doping effect. As shown in Figure S7, the reprocessed monolayer WSe2 showed low PL intensity due to the redoping of the WSe2. These data further indicate the exposure of the original area, and local carrier doping is exhibited by the spontaneous pattern formation.

CONCLUSION

We demonstrated the spontaneous pattern formation of electron donor BV molecules on monolayer and bulk TMDCs (MoS2 and WSe2). AFM images showed that the flow rate of the BV molecules is highly dependent on the number of TMDC layers due to the different electron transfer interactions between the BV film and each layer of the TMDCs. Analysis of the relationships between the AFM images and PL spectra indicates that the patterned BV molecules inject electrons into the local region of WSe2, which generates the mosaic electronic structure of WSe2. Control of the position of the patterns will be important for


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applications in the future. It may be possible to generate a controlled pattern by applying external stimuli, such as shear stress or an electric field. Our results show that the spontaneous dynamic patterning process would be useful in making electronic junctions such as multiple p/n junctions on semiconducting materials on a nanometer scale uniformly and simultaneously.

EXPERIMENTAL SECTION

Sample Preparation Method

The donor molecule, benzyl viologen (BV), was prepared according to the previous reports.7 The detailed procedure was that 25 mg of benzyl viologen dichloride (SigmaAldrich) was dissolved in Milli-Q water, then 5 ml toluene (Sigma-Aldrich) was poured on top to make a binary solution layer. Sodium borohydride (NaBH4, ~3.7 g, SigmaAldrich) was poured on top of the solution. The solution was kept in ambient more than


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12 hours, and the toluene layer (yellow color) was extracted and applied to the transition metal dichalcogenides (TMDCs). MoS2 (SPI Supplies) and WSe2 (HQ Graphene) were exfoliated via a mechanical process and transferred onto p+-Si wafers covered with thermally grown SiO2 (Silicon Valley Microelectronics, Inc.). The BV solution was spincast onto the TMDCs using a spin coater (MIKASA 1H-D7). The pattern-formation process followed the previous report, and all procedures were under Milli-Q water at 50 ℃.9 For the experiment in Figure S6, the BV toluene solution was diluted with neat toluene to reduce the concentration to 1/10 and 1/100 to form the prepared solution.

Physical measurement

To obtain the topographic images, an AFM (SII instruments) with dynamic force mode or sampling intelligent scan mode was used. Image J software was used to analyze the BV-covered area shown in Figure S4. PL and Raman spectroscopy were carried out using a LabRAM HR800 microspectrometer equipped with an EMCCD camera


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(HORIBA Scientific). The excitation intensities were ~2.7 W cm-2 for PL measurements and 27 W cm-2 for Raman measurements.

Analysis of the nanoscopic flow and patterns of BV molecules on TMDCs

To calculate the delta interface area shown in Figure 3e, the topographic area of the BV film was obtained using SII and Gwyddion softwares. The nanoscopic flow rates of the BV molecular film on MoS2 were calculated under the hypothesis that uniform bumps were generated by the lateral flow of the BV molecules. Using an average height of the patterned BV molecular film and hypothesizing the initial state (before contact with water) as a flat surface, the volume flow rate could be estimated as shown in Figure S2. In the experiment, monolayer and bulk MoS2 flakes were close to each other (less than 30 m distance), as shown in Figure S1; therefore, all conditions (temperature of water, water fluidic condition, etc.) would be the same on all the MoS2 flakes, except for the thickness of each flake, which changes the electronic structure of the MoS2 and the electron transfer interaction between the BV molecules and the MoS2. To calculate the


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occupancy of the BV molecules in Figure 4b, the AFM images were converted into binary images shown in Figure S4. The white area (BV molecular area) was evaluated using Image J software. To estimate the characteristic length of the patterned BV molecules in Figures 4b and S4, fast Fourier transform analysis was applied on the low and high threshold binary images in Figure S4. All the films show a peak characteristic length of 100 to 200 nm.


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FIGURES

Figure 1. Schematic illustration of the spontaneous pattern formation of the electron donor BV molecule and local carrier injection on 2D semiconducting materials, TMDCs.


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Figure 2. (a,c) Raman and (b,d) PL spectra for the original and BV-treated monolayer MoS2 (a, b) and WSe2 (c, d). The inset graph in Figure 2d shows the normalized PL spectra.

Figure 3. (a) Illustrations of the spontaneous pattern formation process. Spontaneous pattern formation of (b) monolayer (ML), bilayer (2L), 10-layer (10L) and bulk MoS2, and (c) ML and bulk WSe2. (d) Illustrative image of the energy alignment between a BV molecule and monolayer and bulk MoS2. (e) Calculated delta interface area between


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water and BV film by the spontaneous pattern formation in Figure 3b. All scale bars are 400 nm.

Figure 4. Experimental analysis of the local carrier doping. (a) PL behavior (original: blue; BV-treated: red; the patterned: green) for each BV-patterned WSe2 and (b) AFM images of the original (no BV molecules) and of the tiny-hole-, network- and islandpatterned BV molecules on monolayer WSe2. The BV areal occupancy is presented


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under the images. An AFM image (86.7 ± 5.6% BV areal occupancy) is identical to the image in Figure 3c. All scale bars are 400 nm. (c) The estimated BV area relationship obtained from PL spectra in Figure 4a and AFM images in Figure 4b. The blue region indicates the low and high thresholds to evaluate from the AFM images. The dashed line is the guide to clarify the relationship. Inset is an illustration. (d) Illustrative image of the estimation of the depletion width (Wd) in the p/n junctions obtained by the pattern formation.

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website. Microscope image (Figure S1), calculated flow rate (Figure S2), original PL spectra of


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Figure 4a (Figure S3), binary images shown in Figure 4b (Figure S4), calculated depletion width (Figure S5), PL spectra treated with the diluted BV solution (Figure S6), PL spectra along the re-doping process to the BV-patterned WSe2 (Figure S7).

AUTHOR INFORMATION

Corresponding Author *[email protected]

Author Contributions The manuscript was written by H.I. and D.K. The design of the experiment was done by H.I. and D.K. All researchers continuously discussed the manuscript.

ACKNOWLEDGMENT This work was supported by JST PRESTO Grant No. JPMJPR1663 and Research Foundation for Opto-Science and Technology.


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Electronic Structure Mosaicity of Monolayer Transition Metal

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