Efficient Defect Healing of Transition Metal Dichalcogenides by

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Efficient Defect Healing of Transition Metal Dichalcogenides by Metallophthalocyanine Hyeyoung Ahn, Yu-Chiao Huang, Chang-Wei Lin, and Yi-Hsien Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09378 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Applied Materials & Interfaces

Efficient Defect Healing of Transition Metal Dichalcogenides by Metallophthalocyanine Hyeyoung Ahn,*1 Yu-Chiao Huang,1 Chang-Wei Lin,1 and Yi-Hsien Lee2 1

Department of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan.

2

Institute of NanoEngineering and MicroSystems, National Tsing-Hua University, Hsinchu

30010, Taiwan. KEYWORDS. Two-dimensional materials, Transition Metal Dichalcogenides, Molybdenum Diselenide, Phthalocyanine, Defects.

ABSTRACT. Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention as alternatives to graphene with semiconducting bandgaps. Mono- or few-layer TMDCs can be prepared by various methods, but regardless of fabrication methods (such as mechanical exfoliation and chemical vapor deposition (CVD)), TMDCs contain many structural defects which significantly affect their physical properties and limit their performance in applications. Metallophthalocyanines (MPcs) are organic semiconductors and as dopants, they are capable to modulate the optical and electrical properties of other semiconducting materials. Here we report that besides the ability to modulate the optoelectronic properties of 2D TMDCs, MPc molecules can be used to heal defects and improve physiochemical properties of TMDCs. Doping of planar MPc molecules to TMDCs is achieved by a simple solution dip coating

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method, but results in significant improvement in the optical properties and thermal responses of CVD-grown TMDCs, even comparable to those of mechanically exfoliated counterpart. Study of carrier dynamics shows that the adsorption of MPc on the TMDC surface leads to the complete suppression of the mid-gap defect-induced absorption in TMDCs. Furthermore, MPc molecules with large lateral size are found to effectively reduce the point defects in mechanically exfoliated TMDCs introduced during the preparation process. Our results not only clarify the optoelectronic modulation mechanism of chemical doping but also offer a simple control method of nano-sized defects in 2D TMDCs. INTRODUCTION Chemical vapor deposition (CVD)-grown transition metal dichalcogenides (TMDCs) have many advantages over exfoliated counterparts including the development of large-scale devices and vertical heterostructures. But the optoelectronic performance of CVD-grown TMDCs is significantly limited by high density of defects, such as topological defects and vacancies.1-3 Compared to mechanically exfoliated TMDCs, CVD-grown two-dimensional (2D) TMDCs show

particularly poor

optical

performance,

such

as

relatively

weak

and

broad

photoluminescence (PL). Recently, much effort has been directed to identify these defects and reduce them from 2D TMDCs.4-7 Vertical stacking of different kind of TMDC layers can not only tune the electronic band structure and generate new excitonic transition, but also offer a possibility to complement defects in 2D layers.8 For example, efficient elimination of defect bound excitons was achieved in sandwiched TMDC trilayer heterostructures.9 Surface treatment with strong acids was also shown to have a suppression effect on the point defects and surface impurities.4,6 Meanwhile, functionalization of 2D TMDCs with organic dopants, such as metallophthalocyanines (MPcs), offers another way to rationally create and control the basal


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plane defects. MPcs are two-dimensional aromatic molecules with a central metal atom in the inner ring and have great importance for organic electronic devices because of their good chemical and thermal stabilities. In spite of large dimension of each molecule (~1.5 nm), the macrocycle of MPc is planar to within 0.03 nm,10 which makes it possible for MPc molecules to easily adsorb on planar TMDC surfaces and reduce nanosized structural defects. For graphene or TMDCs with no dangling bonds, surface adsorbed MPcs can also work as an effective seed layer for the direct deposition of gate dielectrics on 2D surfaces.11 While most of MPcs share similar structural properties, their optical and electrical properties vary with the central metal atoms and various MPcs and their derivatives are widely used as effective dopants. When these MPcs are coupled with TMDCs, preferential and selective charge transfer between TMDCs and MPcs can be achieved depending on the relative difference in potential energies. Previously reported studies of dopant-coupled TMDCs have been conducted using either MPc molecules with different central atoms or other organic dopants with completely different molecular structures,12 which may obscure the correct understanding of the charge transfer mechanism. Here, we use zinc phthalocyanine (ZnPc) and its fluorinated derivative, zinc hexadecafluoro phthalocyanine (F16ZnPc), as n-type and p-type dopants and their influence on the optical and physical properties of CVD-grown few-layer molybdenum diselenide (MoSe2). Since ZnPc and F16ZnPc molecules share the same central metal ion (Zn), they have similar interactions with underlying 2D materials which is sensitive to the filling nature of d-band in the central metal atoms,13 and moreover, they have the same planar structures to similarly stack on the surface of 2D MoSe2 flakes. We observed the large quenching of photoluminescence (PL) from MoSe2 when functionalized with either ZnPc or F16ZnPc. But in opposite to the previous reports, the PL quenching assisted by p-type F16ZnPc was much less pronounced than that by n-type ZnPc.


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Besides the modulation of optoelectronic property, surface-adsorbed ultrathin MPc layer on CVD-grown MoSe2 flakes exhibits the defect healing effect, which is evidenced by the temperature-dependent large blueshift of bandgap, narrow PL bandwidth, and the suppression of mid-gap defect-induced absorption in the transient transmittance measurement. Significant recovery of optical properties approaching the quality of exfoliated MoSe2 is achieved by a simple dip coating method, which enhances the immense potential of CVD-grown TMDCs for further development in large-area optoelectronic and photovoltaic devices. RESULTS AND DISCUSSION In Figure 1a, we show the schematic of the MPc–TMDC structure for manipulation of photoexcited carrier transfer. For this work, 2D MoSe2 flakes were grown by CVD process and the details on the synthesis can be found in the Experimental Section. Upper panels in Figure 1b and c are the optical microscope images of CVD-grown MoSe2 flakes on SiO2/Si substrates. Despite slight difference in flake morphology caused during the growth process, both MoSe2 flakes show hexagonal shapes with the lateral size over 15 µm and the typical thickness of ~1.7 nm, indicating a bilayer of MoSe2. In order to investigate the influence of thickness and fabrication method of MPc layer on the interaction with TMDCs, we deposited the MPc layers on TMDC by using two different methods; solution dip coating (SDC)12 and physical vapor deposition (PVD) methods. Low panels in Figure 1b and c exhibit the images of the same MoSe2 flakes after the deposition of ZnPc molecules (MoSe2+ZnPc) using SDC and PVD methods, respectively. Apparently, PVD method results in the non-uniform and the preferential deposition of ZnPc at the flake boundaries of MoSe2, while the SDC method leads to a relatively uniform deposition of ZnPc on MoSe2 flakes. It has been known that during the SDC process, MPc molecules are preferentially adsorbed on the surface of TMDCs, not on the SiO2.12 Therefore, the


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difference in thickness measured by atomic force microscopy (AFM) in Figure 1c corresponds to the thickness of ZnPc layer, which is ~1.85 nm for SDC-covered ZnPc. The film thickness of F16ZnPc deposited in the same method is ~1.08 nm (not shown) and that of PVD-covered ZnPc is estimated to be ≥6 nm. The success of chemical doping of MPcs in MoSe2 can be realized by measuring PL which illustrates the charge transfer effect between them.

a

Figure 1. a. Schematic of the MPc–TMDC structure. b. and c. Optical microscopic images of MoSe2 flakes on SiO2/Si (Upper panel) and those of the same flakes after the deposition of ZnPc by SDC and PVD methods (Lower panel). Second column in c shows the thickness of the asgrown and the functionalized MoSe2 using SDC method, analyzed by AFM.


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Figure 2a and b exhibit the room-temperature PL spectra of as-grown MoSe2, before and after deposition of ZnPc and F16ZnPc by using SDC process. Under photoexcitation at 400 nm, the PL peaks centered at ~1.56 eV for as-grown MoSe2 redshift to 1.53 eV for both MoSe2+ZnPc and MoSe2+F16ZnPc. The band edge emission of ZnPc at ~1.65 eV is not observed, possibly due to its ultrathin thickness of SDC ZnPc. The PL signals of functionalized MoSe2 show the intensity quenching, but the quenching rate (QR= ⁄ ) of MoSe2-ZnPc is more significant than that of MoSe2+F16ZnPc. The observed PL quenching in MoSe2-ZnPc is attributed to the photoexcited carrier transfer from MoSe2 to ZnPc acceptors whose reduction potential (–4.0 eV) is positioned slightly below the conduction band minimum (CBM) of MoSe2 (–3.9 eV).12 Similar quenching rate of 0.2~0.3 was measured at different areas (six white dots in Figure 1a and actual beam spot size is ~2 µm in diameter) in the same MoSe2 flake, indicating that thin SDCdeposited ZnPc layer uniformly covers the surface of the MoSe2 flake. For comparison purposes, we also measured the PL of the CVD-grown MoS2 [CBM at –4.27 eV] functionalized with ZnPc and found that it remains nearly unchanged as expected. (see Figure S1) Figure 2c illustrates the PL response of MoSe2 functionalized with ZnPc via PVD method, compared with that of as-grown MoSe2. The PL signals of as-grown MoSe2 (black curve) and MoSe2+ZnPc (red curve) are measured at the red dot areas in Figure 1b, where relatively thin and uniform ZnPc covers the MoSe2 flake. For MoSe2+ZnPc, similarly large quench of the MoSe2 peak is observed, and there is an additional weak peak at 1.65 eV, corresponding to ZnPc emission. Relatively weak PL intensity of ZnPc peak may be attributed to small optical absorption of ZnPc at the optical pump wavelength.14,15 Meanwhile, PL spectrum from the thick ZnPc covered area (yellow dot in Figure 1b) shows only weak ZnPc emission and no noticeable emission from MoSe2 is observed. These results reveal that modulation of photoelectric


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properties of TMDCs without the undesired Pc peak can be achieved by ultrathin layer of MPcs (

Efficient Defect Healing of Transition Metal Dichalcogenides by

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