Local Enhancement of Exciton Emission of Monolayer MoS2 by

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C: Plasmonics, Optical Materials, and Hard Matter

Local Enhancement of Exciton Emission of Monolayer MoS by Copper Phthalocyanine (CuPc) Nanoparticles 2

Ganesh Ghimire, Subash Adhikari, Seong Gi Jo, Hyun Kim, Jinbao Jiang, Jinsoo Joo, and Jeongyong Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00092 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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The Journal of Physical Chemistry

Local Enhancement of Exciton Emission of Monolayer MoS2 by Copper Phthalocyanine (CuPc) Nanoparticles Ganesh Ghimire,a, b Subash Adhikari,a, b Seong Gi Jo,c Hyun Kim,a, b Jinbao Jiang,a, b Jinsoo Joo,*, c and Jeongyong Kim *, a, b a

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of

Korea, Tel: +82–31–299–4054; Fax: +82–31–299–6505 b

IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Suwon 16419,

Republic of Korea c

Department of Physics, Korea University, Seoul 02841, Republic of Korea, Tel: +82–2–3290

–3103; Fax: +82–2–927–3292 ABSTRACT:

Monolayer

transition-metal dichalcogenides (1L-TMDs)

provide

ideal

platforms to study light emission using two-dimensionally confined excitons. Recent studies have shown that the exciton emissions of 1L-TMDs can be conveniently modulated by developing heterostructures with zero-dimensional nanoparticles (NPs) or quantum dots. In this study, we synthesized organic semiconducting copper-phthalocyanine (CuPc) NPs with sizes in the range of 30–70 nm by re-precipitation method, and decorated on the chemical vapor deposition (CVD)-grown 1L-MoS2 with these NPs. This hybrid system exhibited a 6 times larger local photoluminescence (PL) at the positions of the CuPc NPs, compared with the pristine 1L-MoS2 sample. The PL enhancement and spectral modification of the 1L-MoS2 decorated with CuPc NPs were attributed to the p-doping effect of the CuPc NPs, confirmed by spectral analysis and field-effect transistor (FET) measurements. 1

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█ INTRODUCTION Monolayers of transition-metal dichalcogenides (1L-TMDs) have recently attracted significant attention for the development of flexible and transparent optical and electronic devices, owing to their ultra-thin thickness and direct bandgap.1–6 The optical properties of 1L-TMDs are dominated by excitonic transitions, which provides the ample variety of engineering their optical properties for nanophotonic applications.

7–10

Among 1L-TMDs,

monolayer molybdenum disulfide (1L-MoS2) has been the most promising two-dimensional (2D) semiconductor, as an ideal platform for hybridization with zero-dimensional quantum dots (QD) and nanoparticles (NPs) to enhance the optical and electrical properties. 11, 12 However, as the 1L-MoS2 is only ~0.7 nm thick, its absorption in the visible-wavelength range is only a few percent and, in addition, the quantum yield of pristine 1L-MoS2 is very low owing to the high density of defects such as sulfur vacancies 13–15, which have been the major drawbacks for their applications. Various approaches have been developed for the local enhancement of light emission and modulation of the optical properties of 1L-TMDs. Enhancement of photoluminescence (PL) of 1L-TMDs was achieved using plasmonic structures, such as metal NPs and core-shell QDs on 1L-TMDs.

16,17

The

formation of a heterostructure by stacking different 1L-TMDs also leads to a large PL enhancement owing to the interlayer charge transfer and resultant p-doping effect.17 Furthermore, laser treatment has been shown to be an effective approach to increase the light emission of 1L-TMDs. The origin of this light emission was attributed to the p-doping provided by adsorbed oxygen molecules on the surface of the TMDs. 18, 19 Organic semiconductors composed of π-conjugated molecules exhibit a high carrier mobility and quantum yield.

20, 21

Crystalline organic nanostructures composed of highly

ordered π-conjugated molecules can generate excitons and, in addition, they have a good 2


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stability and active charge transfer properties.

22, 23

The low synthesis cost and structural

flexibility of organic semiconductors make them promising for applications in photovoltaic devices. Metal phthalocyanines (MPcs) are porphyrinoid macrocyclic complexes (compounds containing rings of eight or more atoms)-based organic semiconductors; macrocyclic complexes provide a high symmetry to MPcs semiconductors. In MPcs semiconductors, holes are the majority charge carriers24; and they also provide high chemical and thermal stability

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, desirable for light-emitting and photovoltaic devices.26 The physiochemical and

optical properties of MPcs can be easily modified by changing the positions of the central atoms and by substituting other molecules at the axial and peripheral positions.24, 25 Among the MPcs, copper phthalocyanine (CuPc) can be synthesized using a simple process, and it exhibits an excellent p-type semiconductor behavior.27 CuPc is used in various optoelectronic devices such as organic field-effect transistors (OFETs), non-linear optical devices, and photovoltaic devices.28, 29 Recently, the hybridization of MPcs with 1L-MoS2 was theoretically and experimentally studied, which showed an efficient charge transfer and improved photoswitching, respectively, suggesting potentials candidate for photomediated applications. 24 30

.

The p-doping of 1L-WSe2 with a deposition of a thin organic layer was also reported,

showing that there is a charge transfer within the hybrid structure.31 However, while organic NPs provides the advantages such as the stability and the convenient control of the density,32 there are no reports on the hybridization of 1L-TMDs with organic NPs to enhance their light emission. In this study, we propose a unique hybrid system that consists of p-type organic semiconducting CuPc NPs and chemical vapor deposition (CVD)-grown inorganic 1L-MoS2. We revealed a localized PL-enhancement of the 1L-MoS2 on the location of single CuPc NPs. 3



Optical and electrical measurements indicated that a local charge transfer to the CuPc NPs was responsible for the local enhancement of the excitonic emission. `

█ MATERIALS AND METHODS Growth of 1L-MoS2 and transfer onto cover glass substrate. 1L-MoS2 was grown on a SiO2/Si substrate using the CVD method. The growth was promoted by spin-coating with perylene-3, 4, 9, 10-tetracarboxylic acid tetrapotassium salt (PTAS, 2D semiconductor) on a SiO2/Si wafer with 2,600 revolutions per minute (rpm) for 1 min. Then, 200 mg of sulfur (S, Sigma-Aldrich) and 10 mg of MoO3 were placed in the furnace. The promoter-coated substrate was suspended above an Al2O3 crucible boat containing a MoO3 powder. Then, the top side was heated to 210°C at a ramping rate of 42°C/min, whereas the temperature of the bottom side was ramped up to 780°C. The whole process was performed under an N2 flow of 500 sccm for 15 min. The CVD-grown MoS2 was transferred on a clean cover glass using the wet-transfer method.33 The transferred MoS2 was cleaned with acetone, isopropanol, and ethanol to remove organic/inorganic residues. The formation of 1L-MoS2 on the cover glass was confirmed by measuring the Raman frequency differences of the A1g and E12g peaks. 34,35 CuPc NPs synthesis and decoration of CuPc NPs on the CVD 1L-MoS2. A commercially available CuPc powder was purchased from Sigma-Aldrich and used without further purification. We used the re-precipitation method for the preparation of NPs from the CuPc powder.36,37 Figure 1(a) illustrates the preparation of the CuPc NPs. First, a 200-µL solution (micromolar) of the CuPc powder was added to vigorously stirred deionized water for approximately 10 min, followed by ultrasonication (20 min); after a short period of time, re-precipitation occurred. The obtained solution contained CuPc NPs. In order to obtain 4


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residue-free CuPc NPs, we performed filtration using a Whatman filter paper with a pore size of 0.1 µm. A field-emission scanning electron microscopy (FESEM) image (Figure 1(b)) shows that the CuPc NPs were distributed uniformly on the substrate and had a size in the range of 30–70 nm. More details of the sample preparation are provided in the supporting information (SI) Figure S1. For the decoration with CuPc NPs on top of the 1L-MoS2, a short O2-plasma treatment was applied to obtain a hydrophilic 1L-MoS2 surface using a radio frequency (RF) plasma cell. The PL and Raman measurements results of the 1L-MoS2 before and after the plasma treatment were the same, which indicated that there was no physical damage. The O2-plasma treatment 38 was performed under an operating pressure of 1.3×10-3 Pa, RF power of 50 W, and oxygen gas flow of 5 sccm for 5 s. The CuPc NPs were dispersed on the 1L-MoS2 by spin coating. Confocal PL and Raman spectral mapping, Absorption measurements and Electrical measurements. Confocal PL and Raman spectroscopy measurements were performed using a laboratory-made confocal microscope system with a 514-nm laser excitation source. The laser powers for the PL and Raman measurements were 150 µW and 300 µW, respectively. An objective lens with a numerical aperture of 0.95 was used to illuminate the samples and collect

the

scattered

light.

The

collected

light

was

guided

to a

30-cm-long

monochromator, equipped with a cooled charge-coupled device (CCD), through an optical fiber with a core diameter of 50 µm, which acted as a confocal detection pinhole. Diffraction gratings with 150 grooves mm-1 and 1800 grooves mm-1 were used for the PL and Raman measurements, respectively. For the electrical measurement, the source and drain electrodes of the field-effect transistor (FET) devices were fabricated by depositing a 5-nm Cr and 50-nm Au layers using electron-beam evaporation.39,40 FET measurements were performed

5



using a four-probe station equipped with a current detector (SourceMeter 2400, Keithley) at ambient conditions. █ RESULTS AND DISCUSSION

Figure 2(a) shows PL intensity images of the CuPc-NPs-decorated 1L-MoS2 and pristine 1L-MoS2. The comparison between the two images (same intensity scale) reveals a PL enhancement of the CuPc-NPs-decorated 1L-MoS2 throughout the triangular 1L-MoS2 grains. As CuPc NPs does not exhibit fluorescence, there are no emissions from the CuPc NPs outside the 1L-MoS2 grains. Certain locations in the hybrid sample exhibit a significantly high emission (dotted circles in Figure 2(a) (i)). By comparing optical images (SI Figure S1), we revealed that these local maxima of exciton emission corresponded to the CuPc NPs positions on the 1L-MoS2. There was an overall increase of the PL owing to the presence of the small CuPc NPs dispersed throughout the 1L-MoS2 grains. The enhancement of light emission at the positions of the CuPc NPs is schematically illustrated in Figure 2(b).

We obtained the average PL spectra of the pristine and hybrid samples for a quantitative spectral comparison of their light emission, which is modified by the CuPc NPs, as shown in Figure 2c. In addition, we selected a few locations (dotted circles in Figure 2a) to obtain the average PL spectra of PL-maxima locations, as shown in Figure 2c. The PL intensity of the selected PL-maxima locations of the hybrid sample was 6 times larger than the average intensity of the PL spectra of the pristine sample, which demonstrates the local enhancement of exciton emissions of the 1L-MoS2 decorated with CuPc NPs. The average PL spectra of the 1L-MoS2/CuPc-NPs hybrid sample (excluding the local maxima points) show double the PL intensity of the pristine sample. The PL spectra of the hybrid sample changed 6


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with the CuPc-NPs decoration; the spectral width of the PL curve became smaller, and the peak position was blue-shifted by ~10 nm, compared with the pristine 1L-MoS2 sample. In order to investigate the origin of the local PL enhancement and spectral modification of the 1L-MoS2 decorated with the CuPc NPs, we fitted the PL spectra with three Lorentzian peaks that correspond to a neutral exciton (A0), trion (A-), and B-exciton (B) peaks, using the peak positions reported previously, 34,41 as shown in Figure 2(d). The pristine 1L-MoS2 sample exhibited a trion spectral weight of 55% and neutral exciton spectral weight of 40%, typical for a n-type CVD-grown pristine 1L-MoS2.33 The average PL spectra of the hybrid sample showed a relative increase of the neutral exciton emission and decrease of the trion emission (trion: 50%; neutral exciton: 45%). The spectra from the local maxima locations of the hybrid sample (dotted circles in Figure 2 (a)) showed that the spectral weight of the neutral exciton increased to 75%, whereas the trion population decreased to 20%, as shown in Figure 2d (iii). The blue shift of the PL peak position and increased spectral weight of the neutral exciton emission, compared with the trion emission, are typical characteristics of the electron depletion or p-doping effect of 1L-MoS2 41 which led to a PL enhancement owing to the reduced charge screening and relaxation of the Pauli blocking.42,43 We believe that the observed p-doping effect originates from the CuPc NPs dispersed on the 1L-MoS2. CuPc tends to withdraw electrons from the 1L-MoS2 surface, as it is a strong electronwithdrawing semiconductor.

The p-doping by the CuPc NPs was also manifested in the Raman spectra of the pristine and hybrid sample. The Raman spectra of 1L-MoS2 were assigned by two major peaks at 385 cm-1 and at 405 cm-1, which correspond to the in-plane (E12g) and out-of-plane (A1g) vibrational modes, respectively.44,45 Raman spectroscopy has been used to evaluate the 7



electron density of 1L-MoS2 owing to the strong electron-phonon interactions, which are correlated with the peak position of the A1g mode.34,35,41 Figure 3(a) shows the comparison between the average Raman spectra of the pristine and CuPc-NPs-decorated 1L-MoS2. We fitted the average Raman spectra of the pristine and hybrid samples (as in Figure 3(a)), which reveals that the A1g Raman mode was blue-shifted by 2.7 cm-1 in the CuPc-NPs-decorated region; no noticeable change was observed for the E12g Raman mode. The blue shift of the A1g Raman mode emerges owing to electron depletion,46 which indicates the p-doping of the 1L-MoS2 sample decorated with CuPc NPs. While the p-doping effect on 1L-MoS2 by CuPc NPs is clearly observed, we note that surface passivation by the presence of CuPc NPs could have also contributed to the observed local PL enhancement of 1L-MoS2. Previous study showed that thin layer of CuPc prevented the oxygen adsorption to MoS2 surface and reduced the trapping of photoexcited carriers, resulting in improved photoswitching response of 1LMoS2 FET30. We further investigated the effect of the CuPc NPs on the 1L-MoS2 by confocal absorption measurements.34 The comparison between the average optical absorption spectra of the pristine and CuPc-NPs-decorated hybrid samples showed an increase of the absorption peaks of 1L-MoS2 after the deposition of the CuPc NPs, as shown in Figure 3(b). We measured the absorption of CuPc NPs dispersed on the substrate as the result is shown in SI Figure S2. The absorption of CuPc NPs was negligible in intensity compared to that of 1LMoS2 due to the low absorption cross section of scattered CuPc NPs, confirming that observed increase of absorption is not the result of direct absorption by CuPc NPs themselves but originates from the improved absorption of 1L-MoS2 hybridized with CuPc NPs. The overall increase of the absorption in the hybrid 1L-MoS2 sample was caused by the increase 8


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of both A and B exciton peaks. In addition, we observed small red-shifts of both (A and B) exciton peaks after the CuPc-NPs-decoration. The p-doping led to the increase of the exciton peak intensity of the absorption spectra of 1L-MoS2, consistent with results in a previous study. 34,47 It is likely that the red-shift of the A- and B-exciton peaks of the CuPcNPs-decorated 1L-MoS2 is caused by the increase of the exciton binding energy of 1L-MoS2 after the decoration with the CuPc NPs owing to the reduced charge screening.48 The confocal absorption spectra results further confirm the p-doping effect and locally enhanced optical properties of the 1L-MoS2 after the decoration with the CuPc NPs. The observed p-doping of 1L-MoS2 owing to the CuPc NPs can be attributed to a charge transfer between the 1L-MoS2 and CuPc NPs. In order to illustrate the charge transfer, the energy-band alignment between the CuPc NPs and 1L-MoS2 is illustrated in Figure 3(c). The work function of CuPc is φ = 4.6 eV; its valance band maximum (VBM) is located at 5.2 eV, while its band gap is 1.7 eV.

24,49

For a CVD-grown 1L-MoS2,, the work function is

φ = 4.35 eV, VBM is located at 5.95 eV, and the optical band gap is 1.9 eV.1,50,51 As the work function of 1L-MoS2 is smaller than that of CuPc, electrons from 1L-MoS2 can be effectively transferred to CuPc NPs. Our observation of p-doping provided by CuPc NPs in the hybrid sample is contrary to a previous density function theory (DFT) prediction for a system of 1LMoS2 with adsorbed CuPc molecules, where the transfer of electrons from the CuPc molecules to the 1L-MoS2 within the hybrid region leads to an n-doping of the 1L-MoS2.24 In their DFT study, the 1L-MoS2 was assumed to be perfectly neutral, and the Fermi-level was at the middle of the band gap, therefore the Fermi-level of the 1L-MoS2 was lower than that of the CuPc molecules, favorable for the transfer of electrons from the CuPc molecules to the 1L-MoS2. However, in our study, we used a CVD-grown 1L-MoS2 which is known to be n9



type due to the ample presence of negatively charged defects in the MoS2 lattice, and thus the Fermi-level is close to the conduction band edge. 52-55

The density of the CuPc NPs on the 1L-MoS2 was controlled through the spin-coating process; a high density of CuPc-NPs decoration is demonstrated in Figure 4. A dark-field and atomic force microscopy (AFM) images in Figure 4(a) show that the 1L-MoS2 is uniformly covered with CuPc NPs. We performed PL and Raman measurements on these samples. The average PL spectra of the pristine 1L-MoS2 and CuPc-NPs-covered 1L-MoS2 samples are shown in Figure 4(b). The comparison between the PL spectra showed that the average PL intensity of the CuPc-NPs-covered MoS2 is 6 times larger, which is larger than the average enhancement obtained with the lower density of CuPc NPs shown in Figure 2, which increased only twice. PL intensity maps of the pristine and fully covered hybrid samples are provided in the SI Figure S3. The Raman measurements of the fully covered 1LMoS2 sample show a blue-shift of the A1g Raman mode, which confirms the p-doping caused by the charge transfer to the CuPc NPs (SI Figure S4).

FET measurements of the pristine and hybrid samples were performed after the deposition of Cr/Au (5/45 nm) electrodes on the samples, which were placed on a SiO2/Si substrate. The inset of Figure 5 shows optical images of these devices, before and after the dispersion of the CuPc NPs. Plots of the source drain current (IDS) versus gate bias (VGS) with a drain voltage of 2 V for both pristine and hybrid 1L-MoS2 FETs are shown in Figure 5. The pristine 1L-MoS2 is intrinsically n-type and showed a threshold voltage (VTh) of approximately –30 V, while the sample spin-coated with CuPc NPs exhibited drastic changes in threshold voltage and current level. After the decoration with CuPc NPs, VTh was shifted to +30 V, from the value of –30 V of the pristine 1L-MoS2, which demonstrates that the n-type 10


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characteristic of the 1L-MoS2 was suppressed after the decoration with CuPc NPs.56 The decrease in drain current after the deposition of CuPc NPs by 104 times (Figure 5) can be attributed to the decrease in the number of electrons in 1L-MoS2 owing to the electron transfer to the CuPc NPs, which is consistent with the result obtained from the heterostructure of CuPc layer and multilayer MoS230. █ CONCLUSIONS We synthesized organic CuPc NPs using the reprecipitation method, and prepared a hybrid structure of CuPc-NPs/1L-MoS2. Nanoscale PL measurements showed local PL enhancements of the 1L-MoS2 at the locations of the CuPc NPs. The optical and FET measurements revealed that a charge transfer between the CuPc NPs and 1L-MoS2 was the origin of the PL enhancement of the 1L-MoS2. Our finding implies that arbitrary patterning of light emission on the two-dimensional 1L-TMDs is possible, if the locations of NPs can be controlled. The demonstrated local PL enhancement of the proposed hybrid system is of key importance for the development of 1L-TMDs/0D-organic-semiconducting-NPs hybrid structures to modulate the optoelectronic properties of 1L-TMDs.

█ ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publication website at DOI: Description of the sample preparation method (S1); Absorption measurement of CuPc solution, CuPc NPs dispersed on the substrate (S2) Photoluminescence (PL) images of the fully covered hybrid sample (S3); Raman spectra and images of the fully covered hybrid 11



sample (S4). █ AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected] and [email protected] Conflict of Interest The authors declare no competing financial interests █ ACKNOWLEDGEMENTS This study was supported by the IBS-R011-D1 project. J. J. acknowledges the support from the National Research Foundation of Korea (NRF) (No. 2015R1A2A2A01003805), grant funded by the Korean government.

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Figure 1: (a) Schematic diagram of the preparation of the CuPc NPs using the reprecipitation method. (b) FESEM image of the CuPc NPs on a glass substrate; the scale-bar corresponds to 1 µm.

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Figure 2: (a) PL intensity maps of the A-exciton emission of the (i) CuPc-NPs/1L-MoS2 and (ii) pristine 1L-MoS2 samples; the scale-bar corresponds to 2 µm. (b) Illustration of the enhanced light emission from the CuPc-NPs-decorated 1L-MoS2 after a laser excitation. (c) Comparison between the average PL spectra obtained from maps of the (i) pristine, (ii) hybrid, and (iii) local maxima points of the CuPc-NPs/1L-MoS2 sample. (d) (i, ii, and iii) show the fitting with three Lorentzian peaks of the obtained PL spectra in (c), where A0, A-, and B, denote the neutral exciton, trion, and B-exciton peaks, respectively.

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(a) Hybrid

(b) A1g

E12g

Pristine E12g

A1g

(c) Evac

Pristine Hybrid Absorption

Ranam Intensity

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


Evac

C B

A

Φ=4.6 eV Ec 1.7eV Ef Ev

380 390 400 410-1 420 Raman Shift (cm )

Φ=4.35 eV

400 450 500 550 600 650 Wavelength (nm)

0.3 eV

0.6 eV

Ec Ef 1.9 eV Ev

CuPC

1L-MoS2

Figure 3: (a) Fitted Raman curves of the average Raman spectra of the pristine and CuPc-NPs-decorated 1L-MoS2 samples with two peaks (E12g and A1g Raman modes); the lower panel shows the results for the pristine, while the upper panel for the CuPc-NPs/1L-MoS2 sample. (b) Average absorption spectra of the pristine and CuPc-NPs/1L-MoS2 samples, where A, B, and C, denote three absorption peaks of 1L-MoS2. (c) Schematic of the band diagram of CuPc and 1L-MoS2 before alignment, where Evac, Ec, Ef, and Ev represent the energies that correspond to the vacuum level, conduction band, Fermi-level, and valance band, respectively.

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Figure 4: (a) (i) Dark-field and (ii) AFM images of the CuPc-NPs-decorated 1L-MoS2 sample showing the distribution of the CuPc NPs on the 1L-MoS2 regions; the scale-bars in (i) and (ii) correspond to 10 µm and 2 µm, respectively. (b) Comparison between the average PL spectra obtained from the pristine and CuPc-NPs/1L-MoS2 samples.

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Figure 5: Plot of the drain current (IDS) versus gate voltage (VGS) at dark conditions at VDS = 2 V; the inset shows optical images of the (i) pristine and (ii) CuPc-NPs-decorated 1L-MoS2 (hybrid) FET devices.

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█ TABLE OF CONTENT (TOC) GRAPHIC

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Local Enhancement of Exciton Emission of Monolayer MoS2 by

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