Encapsulation of a Monolayer WSe2 Phototransistor with

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Functional Nanostructured Materials (including low-D carbon) 2

Encapsulation of Monolayer WSe Phototransistor with Hydrothermally Grown ZnO Nanorods Kang-Nyeoung Lee, Seungho Bang, Ngoc Thanh Duong, Seok Joon Yun, Dae Young Park, Juchan Lee, Young Chul Choi, and Mun Seok Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03508 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Encapsulation of Monolayer WSe2 Phototransistor with Hydrothermally Grown ZnO Nanorods Kang-Nyeoung Lee †, §, #, Seungho Bang †, ‡, #, Ngoc Thanh Duong †, Seok Joon Yun †, ‡, Dae Young Park †, ‡, Juchan Lee†, Young Chul Choi *, § and Mun Seok Jeong *, †, ‡ † Department

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

Korea ‡ Center

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

Republic of Korea § Korea

Institute of Carbon Convergence Technology, Jeonju 54853, Republic of Korea

KEYWORDS: encapsulation, tungsten diselenide, zinc oxide, p-doping, antenna effect, charge transfer

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ACS Paragon Plus Environment

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ABSTRACT Transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials for realizing next-generation electronics and optoelectronics with attractive physical properties. However, monolayer TMDCs (1LTMDCs) have various serious issues, such as instability under ambient conditions and low optical quantum yield from their extremely thin thickness of ~0.7 nm. To overcome these issues, we constructed a hybrid structure (HS) by growing zinc oxide nanorods (ZnO NRs) on a monolayer tungsten diselenide (1LWSe2) using the

hydrothermal

method.

Consequently,

we

confirmed

not

only

enhanced

photoluminescence of 1LWSe2 but also improved optoelectronic properties by fabricating the HS-phototransistor. Through various investigations, we found that these phenomena were due to the antenna effect and p-type doping effect attributed to the ZnO NRs. In addition, we verified that the optoelectronic properties of

1LTMDCs

are maintained for two weeks in the

ambient condition through the sustainable encapsulation effect induced by our HS. This encapsulation method with inorganic materials is expected to be applied to improve the stability and performance of various emerging 2D material-based devices.

INTRODUCTION Transition metal dichalcogenides (TMDCs), unlike graphene, are promising two-dimensional (2D) materials that can be used in various applications such as electronics and optoelectronics owing to their large bandgap in the visible range.1-4 In particular, as the number of layers decreases, the bandgap changes from an indirect to a direct bandgap.5-12 Hence, most studies on optoelectronic devices focus on monolayer TMDCs (1LTMDCs).13-14 Among them, 2


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tungsten diselenide (WSe2) has been widely researched in TMDCs because the monolayer WSe2 (1LWSe2) is an outstanding p-type material governed by holes as the majority carrier.1520

Figure 1. (a) Configurations of the HS-phototransistor. (b) Schematics of hydrothermal process to grow ZnO NRs. (c) Tilted SEM image of the ZnO NRs. Inside of the dashed line is the HS region. Outside of the dashed line is the as-grown ZnO NRs region. The scale bar is 10 µm (d) Magnified SEM image of the red dashed circle in Figure 1c. Average diameter of the ZnO NRs is ~40 nm. The scale bar is 300 nm. (e) Raman spectrum of as-grown ZnO NRs. However, the optoelectronic properties of pristine

1LTMDCs

are still unsatisfactory for

commercial applications owing to their low quantum yield.21-22 Thus, doping has been actively studied for enhancing the properties.23-26 In addition, several researchers have attempted to solve the instability issue of TMDCs in the ambient condition for advanced 3



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TMDCs applications.27-28 The 1LTMDCs are easily oxidized by oxygen (O2) and water (H2O) in the air, crucially affecting their optoelectronic properties including quantum yield, conductivity, and even energy band structure.29-33 To prevent these obstacles, encapsulation with organic or inorganic materials has been used. In previously reported studies on encapsulation, poly(methyl methacrylate) (PMMA) has been used typically and parylene-C has recently attracted attention as a new material for this purpose.33-34 However, when these polymeric materials are used for encapsulation, unpredictable and troublesome problems are induced from the low glass-transition temperature, such as hardening.35-37 Therefore, as an alternative material, hexagonal boron nitride (h-BN) or the deposition of inorganic materials by atomic layer deposition (ALD) have been used.38-41 The use of h-BN requires a limited scale and an imprecise transfer process. Additionally, aluminum oxide (Al2O3) or hafnium dioxide (HfO2) using ALD represent inorganic passivation materials. However, TMDCs are generally known to be n-type doped by the inorganic passivation materials.42 This characteristic is not suitable for the p-type

1LWSe . 2

Therefore, we need to seek another

efficient method. From this perspective, zinc oxide (ZnO) can be a fascinating proposition because it is strong against oxidation and can be deposited selectively.43-46 Moreover, the hydrothermal method is suitable for synthesizing materials because it exhibits many advantages such as a low reaction temperature, inexpensiveness, mass production with highquality crystallinity, and manufacturing in various forms.47 In this study, we confirmed the encapsulation effect of nanorods (ZnO NRs) on the p-type

1LWSe 2

1LTMDCs

through depositing ZnO

to fabricate a hybrid structure (HS)-

phototransistor using a simple hydrothermal method. The HS-phototransistor showed not only an enhanced air stability of two weeks in the ambient condition, but also a higher 4


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photoresponsivity than that of a pristine

1LWSe 2

phototransistor. In addition, the

1LWSe 2

of

HS (HS-1LWSe2) surprisingly showed up to ~80 % enhanced photoluminescence (PL) compared to that of pristine 1LWSe2. Our HS is a new way to encapsulate 2D material-based optoelectronic devices with inorganic materials by a simple method for air stability and improved performance.

RESULTS AND DISCUSSIONS PL and Raman analyses were performed to investigate the difference in optical properties between pristine 1LWSe2 and HS-1LWSe2. Figures 2a and 2b show the integrated PL intensity mapping images of pristine

1LWSe 2

and HS-1LWSe2 using a 532 nm laser as an excitation

source, respectively. In the PL mapping images, we exported the PL spectrum of regions Ⅰ (core region), Ⅱ (central region), and Ⅲ (edge region), as shown in Figure S9. The overall PL intensity of the HS-1LWSe2 was notably enhanced compared to that of pristine 1LWSe2.

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Figure 2. Integrated PL intensity mapping images of (a) pristine 1LWSe2 and (b) HS-1LWSe2 at ~1.67 eV. (c) Integrated PL intensity mapping image of as-grown ZnO NRs (outside of the dashed line) and HS-ZnO NRs (inside of the dashed line) at ~3.26 eV. (d) PL spectra extracted from Figures 2a and 2b. The PL peaks A of pristine

1LWSe 2

and HS-1LWSe2 are

located at ~1.67 eV and ~1.64 eV, respectively. Embedded neutral exciton (X*, purple), positive trion (X+, green) and biexciton (XX, yellow) in peak A are revealed by fitting. (e) PL spectra extracted from outside and inside of the dashed line of Figure 2c. The near-band-edge emissions (NBE) of as-grown ZnO NRs and HS-ZnO NRs are located at ~3.26 eV. The deep level emissions (DL) of as-grown ZnO NRs and HS-ZnO NRs are located at ~2.25 eV. (f) Illustration of band offset between p-type 1LWSe2 and n-type ZnO. In this illustration, CBM, VBM, Eg, and χ indicate the conduction band minimum, valence band maximum, bandgap energy, and electron affinity, respectively. To clearly observe the change in PL, the PL peaks A of pristine

1LWSe 2

(orange) and HS6


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1LWSe 2

(blue) were fitted with a neutral exciton (X*, purple), positive trion (X+, green), and

biexciton (XX, yellow), as shown in Figure 2d.48-51 In pristine 1LWSe2, peak A is governed by neutral excitons. Meanwhile, in the spectrum of HS-1LWSe2, peak A is dominated by positive trions and biexcitons due to hole injection from the ZnO NRs of the HS (HS-ZnO NRs), resulting in the redshift of peak A, a critical clue of p-type doping.48, 52. Generally, the p-type doping effect induces a decrease in PL intensity.53-54 In our results, however, the PL intensity remarkably increased up to ~80 % (see Figure S10). The huge PL enhancement can be explained with the antenna effect by the ZnO NRs, although the structure is p-type doped.55-57 Figure 2c shows an integrated PL intensity mapping image using a 355 nm laser as an excitation source, with regions dividing the as-grown ZnO NRs (outside of the dashed line) and the HS-ZnO NRs (inside of the dashed line). The PL of the ZnO NRs was well detected by the entire region. In the HS-ZnO NRs region, an obvious decrease in the PL intensity was observed compared to the region of the as-grown ZnO NRs. In Figure 2e, we can observe this phenomenon more clearly with the average PL spectra of the as-grown ZnO NRs (pink) and the HS-ZnO NRs (navy) extracted from the mapping result of Figure 2c. The PL of ZnO is generally known to have near-band-edge emission (NBE) and deep level emission (DL) generated from the oxygen defect.58 As shown in Figure 2e, the NBE of the HS-ZnO NRs decreased more than that of the as-grown ZnO NRs. The reason why the PL intensity of HSZnO NRs is decreased is because the hole carriers of the HS-ZnO NRs are transferred to HS1LWSe 2

due to the large band offset as described in the Figure 2f which illustrates the carrier

dynamics in our HS with the band diagram.59-61

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Figure 3. FWHM mapping images of the A1g mode of (a) pristine 1LWSe . 2

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1LWSe 2

and (b) HS-

(c) Raman spectra extracted from Figures 3a and 3b. Embedded E12g mode (pink),

A1g mode (sky blue), and 2LA mode (red) are revealed by fitting. (d) Differences of various phonon modes between pristine 1LWSe2 and HS-1LWSe2 extracted from Figure 3c. The p-type doping effect of 1LWSe2 is generally considered by the softening and broadening of the out-of-plain vibrational mode (A1g) in Raman spectroscopy.62-64 To identify the p-type doping of HS-1LWSe2 by HS-ZnO NRs, the full width at half maximum (FWHM) mapping images of the A1g mode of pristine

1LWSe 2

and HS-1LWSe2, which were measured using a

532 nm laser as an excitation source, are presented in Figures 3a and 3b, respectively. In Figure 3a, we observed a crack-like region (green on the edge region). As can be seen from the PL mapping image, the edge region seems to be degraded by oxygen because the edge region (region III in Figure 2a) shows a weaker PL intensity than the central region (region II in Figure 2a). Therefore, although the ZnO NRs were grown on pristine

1LWSe , 2

the PL 8


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intensity could not be increased in the edge region (region Ⅲ of Figure 2b). We additionally prepared an etched the etched

1LWSe 2

1LWSe 2

without the degraded region, as shown in Figure S6. The PL of

entirely increased after growing the ZnO NRs without the degraded

region. In Figure 3b, the FWHM of the A1g mode of HS-1LWSe2 was clearly broadened at all regions of HS-1LWSe2, in contrast with pristine Raman spectrum of the pristine

1LWSe 2

1LWSe . 2

Figure 3c shows that each average

(top, orange) and the HS-1LWSe2 (bottom, blue) is

fitted by the in-plain vibrational mode (E12g, pink) at ~247 cm-1, out-of-plain vibrational mode (A1g, sky blue) at ~250 cm-1, and second-order longitudinal acoustic mode (2LA, red) at ~260 cm-1.65-66 The movement of the overlapped mode (E12g mode + A1g mode) indicates the p-type doping effect because the A1g mode is sensitive to the doping of TMDCs.18-19 The overlapped mode of pristine

1LWSe 2

at ~249.23 cm-1 was uniformly detected in the entire

region. HS-1LWSe2 shows the overlapped mode at ~248.99 cm-1. This redshift of the overlapped mode indicates that HS-1LWSe2 was p-type doped after depositing the ZnO NRs on the pristine 1LWSe2. In addition, the lattice constants of WSe2 and ZnO are a = 3.34 Å and a = 3.29 Å, respectively.67-68 Generally, strain is inevitably generated at the interface between two materials with different lattice constants, causing a compressive strain in a material having a large lattice constant. According to the report, compressive strain decreases the intensity of the E12g mode.69 Therefore, as shown in Figure 3c, the lattice constant of HS1LWSe 2

is larger than that of HS-ZnO NRs, resulting in compressive strain and the E12g/A1g

mode intensity ratio of HS-1LWSe2 is reduced compared to that of pristine

1LWSe . 2

Furthermore, we clearly present the movements and FWHM of several phonon modes in Figure 3d. After growing the ZnO NRs, the A1g mode (sky blue), 2LA mode (red), E12g mode (pink), and overlapped mode (green) of pristine

1LWSe 2

were shifted from ~251.08 cm-1 to 9



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~250.07 cm-1, ~259.30 cm-1 to ~257.87 cm-1, ~247.10 cm-1 to ~247.15 cm-1, and ~249.23 cm1

to ~248.99 cm-1, respectively. The A1g mode and 2LA mode related to the p-type doping

effect indicate relatively large phonon softening. In particular, the phonon softening of the A1g mode is critical evidence of the p-type doping effect that corresponds to the redshift in the PL spectrum, as described in Figure 2d. Additionally, the FWHM of the A1g mode (dark green) and E12g mode (purple) of pristine 1LWSe2 were changed from ~2.05 cm-1 to ~4.22 cm1

and ~2.09 cm-1 to ~3.01 cm-1, respectively. The change in the FWHM of the A1g mode not

only showed a relatively larger broadening than that of the E12g mode, but also corresponded well to the FWHM mapping images in Figures 3a and 3b. Thus, we can conclude that HS1LWSe 2

was p-type doped from the evidence, such as broadening of the FWHM and phonon

softening of the A1g mode. The pristine

1LWSe 2

phototransistor and the HS-phototransistor were fabricated by e-beam

lithography (detailed information in Figure S7). Figure 4a shows the 3D configurations of the pristine

1LWSe 2

phototransistor and HS-phototransistor. The blue and red dots signify the

pollutants in atmosphere. Figure 4b shows the gate-dependent I–V curves of the pristine 1LWSe 2

phototransistor (black) and HS-phototransistor (red). The figure inset presents the

logarithmic scale graph of Figure 4b. We extract the field-effect mobility (μ) using the following equation:

𝜇 =

d𝐼𝑑𝑠 dV𝑏𝑔

×

𝐿 𝑊𝐶𝑖𝑉𝑑𝑠

where L is the channel length (4.5 μm), W is the channel width (11 μm), and Ci (= ε0∙εr/d; εr = 3.9 for SiO2) is the capacitance per unit area of the thickness of SiO2 (285 nm); additionally, the source–drain voltage is 0.1 V. 10


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Figure 4. (a) Configurations of the pristine

1LWSe 2

phototransistor and HS-phototransistor.

Insets are optical microscope images. (left; pristine

1LWSe 2

phototransistor, right; HS-

phototransistor, scale bars are 5 μm) (b) Gate-dependent I–V curves of the pristine

1LWSe 2

phototransistor (black) and the HS-phototransistor (red). Inset is a logarithmic scale graph. (c) Source–drain I–V curves of the pristine

1LWSe 2

phototransistor (bottom) and the HS-

phototransistor (top) depending on the back gate voltages from +30 V to -60 V. The transfer curves (drain current versus back gate voltage) of (d) the pristine 1LWSe2 phototransistor and (e) the HS-phototransistor with a 638 nm excitation power (none (green), 1000 µW (blue)). Insets are logarithmic scale graphs. (f) Gate-dependent I–V curves of the HS-phototransistor under ambient condition over 14 days. The field-effect mobility of the HS-phototransistor is 0.752 cm2/Vs, higher than that of the pristine

1LWSe 2

phototransistor (0.076 cm2/Vs). A positive shift in threshold voltage is

observed at the HS-phototransistor compared with the pristine

1LWSe 2

phototransistor, as

shown in Figure S11. The on/off ratio of the HS-phototransistor is observed to be ~665, 11



which is higher than that of the pristine

1LWSe 2

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phototransistor (353). Moreover, the

hysteresis (ΔVth) is reduced from 16 to 14 V, as shown in Figure S12. Figure 4c shows the source–drain I–V curves (-1 to +1 V) depending on the back gate voltages from +30 to -60 V. The HS-phototransistor (top) shows a high current value compared to the pristine

1LWSe 2

phototransistor (bottom). We confirmed the p-type characteristics of the pristine

1LWSe 2

phototransistor and HS-phototransistor by different back gate voltages from +30 to -60 V. To observe the optoelectronic properties, we utilized the 638 nm excitation source to excite the photogenerated carriers in the pristine

1LWSe 2

phototransistor and HS-phototransistor.

Figures 4d and 4e show the gate-dependent I–V curves (dark current; green, photocurrent; blue) of the pristine 1LWSe2 phototransistor and HS-phototransistor, respectively. When a 638 nm laser (Pex ~1000 μW) is irradiated to each phototransistor, the overall photocurrent of the HS-phototransistor is significantly improved with respect to the pristine

1LWSe 2

phototransistor. Interestingly, the photoresponsivity and photodetectivity of the HSphotodetector are ~0.541 A/W and 42.37 × 108 Jones, respectively, which are greater than those of the pristine 1LWSe2 phototransistor (0.147 A/W and 5.01 × 107 Jones, respectively), as calculated from Figure S8. Furthermore, we confirmed the air stability of the HSphototransistor for 14 days. After two weeks, the current value decreased from 6.07 x 10-9 A to 4.11 x 10-9 A, indicating that our HS structure is stable under ambient condition. Notably, the reduction rate of hole mobility is calculated as 13.3 % after 14 days as shown in Figure S13. We observed that the HS-ZnO NRs shows a highly efficient encapsulation effect by increasing the air stability of HS-1LWSe2 from contaminants such as O2 and H2O, as well as improved electrical properties by p-type doping effect.

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CONCLUSIONS In this study, we fabricated the HS comprising 1LWSe2 and ZnO NRs using the hydrothermal method. By analyzing the PL of HS-1LWSe2, we observed ~80 % PL enhancement compared with pristine

1LWSe . 2

These phenomena were attributed to antenna effect by ZnO NRs. In

addition, we fabricated a HS-phototransistor to investigate the electrical properties. The HSphototransistor demonstrated excellent properties including enhanced photoresponsivity, photodetectivity, and hole mobility. Interestingly, we observed an encapsulation effect from hydrothermally grown ZnO NRs on pristine

1LWSe . 2

As a result, the lifetime of HS-1LWSe2

persisted for two weeks because the upper ZnO NRs protected the HS-1LWSe2 from contaminants in the air. These advantages are expected to be applied as new methods to improve manufacturing efficiency and device performance in various 2D device fields.

EXPERIMENTAL SECTION Synthesis of ZnO NRs using hydrothermal method ZnO NRs were prepared using the hydrothermal method illustrated in Figure 1b. Initially, zinc acetate dihydrate (C4H6O4Zn ∙ 2H2O) (SIGMA-ALDRICH) and deionized water (DI water) were mixed to 0.05 M and stirred for 20 min to obtain the main solution. Simultaneously, C4H6O4Zn ∙ 2H2O and ethyl alcohol, Pure (C2H6O) (SIGMA-ALDRICH) were mixed to 5 mM and stirred for 20 min to obtain the seed solution. Next, the stirred seed solution was dropped on a cleaned SiO2/Si substrate. The sample was dried using a N2 gun after rinsing it in ethanol. Subsequently, the prepared sample was annealed using a hot plate of 300 °C for 20 min.70-71 The pH of the main solution was adjusted by a strong ammonia 13



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solution (NH4OH) (DAE JUNG) to pH.9. Lastly, the annealed sample was immersed in the prepared main solution and placed in an autoclave. The autoclave was maintained at 150 °C for 2 h and subsequently, cooled at room temperature. Figures 1c and 1d show the tilted and magnified top-view scanning electron microscope (SEM) images of well-grown ZnO NRs with average diameters of ~40 nm, respectively. As shown in Figure 1e, Raman spectroscopy using a 532 nm laser as an excitation source was used to confirm a clear E2 mode at ~438 cm1,

which indicates the crystallinity of ZnO.72 These synthesis conditions were obtained from

various experimental trials to fabricate the best ZnO NRs, as shown in Figure S2–S5 and Table S1. Fabrication of HS-phototransistor An illustration of the fabrication of the HS-phototransistor is shown in Figure S7. The chemical vapor deposition (CVD)-grown

1LWSe 2

flake was transferred by a conventional

wet-transfer method onto a cleaned SiO2/Si substrate (SiO2: 285 nm), as shown in Figures S1a and S1b.73-74 The observed average flake size was 100 µm  100 μm. Subsequently, ebeam evaporation was employed to deposit gold electrodes (Au: 100 nm). 1LWSe2 was etched into a rectangle using sulfur hexafluoride (SF6) and e-beam lithography to measure exact electrical properties. Subsequently, the PMMA was spin-coated on the prepared sample. Next, the open area was fabricated using e-beam lithography to grow ZnO NRs onto the pristine

1LWSe 2

phototransistor. Subsequently, we grew the ZnO NRs onto the prepared

sample by the hydrothermal method. The remaining PMMA was removed using acetone. Finally, the HS-phototransistor was completed, as shown in Figure 1a. Characterization methods 14


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Optical properties, such as Raman and PL, were analyzed by an optical microscopy system (NTEGRA SPECTRA, NT-MDT) using a 532 nm laser with a power of 0.18 mW, to avoid laser-assisted oxidation, and a 355 nm laser as the excitation source. The electrical measurements were performed using a vacuum probe system (4200, Keithley). To measure the photocurrents, a 638 nm excitation source was used. The surface morphology and structural properties of the samples were investigated by atomic force microscopy (AFM) (Esweep, Hitachi-Hightech), field-emission scanning electron microscopy (SEM) (JSM7000F, JEOL), and X-ray diffraction (XRD) using an X-ray diffractometer (Rigaku, SmartLab) with Cu-K radiation ( = 1.54059 Å) at 45 kV and 200 mA in a tube, respectively.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Author Contributions # Kang-Nyeoung

Lee and Seungho Bang contributed equally to this work.

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Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENT This work was supported by the Institute for Basic Science (IBS-R011-D1), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2016R1A2B2015581).

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Encapsulation of a Monolayer WSe2 Phototransistor with

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