Dependence of Photoresponsivity and On/Off Ratio on Quantum Dot Density in Quantum Dot Sensitized MoS2 Photodetector

Abstract

Non-radiative energy transfer (NRET) from quantum dots (QDs) to monolayer MoS2 has been shown to greatly enhance the photoresponsivity of the MoS2 photodetector, lifting the limitations imposed by monolayer absorption thickness. Studies were often performed on a photodetector with a channel length of only a few μm and an active area of a few μm2. Here, we demonstrate a QD sensitized monolayer MoS2 photodetector with a large channel length of 40 μm and an active area of 0.13 mm2. The QD sensitizing coating greatly enhances photoresponsivity by 14-fold at 1.3 μW illumination power, as compared with a plain monolayer MoS2 photodetector without QD coating. The photoresponsivity enhancement increases as QD coating density increases. However, QD coating also causes dark current to increase due to charge doping from QD on MoS2. At low QD density, the increase of photocurrent is much larger than the increase of dark current, resulting in a significant enhancement of the signal on/off ratio. As QD density increases, the increase of photocurrent becomes slower than the increase of dark current. As a result, photoresponsivity increases, but the on/off ratio decreases. This inverse dependence on QD density is an important factor to consider in the QD sensitized photodetector design.

WS2/WSe2 Nanodot Composite Photodetectors for Fast and Sensitive Light Detection

“…The WS 2 /WSe 2 composite photodetector exhibits a high maximum I on / I off of 2.5 × 10 5 , which is 10 3 times higher than that of the pristine WS 2 photodetector under the same applied V gs and irradiance, designated as a sensitive switch for the practical applications. The enhanced I on / I off ratio is due to the suppressed dark current by the WS 2 /WSe 2 nanodot heterostructure, which is different from photodetectors with a QD coating, where the I on / I off ratio decreases with the increase in QD density . Most importantly, the low off-current level in the absence of light can enhance the sensitivity of the photodetector in view of detectivity D * while operating in the depletion mode.…”

“…The enhanced I on /I off ratio is due to the suppressed dark current by the WS 2 /WSe 2 nanodot heterostructure, which is different from photodetectors with a QD coating, where the I on /I off ratio decreases with the increase in QD density. 30 Most importantly, the low off-current level in the absence of light can enhance the sensitivity of the photodetector in view of detectivity D* while operating in the depletion mode. The detectivity of a photodetector is express as…”

“…27,28 It is known that the excitonic behaviors of the 0D-2D heterostructure are affected by the size, density, and quality of the QDs, which determine the performance of the photodetector. 29,30 Although the large-area QDs can be synthesized by the sonication method, intercalation reaction, and hydrothermal method, 31,32 the toxicity, stability, and timeconsuming process of the QDs limit its application for optical devices. 33,34 In addition, the increased dark current, owing to the leakage path induced by QDs, detriments the detectivity of the photodetector.…”

“…The QDs function as a photosensitizer, which transfers massive photoexcited electrons to the channel and simultaneously photogates the underlying channel by means of the trapped photoexcited holes. MoS 2 photodetectors thus made have shown responsivity as high as 10 6 A/W. , Yet, some QDs/TMD hybrid photodetectors suffer from degraded operating speed, depending on the trap states introduced by QDs. , It is known that the excitonic behaviors of the 0D-2D heterostructure are affected by the size, density, and quality of the QDs, which determine the performance of the photodetector. , Although the large-area QDs can be synthesized by the sonication method, intercalation reaction, and hydrothermal method, , the toxicity, stability, and time-consuming process of the QDs limit its application for optical devices. , In addition, the increased dark current, owing to the leakage path induced by QDs, detriments the detectivity of the photodetector. Even this issue can be solved by the isolation process, surface damages might be induced during the patterning procedure. , Thus, chemical vapor deposition (CVD) seems to be the proper way to synthesize large-scale QDs with controllable size and density, which is suitable for integration with high yields. , A graphene-Bi 2 Te 3 heterostructure photodetector with Bi 2 Te 3 nanocrystals directly grown on monolayer graphene has been reported, which exhibits an enhanced photoconductive gain .…”

Interfacial Charge Transfer Induced Enhanced Near-Infrared Photoluminescence and Enhanced Visible Photodetection in Two-Dimensional/Zero-Dimensional Bi2Se3/CsPbBr2I Heterojunctions with Type-I Band Alignment

“…The on/off ratio does not increase significantly as the dark current is also enhanced because of charge transfer from the perovskite NCs to Bi 2 Se 3 NSs. 43 To demonstrate the broadband photoactivity of the hybrid device, have studied the temporal response with 532, 640, and 808 nm laser excitations. The on/off ratios as seen from the data in Figure S6a (SI) are ∼9, 5, 3.…”