Ultrasensitive Hybrid MoS2âZnCdSe Quantum Dot Photodetectors

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Surfaces, Interfaces, and Applications

Ultrasensitive hybrid MoS2-ZnCdSe Quantum Dots Photodetectors with High-Gain Shukui Zhang, Xudong Wang, Yan Chen, Guangjian Wu, Yicheng Tang, Liqing Zhu, Haoliang Wang, Wei Jiang, Liaoxin Sun, Tie Lin, Hong Sheng, Weida Hu, Jun Ge, Jianlu Wang, Xiangjian Meng, and Junhao Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03971 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Applied Materials & Interfaces

Ultrasensitive hybrid MoS2-ZnCdSe Quantum Dots Photodetectors with HighGain Shukui Zhang1,2, Xudong Wang1, Yan Chen1,2, Guangjian Wu1, Yicheng Tang1, Liqing Zhu1, Haoliang Wang1, Wei Jiang1,2, Liaoxin Sun1, Tie Lin1, *, Hong Sheng1, Weida Hu1, Jun Ge1, Jianlu Wang1, *, Xiangjian Meng1, *, and Junhao Chu1. 1State

Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics,

Chinese Academy of Sciences, 500 Yu Tian Road, Shanghai 200083, China 2University

of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China

*Corresponding author email: J. W. (email: [email protected]) T. L. (email: [email protected]) X. M. (email: [email protected])

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Abstract: Recently,

the

two-dimensional

materials,

especially

transition

metal

dichalcogenides (TMDCs) have attracted extensive interest for their potential applications in optoelectronics. Here, we demonstrate a hybrid 2D-0D photodetector, which consists of a single layer or few-layer molybdenum disulfide (MoS2) thin film and a thin layer of core/shell zinc cadmium selenide/zinc sulfide (ZnCdSe/ZnS) colloidal quantum dots (QDs). It is worthwhile mentioning that the photoresponsivity of the hybrid 2D-0D photodetector is three orders of magnitude larger than the TMDCs photodetector (from 10 AW−1 to 104 AW−1). The detectivity of the hybrid structure detector is up to 1012 Jones. And the gain is up to 105. Due to an effective energy transfer from photoexcite QDs sensitizing layer to MoS2 films, light absorption is enhanced and more excitons are generated. Thus, this hybrid 2D-0D photodetector takes advantage of high charge mobility in the MoS2 layer and efficient photon absorption/exciton generation in the QDs. Which suggests their promising applications in the development of TMDC-based optoelectronic devices. Keywords: MoS2, transition metal dichalcogenides, quantum dots, hybrid 2D-0D photodetector, energy transfer


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Introduction In the last decade, considerable attention has been paid to TMDCs results from their excellent electronic, mechanical, optical, thermal, and chemical properties like high in-plane carrier mobility, tunable bandgap that varies with thickness, mechanical flexibility, and photosensitivity.1-4 Such unique properties make the TMDCs becoming a promising nanomaterial for applications in the next-generation photodetectors5 and phototransistors.6-7 One typical semiconductor of TMDCs is MoS2, which can adjust its indirect bandgap of 1.2 eV to the direct bandgap of 1.8 eV by decreasing the thickness from bulk to monolayer, on the basis of quantum confinement.4 The properties of single layer MoS2 suggests that it has great potential as optoelectronic applications material. And photodetectors based on MoS2 have been intensively investigated.6-11 As a result of the low optical absorptivity for single layer and multi-layer MoS2, the atomically-thin thickness limits their optoelectronic performance and it is worthwhile devoting much effort to this. A common method of increasing the absorption of incident light is to couple this atomically thin material to various sensitive nanostructures. Colloidal QDs are a class of such excitonic nanomaterials. QDs are particularly interesting for light harvesting, as they exhibit large absorption covering the broad spectrum from the UV–vis to the NIR. The size-dependent bandgap of QDs can be matched with that of the TMDC for better energy transfer. And large photoluminescence (PL) emission quantum yields mean that the generation of exciton would be efficient.12-13 However, the confinement of charge carriers in QDs blocks their applications for photodetectors.14 Thus, the TMDCs-QDs hybrid combination (2D-0D) 3 / 27 ACS Paragon Plus Environment


integrates the advantages of strong light harvesting of the QDs and high carriers mobility of the TMDCs, paving the way for the next generation of the photodetector. However, the energy transfer (ET) mechanisms between QDs and 2D semiconductors are still not clear and considerable research efforts have been devoted to it.15-23 When electronically excited species, for instance, semiconductor nanocrystals, close to 2D semiconductors can thus be quenched result from the energy transferring to the 2D semiconductors, exciting electron-hole pairs.15 Generally, the transfer of energy between QDs and 2D semiconductor materials undergoing photoexcitation is possible driven by two interaction mechanisms: non-radiative energy transfer (NRET)15-21 and/or photoinduced charge transfer (CT).15, 22-23 Thus, combining QDs with TMDCs not only introduce new interactions at the TMDCs-QD interface such as NRET or CT from photoexcited QDs to the TMDCs, but it may also make for an enhanced optical absorptivity with extended spectral coverage. The interfacial ET between QDs and TMDCs can increase the generation of photocurrent in hybrid TMDCs-QDs photodetectors compared with TMDCs-only devices. Recently, photodetectors with a very high gain based on monolayer or few-layer graphene nanosheets and lead chalcogenide (PbS) QDs were reported.24-25 But graphene-based photodetectors have a high dark current problem due to its semi-metallic character. TMDCs, especially MoS2, doping with colloidal QDs for enhancing the light harvesting of the photodetectors have been intensively investigated.26-31 And the operation of these hybrid TMDCs-QDs photodetectors may achieve low dark current when the channel is in the depletion mode. Visible-wavelength optoelectronics devices usually use CdSe-based core-shell 4 / 27 ACS Paragon Plus Environment

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QDs,15, 17-23, 30 and NIR detectors are dominated by PbS QDs. 24-26, 31 In this letter, we study a hybrid TMDCs-QD photodetector consisting of monolayer or few-layers MoS2 and core/shell ZnCdSe/ZnS QDs. We find that MoS2 effectively quenches QD fluorescence in this hybrid device. And the hybrid MoS2-QDs devices responsivity are three of magnitude higher than pristine MoS2 nanosheet devices. We also investigated the relationship between the performance of hybrid MoS2-QDs devices and the thickness of MoS2 channels. Then we found the photocurrent enhancement of all the original single layer and multilayer photodetectors after adding QDs sensitizing layer. However, the photocurrent enhancements decrease as the thickness increases. This indicates that the efficiency of NRET or CT from the QDs is inversely proportional to the thickness of MoS2. Results and Discussion Devices based on a single layer and few-layer MoS2 were fabricated on Si/SiO2 (280nm) substrates using microexfoliation techniques. The single layer and few-layer MoS2 flakes were first identified by optical contrast measurements, then confirmed with Raman spectroscopy and atomic force microscopy (AFM). The source and drain chromium /gold electrodes were evaporated in thicknesses of 5 nm for Cr and 45 nm for Au. The following step was coating the QDs film on MoS2 nanosheet devices and then the QDs-oleic acid ligands were exchanged by tetrabutylammonium iodide (TBAI). The device fabrication is described in detail in the Experimental Section. The optical images of hybrid monolayer MoS2-QDs photodetector can be seen from Figure 1a. The height profile indicates that the thickness of a single layer MoS2 is around 0.65 5 / 27 ACS Paragon Plus Environment


nm.4 The uniform color distribution of hybrid system shows the conformal coating of QDs over the entire MoS2 photodetector. The inset in Figure 1a is the optical images of monolayer MoS2 photodetector before adding QDs sensitizing layer. The 3D schematic view of the hybrid 2D-0D photodetector device structure is shown in Figure 1b. The SEM images of hybrid 2D-0D device and pristine MoS2 device can be seen in supporting information Figure S2. Figure 1c shows the schematic of the MoS2-QD interface. The diameter of ZnCdSe QDs cores is 5.5 nm and the thickness of ZnS shell is 1.5 nm (according to the manufacturer). In this hybrid device, QDs are light harvesters and MoS2 is carrier transport channel. The reason why QDs sensitizing layer enhances the photocurrent of the photodetector is speculated to be driven by CT (Figure 1c, the black arrow), NRET (Figure 1c, the red dashed arrow), or both. In core-shell systems we hope that the shell acts as a barrier can be overcome by tunneling so that the injection can take place.32 In term of CT, electrons transfer from photoexcitation QDs sensitizing layer to MoS2 through tunneling, generating the photocurrent. Another viable mechanism between donor (QDs) and acceptor (MoS2) is NRET. Under these circumstances, excitons are first generated on the QDs sensitizing layer, and then the exciton’s energy is nonradiatively transferred to MoS2 film through a dipole-dipole coupling,15-21 creating new electron-hole pair on the MoS2 film. As shown in Figure 2a, the Raman spectra of all film thicknesses show that there are two peaks represent the in-plane vibration mode (E12g) and the out-of-plane vibration mode (A1g) respectively.33-34 The difference (Δ = A1g – E12g) between these 6 / 27 ACS Paragon Plus Environment

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modes in the MoS2 monolayer, bilayer, and tri-layer are 19.1cm−1, 22.1 and 23.6 cm−1 respectively. We find that the E12g vibration softens (redshifts) while the A1g vibration stiffens (blueshifts) with increasing the number of layers. This observation results from Coulomb interactions and possible intra-layer bonding changes caused by stacking. This phenomenon can be used as an indicator to confirm the sample thickness. As shown in Figure 2b, PL spectra indicate that MoS2 has a strong inhibition of photoluminescence on QDs when this 2D material close to QDs. Here, we introduce a quenching factor Q to quantify the quenching effect, which is given by Q = IQD/IMoS229

QD.

IMoS2-QD is the height of the QD PL peak at 1.987 eV for the hybrid monolayer

MoS2-QD device (acquired at a point as the magenta arrow marked in Figure 2b), and IQD is the height of the same peak from QDs away from MoS2. We calculate Q ∼ 30.2 from the data shown in Figure 2b. The quenched PL indicates that the QDs next to MoS2 exists an additional nonradiative relaxation channel. We believe that this pathway is attributed to NRET. In principle, CT can also cause nonradiative relaxation. During the spin-coating step, the QDs underwent ligand exchange in order to let electrons easier transfer from QDs to the MoS2 film by tunneling. It is worthwhile mentioning that the transfer characteristics of the monolayer MoS2-QDs photodetector have changed dramatically, as illustrated in Figure 2d. The threshold voltage becomes lower and Isd is 55 times higher than before at Vbg = 30 V. MoS2 is an n-type semiconductor, after adding QDs sensitizing layer the dark current increases indicating that CT can happen on the MoS2-QD interface. As shown in Figure 2c, the bandgap of the monolayer nanosheet determines the spectral responsivity of a 7 / 27 ACS Paragon Plus Environment


pristine MoS2 phototransistor spectral sensitivity. The hybrid detector enhanced responsivity following the absorption spectrum of MoS2. To evaluate the optoelectronic performance of hybrid devices compared with the pristine MoS2 devices, we measured the performances of monolayer MoS2 device before and after adding QDs sensitizing layer. The electrical and photodetection properties were measured in ambient conditions. Figure 3a shows the cross-sectional schematic diagram of this hybrid device. Under the same optical power illumination (λ = 450 nm, P = 400 nW), the photocurrent of the hybrid monolayer MoS2-QDs photodetector is 32 times (from 0.56 µA to 17.86 µA, Figure 3b) higher than the pristine monolayer MoS2 device. As can be seen from parts c and d of Figure 3, the rise time of the hybrid devices is reduced from 15 s to 0.3s after adding QDs. The increasing rate (ΔIph/∆t) of drain current possibly results from carrier concentration increasing. And the fall time is also reduced. It is commonly known that photoexcited carrier direct recombination makes the decay time faster. At the same time, sub-bandgap emissions, which is on account of the charged impurity and trap states inside the bandgap of MoS2, lead to slower decay time.7, 35 Due to the efficient CT between MoS2 and QDs, the decay time is greatly shortened. We then study the performance of hybrid MoS2-QDs devices with different thickness MoS2 channels. We first fabricate four difference thickness MoS2 devices (D1, D2, D3, and D4), including monolayer MoS2 device (D1), bilayer MoS2 device (D2), tri-layer MoS2 device (D3) and multilayer MoS2 device (D4), respectively. The thicknesses of MoS2 layers were established by AFM (Figure S1 a-d, Supporting 8 / 27 ACS Paragon Plus Environment

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Information). We investigated the Isd – Vsd curves for dark and under the illumination (over a range of optical powers from 10-11 W to 10-6 W). And the Isd – Vsd curves of all devices are linear or close to linear. (Figure S1 e-f, Supporting Information). The comparison of photoelectric detection performance between hybrid devices and original MoS2 devices were described in Figure 4. As a function of laser excitation power, the photocurrent (Iph) of all devices was plotted in Figure 4a. It is clear that the photocurrent of pristine MoS2 devices increases with the increase of MoS2 thickness. The reason for this phenomenon is that the light absorption rate of MoS2 is positively correlated with the thickness of MoS2.36 That is to say, under the same illumination conditions, thick pristine MoS2 devices can absorb more photons and produce more electron-hole pairs than thin devices. It is worth pointing out that after combining with QDs, the photocurrent of all hybrid photodetectors is three orders of magnitude larger than the pristine MoS2 photodetectors (from 10-9 A to 10-6 A). Obviously, we can obtain the largest photocurrent enhancement from the D1, which uses a single layer MoS2 as the channel. And the photocurrent enhancement of increase in D2, D3, and D4 decreases in turn. Furthermore, we calculated the responsivity of all devices given by R = Iph/P, where Iph is the photocurrent and P is the effective incident laser power. 9 In addition, the detectivity, assuming that noise from dark current is the major factor, it is given by D* =RA1/2/(2eIdark)1/2, where A is the effective detection area of the device, e is the unit charge, and Idark is the dark current. 6-11 Thus, the laser power dependence of R and D* are shown in Figure 4b, c, respectively. We found that the responsivity of the hybrid 9 / 27 ACS Paragon Plus Environment


MoS2-QDs devices decreases with the increase of irradiation. It is clear that the largest responsivity for all devices is achieved at the lowest excitation power (~7×10-11 W) for both the MoS2 devices only (R ~10 AW−1) and the hybrid 2D-0D devices (R ~ 104 AW−1). The calculated R of all hybrid devices give more than 1000-fold enhancement in responsivity compared to that of the pristine MoS2 devices (from 10 AW−1 to 104 AW−1). The reason why R exponentially decreases with the laser power increases is that the scattering and recombination increase. Similar to R, the maximum value of D* is also obtained on D1 after adding QDs sensitizing layer, which is 206 times that of the pristine MoS2 counterpart (from 4.6 × 109 Jones to 1.0×1012 Jones). And the D* of the D4 is enhanced by a factor of 11 (from 8.7 × 109 Jones to 9.9 × 1010 Jones). The gain G is calculated in order to evaluate the performance of the devices further, which is given by R = Iph/Peff = ηGq/hυ,7 where q is the electron charge, η is the external quantum efficiency, υ is the frequency of the incident laser and h is Planck's constant. Assuming η = 100%, the calculated Gbefore value of varying layer thicknesses pristine MoS2 devices and after adding QDs Gafter are as shown in Table 1. From Table 1, we can see that the Gafter values are ultrahigh compared to Gbefore, which may primarily due to the ET effect. We can see that QDs as light harvesters can highly enhance the performance of different thickness of MoS2 phototransistors. To calculate the performance enhancement of each device, we introduce two enhancement factors. The enhancement factors are then calculated as RMoS2+QDs/RMoS2 and D*MoS2+QDs/ D*MoS2. As can be seen from Figure 4d, the largest enhancement in R and D* is D1, the monolayer device. And 10 / 27 ACS Paragon Plus Environment

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the enhancement of R and D* for D2, D3, and D4 decrease in turn. It is important to highlight that performance enhancements decrease as the thickness increase. This indicates that monolayer devices doping the QDs is most effective while benefiting from the QD sensitive layer decreases with the number of layers of MoS2 increases. Comparing with reported device performance with similar device structure, our data were among the top level and the related works were summarized in Table 2. The core/shell QDs energy band is type I, and the core main ingredient is CdSe (according to the manufacturer). According to ref 37, the ECB and EVB of 5.5nm size QDs

core CdSe are −3.4 eV and −5.4 eV, respectively. Figure 5a shows the schematic of a core/shell heterostructure and the energy band diagram of core/shell QDs type I. And the schematic band diagrams of monolayer, bilayer, tri-layer and bulk MoS2 are shown in Figure 5b.8 The conduction and valence band edge of monolayer MoS2 locations of ECB = −4.27 eV and EVB = −6.12 eV, respectively29. It is complicated to understand the light-matter interaction between MoS2 and QDs because the physical processes of strongly absorbing and emitting light can both occur in MoS2 and QDs. There are at least three mechanisms at the interface (Figure 5c). First, under the luminescence, both QDs sensitizing layer and MoS2 films absorb photons, creating electron-hole pairs. Subsequently, the photoexcited carriers are efficiently generated in the QDs layer and separated on the van der Waals heterostructure between MoS2 and QDs. The photogenerated elections in QDs layer transfer to MoS2 through a tunneling process (process II) and is cycled under the drive of an applied electric field of Vsd, which may contribute more carriers to increase the photocurrent. In the meantime, the excitons can 11 / 27 ACS Paragon Plus Environment


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transfer from QDs to MoS2 via NRET processes, adding the MoS2 channel’s electronhole pairs. We turn now to the layer thickness dependence of enhancement. The thin film geometry enhanced NRET because of reduced dielectric screening of in-plane components of the donor dipole field38. This geometric effect is accentuated in acceptor media with in-plane oriented transition dipole moments39. This effect certainly exists in the atomic thickness MoS2 nanosheets. So NRET from the QDs sensitive layer to a MoS2 monolayer is more efficient than NRET to the bulk. Conclusion In

summary,

we

have

demonstrated

high-performance

hybrid

2D-0D

photodetectors based on MoS2 and ZnCdSe/ZnS QDs. The designed monolayer hybrid MoS2-QD photodetector performs most effective compared with other thickness devices, with a gain >105, specific detectivity of 1.0 × 1012 Jones, and photoresponsivity up to 3.7 × 104 AW–1. After combining with QDs, photocurrent enhancements are 4620 times (from 7.64 × 10-10 to 2.74 × 10-6) more than the pristine monolayer MoS2 device and about as large as 830-fold (from 9.49 × 10-9 to 7.89 × 10-6) enhancement for bulklike MoS2 device. Furthermore, the photocurrent bilayer and tri-layer hybrid devices are 1347 and 1142 times larger than the pristine MoS2 device, respectively. It indicates that the performance of the hybrid MoS2-QDs photodetector becomes better while the MoS2 thickness decreases. More importantly, we have demonstrated a low-cost way to optimize the photodetection performance of 2D materials with the perfect optoelectronic advantages, took from TMDCs based photodetectors.


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EXPERIMENTAL SECTION Device

Fabrication:

The

monolayer

and

few-layer

MoS2

flakes

were

micromechanically exfoliated with polydimethylsiloxane (PDMS) tape on top of a Si/SiO2 (285nm). Metal contacts were fabricated by electron beam lithography, Cr (15 nm) and Au (45 nm) electrodes were evaporated by thermal evaporation. The devices were then annealed at 200 °C in the vacuum with 100 sccm Argon atmosphere for 2 h to release the adsorbate and decrease contact resistance. QD Film Deposition: Alloyed ZnCdSe/ZnS QDs solution (30 mg mL-1) in normal octane were purchased from WuHan JiaYuan Quantum Dots Corporation., LDT. The ZnCdSe/ZnS QDs thin film was spin-coated at a rotation speed of 2500 rpm. TBAI was used as ligand exchange. Normal octane and acetonitrile were used to rinse the device after QDs and TBAI deposition. It should be noted that the spin-coated QDs and ligand exchange processes were accomplished inside an N2-filled glove box. Photoelectrical measurements: The electric and optoelectric measurements were performed in ambient conditions by using an Agilent B2902A semiconducting device analyzer. The Raman spectra and PL spectra were acquired by the Lab Ram HR800.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supporting information includes drain-source characteristic of hybrid photodetectors 13 / 27 ACS Paragon Plus Environment


under different illuminating light powers (Figure S1), the SEM images of pristine MoS2 photodetector and hybrid MoS2-QDs photodetector (Figure S2). AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was partially supported by the Natural Science Foundation of China (Grant Nos. 61674158, 61722408, 61835012, 61474131 and 61574152), the Major State Basic Research Development Program (Grant Nos. 2016YFB0400801, 2016YFA0203900), Key Research Project of Frontier Sciences of Chinese Academy of Sciences (QYZDY-SSW-JSC042, QYZDB-SSW-JSC016) and Natural Science Foundation of Shanghai (Grant No. 16ZR1447600, 17JC1400302)


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F. J.; Koppens, F. H., Universal Distance-Scaling of Nonradiative Energy Transfer to Graphene. Nano Lett 2013, 13, 2030-2035. 17. Raja, A.; Montoya Castillo, A.; Zultak, J.; Zhang, X. X.; Ye, Z.; Roquelet, C.; Chenet, D. A.; van der Zande, A. M.; Huang, P.; Jockusch, S.; Hone, J.; Reichman, D. R.; Brus, L. E.; Heinz, T. F., Energy Transfer from Quantum Dots to Graphene and MoS2: The Role of Absorption and Screening in Two-Dimensional Materials. Nano Lett 2016, 16, 2328-2333. 18. Goodfellow, K. M.; Chakraborty, C.; Sowers, K.; Waduge, P.; Wanunu, M.; Krauss, T.; Driscoll, K.; Vamivakas, A. N., Distance-Dependent Energy Transfer between CdSe/CdS Quantum Dots and a Two-Dimensional Semiconductor. Applied Physics Letters 2016, 108, 021101. 19. Prasai, D.; Klots, A. R.; Newaz, A. K.; Niezgoda, J. S.; Orfield, N. J.; Escobar, C. A.; Wynn, A.; Efimov, A.; Jennings, G. K.; Rosenthal, S. J.; Bolotin, K. I., Electrical Control of near-Field Energy Transfer between Quantum Dots and Two-Dimensional Semiconductors. Nano Lett 2015, 15, 4374-4380. 20. Prins, F.; Goodman, A. J.; Tisdale, W. A., Reduced Dielectric Screening and Enhanced Energy Transfer in Single- and Few-Layer MoS2. Nano Lett 2014, 14, 60876091. 21. Zang, H.; Routh, P. K.; Huang, Y.; Chen, J. S.; Sutter, E.; Sutter, P.; Cotlet, M., Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to SingleLayer and Few-Layer Tin Disulfide. ACS Nano 2016, 10, 4790-4796. 22. Boulesbaa, A.; Wang, K.; Mahjouri-Samani, M.; Tian, M.; Puretzky, A. A.; Ivanov, 17 / 27 ACS Paragon Plus Environment


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Figures and captions

Figure 1. Fabrication and structure of hybrid MoS2-QDs photodetector. (a) Optical image of monolayer MoS2-QDs photodetector, QDs cover the entire device surface. The inset in Figure1a is the optical images of pristine MoS2 photodetector before adding QDs. (b) Three-dimensional schematic view of hybrid MoS2-QDs photodetector. (c) Schematic of MoS2-QDs interface. Under illumination, electrons transfer from QDs to MoS2 via tunneling (black arrow) and excitons transfer from QDs to MoS2 via NRET processes (red dash arrow).



Figure 2. The photodetector performance comparison of hybrid MoS2-QDs device and pristine MoS2 device. (a) Raman spectra of the monolayer (1L), bilayer (2L), trilayer (3L) and multilayer (mL) MoS2 on SiO2/Si substrates. (b) PL spectra of 1L MoS2-QD hybrid (red) and of bare QD film (blue). The spectra were recorded from the same device. (c) Response to illumination as a function of the wavelength of monolayer pristine MoS2 device and monolayer hybrid MoS2-QDs device (Vsd = 1V, Vbg = 0 V and P = 4 µW). (d) Typical transfer curves of MoS2 and MoS2-QDs transistor devices in the dark.


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Figure 3. (a) Cross-sectional view of the device operation under illumination. (b) I−V curves of monolayer pristine MoS2 device and monolayer MoS2-QDs hybrid device with and without 450 nm illumination under Vbg = 0 V. (c, d) Time-dependent photoresponse of pristine MoS2 device and hybrid MoS2-QDs devices.



Figure 4. (a-c) Photocurrent, responsivity and detectivity as a function of excitation power for the four devices (D1-red line, D2-green line, D3-blue line, D4-magenta line) before and after the addition of the QD sensitizing layer (before-open symbol, aftersolid symbol). (d) Responsivity (blue symbol) and Detectivity (red symbol) enhancement for each thickness. The dashed line in (d) is a guide to the eye.


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Figure 5. (a) Schematic of a core-shell heterostructure and type I core-to-shell bandedge alignments. (b) The schematic band diagrams of monolayer, bilayer, tri-layer, and bulk MoS2. (c) Energy diagram of the interface between MoS2 and QDs after the formation of a heterojunction. Three photoelectrical processes are proposed: I, Photons excitation in MoS2 and the QDs; II, Electrons transfer from the QDs to MoS2 via tunneling; III, Excitons transfer from the QDs to MoS2 via NRET processes.



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Table 1. The gain of each device before and after adding QDs Device

D1

D2

D3

D4

Gbefore

22.1

80.1

82.9

66.3

Gafter

1.02 ×105

1.08 ×105

9.47 ×104

4.65 ×104

Table 2. Performance comparisons for 0D/2D hybrid phototransistors and their individual counterparts Reference

Dark current

Spectral coverage

Responsivity

Decay time

[A]

[nm]

[A/W]

[s]

MoS2

[5]

2×10-12[VG=-70V]

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