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Balanced Photodetection in Mixed-Dimensional Phototransistors Consisting of CsPbBr Quantum Dots and Few-Layer MoS 3

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Richeng Lin, Xubiao Li, Wei Zheng, and Feng Huang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00558 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Nano Materials

Balanced Photodetection in Mixed-Dimensional Phototransistors Consisting of CsPbBr3 Quantum Dots and Few-Layer MoS2 Richeng Lin, Xubiao Li, Wei Zheng*, Feng Huang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials, Sun Yat-sen University, Guangzhou 510275, China. E-mail: [email protected].; [email protected] Abstract:

Balanced

photodetection

endows

photoconductive

detectors

with

high

photoresponsivity and fast photoresponse speed. Here, we report a mixed-dimensional phototransistor consisting of CsPbBr3 colloidal quantum dots and few-layer MoS2, which is capable of balanced photodetection. In the heterojunction, the generation region and the transport channel of carriers are separated by the built-in electric field. Thus, the device exhibits a high photoresponsivity of 104 A/W and a fast photoresponse speed of 8 ms. All these results demonstrate that the mixed-dimensional phototransistor is successful in balanced photodetection, and it has great application potential in communication and imaging. KEYWORDS: balanced photodetection, colloidal quantum dots, molybdenum disulfide, cesium lead bromine, phototransistor. Balanced photodetection refers to the balance between high photoresponsivity and fast photoresponse speed in a photoconductive detector, which is required for practical communication and imaging devices.1 Generally speaking, high photoresponsivity (gain or quantum efficiency) is conducive to the detection of weak signals, while fast photoresponse speed ensures that the output of detection signals is undistorted. Traditional photoconductive detectors generally have high photoresponsivity but slow photoresponse speed, which results from the persistent photoconductive effect, that is, when holes (electrons) are trapped, the lifetime of electrons (holes) will be longer and thus the photocurrent can be increased by multiple electron (hole) circulations in the circuit, leading to high quantum efficiency (high photoresponsivity). However, this process inevitably causes longer decay time of photocurrent, which means slower photoresponse speed.2 Recently, two-dimensional (2D) crystals with high carrier mobility (e.g., graphene, TMDs and black phosphorus) have achieved rapid photoresponse speed in various optoelectronic ACS Paragon Plus Environment

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devices. However, due to the limited optical absorption of nanometer-thickness 2D crystals, the photoresponsivity is generally not high enough.3-5 Surface treatment of 2D crystals (introducing appropriate surface trap states) and fabrication of heterojunction are two of the effective methods to enhance the photoresponsivity.6-8 If we introduce zero-dimensional (0D) colloidal quantum dots (CQDs, with long carrier lifetime) as absorbers to increase the optical absorption of 2D photodetectors, and simultaneously ensure that the sacrifice of photoresponse speed is as small as possible, then the balanced photodetection will be achieved. In this work, 2D MoS2 crystals with high mobility and CsPbBr3 CQDs with high absorption coefficient are used to construct a mixed-dimensional phototransistor. The MoS2/CsPbBr3 heterojunction separate the generation region and the transport channel of carriers.9 In photosensitive CsPbBr3 CQDs, the lifetime of photo-generated carriers can reach 200-300 ns, which makes the carriers accumulate more effectively. These photo-generated carriers can be further separated by the built-in electric field in the heterojunction, and then injected into the MoS2 transport channel, and finally collected by electrodes. On this basis, the device successfully achieves balanced photodetection with high photoresponsivity of 104 A/W and fast photoresponse speed with decay time of 8 ms, comparable to the best value reported.1013

These results demonstrate that the mixed-dimensional phototransistor is able to meet the

balance between high photoresponsivity and fast photoresponse speed (balanced photodetection), and thus provides a new strategy for preparing practical communication and imaging devices. The stable CsPbBr3 CQDs with strong light emission are synthesized by means of typical solution injection. We note that both synthesis and characterization are conducted under atmospheric environment (Figure 1a). In order to observe the crystal structure of synthesized CQDs, we cast the sample on a copper net and thereby obtain the high resolution transmission electronic microscope (HRTEM) images. As shown in Figure 1b, the CQDs have a typical tetragonal crystal structure which has a plane distance of about 0.45 nm, corresponding to the (001) plane of CsPbBr3. Also, the X-ray diffraction (XRD) patterns of CQDs confirm that the sample is a tetragonal CsPbBr3 nanocrystal, which shows that the signal peaks correspond to characteristic (001) and (002) planes of CsPbBr3 (Figure 1c). Figure 1d clearly shows that the size of CsPbBr3 CQDs are about 10 nm. The statistics analysis of the size of CQDs reflected in

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TEM images presents a quasi-gauss function, indicating that the synthesized CsPbBr3 CQDs are uniform in size (Figure 1e). It is known that the CQDs-based photodetectors usually possess ultrahigh gain owing to the long lifetime of photo-generated carriers.14 The mechanism is described in Figure 1f. For a nanoscale QD, its electronic structure can be descripted to a quantum-confined bandgap. Surface effects produce abundant surface states in the inner bandgap, resulting in a long-lived net charge of the QD.15 When light irradiates the device, the incident photons excite electronhole pairs in the CQDs. Due to the quantum-confined bandgap, only one kind of the carriers (e.g. electrons) can escape from the QDs under the external electric field, while other kinds of the carriers (e.g. holes) are captured by the surface states in the QDs, also similar to band-like transport. The effective separation reduces the recombination probability of excited electronhole pairs, as well as significantly increases the lifetime of the carriers. When the CsPbBr3 CQDs are irradiated by ultraviolet light, the CQDs have bright and stable luminescence (Figure 1g). The photoluminescence peak of CsPbBr3 CQDs is located at 514 nm. From absorption spectrum, the band gap of the QDs can be estimated to be about 2.4 eV, which is consistent with that in previous reports. In Figure 1h, the time-resolution photoluminescence spectrum (TRPL) shows the long lifetime (~12 ns) of the CsPbBr3 CQDs solid film, which also agrees with the above discussion. Figure 2a is a schematic diagram of CQDs device fabrication. The obtained CQDs are coated on a field effect transistor which has a back-gate structure with few-layer MoS2 as the channel. The CQDs film is subsequently annealed to form a stable and solid photosensitive thin film. Figure 2b is a picture of the devices before and after coating the CsPbBr3 CQDs, presenting an obvious color change. The Raman spectrum of 2D MoS2 excited by a 633 nm laser shows typical E2g and A1g model peaks of MoS2 crystal (Figure 2c).16 Figure 2d and 2e are the comparison of I-V characteristics between the MoS2 transistor and the CsPbBr3-CQDs/MoS2 device. In Figure 2d, the I-V characteristics show that the MoS2 transistor has an on/off ratio of ~ 103, and MoS2 has n-type conductivity. The on/off ratio is suppressed by increasing the source-drain voltage (Vds), and the source-drain current (Ids) is correspondingly enhanced. When the device is under negative gate voltage (Vgs), electrons in MoS2 channel are depleted by the gate electric field and the MoS2 transistor obtains a large



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device resistance and a low cut-off current. The field-effect electronic motilities of MoS2 is extracted from the above I-V characteristics to obtain a maximum value of about 0.13 cm2V1s-1,

which is larger than that of CQDs in previous reports.14,

17

Figure 2e shows the I-V

characteristics of the composite CsPbBr3 CQDs/MoS2 device. It can be seen that the switch ratio of the device is about 102, which is slightly lower than that before the deposition of CQDs. This is because the CsPbBr3 CQDs solid film has increased the thickness of depletion layer, which makes it difficult to deplete the electrons in MoS2 channel. The transfer curve of the CQDsMoS2 device shows that the Ids obviously changes with Vgs. It can be seen that the cut-off, linear and saturation characteristics of the device are significant, which may be attributed to the passivation of the MoS2 surface induced by the coating CsPbBr3 CQDs. Judging from the I-V characteristics, the performance of the device coated with the CsPbBr3 CQDs has been potentially improved for photon detection. To observe detection performance of the device, photoresponse measurement of the composite CsPbBr3-CQDs/MoS2 device is conducted through a source meter and a 405 nm laser excitation. Figure 3a shows the sectional operation view of the device. When the device is under the illumination of 0.5 μW, the photocurrent mapping with Vds and Vgs shows typical photoresponse of a phototransistor, as shown in Figure 3b. The inset shows the mapping image of the dark current. The Ids-Vgs curves of the device under illumination of different optical power show that the switch ratio of the composite device still reaches 3.8×102 under illumination (Figure 3c). Under the bias Vg of -20 V and optical power of 12.8 μW, the maximum photocurrent of the device is about 120 nA (Figure 3d). The photoresponsivity (R) is an important figure of merit that characterizes the detection capability of a detector. It can be calculated from the formula R=Iph/PS, in which Iph refers to the photocurrent, P is the light power density and S the active area.18-19 In the case of our device, the incident light is a circular spot with diameter of 3 mm and power density of 12.8 μW/cm2. The active area is a rectangular flake with 4 μm (wide)×5 μm(length), as shown in Figure 2b. Thus, the photoresponsivity of the device is calculated to be 4.68×104 A/W (see Table 1). The gain of the device is up to 1.4×105, which can be compared with that of the best photoconductive device.6-7, 11, 20 Such ultrahigh photoresponsivity is much larger than that of photovoltaic detectors.18, 21 The high gain can be attributed to the long carrier lifetime of the CsPbBr3 CQDs absorber and the fast


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carrier transport in the high-mobility MoS2 channel. In order to clearly illustrate the photoresponse of the device, the relationship between the photocurrent (∆I) /photovoltage (∆V) and the light power (see Figure 3e and 3f) are analyzed.22 In Figure 3e, the extracted photocurrent ∆I at different light power and the fixed Vg can be well fitted by a logistic function, which means that the device has favorable linear performance for light power. Similarly, the extracted photovoltages ∆V under the fixed Ids show a weak linear relationship with light power, which includes a linearly increasing process and a gradually saturated process. The results are consistent with the performance of a typical photovoltaic device. In fact, both gain and photoresponse speed are needed in practical photodetection application. The gain is defined by the ratio of carrier lifetime to transit time, which is calculated from the formula G = lifetime (τ)/ transit (t).23 From the equation, the gain is proportional to the carrier lifetime, and is opposite to the carrier transit time. Usually, the CQDs have long carrier lifetime, which benefits from the accumulation of trap assisted carriers. Although the long carrier lifetime contributes to a high gain, it also leads to the persistent photoconductive effect, which dramatically reduces the response speed. High-density grain boundaries of the CQDs solid photosensitive film usually cause strong electron scattering effects, which reduces the carrier mobility and increases the transit time. These properties make the photoconductors usually have slow response speed. Figure 4a shows a design concept for the balance between gain and response speed: the mixed-dimensional heterojunction is constructed by zero-dimensional CQDs with long carrier lifetime and two-dimensional MoS2 with high mobility. CQDs and MoS2 are used as photon absorber and carrier transport channel respectively. Such a heterojunction can effectively separate the generation process and the transport process of carriers, which is described in the energy band alignment diagram (Figure 4b). Thus, long carrier lifetime and short transit time are acquired, and the photoconductive gain has been significantly improved. The photoresponsivity of the CsPbBr3-CQDs/MoS2 device is obvious larger than that of the MoS2 device (Figure 4c), which is consistent with the above discussions. Response speed is one of important indexes of photodetectors. In Figure 4d and 4e, the time-dependent current curves of the CsPbBr3-CQDs/MoS2 device show rapid transient



response with a rise time of 7.5 ms and a decay time of 8.0 ms. Such response speed is two orders of magnitude faster than that in previous reports.11 Compared with traditional photoconductive devices, the device also has faster photoresponse speed. In the meantime, the continuous transient response of the device shows excellent stability and repeatability. The fast photoresponse speed can be attributed to the subtle device design, which adopts the CQDs/MoS2 composite heterostructure. Benefiting from the built-in electric field between CsPbBr3 CQDs and MoS2, photo-generated electrons can be effectively separated from the QDs and injected to the MoS2 channel, and finally be collected by electrodes. The high-mobility MoS2 provides a fast transport channel for injected electrons, which makes the device have fast photoresponse speed. In summary, a mix-dimensional phototransistor with high photoresponsivity and fast photoresponse speed is fabricated. Such excellent performances benefit from the 0D-2D heterojunction of the device, which is constructed by CsPbBr3 CQDs and few-layer MoS2. The heterojunction enables the device to take full advantage of high carrier mobility of 2D MoS2 and large absorption of 0D quantum dots. Therefore, the device achieves a balance between photoresponsivity and photoresponse speed, which has a high photoresponsivity of 4.68×104 A/W (corresponding gain of 1.4×105) and a fast photoresponse speed of 8 ms, which is two orders of magnitude faster than that in previous reports. All the above results demonstrate that the mixed-dimensional phototransistor can achieve balanced photodetection, and it also has great potential to meet the requirements of practical low-cost photodetectors, making the applications possible in environmental monitoring, spectroscopy, imaging and communication. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details of CsPbBr3 CQDs synthesis and characterization, devices fabrication and measurements. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. ACS Paragon Plus Environment

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ORCID Wei Zheng: 0000-0003-4329-0469 Feng Huang: 0000-0002-4623-2216 NOTES There are no conflicts to declare. ACKNOWLEDGEMENTS The authors acknowledge the financial support from the Major Research Plan of the National Natural Science Foundation of China (No. 91333207, 91833301, U1505252, 61427901 and 61604178), the Guangdong Natural Science Foundation (No. 2014A030310014), and the China Postdoctoral Science Foundation (No. 2015M580752, 2018M643305). The authors would like to thank Dr. Zhaojun Zhang for writing assistance, and thank Mr. Hao Zhang at Sun Yat-sen University for TEM measurements.

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(12) Song, X.; Liu, X.; Yu, D.; Huo, C.; Ji, J.; Li, X.; Zhang, S.; Zou, Y.; Zhu, G.; Wang, Y.; Wu, M.; Xie, A.; Zeng, H., Boosting Two-Dimensional MoS2/CsPbBr3 Photodetectors via Enhanced Light Absorbance and Interfacial Carrier Separation. ACS Appl. Mater. Interfaces 2018, 10, 2801-2809. (13) Zhang, Z.; Zhu, Y.; Wang, W.; Zheng, W.; Lin, R.; Huang, F., Growth, characterization and optoelectronic applications of pure-phase large-area CsPb2Br5 flake single crystals. J. Mater. Chem. C 2018, 6, 446-451. (14) Kagan, C. R.; Murray, C. B., Charge transport in strongly coupled quantum dot solids. Nat. Nanotechnol. 2015, 10, 1013-1026. (15) Sukhovatkin, V.; Hinds, S.; Brzozowski, L.; Sargent, E. H., Colloidal Quantum-Dot Photodetectors Exploiting Multiexciton Generation. Science 2009, 324, 1542. (16) Yu, H.; Liao, M.; Zhao, W.; Liu, G.; Zhou, X. J.; Wei, Z.; Xu, X.; Liu, K.; Hu, Z.; Deng, K.; Zhou, S.; Shi, J. A.; Gu, L.; Shen, C.; Zhang, T.; Du, L.; Xie, L.; Zhu, J.; Chen, W.; Yang, R.; Shi, D.; Zhang, G., Wafer-Scale Growth and Transfer of Highly-Oriented Monolayer MoS2 Continuous Films. ACS nano 2017, 11, 12001-12007. (17) Lee, J.-S.; Kovalenko, M. V.; Huang, J.; Chung, D. S.; Talapin, D. V., Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays. Nat. Nanotechnol. 2011, 6, 348-352. (18) Zheng, W.; Lin, R.; Ran, J.; Zhang, Z.; Ji, X.; Huang, F., Vacuum-Ultraviolet Photovoltaic Detector. ACS nano 2018, 12, 425-431. (19) Zhang, D.; Zheng, W.; Lin, R. C.; Li, T. T.; Zhang, Z. J.; Huang, F., High quality β -Ga 2 O 3 film grown with N 2 O for high sensitivity solar-blind-ultraviolet photodetector with fast response speed. J. Alloys Compd. 2018, 735, 150-154. (20) Zheng, W.; Lin, R.; Zhang, Z.; Liao, Q.; Liu, J.; Huang, F., An ultrafast-temporally-responsive flexible photodetector with high sensitivity based on high-crystallinity organic-inorganic perovskite nanoflake. Nanoscale 2017, 9, 12718-12726. (21) Zheng, W.; Lin, R.; Zhang, D.; Jia, L.; Ji, X.; Huang, F., Vacuum-Ultraviolet Photovoltaic Detector with Improved Response Speed and Responsivity via Heating Annihilation Trap State Mechanism. Advanced Optical Materials 2018, 0, 1800697. (22) Kufer, D.; Konstantatos, G., Photo-FETs: Phototransistors Enabled by 2D and 0D Nanomaterials. ACS Photonics 2016, 3, 2197-2210. (23) Konstantatos, G.; Sargent, E. H., Nanostructured materials for photon detection. Nat. Nanotechnol. 2010, 5, 391-400. (24) Tao, L.; Chen, Z.; Li, X.; Yan, K.; Xu, J.-B., Hybrid graphene tunneling photoconductor with interface engineering towards fast photoresponse and high responsivity. npj 2D Materials and Applications 2017, 1, 19. (25) Tsai, D.-S.; Liu, K.-K.; Lien, D.-H.; Tsai, M.-L.; Kang, C.-F.; Lin, C.-A.; Li, L.-J.; He, J.-H., Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments. ACS nano 2013, 7, 3905-3911. (26) Suess, R. J.; Leong, E.; Garrett, J. L.; Zhou, T.; Salem, R.; Munday, J. N.; Murphy, T. E.; Mittendorff, M., Mid-infrared time-resolved photoconduction in black phosphorus. 2D Materials 2016, 3, 041006. (27) Li, Y.; Shi, Z.; Lei, L.; Zhang, F.; Ma, Z.; Wu, D.; Xu, T.; Tian, Y.; Zhang, Y.; Du, G.; Shan, C.; Li, X., Highly Stable Perovskite Photodetector Based on Vapor-Processed Micrometer-Scale CsPbBr3 Microplatelets. Chemistry of Materials 2018, 30, 6744-6755. (28) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H., Ultrasensitive solution-cast quantum dot photodetectors. Nature 2006, 442, 180.


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Figure 1. (a) Solution synthesis schematic diagram of CsPbBr3 CQDs. The pre-prepared CsOA is mixed with the PbBr2 ODE solution maintained at 180℃ for 5s, and then the mixed solution is cooled down in an ice-water bath. Finally, the CsPbBr3 CQDs are obtained by centrifugation. (b) High-resolution transmission electronic microscope (HRTEM) images of the CsPbBr3 CQDs. (c) X-ray diffraction (XRD) patterns of the CsPbBr3 CQDs, which are coated on a sapphire substrate. (d) and (e) TEM image and statistic size distribution of CsPbBr3 CQDs. (f) Working mechanism of quantum dots. The election-hole pairs are generated under the illumination. Under an external electric field, electrons can escape from the quantum dots while holes are trapped. (g) Photoluminescence and absorption spectra of CsPbBr3 CQDs. The inset shows the photographs of CsPbBr3 CQDs under ultraviolet illumination, which show a bright green emitting. (h) Time-resolution photoluminescence spectra (TRPL) of the CQDs solid film, showing decay time of ~12 ns.



Figure 2. (a) Manufacturing schematic of the mixed-dimensional heterostructured device. The 2D back-gate field effect transistor (FET) is further coated with 0D perovskite QDs. (b) Photographs of 2D MoS2 FET before and after coating CsPbBr3 CQDs. (c) Raman spectra of the MoS2 by a 633 nm laser excitation. (d) I-V characteristics of MoS2 FET. The device shows a N-type depletion mode, and field-effect mobility of 0.13 cm2V-1s-1 (e) I-V characteristics of the mixed-dimensional heterostructured (CsPbBr3-CQDs/MoS2) device.


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Figure 3. (a) Sectional operation view of the CsPbBr3 CQDs/MoS2 heterostructured device. (b) Photocurrent mapping of the device under 405 nm illumination. The inset shows the current mapping of the device under dark state. (c) I-V characteristics of the CsPbBr3 CQDs/MoS2 heterostructure device under different illumination intensities. (d) Photocurrent as a function of the back-gate voltage (Vg). (e) Extracted photocurrent (∆I) and (f) photovoltage (∆V) of the device as a function of the light power density.



Figure 4. (a) Carrier transport mechanism of the CsPbBr3 CQDs/MoS2 heterostructure device under illumination. The incident photons excite electron-hole pairs in CsPbBr3 CQDs. The electrons can be separated from QDs by the built-in electric field of the CsPbBr3 CQDs/MoS2 heterojunction while the holes are trapped. The separated electrons are transported in the MoS2 channel, and collected by the electrodes. (b) Energy band diagram of the CsPbBr3 CQDs/MoS2 heterojunction. (c) Photoresponsivity of MoS2 and CsPbBr3 CQDs/MoS2 heterostructure devices. (d) and (e) Time-dependent current curve of the CsPbBr3 CQDs/MoS2 device, which shows fast photoresponse speed with rising time of 7.5 ms and decay time of 8.0 ms.


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Table 1. Figures of merits for 0D, 2D and 0D-2D photodetectors. Materials Graphene MoS2 Black Phosphorus CsPbBr3 PbS PbS/Graphene HgTe/MoS2 CsPbBr3/MoS2

Dimensional 2D 2D 2D 2D 0D 0D/2D 0D/2D 0D/2D

Photoresponsivity (A/W) ~3×104 0.57 1.33 2.7×103 ~5×107 ~106 4.68×104

Photoresponse speed rise time decay time ~17 ns 1.1 μs 70 μs 110 μs 65 ps ~3 ns 20.9 ms 24.6 ms 70 ms 10 ms 10 ms < 1 ms 7.5 ms 8 ms


Ref. 24 25 26 27 28 6 7 This work


For Table of Contents only.

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