Enhanced Performance of a CVD MoS2 Photodetector by Chemical in

Subscriber access provided by UNIV OF TEXAS DALLAS

Functional Inorganic Materials and Devices 2

Enhanced Performance of CVD MoS Photodetector by Chemically In-situ n-Type Doping Songyu Li, Xiaoqing Chen, Fa-Min Liu, Yongfeng Chen, Beiyun Liu, Wenjie Deng, Boxing An, Feihong Chu, Guoqing Zhang, Shanlin Li, Xuhong Li, and Yongzhe Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00856 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 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

ACS Applied Materials & Interfaces

Enhanced Performance of CVD MoS2 Photodetector by Chemically In-situ n-Type Doping Songyu Li†,‡, Xiaoqing Chen‡, Famin Liu*,†, Yongfeng Chen‡, Beiyun Liu‡, Wenjie Deng‡, Boxing An‡, Feihong Chu§, Guoqing Zhang§, Shanlin Li§, Xuhong Li†,‡, Yongzhe Zhang*,‡ †School

‡Key

of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China

Laboratory of Advanced Functional Materials, Ministry of Education, College of Materials

Science and Engineering, Beijing University of Technology, Beijing 100124, China §Institute

of Microstructure and Property of Advanced Materials, Beijing University of

Technology, Beijing 100124, China

KEYWORDS: MoS2, Chemical vapor deposition (CVD), In-situ n-type doping, Photodetector, Photogating effect

ABSTRACT: Transition metal dichalcogenides (TMDs) are a category of promising 2D materials for the optoelectronic devices, while the unique characteristics include tunable band-gap, nondangling bonds as well as compatibility to large-scale fabrication, for instance, chemical vapor

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 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

Page 2 of 30

deposition (CVD). MoS2 is one of the first TMDs which is well-studied in the photodetection area widely. However, low photoresponse restricts its applications in photodetectors, unless the device is applied with ultrahigh source-drain voltage (VDS) and gate voltage (VGS). In this work, the photoresponse of MoS2 photodetector were improved by a chemically in-situ doping method using gold chloride hydrate. The responsivity and specific detectivity were increased to 99.9 A/W and 9.4×1012 Jones under low VDS (0.1 V) and VGS (0 V), which are 14.6 times and 4.8 times higher than those of pristine photodetector, respectively. Since, the chlorine n-type doping in CVD MoS2 reduces the trapping of photoinduced electrons and promotes the photogating effect. This novel doping strategy provides a great applications of high performance MoS2 photodetectors potentially, and lead a new avenue to enhance photoresponse for other 2D materials.

INTRODUCTION Transition metal dichalcogenides (TMDs) is a novel and important category of 2D materials, and widely-applied in optoelectronic devices1-4 considering a variety of excellent characteristics. For instance, broadband optical sensitivity5-6, tunable band-gap7-9, absence of dangling bonds10, large-scale fabrication capability via chemical vapor deposition (CVD)9, 11 and compatibility to silicon complementary metal oxide semiconductor (CMOS)12-14. In the optoelectronic area, novel photodetector is an important part and attracts the interest of worldwide researchers.15-19 The MoS2-based photodetector (one of the earliest TMDs) has also been widely studied in both theory and experiments.3, 20-24 However, photoresponse of MoS2 photoconductive detectors is usually low when the devices are measured under moderate source-drain voltages (VDS) and gate voltages (VGS).25-28 Indeed, the low photoresponse could be compensated by applying ultrahigh VDS and VGS, because the photoresponse is influenced by VDS and VGS values strongly.20,

27, 29

The


2

Page 3 of 30 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


responsivity can be raised up when increasing VDS, because high VDS can extract more photoinduced carriers. Then, the photoresponse of 2D material photodetectors can be promoted significantly by photogating effect (PG effect) as well. The simplest method to enhance PG effect is to increase positive VGS.30-31 However, such ultrahigh voltages will consume huge amount of electric power and are unpractical in real photodetecting applications. Beyond ultrahigh voltages, the other approaches to enhance photoresponse were reported by previous studies, such as quantum dot combinations32, surface plasmon resonance via nanostructures28,

33

and fabricating

heterojunctions34-37. Unfortunately, those approaches suffer from their own shortcomings. For instance, insufficient-stability of quantum dots; weak enhancing performance of surface plasmon resonance; and complex preparations of heterojunctions. An alternative method to enhance the photoresponse of MoS2 photodetectors is via chemical doping, which plays a significant role in tuning carrier concentration and fermi levels.38 A variety of chemical doping methods are reported to modify the electronic properties and photodetecting performance of MoS2. For example, Choi et al.39 fabricated a lateral MoS2 p-n homojunction doping by gold chloride which exhibits a responsivity of 5.07 A/W and a specific detectivity (D*) of 3×1010 Jones at VDS=1.5 V. Li et al.40 and Huo et al.21 reported two kinds of out-of-plane p-n homojunction MoS2 devices through gold chloride and benzyl viologen doping. The responsivities of these two devices are 30 mA/W (at VDS=1 V and VGS=60 V) and 7×104 A/W (under an extreme low illumination power intensity of 73 pW/cm2, VDS=10V and VGS=60 V) respectively. However, the performance of these devices will degrade quickly, since the gold chloride (adsorbed on MoS2 surface) is unstable in the air. Therefore, it is much necessary to further develop the optoelectronic properties of MoS2 photodetectors based on chemical doping.


3


Page 4 of 30

In this work, we present an in-situ n-type doping method to enhance the photoresponse of MoS2 photodetector. Compared with chemical treatment following CVD growth21, 39-40, in-situ doping is a more stable41-43 and simpler one-step doping method. Before CVD growth of MoS2, the dopant of gold chloride hydrate (GCH) was spin-coated onto the Si/SiO2 substrate. Then, during the CVD reaction process, chlorine (Cl) atoms (from GCH) bonded with Mo atoms 44 and are considered as n-type dopants in MoS245. The photodetector with a bottom gate field effect transistor (FET) structure was fabricated after synthesizing CVD MoS2 grown on the above mentioned chemically processed substrate (named as doped MoS2 in this work). A high responsivity of 99.9 A/W is obtained and 14.6 times higher than that of the pristine CVD MoS2 photodetector (named as pristine MoS2 in this work) under the illumination power intensity of 0.15 mW/cm2 at VDS=0.1 V and VGS=0 V. Meanwhile, this photodetector exhibits a high D* of 9.4×1012 Jones and 4.8 times higher than that of pristine MoS2. Furthermore, the mechanism of photoresponse enhancement were analyzed. It is that Cl n-type doping in MoS2 reduces trapping of photoinduced electrons and thus boosts the photogating effect. Our finding exhibits a great probability for photodetecting applications of MoS2. Furthermore, this doping strategy offers a new approach to enhancing the performance of other 2D material photodetectors.

EXPERIMENTAL SECTION Synthesis of the doped MoS2. GCH was used in this study for pretreatment of Si /SiO2 substrate before MoS2 growth by CVD. First, the pretreated substrates were prepared by spin coating GCH solution (20 mM) on to the Si/SiO2(285 nm) substrates (cleaned with Ar plasma etching) at 2000 rpm for 1 min. Then the substrates were dried on the hotplate at 80℃ for 10 min. All the operations related to GCH were performed in glove box to protect the reagents from the ambient air. During


4

Page 5 of 30 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


the CVD growth of MoS2, the chemically processed substrates and sulfur powder were placed in a quartz tube at the high temperature region (850℃) and the air intake of low temperature region (200℃) respectively. A piece of molybdenum foil (oxidized by electrochemical workstation) were folded into triangle and placed standing over the substrates. Ar was used as the carrier and protective gas during the entire deposition process. The furnace maintained at 850℃ for 1 min and then cooled naturally to room temperature. Device fabrication. The photodetectors were fabricated using bottom gate FET structure. First, metal contact patterns were defined by UV lithography (SUSS MicroTec MJB4). Then, Ti/Au (10 nm/80 nm) electrodes were deposited by e-beam evaporation (HHV FL400). Finally, the devices were annealed at 300℃ for 30 min under high purity of H2 and Ar atmosphere to improve the contact quality. Material and device characterizations. The material characterizations were measured by atomic force microscopy (AFM, Bruker MultiMode 8), confocal Raman and photoluminescence (PL) spectroscopic system (WITec Alpha 300R), transmission electron microscopy (TEM, Titan G2 60-300 Cs-corrected TEM with energy dispersive spectrometer (EDS) mapping) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250Xi). All the electrical measurements were performed using semiconductor device analyzer (Keysight B1500A). For optoelectronic measurements, the devices were illuminated with supercontinuum laser (SuperK EXTREME, NKT Photonics).

RESULTS AND DISCUSSION Figure 1a shows the schematic diagram of the chemically pretreated Si/SiO2 substrate and onestep synthesis method of CVD MoS2. After spin-coated with GCH, the substrate was loaded into


5


Page 6 of 30

the furnace to synthesize MoS2 at a high temperature of 850℃. Then, atomic force microscopy (AFM) was employed to measure the height profile of the doped MoS2 (Figure 1b). The color of AFM image from dark to bright represents relative height of sample. The red straight line is the linear scanning path. The height profile of the sample is 0.77 nm suggesting that the MoS2 is monolayer.46 To characterize the quality of the doped MoS2, its Raman and photoluminescence (PL) spectra (measured with the laser wavelength of 532 nm) are compared with those of pristine MoS2 (Figure 1c and 1d). The doped MoS2 (red curve) and pristine MoS2 (blue curve) exhibit almost the same Raman peak positions (385 cm-1 for E2g and 385 cm-1 for A1g) and PL peak positions (690 nm). The difference between A1g and E2g peak positions is 17 cm-1 which again confirms the monolayer structure of MoS247. Besides, the single-crystal nature and good uniformity of the doped MoS2 could be validated by selected area electron diffraction (SAED) and Raman mapping respectively, as depicted in Figure S1. In order to study and differentiate in electrical properties of both doped and pristine MoS2, they were fabricated using bottom gate FET structure. Figure 2a shows the schematic diagram of the structure and electrical connections for the back gate MoS2 photodetector. All measurements of electrical performances were carried out in dark. As shown in the inset of Figure 2b, the optical microscopy image demonstrates the structure of doped MoS2 photodetector. The doped MoS2 (red equilateral triangular shape) is covered by two electrodes (white), -the source and drain. Output curves of the doped MoS2 device were measured at VGS range from -50V to 50V. A clear linear relationship between ID and VDS is observed which indicates good ohmic contact between doped MoS2 and two electrodes. To highlight the difference in electrical performance between the doped MoS2 and the pristine MoS2, output and transfer curves of the two FETs are compared in Figure 2c and 2d, respectively. Drain current is rescaled to drain current density so that the current of the


6

Page 7 of 30 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


two devices are comparable. As shown in Figure 2c, the drain current density of doped MoS2 is 32 times higher than that of pristine MoS2 at VDS=1.0 V. From transfer curves in Figure 2d, the mobility and ON/OFF ratio of two FETs can be extracted, as exhibited in Table S1. Meanwhile, threshold voltage (VT) can be calculated using the following formula48 at a condition of 𝑉DS ≪ 𝑉GS: 1

𝑉T = 𝑉𝑦 = 0 ― 2𝑉DS

(1)

where, 𝑉𝑦 = 0 is the intercept of tangent line of transfer curve (dash line on Figure 2d). The VT of doped MoS2 FET (12.7 V) is downshifted compared with pristine MoS2 FET (25.1 V). The result demonstrates that the fermi level of doped MoS2 is higher than that of pristine MoS2. This is further verified by energy dispersive spectrometer (EDS) mapping and X-ray photoelectron spectroscopy (XPS) in Figure 3. As for transmission electron microscopy (TEM) image and EDS mapping, doped MoS2 was first transferred onto TEM grid as shown in Figure 3a. The outline of doped MoS2 on TEM grid is marked with colored dash lines. A part of doped MoS2 wrinkled and folded unintentionally during the transfer process. In the images of EDS mapping, colored spots of elements Cl, Mo and S distribute uniformly inside the shape of doped MoS2 and no spot appears outside the shape. XPS spectrum of doped MoS2 (Figure 3b) also shows the presence of Cl 2p peak. Both measurements confirmed the existence of Cl element in doped MoS2. The peak position of Cl 2p binding energy is 199.6 eV which is consistent with the XPS peak of a bond connection between Cl atoms and Mo atoms44. It is the low formation energy of Cl dopant49 and many S vacancies50 that make Cl easy to interact chemically with MoS2. Besides, in both previous theoretical and experimental studies, Cl has a feature of n-type doping for MoS2, concluded based on the first principle simulation49 and experimental measurement.45 Binding energy shift in XPS spectra which is related to the core level electrons has been considered as an indicator for the doping type of


7


Page 8 of 30

MoS2.43, 45, 51-52 The binding energy shifts of Mo 3d peaks (Mo 3d3/2 and Mo 3d5/2) and S 2p peaks (S 2p1/2 and S 2p3/2) for doped MoS2 are compared with those of pristine MoS2 as shown in Figure 3c and 3d. Compared with pristine MoS2, all peak positions of doped MoS2 shifts to high binding energy side. The upshift energy values of Mo 3d and S 2p are 0.29 eV and 0.28 eV, respectively. This result again confirms the rising of fermi level towards conduction band in doped MoS2 which agrees with the VT shift in Figure 2d due to Cl atoms n-type doping. Optoelectronic characteristics of doped MoS2 photodetector were then measured under different VGS and different laser illumination powers. Figure 4a shows the output curves of doped MoS2 photodetector at various VGS under the illumination power intensity of 68 mW/cm2. Upon laser illumination, massive photoinduced excess electrons and holes were generated, separated and transported between source and drain electrodes. So, the drain current increased substantially compared to that in dark condition. Transfer curves of doped MoS2 photodetector were measured under dark and several illumination powers (Figure 4b). Drain current rises up with increase of bottom gate voltage. In general, two mechanisms can give rise to photoconductivity for a detector: photoconductive effect (PC effect, 𝐼ph,PC =

(𝑊𝐿)𝑉D𝛥𝜎) and PG effect (𝐼ph,PG = 𝑔m𝛥𝑉T). PC effect

is the process of photoinduced carriers generated by illumination and their consequent drift under applied bias voltage. PG effect is the photoinduced additional effective gate voltage owing to the trapping and slowly detrapping of photoinduced carriers. Therefore, the VT shift observed in Figure 2d could only be from the PG effect rather than the PC effect.53-56 Moreover, after extracting VT under different illumination power (inset of Figure 4b), a more obvious trend of VT downshift with increasing power intensity is observed, which proves the contribution of PG effect for doped MoS2 photodetector.57 Under low light intensities, ΔVT rapidly decreases with increasing illumination


8

Page 9 of 30 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


power intensity, while under high light intensities ΔVT tends to saturate. The saturation of ΔVT is due to the saturation of the trap states under high illumination power.53, 57-58 Time-resolved photoresponse was measured via switching ON or OFF the 532 nm laser at VDS=0.1 V and VGS=0 V. When the laser is switched ON or OFF, the photocurrent rises or falls. Figure 4c shows time-resolved photocurrent for the doped MoS2 photodetector under different illumination power intensities. The rise time (tr) or fall time (tf) is the time when the photocurrent rises up or falls down by 63% of its maximum value. tr and tf of the doped MoS2 photodetector are calculated to be 16.6 s and 5.2 s respectively. It is known that due to the atomic thickness of MoS2, response time is easily affected by surface interaction and trap states.50, 54, 59 In the cycle periods of light, photoinduced carriers are captured first by the trap states when light is ON. Then, they are released gradually to recombine with each other when the light is OFF. Thus, the photocurrent changes slowly. In order to realize faster photoresponse, this slow response time can be shortened by a pulsed VGS at the edge time of lighting off.20, 36 Figure 4d is the time-resolved photocurrent for the doped and pristine MoS2 photodetectors under the illumination power intensity of 2 mW/cm2. Photocurrent of the doped MoS2 photodetector is 7 times higher than that of pristine MoS2 photodetector. Besides, the photocurrent of the doped MoS2 photodetector maintained 94% of the initial value after 9 months (Figure S3) which confirms the good stability of the doped MoS2. To evaluate the enhancement of optoelectronic performance for the doped MoS2 photodetectors, photocurrent for the doped and pristine MoS2 photodetectors was measured with different illumination power intensities and laser wavelengths. The key parameters of photodetector, responsivity (R) and special detectivity (D*) are defined as follow: 𝐼ph

𝑅 = 𝑃in

(2)


9


Page 10 of 30

where, Iph is the photocurrent which can be calculated by 𝐼ph = 𝐼illuminaiton ― 𝐼dark (𝐼illuminaiton is the stable drain current under illumination and 𝐼dark is the dark current); 𝑃in is the illumination power of laser on the photosensitive area. 𝐷∗ = 𝑅

𝐴∆𝑓 𝑖n2

(3)

where, 𝐴 is the photosensitive area, ∆𝑓 is the electrical bandwidth, and 𝑖n2 is the total noise current at the same measuring condition. Assuming that shot noise denotes the major contribution to the total noise, Eq. (3) could be simplified to 𝐷 ∗ = 𝑅 𝐴 2𝑒𝐼dark.37, 60 Figure 5a and 5b are the power intensity dependence of responsivity and D* for the doped MoS2 and pristine MoS2 photodetectors under 532 nm laser at VDS=0.1 V and VGS=0 V. Result shows that responsivity and D* of the doped MoS2 photodetector are much higher than those of the pristine MoS2 photodetector. Highest responsivity of 99.9 A/W and D* of 9.4×1012 Jones are obtained under the power intensity of 0.15 mW/cm2. Responsivity and D* decreases with increasing power intensity. Furthermore, there are different exponents (α) in the power law relationships between responsivity and illumination power (R ∝ Pα ― 1) for the two photodetectors. Extracting from the fitting curve of power law, the power exponents are αdoped=0.69 and αpristine =0.89. Both PC and PG effects contribute to the photodetecting performance. The power exponents indicate that enhanced photosensitivity of the doped MoS2 originate from the contribution of PG effect.54-55, 58 The relative enhancements of responsivity and D* out of replacing pristine MoS2 with doped MoS2 are shown in Figure 5c. The highest enhancement of the doped MoS2 photodetector compared to the pristine MoS2 device is 14.6 times in responsivity and 4.8 times in D* under the illumination power intensity of 0.15 mW/cm2. The enhancement of D* is not as much as the responsivity, because the dark current of the doped MoS2 photodetector is higher than that of pristine MoS2 photodetector, as depicted in Figure 2c.


10

Page 11 of 30 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


The power exponent of responsivity increment is Δα=0.20. Figure 5d shows the wavelength dependence of responsivity and D* for the two photodetectors at VDS=0.1 V and VGS=0 V. The sampling wavelength covers the range of visible light. To quantitively compare the photoresponse under different wavelengths, power intensity is fixed to 7 mW/cm2. As shown in Figure 5d, two photodetectors almost have the same range of photoresponse wavelength. The responsivity maintains about 45 A/W (for doped MoS2 photodetector) and fluctuates slightly before wavelength=600 nm. Then, it decreases rapidly until 750 nm. This wavelength-based photoresponse change is related to MoS2 photon energy band of ~1.8 eV which is validated by PL spectra in Figure 1d. Additionally, the doped MoS2 photodetector exhibits high external quantum efficiency (EQE), as depicted in Figure S4. In order to elucidate the mechanism of photoresponse enhancement by Cl atoms n-type doping, a bandgap model with continuous bandtail states for electrons and holes30-31, 53-55 is employed to explain it. These bandtail states can result from a distribution of trap charges introduced by the MoS2-SiO2 interfaces, vacancies or dislocations in MoS2.53, 61-65 The energy band diagrams of this model are shown in Figure 6. These diagrams include the PG effect mechanism owing to trap states in pristine MoS2 and photocurrent promotion for PG effect induced by Cl dopants. As for pristine MoS2 photodetector in dark, electron trap states below fermi level (EF) are filled with electrons and the trap states above EF stay empty. By contrast, majority of hole trap states are vacant. Upon illumination, massive photoinduced holes and electrons are generated (depicted by red dots) and then captured by vacant trap states (depicted by up arrows for holes and down arrows for electrons). This process extends the lifetime of remaining photoinduced electrons in conduction band and thus generates the photocurrent (i.e. PG effect). As for doped MoS2, its fermi level is higher than that of pristine MoS2 due to Cl atoms n-type doping. More electron trap states are


11


Page 12 of 30

occupied naturally in dark. It indicates that fewer vacant electron trap states are remained to capture photoinduced electrons. Thus, there are more free photoinduced electrons in the conduction band and subsequently increase the photocurrent compared with pristine MoS2. In one word, photoresponse enhancement results from the boost of PG effect due to the n-type doping.

CONCLUSION In summary, this work demonstrated a novel in-situ n-type doping method for CVD MoS2, and the new technique enhanced the devices performance of MoS2 photodetector. Chlorine atoms were introduced to MoS2 successfully and bonded with Mo atoms as n-type dopants, which observed from both energy dispersive spectrometer mapping and X-ray photoelectron spectroscopy. The chlorine-doped photodetector exhibits a high responsivity of 99.9 A/W and a high specific detectivity of 9.4×1012 Jones (measured at VDS=0.1 V and VGS=0 V), respectively. They are 14.6 times and 4.8 times higher than those of pristine photodetector. A bandgap model with continuous bandtail states was employed to study the dopant-induced photoresponse enhancement in theory, since n-type doping reduces trapping of the photoinduced electrons and promotes the photogating effect. This doping strategy provides a simple and efficient route to realize high performance MoS2 photodetectors and can expand to other 2D materials application widely.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:


12

Page 13 of 30 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


SAED pattern, Raman mapping image (Figure S1); extraction of mobilities and current ON/OFF ratios, transfer curves on logarithmic scale (Table S1, Figure S2); stability of photocurrent (Figure S3); wavelength dependence of EQE (Figure S4).

AUTHOR INFORMATION Corresponding Author *E-mails: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grant Nos. 61575010 and 51671006, the Beijing Municipal Natural Science Foundation under Grant Nos. 4162016 and Z2009001201703, the Beijing Municipal Science and Technology Commission under

Grant

No.

Z141109001814053,

Beijing

Nova

Program

under

Grant

No.

Z151100003315018, Equipment Pre-research Project of China Electronics Technology Group Corporation (CETC) under Grant No. 6141B08110104. The authors would like to thank Prof. Zilong Zheng and Mr. Faizan Ahmad from Beijing University of Technology for the discussions.


13


Page 14 of 30

REFERENCES (1) Koppens, F. H.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Photodetectors based on graphene, other two-dimensional materials and hybrid systems. Nat. Nanotechnol. 2014, 9, 780-793. (2) Xia, F. N.; Wang, H.; Xiao, D.; Dubey, M.; Ramasubramaniam, A. Two-dimensional material nanophotonics. Nat. Photonics 2014, 8, 899-907. (3) Xie, C.; Mak, C.; Tao, X. M.; Yan, F. Photodetectors Based on Two-Dimensional Layered Materials Beyond Graphene. Adv. Funct. Mater. 2017, 27, No. 1603886. (4) Pu, J.; Takenobu, T. Monolayer Transition Metal Dichalcogenides as Light Sources. Adv. Mater. 2018, 30, No. 1707627. (5) Yu, X.; Yu, P.; Wu, D.; Singh, B.; Zeng, Q.; Lin, H.; Zhou, W.; Lin, J.; Suenaga, K.; Liu, Z.; Wang, Q. J. Atomically thin noble metal dichalcogenide: a broadband mid-infrared semiconductor. Nat. Commun. 2018, 9, No. 1545. (6) Yim, C.; McEvoy, N.; Riazimehr, S.; Schneider, D. S.; Gity, F.; Monaghan, S.; Hurley, P. K.; Lemme, M. C.; Duesberg, G. S. Wide Spectral Photoresponse of Layered Platinum DiselenideBased Photodiodes. Nano Lett. 2018, 18, 1794-1800. (7) Zheng, J.; Yan, X.; Lu, Z.; Qiu, H.; Xu, G.; Zhou, X.; Wang, P.; Pan, X.; Liu, K.; Jiao, L. High-Mobility Multilayered MoS2 Flakes with Low Contact Resistance Grown by Chemical Vapor Deposition. Adv. Mater. 2017, 29, No. 1604540.


14

Page 15 of 30 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


(8) Wang, C.; He, Q.; Halim, U.; Liu, Y.; Zhu, E.; Lin, Z.; Xiao, H.; Duan, X.; Feng, Z.; Cheng, R.; Weiss, N. O.; Ye, G.; Huang, Y. C.; Wu, H.; Cheng, H. C.; Shakir, I.; Liao, L.; Chen, X.; Goddard, W. A., III; Huang, Y.; Duan, X. Monolayer atomic crystal molecular superlattices. Nature 2018, 555, 231-236. (9) Jeon, J.; Jang, S. K.; Jeon, S. M.; Yoo, G.; Jang, Y. H.; Park, J. H.; Lee, S. Layer-controlled CVD growth of large-area two-dimensional MoS2 films. Nanoscale 2015, 7, 1688-1695. (10) Liu, Y.; Guo, J.; Zhu, E.; Liao, L.; Lee, S. J.; Ding, M.; Shakir, I.; Gambin, V.; Huang, Y.; Duan, X. Approaching the Schottky-Mott limit in van der Waals metal-semiconductor junctions. Nature 2018, 557, 696-700. (11) Yang, P.; Zou, X.; Zhang, Z.; Hong, M.; Shi, J.; Chen, S.; Shu, J.; Zhao, L.; Jiang, S.; Zhou, X.; Huan, Y.; Xie, C.; Gao, P.; Chen, Q.; Zhang, Q.; Liu, Z.; Zhang, Y. Batch production of 6inch uniform monolayer molybdenum disulfide catalyzed by sodium in glass. Nat. Commun. 2018, 9, No. 979. (12) Sangwan, V. K.; Beck, M. E.; Henning, A.; Luo, J.; Bergeron, H.; Kang, J.; Balla, I.; Inbar, H.; Lauhon, L. J.; Hersam, M. C. Self-Aligned van der Waals Heterojunction Diodes and Transistors. Nano Lett. 2018, 18, 1421-1427. (13) Engel, M.; Farmer, D. B.; Azpiroz, J. T.; Seo, J. T.; Kang, J.; Avouris, P.; Hersam, M. C.; Krupke, R.; Steiner, M. Graphene-enabled and directed nanomaterial placement from solution for large-scale device integration. Nat. Commun. 2018, 9, No. 4095.


15


Page 16 of 30

(14) Yoo, H.; Hong, S.; On, S.; Ahn, H.; Lee, H. K.; Hong, Y. K.; Kim, S.; Kim, J. J. Chemical Doping Effects in Multilayer MoS2 and Its Application in Complementary Inverter. ACS Appl. Mater. Interfaces 2018, 10, 23270-23276. (15) Dhar, S.; Chakraborty, P.; Majumder, T.; Mondal, S. P. Acid-Treated PEDOT:PSS Polymer and TiO2 Nanorod Schottky Junction Ultraviolet Photodetectors with Ultrahigh External Quantum Efficiency, Detectivity, and Responsivity. ACS Appl. Mater. Interfaces 2018, 10, 41618-41626. (16) Abbas, S.; Kumar, M.; Kim, H. S.; Kim, J.; Lee, J. H. Silver-Nanowire-Embedded Transparent Metal-Oxide Heterojunction Schottky Photodetector. ACS Appl. Mater. Interfaces 2018, 10, 14292-14298. (17) Li, F.; Chen, Y.; Ma, C.; Buttner, U.; Leo, K.; Wu, T. High-Performance Near-Infrared Phototransistor Based on n-Type Small-Molecular Organic Semiconductor. Adv. Electron. Mater. 2017, 3, No. 1600430. (18) Alwadai, N.; Haque, M. A.; Mitra, S.; Flemban, T.; Pak, Y.; Wu, T.; Roqan, I. HighPerformance Ultraviolet-to-Infrared Broadband Perovskite Photodetectors Achieved via Inter/Intraband Transitions. ACS Appl. Mater. Interfaces 2017, 9, 37832-37838. (19) Yu, X.; Shen, Y.; Liu, T.; Wu, T. T.; Wang, Q. J. Photocurrent generation in lateral graphene p-n junction created by electron-beam irradiation. Sci. Rep. 2015, 5, No. 12014. (20) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive photodetectors based on monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497-501. (21) Huo, N.; Konstantatos, G. Ultrasensitive all-2D MoS2 phototransistors enabled by an outof-plane MoS2 PN homojunction. Nat. Commun. 2017, 8, No. 572.


16

Page 17 of 30 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


(22) Wang, J.; Fang, H.; Wang, X.; Chen, X.; Lu, W.; Hu, W. Recent Progress on Localized Field Enhanced Two-dimensional Material Photodetectors from Ultraviolet-Visible to Infrared. Small 2017, 13, No. 1700894. (23) Chen, Y.; Wang, X.; Wang, P.; Huang, H.; Wu, G.; Tian, B.; Hong, Z.; Wang, Y.; Sun, S.; Shen, H.; Wang, J.; Hu, W.; Sun, J.; Meng, X.; Chu, J. Optoelectronic Properties of Few-Layer MoS2 FET Gated by Ferroelectric Relaxor Polymer. ACS Appl. Mater. Interfaces 2016, 8, 3208332088. (24) Wang, X.; Wang, P.; Wang, J.; Hu, W.; Zhou, X.; Guo, N.; Huang, H.; Sun, S.; Shen, H.; Lin, T.; Tang, M.; Liao, L.; Jiang, A.; Sun, J.; Meng, X.; Chen, X.; Lu, W.; Chu, J. Ultrasensitive and Broadband MoS(2) Photodetector Driven by Ferroelectrics. Adv. Mater. 2015, 27, 6575-6581. (25) Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-layer MoS2 phototransistors. ACS Nano 2012, 6, 74-80. (26) Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G. B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.; Kim, S. High-detectivity multilayer MoS(2) phototransistors with spectral response from ultraviolet to infrared. Adv. Mater. 2012, 24, 5832-5836. (27) Wang, Q. S.; Lai, J. W.; Sun, D. Review of photo response in semiconductor transition metal dichalcogenides based photosensitive devices. Opt. Mater. Express 2016, 6, 2313-2327. (28) Miao, J.; Hu, W.; Jing, Y.; Luo, W.; Liao, L.; Pan, A.; Wu, S.; Cheng, J.; Chen, X.; Lu, W. Surface Plasmon-Enhanced Photodetection in Few Layer MoS2 Phototransistors with Au Nanostructure Arrays. Small 2015, 11, 2392-2398.


17


Page 18 of 30

(29) Zhang, W.; Huang, J. K.; Chen, C. H.; Chang, Y. H.; Cheng, Y. J.; Li, L. J. High-gain phototransistors based on a CVD MoS(2) monolayer. Adv. Mater. 2013, 25, 3456-3461. (30) Kufer, D.; Konstantatos, G. Highly Sensitive, Encapsulated MoS2 Photodetector with Gate Controllable Gain and Speed. Nano Lett. 2015, 15, 7307-7313. (31) Guo, Q.; Pospischil, A.; Bhuiyan, M.; Jiang, H.; Tian, H.; Farmer, D.; Deng, B.; Li, C.; Han, S. J.; Wang, H.; Xia, Q.; Ma, T. P.; Mueller, T.; Xia, F. Black Phosphorus Mid-Infrared Photodetectors with High Gain. Nano Lett. 2016, 16, 4648-4655. (32) Kufer, D.; Nikitskiy, I.; Lasanta, T.; Navickaite, G.; Koppens, F. H.; Konstantatos, G. Hybrid 2D-0D MoS2 -PbS quantum dot photodetectors. Adv. Mater. 2015, 27, 176-180. (33) Wu, Z. Q.; Yang, J. L.; Manjunath, N. K.; Zhang, Y. J.; Feng, S. R.; Lu, Y. H.; Wu, J. H.; Zhao, W. W.; Qiu, C. Y.; Li, J. F.; Lin, S. S. Gap-Mode Surface-Plasmon-Enhanced Photoluminescence and Photoresponse of MoS2. Adv. Mater. 2018, 30, No. 1706527. (34) Rathi, S.; Lee, I.; Lim, D.; Wang, J.; Ochiai, Y.; Aoki, N.; Watanabe, K.; Taniguchi, T.; Lee, G. H.; Yu, Y. J.; Kim, P.; Kim, G. H. Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices. Nano Lett. 2015, 15, 50175024. (35) Lee, C. H.; Lee, G. H.; van der Zande, A. M.; Chen, W.; Li, Y.; Han, M.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; Guo, J.; Hone, J.; Kim, P. Atomically thin p-n junctions with van der Waals heterointerfaces. Nat. Nanotechnol. 2014, 9, 676-681.


18

Page 19 of 30 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


(36) Deng, W. J.; Chen, Y. F.; You, C. Y.; Liu, B. Y.; Yang, Y. H.; Shen, G. L.; Li, S. Y.; Sun, L.; Zhang, Y. Z.; Yan, H. High Detectivity from a Lateral Graphene-MoS2 Schottky Photodetector Grown by Chemical Vapor Deposition. Adv. Electron. Mater. 2018, 4, No. 1800069. (37) Xu, H.; Han, X.; Dai, X.; Liu, W.; Wu, J.; Zhu, J.; Kim, D.; Zou, G.; Sablon, K. A.; Sergeev, A.; Guo, Z.; Liu, H. High Detectivity and Transparent Few-Layer MoS2 /Glassy-Graphene Heterostructure Photodetectors. Adv. Mater. 2018, 30, No. 1706561. (38) Pham, V. P.; Yeom, G. Y. Recent Advances in Doping of Molybdenum Disulfide: Industrial Applications and Future Prospects. Adv. Mater. 2016, 28, 9024-9059. (39) Choi, M. S.; Qu, D.; Lee, D.; Liu, X.; Watanabe, K.; Taniguchi, T.; Yoo, W. J. Lateral MoS2 p-n junction formed by chemical doping for use in high-performance optoelectronics. ACS Nano 2014, 8, 9332-9340. (40) Li, H. M.; Lee, D.; Qu, D.; Liu, X.; Ryu, J.; Seabaugh, A.; Yoo, W. J. Ultimate thin vertical p-n junction composed of two-dimensional layered molybdenum disulfide. Nat. Commun. 2015, 6, No. 6564. (41) Zhang, K. H.; Bersch, B. M.; Joshi, J.; Addou, R.; Cormier, C. R.; Zhang, C. X.; Xu, K.; Briggs, N. C.; Wang, K.; Subramanian, S.; Cho, K.; Fullerton-Shirey, S.; Wallace, R. M.; Vora, P. M.; Robinson, J. A. Tuning the Electronic and Photonic Properties of Monolayer MoS2 via In Situ Rhenium Substitutional Doping. Adv. Funct. Mater. 2018, 28, No. 1706950. (42) Xu, E. Z.; Liu, H. M.; Park, K.; Li, Z.; Losovyj, Y.; Starr, M.; Werbianskyj, M.; Fertig, H. A.; Zhang, S. X. p-Type transition-metal doping of large-area MoS2 thin films grown by chemical vapor deposition. Nanoscale 2017, 9, 3576-3584.


19


Page 20 of 30

(43) Suh, J.; Park, T. E.; Lin, D. Y.; Fu, D. Y.; Park, J.; Jung, H. J.; Chen, Y. B.; Ko, C.; Jang, C.; Sun, Y. H.; Sinclair, R.; Chang, J.; Tongay, S.; Wu, J. Q. Doping against the Native Propensity of MoS2: Degenerate Hole Doping by Cation Substitution. Nano Lett. 2014, 14, 6976-6982. (44) Zhou, J.; Lin, J.; Huang, X.; Zhou, Y.; Chen, Y.; Xia, J.; Wang, H.; Xie, Y.; Yu, H.; Lei, J.; Wu, D.; Liu, F.; Fu, Q.; Zeng, Q.; Hsu, C. H.; Yang, C.; Lu, L.; Yu, T.; Shen, Z.; Lin, H.; Yakobson, B. I.; Liu, Q.; Suenaga, K.; Liu, G.; Liu, Z. A library of atomically thin metal chalcogenides. Nature 2018, 556, 355-359. (45) Yang, L.; Majumdar, K.; Liu, H.; Du, Y.; Wu, H.; Hatzistergos, M.; Hung, P. Y.; Tieckelmann, R.; Tsai, W.; Hobbs, C.; Ye, P. D. Chloride molecular doping technique on 2D materials: WS2 and MoS2. Nano Lett. 2014, 14, 6275-6280. (46) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nat. Nanotechnol. 2011, 6, 147-150. (47) Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D. From Bulk to Monolayer MoS2: Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22, 1385-1390. (48) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, Third ed.; Wiley: Hoboken, New Jersey, 2006; pp 312-313. (49) Ma, J.; Yu, Z. G.; Zhang, Y.-W. Tuning deep dopants to shallow ones in 2D semiconductors by substrate screening: The case of XS (X = Cl, Br, I) in MoS2. Phys. Rev. B 2017, 95, No. 165447.


20

Page 21 of 30 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


(50) Hong, J.; Hu, Z.; Probert, M.; Li, K.; Lv, D.; Yang, X.; Gu, L.; Mao, N.; Feng, Q.; Xie, L.; Zhang, J.; Wu, D.; Zhang, Z.; Jin, C.; Ji, W.; Zhang, X.; Yuan, J.; Zhang, Z. Exploring atomic defects in molybdenum disulphide monolayers. Nat. Commun. 2015, 6, No. 6293. (51) Lin, J. D.; Han, C.; Wang, F.; Wang, R.; Xiang, D.; Qin, S.; Zhang, X. A.; Wang, L.; Zhang, H.; Wee, A. T.; Chen, W. Electron-doping-enhanced trion formation in monolayer molybdenum disulfide functionalized with cesium carbonate. ACS Nano 2014, 8, 5323-5329. (52) Nipane, A.; Karmakar, D.; Kaushik, N.; Karande, S.; Lodha, S. Few-Layer MoS(2) p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation. ACS Nano 2016, 10, 2128-2137. (53) Furchi, M. M.; Polyushkin, D. K.; Pospischil, A.; Mueller, T. Mechanisms of photoconductivity in atomically thin MoS2. Nano Lett. 2014, 14, 6165-6170. (54) Huang, H.; Wang, J.; Hu, W.; Liao, L.; Wang, P.; Wang, X.; Gong, F.; Chen, Y.; Wu, G.; Luo, W.; Shen, H.; Lin, T.; Sun, J.; Meng, X.; Chen, X.; Chu, J. Highly sensitive visible to infrared MoTe2 photodetectors enhanced by the photogating effect. Nanotechnology 2016, 27, No. 445201. (55) Fang, H.; Hu, W. Photogating in Low Dimensional Photodetectors. Adv. Sci. 2017, 4, No. 1700323. (56) Huang, L.; Tan, W. C.; Wang, L.; Dong, B.; Lee, C.; Ang, K. W. Infrared Black Phosphorus Phototransistor with Tunable Responsivity and Low Noise Equivalent Power. ACS Appl. Mater. Interfaces 2017, 9, 36130-36136.


21


Page 22 of 30

(57) Island, J. O.; Blanter, S. I.; Buscema, M.; van der Zant, H. S.; Castellanos-Gomez, A. Gate Controlled Photocurrent Generation Mechanisms in High-Gain In(2)Se(3) Phototransistors. Nano Lett. 2015, 15, 7853-7858. (58) Wu, J. Y.; Chun, Y. T.; Li, S.; Zhang, T.; Wang, J.; Shrestha, P. K.; Chu, D. Broadband MoS2 Field-Effect Phototransistors: Ultrasensitive Visible-Light Photoresponse and Negative Infrared Photoresponse. Adv. Mater. 2018, 30, No. 1705880. (59) Chen, Y.; Huang, S.; Ji, X.; Adepalli, K.; Yin, K.; Ling, X.; Wang, X.; Xue, J.; Dresselhaus, M.; Kong, J.; Yildiz, B. Tuning Electronic Structure of Single Layer MoS2 through Defect and Interface Engineering. ACS Nano 2018, 12, 2569-2579. (60) Gong, X.; Tong, M.; Xia, Y.; Cai, W.; Moon, J. S.; Cao, Y.; Yu, G.; Shieh, C. L.; Nilsson, B.; Heeger, A. J. High-detectivity polymer photodetectors with spectral response from 300 nm to 1450 nm. Science 2009, 325, 1665-1667. (61) Ghatak, S.; Pal, A. N.; Ghosh, A. Nature of electronic states in atomically thin MoS(2) fieldeffect transistors. ACS Nano 2011, 5, 7707-7712. (62) Qiu, H.; Xu, T.; Wang, Z.; Ren, W.; Nan, H.; Ni, Z.; Chen, Q.; Yuan, S.; Miao, F.; Song, F.; Long, G.; Shi, Y.; Sun, L.; Wang, J.; Wang, X. Hopping transport through defect-induced localized states in molybdenum disulphide. Nat. Commun. 2013, 4, No. 2642. (63) Yu, Z.; Pan, Y.; Shen, Y.; Wang, Z.; Ong, Z. Y.; Xu, T.; Xin, R.; Pan, L.; Wang, B.; Sun, L.; Wang, J.; Zhang, G.; Zhang, Y. W.; Shi, Y.; Wang, X. Towards intrinsic charge transport in monolayer molybdenum disulfide by defect and interface engineering. Nat. Commun. 2014, 5, No. 5290.


22

Page 23 of 30 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


(64) He, G.; Ghosh, K.; Singisetti, U.; Ramamoorthy, H.; Somphonsane, R.; Bohra, G.; Matsunaga, M.; Higuchi, A.; Aoki, N.; Najmaei, S.; Gong, Y.; Zhang, X.; Vajtai, R.; Ajayan, P. M.; Bird, J. P. Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation. Nano Lett. 2015, 15, 5052-5058. (65) Zhu, W.; Low, T.; Lee, Y. H.; Wang, H.; Farmer, D. B.; Kong, J.; Xia, F.; Avouris, P. Electronic transport and device prospects of monolayer molybdenum disulphide grown by chemical vapour deposition. Nat. Commun. 2014, 5, No. 3087.


23


Page 24 of 30

Figure 1. Synthesis method and characterizations of the doped MoS2. (a) Schematic diagram of the chemically pretreated Si/SiO2 substrates and synthesis method of CVD MoS2. (b) AFM image and the height profile of the doped MoS2 that the 0.77 nm indicates the monolayer structure of MoS2. (c) Raman and (d) PL spectra of the doped MoS2 and pristine MoS2.


24

Page 25 of 30 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


Figure 2. Electrical characteristics of the MoS2 photodetectors. (a) Schematic diagram of structure for the back gate MoS2 photodetector together with electrical connections to characterize the device. (b) Output curves for the doped MoS2 FET at various VGS from -50 V to 50 V in dark, which shows a linear relationship between ID and VDS. Inset shows optical microscopy image of the doped MoS2 photodetector, where the red equilateral triangle is the outline of the doped MoS2. Scale bar (red) is 10 μm. (c) Output curves at VGS=0 V and (d) transfer curves at VDS=1.0 V for the doped and pristine MoS2 FET in dark. Drain currents in (c) and (d) are rescaled to drain current density so that the current can be compared between two devices.


25


Page 26 of 30

Figure 3. Element analyses of doped MoS2 and pristine MoS2. (a) TEM image (greyscale) and EDS mapping images (in color) of doped MoS2. Outline of the doped MoS2 is marked with colored dash lines. Scale bar (white) is 1 μm. (b) XPS spectrum for doped MoS2 showing the presence of Cl 2p peak. XPS spectra of (c) Mo 3d peaks and (d) S 2p peaks showing an upshift in binding energy (indicated by the green arrows in the lower panels) for doped MoS2. In (b), (c) and (d), symbols are the experiment data of XPS spectra, solid lines are the fitting curves while the green dash lines are the baselines.


26

Page 27 of 30 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


Figure 4. Optoelectronic characteristics of the doped MoS2 photodetector. (a) Output curves for the doped MoS2 photodetector at various VGS from -50 V to 50 V under the illumination power intensity of 68 mW/cm2. (b) Transfer curves for the doped MoS2 photodetector under dark and different illumination powers at VDS=0.2 V. Inset shows the shift of the VT with the increase of illumination power intensity. (c) Time-resolved photocurrent for the doped MoS2 photodetector under different illumination powers at VDS=0.1 V and VGS=0 V, which shows the rise time and the fall time during the light was ON and OFF. (d) Time-resolved photocurrent for the doped and pristine MoS2 photodetectors under the illumination power intensity of 2 mW/cm2 at VDS=0.1 V and VGS=0 V. Laser wavelength is 532 nm in the measurement of (a-d).


27


Page 28 of 30

Figure 5. Photoresponse enhancement, and spectrum of the doped MoS2 photodetector at VDS=0.1 V and VGS=0 V. (a) Power intensity dependence of responsivity for doped and pristine MoS2 photodetectors. The power law exponents (α, R ∝ Pα ― 1) are fitted to be 0.69 and 0.89 for doped and pristine MoS2 photodetectors, respectively. (b) Power intensity dependence of D* for the two photodetectors. (c) Power intensity dependence of responsivity ratio and D* ratio between the two photodetectors. A high responsivity ratio up to 14.6 and a high D* ratio up to 4.8 are acquired under power intensity of 0.15 mW/cm2. Laser wavelength is 532 nm in the measurement of (a-c). (d) Wavelength dependence of responsivity and D* from 450 nm to 750 nm for the two photodetectors with the illumination power intensity of 7 mW/cm2.


28

Page 29 of 30 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


Figure 6. Energy band diagrams of enhanced mechanism for doped MoS2 photodetector. Energy band diagrams for (a) pristine MoS2 and (b) doped MoS2 under dark and illumination environment. Focused on each energy band, a bandgap model of density of states (DOS) is on the left of energy axis. A discrete band model is on the right of energy axis to depict the process of carrier excitation and trapping dynamics.


29


Page 30 of 30

TOC


30

Enhanced Performance of a CVD MoS2 Photodetector by Chemical in

Recommend Documents