Abstract
MoS2, as a typical representative of two-dimensional semiconductors, has been explored extensively in applications of optoelectronic devices because of its adjustable bandgap. However, to date, the performance of the fabricated photodetectors has been very sensitive to the surrounding environment owing to the large surface-to-volume ratio. In this work, we report on large-scale, high-performance monolayer MoS2 photodetectors covered with a 3-nm Al2O3 layer grown by atomic layer deposition. In comparison with the device without the Al2O3 stress liner, both the photocurrent and responsivity are improved by over 10 times under 460-nm light illumination, which is due to the tensile strain induced by the Al2O3 layer. Further characterization demonstrated state-of-the-art performance of the device with a responsivity of 16.103 A W−1, gain of 191.80, NEP of 7.96 × 10−15 W Hz−1/2, and detectivity of 2.73 × 1010 Jones. Meanwhile, the response rise time of the photodetector also reduced greatly because of the increased electron mobility and reduced surface defects due to the Al2O3 stress liner. Our results demonstrate the potential application of large-scale strained monolayer MoS2 photodetectors in next-generation imaging systems.
1 Introduction
Ever since the discovery of graphene in 2004, two-dimensional (2D) materials have become a hot topic of research owing to their special properties such as the absence of surface dangling bonds, large surface-to-volume ratio, energy bandgap tunable with the number of layers, as well as adjustable chemical and physical properties [1], [2], [3], [4], [5]. Among them, semiconducting transition-metal dichalcogenides (TMDCs) have been considered as promising materials for future and emerging electronic, optoelectronic, and biological devices [6], [7]. MoS2, a typical representative of TMDCs, possesses a bandgap ranging from 1.80 eV (monolayer) to 1.20 eV (bulk), high mobility, high light absorptivity, and thermal stability (~1100°C), which facilitate its applications in field-effect transistors (FETs), photodetectors, and sensors [1], [8], [9], [10], [11]. For photodetectors, monolayer or multilayer MoS2-based phototransistors have made significant progress. The exfoliated monolayer MoS2 phototransistor shows a responsivity of up to ~880 A W−1 [12], while the detectivity and responsivity of a multilayer MoS2 photodetector reach 1012 Jones and 2570 A W−1 [13], respectively. However, it is still a great challenge to achieve large-area, high-quality monolayer MoS2 films. Chemical vapor deposition (CVD) as a promising method has been widely employed. Yore et al. have successfully grown large-area monolayer MoS2 films on sapphire substrates and made them into photodetectors with a responsivity of 0.8 mA W−1 [14]. Obviously, such low responsivity is unable to meet the requirements for detection capabilities. Thus, finding ways to improve the responsivity of MoS2-based photodetectors is an urgent need. In the literature, a common method to improve the performance of photodetectors is to form a heterojunction with grapheme [15], [16], [17], [18], [19], [20]. Fazio et al. prepared a graphene/MoS2 heterojunction photodetector on a flexible substrate, giving a responsivity of up to 45.5 A W−1 [21]. In addition, the MoS2/TiO2 p-n heterojunction and the n-doped MoS2 photodetector achieved responsivity values of 35.9 [22] and 99.9 A W−1 [23], respectively. Another efficient way is to protect the MoS2 from the environment by covering it with Al2O3, HfO2, or BN layers [1], [24], [25], [26]. Over 20-fold enhancement of photoresponsivity of multilayer MoS2 transistors using a high-work-function MoOx overlayer was also demonstrated [27]. Highly stable and high-performance monolayer/bilayer MoS2 photodetectors covered with atomic layer deposited (ALD) HfO2 exhibited zero hysteresis and low device resistance [28]. FETs fabricated on exfoliated MoS2 flakes capped with a 15-nm Al2O3 film achieved an effective carrier mobility of 700 cm2 V−1 s−1 at room temperature [24], which is primarily attributed to the screening effect on charged-impurity scattering. In addition, Yu et al. studied the interface effect between Al2O3 and monolayer MoS2 by first-principles calculation, which indicated that the 3-nm Al2O3 can introduce about 0.41% tensile strain on monolayer MoS2 [29]. Large changes in the electrical properties of monolayer MoS2 caused by doping and strain also affect its application in nanoelectronics [30]. The electron mobility of the monolayer MoS2 effectively increases by over 10 times by the biaxial strain effect [31]. The strain in monolayer MoS2 can be detected by Raman spectroscopy, where the
We have successfully grown high-quality, large area, and continuous monolayer MoS2 on sapphire substrates using CVD, and deposited a 3-nm-thick Al2O3 stress liner grown by ALD to isolate the MoS2 layer from the environment. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, and absorption techniques were carried out to characterize the material. Then MoS2-on-sapphire photodetectors with and without the Al2O3 stress liner were fabricated using typical semiconductor fabrication processes. After applying the Al2O3 stress liner, not only the photocurrent and photoresponsivity of the photodetector increased profoundly, but also the response time reduced substantially.
2 Experimental section
2.1 Materials
Acetone (CH3COCH3, 99.5%), isopropyl alcohol (C3H8O, 99.5%), and alcohol (C2H6O, 99.7%) were purchased from Aladdin Co., Ltd (Shenzhen, China). Molybdenum trioxide powder (MoO3, 99.95%) and sulfur powder were purchased from Aladdin Co., Ltd. Trimethylaluminum (TMAl, 99.9999%) was purchased from Fornano Co., Ltd (Suzhou, China). Titanium (Ti, 99.999%) and gold (Au, 99.999%) were purchased from Tim Co., Ltd (Beijing, China). Double-side-polished sapphire substrates (430 μm thickness) with (0001) orientation were purchased from Xiamen Zhonghe Optoelectronics Co., Ltd (Xiamen, China). The photoresist (AZ5214) and developer (ZX238) were purchased from AZ Co., Ltd (Shanghai, China). Deionized water (DI water) was prepared in the laboratory.
2.2 Preparation of the MoS2 on sapphire, Al2O3, and photodetectors
Large-area, continuous monolayer MoS2 were grown on (0001)-oriented sapphire substrates by a CVD system with two zones. Prior to the growth, the substrates were first cleaned using acetone, isopropanol, and DI water. After the treatment, they were placed face down on a crucible containing MoO3 and loaded into a heating furnace with a quartz tube of 100 mm outer diameter. Ultrahigh-purity Ar gas was employed as the carrier gas, and the growth pressure was kept at atmospheric. Another crucible containing 0.8 g sulfur powder was located in the adjacent zone, upstream from the sapphire substrates. Before heating, pure Ar gas with a flow rate of 100 sccm was used to purge the tube in order to drive away other gases (e.g. O2, CO2 etc.), and it was then maintained at a flow rate of 50 sccm during the growth. The MoO3 powder was heated up to 750°C over 120 min, while the sulfur powder was first maintained at room temperature for 70 min and then heated to 200°C in 50 min. Subsequently, the growth temperature was kept at 750°C for 15 min and then naturally cooled down to room temperature. Ti(5 nm)/Au(50 nm) was selected as the electrode, which was deposited on monolayer MoS2 by the UV lithography and e-beam evaporation and lift-off processes. Then, Al2O3 oxide was deposited on MoS2 by an ALD system, using TMAl and H2O. TMAl and H2O produced sufficient vapor pressure at 300°C, and pure nitrogen with a flow rate of 50 sccm was injected into the reaction chamber. The flow rate of pure nitrogen in the reaction chamber was 200 sccm. After the temperature of the reaction chamber reached 300°C for 10 min, the reaction was carried out, during which the temperature was maintained at 300°C throughout the process. In order to deposit 3 nm Al2O3 on the surface of MoS2, the deposition period was selected as 16 cycles.
2.3 Characterization and measurement
The room-temperature and temperature-dependent Raman spectra were recorded using a confocal Renishaw system with a wavelength of 514.5 nm and a power of 0.25 mW for excitation and 2400 grooves/mm gratings for high resolution. XPS spectra were measured using a VG Escalab 220i-XL system with a constant pass energy of 20 eV and a monochromatic AlKα (1486.6 eV) X-ray source. The C 1s peak (284.8 eV) was used to correct the core-level binding energy in order to eliminate the differential charging effect on the sample surface. Absorbance spectra were measured using a Shimadzu-2450 UV-visible spectrometer in the spectral range 400–800 nm. The I-V characteristics and responsivity measurements of the fabricated photodetectors were carried out by a Keithley 4200-SCS semiconductor analyzer.
2.4 Theoretical simulation
First-principles calculations involved in this work were carried out using Material Studio, as implemented in the Cambridge Sequential Total Energy Package (CASTEP) based on density functional theory (DFT) [33]. The super-soft pseudopotential was used to describe the interaction between ions and the valence electrons. The exchange correlation used was the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) [34], together with the projector augmented wave potential (PAW) to treat ion-electron interactions [35]. The maximum Hellmann-Feynman force was 0.05 eV Å−1, which is sufficient to obtain relaxed structures [34]. The electronic wave functions were expanded in a plane wave basis set with an energy cut-off of 650 eV to ensure convergence, and a 5×5×1 Monkhorste-Pack mesh was used in the Brillouin zone sampling [36]. The convergence criteria for the geometric optimization of the system were as follows: total energy changes during the optimization finally converged to less than 10−6 eV/atom, maximum force on each atom in the crystal was less than 0.03 eV nm−1, the maximum stress in the crystal was less than 0.05 GPa, and the maximum displacement of the atom was less than 10−4 nm.
3 Results and discussion
Double-polished sapphire with surface orientation (0001) was used as the substrate for the growth of monolayer MoS2 films. In order to characterize the film thickness and uniformity, room-temperature Raman spectroscopy and TEM were carried out. The Raman spectra of the sample measured at four different positions (marked by 1, 2, 3, and 4 shown in Figure 1A) confirmed that the two main peaks at 384.3 and 403.4 cm−1 belonged to the
In order to further understand the effects of the Al2O3 stress liner on the thermal properties of monolayer MoS2, temperature-dependent Raman spectra were recorded to investigate the lattice vibration properties, which are important for the related devices. Figure S4A,B shows the Raman spectra of both the control sample and the sample with the Al2O3 stress liner sample as a function of the temperature, which was varied from 300 to 500 K in steps of 20 K. Meanwhile, Figure 1C shows the peak positions of the
where ω0 is the mode frequency measured at room temperature (300 K), χT is the first-order temperature coefficient, and ΔT is the temperature difference relative to 300 K. The measured χT values for
In contrast, the Raman spectra of the control and stress liner samples are shown in Figure 1D. An obvious red shift of ω=−1.22 cm−1 in the stress liner sample can be seen on the
To simulate the strain effect of Al2O3 on monolayer MoS2 in a photodetector, Figure 2A shows the stress distribution of a monolayer MoS2 photodetector with the 3-nm Al2O3 stress liner. The MoS2 layer in contact with Al2O3 is stretched, whereas that in contact with the Ti/Au is compressed. From the stress distribution of the MoS2 layer along the horizontal direction, shown in Figure 2B, the tensile stress is higher near the intersection of the monolayer MoS2, Al2O3, and Ti/Au (located at 485 nm) than at other locations. The tensile strain produced by the oxide has a significant effect on the electronic structure of monolayer MoS2. Figure S5A–E shows, respectively, the first-principles calculation results of the band structures with the tensile strain ranging from 0% to 2% for monolayer MoS2. The down-shift and up-shift can be observed at the K and Γ points of the reciprocal space of the conduction and valence bands, respectively, with increase in the tensile strain. As a reference, the monolayer MoS2 without applied strain shows a bandgap of 1.73 eV, which is slightly less than the experimental value of 1.8 eV [51]; this is because the first-principles calculation has the disadvantage of underestimating the bandgap. The bandgap of monolayer MoS2 has decreased with tensile strain, changing it from direct bandgap to indirect bandgap, which is consistent with the results reported by Sun and Lin [52]. In addition, the effective mass of the lowest conduction band that the electrons occupy can be calculated through the relation
where ħ is the Planck constant. As shown in Figure 2C, compared with the unstrained monolayer MoS2, in the strained layer the electron effective mass of MoS2 is reduced due to the introduction of the tensile strain, which is beneficial for enhancing the electron mobility of MoS2. Simultaneously, Figure 2D shows that the simulated absorption coefficient of monolayer MoS2 varies with tensile strain in the wavelength range 440–500 nm. The increase in absorption coefficient with the increase in the tensile strain implies that the tensile strain existing in the MoS2 layer can effectively enhance the absorption of light.
On the basis of the simulation, the absorbance of the control and stress liner samples was also measured in the experiment. Figure 2E shows the absorbance curves of both samples, which were obtained by subtracting the absorbance of the blank sapphire substrate and plotting it as a function of the incident wavelength. In the absorbance curve of pure MoS2, two obvious excitonic absorption peaks, labeled as A and B, corresponding to band edges between 600 and 700 nm exist, which originate from the K point of the Brillouin zone. Their energy difference is due to the spin-orbital splitting of the valence band [53]. In addition to excitons associated with the band edges, there is another excitation absorption peak C at 440 nm in Figure 2E. This peak is due to the parallel band or the so-called band nesting effect in the density of states, which produces a strong optical response even though the excitation energy is far beyond the bandgap [54]. Because the “absorbance A” is defined as A=log10(I0/It), where It is the intensity of the transmitted light and I0 is the intensity of the incident light, the absorption rate can be obtained by (I0 –It)/I0=1–1/10A. In this work, 460 nm was chosen as the wavelength to study the photoelectric performance of the photodetectors. Therefore, the absorption rate of the control sample is determined to be 17.09% at 460 nm incident wavelength, taking A460nm=0.0814 into account. For the stress liner sample, the absorption rate is 22.83% with A460nm=0.1125. The absorption ratio increased significantly since Al2O3 was deposited onto the MoS2. According to the Beer-Lambert law
where t is the thickness of MoS2, the absorption coefficient α can be calculated by the following formula:
Thus, the calculated value of absorption coefficient for the control sample is 3.67×106 cm−1 at 460 nm. The corresponding value increases to 6.20×106 cm−1 for the stress liner sample, which is due to the effect of the tensile stress in monolayer MoS2 [55]. Higher absorption of the stress liner sample induced by the Al2O3 stress liner can directly result in better performance of the photodetector, especially due to the increased photocurrent.
Transparent arrays of monolayer MoS2 photodetectors were fabricated with the size 1 cm×1 cm. Figure 3A shows a 3D schematic view of monolayer MoS2 photodetectors with the Al2O3 stress liner, where a 3-nm Al2O3 capping layer was grown using ALD. For the contacts, (5 nm)Ti/(50 nm)Au was deposited as the electrode, which may not be optimal. The contact can be formed by a low-work-function metal such as Sc, which can result in high carrier injection and low contact resistance, since the Fermi level can be pinned close to the conduction band of MoS2 [24]. Figure 3B shows the current (I) as a function of voltage (V) for both devices plotted under incident light of 460 nm wavelength (1.964 μW) with a laser spot diameter of 1.5 cm. For the control device, the value of the current is 7.1 nA in dark condition at 20 V, suggesting that the monolayer MoS2 film is undoped and that the Fermi level resides in the bandgap [56]. Meanwhile, the current under illumination is as high as ~1.384 μA at 460 nm under an applied voltage of 20 V. When the sample is impinged by a laser, this kind of the observed conductive behavior is due to the generation of photocarriers due to the doping, a process known as optical doping [57]. Under the same condition, the dark current of the device with Al2O3 stress liner device increased by two orders of magnitude. The increase in dark current is due to the large tensile strain of MoS2 near the electrode or to the strain-induced narrowing of the bandgap, as shown in the Figure S5. Similar to the 460 nm in this work, a 660 nm light source was used to perform the same measurement on the photodetector shown in Figure S6. Performance improvement due to the Al2O3 stress liner is also seen when the monolayer MoS2 photodetector was operated under a light source of 660 nm, with the maximum measured photocurrent of 6.387 μA.
The laser-power-dependent I-V curves for the control and stress liner devices under 460 nm illumination are shown in Figure S7A,B. In order to quantify the photosensitive performance of the photodetector, the photocurrent IPh is extracted by the equation
where Idark and Ilight are the current in dark condition and under illumination, respectively. The extracted photocurrent IPh is plotted as a function of the effective power with 460 nm wavelength at a voltage of 20 V in Figure 4A. The curve follows a power law IPh∝Pα, where P is the laser power. For the control device, a linear behavior with α=2.31 can be deduced in the low incident power range of Pin<1.580 μW, and IPh increases rapidly with the increase of the power in this range. However, the upward trend of photocurrent is slowed down with α=1.24 in the range Pin>1.580 μW. There are several possible reasons for this slowdown: (i) absorption reaches saturation [58], [59], [60]; (ii) photo-generated carriers are recombined because of existing defects and charge impurities [61]; (iii) recombination of electrons and holes [62]; (iv) exciton-exciton annihilation [63], [64]; and (v) carrier-carrier chattering [59]. Similar behavior for MoS2 and other semiconductor-based photodetectors has been reported recently [58], [64], [65], [66]. In comparison with that of the control device, the “saturation point” is found at a lower power level Pin<1.454 μW for the device with the stress liner.
The photoresponsivity R is calculated by the expression
where Pin is the effective power of the incident light normalized by the equation
where P0 is the laser output power. To further evaluate the performance of our photodetectors, photoconductive gain G, indicating the number of carriers of the photocurrent that can be generated by one absorbed photon, is also calculated by the equation
where Pabs is the absorbed power, which is defined as Pabs=μPin, with μ absorption percentage calculated above and Pin the incident power, h is Plank’s constant, v is the frequency of the incident laser, and q is the elementary charge. The calculated photoresponsivity R and photoconductive gain G of both devices are plotted as a function of the effective power at 460 nm wavelength under a bias voltage of 20 V in Figure 4B. For the control device, both R and G show an increasing trend as the irradiation power increases, while for the stress liner device, an increasing trend is seen first below 1.454 μW, and then a decreasing trend above this value. The extracted highest achievable responsivity value in stress liner device is 16.103 A W−1 (1.454 μW laser power), which is more than 16 times higher than that of the control device, which is due to the increased absorption capacity of monolayer MoS2 with the Al2O3 stress liner. In addition, the corresponding photoconductive gain G also increases from 14.20 to 191.80, indicating that the Al2O3 stress liner successfully protects the MoS2 layer from environmental contamination, such as from O2, H2O molecules, which in turn reduces the nonradiative recombination ratio [67], [68], [69]. In order to further understand the detection limit and signal-to-noise ratio of the photodetectors, the noise equivalent power (NEP) and normalized detectivity (D*) are shown in Figure 4C. NEP is estimated by the equation
where Idark is the current in the absence of light, R is the responsivity, and f is the bandwidth (here we use Δf=1). A photodetector with stronger ability to detect weak signals has a smaller NEP. D* is defined by
where A is the device active area. In comparison, the lowest NEP=7.96×10−15 W Hz1/2 and highest D*=2.73×1010 at 460 nm are obtained in the stress liner device, which are one order of magnitude lower and higher, respectively, than the corresponding values of the control device.
In order to understand the reproducibility and speed of the control and stress liner devices, we also measured the temporal response of the photodetectors under a fixed voltage 20 V at 460 nm at ambient air environment. Light with power 1.964 μW is switched on/off alternately and the time with/without illumination is set to 40 s. As shown in Figure 5A,B, both devices show good repeatability and stability without degradation of the photoresponse during the cyclic test. In addition, the corresponding rise time is taken as time to increase from 0% to 90% of the maximum photocurrent and the fall time is taken as that to decrease from 100% to 0% of maximum photocurrent, which are displayed in the inset. The decreasing maximum photocurrent per cycle shown in Figure 5B could be caused by the recombination of electrons and holes due to the defects within the Al2O3 layer. The rise time of the control device is 19.0 s, as shown in Figure 5A. In comparison, the corresponding values of the stress liner device is reduced to 12.9 s, as shown in Figure 5B, which is mainly due to the improvement of electron mobility induced by the Al2O3 stress liner, but the fall time shows no significant change [70], [71]. Overall, the performance of monolayer MoS2-based photodetector in this work can be further enhanced by integrating a ferroelectric film or employing a few MoS2 layers [13], [72].
4 Conclusion
In summary, we successfully fabricated high-quality large-area monolayer layer MoS2 on transparent sapphire substrates, which was confirmed by Raman, XPS, and TEM characterizations. Temperature-dependent Raman spectra showed that the thermal stability of monolayer layer MoS2 was improved because of the Al2O3 stress liner. A tensile strain of 0.58% was introduced by depositing 3 nm Al2O3 with ALD. First-principles calculation based on DFT and the measured absorbance verified the effect of the strain introduced by Al2O3 on the electron effective mass and light absorption. In comparison with that of the control device, the photocurrent of the stress liner device improved by 20 times and the responsivity by 15 times under illumination of light at 460 nm. A state-of-the-art performance was demonstrated in our stress liner photodetector, leading to a responsivity of 16.103 A W−1, gain of 191.80, NEP of 7.96×10−15 W Hz−1/2, and detectivity of 2.73×1010 Jones. The rise time of the photodetector was also shortened because of the increased electron mobility introduced by the Al2O3 stress liner. This work demonstrates the potential application of large-scale, strained monolayer MoS2 photodetectors in next-generation imaging systems.
Funding source: National Natural Science Foundation of China
Award Identifier / Grant number: 61974144
Award Identifier / Grant number: 51872187
Award Identifier / Grant number: 61674161
Funding statement: This work was supported by the National Natural Science Foundation of China (61974144, Funder Id: http://dx.doi.org/10.13039/501100001809, 51872187, 61674161) and the Open Project of State Key Laboratory of Functional Materials for Informatics.
References
[1] Radisavljevic B, Radenovic A, Brivio J, et al. Single-layer MoS2 transistors. Nat Nanotechnol 2011;6:147.10.1038/nnano.2010.279Search in Google Scholar PubMed
[2] Fiori G, Bonaccorso F, Iannaccone G, et al. Electronics based on two-dimensional materials. Nat Nanotechnol 2014;9:768.10.1038/nnano.2014.207Search in Google Scholar PubMed
[3] Wang QH, Kalantar-Zadeh K, Kis A, et al. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 2012;7:699.10.1038/nnano.2012.193Search in Google Scholar PubMed
[4] Ganatra R, Zhang Q. Few-layer MoS2: a promising layered semiconductor. ACS Nano 2014;8:4074–99.10.1021/nn405938zSearch in Google Scholar PubMed
[5] Zheng Z-B, Li J-T, Ma T, et al. Tailoring of electromagnetic field localizations by two-dimensional graphene nanostructures. Light: Sci Appl 2017;6:e17057.10.1038/lsa.2017.57Search in Google Scholar PubMed PubMed Central
[6] Wang H, Yu L, Lee Y-H, et al. Integrated circuits based on bilayer MoS2 transistors. Nano Lett 2012;12:4674–80.10.1021/nl302015vSearch in Google Scholar PubMed
[7] Zhang W, Huang Z, Zhang W, et al. Two-dimensional semiconductors with possible high room temperature mobility. Nano Res 2014;7:1731–7.10.1007/s12274-014-0532-xSearch in Google Scholar
[8] Kim S, Konar A, Hwang W-S, et al. High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals. Nat Commun 2012;3:1011.10.1038/ncomms2018Search in Google Scholar PubMed
[9] Wang T, Zhu R, Zhuo J, et al. Direct detection of DNA below ppb level based on thionin-functionalized layered MoS2 electrochemical sensors. Anal Chem 2014;86:12064–9.10.1021/ac5027786Search in Google Scholar PubMed
[10] Wang L, Chen L, Wong SL, et al. Electronic devices and circuits based on wafer-scale polycrystalline monolayer MoS2 by chemical vapor deposition. Adv Electron Mater 2019;5:1900393.10.1002/aelm.201900393Search in Google Scholar
[11] Huang Z, Han W, Tang H, et al. Photoelectrochemical-type sunlight photodetector based on MoS2/graphene heterostructure. 2D Mater 2015;2:035011.10.1088/2053-1583/2/3/035011Search in Google Scholar
[12] Xia F, Mueller T, Lin Y-M, et al. Ultrafast graphene photodetector. Nat Nanotechnol 2009;4:839.10.1364/CLEO.2010.CMV1Search in Google Scholar
[13] Wang X, Wang P, Wang J, et al. Ultrasensitive and broadband MoS2 photodetector driven by ferroelectrics. Adv Mater 2015;27:6575–81.10.1002/adma.201503340Search in Google Scholar PubMed
[14] Yore AE, Smithe KKH, Jha S, et al. Large array fabrication of high performance monolayer MoS2 photodetectors. Appl Phys Lett 2017;111:043110.10.1063/1.4995984Search in Google Scholar
[15] Meng J, Song H-D, Li C-Z, et al. Lateral graphene p–n junctions formed by the graphene/MoS2 hybrid interface. Nanoscale 2015;7:11611–9.10.1039/C5NR02552DSearch in Google Scholar
[16] Choi M S, Qu D, Lee D, et al. Lateral MoS2 p–n junction formed by chemical doping for use in high-performance optoelectronics. ACS Nano 2014;8:9332–40.10.1021/nn503284nSearch in Google Scholar PubMed
[17] Sun B, Shi T, Liu Z, et al. Large-area flexible photodetector based on atomically thin MoS2/graphene film. Mater Des 2018;154:1–7.10.1016/j.matdes.2018.05.017Search in Google Scholar
[18] Chen T, Zhou Y, Sheng Y, et al. Hydrogen-assisted growth of large-area continuous films of MoS2 on monolayer graphene. ACS Appl Mater Interf 2018;10:7304–14.10.1021/acsami.7b14860Search in Google Scholar PubMed
[19] Deng W, Chen Y, You C, et al. Visible-infrared dual-mode MoS2-graphene-MoS2 phototransistor with high ratio of the I ph/I dark. 2D Mater 2018;5:045027.10.1088/2053-1583/aadc79Search in Google Scholar
[20] Liu B, Chen Y, You C, et al. High performance photodetector based on graphene/MoS2/graphene lateral heterostrurcture with Schottky junctions. J Alloys Compd 2019;779:140–6.10.1016/j.jallcom.2018.11.165Search in Google Scholar
[21] De Fazio D, Goykhman I, Yoon D, et al. High responsivity, large-area graphene/MoS2 flexible photodetectors. ACS Nano 2016;10:8252–62.10.1021/acsnano.6b05109Search in Google Scholar PubMed PubMed Central
[22] Paul KK, Mawlong LPL, Giri PK. Trion-inhibited strong excitonic emission and broadband giant photoresponsivity from chemical vapor-deposited monolayer MoS2 grown in situ on TiO2 nanostructure. ACS Appl Mater Interf 2018;10:42812–25.10.1021/acsami.8b14092Search in Google Scholar PubMed
[23] Li S, Chen X, Liu F, et al. Enhanced performance of a CVD MoS2 photodetector by chemical in situ n-type doping. ACS Appl Mater Interf 2019;11:11636–44.10.1021/acsami.9b00856Search in Google Scholar PubMed
[24] Das S, Chen H-Y, Penumatcha AV, et al. High performance multilayer MoS2 transistors with scandium contacts. Nano Lett 2013;13:100–5.10.1021/nl303583vSearch in Google Scholar PubMed
[25] Na J, Joo M-K, Shin M, et al. Low-frequency noise in multilayer MoS2 field-effect transistors: the effect of high-k passivation. Nanoscale 2014;6:433–41.10.1039/C3NR04218ASearch in Google Scholar PubMed
[26] Late DJ, Liu B, Matte HSSR, et al. Hysteresis in single-layer MoS2 field effect transistors. ACS Nano 2012;6:5635–41.10.1021/nn301572cSearch in Google Scholar PubMed
[27] Yoo G, Hong S, Heo J, et al. Enhanced photoresponsivity of multilayer MoS2 transistors using high work function MoOx overlayer. Appl Phys Lett 2017;110:053112.10.1063/1.4975626Search in Google Scholar
[28] Kufer D, Konstantatos G. Highly sensitive, encapsulated MoS2 photodetector with gate controllable gain and speed. Nano Lett 2015;15:7307–13.10.1021/acs.nanolett.5b02559Search in Google Scholar PubMed
[29] Yu S, Ran S, Zhu H, et al. Study of interfacial strain at the α-Al2O3/monolayer MoS2 interface by first principle calculations. Appl Surf Sci 2018;428:593–7.10.1016/j.apsusc.2017.09.203Search in Google Scholar
[30] Yu S, Zhu H, Eshun K, et al. Strain-engineering the anisotropic electrical conductance in ReS2 monolayer. Appl Phys Lett 2016;108:191901.10.1063/1.4947195Search in Google Scholar
[31] Yu S, Xiong HD, Eshun K, et al. Phase transition, effective mass and carrier mobility of MoS2 monolayer under tensile strain. Appl Surf Sci 2015;325:27–32.10.1016/j.apsusc.2014.11.079Search in Google Scholar
[32] Liu K-K, Zhang W, Lee Y-H, et al. Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates. Nano Lett 2012;12:1538–44.10.1021/nl2043612Search in Google Scholar PubMed
[33] Segall MD, Lindan PJD, Probert MJ, et al. First-principles simulation: ideas, illustrations and the CASTEP code. J Phys Condens Matter 2002;14:2717–44.10.1088/0953-8984/14/11/301Search in Google Scholar
[34] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865–8.10.1103/PhysRevLett.77.3865Search in Google Scholar PubMed
[35] Blöchl PE. Projector augmented-wave method. Phys Rev B 1994;50:17953–79.10.1103/PhysRevB.50.17953Search in Google Scholar PubMed
[36] Monkhorst HJ, Pack JD. Special points for Brillouin-zone integrations. Phys Rev B 1976;13:5188–92.10.1103/PhysRevB.13.5188Search in Google Scholar
[37] Baker MA, Gilmore R, Lenardi C, et al. XPS investigation of preferential sputtering of S from MoS2 and determination of MoSx stoichiometry from Mo and S peak positions. Appl Surf Sci 1999;150:255–62.10.1016/S0169-4332(99)00253-6Search in Google Scholar
[38] Kong J, Park KT, Miller AC, et al. Molybdenum disulfide single crystal (0002) plane XPS spectra. Surf Sci Spectra 2000;7:69–74.10.1116/1.1287819Search in Google Scholar
[39] Ruzmetov D, Zhang K, Stan G, et al. Vertical 2D/3D semiconductor heterostructures based on epitaxial molybdenum disulfide and gallium nitride. ACS Nano 2016;10:3580–8.10.1021/acsnano.5b08008Search in Google Scholar PubMed
[40] Calizo I, Balandin AA, Bao W, et al. Temperature dependence of the Raman spectra of graphene and graphene multilayers. Nano Lett 2007;7:2645–9.10.1021/nl071033gSearch in Google Scholar PubMed
[41] Su L, Zhang Y, Yu Y, et al. Dependence of coupling of quasi 2-D MoS2 with substrates on substrate types, probed by temperature dependent Raman scattering. Nanoscale 2014;6:4920–7.10.1039/C3NR06462JSearch in Google Scholar PubMed
[42] Lei S, Ge L, Najmaei S, et al. Evolution of the electronic band structure and efficient photo-detection in atomic layers of InSe. ACS Nano 2014;8:1263–72.10.1021/nn405036uSearch in Google Scholar PubMed
[43] Late DJ. Temperature dependent phonon shifts in few-layer black phosphorus. ACS Appl Mater Interf 2015;7:5857–62.10.1021/am509056bSearch in Google Scholar PubMed
[44] Pawbake AS, Pawar MS, Jadkar SR, et al. Large area chemical vapor deposition of monolayer transition metal dichalcogenides and their temperature dependent Raman spectroscopy studies. Nanoscale 2016;8:3008–18.10.1039/C5NR07401KSearch in Google Scholar
[45] Li K, Ang K-W, Lv Y, et al. Effects of Al2O3 capping layers on the thermal properties of thin black phosphorus. Appl Phys Lett 2016;109:261901.10.1063/1.4973363Search in Google Scholar
[46] Feng X, Kulish VV, Wu P, et al. Anomalously enhanced thermal stability of phosphorene via metal adatom doping: an experimental and first-principles study. Nano Res 2016;9:2687–95.10.1007/s12274-016-1156-0Search in Google Scholar
[47] Ding Z, Pei Q-X, Jiang J-W, et al. Manipulating the thermal conductivity of monolayer MoS2 via lattice defect and strain engineering. J Phys Chem C 2015;119:16358–65.10.1021/acs.jpcc.5b03607Search in Google Scholar
[48] Li H, Tsai C, Koh AL, et al. Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies. Nat Mater 2015;15:48.10.1038/nmat4465Search in Google Scholar PubMed
[49] Christopher JW, Vutukuru M, Lloyd D, et al. Monolayer MoS2 strained to 1.3% with a microelectromechanical system. J Microelectromech Syst 2019;28:254–63.10.1109/JMEMS.2018.2877983Search in Google Scholar
[50] Bertolazzi S, Brivio J, Kis A. Stretching and breaking of ultrathin MoS2. ACS Nano 2011;5:9703–9.10.1021/nn203879fSearch in Google Scholar PubMed
[51] Mak KF, Lee C, Hone J, et al. Atomically thin MoS2: a new direct-gap semiconductor. Phys Rev Lett 2010;105:136805.10.1103/PhysRevLett.105.136805Search in Google Scholar PubMed
[52] Sun Y, Liu K. Strain engineering in functional 2-dimensional materials. J Appl Phys 2019;125:082402.10.1063/1.5053795Search in Google Scholar
[53] Choi W, Cho MY, Konar A, et al. High-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infrared. Adv Mater 2012;24:5832–6.10.1002/adma.201201909Search in Google Scholar PubMed
[54] Wang L, Wang Z, Wang H-Y, et al. Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer. Nat Commun 2017;8:13906.10.1038/ncomms13906Search in Google Scholar PubMed PubMed Central
[55] Deng S, Zhang Y, Li L. Study on electronic and optical properties of the twisted and strained MoS2/PtS2 heterogeneous interface. Appl Surf Sci 2019;476:308–16.10.1016/j.apsusc.2019.01.097Search in Google Scholar
[56] Klots AR, Newaz AKM, Wang B, et al. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy. Sci Rep 2014;4:6608.10.1038/srep06608Search in Google Scholar PubMed PubMed Central
[57] Lee HS, Kim MS, Kim H, et al. Identifying multiexcitons in MoS2 monolayers at room temperature. Phys Rev B 2016;93:140409.10.1103/PhysRevB.93.140409Search in Google Scholar
[58] Massicotte M, Schmidt P, Vialla F, et al. Picosecond photoresponse in van der Waals heterostructures. Nat Nanotechnol 2015;11:42.10.1038/nnano.2015.227Search in Google Scholar PubMed
[59] Nie Z, Long R, Sun L, et al. Ultrafast carrier thermalization and cooling dynamics in few-layer MoS2. ACS Nano 2014;8:10931–40.10.1021/nn504760xSearch in Google Scholar PubMed
[60] Wang K, Wang J, Fan J, et al. Ultrafast saturable absorption of two-dimensional MoS2 nanosheets. ACS Nano 2013;7:9260–7.10.1021/nn403886tSearch in Google Scholar PubMed
[61] Zhang W, Huang J-K, Chen C-H, et al. High-gain phototransistors based on a CVD MoS2 monolayer. Adv Mater 2013;25:3456–61.10.1002/adma.201301244Search in Google Scholar PubMed
[62] Gabor NM, Zhong Z, Bosnick K, et al. Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube p−i−n photodiodes. Phys Rev Lett 2012;108:087404.10.1103/PhysRevLett.108.087404Search in Google Scholar PubMed
[63] Mouri S, Miyauchi Y, Toh M, et al. Nonlinear photoluminescence in atomically thin layered WSe2 arising from diffusion-assisted exciton-exciton annihilation. Phys Rev B 2014;90:155449.10.1103/PhysRevB.90.155449Search in Google Scholar
[64] Wu C-C, Jariwala D, Sangwan VK, et al. Elucidating the photoresponse of ultrathin MoS2 field-effect transistors by scanning photocurrent microscopy. J Phys Chem Lett 2013;4:2508–13.10.1021/jz401199xSearch in Google Scholar
[65] Lähnemann J, Den Hertog M, Hille P, et al. UV photosensing characteristics of nanowire-based GaN/AlN superlattices. Nano Lett 2016;16:3260–7.10.1021/acs.nanolett.6b00806Search in Google Scholar PubMed
[66] González-Posada F, Songmuang R, Den Hertog M, et al. Room-temperature photodetection dynamics of single GaN nanowires. Nano Lett 2012;12:172–6.10.1021/nl2032684Search in Google Scholar PubMed
[67] Yue Q, Shao Z, Chang S, et al. Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field. Nanoscale Res Lett 2013;8:425.10.1186/1556-276X-8-425Search in Google Scholar PubMed PubMed Central
[68] Tongay S, Zhou J, Ataca C, et al. Broad-range modulation of light emission in two-dimensional semiconductors by molecular physisorption gating. Nano Lett 2013;13:2831–6.10.1021/nl4011172Search in Google Scholar PubMed
[69] Zhang H, Hu Y, Wang Z, et al. Performance boosting of flexible ZnO UV sensors with rational designed absorbing antireflection layer and humectant encapsulation. ACS Appl Mater Interf 2016;8:381–9.10.1021/acsami.5b09093Search in Google Scholar PubMed
[70] Mcdonnell S, Brennan B, Azcatl A, et al. HfO2 on MoS2 by Atomic layer deposition: adsorption mechanisms and thickness scalability. ACS Nano 2013;7:10354–61.10.1021/nn404775uSearch in Google Scholar PubMed
[71] Cui Y, Xin R, Yu Z, et al. High-performance monolayer WS2 field-effect transistors on high-κ dielectrics. Adv Mater 2015;27:5230–4.10.1002/adma.201502222Search in Google Scholar PubMed
[72] Chen Y, Wang X, Wang P, Huang H, et al. Optoelectronics properties of few layer MoS2 FET gated by ferroelectric relaxor polymer. ACS Appl Mater Interf 2016;8:32083–8.10.1021/acsami.6b10206Search in Google Scholar PubMed
Supplementary Material
The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2019-0515).
© 2020 Xinke Liu et al., published by De Gruyter, Berlin/Boston
This work is licensed under the Creative Commons Attribution 4.0 International License.