† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 11874423).
The grain boundaries of graphene are disordered topological defects, which would strongly affect the physical and chemical properties of graphene. In this paper, the spectral characteristics and photoresponse of MoS2/graphene heterostructures are studied. It is found that the blueshift of the G and 2D peaks of graphene in Raman spectrum is due to doping. The lattice mismatch at the graphene boundaries results in a blueshift of MoS2 features in the photoluminescence spectra, comparing to the MoS2 grown on SiO2. In addition, the photocurrent signal in MoS2/hexagonal single-crystal graphene heterostructures is successfully captured without bias, but not in MoS2/polycrystalline graphene heterostructures. The electron scattering at graphene grain boundaries affects the optical response of MoS2/graphene heterostructures. The photoresponse of the device is attributed to the optical absorption and response of MoS2 and the high carrier mobility of graphene. These findings offer a new approach to develop optoelectronic devices based on two-dimensional material heterostructures.
The discovery of graphene has opened the door to study other two-dimensional (2D) materials,[1–5] such as 2D metal chalcogenides (2DMCs), black phosphorus, and so on.[6] These materials have attracted great interest owing to their completely different electronic structures from those of bulk materials. Furthermore, novel electronic properties and photoresponse can be achieved, due to the integration of two or more 2D materials.[7,8] On account of its atomic-level thickness, the layered heterostructure also provides excellent optical transparency and good mechanical flexibility.[9]
Graphene is expected to be used in many electronic devices due to its high conductivity and high carrier mobility. However, its low photoresponse limits its potential.[10–12] Single layer MoS2 has high optical absorption and distinct photoluminescence (PL). Nonetheless, it has low carrier mobility, which affects its responsivity. Therefore, the two materials can complement each other in the MoS2/graphene heterostructures. The single layer MoS2 can enhance the optical absorption, while transfer photo-generated carriers effectively to graphene.[13] New photoelectric devices have been developed based on these heterostructures, such as high gain photodetectors and optical responding memories.[14] MoS2 is used as the sensitizer of light, and graphene with high mobility is used as highway to transport the photo-excited carrier. In this case, the combination of high optical absorption and fast electron transportation makes the MoS2/graphene phototransistors more responsive (up to 108 A/W) than those with only graphene or MoS2.[15,16]
Nonetheless, the existence of grain boundaries (GBs) leads to the deterioration of physical properties of graphene, such as the decrease of chemical reactivity,[17] thermal conductivity,[18] and carrier mobility.[19] This, means that growing single-crystal graphene (SCG), identifying and modifying its GBs are very important for practical applications. The GBs of graphene are disordered topological defects, which strongly affect the physical and chemical properties of graphene,[20] therefore, its application in devices. Recent research shows that the distribution of defects at grain boundaries is linear periodic.[21] In general, nonhexagonal rings (such as five-, seven-, and eight-membered rings) exist at the grain boundaries to mediate the lattice mismatch between two graphene domains.[22] The inherent topological defects can accommodate lattice defects, but retain three folds of sp2 bonding of carbon atoms.[21] Topological defects are also the basis of polycrystalline graphene. Such defects may adversely affect the thermal and electronic properties of graphene, such as reducing the electronic mobility. The growth of MoS2 on graphene depends on van der Waals interaction, which can be used to mark the graphene GBs.[23] Therefore, it is crucial to investigate the influence of graphene GBs on the photoelectric response of MoS2/graphene heterostructures.
In this paper, single crystal MoS2 is deposited on hexagonal single-crystal graphene (HSCG) by molecular beam epitaxy (MBE). There is only one orientation of MoS2 on SCG. The spectral characteristics and photoresponse of the samples are measured. The G and 2D peaks of graphene demonstrate a blueshift after growing a monolayer of MoS2. The quenching of PL of MoS2 is remarkable, while the PL peaks at the MoS2/graphene GBs shift toward short wavelength. Besides, the photocurrent signal in the MoS2/HSCG heterostructures is successfully measured without bias.
The HSCGs were grown by inserting transition metal into Cu pocket.[24,25] SLG was grown on the Cu pocket via the low-pressure-chemical-vapor-deposition process.[23] After that, the polymethyl methacrylate (PMMA)-assisted method was used to transfer the graphene onto SiO2/Si substrates[26] for further synthesis. MoS2 crystals were grown by an MBE system using MoO3 and S powder as precursors.[23] The aqueous solution (2 M NaOH) at 90 °C was used to transfer the MoS2/graphene onto Cu grids. Finally, electron beam lithography was used to prepare a pair of Ti/Au electrodes.[27]
The surface morphology of materials was characterized by a scanning electron microscope (SEM) (Zeiss Sigma). A Raman spectrometer (Alpha 300, WITec) with a 488 nm laser excitation source was used to measure Raman and PL spectra. The photocurrent was measured using the Raman spectrometer with a 532 nm laser. TEM (JEM-2100, operated at 200 keV with a point-to-point resolution of 0.19 nm) was used to probe the heterostructures jointly with selected area electron diffraction (SAED) patterns.
A few number of HSCG domains on Cu pocket are observed by SEM, as shown in Fig.
To confirm the orientation of the MoS2/graphene heterostructures, high resolution TEM (HR-TEM) images of the heterostructures and corresponding diffraction patterns were obtained by using TEM and SAED. Figure
Figure
It is known that elevated temperature, charge transfer, and strain affect the position of the 2D peak of graphene. Since the spectrum is measured at room temperature, the blueshifts of the G and 2D peaks are not due to increasing temperature. On the other hand, the van der Waals contact between graphene and MoS2 tends to change the doping level,[31] while the increase of the doping concentration generally leads to a reduction of the I2D/IG ratio.[5] Therefore, the blueshift in this case may due to doping.
The Raman spectra with characteristic MoS2 features are measured and shown in Fig.
Figures
The photocurrent signal is successfully captured by irradiation of a 532 nm laser without bias, which proves the presence of photoinduced electron transfer from MoS2 to graphene. Figure
The photocurrent near the MoS2/graphene interface in Fig.
The photothermoelectric effect (PTE) will form a photo-generated voltage VPTE and generate a photocurrent in graphene,[35,36] due to the interaction between electrons. The photoresponse can be calculated by VPTE = (S2 – S1)ΔT, where S1 and S2 are the Seebeck coefficients, and ΔT is the temperature difference. Therefore, the photothermoelectric effect is one of the causes of the photocurrent in this case.
As shown in Fig.
In summary, the MoS2/graphene heterostructures are prepared and the spectra and photoresponse are systematically investigated. It is observed that MoS2 can mark the position of graphene grain boundaries, and the PL of MoS2 is quenched after growing on graphene. The PL peak position of MoS2 grown on graphene grain boundaries shows a blue-shift due to the lattice mismatch. Besides, the photocurrent signal is observed in MoS2/hexagonal single-crystal graphene heterostructures without a bias, but not in MoS2/polycrystalline graphene heterostructures. The electron scattering at the graphene grain boundaries affects the optical response of MoS2/graphene heterostructures. The effect of graphene GBs on MoS2/graphene heterostructures may have wide applications in optoelectronics devices.
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