Recently, the Chinese Academy of Sciences Institute of semiconductor superlattice national key laboratories, Li Jingbo researcher, Wei Zhongming researcher and Li Shu's research team, in the new two-dimensional semiconductor heterojunction and its optoelectronic devices based on a series of important progress.
Two dimensional atomic crystals, including graphene and transition metal sulfide (TMDs), such as MoS2, WS2, and WSe2, have been shown to have a unique and excellent optical properties, and thus have gained wide attention. In the last two years, Van Der Waals has been found to be a new material system, which combines the advantages of the individual components, and provides a flexible and broad platform for the study of novel physical phenomena and device functions. Which, based on the two-dimensional TMDs (MX2), has a wide range of optical band gap and strong light material interaction, in the future, the field of optoelectronic devices has an important application prospect. In theory, the MX2 heterojunction tends to have a II type with order arrangement, which can promote the effective separation of the electron hole pairs. In the experiment, the atomic layer thickness of the MX2 heterojunction can also exhibit ultrafast charge transfer and can be controlled by the interlayer coupling effect.
In such a research background, the research team and the United States renewable energy National Laboratory Wei Suhuai group cooperation, found that the two-dimensional semiconductor heterojunction based on WSe2/WS2 showed a unique polarity adjustable phenomenon and self driven optical switching characteristics, the work was completed by Dr. Huo Nengjie. They were prepared by the method of transfer of the WSe2 type P and N type WS2 thin layer structure of the Van Der Waals heterojunction, and detailed study of their optical and electrical transport properties, as shown in figure 1.
Fig. 1 the transfer curves, device diagrams, and optical switching characteristics of the polarity of the WSe2/WS2 heterojunction under different bias voltages are shown.
They were observed in the P-N WSe2/WS2 heterojunction tube with a clear rectification effect and a bipolar behavior, and the rectifying ratio reached 100, and the electron and hole field effect switch ratio reached 1000. Due to the charge trapping at the surface and interface of the heterojunction system, it also shows the hysteresis phenomenon of a large memory window (the maximum voltage shift caused by the gate voltage in the forward scan and reverse scan). It is interesting that this heterojunction can be transformed from the N or P to bipolar or bipolar type with the modulation of an external bias. This is because the junction effect of the heterojunction, the P channel WSe2 and the N channel WS2 are alternately controlled by the electric transport properties of the whole system. At the same time, due to the presence of the inner construction and the II type band, the heterojunction also exhibits a good photovoltaic property and a large amplitude increase of the self driven optical switching characteristics. Under zero bias voltage, the photocurrent can still be generated at the switch of the incident light source, the response time is less than 20 ms and the optical switch ratio is 400. These novel physical phenomena and device functions show that the WSe2/WS2 and other two-dimensional heterojunction in the future of nano electronic devices and optoelectronic devices, including the field effect tube, detector, optical switch, off battery and memory, and other fields has a broad application value. Related papers published online in the Wiley Publishing Group's "Small" magazine, the paper was published in the Views China Materials Wiley website was published in the title of the "polarity can be adjusted to the new two-dimensional semiconductor heterojunction" hot spots.
The research team also used the graphene oxide (rGO) thin film, together with the MoS2 layer to build a Van Edward Mars heterojunction, and the photoelectric device system research, the work was completed by Dr. Yang Juehan. In comparison, the preparation and device testing of a lot of graphene /BN and graphene /MX2 heterojunction have been reported in the literature, and the high transport properties can be obtained under the single structure, and the preparation method is more complex. Based on previous work, the research team successfully introduced rGO thin film (5 to 10nm thick), which was prepared by a simple solution method, which can be used to obtain the typical bipolar transport and high gate controlled light response when it was combined with MoS2.
Raman and photoluminescence tests show that the MoS2/rGO heterojunction can have a strong interface coupling effect, which can lead to the formation of band bending. Field-effect transistor based on heterostructures of the MoS2/rGO thin layer, the Schottky barrier at the interface can be gate voltage control, so under the different gate voltage and source drain voltage can be observed MoS2 conduction band and valence band to participate in the carrier transport, and obtain the typical bipolar transport properties. Because the barrier is effective in the light induced electron hole pairs, the MoS2/rGO thin film exhibits a high optical response rate (2.4 * 104 A/W) and Guang Zengyi (4.7 x 104), and the optical response is also shown to be clearly regulated by the gate (Figure 2). The experimental data obtained from the optical response are also higher than those reported in the single layer (Bo Ceng) of pure MoS2 by one to two orders of magnitude. Related results recently published online in the Advanced Materials Wiley group's magazine Electronic. This work shows the potential applications of MoS2/rGO thin layer heterojunction in the multi - functional optoelectronic devices, and effectively extends the new method for the fabrication of heterojunction.
Fig. 2 the grid controlled light response and the structure of the device in the MoS2/rGO thin layer heterojunction
The work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences and the National Bureau of foreign experts. Related papers
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