The development of inkjet-printed 2D crystal inks offers the ability to print different 2D materials on various substrates to form vertical heterostructures. However, the detailed characterization of the atomic structures of the inkjet-printed MoTe2 nanosheets has been rarely reported. In this work, water-based 2D crystal inks of MoTe2, WS2, and graphene have been prepared and printed to obtain the flexible photodetectors. The absorption coefficient of MoTe2 has been estimated as α (500 nm) = 925 ± 47 lg−1 m−1 using the gravimetric method. Intriguingly, the inkjet-printed MoTe2 nanosheets down to 4 nm show both the semiconducting 2H and metallic 1T′ phases. The responsivities of the photodetectors based on MoTe2/graphene and WS2/graphene heterostructures can reach 120 mA/W and 2.5 A/W at 532 nm, respectively. Moreover, the inkjet-printed MoTe2/graphene shows a responsivity of 7.7 mA/W at 940 nm. The fabrication technique of inkjet printing will help design flexible optoelectronic devices based transition metal dichalcogenide–graphene heterostructures for the near-infrared photo detection.
Ultrathin hexagonal boron nitride (h-BN) has recently attracted a lot of attention due to its excellent properties. With the rapid development of chemical vapor deposition (CVD) technology to synthesize wafer-scale single-crystal h-BN, the properties of h-BN have been widely investigated with a variety of material characterization techniques. However, the electronic properties of monolayer h-BN have rarely been quantitatively determined due to its atomically thin thickness and high sensitivity to the surrounding environment. In this work, by the combined use of AFM (atomic force microscope) PeakForce Tunneling (PF-TUNA) mode and Kevin probe force microscopy (KPFM) model, both the electrical resistivity (529 MΩ cm) and the inherent Fermi level (∼4.95 eV) of the as-grown monolayer h-BN flakes on the copper substrate have been quantitatively analyzed. Moreover, direct visualization of the high-temperature oxidation-resistance effect of h-BN nanoflakes has been presented. Our work demonstrates a direct estimation of the electronic properties for 2D materials on the initial growth substrate without transfer, avoiding any unwanted contaminations introduced during the transfer process. The quantitative analysis by state-of-the-art atomic force microscope techniques implies that monolayer h-BN can be employed as an atomically thin and high-quality insulator for 2D electronics, as well as a high-temperature antioxidation layer for electronic device applications.
Two-dimensional (2D) lateral heterostructures have shown promising device applications. Although the diodelike responses across the 2D lateral heterostructure have been widely reported, the essential electrical properties, such as the Fermi levels and the lateral built-in potential, have been barely studied, especially with the applied gate voltage. In this work, we report the highly gate-tunable junction within monolayer MoS 2 −WS 2 lateral heterostructures synthesized by our developed shortcut growth strategy. The quantitative determination between the gate voltage and the built-in potential has been revealed by the combined use of Kelvin probe force microscopy and I−V characteristics, in good agreement with the theoretical calculation results. A built-in potential up to 262 meV is observed at V g = 40 V, which is three times larger than that at V g = 0 V. A trap density mediated mechanism is proposed to explain the highly gatetunable built-in potential. Based on the band-discontinuity mode, the energy-band diagram of the monolayer MoS 2 −WS 2 lateral heterostructures is presented, which belongs to the 2D type II n−n heterojunction. Our findings demonstrate the strongly gate voltage-dependent and highly tunable built-in potential within the 2D type II heterojunction, which will become the novel building blocks for 2D optoelectronic, photodetection, and photovoltaic devices.
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