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van der Waals (vdW) heterostructures based on two-dimensional (2D) materials holding design-by-demand features offer astonishing opportunities to construct novel electronics and optoelectronics devices due to the vdW force interaction between their stacked components. At the atomically thin confinement, vdW heterostructure not only exhibits unprecedented properties as an entire counterpart, but also provides unique platforms to manipulate the vdW interfacial behaviors. Therefore, developing characterization techniques to comprehensively understand the coupling effect on structure-property-performance relationship of vdW heterostructures is crucial for fundamental science and practical applications. Here, we focus on the most widely studied 2D semiconductor transition metal dichalcogenides (TMDCs) and systematically review significant advances in characterizing the material and interfacial coupling effect of the related vdW heterostructures. Specially, we will discuss microscopy techniques for unveiling the structure-property relationship of vdW heterostructures and optical spectroscopy measurements for analyzing vdW interfacial coupling effect. Finally, we address some promising strategies to optimize characterization technologies for resolving vdW heterostructures, including coupling multiple characterization technologies, improving temporal and spatial resolution, developing fast, efficient, and non-destructive techniques and introducing artificial intelligence.

Van der Waals (vdWs) heterostructure is emerging as one of the most interesting systems in next-generation flexible optoelectronics. Furthermore, the gate-tunability of vdWs heterostructure is widely applied to enhance the performance of various devices, which is generally achieved by external electric field. However, in-plane cracking and slippage of gate electrode restricted this external electric field modulation in flexible devices. Here, we propose a distinctive strain-gating method to manipulate 2D WSe2-1D ZnO vdWs interfacial charge and modulate its photosensing performance by tuning electronic states of WSe2. With increasing tensile strain, the device shows obvious enhancing photocurrent and the corresponding photoresponsivity reaches up to 394 mA W−1 under white light illumination. Such performance enhancement can be attributed to strain-induced piezopolarization charges on ZnO nanobelt polar surface, which function as "gate" to tune the local transport of photogenerated carriers at the WSe2-ZnO vdWs interface. This work provides a new strategy to achieve interaction between vdWs interface and strain stimuli, which broadens applications of functional vdWs heterostructure for next-generation photodetection or imaging.

The mixed-dimensional van der Waals (vdW) heterostructure is a promising building block for strained electronics and optoelectronics because it avoids the bond fracture and atomic reconstruction under strain. We propose a novel mixed-dimensional vdW heterostructure between two-dimensional graphene and a one-dimensional ZnO nanowire for high-performance photosensing. By utilizing the piezoelectric properties of ZnO, strain modulation was accomplished in the mixed-dimensional vdW heterostructure to optimize the device performance. By combining the ultrahigh electrons transfer speed in graphene and the extremely long life time of holes in ZnO, an outstanding responsivity of 1.87 × 105 A/W was achieved. Under a tensile strain of only 0.44% on the ZnO nanowire, the responsivity was enhanced by 26%. A competitive model was proposed, in which the performance enhancement is due to the efficient promotion of the injection of photogenerated electrons from the ZnO into the graphene caused by the strain-induced positive piezopotential. Our study provides a strain-engineering strategy for controlling the behavior of the photocarriers in the mixed-dimensional vdW heterostructure, which can be also applied to other similar systems in the future.

Monolayer 2D semiconductors (e.g., MoS2 ) are of considerable interest for atomically thin transistors but generally limited by insufficient carrier mobility or driving current. Minimizing the lattice defects in 2D semiconductors represents a common strategy to improve their electronic properties, but has met with limited success to date. Herein, a hidden benefit of the atomic vacancies in monolayer 2D semiconductors to push their performance limit is reported. By purposely tailoring the sulfur vacancies (SVs) to an optimum density of 4.7% in monolayer MoS2 , an unusual mobility enhancement is obtained and a record-high carrier mobility (>115 cm2 V-1 s-1 ) is achieved, realizing monolayer MoS2 transistors with an exceptional current density (>0.60 mA µm-1 ) and a record-high on/off ratio >1010 , and enabling a logic inverter with an ultrahigh voltage gain >100. The systematic transport studies reveal that the counterintuitive vacancy-enhanced transport originates from a nearest-neighbor hopping conduction model, in which an optimum SV density is essential for maximizing the charge hopping probability. Lastly, the vacancy benefit into other monolayer 2D semiconductors is further generalized; thus, a general strategy for tailoring the charge transport properties of monolayer materials is defined.

The atomic-thin 2D semiconductors have emerged as plausible candidates for future optoelectronics with higher performance in terms of the scaling process. However, currently reported 2D photodetectors still have huge shortcomings in ultraviolet and especially visible-blind wavelengths. Here, a simple and nontoxic surfactant-assisted synthesis strategy is reported for the controllable growth of atomically thin (1.5 to 4 nm) ZnO nanosheets with size ranging from 3 to 30 µm. Benefit from the short carbon chains and the water-soluble ability of sodium dodecyl sulfate (SDS), the synthesized ZnO nanosheets possess high crystal quality and clean surface, leading to good compatibility with traditional micromanufacturing technology and high sensitivity to UV light. The photodetectors constructed with ZnO demonstrate the highest responsivity (up to 2.0 × 104 A W-1 ) and detectivity (D* = 6.83 × 1014 Jones) at a visible-blind wavelength of 254 nm, and the photoresponse speed is optimized by the 400 °C annealing treatment (τR = 3.97 s, τD = 5.32 s), thus the 2D ZnO can serve as a promising material to fill in the gap for deep-UV photodetection. The method developed here opens a new avenue to controllably synthesize 2D nonlayered materials and accelerates their applications in high-performance optoelectronic devices.

Special surface plays a crucial role in nature as well as in industry. Here, the surface morphology evolution of ZnO during wet etching is studied by in situ liquid cell transmission electron microscopy and ex situ wet chemical etching. Many hillocks are observed on the (000 1¯ ) O-terminated surface of ZnO nano/micro belts during in situ etching. Nanoparticles on the apex of the hillocks are observed to be essential for the formation of the hillocks, providing direct experimental evidence of the micromasking mechanism. The surfaces of the hillocks are identified to be {01 1¯3¯ } crystal facets, which is different from the known fact that {01 1¯1¯ } crystal facets appear on the (000 1¯ ) O-terminated surface of ZnO after wet chemical etching. O2 plasma treatment is found to be the key factor for the appearance of {01 1¯3¯ } instead of {01 1¯1¯ } crystal facets after etching for both ZnO nano/micro belts and bulk materials. The synergistic effect of acidic etching and O-rich surface caused by O2 plasma treatment is proposed to be the cause of the appearance of {01 1¯3¯ } crystal facets. This method can be extended to control the surface morphology of other materials during wet chemical etching.

In this work, we fabricated a strain-gated piezoelectric transistor based on single In-doped ZnO nanobelt with ±(0001) top/bottom polar surfaces. In the vertical structured transistor, the Pt tip of the AFM and Au film are used as source and drain electrode. The electrical transport performance of the transistor is gated by compressive strains. The working mechanism is attributed to the Schottky barrier height changed under the coupling effect of piezoresistive and piezoelectric. Uniquely, the transistor turns off under the compressive stress of 806 nN. The strain-gated transistor is likely to have important applications in high resolution mapping device and MEMS devices.