Search

The remarkable in-plane anisotropy of anisotropic two-dimensional materials (ATDMs), such as black phosphorus (BP), ReS2, and ReSe2, provides a new degree of freedom for the design of novel orientation-based optical and electronic devices. Therefore, visualization of the anisotropy and determination of the crystal orientation (CO) of ATDMs is of fundamental importance. Herein, using a setup equipped with a non-polarized beam splitter, a four-dimensional anisotropic imaging technique (AIT) based on the anisotropic optical response of ATDMs was proposed, by which the COs of many different ATDMs on the same substrate could be efficiently (

van der Waals p-n heterostructures based on p-type black phosphorus (BP) integrated with other two-dimensional (2D) layered materials have shown potential applications in electronic and optoelectronic devices, including logic rectifiers and polarization-sensitive photodetectors. However, the engineering of carriers transport anisotropy, which is related to the linear dichroism, have not yet been investigated. Here, we demonstrate a novel van der Waals device of orientation-perpendicular BP homojunction based on the anisotropic band structures between the armchair and zigzag directions. The structure exhibits good gate-tunable diode-like rectification characteristics caused by the barrier between the two perpendicular crystal orientations. Moreover, we demonstrate that the unique mechanisms of the polarization-sensitivity properties of this junction are involved with the linear dichroism and the anisotropic carriers transport engineering. These results were verified by the scanning photocurrent images experiments. This work paves the way for 2D anisotropic layered materials for next-generation electronic and optoelectronic devices.

Anisotropic two-dimensional materials, such as black phosphorus (BP) and rhenium disulfide (ReS2), possess intrinsically in-plane anisotropic properties that can be tuned for advanced applications, like in thin-film polarizers or polarization-sensitive photodetectors. The crystal orientations should be accurately determined before these materials are applied. However, current methods for measuring the crystalline orientation, such as reflection and transmission spectra, and polarized Raman-related techniques, suffer from significant uncertainty and unreliability because of complex interference, the sample thickness and background signals, which can be particularly strong on transparent substrates. Herein, for the first time, we present a new high-accuracy and background-free method, photothermal detection (PTD), for the identification of the crystalline orientation of BP and ReS2 on a transparent substrate. PTD is a refractive index sensing optical technique, in which a modulated pump beam is absorbed by BP or ReS2, leading to a local change in the refractive index of the photothermal medium. The propagation of the probe beam at different wavelengths is modified by the produced periodical change of the refractive index. PTD could be utilized here to identify the crystalline orientation explicitly and accurately. Taking into account the signal generated by the anisotropic optical absorption of the pump beam and the resulting heating of the liquid medium, PTD has no background interference regardless of the excitation wavelength and samples thickness. By analyzing the relationship between the polarization angle of the pump beam and the strength of the photothermal signal, the crystalline orientation can be accurately distinguished. Therefore, PTD overcomes typical challenges associated with transparent substrates, including insulating and rough surfaces, and enables the unambiguous identification of crystalline orientation.

Manipulating the polarization of an incident beam using two-dimensional materials has become an important research direction towards the development of nano-optical devices. Black phosphorus (BP) and rhenium diselenide (ReSe2) possess excellent in-plane optical anisotropy with optical birefringence in the visible region, which has led to novel applications in polarizing optics and optoelectronics. Herein, the polarization-dependent absorption of BP and ReSe2 and a modulated pump beam is utilized to obtain the photothermal signal from them. The photothermal anisotropy of BP and ReSe2 has been explored using photothermal detection. Then we have defined the photothermal contrast using the ratio of the maximum to the minimum of the photothermal signal. The photothermal contrast of BP and ReSe2 can be obtained accurately by the relationship between the polarization angle of the pump light and the photothermal signal. We demonstrate that a layered BP with different thicknesses can remarkably change the photothermal contrast. In contrast, the photothermal contrast of ReSe2 does not change with the different thicknesses of the samples. Further, the photothermal anisotropies of BP/ReSe2 heterostructures were also explored. The photothermal contrasts of samples were observed to change with different stacking angles indicating that the photothermal anisotropy of heterostructures is dependent on the stacking angle. Our findings provide new prospects for designing novel optical devices based on two-dimensional anisotropic materials, with potential applications in electronics, photonics, and optoelectronics.

Despite many decades of research of diodes, which are fundamental components of electronic and photoelectronic devices with p-n or Schottky junctions using bulk or 2D materials, stereotyped architectures and complex technological processing (doping and multiple material operations) have limited future development. Here, a novel rectification device, an orientation-induced diode, assembled using only few-layered black phosphorus (BP) is investigated. The key to its realization is to utilize the remarkable anisotropy of BP in low dimensions and change the charge-transport conditions abruptly along the different crystal orientations. Rectification ratios of 6.8, 22, and 115 can be achieved in cruciform BP, cross-stacked BP junctions, and BP junctions stacked with vertical orientations, respectively. The underlying physical processes and mechanisms can be explained using "orientation barrier" band theory. The theoretical results are experimentally confirmed using localized scanning photocurrent imaging. These orientation-induced optoelectronic devices open possibilities for 2D anisotropic materials with a new degree of freedom to improve modulation in diodes.