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Few‐layered black phosphorus (BP) is known to be oxidized easily in the air which causes property degradation. However, the optical anisotropy of oxidized BP is yet to be well determined. Here, the optical anisotropy of the oxidized BP is revealed by continuous measurement of ambient exposed BP flake over a month using azimuth‐dependent reflectance difference microscopy. The isotropic oxidization process is elucidated by both the disappeared optical anisotropy of oxidized residuals and the unchanged crystalline orientation during BP oxidization. Unlike the severe degradation of electrical conductivity for oxidized BP within hours, the optical anisotropy is rather insensitive to the oxidization. The optical anisotropy of a oxidized BP could remain constant even days because of the isotropic optical properties and low coverage of the oxidized species at initial stage. Furthermore, when the oxidization spots grow, forming an effective layer, the oxidization layer modulates the total optical anisotropy via optical interference, which might result in an increased or decreased optical anisotropy compared with the fresh one. These results shed light on the oxidization impact on optical anisotropy and suggest a new method of manipulating optical anisotropy by controlling the degree of oxidization. [ABSTRACT FROM AUTHOR]

Germanium diselenide (GeSe2) has recently emerged as a new member of in‐plane anisotropic 2D materials, notable for its wide bandgap of 2.74 eV, excellent air stability, and high performance in polarization‐sensitive photodetection. However, the interlayer interaction in GeSe2 has never been reported, which usually plays an important role in layer‐number‐dependent physical properties. Here, the interlayer coupling in GeSe2 is systematically investigated from theory to experiment. Unexpectedly, all of density functional theory (DFT) calculations about layer‐dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density demonstrate the much weaker interlayer interaction in GeSe2 when compared with black phosphorus (BP). Furthermore, both thickness‐dependent and temperature‐dependent Raman spectra of GeSe2 flakes, which exhibit no detectable changes of Raman peaks with the increase in thickness and a small first‐order temperature coefficient of −0.0095 cm−1 K−1, respectively, experimentally confirm the weakly coupled layers in GeSe2. The results establish GeSe2 as an unusual member of in‐plane anisotropic 2D materials with weak interlayer interaction. The ultraweak interlayer interaction in anisotropic GeSe2 is reported here. Azimuth‐dependent reflectance difference microscopy (ADRDM) verifies in‐plane anisotropy of GeSe2. DFT calculations including layer‐dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density confirm weak interlayer interaction in GeSe2, which is further demonstrated by experimental studies, such as thickness‐dependent and temperature‐dependent Raman spectra. [ABSTRACT FROM AUTHOR]

As a new member of 2D materials, GeSe has attracted considerable attention recently due to its fascinating in‐plane anisotropic vibrational, electrical, and optical properties originating from the low‐symmetry crystal structure. Among these anisotropic properties, the anisotropic optical property, as a new degree of freedom to manipulate optoelectronic properties in 2D materials, is of great importance for practical applications. However, the fundamental understanding of the optical anisotropy of GeSe is still under exploration, severely restricting its utility in polarization‐sensitive optical systems. Here, a systematic study about the in‐plane optical anisotropy of GeSe is reported, including its anisotropic optical absorption, reflection, extinction, and refraction. The anisotropic band structure of GeSe is experimentally observed for the first time through angle‐resolved photoemission spectroscopy, explaining the origin of the optical anisotropy. The anisotropic reflection and refraction of GeSe are further directly visualized through the angle‐dependent optical contrast of GeSe flakes by azimuth‐dependent reflectance difference microscopy and polarization‐resolved optical microscopy, respectively. Finally, GeSe‐based photodetectors exhibit a polarization‐sensitive photoresponsivity due to the intrinsic linear dichroism. This study provides fundamental information for the optical anisotropy of GeSe, forcefully stimulating the exploration of novel GeSe‐based optical and optoelectronic applications. In‐plane optical anisotropy of GeSe is systematically studied by angle‐resolved photoemission spectroscopy, azimuth‐dependent reflectance difference microscopy, and polarization‐resolved optical microscopy, thus providing fundamental information for the optical anisotropy of GeSe and stimulating the exploration of novel GeSe‐based optical and optoelectronic applications. [ABSTRACT FROM AUTHOR]

Germanium diselenide (GeSe2) has recently emerged as a new member of in‐plane anisotropic 2D materials, notable for its wide bandgap of 2.74 eV, excellent air stability, and high performance in polarization‐sensitive photodetection. However, the interlayer interaction in GeSe2 has never been reported, which usually plays an important role in layer‐number‐dependent physical properties. Here, the interlayer coupling in GeSe2 is systematically investigated from theory to experiment. Unexpectedly, all of density functional theory (DFT) calculations about layer‐dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density demonstrate the much weaker interlayer interaction in GeSe2 when compared with black phosphorus (BP). Furthermore, both thickness‐dependent and temperature‐dependent Raman spectra of GeSe2 flakes, which exhibit no detectable changes of Raman peaks with the increase in thickness and a small first‐order temperature coefficient of −0.0095 cm−1 K−1, respectively, experimentally confirm the weakly coupled layers in GeSe2. The results establish GeSe2 as an unusual member of in‐plane anisotropic 2D materials with weak interlayer interaction. The ultraweak interlayer interaction in anisotropic GeSe2 is reported here. Azimuth‐dependent reflectance difference microscopy (ADRDM) verifies in‐plane anisotropy of GeSe2. DFT calculations including layer‐dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density confirm weak interlayer interaction in GeSe2, which is further demonstrated by experimental studies, such as thickness‐dependent and temperature‐dependent Raman spectra. [ABSTRACT FROM AUTHOR]