Short-Wave Near-Infrared Linear Dichroism of Two-Dimensional Germanium Selenide

Abstract

Polarized detection has been brought into operation for optics applications in the visible band. Meanwhile, an advanced requirement in short-wave near-infrared (SW-NIR) (700-1100 nm) is proposed. Typical IV-VI chalcogenides-2D GeSe with anisotropic layered orthorhombic structure and narrow 1.1-1.2 eV band gap-potentially meets the demand. Here we report the unusual angle dependences of Raman spectra on high-quality GeSe crystals. The polarization-resolved absorption spectra (400-950 nm) and polarization-sensitive photodetectors (532, 638, and 808 nm) both exhibited well-reproducible cycles, distinct anisotropic features, and typical absorption ratios α/α ≈ 1.09 at 532 nm, 1.26 at 638 nm, and 3.02 at 808 nm (the dichroic ratio I/I ≈ 1.09 at 532 nm, 1.44 at 638 nm, 2.16 at 808 nm). Obviously, the polarized measurement for GeSe showed superior anisotropic response at around 808 nm within the SW-NIR band. Besides, the two testing methods have demonstrated the superior reliability for each other. For the layer dependence of linear dichroism, the GeSe samples with different thicknesses measured under both 638 and 808 nm lasers identify that the best results can be achieved at a moderate thickness about 8-16 nm. Overall, few-layer GeSe has capacity with the integrated SW-NIR optical applications for polarization detection.

Highly Anisotropic GeSe Nanosheets for Phototransistors with Ultrahigh Photoresponsivity

“…Figure 1d illustrates the temperature‐dependent Raman spectra measured at the temperature range from 80 to 500 K, which is a practical way for uncovering the atomic bonds, thermal expansion, and phonon vibration properties of 2D materials 21, 29, 30, 31, 32. The peak positions around 82.5, 149.2, and 185.9 are corresponding to the A 1 g , B 3g , and A 3 g modes, respectively, in good agreement with the previous reports 13, 33. As demonstrated in Figure 1d, all these visible peaks exhibited redshift and softening with increasing temperature due to the anharmonic electron–phonon coupling, phonon–phonon interactions, or thermal expansion 32.…”

“…2D GeSe shows excellent air stability and highly in‐plane anisotropic properties due to the low symmetry of Pnma space group 13. GeSe is a p‐type semiconductor with closely located direct and indirect band gaps in the range of 1.1–1.2 eV,13, 14 which matches well with solar spectrum 15.…”

“…The optimized photoresponsivity can reach 1.6 × 10 5 A W −1 (under illumination of 532 nm light), which is about five orders of magnitude higher than those of previously reported GeSe‐based photodetectors13, 22 and 100 times higher than that of monolayer MoS 2 23. As far as we know, this is better than most reported responsivities of similarly structured phototransistors based on single pristine 2D materials13, 22, 23, 24, 25, 26, 27, 28 without further hybridization and functionalization. The high responsivity can be attributed to the highly efficient light absorption as well as the enhanced photoconductive gain resulting from the existence of trap states.…”

“…Commonly, the c ‐axis is named as the armchair direction, and the b ‐axis is designated as the zigzag direction 13. The 2D GeSe crystals were exfoliated from bulk GeSe.…”

“…Thus the Raman scattering intensities of A g and B 3g modes can be further expressed by13, 37 $I({\mathrm{normalA}}_{\mathrm{normalg}},//)={(B{\cos }^2\theta +C{\sin }^2\theta )}^2$ $I({\mathrm{normalB}}_{3g},//)=F^2{\sin }^{2}2\theta$ $I({\mathrm{normalA}}_g,\perp )=\frac{{(C-B)}^2}{4}{\sin }^{2}2\theta$ $I({\mathrm{normalB}}_{3g},\perp )=F^2{\cos }^{2}2\theta$ …”