Anisotropic Infrared Response and Orientation-Dependent Strain-Tuning of the Electronic Structure in Nb₂SiTe₄

Abstract

Two-dimensional materials with tunable in-plane anisotropic infrared response promise versatile applications in polarized photodetectors and field-effect transistors.Black phosphorus is a prominent example. However, it suffers from poor ambient stability. Here, we report the strain-tunable anisotropic infrared response of a layered material Nb2SiTe4, whose lattice structure is similar to the 2H-phase transition metal dichalcogenides (TMDCs) with three different kinds of building units. Strikingly, some of the strain-tunable optical transitions are crystallographic axis-dependent, even showing opposite shift when uniaxial strain is applied along two in-plane principal axes.Moreover, G0W0-BSE calculations show good agreement with the anisotropic extinction spectra. The optical selection rules are obtained via group theory analysis, and the strain induced unusual shift trends are well explained by the orbital coupling analysis. Our comprehensive study suggests that Nb2SiTe4 is a good candidate for tunable polarization-sensitive optoelectronic devices.

Optical properties and polaritons of low symmetry 2D materials

“…Calculation results by Chaves et al showed that, although larger than the usual trion binding energy in TMDCs, even the trion binding energy in suspended monolayer BP is only ∼51 meV, and decreases to ∼34 meV in supported BP, which is much smaller than the claim by Yang et al [138] . Due to the discrepancy, the validity [41,42,125,126] 8.2(x), 2.3(y) [133] AC(x) Multi-interband w 0.3-0.45 [102,106,108] , 0.9 [104] Bulk / MX (M = Ge, Sn; X = S, Se) GeS 1L w/o ∼0.7-1.2 [34,128,129] 6.7(x), 4.5(y) [34] AC(x) Multi-valley exciton w ∼0.6 [129] Bulk / <0.1 GeSe 1L w/o ∼0.3-0.4 [34,130,129] 21.2(x), 15.4(y) [34] w ∼0.2 [129] Bulk / <0.1 SnS 1L w/o ∼0.5-0.9 [34,128] 15.1(x), 15.1(y) [34] Bulk / <0.1 SnSe 1L w/o ∼0.3 [34,130] 27.3(x), 25(y) [34] Bulk / <0.1…”

“…(c) Illustration of interband transitions at normalΓ point of BP. (d) Photocurrent spectra of thin TiS3 (∼15 nm) (left) [99] and absorption spectra of thin Nb2SiTe4 (∼18 nm) (right) [102] . (e) Layer-dependent bandgap and interband transitions.…”

“…Bulk layered TiSe3 and Ta2NiS2 were reported with narrow bandgaps of ∼0.2 and 0.3 eV in MIR, respectively [93,100] , while the optical properties of their 2D counterparts are hardly known. Recently, Nb2SiTe4 (NST) was discovered as an IR anisotropic 2D material [101,102] . NST belongs to the monoclinic crystal system, possessing in-plane anisotropic atomic structure formed by a Te-(Nb, Si)-Te sandwich layer, which is similar to the 2H-TMDCs such as MoS2 but with two types of atoms (Nb and Si) in the inner layer instead of one (Mo) in the MoS2 (as shown in Fig.…”

“…1). Theoretical calculations suggest that NST has a narrow indirect bandgap ranging from 0.89 eV in the monolayer to 0.39 eV in the bulk, situating in the NIR to MIR regime [102] . Devices based on few-layer NST show a relatively high carrier mobility of prefix∼100 cm2false/false(normalV normalsfalse) and photon responsivity of 0.66 A/W.…”

“…The different changes in monolayer puckered height d cancel the difference between the strain effect along AC and ZZ directions. Recently, Wang et al reported that the strain effect for interband absorption in few-layer Nb2SiTe4 exhibits strong orientation dependence [102] , in particular, for transitions B and B*, which blueshift (redshift) with tensile strain along the a axis (b axis). Other interband absorption peaks exhibit anisotropic strain effects as well, manifested by the different shift rates of the absorption peak with strain along different axes.…”