Electronic Properties of Quasi-1D Materials, TiS₃ and ln₄Se₃

Two-dimensional (2D) layered materials have received much interest in recent years due to their ease of miniaturization by exfoliation techniques and very diverse physical properties. Quasi-one-dimensional (quasi-1D) materials, while seeing considerably less interest, can express many of the same desirable properties as conventional layered materials, with an added dimension of anisotropy. A representative example of quasi-1D materials is titanium trisulfide (TiS₃), a layered n-type semiconductor composed of chains of trigonal sulfur prisms with Ti⁴⁺ centers. Because of its moderate bandgap of about 1 eV, which is comparable to that of silicon, and theoretically predicted electron mobilities of up to 10,000 cm² V^-1 s^-1 , TIS₃ is a promising material for electronic applications.
We demonstrate the ease of mechanical exfoliation of bulk TiS₃ crystals accompanied with theoretical calculations and show that these materials exfoliate into few-atomic-layer nanoribbons with very smooth edges. Their characterization by Raman spectroscopy shows a reliable, internally standardized shift of a few cm^-1 from monolayer to bulk demonstrating tunability typical of conventional layered materials. The TiS₃ field-effect transistors showed an n-type electronic transport with characteristics comparable to those of MoS₂, a popular 2D semiconductor. Their room-temperature mobilities of about 30 cm² V^-1 s^-1 were two orders of magnitude smaller than predicted theoretically, which we quantitatively explain by polar-optical phonon scattering in TIS₃. We demonstrate that TiS₃ is compatible with the conventional atomic layer deposition procedure for Al₂O₃, and the encapsulation of TiS₃ with alumina resulted in the mobility increase up to 43 cm² V^-1 s^-1. The quasi-1D TiS₃ chains exhibit an anisotropic photocurrent response to polarized light as well as a gate-tunable metal-insulator transition (MIT) and an access to the charge density wave (CDW) physics.
Many of the conclusions drawn for TiS₃ can be extended to other quasi-1D materials, such as ln₄Se₃. ln₄Se₃ has a different crystal structure, but it also features covalently bonded quasi-1D chains as its basic building blocks. We successfully exfoliated bulk ln₄Se₃ crystals into few-nm-thick flakes with visible signatures of quasi-1D chains, similar to the exfoliated TiS₃ flakes. The ln₄Se₃ flakes exfoliated on Si/SiO₂ have anisotropic electronic properties and exhibit field-effect electron mobilities of about 50 cm² V^-1 s^-1 at room temperature, as well as a polarization dependent photoresponse on a timescale of less than 30 ms. These two examples, TiS₃ and ln₄Se₃, demonstrate the promise of quasi-1D materials for emerging electronic applications.