Electric-Field Modulated Optical Transitions in Monolayer CrI3 and Its Nanoribbons

TL;DR

This work develops an analytical 28-band tight-binding framework to study electric-field modulation of electronic and optical properties in monolayer CrI3 and CrI3 nanoribbons. Using linear response and the Kubo formalism, it reveals spin-polarized band evolution under a perpendicular field, including field-induced half-metallicity, and shows edge-configuration–dependent edge states that tailor optical transitions. The approach provides a fast, interpretable alternative to first-principles methods and offers design principles for electric-field–controlled spintronic and optoelectronic devices in CrI3 systems.

Abstract

The successful synthesis of few-layer CrI3 has opened new avenues for research in two-dimensional magnetic materials. Owing to its simple crystal structure and excellent physical properties, layered CrI3 has been extensively studied in magneto-optical effects, excitons, tunneling transport, and novel memory devices. However, the most current theoretical studies rely heavily on the first-principles calculations, and a general analytical theoretical framework, particularly for electric-field modulation and transport properties, is still lacking. In this work, using a 28-band tight-binding model combined with linear response theory, we systematically investigate the optoelectronic response for monolayer CrI3 and its nanoribbons. The results demonstrate that: (1) a vertical electric field can selectively close the band gap of one spin channel while the other remains insulating, resulting a transition to an half-metallic state; (2) the electric field dynamically shifts the optical transition peaks, providing a theoretical basis for extracting band parameters from experimental photoconductivity spectra; (3) nanoribbons with different edge morphologies exhibit distinct edge-state distributions and electronic properties, indicating that optical transition can be dynamically modualted through edge design. The theoretical model developed in this study, which can describe external electric field effect, offers an efficient and flexible approach for analytically investigating the CrI3 family and related materials. This model overcomes the limitations of first-principles methods and provides a solid foundation for designing spintronic and optoelectronic devices controlled by electric fields and edge effect.

Figures (6)

• Figure 1: Schematic illustrations of (a) the primitive cell of monolayer CrI$_3$, (b) the Brillouin zone, and (c) the atomic structures of CrI$_3$ nanoribbons with the four distinct boundary types marked in which are defined by different edge-terminating atoms. Different combinations of these boundary types form nanoribbons with various edge configurations, as detailed in the main text. Cr and I atoms are represented by light purple and deep purple spheres, respectively.
• Figure 2: Evolution of band structures in monolayer CrI$_3$ under applied vertical electric field. Red and blue curves denote spin-up and spin-down bands, respectively.
• Figure 3: (Color online) Optical conductivity of monolayer CrI$_3$ under different strengths of vertical electric field. The transition peaks marked by green arrows correspond to inter-subband transitions shown in Fig.2.
• Figure 4: (Color online) Band structures of monolayer CrI$_3$ nanoribbons with distinct edge configurations. Green solid lines and red dashed lines represent edge states, which are contributed by spin-down electronic states. The arrows in the figures correspond to the optical transition processes shown in Fig. 6.
• Figure 5: The LDOS for edge atoms of monolayer CrI$_3$ nanoribbons: (a) Ribbon_1, (b) Ribbon_2, and (c) Ribbon_4.