Charge polarization-driven type-II band alignment and enhanced piezoelectricity in tin nitride halide heterostructures

In the quest for efficient energy conversion materials, we investigate piezoelectric properties of tin nitride halide (SnNX) through strategic design of vertical and lateral SnNCl/SnNBr heterostructures, using first-principles calculations. Two vertical configurations (HB-I and HB-II), based on the choice of the basal atomic layer (SnNBr or SnNCl), are studied with six stacking sequences featuring parallel and antiparallel orientations. The interlayer registry index highlights the dominant role of interface interactions in determining the energy landscape. HB-II configuration exhibits a type-II band alignment for all stacking orders (AA, AB, AC). While only the AC stacking order of HB-I displays a type-II band alignment, which correlates with the reversal in the direction of charge polarization. Lateral heterostructures composed of eight-unit cells of SnNCl and SnNBr [(SnNCl)₈/(SnNBr)₈] are also constructed along armchair and zigzag directions, revealing mixed band alignment at the interfaces. A comprehensive analysis indicates that interfacial charge polarization critically determines the piezoelectric response. The out-of-plane piezoelectric strain coefficient, $d_{33}$ reaches 90 p.m./V in the vertical heterostructure, comparable to leading bulk perovskites. Our findings provide a deeper understanding of band alignment and piezoelectricity in SnNCl/SnNBr heterostructures, paving the way for future experimental efforts to design advanced 2D energy conversion materials with tailored properties.