Tunable Band Alignment and Optical Properties in Van der Waals Heterostructures based on two-dimensional materials Janus-MoSSe and C3N4

van der Waals heterostructures with tunable band alignments are the promising candidates for the fabrication of high-performance multifunctional nano-optoelectronic devices. In this work, we investigate the band alignments and optical properties of two-dimensional MoSSe/C{$_{3}$}N{$_{4}$} and C{$_{3}$}N{$_{4}$}/MoSSe heterostructures using first-principles methods. The two most stable MoSSe/C{$_{3}$}N{$_{4}$} (C{$_{3}$}N{$_{4}$}-Se) and C{$_{3}$}N{$_{4}$}/MoSSe (C{$_{3}$}N{$_{4}$}-S) heterostructures (labeled as A2 and B2, respectively) out of the twelve possible heterostructures are selected for the corresponding properties research. It is found that the A2 exhibits type-I band alignment, making it suitable for light-emitting applications, while the B2 exhibits typical type-II band alignment, which is favorable for carrier separation. Moreover, the band alignment of the two heterostructures can be modulated by the external electric fields, that is, band alignment transition between type-I and type-II. In addition, the main absorption peaks of both heterostructures in their pristine state are located in the visible light region (approximately 2.9 eV), and the peak values of the absorption peaks can be enhanced (weaken) via applying positive (negative) external electric fields. Our findings demonstrate that the C{$_{3}$}N{$_{4}$}/C{$_{3}$}N{$_{4}$} and C{$_{3}$}N{$_{4}$}/MoSSe heterostructures hold significant potential for applications in multifunctional electronic devices including light-emitting, carrier separation, optical modulators, etc.