PHYSICAL REVIEW MATERIALS 6, 054001 (2022) First-principles investigation of the role of Cr in the electronic properties of the two-dimensional Mo x Cr 1-x Se 2 and W x Cr 1-x Se 2 alloys A. C. Dias , 1 , * Helena Bragança , 2, † Matheus P. Lima , 3 , ‡ and Juarez L. F. Da Silva 1 , § 1 São Carlos Institute of Chemistry, University of São Paulo, P.O. Box 780, 13560-970 São Carlos, São Paulo, Brazil 2 Physical Institute and International Center for Physics, Universidade de Brasília, Brasília 70919-970, Distrito Federal, Brazil 3 Department of Physics, Federal University of São Carlos, 13565-905 São Carlos, São Paulo, Brazil (Received 11 December 2021; revised 2 April 2022; accepted 22 April 2022; published 12 May 2022) The tuning of the structural and electronic properties of two-dimensional semiconductor monolayers is highly desirable for designing van der Waals heterostructures, which can be employed for several optoelectronic applications. Here, we report a theoretical investigation based on the combination of spin-polarized density functional theory calculations and alloying structures generated by the special quasirandom structure method to investigate the energetic stability and band gap engineering of the compounds Mo x Cr 1−x Se 2 and W x Cr 1−x Se 2 as a function of the Cr composition for x = 0 up to 1. We found that even a small concentration of Cr already flattens the low-energy electronic bands and decreases the fundamental electronic band gap. Due the lattice mismatch of the compounds CrSe 2 and Mo(W)Se 2 , the renormalization of the electronic properties is nonlinear as a function of the Cr composition. We found bowing parameters for the work function and band gap that change in magnitude from 0.066 to 1.178eV, respectively. From our analyses, Cr alloying decreases the band gap of these monolayers in the direction of the maximum performance band gap predicted by the Shockley-Queisser limit for photovoltaic applications. Band alignment analysis reveals that stacks of Mo(W) x Cr 1−x Se 2 monolayers with particular compositions x can form type II heterojunctions with a high solar harvesting efficiency. DOI: 10.1103/PhysRevMaterials.6.054001 I. INTRODUCTION A new frontier opened in the study of two-dimensional (2D) materials after the experimental isolation of graphene in 2004 [1]; however, the semimetal nature of graphene hin- ders its direct application in semiconductor nanodevices [2]. Thus, alternative 2D materials have been investigated [3–5] with the aim to identify candidates for real-life applications. Among several 2D materials, 2D transition-metal dichalco- genide (TMD) monolayers have attracted great interest due to their intrinsic electronic and optical properties [6,7], espe- cially their higher optical absorbance despite their few-atom thickness [8], and valley optical selection rules [9], which make 2D TMDs potential candidates for applications in na- noelectronic [10] and photovoltaic [11] devices. Two-dimensional TMD monolayers are formed by the combination of a transition-metal M layer (M = Mo, W, Cr, etc.) sandwiched between two chalcogen X layers (X = S, Se, Te), which are represented by the chemical formula MX 2 . These monolayers can be found in two structures, namely, trigonal prismatic (2H ) and octahedral (T ), which is subdi- vided into three variants (1T ,1T ′ ,1T d ) due to the possibility of structure Peierls distortions [4,5,12]. TMD monolayers or stacked 2D monolayers [13] can be found in metallic, semi- * alexandre.dias@unb.br † helena.braganca@unb.br ‡ mplima@df.ufscar.br § juarez_dasilva@iqsc.usp.br conductor, and even superconductor phases [4,5,14], which opens the possibility of designing van der Waals (vdW) het- erostructures [13,15] with a wide variety of properties. Several potential technological applications have a close relationship with the nature of the fundamental electronic band gap and its absorption spectrum [5,8,16]. Thus, the engi- neering of the electronic structure is fundamental to design TMDs for real-life applications. For example, the alloying mechanism can be employed to tailor their electronic prop- erties and realize their full potential. Several experimental studies focused on the synthesis and characterization of the Mo x W 1−x S 2 and MoS 2x Se 2(1−x ) [17,18] alloy compounds, which yielded the following trends: (i) chemical vapor depo- sition synthesis with excellent uniformity and a controllable composition, (ii) high alloy stability against phase separation, (iii) alloys retaining a direct band gap and high photolumines- cence intensity, and (iv) broadening of the optical band gap in the range from 1.83 to 1.55 eV (1.83 up to 1.97 eV) for MoS 2x Se 2(1−x ) (Mo x W 1−x S 2 ) by controlling the ratio of S/Se (Mo/W) in the composition [17]. Complementary to the experimental studies, theoretical calculations based on density functional theory (DFT) have found good thermodynamic stability in those systems, and the range of values of the electronic band gap is consistent with experimental results [19,20]. Despite several studies inves- tigating the applicability of TMDs in photovoltaics [21,22], the electronic band gaps obtained with the S/Se and Mo/W composition modulation are still far from the ideal band gap for photovoltaic applications, i.e., close to 1.34 eV [23]. Hybrid DFT calculations demonstrated that Cr-based TMDs 2475-9953/2022/6(5)/054001(11) 054001-1 ©2022 American Physical Society