Synthesis and Characterization of the 2D Layered Heterostructure Bismuth Triiodide—Graphene

Among the new emerging 2D materials, bismuth tri-iodide-graphene heterostructure has been reported both theoretical and experimentally, displaying intriguing properties. These findings suggest potential photovoltaic applications for BiI₃ layers and van der Waals superstructures. In this context, the present work reports the growth and characterization of the heterostructure BiI₃–graphene. The growth was performed in a chamber designed and built in our laboratory, through physical vapor deposition, by first nucleating BiI₃ on graphene-covered substrates pre-treated with UV/O₃, then allowing it to grow by non-classical attachment mechanisms followed by thermal annealing and post growth, up to three times (initial pressure 0.001-0.0001 Pa, sublimation temperature of BiI₃ for nucleation and post-growth 262±2°C, substrate temperature 40±1°C, thermal annealing at 150±1°C during 3600 s, post-growth (after annealing) from 120 to 600 s). The heterostructure was characterized by Bragg-Brentano X-Ray Diffraction (XRD), Grazing Incidence XRD (GIXRD), X-Ray reflectometry (XRR), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive Spectroscopy (EDS), Selected Area Electron Diffraction (SAED), Fast Fourier Transform (FFT) analysis, High Resolution Scanning Electron Microscopy (HR-SEM), Atomic Force Microscopy (AFM), X-Ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy and diffuse reflectance spectroscopy with integrating sphere. TEM grids and ultra-flat silicon substrates, both covered with graphene, were used as substrates, according to the characterization needs. Results show that nucleation proceeds through a non-classical mechanism with metastable amorphous entities that later pass to crystalline forms about 4 nm in size, growing through non-classical mechanisms of “oriented attachment” and “amorphous addition”. Entities are oriented with their <i>c</i>-axis perpendicular to the substrate. After the annealing and post-growth, structures obtained on substrates result in layers, with thicknesses in the range 7.6 to 35 nm, with roughness in the range 0.4 to 6 nm. The obtained heterostructures are uniform and aligned, without twisting (no Moire interference observed neither in HR-TEM images nor SAED diagrams). As the thickness decreases, the position of the (<i>0 0 3</i>) reflection peak in the diffraction pattern shifts to lower values and broadens, indicating an expansion of the cell along the <i>c</i>-axis. XRD and HR-TEM verify BiI₃ composition as well as the layer orientation with their <i>c</i>-axis perpendicular to the substrate. HR-TEM images of the structures show lattice planes with nearly perfect atomic arrangement, suggesting a single-crystalline nature, with a predominantly lattice spacing of 0.35 nm, which can be attributed to the (<i>1 1 0</i>) planes of BiI₃ hexagonal structure The FFT of the structure further confirms its single crystal nature and displays spots in a hexagonal symmetry, which can be indexed to the BiI₃ hexagonal structure along [<i>0 0 l</i>] zone axis. HR-SEM images verify the substrate continuous coverage, and AFM measurements confirm the roughness of the layers. XPS analysis gives peaks for Bi 4f_5/2 (165.2 eV) and 4f_7/2(159.8 eV), I 3d_3/2(631.0 eV) and 3d_5/2 (619.3 eV) which can be attributed to the characteristic signals from I^- and Bi³⁺ species, respectively, as far as the C-C from C sp₂ bond (284.7 eV), showing there are no chemical reactions between graphene and BiI₃ which then only interact through weak van der Waals interaction. Raman spectra show a peak around 115.2 cm^-1, which represents out-plane Ag Raman mode. All these results agree with previously reported values for layers of this heterostructure. The obtained bandgap values, ranging from 1.55 to 1.62eV, closely align with theoretical predictions. The obtained results pose these structures as part of the 2D universe of other similar materials, such as TMDs-graphene, and open up exciting possibilities for new properties and applications.