2D van der Waals Molecular Crystal β‐HgI₂: Economical, Rapid, and Substrate‐Free Liquid‐Phase Synthesis and Strong In‐Plane Optical Anisotropy

Abstract

Abstract2D materials have a great potential for wide‐range applications due to their adjustable bandgap characteristics and special crystal structures. β‐HgI2 is a new 2D van der Waals inorganic molecular crystal material with a wide bandgap of 4.03 eV, on whose preparation and properties there are few relevant reports due to the feature of instability of molecular crystals. Here, an economical method to control the synthesis of large‐size 2D β‐HgI2 single crystal by using a mineralizer‐assisted solution is reported. According to angle‐resolved polarization Raman spectroscopy and first‐principles optical absorption calculation, 2D β‐HgI2 flake has a strong in‐plane anisotropic light scattering characteristic and high optical absorption dichroism (az/ay = 3.4), which is due to a low in‐plane symmetry of the orthorhombic structure of β‐HgI2. More importantly, due to the molecular crystal structure of β‐HgI2, its sensitivity to temperature is less than that of 2D materials such as MoS2, which has been confirmed by temperature‐dependent Raman spectroscopy. In the work, more 2D inorganic molecular crystals are studied in the aspect of growth, which provides a theoretical basis for 2D molecular crystal optoelectronic devices’ potential applications.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals

“…The long-range ordered stacking of inorganic molecules and intermolecular interactions directly lead to the formation of lattice phonons and Davydov-splitting effects. , At the limit of vanishing intermolecular interactions, the lattice phonons fall into zero frequency and the splits of Raman internal modes (Davydov splitting) disappear . In addition, the first-order temperature coefficient (χ) of Raman modes can effectively reflect the strength of the intermolecular coupling. − Figure c shows the Raman spectra of a P 4 Se 3 nanoflake at temperatures ranging from 80 to 300 K, and the Raman spectrum under 80 K is displayed in Figure S6a. As the temperature is decreased, the Raman peaks around 320, 365, and 485 cm –1 exhibit conspicuous Davydov splitting due to the stronger intermolecular interactions at low temperature.…”

“…Though the splitting is usually expected to be small, surprisingly, the Davydov splitting for the P 4 Se 3 nanoflake is larger than those for the isoelectronic P 4 S 3 and As 4 S 3 . ,,, The notable Davydov splitting reveals the strong intermolecular interactions of α-P 4 Se 3 nanoflakes. , To further analyze the intermolecular interactions between P 4 Se 3 molecules, the first-order temperature coefficients (χ) of the Raman modes were extracted (shown in Figure d and Figure S6b,c) by fitting the peak position with the equation: ω­( T ) = ω 0 + χ T , where T is the Kelvin temperature of the sample and ω 0 is the Raman frequency with the temperature extrapolated to 0 K. , As shown in Table S3, the first-order temperature coefficient of α-P 4 Se 3 nanoflakes at 489 cm –1 (Figure d) is comparable to or slightly larger than those of common 2D van der Waals solids, such as MoS 2 , WS 2 , RhI 3 , and MnPSe 3 . In addition, in comparison to the previously reported molecular crystal β-HgI 2 , the first-order temperature coefficients of α-P 4 Se 3 nanoflakes at 367 and 489 cm –1 are both much larger, which confirms the strong intermolecular interactions in 2D P 4 Se 3 nanoflakes as well.…”

Recent Advances in Growth, Properties and Applications of 2D Inorganic Molecular Crystals†

“…The second method of synthesizing 2DIMC involves mineralizer‐assisted solutions. [ 6 ] In this work, 2D HgI 2 was synthesized by liquid phase method, and the schematic diagram is shown in Figure 7d. This method is simpler and faster than the traditional 2D material growth method (CVD) because it does not require vacuum, high growth temperatures and long growth cycles.…”

“…Generally, for 2DIMCs, high stability and high carrier mobility are obtained by improving intermolecular coupling. [ 6‐8 ] The existing problem is that strong intermolecular interactions caused by π‐π stacking in 2D organic crystals have been extensively studied, [ 9‐12 ] however, there are few studies on strong intermolecular interactions in 2DIMCs. [ 13‐14 ] Therefore, the lack of understanding of the strong intermolecular interactions in 2DIMCs has greatly hindered its development in expanding new members, developing new properties and exploring new applications.…”

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

“…[ 52 ] In particular, a remarkable photoelectric power conversion efficiency of up to 23.4% can be achieved based on the optimally designed heterostructure. It is noteworthy that a number of 2DLMs share a common stoichiometric formula (e.g., MX, [ 53 , 54 , 55 , 56 , 57 , 58 , 59 ] MX 2 , [ 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 ] MX 3 , [ 71 , 72 , 73 , 74 ] M 2 X 3 , [ 75 , 76 , 77 , 78 , 79 , 80 ] ABX 2 , [ 81 , 82 ] ABX 3 , [ 35 , 83 , 84 , 85 ] etc.) and analogous crystal structures.…”