Graphene quantum dots (GQDs) have attracted significant interest as synthetically tunable optoelectronic and photonic materials that can also serve as model systems for understanding size-dependent behaviors of related graphene structures such as nanoribbons. We present a Raman spectroscopy study of bottom-up synthesized GQDs with lateral dimensions between 0.97 to 1.62 nm, well-defined (armchair) edge type, and fully benzenoid structures. For a better understanding of observed size-dependent trends, the study is extended to larger graphene structures including nano-graphene platelets (>25 nm) and large-area graphene. Raman spectra of GQDs reveal the presence of D and G bands, as well as higher order modes (2D, D + G, and 2G). The D and G band frequencies and intensity were found to increase as GQD size increases, while higher order modes (2D, D + G, and 2G) also increased in intensity and became more well-defined. The integrated intensity ratios of D and G bands (I_D/I_G) increase as the size of the GQDs approaches 2 nm and rapidly decrease for larger graphene structures. We present a quantitative comparison of I_D/I_G ratios for the GQDs and for defects introduced into large area graphenes through ion bombardment, for which inter-defect distances are comparable to the sizes of GQDs studied here. Close agreement suggests the I_D/I_G ratio as a size diagnostic for other nanographenes. Finally, we show that Raman spectroscopy is also a good diagnostic tool for monitoring the formation of bottom-up synthesized GQDs.

Chalcogenide perovskites (CPs), with the general composition ABX₃, where A and B are metals and X = S and Se, have recently emerged as promising materials for application in photovoltaics. However, the development of CPs and their applications has been hindered by the limitations of available preparation methods. Here we present a new approach for the synthesis of CPs, based on the sulfurization of ternary and binary oxides or carbonates with in situ formed boron sulfides. In contrast to the previously described approaches, the method presented here uses chemically stable starting materials and yields pure-phase crystalline CPs within several hours, under low hazard conditions. CP yields over 95% are obtained at temperatures as low as 600 °C. The generality of the approach is demonstrated by the preparation of CPs with compositions BaZrS₃, β-SrZrS₃, BaHfS₃, SrHfS₃, and EuHfS₃. Mechanistic insights about the formation of CPs are discussed.

The effects of temperature and composition on the structural and electronic properties of chalcogenide perovskite (CP) materials AZrX₃ (A = Ba, Sr, Ca; X = S, Se) in the distorted perovskite (DP) phase are investigated using ab initio molecular dynamics (AIMD) accelerated by machine-learned force fields. Long-range van der Waals (vdW) interactions, incorporated into the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional using the DFT-D3 scheme, are found to be crucial for achieving correct predictions of structural parameters. Our calculations show that the distortion of the DP structure with respect to the parent cubic (C) phase, realized in the form of interoctahedral tilting, decreases with the increasing size of the A cations. The tendency for a gradual transformation of the DP-to-C phase with increasing temperature is shown to be strongly composition-dependent. The transformation temperature decreases with the size of cation A and increases with the size of anion X. Thus, within the range of the temperatures considered here (300-1200 K), a complete transformation is observed only for BaZrS₃ (∼600 K) and BaZrSe₃ (∼900 K). The computed band gap of CPs is shown to monotonically decrease with increasing temperature, and the magnitude of this decrease is found to be proportional to the extent of the thermally induced changes in the internal structure. Diverse factors affecting the magnitude of band gaps of CP materials are analyzed.