Correlation between crystal purity and the charge density wave in 1$T$-VSe$_2$

We examine the charge density wave (CDW) properties of 1$T$-VSe$_{2}$ crystals grown by chemical vapour transport (CVT) under varying conditions. Specifically, we find that by lowering the growth temperature ($T_{\mathrm{g}}$ $<$ 630$^{\circ}$C), there is a significant increase in both the CDW transition temperature and the residual resistance ratio (RRR) obtained from electrical transport measurements. Using x-ray photoelectron spectroscopy (XPS), we correlate the observed CDW properties with stoichiometry and the nature of defects. In addition, we have optimized a method to grow ultra-high purity 1$T$-VSe$_{2}$ crystals with a CDW transition temperature, $T_{\mathrm{CDW}}$ = (112.7 $\pm$ 0.8) K and maximum residual resistance ratio (RRR) $\approx$ 49, which is the highest reported thus far. This work highlights the sensitivity of the CDW in 1$T$-VSe$_{2}$ to defects and overall stoichiometry, and the importance of controlling the crystal growth conditions of strongly-correlated transition metal dichalcogenides.