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Sub-μm thin samples are essential for spectroscopic purposes. The development of flat micro-jets enabled novel spectroscopic and scattering methods for investigating molecular systems in the liquid phase. However, the temperature of these ultra-thin liquid sheets in vacuum has not been systematically investigated. Here, we present a comprehensive temperature characterization using optical Raman spectroscopy of sub-micron flatjets produced by two different methods: colliding of two cylindrical jets and a cylindrical jet compressed by a high pressure gas. Our results reveal the dependence of the cooling rate on the material properties and the source characteristics, i.e., nozzle-orifice size, flow rate, and pressure. We show that materials with higher vapor pressures exhibit faster cooling rates, which is illustrated by comparing the temperature profiles of water and ethanol flatjets. In a sub-μm liquid sheet, the temperature of the water sample reaches around 268 K and the ethanol around 253 K close to the flatjet's terminus.