Single-crystal erbium silicate nanowires have attracted considerable attention because of their high optical gain. In this work, we report the controlled synthesis of silicon-erbium ytterbium silicate coreshell nanowires and fine-tuning the erbium mole fraction in the shell from x=0.3 to x=1.0, which corresponds to changing the erbium concentration from 4.8×10²¹ to 1.6×10²² cm⁻³. By controlling and properly optimizing the composition of erbium and ytterbium in the nanowires, we can effectively suppress upconversion photoluminescence while simultaneously enhancing near-infrared emission. The composition-optimized nanowires have very long photoluminescence lifetimes and large emission crosssections, which contribute to the high optical gain that we observed. We suspended these concentrationoptimized nanowires in the air to measure and analyze their propagation loss and optical gain in the near-infrared communication band. Through systematic measurements using wires with different core sizes, we obtained a maximum net gain of 20±8 dB·mm⁻¹, which occurs at a wavelength of 1534 nm, for a nanowire with a diameter of 600 nm and a silicon core diameter of 300 nm.