<p>Spin-1/2 Heisenberg antiferromagnetic chains are one-dimensional platforms that are excellent for exploring quantum magnetic states and quasiparticle fractionalization. Understanding quantum magnetism and quasiparticle excitation at the atomic scale is crucial for manipulating the quantum spin systems. Here we report the fabrication of spin-1/2 Heisenberg chains through on-surface synthesis and in situ reduction. A closed-shell nanographene is employed as a precursor for Ullmann coupling to avoid side reactions, thus obtaining oligomer chains. Following exposure to atomic hydrogen and tip manipulation, the closed-shell polymers are transformed into spin-1/2 chains with controlled lengths by reducing ketone groups and subsequent hydrogen desorption. The spin excitation gaps are found to decrease in a power law as the chain lengthens, suggesting a gapless feature in the thermodynamic limit. Interestingly, the spinon dispersion is extracted from the inelastic spectroscopic spectra, agreeing well with calculations. Our results demonstrate the great potential of fabricating desired quantum systems through a combined on-surface synthesis and reduction approach.</p><p></p>