Optimization results display several trends that are well established in the experimental literature on UCNP heterostructures. Optimized UCNPs generally achieve higher absolute brightness at larger diameters, presumably because they are able to host a greater number of absorbing and emitting dopants. The optimized heterostructures for several representative sizes (Fig. ) show that the domains of these champion UCNPs are in fact heavily doped -or rather, alloyed, with up to 100% lanthanide substitution -to maximize absorption and emission throughput. Rather than spreading dopants homogeneously through UCNPs, the brightest structures partition Er and Nd dopants into separate shells, reflecting the established knowledge that Er and Nd are prone to quench each other via cross-relaxation. . Since the energy absorbed by Nd must be transferred to Er for upconversion, the optimizer produced structures that separate Nd-and Er-rich domains by a thin shell heavily doped with only Yb. . Such layers transmit the energy absorbed by Nd dopants to the upconverting Er dopants via rapid energy migration through the Yb sublattice. To maximize Nd-Yb and Er-Yb energy transfer, the Nd-and Er-containing shells are also heavily doped with Yb. Many of the GNN-optimized structures, particularly ones with fewer layers, are reminiscent of the 3-layered heterostructure refined by Zhong et al. and others. Nd-rich domains are located in outer shells to maximize absorption by a larger number (volume) of the sensitizers. Meanwhile, an Er-rich core is used to promote upconversion by concentrating absorbed energy into a smaller volume and smaller number of Er activators. The fact that gradient-based optimization of the hetero-GNN surrogate model can rapidly learn design rules that have been developed over decades of UCNP research suggests its potential to discover even more complex and functional nanostructures.