Heat-trapping bulk layers or thermal-boundary film stacks are formed between a heat-assisted active layer and an associated electrode to confine such transient heat to the active layer in a heat-assisted device (e.g., certain types of resistance-switching and selector elements used in non-volatile memory. Preferably, the heat-trapping layers or thermal-boundary stacks are electrically conductive while being thermally insulating or reflective. Heat-trapping layers use bulk absorption and re-radiation to trap heat. Materials may include, without limitation, chalcogenides with Group 6 elements. Thermal-boundary stacks use reflection from interfaces to trap heat and may include film layers as thin as 1-5 monolayers. Effectiveness of a thermal-boundary stack depends on the thermal impedance mismatch between layers of the stack, rendering thermally insulating bulk materials optional for thermal-boundary stack components.