NOVELTY - The method (200) involves providing (202) a monocrystalline silicon substrate by creating open volume defects within the substrate and implanting (204) high-energy ions into the substrate where the volume defects are to be created. The open volume defects are grown (206) by defect annealing at a temperature of 950 degrees Celsius for one to two hours. An immiscible metal is deposited (208) on a surface of the substrate. Metal nanoparticles are formed (210) by diffusing the immiscible metal from the surface onto the inner surface of the open volume defects using a heat treatment.    USE - Method for forming metal nanoparticles e.g. gold nanoparticle, silver nanoparticle, platinum nanoparticle, copper nanoparticle, ruthenium nanoparticle, rhodium nanoparticle, palladium nanoparticle, osmium nanoparticle, iridium nanoparticle, mercury nanoparticle and cobalt nanoparticle, onto an inner surface of an open volume defect within a monocrystalline silicon substrate of an apparatus e.g. solar cell, optical switch and radiation detector (all claimed). Uses include but are not limited to void, bubble, cavity and platelet.    ADVANTAGE - The method enables providing a structure which forms pure metal nanoparticles for maximized photoemission such that the nanoparticles are formed with minimized defects to avoid leakage current and well-ordered metal-silicon interface to minimize electron trapping.    DETAILED DESCRIPTION - The high-energy ions comprise helium or hydrogen ions. INDEPENDENT CLAIMS are also included for the following:    (1) an apparatus comprising a substrate with an open volume defect    (2) a process for fabricating a substrate    (3) a process for fabricating an apparatus including a substrate.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram illustrating a method for forming metal nanoparticles onto an inner surface of an open volume defect within a monocrystalline silicon substrate.    Method for forming metal nanoparticles onto inner surface of open volume defect within monocrystalline silicon substrate (200)    Step for providing substrate (202)    Step for implanting high-energy ions into substrate (204)    Step for growing open volume defects by defect annealing (206)    Step for depositing immiscible metal on surface of substrate (208)    Step for forming metal nanoparticles by diffusing immiscible metal from surface onto inner surface of open volume defects using heat treatment (210)