The present invention provides methods, systems, and devices for performing photothermal therapy (e.g., to treat cancer) using photothermal nanoparticles with a biocompatible coating surrounding a highly crystallized Fe3O4 core. In certain embodiments, the highly crystallized Fe3O4 core of the photothermal nanoparticles has an X-ray diffraction (XRD) pattern where the brightest diffraction ring is from the 440 plane. In some embodiments, the photothermal therapy is conducted with a device configured to emit electromagnetic radiation in the wavelengths between about 650 nm and 1000 nm, wherein the device further comprises a visible light source that allows a user to determine where the electromagnetic radiation is contacting a subject.