[en] Fresnel zone plates have recently been used as the focusing optic for hard x-ray (5--11 keV) microscopy techniques. Fresnel zone plates used in the hard x-ray regime focus by constructive interference effects based on the phase modulation of the incident x-ray beam and have experimentally been shown to focus 20--30% of the incident photons to less then a one-micron focal spot. The materials of choice for these zone plates have been Al, Cu, Ni, and Au. The focus of this work is the theoretical optimization of the focusing efficiency of phase-modulating Fresnel zone plates in the hard x-ray regime by appropriate material selection. The optimal materials for three different energy ranges will be examined (1--5 keV, 5--20 keV, and > 20 keV and a discussion of the selection criteria involved will be presented. The current zone plate fabrication techniques will be discussed as they pertain to the physical aspects of the zone plates such as thickness, finest zone width, and aspect ratio