It is highly desirable to design a monolithic image sensor (and array), which could offer high quantum efficiency over broad spectral ranges, and the possibility to rapidly and randomly address any element in the array. This invention utilizes the growth of semiconductor nanowires such as Si, Ge, Si:Ge, ZnO, or their alloys based nanowires on standard substrates to create multispectral image sensors and photovoltaic cells having these highly desirable features.