The top-down approach limits the dimensions of devices to what is technically achievable using

Fig. 13 Realization of optically active InGaA QDs inside GaAs nanowires with sharp single
exciton emission lines for the weakest level of excitation and with biexcitons and excited
states contributing for higher excitation levels is shown in (a) and (b). (c) and (d)
illustrate the possibility of forming perfect periodic arrays of identical nanowires using
lithographically seeded growth. Such structures are interesting for the realization of arrays
of active nanowire devices, field-emission arrays, or photonic band gap structures. 
Fig. 14 An ‘artist's view’ of what may be possible in the future in terms of highly parallel
fabrication of large numbers of complex nanowire devices by seeded bottom-up
fabrication. (Courtesy of Martin Persson, Lund University.)