Nanoscale X-ray imaging

NATURE PHOTONICS | VOL 4 | DECEMBER 2010 | www.nature.com/naturephotonics X-ray imaging at the nanoand microscale is of great interest for applications in the physical and life sciences. !e associated wavelength range1 extends from a few nanometres to a small fraction of a nanometre, o"ering the potential to image objects at this spatial resolution. !e concomitant photon energies of such wavelengths extend from a few hundred electronvolts to several tens of kilo-electronvolts, spanning the spectroscopic features of essentially all elements and thereby providing a simple mechanism for identifying elements and probing chemical bonds by imaging with photon energies near an absorption edge. Furthermore, the large penetration depth of X-rays allows relatively thick samples to be imaged. !e early visionaries of X-ray microscopy saw the opportunity to extend microscopic imaging by utilizing the short wavelengths, elemental speci#city and penetration depth of X-rays. !ey also saw the need to develop appropriate X-ray optics to realize this goal. Pioneering e"orts were made by Kirkpatrick and Baez2, who pursued the development of crossed re$ective optics operating at glancing incidence, and Baez3, who suggested the use of Fresnel zone plates. Reviews of the early history of X-ray imaging are given in several papers4–9. For so% X-rays, signi#cant progress towards nanoscale imaging began in the 1970s and 1980s with the work of Schmahl and colleagues, who pursued the development of full-#eld transmission X-ray microscopy10,11, and Kirz and colleagues, who investigated scanning transmission X-ray microscopy12,13. Both groups employed zone plate optics14 (Fig. 1a) and synchrotron radiation. In this early work, the emphasis was on biological imaging in the ‘water window’ — between the absorption K-edges of carbon at 284 eV and oxygen at 543 eV — where water is relatively transparent and absorption by carbon-containing cellular sub-substructure provides a natural contrast mechanism. Spatial resolutions were around 100– 200 nm in this early work, moving towards 50 nm in the late-1980s, and #nally towards 10 nm today15,16. In addition to di"ractive optics, normal-incidence re$ective optics with multilayer coatings such as Schwarzschild optics17 (Fig. 1b) were pursued by Cerrina et al.18 in the 1980s, primarily for the scanning photoemission microscopy of material surfaces. Such optics have the advantages of a long working distance for photoemission studies, generally operating at photon energies of the order of 100 eV, and now being capable of achieving spatial resolutions down to tens of nanometres. For harder X-rays — several to tens of kilo-electronvolts — early X-ray microscopy e"orts were based largely on the use of curved re$ective optics at near-glancing angles of incidence, at which the total external re$ection of X-rays provides high re$ectivity Nanoscale X-ray imaging