Introduction. X-ray microscopy and coherent diffractive imaging (CDI; Miao et al.
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1999 ; Nugent et al. 2003 ; Eisebitt et al. 2004 ; Pfeifer et al.
2006 ; Quiney et al.
2006 ; Chapman et al. 2006 ; Chapman & Nugent, 2010 ) and their modifications are rapidly developing as ultra-high spatial-resolution imaging techniques that exploit coherent, ultra-bright X-ray sources. To visualize an object at a nanoscale resolution, a significant amount of X-ray photons must be delivered to a very small volume. A modern synchrotron ( i.
6–8 GeV third-generation machine) typically delivers approximately 10 10 –10 12 photons s −1 mm −2 at 8–20 keV within the coherence volume (Nikulin et al. 2008 ). A 100 nm 3 cube within the sample will scatter 10 2 –10 4 photons per second at best. Since the diffracted intensity contrast is proportional to the product of the feature's thickness and the refractive index difference at the boundary between sample and its environment, soft X-rays are much better suited to image materials with low electron densities (Sayre & Chapman, 1995 ; Chapman et al.
2006 ). However, their use is limited owing to high vacuum requirements, so hard X-rays are preferred (Chapman et al.
2006 ). The real part of the refractive index at ≥10 keV photon energy ranges between 10 −5 and 10 −8 in heavy metals and light elements, respectively, so that a much brighter source is required to visualize low-atomic-number samples at the true nanoscale, e. polymers or biological membranes with a spatial resolution of <10 nm (Chapman et al. 2006 ; Nikulin et al.
2008 ). However, when the required density of photons increases as we approach a true nanoscale imaging, so does the radiation damage to the specimen (Sayre & Chapman, 1995 ). The ionizing nature of X-rays results in various damaging consequences to samples, which are serious limiting factors in macromolecular crystallography. Systematic studies on the dose dependence of specific types of radiation damage to certain classes of crystalline samples have been conducted.
A so-called `Henderson limit', H = 2 × 10 7 Gy, introduced in macromolecular crystallography (Henderson, 1990 ), defines the dose at which the intensity of the diffraction pattern of a typical macromolecular crystalline sample is predicted to be halved. The macromolecular crystallography data consist of initially very strong peaks, which are Bragg reflections from a crystal lattice. The deterioration of the Bragg diffraction contrast is a result of many complex processes which happen within the macromolecular crystal during its X-ray exposure (Weik et al. 2000 ).
The primary effect of X-rays is the photoionization of preferentially core levels, followed by secondary processes like the emission of Auger electrons leading finally to conformational modifications of active centres, cleavage and re-arrangement of bonds (Weik et al. 2000 ; Murray et al. 2004 ; in `polymer language' for PMMA, for instance, cleavage and re-arrangement correspond to main chain scission and cross linking) and heat.
In absorption-, transmission- and CDI-based X-ray microscopy of organic samples, radiation damage is widely acknowledged as a major problem and subjected to rigorous studies (Howells, Hitchcock & Jacobsen, 2009 ; Howells, Beetz et al. 2009 ; Schafer et al.
2009 ). The CDI schemes present an opportunity for the diffraction-limited three-dimensional structure determination of non-periodic objects, such as biological cells and nanocrystals.
In practice, the resolution attained in CDI arises from a fine balance between fluence (the total number of photons per unit area) and dose (absorbed energy per unit mass; Howells, Beetz et al. 2009 ; Marchesini et al. 2003 ). In contrast to crystallographic diffraction, in the case of coherent diffractive imaging (Sayre & Chapman, 1995 ; Jacobsen & Kirz, 1998 ; Larson et al.
2002 ; Chao et al. 2005 ; Miao et al. 1999 ; Nugent et al.
2003 ; Eisebitt et al. 2004 ; Pfeifer et al. 2006 ; Quiney et al. 2006 ; Chapman et al.
2006 ), the data essentially represent a weak Fraunhofer diffraction pattern. For a given resolution, the non-periodic character of samples in CDI imposes more stringent conditions on coherence properties of the source and dose–fluence penalty relations (Howells, Beetz et al. 2009 ; Marchesini et al.
2003 ) than in conventional crystallographic schemes. However, from an analysis of maximum tolerable doses in both the CDI-based X-ray microscopy and macromolecular crystallography, Howells, Beetz et al. (2009 ) predicted that a particular feature of biological protein can be imaged with 10 nm resolution at a dose ∼10 9 Gy. Based on the assumption that the material science samples have higher radiation tolerance, the authors (Howells, Beetz et al. 2009 ) also predicted the possibility of coherent diffraction imaging of such samples with 1 nm resolution.
However, the assumption of higher tolerance to radiation damage of inorganic samples has to be tested for nanostructured materials. The physical properties of nanoscale materials differ from those in bulk owing to a larger surface/volume ratio and lower atomic coordination (Marks, 1994 ; Huang et al. 2008 ).
Noticeable effects of collective excitations (electronic confinement) also play an important role in the responses of nanostructured materials to external perturbations. These effects often result in the lower thermodynamic stability of nanomaterials in comparison with the bulk, and a spontaneous change of phase ( e. quasimelting) has been observed even at low temperatures (Ajayan & Marks, 1988 ).
For example, the quasimelting state of very small gold (∼1 nm) nanoclusters has been observed directly under an electron microscope (Marks, 1994 ). In CDXI experiments, even for larger nanostructures, the lowered stability could place serious limits on resolution owing to lowering the dose thresholds (Robinson, 2008 ; Marchesini et al.
2003 ). However, there are almost no publications with quantitative data addressing the stability of material science nanosamples exposed to intense synchrotron radiation. Whether the Henderson limit is applicable for inorganic structures which do not contain carboxyl groups or sulfur bridges is an open question (Favre-Nicolin et al. 2009 ). An important problem in X-ray diffraction studies is the temperature effect on the radiation dose tolerance.
In biomolecular crystallography, cryocooling down to liquid-helium temperatures can prove to be advantageous against secondary radiation damage effects. However, for electron tomography imaging of single frozen-hydrated biological objects such as large protein–membrane complexes, organelles and small cells with lower than atomic resolution (4–20 Å), a liquid-helium environment at 4–12 K did not provide any improvement in comparison with that of liquid nitrogen at ∼100 K (Iancu et al. 2006 ; Bammes et al. 2009 ).
Systematic studies have shown that dose/damage relationships caused by either soft X-rays or electron beams in the polyethylene derivative samples are comparable (Wang et al. 2009 ). Nevertheless, in X-ray imaging experiments the optimal experimental environment ( e. high vacuum or a particular gas/liquid atmosphere, forced or natural convection) must be individually attuned with respect to the experimental method, sample material and target resolution.
In this paper we present experimental evidence for the destructive influence of synchrotron X-rays on nanoscale samples of both organic and metallic nature, show the role of heat loading in each case, and propose a tentative scenario to explain the observations. The experiments were performed at the BL13XU beamline at SPring-8, Japan. Synchrotron radiation energy of 12. 4 keV was selected using a primary, tunable, double-crystal Si(111) beamline monochromator.
Further angular collimation was performed using a double-crystal channel-cut Si(400) monochromator placed in non-dispersive mode. The beam was then spatially collimated by two pairs of slits defining a 0. 3 mm (height) × 0. 2 mm (width) beam incident on the sample. Samples were placed on a linear motion stage downstream immediately beyond the slits in such a way that the X-ray diffraction from it occurred in the vertical plane coinciding with the diffraction plane of the X-ray optics. A Si(400) crystal analyzer and a scintillation detector were placed downstream from the sample to collect the diffracted intensity from the sample as a function of the angular position of the analyzer.
The sample was then scanned across the collimating slits to expose different nanostructures to the incident beam. The experimental chamber was kept under ambient conditions, e. the sample was cooled by natural convection of air under normal pressure and room temperature. Samples of known geometry composed of 200 nm-thick PMMA resist were deposited on ∼5 mm × 5 mm-wide 1 µm-thick Si 3 N 4 membranes held by a thicker silicon window-frame and consisted of 3 × 3 fields of 500 µm × 500 µm areas, which were filled with various patterns including holes, posts and lines and spaces. The characteristic pattern sizes were 100, 200 and 500 nm. We also examined a sample which included 50 nm-diameter gold nanoparticles which were dispersed densely, but not uniformly, in a 1 µm gap between two 50 µm-thick kapton sheets.
The estimated volume fraction filled by gold nanoparticles was ∼45–50%.