Coherent Surface X-ray Diffraction investigation of Thiol-induced structural changes in Gold

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology


We propose to investigate the interactions of thiols (organic molecules containing sulphur) directly on the surfaces of gold nanocrystals. Nanocrystals offer a number of strong advantages over extended surfaces. Only for a model system of nanoparticles, the surface chemistry of the stress-generating mechanism can be detected as a pattern of strains. A nanocrystal necessarily has small regions of its surface with different orientations in close proximity, which allows the pattern of strain to develop in a finite amount of time. We are proposing to image these strains quantitatively using a coherent version of Surface X-ray Diffraction (SXRD) at the I-07 beamline of Diamond. The nanocrystal format of the experiment allows the support-based methods of CXD imaging to be employed, which would not be possible for extended surfaces.In the proposed work, we will grow nanocrystals of gold (and other elements) by dewetting . We will make the Au nanocrystals by evaporation onto Si wafers with a thick oxide and no adhesion layer. Annealing in a furnace at 1100C in air for several hours causes dewetting. At first, the film breaks up into connected ribbons of particles that are obviously crystalline (as seen by the grain boundaries emerging to the surface), then the boundaries split apart to leave isolated crystals, as seen by Scanning Electron Microscopy. Control of the nanocrystal size is possible by varying the thickness of the initial metal film before it is annealed. We have already prepared Au/Si samples with a thickness of the deposited film 'ramped' from 0 to 50nm. There appears to be a straightforward linear scaling of the size and spacing of the resulting crystals with the thickness of the initial deposit. The result is self-similar arrangements, with a crystal spacing always about three times the crystal size and a height about ten times the original thickness. This empirical scaling rule is roughly consistent with conservation of the volume of deposited material. Once we have developed a reliable source of Au nanocrystals, we will undertake systematic investigation of the effects of thiol adsorption. We will employ a 'kinematic mount' on the hexapod of the I-07 diffractometer so that we can remove an array of crystals and later return to the exactly the same crystal after some chemical treatment. Short-chain alkyl thiols will be deposited by evaporation inside a fume cupboard. Heavier thiols will be deposited from alcohol solution. Later on, it is planned that the student will develop a flow cell, similar to those used in the studies described in the full case, for in situ modification, whereby thiols in solution can be admitted directly to a thin-layer cell. The evolution of the nanocrystal diffraction patterns will be monitored by the inversion of its CXD pattern, as demonstrated in some of our earlier work.


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Description This grant was the second to develop X-ray imaging technology at the Diamond Light Source. It developed the theme of imaging strains in crystals using the relationship between phase and a projection of strain. Applied to nanocrystals this leads to many interesting conclusions about their structure.
Exploitation Route Potential for imaging nanoparticles relevant to magnetic recording media. Potential applications in cantilever-based medical sensors. The new methodology of Coherent Diffraction Imaging (CDI) was used in the design of instrumentation for the I-13 beamline of Diamond, which is the longest in Europe.
Sectors Education,Electronics,Energy,Environment,Healthcare

Description Numerous invitations to give seminars and conference talks. The work has been used to motivate the construction of new facilities and the upgrading of existing ones.
First Year Of Impact 2008
Sector Education,Electronics,Energy,Environment,Healthcare
Impact Types Cultural,Societal,Economic

Description Chair of Royal Society Conference "Real and reciprocal space X-ray imaging", 2013 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A double event, supported as part of the Royal Society scientific meetings, was organized in February 2013 in London and at Chicheley Hall in Buckinghamshire by Dr A. Olivo and Prof. I. Robinson. The theme that joined the two events was the use of X-ray phase in novel imaging approaches, as opposed to conventional methods based on X-ray attenuation. The event in London, led by Olivo, addressed the main roadblocks that X-ray phase contrast imaging (XPCI) is encountering in terms of commercial translation, for clinical and industrial applications. The main driver behind this is the development of new approaches that enable XPCI, traditionally a synchrotron method, to be performed with conventional laboratory sources, thus opening the way to its deployment in clinics and industrial settings. The satellite meeting at Chicheley Hall, led by Robinson, focused on the new scientific developments that have recently emerged at specialized facilities such as third-generation synchrotrons and free-electron lasers, which enable the direct measurement of the phase shift induced by a sample from intensity measurements, typically in the far field. The two events were therefore highly complementary, in terms of covering both the more applied/translational and the blue-sky aspects of the use of phase in X-ray research. optics, image processing

Since Roentgen's discovery well over a century ago, X-ray imaging has been based on attenuation. While this is an effective and reliable approach, it is known to have limitations whenever objects with similar attenuation characteristics have to be distinguished, as this leads to poor image contrast. One possible solution is to generate image contrast by exploiting phase changes in the X-ray wavefront rather than attenuation: this approach is called X-ray phase contrast imaging (XPCI). Its main advantage is that the term responsible for phase effects (the unit decrement of the real part of the refractive index) is much larger than the imaginary part, responsible for attenuation. In many practical cases, this difference can be up to 1000-fold. Hence, albeit that it can be difficult to detect, phase is intrinsically a much stronger effect and can therefore lead to much higher image contrast if appropriately exploited.

The first X-ray phase contrast image dates back to 1965 and was acquired by Bonse & Hart [1] with a crystal interferometer, which has been bearing their names since. We were fortunate enough to have Prof. Hart attending our conference. Following a significant attempt by Ando & Hosoya [2] in 1972, the idea of using a crystal-based X-ray interferometer was picked up again in the 1990s, in particular by Momose's group. XPCI exploded in the mid-1990s, and, to the best of our knowledge, the first of a long series of papers published from 1995 onwards is Momose's [3], published on 1 January 1995, on results already presented in the previous year. Momose and his group continued to explore medical and biological applications of XPCI implemented with the Bonse-Hart interferometer, as reported, for example, in their 1996 Nature Medicine paper [4].

Almost simultaneously, researchers started to explore a different approach to the use of crystals in XPCI-namely the use of a perfect crystal as an 'angular analyser'. It was in fact observed that the faint distortions in the wavefront caused by phase changes translate directly into slight changes in the X-ray direction (X-ray refraction), which could therefore be picked up by a system with sufficient angular sensitivity like a perfect crystal. The fact that the refraction angle is proportional to the gradient of the phase shift gave rise to the name 'differential' XPCI, which is common to other methods listed below, such as edge-illumination and Talbot interferometry. The most famous paper pioneering this approach is the 1995 Nature letter by Davis et al. [5]. It has to be said, however, that a few known examples exist that pre-date that deservedly famous paper, where effectively the same method had been used to address specific problems, and the results were published in journals with relatively limited diffusion [6,7]. Another paper on the crystal method appeared later in the same year [8] and the method continued to be explored later, leading to some of the first simplified phase retrieval algorithms [9] and extraction of the first quantitative X-ray 'dark-field' images [10-12] (qualitative dark-field images had already been presented in [5]).

Still in 1995, a much simpler implementation, based on free-space propagation, was proposed by Snigirev et al. [13]. Through this approach, phase contrast images are acquired without the need for any additional optical element, simply by increasing the distance between sample and detector. If the source possesses enough coherence, this is sufficient to detect phase-induced interference fringes at the boundaries of the imaged objects. In the following year, Wilkins et al. [14] demonstrated that this approach works also with polychromatic radiation, something that was impossible to observe before as the use of a crystal automatically renders the beam monochromatic. Interestingly, similar images had been obtained before, but the substantially improved image contrast had not been interpreted on the basis of phase effects (e.g. [15]). Owing to its simplicity of implementation, the free-space propagation technique is still widely used, and it is the only one to date to have reached the in vivo stage for human patients [16].

A method that combines the angular selectivity principle exploited with crystal 'analysers' and the simplicity of free-space propagation, called edge-illumination XPCI, was developed in the late 1990s [17]. As the name suggests, it performs a fine selection on X-ray direction by illuminating only the physical edge of the detector pixels, and by doing so it eliminates the need for a perfect crystal, opening the way to polychromatic and divergent X-ray beams. This was later exploited to adapt the method for use with conventional X-ray sources [18], which also led to the first quantitative phase retrieval performed with incoherent illumination of the sample [19].

In the early 2000s, a further approach was introduced, based on the adaptation to X-rays of Talbot and Talbot-Lau interferometers, well-known methods among the optics community. This adaptation was made possible by novel nanofabrication techniques, which enabled the development of gratings with a pitch sufficiently small to produce Talbot 'self-images' [20] at X-ray wavelengths when coherently illuminated. The approach was first demonstrated in the 'Talbot' set-up by David et al. [21] and by Momose's group shortly afterwards [22]; a few years later, Pfeiffer et al. [23] demonstrated that the Talbot-Lau arrangement could also be implemented with X-rays, opening the way to the use of laboratory sources. In a further study, it was shown that the same set-up could also be used to obtain quantitative dark-field images [24], along the lines of what was previously done with analyser crystals [10-12]. To extract phase and dark-field signals, a technique called phase stepping is employed, in which one of the two (Talbot) or three (Talbot-Lau) gratings is scanned with respect to the others, and individual images are acquired at each scanning position. Later on, Wen et al. [25] developed a similar approach, which, however, employed a single grating, and, instead of using a phase-stepping method to isolate the different signals, did so by applying a Fourier-based analysis method to a single frame.

Quite remarkably, most of the leading figures behind the above developments agreed to participate in the conference, which meant that most of the above topics were covered in the various talks. Another interesting aspect that emerged is that the field is continuing to evolve, with new examples of implementation already being presented at this same meeting (e.g. Wen et al. [26] realized gratings with nanometric pitch which led to a hybrid between Talbot and Bonse-Hart interferometry), new implementation schemes (e.g. [27]), use of materials as common as paper to replace gratings or analysers [28] and much more. This extreme liveliness indicates a prosperous future for the area, both in terms of new methods and new applications, and definitely looks encouraging in terms of ultimately reaching the 'translation into mainstream application' goal which inspired this meeting.
Year(s) Of Engagement Activity 2013
Description Invited talks at conferences: 20 per year 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Invitation accepted to travel to international conference, give invited talk and discuss with participants at length.
Year(s) Of Engagement Activity 2006,2007,2008,2009,2010