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

我们建议直接研究硫醇(含硫的有机分子)在金纳米晶表面的相互作用。与延伸表面相比，纳米晶具有许多强大的优势。只有对于纳米颗粒的模型系统，才能将应力产生机制的表面化学检测为应变模式。纳米晶体的表面必然有不同取向的小区域，这使得应变模式可以在有限的时间内发展。我们建议在钻石的I-07光束线上使用表面X射线衍射仪的相干版本对这些应变进行定量成像。实验的纳米晶体形式允许使用基于载体的CXD成像方法，这对于扩展的表面是不可能的。在拟议的工作中，我们将通过去湿法生长金(和其他元素)的纳米晶体。我们将通过蒸发在没有粘附层的厚氧化层的硅片上来制备Au纳米晶。在空气中1100摄氏度的炉子中进行几个小时的退火会导致去湿。首先，如扫描电子显微镜所见，薄膜分解成明显是结晶的相连的颗粒带(从表面出现的晶界可以看到)，然后晶界分裂，留下孤立的晶体。通过改变初始金属膜的厚度来控制纳米晶尺寸是可能的。我们已经制备了沉积厚度从0到50 nm的Au/Si样品。由此产生的晶体的大小和间距与初始沉积物的厚度似乎存在直接的线性比例关系。结果是自相似排列，晶体间距总是大约是晶体大小的三倍，高度大约是原始厚度的十倍。这一经验标度规则与沉积物质体积守恒大体一致。一旦我们开发出金纳米晶的可靠来源，我们将对硫醇吸附的影响进行系统的研究。我们将在I-07衍射仪的六脚上安装一个“运动学支架”，这样我们就可以移除一组晶体，然后在经过一些化学处理后返回到完全相同的晶体。短链烷基硫醇将通过蒸发在通风柜内沉积。较重的硫醇将从酒精溶液中沉淀出来。随后，学生计划开发一种流动池，类似于完整案例中描述的研究中使用的流动池，用于原位修饰，从而使溶液中的硫醇可以直接进入薄层池。纳米晶体衍射图的演变将通过其CXD图样的反转来监测，正如我们早期的一些工作所证明的那样。

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.