Development of an atom-cluster generator for an ultra-high voltage electron microscope

当固体的尺寸减小到几纳米时，其物理和化学性质与大块固体相比发生了显著变化。此后，这种超细粒子被称为原子团簇或简称团簇。然而，对原子团簇中的扩散特性研究较少。在此前提下，本文研制了一种超高压超导原子团簇发生器，并利用它对原子团簇中的合金化行为进行了研究。发电机由两个螺旋形的钨丝组成。发电机设置在3MV HVEN的试样室中。细丝与原子团簇的支撑膜（衬底）之间的距离约为100毫米。采用非晶碳膜作为支撑膜。利用发生器首先在支撑碳膜上沉积了-3nm大小的金团簇。然后在同一层薄膜上形成铜簇。用透射电镜原位观察了铜簇沉积过程中簇的形貌和晶格常数的变化。结果表明，在沉积铜时，铜迅速溶解成金团簇。观察到的铜在金团簇中的扩散率比在大块金中的扩散率大约快9个数量级。在合金化过程中，金团簇的重心没有明显的平移运动。这些观察结果清楚地表明，沉积的铜直接或经过迁移到基体上，与金团簇碰撞并结合，并迅速溶解到金团簇内部，形成均匀混合的金铜合金团簇。这种溶解是在没有可见的支撑膜上单个团簇的平移运动的情况下发生的。

When the size of a solid decreases to as small as several nm, its physical and chemical properties change remarkably as compared to those of a bulk solid. Hereafter such ultrafine particles are designated as atom clusters or simply clusters. However, studies on the diffusional properties in atom clusters are lacking. Based on this premise, in the present work, an atom-cluster generator for UHVEM was developed, and with the use of it, alloying behaviour in atom clusters was examined. The generator consisted of two spiral-shaped filaments made of tungsten. The generator was set in the specimen chamber of a 3MV HVEN. The distance between the filaments and a supporting film (substrate) for atom clusters was approximately 100 mm. An amorphous cabon film was used as the supporting film. With the use of the generator gold clusters of -3nm in size were first deposited on the supporting carbon film. Copper clusters were then formed on the same film. Change in the morphology and lattice constant of the clusters associated with the copper cluster deposition were studied in situ by transmission electron microscopy. It is revealed that upon the deposition of copper a rapid dissolution of copper into gold clusters has taken place. The observed copper diffusivity in gold clusters is approximately 9 orders of magnitude faster than that in bulk gold. It is also revealed that the center of gravity of the gold clusters has shown no visible translational motion during the alloying. These observations clearly indicate that deposited copper has either directly or after migration on the substrate, impinged upon and joined with gold clusters and has dissolved quickly into the interior of the gold clusters to form homogeneously mixed Au-Cu alloy clusters. This dissolution has taken place without visible translational motionof individual clusters on the supporting film.