Growth and Characterization of Crystalline Silica Films on Pd(100)
Growth and Characterization of Crystalline Silica Films on Pd(100)
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DOI：
10.1021/jp4101152
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发表时间：
2013-12-12
影响因子：
3.7
通讯作者：
Schwarz, Udo D.
中科院分区：
文献类型：
--
作者：
Altman, Eric I.;Goetzen, Jan;Schwarz, Udo D.
用俄歇电子能谱、低能电子衍射（LEED）和扫描隧道显微镜（STM）对Pd（100）衬底上生长的SiO2薄膜进行了表征。虽然没有证据表明长程有序可以检测到低于600 K生长的薄膜，但这些薄膜的STM图像显示平坦的表面，通过该表面可以看到衬底的台阶结构。在975 K以上的10（-6）Torr的O-2中退火膜导致产生六角LEED图案的结晶双层，其周期性为衬底的两倍，并且其中一个覆盖层紧密堆积方向平行于Pd[011]。结晶域的范围通常限于沿膜的三个紧密堆积方向中的两个方向的沿着五个重复单元，但沿第三个方向的长度为沿着数十个重复单元。与在其他衬底上观察到的块状晶体SiO2和双层相比，与衬底匹配的晶格扩展了Pd（100）上双层中的间距;因此，建议形成的规则域边界有助于缓解应力。高分辨率STM图像中的主要特征是被其他六个孔包围的暗孔;与先前的研究一致，这些特征被分配给角共享SiO 4四面体的六元环。在域边界处的孔的伸长归因于边缘共享四面体插入到环中。从头计算上的独立的双层膜进行了解的影响，大量的应变上的薄膜的生长和结构。结果表明，松弛正交的公度方向可以大大降低应变能，因此，正方形衬底通过提供一个沿着的薄膜可以放松的非公度方向，促进晶体SiO2的外延生长。
Silica films grown on Pd(100) were characterized by Auger electron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). While no evidence of long-range order could be detected for films grown below 600 K, STM images of these films nevertheless revealed flat surfaces through which the step-terrace structure of the substrate could be seen. Annealing the films in 10(-6) Torr of O-2 above 975 K resulted in crystalline bilayers that produced hexagonal LEED patterns with a periodicity twice that of the substrate and with one of the overlayer close-packed directions paralleling Pd[011]. The extent of the crystalline domains was limited to typically five repeat units along two of the three close-packed directions of the film but was tens of repeat units long along the third. The lattice matching to the substrate expands the spacing in the bilayer on Pd(100) compared to bulk crystalline SiO2 and bilayers observed on other substrates; as a consequence, it is suggested that the regular domain boundaries that form help relieve stress. The dominant features in high-resolution STM images were dark pores surrounded by six other pores; consistent with prior studies, these features are assigned to six-membered rings of corner-sharing SiO4 tetrahedra. Elongation of the pores at the domain boundaries is attributed to insertion of edge-sharing tetrahedra into the rings. Ab initio calculations on freestanding bilayers were performed to understand the effect of the substantial strain on the growth and structure of the film. The results indicate that relaxation orthogonal to the commensurate direction can greatly reduce the strain energy; as a consequence, the square substrate promotes epitaxial growth of crystalline SiO2 by providing an incommensurate direction along which the film can relax.