Submonolayer and Monolayer Sn Adsorption and Diffusion Behavior on Oxidized Nb(100)
Submonolayer and Monolayer Sn Adsorption and Diffusion Behavior on Oxidized Nb(100)
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DOI：
10.1021/acs.jpcc.2c08458
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发表时间：
2023-01
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影响因子：
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通讯作者：
Sarah A. Willson;R. G. Farber;A. Hire;R. Hennig;S. Sibener
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作者：
Sarah A. Willson;R. G. Farber;A. Hire;R. Hennig;S. Sibener
目前，产生更明亮的带电粒子束的努力集中在开发用于超导加速器设施的Nb 3Sn涂层超导射频（SRF）腔。驱动形成的Nb 3Sn膜上预先存在的铌SRF腔的生长机制，但是，没有完全理解。为了理解金属Sn和氧化Nb表面之间复杂的相互作用导致Nb 3Sn合金的形成，我们研究了氧化Nb（100）单晶的结构和化学组成如何影响Sn的吸附和扩散行为在亚单层和单层Sn覆盖。Sn沉积在NbO表面上，并在与Nb 3Sn生长程序相关的温度下退火，然后通过原位扫描隧道显微镜（STM）进行分析。实验数据，沿着与支持模拟STM和计算得到的结合能，使用密度泛函理论，揭示了热力学和动力学驱动的扩散途径，优选的结合位点，新颖的Sn吸附层结构，以及底层的NbO基板如何演变，以适应Sn扩散的Sn覆盖和退火温度的影响。这种新认识的吸附锡和Nb表面位点之间的界面化学相互作用的理解是必不可少的，以开发预测生长模型Nb 3Sn膜用于未来的SRF腔。现代超导加速器设施依赖于Nb超导射频（SRF）腔体来有效地产生包括X射线自由电子（FEL）激光辐射在内的高能应用所需的光束。Nb是这种SRF腔的当前标准，这是由于几种固有的材料特性，包括其临界温度（Tc）为109.8K，最高的元素Tc，以及其在102 K的工作温度下的高品质因数（Q）。然而，Nb SRF腔的相关低温基础设施、能耗和运行成本确实限制了超导技术的使用。为了增加超导科学和技术的可及性，已经致力于确定可以在1002 K以上操作的超导材料，其具有与Nb SRF腔相当的Q。最有前途的材料之一是Nb 3Sn，这是一种A15合金，其Tc为1018 K，已被证明能够在4 K下工作，Q值与在1022 K下工作的Nb SRF腔相当。这种工作温度的提高显著降低了低温基础设施的尺寸和成本，最近的工作显示了通过低温冷却器传导冷却的Nb 3Sn SRF腔的成功运行。尽管有这些技术进步，但目前的Nb 3Sn SRF腔并没有以预测的指标运行。最近的工作已经将SRF性能与微米级Nb 3Sn膜的形态特征（诸如SRF腔体内的不均匀Sn浓度梯度）相关联。虽然Nb SRF腔体可以机械地成形为期望的腔体形状，但Nb 3Sn非常脆并且不能承受形成SRF腔体所需的物理操作。相反，通常通过伍珀塔尔工艺制造Nb 3Sn SRF腔，其中Nb 3Sn膜经由Sn气相沉积和注入在预先存在的Nb SRF腔上生长。已经对伍珀塔尔生长程序进行了修改，以优先选择A15 Nb 3Sn相的生长。Posen等人最近的工作说明了Nb 3Sn膜质量作为Sn熔剂的函数的显著差异。在这些实验中，Posen等人在500 °C的Sn成核过程中增加了SnCl 2的流量，在1100 °C的涂覆阶段增加了Sn蒸气的流量。与具有较大、较粗糙晶粒的视觉上无光泽的试样相比，该新工艺产生视觉上有光泽并且由较小、较光滑晶粒组成的Nb 3Sn。进一步研究视觉上有光泽的接收时间：2022年12月2日修订时间：2023年1月11日发布时间：2023年1月31日文章pubs.acs.org/JPCC © 2023 American Chemical Society 3339 https://doi.org/10.1021/acs.jpcc.2c08458 J. Phys. Chem. C 2023，127，3339−3348 D ow nl oa de d vi a U N IV O F C H IC A G O o n Fe br ua ry 1 7，2 02 3 at 1 9：40：4 8（U T C）。See e e ht tp s：//p ub s. AC S。或g/ sh ar in gg ui de lines for or o pt io ns on how to le gi tim at el y sh ar e publ is he dartic le s.
Current efforts to produce brighter beams of charged particles are focused on developing Nb3Sn-coated superconducting radio frequency (SRF) cavities for use in superconducting accelerator facilities. The growth mechanisms driving the formation of Nb3Sn films on preexisting Nb SRF cavities are not, however, fully understood. In order to understand the complex interplay between metallic Sn and the oxidized Nb surface leading to Nb3Sn alloy formation, we have examined how the structural and chemical composition of an oxidized Nb(100) single crystal influences Sn adsorption and diffusion behavior at submonolayer and monolayer Sn coverages. Sn was deposited on an NbO surface and annealed at temperatures relevant to Nb3Sn growth procedures before analysis via in situ scanning tunneling microscopy (STM). Experimental data, along with supporting simulated STM and calculated binding energies obtained using density functional theory, revealed the influence of Sn coverage and annealing temperatures on thermodynamic and kinetically driven diffusion pathways, preferred binding sites, novel Sn adlayer structures, and how the underlying NbO substrate evolves to accommodate Sn diffusion. This newly realized understanding of the interfacial chemical interactions between adsorbed Sn and the Nb surface sites is essential to develop predictive growth models for Nb3Sn films for use in future SRF cavities. ■ INTRODUCTION Modern superconducting accelerator facilities rely on Nb superconducting radio frequency (SRF) cavities to effectively generate beams necessary for high-energy applications including X-ray free electron (FEL) laser radiation. Nb is the current standard for such SRF cavities due to several intrinsic material properties including its critical temperature (Tc) of ∼9.8 K, the highest elemental Tc, and its high quality factor (Q) at operating temperatures of ∼2 K. The associated cryogenic infrastructure, energy consumption, and operating costs of Nb SRF cavities do, however, limit access to superconducting technology. To increase the accessibility of superconducting science and technology, significant effort has been dedicated to identifying superconducting materials that can operate above ∼2 K with comparable Q to Nb SRF cavities. One of the most promising materials identified is Nb3Sn, an A15 alloy with a Tc of ∼18 K that has been shown to be capable of operating at 4 K with Q comparable to Nb SRF cavities operating at ∼2 K. This increase in operating temperature significantly reduces the size and cost of cryogenic infrastructure, with recent work showing the successful operation of Nb3Sn SRF cavities cooled via cryocooler conduction. Despite these technological advancements, current Nb3Sn SRF cavities are not operating at predicted metrics. Recent work has associated SRF performance with micron-scale Nb3Sn film morphological features such as inhomogeneous Sn concentration gradients within the SRF cavity. While Nb SRF cavities can be mechanically shaped into the desired cavity shape, Nb3Sn is extremely brittle and cannot withstand the physical manipulation needed to form SRF cavities. Instead, Nb3Sn SRF cavities are fabricated, typically through the Wuppertal process, in which a Nb3Sn film is grown on a preexisting Nb SRF cavity via Sn vapor deposition and infusion. There have been modifications to the Wuppertal growth procedure to preferentially select for growth of the A15 Nb3Sn phase. Recent work by Posen et al. illustrates a significant difference in Nb3Sn film quality as a function of Sn flux. In these experiments, Posen et al. increased the flux of SnCl2 during Sn nucleation at 500 °C and Sn vapor during the coating stage at ∼1100 °C. This new procedure results in Nb3Sn that is visually shiny and composed of smaller, smoother grains compared to visually matte coupons with larger, rougher grains. Further studies of the visually shiny Received: December 2, 2022 Revised: January 11, 2023 Published: January 31, 2023 Article pubs.acs.org/JPCC © 2023 American Chemical Society 3339 https://doi.org/10.1021/acs.jpcc.2c08458 J. Phys. Chem. C 2023, 127, 3339−3348 D ow nl oa de d vi a U N IV O F C H IC A G O o n Fe br ua ry 1 7, 2 02 3 at 1 9: 40 :4 8 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.