The Metallicity of Surfaces and Metal Thin Film Overlayers

本研究计划将探讨为什么一些材料在金属基底上沉积时会表现出非金属行为。这种不寻常的行为最近被几种不同的光谱所证实，包括实验带结构映射、共振光发射、核心能级光谱和电子能量损失。非金属覆盖层经历非金属到金属的转变，随着覆盖范围的增加，非金属到金属的转变通常伴随着表明一级转变的结构转变。大学与工业的合作将有助于表征表现出激子绝缘行为的非金属覆层。微观结构探针将与更传统的光发射和共振光发射技术相关联，用于研究电子跃迁。非金属在自由簇和金属表面向金属的转变之间的相似之处是显著的，特别是金属汞。金属丰度随配位数和簇大小（薄膜覆盖或厚度）的相似变化的根本原因只能通过进一步的电子和结构研究来确定。二价金属覆盖层如Mg， Ba和Hg将被研究并与其他没有非金属到金属转变的覆盖层进行比较。这个研究项目解决了一个非常基本的问题：金属的表面是金属吗？测定金属特性的传统测量方法，例如电导率测量，很难应用于沉积在金属衬底上的一两个原子厚的材料薄膜。已经开发出新的方法来确定金属是金属的意义，现在可以应用于通常被认为是金属的材料。有必要探索为什么某些材料的表面与通常与该材料相关的表面具有如此根本不同的性质。在金属表面或金属衬底上观察到的这种非金属行为现象的例子越来越多；然而，指导我们理解的一般规则或原则仍然难以捉摸。

This research program will investigate why some materials exhibit nonmetallic behavior when deposited on metallic substrates. This unusual behavior has been confirmed recently by several different spectroscopies including experimental band structure mapping, resonant photoemission, core level spectroscopy and electron energy loss. Nonmetallic overlayers undergo a nonmetal to metal transition with increasing coverage and the nonmetal to metal transition is typically accompanied by a structural transition indicative of a first order transition. University-industry collaborations will assist in characterizing nonmetallic overlayers which exhibit an excitonic insulating behavior. Microscopic probes of structure will be correlated with the more conventional photoemission and resonant photoemission techniques for studying electronic transitions. The similarities between the nonmetal to metal transition in free clusters and at metal surfaces are remarkable, particularly for the metal mercury. The fundamental reasons for the similar variations in metallicity with coordination number, and cluster size (film coverage or thickness) can only be established with further electronic and structural studies. Divalent metal overlayers such as Mg, Ba, and Hg will be investigated and compared with other overlayers exhibiting no nonmetal to metal transition. %%% This research program addresses a very basic and fundamental question: are the surfaces of metals metallic? Conventional measurements to determine metallic character, such as conductivity measurements for example, are difficult to apply to a thin film of material one or two atoms thick deposited on a metal substrate. New approaches to establishing what it means for a metal to be a metal have been developed and can now be applied to materials normally considered metals. There is a need to explore why the sufaces of some material have such fundamentally different properties form those normally associated with that material. Increasing number of examples of this phenomenon of nonmetallic behavior at metal surfaces or on metal substrates have been observed; however, general rules or principles to quide our understanding remain elusive.