Mechanochemistry at the Single Bond Limit: Towards "Deterministic Epitaxy" [Resubmission]

我们能像控制计算机中的信息一样操纵原子吗？我们能不能建造一个物质编译器--一个把原子一个接一个地定位的装置，来构造一个宏观的产品，比如一张桌子、一台计算机，甚至一栋建筑物？换句话说，我们最终能否将3D打印一直推进到原子水平？这是埃里克·德雷克斯勒在80年代提出的极具争议的“分子制造”概念的本质，最初的灵感来自于理查德·费曼在50年代后期对生物质化极限的思考。德雷克斯勒的想法曾经并将继续受到许多人（包括这个提议的作者）的广泛批评和批评，但在他的分子制造计划的核心是一个明显有效的过程：计算机控制和原子精确的化学纯粹由机械力驱动。这种类型的机械化学现在由世界各地的少数研究小组在实验室中实施（并在理论上进行研究），包括参与这项提议的研究小组。我们的核心目标是一个宏观的制造，或者说，微观物体使用单原子操纵少一点宏伟。尽管如此，这是一个非常具有挑战性的目标：制造3D物体-纳米颗粒-一次一个原子。虽然现在有许多令人印象深刻的例子，单原子控制被用来在表面形成各种人工结构-IBM最近的“一个男孩和他的原子”视频，现在已经积累了超过500万的浏览量，是一个特别优雅的演示-到目前为止，3D物体还没有被构造出来。这样做有很好的理由;将原子操纵和定位扩展到第三维空间将涉及与原子和分子相互作用的非常不同的方法。开发这些协议形成了我们的建议的核心。这是一个完全不同类型的显微镜的发明和随后的应用称为原子力显微镜（AFM），使计算机控制的单原子机械化学（德雷克斯勒设想的类型）得以实现。AFM是一种与众不同的显微镜--它不使用透镜、镜子或任何类型的光学元件来生成图像。相反，原子级锋利的尖端靠近表面（在几个原子直径内）。在这个距离上，许多重要的力和相互作用开始起作用，包括在最小的分离处，针尖末端的原子和探针正下方的原子之间形成化学键。通过在表面上来回扫描针尖，同时监测化学力如何变化，可以建立一个不仅具有原子分辨率，而且具有单键分辨率的表面图像。然而，原子力显微镜的成像能力远不止于100倍分辨率。尖端-样品力场可以被映射，单键的强度可以被测量，并且，对这个提议至关重要的是，单个原子可以单独通过化学机械力来操纵。与其前身扫描隧道显微镜不同，AFM -特别是我们在研究中使用的变体，动态力显微镜（DFM）-不依赖于针尖和样品之间的电流流动。利用DFM，原子可以仅通过化学力移动，并且这沿着DFM对单个化学键的取向和强度的高得多的灵敏度，具有提供制造3D纳米结构所需的异常高水平的原子级控制的潜力。

Can we manipulate atoms just like we control bits of information in a computer? Could we ever build a matter compiler - a device that positions atoms, one by one, to construct a macroscopic product like a table, a computer, or even a building? In other words, could we ultimately push 3D printing all the way down to the atomic level?This is the essence of the highly controversial "molecular manufacturing" concept put forward by Eric Drexler in the eighties, originally inspired by Richard Feynman's thoughts on the ultimate limits of miniaturisation back in the late fifties. Drexler's ideas were, and continue to be, widely critiqued and criticised by many (including the authors of this proposal) but at the core of his molecular manufacturing scheme is a demonstrably valid process: computer-controlled and atomically precise chemistry driven purely by mechanical force. This type of mechanochemistry is now implemented in the lab (and studied theoretically) by a small number of research groups across the world, including those involved in this proposal. Our core objective is a little less grandiose than the fabrication of a macroscopic or, indeed, microscopic object using single atom manipulation. Nonetheless, it is an exceptionally challenging goal: the fabrication of a 3D object -- a nanoparticle -- an atom at a time. Although there are now many impressive examples of single atom control being used to form a variety of artificial structures at surfaces -- with IBM's recent "A Boy And His Atom" video, which has now amassed over 5M views, being a particularly elegant demonstration -- to date a 3D object has not been constructed. There are very good reasons for this; extending atomic manipulation and positioning to the third spatial dimension will involve a very different approach to interacting with atoms and molecules. Developing those protocols forms the core of our proposal.It was the invention and subsequent application of a radically different type of microscope called the atomic force microscope (AFM) which enabled computer-controlled single atom mechanochemistry (of the type envisaged by Drexler) to be realised. The AFM is a microscope like no other -- it doesn't use lenses, mirrors, or any type of optical element to generate an image. Instead, an atomically sharp tip is brought close (within a few atomic diameters) to a surface. At this distance a number of important forces and interactions kick in, including, at the smallest separations, the formation of a chemical bond between the atom at the end of the tip and an atom directly underneath the probe. By scanning the tip back and forth across the surface whilst monitoring how the chemical force changes it's possible to build up an image of a surface with not only atomic, but single bond, resolution. AFM is capable of a lot more than 'just' ultrahigh resolution imaging, however. The tip-sample force field can be mapped, the strength of single bonds measured, and, of key importance to this proposal, single atoms can be manipulated via chemomechanical force alone. Unlike its predecessor, the scanning tunnelling microscope, the AFM -- particularly the variant we use in our research, dynamic force microscopy (DFM) -- does not rely on the flow of an electrical current between tip and sample. With DFM, atoms can be moved through chemical force alone and this, along with the much higher sensitivity of DFM to the orientation and strength of single chemical bonds, has the potential to provide the exceptionally high levels of atomic-level control required to fabricate 3D nanostructures.