纳米：改变我们对物质的认识

文 / 穆罕穆德·沙法（Muhammad Shafa，巴基斯坦） 译/王受信

我们为什么应该关注纳米科学？因为它将改变我们的生活，改变我们对物质的认知。1999年由美国国家科学基金会召集的顶级科学家曾说：“纳米技术对21世纪人类健康、财富和生活水平的影响，至少可以与20世纪发展的微电子、医学成像、计算机辅助工程和人造聚合物的综合影响一样重要。”

纳米科学涉及对极小尺度——10-7米（100纳米）到10-9米（1纳米）现象的研究。在这个尺度下物质的性能完全不同于更大范围的性能。纳米科学是一门多学科领域，不仅聚焦于化学，还有生物学、物理学、工程学和计算科学等。正由于它的多学科性，纳米科学需要我们去关注不熟悉学术领域的知识。

金子总是金色吗？

想象一下当你切东西时会发生什么。在哪一刻你会切下单个原子？在哪一刻“颜色”会变化并消失？其实单个原子并没有颜色。一种物质的颜色由从它反射回来光的波长所决定，但一个原子由于太小而不能自己反射光。只有当你有了足够大的原子集合体（一堆原子）时，你才能开始分辨出某种接近“颜色”的东西。例如，一堆盐晶体在一起看起来是白色的，但一个单独的盐晶体却是无色的。这一类比使人们认识到，不同厚度的材料产生不同的颜色。例如，水上的油会根据油膜的厚度产生不同的颜色。在浮油中原子是不变的，只是不同的厚度（原子的数量）反射出不同的颜色。树叶之所以看起来是绿色的，是因为树叶表面的原子结构反射出绿色的波长，并吸收了其他的波长。随着叶子枯萎，原子结构会发生变化，因此叶绿素分解后你看到的是反射回来的棕色。对于金子而言，颜色取决于纳米尺度的晶体或者原子结构：光吸收根据晶体的厚度而不同。在《个人触摸》的故事中，桑德拉的礼服颜色变化是因为她能改变礼服原子的排列，从而反射出不同的颜色。

如何构建纳米结构？

碳纳米管是最近创造的结构，并且具有许多新奇性能。它非常轻且坚固，可以添加到各种材料中以增加强度而不会增加太多重量。与此同时它也有许多有趣的电导（电学）特性。碳巴基球（足球烯）基于其交错“足球”形状是一种非常坚固的结构。它有一个独特的性能，能够携带一些东西穿透细胞壁，然后把东西送到细胞内。一般来说，我们的身体对此没有反应，所以你的身体不会试图攻击它，它可以很容易地随着血液流动。

穆罕穆德·沙法（Muhammad Shafa），西安交通大学材料科学与工程学院助理教授，2016—2017年在阿联酋大学做博士后。沙法博士在实验凝聚态物理学、半导体纳米线和薄膜合成、制造纳米器件等领域作出了重要贡献并发表了若干篇高质量文章。

构建纳米结构。我们如何构建如此小的东西呢？目前主要有三种方法。首先，扫描探针显微镜的尖端可以与它们所扫描材料的原子成键并移动原子。1990年，IBM用这种方法操作氙原子制作出了有史以来最小的商标。另外，科学家可以从表面刻材料，直到出现所需的结构。这是计算机工业用来制造集成电路的过程。最后，自组装是分子构件自然“组装”形成有用产品的过程。分子试图通过将自己排列在特定位置来最小化它们的能级。如果与相邻的分子成键能达到较低的能量状态，则会键合。我们可以在自然界的许多地方看到这种情况。例如，气泡的球形或雪花的形状是分子最小化其能级的结果。

晶体生长的自组装。晶体生长是一种特殊的自组装类型。这种技术被用来“生长”纳米管。在这种方法中，“种子”晶体被放置在某些表面，引入一些其他原子或分子，这些粒子模仿小种子晶体的模式。例如，制造纳米管的一种方法是在硅之类的材料上制造一组铁纳米粉颗粒，把这些阵列放在一个腔室中，然后往腔室中加入一些含碳的天然气。碳与铁发生反应，并使其过饱和，形成碳沉淀，随后析出。通过这种方式，可以生长出像树一样的纳米管!

自然界中的生物纳米机器。在我们的生物世界中存在着许多天然的纳米级器件。生命始于纳米！例如，在所有细胞内部，各种大小的分子和粒子都必须四处移动。一些分子可以通过扩散移动，但离子和其他带电粒子必须在细胞周围和细胞膜之间进行特异性运输。生物学中有大量蛋白质可以自组装成纳米级结构。

显微镜的发展

科学中的一个重要思想是，创造工具或仪器来提高我们收集数据的能力，往往伴随着新的科学理解。科学是动态的。科学仪器的创新伴随着对科学的更好理解，并与创造创新的技术应用相关联。传统的光学显微镜在许多生物学相关的应用中仍然非常广泛，因为使用该工具可以很容易地看到细胞和细菌等。它们也相当便宜，易于安装。

光学显微镜。纳米有多大？你可以用肉眼看到大约1000微米，而生物课上使用的典型显微镜可以让你看到大约10微米。更先进的显微镜，如扫描电子显微镜，可以获得相当好的分辨率（1微米）范围。更新的技术（在过去20年左右）允许我们“看到”100纳米到1纳米范围。

电子显微镜。扫描电子显微镜和标准光学显微镜的区别在于，电子而不是各种波长的光从被观察物体的表面“反弹”，由于电子体积小，所以可以获得更高的分辨率。打一个比方，你可以在一个表面上拍沙滩球来确定其表面是否平坦（球在各个不同方向上的散射）。

原子力显微镜。原子力显微镜利用针尖与样品表面原子发生相互作用，从而探测样品表面信息，具有原子级的分辨率。人们能做到的最小的针尖必须由原子制成。针尖与要观测的材质表面会相互作用，因此针尖越小，分辨率越高。但是因为针尖是由原子组成的，它不能比你看到的原子小。针尖由多种材料制成，如硅、钨，甚至碳纳米管。

原子力显微镜（AFM）和扫描隧道显微镜（STM）之间的区别在于AFM依靠原子间电磁力的运动，而STM则依靠针尖和表面之间的电流。需要注意的是AFM正是为了克服STM的基本缺点而发明的，即STM只能用来测量导体因为它依赖于针尖和表面之间电流的产生。AFM依靠实际接触而不是电流，因此它可以用来探测几乎任何类型的材料，包括聚合物、玻璃和生物样品。人们利用这些仪器的信号（力或电流）来推断原子的图像。针尖的波动被记录下来并输入计算机模型中，计算机模型根据数据生成图像。这些图像给我们提供了原子尺度的大致情况。

纳米科学的特殊之处在于，在如此小的尺度下，不同的物理定律占据主导地位，材料的性质也会发生变化。

Why should we care about nanoscience? It will change our lives and change our understanding of matter.A group of leading scientists gathered by the National Science Foundation in 1999 said, “The effect of nanotechnology on the health, wealth and standard of living for people in this century could be at least as significant as the combined influences of microelectronics,medical imaging, computer-aided engineering and man-made polymers developed in the past century.”

Nanoscale science deals with the study of phenomena at a very small scale—10-7m (100 nm) to 10-9m (1 nm)—where properties of matter differ significantly from those at larger scales.This very small scale is difficult for people to visualize.Nanoscale science is a multidisciplinary field and draws on areas outside of chemistry, such as biology, physics, engineering and computer science.Because of its multidisciplinary nature, nanoscience may require us to draw on knowledge in potentially unfamiliar academic fields.

Is Gold Always Gold?

Think about what happens when you keep cutting something down.At what point will you get down to the individual atoms,and at what point does “color” change and go away? Remind that individual atoms do not have color.The color of a substance is determined by the wavelength of the light that bounces off it, and one atom is too small to reflect light on its own.Only once you have an aggregate (a bunch) of atoms big enough can you begin to discern something approaching “color”.For example, a bunch of salt crystals together look white, but an individual salt crystal is colorless.Which analogies to drive home the concept that different thicknesses of a material can produce different colors.For example, oil on water produces different colors based on how thin the film of oil is.In an oil slick the atoms aren’t changing;there are just different thicknesses (numbers of atoms) reflecting different colors.Leaves on a tree look green because the atomic structure on surface of leave reflects back green wavelength and absorbs all others.As leaves die, the atomic structure changes so you get brown reflected back as the chlorophyll breaks down.For gold, color is based on the crystalline or atomic structure at the nanoscale: light absorbs differently based on the thickness of the crystal.In thePersonal Touchstory, Sandra’s dress changes color because she can change the arrangement of atoms in her dress,which will then reflect different colors.

How to Build Nanostructures?

Carbon Nanotubes: This describes a recently-created structure that has some amazing properties.Nanotubes are very light and strong and can be added to various materials to give them added strength without adding much weight.Nanotubes also have interesting conductance (electrical) properties.Carbon Buckyballs: Buckyballs are another very strong structure based on its interlaced “soccer ball” shape.It has the unique property of being able to carry something inside of it, penetrate a cell wall, and then deliver the package into the cell (not sure how you “open” the buckyball).It is also non-reactive in general to the body, so your body will not try to attack it and it can travel easy in the bloodstream.

Building Nanostructures.How we build things that are so small? There are three main methods that are used to make nanoscale structures.First, the tips of scanning probe microscopes can form bonds with the atoms of the material they are scanning and move the atoms.Using this method with xenon atoms, IBM created the tiniest logo ever in 1990.Alternately, scientists can chisel out material from the surface until the desired structure emerges.This is the process that the computer industry uses to make integrated circuits.Finally, self-assembly is the process by which molecular building blocks “assemble” naturally to form useful products.Molecules try to minimize their energy levels by aligning themselves in particular positions.If bonding to an adjacent molecule allows for a lower energy state, then the bonding will occur.We see this happening in many places in nature.For example, the spherical shape of a bubble or the shape of snowflake are a result of molecules minimizing their energy levels.

Self-Assembly by Crystal Growth.One particular type of selfassembly is crystal growth.This technique is used to “grow”nanotubes.In this approach, “seed” crystals are placed on some surface, some other atoms or molecules are introduced, and these particles mimic the pattern of the small seed crystal.For example, one way to make nanotubes is to create an array of iron nanopowder particles on some material like silicon, put this array in a chamber, and add some natural gas with carbon to the chamber.The carbon reacts with the iron and supersaturates it, forming a precipitate of carbon that then grows up and out.In this manner, you can grow nanotubes like trees!

Biological Nanomachines in Nature.There are many natural nanoscale devices that exist in our biological world.Life begins at the nanoscale! For example, inside all cells, molecules and particles of various sizes have to move around.Some molecules can move by diffusion, but ions and other charged particles have to be specifically transported around cells and across membranes.Biology has an enormous number of proteins that self-assemble into nanoscale structures.

The Evolution of Microscopes

One of the big ideas in science is that the creation of tools or instruments that improve our ability to collect data is often accompanied by new science understandings.Science is dynamic.Innovation in scientific instruments is followed by a better understanding of science and is associated with creating innovative technological applications.Traditional light microscopes are still very useful in many biology-related applications since things like cells and bacteria can readily be seen with this tool.They are also fairly inexpensive and are easy to set up.

Optical Microscope.How big is a nanometer? You can see down to about 1000 microns with the naked eye, and a typical microscope as used in biology class will get you down to about 10 microns.More advanced microscopes, such as scanning electron microscopes can get you pretty good resolution (1 micron) range.Newer technologies (within the last 20 years or so) allow us to“see” in the range of 100 nanometers to 1 nanometer.

Electron Microscope.The difference between the standard light microscope and the scanning electron microscope is that electrons, instead of various wavelengths of light, are“bounced” off the surface of the object being viewed, and that electrons allow for a higher resolution because of their small size.You can use the analogy of bouncing beach ball on a surface to find out if it is uneven (beach ball scattering in all different directions).

Atomic Force Microscope.The atomic force microscope uses the tip to interact with the atoms on the surface of the sample to detect information with atomic resolution.The smallest tip you can possibly make has to be made from atoms.The tip interacts with the surface of the material you want to look at, so the smaller the tip, the better the resolution.But because the tip is made from atoms, it can’t be smaller than the atoms you are looking at.Tips are made from a variety of materials, such as silicon, tungsten, and even carbon nanotubes.

The difference between the Atomic Force Microscope (AFM)and the Scanning Tunneling Microscope (STM) is that the AFM relies on movement due to the electromagnetic forces between atoms, and the STM relies on electrical current between the tip and the surface.Mention that the AFM was invented to overcomes the STM’s basic drawback: it can only be used to sense the nature of materials that conduct electricity, since it relies on the creation of a current between the tip and the surface.The AFM relies on actual contact rather than current flow, so it can be used to probe almost any type of material, including polymers, glass, and biological samples.The signals (forces or currents) from these instruments are used to infer an image of the atoms.The tip’s fluctuations are recorded and fed into computer models that generate images based on the data.These images give us a rough picture of the atomic landscape.

What makes the science at the nanoscale special is that at such a small scale, different physical laws dominate and properties of materials change.