納米：改變我們對物質的認識

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

我們為什么應該關注納米科學？因為它將改變我們的生活，改變我們對物質的認知。1999年由美國國家科學基金會召集的頂級科學家曾說：“納米技術對21世紀人類健康、財富和生活水平的影響，至少可以與20世紀發展的微電子、醫學成像、計算機輔助工程和人造聚合物的綜合影響一樣重要。”

納米科學涉及對極小尺度——10-7米（100納米）到10-9米（1納米）現象的研究。在這個尺度下物質的性能完全不同于更大范圍的性能。納米科學是一門多學科領域，不僅聚焦于化學，還有生物學、物理學、工程學和計算科學等。正由于它的多學科性，納米科學需要我們去關注不熟悉學術領域的知識。

金子總是金色嗎？

想象一下當你切東西時會發生什么。在哪一刻你會切下單個原子？在哪一刻“顏色”會變化并消失？其實單個原子并沒有顏色。一種物質的顏色由從它反射回來光的波長所決定，但一個原子由于太小而不能自己反射光。只有當你有了足夠大的原子集合體（一堆原子）時，你才能開始分辨出某種接近“顏色”的東西。例如，一堆鹽晶體在一起看起來是白色的，但一個單獨的鹽晶體卻是無色的。這一類比使人們認識到，不同厚度的材料產生不同的顏色。例如，水上的油會根據油膜的厚度產生不同的顏色。在浮油中原子是不變的，只是不同的厚度（原子的數量）反射出不同的顏色。樹葉之所以看起來是綠色的，是因為樹葉表面的原子結構反射出綠色的波長，并吸收了其他的波長。隨著葉子枯萎，原子結構會發生變化，因此葉綠素分解后你看到的是反射回來的棕色。對于金子而言，顏色取決于納米尺度的晶體或者原子結構：光吸收根據晶體的厚度而不同。在《個人觸摸》的故事中，桑德拉的禮服顏色變化是因為她能改變禮服原子的排列，從而反射出不同的顏色。

如何構建納米結構？

碳納米管是最近創造的結構，并且具有許多新奇性能。它非常輕且堅固，可以添加到各種材料中以增加強度而不會增加太多重量。與此同時它也有許多有趣的電導（電學）特性。碳巴基球（足球烯）基于其交錯“足球”形狀是一種非常堅固的結構。它有一個獨特的性能，能夠攜帶一些東西穿透細胞壁，然后把東西送到細胞內。一般來說，我們的身體對此沒有反應，所以你的身體不會試圖攻擊它，它可以很容易地隨著血液流動。

穆罕穆德·沙法（Muhammad Shafa），西安交通大學材料科學與工程學院助理教授，2016—2017年在阿聯酋大學做博士后。沙法博士在實驗凝聚態物理學、半導體納米線和薄膜合成、制造納米器件等領域作出了重要貢獻并發表了若干篇高質量文章。

構建納米結構。我們如何構建如此小的東西呢？目前主要有三種方法。首先，掃描探針顯微鏡的尖端可以與它們所掃描材料的原子成鍵并移動原子。1990年，IBM用這種方法操作氙原子制作出了有史以來最小的商標。另外，科學家可以從表面刻材料，直到出現所需的結構。這是計算機工業用來制造集成電路的過程。最后，自組裝是分子構件自然“組裝”形成有用產品的過程。分子試圖通過將自己排列在特定位置來最小化它們的能級。如果與相鄰的分子成鍵能達到較低的能量狀態，則會鍵合。我們可以在自然界的許多地方看到這種情況。例如，氣泡的球形或雪花的形狀是分子最小化其能級的結果。

晶體生長的自組裝。晶體生長是一種特殊的自組裝類型。這種技術被用來“生長”納米管。在這種方法中，“種子”晶體被放置在某些表面，引入一些其他原子或分子，這些粒子模仿小種子晶體的模式。例如，制造納米管的一種方法是在硅之類的材料上制造一組鐵納米粉顆粒，把這些陣列放在一個腔室中，然后往腔室中加入一些含碳的天然氣。碳與鐵發生反應，并使其過飽和，形成碳沉淀，隨后析出。通過這種方式，可以生長出像樹一樣的納米管!

自然界中的生物納米機器。在我們的生物世界中存在著許多天然的納米級器件。生命始于納米！例如，在所有細胞內部，各種大小的分子和粒子都必須四處移動。一些分子可以通過擴散移動，但離子和其他帶電粒子必須在細胞周圍和細胞膜之間進行特異性運輸。生物學中有大量蛋白質可以自組裝成納米級結構。

顯微鏡的發展

科學中的一個重要思想是，創造工具或儀器來提高我們收集數據的能力，往往伴隨著新的科學理解?？茖W是動態的?？茖W儀器的創新伴隨著對科學的更好理解，并與創造創新的技術應用相關聯。傳統的光學顯微鏡在許多生物學相關的應用中仍然非常廣泛，因為使用該工具可以很容易地看到細胞和細菌等。它們也相當便宜，易于安裝。

光學顯微鏡。納米有多大？你可以用肉眼看到大約1000微米，而生物課上使用的典型顯微鏡可以讓你看到大約10微米。更先進的顯微鏡，如掃描電子顯微鏡，可以獲得相當好的分辨率（1微米）范圍。更新的技術（在過去20年左右）允許我們“看到”100納米到1納米范圍。

電子顯微鏡。掃描電子顯微鏡和標準光學顯微鏡的區別在于，電子而不是各種波長的光從被觀察物體的表面“反彈”，由于電子體積小，所以可以獲得更高的分辨率。打一個比方，你可以在一個表面上拍沙灘球來確定其表面是否平坦（球在各個不同方向上的散射）。

原子力顯微鏡。原子力顯微鏡利用針尖與樣品表面原子發生相互作用，從而探測樣品表面信息，具有原子級的分辨率。人們能做到的最小的針尖必須由原子制成。針尖與要觀測的材質表面會相互作用，因此針尖越小，分辨率越高。但是因為針尖是由原子組成的，它不能比你看到的原子小。針尖由多種材料制成，如硅、鎢，甚至碳納米管。

原子力顯微鏡（AFM）和掃描隧道顯微鏡（STM）之間的區別在于AFM依靠原子間電磁力的運動，而STM則依靠針尖和表面之間的電流。需要注意的是AFM正是為了克服STM的基本缺點而發明的，即STM只能用來測量導體因為它依賴于針尖和表面之間電流的產生。AFM依靠實際接觸而不是電流，因此它可以用來探測幾乎任何類型的材料，包括聚合物、玻璃和生物樣品。人們利用這些儀器的信號（力或電流）來推斷原子的圖像。針尖的波動被記錄下來并輸入計算機模型中，計算機模型根據數據生成圖像。這些圖像給我們提供了原子尺度的大致情況。

納米科學的特殊之處在于，在如此小的尺度下，不同的物理定律占據主導地位，材料的性質也會發生變化。

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.