Nanoscale electro-optics of metals and molecules using UHV-STM

想象一个光源如此之小，以至于它能够从单个分子中产生信息。这就是这个项目背后的想法。但我们为什么要这么做呢？背景简介是这样运行的。电子设备的尺寸不断缩小，以便在更小的空间内以更快的速度产生更大的处理能力；数据存储设备中基本内存元素的大小也是如此。然而，向100纳米以下尺寸（1纳米是1毫米的百万分之一）的驱动是基于将传统的图案技术推向新的极限；它基本上是十多年前用于生产微米级产品的旧技术的粉饰版。与其一点一点地解决尺寸问题，难道不能用一种更具革命性的方式来处理吗？例如，是否有可能制造基于单分子的加工元件？这将使处理元件的尺寸降至1纳米左右。但我们如何评估这种可能性呢？实际上，这不是很好的思想实验。要做到这一点，我们需要处理单个分子并从中读取信息。用来做这项工作的仪器被称为扫描隧道显微镜，简称STM。它可以将导电尖端与导电表面保持极短的距离（1纳米或更短）。当在尖端和表面之间施加偏压时，一些电子穿过间隙-小电流流动（pA到nA刻度）。这是通过一种被称为隧道效应的量子力学过程发生的——从“经典”物理学的角度来看，没有电流应该流过，因为缝隙是电绝缘的！正如你可能猜到的，有一些非常漂亮的反馈涉及到保持尖端在样品上方1纳米的固定距离。然后STM可以以一种可控的方式移动尖端，以原子分辨率建立样品表面的图像-你可以看到原子就在那里！显然，它会“看到”放置在表面上的任何分子。这就解决了分子的寻址问题。这种方法的下一个非常简洁的方面是信息输出可以是光学的。当电子在尖端和样品之间流动时（任何一个方向都可以），就会产生一些光。如果有一个分子在那里，那么光输出与分子的关系可能比尖端或底层基质的性质更大。然后，该项目将尝试解决两个进一步的步骤。首先，我们希望在表面上有一个“智能”分子，最好是能够在两种不同的稳定状态之间改变形状和性质的分子。这样的分子确实存在，我们打算使用一种非常简单的分子，叫做偶氮苯。这个想法是通过施加在尖端的电（电压）脉冲来改变分子的状态，并观察光输出是否发生变化。（或者，反过来，用外部光脉冲处理分子并监测其电导率的变化。）第二件事是提高光输出，这里我们有一些关于微型天线的想法。要感受一下这一点，想想电视上的接收天线或ariel。它的尺寸在0.1到1米之间，这是因为它的设计是为了匹配电视载波信号的波长——频率为750兆赫的信号约为0.4米。缩放到光学/近红外区域（波长400- 1000nm）意味着天线长度只有几个100nm。这种天线将在被称为碳纳米管的迷人实体的基础上实现。它们本身就是一个完整的故事，但与这项工作相关的重要一点是，它们可以从表面垂直生长。我们将使用的纳米管直径为25-50纳米，长100纳米。这个想法是定制制造STM尖端作为光学天线，这样它就能有效地传输来自单独定位分子的光学信息。

Imagine a light source so small that it is able to generate information from just a single molecule. That's the idea behind this project. But why should we want to do this? The background blurb runs something like this. Electronic device sizes are continually being reduced to yield greater processing power in a smaller space at greater speed; the same goes for the size of the basic memory element in data storage devices. However, the drive to sub-100 nm sizes (1 nm is one millionth of 1 mm) is based on pushing conventional patterning technology to new limits; it's basically a glossed-up version of the same old technology that was used to produce micron scale stuff a decade or more ago. Instead of chipping away bit by bit at the size thing, could it not be treated in a more revolutionary fashion? Is it possible, for example, to make processing elements based on single molecules? That would knock the processing element size down to 1 nm or so. But how do we go about assessing this possibility? In real terms that is, not nice thought experiments. To do this we need to address single molecules and read information from them.The instrument that will be used to do this is called a scanning tunnelling microscope or STM for short. It can hold an electrically conducting tip an incredibly small distance (1 nm or less) above a conducting surface. When a bias is applied between tip and surface some electrons cross the gap - a small current flows (pA to nA scale). This happens by a quantum mechanical process called tunnelling - in terms of 'classical' physics, no current should flow because the gap is electrically insulating! As you might guess, there is some pretty nifty feedback involved in keeping the tip a fixed distance of just 1 nm above the sample. The STM can then move the tip in a controlled way to build up an image of the sample surface with atomic resolution - you can see the atoms poking right out there! Obviously, it will 'see' any molecules that are placed on the surface.That takes care of the molecular addressing. The next and really neat aspect of this approach is that the information output can be optical. When electrons flow between the tip and the sample (either direction is OK), some light is generated. If a molecule is sitting in there, the light output can have more to do with the molecule than the properties of either the tip or the underlying substrate. There are then two further steps that the project will try to address. First, we would like a 'smart' molecule on the surface, preferably one that can change shape and properties between two different, stable states. Such molecules actually exist and we intend using a remarkably simple one called azobenzene. The idea is to change the state of the molecule by an electrical (voltage) pulse applied to the tip and see if the light output changes. (Or, conversely, address the molecule with an external light pulse and monitor a change in its conductivity.) The second thing is to improve the optical output and here we have some ideas on really tiny antenna. To get a feel for this, think about the receiving antenna or ariel on your TV. It's size is in the 0.1 to 1 m range and that's because it's designed to match the wavelength of the TV carrier signal - about 0.4m for a signal of frequency 750 MHz. Scaling to the optical/near-infrared region (wavelength 400-1000 nm) implies an antenna length of a few 100 nm. Such antenna will be realized on the basis of fascinating entities known as carbon nanotubes. These are a whole story in themselves, but the important point in relation to this work is that they can be grown perpendicularly out of a surface. The nanotubes we will use will be 25-50 nm in diameter and several 100 nm long. The idea is to custom manufacture the STM tip as an optical antenna so that it efficiency transmits optical information from individually addressed molecules.