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

想象一下，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。但我们为什么要这样做呢？背景简介大概是这样写的。电子设备的尺寸正在不断减小，以在更小的空间中以更快的速度产生更大的处理能力；数据存储设备中基本存储元件的大小也是如此。然而，对100纳米以下尺寸(1纳米是1毫米的百万分之一)的推动是基于将传统的图案化技术推向新的极限；它基本上是十年或更长时间前用于生产微米级材料的相同旧技术的改进版本。与其一点点地削弱规模，难道不能以更具革命性的方式对待它吗？例如，有没有可能制造基于单分子的加工元件？这将使处理单元的大小降至1纳米左右。但我们如何评估这种可能性呢？实际上，这不是很好的思维实验。要做到这一点，我们需要处理单分子并从它们读取信息。用来做这件事的仪器被称为扫描隧道显微镜，简称STM。它可以将导电尖端保持在导电表面上方非常小的距离(1 nm或更短)。当在尖端和表面之间施加偏置时，一些电子穿过间隙--一个小电流流动(pA到nA尺度)。这是通过一种称为隧道效应的量子力学过程来实现的--从经典物理学的角度来看，不应该有电流流动，因为缝隙是电绝缘的！正如你可能猜到的，有一些相当巧妙的反馈涉及到将尖端保持在样品上方仅1 nm的固定距离。然后，扫描隧道显微镜可以以受控的方式移动尖端，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戳！显然，它会‘看到’放置在表面上的任何分子。这就负责了分子寻址。这种方法的下一个真正巧妙的方面是，信息输出可以是光学的。当电子在针尖和样品之间流动时(任何方向都可以)，就会产生一些光。如果一个分子坐在那里，光输出可能更多地与分子有关，而不是尖端或底层基质的性质。然后，该项目将尝试解决另外两个步骤。首先，我们希望表面有一个“智能”的分子，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叫做偶氮苯的分子。这个想法是通过在尖端施加一个电脉冲(电压)来改变分子的状态，并观察光输出是否改变。(或者，相反，用外部光脉冲寻址分子，并监测其导电性的变化。)第二件事是改善光学输出，这里我们有一些关于非常微小的天线的想法。要了解这一点，可以考虑电视上的接收天线或Ariel。它的尺寸在0.1到1米的范围内，这是因为它的设计与电视载波信号的波长相匹配--对于频率为750 MHz的信号，约为0.4m。缩放到光学/近红外区(波长400-1000 nm)意味着天线长度为100 nm。这种天线将在被称为碳纳米管的迷人实体的基础上实现。这些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出来。我们将使用的纳米管的直径为25-50 nm，长度为100 nm。这个想法是定制制造STM尖端作为光学天线，这样它就可以有效地传输来自单独寻址的分子的光学信息。

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.