There was a pretty jovial guy in grad school who was working on surface analysis of materials. There was a problem. His initial tasks all had to do with creating a sufficient vacuum so that he could do his spectroscopy (I’m not sure which of the surface analysis techniques he was attempting; generally, I was too busy reading organic chemistry literature and trying to create new types of metal ligands, but (again) I digress). Every time I’d see Mr. Smythe-Terwilliger (a fictional name to protect his innocence), I’d ask him how it was going. “Oh, I got the apparatus down to about 1×10-15 atmospheres (a measurement of pressure; at sea level, we experience about 1 atmosphere or 760 torr) and it sprung a leak.” He was achieving REALLY low pressures, and it would take him days to achieve those, but they would get stuck at some pressure above where he needed to be.
If somehow, we were to instantly find ourselves at a pressure of 1×10-15 atm (the abbreviation) we would go through explosive decompression. Whenever he would get close to his goal, some tiny, previously undetectable pore between the metal gaskets or flanges he had tightened down with impressive looking bolts would start inhaling molecules of air into his nice vacuum. Air, being a collection of elements, is among the many substances you do not want around when you are trying to characterize the surface of a material. The material you hope to study is already going to be far from perfect and that will be a nice problem to have, but until that happens, he would have to shut off the valves to the huge vacuum pumps (pumps that pull most of the gas molecules out of his instrument), then shut off the pumps themselves. If he shut off the pump first, its oil would go slithering up the vacuum hose into his relatively pristine, ultra-high vacuum instrument and he would probably spend his remaining years on earth swabbing oil out before he could pump it down again (and find yet another leak). Here is a chart of relative pressures achieved or observed and a paper discussing how to achieve ultra- and extremely high vacuum conditions.
|Low vacuum||760 to 25||1×105 to 3×103||9.87×10−1 to 3×10−2|
|Medium vacuum||25 to 1×10−3||3×103 to 1×10−1||3×10−2 to 9.87×10−7|
|High vacuum||1×10−3 to 1×10−9||1×10−1 to 1×10−7||9.87×10−7 to 9.87×10−13|
|Ultra high vacuum||1×10−9 to 1×10−12||1×10−7 to 1×10−10||9.87×10−13 to 9.87×10−16|
|Extremely high vacuum||< 1×10−12||< 1×10−10||< 9.87×10−16|
|Outer space||1×10−6 to < 1×10−17||1×10−4 to < 3×10−15||9.87×10−10 to < 2.96×10−20|
|Perfect vacuum||0 (theoretical)||0 (theoretical)||0 (theoretical)|
The whole exercise seemed very exciting in a way, but in slow motion. I’m too impatient to do this kind of thing, but Mr. S-T was always fairly cheery about it all. He reminded me, I suppose, of a well-adjusted Sisyphus, the tricky Greek king punished for pissing off Hades and condemned to roll a large boulder up a mountain every day, just to watch it roll down the other side. “Repeat for eternity, please, Sisy old boy. Do not call, do not write. Thank you, Hades.” There is something very prescient in this myth as much of what scientists do on most days is do experiments that do not result in any useful advancement of knowledge. Experiments fail, fail often (just to make sure the first ten were true failures), are redesigned, refocused, fail again and often, and are redesigned until they succeed or that path to new knowledge is abandoned to other groups and/or other future times or altogether by everybody previously working on it. The experiment is rolled up the “hill,” falls down the other side, and is dutifully pushed again, this time with a few micro-grains of stone polished away. It is kind of crazy to do… until it rewards persistence with the tiny gleam of something new, something to be affirmed and passed into the literature to be replicated, confirmed, or rejected by others.
There are many instrumental methods for materials analysis. There are a substantial number of surface analysis methods that require insanely high vacuums and all of the asymptotic refinements in technology (pumps, metal, plastic, ceramic, and adhesive purities, for instance) that must improve to get there. The rapid evolution of nanoelectronics and the desire to create quantum computers have been critical driving forces towards improvements in these methods but before there is any application in science there is an exploration of theories and techniques. What is possible? How can the formerly impossible be attained?
To convince you further of the truth found in my lede for The Mess (Part 1: Too Much Mess! (to wit, “There is nothing superficial about surfaces”), let us explore three of those methods: X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), along with its quantum-empowered sister scanning tunneling microscopy (STM). This stuff gets heady very quickly so there will be much generalization and extreme hand-waving.
The XPS (aka ESCA) system is illustrated below, along with a cartoon of what is happening in the surface atoms (blue box) and the resulting graphical output showing the number of electrons blasted out of their energy levels. It shares characteristics with many electromagnetic analytical techniques in that a beam (“ray”) of some chosen energy is aimed at a sample, the sample responds in some way, and a graph of response versus physicochemical property is gained. The particular sample of copper illustrated seems to be quite pure as electrons from other elements are not produced during the experiment. If there had been other metallic or organic impurities present, they could have been revealed in the spectrum as well. This method is able to detect impurities in the parts per thousand (ppt) and parts per million (ppm) range. Purified silicon is required to make quality semiconductors but to get this material to function as a conductor it must be purposely contaminated with trace phosphorus, boron, and other “dopants.” XPS is one means of determining whether the silicon is adequately pure and appropriately doped.
To give the truly inspired reader something to ponder, I have included the following video in which the contours of a typical XPS instrument are lovingly caressed by several supplicants. While I have never known the fickle love of an XPS, I have appreciated that of other large, expensive instruments. Alas! No more!
But XPS (aka ESCA) and its affiliated methods provide data that is deconvoluted and interpreted to arrive at an insight into some aspect of a material’s quality. The really mind-blowing achievements in surface analysis (to the extent that I can cover in this post) come with SEM and STM. Let’s start with SEM.
As with XPS, a beam is propagated from an electron-producing device and focused through a range of lenses, apertures, and magnetic deflection coils to optimize and narrow the beam as it speeds towards the sample. Once the beam strikes the sample, electrons are scattered off the sample and detected by various types of detectors, depending on the configuration of the microscope. This type of microscope is responsible for providing some of the most intriguing views into many of the tiny realities among which we stomp, giants in a universe of delicate details. I am particularly intrigued by electron photomicrographs of bug heads. Well, any bug part, really.
To extend this fun further, please watch the following portion of a much longer video about microscopy in general.
STM takes these explorations down to individual atom resolution. It works by achieving just the right voltage difference between the scanning tip and the sample surface to cause electrons to quantum tunnel their way through the vacuum between them. This makes the atoms in the surface develop a three-dimensional aspect that allows very close inspection of materials. The truly remarkable feat that STM allows is the rearrangement of individual atoms on the surface of – or within – a material! Pick it up, put it down “here, please!”
The following video goes into greater detail as to how this is accomplished. Please enjoy.
Archimedes animated this film for the Max Planck Institute of Microstructure Physics. The film explains, how scientists observe surfaces at the atomic level with a scanning tunnelling microscope. An extremely fine tip “feels” its way over a surface at a constant distance of a few atomic diameters. The distance between the tip and the surface is regulated with the aid of what is called a tunnelling current, which flows between the tip and the sample when a voltage is applied between them.
There’s an incredible array of these electron photomicrographs (and even just light photomicrographs) out there on the web. Here are two sites that run annual competitions to select the best images from the previous year. It is important to note that all of the SEM and STM images and perhaps some of the light images use the art of false color imaging, a technique also used to great effect in the Hubble, and other satellite images. In effect, it takes images produced from non-visible portions of the electromagnetic (EM) spectrum, or from black and white images, and assigns colors within the visible spectrum to them so we can have some comprehension of their appearance. It’s a useful illusion but we would only be able to see the real thing if, somehow, our eyes and the brain to which they’re tethered could see the entire EM spectrum. It would probably result in minds shattering into some huge number of pieces, so false color it is!
To wrap, I provide a few SEM and STM videos, plus a playful explanatory video about the size of atoms to give some general perspective.
A link to a whole bunch of other “Zoom into…” videos:
Plus a mosquito eye!
A cluster of atoms!
And the promised explanatory video!
To conclude, there is nothing superficial about any of what you see around you. Do your own searches using some of the key words and phrases provided above. Discover more about the world you live in than you may have ever imagined before. It’s pretty beautiful, particularly if we are safe from the pincers, proboscides, and mandibles of all those tiny insects. Have a look around!
Featured image attribution: By Heiti Paves (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons