20.109(F15):TEM (Day5)

Introduction

The Transmission Electron Microscope (TEM) achieves its remarkable resolution by “illuminating” samples using an electron beam in a vacuum rather than using a conventional light source in air. Since the electron beam passes through the sample that is being examined, the sample must be sufficiently thin and sufficiently sturdy to be hit by electrons in a vacuum. It’s important to remember that many biological materials are damaged or destroyed by the incoming electrons and that the TEM can image only the species that survive this harsh treatment. The denser parts of the sample will absorb or scatter some of the electron beam, and it’s the scattered electrons or those that pass through the sample that are focused using an electromagnetic lens. This “electron shadow” then strikes a fluorescent screen, giving rise to an image that varies in darkness according to the sample's density. For samples that are amenable to TEM, this form of examination can allow observation of angstrom-sized objects and of cellular details down to near atomic levels.

Copper TEM grid with carbon mesh, photographed at 60X magnification

Samples are applied to a wafer-thin "grid" before being loaded into the TEM and placed under vacuum. The grid can be made of many kinds of materials. All have lines of a conductive metal, in our case copper, that disperse the electron beam and thereby help keep the sample from being blown to bits by the energy in the beam. A carbon mesh is strung between the metal lines. Once a sample has been applied to the grid, it's only the portions that come to rest on the carbon mesh can be visualized, along with any imperfections in the carbon mesh itself.

Today you'll wash some of the nanowires you synthesized on M3D1 and M3D2 and visualize a sample of them by TEM. The rest of the nanowires was ground in a mortar with some carbon and teflon for assembly into your battery.

Protocols

Part 1: Examining TEM images

1. Today you will image the TEM grids that you prepared on Day 2. Due to space limitations, only 2-3 groups can join Angie for the imaging at a time.
2. Use the downtime to work on your Biomaterials engineering mini-report or your Research proposal presentation.

For next time

1. The Research proposal presentation slides are due by 1 pm on Tuesday, December 8 or Wednesday, December 9, according to which day you have lab.

   Assignment is to be submitted on Stellar.