11. History The principle of electron tunneling was proposed by Giaever I. Giaever: Energy gap in superconductors measured by electron tunneling, Phys. Rev. Lett. 5 (1960) 147–148 He envisioned that if a potential difference is applied to two metals separated by a thin insulating film, a current will flow because of the ability of electrons to penetrate a potential barrier. To be able to measure a tunneling current, the two metals must be spaced no more than 10 nm apart.
12. Principle of STM The principle of the STM is straightforward. A sharp metal tip (one electrode of the tunnel junction) is brought close enough (0.3–1 nm) to the surface to be investigated (the second electrode) that, at a convenient operating voltage (10 mV–1 V), the tunneling current varies from 0.2 to 10 nA which is measurable. The tip is scanned over a surface at a distance of 0.3–1 nm, while the tunneling current between it and the surface is measured.
16. Scanning Tunneling Microscope (STM) This method uses an electric current (tunneling current) that begins to flow when a very sharp tip moves near to a conducting surface and hovers at about one nanometer away.
17. The tip (about the size of a single atom) sits on a piezoelectric tube. When you apply voltage to electrodes attached to this tube, you can make teensy adjustments to keep the tunneling current constant — which also keeps the tip at a constant distance from the sample while an area is scanned. The movement of the piezoelectric tube is recorded and displayed as an image of the sample surface.
18. Binnig et al.’s Design VT = bias voltage Ø = average barrier height (work function) JT = tunnel current A = constant 1.025 eV−1/2Å−1.
19. STM Operation STM for operation in ambient air, the sample is held in position while a piezoelectric crystal in the form of a cylindrical tube (referred to as PZT tube scanner) scans the sharp metallic probe over the surface in a raster pattern while sensing and outputting the tunneling current to the control station. The digital signal processor (DSP) calculates the desired separation of the tip from the sample by sensing the tunneling current flowing between the sample and the tip. The bias voltage applied between the sample and the tip encourages the tunneling current to flow. The DSP completes the digital feedback loop by outputting the desired voltage to the piezoelectric tube.
20. constant-current &constant-height mode STM can be operated in either the constant-current or the constant-height mode.The images are of graphite in air. Source: Springer Handbook of Nanotechnology by B. Bhushan
21. STM cantilever / tip Typically fabricated from metal wires of tungsten (W), platinum-iridium (Pt-Ir), or gold (Au). sharpened by grinding,cutting with a wire cutter or razor blade, field emission/evaporator, ionmilling, fracture, or electrochemical polishing/etching Schematics of a) CG Pt-Ir probe, and (b) CG Pt-Ir FIB milled probe
22. STM Tips The two most commonly used tips are Pt-Ir (80/20) Iridium: tips are generally mechanically formed and are readily available. provide better atomic resolution than tungsten tips. Tungsten wire: are etched from tungsten wire with an electrochemical process. Tungsten tips are more uniformly shaped and may perform better on samples with steeply sloped features.
23. Mechanically cut and electrochemically etched STM tips A mechanically cut STM tip (left) and an electrochemically etched STM tip (right), p.383 Springer Handbook of Nanotechnology
24. Sample should be conductive Samples to be imaged with the STM must be conductive enough to allow a few nanoamperes of current to flow from the bias voltage source to the area to be scanned. In many cases, nonconductive samples can be coated with a thin layer of a conductive material to facilitate imaging. The bias voltage and the tunneling current depend on the sample.
25. piezoelectric tube If you put electrodes on the opposite sides of some crystals — quartz or topaz, for example — and apply a voltage across the crystal, it will expand or contract. Any movement of the crystal in response to a voltage is called the piezoelectric effect. The piezoelectric tube used in the scanning tunneling microscope is simply a crystal that expands or contracts depending upon the voltage you apply to it.
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27. Scanning electron microscope (SEM) An SEM shoots a beam of electrons at whatever you’re examining, transferring energy to the spot that it hits. The electrons in the beam (called primary electrons) break off electrons in the specimen. These dislodged electrons (called secondary electrons) are then pulled onto a positively charged grid, where they’re translated into a signal. Moving the beam around the sample generates a whole bunch of signals, after which the SEM can build an image of the surface of the sample for display on a computer monitor.
28. Using SEM SEMs can ferret out quite a bit of information about the sample: Topography: surface features such as texture Morphology: shape, size, and arrangements of the particles that compose the object’s surface Composition: elements that make up the sample (This can be determined by measuring the X-rays produced when the electron beam hits the sample.)
29. Transmission electron microscope (TEM) It’s a kind of nano-scale slide projector: Instead of shining a light through a photographic image the TEM sends a beam of electrons through a sample. The electrons that get through then strike a phosphor screen, producing a projected image: Darker areas indicate that fewer electrons got through; lighter areas are where more electrons got through A TEM can achieve a resolution of approximately 0.2 nanometers, roughly the size of many atoms. A TEM can produce images that show you just how the atoms are arranged in a material.
