Editing Scanning tunneling microscope (section)

== Procedure ==
[[File:Scanning Tunneling Microscope schematic.svg|thumb|300x300px|Schematic view of an STM]]
The tip is brought close to the sample by a coarse positioning mechanism that is usually monitored visually. At close range, fine control of the tip position with respect to the sample surface is achieved by [[Piezoelectricity|piezoelectric]] scanner tubes whose length can be altered by a control voltage. A bias [[electric tension|voltage]] is applied between the sample and the tip, and the scanner is gradually elongated until the tip starts receiving the tunneling current. The tip–sample separation ''w'' is then kept somewhere in the 4–7&nbsp;[[Angstrom|Å]] (0.4–0.7&nbsp;[[Nanometre|nm]]) range, slightly above the height where the tip would experience repulsive interaction {{nobr|(''w'' < 3 Å),}} but still in the region where attractive interaction exists {{nobr|(3 < ''w'' < 10 Å).<ref name="Chen"/>}} The tunneling current, being in the sub-[[nanoampere]] range, is amplified as close to the scanner as possible. Once tunneling is established, the sample bias and tip position with respect to the sample are varied according to the requirements of the experiment.

As the tip is moved across the surface in a discrete ''x''–''y'' matrix, the changes in surface height and population of the electronic states cause changes in the tunneling current. Digital images of the surface are formed in one of the two ways: in the ''constant-height mode'' changes of the tunneling current are mapped directly, while in the ''constant-current mode'' the voltage that controls the height (''z'') of the tip is recorded while the tunneling current is kept at a predetermined level.<ref name="Chen" />

In constant-current mode, feedback electronics adjust the height by a voltage to the piezoelectric height-control mechanism. If at some point the tunneling current is below the set level, the tip is moved towards the sample, and conversely. This mode is relatively slow, as the electronics need to check the tunneling current and adjust the height in a feedback loop at each measured point of the surface. When the surface is atomically flat, the voltage applied to the ''z''-scanner mainly reflects variations in local charge density. But when an atomic step is encountered, or when the surface is buckled due to [[Surface reconstruction|reconstruction]], the height of the scanner also have to change because of the overall topography. The image formed of the ''z''-scanner voltages that were needed to keep the tunneling current constant as the tip scanned the surface thus contain both topographical and electron density data. In some cases it may not be clear whether height changes came as a result of one or the other.

In constant-height mode, the ''z''-scanner voltage is kept constant as the scanner swings back and forth across the surface, and the tunneling current, exponentially dependent on the distance, is mapped. This mode of operation is faster, but on rough surfaces, where there may be large adsorbed molecules present, or ridges and groves, the tip will be in danger of crashing.

The [[raster scan]] of the tip is anything from a 128×128 to a 1024×1024 (or more) matrix, and for each point of the raster a single value is obtained. The images produced by STM are therefore [[grayscale]], and color is only added in post-processing in order to visually emphasize important features.

In addition to scanning across the sample, information on the electronic structure at a given location in the sample can be obtained by sweeping the bias voltage (along with a small AC modulation to directly measure the derivative) and measuring current change at a specific location.<ref name="Bai" /> This type of measurement is called [[scanning tunneling spectroscopy]] (STS) and typically results in a plot of the local [[density of states]] as a function of the electrons' energy within the sample. The advantage of STM over other measurements of the density of states lies in its ability to make extremely local measurements. This is how, for example, the density of states at an [[impurity]] site can be compared to the density of states around the impurity and elsewhere on the surface.<ref name="Pan">{{cite journal | vauthors = Pan SH, Hudson EW, Lang KM, Eisaki H, Uchida S, Davis JC | title = Imaging the effects of individual zinc impurity atoms on superconductivity in Bi2Sr2CaCu2O8+delta | journal = Nature | volume = 403 | issue = 6771 | pages = 746–750 | date = February 2000 | pmid = 10693798 | doi = 10.1038/35001534 | arxiv = cond-mat/9909365 | bibcode = 2000Natur.403..746P | s2cid = 4428971 }}</ref>