Scanningprobe microscope (SPM):An SPM is an instrument which is used for studying surfaces at ananoscale level. They create images of surfaces with the use of a physicalprobe that touches the surface of a sample in order to scan the surface andcollect data, they’re typically obtained as a two dimensional grid of datapoints and are displayed as a computer image.The tracing of the surface of a specimen is carried out throughthe use of a sharp, electrically charged probe. The SPM probe does not touchthe surface but instead traces the specimen nanometres above the surface level.The probe can be used to interact with a specimen enabling researchers toexamine how a substance attracts or detracts and also how it responds toelectrical currents. Seeing as SPM technology can operate in a various amountof environments, even that of non-conductive specimens can be observed andmanipulated.
An SPM includes a probe tip which is mounted on the end of acantilever. The tip is sharp and can be as sharp as a single atom. It can bemoved precisely and accurately back and forth across the surface, even atom byatom. When the tip is near the sample surface, the cantilever is deflected by aforce. SPMs can measure deflections caused by many kinds of forces, includingmechanical contact, electrostatic forces, magnetic forces, chemical bonding,van der Waals forces, and capillary forces. The distance of the deflection ismeasured by a laser that is reflected off the top of the cantilever and into anarray of photodiodes.
SPMs can detect differences in height that are a fractionof a nanometer, about the diameter of a single atom. The tip is moved acrossthe sample multiple times. This is why they are known as ‘scanning’microscopes. A computer combines the data to create an image. The images are mostlycolourless because they measure properties other than the reflection of light.However, the images are often colourised, with different colours whichrepresent various properties for example height along the surface. Scientistsuse SPMs in a number of different ways, depending on the information they’retrying to gather from a sample.
The two primary modes are contact mode andtapping mode. In contact mode, the force between the tip and the surface iskept constant. This allows a scientist to rapidly image a surface. In tappingmode, the cantilever oscillates, intermittently touching the surface. Thetapping mode is especially useful when imaging a soft surface.
There are multiple types of SPMs. There is the Atomic forcemicroscope (AFM) which measures the electrostatic forces between the cantilevertip and the sample. The Magnetic force microscope (MFM) which measures magneticforces.
The Scanning tunnelling microscope (STM) which measures the electricalcurrent flowing between the cantilever’s tip and the sample. What arethe advantages of SPM technology?SPM technology provides researchers with a larger variety ofspecimen observation environments using the same microscope and the samespecimen, in turn reducing the time required to prepare and study specimens.Specialised probes, improvements and modifications to scanning probe equipmentcontinues to provide faster, more efficient, revealing specimen images withlittle effort and modification.What arethe disadvantages of SPM technology?One of the disadvantages of a scanning probe microscope is thatthe images in which they form in black, white or greyscale can in someinstances exaggerate a specimen’s shape or size. Computers are used tocompensate for these exaggerations and produce real time colour images whichfacilitate researchers with information in real time including interactionswithin cellular structures, harmonic responses and magnetic energy.As researchers continue with improvement of techniques andexpanding the abilities of scanning probe microscopes, the technologicalevolution will include better observation instruments, better quality dataanalysis, and better processing equipment. Also, micro-manipulation ofmolecules, DNA, biological and organic specimens incorporating this precisionequipment will produce a better understanding of and new methods for:· Treating disease· Manufacturing· Astronomy· Physics· EnergyThe evolution of SPMs has resulted in both scientists andengineers being able to observe structure and detail with unprecedentedresolution, without the need for rigorous sample preparations.Technical advances along with the development of sophisticatedtechniques, have undoubtedly extended the capabilities of SPMs, in particularAFMS, across a broad range of research into both materials and life sciences.
The AtomicForce Microscope:The Atomic Force Microscope (AFM) is a form of scanning probemicroscopy (SPM) involving the detection of interatomic forces that happenbetween a probe tip and a sample. The AFM consists of a cantilever with a sharptip (probe) at its end that is used to scan the specimen surface. Thecantilever is typically silicon or silicon nitride and is used to scan across asample in an effort to obtain information about its surface (topography). Thetip is integrated into a cantilever which moves up and down tracing theinteraction of the surface of a sample.
When the tip is brought into proximityof a sample surface, forces between the tip and the sample lead to a deflectionof the cantilever according to Hooke’s law. The probe is scanned in a similarpattern to the SEM where it moves in a raster pattern across the sample inorder to generate an image in an x, y and z pattern. The AFM can either use theprobe in a contact or non-contact mode.
A great advantage of the AFM is thatthe specimen being examined can be non-conducting, unlike for the SEM. In thismanner, the AFM can be used to study almost any sample.The AFM generates resolution by calculating the vertical andlateral deflections of the cantilever. This is completed by reflecting a laserbeam off the cantilever which is reflected to a position sensitive photodiode(PSPD) that consists of two sections. Consequently, any small change indeflection of the cantilever produces a magnified reflection of displacement onthe photodiode relative to the cantilever (American society for Microbiology,2002).AFM has considerable advantages over the SEM.
The ATM providesmore information with regard to the 3D surface of the specimen. Another majoradvantage of the AFM is that it does not require any pre-treatment that may causedamage or disfigurement to the sample. AFM also does not require a vacuum andcan be performed in air and liquid environments. Disadvantages of the AFMinclude the single scan image size produced. The AFM has a lower depth of fieldcompared to the SEM because it only scans an area of micrometres, notmillimetres like the SEM. During analysis, the AFM has a slower scanning speedto the SEM. Due to the nature of AFM probes, they cannot normally measure steepwalls or overhangs.
Specially made cantilevers and AFMs can be used to modulatethe probe sideways as well as up and down to measure sidewalls. Thesecantilevers however are considerably more expensive and have a lower lateralresolution. What isAFM used for?· Inorganics, polymers, coatings and bio-samples· Personal care products, the measuring of the change in nanoscalemechanical properties (modulus and friction) of hair, teeth and skin.· Investigation of the force in which is required to removenanoparticles from a surface.· The topography and nano mechanical properties of coatings.Thechallenges faced with measurement· Calibration, quantification and understanding of AFM modes(including that of force spectroscopy,multi-frequency modes, frequency modulation mode, lateral forceand amplitude modulation mode.· Obtaining important and additional information from AFM(mechanical, chemical, electrical).· Imaging soft samples at a high resolution whilst working onminimising damage.