Atomic Force Microscope

The atomic force microscope (AFM) is a very powerful microscope invented by Binnig, Quate and Gerber in 1986.

Besides imaging it is also one of the foremost tools for the manipulation of matter at the nanoscale.

The AFM consists of a cantilever with a sharp tip at its end, typically composed of silicon or silicon nitride with tip sizes on the order of nanometers.

The tip is brought into close proximity of a sample surface.

The Van der Waals force between the tip and the sample leads to a deflection of the cantilever according to Hooke's law, where the spring constant of the cantilever is known.

Typically, the deflection is measured using a laser spot reflected from the top of the cantilever into an array of photodiodes.

However a laser detection system can be expensive and bulky;

an alternative method in determining cantilever deflection is by using piezoresistive AFM probes.

These probes are fabricated with piezoresistive elements that act as a strain gage.

Using a Wheatstone bridge, strain in the AFM probe due to deflection can be measured, but this method is not as sensitive as the laser deflection method.

If the tip were scanned at constant height, there would be a risk that the tip would collide with the surface, causing damage.

Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to keep the force between the tip and the sample constant.

Generally, the sample is mounted on a piezoelectric tube, which can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample.

The resulting map of s(x,y) represents the topography of the sample.

Over the years several modes of operation have been developed for the AFM.

The primary modes of operation are contact mode, non-contact mode, and dynamic contact mode.

In the contact mode operation, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection.

In the non-contact mode, the cantilever is externally oscillated at or close to its resonance frequency.

The oscillation gets modified by the tip-sample interaction forces;

these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics.

Because most samples develop a liquid meniscus layer, keeping the probe tip close enough to the sample for these inter-atomic forces to become detectable while preventing the tip from sticking to the surface presents a major hurdle for non-contact mode in ambient conditions.

Dynamic contact mode was developed to bypass this problem (Zhong et al).

In dynamic contact mode, the cantilever is oscillated such that it comes in contact with the sample with each cycle, and then enough force is applied to detach the tip from the sample.

Schemes for non-contact and dynamic contact mode operation include frequency modulation and the more common amplitude modulation.

In frequency modulation, changes in the oscillation frequency provide information about a sample's characteristics.

In amplitude modulation (better known as intermittent contact or tapping mode), changes in the oscillation amplitude yield topographic information about the sample.

Additionally, changes in the phase of oscillation under tapping mode can be used to discriminate between different types of materials on the surface.

The AFM has several advantages over the electron microscope.

Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile.

Additionally, samples viewed by an AFM do not require any special treatment that would actually destroy the sample and prevent its reuse.

While an electron microscope needs an expensive vacuum environment for proper operation, most AFM modes can work perfectly well in an ambient or even liquid environment.

The main disadvantage that the AFM has compared to the scanning electron microscope (SEM) is the image size.

The SEM can show an area on the order of millimetres by millimetres and a depth of field on the order of millimetres.

The AFM can only show a maximum height on the order of micrometres and a maximum area of around 100 by 100 micrometres.

Additionally, the AFM cannot scan images as fast as an SEM.

It may take several minutes for a typical region to be scanned with the AFM, however an SEM is capable of scanning at near real-time (although at relatively low quality).

The atomic force microscope (AFM) is a very powerful microscope invented by Binnig, Quate and Gerber in 1986. Besides imaging it is also one of the foremost tools for the manipulation of matter at the nanoscale. The AFM consists of a cantilever with a sharp tip at its end, typically composed of silicon or silicon nitride with tip sizes on the order of nanometers. The tip is brought into close proximity of a sample surface. The Van der Waals force between the tip and the sample leads to a deflection of the cantilever according to Hooke's law, where the spring constant of the cantilever is known. Typically, the deflection is measured using a laser spot reflected from the top of the cantilever into an array of photodiodes. However a laser detection system can be expensive and bulky; an alternative method in determining cantilever deflection is by using piezoresistive AFM probes. These probes are fabricated with piezoresistive elements that act as a strain gage. Using a Wheatstone bridge, strain in the AFM probe due to deflection can be measured, but this method is not as sensitive as the laser deflection method. If the tip were scanned at constant height, there would be a risk that the tip would collide with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to keep the force between the tip and the sample constant. Generally, the sample is mounted on a piezoelectric tube, which can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample. The resulting map of s(x,y) represents the topography of the sample. Over the years several modes of operation have been developed for the AFM. The primary modes of operation are contact mode, non-contact mode, and dynamic contact mode. In the contact mode operation, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection. In the non-contact mode, the cantilever is externally oscillated at or close to its resonance frequency. The oscillation gets modified by the tip-sample interaction forces; these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. Because most samples develop a liquid meniscus layer, keeping the probe tip close enough to the sample for these inter-atomic forces to become detectable while preventing the tip from sticking to the surface presents a major hurdle for non-contact mode in ambient conditions. Dynamic contact mode was developed to bypass this problem (Zhong et al). In dynamic contact mode, the cantilever is oscillated such that it comes in contact with the sample with each cycle, and then enough force is applied to detach the tip from the sample. Schemes for non-contact and dynamic contact mode operation include frequency modulation and the more common amplitude modulation. In frequency modulation, changes in the oscillation frequency provide information about a sample's characteristics. In amplitude modulation (better known as intermittent contact or tapping mode), changes in the oscillation amplitude yield topographic information about the sample. Additionally, changes in the phase of oscillation under tapping mode can be used to discriminate between different types of materials on the surface. The AFM has several advantages over the electron microscope. Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile. Additionally, samples viewed by an AFM do not require any special treatment that would actually destroy the sample and prevent its reuse. While an electron microscope needs an expensive vacuum environment for proper operation, most AFM modes can work perfectly well in an ambient or even liquid environment. The main disadvantage that the AFM has compared to the scanning electron microscope (SEM) is the image size. The SEM can show an area on the order of millimetres by millimetres and a depth of field on the order of millimetres. The AFM can only show a maximum height on the order of micrometres and a maximum area of around 100 by 100 micrometres. Additionally, the AFM cannot scan images as fast as an SEM. It may take several minutes for a typical region to be scanned with the AFM, however an SEM is capable of scanning at near real-time (although at relatively low quality). °	The atomic force microscope (AFM) is a very powerful microscope invented by Binnig, Quate and Gerber in 1986.	Il microscopio ad interazione atomica (AFM) è un potentissimo microscopio inventato da Binning, Quate e Gerber nel 1986.
	Besides imaging it is also one of the foremost tools for the manipulation of matter at the nanoscale.	Oltre a memorizzare immagini è anche uno dei principali strumenti di manipolazione della materia su scala nanometrica.
	The AFM consists of a cantilever with a sharp tip at its end, typically composed of silicon or silicon nitride with tip sizes on the order of nanometers.	Il microscopio ad interazione atomica (AFM) consiste di una microleva alla cui estremità è montata una punta acuminata, tipicamente composta di silicio o nitruro di silicio, che ha misure dell’ordine dei nanometri.
	The tip is brought into close proximity of a sample surface.	La punta viene collocata nelle strette vicinanze di una superficie campione.
	The Van der Waals force between the tip and the sample leads to a deflection of the cantilever according to Hooke's law, where the spring constant of the cantilever is known.	La forza di Van der Waals che agisce tra la punta ed il campione provoca una deflessione della microleva (la cui costante elastica è nota) in accordo con la legge di Hooke,
	Typically, the deflection is measured using a laser spot reflected from the top of the cantilever into an array of photodiodes.	Tipicamente, la deflessione è misurata utilizzando un punto laser riflesso dalla sommità della microleva verso una matrice di fotodiodi.
	However a laser detection system can be expensive and bulky;	Tuttavia, un sistema di rilevamento laser può essere costoso ed ingombrante;
	an alternative method in determining cantilever deflection is by using piezoresistive AFM probes.	un metodo alternativo per determinare la deflessione della microleva consiste nell’utilizzare sonde AFM piezoresistive.
	These probes are fabricated with piezoresistive elements that act as a strain gage.	Queste sonde sono fabbricate con elementi piezoresistivi che fungono da estensimetri a resistenza.
	Using a Wheatstone bridge, strain in the AFM probe due to deflection can be measured, but this method is not as sensitive as the laser deflection method.	Le deformazioni nella sonda del microscopio ad interazione atomica (AFM) dovute alla deflessione possono essere misurate utilizzando un ponte di Wheatstone, ma questo metodo non è altrettanto preciso di quello a deflessione laser.
	If the tip were scanned at constant height, there would be a risk that the tip would collide with the surface, causing damage.	Se la punta fosse esplorata ad altezza costante, si correrebbe il rischio che essa possa collidere con la superficie danneggiandola.
	Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to keep the force between the tip and the sample constant.	Di conseguenza, nella maggior parte dei casi viene utilizzato un meccanismo di retroazione per regolare la distanza tra la punta e il campione al fine di mantenere costante la forza che agisce tra loro.
	Generally, the sample is mounted on a piezoelectric tube, which can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample.	Generalmente il campione è collocato su un tubo piezoelettrico che può spostarlo in direzione perpendicolare (direzione z) per mantenere una forza costante e nel piano (direzioni x ed y) per analizzarne la superficie.
	The resulting map of s(x,y) represents the topography of the sample.	La mappa risultante s(x,y) rappresenta la topografia della superficie campione.
	Over the years several modes of operation have been developed for the AFM.	Nel corso degli anni sono stati sviluppati diversi metodi di funzionamento per il microscopio ad interazione atomica (AFM) .
	The primary modes of operation are contact mode, non-contact mode, and dynamic contact mode.	I principali metodi di funzionamento sono: a contatto (contact mode), ad assenza di contatto (non-contact mode) e a contatto dinamico (dynamic mode).
	In the contact mode operation, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection.	Nel funzionamento a contatto, la forza agente tra la punta e la superficie campione viene mantenuta costante durante la scansione mantenendo una deflessione costante.
	In the non-contact mode, the cantilever is externally oscillated at or close to its resonance frequency.	Nella modalità ad assenza di contatto, la microleva viene fatta oscillare esternamente alla, o in prossimità, della sua frequenza di risonanza.
	The oscillation gets modified by the tip-sample interaction forces;	L’ oscillazione viene modificata dalle forze di interazione tra la punta e la superficie campione;
	these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics.	queste variazioni dell’oscillazione in rapporto all’oscillazione esterna di riferimento forniscono informazioni riguardo le caratteristiche del campione.
	Because most samples develop a liquid meniscus layer, keeping the probe tip close enough to the sample for these inter-atomic forces to become detectable while preventing the tip from sticking to the surface presents a major hurdle for non-contact mode in ambient conditions.	Poiché la maggior parte dei campioni sviluppa uno strato di menisco liquido, mantenere la punta della sonda abbastanza vicina al campione, in modo da rendere queste forze interatomiche rilevabili evitando allo stesso tempo che la punta si blocchi sulla superficie, rappresenta un ostacolo non irrilevante per la modalità ad assenza di contatto in condizioni ambientali normali.
	Dynamic contact mode was developed to bypass this problem (Zhong et al).	La modalità a contatto dinamico è stata sviluppata al fine di bypassare il problema. (Zhong et al)
	In dynamic contact mode, the cantilever is oscillated such that it comes in contact with the sample with each cycle, and then enough force is applied to detach the tip from the sample.	Nel funzionamento a contatto dinamico, la microleva viene fatta oscillare così che possa venire a contatto con il campione ad ogni ciclo e, successivamente, viene applicata la forza necessaria per staccare la punta dal campione.
	Schemes for non-contact and dynamic contact mode operation include frequency modulation and the more common amplitude modulation.	Gli schemi per i funzionamenti ad assenza di contatto e a contatto dinamico includono la modulazione di frequenza e la più comune modulazione di ampiezza.
	In frequency modulation, changes in the oscillation frequency provide information about a sample's characteristics.	Nella modulazione di frequenza le variazioni della frequenza di oscillazione forniscono informazioni riguardo alle caratteristiche della superficie campione.
	In amplitude modulation (better known as intermittent contact or tapping mode), changes in the oscillation amplitude yield topographic information about the sample.	Nella modulazione di ampiezza (meglio nota come contatto ad intermittenza o modalità tapping), le variazioni nell’ ampiezza di oscillazione producono informazioni topografiche della superficie campione.
	Additionally, changes in the phase of oscillation under tapping mode can be used to discriminate between different types of materials on the surface.	In aggiunta, le variazioni di fase delle oscillazioni nella modalità tapping possono essere usate per discriminare tra differenti tipologie di materiali sulla superficie.
	The AFM has several advantages over the electron microscope.	Il microscopio ad interazione atomica (AFM) presenta diversi vantaggi rispetto al microscopio elettronico.
	Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile.	Diversamente da quest ultimo, che fornisce una proiezione bidimensionale o un’immagine bidimensionale di un campione, l’ AFM produce un reale profilo tridimensionale della superficie.
	Additionally, samples viewed by an AFM do not require any special treatment that would actually destroy the sample and prevent its reuse.	Inoltre i campioni analizzati da un microscopio ad interazione atomica (AFM) non richiedono nessun trattamento speciale che in realtà distruggerebbe il campione impedendone il riutilizzo.
	While an electron microscope needs an expensive vacuum environment for proper operation, most AFM modes can work perfectly well in an ambient or even liquid environment.	Mentre un microscopio elettronico necessita di un costoso ambiente sotto vuoto per un funzionamento corretto, la maggior parte delle modalità operative del microscopio ad interazione atomica (AFM) funzionano perfettamente nell’ambiente normale o perfino in un ambiente liquido.
	The main disadvantage that the AFM has compared to the scanning electron microscope (SEM) is the image size.	Il principale svantaggio che ha il microscopio ad interazione atomica(AFM) paragonato al microscopio elettronico a scansione (SEM) consiste nella dimensione dell’immagine.
	The SEM can show an area on the order of millimetres by millimetres and a depth of field on the order of millimetres.	Il microscopio elettronico a scansione (SEM) è in grado di mostrare un'area dell’ordine del millimetro quadro ed una profondità di campo dell’ordine del millimetro.
	The AFM can only show a maximum height on the order of micrometres and a maximum area of around 100 by 100 micrometres.	Il microscopio ad interazione atomica (AFM) può invece riprodurre solo un’altezza dell’ordine del micrometro ed un’area massima di circa 100 per 100 micrometri.
	Additionally, the AFM cannot scan images as fast as an SEM.	Inoltre il microscopio ad interazione atomica (AFM) non è in grado di analizzare le immagini altrettanto velocemente di un microscopio elettronico a scansione (SEM).
	It may take several minutes for a typical region to be scanned with the AFM, however an SEM is capable of scanning at near real-time (although at relatively low quality).	Per effettuare la scansione di un'area ci vogliono tipicamente diversi minuti con un microscopio ad interazione atomica (AFM), mentre un microscopio elettronico a scasione (SEM) è in grado di esplorarla quasi in tempo reale (anche se con una qualità relativamente bassa).