Scanning Probe MicroscopyScanning Probe Microscopy

Scanning Tunneling Microscope Scanning Tunneling Microscope STMSTM

Atomic Force Microscope Atomic Force Microscope AFMAFM

HEINRICH ROHRERHEINRICH ROHRER GERD BINNIGGERD BINNIG

SharedShared** the 1986 Nobel prize in Physics for the 1986 Nobel prize in Physics for their invention of the scanning tunneling their invention of the scanning tunneling microscope microscope

““IBM” IBM” spelled in spelled in Xenon atomsXenon atoms

HEINRICH ROHRERHEINRICH ROHRER GERD BINNIGGERD BINNIG

SharedShared** the 1986 Nobel prize in Physics for the 1986 Nobel prize in Physics for their invention of the scanning tunneling their invention of the scanning tunneling microscope microscope

* Ernst Ruska was the other winner* Ernst Ruska was the other winner

““IBM” IBM” spelled in spelled in Xenon atomsXenon atoms

Electron Tunneling:Electron Tunneling:In scanning tunnneling In scanning tunnneling microscopy a small bias microscopy a small bias voltage V is applied so voltage V is applied so that due to the electric that due to the electric field the tunneling of field the tunneling of electrons results in a electrons results in a tunneling current I. The tunneling current I. The height of the barrier can height of the barrier can roughly be roughly be approximated by the approximated by the average workfunction of average workfunction of sample and tip. sample and tip.

When the tip of the STM probe is sufficiently When the tip of the STM probe is sufficiently close to the surface of the specimen (~ 1nm) a close to the surface of the specimen (~ 1nm) a tunneling current can become establishedtunneling current can become established

Ideally a STM probe tip is very pointed (1-2 Ideally a STM probe tip is very pointed (1-2 atoms at the end) and has a relatively low work atoms at the end) and has a relatively low work function. Etched tungsten crystals are ideal and function. Etched tungsten crystals are ideal and are nearly identical to field emitters.are nearly identical to field emitters.

The tunneling current is exponentially The tunneling current is exponentially proportional to the distance and thus via a proportional to the distance and thus via a feedback loop the tip can be maintained at a feedback loop the tip can be maintained at a constant distance from the surface by constant distance from the surface by maintaining a constant tunneling current.maintaining a constant tunneling current.

If the tunneling current is kept constant the Z If the tunneling current is kept constant the Z position of the tip must be moved up and position of the tip must be moved up and down. If this movement is recorded then the down. If this movement is recorded then the topography of the specimen can be inferred. topography of the specimen can be inferred.

Alternatively if the Z position of the tip is Alternatively if the Z position of the tip is kept constant the tunneling current will kept constant the tunneling current will change as it moves across the surface. If the change as it moves across the surface. If the changes in current are recorded the then the changes in current are recorded the then the topography of the specimen can be inferred. topography of the specimen can be inferred.

The probe is The probe is scanned over the scanned over the surface in a raster surface in a raster pattern similar to pattern similar to that of a SEM or that of a SEM or Confocal. Each Confocal. Each coordinate (X,Y, & coordinate (X,Y, & Z) is recorded by a Z) is recorded by a computer.computer.

The ability to precisely position the probe of an The ability to precisely position the probe of an STM is made possible by an XYZ Piezo-STM is made possible by an XYZ Piezo-Scanner which coupled to a feedback regulator Scanner which coupled to a feedback regulator keeps track of the tunneling current and keeps track of the tunneling current and precisely positions the tip accordingly.precisely positions the tip accordingly.

Crystals which acquire a charge when compressed, Crystals which acquire a charge when compressed, twisted or distorted are said to be piezoelectric. twisted or distorted are said to be piezoelectric. Piezoelectric ceramic materials have found use in Piezoelectric ceramic materials have found use in producing motions on the order of nanometers in the producing motions on the order of nanometers in the control of STMs and other devices.control of STMs and other devices.

The Piezoelectric Effect:The Piezoelectric Effect:

Forces applied to a segment of material lead to the Forces applied to a segment of material lead to the appearance of electrical charge on the surfaces of appearance of electrical charge on the surfaces of the segment. The specific distribution of electric the segment. The specific distribution of electric charges in the unit cell of a crystal is the source of charges in the unit cell of a crystal is the source of this phenomenon.this phenomenon.

Contact Modes

Contact-mode

• Interaction between tip and sample in physical contact

• Can damage sample, esp bio materials• Forces in nano Newtons

Tapping Mode

Vibrate tip, measure freq of oscillation

Surface of PlatinumSurface of Platinum

Iron corrals on CuIron corrals on Cu

Positioning of atoms for a Positioning of atoms for a mass data storage systemmass data storage system

The atomic force microscope (AFM), uses a sharp tip attached to the end of a cantilever rasters across an The atomic force microscope (AFM), uses a sharp tip attached to the end of a cantilever rasters across an area while a laser and photodiode are used to monitor the tip force on the surface. A feedback loop between area while a laser and photodiode are used to monitor the tip force on the surface. A feedback loop between the photodiode and the piezo crystal maintains a constant force during contact mode imaging and constant the photodiode and the piezo crystal maintains a constant force during contact mode imaging and constant amplitude during intermittent contact mode imaging.amplitude during intermittent contact mode imaging.

~ ATOMIC FORCE MICROSCOPE ~

HOW DOES IT WORK?HOW DOES IT WORK?

ThermoMicroscopes Explorer AFM

V

Photodiode

Mirror

Laser

Tip

PiezoCrystal

Feedback Loop

Substrate

As with the STM As with the STM the probe tip of an the probe tip of an AFM must be very AFM must be very small but because small but because there is no need to there is no need to establish a establish a tunneling current tunneling current one can use a one can use a variety of materials, variety of materials, not just those with a not just those with a low workfunction.low workfunction.

Similar to a phonograph needle the probe Similar to a phonograph needle the probe is actually in contact with the specimen and is actually in contact with the specimen and is physically moved up and down due to the is physically moved up and down due to the repulsion of van der Waals forcesrepulsion of van der Waals forces

The AFM records the position of the probe The AFM records the position of the probe by bouncing a laser off the back surface of by bouncing a laser off the back surface of the probe and recording how the light is the probe and recording how the light is deflected deflected

By using a four By using a four quadrant detector the quadrant detector the relative amount of laser relative amount of laser light hitting each light hitting each quadrant can be used to quadrant can be used to determine how the tip determine how the tip has been deflected as it has been deflected as it moves over the surface moves over the surface of the specimen of the specimen

AFM of ChromosomeAFM of Chromosome

AFM derived models of AFM derived models of nuclear pore complexnuclear pore complex

Since an AFM relies on Since an AFM relies on contact rather than contact rather than current many current many nonconductive materials nonconductive materials can be examinedcan be examined

Since the contact of the tip with the Since the contact of the tip with the specimen can cause physical damage to the specimen can cause physical damage to the specimen many AFMs employ a specimen many AFMs employ a “tapping”“tapping” mode in which the probe vibrates up and mode in which the probe vibrates up and down as the sample is moved.down as the sample is moved.

~ ATOMIC FORCE MICROSCOPE ~

Friction

Elasticity

Binding

WHAT CAN WE LEARN?

Imaging direction

Height

Width

Rc

TIP

AFM Image and manipulation of an Adenovirus.AFM Image and manipulation of an Adenovirus.

A recent development uses an AFM to A recent development uses an AFM to “write” with biomolecules such as DNA “write” with biomolecules such as DNA sequences. This will allow for the creation sequences. This will allow for the creation of micro DNA chips which can be used a of micro DNA chips which can be used a wide variety of applicationswide variety of applications

There are now a number of systems that combine There are now a number of systems that combine an AFM with a conventional inverted microscope an AFM with a conventional inverted microscope so that light and surface information can be so that light and surface information can be collected from the same samplescollected from the same samples

AFM vs. STMAFM vs. STMIn all three resolution is largely dependent on In all three resolution is largely dependent on probe size and the ability to control scanning.probe size and the ability to control scanning.

STM requires a conductive specimen, AFM STM requires a conductive specimen, AFM does not and this can be used in air, vacuum, does not and this can be used in air, vacuum, or in liquids. or in liquids.

AFM physically contact the specimen but AFM physically contact the specimen but STM does not.STM does not.