Scanning Probe Microscopy – the Nanoscience Tool NanoScience & NanoTechnology NanoScience & NanoTechnology Tools that operate in real space with Ångstrom to nanometer spatial resolution, in contrast to scattering techniques, such as for instance the SEM (scanning electron microscope), that operate in the reciprocal space. In principle, SPM systems consist of probes that are nanosized (accomplished microlithographically), scanning and feedback mechanisms that are accurate to the subnanometer level (achieved with piezoelectric material), and highly sophisticated computer controls (obtained with fast Field or Perturba tion Sample Material SPM Probe Piezo Scanner Feedback Signal
Tools that operate in real space with Ångstrom to nanometer spatial resolution, in contrast to scattering techniques, such as for instance the SEM (scanning electron microscope), that operate in the reciprocal space.
In principle, SPM systems consist of
probes that are nanosized (accomplished microlithographically),
scanning and feedback mechanisms that are accurate to the subnanometer level (achieved with piezoelectric material), and
highly sophisticated computer controls (obtained with fast DACs (digital analog converters, etc.).
In 1981, G. Binnig, H. Rohrer, Ch. Gerber and J. Weibel observed vacuum tunneling of electrons between a sharp tip and a platinum surface. The tunnel current is strongly distance, z, dependent; i.e.,
A=4(2m)1/2/h, with the tip-sample applied bias voltage, Vbias, and the average potential barrier height .
Tunneling occurs in the low bias voltage regime, i.e., ~0.1 V. At high bias voltage, i.e., Vbias>/e, the current flow is due to field emission (FE), i.e.,
Constant height imaging or variable current mode (fast scan mode). The scan frequency is fast compared to the feedback response, which keeps the tip in an average (constant) distance from the sample surface. Scanning is possible in real-time video rates that allow, for instance, the study of surface diffusion processes.
Differential tunneling microscopyTip is vibrated parallel to the surface, and the modulated current signal is recorded with lock-in technology.
Tracking tunneling microscopyScanning direction is guided by modulated current signal (e.g., steepest slope).
Scanning noise microscopyUse current noise as feedback signal at zero bias.
Nonlinear alternating-current tunneling microscopyConventionally, STM is restricted to non-conducting surfaces. A high frequency AC driving force causes a small number of electrons to tunnel onto and off the surface that can be measured during alternative half-cycles (third harmonics).
SAMPLE
CANTILEVER
PIEZO
Scanning Force Microscopy (SFM)
Photodiode LASER
Topography
NanoScience ToolNanoScience Tool
AFM
Friction
Material Distinction
100 µm
0 µm
50 µm
100 µm0 µm 50 µm
A
B32.33 µm
0 µm
16.17 µm
32.33 µm0 µm 16.17 µm
ElasticityTg = 374K
Glass Transition
Environmental chamber and heating /cooling stage for scanning probe microscope.
R.M. Overney et al., Phys. Rev. Lett. 76, 1272-1275 (1996).
F(D)
0
xresp(D)
0
Rheological Boundary Regime
F
D 0
xresp
PS grafted Silicon in (a) water (poor solvent)(b) toluene (good solvent)
Entropic Structuring of Simple LiquidsPrinciple
Water no boundary layerOMCTS “monolayer"n-C16H34 ~ 3 layers
M. He et al., Phys. Rev. Lett. 88 p.154302/1-4 (2002).
X - ray study: H. Kim et al., in Dynamics in Small Confining Systems V, edited by J.M. Drake et al., (Mat. Res. Soc. Symp. Proc. 2001) Vol 651, p T2.1
OMCS measurements are in agreement with x-ray reflectivity results (S. Sinha):
xmod
Shear Response
ShearDisplacement
Sample
CantileverTip
xL
Heating/Cooling Stage
250 350 360 370 380 400
0
Tg
xL,
Contact Thermal Shear Modulation Analysis
SFM Modes of Operation
85
90
95
100
105
0 50 100 150 200 250 300FILM THICKNESS, ( nm )
Tg (
oC
)
12.0 kDa PS
FOX-FLORY (BULK)
7
27
87
67
47
127
107
MOLECULAR WEIGHT [Mn]103 104 105 106 107
Tg
( O
C )
C. Buenviaje, et al., ACS symposium series; 781 (2001): 76-92
NanoScience ToolNanoScience Tool
SFM: Other Modes of Operation- Electrostatic Force Microscopy (EFM)
Application: Study of the location and lifetime of surface charges on insulating surfaces.
Procedure: Long-range electrostatic Coulombic forces are measured with a mechanically modulated conductive or clean silicon cantilever tip. An AC voltage is applied between the tip and the sample with a frequency w2 that is smaller than the mechanical modulation frequency w1 but larger than the gain of the feedback response. The AC voltage causes a charge and a mirror charge on the tip and the sample, respectively. The mechanically modulating tip is experiencing a Coulombic force gradient. For an uncharged surface the force gradient will oscillate at 22, whereas for a charged surface, the force gradient will be modulated at 2. A charge signal can be extracted by measuring the f and 2f signal with lock-in technique. The phase of that signal corresponds to the sign of the surface charge.
- Magnetic Force Microscopy (MFM)
Application: Measuring of surface magnetic structures
Procedure: Using the non-contact mode with magnetically coated cantilever tips.
Environmental chamber and heating /cooling stage for scanning probe microscope (SFM).
AFM/SFM Environment
C
FORWARDSCAN
REVERSESCAN
SAMPLE
PHOTODIODE
xREFLECTED LASER BEAM
CANTILEVER WITH LATERAL SPRING
CONSTANT KL
Friction Force Microscopy
F
0
Fstatic Fdynamic
xFL
FORWARD
REVERSE
Molecular Resolution Lateral Force Image (left) with Molecular Stick Slip behavior (below)
Lateral force images on smooth surfaces may be used to distinguished materials displaying different coefficients of friction.
100 µm
0 µm
50 µm
100 µm0 µm 50 µm
100 µm
0 µm
50 µm
100 µm0 µm 50 µm
FORWARD
REVERSE
SFM ModesSFM Modes
D
linearly ramped z-Piezo
xresp
Probing Distance (D)
x-modulation input
Rheological Boundary Regime
Normal Force Response
F(D)
R.M. Overney et al., Phys. Rev. Lett. 76, 1272-1275 (1996).
F(D)
0
xresp(D)
0
Rheological Boundary Regime
F
D 0
xresp
PS grafted Silicon in (a) water (poor solvent)(b) toluene (good solvent)
SFM ModesSFM Modes
Force Approach Spectroscopy
Tomlinson Model Molecular Stick-Slip Model (1920)
R. M. Overney et al., Phys. Rev. Lett. 72, 3546 (1994)
