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Scanning Probe Techniques

 

Advanced non-contact AFM techniques

Advanced noncontact AFM techniques

Kelvin probe + 2nd harmonic instrumentation

Omicron + Nanonis Instrumentation


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Interpreting EFM and SSPM Images of Interfaces Intersecting a Surface

Create a model

1. Create a model of charge distribution in material:

For example, a charged grain boundary normal to the surface can be modeled as a plane with a given interface charge density compensated by two uniformly charged depletion regions (abrupt junction approximation). A double Schottky barrier  model allows an analytical expression for the stray fields in air above surface interface junction.  A similar example is a ferroelectric surface, which is traditionally represented as a constant charge density (unscreened surface) or a constant dipole moment density (completely screened surface) within domains with rapid variations of surface properties at the domain boundary.

Create a model

2. Describe stray fields in air:

Once the interface model is established, it can be used to calculate the field distribution above the surface. For some cases this can be done analytically; however, in general  it requires numerical integration or Finite Element Analysis (FEA). For surfaces with lateral inhomogeneities, first order perturbation theory can often be used. In this particular case, the potential above the grain boundary-surface junction is

formula

where dsc is the depletion width and z is the distance from the surface. j0 is the potential at the grain boundary-surface junction, which is equal to grain boundary potential in the bulk for an unscreened surface if a high dielectric constant   material.

Calculate the surface-tip interaction

3. Calculate the surface-tip interaction:

Provided the potential and/or electric field in air above the surface is known (step 2), its effect on the static and dynamic properties of a cantilever can be established. Different non-contact SPM techniques are sensitive to different interactions, e.g. Electrostatic Force Microscopy (EFM) detects the force gradient acting on the tip, while Scanning Surface Potential Microscopy (SSPM) detects the first harmonic of the force. The difference in imaging mechanisms provides independent sets of data. Unfortunately, contrast in SSPM can be sensitive to imaging parameters, e.g. driving voltage in SSPM, feedback gains, etc. Hence the influence of all parameters on SSPM image must be taken into account to be quantitative. For an electroactive grain boundary, the total force gradient acting on the tip is

formula

where the first term is the capacitive force between the tip and dielectric surface  

formula

and the second term is coulombic term due to grain boundary charges  

formula

From these formulae the total force and force gradient acting on the tip can be calculated and used to describe experimental data.

Create a model

4. Quantification of SSPM images in terms of the model and extraction of model parameters

If the stray field distribution in air and the probe response to the fields is known (step 2 and 3), SSPM images can be   interpreted in terms of the local field. Acquisition of several images at different tip biases (EFM) allows different contributions into image contrast, i.e.  capacitive (parabolic in bias) and Coulombic forces (linear in bias), to be distinguished. Performing EFM and SSPM imaging at different tip-surface separations allows reconstruction of the distance dependence of the forces. Clearly, the decay length of the stray fields is determined by the specifics of the charge distribution in the material. For example, for charged grain boundaries the decay length is comparable to the depletion width. Mathematical analysis of distance dependence yields model parameters and  the local material properties.

5. Environmental conditions:

Presence of mobile surface charges can effectively screen the bulk charges. The most spectacular example of such behavior is on ferroelectric surfaces, but screening can occurs in other systems as well and we believe it to be a rather universal phenomenon.


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Nano-impedence Spectroscopy

Many Scanning Probe based measurements have been developed to characterize local properties related to transport. Some of the more interesting access systems in the presence of perturbations, such as with temperature or field variations or during current flow. However, there exists a potential to examine fundamental mechanisms of transport if the frequency dependence of local properties can be quantified. The approach outlined here utilizes principles of impedance spectroscopy and is performed with a conductive AFM tip.

Nano-impedence

Nanoimpedance Microscopy/Spectroscopy (NIM) has both imaging and spectroscopic modes that differentiate processes at interfaces and defects by the time constants associated with local relaxations.An equivalent circuit incorporating all possible relaxation processes in the system is constructed and is used to fit the experimental data.

Click the poster below to open the PDF file:

Nano-impedence Poster


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Scanning Impedance Microscopy

Scanning Impedance Microscopy (SIM) is a novel SPM technique based on the on the detection of the phase change of cantilever oscillations induced by a lateral ac bias applied to the sample. The bias induces oscillations in surface potential and, therefore, results in the periodic force acting on the dc biased tip. The lock-in system detects phase and amplitude of tip vibration as shown on the image below. The phase and amplitude of the cantilever oscillation is comprised of SIM phase and amplitude images. The phase of the surface potential oscillations is related to the local transport properties. For example, phase and amplitude change abruptly at the electroactive interfaces such as metal-semiconductor interfaces or grain boundaries. For the measurements performed far  (>5kHz) from the resonant frequency of the cantilever (60-70kHz) the phase lag between the surface potential oscillations and cantilever oscillations is constant (albeit frequency dependent). Therefore, variation of measured cantilever oscillation phase is equal to voltage phase angle variation across the surface.

Scanning Impedance Microscopy

This technique bears certain similarity to Scanning Surface Potential Microscopy (SSPM) under lateral bias, but the former allows to study dc transport properties of complex microstructures while SIM directly addresses ac transport properties. Currently we are able to perform SIM imaging in the frequency range from 300 Hz to 100kHz and obtain quantitative information on the transport properties of the electroactive interfaces. The lower limit is imposed by the pixel acquisition time, while the upper is given by lock-in range. Spectroscopic variants of SIM can be extended to dc frequency limit, while the high frequency limit is limited by cantilever dynamic properties (the response amplitude is small above the resonant frequency, hence phase and amplitude error increases). Shown below is the example of SSPM and SIM imaging of metal-semiconductor interface (Schottky diode).

SSPM and SIM imaging of metal-semiconductor interface (Schottky diode)

Reconstruction of dc and ac transport properties from SSPM and SIM data requires the overall circuit topology. In experimental setup, R is known current limiting resistors in the circuit. Variation of R allows correctness of the chosen equivalent circuit to be verified (e.g. presence of additional resistive or capacitive elements can be detected) and parameters of the interface can be reconstructed. Measured  in SSPM under lateral bias is potential drop at the interface as a function of external lateral bias (voltage characteristic of the interface). Measured in SIM is phase shift across the interface and ratio of oscillation amplitudes across the interface. Both these quantities are independent of the cantilever and tip properties and are dependent solely by frequency dependent impedance of the interface and the circuit. Shown below is surface topography (left) and surface potential (right) of the Schottky diode under slow (~2mHz) triangular voltage ramp. Note that under forward bias conditions there is no potential drop at the interface, which however develops under reverse bias. Also shown is potential profiles across the interface for low (R = 500 Ohm) and high (R = 100kOhm) resistivity circuit terminations.

surface topography (left) and surface potential (right) of the Schottky diode under slow (~2mHz) triangular voltage ramp

Note that onset of reverse bias condition shifts to the negative voltages for high-resistivity terminations. Voltage characteristic of the interface are shown below. Analysis of these characteristic allows to obtain both saturation current density and leakage resistivity of the diode under reverse bias.

Scanning Surface Potential Microscopy: Schottky Diode

Note that these values are extremely close to saturation current density from conventional I-V measurements and leakage resistance (600 kOhm). On the next step, SIM phase and amplitude images were acquired in the frequency range from 3 to 100 kHz. Shown below is phase (left) and amplitude (right) of cantilever oscillations to the left and right of the interface for two different circuit terminations.

phase (left) and amplitude (right) of cantilever oscillations to the left and right of the interface for two different circuit terminations

Note that absolute values of measured phase and amplitude are the convolution of tip dynamics (harmonic oscillator) and lateral surface transport. However, phase shift cross the interface and ratio of the oscillation amplitudes are independent on the cantilever properties and interface capacitance can be calculated from SIM phase data as shown below.


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Polarization and dielectric function of molecules

Polarization and dielectric function of moleculesSub monolayer deposition of this porphyrin on HOPG results in the formation of islands in which the molecules adopt two different orientations. In one case the porphyrin ring is oriented parallel to the substrate, in the other it is perpendicular to the surface. The different orientations can be distinguished by the height, as shown in figure 2, which also illustrates the structure of the molecules.  The detailed structures of  various  monolayers are treated in ref. Here the porphyrin islands with the rings parallel to the substrate and a height of 1.5 nm are examined.

The topographic structure and dependence of the s-NSOM signal on the polarization of the incoming light at multiple harmonics is shown in figure. The porphyrin island is 1.5 nm in height, confirming that the molecules align with porphyrin ring parallel to the HOPG surface18. s-NSOM images were obtained for the 4 harmonics of the scattered light. The response functions and therefore the properties of the graphite surface and the porphyrin/graphite complex are compared.   In order to quantify the properties, histograms of the optical amplitudes backscattered from areas with and without monolayers are shown in figure b-f. The area marked with the red square corresponds to the substrate (HOPG) and that with the blue square corresponds to the porphyrin monolayer. Clearly when the incident light is polarized perpendicular to the surface there is almost no difference between the backscattered light amplitude at all frequencies. In contrast, the when the light is polarized parallel to the surface the difference in contrast is obvious. Increasing the harmonic order from 1 to 3 results in the increase of the difference in optical amplitude between HOPG surface and porphyrin island  because the sensitivity of s-NSOM to dielectric function difference increases with the harmonic order. There is no increase in contrast for the 4th harmonic probably because of the lower signal-to-noise ratio for this signal due to the weakness of the signal.

In the conclusion that current work is the first measurement of polarization dependence of light scattering on the optically active molecules (porphyrin). This is the first time when high spatial resolution measurements (resolution is better then 50 nm) were performed on organic molecules. From this kind of measurements information about lateral distribution of dielectric constant of the sample could be extracted.


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Bio-related Scanning Probe Techniques

DNA Project

DNA project was a collaboration with a professor in the Vet School. He was creating patterns with DNA and RNA.

DNA figure

DNA images are AFM of DNA on mica. This was a collaboration with Prof Kaufman from the Vet School.

 

Muscle Project

Muscle project was developing methods of characterizing tissue with AFM
heart are from the muscle project. Wu group in the Surgery department at the med school.

measurement project 1

measurement technique -- we measured the change in stiffness as we penetrated into the tissue to calculate the elastic modulus.

measurement project 2

results -- control tissue was soft, untreated infarcted were stiff, and treating the infarcted tissue reduced stiffness.


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Capacitance Measurement

Si-SiO2 was developing technique to measure capacitance with 5-10 nanometer resolution. 
Si-SiO2 is 2nd capacitance measurement. Bare (111)Si-H demonstrates greater capacitance than 100nm thick SiO2 on silicon. Increased frequency shift (measured in Hz) indicates a stronger capacitive field  between conductive AFM tip and the sample.

Capacitance Measurement

results -- control tissue was soft, untreated infarcted were stiff, and treating the infarcted tissue reduced stiffness.


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In-Situ High Temperature Scanning for Fuel Cells

Many of the fundamental processes that underlie the function fuel cells occur at or near interfaces. To date, idealized experiments, ex-situ analysis, operando spectroscopy,   and macroscopic electrical property measurements, have driven advancements in SOFCs, however, in-situ (operando) imaging techniques capable of locally resolving the governing electrochemical phenomena under realistic operating conditions have remained largely elusive.

The scanning probe approach presented here demonstrates, for the first time, direct imaging of local interfacial potential perturbations across electrode-electrolyte interfaces (Fig. a).  By comparing fuel cells based on two common electrode materials; lanthanum-strontium-ferrite (LSF) and lanthanum-strontium-manganite (LSM), we also demonstrate the ability to image and distinguish between bulk-mediated and surface mediated transport mechanisms, identify both the active zone and triple phase boundary regions, and directly estimate activation barrier changes in these systems.

in-situ

The development of miniature sample chambers enables high temperature scanning of SOFCs in cross sectional geometries.  The images shown here are of LSF-YSZ-LSF symmetrical fuel cells, under operation, at 600°C.  Topographic (Fig. d) and scanning surface potential are shown for the cathode (Fig. g), electrolyte (Fig. c), and anode (Fig. e) cell constituents, respectively.  Line profile analysis averaged between the dotted red lines display a distinct, localized perturbation occurring along the electrolyte/electrode interface (Figs. b,i).  This result demonstrates the influence of trapped ionic oxygen at the interface, which perturbs the local chemical potential and subsequently the surface potential.  This phenomenon is supported by the presence of an Ohmic-like response within the yttria-stabilized-zirconia (YSZ) electrolyte, which conducts only ions, not electrons.  These results also pose significant implications for direct identification of active zones within the electrodes, as evident by the distance over which the perturbations occur.

 

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