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How to Select Atom Force Microscope (AFM)

How to Select Atom Force Microscope (AFM)?


 
Brief History of Development of Microscopes

• First generation: Optical Microscope (invented in the 17th century)

• Second Generation: Electron Microscope (1932)

In 1933, Ruska and Knoll, Germany: the first scanning electron microscope (SEM), which required high vacuum and could not distinguish atoms;
Transmission electron microscopy (TEM), high vacuum, bulk and interface studies of thin-layer samples, high price;
Field electron microscope (FEM) and field ion microscope (FEM) detect the atomic structure on the tip of r<100nm, and the sample preparation is complicated;

• Third Generation: Scanning Probe Microscopy(SPM) (1982)

1982 IBM Zurich Laboratory and Binnig Rohrer, the first new type of surface analysis instrument - Scanning Tunneling Microscope (STM);
1986 Binnig, Quate and Gerber invent the first atomic microscope (AFM)


 
Scanning Probe Microscope (SPM) Compared to Other Microscopies

The main microscopic analysis techniques at present:

First generation: Optical Microscope (Including Interference, Confocal and Digital Holography)
Second Generation: Electron Microscope (Mainly SEM and TEM)
Third Generation: Scanning Probe Microscopy(SPM)

Comparison of Resolution of Three Generations of Microscopes
 
What Is Scanning Probe Microscopy (SPM)

Scanning Probe Microscope (SPM), is the third generation microscope, as the name suggests, it is scanning + probe + microscope. The principle of SPM is to use a probe to scan the surface of the sample with a raster line, and to record the surface features of the sample and obtain an image by detecting the interaction between the probe and the sample.

 
Classification of Scanning Probe Microscope (SPM)

According to the difference in the detection force between the probe and the sample, Scanning Probe Microscopy (SPM) can be divided into two categories, namely, Scanning Tunneling Microscopy (STM), which uses current to detect, and Atomic Force Microscope (AFM), which uses different forces to detect.

AFM can also be divided into magnetic force microscope, friction force microscope, lateral  force microscope, chemical force microscope, electrostatic force microscope and other types according to the different forces.

 
Differences between STM and AFM applications:

Scanning Tunneling Microscope (STM) is mainly used for the study of conductors,
Atomic Force Microscopy (AFM) can be used not only for the study of conductors, but also for the study of non-conductors.


Depth of Field, Sample Preparation of Different Microscopes

 
Advantages and Disadvantages of Scanning Probe Microscope (SPM)

As a new type of microscope tool, SPM has obvious advantages compared with previous microscope analysis instruments:

• SPM has extremely high resolution, such as STM parallel and perpendicular to the sample surface direction can reach 0.1nm and 0.01nm.

• SPM obtains a real-time, high-resolution 3D image of the sample surface, which is different from some analytical instruments through indirect or computational methods to estimate the surface structure of the sample.

• SPM enables direct observation of surface defects, surface remodeling, morphology and location of surface adsorbates, and surface remodeling induced by adsorbents.

• The use environment of SPM is loose, it can work not only in vacuum, but also in atmosphere, low temperature, normal temperature, high temperature, and even in solution.

• SPM does not require special sample preparation techniques, and the detection process is basically harmless to the sample.

• In conjunction with scanning tunneling spectroscopy, information about the surface structure can be obtained, such as the density of states at different levels of the surface, surface electron traps, changes in surface potential barriers, etc.

• At the same time, SPM also has some disadvantages

• Scanning speed. Since its working principle is to control a probe with a certain quality for scanning imaging, the scanning speed is limited, and the detection efficiency is lower than other microscopy methods.

• Scan range. Limited by the stretching range of piezoelectric ceramics, the maximum scanning range of SPM is in the range of tens to hundreds of microns.

• positioning accuracy. Because piezoelectric ceramics have a small range of motion under the premise of ensuring positioning accuracy, and the mechanical adjustment accuracy cannot be connected with it, it cannot achieve large-scale continuous zooming such as electron microscopes, and it is difficult to locate and find characteristic structures.

• Sample surface roughness. Tubular piezoelectric scanners are widely used at present, and their vertical stretching range is 1/10 of the planar scanning range. If the surface undulation of the sample to be tested exceeds the expansion and contraction range of the scanning tube, it will cause the system to malfunction or even damage the probe.

• Needle sharpness. Since the system infers the surface topography by detecting the motion trajectory of the probe when it scans the sample, the geometric width, curvature radius and anisotropy of the probe will cause image distortion.


Basic Principles of Atomic Force Microscope (AFM)

One end of a cantilever, which is extremely sensitive to weak force, is fixed, and the other end has a tiny needle tip. The needle tip is in light contact with the sample surface. Due to the extremely weak repulsive force between the atoms at the tip of the needle tip and the atoms on the sample surface, by controlling this during scanning. When the force is constant, the cantilever with the tip will fluctuate in the direction perpendicular to the surface of the sample corresponding to the isoplane of the force between the tip and the surface atoms of the sample. Using the optical detection method or the tunnel current detection method, the position change of the microcantilever corresponding to each scanning point can be measured, so that the information of the surface topography of the sample can be obtained.

 
Composition & Working Diagram of Atomic Force Microscope (AFM)


 
The atomic force microscope AFM system is divided into four parts: force detection part, position detection part, feedback electronic system, piezoelectric scanning system.

1. Force Detection Cantilever

In an atomic force microscope (AFM) system, the force to be detected is the van der Waals force between atoms. So in this system, a tiny cantilever (Cantilever) is used to detect the change of the force between atoms. The tiny cantilever has certain specifications, such as: length, width, elastic modulus and shape of the tip, and the selection of these specifications is based on the characteristics of the sample and the different operation modes, and different types of probes are selected.

2. Position Sensitive Photodetector

When the needle tip interacts with the sample, the cantilever will swing, so when the laser irradiates the end of the cantilever, the position of the reflected light will also change due to the swing of the cantilever. , which results in an offset. In the whole system, the laser spot position detector is used to record the offset and convert it into an electrical signal for the controller to do signal processing.

3. Feedback Electronic System: Feedback Loop

 AFM feedback control is completed by computer system and electronic circuit. The operation of AFM is realized under the control of high-speed and powerful computer. The control system has two main functions: (1) Provide the driving voltage to control the X-Y direction scanning of the piezoelectric transducer; (2) Maintain a constant value of the input analog signal from the microscope detection loop in the constant force mode. The computer reads and compares the loop voltage (ie the difference between the set value and the actual measured value) through A/D conversion. According to the different voltage values, the control system continuously outputs the corresponding voltage to adjust the expansion and contraction of the piezoelectric sensor in the Z direction to correct the deviation read into the A/D converter, thereby maintaining the output voltage of the comparison loop constant.
The electronic circuit system plays the role of connecting the computer and the scanning system. The electronic circuit provides voltage for the piezoelectric ceramic tube, receives the signal from the position sensitive device, and forms a feedback system for controlling the distance between the needle tip and the sample. 

4. Piezoelectric Scanner

 The precise control of the AFM scan of the sample is achieved by the scanner. A piezoelectric transducer is installed in the scanner, and the piezoelectric device precisely controls the position of the sample or probe in the three directions of X, Y, and Z.
At present, the matrix material of the scanner is mainly a piezoelectric ceramic material made of lead zirconate titanate [Pb(Ti,Zr)O3]. Piezoelectric ceramics have piezoelectric effect, that is, they have shrinkage characteristics when a voltage is applied, and the degree of shrinkage is proportional to the applied voltage. Piezoelectric ceramics can convert voltage signals ranging from 1mV to 1000V into displacements ranging from tenths of a nanometer to several micrometers.


Force & Working Mode of Atomic Force Microscope (AFM)

There are several different interaction forces between the tip of the AFM and the sample, which are listed in the following table. According to different forces, AFM can be extended to a variety of working modes: phase mode, magnetic mode, electrostatic force mode, extended resistance, lateral force, force modulation, etc.


 
The working modes of AFM are classified in the form of the force between the probe and the sample. There are three main operating modes: Contact Mode, Non-Contact Mode and Tapping Mode.

Contact Mode

Contact mode is the most direct imaging mode of AFM. During the entire scanning imaging process, the probe tip is always in close contact with the sample surface, and the interaction force between the two is a repulsive force, and the magnitude of the force ranges from 10-10 to 10-6N. During scanning, the force exerted by the cantilever on the tip has the potential to damage the surface structure of the specimen, therefore. Contact mode is not suitable for testing samples with soft surfaces.

Advantages: Fast scanning speed, the only mode that can obtain "atomic resolution" images.
Disadvantage: Lateral force affects image quality. In the air, because of the capillary action of the adsorbed liquid layer on the surface of the sample, the adhesion between the needle tip and the sample is very large. The combined force of the lateral force and the adhesive force leads to a decrease in the spatial resolution of the image. Also, scratching the sample with the tip can damage soft samples (such as biological samples, polymers, etc.).

Non-Contact Mode

When the sample surface is probed in non-contact mode, the cantilever oscillates at a distance of 5–10 nm above the sample surface. At this time, the interaction between the sample and the needle tip is controlled by the van der Waals force, usually 10 - 12 N, the sample will not be destroyed, and the needle tip will not be contaminated, especially suitable for studying the surface of soft objects. The disadvantage of this mode of operation is that it is very difficult to achieve this mode in a room temperature atmosphere. Because a thin layer of water inevitably accumulates on the surface of the sample, it creates a small capillary bridge between the sample and the tip, drawing the tip and surface together, increasing the pressure of the tip against the surface.

Advantage: No force acts on the sample surface.
Disadvantage: Usually only used for samples that are very afraid of water. Because the needle tip is separated from the sample, the lateral resolution is low; in order to avoid contact with the adsorption layer and cause the needle tip to stick, the scanning speed is lower than that of Tapping Mode and Contact Mode AFM, and the adsorption liquid layer on the sample surface must be very thin, if it is too thick, the needle tip Can sink into the liquid layer, causing feedback instability and scratching the sample.

Tapping Mode

The tapping mode is between the contact mode and the non-contact mode, which is a hybrid concept. The cantilever oscillates at its resonant frequency above the sample surface and the tip only touches/taps the sample surface briefly and periodically. This means that the lateral force generated by the tip contacting the sample is significantly reduced. Therefore, the tapping mode of AFM is one of the best choices when detecting tender samples. Once the AFM starts to image and scan the sample, the device will then input relevant data into the system, such as surface roughness, average height, maximum distance between peaks and valleys, etc., for surface analysis of the object. At the same time, AFM can also complete the force measurement work, measuring the bending degree of the cantilever to determine the force between the tip and the sample.

Advantages: The influence of lateral force is well eliminated, the force caused by the adsorption liquid layer is reduced, and the image resolution is high. Ideal for viewing soft, brittle, or sticky samples without damaging the surface.
Disadvantage: slower scanning speed than contact mode.


Application Areas of Atomic Force Microscope (AFM)

Materials Science: Surface morphology, roughness, mechanical properties, local electrical properties, etc. of micro-nano materials, mainly including metals, alloys, thin films, crystals, ceramics, polymers, composite materials, etc.;

Semiconductor and microelectronics: 3D structure of chip or wafer surface, roughness, defect detection, local electrical properties, etc.;

Biology and medicine: DNA, protein, cell, virus, biological macromolecular in situ imaging and research, etc.;
Physics and Chemistry: Detect the atomic structure of the material surface, study the interaction force between atoms, surface chemical reactions, observe the atomic-level changes of chemical reactions, etc.;

Nano-teaching education: teaching of nanotechnology, observing nanostructures on the surface of materials, learning the basic theoretical knowledge of nanometers, and cultivating students' interest in scientific research, etc.


 
Advantage of Atomic Force Microscope (AFM)

• AFM can perform three-dimensional imaging, and has extremely high resolution in both lateral and vertical directions, which can reach 0.2 nm in the lateral direction and 0.05 nm in the vertical direction;

• AFM obtains a real-time, real three-dimensional image of the sample surface, which is different from some analytical instruments that deduce the surface structure of the sample through some software algorithms;

• AFM does not have any restrictions on the reflectivity and conductivity of the observed samples. It can measure conductive samples, non-conductive samples, and biological samples.

• The use environment of AFM is loose, it can work not only in vacuum, but also in atmosphere, low temperature, normal temperature, high temperature, and even in solution;

• AFM does not require special sample preparation technology, and the detection process basically does not damage the sample;

• In addition to observing the three-dimensional morphology, roughness, particle size, film thickness, surface defects, etc. of the sample surface, AFM can also detect the mechanical, electrical, and thermal properties of microscopic substances.


• Opto-Edu Atomic Force Microscope (AFM) Models


 
A64.4500 Teaching Level, Atomic Force Microscope (AFM)

 
Teaching Level Separate controller & main body design, with Tapping Mode, 4x Objective, economic model for teaching.

◆ Miniaturized and detachable design, very easy to carry and teach

◆ The laser detection head and the sample scanning stage are integrated, the structure is very stable, and the anti-interference is strong

◆ Precision probe positioning device, laser spot alignment adjustment is very easy

◆ The single-axis drive sample automatically approaches the probe vertically, so that the needle tip is perpendicular to the sample scan

◆ The intelligent needle feeding method of motor-controlled pressurized piezoelectric ceramic automatic detection protects the probe and the sample

◆ Automatic optical positioning, no need to focus, real-time observation and positioning of the probe sample scanning area

◆ Spring suspension shockproof method, simple and practical, good shockproof effect

◆ Integrated scanner nonlinear correction user editor, nanometer characterization and measurement accuracy better than 98%

Click to see details of A62.4500

A62.4501 Basic Level, Atomic Force Microscope (AFM)

 
Basic Level, Separate controller & main body design, with Contact Mode, Tapping Mode, 4x Objective.

1. The scanning probe and the sample stage are integrated, and the anti-interference ability is strong;

2. Precision laser and probe positioning device, it is simple and convenient to replace the probe and adjust the spot;

3. Use the sample approaching probe method, so that the needle tip is perpendicular to the sample scanning;

4. The automatic pulse control of the motor drives the sample to approach the probe vertically to achieve precise positioning of the scanning area;

5. 4X objective lens optical positioning, no need to focus, real-time observation and positioning of the probe sample scanning area;

6. The spring suspension shockproof method is simple and practical, and has strong anti-interference ability.

Click to see details of A62.4501

A62.4503 Research Level, Atomic Force Microscope (AFM), All-in-One Design

 
Research Level, Combined controller & main body design, with Contact Mode, Tapping Mode, 10x Objective.

1. The scanning probe and the sample stage are integrated, and the anti-interference ability is strong;

2. Precision laser and probe positioning device, it is simple and convenient to replace the probe and adjust the spot;

3. Use the sample approaching probe method, so that the needle tip is perpendicular to the sample scanning;

4. High-precision and large-scale piezoelectric ceramic scanners can be selected according to different accuracy and scanning range requirements;

5. Optical positioning of 10X APO objective lens, no need to focus, real-time observation and positioning of the probe sample scanning area;

6. The spring suspension shockproof method is simple and practical, and has strong anti-interference ability.

Click to see details of A62.4503

A62.4505 Optical + Atomic Force Microscope


 
Combined optical microscope + AFM All-in-One, A62.4505 has Infinity Plan LWD APO objective 5x10x20x50x, Eyepiece 10x, 5.0M Digital Camera, 10" LCD Monitor with measuring function, LED kohler illumination to watch the sample under both optical mode & AFM mode. 

1. Integrated design of optical microscope and atomic force microscope, integrated photoelectric control;

2. It has both optical and atomic force microscope imaging functions, both of which can be scanned and imaged at the same time without affecting each other;

3. At the same time, it has the functions of optical two-dimensional measurement and atomic force microscope three-dimensional measurement;

4. The vertical optical path design is adopted, and the gas-liquid dual-purpose probe holder can be used in air or liquid at the same time;

5. High-magnification optical positioning system is used to achieve precise positioning of probe and sample scanning area;

6. The portable laser spot adjustment method can observe and adjust the spot in real time through the optical CCD window;

7. Equipped with a pneumatic shock-absorbing table, which has strong anti-interference ability and does not affect the operation of the instrument;

Click to see details of A62.4505


Opto-Edu Gantry Type Atomic Force Microscope (AFM)


 
A62.4510 Probe Scanning Atomic Force Microscope


 
A62.4511 Plane Scanning Atomic Force Microscope


 
A62.4510 + Piezoelectric Shift Scanning Stage =A62.4511

◆The first atomic force microscope in China to keep the sample still and the probe moving and scanning;

◆The first in China to use a closed-loop three-axis independent pressure shift scanner, which can scan with high precision in a wide range; 

◆The sample size and weight are basically unlimited, especially suitable for the detection of large samples; sample area of interest;

◆Gantry scanning head design, marble base, vacuum adsorption stage; 

◆Intelligent needle feeding method with automatic detection of motor-controlled piezoelectric ceramics to protect probes and samples; 

◆Automatic optical positioning, no need to adjust focus, real-time observation and positioning probe sample scanning area; 

◆Equipped with closed metal shield, pneumatic shock-absorbing table, strong anti-interference ability; 

◆Integrated scanner nonlinear correction user editor, nanometer characterization and measurement accuracy is better than 98%.

Click to see details of A62.4510
Click to see details of A62.4511


Sample Pictures of Atomic Force Microscope (AFM) 

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