SPM test for characterization of nanoparticles
- 1. Scanning Probe Microscopy Scanning probe microscopy (SPM) is a technique to examine materials with a solid probe scanning the surfaces. SPM examines surface features whose dimensions range from atomic spacing to a tenth of a millimeter. The SPM is considered the most powerful tool for surface structure examination currently. SPM started with the scanning tunneling microscope (STM) invented in 1982. An STM uses a tunneling current, a phenomenon of quantum mechanics, to examine material surfaces. The tunnelling current flows through an atomic-scale gap between a sharp metallic tip and conducting surface atoms. An SFM is commonly called an atomic force microscope (AFM). It uses near-field forces between atoms of the probe tip apex and the surface to generate signals of surface topography. The AFM is more widely used than the STM because it is not restricted to electrically conducting surfaces
- 2. Instrumentation A sharp probe tip that scans a sample surface. The tip must remain in very close proximity between the tip and a sample surface Comparison of surface microscopy techniques.
- 3. Figure: The main components of a scanning probe microscope (SPM). Probe and Scanner Control and Vibration Isolation STM-the probe is commonly made from tungsten wire. AFM-the probe tip is commonly made from SiO2 or Si3N4
- 4. Scanning Tunneling Microscopy Tunneling Current The tunneling current is a phenomenon of quantum mechanics. It results from electrons penetrating an energy barrier larger than the electron energy. To generate the tunneling current (It), a bias voltage is applied across a gap between the tip and the sample when the tip is kept near the sample. Without much knowledge of quantum mechanics, we may quantify the tunneling current through the following equation Vb is the bias voltage applied between the tip and the surface (1 mV and 4 V), C is a constant that depends on the nature of the sample material, and d is the nearest distance between the tip and the sample surface. The tunneling current (It) is between 10 pA and 10 nA Probe Tips and Working Environments A probe tip is made from a wire of tungsten or Pt–Ir alloys by an electrochemical etching method produces a sharp tip of tungsten wire. Ideally, for atomic resolution, a smooth tip should have a radius of a single atom.
- 5. Operational Modes Four operational modes in the STM: constant current, constant height, spectroscopic, and manipulation modes. The most commonly used mode is the constant current mode The constant-height mode can provide much higher scanning rates than that of constant current. It is useful for observing dynamic processes in which timing is important. The spectroscopic mode refers to the operation of recording the tunneling current as a function of either tip–sample spacing or bias voltage. The spectroscopic mode is useful for studying surface properties such as super conduction and molecular adsorption on metal. The manipulation mode refers to operations of relocating or removing atoms on a surface.
- 6. Typical Applications The most exciting function of an STM is its ability to image atoms. used to produce atomic images of various materials including metals (such as gold, copper, platinum, and nickel) and layered crystals (such as graphite and MoS2) The STM can image adsorbed atoms and single molecules on surfaces. An STM image of a 32 nm × 36 nm area on the Si (111) planes. Surface atoms have been reconstructed, showing 7 × 7 symmetry. Three atomic steps separate terraces Oxygen adsorbed onto a GaAs surface: (a) the sample is under negative bias voltage ( 2.6 V with respect to the tip) and (b) the − sample is under positive bias voltage (+1.5 V with respect to the tip