8.4 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Probe tip | 2/5 | https://en.wikipedia.org/wiki/Probe_tip | reference | science, encyclopedia | 2026-05-05T03:41:57.526681+00:00 | kb-cron |
=== Non-conductive probe tip === Non-conductive nanoscale tips are widely used for AFM measurements. For non-conducting tip, surface forces acting on the tip/cantilever are responsible for deflection or attraction of tip. These attractive or repulsive forces are used for surface topology, chemical specifications, magnetic and electronic properties. The distance-dependent forces between substrate surface and tip are responsible for imaging in AFM. These interactions include van der Waals forces, capillary forces, electrostatic forces, Casimir forces, and solvation forces. One unique repulsion force is Pauli Exclusion repulsive force, which is responsible for single-atom imaging as in references and Figures 10 & 11 (contact region in Fig. 1).
== Fabrication methods == Tip fabrication techniques fall into two broad classifications, mechanical and physicochemical. In the early stage of the development of probe tips, mechanical procedures were popular because of the ease of fabrication.
=== Mechanical methods === Reported mechanical methods in fabricating tips include cutting, grinding, and pulling.; an example would be cutting a wire at certain angles with a razor blade, wire cutter, or scissors. Another mechanical method for tip preparation is fragmentation of bulk pieces into small pointy pieces. Grinding a metal wire or rod into a sharp tip was also a method used. These mechanical procedures usually leave rugged surfaces with many tiny asperities protruding from the apex, which led to atomic resolution on flat surfaces. However, irregular shape and large macroscopic radius of curvature result in poor reproducibility and decreased stability especially for probing rough surfaces. Another main disadvantage of making probes by this method is that it creates many mini tips which lead to many different signals, yielding error in imaging. Cutting, grinding and pulling procedures can only be adapted for metallic tips like W, Ag, Pt, Ir, Pt-Ir and gold. Non-metallic tips cannot be fabricated by these methods. In contrast, a sophisticated mechanical method for tip fabrication is based on the hydro-gel method. This method is based on a bottom-up strategy to make probe tips by a molecular self-assembly process. A cantilever is formed in a mould by curing the pre-polymer solution, then it is brought into contact with the mould of the tip which also contains the pre-polymer solution. The polymer is cured with ultraviolet light which helps to provide a firm attachment of the cantilever to the probe. This fabrication method is shown in Fig. 2.
=== Physio-chemical procedures === Physiochemical procedures are fabrication methods of choice, which yield extremely sharp and symmetric tips, with more reproducibility compared to mechanical fabrication-based tips. Among physicochemical methods, the electrochemical etching method is one of the most popular methods. Etching is a two or more step procedure. The "zone electropolishing" is the second step which further sharpens the tip in a very controlled manner. Other physicochemical methods include chemical vapor deposition and electron beam deposition onto pre-existing tips. Other tip fabrication methods include field ion microscopy and ion milling. In field ion microscopy techniques, consecutive field evaporation of single atoms yields specific atomic configuration at the probe tip, which yields very high resolution.
==== Fabrication through etching ==== Electrochemical etching is one of the most widely accepted metallic probe tip fabrication methods. Three commonly used electrochemical etching methods for tungsten tip fabrication are single lamella drop-off methods, double lamella drop-off method, and submerged method. Various cone shape tips can be fabricated by this method by minor changes in the experimental setup. A DC potential is applied between the tip and a metallic electrode (usually W wire) immersed in solution (Figure 3 a-c); electrochemical reactions at cathode and anode in basic solutions (2M KOH or 2M NaOH) are usually used. The overall etching process involved is as follows: Anode;
W
(
s
)
+
8
OH
−
⟶
WO
4
+
4
H
2
O
+
6
e
−
(
E
=
1
⋅
05
V
)
{\displaystyle {\ce {W (s) + 8OH- -> WO4 + 4H2O + 6e- (E= 1.05V)}}}
Cathode:
6
H
2
O
+
6
e
−
⟶
3
H
2
+
6
OH
−
(
E
=
−
2
⋅
48
V
)
{\displaystyle {\ce {6H2O + 6e- -> 3H2 + 6OH- (E=-2.48V)}}}
Overall:
W
(
s
)
+
2
OH
−
⟶
WO
4
2
−
+
2
H
2
O
(
l
)
+
6
e
−
+
3
H
2
(
g
)
(
E
=
−
1
⋅
43
V
)
{\displaystyle {\ce {W (s) + 2OH- -> WO4^2- + 2H2O (l) + 6e- + 3H2 (g) (E= -1.43V)}}}
Here, all the potentials are reported vs. SHE.
The schematics of the fabrication method of probe tip production through the electrochemical etching method is shown in Fig. 3. In the electrochemical etching process, W is etched at the liquid, solid, and air interface; this is due to surface tension, as shown in Fig. 3. Etching is called static if the W wire is kept stationary. Once the tip is etched, the lower part falls due to the lower tensile strength than the weight of the lower part of the wire. The irregular shape is produced by the shifting of the meniscus. However, slow etching rates can produce regular tips when the current flows slowly through the electrochemical cells. Dynamic etching involves slowly pulling up the wire from the solution, or sometimes the wire is moved up and down (oscillating wire) producing smooth tips.
==== Submerged method ==== In this method, a metal wire is vertically etched, reducing the diameter from 0.25 mm ~ 20 nm. A schematic diagram for probe tip fabrication with submerged electrochemical etching method is illustrated in Fig 4. These tips can be used for high-quality STM images.