Instrument used to physically scan the surface of a sample
A probe tip is an instrument used in scanning probe microscopes (SPMs) to scan the surface of a sample and make nano-scale images of surfaces and structures. The probe tip is mounted on the end of a cantilever and can be as sharp as a single atom. In microscopy, probe tip geometry (length, width, shape, aspect ratio, and tip apex radius) and the composition (material properties) of both the tip and the surface being probed directly affect resolution and imaging quality. Tip size and shape are extremely important in monitoring and detecting interactions between surfaces. SPMs can precisely measure electrostatic forces, magnetic forces, chemical bonding, Van der Waals forces, and capillary forces. SPMs can also reveal the morphology and topography of a surface.
The use of probe-based tools began with the invention of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), collectively called scanning probe microscopy (SPM) by Gerd Binnig and Heinrich Rohrer at the IBM Zurich research laboratory in 1982. It opened a new era for probing the nano-scale world of individual atoms and molecules as well as studying surface science, due to their unprecedented capability to characterize the mechanical, chemical, magnetic, and optical functionalities of various samples at nanometer-scale resolution in a vacuum, ambient, or fluid environment.
The increasing demand for sub-nanometer probe tips is attributable to their robustness and versatility. Applications of sub-nanometer probe tips exist in the fields of nanolithography, nanoelectronics, biosensor, electrochemistry, semiconductor, micromachining and biological studies.
History and development.
Increasingly sharp probe tips have been of interest to researchers for applications in the material, life, and biological sciences, as they can map surface structure and material properties at molecular or atomic dimensions. The history of the probe tip can be traced back to 1859 with a predecessor of the modern gramophone, called the phonautograph. During the later development of the gramophone, the hog's hair used in the phonautograph was replaced with a needle used to reproduce sound. In 1940, a pantograph was built utilizing a shielded probe and adjustable tip. A stylus was free moving allowing it to slide vertically in contact with the paper. In 1948, a circuit was employed in the probe tip to measure peak voltage, creating what may be considered the first scanning tunneling microscope (STM). The fabrication of electrochemically etched sharp tungsten, copper, nickel and molybdenum tips were reported by Muller in 1937. A revolution in sharp tips then occurred, producing a variety of tips with different shapes, sizes, and aspect ratios. They composed of tungsten wire, silicon, diamond and carbon nanotubes with Si-based circuit technologies. This allowed the production of tips for numerous applications in the broad spectrum of nanotechnological fields.
Following the development of STM, atomic force microscopy (AFM) was developed by Gerd Binnig, Calvin F. Quate, and Christoph Gerber in 1986. Their instrument used a broken piece of diamond as the tip with a hand-cut gold foil cantilever. Focused ion and electron beam techniques for the fabrication of strong, stable, reproducible Si3N4 pyramidal tips with 1.0 μm length and 0.1 μm diameter were reported by Russell in 1992. Significant advancement also came through the introduction of micro-fabrication methods for the creation of precise conical or pyramidal silicon and silicon nitride tips. Numerous research experiments were conducted to explore fabrication of comparatively less expensive and more robust tungsten tips, focusing on a need to attain less than 50 nm radius of curvature.
A new era in the field of fabrication of probe tips was reached when the carbon nanotube, an approximately 1 nm cylindrical shell of graphene, was introduced. The use of single wall carbon nanotubes makes the tips more flexible and less vulnerable to breaking or crushing during imaging. Probe tips made from carbon nano-tubes can be used to obtain high-resolution images of both soft and weakly adsorbed biomolecules like DNA on surfaces with molecular resolution.
Multifunctional hydrogel nano-probe techniques also advanced tip fabrication and resulted in increased applicability for inorganic and biological samples in both air and liquid. The biggest advantage of this mechanical method is that the tip can be made in different shapes, such as hemispherical, embedded spherical, pyramidal, and distorted pyramidal, with diameters ranging from 10 nm – 1000 nm. This covers applications including topography or functional imaging, force spectroscopy on soft matter, biological, chemical and physical sensors. Table 1. Summarizes various methods for fabricating probe tips, and the associated materials and applications.
Tunneling current and force measurement principle.
The tip itself does not have any working principle for imaging, but depending on the instrumentation, mode of application, and the nature of the sample under investigation, the probe's tip may follow different principles to image the surface of the sample. For example, when a tip is integrated with STM, it measures the tunneling current that arises from the interaction between the sample and the tip. In AFM, short-ranged force deflection during the raster scan by the tip across the surface is measured. A conductive tip is essential for the STM instrumentation whereas AFM can use conductive and non-conductive probe tip. Although the probe tip is used in various techniques with different principles, for STM and AFM coupled with probe tip is discussed in detail.
Conductive probe tip.
As the name implies, STM utilizes the tunneling charge transfer principle from tip to surface or vice versa, thereby recording the current response. This concept originates from a particle in a box concept; if potential energy for a particle is small, the electron may be found outside of the potential well, which is a classically forbidden region. This phenomenon is called tunneling.
Expression derived from Schrödinger equation for transmission charge transfer probability is as follows:
formula_1
where
 formula_2
 formula_3
 formula_4 is the Planck constant
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 &amp; 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;
 ;chem ;W (s) + 8OH- - ; WO4 + 4H2O + 6e- (E= 1.05V) ;/chem ;
Cathode:
 ;chem ;6H2O + 6e- - ; 3H2 + 6OH- (E=-2.48V) ;/chem ;
Overall:
 ;chem ;W (s) + 2OH- - ; WO4^2- + 2H2O (l) + 6e- + 3H2 (g) (E= -1.43V) ;/chem ;
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.
Lamella method.
In the double lamella method, the lower part of the metal is etched away, and the upper part of the tip is not etched further. Further etching of the upper part of the wire is prevented by covering it with a polymer coating. This method is usually limited to laboratory fabrication. The double lamella method schematic is shown in Fig. 5.
Single atom tip preparation.
Transitional metals like Cu, Au and Ag adsorb single molecules linearly on their surface due to weak van der Waals forces. This linear projection of single molecules allows interactions of the terminal atoms of the tip with the atoms of the substrate, resulting in Pauli repulsion for single molecule or atom mapping studies. Gaseous deposition on the tip is carried out in an ultrahigh vacuum (5 x 10−8 mbar) chamber at a low temperature (10K). Depositions of Xe, Kr, NO, CH4 or CO on tip have been successfully prepared and used for imaging studies. However, these tips preparations rely on the attachment of single atoms or molecules on the tip and the resulting atomic structure of the tip is not known exactly. The probability of attachment of simple molecules on metal surfaces is very tedious and required great skill; as such, this method is not widely used.
Chemical vapor deposition (CVD).
Sharp tips used in SPM are fragile, and prone to wear and tear under high working loads. Diamond is considered the best option to address this issue. Diamond tips for SPMs are fabricated by fracturing, grinding and polishing bulk diamond, resulting in a considerable loss of diamond. One alternative is depositing a thin diamond film on Silicone tips by CVD. In CVD, diamond is deposited directly on silicon or W cantilevers. A is shown in Fig. 6.