In general SPM relies of the interaction of a sharp tip (probe) with the sample surface. The interaction is mapped as a function of tip position over the sample resulting in an image of the sample surface. Mapping is done by scanning the tip over the surface and depending of the technique, different interactions are used for image formation. The best know are a tunneling current (STM) and the van-der-Waals interactions (AFM). But also a large variate of other interaction can be used, e.g. magnetic forces (MFM) or electric forces (EFM). The realization of the signal detection depends from instrument to instrument and similar techniques are known by different names from various vendors.
The LCS can help learning specific techniques in our facilities or implement certain techniques, if required by a user. Therefore, a proposal for using the instrument has to be submitted over the user portal. We also help to implement techniques at instruments owned by the users.
The following scanning probe microscopy techniques were implemented at the equipment of the LCS. To provide an idea of the technique, a small description is provided. Images are made with our instruments and copyrighted by the LCS/LNNano/CNPEM.
Scanning Tunneling Microscopy (STM)
Scanning tunneling microscopy (STM) was the first SPM method to be developed. The surface of the sample is imaged by a small tunneling current between the sample and an atomic sharp tip. The technique allows to image the sample surface with atomic resolution, provides possibility to map the electron density of state of tip and sample or “see” the wave function of an electron on the surface. We have a scanning tunneling microscopy head for one of our AFMs operating at air and ambient conditions. In this way, we can obtain topography images from conduction samples, but resolution is rather limited.
Atomic Force microscopy (AFM)
After STM, atomic force microscopy was the second SPM technique developed. Its main strength is ability to work in air and to provide good topographical resolution under such conditions. Using the van der Walls interaction between sample and a sharp tip (tip radius in our days better than 10 nm), the sample surface is mapped keeping tip-sample interaction constant. This can either be done in contact mode (tip touches sample) or in non-contact techniques implemented in various ways by the instrument vendors. Under good conditions with the appropriated setup, atomic roughness – as demonstrated in the images – above can be measured at highly ordered surfaces like mica or HOPG. AFM might be the most important surface characterization technique in our days, addressing and solving problems of solid state physics, material science to life science and biological systems. Measurements can be carried out in air, water or vacuum.
Electric Force Microscopy (EFM)
Electric force microscopy (EFM) is a sister technique to AFM. Instead of analyzing the van der Waals interaction, long range electrical forces are analyzed. In a qualitative way, sample charges or differences in surface potential are detectable. In more sophisticated setups, quantitative measurement of the surface work function is possible. Implementation vary from vendor to vendor, and EFM can be carried out as single or double pass technique. Normally, the tip sample interaction is modeled as capacitor formed by tip and sample. Changes in the charge or the capacitance of the tip sample system are detected either static or by applying external AC signals. Care has to be taken as electrical forces are long rage forces. Resolution is therefore limited to ca. 100 nm depending on samples and measurement environment. EFM is highly related to KPFM or KFM as described below and separation between the techniques is not sharp and depends on the scientific communities.
See also: EFM
Kelvin Probe Force Microscope (KPFM/KFM)
Kelvin probe force microscopy (KPFM) is highly related to EFM. In a physical definition, KPFM is a quantitative EFM determining the work function of the material. This is normally done in a single pass technique applying an external DC voltage to nullify any electrical signal measured in dynamic EFM using an AC voltage applied between tip and sample. For KPFM, the existent of no surface charges is assumed and differences in the tip sample voltage measured arises from a difference in the work function of the tip and the sample surface – the contact potential difference (CPD). In this way, the tip sample interaction can be completely modeled and quantitative numbers are determined. In addition, we tend to monitor the capacitive coupling of the tip and the sample (dC/dz), which for flat samples can purely be modeled as a change of the local dielectric function of the sample.
See also: KPFM/KFM
Scanning capacitance microscopy (SCM)
Scanning capacitance microscopy (SCM) as implemented in our Park systems, uses a microwave circuit to directly determine the tip sample capacitance. Hence, unlike in the KPFM method, one can directly determine a change in the local dielectric function, e.g. to see the doping profile of a semiconductor.
See also: SCM
Magneto Force Microscopy (MFM)
Like EFM, magnetic force microscopy (MFM) is an AFM based technique. Instead of an Si cantilever, a magnetic tip is used. Normally, MFM is carried out in a two pass technique: in a first pass close to the sample, an AFM image of the sample is obtained. In a second pass, the tip follows the topography in a long distance from the sample surface. Taking advantage that magnetic forces are long range, whereas van der Walls forces are short range, we obtain a sole interaction of the magnetic tip with the magnetic domains of the surface. due to the long range nature, the technique is hard to model and only qualitative information is gathered. Never the less, MFM is a powerful to to map the magnetic domains of a material providing deep inside not only into the magnetism of samples, but also can show non-magnetic magnetic phase transitions e.g. in steel.
See also: MFM
Scanning Nearfield Optical Microscopy
Scattering scanning near-field optical microscopy (NSOM/SNOM) is an AFM based technique to obtain an optical image in the nano-meter rage. Therefore, a metalized tip is brought close to the sample surface (less than the wavelength of the used light) that it can interact with the optical near field. The tip acts as a kind of nano-anntena transferring the nearfield signal (that does not propagate, but contains lateral optical information below the classic far field diffraction lime of ca. half the wavelength) into a far field signal. To obtain the optical information, the tip needs to be illuminated with a powerful light source – in the case of the table top instrument of the LCS a CO2 laser, in the case of the LNLS IR beamline, the sychrotron IR broadband beam extracted from a bending magnet. Both instruments work after the same amplification principle taking advantage that the weak near field signal emitted by the tip will be modulated by the AFM cantilever frequency. Both setup demonstrated lateral resolution of ca. 20 nm. Whereas the synchrotron with the broadband source covers a large wavelength range, the LCS instrument allows detection of amplitude and phase of the emitted signal. Good signals are expected for strong optical oscillators and absorption process, e.g. phonons in inorganic matter, near fields of gold nano-antennas and graphene plasmons (as demonstrate in the image above showing the SPP coupling between graphene and SiO2).
See also: SNOM
Atomic Force Microscopy based Infrared Spectroscopy
AFMIR combines optical excitation of the sample with a contact mode AFM detection – it is in some ways a complementary technique for the SNOM approach. Using a tunable laser source, the sample is locally heated. The tip-sample system is excited to vibrate by the local thermal expansion of the sample. The amplitude of this tip vibration will be proportional to the light absorption at this specific wavelength. Analyzing the tip vibration as a function of excitation wavelength, we can obtain an infrared spectra of the sample material. Analytically, this offers two possibilities: (1) obtain point IR spectra at specific sample locations (2) mapping the sample reaction at a specific wavelength to obtain chemical composition maps. Higher sensitivity can be achieved by pulsing the laser excitation at the mechanical resonance frequency of the tip sample system. Good signal is expected for material with a high absorption and thermal expansion, e.g. polymers which are hard to analyses with SNOM.
See also: AFMIR Wikipedia
Local oxidation of the sample surface can be carried out at various instruments. This allows the fabrication of arbitrary patterns on the surface, e.g. for electrical devices. Normally, to achieve good results, local anode oxidation is deducted inside a controlled environment especially the relative humidity has to be adjusted.
“Nanoindentation is a variety of indentation hardness tests applied to small volumes. Indentation is perhaps the most commonly applied means of testing the mechanical properties of materials. The nanoindentation technique was developed in the mid-1970s to measure the hardness of small volumes of material.”
See also: Nanoindentation
Working in liquid
We have liquid cells for two instruments allowing to work in water, e.g. to investigate cellulose before drying out.
For data evolution, we work mainly with the Gwiyddion software packages preferring this open source solution. The package can read the data from all instruments and allows user data evaluation after leaving the facilities. Users can get an introduction to the software by us.