National Physical Laboratory

Characterisation of 2-D materials in different environments using Scanning Probe Microscopy (SPM) Techniques


Studies under various atmospheres

Scanning Probe Microscope

Topography and electrical measurements can be performed on samples while varying conditions such as gas atmosphere, including different humidity levels and liquid environments. Changes in structural, electronic and magnetic properties in different and/or changing gas environments can be monitored in the temperature range -30 °C to +300 °C. Adsorption of gases or chemical reactions at the surface can be quantified, which is useful for sensing and catalysis studies.

The NTEGRA Aura (NT-MDT) (right) is a unique Scanning Probe Microscope for studies in controlled environment conditions: vacuum, temperature and in an external magnetic field.

Below are examples of topography (left column) and associated surface potential maps (right column) of graphene under various environments. In surface potential maps the background corresponds to one-layer graphene and the stripes along terrace edges to two-layer graphene. (Samples courtesy of Dr Kurt Gaskill, Naval Research Laboratory, Washington, USA.)

Sample in ambient conditions:

Sample in ambient conditions

Sample in vacuum (P=4.7x10-6 mbar):

Sample in vacuum

Sample in vacuum and at variable temperature (65 °C beginning of the scan to 40 °C end of the scan)

Sample in vacuum and variable temp

Sample under nitrogen environment:

Sample in nitrogen

Sample under high humidity environment (RH = 85%):

Sample in high humidity

Measuring modes and techniques

  • Atomic Force Microscopy gives information about surface topography with nanometre resolution. This mode is widely used to study characteristics of surface features for semiconductors, metallic film coatings, polymers, biological materials, etc.
  • Lateral Force Microscopy (LFM) mode facilitates discrimination between areas with different friction coefficients. The LFM mode is particularly useful in studies concerning deterioration of physical and chemical properties of surfaces and tribology.
  • Piezoresponse Force Microscopy is typically used for studies of ferroelectrics for the investigation of their domain structures. The mode is of special interest for the development of ferroelectrics data storage devices, as well as associated data reading/writing.
  • Force Modulation is a mode that facilitates measurements of local elasticity of a sample, highlighting softer and harder areas of the sample surface. The mode is largely used in the study of biological entities, polymers and especially composite materials.
  • Electrostatic Force Microscopy is a semi-contact technique used for qualitative imaging of charge inhomogeneities in conductors and insulators.
  • Kelvin Probe Microscopy is a semi-contact technique used for mapping the electric surface potential and thus the work function of materials.
  • Scanning Capacitance Microscopy is a contact technique used for mapping the probe-sample electrostatic capacitance using a high-frequency resonant circuit. The technique is useful for mapping the local carrier concentration in semiconductors.
  • Spreading Resistance Imaging is a contact technique used for mapping the spreading resistance and thus the local resistivity of conductor and semiconductor samples.
  • Scanning Tunnelling Microscopy is intended for investigations of conductive surfaces with resolution down to atomic level. Application of Scanning Tunnelling Spectroscopy allows measuring the surface distribution of the local electronic density of states or of the work function of the studied material.
  • Magnetic Force Microscopy is a semi-contact technique used for imaging magnetisation properties of magnetic samples. The perpendicular magnetic anisotropy of the sample can be studied by applying out-of-plane magnetic fields, whereas the magnetic domain wall trapping inside structures can be imaged by applying in-plane magnetic fields.

Topography and MFM of permalloy

Evolution of a permalloy when an in-plane magnetic field is ramped up and down.
Topography (left) and MFM image (right).

Lithography

  • In Force lithography, the pressure exerted by the probe on the surface is used to cause plastic deformations of the surface.
  • Charge Lithography is based on the effect of the electrical field between the probe and the sample to locally modify the sample surface through local heating, electric-field evaporation/deposition of material, etc. The probe-sample voltage is maintained constant in this mode.
  • Current Lithography operates with the probe-sample current maintained at a constant level. When a sufficiently large voltage is applied between the tip and the sample, the electrical field can locally modify the surface.

To find out more, please contact Olga Kazakova

Last Updated: 11 Nov 2013
Created: 14 Oct 2013

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