Methods of nanoscopy for directional modification of nanoscale 2D structures and determination of their physical-chemical and electrophysical characteristics
canning probe microscopy involves a large number of methods both in the study of nanostructures and in the directional modification of their physical-chemical and electrophysical properties. Nanoscale 2D structures are a promising basis for the creation of high-performance energy storage and catalysts using carbon materials and biopolymers.
For directional modification of the carbon surface, the authors used atomic force microscopy in a moist medium and scanning ion conductance microscopy in an electrolyte. Modification of the carbon surface under the influence of a negative electrical voltage applied to the probe of an atomic force microscope occurs due to local anodic oxidation . The effectiveness of this process depends largely on the environmental conditions, mainly on the humidity of the air. In drier air, to initiate local anodic oxidation, it is necessary to increase the potential difference between the probe and the surface of the carbon material. A typical value of the operating voltages for typical laboratory conditions is in the range from 2 to 8 V.
An important interest for the problems of energy-efficient technologies is the creation by lithography method of a large-scale nanostructured surface consisting of graphite and graphite oxide . For this purpose, we have created an experimental unit for chemical modification of the surface of carbon materials by the method of local anodic oxidation in a system with controlled external parameters (temperature, humidity, air atmosphere composition) using the FemtoScan atomic force microscope .
Examples of lithographic patterns are shown in Fig.1.
A new area of chemical modification of carbon materials (graphite, graphene) is scanning capillary microscopy. In this case, targeted delivery of reagents can be carried out using a multichannel nanocapillary .
A study of the kinetics of intercalation and deintercalation of lithium ions from a battery cathode material was carried out using a scanning capillary microscope in which lithium ions were fed through a nanocapillary.
Earlier, an atomic force microscopy method was used in  to observe the intercalation of sulfuric acid with the formation of blisters on the surface of highly oriented pyrolytic graphite (HOPG) and highly annealed pyrolythic graphite (HAPG). The difference between HOPG and HAPG is the technology of obtaining the material. As a result, HAPG has a more perfect structure with a smaller content of defects on the surface, including the steps of cleavage. After chemical oxidation according to the Hammers method, blisters are formed on the surface of the HAPG, unlike HOPG.
A significant advantage of scanning capillary microscopy is that the chemical oxidation process can be carried out locally using an additional channel of the nanocapillary. In this case, the main channel of the nanocapillary serves as a measuring probe for recording the topographic relief of the oxidized region.
Earlier, we obtained results on recording the local distribution of the surface potential of highly oriented pyrolytic graphite by the method of scanning resistance microscopy . The data of scanning Kelvin microscopy confirm this result.
Determination of the relationship between surface topography and its electrophysical properties with spatial resolution at the level of nanometer units is possible when using combined modes of scanning probe microscopy: simultaneous measurement of the sample topography and distribution of several local parameters of its surface (spreading resistance, surface charge, electric current, etc.).
Determination of the local charge/discharge current is carried out with the help of scanning capillary (ion conductance) microscopy. An estimate of the total number of charge/discharge cycles for local cathode regions by means of scanning probe microscopy shows that this value is 100 or more times for the case when the current decrease does not exceed 60%.
With the help of the different types of scanning probe microscopy – atomic force microscopy, scanning resistive microscopy, Kelvin microscopy, scanning capillary microscopy – the following information can be obtained: topographic relief, local electrical conductivity distribution map, surface charge distribution, dielectric inclusions. At the same time, the spatial resolution is achieved at the level of units and fractions of a nanometer. Modification of the carbon surface is carried out locally in a moist air medium with the help of an atomic force microscope probe when a negative potential is applied to it. Local modification of the surface of carbon materials in solutions of electrolytes is carried out with the help of a scanning capillary microscope. ■
The study was carried out with the financial support of the Russian Foundation for Basic Research (project No. 16-29-06290).