Combined capillary and probe microscopy
Within the framework of the project "Initiation of local chemical reactions in deposited thin films using scanning probe microscopy" a capillary microscopy mode was implemented in the FemtoScan scanning probe microscope. With the help of a combined tool, experiments were conducted on the nanolithography of carbon materials and semiconductors.
Thanks to the use of replaceable heads, the FemtoScan scanning probe microscope allows the use of scanning tunneling and atomic force microscopy. The task of integrating the capillary microscopy mode was solved using a scanning tunneling microscope head and a nanocapillary holder: a nanocapillary was used instead of the STM probe.
For the manufacture of capillaries borosilicate glass tubes with outer and inner diameters of 1.0 mm and 0.58 mm, respectively, are used. Depending on the heating conditions, speed and drawing force, it is possible to obtain capillaries with an outlet opening of 5–100 nm. When using a two-channel capillary, one of the channels serves for delivery of the reagent, and the other one – for positioning the capillary above the surface of the sample .
The uniqueness of the experimental system lies in the accuracy and reliability of measurements, compactness and ergonomics of the device, which allows to change the measurement modes quite quickly and simply. According to the main technical characteristics, the installation surpasses foreign analogues.
In the course of the project, the FemtoScan scanning probe microscope was used to perform nanolithography on the surface of carbon materials, metal films (aluminum, titanium) and semiconductors (doped silicon) (Fig.2). Nanolithography was first described in 1990 on the example of local oxidation of silicon under the influence of an electric field in the region of the tunnel junction . Local anodic oxidation has become a useful tool for manufacturing complex devices in order to study various quantum phenomena, for example, Coulomb blockade, quantum conductivity, and the like .
The main advantage of nanolithography by the method of local anodic oxidation is the ability to accurately control the electrical and topographic characteristics of nanoscale structures. At the first stage, a lithographic pattern is created using a conductive probe, and then, using the same probe, the relief and map of the distribution of the local electrical resistance of the selected area are determined.
The disadvantage of the method is the possibility to use it only for a limited class of materials (anodically oxidizable metals and heavily doped semiconductors, as well as hydrogenated silicon) . We conducted experiments on local anodic oxidation of the graphite surface with the help of a conducting cantilever (Fig.3). Under the influence of the potential difference in the presence of moisture, graphite oxide forms on the surface.
In , the etching of a graphite surface by an STM probe and the effects associated with the peculiarities of the feedback of a microscope are considered. On the basis of the experimental data, it was suggested that the etching of graphite by STM under the conditions of the presence of water vapor in the atmosphere has a mixed mechanism combining the destruction of the surface by electrons emitted from the needle and its local anodic oxidation. In , results were published on the measurement of the local electrical conductivity of graphene oxides grown by the method of local anodic oxidation on the graphite surface and obtained by the chemical method.
Scanning capillary microscopy is an effective tool for both determining topography with nanometer-scale spatial resolution and selective surface modification. Fig.4 shows the surface topography of a DVD obtained in the scanning capillary microscopy mode. A 3D image of the same area is shown in Fig.5. The images were obtained in a solution of common salt with a concentration of 0.9 M. The current through the nanocapillary is 100 pA. The depth of the cavities corresponding to single pits is about 97 nm. To evaluate it, we used a histogram – the distribution of the surface regions by the height, as shown in Fig.6.
In the first year of cooperation, the main objectives of the project "Initiation of local chemical reactions in deposited thin films using scanning probe microscopy" were achieved. At the next stages of the work new tasks will be solved. ■
The study was carried out with the financial support of the Russian Foundation for Basic Research within the framework of the scientific project 17-52-560001. The authors are sincerely grateful to the Government of Moscow, the Department of Science, Industrial Policy and Entrepreneurship of Moscow, the Ministry of Economic Development of Russia (Agreement No. 8 / 3-63in-16 of August 22, 16) for financial support of the projects of the Nanotechnologies YICC.