The Design And Application Of High-stability Scanning Tunneling Microscope

Scanning tunneling microscope (STM) is an important member in the scanning probe microscope family. According to the quantum tunneling effect, it can be used to study the surface morphology and the electron density of samples. STM has been widely applied in various fields, such as material characterization, electrochemical measurement and biological imaging, because of its ultra high real space resolution.Up to now, a large number of designs of STM with special functions have been developed, such as low temperature ultra-high vacuum scanning tunneling microscopy and electrochemical scanning tunneling microscopy. Nevertheless, the stability remains a key problem and the vibration and sound isolations setups are still essential in the STM systems. To improve the stability of STM, we start by designing a new coarse approach motor, which is the core component of STM.Most of the coarse approach motors used in STM are piezoelectric motors. However, none of the designs of the piezoelectric motor meet the requirement so far. So, we develop a novel piezoelectric motor named GeckoDrive with new principle and structure. The working prinple of GeckoDrive is simple with the expression "the sum of two frictions is greater than the third". The GeckoDrive owns several important features not simultaneously owned by other piezomotors, including:large step size, small size, very rigid, simple in structure and operation, very large temperature range, easy to make and loose machining tolerance, etc. This work has been published in the journal Review of Scientific Instruments, the paper published in which is equal to SCI-Ⅱ journal article in Hefei national laboratory for physical sciences at the microscale. The reviewer’s comments are favorable:" The design and idea is new."After that, we have modified the structure of GeckoDrive. The new structure GeckoDrive is more rigid and stable and can be built easily by one piezoelectric tube, two metal rings, one rigid shaft, and one spring strip. This work is also published in the journal Review of Scientific Instruments. We have applied for four national invention patents and two international patents about the GeckoDrive, and two of the national invention patents have authorized.By using the improved GeckoDrive, we have designed and built a stable STM. The scanner of homebuilt STM is a small piezoelectric scanning tube with the size8 ×0.5×7mm (outer diameter×wall thickness×length, EBL#3, EBL Products Inc.). The GeckoDrive is fixed on the scanning tube at one end coaxially. The tip holder is glued at the shaft of the GeckoDrive. The scanning tube will drive the GeckoDrive and the tip to scanning on the sample surface with all low voltage control signals (-10V～+10V) when the STM is working. Using this STM, we have obtained high quality atomic images of highly oriented pyrolytic graphite (HOPG). The successful manufacture of the homebuilt STM further confirms that small size scanner and all low control voltage enhance the precision and the stability of STM.To protect the scanner from the impact from coarse approach motor, we have designed a novel STM, the scanner of which is able to detach from the coarse approach motor. Different from the structure of traditional STM, the scanner in our STM is isolated from the coarse approach motor during imaging. As a result, the scanner will not be disturbed by the instability of the motor caused by external vibration, sound, thermal drift, and electric noise in the control signal, etc. Furthermore, the scanner adopts a clamping structure which is compact and firm. The scanning tube used in the scanner is tiny with the size3.2×0.5×7mm (outer diameter×wall thickness×length) and the component materials is a type of piezoelectric ceramics with high locating accuracy and stability (EBL#4, EBL Products Inc., equal to the industry type PZT-8). The scanner is still controlled by all low voltage signals. These advantages finally lead to a highly stable STM capable of obtaining high quality atomic resolution images of HOPG even without vibration and sound isolations setups. The lateral drift and the Z drift of our STM are about22.6and lower than35.3pm/min, respectively. All the drift tests are performed in air at room temperature without any auxiliary softwares or hardwares which suppress the drift artificially (for example, air conditioner). The performance of our STM is far better than the commercial ones. The study on high-stability scanning tunneling microscope has been published in the journal Review of Scientific Instruments and applied for two national invention patents.In the first part of this thesis, we briefly introduced the structure and the application of STM. After that, we discribed some technologies of preparing tungsten tips and platinum-iridium tips used in our experiments. In the next part, we detailly elucidated the process of developing GeckoDrive and the corresponding STM. Then we discussed and solved the problems we encountered. At last, we have made a summary of all the work shown above and proposed a further design and the expection of our work in the furture.