A Probe-scanning Type Atomic Force Microscope For Observation In Liquid Environment

Nanotechnology opens new frontier in scientific research. Atomic force microscope (AFM). a high-resolution imaging technique that can resolve atomic features, can be operated in various environments, such as ambient air and liquids. It makes AFM an important analytic tool for nanotechnology. Traditional liquid-AFMs usually adopt sample-scanning method, which promises a high stability and sensitivity but encounters challenges when imaging large and massive samples. Moreover, the liquid cell of commercial AFMs is available to immerse samples in liquid but only maintain a limited area of sample surface for their limited volume and closed design. In contrast, probe-scanning type AFM is not subject to this limitation and can obtain images with high speed either in air or in liquid.The objective of this dissertation is to develop a novel liquid-AFM system capable of imaging large samples in liquid environment with a high resolution. With a unique optical detection method, the laser beam can reflect from the same point on the cantilever during scanning. It can overcome the limitations of current liquid-AFM systems described above, and shows its broader potential applications in the fields of physics, chemistry, electrochemistry, materials and biology.Firstly, the principle, features and state-of-the-art of liquid AFM system are briefly reviewed and presented. Then the probe-sample interactions in liquid environment and their influences have been detailed analyzed. Besides, the scanning-and feedback-induced errors in traditional probe-scanning AFMs and their influences on AFM measurements have been theoretical analyzed and calculated. Based on these theoretical analyses, the research plan for this project will be demonstrated.Secondly, a novel optical detection method for probe-scanning AFM is designed. With a new beam tracking method, the laser beam can reflect from the same point on the cantilever throughout raster scan over the entire scan area. This method has enabled elimination of the scanning- and feedback-induced errors in the optical path of current probe-scanning AFMs. To estimate the scanning-induced error, a detection method based on position sensitive detector (PSD) is developed. With the aid of this method, the optical path of the detection system can be easily adjusted.On the basis of the new optical detection method, a liquid-AFM probe unit has been designed and fabricated. In the liquid-AFM head, the laser adjustment can be eliminated regardless of the measuring environment. Thanks to the special designed window, the beam shift that normally occurs upon immersion into liquid is completely absent in the AFM head, which makes the measurements in liquid environment as simple as measuring in air. Additionally, a new kind of three-dimensional piezoelectric scanner is designed. The new compound scanner uses two stacked piezoelectric actuators in the X and Y directions for their large displacements, and a piezoelectric tube in Z-axis for its fast response. It combines the advantages of these two kind of piezoelectric actuators. With the help of the two-dimensional step scanner, our AFM probe unit is able to locate on the sample surface rapidly and scan AFM images in adjacent regions successively.Based on above theoretical analyses and developed novel methods, a liquid-AFM system is established to address measurements in liquid environment of both the research lab and industrial applications. It enables imaging large sample in liquid with high resolution and high performances. Furthermore, image stitching method has been developed to build a wider AFM image with range from micrometers to millimeters.Below is a summary of the key features of this AFM. The system is designed to image samples with size up to 300mm×300mm and weight up to 10kg. To satisfying different measurement needs, two kinds of piezo scanner are fabricated. The smaller scanner has a scan range of 4μm×4μm with spatial resolution of 0.1 nm. The compound scanner, which combines the advantages of stacked piezo and piezo tube, has a scan range of 20μm×20μm and spatial resolution of nanometer. The two-dimensional step scanner with an X-Y travel range of 30mm×30mm, allows fast travelling and location of AFM probe on large sample surface in X-Y plane and scanning AFM images in adjacent regions successively. Combined with the Z-axis piezo tube, it can also realize larger-size AFM measurements. In addition, an image stitching method is utilized to build a broad merged microscopic image with range up to millimeters while keeping nanometer order resolution. This function enables us to obtain wide AFM images with high resolution, which satisfies the rising scientific and industrial demands in micro- and nano-measurements.To demonstrate the imaging ability and stability of the developed liquid-AFM system, serials of experimental measurements were carried out. Results show that the overall shape, size, and contrast are nearly identical in those images. Furthermore, by studying the corrosion behavior of aluminum surface in NaOH solution, as well as copper plating on silicon wafer in real time with the developed liquid-AFM system, we also show that the system can observe the solid-liquid interface of the electrochemical reactions and processes in the atomic level. The superior performance of the system renders its great prospect in research and industry applications among chemical, electrochemical and biological studies, where large-size scanning and high-resolution imaging are required for large, heavy samples in liquid environment.