Advanced Scanning Probe Microscopy for Materials Research

Scanning probe microscopy (SPM) encompasses a set of advanced techniques for mapping the structure and properties of the surfaces of materials from the atomic to micro scales. The most widely used SPM technique is atomic force microscopy (AFM), in which forces exerted between the tip of a needle probe and the sample surface can be measured with extremely high precision. By recording these forces as the tip rasters across the surface, an image of the sample surface topography is obtained. Beyond the surface topography, several SPM techniques can provide quantitative information about the properties of a material’s surface. These include scanning Kelvin probe force microscopy (KPFM) for surface potential measurements, scanning capacitance microscopy (SCM) for surface capacitance mapping, conductive and tunneling AFM (C-AFM and TUNA) for imaging the electrical conductance of a surface, as well as several techniques for imaging the mechanical properties of a surface. These advanced SPM techniques provide tools for direct structure-property correlations in materials at the nanoscale and are powerful capabilities for materials research, especially when co-located with other surface analytical techniques. Each of these advanced SPM techniques is available for materials research in the Boise State University Surface Science Laboratory

TiO2 Coatings Formed by Atomic Layer Deposition for Enhanced Corrosion Performance of Mg-biomaterials

Magnesium (Mg) alloys have experienced increased attention in the area of biomaterials due to Mg being considered a resorbable biomaterial. Mg alloy implants can potentially be designed to degrade in the body, thus an implant would not remain in the body for longer than is needed to perform its task. Mg and many of its alloys are considered to be biocompatible and non-toxic in the body; however, due to the high rate at which Mg degrades a negative host response is expected. A novel approach to inhibit corrosion rate using thin film coatings on a Mg alloy (AZ31B) via atomic layer deposition (ALD) is proposed. ALD is based on saturated surface reactions on the substrate unlike other thin film deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD). Sequentially-performed surface reactions between the substrate and precursor molecules water and titanium tetrachloride (H2O , TiCl4) result in thin film growth of amorphous titanium dioxide (TiO2). TiO2 is grown in an atomic layer-by-layer fashion during ALD allowing sub-nanometer thickness control of growth, with excellent coating uniformity and step coverage. TiO2 coatings and their impact on the corrosion resistance of AZ31B were characterized using atomic force microscopy, scanning electron microscopy, electrochemical impedance spectroscopy, and linear polarization measurements

Advanced Atomic Force Microscopy for BioMaterials Research

Optical microscopy uses the interactions between light and materials to provide images of the microscopic world. It is widely employed in science to study the behavior and properties of microscopic organisms and cells. Atomic force microscopy (AFM) is a technique for obtaining images of the surfaces of materials at the atomic to micrometer scales. AFM operates by rastering an ultra-sharp needle across a sample surface and recording the height of the needle at each position. While AFM can provide atomic resolution images of the contours (topography) of a surface, it can also perform extremely sensitive measurements of surface mechanical properties. By fabricating custom AFM probes, the mechanical properties of specific locations of living cells can be studied and manipulated. In addition, high-speed imaging of biological materials can provide images of changes to cellular surfaces in response to chemical or electrical signals. This poster will present examples and applications of advanced AFM capabilities for research in biomaterials available in the Boise State University Surface Science Laboratory

Fabrication of a Nanoscale Electrical Thermometry Platform for 2D Materials

Nanoscale Electrical Thermometry is a technique for characterizing the thermal properties of 2D materials. Fabrication of a nanoscale electrical thermometry platform requires the use of nanoscale processing techniques, such as photolithography and electron beam lithography (EBL). Photolithography is the most common method to create patterns on thin films for integrated circuits and microelectromechanical systems. Using silicon as a semiconductor substrate, we are able to add a single layer of positive photoresist (such as poly(methyl methacrylate), PMMA) and cover the resist with a photomask. By subsequent fixing and developing, the pattern in the resist layer can be transferred into the underlying substrate by etching or deposition processes. The resolution of patterns formed by photolithography is determined by the diffraction limit of light. Another form of lithography is called Electron Beam Lithography, which is a maskless technique that uses a fine beam of electrons to write a pattern directly into a resist. The control system called Nanometer Pattern Generation System (NPGS) is used to write the 2-D pattern as part of a scanning electron microscope. The resolution of EBL is determined by the size of the electron beam which can be as small as 10 nm. The use of these two forms of lithography to fabricate an electrical thermometry platform for 2D materials will be described

Scanning Tunneling Microscope: 3D Imaging on the Atomic Level

A Scanning Tunneling Microscope (STM) is a very useful tool in Physics and Material Science with its ability to image surfaces with atomic-scale resolution making it a critical instrument for the surface analysis of conductive materials. In operation, the STM utilizes a fluctuation in tunneling current between the tip of a needle and surface atoms to track the contours of the electron density of surface atoms. The needle and surface are so close that electrons can quantum mechanically tunnel (hence the name tunneling current) across the classically forbidden vacuum barrier between the two generating a tiny, but measurable, current. As the probe tip moves along a surface, the sample’s change in topography (or composition) will affect the tunneling current. If the tip is raised or lowered to maintain a constant tunneling current, then the change in the tip position may be processed into a contour map of the surface. The ability to visualize surface atoms provides valuable information for materials science, semiconductor physics, and microelectronics and has been used to determine the atomic structure of semiconductor surfaces, visualize atomic and molecular orbital structures, and even manipulate single atoms

Pretend I’m Not Here: Minimally-Interfering Fluorescent Dye-Quenchers for DNA Reaction Networks

Many disease-related biomarkers have been detected in the blood stream. However, detection of such low-concentration molecules requires expensive and time-consuming analytical techniques. In the past decade, the programmability of Watson-Crick base-pairing has been used to develop DNA-based molecular circuits that can detect and amplify low-concentration, disease-related biomarkers. These DNA-based circuits consist of coupled reaction networks, and the interactions between network components can be difficult to predict. Operation of such networks is normally monitored by measuring the fluorescence of separate molecular probes. Such probes are used to minimized the interference of fluorescent dye-quencher pairs, which may bind strongly and impact network operation; however, the probe sequences may also interfere with network operation. To simplify amplification circuitry and improve network performance, we performed of a series of experiments to establish a fluorescent dye-quencher system that can be directly incorporated within a DNA-based amplifying circuit with minimal impact on circuit performance

Colorimetric Detection of DNA via Catalytic Disassembly of Gold Nanoparticles

Gold nanoparticles exhibit colorimetric properties due to plasmonic coupling that can be utilized for DNA sensing. We are attempting to incorporate an autocatalytic DNA reaction mechanism to induce the separation of gold nanoparticle aggregates. The catalytic disassembly nature of the network allows for separation of multiple aggregated particles, which enhances target DNA detection sensitivity and yields faster sensing than an aggregation-based colorimetric detection system. The network includes two particles functionalized with complementary strands of DNA, the target strand of interest, and a fuel strand to complete the disassembly. To induce the aggregation, the functionalized particles were combined and vortexed. Different concentrations of the target and fuel strands were added to the aggregates and their absorbance was monitored over time. Results have shown a clear difference in absorbance of solutions with target and fuel compared to those without, confirming that the particles are separating to an extent. In order to achieve better performance of the disassembly, the effects of ionic strength and stoichiometry of particles are being investigated

Towards Plasma Enhanced Atomic Layer Deposition

The fabrication of microelectronics relies on thin film technologies. As the demand for improved performance of microchips continues to escalate, atomic layer deposition (ALD) has emerged as a crucial technique in enabling precise and controlled thin film deposition. Plasma-enhanced ALD in particular is an energy-enhanced method for synthesizing thin films with mono-layer resolution. Unlike conventional thermal ALD processes where chemical precursors react with a heated substrate to deposit the thin film, forming a plasma of the ALD precursors allows for alternate reaction paths, potentially leading to improved film density, crystallinity, and mechanical properties at lower deposition temperatures. Plasma exposure during ALD can also assist in the removal of surface contaminants during the deposition process. In this project, a capacitively coupled plasma is generated by applying a direct current (DC) bias between the powered and grounded electrodes in a quartz chamber, inducing an electric field. This couples with the precursor gas to create a high-energy plasma that produces energetic reactive species, which can then be directed to the substrate, providing novel reaction pathways having a more desired thermal budget to achieve the desired film properties. This chamber will be integrated into an ALD system to enable plasma-enhanced ALD. The chemical, physical, and electrical properties of the resulting films will be characterized