Scanning Probe Microscopy of Functional Materials

Scanning tunneling microscopy (STM) achieves atomic-scale resolution due to the exponential dependence of the tunneling current on the distance from the tip to the surface. The majority of tunneling electrons traverse the junction elastically via coherent quantum mechanical coupling between the electronic states of the tip and the conducting substrate. However, a small fraction of tunneling electrons undergoes inelastic scattering, losing parts of their energy to available dynamic modes in the junction with the energy that is less or equal to the electrochemical potential of one of the tunneling leads relative to the Fermi level of the other. Depending on the atomic electronic structure of the tunneling junction and the tunneling conditions, the excited processes may include localized plasmons with subsequent photon emission [1x2013;4], frustrated [5] and free [6] adsorbate motion, formation of charged species [7], molecular fluorescence [8], rotation [9], vibration [10], bond breaking [11, 12], and isomerization [13, 14]. The STM can therefore glimpse far beyond the local electronic structure of the junction and it has been extensively used to explore the dynamic functionality of surfaces, nanoparticles, and single molecules.

One of the great scientific challenges at the intersection of chemistry, biology and materials science is to define the biophysical pathways of cellular life, and in particular, to elucidate the complex molecular machines that carry out cellular and microbial function and propagate the disease. To study this in a comprehensive way, the fundamental understanding of the principal mechanisms by which cellular systems are ultimately linked with their chemical, physical, and biological environment are required. Complete genome sequences are often available for understanding biotransformation, environmental resistance, and pathogenesis of microbial and cellular systems. The present technological and scientific challenges are to unravel the relationships between the organization and function of protein complexes at cell, microbial, and pathogen surfaces, to understand how these complexes evolve during the bacterial, cellular, and pathogen life cycles, and how they respond to environmental changes, chemical stimulants, and therapeutics. Development of atomic force microscopy (AFM) for probing the architecture and assembly of single microbial surfaces at a nanometer scale under native conditions, and unraveling of its structural dynamics in response to changes in the environment has the capacity to significantly enhance the current insight into molecular architecture, structural and environmental variability of cellular and microbial systems as a function of spatial, developmental, and temporal organizational scales.

In this chapter, we review the fundamentals of dynamic force microscopy (DFM) and dynamic force spectroscopy (DFS) focusing on applications in ambient conditions. More specifically, we analyze the basic principles of the two important driving mechanisms that are used in AFM imaging modes:the amplitude-modulation technique (“tapping mode”) and the frequency-modulation technique. From this starting point, analytical descriptions of the two modes are developed. The theory is then applied in conjunction with numerical simulations to various situations occurring while imaging in ambient conditions. Finally, we present methods for the continuous measurement of the tip–sample interaction forces during the approach towards the sample surface using DFS.

The superb spatial resolution and imaging capability of atomic force microscopy (AFM) make it an attractive tool for investigating nanoscale mechanical properties. One AFM method that shows promise for quantitative property data is contact resonance force microscopy (CR-FM). In this approach, the cantilever’s resonant frequencies are measured while the tip is in contact and are used to determine the local contact stiffness. Nanomechanical information is obtained from the contact stiffness with an appropriate contact mechanics model. Here, we describe our work to develop quantitative CR-FM metrology and apply it to material systems. We explain the basic theoretical and experimental concepts and show ways to implement them for accurate, reliable measurements in specific applications. New extensions of the original approach that enable measurements of viscoelastic and shear elastic properties are demonstrated. Work on quantitative CR-FM imaging is also discussed. Contact resonance frequency images enable maps of the spatial distribution in properties such as the elastic modulus of small-scale structures and the interfacial adhesion of buried interfaces. Such mapping capabilities facilitate new studies of nanoscale mechanical behavior in a variety of emerging applications.

The atomic force microscope (AFM) was invented in 1986 [1], a close relative of another instrument, the scanning tunneling microscope (STM), invented in 1981 [2]. Both fall under the umbrella of techniques and instruments referred to as scanning probe microscopes (SPMs), with the common thread being that a sharp probe is scanned in a regular pattern to map some sample characteristic. Unlike the STM, the AFM can readily image insulating surfaces. Combined with the ability to study a wide variety of samples and sample environments – ambient, liquid, and vacuum – has made AFM the technique of choice for many high resolution surface imaging applications, including imaging with atomic resolution. Since those early days, AFM techniques have become the mainstay of nanoscience and nanotechnology by providing the capability for structural imaging and manipulation on the nanometer and atomic scales. Beyond simple topographic imaging, AFMs have found an extremely broad range of applications for probing electrical, magnetic, and mechanical properties – often at the level of several tens of nanometers.

Macroscopic behavior of materials, whether synthetic or biological, depends on the morphology and characteristics of their microscopic constituents. Improving the performance of engineered materials and understanding the design principles of biomaterials demand tools that can characterize material properties with nanoscale resolution. What is the spatial arrangement of the components of a heterogeneous material? Are the material properties of those components different from their respective bulk properties? How do material properties change near the interfaces? What is the influence of temperature, electric or magnetic fields, or solvents? Answering these questions is of critical importance to the rational design of advanced materials and to the analysis of biological materials. In this chapter, we focus on the recent advances in the measurement and characterization of dynamic nanomechanical properties with high spatial resolution using specially designed atomic force microscope cantilevers. We will first describe the basic operation principles of this method and present results to judge its performance on various material systems.

Advances in analytical technology have pushed the limits of our understanding of chemical and physical phenomena as new tools create new opportunities for discovery. The levels of sensitivities and signal-to-noise levels of atmospheric pressure mass spectrometry techniques have increased to the point that the chemical content of nanometer-sized volumes of material can be determined. Advances in scanning probe technology, which have resulted in improved stability and material property determination, make interfacing with mass-spectrometry possible. Innovations in probe technology to couple or focus intense light or heat at the probe tip enable new ways to remove and transfer specific and highly localized material from the sample surface to the mass spectrometer. The marrying of these two previously separate fields of study creates a viable pathway for true nanoscale chemical imaging. This chapter will cover the recent advances in mass spectrometry that can most readily be coupled with ambient scanning probes and discuss the state-of-the-art efforts to combine these techniques.

Thermo-mechanical properties of materials determine whether they will be useful or not. Measurement techniques for bulk mechanical parameters (Young’s modulus, loss modulus, Poisson ratio, etc.) are well established and regulated by ASTM standards [1 – 14]. As length scales shrink, Nanoindentation (NI) and Atomic Force Microscopy (AFM) techniques allow sub-1,000nm spatial resolution thermo-mechanical properties measurements. These techniques started becoming available less than two decades ago [5] and are evolving rapidly. Examples of nanoscale property measurements include subsurface delamination in thin films [6], the structure of the cytoskeleton of a single cell [7] and composition of polymer blends [8]. As the spatial resolution increases, the variability in the measurements also increases. Part of this increase in variability is real in the sense that as one approaches characteristic length scales of a material, one begins to sample domains with very different thermal and mechanical properties.

Multifrequency measurements in atomic force microscopy (AFM) are one of the main techniques advancing this method. Detection of the AFM probe response at different frequencies enables simultaneous and independent studies of individual constituents of overall tip–sample force and, therefore, begins to empower the advanced compositional mapping and quantitative examination of local mechanical, electromagnetic, and other properties of materials. This chapter describes the practical implementation of multifrequency measurements with a commercial instrument and, particularly, their use in AFM-based electric techniques (electric force microscopy (EFM), Kelvin force microscopy (KFM), and piezoresponse force microscopy (PFM)). One of the peculiarities of the multifrequency approach is multiple choices for a particular type of measurement. This demands a thorough evaluation of different permutations for finding the most sensitive and reliable experimental procedure. In case of EFM and KFM, the evaluation of amplitude modulation and frequency modulation detection of tip–sample electrostatic force during intermittent contact imaging revealed the more precise nature and higher spatial resolution of the frequency modulation studies. This technique has been applied for EFM and KFM imaging of various samples (metals, semiconductors, and organic self-assemblies) that have heterogeneities related to variations of work functions, strength and orientation of molecular dipoles and to a presence of surface charges. The presented results demonstrate the advanced capabilities of multifrequency measurements that are improving the nanoscale characterization of electric properties of materials.

We present quantitative experiments, analytical theory and finite element modeling­ (FEM) of vertical and lateral piezoresponse force microscopy (PFM) across a single antiparallel (180°) ferroelectric domain wall. There are three important aspects in making quantitative measurements. (1) Calibration and background subtraction of PFM displacements; (2) characterization of the tip shape and contact area; and (3) analytical theory and numerical simulations that incorporate all the relevant property tensors (dielectric, piezoelectric, and ferroelectric), tip shape, contact geometry, and the relevant physics of the feature being studied, such as the width of the wall. By calibrating the displacement of the tip, and using a reference sample, one can measure nanoscale piezoelectric coefficients, which are shown to be independent of tip size for a uniform sample. The shape of the contact area of a tip with the sample is characterized by field emission scanning electron microscopy (FE-SEM) to be disk-like. Only a true-contact with zero dielectric gap between the tip and the sample can explain the experimental PFM wall width versus tip radius measurements. Finally, in the limit of the tip disk-radius approaching zero, one can estimate the ferroelectric wall width from the vertical PFM profiles across the wall. The most complete analytical theory and finite element modeling to date are presented that can realistically simulate the PFM profile across a single wall. While vertical PFM signal agrees well with theory and simulations, the lateral PFM signal shows excellent qualitative agreement only. The experimental width of the lateral PFM signal across a wall is significantly wider than that predicted by FEM, suggesting elements of surface physics that are not captured in the current electromechanical theory of PFM.

High-speed piezo force microscopy (HSPFM) has been developed to map ferroelectric properties with imaging rates beyond 1frame/s. In addition to efficient measurements of large areas, multiple samples, and various experimental conditions, this capability is particularly advantageous for monitoring ferroelectric domain poling dynamics. As discussed, this includes identifying switching mechanisms, elucidating the influence of structural defects, and especially quantifying and mapping nucleation times and growth rates. Domains written with tip speeds beyond 1cm/s are also presented and analyzed.

This chapter introduces the structural background of the appearance of polar nanoregions in relaxors, describes their major macroscopic dielectric and ferroelectric properties, and presents the results of recent investigations of local polar structures by piezoresponse force microscopy (PFM). Statistical analysis of observed nanodomains, their temperature evolution, and polarization switching results are given for several relaxor families based on Sr _x Ba_1–x Nb₂O₆ (SBN), PbMg_1/3Nb_2/3O₃ (PMN), PbZn_1/3Nb_2/3O₃ (PZN), and (Pb_1–x La _x )(Zr_1–y Ti _y )O₃ (PLZT). The PFM technique has proved to be a powerful tool for the investigation of local properties of relaxors where optical techniques obviously fail because of their lack of resolution. In addition, size- and grain boundary-dependent phenomena in relaxors are reviewed for representative PLZT compositions. Examples of investigations of polar structures in relaxor films and ceramics and single are also presented.

The tremendous success of piezoresponse force microscopy (PFM) in various fields of material research has been driven by its promise to locally reconstruct the piezoelectric tensor and thus the polarization state by value and orientation. In order to achieve such a reconstruction, PFM generally allows for the collection of five different signals at once:topography, vertical amplitude, vertical phase, lateral amplitude, and lateral phase.

The achievement of nanometer spatial resolution with direct elemental selectivity would have a tremendous impact on our ability to probe and understand complex phenomena occurring at the nanoscale. The combination of synchrotron-based X-ray spectroscopy with the high spatial resolution of scanning tunneling microscopy (STM) has the potential to help attain this goal. In this chapter we show how synchrotron X-ray-enhanced scanning tunneling microscopy (SXSTM) has evolved from the very early days of photo-assisted STM to become a promising spectroscopy and imaging technique in nanoscience and nanotechnology. The basic principles of SXSTM are discussed accompanied by a presentation of recent experiments.

In 1981, the age of the scanning probe microscopes (SPMs) began when Binnig, Rohrer, and cowokers developed the first scanning tunneling microscope (STM) [1]. Their setup was based on measuring an electrical tunneling current between a sharp metal tip and a conducting sample. For the first time, a sample surface could be imaged with true atomic resolution in real space. The STM launched the development of several other types of SPMs. In general, these microscopes consist of a small, submicrometer probe, which senses a certain physical interaction with the sample and which is scanned over the sample to generate an image. For example, Pohl et al. invented the scanning near-field optical microscope (SNOM) in 1984 [2], which uses an evanescent electromagnetic field in the subwavelength range to image the sample. In 1986, Binnig and co-workers developed the atomic force microscope (AFM), which is based on measuring the mechanical forces between a sharp tip and the sample [3]. The AFM is not limited to conducting or transparent samples and has become one of the most important tools in nanoscale science. The AFM also works in aqueous environments, such as buffer solutions and so is well suited for biological samples [4]. Since then, several related SPMs have been developed, such as the magnetic force microscope [5,6] the electrical force microscope [7], and the scanning electrochemical force microscope (SECM) [8].

Electromechanical and mechanoelectrical transduction (MEM) by membranes are general properties and that have probably been utilized by evolution in the design of sensors. To obtain a high-resolution record of MEM with a direct recording of movement, we combined a voltage-clamp and an atomic force microscope (VC-AFM). We measured the voltage-induced movement of membranes of wild-type cultured human embryonic kidney cells (wtHEK) and those transfected with voltage-gated potassium selective channels of the Shaker family (ShHEK). The presence of the channels introduced a striking asymmetry into the MEM that probably reflects shape changes during gating.

Membranes of wtHEK cells move outward, linearly with voltage, in the physiologic voltage range and the change in force also scaled linearly with the set force. However, when HEK cells were transfected with Shaker channels (ShHEK), the membranes showed a pronounced saturation of MEM in the voltage range of channel opening. The saturation was not caused by ion fluxes because saturation was independent of the current. The onset of saturation was correlated with channel opening and not voltage sensing. We tested this dependence with the IL mutant where voltage sensing and channel opening occurs at distinctly different voltages. This nonlinearity changed with time and relaxed to background MEM after about 20 msec. There proved to be few models of the saturation that could incorporate all the constraints posed by the data, notably a channels density of ∼ 1 per μm2, but we could explain the data if channel opening splays the interior half of the channel and generates torque that causes buckling of the local membrane.

These experiments demonstrate the feasibility of VC-AFM for time resolved measurement of MEM in biological systems at atomic resolution (∼ 1 kHz & and 0.1 nm) primarily through the use of ensemble averaging. We discuss some of the experimental problems and their solutions, and provide guidance for advancing the technique.

Nonvolatile bias-controlled polarization states in ferroelectric materials offer unique opportunities for information technology and data storage applications. The ability to probe ferroelectric properties at the nanoscale by piezoresponse force microscopy (PFM) has enabled fundamental studies of polarization dynamics and the role of defects and disorder on domain nucleation and wall motion and has led to advances in the design and implementation of such applications. This has resulted in the development of fast spectroscopic modes to collect polarization switching data from every point in an image. The emergence of fast, configurable data processing electronics has prompted the development of dynamic and nonsinusoidal data acquisition methods for PFM. Further, the recent synergy of spectroscopic and dynamic modes has necessitated the development of multivariate data analysis and processing in PFM. These recent advances in the applications of PFM for imaging and spectroscopy of the ferroelectric switching processes will be discussed.

A physical principle of most of ferroelectric-based devices is electrically induced polarization reversal, which on a microscopic level occurs via the nucleation and growth of a large number of domains. The dynamic characteristics of domain growth as well as static properties of domain structure to a large extent determine the ferroelectric device performance. Recent advances in the synthesis and fabrication of micro- and nanoscale ferroelectric structures [1–4] make it imperative to understand the domain switching behavior at this scale. A major limitation in acquiring this crucial information is the lack of experimental methods to characterize the domain kinetics with the nanometer length and nanosecond time resolution. The most effective approach to visualization of domain kinetics is based on linear coupling between ferroelectric and piezoelectric parameters, which on the experimental level can be detected either by X-ray scattering or by scanning force microscopy. High-resolution studies using time-resolved X-ray microdiffraction imaging [5–7] have demonstrated reproducible switching behavior of polarization from cycle to cycle and allowed direct measurements of domain wall velocity at high electric fields.