Chapter 1

Chemical Imaging Introduction

Freddy Adams and Carlo Barbante

Abstract

While as humans, we image the world by observing interactions of matter with light in a narrow wavelength range, technology has increased our ability to use a greatly expanded portion of the electromagnetic spectrum and higher-order interactions of light with matter. Imaging techniques provide chemical information about a subject, in some cases even detailed chemical composition. The purpose of this chapter is to provide the basic information common to the up-to-date methodologies for chemical imaging analysis. Different methods are then covered in more detail in the following chapters of this book.

Keywords

Analytical chemistry and nanoanalysis; Hyperspectral imaging; Multispectral imaging; Nanotechnology; Surface analysis

.sustained efforts are needed to facilitate understanding and manipulation of complex chemical structures and processes. Chemical imaging offers a means by which this can be accomplished by allowing the acquisition of direct, observable information about the nature of these chemistries. By linking technological advances in chemical imaging with a science-based approach to using these new capabilities, it is likely that fundamental breakthroughs in our understanding of basic chemical processes in biology, the environment, and human creations will be achieved.

Committee on Revealing Chemistry through Advanced Chemical Imaging, National Research Council (USA), Visualizing Chemistry: The Progress and Promise of Advanced Chemical Imaging (2006). Available from the National Academies Press at: http://www.nap.edu/catalog/11663.htm

While as humans, we image the world by observing interactions of matter with light in a narrow wavelength range, technology has increased our ability to use a greatly expanded portion of the electromagnetic spectrum and higher-order interactions of light with matter. Imaging techniques provide chemical information about a subject, in some cases even detailed chemical composition. The purpose of this chapter is to provide the basic information common to the up-to-date methodologies for chemical imaging analysis. Different methods are then covered in more detail in the following chapters of this book.

1.1. Introduction

Microscopic and submicroscopic imaging methods capable of identifying specific atoms and molecules have significantly improved our understanding of nature, technological objects and processes on the microscopic scale. Optical imaging techniques have revolutionised our ability to study the microscopic world. Simple optical microscopy methods have played a large role in scientific fields such as geology and cellular and molecular biology; unfortunately, they do not provide sufficient chemical specificity to describe structural and compositional identity of the material studied. They also have limitations in spatial resolution and conventionally cannot go beyond a given limit of lateral resolution, the diffraction limit of light or, more generally, electromagnetic radiation. In this book, we will show what kind of solutions were elaborated for both limitations, the specificity in chemical detection and the spatial one. Analytical techniques that provide elemental and molecular information with high spatial resolution are becoming increasingly important for the development of nanotechnology and nanoscience. The techniques available for the characterisation of the atomic and molecular composition and structure at the bulk level often fail when applied to the quantitative analysis of such materials. In this context, chemical imaging concerns the analytical technique that couples spatial and chemical characterisation. In general, four factors are of importance for the various techniques that fall in this realm: spatial resolution, spectral resolution, field of view (the area probed for analysis) and magnification. Generally speaking, chemical imaging instrumentation has three components: a radiation source to illuminate the sample, a spectrally selective element, and usually a detector or a detector array (the camera) to collect the images. Spectroscopy encompasses the study of the interaction between electromagnetic radiation and matter and is a vital component of nearly all chemical imaging systems. The large variety of available spectroscopic techniques enables the investigation at different length and timescales (Table 1.1). Spectroscopy is at the centre of modern research in chemical, physical and materials sciences. It is at the heart of most modern tools for structural and chemical characterisation and offers imaging tools for studying materials down to the nanoscale, in the range between 1 and 100 nm. In this introduction, we give an overview of the methods currently used for studying composition and structure at the nanoscale. The successful development of nanostructured materials involves the simultaneous use of several disciplines (chemistry, physics, materials science, computer science, mathematics, statistics and metrology) [1]. In addition, challenges in nanotechnology lie on a more practical level such as reducing costs, scaling up production and avoiding work in low-temperature, ultrahigh vacuum and dust-free conditions.

Table 1.1

Overview of the Electromagnetic Spectrum and Its Respective Molecular Effects

?E (kJ/mol) - 108 106 104 100 1 10-2 Spectral range Gamma-rays X-rays UV VIS Infrared Microwave EPR NMR The atomic order of many complex functional materials is usually limited to the nanoscale. Microstructures, electronics and nanotechnology are vast fields of research that are growing together as the size gap narrows. Current research developments, recent achievements in engineering and newly commercialised products indicate that there is an immense innovative potential for numerous future applications. Examples include nanoparticles, specific structures encapsulated in a porous host, and bulk crystals with intrinsic order in composition and structure on the nanoscale. For nanomaterials, there is often a close relation between the composition and structure on the one hand, and the properties and function on the other [2]. The properties of nanomaterials significantly depend on their three-dimensional (3-D) morphology (size, shape, surface topography) and on the heterogeneity of their composition. These parameters must be precisely correlated with the properties for the design and fabrication of new materials, the discovery of their quite often unexpected and unusual properties and their ensuing potential for applications [3]. Heterogeneity on the nanolevel also plays an important role in the natural world. For instance, in environmental samples, heterogeneity can result from numerous different processes. In the case of materials such as minerals, soils or sediments, heterogeneity is the result of diverse biogeochemical processes that operate over long time periods. In the case of biological samples, heterogeneity is the result of the development processes responsible for the differentiation of tissues or subcellular compartments. The overall result of all these processes is an increased system complexity, leading to inherent instability and lack of robustness. Irrespective of the mechanism driving heterogeneity, the result requires the acquisition of large data sets in order to be properly understood. Nanometrology requirements in industry are different from those in fundamental research. In research-oriented nanometrology, resolution is the most important feature, whereas in industrial nanometrology accuracy is given precedence over resolution. Various measurement techniques available today require a controlled environment such as in vacuum, vibration- and noise-free environments. Also, industrial nanometrology requires that the measurements be more quantitative, with a minimum number of parameters. The tools required to ensure traceability have not yet been developed. The methodologies generally used for traceability are miniaturisations of traditional metrology standard procedures, hence there is a need for establishing nanoscale standards. There is also a need to establish some kind of uncertainty estimation model. Traceability is one of the fundamental requirements for the manufacturing and assembly of products when multiple producers are involved. Because of the importance of nanotechnology in the future, programmes run by national standard agencies have been set up in their respective countries to establish national standards for nanometrology and nanotechnology. Many definitions of nanotechnology refer to the dimensions involved: according to the National Nanotechnology Initiative (NNI) in the United States,for instance, 'nanotechnology is the understanding and control of matter at dimensions between approximately 1 and 100 nanometres, where unique phenomena enable novel applications'. Often only one or two dimensions are in the nanoregime, as in quantum wells and nanowires, but sometimes all three dimensions are at nanoscale, as in quantum dots and nanocrystals. A challenge is to make every dimension as small as possible, as in nanoelectronics, but other times the aim is to make at least one dimension as large as possible, as in carbon...