Page 48 - Nanotechnology an introduction

Chapter Contents

5.1 Topography 75
5.1.1 Contact Methods 75
5.1.2 Noncontact (Optical) Methods 79
5.2 Chemical Surface Structure (Chemography) 82
5.3 The Metrology of Self-Assembly 84
5.4 The Representation of Texture 85
5.4.1 Roughness 86
5.4.2 One-Dimensional Texture 87
Markov Chains 88
Runs 89
Genetic Sequences 90
Algorithmic Information Content 90
Effective Complexity 91
5.4.3 Two-Dimensional Texture: Lacunarity 91
5.5 Metrology of the Nano/Bio Interface 92
5.5.1 Determining the Nanostructure of Protein Coronas 92
5.5.2 Measuring Cell Adhesion: the Interaction of an Evanescent Field with a Cell 93
5.5.3 Optical Measurement Schemes 95
5.5.4 Reverse Waveguides 96
5.5.5 The Interpretation of Effective Refractive Index Changes 97
5.5.6 The Interpretation of Coupling Peak Width Changes 98
5.6 Summary 99
5.7 Further Reading 100
Metrology at the nanoscale imposes stringent requirements of accuracy and precision. A particular challenge is the fact that the size of the measuring instrument and the feature being measured become comparable. Should the nanoscale
feature be absolutely small in the quantum sense, then its state is destroyed by the measurement. Approaches can be categorized as imaging or nonimaging, and within each category there are methods that can be categorized as contact or
noncontact. As one approaches the nanoscale, however, the distinction becomes harder to make. One can distinguish between topographical and chemical features; some techniques yield both. Techniques able to make time-resolved
measurements in situ are very useful for monitoring actual processes of assembly and operation. The ultimate goal of nanometrology is to provide a list of the coordinates and identities of all the constituent atoms of a nanoscale structure,
but this creates vast datasets. Therefore, there is considerable interest in capturing the salient features of topography and chemistry by identifying regularities, possibly statistically. The nano/bio interface presents great challenges to
nanometrology. Instrumentation able to combine two or more techniques, enabling the simultaneous observation of multiple parameters on the same sample, is beneficial.
Keywords: contact, noncontact, topography, chemography, texture, roughness, lacunarity, cell adhesion, ultramicroscopy, scanning probes
The ultimate goal of nanometrology is to provide the coordinates and identity of every constituent atom in a nano-object, nanostructured material or
nanodevice. This goal raises a number of problems, not least in the actual representation and storage of the data. Thanks to techniques such as X-
ray diffraction (XRD), one can readily determine atomic spacings to a resolution of the order of 0.1 nm and this information, together with the
external dimensions of an object, effectively achieves our goal provided the substance from which the object is made is large, monolithic and
regular. This achievement must, however, be considered as only the beginning since nanomaterials (and nanodevices considered as complex
nanomaterials) may be fabricated from a multiplicity of substances, each one present as a domain of unique and irregular shape. Techniques such
as XRD require averaging over a considerable volume to achieve an adequate signal to noise ratio for the finest resolution—hence the stipulations
that the object being examined be large and regular. As an illustration, consider the challenge of reverse engineering a very large-scale integrated
circuit on a “chip”—bearing in mind that this is essentially a two-dimensional structure. X-ray diffraction of the chip could not yield useful information
for the purpose of making an exact replica of the circuit. The challenge of nanometrology is to achieve atomic scale resolution for such arbitrary
structures (the chip is not of course truly arbitrary—the structures form a functional circuit; but knowledge of the function alone would not suffice to
help one reconstruct the details, not least because there are likely to be several hardware routes to achieving comparable function).
Any successful manufacturing technology requires appropriate metrology, and the atomic-scale precision implicit in nanotechnology places new
demands on measuring instruments. Reciprocally, the development of the quintessential nanometrology tool, namely the scanning probe
microscope (better called an ultramicroscope or nanoscope, see Appendix, p. 248-9) has powerfully boosted nanotechnology itself, since these
instruments have become the backbone of efforts to develop bottom-to-bottom manufacturing procedures (Section 8.3). The focus of this chapter is
the nanometrology of surfaces. For more general issues regarding ultraprecision measurement instrumentation, the reader may refer to the recent
book by Leach (see Section 5.7).

Morphology and chemistry are not independent at the nanoscale. Depending on how it is cut, even the planar face of a crystal of a binary compound
MX can vary dramatically from pure M to pure X. “Roughness” or texture at the nanoscale may actually be constituted from an intricate array of
different crystal facets. The chemical effect of this morphology depends on the characteristic length scale of the phenomenon being investigated.
Living cells, for example, are known to be highly sensitive to the crystallographic orientation of a substratum. This has been demonstrated by cell
growth experiments on single crystals: epithelial cells attached themselves and spread only on the (011) faces of calcium carbonate tetrahydrate
and not on the (101) faces within tens of minutes following initial contact, but after 72 hours all cells on the (011) faces were dead, but well-spread
and living on the (101) faces [70]. However, it should be noted that these two faces mainly differ in the surface distribution of the lattice water
molecules, to which the living cell may be especially sensitive. Note that cells actively secrete extracellular matrix (ECM) proteins when in contact
with a substratum, which are then interposed to form a layer between the cell and the original substratum material; hence the observed effects could
have been due to the different conformations adopted by these ECM proteins due to the different chemistries and morphologies of the different