move along the long axis of the nanotube and exhibits 1D translational
dimensionality. Similarly, rotational dimensionality, or rotational degrees
of freedom, quantifies the extent to which a molecule or nanostructure
may change the direction in which it is oriented. For example, a molecule
bound to a surface or confined between two surfaces is only able to rotate
within a plane and exhibits 2D rotational dimensionality, with the latter
also exhibiting 2D translational dimensionality. The concept of 1D rotational dimensionality is less intuitive; it refers to a structure that may only
be pointing one of two directions along a single axis (e.g., up or down).
Finally, systems have vibrational degrees of freedom, which can store
significant amounts of energy and which can be examined using techniques such as infrared and Raman spectroscopy.
1.5 SELF-ASSEMBLY
In general, there are two strategies for designing nanoscale systems. A
“top-down” approach describes constructing a material of interest by
breaking down a larger structure in an organized manner or by using
larger tools to organize molecules. Nanoscale materials can be carved into
shape by physical methods such as lithography or placing molecules on a
surface using the tip of an atomic force microscope (AFM, discussed in
Chapter 8) or similar instrument. In contrast, a “bottom-up” approach
describes forming a nanosystem from smaller subunits (e.g., molecules in
solution), driven by the physical and chemical properties of the subunits
themselves. While nanomaterials can be formed by both top-down and
bottom-up approaches, nanotechnology more often refers to being able
to synthesize and manipulate materials using a bottom-up approach. The
bottom-up approach may take advantage of specific chemical reactions
or may involve intermolecular interactions between molecular fragments.
0D
1D
2D
3D
Figure 1.4 Examples of nanomaterials of different spatial
dimensionalities, including a
fullerene (0D), a carbon nanotube (1D), a sheet of graphene
(2D), and a nanoparticle lattice (3D). The gray rectangles
extending beyond the system
indicate the directions in which
the system repeats to form an
extended structure.
SELF-ASSEMBLY
9
dimensionality. Similarly, rotational dimensionality, or rotational degrees
of freedom, quantifies the extent to which a molecule or nanostructure
may change the direction in which it is oriented. For example, a molecule
bound to a surface or confined between two surfaces is only able to rotate
within a plane and exhibits 2D rotational dimensionality, with the latter
also exhibiting 2D translational dimensionality. The concept of 1D rotational dimensionality is less intuitive; it refers to a structure that may only
be pointing one of two directions along a single axis (e.g., up or down).
Finally, systems have vibrational degrees of freedom, which can store
significant amounts of energy and which can be examined using techniques such as infrared and Raman spectroscopy.
1.5 SELF-ASSEMBLY
In general, there are two strategies for designing nanoscale systems. A
“top-down” approach describes constructing a material of interest by
breaking down a larger structure in an organized manner or by using
larger tools to organize molecules. Nanoscale materials can be carved into
shape by physical methods such as lithography or placing molecules on a
surface using the tip of an atomic force microscope (AFM, discussed in
Chapter 8) or similar instrument. In contrast, a “bottom-up” approach
describes forming a nanosystem from smaller subunits (e.g., molecules in
solution), driven by the physical and chemical properties of the subunits
themselves. While nanomaterials can be formed by both top-down and
bottom-up approaches, nanotechnology more often refers to being able
to synthesize and manipulate materials using a bottom-up approach. The
bottom-up approach may take advantage of specific chemical reactions
or may involve intermolecular interactions between molecular fragments.
0D
1D
2D
3D
Figure 1.4 Examples of nanomaterials of different spatial
dimensionalities, including a
fullerene (0D), a carbon nanotube (1D), a sheet of graphene
(2D), and a nanoparticle lattice (3D). The gray rectangles
extending beyond the system
indicate the directions in which
the system repeats to form an
extended structure.
SELF-ASSEMBLY
9