11.8 Studying Biological Systems at the Nanoscale
537
of symmetry, has 3N − 5 = 4 normal modes of vibration. If we take the z-axis
along the symmetry axis, there is a bending mode in the z-x plane at 667 cm −1 , a
second bending mode in the z − y plane also at 667 cm −1 , a symmetric stretch at
1333 cm −1 , and an antisymmetric stretch at: 2349 cm −1 .
Normal mode vibrations will be excited by resonant absorption of a photon which
happens to have a frequency at or near a vibrational frequency of the molecule,
provided the vibration generates a charge separation of some multipolar character in
the molecule, since photons are absorbed only by acceleration of charges within the
molecule. Moreover, in the photon absorption process, the total angular momentum
of the whole molecule, made up from orbital angular momentum and intrinsic spins
of the electrons and nuclei in the molecule, J = L + S, must change by at least one
unit of angular momentum, representing the intrinsic spin of the photon. Larger
changes in J are possible if the photon carries into the reaction orbital angular
momentum as well as spin. From the quantum theory of light, the photon spin is
either aligned with its direction of motion, or anti-aligned, corresponding to the two
possible helicity states of a photon.
11.8.2 Scanning Probe Microscopy
Scanning Tunneling Microscopy Pictures of individual molecules and atoms have
been generated by using quantum tunneling. A sharp conducting needle is carried
back and forth over the surface of an atomically smooth surface on which the
molecules have been laid. The needle is charge relative to the surface. When the
needle comes close to a molecule, electrons tunnel across the gap. This small current
is used to control the height of the needle, and to monitor the topology of the
surface being explored. The system forms a scanning tunneling microscope. Lateral
resolutions the diameter of hydrogen atoms have been achieved. Depth resolutions
can be one tenth of a hydrogen atom diameter.
Scanning Atomic Force Microscope An atomic force microscopy uses a needle
placed on the end of a flexible small-mass cantilever. The tip of the needle
(the ‘probe’) can be sharpened to about 100 nm. The force between the needle
and a surface deflects the cantilever. The displacement of the cantilever is often
determined by the deflection of a laser beam from a mirror on the end of the
cantilever. To control the needle distance from the surface being scanned, there is a
feedback system to adjust this distance according to the measured force. Scanning
is usually done by mounting the sample on a piezoelectric material which can be
moved in the plane of the sample surface by a change in the electric field in these
directions.
This kind of microscope can show individual strands of DNA lying on the surface
of a selected substrate. (See Fig. 11.4.)
A variant is the Near-Field Scanning Microwave Microscope, which can probe
the electric properties of flat materials at the nanoscale. The microwave signal
537
of symmetry, has 3N − 5 = 4 normal modes of vibration. If we take the z-axis
along the symmetry axis, there is a bending mode in the z-x plane at 667 cm −1 , a
second bending mode in the z − y plane also at 667 cm −1 , a symmetric stretch at
1333 cm −1 , and an antisymmetric stretch at: 2349 cm −1 .
Normal mode vibrations will be excited by resonant absorption of a photon which
happens to have a frequency at or near a vibrational frequency of the molecule,
provided the vibration generates a charge separation of some multipolar character in
the molecule, since photons are absorbed only by acceleration of charges within the
molecule. Moreover, in the photon absorption process, the total angular momentum
of the whole molecule, made up from orbital angular momentum and intrinsic spins
of the electrons and nuclei in the molecule, J = L + S, must change by at least one
unit of angular momentum, representing the intrinsic spin of the photon. Larger
changes in J are possible if the photon carries into the reaction orbital angular
momentum as well as spin. From the quantum theory of light, the photon spin is
either aligned with its direction of motion, or anti-aligned, corresponding to the two
possible helicity states of a photon.
11.8.2 Scanning Probe Microscopy
Scanning Tunneling Microscopy Pictures of individual molecules and atoms have
been generated by using quantum tunneling. A sharp conducting needle is carried
back and forth over the surface of an atomically smooth surface on which the
molecules have been laid. The needle is charge relative to the surface. When the
needle comes close to a molecule, electrons tunnel across the gap. This small current
is used to control the height of the needle, and to monitor the topology of the
surface being explored. The system forms a scanning tunneling microscope. Lateral
resolutions the diameter of hydrogen atoms have been achieved. Depth resolutions
can be one tenth of a hydrogen atom diameter.
Scanning Atomic Force Microscope An atomic force microscopy uses a needle
placed on the end of a flexible small-mass cantilever. The tip of the needle
(the ‘probe’) can be sharpened to about 100 nm. The force between the needle
and a surface deflects the cantilever. The displacement of the cantilever is often
determined by the deflection of a laser beam from a mirror on the end of the
cantilever. To control the needle distance from the surface being scanned, there is a
feedback system to adjust this distance according to the measured force. Scanning
is usually done by mounting the sample on a piezoelectric material which can be
moved in the plane of the sample surface by a change in the electric field in these
directions.
This kind of microscope can show individual strands of DNA lying on the surface
of a selected substrate. (See Fig. 11.4.)
A variant is the Near-Field Scanning Microwave Microscope, which can probe
the electric properties of flat materials at the nanoscale. The microwave signal