17.4
Chemical Reactions Induced by Confined Electrons
In the examples described above single chemical bonds of reactants and products
are activated, dissociated and formed by interacting with the electronic states of
confined valence electrons of nanoscale systems such as point defects, free or supported atoms or clusters. In a completely different approach single chemical bonds
are manipulated by nanoscale devices. Such a device can be a scanning tunneling
microscope (STM) where tunneling electrons resonate with single electronic states
of molecules to induce chemistry by making and breaking bonds [78]. These two
methods for doing chemistry on the nanoscale differ completely. In the first example confinement is achieved by the system (point defects, atoms, clusters) itself
and in the best case it may act as a real nanocatalyst. The second method uses a
nanoscale device, which can be applied to non-localized systems to induce chemistry on the nanoscale.
The use of nanoscale devices for inducing chemical reactions was first introduced by the group of Somorjai in 1994, when a platinum–rhodium tip of a STM
operating inside an atmospheric-pressure chemical reactor cell was used to locally
rehydrogenate carbonaceous fragments deposited on the (111) surface of platinum
[79]. Recently the groups of Rieder (FU-Berlin) [80] and Ho (Cornell) [81] used a
tip of an STM as a local electron source to induce bond-selected chemistry with a
spatial resolution on the atomic scale (@1 A ˚ ). With the same device the induced
changes can be imaged with similar spatial resolution. The sharp metal tip of an
STM is held in close proximity to a conducting surface. The tip is then positioned
above a selected bond of a reactant molecule and by applying different bias voltages
the energy of the tunneling electrons is tuned to resonate with single bonds. At a
tip–surface distance of about 6 A ˚ this process is dominant and the overlap of the
wavefunctions of the tip and the reactant molecule is negligible. In such a way a
chemically bound CO molecule on Cu(111) could e.g. be transferred to and from
the tip reversibly [82] or in another example tunneling-electron-stimulated desorption of H atoms from a Si(100)–2 Â 1 surface could be successfully used for nanopatterning [83]. They observed that for localized and controlled reactions the energies of the tunneling electrons ought to be below 1 eV (for bias voltages below
1 V). For example, the dissociation of a single O 2 molecule on Pt(111) is confined to
the molecule if the bias is less than 0.5 V [84]. However, when the bias is increased
to 1.0 V, molecules as far away as 100 A ˚ are also dissociated, even though the electrons are confined to atomic dimensions. The detailed mechanism for this nonlocal effect is not yet understood. In general, they conclude that for bias voltages
larger than 2.5 V non-local chemistry is dominant, whereas for voltages up to about
1–2 V local, single-molecule chemistry takes place.
The Cornell group studied the local dissociation of a chemisorbed, single O 2
molecule on Pt(111) in great detail. Obviously this system is of relevance in catalysis. It is known that oxygen adsorbs molecularly on Pt(111) at temperatures below
100 K [85]. When the temperature is increased to about 150 K, some of the O 2
molecules desorb and the remaining molecules dissociate and recombine and de17 Nanocatalysis
582
Chemical Reactions Induced by Confined Electrons
In the examples described above single chemical bonds of reactants and products
are activated, dissociated and formed by interacting with the electronic states of
confined valence electrons of nanoscale systems such as point defects, free or supported atoms or clusters. In a completely different approach single chemical bonds
are manipulated by nanoscale devices. Such a device can be a scanning tunneling
microscope (STM) where tunneling electrons resonate with single electronic states
of molecules to induce chemistry by making and breaking bonds [78]. These two
methods for doing chemistry on the nanoscale differ completely. In the first example confinement is achieved by the system (point defects, atoms, clusters) itself
and in the best case it may act as a real nanocatalyst. The second method uses a
nanoscale device, which can be applied to non-localized systems to induce chemistry on the nanoscale.
The use of nanoscale devices for inducing chemical reactions was first introduced by the group of Somorjai in 1994, when a platinum–rhodium tip of a STM
operating inside an atmospheric-pressure chemical reactor cell was used to locally
rehydrogenate carbonaceous fragments deposited on the (111) surface of platinum
[79]. Recently the groups of Rieder (FU-Berlin) [80] and Ho (Cornell) [81] used a
tip of an STM as a local electron source to induce bond-selected chemistry with a
spatial resolution on the atomic scale (@1 A ˚ ). With the same device the induced
changes can be imaged with similar spatial resolution. The sharp metal tip of an
STM is held in close proximity to a conducting surface. The tip is then positioned
above a selected bond of a reactant molecule and by applying different bias voltages
the energy of the tunneling electrons is tuned to resonate with single bonds. At a
tip–surface distance of about 6 A ˚ this process is dominant and the overlap of the
wavefunctions of the tip and the reactant molecule is negligible. In such a way a
chemically bound CO molecule on Cu(111) could e.g. be transferred to and from
the tip reversibly [82] or in another example tunneling-electron-stimulated desorption of H atoms from a Si(100)–2 Â 1 surface could be successfully used for nanopatterning [83]. They observed that for localized and controlled reactions the energies of the tunneling electrons ought to be below 1 eV (for bias voltages below
1 V). For example, the dissociation of a single O 2 molecule on Pt(111) is confined to
the molecule if the bias is less than 0.5 V [84]. However, when the bias is increased
to 1.0 V, molecules as far away as 100 A ˚ are also dissociated, even though the electrons are confined to atomic dimensions. The detailed mechanism for this nonlocal effect is not yet understood. In general, they conclude that for bias voltages
larger than 2.5 V non-local chemistry is dominant, whereas for voltages up to about
1–2 V local, single-molecule chemistry takes place.
The Cornell group studied the local dissociation of a chemisorbed, single O 2
molecule on Pt(111) in great detail. Obviously this system is of relevance in catalysis. It is known that oxygen adsorbs molecularly on Pt(111) at temperatures below
100 K [85]. When the temperature is increased to about 150 K, some of the O 2
molecules desorb and the remaining molecules dissociate and recombine and de17 Nanocatalysis
582