Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 43

Fig. 10. (a) Three dots, two of which look darker indicating greater negative charge
localization. (b) Upon adding a fourth dot in the upper right region the previously
sites which previously looked dark become relatively light in appearance. This is due
to the electrons attaining a lower energy configuration along the newly available longer
diagonal.
but not so close as to be tunnel coupled and a direct participant in the molecule.
It is clear that the perturbation creates an analog of a heteronuclear diatomic
molecule. The bond is polarized, it has ionic character. The perturbation con-
trollably positions the electron. This is a key result. It shows that the type of
coupling required between QCA cells, and between an electrode and a QCA cell
is possible by this new approach. And all of this is possible at room temperature
with an all silicon system.
Figure 12 shows the result of placing two perturbing electrons along one diag-
onal to place a 4 dot, 2 electron cell into one polarized binary state [ 14 ]. It is
stressed again that this result was obtained at room temperature.
One could wonder about ways to increase the chemical stability of such com-
plex QCA structures against environmental damage. As already discussed, for
certain species such as styrene, there are known stable attachment mechanisms
that require only a single DB. Such reactions can lead to molecular line growth
on the surface. However the class of molecules that undergo such a process
is rare. A passivating layer can be formed of molecules containing functional
groups known not to react with a single DB. A yet broader class of molecules
chemisorbs when two closely spaced DBs are provided. However, in order to
cooperatively react with an incoming molecule, two DBs must be immediately
adjacent, that is, co-located on the same underlying silicon dimer unit. Such DBs
are separated by approximately 2.3 A. After reaction initiates at one of the two
DBs, the second DB is just within reach by a C atom centered radical, allowing a
second Si-C bond to form and resulting in a stable species and DB annihilation.
When a second DB is not within reach, the single bonded species very quickly
releases its grip because the interaction strength is only of order 0.1 eV. The DB
in that circumstance is left unchanged. There is evidence of reactivity involving
paired DBs separated by 3.84 A. The particular molecular functions able to form
such a bond are readily avoided. In any case, none of the QCA patterns to be