Page 8 - Surface-Confined Assemblies and Polymers for Molecular Logic

Surface-Confined Assemblies and Polymers de Ruiter and van der Boom

FIGURE 13. Retention times of the absorbance band at λ = 630 nm of the PEDOT-coated ITO after applying a multipotential step with 3 s intervals. (A)
Quaternary memory and (B) quinary memory. Adapted with permission from ref 40. Copyright 2010 American Chemical Society.

oxidation states. Nevertheless, our SPMA is able to achieve
the same effect. In the absence of any inputs, the oxidation
state of the SPMA is preserved within a certain time period
and within predefined threshold values, and hence, no
continuously applied potential is needed to maintain the
current state. As indication, it takes ∼25 min for full conver-
sion from Os 3þ to Os 2þ , which can be extended by avoiding
trace amounts of H 2 O. 39 The observed retention times of the
1, 0, and 1 states of the ternary memory are 75, 110, and ¥
s, respectively. The electrical addressability is an improve-
ment over our chemical addressable binary memory (vide
supra) 32 and en route toward all solid-state systems. However,
FIGURE 14. Generalized memory circuit capable of storing up to N the maximum time (180 ms) it takes for the SPMA to change its
different states in a single setup. Adapted with permission from ref 40.
Copyright 2010 American Chemical Society. output from high-to-low or from low-to-high is slow. Although,
this propagation delay of the electrical addressable SPMAs has
applying a potential within the range of 0.601.30 V. The 3
been decreased by a factor of 2.0 10 compared with the
presence or absence of an applied potential is therefore
chemically addressable monolayers, a conventional logic gate
defined as a logical 1 or 0, respectively. Although modula-
has a propagation delay of nanoseconds or lower.
tion of the oxidation state is binary in nature, the absorbance
The observed dependence of the absorbance upon chan-
is a precise function of the applied voltage and can be used
ging the potential is represented by a sigmoidal shape.
to create multiple states (Figure 10A). However, the multiple
Differentiating the obtained function results in a normal
states are generated by the assembly as a whole, rather than
distribution centered on the E 1/2 of the electroactive materi-
by an individual molecule, which is binary.
al, which is expected (Figure 10B). Within this, the full-width
The SPMA was used to demonstrate binary and ternary
at half-maximum (fwhm) of the responsepotential char-
memory. 38 If the input potentials were chosen at 0.60 and
acteristics is a useful benchmark. The fwhm describes the
1.30 V, the assembly was cycled between its two oxida-
potential range in which the intended material is functional.
tion states (Os 2þ/3þ ) and binary memory was created
If the range is too narrow, a small change in the potential
(Figure 11A). In contrast, if a third input potential at 0.91 V leads to a large optical change. This is undesirable, since it
is introduced, an intermediate state is accessible in which the introduces errors, making it difficult to differentiate between
assembly is not fully oxidized or reduced and the assembly is states. In contrast, a large fwhm makes each state easy to
of mixed valency (Figure 11B). In this way, three accessible distinguish, although the potential range might be too large
states are generated that allow the formation of ternary for practical applications. The SPMA has a relatively small
memory. A ternary device operating with individually ad- fwhm of 0.17 V. In combination with the short retention times
dressable redox-active complexes would require three of the assembly, we were able to generate dynamic random

570 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 563–573 ’ 2011 ’ Vol. 44, No. 8