Page 8 - Molecules for Charge-Based Information Storage

Molecules for Charge-Based Information Storage Lindsey and Bocian

adsorption geometry is primarily controlled by properties
intrinsic to the porphyrin macrocycle rather than by proper-
ties of the tether or the surface.
The read/write speed of a memory cell and the rate at
which charge dissipates in the absence of applied potential
are important characteristics that affect overall device per-
formance. Consequently, we examined the redox kinetics of
a large number of porphyrin-containing monolayers on
various metal and semiconductor surfaces. 3,15,16,30,32,35
The measurements probed both the rate of electron-transfer
0
(k ) for oxidation (in the presence of applied potential) and
the rate of charge dissipation after the applied potential is
disconnected (characterized by a charge-retention half-life
0
(t 1/2 )). The studies revealed that the k values for all the
5 1
4
porphyrin monolayers are in the range of 10 10 s . The
exact rate depends on the length of the tether and the extent
of electronic communication between the porphyrin and the
tether;the shorter the tether and greater the electronic
communication, the faster the rate. The trends observed in
the k 0 values are paralleled in the t 1/2 values, that is,
monolayers that exhibit relatively faster electron-transfer
rates also exhibit faster charge-dissipation rates (shorter t 1/2
values). However, the charge-dissipation rates (no applied
potential) are approximately 6 orders of magnitude slower
than the electron-transfer rates (applied potential). Both the
0
k and t 1/2 values for the porphyrin monolayers are sensitive
to the surface coverage of the molecules. The rates for both
processes decrease as the monolayers become more den-
sely packed. This behavior was shown to be due to exclusion
of solvent/counterions or spacecharge effects. 47 The effect
of surface coverage on rates overshadows differences that
result from differences in linker length and type.
FIGURE 8. SEM images following polymerization of
A key aspect of the electron-transfer characteristics of the porphyrinethynes.
porphyrin monolayers is that the rates are in the 10
100 kHz regime. These rates are far slower than those of are in the range of tens of seconds to minutes. These times
modern DRAM chips, which lie in the gigahertz regime, thus are much longer than the tens of milliseconds charge-reten-
2
signaling a potentially serious limitation in the implementa- tion times exhibited by typical DRAM memory cells. Accord-
tion of porphyrin charge-storage elements in memory ingly, memory cells based on porphyrin charge storage
2
devices. However, other considerations suggest that the could be refreshed much less frequently than those in a
observed rates may not be as large a constraint as might be typical DRAM device.
0
implied. In particular, the k values are determined at the
standard redox potential of the monolayer; thus, minimal Integration of Molecules with Advanced
driving force is being applied to induce the redox event. Semiconductor Technology
Application of an overpotential is expected to increase the The integration of molecular charge-storage molecules with
rates exponentially; a 500 mV overpotential could theore- advanced semiconductor technology required meeting mul-
tically increase the rates to the gigahertz regime. tiple objectives, including achieving higher charge density
The most important aspect of the charge-retention char- and developing approaches for depositing a (metal) counter-
acteristics of the porphyrin monolayers is that the t 1/2 values electrode on top of the molecules. The first objective was

Vol. 44, No. 8 ’ 2011 ’ 638–650 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 645