MOLECULAR COMPUTING: INTEGRATION OF MOLECULES FOR NANOCOMPUTING - Bio-Inspired and Nanoscale Integrated Computing - page 302

Au

1.2

I peak = 1.03 nA

1.0

O 2 N

0.8

NH 2

0.6

0.4

0.2

I valley = 1 pA

T= 60 K

S

0.0

Au

0.0 0.5 1.0 1.5 2.0 2.5

Voltage (V)

Figure 11.3. I(V) characteristics of an Au-(nitroaniline OPV)-Au device at 60K in
the nanopore.

dropped across a resistor, sent to a comparator, inverted, and gated with the
''read'' pulse. The upper trace shown in Figure 11.5 is an input waveform applied,
and the lower is the mDRAM cell output. The first positive pulse configures the
state of the cell by writing a bit, and the second and third positive pulses read the
cell. The third pulse (and subsequent read pulses, not shown here for simplicity)
demonstrates that the cell is robust and continues to hold the state (up to the limit
of the bit retention time). The negative pulse erases the bit, resetting the cell. The
second set of four pulses repeats this pattern, and many months of continuous
operation have been observed with no degradation in performance [25].
This memory can be rationalized based upon conduction channels that
change upon charge injection as studied by density functional theory (DFT)
[26, 27]. These DFT studies further corroborate with the experimental results in
that the unsubstituted OPE and the amine-substituted OPE would be inactive as
devices (having linear I(V) curves) while nitroaniline and nitro OPE would both
have switching states (exhibited by sharp nonlinear I(V) characteristics) due to the
accepting of electrons during voltage application. Furthermore, the DFT calcula-
tions showed that nitroaniline would need to receive one electron in order to
become conductive whereas the nitro OPE would be initially conductive (''on'' in
the mDRAM) and then become less conductive, ''off '', upon receipt of one
electron [26, 27]. This is precisely the effect observed in the experiment [25].
Stoddart and Heath have synthesized molecular devices that would bridge the
crossed nanowires and act as switches in memory and logic devices [28, 29]. The
UCLA researchers have synthesized catenanes and rotaxanes [30] that can be
switched between states using redox chemistry. For instance, Langmuir-Blodgett
(LB) films were formed from the catenane, and the monolayers were deposited on
polysilicon nanowires etched onto a silicon wafer through photolithography. A
second set of orthogonal titanium nanowires were deposited through a shadow