Biswanath Mukherjee

Multilevel conductance and memory in ultrathin organic films

The present letter reports conductance switching in Langmuir–Blodgett films of an organic semiconductor. We have achieved multiple conducting levels in devices based on ultrathin films for increased density of memory bits in the same space. We have shown that multiple conducting levels in a device can be achieved by controlling the density of high-conducting molecules in the structures. We have observed one low- and three high-conducting states of the devices. All four states have associated memory for data-storage applications. Any of the four states, namely 00, 01, 10, and 11, can be “read” for several hours for read-only memory applications. We could “erase” a state, “write” another, and “read” the state for multibit random-access-memory applications.

A simply constructed lead phthalocyanine memory diode

Random-access memory switches were fabricated by using 70 nm thick spun films of nonperipherally octahexyl substituted lead phthalocyanine (PbPc) molecules sandwiched between indium tin oxide (ITO) substrates and vacuum-deposited aluminum (Al) top electrodes. The reproducible hysteresis behavior in terms of high and low conductance states was observed in the current-voltage characteristics recorded for the device at room temperature, and the on/off ratio for this single layered device was estimated to be as large as 103. The bistable electrical switching effects were attributed to the existence of the depletion layer at the ITO/PbPc and energetically exponential distribution in energy of traps in nonactive region of PbPc films.