posted on 2024-01-04, 09:03authored byKarl Griffin, Gareth Redmond
In this work, the discovery of volatile memristive devices
that
exhibit analog resistive switching (RS) and synaptic emulation based
on squaraine materials is presented. Specifically, organic microtubes
(MTs) based on 2,4-bis[(4-(N,N-diisobutyl)-2-6-hydroxyphenyl]squaraine
(SQ) are prepared by evaporation-induced self-assembly (EISA). The
MTs are ca. 2 μm in diameter (aspect ratio: 10–130).
While powder X-ray diffraction data for MTs identify monoclinic and
orthorhombic polymorphs, optical data report the monoclinic phase
with energetic disorder. By favorable energetic alignment of the Au
work function with the SQ HOMO energy, unipolar (hole-only) symmetric
metal–insulator–metal devices are formed by EISA of
MT meshes on interdigitated electrodes. The DC I–V characteristics acquired exhibit pinched hysteretic I–V loops, indicative of memristive behavior. Analysis indicates Ohmic
transport at low bias with carrier extraction by thermionic emission.
At high bias, space-charge-limited conduction in the presence of traps
distributed in energy, enhanced by a Poole-Frenkel effect and with
carrier extraction by Fowler-Nordheim tunneling, is observed. These
data indicate purely electronic conduction. I–V hysteresis attenuates at smaller voltage windows, suggesting that
carrier trapping/detrapping underpins the hysteresis. By applying
triangular voltage waveforms, device conductance gradually increases
sweep-on-sweep, with wait-time-erase or voltage-erase options. Using
square waveforms, repeated erase-write-read of multiple distinct conductance
states is achieved. Such analog RS behavior is consistent with trap
filling/emptying effects. By waveform design, volatile conductance
states may also be written so that successive conductance states exhibit
identical current levels, indicating forgetting of previously written
states and mimicking the forgetting curve. Finally, advanced synaptic
functions, i.e., excitatory postsynaptic current, paired-pulse facilitation,
pulse-dependent plasticity, and a transition from short- to long-term
memory driven by post-tetanic potentiation, are demonstrated.