Photochromic Dithienylethene Monolayer-Modified Gold Nanoparticles as a Tunable Floating Gate in the Fabrication of Nonvolatile Organic Memory

Nonvolatile memory (NVM) devices were fabricated by implanting a self-assembled monolayer (SAM) of functional dithienylethene (DTE) derivative on the gold nanoparticle (Au-NP) surface in a pentacene-based organic transistor. The Au-NPs and DTE served as a charge-trapping medium and tunneling barrier layer, respectively. The transfer characteristic of the NVM device showed a narrow hysteresis window and wide memory window, indicating that the DTE-SAM served as a variable barrier layer to regulate the trapping and detrapping of external free charges at the Au-NPs. The energy gap introduced by the DTE-SAM is modulated through photoisomerization between a ring-open form and a ring-closed form by absorbing UV or visible light. For a memory device, the ring-closed DTE allows more free charge injection into the trapping sites, and the ring-open one better retains the trapped charges. A longer anchoring alkanethiol chain at the DTE moiety can further extend the device's retention time. For the NVM operation, programming with the ring-closed DTE and then switching the DTE structure to the ring-open form for erasing can facilitate the charge trapping and charge retention with the same molecule compared to operating all in the ring-open form or all in the ring-closed form of DTE. The structural characterization and electronic characteristics of these devices are discussed in detail.