Switching between different conformers of a molecule: Multilevel memory elements

doi:10.1016/j.orgel.2007.07.008

Organic Electronics

Volume 9, Issue 1, February 2008, Pages 39-44

https://doi.org/10.1016/j.orgel.2007.07.008 Get rights and content

Abstract

We report voltage-driven electrical bistability in an organic semiconductor, namely Ponceau SS. Conductance switching to different levels or “multilevel switching” in devices based on thin-films is due to different density of high-conducting molecules. In a monolayer of Ponceau SS, we have observed one low-conducting and two high-conducting states. This is due to three configurable planes of the molecule exhibiting at least two stable high-conducting conformers. Apart from establishing conductance switching to be a molecular phenomenon, the multilevel conductance in a monolayer shows that a single molecule can exhibit multilevel memory application.

Introduction

With the success of organic semiconductors in thin-film transistors [1], light-emitting devices [2], sensors [3] and photovoltaic solar cells [4], the materials are being considered for electronic memory elements [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Apart from acting as switching elements in integrated-circuits, the conjugated organics are expected to offer high-density memory applications to meet the need of the future. During the last few years, several classes of organic materials exhibited such applications, which occurred due to electrical bistability. In general, a suitable voltage changes the conformer of a molecule [5], [15]. When both the conformers are stable with a large difference in their conductivities, the molecules exhibit electrical bistability. It is manifested as two current values at a voltage, with the preceding voltage pulse determining the conducting state. By controlling the density of high-conducting molecules in a device, multilevel conductivity has also been achieved in organic memory devices [13], [14].

In devices based on thin-films of organic materials with electrode metals having low electronegativity, the bistability sometimes arises due to reversible growth of metal filaments through redox reactions [16]. When individual molecules (or a 2D molecular layer) with noble metal electrodes exhibit electrical bistabilty, the phenomenon is explained in terms of conformational change [5] and/or electroreduction of the molecule [17]. The bistability in such cases is hence a molecular phenomenon. With a suitable material, such systems may yield multilevel conductivity for multibit memory storage. The chosen molecules will then have to possess several stable conformers. In this article, we introduce such a molecule, which, due to its multiplaner structure, yields multilevel conductivity with associated memory phenomenon in the molecular scale.

Section snippets

Experimental

The molecule for the present work is Ponceau SS (Acid red 150), which was purchased from Aldrich Chemical Co. Molecular structure of the molecule is shown in the inset of Fig. 1. Both monolayer and spin-cast thin-films of the material were deposited and characterized. Devices based on spun-cast films of Ponceau SS were fabricated on indium tin oxide (ITO) coated glass substrates (sheet resistance = 12 Ω/□). Ponceau SS in methanol (2 mg/ml) was spun at a speed of 1000 rpm resulting in a film

Conductance switching

Fig. 1 shows a typical I–V plot for a spun-cast film of Ponceau SS sandwiched between ITO and Al electrodes. Ponceau SS exhibits electrical bistability. The I–V characteristics depend on the voltage sweep direction. The magnitude of device current at a voltage is higher during the sweep from a positive voltage as compared to that from a negative one. In other words, a suitable positive bias induces a higher conducting state. The higher state is retained even when the bias is removed from the

Conclusions

In summary, we have shown that Ponceau SS molecules exhibit multilevel memory-switching property. When a monolayer of the molecule is characterized by STM, the molecules exhibit one low- and two high-conducting states. The three states arise due to different conformers of the molecules. Amplitude of voltage pulse determines the conformer or corresponding high-state of the molecule. The width of the pulse has little or no effect in the 1–1000 ms range. In thin-film based devices, amplitude of

Acknowledgments

B.C.D. acknowledges CSIR Junior Research Fellowship No. 09/080(0504)/2006-EMR-I, Roll No. 503982. The Department of Science and Technology, Government of India, financially supported the work through Ramanna Fellowship SR/S2/RFCMP-02/2005.

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Switching between different conformers of a molecule: Multilevel memory elements

Abstract

Introduction

Section snippets

Experimental

Conductance switching

Conclusions

Acknowledgments

Mater. Today

Science

J. Mater. Chem.

J. Mater. Res.

Science

Adv. Mater.

J. Phys. Chem. B

Jpn. J. Appl. Phys., Part 2