Designing Devices with Organic-Based Materials

The various examples described above show that adapting organic molecules for electronic and spintronic applications can have many advantages. The potential for switches, transistors, and molecular magnets, can all be realized through the design and implementation of organic-based molecular devices. As may also be apparent, one of the largest challenges with molecular electronics/ spintronics and the implementation of these promising effects in

functional devices is actually fabricating devices with single/ few numbers of molecules. Many of the examples above describe thick organic films which can often utilize the same fabrication techniques – such as metal evaporation and sputtering to attach electrodes – as their inorganic counterparts. Adapting these processes by using metals such as copper or lead that have exceedingly low interactions with substrates helps mitigate metal penetration and subsequent device short circuiting.71 Despite this, pinholes and defects are common to certain systems, and metal penetration can lead to poor interfaces and, in some cases, erroneous results. Moreover, many of the effects offered by these organic based materials – switching events, spin

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polarization, and electronic/ spintronic coupling – have been seen or are expected to be most pronounced in monolayers, wires, or single/few number of molecule devices, where these defects, pinholes, and other fragilities will be most pronounced. Altogether, reproducing organic devices with inorganic fabrication processes is not always a perfect process. For both the efforts of scaling down device size and for seeing potentially more dramatic properties from out

systems, especially at the single molecule level, special consideration must be made in device fabrication in order to realize the effects, especially in a practical setting.

1.5.1 Techniques for Designing Molecular Electronic Devices

Figure 1.14 – Molecular Electronic Junctions

Several examples for forming direct contact to an organic monolayer, including A) break junctions (Reprinted with

permission from Ref. 72. Copyright 2004 American Chemical Society), B) conductive-atomic force microscopy

(cAFM) (Reprinted with permission from Ref. 73. Copyright 2011 American Chemical Society), C) gallium-indium

drop junctions (EGaIn) (Reprinted with permission from Ref. 74. Copyright 2014 American Chemical Society), D)

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Much effort has been devoted to designing molecular junctions to realize the unique electrical and magnetic properties of organic based molecules. A host of techniques such as break junctions,72 atomic force microscopy with conductive cantilevers,73 mercury-drop electrodes, and gallium-indium drop junctions (EGaIn)74 _{have been used to analytically study} organic systems, incorporating them into non-destructive electrical junctions and seeing the desired effects (Figure 1.14). However, these techniques are purely analytic and, at present, restricted in their possible use due to requiring either dangerous materials or highly specialized equipment for their implementation.

Figure 1.15 – Molecular Electronics Devices Prepared with Buffer Layers.

A) Devices incorporating solution processable PEDOT:PSS as a buffer layer. Reprinted by permission from

Macmillan Publishers Ltd: Nature, Ref. 75, Copyright 2006. B) Device architecture incorporating solution

processable reduced graphene oxide (rGO) for transparent electrodes.76 _{Copyright © 2013, Wiley-VCH Verlag}

GmbH & KGaA.

Techniques to get beyond these limitations have been pursued, a large subset involving depositing buffer layers onto organic thin films or monolayers. These buffers, which include PEDOT:PSS75 _{and single-sheet graphene}76 _{or graphene derivatives}77 _{can withstand many of the} fabrication techniques used in the semiconductor industry. Unfortunately, buffer layers introduce additional interfaces in these devices, which, as described previously, may limit the ability to

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interact metal and molecule directly at interfaces to exhibit unique effects in certain systems. For full realization of the effects organic materials can bring into the electronic/spintronic realm, the interface must be preserved and carefully tuned, and adding buffer layers only reduces the possibilities and benefits that exist from this exercise. Utilizing techniques that allow electrode deposition directly onto our systems without destroying them will offer the largest possible benefit toward dynamic single molecule systems.

1.5.2 Soft Lithography and Transfer Printing in Molecular Electronics

A recent focus for designing molecular electronic architectures with chemically bound metal contacts falls under the general title of soft lithography. The basic premise of these techniques involves some form of a transfer substrate, often times a polymer, that brings a metallic electrode into ‘soft’ contact (i.e. physical contact at ambient conditions) with an organic layer. The transfer layer may be sacrificial, as is typical in transferring plasma-enhanced

chemical vapor deposition (PECVD) grown graphene where a poly(methyl methacrylate) (PMMA) layer or other polymer layer is dissolved to expose the graphene,78 or it may be reusable in the case of a transfer process such as developed by John Rogers’ group.79 _{In either} case, the technique usually relies on standard lithographic patterning of metal contacts on robust films and materials – polymers or silicon wafers are typical – and transferring these metals and materials using soft lithographic techniques onto the more ‘fragile’ organic monolayers and films.

The advantage here is obvious – deposition of a metal contact directly onto an arbitrary substrate without fear of thermally penetrating metal into the film. Transfer printed methods have been shown to transfer features of various shapes and sizes onto organic monolayers, sustaining the films such that their charge transport properties could be measured.80 If used in conjunction with metal binding groups on the target substrate, a chemical bond can form the metal-molecule

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interface and allow for strong coupling that can potentially lead to strong electron (hole) injection and effects such as the “spinterface” effects described previously. It should be noted that there is evidence of size limitations and effects of even these soft lithographic processes due to the more frail nature and other properties of certain organic systems.81 _{This fact will motivate} studies throughout this work in analyzing the effects of different architectures and fabrication techniques on organic systems.