Importance of Oxides in Carbon/Molecule/Metal Molecular Junctions with Titanium and Copper Top Contacts

McGovern, William R.; Anariba, Franklin; McCreery, Richard L.

doi:10.1149/1.1888369

Cited by 38 publications

(84 citation statements)

References 34 publications

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“…In addition, Ti can oxidize during deposition, resulting in a molecule/TiOx/Ti (TiOx ¼ titanium oxide) structure with different properties from that of a molecule/metal device. [126,127] While TiO 2 has proven to be a useful component of molecular junctions used for memory applications, [10,74,121,128,129] any user of Ti for fabrication should be aware of possible artifacts from the high reactivity of vapor-deposited Ti atoms. Several other metals have been vapor-deposited on molecular mono-and multilayers, including Au, Ag, Al, and Cu, and in many cases the metal penetrates the monolayer to form metallic ''shorts.''…”

Section: Progress With Ensemble Molecular Junctionsmentioning

confidence: 99%

“…[79,169] In addition, theory showed that the high and low conductance states of the rotaxane devices were consistent with the predictions of density functional theory. [170] Although the rotaxane junctions showed great promise as molecular memory devices, the mechanistic picture was convoluted by subsequent reports from other laboratories that demonstrated switching behavior for similar devices containing non-redox-active molecules, [171] and for molecular junctions known to contain titanium oxide [74,121,126] (titanium was used as an adhesion layer in the rotaxane devices, and partial oxidation of the Ti during evaporation is likely at the backpressures typically used in metal deposition). An example of conductance switching in a molecule/TiO 2 junction is shown in Figure 18.…”

Section: Progress With Conductance Switchingmentioning

confidence: 99%

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Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

2009

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Molecular electronics seeks to incorporate molecular components as functional elements in electronic devices. There are numerous strategies reported to date for the fabrication, design, and characterization of such devices, but a broadly accepted example showing structure-dependent conductance behavior has not yet emerged. This progress report focuses on experimental methods for making both single-molecule and ensemble molecular junctions, and highlights key results from these efforts. Based on some general objectives of the field, particular experiments are presented to show progress in several important areas, and also to define those areas that still need attention. Some of the variable behavior of ostensibly similar junctions reported in the literature is attributable to differences in the way the junctions are fabricated. These differences are due, in part, to the multitude of methods for supporting the molecular layer on the substrate, including methods that utilize physical adsorption and covalent bonds, and to the numerous strategies for making top contacts. After discussing recent experimental progress in molecular electronics, an assessment of the current state of the field is presented, along with a proposed road map that can be used to assess progress in the future.

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Section: Progress With Ensemble Molecular Junctionsmentioning

confidence: 99%

Section: Progress With Conductance Switchingmentioning

confidence: 99%

Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

2009

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“…A simplified memristor device physics model [1] matched the experimental behavior [2] of nanocrossbar Pt/TiO 2 /Pt devices. Resistive switching in metal oxides has in fact seen more than four decades of scientific research [4][5][6][7][8][9][10][11][12][13][14][15], motivated in large part by that prospect of a fast and high density NVRAM technology [16][17][18][19][20][21][22][23]. The continuous resistance change exhibited by oxide switches also appears to meet analog switch requirements for neuromorphic computing [24][25][26].…”

Section: Introductionmentioning

confidence: 99%

The mechanism of electroforming of metal oxide memristive switches

et al. 2009

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Metal and semiconductor oxides are ubiquitous electronic materials. Normally insulating, oxides can change behavior under high electric fields-through 'electroforming' or 'breakdown'-critically affecting CMOS (complementary metal-oxide-semiconductor) logic, DRAM (dynamic random access memory) and flash memory, and tunnel barrier oxides. An initial irreversible electroforming process has been invariably required for obtaining metal oxide resistance switches, which may open urgently needed new avenues for advanced computer memory and logic circuits including ultra-dense non-volatile random access memory (NVRAM) and adaptive neuromorphic logic circuits. This electrical switching arises from the coupled motion of electrons and ions within the oxide material, as one of the first recognized examples of a memristor (memory-resistor) device, the fourth fundamental passive circuit element originally predicted in 1971 by Chua. A lack of device repeatability has limited technological implementation of oxide switches, however. Here we explain the nature of the oxide electroforming as an electro-reduction and vacancy creation process caused by high electric fields and enhanced by electrical Joule heating with direct experimental evidence. Oxygen vacancies are created and drift towards the cathode, forming localized conducting channels in the oxide. Simultaneously, O 2− ions drift towards the anode where they evolve O 2 gas, causing physical deformation of the junction. The problematic gas eruption and physical deformation are mitigated by shrinking to the nanoscale and controlling the electroforming voltage polarity. Better yet, electroforming problems can be largely eliminated by engineering the device structure to remove 'bulk' oxide effects in favor of interface-controlled electronic switching.

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“…Although each method has certain advantages and disadvantages, it is clear that the entire system must be considered in order to delineate the main factors influencing molecular conduction. Whereas many groups employ thiolate-based self-assembled monolayers (SAMs) on metallic substrates as a base system, we have taken an alternative approach using carbon electrodes (6,(14)(15)(16)(17)(18)(19). The foundation of this paradigm is a flat carbon electrode composed of pyrolyzed photoresist films (PPF) with covalently bonded nanoscopic molecular layers deposited using the electrochemical reduction of diazonium reagents.…”

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confidence: 99%

Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime

Sayed

Fereiro²,

Yan³

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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140

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Molecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting "contact." It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, we show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to determine the impact of a variety of electronic factors that act to determine the tunnel barrier.energy alignment | molecular electronics | electronic coupling | charge transport | Fermi-level pinning T he conductance of electrical charge through and across molecular entities is the basis of molecular and organic electronics (1, 2). Understanding, controlling, and designing electronic circuits using organic molecules as components is a major goal of molecular electronics (3); however, it has been a challenge to identify all of the factors that govern the conductance of a molecular junction. Rather than being a simple property of the molecule itself, many circumstances contribute to the measured electronic properties of the junction. Some of the important features include the nature of the molecule-contact bonding (4), the properties of the contact materials (5, 6), the orientation of the molecules relative to the contacts (7), and the structure of the molecule (5,8,9). Although there is no general consensus on exactly how each of these features affects the conductance of the junction, it is generally agreed that the alignment of the molecular and contact energy levels is an important factor (10-13). The offset between the substrate Fermi energy (E f ) and the molecular orbital closest in energy to E f is often used to estimate charge transport barriers in the context of tunneling or charge injection models; however, it is increasingly clear that the situation is complex and that there is no simple meth...

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Importance of Oxides in Carbon/Molecule/Metal Molecular Junctions with Titanium and Copper Top Contacts

Cited by 38 publications

References 34 publications

Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

The mechanism of electroforming of metal oxide memristive switches

Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime

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