Electronic characteristics of fluorene/TiO2 molecular heterojunctions

Abstract

The electronic properties of molecular junctions of the general type carbon/molecule/TiO2Au were examined as examples of "molecular heterojunctions" consisting of a molecular monolayer and a semiconducting oxide. Junctions containing fluorene bonded to pyrolyzed photoresist film (PPF) were compared to those containing Al2O3 instead of fluorene, and those with only the TiO2 layer. The responses to voltage sweep and pulse stimulation were strongly dependent on junction composition and temperature. A transient current response lasting a few milliseconds results from injection and trapping of electrons in the TiO2 layer, and occurred in all three junction types studied. Conduction in PPFTiO2Au junctions is consistent with space charge limited conduction at low voltage, then a sharp increase in current once the space charge fills all the traps. With fluorene present, there is a slower, persistent change in junction conductance which may be removed by a reverse polarity pulse. This "memory" effect is attributed to a redox process in the TiO2 which generates TiIII and/or TiII, which have much higher conductance than TiO2 due to the presence of conduction band electrons. The redox process amounts to "dynamic doping" of the TiO2 layer by the imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration.

Observation, Characterization and Modeling of Memristor Current Spikes

“…It has been suggested that these spikes are capacitance; however the timescale is too long. The spikes have been reported by other groups in their memristors (see for example, [14]), however they are usually overlooked or attributed to artefacts arising from the experimental set-up or not reported at all (many researchers only report the I − V curves to demonstrate that they have a memristor). However, the current spike is an equilibrating process that is responsible for the frequency dependence of the I − V curves.…”

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

“…Some techniques for achieving sub-nanometer substrate flatness include the Au deposition on a thiol primer on silicon [120] and the pyrolysis of photoresist in a reducing atmosphere, resulting in a substrate dubbed ''pyrolyzed photoresist film,'' or PPF. [121,122] Both substrate flatness and defect occurrence favor the use of small junction areas where possible. [59,60,63,66] Making a second, or top, contact between a conductor (usually metallic) and a molecular monolayer over an area covering at least thousands of molecules has proven challenging.…”

“…An obvious alternative is the use of bonding schemes that result in stronger, less labile surface bonds to provide a stable template for the junction. Examples include Si-C bonds resulting from alkene attachment to H-terminated silicon [84,145] and the use of aromatic diazonium reagents to result in SiÀC [66,67] and CÀC [10,72,121] bonds after electrochemical reduction of the diazonium ion. SiÀC and CÀC bond strengths are $4 eV, compared to $1.6 eV for AuÀS, making lateral motion of the molecules on the Si or C surface impossible at temperatures commonly experienced in top contact deposition.…”

“…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.''…”

The mechanism of electroforming of metal oxide memristive switches

“…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].…”