Hylke B. Akkerman

Towards molecular electronics with large-area molecular junctions

Electronic transport through single molecules has been studied extensively by academic and industrial research groups. Discrete tunnel junctions, or molecular diodes, have been reported using scanning probes, break junctions, metallic crossbars and nanopores. For technological applications, molecular tunnel junctions must be reliable, stable and reproducible. The conductance per molecule, however, typically varies by many orders of magnitude. Self-assembled monolayers (SAMs) may offer a promising route to the fabrication of reliable devices, and charge transport through SAMs of alkanethiols within nanopores is well understood, with non-resonant tunnelling dominating the transport mechanism. Unfortunately, electrical shorts in SAMs are often formed upon vapour deposition of the top electrode, which limits the diameter of the nanopore diodes to about 45 nm. Here we demonstrate a method to manufacture molecular junctions with diameters up to 100 µm with high yields (> 95 per cent). The junctions show excellent stability and reproducibility, and the conductance per unit area is similar to that obtained for benchmark nanopore diodes. Our technique involves processing the molecular junctions in the holes of a lithographically patterned photoresist, and then inserting a conducting polymer interlayer between the SAM and the metal top electrode. This simple approach is potentially low-cost and could pave the way for practical molecular electronics.

Electrical conduction through single molecules and self-assembled monolayers

Although research on molecular electronics has drawn increasingly more attention in the last decade, the large spread in obtained results for the conduction rescaled to a single molecule indicates a strong dependence of the measured data on the experimental testbed used. We subdivided a generalized metal-molecule-metal junction into different components and discuss their influence on electrical transport measurements of a single organic molecule or an assembly of molecules. By relating the advantages and disadvantages of different experimental testbeds to the more general view of a molecular junction, we strive to explain the discrepancies between the obtained results on molecular conduction. The reported results on molecular conduction of molecules with an alkane backbone can be categorized into three groups with different resistance values, depending on the device area of the molecular junction and the nature of the contacts.

Electron tunneling through alkanedithiol self-assembled monolayers in large-area molecular junctions

The electrical transport through self-assembled monolayers of alkanedithiols was studied in large-area molecular junctions and described by the Simmons model [Simmons JG (1963) J Appl Phys 34:1793–1803 and 2581–2590] for tunneling through a practical barrier, i.e., a rectangular barrier with the image potential included. The strength of the image potential depends on the value of the dielectric constant. A value of 2.1 was determined from impedance measurements. The large and well defined areas of these molecular junctions allow for a simultaneous study of the capacitance and the tunneling current under operational conditions. Electrical transport for octanedithiol through tetradecanedithiol self-assembled monolayers up to 1 V can simultaneously be described by a single effective mass and a barrier height. There is no need for additional fit constants. The barrier heights are in the order of 4–5 eV and vary systematically with the length of the molecules. Irrespective of the length of the molecules, an effective mass of 0.28 was determined, which is in excellent agreement with theoretical predictions.

Upscaling, integration and electrical characterization of molecular junctions

The ultimate target of molecular electronics is to combine different types of functional molecules into integrated circuits, preferably through an autonomous self-assembly process. Charge transport through self-assembled monolayers has been investigated previously, but problems remain with reliability, stability and yield, preventing further progress in the integration of discrete molecular junctions. Here we present a technology to simultaneously fabricate over 20,000 molecular junctions-each consisting of a gold bottom electrode, a self-assembled alkanethiol monolayer, a conducting polymer layer and a gold top electrode-on a single 150-mm wafer. Their integration is demonstrated in strings where up to 200 junctions are connected in series with a yield of unity. The statistical analysis on these molecular junctions, for which the processing parameters were varied and the influence on the junction resistance was measured, allows for the tentative interpretation that the perpendicular electrical transport through these monolayer junctions is factorized.

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

We have studied the electrical characteristics of close-packed monolayers of conjugated para-phenylene oligomers as a function of molecular length in large-area molecular junctions. An exponential increase in resistance with molecular length is observed, Rexp (βL), with β=0.26±0.04 A-1 and β=0.20±0.06 A-1 for dithiol and monothiol derivatives, respectively. The decay coefficients are lower than previously determined experimentally using scanning probe or breakjunction techniques. We tentatively explain the low values by the forced planer geometry of the self-assembled molecules.

Solving the technology barriers in flexible AMOLED displays

In this paper, we present some of the technology challenges and process temperature trade-offs when realizing AM OLED displays on thin flexible plastic films that can be mechanically bent to a roll radius of ~1 cm. We furthermore present complementary approaches to realize low-power, high resolution OLED displays using self-aligned IGZO TFT architecture; a novel driving method using a compact 2T-1C pixel engine.

Molecular Electronics with Large-area Molecular Junctions

A technology is demonstrated to fabricate reliable metal-molecule-metal junctions with unprecedented device diameters up to 100 μm. The yield of these molecular junctions is close to unity. Preliminary stability investigations have shown a shelf life of years and no deterioration upon cycling. Key ingredients are the use of a conducting polymer layer (PEDOT:PSS) sandwiched between a bottom electrode with a self-assembled monolayer (SAM) and the top electrode to prevent electrical shorts, and processing in lithographically defined vertical interconnects (vias) to prevent both parasitic currents and interaction between the environment and the SAM [1]. Modeling the current–voltage ( I–V ) characteristics of alkanedithiols with the Simmons model showed that the low dielectric constant of the molecules in the junction results in a strong image potential that should be included in the tunneling model. Including image force effects, the tunneling model consistently describes the current-voltage characteristics of the molecular junctions up to 1 V bias for different molecule lengths [2]. Furthermore, we demonstrate a dependence of the I–V characteristics on the monolayer quality. A too low concentration of long alkanedithiols leads to the formation of looped molecules, resulting in a 50-fold increase of the current through the SAM. To obtain an almost full standing-up phase of 1,14-tetradecanedithiol (C14) a 30 mM concentration is required, whereas a 0.3 mM concentration leads to a highly looped monolayer. The conduction through the full standing-up phase of C14 and C16 is in accordance with the exponential dependence on molecular length as obtained from shorter alkanedithiols [3]. Finally, a fully functional solid-state molecular electronic switch is manufactured by conventional processing techniques. The molecular switch is based on a monolayer of photochromic diarylethene molecular switches. The monolayer reversibly switches the conductance by more than one order of magnitude between the two conductance states via optical addressing. This reversible conductance switch operates as an electronic ON/OFF switch (or a reprogrammable data storage unit) that can be optically written and electronically read [4].