Paul A. van Hal

Bottom-up organic integrated circuits

Self-assembly—the autonomous organization of components into patterns and structures—is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom-up approach involving self-assembling molecules was proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In the SAMFETs fabricated so far, current modulation has only been observed in submicrometre channels, the lack of efficient charge transport in longer channels being due to defects and the limited intermolecular π–π coupling between the molecules in the self-assembled monolayers. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. Here we demonstrate SAMFETs with long-range intermolecular π–π coupling in the monolayer. We achieve dense packing by using liquid-crystalline molecules consisting of a π-conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk-like carrier mobility, large current modulation and high reproducibility. As a first step towards functional circuits, we combine the SAMFETs into logic gates as inverters; the small parameter spread then allows us to combine the inverters into ring oscillators. We demonstrate real logic functionality by constructing a 15-bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional self-assembled integrated circuits puts bottom-up electronics in a new perspective.

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.

Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors

The mobility of self-assembled monolayer field-effect transistors (SAMFETs) traditionally decreases dramatically with increasing channel length. Recently, however, SAMFETs using liquid-crystalline molecules have been shown to have bulk-like mobilities that are virtually independent of channel length. Here, we reconcile these scaling relations by showing that the mobility in liquid crystalline SAMFETs depends exponentially on the channel length only when the monolayer is incomplete. We explain this dependence both numerically and analytically, and show that charge transport is not affected by carrier injection, grain boundaries or conducting island size. At partial coverage, that is when the monolayer is incomplete, liquid-crystalline SAMFETs thus form a unique model system to study size-dependent conductance originating from charge percolation in two dimensions.

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.

Revealing Buried Interfaces to Understand the Origins of Threshold Voltage Shifts in Organic Field‐Effect Transistors

The semiconductor of an organic field-effect transistor is stripped with adhesive tape, yielding an exposed gate dielectric, accessible for various characterization techniques. By using scanning Kelvin probe microscopy we reveal that trapped charges after gate bias stress are located at the gate dielectric and not in the semiconductor. Charging of the gate dielectric is confirmed by the fact that the threshold voltage shift remains, when a pristine organic semiconductor is deposited on the exposed gate dielectric of a stressed and delaminated field-effect transistor.

Ordered Semiconducting Self-Assembled Monolayers on Polymeric Surfaces Utilized in Organic Integrated Circuits

We report on a two-dimensional highly ordered self-assembled monolayer (SAM) directly grown on a bare polymer surface. Semiconducting SAMs are utilized in field-effect transistors and combined into integrated circuits as 4-bit code generators. The driving force to form highly ordered SAMs is packing of the liquid crystalline molecules caused by the interactions between the linear alkane moieties and the pi-pi stacking of the conjugated thiophene units. The fully functional circuits demonstrate long-range order over large areas, which can be regarded as the start of flexible monolayer electronics.

Manipulating the local light emission in organic light-emitting diodes by using patterned self-assembled monolayers

Patterned organic light-emitting diodes are fabricated by using microcontact- printed self-assembled monolayers on a gold anode (see background figure). Molecules with dipole moments in opposite directions result in an increase or a decrease of the local work function (foreground picture), providing a direct handle on charge injection and enabling local modification of the light emission.

Universal Scaling of the Charge Transport in Large-Area Molecular Junctions

Charge transport through alkanes and para-phenylene oligomers is investigated in large-area molecular junctions. The molecules are self-assembled in a monolayer and contacted with a top electrode consisting of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonic acid) (PEDOT:PSS). The complete set of J(V,T) characteristics of both saturated and π-conjugated molecules can be described quantitatively by a single equation with only two fit parameters. The derived parameters, in combination with a variation of the bulk conductivity of PEDOT:PSS, demonstrate that the absolute junction resistance is factorized with that of PEDOT:PSS.

Extending the voltage window in the characterization of electrical transport of large-area molecular junctions

A large bias window is required to discriminate between different transport models in large-area molecular junctions. Under continuous DC bias, the junctions irreversibly break down at fields over 9 MV/cm. We show that, by using pulse measurements, we can reach electrical fields of 35 MV/cm before degradation. The breakdown voltage is shown to depend logarithmically on both duty cycle and pulse width. A tentative interpretation is presented based on electrolysis in the polymeric top electrode. Expanding the bias window using pulse measurements unambiguously shows that the electrical transport exhibits not an exponential but a power-law dependence on bias.

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].