Photoswitchable Hopping Transport in Molecular Wires 4 nm in Length

We report the synthesis and electrical characterization of photoswitchable π-conjugated molecular wires. The wires were designed based on the previously reported oligophenyl­eneimine (OPI) wires [Frisbie Science 2008, 320, 1482] with a slight modification to incorporate the dithienylethene linker (the “photoswitch”) into the wire backbone (e.g., PS3-OPI 5; PS stands for the photoswitch, and the number following “PS’’ indicates its position within the OPI chain). Stepwise arylimine condensation reaction between 1,4-diamino­benzene and terephthalaldehyde (1,4-benzene­dicarbaldehyde) was employed to grow these wires from Au surfaces. To insert the “photoswitch” into the wire, 1,4-diaminobenzene was replaced with perfluoro-1,2-bis­(2-(4-amino­phenyl)-5-methyl­thien-4-yl)­cyclo­pentene (PS) at specific steps during the wire growth. A variety of surface characterization techniques were employed to investigate the structure of the wires including FT-IR spectroscopy, ellipsometry, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy. The current–voltage (<i><i>I</i>−<i>V</i></i>) characteristics and resistances of the wires were acquired using conducting probe atomic force microscopy (CP-AFM). It was observed that all of the wires switch between high and low conductance modes (“ON” and “OFF” states corresponding to “closed” and “open” forms of the dithienyl­ethene linker, respectively) when irradiated by UV and visible light, respectively. Measuring the temperature dependence of the resistance revealed that the charge transport mechanism in the PS3-OPI 3 wire is tunneling (temperature independent) whereas longer PS3-OPI 5 and PS5-OPI 5 showed Arrhenius temperature dependence which is characteristic of a hopping mechanism. These experiments demonstrate light-based control of transport in molecular wires in the hopping regime, which ultimately may be useful for switching applications in molecular electronics

Four-Terminal Electrochemistry: A Back-Gate Controls the Electrochemical Potential of a 2D Working Electrode

We demonstrate that ultrathin semiconductor working electrodes integrated into metal–insulator–semiconductor (MIS) stacks are an enabling platform for understanding non-Faradaic semiconductor electrochemistry. Here, 5 nm thick ZnO electrodes were deposited on 30 nm HfO2 dielectric on a Pd “gate” electrode. Application of a bias VG between the Pd gate and the ZnO electrode causes electrons to accumulate in the ZnO layer as measured by recording the in-plane sheet conductance. By contacting the top surface of the ZnO layer with the electrolyte in a conventional three-electrode electrochemical cell, we show that the gate voltage VG modulates the electrochemical potential VZnO of the ZnO film with respect to a reference electrode. Electrochemical potential changes ΔVZnO up to −1 V vs Ag/Ag+ are achieved for VG = +7 V. Furthermore, by measuring VZnO vs VG, we extract the quantum capacitance CQ of the ZnO film as a function of the Fermi-level position, which provides a direct measure of the ZnO electronic density of states (DOS). Finally, we demonstrate that the gated ZnO working electrodes can disentangle the two principal components of electrochemical potential, namely, the Fermi-level shift Δδ and the double-layer charging energy eΔϕEDL. This disentanglement hinges on a fundamental difference between back-gating and normal electrochemical control, namely, that electrochemical control requires double-layer charging, while back-gate control does not. Collectively, the results show that the backside gate electrode is an effective fourth terminal that enables measurements that are difficult to achieve in conventional three-terminal electrochemical setups

Determination of Quantum Capacitance and Band Filling Potential in Graphene Transistors with Dual Electrochemical and Field-Effect Gates

We report here an investigation of graphene field-effect transistors (G-FETs) in which the graphene channel is in contact with an electrolyte phase. The electrolyte and the ultrathin nature of graphene allow direct measurement of the channel electrochemical potential versus a reference electrode also in contact with the electrolyte. In addition, the electrolyte can be used to gate the graphene; i.e., a dual-gate structure is realized. We employ this electrolyte modified G-FET architecture to (1) track the Fermi level of the graphene channel as a function of gate bias, (2) determine the density of states (i.e., the quantum capacitance <i>C</i>_Q) of graphene, and (3) separate the gate induced band filling potential δ from the electrochemical double-layer charging potential Δϕ_EDL. Additionally, we are able to determine the electric double-layer capacitance <i>C</i>_EDL for the graphene/electrolyte interface, which is ∼5 μF/cm², the same order of magnitude as <i>C</i>_Q. Overall, the electrolyte modified G-FETs provide an excellent model system for probing the electronic structure and transport properties of graphene and for understanding the differences between the two gating mechanisms

Dependence of Conductivity on Charge Density and Electrochemical Potential in Polymer Semiconductors Gated with Ionic Liquids

We report the hole transport properties of semiconducting polymers in contact with ionic liquids as a function of electrochemical potential and charge carrier density. The conductivities of four different polymer semiconductors including the benchmark material poly(3-hexylthiophene) (P3HT) were controlled by electrochemical gating (doping) in a transistor geometry. Use of ionic liquid electrolytes in these experiments allows high carrier densities of order 10²¹ cm^–3 to be obtained in the polymer semiconductors and also facilitates variable temperature transport measurements. Importantly, all four polymers displayed a nonmonotonic dependence of the conductivity on carrier concentration. For example, for P3HT in contact with the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]), the hole conductivity reached a maximum of 85 S/cm at 6 × 10²⁰ holes cm^–3 or 0.16 holes per thiophene ring. Further increases in charge density up to 0.35 holes per ring produced a reversible drop in film conductivity. The reversible decrease in conductivity is due to a carrier density dependent hole mobility, which reaches 0.80 ± 0.08 cm² V^–1 s^–1 near the conductivity peak. The conductivity behavior was qualitatively independent of the type of ionic liquid in contact with the polymer semiconductor though there were quantitative differences in the current versus gate voltage characteristics. Temperature dependent measurements of the mobility in P3HT revealed that it is activated over the range 250–350 K. Both the pre-exponential coefficient μ₀ and the activation energy <i>E</i>_A depend nonmonotonically on carrier density with <i>E</i>_A becoming as small as 20 meV at the conductivity peak. Overall, the peak in conductivity versus carrier density appears to be a general result for polymer semiconductors gated with ionic liquids

Field Effect Modulation of Outer-Sphere Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes

Here we report field-effect modulation of solution electrochemistry at 5 nm thick ZnO working electrodes prepared on SiO₂/degenerately doped Si gates. We find that ultrathin ZnO behaves like a 2D semiconductor, in which charge carriers electrostatically induced by the back gate lead to band edge shift at the front electrode/electrolyte interface. This, in turn, manipulates the charge transfer kinetics on the electrode at a given electrode potential. Experimental results and the proposed model indicate that band edge alignment can be effectively modulated by 0.1–0.4 eV depending on the density of states in the semiconductor and the capacitance of the gate/dielectric stack

Parasitic Capacitance Effect on Dynamic Performance of Aerosol-Jet-Printed Sub 2 V Poly(3-hexylthiophene) Electrolyte-Gated Transistors

Printed, low-voltage poly­(3-hexylthiophene) (P3HT) electrolyte-gated transistors (EGTs) have favorable quasi-static characteristics, including sub 2 V operation, carrier mobility (μ) of 1 cm²/(V s), ON/OFF current ratio of 10⁶, and static leakage current density of 10^–6 A/cm². Here we study the dynamic performance of P3HT EGTs in which the semiconductor, dielectric, and gate electrode were deposited using aerosol-jet printing; the source and drain electrodes were patterned by conventional microlithography. With a source-to-drain separation of 2.5 μm, the highest theoretical achievable switching frequency is ∼10 MHz, assuming the movement of charge through the semiconductor is the limiting step. However, the measured maximum switching frequency of P3HT EGTs to date is ∼1 kHz, implying that another process is slowing the response. By systematically varying the device geometry, we show that the frequency is limited by the capacitance between the gate and drain (i.e., parasitic capacitance). The traditional scaling of switching time with the square of channel length (<i>L</i>) does not hold for P3HT EGTs. Rather, minimizing the size of the drain electrode increases the maximum switching speed. We achieve 10 kHz for P3HT EGTs with source/drain electrode dimensions of 2.5 μm × 50 μm and channel dimensions of 2.5 μm × 50 μm. Further improvements will require additional shrinkage of electrode dimensions as well as consideration of other factors such as ion gel thickness and carrier mobility

Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure

Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by scanning Kelvin probe microscopy (SKPM) of surface potentials and CPDs across ultrathin (1–2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly­(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of −10 to +50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are ∼15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in interfacial polarization and lateral electrostatic heterogeneity; these factors ultimately should affect the performance of these interfaces in devices

Growth of Thin, Anisotropic, π‑Conjugated Molecular Films by Stepwise “Click” Assembly of Molecular Building Blocks: Characterizing Reaction Yield, Surface Coverage, and Film Thickness versus Addition Step Number

We report the systematic characterization of anisotropic, π-conjugated oligophenyleneimine (OPI) films synthesized using stepwise imine condensation, or “click” chemistry. Film synthesis began with a self-assembled monolayer (SAM) of 4-formylthiophenol or 4-aminothiophenol on Au, followed by repetitive, alternate addition of terephthalaldehyde (benzene-1,4-dicarbaldehyde) or 1,4-benzenediamine to form π-conjugated films ranging from 0.6–5.5 nm in thickness. By systematically capping the OPI films with a redox or halogen label, we were able to measure the relative surface coverage after each monomer addition via Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, reflection–absorption infrared spectroscopy, and cyclic voltammetry. Nuclear reaction analysis was also employed for the first time on a SAM to calculate the surface coverage of carbon atoms after each stepwise addition. These six different analysis methods indicate that the average extent of reaction is 99% for each addition step. The high yield and molecular surface coverage confirm the efficacy of Schiff base chemistry, at least with the terephthalaldehyde and 1,4-benzenediamine monomers, for preparing high-quality molecular films with π conjugation normal to the substrate

Optimization of Aerosol Jet Printing for High-Resolution, High-Aspect Ratio Silver Lines

Aerosol jet printing requires control of a number of process parameters, including the flow rate of the carrier gas that transports the aerosol mist to the substrate, the flow rate of the sheath gas that collimates the aerosol into a narrow beam, and the speed of the stage that transports the substrate beneath the beam. In this paper, the influence of process parameters on the geometry of aerosol-jet-printed silver lines is studied with the aim of creating high-resolution conductive lines of high current carrying capacity. A systematic study of process conditions revealed a key parameter: the ratio of the sheath gas flow rate to the carrier gas flow rate, defined here as the focusing ratio. Line width decreases with increasing the focusing ratio and stage speed. Simultaneously, the thickness increases with increasing the focusing ratio but decreases with increasing stage speed. Geometry control also influences the resistance per unit length and single pass printing of low-resistance silver lines is demonstrated. The results are used to develop an operability window and locate the regime for printing tall and narrow silver lines in a single pass. Under optimum conditions, lines as narrow as 20 μm with aspect ratios (thickness/width) greater than 0.1 are obtained

Facile Method for Fabricating Flexible Substrates with Embedded, Printed Silver Lines

Insertion, curing and delamination is presented as a simple and scalable method for creating flexible substrates with embedded, printed silver lines. In a sequential process, aerosol-jet printed silver lines are transferred from a donor substrate to a thin reactive polymer that is directly adhered to a flexible substrate. Due to the unique ability of the aerosol jet to print continuous lines on a low energy surface, a 100% transfer of the printed electrodes is obtained, as confirmed by electrical measurements. Moreover, the root-mean-square roughness of the embedded electrodes is less than 10 nm, which is much lower than that for their as-printed form. The embedded electrodes are robust and do not show a significant degradation in electrical performance after thousands of bending cycles