Control Charge Transport Research Articles

Constructing Dual-Molecule Junctions to Probe Intermolecular Crosstalk.

Understanding and controlling charge transport across multiple parallel molecules are fundamental to the creation of innovative functional electronic components, as future molecular devices will likely be multimolecular. The smallest possible molecular ensemble to address this challenge is a dual-molecule junction device, which has potential to unravel the effects of intermolecular crosstalk on electronic transport at the molecular level that cannot be elucidated using either conventional single-molecule or self-assembled monolayer (SAM) techniques. Herein, we demonstrate the fabrication of a scanning tunneling microscopy (STM) dual-molecule junction device, which utilizes noncovalent interactions and allows for direct comparison to the conventional STM single-molecule device. STM-break junction (BJ) measurements reveal a decrease in conductance of 10% per molecule from the dual-molecule to the single-molecule junction device. Quantum transport simulations indicate that this decrease is attributable to intermolecular crosstalk (i.e., intermolecular π-π interactions), with possible contributions from substrate-mediated coupling (i.e., molecule-electrode). This study provides the first experimental evidence to interpret intermolecular crosstalk in electronic transport at the STM-BJ level and translates the experimental observations into meaningful molecular information to enhance our fundamental knowledge of this subject matter. This approach is pertinent to the design and development of future multimolecular electronic components and also to other dual-molecular systems where such crosstalk is mediated by various noncovalent intermolecular interactions (e.g., electrostatic and hydrogen bonding).

Evaluation of brass electrodeposition at RDE from cyanide-free bath using EDTA as a complexing agent

The use of cyanide-free complexing agents in brass electroplating has been tested in recent years and EDTA has shown to be a promising alternative. Herein, a rotating disc electrode was used to construct voltammetric curves and to evaluate the influence of the rotation speed on the quality of brass electrodeposition with EDTA on steel. The reference and counter electrodes were made of Ag/AgCl and platinum, respectively. The influence of the galvanostatic/potentiostatic mode, charge density and bath concentration were also studied. By the voltammetric curves and the Koutecky-Levich equation, the diffusion coefficient of copper-EDTA was determined (2.9 × 10−6 cm2/s) and it was verified that the electrodeposition of this metal is controlled by mass transport. For Zn-EDTA deposition, a mixed control (charge and mass transport) was suggested. The agitation generally darkened the deposits due to the hydrogen evolution and lower Cu/Zn proportions. However, for agitated solutions at −1.3 V, very good deposits were obtained with brightness and typical color of brass. Therefore, the operation at −1.3 V with agitation may be more interesting than at higher potentials. Charge densities between 0.5 and 1.0C/cm2 must be used. Finally, the effluent generated was treated, by electrodialysis, and the recovery of the metals and EDTA on the bath was studied. In general, uniform and bright deposits were obtained.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Orbital Control and Coherent Charge Transport in Transition Metal Platinum(II)–Platinum(II) Lantern Complexes in Molecular Junctions

Platinum (II) dimers in organometallic complexes exhibit oscillatory vibrational charge transfer with optical excitation and whose nature depends strongly on the Pt-Pt distance. Yet the electronic transport of such devices has not been studied. We take a well-reported Pt(II) dimer complex that is ideal for transport in a molecular junction and subject it to a battery of electronic structure property calculations. With DFT, we find the orbital nature of the HOMO level to not be solely induced by the 5dz2 orbital as long thought, but rather a complex mixture with significant contributions from 6s orbital states from the Au electrodes that have not been considered before. We probe how the chemical tuning of the ligand affects the Pt-Pt bond length and thus conduction, by systematically modifying both the bridging ligands (BL) and cyclometallating ligands (CMLs) with donor (methyl) and acceptor (fluorine) derivatives. With nonequilibrium green’s functions (NEGF-DFT), we find patterns between the conductivity,...

Controlling the Microstructure of Conjugated Polymers in High‐Mobility Monolayer Transistors via the Dissolution Temperature

AbstractIt remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (Tdis) is investigated as a powerful strategy for morphology control. Low Tdis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π–π stacking. The field‐effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high Tdis with a value approaching 1 cm2 V−1 s−1. Besides that, the solution kinetics reveal a higher growth rate of aggregates at low Tdis, and filtration experiments further confirm that the dependence of the fibril width in monolayers on Tdis is consistent with the aggregate size in solution. The generalizability of the Tdis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high‐mobility monolayer transistors.

Controlling the Microstructure of Conjugated Polymers in High-Mobility Monolayer Transistors via the Dissolution Temperature.

It remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (T dis) is investigated as a powerful strategy for morphology control. Low T dis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π–π stacking. The field‐effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high T dis with a value approaching 1 cm2 V−1 s−1. Besides that, the solution kinetics reveal a higher growth rate of aggregates at low T dis, and filtration experiments further confirm that the dependence of the fibril width in monolayers on T dis is consistent with the aggregate size in solution. The generalizability of the T dis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high‐mobility monolayer transistors.

Electrochemical electron transfer and its relation to charge transport in single molecule junctions

Abstract Ability to control charge transport at nanometer scale lies in the heart of design of fast reliable electronic devices. Molecular electronics thrive to use functional molecules for such transport. If the molecule contains redox center(s), a diode-like or transistor-like behavior can be easily explored by controlling not only the voltage difference between two metallic contacts of the molecular junction but also the potential of one of the contacting electrodes with respect to some reference. Thus, one needs to understand the relationship between electrochemical electron transfer and charge transport in metal–molecule–metal junctions. This review presents latest theoretical approaches toward understanding of such relationship and discusses pivotal experimental works to validate them. Tunneling and hopping pathways may operate in parallel (two-channel model), but experimental conditions dictate the channel preference.

Electronegativity, symmetry, and bond strength intrinsically control charge transport through five-membered single-molecule junction

Density functional theory and Green’s function incorporated tight binding approach have been implemented to investigate the role of electronegativity, symmetry, and bond strength in charge transport of five-membered aromatic molecules, namely cyclopentadiene, thiophene, furan, pyrrole, and 5,5-dimethylcyclopenta-1,3-diene. A novel dependence of charge transport and band gap on the bond strength of the intact segment of molecule was revealed. It is found that decreasing the strength (increasing the length) of the bonds of the sound branch would enhance the electronic transmission and narrow the band gap. In addition, the electronic transmission of the symmetric systems is higher than the asymmetric counterparts and it increases (decreases) with onsite energy of the hetero-site of the symmetric (asymmetric) system.

Photomultiplying Visible Light Detection by Halide Perovskite Nanoparticles Hybridized with an Organo Eu Complex.

A molecular layer of an organo europium (Eu) complex was hybridized with the surface of halide perovskite (MAPbI3) nanocrystals to control charge transport between photoexcited perovskites and a metal electrode. A thin hybridized nanocrystalline film was found to cause an efficient photomultiplication reaction based on hole accumulation at photoexcited perovskite nanoparticles and charge tunneling across the Eu complex molecules under application of a low external bias (-0.5 V). A photodetecting device based on the Eu complex and perovskite absorber demonstrated photoinduced current generation at extremely high quantum conversion efficiency of 290 000% for weak monochromatic light (<1 mW cm-2), yielding a power responsivity value up to 1289 A W-1. High sensitivity of photodetection was enabled by using nanosized particles (<5 nm) of perovskite having a high surface area in contact with the Eu complex, while sensitivity was reduced by use of thick and large particle perovskite layers.

Nanoelectrochemistry Concepts in Desalination Energy Harvesting and Analysis

This talk will provide an overview of the research in GWang’s group at Georgia State centered around the electrochemical processes at the nanometer scale. Three examples will be discussed in two research topics. The first is the development of new analytical method/toolbox for actively controlled nucleation and crystal growth. By controlling charge transport through a single nanopore, a single crystal is synthesized while the whole process is monitored by combined electroanalytical and optical methods. In the second topic, ion transport through a single asymmetric nanopipette is studied by experiments and simulation. The solution-substrate interface confined inside the conical nanopore is viewed as structurally-defined unit elements in ensemble porous membrane/electrode/materials. The surface electric field direction is separated from the through-transport ion flux which affects both the structure and dynamics of the electrical double layer (EDL). Selective and efficient transport of ions is the fundamental process in 1. the desalination with energy input; and a retrospective process of 2. energy harvesting from salinity when fresh and brackish/sea water mix. Novel time-dependent current-potential features are captured in electrochemical experiments due to the polarization of the nanoscale EDL or selective transport of ions. Characteristic signatures include a non-zero cross point in I-V curves separating pinched hysteresis loops. Finite element simulations by solving the Poisson, Nernst-Planck and Navier-Stokes equations reveal the respective transport of cations and anions driven by migration, diffusion and electroosmotic flow. The transport selectivity and efficiency under time- and space- confined conditions provide fundamental insights to better transport related applications such as desalination separation and energy harvesting from salinity gradients. Financial supports from NSF (CHE-1610616) and DOE (DE-SC0019043) are acknowledged.

Polyacrylate polymer assisted crystallization: Improved charge transport and performance consistency for solution-processable small-molecule semiconductor based organic thin film transistors

Abstract In this study, we report on an effective approach to modulate crystallization, control charge transport and enhance performance consistency of small-molecule semiconductor based organic thin film transistors (OTFTs) with the addition of a polyacrylate polymer additive poly(2-ethylhexyl acrylate) (P2EHA). 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a benchmark semiconductor to blend with the P2EHA additive, leading to a vertical phase separation between these two components. The resultant TIPS pentacene film exhibited greatly reduced crystal misorientation, enlarged grain width and enhanced film coverage. Bottom-gate, bottom-contact OTFTs based on the TIPS pentacene/P2EHA blends were fabricated and showed an increased average hole mobility of 0.317 ± 0.047 cm2/V, as well as a performance consistency factor of 6.72, which is defined as the ratio of the average hole mobility to the standard deviation of mobility. Notably, it leads to a 10-fold and 7-fold enhancement of average mobility and performance consistency as compared to the pristine TIPS pentacene OTFTs. This great improvement of device performance can be attributed to the reduced crystal misorientation, less defects and trap centers at the grain boundaries as a result of the enlarged grain width, as well as increased film coverage, due to the addition of the P2EHA polyacrylate polymer additive.

Light‐Driven Reversible Intermolecular Proton Transfer at Single‐Molecule Junctions

AbstractPhotoresponsive molecular systems are essential for molecular optoelectronic devices, but most molecular building blocks are non‐photoresponsive. Employed here is a photoinduced proton transfer (PIPT) strategy to control charge transport through single‐molecule azulene junctions with visible light under ambient conditions, which leads to a reversible and controllable photoresponsive molecular device based on non‐photoresponsive molecules and a photoacid. Also demonstrated is the application of PIPT in two single‐molecule AND gate and OR gate devices with electrical signal as outputs.

Light-Driven Reversible Intermolecular Proton Transfer at Single-Molecule Junctions.

Photoresponsive molecular systems are essential for molecular optoelectronic devices, but most molecular building blocks are non-photoresponsive. Employed here is a photoinduced proton transfer (PIPT) strategy to control charge transport through single-molecule azulene junctions with visible light under ambient conditions, which leads to a reversible and controllable photoresponsive molecular device based on non-photoresponsive molecules and a photoacid. Also demonstrated is the application of PIPT in two single-molecule AND gate and OR gate devices with electrical signal as outputs.

Frontispiece: Molecular Design towards Controlling Charge Transport

The field of single-molecule electronics today presents innumerable opportunities for basic research in both synthesis as well as physical and theoretical studies. However, such systems are still quite far from actual commercialization of these systems because of a variety of challenges, including charge transport. In their Concept article on page 17180 ff., L. Yu et al. discuses recent advances on how to control charge transport in single-molecule systems.

Tyrosine-Rich Peptides as a Platform for Assembly and Material Synthesis.

The self‐assembly of biomolecules can provide a new approach for the design of functional systems with a diverse range of hierarchical nanoarchitectures and atomically defined structures. In this regard, peptides, particularly short peptides, are attractive building blocks because of their ease of establishing structure–property relationships, their productive synthesis, and the possibility of their hybridization with other motifs. Several assembling peptides, such as ionic‐complementary peptides, cyclic peptides, peptide amphiphiles, the Fmoc‐peptide, and aromatic dipeptides, are widely studied. Recently, studies on material synthesis and the application of tyrosine‐rich short peptide‐based systems have demonstrated that tyrosine units serve as not only excellent assembly motifs but also multifunctional templates. Tyrosine has a phenolic functional group that contributes to π–π interactions for conformation control and efficient charge transport by proton‐coupled electron‐transfer reactions in natural systems. Here, the critical roles of the tyrosine motif with respect to its electrochemical, chemical, and structural properties are discussed and recent discoveries and advances made in tyrosine‐rich short peptide systems from self‐assembled structures to peptide/inorganic hybrid materials are highlighted. A brief account of the opportunities in design optimization and the applications of tyrosine peptide‐based biomimetic materials is included.

Highly Efficient Organic Blue Electroluminescent Materials and Devices with Mesoscopic Structures.

Due to the difficulty in achieving high efficiency and high color purity simultaneously, blue emission is the limiting factor for the performance and stability of OLEDs. Since 2003, we have been working on organic light-emitting diodes (OLEDs), especially on blue light. After a series of molecular designs, novel strategies have been proposed from different aspects. At first, highly efficient deep blue emission could be achieved through molecular design with highly twisted structure to suppress fluorescence quenching and redshift. Deep blue emitters with high efficiency in solid state, a twisted structure with aggregation induced emission (AIE) characteristics was incorporated to inhibit molecular aggregation, and triplet-triplet fusion (TTF) and hybridized localized charge transfer (HLCT) were adopted to increase the ratio of triplet exciton used. Secondly, a highly efficient blue OLED could be achieved through improving charge transport. New electron transport materials (ETMs) with wide band gap were developed to control charge transport balance in devices. Thirdly, a highly efficient deep blue emission could be achieved through a mesoscopic structure of out-coupling layer. A mesoscopic photonic structured organic thin film was fabricated on the top of metal electrode by self-aggregation in order to improve the light out-coupling efficiency.

Molecular Design towards Controlling Charge Transport

Organic electronics generally deals with bulk, statistically averaged electrical properties of organic materials. Single-molecule electronic devices can then be viewed as linkers to these bulk properties. However, controlling charge transport in single-molecule systems still remains a formidable task and has prevented proliferation of these systems from research laboratories to household electronics. This Concept article provides a general overview of the recent advances made by our group in the field. We will describe several concepts in designing molecular functions towards controlling charge transport through molecular systems. It should be noted, however, that this Concept article is by no means comprehensive and readers should look elsewhere for a more comprehensive picture of molecular electronics.

Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions.

Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring ( para-OPE3) and meta-connected central ring ( meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions.

Freestanding Organic Charge-Transfer Conformal Electronics.

Wearable conformal electronics are essential components for next-generation humanlike sensing devices that can accurately respond to external stimuli in nonplanar and dynamic surfaces. However, to explore this potential, it is indispensable to achieve the desired level of deformability and charge-transport mobility in strain-accommodating soft semiconductors. Here, we show pseudo-two-dimensional freestanding conjugated polymer heterojunction nanosheets integrated into substrate-free conformal electronics owing to their exceptional crystalline controlled charge transport and high level of mechanical strength. These freestanding and mechanical robust polymer nanosheets can be adapted into a variety of artificial structured surfaces such as fibers, squares, circles, etc., which produce large-area stretchable conformal charge-transfer sensors for real-time static and dynamic monitoring.

Acceptor Percolation Determines How Electron-Accepting Additives Modify Transport of Ambipolar Polymer Organic Field-Effect Transistors.

Organic field-effect transistors (OFETs) that utilize ambipolar polymer semiconductors can benefit from the ability of both electron and hole conduction, which is necessary for complementary circuits. However, simultaneous hole and electron transport in organic field-effect transistors result in poor ON/OFF ratios, limiting potential applications. Solution processing methods have been developed to control charge transport properties and transform ambipolar conduction to hole-only conduction. The electron-acceptor phenyl-C61-butyric acid methyl ester (PC61BM), when mixed in solution with an ambipolar semiconducting polymer, can reduce electron conduction. Unipolar p-type OFETs with high, well-defined ON/OFF ratios and without detrimental effects on hole conduction are achieved for a wide range of blend compositions, from 95:5 to 5:95 wt % semiconductor polymer:PC61BM. When introducing the alternative acceptor N, N'-bis(1-ethylpropyl)-3,4:9,10-perylenediimide (PDI), high ON/OFF ratios are achieved for 95:5 wt % semiconductor polymer:PDI; however, electron conduction increases for 50:50 and 5:95 wt % semiconductor polymer:PDI. As described within, we show that electron conduction is practically eliminated when additive domains do not percolate across the OFET channel, that is, electrons are "morphologically trapped". Morphologies were characterized by optical, electron, and atomic force microscopy as well as X-ray scattering techniques. PC61BM was substituted with an endohedral Lu3N fullerene, which enhanced contrast in electron microscopy and allowed for more detailed insight into the blend morphologies. Blends with alternative, nonfullerene acceptors further emphasize the importance of morphology and acceptor percolation, providing insights for such blends that control ambipolar transport and ON/OFF ratios.