Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

We compare the charge transport characteristics of heavy doped p- and n-Si-alkyl chain/Hg junctions. Photoelectron spectroscopy (UPS, IPES and XPS) results for the molecule-Si band alignment at equilibrium show the Fermi level to LUMO energy difference to be much smaller than the corresponding Fermi level to HOMO one. This result supports the conclusion we reach, based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction, that for both p- and n-type junctions the energy difference between the Fermi level and LUMO, i.e., electron tunneling, controls charge transport. The Fermi level-LUMO energy difference, experimentally determined by IPES, agrees with the non-resonant tunneling barrier height deduced from the exponential length-attenuation of the current

Nanostructured ultra thin films by covalent molecular assembly in supercritical carbon dioxide

Ph.DDOCTOR OF PHILOSOPH

Tuning the optoelectronic properties of dual-acceptor based low-bandgap ambipolar polymers by changing the thiophene-bridge length

Three very-low-bandgap dual-acceptor based polymers containing diketopyrrolopyrrole (DPP) and a thiadiazoloquinoxaline (TQ) derivative were prepared. Both acceptors in these polymers were separated by one, two and three thiophenes. By only inserting one thiophene between DPP and benzodithiophene condensed TQ, the polymer PDPP-T-TQ shows a very low optical bandgap of 0.60 eV with an electron affinity of -4.23 eV. Optical and electrochemical bandgaps of the polymers were enlarged with increasing the thiophene-bridge length between both acceptors. GIWAXS measurements confirmed that the polymer with three thiophene bridges (PDPP-3T-TQ) showed an ordered arrangement of the crystallites, providing the best ambipolar device performance among these polymers

Benzotrithiophene-Based Donor–Acceptor Copolymers with Distinct Supramolecular Organizations

Two donor–acceptor copolymers, <b>P1</b> and <b>P2</b>, containing the novel donor component benzo­[2,1-<i>b</i>:3,4-<i>b</i>′:5,6-<i>c</i>″]­trithiophene were synthesized. Both polymers show small π-stacking distances (0.35 nm for <b>P1</b> and 0.37 nm for <b>P2</b>) due to the use of the disklike-shaped donor unit. However, they exhibit remarkable differences in supramolecular organization, film microstructure, and transistor performance. Indeed, <b>P1</b> reveals a distinct supramolecular organization in the bulk in comparison to conventional conjugated polymers, including <b>P2</b>. Interestingly, no charge carrier transport was observed for <b>P1</b> in field-effect transistors, while <b>P2</b> exhibited a hole mobility of up to 0.04 cm² V^–1 s^–1. This variation in device behavior is attributed to the evidently different degree of curvature in the polymer backbone induced by the introduction of two additional thiophene units in <b>P2</b>

Stable Organic Monolayers on Oxide-Free Silicon/Germanium in a Supercritical Medium: A New Route to Molecular Electronics

Oxide-free Si and Ge surfaces have been passivated and modified with organic molecules by forming covalent bonds between the surfaces and reactive end groups of linear alkanes and aromatic species using single-step deposition in supercritical carbon dioxide (SCCO₂). The process is suitable for large-scale manufacturing due to short processing times, simplicity, and high resistance to oxidation. It also allows the formation of monolayers with varying reactive terminal groups, thus enabling formation of nanostructures engineered at the molecular level. Ballistic electron emission microscopy (BEEM) spectra performed on the organic monolayer on oxide-free silicon capped by a thin gold layer reveals for the first time an increase in transmission of the ballistic current through the interface of up to three times compared to a control device, in contrast to similar studies reported in the literature suggestive of oxide-free passivation in SCCO₂. The SCCO₂ process combined with the preliminary BEEM results opens up new avenues for interface engineering, leading to molecular electronic devices

Combination of Two Diketopyrrolopyrrole Isomers in One Polymer for Ambipolar Transport

Combination of Two Diketopyrrolopyrrole Isomers in One Polymer for Ambipolar Transpor

Imprinting of metal receptors into multilayer polyelectrolyte films: fabrication and applications in marine antifouling

Polymeric films constructed using the layer-by-layer (LbL) fabrication process were employed as a platform for metal ion immobilization and applied as a marine antifouling coating. The novel Cu2+ ion imprinting process described is based on the use of metal ion templates and LbL multilayer covalent cross-linking. Custom synthesized, peptide mimicking polycations composed of histidine grafted poly(allylamine) (PAH) to bind metal ions, and methyl ester containing polyanions for convenient cross-linking were used in the fabrication process. Two methods of LbL film formation have been investigated using alternate polyelectrolyte deposition namely non-imprinted LbLA, and imprinted LbLB. Both LbL films were cross linked at mild temperature to yield covalent bridging of the layers for improved stability in a sea water environment. A comparative study of the non-imprinted LbLA films and imprinted LbLB films for Cu2+ ion binding capacity, leaching rate and stability of the films was performed. The results reveal that the imprinted films possess enhanced affinity to retain metal ions due to the preorganization of imidazole bearing histidine receptors. As a result the binding capacity of the films for Cu2+ could be improved by seven fold. Antifouling properties of the resulting materials in a marine environment have been demonstrated against the settlement of barnacle larvae, indicating that controlled release of Cu ions was achieved

Tuning Packing and Solubility of Donor (D)–Acceptor (A) Polymers by <i>cis</i>–<i>trans</i> Isomerization within Alkenyl Side Chains

The impact of alkenyl substituents on the behavior of cyclopentadithiophene–benzothiadiazole (CDT–BTZ) donor (D)–acceptor (A) polymers in organic field-effect transistors (OFETs) and on the supramolecular organization was investigated. Linear <i>cis</i>- and <i>trans-</i>alkenes were attached to the donor unit of CDT–BTZ polymers to demonstrate the dependence of supramolecular ordering and solubility in organic solvents on chemical conformation. The layer interdigitation of the substituents differed due to shape disparities between <i>cis-</i> and <i>trans-</i>alkenes. While <i>trans-</i>alkenes exhibit zigzag structures that are beneficial for close packing, <i>cis</i>-alkenes are curved and thus possess a less regular shape that is disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies, which revealed shorter intermolecular distances for the polymer with <i>trans-</i>alkene substituents even in comparison to analogous polymers with saturated alkyl substituents. Furthermore, the isomerization of the <i>cis</i>-substituents toward their <i>trans-</i>conformers allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies

Benzodithiophene–Thiadiazoloquinoxaline as an Acceptor for Ambipolar Copolymers with Deep LUMO Level and Distinct Linkage Pattern

Two new conjugated copolymers, <b>PBDTTQ-1</b> and <b>PBDTTQ-2</b>, with a distinct linked pattern between benzodithiophene–thiadiazoloquinoxaline (<b>BDTTQ</b>) as acceptor and bithiophene as donor were synthesized and characterized. The difference in the linkage between donor and acceptor exerts great influence on the optoelectronic properties of the two polymers. The optical band gap decreases from 1.18 eV for <b>PBDTTQ-1</b> to 1.03 eV for <b>PBDTTQ-2</b>, due to the lower LUMO energy level (−4.01 eV) of the latter. Moreover, density functional theory calculations demonstrate that the electron density is mainly confined on the acceptor unit in both HOMO and LUMO of <b>PBDTTQ-1</b>, while the electronic densities almost delocalize along the entire backbone of <b>PBDTTQ-2</b>, which facilitates the charge transport within the polymer chain. In contrast to <b>PBDTTQ-1</b> missing any field-effect characteristics, <b>PBDTTQ-2</b> exhibits ambipolar charge transporting behavior with mobilities of 1.2 × 10^–3 cm²/(V s) for holes and 6.0 × 10^–4 cm²/(V s) for electrons