Controlling semiconductor/metal junction barriers by incomplete, nonideal molecular monolayers

We study how partial monolayers of molecular dipoles at semiconductor/metal interfaces can affect electrical transport across these interfaces, using a series of molecules with systematically varying dipole moment, adsorbed on n-GaAs, prior to Au or Pd metal contact deposition, by indirect evaporation or as "ready-made" pads. From analyses of the molecularly modified surfaces, we find that molecular coverage is poorer on low- than on high-doped n-GaAs. Electrical charge transport across the resulting interfaces was studied by current-voltage-temperature, internal photoemission, and capacitance-voltage measurements. The data were analyzed and compared with numerical simulations of interfaces that present inhomogeneous barriers for electron transport across them. For high-doped GaAs, we confirm that only the former, molecular dipole-dependent barrier is found. Although no clear molecular effects appear to exist with low-doped n-GaAs, those data are well explained by two coexisting barriers for electron transport, one with clear systematic dependence on molecular dipole (molecule-controlled regions) and a constant one (molecule-free regions, pinholes). This explains why directly observable molecular control over the barrier height is found with high-doped GaAs: there, the monolayer pinholes are small enough for their electronic effect not to be felt (they are "pinched off"). We conclude that molecules can control and tailor electronic devices need not form high-quality monolayers, bind chemically to both electrodes, or form multilayers to achieve complete surface coverage. Furthermore, the problem of stability during electron transport is significantly alleviated with molecular control via partial molecule coverage, as most current flows now between, rather than via, the molecules.     

Influence of binding groups on molecular junction formation

We study the formation mechanism of molecular junctions using break-junction experiments. We explore the contribution of gold-atom rearrangements in the electrodes by analyzing the junction stretching length, the length of individual plateaus, and the length of the gold one-atom contacts. Comparing the results for alkane dithiols and diamines, we conclude that thiols affect gold electrode dynamics significantly more than amines. This is a vital factor to be considered when comparing different binding groups.     

Ratiometric,filter-free optical sensor based on a complementary metal oxide semiconductor buried double junction photodiode

We report a complementary metal oxide semiconductor integrated circuit (CMOS IC) with a buried double junction (BDJ) photodiode that (i) provides a real-time output signal that is related to the intensity ratio at two emission wavelengths and (ii) simultaneously eliminates the need for an optical filter to block Rayleigh scatter. We demonstrate the BDJ platform performance for gaseous NH₃ and aqueous pH detection. We also compare the BDJ performance to parallel results obtained by using a slew scanned fluorimeter (SSF). The BDJ results are functionally equivalent to the SSF results without the need for any wavelength filtering or monochromators and the BDJ platform is not prone to errors associated with source intensity fluctuations or sensor signal drift.     

Cooperative self-assembly and molecular binding behavior of cyclodextrin-crown ether conjugates mediated by alkali metal ions

In order to quantitatively investigate their molecular binding ability, a series of cyclodextrin-crown ether conjugates containing beta-cyclodextrin (beta-CyD) and crown ether units, i.e.N-(benzoaza-15-crown-5)acylaminomethylene tethered 6-diethylenetriamino-6-deoxy-beta-CyD, N-(benzoaza-15-crown-5)acylaminomethylene tethered 6-triethylenetetraamino-6-deoxy-beta-CyD and 4',5'-dimethylene-benzo-15-crown-5 tethered 6-diethylenetriamino-6-deoxy-beta-CyD, have been prepared as ditopic molecular receptors. Their inclusion complexation behavior with four representative fluorescent dyes, i.e. ammonium 8-anilino-1-naphthalenesulfonate (ANS), sodium 6-toluidino-2-naphthalenesulfonate (TNS), acridine red (AR) and rhodamine B (RhB), has been comprehensively investigated in aqueous NaH2PO4/Na2HPO4 or KH2PO4/K2HPO4 buffer solution (pH 7.20) by means of circular dichroism, fluorescence, and 2D NMR spectra. The results indicate that the self-assembly of crown ether modified beta-CyD mediated by potassium ion exhibits a dimeric structure, which significantly enhances the original binding ability and molecular selectivity of parent beta-CyD and its derivatives towards guest molecules through the cooperative binding of two hydrophobic CyD cavities with one guest. This cooperative binding mode of K+/CyD-crown ether systems are further confirmed by Job's experiments and 2D NMR investigations. Attributed to the positive contributions from the metal-ligated crown ether cap and K+-mediated dimerization of CyDs, the binding constant (Ks) values of CyD-crown ether conjugates toward ANS are 10-83 times higher than that of beta-CyD. The increased binding ability and molecular selectivity of CyD-crown ether conjugates are discussed from the viewpoints of size/shape-fit and multiple recognition mechanism.     

Electronic structure of metal/molecule//metal junctions: a density functional theory study of the influence of the molecular terminal group

We report on density functional theory calculations of the electronic structure of Au(111)/molecule//Au(111) junctions in which thiol molecules are chemically bound at one end to a gold electrode (the "substrate"), while the other end has a separation of a few to several angstroms from a second gold electrode (the "tip"). Our goal is to investigate the role of different molecular terminal groups and of the tip-molecule distance either on the spatial dependence of the local density of states (LDOS) at the Fermi energy E(f) or on the energy dependence of the projected density of states onto different molecular subunits. We consider conjugated diphenylthiol (SPh2R) molecules with terminal groups R = H, SH, CH3, or CF3 as well as "mixed" conjugated-saturated phenylthiol-pentane (SPhC4CH3) and butanethiol-toluene (SC4PhCH3) molecules. For SPh2R molecules, the LDOS at E(f) exhibits an oscillatory exponential decay along the molecule, with an average decay constant that depends weakly on the R terminal group. For the mixed aromatic-aliphatic molecules instead, there are large differences in the LDOS at E(f), with SC4PhCH3 showing a much larger LDOS in the proximity of the terminal CH3 group than SPhC4CH3.     

Surface-assisted one-dimensional self-assembly of a perylene based semiconductor molecule

Chloroform-vapor annealing of thin films of propoxyethyl perylene tetracarboxylic diimide (PE-PTCDI, an n-type semiconductor) deposited on glass or mica leads to formation of well-defined one-dimensional self-assemblies (e.g. nanobelts), which show optically uniaxial properties as demonstrated by the linearly polarized emission.     

Plasmonic metal/semiconductor hybrid nanomaterials for solar to chemical energy conversion

Plasmonic nanomaterials are sources of light,heat and electrons at nanometer scale.Given the outstand-ing performance in harvesting and converting solar energy ...     

Dependence of single-molecule conductance on molecule junction symmetry

The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.     

A molecular simulation based assessment of binding of metal ions on micelles

An assessment of the ability of a micellar surface to bind different metal ions using molecular simulation is presented in this study. Sodium dodecyl sulfate (SDS) is considered as the anionic surfactant. Various relevant characteristics of SDS-metal ion systems are estimated to quantify preferential binding of metal ions. These are electrostatic energy, total potential energy of the system, radial distribution function, and entropy and free energy change of the system. By examining these parameters, the relative extents of binding of different metal ions to the micellar surface are assessed.     

Diastereomeric molecular recognition and binding behavior of bile acids by L/D-tryptophan-modified beta-cyclodextrins

[reaction: see text] Binding behavior of L- and D-tryptophan-modified beta-cyclodextrins (L/D-Trp-beta-CD) (1 and 2) with four bile acids, i.e., cholate (CA), deoxycholate (DCA), glycocholate (GCA), and taurocholate (TCA), has been investigated by fluorescence, circular dichroism, and 2D-NMR spectroscopies and fluorescence lifetime measurement, as well as isothermal titration microcalorimetry. From the induced circular dichroism (ICD) and 2D NMR spectra, it is deduced that the D-Trp moiety of 2 attached to beta-CD is more deeply self-included in the cavity than that of the antipodal L-Trp moiety of 1, indicating appreciably enantioselective binding of the chiral sidearm by beta-CD. Interestingly, the original difference in conformation between 1 and 2 led to quite a large difference in affinity toward DCA, giving 3.3 times higher binding ability for 2 than for 1. Thermodynamically, the inclusion complexation of 1 and 2 with bile acids was entirely driven by favorable enthalpy change (DeltaH degrees) with accompanying negative entropy change (DeltaS degrees). The stronger binding of bile acids by L/D-Trp-beta-CD is attributable to higher enthalpic gains. The combined use of the calorimetric and NMR ROESY spectral examinations revealed the correlation between the thermodynamic parameters and the role of sidearm conformation in modified beta-cyclodextrins.     

The influence of water molecule coordination to a metal ion on water hydrogen bonds

The geometry of hydrogen bonds in the crystal structures from the Cambridge Structural Database and calculated data show that water coordination to a metal ion has a remarkable influence on hydrogen bonds. The calculated energies of hydrogen bonds of coordinated water are much stronger, even if the aqua complex is neutral.     

Effect of ultraviolet radiation on a metal/polyamidine system

An increase in the optical transmission in electronic spectra for metal/poly(1,10-decamethylene-acetamidine) systems after UV irradiation is observed, in contrast to polymer and metal films investigated separately where there was no such effect. The increase is explained by changes in the distribution of the electric charge induced by UV irradiation at the metal/polymer interface in a polymer film.     

The effect of molecular conformation on single molecule conductance: measurements of pi-conjugated oligoaryls by STM break junction

Measurements of molecular break junction reveal quantitatively the correlation between the single-molecule conductance and the conformation of pi-conjugated molecules with 6-18 conjugated double bonds.     

Effect of carbodiimide as a coupling reagent on the metal binding properties of desferrioxamine

The effect of a carbodiimide on the gallium binding properties of desferrioxamine as a model for interpreting the low specific activity of gallium-labelled desferrioxamine-human serum albumin conjugate prepared using carbodiimide as the coupling reagent was studied. Desferrioxamine was treated with carbodiimide at various mole ratios and pH, before and after labeling with⁶⁷Ga. Substantial loss of gallium binding capacity resulted when desferrioxamine was treated with carbodiimide prior to labeling, suggesting that the carbodiimide reacts with the hydroxamic acid groups of desferrioxamine and destroys their ability to bind gallium. No significant metal release occurred when desferrioxamine was labeled with gallium prior to carbodiimide treatment, indicating that the presence of the gallium protects the hydroxamic acid troups from the effects of carbodiimide. These results have important implications for preparing high-specific-activity radiopharmaceuticals using bifunctional chelation.     

Major versus minor groove DNA binding of a bisarginylporphyrin hybrid molecule: A molecular mechanics investigation

On the basis of theoretical computations, we have recently synthesised [Perrée-Fauvet, M. and Gresh, N., Tetrahedron Lett., 36 (1995) 4227] a bisarginyl conjugate of a tricationic porphyrin (BAP), designed to target, in the major groove of DNA, the d(GGC GCC)2 sequence which is part of the primary binding site of the HIV-1 retrovirus site [Wain-Hobson, S. et al., Cell, 40 (1985) 9]. In the theoretical model, the chromophore intercalates at the central d(CpG)2 step and each of the arginyl arms targets O6/N7belonging to guanine bases flanking the intercalation site. Recent IR and UV-visible spectroscopic studies have confirmed the essential features of these theoretical predictions [Mohammadi, S. et al., Biochemistry, 37 (1998) 6165]. In the present study, we compare the energies of competing intercalation modes of BAP to several double-stranded oligonucleotides, according to whether one, two or three N- methylpyridinium rings project into the major groove. Correspondingly, three minor groove binding modes were considered, the arginyl arms now targeting N3, O2 sites belonging to the purine or pyrimidine bases flanking the intercalation site. This investigation has shown that: (i) in both the major and minor grooves, the best-bound complexes have the three N-methylpyridinium rings in the groove opposite to that of the phenyl group bearing the arginyl arms; (ii) major groove binding is preferred over minor groove binding by a significant energy (29 kcal/mol); and (iii) the best-bound sequence in the major groove is d(GGC GCC)2 with two successive guanines upstream from the intercalation. On the other hand, due to the flexibility of the arginyl arms, other GC-rich sequences have close binding energies, two of them being less stable than it by less than 8 kcal/mol. These results serve as the basis for the design of derivatives of BAP with enhanced sequence selectivities in the major groove.     

Probing small molecule binding to amyloid fibrils

Much effort has focussed in recent years on probing the interactions of small molecules with amyloid fibrils and other protein aggregates. Understanding and control of such interactions are important for the development of diagnostic and therapeutic strategies in situations where protein aggregation is associated with disease. In this perspective article we give an overview over the toolbox of biophysical methods for the study of such amyloid-small molecule interactions. We discuss in detail two recently developed techniques within this framework: linear dichroism, a promising extension of the more traditional spectroscopic techniques, and biosensing methods, where surface-bound amyloid fibrils are exposed to solutions of small molecules. Both techniques rely on the measurement of physical properties that are very directly linked to the binding of small molecules to amyloid aggregates and therefore provide an attractive route to probe these important interactions.     

Influences of the molecule–electrode interface structure on the conducting characteristics of the gold-4,4 bipyridine-gold molecular junction

Two popular models of the gold-4,4 bipyridine (44BPD)-gold molecular junction, i.e., the direct contact of the 44BPD molecule with the Au(1 1 1) surface and the intermediary contact through one extra gold atom on each side, were studied using density functional theory calculations under periodic boundary conditions. The relative position of the Fermi level is changed by the extra gold atom from well below the LUMO (lowest unoccupied molecular orbital) of the 44BPD molecule in the direct contact model to within the energy range of the LUMO in the intermediary contact model, indicating that the local structure of the contact can significantly affect the conducting characteristics of the junction. The dependence of the molecule–electrode interaction on the interface structure was also investigated in details.