Biomedical subjects

David L Allara

Publications and source records attributed to David L Allara.

16 recordsLinked to original sources

PubMed2007

Effects of molecular structure and interfacial ligation on the precision of Cu-bound alpha,omega-mercaptoalkanoic acid "molecular ruler" stacks.

Nanolithography processes based on designed, precision thickness multilayer thin films (molecular rulers) have been reported that enable patterning of features on surfaces from a few to the hundred nanometer range. These strategies are unique in their potential ability to enable wafer scale patterning of features of just a few nanometers. If these techniques could be developed to be sufficiently precise and generally applicable, they would fill a long-standing need in nanoscience. In this study a systematic and detailed analysis of the growth mechanisms and molecular layer structures has been carried out for the mercaptoalkanoic acid-copper ion multilayer thin film system currently used as the standard nanolithography resist. Our results show these films form via a redox reaction of thiol groups with surface-ligated Cu(II) ions to form adlayers at only approximately 50% coverage with islanding of the alkyl chains, thereby leading to rough topographies and less than theoretical thicknesses based on a 1:1 ideal adlayer. Strategies are suggested to help overcome these issues for molecular resist applications in nanolithographic processing.

Journal Article↗

PubMed2006

Controlling gold atom penetration through alkanethiolate self-assembled monolayers on Au{111} by adjusting terminal group intermolecular interactions.

The penetration behavior of thermally evaporated Au on S(CH(2))(15)CH(3), S(CH(2))(15)CO(2)CH(3), S(CH(2))(15)CO(2)H, K-modified S(CH(2))(15)CO(2)CH(3), and K-modified S(CH(2))(15)CO(2)H self-assembled monolayers (SAM) on Au substrates is investigated. Gold is a particularly interesting metal since vapor-deposited Au atoms are known to pass through alkanethiolate SAMs on Au{111} substrates at room temperature. Here we show that it is possible to control Au penetration by adjusting the interactions between terminal groups. It is found that Au atoms evenly penetrate into the CH(3) and CO(2)CH(3) films, forming smooth buried layers below the organic thin films. For the CO(2)H film, although Au atoms can still penetrate through it, filaments and mushroomlike clusters form due to H-bonding between film molecules. In the case of the K-modified CO(2)CH(3) or CO(2)H films, however, most Au atoms form islands at the vacuum interface. These results suggest that van der Waals forces and H-bonds are not strong enough to block Au from going through but that ionic interactions are able to block Au penetration. The measurements were performed primarily using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM). The combination of these highly complementary probes provides a very useful strategy for the study of metal atom behavior on SAMs.

Journal Article↗

PubMed2006

A simple technique to grow polymer brushes using in situ surface ligation of an organometallic initiator.

A simple approach is described here for the in-place synthesis of polymer brushes by surface-initiated polymerization. A cyano-terminated self-assembled monolayer on a gold surface was used to anchor a highly active cationic Pd organometallic initiator by ligand exchange. We grew ultrasmooth patterned poly(4-methoxystyrene) brushes with excellent thickness control at room temperature.

Catalysis↗

PubMed2006

Molecular self-assembly at bare semiconductor surfaces: preparation and characterization of highly organized octadecanethiolate monolayers on GaAs(001).

Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 +/- 1 degrees from the surface normal with a 43 +/- 5 degrees twist, a highly oleophobic and hydrophobic ambient surface, and direct S-GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to approximately 100 degrees C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au[111] surfaces. A two-step solution assembly process is observed: rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densification and asymptotic approach to final ordering. This process, while similar to the assembly of alkanethiols on Au[111], is nearly 2 orders of magnitude slower. Finally, despite differences in assembly rates and the thermal stability, exchange experiments with isotopically tagged molecules show that the octadecanethiol on GaAs(001) monolayers undergo exchange with solute thiol molecules at roughly the same rate as the corresponding exchanges of the same monolayers on Au[111].

Journal Article↗

PubMed2006

Ice nucleation and phase behavior on oligo(ethylene glycol) and hydroxyl self-assembled monolayers: simulations and experiments.

The nucleation and phase behavior of ultrathin D2O-ice overlayers have been studied on oligo(ethylene glycol) (OEG)-terminated and hydroxyl self-assembled monolayers (SAMs) at low temperatures in ultrahigh vacuum. Infrared reflection-absorption spectroscopy (IRAS) is used to characterize the ice overlayers, the SAMs, and the interactions occurring between the ice and the SAM surfaces. Spectral simulations, based on optical models in conjunction with Maxwell Garnett effective medium theory, point out the importance of including voids in the modeling of the ice structures, with void fractions reaching 60% in some overlayers. The kinetics of the phase transition from amorphous-like to crystalline-like ice upon isothermal annealing at 140 K is found to depend on the conformational state of the supporting OEG SAM surface. The rate is fast on the helical OEG SAMs and slow on the corresponding all-trans SAMs. This difference in kinetics is most likely due to a pronounced D2O interpenetration and binding to the all-trans segments of the ethylene glycol portion of the SAM. No such penetration and binding was observed on the helical OEG SAM.

Computer Simulation↗

PubMed2006

Single-molecule electrical junctions.

The objective of this review is to describe current experimental research of single-molecule electrical junctions in the context of various theoretical frameworks, with emphasis on the application of single-electron transistor theory to molecular junctions. Molecule quantum dots are at least an order of magnitude smaller than semiconductor quantum dots, which allows the study of many of the same mesoscopic and many-body effects at far higher temperatures. We discuss processes such as cotunneling, sequential tunneling, and incoherent tunneling, as well as the Kondo effect, Zeeman splitting, and the Coulomb diamond. Goals for future experimental work are outlined.

Journal Article↗

PubMed2005

Self-assembly, characterization, and chemical stability of isocyanide-bound molecular wire monolayers on gold and palladium surfaces.

Self-assembled monolayers (SAMs) of the isocyano derivative of 4,4'-di(phenylene-ethynylene)benzene (1), a member of the "OPE" family of "molecular wires" of current interest in molecular electronics, have been prepared on smooth, {111} textured films of Au and Pd. For assembly in oxygen-free environments with freshly deposited metal surfaces, infrared reflection spectroscopy (IRS) indicates the molecules assume a tilted structure with average tilt angles of 18-24 degrees from the surface normal. The combination of IRS, X-ray photoelectron spectroscopy, and density functional theory calculations all support a single sigma-type bond of the -NC group to the Au surface and a sigma/pi-type of bond to the Pd surface. Both SAMs show significant chemical instability when exposed to typical ambient conditions. In the case of the Au SAM, even a few hours storage in air results in significant oxidation of the -NC moieties to -NCO (isocyanate) with an accompanying decrease in surface chemical bonding, as evidenced by a significant increase in instability toward dissolution in solvent. In the case of the Pd SAM, similar air exposure does not result in incorporation of oxygen or loss of solvent resistance but rather results in a chemically altered interface which is attributed to polymerization of the -NC moieties to quasi-2D poly(imine) structures. Conductance probe atomic force microscope measurements show the conductance of the degraded Pd SAMs can diminish by approximately 2 orders of magnitude, an indication that the SAM-Pd electrical contact has severely degraded. These results underscore the importance of careful control of the assembly procedures for aromatic isocyanide SAMs, particularly for applications in molecular electronics where the molecule-electrode junction is critical to the operational characteristics of the device.

Journal Article↗

PubMed2005

Evolution of the interface and metal film morphology in the vapor deposition of Ti on hexadecanethiolate hydrocarbon monolayers on Au.

The combination of in situ X-ray photoelectron spectroscopy, infrared reflection spectroscopy, atomic force microscopy, and time-of-flight secondary ion mass spectrometry are used to probe the nature of the evolving interface chemistry and metal morphology arising from Ti vapor deposition onto the surface of a CH(3)(CH(2))(15)S/Au{111} self-assembled monolayer (SAM) at ambient temperature. The results show that for a deposition rate of approximately 0.15 Ti atom.nm(-2).s(-1) a highly nonuniform Ti overlayer is produced via a process in which a large fraction of impinging Ti atoms do not stick to the bare SAM surface. The adsorbed atoms form isolated Ti clusters and react with CH(3) groups to form carbide products at the cluster-SAM interfaces. Further growth of Ti clusters appears to be concentrated at these scattered reaction centers. The SAM molecules in the local vicinity are subsequently degraded to inorganic products, progressing deeper into the monolayer as the deposition proceeds to give an inorganic/organic nanocomposite. A continuous overlayer does not form until metal coverage approaches approximately 50 Ti atoms per SAM molecule. These data indicate that for applications such as molecular device contacts the use of Ti may be highly problematic, suffering from both a highly nonuniform contact area and the presence of extensive inorganic products such as nonstoichiometric carbides and hydrides.

Journal Article↗

PubMed2005

Relative conductances of alkaneselenolate and alkanethiolate monolayers on Au{111}.

The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.

Journal Article↗

PubMed2005

A perspective on surfaces and interfaces.

The importance of surfaces and interfaces cannot be overstated, with their reach extending from the hardware of the digital age to the processes of life. The past half-century has seen the development of a full and varied toolkit for characterizing them. This toolkit is now serving a growing interdisciplinary community and is providing a powerful platform for scientific research and manufacturing technology.

Microscopy, Scanning Probe↗

PubMed2005

Effect of local environment on molecular conduction: isolated molecule versus self-assembled monolayer.

Developing a fundamental understanding of molecular conduction in different device environments is essential to the advance of molecular electronics. We show through a quantitative comparison of two types of junctions with the same molecule - one based on an isolated individual molecule and the other on a self-assembled monolayer - that intrinsic differences in the conduction per molecule as large as several orders of magnitude can exist simply as a function of the presence or absence of neighboring molecules. This behavior can be understood on the basis of thermal and electrostatic effects that depend critically on the local molecular environment. These results will help to unify data obtained from disparate device structures and to provide an improved basis for designing future molecular electronic devices.

Journal Article↗

PubMed2005

Reversible bistable switching in nanoscale thiol-substituted oligoaniline molecular junctions.

Single molecular monolayers of oligoaniline dimers were integrated into sub-40-nm-diameter metal nanowires to form in-wire molecular junctions. These junctions exhibited reproducible room temperature bistable switching with zero-bias high- to low-current state conductance ratios of up to 50, switching threshold voltages of approximately +/-1.5 V, and no measurable decay in the high-state current over 22 h. Such switching was not observed in similarly fabricated saturated dodecane (C12) or conjugated oligo(phenylene ethynylene) (OPE) molecular junctions. The low- and high-state current versus voltage was independent of temperature (10-300 K), suggesting that the dominant transport mechanism in these junctions is coherent tunneling. Inelastic electron tunneling spectra collected at 10 K show a change in the vibrational modes of the oligoaniline dimers when the junctions are switched from the low- to the high-current state. The results of these measurements suggest that the switching behavior is an inherent molecular feature that can be attributed to the oligoaniline dimer molecules that form the junction.

Aniline Compounds↗

PubMed2004

Thermally activated conduction in molecular junctions.

We report temperature dependence measurements on the conductance of individual molecular wires. The results show for the first time in a molecular junction the theoretically predicted transition from coherent superexchange tunneling conductance to an activated hopping mechanism as temperature is increased.

Journal Article↗

PubMed2004

The dynamics of noble metal atom penetration through methoxy-terminated alkanethiolate monolayers.

We have studied the interaction of vapor-deposited Al, Cu, Ag, and Au atoms on a methoxy-terminated self-assembled monolayer (SAM) of HS(CH(2))(16)OCH(3) on polycrystalline Au[111]. Time-of-flight secondary ion mass spectrometry, infrared reflection spectroscopy, and X-ray photoelectron spectroscopy measurements at increasing coverages of metal show that for Cu and Ag deposition at all coverages the metal atoms continuously partition into competitive pathways: penetration through the SAM to the S/substrate interface and solvation-like interaction with the -OCH(3) terminal groups. Deposited Au atoms, however, undergo only continuous penetration, even at high coverages, leaving the SAM "floating" on the Au surface. These results contrast with earlier investigations of Al deposition on a methyl-terminated SAM where metal atom penetration to the Au/S interface ceases abruptly after a approximately 1:1 Al/Au layer has been attained. These observations are interpreted in terms of a thermally activated penetration mechanism involving dynamic formation of diffusion channels in the SAM via hopping of alkanethiolate-metal (RSM-) moieties across the surface. Using supporting quantum chemical calculations, we rationalized the results in terms of the relative heights of the hopping barriers, RSAl > RSAg, RSCu > RSAu, and the magnitudes of the metal-OCH(3) solvation energies.

Journal Article↗

PubMed2004

Direct covalent grafting of conjugated molecules onto Si, GaAs, and Pd surfaces from aryldiazonium salts.

Using aryldiazonium salts that are air-stable and easily synthesized, we describe here a one-step, room-temperature route to direct covalent bonds between pi-conjugated organic molecules on three material surfaces: Si, GaAs, and Pd. The Si can be in the form of single crystal Si including heavily doped p-type Si, intrinsic Si, heavily doped n-type Si, on Si(111) and Si(100), and on n-type polycrystalline Si. The formation of the aryl-metal or aryl-semiconductor bond attachments was confirmed by corroborating evidence from ellipsometry, reflectance FTIR, XPS, cyclic voltammetry, and AFM analyses of the surface-grafted monolayers. A data-encompassing explanation for the mechanism suggests a diazonium activation by reduction at the open circuit potential, with aryl radical secondary products bonding to the surface. The synthetic details are included for preparing the surface-grafted monolayers and the precursor diazonium salts. This spontaneous diazonium activation reaction offers an attractive route to highly passivating, robust monolayers and multilayers on many surfaces that allow for strong bonds between carbon and surface atoms with molecular species that are near perpendicular to the surface.

Journal Article↗

PubMed2002

Bond insertion, complexation, and penetration pathways of vapor-deposited aluminum atoms with HO- and CH(3)O-terminated organic monolayers.

The interaction of vapor-deposited Al atoms with self-assembled monolayers (SAMs) of HS-(CH(2))(16)-X (X = -OH and -OCH(3)) chemisorbed at polycrystalline Au[111] surfaces was studied using time-of-flight secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, and infrared reflectance spectroscopy. Whereas quantum chemical theory calculations show that Al insertion into the C-C, C-H, C-O, and O-H bonds is favorable energetically, it is observed that deposited Al inserts only with the OH SAM to form an -O-Al-H product. This reaction appears to cease prior to complete -OH consumption, and is followed by formation of a few overlayers of a nonmetallic type of phase and finally deposition of a metallic film. In contrast, for the OCH(3) SAM, the deposited Al atoms partition along two parallel paths: nucleation and growth of an overlayer metal film, and penetration through the OCH(3) SAM to the monolayer/Au interface region. By considering a previous observation that a CH(3) terminal group favors penetration as the dominant initial process, and using theory calculations of Al-molecule interaction energies, we suggest that the competition between the penetration and overlayer film nucleation channels is regulated by small differences in the Al-SAM terminal group interaction energies. These results demonstrate the highly subtle effects of surface structure and composition on the nucleation and growth of metal films on organic surfaces and point to a new perspective on organometallic and metal-solvent interactions.

Alcohols↗