Total since November 1999
American Vacuum Society 43rd National Symposium,
14-18 October 1996, Philadelphia, PA.
Molecular and Nanoscale Electronic Properties of Organic Thin Films.
L. A. Bumm, J. J. Arnold, M. T. Cygan, D. L. Allara, J. M. Tour,
and P. S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
The molecular and nanoscale electronic structure of organic
thin films is studied using scanning tunneling microscopy (STM).
Thiols adsorbed onto Au(111) are an ideal model system for
studying molecular and nanoscale electronic structure. We use
self-assembled monolayers of alkanethiolates as a matrix to
dilute and to isolate "guest" molecules in the film. STM
can distinguish the isolated guest molecules from the
alkanethiolate matrix even for guest-matrix differences as
slight as one methylene unit in alkyl chain length.
The microwave frequency alternating current STM (ACSTM)
provides additional contrast which complements
conventional STM topographic images. The combination of STM
and ACSTM are used to differentiate between molecular wire
candidates and the surrounding alkanethiolate matrix.
These techniques allow us to measure the electronic properties
of single molecules isolated by the matrix.
American Vacuum Society 43rd National Symposium,
14-18 October 1996, Philadelphia, PA.
Session: Self-Assembled Monolayers
Molecular Dynamics Simulations of Alkanethiols on
Stepped Au(111).
K. R. Krom, R. Bhatia, B. J. Garrison, and P. S. Weiss,
Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Molecular dynamics are used to study the effects of monatomic substrate steps on self-assembled monolayers of alkanethiols on Au(111). Our calculations reveal a mechanism of molecular reorientation at the step edges, leading to long-range structural ordering of adsorbates. The azimuthal tilt angle of the molecular chains appears to be determined by the direction of the step edge. Further, migration of adsorbates to the step produces defects and local disorder in the monolayers which mirrors our previous experimental STM images of these monolayers at substrate step edges. Adsorbate molecules at the step edges also produces kinks in the steps by a concerted mechanism.
American Vacuum Society 43rd National Symposium,
14-18 October 1996, Philadelphia, PA.
Controlling Surface Molecular Structure: Step by Step Construction of
Multi-Component Organic Monolayers.
M. T. Cygan,
L. A. Bumm, J. J. Arnold, T. D. Dunbar, D. L. Allara,
and P. S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Precise control of molecular placement and structure is one of the most important prerequisites for nanofabrication on surfaces. We explore methods of designing and building surface film structures using organic thiolates attached to Au(111) substrates. We have examined the insertion of single aromatic oligomers into sites in a nominally complete alkanethiol lattice. We have also examined the replacement of significant portions of one type of alkanethiol lattice with molecules of a second type. Using high resolution scanning tunneling microscopy, we find in each of these studies that molecules preferentially adsorb or exchange at defect sites in the film (such as step edges) which offer increased access to the underlying Au substrate. We discuss the structure, stability, and lattice ordering of these monolayers as compared to similar films formed by coadsorption. We discuss how control of the number and disctribution of defects can be used to control the film structure and local composition.
American Vacuum Society 43rd National Symposium,
Monday-Friday 14-18 October 1996, Philadelphia, PA.
Surface Science Poster Session
Substrate-Mediated Interactions and Their Role in
Nanoscale Processes.
M. M. Kamna, T. M. Graham, S. J. Stranick, and P. S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Surface features and adsorbates perturb the electronic properties of the surrounding substrate surface. This modifies the adsorption, dynamics, and chemistry of nearby atoms and molecules. Using scanning tunneling microscopy we have imaged these perturbations at low temperature on Cu(111) and have observed their effects on the ordering and dynamics of adsorbed molecules. We discuss the important implications that substrate-mediated interactions can have on the atomic-scale mechanisms of film growth and heterogeneous selective catalysis. We speculate on how such interactions can be exploited in the design and creation of atomically precise self-assembling structures at the nanometer scale and beyond.
American Vacuum Society 43rd National Symposium,
14-18 October 1996, Philadelphia, PA.
Surface Science Poster Session
The Dynamics of Benzene Adsorption on Ni(110).
J. H. Ferris, J.G. Kushmerick, J. A. Johnson,
and P. S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
The low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) offers the unique opportunity to study single adsorbates and surface structures of small numbers of adsorbates. We use a low-temperature UHV STM to study the dynamics of adsorbates on single crystal metal surfaces in the dilute coverage regime. We dose a cold Ni(110) crystal with benzene molecules from the room temperature UHV chamber above the STM. At 4K diffusion is sufficiently slow to allow imaging of isolated molecules. For lateral motion, i.e. transient mobility, induced by surface processes such as adsorption we are able to analyze the final positions of the adsorbates in order to measure the distances covered and to elucidate the means by which energy is accommodated to the surface. When dosing, we align the crystal on a goniometer in a cooled chamber so that molecules adsorbing from the gas phase have surface plane projections of their momentum vectors pointing toward surface step risers at an oblique angle, thus increasing the effective terrace width. Finite amounts of adsorbed hydrogen induce the formation of line defects on the Ni(110) surface. These line defects form comb-like structures originating at step edges. We use these to investigate two-dimensional scattering and the probability with which molecules stick at defects.
American Vacuum Society 43rd National Symposium,
14-18 October 1996, Philadelphia, PA.
Molecular Contrast within Organic Films with Scanning Tunneling
Microscopy.
J. J. Arnold, L. A. Bumm, M. T. Cygan,
and P. S. Weiss, D. L. Allara, J. M. Tour, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Scanning tunneling microscopy (STM) has become a valuable tool in the study of organic thin films. Although widely studied, the question of how these molecules contribute to tunneling or conduction remains unanswered. We have grown and modified multi-component alkanethiols on Au(111) by a combination of deposition and processing techniques and have studied them using high-resolution STM. We have been able to show STM can distinguish between molecules of varying chain length and composition. By analyzing images and local spectra of isolated and aggregated alkanethiols and/or conjugated thiols, we have been able to obtain insight into the mechanism by which these films can be imaged and the extent to which organic molecules conduct.
National Science Foundation Materials Chemistry Workshop, Philadelphia, PA.
Issues and Opportunities in Scnning Probe Microscopies.
Paul S. Weiss,
Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Scanning probe microscopies offer unprecedented opportunities to explore the surfaces of materials at the atomic scale. With these tools, we can measure the structures, dynamics, spectra, and selected properties of surfaces and even of single molecules on surfaces. However, the results obtained with such instruments can be misleading for three reasons. First and foremost, we have no predictive understanding of images, spectra, force curves, and other data obtained from these microscopes. Second, typically only small areas which are amenable to imaging are studied, and these may not be representative of the entire sample. Third, we all respond strongly to visual cues and tend to recognize patterns (even in noise!). While we will focus on the first issue, we will touch on all three topics.
The meaning of much of the data recorded with scanning probe microscopes remains undetermined. Surprisingly little work has been done to remedy this situation. Here, we will discuss some areas which are being actively pursued. For scanning tunneling microscopy, these include: interpreting images, measuring the spectra and conductivity of single molecules, and understanding the contributions of parts of molecules in the tunnel junction to the tunneling current.
In all such microscopes, gaining a better understanding of what is imaged requires extracting complementary information from the microscope and ancillary tools. This situation is complicated by the fact that the areas of interest may be in the minority on the surface by orders of magnitude. Thus, we are commonly faced with the situation where only a scanned probe microscope and related calculations and analyses can yield useful information.
This is particularly true in the case of using scanned probe microscopes to analyze quantum structures such as single molecular electronics. We will discuss the preliminary experiments along these lines and the many upcoming determinations of the properties of nanometer-scale structures created. Scanning probe microscopies will remain key tools in this area where our ability to create structures and devices often exceeds our ability to place and to study them. The microscope serves both to find and to probe the structure. We are also able to screen potentially useful materials and structures rapidly without the need to fabricate macroscopic connections and devices.
There remain many opportunities for extracting more and better information from scanned probe microscopes.
Manhattan Poster Project 1996, University of Delaware, Newark, DE.
Transient Mobility and 2D Scattering of Benzene on Ni{110} at 4K.
J.G. Kushmerick, J. H. Ferris, J. A. Johnson,
and P. S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Transient mobility and two-dimensional (2-D) scattering of benzene on the Ni{110} surface at 4K is observed by scanning tunneling microscopy (STM). Room temperature benzene molecules impinge upon a cryogenically cold Ni{110} surface with momentum parallel to the surface plane. The final adsorption site of benzene molecules is determined by STM. Surface sites devoid of adsorbed benzene at the top of the step riser yield a lower limit of 7 atomic sites for the accommodation length (length of transient mobility). Hydrogen-induced added rows and the naturally occurring step edges combine to form a comb structure which allows us to investigate 2-D scattering.
University of Washington, Condensed Matter Physics Seminar, Department of Physics, Seattle, WA.
Atomic-Scale Views of Interactions and Dynamics of Single Molecules on Surfaces.
Paul S. Weiss,
Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
University of Washington, Colloquium, Department of Molecular Biotechnology, Seattle, WA.
Manipulating Single Molecules on Surfaces.
Paul S. Weiss,
Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Sandia National Laboratories, Livermore, CA.
Probing Surface Interactions and Dynamics with the Scanning Tunneling Microscope.
M. M. Kamna, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Naval Research Laboratories, Arlington, VA.
Controlling Surface Molecular Structure: Step by Step Construction of Multi-Component Organic Monolayers.
M. T. Cygan, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
University of Washington, Surface Science Seminar, Departments of Chemistry and Physics, Seattle, WA.
Controlling the Structure and Properties of Self-Assembled Monolayers.
Paul S. Weiss,
Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
1996 Fall Meeting of the Materials Research Society, Boston, MA.
(Symposium C Thin Films and Surfaces: Structure and Morphology)
Growth, Modification, and Control of the Structures of Mixed Composition Organic Monolayers.
L. A. Bumm,
J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P. Burgin, L. Jones
II, D. L. Allara,* J. M. Tour,* and P. S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Control of the molecular-scale structure of multi-component self- assembled of organic thiols on Au{111} can be achieved by selecting a combination of deposition and processing techniques. These include competitive adsorption (growth) and subsequent exchange (modification). The effects of this processing on the molecular-scale structure have been studied by conventional and AC scanning tunneling microscopy.
Lateral epitaxy has been observed where a growing domain of one molecular species is grafted onto an existing crystalline lattice of a different molecular species with no lattice mismatch. In other examples, the limited substrate access afforded by structural defects in the films has been utilized to insert single molecules for further use or study.
We also use these mixed composition monolayers to gain insight into the mechanism by which these films can be imaged and the extent to which organic molecules conduct. By analyzing images and local spectra of isolated and aggregated molecules, we can determine the extent to which neighboring molecules contribute to these processes.
We believe that none of the structures we obtain are equilibrium structures. We discuss relevant considerations for stabilizing the nanometer-scale structures created.
1996 Fall Meeting of the Materials Research Society, Boston, MA.
(Symposium D-Properties and Applications of Electronic
Organic Materials and Fullerenes)
Self-Assembly Strategies for Fabrication of Molecule-Based Devices.
D. Allara, T. Dunbar, P. Weiss, L. Bumm and M. Cygan, Dept. of Chemistry,
Pennsylvania State University, University Park, PA; J. Tour, T. Burgin and
L. Jones, II, Dept. of Chemistry and Biochemistry, University of South
Carolina, Columbia, SC; M. Reed and R. Lombardi, Dept. of Electrical
Engineering, Yale University, New Haven, CT.
In order to favorably arrange device candidate molecules at contacting electrode surfaces, we have developed solution self-assembly methods involving mixed composition monolayers which isolate individual candidate molecules pointing vertically outward from the surface in a background of low conductivity alkyl chain molecules. Typical candidate molecules include sulfur group terminated oligomers constituted from rigid rod phenylene-ethynylene and thiophene units and the self-assembly was carried out on gold surfaces. The overall structures have been characterized by XPS, IR, Auger and ellipsometry while the molecular distributions and individual electronic responses of the molecules have been characterized by molecular resolution STM. These types of structures are being used as a basis for incorporation of candidate molecules into nanometer-scale gaps for electrical testing of device characteristics.
University of Washington, Physical Chemistry Seminar, Department of Chemistry, Seattle, WA.
Growth, Modification, and Control of the Structures of Mixed Composition Organic Monolayers.
L. A. Bumm,
Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Penn State University, Analytical Chemistry Seminar, University Park, PA.
The Electronic Nose.
J. A. Johnson, Department of Chemistry, The Pennsylvania State University,
University Park, PA 16802-6300
Twelfth Annual Graduate Research Exhibition, University Park, PA.
Adsorption Dynamics of Benzene on Ni(110) at 4K.
J. G. Kushmerick, J. H. Ferris, J. A. Johnson, P. S. Weiss, Department of Chemistry, The Pennsylvania State University,
University Park, PA 16802-6300
The adsorption dynamics of benzene on Ni{110} at 4K are investigated with a
low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM).
The low-temperature UHV STM offers the unique opportunity to study surface
processes on the atomic scale. We dose a 4K Ni{110} single crystal with
room temperature benzene molecules. The time scale for adsorption is too
rapid to allow real-time monitoring with the STM. At 4K thermal diffusion
is frozen out, allowing imaging of isolated molecules. The final adsorption
sites of benzene molecules are mapped, enabling a statistical analysis of
adsorption site distribution. Since thermal diffusion is quenched, any
non-random distribution of benzene across adsorption sites is attributed to
the effects of motion during energy accommodation and trapping of benzene at
favorable adsorption sites.
Brian Bent Memorial Symposium.
American Chemical Society Meeting, San Francisco, CA, Sunday-Thursday 13-17 April 1997.
Surface Chemistry: Following Brian Bent.
Paul S. Weiss, M. M. Kamna, S. J. Stranick,*
and T. M. Graham, Department of Chemistry, The Pennsylvania State University,
University Park, PA 16802-6300
We describe two experiments inspired by the work of Brian Bent. In both cases, following Brian's key work, we were able to take aim at fundamental microscopic issues brought out in his work. For benzene on Cu{111}, using his thermodynamic and kinetic measurements and with Brian's encouragement and assistance, we were able to deposit a fraction of monolayer of the molecules in order to look at the molecular-scale dynamics at low temperature and dilute coverage using scanning tunneling microscopy. For iodobenzene on Cu{111}, we were able to image the biphenyl recombination products and once again to follow the dynamics of these molecules. *Present address: Chemistry Sciences & Technology Laboratory; Surface & Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899.
American Chemical Society Meeting, San Francisco, CA, Sunday-Thursday 13-17 April 1997.
Atomic-Scale Views of the Dynamics, Interactions, and Properties
of Single Adsorbed Molecules.
Paul S. Weiss, J. J. Arnold, L. A. Bumm,
M. T. Cygan, J. H. Ferris, J. A. Johnson, M. M. Kamna, and J. G. Kushmerick,
Department of Chemistry, The Pennsylvania State University, University
Park, PA 16802-6300
Scanning probe microscopes allow unprecedented views of the dynamics
of single adsorbates and the effects that they have on their surface environment.
We are able to deduce the trajectories followed by individual adsorbates
in the adsorption process and to infer how quickly they accommodated their
incident kinetic energy to the surface. We discuss the motion of individual
adsorbates at reduced temperature and the motion and electronic properties
of constrained "marker" molecules in densely packed films.
__________
This work has been supported by the Alfred P. Sloan Foundation, the National
Science Foundation and the Office of Naval Research.
University of Oregon, Materials and Physical Chemistry and Condensed Matter Physics Seminar, Departments of Chemistry and Physics, Eugene, OR.
Atomic-Scale Views of Interactions and Dynamics of Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
University of Chicago, Chemistry Colloquium, Department of Chemistry, Chicago, IL.
Atomic-Scale Views of Interactions and Dynamics of Single Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
University of Pennsylvania, Physical Chemistry Seminar, Department of Chemistry, Philadelphia, PA.
Atomic-Scale Views of Interactions and Dynamics of Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
University of Washington, Department of Chemistry, Seattle, WA.
Atomic-Scale Views of Interactions and Dynamics of Single Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Charles Evans and Associates, Redwood City, CA.
Atomic-Scale Views of Atoms and Molecules on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
University of Washington, Science in Progress Seminar, Department of Molecular Biotechnology, Seattle, WA.
Marrying Techniques for Manipulating and Probing Single Molecules: Something Old, Something New, Something Borrowed , Something Blue.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300 and Department of Molecular Biotechnology, University of Washington, Seattle, WA 98195
California Institute of Technology, Inorganic Electrochemistry Seminar, Department of Chemistry, Pasadena, CA.
Electron Tunneling Measurements Through and Near Single Atoms and Molecules.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
The motion of benzene molecules across the Ni{110} surface was studied with a low temperature, ultrahigh vacuum scanning tunneling microscope (STM). A method for studying motion on surfaces using the distribution of final adsorption sites was developed. Using small amounts of a H/Ni added row structure combined with the natural barrier of the step risers, a comb-like structure was grown on the terraces. To investigate the role of momentum transfer in directing transient mobility, the Ni{110} crystal was aligned such that the impinging molecules would travel across the terraces towards the step risers and the comb-like H/Ni structure. Using a statistical method, we demonstrated that benzene molecules were preferentially bound at step edges. We determined that the initial tangential momentum did not contribute significantly to the motion of benzene molecules towards the step edges. Rather, these measurements indicated that a perturbation of the surface electronic structure, caused by the presence of step edges, created an attractive interaction between step edges and benzene molecules. This effect was limited to a narrow zone along the step edge which extended two sites onto the Ni{110} terraces. We also evaluated the potential for two dimensional scattering of benzene molecules from atomic-scale structures.
57th Annual Physical Electronics Conference, 18-21 June 1997, Eugene, OR.
Interactions and Dynamics of Adsorbed Aromatic Molecules and Radicals.
Paul S. Weiss, P. S. Weiss, M. M. Kamna,* and T. M. Graham,** Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
stm@psu.edu http://stm1.chem.psu.edu/
We have used low temperature scanning tunneling microscopy to probe the interactions and dynamics of benzene [1] and 7,7',8,8'-tetracyanoquinodimethane (TCNQ) molecules and of phenyl radicals on the Cu{111} surface. We have found that each of these species perturb the electronic structure of the surrounding surface. The induced modulations of the surface electronic structure are the substrate-mediated interactions that determine the adsorbate dynamics and structures.
The TCNQ molecules greatly perturb the surface electronic structure as they are strong electron acceptors. This electrophilicity determines where the molecules initially adsorb at dilute coverage and how they nucleate and grow complex structures at submonolayer coverages. The growth front of the submonolayer films have well defined electronic structures which key subsequent adsorbates into place.
The phenyl radicals (C6H5.) align into complex pairs on the surface as a prelude to forming biphenyl molecules (at higher temperatures). Individual radicals can be found moving in and out of well defined positions. These are aligned by the partner with which the radical complexes. Phenyl radicals have covalent interactions with the substrate and thus diffuse more slowly than the benzene molecules that we have previously studied [1].
1. S. J. Stranick, M. M. Kamna, and P. S. Weiss, Science 266, 99 (1994);
S. J. Stranick, M. M. Kamna, and P. S. Weiss, Surface Science 338, 41 (1995);
M. M. Kamna, S. J. Stranick, and P. S. Weiss, Science 274, 118 (1996).
JASON, La Jolla, CA.
Technologies for Genomics.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300, and Department of Molecular Biotechnology, University of Washington, Seattle, WA 98195-7730
University of Cambridge, Physical Chemistry Seminar, Cambridge, England.
Atomic-Scale Views of Interactions and Dynamics of Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300, USA and
Department of Molecular Biotechnology, The University of Washington,
Seattle, WA 98115-7730, USA
Scanning probe microscopes allow unprecedented views of the site-specific interactions and dynamics of adsorbates. We are able to measure directly the dramatic changes in these dynamics due to surface features such as steps, defects, and co-adsorbates. In the case of dilute coverage, we can observe the origins of these local changes by observing the perturbations in the surface electronic structure using low temperature ultrahigh vacuum scanning tunneling microscopes. We discuss the chemical consequences of these perturbations and how they may be exploited to enhance reactivity and the atomic-scale precision of film growth. We are also able to measure the electronic properties of single adsorbed atoms and molecules. We discuss how we understand electron transmission through a broad range of adsorbates.
STM'97: the 9th Scanning Tunneling Microscopy Conference, 20-25 July 1997, Hamburg, Germany.
Poster Tu18.2P04IMAGING ADSORBED MOLECULES AND THEIR INTERACTIONS WITH SCANNING TUNNELING MICROSCOPY. M. M. Kamna, T. M. Graham, and Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA.
Surface features and adsorbates perturb the
electronic properties of the surrounding substrate surface.
This modifies the adsorption, dynamics, and chemistry of
nearby atoms and molecules. Using scanning tunneling
microscopy we have imaged these perturbations at low
temperature and low coverage on Cu(111) and have observed their effects on
the ordering and dynamics of adsorbed benzene molecules, phenyl radicals, and
7,7',8,8'-tetracyanoquinodimethane (TCNQ) molecules. The adsorbates adopt well defined positions and orientations and have residence times at these sites determined by the electronic perturbations of the surface and co-adsorbates. We discuss
the important implications that substrate-mediated
interactions can have on the atomic-scale mechanisms of
film growth and heterogeneous selective catalysis. We
speculate on how such interactions can be exploited in the
design and creation of atomically precise self-assembling
structures at the nanometer scale and beyond.
This work was supported by the National Science Foundation and the Office of Naval Research.
STM'97: the 9th Scanning Tunneling Microscopy Conference, 20-25 July 1997, Hamburg, Germany.
We17.2MO3IMAGING AND PROBING SELF-ASSEMBLED MONOLAYERS AND MOLECULES INSERTED IN THESE ORGANIC FILMS. L. A. Bumm, J. J. Arnold, M. T. Cygan, J. D. Shore, L. F. Charles, T. D. Dunbar, D. L. Allara, L. Jones,1 T. P. Burgin,1 J. M. Tour,1 and Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA.
We discuss recent experiments which shed light upon how it is we can image organic films tens of Ångströms thick with the scanning tunneling microscope. In lattice-resolved images of mixed composition alkanethiols on Au(111, we differentiate the chemical species present on the surface. We exert chemical control of these films to separate the component molecules or to leave them randomly mixed. By inserting single molecules or bundles of molecules, we use these films for isolating the inserted molecules for further study. We then probe the inserted molecules and bundles using conventional and novel tunneling spectroscopies. The images and spectra of these inserted conjugated and functionalized molecules are interpreted within the same framework we use to understand the alkanethiol films.
8th International Conference on Organized Molecular Films, 24-29 August 1997, Asilomar, CA.
Control of Structures and Properties in Self-Assembled Monolayers.
Paul S. Weiss, L. A. Bumm, J. J. Arnold, L. F. Charles, M. T. Cygan, T. D. Dunbar, J. J. Jackiw, and D. L. Allara, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Control of the molecular-scale structures of multi-component self-assembled films of organic thiols and selenols on Au{111} can be achieved by selecting a combination of deposition and processing techniques. These include competitive adsorption and subsequent exchange. The effects of this processing on the molecular-scale structures have been studied using conventional and AC scanning tunneling microscopy as well as a host of macroscopic spectroscopies. A growing domain of one alkanethiolate species can be grafted onto an existing crystalline lattice of a different molecular species with no lattice mismatch. In other examples, the limited substrate access afforded by structural defects in the films has been utilized to insert single molecules for further use and study. None of the films we obtain are equilibrium structures. We discuss relevant considerations for stabilizing the nanometer-scale structures created. We also use mixed composition monolayers to gain insight into the mechanisms by which these monomolecular films can be imaged and the extent to which organic molecules conduct. By analyzing images and local spectra of isolated and aggregated molecules, we can determine the contributions of the alkyl chains to the tunneling. We have self-assembled mixed composition films containing both alkanethiolates and alkaneselenates. These should provide insight into the contributions of the substrate-molecule bonds to the tunneling.
Relevant references:
1) L. A. Bumm et al., Science, 271 (1996) 1705.
2) S. J. Stranick et al., J. Phys. Chem., 98 (1994) 7636.
Symposium on Nanoscale and Patterned Assemblies
American Chemical Society Meeting, Las Vegas, NV, 7-11 September 1997.
Patterning Molecules on Surfaces at the Nanometer Scale Through Self-Assembly.
P. S. Weiss, J. J. Arnold, L. A. Bumm, L. F. Charles,
M. T. Cygan, T. D. Dunbar, J. J. Jackiw, D. L. Allara, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Symposium on Nanoscale and Patterned Assemblies
American Chemical Society Meeting, Las Vegas, NV.
Combining IR Spectroscopy and STM for Probing Inserted Molecules in Monolayer Assemblies.
Dave L. Allara, P.S. Weiss, T.D. Dunbar, M. T. Cygan, J. Arnold and L.A. Bumm, Pennsylvania State University, University Park, PA 16802
We have been developing strategies for the insertion and characterization of
chemically and electronically interesting molecules into a uniform,
relatively inactive molecular background of self-assembled alkanethiolate
chains on Au(111) textured surfaces. We have found that IR can be used to
follow the concentration, average geometry and local environment of the
inserted molecules while STM can be used to probe the surface locations at
molecular resolution as well as the electronic properties of individual
molecules. Examples will be shown for various linear molecules inserted
into varying chain length alkanethiolate lattices.
FOM - Institute for Atomic and Molecular Physics, Amsterdam, The Netherlands.
Atomic-Scale Views of Interactions and Dynamics of Molecules on Surfaces.
19th IUVSTA-Workshop : Surface Chemistry on the Nanoscopic Scale, 11-13 September 1997, Oegstgeest, The Netherlands.
Atomic-Scale Measurements, Interactions, and Manipulation of Molecules.
Scanning probe microscopes can be used to manipulate individual atoms and molecules, but it remains difficult to make substantial structures in this way or to make many copies of a specific structure in parallel. We discuss the use of interactions between adsorbates, self-assembly, and chemical processing in order to create structures with atomic-scale precision. We then use such structures to probe the chemical, physical, and electronic behavior at the nanometer scale.
Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Physical Chemistry Seminar, Berlin, Germany.
Atomic-Scale Views of Interactions and Dynamics of Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300, USA and
Department of Molecular Biotechnology, The University of Washington,
Seattle, WA 98115-7730, USA
Scanning probe microscopes allow unprecedented views of the site-specific interactions and dynamics of adsorbates. We are able to measure directly the dramatic changes in these dynamics due to surface features such as steps, defects, and co-adsorbates. In the case of dilute coverage, we can observe the origins of these local changes by observing the perturbations in the surface electronic structure using low temperature ultrahigh vacuum scanning tunneling microscopes. We discuss the chemical consequences of these perturbations and how they may be exploited to enhance reactivity and the atomic-scale precision of film growth. We are also able to measure the electronic properties of single adsorbed atoms and molecules. We discuss how we understand electron transmission through a broad range of adsorbates.
Monday-Wednesday 6-8 October 1997
Office of Naval Research, Solid State and Surface Chemistry Program Contractors Meeting, Alexandria, VA.
Molecular Electronics
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Naval Research Laboratory, Washington, DC.
Atomic-Scale Views of Interactions and Dynamics of Molecules
on Surfaces.
Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
Scanning probe microscopes allow unprecedented views of the site-specific interactions and dynamics of adsorbates. We are able to measure directly the dramatic changes in these dynamics due to surface features such as steps, defects, and co-adsorbates. In the case of dilute coverage, we can observe the origins of these local changes by observing the perturbations in the surface electronic structure using low temperature ultrahigh vacuum scanning tunneling microscopes. We discuss the chemical consequences of these perturbations and how they may be exploited to enhance reactivity and the atomic-scale precision of film growth. We are also able to measure the electronic properties of single adsorbed atoms and molecules. We discuss how we understand electron transmission through a broad range of adsorbates.
Defense Science Study Group V, Study Papers, IDA, Alexandria, VA, Monday-Wednesday 17-19 November 1997.
A Survey of Issues, Components, Systems, and Opportunities for Detectors of Biological and Chemical Warfare Agents
Geoffroy A. Blake, California Institute of Technology, Pasadena, CA, Mara Prentiss, Department of Physics, Harvard University, Cambridge, MA, and Paul S. Weiss, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
International Conference on Molecular Electronics --- Science and Technology, Palmas del Mar, Puerto Rico, Sunday-Thursday 14-18 December 1997.
Electron Transport through Organic Molecules Isolated or Organized in Monomolecular Films.
P. S. Weiss, L. A. Bumm, L. F. Charles, T. D. Dunbar,
J. J. Jackiw, J. A. Johnson, and D. L. Allara, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300
We measure electron transport through the lengths of single and bundled
molecules. We compare organic molecules which are fully conjugated
and those with saturated alkyl chains. We also study the effects
of chemical substitution on electron transfer. By processing self-assembled
monolayers we use film defects to insert single molecules, to insert
bundles of molecules, or to graft new molecular terraces onto existing
domains. We connect our scanning tunneling microscopy measurements
to ubiquitous electron transfer phenomena in such areas as biochemistry
and electrochemistry by separating the transconductance into components
arising from transport through the molecule vs. the tunneling gap
outside the film. We show how these components can be measured independently.
International Conference on Molecular Electronics --- Science and Technology, Palmas del Mar, Puerto Rico, Sunday-Thursday 14-18 December 1997.
Strategies for Self-Assembly And Hookup of Molecule-Based Devices.
D. Allara, T. Dunbar, P. Weiss, L. Bumm, M. Cygan, Department of Chemistry,
The Pennsylvania State University, University Park, PA 16802-6300, and J. Tour, T. Burgin and L. Jones, Dept. of Chemistry and Biochemistry, University of South Carolina,
Columbia, SC
In order to favorably arrange device candidate molecules at contacting electrode surfaces, we have developed solution self-assembly methods involving mixed composition monolayers which isolate individual candidate molecules pointing vertically outward from the surface in a background of low conductivity alkyl chain molecules. Typical candidate molecules include sulfur group terminated oligomers constituted from rigid rod phenylene-ethynylene and thiophene units and the self-assembly was carried out on gold surfaces. The overall structures have been characterized by XPS, IR, Auger and optical probes while the molecular distributions and individual electronic responses of the molecules have been characterized by molecular resolution STM. These types of structures are being used as a basis for incorporation of candidate molecules into nanometer-scale gaps for electrical testing of device characteristics. Recent work has focussed on the issue of metallization-induced effects which arise in the fabrication of hybrid devices with vapor-deposited metal contacts and on altering oligomer-electrode interface states by attachment with S, Se or Te to gold as well as III-V substrates.
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6 January 1998
psw