You missed us!
Here are some abstracts of talks that we gave in 2000 and 2001.

Effect of Curvature on the Control of Local Structure of Model Membranes
A. Hatzor, T. G. D'Onofrio, C. D. Keating, M. J. Natan, and P. S. Weiss
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The physical properties and the biological functions of lipid bilayers are dependent on the local structure of the membrane. We describe methods to modulate the local structure of model membranes through control of membrane curvature. Our model membranes of choice are giant unilamellar vesicles prepared from multi-component lipid mixtures. We use lipid components known to phase segregate into domains large enough to resolve with optical microscopy. Fluorescence microscopy techniques indicate properties of the local structure and enable us to observe domain rearrangement as a function of curvature. Methods to manipulate the curvature include the polymerization of tubulin encapsulated within the vesicle and "stretching" the membrane in the grasp of an optical trap. The combination of the optical trap with a flowcell enables us to control the forces used to modulate the curvature. We will measure the local composition dependence of these physical properties, correlating local structure and models of biological function.

Placement, Control and Isolation of Molecules via Directed Assembly
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We manipulate and measure the structures of monolayer films in order to tune their properties. This is accomplished by controlling the defect type and density in the films. We then process the films to insert single molecules, to insert bundles of molecules, or to graft new molecular terraces onto existing domains by using these defects to advantage. The inserted molecules can serve as the anchor points for polymerization; this allows us to choose to produce single polymer dots or isolated polymer brushes. We connect our scanning tunneling microscopy measurements to electron transfer phenomena which are ubiquitous 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. We prepare films predicted to have many equivalent defect sites so as to provide identical matrix isolation environments for single molecular wire candidates. We also prepare films with well defined interfaces between separated components so that insertion, deposition, or reaction can be directed to these molecularly sharp boundaries.

STM Investigation of Benzene Adsoprtion on Ag(110)
K. F. Kelly, J. J. Jackiw, J. I. Pascual, H. Conrad, H.-P. Rust, and P. S. Weiss, Fritz-Haber Institut and Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We have investigated the adsorption of benzene on Ag(110) using the scanning tunneling microscope. We found that the molecules preferentially adsorb above step edges at 66 K. The preference for step edge adsorption is attributed to the Smoluchowski effect enhancing the empty states to which charge is donated from the \pi orbitals of the benzene. However, there is no adsorption at the [001] steps. A lack of free charge due to a gap in the Ag Fermi surface along that direction reduces the Smoluchowski effect and thus the adsorption at these steps. After further deposition, we find that benzene forms a weakly adsorbed hexagonal monolayer. The monolayer is imaged at large tip-sample separations and is transparent upon closer approach. This transparency is reduced near steps and point defects.

Directing Assembly in Monolayers by Controlling Exchange Kinetics
A. L. Bross, A. Hooper, L. A. Bumm, D. L. Allara, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The ability to exchange molecules into self-assembled monolayers allows local selective control of the physical, electronic, and chemical properties of thin films. We study the kinetics of this exchange and the resulting structures in order to gain this level of control. The exchange of deuterium-labeled dodecanethiolate self-assembled monolayers on gold with millimolar decanethiol solutions in ethanol was monitored using external reflectance infrared spectroscopy. The deuterium-labeled monolayers were immersed into the perhydro thiols for times ranging from minutes to days at temperatures ranging from -5 to 25°C. Surface concentrations of the two thiolate species were calculated from the relative areas of the C-H and C-D vibrational modes. The temperature of the exchange solution determines the total amount of exchange. In addition, scanning tunneling microscopy and infrared spectroscopy results show that heating the monolayers in the original thiol solution at 78°C for one hour restructures the films. No significant difference in exchange as measured by infrared spectroscopy was observed between annealed and untreated samples. This suggests that the defects that are removed or rearranged by annealing are not the primary regions for exchange.

Nobel Laureate Signature Award Address (title TBA)
S. A. Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Atomic-Scale Insights into Hydrodesulfurization
P. S. Weiss, S. A. Kandel, P. Han, J. G. Kushmerick, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We have used low temperature ultrahigh vacuum scanning tunneling microscopy to gain atomic-scale insights into the hydrodesulfurization process. Investigations of Ni adsorbed on the basal plane of MoS₂ revealed that the Ni adatoms are highly mobile due to their weak interaction with the exposed sulfur atoms. The Ni adatoms rapidly diffuse about the surface down to 77 K, and even at 4 K are easily manipulated with the STM probe tip. New roles for metal promoter atoms in hydrodesulfurization catalysis, based on these results, are suggested. Spectroscopic imaging of Ni clusters adsorbed on the MoS₂ basal plane reveals that their electronic structure is well suited to bind nucleophilic reactant species. From low temperature STM images of thiophene adsorbed on Ni{110} a bonding geometry is proposed. We are now using local spectroscopic probes to understand the bonding changes induced in adsorbed thiophenes on HDS-active surfaces and particles.

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Scanning probe microscopes enable unprecedented views of the site-specific interactions and dynamics of atoms and molecules on surfaces. We are able to manipulate these adsorbates chemically or with our microscopes to form structures that we can in turn probe. We are able to measure directly the dramatic chemical changes and the origins of these changes due to surface features such as steps, defects, and other molecules. In the case of dilute coverage, we can also observe the origins of these local changes by measuring the perturbations in the local surface electronic structure using low temperature ultrahigh vacuum scanning tunneling microscopes. We are able to use these effects and also film defects to control and to direct the placement of molecules on the surface as well as in and out of monolayer films. We discuss the chemical consequences of these changes and how they may be exploited to enhance reactivity and film growth with atomic-scale precision. We discuss new tools for measuring environment-dependent electronic structure, bonding, photon emission, conductivity, and other properties.

We are now trying to extend to biological interfaces our ability to probe and to control at the molecular scale. To do so, we can no longer stay far from equilibrium nor can we use scanning probe methods. Thus, we must develop new strategies and tools. I will briefly discuss our first primitive efforts along these lines -- to direct the placement of molecules and to measure the resulting properties of fluid lipid bilayers in vesicles and cell walls.

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Opportunities for Exploring Surfaces with Scanning Probe Microscopes
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Scanning probe microscopes allow unprecedented views of surfaces and the site-specific interactions and dynamics of adsorbates. We discuss our efforts to identify and to characterize atoms and molecules on surfaces and how it is that the scanning tunneling microscope images these adsorbates. We extend the capabilities of scanning probe microscopes in several ways. Recent advances in tunable microwave frequency AC scanning tunneling microscopy allow dopant profiling at unprecedented resolution as well as measurements of the conductances of molecular wires and switches. We are also able to map the emission of photons induced by tunneling electrons with nanometer resolution. We are developing these and other local spectroscopic tools to determine the chemical identity and modifications of surface features and adsorbates given their measured chemical environment.

Atomic-Scale Views Insights into Hydrodesulfurization Catalysis
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Tunable Microwave Frequency AC Scanning Tunneling Microscopy for Dopant Profiling
G. S. McCarty,^1,2 Z. J. Donhauser,¹ P. S. Weiss,^1,2
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Atolytics, Inc., State College, PA 16803-3479, USA

We apply nonlinear tunable microwave frequency AC scanning tunneling microscopy and spectroscopy to profiling dopants at ultrahigh resolution in semiconductors. With microwave difference frequency (MDF) measurements of uniformly doped Si, we have shown that we are sensitive to both dopant type and density. The MDF signal vs. applied bias voltage gives a spectral signature characteristic of dopant type and density. We are then able to use a spectroscopic imaging mode to map the dopant density at ultrahigh resolution.

The MDF signal is convenient in that while two (or more) microwave frequencies are applied to the AC scanning tunneling microscope (ASCTM) probe tip, a much lower mixed frequency is recorded. This is typically accomplished using a lock-in amplifier tuned to a difference frequency at a few kHz. We are also able to look simultaneously at higher order difference frequencies and combinations. These may prove useful in further mapping and characterizing devices at high resolution.

Tunable Microwave Frequency AC Scanning Tunneling Microscopy for Dopant Profiling
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Short-range Correlations in Binary Self-assembled Monolayers of Alkanethiolates on Au{111} Using Scanning Tunneling Microscopy
Mary E. Anderson^1,2 and P. S. Weiss^1
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Department of Chemistry, Samford University, Birmingham, AL, USA

Binary mixtures of alkanethiols with slightly different chain lengths (1-decanethiol and 1-dodecanethiol) are known to self-assemble onto Au{111} forming structural domains where the component molecules are mixed at the molecular level. The longer chain length alkanethiol (1-dodecanethiol) has a slightly larger Van der Waals interaction energy, which could drive the system to a spatially ordered distribution of the components. We have imaged self-assembled monolayers with different ratios of 1-decanethiol and 1-dodecanethiol by scanning tunneling microscopy with molecular resolution. We have analyzed these images to determine radial distribution functions to ascertain if the intermixing of the molecular components is random or if there are spatial correlations between the component molecules. Correlations would be evidence that subtle differences in the Van der Waals interaction energy between components is significant in their spatial ordering.

Mechanical Studies of Multiphase Model Membranes Using Micropipette Aspiration
E. H. Muth, T. G. D'Onofrio and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The lipid bilayer structure found in cell membranes can be modeled using giant unilamellar vesicles (GUVs), liposomes with a diameter greater than 10 microns. We utilize sensitive micropipette manipulation tools and techniques that have been developed elsewhere to probe and quantitatively measure the unique mechanical properties of GUVs. Here we use micropipette aspiration to study multicomponent membranes, which make deformed (nonspherical) vesicles due to the presence of liquid and gel phase microdomains. We determine membrane rigidity, which is characterized by a vesicle's modulus of expansion, by applying pipette suction pressures to a vesicle and measuring its resultant deformation. We also observe vesicle deformation-both natural and pipette-induced-with an optical microscope capable of differential interference contrast and fluorescence imaging, which is coupled to the micromanipulator. These methods will be used to determine the relationship between vesicle rigidity and degree of natural deformation.

Probing the Local Structure of Membranes Using Double Labeled Lipid Vesicles
C. W. Binns^1,2, T. G. D'Onofrio,¹ and P. S. Weiss^1
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Department of Chemistry, Ithaca College, Ithaca, NY, USA

Lipid vesicles work as suitable models for biological membranes. We look to control the local structure through manipulation of the membrane curvature. We choose giant unilamellar vesicles prepared from multi-component lipid mixtures, known to phase segregate into domains large enough to resolve with optical microscopy. Using fluorescence microscopy techniques we are able to image phase segregation and probe the local structure of the membranes. Rapid photobleaching and weak fluorescence proved to be problematic in studies with 6-(9-anthroloxy)stearic acid, a fluorescent probe that partitions into the fluid phase.. Current experiments are exploring the use of combinations of 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI) and 1,1'-dioctadecyl-3,3,3',3'-tetramehtylindocarbocyanine (DiD) to label the fluid and gel phases respectively. Optimal lipid to dye concentrations are being explored, along with proper imaging parameters. DiI and DiD both yield brighter fluorescence, and appear to photobleach slower than anthroloxy stearic acid. Double labeling of membranes is also possible for sequential imaging of different phases. This double labeling will more accurately map the local structure and phase segregation as their shape and curvature are manipulated.

Coverage Analysis of Mixed Monolayers of 3-Mercapto-n-nonylpropionamide and Decanethiol on Au{111}
Nancy Santagata^1,2 and P. S. Weiss^1
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Department of Chemistry, Monmouth College

The ability to control the placement of molecules is essential to the fabrication and patterning of nanoscale devices. In this study, mixed self-assembled monolayers of an amide-containing alkanethiol (3-mercapto-N-nonylpropionamide) and n-alkanethiols were coadsorbed onto a Au{111}surface and examined using scanning tunneling microscopy. Samples self-assembled from solutions of varying molar ratios were analyzed and it was found that the molecules phase separate into discrete, nanometer-scale domains. This leads to the formation of a novel system in which spontaneous, room-temperature separation of a multi-component monolayer is observed. The domain formation is attributed to the hydrogen bonds that occur between the amide molecules. The percent surface composition for each molecule was measured, and it was found that the films studied do not necessarily reflect the makeup of the deposition solutions. Surface reconstructions of the amide domains were also analyzed and compared to those of the n-alkanethiol regions.

Coverage Analysis of Mixed Monolayers of 3-Mercapto-n-nonylpropionamide and Decanethiol on Au{111}
Nancy Santagata^1,2 and P. S. Weiss^1
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Department of Chemistry, Monmouth College

The ability to control the placement of molecules is essential to the fabrication and patterning of nanoscale devices. In this study, mixed self-assembled monolayers of an amide-containing alkanethiol (3-mercapto-N-nonylpropionamide) and n-alkanethiols were coadsorbed onto a Au{111}surface and examined using scanning tunneling microscopy. Samples self-assembled from solutions of varying molar ratios were analyzed and it was found that the molecules phase separate into discrete, nanometer-scale domains. This leads to the formation of a novel system in which spontaneous, room-temperature separation of a multi-component monolayer is observed. The domain formation is attributed to the hydrogen bonds that occur between the amide molecules. The percent surface composition for each molecule was measured, and it was found that the films studied do not necessarily reflect the makeup of the deposition solutions. Surface reconstructions of the amide domains were also analyzed and compared to those of the n-alkanethiol regions.

STM investigation of benzene adsorption on Ag(110)
Kevin F. Kelly,¹ Jennifer J. Jackiw,¹ Jose I. Pascual,² Horst Conrad,² Hans-Peter Rust,² and P. S. Weiss,^1
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Department of Surface Physics, Fritz Haber Institute, Berlin, Germany

We have investigated the adsorption of benzene on Ag(110) using the scanning tunneling microscope. We found that the molecules preferentially adsorb above step edges at 66 K. The preference for step edge adsorption is attributed to the Smoluchowski effect enhancing the empty states to which charge is donated from the pi orbitals of the benzene. However, there is no adsorption at the [001] steps. A lack of free charge due to a gap in the Ag Fermi surface along that direction reduces the Smoluchowski effect and thus the adsorption at these steps. After further deposition, we find that benzene forms a weakly adsorbed hexagonal monolayer. The monolayer is imaged at large tip-sample separations and is transparent upon closer approach. The interaction of benzene molecules with steps and point defects reduces this transparency.

STM Characterization of Molecular Switches
J. D. Monnell¹, Z. J. Donhauser¹, L. A. Bumm¹, P. A. Lewis¹, K. F. Kelly², B. A. Mantooth¹, A. M. Rawlett², D. W. Price², James M. Tour², David L. Allara³, and P. S. Weiss¹.
(1) Department of Chemistry, Pennsylvania State University, Box 53, 152 Davey Lab, University Park, PA 16802
(2) Department of Chemistry and Center for Nanoscale Science and Technology, Rice University
(3) Department of Chemistry and Materials Science, Pennsylvania State University

Molecular switches are an essential component of molecular-level devices, and several such structures have been synthesized. In order to ascertain the electronic properties of these switches, we have inserted them into alkanethiol monolayers and probed them using scanning tunneling microscopy and spectroscopy. These "switches" selectively bind in monolayer defect sites and switch between conductive and non-conductive states. In order to understand and control the switching between high and low conductive states, we have recorded switching over time, and measured their electronic spectra and non-linear microwave difference frequency. We present images, spectra, and movies of these switches in action.

Atomic Insight into Hydrodesulfurization
Patrick Han, S. Alex Kandel, J. G. Kushmerick, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We have dosed and imaged Ni atoms on single crystal MoS₂ at 4 K using a low temperature ultrahigh vacuum scanning tunneling microscope. This system was used to study fundamental aspects of the mechanism of hydrodesulfurization. STM images have shown that Ni atoms remains highly mobile at 77 K and still can be easily manipulated by the STM tip at 4 K. Spectroscopic imaging has shown that adsorbed Ni clusters display empty orbitals that are well suited to bind reactant molecules. STM studies on the adsorption and chemical changes of thiophenes on single crystal metal surfaces will also be presented.

Measuring and Controlling Properties at the Nanometer Scale
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We measure electronic, chemical, optical, and mechanical properties of nanostructures using scanning probe microscopies and spectroscopies. These span the frequency range from dc to beyond the visible. These measurements afford unique views that allow us to tune properties so as to access and to optimize single and distributed nanostructures. We are able to construct assemblies for study and to change them systematically so as to see how structural variations determine properties. We can also exploit combinations of materials properties and variations at this scale to understand phenomena such as photon emission, electronic coupling, conductance changes, and ferroelectricity. Such measurements enable us to target optimized structures for subsequent synthesis and assembly.

Interdependence of curvature and the local structure of model membranes
T. G. D'Onofrio,¹ A. Hatzor,¹ R. K. Smith,¹ C. D. Keating,¹ M. Natan,² and P. S. Weiss,^1
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Surromed, Inc. Palo Alto, CA

The physical properties and the biological functions of membranes are dependent on the local structure of the lipid bilayer. The curvature of the membrane is dependent on the local structure and composition and vice versa. We describe methods to modulate the local structure of model membranes through control of membrane curvature. Our model membranes of choice are giant unilamellar vesicles prepared from multi-component lipid mixtures. We use lipid components known to phase segregate into domains large enough to resolve with optical microscopy. Fluorescence microscopy techniques probe the local structure and enable us to observe domain rearrangement as a function of curvature. Methods to manipulate the curvature include the polymerization of tubulin encapsulated within the vesicle and "stretching" the membrane in the grasp of an optical trap using a bead as a "handle". We measure the local composition dependence of these mechanical properties, seeking to correlate local structure with models of biological function.

Atomic Insight into Hydrodesulfurization
P. Han, S. A. Kandel, J. G. Kushmerick, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We have dosed and imaged Ni atoms on single crystal MoS₂ at 4 K using a low temperature ultrahigh vacuum scanning tunneling microscope. This system was used to study fundamental aspects of the mechanism of hydrodesulfurization. STM images have shown that Ni atoms remains highly mobile at 77 K and still can be easily manipulated by the STM tip at 4 K. Spectroscopic imaging has shown that adsorbed Ni clusters display empty orbitals that are well suited to bind reactant molecules. STM studies on the adsorption and chemical changes of thiophenes on single crystal metal surfaces will also be presented.

2D Dopant Profiling at High Spatial Resolution Using a Tunable Microwave Frequency AC Scanning Tunneling Microscope
G. S. McCarty, Z. J. Donhauser, B. A. Mantooth, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We have shown that microwave difference-frequency imaging with a tunable microwave frequency AC scanning tunneling microscope can differentiate between p- and n- doped silicon. Using difference-frequency spectroscopy at varying biases, the dopant types and density for various standard test structures and transistors were determined.

Nonlinear Mixing at High Frequency in the AC STM Tunnel Junction as a Molecular-Scale Electronic Probe
L. A. Bumm, Z. J. Donhauser, G. S. McCarty, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The high spatial resolution of the scanning tunneling microscope (STM) is exploited to measure the high frequency (DC to 20 GHz) electronic characteristics of the tunnel junction. Conventional high frequency measurements are not possible at the nanometer scale due to parasitic capacitive coupling of the probe and probe structures. We circumvent this problem by exploiting the inherent nonlinear characteristics of the STM tunnel junction to spatially localize the measurement. We measure 2nd and 3rd order mixing products (f₁ - f₂ and f₁ - f₂ - f₃, respectively) of the applied frequencies. Each mixing process measures a different aspect of the tunnel junction electronic characteristic, where the symmetry of the measured response, with respect to the tunnel junction bias voltage, is determined by the order of that mixing process (2nd order, odd and 3rd order, even). The applied frequency range determines time scale of the nonlinear response. The measurement of the phase of the nonlinear response can be used to further characterize the response. At low frequency the mixing can be treated as a quasi-static interaction. At higher frequency some systems studied do show a marked dependence on the applied frequency. We show results for diverse systems which have different mixing product frequency and phase characteristics.

Molecular Nano-scale Science and Technology
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Adventures in Nano
K. F. Kelly, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We have investigated the adsorption of benzene on Ag(110) using the scanning tunneling microscope. We found that the molecules preferentially adsorb above step edges at 66 K. The preference for step edge adsorption is attributed to the Smoluchowski effect enhancing the empty states to which charge is donated from the ? orbitals of the benzene. However, there is no adsorption at the [001] steps. A lack of free charge due to a gap in the Ag Fermi surface along that direction reduces the Smoluchowski effect and thus the adsorption at these steps. After further deposition at 4 K, we find that benzene forms a weakly adsorbed hexagonal monolayer. The monolayer is imaged at large tip-sample separations and is transparent upon closer approach. The interaction of benzene molecules with steps and point defects reduces this transparency.

We present the results of an ultrahigh vacuum, low temperature scanning tunneling microscopy investigation of cobalt and nickel clusters adsorbed on the surface of single-crystal molybdenum disulfide. This system is of interest as a model for understanding industrial hydrotreating catalysts. We observe small metal clusters that bind on the MoS₂ basal plane, and determine from atomically resolved images that Co clusters bind exclusively at three-fold hollow sites between surface sulfur atoms. This surprising observation (in view of the fact that the MoS₂ surface has fully saturated bonding) has important implications for the structure and behavior of the catalyst. In addition, we observe metal clusters bound at MoS₂ steps, which are believed to be the active catalytic site for this system. Metal clusters on MoS₂ are extremely mobile, changing their size and shape on the timescale of imaging; we have succeeded in resolving individual atoms adhering to and detaching from clusters. Furthermore, under appropriate conditions, the STM tip can manipulate atoms and clusters on the surface. All of these experimental results are valuable in drawing new conclusions about the action of promoter species in hydroprocessing catalysis.

Measuring and Controlling Atomic-Scale Properties in Catalysis and Molecular Electronics
Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Biological Microcavity Laser Imaging for the Analysis of Cell Morphology
Rachel K. Smith, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Manipulating and Measuring Local Composition and Properties on Surfaces and in Lipid Bilayer Membranes
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Placement, Control and Isolation of Molecules via Directed Assembly
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We manipulate and measure the properties of single, isolated molecules and of bundles of molecules, selectively placed into monolayer films. The structure of these films is determined by controlling the defect type and density in the films in order to tune their properties. We then process the films to insert single molecules, to insert bundles of molecules, or to graft new molecular terraces onto existing domains by using these defects to advantage. The inserted molecules can function as molecular switches or serve as the anchor points for polymerization. We also prepare films with well defined interfaces between separated components so that insertion, deposition, or reaction can be directed to these molecularly sharp boundaries.

We connect our scanning tunneling microscopy measurements to electron transfer phenomena that are ubiquitous 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. We switch the conductance states of measured numbers of molecular switches using the electric field applied by the scanning tunneling microscope. We demonstrate how proximity can affect electronic structure, potentially limiting ultimate device densities or providing new opportunities for coupling and tuning devices or components.

Extending Nanofabrication via Chemical Manipulation
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Atomically Resolved Cobalt and Nickel Clusters on Molybdenum Disulfide
S. Alex Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Controlling the Placement and Properties of Molecules in Self- and Directed Assembly
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We manipulate and measure the properties of single, isolated molecules and of bundles of molecules, selectively placed into monolayer films. The structure of these films is determined by controlling the defect type and density in the films in order to tune their properties. We then process the films to insert single molecules, to insert bundles of molecules, or to graft new molecular terraces onto existing domains by using these defects to advantage. The inserted molecules can function as molecular switches or serve as the anchor points for polymerization. We also prepare films with well defined interfaces between separated components so that insertion, deposition, or reaction can be directed to these molecularly sharp boundaries.

We connect our scanning tunneling microscopy measurements to electron transfer phenomena that are ubiquitous 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. We switch the conductance states of measured numbers of molecular switches using the electric field applied by the scanning tunneling microscope. We demonstrate how proximity can affect electronic structure, potentially limiting ultimate device densities or providing new opportunities for coupling and tuning devices or components.

Measuring and Controlling Atomic-Scale Properties for Molecular Electronics and Molecular Motors
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We manipulate and measure the properties of single, isolated molecules and of bundles of molecules, selectively placed into monolayer films. The structure of these films is determined by controlling the defect type and density in the films in order to tune their properties. We then process the films to insert single molecules, to insert bundles of molecules, or to graft new molecular terraces onto existing domains by using these defects to advantage. We also prepare films with well defined interfaces between separated components so that insertion, deposition, or reaction can be directed to these molecularly sharp boundaries.

We connect our scanning tunneling microscopy measurements to electron transfer phenomena that are ubiquitous 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. We switch the conductance states of measured numbers of molecular switches using the electric field applied by the scanning tunneling microscope. We demonstrate how proximity can affect electronic structure, potentially limiting ultimate device densities or providing new opportunities for coupling and tuning devices or components.

We have designed and begun to construct synthetic molecular motors. These are different than any previous synthetic motors in that they have no unknown interfaces. Thus, we can know the position of every atom, calculate the motors' properties, behavior, and operation, and then compare these results to experiments. We will discuss the parts of the motors synthesized and measured thus far, and what we hope to learn from these studies.

Measuring and Controlling Atomic-Scale Properties in Catalysis and Molecular Electronics
Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We measure electronic, chemical, optical, and mechanical properties of nanostructures using scanning probe microscopies and spectroscopies. These span the frequency range from dc to beyond the visible. These measurements afford unique views that allow us to tune properties so as to access and to optimize single and distributed nanostructures. I will cover recent developments in our laboratory studying Ni and Co promoters on MoS₂, a model system for hydrodesulfurization catalysis, on single molecule motion, and on single molecular switches in operation.

Atomically Resolved Cobalt and Nickel Clusters on Molybdenum Disulfide
S. Alex Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Atomically Resolved Cobalt and Nickel Clusters on Molybdenum Disulfide
S. Alex Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Atomically Resolved Cobalt and Nickel Clusters on Molybdenum Disulfide
S. Alex Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Atomically Resolved Cobalt and Nickel Clusters on Molybdenum Disulfide
S. Alex Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Controlling the Placement and Properties of Molecules in the Self- and Directed Assembly of Organic Monolayers
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We measure electronic, chemical, optical, and mechanical properties of nanostructures using scanning probe microscopies and spectroscopies. These span the frequency range from dc to beyond the visible. These measurements afford unique views that allow us to tune properties so as to access and to optimize single and distributed nanostructures. We are able to construct assemblies for study and to change them systematically so as to see how structural variations determine properties. We can also exploit combinations of materials properties and variations at this scale to understand phenomena such as photon emission, electronic coupling, conductance changes, and ferroelectricity. Such measurements enable us to target optimized structures for subsequent synthesis and assembly.

Atomically Resolved Cobalt and Nickel Clusters on Molybdenum Disulfide
S. Alex Kandel, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

TBA
Lloyd A. Bumm, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Chemical Identification via Nanomechanical Response
Brent A. Mantooth, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Molecular Insulators, Wires and Switches
Lloyd Bumm, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Optical Biosensors for Intracellular Analysis
Penelope A. Lewis, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Shape Deformation and Membrane Rigidity in Two-Component Lipid Vesicles
E. H. Muth, T. G. D'Onofrio, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We investigate the interdependence of shape deformation, local phase concentration and membrane rigidity in two-component lipid membranes. We have observed the presence of deformed (nonspherical) vesicles due to the arrangement of liquid and gel phase microdomains within their membranes. We quantitatively characterize the relationship between shape deformation and membrane rigidity for these vesicles. We determine the moduli of elastic expansion by applying micropipette suction to the vesicles and measuring their resultant deformation. Vesicle deformation-both natural and mechanically induced-is viewed through an optical microscope capable of differential interference contrast and fluorescence imaging. We will correlate our experimental data with mathematical models that theoretically describe simple, axially symmetric 3D geometries. Our experiments test the predicted inverse quadratic relationship between vesicle deformation and rigidity.

Manipulation of the Local Structure and Composition of Model Membranes
T. G. D'Onofrio, C. D. Keating, E. H. Muth, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The physical properties and biological functions of membranes are dependent on the local structure of the lipid bilayer. The dependence of the membrane curvature on the local structure and composition has been well documented. We hypothesize that this relationship is interdependent. We describe methods to modulate the local structure of model membranes through control of membrane curvature. Our model membranes of choice are giant unilamellar vesicles prepared from multi-component lipid mixtures. We use lipid components known to phase segregate into domains large enough to resolve with optical microscopy. Dual-color fluorescence microscopy techniques probe the local structure and enable us to map domain rearrangement as a function of curvature. Our methods to manipulate the curvature include "stretching" the membrane in the grasp of an optical trap, using a bead as a "handle." Tension and corresponding mechanical properties may be measured with micropipette aspiration techniques. We measure the local composition dependence of these mechanical properties, seeking to correlate local structure with models of biological function.

Molecular Insulators, Wires and Switches
Lloyd Bumm, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Shape Deformation and Membrane Rigidity in Two-Component Lipid Vesicles
E. H. Muth, T. G. D'Onofrio, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We investigate the interdependence of shape deformation, local phase concentration and membrane rigidity in two-component lipid membranes. We have observed the presence of deformed (nonspherical) vesicles due to the arrangement of liquid and gel phase microdomains within their membranes. We quantitatively characterize the relationship between shape deformation and membrane rigidity for these vesicles. We determine the moduli of elastic expansion by applying micropipette suction to the vesicles and measuring their resultant deformation. Vesicle deformation-both natural and mechanically induced-is viewed through an optical microscope capable of differential interference contrast and fluorescence imaging. We will correlate our experimental data with mathematical models that theoretically describe simple, axially symmetric 3D geometries. Our experiments test the predicted inverse quadratic relationship between vesicle deformation and rigidity.

High Resolution Dopant Profiling with AC Scanning Tunneling Microscopy
R. Y. Wang, Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802-6300, USA

Moving Toward Nano-scale Electronics
G. S. McCarty, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA & Atolytics, Inc, State College, PA 16803, USA

Synthetic Molecular Motors
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA and J. M. Tour, Department of Chemistry, Rice University, Houston, TX, USA

We have designed and begun to construct synthetic molecular motors. These are different than any previous synthetic motors in that they have no unknown interfaces. Thus, we can know the position of every atom, calculate the motors' properties, behavior, and operation, and then compare these results to experiments. We will discuss the parts of the motors synthesized and measured thus far, and what we hope to learn from these studies.

Measuring and controlling molecular-scale properties for molecular electronics
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We isolate single molecules and bundles of molecules in two-dimensional matrices for study with scanning probe microscopy. We measure and control the conductance of these molecules. We relate our scanning tunneling microscopy measurements to measurements in test structures that can be fashioned into molecular electronic logic and memory. From these measurements we seek to determine and to optimize the mechanisms of switching.

Clusters of nickel and cobalt promoters on molybdenum disulfide
P. S. Weiss, S. A. Kandel, and P. Han, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We present the results of a low temperature scanning tunneling microscopy/spectroscopy investigation of cobalt and nickel clusters adsorbed on the surface of single-crystal molybdenum disulfide. This system is of interest as a model for understanding industrial hydrotreating catalysts. We observe small metal clusters that bind on the MoS₂ basal plane, and determine from atomically resolved images that Co clusters bind exclusively at lattice sites and spacings. This is surprising in that the MoS₂ surface has fully saturated bonding, and has important implications for the preparation, structure, and behavior of the catalyst. In addition, we observe metal clusters bound at MoS₂ steps, which are believed to be the active catalytic site for this system. Metal clusters on MoS₂ are easily manipulated or separated while imaging; we have succeeded in resolving individual atoms adhering to and detaching from clusters. All of these experimental results are valuable in considering potential roles of these promoter species in hydroprocessing catalysis.

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Molecular Self-Assembly as a Tool for Metallic Nanowire Fabrication
Anat Hatzor, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Measuring and Controlling Molecular-Scale Properties for Molecular Electronics
P. S. Weiss and D. L. Allara, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA,
and
J. M. Tour, Department of Chemistry, Rice University, Houston, TX 77005, USA

We isolate single molecules and bundles of molecules in two-dimensional matrices for study with scanning probe microscopy. We measure and control the conductance of these molecules. We relate our scanning tunneling microscopy measurements to measurements in test structures that can be fashioned into molecular electronic logic and memory. From these measurements we seek to determine and to optimize the mechanisms of switching. One focus of our work has been on functionalized oligo(1,4-phenyleneethynylene)s, as members of this family of molecules have been proposed as both molecular wires and molecular switches. We measure the dependence of the switching on the bias conditions and on the number of molecules in the bundle. We use the persistence time in a conductance state given these conditions and the molecules' environment to elucidate the switching mechanism.

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Placement, Control and Isolation of Molecules via Directed Assembly
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

We manipulate and measure the properties of single, isolated molecules and of bundles of molecules, selectively placed into monolayer films. The structure of these films is determined by controlling the defect type and density in the films in order to tune their properties. We then process the films to insert single molecules, to insert bundles of molecules, or to graft new molecular terraces onto existing domains by using these defects to advantage. The inserted molecules can function as molecular switches or serve as the anchor points for polymerization. We also prepare films with well defined interfaces between separated components so that insertion, deposition, or reaction can be directed to these molecularly sharp boundaries.

We connect our scanning tunneling microscopy measurements to electron transfer phenomena that are ubiquitous 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. We switch the conductance states of measured numbers of molecular switches using the electric field applied by the scanning tunneling microscope. We demonstrate how proximity can affect electronic structure, potentially limiting ultimate device densities or providing new opportunities for coupling and tuning devices or components.

Tunable Microwave Frequency AC Scanning Tunneling Microscopy for Dopant Profiling
G. S. McCarty,^1,2 Z. J. Donhauser,¹ and P. S. Weiss,^1,2
1Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
²Atolytics, Inc., State College, PA 16803, USA

A Step Towards Nanoscale Electronics: Instrumentation and Characterization
G. S. McCarty, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Combining Self- and Directed Assembly with Nanofabrication to Connect Molecular Devices
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Combining Self- and Directed Assembly with Nanofabrication to Connect Molecular Devices
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Measuring and Controlling Molecular-Scale Properties for Molecular Electronics
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

As the scales of devices shrink or change more disruptively, so must the resolution and capabilities of analytical tools. Tremendous opportunities exist in developing the sensitivity to probe chemical, physical, electronic, optical, mechanical, and other properties at the nanometer scale and below. I will show some of the capabilities that have been developed and how we have applied these in molecular electronics and in related areas. Our ability to make variations at the atomic scale, chemically and otherwise, gives us the capability to determine the sensitivity of properties to precise structures and thus to tailor these properties. Since measurements with scanning probe microscopes, the tools of choice, can (must) be made on a single nanostructure at a time, we are able to measure monodisperse samples and to guide the selection, synthesis, and assembly of optmized structures.

Manipulating Phase Behavior by Varying Buried Functionality in Monolayers: STM Investigation of Alkanethiols and Amide-Containing Alkanethiols
P. S. Weiss,* P. A. Lewis, R. K. Smith, B. A. Mantooth, L. A. Bumm, and K. F. Kelly, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
S. M. Reed, R. S. Clegg, and J. E. Hutchison, Department of Chemistry, University of Oregon, Eugene, OR 97403

The ability to control the placement of molecules is essential for the patterning and fabrication of nanoscale devices. For that reason, there have been many studies aimed at extending our understanding of the atomic-scale chemical dynamics on surfaces. Towards this end, we have imaged a variety of alkanethiol and internally amide-functionalized alkanethiol self-assembled monolayers with the scanning tunneling microscope (STM).

We have studied self-assembled monolayers (SAMs) with different ratios of decanethiol and dodecanethiol. Figure 1 shows an STM image of a 75% decanethiol / 25% dodecanethiol SAM that appears randomly ordered. We have analyzed the spatial correlations between the component molecules in these images using pair correlation functions, which are compared to Monte Carlo simulations of randomly mixed monolayers. Deviations would indicate that the adsorbates are more or less dispersed (more ordered) than those resulting from random mixing alone. We find that the short-range spatial correlation between the components is consistent with that expected of random mixing. Hence the subtle differences in the van der Waals interaction energy between the different alkyl chain length molecules are not large enough to effect the short-range spatial ordering.

In comparison, we have recently investigated the role of internal functionality in SAMs of a family of amide-containing alkanethiol molecules on Au{111} using STM. The introduction of an amide-functionality induces spontaneous, room-temperature separation between these molecules and alkanethiols. Figure 2 is an STM image illustrating the phase separation that occurs in a mixed monolayer containing equal amounts of decanethiol and an amide-containing alkanethiol. In addition to van der Waals interactions that are present within alkanethiol SAMs, hydrogen bonding between adjacent buried amide groups contributes substantially to the intermolecular interaction strength and thus to the stability of the molecules on the surface, causing spontaneous phase separation when coadsorbed with alkanethiols.

Conductance Switching in Single Molecules through Conformational Changes
K. F. Kelly, Z. J. Donhauser, B. A. Mantooth, J. D. Monell, L. A. Bumm, J. J. Stapleton, D. L. Allara and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
D. W. Price, Jr., and J. M. Tour, Department of Chemistry, Rice University, Houston, TX 77005

The viability of molecular electronics is being considered as a means to create inexpensive, ultra-dense, high-capacity electronic devices. Conjugated phenylene-ethynylene oligomers have been extensively studied as candidate molecular devices. However, most experiments have required the assembly and study of these molecules in groups of thousands. Contradictory theoretical studies have ascribed the conductance switching to a variety of effects, including internal rotations, concerted motions of neighboring molecules, and electrochemical reduction of the switch molecules. We utilize self-assembly techniques in combination with STM to study candidate molecular switches individually and in small bundles. We find that none of the previous explanations are likely to be the origin of switching. In contrast to some theoretical predictions, individual, isolated molecules do switch.

Alkanethiol self-assembled monolayers (SAMs) on gold are used as a host two-dimensional matrix to isolate and to insulate electrically the molecular switches. The candidate molecules selectively adsorb into existing defect sites and at step edges. The molecules bind with a sulfur "alligator clip" to the underlying gold substrate, and the ordered SAM causes the molecules to stand in a position nearly normal to the substrate. We then individually address and electronically probe each molecule using STM. The conjugated molecules exhibit reversible conductance switching, manifested as a change in the topographic height in STM images, as seen in Figure 1. The observed switching occurs randomly and reversibly, with persistence times for each state ranging from seconds (or less) to hours. Both individual molecules and bundles of molecules switch.

Exploration of Nanoscale Structures and Properties
J. J. Jackiw, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA
(Ph.D. Thesis Defense)

Opportunities for Exploring Surfaces with Scanning Probe Microscopes
K. F. Kelly, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Controlling the Switching Behavior of Single Molecules
Z.J. Donhauser, B. A. Mantooth, K. F. Kelly, L. A. Bumm, J. D. Monnell, J. J. Stapleton, D. L. Allara, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA,
and
D. W. Price, Jr., and J. M. Tour, Department of Chemistry, Rice University, Houston, TX 77005, USA

Molecular electronics is quickly emerging as one of the most viable possibilities for the creation of inexpensive, ultra-dense, high-capacity electronic devices. Conjugated phenylene-ethynylene oligomers have been extensively studied as candidate molecular devices, with functionality already observed for molecular wires, diodes, and switches. However, most experiments have required the assembly and study of these molecules in groups of thousands. We utilize self-assembly techniques in combination with STM to study candidate molecular switches individually and in small bundles.
Alkanethiol self-assembled monolayers (SAMs) on gold are used as a host matrix to isolate and electrically insulate the molecular switches. The candidate molecules selectively adsorb at existing defect sites in the host SAM and at substrate step edges. The molecules bind with a sulfur "alligator clip" to the underlying gold substrate, and the ordered SAM causes the molecules to stand in a position nearly normal to the substrate. We can individually address and electronically probe each molecule using STM imaging and spectroscopy.
The guest molecules exhibit reversible conductance switching, manifested in a change in the topographic height in STM images, as seen in figure 1. The observed height change is roughly 3 Ĺ, with the topographically higher state corresponding to a high conductance ON state, and the topographically lower state corresponding to a low conductance OFF state. The observed switching occurs randomly and reversibly, with persistence times for each state ranging from minutes to hours.
We have shown the ability to control the amount and rate of active switching by controlling the local environment of the guest molecules. Inserting the guest molecules into poorly-ordered matrix films results in increased switching activity when compared to well-ordered films. Similarly, annealing the SAM after inserting the guest molecules results in decreased switching activity, when compared to unannealed SAMs.
We also have shown the ability to switch the molecules controllably between the ON and OFF states using the STM tip. Voltage pulses and ramp programs applied to the STM tip cause the molecules to switch into the OFF state. After controllably switching the molecules into the OFF state, they are seen to relax back into the ON state.
This work demonstrates the ability to control stochastic switching activity using self-assembly techniques, and the ability to select and to induce the switching of single molecules using the STM tip.

Controllable Patterning of Self-Assembled Monolayers Using Buried Functionality
P. A. Lewis, R. K. Smith, K. F. Kelly, L. A. Bumm, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802
S. M. Reed, R. S. Clegg, J. D. Gunderson, and J. E. Hutchison, Department of Chemistry, University of Oregon, Eugene, OR 97403

The ability to control the local placement of atoms and molecules within larger systems is a crucial factor in innovative technologies en route to the fabrication of molecular-scale electronic devices. Current lithographic techniques are limited in their resolution and cannot reproducibly achieve patterns with dimensions at the nanometer scale. On the other end of the spectrum, single-molecule manipulation has been successfully demonstrated using scanning probe microscopy, but is unable to produce devices in parallel and is still too time-consuming to be practical as a fabrication technique. We anticipate the need to combine the speed and versatility of lithographic techniques with the resolution of single-molecule manipulation in order to construct commercially viable molecular devices.

We have explored possible methods for nanoscale patterning by capitalizing upon the inherent chemical and physical properties of molecules to control the placement and density of these molecules within organic monolayers. Toward this end, we have recently investigated the role of internal functionality in self-assembled monolayers (SAMs) of a family of amide-containing alkanethiol molecules on Au{111} using scanning tunneling microscopy. In addition to van der Waals interactions that are present within n-alkanethiol SAMs, hydrogen bonding between adjacent buried amide groups contributes substantially to the intermolecular interaction strength and thus to the stability of the molecules on the surface, causing spontaneous phase separation when coadsorbed with n-alkanethiols. This differs from previous studies of binary component n-alkanethiol SAMs in which intermixing of the two components in the monolayer occurred.

We have observed different packing structures within these amide-containing alkanethiol monolayers compared to n-alkanethiol films. These observed superlattice structures appear more linear than those typically attributed to n-alkanethiol monolayers. We hypothesize that the hydrogen bonding of adjacent amide subunits contributes to the observed linearity within the films. We foresee these structures as effective in patterning stable walls or lines within monolayers where this type of structure is desired. The internal functionality of these monolayers can be used to control film formation and stability and has important implications in nanoscale patterning.

Controlling and Measuring Local Composition and Properties in Lipid Bilayer Membranes
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Measuring and Controlling Molecular-Scale Properties for Molecular Electronics
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

I will describe our research in nanoscience and the coordinated, interdisciplinary approach that we take. We explore new phenomena in the function and control of molecules at the nanometer scale and smaller. We seek to exploit these phenomena by developing the means to connect our molecules to the microscopic and macroscopic worlds. I will illustrate this multifaceted approach with examples from our work in molecular electronics.

We use intermolecular interactions to direct molecules into desired positions. We use and develop scanning probe microscopes to determine both structures and the electronic, optical, chemical, physical, mechanical, and other local properties. The tools developed allow us to measure the environment-dependent electronic structure, bonding, photon emission, high frequency conductivity, and ferroic response with unprecedented resolution.

Exploring the Surface Chemistry of Colloidal Gold
Read T. Langlois, Rachel, K. Smith, and Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Tailoring the optical and electronic properties of nanoparticles is a vital step towards fabricating novel nanoelectronic devices. It is possible to synthesize these materials in a variety of sizes, compositions, and shapes. Size and composition control electronic and optical properties of nanoparticles. These properties are characterized by studying a population of nanoparticles varying both in size and composition. Through the chemical process of ligand exchange, the surface chemistry of a nanoparticle can be used to control passivating layer around the metal nanoparticle, thus controlling the solubility, reactivity and dielectric properties of the particle. Two ligand exchange systems were developed for passivating nanoparticles, with n-dodecanethiol and DNA. These particles were characterized using UV-Visible spectroscopy and transmission electron microscopy. By varying the passivating layer through ligand exchange, interparticle and particle/substrate interactions can be controlled. Variation in ligand shells allows the chemical properties of nanoparticles to be customized for incorporation into nanoelectronic materials and devices.

Probing the Local Structure of Membranes Using Fluorescence Microscopy
C. W. Binns, T. G. D'Onofrio, and P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Lipid vesicles work as suitable models for biological membranes. We look to control the local membrane structure through manipulation of the membrane curvature. We choose giant unilamellar vesicles prepared from multi-component lipid mixtures, known to phase segregate into domains large enough to be resolved with optical microscopy. Using fluorescence microscopy we are able to image phase segregation and probe the local structure of the membranes. Currently we use a combination of 1,1’-didodecyl-3, 3,3’,3’-tetramethylindocarbocyanine (DiI) and 1,1’-dioctadecyl-3,3,3’,3’-tetramehtylindocarbocyanine (DiD) to label the fluid and gel phases, respectively. Our past methods of imaging phase segregation allowed us only to view the domains around the equator of the membrane, leaving us with little information about the remainder of the membrane’s composition. Other researchers have demonstrated the ability to image phase segregation over the entire membrane using confocal microscopy, but this method is costly and slow. We are developing the capability of using fluorescence microscopy to image the entire membrane at higher data acquisition rates so as to record intramembrane dynamics. We acquire a series of images in successive focal planes and later combine them to elucidate a better three-dimensional understanding of the membrane.

Molecular Graph Paper
Alex Wissner-Gross, Anat, Hatzor, and Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The recent development of the molecular ruler nanofabrication process has enabled the creation of very closely spaced metal structures with positional accuracies of 1 nm. This process may be useful for molecular electronic measurements, as it could allow the fabrication of electrodes with spacing that precisely matches the length of a particular functional molecule.

This paper presents a new method for creating a "graph paper" of gaps between Au and Ti electrodes on an oxided silicon substrate. A hexagonally-packed monolayer of polystyrene microspheres was used as a mask for metal evaporation onto the substrate. After evaporation, CH2Cl2 dissolution of the microspheres left an array of triangular metal particles on the silicon, which were then used as parent structures for the molecular ruler process. The graph paper technique has potential applications for organizing arrays of molecular electronic devices.

Controlling and Measuring Local Composition and Properties in Lipid Bilayer Membranes
P. S. Weiss, T. G. D’Onofrio, E. H. Muth, and S. M. Ponder, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

The focus of our research is to measure, to control, and to understand the local structure of model and real biological membranes at the molecular scale. The local composition of these membranes is a critical element of cellular processes such as transport, infection, and uptake. We describe methods to modulate the local structure of model membranes through control of membrane curvature. This requires analytical chemical methods capable of probing intact membranes. Our tools include multibeam infrared optical tweezers, dual micropipette aspiration, fluorescent molecular probes, and simultaneous high-resolution multi-color fluorescence imaging. Our model membranes of choice are giant unilamellar vesicles prepared from multi-component lipid mixtures. We use lipid components known to phase segregate into domains large enough to resolve with optical microscopy. Dual-color fluorescence microscopy techniques probe the local structure and enable us to map domain rearrangement as a function of curvature.

We are analyzing the effects of membrane curvature on the local composition of multi-component lipid bilayers. Curved membrane surfaces are prevalent in biological systems, due to the high radius of curvature necessary for many functions including exocytosis, endocytosis, and adhesion. Preliminary evidence supports our hypothesis that there is an inter-dependent relationship between local structure, composition, and curvature. We obtain quantitative measurements of the membrane components by perturbing the membrane surface and monitoring the corresponding change in the local structure with fluorescence techniques. Such systematic measurements are key to understanding the roles of individual membrane components and their aggregate effects on the behavior of the whole system.

Another means of preparing membranes with varying curvature is to build upon high aspect ratio nanoparticles and micron-sized particles. We have developed the capability to do this and shown how to optimize these structures. In so doing, we touch on the potentially important role of encapsulation in infection. Our hypotheses and test measurements in this area will be discussed.

In a related thrust, we are investigating the interdependence of shape deformation, local phase concentration, and membrane rigidity in two-component vesicles. We have observed the presence of naturally deformed (non-spherical) vesicles due to the phase segregation of the components. We quantitatively characterize this relationship by comparing the shape deformation with the elastic modulus, a measure of the membrane rigidity obtained via micropipette aspiration. We correlate our measurements to existing and developing mathematical models, which describe axially symmetric three-dimensional geometries.

Directed Nanosphere Lithography
Alex Wissner-Gross, Anat Hatzor, and Paul S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Nanosphere monolayers have been previously used as lithographic masks for regular arrays of sub-100nm metal nanoparticles. Patterns produced by nanosphere lithography have interesting applications for optics and efficient creation of magnetic storage media. However, the technique lacks positional specificity and is restricted to making periodic patterns. In this paper, we present a method for adding local positional control to nanosphere lithography. Patterned gaps in an oxidized silicon layer etched by standard photolithography were used to align the orientations of defect-free monolayer domains. Positional control over domain walls is demonstrated, as well as controlled formation of "strings" of gold nanoparticles by metal evaporation through the domain walls.

Measuring and Controlling Molecular-Scale Properties for Molecular Electronics
P. S. Weiss and D. L. Allara, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA,
and
J. M. Tour, Department of Chemistry, Rice University, Houston, TX 77005, USA

We isolate single molecules and bundles of molecules in two-dimensional matrices for study with scanning probe microscopy. We measure and control the conductance of these molecules. We relate our scanning tunneling microscopy measurements to measurements in test structures that can be fashioned into molecular electronic logic and memory. From these measurements we seek to determine and to optimize the mechanisms of switching. One focus of our work has been on functionalized oligo(1,4-phenyleneethynylene)s, as members of this family of molecules have been proposed as both molecular wires and molecular switches. We measure the dependence of the switching on the bias conditions and on the number of molecules in the bundle. We use the persistence time in a conductance state given these conditions and the molecules' environment to elucidate the switching mechanism.

Distributed and Coupled Nanostructures
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World: Graduate Research Opportunities in the Weiss Group
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Measuring and Controlling Molecular-Scale Properties for Molecular Electronics
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

As the scales of devices shrink or change more disruptively, so must the resolution and capabilities of analytical tools. Tremendous opportunities exist in developing the sensitivity to probe chemical, physical, electronic, optical, mechanical, and other properties at the nanometer scale and below. I will show some of the capabilities that have been developed and how we have applied these in molecular electronics and in related areas. Our ability to make variations at the atomic scale, chemically and otherwise, gives us the capability to determine the sensitivity of properties to precise structures and thus to tailor these properties. Since measurements with scanning probe microscopes, the tools of choice, can (must) be made on a single nanostructure at a time, we are able to measure monodisperse samples and to guide the selection, synthesis, and assembly of optmized structures.

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

As the scales of devices shrink or change more disruptively, so must the resolution and capabilities of analytical tools. Tremendous opportunities exist in developing the sensitivity to probe chemical, physical, electronic, optical, mechanical, and other properties at the nanometer scale and below. I will show some of the capabilities that have been developed and how we have applied these in molecular electronics and in related areas. Our ability to make variations at the atomic scale, chemically and otherwise, gives us the capability to determine the sensitivity of properties to precise structures and thus to tailor these properties. Since measurements with scanning probe microscopes, the tools of choice, can (must) be made on a single nanostructure at a time, we are able to measure monodisperse samples and to guide the selection, synthesis, and assembly of optmized structures.

Ultrahigh Spatial Resolution Scanning Probe Spectroscopies
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Scanning probe microscopes can be used to measure spectroscopic information with extremely high spatial resolution. Local spectroscopies that cover the near infrared through the visible into the near ultraviolet, as well as in the microwave and vibrational regions will be described. These spectroscopies can be used to record chemical, physical, electronic, and optical information and couplings at unprecedented scales. Such spectroscopic capabilities allow the measurement of uniquely prepared distributed nanostructures. Each sample is monodisperse in that only one at a time is measured; in some cases only part of one molecule, nanoparticle, or nanostructure can be probed.

Molecular Electronics: Characterizing Single Molecule Switches with the Scanning Tunneling Microscope
J. D. Monnell, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

I will describe our research in nanoscience and the coordinated, interdisciplinary approach that we take. We explore new phenomena in the function and control of molecules at the nanometer scale and smaller. We seek to exploit these phenomena by developing the means to connect our molecules to the microscopic and macroscopic worlds. I will illustrate this multifaceted approach with examples from our work in molecular electronics.

We use intermolecular interactions to direct molecules into desired positions. We use and develop scanning probe microscopes to determine both structures and the electronic, optical, chemical, physical, mechanical, and other local properties. The tools developed allow us to measure the environment-dependent electronic structure, bonding, photon emission, high frequency conductivity, and ferroic response with unprecedented resolution.

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Molecular Electronics and Self-Assembly
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Controlling and Measuring Molecular-Scale Properties for Molecular Electronics
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

As the scales of devices shrink or change more disruptively, so must the resolution and capabilities of analytical tools. Tremendous opportunities exist in developing the sensitivity to probe chemical, physical, electronic, optical, mechanical, and other properties at the nanometer scale and below. I will show some of the capabilities that have been developed and how we have applied these in molecular electronics and in related areas. Our ability to make variations at the atomic scale, chemically and otherwise, gives us the capability to determine the sensitivity of properties to precise structures and thus to tailor these properties. Since measurements with scanning probe microscopes, the tools of choice, can (must) be made on a single nanostructure at a time, we are able to measure monodisperse samples and to guide the selection, synthesis, and assembly of optimized structures.

We use intermolecular interactions to direct molecules into desired positions to create nanostructures, to connect functional molecules to the outside world, and to serve as test structures for measurements on single or bundled molecules. We use and develop scanning probe microscopes to determine both local structures and the electronic and other local properties. We have applied these to isolate molecules with electronic function to determine the mechanisms of function, and the relationships between molecular structure, environment, connection, coupling, and function. We have been able to demonstrate that single molecules can function as multistate switches, and have determined important aspects of the mechanism, function, and persistence of switching. We will discuss the origins of switching and the relevant aspects of the molecular structure and environment required.

Molecular Electronics: Characterizing Single Molecule Switches with the Scanning Tunneling Microscope
B. A. Mantooth, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Nano-Scale Measurements and Manipulation
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

Exploring and Controlling the Atomic-Scale World
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

I will describe our research in nanoscience and the coordinated, interdisciplinary approach that we take. We explore new phenomena in the function and control of molecules at the nanometer scale and smaller. We seek to exploit these phenomena by developing the means to connect our molecules to the microscopic and macroscopic worlds. I will illustrate this multifaceted approach with examples from our work in molecular electronics.

We use intermolecular interactions to direct molecules into desired positions. We use and develop scanning probe microscopes to determine both structures and the electronic, optical, chemical, physical, mechanical, and other local properties. The tools developed allow us to measure the environment-dependent electronic structure, bonding, photon emission, high frequency conductivity, and ferroic response with unprecedented resolution.

Single Molecular Switches
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

As the scales of devices shrink or change more disruptively, so must the resolution and capabilities of analytical tools. Tremendous opportunities exist in developing the sensitivity to probe chemical, physical, electronic, optical, mechanical, and other properties at the nanometer scale and below. I will show some of the capabilities that have been developed and how we have applied these in molecular electronics and in related areas. Our ability to make variations at the atomic scale, chemically and otherwise, gives us the capability to determine the sensitivity of properties to precise structures and thus to tailor these properties. Since measurements with scanning probe microscopes, the tools of choice, can (must) be made on a single nanostructure at a time, we are able to measure monodisperse samples and to guide the selection, synthesis, and assembly of optimized structures.

We use intermolecular interactions to direct molecules into desired positions to create nanostructures, to connect functional molecules to the outside world, and to serve as test structures for measurements on single or bundled molecules. We use and develop scanning probe microscopes to determine both local structures and the electronic and other local properties. We have applied these to isolate molecules with electronic function to determine the mechanisms of function, and the relationships between molecular structure, environment, connection, coupling, and function. We have been able to demonstrate that single molecules can function as multistate switches, and have determined important aspects of the mechanism, function, and persistence of switching. We will discuss the origins of switching and the relevant aspects of the molecular structure and environment required.

Single Molecule Electronics
P. S. Weiss, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802-6300, USA

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7 February 2016
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