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ABSTRACT: Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the
energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred
in a number of photoemission experiments. We present electronic transport measurements in very short channel (L < 100 nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact
resistance and the zero gate voltage conductance are consistent with such charge transfer. Short channel conduction in devices
with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations.
Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into
P3HT, while much less charge transfer takes place at the Au/P3HT interface.
KeywordsOrganic semiconductors-band alignment-charge transfer-organic field-effect transistor-scanning tunneling microscopy
Nano Research 04/2012; 1(4):341-350. · 6.97 Impact Factor
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ABSTRACT: The instrumental synthesis of high resolution scanning tunneling microscopy (STM) with the ability to measure differential capacitance with atomic scale resolution is highly desirable for fundamental metrology and for the study of novel physical characteristics. Microwave frequency radiation directed at the tip-sample junction in an STM system allows for such high-resolution differential capacitance information. This ability is particularly critical in ultrahigh vacuum environments, where the additional parameter space afforded by including a capacitance measurement would prove powerful. Here we describe the modifications made to a commercial scanning tunneling microscope to allow for broad microwave frequency alternating current scanning tunneling microscopy (ACSTM) in ultrahigh vacuum conditions using a relatively simple loop antenna and microwave difference frequency detection. The advantages of our system are twofold. First, the use of a removable antenna on a commercial STM prevents interference with other UHV processes while providing a simple method to retrofit any commercial UHV-STM with UHV-ACSTM capability. Second, mounting the microwave antenna on a translator allows for specific tuning of the system to replicate experimental conditions between samples, which is particularly critical in sensitive systems like organic thin films or single molecules where small changes in incident power can affect the results. Our innovation therefore provides a valuable approach to give nearly any commercial STM, be it an ambient or UHV system, the capability to measure atomic-scale microwave studies such as differential capacitance or even single molecule microwave response, and it ensures that experimental ACSTM conditions can be held constant between different samples.
The Review of scientific instruments 05/2011; 82(5):053710. · 1.52 Impact Factor
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ABSTRACT: The synthesis and imaging by scanning tunneling microscopy of a mixed wheeled nanovehicle composed of a p-carborane small-wheeled short front axle and a C(60) large-wheeled long rear axle that has been termed a nanodragster due to the structural relation to a dragster are reported. This nanodragster is expected to exhibit motion at a lower temperature than pure C(60)-wheeled nanocars and should allow the investigation of the role played by p-carborane wheels in directional motion.
Organic Letters 12/2009; 11(24):5602-5. · 5.86 Impact Factor
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ABSTRACT: Scanning tunneling microscopy (STM) has been used to study polydiacetylene (PDA) nanowires and their electronic coupling with the surface. PDA nanowires exhibit intriguing substrate-dependent electronic effects when probed at varying sample bias voltage conditions on different substrate electrode materials, in this case, highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS(2)). An analysis of nanowire heights over a wide range of bias voltages shows strong polymer-substrate contact effects, the strength of which is reflected in the asymmetry of the height-voltage data on each substrate. On HOPG, PDA nanowires exhibit a decrease in height as the bias voltage magnitude is reduced, and the height is substantially greater at negative voltages than at positive voltages. On MoS(2), PDA nanowires appear with much higher contrast than on HOPG when imaged at the same negative bias conditions. At positive bias voltages on MoS(2), the nanowires are invisible in all STM images, yet the unpolymerized molecules can still be imaged. These effects are necessarily electronic in origin. Surprisingly, only the polymer nanowires exhibit any bias-dependent change; the unpolymerized molecules are imaged at all bias voltages on both substrates. Additionally, the substrate affects how the unpolymerized molecules are ordered. In some areas, the molecules are arranged such that part of the monolayer is offset from the correct threefold symmetry direction by a slight misfit angle. On HOPG, this misfit is approximately 6 degrees, while on MoS(2), it is approximately 11 degrees. Interactions with the substrate thus play a role both in electronic structure and in molecular alignment.
ACS Nano 09/2008; 2(8):1571-80. · 10.77 Impact Factor
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ABSTRACT: Rapid progress in graphene-based applications is calling for new processing techniques for creating graphene components with different shapes, sizes, and edge structures. Here we report a controlled cutting process for graphene sheets, using nickel nanoparticles as a knife that cuts with nanoscale precision. The cutting proceeds via catalytic hydrogenation of the graphene lattice, and can generate graphene pieces with specific zigzag or armchair edges. The size of the nanoparticle dictates the edge structure that is produced during the cutting. The cutting occurs along straight lines and along symmetry lines, defined by angles of 60 degrees or 120 degrees, and is deflected at free edges or defects, allowing practical control of graphene nano-engineering.
Nano Research 07/2008; 1(2):116-122. · 6.97 Impact Factor
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ABSTRACT: We disclose the synthesis of a porphyrin-fullerene pinwheel that was subsequently observed by scanning tunneling microscopy. The molecule was designed to further our understanding of fullerene-surface interactions, directional control, and surface-rolling versus pivoting capabilities of this class of nanomachines. The inner porphyrin provides the square planar configuration that might lead to realization of the pinwheel spiraling motion on surfaces.
Organic Letters 05/2008; 10(7):1377-80. · 5.86 Impact Factor
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ABSTRACT: Oxidation of graphite may be carried out by reaction with meta-chloroperoxybenzoic acid to yield graphite epoxide. Scanning tunneling microscopy (STM) showed that the functionalization occurs at the edges rather than on the basal plane of the graphite. Quantification of the epoxide content is possible through the deepoxidation reaction using MeReO3/PPh3.
Journal of the American Chemical Society 05/2008; 130(16):5414-5. · 9.91 Impact Factor
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ABSTRACT: Nanocars with an angled chassis have been synthesized and imaged using scanning tunneling microscopy. These angled chassis nanocars were designed to further our understanding of the directional control and surface-rolling capabilities of this class of nanomachines. The alkylated carbazole inner core might enable the molecular scaffold to produce circular rolling motions of the nanovehicles on surfaces.
Organic Letters 02/2008; 10(2):229-32. · 5.86 Impact Factor
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ABSTRACT: The desorption of polydiacetylene nanowires has been studied using scanning tunneling microscopy at both solid−air and solid−vacuum interfaces. Langmuir-Schaefer films of 10,12-pentacosadiyonic acid on highly ordered pyrolytic graphite can be polymerized to form well-ordered polydiacetylene nanowires in ambient and ultrahigh vacuum conditions. These nanowires can either fully desorb or be cut into shorter segments depending on the strength of the interaction with the scanning tunneling microscope (STM) tip. A single disruption usually causes the nanowire to completely desorb. In both desorption cases, the surrounding monolayer order is fully restored within a single line acquisition (approximately 100 ms). The subsequent monolayer reordering, though expected in a solid−liquid interface experiment, is unexpected at solid−air and solid−vacuum interfaces and may be due to a molecular cascade of the diacetylene derivative molecules. Desorption has been observed over numerous sample imaging conditions on both blue-phase and red-phase polydiacetylene nanowires. The results reported here shed light on the relative interaction strengths between the scanning tip, the polymer backbone, and the graphite substrate. Observations of desorption also imply that polydiacetylene nanowires are sensitive to perturbation by the local scanning probe, which is a critical factor to consider in evaluating tunneling spectroscopy data.
04/2007;
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Yasuhiro Shirai,
Andrew J Osgood,
Yuming Zhao,
Yuxing Yao,
Lionel Saudan,
Hanbiao Yang,
Chiu Yu-Hung,
Lawrence B Alemany,
Takashi Sasaki,
Jean-François Morin,
Jason M Guerrero, Kevin F Kelly,
James M Tour
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ABSTRACT: Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely, nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic components that include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use of spherical wheels based on C60 and freely rotating axles based on alkynes permits directed nanoscale rolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles the same motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A new synthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, and the latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provided definitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. The studies here underscore the ability to control directionality of motion in molecular-sized nanostructures through precise molecular design and synthesis.
Journal of the American Chemical Society 05/2006; 128(14):4854-64. · 9.91 Impact Factor
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ABSTRACT: Thiol- and thiophene-functionalized SWNTs prepared via the reaction of a substituted amine with fluoronanotubes show similar levels of sidewall functionalization, however, the use of Au nanoparticles as chemical markers for AFM gives misleading results for substituent distribution since STM shows the thiol substituents grouped in bands while the thiophene substituents uniformly distributed along the SWNTs.
Chemical Communications 12/2005; · 6.17 Impact Factor
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ABSTRACT: With the hope of directing future bottom-up fabrication through bulk external stimuli (such as electric fields) on nanometer-sized transporters, we sought to study controlled molecular motion on surfaces through the rational design of surface-capable molecular structures called nanocars. Here we show that the observed movement of the nanocars is a new type of fullerene-based wheel-like rolling motion, not stick-slip or sliding translation, due to evidence including directional preference in both direct and indirect manipulation and studies of related molecular structures.
Nano Letters 12/2005; 5(11):2330-4. · 13.20 Impact Factor
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ABSTRACT: Single-wall carbon nanotubes (SWNTs) were arranged in a membrane similar to a "bed-of-nails", in which a single layer of parallel SWNTs was densely packed and aligned along the normal to the membrane. The planar, free-standing, ultrathin SWNT membranes were prepared by milling a neat SWNT fiber with a gallium focused ion beam. The approach is readily applicable to cutting nanotubes to a desirable and precise length and enables further fabrication of devices using the "bed-of-nails" membranes to test the transport properties of SWNTs.
Journal of the American Chemical Society 09/2004; 126(31):9502-3. · 9.91 Impact Factor
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ABSTRACT: This dissertation analyzes the feasibility and advantages of molecular functionalization of scanning tunneling microscope (STM) probes with buckminsterfullerenes. The C 60 molecules are adsorbed onto the tunneling region of a STM tip by rastering the tip in a thin film of C 60 vacuum deposited on graphite. The individual tip-adsorbed molecules are subsequently imaged in situ by scanning the fullerene-adsorbed tip over a defect covered graphite surface. These nanometer-size defects serve as a surface tip array which images the molecules adsorbed to the tip when the surface is scanned. It is then demonstrated that the fullerene-adsorbed STM tips can be used to observe electron scattering from point defects and steps on graphite. This cannot be observed using bare metal tips. Molecular functionalization adds a new dimension to scanning tunneling microscopy by allowing greater control of the electronic interactions between the tip and sample. Acknowledgments In graduate school, as in the...
04/2000;
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ABSTRACT: Investigating the Electronic Properties of Carbon Nanostructures with Scanning Tunneling Microscopy Kevin F. Kelly Abstract Utilizing the scanning tunneling microscope, the electronic and structural properties of a variety of carbon based nanostructures are explored. Among these, threefold electron scattering on graphite generated byvacancies and adsorbates is imaged using C 60 -functionalized STM tips. By comparing a simple theoretical model to the experimental results, it is possible to identify the exact location of the defect in the graphite lattice. It is also possible to qualitatively identify the bonding strength of various adsorbates. In addition to graphite, monolayers of disulfide derivatized fullerenes are probed with the STM. The self-assembling and electronic properties of these molecules are investigated. Lastly, comparisons are drawn from these experiments to similar investigations of functionalized carbon nanotubes. Contents Abstract ii Abstract iii List of Illus...
04/2000;
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ABSTRACT: The thiol and thiophene sidewall functionalized single-walled carbon nanotubes have been investigated by scanning tunneling microscopy. By exploiting the well-established Au-S chemistry, the thiol-and thiophene-modified nanotubes were self-assembled and anchored on bare gold surfaces as well as inserted into hexanethiol self-assembled monolayers. The positions of functional groups on the nanotube sidewall are revealed during imaging. From these measurements we have determined the size and spatial distribution of the sulfur-functionalized sidechains along the nanotube sidewalls.
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ABSTRACT: Reduction of graphite by lithium in liquid ammonia yields graphite salts that can be reacted with dodecyl iodide to yield soluble dodecylated graphite. The height of the soluble graphite nanoplatelets was determined by scanning tunneling microscopy (STM) to be 3.5 nm, corresponding to approximately 10 layers of graphene. Solubility in organic solvents was determined to be 20 mg/L in chloroform and 70 mg/L in 1,2,4-trichlorobenzene.
Carbon. 47(13):2945-2949.
