- [Show abstract] [Hide abstract] ABSTRACT: This article focuses on surfaces containing nanoparticles and self-assembled monolayers (SAMs). These surfaces provide a simple and reliable platform for measurements of single electron tunneling (SET) properties of metal nano-particles at room temperature. This approach of interfacial chemistry allows for the elimination of lateral motion of the individual nanoparticles during electronic property studies. The scanning tunneling microscopy (STM) in ultra-high vacuum is used as an accurate and reproducible probe for imaging and I–V characterization of individual or aggregated Au nanoparticles, revealing a large Coulomb gap (1.0 eV) and fine Coulomb staircases (0.2–0.3 eV) at room tempera-ture. The surrounding decanethiol SAM provides an ideal reference for the imaging and I–V measurements of nano-particles. These measurements provide a quantitative guide for regulating current and voltage, at which individual Au nanoparticles may be detached and manipulated with the STM tip.
- [Show abstract] [Hide abstract] ABSTRACT: A molecular-level approach is developed to prevent or inhibit the degradation processes of alkanethiol self-assembled monolayers (SAMs). Previous studies revealed two degradation pathways: direct desorption and oxidation-desorption. By use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in situ and time-dependent imaging reveals and confirms that degradations of alkanethiol SAMs on gold mainly initiate at defect sites, such as domain boundaries and vacancy islands, and then propagate into the ordered domains. Our approach targets at attaching small molecules with preferred adhesion to the defects. The best candidates are aqueous media containing a small amount of amphiphilic surfactant molecules, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). High-resolution studies demonstrate that DMSO and DMF molecules attach to SAM surfaces and more favorably at defect sites, forming relatively stable adsorbates. This attachment increases the activation energy sufficiently to inhibit both degradation pathways. The robustness of this approach has been investigated as a function of surfactant concentration, solution temperature, and the stirring condition. Molecular-level mechanisms and energetics for degradation inhibition of SAMs are also discussed in detail.
- [Show abstract] [Hide abstract] ABSTRACT: Systematic studies on scanning probe lithography (SPL) methodologies have been performed using self-assembled monolayers (SAMs) on Au as examples. The key to achieving high spatial precision is to keep the tip-surface interactions strong and local. Approaches include three atomic force microscopy (AFM) based methods, nanoshaving, nanografting, and nanopen reader and writer (NPRW), which rely on the local force, and two scanning tunneling microscopy (STM) based techniques, field-induced desorption and electron-induced desorption, which use electric field and tunneling electrons, respectively, for nanofabrication. The principle of these procedures, the critical steps in controlling local tip-surface interactions, and nanofabrication media will be discussed. The advantages of SPL will be illustrated through various examples ofproduction and modification of SAM nanopatterns.
- [Show abstract] [Hide abstract] ABSTRACT: In the past decade, scanning tunneling microscopy (STM) has revealed new information regarding self-assembled monolayers (SAMs) of organothiols on Au(111) at the molecular level. The periodicity, defects, morphology, and various phases during the self-assembly process have been visualized with unprecedented detail. Using STM under ultrahigh vacuum, new insights regarding SAMs have been revealed from the perspective of potential applications in molecular devices. This article focuses on a molecular-level understanding of the formation of adatom and vacancy islands and reveals how the structure is impacted by introducing aromatic termini. The thermal stability and thermally induced structural evolution of SAMs are monitored in situ. The behavior of alkanethiol molecules under local electric field and tunneling current are studied with molecular resolution. Molecular-level insight regarding negative differential resistance of SAMs is also discussed.
- [Show abstract] [Hide abstract] ABSTRACT: While the structures of self-assembled monolayers (SAMs) of alkanethiols on Au(111) are extensively studied and well-known, new structures and complex phase behavior have been progressively discovered when coverage of these layers falls below saturation. Structures and phase transitions of annealed decanethiol monolayers on Au(111) surfaces were systematically investigated using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Rich structures were revealed as a result of annealing in UHV. At temperatures below 345 K, no significant changes in coverage were observed, although the size of two-dimensional crystalline c(4√3 × 2√3)R30° domains increases as annealing progresses. A two-dimensional melting occurs at 345 ± 5 K and was captured in situ from time-dependent STM studies. Above 400 K, significant desorption takes place. In the temperature range of 345−400 K, within which desorption progresses to gradually decrease the surface coverage, a variety of striped phases have been observed, each having distinct molecular-level packing and unit cells. Well-known striped phases have been confirmed: (p × √3), with p values (integer or half-integer multiples of the Au(111) periodicity) of 7.5, 9, and 11. In addition, new structures such as mixed striped phases and mesh-like structures are revealed, which are often found to coexist with the regions of pure striped phases. The systematic investigations of the structural and phase evolution shed light on the SAM desorption process at the molecular level.
- [Show abstract] [Hide abstract] ABSTRACT: Self-assembled monolayers (SAMs) of 4-[4‘-(phenylethynyl)-phenylethynyl]-benzenethiols on Au(111) surfaces are investigated by scanning tunneling microscopy (STM) under ultrahigh vacuum. STM images reveal long-range order in these SAMs with a rectangular unit cell containing two molecules. In high-resolution images, two new structural features are resolved, which cannot be explained by the previously proposed (√3 × 2√3)R30° commensurate structure which consists of three equivalent domains. First, six equivalent domains are present, and the orientations of these domains with respect to the three 121 directions of Au(111) are ±5°. Second, superstructures are observed. Periodical ridges are observed as a modulation of the STM imaging contrast within the ordered domains. A new model is proposed, which is very similar to the crystalline structure of p-terphenyl. The closest-packed row is aligned along the next-nearest neighbor or 121 direction of Au(111) with phenyl planes arranged in a herringbone fashion. The lack of simple commensurate structure of arenethiol SAMs is mainly attributed to intermolecular interactions.
University of California, DavisDavis, California, United States