266 research outputs found
Bowl Inversion of Surface-Adsorbed Sumanene
No full textBowl-shaped π-conjugated compounds offer the possibility to study curvature-dependent host–guest interactions and chemical reactivity in ideal model systems. For surface-adsorbed π bowls, however, only conformations with the bowl opening pointing away from the surface have been observed so far. Here we show for sumanene on Ag(111) that both bowl-up and bowl-down conformations can be stabilized. Analysis of the molecular layer as a function of coverage reveals an unprecedented structural phase transition involving a bowl inversion of one-third of the molecules. On the basis of scanning tunneling microscopy (STM) and complementary atomistic simulations, we develop a model that describes the observed phase transition in terms of a subtle interplay between inversion-dependent adsorption energies and intermolecular interactions. In addition, we explore the coexisting bowl-up and -down conformations with respect to host–guest binding of methane. STM reveals a clear energetic preference for methane binding to the concave face of sumanene
On-Surface Synthesis of Atomically Precise Graphene Nanoribbons
No full textThe surface-assisted polymerization and cyclodehydrogenation of specifically designed organic precursors provides a route toward atomically precise graphene nanoribbons, which promises to combine the outstanding electronic properties of graphene with a bandgap that is sufficiently large for room-temperature digital-logic applications. Starting from the basic concepts behind the on-surface synthesis approach, this report covers the progress made in understanding the different reaction steps, in synthesizing atomically precise graphene nanoribbons of various widths and edge structures, and in characterizing their properties, ending with an outlook on the challenges that still lie ahead
Electronic Structure of Atomically Precise Graphene Nanoribbons
No full textSome of the most intriguing properties of graphene are predicted for specifically designed nanostructures such as nanoribbons. Functionalities far beyond those known from extended graphene systems include electronic band gap variations related to quantum confinement and edge effects, as well as localized spin-polarized edge states for specific edge geometries. The inability to produce graphene nanostructures with the needed precision, however, has so far hampered the verification of the predicted electronic properties. Here, we report on the electronic band gap anddispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated via on-surface synthesis. Angle-resolved photoelectron spectroscopy and scanning tunnelingspectroscopy data from armchair graphene nanoribbons of width N = 7 supported on Au(111) reveal a band gap of 2.3 eV, an effective mass of 0.21 m0 at the top of the valence band, and anenergy-dependent charge carrier velocity reaching 8.2 105 m/s in the linear part of the valence band. These results are in quantitative agreement with theoretical predictions that include image charge corrections accounting for screening by the metal substrate and confirm the importance of electron-electron interactions in graphene nanoribbon
Exciton-dominated optical response of ultra-narrow graphene nanoribbons
No full textNarrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths
Molecular heterostructure by fusing graphene nanoribbons of different lengths through a pentagon ring junction
No full textGraphene nanoribbons (GNRs) have attracted great research interest because of their widely tunable and unique electronic properties. The required atomic precision of GNRs can be realized via on-surface synthesis method. In this work, through a surface assisted reaction we have longitudinally fused the pyrene-based graphene nanoribbons (pGNR) of different lengths by a pentagon ring junction, and built a molecular junction structure on Au (111). The electronic properties of the structure are studied by scanning tunneling spectroscopy (STS) combined with tight binding (TB) calculations. The pentagon ring junction shows a weak electronic coupling effect on graphene nanoribbons, which makes the electronic properties of the two different graphene nanoribbons connected by a pentagon ring junction analogous to type I semiconductor heterojunctions
A thermistor-based temperature sensor for a real-time clock with ±2 ppm frequency stability
No full textThis paper describes the design of a temperature sensor based on integrated poly-silicon thermistors. The thermistors are incorporated in a Wien-bridge RC filter, which, in turn, is embedded in a frequency-locked loop. The loop’s output frequency is then determined by the filter’s temperature-dependent phase shift, thus realizing an energy-efficient and high resolution temperature sensor. After a 3-point calibration, the sensor achieves an inaccuracy of less than ±0.12°C (min-max) from -40°C to 85°C. This translates into a frequency stability ofbetter than ±2ppm from -40°C to 85°C when the sensor is used to temperature compensate the quartz-crystal oscillator of a 32kHz real-time clock. The 0.09mm2 sensor also achieves 2.8mK (rms) resolution in a 32ms conversion time while dissipating only 31μW.Accepted Author ManuscriptElectronic Instrumentatio
Do They Always Say No? German Consumers and Second-Generation GMO Foods
No full textEuropean consumers and, in particular, German consumers are known to be very critical towards the introduction of genetically modified (GM) foods. It is analyzed here whether German consumers do reject second-generation GMO foods, too. Whereas first-generation GM crops induced producer-related benefits, second-generation GM crops are associated with consumer-oriented benefits like an improvement of nutritional quality. The determinants of demand for second-generation GM rapeseed oil are investigated within an online survey of 1556 German consumers. It is elaborated how two functional properties of that product matter; i.e. long-chain ϖ3 fatty acids and the cholesterol-lowering effect of phytosterols. It turns out that GMO rapeseed oil is neglected by 74 % of all respondents. Output traits, however, will increase the probability of purchases of GMO rapeseed oil. This is more the case for long-chain ϖ3 fatty acids than for phytosterols.consumer behavior, second generation, GMO foods, rapeseed oil, Consumer/Household Economics, Food Consumption/Nutrition/Food Safety,
Probing optical excitations in chevron-like armchair graphene nanoribbons
No full textThe bottom-up fabrication of graphene nanoribbons (GNRs) has opened new opportunities to specifically
tune their electronic and optical properties by precisely controlling their atomic structure. Here, we
address excitation in GNRs with periodic structural wiggles, the so-called chevron GNRs. Based on reflectance difference and high-resolution electron energy loss spectroscopies together with ab initio simulations, we demonstrate that their excited-state properties are of excitonic nature. The spectral fingerprints corresponding to different reaction stages in their bottom-up fabrication are also unequivocally
identified, allowing us to follow the exciton build-up from the starting monomer precursor to the final
GNR structur
Detachment Dynamics of Graphene Nanoribbons on Gold
No full textMetal-surface physisorbed graphene nanoribbons (GNRs) constitute mobile nanocontacts whose interest is simultaneously mechanical, electronic, and tribological. Previous work showed that GNRs adsorbed on Au(111) generally slide smoothly and superlubrically owing to the incommensurability of their structures. We address here the nanomechanics of detachment, as realized when one end is picked up and lifted by an AFM cantilever. AFM nanomanipulations and molecular-dynamics (MD) simulations identify two successive regimes, characterized by (i) a progressively increasing local bending, accompanied by the smooth sliding of the adhered part, followed by (ii) a stick-slip dynamics involving sudden bending relaxation associated with intermittent jumps of the remaining adhered GNR segment and tail end. AFM measurements of the vertical force exhibit oscillations which, compared with MD simulations, can be associated with the successive detachment of individual GNR unit cells of length 0.42 nm. Extra modulations within one single period are caused by steplike advancements of the still-physisorbed part of the GNR. The sliding of the incommensurate moiré pattern that accompanies the GNR lifting generally yields an additional long-period oscillation: while almost undetectable when the GNR is aligned in the standard "R30" orientation on Au(111), we predict that such feature should become prominent in the alternative rotated "R0" orientation on the same surface, or on a different surface, such as perhaps Ag(111)
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