1,720,998 research outputs found
Improving the resolution of lock-in measurements by tailoring the modulation
No full textPhase-sensitive detection with a lock-in amplifier is a powerful, general measurement scheme finding widespread applications in science and technology. In several cases, however, its performance is limited by the modulation broadening, i.e. the resolution loss due to the finite amplitude of the modulation that, in this technique, is applied to the system under study. We demonstrate that this broadening can be reduced by employing a modulation function different from the commonly used sinusoid, which is non-optimal in this respect. On the basis of a mathematical analysis of the lock-in output, we provide a modulation function optimized for the detection at twice its frequency, testing the resulting performance in an STM–IETS experiment. Our findings can be directly applied whenever a lock-in is used to measure a second derivative, and are easily extendible to other derivatives as well
How to select fast scanning frequencies for high-resolution fast STM measurements with a conventional microscope
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Method for driving a scanning probe microscope at elevated scan frequencies
No full textA method for operating a scanning probe microscope at elevated scan frequencies has a characterization stage of sweeping a plurality of excitation frequencies of the vertical displacement of the scanning element; measuring the value attained by the reading parameter at the excitation frequencies; and identifying plateau regions of the response spectrum of the reading parameter. The reading parameter variation is limited within a predetermined range over a predefined frequency interval, thereby defining corresponding fast scanning frequency windows in which the microscope assembly is sufficiently stable to yield a lateral resolution comparable to the one obtained during slow measurements. The measurement stage includes driving the scanning element along at least a scanning trajectory over the surface of the specimen at a frequency selected among the frequencies included in a fast scanning frequency window
Growth of regular nanometric molecular arrays on a functional 2D template based on a chemical guest-host approach
No full textA regular 2D array of crown molecules, which would spontaneously self-assemble into disordered molecular
clusters, is obtained by exploiting a guest–host process, based on the chemical affinity between
amino and carboxylic groups on a gold surface. First a carboxylic organic template is formed, which then
serves as a host for amino-functionalized crown molecules. The amino-carboxylic interaction thereby
drives the formation of a monolayer of guest molecules, regularly distributed at the nanometer scale, preventing
their aggregation in unordered clusters observed on a bare gold surface. This method, which can
be applied to other guest molecules, represents a novel route to overcome the shape-matching requirements
of the standard guest–host architectures. Furthermore, it is intrinsically selective, due to the chemical
nature of the anchoring process
Effects of the Lattice Expansion on the Reactivity of a 1D Oxide
No full textBy means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we characterize at the single-atom level the mechanism of the water formation reaction on the (10 × 2)-O/Rh(110) surface, a prototype of a one-dimensional (1D) oxide where the lattice expansion and the segmentation of the surface play a fundamental role. When the reaction is imaged in the 238−263 K temperature range (35 s/image acquisition time), a peculiar comblike propagation mechanism for the reaction front is found. Fast STM measurements (33 ms/image) prove that this mechanism holds also at room temperature, being therefore an intrinsic characteristic of the reaction on the 1D oxide. DFT calculations explain the observed behavior as due to the interplay between the lattice expansion in the initial surface and its relaxation during the reaction that leads to varying configurations for the reactants
Binary Conformational Switches in a Porphyrin Chain: Tautomerization and Stereoisomerization
Full text linkIn the last decade, hydrogen (H-)tautomerization, that is, a reaction that involves simple intramolecular proton transfer, has been studied in single phthalocyanine, porphyrin, and porphycene derivatives as a prototypical single molecular conductance switch. Here, by means of low-temperature scanning tunneling microscopy and density functional theory calculations, we report a binary H-tautomerism and stereoisomeric conformational switch in (amino-functionalized) porphyrins assembled in molecular chains on a gold surface. We show that the formation of the chain is crucial for the binary tautomeric switch mechanism as the single molecule switches differently. Our findings suggest that the (amino-)functionalization of molecules can be exploited not only to drive the formation of molecular self-assemblies but also to steer their switching properties
A competitive amino-carboxylic hydrogen bond on a gold surface
No full textAn amino-carboxylic motif is identified as a novel synthon in the formation of 2D hetero-organic architectures at surfaces. The well-defined interacting scheme we describe herein represents an ideal prototypical system for further investigation of the interaction at surfaces of the two functional groups
Spatial periodicity in molecular switching
No full textThe ultimate miniaturization of future devices will require the use of functional molecules at the nanoscale and their integration into larger architectures. Switches represent a prototype of such functional molecules because they exhibit characteristic states of different physical/chemical properties, which can be addressed reversibly. Recently, various switching entities have been studied and switching of single molecules on surfaces has been demonstrated. However, for functional molecules to be used in a future device, it will be necessary to selectively address individual molecules, preferentially in an ordered pattern. Here, we show that azobenzene derivatives in the trans form, adsorbed in a homogeneous two-dimensional layer, can be collectively switched with spatial selectivity, thus forming a periodic pattern of cis isomers. We find that the probability of a molecule switching is not equally distributed, but is strongly dependent on both the surrounding molecules and the supporting surface, which precisely determine the switching capability of each individual molecule. Consequently, exactly the same lattices of cis isomers are created in repeated erasing and re-switching cycles. Our results demonstrate a conceptually new approach to spatially addressing single functional molecules
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