Dynamic Scanning Tunneling Research Articles

Distinct armchair and zigzag charge transport through single polycyclic aromatics.

In aromatic systems with large π-conjugated structures, armchair and zigzag configurations can affect each material's electronic properties, determining their performance and generating certain quantum effects. Here, we explore the intrinsic effect of armchair and zigzag pathways on charge transport through single hexabenzocoronene molecules. Theoretical calculations and systematic experimental results from static carbon-based single-molecule junctions and dynamic scanning tunneling microscope break junctions show that charge carriers are preferentially transported along the hexabenzocoronene armchair pathway, and thus, the corresponding current through this pathway is approximately one order of magnitude higher than that through the zigzag pathway. In addition, the molecule with the zigzag pathway has a smaller energy gap. In combination with its lower off-state conductance, it shows a better field-effect performance because of its higher on-off ratio in electrical measurements. This study on charge transport pathways offers a useful perspective for understanding the electronic properties of π-conjugated systems and realizing high-performance molecular nanocircuits toward practical applications.

Straight imaging and mechanism behind grain boundary electron emission in Pt-doped ultrananocrystalline diamond films

A detailed scanning tunneling microscopic (STM) investigation is carried out to directly image and understand the mechanism behind enhanced conductivity and electron field emission (EFE) properties for platinum (Pt) ion doped/post-annealed ultrananocrystalline diamond (UNCD) films. Straight imaging of conducting/non-conducting sites is mapped by dynamic scanning tunneling microscopy (D–STM). Energy dissipation mapping and the local current–voltage measurements illustrate that grain boundaries are the conducting/emitting sites. Further, this fact is supported by the high resolution current imaging tunneling spectroscopy (CITS). The formation of abundant conducting sp2 nanographitic phases along the diamond grain boundaries, confirmed from transmission electron microscopic examinations, is believed to be the genuine factor that helps for the easy transport of electrons and hence enhanced the conductivity/emission properties. The fabrication of these films with high conductivity and superior EFE properties is a direct and simple approach which opens up new prospects in flat panel displays and high brightness electron sources. Moreover, we expect the energy dissipation associated with assisted tunneling process of electrons from the tip of STM to sample surface, can be a very useful technique for imaging heterostructured semiconducting surfaces due to their electrical conductivity differences in nanometer scale, which can explain many physical and chemical properties.

Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H–SiC(0001) surface during scanning tunneling and atomic force microscopy studies

The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H–SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H–SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15–20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H–SiC(0001).

Fabrication of ultra-sharp tips from carbon fiber for scanning tunneling microscopy investigations of epitaxial graphene on 6H-SiC(0 0 0 1) surface

The fabrication of ultra-sharp tips from carbon fiber (CF), which are mounted on a qPlus probe for combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM) experiments, is reported. The carbon fiber tips were electrochemically etched in a KOH or NaOH solution, using different electronic devices. CF tips with an apex radius ∼10nm, as deduced from the measured slopes of the Fowler–Nordheim plots (kR<70nm for k∼6), were routinely obtained with a homemade electronic device that controls the intensity of the etching current. Then, these conductive CF tips were also characterized by imaging the 6H-SiC(0001) surface covered by an epitaxial graphene layer in ultra-high vacuum (UHV). The lattice of the (63×63) R30° reconstruction was regularly imaged by STM working either in non-oscillating mode or in dynamic mode, which also maps the variations of the force gradient. From these measurements with a constant mean-tunneling-current of 20pA, it was found that the STM tip suffered variations of the tip/surface force gradient in between 8.25 and 16.50N/m when it scanned the epitaxial graphene layer on the reconstructed 6H-SiC(0001) surface.

Combined dynamic scanning tunneling microscopy and frequency modulation atomic force microscopy investigations on polythiophene chains on graphite with a tuning fork sensor

Polythiophene molecules adsorbed on a highly oriented pyrolytic graphite surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM) with a quartz tuning fork sensor operating in Qplus mode and equipped with a Pt/Ir tip. Upon completing a careful sub-angström oscillation amplitude calibration of the probe, experiments were conducted in an ultra high vacuum at room temperature. By selecting the tip/surface distance regulation parameter, one can select the type of simultaneous information obtained in an area. For distance regulation based on the mean tunneling current, dynamic STM images together with maps of tip/surface force gradient were obtained. FM-AFM images with maps of the tunneling current were also acquired when the distance regulation was based on the frequency shift. Comparison between these images reveals interesting features. For example the tip which operates in STM mode with ultra low current (&amp;lt;10 pA) generates different interaction forces above molecules or graphite. Changes in energy dissipation processes as small as tens of millielectronvolts per cycle were recorded when the tip oscillates above the polymer or on the graphite surface. Hence data demonstrates that a stiff piezoelectric tuning fork of several kilonewtons/meters working as an AFM/STM probe with sub-angström amplitude can characterize weakly adsorbed molecules.

Dynamic scanning probe microscopy of adsorbed molecules on graphite

We have used a combined dynamic scanning tunneling and atomic force microscope to study the organization of weakly bound adsorbed molecules on a graphite substrate. These weakly bound molecules may be imaged in dynamic scanning tunneling microscopy (STM) mode in which the probe is oscillated above the surface. We show that molecular resolution may be readily attained and that a similar mode of imaging may be realized using conventional STM arrangement. We also show, using tunneling spectroscopy, the presence of an energy gap for the adsorbed molecules confirming a weak molecule-substrate interaction.

Structure and Energetics of Diphenylalanine Self-Assembling on Cu(110)

We investigate the dynamical features of the adsorption of diphenylalanine molecules on the Cu(110) surface and of their assembling into supramolecular structures by a combination of quantum and classical atomistic modeling with dynamic scanning tunneling microscopy and spectroscopic experiments. Our results reveal a self-assembling mechanism in which isolated adsorbed molecules change their conformation and adsorption mode as a consequence of their mutual interactions. In particular, the formation of zwitterions after proton transfer between initially neutral molecules is found to be the key event of the assembling process, which stabilizes the supramolecular structures. Because of the constraints on the intermolecular bonds exerted by the surface-molecule interactions, the assembly process is strictly stereoselective, and may suggest a general model for patterning and functionalization of bare metal surfaces with short chiral peptides.

Simultaneous current-, force-, and work-function measurement with atomic resolution

The local work function of a surface determines the spatial decay of the charge density at the Fermi level normal to the surface. Here, we present a method that enables simultaneous measurements of local work-function and tip-sample forces. A combined dynamic scanning tunneling microscope and atomic force microscope is used to measure the tunneling current between an oscillating tip and the sample in real time as a function of the cantilever’s deflection. Atomically resolved work-function measurements on a silicon (111)−(7×7) surface are presented and related to concurrently recorded tunneling current and force measurements.