PTCDA growth on Ge(111)-[Formula: see text] surfaces: a scanning tunneling microscopy study.

The initial stages of growth of PTCDA (3,4,9,10 perylene tetracarboxylic dianhydride) at room temperature (RT) on Ge(111)-[Formula: see text] surfaces have been studied by means of scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. The results show that PTCDA molecules have a high mobility at RT on the well ordered areas of the semiconductor substrate, since nucleation is only observed in domain walls, steps and surface defects. However, no molecular ordering has been detected at submonolayer coverage. For higher coverages, the formation of three-dimensional (3D) molecular islands has been observed. These 3D islands present a crystalline nature as demostrated by molecularly resolved STM images. According to these STM measurements, PTCDA molecules are ordered in a herringbone structure, similar to the one observed in PTCDA bulk crystals. Moreover, the 3D crystallites are grown on top of a disordered molecular layer, which acts as a passivating layer.

Visualizing the interface state of PTCDA on Au(111) by scanning tunneling microscopy.

We have investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS) the electronic structure of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) molecular monolayers grown on Au(111). Thanks to our STM/STS measurements, performed under ultra-high vacuum conditions and low temperature, an interface state directly derived from the Shockley-type surface state of pristine Au(111) has been detected. Low bias voltage STM images show the formation of standing wave patterns both on Au(111) and on Au(111) covered by a PTCDA monolayer. These patterns result from the scattering of quasi-free 2D electron surface states with surface defects. By Fourier transforming STM images, the corresponding wavevectors have been extracted. In particular, the simultaneous imaging of both pristine and PTCDA covered Au(111) areas has allowed to measure the Fermi contours and the Fermi wavevectors of both systems. These measurements show that one monolayer PTCDA on Au(111) presents an interface state with an isotropic circular Fermi contour and smaller Fermi wavector ([Formula: see text]) than the corresponding Fermi wavector of pristine Au(111) ([Formula: see text]). This picture is consistent with an upward shift of the Shockley-type surface state due to the presence of the molecular monolayer.

Growth of ordered molecular layers of PTCDA on Pb/Si(111) surfaces: a scanning tunneling microscopy study.

The growth of well-ordered layers of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) molecules on Pb/Si(111) surfaces has been investigated by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. These Pb/Si(111) substrates, which present several distinct phases with different reconstructions, have allowed the exploration of new passivation schemes for the growth of ordered organic layers on Si(111) surfaces. According to our STM measurements, the higher Pb coverage phases (namely the so-called hexagonal incommensurate and [Formula: see text] reconstructions) present rather inert surfaces that allow easy diffusion of PTCDA molecules at room temperature and the formation of a well ordered first molecular layer which displays a herringbone reconstruction. For multilayer PTCDA coverage on these Pb/Si(111) phases, the formation of three-dimensional crystallites, with structure similar to that of the bulk PTCDA crystal, has been observed, indicating that a Stranski-Krastanov growth mode is dominant. On lower Pb coverage substrates (presenting the defective [Formula: see text] and mosaic [Formula: see text] reconstructions) no long range PTCDA order has been obtained. The systematic variation of the substrate reconstruction has allowed in the present work the relation of the surface reactivity of each reconstruction to the formation of ordered layers of PTCDA on Pb/Si(111) substrates.

Atomic-Scale Variations of the Mechanical Response of 2D Materials Detected by Noncontact Atomic Force Microscopy.

We show that noncontact atomic force microscopy (AFM) is sensitive to the local stiffness in the atomic-scale limit on weakly coupled 2D materials, as graphene on metals. Our large amplitude AFM topography and dissipation images under ultrahigh vacuum and low temperature resolve the atomic and moiré patterns in graphene on Pt(111), despite its extremely low geometric corrugation. The imaging mechanisms are identified with a multiscale model based on density-functional theory calculations, where the energy cost of global and local deformations of graphene competes with short-range chemical and long-range van der Waals interactions. Atomic contrast is related with short-range tip-sample interactions, while the dissipation can be understood in terms of global deformations in the weakly coupled graphene layer. Remarkably, the observed moiré modulation is linked with the subtle variations of the local interplanar graphene-substrate interaction, opening a new route to explore the local mechanical properties of 2D materials at the atomic scale.

Tug-of-war between corrugation and binding energy: revealing the formation of multiple moiré patterns on a strongly interacting graphene-metal system.

The formation of multidomain epitaxial graphene on Rh(111) under ultra-high vacuum (UHV) conditions has been characterized by scanning tunnelling microscopy (STM) measurements and density functional theory (DFT) calculations. At variance with the accepted view for strongly interacting graphene-metal systems, we clearly demonstrate the formation of different rotational domains leading to multiple moiré structures with a wide distribution of surface periodicities. Experiments reveal a correlation between the STM apparent corrugation and the lattice parameter of the moiré unit cell, with corrugations of just 30-40 pm for the smallest moirés. DFT calculations for a relevant selection of these moiré patterns show much larger height differences and a non-monotonic behaviour with the moiré size. Simulations based on non-equilibrium Green's function (NEGF) methods reproduce quantitatively the experimental trend and provide a detailed understanding of the interplay between electronic and geometric contributions in the STM contrast of graphene systems. Our study sheds light on the subtle energy balance among strain, corrugation and binding that drives the formation of the moiré patterns in all graphene/metal systems and suggests an explanation for the success of an effective model only based on the lattice mismatch. Although low values of the strain energy are a necessary condition, it is the ability of graphene to corrugate in order to maximize the areas of favourable graphene-metal interactions that finally selects the stable configurations.

Towards scalable nano-engineering of graphene.

By merging bottom-up and top-down strategies we tailor graphene's electronic properties within nanometer accuracy, which opens up the possibility to design optical and plasmonic circuitries at will. In a first step, graphene electronic properties are macroscopically modified exploiting the periodic potential generated by the self assembly of metal cluster superlattices on a graphene/Ir(111) surface. We then demonstrate that individual metal clusters can be selectively removed by a STM tip with perfect reproducibility and that the structures so created are stable even at room temperature. This enables one to nanopattern circuits down to the 2.5 nm only limited by the periodicity of the Moiré-pattern, i.e., by the distance between neighbouring clusters, and different electronic and optical properties should prevail in the covered and uncovered regions. The method can be carried out on micro-meter-sized regions with clusters of different materials permitting to tune the strength of the periodic potential.

Unraveling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis.

Extensive scanning tunneling microscopy and spectroscopy experiments complemented by first-principles and parametrized tight binding calculations provide a clear answer to the existence, origin, and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1° to 10°) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and by calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the role of the layer topographic corrugation and of electronic effects in the apparent moiré contrast measured on the STM images.

Comparative structural and electronic studies of hydrogen interaction with isolated versus ordered silicon nanoribbons grown on Ag(110).

We have investigated the geometry and electronic structure of two different types of self-aligned silicon nanoribbons (SiNRs), forming either isolated SiNRs or a self-assembled 5 × 2/5 × 4 grating on an Ag(110) substrate, by scanning tunnelling microscopy and high resolution x-ray photoelectron spectroscopy. At room temperature we further adsorb on these SiNRs either atomic or molecular hydrogen. The hydrogen absorption process and hydrogenation mechanism are similar for isolated or 5 × 2/5 × 4 ordered SiNRs and are not site selective; the main difference arises from the fact that the isolated SiNRs are more easily attacked and destroyed faster. In fact, atomic hydrogen strongly interacts with any Si atoms, modifying their structural and electronic properties, while molecular hydrogen has first to dissociate. Hydrogen finally etches the Si nanoribbons and their complete removal from the Ag(110) surface could eventually be expected.

Point defects on graphene on metals.

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural, and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in freestanding graphene layers.

Missing atom as a source of carbon magnetism.

Atomic vacancies have a strong impact in the mechanical, electronic, and magnetic properties of graphenelike materials. By artificially generating isolated vacancies on a graphite surface and measuring their local density of states on the atomic scale, we have shown how single vacancies modify the electronic properties of this graphenelike system. Our scanning tunneling microscopy experiments, complemented by tight-binding calculations, reveal the presence of a sharp electronic resonance at the Fermi energy around each single graphite vacancy, which can be associated with the formation of local magnetic moments and implies a dramatic reduction of the charge carriers' mobility. While vacancies in single layer graphene lead to magnetic couplings of arbitrary sign, our results show the possibility of inducing a macroscopic ferrimagnetic state in multilayered graphene just by randomly removing single C atoms.

WSXM: a software for scanning probe microscopy and a tool for nanotechnology.

In this work we briefly describe the most relevant features of WSXM, a freeware scanning probe microscopy software based on MS-Windows. The article is structured in three different sections: The introduction is a perspective on the importance of software on scanning probe microscopy. The second section is devoted to describe the general structure of the application; in this section the capabilities of WSXM to read third party files are stressed. Finally, a detailed discussion of some relevant procedures of the software is carried out.

Adatom-adatom interaction mediated by an underlying surface phase transition.

Low temperature scanning tunneling microscopy measurements on the adsorption of single Pb adatoms on Si(111)-(square root 3 x square root 3)-Pb surfaces reveal the vertical displacement patterns induced on the substrate by these Pb adatoms as well as a novel adatom-adatom interaction. The origin of both can be traced back to the (square root 3 x square root 3)<-->(3 x 3) phase transition taking place at lower temperatures. A Landau-like approach explains the displacement patterns as due to the corresponding order parameter and shows that the vicinity of a surface phase transition gives rise to a nonmonotonic adatom-adatom interaction.

Direct observation of a (3 x 3) phase in alpha-Pb/Ge(111) at 10 K.

We have investigated the recently reported structural phase transition at low temperature (LT) for alpha-Pb/Ge(111) [from a (3 x 3) symmetry to a disordered phase] using scanning tunneling microscopy (STM). By tracking exactly the same surface regions with atomic resolution while varying the sample temperature from 40 to 140 K, we have observed that substitutional point defects are not mobile, in clear contrast to previous assumptions. Moreover, STM data measured at the lowest temperatures ever reported for this system (10 K) show that while filled-state images display the apparent signature of a glassy phase with no long-range order, in empty-state images honeycomb patterns with (3 x 3) periodicity, and not distinguishable from data measured at much higher temperatures, are clearly resolved. These new observations cast serious doubts on the nature and/or on the existence of a disordered phase at LT.

Intrinsic character of the (3 x 3) to (square root 3 x square root 3) phase transition in Pb/Si(111).

We have investigated the (3 x 3) to (square root 3 x square root 3) reversible phase transition in Pb/Si(111) by means of variable temperature scanning tunneling microscopy and density functional first-principles calculations. By tracking exactly the same regions of the surface with atomic resolution in a temperature range between 40 and 200 K, we have observed the phase transition in real time. The ability to prepare and track exceptionally large domains without defects has allowed us to detect the intrinsic character of the phase transition at temperatures around 86 K. This intrinsic character is in full agreement with our first-principles calculations. Moreover, our results show that the hypothesis that point defects play a fundamental role as the driving force, reported for similar systems, can be discarded for Pb/Si(111).