By combining non-contact atomic force microscopy (nc-AFM) and Kelvin probe microscopy (KPFM) in ultra high vacuum environment (UHV), we directly measure the height and work function of graphene monolayer on the Si-face of 6H-SiC(0001) with a precision that allows us to differentiate three different types of graphene structures : zero layer graphene (ZLG), Quasi free-standing monolayer graphene (QFMLG) and bilayer graphene (BLG). The height and work function of ZLG are 2.62 ± 0.22 Å and 4.42 ± 0.05 eV respectively, when they are 4.09 ± 0.11 Å and 4.63 ± 0.05 eV for QFMLG. The work function is 4.83 ± 0.05 eV for the BLG. Unlike any other available technique, the local nc-AFM/KPFM dual probe makes it possible to directly identify the nature of nanometer-sized graphene islands that constitute the early nuclei of graphene monolayer grown on 6H-SiC(0001) by chemical vapor deposition.

The study revealed that the nature of the AlN (0001) surface could play a fundamental role in the growth mode of metals. In the case of silver (Ag) deposited on the AlN(0001) (2×2)‐Nad reconstructed surface, the growth mode was Volmer–Weber. Deposition of indium (In) on a contaminated AlN(0001) surface leads to the formation of a wetting layer, followed by a 3D growth mode. For magnesium (Mg), a wetting layer was observed on the clean and contaminated surface followed by a 2D growth mode. Finally, the Au case was the most interesting since on the contaminated surface and the (2×2)‐Nad reconstructed surface, the growth mode was 3D and 2D, respectively, without the formation of a wetting layer. The Kelvin probe force microscopy (KPFM) study allowed to get qualitative and quantitative information about the work function of cluster and islands observed for the different metals on the AlN(0001) surface.

Charging of 2D Au nanoplatelets deposited on an insulating SiO2 substrate to or from the tip of a non-contact atomic force microscope (nc-AFM) is demonstrated. Charge transfer is controlled by monitoring the resonance frequency shift Δf(V) during the bias voltage ramp V applied to the tip-back electrode junction. The onset of charge transfer is revealed by a transition from a capacitive parabolic behavior to a constant Δf(V) region for both polarities. An analytical model, based on charging by electron field emission, shows that the field-emitted current saturates shortly after the onset of the charging, due to the limiting effect of the charge-induced rise of the Au platelet potential. The value of this current plateau depends only on the rate of the bias voltage ramp and on the value of the platelet/SiO2/back electrode capacitance. This analysis is confirmed by numerical simulations based on a virtual nc-AFM model that faithfully matches the experimental data. Our charging protocol could be used to tune the potential of the platelets at the single charge level.

Silicon carbide (SiC) is nowadays a major material for applications in high power electronics, quantum optics, or nitride semiconductors growth. Mastering the surface of SiC substrate is crucial to obtain reproducible results. Previous studies on the 6H-SiC(0001) surface have determined several reconstructions, including the (root 3x root 3)-R30 degrees and the (3x3). Here, we introduce a process of progressive Si enrichment that leads to the formation of two reconstructions, the giant (12x12) and the (4x8). From electron diffraction and tunneling microscopy completed by molecular dynamics simulations, we build models introducing a type of Si adatom bridging two Si surface atoms. Using these Si bridges, we also propose a structure for two other reconstructions, the (2 root 3x2 root 3)-R30 degrees and the (2 root 3x2 root 13). We show that five reconstructions follow each other with Si coverage ranging from 1 and 1.444 monolayer. This result opens the way to greatly improve the control of 6H-SiC(0001) at the atomic scale.

Noncontact atomic force microscopy images show that gold grows on the (2 x 2)-N-ad reconstructed polar (0001) surface of AlN insulating films, in the form of large monatomic islands. High-resolution images and in situ reflection high-energy electron diffraction spectra reveal two moire patterns from which an atomic model can be built. Density functional theory calculations confirm this model and give insight into the mechanisms that lead to the stabilization of the monolayer. Gold adsorption is accompanied, first, by a global vertical charge transfer from the AlN substrate that fulfills the electrostatic stability criterion for a polar material, and second, by lateral charge transfers that are driven by the local chemical properties of the (2 x 2)-N-ad reconstruction. These results present alternative strategies to grow metal electrodes onto nitride compounds with a better controlled interface, a crucial issue for applications.

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2 x 2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications.

The adsorption of single hexamethoxyltriphenylene molecules on KBr(001) at low temperature (T < 10 K) was studied by low-temperature (T = 5 K) noncontact atomic force microscopy in ultrahigh vacuum. Images of the frequency shift, Measured at constant height, show intramolecular features that are interpreted on the basis of molecular force-field calculations. The main conclusion is that the experimental contrast originates from the electrostatic interaction between a negatively terminated ionic tip and the atomic charges of the molecule. The main trends of the experimental observations of lateral manipulations could also be reproduced by the same method, giving some insight into the manipulation mechanisms. In particular, it is shown that the strong interaction of the oxygen atoms of the molecule with positive ions of the tip plays a major role by facilitating the partial lifting of the molecule from the substrate during the intermediate stages of these manipulation processes.

The performances of the new ScientaOmicron LT-UHV 4-STM microscope have been certified by a series of state-of-art STM experiments on an Au(1 1 1) surface at 4.3 K. During the STM operation of the 4 STM scanners (independently or in parallel with an inter tip apex front to front distance down to a few tens of nanometers), a ΔZ stability of about 2 pm per STM was demonstrated. With this LT-UHV 4-STM stability, single Au atom manipulation experiments were performed on Au(1 1 1) by recording the pulling, sliding and pushing manipulation signals per scanner. Jump to contact experiments lead to perfectly linear low voltage I-V characteristics on a contacted single Au ad-atom with no need of averaging successive I-V ’s. Our results show how this new instrument is exactly 4 times a very precise single tip LT-UHV-STM. Two tips surface conductance measurements were performed on Au(1 1 1) using a lock-in technique in a floating sample mode of operation to capture the Au(1 1 1) surface states via two STM tips dI /dV characteristics.

Nitride wide-band-gap semiconductors are used to make high power electronic devices or efficient light sources. The performance of GaN-based devices is directly linked to the initial AlN buffer layer. During the last twenty years of research on nitride growth, only few information on the AlN surface quality have been obtained, mainly by ex-situ characterization techniques. Thanks to a Non Contact Atomic Force Microscope (NC-AFM) connected under ultra high vacuum (UHV) to a dedicated molecular beam epitaxy (MBE) chamber, the surface of AlN(0001) thin films grown on Si(111) and 4H-SiC(0001) substrates has been characterized. These experiments give access to a quantitative determination of the density of screw and edge dislocations at the surface. The layers were also characterized by ex-situ SEM to observe the largest defects such as relaxation dislocations and hillocks. The influence of the growth parameters (substrate temperature, growth speed, III/V ratio) and of the initial substrate preparation on the dislocation density was also investigated. On Si(111), the large in-plane lattice mismatch with AlN(0001) (19%) induces a high dislocation density ranging from 6 to 12×1010/cm2 depending on the growth conditions. On 4H-SiC(0001) (1% mismatch with AlN(0001)), the dislocation density decreases to less than 1010/cm2, but hillocks appear, depending on the initial SiC(0001) reconstruction. The use of a very low growth rate of 10 nm/h at the beginning of the growth process allows to decrease the dislocation density below 2 × 109/cm2.

The results of the study of the adsorption of 2,3,6,7,10,11-hexacyano-propyloxytriphenylene on KBr(001) by non-contact atomic force microscopy (nc-AFM) coupled to Kelvin probe force microscopy (KPFM) are presented. A particular emphasis is put on the KPFM measurements. In particular, the contrast in the Kelvin potential between a molecular monolayer and the KBr substrate is interpreted on the basis of molecular modeling and classical electrostatic calculations using a spherical tip model. It is demonstrated that this contrast originates from the polarization of the molecule induced by its adsorption conformation where five polar CN groups are adsorbed above five surface K+ ions. The calculated value of the Kelvin potential difference between the molecular monolayer and KBr is in good agreement with the experimental value.

Cantilevers with resonance frequency ranging from 1 MHz to 100 MHz have been developed for dynamic atomic force microscopy. These sensors are fabricated from 3C-SiC epilayers grown on Si(100) substrates by low pressure chemical vapor deposition. They use an on-chip method both for driving and sensing the displacement of the cantilever. A first gold metallic loop deposited on top of the cantilever is used to drive its oscillation by electrothermal actuation. The sensing of this oscillation is performed by monitoring the resistance of a second Au loop. This metallic piezoresistive detection method has distinct advantages relative to more common semiconductor-based schemes. The optimization, design, fabrication, and characteristics of these cantilevers are discussed.

We have developed an energy-filtering device coupled to a quadrupole mass spectrometer to deposit ionized molecules on surfaces with controlled energy in ultra high vacuum environment. Extensive numerical simulations as well as direct measurements show that the ion beam flying out of a quadrupole exhibits a high-energy tail decreasing slowly up to several hundred eV. This energy distribution renders impossible any direct soft-landing deposition of molecular ions. To remove this high-energy tail by energy filtering, a 127° electrostatic sector and a specific triplet lenses were designed and added after the last quadrupole of a triple quadrupole mass spectrometer. The results obtained with this energy-filtering device show clearly the elimination of the high-energy tail. The ion beam that impinges on the sample surface satisfies now the soft-landing criterion for molecular ions, opening new research opportunities in the numerous scientific domains involving charges adsorbed on insulating surfaces.

The scientific and technical challenges involved in the building of the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) on insulating substrates are presented. In the Nanoscience group of Toulouse, the UHV factory has been developed since ten years in order to realize under UHV the five levels of interconnections on insulating substrate, to characterize by NC-AFM the different steps and to measure the electrical properties of the realized device.

The adsorption on KBr(001) of a specially designed molecule, consisting of a flat aromatic triphenylene core equipped with six flexible propyl chains ending with polar cyano groups, is investigated by using atomic force microscopy in the noncontact mode (NC-AFM) coupled to Kelvin probe force microscopy (KPFM) in ultrahigh vacuum at room temperature. Two types of monolayers are identified, one in which the molecules lie flat on the surface (MLh) and another in which they stand approximately upright (MLv). The Kelvin voltage on these two structures is negatively shifted relative to that of the clean KBr surface, revealing the presence of surface dipoles with a component pointing along the normal to the surface. These findings are interpreted with the help of numerical simulations. It is shown that the surface–molecule interaction is dominated by the electrostatic interaction of the cyano groups with the K + ions of the substrate. The molecule is strongly adsorbed in the MLh structure with an adsorption energy of 1.8 eV. In the MLv layer, the molecules form π-stacked rows aligned along the polar directions of the KBr surface. In these rows, the molecules are less strongly bound to the substrate, but the structure is stabilized by the strong intermolecular interaction due to π-stacking.

One way to improve the force sensitivity of Atomic Force Microscopy (AFM) cantilevers is to increase their resonance frequency. SiC is an excellent material for that purpose due to its high Young’s modulus and low mass density. This size reduction makes conventional optical motion detection methods inappropriate. Here, we introduce self-sensing, self-excited high frequency AFM cantilevers. The motion detection is based on the measurement of a metallic piezoresistor incorporated in the cantilever. The motion excitation is performed by electrothermal actuation using another metallic circuit. Cantilevers with sizes as low as 4 μm in length, 1.2 μm in width and 0.5 μm in thickness were realized by using different steps of e-beam lithography, deposition of thin gold films to pattern the piezoresistor and the electrothermal actuation electrode. Dry etching SF 6 plasma was used for etching the SiC cantilever and TMAH solution heated to 80°C to release the cantilever. In this case, a thigh control of underetching, which reduces the cantilever resonance frequency was required.

Molecular adsorption studies on atomically clean, well-defined surfaces of bulk insulators are still very scarce and have been restricted to a small number of molecules. In this study, the adsorption of 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP) on KBr(001) was investigated by atomic force microscopy in noncontact mode (NC-AFM) under ultrahigh vacuum at room temperature from submonolayer to multilayer coverage. This molecule is the first member of a new family of molecules, designed and synthesized for adsorption studies on alkali halide surfaces. They were built around a flat aromatic triphenylene core equipped with peripheral polar groups to enhance the interaction of the molecule with the surface in order to limit molecular diffusion at room temperature. Constant-frequency-shift NC-AFM imaging established the following adsorption sequence: The molecules decorate the KBr steps before forming two-molecular-layer-thick islands with a distorted hexagonal arrangement. At higher coverage, a second generation of much taller islands with the structure of the bulk HMTP crystal appears as a consequence of a dewetting transition, driven by the deformation energy accumulated in the first-generation islands during growth. High-resolution images obtained on top of these islands show submolecular contrast. These results will be useful in optimizing the next generation of molecules of this new family.

The scientific and technical challenges involved in building the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) are discussed. The practical, laboratory scale approach explored today to assemble a multi-access atomic scale precision interconnection machine is presented. Depending on the surface electronic properties of the targeted substrates, two types of machines are considered: on moderate surface band gap materials, scanning tunneling microscopy can be combined with scanning electron microscopy to provide an efficient navigation system, while on wide surface band gap materials, atomic force microscopy can be used in conjunction with optical microscopy. The size of the planar part of the circuit should be minimized on moderate band gap surfaces to avoid current leakage, while this requirement does not apply to wide band gap surfaces. These constraints impose different methods of connection, which are thoroughly discussed, in particular regarding the recent progress in single atom and molecule manipulations on a surface.

Buckybowls: The adsorption of penta-tert-butylcorannulene, a molecule with fivefold symmetry, on Cu(111), a surface with threefold symmetry, is investigated by scanning tunneling microscopy complemented by structure calculations. The symmetry mismatch is resolved by the formation of threefold-symmetric subunits consisting of three molecules, which combine with single molecules to form a nearly perfect filling of the plane (see picture).

Technology continues to produce functioning transistors on ever smaller scales. The day will come soon, however, when there will not be enough atoms on the surface of a semi-conductor to define the structure of a transistor and, consequently, of complex electronic circuits. At this stage, new approaches and new technologies are necessary for building computers, memory or telecommunication devices [1]. Anticipating this challenge, researchers in a few laboratories around the world are now looking for the maximum number of atoms required to fabricate, for example, a calculating unit able to perform a computation by itself. This problem of creating an atom based technology is not limited to electronics or to telecommunication and encompasses all types of devices, including mechanical machines and transducers.