Agnès Piednoir

Atomic resolution on small three-dimensional metal clusters by STM

Abstract Small, three-dimensional Pd clusters ( d 2 ) are imaged with atomic resolution, both in air and in UHV, using scanning tunneling microscopy (STM). We show that a complete characterization of the morphology of the cluster can be achieved: three-dimensional shape; azimuthal orientation on the substrate; and arrangement of atoms on lateral facets. These results are in good agreement with those obtained from TEM observations on large particles ( d > 10 nm). Some limitations of STM for such studies are discussed.

Nanostenciling for fabrication and interconnection of nanopatterns and microelectrodes

Stencil lithography is used for patterning and connecting nanostructures with metallic microelectrodes in ultrahigh vacuum. Microelectrodes are fabricated by static stencil deposition through a thin silicon nitride membrane. Arbitrary nanoscale patterns are then deposited at a predefined position relative to the microelectrodes, using as a movable stencil mask an atomic force microscopy (AFM) cantilever in which apertures have been drilled by focused ion beam. Large scale AFM imaging, combined with the use of a high precision positioning table, allows inspecting the microelectrodes and positioning the nanoscale pattern with accuracy better than 100nm.

STM and TEM studies of a model catalyst: Pd/MoS2(0001)

Abstract Combined in situ scanning tunnelling microscopy (STM) and ex situ transmission electron microscopy (TEM) experiments performed on the same samples have been used to characterize in detail a model catalyst: Pd/MoS 2 (0001). The Pd clusters were epitaxially grown under ultrahigh vacuum conditions by condensing a calibrated beam of atoms on a natural single crystal of MoS 2 kept at high temperature ( T =220°C). Atomic-resolution STM images of the clean MoS 2 (0001) surface were found to vary with the tip to sample distance, in good agreement with the theoretical predictions of Altibelli et al. [Surf. Sci. 367 (1996) 209] for the origin of contrast in such images. TEM and STM investigations of the statistical properties of the Pd clusters (spatial and size distributions, epitaxial orientation) are shown to be coherent and complementary.

STM studies : spatial resolution limits to fit observations in nanotechnology

Atomically resolved Scanning Tunneling Microscopy (STM) images are nowadays currently obtained on flat surfaces without any special STM tip preparation. On the contrary, imaging of three-dimensional nanometer-scale objects exhibiting very much inclined facets, requires the use of STM tips with an appropriate geometry. The tip shape has to be controlled over the whole length that corresponds to the height of the object to be imaged. The cone angle of the tip has to be smaller than the tilt angle of the imaged facets. These criteria are also required when STM tips are used for patterning of nanometer-scale dots. In this paper, we will present the spatial resolution limits determined for these tips on different calibration gratings as well as first results obtained on nanometer scale patterning.

Experimental investigation of resonance curves in dynamic force microscopy

A precise experimental investigation of the amplitude and phase resonance curves of a driven dynamic force microscope (DFM) cantilever interacting with an Al2O3(0001) surface in ultra-high vacuum is reported. The large amplitude (a few tens of nanometres), high cantilever stiffness (25 N m−1) and high quality factor (a few 104) characterizing these experiments are typical of the frequency modulation (FM) mode of DFM. The whole range of tip–substrate distances where a stationary oscillation of the cantilever can be maintained is explored. It covers two different regimes: a large distance regime where only long range conservative van der Waals interactions contribute and a small distance regime where short range interactions play a significant role. A comparison is made with frequency shift and excitation amplitude curves as a function of the distance acquired in the FM mode. It is also shown that approach–retract amplitude and phase curves usually obtained in the amplitude modulation mode can be extracted from these data. These experimental results are compared with analytical predictions reported in the literature. An excellent agreement is found in the van der Waals domain, allowing us to evaluate the Hamaker constant for the alumina–vacuum–silicon system.

An experimental investigation of resonance curves on metallic surfaces in dynamic force microscopy: the influence of frozen versus mobile charges

Amplitude resonance curves of a driven dynamic force microscope cantilever interacting with a Cu(100) substrate in ultra high vacuum are derived and analysed, extending a previous study on Al2O3(0001) (Polesel-Maris et al 2003 Nanotechnology 14 1036). It is shown that the charges that are trapped on the oxidized n+-doped silicon tip give rise to long-range electrostatic forces that dominate the van der Waals forces, due to the metallic nature of the substrate. These electrostatic forces cannot be compensated by the usual procedure consisting of applying a constant bias voltage between the tip and the sample. Indeed, the trapped charge does not remain constant on the timescale of the experiment due to charge leakage across the tip oxide layer. Removal of this oxide by electron field emission solves this problem and allows the access of a pure van der Waals tip–substrate interaction regime.

The DUF Project: A UHV Factory for Multi-Interconnection of a Molecule Logic Gates on Insulating Substrate

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.