Development of a shear-force scanning near-field cathodoluminescence microscope for characterization of nanostructures' optical properties.

An original scanning near-field cathodoluminescence microscope for nanostructure characterization has been developed and successfully tested. By using a bimorph piezoelectric stack both as actuator and detector, the developed setup constitutes a real improvement compared to previously reported SEM-based solutions. The technique combines a scanning probe and a scanning electron microscope in order to simultaneously offer near-field cathodoluminescence and topographic images of the sample. Share-force topography and cathodoluminescence measurements on GaN, SiC and ZnO nanostructures using the developed setup are presented showing a nanometric resolution in both topography and cathodoluminescence images with increased sensitivity compared to classical luminescence techniques.

Is the exponent 3/2 justified in analysis of loading curve of pyramidal nanoindentations?

Kaupp and Naimi-Jamal (2010) claimed that the analysis of published loading curves reveals the exponent 3/2 to the depth for nanoindentations with sharp pyramidal or conical tips. To demonstrate this, they plotted the load vs. the penetration depth to the power 3/2. We show, through examples, the authors' assertion is not credible because the methodology used is misleading and it cannot be asserted that the exponent 3/2 has a universal validity that applies to all kinds of materials.

Conductive AFM microscopy study of the carrier transport and storage in Ge nanocrystals grown by dewetting.

A combined conductive atomic force microscope (C-AFM)/scanning electron microscope (SEM) has been used to study the electric transport and retention mechanisms through Ge nanocrystals (NCs). The NCs were formed by a two-step dewetting/nucleation process on a silicon oxide layer grown on n-doped 001 silicon substrate. Without preliminary e-beam irradiation, electric images are obtained only with bias voltages larger than 8 V. This is due to the barrier height introduced by the presence of the native oxide on NCs and of the oxide layer on which the NCs are grown. After acquisition of an e-beam-induced current image, electric images (e-beam off) can be easily obtained at low bias voltages because of the trap creation in the oxide layer. We show that the critical threshold voltage to detect a current through the NCs decreases with NCs size. The band diagram of the contact in the presence of a p-doped diamond coated tip shows that the conduction mechanism is dominated by holes. At last we show a good memory effect with charge/discharge in the NCs resulting in a long retention time.

Template assisted electrodeposition of germanium and silicon nanowires in an ionic liquid.

In this paper we report for the first time on the room temperature template synthesis of germanium and silicon nanowires by potentiostatic electrochemical deposition from the air- and water stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) containing GeCl(4) and SiCl(4) as a Ge and Si source, respectively. Commercially-available track-etched polycarbonate membranes (PC) with an average nominal pore diameter of 90-400 nm were used as templates. Ge and Si nanowires with an average diameter corresponding to the nanopores' diameter and lengths of a few micrometres were reproducibly obtained. Structural characterization of the nanowires was performed by EDX, TEM, HR-SEM and Raman spectroscopy. Despite the rough surface of the nanowires, governed mostly by the original shape of the nanopore's wall of the commercially-available PC membrane, preliminary structural characterizations demonstrate the promising prospective of this innovative elaboration process compared to constraining high vacuum and high temperature methods.

Growth of silicon nanowires of controlled diameters by electrodeposition in ionic liquid at room temperature.

Silicon nanowires were fabricated for the first time by electrochemical template synthesis at room temperature. This innovative, cheap, and simple process consists of electroreduction of Si ions using a nonaqueous solvent and insulating nanoporous membranes with average pore diameters from 400 to 15 nm which fix the nanowires diameters. Characterization techniques such as scanning and transmission electron microscopies, infrared absorption measurements, X-ray diffraction experiments, energy dispersive X-ray, and Raman spectrometries show that the as-deposited silicon nanowires are amorphous, composed of pure Si and homogeneous in sizes with average diameters and lengths well matching with the nanopores' diameters and the thicknesses of the membranes. Thanks to annealing treatments, it is possible to crystallize the Si nanowires, demonstrating the potentiality for this innovative electrochemical process to obtain a wide range of Si nanowires with well controlled diameters and lengths.

Electrodeposition and characterization of CdSe semiconducting nanowires.

In this paper, we present our work on the electrodeposited CdSe semiconducting nanowires. Using a low cost and low temperature approach by electrochemistry, CdSe nanowires were successfully grown using polycarbonate template. Depending on the host pore dimension of the substrate, wire diameter can be varied from 400 nm down to 30 nm and wire length from a few microns to tens microns. The as-deposited nanowires exhibit predominantly metastable zinc blende (ZB) structure but after the heat treatment they become wurtzite (W) structure. A combination of different characterization techniques, such as X-ray diffraction, SEM, TEM-HRTEM and EDXS, was used to investigate the growth morphology, crystalline structure and defects in the nanowires. The luminescent properties of CdSe nanowires have also been studied by means of photoluminescence.

High-resolution scanning near-field EBIC microscopy: application to the characterisation of a shallow ion implanted p+-n silicon junction.

High-resolution electron beam induced current (EBIC) analyses were carried out on a shallow ion implanted p+-n silicon junction in a scanning electron microscope (SEM) and a scanning probe microscope (SPM) hybrid system. With this scanning near-field EBIC microscope, a sample can be conventionally imaged by SEM, its local topography investigated by SPM and high-resolution EBIC image simultaneously obtained. It is shown that the EBIC imaging capabilities of this combined instrument allows the study of p-n junctions with a resolution of about 20 nm.

Application of nano-EBIC to the characterization of GaAs and InP homojunctions.

High resolution electron-beam-induced current (EBIC) analyses were carried out on InP and GaAs substrates with a home-made atomic force microscope (AFM) combined with a scanning electron microscope (SEM). With this scanning nano-EBIC microscope, a sample can be conventionally imaged by SEM, its local topography investigated by AFM and nano-EBIC image simultaneously obtained. In this study, we report the utilization of nano-EBIC microscopy for imaging and characterizing GaAs and InP homojunctions. I-V characteristic measurements allow understanding of the electrical behavior of the AFM tip-sample contact. The electron probe intensity must be larger than about 100 pA to be able to generate an induced current because of the surface states which act as non-radiative recombination centers. The minority carrier diffusion length of InP and GaAs is measured and compared for different electron probe currents and it is shown that the measurements are not perturbed by photon recycling, i.e. the self-absorption of photons that gives rise to an extra generation of electron-hole pairs.

Influence of electron irradiation on the electronic transport mechanisms during the conductive AFM imaging of InAs/GaAs quantum dots capped with a thin GaAs layer.

We have used conductive atomic force microscopy (C-AFM) to study the electronic transport mechanisms through InAs quantum dots (QDs) grown by molecular beam epitaxy on an n-type GaAs(001) substrate and covered with a 5 nm thick GaAs cap layer. The study is performed with a conductive atomic force microscope working inside a scanning electron microscope. Electric images can be obtained only if the sample is preliminarily irradiated with an electron probe current sufficiently high to generate strong electron beam induced current. In these conditions holes are trapped in QDs and surface states, so allowing the release of the Fermi level pinning and thus conduction through the sample. The electronic transport mechanism depends on the type of AFM probe used; it is explained for a metal (Co/Cr) coated probe and p-doped diamond coated probe with the aid of energy band diagrams. The writing (charge trapping) and erasing (untrapping) phenomena is conditioned by the magnitude of the electron probe current. A strong memory effect is evidenced for the sample studied.

Development of a shear force scanning near-field fluorescence microscope for biological applications.

In this paper, a shear force scanning near-field fluorescence microscope combined with a confocal laser microspectrofluorometer is described. The shear force detection is realized based on a bimorph cantilever, which provides a very sensitive, reliable, and easy to use method to control the probe-sample distance during scanning. With the system, high-quality shear force imaging of various samples has been carried out. Furthermore, simultaneous shear force and near-field fluorescence imaging of biological cells has also been realized. As an example, we especially present the result on the distribution of P-glycoprotein in the plasma membrane of human small cell lung cancer cells, suggesting that the system would be a promising tool for biological applications.

Shear force near-field optical microscope based on Q-controlled bimorph sensor for biological imaging in liquid.

Shear force near-field microscopy on biological samples in their physiological environment loses considerable sensitivity and resolution as a result of liquid viscous damping. Using a bimorph-based cantilever sensor incorporating force feedback, as recently developed by us, gives an alternative force detection scheme for biological imaging in liquid. The dynamics and sensitivity of this sensor were theoretically and experimentally discussed. Driving the bimorph cantilever close to its resonance frequency with appropriate force feedback allows us to obtain a quality factor (Q-factor) of up to 10(3) in water, without changing its intrinsic resonance frequency and spring constant. Thus, the force detection sensitivity is improved. Shear force imaging on mouse brain sections and human skin tissues in liquid with an enhanced Q-factor of 410 have shown a high sensitivity and stability. A resolution of about 50 nm has been obtained. The experimental results suggest that the system is reliable and particularly suitable for biological cell imaging in a liquid environment.

Non-optical bimorph-based tapping-mode force sensing method for scanning near-field optical microscopy.

A non-optical bimorph-based tapping-mode force sensing method for tip-sample distance control in scanning near-field optical microscopy is developed. Tapping-mode force sensing is accomplished by use of a suitable piezoelectric bimorph cantilever, attaching an optical fibre tip to the extremity of the cantilever free end and fixing the guiding portion of the fibre to a stationary part near the tip to decouple it from the cantilever. This method is mainly characterized by the use of a bimorph, which carries out simultaneous excitation and detection of mechanical vibration at its resonance frequency owing to piezoelectric and anti-piezoelectric effects, resulting in simplicity, compactness, ease of implementation and lack of parasitic optical background. In conjugation with a commercially available SPM controller, tapping-mode images of various samples, such as gratings, human breast adenocarcinoma cells, red blood cells and a close-packed layer of 220-nm polystyrene spheres, have been obtained. Furthermore, topographic and near-field optical images of a layer of polystyrene spheres have also been taken simultaneously. The results suggest that the tapping-mode set-up described here is reliable and sensitive, and shows promise for biological applications.

Cathodoluminescence imaging and spectroscopy by near-field detection

Using near-field techniques, we have developed an experimental set-up for spatially resolved cathodoluminescence (CL) spectroscopy and monochromatic imaging. It combines a scanning near-field optical/force microscope with a scanning electron microscope equipped with a field emission gun. The potentialities of this scanning near-field cathodoluminescence microscope are demonstrated on two kinds of sample: an indented MgO crystal and AlGaN/GaN quantum wells grown on GaN/sapphire. Monochromatic CL imaging allows a clear distinction between the emission of quantum wells and the GaN substrate, and for the MgO crystal, the localization on the slip bands, near the indentation, of luminescent centres emitting at 450 nm.

General Equations Describing Elastic Indentation Depth and Normal Contact Stiffness versus Load.

Continuum mechanics models describing the contact between two adhesive elastic spheres, such as the JKR and DMT models, provide a relationship between the elastic indentation depth and the normal load, but the general intermediate case between these two limiting cases requires a more complex analysis. The Maugis-Dugdale theory gives analytical solutions, but they are difficult to use when comparing to experimental data such as those obtained by scanning force microscopy. In this paper we propose a generalized equation between elastic indentation depth and load that approximates Maugis' solution very closely. If the normal contact stiffness can be described as the force gradient, that is the case of the force modulation microcopy, then a generalized equation between normal contact stiffness and load can be deduced. Both general equations can be easily fit to experimental data, and then interfacial energy and elastic modulus of the contact can be determined if the radius of the indenting sphere is known. Copyright 2000 Academic Press.

Magnetic field emission gun with zirconiated emitter.

A magnetic-field-superimposed field emission gun with low aberrations and equipped with a zirconiated tungsten emitter has been developed for applications where very stable high probe currents are required. It has been tested on a conventional electron microscope at 10 kV and on an electron beam testing system at 1 kV. Probe current i = 250 nA in a probe size d = 0.4 micron is obtained at 10 kV; at 1 kV the resolution is 0.1 micron with i = 5 nA, and 0.4 micron with i = 30 nA. For these probe currents, the spatial broadening effect due to electron-electron interactions in the beam is the preponderant factor limiting the probe size.