Hervé Dallaporta

Growth of silicon oxide on hydrogenated silicon during lithography with an atomic force microscope

We present an experimental study of growth of silicon oxide strips drawn on hydrogenated silicon under the voltage biased tip of an atomic force microscope operating in ambient atmosphere. Oxide formation was found to occur at negative tip biases above a voltage threshold around |−2|V, corresponding to the minimum electric field required for hydrogen removal from the substrate surface. We show the influence of tip-sample distance and of the chemical composition of the atmosphere on the growth. An ozone enriched atmosphere leads to a growth kinetics enhancement.

Plasmonic resonances in diffractive arrays of gold nanoantennas: near and far field effects

We examine the excitation of plasmonic resonances in arrays of periodically arranged gold nanoparticles placed in a uniform refractive index environment. Under a proper periodicity of the nanoparticle lattice, such nanoantenna arrays are known to exhibit narrow resonances with asymmetric Fano-type spectral line shape in transmission and reflection spectra having much better resonance quality compared to the single nanoparticle case. Using numerical simulations, we first identify two distinct regimes of lattice response, associated with two-characteristic states of the spectra: Rayleigh anomaly and lattice plasmon mode. The evolution of the electric field pattern is rigorously studied for these two states revealing different configurations of optical forces: the first regime is characterized by the concentration of electric field between the nanoparticles, yielding to almost complete transparency of the array, whereas the second regime is characterized by the concentration of electric field on the nanoparticles and a strong plasmon-related absorption/scattering. We present electric field distributions for different spectral positions of Rayleigh anomaly with respect to the single nanoparticle resonance and optimize lattice parameters in order to maximize the enhancement of electric field on the nanoparticles. Finally, by employing collective plasmon excitations, we explore possibilities for electric field enhancement in the region between the nanoparticles. The presented results are of importance for the field enhanced spectroscopy as well as for plasmonic bio and chemical sensing.

Variable structure control of a piezoelectric actuator for a scanning tunneling microscope

Scanning probe microscopes are now widely used in the field of material science and engineering for surface imaging at atomic scale. Their principle is based on the surface probing by a sharp tip approached at a nanometric distance of the surface. The probe is fixed to piezoelectric actuators allowing its displacement above the surface. An electronic command of a scanning tunneling microscope (STM) has been designed and tested. The regulation feedback loop of the tunnel current includes an integral controller, as is the case in commercial equipment. An extra control by variable-structure system has been implemented on this electronic command. Its principle is based on the commutation of the feedback sign. The effect on the system performance of the variable structure control is presented and discussed. An STM head has been modeled and all the model parameters have been determined. The model has been validated by comparison of the experimental and simulated responses of the system under excitation.

Narrow plasmon resonances in diffractive arrays of gold nanoparticles in asymmetric environment: Experimental studies

We examine experimentally the excitation of plasmon resonances in ordered two-dimensional diffractive arrays of gold nanoparticles placed in asymmetric refractive index environment. The excitation of the plasmon modes with narrow spectral profile in asymmetric environment was experimentally verified. The ability to tune the wavelength of these resonances in the near infrared range by varying the structural parameters of the periodic arrays in combination with size and geometry of the constituent nanoparticles is discussed.

Electronic transport properties of single-crystal silicon nanowires fabricated using an atomic force microscope

We present electrical characterization of silicon nanowires made from ultrathin silicon-on-insulator (SOI) using a lithography process based on an atomic force microscope (AFM). SOI wafers were first thinned, prepatterned and doped using conventional microelectronics processes in order to elaborate contact leads and pads. Between contacts, the upper Si was further thinned down to 15nm and n-doped by arsenic implantation. The Si top layer is then locally patterned using local oxidation induced under the biased tip of the AFM. The active part of the device is finally obtained by silicon selective wet etching using the AFM-made oxide pattern as a mask. This technique was used to study electrical transport through silicon wires with sub-1000nm2 cross-section. The implementation of both side gates and backgate control allowed to test a full device which acts at room temperature as a field effect transistor. Current densities as high as 2×105A/cm2 can be switched off by lateral gate control. At low temperatures, aperiodic oscillations of the nanowire current are observed while the gate voltage is swept. This behavior is attributed to potential variations along the wire caused by random fluctuations of dopants.

Direct patterning of nanostructures by field-induced deposition from a scanning tunneling microscope tip

The process of local-field-induced deposition on a surface facing a scanning tunneling microscope (STM) tip has been investigated for several tip-sample systems. Applying negative voltage pulses, atoms can be transferred from the STM tip to the surface and, for example, platinum dots and lines have been drawn on gold or silicon samples by this technique. In this latter case, a discussion is proposed on growth mechanisms involved in field-induced deposition processes on the basis of growth kinetics studies. When positive voltage pulses are applied to a silicon sample placed in tunneling conditions with a STM tip, silicon nanofeatures are elaborated on the substrate surface by field-enhanced surface diffusion of silicon atoms.

Direct patterning of noble metal nanostructures with a scanning tunneling microscope

We demonstrate in this article the controlled deposition of noble metal dots and lines using local chemical vapor deposition in the tip–sample gap of a scanning tunneling microscope. 3 nm diam rhodium dots have been patterned by local decomposition of an inorganic precursor, which was synthesized on purpose. Deposition is obtained on gold surfaces by applying a series of negative voltage pulses on the sample exceeding a voltage threshold of 1.9 V. The influence of kinetics parameters (pulse voltage duration and number, as well as the effect of gas pressure) are presented. In a second step, the deposition process has been applied on hydrogenated silicon (100) surfaces. These samples were previously hydrogen passivated using two different wet etching operations, leading surface dangling bonds saturated by either mono- or di-hydride bonds. The difference in the deposition processes observed in both cases is discussed.

Label free femtomolar electrical detection of Fe(III) ions with a pyridinone modified lipid monolayer as the active sensing layer

An innovative MOS-type field effect transistor was developed for the electrical detection of ferric ions. The sensing assays clearly show a specific detection with a gate-source voltage shift of up to 200 mV and a wide linear detection range (5 × 10-14 to 5 × 10-5 M) associated with good stability, selectivity and reproducibility.

Nanometer scale patterning by scanning tunneling microscope assisted chemical vapour deposition

Single electron devices are of great interest for their possible replacement of transistors in memories. The key to the preparation of these components is the production of low capacitance dots, which requires a lithography step at nanometric scale. Direct patterning of metallic features at nanometric scale is possible by local decomposition of gaseous molecules under a scanning tunneling microscope (STM) tip, by application of a voltage of a few volts on the sample (STM assisted chemical vapour deposition). The gaseous molecules are dissociated by the high electric field (about 107 V/cm) within the tip–sample gap. Rhodium lines and dots have been deposited on gold or silicon surfaces by decomposition of [Rh(PF3)2Cl]2. The influence of the sample voltage was studied and the resolution limit of the technique was investigated.

High aspect ratio nano-oxidation of silicon with noncontact atomic force microscopy

We report the formation of high aspect ratio ∼0.3 (height/width) oxide features with noncontact mode atomic force microscopy assisted lithography. The process requires high humidity levels, series of short pulses 25 V, a tip oscillation amplitude ∼20 nm, and feedback “on.” We also show that the application of a voltage at magnitude higher than a certain limit damages the surface.