Mihai N. Mihaila

Surface acoustic wave CO 2 sensing with polymer-amino carbon nanotube composites

The synthesis of two new types of nanocomposite matrices, the first based on polyallylamine (PAA) and aminocarbon nanotubes, the second on polyethyleneimine (PEI) and aminocarbon nanotubes, are reported. The surface acoustic wave (SAW) sensors, coated with the two selected nanocomposites, showed good sensitivities when varying the CO2 concentrations in the range (500-5000) ppm. The sensor sensitivity is larger when using polyethyleneimine aminocarbon nanotubes than in the case when only a pure polyethyleneimine layer is considered for coating.

Nano-scale resonant sensors for gas and bio detection: Expectations and challenges

It is the purpose of this paper to show expectations, challenges and initial steps concerning the realization of resonant chemical NEMS sensors able to meet the needs of future applications. Here, we focus on the functionalization principles of the sensing Self-Assembled Monolayer (SAM), modeling and simulation of CMOS-SOI resonant NEMS sensor with electrostatic actuation and MOSFET detection, first CMOS-SOI experimental results for making Si nano wire for piezoresistive detection schemes, noise limits of the resonant nano-sensors, challenges for the design of the on-chip readout circuitry, and the specific reliability issues of resonant NEMS. Some of the simulated sensitivity results of about 5 Hz/zg at 433 MHz and MOSFET detection are close to the best state- of- art experimental data from literature of 0.7 Hz/zg at 127 MHz. It is our challenge to pursuit at experimental level with our nanosensor concepts for making reliable nanodevices addressing the needs of integrated sensing.

SiO2/4H-SiC interface states reduction by POCl3 post-oxidation annealing

Interface state density (D_it) at SiO₂/4H-SiC interfaces is investigated using capacitance-voltage (C-V) characterization. Two different measurement methods for D_it determination (both C-V at different temperatures in the range of 80-300K and high frequency (HF) vs quasi static (QS) characteristics) have been used. A significant reduction of D_it is observed, almost one order of magnitude, after a post oxidation annealing (POA) in POCl₃ ambient, compared to as-grown dry oxidized sample.

New ligand selection rule for quantum dot functionalization

Hard Soft Acid Base (HSAB) theory is introduced as a new tool to select ligands (molecules) for quantum dot covalent functionalization. According to HSAB, only soft acid-soft base bond is covalent. Since most of the transition metal semiconductor cations on the surface of the quantum dots are soft acids, the approach we propose is to select anchors which are soft bases. Following this strategy, the suitable anchors for surface modification of the quantum dots are selected. Moreover, a plethora of bifunctional ligands for assembling quantum dots onto the surface of titania are identified among the existing molecules. The functionalization of a semiconducting organic polymer backbone with HSAB-adequate anchors for the design of polymer-quantum dot hybrid interface is presented. In addition, our new approach coherently explains the heuristic approaches described in the literature.

A new sensitizer containing dihexyloxy-substituted triphenylamine as donor and a binary conjugated spacer for dye-sensitized solar cells

The synthesis and application to dye-sensitized solar cells (DSSC) of a new dihexyloxy-substituted triphenylamine-based organic dye is reported. The dye design, based on the push–pull concept, consists of dihexyloxy-substituted triphenylamine as an electron donor and a cyanoacrylic acid as an anchoring group and electron acceptor connected through a π-conjugated bridge. The electron spacer containing furan and thiophene moieties was employed in the dye sensitizer for expansion of the π-conjugated fragment to adjust the absorption spectra and HOMO–LUMO levels of the dye. The relationship between the dye chemical structure and the photophysical, electrochemical, and photovoltaic properties determined by spectral, electrochemical, photovoltaic experiments, and density functional theory calculations was thoroughly investigated. The photovoltaic performance of the dye as sensitizer was assessed in DSSC cells realized using electrolytes containing the iodide/triiodide redox couple. The cells obtained with the new dye show a power conversion efficiency of 5.14%, without using any coadsorbant and optimization.

Low-frequency noise in nanomaterials and nanodevices

The status of low frequency noise in nanodevices and nanomaterials is briefly surveyed. The very old yet hot problematics of the 1/f noise is discussed in the light of some new achievements in the nanoscience. It is shown that the very old question regarding the topology of the noise sources (surface or bulk origin) of this phenomenon is clarified, while the microscopic origin of the phenomenon cannot be unambiguously answered. Although present even in the quantum dots, the random telegraph signal (RTS) noise as a fundamental component of the 1/f spectrum seems to be a non sequitur. McWhorter model cannot explain the occurrence of the 1/f noise in carbon nanotubes. Surface anharmonicity is proposed as a possible ingredient in the generation of the very high noise level observed in carbon nanotubes, while the increased phonon density of states in nanomaterials seems to be a source of higher noise levels. Low frequency noise appears as a powerful tool to characterize nanomaterials and nanodevices.

Atomic vibration-induced 1/ƒ noise in sensing nanomaterials

Results demonstrating that the temperature dependence of the 1/f noise intensity in a gold nanoparticle-chemosensor and metallic nanowires is the image of the atomic vibration (phonon) spectrum of the metals they were made of are presented. To this purpose, existing noise data for gold nanoparticles (C. Kurdak et al., Appl. Phys. Lett. 86, 073506, 2005 [1]) and silver and copper nanowires (A. Bid, A. Bora and A. K. Raychaudhuri, Phys. Rev. B 72, 113415, 2005 [2]) are compared with the phonon density of states or phonon dispersion curves of gold, silver and copper, respectively, as determined by neutron/(helium atoms) inelastic scattering (HAS) or high resolution electron energy loss spectroscopy (EELS). It is shown that the noise structure in gold nanoparticles is the image of the gold bulk phonon density of states. In silver nanowires, a noise maximum at 220K is in excellent correspondence with the bulk longitudinal phonon density of states of silver. Other weaker noise peaks are correlated with the surface phonons, including Rayleigh modes. In the case of the copper nanowires, the highest noise peak at 260K is correlated with a transversal bulk phonon, while other peaks are in good agreement with the energy of some Rayleigh modes and surface resonances, at different points of symmetry of the copper surface Brillouin zone. These observations strongly support surface and bulk atomic thermal motion as fundamental sources of 1/f noise in nanoparticles and nanowires, with direct consequences on the physics, design and technology of nanosensors and other nanoelectronic devices.

Nonequilibrium 1/f noise in platinum nanoparticle films

1/f noise spectra have been observed in platinum nanoparticle films deposited on SiO/sub 2//Si substrates by laser ablation. Although all noise measurements were done in the linear region of the I-V characteristics, both equilibrium (ohmic) and nonequilibrium (nonohmic) behavior was observed in the dependence of the noise intensity on voltage (V). The nonequilibrium 1/f noise was found in some samples of smaller conductivity (small coverage). The sample with the smallest noise intensity features strong sublinear (V/sup 0.2-0.37/) nonequilibrium 1/f noise for voltages V<0.5V, while for V>0.5V the noise intensity is superlinear (V3.2). In the (20-300)K temperature range, the film resistance follows a T/sup -1/4/ dependence, specific to Motts phonon-assisted variable range hopping conduction.

Nonlinear conduction in platinum nanoparticle films

Platinum (deca)nanoparticles with a size between (25-150) nm have been deposited on a SiO/sub 2//Si substrate by laser ablation. Roughly estimated, the particle distribution deviates from lognormal distribution at larger particle size. Electrical measurements on nanoparticle films with different surface coverage revealed that the films start conducting at a given percolation threshold. As expected, ohmic I-V characteristics were found in all investigated structures. However, below a voltage threshold, a transition from linear to pronounced nonlinear behaviour was observed in the samples with a higher surface coverage. A tunneling mechanism between the metal islands partially embedded into the substrate is proposed as a source of nonlinearity. This indicates an interaction between the nanoparticle film and the substrate, a conjecture strongly supported by the observation of some Pt/sub 3/Si lines in the X-ray diffractogram. Other possible causes of nonlinear conduction mechanisms are heuristically discussed.