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In this paper we will report about the development of an innovative way to self-assemble aromatic molecules as monolayers on a Si(100) surface through the formation of a direct Si-C bond, these systems acting as chemical sensors for gases. Specifically, we will present an investigation on how donor/acceptor groups substituted on the benzene ring tailor the gas-surface interaction. The new sensors were obtained by wet chemistry nucleophilic attack onto halogenated silicon surfaces. Infrared multiple internal reflection (MTR) experiments and high-resolution electron energy loss spectroscopy analyses confirmed the formation of Si-C bonds. Gases were detected by measuring the electrical conductivity changes at 308 K in the presence of trace amounts of oxidants (SOx, CO and NOx, 0.5-5.0 ppm) in Ar. An analysis of the sensor response showed that gas-ring interactions actually modulate the pi -system of the aromatic ring. A correlation between gas composition and surface conductance was found. In order to provide a consistent interpretation of these resistivity changes, computer simulations have been carried out. We successfully modeled the supramolecular interactions between the organic fragments and the gas, finding that this interaction unbalances the charge distribution in the aromatic fragment, creating in turn a depletion/accumulation layer at the Si surface which can be held responsible for the conductivity modulation experimentally observed

The relaxations of the first three interlayer distances in the H-covered Si(111)(1 x 1) surface were calculated using a fully periodic Hartree-Fock approach and a finely tuned slab model. All computed relaxations fall well within the error bounds of the experiment, provided the relevant geometrical parameters and the basis set of the first layer Si atoms (Si1) are both optimized. The quantum theory of atoms in molecules is applied on the wave functions of Si bulk and of H-covered or clean Si(111)(1 x 1) slabs so as to shed light on how the electronic perturbation caused by H adsorption and surface formation propagates and dampens through the first Si atoms layers. In the H-covered surface, the large charge transfer from Si1 to H induces a noticeable asymmetry in and strengthening of the surface Si1-Si2 back bonds, whereas in the clean slab the same bonds are found to be weakened compared to the bulk in agreement with the well-known tendency of this system to evolve in favor of other reconstructions. The negatively charged hydrogen layer in the Si(111)(1 x 1)-H slab is almost entirely counterbalanced by the first two silicon layers with the Si1 atoms bearing more than 94 percent of the compensating positive charge. The hydrogen and Si1 atoms in the H-covered surface polarize in such a way as to oppose the electric field created by charge transfer into the surface double layer. The effect of H-coverage is to reverse the outwards polarization of Si1 atoms present in the clean system and to enhance its magnitude. Due to the surface electric field, the atomic energies in both slabs are not found to converge towards bulk values even for the atoms of the innermost layers, although the other calculated local and integrated properties exhibit an almost perfect convergence beyond the first two or three atomic layers. In the H-covered slab, the Si1 atoms have their interatomic surface completely isolated from the outside through their interaction with H atoms, while Si2 are fo

Transmission electron microscopy analysis of tin dioxide films grown by aerosol-assisted chemical vapor deposition onto oxidized or etched silicon displayed the formation of a sub-oxide phase that was identified as Sn2O3. Such a phase is observed to disappear upon heat treatment, and is believed to be one of the factors responsible for the instability of tin dioxide films used as gas sensing layers. © 1996 American Institute of Physics.

Using two different hydrosilylation methods, low temperature thermal and UV initiation, silicon (111) hydrogenated surfaces were functionalized in presence of an OH-terminated alkyne, a CF 3 -terminated alkyne and a mixed equimolar ratio of the two alkynes. XPS studies revealed that in the absence of premeditated surface radical through low temperature hydrosilylation, the surface grafting proceeded to form a Si-O-C linkage via nucleophilic reaction through the OH group of the alkyne. This led to a small increase in surface roughness as well as an increase in hydrophobicity and this effect was attributed to the surficial etching of silicon to form nanosize pores (∼1-3 nm) by residual water/oxygen as a result of changes to surface polarity from the grafting. Furthermore in the radical-free thermal environment, a mix in equimolar of these two short alkynes can achieve a high contact angle of ∼102°, comparable to long alkyl chains grafting reported in literature although surface roughness was relatively mild (rms = ∼1 nm). On the other hand, UV initiation on silicon totally reversed the chemical linkages to predominantly Si-C without further compromising the surface roughness, highlighting the importance of surface radicals determining the reactivity of the silicon surface to the selected alkynes.

Thermal matching of thermoelectric generators (TEGs) to their heat sinks is well known to be a critical issue in the development of efficient harvesters. It is slightly less obvious that the development of suitable novel materials would possibly take advantage of their contextualization to prospective scenarios of deployment. In this communication a novel unified analysis of the heat equation is proposed. The thermal system was embedded into a thermostat while the heat source was taken as a part of the system itself, also explicitly accounting for the heat dissipation. A continual transition of thermal matching conditions from those predicted under Dirichlet boundary conditions (BCs) to those obtained under Neumann BCs was found, depending on the strength of the heat source and on the thermal resistances of the circuit branches. In all cases it was found that, independently of the thermal or thermoelectric circuit geometry, no single material may provide optimal TEG power density for any heat source strength and branch resistance.

In this letter, we report results of a hydrosilylation carried out on bifunctional molecules by using two different approaches, namely through thermal treatment and photochemical treatment through UV irradiation. Previously, our group also demonstrated that in a mixed alkyne/alcohol solution, surface coupling is biased towards the formation of Si-O-C linkages instead of Si-C linkages, thus indirectly supporting the kinetic model of hydrogen abstraction from the Si-H surface (Khung, Y. L. et al. Chem. - Eur. J. 2014, 20, 15151-15158). To further examine the probability of this kinetic model we compare the results from reactions with bifunctional alkynes carried out under thermal treatment (<130°C) and under UV irradiation, respectively. X-ray photoelectron spectroscopy and contact angle measurements showed that under thermal conditions, the Si-H surface predominately reacts to form Si-O-C bonds from ethynylbenzyl alcohol solution while the UV photochemical route ensures that the alcohol-based alkyne may also form Si-C bonds, thus producing a monolayer of mixed linkages. The results suggested the importance of surface radicals as well as the type of terminal group as being essential towards directing the nature of surface linkage.

In the past decade, zwitterionic-based anti-biofouling layers had gained much focus as a serious alternative to traditional polyhydrophilic films such as PEG. In the area of assembling silica nanoparticles with stealth properties, the incorporation of zwitterionic surface film remains fairly new but considering that silica nanoparticles had been widely demonstrated as useful biointerfacing nanodevice, zwitterionic film grafting on silica nanoparticle holds much potential in the future. This review will discuss on the conceivable functional chemistry approaches, some of which are potentially suitable for the assembly of such stealth systems.

A possible approach to raise the efficiency of single-junction solar cells is to couple them with thermoelectric generators (TEGs). It was shown that TEG contribution to the output power is basically ruled by the characteristics of the photovoltaic (PV) material. In this study, we present a quantitative model that correlates the efficiency of the hybrid thermoelectric-photovoltaic (HTEPV) device with the energy gap and the working temperature of the solar cell. Two HTEPV structures are discussed, one capable only to recover the heat released by relaxation of hot electron-hole pairs; and a second one also capturing the low-energy part of the solar spectrum. We show that in the second case the increase of the conversion efficiency could justify the effort needed to add a TEG stage to the PV device. HTEPV constructions are also shown to enable the use of wide-gap materials that are not currently considered in PV applications.

Exploitation of solar energy conversion has become a fundamental aspect of satisfying a growing demand for energy. Thus, improvement of the efficiency of conversion in photovoltaic (PV) devices is highly desirable to further promote this source. Because it is well known that the most relevant efficiency constraint, especially for single-junction solar cells, is unused heat within the device, hybrid thermo-photovoltaic systems seem promising. Among several hybrid solutions proposed in the literature, coupling of thermoelectric and PV devices seems one of the most interesting. Taking full advantage of this technology requires proper definition and analysis of the thermal losses occurring in PV cells. In this communication we propose a novel analysis of such losses, decoupling source-dependent and absorber-dependent losses. This analysis enables an evaluation of the actual recoverable amount of energy, depending on the absorber used in the PV cell. It shows that for incoming solar irradiation of 1000 W/m2, and depending on the choice of material, the maximum available thermal power ranges from 380 W/m2 (for single-crystal silicon) to 130 W/m2 (for amorphous silicon).

Second-phase precipitation in nanocrystalline silicon may lead to improved thermoelectric power factors (up to ≈15 mW K-2 m-1 at room temperature) and high figures of merit. In this review, energy filtering is shown to provide an explanatory framework for these findings, helping predict strategies to further improve nanocrystalline silicon efficiency in thermal harvesters.

We have modeled the thermoelectric power factor enhancement by energy barriers in a one-dimensional composite semiconductor using the Boltzmann transport equation within the relaxation time approximation. The effective transport coefficients of the composite structure are calculated assuming two-phases connected in series: (i) the grain and (ii) the barrier. For the grains, bulk properties are assumed. For the barriers, the transport coefficients are calculated within Boltzmann transport equationand assuming thermionic emission over an energy barrier. The power factor enhancement is calculated as function of the relevant parameters of the material. It is found that power factor enhancement is possible in the presence of energy barriers. The role of the non-uniformity in the thermal conductivity is considered. Moreover, the occurrence of simultaneous increase of the conductivity and Seebeck with carrier density and its effect on the power factor enhancement is discussed.

We have investigated the behavior of one B impurity in the presence of dislocations, nanovoids and grain boundaries (GBs) in Si. Our study has been motivated by recent experiments on highly B-doped polycrystalline Si where a very significant thermoelectric power factor enhancement was observed. We study the influence of nanovoids on B diffusivity and activation in Si. We compare with the grain-boundary of pure tilt Si[001](120) poly-Si, where the presence of dislocations is dominant, similarly as in the case of nanovoids in c-Si. We performed DFT calculations to investigate minimum energy configurations, energy barriers and activation or deactivation of one B impurity in the proximity of the above structures. We found that, B preferably occupies substitutional lattice sites at the edge of the cavities. Thus the presence of small voids reduces its diffusivity and mobility, but the electrical activation of B is not significantly influenced. In the case of Si[001](120) GB, B is found to form Boron interstitials clusters in the close vicinity of a GB. The formation energy of those clusters is lower when the cluster is closer to center of the GB. Our results are in agreement with previously reported experimental data. It is provided insight to the effect of structural inhomogeneity on the dopant properties in Si structures that are currently of interest for thermoelectric applications.

The thermoelectric properties of doped polycrystalline silicon nanowires have been investigated using doping techniques that impact grain growth in different ways during the doping process. In particular, As- and P-doped nanowires were fabricated using a process flow which enables the manufacturing of surface micromachined nanowires contacted by Al/Si pads in a four-terminal configuration for thermal conductivity measurement. Also, dedicated structures for the measurement of the Seebeck coefficient and electrical resistivity were prepared. In this way, the thermoelectric figure of merit of the nanowires could be evaluated. The As-doped nanowires were heavily doped by thermal doping from spin-on-dopant sources, whereas predeposition from POCl3 was utilized for the P-doped nanowires. The thermal conductivity measured on the nanowires appeared to depend on the doping type. The P-doped nanowires showed, for comparable cross-sections, higher thermal conductivity values than As-doped nanowires, most probably because of their finer grain texture, resulting from the inhibition effect that such doping elements have on grain growth during high-temperature annealing.

An efficient use of solar energy as a renewable power source has been the subject of an enduring research effort, still challenging scientists and technologists worldwide. Beyond standard photovoltaic (PV) and thermodynamic solar power plants, the increase of conversion efficiency achieved in thermoelectric generators (TEGs) has opened a new, lively research front aimed at the direct conversion of solar power into electricity. Although the efficiency of thermoelectric (TE) materials cannot rival that of PV materials yet, TEGs allow concentrating solar power without requiring bulky and expensive optical devices. Also, TEGs may be deployed in harsh environments (e.g. deserts) where maintenance of PV cells is often unaffordable. However, it is realistic to consider today TEGs as tools to improve, not to replace, PV generators. PV efficiency losses will be analyzed in details by decoupling source- and absorber-dependent losses, showing that the implementation of a tandem PV-TE device might lead to a substantial increase of available power densities. Specifically, it will be shown that the proper choice of the TEG geometry and layout remarkably depends on the PV material. In addition, since the efficiency of the tandem device depends on the energy gap and then on the working temperature of its PV stage, a major increase of the conversion efficiency can be achieved by adding an intermediate layer between the PV an the TE stage capable of absorbing the under-the-gap fraction of the solar spectrum. This increase of efficiency will be discussed and commented upon also in view of the constraints they impose to the TEG layout and to its material characteristics. The critical importance of effective heat dissipation will also be addressed. The conclusion will be reached that not only could tandem PV-TE cells improve the conversion rate of existing solar cells but also that they could enable the use of lower cost PV materials, currently not considered because of their marginal P

The coordination of Zn2+ and Cu2+ ions in semiconducting phosphate glasses was investigated by EXAFS spectroscopy. A change in Zn coordination occurs when the glass composition deviates from metaphosphate. The results are consistent with the hypothesis that Cu(II) and Cu(I) occupy sites with different coordination geometries, but with entire coordination polyhedra similar for all copper atoms. The consistency of the structural model with the semiconducting properties of the glasses is examined. © 1991.

Boron diffusivity in single-crystal diamond has been investigated. To this aim, a novel method using impedance spectroscopy for the study of the atomic diffusivity in wide-gap semiconductor has been developed, along with a model for the analysis of the dielectric response function. The advantages of this procedure are discussed. Boron diffusivity in diamond has been determined to be 6.9×10-20 cm2 s-1 at 800°C. A discussion of the results and a comparison with previous estimates are presented.

Helium implantation in single crystal silicon is known to lead, after a proper thermal treatment, to the formation of voids with diameters ranging between 10 nm and 30 nm. Formation of voids is governed by the coalescence of vacancies created by implantation, initially trapping helium atoms. At high temperatures ((Formula presented.)), helium leaves the nanobubbles and outdiffuses, while the now empty voids grow in size and eventually change their shape to form tetrakaidecahedra (Wulff construction). In this communication, we report how He+ implantation in heavily boron-doped nanocrystalline silicon shows a completely different dynamics. Annealing at (Formula presented.) leads to the formation of large voids, located around grain boundaries, along with a large number of nanovoids with an average diameter of 2-4 nm and an estimated density of (Formula presented.) distributed throughout the grains. Annealing at higher temperature (up to (Formula presented.)) also induces a decrease of the void size with a change in their density, finally accounting to (Formula presented.). The high temperature annealing also causes vacancy evaporation down to a depth of 80-100 nm from the outer surface. The possibility of obtaining a stable, uniform distribution of nanometer-sized voids is of major relevance as a novel tool for phonon and electron engineering in thermoelectric materials. © 2014 TMS.

In a stringent and near oxygen-free environment, Si-H surfaces were introduced to a trifluoroalkyne, an alcohol-derivatized alkyne, as well as an equal mixture of both alkynes at a temperature of 130°C. Contact angle measurements, high-resolution X-ray photoelectron spectroscopy (XPS), and angle-resolved XPS were performed to examine the system. Si-H surfaces were found to have a strong preference towards the formation of Si-O-C rather than Si-C bonds when the alcohol and alkyne reactivities were compared.

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mW K−2 m−1 (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He+ ion implantation. In contrast with single-crystalline silicon in which He+ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000°C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 1019 cm−3 with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiB x . We recently reported that PFs up to 15 mW K−2 m−1 could be achieved for silicon–boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiB x precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF.

In this work, we describe a novel idea that allows for high thermoelectric power factors in two-phase materials that are heavily doped with an inhomogeneous distribution of dopants. We show that a concurrent increase of the electrical conductivity and Seebeck coefficient and a consequent increase of the power factor can be achieved in such systems. To explain the concept, we employ a semiclassical one-dimensional model that considers both electron and phonon transport through a series connection of two-phases of the material. We discuss microscopic characteristics of the material and the formation of the two phases (grains and grain boundaries in our case) by the inhomogeneous distribution of dopants in the polycrystalline material. Our theoretical investigation reveals that: (1) the improvement in the Seebeck coefficient can be attributed to carrier filtering due to the energy barriers at the grain boundaries, and to the difference in the lattice thermal conductivity of the grains and grain boundaries, and (2) the improvement in the electrical conductivity is a result of a high Fermi level in the grains. This allows high energy carriers to contribute to transport, which increases the impurity scattering limited mean-free-path, and increases the conductivity in the grains and thus in the whole material. Such an unexpected concurrent in- crease of the electrical conductivity and the Seebeck coefficient was recently observed in heavily boron-doped polycrystalline silicon of grain sizes<100 nm in which a silicon-boride phase is formed around the grain boundaries. We provide a simple 1D model that explains the behavior of this system, indicating processes that can take place in heavily doped nanocrystalline materials.

Since the introduction by Gold et al. in 1990, nucleic acid aptamers had evolved to become a true contender in biosensors for protein and cell detections. Aptamers are short strands of synthetically designed DNA or RNA oligonucleotides that can be self-assembled into unique 3-dimensional structures and can bind to different proteins, cells or even small molecules at a high level of specificity and affinity. In recent years, there had been many reports in literature in using aptamers in place of conventional antibodies as capture biomolecules on the surface. This is mainly due to the better thermal stability properties and ease in production. Consequently, also these characteristics allowed the aptamers to find use in field effect transistors (FETs) based upon 1D nanostructured (1D-NS) as label-free biosensing. In terms of designing label-free platforms for biosensors applications, 1D-NS FET had been an attractive option due to reported high sensitivities toward protein targets arising from the large surface area for detection as well as to their label-free nature. Since the first aptamer-based 1D-NS FET biosensor had surfaced in 2005, there had been many more improvements in the overall design and sensitivity in recent years. In this review, the latest developments in synergizing these two interesting areas of research (aptamers and 1D-NS FET) will be discussed for a range of different nanowire types as well as for the detection results.

A large thermoelectric power factor in heavily boron-doped p-type nanograined Si with grain sizes ∼30 nm and grain boundary regions of ∼2 nm is reported. The reported power factor is ∼5 times higher than in bulk Si. It originates from the surprising observation that for a specific range of carrier concentrations, the electrical conductivity and Seebeck coefficient increase simultaneously. The two essential ingredients for this observation are nanocrystallinity and extremely high boron doping levels. This experimental finding is interpreted within a theoretical model that considers both electron and phonon transport within the semiclassical Boltzmann approach. It is shown that transport takes place through two phases so that high conductivity is achieved in the grains, and high Seebeck coefficient by the grain boundaries. This together with the drastic reduction in the thermal conductivity due to boundary scattering could lead to a significant increase of the figure of merit ZT. This is one of the rare observations of a simultaneous increase in the electrical conductivity and Seebeck coefficient, resulting in enhanced thermoelectric power factor.

In this work we perform a theoretical analysis of the thermoelectric performance of polycrystalline Si nanowires (NWs) by considering both electron and phonon transport. The simulations are calibrated with experimental data from monocrystalline and polycrystalline structures. We show that heavily doped polycrystalline NW structures with grain size below 100 nm might offer an alternative approach to achieve simultaneous thermal conductivity reduction and power factor improvements through improvements in the Seebeck coefficient. We find that deviations from the homogeneity of the channel and/or reduction in the diameter may provide strong reduction in the thermal conductivity. Interestingly, our calculations show that the Seebeck coefficient and consequently the power factor can be improved significantly once the polycrystalline geometry is properly optimized, while avoiding strong reduction in the electrical conductivity. In such a way, ZT values even higher than the ones reported for monocrystalline Si NWs can be achieved

It is a common belief that in composites the thermoelectric figure of merit ZT is never larger than its value in the most efficient of its components. This criterion was demonstrated in 1991 in a classical paper by Bergman and Levy [1] based upon a variational analysis of two-component mixtures. The Bergman−Levy limit may be violated (and has been experimentally verified not to hold) in nanostructured systems, namely wherever interfaces play a significant role on the composite transport properties, but has always been considered an unsurpassable threshold in macroscopic systems. Very recently, however, Yang and coworkers [2] argued that heat fluxes in thermoelectric systems are not necessarily divergence-free, so that the use of Laplace equation made by Bergman and Levy may not be appropriate to compute the effective value of the transport properties of any composite. Specifically, Yang et al. showed that in a one-dimensional system ZT may actually exceed its value in both components – although the improvement is small and achievable only under an external bias. In any case and beyond the practical relevance of the reported result, the theoretical criticism to the Bergman−Levy limit appears founded, motivating an investigation of the possibility of designing composites with enhanced thermoelectric efficiency with respect to their components. In this communication analytical and numerical analyses concerning the thermoelectric properties of two-phase macrostructured composites will be reported. Preliminary results confirmed the claim of Yang and coworkers concerning the violation of the Bergman−Levy limit in one-dimensional systems. An extension of the analysis to periodically layered systems will be presented, allowing for conjectures on the criteria to be followed in order to maximize ZT in such a class of structures

An experimental investigation about the thermoelectric properties of heavily doped p ad n-type nanocrystalline silicon nanowires (NWs) is described. The NWs are produced with low cost CMOS compatible processes, highly customizable in terms of cross-section and placement, which enables the fabrication of both stacked NWs in nearly vertical arrays within nanostructured templates built with SiO 2 /Si 3 N 4 thin films and individual, freestanding NWs suited for thermal conductivity measurements. The cross-section dimensions of the investigated NWs range between 30 and 120 nm in size and up to about 2 cm in length. The structure of the NWs, as shown by SEM/TEM observations, is nanocrystalline with average size of the nanocrystals in one dimension that is comparable with the nanowire diameter. On the NWs, Seebeck coefficient, electrical resistivity and thermal conductivity have been measured, yielding thermoelectric figure of merit (ZT) values of 0.2 at 300 K for the best case.

Heavily boron-doped polycrystalline silicon has been reported to be characterized by somewhat unexpectedly high power factor. High Seebeck coefficients are however unexpected in materials with high carrier densities. A semiquantitative model was proposed, showing that the potential barrier structure at grain boundaries, along with the nanometric grain size, leads to an unusual mechanism of carrier filtering, named adiabatic energy filtering. Actually, the presence of potential barriers associated with segregated boron disables charge transport by holes in the band deep tail. This leads to a decrease of the actual carrier density, as in the case of standard energy filtering. However, the nanometric grain size along with the inefficiency of the hole-hole relaxation mechanism in degenerate semiconductors actually prevents carriers from relaxing, causing an increase of the average (macroscopic) drift mobility. Thus, in spite of the decrease of drifting hole density the electrical conductivity is found to increase. In this communication a refinement of the model is presented, that will be discussed and corroborated with an extended body of experimental data gathered by several authors on degenerate polycrystalline silicon films. © 2012 American Institute of Physics.

Internal surfaces of nanocavities are an exceptionally useful laboratory wherein one can spotlight the factors ruling the intricate interplay between morphology and chemistry at silicon surfaces. At the same time, they offer unparalleled opportunities to validate the assignment of vibrational signals of silicon-terminating species under almost ideal experimental conditions. In the case of hydrogen, evidence will be provided of the detailed evolution of H-related species at surfaces depending on their orientation. Also, preliminary results concerning nitrogen at and around nanocavity surfaces will be reported

Energy filtering has been widely considered as a suitable tool to increase the thermoelectric performances of several classes of materials. In its essence, energy filtering provides a way to increase the Seebeck coefficient by introducing a strongly energy-dependent scattering mechanism. Under certain conditions, however, potential barriers may lead to carrier localization, that may also affect the thermoelectric properties of a material. A model is proposed, actually showing that randomly distributed potential barriers (as those found, e.g., in polycrystalline films) may lead to the simultaneous occurrence of energy filtering and carrier localization. Localization is shown to cause a decrease of the actual carrier density that, along with the quantum tunneling of carriers, may result in an unexpected increase of the power factor with the doping level. The model is corroborated toward experimental data gathered by several authors on degenerate polycrystalline silicon and lead telluride.

This paper will show that, while ZT is an appropriate performance index when optimizing the heat conversion rate, it may mislead research in view of applications aiming at large electric power production. This is of special relevance when related to the surge of research in the area of low-dimensionality semiconductors where ZT is increased by lowering the thermal conductivity . It will be shown that, when operating between sources at fixed temperature, the highest power output can be obtained by increasing , not decreasing it, the larger electric power output economically enabling thermoelectric generators for massive electric power production. © 2011 American Institute of Physics.

Silicon has been, and continues to be, the material support of integrated circuit (IC) technology-the enabling tool of one of the most impressive technological, industrial and social revolution of mankind. Silicon (both in monocrystalline and polycrystalline form as well as epitaxial silicon, silicon on insulator and silicon on nothing) has grown with IC technology for its ability to match the numerous and complex needs of microelectronics. In recent years, however, IC technology is experiencing problems of maturity that are expected to be soluble only by huge investment in fabrication plants. This state of affairs has motivated the search for potentially cheaper alternatives to the basic IC element-the metal-oxide-semiconductor (MOS) field-effect transistor (FET). Solutions involving the use of low dimensional structures (like silicon nanowires for the crossbar structure, or silicon quantum dots for single-electron transistors) instead of conventional MOS-FETs have been proposed. Remarkably enough, such a nano-engineered silicon has been proved to be of interest not only in electronics but also in photonics, energetics, and biomedicine.