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In this work two alternative approaches are introduced for automatically integrating large numbers of several μms long Si NWs in lateral thermoelectric devices. Both cases being planar require a thermally isolated platform to be defined by silicon micromachining. This opens the path to all-silicon thermal harvesters, which can be built in large volumes offering a cost-effective energy harvesting solution where thermal gradients are present.

A 4 × non-imaging optically concentrated solar thermoelectric generator (STEG) was simulated and its layout was optimized depending on materials characteristics. The performances of seven state-of-the-art thermoelectric materials were realistically compared considering direct normal irradiances (DNI) between 400 and 900 W/m2 and temperature dependence of the thermoelectric parameters. The model was tested with experimental data from literature and leg aspect ratios, fill factor (or thermal concentration), and leg number per STEG unit area were also used as variables. Due to the high values of thermal concentrations at maximum efficiency, different materials filling the gap among STEG legs were also considered. Maximum efficiency weakly decreases for filler thermal conductivities typical of common insulating materials, opening novel opportunities for STEGs not requiring vacuum. Results of the analysis show that skutterudites, lead telluride and bismuth telluride exhibit the highest efficiencies (≈7%) in the studied range of thermal concentrations and for a DNI equal to 900 W/m2. However, skutterudites and lead telluride were found to be very sensitive on the DNI level, differently from bismuth telluride, which therefore qualifies as the best solution for energy conversion. Moreover, optimal layouts for STEGs based on bismuth telluride more easily meet manufacturing constraints.

The possibility of recovering heat released by photovoltaic (PV) solar cells into electric power by using thermoelectric generators has attracted a remarkable research effort over the last two decades. Views and results are not fully converging, however, ranging from overoptimistic estimates of power gains to fully negative opinions about the convenience of hybridization. The aim of this paper is to review both energetic and economic profitability of photovoltaic-thermoelectric hybridization, as both issues are to be considered in the design of hybrid solar harvesters. It will be shown how the PV material rules the total system efficiency and its economic competitiveness compared to standard PV modules. Marginal opportunities are reported for roof-top concentrated solar harvesters. Yet, hybridization may leverage novel PV materials, currently not considered due to their lower efficiency compared to polycrystalline silicon. Much more significant is instead the window of opportunity when thermoelectric generators are coupled to perovskite solar cells.

A simultaneous increase of the Seebeck coefficient and the electrical conductivity resulting in a remarkable enhancement of the thermoelectric power factor was observed in heavily boron doped nanocrystalline silicon films. To understand the underlying mechanisms for this behavior, we explored the thermoelectric transport properties with a theoretical model based on Boltzmann transport equation in the relaxation time approximation. Energy filtering has been included in the model assuming an energy threshold in the transmission coefficient of holes. The relevant scattering mechanisms for holes in bulk silicon have been taken into account. It has been found that the simultaneous increase of S and σ can be interpreted by the synergy of two processes that take place upon annealing: the redistribution of ionized scatterers and the formation of energy barriers. The redistribution of ionized scatterers forms energy barriers at the grain boundaries and increases Seebeck coefficient. Moreover, it decreases the density of ionized scatterers in the grains and increases the mobility. The increase of the mobility is such that over-compensates the decrease of the conductivity due to energy filtering of carriers by the energy barriers. Hence, the conductivity increases simultaneously with the Seebeck coefficient as the annealing process progresses

In this work we study the effect of different synthetic conditions on thermal transport properties of poly(3,4-ethylenedioxythiophene) (PEDOT) by focusing in particular on the role of proton scavengers. To this aim, different PEDOT samples were generated in silico using a novel computational algorithm based on a combination of first-principles density functional theory and classical molecular dynamics simulations. The corresponding thermal conductivities were then estimated using the approach to equilibrium molecular dynamics methodology. The results show that the initial synthetic conditions strongly affect the corresponding thermal conductivities, which display variations up to a factor of $\ensuremath{\sim}2$ depending on the proton scavenger. By decomposing the thermal conductivity tensor along the direction of maximum chain alignment and the corresponding perpendicular directions, we attribute the thermal conductivity differences to the variations in the average polymer chain length ${\ensuremath{\lambda}}_{\mathrm{ave}}$. A dependence of the thermal conductivity with the polydispersity index was finally observed, suggesting a possible role of intercrystallite chains in enhancing thermal transport properties. By means of the Green-Kubo modal analysis, we eventually characterize the vibrational modes involved in PEDOT thermal transport and investigate how they are related to the thermal conductivity values of the samples.

Silicon nanowires, whose thermal conductivity is strongly reduced with respect to that of the bulk silicon, are very promising for high-efficient thermoelectric conversion. This work focuses on the development of a technique for the fabrication of thermoelectric generators which are based on vertical silicon nanowire forests, achieved through a metal-assisted chemical etch. As heavily doped nanowires are essential in thermoelectric applications, this chemical process has been applied both on lightly and on highly doped (> 1019 cm−3) silicon substrates. A comparison of the results shows that the etch behaves in a completely distinct way when applied to the differently doped substrates. The results of this comparison and a preliminary insight into the diverse behavior occurred are reported. The different initial nucleation of silver, which determines the hole injection, essential to the etching of silicon, seems to be the key point of this different behavior.

PdGe contact fabrication on Ge(001) wafers doped with Ga is investigated using conventional complementary metal-oxide-semiconductor processes. Despite a p-type doping level of ~1.4 × 1020 cm−3, the resistivity of the PdGe contact is found to be twice higher than that of undoped Ge. Ga doping has no influence on the Pd reaction with Ge. However, the doping process and the Salicide process led to the formation of Ga-Pd defects in both sides of the PdGe/Ge interface, resulting from Ga and Pd co-segregation on Ge dislocation loops.

The decrease of solar cell efficiency with temperature is a known problem for photovoltaics (PV). Temperature sensitivity can lead to a considerable amount of energy losses over the lifetime of solar panels. In this perspective Hybrid Thermoelectric-Photovoltaic (HTEPV) systems, which recover solar cell heat losses to produce an additional power output, can be a suitable option. However only hybridization of wide-gap solar cells is convenient in terms of efficiency gains and deserves investigation to evaluate HTEPV devices effectiveness. In this work we report the modeling and the development of customized bismuth telluride thermoelectric generators, optimized to be hybridized with amorphous silicon (aSi), Gallium Indium Phosphide (GaInP) or Perovskites solar cells. The model results showed in all three cases efficiency gains with a maximum of +3.1% for Perovskites (from 16.4% to 19.5%). These enhancements were then experimentally validated for the case of Perovskites solar cells, for which maximum gains were found to occur at typical operating temperatures of conventional PVs. This experimental evaluation demonstrated in an accurate fashion the real potential of thermoelectric hybridization of solar cells.

The development and commercialization of Photovoltaic (PV) cells with good cost-efficiency trade-off not using critical raw materials (CRMs) is one of the strategies chosen by the European Community (EC) to address the Energy Roadmap 2050. In this context Cu2ZnSnS4 (CZTS) solar cells are attracting a major interest since they have the potential to combine low price with relatively high conversion efficiencies. Although a ≈9% lab scale efficiency has already been reported for CZTS this technology is still far from being competitive in terms of cost per peak-power (€/Wp) with other common materials. One possible near-future solution to increase the CZTS competiveness comes from thermoelectrics. Actually it has already been shown that Hybrid Thermoelectric-Photovoltaic Systems (HTEPVs) based on CIGS, another kesterite very similar to CZTS, can lead to a significant efficiency improvement. However it has been also clarified how the optimal hybridization strategy cannot come from the simple coupling of solar cells with commercial TEGs, but special layouts have to be implemented. Furthermore, since solar cell performances are well known to decrease with temperature, thermal decoupling strategies of the PV and TEG sections have to be taken. To address these issues, we developed a model for two different HTEPV solutions, both coupled with CZTS solar cells. In the first case we considered a Thermally-Coupled HTEPV device (TC-HTEPV) in which the TEG is placed underneath the solar cell and in thermal contact with it. The second system consists instead of an Optically-Coupled but thermally decoupled device (OC-HTEPV) in which part of the solar spectrum is focused by a non-imaging optical concentrator on the TEG hot side. For both solutions the model returns conversion efficiencies higher than that of the CZTS solar cell alone. Specifically, increases of ≈30% are predicted for both kind of systems considered. Copyright © 2017 American Scientific Publishers All rights reserved.

Breaking the conventional decrease of the Seebeck coefficient with increasing conductivity would be a significant advancement towards large thermoelectric power factor enhancement and high thermoelectric efficiency. We report on a mechanism identified in hyper-doped nanocrystalline Si films that can lead to this task: a transition from dominant ionized impurity scattering to dominant phonon scattering upon thermal annealing at a high annealing temperature T a that takes place to fulfill charge neutrality. We show that the synergy between charge neutrality and energy filtering activated by thermal annealing of the originally defective nanocrystalline sample leads to high mobility, simultaneous increase of the conductivity and the Seebeck coefficient and large enhancement of the thermoelectric power factor. This is demonstrated by means of advanced theoretical modeling and excellent quantitative agreement with the experiment. Our work provides interpretation of so far not understood observations in nanocrystalline Si and indicates a new route for engineering Si as well as other nanostructured materials for high thermoelectric efficiency.

In this chapter we will present the full-thermal approach to thermoelectric solar harvesting. Analysing the state of the art of this field we will report on its historical development, showing its advantages. Technical and technological issues solved and yet to be solved will be addressed as well. Starting from a description of the main system components we will analyse the literature and the strategies reported so far. Then we will discuss how a solar thermoelectric genenerator (STEG) may be modeled, quantitatively predicting their final efficiency. This analysis will show which are the main parameters influencing STEG performances, suggesting which are the best solutions to achieve efficiencies competitive with other solar strategies.

This chapter is devoted to provide the general theory describing the hybridization of solar cells with thermoelectric generators. Moving from a description of the system, its main components will be introduced and analysed. Their characteristics and their impact on the final system efficiency will be scrutinised. Specifically, the heat generation within solar cells will be detailed considering the main losses occurring in a PV cell. This will bring to an evaluation of the temperature sensitivity of solar cells, which is one of the most important parameter to be considered when pairing PV cells and TEGs. In addition, we will introduce the concept of fully hybridized systems, where the thermoelectric and PV devices are both thermally and electrically connected to each other.

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 (

This chapter focuses on the ways ZT may be improved in silicon by nanotechnology. It describes the profound interplay between preparation methods and thermoelectric performances will be reviewed; both top-down and bottom-up methods. Thermoelectricity is the onset of transport phenomena in which heat or charge current are cross-driven by temperature gradients or electric fields. The most common thermoelectric phenomena are heat currents flowing as a result of the application of an electric field, and electric fields setting up as a result of a temperature difference. In addition, a major side effect of the discovery of high ZT in dimensionally constrained systems has been the attempt to also use nanotechnology to enhance the thermoelectric figure of merit in bulk materials. After providing an overview of the thermoelectric theory in nondimensionally constrained systems that will be extended to one-dimensional (1D) and two-dimensional materials, the state of the art of silicon nanowires and nanolayers for thermoelectric 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 $$1000\,\hbox {W}/\hbox {m}^{2}$$ , and depending on the choice of material, the maximum available thermal power ranges from $$380\,\hbox {W}/\hbox {m}^{2}$$ (for single-crystal silicon) to $$130\,\hbox {W}/\hbox {m}^{2}$$ (for amorphous silicon).

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 1000A degrees C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 10(19) 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.

The spectacular growth of networks of intercommunicating sensing nodes has generated a request for alternate, renewable power sources. Thermoelectric generators (TEGs), either conventional or integrated, are possible candidates. This paper analyzes the usability of TEGs as alternate power sources for wireless sensor network. It is shown how TEGs meet power requirements of low-power sensing nodes and how they outperform batteries as of the installation costs. Factors still hampering TEG wider use are also reviewed and commented upon, and an outlook at specific applications where TEGs might be rapidly deployed is provided.

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.

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.

This paper will address the use of the thermoelectric figure of merit ZT and the related concept of efficiency in low temperature heat conversion. It will be shown that, while ZT is an appropriate performance index when optimizing the rate of heat conversion, it may be misleading for actual applications. This is of special relevance when related to the recent surge of research effort in the area of low-dimensionality semiconductors (e.g., Si nanowires) where the increase of the figure of merit is mostly due to their lowered thermal conductivity κ. Under fixed heat flow conditions the optimization of ZT may actually proceed only by maximizing the power factor, since materials with low κ may be unable to duly dissipate heat. Also when operating between sources at fixed temperature it will be shown that at any given value of power factor the highest output can be obtained by increasing κ, not decreasing it. A brief analysis of the economic factors concurring to define mature scenarios of application will also be provided.

While the analysis of the spectra resulting from energy- or angle-resolved X-ray photoelectron spectroscopy allows the in-depth atomic composition in the probed region to be determined even for complex samples, the determination of the bonding configuration is less trivial. In this paper it is shown that a description of the chemical shift in terms of partial charge and Madelung potential (as results from local modelling of the atom) can provide information even in complicate situations, like that characterizing the hydrogen-terminated (1 0 0) Si prepared by HF aq etching of the native oxide.

Silicon nanocavities can be terminated with hydrogen by wet chemical etching. Their infrared spectra can to a large extent be interpreted in terms of silicon monohydrides on H(7 × 7)Si(111), H(1 × 1)Si(111) and H(2 × 1)Si(100), and of silicon dihydrides on H(1 × 1)Si(100). The time evolution under isothermal conditions (600 °C) of the (1 0 0) faces admits a description in terms of transformation from H(1 × 1)Si(100) into (2 × 1)Si(100) with simultaneous H2 adsorption onto (2 × 1)Si(100) neat dimers. In so doing the inner H2 pressure decreases by about one order of magnitude from the initial value of 3 × 103 Torr. The unique properties of nanocavities allow their use as nanoreactors; this has led to the determination of the sticking coefficient for H2 adsorption in conditions of pressure, temperature and adventitious contamination otherwise not achievable. © 2010 Elsevier B.V. All rights reserved.

Metal sputtering is known to affect metal-insulating-semiconductor (MIS) devices where the insulator is an organic monolayer grafted onto crystalline substrates. We comparatively discuss current–voltage characteristics in MIS devices, where the insulating layer is either a thin oxide layer or an organic monolayer covalently grafted onto single-crystal silicon. Variation of the sputtering geometry from on-axis to off-axis configuration is analyzed to compare differences between them, obtaining the reduction of damages in the oxide layer accordingly to the supposed conduction mechanism, but no changes in organic layer of aliphatic molecules. Effects of ultraviolet radiations, already present during metal deposition, are also discussed.

Aim of this paper is to validate a modified Schottky barrier model accounting for the electrical properties of metal - self-assembled layer - semiconductor structures. To this end, the effect of the dynamic modulation of the dipole moment of the organic layer was studied. The system was a junction built on Si(1 0 0) surfaces modified by grafting an organic layer by wet chemistry reactions. As the metallic electrode, a thin, porous gold layer was deposited, enabling gas diffusion through it. In such a geometry, a polar gas was allowed to adsorb onto the Si surface, and the variation of the barrier height could be measured and correlated with the dipole moment of the gas molecule and its partial pressure. © 2006 Elsevier B.V. All rights reserved.

In this communication we report a novel, technologically promising procedure able to graft alkynes onto Si(1 0 0) through the formation of direct Si-C bond. It results in the formation of grafted layers as FTIR measurements demonstrates. The advantages of this procedure compared to other available routes are that chemicals used are relatively harmless compared to those used in other wet chemical procedures; and that no surface pre-activation is required, the overall process being surface preserving, i.e. leading to no increase of surface roughness. Furthermore, the reaction mechanism guarantees full stereoselectivity. Therefore, gas sensors built using such a procedure are expected to show higher reliability and more stable electrical behavior. © 2006 Elsevier B.V. All rights reserved.

Over the last years interest on thermoelectricity for microharvesting or for prospective large-scale conversion of heat into electric power has revamped. This was mostly due to the tremendous impact of nanotechnology onto the conversion rate achievable by thermoelectric generators [1], grown up by almost a factor three over the last two decades. Novel classes of thermoelectric materials have appeared – but also old materials have rejuvenated. Nanowires and nanolayers, along with multilayered structures, have imparted new improved characteristics to many age-old materials [2–4]. Two avenues were actually explored, namely reducing the thermal conductivity κ and increasing the power factor σα (where σ is the electrical conductivity and α is the Seebeck coefficient). Both approaches lead to an increase of the thermoelectric figure of merit ZT = σαT/κ (where T is the absolute temperature) – and thus of the thermoelectric efficiency with which heat is converted into electric energy [5]. As thermoelectric performances have remarkably enhanced, much attention is now dedicated to selecting materials and technologies that could be scaled up and brought to bulk production. On this avenue, geo-abundance and low material costs are key issues. Thus, it is not surprising that thermoelectric properties of silicon, silicon-germanium alloys, and silicides had been the subject of more than 700 publications over the last five years. This Special Issue is devoted to cover the most important advances in this field through some mini-review papers and a selected number of research contributions, spanning from theoretical and experimental research on silicon-based nanostructured materials to the technology enabling their implementation in actual thermoelectric generators. Possibly, a key date in the history of nanostructured thermoelectric silicon may be set at 2008, when Nature published back-to-back two seminal papers by Hochbaum et al. [6] and by Boukai et al. [7] on nanowires. Both

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.

Protection of silicon surface toward oxidation represents a relevant issue in view of many microelectronic applications. In 2000, it has been shown how molecular grafting with methyl groups could efficiently passivate atomically flat Si(1 1 1). However, neither data are currently available on (0 0 1) surfaces nor systematic analyses of the role played by the organic fragment on the protection efficiency have been reported yet. The aim of this paper is to present data on the passivation of (0 0 1) silicon surface by different aryl fragments self-assembled onto it. Protection was carried out using nucleophilic organic species that were self-assembled on a previously brominated Si surface. Specifically, five differently substituted phenyl groups were considered. These samples were characterized by specular reflection infrared spectroscopy to detect the presence of arylic-terminating groups. The kinetics of the re-oxidation reaction was then monitored over a time period of 2 months. We found qualitatively different mechanisms of re-oxidation, depending on the structure of the organic fragment. The first mechanism is compatible with a model where the aryl group simply hinders the surface Si sites, disabling the access of oxidizing species to the surface itself. The second process appears to be more complex, with the aryl group interacting with the oxidizing species. (C) 2003 Elsevier Science B.V. All rights reserved.

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.

In this Chapter it will be shown how surface chemistry can master silicon surface properties. Major differences between gas phase and solution chemistry will be remarked. The microscopic interaction of small molecules (either charged or neutral) with the Si surface will be shown to be based onto two completely different mechanisms. Gas phase (or plasma) chemistry relies upon high energies and on non-polar reaction paths. Instead, solution chemistry is ruled by low energies and by polar mechanisms. This is further complicated by the simultaneous presence of species in equilibrium with each other, and by complex phenomena such as solvatation. In spite of these differences, many structural aspects of chemically reconstructed Si surfaces will be shown to be similar, since the same static quantities rule dissimilar dynamic scenarios. Finally, recent areas of research will be commented upon, mainly relating to the interaction of organic molecules with silicon, either in view of the need of providing efficient methods of ultracleaning or of that of engineering surfaces to devise novel Si-based devices.

A light-beam induced current (LBIC) in a planar configuration has been coupled to multiple-internal-reflection (MIR) spectroscopy in order to correlate evidence of the surface recombination velocity changes to contaminants on the silicon surface. We verified preliminarily the chemical evolution of HF-treated silicon surfaces from the hydrophobic to the hydrophilic condition after ageing in the laboratory. We then carried out the analysis of silicon surfaces deliberately contaminated with volatile organie molecules using both LBIC and MIR. The results show that the absorption of organics at the surface involves a significant effect on the surface recombination rate.

In this article, evidence will be put forward for the first time about modifications of the Si (100) surface induced by standard SC1/SC2 etching cycles. SC1/SC2 etching (also known as RCA cleaning) submits silicon wafers to oxidation by NH3:H2O2:H2O mixtures, oxide removal in diluted HF, further oxidation by HCl:H2O2:H2O mixtures, and final etching in diluted HF. Samples were analysed using high-resolution transmission electron microscopy (HRTEM)-parallel electron energy loss spectroscopy (PEELS) and Low-energy electron diffraction (LEED) techniques. HRTEM-PEELS analyses were carried out adopting a special cross-section geometry enhancing HRTEM sensitivity to surface species. Atomic-resolution HRTEM micrographs displayed a partial loss of the crystalline order in a 4 nm layer at the Si surface only when samples had undergone SC1/SC2 cycles. No (2×1) reconstruction pattern could be observed by either HRTEM or LEED. PEELS analyses allowed one to rule out the presence of either oxygen, carbon or fluorine at the surface or within the disordered layer, thereby leading to the conclusion that oxidation treatments yield to a modification of the crystalline structure at the Si (100) surface.

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 found to be the only surface silicon atoms in agreement with the experimental observation that passivant substitution or oxidation are mediated by Si2 and never occur directly at Si1 atoms. (C) 2000 American Institute of Physics. [S0021-9606(00)70202-9]

High yield catalytic growth of carbon nanotubes (NT's) at extremely low temperatures (

The ability to tune the thermal conductivity of semiconductor materials is of interest for thermoelectric applications, in particular, for doped silicon, which can be readily integrated in electronic microstructures and have a high thermoelectric power factor. Here, we examine the impact of nanovoids on the thermal conductivity of highly doped, high-power factor polysilicon thin films using time-domain thermoreflectance. Voids are formed through ion implantation and annealing, evolving from many small (∼4 nm mean diameter) voids after 500 °C anneal to fewer, larger (∼29 nm mean diameter) voids with a constant total volume fraction after staged thermal annealing to 1000 °C. The thermal conductivity is reduced to 65% of the non-implanted reference film conductivity after implantation and 500 °C anneal, increasing with anneal temperature until fully restored after 800 °C anneal. The void size distributions are determined experimentally using small-angle and wide-angle X-ray scattering. While we believe multiple physical mechanisms are at play, we are able to corroborate the positive correlation between measurements of thermal conductivity and void size with Monte Carlo calculations and a scattering probability based on Matthiessen's rule. The data suggest an opportunity for thermal conductivity suppression combined with the high power factor for increased material zT and efficiency of nanostructured polysilicon as a thermoelectric material.

This special issue of Nanoscale Research Letters is a collection of selected papers presented at the Nano and Giga Challenges in Electronics, Photonics and Renewable Energy (NGC2011) conference in Moscow and Zelenograd which addresses both theoretical and experimental achievements and provide a stimulating outlook for technological developments in these highly topical fields of research.

The possibility of enhancing the thermoelectric figure of merit ZT by damping the material thermal conductivity (e.g. by quantum confinement) rather than by increasing its power factor has generated a surge of interest toward the use of nanostructures in thermoelectric generators (TEGs). Actually, while ZT is an appropriate performance index when optimizing the rate of heat conversion in the presence of small thermal power inputs, it may turn out to be misleading when used as a general criterion. Under fixed heat flow conditions, the optimization of ZT may actually proceed only by maximizing the power factor, since materials with low κ may be unable to duly dissipate heat. However, also when operating between heat reservoirs at fixed temperature it is shown that the highest electric power a TEG can output may be obtained by increasing κ, not decreasing it. In addition, it will be presented an equation allowing to determine the TE efficiency over temperature ranges where transport parameters are no longer temperature-independent. © 2012 American Institute of Physics.

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.

We report the study of the thermoelectric properties of degenerate, boron-doped polycrystalline silicon on insulator structures. The occurrence of a regime where both the Seebeck coefficient and the conductivity increase is confirmed. This results in a power factor P of 13 mW K-2 m-1. We propose that such high values of P may be determined by adiabatic energy filtering occurring at grain boundaries decorated by segregated boron.

We will present the first evidence ever reported in literature of modifications of the carrier surface density in a semiconductor due to weak interactions of gaseous species with self-assembled monolayers (SAMs). SAMs were obtained by nucleophilic reaction onto halogenated Si(100) surfaces, leading to the formation of an array of aromatic rings directly bonded to silicon through a covalent Si-C bond. The modulation of the Si surface conductance was studied as a function of temperature (340-570 K) in the presence of trace amounts of oxidants (CO, SO, and NO,) in Ar. A correlation between gas composition and the surface conductance was found and is modelled accounting for weak, reversible interactions between the aromatic ring and the gas molecule. (C) 2001 Elsevier Science B.V. All rights reserved.

The response of a potentiometric sensor having a silver chloride layer interposed between the electronic conductor and the β″-alumina electrolyte is analysed. The open-circuit voltage is found to be Nernstian in the range 10–10 000 Pa of chlorine, with slight deviations from the theoretically expected values. The setting-up of a diffusion potential due to the formation of a new phase or a solid solution might account for these departures from theory. A normalized function of the output for step-like changes of chlorine pressure is defined. The approach to steady-state e.m.f. values is found to be linear with the square root of time.

Heavily doped polycrystalline thin diamond films were prepared by chemical vapor deposition. Boron was introduced in the films during the deposition. The electrical activity of acceptors was studied using dc polarization technique and bulk admittance spectroscopy. In the concentration range of 1017–1020cm-3 we have observed the occurrence of Hubbard interaction and band splitting. The conductivity of the films is mobility limited, and the pre-exponential factor of σ(T) decreases as the total concentration of boron increases. The occurrence of boron-defect clustering has been detected, and its effect on the majority-carrier injection level is discussed. © 1991, The Electrochemical Society, Inc. All rights reserved.

Potentiometric solid state gas sensors have been developed for CO monitoring in air. The following galvanic cell was investigated: Pt|M, (CO+O2)|ZrO2-Y2O3|(O2), Pt|Pt where M is either a Pt or an RuO2 layer. Sensors were characterized between 350 and 650 °C with carbon monoxide partial pressures ranging from 1 to 80 Pa. A linear relationship between the electromotive forces (e.m.f.s) and log(p co ) was observed. The e.m.f. versus time curve can be described by: where E(0) and E(∞) are the values at the time at which the pressure of CO is abruptly changed (taken as t=0) and that at which steady-state conditions are reached. τ is a time constant and n is an adimensional exponent. Results obtained with Pt and RuO2 electrode materials are compared and discussed.

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.