Fabrication of Metal Contacts on Silicon Nanopillars: The Role of Surface Termination and Defectivity.

The application of nanotechnology in developing novel thermoelectric materials has yielded remarkable advancements in material efficiency. In many instances, dimensional constraints have resulted in a beneficial decoupling of thermal conductivity and power factor, leading to large increases in the achievable thermoelectric figure of merit (ZT). For instance, the ZT of silicon increases by nearly two orders of magnitude when transitioning from bulk single crystals to nanowires. Metal-assisted chemical etching offers a viable, low-cost route for preparing silicon nanopillars for use in thermoelectric devices. The aim of this paper is to review strategies for obtaining high-density forests of Si nanopillars and achieving high-quality contacts on them. We will discuss how electroplating can be used for this aim. As an alternative, nanopillars can be embedded into appropriate electrical and thermal insulators, with contacts made by metal evaporation on uncapped nanopillar tips. In both cases, it will be shown how achieving control over surface termination and defectivity is of paramount importance, demonstrating how a judicious control of defectivity enhances contact quality.

Roadmap on thermoelectricity.

The increasing energy demand and the ever more pressing need for clean technologies of energy conversion pose one of the most urgent and complicated issues of our age. Thermoelectricity, namely the direct conversion of waste heat into electricity, is a promising technique based on a long-standing physical phenomenon, which still has not fully developed its potential, mainly due to the low efficiency of the process. In order to improve the thermoelectric performance, a huge effort is being made by physicists, materials scientists and engineers, with the primary aims of better understanding the fundamental issues ruling the improvement of the thermoelectric figure of merit, and finally building the most efficient thermoelectric devices. In this Roadmap an overview is given about the most recent experimental and computational results obtained within the Italian research community on the optimization of composition and morphology of some thermoelectric materials, as well as on the design of thermoelectric and hybrid thermoelectric/photovoltaic devices.

Magnetic moment impact on spin-dependent Seebeck coefficient of ferromagnetic thin films.

Magnetic materials may be engineered to produce thermoelectric materials using spin-related effects. However, clear understanding of localized magnetic moments (µI), free carriers, and Seebeck coefficient (S) interrelations is mandatory for efficient material design. In this work, we investigate µI influence on the spin-dependent S of model ferromagnetic thin films, allowing µI thermal fluctuations, ordering, and density variation influence to be independently investigated. µI influence on free carrier polarization is found to be of highest importance on S: efficient coupling of free carrier spin and localized magnetic moment promotes the increase of S, while spin-dependent relaxation time difference between the two spin-dependent conduction channels leads to S decrease. Our observations support new routes for thermoelectric material design based on spin-related effects in ferromagnetic materials.

Recent Advances on Thermoelectric Silicon for Low-Temperature Applications.

Silicon is the most widely used functional material, as it is geo-abundant and atoxic. Unfortunately, its efficiency as a thermoelectric material is very poor. In this paper, we present and discuss advances of research on silicon and related materials for thermoelectric applications, mostly focusing on the comparison between the two strategies deployed to increase its performance, namely either reducing its thermal conductivity or, in polycrystalline materials, increasing its power factor. Special attention will be paid to recent results concerning silicon thin films. The enhancement of Si performances has motivated efforts to develop integrated heat microharvesters operating around room temperature, which will be reviewed also in view of their applications to power wireless sensors for the Internet of Things.

Economic Convenience of Hybrid Thermoelectric-Photovoltaic Solar Harvesters.

Over the last few years, a growing interest has surfaced about the possibility of enhancing solar harvester efficiency by coupling photovoltaic (PV) cells with thermoelectric generators (TEGs). To be effective solutions, hybrid thermoelectric-photovoltaic (HTEPV) solar harvesters must not only increase the solar conversion efficiency but should also be economically competitive. The aim of this paper is to estimate the profitability of HTEPV solar harvesters with no reference to specific materials, relating it instead to their physical properties only and thus providing a tool to address research effort toward classes of HTEPV systems able to compete with current PV technologies. An economic convenience index is defined and used to assess the economic sustainability of hybridization. It is found that, although hybridization often leads to enhanced solar power conversion, power costs (USD/W) may not always justify HTEPV deployment at the current stage of technology. An analysis of the cost structure shows that profitability requires largely enhanced thermoelectric stages, concentrated solar cells, or PV materials with favorable temperature efficiency coefficients, such as perovskite solar cells.

On the mechanism ruling the morphology of silicon nanowires obtained by one-pot metal-assisted chemical etching.

One-pot Ag-assisted chemical etching (SACE) of silicon provides an effective, simple way to obtain Si nanowires (NWs) of potential interest for technological applications ranging from photovoltaics to thermoelectricity. The detailed mechanism ruling the process has not been yet fully elucidated, however. In this paper we report the results of an extended analysis of the interplay among doping level and type of silicon, nanowire nanomorphology and the parameters controlling the chemistry of the etching process. We provide evidence that the SACE mechanism entirely occurs at the interface between the etching solution and the Si substrate as a result of Si extrusion by sinking self-propelled Ag particles. Also, a rationale is advanced to explain the reported formation of (partially) porous NWs at high doping levels in both p- and n-type Si. A model not relying on the asserted formation of potential barriers enables to recover full consistency between SACE electrochemistry and the mechanism of formation of porous silicon in electrochemical cells.

High Power Thermoelectric Generator Based on Vertical Silicon Nanowires.

Thermoelectric generators, which convert heat directly into electrical power, have great potentialities in the energy harvesting field. The exploitation of these potentialities is limited by the materials currently used, characterized by good thermoelectric properties, but also by several drawbacks. This work presents a silicon-based thermoelectric generator, made of a large collection of heavily p-doped silicon nanostructures. This macroscopic device (area of several mm2) collects together the good thermoelectric features of silicon, in terms of high power factor, and a very reduced thermal conductivity, which resulted in being exceptionally low (1.8 W/(m K), close to the amorphous limit). The generated electrical power density is remarkably high for a Si-based thermoelectric generator, and it is suitable for scavenging applications which can exploit small temperature differences. A full characterization of the device (Seebeck coefficient, thermal conductivity, maximum power output) is reported and discussed.

Synergy between defects, charge neutrality and energy filtering in hyper-doped nanocrystalline materials for high thermoelectric efficiency.

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 Ta 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.

Tuning PEDOT:Tos Thermoelectric Properties Through Nanoparticle Inclusion.

Thermoelectric application of conjugated polymers has recently become a subject of scientific interest. This is due to the peculiar features of these organic materials, such as low cost, safety, abundance of atomic components, and easy processing, which make them an interesting alternative to inorganic materials commonly used for this application in the room temperature range, i.e., tellurides derivatives, such as Bi2Te3 and Sb2Te3. Two are the main drawbacks of organic materials employment: the first is their poor thermoelectrical performance, which is still low in comparison with inorganic benchmark, the second is the scarcity of stable and easy-to-dope n-type polymers. In order to address the first issue, we tried to obtain a further and crucial efficiency improvement, developing a nanocomposite embedding inorganic nanoparticles in a matrix of conjugated polymer. A hybrid film of poly(3,4-ethylenedioxithiophene):Tosylate (PEDOT:Tos) and Mn3O4 nanoparticles have been achieved through a novel strategy, involving nanoparticle functionalization and in situ polymerization. The purpose is to enable energy filtering thanks to the presence of the NPs so as to extend this beneficial effect already been observed in inorganic semiconductor to polymers. Our study indicates a new path to obtain PEDOT-based nanocomposite and enlightens the peculiar behaviour of this hybrid material.

Theoretical Analysis of Two Novel Hybrid Thermoelectric-Photovoltaic Systems Based on Cu2ZnSnS4 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.

Annealing of Heavily Boron-Doped Silicon: Effect on Electrical and Thermoelectric Properties.

In previous studies it was shown that heavily boron-doped nanocrystalline silicon submitted to thermal treatments at temperatures ≥800 °C is characterized by an anomalously high thermoelectric power factor. Its enhanced performances were ascribed to the formation of SiBx precipitates at grain boundary, leading to the formation of potential barriers that filter out low-energy carriers, then causing a simultaneous enhancement of the Seebeck coefficient and of the electrical conductivity. To further investigate the effect of thermal treatment on boron-doped nanocrystalline silicon, samples were submitted to a host of annealing processes or of sequences of them at temperatures between 900 and 1000 °C and for various amounts of time. Electrical conductivity and Hall effect measurements were carried out after each thermal treatment over the temperature range 20­300 K. They provided evidence of the formation of an impurity band, and of hopping conduction at very low temperatures. Hall resistivity data versus temperature provided therefore important insights in the electronic structure of the system, which will enable a more complete understanding of the factors ruling energy filtering in this class of materials.

Energy Filtering and Thermoelectrics: Artifact or Artifice?

The possibility of selecting carrier energies by using suitable potential barriers has played a long-standing role in the physics of thermionic devices. It entered instead the arena of thermoelectricity only in the Nineties through the pioneering work of Rowe and Min and of Nishio and Hirano. Since then, the virtuous use of energy barrier in thermoelectricity has gone through alternating fortunes, with analyses sustaining its capacity as a tool to decouple the adverse interdependency between Seebeck coefficient and electrical conductivity; and papers disproving instead such a possibility. In spite of a yet uncomplete theoretical framework, over the last years an impressive number of papers has been published reporting unusual dependencies of thermopower and conductivity upon carrier densities, mostly in nanocomposites­and attributing them to energy filtering. Aim of this paper is to discuss to which extent and under which physical constraints energy filtering may be actually invoked to explain enhanced power factors­and which alternate possibilities of explanation may be considered instead.

Surface modification strategies on mesoporous silica nanoparticles for anti-biofouling zwitterionic film grafting.

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.

Formation of stable Si-O-C submonolayers on hydrogen-terminated silicon(111) under low-temperature conditions.

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.

Preferential formation of Si-O-C over Si-C linkage upon thermal grafting on hydrogen-terminated silicon (111).

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.

Synergizing nucleic acid aptamers with 1-dimensional nanostructures as label-free field-effect transistor biosensors.

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.

Strain-induced generation of silicon nanopillars.

Silicon metal-assisted chemical etching (MACE) is a nanostructuring technique exploiting the enhancement of the silicon etch rate at some metal-silicon interfaces. Compared to more traditional approaches, MACE is a high-throughput technique, and it is one of the few that enables the growth of vertical 1D structures of virtually unlimited length. As such, it has already found relevant technological applications in fields ranging from energy conversion to biosensing. Yet, its implementation has always required metal patterning to obtain nanopillars. Here, we report how MACE may lead to the formation of porous silicon nanopillars even in the absence of gold patterning. We show how the use of inhomogeneous yet continuous gold layers leads to the generation of a stress field causing spontaneous local delamination of the metal-and to the formation of silicon nanopillars where the metal disruption occurs. We observed the spontaneous formation of nanopillars with diameters ranging from 40 to 65 nm and heights up to 1 µm. Strain-controlled generation of nanopillars is consistent with a mechanism of silicon oxidation by hole injection through the metal layer. Spontaneous nanopillar formation could enable applications of this method to contexts where ordered distributions of nanopillars are not required, while patterning by high-resolution techniques is either impractical or unaffordable.

Simultaneous increase in electrical conductivity and Seebeck coefficient in highly boron-doped nanocrystalline Si.

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.