Over the past decade, electroactive conducting polymers (ECPs) are among the most potential pseudocapacitive materials for the electrodes of microsupercapacitors. ECPs exhibit several advantages, such as good conductivity (up to 103 S cm-1), flexibility, high specific capacitance (values ranging from 300 to 800 F.g-1 depending on synthesis and electrochemical performance conditions), relatively cheap and ease of synthesis [4]. Therefore, the electrochemical deposition of ECPs revealed a new strategy to design a great variety of various polymer morphologies as an innovative pseudocapacitive materials. Based on the different storage mechanism, electrochemical supercapacitors can be divided into three types, namely electrochemical double layer capacitors (EDLCs), pseudosupercapacitors and hybrid supercapacitors in which ECPs and other pseudocapacitive materials such as carbon materials, transition metal oxides, nitride complexes allow to store higher amount of capacitance per gram (faradic reactions) than EDLCs (pure capacitive energy storage reaction). In this work, we have elaborated new n-doped conducting polymers Poly[3,6-bis(2-thienyl)pyridazine] and Poly[3,6-bis(2-(3-n-hexylthienyl))pyridazine] by making electropolymerization on Pt disk electrode and 3 nm alumina coated silicon nanowires in acetonitrile organic electrolyte. The comonomers 3,6-bis(thienyl)pyridazine and (3,6-bis(2-(3-nhexylthienyl))pyridazine) have been successively synthesized. The produced polymeric materials Poly[3,6-bis(2-thienyl)pyridazine] and Poly[3,6-bis(2-(3-n-hexylthienyl))pyridazine] during n-doping process on Pt working electrode in acetonitrile solution of TBAPF6 (0.1 M) expose very low potential value of -2.23 V and -2.19 V at current density of -0.25 mA.cm-2 and -0.06 mA.cm-2, respectively. The elctropolymerization of synthesized comonomer 3,6-bis(thienyl)pyridazine has been done on Al2O3@SiNWs and resulted in thin layer of polymer film with micropores. The experimental results are compared to electropolymerization study of thiophene monomer on Pt disk and Al2O3@SiNWs electrodes to highlight the similarities and electrochemical advancements.

Developing flexible solid-state micro-supercapacitors with large potential window is crucial for many technologies. The present study focuses on the synthesis and elaboration of quasi-solid-state (QSS) electrolytes with an electrochemical stability greater than 3.0 V based on functionalized polysiloxane. Their electrochemical behavior was studied with silicon nanowires electrodes. The polysiloxane was functionalized with both ammonium and cross-linkable functions. The nanostructured electrodes were impregnated with a mixture of ionic liquid and functionalized polysiloxane. The crosslinking of the polymer electrolyte was then performed by thermal curing directly within the nanostructured electrodes. The quasi-solid system exhibits very high stability upon galvanostatic cycling i.e. during 100 000 cycles with a cell potential of 3.0 V only a capacitance loss of 2.10-5% per cycle was registered. The performance of this QSS with safe electrolyte is remarkable.

Conducting polymers show attractive characteristics as electrode materials for micro-electrochemical energy storage (MEES). However, there is a lack of characterization techniques to study conjugated/conducting polymer-based nanostructured electrodes. Here, scanning electrochemical microscopy (SECM) is introduced as a new technique for in situ characterization and acceleration of degradation processes of conducting polymers. Electrodes of PEDOT:PSS on flat silicon, silicon nanowires (SiNWs) and silicon nanotrees (SiNTrs) are analyzed by SECM in feedback mode with approach curves and chronoamperometry. The innovative degradation method using SECM reduces the time required to locally degrade polymer samples to a few thousand seconds, which is significantly shorter than the time usually required for such studies. The degradation rate is modeled using Comsol Multiphysics. The model provides an understanding of the phenomena that occur during degradation of the polymer electrode and describes them using a mathematical constant A0 and a time constant τ.

Photoelectrochemical water splitting is a sustainable and environmentally friendly way to produce green hydrogen, for which the practical fabrication of low-cost photoelectrodes remains a challenge. Here we report a porous photocathode assembly based on silicon nanowires (SiNW) as a light absorber and non-precious molybdenum sulfide (MoSx) as a hydrogen-evolution catalyst. Tuning the SiNW diameter is key to select the light absorption wavelength range of the system. We demonstrate a facile and robust route to synthesize SiNWs with a controlled diameter from 13 to 48 nm directly on a porous conductive support. The high quality and homogeneity of the SiNWs grown by this method also allowed drawing unprecedented conclusions on the growth process, hinting towards a silylene path. Photocathodes baring SiNWs covered with MoSx perform photoelectrocatalytic production of hydrogen for several hours with a faradaic yield over 98%. Building a porous photocathode for solar hydrogen production with earth-abundant materials: silicon nanowires to harvest light, molybdenum sulfide to turn protons into hydrogen.

We report on the influence of the liquid droplet composition on the Sn incorporation in GeSn nanowires (NWs) grown by the vapor-liquid-solid (VLS) mechanism with different catalysts. The variation of the NW growth rate and morphology with the growth temperature is investigated and 400 °C is identified as the best temperature to grow the longest untapered NWs with a growth rate of 520 nm min-1. When GeSn NWs are grown with pure Au droplets, we observe a core-shell like structure with a low Sn concentration of less than 2% in the NW core regardless of the growth temperature. We then investigate the impact of adding different fractions of Ag, Al, Ga and Si to Au catalyst on the incorporation of Sn. A significant improvement of Sn incorporation up to 9% is obtained using 75:25 Au-Al catalyst, with a high degree of spatial homogeneity across the NW volume. Thermodynamic model based on the energy minimization at the solid-liquid interface is developed, showing a good correlation with the data. These results can be useful for obtaining technologically important GeSn material with a high Sn content and, more generally, for tuning the composition of VLS NWs in other material systems.

This work deals with electroactive conducting polymers (ECPs) used as a complementary component on purely capacitive silicon nanowires protected by a 3 nm alumina layer. Accordingly, in this work, we use a fast and simple deposition method to create a pseudocapacitive material based on the electropolymerization in aqueous micellar media (SDS and SDBS 0.01 M) of hydroxymethyl-EDOT (EDOT-OH) onto 3 nm alumina-coated silicon nanowires (Al3@SiNWs). The composite material displays remarkable capacitive behavior with a specific capacitance of 4.75 mF·cm−2 at a current density of 19 µA·cm−2 in aqueous Na2SO4 electrolyte.

Here, we use electron beam induced current (EBIC) in a scanning transmission electron microscope to characterize the structure and electronic properties of Al/SiGe and Al/Si-rich/SiGe axial nanowire heterostructures fabricated by thermal propagation of Al in a SiGe nanowire. The two heterostructures behave as Schottky contacts with different barrier heights. From the sign of the beam induced current collected at the contacts, the intrinsic semiconductor doping is determined to be n-type. Furthermore, we find that the silicon-rich double interface presents a lower barrier height than the atomically sharp SiGe/Al interface. With an applied bias, the Si-rich region delays the propagation of the depletion region and presents a reduced free carrier diffusion length with respect to the SiGe nanowire. This behaviour could be explained by a higher residual doping in the Si-rich area. These results demonstrate that scanning transmission electron microscopy EBIC is a powerful method for mapping and quantifying electric fields in micrometer- and nanometer-scale devices.

Dielectric materials have been used for decades for energy applications where their insulation and polarizability properties are critical. In the energy storage field, most material scientists envision high-k dielectric layers in contact with an active material only as an insulating passivation layer. In microelectronics, this concept has been modified with the study of dielectrics at nanoscale level revealing interesting properties scarcely known by other fields. We propose to reconsider the vision of high-k dielectric materials for energy at nanoscale specifically. Based on microelectronic measurement techniques and nanometric control of dielectric thickness by Atomic Layer Deposition (ALD), an ultra-thin pinhole-free alumina (Al2O3) layer on a silicon nanowire (SiNW) is shown to display thickness dependent tunneling electrical conduction. This result brings a new light on this material class in the energy field and allows original approaches toward achieving scientific leaps. As an illustrative application, a silicon based micro-supercapacitor (MSC) protected by 3 nm of alumina dielectric layer exhibits Electrical Double Layer Capacitance (EDLC) by means of tunneling current in aqueous electrolyte, an unprecedented result for this material, with outstanding lifetime capacity retaining 99% of its initial capacitance after 2 million cycles. Extended to multiple energy materials, such method could lead to notable progress. (C) 2021 Elsevier Ltd. All rights reserved.

While reversibility is a fundamental concept in thermodynamics, most reactions are not readily reversible, especially in solid state physics. For example, thermal diffusion is a widely known concept, used among others to inject dopant atoms into the substitutional positions in the matrix and improve the device properties. Typically, such a diffusion process will create a concentration gradient extending over increasingly large regions, without possibility to reverse this effect. On the other hand, while the bottom up growth of semiconducting nanowires is interesting, it can still be difficult to fabricate axial heterostructures with high control. In this paper, we report a reversible thermal diffusion process occurring in the solid-state exchange reaction between an Al metal pad and a Si$_x$Ge$_{1-x}$ alloy nanowire observed by in-situ transmission electron microscopy. The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted SixGe1-x part, forming an axial Al/Si/Si$_x$Ge$_{1-x}$ heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the Si$_x$Ge$_{1-x}$ alloy nanowire while maintaining the rod-like geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts, as well as Si/Si$_x$Ge$_{1-x}$/Si heterostructures for near-infrared sensing applications.

It is well-known that the chemical potential which drives the vapor-liquid-solid growth of semiconductor nanowires is strongly affected by the liquid phase composition. Here, we investigate theoretically how the droplet composition influences the nucleation of Au-catalyzed GeSn nanowires on Ge(111) and Si(111) substrates. We compare the chemical potentials in an Au-Ge-Sn catalyst droplet before and after adding Ga and/or Si atoms. It is found that the presence of these atoms enhances the nucleation rate of nanowires on both substrates. Theoretical results are compared to experimental data on GeSn nanowires grown in a hot-wall reduced pressure chemical vapor deposition reactor. It is shown that the intentional addition of Ga in the de-wetting step improves the uniformity of the nanowire dimensions and yields higher density of nanowires over Ge(111) substrates. The nanowire growth on Si(111) substrate occurs only when Ga and/or Si are added to Au droplets. These results show that controlling the composition of the catalyst droplet is crucial for improving the quality of GeSn nanowires.

While reversibility is a fundamental concept in thermodynamics, most reactions are not readily reversible, especially in solid-state physics. For example, thermal diffusion is a widely known concept, used among others to inject dopants into the substitutional positions in the matrix and improve device properties. Typically, such a diffusion process will create a concentration gradient extending over increasingly large regions, without possibility to reverse this effect. On the other hand, while the bottom-up growth of semiconducting nanowires is interesting, it can still be difficult to fabricate axial heterostructures with high control. In this paper, we report a thermally assisted partially reversible thermal diffusion process occurring in the solid-state reaction between an Al metal pad and a SixGe1–x alloy nanowire observed by in situ transmission electron microscopy. The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted SixGe1–x part, forming an axial Al/Si/SixGe1–x heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the SixGe1–x alloy nanowire while maintaining the rodlike geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si-based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts as well as Si/SixGe1–x/Si heterostructures for near-infrared sensing applications.

The capacitive properties of electrodes elaborated from the electrochemical deposition of poly(3,4-ethy lenedioxythiophene) (PEDOT) coatings onto chemical vapor deposition (CVD)-grown silicon nanowires (SiNWs) were investigated for supercapacitor applications. A high areal capacitance value of 17 mF cm(-2) (50 F g(-1)) was obtained at a scan rate of 100 mV s(-1) in a 3-electrode cell configuration. Furthermore, this 3D hybrid nanostructure was analyzed by electrochemical quartz crystal microbalance (EQCM) to correlate the interfacial ionic exchange mechanisms to their electrochemical performance. It was demonstrated that both anions (BF4) and cations (TBA(+)) were simultaneously involved in the charge compensation during the oxidation-reduction scans of cyclic voltammetry measurements. (C) 2018 Elsevier B.V. All rights reserved.

We report on the ultrafast vibrational response of single Ge-Si core-shell nanowires obtained by epitaxial growth and investigated by femtosecond transient reflectivity and coherent x-ray diffraction measurements. The oscillations of the sample reflectivity are correlated with the fundamental breathing mode for wires with a diameter ranging from 150 to 350 nm and compared with solutions of the Navier equation. Taking advantage of a free standing geometry, we are able to get a mechanical quality factor of higher than 80. Coupling electron microscopy and pump and probe investigations with a very high spectral resolution performed on the same wire, we demonstrate that both shell and core diameter fluctuations are revealed and quantified. X-ray coherent diffraction measurements on individual nanowires evidence changes in the Ge-core diameter and different strain states along a single structure.

Herein, we propose a fast and simple deposition method of a highly robust pseudocapacitive material based on a straightforward drop-cast of a commercial PEDOT:PSS solution onto 3 nm alumina-coated silicon nanowires. The composite material produced (PPSS-A@SiNWs) displays, remarkable capacitive behavior with a specific capacitance of 3.4 mF.cm(-2) at a current density of 2 A.g(-1) in aqueous Na2SO4 electrolyte. Moreover microsupercapacitor (MSC) devices based on this material exhibits outstanding lifetime capacity retaining 95% of its initial capacitance after more than 500 000 cycles at a current density of 0.5 A.g(-1), a specification which exceeds by far most of the stability of conducting polymers previously reported in the literature. In term of pure energy storage performances, the system is able to reach excellent specific energy and power values of 8.2 mJ.cm(-2) and of 4.1 mW.cm(-2), respectively, at a high current density of 2 A.g(-1). Results are systematically compared to both the state-of-the-art silicon based aqueous on-chip supercapacitors and to that of the pristine alumina-coated silicon nanowires (A@SiNWs) to highlight the contribution of the conductive PEDOT:PSS polymer (PPSS in this study). Hence, the aforementioned one-step deposition represents a simple, cheap and scalable method to thoroughly increase the cycling stability of a well-known conductive polymer, PEDOT PSS, while drastically increasing the electrochemical performances of an existing technology, the Si NW-based MSCs using aqueous electrolytes.

In this work we report on the elaboration and characterization of Ge1−xSnx nanowires synthetized by chemical vapor deposition (CVD) via vapor–liquid–solid (VLS) mechanism using GeH4 and SnCl4 as precursors. We have investigated tin incorporation in Ge as a function of experimental growth conditions such as growth temperature and Sn precursor partial pressure (PSnCl4/PGeH4 ratio). We have demonstrated Ge1−xSnx nanowires with Sn incorporation around 1 at.% in the core with a thin Sn‐rich shell with up to 10 at.% Sn well beyond the equilibrium solubility of Sn in bulk Ge.

The 3ω method is a dynamic measurement technique developed for determining the thermal conductivity of thin films or semi-infinite bulk materials. A simplified model is often applied to deduce the thermal conductivity from the slope of the real part of the ac temperature amplitude as a function of the logarithm of frequency, which in-turn brings a limitation on the kind of samples under observation. In this work, we have measured the thermal conductivity of a forest of nanowires embedded in nanoporous alumina membranes using the 3ω method. An analytical solution of 2D heat conduction is then used to model the multilayer system, considering the anisotropic thermal properties of the different layers, substrate thermal conductivity, and their thicknesses. Data treatment is performed by fitting the experimental results with the 2D model on two different sets of nanowires (silicon and BiSbTe) embedded in the matrix of nanoporous alumina templates, having thermal conductivities that differ by at least one order of magnitude. These experimental results show that this method extends the applicability of the 3ω technique to more complex systems having anisotropic thermal properties.

Key words: Microsupercapacitors, Silicon nanotrees, High-k dielectrics, aqueous electrolytes, PEDOT-PSS. In recent years, significant attention has been paid to the development of micro-devices as innovative energy storage solutions. For instance micro-sensor networks such as sensors actuators or implantable medical devices require power densities and cyclability that are several orders of magnitude higher than those of conventional Lithium-Ion batteries. For such applications, Microsupercapacitors (MSCs), a developing novel class of micro/nanoscale power source are rising alternatives, and their integration “on-chip” could allow significant innovations to emerge.1 Therefore, a great deal of attention has been focused on MSCs, for which large series of nanostructured active materials have been developed. Following this trend, we have demonstrated through comprehensive investigations the interest of silicon nanostructures grown by Chemical Vapor Deposition (CVD) as electrodes materials for MSCs using ionic liquid electrolytes2,3. The fine morphological tuning of the nanostructure allowed by the bottom-up approach enables specific designs of electrode architectures, with a considerable leeway compared to other techniques. Such latitude allows optimizing porosity and ionic and electronic pathways while keeping robust mechanical and thermal performances, depending on the target application. Nanostructures such as SiNWs and SiNTrs have displayed excellent electrochemical performances being stable over more than 1 million cycles of galvanostatic charge/discharge under a 4 V wide electrochemical windows in EMI-TFSI ionic liquid, with large power densities of 10 mW.cm-2 and good capacitance values of 0.5 mF.cm-2 at high current density of 0.5 mA.cm-2.4 However a major silicon weakness which was still hindering its use with acqueous electrolytes is the native uncontrolled growth of silica when subjected to ambient atmosphere. Here we have developed a highly conformal passivation coating of a nanometric high-k dielectric layer of Al2O3 based on the rising Atomic Layer Deposition (ALD) technique. ALD has proven to allow a nanometric thickness control of the deposited layer while being highly conformal and covering. Electrochemical stability performances in ionic liquid, were enhanced allowing symmetric 2 electrode devices to reach an unprecedented cell voltage of 5.5 V5, improving energy and maximum power densities compared to pristine nanostructured silicon. The cyclability was also largely enhanced, with only 3% capacitance fade after 106 galvanostatic charge/discharge cycles at 4 V, and no degradation even after several 105 resilience cycles over a 5 V window5. Moreover, the protective alumina layer enables the use of aqueous electrolytes for nanostructured Si based MSCs, which significantly increases the specific power of the devices up to 200 mW.cm-2 at 0.5 mA.cm-2 while keeping the capacitance performances at 0.5 mF.cm-2. Furthermore the system is remarkably able to retain 99% of its initial capacitance after 2 billion galvanostatic charge-discharge cycles at high current density of 0.5 mA.cm-2 in an aqueous electrolyte of Na2SO4. Eventually we have investigated the pseudocapacitive response of such MSCs in aqueous eletrolytes by a simple drop-cast method of a PEDOT-PSS film. The device exhibited promising performances with a specific energy of 2 Wh.kg-1 and a power density of 300 W.kg-1 at a current density of 1 A.g-1. The MSCs was able to retain 80% its initial capacitance after 50,000 galvanostatic charge-discharge cycles at 0.5 A.g-1. [1] Beidaghi, M. Gogotsi, Y. Energy & Environ. Sci. 2014, 7 (3), 867-884 [2]Thissandier, F. ; Gentile, P. ; Pauc, N. ; Brousse, T. ; Bidan, G. ; Sadki, S. Nano Energy 2014, 5, 20-27 [3]Thissandier, F. Gentile, P. Sadki, S., 2014, Journal of Power Sources 269, 740-746 [4]Gaboriau, D. Aradilla, D. Gentile, P. Sadki, S., RSC Advances, 2016, 6, 81017-81027 [5]Dorian Gaboriau, Maxime Boniface, Anthony Valero, Dmitry Aldakov, Thierry Brousse, Pascal Gentile, and Said Sadki Acknowledgements The authors would like to thank the DGA, Direction General de l’Armement, and the CEA for their financial and technical support throughout the PhD thesis of A. Valero. Figure 1

Selective oxidation of the silicon element of silicon germanium (SiGe) alloys during thermal oxidation is a very important and technologically relevant mechanism used to fabricate a variety of microelectronic devices. We show a two-step epitaxy/condensation process at low temperature to produce atomically flat Ge rich layer fully strained and free of defects. We demonstrate that the condensation based process enables the total inhibition of the classical ATG morphological instability, together with the hindering of dislocations for critical thickness much greater than those commonly obtained by direct deposition. Those behaviors could be explained by the injection of self-interstitials in the Ge-rich layers during condensation. An integrative approach involving vapor–liquid–solid (VLS) growth followed by selective oxidation steps to the construction of core–shell nanowires and higher-level ordered systems with scalable configurations is used. We contrast this strategy that uses reaction-diffusion-segregation mechanisms to produce coherently strained structures with highly configurable geometry and abrupt interfaces with growth-based processes which lead to low strained systems with non uniform composition, three-dimensional morphology, and broad core–shell interface. We specially focus on SiGe small core–shell nanowires and demonstrate that they can have up to 70% Ge-rich shell and 2% homogeneous strain with core diameter as small as 14 nm. Key elements of the building process associated with this approach are identified with regard to existing theoretical models. Moreover, starting from results of ab initio calculations, we discuss the electronic structure of these novel nanostructures as well as their wide potential for advanced device applications. Similar core-shell structures can be formed around small nanocrystals for photonic applications.

The use of a mixture of propylene carbonate (PC) and N1114 TFSI ionic liquid (50:50% w.t) has been investigated as an optimal electrolyte for symmetric micro-supercapacitors based on SiNWs using a large and stable cell voltage of 3.5V. The device showed an areal capacitance of 150μFcm−2, an energy density of 1mJcm−2 and a power density of 16mWcm−2 maintaining an outstanding cycling stability after 3·106 galvanostatic charge-discharge cycles at room temperature. Such properties were comparable to those obtained using the pure ionic liquid. Additionally, the excellent electrochemical performances reported in this study reflect the potential of such mixture to be employed as a promising electrolyte in wide operating temperatures ranging from 0 to 80°C at large electrochemical windows.