Oxidation States Of Silicon Research Articles

Binder Effect on Electrochemical Performance of Silicon Anodes

Binder is a crucial component of high-capacity silicon anodes. It allows the active material to cohere to itself and surrounding materials. In the past few years, there have been divergent conclusions on the cell performance of anodes using water-based PAA vs. LiOH-PAA binder [1-3]. While there is agreement that LiOH-PAA facilitates the coating process, cell performance between these two binders is not settled.The difference in cell performance may be due to the pH value or the extra Li+ in the binder, both of which change when LiOH is added to PAA. In this work we examined these two factors separately to determine their effect on cell performance. Our hypothesis is that the extra Li+ could facilitate ion transfer and the low pH could affect current collector corrosion and silicon oxidation. NH3(aq) is used to adjust the binder slurry pH and experimental results are compared between NH3-PAA and LiOH-PAA electrodes otherwise made with the same pH value. Similarly, we compared performance of NH3-PAA anodes with different pH values.Multiple experiments were done to compare aspects of cell performance. Overall electrochemical performance was examined by cell cycling. Ionic conductivity was determined by an impedance test. Contact resistances to the current collector were measured with an apparatus developed in our lab. Silicon oxidation state was determined by XRD. Morphology of the anode can be compared by SEM.In our cycling results silicon half cells with LiOH-PAA binder had a 10% higher capacity than those with PAA binder (Fig. 1), all other factors being constant. And the LiOH-PAA film had a lower impedance value which confirms the hypothesis that extra Li+ could facilitate ion transfer. The PAA anode had a lower contact resistance to the current collector than did the LiOH-PAA anode. This result agrees with Xiong’s work [3], which claims that the corrosion of current collector when using PAA slurry could form a more rigid coating surface and thus lead to the surface cracking and lower the capacity. Other parts of this work including cycling performance and XRD are still in progress and will be reported. Hays, K.A., et al., Journal of Power Sources, 2018. 384: p. 136-144. Hu, B., et al., Journal of Power Sources, 2019. 416: p. 125-131. Xiong, J., et al., ACS Applied Materials & Interfaces, 2021. 13(24): p. 28304-28323. Figure 1

Surface and interface effects on the current–voltage characteristic curves of multiwall carbon nanotube-Si hybrid junctions selectively probed through exposure to HF vapors and ppm-NO2

The possibility to increase the efficiency of photovoltaic (PV) cells based on hybrid carbon nanotube (CNT)–Si heterojunctions is related to the ability to control the chemical properties of the CNT–Si interface and of the CNT bundle layer. In spite of the encouraging performances of PV cells based on multiwall (MW) CNT, so far few efforts have been made in the study of this device compared to single wall (SW) CNT–Si interfaces. Here, surface and interface effects on the current–voltage characteristic curves of MW CNT–Si hybrid junctions are investigated through exposure to HF vapors and to 10 ppm-NO2 and compared to the effects detected in SW CNT–Si junctions. Quite similar results in terms of open circuit voltage, short circuit current density, and efficiency are found for both cells, suggesting that exposure to HF vapors mostly affects the interface chemical properties, i.e., the silicon oxidation state, that in both junctions reach an optimal state about 50 h after etching. In turn, NO2 exposure has larger effects on the SW-based cell, consistently with the larger surface-to-volume ratio of SW with respect to MW. In both cases, the efficiency value reaches a maximum after 28 min, before dropping when the NO2 molecules desorb from the surface. A combined analysis of current–voltage curves and photoemission data collected along the different phases of gas exposures allowed us to relate changes in the electrical properties to the chemistry of Si at the interface.

Study of the properties of SiOx layers prepared by different techniques for rear side passivation in TOPCon solar cells

Passivation of n-type and p-type monocrystalline CZ Si wafers (both polished and textured) with silicon oxide layers prepared by thermal (TO), chemical (CO) and plasma (PO) techniques have been extensively investigated from the measurement of minority carrier lifetime (τ) by transient electrical photoresponse method, density of interface states (NSS) measurement by capacitance – voltage study and silicon oxidation states by X-ray photoelectron spectroscopy (XPS) study of the SiO2/Si interface. It has been observed that NSS and τ have an inverse relation but the dependence is not linear. The method (TO, CO or PO) of oxide layer development has been found to play a crucial role to control the passivation of the c-Si wafer surface. It has been observed that the thermally grown oxide layer (TO) is superior among three oxide layers for all the different c-Si surfaces. Very low density of interface states (<5 × 1011) were found in both p-type and n-type polished wafer passivated with TO layer. Highest lifetimes of 170 μs for n-type polished wafer and 102.74 μs for p-type polished wafer were obtained with TO. The amount of sub oxide formed at the interface of both n- and p-type of wafers during different oxidation process, may have some correlation with Nss which in turn determines the passivation quality of the wafers. Improvement of implied Voc for both polished and textured wafers (n-type and p-type) was found using thermally grown oxide with respect to others.

Stability of Chemically Doped Nanotube–Silicon Heterojunction Solar Cells: Role of Oxides at the Carbon–Silicon Interface

Heterojunctions of carbon nanotubes interfaced with silicon and doped with AuCl3 can achieve attractive power conversion efficiencies when operated in the photovoltaic regime; however, the cost and long-term stability of such devices must be improved before they could become commercially viable. Here, we investigate the role of chemical treatment of the carbon nanotube/silicon interface with either SOCl2 or HNO3, prior to AuCl3 doping, on the stability of the photovoltaic devices. We find that while both treatments initially lead to similar device performance, devices treated with HNO3 are significantly more stable. Using X-ray photoemission spectroscopy, we demonstrate that pretreatment with the powerful organic oxidant SOCl2 generates a variety of low-oxidation-state silicon species at the nanotube–silicon interface that are not generated by exposure to HNO3. These species and their evolution over time are implicated in the reduced device stability, highlighting the importance of silicon oxidation states in determining the stability of carbon nanotube–silicon photovoltaic devices.

A high-capacity cathode based on silicates material for advanced lithium batteries

Silicate materials have been proposed as alternative cathodes for Li-ion battery applications. A novel mixture of silicates, labelled Li6MnSi5, based on the molar ratio among the Li/Mn/Si precursors, with promising electrochemical properties as positive electrode material is synthesized through a solid-state reaction. The results indicate the proposed synthetic method as effective for preparation of nanostructured silicate powders with average particle diameter of 30 nm. Structural morphology of the samples was determined using X-ray powder diffraction (XRPD), XPS and FESEM analysis. A joint analysis by XRPD data and by density functional theory (DFT) identified LiHMn4Si5O15, Li2Mn4Si5O15, Li2Si2O5 and Li0.125Mn0.875SiO4 as components of Li6MnSi5 mixture. The electrochemical performance of Li6MnSi5 was evaluated by charge/discharge testing at constant current mode. Li6MnSi5 discharge behaviour is characterized by high capacity value of 480 mA h g−1, although such capacity fades gradually on cycling. Ex situ XPS studies carried out on the electrode in both full charged and discharged states pointed out that Li2Si2O5 is decisive for achieving such high capacity. The discharge/charge plateau is most probably related to the change in the oxidation state of silicon at the surface of the silica material.

Growth kinetics and complex characterization of PECVD SiOxNy dielectric films

This paper is devoted to the study of patterns of dielectric film growth depending on the parameters of the plasma chemical deposition process. The study has revealed the influence of the basic reagents’ content on the changes in surface morphology, breakdown voltage and stoichiometry (transition to the intermediate oxidation states of silicon) of dielectric films. Furthermore, an exponential pattern of the change in the films’ growth rate has been registered. When increasing the N/Si ratio, a nonlinear dependence of the change in the dielectric films’ band gap has been recorded. When increasing the SiH4/N2O ratio, a shift of the peak positions of the interband interactions relative to the band gap boundaries has been detected. The dataset on breakdown voltage and band structure suggests a certain optimum of barrier properties of SiOxNy dielectric films at the basic reagents’ ratio close to 0.3–0.4.

The effect of oxide shell thickness on the structural, electronic, and optical properties of Si-SiO2 core-shell nano-crystals: A (time dependent)density functional theory study

Due to their tunable properties, silicon nano-crystals (NC) are currently being investigated. Quantum confinement can generally be employed for size-dependent band-gap tuning at dimensions smaller than the Bohr radius (∼5 nm for silicon). At the nano-meter scale, however, increased surface-to-volume ratio makes the surface effects dominant. Specifically, in Si-SiO2 core-shell semiconductor NCs the interfacial transition layer causes peculiar electronic and optical properties, because of the co-existence of intermediate oxidation states of silicon (Sin+, n = 0–4). Due to the presence of the many factors involved, a comprehensive understanding of the optical properties of these NCs has not yet been achieved. In this work, Si-SiO2 NCs with a diameter of 1.1 nm and covered by amorphous oxide shells with thicknesses between 2.5 and 4.75 Å are comprehensively studied, employing density functional theory calculations. It is shown that with increased oxide shell thickness, the low-energy part of the optical transition spectrum of the NC is red shifted and attenuated. Moreover, the absorption coefficient is increased in the high-energy part of the spectrum which corresponds to SiO2 transitions. Structural examinations indicate a larger compressive stress on the central silicon cluster with a thicker oxide shell. Examination of the local density of states reveals the migration of frontier molecular orbitals from the oxide shell into the silicon core with the increase of silica shell thickness. The optical and electrical properties are explained through the analysis of the density of states and the spatial distribution of silicon sub-oxide species.

High-resolution structural investigation of passivated interfaces of silicon solar cells

High-efficiency solar cell technologies rely on a sound knowledge of interface engineering and characterization on an atomic scale. Electrical interface recombination losses are closely related to structural and chemical properties of passivation layers. Ultra-thin silicon oxide layers (thickness about 1nm, often referred to as tunnel oxide layers) applied in the recently introduced TOPCon cell concept and ALD passivation techniques require advanced techniques in characterization of layer thickness and homogeneity. Furthermore, the interface properties seem to play an important role for PID solar cell reliability. Within this paper, we have a methodological focus on ultrathin silicon oxide layer characterization both from a preparative and analytical point of view. We show results on atomically resolved imaging of passivation layers and related chemical and electrical properties. The thickness of silicon oxide layers which lies in the low nm range is measured by TEM and angle-resolved XPS. The preparation of TEM samples was done by wedge polishing including final Ar+ ion polishing. Further preparation steps are required to inhibit further oxidation. From the comparison of TEM and XPS for thickness measurement results that layer thickness can be determined with an accuracy of 0.35nm. These thickness values are compared to data obtained by spectral ellipsometry. The chemical composition of interfaces and stoichiometric composition of SiOx interface layers was analyzed by means of TEM, XPS and ToF-SIMS. The elemental depth resolution of all methods was compared. Finally, the silicon oxidation state was investigated by XPS as a function of layer thickness addressing both interfacial and thin film oxidation states. The presence of subsurface oxides is investigated depending on annealing processes.

Influence of silicon dangling bonds on germanium thermal diffusion within SiO2 glass

We study the influence of silicon dangling bonds on germanium thermal diffusion within silicon oxide and fused silica substrates heated to high temperatures. By using scanning electron microscopy and Rutherford backscattering spectroscopy, we determine that the lower mobility of Ge found within SiO2/Si films can be associated with the presence of unsaturated SiOx chemical bonds. Comparative measurements obtained by x-ray photoelectron spectroscopy show that 10% of silicon dangling bonds can reduce Ge desorption by 80%. Thus, the decrease of the silicon oxidation state yields a greater thermal stability of Ge inside SiO2 glass, which could enable to considerably extend the performance of Ge-based devices above 1300 K.

Initial Products in the Oxidation of Si(111)-7×7 Surface Using O2 at 300 K and Those Formation Processes Observed by Real-time Synchrotron Radiation X-ray Photoelectron Spectroscopy

To understand reaction mechanism of the oxidation at an Si(111)-7×7 surface using O2 at 300 K, the determination of initial products is essentially important. Oxygen bonding configurations and silicon oxidation states were observed using real-time X-ray photoelectron spectroscopy for O 1s together with Si 2p. It is found that ins structure initially forms, where one oxygen atom inserts in the backbond of Si adatom. It is ascertained that the chemisorbed molecular oxygen, so-called paul oxygen, is the adsorbate on top of the ins structure. It is also clarified that ad-ins structure and ins-tri structure, where ad means an oxygen atom on top of Si atom and tri means the interstitial oxygen atom, appear after a short time. Our results imply the presence of a mobile O2 on the surface.

Interactive metal ion–silicon oxidation/reduction processes on fumed silica

Fumed silica is shown to represent an active oxidation/reduction site. Arresting results demonstrate that the average oxidation state of silicon, at least at the surface of fumed silica, is +1 in contrast to the assumed value of +4, thus allowing the preparation of silica with desired reduced silicon surface oxidation states without post-synthesis treatment. The nature of this silica interface is demonstrated by preparing Fe/silica and Cu/silica materials with unprecedented control of the transition metal oxidation state. Supported iron or copper catalysts were prepared by contacting anhydrous iron(III) chloride, iron(III) nitrate nonahydrate, or hydrated copper(II) chloride with fumed, amorphous silica (CAB-O-SIL®) in dry methanol at room temperature. Subsequently, the solids were vacuum-dried (∼10−3 Torr) at room temperature for two hours. These solids, and in particular the iron-silica interface, were examined by X-ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FTIR) spectroscopy, UV-vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), transmission electron microscopy (TEM), colour analysis, and water contact angle analysis. No discernible evidence was found that indicated the formation of large crystallites of the transition metals. The products of iron(III) interaction at the interface with amorphous silica were also investigated using phenanthroline complexation to confirm the presence of Fe(II) ions. This body of data showed compelling evidence that a portion of the transition metal ions in contact with the fumed silica were reduced to lower oxidation states while some of the silicon ions were observed to be “oxidized” to higher oxidation states. The ratio of Fe(II) over Fe(III) found by XPS deconvolution for the chloride spectra matches well with theoretical prediction based upon a simple surface reaction between the Fe(III) ions and the lower valent Si ions. The Fe doping was deduced to be more likely at the axial position of the Si–O bond rather than the equatorial. It is remarkable that these observed transitions in the metal ion oxidation states occurred at room temperature. The inherent simplicity of this technique is general to many reducible metal oxides, and thus, its use in preparations may provide a new way of controlling the ratio of various oxidation states of metal elements.

Effect of UV/ozone irradiation on the surface properties of electrospun webs and films prepared from polydimethylsiloxane–urea copolymers

Highly hydrophobic surfaces of silicone–urea copolymers were transformed into hydrophilic ones upon UV/ozone treatment. The extent of surface modification was strongly dependent on the sample preparation method and the exposure time. The physical and chemical changes at the copolymer surfaces were analyzed by spectroscopic (XPS, ATR-FTIR), microscopic (SEM) techniques and static water contact angle measurements. ATR-FTIR spectra clearly showed the dramatic change in the strongly hydrogen bonded urea hard segments and the degradation of dimethylsiloxane units in silicone–urea copolymers. XPS results revealed the formation of SiOx on the surface, which gradually increased with exposure time. After 3h of UV/ozone exposure, Si(2p) binding energy shifted from 101.9 to 102.85eV, which is a clear indication of an increase in the oxidation state of silicon. The deterioration of microroughness of the electrospun webs upon UV/ozone exposure, which was revealed by SEM, resulted in a dramatic decrease in the static water contact angle values from 129 to 62°. These results clearly show that UV/ozone process is a very simple and facile method to transform hydrophobic silicone–urea copolymer surfaces into fairly hydrophilic ones.

The surprising oxidation state of fumed silica and the nature of water binding to silicon oxides and hydroxides

X-ray photoelectron spectroscopy (XPS) data demonstrate that the average formal oxidation state of silicon in fumed silica (CAB-O-SIL®) is+1 as opposed to the usual+4. Electronic structure calculations on model molecular clusters and comparison with XPS data for silicon compounds in known oxidation states suggest less hydrophilic character for CAB-O-SIL® than the oxides of silicon with Si in an average formal+3 or+4 oxidation state. Once the+3 oxidation state is formed, water on the fumed silica surface facilitates the ready conversion of the Si in an average+3 oxidation state to an average+4 oxidation state.

Reactions of a Bis-silylene (LSi−SiL, L = PhC(NtBu)2) and a Heteroleptic Chloro Silylene (LSiCl) with Benzil: Formation of Bis(siladioxolene) and Monosiladioxolene Analogue with Five-Coordinate Silicon Atoms in Both Ring Systems

LSi−SiL (L = PhC(NtBu)2) (4) was reacted with benzil in toluene at room temperature to afford a compound that contains two siladioxolene rings connected by a Si−Si bond. This is the first reactivity study of 4 where the central Si−Si bond was not cleaved during the reaction. Furthermore, LSiCl (L = PhC(NtBu)2) (6) was treated with benzil at ambient temperature in toluene to yield 2,5-dioxa-1-silacyclopent-3-ene (7). Both 5 and 7 were characterized by single-crystal X-ray diffraction and additionally by NMR spectroscopy, EI-MS spectrometry, and elemental analysis. Investigation of the molecular structure shows that the Si−Si bond length in 5 is 2.36 Å, which is more than 2% shorter than the Si−Si bond length in 4. The formal oxidation state of silicon in 5 is +3, which is in agreement with its structure. The existence of 5 and 7 as stable compounds indicates that siladioxolenes can be electronically stabilized by σ-donor ligands.

Oxidation study of silicon nanoparticle thin films on HOPG

Thin films of silicon nanoparticles (diameter 5–10 nm) were deposited on highly oriented pyrolytic graphite (HOPG) by low-pressure DC magnetron sputtering. The effect of different room-temperature oxidation techniques was investigated using XPS sputter-depth profiling. Both oxygen treatment during deposition (using an argon–oxygen mixture in the sputter gas) as well as post-deposition oxidation techniques (exposure to oxygen plasma beam, ambient air conditions) were studied. In all cases oxidation was found to involve the whole film down to the film/substrate interface, indicating a network of open pores. Depending on the type of oxidation between 15 and 25 at% of oxygen, mostly associated with low oxidation states of silicon, were detected in the interior of the film and attributed to oxidized surfaces of the individual silicon nanoparticles. The highest oxygen concentrations were found at the very film surface, reaching levels of 25–30% for films exposed to air or prepared by reactive magnetron sputtering. For the oxygen plasma-treated films even oxygen surface concentrations around 45% and fully oxidized silicon (i.e., SiO 2) were achieved. At the Si/HOPG interface formation of silicon carbide was observed due to intermixing induced by Ar-ion beam used for sputter-depth profiling.

Evolution of Si suboxides into Si nanocrystals during rapid thermal annealing as revealed by XPS and Raman studies

Si nanocrystals (nc-Si) were synthesized by rapid thermal annealing of magnetron-sputtered amorphous SiO x films. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were employed to study the evolution of chemical structures and the rapid growth of nc-Si. XPS results reveal five chemical structures of Si in the as-deposited films corresponding to silicon oxidation states: Si n+ ( n=0, 1, 2, 3 and 4). Their bonding structures are found to be inhomogeneous, and far from that predicted by the random-bonding model. XPS and Raman spectra suggest that the formation of amorphous Si nanoclusters had already taken place in the as-deposited samples. Formation of nc-Si was realized through a combination of thermal decomposition and the diffusion process. Upon thermal annealing, Si suboxides (Si 2O, SiO and Si 2O 3) decompose into Si and SiO 2, resulting in the rapid growth of nc-Si.

Active oxidation: Silicon etching and oxide decomposition basic mechanisms using density functional theory

The etching of silicon atom from the Si(1 0 0)- p(2 × 2) surface, i.e. the desorption of SiO molecules from this surface, either clean or pre-oxidized, is investigated at the density functional theory level. The reaction paths for desorption are given as a function of the initial oxidation state of the extracted silicon atom. The associated activation energies and the atomic configurations are discussed. Particularly, it is shown that desorption of SiO molecules takes place during conventional thermal oxide growth (∼2 eV activation) via non-oxidized silicon atoms. Further SiO extraction mechanisms of higher silicon oxidation states required higher temperatures. In particular, doubly oxidized silicon atoms (Si 2+) are able to decompose with an activation of ∼4 eV which corresponds to the actual temperature where decomposition of oxides is observed. This confirms the statement that decomposition of oxide layer nucleates at the interface with silicon where Si 2+ has been detected thanks to XPS experiment.

An XPS study on ion beam induced oxidation of titanium silicide

Titanium silicides (TiSi 2) films grown on Si(1 0 0) substrate were investigated by ex situ XPS depth profiling after athermal ion beam induced oxidation (IBO) at 12 keV O 2 + incident energy and normal incidence. The composition and stoichiometry of these films were quantitatively determined as chemical state relative concentrations versus sputter time. “In depth” silicon and titanium oxidation states have been obtained after spectra deconvolution, showing a mixture of silicon dioxide, titanium dioxide, titanium suboxides, elemental titanium and residual traces of titanium nitride. Thermochemical data based on the corresponding enthalpies of formation of the oxides cannot explain our experimental results as in the case of low energy IBO, an oxygen defective altered layer is formed, presenting features of a reduced TiO x phase.

Transmission electron microscopy characterization and sculpting of sub-1nm Si–O–C freestanding nanowires grown by electron beam induced deposition

The authors present a transmission electron microscopy characterization and sculpting of freestanding Si–O–C nanowires, fabricated by electron beam induced deposition from a tetraethylorthosilicate precursor, in a dual beam system. Electron energy loss spectroscopy and near edge structure analyses performed on as deposited wires show the formation of amorphous silicon dioxide with extra oxygen and carbon content. Subsequent electron beam sculpting by 200keV transmission electron microscope irradiation decreases carbon and oxygen contents leaving the silicon oxidation state unchanged and narrows Si–O–C wire width to less than 1nm.

Depth profiling the electronic structures at HfO2∕Si interface grown by molecular beam epitaxy

The depth profile of the HfO2∕Si interface grown by molecular beam epitaxy (MBE) has been investigated by high-resolution x-ray photoelectron spectroscopy using synchrotron radiation beam and low energy Ar+ sputtering. At the interfacial layer, the Hf 4f spectra show a Hf silicide state without a Hf silicate signal. The O 1s and Si 2p spectra show weakly two silicon oxidation states of SiO and SiO2. The data suggest an inward reaction of the Hf and O species into the Si substrate during the MBE growth. The valence band offset (ΔEV) of ∼3.5eV is obtained for the HfO2∕Si interface by measuring the valence-band edges of HfO2 and Si.