Quantifying the Short-Range Order in Amorphous Silicon by Raman Scattering

Developments in Raman Spectromicroscopy for Strengthening Materials and Natural Science Research: Shaping the Future of Physical Chemistry

Spectroscopic techniques, especially Raman spectroscopy, cover a large subset in the teaching and research domain of physical chemistry. Raman spectroscopy, and other Raman based techniques, establishes itself as a powerful analytical tool with diverse applications across scientific, industrial, and natural science (including biology and pharmacy) fields and helps in the progress of physical chemistry. Recent advancements and future prospects in Raman spectroscopy, focusing on key areas of innovation and potential directions for research and development, have been highlighted here along with some of the challenges that need to be addressed to prepare Raman based techniques for the future. Significant progress has been made in enhancing the sensitivity, spatial resolution, and time resolution of Raman spectroscopy techniques. Raman spectroscopy has applications in all areas of research but especially in biomedical applications, where Raman spectroscopy holds a great promise for noninvasive or minimally invasive diagnosis, tissue imaging, and drug monitoring. Improvements in instrumentation and laser technologies have enabled researchers to achieve higher sensitivity levels, investigate smaller sample areas with improved spatial resolution, and capture dynamic processes with high temporal resolution. These advancements have paved the way for a deeper understanding of molecular structure, chemical composition, and dynamic behavior in various materials and biological systems. It is high time that we consider whether Raman based techniques are ready to be improved based on the strength of the current era of AI/ML and quantum technology.

Effect of dimensionality on the excitation wavelength dependence of the Fano-Raman line-shape: a brief review

The already existing heterogeneity in nanomaterials makes it an intriguing yet complex system to study size effect vis-à-vis other external perturbations and thereby local modifications at the nanoscale, thus demanding an improved tool and analysis for the choice of study. The analysis of existential subtle perturbations and interactions in a wide class of materials using Raman spectromicroscopy has proved to be of utmost importance, and various phenomena such as quantum confinement and its interplay with Fano resonance have already been investigated in nanomaterials, including the role of various perturbations such as temperature, pressure, doping, bias, and excitation wavelength on Raman spectral line shape parameters. Amongst different perturbations that cause a change in the spectral profile of Fano resonance, the gray area of wavelength dependence of Fano Raman line shape profiles has been least analysed in the literature. Moreover, the true signature of Fano resonance in nanoscaled systems, which is the wavelength dependence of Fano interaction, remains the least discussed. This review summarises the wavelength dependent correlation of Fano resonance and its effect on the Raman spectral line-shape parameters in some bulk materials, nanomaterials, and molecular systems involving heavily doped p-type crystalline silicon, 2-D MoS2, graphene, WS2, single walled carbon nanotubes, etc. A brief overview of Fano resonance in metamaterials and photonic systems is also provided.

Formation Processes of a Solid Electrolyte Interphase at a Silicon/Sulfide Electrolyte Interface in a Model All-Solid-State Li-Ion Battery

Understanding the electrochemical reactions at the interface between a Si anode and a solid sulfide electrolyte is essential in improving the cycle stabilities of Si anodes in all-solid-state batteries (ASSBs). Highly dense Si films with very low roughnesses of <1 nm were fabricated at room temperature via cathodic arc plasma deposition, which led to the formation of a Si/sulfide electrolyte model interface. Li (de)alloying through the model interface hardly occurred during the first cycle, whereas it proceeded stably in subsequent cycles. Hard X-ray photoelectron spectroscopy and neutron reflectometry directly revealed that the reduction or oxidation of the interfacial component or Li3PS4 electrolyte occurred during the first cycle. Consequently, an interfacial layer with a thickness of 13 nm and primarily composed of Li2S, SiS2, and P2S5 glasses was formed during the first cycle. The interfacial layer acted as a Li-conductive, electron-insulating solid electrolyte interphase (SEI) that provided reversible (de)lithiation. Our model interface directly demonstrates the electrochemical reaction processes at the Si/Li3PS4 interface and provides insights into the structures and electrochemical properties of SEIs to activate the (de)lithiation of Si anodes using a sulfide electrolyte.

Not all silicon quantum dots are equal: photostability of silicon quantum dots with and without a thick amorphous shell

Luminescent colloidal silicon quantum dots (SiQDs) are sustainable alternatives to metal-based QDs for various optical applications. While the materials are reliant on their photoluminescence efficiency, the relationship between the structure and photostability of SiQDs is yet to be well studied. An amorphous silicon (a-Si) shell was recently discovered in SiQDs prepared by thermally-processed silicon oxides. As a-Si is known as a source of defects upon UV irradiation, the disordered shell could potentially have an adverse effect on the optical properties of nanoparticles. Herein, the photostability of ∼5 nm diameter SiQDs with an amorphous shell was compared with that of over-etched SiQDs of equivalent dimensions that bore an a-Si shell of negligible thickness. An UV-induced degradation study was conducted by subjecting toluene solutions of SiQDs to 365 nm light-emitting diodes (LEDs) under an inert atmosphere for predetermined times up to 72 hours. The structure, composition, and optical responses of the exposed SiQDs were evaluated.

Nonlinear Temperature-Dependent Phonon Decay in Heavily Doped Silicon: Predominant Interferon-Mediated Cold Phonon Annihilation

A nonlinear Fano interaction has been reported here which is manifest in terms of a parabolic temperature-dependent phonon decay process observable in terms of a Raman spectral parameter. Temperature-dependent Raman spectroscopic studies have been carried out on heavily and moderately doped crystalline silicon to investigate the behavior of anharmonic phonon decay in semiconductor systems where Fano interactions are present inherently. Systematic study reveals that in heavily doped systems an interferon-mediated decay route exists for cold phonons present at lower temperatures (<475 K) where Fano coupling is stronger and dominates over the typical multiple-phonon decay process. On the other hand, the anharmonic phonon decay remains the predominant process at higher temperatures irrespective of the doping level. Temperature-dependent phonon self-energy has been calculated using experimentally observed Raman line-shape parameters to validate the fact that the nonlinear decay of phonons through interferon mediation is a thermodynamically favorable process at low temperatures.

Synthesizing Luminescent Carbon from Condensed Tobacco Smoke: Bio-Waste for Possible Bioimaging

Used cigarette filters, a waste material and a major source of land pollution, has been used as a raw material to study the nature of condensed tobacco smoke (tar) using microscopy, optical, IR, photoluminescence and Raman spectroscopy as well as X-ray diffraction and electron & fluorescence microscopy. The tar present in the cigarette filter bud has been used to synthesize luminescent low dimensional carbon using a simple methanol extraction technique. The collected material shows light blue emission under UV excitation with emission peak energy depending strongly on the excitation wavelength. Such excitation energy dependent emission is observed from the extract solution as well as dried film. Careful analysis has been carried out to understand its origin which reveals the presence of giant red-edge effect in the samples. A correlation between room temperature photoluminescence spectroscopy and fluorescence microscopy has also been carried out. Presence of amorphous phase carbon has been established using Raman spectroscopy and a quantum yield of more than 9% has been estimated which is moderately high in comparison with the one shown by carbon dots prepared by using other sources and can be used for bioimaging applications.

Application of Advanced Vibrational Spectroscopy in Revealing Critical Chemical Processes and Phenomena of Electrochemical Energy Storage and Conversion

The future of the energy industry and green transportation critically relies on exploration of high-performance, reliable, low-cost, and environmentally friendly energy storage and conversion materials. Understanding the chemical processes and phenomena involved in electrochemical energy storage and conversion is the premise of a revolutionary materials discovery. In this article, we review the recent advancements of application of state-of-the-art vibrational spectroscopic techniques in unraveling the nature of electrochemical energy, including bulk energy storage, dynamics of liquid electrolytes, interfacial processes, etc. Technique-wise, the review covers a wide range of spectroscopic methods, including classic vibrational spectroscopy (direct infrared absorption and Raman scattering), external field enhanced spectroscopy (surface enhanced Raman and IR, tip enhanced Raman, and near-field IR), and two-photon techniques (2D infrared absorption, stimulated Raman, and vibrational sum frequency generation). Finally, we provide perspectives on future directions in refining vibrational spectroscopy to contribute to the research frontier of electrochemical energy storage and conversion.

Raman area- and thermal- mapping studies of faceted nano-crystalline α-Fe2O3 thin films deposited by spray pyrolysis

Different advanced techniques including Raman area mapping and Raman thermal imaging has been used to investigate various properties of large area iron oxide thin films deposited by spray pyrolysis, on a large area of crystalline silicon substrates under controlled external parameters. Morphological studies reveal that the obtained films acquire lateral faceted crystalline structure of iron oxide. The Raman and SEM images, in unison, confirm the presence and large area distribution of the nano crystals of Fe2O3 phase. Thermal Raman imaging reveals that the obtained iron oxide thin films are robust and thus can be used for appropriate technological applications like electromagnetic shielding.

Parallel or interconnected pores’ formation through etchant selective silicon porosification

The effect of the oxidizer present in the etching solution on the surface morphology and microstructure obtained after porosifying a p-type silicon wafer using metal-assisted chemical etching was studied. The morphologies of Si wafers porosified using two different solutions, HF/H2O2 and HF/KMnO4, were compared to establish how either of the oxidizers (H2O2 or KMnO4) should be chosen depending on the desired application. A comparative study revealed that parallel pores with wire-like structures or interconnected pores with cheese-like structures can be obtained when H2O2 or KMnO4 are chosen, respectively. Careful analysis of the SEM images was carried out using ImageJ to establish that the samples prepared using KMnO4 are more porous due to aggressive etching. Additionally, experimental and theoretical Raman spectroscopic studies have been utilized to study the presence of low-dimensional Si nanostructures, which are a few nanometers in size, at the microscopic level in porosified silicon.

Pseudo-Anomalous Size-Dependent Electron-Phonon Interaction in Graded Energy Band: Solving the Fano Paradox

Quantum size effects on interferons (electron-phonon bound states), confined in fractal silicon (Si) nanostructures (NSs), have been studied by using Raman spectromicroscopy. A paradoxical size dependence of Fano parameters, estimated from Raman spectra, has been observed as a consequence of longitudinal variation of nanocrystallite size along the Si wires leading to local variations in the dopants' density which actually starts governing the Fano coupling, thus liberating the interferons to exhibit the typical quantum size effect. These interferons are more dominated by the effective reduction in dopants' density rather than the quantum confinement effect. Detailed experimental and theoretical Raman line shape analyses have been performed to solve the paradox by establishing that the increasing size effect actually is accompanied by receding Fano coupling due to the weakened electronic continuum. The latter has been validated by observing a consequent variation in the Raman signal from dopants which was found to be consistent with the above conclusion.

A Functionalized Silicate Adsorbent and Exploration of Its Adsorption Mechanism

Active silicate materials have good adsorption and passivation effects on heavy metal pollutants. The experimental conditions for the preparation of active silicate heavy metal adsorbent (ASHMA) and the adsorption of Cu(II) by ASHMA were investigated. The optimum preparation conditions of ASHMA were as follows: 200 mesh quartz sand as the raw material, NaOH as an activating agent, NaOH/quartz sand = 0.45 (mass fraction), and calcination at 600 °C for 60 min. Under these conditions, the active silicon content of the adsorbent was 22.38% and the utilization efficiency of NaOH reached 89.11%. The adsorption mechanism of Cu(II) on the ASHMA was analyzed by the Langmuir and Freundlich isotherm models, which provided fits of 0.99 and 0.98, respectively. The separation coefficient (R_L) and adsorption constant (n) showed that the adsorbent favored the adsorption of Cu(II), and the maximum adsorption capacity (Q_max) estimated by the Langmuir isotherm was higher than that of 300 mg/L. Furthermore, adsorption by ASHMA was a relatively rapid process, and adsorption equilibrium could be achieved in 1 min. The adsorbents were characterized by FT-IR and Raman spectroscopy. The results showed that the activating agent destroyed the crystal structure of the quartz sand under calcination, and formed Si-O-Na and Si-OH groups to realize activation. The experimental results revealed that the adsorption process involved the removal of Cu(II) by the formation of Si-O-Cu bonds on the surface of the adsorbent. The above results indicated that the adsorbent prepared from quartz sand had a good removal effect on Cu(II).

Si nanocrystal solution with stability for one year

Colloidal silicon nanocrystals (SiNCs) are a promising material for next-generation nanostructured devices. High-stability SiNC solutions are required for practical use as well as studies on the properties of SiNC. Here, we show a solution of SiNCs that was stable for one year without aggregation. The stable solution was synthesized by a facile process, i.e., pulsed laser ablation of a Si wafer in isopropyl alcohol (IPA). The long-term stability was due to a large ζ-potential of −50 mV from a SiNC passivation layer composed of oxygen, hydrogen, and alkane groups, according to the results of eight experiments and theoretical calculations. This passivation layer also resulted in good performance as an additive for a conductive polymer film. Namely, a 5-fold enhancement in carrier density was established by the addition of SiNCs into an organic conductive polymer, poly(3-dodecylthiophene), which is useful for solar cells. Furthermore, it was found that fresh (<1 day) and aged (4 months) SiNCs give the same enhancement. The long-term stability was attributed to a great repulsive energy in IPA, whose value was quantified as a function the distance between SiNCs.

A stable nanocrystal for one year without aggregation in a liquid is synthesized by one-step, one-pot, and one-hour process.