Reflectance Anisotropy Spectroscopy Research Articles

Operando Investigation of Al Plating Regimes on HOPG in [EMImCl]:AlCl3 by Electrochemical Reflection Anisotropy Spectroscopy

Rechargeable aluminium batteries show promise as next‐generation systems with a more abundant material base than lithium technology. However, the stable native oxide on top of aluminium metal electrodes leads to poor cell performance. Graphite, on the other hand, is a so far rarely investigated alternative that can be used as both the anode and cathode. Here, metallic aluminium is deposited at the anode, while AlCl4– is intercalated at the cathode. For both cases, understanding the electrode–electrolyte interface is crucial for improving the performance of the battery. In this work, we use reflection anisotropy spectroscopy to study the evolution of the interface under applied potentials. We find that the cathode exhibits an irreversible swelling of the top‐most graphite layer due to AlCl4– intercalation as well as the formation of an SEI during the first voltammetry cycle. On the anode, the electrodeposition of aluminium is initially well‐ordered. However, the evolution of the surface morphology depends on the applied potential, with island‐like growth at less cathodic potentials, and layer‐by‐layer growth at more anodic potentials. With the optical operando spectroscopy, we can follow these qualitatively different plating and stripping regimes in a time‐resolved manner.

In Situ Studies on the Influence of Surface Symmetry on the Growth of MoSe2 Monolayer on Sapphire Using Reflectance Anisotropy Spectroscopy and Differential Reflectance Spectroscopy

The surface symmetry of the substrate plays an important role in the epitaxial high-quality growth of 2D materials; however, in-depth and in situ studies on these materials during growth are still limited due to the lack of effective in situ monitoring approaches. In this work, taking the growth of MoSe2 as an example, the distinct growth processes on Al2O3 (112¯0) and Al2O3 (0001) are revealed by parallel monitoring using in situ reflectance anisotropy spectroscopy (RAS) and differential reflectance spectroscopy (DRS), respectively, highlighting the dominant role of the surface symmetry. In our previous study, we found that the RAS signal of MoSe2 grown on Al2O3 (112¯0) initially increased and decreased ultimately to the magnitude of bare Al2O3 (112¯0) when the first layer of MoSe2 was fully merged, which is herein verified by the complementary DRS measurement that is directly related to the film coverage. Consequently, the changing rate of reflectance anisotropy (RA) intensity at 2.5 eV is well matched with the dynamic changes in differential reflectance (DR) intensity. Moreover, the surface-dominated uniform orientation of MoSe2 islands at various stages determined by RAS was further investigated by low-energy electron diffraction (LEED) and atomic force microscopy (AFM). By contrast, the RAS signal of MoSe2 grown on Al2O3 (0001) remains at zero during the whole growth, implying that the discontinuous MoSe2 islands have no preferential orientations. This work demonstrates that the combination of in situ RAS and DRS can provide valuable insights into the growth of unidirectional aligned islands and help optimize the fabrication process for single-crystal transition metal dichalcogenide (TMDC) monolayers.

A versatile system for the growth of porphyrin films via electrospray and molecular sublimation in vacuum and their multi-technique characterization.

We present a system for the growth of molecular films in vacuum that exhibits high versatility with respect to the choice of molecular species. These can be either evaporated from powders or injected from solutions using an electrospray system, making it possible to handle particularly large and/or fragile molecules in a controlled environment. The apparatus is equipped with a reflectance anisotropy spectroscopy system for the in situ characterization of the optical response of the films and can be directly connected to a photoelectron spectrometer without breaking the vacuum. The system is conceived for the study and characterization of porphyrin films. Here, to showcase the range of possible analyses allowed by the experimental setup and test the operation of the system, novel results are provided on electrospray deposition on highly oriented pyrolytic graphite of Zn tetraphenyl porphyrins and Zn proto porphyrins, the latter featuring fragile side groups that make deposition from solution more attractive. In situ characterization is complemented by ex situ atomic force microscopy. Thanks to this multi-technique approach, changes in the film morphology and spectroscopic response are detected and directly related to the choice of the molecular moiety and growth method.

Bismuth Ordering and Optical Anisotropy in GaAsBi Alloys

Reflectance anisotropy spectroscopy (RAS) is applied to investigate GaAsBi samples grown by molecular beam epitaxy on (001)‐oriented GaAs substrates with GaAs or InGaAs buffer layers, resulting in nearly lattice‐matched or compressive strain conditions, with Bi concentration in the alloy in the range 2–5%. These new samples allow to bridge the gap in the Bi concentration values of previous RAS experiments (C. Goletti et al., Appl. Phys. Lett. 2022, 120, 031902), confirming the [110]‐polarized Bi‐related anisotropy in optical spectra below 3 eV and the linear dependence of its amplitude on Bi concentration. The characterization of the grown GaAsBi samples by X‐Ray diffraction and transmission electron microscopy clearly demonstrates the presence of CuPt‐like ordering in the bulk. CuPt structure is the primary origin of the optical anisotropy measured by RAS and by polarized photoluminescence, due to the anisotropic strain produced in the bulk crystal lattice. The lineshape of the RAS spectra above 3 eV, with its overall and characteristic positive convexity, confirms this conclusion.

Circular Dichroism Reflectance Anisotropy of Chiral Atomically Thin Films

Recently, a technical modification of a Reflectance Anisotropy Spectroscopy (RAS) spectrometer has been proposed to investigate the circular dichroism (CD) of samples instead of the normally studied linear dichroism. CD-RAS measures the anisotropy of the optical properties of a sample under right-handed and left-handed circularly polarized light. Here, we present the application of CD-RAS to measure the circular dichroism of a twisted bilayer of graphene, purposely prepared as a possible substrate for the adsorption of thin molecular layers, in air, in liquid or in a vacuum. This result demonstrates the performance of the apparatus and shows interesting perspectives for the investigation of chiral organic assemblies deposited in solid film.

Surface structure of MOVPE-prepared As-modified Si(100) substrates

In the pursuit of high-efficiency tandem devices for solar energy conversion based on III-V-semiconductors, low-defect III-V nucleation on Si(100) substrates is essential. Here, hydrogen and arsenic are key ingredients in all growth processes with respect to industrially scalable metalorganic vapor phase epitaxy. Our study provides insight into Si(100) surface preparation for the initial stage of III-V nucleation. The samples investigated, prepared on substrates with different offcut angles, show single domain surfaces consisting of rows of preferentially buckled dimers. Low energy electron diffraction and reflection anisotropy spectroscopy confirm well-defined (1 × 2)/(2 × 1) majority domains. Fourier-transform infrared spectroscopy revealed hydrogen bonding to the surface dimers, while no impurities were found by XPS. Density functional theory calculations support the experimental results and reveal a novel surface motif of H-passivated Si-As mixed dimers.

Strain mapping in amorphous germanium thin films with scanning reflectance anisotropy microscopy

Strain imaging is a critical aspect in the design and characterization of opto-electronics, microelectronics, flexible electronics, and on-chip photonics. However, strain mapping techniques are often material specific and strain measurements in amorphous materials remain a challenge. Here, we demonstrate strain mapping and optical characterization of an amorphous semiconductor using scanning reflectance anisotropy microscopy. Using reflection anisotropy spectroscopy and finite element simulations on evaporated amorphous germanium films, we showcase the strain sensitivity of the ellipsometric parameters. We demonstrate nondestructive mapping for simple and complex strain states in amorphous systems. The sub-degree phase and amplitude sensitivity of the microscope is able to determine strain states on the order of 10−3.

Interfacial optical anisotropy of Cu(110) electrochemistry using singular value decomposition

Understanding surface/interface states (SIs) in metal–electrolyte environments is crucial for diverse scientific and technological fields. This work investigates the reflectance anisotropy spectroscopy (RAS) of Cu(110) in HCl solution to gain deeper insights. Using the singular-value decomposition (SVD) method, we extract three distinct RAS features that track electrochemical changes. We argue that these features stem from surface-induced bulk optical anisotropies and Cl− adsorption. Their correlation with cyclic voltammetry (CV) further supports this interpretation. Specifically, we link these lineshapes to HCl adsorption, and microscale surface roughness resembling a lamellar grating, as verified by electrochemical scanning tunneling microscopy (EC-STM), and another one correlated to ab initio calculations bearing transitions between bulk and surface states of Cu(110). Our findings shed light on SIs behavior in metal–electrolyte interfaces. The combination of SVD and RAS establishes a robust methodology for studying metallic surfaces in electrochemical systems, enabling comparisons with ab initio predictions.

The relevance of structural variability in the time-domain for computational reflection anisotropy spectroscopy at solid–liquid interfaces

In electrochemistry, reactions and charge-transfer are to a large extent determined by the atomistic structure of the solid–liquid interface. Yet due to the presence of the liquid electrolyte, many surface-science methods cannot be applied here. Hence, the exact microscopic structure that is present under operating conditions often remains unknown. Reflection anisotropy spectroscopy (RAS) is one of the few techniques that allow for an in operando investigation of the structure of solid–liquid interfaces. However, an interpretation of RAS data on the atomistic scale can only be obtained by comparison to computational spectroscopy. While the number of computational RAS studies related to electrochemical systems is currently still limited, those studies so far have not taken into account the dynamic nature of the solid–liquid interface. In this work, we investigate the temporal evolution of the spectroscopic response of the Au(110) missing row reconstruction in contact with water by combining ab initio molecular dynamics with computational spectroscopy. Our results show significant changes in the time evolution of the RA spectra, in particular providing an explanation for the typically observed differences in intensity when comparing theory and experiment. Moreover, these findings point to the importance of structural surface/interface variability while at the same time emphasising the potential of RAS for probing these dynamic interfaces.

Creation and plasmon anisotropy spectroscopy of wedge-shaped gold nanoclusters conditioned by GaAs(001) surface

Wedge-shaped nanoclusters of gold (Au2Ga) are fabricated by high-temperature annealing of a gold nanofilm deposited onto (001) surface of p-doped GaAs crystal with a very thin overlayer of natural oxide. The data of diagnostics confirm the presence in prepared Au/p-GaAs(001) structures of the wedge-shaped Au-intermetallic nanoclusters elongated in [110] direction at GaAs surface. A crystallographic model of the wedge-shaped Au (Au2Ga) nanoclusters conditioned by GaAs(001) surface is discussed in relation with their physicochemical nature. Anisotropic plasmons localized on equally oriented Au-based nanoclusters are detected optically with the reflectance anisotropy spectroscopy and investigated thoroughly with the spectroscopy of polarized light reflection. It is proved experimentally and theoretically that the inhomogeneously broadened infrared spectral peak at the energy about 1.1 eV is associated with plasmons polarized along the wedge-shaped clusters in [110] crystal direction. Another peak - at the energy approximately of 1.8 eV - is due to plasmons having orthogonal polarization in direction [11¯0].

Identification and control of crystalline nuclei facets imparting to the breaking of symmetry selection rules of optical phonon modes in GaP/Si (001)

The complex interfacial structural issues associated with hetero-polar epitaxial integration of GaP on Si(001) are resolved by improving the kinetics of gallium and phosphorous adatoms during the initial nucleation process. This is achieved through the use of Raman spectroscopy as a feedback mechanism. The peak intensity ratio of forbidden (transverse optical)-to-allowed (longitudinal optical) phonons of GaP under unpolarized Raman scattering is contemplated as an initial guide to ascertain the relative density of defect-mediated non-(001) GaP facets. The azimuthal-angle dependent polarized Raman spectroscopy is then applied to identify and quantify the nucleation of higher-index non-(001) ({111} and {112}) faceted islands in the nucleation layer and their propagation in the overgrown structures. The nucleation of GaP adatoms on the Silicon surface at ∼525 °C leads to a smooth surface and substantially suppresses the defect-mediated facets. The optimal mobility and controlled adsorption of adatoms of Ga and P on the substrate surface under intermediate nucleation kinetics foster the charge-neutral interface, and hence the defect-suppressed two-dimensional layer growth of GaP. The proposed Raman spectroscopy methodology emerges as a rapid and economical alternative to existing in situ reflection anisotropy spectroscopy and high-resolution transmission electron microscopy techniques for structural identification. This method of growth optimization can also be applied to other III-V polar semiconductors to achieve high-quality and efficient monolithic integration of devices on existing Si technology.

In Situ Monitoring of the Al(110)‐[EMImCl] : AlCl3 Interface by Reflection Anisotropy Spectroscopy

AbstractRecently, Al‐batteries (AlBs) have become promising candidates for post‐lithium batteries, with [EMImCl] : AlCl3 (1 : 1.5) as the most commonly used electrolyte. However, progress in the development of AlBs is currently hindered by the lack of understanding of its solid‐electrolyte interface. Monitoring the structure of this interface under operational conditions by complementary spectroscopy could help to identify and overcome bottlenecks of the system. Reflection anisotropy spectroscopy (RAS), an optical in situ technique, provides access to physical and chemical properties of electrochemical interfaces on an atomistic level. Herein, we report the first example of RAS as an in situ characterization technique for non‐aqueous battery systems, investigating an Al(110)‐based model system. During chemical pre‐treatment in [EMImCl] : AlCl3, the Al(110) surface passivation film is modified. The oxide film is partially etched while an inhomogeneous passivation layer forms, increasing the surface roughness. Upon electrochemical cycling, applied potential‐dependent oscillations of the anisotropy are observed and demonstrate the applicability of RAS to monitor phenomena such as plating/stripping and surface passivation in real‐time.

Understanding growth‐faulted GaAsBi samples by Reflectance Anisotropy Spectroscopy.

Reflectance Anisotropy Spectroscopy (RAS) has been recently applied to Molecular Beam Epitaxy (MBE) of GaAsBi alloys. The presence of the voluminous Bi atoms induces strain in the crystal lattice, modifying the substrate symmetry of the centrosymmetric GaAs(001) and then producing clear signatures in the anisotropy spectra of the GaAsBi layers. In particular, the amplitude of the characteristic structure measured below 2.5 eV has been shown to be directly related to the Bi concentration, while the sign has a meaningful correlation to the strain conditions present in the sample. In this paper, we extend the application of RAS to “faulted” GaAsBi samples, i.e. samples that after growth result not satisfactory for research because of problems or errors risen during the complex deposition process (wrong growth temperature, excess or deficiency of Bi flux, formation of dislocations, etc.). We demonstrate that also in these cases RAS offers a useful characterization of the sample, possibly (if RAS runs during the deposition) singling out the occurrence of faults eventuality, and thus validating its potential applicability to an all‐optical real time monitoring of the deposition process.This article is protected by copyright. All rights reserved.

Reflectance anisotropies of polycrystalline Ce[formula omitted] Gd[formula omitted] O[formula omitted]/ Si[formula omitted] interfaces

Ce1−xGdxO2−x/2 thin films were deposited by spin coating on oxidized Si(001) substrates. Two strain regimes are observed by μ− Raman spectroscopy: for x< 0.1 the films are tensile strained, whereas a strain component of the wave number shift of −71.2x cm−1 compensates with a 32.6x cm−1 bond component for x> 0.1. Reflectance anisotropy spectroscopy (RAS) measurements were carried out on these films in which interface-related optical anisotropies signatures appearing in the 1.5 eV to 3.6 eV photon energy range are found dependent on the incorporation of Gd in the lattice. To explain the observed RAS signatures with a complex reflectance three-phase model, we resorted to spectroscopic ellipsometry (SE) to retrieve the interface optical anisotropies (IOA) related to the changes in refractive index, <Δn>, between interface formed by both the oxidized Si(001) and polycrystalline CeO2:Gd films, and to assess the CeO2 energy gap variation from 3.2 to 2.8 eV in the 0 <x< 0.4 composition range. This study renders the combination of RAS, Raman, and SE versatile tools to optimize the growth parameters during the fabrication of devices based on polycrystalline CeO2 on a quantitative basis.

Exploring the range of applicability of anisotropic optical detection in axially coordinated supramolecular structures

Ensuring the highest accuracy in determining the molecular assembling and the preservation of the chemical and physical properties of the molecules during the growth of organic layers requires a real-time monitoring of film formation. In this respect, optical techniques are preferred since they result in minimum organic film damage. Among these techniques, Reflectance Anisotropy Spectroscopy (RAS) proved to be the one with the highest sensitivity due to the development of intrinsic anisotropies in the optical response of molecular films, where molecular packing is driven by Van der Waals interactions. Recently, we proposed an original strategy to enable the growth of organic films via molecular self-assembly through highly directional coordination bonds. Herein, we consider a straightforward supramolecular structure employing axial bonds, based on the 1:1 assembly of a CoII porphyrin (CoTPP) and a linear ligand (DPNDI). This system is characterized by a rather isotropic structure that might, in principle, result in a weak RAS signal. Therefore, in the present work, we critically assess the range of applicability of such a spectroscopy. A number of other surface science techniques, including Low Energy Electron Diffraction (LEED), photoemission spectroscopies (PES and IPES) and Atomic Force Microscopy (AFM) is employed to fully characterize the axially coordinated molecular film.

Chiral Porphyrin Assemblies Investigated by a Modified Reflectance Anisotropy Spectroscopy Spectrometer.

Reflectance anisotropy spectroscopy (RAS) has been largely used to investigate organic compounds: Langmuir-Blodgett and Langmuir-Schaeffer layers, the organic molecular beam epitaxy growth in situ and in real time, thin and ultrathin organic films exposed to volatiles, in ultra-high vacuum (UHV), in controlled atmosphere and even in liquid. In all these cases, porphyrins and porphyrin-related compounds have often been used, taking advantage of the peculiar characteristics of RAS with respect to other techniques. The technical modification of a RAS spectrometer (CD-RAS: circular dichroism RAS) allows us to investigate the circular dichroism of samples instead of the normally studied linear dichroism: CD-RAS measures (in transmission mode) the anisotropy of the optical properties of a sample under right and left circularly polarized light. Although commercial spectrometers exist to measure the circular dichroism of substances, the "open structure" of this new spectrometer and its higher flexibility in design makes it possible to couple it with UHV systems or other experimental configurations. The importance of chirality in the development of organic materials (from solutions to the solid state, as thin layers deposited-in liquid or in vacuum-on transparent substrates) could open interesting possibilities to a development in the investigation of the chirality of organic and biological layers. In this manuscript, after the detailed explanation of the CD-RAS technique, some calibration tests with chiral porphyrin assemblies in solution or deposited in solid film are reported to demonstrate the quality of the results, comparing curves obtained with CD-RAS and a commercial spectrometer.

Experimental and Computational Aspects of Electrochemical Reflection Anisotropy Spectroscopy: A Review

AbstractElectrode/electrolyte interfaces play a crucial role in many electrochemical energy conversion and storage technologies. Hence, a deep understanding of the interfacial structure, energetic alignment and processes is of high relevance and has triggered the development of a number of in situ and operando techniques. One approach for gaining information about the change in surface chemistry and structure on an atomic scale is reflection anisotropy spectroscopy (RAS). This review presents and discusses the continuing effort to develop RAS as an in situ optical probe for solid‐liquid interfaces under applied potentials. Experimental and computational basic principles are presented and key challenges of electrochemical RAS are highlighted. Furthermore, we exemplarily demonstrate the potential of the method for spectroelectrochemistry, focusing on indium phosphide‐ and gold‐aqueous electrolyte interfaces as exemplary case studies, and outline research directions for battery systems.

Fourier Transform Infrared Reflection Anisotropy Spectroscopy of Semiconductor Crystals and Structures: Development and Application in the Mid-Infrared.

A new method of reflection anisotropy spectroscopy (RAS) with increased mid-IR efficiency owing to the use of a Fourier transform infrared (FT-IR) spectrometer has been developed. An optical setup was implemented using a photoelastic modulator (PEM) to modulate the direction of linear polarization of the probe beam originating from the Michelson interferometer. An original measurement algorithm was proposed to eliminate the influence of spectral inhomogeneity of the PEM efficiency on the obtained spectra using appropriate calibration. It was shown that to preserve the sign of the RAS signal, it is necessary to use a specialized procedure for phase correction of the interferogram registered by the FT-IR spectrometer. In the visible range, good agreement was confirmed between the obtained reflection anisotropy (RA) spectra of a semiconductor crystal and the results of independent measurements using a conventional diffraction-grating spectrometer-based setup. The RA spectrum of a III-V semiconductor heterostructure in the mid-infrared range (λ up to 8 µm) is demonstrated. Application of the developed FT-IR RAS method to layered black phosphorus has enabled characterization of anisotropic interband transitions in this graphene-like semiconductor crystal.

Chemical modification of the GaP(0 0 1) surface electric field with sulfide solutions

The surface electric field in GaP(001) passivated with aqueous and alcoholic sulfide solutions is studied by reflectance anisotropy spectroscopy using data on the surface chemical and electronic structure obtained by synchrotron-radiation X-ray photoelectron spectroscopy. Treatment of n-GaP(001) surface with a sulfide solution decreases the surface electric field depending on sulfide solution composition. In particular, passivation with the solution of ammonium sulfide in 2-propyl alcohol turns the total surface electric field almost to zero, while passivation with the aqueous ammonium sulfide solution reduces the surface electric field in a lesser extent. On the other hand, sulfide treatment of p-GaP(001) surface enhances the surface electric field dramatically and this enhancement is nearly independent of the sulfide solution composition. Such changes in the surface electric field can be related to the modification of the dipole associated with chemical bonds in the passivating layer.

Reflectance anisotropy spectroscopy (RAS) for in-situ identification of roughness morphologies evolving during reactive ion etching (RIE)

When using reactive ion etching (RIE) as a semiconductor processing tool, techniques to recognize upcoming roughness and to identify the roughness morphologies at the etch front in-situ and in real-time can significantly increase yield and thus productivity of the technological processes. Here, reflectance anisotropy spectroscopy (RAS) is applied in combination with sophisticated statistical data analyses to recognize or even define the possible roughness morphologies on RIE-etched monocrystalline substrates in real time. We exemplarily use GaSb, but our approach will also work for other III/V semiconductors and even silicon, as long as the material is monocrystalline. In the first step of the evaluation of RAS spectra, principal component analysis (PCA) and linear discriminant analysis (LDA) are performed. The algorithms are capable of calculating eigenvectors that produce the maximum variance from the spectroscopic data, thus allowing for a reduction of dimensionality of the dataset and for a distinction of different roughness morphologies. In a second step, consulting a base of earlier RAS data from former own samples, such results can be employed to identify the current surface morphology based on its spectra in-situ and in real-time.