Reflectance Anisotropy Spectroscopy Research Articles

Anisotropic local-field effects of nanoparticles in plasmonic optics and magneto-optics

A theory of anisotropic optical local-field effects caused by resonantly polarizable small particles in multilayer polarizable media is developed. Considered is the model of a rectangular lattice of ellipsoidal nanoparticles with taking account of “image forces” at an interface in a layered medium. The lattice sums for anisotropic dipolar interactions are found using the Green’s function method in the quasi-point dipole approximation, and the effective polarizabilities of particles in a layer located near an interface are calculated self-consistently. The manifestation of an anisotropic local field of nanoparticles in optical radiation and propagation of evanescent waves responsible for optical near-field effects is investigated. Applications of the obtained results in the polar magneto-optical Kerr effect and reflectance anisotropy spectroscopy in propagating the polarized light along the normal to layers are considered. The resonant features in the spectra due to enhancement of the optical effects under excitation of surface (local) plasmons in nanoparticles of a noble metal are studied.

Anomalous double-layer step formation on Si(100) in hydrogen process ambient

We prepared Si(100) surfaces with anomalous atomic double-layer steps grown via chemical vapor deposition. Scanning tunneling microscopy resolved ${D}_{A}$-type steps, supported by low-energy electron diffraction, Fourier-transform infrared spectroscopy, and in situ reflection anisotropy spectroscopy, which enabled direct control of majority domain formation. We attribute the energetically unfavorable step structure to interaction of the surface with the H${}_{2}$ ambient, driving a dynamic step formation process governed by surface vacancy generation, diffusion, and annihilation at step edges.

Electron auger spectroscopy and reflectance anisotropy spectroscopy of monolayer nitride films on (001) surfaces of GaAs and GaSb crystals

The methods of electron Auger spectroscopy and reflectance anisotropy spectroscopy are used to study monolayer films of gallium nitride formed on the (001) surface of GaAs by chemical nitridization in hydrazine-sulfide solutions. It is found that the Auger signal for nitrogen N KLL from the nitride film is shifted to higher kinetic energies by ∼17.2 eV in comparison with its position for the same signal for a bulk GaN crystal. The observed shift is caused by the specific configuration of the valence orbitals of nitrogen atoms terminating the nitridized GaAs (001) surface. One of the valence orbitals for these atoms does not form a chemical bond and is occupied by an uncoupled pair of electrons. The suggested configuration is confirmed by the results of an analysis of the spectra of anisotropic reflectance from the nitridized GaAs (001) surface. Experiments with chemical nitridization of a GaSb surface have been performed for the first time. The Auger spectra for a nitridized GaSb (001) surface are found to be similar to those for a nitridized GaAs (001) surface. This is indicative of the similar character of chemical processes on these surfaces and the formation of a monolayer nitride film on the GaSb surface.

The nature and stability of the Au(110)/electrochemical interface produced by flame annealing

It is shown using reflection anisotropy spectroscopy (RAS) that following flame annealing and immersion in pure water, the Au(110) surface adopts a (1×1) structure and that this structure is preserved in a 0.1 M H2SO4 environment. The surface transforms to the (1×2) reconstruction following the application of a potential of 0.0 V versus SCE (a saturated calomel electrode). This surface is unstable and the RAS profile changes over periods of 15 min and 1 h in a manner which suggests that changes are occurring in the structure and distribution of steps. Over longer periods the RAS transforms towards a profile attributed to a surface associated with the specific adsorption of anions.

Structural, electronic and optical properties of the two isomers of Si(111)2×1

The Si(111)2×1 is one of the most studied surfaces. Its reconstruction is described by the Pandey model with a buckling of the topmost atoms. With relation to the sign of the buckling, there are two slightly different geometric structures (isomers), conventionally named positive buckling and negative buckling. STM measurements suggest that the positive buckling isomer is the stable configuration at room temperature, but a recent work, involving STS measurements, has shown the coexistence of both the isomers, at very low temperature, for highly n-doped Si(111)2×1 specimens. There is hence the necessity to deepen the study of the negative buckling isomer, almost completely neglected in literature especially for what concerns its optical properties. In this work we have studied the structural, electronic and optical properties of both the isomers of Si(111)2×1 within the Density Functional Theory in the Local Density Approximation. Our results show that the response to light of the two isomers is sizearly different; Reflectance Anisotropy Spectroscopy would be the right experimental tool to investigate the coexistence of the two isomers.

Growth mode and self-organization of LuPc2on Si(001)-2×1vicinal surfaces: An optical investigation

We report an investigation of the initial growth and of the self-organization of lutetium biphthalocyanine LuPc2 on Si(001)-2 x 1 vicinal surfaces. Using surface-sensitive optical spectroscopies, namely, surface-difference-reflectance spectroscopy (SDRS) and reflectance-anisotropy spectroscopy (RAS), together with local-probe microscopies, we are able to propose a scenario for the growth mode up to about 20 nm. We demonstrate that the growth mode initially proceeds through the formation of a wetting layer, followed by the formation of clusters whose sizes increase while keeping a constant shape in which the molecules are inclined. Moreover, the LuPc2 molecules are self-organized along the step edges, and we are able to estimate that about 30% to 100% of the molecules are aligned when considering that the molecules are tilted by 45 degrees to 63 degrees with respect to the surface normal.

In situ study of Ge(100) surfaces with tertiarybutylphosphine supply in vapor phase epitaxy ambient

GaInP nucleation on Ge(100) often starts by annealing of the Ge(100) substrates under supply of phosphorus precursors. However, the influence on the Ge surface is not well understood. Here, we studied vicinal Ge(100) surfaces annealed under tertiarybutylphosphine (TBP) supply in MOVPE by in situ reflection anisotropy spectroscopy (RAS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). While XPS reveals a P termination and the presence of carbon on the Ge surface, LEED patterns indicate a disordered surface probably due to by-products of the TBP pyrolysis. However, the TBP annealed Ge(100) surface exhibits a characteristic RA spectrum, which is related to the P termination. RAS allows us to in situ control phosphorus desorption dependent on temperature.

Controlling the formation of a monolayer of cytochrome P450 reductase onto Au surfaces

The conditions necessary for the formation of a monolayer and a bilayer of a mutated form (P499C) of human cytochrome P450 reductase on a Au(110)/electrolyte interface have been determined using a quartz crystal microbalance with dissipation, atomic force microscopy, and reflection anisotropy spectroscopy (RAS). The molecules adsorb through a Au-S linkage and, for the monolayer, adopt an ordered structure on the Au(110) substrate in which the optical axes of the dipoles contributing to the RAS signal are aligned roughly along the optical axes of the Au(110) substrate. Differences between the absorption spectrum of the molecules in a solution and the RAS profile of the adsorbed monolayer are attributed to surface order in the orientation of dipoles that contribute in the low energy region of the spectrum, a roughly vertical orientation on the surface of the long axes of the isoalloxazine rings and the lack of any preferred orientation in the molecular structure of the dipoles in the aromatic amino acids. Our studies establish an important proof of principle for immobilizing large biological macromolecules to gold surfaces. This opens up detailed studies of the dynamics of biological macromolecules by RAS, which have general applications in studies of biological redox chemistry that are coupled to protein dynamics.

Studying structure of thin copper phthalocyanine films by reflectance anisotropy spectroscopy

The structure of thin molecular films of copper phthalocyanine (CuPc) on crystalline GaAs(001) and glass substrates has been studied by the method of reflectance anisotropy spectroscopy. It is shown that, using this method, it is possible to determine the orientational anisotropy in CuPc layers. It is established that the GaAs crystal surface produces orienting action on the molecular structure of CuPc films, while a glass substrate does not exhibit this effect. A model of the anisotropic reflection of light is considered that provides satisfactory description of the orientation-related features in the spectra of thin CuPc films as dependent on their thicknesses.

Ge(100) surfaces prepared in vapor phase epitaxy process ambient

AbstractGe(100) substrates essential for subsequent III–V integration were studied in the hydrogen ambient of a metalorganic vapor phase epitaxy reactor. We confirmed thermal oxide and carbon removal by X‐ray photoelectron spectroscopy, characterized the (2 × 1)/(1 × 2) surface reconstruction by low energy electron diffraction, and employed reflection anisotropy spectroscopy for optical in situ analysis. Our Ge(100) spectra de‐ viate from reference data of clean surfaces prepared in ultra‐high vacuum, most probably due to the presence of hydrogen bonds. The observation was correlated with Fourier‐transform infrared spectroscopy showing coupled H–Ge–Ge–H stretch modes associated with a monohydride termination of the Ge(100) surface. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pyrrole adsorption on GaAs(001)-c(4×4): The role of surface defects

We report on the adsorption mechanism of pyrrole on the As-rich GaAs$(001)$-$c(4\ifmmode\times\else\texttimes\fi{}4)$ surface. We present measurements with reflectance anisotropy spectroscopy, x-ray photoelectron spectroscopy, and high-resolution scanning tunneling microscopy and spectroscopy (STS) and our experiments show that the surface defects play a crucial role for the adsorption configuration. We found that pyrrole physisorbs on the ideal $c(4\ifmmode\times\else\texttimes\fi{}4)$ surface and does not form any covalent bonds to the surface atoms. At submonolayer coverage, however, we found evidence for single molecules that are chemisorbed at surface defects. We could identify the molecular electronic states with single molecule STS and could distinguish the chemisorbed and physisorbed molecular species in the STS spectra.

The linear and nonlinear optical response of native‐oxide covered rippled Si templates with nanoscale periodicity

AbstractFabrication of dense arrays of nanowires by growth on nanostructured substrates appears to be a promising approach for producing functional nanoscale devices. Ion beam irradiation under carefully controlled conditions produces self‐organized ripple patterns on silicon substrates, on which Ag nanowire arrays with nanoscale periodicity have been grown successfully. Here, the linear and nonlinear optical response from native‐oxide‐covered Si(001) templates, with ripple periodicity between ∼20 and ∼50 nm, is reported. Reflection anisotropy spectroscopy (RAS) shows a small, broad peak at ∼2.5 eV of ∼1 × 10−3 amplitude. The RAS response decreases as the periodicity of the ripple structure increases. The nonlinear second‐harmonic (SH) response from the samples was also measured, using unamplified femtosecond excitation at 800 nm, by rotating the linear polarized input and detecting either the s‐ or p‐polarized SH output. A decrease in response with increased ripple structure periodicity was observed for the p‐in/p‐out configuration, when the plane of incidence was orthogonal to the average ripple orientation. Possible origins of the response and future experiments are discussed.

Anisotropic optical response of elongated Pb islands in the infrared spectral region

AbstractLead forms elongated islands when grown on vicinal Si(111) surfaces. Polarized infrared transmittance studies have shown a strong anisotropic optical response associated with antenna‐like plasmonic resonances, whose spectral position in the region of 0.25 eV is sensitive to the length of the islands. Reflection anisotropy spectroscopy (RAS) using a photoelastic modulator (PEM) should be more sensitive to such optical anisotropies, but becomes difficult below ∼0.5 eV for instrumental reasons. Measurements of the anisotropic response, in reflectance, of Pb islands grown on Si(557)‐5 × 1–Au are extended down to ∼0.12 eV by combining sample rotation with tuneable femtosecond laser irradiation from a difference frequency generator. The extended RAS spectral range allows the full anisotropic nanoparticle plasmon‐polarition optical response in the surface plane to be explored for this type of material system. Reasonable agreement with a simple nanoantenna model of the resonance maximum is obtained, but calculating the full line profile of the RAS response of supported nanoparticles remains challenging.

The self assembly of thymine at Au(110)/liquid interfaces

AbstractWe show that thymine self‐assembles into an ordered structure when adsorbed at a Au(110)/liquid interface. Reflection anisotropy spectroscopy (RAS) shows that as found for cytosine and adenine the adsorbed thymine molecules are oriented essentially vertically on the Au(110) surface with the molecule aligned along one of the principal axes of the Au(110) surface. Simulations of the RA spectra to an empirical model indicates that as found for adsorbed cytosine and adenine, thymine is aligned along the [$1{\bar {1}}0$] direction on the Au(110) surface.

Self-assembled broadband plasmonic nanoparticle arrays for sensing applications

Highly ordered noble metal nanoparticle (NP) arrays are produced using a glancing angle deposition on stepped substrates. The versatility of the technique is demonstrated by depositing different metals, resulting in shifts of the resonance positions. The behaviour of the NP arrays grown is predicted by a dipolar model, and it is measured using reflectance anisotropy spectroscopy (RAS). Fine tuning of the resonances can be finally realised by selecting the deposition parameters. The combined application of both RAS and deposition at glancing angles provides a unique tool to grow NP arrays with the tunable plasmonic resonances in the entire visible range.

In-Situ Monitoring the Growth of Polypyrrole Films at Liquid/Solid Interface Using a Combination of Polarized Infrared Spectroscopy and Reflectance Anisotropy Spectroscopy

Polarized infrared reflection measurements in an ellipsometric set-up and reflectance anisotropy spectroscopy (RAS) were used for the first time for combined in-situ characterization of the electrochemical growth (potentiostatic pulse method) of thin polypyrrole (PPy) films on Si(110). Both methods can monitor material and thickness related changes during deposition and are shown as sensitive tools for studying time development of PPy growth under varying potential. In detail polarized infrared spectroscopy delivered information on the chemical structure of the as-deposited film and RAS gave information on the quality of the interface and deposition rate. The identified polymerisation mechanism is in agreement with literature and valid for all times of deposition. For deposited films rough surfaces and partially inhomogeneous thicknesses (35 ± 10 nm and 120 ± 10 nm) were found. At present state it is not possible to simulate the in-situ RAS and IR-spectra in simple coherent optical layer models.

Multi-phase model for reflection anisotropy spectra of copper phthalocyanine films on anisotropic silicon substrates

Reflection anisotropy spectroscopy (RAS) and spectroscopic ellipsometry (SE) have extensively been applied to inorganic and organic structures and, because of the similarity of these two techniques, the evaluation procedure of RA spectra can be performed in a similar way as for SE. Especially for thin films, RA spectra are often strongly enhanced in the spectral region where optical interference occurs and the superposition of interference features can lead to an incorrect interpretation of RA spectra. By simulation of the origin of each RAS feature it is possible to distinguish interference from true film anisotropy. Also, the effect of surface roughness on the RA spectra is discussed.

Fundamental differences in model cell-surface polysaccharides revealed by complementary optical and spectroscopic techniques

The propensity of two classes of polysaccharides, charged heparins and uncharged dextrans serving as models of those found in the extracellular matrix, to form ordered arrangements in solids and in solution, were explored employing polarising optical microscopy and reflection anisotropy spectroscopy. The fundamental modes of molecular vibration in the solid state, which relate to the occupancy of conformational states at ambient temperatures, were also investigated using terahertz (1–15 THz) spectroscopy on purpose built beam lines at SRS, Daresbury and ANKA, Karlsruhe. In the solid state, evidence for the anisotropic arrangement of ion centres was observed for the Na, Ca and Mg ion forms of heparin by RAS but, this was absent in aqueous solution at 100 mg ml−1 and the absence of molecular ordering in solution was confirmed by polarised optical microscopy. The ion charged heparin specimens showed very strong absorption of THz radiation indicating the accessibility of unoccupied higher energy conformational states in these materials whereas the uncharged dextrans showed only weak THz absorption, implying that the majority of conformational states are occupied and there are few unoccupied higher energy states in these materials. This suggests the two classes of material will have very different conformational properties, particularly in response to temperature changes in the extracellular matrix.

反射各向异性谱在线监测852 nm半导体激光器AlGaInAs/AlGaAs量子阱的MOCVD外延生长

反射各向异性谱在线监测852 nm半导体激光器AlGaInAs/AlGaAs量子阱的MOCVD外延生长

Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays

Self-assembled silver nanoparticle (NP) arrays were produced by deposition at glancing angles on transparent stepped Al2O3 templates. The evolution of the plasmonic resonances has been monitored using reflection anisotropy spectroscopy (RAS) during growth. It is demonstrated that the morphology of the array can be tailored by changing the template structure, resulting in a large tunability of the optical resonances. In order to extract detailed information on the origin of the measured dichroic response of the system, a model based on dipolar interactions has been developed and the effect of tarnishing and morphological dispersion addressed.