Surface Differential Reflectivity Research Articles

Change of composition and surface plasmon resonance of Pd/Au core/shell nanoparticles triggered by CO adsorption.

Controlling composition and plasmonic response of bimetallic nanoparticles (NPs) is of great relevance to tune their catalytic activity. Herein, we demonstrate reversible composition and plasmonic response transitions from a core/shell to a bimetallic alloyed palladium/gold NP triggered by CO adsorption and sample temperature. The use of self-organized growth on alumina template film allows scrutinizing the impact of core size and shell thickness onto NP geometry and plasmonic response. Topography, molecular adsorption, and plasmonic response are addressed by scanning tunneling microscopy, vibrational sum frequency generation (SFG) spectroscopy, and surface differential reflectance spectroscopy, respectively. Modeling CO dipolar interaction and optical reflectivity corroborate the experimental findings. We demonstrate that probing CO adsorption sites by SFG is a remarkably sensitive and relevant method to investigate shell composition and follow in real-time Pd atom migration between the core and the shell. Pd-Au alloying is limited to the first two monolayers of the shell and no plasmonic response is found, while for a thicker shell, a plasmonic response is observed, concomitant with a lower Pd concentration in the shell. Above 10-4mbar, at room temperature, CO adsorption triggers the shell restructuration, forming a Pd-Au alloy that weakens the plasmonic response via Pd migration from the core to the shell. NP annealing at 550K, after pumping CO, leads to the desorption of remaining CO and gives enough mobility for Pd to migrate back inside the core and recover a pure gold shell with its original plasmonic response. This work demonstrates that surface stoichiometry and plasmonic response can be tuned by using CO adsorption and NP annealing.

A microprocessor-aided platform enabling surface differential reflectivity and reflectance anisotropy spectroscopy

Surface differential reflectivity (SDR) and reflectance anisotropy spectroscopy (RAS) [sometimes known as reflectance difference spectroscopy] are two well-known optical spectroscopies used in the investigation of surfaces and interfaces. Their adaptability on different experimental conditions (vacuum, controlled atmosphere and liquid environment) allows for the investigation not only of surface states and/or ultra-thin films but also of more complex interfaces. In these circumstances, the analysis of the sample with both techniques is decisive in view of obtaining a correct picture of the sample optical properties. In this work, we show a microelectronic hardware solution useful to control both a SDR and a RAS apparatus. We describe an electronic architecture that can be easily replicated, and we applied it to a representative sample where the interpretation of the optical properties requires an analysis by both SDR and RAS.Graphic abstract

Theoretical Studies of Sum Optical Properties for InAs (001) by Surface Differential Reflectivity

The real and imaginary part of complex dielectric constant for InAs(001) by adsorption of oxsagen atoms has been calculated, using numerical analysis method (non-linear least square fitting). As a result a mathematical model built-up and the final result show a fairly good agreement with other genuine published works.

Porphycene Protonation: A Fast and Reversible Reaction Enabling Optical Transduction for Acid Sensing

AbstractNano‐sensor materials, especially if they target biological purposes, require fine control and tuning of the properties at the molecular scale when intramolecular properties have to be exploited, for example, in optical sensors. By means of optical spectroscopies (namely, spectrophotometry, spectrofluorimetry, and surface differential reflectivity), we demonstrate that thin films of porphycene show a spectacular chromatic change when exposed to acid vapors (specifically to hydrochloric acid). After exposure, the original optical properties of the porphycene films are recovered within a few seconds and without any thermal annealing. In addition, since clear spectroscopic signatures are observed both in absorption and in reflectivity, the parent porphycene is shown to be suitable even for deposition of films onto opaque substrates. These findings are of significant interest in view of potential engineering of the molecules and implementation in devices.

Physical Synthesis and Study of Ag@CaF2 Core@Shell Nanoparticles: Morphology and Tuning of Optical Properties

Pre‐formed Ag nanoparticles (NPs) and Ag@CaF2 core–shell NPs are physically synthesized using DC magnetron‐based NP source and deposited on Si‐SiOx wafers. The samples are prepared by co‐depositing Ag nanoparticles and CaF2 produced by an evaporation source, or by sequential deposition method, i.e., by depositing in a sequence a CaF2 buffer layer, the Ag NPs generated by the NP source and a capping CaF2 layer. The supported films are characterized by Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS), and Surface Differential Reflectivity (SDR). SEM shows that Ag NPs deposited directly on Si‐SiOx tend to diffuse and to agglomerate, affecting the size distribution of the nanostructures. The presence of a CaF2 buffer layer between Ag and Si‐SiOx limits this effect, while XPS reveals electrical charging, caused by the insulating nature of the CaF2 continuous film. The surface plasmon resonance behavior for different samples is analyzed using SDR with p‐polarized light. There is a clear evidence of a blue shift in the plasmon excitation due to the presence of CaF2 on Si, which can represent a potential advantage for the technological applications in photovoltaics and optoelectronics.

Plasma emission correction in reflectivity spectroscopy during sputtering deposition

Surface differential reflectivity spectroscopy is a fast non-destructive in situ and real-time measurement technique which allows following the first stages of thin film deposition. However, when applied to sputtering technique, spectra can strongly be distorted by residual light coming from plasma in a way, as shown herein, that depends on sample reflectivity. Thanks to suitable measurements, before and after growth with and without plasma or illumination lights, a protocol of signal correction is proposed to get rid of the spurious plasma contribution. The interest of the method is illustrated in the case of silver deposition on a silicon substrate.

Impact of Ge alloying on the early growth stages, microstructure and stress evolution of sputter-deposited Cu-Ge thin films

The effect of Ge alloying on the growth morphology and intrinsic stress evolution during deposition of Cu thin films is investigated using in situ and real time diagnostics, complemented by ex situ microstructural characterizations. Cu1-xGex alloys with x up to 0.15 were sputter-deposited at room temperature on Si substrates, while measuring simultaneously the substrate curvature evolution and surface differential reflectance spectroscopy during the early stages of growth. All films exhibit a typical compressive-tensile-compressive stress behavior, which underpins the distinct stages of a Volmer-Weber growth mode; however, it is revealed that the Ge content plays a decisive role on the nucleation and coalescence stages. This results in a grain size refinement and development of higher compressive stresses for Ge solute additions up to x = 0.07. At higher Ge contents, the strong chemical affinity between Cu and Ge atoms leads to a germanide compound (Cu5Ge) formation and even higher compressive stress. The stress evolutions are discussed considering the surfactant character of Ge atoms affecting Cu adatom mobility and growth morphology. This study highlights the possibility to tailor the early stages of growth and film microstructure (grain size, texture, surface roughness) by introducing solute concentrations of Ge in Cu films. We also determined for the elemental Cu film the percolation and film continuity thicknesses and discussed theses values with respect to those reported for other metallic films.

Optical properties of silicene, Si/Ag(111), and Si/Ag(110)

We present a state-of-the-art study of the optical properties of free-standing silicene and of single-layer Si 1D and 2D nanostructures supported on Ag(110) and Ag(111) substrates. Ab initio simulations of reflectance anisotropy spectroscopy (RAS) and surface differential reflectivity spectroscopy (SDRS) applied to the clean Ag surface and Si/Ag interfaces are compared with new measurements. For Si/Ag(110) we confirm a pentagonal nanoribbon geometry, strongly bonded to the substrate, and rule out competing zigzag chain and silicenelike models. For Si/Ag(111) we reproduce the main experimental features and isolate the optical signal of the epitaxial silicene overlayer. The absorption spectrum of a silicene sheet computed including excitonic and local field effects is found to be quite similar to that calculated within an independent particle approximation, and shows strong modifications when adsorbed on a Ag substrate. Important details of the computational approach are examined and the origins of the RAS and SDRS signals are explained in terms of the interface and substrate response functions. Our study does not find any evidence for Si adlayers that retain the properties of freestanding silicene.

Reflectance calculations of anisotropic dielectric constants of graphene-like two-dimensional materials.

A new optical method for determining anisotropic dielectric constants of graphene-like two-dimensional materials on semiconductor or metal substrates is developed. The method is based on the surface differential reflectance measurements at three different incident angles. The inversion problem is resolved analytically in a long-wavelength approximation, where graphene is discussed within the framework of macroscopic electrodynamics as a uniaxially anisotropic film with the optical axis perpendicular to the film surface. The method is fast and has no need for the initial guesses of the desired parameters. It also offers an effective technique to get good start points for iterative numerical methods to improve the preliminary results. The method is unique in its simplicity and, therefore, important for graphene research.

Plasmonic properties of gold nanoparticles on silicon substrates: Understanding Fano-like spectra observed in reflection

Gold nanoparticles (AuNPs) are known for their localized surface plasmon resonance (LSPR) that can be measured with UV-visible spectroscopy. AuNPs are often deposited on silicon substrates for various applications, and the LSPR is measured in reflection. In this case, optical spectra are measured by surface differential reflectance spectroscopy (SDRS) and the absorbance exhibits a negative peak. This article studies both experimentally and theoretically on the single layers of 16 nm diameter spherical gold nanoparticles (AuNPs) grafted on silicon. The morphology and surface density of AuNPs were investigated by atomic force microscopy (AFM). The plasmon response in transmission on the glass substrate and in reflection on the silicon substrate is described by an analytical model based on the Fresnel equations and the Maxwell-Garnett effective medium theory (FMG). The FMG model shows a strong dependence to the incidence angle of the light. At low incident angles, the peak appears negatively with a shallow intensity, and at angles above 30°, the usual positive shape of the plasmon is retrieved. The relevance of the FMG model is compared to the Mie theory within the dipolar approximation. We conclude that no Fano effect is responsible for this derivative shape. An easy-to-use formula is derived that agrees with our experimental data.

Excitation spectra of Bi/Si(001) interfaces

Present article deals with the theoretical and experimental investigation of optical properties of Bi/Si(001) interfaces in a wide spectral range (1–4 eV). Theoretical excitation spectra of vicinal Si(001) surface, Bi/Si(001)–0.5 monolayer and Bi/Si(001)–1 monolayer interfaces are calculated by time‐dependent density functional theory with the hybrid Becke and Lee, Yang and Parr pseudopotential. Calculations and experimental studies of the absorption spectrum of the samples revealed the blue shift of the optical absorption edge with increasing of the bismuth covering degree. Features of the experimentally obtained reflectance anisotropy spectra (RAS) and surface differential reflectance spectra (SDRS) concerned with changing of the silicon surface reconstruction from 2 × 1 to 1 × 1 with the increasing of the bismuth covering degree from 0.5 to 1 monolayer. It is shown that the correlation between the theoretically calculated and experimental data makes it possible to identify the electronic transitions in the surface region.

Volmer-Weber growth stages of polycrystalline metal films probed by in situ and real-time optical diagnostics

The Volmer-Weber growth of high-mobility metal films is associated with the development of a complex compressive-tensile-compressive stress behavior as the film deposition proceeds through nucleation of islands, coalescence, and formation of a continuous layer. The tensile force maximum has been attributed to the end of the islands coalescence stage, based on ex situ morphological observations. However, microstructural rearrangements are likely to occur in such films during post-deposition, somewhat biasing interpretations solely based on ex situ analysis. Here, by combining two simultaneous in situ and real-time optical sensing techniques, based on surface differential reflectance spectroscopy (SDRS) and change in wafer curvature probed by multibeam optical stress sensor (MOSS), we provide direct evidence that film continuity does coincide with tensile stress maximum during sputter deposition of a series of metal (Ag, Au, and Pd) films on amorphous SiOx. Stress relaxation after growth interruption was testified from MOSS, whose magnitude scaled with adatom mobility, while no change in SDRS signal could be revealed, ruling out possible changes of the surface roughness at the micron scale.

Influence of size, shape and core-shell interface on surface plasmon resonance in Ag and Ag@MgO nanoparticle films deposited on Si/SiO x.

Ag and Ag@MgO core–shell nanoparticles (NPs) with a diameter of d = 3–10 nm were obtained by physical synthesis methods and deposited on Si with its native ultrathin oxide layer SiOx (Si/SiOx). Scanning electron microscopy and transmission electron microscopy (TEM) images of bare Ag NPs revealed the presence of small NP aggregates caused by diffusion on the surface and agglomeration. Atomic resolution TEM gave evidence of the presence of crystalline multidomains in the NPs, which were due to aggregation and multitwinning occurring during NP growth in the nanocluster source. Co-deposition of Ag NPs and Mg atoms in an oxygen atmosphere gave rise to formation of a MgO shell matrix surrounding the Ag NPs. The behaviour of the surface plasmon resonance (SPR) excitation in surface differential reflectivity (SDR) spectra with p-polarised light was investigated for bare Ag and Ag@MgO NPs. It was shown that the presence of MgO around the Ag NPs caused a red shift of the plasmon excitation, and served to preserve its existence after prolonged (five months) exposure to air, realizing the possibility of technological applications in plasmonic devices. The Ag NP and Ag@MgO NP film features in the SDR spectra could be reproduced by classical electrodynamics simulations by treating the NP-containing layer as an effective Maxwell Garnett medium. The simulations gave results in agreement with the experiments when accounting for the experimentally observed aggregation.

Large differences in the optical properties of a single layer of Si on Ag(110) compared to silicene

The optical properties of Si nanoribbons grown on Ag(110) under ultrahigh vacuum have been experimentally determined by use of in situ surface differential reflectance spectroscopy. Real-time measurements showed a clear transition of the optical response of the Si deposit at full coverage of the Ag(110) surface, corresponding to 0.8 monolayer of silicon. The spectra measured for the complete self-assembled nanoribbon layer are different from the reflectance spectrum calculated for a layer of silicene on silver, which rules out the possible silicenelike character of this layer. Furthermore, the dielectric function of the nanoribbons is calculated from the experimental data and is similar to the one of amorphous silicon, with a red shift of about 0.6 to 0.8 eV of the main absorption feature. This result indicates that the Si nanoribbons display a preferential $s{p}^{3}$ hybridization as in amorphous Si and not a partial $s{p}^{2}$ hybridization as expected for silicene.

Mechanism of Benzene Monolayer Formation on Si(100)-2×1 Studied by Surface Differential Reflectance Spectroscopy

The kinetics of benzene adsorption on a Si(100)-2×1 surface has been monitored using real-time surface differential reflectance spectroscopy. A model with two adsorption modes is confirmed where benzene binds to the silicon surface in tight-bridge (TB) and butterfly (BF) configurations. For the first time, the balance between these modes is probed continually as the adsorption progressed and the resulting kinetics is discussed. In a first stage, benzene adsorbs in the TB configuration until it reaches 17% of a monolayer. In the second stage, additional molecules adsorb in the BF configuration and TB gradually converts into BF. At saturation, about 50% of the dimers are occupied by BF. Moreover, a Monte Carlo simulation provides a proper description of the adsorption kinetics.

Model-Free Unraveling of Supported Nanoparticles Plasmon Resonance Modes

Plasmonics of Ag, Au, and Zn nanoparticles supported on Al2O3(0001), TiO2(110), and ZnO(0001) substrates has been probed by surface differential reflectivity spectroscopy (SDRS) during vapor deposition growth. Parallel and perpendicular interfacial susceptibilities (ISs), or “optical thicknesses”, which characterize only the dielectric response of the film, are derived from experimental spectra in p- and s-polarization using an inversion procedure based on Kramers–Kronig transform. The consistency of the approach is checked against sum rules. Plasmonic contributions are unraveled by decomposing ISs into damped oscillators and identified with the help of dielectric simulations of truncated supported spheres or spheroids. Beyond the common Drude behavior of Ag, Au, and Zn, the comparison between the three metals demonstrates the paramount role of interband transitions in the ISs profiles. While gold and silver show free electron plasmon modes, zinc exhibits polarization modes of bound electrons. However, despite those differences, the resonant modes that are identified herein are universal for supported particles. Particle shape, equilibrium aspect ratio, image field, polydispersity, and interface-induced damping are discussed by analyzing changes in frequencies, oscillator strengths, and broadenings. Deposit-induced band gap absorption for semiconductor substrate and switches from growth to coalescence regimes are evidenced. Static and dynamic coalescence are characterized by power law exponents as a function of particle size. Therefore, the unique framework that is proposed opens strong prospects in the optical characterization of growth, metal/semiconductor interfaces, and gas adsorption.

Real-time curvature and optical spectroscopy monitoring of magnetron-sputtered WTi alloy thin films

WTi thin films are known as potential adhesion promoters and diffusion barriers. WTi thin films were deposited by magnetron sputtering from an alloyed target (W:Ti~70:30at.%). Real-time surface differential reflectance (SDR) spectroscopy and wafer-curvature measurements were performed during deposition to study the growth and the film continuity threshold. SDR measurements during WTi deposition allow the determination of the change in reflectivity of p-polarized light (at Si substrate Brewster's angle) between WTi film and Si substrate in order to monitor layer growth. The comparison between experimental and simulated WTi SDR signals assuming a homogeneous and continuous layer growth shows that film continuity is ensured beyond a thickness of 4.5±0.2nm. Real-time wafer-curvature measurements allow the determination of the intrinsic stress development in the film. Two regimes are noticed during the growth up to the development of a compressive steady state stress. The early stages of growth are rather complicated and divided into sub-regimes with similar boundaries revealed by both in situ techniques. Deposition of an interfacial continuous layer different from WTi bulk is suggested by both in situ techniques below a thickness of 4.5nm.

Real-time monitoring of nanoparticle film growth at high deposition rate with optical spectroscopy of plasmon resonances

In the thin film area, it is well-known that deposition rate impacts the morphology but tools to quantify online fast growth processes are scarce. Here we show that surface differential reflectivity spectroscopy (SDRS) can be used in real time to follow silver nanoparticle growth during sputtering deposition. The main experimental challenge was to avoid noise and saturation due to the plasma emission and to obtain a reasonable signal/noise ratio to monitor fast deposition. A specific setup was designed resulting in an acquisition speed in the range of hundreds of milliseconds and used to investigate the growth of silver on alumina as a test example. The evolution of the size, density and aspect ratio of growing silver islands were determined by modelling their plasmonic response and compared with previous results obtained at a much lower growth rate using physical vapour deposition (Lazzari and Jupille 2012 Nanotechnology 23 135707). During room-temperature sputter deposition, coalescence leads to significantly larger and flatter aggregates compared to evaporation at the same coverage. However, both deposition techniques lead to similar nucleation and growth behaviours. Higher substrate temperature (575 K) did not change the trend and a sticking coefficient close to one was found. The observed evolution of the particle aspect ratio is discussed in terms of supersaturation and flux driven hindrance of the coalescence.

Unusual Two-Stage Kinetics of Ethylene Adsorption on Si(100) Unraveled by Surface Optical Spectroscopy and Monte Carlo Simulation

The adsorption of ethylene on a Si(100)-2×1 surface in an ultrahigh vacuum has been monitored at room temperature by use of real-time surface differential reflectance spectroscopy, which clearly demonstrated that the adsorption follows a two-stage process. About half a monolayer is obtained for 1L, while the second stage is much slower, yielding the complete monolayer for an exposure of ∼400 L. The kinetics over the full range has been successfully reproduced by a MonteCarlo calculation. The key point of this two-stage adsorption kinetic lies in the reduced adsorption probability (by a factor of several hundreds) on the Si dimers, neighbors of dimers which have already reacted, with respect to the adsorption probability on isolated dimers. This new kind of adsorption kinetics, due to a repulsion between already adsorbed molecules and additional molecules impinging on the surface, makes it a textbook case for a "cooperative" adsorption process.

Determination of molecular orientation of α-sexithiophene on passivated Si(001) by means of optical reflectance spectroscopic methods

We used reflectance difference spectroscopy (RDS) and surface differential reflectance spectroscopy (SDRS) to investigate the molecular orientation of ultrathin α-sexithiophene (α-6T) films on four passivated Si(100)-(2×1) surfaces, oxidized Si(001), water-adsorbed Si(001), hydrogen-adsorbed Si(001) and ethylene-adsorbed Si(001), in order to study the substrate dependence of the molecular orientation of isolated molecules and molecules located in islands or films. Strong in-plane anisotropy was observed on ethylene-adsorbed Si(001) even at 5nm film thickness, which was not the case for the other substrates.