Manipulation of thin metal film morphology on weakly interacting substrates via selective deployment of alloying species

We demonstrate a versatile concept for manipulating morphology of thin (& LE;25 nm) noble-metal films on weakly interacting substrates using growth of Ag on SiO2 as a model system. The concept entails deployment of minority metallic (Cu, Au, Al, Ti, Cr, and Mo) alloying species at the Ag-layer growth front. Data from in situ and real-time monitoring of the deposition process show that all alloying agents-when deployed together with Ag vapor throughout the entire film deposition-favor two-dimensional (2D) growth morphology as compared to pure Ag film growth. This is manifested by an increase in the substrate area coverage for a given amount of deposited material in discontinuous layers and a decrease of the thickness at which a continuous layer is formed, though at the expense of a larger electrical resistivity. Based on ex situ microstructural analyses, we conclude that 2D morphological evolution under the presence of alloying species is predominantly caused by a decrease of the rate of island coalescence completion during the initial film-formation stages. Guided by this realization, alloying species are released with high temporal precision to selectively target growth stages before and after coalescence completion. Pre-coalescence deployment of all alloying agents yields a more pronounced 2D growth morphology, which for the case of Cu, Al, and Au is achieved without compromising the Ag-layer electrical conductivity. A more complex behavior is observed when alloying atoms are deposited during the post-coalescence growth stages: Cu, Au, Al, and Cr favor 2D morphology, while Ti and Mo yield a more pronounced three-dimensional morphological evolution. The overall results presented herein show that targeted deployment of alloying agents constitutes a generic platform for designing bespoken heterostructures between metal layers and technologically relevant weakly interacting substrates.& nbsp;Published under an exclusive license by the AVS.Peer reviewe

Clustering and Morphology Evolution of Gold on Nanostructured Surfaces of Silicon Carbide: Implications for Catalysis and Sensing

A fundamental understanding of the behavior of gold (Au) nanostructures deposited on functional surfaces is imperative to discover and leverage interface-related phenomena that can boost the efficiency of existing electronic devices in sensorics, catalysis, and spintronics. In the present work, Au layers with nominal thickness of 2 nm were sputter-deposited on graphenized SiC substrates represented by buffer layer (BuL)/4H-SiC and monolayer epitaxial graphene (MLG)/4H-SiC. Morphometric analysis by means of scanning electron microscopy shows that Au on BuL self-assembles in nearly round-shaped plasmonically active islands, while on MLG, a fractal growth of considerably larger and ramified islands is observed. To correlate the experimentally established differences in surface morphology on the two types of graphenized substrates with energetics and kinetics of Au nanostructure growth, the deposit-substrate interaction strength was studied using density functional theory (DFT) calculations, molecular dynamics simulations, and optical measurements. The theoretical considerations involve participation of Au clusters with different sizes and energetics at the initial stages of the metal nanostructure formation. The results indicate that gold exhibits a considerably stronger interaction with BuL than with MLG, which can be considered as a key aspect for explaining the experimentally observed morphological differences. From the statistical analysis of Raman spectra, indications of Au intercalation of MLG are discussed. The current research shows that, due to its unique surface chemistry, buffer layer has peculiar affinity to gold when compared to other atomically flat surfaces, which is beneficial for boosting high-performance catalytic and sensing technologies based on low-dimensional materials

Ανάπτυξη και αυτο - οργάνωση πλασμονικών σωματιδίων με εφαρμογές τους σε μοριακούς αισθητήρες

The aim of the thesis was the study, development, formation by self-assembly and characterization of plasmonic nanostructures, namely nanoparticles of metals, and devices for optical and photonic applications. In particular, in this thesis, nanostructures of silver and gold nanoparticles were developed and studied on silicon substrates. The metal nanoparticles were developed by: (a) chemical methods such as Electroless Metal Deposition, a fast, reproducible and low-cost process, and (b) physical deposition methods, such as Magnetron Sputtering followed by a pulse laser annealing process, that enabled the degree of modification and self-assembly of nanoparticles of different sizes and distributions. The metal nanoparticles structures showed plasmonic behavior with the Local Surface Plasmon Resonance at frequencies covering the entire visible spectrum. The devices were successfully tested for their suitability in the detection and trapping of low concentration rhodamine molecules as Surface Enhanced Raman Scattering and Surface Enhanced Photoluminescence templates. Additionally, plasmonic nanostructures were developed on top of titanium oxide templates and their efficiency in photocatalytic detection and trapping of heavy metal ions such as manganese in aqueous solutions were tested. The results showed an enhancement of this process in order to be applied by using consumer LED lamps and in realistic detection time for industrial use.Σκοπός της διατριβής ήταν η μελέτη, ανάπτυξη, μορφοποίηση με αυτό-οργάνωση και ο χαρακτηρισμός πλασμονικών νανοδομών, δηλαδή νανοσωματιδίων μετάλλων, και συσκευών για οπτικές και φωτονικές εφαρμογές. Συγκεκριμένα, στην παρούσα διδακτορική διατριβή αναπτύχθηκαν και μελετήθηκαν συστήματα νανοδομών από μεταλλικά νανοσωματίδια αργυρού και χρυσού ανεπτυγμένα πάνω σε υποστρώματα πυριτίου και τιτανίας. Η ανάπτυξη των μεταλλικών νανοσωματιδίων έγινε: (α) με χημικές μεθόδους όπως η Ηλεκτρολυτική Εναπόθεση Μετάλλου, μία γρήγορη και επαναλήψιμη χαμηλού κόστους διαδικασία, και (β) με φυσικές μεθόδους εναπόθεσης ατμών, όπως η Ιοντοβολή Εκκένωσης Αίγλης για την ανάπτυξη αρχικά λεπτών υμενίων μετάλλων και στην συνεχεία τον σχηματισμό των μεταλλικών νανοσωματιδίων με ανόπτηση με τη χρήση παλμικού λέιζερ, η οποία μας επέτρεψε να δημιουργήσουμε νανοσωματίδια διαφορετικών μεγεθών και κατανομών που εμφανίζουν το φαινόμενο του Τοπικού Επιφανειακού Πλασμονικού Συντονισμού σε συχνότητες που καλύπτουν όλο το φάσμα του ορατού φωτός. Οι πλασμονικές νανοδομές δοκιμάστηκαν επιτυχώς για την καταλληλότητα τους στην ανίχνευση και παγίδευση χαμηλής συγκέντρωσης μορίων ροδαμίνης ως υποστρώματα επιφανειακής ενίσχυσης σκέδασης Ράμαν μέσω της Φασματοσκοπίας Ράμαν και σε πειράματα Επιφανειακής Ενίσχυσης Φωταύγειας. Επιπροσθέτως, αναπτύχθηκαν πλασμονικές νανοδομές στην επιφάνεια τροποποιημένης τιτανίας και ελέγθηκε η απόδοσής τους στην φωτοκαταλυτική ανίχνευση και παγίδευση ιόντων βαρέων μετάλλων (ιόντων μαγγανίου), και επιτεύχθηκε η αύξηση της απόδοσης, ώστε η διαδικασία αυτή να επιτυγχάνεται με την χρήση απλών οικιακών λαμπτήρων τύπου φωτοδιόδου σε μικρό χρονικό διάστημα κατάλληλο για τη χρήση τους στη βιομηχανία

On the effect of copper as wetting agent during growth of thin silver films on silicon dioxide substrates

We study the effect of Cu incorporation on the morphological evolution and the optoelectronic properties of thin Ag films deposited by magnetron sputtering on weakly-interacting SiO2 substrates. In situ and real time spectroscopic ellipsometry data show that by adding up to 4at.% Cu throughout the entire film deposition process, wetting of the substrate by the metal layer is promoted, as evidenced by a decrease of the thickness at which the film becomes continuous from 19.5nm (pure Ag) to 15nm (Ag96Cu4). The in situ data are consistent with ex situ x-ray reflectometry analyses which show that Cu-containing films exhibit a root mean square roughness of 1.3nm compared to the value 1.8nm for pure Ag films, i.e., Cu leads to smoother film surfaces. These morphological changes are coupled with an increase in continuous-layer electrical resistivity from 1.0×10-5Ωcm (Ag) to 1.25×10-5Ωcm (Ag96Cu4). Scanning electron microscopic studies of discontinuous layers reveal that the presence of Cu at the film growth front promotes smooth surfaces (as compared to pure Ag films) by hindering the rate of island coalescence. To further understand the effect of Cu on film growth and electrical properties, in a second set of experiments, we deploy Cu with high temporal precision to target specific film-formation stages. The results show that longer presence of Cu in the vapor flux and the film growth front promote flat morphology. However, both a flat surface and a continuous-layer electrical resistivity that is equal to that of pure Ag films can only be achieved when Cu is deployed during the first 2.4nm of film deposition, during which morphological evolution is, primarily, governed by island coalescence. Our overall results highlight potential pathways for fabricating high-quality multifunctional metal contacts in a wide range of optoelectronic devices based on weakly-interacting oxides and van der Waals materials.Funding agencies: The French Government program "Investissements d’Aveni"r (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01), Linköping University ("LiU Career Contract, Dnr-LiU-2015-01510, 2015-2020"), The Swedish research council (contract VR-2015-04630), The ÅForsk foundation (contracts ÅF 19-137 and ÅF 19-746), The Olle Engkvist foundation (contract SOEB 190-312), The Wenner-Gren foundations (contracts UPD2018-0071 and UPD2019-0007)</p

The effect of kinetics on intrinsic stress generation and evolution in sputter-deposited films at conditions of high atomic mobility

Vapor-based metal film growth at conditions that promote high atomic mobility is typically accompanied by compressive stress formation after completion of island coalescence, while an apparent stress relaxation is observed upon deposition interruption. Despite numerous experimental studies confirming these trends, the way by which growth kinetics affect postcoalescence stress magnitude and evolution is not well understood, in particular, for sputter-deposited films. In this work, we study in situ and in real-time stress evolution during sputter-deposition of Ag and Cu films on amorphous carbon. In order to probe different conditions with respect to growth kinetics, we vary the deposition rate F from 0:015 to 1:27 nm/s, and the substrate temperature T-S from 298 to 413 K. We find a general trend toward smaller compressive stress magnitudes with increasing T-S for both film/substrate systems. The stress-dependence on F is more complex: (i) for Ag, smaller compressive stress is observed when increasing F; (ii) while for Cu, a nonmonotonic evolution with F is seen, with a compressive stress maximum for F = 0.102 nm/s. Studies of postdeposition stress evolution show the occurrence of a tensile rise that becomes less pronounced with increasing T-S and decreasing F, whereas a faster tensile rise is seen by increasing F and T-S. We critically discuss these results in view of ex situ obtained film morphology which show that deposition-parameter-induced changes in film grain size and surface roughness are intimately linked with the stress evolution. (c) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Funding Agencies|French Government program "Investissements dAvenir" (LABEX INTERACTIFS)French National Research Agency (ANR) [ANR-11-LABX-0017-01]; Linkoping University ("LiU Career Contract) [LiU-2015-01510]; Swedish Research CouncilSwedish Research Council [VR-2015-04630]; Aforsk Foundation [AF 19-137]; Olle Engkvist Foundation [SOEB 190-312]; Wenner-Gren Foundations [UPD2018-0071, UPD2019-0007]</p

Manipulation of thin silver film growth on weakly interacting silicon dioxide substrates using oxygen as a surfactant

The authors study the morphological evolution of magnetron-sputtered thin silver (Ag) films that are deposited on weakly interacting silicon dioxide (SiO2) substrates in an oxygen-containing (O-2) gas atmosphere. In situ and real-time monitoring of electrically conductive layers, along with ex situ microstructural analyses, shows that the presence of O-2, throughout all film-formation stages, leads to a more pronounced two-dimensional (2D) morphology, smoother film surfaces, and larger continuous-layer electrical resistivities, as compared to Ag films grown in pure argon (Ar) ambient. In addition, the authors data demonstrate that 2D morphology can be promoted, without compromising the Ag-layer electrical conductivity, if O-2 is deployed with high temporal precision to target film formation stages before the formation of a percolated layer. Detailed real-space imaging of discontinuous films, augmented by in situ growth monitoring data, suggests that O-2 favors 2D morphology by affecting the kinetics of initial film-formation stages and most notably by decreasing the rate of island coalescence completion. Furthermore, compositional and bonding analyses show that O-2 does not change the chemical nature of the Ag layers and no atomic oxygen is detected in the films, i.e., O-2 acts as a surfactant. The overall results of this study are relevant for developing noninvasive surfactant-based strategies for manipulating noble-metal-layer growth on technologically relevant weakly interacting substrates, including graphene and other 2D crystals.Funding Agencies|Linkoping University ("LiU Career Contract) [Dnr-LiU-2015-01510]; Swedish Research CouncilSwedish Research Council [VR-2015-04630]; Olle Engkvist Foundation [SOEB 190-312]; Wenner-Gren Foundations [UPD2018-0071, UPD2019-0007]; French Government Program "Investissements dAvenir" (LABEX INTERACTIFS)French National Research Agency (ANR) [ANR-11-LABX-0017-01]; Aforsk Foundation; European Consortium of Innovative Universities (ECIU)</p

In Situ and Real-Time Nanoscale Monitoring of Ultra-Thin Metal Film Growth Using Optical and Electrical Diagnostic Tools

Continued downscaling of functional layers for key enabling devices has prompted the development of characterization tools to probe and dynamically control thin film formation stages and ensure the desired film morphology and functionalities in terms of, e.g., layer surface smoothness or electrical properties. In this work, we review the combined use of in situ and real-time optical (wafer curvature, spectroscopic ellipsometry) and electrical probes for gaining insights into the early growth stages of magnetron-sputter-deposited films. Data are reported for a large variety of metals characterized by different atomic mobilities and interface reactivities. For fcc noble-metal films (Ag, Cu, Pd) exhibiting a pronounced three-dimensional growth on weakly-interacting substrates (SiO2, amorphous carbon (a-C)), wafer curvature, spectroscopic ellipsometry, and resistivity techniques are shown to be complementary in studying the morphological evolution of discontinuous layers, and determining the percolation threshold and the onset of continuous film formation. The influence of growth kinetics (in terms of intrinsic atomic mobility, substrate temperature, deposition rate, deposition flux temporal profile) and the effect of deposited energy (through changes in working pressure or bias voltage) on the various morphological transition thicknesses is critically examined. For bcc transition metals, like Fe and Mo deposited on a-Si, in situ and real-time growth monitoring data exhibit transient features at a critical layer thickness of similar to 2 nm, which is a fingerprint of an interface-mediated crystalline-to-amorphous phase transition, while such behavior is not observed for Ta films that crystallize into their metastable tetragonal beta-Ta allotropic phase. The potential of optical and electrical diagnostic tools is also explored to reveal complex interfacial reactions and their effect on growth of Pd films on a-Si or a-Ge interlayers. For all case studies presented in the article, in situ data are complemented with and benchmarked against ex situ structural and morphological analyses.Funding Agencies|French Government program "Investissements dAvenir" (LABEX INTERACTIFS)French National Research Agency (ANR) [ANR-11-LABX-0017-01]; French Government program "Investissements dAvenir" (EUR INTREE)French National Research Agency (ANR) [ANR-18-EURE-0010]; Linkoping University ("LiU Career Contract) [Dnr-LiU-2015-01510]; Swedish research councilSwedish Research Council [VR-2015-04630]; AForsk foundation [AF 19-137, AF 19-746]; Olle Engkvist foundation [SOEB 190-312]; Wenner-Gren foundations [UPD2018-0071, UPD2019-0007]</p

Exploring the Interface Landscape of Noble Metals on Epitaxial Graphene

Understanding the interaction between noble metals (NMs) and epitaxial graphene is essential for the design and fabrication of novel devices. Within this framework, a combined experimental and theoretical investigation of the effect of vapor-deposited NM (silver [Ag] and gold [Au]) nanostructures on the vibrational and electronic properties of monolayer epitaxial graphene (MLG) on 4H-SiC is presented. Large sets of Raman scattering data are analyzed using supervised classification and statistical methods. This analysis enables identification of the specific Raman fingerprints of Au- and Ag-decorated MLG originating from different dispersion interactions and charge transfer at the metal nanostructure/MLG interface. It is found that Raman scattering spectra of Au-decorated MLG feature a set of allowed phonon modes similar to those in pristine MLG, whereas the stronger Ag physisorption triggers an activation of defect-related phonon modes and electron doping of MLG. A principal component analysis (PCA) and linear discriminant analysis (LDA) are leveraged to highlight the features in phonon dispersion of MLG that emanate from the NM deposition process and to robustly classify large-scale Raman spectra of metal-decorated graphene. The present results can be advantageous for designing highly selective sensor arrays on MLG patches decorated with different metals.Funding Agencies|Angpanneforeningens Forskningsstiftelse [16-541]; Bulgarian National Science FundNational Science Fund of Bulgaria [DN 18/6]; VRSwedish Research Council [2018-04962, 2016-05362]; SSFSwedish Foundation for Strategic Research [RMA 15-0024]; Linkoping University ("LiU Career Contract") [Dnr-LiU-2015-01510]; Swedish research councilSwedish Research CouncilEuropean Commission [VR-201504630]; Olle Engkvist foundation [SOEB 190-312]; Wenner-Gren foundations [UPD2018-0071, UPD2019-0007]; AForsk foundation [AF 19-137]</p

Probing the uniformity of silver-doped epitaxial graphene by micro-Raman mapping

We present a Raman spectroscopy study on epitaxial graphene decorated with thin Ag films (2-15 nm), which are deposited using magnetron sputtering. We find that the presence of Ag on the graphene surface induces doping, the uniformity and efficiency of which is determined by Ag nominal thickness. Deposition of Ag films with thicknesses up to 5 nm favors the effective electron transfer from Ag to epitaxial graphene. A significant redshift and broadening of the 2D peak are observed with increasing the Ag-layer thickness above 5 nm, which is indicative of large strain and doping fluctuations. We also observe a non-trivial linear growth of 2D/G peak intensity ratio with increasing D/G ratio for all Ag-decorated samples, which is explained by increase of peak amplitude due to surface enhanced Raman scattering and charged impurity-induced screening caused by the presence of Ag on the graphene surface.Funding Agencies|VR grant [2018-04962, 302791]; SSF grants [SSF GMT14-0077, SSF RMA15-0024]; Angpanneforeningens Forskningsstiftelse [16-541]; Linkoping University ("LiU Career Contract) [Dnr-LiU-2015-01510]; Swedish research councilSwedish Research Council [VR-2015-04630]; Olle Engkvist foundation [SOEB 190-312]; Wenner-Gren foundations [UPD2018-0071]</p

Chiral Metal-Oxide Nanofilms by Cellulose Template Using Atomic Layer Deposition Process

In this article, we describe an advance approach for the fabrication of chiral metal-oxide nanofilms. Our approach is based on the atomic layer deposition of titania and alumina nanofilms onto cellulose microfibers, used as chiral templates, leading to the formation of chiral nanofilms with a spatial fibrous structure. The chiral nanofilms were extensively characterized by X-ray photoelectron spectroscopy and high-resolution electron microscopy. The chiral property of the produced titania nanofilms was studied by enantioselective adsorption experiments using circular-dichroism spectroscopy and chiral high-performance liquid chromatography. We demonstrate the application of the titania chiral nanofilms for enantioselective crystallization. Overall, the basic principle for the preparation of chiral nanofilms by atomic layer deposition is demonstrated, as well as their uses for several enantioselective applications