Spatially-controlled laser-induced decoration of 2D and 3D substrates with plasmonic nanoparticles

Abstract

We demonstrate a new approach which can be used for targeted imparting of plasmonic properties for a wide range of different substrates (transparent and non-transparent) which may have any 2D or 3D topological structure created independently in a prior step with some other technology.

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1 Introduction

The need for deploying patterns based on plasmonic nanoparticles on diverse surfaces has driven extensive efforts towards developing and improving the production methods.¹ Most of the available patterning or printing methods rely on the delivery of “chemical inks” and the application of planar patterned surface geometries.² However, advanced patterned methods require the following: (i) addressable deposition; (ii) patterning over 2D or 3D substrates, (iii) patterning over a variety of plasmonic materials, and (iv) patterning over large areas at low costs.

We succeeded in developing an approach that can ensure the listed requirements. The method can be called direct Laser-Induced Deposition (LID); it consists of a photo-reduction process followed by heterogeneous nucleation of metal nanoparticles (NPs) onto the surface of a substrate immersed into the solution of organometallic complexes. The chemical composition of deposited NPs is predetermined by the used organometallic complexes; as a result not only monometallic but even bimetal NPs can be obtained. The described approach was successfully demonstrated for decoration of planar transparent substrates.³ The properties of the deposited plasmonic NPs (chemical composition, morphology, compactness of arrangement) can be controlled by the deposition parameters, such as laser power, composition of the liquid phase (composition and concentration of organometallic complexes, kind of solvent), and dwell time during the LID process. The area of plasmonic patterning is determined by laser-affected zone. The suggested LID process avoids harsh chemical treatments in contrast to the conventional photolithographic methods.⁴ Up to date inks based on bio-organic chemistry involving organic molecules, alkylthiols, polymers, DNA, proteins, peptides, click-chemistry or even metal ions were used but not plasmonic nanoparticles posing a severe limitation on such printing or patterning combinations methods.⁵

We herein address this challenges by developing a novel generalizable photo-reduction protocol at addressable locations with high resolution based on LID without prior surface treatment.⁶ Substrates can be transparent or non-transparent, conductive or insulating and having complex geometries such as 3D substrates (such as nanowire and capillaries). The plasmonic nanoparticles are deposited by photo-reduction of an organometallic complex at the solution/substrate interface; the size of the deposited nanoparticles is 6 ± 2 nm in diameter. The LID process tends to maintain the deposition patterns as determined by the photo-induced process without side thermal effects.

The LID method can be used not only for surface patterning based on mono-metallic nanoparticle but also for multi-metallic particles with a wide range of metal components and ratios. Prior to the LID process, a solution containing a supra-molecular organometallic (SM) complex was prepared. Briefly, the SM complex i.e. [{Au₁₀Ag₁₂(C₂Ph)₂₀}Au₃(PPh₂(C₆H₄)₃PPh₂)₃][PF₆]₅, was synthesized by the reaction of the terphenyl-based gold complex [Au₂(CtCPh)₂(μ-4,4′′-PPh₂(C₆H₄)₃PPh₂)] with AgPF₆ in tetrahydrofuran (THF) and isolated after repeated recrystallization.⁷ More information can be found in the experimental section. The SM complex consists of a central bi-metallic cluster core, stabilized with alkynyl and phenyl ligands. The central core is stabilized by [Au₃(PP)₃]³⁺ belt as schematically illustrated in the inset in Fig. 1. The absorbance spectrum of the SM complex (see ESI Fig. S1†) has three main absorption regions: (i) 200–300 nm, the photon absorption via the ligand–ligand electron transition; (ii) 300–350 nm, the absorption via the ligand–metal electron transition; and (iii) 350–450 nm, via the transition between the orbitals of the complex core.⁸

Fig. 1 Schematic diagram the LID process. The schematic structure down-left is the SM complex [{Au₁₀Ag₁₂(C₂Ph)₂₀}Au₃(PPh₂(C₆H₄)₃PPh₂)₃][PF₆]₅.

2 Results and discussion

The solution was prepared by dissolving the SM complex in acetone (5 mg ml⁻¹). To start the patterning process, the substrate was immersed into the solution forming a solution/substrate interface and irradiated with CW laser for 10 min. The laser wavelength of 325 nm was chosen for laser-induced deposition as it corresponds to spectral region (ii) of the SM complex absorption. Region (i) is not favorable due to the high UV-absorption of the LID system components such as solvents, and reaction vessel walls.⁹ Cross-section of band (iii) is low that results in low efficiency of energy absorption. To realize the controlled patterns which consist of monodispersed nanoparticles with controlled shape and size, the solution was irradiated with low laser power (10 mW) as illustrated in Fig. 1. As a result, the plasmonic nanoparticles are formed by photo-reduction process exactly onto the irradiated area of the substrate without thermal effects.¹⁰ The temperature profile in the irradiated area of the solution/substrate interface was modeled and estimated to be (∼23 °C) which is below the melting point of the SM complex (∼95 °C), see ESI† for details.

All the different 2D and 3D substrates were cleaned before use following a standard recipe: sonication for 5 min in ethanol and drying with N₂. Subsequently, the ability of these substrates to support LID nano-patterning was confirmed for differently designed substrates as illustrated in Fig. 2: (A and B) transparent, (C and D) non-transparent 2D substrates (i.e. planar surface) and 3D substrates such as vertical nanowires and capillaries (G and H).

Fig. 2 Schematic transparent 2D substrates with focused (A) and unfocussed LID (B). Non-transparent substrates with focused (C) and unfocussed LID (D). SEM images of deposited nanoparticles with focused spot (inset is HRSEM) of a selected area (E), and unfocused laser (F). Schematic 3D substrates: Si NWs (G) representing the non-transparent case and tubes (H) representing the transparent case. (I) and (J), respectively, are the corresponding SEM images after deposition.

To start the decoration process the substrate interface should be in contact with the SM complex solution. Thus, we developed two deposition geometries for forming such patterns: (A and B) substrate above the solution for transparent substrates, (C and D) substrate inside or below the solution for (non)-transparent substrates. In these two geometries, which are suitable for 3D substrates as well, the laser beam is directed via beam splitter to the micro objective and focused on the substrate/solution interface through a transparent substrate (case A) or through the solution (case C). In all the listed cases the formation of plasmonic nanoparticles is observed in the laser-affected area of the substrates (Fig. 2E, F, I and J).

Moreover, patterned structures on addressable locations can be realized with a focused beam, while random decoration can be realized by using an unfocussed laser beam. The latter case may be useful for covering large substrate areas with a homogenous plasmonic nanoparticle distribution. Patterns of specific shapes have been realized by the focused laser beam scanning. As an example, Fig. 3 demonstrates a rectangular-shaped pattern deposited onto a 3D sample (silicon nanowires). The plasmonic patterning was realized by the constant laser scanning with a beam of 2 μm diameter at a speed of 6 μm s⁻¹ and an intensity of 25 W cm⁻² so that an areas of 30 μm × 30 μm was covered. The density of the nanoparticles in the decorated area was controlled through the laser intensity, scanning speed and occasionally by scanning repeatedly. The effective exposure time varied between 0.5 and 10 min resulting in a deposition density (number of NPs per unit square) ranging from 700 to 3500 μm⁻². It worth noting that short exposure times give a homogenous deposition while longer exposure times show formation of NPs agglomerates. The sharp edges of the rectangular deposition area are an indication for the high spatial resolution estimated to be 100 nm.

Fig. 3 Exposure time variations between 30 s (A), 5 min (B) and 10 min (C) were used to decorate 3D substrate (Si NWs arrays) with AgAu nanoalloys in a rectangular-shape. The homogeneity of the deposited sample was achieved by tight focusing and scanning the laser spot. The deposition density was found to be 700 μm⁻² after 30 s (D), 2100 μm⁻² after 5 min (E) and 3500 μm⁻² after 10 min (F). The EDX spectrum of the deposited AgAu plasmonic particles is presented in (G). Making scans recording either of the two lines marked Au and Ag reveals the distribution of gold (H) and silver (I). The crystallinity of a single nanoparticle is illustrated in (J).¹¹ The absorbance of the AgAu nanoparticles gives rise to a single plasmon peak at 420 nm in the optical response (K).

The chemical composition of the decorated areas was investigated with EDX (energy dispersive X-ray) and XPS (X-ray photoelectron spectroscopy). EDX measurements of the patterned area shows the homogenous distribution of the metals (silver, gold) and carbon while the averaged chemical composition was realized by XPS showing two spin–orbit peaks of Au4f (80–92 eV) and Ag3d (365–380 eV), see ESI Fig. S3.† Comparing the data obtained from EDX and XPS suggests the bi-metal composition of the nanoparticles. STEM measurements were preformed to study the single nanoparticles. However, STEM imaging could not help to distinguish between Ag and Au, even though the atomic numbers are clearly different (ZAg = 47 and ZAu = 79). Hence, a combined HAADF-STEM and EDX point measurement analysis was performed, which clearly reveals the presence of AgAu nanoalloys rather than core–shell or individual nanoparticles (more details involving a selected area electron diffraction (SEAD) analysis can be found in the ESI† and our previous paper).¹¹ The EDX spot analysis on several nanoalloys shows characteristic peaks at 2.1 keV for gold and at 2.9 keV and 3.2 keV for silver as schematically illustrated in Fig. 3G. The absorption spectra reveal that the obtained nanoparticles are Ag–Au nanoalloys as confirmed by a single peaked optical absorption at 420 ± 10 nm (Fig. 3K). The maximum absorption is due to the dipolar plasmon absorption of AgAu nanoalloys in agreement with Mie theory.¹² It is worth mentioning that the absorbance shows one peak (Fig. 3K) which suggests alloy composition rather than a mixture of AgNPs and AuNP that is known to exhibit two characteristic absorbance peaks (one of Au at ∼500 nm and other one of Ag at ∼400 nm).¹³ Core–shell structure of metal phase is not supported by the TEM results. The luminescence spectroscopy of the SM solution demonstrated the intense luminescence in the spectral region 500–700 nm with the maximum being at 570 nm (see ESI Fig. S4†). Interestingly, forming the nanoalloys in the course of laser irradiation of the SM complexes results in a decrease of the luminescence intensity with irradiation time. The photoluminescence (PL) intensity was measured for the untreated solution and for solutions irradiated for 5, 10, 15, and 20 min during the LID process. The luminescence intensity decrease can be considered as evidence of the LID process and the promotion of the nanoalloys formation. Obviously, the SM complex concentration in the solution is decreased during the irradiation time which is ascribed to the organometallic decomposition and nanoalloy formation and, as a result, to the surface decoration with plasmonic NPs. It should be noted that the demonstrated process of the plasmonic NPs formation in course of laser irradiation of precursor heterometallic complexes can be expanded towards precursors of other families like metal–organic frameworks, multimetallic nanoclusters, etc.¹⁴

3 Conclusions

In summary, a protocol for plasmonic patterning of 2D or 3D substrates based on LID-based photo-decomposing of SM complexes of [{Au₁₀Ag₁₂(C₂Ph)₂₀}Au₃(PPh₂(C₆H₄)₃PPh₂)₃][PF₆]₅ in solution has been established. This simple, yet versatile approach is based on the use of laser irradiation of solution/substrate interfaces. Beyond the AgAu nanoparticles, other plasmonic nanostructures with a wide range of metallic or magnetic nanoparticles can potentially be realized. Moreover, the method opens up a new route towards fast nano-metal synthesis (irradiation time less than 10 min) without involving any reducing agents or high temperature treatment. In this way, we can eliminate the role of surfactant on the surface for subsequent chemical manipulations (which required for special decoration techniques such as dip-pen-nanolithography).¹⁵ This important detail highlights the possibility of extending LID as a writing tool for plasmonic structures and patterns.

Acknowledgements

The authors greatly appreciate the financial support of the Ministry of Education and Science of Russia through project 14.604.21.0078 (registration number RFMEFI60414X0078). The UV/VIS absorption and luminescence spectra were measured at the Center for Optical and Laser Materials Research, Saint-Petersburg State University, The SEM analysis was carried out at the Interdisciplinary Resource Center for Nanotechnology, (St. Petersburg State University). M. Y. B. gratefully acknowledges the Max Planck Society for a post-doctoral fellowship. S. H. C. acknowledges the financial support by the FP7 EU projects LCAOS (nr. 258868, HEALTH priority) and Univsem (nr. 280566, NMP priority), the NAWION project funded by the German Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) within the cluster of excellence ‘Engineering of Advanced Materials’ (EAM) at the University of Erlangen-Nuremberg, Germany.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16585k

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