Letter pubs.acs.org/JPCL
Single Nanoparticle-Based Heteronanojunction as a Plasmon Ruler for Measuring Dielectric Thin Films Li Li,*,†,‡ Tanya Hutter,§ Wenwu Li,∥ and Sumeet Mahajan*,‡ †
School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China Institute for Life Sciences and Department of Chemistry, Highfield Campus, University of Southampton, Southampton SO17 1BJ, U.K. § Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. ∥ Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China ‡
S Supporting Information *
ABSTRACT: Nondestructive, noninvasive and accurate measurement of thin film thicknesses on dielectric substrates is challenging. In this work a ruler for measuring thin film thicknesses utilizes the heteronanojunction construct formed between a plasmonic nanoparticle and a high refractive index nonplasmonic substrate. The high near-field sensitivity in the nanojunction renders it suitable for measuring the thickness of intervening dielectric thin films. We demonstrate this by controlling the thickness of dielectric spacer layers created by overgrowing SiO2 thin films on commercially available silicon substrates. While Rayleigh (using dark-field) scattering measurements show that the spectral response is well correlated to the thickness of SiO2 spacer layers the distance-dependence is much steeper with surface-enhanced Raman scattering (SERS). Good agreement between 3D simulations and experimental results confirm the plasmon ruler construct’s sensitivity to the dielectric thin film spacing. Thus, we postulate that this single nanoparticle based heteronanojunction configuration can serve as a convenient and simple ruler in metrology of thin films as well as a platform for SERS-based detection even in cases where plasmonically active films are not a suitable substrate. hin film dielectric materials, such as SiO2 and Al2O3, are commonly used in electronic devices, such as field-effect transistors and photovoltaics.1,2 The precise measurement and control of the thickness of dielectric layer is therefore of great scientific and technological interest. Traditionally, atomic force microscopy (AFM) or spectroscopic ellipsometry (SE) have been commonly used to measure the dielectric layer thickness. AFM usually measures trench (scratch) depth, which inevitably compromises the integrity of the thin film, while SE, although nondestructive, measures a change in optical polarization upon reflection/transmission and compares it to a physical model, which can be a source of ambiguity for metrology of thin films. In this work we offer a simple and convenient ruler for such measurements based on single nanoparticle (NP)-based heteronanojunctions. In recent years, plasmon rulers typically constructed from coupled coinage metal (gold, silver, and copper) NP dimers have been developed for minute distance and orientation measurements.3−6 They are based on the distance-dependent wavelength shift of the localized surface plasmon resonance (SPR), which also allows monitoring of DNA hybridization7 and biological activities in living systems.8,9 Recent demonstrations of plasmon rulers have made use of the so-called nanoparticle-on-mirror (NPOM) configuration,10,11 in which a metallic NP is brought into the vicinity of a flat metallic film. In such a configuration, the hybridization between the localized
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SPR of the metallic NP and the propagating surface plasmon polariton of the metallic substrate is remarkably sensitive to the gap distance. Therefore, small variations in the separation distance can be visualized by consistent SPR shifts in the corresponding dark-field scattering spectra.12 This NPOMbased plasmon ruler has also been experimentally characterized using a series of ultrathin molecular spacers in combination with layer-by-layer deposited polyelectrolyte layers ranging from 0.5 to 27 nm in thickness12,13 for sensing dynamic polyelectrolyte swelling in response to pH.14 While the characterization of dielectric thin-film thicknesses, such as polyelectrolyte layers, has been demonstrated using NPOMbased plasmon ruler, to the best of our knowledge, nonplasmon active substrates, for instance Si, have never been employed in such a plasmon ruler system. Moreover, the measurement of the thicknesses of the thin-film dielectric oxide materials, for instance, SiO2, which has a huge impact on electronic device performance, has not yet been demonstrated in such plasmon ruler system. Thus, in this work, a heteronanojunction formed between a nonplasmon active substrate and a single gold Received: April 19, 2015 Accepted: June 2, 2015
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DOI: 10.1021/acs.jpclett.5b00806 J. Phys. Chem. Lett. 2015, 6, 2282−2286
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a−e) Schematic illustrating the fabrication process of the AuNP on Si substrate with variable thickness of SiO2 spacer layer referred to as the SiO2@Si substrate. (f) Thickness of SiO2 spacer layer as a function of O2 plasma treatment time. (g) Representative SEM image showing several well-separated single AuNPs placed on top of the dithiol-functionalized SiO2@Si substrate.
for AuNPs on a gold substrate by other research groups.20,21 The decrease in the near-field observed with further increasing size of AuNPs in our case is attributed to the retardation effect, which is often found in metallic plasmonic systems.22 Moreover, our simulation study suggests that a variety of dielectric substrates can be applied in constructing such heteronanojunctions and the strength of the field generated in the gap almost linearly correlates with the refractive index of some common real materials. It increases compared to an isolated NP for SiO2, SiN, SiC, TiO2 and Si (Figure S2 in the Supporting Information). Among them, the Si substrate is predicted to generate the highest electromagnetic field enhancement of ∼70 times. This suggested that the AuNP on Si substrate configuration could potentially be a good candidate material for constructing an effective heteronanojunction. Consequently the subsequent experiments were carried out using 100 nm AuNPs and Si substrates to create and test the resulting heteronanojunction geometry. To characterize the heteronanojunction plasmon ruler, we created thin spacer layers on Si by overgrowing SiO2 through the O2 plasma-assisted oxidation process.23 The fabrication process is schematically presented in Figure 1a−e. A bare Si substrate was obtained by removing the thin native SiO2 layer (∼2.3 nm) in HF. The Si substrate was then immediately immersed in the tetrahydrofuran (THF) solution containing 1 mM 4,4′-dimercaptostilbene (dithiol). It is noted that because of the extreme tendency of the oxidation of Si substrate, the thiol functionalization was carried out, which also created a separation distance of 1.3 nm, corresponding to the length of the dithiol molecule.24 Although silane is most commonly used for the surface functionalization of Si and SiO2, previous studies have demonstrated that thiol molecules can also chemisorb on a Si surface, predominantly through the sulfur atom.25 Moreover, dithiol was preferred over a silane, as it helps controlling the AuNP density on the silica surface to avoid interparticle coupling due to the lower quality of the selfassembled monolayer produced. The dithiol acts as a molecular linker for anchoring AuNPs and also as a Raman probe for characterizing the local electromagnetic field in the gap of the nanojunction, and as our results show, they locate in the gap of all the nanojunctions constructed, as proved by the intense SERS signals obtained, regardless of the quality of SAM on the silicon or silica surface. The thickness of the SiO2 layer was controlled by the O2 plasma treatment time (Figure 1f). The
nanoparticle (AuNP) is used to measure the thickness of dielectric spacer layers. Conventional NPOM configurations use the interaction between metallic (typically silver or gold) NP and its surface charge image, which results in the confinement of the electromagnetic field at the interface.15 The strength of this interaction is highly sensitive to the separation distance as well as the permittivity of the substrate.15−17 The field gets concentrated and modulated as a result of the asymmetric distribution of the dielectric environment around the plasmonic NP and therefore highly dependent on the refractive index of the substrate. Also, we have shown that a nonconventional NPOM configuration between a AuNP and a dielectric metal oxide is highly sensitive to allow monitoring of surface phenomena on electrodes by surface-enhanced Raman scattering (SERS).18 Furthermore, we predicted that dielectric substrates such as silicon with high real and imaginary components of the refractive index can create high electricfield enhancements in the gap in a heteronanojunction configuration.17 Herein, we report the exploitation of this heteronanojunction configuration as a plasmon ruler that enables nondestructive and sensitive measurement of dielectric layer thickness on a nonplasmon active substrate and as a convenient SERS sensor. The heteronanojunction-based ruler is constructed by simply placing a single AuNP on silicon (high refractive index substrate), which thus becomes sensitive to the gap distance set by the intervening thin film between the NP and the substrate. This system is characterized by correlating the scattering behavior with the thickness of intervening SiO2 layer. The distance-dependent near-field enhancement is further mapped by highly sensitive single-NP SERS measurements and is verified by 3D simulations, which correlate well with experimental results. In addition, we show that this singleNP based heteronanojunction ruler system serves as a convenient SERS sensor for the detection of biomolecules, making it useful for both metrology and analytical applications. At the outset we sought to gain an understanding of the nearfield behavior of our configuration (Figure 1), wherein each single immobilized AuNP couples to the underlying nonplasmonic substrate and functions as a ruler. We carried out 3D simulations using COMSOL Multiphysics.19 We find that the maximum field enhancement is achieved for the size of 100− 120 nm (see Materials and Methods and Figure S1, Supporting Information). This observation is in line with results observed 2283
DOI: 10.1021/acs.jpclett.5b00806 J. Phys. Chem. Lett. 2015, 6, 2282−2286
Letter
The Journal of Physical Chemistry Letters
Figure 2. (a) Dark-field scattering spectra of individual AuNPs on Si substrates with increasing thickness of intervening SiO2 spacer layer in this heteronanojunction. The intensities are normalized. The red dotted line shows the laser excitation in both panels a and b. (b) Dipolar peak positions as a function of the SiO2 layer thickness are plotted and fitted by an exponential function. The analysis was carried out on 15 AuNPs at each SiO2 thickness; error bars showing the standard deviation are included. The shaded boxes indicate the full width at half-maximum (FWHM) of dipolar plasmon resonance peak of the spectra in panel a. The fwhm was calculated by fitting the dipolar peaks with a Lorentzian. (e−i) The corresponding dark-field scattering images of individual AuNPs on Si substrates with an increasing thickness of SiO2 spacer layer are shown. The thickness mentioned above each image is the thickness of the SiO2 layer only and does not include the molecular layer (∼1.3 nm), which is present in all cases (e−i).
growth in film thickness was confirmed by an ellipsometer to correlate results obtained with our plasmon ruler. The Si substrates with the thin defined silica layers (SiO2@Si substrates) were immersed in 1 mM solution of dithiol in THF for 1 h to allow the formation of a monolayer of dithiol molecules and subsequently rinsed with THF to remove the excess molecules from the surface. Then, a droplet containing AuNPs was cast onto the dithiol functionalized SiO2@Si substrates for 30 min, followed by rinsing and air-drying to remove unattached AuNPs. A representative SEM image is shown in Figure 1g; the NPs were dispersed on the substrate, with distances usually larger than 1 μm, and thus any electromagnetic coupling between neighboring AuNPs can be ignored. Moreover, the AuNPs (BBI solutions) were relatively monodisperse (
