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Interstitial Dependent Enhanced Photoluminescence: A Near-field Microscopy on Single Spheroid to Dimer, Tetramer and Few Particles Gold Nanoassembly Mohammad Kamal Hossain, Masahiro Kitajima, Kohei Imura, and Hiromi Okamoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10452 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017
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The Journal of Physical Chemistry
Interstitial
Dependent
Enhanced
Photoluminescence:
A
Near-Field
Microscopy on Single Spheroid to Dimer, Tetramer and Few Particles Gold Nanoassembly
Mohammad Kamal Hossain1,2,*, Masahiro Kitajima3, Kohei Imura4, and Hiromi Okamoto5
1
University of Tsukuba, Tsukuba, Ibaraki 305-8571 and National Institute for Materials Science,
Sengen, Tsukuba, Ibaraki 305-0047, Japan. 2
Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and
Minerals, Dhahran 131261, Kingdom of Saudi Arabia. 3
Department of Applied Physics, National Defense Academy, Hashirimizu 1-10-20, Japan
4
Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan.
5
Institute for Molecular Science and The Graduate University for Advanced Studies, Okazaki
444-8585, Japan.
*Correspondence to: Mohammad Kamal Hossain, Dr. Res. Prof. (RE-III) Center of Research Excellence in Renewable Energy (CoRERE) King Fahd University of Petroleum and Minerals (KFUPM) PO Box 5040, Dhahran 31261, Dammam Kingdom of Saudi Arabia Tel: 013 860 1058, Fax: 013 860 7321, Email:
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Abstract Interstitials to be active or inactive depend on the characteristics of nanoaggregate. In this perspective, near-field scanning optical microscopy has been utilized to probe well-defined monomer, dimer, tetramer and small assembly of gold nanoparticles of 60 nm diameter with spatial resolution of ~35 nm. Confinement of localized surface plasmon resonances (LSPRs)mediated near electromagnetic (EM) field distribution was observed through two-photon induced photoluminescence (TPI-PL), since TPI-PL emission is proportional to fourth power of induced EM-field. It was revealed that the intensity of TPI-PL observed in closely connected nanoparticles was very strong with reference to those obtained in presence of isolated nanoparticles of the same size. Amongst the closely connected nanoparticles, interparticle gap and interparticle axes played a vital role in TPI-PL emission enhancement, as was evident and observed directly through TPI-PL intensity distribution. Correlated local geometry and emission distribution confirmed that interstitials along the active interparticle axes exhibits confined and enhanced TPII-PL emission. The emission intensity for dimers was found higher than that of tetramer of gold nanoparticles. In case of small assembly of same gold nanoparticles, intensity was observed to be the highest of all along with wider full width at half maximum of 188.09 nm, more than double of those obtained at dimers and tetramer. The observations were further demonstrated as a function of EM near-field distribution, as it is well-known that TPI-PL emission is enhanced by LSPRs-assisted EM field localization. Model systems were designed along with geometries and parameters very similar to those under investigation. Finite-difference time-domain analysis was carried out to deduce near EM-field distributions at various deterministic factors such as size, interparticle gap, incident polarization etc. A plausible explanation of enhanced and confined TPI-PL emissions in archetype interstitials of well-known plasmonic systems was highlighted.
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1. Introduction Plasmonic nanoparticles exhibit exciting and unique optical properties that are resulted from coherent oscillation of free or loosely-bonded electrons located near the surface1–10. This electronic feature becomes prominent in presence of incident photon of appropriate energy and as a consequence, induced localized surface plasmon resonances (LSPRs) and surface plasmon polariton (SPP) intervene in the play. Confinement of such resonant oscillation is the key to achieve giant electromagnetic (EM) near-field enhancements usually observed at the hotsites of constituent nanoparticles. The intensity and distribution of localized EM near-field have been known to be dependent on a variety of factors viz. shape, size, composition of nanoparticles, surrounding medium, underneath substrate, local geometry of nanoaggregate, etc10–13. Whilst all of these factors are important to achieve effective plasmonic characteristics, as for EM near-field enhancement, proximity of two or more plasmonic nanoparticles surpasses inherent characteristics and therefore has recently emerged as an increasingly important aspect in the field of nanoplasmonics14–18. EM near-field between two nanoparticles begins to collapse and LSPR-mediated EM-field turns into coalescence and hybridization through SPP when the separation distance of the archetype dimer is much smaller than the size of the individual nanoparticle and mismatch happens between interparticle axes and incident polarization19–25. In such scenario, there is phenomenal alteration in both the near-field coupling of dimer and SPR related characteristics26–28. It has been evident that EM near-field at the interstitials increases with decreasing separation distance between two adjacent nanoparticles provided that polarization direction is in-plane with interparticle axis. At this very optimum condition, a fraction of variation in local geometry and interparticle axes affect tremendously in surface plasmon coupling and hence near-field phenomena can be tailored. Strong EM near-field and the subsequent unique optical properties associated with two or more plasmonic nanoparticles in close proximity to each other have proven to be useful for a wide range of features29–38. Amongst many characteristics, exceptional transmission, EM field confinement at interstitials, coalescence, energy percolation, superlens etc. are few that have already been reported39–41. Any optical process, such as surface-enhanced Raman scattering (SERS), photoluminescence (PL), surface-enhanced fluorescence (SEF), absorption, etc. will be greatly affected under such scenario. Despite the significant progress on
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the theory side, experimental measurement of LSPR related phenomena is very challenging38,42– 46
. Two major obstacles are the difficulties in preparing the interstitials with distances down to
the nanometer range and measuring the correlated characteristics with meticulous accuracy. Self-assembled colloidal assembly has been well-known for devising controlled interstitials whilst near-field scanning optical microscopy (NSOM) has the ability to detect optical characteristics beyond the diffraction limits and simultaneously capture corresponding interstitials through shear-force topography47–49. PL is an interesting process that can be involved with single photon or multi-photon excitations. Although PL emission is of great interest in many fields, its applicability is limited due to low quantum efficiency, ~10-10. An extensive report by Boyd et al. in 1986 revealed how the corrugated gold film enhanced PL with reference to that of smooth surface50. It was welldemonstrated that PL emission not only depends on the metal studied but also on LSPR-mediate EM field induced at corrugated surface. PL emission is generally classified based on number of electron excitation, such as, single-photon-induced photoluminescence (SPI-PL), two-photoninduced photoluminescence (TPI-PL) and three-photon-induced photoluminescence. Generally speaking, three steps are involved in PL process; (i) excitation of d-electron to sp-band to generate electron-hole pairs, (ii) partial relaxation to phonon lattice or energy transfer through plasmonic mode and (iii) photon emission due to electron-hole recombination near the fermi level. Most of the reports available till date confirmed PL emission of gold at around 2.2 eV implying the fact that the peak at 540 nm does match with energy gap between the upper d-band and Fermi level. It is to be noted that nonradiative relaxation process in metals is faster than that of radiative electron-hole recombination. We reported previously that the NSOM setup used in this investigation is capable enough to induce TPI-PL from gold nanoparticles and nanostructures. Thanks to the NSOM facility, correlated metrology in nanoscale opened up new avenues to explore plasmon-assisted optical characteristics observed in colloidal gold nanoassembly. H. Okamoto group has been able to detect week TPI-PL using state-of-art facility like NSOM49,51–54. While in plasmonic system SPI-PL remained less considered due to low quantum efficiency, TPI-PL was observed to be several orders of magnitude higher mainly due to resonant coupling with LSPRs. Multi-photon-induced photoluminescence, such as TPI-PL, is of great interest due to its vital role in oncology, for instance, early cancer diagnosis through noninvasive and deep in vivo imaging55,56. Therefore, it is inevitable to explore the behavior of 4 ACS Paragon Plus Environment
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TPI-PL in presence of well-defined plasmonic system, such as, dimer, tetramer, and small assembly of gold nanoparticles. Although correlated topography and metrology in nanoscale for simple interstitials happened to be in well-defined dimer and trimer have been demonstrated, a further insights and critical look is required for advanced plasmonic systems47,51. In this communication, archetype interstitials obtained in well-defined dimer, tetramer and small assembly of gold nanoparticles were probed and characterized with aperture-NSOM setup of sufficiently high spatial resolution. TPI-PL measurements confirmed enhanced and confined emission distributions at the interstitials of closely connected nanoparticles with reference to those obtained in presence of isolated nanoparticles of the same size. TPI-PL emission was affected by interparticle gaps and interparticle axes, as was evident and observed directly through TPI-PL intensity distribution. A plausible validation of such observation was further demonstrated in light of EM near-field distribution, as TPI-PL is reported to be proportional to the fourth power of localized EM field induced in respective plasmonic system.
Finite-
difference time-domain (FDTD) analysis was carried out using model systems considering the geometry and parameter very similar to those under investigation. Near EM-field distributions were deduced at various deterministic factors such as size, interparticle gap, incident polarization etc. It is inevitable to understand such correlated nanoscale topology and corresponding metrology for moving forward in various applications that get affected by localized plasmons and utilize deep in-vivo and non-invasive technique such as TPI-PL.
2. Experimental 2.1 Sample preparation A cover-slip (thickness ca. 0.1 mm) pretreated with trimethoxy-[3-(methylamino)propyl] silane was used as substrate and subsequently gold nanoparticles were immobilized atop. Colloidal gold (diameter ca. 60 nm) received from BBInternational (Cardiff, UK) was used without any further treatment. Since pre-treated cover-slip contained S+ terminals available at the surface, the gold colloids with negative charges got immobilized easily on the substrate. The immobilization might further be controlled by tuning colloids’ surface charge and external local-forces such as Vander-Waals attraction, electrostatic repulsion, evaporation-assisted convection, surface tension at meniscus, etc48,57–59. Morphology of the sample was verified by scanning electron microscope (SEM, JEOL FE6500) and topographic measurements of the NSOM. 5 ACS Paragon Plus Environment
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2.2 NSOM setup and measurements A Ti:sapphire laser (λ = 800 nm,
