Article pubs.acs.org/JPCC
Quantum Size Effects in the Optical Properties of Ligand Stabilized Aluminum Nanoclusters Sukhendu Mandal,† Juan Wang,‡ Randall E. Winans,‡ Lasse Jensen,*,† and Ayusman Sen*,† †
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
S Supporting Information *
ABSTRACT: Here we describe an approach to the synthesis of small ligand stabilized Al nanoclusters by catalytic decomposition of alane using Ti(OiPr)4 as catalyst. The selected area electron diffraction (SAED) and elemental analysis are consistent with the presence of Al in the clusters. The cluster sizes are measured by the small-angle X-ray scattering method in air-free conditions. The absorption maximum exhibits red shifts when cluster sizes decrease from 4 to 1.5 nm. A two-layer Mie theory model indicates that the electron conductivity in the Al core is reduced due to a combination of quantum size effects and chemical interaction with the ligand shell resulting in the observed red shift with decreasing size. The red shift is shown to scale with the inverse radius in good agreement with a spill-out model. Furthermore, the results are consistent with time-dependent density functional simulations for a small ligand stabilized Al cluster. Remarkably, we find that the absorption maximum is significantly red-shifted compared with that expected from simulations based on the bulk dielectric constant. This is true even for the larger nanoclusters with diameters of 4 nm. This indicates that small ligand protected Al clusters behave significantly different from similar Ag and Au clusters.
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INTRODUCTION There is a significant interest in understanding and controlling the properties of metal clusters with dimensions of a few nanometers. Of particular interest are small monolayer protected clusters, which can be synthesized with precise control of the number of metal atoms and ligands.1−3 These small clusters exhibit properties that depend strongly on the size of the cluster, which provides a unique opportunity to fabricate novel materials with desired properties. Small Ag and Au clusters stabilized by encapsulation in organic dendrimers have been shown to exhibit strong fluorescence at room temperature.4,5 Furthermore, small Ag clusters have been shown to produce enhanced Raman signals characteristic of the scaffold used to stabilize the clusters in solution.6,7 Thus, these nanoclusters are of great interest due to their unique optical and electronic properties, and their small size makes them particularly suitable for biological sensing. The optical properties of large (>20 nm) metal nanoparticles are characterized by the plasmon excitation which is a collective oscillation of the conduction electrons. For smaller nanoparticles the optical properties become strongly dependent on the size of the metal nanoparticle. This is especially true for particles smaller than 5 nm due to quantum size effects.8−10 Quantum effects become prevalent when the dimension of a nanoparticle is decreased further to dimensions comparable with the Fermi wavelength of the electron. This smaller size is associated with optical, electronic, and chemical properties differing from those of the larger nanoparticles. Due to © 2013 American Chemical Society
quantum confinement, these small metal clusters show molecule-like electronic structures, and the characteristic plasmon bands are replaced with discrete electronic transitions. Recently, electron energy-loss spectroscopy (EELS) has illustrated that for ligand-free Ag nanoparticles the plasmon resonance continuously blue shifts by around 0.5 eV as the size reduces from 20 nm to less than 2 nm.11 In contrast, a recent study on ligand protected Ag nanoparticles showed an initial blue shift as the size was reduced below 20 nm; however, a turnover occurred at around 12 nm, and a strong red shift was observed.12 This red shift was attributed to a significant reduction in the conductivity of the outer layer of metal atoms due to the ligands. This shows that for a small metal nanoparticle the optical properties depend sensitively on the size, stabilizing ligands, and for very small clusters on the exact atomic arrangements. Thus, investigating small metal clusters by optical spectroscopy provides a way of understanding the electronic structure of these systems and allows for developing size-tunable properties. Although plasmonic properties of Au and Ag have been investigated extensively, the literature on Al is surprisingly limited. Au and Ag exhibit plasmon excitations at wavelengths longer than 590 and 350 nm, respectively,13 while Al should be plasmonically active from 200 nm to just below 800 nm due to Received: October 23, 2012 Revised: February 20, 2013 Published: March 11, 2013 6741
dx.doi.org/10.1021/jp310514z | J. Phys. Chem. C 2013, 117, 6741−6746
The Journal of Physical Chemistry C
Article
precipitation. Elemental analysis showed the presence of Al and Si (molar ratio: 1.0:0.73). Aluminum Nanoparticles of 2.5 and 4 nm. Analogous procedures were used for two other particle sizes of 2.5 and 4 nm, with different alane to ligand ratios. For particles with 2.5 nm, the alane to LiN{Si(CH3)2}2 ratio was 7:1, and for 4 nm particles it was 20:1. Unfortunately, with the current synthetic strategy, attempts to synthesize Al nanoparticles larger than 4 nm were unsuccessful because of significant and irreversible aggregation during synthesis with higher alane concentrations, which resulted in mirror-like precipitation. Characterization Techniques. Electron Microscopy. Samples for transmission electron microscopy (TEM) analysis were prepared by adding droplets of a dilute dispersion of the Al nanoparticles on amorphous carbon-coated copper grids. The samples were then dried in a glovebox. TEM images (including selected area electron diffraction (SAED)) were collected by a JEOL 2010F Field Emission TEM at 200 kV. UV−vis Absoprtion Spectra. UV−vis absoprtion spectra were collected on a Hewlett-Packard 8453 diode-array UV/ visible spectrometer. Small-Angle X-ray Scattering (SAXS). The SAXS data were obtained at the beamlines 12-ID-B and 12-ID-C at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). SAXS data of the samples were measured for 0.5 s exposure time, and the scattered X-rays (12 keV) are detected by a MAR 165 CCD detector. The scattered intensity has been corrected for absorption, the empty cell scattering, and instrument background, and the differential scattering cross section was expressed as a function of the scattering data Q, which is defined as: Q = (4π/λ)sin θ, where λ is the wavelength of the X-rays and θ is the half angle of the scattering. Theoretical Methods. The absorption spectra of the Al nanoparticles was simulated using a multilayer Mie model.12,24,25 The nanoclusters were modeled as consisting of two layers. The core layer representing Al atoms where the dielectric constant was described using a size-corrected Drude model
its free-electron-like character and high bulk-plasmon frequency.14 The accessibility of the electromagnetic spectrum in the 200−800 nm range makes Al an excellent substrate for surface-enhanced fluorescence15 and surface-enhanced Raman spectroscopy16 in this region. However, Al rapidly oxidizes when exposed to air, forming a thin Al2O3 layer.17 The presence of the Al2O3 layer is expected to affect its plasmonic properties. Chan and co-workers have demonstrated that the presence of a thin native Al2O3 layer leads to a red shift in the surface plasmon reseonance.18 Consequently, the plasmonic properties of Al have just recently received attention.18,19 Al clusters have also been studied owing to their electronic structure and possible applications as cluster assemblies with tunable properties or as catalysts.20,21 Experimentally deducing the correct plasmon size dependence of the metal nanoparticles in the nanoregime requires synthesis of nanoparticles with (i) narrow size distribution, (ii) high uniformity in morphology and surface environment, and (iii) excellent dispersibility. The catalytic decomposition of alane by titanium isopropoxide is a well-known approach to the synthesis of aluminum nanoparticles.22,23 Here, we report the synthesis of Al nanoparticles with sizes ranging from 1.5 to 4 nm that meet the above criteria. Optical absorption spectra show a red shift as the particle size decreases which is attributed to a combination of quantum size effects and chemical interaction with the ligand shell. The phenomenon was explained using multilayer Mie theory. The red shift is shown to scale with the inverse radius in good agreement with a spill-out model, and the results are consistent with time-dependent density functional theory.
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EXPERIMENTAL SECTION Materials. Alane:N,N-dimethylethylamine complex solution in 0.5 M toluene and titanium isopropoxide were purchased from Aldrich. Lithium−bis(trimethylsilyl)amide was purchased from Alfa Aesar. Toluene was dried by passing through an activated alumina column followed by deoxygenation by passing over a copper catalyst. Carbon film supported copper TEM grids (200 mesh) were purchased from Electron Microscope Sciences. All glassware (oven-dried), reactants, and solvents were stored in a glovebox filled with argon. All reactions and product manipulations were carried out under an inert atmosphere using glovebox techniques (MBraun UNIlab glovebox maintained at
