Modulating Up-Energy Transfer and Violet-Blue Light Emission in

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Modulating Up-Energy Transfer and Violet-Blue Light Emission in Gold Nanoparticles with Surface Adsorption of Poly(vinyl pyrrolidone) Molecules S. Ram†,* and H.-J. Fecht‡ † ‡

Materials Science Centre, Indian Institute of Technology, Kharagpur-721 302, India Institut f€ur Mikro- und Nanomaterialien, Universit€at Ulm, Albert-Einstein Allee-47 D-89081, Ulm, and Forschungszentrum Karlsruhe, Institute of Nanotechnology, Karlsruhe, D-76021, Germany ABSTRACT: Au-nanoparticles (Au-NPs) with a polymer surface layer of poly(vinyl pyrrolidone) (PVP) exhibit a combined Stoke emission of an intense violet-blue band 300550 nm in a coupled polaritonplasmon structure. Nascent Au-NPs, mostly polygonal shapes (1045 nm widths), cobridge an adhesive PVP surface layer (23 nm thickness) that congregates the Au-surface-plasmons (SPs) in a localized state, showing a marked blue shift of Au-SPs band 450 nm from 530 nm in bared Au-NPs. In an up-energy transfer, the SPs recombine coexcited n f nπ* (PVP) photons (∼412 nm) and ultimately emit two 260 and 345 nm anti-Stoke bands. When increasing PVP concentration to a critical value of 11.0 g L1 (0.1 mol L1) in a rheo-optical Au-PVP nanofluid, the first band grows in intensity markedly at the expense of the other band like the Stoke bands. Dielectric field and viscosity of the fluid mediate the Au r PVP energy transfer useful for biological and optical probes.

1. INTRODUCTION Polar polymer molecules when adsorbed on nascent gold nanoparticles (Au-NPs) of selective sizes and/or shapes often tailor light emission of the Au-NPs over a wide spectrum from ultravioletvisible to near-infrared (IR) region useful for optical probe and other devices.17 Receptor-mediated targeting with surface enhanced Raman scattering allows sensitive detection of living biological cells or tissues. Dielectric field near the metalinsulator interface in conjunction with correlated polymer molecular surfaces in examples such as surface-modified AuNPs helps the surface plasmons (SPs) from converging at the interface.810 Such kinds of surface designed Au-NPs, often called labeled Au-NPs, with biocompatible molecules of large polymer molecular surfaces in general need elaborate studies of optical properties to make use of them in biological sensors,4,10 optical imaging of cancer,3,11 drugs and drug-delivery,3,7 nonlinear optics,1,12 catalysis,17 and microelectronics.12,13 A metal particle with a dielectric molecular surface yields multiple light interaction events of the dielectric and metallic scatterers, with a closed loop of the scattered light.14,15 A photon absorbed at the interface shares radiative energy with localized SPs in the metal NPs when coupled in-phase with the incident light. As a result, resonating structures arise with a modulated quantum efficiency of the light emission in localized SPs in surface-confined metal NPs such as gold. Enhanced light emission depends on the loop of the scattered light and the medium scattering strength,14,15 which is inversely proportional to the photon mean free path.15 Au-NPs scatter and absorb light strongly at the oscillation wavelengths of the SPs. Popov et al.15 observed marked amplification of visible light emission in r 2011 American Chemical Society

polymer films of poly(vinyl alcohol) with Rhodamine dyes and Au-NPs. Overlap in the emission bands in the SPs and dye controls the output intensity. In a resonance, an emitting photon of SPs gains energy transferred from coexcited dye molecules, with random lasing.15,16 Grafting Au-NPs with an adhesive molecular layer of a polymer embeds and immobilizes localized SPs in the individual Au-NPs. Otherwise, the Au-SPs band diverges widely in bare Au-NPs from the visible to near IR range depending on their effective size and/or shape.17,18 In this Article, we describe an up-energy transfer of light emission in Au-NPs of a grafted surface with a correlated molecular surface layer from a reactive polymer such as poly(vinylpyrrolidone) (PVP) of small molecules and dispersed in form of a rheo-optical nanofluid in water. We studied the Stoke (300560 nm) and anti-Stoke (200400 nm) emissions by varying rheology of the nanofluid in terms of PVP molecules (0.1100 g/L) with a fixed value of 0.001 mM of Au-NPs, which were created in situ in a specific synthesis process. Co-excited nonbonding CdO electrons (n) of PVP molecules transfer energy to the localized SPs at a resonance. Selective excitations describe a maximum effect with energy loss in the n r nπ* band at a critical percolation value of 11 g/L PVP (0.1 mol/L monomers). Biocompatible PVP molecules are widely used in medicals, for example, targeted drug delivery, therapeutic agents, tablet disintegrating agents, lubricator and antitoxic assistant for drugs, bladder cancer detection and diagnosis, and artificial cartilages in Received: June 28, 2010 Revised: March 14, 2011 Published: April 01, 2011 7817

dx.doi.org/10.1021/jp105941h | J. Phys. Chem. C 2011, 115, 7817–7828

The Journal of Physical Chemistry C

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cartilages replacement.1922 Modulated rheology, energy transfer, and optical properties in the presence of Au-NPs are highly demanded in such applications of nontoxic polymer nanofluids.

loaded on a specific carbon-coated copper grid. X-ray photoelectron spectroscopy (XPS) was studied with a PHI 5800 XPS spectrophotometer in analyzing the particle surfaces.27,28 Stoke and anti-Stoke light emissions were studied by exciting Au-SPs and CdO n-electrons from the Au-NPs with a surface PVP layer at selective wavelengths by a pulsed xenon lamp (7.3 W power at 50 Hz, 10 nm widths for the entrance or the emission slit) in a Perkin-Elmer model-LS 55 luminescence spectrometer.

2. EXPERIMENTAL DETAILS A simple reaction Au3þ f Au with a dilute sample of PVP molecules in hot water is used to create Au-NPs and in situ grafting of a surface PVP layer at an early stage of growth of a stable sample. In hot water, dispersed PVP molecules serve not only as a model reductant but also govern clustering and growth of Au-NPs in small templates of a polymer complex.23,24 A polymer complex passes through transient stages25,26 before it thermally disintegrates into Au-NPs at modest temperature. As given in Table 1, in the synthesis, a fixed 0.1 mL volume of a dilute HAuCl4 3 3H2O solution (0.01 mol/L) was added slowly to selective batches (5.0 mL) of 0.1100 g/L PVP (average molecular weight Wp = 40 000) in water at 5560 C with magnetic stirring. Dispersed PVP molecules induce and drive a reaction Au3þ f Au so that the sample color turns gradually from a pale yellowish f a reddish f a faint violet bluish (grayish on 1.0 g/L or smaller PVP). On 2025 min stirring in a hot condition, a byproduct acid evaporates out, and meanwhile PVP molecules bridge a surface layer with Au-atoms on the native Au-NPs surfaces. Au-NPs immobilized on grafting disperse easily via residual PVP molecules in the form of an Au-PVP nanofluid in water. Next, by subjecting to an ultasonication (at 20 kHz frequency and 250 W power of the ultrasonics by a specific sonicator) for 10 min, soft agglomerates break down in singly dispersed Au-NPs with an adhesive PVP layer, showing a deepened color to the naked eye. A sonication not only leads to sever soft agglomerate but also helps grafting Au-NPs in a specific cross-linked structure via the polymer molecules of small assemblies. Stable samples when cooled at room temperature were studied in terms of the light emission in correlation to the rheology and microstructure as follows. The shear viscosity (η) was studied at 10100 s1 shear rates using a rotational rheometer (TA Instruments, model AR-1000) in parallel plate geometry as described elsewhere.24 This method measures a precise η-value at a shear rate from a nanofluid of few drops on the plate, accounting for how PVP molecules interact with Au-NPs. Images of the Au-NPs were studied with a field emission scanning electron microscope (FESEM) of ZEISS SUPRA-40 and a transmission electron microscope (TEM) of JEOL JEM-2100. In the SEM studies, the samples were spincoated on a (110) silicon plate, and then a thin carbon coating was sputtered to make a conducting surface. TEM images were taken from a sample dispersed in acetone by sonicating and then

3. RESULTS AND DISCUSSION 3.1. Formation of Au-NPs with a Surface Adsorption Layer. A dilute PVP sample prepared freshly in hot water occurs

in dispersed molecules of thin layers via polar H2O molecules. Mechanochemical stretching27,29 when stirring cuts down a sample as thin as single molecules. Refreshed CdO groups free from H-bonding design such PVP molecules both as a model surfactant and as a reductant. When adding HAuCl4 f AuCl4 þ Hþ, they adsorb AuCl4 in head-groups and form an ionpolymer complex. A destabilized complex in hot condition mitigates Au3þf Au atoms in a local heat-induced transfer reaction at low temperature 5560 C in ambient air. No reaction begins unless using a refreshed PVP surface with CdO groups in a hot water. Reaction species in small groups (templates) control a local reaction without adding any other activator at any stage in this simple reaction. As expressed in Figure 1, Au atoms in a redox reaction of AuCl4 with a PVP molecule cluster and grow in support over a planar PVP template.

Figure 1. Proposed chemical reaction steps in reducing gold ions via a polymer complex from dispersed PVP molecules in a hot water. R: Carbon chain in part of a PVP molecule.

Table 1. Surface-Modified Au-NPs with a Polymer Layer Dispersed in a Rheological Medium of PVP Molecules of Selective Concentrations in Watera initial concentrations PVP (g/L)b

volumes taken (mL)

Au-salt (M)

PVPc

Au-saltc

Au-content (wt %)d

monomer/Au ratio

100 (0.90)

0.01

5.0 (4.50)

0.1 (0.001)

0.001

4500

50 (0.45)

0.01

5.0 (2.25)

0.1 (0.001)

0.002

2250

20 (0.18)

0.01

5.0 (0.90)

0.1 (0.001)

0.005

900

10 (0.09)

0.01

5.0 (0.45)

0.1 (0.001)

0.010

450

1.0 (0.009)

0.01

5.0 (0.045)

0.1 (0.001)

0.100

45

0.1 (0.0009)

0.01

5.0 (0.0045)

0.1 (0.001)

1.000

4.5

a

The reaction was carried out in a batch of 5.1 mL of total sample. b Values in the parentheses refer to the values in mol/L of monomers. c Values in the parentheses refer to the values in mM. d Au-content as per the Au-PVP chemical composition. 7818

dx.doi.org/10.1021/jp105941h |J. Phys. Chem. C 2011, 115, 7817–7828

The Journal of Physical Chemistry C A byproduct chlorinated PVP molecule can be analyzed with O1s XPS band and CdO stretching vibration when part of the CdO groups converts to “COCl” in driving the reaction. As usual,28,30 the O1s XPS band upshifts by 0.41.0 eV from an initial value of 529.54 eV as per the “COCl” yield. The counterpart CdO stretching band (infrared) is decreased from 1640 to 1630 cm1 with concomitantly reduced intensity. As also Longenberger and Mills25 proposed in a reaction Au3þ f Au with poly(vinyl alcohol), a metal ionpolymer complex degrades in steps Au3þ f Au2þ f Auþ f Au only if AuCl4 chlorinates PVP molecule on a counterpart reaction. PVP molecules revert back when chlorine desorbs off during a prolong heating. As can be seen from SEM images, Au atoms, which mainly confine onto a polymer complex surface, grow in a planarshaped particle as long as adhering to the parent surfaces (planar). As demonstrated in Figure 2a,b, an Au-particle when growing to a critical value offers an electrode so that PVP molecules in an intimate contact cobridge a surface layer via the Au-atoms on a native metal surface. In a solution, PVP molecules dispersed around in thin layers mediate such Au-NPs in cross-linking in a network, that is, a rheological fluid of an enhanced η-value as will be discussed later. In this model reaction, residual Au3þ f Au species, if any, thus grow in smaller particles of highly reactive nature. Immobilized Au-NPs with a surface polymer layer exhibit effectively large surfaces to account for a surface-enhanced absorption, scattering, or emission of light shown in both the Au-NPs and the surface molecules. As depicted in Figure 2c, on an extended metal surface, PVP molecules line-up one another in a molecularly ordered layer. A coplanar pyrrolidone ring with the side groups (Figure 2a) can design a coplanar layer onto a particle in several ways. Strong intermolecular intercalations can support a model conformer in

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Figure 2c not only on the stereosequence but also in view of the side group orientations along an extended network layer. In Figure 2c, as marked by the area ABCD in a group of m = 4 monomer units, a PVP molecular layer bears a surface charge layer of localized 2p2(O) and 2s2(N) n-electrons. Nearly equal surfaces in the CdO and CN moieties support a resonance electronic structure in the n-electrons.29,31 As electron donor, the CdO donates an n-electron (n1) to the neighboring N atom (electron acceptor) in a >NCdO moiety in an intramolecular charge transfer. High polarity developed in monomers in a resonance structure eases the grafting of a polymer layer, with a highly localized density state of n-electrons. Average molecular center (*) and charge center (•), staying well apart, are a source for inducing a perturbation in the n-electron density subject to either a change in a local atomic configuration or a change in a local charge distribution. Conformers sequence one over others (Figure 2d) perpendicular to the metal surface (z-axis) with n-electrons embedded in parallel arrays (Figure 2e). 3.2. Microstructure in Au-NPs in Nanofluids. Figure 3a shows FESEM images from a nanofluid, 0.001 mM Au-NPs in 10 g/L PVP in 5.1 mL of water, that is, a sample of 0.01 wt % AuNPs with an average number density of Fp ≈ 450 PVP-monomers per Au atom (Table 1), which exhibits desirably a highly intense light emission. Au-NPs in this example are embedded in PVP molecules such that they appear of oval shapes, with 2550 nm widths and 510 nm thickness. More realistic shapes of them of thin plates (1245 nm widths), mostly pentagon, hexagon, or octagon (marked in the letters A, B, and C in Figure 3b), could be observed in terms of TEM images when the measured part of Au-NPs dispersed from a dilute nanofluid in acetone by a sonication. As usual, the excess polymer layer piles off the

Figure 2. Adsorption and cobridging of (a) PVP molecules in (b) a surface layer via Au-atoms on a gold particle, showing (c) a planar cross-section of a model polymer configuration with aligned molecules, and (d) polymers sequenced one over others along the z-axis (perpendicular to the metal surface) with (e) n1 and n2 electrons embedded in parallel arrays. 7819



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Figure 3. (a) SEM and (b) TEM images in 0.001 mM Au-NPs dispersed in 10 g/L PVP in 5.1 mL of water, with PVP-modified model polygons in the right. A magnified TEM image in the top shows a PVP-modified surface layer of a distinct whitish contrast.

Figure 4. TEM images and SAED patterns from 0.001 mM Au-NPs dispersed in (a,b) 100 g/L and (c,d) 0.1 g/L PVP in 5.1 mL of water, with (e) a magnified image of (c) a surface layer.

Au-NPs and ultimately gets dissolved in the acetone. This is a routine practice of preparing a sample suitable for TEM imaging. TEM images collected in this way measure Au-NPs with an

inherent polymer surface layer, if any, very closely. Consistent with model polygons given in the bottom of Figure 3b, magnified images, for example, a selective one is given on the top, reveal a 7820


The Journal of Physical Chemistry C distinct surface layer of a whitish contrast (2.5 nm thickness), which adheres very intimately to the darkish regions of the Au-NPs. To analyze how Fp-value affects average size and/or shape of Au-NPs, in Figure 4 we compare TEM images from the samples processed using either a large Fn = 4500 value (0.001 wt % Au-NPs) or a small Fp-value 4.5 (1.0 wt % Au-NPs). The first sample (Figure 4a) displays mostly plates of rounded polygons with effectively smaller sizes (1025 nm widths) from those shown above in Figure 3b, indicating an obvious effect of a rheological fluid on creating small Au-NPs. Bigger polygons, 4590 nm widths, are observed in the other sample (Figure 4c). All of these polygons look alike near spheroids, discs, or prisms on a sufficiently thick surface coverage layer, for example, a thickness of 3.1 nm seen in an image in Figure 4e. A more viscous sample 10 g/L PVP (Figure 3b) stops it from growing beyond 2.5 nm.

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In Figure 4b, selected area electron diffraction (SAED) has (111), (200), (220), (311), and (420) reflections of interplanar distances (dhkl) 0.2315, 0.2010, 0.1420, 0.1210, and 0.0897 nm, with 1.6% smaller size a = 0.4014 nm from an Fm3m fcc lattice of pure gold of 0.4079 nm.32 The diffraction ring in the (311) reflection is widened in overlapping a broad reflection from a surface layer (with an interlayer distance ∼0.121 nm) on the Au-NPs. Comparing to an interlayer (006) distance of 0.140 nm in graphene oxide (0.13 nm in graphene),33,34 a polymer layer rears a reasonably smaller value. As shown in Figure 4d, well-resolved (111) diffraction arrays arise in Au-NPs of effectively larger polygons, characteristically single crystallites. A surface enforced structure on a surface layer breeds a smaller lattice in a high pressure surface effect.35,36 Figure 5A shows high-resolution TEM (HRTEM) images of Au-NPs (thin plates) that display a discrete boundary layer

Figure 5. (A) TEM images in polymer embedded Au-NPs of (a) a surface layer bounding (b) a lattice grown in (111) planes, with (c) SAED pattern in the layer, after a sample of 0.001 mM Au-NPs in 0.1 g/L PVP in 5.1 mL of water. (B) Broad fringes along with the (200) and (220) images bounded by the (111) plane, with (C,D) magnified P and Q regions. 7821



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(Figure 5a) even when embedded in a polymer. Arrays of lattice images (Figure 5b) arise from the central zone in the (111) planes, 0.2315 nm dhkl-value. This means the thin single plates are grown in the [111] directions. In Figure 5c, a surface region exhibits two broad diffraction rings of 0.2395 nm (r1) and 0.1210 nm (r2) radii presumably in a graphene-like amorphous layer. As usual,6,37 the (220) or (200) planes bound the {111} surfaces (Figure 5B). A magnified region P (Figure 5C) of an interfacial boundary smears an angular gap ∼11 between the (111) and (220) images. Region Q (Figure 5D) briefs how two Au-NPs form a conjunction if grow even at an angle in the (110) and (220) planes. Diffuse fringes (∼1.065 nm separation) seen in Figure 5B arise in a surface layer. Lattice reflections often rear inherently different Moire fringes.37,38 As shown from the schematic diagram in Figure 6A, a surface layer PQ bounding a lattice if reflects an electron beam, passing through lattice planes AB, from its top and bottom facades (or sublayers) rears such fringes on the beam returns from a similar layer. In other words, simply surface vibes emerge in shear bands and could span over as of a large space as the effective surface. To confirm a bonding “AuOC” in an interface layer, as proposed above in Figure 2b, we studied Au4f XPS bands (a) before and (b) after cleaving out the layer by heating a dried sample at ∼450 C for 2 h in air. A surface layer yields 810% mass loss over 300450 C in a thermogravimetric analysis, as we studied earlier with series of polymers.28 As shown in Figure 6B, a weak doublet band-2 is developed beside the

primary 4f7/2 and 4f5/2 doublet-1 on a layer present, showing an average 0.79 eV larger binding energy (Eb). The doublet-1 bands 83.30 and 86.95 eV are ascribed to the core Au atoms, while the doublet-2 bands 84.21 and 87.92 eV are ascribed to the surface atoms in the “AuOC” bonds. When a pure gold sample turns up free from such layer, only the doublet-1 retains with reasonably up-shifted peak values of 84.15 and 87.82 eV, closer to the bulk 4f7/2 and 4f5/2 levels 84.0 and 87.7 eV, respectively.28,39 When the layer is piled off, the doublet separation 3.65 eV (3.71 eV in doublet-2) is enhanced a bit to 3.67 eV, that is, very close to the bulk value 3.70 eV. A decreased Eb-value in Au-NPs anticipates that 4f7/2 and 4f5/2 levels converge with high-energy particle surface above the bulk value. Gold metallized polycarbonate gives similarly surface-modified 4f-levels with an Eb-value suppressed by 0.3 eV.40 A shift () 0.40 eV is found in thin Au-films.41 Electrons in such examples adopt a transitional state between the discrete levels of free atoms and the continuous bulk structure. It affects the localized SPs bands as will be discussed later with light emission in the surface-modified Au-NPs. 3.3. Rheology of Au-NPs in Nanofluids. In a dilute Au-PVP nanofluid in water, the Au-NPs with a stable adhesive surface PVP layer promote the base η-value of the parent PVP solution. This is feasible only when such Au-NPs mediate a cross-linked structure in dispersed polymer molecules. PVP molecules, which graft a cosurface layer on Au-NPs, likely interlink them in a chain via the host molecules. In Figure 7ac, the η-value plotted against the PVP-content in selective solutions at different shear rates r_ = 1050 s1 confers a unique rheological behavior of a polymer liquid. Irrespective of the r_ -value, which just stifles the final viscosity, the η-value increases slowly by increasing the PVP-content from 0.1 to 100 g/L used here. As expected, an order of enhanced η-value invades a markedly modified profile when Au-NPs present, showing a peak at a critical 11 g/L PVP value, that is, an optical region (marked by the strips in Figure 7dg) of an intense light emission. In optimizing the effective interactions of PVP molecules with in situ built Au-NPs in these nanofluids, we have varied only the PVP-content relative to a common value 0.001 mM of Au-NPs in a 5.1 mL volume.

Figure 6. (A) Model of multiple reflections of an electron beam AB (passing through a lattice pattern) from the front and rear parts of two layers PQ and P*Q* bounding a lattice, forming images of the surface layers, with (B) XPS bands (a) before and (b) after removing a polymer PVP surface layer in Au-NPs.

Figure 7. Shear viscosity in PVP molecules in water at (a) 10, (b) 30, and (c) 50 s1 shear rates. The inset plots a peak near 11 g/L PVP in the presence of Au-NPs (0.001 mM) with a PVP polymer layer with € n-electrons (shown in the balls); (d) 10, (e) 30, (f) localized CdO 50, and (g) 70 s1 shear rates. The strips mark the optical regions. 7822



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Despite that the peak η-value drops regularly by increasing the r_ -value over 1070 s1, the average peak position does not shift much with the PVP-content in a predominant effect of the surface PVP molecules, which are effectively fixed unless varying contact area with the Au-NPs. A larger r_ -value beyond 70 s1 brings down a steady η-value independent of flocculates out of a dilute medium of nearly the base-value. To confer the effects of shape, size, or number density of Au-NPs on the η-profile demands more experiments on different rheo-optical fluids for applications. Phenomenologically, in Au-PVP nanofluids, a macroscopic interaction between Au-NPs and PVP polymer molecules reaches a maximum at the peak η-value. As one can infer from the Au-PVP compositions used (Table 1), the effective number density varies in Au-NPs as per effective size, shape, and surface topology of them in different samples. A maximum interaction is expected on nearly equal number densities in the two kinds of the geometrical entities. In this conjuncture, a peak η-value observed at a specific content of 0.01 wt % Au-NPs and 11 g/L PVP molecules (Fp ≈ 450 monomers per Au atom) implies a number density Fp* ≈ Fp ÷ Wp/Wm ≈ 1.3 for polymer molecules per Au atom, with Wp = 40 000 and Wm = 111 (molecular weight in the monomers). Because the Au-atoms exist in particles in an order of 103, as many PVP molecules as this big number are interacting on average with one Au-particle. A larger η-value is thus plausible with Au-NPs on a still larger number density if they do not coalesce and separate out of polymer molecules. At an applied r_ -value, the Au-NPs redistribute through mobile polymer molecules of concentration C (in g/L) to reassume the equilibrium. A model η-value can be expressed in terms of a mathematical relation as follows: η ¼ ARΩβ expð ΩCβ ÞCðβ 1Þ ( ) p p C C p ð1 RÞ exp C τ τ

ð2Þ

This relation is derived assuming FðC, r_ Þ ¼ A

DN DC

ð3Þ

where N(C, r_ ) describes a distribution function of the geometrical entities in the sample. In a simple case, we can write: ( ) C p NðC, r_ Þ ¼ Rf1 expð ΩCβ Þg þ ð1 RÞ exp τ ð4Þ Here, R and (1 R) share N(C, r_ ) in two parts in redistribution of Au-NPs followed by a structural relaxation of r_ -induced configuration to its equilibrium value. As portrayed in Figure 8, the η-value in eq 2 fairly retraces the experimental curve, assuming model values A = 1.815 Pa 3 s 3 g L1, R = 0.9995, damping constant Ω = 2.42 103 (g L1)β, exponent factors β = 2.272, p = 2.00, and relaxation constant τ = 0.05 g L1. The units for b and τ are taken such that the exponential terms are unitless. As expected, the majority of Au-NPs endure redistribution via polymer molecules without a significant change in the basic structure as conferred by a relatively small level of the structural relaxation.

Figure 8. Variation of shear viscosity in PVP molecules in the presence of Au-NPs (0.001 mM) with a grafted surface PVP layer in water; (a) an experimental curve measured at 10 s1 shear rate and (b) a shearinduced redistribution of Au-NPs through PVP molecules in a rheological fluid followed by a local structural relaxation in a model in eq 2.

As will be described later, the colored strips in Figure 7ac mark that, in the absence of Au-NPs, bare PVP molecules emit an n1 r n1π* (CdO) band at smaller number densities. We can argue that an increased η-value in PVP molecules in a water like polar liquid leads to delocalize the CdO n1-electrons on a nonplanar molecular shape so that they hardly exhibit a cumulative n1 r n1π* emission. In an Au-PVP nanofluid, the viscosity seems to be helping a light emission merely on a reason that it designs and holds a dispersed structure of active optical species of surface-modified Au-NPs via polymer PVP molecules of the medium. As shown with a model in Figure 7, a surface PVP molecule, which adheres to an Au-NP gets molecularly stretched thereon in a planar configuration, serves as a planar bed with selflocalized n1-electrons to be active carriers of radiative photons. 3.4. Light Emission in Au-NPs with a Polymer Surface Layer. In a nanofluid, Au-NPs dispersed in an aqueous PVPmedium exhibit modified light emission of Au-SPs with surface PVP-molecules, or vice versa, out of an exchange coupled system. In grafting a surface layer on Au-NPs, the surface PVP molecules € bonds and rebond the Au-atoms. A partially open in the CdO ionic state of Au-atoms, which cobond the PVP-molecules, forms a plasmonpolariton structure, cropping a marked blue-shift of the parent Au-SPs band. Excited π and/or n1-electrons in part from the PVP-layer share photon-energy with coexcited Au-SPs in an electronic state S1 above the ground-state S0. This is studied by exciting different samples in the Stoke light emission with a resonance wavelength (λex) in one of the excited levels of PVP π and n1-electrons, the conduction band, or the Au-SPs. Eventually, the π f π* (PVP)24 and 5d106s1 f 5d96s1p1 (Au) interband transitions18,24,42 overlap each other over 200300 nm in a broad band. As shown in Figure 9A, two separate bandgroups 300550 and 550900 nm arise in the Stoke emission when exciting in these λex levels. Both of the groups contain two distinct overlapping bands, say band-1 and band-2 on ascending order of the emission maximum wavelengths (λem), with progressively increasing intensity in dilute Au-PVP nanofluids with PVP-content up to a critical 11 g/L value, which causes an enhanced η-value on a cross-linked structure of Au-NPs and PVP molecules (Figure 7df). Intensity, for example, see the inset in 7823



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Figure 9. The Stoke light emission in PVP-modified Au-NPs dispersed in PVP solutions in water and measured by exciting in (A) the 5d106s1 r 5d96s1p1 Au (λex = 260 nm) and (B) the n1 r n1π* PVP band (λex = 345 nm); (a) 100, (b) 50, (c) 20, (d) 10, and (e) 1.0 g/L PVP. The insets plot PVP-dependent peak intensities in the two bandgroups. Modifying Au-NPs with a PVP surface layer renders a blue-shift of Au-SPs band-2.

Figure 9A with the peak value I1 in the 300550 nm group, peaks up against PVP-content at this specific value. It is a percolation effect43,44 of the polymer liquid on light emission in the localized Au-SPs and CdO n1-electrons in the interface layer (Figure 2b). Both the surface layer and the shear viscosity tailor I1-value in a sample in the 300550 nm emission band (Figure 9A). It is described well by invoking a model relation: I1 ðε, μÞ¼ I1 ðε0 , μ0 Þ þ I1 ðεs , μs Þ I1 ðεm , γm Þ "

( )# q q #" a1 b1 C r ¼ I1 ðε0 , μ0 Þ þ 1 exp b C C "

( )# q #" a C r = I1 ðε0 , μ0 Þ þ 1 exp C b

ð5Þ

In general, a perturbation coefficient a1 with a power q describes a contribution I1(εs, μs) of the local dielectric field εs and chemical potential μs caused by the surface charges localized on the Au-NPs, while the coefficient b1 with a power q* describes an intensity loss I1(εm, γm) on an excited photon interacts with the surroundings with a damping parameter γm (average dielectric field εm). Replacing a1 ≈ b1 a with q* = q simplifies a working model. As plotted in Figure 10, following the experimental profile, the I1-value traces a peak at C ≈ 11 g/L PVP on a percolation threshold, assuming an equilibrium value I1(ε0, μ0) = 570 units with a = 1.75 103 g/L, a relaxation constant b = 1.85 g/L, and q ≈ r = 1. Likely, when exposing to an ultraviolet light 200300 nm, the sample feels a local perturbation in terms of excited photons of Au-SPs as well as the valence electrons, and those likely induce a shear wave, or simply a perturbation of the local Au-PVP surface interface and nearby PVP molecules. As a result, an induced shear wave or viscosity attenuates the photon dynamics. Light emission studied on exciting at different λex-values (220450 nm) infers that band-2 (Figure 9) arises in a modified S1 r S0 transition of Au-SPs by a surface PVP layer. An λem-value 455 nm has shown that this band in a 100 g/L PVP sample (a) blue-shifts in further dilute samples such as 430 nm at 1.0 g/L PVP in sample (e). The corresponding absorption band is so weak that it is masked in the parent Au-SPs band; the peak shifts from 530 to 580 nm in dilute PVP samples.23,24 It means a good absorber of Au-SPs is not necessarily a good emitter. To resolve

Figure 10. PVP-dependent peak intensity in the 300550 nm Stoke emission in surface-modified Au-NPs in aqueous PVP solutions, (a) an experimental curve measured by exciting in 5d106s1 r 5d96s1p1 Au band (λex = 260 nm) and (b) an Au r PVP energy transfer-induced profile in a rheo-optical nanofluid in a model in eq 5.

the origin of band-1, we lighted the sample in band-2 (extends deeply in the band-1) so that it knocks out selectively an n1 r n1π* emission (weaker intensity) from modified PVP molecules of a surface layer. At λex = 345 nm, a doublet band different from those in Figure 9A incurs in part with the n1 r n1π* band-1 of λem ≈ 412 nm in Figure 9B. As coexcited Au-SPs pump up the energy to correlated Au-5d96s1p1 level of the valence electrons in a conjugated plasmonpolariton system, it loses intensity in dilute samples. Linearly decreasing the peak value I2 in this band reveals (see the inset in Figure 9B) a markedly increased slope on decreasing PVP-content in a sample near 21 g/L. A physical significance is that an n1f Au electron transfer dominates at the expense of the n1 r n1π* transition in surface adsorbed PVP molecules on Au-NPs in dilute media below 21 g/L PVP. In the other words, Au-NPs serve a strong electron acceptor to correlated CdO n1-electrons. A band n1 r n1π*, which lies over 300600 nm (average λem = 427 nm) in a dilute PVP solution,31 is confined over 360540 nm in PVP molecules of in-built surface layer on Au-NPs in nanofluids. The average shear viscosity in an Au-PVP nanofluid seems to stifle down an electron-transfer n1 f Au in Au-NPs with a 7824



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polymer surface layer. It imposes a damping coefficient a2, which leads to modulate a progressively increasing I2-value in counterpart surface PVP molecules in the n1 r n1π* transition, assuming a simple model relation: I2 ðε, μÞ¼ I2 ðε0 , μ0 Þ þ I2 ðεs , μs Þ I2 ðεm , γm Þ "

( ) ( )# C r1 C r2 ¼ I2 ðε0 , μ0 Þ 1 þ f exp exp a2 b2 C = I2 ðε0 , μ0 Þ 1 ψ exp a2

ð6Þ

when b2 ≈ a2, r1 ≈ r2 ≈ 1, and ψ = 1 f. Other symbols have the same meanings as those in eq 5. A factional value f e 1 poises effective I2(εs, μs) contribution over the I2(εm, γm) value. In Figure 11ac, a plot (b) made with an arbitrary a2 = 39.0 g/L value with ψ = 0.95 provides a best fit to (a) the observed data,

Figure 11. PVP-dependent peak intensity in the n1 r n1π* PVP emission band (360560 nm) in surface-modified Au-NPs in aqueous PVP solutions, (a) an experimental curve measured by exciting in the Au-SPs band (λex = 345 nm) and (b,c) a local dielectric field assisted profile in a rheo-optical nanofluid in a model in eq 6.

assuming an equilibrium value, I2(ε0, μ0) = 540 units. Only a small change in ψ-value, such as 1, leads to tailor (c) a model curve sensitively from the experimental profile especially in dilute samples (PVP content below 21 g/L). The red-emission group 550900 nm, that is, the first harmonic n1 r n1π* band-1,31 is promoted when a plasmonpolariton is built-up in PVP surface-modified Au-NPs € groups in the surface PVP molecules extend as if CdO € “COAu” bonds in cross-linking to the surface Au-atoms. According to the model in Figure 7, an increased effective density of localized n1-electrons in the surface regime on conjunction € with a “COAu” moiety amplifies such a transition; otherwise, it is a highly forbidden band difficult to observe in a measurable intensity in a bulk PVP sample. A PVP-dependent rheo-optical medium, or electromagnetic field, describes progressively enhanced intensity of this band in dilute samples at the outset of a united system of localized n1-electrons with coexcited Au-SPs until a PVP content reaches a critical threshold value 11 g/L. There is a report that a red-emission improves by a factor as large as “7” when Si wafers,45 or n-CdSe/p-Si diodes,4 form an intermediate surface AuSi bond, which creates a plasmonpolariton structure in part of a surface interface. With a plasmonpolariton film, PVP surface-modified AuNPs render a promptly strong up-energy transfer of excited AuSPs in terms of the anti-Stoke 5d106s1 r 5d96s1p1, n1 r n1π*, and S0 r S1 bands. In Figure 12, two distinct bandgroups of the anti-Stoke emission 225300 nm (1) and 300400 nm (2) evolve when irradiating such Au-NPs in form of Au-PVP nanofluids in part of the S0 r S1 Au-SPs band-2 in the Stoke-emission such as at 430 nm of an λem-value. This specific λem-value was chosen as per a maximum emission shown in band-2 (Figure 9A) in the dilute samples so that it induces reasonably intense and interdependent resonance spectra out of exchange coupled 5d106s1 r 5d96s1p1 and n1 r n1π* photons. In these samples, as the PVP content decreases, the 5d106s1 r 5d96s1p1 band-1 intensity I3 increases at the expense of the value I4 in the other n1 r n1π* band-2, that is, the same trend as shown in a PVP n1 r n1π* modified Stoke band-2 of Au-SPs in Figure 9A. A correlation that lightening the anti-Stoke emission band-1 induces the Stoke band-2 and vice versa confers their origins of the PVP-modified energy levels of the valence electrons and Au-SPs. As described in the schematic diagram in Figure 12,

Figure 12. The anti-Stoke emission in PVP surface-modified Au-NPs dispersed in PVP solutions in water and measured by irradiating in the Stoke band at 430 nm; (a) 100, (b) 50, (c) 20, (d) 10, and (e) 1.0 g/L PVP, with energy levels describing the transitions on the right. 7825



Figure 13. The variation of peak intensities in the anti-Stoke emission bands of 220300 and 300400 nm relative to the Stoke bands in Figure 9 showing the effect of the medium on them in PVP surfacemodified Au-NPs dispersed in PVP solutions in water.

Figure 14. Schematic energy-level diagram describing the Stoke light emission in two primary bands (1 and 2) in surface-modified Au-NPs dispersed in a polymer medium of PVP molecules in water when lightening the Au-conduction band. A weak n1 r n1π* band (0) in part of PVP molecules from the medium perceives up-energy transfer from coupled Au-SPs band-2 with n1 r n1π* band-1 in surface PVP molecules in an anti-Stoke emission.

coexcited S0 r S1 (∼430 nm) and n1 r n1π* (∼345 nm) photons recombine each other in a promoted energy state AB and then ultimately emit in two kinds of up-energy converted optical signals of the anti-Stoke bands, that is, the interband transition 5d106s1 r 5d96s1p1 near 263 nm superposing the π r π* band at shorter values and the n1 r n1π* band at ∼345 nm, after part of the energy loss on interactions with surrounding of a rheooptical medium. In Figure 13, the values I3/I1 and I4/I2 vary in opposite trends to each other in nonlinear plots over the PVP contents. A wide plateau AB stays in prominent I3/I1 values over 2759 g/L PVP molecules before they drop on the larger PVP values, while the I4/I2 value, prominent in the PVP-rich samples, traces sharp peaks shown in the η and I1-values near 11 g/L PVP molecules. 3.5. Energy Levels and Transitions in Au-NPs with a Surface Layer. An energy-level diagram proposed in Figure 14 on the basis of the Stoke and anti-Stoke emissions observed in

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Figures 9 and 12 describes how effectively the surface molecules in PVP surface-modified Au-NPs coexcite with the metallic core in a single united system of photons in terms of the π f π* and 5d106s1 f 5d96s1p1 electrons in a common energy level AB in the conduction band. A direct process to take place needs a light pumping-up by a photon of a sufficiently large energy at 220300 nm λex-value. An electron so excited in the conduction band recombines to a newly born hole in the valence band so rapidly (at a time scale faster than 100 fs known in Au-NPs dispersed in water)46 that the excited Au-atoms decouple from the π*-PVP species. Separate energy-levels A and B copopulate € n1 f n1π* electrons and Au-SPs (S1), in terms of the CdO respectively. In-situ populated levels n1π* and S1, lying just below the Fermi level EF, thus emit two distinct bands following a single excitation in the AB level. The n1 r n1π* band-1 shown of a prominent emission in Figure 9A thus accords to a promoted € n1-electrons in the surface effective density of localized CdO when they bridge a polymer bed of PVP molecules on a highelectron-density metallic surface of Au-NPs. € Merely regenerated photon carriers by excited CdO n1-electrons divide light-emitting into two steps in a different energy-exchange process if irradiating the sample at energy below the EF-value in the vicinity of point A (e.g., ∼345 nm as the λex-value) in part of the n1 f n1π* band-1. In this case, excited n1π* electrons in level A transfer their energy to correlated AuSPs in PVP surface-modified Au-NPs so that a red-shifted band-2 (S0 r S1) coemits from level B, which is observed in Figure 9B. As per a PVP-content-dependent value I2 observed in the emission band-1, such energy transfer PVP f Au is effective only in dilute Au-PVP nanofluids over less than 21 g/L PVP. Both of the bands decrease in intensity when the medium (H2O) interacts strongly with dispersed Au-NPs in dilute samples. Surface PVP molecules modified on bridging the Au-atoms are effectively quenching the light emission in water. Bare PVP molecules hardly lose light emission in water.31 Furthermore, when irradiating in the Au-SPs band-2 at level B in Figure 14, PVP surface-modified Au-NPs have an anti-Stoke light emission in both the n1 r n1π* and the 5d106s1 r 5d96s1p1 transitions as shown in Figure 12. This reverse energy transfer PVP r Au observed here is feasible only in a highly exchanged coupled system. Correlated PVP molecules thus coexcite to the n1π* level A, while the Au-valence electrons coexcite to that in the conduction band as long as the S0 f S1 photons transfer their energy pumping the CdO n1-electrons up in the n1 f n1π* transition. A conjugated system of Au-SPs, Au-valence electrons, and CdO n1-electrons makes it feasible on a cross-linked polymer structure of a high-electron-density metal and a conjugate dye molecule such as PVP used here in a surface layer on Au-NPs. It is known that Au-NPs in assembly promote local field amplitudes of SPs exceeding those of single Au-NPs.18,42 In these examples, it is the surface plasmonpolariton that probes energy transfer on a two-energy-levels system of coupled electric-dipoles from the two phases. Mixing of local dipoles in two phases monitors energy transfer in a combined optical signal. Such Au-NPs can carry the features if transferred to other hosts useful for devices such as optical switching, optical probes, biosensors, or molecular sensors. 3.6. Model Electronic Transitions in Au-LSPS with CdO n1Electrons. Localized SPs on a given Au-fcc lattice bounded by a specific particle shape, which is itself bounded by CdO πn1-electrons of a thin PVP surface layer, grow a large number of localized electric dipoles Ns competing with the total number of 7826


The Journal of Physical Chemistry C dipoles N0 in the particle in form of an electrostatic surface charge. A locally enhanced dielectric filed in a charge capacitor affects the light emission process. In a simple relation Ns = {j1 þ 2R1}N0Δt, a model circular Au-plate with thickness j ≈ 5 nm and diameter R ≈ 10 nm bears Ns ≈ 12%, assuming an average Δt ≈ 0.3 nm distance between dipoles. To derive this relation, we discretized volume V in a plate in N0 spherical entities with radius 1/2Δt, having V = N0v, with v = 4/3π(1/ 2Δt)3 as the volume in a dipole. Here, the Δt-value can be treated as the effective thickness in a single CdO layer carting πn1electrons. As many as N0 ≈ 105 Au atoms comprise such small particle, that is, equivalent to a sphere with R ≈ 10 nm but a lower Ns ≈ 6.3%.47 Interaction with πn1-electrons leads the d-band center Ed to shift up closer to the EF level in the localized Au-SPs on hybridizing the 5d106s1 Au-electrons. With a DFT (density functional theory) calculation, Hammer et al.48 used a relation, Ed-hyb (eV) = 20.1 ÷ (2.5 Ed) þ 3.41, to compute contribution of d-band hybridization energy Ed-hyb in chemisorption of CO on a gold surface. In Au13 clusters,49 as large of a value as Ed-hyb ≈ (-)1.3 eV arises as compared to the sp-contribution, that is, merely ()0.40 eV, showing ∼1 eV higher Ed-value above bulk gold of ()3.56 eV below the EF level. A combined value reaps when also the EF level down-shifts. Eventually, in a dispersed sample of Au-NPs with countable atoms in a liquid medium with PVP like polymer molecules, the transition energy ΔEirn in an electronic transition from an excited state Ei (emissive) to the ground-state En is governed with four major physical parameters: (i) the surface confinement of the conduction 5d106s1 electrons, (ii) the chemical bonding with surface molecules, (iii) the local dielectric field imposed on the Au-NPs when bridging a surface charge layer, and (iv) the microscopic interactions with the surroundings. As a result, we can express the effective ΔEirn as follows: R31 A1 R22 A2 R23 A3 R4 A4 þ þ V V V V ( ) R31 þ R22 þ R23 R4 0 ¼ ΔEi r n 1 þ ð7Þ jðΔE0i r n Þ

ΔEi r n ¼ ΔE0i r n þ

with A1 = A2 = A3 = A4 as the effective particle surface areas in the individual contributions in terms of the coefficients R1, R2, R3, and R4, respectively. The powers rationalize the cubic, quadratic, and linear contributions. In a workable model, we replace R2 þ R3 ≈ R23, in fact of a common reserve, and resume a more feasible variable Ædæ = (βR)1/2: ( ) R31 þ R223 R4 0 ΔEi r n ¼ ΔEi r n 1 þ ð8Þ ÆdæðΔE0i r n Þ As Ædæ f ¥, ΔEirn f the bulk value ΔE0irn, that is, ∼2.23 eV at Ædæ above 25 nm.49,50 Including a model value R1 = 0.93 (eV/nm)1/3 in a sample Ædæ = 4 nm adds a value 2.43 eV as also obtained from the DFT calculations on spheroids (R = 4 nm).50 R23 augments it progressively, a value 2.76 eV at R23 = 0.97 (eV/nm)1/2 with R4 = 0.13 eV/nm and modified R1 = 0.95 (eV/nm)1/3 in surface-modified Au-NPs, Ædæ ≈ 5 nm. The average ΔEirn value obtained using eq 8 is used to setup the band position with a DFT simulation, which otherwise underestimates the bandgap energy. Figure 15 shows simulated bands (a Gaussian profile) in Au-NPs of two different ΔEirn values (A) before and (B) after a surface layer in a

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Figure 15. Simulated electronic bands with different ωp-values (A) 5.37.3 eV (pγ = 2.4 eV) and (B) 4.866.86 eV (pγ = 3.0 eV) in LSPs in Au-NPs, showing a blue shift in average peak position by 0.33 eV in (B) due to a rigid surface layer.

model relation: IðωÞ ¼

ω2p γω ðω20 ω2 Þ2 þ ðγωÞ2

ð9Þ

often used in describing dielectric spectrum in localized SPs.50,51 The plasma frequency ωp = (N~e2/ε0m0)1/2, N~ = concentration of effective conduction electrons of electron mass m0 and charge e, and ε0 = dielectric constant of the vacuum, is varied in a narrow range (A) 5.86 ( 1 eV with a dapping energy pγ = 3.00 eV and (B) 6.30 ( 1 eV with pγ = 2.40 eV to get courier bands at an energy scale. Surface states on surface-modified Au-NPs by a surface layer rear a visible band widening. An increase in ωp-value not only breeds progressively increasing I(ω) in the band but also leads to shift its center ω0 marginally over lower values.

4. CONCLUSIONS A surface design of Au-NPs (polygons of 1245 nm widths) with a polymer surface layer of PVP molecules via an in situ method in aqueous medium makes violet-blue light emission to vary in coexcited photon carriers, that is, Au-SPs, Au-valence electrons, or CdO (PVP) n1-electrons out of a single united system. The Stoke and anti-Stoke bands studied of such Au-NPs in aqueous solutions (0.1100 g/L PVP) with optical pumpingup of selective photon-carriers into (i) the conduction band, (ii) an Au-SPs band, and/or (iii) a PVP-band in near resonance with an elevated light-source reveal PVP-dependent correlated features. Surface PVP molecules, which bridge a polymer bed with Au-atoms on a metal surface (high-electron-density) in such Au-NPs, invade localized n1-electrons of a manifested density on aligned CdO groups that promote the Stoke emission into two groups 300550 and 550900 nm (weak). Localized Au-SPs with a PVP-surface layer in Au-NPs have a marked blue shift of their emission band ∼450 nm (relative to 530 nm in bare AuNPs)23,24 in overlap with the n1 r n1π* band at lower side. An Au r PVP energy transfer tailors light emission with a maximum intensity at 11 g/L PVP in a rheological nanofluid. At a small shear rate of 1050 s1, the shear viscosity arises of a peak at this 7827


The Journal of Physical Chemistry C specific ratio of PVP molecules with 0.001 mM Au-NPs, that is, a cross-linked structure of flocculates Au-NPs and PVP molecules. An excitation in the emission band of Au-SPs such as at ∼430 nm leads to share the energy with a coexcited n1 r n1π* transition, which results in a single band at 345 nm and ultimately knocks out a valence electron 5d106s1 r 5d96s1p1 transition in part of the metal in a prominent anti-Stoke emission 225300 nm with an up-energy transfer. An opposite trend to the n1 r n1π* anti-Stoke or Stoke emission, an energy transfer Au r PVP describes a progressively increasing intensity in the 225300 nm band in dilute Au-PVP samples, with a maximum value achieved at a specific 11 g/L PVP content, that is, a percolation threshold value out of a hybrid nanocomposite. A dielectric polymer medium, or a surface layer, not only converges the Au-SPs or n1-electrons on the Au-NPs but also imposes a dielectric field that amplifies the light emission useful for probes and other possible devices.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Part of this work was supported by a research grant (to S.R.) from DAE-BRNS, Government of India. ’ REFERENCES (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (2) Kneipp, Z.; Kneipp, H.; McLaughlin, M.; Dennis, B.; Kneipp, K. Nano Lett. 2006, 6, 2225. (3) Wang, Y.; Li, D.; Li, P.; Wang, W.; Ren, W.; Dong, S.; Wang, E. J. Phys. Chem. C 2007, 111, 16833. (4) Konda, R. B.; Mundle, R.; Mustafa, H.; Bamiduro, O.; Pradhan, A. K.; Roy, U. N.; Cui, Y.; Burger, A. Appl. Phys. Lett. 2007, 91, 191111. (5) Tang, X. L.; Jiang, P.; Ge, G. L.; Tsuji, M.; Xie, S. S.; Guo, Y. J. Langmuir 2008, 24, 1763. (6) Kemal, L.; Jiang, X. C.; Wong, K.; Yu, A. B. J. Phys. Chem. C 2008, 112, 15657. (7) Lee, J. S.; Green, J. J.; Love, K. T.; Sunshine, J.; Langer, R.; Anderson, D. G. Am. Chem. Soc. 2009, 9, 2402. (8) Nam, J. M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884. (9) Tao, A.; Sinsermsuksakul, P.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 4597. (10) Lin, H. Y.; Chen, Y. F.; Wu, J. G.; Wang, D. I.; Chen, C. C. Appl. Phys. Lett. 2006, 88, 161911. (11) Benjamin, J. W.; Chen, Y.; McLellan, M. J.; Xiong, Y.; Li, Y. Z.; Ginger, D.; Xia, Y. Nano Lett. 2007, 4, 1032. (12) Kim, K.; Lee, H. B.; Lee, J. W.; Park, H. K.; Shin, K. S. Langmuir 2008, 24, 7178. (13) Zhang, Y.; Peng, H.; Huang, W.; Zhou, Y.; Zhang, X.; Yan, D. J. Phys. Chem. C 2008, 112, 2330. (14) Florescu, L.; John, S. Phys. Rev. Lett. 2004, 93, 013602. (15) Popov, O.; Zilbershtein, A.; Davidov, D. Appl. Phys. Lett. 2006, 89, 191116. (16) Dice, G. D.; Majumdamar, S.; Elezzabi, A. Y. Appl. Phys. Lett. 2005, 86, 131105. (17) Dulkeith, E.; Niedereichholz, T.; Klar, T. A.; von Plessen, J. F. G.; Gittins, D. I.; Mayya, K. S.; Caruso, F. Phys. Rev. B 2004, 70, 205424. (18) Zheng, J.; Zhang, C.; Dickson, R. M. Phys. Rev. Lett. 2004, 93, 077402.

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(19) Kamada, H.; Tsutsumi, Y.; Kamada, K. S.; Yamamoto, Y.; Yoshioka, Y.; Okamoto, T.; Nakagawa, S.; Nagata, S.; Mayumi, T. Nat. Biotechnol. 2003, 21, 399. (20) D’Souza, A. J. M.; Schowen, R. L.; Topp, E. M. J. Controlled Release 2004, 94, 91. (21) Kubin, A.; Meissner, P.; Wierrani, F.; Burner, U.; Bodenteich, A.; Pytel, A.; Schmeller, N. Photochem. Photobiol. 2008, 84, 1560. (22) Zheng, Y.; Huang, X.; Wang, Y.; Xu, H.; Chen, X. J. Appl. Polym. Sci. 2009, 113, 736. (23) Mishra, A.; Tripathy, P.; Ram, S.; Fecht, H. J. J. Nanosci. Nanotechnol. 2009, 9, 4342. (24) Mishra, A.; Ram, S.; Ghosh, G. J. Phys. Chem. C 2009, 113, 6976. (25) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475. (26) Hoppe, C. E.; Lazzari, M.; Blanco, I. P.; Quintela, M. A. L. Langmuir 2006, 22, 7027. (27) Gautam, A.; Tripathy, P.; Ram, S. J. Mater. Sci. 2006, 41, 3007. (28) Tripathy, P.; Ram, S.; Fecht, H. J.; Bansmann, J.; Behm, R. J. Nanotechnology 2009, 20, 07501. (29) Mishra, A.; Ram, S. J. Phys. Chem. A 2009, 113, 14067. (30) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; Wiley: New York, 1992; p 96. (31) Mishra, A; Ram, S. J. Chem. Phys. 2007, 126, 084902. (32) McClume, W. F. Powder Diffraction File 04-0784 JCPDS; International Centre for Diffraction Data: Swarthmore, PA, 1979. (33) Yoon, S. B.; Chai, G. S.; Kang, S. K.; Yu, J. S; Gierszal, K. P.; Jaroniec, M. J. Am. Chem. Soc. 2005, 127, 4188. (34) Shen, J.; Hu, Y.; Shi, M.; Li, N.; Ma, H.; Ye, M. J. Phys. Chem. C 2010, 114, 1498. (35) Tripathy, P.; Mishra, A.; Ram, S. Mater. Chem. Phys. 2007, 106, 379. (36) Anurag, G.; Singh, G. P.; Ram, S. Synth. Met. 2007, 157, 5. (37) Chen, H.; Gao, Y.; Zhang, H.; Liu, L.; Yu, H.; Tian, H.; Xie, S.; Li, J. J. Phys. Chem. B 2004, 108, 12038. (38) Edington, J. W. Practical Electron Microscopy in Materials Science; Techbooks: Herndon, 1976. (39) Shanmugam, S.; Viswanathan, B.; Varadarajan, T. K. Nanoscale Res. Lett. 2007, 2, 175. (40) Patnaik, A.; Li, C. J. Appl. Phys. 1998, 83, 3049. (41) Citrin, P. H.; Wertheim, G. K.; Baer, Y. Phys. Rev. Lett. 1978, 41, 1425. (42) Balamurugan, B.; Maruyama, T. Appl. Phys. Lett. 2005, 87, 143105. (43) Wang, L.; Dang, Z. M. Appl. Phys. Lett. 2005, 87, 042903. (44) Yao, S. H.; Dang, Z. M.; Jiang, M. J.; Bai, J. Appl. Phys. Lett. 2008, 93, 182905. (45) Qiu, T.; Wu, X. L.; Siu, G. G.; Chu, P. K. Appl. Phys. Lett. 2005, 87, 223115. (46) Varnavski, O. P.; Mohamed, M. B.; EI-Sayed, M. A.; Goodson, T., III. J. Phys. Chem. B 2003, 107, 3101. (47) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6275. (48) Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. Rev. Lett. 1996, 76, 2141. (49) Phala, N. S.; Steen, E. V. Gold Bull. 2007, 40, 150. (50) Gharibshahi, E.; Saion, E. Phys. Int. 2010, 1, 57. (51) He, Y.; Zeng, T. J. Phys. Chem. C 2010, 114, 1823.

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Modulating Up-Energy Transfer and Violet-Blue Light Emission in

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