Article pubs.acs.org/JPCC
Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function Anup L. Dadlani,† Peter Schindler,‡ Manca Logar,‡ Steve P. Walch,‡ and Fritz B. Prinz*,‡,§ †
Department of Chemistry, ‡Department of Mechanical Engineering, and §Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: The work function (WF) and surface plasmon resonance (SPR) of organic ligand capped Ag nanoparticles (NPs) have been studied experimentally and computationally. Experimental observations reveal a significant increase in WF as the size of ligand-capped Ag NPs increases, a trend contrary to that previously observed for bare Ag NPs. Computational results confirm the effect on the WF from simplified ligand molecules and relate it to charge transfer between the Ag core and surrounding ligands. We also observe a possible coupling between increases in WF and decreases in SPR transition energy, supported by computational results and attributed to the interplay between the 4d and 5s electron states of the system. These results, along with our observations of WF dependence on ligand choice, indicate the ability to strongly engineer the electronic structure of metal NPs through size and ligand control.
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INTRODUCTION Metal nanoparticles (NPs) have recently attracted significant attention due to their promise for a growing number of applications, including biomedicine,1 catalysis,2 optical sensors,3,4 and surface-enhanced Raman spectroscopy.5 However, better understanding and control of the electron energy structure of these particles are needed to realize this promise. For example, in electrical devices correct band structure is essential for efficient charge transport, and in electro-catalysis the ease of charge extraction from surfaces is a major determiner of reaction rates. These properties are reflected in the work function (WF) of the material system employed. In many frontier applications, such as LEDs, PVs, and FETs, the NPs used are typically capped by ligands, which modify the electron energy levels at the metal−organic interface.6,7 Better understanding of the effect of capping ligands is needed in order to exploit the benefits offered by NPs. Additionally, the burgeoning field of plasmonics depends on the nature of the surface plasmon resonances (SPRs) supported by NPs. SPRs stem from the physical interaction between light and metal NPs, which induces a collective oscillation of the conduction electrons due to the presence of an electromagnetic field.8−10 This interaction is strongly affected by the presence of capping ligands, and much remains to be learned about the nature of their influence. In this paper, we report observations of trends in the WF and SPR of Ag NPs capped by different ligands as a function of NP size and ligand type. WF measurements were taken using ultraviolet photoelectron spectroscopy (UPS), and SPR measurements were taken using ultraviolet−visible spectrosco© 2014 American Chemical Society
py (UV−vis) and electron energy loss spectroscopy (EELS) by scanning transmission electron microscopy (STEM). Additionally, we present the results of a simplified ab initio quantum model of the systems under study, gleaning insight into the trends observed. Our experimental and simulated results show consistent and correlated trends in the WF and SPR of Ag NPs.
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EXPERIMENTAL METHODS
WF and SPR Measurements. WF and SPR measurements were carried out on 5 mg/mL (Econix) Ag NPs capped with polyvinylpyrrolidone (PVP) dispersed in water, procured from NanoComposix. The core diameters were (5.5 ± 1.6) nm, (23.1 ± 6.9) nm, (54.8 ± 10.1) nm, and (74.5 ± 11.8) nm (see Figure S1 in the Supporting Information for size distributions). For ligand comparison, a Sigma-Aldrich sample of (5 ± 2) nm dodecanethiol (DDT)-capped Ag NPs dispersed in hexane was also measured. WFs were measured by UPS with a model AC-2 Photoelectron Spectrometer at atmospheric pressure, using a deuterium UV source in air. UPS samples were drop-casted via pipet on quartz wafers cleaned with acetone, methanol, and isopropyl alcohol. Measurements were repeated over at least three different areas for each sample. Fowler’s hypothesis for metals was used in determining the WF of the NPs. Straight lines were fitted to plots of quantum yield to the half power Received: July 22, 2014 Revised: September 19, 2014 Published: September 29, 2014 24827
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versus photon energy then extrapolated to the x axis to give the photo threshold.11 Optical SPR measurements were performed in solution in a 1 × 1 cm quartz cuvette on a PerkinElmer Lambda 1050 UV− vis/NIR Spectrophotometer. STEM-EELS SPR measurements were performed on an FEI spherical aberration-corrected (Cs image corrector) 80−300 kV Titan environmental STEM equipped with a Gatan Tridiem 866 EEL spectrometer operated at 300 kV in monochromated mode. TEM samples were prepared by drop casting various dispersions of NPs on ultrathin silicon oxide TEM supports. Further details regarding the STEM-EELS measurement procedure are presented in the Supporting Information. Ab Initio Calculations. Ab initio calculations were performed using Gaussian0912 for small Ag clusters (Agn, n = 6, 12, 19, and 26) with and without ligands for comparison to the experimental results. Only trends as a function of size and ligand type are compared (as opposed to absolute energy values), since the simulated clusters are much smaller than the measured NPs due to computational limitations. In addition, PVP and DDT ligands were approximated by simpler molecules to reduce computational load, as described in the Results and Discussion. Ionization potentials (IP) were obtained for comparison with WF measurements using density functional theory with the B3LYP hybrid functional.13,14 The cc-pvdz basis set15 was used for ligand atoms, and the LANL2dz basis set16 was used for the Ag cores. [Similar calculations of electron affinity (EA) were also performed and are presented in the Supporting Information.] The IP and EA results are obtained as vertical excitation energies (ions were not allowed to relax) using the optimal geometry for the neutral structure. The ground states of the Ag clusters are low spin (singlet for an even number of atoms and doublet for an odd number of atoms).
WF measurements are displayed in Figure 2, showing cubic fcc crystal structure. We find that the WFs of PVP-capped NPs increase with increasing particle size, with a difference of 0.43 eV between the 5 and 75 nm groups. The average WF values obtained for the 5, 25, 50, and 75 nm groups are 4.78, 4.77, 4.88, and 5.21 eV, respectively. Each of these values is higher than the value measured for bare sputtered Ag (4.55 eV), whereas the WF measured for the DDT-capped 5 nm NPs is lower (4.47 eV). Our measurement for bare sputtered Ag is consistent with previous measurements of bare Ag surfaces.17−19 It has also been previously observed that self-assembled monolayers of thiolated hydrocarbons decrease the WF from the bare Ag value.17 In that study, longer chain ligands (C2) had a smaller effect on the WF than shorter chain ligands (C1) due to the smaller molecular dipole induced. It is thus expected that the thiolated ligand DDT, which has 12 carbons, should cause only a slight decrease in the WF as compared to bare Ag, in agreement with our observation (0.08 eV lower). In the case of the sulfur-free PVP ligand, the dipoles introduced act to raise the WF. Chae et al. found a WF = 4.7 eV for 20 nm Ag NPs capped by PVP, in agreement with our data.20 The increase in WF over bare Ag can be explained in terms of the electronegativity of the atom directly bonded to the surface. Similar interfacial and molecular dipoles are present in the PVP- and DDT-capped NPs. In both cases, the molecular dipole of the ligand points toward the surface, reducing the WF, but the interfacial dipole in the case of PVP has a stronger effect in the opposite direction due to the greater electronegativity of the binding oxygen atom as compared to sulfur. The difference in interfacial dynamics between different ligand types and Ag surface provides the means to exert significant control over metal NP WFs, as evidenced by the 0.31 eV difference we observe between our 5 nm particles capped with DDT and those capped with PVP. Notably, the size trend we observe in our PVP-capped NPs is in opposition to the well-documented trend in bare metal NPs,21 in which WF decreases by a small amount with increasing size.22−24 That effect can be understood through classical electrostatics, combining the effects of the spherical geometry of the NPs and interaction of the charged NPs with the removed electrons.21 (The model holds for Ag NPs until they are smaller than 1.5 nm, where a steep increase in WF arises from quantum size effects.18,25) In our case, the ligands capping our colloidal NPs appear to reverse and magnify the size trend observed in bare Ag NPs. Though our WF curve looks like there is an exponential increase, the WF is not expected to increase much more than the value for the 75 nm batch. As the diameter increases from 5 to 75 nm, the ratio of the surface area of the subsequent size to the previous is reduced in the following trend (252/52 = 25, 502/252 = 4, and 752/502 = 2.25). Thus, it is anticipated that the value of the WF would approach the limit of a planar surface with PVP adsorbed on it. To gain further physical insight into the WF trends observed in our experiments, we performed ab initio quantum calculations on simulated clusters of Ag atoms with representative organic ligands. Visualization of the Ag atomic arrangements in the Ag6, Ag12, Ag19, and Ag26 clusters are shown in Figure 3a. The simulated clusters were limited by computational constraints to sizes an order of magnitude smaller than the NPs observed experimentally, so comparisons are made between the trends observed rather than absolute
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RESULTS AND DISCUSSION The WFs of all samples as measured by UPS are presented in Figure 1. High-resolution TEM micrographs of selected NPs from the 5, 25, and 50 nm PVP ligand-capped groups used for
Figure 1. WF as a function of diameter of Ag NPs, measured by UPS. PVP capped, DDT capped, and a pure silver surface sputtered on quartz are compared. Error bars indicate standard deviations. 24828
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Figure 2. High-resolution TEM micrographs of individual PVP ligand-capped Ag NPs from the (a) 5, (b) 25, and (c) 50 nm groups used for the experiment.
structure obtained by removing a hydrogen atom from the αCH2 group. As a result, it still leaves the amide moiety partially intact. (See the Supporting Information for discussion of the other two candidates and reasoning for the choice.) PVP is believed to bond to the surface through the carbonyl oxygen atoms, and the bonding locations and orientations chosen for the ligand groups in the simulation are indicated in Figure S3 of the Supporting Information. Simulations were performed in 3 cases for each cluster size: bare Ag, SCH3-capped, and PVP-R1-capped. In the ligandcapped cases, the ligands were found to withdraw electrons from the Ag core, as shown in Figure 4a. [The dashed lines are included to visualize the trends, noting that points in between have an underlying oscillatory behavior which arises depending on state (whether singlet or doublet) from which the electron is removed or added.26] The curves there suggest an explanation for the observed trend that larger particles have higher WFs (Figure 1). Both the SCH3 and PVP-R1 groups are seen to withdraw more electrons with increasing cluster size. The PVPR1 effect is larger than for the SCH3 group, as expected, given the greater electronegativity of oxygen as compared to sulfur. (The charge donation seen for PVP-R1 on Ag6 may indicate the lack of an Ag−O chemical bond at that size.) The experimentally observed increase in WF (decrease in Fermi level or 4d orbital energy of Ag) with increasing NP size in the
Figure 3. (a) Positions of Ag atoms in simluated clusters, based on Ag (111) planes. (b) Simple SCH3 group used to represent the DDT ligand. (c) Simple PVP-R1 structure used to represent the PVP ligand.
values of calculated properties. Computational constraints also required simplification of the ligand groups used in the experiments. The DDT molecule was simplified to SCH3, as shown in Figure 3b. Due to the complex structure of the polymeric PVP ligand, three possible representative monomers were used. The one chosen as most accurately representative, labeled PVP-R1, is depicted in Figure 3c. PVP-R1 is an allylic
Figure 4. Simulation results regarding WF. (a) Total charge on the Ag cluster as a function of cluster size for both ligand models. (b) IPs as a function of cluster size for each ligand model, including bare Ag. 24829
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Figure 5. Experimental SPR and computed polarizability results. (a) UV−vis spectra of colloidal solutions of each PVP-capped NP size group. (b) Trace of calculated polarizability tensor of bare Ag clusters as a function of cluster size.
Figure 6. (a−c) Dark field micrographs of PVP-capped Ag NPs from the 5, 25, and 50 nm groups, respectively. Corresponding EELS signal intensity maps are shown in (d−f) integrated over the energy ranges of 3.2−3.5, 3.0−3.3, and 2.8−3.1 eV (marked as yellow, red, and green, respectively). (g) Normalized and deconvoluted EEL spectra taken near the edge of each NP, as indicated by the colored markers in the dark field images.
significantly red-shifted SPR absorption peak compared to smaller ones. Retardation in bigger NPs results in red-shifting of the SPR and broadening due to higher order oscillation modes that are lower in energy.9 In our case, 75 nm Ag NPs exhibit effects (significant red-shifting and a broadening bandwidth) due to retardation. We speculate that the WF is unaffected by retardation because it increases as particle size gets larger, contrary to expectation. The peak wavelengths λmax for the 5, 25, 50, and 75 nm PVP-capped groups are 406 nm (3.05 eV), 402 nm (3.08 eV), 433 nm (2.86 eV), and 456 nm (2.72 eV). This trend has been previously observed in bare metal NPs28 and is to be expected according to the simple particle in a box model, in which the energy levels are more closely spaced as the cluster gets larger, resulting in a smaller SPR transition energy. The inflection point in the trend as the NPs shrink below 25 nm is explained by a multilayer Mie theory model which indicates that lowered electron conductivity in the outermost atomic layer, induced by chemical interactions, decreases the SPR transition energy.29 Figure 5b shows the polarizability results from our ab initio calculations on bare Ag clusters. Polarizability increases with
PVP case may be explained by the greater positive charge induced by the ligands. Ab initio calculations were also performed for the IP of the clusters, which are analogous to the WFs of larger systems. Results are presented in Figure 4b. The bare Ag cluster shows the often seen trend in literature where the IP decreases with increasing particle size,21 but this trend is significantly mitigated in the SCH3 and PVP-R1 cases. The substantial effect of these simplified ligands provides evidence for the ligand-based mechanism proposed to explain our experimental observation, in which the PVP capping is seen to reverse the established trend of decreasing WF with increasing size in bare Ag NPs. As computational capability improves in the future, it will be interesting to see whether a simulation of the complete PVP ligand shows a full reversal of the bare Ag trend. In addition to WF, SPR was measured on the PVP-capped Ag NPs using UV−vis spectroscopy. (Results for the DDTcapped NPs are shown in Figure S6 of the Supporting Information.) The SPR of Ag NPs causes a pronounced absorption in the visible range of the electromagnetic spectrum.27 As shown in Figure 5a, larger particles show a 24830
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energy decreases. This symmetry suggests a link between WF and SPR in our NPs that may extend to other organic polymer capped metallic NP systems. In our system, the link may be explained by the roles of the Ag 4d and 5s orbitals in these two properties. In Ag NPs, the 4d state is higher in energy than the 5s state and corresponds to the Fermi level of the system. The increase in WF observed with increasing NP size due to the charge-extracting tendency of the ligands corresponds to a lowering in energy of the 4d state as the NPs get larger. This, in addition to the standard particle-in-a-box size effect, results in a smaller energy spacing and stronger interaction between the 4d and 5s states. As described by Mie resonance theory, more strongly interacting states give rise to collective electron oscillation (i.e., SPR) with smaller excitation energies.28 In smaller particles, where the 5s wave function begins to spill out28 of the NP boundaries, the two states are less coupled, resulting in a higher SPR transition energy and lower WF.
increasing cluster size. An increase in polarizability reduces the energy necessary to excite SPR, which is in accordance with the trend of decreasing SPR energy with increasing particle size. Since the simulation for polarizability was done on single particles, an additional test measured the SPR in our PVPcapped NPs, by performing STEM-EELS measurements on individual particles from each size group. Local valence EEL spectra taken over 0.3 nm wide spots near the edges of NPs from the 5, 25, and 50 nm groups are shown in Figure 6g. (NPs from the 75 nm group were too thick to be suitable for these measurements.) The locations where the spectra were taken are indicated on the dark field TEM micrographs of the particles in Figure 6 (a−c). The peak positions, corresponding to the SPR transition energies of the NPs, occur at 3.35, 3.1, and 2.95 eV for the 5, 25, and 50 nm groups, respectively. Figure 6 (d−f) show integrated EELS intensity maps of the three NPs over the energy range of their individual plasmon resonances: 3.2−3.5, 3.0−3.3, and 2.8−3.1 eV. SPR signal is absent in the silicon oxide surrounding the dots and is most apparent near the edges of the NPs, where the surface cross section of electron beam is greatest. (The lack of signal near the center of the dots is an experimental artifact arising from the combination of their thickness in that area and the lower surface cross section of the beam.) A distinct downward trend in the SPR transition energy with increasing particle size is observed, in agreement with our UV− vis measurements. Discrepancies in the absolute values measured in the two experiments may result from the fact that individual particles were measured in the EELS case, whereas UV−vis measurements were taken over the full distribution of NPs in a group. In addition to the decrease in peak energy, a narrowing of the peak from 0.3 to 0.1 eV is observed (5 to 50 nm, respectively), in agreement with predictions of both classical and quantum mechanical theories that show an inverse dependence of particle size and plasmon bandwidth.30 In Figure 7, we compare the results of our WF (UPS) and SPR (UV−vis) measurements on PVP-capped Ag NPs from each size group. We note that the curves appear nearly symmetric, with WF increasing with size as SPR transition
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CONCLUSIONS In summary, we have demonstrated that capping Ag NPs with ligands containing electronegative atoms directly bonded to the metal surface reverses the WF size trend found in bare particles, causing the WF to increase with increasing particle size. A significant change in WF (ΔE = 0.43 eV) in the approximate size range from 5 to 75 nm is observed. In addition to size, a pronounced effect on WF is seen from the ligand choice, causing the WF to either decrease or increase from the bare Ag value, depending on the chosen ligand. We also find an inverse correlation between the WF and SPR as a function of size in such NPs. Such a link may allow for the inference by proxy of one quantity from the other with only a single measurement of either. Our quantum ab initio simulation of similar, smaller systems supports the trends we observe in WF and SPR. Our results demonstrate the ability to strongly engineer the WF and SPR through a combination of size and capping ligand control.
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ASSOCIATED CONTENT
* Supporting Information S
Details of the computational simulations and experimental measurements. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*F. B. Prinz. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank SNL for use of model AC-2 UPS instrument for WF measurements and SNC for FEI Titan 80-300 kV Environmental Transmission Electron Microscope for EELS measurements. We would also like to thank both Samsung Corporation and Honda. We gratefully acknowledge partial support from Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001060. Lastly, we would also like to thank Phil Van Stockum for proof-reading and editing the manuscript.
Figure 7. Comparison of measured size trends in WF and SPR transition energy for PVP-capped NPs. WF numbers are taken from UPS measurements, and SPR numbers are taken from UV−vis measurements. Error bars indicate standard deviations. 24831
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