X-Ray Photoelectron Spectroscopy of Core (Silver)–Shell (Polydiacetylene) Type Hybridized Nanocrystals

In the core-shell type hybridized nanocrystals composed of silver (Ag)-core and polydiacetylene (PDA)-shell, the localized surface plasmon (LSP) from Ag-core disappeared during solid-state polymerization in the coreprecipitation method. From high-energy X-ray photoelectron spectroscopy measured with synchrotron radiation X-ray in SPring-8, it has become apparent that a Ag-core is composed of nano-sized conductive domains. Domain boundaries may cause the reduction of the conduction electron mean free path in a Ag-core, and may be responsible for LSP damping without changing the plasmon frequency. [DOI: 10.1380/ejssnt.2009.711]


I. INTRODUCTION
A π-conjugated polymer, polydiacetylene (PDA), is one of the most promising organic third-order nonlinear optical (NLO) materials [1] owing to high NLO susceptibility χ (3) (ω), fast optical responsibility, and easy processability, in comparison with inorganic materials [2].The magnitude of χ (3) (ω), however, is not still enough to realize NLO device applications.Neeves et al. theoretically predicted the enlargement of χ (3) (ω) value, due to optical electrical field enhanced by localized surface plasmon (LSP) at the interface in a core-shell type nano-structured hybrid system [3].
We have already established the co-reprecipitation method to fabricate successfully a core-shell type hybridized nanocrystal (NC), which was composed of silver (Ag) nanoparticle (NP) core [Ag-core] with PDA NC shell [PDA-shell] [4].The resulting hybridized NC exhibited the peculiar visible (VIS) absorption spectral changes during the solid-state polymerization that is the final process to convert diacetylene (DA)-shell into PDA-shell in the co-reprecipitation method [4].Namely, the intensity of LSP from Ag-core was reduced gradually without the shift of LSP frequency, and then disappeared [4].It was reported so far that LSP diminished in metal NP embedded in silicon-oxide glassy matrix [5], and in metal-metal core-shell type nanostructure [6] with the shift of LSP frequency.Usually, the shift of LSP frequency is induced by the surrounding dielectric medium.Previous report looks different from our LSP damping phenomena.
In the present article, high energy X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) for Ag-core and PDA-shell hybridized NC were measured using synchrotron radiation (SR) in SPring-8 to discuss in detail LSP damping.
XPS and XANES measurements were carried out with the beamline BL15XU in SPring-8, which is a contract beamline of National Institute for Materials Science.This beamline has a revolver-type (planar or helical) undulator X-ray sources and Si double crystal monochromator of which energy resolution is about ∆E/E = 10 −4 .XPS analyzer is modified version based on ULVAC-PHI 10-360 type hemispherical analyzer [7].SR energy was set 4750 eV, close to maximum analyzing energy of this analyzer.High energy XPS is desirable to evaluate the electronic structure of Ag-core covered with PDA-shell because of its long observation depth.For example, an inelastic mean free path of C 1s electron excited by 4750 eV X-ray is calculated to be 8.6 nm for PDA [8].The binding energy in XPS was calibrated not with the Fermi edge but with the corresponding C 1s = 284.8eV, since there might be no good metallic contact between PDA-shell and a specimen holder.In XANES measurement of Ag-core, the intensity of L 3 M 4,5 M 4,5 Auger process was monitored as a function of X-ray energy.This is a special mode of XANES, which is sensitive to thin layer.On the contrary, simple X-ray transmission mode XANES is sensitive to thick bulk and could not be employed in the present case where the subtle hybridized NC was deposited on a substrate.

III. RESULTS AND DISCUSSION
First, the chemical state of Ag-core should be investigated whether or not Ag-core still remains metallic before and after solid-state polymerization of DA-shell.It can be easily imagined that the disappearance of LSP is caused by the alteration of Ag-core.To evaluate the chemical state of Ag-core, XANES spectra have been measured.In usual, XPS is utilized to evaluate the chemical states of materials, but the chemical shift of XPS spectrum of Ag is exceptionally small and is not necessarily sensitive to chemical states [9].XANES spectra at L 2,3 edge for transition metal mainly probes the local density of unoccupied d states, and the first XANES peak within 20 eV above the absorption edge, that is "white line", is sensitive to the chemical reaction of transition metal [10,11].
Figure 1(a) shows the XANES of Ag-core in the hybridized NC.The L 3 M 4,5 M 4,5 Auger intensity in L 3absorption edge grew up remarkably above 3350 eV.There was scarcely the difference in XANES before and after solid-state-polymerization.In addition, these spectra were in almost agreement with that of reference bulk-Ag, and obviously different from that of silver oxide as shown in Fig. 1(b) [12].Namely, this fact describes that no chemical changes such as oxidation occur in Ag-core during solid-state polymerization to form PDA-shell in the co-reprecipitation processes.
XANES in Fig. 1 demonstrated that Ag-core was metallic state, but it is of much interest whether or not Ag-core is simply the same as bulk Ag, relating to the disappearance of LSP from Ag-core. Figure 2 indicates the valence band (VB) XPS spectra of Ag-core, i.e., Ag 4d and Ag 5s bands, before and after solid-state polymerization.VB of bulk Ag metal is also overlapped in Fig. 2, and the top of its 4d band is artificially aligned with that of Ag core.The positions of Ag 3d 5/2 and Ag 3d 3/2 peaks from Ag-core were, respectively, 367.7 eV and 373.7 eV, since binding energy was calibrated with C 1s = 284.8eV.It is well-known that the VB of Ag metal is composed of hybridized 4d and 5s bands [13].The slope at Fermi-edge became relatively gentle.In addition, the positions of Ag 4d band around 4 eV to 6 eV were clearly shifted by ca.0.4 eV to the high binding energy region, and the Ag 4d band width became narrow, compared with bulk-Ag.
In general, it is said that such the behaviors of Ag 5s and Ag 4d bands of Ag-core would be due to the size effect or to chemical changes like oxidation.However, the possibility of the later case could be excluded on the basis of the above discussion in Fig. 1, and the present tendency is similar to that often-observed in Ag-cluster with several nm in size as follows.Tanaka et al. reported that the Fermi edge becomes broad for Ag-cluster, which size was about 4 nm, on HOPG [14].It was concluded that this broadening of Fermi edge is caused by the final state effect, originating from the positively charged hole created by photoionization, which depends mainly on cluster size.On the other hand, the d valence-band width of XPS spectra for Au-cluster on NaCl (100) substrate became also narrow with decreasing cluster size, and the critical size for band narrowing was 4 nm (about 10 3 atoms in a cluster) [15].In other words, d band width of large Au-cluster above the critical size is the same as that of bulk-Au, and the narrowing of d band width takes place below this critical size.Similar d band narrowing has been already reported for Ag-cluster [13,16,17].Moreover, these reports mentioned that the position of the midpoint of 4d band were also shifted to the high binding energy region, depending on Ag-cluster size.These experimental results are consistent with the present case shown in Fig. 2. Anyway, these facts suggest that the "electronic http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology conductive domain" observed from XPS is less than about 4 nm, which is obviously smaller than the overall size of the hybridized NC with ca.20 nm in size evaluated by TEM observation [4,18].There might be some resistive boundaries in Ag core.Small-sized electronic conductive domain in Ag core is a disadvantage to keep the intensity of LSP as follows.
Origin of LSP is a collective oscillation of the conduction electrons, which needs two-dimensional sufficient space for the oscillatory plasma.The intensity of LSP is closely related to mean free path of conduction electrons, the reduction of which leads to the strong damping of LSP [19].This fact means that the shortening of electron mean free path on the surface of Ag-core results in the strong damping of LSP.In addition, it should be noted that such kind of LSP damping occurs, accompanied by vanishingly small shift of LSP peak, and enables an annihilation of LSP as a consequence [19].Our optical measurement detecting LSP damping was this type of LSP damping.
Some non-uniformity in Ag core can be an important factor behind LSP damping.Solid-state polymerization of DA suggests an additional direct cause of LSP damping.We observed the characteristics of carbon after polymerization as follows.Figure 3 displays the carbon 1s XPS peak for PDA-shell in the hybridized NC under the condition that SR energy is 4750 eV and 3000 eV.The inelastic mean free paths of C 1s photoelectrons excited by 4750 eV and 3000 eV X-rays are estimated to 8.6 nm and 5.6 nm, respectively.By reference to XPS data of HOPG, one can see the small satellite peak of C 1s around binding energy 292 eV, due to π-π* transition of π-conjugated PDA backbone [20].The presence of π-π* transition in PDA is consistent with the growth of exciton by the solidstate polymerization of DA (see the inset of Fig. 3).On another note, changing XPS observation depth of carbon in PDA by use of 4750 eV and 3000 eV X-rays resulted in no change of C 1s XPS spectra for PDA.This concludes that PDA in hybridized NC is almost uniform in depth.We also found that Ag-core was not completely covered with PDA as follows: Ag MVV Auger peaks (kinetic energy 351.6 eV) excited by X-ray is visible for hybridized NC.If thick PDA shell is tightly packed on Ag core surface, Ag MVV Auger peak with short observation depth should be invisible.We can conclude that there is an interspace between PDA shell and some part of Ag core is visible through this interspace.It should be noted that the projective transmission image by TEM indicated that Ag core is entirely covered with PDA shell-layer.Therefore, an interspace between PDA shell can be caused not by the imperfect solid-state polymerization (that means locally partial bald area of Ag-core) but by the essential difficulty of stacking PDA chains (in PDA NC shell) closely on a steeply curved surface of Ag core, because PDA is one-dimensional conjugated chain in the essentially crystal state of PDA shell.These results let us have an image that π-conjugated PDA chains contact with some points on Ag-core surface.These Ag-PDA heterojunction points scattered on Ag-core surface might be anchors to block the plasma oscillation and decrease the LSP domain.Further research on nano-level hetero-junction of core-shell NC will reveal LSP mechanism in the future.
In conclusion, the LSP from Ag-core is considered to be damped without changing the frequency during the solid-state polymerization to form PDA-shell, owing to the reduction of the conduction electron mean free path in Ag-core induced by locally physicochemical interfacial interaction.
FIG.1: X-ray absorption near-edge structure (XANES) of Ag L3 edge for Ag-core: (a) hybridized NC before (red line) and after (blue line) solid-state polymerization, and (b) Reference XANES spectra for bulk Ag metal (purple line) and Ag oxide (green line)[12].

FIG. 2 :
FIG.2: XPS valence band spectra for Ag-core in hybridized NC: before (red line) and after (blue line) solid-state polymerization, and bulk-Ag (black line).

FIG. 3 :
FIG.3: XPS Carbon 1s spectra for PDA-shell in hybridized NC: SR energies are 3000 eV (green line) and 4750 eV (red line).The black line is for HOPG (4750 eV) as a reference.The inset shows the magnified spectra in the high binding energy region, which indicate the energy loss structure.