Silver Nanocrystals: Self-Organization and Collective Properties

J. Phys. Chem. C 2010, 114, 3719–3731

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Silver Nanocrystals: Self-Organization and Collective Properties A. Courty* Laboratoire des Mate´riaux Me´soscopiques et Nanome´triques, UMR-CNRS 7070, UniVersite´ Pierre et Marie Curie, BP 52, 4 place Jussieu, 75252 Paris Cedex 05, France ReceiVed: September 16, 2009; ReVised Manuscript ReceiVed: December 18, 2009

In this paper, the main parameters, which dictate the supracrystalline structure of silver nanocrystals selfassembled at the micrometer scale, are evidenced. They are the alkyl-chain length, the deposition temperature, the concentration of the colloidal solution and the nature of the substrate. By varying the deposition temperature (from 15° to 50 °C), from the same batch of nanocrystals (same size, same coating agent), dense (fcc and hcp) and loose (bcc) supracrystals or disordered arrangements (rcp) are produced. By tuning the nature of the substrate (HOPG or amorphous carbon) and its temperature, the final size of the supracrystals is controlled. Collective properties due to either nanocrystals ordering in 2D superlattices or in supracrystals (3D) are pointed out. These properties are either due to dipolar interactions with the appearance of coupled plasmon modes or to the attraction between nanocrystals, self-assembled via interdigitation of the alkylchains used as coating agents, which leads to coherent vibrations of nanocrystals in a supracrystal. I. Introduction During the past decade, due to the emergence of a new generation of high technology materials, the number of groups involved in nanomaterials has increased exponentially. Fabrication of nm-orders at the mesoscopic scale is considered as the key for applications in data storage, functional devices, communications, and technology. A new field of research has thus emerged in the use of individual nanocrystals for growing 2D and 3D superstructures and investigation of their collective properties.1-3 The first self-organizations of particles having a diameter of a few nanometers ( λexc) (Figure 16, panels e and f). In order to explain this change in the behavior with the ordering of nanocrystals, two effects have to be taken into account: (i) the effect of the Lorentz field (the electromagnetic field, that is induced on each nanocrystal by the electric dipoles of the neighboring nanocrystals organized in fcc supracrystals) and (ii) that of vibrational coherence in a supracrystal. (i) The Lorentz field acts through the polarizability R(D) of the nanoparticles of diameter D. The self-organization in supracrystals implies that in presence of a size distribution, the nanoparticles of the same diameter D self-organize in order to form well-ordered assemblies. This induces a local enhancement of the Lorentz field, that increases rapidly with D. According to eq 2, the Raman diffusion of the large-size particles is thus shifted toward a lower frequency. The polarizability of the nanocrystals in the supracrystals is then given by the following expression:

R(ν)L(ν)

(3)

where L(ν) is the Lorentz field factor and R(ν) is the polarizability of the isolated nanoparticle. (ii) The van der Waals bonding between thiol chains is sufficient to establish a correlation between the vibrating nanocrystals so that they vibrate coherently in the supracrystals. The light is scattered by stationary modes (Q ) 0) when the size of the supracrystals is small compared to the laser excitation wavelength, The Raman scattered intensiy is then35 (3) ISmallsupra (ν) ∝ L2(ν)[I (1)(ν)]2

(4)

The quadratic dependence in I(1)(ν) (Raman scattering of disordered arrangements of nanocrystals) implies a narrowing of the Raman peak, as is experimentally observed (Figure 16d). This narrowing is thus the direct expression of the interparticle coherence. When this narrowing is observed, the line profile is given by the square of that corresponding to a disordered arrangement of nanocrystals. This is illustrated in the Figure 16d inset and is the proof that the silver nanocrystals vibrate coherently in a supracrystal as atoms do in a nanocrystals. When the size of the supracrystals is large compared to the laser excitation wavelength, the light is scattered by propagative modes of the wave vector Q. The Raman scattering intensity is then given by35

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Figure 16. (a) Low frequency Raman spectrum of disordered assemblies of silver nanocrystals. (b) Comparison of the Stokes lineshapes with the inverse nanocrystal diameter distribution F(D-1). (c) Low-frequency Raman spectra of silver nanocrystals deposited on a HOPG substrate at 10 °C (solid line) and at 22 °C (dashed line). (d) The superposition of both lineshapes after horizontal shifting. (e) Low-frequency Raman spectra of silver nanocrystals deposited on a HOPG substrate at 30 °C (dashed line), in inset the comparison of the Raman scattered intensity I(ν) from silver nanocrystals organized in a “supra” crystals with the product I(ν)2 where I(ν) is the Raman scattered intensity of a disordered assembly of nanocrystals. (f) The superposition of both line shapes after horizontal shifting. The sharp band (dotted lines) in all the LFRS spectra at -13 cm-1 is plasma laser line reflected by the sample.

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(4) Ilargesupra (V) )

n(V) + 1 V

NxD

∑ exp[-ikb(br i - br j)]|δR(ν)|2expi[Qb(br i - br j)] i,j

(5) The result is that the Raman scattered intensity is proportional to the Raman intensity scattered by a disordered assembly of nanoparticles (4) Ilargesupra (ν) ∝ L2(ν)I (1)(ν)

(4a)

The Raman peak of the supracrystal thus overlaps that of the disordered assembly and is shifted to a lower frequency (Figure 16, panels e and f). The LFRS experiments are not a direct proof of internanocrystal coherence in fcc silver supracrystals. Nevertheless, very recently, they have been confirmed by the observation of vibrational coherence in fcc supracrystals, made of cobalt nanocrystals, by time-resolved pump-probe spectroscopy.36 The long-range ordering in the supracrystal makes possible the launching of coherent vibrations of the nanocrystals when suddenly heated by femtosecond laser pulses. IV. Conclusion Alkyl chain length and deposition temperature are shown to dictate the crystalline structure (from fcc, hcp to bcc) of Ag nanocrystal superlattices, that correspond to equilibrium states. The interaction forces are dominated by alkylthiol interactions. The balance between the hard-core repulsion and the short-range repulsive interaction of the alkyl chains (which can be tuned by their length) determines whether silver nanocrystals form dense or loose lattices. Furthermore, by tuning the nature of the substrate and its temperature, the size of the fcc “supra” crystals can be controlled. Specific, unexpected and remarkable properties are found on comparing the physical properties of disordered and ordered assemblies prepared from the same batch of nanocrystals. Collective optical properties of self-organizations of silver nanocrystals in 2D hexagonal networks induce the appearance of coupled plasmon modes due to induced dipole-dipole interactions. Furthermore the strength of the network produced with formation of fcc “supra” crystals induces vibrational coherence which is an intrinsic property of the self-organization. V. Experimental Method Section Synthetic Procedures for Silver Nanocrystals. Silver nanocrystals are synthesized in functionalized mixed reverse micelles as described previously7,12 and are obtained by mixing two reverse micellar solutions. The first is (made of) 60% 0.1 M functionalized surfactant Ag(AOT) (silver di(2-ethyl-hexyl) sulfosuccinate) and 40% 0.1 M Na(AOT) dissolved in isooctane. The water content w ) [H2O]/[AOT] is kept at 2. The second solution is 0.1 M Na(AOT) in isooctane with water replaced by hydrazine. Hydrazine is in excess and its content, defined as R ) [N2H4]/ [AOT], is kept at 1.44. The silver ion reduction starts as soon as the two solutions are mixed, and is stopped several hours later by the addition of dodecanethiol (2 µL/mL) or decanethiol to the solution (3 µL/mL). The remaining surfactant is removed by ethanol addition inducing flocculation of the coated (Ag-C12 or Ag-C10) nanocrystals. This precipitate is then dispersed in hexane. In order to reduce the size distribution, a size selective precipitation process is carried out by progressively adding pyridine to the colloidal solution. An ag-

glomeration of the largest particles takes place, which allows them to be collected as a precipitate after centrifugation. The latter is then dispersed in hexane (BP ) 68 °C) or decane (BP ) 174 °C) leading to a homogeneous clear solution of nanocrystals. These silver nanocrystals are characterized by a mean diameter of 5.0 nm and a size distribution of 12% and consist of a mixture of cuboctahedral, decahedral and icosahedral nanocrystals.33 Small Angle X-ray Diffraction (SAXRD). Small angle X-ray diffraction (SAXRD) experiments were performed with a rotating anode generator with a small size focus 0.2 mm × 0.2 mm (50 kV, 30 mA). The optics consisted of a parabolic multilayer graded mirror followed by a bent nickel-coated mirror at right angles. It delivered a well-defined and intense parallel monochromatic beam. The sample was mounted on a rotation stage and diffraction pattern is recorded on photostimulable imaging plates. Vacuum pipes were inserted between the sample and the imaging plates. The comparison of observed and calculated diffraction spot coordinates in the small-angle X-ray diffraction pattern enables determining the crystal structure of the 3D nanocrystal assemblies. From the experimental spot coordinates and the sample-detector distance, we can determine the Bragg angle 2θ and thus the modulus of the diffracted vector q from the Bragg equation. We can also identify the center-to-center interparticle distance from the position of the intense and welldefined Bragg reflections. The distances in the reciprocal space are converted into dhkl spacings using the formula q ) 2π/dhkl. The projected qx and qy values are expressed as


38 and

x qhkl )

2π l D

y ) qhkl

2π D

4(h2 + k2 + hk) for a hcp packing 3




2π h2 + k2 + l2 - hk - kl - hl and D 3 2π h + k + l ) for a fcc packing, and D √6

x ) qhkl y qhkl




x ) qhkl

2π h + k D 2

y ) qhkl

2π D





32 and

l2 +




(h - k2) 3 2 2 for a bcc packing, with a coated particle

diameter D. From the agreement between expected and measured diffraction spots for a given nanoparticle diameter, we deduce the indexation of the diffraction spots. Low Frequency Raman Scattering Experiments (LFRS). The Raman measurements for 5 nm and 6 nm nanocrystals organized at 2D were performed using a CODERG T800 triple monochromator setup coupled with a conventional monochanel detector IN the laboratory LPST in Toulouse (France). For the 3D silver nanocrystals assemblies, the Stokes-anti-Stokes Raman spectra were recorded with a five-grating monochromator in the laboratory LPCML in Lyon (France). The high resolution and rejection rate of these setup permit to observe low-frequency Raman signals close to the Rayleigh line down to less than 2 cm-1. The samples were illuminated with a 488 or 514.5 nm line of an Ar laser, with an incidence

Feature Article angle close to π/3. The excitation wavelength is thus close to the resonance of the broad dipolar plasmon excitation (2.9 eV for nanocrystals in hexane solution (see inset Figure 15)), thereby ensuring plasmon resonant Raman scattering. The scattered light was detected at about π/2 with respect to the incoming laser beam. The spectra where only the quadrupolar mode are observed (Figure 16, panels a, c and e), are recorded in crossed (HV) polarization mode. Acknowledgment. For their participation in this work, I would like to thank Prof. M. P. Pileni and Drs. A. Brioude. A.-I. Henry, M. Maillard, N. Pinna, V. Russier, and A. Taleb, from the LM2N Laboratory in Paris and also Dr. P. A. Albouy from the LPS in Orsay and Prof. E. Duval and Dr. A. Mermet from the LPCML in Lyon. References and Notes (1) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Nanocrystal superlattices. Annu. ReV. Phys. Chem. 1998, 49, 371–404. (2) Pileni, M. P. J. Phys. Chem. 2001, 105, 3358. (3) Pileni, M. P. Acc. Chem. Res. 2007, 40, 685–693. (4) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices. Science 1995, 270, 51338. (6) Harfenist, S. A.; Wang, Z. L.; Whetten, R. L.; Vezmar, I.; Alvarez, M. M. AdV. Mater. 1997, 9, 817. (7) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (8) Vijaya Sarathy, K.; Raina, G.; Yadav, R. T.; Kullkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (9) Wang, Z. L. Structural analysis of self-assembling nanocrystal superlattices. AdV. Mater. 1998, 1, 13–30. (10) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. Bundling and interdigitation of adsorbed thiolate groups in selfassembled nanocrystal superlattices. J. Phys. Chem. 1998, 102, 3068. (11) Korgel, B. A.; Fitzmaurice, D. Small-angle x-ray scattering study of silver- nanocrystal disorder-order phase transitions. Phys. ReV. B 1999, 59, 14191. (12) Courty, A.; Fermon, C.; Pileni, M. P. AdV. Mater. 2001, 13, 254. (13) Courty, A.; Araspin, O.; Fermon, C.; Pileni, M. P. Langmuir 2001, 17, 1372. (14) Courty, A.; Mermet, A.; Albouy, P. A.; Duval, E.; Pileni, M. P Self-organized Ag-nanocrystals in fcc “supra” crystals. Vibr. Coherence Nat. Mater. 2005, 4, 395–4006. (15) Petit, C.; Taleb, A.; Pileni, M. P. AdV. Mater. 1998, 10, 259. (16) Lisiecki, I.; Albouy, P. A.; Pileni, M. P. AdV. Mater. 2003, 15, 712. (17) Yang, H. T.; Shen, C. M.; Su, Y. K.; Yang, T. Z.; Gao, H. J. Appl. Phys. Lett. 2003, 82, 4729. (18) Lisiecki, I.; Albouy, P. A.; Pileni, M. P. J.Phys.chem.B 2004, 108, 20050. (19) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. AdV. Mater. 1995, 9, 797. (20) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. ReV. Lett. 1995, 75, 3466. (21) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 44. (22) Whetten, R. L.; Shafgullin, M. N.; Khoury, J. T.; Shaaf, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A.; et al. Crystal structures of molecular gold nanocrystal arrays. Acc. Chem. Res. 1999, 32, 397. (23) Zeng, N.; fan, J.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 6550. (24) Zaitseva, N.; Rong Dai, Z.; Leon, F. R.; Krol, D. Optical properties of CdSe superlattices. J. Am. Chem. Soc. 2005, 10221–10226. (25) Harfenist, S. A.; Wang, Z. L.; Whetten, R. N.; Vezmar, I.; Alvarez, M. M. Three-dimensional hexagonal close-packed superlattice of passivated Ag nanocrystals. AdV. Mater. 1997, 9, 817–822. (26) Stoeva, S. I.; Prasad, B. L. V.; Uma, S.; Stoimenov, P. K.; Zaikovski, V.; Sorensen, C. M.; Klabunde, K. J. Face-centered cubic and hexagonal closed-packed nanocrystal superlattices of gold nanoparticles prepared by different methods. J. Phys. Chem. B 2003, 107, 7441. (27) Henry, A. I.; Courty, P. A.; Albouy, A.; Israelachvili, J.; Pileni, M. P. Nano Lett. 2008 (28) Park, S. Y.; Lytton-Jean, A. K. R.; Lee, B.; Weigand, S.; Schatz, G. C.; Mirkin, C. A. Nature 2008, 451 (7178), 553.

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