Concluding Remarks

Nanoparticle assemblies: state-of-the-art and future challenges

Making a synopsis of the past two days is a daunting task, which I will approach from the perspective of a materials scientist. What follows is a rather subjective account that inevitably reflects my own tastes, predilections and preferences, even though I have conscientiously tried to reflect a balanced view.

My interest in the self-assembly of nanoparticles was aroused in 1994 when an eager PhD student (Mathias Brust) and his equally enthusiastic PhD supervisor (David Schiffrin) asked me to examine some thiol-stabilized Au nanoparticles they had just synthesized by a new phase transfer process.¹ On viewing their material in the electron microscope I was immediately fascinated by the way natural chemical interactions could self-assemble the particles into beautiful regular rafts on the microscope grid. They reminded me of the bubble rafts that materials scientists regularly use to show crystal structures and their imperfections such as grain boundaries, dislocations and point defects. Over the past decade it has been exciting to see how these self-assembled nanoparticle systems have exploded onto the research scene, stimulating the interest and fuelling the imagination of chemists, physicists, biologists, materials scientists and engineers worldwide. It is therefore very fitting that the 125th Faraday Discussion meeting on ‘Nanoparticle Assemblies’ should be hosted by Professor Schiffrin and Dr Brust at Liverpool University, since their work has been so influential in this field.

The twenty-two papers presented here, the stimulating and informative discussion of these papers, and the forty-five posters have all served to reassure me that that the science of nanoparticle assemblies is maturing well. No longer is it sufficient to synthesize a batch of nanoparticles, disperse them on a grid, and simply take a representative electron micrograph in order to write a paper. There is now much greater emphasis being placed on the detailed understanding of the physical and chemical properties of these nanoparticle systems. This is a step in the right direction and should help the nanotechnology community to fulfill the high expectations being placed upon it by governments, industry and venture capitalists. In order to maintain their future financial support for our research, it is important that we realize the full technological potential of some of these nanoscale materials.

About two-thirds of the papers presented here relate to discrete nanoparticles and their assembly into superstructures. Table 1 summarizes the distribution of assembly motifs (i.e. 0-D nanoparticles, 1-D strings, 2-D films, 2-D multilayers and 3-D supercrystals) and materials systems discussed at this meeting. It is clear that Au nanoparticles, perhaps because of the ease of fabrication and high stability, are still a strong favourite within our research community since they feature in 13 of the 22 discussion papers. Several other metals also featured prominently, namely Ag for its optical properties, Co for its magnetic properties, and Pt,Pd for their catalytic properties. What is surprising to me is the relative scarcity of papers dealing with semiconductor, oxide and polymeric nanoparticles. Maybe this reflects a real bias towards metal nanoparticle research in the European ‘nanotech’ community. If this meeting had taken place in the USA for instance, there would certainly have been a more even distribution of contributions between semiconductors/oxides and metal systems. Another interesting observation is that although there were papers discussing nanoparticle self-assembly into 2-D films, 2-D multilayers and 3-D supercrystals, there were none on forming 1-D nanoparticle strings. Maybe this is understandable since the string motif is the most difficult to self-assemble. Such string structures however should not be forgotten because of their potential for conversion into ultra-fine nanowires.²

Table 1 Distribution of papers with respect to nanoparticle self-assembly motif

Assembly motif	Authors	Material System
0-D nanoparticles	Banin et al.	Au, Ga, In
	Bowker et al.	Pt, Ru, PtRu
	Chechik et al.	Au
	Horrocks et al.	Si
	Liz-Marzán et al.	AuAg
	Nichols et al.	Au
	Savinova et al.	Pt
1-D strings	—	—
2-D layers	Amiens et al.	Co, NiFe
	Bartlett et al.	Polystyrene
	Becker et al.	Pd
	Bjornholm et al.	Au
	Fermin et al.	Au
2-D multilayers	Uosaki et al.	CdS
	Vossmeyer et al.	Au
3-D supercrystals	Landman	Au
	Murray et al.	Au
	Pileni et al.	Ag, Co, Fe2O3
	Vanmaeckelbergh et al.	ZnO

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The remaining one third of the papers presented and discussed at the meeting concerned the highly complementary areas of nanorods, nanowires and nanoporous materials as summarized in Table 2. Curiously, in this area, papers on semiconductor and oxide materials outweighed those on metallic systems.

Table 2 Paper distribution with respect to nanorod, nanowire and nanoporous systems

Nanostructure	Authors	Materials System
Nanorods	Banin et al.	Au
	Brevet et al.	Au
	Pileni et al.	Co
Nanowires	Banin et al.	InAs, InP, GaAs
Nanowire arrays	Benfield et al.	Fe, Co, Sn, GaN
	Morris et al.	Ge, Co, Cu
Porous membrane	Bartlett et al.	Au, Pt, Ag
	Benfield et al.	Mesoporous alumina
Channel membrane	Morris et al.	Aluminosilicate
Random porosity	Horrocks et al.	Porous Si

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In terms of the distribution of meeting participants, experimentalists far outnumbered theorists. I believe this is indeed a realistic snapshot of the ratio of experimental-to-theoretical participation in our research field at the moment. It would be a very healthy development if we could all encourage more theoretical participation by creating research links to the very talented pool of theorists that exists out there!

Several important themes evolved during the course of the lively discussion sessions. I will attempt to briefly summarize the most significant points raised in the next few paragraphs.

Most of the nanoparticle systems currently in vogue within our research community are ligand stabilized. Whilst we have an armoury of techniques available for studying the (usually) nanocrystalline core, the number of methods available for characterizing the soft ligand shell is much more limited. It is vital that we should make a real effort to understand the ligand shell structure, as it can in fact make up 30–40% of the material volume in our self-assembled nanocomposite systems.

It was both refreshing and exciting to see that the subject of ligand geometry and ligand dynamics featured prominently in the discussion. Landman, in his introductory lecture, presented a new model showing that 3-D supercrystal packing is strongly influenced by the ratio of extended chain length to ligand footprint radius. Bjornholm used WAXS to convincingly show that dynamic ligand rearrangements can occur in mixed ligand systems in response to external factors such as Langmuir pressure or hydrophilic liquid surfaces. Chechik demonstrated the use of EPR on spin labeled ligands for dynamical and interaction studies. The presentations of Bjornholm and Chechik were indeed excellent examples of how existing characterization techniques can be adapted for use to study the elusive ligand shell structure. Questions to be resolved concerning ligand structure and dynamics include; What is the typical surface coverage of ligands on the nanoparticle surface? Do sufficient vacant sites exist to allow ligand exchange or diffusion on the surface? When particles self-assemble into superstructures, do individual ligands or bundles of ligands interdigitate?

The collective electronic properties of 3-D Au nanoparticle assemblies were considered by a number of speakers. This is an important physical aspect to understand since it these collective properties that are likely to be exploited in various sensing applications. Murray surprisingly showed that the tunneling pathway for electron hopping from particle-to-particle is via non-bonded rather than cross-bonded ligands. Vossmeyer introduced the novel idea of using dendrimer linkers between the Au particles to improve analyte absorption and mechanical stability in vapour sensing applications. The ensuing discussion identified the need for frequency dependant conduction measurements on such systems and questioned whether or not the complex interaction of specific analytes with ligand molecules can be easily deconvoluted. Vanmaeckelbergh presented somewhat controversial ideas of conduction through ZnO nanoparticle arrays permeated either with aqueous or organic solutions. In particular, he considered the effects of electron–electron repulsion screening and proton compensation effects in this potentially important system.

More fundamental aspects of molecular conduction through complex ligands were described by Schmickler and Nichols. Schmickler modeled electron exchange between two electrodes mediated by self-assembled monolayer (SAM) adsorbates which prompted questions on the long term stability of such SAM's. Nichols presented STM conductance measurements through complex molecules via nanoparticle contacts. The complexities of such molecular conduction measurements were highlighted in the discussion. For instance, what are the contact resistances in such systems and how can we ensure contact to just one, rather than several molecules? In addition, how can we determine the carrier residence time within the molecule under investigation?

A number of speakers concerned themselves with the optical properties displayed by nanostructured metallic systems. Bartlett described the impressive designer optical properties that can created in metallic films containing very regular spherical nanocavity arrays. Such films were formed by electrodepositing metal on templates consisting of highly regular self-assembled arrays of polystyrene latex nanoparticles that were subsequently removed to leave the nanocavities. Liz-Marzán showed that it is possible to tailor the plasmon resonance frequency of AuAg alloy particles by systematically varying the alloy composition. His presentation raised the general following question: Is 100 year old Mie theory, which is insensitive to particle shape effects and polydispersity, really the best model for analysing plasmon resonances in metallic nanoparticle systems? Brevet demonstrated that there is considerable merit in applying the technique of hyper-Raleigh scattering for studying the polarisability properties of Au nanorods. Fermin studied the electrochemical and optical properties of 2-D electrostatically assembled arrays of Au nanoparticles using a variety of techniques ranging from Kelvin probe measurements to electrochemical impedance spectroscopy.

The optoelectronic properties of semiconductor nanoparticle systems also came under scrutiny during the meeting. Banin described elegant experiments that for the first time begin to explore the important optical transition regime between 0-D nanoparticles and 1-D nanorods. To achieve this he used InAs nanorods prepared by a new catalytic seeding technique. Uosaki probed the UV absorption properties of 2-D multilayers of CdS nanoparticles and measured their quantum efficiency as a function of the number of layers in the stack. By clever use of visible pump–IR absorption he also demonstrated that the decay behaviour of luminescence and IR absorption signals are fundamentally different.

A particularly lively session was that concerning the magnetic properties of nanoparticle assemblies. Pileni described how the application of an external magnetic field during the solvent evaporation process could strongly influence the self-assembly behaviour of magnetic nanoparticles. By applying a magnetic field she could modify the gross morphology of the 3-D supercrystal of ferrite nanoparticles from blocks to cylinders, each of which display different characteristic cracking patterns on drying out. Amiens studied the self-assembly and magnetic properties of 2-D arrays of Co and NiFe nanoparticles and raised the important general question as to whether or not the attachment of ligands affects the magnetic properties of the outermost layer of atoms on the nanoparticle surface. It was agreed that in order for magnetic nanoparticles to make a significant impact in the data storage arena, there is a need for further considerable optimization of particle size, shape, separation and anisotropy. Finally, Landman in an unofficial presentation, raised the intriguing possibility that Pd nanoparticles within a very narrow size range can exhibit large magnetic moments. If verified this would be the first instance where a metal, which is not magnetic in either its bulk or elemental form, could by virtue of its nanoscale icosahedral structure, exhibit a window of strong magnetic behaviour.

The use of nanoparticles or their assemblies as templates for the growth of other nanostructured materials was not a particularly dominant theme in this meeting, although as mentioned previously, the principle has been used to great effect by Bartlett in generating metal films containing regular arrays of spherical nanocavities. On this templating theme, Horrocks presented a preliminary comparison of DNA synthesis on H-terminated bulk Si(111) surfaces, on nanoporous Si and on a random dispersion of 0-D Si nanofragments. Becker used another approach whereby he used the regular structure presented by a model Ni₃Al/Al₂O₃ surface to template the vapour phase deposition of naked Pd clusters. The highly ordered and spectacular hexagonal arrays of Pd particles achieved were an unprecedented achievement for non-ligand stabilized clusters. If a suitable stabilization strategy for these bare particles could be devised, it would herald a new era in model catalyst experiments.

The chemical reactivity theme of nanoparticles, which is particularly relevant to catalysis, was explored by Savinova and Bowker. Savinova studied Pt nanoparticles supported on glassy carbon as a model catalyst for CO oxidation. She modeled the size dependant catalytic activity in terms of the restricted mobility of CO molecules on nanoparticle surfaces. This raised the fundamental question as to whether or not Fick's laws have any physical meaning in such physically constrained systems where the diffusion lengths are so small. Bowker presented an elegant STM study of Pt, Ru and PtRu alloy particles on the TiO₂ surface. The nanoparticle sintering behaviour and the propensity to locally re-oxidize the TiO₂ surface for these three nanoparticle types were compared. Bowker's work graphically reminded us that we should not think of the nanoparticle support material as being benign, since the nanoparticle and the support can often strongly interact.

The final theme of the meeting was nanowires and how in particular to assemble them into organized arrays. Benfield concentrated on the preparation and detailed structural characterization of Fe, Co, Sn and GaN nanowires inside mesoporous alumina templates. Morris presented results on Ge, Co and Cu nanowire formation by infiltrating the precursor under supercritical conditions into mesoporous aluminosilicate thin films. Issues arising pertaining primarily to the electrical characterization of nanowires included: How do we reproducibly make reliable contacts to nanowires? What are the effects of (i) grain boundaries, (ii) planar defects, (iii) surface structure and (iv) surface adsorbates on the electrical conductivity of nanowires?

At this point, I would like to do some crystal ball gazing and predict what would be the major new research themes if there were to be another Faraday Discussion meeting on Nanoparticle Assemblies in three years time. I would certainly expect to see more papers on the structure and properties of metamaterials comprised of combinations of different nanoparticles. In 1998 it was shown that Au nanoparticles of two different sizes could spontaneously self-assemble into ordered bimodal rafts³ that are analogous to ordered metal alloy systems on the atomic scale and colloidal crystals on the micron scale. Shortly after this, it was demonstrated that mixtures of thiol-capped Au and Ag nanoparticles could be self-assembled into true colloidal nanoalloys.⁴ More recently, just a couple of weeks before this meeting, Redl et al.⁵ published a Nature report of a metamaterial they had fabricated comprised of ordered magnetic (iron oxide) and semiconducting (lead selenide) nanoparticles. In principle, there are an infinite number of combinations of different nanoparticle types that could be self-assembled into new metamaterials, provided that in each case, the ligand shell and solvent chemistry of both species can be made compatible. This opens up a whole new vista of designer metamaterial combinations (e.g, metals with semiconductors, magnetic and non-magnetic metals, ceramics with polymers etc.). Each new metamaterial will display physical properties determined by the nature of its nanoparticle constituents and the linker ligand molecules. Furthermore, in three years I would expect there to be reports of even more complex metamaterials built up from three or more different nanoparticle types or combinations of different shaped nanoparticles (e.g. rods, spheres and prisms).

A particularly challenging motif to fabricate would be 1-D chains of nanoparticles in which the nanoparticle identity alternates regularly along the length of the string. It is probable that some kind of directed self-assembly will be required in order to achieve this goal. In this meeting there have been only a couple of examples of directed self-assembly (e.g. the application of external magnetic fields to ferrite nanocrystals while organizing into 3-D supercrystals). I anticipate that over a three year timespan the topic of directed nanoparticle self-assembly will become much more prominent. There is enormous potential for using DNA oligomers, substrate topology, substrate chemistry (e.g. hydrophilic/hydrophobic patterned surfaces, diblock copolymers) and dip-pen lithography to influence nanoparticle positioning.

The number of characterization techniques applied to the nanoparticle systems discussed at this meeting is truly impressive (see Table 3). As mentioned earlier, the majority of these give information on the crystalline core rather than the soft ligand shells. However, we have heard several examples where techniques have been ingeniously adapted to study the geometry, interactions and properties of the ligand component. I anticipate that even more techniques for ligand characterisation will be developed over the next three years. I also hope that some of the techniques currently being used for studying the crystalline component will be pushed more towards their limits. For instance, in my own field of transmission electron microscopy, several advances are now occurring which should have a considerable impact on nanocrystal characterization. High angle annular dark field (HAADF) tomography⁶ offers the possibility of directly visualizing the 3-D topography of individual nanoparticles. Aberration corrected scanning transmission electron microscopes⁷ afford the possibility of performing atomic column-by-atomic column XEDS and EELS spectroscopy of nanomaterials, allowing for instance the location of one dopant atom within an individual semiconductor nanoparticle. Other characterization areas (e.g. SPM) are also undoubtedly experiencing similar advances that will enhance their applicability to nanoscale materials.

Table 3 Summary of characterization techniques employed

Structural	Chemical	Electrical	Optical	Other
TEM	XPS	I–V	UV–vis	Kelvin probe
AFM	EDS	C–V	IR	FIB
STM	EPR	Cyclic voltammetry	FTIR	BET
XRD	STS	Chronamperometry	Optical reflectance spectroscopy	Microgravimetry
SAXS	EXAFS	Photoelectrochemistry	Fluorescence spectroscopy
WAXS	XANES	Impedance spectroscopy	Luminescence lifetime
LEED	NMR	Scanning electrochemical spectroscopy	Hyper-Raleigh scattering
Electron diffraction	ICP		Ellipsometry
LV-SEM			Raman spectroscopy
HREM

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Overall this has been an excellent and very useful Discussion. We leave knowing and understanding more than when we came, while also recognizing a little more clearly that which we do not understand. It is obvious that we as researchers are making rapid and substantial progress in the field of Nanoparticle Self-Assembly. Long may this continue!

References

1. M. Brust, M. Walker, R. M. Whyman, D. Bethell and D. J. Schiffrin, J. Chem. Soc. Chem. Commun., 1994, 801.
2. T. Hutchison, Y. P. Liu, C. Kiely, C. J. Kiely and M. Brust, Adv. Mater., 2001, 13, 1800.
3. C. J. Kiely, J. Fink, M. Brust, D. Bethell and D. J. Schiffrin, Nature, 1998, 396, 444.
4. C. J. Kiely, J. Fink, J. G. Zheng, M. Brust, D. Bethell and D. J. Schiffrin, Adv. Mater., 2000, 12, 640.
5. F. Redl, K. S. Cho, C. B. Murray and S. O'Brien, Nature, 2003, 423, 968.
6. P. Midgley, M. Weyland, R. Dunin-Borkowski, L. Laffont, J. M. Thomas and T. Yates, Microsc. Microanal., 2003, 9(2), 4.
7. S. Pennycook, A. R. Lupini, M. Varela, A. Borisevich, Y. Peng, R. Buckzo, X. Fan, J. R. McBride, T. C. Kipeny, S. J. Rosenthal, A. Franceschetti and S. T. Pantelides, Microsc. Microanal., 2003, 9(2), 2.

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