PARTICLES AS MOLECULES
C. M. S ORENSEN
3.1 INTRODUCTION
3
ligands cause colloids of the nanoparticles to be stable against irreversible aggregation.
Often these colloids act as solutions, with the nanoparticles displaying temperature-and solvent-dependent solubility. Also in many cases the precipitating solid is a two- or three-dimensional superlattice of the nanoparticles.
This chapter promotes the perspective that this new class of nearly monodisperse nanoparticles, which have been called quantum dots, nanoclusters, monolayer protec-ted nanoparticles, etc., can be viewed and described as large molecules. I will give justification for this perspective in terms of stoichiometry, properties, their solutions, and their assemblies. I will not describe their electronic and optical properties, which are very fascinating and with which further support for their classification as molecules can be found.
3.2 NANOPARTICLE MOLECULES
I start by asking: what is a molecule? I answer that a molecule is a discrete, complete entity, the smallest piece of a compound. A compound is a combination of elements whose atoms are bound together with the same definite proportions, that is, the same stoichiometry. This is different than a mixture, where the proportions are indefinite and the atomic binding is weak. A molecule if further divided would have radically different properties. Moreover, molecules tend to keep their identities when the compound is melted or vaporized or dissolved. In the solid the molecules often sit as discrete entities either at lattice points of a crystal or at random in the amor-phous solid.
As an example, consider carbon tetrachloride, CCl4. Carbon tetrachloride is a compound because we always find it with the same stoichiometeric ratio C/Cl of 1/4. The atoms are covalently bonded together so that the carbon tetrachloride mol-ecule is a definite entity. We can boil CCl4and the molecules will fill space keeping their integrity as CCl4units, that is, molecules. If we freeze CCl4, we will find the molecules arranged as entities in a lattice. If we dissolve CCl4 in acetone once again we will find that the molecules keep their identities. Yet if we look wider, we find this perfect example of molecular definition and identity is often broken by some of our favorite molecules. Certainly water is a compound because of its definite stoichiometry, but the water molecule looses its integrity in the many forms of solid ice in which the hydrogens are shared via hydrogen bonds so that the identity of the H2O molecule is confused. These ices can be thought of as huge entities (huge mol-ecules?) bound together via a combination of covalent and hydrogen bound linkages.
Similarly salt, NaCl, is a compound and its molecule is a single NaCl unit; yet dissolve salt in water and it dissociates, and the salt crystal is one huge macro-continuum of an ionic lattice. More relevant to nanoparticles, consider polymer molecules. These have the requisite stoichiometry yet can be huge, with essentially any molecular weight the synthesizer chooses.
Here we contend that nearly monodisperse nanoparticles can be fit under the rather broad definition of molecule. From the discussion above we see that all molecules have stoichiometry. The near monodispersity of the nanoparticles of the last two decades (indeed, the reason they have become so interesting) is the analogue to compound
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stoichiometry. For example, a 5.0-nm diameter gold particle ligated with dodecane thiol can be written
Au3870ðC12SHÞ365 (3:1)
where the standard deviation on the numbers is about 10%. This is close to the uniformity in mass of most compounds given the variety of stable isotopes most elements enjoy. We think of molecules as having an exact stoichiometry but then we remember that this is hard to achieve for polymeric molecules where a spread in polydispersity of 5% is considered very narrow. NPs are entities which if divided would not only be smaller but in some manner would be different than before division because of the well known size-dependent properties of these materials. True, the change would not be as drastic a change as one would get dividing H2O. Consider, however, that division of a polymer molecule changes the properties but, like the nano-particle, in a gradual manner. Moreover, the relatively large size of the NP of Equation (3.1) should not exclude it from the molecule club since we readily recall that polymer molecules can have large molecular weights, well beyond 106, and hence large phys-ical dimension. The nanoparticle in Equation (3.1) has a molecular weight of approxi-mately 800,000. The NP of Equation (3.1) can be dissolved in toluene, and when it is, it keeps its identity (unlike any ionic salt molecule when dissolved in water). Careful drying of such a solution will yield two- and three-dimensional superlattices analo-gous to the molecular lattices of “normal” molecules like CCl4. Other properties of such particle molecules continue the analogy. Thus, the electronic properties evolve from the bonding-antibonding orbitals of the simple diatomic molecule to the energy bands of the bulk solid with the HOMO-LUMO gap becoming the band gap of the solid. Many polymers and biomolecules such as proteins have solvent-and temperature-induced conformational changes. In analogy the ligsolvent-ands of NPs such as in Equation (3.1) have been shown to have similar changes.
The analogies of nanoparticles to molecules can be classified into two categories:
1. Truly stoichiometric cluster compounds. These have between several tens to a thousand atoms, hence sizes in the range of one to a few nanometers.
2. Nanoparticles with narrow size distributions. Here the stoichiometry is approxi-mate. The size ranges from a nanometer to typically several nanometers although examples exist to tens of nanometer scales. The number of atoms involved ranges from approximately 100 to 10,000.
One might say that chemistry rules the first category and physics the second. We say this because cluster compounds are just that, true compounds, with composition and size determined by the chemical reactivities and propensities of the constituent atoms. On the other hand, if the nanoparticles of the second category are to be viewed as molecular analogues, the first requirement is near stoichiometry, which is achieved by the narrow distributions of sizes. This in turn is achieved physically by one of three methods:
1. Nucleation kinetics that forces a sudden burst of particle creation from which a narrow distribution can ensue.
3.2 NANOPARTICLE MOLECULES 39
2. Digestive ripening, a heat treatment during which the particle size distribution narrows for poorly understood yet physical reasons.
3. Size selective precipitation/chromatography.
In category 2 above we have a new class of molecules; we can call them “nanopar-ticle molecules” and their compounds “stoichiometric par“nanopar-ticle compounds (SPC).”
Yet, notwithstanding the arguments above, there are differences with the canonical picture of molecules and vive la difference! Unlike the molecules of the past, nano-particle molecules are not tightly bound by rules of valency. Thus, their stoichiometry can range continuously over broad ranges of composition. Moreover, this freedom allows nanoparticle molecules to range broadly over size as well. And it is here that Nature has been kind because nanoparticles display size-dependent properties. Thus, we find ourselves with a new molecular category, a category with stoichiometries, sizes, and properties that we can choose and tailor to our needs, and a category with which we can create a whole new universe of materials. With these nanoparticle molecules, we have a new kind of chemical matter.
3.3 STOICHIOMETRIC CLUSTER COMPOUNDS
3.3.1 Semiconductor Cores
Stoichiometric cluster compounds involving II-VI semiconductor materials have a long history. For example, [Cd10(SCH2CH2OH)16]þ4 (5), [Cd4S10(SPh)16]42, [Cd8S(CH2CHCH3CH3)]2þ, Cd17S4(PhS)28]22 (6, 7), Cd10S4(SPh)12 (8), Cd32S14 (SPh)36. DMF4 (9), and Cd17S4(SCH2CH2OH)26 (10) have been synthesized.
These cluster compounds have approximately 1.0- to 1.5-nm cores that are essentially chunks of the corresponding bulk material, for example, CdS with wurtzite, zinc blende, or cubic sphalerite structures. These cores are covalently bonded to their ligand shells which via the sulfur are organic extensions of those cores. As such they are complete entities with a definite stoichiometry. They are soluble in organic solvents and display emission spectra blue shifted from the bulk. Other examples include (11, 12) stoichiometric copper-selenium compounds with cores of Cu2Se7, Cu50Se20, Cu73Se35, and Cu140Se70 and similarly Ni34Se22 with stoichiometric butyl and alkylphosphine ligands and Cu146Se73(PPh3)30. The colors of these com-pounds range from red to brown to black with increasing core size. These systems are also soluble in organics. A great many other examples exist, see Reference 13.
3.3.2 Metallic Cores
Another important class of cluster compounds is those with metallic cores like gold, platinum, and palladium, see the review by Schmid (14). The stability and hence the stoichiometry of these materials relies to first order on a magic number of atoms in the core. These magic numbers are based on close packing of hard spheres around a central sphere in successive layers. These structures can be created for
PARTICLES AS MOLECULES 40
finite-sized clusters by starting with a single particle and then surrounding it with 12 other same-sized particles, each touching the first, now central, particle to create a cluster with 13 total spheres. Given this 13-sphere cluster, another layer of particles can be added such that the additional spheres all fit in threefold coordination sites of the underlayer. This second layer would have 42 spheres to yield a 55-particle cluster. This process can be continued as illustrated in Figure 3.1. For small clusters this arrangement yields energetically favorable clusters especially when the shells are full and hence the numbers 13, 55, 147, 309. . . are “magic” numbers.
Perhaps the best examples are the cluster compounds synthesized by Schmid Au55(PPh3)12Cl6(15, 16), which have a metal core diameter of 1.4 nm and a total overall diameter of 2.3 nm. These clusters are soluble in organic liquids of low polarity. Since the triphenylphosphines are weakly ligating, they may be replaced.
Thus, the clusters may be made water soluble by exchanging the triphenylphosphine ligand with Ph2PC6H4SO3Na.
Clusters with larger magic numbers are increasingly hard to make; however, clus-ters of Pt309Phen36O30, Pd561Phen36O200, Pd1415Phen54O1000, and Pd2057Phen78O1600 with a variety of aromatic ligands and anions have been reported. Now recognize that the stoichiometry becomes inexact with a typical spread of+5% in the numbers of atoms in the core, ligands around the core, and anions associated with the ligands.
The numbers reported for the core favor the magic numbers out of a reasonable sense that they might represent the most stable clusters. This polydispersity in number may be joined by polydispersity in structure within the core. For example, Moiseev’s group (17, 18) reports evidence for at least three kinds of metal cores in Pd561phen60Oac180, nearly perfect packed fcc metal, icosohedral multiple twinned structures and roughly amorphous. Given all this, it has been argued that these cluster compounds cannot be considered as stoichiometrically well-defined molecules. Moreover, the ligands are no longer an integral part of the particle as they were for the semiconductor particles described above and hence are more labile and their number less certain.
We see a general trend that as the clusters become larger, the stoichiometry becomes less exact and the ligands become more independent of the core.
Figure 3.1 Hexagonal close-packed full-shell “magic number” clusters of spherical atoms.
The series starts as a single atom and then 12 others can be placed around it all touching the central atom, thus giving 12-fold coordination. This is the first shell. The number of atoms in the nth shell is 10n2þ 2. The number of atoms in the total cluster is (5/3)n(n þ 1) (2nþ 1) þ 2n þ 1.
3.3 STOICHIOMETRIC CLUSTER COMPOUNDS 41
Recently Jadzinski et al. (19) reported x-ray crystallography analysis of a thiolated gold molecular cluster with an exact stoichiometry. The cluster was made via borohy-dride reduction of gold chloride in the presence of p-thiobenzene acid, which became the ligand. The molecule contained 102 gold atoms and 44 ligands. It was shown that 79 of the gold atoms formed a truncated-decahedron inner core. Beyond that there were layers of gold atoms singly coordinated and then doubly coordinated with the ligand. The ligands not only interacted with the gold but with each other via both parallel and perpendicular ring stacking and the sulfur interacting with the ring. Most of the ligands assembled into chains extending from one pole of the roughly spherical molecule to the other. Finally the existence of this particular conformation for the molecule was ascribed to an electronic shell closing in which the 109 gold atoms donated 109 electrons, 44 of which engaged in bonding one of the 44 ligands.
This leaves 58 electrons free to pair up and occupy 29 delocalized orbitals to yield a stable zero angular momentum situation. This cluster compound is a small version of gold nanoparticles that we will discuss below.