Welcome to one of our guest columns, where active researchers can share their views on topics relevant to materials science. Professor Geoffrey Ozin from the University of Toronto gives us his thoughts on nanochemistry and photonic metamaterials.
Many exciting chapters in the annals of twentieth-century chemistry are traceable to breakthroughs in experimental and theoretical physics like the discoveries of X-ray diffraction and electron optics that revealed the geometric arrangement of atoms in molecules and materials, and the formulations of quantum mechanics, molecular orbital theory and band theory which enabled our understanding of the structure, bonding, optical, electrical and magnetic properties of molecules and materials.
Nowadays, one of the hottest areas of physics research is photonics: the control of light by structures fabricated at the wavelength and sub-wavelength scales. The field of photonics really took off with the theoretical prediction of an omni-directional photonic bandgap in certain 3D periodic dielectric lattices, now known as photonic crystals. Chemists and materials scientists have been involved in efforts to fabricate photonic crystals for over a decade, and their main successes have been with the self-assembly of spherical colloids into opals, and the use of these structures as starting points for replicating other, more complex, materials. Chemists working in the field have made contributions to many of the current and emerging applications of these materials, such as full colour reflective displays, security devices, and sensors.
Now I am wondering what chemistry can do for the rapidly emerging and intriguing field of metamaterials that provide a materials platform for tuning optical space through command of local electric and magnetic fields in ways not previously imagined possible (Wegener, M., Linden, S., Physics Today, October 2010). The first example of a metamaterial with designed structural properties was the split-ring resonator (a miniature LC circuit wherein an electromagnetic field can induce a circulating and oscillating electric current making it act as a miniature planar electromagnet in which the electric dipole and magnetic dipole moments are respectively disposed within and normal to the plane of the split-ring – see illustration courtesy of Martin Wegener and Physik in unserer Zeit (2007, 1, 24)), which was first demonstrated in the microwave region but with advances in nanofabrication techniques can now be applied to near-IR frequencies. The ability to fabricate optical metamaterials, that is, metamaterials operating at or near to visible frequencies, also referred to as photonic metamaterials, has excited enormous interest in the optical physics community not just because of the potential to discover new physics but also the opportunity to reduce to practice some of the promises of metamaterials that operate at visible wavelengths, such as the perfect lens, the invisibility cloak, and the perfectly absorbing optical black hole.
All of this excitement in physics makes me wonder, what’s in this new class of photonic metamaterials for chemistry?
To understand this, we need to answer a number of fundamental questions which include:
First, what is it that is so amazing about metamaterials? Although the term ‘metamaterial’ in its broadest sense refers to any material displaying optical properties not readily available in nature, it’s most common association is with materials having negative refractive indices. The simplest physical manifestation of a negative refractive index would be that when light enters a negative index material the resulting refracted beam would appear as the reflection of the refracted beam for a material of positive index of the same magnitude (see image, also from Physik in unserer Zeit 2007, 1, 24). Although it may be difficult to immediately see how these effects could be of practical value and not just cute physics demonstrations, it turns out that they actually have extremely powerful applications to the control of the flow of light.
Second, what material properties do metamaterials need to have in order to display their incredible optical properties? The refractive index of a material may be written as n=±(εμ)1/2 where ε is the material’s relative electric permittivity and μ its relative magnetic permeability – terms which describe a material’s response to imposed electric and magnetic fields. In order for a material to possess a negative refractive index it turns out that both ε and μ must be simultaneously negative. Achievement of this poses a significant challenge, and has only been possible in recent years, firstly with the split-ring resonator in the microwave region, albeit with a slightly different double-ring configuration, as opposed to the single ring-design shown in the illustration, used for the visible region.
Achieving negative ε, even at visible wavelengths is not difficult. Metals are the typical example of materials displaying this property. Negative μ and n are typically achieved by fashioning a material with negative ε, such as gold, silver, or copper into a structure which displays negative μ. In the split-ring resonator the size of the metal ring is designed to be resonant with electromagnetic radiation of the target frequency, that is, the frequency at which we intend to demonstrate a negative index. The slit in the ring allows it to resonate on exposure to radiation whose wavelength is much larger than the size of the ring – this is critical because when we design such a material we want the radiation to have uniform optical properties at the frequency of interest, which requires that the spatial period of the material be much less than that of the radiation. The optical physics analysis is complicated, but it turns out that a properly designed split-ring resonator can display a negative permeability at its resonant frequency, and thus can be used as an element in fabricating negative-index materials. Since the advent of the split-ring resonator other structures have been shown to function as metamaterials, such as gold helices.
Photonic crystals can provide control over the electric component of light while photonic metamaterials can control both electric and magnetic components of light.
Third, and a central feature of this column, how does one make photonic metamaterials? All reports so far of photonic metamaterial fabrication have relied on top-down methods such as electron beam lithography, direct laser writing and nanoskiving. As nanochemists, is there a way for us to bring the same bottom-up techniques that we successfully applied to photonic crystal fabrication to the world of metamaterials? What molecular or materials structures and compositions should be synthetic targets for photonic metamaterial building blocks and their arrays besides gold? Also what properties of photonic metamaterials should be sought after and what are the best ways for evaluating these properties? And what properties of photonic metamaterials can be usefully exploited in chemistry and biochemistry, biology and medicine?
As far as I can judge, a bottom-up approach to photonic metamaterials is proving to be quite a challenge. Gaining synthetic access to building blocks with complex shapes like split-rings and spirals having sub-100 nm dimensions and self-assembling them into periodic arrays with predefined geometries is not easy. It is at the cutting edge of nanochemistry! In this context, a couple of bottom-up inroads to large-area split-ring arrays could make use of multi-tip dip pen nanolithography coupled with metal electrodeposition or template directed self-assembly using nanodroplet wettability arrays and metal nanocrystals.
With respect to possible applications one can seek to exploit the unique properties of photonic metamaterials, namely that conduction electric and magnetic resonances both become observable at optical wavelengths. This in principle provides an extra degree of analytical freedom beyond that offered by plasmon resonance spectroscopy for monitoring local changes in the environment of split-rings caused by chemical, biochemical, thermal, photochemical and electrochemical stimuli.
In this context one can imagine dynamic tuning of the photonic properties of gold metamaterials by selective chemical etching or galvanostatic reconstruction of the split-ring units in an array to change the optical resonant frequencies. One can also envision gold metamaterial arrays coated with self-assembled monolayers based upon alkanethiolates with terminal groups designed to recognize DNA, proteins and peptides, viruses, bacteria and cells through changes in the electric and magnetic optical resonances of the metamaterials that might outperform straight plasmonic probes. Gain materials like dyes, polymers and quantum dots chemically tethered to metamaterials could help ameliorate optical absorption losses that plague metals like gold. Another interesting study would be to gradually bend a gold nanorod into a split-ring configuration to see when the diagnostic transverse and longitudinal plasmon resonances of the nanorod transform into the electrical and magnetic resonances of the metamaterial. And as it is well documented that gold nanorods can be functionalized on their ends and sides with different kinds of self-assembled monolayers, one can imagine the same being accomplished with the gold split-ring and this attribute used to selectively tune the electric and magnetic resonances through end- and side-selective chemical recognition or by altering the capacitance across the ends of the ring.
Also what materials besides gold are useful for other photonic metamaterials objectives? Can one for instance build solid-state properties like piezoelectricity, ferromagnetism or photoconductivity into photonic metamaterials to create a whole new world of multifunctional photonic metamaterials? It is interesting to ask, of the many perceived applications of metamaterials which one could make it first as a real product to the marketplace? How about the perfectly absorbing optical black hole solar cell?
If some of these ideas could be reduced to practice it seems to me that nanochemistry has a lot to offer photonic metamaterials.
Do you think nanochemistry is the way to go for making these photonic metamaterials? Let us know in the comments below!