Feb 22 2017
When John Crocker - now a professor of chemical and biomolecular engineering at the School of Engineering and Applied Science of the University of Pennsylvania - was a graduate student, his mentor called everyone to his lab to “throw down the gauntlet” on a new problem in the field.
Speculations were rife that if colloidal crystals with a structure similar to that of carbon atoms in diamond were grown, the crystals would possess exceptional optical characteristics with the ability to dramatically transform the field of photonics. In such a material, known as the photonic bandgap material (PBM), the behavior of light will be mathematically analogous to the movement of electrons in a semi-conductor.
The technological implication is that such materials would allow for the construction of ‘transistors’ for light, the ability to trap light at specific locations and build microcircuits for light and more efficient LEDs and lasers.
John Crocker, Professor, University of Pennsylvania
During that period, Crocker resolved to work on his own projects, and left the research on PBM to other researchers.
Nearly two decades later, Yifan Wang, Crocker’s graduate student, accidentally developed the elusive diamond structure when he was working on another challenge. This encouraged them to work for developing PBMs, the “holy grail of directed particle self-assembly,” stated Crocker.
“It’s a classic story of serendipity in scientific discovery. You can’t anticipate these things. You just get lucky sometimes and something amazing comes out.”
Crocker, Wang, Talid Sinno, a professor at SEAS, and Ian Jenkins, a graduate student, headed the study. The outcome of the research were published in the journal Nature Communications.
In order to be classified as a PBM, a material should have a crystal-like structure on the length scale of light wavelength, and not on the atomic scale.
In other words, you need to sculpt or arrange some transparent material into an array of spheres with a particular symmetry, and the spheres or holes need to be hundreds of nanometers in size.
John Crocker, Professor, University of Pennsylvania
According to Crocker, in the 1990s, researchers were convinced that the spheres could be arranged in various probable ways to grow the desired structure by making use of colloid crystals, identical to the way in which semi-conductor crystals are grown—spontaneous self-arrangement of colloidal spheres into disparate crystal lattices.
One of the natural examples for this is opals, which are developed during the formation microscopic spheres by silica in groundwater. The microscopic spheres are crystallized underground and are then fossilized in solids.
Despite the fact that opals do not possess the correct symmetry to be classified as PBM, the iridescent appearance of the opals is the outcome of the point that their periodic crystal structure is on scales when compared to the wavelength of light.
The main aim while forming a PBM is to arrange transparent microscopic spheres in a three-dimensional (3D) pattern identical to the arrangement of carbon atoms in a diamond lattice. In contrast to other crystals, this structure is deprived of specific symmetry directions seen in other crystals, in which the behavior of light is normal, thus enabling the diamond structure to preserve the PBM effect.
The earlier assumptions of researchers - that synthetic opals can be developed with different structures by using disparate materials to synthesize PBMs - turned out to be highly challenging; and nearly two decades after that, it is yet to be achieved.
The Penn scientists made use of DNA-covered microspheres with somewhat different sizes to eventually develop the diamond lattices.
These spontaneously form colloidal crystals when incubated at the correct temperature, due to the DNA forming bridges between the particles. Under certain conditions, the crystals have a double diamond structure, two interpenetrating diamond lattices, each made up by one size or ‘flavor’ of particle.
John Crocker, Professor, University of Pennsylvania
Then, the researchers carried out crosslinking of the crystals to make a solid.
Crocker outlined the accomplishment as a stroke of good luck. Originally, the researchers did not work to develop the diamond structure, but they were carrying out a “mix and pray” experiment - that is, in order to investigate the parameter space, Wang adjusted five material variables. Until now, the method has yielded 11 different types of crystals, one of them being the astonishing double diamond structure.
“Often times when something unexpected happens, it opens up a door to a new technological approach,” stated Sinno. “There could be new physics as opposed to dusty old textbook physics.”
As the team has now overcome one of the major difficulties in developing PBM, the next aim of the researchers would be to find a way to switch out materials for high index particles and to dissolve one species in a selective manner to create one self-assembled diamond lattice containing colloidal microspheres.
The successful creation of a PBM will lead to a material which will be similar to a “semi-conductor for light,” possessing exceptional optical characteristics that have not been observed in any natural material. While the index of refraction of normal transparent materials is 1.3 to 2.5, the index of refraction of PBM could be very high. Conversely they can also have a negative index of refraction which leads to backward refraction of light.
Cameras, lenses, microscopes with higher performance can be made using this material. They can be probably also be used to make “invisibility cloaks,” that is, solid objects with the ability to redirect all light rays surrounding a central compartment, thus making the objects invisible.
Despite the fact that PBM could be experimentally reproduced over a dozen times, Sinno and Jenkins were unsuccessful in reproducing the results in simulation. Specifically, they have been unable to replicate the structure with 11 crystals, produced by Wang, in simulation.
“This is the one structure we’ve found so far that we can’t explain which is probably not unrelated to the fact that nobody predicted that you could form it with this system,” stated Sinno. “There are several other papers we’ve had in the past that really show how powerful our approaches are in explaining everything. In a way, the fact that none of this worked adds evidence that something fundamentally different is taking place here.”
Currently, the researchers feel that an unknown, distinct crystal grows and gets transformed into the double diamond crystals; however, this concept has been highly challenging to confirm.
You’re used to writing papers when you understand something. So we had a dilemma. Normally when we find something we chew on it for a while, we do simulations and then when it all makes sense we write it up. In this case, we had to triple-check everything and then make a judgment call to say that this is an exciting discovery and other people beyond us can also work on this and think about and help us try to solve this mystery.
John Crocker, Professor, University of Pennsylvania