Zhiqun Lin didn’t spend last summer lying in the sun. While school may have been out, the young materials scientist gave five talks on his work at five leading Chinese universities: two in Beijing, two in Shanghai, and one in his home province of Fujian—in addition to the rigorous research and writing schedule he maintains year round.
Still, the sun is shining on Lin’s career. His work in creating novel self-assembling micro- and nanoscale structures has achieved international recognition since he first came to Iowa State five years ago. And, fittingly, his recent project seeks to apply his unique insights into materials structures to the use of quantum dots in titanium dioxide (TiO2) nanotubes for solar energy technologies.
A multidimensional architecture
These are just the latest in a series of innovations Lin has recently made, breakthroughs that have seen his work featured in the pages—even on the covers—of some of the leading journals in his field.
Barely into the first of a five-year National Science Foundation CAREER Award, Lin’s pioneering techniques for creating hierarchically ordered structures through the synergy of evaporation-driven self-assembly at the microscopic scale and spontaneous self-assembly at the nanoscopic level have won the covers of Soft Matter and Angewandte Chemie International Edition, which designated his contribution a “very important paper.”
Lin’s one-step method confines a polymer or nanocrystal solution in a geometry consisting of a curved upper surface on a flat lower substrate, yielding a confined microfluid. The evaporation of solvent at the capillary edge triggers the “stick-slip” motion of the microfluid contact line, depositing hundreds of highly ordered concentric polymer or nanocrystal patterns—“coffee ring”-like deposits, Lin calls them—over the substrate.
“These structures,” Lin observes, “consist of whatever nanoscale materials we use as building blocks—block copolymers, conjugated polymer nanofibers, quantum dots, DNA-based nanocomposites—thereby exhibiting two or more independent characteristic dimensions: microscale structures with self-organized nanoscopic constituents residing along these various dimensions, a hierarchically ordered structure.”
Depending upon their constituents and architecture, Lin adds, the structures can demonstrate varying degrees of conductivity and optoelectronic properties, with potential applications in sensors, processing, and data storage. Recently, Lin used the technique to develop a “snakeskin” architecture incorporating conjugated polymer nanofibers he says shows significant promise for light-emitting diodes and thin-film transistors.
“People are using conjugated polymers for electronic materials,” says Lin. “But when the thickness of the polymer deposit on the transparent substrate is over a certain limit, the film becomes opaque, and light will not pass through.”
By contrast, the open spaces in Lin’s snakeskin structure expose the glass substrate. Besides requiring fewer materials to fabricate, he says, such a structure is more transparent and, depending upon its components, potentially more conductive.
Probing the theoretical bases
To date, Lin’s work in self-assembling nanoscale structures has been only a prelude to their application across a range of technologies. To achieve that goal, he must better understand the theoretical bases of his techniques in order to reproduce a given architecture uniformly and over much larger scales than with his lab-scale prototypes.
“In order to understand how these complex structures form,” Lin says, “you have to have very good knowledge of fluid dynamics, surface and interface science, colloidal chemistry, and polymer physics. You have to know why you have these ordered structures and be able to predict their length scale of periodicity and dimension.”
Lin is convinced that his CAREER project will develop that theory and so is already investigating potential applications in sensors. And, significantly, he’s busy developing new approaches to fabricating photovoltaic cells to harness solar energy more efficiently and cheaper than today’s silicon-based arrays.
Currently, Lin is in the midst of a three-year NSF project to produce an efficient nanocomposite of quantum dots within a conjugated polymer matrix for solar applications. But instead of physically “mixing” quantum dots with conjugated polymers, Lin has devised an approach for controlling the interface between these two semiconductors that promotes faster electron transfer from one to the other.
“I want to directly graft this very long chain of polymeric ‘spaghetti’ onto quantum dot surfaces,” he says. “By grafting the conjugated polymers onto quantum dots, rods, or wires, we can better exploit the polymer’s semiconductor-like optical and electronic properties for use in solar cells.”
Turning new techniques toward the sun
Lin’s larger ambition is to replace the organic dyes currently used in dye-sensitized solar cells with TiO2 nanotubes “sensitized” with inorganic quantum dots.
Dye-sensitized cells, Lin notes, are relatively recent innovations that typically use ruthenium-based dyes to generate the electron-hole pairs known as “excitons” upon absorbing the sun’s rays. In turn, the electrons must travel rapidly to the TiO2 photo anode to generate photocurrent before they recombine with their holes, after which the exciton would rapidly dissipate.
Yet current approaches are compromised by the need for electrons to “hop” between the randomly distributed TiO2 nanoparticles as they make their way to the electrode. This raises the possibility of increased scattering of free electrons and electron trapping at the interfaces, thus reducing electron mobility.
Instead, Lin seeks to replace the TiO2 nanoparticles with uniformly arrayed nanotubes, seeding these with highly semiconducting materials to act as a “bridge” to rapidly transmit electrons directly to the electrode without hopping. “That increases transfer efficiency,” Lin says, “which in turn increases power conversion efficiency.”
Next, Lin is refining techniques to “seed” the nanotubes with quantum dots. These, he says, offer multiple advantages, including their ability to absorb a wide spectrum of sunlight due to optical properties that are “tunable” as a function of particle size. Also, whereas organic dyes can produce only one exciton for each photon absorbed, quantum dots can generate multiple excitons from a single photon when incident energy is higher than the bandgap of quantum dots, thereby enhancing a cell’s solar conversion efficiency.
‘It’s quite good’
While today he better understands the fundamental photo physics of these varied architectures with regard to light harvesting, charge injection, and charge collection, Lin acknowledges that his approach has some distance to travel to reach the 11% conversion efficiency of today’s best dye-sensitized cells, let alone the 12–18% of silicon cells. But his early efforts have already increased the efficiency of a dye-sensitized TiO2 nanotube solar cell from 4.34% to 5.24%—a 20% increase. And, just this spring, one of his students used an oxygen plasma treatment to increase that gain to 7.37%.
“We’re still in the early stage of developing this type of cell,” Lin reminds. “So I am confident we can go even higher than 11% over time.”
Lin’s is a confidence born of a career that by analogy mirrors the self-assembled, highly ordered structures he has pioneered, and now seeks its application to real-world challenges. For there’s both elegance and inevitability to his ambitions, as he describes his creation.
“You can see this beautiful structure here,” Lin says as he traces a finger over a photo of his TiO2 nanotubes. “The bottom of the tube is closed; the top is open. Some of this is broken, but you can see it’s really straight and perpendicular to the membrane surface.
“This is the work we published,” he offers. “And it’s quite good.”