Chlorine and Graphene Combine


Nanotubes imagesResearchers at the Massachusetts Institute of Technology in the US are reporting on a new way to p-dope graphene that does not sacrifice its excellent electronic properties too much – something that has proved to be somewhat of a challenge until now. The resulting material could be ideal for making all-graphene integrated circuits on a chip, radio-frequency transistors and nanoelectronic circuit interconnects to name a few examples.

In the lab

Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycombed lattice. Since the material was first isolated in 2004, its unique electronic and mechanical properties, which include extremely high mobility, and high strength, have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future according to some. This is because electrons whiz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass, a property that could allow for transistors that are faster than any existing today.

However, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. One way of introducing a bandgap into graphene is to chemically dope it, but this has to be done carefully so as not to destroy graphene’s unique electronic properties too much.

Plasma-based surface functionalization technique

A team led by Mildred Dresselhaus and Tomas Palacios has now succeeded in p-doping graphene with chlorine using a plasma-based surface functionalization technique. “Compared with other chemical doping methods, the advantages of our approach are very significant,” says team member Xu Zhang. “First and foremost, the chlorine-doped graphene keeps a high charge mobility of around 1500 cm2/Vs after the hole doping. This value is impressively high compared to those obtained with other chemical species previously.”

The chlorine can also cover over 45% of the graphene sample surface, he adds. This is the highest surface coverage area reported for any graphene doping material until now, according to the researchers.

Density functional theory predicts that a bandgap of up to 1.2 eV can be opened up in graphene if both sides of the sample are chlorinated, and if the amount of chlorine on each side covers 50% of the total sample area. “The 45.3% coverage in single-sided chlorinated graphene observed in our work is thus important and paves the way to ultimately opening up a sizeable bandgap in the material while maintaining a reasonably high mobility,” Zhang told nanotechweb.org.

In their work, the researchers studied both “exfoliated” graphene and that obtained using chemical vapour deposition (CVD). They performed the chlorine plasma treatments in an Electron Cyclotron Resonance Reactive Ion Etcher (ECR/RIE) in which chlorine gas was excited into the plasma state by absorbing energy from an in-phase electromagnetic field at a certain frequency. The chlorine plasma was accelerated by applying a DC bias relative to the sample stage. “We carefully optimized both the ECR power and DC bias to control the reaction conditions,” explained Zhang, “and the experiments were performed at room temperature.”

The p-doped material produced could be used to make all-graphene integrated circuits on a chip and RF transistors, he added. Doping the graphene with chlorine also reduces its sheet resistance, making it suitable for use in electronic circuit interconnects.

The team now plans to dope suspended samples of graphene with chlorine – to access both sides of a sample – and so open up an even bigger electronic band gap.

The present work is detailed in ACS Nano DOI: 10.1021/nn4026756.

Advertisements

Flexible, light solar cells could provide new opportunities


QDOTS imagesCAKXSY1K 8David L. Chandler, MIT News Office
MIT researchers develop a new approach using graphene sheets coated with nanowires.
MIT researchers have produced a new kind of photovoltaic cell based on sheets of flexible graphene coated with a layer of nanowires. The approach could lead to low-cost, transparent and flexible solar cells that could be deployed on windows, roofs or other surfaces.

The new approach is detailed in a report published in the journal Nano Letters, co-authored by MIT postdocs Hyesung Park and Sehoon Chang, associate professor of materials science and engineering Silvija Gradečak, and eight other MIT researchers.Flexible, light solar cells could provide new opportunities

While most of today’s solar cells are made of silicon, these remain expensive because the silicon is generally highly purified and then made into crystals that are sliced thin. Many researchers are exploring alternatives, such as nanostructured or hybrid solar cells; indium tin oxide (ITO) is used as a transparent electrode in these new solar cells.

“Currently, ITO is the material of choice for transparent electrodes,” Gradečak says, such as in the touch screens now used on smartphones. But the indium used in that compound is expensive, while graphene is made from ubiquitous carbon.

The new material, Gradečak says, may be an alternative to ITO. In addition to its lower cost, it provides other advantages, including flexibility, low weight, mechanical strength and chemical robustness.

Building semiconducting nanostructures directly on a pristine graphene surface without impairing its electrical and structural properties has been challenging due to graphene’s stable and inert structure, Gradečak explains. So her team used a series of polymer coatings to modify its properties, allowing them to bond a layer of zinc oxide nanowires to it, and then an overlay of a material that responds to light waves — either lead-sulfide quantum dots or a type of polymer called P3HT.

Despite these modifications, Gradečak says, graphene’s innate properties remain intact, providing significant advantages in the resulting hybrid material.

“We’ve demonstrated that devices based on graphene have a comparable efficiency to ITO,” she says — in the case of the quantum-dot overlay, an overall power conversion efficiency of 4.2 percent — less than the efficiency of general purpose silicon cells, but competitive for specialized applications. “We’re the first to demonstrate graphene-nanowire solar cells without sacrificing device performance.”

In addition, unlike the high-temperature growth of other semiconductors, a solution-based process to deposit zinc oxide nanowires on graphene electrodes can be done entirely at temperatures below 175 degrees Celsius, says Chang, a postdoc in MIT’s Department of Materials Science and Engineering (DMSE) and a lead author of the paper. Silicon solar cells are typically processed at significantly higher temperatures.

The manufacturing process is highly scalable, adds Park, the other lead author and a postdoc in DMSE and in MIT’s Department of Electrical Engineering and Computer Science. The graphene is synthesized through a process called chemical vapor deposition and then coated with the polymer layers. “The size is not a limiting factor, and graphene can be transferred onto various target substrates such as glass or plastic,” Park says.

Gradečak cautions that while the scalability for solar cells hasn’t been demonstrated yet — she and her colleagues have only made proof-of-concept devices a half-inch in size — she doesn’t foresee any obstacles to making larger sizes. “I believe within a couple of years we could see [commercial] devices” based on this technology, she says.

László Forró, a professor at the Ecole Polytechnique Fédérale de Lausanne, in Switzerland, who was not associated with this research, says that the idea of using graphene as a transparent electrode was “in the air already,” but had not actually been realized.

“In my opinion this work is a real breakthrough,” Forró says. “Excellent work in every respect.”

He cautions that “the road is still long to get into real applications, there are many problems to be solved,” but adds that “the quality of the research team around this project … guarantees the success.”

The work also involved MIT professors Moungi Bawendi, Mildred Dresselhaus, Vladimir Bulovic and Jing Kong; graduate students Joel Jean and Jayce Cheng; postdoc Paulo Araujo; and affiliate Mingsheng Wang. It was supported by the Eni-MIT Alliance Solar Frontiers Program, and used facilities provided by the MIT Center for Materials Science Engineering, which is supported by the National Science Foundation.