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Summer 2016

Controlling Surfaces with a TAD of Material

A new technique could enhance materials used in a broad spectrum of technologies

Robert Call

Targeted atomic deposition (TAD) eliminates surface defects that can have large effects on material properties. In this example, a cycle of TAD bleaches a NiO thin film indicating a large change from a small amount of material. Image credit: TA Celano and CJ Flynn

From electronics to healthcare to industrial applications, nanomaterials have become an essential element of innovation. Consequently, development of new and better nanomaterials is a vital part of advancing technology. Controlling the surfaces of nanomaterials is key to their development because the surface determines how a material interacts with its environment. Surface structures and defects can significantly alter a material's behavior. Special fabrication and treatment techniques are required to precisely control the surface. Scientists at the Center for Solar Fuels (UNC), an Energy Frontier Research Center, have developed one such technique. Specifically, the UNC researchers have developed a technique that eliminates surface defects on thin films of nickel oxide (NiO). The surface defects on NiO arise from missing nickel atoms (nickel vacancies) on the film surface that result in dangling oxygen bonds. These defects are partially responsible for the poor performance of NiO films in some of their applications.

Nickel oxide films are often used in energy applications to control how electric charges move and where they accumulate. In particular, NiO films have been studied extensively as electrodes for dye-sensitized solar cells (DSSCs). These cells function like most solar cells by turning light into voltage. The difference from common solar cells is that DSSCs use a semiconductor film with light-absorbing molecules on the surface to accomplish this conversion. These devices are more flexible than common solar cells because the molecules on the surface can be tuned to absorb different wavelengths of light. DSSCs also provide important insight for building solar cells that can use sunlight to catalyze chemical reactions resulting in solar fuels.

The UNC team that developed the procedure, led by Jim Cahoon and Cory Flynn, was investigating films composed of tiny platelets of NiO for use in DSSCs. The team was examining the effects of depositing aluminum oxide (Al2O3) onto the NiO films using atomic layer deposition (ALD). Aluminum oxide is often used as a buffer layer to protect surfaces in chemical environments. In their study, the best performance coincided with very small amounts of Al2O3. By adjusting the pulse length and growth temperature of the ALD process, they were able to deposit 75 percent less material than the amount put down by a single cycle of ALD deposition. This is significantly less material than is needed to form a one-atom-thick Al2O3 layer on the surface. They named their revised procedure targeted atomic deposition (TAD).

That such a small amount of material could have a large effect implied that the Al2O3 was reacting to specific surface sites on the NiO film. NiO films that were TAD treated were visibly bleached compared to the initial films. This change in color, combined with other information, suggested that the TAD of alumina was reacting with (passivating) the NiO surface defect sites. First principles calculations confirmed that the observed changes corresponded to passivation of surface nickel-vacancy sites.

The key to TAD is using a high-energy vapor-phase precursor that selectively reacts with the target defects. This requires a precise understanding of what the defect is and what precursor will react with the defect site. The growth conditions—flow rate, exposure time, pressure, temperature, etc.—of the TAD process also play a critical role. For example, if the flow rate or temperature is too high, the process can result in material piling up on the surface instead of reacting with the specific defect sites.

Continued work by the UNC team has shown that the TAD process is not limited to alumina deposition on NiO. They have used a boron precursor in a chemical vapor deposition (CVD) setup to achieve the same result. Additional work needs to be done, but the ability to use a different precursor and delivery method (CVD uses continuous flow as opposed to the pulsed flow of an ALD system) suggests the TAD technique is flexible enough to use on other materials and setups.

The ability to selectively modify surface defects on nanomaterials is an important advantage in technological progress. In this case, a TAD treatment on NiO films led to higher-efficiency NiO-based DSSCs. The UNC team is working to develop TAD as a general strategy to passivate defect sites. This capability would benefit a wide variety of nanomaterials with applications in many different fields.

Acknowledgments

This work was supported as part of the Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Flynn CJ, SM McCullough, EB Oh, L Li, CC Mercado, BH Farnum, W Li, CL Donley, W You, AJ Nozik, JR McBride, TJ Meyer, Y Kanai, JF Cahoon. 2016. “Site-Selective Passivation of Defects in NiO Solar Photocathodes by Targeted Atomic Deposition.” ACS Applied Materials Interfaces 8:4754-4761. DOI: 10.1021/acsami.6b01090

Flynn CJ, SM McCullough, L Li, CL Donley, Y Kanai, JF Cahoon. 2016. “Passivation of Nickel Vacancy Defects in Nickel Oxide Solar Cells by Targeted Atomic Deposition of Boron.” Journal of Physical Chemistry C Just Accepted. DOI: 10.1021/acs.jpcc.6b06593

About the author(s):

A TAD Closer to Perfection

New technique fills in defects that hamper solar cells

The TAD technique fills in missing nickel atoms (red box) on the surface. This technology gives scientists the precise control they need to implement new materials into technology. Modified from image, credit TA Celano and CJ Flynn.

Sometimes it’s the little things that get you down. For devices that use nanomaterials, the little thing can be atom-sized defects on a material’s surface. Scientists need closer-to-perfect surfaces on nanomaterials. Why? Because these materials are essential to technological advancement in many fields and surfaces effect how these materials interact with their environment. At the Center for Solar Fuels, scientists developed a new technique, nicknamed TAD. This technique selectively fills in defects on nickel oxide films. Removing defects could help these films perform better in specialized solar cells and other devices. But it isn’t a one-trick pony. Scientists used TAD on other materials as well. They believe that if this technique is developed further, it could allow for the careful control of nanomaterial surfaces necessary for innovations in energy and beyond. The Center for Solar Fuels is led by the University of North Carolina, Chapel Hill.

More Information

Flynn CJ, SM McCullough, EB Oh, L Li, CC Mercado, BH Farnum, W Li, CL Donley, W You, AJ Nozik, JR McBride, TJ Meyer, Y Kanai, JF Cahoon. 2016. “Site-Selective Passivation of Defects in NiO Solar Photocathodes by Targeted Atomic Deposition.” ACS Applied Materials Interfaces 8:4754-4761. DOI: 10.1021/acsami.6b01090

Flynn CJ, SM McCullough, L Li, CL Donley, Y Kanai, JF Cahoon. 2016. “Passivation of Nickel Vacancy Defects in Nickel Oxide Solar Cells by Targeted Atomic Deposition of Boron.” Journal of Physical Chemistry C Just Accepted. DOI: 10.1021/acs.jpcc.6b06593

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.