Apr 10, 2012
Boron nitride nanotube analysis links structural, mechanical and electronic properties
Boron nitride nanotubes (BNNTs) were discovered a few years after carbon nanotubes and offer many unique properties. These nanotubes have structural and mechanical properties similar to their counterpart carbon nanotubes, but due to their large band gap (~6 eV) they are considered to be electrically insulating. Altering the electrical conductivity of BNNTs by mechanical deformation has attracted some theoretical and experimental interest, but a quantitative correlation between electrical conductivity and mechanical straining has yet to be determined. On the other hand, previous studies show that BNNTs are good candidates for electron field emission (FE) displays thanks to the chemical stability of the material at high temperatures. However, the FE cycling stability of the nanotubes remains unclear. A better understanding of the electrical properties of BNNTs could yield many new applications. Uses include nanoelectronic switches and FE displays.
To find out more about the material, researchers from the Mechanical Engineering and Physics departments at Michigan Technological University (MTU), US, have used a novel scanning probe microscopy (SPM) stage that is located inside a transmission electron microscopy (TEM) system. “The set-up enables in situ monitoring of the structural changes of nanotubes under field emission cycles as well as mechanical straining,” explained Hessam Ghassemi, first author of the study, which has just been published in the journal Nanotechnology.
The team’s work showed that the electrical conductivity of BNNTs increases as a function of mechanical bending deformation. By straining the samples up to 2.5%, the resistance of individual BNNTs reduced from 2000 to 769 MΩ, the carrier concentration increased from 0.35 × 1017 /cm3 to 1.1 × 1017 /cm3 and the carrier mobility decreased from 2 to 0.4 cm2/(Vs). The reduction in carrier mobility was discussed in the context of the scattering between electrons and phonons under the applied electric fields and mechanical strains.
In the study, the researchers also showed that the field emission properties of BN nanotubes are sensitive to the number of FE cycles. For instance, in the second cycle, the turn-on voltage increased from 65 to ~80 V and the emission current dropped from 140 to ~120 nA. Further degradation of emission current as well as higher turn-on voltages were observed in subsequent cycles. Eventually, the amount of emission current dropped by almost 90% in comparison with the first cycle. The in situ TEM analysis of the FE cycled nanotubes revealed that structural decomposition can be responsible for the degradation of field emission properties.
The team’s goal is to map a precise correlation between the chemistry, structure, processing and electrical properties of BNNTs. “In the next step, we would like to study the effect of chemical dopants and impurities on the electrical properties of BNNTs,” revealed Reza Yassar.
BNNTs are unique materials that enable the study of band structure modulation by mechanical straining. “This may lead to rational control of the electrical properties of novel nanostructures in the future,” commented Yoke Yap.
Back in the lab, the researchers are busy making progress with their real time nanoscale characterization programme.
Further information on the study can be found in the journal Nanotechnology.
About the author
The lead author, M S H Ghassemi, was a PhD student at the Department of Mechanical Engineering-Engineering Mechanics and conducted this work under the supervision of Prof. Reza S Yassar who leads a research group on in situ TEM characterization of nanoscale materials for energy systems and nanoelectronics. Prof. Yoke Y Yap at the Physics Department investigates the synthesis of novel nanotubes and nanowires. The authors are based at Michigan Technological University in Houghton, MI, US.