Researchers characterize individual defects inside a bulk insulator using scanning tunneling microscopy

September 3, 2015 by Rachel Berkowitz, Lawrence Berkeley National Laboratory
(a) STM topographic image of a clean graphene/BN area (b) dI/dV map acquired simultaneously with (a) exhibits new features including bright dots, a dark dot and a ring.

Nanoscale defects are enormously important in shaping the electrical, optical, and mechanical properties of a material. For example, a defect may donate charge or scatter electrons moving from one point to another. However, observing individual defects in bulk insulators, a ubiquitous and essential component to almost all devices, has remained elusive: it's far easier to image the detailed electrical structure of conductors than insulators.

Now, Berkeley Lab researchers have demonstrated a new method that can be applied to study individual defects in a widely used bulk insulating material, hexagonal boron nitride (h-BN), by employing scanning tunneling microscopy (STM).

"Normally, STM is used to study conductors and cannot be used to study bulk insulators, since electrical current does not typically flow through an insulator," explains Mike Crommie, physicist at Berkeley Lab's Materials Sciences Division and professor at UC Berkeley, in whose lab this work was conducted. His team overcame this obstacle by capping the h-BN with a single sheet of graphene.

"This permits us to visualize the charged defects embedded in the underlying BN crystal," Crommie says. "Essentially, we use graphene as a window to look into the insulator."

Adds Jairo Velasco Jr, also a member of the Materials Sciences Division and a lead co-author of this work, "In contrast to previous studies that were limited to spatially averaging defect behavior, our experiment visualizes individual embedded inside a BN crystal with nanoscale precision. The STM allows details of a defect's electronic properties to be extracted by directly detecting how electrons in graphene respond to the defect in the underlying bulk insulator."

Graphene synthesis and characterization, performed at the Molecular Foundry, a DOE Office of Science User Facility, aided the researchers in visualizing and even manipulating individual defects in the underlying bulk BN insulator. New features in STM topographic and energy-dependent electron density images included randomly distributed dots and rings of varying intensities.

"We discovered that it is possible to selectively manipulate the charge states of individual BN by applying voltage pulses with our STM tip," Velasco says.

The new technique provides a valuable tool for the many scientists in the 2D materials community who use h-BN. It may also be used to study other insulators such as diamond with nitrogen-vacancy centers—a popular system for nanoscale sensing.

Explore further: Scientists use particle accelerator to visualize properties of nanoscale electronic materials

More information: "Characterization and manipulation of individual defects in insulating hexagonal boron nitride using scanning tunnelling microscopy." Nature Nanotechnology (2015) DOI: 10.1038/nnano.2015.188

Related Stories

STM of individual grains in CVD-grown graphene

June 24, 2011

Users from Purdue University, working collaboratively with staff in the CNM Electronic & Magnetic Materials & Devices Group, studied CVD-grown graphene on polycrystalline copper foil for the first time at the atomic-scale. ...

Tough foam from tiny sheets

July 29, 2014

Tough, ultralight foam of atom-thick sheets can be made to any size and shape through a chemical process invented at Rice University.

Trapping vortices key to high-current superconductors

July 2, 2015

If we are to see the promised benefits of high-temperature superconductors, such as low-loss motors and generators or maglev trains, we will need superconductors that can carry very large currents.

Recommended for you


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.