New path to flex and stretch electronics

December 13, 2011

Imprinting electronic circuitry on backplanes that are both flexible and stretchable promises to revolutionize a number of industries and make "smart devices" nearly ubiquitous. Among the applications that have been envisioned are electronic pads that could be folded away like paper, coatings that could monitor surfaces for cracks and other structural failures, medical bandages that could treat infections and food packaging that could detect spoilage. From solar cells to pacemakers to clothing, the list of smart applications for so-called "plastic electronics" is both flexible and stretchable. First, however, suitable backplanes must be mass-produced in a cost-effective way.

Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a promising new inexpensive technique for fabricating large-scale flexible and stretchable backplanes using semiconductor-enriched carbon nanotube solutions that yield networks of thin film transistors with superb electrical properties, including a charge carrier mobility that is dramatically higher than that of organic counterparts. To demonstrate the utility of their carbon nanotube backplanes, the researchers constructed an artificial electronic skin (e-skin) capable of detecting and responding to touch.

"With our solution-based processing technology, we have produced mechanically flexible and stretchable active-matrix backplanes, based on fully passivated and highly uniform arrays of thin film transistors made from single walled carbon nanotubes that evenly cover areas of approximately 56 square centimeters," says Ali Javey, a faculty scientist in Berkeley Lab's Materials Sciences Division and a professor of electrical engineering and computer science at the University of California (UC) Berkeley. "This technology, in combination with inkjet printing of metal contacts, should provide lithography-free fabrication of low-cost flexible and stretchable electronics in the future."

Javey is the corresponding author of a paper in the journal NanoLetters that describes this work titled "Carbon Nanotube Active-Matrix Backplanes for Conformal Electronics and Sensors." Co-authoring this paper were Toshitake Takahashi, Kuniharu Takei, Andrew Gillies and Ronald Fearing.

With the demand for plastic electronics so high, research and development in this area has been intense over the past decade. Single walled carbon nanotubes (SWNTs) have emerged as one of the top contending semiconductor materials for plastic electronics, primarily because they feature high mobility for electrons - a measure of how fast a semiconductor conducts electricity. However, SWNTs can take the form of either a semiconductor or a metal and a typical SWNT solution consists of two-thirds semiconducting and one-third metallic tubes. This mix yields nanotube networks that exhibit low on/off current ratios, which poses a major problem for electronic applications as lead author of the NanoLetters paper Takahashi explains.

"An on/off current ratio as high as possible is essential for reducing the interruption from pixels in an off-state," he says. "For example, with our e-skin device, when we are pressure mapping, we want to get the signal only from the on-state pixel on which pressure is applied. In other words, we want to minimize the current as small as possible from the other pixels which are supposed to be turned off. For this we need a high on/off current ratio."

To make their backplanes, Javey, Takahashi and their co-authors used a SWNT solution enriched to be 99-percent semiconductor tubes. This highly purified solution provided the researchers with a high on/off ratio (approximately 100) for their backplanes. Working with a thin substrate of polymide, a high-strength polymer with superior flexibility, they laser-cut a honeycomb pattern of hexagonal holes that made the substrate stretchable as well. The holes were cut with a fixed pitch of 3.3 millimeters and a varied hole-side length that ranged from 1.0 to 1.85 millimeters.

"The degree to which the substrate could be stretched increased from 0 to 60-percent as the side length of the hexagonal holes increased to 1.85 mm," Takahashi says. "In the future, the degrees of stretchability and directionality should be tunable by either changing the hole size or optimizing the mesh design."

Backplanes were completed with the deposition on the substrates of layers of silicon and aluminum oxides followed by the semiconductor-enriched SWNTs. The resulting SWNT thin film transistor backplanes were used to create e-skin for spatial pressure mapping. The e-skin consisted of an array of 96 sensor pixels, measuring 24 square centimeters in area, with each pixel being actively controlled by a single thin film transistor. To demonstrate pressure mapping, an L-shaped weight was placed on top of the e-skin sensor array with the normal pressure of approximately 15 kilo Pascals (313 pounds per square foot).

"In the linear operation regime, the measured sensor sensitivity reflected a threefold improvement compared with previous nanowire-based e-skin sensors reported last year by our group," Takahashi says. "This improved sensitivity was a result of the improved device performance of the SWNT backplanes. In the future we should be able to expand our backplane technology by adding various sensor and/or other active device components to enable multifunctional artificial skins. In addition, the SWNT backplane could be used for flexible displays."
-end-
This research was supported in part by the DOE Office of Science and in part by the National Science Foundation.Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov.

DOE/Lawrence Berkeley National Laboratory

Related Semiconductor Articles from Brightsurf:

Blue phosphorus: How a semiconductor becomes a metal
Blue phosphorus, an atomically thin synthetic semiconductor, becomes metallic as soon as it is converted into a double layer.

A new method to measure optical absorption in semiconductor crystals
Tohoku University researchers have revealed more details about omnidirectional photoluminescence (ODPL) spectroscopy - a method for probing semiconducting crystals with light to detect defects and impurities.

Medical robotic hand? Rubbery semiconductor makes it possible
A medical robotic hand could allow doctors to more accurately diagnose and treat people from halfway around the world, but currently available technologies aren't good enough to match the in-person experience.

Laser allows solid-state refrigeration of a semiconductor material
A team from the University of Washington used an infrared laser to cool a solid semiconductor by at least 20 degrees C, or 36 F, below room temperature, as they report in a paper published June 23 in Nature Communications.

Scientists create smallest semiconductor laser
An international team of researchers announced the development of the world's most compact semiconductor laser that works in the visible range at room temperature.

Clemson researcher's novel MOF is potential next-gen semiconductor
Clemson professor Sourav Saha demonstrated a novel double-helical metal organic framework architecture in a partially oxidized form that conducts electricity, potentially making it a next-generation semiconductor.

A gold butterfly can make its own semiconductor skin
A nanoscale gold butterfly provides a more precise route for growing/synthesizing nanosized semiconductors that can be used in nano-lasers and other applications.

Scientists pioneer new generation of semiconductor neutron detector
In a new study, scientists have developed a new type of semiconductor neutron detector that boosts detection rates by reducing the number of steps involved in neutron capture and transduction.

Scientists see defects in potential new semiconductor
A research team has reported seeing, for the first time, atomic scale defects that dictate the properties of a new and powerful semiconductor.

Bending an organic semiconductor can boost electrical flow
Slightly bending semiconductors made of organic materials can roughly double the speed of electricity flowing through them and could benefit next-generation electronics such as sensors and solar cells, according to Rutgers-led research.

Read More: Semiconductor News and Semiconductor Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.