In United States Patent 7,595,260, Harvard College (Cambridge, MA) Hyman Professor of Chemistry Charles M. Lieber with Yi Cui, Xiangfeng Duan and Yu Huang reveal bottom-up assembly of nanoscale electronic and optoelectronic devices including LEDs. The researchers demonstrated the ability to assemble active devices in the absence of multi-billion dollar fabrication lines which is of critical importance to the semiconductor and photonic fields and which they believe "augers well for the immediate and longer-term advances."
Lieber, Cui, Duan and Huang believe that the broad range of nanowire (NW) materials now available and the clearly defined ability to control their electronic properties will make possible nanoscale LEDs that cover the entire visible and near infrared range (e.g., GaN NWs for blue color). Such nanoscale light sources might be useful in creating new types of highly parallel optical sensors and for optical inter-connects in nanoelectronics. Moreover, the assembly of doped NW building blocks clearly has great potential for creating many other types of electronic devices and possibly even lasers.
With its extensive expertise in nanowire technologies, the laboratory of Charles Lieber has developed a novel high-performance memory device. Nonvolatile crossbar switches are memory elements created by crossing semiconductor and metal nanowires. Results have already shown bistable switching between ON and OFF states with ON/OFF ratios comparable to conventional devices (>104) and a well-defined, stable threshold voltage. Bit sizes are smaller than 20 nm by 20 nm, allowing for extremely high memory densities.
Additionally, writes times of less than 100 nanoseconds have been demonstrated. Fully addressable memory arrays have been created exhibiting no cross-talk between individual elements, minimal degradation after thousands of cycles, and long memory retention times. In addition to operation on conventional substrates, the technology has been demonstrated on flexible plastic substrates, possibly allowing for low-cost manufacturing methods to be utilized for the creation of high-density, high-speed, stable and robust memory. The technology is available for licensing from the Harvard Office of Technology Development.
The microelectronics industry's continued success with scaling devices faces a number of physical limitations using current materials and device configurations. As demand surges for portable, high-density, low-cost memory, new technologies become increasingly attractive compared to the billion-dollar manufacturing requirements for conventional silicon. With revenues already in the tens of billions, memory for consumer electronics and PCs is a rapidly growing market demanding stable, robust, high-speed, and high-density technologies
Taking nanomaterials to a new level of structural complexity, Lieber's team has determined how to introduce kinks into arrow-straight nanowires, transforming them into zigzagging two- and three-dimensional structures with correspondingly advanced functions. This work was described in October in a letter in the journal Nature Nanotechnology by scientists led by research assistant Bozhi Tian and Professor Lieber.
Among possible applications, the authors say, the new technology could foster a new nanoscale approach to detecting electrical currents in cells and tissues. Nanowires for cancer detection have also been developed.
Lieber and Tian’s approach involves the controlled introduction of triangular “stereocenters” – essentially, fixed 120-degree joints – into nanowires, structures that have previously been rigidly linear. These stereocenters, analogous to the chemical hubs found in many complex organic molecules, introduce kinks into 1-D nanostructures, transforming them into more complex forms.
The researchers halted growth of the 1-D nanostructures for 15 seconds by removing key gaseous reactants from the chemical brew in which the process was taking place, replacing these reactants after joints had been introduced into the nanostructures. This approach resulted in a 40 percent yield of bent nanowires, which can then be purified to achieve higher yields.
"The stereocenters appear as kinks, and the distance between kinks is completely controlled,” said Tian, a research assistant in Harvard’s Department of Chemistry and Chemical Biology. “Moreover, we demonstrated the generality of our approach through synthesis of 2-D silicon, germanium, and cadmium sulfide nanowire structures.”
The research by Lieber and Tian is the latest in the years-long efforts by scientists to control the composition and structure of nanowires during synthesis. Despite advances in these areas, the ability to control the design and growth of self-assembling nanostructures has been limited. Lieber and Tian’s work takes the formation of 2-D nanostructures a step further by enabling the introduction of electronic devices at the stereocenters.
“An important concept that emerged from these studies is that of introducing functionality at defined nanoscale points for the first time – in other words, nanodevices that can ‘self-label,’ ” Lieber said. “We illustrated this novel capability by the insertion of p–n diodes and field-effect transistors precisely at the stereocenters.”
Such self-labeled structures could open up the possibility of introducing nanoelectronics, photodetectors, or biological sensors into complex nanoscale structures.
Lieber and Tian’s co-authors are Ping Xie and Thomas J. Kempa of Harvard’s Department of Chemistry and Chemical Biology and David C. Bell of Harvard’s Center for Nanoscale Systems. Their work was funded by the National Institutes of Health, the McKnight Foundation, the MITRE Corp., and the National Science Foundation.
“We are very excited about the prospects this research opens up for nanotechnology,” has said Lieber, Mark Hyman Jr. Professor of Chemistry in Harvard’s Faculty of Arts and Sciences. “For example, our nanostructures make possible integration of active devices in nanoelectronic and photonic circuits, as well as totally new approaches for extra- and intracellular biological sensors. This latter area is one where we already have exciting new results, and one we believe can change the way much electrical recording in biology and medicine is carried out.”
In 2002, Professor Charles Lieber and his students have made nanowires that allowed Lieber's team to develop what is likely to be an important scientific tool, a coated wire capable of detecting low levels of a protein that marks the presence or recurrence of prostate cancer.
