If you think that building an artificial human brain is science fiction, you are probably right - for now. But don't think for a moment that researchers are not working hard on laying the foundations for what is called neuromorphic engineering - a new interdisciplinary discipline that includes nanotechnologies and whose goal is to design artificial neural systems with physical architectures similar to biological nervous systems. One of the key components of any neuromorphic effort is the design of artificial synapses. The human brain contains vastly more synapses than neurons - by a factor of about 10,000 - and therefore it is necessary to develop a nanoscale, low power, synapse-like device if scientists want to scale neuromorphic circuits towards the human brain level. New research now suggests that memristor devices are capable to emulate the biological synapses with properly designed CMOS neuron components.
Researchers have been exploring boron nitride (BN) nanomaterials - from nanotubes to nanosheets and nanoribbons - and found similarities, but also differences to the the properties of the corresponding carbon nanomaterials. Motivated by the outstanding properties of graphene, the boron nitride single layer - a structural analogy of graphene - has been extensively studied both theoretically and experimentally, and has been experimentally realized. However, BN nanomaterials are wide-band-gap semiconductors, and their band structures are rather robust, and difficult to modulate, which is a substantial obstacle for their applications in nanoelectronic devices. Researchers have now shown that hydrogenation might be a simple approach to tune the band structure of graphene-like boron nitride structures.
Continuing miniaturization has moved the semiconductor industry well into the nano realm with leading chip manufacturers on their way to CMOS using 22nm process technology. With transistors the size of tens of nanometers, researchers have begun to explore the interface of biology and electronics by integrating nanoelectronic components and living cells. While researchers have already experimented with integrating living cells into semiconductor materials other research is exploring the opposite way, i.e. integrating nanoelectronics into living cells. Researchers in Spain have demonstrated that silicon chips smaller than cells can be produced, collected, and internalized inside living cells by different techniques (lipofection, phagocytosis or microinjection) and, most significantly, they can be used as intracellular sensors.
All existing transistors are based on junctions - obtained by changing the polarity of silicon from positive to negative. Researchers have now demonstrated a new type of transistor in which there are no junctions and no doping concentration gradients. The key to fabricating a junctionless gated resistor is the formation of a semiconductor layer that is thin and narrow enough to allow for full depletion of carriers when the device is turned off - something that was achieved by fabricating silicon nanowires with a diameter of a few dozens of atomic planes. The electrical current flows in this silicon nanowire, and the flow of current is perfectly controlled by a ring structure that electrically squeezes the silicon wire in the same way that you might stop the flow of water in a hose by squeezing it.
Materials that can produce electricity are at the core of piezoelectric research and the vision of self-powering machines and devices. Nanotechnology researchers are even pursuing nanopiezotronics devices that have the potential of converting biological mechanical energy, acoustic/ultrasonic vibration energy, and biofluid hydraulic energy into electricity, demonstrating a new pathway for self-powering of wireless nanodevices and nanosystems. In addition to miniaturizing piezoelectric devices down to the nanoscale, nanotechnology is also contributing to making next-generation devices more effective. Piezoelectric ceramics for instance generate electrical charge or voltage when they experience stress/strain, and thus are highly efficient at converting mechanical energy into electrical energy. However, ceramics are rigid, which greatly limits the applicability of the energy harvesting. Researchers have now demonstrated that high performance piezoelectric ceramics can be transferred in a scalable process onto rubber or plastic, rendering them flexible without any sacrifice in energy conversion efficiency.
Scientists have great expectations that nanotechnologies will bring them closer to the goal of creating computer systems that can simulate and emulate the brain's abilities for sensation, perception, action, interaction and cognition while rivaling its low power consumption and compact size. DARPA for instance has a program called SyNAPSE that is trying to develop electronic neuromorphic machine technology that scales to biological levels. Started in late 2008 and funded with $4.9 million, the goal of the initial phase of the SyNAPSE project is to 'develop nanometer scale electronic synaptic components capable of adapting the connection strength between two neurons in a manner analogous to that seen in biological systems, as well as, simulate the utility of these synaptic components in core microcircuits that support the overall system architecture.' Independent from this military-inspired research, nanotechnology researchers in France have developed a hybrid nanoparticle-organic transistor that can mimic the main functionalities of a synapse. This organic transistor, based on pentacene and gold nanoparticles and termed NOMFET (Nanoparticle Organic Memory Field-Effect Transistor), has opened the way to new generations of neuro-inspired computers, capable of responding in a manner similar to the nervous system.
The key for most visionary electronic applications will be printability, i.e. that the circuits can be applied to any material, and flexibility, i.e. that they can adhere to any shape or form - even body parts. Imagine an ultrathin film of electronic circuits attached to internal organs like your heart to monitor vital functions. All existing forms of electronics are built on the two-dimensional, planar surfaces of either semiconductor wafers or plates of glass. Mechanically flexible circuits based on organic semiconductors are beginning to emerge into commercial applications, but they can only be wrapped onto the surfaces of cones or cylinders - they cannot conform to spheres or any other type of surface that exhibits non-Gaussian curvature. Applications that demand conformal integration, e.g. structural or personal health monitors, advanced surgical devices, or systems that use ergonomic or bio-inspired layouts, etc., require circuit technologies in curvilinear layouts.
Chip structures already have reached nanoscale dimensions but as they continue to shrink below the 20 nanometer mark, ever more complex challenges arise and scaling appears not to be economically feasible any more. And below 10 nm, the fundamental physical limits of CMOS technology will be reached.
One promising material that could enable the chip industry to move beyond the current CMOS technology is graphene, a monolayer sheet of carbon. Notwithstanding the intense research interest, large scale production of single layer graphene remains a significant challenge. Researchers at Cornell University have now reported a new technique for producing large scale single layer graphene sheets and fabricating transistor arrays with uniform electrical properties directly on the device substrate.