Molecular electronics www.scientificamerican.com
Molecular electronics Biological Systems Molecular Electronics Devices Use molecular electronics to study biological systems.
Molecular electronics • Incentives • Molecules are nano-scale • Self assembly is achievable • Very low-power operation • Highly uniform devices • Quantum Effect Devices • Building quantum wells using molecules • Electromechanical Devices • Using mechanical switching of atoms or molecules • Electrochemical Devices • Chemical interactions to change shape or orientation • Photoactive Devices • Light frequency changes shape and orientation.
Molecular electronics Definition is a field emerging around the premise that it is possible to build individual molecules that can perform functions identical to those of the key components of today’s microcircuits.
Why molecular electronics? Chip-fabrication specialists will find it economically infeasible to continue scaling down microelectronics. • stray signals on the chip • the need to dissipate the heat from so many closely packed devices • the difficulty of creating the devices in the first place
Molecular electronics, any better? • Modern technologies can only go so far. • Solution (new development) • DNA - It is promising to achieve super-high density memory and high sensitive detection technology. • Cell Computing • Silicon transistors at 120 nm in length will still be 60,000 times larger in area than molecular electronic devices.
Recent research • Recent studies have shown that individual molecules can conduct and switch electric current and store information. • July of 1999 – HP and the University of California at Los Angeles build an electronic switch consisting of a layer of several million molecules of an organic substance called rotaxane. Linking a number of switches - a version of an AND gate is produced.
Recent research June 2002 - Fuji Xerox biotechnology made a prototype transistor of DNA from salmon sperm. • Researchers successfully passed an electric current through the DNA-transistor. • This demonstrates that the chain behaves in a similar fashion to semiconductor. • Super smaller chip in 10 years.
Recent research Atomic force microscope image of semi-conductive DNA compound http://www.fujixerox.co.jp/research/eng/category/inbt/m_electronics/index.html
Self assembly Molecular self-assembly • the autonomous organization of components into patterns or structures without human intervention (Whitesides 2002) • Current Problem: Forming electrical interconnects between molecules
Self assembly www.scientificamerican.com
Molecular electronics Thiol Acetylene linkage Benzene ring
Molecular electronics • Mechanical synthesis • Molecules aligned using a scanning tunneling microscope (STM) • Fabrication done molecule by molecule using STM • Chemical synthesis • Molecules aligned in place by chemical interactions • Self assembly • Parallel fabrication
Well known from biology Forms predictable structure Controllable self assembly through base pair sequences May be selectively processed using restriction enzymes DNA http://www.chemicalgraphics.com/
As the major component in a Single Electron Tunneling (SET) Transistor As tags to connect up nano-circuitry including wires and nanoparticles (taking advantage of DNA selectivity) As basis for a Qubit (for quantum computation) DNA in microelectronics
DNA SET transistor Main strand Main strand Gate strand DNA Single electron transistor Equivalent Electrical Circuit E. Ben-Jacob , Phys. Lett. A 263, 199 (1999).
Assumptions • Chemical bonds(in DNA) can act as tunnel junctions in the coulomb blockade regime, could emit electricity, given a proper coating. • Has the ability to coat a DNA strand with metal in nanometer scale.
Operation Schematic image with 2 grains in DNA connected by P-bond. Dark circle->carbon atoms, white circles->oxygen atoms.
DNA pairs • P-bond -> tunneling junction. • H-bonds -> capacitor. • The grain itself -> inductive properties.
DNA pairs • P bond: Has 2 bonds, 1 bond. • The electron can be shared with 2 oxygen, resembles an electron in well, put it on the lowest level. • When electron enters, it meet the barrier set by energy gap. • But the gap is narrow and small so the electron can walk trough.
DNA pairs • H-bonds: Can be the capacitor. • The proton in the h-bond can screen a net charge density on either side, by movement. • Thus the net charge could be in the side of the h-bond. • The grains: Can be the inductive properties. • Due to the hopping of additional electrons. • But can be ignored (L & Lo is small, consistent to the usual SET)
DNA pairs • Consist of 2 strands (1 main, 1 gate) • Connect the end base of the gate strand with a complimentary strand. • Both strands should be metal-coated, except (a) the grain in the main strand, which connect to the gate strand, the 2 adjacent P-bonds, (b) the connective h-bond. • Connect the main strand with voltage source (V)
DNA pairs The end of the gate strand with another voltage source (Vg) that acts as gate source.
DNA conductance • Double helix – a backbone and base pairs • Building blocks are the base pairs: A, T, C & G • Example: 10 base pairs per turn, distance of 3.4 Angstroms between base pairs. • Arbitrary sequences possible • A challenge for nanotechnology is controlled / reproducible growth. DNA is an example with some success. However, there are many copies in a solution! • 2D and 3D structures with DNA base pairs as a building block have been demonstrated • Lithography? Not yet.
DNA conductance • Conductivity in DNA has been controversial • Electron transfer experiments (biochemistry) / possible link to cancer • Transport experiments (physics)
DNA conductance Semiconducting / Insulating Metallic, No gap ~ 10nA ~ 1nA Current Current Voltage (V) Voltage 20mV Porath et. al, Nature (2000) Fink et. al, Science (1999)
Counter-ions • Is conduction through the base pair or backbone? - Basepair • When DNA is dried, where are the counter ions? • Crystalline / non crystalline? • Counter ions significantly modify the energy levels of the base pairs • Counter-ion species is also important • Resistance increases with the length of the DNA sample (exponential within the context of simple models) Counter-ions
DNA-based metalised nanowires 10 nm wires: AuPd on DNA
Methods Schematic of undercut trench
Set-up Schematic of electrode overlaying wire
Metalised DNA-wires • Longest wire to date: 960 nm (~30 nm thick) • Appearance of multi-strand “Ropes” Variable width cuts in membrane, made by focused ion beam. DNA bridges the cuts.
Metalised DNA-wires Multi-strand “rope,” 3 nm AuPd coating, total thickness: 30-40 nm Length: 960 nm Two wires connected by “rope” visible on surface of membrane, length: 550 nm on right, 670 nm on left
Sequence specific molecular lithography RecA polymerised on DNA (cryo-TEM)
Sequence specific molecular lithography patterning of DNA metallization
Sequence specific molecular lithography RecA nuleoprotein filament localised on aldehyde- derivatized DNA sample after silver deposition AFM sample after gold deposition SEM
Sequence specific molecular lithography optical lithography molecular lithography
Carbon nanotubes The device - which consists of a single-walled carbon nanotube sandwiched between two gold electrodes - operates at extremely fast microwave frequencies. The result is an important step in the effort to develop nanoelectronic components that could be used to replace silicon in a range of electronic applications (S Li et al. 2004 Nano Lett. 4 753). http://physicsweb.org/article/news/8/4/15
Superconductivity in nanotubes • Left red data show insulating like behavior with resistance upturns at the lowest temperatures, blue data show superconducting behavior • Right V-I data for a strongly superconducting sample at various temperatures. Courtesy, A. Bollinger
Buckyball www.osti.gov/accomplishments/ smalley.html