Nanowires

1. Nanowires

2. Course outline

3. Introduction

4. Molecular electronics www.scientificamerican.com

5. Molecular electronics Biological Systems Molecular Electronics Devices Use molecular electronics to study biological systems.

6. 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.

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. Recent research Atomic force microscope image of semi-conductive DNA compound http://www.fujixerox.co.jp/research/eng/category/inbt/m_electronics/index.html

13. 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

14. Self assembly www.scientificamerican.com

15. Molecular electronics Thiol Acetylene linkage Benzene ring

16. 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

17. an atomic relay

18. Electronics course A very short

19. Transistors A device composed of semiconductor material that amplifies a signal or opens or closes a circuit. Invented in 1947 at Bell Labs, transistors have become the key ingredient of all digital circuits, including computers. Today's microprocessors contains tens of millions of microscopic transistors.

20. Transistors Transistors consist of three terminals; the source, the gate, and the drain.

21. Transistors In the n-type transistor, both the source and the drain are negatively-charged and sit on a positively-charged well of p-silicon.

22. Transistors When positive voltage is applied to the gate, electrons in the p-silicon are attracted to the area under the gate forming an electron channel between the source and the drain.

23. Transistors When positive voltage is applied to the drain, the electrons are pulled from the source to the drain. In this state the transistor is on.

24. Transistors If the voltage at the gate is removed, electrons aren't attracted to the area between the source and drain. The pathway is broken and the transistor is turned off.

25. DNA wires

26. 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/

27. 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

28. 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).

29. 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.

30. Operation Schematic image with 2 grains in DNA connected by P-bond. Dark circle->carbon atoms, white circles->oxygen atoms.

31. DNA pairs • P-bond -> tunneling junction. • H-bonds -> capacitor. • The grain itself -> inductive properties.

32. 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.

33. 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)

34. 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)

35. DNA pairs The end of the gate strand with another voltage source (Vg) that acts as gate source.

36. Functionalisation of nanoparticles • DNA may be attached to surface area of nanoparticles to construct desired assemblies. • May provide insight to possible solution to connecting transistors

37. Functionalisation of nanoparticles Mirkin et al.: Nature, 1996, 382, 607

38. Functionalisation of nanoparticles Mirkin et al.: Nature, 1996, 382, 607

39. Functionalisation of nanoparticles 8 nm gold particles attached to a 31 nm gold particle with DNA http://www.chem.nwu.edu/~mkngrp/dnasubgr.html

40. 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.

41. DNA base-pairs

42. DNA conductance • Conductivity in DNA has been controversial • Electron transfer experiments (biochemistry) / possible link to cancer • Transport experiments (physics)

43. 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)

44. 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

45. DNA-based metalised nanowires 10 nm wires: AuPd on DNA

46. Needed • Smaller wires and constructs • Difficult to make wires this scale by conventional means • Find if DNA is a good substrate for metalisation (and for which metals) • Conducting and superconducting wires

47. Which DNA? • λ-DNA: double-stranded, 2 nm width, 16 micron length • Poly-C, Poly-A, etc.: Single-stranded, all same base, 1 nm width • Designed, complementary strands: Self assembly presents possibility for complex structures λ-DNA, uncoated: ~5 nm wires

48. Metalised DNA • Earlier construction of DNA-templated nanowires • Braun 1: 100 nm thick wires, Ag on DNA • Richter 2: 50 nm thick wires, Pd on DNA • Nanotubes, other substrates 1) E. Braun, Y.Eichen, U. Sivan, and G. Ben-Yoseph, Nature (London) 391, 775 (1998). 2) J. Richter et al. Appl. Phys. Lett. 78, 536 (2001)

49. Methods • Suspend DNA across undercut 100 nm trench -or- • Suspend across cuts in thin (60 nm) membrane –variable width carved by focused ion beam • Metalize by sputtering or evaporation • Image with scanning electron microscope • Make electrical measurements

50. Methods Schematic of undercut trench