1. Controlled Coupling and Occupation of Silicon Atomic Quantum Dots
At room temperature
M. Baseer Haider, Jason L Pitters, Gino A. DiLabio, Lucian Livadaru, Josh Y Mutus, Robert A. Wolkow

2. NINT Scientists
Gino DiLabio
Jason Pitters
NINT Scientist dedicated to commercialization ventures
Stas Dogel
NINT Instrument design Engineer
Mark Salomons
Technician
Martin Cloutier
Postdocs
Baseer Haider
Lucian Livadaru
Radovan Urban
Peter Ryan
Paul Piva
Students
Manuel Smeu, Co-supervised with Hong Guo/McGill
Janik Zikovsky
Shoma Sinha
Cristian Vesa
Marco Taucer

3. Single, small ensembles, and large arrays of Dangling Bonds are wonderful – let’s discuss small groups of Si DBs today

4. Si (100)-H, 2x1
3.84 Å
7.68 Å
2.25 Å

5. STM DB (Dangling Bond) Creation

6. 10 nm Just a demo But interesting in itself
Can for example decorate each point with a molecule
Or with a metal atom

7. Neutral DBs Negative DBs 35x35 nm, 2V, 0.1nA 35x35 nm, 2.2 V, 0.1nA
Low doped n-type Silicon
Neutral DBs
High doped n-type Silicon
Negative DBs
One electron per DB
Two electrons per DB e-
tip CB VB EF CB VB EF
tip
1 e-
= neutral
2 e-
= 1 neg
charge

8. e
Field regulation of single-molecule conductivity by a charged surface atom
Paul G. Piva, Gino A. DiLabio, Jason L. Pitters, Janik Zikovsky, Moh’d Rezeq, Stanislav Dogel, Werner A. Hofer & Robert A. Wolkow
Nature 435, (2005)
Lopinski, Wayner, and Wolkow, Nature 406, 48 (2000)

9. Pitters, J. L. ; Piva, P. G. ; Tong, X. ; Wolkow, R. A. , Nano Lett
Pitters, J. L. & Wolkow, R. A., J. Am. Chem. Soc. 127, 48–-49 (2005).
Dangling bond capping => Charge elimination and therefore Field elimination
Also single molecule sensing

10. All that was an aside
Showing Dangling Bond (DB) as a point charge
Returning now to interactions among DBs

11. 1e- Not 2e- DB1 DB2 DB1 DB2 NEW DB3 DB distance is 8.2Å
10x10nm, 2V, 0.2nA

12. Coulombic repulsion limits filling of DB’s
Coupled DB’s are “self-biased”
CBM
VBM
Vel/2
R12 
R d E0
(a)
(b)
Unfavourable  
E0  t U/2

13. Rare tunneling
Very High
Tunnel Rate

14. Distance dependent coupling
III II I
23.2 Å
15.6 Å
11.5 Å
2e-
1e-
Bandwidth of amplifier is ~5kHz.

15. Charging state probabilities for a 4DB cell
6x6 nm, 2V, 0.08 nA
Room temp
Low temp

17. 6x6 nm, 2V, 0.08 nA

18. Quantum-Dot Cellular Automata
High Density
Low power consumption
Patterned Q-dot clusters using
e-beam lithography prepared samples
Typically ~10 nm clusters spaced by tens of nm
Operated at mK temperatures and local electrostatic
tuning in order to achieve the appropriate filling.
wire
majority gate
fanout
inverter
Lent, C. S.,Tougaw, P. D., Porod, W. & Bernstein, G. H. Nanotechnology 4, 49-57, (1993).

19. Tilting the potential
CBM
VBM
Vel/2
R12 
R d E0
E0  t U/2
(a)
(b)
(c)
Unfavourable
Vpert  
This creates a situation where the forward and reverse tunnel rates are not equivalent

20. An H-terminated Silicon surface
~ 3 nm
A 3rd DB acts as an electrostatic perturbation – it shifts charge in pre-existing coupled pair
This is single electron state control
The Si DBs are atomic quantum dots
The grouping is an artificial molecule
A 2nd dangling bond is about to be created
Watch:
Symmetry breaking will occur when a 3rd DB added
Watch:
This DB will brighten when a nearby DB added
One H atom removed with STM tip
the pre-existing and the new DB are lighter in appearance – evidence of rejection of charge
The resulting silicon “dangling bond” is negatively charged with one electron
The empty state allows electron tunneling between the two atoms!
Resulting in local energy level shifts, manifest as a dark feature in STM

21. These entities are small.

23. play fun movie QCA/computer

24. though enmeshed in silicon lattice, single Si atoms can stand out to act as quantum dots (remarkably like a dopant)
Ultimate small dot –> wide level spacing -> Room Temperature
Qdots are identical
multiple dots can be tunnel coupled
electron filling is geometry controlled - “self-biased”
can electrostatically control electronic configuration
immune to stray charge (beyond ~3 nm)
QCA cells have been electrostatically set in one binary state – not yet dynamically
2 coupled dots are candidate charged qubit
PRL 102, (2009)

27. the end