Molecular Quantum-dot Cellular Automata (QCA): Beyond Transistors Craig S. Lent University of Notre Dame ND Collaborators : Greg Snider, Peter Kogge, Mike Niemier, Marya Lieberman, Thomas Fehlner, Alex Kandel, Alexei Orlov, Mo Liu,Yuhui Lu Supported by National Science Foundation Center for Nano Science and Technology
Outline of presentation • Introduction and motivation • QCA paradigm • QCA implementations – Metal-dot – Semiconductor-dot – Magnetic – Molecular • Circuit and system architecture • Summary Center for Nano Science and Technology
Goal: Electronics at the single-molecule scale Center for Nano Science and Technology
The Dream of Molecular Transistors Why don’t we keep on shrinking transistors until they are each a single molecule? Center for Nano Science and Technology
Dream molecular transistors V V on off I 1 nm f max =1 THz low conductance high conductance state state Molecular densities: 1nm x 1nm � 10 14 /cm 2 Center for Nano Science and Technology
Transistors at molecular densities V Suppose in each clock cycle a single electron moves from power supply (1V) to ground. Power dissipation (Watts/cm 2 ) 10 14 devices/cm 2 10 13 devices/cm 2 10 12 devices/cm 2 10 11 devices/cm 2 Frequency (Hz) 10 12 16,000,000 1,600,000 160,000 16,000 10 11 1,600,000 160,000 16,000 1,600 10 10 160,000 16,000 1,600 160 10 9 16,000 1600 160 16 10 8 1600 160 16 1.6 10 7 160 16 1.6 0.16 10 6 16 1.6 0.16 0.016 ITRS roadmap: 7nm gate length, 10 9 logic transistors/cm 2 @ 3x10 10 Hz for 2016 Center for Nano Science and Technology
Center for Nano Science and Technology
The Dream of Molecular Transistors Center for Nano Science and Technology
New paradigm: Quantum-dot Cellular Automata Represent information with molecular charge configuration. Zuse’s paradigm � • Binary � � • charge configuration • Current switch Revolutionary, not incremental, approach Beyond transistors – requires rethinking circuits and architectures Use molecules, not as current switches, but as structured charge containers . Center for Nano Science and Technology
Quantum-dot cellular automata Represent binary information by charge configuration of cell. active QCA cell “1” “0” • Dots localize charge • Two mobile charges • Tunneling between dots • Clock signal varies relative energies of “active” and “null” dots “null” Clock need not separately contact each cell. Center for Nano Science and Technology
Quantum-dot cellular automata Neighboring cells tend to align in the same state. “1” “null” Center for Nano Science and Technology
Quantum-dot cellular automata Neighboring cells tend to align in the same state. “1” “1” Center for Nano Science and Technology
Quantum-dot cellular automata Neighboring cells tend to align in the same state. “1” “1” This is the COPY operation. Center for Nano Science and Technology
QCA cell-cell response function Clock: off Clock: on Cell Polarization Cell Polarization Neighbor Polarization Neighbor Polarization Center for Nano Science and Technology
Majority Gate “0” “null” “1” “1” Center for Nano Science and Technology
Majority Gate “0” “1” “1” “1” Center for Nano Science and Technology
Majority Gate “B” A M B C “out” “A” “C” Three input majority gate can function as programmable 2-input AND/OR gate. Center for Nano Science and Technology
QCA single-bit full adder result of SC-HF calculation with site model Hierarchical layout and design are possible. Simple-12 microprocessor (Kogge & Niemier) Center for Nano Science and Technology
Outline of presentation • Introduction and motivation • QCA paradigm • QCA implementations – Metal-dot – Semiconductor-dot – Magnetic – Molecular • Circuit and system architecture • Summary Center for Nano Science and Technology
QCA devices exist Metal-dot QCA implementation Al/AlO x on SiO 2 electrometers 70-300 mK “dot” = metal island Greg Snider, Alexei Orlov, and Gary Bernstein Center for Nano Science and Technology
Metal-dot QCA cells and devices • Majority Gate A M B C Amlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science 284 , pp. 289-291 (1999). Center for Nano Science and Technology
QCA Shift Register Center for Nano Science and Technology
QCA Shift Register D4 D1 G top IN + V V V CLK1 CLK2 IN – V G bot electrometers Center for Nano Science and Technology
Metal-dot QCA devices exist • Single electron analogue of molecular QCA • Gates and circuits: – Wires – Shift registers – Fan-out – Power gain demonstrated – AND, OR, Majority gates • Work underway to raise operating temperatures Center for Nano Science and Technology
Power Gain in QCA Cells • Power gain is crucial for practical devices because some energy is always lost between stages. • Lost energy must be replaced. – Conventional devices – current from power supply – QCA devices – from the clock • Unity power gain means replacing exactly as much energy as is lost to environment. Power gain > 3 has been measured in metal-dot QCA. Center for Nano Science and Technology
GaAs-AlGaAs QCA cell • Dots defined by top gates depleting 2DEG • Direct measurement of cell switching Center for Nano Science and Technology
Silicon P-dot QCA cell • Dots defined by implanted phosphorus • Single-donor creation foreseen • Direct measurement of cell switching Center for Nano Science and Technology
Magnetic QCA • Dots defined by magnetic domains • Room temperature operation Center for Nano Science and Technology
Molecular QCA Metal tunnel junctions “dot” = metal island 70 mK “dot” = redox center Mixed valence compounds room temperature+ Key strategy: use nonbonding orbitals ( π or d) to act as dots. Center for Nano Science and Technology
Experiments on molecular double-dot Thomas Fehlner et al. (Notre Dame chemistry group) Journal of American Chemical Society , 125:15250, 2003 Fe Fe Ru Ru “0” “1” trans -Ru-(dppm) 2 (C ≡ CFc)(NCCH 2 CH 2 NH 2 ) dication Fe group and Ru group act as two unequal quantum dots. Center for Nano Science and Technology
Surface attachment and orientation N molecule 2.4 Α Si-N bonds 106 o Si Si 3.8 Α Si(111) PHENYL GROUPS “TOUCHING” SILICON “struts” Molecule is covalent bonded to Si and oriented vertically by “struts.” Center for Nano Science and Technology
Measurement of molecular bistability layer of molecules Applied field equalizes the energy of the two dots 1.7 0.15 Ru Fe Energy Fe Ru Ru Fe C (oxidized) 0.10 1.6 C (reduced) ∆ C 0.05 1.5 Hg Hg Hg 0.00 Fe Fe Fe 1.4 ∆ C (nF) C (nF) applied Ru Ru Ru -0.05 1.3 potential -0.10 1.2 Si Si Si ac Capacitance -0.15 1.1 ground state switching excited state -0.20 1.0 -0.25 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 voltage V g (V) Fe Fe H When equalized, capacitance peaks. 2 counterion charge configurations on surface Ru Ru Center for Nano Science and Technology
Longer molecular double-dot HOMO Isopotential orbital surface Center for Nano Science and Technology
Double-dot click-clack Hua & 0.1 C Oxidized Fehlner C as prepared 3+ 1.3 H 2 N ∆ C Ph 2 Ph 2 P N P Ru P P 1.2 0.0 Ph 2 Ph 2 d m =2.8 nm ∆ C (nF) C (nF) d 1 =1.8 nm 1.1 Ph 2 Ph 2 P P R u -0.1 P P Ph 2 N Ph 2 1.0 NH 2 0.9 -0.2 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 V Hg (V) Center for Nano Science and Technology
Square 4-dot QCA molecules 0.6 nm Center for Nano Science and Technology
Imaging molecular double-dot Kandel group structure toluene solution Goal: single-molecule imaging on surfaces Molecules are pulse-injected from solution into vacuum onto a clean, crystalline gold [Au(111)] surface. Ru-Ru molecule with no surface binding. Not mixed-valence species. Center for Nano Science and Technology
Ru 2 clustering Some clustering and alignment of molecules occurs automatically during deposition. (50 nm image shown.) We should be able to compare isolated molecules with those in larger clusters. Experimental conditions: 0.5 V, 20 pA, 298 K Center for Nano Science and Technology
Molecular motion Changing tunneling conditions (from 1.0 V, 20 pA to 1.0 V, 100 pA) increases tip/molecule interaction. We observe a change in orientation for one Ru 2 molecule. This suggests the possibility of using the STM tip for controlled manipulation of these molecules on the surface. Experimental conditions: 250 × 180 Å, 1.0 V, 20 (100) pA, 298 K Center for Nano Science and Technology
Imaging charge localization Neutral molecules Mixed-valence molecules Preliminary results for Fe-Fe fabricated by Claude Lapinte (Rennes) Center for Nano Science and Technology
Single-atom quantum dots Center for Nano Science and Technology
Outline of presentation • Introduction and motivation • QCA paradigm • QCA implementations – Metal-dot – Semiconductor-dot – Magnetic – Molecular • Circuit and system architecture • Summary Center for Nano Science and Technology
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