ScienceMWNT2005 - REPORTS period oscillations in both...

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period oscillations in both devices, although the period D V g is approximately doubled in the 250- versus 500-nm QD. Because D V g is inversely proportional to the gate capacitance, D V g 0 e / C g , and QD size, this comparison shows that the true size of the confined QD can be controlled in a predictable manner in these modulation-doped nanowires ( 22 ). The potential of our approach for encoding coupled quantum structures has been explored in modulation-doped silicon nanowires that have structures of the form n þ - n 1 - n QD þ - n 2 - n QD þ - n 1 - n þ ,wh e r e n 1 are fixed-width tunnel bar- riers that weakly couple the structure to source and drain electrodes, and n 2 is a variable-width barrier that couples the two QDs (Fig. 4D, left panel). The I V g data recorded from rep- resentative nanowire devices with three dif- ferent n 2 barrier widths coupling the QDs (Fig. 4D, right panel) demonstrate several key points. First, the device with the largest barrier exhibits a single Coulomb oscillation period that yields a capacitance consistent with the size of each individual QD deter- mined from SGM measurements. This result shows qualitatively that the two QDs are weakly coupled, and moreover, have sizes that are similar. Second, the data from the device with an intermediate-width n 2 barrier exhibits a splitting of each of the Coulomb oscillation peaks into doublets, which is the signature of enhanced tunneling conductance between the QDs ( 23 , 24 ). This observation agrees with previous studies ( 23 , 25 , 26 )where coupled dots were defined by lithographically patterned gate electrodes. Last, as the barrier width is reduced further, a single Coulomb oscillation period is again observed, although the capacitance shows that the effective QD size is twice that of the individual n QD þ regions; that is, the structures are fully delocalized. These studies demonstrate the ability to synthesize coupled QDs within nanowires, where the interaction between quantum struc- tures is defined by synthesis not lithography. More generally, this work demonstrates the potential of encoding functional information into nanostructures during synthesis, which we believe will open up opportunities for con- ventional and quantum electronic devices and circuits in the future. Reference and Notes 1. P. L. McEuen, M. S. Fuhrer, H. Park, IEEE Trans. Nanotechnology 1 , 78 (2002). 2. H. Dai, Acc. Chem. Res. 35 , 1035 (2002). 3. C. M. Lieber, Mater. Res. Soc. Bull. 28 , 486 (2003). 4. L. Samuelson et al. , Phys. E 25 , 313 (2004). 5. T. Mokari, E. Rothenberg, I. Popov, R. Costi, U. Banin, Science 304 , 1787 (2004). 6 .M .S
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ScienceMWNT2005 - REPORTS period oscillations in both...

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