Hybrid Devices from Nanowire Assemblies smal wavelength sources, and demonstrates an important step to wards integrated nanoscale photonic circuits. Although lith ography sets the integration scale of these multicolor arrays, it should be possible to create much denser nanoLED arrays via the controlled growth of modulated Nw superlat- n-GaN ice structures, o and/or the selective assembly of different semiconductor materials 22 LED In addition, we have assembled optoelectronic circuits consisting of integrated crossed NW LEd and FET ele e 4 a). in a two-by-one array, one v<0V≤0 Gan NW forms a p-n diode with the SiNW and a second GaN NW functions as a local gate as described previously. S) Measurements of current and emission intensity versus gate oltage(Figure 4b)show that the current decreases rapidly 1500 with increasing voltage, as expected for a depletion mode FET, and that the intensity of emitted light also decreases 8 1000 with increasing gate voltage. When the gate voltage is in- creased from 0 to +3 V, the current is reduced from c2200 nA to an off state, where the supply voltage is -6V. Advantages of this integrated approach include switching with much smaller changes in voltage(0-3 versus 0-6 V) and the potential for much more rapid switching. The ability to use the nanoscale FET to reversibly switch the nanoLED d off is shown clearly in We have also investigated the potential of coupling the bottom-up assembly of nanophotonic devices described above together with top-down fabricated silicon structures, since this could provide a new approach for introducing effi- It photonic capabilities into integrated silicon electronics. Ve implemented this hybrid top-down/bottom-up approach by using lithography to pattern p-type silicon wires on the surface of a silicon-on-insulator (Son) substrate, and ther assembling n-type emissive NWs on top of silicon structures 1oFWWNNMTW to form arrays consisting of p-n junctions at cross points (Figure 5a). Conceptually, this hybrid structure(Figure 5b) Time(s) is virtually the same as the crossed Nw structures described above and should produce EL in forward bias. Notably, I-v LED-FET array is obtained by crossing two n-GaN NWs with one p-si data recorded for a hybrid p-n diode formed between the NW. The p-Si Nw is grounded and the first n-GaN NW is biased nega- p-Si and an n-Cds Nw show clear current rectification(Fig- tively, thus forming a forward-biased p-n diode that functions as an ure 5c)and sharp EL spectrum peaked at 510 nm(Fig- LED. The second n-GaN NW is positively biased, and forms a reverse. ure 5d), which is consistent with CdS band-edge emission. biased p-n diode with the grounded p-Si NW and blocks current To explore the reproducibility of this new hybrid approach through the second GaN Nw. In this way, the positive-biased GaN ve have also characterized arrays. For example, a 1x7 NW can function as a local gate to modulate the current flow through crossed array consisting of a single CdS NW over seven fab. the silicon NW and forms a nanofET to modulate the current flow through the nanoled and emission intensity of the nanoLED Inset: a ricated p-Si wires(Figure 5e)exhibits well-defined emission diagram of the equivalent circuit; b)plots of current and emission from each of the cross points in the array(Figure 5f). Simi- intensity of the nanoled as a function of voltage applied to the Nw lar results were obtained for two-dimensional arrays, and gate at a fixed bias of -6 V Inset: SEM image of a representative demonstrate clearly that bottom-up assembly has the poten- device( scale bar=3 um); c)EL intensity versus time relationship ial to introduce photonic function into integrated silicon when a voltage applied to a NW gate is switched between 0 and microelectronics. +4 V for a fixed bias of -6 V The localized el from crossed Nw nanoLEDs and hybrid LEDs can result in near-field power densities greater the same spectra(solid lines, Figure 6a, b) as those obtained than 100 Wcm", which is sufficient to excite molecular and using much larger conventional excitation source(dashed nanoparticle chromophores. To explore this important possi- lines, Figure 6a, b: see Experimental Section). These results bility we use a Cds-based nanoLEd to excite and record demonstrate that nanoLEDs could function as excitation the emission spectra from CdSe quantum dots(QDs) and sources for integrated chemical and biological analysis. propidium iodide (a fluorescent nucleic acid stain; In summary, we have demonstrated a broad range of Figure 6). Notably, the emission of CdSe QDs and propidi- multicolor and multifunction nanophotonic devices um iodide obtained by nanoLED excitation show essentially small circuits assembled from semiconductor NW building l2005,1,No.1 www.small-journalcom 2005 Wiley-VCH Verlag GmbH Co KGaA, D-69451 Weinheim 45wavelength sources, and demonstrates an important step to￾wards integrated nanoscale photonic circuits. Although lith￾ography sets the integration scale of these multicolor arrays, it should be possible to create much denser nanoLED arrays via the controlled growth of modulated NW superlat￾tice structures,[10] and/or the selective assembly of different semiconductor materials.[22] In addition, we have assembled optoelectronic circuits consisting of integrated crossed NW LED and FET ele￾ments (Figure 4 a). Specifically, in a two-by-one array, one GaN NW forms a p–n diode with the SiNW and a second GaN NW functions as a local gate as described previously.[8] Measurements of current and emission intensity versus gate voltage (Figure 4 b) show that the current decreases rapidly with increasing voltage, as expected for a depletion mode FET, and that the intensity of emitted light also decreases with increasing gate voltage. When the gate voltage is in￾creased from 0 to +3V, the current is reduced from 2200 nA to an off state, where the supply voltage is
6 V. Advantages of this integrated approach include switching with much smaller changes in voltage (0–3versus 0–6 V) and the potential for much more rapid switching. The ability to use the nanoscale FET to reversibly switch the nanoLED on and off is shown clearly in Figure 4 c. We have also investigated the potential of coupling the bottom-up assembly of nanophotonic devices described above together with top-down fabricated silicon structures, since this could provide a new approach for introducing effi￾cient photonic capabilities into integrated silicon electronics. We implemented this hybrid top-down/bottom-up approach by using lithography to pattern p-type silicon wires on the surface of a silicon-on-insulator (SOI) substrate, and then assembling n-type emissive NWs on top of silicon structures to form arrays consisting of p–n junctions at cross points (Figure 5 a). Conceptually, this hybrid structure (Figure 5 b) is virtually the same as the crossed NW structures described above and should produce EL in forward bias. Notably, I–V data recorded for a hybrid p–n diode formed between the p-Si and an n-CdS NW show clear current rectification (Fig￾ure 5 c) and sharp EL spectrum peaked at 510 nm (Fig￾ure 5 d), which is consistent with CdS band-edge emission. To explore the reproducibility of this new hybrid approach we have also characterized arrays. For example, a 1 E 7 crossed array consisting of a single CdS NW over seven fab￾ricated p-Si wires (Figure 5 e) exhibits well-defined emission from each of the cross points in the array (Figure 5 f). Simi￾lar results were obtained for two-dimensional arrays, and demonstrate clearly that bottom-up assembly has the poten￾tial to introduce photonic function into integrated silicon microelectronics. The localized EL from crossed NW nanoLEDs and hybrid LEDs can result in near-field power densities greater than 100 W cm
2 , which is sufficient to excite molecular and nanoparticle chromophores. To explore this important possi￾bility we use a CdS-based nanoLED to excite and record the emission spectra from CdSe quantum dots (QDs) and propidium iodide (a fluorescent nucleic acid stain; Figure 6). Notably, the emission of CdSe QDs and propidi￾um iodide obtained by nanoLED excitation show essentially the same spectra (solid lines, Figure 6 a,b) as those obtained using much larger conventional excitation source (dashed lines, Figure 6 a,b; see Experimental Section). These results demonstrate that nanoLEDs could function as excitation sources for integrated chemical and biological analysis. In summary, we have demonstrated a broad range of multicolor and multifunction nanophotonic devices and small circuits assembled from semiconductor NW building Figure 4. a) Schematic of an integrated crossed-NW FET and LED. The LED–FET array is obtained by crossing two n-GaN NWs with one p-Si NW. The p-Si NW is grounded and the first n-GaN NW is biased nega￾tively, thus forming a forward-biased p–n diode that functions as an LED. The second n-GaN NW is positively biased, and forms a reverse￾biased p–n diode with the grounded p-Si NW and blocks current through the second GaN NW. In this way, the positive-biased GaN NW can function as a local gate to modulate the current flow through the silicon NW and forms a nanoFET to modulate the current flow through the nanoLED and emission intensity of the nanoLED. Inset: a diagram of the equivalent circuit; b) plots of current and emission intensity of the nanoLED as a function of voltage applied to the NW gate at a fixed bias of
6 V. Inset: SEM image of a representative device (scale bar=3 mm) ; c) EL intensity versus time relationship when a voltage applied to a NW gate is switched between 0 and +4 V for a fixed bias of
6 V. small 2005, 1, No. 1 www.small-journal.com < 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim 145 Hybrid Devices from Nanowire Assemblies