复旦大学：《纳米线材料和功能器件》课外阅读内容_纳米光子、等离子材料与器件 2_Lieber_Small05_NW for multicolor nanophotonics

papers C. M. Lieber et al Nanowires for Integrated Multicolor Nanophotonics*r s Nanophoto Yu Huang Xiangfeng Duan, and Charles M. Lieber Nanoscale light-emitting diodes(nano LEDs)with colors spanning from the ultraviolet to near-infrared region of the electromagnetic spectrum Keywords: were prepared using a solution-based approach in which emissive electroluminescence sive hole-doped silicon nanowires in a crossed nanowire architecture electron-doped semiconductor nanowires were assembled with none optoelectronics Single- and multicolor nanoled devices and arrays were made with photonics colors specified in a predictable way by the bandgaps of the Ill-v and II-VI nanowire building blocks. The approach was extended to combine nanoscale electronic and photonic devices into integrated structures where a nanoscale transistor was used to switch the nanoled on and off. In addition, this approach was generalized to hybrid devices consist ing of nanowire emitters assembled on lithographically patterned planar silicon structures, which could provide a route for integrating photonic devices with conventional silicon microelectronics. Lastly, nano LEDs were used to optically excite emissive molecules and nanoclusters, and hence could enable a range of integrated sensor/detection "chips"with multiplexed analysis capabilities. Bottom-up assembly of nanoscale building blocks into in- NW logic gates, &9 and in nanophotonics with the assembly creasingly complex structures offers the potential to produce of, for example, individual light-emitting diodes 5, oand devices with novel function since it is possible to combine laser diodes l1 a general way materials with distinct chemical composition, The assembly of chemically distinct nanoscale building structure, size, and morphology, in contrast to planar device blocks that would otherwise be structurally and/or chemical fabricationI-31 Semiconductor nanowires (NWs)o and ly incompatible in a sequential growth process typical of carbon nanotubes I are especially attractive building blocks planar fabrication has received considerably less attention. for assembling active and integrated nanosystems since the However, this capability of the bottom-up approach should individual nanostructures can function as both device ele- allow for assembly of nanostructures with function not read- ments and interconnects. This concept has been demonstrat- ily obtained by other methods and open new opportunities ed in nanoelectronics with the assembly of a variety of devi- For example, planar silicon, which serves as the foundation es, such as field-effect transistors(FETs)4-7 and integrated for the microelectronics industry, is poorly suited to many photonic applications since it has a poor efficiency for light emission. 2 Here we demonstrate the assembly of a wide Department of Chemistry and Chemical Biology, Division of Engi- nge of efficient direct-gap III-V and II-VI NWs with sili I Dr Y Huang, Dr X. Duan, Prof C M. Lieber con NWs(SiNWs) and planar silicon structures to produce neering and Applied Science multicolor, electrically driven nanophotonic and integrated Harvard University Cambridge, Massachusetts 02138(USA) nanoelectronic-photonic systems. The flexibility of our ap- E-mail: cml@cmliris. harvard. edu proach suggests potential for applications in integrated I We acknowledge discussions with H. Park. This work was sup- sensor/detection systems, information storage, and other ported by the Air Force Office of Scientific Research and Defense areas of photonics. Advanced Research Projects Agency. We thank Andrew Greytak ur approach to nanoscale photonic devices is based for a gift of CdSe quantum dots upon sequential deposition of p-type and n-type NW mate 142 O 2005 Wiley-VCH GmbH &Co. KGaA, D-69451 Weinheim Dol:10,1002/sm2o400030

Nanophotonic assemblies Nanowires for Integrated Multicolor Nanophotonics** Yu Huang, Xiangfeng Duan, and Charles M. Lieber* Nanoscale light-emitting diodes (nanoLEDs) with colors spanning from the ultraviolet to near-infrared region of the electromagnetic spectrum were prepared using a solution-based approach in which emissive electron-doped semiconductor nanowires were assembled with nonemis￾sive hole-doped silicon nanowires in a crossed nanowire architecture. Single- and multicolor nanoLED devices and arrays were made with colors specified in a predictable way by the bandgaps of the III–V and II–VI nanowire building blocks. The approach was extended to combine nanoscale electronic and photonic devices into integrated structures, where a nanoscale transistor was used to switch the nanoLED on and off. In addition, this approach was generalized to hybrid devices consist￾ing of nanowire emitters assembled on lithographically patterned planar silicon structures, which could provide a route for integrating photonic devices with conventional silicon microelectronics. Lastly, nanoLEDs were used to optically excite emissive molecules and nanoclusters, and hence could enable a range of integrated sensor/detection “chips” with multiplexed analysis capabilities. Keywords : · electroluminescence · LEDs · nanowires · optoelectronics · photonics Bottom-up assembly of nanoscale building blocks into in￾creasingly complex structures offers the potential to produce devices with novel function since it is possible to combine in a general way materials with distinct chemical composition, structure, size, and morphology, in contrast to planar device fabrication.[1–3] Semiconductor nanowires (NWs)[1] and carbon nanotubes[3] are especially attractive building blocks for assembling active and integrated nanosystems since the individual nanostructures can function as both device ele￾ments and interconnects. This concept has been demonstrat￾ed in nanoelectronics with the assembly of a variety of devi￾ces, such as field-effect transistors (FETs)[4–7] and integrated NW logic gates,[8, 9] and in nanophotonics with the assembly of, for example, individual light-emitting diodes[5, 10] and laser diodes.[11] The assembly of chemically distinct nanoscale building blocks that would otherwise be structurally and/or chemical￾ly incompatible in a sequential growth process typical of planar fabrication has received considerably less attention. However, this capability of the bottom-up approach should allow for assembly of nanostructures with function not read￾ily obtained by other methods and open new opportunities. For example, planar silicon, which serves as the foundation for the microelectronics industry, is poorly suited to many photonic applications since it has a poor efficiency for light emission.[12] Here we demonstrate the assembly of a wide range of efficient direct-gap III–V and II–VI NWs with sili￾con NWs (SiNWs) and planar silicon structures to produce multicolor, electrically driven nanophotonic and integrated nanoelectronic–photonic systems. The flexibility of our ap￾proach suggests potential for applications in integrated sensor/detection systems, information storage, and other areas of photonics. Our approach to nanoscale photonic devices is based upon sequential deposition of p-type and n-type NW mate￾rials into a crossed NW architecture using directed fluidic assembly[13] (Figure 1 a), where the cross points are electri- [*] Dr. Y. Huang, Dr. X. Duan, Prof. C. M. Lieber Department of Chemistry and ChemicalBiology, Division of Engi￾neering and Applied Sciences Harvard University Cambridge, Massachusetts 02138 (USA) E-mail: cml@cmliris.harvard.edu [**] We acknowledge discussions with H. Park. This work was sup￾ported by the Air Force Office of Scientific Research and Defense Advanced Research Projects Agency. We thank Andrew Greytak for a gift of CdSe quantum dots. Supporting information for this article is available on the WWW under http://www.small-journal.com or from the author. 142 < 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim DOI: 10.1002/smll.200400030 small 2005, 1, No. 1 full papers C. M. Lieber et al

Hybrid Devices from Nanowire Assemblies sma rect-gap SiNW is a passive optical component and used for its well-defined electronic properties. Important features of the crossed Nw nanoLED concept include: a) the emitted colors are limited only by the available direct-gap Nw embly building blocks; b) the active device area is of nanometer dimensions, thus making these point light sources; c)the ubstrate and multicolor arrays and integration of photonic and nano lectronic elements, and d) the nanophotonic devices can be assembled on both rigid and flexible substrates. NW material Ea (ev The Nw building blocks used in these studies, including GaN direct bandgap III-V(e.g, GaN and InP)and II-VI(e.g, CdS and CdSe) materials, were prepared as single crystals y metal-nanocluster-catalyzed growth 1, I5) The bulk band cdsosSeos aps of these semiconductors enable light emission from ul traviolet(UV, Gan) to near infrared (NiR, InP)as con- 1.70 firmed for individual NWs using photoluminescence meas urements. 16, 17 Electrical transport measurements made on 1.14 individual NWs in a FET geometry showed that the GaN, Cds, CdSSe, CdSe, and InP NWs are all n-type with room- temperature electron mobilities (100-5000 cm'v-ls) 7. ls Figure 1. a) Schematic of the assembly of crossed NW heterojunc. that are comparable to bulk materials 9 Previous studies tions. First, a parallel array of A NWs(orange lines) is aligned on the have demonstrated that boron-doped p-type SiN Ws exhibit substrate using a fluidic assembly method and then a secon carrier mobilities comparable to or better than bulk materi lel array of B NWs (green lines) is deposited orthogonally to al. l6b, 8 b)left schematics showing a nanoLED struc ture for a p-siNW and a direct bandgap n-type Nw and Current-voltage (I-V) data for a typical p-si/n-GaN nd diagram. Right: the table lists bandgaps of crossed NW junction(Figure 2 a) shows well-defined current 300 K used in this stud rectification, as expected for a p-n diode with a turn-on oltage of approximately 1 V. These data are consistent with previous studies of n-GaN/p-si diodes used as nanoelectron cally addressable In the crossed Nw p-n structure, the n- ic logic gates. The initial turn-on voltage, which appre type NW is chosen to be a direct bandgap semiconductor mates to the bandgap of Si, corresponds to injection of elec with efficient light emission and the p-type material is sili- trons into the Sinw from the n-Gan(see Supporting Infor- con, which has an indirect bandgap and inefficient light mation). Notably, applying a forward bias to the p-n junc emission(Figure 1 b). The crossed NW heterostructures can tion of greater than bandgap of GaN yields strong electrolu be described qualitatively by a staggered type-II band dia- minescence(EL)at room temperature(Figure 2b). The EL gram(Figure 1b, and Supporting Information), 4I and will spectrum shows a peak maximum at 365 nm consistent emit light characteristic of the n-type NW element when the with the gan band-edge emission, and demonstrates that applied forward bias voltage exceeds the bandgap; the indi- crossed nw devices can function as UV nanoLEDs. Studi of over 30 n-GaN/p-Si nanoLEDs yielded similar I-v and EL results. Data plotted for four representative devices ( Figure 2c)show several important points. First, light emis- Editorial Advisory Board Member sion is detected when the bias voltage exceeds 3.5 V. This Charles M. Lieber is the Mark Hyman threshold is consistent with the 3.36 eV bandgap of GaN, Professor of Chemistry and member of which when exceeded, leads to radiative recombination of the Division of Engineering and Applied electron/hole pairs in GaN. Second, the emission intensity iences at Harvard University. He has increases rapidly with voltage and exhibits a nearly linear dependence on current above the threshold. The estimated range of nanoscale materials, the char- acterization of the unique physical prop- quantum efficiency(electron to photon) is approximately erties of these materials and the devel. 0.1%. This efficiency is less than commercial LEDs, al- opment of methods of hierarchical as- though it could be improved by passivating surface traps sembly of nanoscale wires, together with and using a larger bandgap p-type Nw for hole injection Third, stable (1 h)UV output is observed these materials in nanoelectronics, bio- temperature at drive currents of x2 HA. Lastly, the near- gical and chemical sensing, and nanophotonics. He is a member of field optical power densities, which are estimated 50nm Sciences, and fellow of the american Physical Society. He has pub from a typical UV nanoLED, are the order of shed more than 230 peer-reviewed papers and is the principle in- 100 Wcm.Notably, this value exceeds that needed in ventor on more than 20 patents. He also founded the nanotechnolo. number of optical applications, such as lithography and gy company, Nano Sys, Inc. spectroscop l2005,1,No.1 www.small-journalcom o 2005 Wiley-VCH Verlag gmbH Co KGaA, D-69451 Weinheim 143

cally addressable. In the crossed NW p–n structure, the n￾type NW is chosen to be a direct bandgap semiconductor with efficient light emission and the p-type material is sili￾con, which has an indirect bandgap and inefficient light emission (Figure 1 b). The crossed NW heterostructures can be described qualitatively by a staggered type-II band dia￾gram (Figure 1 b, and Supporting Information),[14] and will emit light characteristic of the n-type NW element when the applied forward bias voltage exceeds the bandgap; the indi￾rect-gap SiNW is a passive optical component and used for its well-defined electronic properties. Important features of the crossed NW nanoLED concept include: a) the emitted colors are limited only by the available direct-gap NW building blocks; b) the active device area is of nanometer dimensions, thus making these point light sources; c) the crossed NW architecture enables the formation of single￾and multicolor arrays and integration of photonic and nano￾electronic elements, and d) the nanophotonic devices can be assembled on both rigid and flexible substrates. The NW building blocks used in these studies, including direct bandgap III–V (e.g., GaN and InP) and II–VI (e.g., CdS and CdSe) materials, were prepared as single crystals by metal-nanocluster-catalyzed growth.[1, 15] The bulk bandg￾aps of these semiconductors enable light emission from ul￾traviolet (UV, GaN) to near infrared (NIR, InP) as con￾firmed for individual NWs using photoluminescence meas￾urements.[16, 17] Electrical transport measurements made on individual NWs in a FET geometry showed that the GaN, CdS, CdSSe, CdSe, and InP NWs are all n-type with room￾temperature electron mobilities (100–5000 cm2V
1 s)[7, 18] that are comparable to bulk materials.[19] Previous studies have demonstrated that boron-doped p-type SiNWs exhibit carrier mobilities comparable to or better than bulk materi￾al.[6b, 8] Current–voltage (I–V) data for a typical p-Si/n-GaN crossed NW junction (Figure 2 a) shows well-defined current rectification, as expected for a p–n diode with a turn-on voltage of approximately 1 V. These data are consistent with previous studies of n-GaN/p-Si diodes used as nanoelectron￾ic logic gates.[8] The initial turn-on voltage, which approxi￾mates to the bandgap of Si, corresponds to injection of elec￾trons into the SiNW from the n-GaN (see Supporting Infor￾mation). Notably, applying a forward bias to the p–n junc￾tion of greater than bandgap of GaN yields strong electrolu￾minescence (EL) at room temperature (Figure 2 b). The EL spectrum shows a peak maximum at 365 nm consistent with the GaN band-edge emission, and demonstrates that crossed NW devices can function as UV nanoLEDs. Studies of over 30 n-GaN/p-Si nanoLEDs yielded similar I–V and EL results. Data plotted for four representative devices (Figure 2 c) show several important points. First, light emis￾sion is detected when the bias voltage exceeds 3.5 V. This threshold is consistent with the 3.36 eV bandgap of GaN, which when exceeded, leads to radiative recombination of electron/hole pairs in GaN.[20] Second, the emission intensity increases rapidly with voltage and exhibits a nearly linear dependence on current above the threshold. The estimated quantum efficiency (electron to photon) is approximately 0.1%. This efficiency is less than commercial LEDs, al￾though it could be improved by passivating surface traps and using a larger bandgap p-type NW for hole injection. Third, stable (1 h) UV output is observed in air at room temperature at drive currents of 2 mA. Lastly, the near￾field optical power densities, which are estimated 50 nm from a typical UV nanoLED, are on the order of 100 W cm
2 . Notably, this value exceeds that needed in a number of optical applications, such as lithography and spectroscopy. Editorial Advisory Board Member Charles M. Lieber is the Mark Hyman Professor of Chemistry and member of the Division of Engineering and Applied Sciences at Harvard University. He has pioneered the synthesis of a broad range of nanoscale materials, the char￾acterization of the unique physicalprop￾erties of these materials, and the devel￾opment of methods of hierarchicalas￾sembly of nanoscale wires, together with the demonstration of applications of these materials in nanoelectronics, bio￾logical and chemical sensing, and nanophotonics. He is a member of the NationalAcademy of Sciences, American Academy of Arts and Sciences, and fellow of the American Physical Society. He has pub￾lished more than 230 peer-reviewed papers and is the principle in￾ventor on more than 20 patents. He also founded the nanotechnolo￾gy company, NanoSys, Inc. Figure 1. a) Schematic of the assembly of crossed NW heterojunc￾tions. First, a parallel array of A NWs (orange lines) is aligned on the substrate using a fluidic assembly method, and then a second paral￾lel array of B NWs (green lines) is deposited orthogonally to achieve a crossed NW matrix ; b) left: schematics showing a nanoLED struc￾ture formed between a p-SiNW and a direct bandgap n-type NW and its corresponding band diagram. Right: the table lists bandgaps of different materials (at 300 K) used in this study. small 2005, 1, No. 1 www.small-journal.com < 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim 143 Hybrid Devices from Nanowire Assemblies

papers C. M. Lieber et al 1200 to p-Si/n-Gan p-n diodes. For example, I-V data recorded from a p-si/n-Cds junction shows clear current rectification (inset, Figure 3 a). When the CdS-Si NW diode forward bias voltage exceeds a2.6V, strong light emission from the cross point is observed at room temperature. The emission maxi- mum at 510 nm corresponds to Cds band-edge emission. Significantly, the light emission is so strong that it can be readily imaged with a color CCD camera and is visible to the naked eye in a dark room. The quantum efficiency of GaN Cds 03.54.04.55.0 400600 300400500600700800 Figure 2. a)I-v data recorded for a p-Si/n-GaN crossed NW junction Inset: scanning electron microscopy( SEM) image of a typical cross Figure 3. a)EL spectra from crossed p-n diodes of p-Si and n-Cds, junction(scale bar=1 um); b)EL spectrum from the crossed p-Si/n- CdSSe, CdSe, and InP, respectively(top to bottom). Insets to the left GaN NW nanoLED with a peak at a365 nm. Inset: EL image showing are the corresponding EL images for CdS, CdSSe, CdSe(all color a spatial map of the intensity with a maximum at the Nw cross CCD), and InP (liquid-nitrogen-cooled CCD) nanoLEDs. The inset top- point; c) emission intensity versus forward bias voltage for four dis- right shows representative l-v and SEM data recorded for a p-Si/n tinct nanoLEDs. Inset: emission intensity versus injection current CdS crossed NW junction(scale bar=l um); spectra and images relationship for the same four nanoLEDs; d)representative SEM were collected at +5 V); b)schematic and corresponding SEM image of 3 p-n diodes formed by crossing three p-Si NWs over an n- image of a tricolor nanoLED The array was obtained by fluidic GaN NW; scale bar is 2 um; e-g) three-dimensional EL intensity plots assembly and photolithography with 5 um separation between NW of the three p-n diodes as a bias of 5.5 V is added sequentially to emitters; c)normalized l spectra and color images from the three each Si NW with the GaN NW grounded. elements nano the reproducibility and stability of the n-GaN/p-Si the Cds nanoled device is estimated to be 0.1-1%,which OLEDs suggests that they could be assembled into ad- is significantly higher than previously reported InP NIR dressable arrays. To test this idea we used fluidic assemblyl) (0.001%)and GaN(see above)nanoLEDs. The enhanced to make three junctions consisting of a n-Gan NW and quantum efficiency may be explained in part by the intrinsi three crossed p-Si NWs(Figure 2d). Transport measure- cally low surface state density for II-VI materials. 11.21 ments show that each of the three nanoscale junctions form Moreover, studies of nanoLEDs based on Cdsse, Case, and independently addressable p-n diodes with clear current InP NWs showed eL peak maxima and images characteri ectification. These p-n diodes each function as UV nano- tic of band-edge emission from the crossed junctions made LEDs, where each source can be individually switched on or with these materials: CdSosSep. s, 600 nm; CdSe, 700 nm; and off Figure 2e-g shows a sequence of EL images where the InP, 820 nm(Figure 3 a) three nanoLEDs were sequentially turned on, and demon- We have exploited the ability to form nanoLEDs with strate that the output intensity is similar for three sources. nonemissive SiNw hole-injectors to assemble multicolor We believe these array results highlight the potential of the arrays consisting of n-type Gan, CdS, and CdSe NWs cross bottom-up approach for assembling integrated photonic de- ing a single p-type SiNW(Figure 3 b) Normalized emission vices, and should be important in exploiting such nanoLEDs spectra recorded from the array demonstrates three spatially in applications where parallel writing(e.g, lithography and and spectrally distinct peaks with maxima at 365, 510, and information storage) and spectroscopy (e. g, integrated sen- 690 nm(Figure 3c) consistent with band-edge emission from sors)could be advantageou GaN, Cds, and CdSe, respectively. In addition, color images In addition, we assembled p-n junctions using p-Si NWs of EL from the array shows the green and red emission and a variety of other n-type Nw materials, including Cds, from p-Si/n-Cds and p-si/n-CdSe crosses, respectively CdSeS, CdSe, and InP to test whether our approach could (inset, Figure 3c). The ability to assemble different materi- yield nanoLEDs with a broad range of outputs. Transport als and independently tune the emission from each measurements show that these p-n diodes behave similarly nanoLED offers substantial potential producing specific 144 O 2005 Wiley-VCH GmbH &Co. KGaA, D-69451 Weinheim www.small-lournalcom

The reproducibility and stability of the n-GaN/p-Si nanoLEDs suggests that they could be assembled into ad￾dressable arrays. To test this idea we used fluidic assembly[13] to make three junctions consisting of a n-GaN NW and three crossed p-Si NWs (Figure 2 d). Transport measure￾ments show that each of the three nanoscale junctions form independently addressable p–n diodes with clear current rectification. These p–n diodes each function as UV nano￾LEDs, where each source can be individually switched on or off. Figure 2 e–g shows a sequence of EL images where the three nanoLEDs were sequentially turned on, and demon￾strate that the output intensity is similar for three sources. We believe these array results highlight the potential of the bottom-up approach for assembling integrated photonic de￾vices, and should be important in exploiting such nanoLEDs in applications where parallel writing (e.g., lithography and information storage) and spectroscopy (e.g., integrated sen￾sors) could be advantageous. In addition, we assembled p–n junctions using p-Si NWs and a variety of other n-type NW materials, including CdS, CdSeS, CdSe, and InP to test whether our approach could yield nanoLEDs with a broad range of outputs. Transport measurements show that these p–n diodes behave similarly to p-Si/n-GaN p–n diodes. For example, I–V data recorded from a p-Si/n-CdS junction shows clear current rectification (inset, Figure 3a). When the CdS–Si NW diode forward bias voltage exceeds 2.6 V, strong light emission from the cross point is observed at room temperature. The emission maxi￾mum at 510 nm corresponds to CdS band-edge emission. Significantly, the light emission is so strong that it can be readily imaged with a color CCD camera and is visible to the naked eye in a dark room. The quantum efficiency of the CdS nanoLED device is estimated to be 0.1–1%, which is significantly higher than previously reported InP NIR (0.001%) and GaN (see above) nanoLEDs. The enhanced quantum efficiency may be explained in part by the intrinsi￾cally low surface state density for II–VI materials.[11, 21] Moreover, studies of nanoLEDs based on CdSSe, CdSe, and InP NWs showed EL peak maxima and images characteris￾tic of band-edge emission from the crossed junctions made with these materials: CdS0.5Se0.5, 600 nm; CdSe, 700 nm; and InP, 820 nm (Figure 3a). We have exploited the ability to form nanoLEDs with nonemissive SiNW hole-injectors to assemble multicolor arrays consisting of n-type GaN, CdS, and CdSe NWs cross￾ing a single p-type SiNW (Figure 3b). Normalized emission spectra recorded from the array demonstrates three spatially and spectrally distinct peaks with maxima at 365, 510, and 690 nm (Figure 3c) consistent with band-edge emission from GaN, CdS, and CdSe, respectively. In addition, color images of EL from the array shows the green and red emission from p-Si/n-CdS and p-Si/n-CdSe crosses, respectively (inset, Figure 3c). The ability to assemble different materi￾als and independently tune the emission from each nanoLED offers substantial potential producing specific Figure 2. a) I–V data recorded for a p-Si/n-GaN crossed NW junction. Inset: scanning electron microscopy (SEM) image of a typical cross junction (scale bar=1 mm) ; b) EL spectrum from the crossed p-Si/n￾GaN NW nanoLED with a peak at 365 nm. Inset: EL image showing a spatial map of the intensity with a maximum at the NW cross point ; c) emission intensity versus forward bias voltage for four dis￾tinct nanoLEDs. Inset: emission intensity versus injection current relationship for the same four nanoLEDs ; d) representative SEM image of 3 p–n diodes formed by crossing three p-Si NWs over an n￾GaN NW; scale bar is 2 mm ; e–g) three-dimensional EL intensity plots of the three p–n diodes as a bias of 5.5 V is added sequentially to each Si NW with the GaN NW grounded. Figure 3. a) EL spectra from crossed p–n diodes of p-Si and n-CdS, CdSSe, CdSe, and InP, respectively (top to bottom). Insets to the left are the corresponding EL images for CdS, CdSSe, CdSe (all color CCD), and InP (liquid-nitrogen-cooled CCD) nanoLEDs. The inset top￾right shows representative I–V and SEM data recorded for a p-Si/n￾CdS crossed NW junction (scale bar=1 mm); spectra and images were collected at +5 V) ; b) schematic and corresponding SEM image of a tricolor nanoLED array. The array was obtained by fluidic assembly and photolithography with 5 mm separation between NW emitters ; c) normalized EL spectra and color images from the three elements. 144 < 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim www.small-journal.com small 2005, 1, No. 1 full papers C. M. Lieber et al.

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 45

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￾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

papers C. M. Lieber et al SOI substrate op-down hybrid structure 600650 00 Wavelength(nm Figure 6. a)Emission spectrum(solid line) from CdSe QDs excited Figure 5. a)Schematic illustrating the fabrication of hybrid structures. using a p-si/n-Cds NW nanoLED; the QD emission A silicon-on-insulator(So0 substrate is patterned by standard elec- 619 nm. The increasing intensity on the shorter wavelength side of tron-beam or photolithography followed by reactive- ion etching the emission peak corresponds to the tail of the cds nanoLED. The Emissive NWs are then aligned on to the patterned sol substrate to form photonic sources; b) schematic of a single LED fabricated by dashed red line is the spectrum of pure Cdse QDs excited with an the method outlined in(a); c)/v behavior for a crossed p-n junc- Ar-ion laser; b)emission spectrum(solid line) from propidium iodide excited using a p-Si/n-CdS NW nanoLED. The dashed red line is the tion formed between a fabricated p+Si electrode and an n-Cds Nw; emission spectrum of propidium iodide obtained in aqueous so- d)EL spectrum from the forward-biased junction; e) SEM image of a lution(Fluorolog, ISA/Jobin Yvon-Spex) CdS NW assembled over seven p+Si electrodes on a Sol wafer scale bar=3 um); f) EL image recorded from an array consisting of a CdS NW crossing seven p*t-Si electrodes. The image was acquired with +5 V applied to each silicon electrodes while the Cds Nw was Experimental Section blocks. We believe that these studies represent a new path Nanowire synthesis: Compound semiconductor NWs(GaN, way towards integrated nanophotonic systems and could CdS, CdSse, CdSe, InP) were synthesized using laser-assisted impact a number or areas including intra-and inter-chip op- catalytic growth(LCG). tSal The LCG target typically consisted of tical interconnects and communications for the next genera- 95% of the respective semiconductor material and 5%Au tion of computing systems, ultrahigh density optical infor- the catalyst. The fumace temperature was set at 700-900oC mation storage, high-resolution microdisplays, and multi- during growth, and the target was placed at the upstream end of plexed chemical/biological analysis. Our studies demonstrate the furnace. A pulsed(8 ns, 10 Hz) Nd-YAG laser (1064 nm)wa the potential of nanoLEDs in this latter area, and we be- used to vaporize the target. Typically, growth was performed lieve this is especially promising since arrays of different 10 min with NWs collected at the downstream, cool end of olor nanolED sources could be combined with microflui- fumace. SiNWs were synthesized using a Au-nanocluster-cata- analytic systems that might enable applications ranging respectively l g silane and diborane as reactant and dopant, dics in lab-on-a-chip systems to produce highly integrated lyzed process usi from high-throughput screening to medical diagnostics. Assembly of crossed Nw devices: The crossed Nw devices were assembled onto Si/Sio, substrates(600 nm oxide)using a layer-by-layer fluidic directed assembly Electrical contact pat- terns were defined using electron-beam lithography (EOL 6400), 46 O 2005 Wiley-VCH Verlag GmbH& Co KGaA, D-69451Weinheim www.small-lournalcom

blocks. We believe that these studies represent a new path￾way towards integrated nanophotonic systems and could impact a number or areas including intra- and inter-chip op￾tical interconnects and communications for the next genera￾tion of computing systems, ultrahigh density optical infor￾mation storage, high-resolution microdisplays, and multi￾plexed chemical/biological analysis. Our studies demonstrate the potential of nanoLEDs in this latter area, and we be￾lieve this is especially promising since arrays of different color nanoLED sources could be combined with microflui￾dics in lab-on-a-chip systems to produce highly integrated analytic systems that might enable applications ranging from high-throughput screening to medical diagnostics. Experimental Section Nanowire synthesis: Compound semiconductor NWs (GaN, CdS, CdSSe, CdSe, InP) were synthesized using laser-assisted catalytic growth (LCG).[15a] The LCG target typically consisted of 95% of the respective semiconductor material and 5% Au as the catalyst. The furnace temperature was set at 700–9008C during growth, and the target was placed at the upstream end of the furnace. A pulsed (8 ns, 10 Hz) Nd-YAG laser (1064 nm) was used to vaporize the target. Typically, growth was performed for 10 min with NWs collected at the downstream, cool end of the furnace. SiNWs were synthesized using a Au-nanocluster-cata￾lyzed process using silane and diborane as reactant and dopant, respectively.[6a, 15c] Assembly of crossed NW devices: The crossed NW devices were assembled onto Si/SiO2 substrates (600 nm oxide) using a layer-by-layer fluidic directed assembly.[13] Electrical contact pat￾terns were defined using electron-beam lithography (JEOL 6400), Figure 5. a) Schematic illustrating the fabrication of hybrid structures. A silicon-on-insulator (SOI) substrate is patterned by standard elec￾tron-beam or photolithography followed by reactive-ion etching. Emissive NWs are then aligned on to the patterned SOI substrate to form photonic sources; b) schematic of a single LED fabricated by the method outlined in (a) ; c) I–V behavior for a crossed p–n junc￾tion formed between a fabricated p+-Si electrode and an n-CdS NW; d) EL spectrum from the forward-biased junction ; e) SEM image of a CdS NW assembled over seven p+-Si electrodes on a SOI wafer (scale bar=3 mm) ; f) EL image recorded from an array consisting of a CdS NW crossing seven p+-Si electrodes. The image was acquired with +5 V applied to each silicon electrodes while the CdS NW was grounded. Figure 6. a) Emission spectrum (solid line) from CdSe QDs excited using a p-Si/n-CdS NW nanoLED ; the QD emission maximum was 619 nm. The increasing intensity on the shorter wavelength side of the emission peak corresponds to the tail of the CdS nanoLED. The dashed red line is the spectrum of pure CdSe QDs excited with an Ar-ion laser; b) emission spectrum (solid line) from propidium iodide excited using a p-Si/n-CdS NW nanoLED. The dashed red line is the emission spectrum of propidium iodide obtained in aqueous so￾lution (Fluorolog, ISA/Jobin Yvon-Spex). 146 < 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim www.small-journal.com small 2005, 1, No. 1 full papers C. M. Lieber et al

Hybrid Devices from Nanowire Assemblies sma and Ni/In/Au contact electrodes were thermally evaporated from [6] a)Y. Cui, C M. Lieber, Science 2001, 291, 851; b)Y. Cui,Z. pure metals. Electrical transport measurements of individual hong, D Wang, W U. Wang, C M. Lieber, Nano Lett. 2003, 3, NWs or crossed Nw junctions were made using a home-built 149 system with <1 pA noise under computer control [7] Y. Huang, X. Duan, Y. Cui, C. M. Lieber, Nano Lett. 2002, 2, 101 Optoelectrical characterization: Photoluminescence (PL)of [8)YHuang,XDuan,Y. Cui,L.Lauhon,K.Kim, CMLieber,Sci- ence2001,294,1313 individual NWs and electroluminescence (ED) of crossed Nw [].Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science 2001, junctions were characterized with a home-built microlumines cence instrument 5.17 PL or EL images were taken with a liquid. [10]M.Gudiksen, L.Lauhon,IWang, D.Smith, CMLieber, Nature nitrogen-cooled CCD camera, and the spectra were obtained by 2002,415,617 dispersing emission with a 150 linesmm-Igrating in a 300 mm (111 X. Duan, Y. Huang, R. Argarawal,C M. Lieber, Nature 2003, [12] P. Zhang, V.H. Crespi, E. Chang, S G. Louie, M. L. Cohen, Nature Quantum dot and dye experiments CdSe QDs were 2001,409,69 persed in hexane and then deposited over the nanoLED devices. [13]Y. Huang, X. Duan, Q. Wei, C.M. Lieber, Science 2001, 291 An aqueous solution of propidium iodide(1 mg mL-; Molecular Probes, Inc. was deposited directly onto a substrate containing [14] a) Heterojunction Band Discontinuities(Ed F. Capasso, G.Mar- p-Si/n-CdS NW nanoLEDs. Spectra were recorded from the Nw garitondo), North-Holland, Amsterdam, 1987; b)M. Leibovitch, cross points as described above and previously 6 17 The emis- L. Kronik, E. Fefer, V. Korobov, Y. Shapiraa, Appl. Phys. Lett 1995,66,457 sion spectrum of the propidium iodide aqueous solution was re- 15] a)X. Duan, C.M. Lieber, Adv Mater. 2000, 12, 298; b)X. Duan, corded using a commercial instrument ( Fluorolog, ISA/Jobir C M. Lieber, / Am. Chem. Soc. 2000, 122, 188; c)Y Cui, LJ Yvon-Spex) auhon, M. s. Gudiksen, ). Wang, C. M. Lieber, Appl. Phys. Lett. 1, [16] Photoluminescence measurements were made on individual NWs using a home-built microluminescence instrument. 5. 17 The ata recorded on GaN, Cds, CdSes, CdSe, and InP Nws typically howed luminescence maxima of≈370,≈510,≈600,≈700, and 820 nm, respectively, which are consistent with the bulk [1] a)I. Hu, T. C M. Lieber. A em.Res.1999,32, emiconductor bandgaps. The emission from InP NWs is blue- 435; b)C M. Lieber, Sci. Am. 2001 (September), 58; c)CM shifted due to quantum confinement and other factors. 5.17 17] M.S. Gudiksen, J. Wang, C M. Lieber, /. Phys. Chem. B 2002, M. Lieber in Molecular Nanoelectronics(Eds M. A. Reed, T. 106,4036 ee), American Scientific Publishers, New York, 2003, pp. 19 [18] Transport measurements were made on GaN, CdS, CdSes 227; e)Y Cui, X Duan, Y. Huang, C M. Lieber in Nanowires and CdSe, and InP NWs in FET geometry with a back gate as descri- Nanobelts-Materials, Properties and Devices(Ed Z L Wang ed previously 5-8 In all cases, positive gate voltage increases Publishers, Dordrecht, 2003, pp the conductance, and negative gate voltages decrease the con- [2] a)A P. Alivisatos, Science 1996, 271, 933; b)Z L Wang, Adv. The carrier mobility of each material is estimated from the trans- Mater. 1998, 10, 13; c)M. NirmaL, L. Brus, Acc. Chem. Res. conductance with values GaN, 150-650 cm'V-ls: CdS. CdSSe 1999,32,407;dC.B. Murray,C.R. n. M.G. Bawendi c.2000,30,545;e)W.J Parak, D Gerion, [1910. Madelung in LANDOLT-BORNSTEIN New Series: Vol /22a, T. Pellegrino, D. Zanchet, C. Micheel, S C. Williams, R. Bou- Semiconductors: Intrinsic properties of Group /V Elements and dreau, M.A. Le gros, C l1l-V and Il-VI and I-Vll Compounds(Ed. 0. Madelung), Spring ology 2003, 14, 15: f M. A. El-Sayed, Acc. Chem. Res. 2004 7,326 [20] Electrons are injected efficiently into the SiNW(from n-GaN NW) [3] a)Z. Yao, C. Dekker, P Avouris pl.Phys.2001,80,147 at the initial diode turm-on voltage of 1 V. Measurements in b)P L. McEuen, M.S. Fuhrer, H. Park, IEEE Trans. Nanotechnol hich the spectrometer detection range was extended to 2002,1,78;c)H.Dai,Ac.Chem.Res.2002,35,1035. 1000 nm showed no evidence for bandgap emission from the [4] a)SJ. Tans, R.M. SiNWs above the e1 V threshold 49; b)R. Martel, T. Schmidt, H. R. Shea, T. Hertel, P. Avouris [21]X. Duan, C. Niu, V. Sahi, 1. Chen, W. Parce, S. Empedocles, ). Appl. Phys. Lett. 1998, 73, 2447. a V. Derycke, R. MartelL, 1. Ap: (22)S RWhaley, D.S. English, E. L. Hu, P. F. Barbara, A M. Belcher Guo, Q Wang, M. Lundstrom, H Dai, Nature 2003, 424, 654. Nature2000,405,665 [5]X. Duan, Y. Huang, Y. Cui, I Wang, 09,66. Published online on October 15, 2004 l2005,1,No.1 www.small-journalcom o 2005 Wiley-VCH Verlag GmbH& Co KGaA, D-69451 Weinheim 147

and Ni/In/Au contact electrodes were thermally evaporated from pure metals. Electrical transport measurements of individual NWs or crossed NW junctions were made using a home-built system with <1 pA noise under computer control. Optoelectrical characterization: Photoluminescence (PL) of individual NWs and electroluminescence (EL) of crossed NW junctions were characterized with a home-built microlumines￾cence instrument.[5, 17] PL or EL images were taken with a liquid￾nitrogen-cooled CCD camera, and the spectra were obtained by dispersing emission with a 150 linesmm
1 grating in a 300 mm spectrometer. Quantum dot and dye experiments: CdSe QDs were dis￾persed in hexane and then deposited over the nanoLED devices. An aqueous solution of propidium iodide (1 mgmL
1 ; Molecular Probes, Inc.) was deposited directly onto a substrate containing p-Si/n-CdS NW nanoLEDs. Spectra were recorded from the NW cross points as described above and previously.[6, 17] The emis￾sion spectrum of the propidium iodide aqueous solution was re￾corded using a commercial instrument (Fluorolog, ISA/Jobin Yvon-Spex). [1] a) J. Hu, T. W. Odom, C. M. Lieber, Acc. Chem. Res. 1999, 32, 435 ; b) C. M. Lieber, Sci. Am. 2001 (September), 58 ; c) C. M. Lieber, MRS Bull. 2003 (July), 486 ; d) X. Duan, Y. Huang, Y. Cui, C. M. Lieber in Molecular Nanoelectronics (Eds.: M. A. Reed, T. Lee), American Scientific Publishers, New York, 2003, pp. 199 – 227; e) Y. Cui, X. Duan, Y. Huang, C. M. Lieber in Nanowires and Nanobelts—Materials, Properties and Devices (Ed.: Z. L. Wang), Kluwer Academic/Plenum Publishers, Dordrecht, 2003, pp. 3 – 68. [2] a) A. P. Alivisatos, Science 1996, 271, 933 ; b) Z. L. Wang, Adv. Mater. 1998, 10, 13 ; c) M. Nirmal, L. Brus, Acc. Chem. Res. 1999, 32, 407; d) C. B. Murray, C. R. Kagan, M. G. Bawendi, Annu. Rev. Mater. Sci. 2000, 30, 545 ; e) W. J. Parak, D. Gerion, T. Pellegrino, D. Zanchet, C. Micheel, S. C. Williams, R. Bou￾dreau, M. A. Le Gros, C. A. Larabell, A. P. Alivisatos, Nanotech￾nology 2003, 14, 15 ; f) M. A. El-Sayed, Acc. Chem. Res. 2004, 37, 326. [3] a) Z. Yao, C. Dekker, P. Avouris, Top. Appl. Phys. 2001, 80, 147; b) P. L. McEuen, M. S. Fuhrer, H. Park, IEEE Trans. Nanotechnol. 2002, 1, 78 ; c) H. Dai, Acc. Chem. Res. 2002, 35, 1035. [4] a) S. J. Tans, R. M. Verschueren, C. Dekker, Nature 1998, 393, 49 ; b) R. Martel, T. Schmidt, H. R. Shea, T. Hertel, P. Avouris, Appl. Phys. Lett. 1998, 73, 2447; c) V. Derycke, R. Martel, J. Ap￾penzeller, P. Avouris, Nano Lett. 2001, 1, 453 ; d) A. Javey, J. Guo, Q. Wang, M. Lundstrom, H. Dai, Nature 2003, 424, 654. [5] X. Duan, Y. Huang, Y. Cui, J. Wang, C. M. Lieber, Nature 2001, 409, 66. [6] a) Y. Cui, C. M. Lieber, Science 2001, 291, 851; b) Y. Cui, Z. Zhong, D. Wang, W. U. Wang, C. M. Lieber, Nano Lett. 2003, 3, 149. [7] Y. Huang, X. Duan, Y. Cui, C. M. Lieber, Nano Lett. 2002, 2, 101. [8] Y. Huang, X. Duan, Y. Cui, L. Lauhon, K. Kim, C. M. Lieber, Sci￾ence 2001, 294, 1313. [9] A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science 2001, 294, 1317. [10] M. Gudiksen, L. Lauhon, J. Wang, D. Smith, C. M. Lieber, Nature 2002, 415, 617. [11] X. Duan, Y. Huang, R. Argarawal, C. M. Lieber, Nature 2003, 421, 241. [12] P. Zhang, V. H. Crespi, E. Chang, S. G. Louie, M. L. Cohen, Nature 2001, 409, 69. [13] Y. Huang, X. Duan, Q. Wei, C. M. Lieber, Science 2001, 291, 630. [14] a) Heterojunction Band Discontinuities (Ed.: F. Capasso, G. Mar￾garitondo), North-Holland, Amsterdam, 1987; b) M. Leibovitch, L. Kronik, E. Fefer, V. Korobov, Y. Shapiraa, Appl. Phys. Lett. 1995, 66, 457. [15] a) X. Duan, C. M. Lieber, Adv. Mater. 2000, 12, 298 ; b) X. Duan, C. M. Lieber, J. Am. Chem. Soc. 2000, 122, 188 ; c) Y. Cui, L. J. Lauhon, M. S. Gudiksen, J. Wang, C. M. Lieber, Appl. Phys. Lett. 2001, 78, 2214. [16] Photoluminescence measurements were made on individual NWs using a home-built microluminescence instrument.[5,17] The data recorded on GaN, CdS, CdSeS, CdSe, and InP NWs typically showed luminescence maxima of 370, 510, 600, 700, and 820 nm, respectively, which are consistent with the bulk semiconductor bandgaps. The emission from InP NWs is blue￾shifted due to quantum confinement and other factors.[5,17] [17] M. S. Gudiksen, J. Wang, C. M. Lieber, J. Phys. Chem. B 2002, 106, 4036. [18] Transport measurements were made on GaN, CdS, CdSeS, CdSe, and InP NWs in FET geometry with a back gate as descri￾bed previously.[5-8] In all cases, positive gate voltage increases the conductance, and negative gate voltages decrease the con￾ductance of the NWs, which is consistent with n-type doping. The carrier mobility of each material is estimated from the trans￾conductance with values: GaN, 150–650 cm2 V
1 s ; CdS, CdSSe, and CdSe, 100–400 cm2 V
1 s ; and InP, 400–4000 cm2 V
1 s. [19] O. Madelung in LANDOLT-BORNSTEIN New Series: Vol III/22a, Semiconductors: Intrinsic properties of Group IV Elements and III–V and II–VI and I–VII Compounds (Ed.: O. Madelung), Spring￾er, Heidelberg, 1987. [20] Electrons are injected efficiently into the SiNW (from n-GaN NW) at the initial diode turn-on voltage of 1 V. Measurements in which the spectrometer detection range was extended to 1000 nm showed no evidence for bandgap emission from the SiNWs above the 1 V threshold. [21] X. Duan, C. Niu, V. Sahi, J. Chen, W. Parce, S. Empedocles, J. Goldman, Nature 2003, 425, 274. [22] S. R. Whaley, D. S. English, E. L. Hu, P. F. Barbara, A. M. Belcher, Nature 2000, 405, 665. Published online on October 15, 2004 small 2005, 1, No. 1 www.small-journal.com < 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim 147 Hybrid Devices from Nanowire Assemblies