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 143cally 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