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-lournalcomThe 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.