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Enhanced DC-operated electroluminescence of forwardly aligned p/MQW/n InGaN nanorod LEDs via DC offset-AC dielectrophoresis Yun Jae Eo, Gang Yeol Yoo, Hyelim Kang, Youngki Lee, Chan Sik Kim, Ji Hye Oh, Keyong Nam Lee, Woong Kim, and Young Rag Do ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09794 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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ACS Applied Materials & Interfaces

Enhanced DC-operated electroluminescence of forwardly aligned p/MQW/n InGaN nanorod LEDs via DC offset-AC dielectrophoresis Yun Jae Eo†, Gang Yeol Yoo‡, Hyelim Kang†, Young Ki Lee†, Chan Sik Kim†, Ji Hye Oh†, Keyong Nam Lee†, Woong Kim‡, and Young Rag Do†,* †

Department of Chemistry, Kookmin University, Seoul 02707, Republic of Korea

‡

Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of

Korea

KEYWORDS: GaN nanorod, light-emtting diode, dielectrophoresis, DC offset AC electric field

Abstract

We introduce orientation-controlled alignment process of p-GaN/InGaN multi quantum-well/nGaN (p/MQW/n InGaN) nanorod light-emitting diodes (LEDs) by applying direct-current (DC) offset alternating-current (AC) or pulsed DC electric fields across interdigitated metal electrodes. The as-forwardly aligned p/MQW/n InGaN nanorod LEDs by a pulsed DC dielectrophoresis (DEP) assembly process improve the electroluminescence (EL) intensities by 1.8 times

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compared to the conventional AC DEP assembly process under DC electric field operation and exhibit an enhanced applied current and EL brightness in the current-voltage and EL intensityvoltage curves that can be directly used as fundamental data to construct DC-operated nanorod LED devices, such as LED areal surface lightings, scalable lightings (micrometers to inches) and formable surface lightings. The enhancement of the applied current, the improved EL intensity, and the increased number of forwardly aligned p/MQW/n InGaN nanorods in panchromatic cathodoluminescence (CL) images confirm the considerable enhancement of forwardly aligned 1D nanorod LEDs between two opposite electrodes using DC offset-AC or a pulsed DC electric field DEP assembly process. These DC offset-AC or pulsed DC electric field DEP assembly process suggests that designing for these types of interactions could yield new ways to control the orientation of asymmetric p/MQW/n InGaN diode-type LED nanorods with a relatively low aspect ratio.

Introduction Since the first appearance of blue InGaN light-emitting diodes (LEDs) in the 1990s,1,2 numerous research activities have been actively performed to enhance the external quantum efficiency (EQE) of InGaN LEDs.3-5 Among them, m-plane (nonpolar) InGaN core/shell nanorod6-9 and axial c-plane nanorod arrayed structures10-12 have been introduced by InGaN LED scientists. These devices reduce the inside strain among the p-GaN layer, the quantum well (QW), and the n-GaN layer in InGaN LEDs to increase the internal quantum efficiency (IQE) of InGaN LEDs. Individually separated p-GaN/InGaN multi-quantum-well/n-GaN (p/MQW/n InGaN) nanorods obtained from two-dimensional (2D) nanorod arrays have also attracted much technological interest as part of the effort to overcome the millimeter or micrometer size


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limitations of conventional LED chips by increasing the electro-optical properties of LEDs and creating more challenging LED applications, such as LED areal surface lightings, scalable lightings (micrometers to inches), formable surface lightings13 and displays.14 To fabricate separate nanorod-based LED devices successfully, it is necessary to position and assemble 1D p/MQW/n InGaN nanorods on predefined and pre-patterned electrodes to develop microelements and to optimize the electroluminescence (EL) emission properties of multiple horizontally aligned nanorods (or nanowires). Very recently, it was reported that individually separated, single nanorods were fabricated by a top-down approach using Ni or SiO2 nanoparticles as etching masks, with the quantum confined Stark effect reduced compared to that in a standard planar counterpart fabricated from the same wafer.15, 16 To date, various assembly technologies, such as the layer-by-layer method,17 transfer printing,18,19 and fluidic flow-assisted20,21 and electric field-assisted techniques22,23 have been developed to position one-dimensional (1D) semiconductor nanowires, nanorods or microchips on predefined electrodes. Among them, dielectrophoresis (DEP), which is related to the alternating-current (AC) electric field-assisted technique, was developed as a strong assembling process candidate to align 1D nanorods or nanowires dispersed in a solvent by applying a nonuniform electric field. Using the same theory and technology, the number of studies regarding AC-field DEP processes has steadily increased over the years, leading to the successful assembly of 1D nanostructures on pre-defined electrodes.24-26 Specifically, GaN-based semiconductor nanowires/nanorods27-29 as well as metallic nanostructures30,31 and carbon nanotubes32,33 can also be assembled on pre-patterned electrodes using the AC-field DEP process, which offers the strong advantage of being able to assemble millions of nanowires on a specified area. Over the last decade, GaN-based nanowires/nanorods were assembled onto electrodes using the DEP


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process, as summarized in Table 1.14,16,27,34-39 As shown in several studies, GaN nanowires/nanorods have a high aspect ratio (length/diameter of nanowires/nanorods) and can be aligned onto electrodes using the DEP process even at low AC voltages. Park et al. fabricated a blue LED device consisting of GaN nanorods LED with a relatively low aspect ratio using a DEP process at a AC voltage of only 6 Vpp (peak-to-peak voltage) at 1 MHz, but it was not a surfaceplanar light source.16 Nonetheless, our group reported a great number of horizontally assembled p/MQW/n InGaN nanorod LEDs with a similar aspect ratio using a high AC voltage DEP process (50 Vpp at 950 kHz) that can be evolved into specified layouts for self-emissive planarsurface lighting, scalable lightings (micrometers to inches) and formable surface lightings.14 However, the assembly directions of LED nanorods, which are fabricated from p/MQW/n InGaN-based LED epi-wafers, are not simple, as LED nanorods consist of three different layers of a p-GaN layer, InGaN quantum wells (QWs), and a n-GaN layer. As is well known, the n-type GaN layer (2–3 µm) is supposed to be much longer than the p-type GaN layer (50–200 nm) layer owing to the faster electron mobility of the n-GaN layer than the hole mobility of the p-GaN layer.40 Therefore, p/MQW/n InGaN-based nanorod LEDs have an asymmetric structure and increase the possibility of non-uniform alignment, resulting in the unlikely identical positions of MQWs. In this study, we fabricated several hundred million individually separated p/MQW/n InGaN nanorod LEDs from a two-inch InGaN-based green wafer using a top-down method which included a combined process of nanosphere lithography and dry etching. We also dispersed numerous p/MQW/n InGaN nanorod LEDs in acetone and assembled nanorods with a relatively low aspect ratio on interdigitated electrodes (0.6 × 0.7 cm2) by applying an AC or direct-current (DC) offset AC electric field using the DEP method. We studied how to characterize the


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orientation of individual p-type heads of p/MQW/n nanorod LEDs between the electrodes by measuring cathodoluminescence (CL) images and the electroluminescence (EL) under AC/DC operational signals. The orientations of the p-type heads of individually separated nanorod LEDs are randomly directed in the horizontally assembled nanorod LEDs under a conventional sinusoidal AC applied field in the DEP process. Almost half of the p-type heads are directed toward the right-sided electrodes while the other half of the heads also directed toward the leftsided electrodes. Thus, only the assembled nanorod LEDs which are properly oriented are turned on under DC operation, while the oppositely aligned nanorod LEDs are turned off. To increase the preferred orientation of nanorod LEDs and the orientation selectivity, we added a DC offset to the AC electrical field. By doing this, we studied how the increment of the preferred orientation of a nanorod LED affects the increase in the EL intensity under DC operation through variations of the electrical field during the DEP assembled process. The results suggested that a DC offset AC or pulsed DC applied field is a promising assembly process to align asymmetrical p/MQW/n InGaN nanorod LEDs preferably with a relatively low aspect ratio and to make DC operation preferable by increasing the EL intensity of the forwardly aligned nanorod LEDs.

Experimental Methods 1. Nanorod LEDs Ink Preparation Conventional p-GaN/MQW/n-GaN green LED structures on patterned sapphire substrates (PSS) were used to prepare the nanorod array. A 150-nm-thick Al layer and a 1.5-µm-thick SiO2 layer were deposited on the LED structure via e-beam evaporation and plasma-enhanced chemical vapor deposition (PECVD), respectively, and a monolayer with a triangular pattern of a polystyrene (PS) nanosphere (Interfacial Dynamics Co.) with a diameter of 960 nm was


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transferred onto the Al/SiO2/GaN substrate using a scooping transfer method. The PS nanospheres were reduced in diameter using an O2 plasma ashing process. Subsequently, the Al layer was etched with Cl2-based inductively coupled plasma (ICP) etching using a PS nanosphere mask, after which the SiO2 layer was etched with CF4-based reactive ion etching (RIE) using an Al nanodot mask. The triangularly patterned GaN nanorod LED array was fabricated by ICP etching the GaN layer using a SiO2 hard mask. The fabricated GaN nanorod LEDs were wet-etched using 2 M KOH (Daejung Chemicals & Metals Co., Ltd., Korea) solution at 80 oC for 10 min to remove the surface defects caused by the dry etching process and to make them cylindrical. Finally, GaN nanorod LEDs with average length of 3.0 µm and average diameter of 500 nm, and an aspect ratio of 6 were individually separated from the array using a diamond cutter. The separated LEDs were then dispersed in acetone (99.9%, Sigma-Aldrich) for use as ink.

2. Nanorod LED Device Fabrication Interdigitated metal electrodes with an active area of 0.6 × 0.7 cm2 were prepared on a glass substrate by conventional photolithography. An interdigitated photoresist (PR) pattern with a 3 µm spacing was formed by selectively ultraviolet (UV) exposure of the negative PR (DNRL300-40, Dongjin Semichem Co., Ltd., Korea) coated glass substrate with an interdigitated Cr photomask. Subsequently, 20-nm-thick Ti as an adhesive layer between the glass substrate and the Au metal and 200-nm-thick Au metal films were sequentially deposited using an e-beam evaporator. Consequently, the interdigitated metal electrodes were obtained by a metal lift-off technique with sonication in acetone.


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A function generator (Handyscope HS5, TiePie Engineering, Netherlands) and a voltage amplifier (A400, FLC Electronics, Sweden) were used to align the individually separated nanorod LEDs on the pre-patterned electrodes. Nanorod LED devices were fabricated by dropping several drops of the ink containing the individually separated nanorod LEDs in acetone under a sinusoidal AC voltage (V = 30 sin ), DC offset AC voltage (V = 25 sin + 5, V = 20 sin + 10 ), or pulsed DC voltage ( V = 15 sin + 15 ), where the angular frequency was determined by = 2 × 950,000 . Rapid thermal annealing (RTA) was performed on the as-assembled nanorod LED device in a N2 atmosphere of 0.5 torr at 810 oC for 2 min to remove any solvent remaining after the alignment process of the nanorod LEDs and to improve the contact between the nanorod LEDs and the electrodes.

3. Measurement and Characterization Processes The material properties of the fabricated nanorod LEDs were confirmed using scanning electron microscopy (SEM, JSM-7610F, JEOL Ltd., Japan) in conjunction with energy dispersive X-ray spectroscopy (EDX) and optical microscopy (PSI 4RT, PSI Trading Co., Ltd., Korea). The orientations of the assembled nanorod LEDs on the pre-patterned electrodes were measured by measuring the CL images of the InGaN-based MQW through panchromatic CL measurements (Mono CL3+, Gatan, Inc., USA) with an electron beam with an acceleration voltage of 15 kV at room temperature. The EL images of nanorod LED device were obtained using a digital camera (NEX-5, Sony, Japan) with a shutter speed of 1/30 sec in the ISO 320 condition. The optical properties, including the EL spectra and voltage-current (V-I) curve, were measured using a spectrophotometer (Darsa II, PSI Trading Co., Ltd., Korea), a


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spectroradiometer (CS-2000, Konica Minolta, Japan), and a digital multimeter (Model 2001, Keithley Instruments Inc., USA).

Results and discussion As previously reported, several hundred million individually separated green-emitting nanorod LEDs were successfully fabricated by a fused process that combines bottom-up and top-down methods. Figure S1 and Figures 1a and b illustrate the fabrication processes of both the 2D nanorod arrays and individually separated, 1D nanorod LEDs. Figure 1c also shows the assembly procedure of the 1D nanorod LEDs between interdigitated metal electrodes. The side-view scanning electron microscopy (SEM) results of the fabricated nanorod LED array show that each nanorod LED is cylindrical after the wet-etch process using KOH (See Figure S2). Moreover, the length and diameter of the individually separated nanorod LEDs are approximately 3 µm and 500 nm, respectively, consisting of 200 nm of a p-type GaN layer, 90 nm of QW, and 2.7 µm of a ntype GaN layer (See Figures S3a and S3c). A SEM-EDX analysis confirms that the miniature nanorod LEDs have a vertical structure and composition identical to those of the original InGaNbased LED wafer (See Figures S3b and S3d). Therefore, individually separated nanorod LED structures with an asymmetrical p-GaN/InGaN QW/n-GaN layered structure can be obtained from a green LED wafer using our combined fabrication process of the nanosphere lithography, dry etching (RIE etching and ICP etching), and KOH wet etching. A nanorod LED suspension containing 5 wt.% of individually separated green-emitting nanorod LEDs and 95 wt.% acetone was dropped onto the pre-defined electrodes during the assembly process. Numerous multiple 1D nanorod LEDs were assembled on the prepared


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electrodes for a comparison under an applied AC field (±30 V at 950 kHz) and AC fields with a DC offset (+30~–20 V, +30~–10 V and +30~0 V at 950 kHz) onto interdigitated metal electrodes, as shown in Figure 2. The DC offset AC voltage means the AC voltage having a certain frequency such as a sinusoidal function with additional DC offset. In contrast, the pulsed DC voltage also has a certain frequency, like DC offset AC voltage, but there is no change in polarity. The interdigitated metal electrodes, which have a line width of ~3 µm and spacing of ~3 µm, were fabricated by photolithography and an Au metal lift-off process, as reported in our previous publication.14 In the conventional AC-field DEP process under a non-uniform electric field, the timeaveraged DEP force on the cylindrical nanorod was estimated by22,23 =

!" Re%& '∇|*|

(1)

with ∗ + ∗ .+/

& = +∗,0 ,

∗ +/

(2),

where 1 and 2 are the corresponding radius and length of the individually separated nanorod LEDs, !" is the dielectric constant of the suspending medium (in this case acetone), * is the electric field, and Re%& ' is the real part of the Clausius-Mosotti (CM) factor. & is related ∗ to the complex dielectric function of the nanorod LEDs !3∗ , and the suspending medium !" , 7

defined as ! ∗ = ! − 5 689, where σ is the conductivity of the nanorod LEDs or medium and ω is the angular frequency of the applied electric field. Figure 3 shows Re%& ', which in our case as a function of the frequency. Given that the value of Re%& ' is greater than 0 at the


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frequency of the electric field of ~1 MHz as used here, the nanorod LEDs move toward the electrodes (positive DEP). Moreover, 1D nanorod LED suspension flows through the space between the electrodes, and the nanorod LEDs can be polarized within the electric field from two electrodes (see Figure 4a). The heads (p-type GaN layer) of the polarized nanorod LEDs are attracted to the both of the interdigitated electrodes using an AC electric field regardless of the orientation. The nanorod LED trapping process can be performed on both electrodes by DEP force at a distance where the van der Waals force dominates.27 The heads of the nanorod LEDs tend toward contact with two opposite interdigitated electrodes while maintaining the initial orientation under a sinusoidal AC field (see Figure 4b). Therefore, when the nanorod LEDs aligned using a sinusoidal AC electric field are driven by DC voltage, the ratio of the nanorod LEDs driven to the aligned nanorod LEDs is low because only the nanorod LEDs with forward bias are turned on, as shown in Figure 4c. As previously reported, the magnitude of the DEP force during the AC-field DEP process depends on the voltage, frequency, material properties, electrode geometry, and other factors.41 Here, we added three types of DC offsets to the AC applied field of the LED nanorod assembly process, as shown in Figures 2b~d. When an AC field with DC offset is applied to both electrodes, the positive charge on one electrode is greater than the charge on the opposite electrodes with an increase in the DC offset (see Figure 4d), as the offset magnitude of the negative sinusoidal field increases with an increase in the DC offset voltage. When additional DC offset is applied, the p/MQW/n nanorod LEDs are affected by electrophoresis (EP) as well as the DEP due to the intrinsic dipole of the nanorod LEDs, and the EP force and torque are defined as follows,42 = *

(3)


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@

? = 6 9

(4),

where < is the elementary charge, = is the number density of either the donors or acceptors, and > and A are the corresponding volume and width of the depletion layer. Therefore, if DC offset exists, the torques (? and ? ) compete to determine the orientation of the nanorod LEDs. According to previous research, is proportional to the DC voltage >B ; as a result, ? increases as >B increases43. Therefore, most of heads of p/MQW/n InGaN nanorod LEDs are asymmetrically trapped on the more positively charged electrodes of the two electrodes at a certain time under an AC field with DC offset (see Figure 4e) owing to the increased asymmetrical van der Waals force with an increase in the DC offset. Accordingly, this unequal orientation of nanorods is realized in the trapping process by applying an AC electrical field with DC offset. Therefore, nearly all of the heads of the resulting assembled nanorod LEDs come into contact with a positively charged electrode. If forward DC current is applied to the assembled nanorod LEDs, the most forwardly positioned nanorod LEDs in contact are turned on and a very small number of reversibly assembled nanorod LEDs are turned off (Figure 4f). Subsequently, the electroluminescence (EL) intensity of the p/MQW/n InGaN nanorod LEDs in contact is significantly increased under the forward DC operation voltage compared to the EL intensity of the nanorod LEDs in contact at the AC operational voltage. To confirm the enhancement of the EL intensity of the AC with DC offset assembled p/MQW/n nanorod InGaN LEDs from AC operation to DC operation, we compared the EL intensity levels and actual emittance photographs of green nanorod assembled LEDs on the planar surface as a variation of the DC offset voltage added to the AC applied voltage. Figures 5a and b show actual photographs of the EL emissions of 0.6 × 0.7 cm2 areal EL devices with conventional AC ( > = 30 sin , see Figure 2a) and pulsed DC by DC offset ( > =


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15 sin + 15, see Figure 2d) assembled nanorod LEDs at voltages of AC 21.0 Vrms at 60 Hz, DC +21.0 V (+DC) and –21.0 V (–DC) in the presence and absence of background light. Consistent with the above-mentioned explanation, the conventional AC-assembled nanorod LEDs show similar EL intensity levels under both AC and –DC operation. Despite the use of sinusoidal AC when assembling the nanorod LEDs, the EL intensity levels at +DC and –DC are considerably different because the structures of the p/MQW/n nanorod LEDs are asymmetrical. On the other hand, the pulsed DC-assembled nanorod LEDs show improved EL intensity by 1.8 times under +DC operation compared to that of the pulsed DC-assembled nanorod LEDs under AC operation (see Figure 5c and 5d). When operated under –DC, the EL intensity of the pulsed DC-assembled nanorods is very low because the nanorod LEDs are inversely aligned by the assembly of an asymmetric field. These enhanced EL intensity levels under +DC and the lower EL intensity levels under –DC can confirm the concept under which most of the head directions of the p/MQW/n InGaN nanorod LEDs are in contact with the upper electrodes of interdigitated electrodes (see Figure 4f). The orientation selectivity of p/MQW/n InGaN nanorod LEDs enables a change of the operational power and circuits from an AC power/operation system to a DC power/operation system and an improvement in the EL efficiency by ~ 1.8 times when changing the nanorod arrays from AC to DC operation. The considerable enhancement of the DC-operated EL intensity caused by the pulsed DC aligned nanorod p/MQW/n InGaN LEDs is due to the increased number density of nanorod LEDs preferably aligned between two opposite electrodes. Figure 6 compares optical microscopy images of the as-assembled nanorod LEDs on the interdigitated electrodes with three different types of applied fields. When the voltage was applied to align the nanorod LEDs with pre-patterned electrode, nanorod LEDs were aligned between the electrodes because a positive DEP applied regardless of the type of AC or DC


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voltage. However, using conventional DC (> = 18.4 (See Figure 6d) with the root-mean-square voltage (>"E ) identical to that of the pulsed DC, electrodes were severely damaged, as shown in Figure 6c. Although the >"E of the conventional DC is lower than that of the conventional AC, this damage issue arises from the constantly high voltage, which directly shorts the electrodes due to the solvent. Previous studies have shown that GaN nanowires when aligned use a relatively low voltage because these nanowires have a larger aspect ratio than ours (See Table 1). However, the p/MQW/n InGaN nanorod LEDs cannot be extended in length due to carrier mobility considerations, as mentioned above, and the use of a high voltage level for alignment was inevitable in order for the nanorod LEDs to be aligned simultaneously for a LED device with a large area. Nevertheless, the use of conventional AC and pulsed DC does not damage the electrodes when the nanorod LEDs are aligned, as the applied voltage was conducted at a certain frequency. In other words, given that the applied voltage varies with time, damage to the electrodes can be avoided. Figure 7 presents the DC voltage-dependent variations of the applied current and the EL luminescence intensity of the forwardly aligned nanorod EL devices by pulsed DC electric field DEP assembly. The applied current and EL intensity of the pulsed DC-assembled nanorod LED device was significantly increased compared to those of the conventional AC-assembled nanorod LED device. This occurred because the number of forwardly assembled LEDs is significantly increased to more than 80% of all nanorod LEDs when using the pulsed DC-assembled DEP assembly process. Therefore, a planar-surface green EL emission from many forwardly aligned luminous p/MQW/n InGaN nanorod LEDs has not been reported in any previously published work. To the best of our knowledge, this is the first demonstration of such collective DCoperational surface green EL emission from many forwardly oriented p/MQW/n nanorod LEDs


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aligned between interdigitated metal electrodes with a large planar area. Our DC offset-AC electric field DEP assembly approach indicates that forwardly orientated tiny lights emitted from p/MQW/n InGaN nanorod LEDs can be realized as an efficient planar surface source under DC operation if most of the forwardly oriented nanorod LEDs are interconnected with electrodes. Figure 8 shows panchromatic images of the cathodoluminescence (CL) emission of both conventional AC and pulsed DC-assembled nanorod LEDs. In the conventional AC assembly process, the panchromatic CL image (See Figure 8b) indicates that the bright QW heads of nanorod LEDs are randomly oriented toward two electrodes. However, the bright QW heads of the nanorod LEDs are directed toward one electrode (the upper or lower electrode), as they were assembled while applying a pulsed DC field (See Figure 8d). In addition, a statistical analysis of the panchromatic CL images was conducted to confirm the assembly ratio of the actual nanorod LEDs with various proposed assembly voltages (See Figure 8e). The forwardly assembly ratio of the nanorod LEDs increased from 40.4% to 74.4% as the DC offset applied when assembling the nanorod LEDs was increased, and the ratio of unaligned nanorod LEDs was similar regardless of the assembly voltage. The orientation ratio of the aligned nanorod LEDs, excluding the unaligned nanorod LEDs, which is the ratio of the tail of the nanorod LEDs (the n-type GaN layer) connected to the ground of the electrode among the total aligned nanorod LEDs, reached 80.2% when using the pulsed DC voltage assembly process (See Figure 8f). This ratio is approximately twice as high as that when using the conventional AC voltage assembly process (43.6%), and this result is similar to the difference in the EL intensity between the +DC operating and the –DC operating cases. These panchromatic CL images confirm that the number density of forwardly aligned nanorod LEDs increases significantly with an increase in the offset


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voltage and that the EL intensity of the aligned nanorod LEDs increases with an increased degree of preferably oriented alignment under DC operation.

Conclusion In the conventional DEP process under an AC electric field, millions of 1D p/MQW/n InGaN/GaN nanorod LEDs are randomly aligned between two metal electrodes. The heads of 1D nanorod LEDs are half in contact with two different electrodes on interdigitated electrodes. Otherwise, nearly all heads of 1D nanorod LEDs are oriented toward one electrode when applying a pulsed DC field. The EL intensity levels of DC-operated 1D p/MQW/n InGaN nanorod LEDs assembled with a pulsed DC field are much greater than those of AC-operated 1D nanorod LEDs at all applied voltages as well as that of AC-operated 1D nanorod LEDs assembled with a conventional AC field. It can be considered that the +DC operation improves the EL intensity of 1D p/MQW/n nanorod LEDs induced by a pulsed DC field by 1.8 times compared to the AC-operated EL intensity of 1D nanorod LEDs induced by the same pulsed DC field. The significant EL enhancement under the DC operation by the pulsed DC aligned nanorod LEDs is due to the increased number density of forwardly aligned 1D nanorod LEDs between two opposite electrodes by the larger EP force and torque of the intrinsic dipole of the nanorod LEDs as compared to the DEP force, as verified in the panchromatic CL images presented in this study. The pulsed DC field assembly process of p/MQW/n InGaN nanorod LEDs can provide the potential to increase the EL efficiency and manage the complexity of the operating system using DC circuits instead of AC circuits. Therefore, the DC offset-AC field or pulsed DC field induced DEP assembly process provides a viable means of preferably assembling 1D nanorod LEDs with a relatively low aspect ratio and potentially represents an important step toward a wide range of


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DC-operated self-emissive surface LED applications, such as surface lighting, scalable lightings (micrometers to inches), formable surface lightings, polarized surface lighting, and other innovative bio-applications.


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ASSOCIATED CONTENT Supporting Information The schematic of fabrication of individually separated nanorod LEDs and SEM/EDX analyses of fabricated nanorod LED and bare GaN substrate are available via the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +82-2-910-4893. Fax: +82-2-910-4415. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT&Future Planning (MSIP) (No. 2016R1A5A1012966) and by the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20163010012570).


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Figure captions

Figure 1. Schematics of the fabrication of the InGaN-based green-emitting nanorod LED device: (a) InGaN nanorod arrays fabricated on a sapphire substrate using NSL and dry etching. The nanorod arrays were treated with KOH in order to remove defects caused by the dry etching process. (b) Individually separated nanorod LEDs from (a). (c) Individually separated nanorod LEDs were aligned on a pre-patterned electrode (0.6 × 0.7 cm2) using dielectrophoresis.


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Figure 2. Sinusoidal voltage for nanorod the LED assembly using dielectrophoresis: (a) V = 30 sin (conventional AC), (b) V = 25 sin + 5 , (c) V = 20 sin + 10 (DC offset AC), and (d) V = 15 sin + 15 (pulsed DC), where = 2π × 950,000 is the angular frequency. The red dash line is the additional DC offset to the sinusoidal voltage.


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Figure 3. Plot of the real part of the CM factor for the InGaN nanorod LED and the suspension in medium acetone as a function of the frequency. The dielectric constant and electrical conductivity of the nanorod LEDs and the medium are !3 = 12.2!G , H3 = 104 S/m, !" = 21.4!G , H" = 20 × 10.I S/m, respectively.27 The red dashed line indicates the frequency used when fabricating the nanorod LED device.


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Figure 4. Schematics of the orientation of nanorod LEDs when the LEDs were aligned onto the electrodes: (a) cross-sectional and (b) top-view images of the green nanorod LED device using an AC electric field for the nanorod LED alignment. The nanorod LEDs were randomly aligned, and (c) only approximately half of the aligned LEDs were turned on when DC voltage was applied. (d) cross-sectional and (e) top-view images of the green nanorod LED device using AC electric field with DC offset for alignment of the nanorod LEDs. Most of the nanorod LEDs were forwardly aligned due to ? by the intrinsic dipole,35 and (f) the aligned LEDs were mostly turned on when DC voltage was applied.


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Figure 5. Electroluminescent properties of the green-emitting nanorod LED device: Photographs of the EL emissions of 0.6 × 0.7 cm2 areal EL devices at a driving voltage of AC 21.0 Vrms at 60 Hz, DC +21 V (+DC), and –21.0 V(–DC) in the presence and absence of background light. The assembly voltages of the nanorod LEDs are (a) V = 30 sin (conventional AC), (b) V = 15 sin + 15 (pulsed DC), where = 2π × 950,000 is the angular frequency. (c) The relative EL intensity at a driving voltage of AC 21.0 Vrms at 60 Hz, DC +21.0 V, and –21.0 V pertaining to the variable assembly voltages of the nanorod LEDs, and (d) the relative EL spectra of green-emitting nanorod LED devices aligned with nanorod LEDs with pulsed DC voltage at AC, +DC, and –DC driving voltages.


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Figure 6. Optical microscopy image of nanorod LEDs aligned onto the pre-patterned electrodes by means of dielectrophoresis. The assembly voltage was (a) V = 30 sin (conventional AC), (b) V = 15 sin + 15 (pulsed DC), where = 2π × 950,000 is the angular frequency, and (c) V = 18.4 (conventional DC). (d) Conventional AC, pulsed DC, and conventional DC assembly voltages for the alignment of the nanorod LEDs.


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Figure 7. DC voltage-dependent variation of (a) the applied current and (b) the EL luminescence intensity of green-emitting nanorod LED devices using conventional AC voltage and pulsed DC voltage to align the nanorod LEDs.


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Figure 8. (a) SEM and (b) panchromatic CL images of a green LED device with individually separated nanorod LEDs aligned using conventional AC. (c) SEM and (d) panchromatic CL images of a green LED device with individually separated nanorod LEDs aligned using pulsed DC. (e) Statistical analysis of panchromatic CL images with the assembly ratio of nanorod LEDs by various assembly voltages. (f) Plot of the orientation ratio of the assembled nanorod LEDs of the green LED device at various assembly voltages.


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Table captions Diameter (nm)

Length (µm)

Aspect ratio

Assembly voltage

Ref.

400

1.0

2.5

AC 6 Vpp @ 1 MHz

16

500

2.5

5

AC 50 Vpp @ 950 kHz

14

40–80

1–1.5

12.5–37.5

AC 14 Vpp @ 1 MHz

34

210–470

11–19

23–90

AC 20 Vpp @ 75 kHz

35

200

8–20

40–100

AC 20 Vpp @ 70 kHz

36

50–300

Enhanced DC-Operated Electroluminescence of ... - ACS Publications

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