Field Emission from Decorated Carbon Nanotube–QDs

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Field Emission from Decorated Carbon Nanotube−QDs Microstructures with a View to the Dominant Electron Paths Sharon Xiaodai Lim,†,‡ Sheh Lit Chang,‡ Fook Chiong Cheong,§,∥ Eng Soon Tok,‡ Zheng Zhang,⊥ Chwee Teck Lim,†,§,∥ and Chorng-Haur Sow*,†,‡ †

NUS Graduate School for Integrative Sciences & Engineering (NGS), Centre for Life Sciences (CeLS), #05-01, 28 Medical Drive, Singapore 117456 ‡ Department of Physics, Blk S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542 § Mechanobiology Institute, 5A Engineering Drive 1, Singapore 117411 ∥ Department of Mechanical Engineering & Department of Bioengineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576 ⊥ Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore S Supporting Information *

ABSTRACT: We present a study on the field emission properties of a hybrid system comprised of carbon nanotube (CNT) micropillars decorated with quantum dots (QDs). With controlled decoration of QDs on the CNT micropillars through a simple assisted self-assembly process, further enhancement in the field emitting property of the hybrid microstructures was detected. Upon irradiation of the hybrid structure with a broad visible-light laser beam, additional enhanced field emission was observed. Analyses using fluorescence and confocal microscopy, as well as ultraviolet photoelectron spectroscopy, suggested that electron transfer from QDs to the CNT strands and the reduced work function of the hybrid system as the contributing factors behind the enhanced field emissions. In addition, we discovered that the field emission process gave rise to lost of the QDs’ fluorescence luminosity on the microstructures in specific patterns attributable to transfers of charge carrier from QDs to the CNTs. This observation provided a new means to understand and to determine the predominant 3D path of the emission of electrons from the sample down to a micrometer scale level.

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semiconducting zinc oxide (ZnO) nanoparticles were coated onto the walls of CNTs. The presence of high local electric field around the ZnO particle on the walls of CNT increases the tunneling probability at the CNTs−ZnO heterojunction, which in turn enhances the field emission property for CNTs.6 With their size tunable band gap, semiconducting quantum dots (QDs) have emerged as potential light harvesters. To improve charge transport/collection efficiency of the charges created in QDs in the event of photoexcitation, researchers have developed various strategies to create hybrid materials with QDs. One such strategy involves the use of nanotube and/ or other one-dimensional (1D) architectures which had exhibited high efficiencies in improving charge transport within the several micrometer thick photoactive film.7,8 In an article by Farrow et al.,9 stacked-cup carbon nanotube network was used

arly experimental work has established the promise of carbon nanotubes (CNTs) for field emission.1 Indeed, CNTs have attracted the attention of many researchers due to its many advantages as cold field emitter. These advantages include the inertness and emission stability of the nanotube tips, low temperature, low threshold voltage for cold field emission, fast respond time, low power, and small size.2 Some researchers have tried to further improve this property by arranging pillar array of aligned CNT bundles with specific interpillar distance to pillar height ratio. In doing so, minimization of field-screening effect (which improves the field emission characteristics) caused by the proximity of neighboring CNTs was achieved.3,4 Others have also tried to enhance the field emitting property of CNTs through the creations of CNTs-based hybrid materials. One such hybrid material involves the fabrication of graphitic CNTs embedded with metallic gold nanoparticles. Field emission properties of this hybrid structure revealed a fairly low threshold voltage and a high enhancement factor.5 In a separate experiment, © 2013 American Chemical Society

Received: April 23, 2013 Revised: June 8, 2013 Published: June 11, 2013 14408

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MWNT films using a 660 nm focused laser beam. By adjusting the laser power, we were able to create MWNT structures of various height and thus creating 3D microstructures. Quantum Dots. Cadmium selenide (CdSe) core−shell quantum dots coated with zinc sulfide (ZnS) capping (CdSe/ ZnS QDs) were purchased from Evident Technologies. The size of QDs used was 4.0 nm (Orange). These QDs came in concentrations of 20.7 nmol/mL. The outer shells were covered with 16 hydrocarbon ligands. The ZnS capping helped to prevent quenching effect of uncoordinated atoms33 on the surfaces of CdSe nanocrystals and hence enhancing the photoluminescence (PL) property.34 All sizes of the QDs mentioned in this report corresponded to the diameter of the CdSe core and the ZnS shell was a few monolayers in thickness. These commercially purchased QDs were suspended in either chloroform or toluene, respectively, which were found to spread across CNTs rapidly. Creation of Hybrid QD−CNT Micropillars. To create the hybrid QD−CNT micropillars (sample II QDs), sample II was densified using toluene with suspension of the QDs. The QDs used were cadmium selenide (CdSe) core−shell quantum dots coated with zinc sulfide (ZnS) capping (CdSe/ZnS QDs). Upon the evaporation of the toluene, the QDs would be arranged along the CNT strands within the micropillar (as a result of capillary induced self-assembly). UPS Characterization. The samples were analyzed by VG ESCALAB 200i-XL X-ray photoelectron spectroscopy (XPS) system equipped with an ultraviolet gas discharge lamp as excitation sources. The electron analyzer was calibrated with polycrystalline gold, silver, and copper standard samples by setting the Au 4f7/2, Ag 3d5/2, and Cu 2p3/2 peaks at binding energies of 83.96 ± 0.02, 368.21 ± 0.02, and 932.62 ± 0.02 eV. The Fermi edge was calibrated with a polycrystalline Ni sample. The measurement resolution from this calibration is better than 0.2 eV. The unfiltered He I (21.2 eV) photons were used for work function measurements under a bias of −5 V. Field Emission Characterization. For the field emission (FE) effect, the measurements were conducted using a twoparallel plate arrangement in a vacuum chamber, at room temperature, with a base pressure of 1 × 10−6 Torr. FE measurements for each sample were conducted by securing the sample to a copper (Cu) substrate cathode using a Cu doublesided tape. In the setup, indium tin oxide (ITO) glass coated with a layer of phosphor acted as the anode. Depending on the sample, a choice of 100 and 200 μm thick polymer film with a square opening was employed as a spacer between the electrodes. The corresponding emission current was measured using a Keithley 237 high-voltage source measure unit (SMU). To conduct laser-enhanced field emission measurements, a broad beam laser was introduced onto the hybrid sample, through a viewing window of the vacuum chamber, while applying an electric field of 3.4 V/μm across the sample. Wavelengths of the lasers used were 405, 532, and 660 nm. The lasers were introduced with laser power of 17 mW at 5 s intervals. Further Characterizations. Further characterizations of the samples were carried out using a scanning electron microscope (SEM, JEOL JSM-6400F), a transmission electron microscope (TEM, JEOL JEM-2010F), a fluorescence microscope (FM, Olympus IX71S1F-3 inverted microscope), a confocal microscope (Nikon D-Eclipse C1) with 405 nm coherent laser, and a photoluminescence microscope (PLM, Rashaw 5161R-G with a Kimmon 1K Series He−Cd laser).

as conducting scaffolds for the anchoring of cadmium selenide, CdSe, quantum dots for light harvesting in a photoelectrochemical solar cell. As a 1D carbon base material with unique electronic and optical properties, CNTs could well serve as building blocks in light harvesting architectures.10,11 Herein, it would be exciting to determine if QD-CNT micropillar will exhibit any enhanced field emission property upon excitation with an external energy source. Wong et al. reported laser-enhanced electron emission from multiwalled CNTs by the irradiation of the second, third, and fourth harmonics of a Q-switched Nd:YAG pulse laser. While result from 266 nm excitation showed a one-photon field assisted photoemission mechanism at low laser energy, and a thermal contribution at higher energy, thermal assisted field emission was observed with excitation using 355 and 532 nm wavelengths.12 By incorporating QDs into patterned array of vertically aligned multiwalled CNTs, we aim to create a hybrid material that would show enhanced field emission under visible light excitation. Given the simplicity of the fabrication process as well as the energy efficiency involving the use of visible light excitation, this hybrid material could well serve as a cost-saving electron source. Possible applications of such laser-controlled electron source ranges from electron beam lithography to electron beam machining and vacuum electronics. In this work, a postsynthesis laser pruning technique13 was employed to fabricate an array of vertically aligned multiwalled CNTs into micropillars. The patterned CNT arrays were found to exhibit enhanced field emitting properties. Through selfassembly of QDs onto the CNTs micropillars, further enhancement in the field emitting property of the hybrid microstructures was detected. As the self-assembly process is solution-based, elastocapillary deformations of the CNT strands were observed. By applying external broad beam visible-light irradiation from a laser onto the hybrid microstructures, further laser-enhanced field emission was observed. Analyses using fluorescence and confocal microscopy, as well as ultraviolet photoelectron spectroscopy, hinted at possible electron transfer from QDs to the CNT strands as the reason behind this phenomenon. Moreover, we chanced upon an interesting and yet unexpected observation that field emission gave rise to lost of QD’s fluorescence luminosity on the microstructures. Remarkably, the “bleached” QDs on the microstructure exhibit unique and specific patterns depending on the shape and dimensions of the micropillars. Careful studies show that the QDs are not destroyed, and this observation is attributed to transfer of charge carriers from QDs to the CNTs in these “bleached” regions. One can adopt such approach to map out the predominant 3D path of the emission of electrons from the sample down to a micrometer scale level in the event of field emission.

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EXPERIMENTAL METHODS MWNTs and Laser Pruning. Aligned multiwalled CNTs (MWNTs) with typical length of 30−40 μm were grown on clean n-type silicon (3 mm by 5 mm, (100) Si) substrates containing native oxide layer. Before growth, a layer of iron film was coated on the substrates as catalyst using a magnetron sputtering system (Model: RF Magnetron Denton Discovery 18). The coating rate was 4 nm/min lasting for 3.25 min. These MWNTs were synthesized using a plasma enhanced chemical vapor deposition (PECVD), and details of the growth process were reported elsewhere.30−32 With the laser pruning technique,13 channels and patterns were created on the 14409

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The overall dimension of the array was maintained at 1000 × 1000 μm2 while varying the number of micropillars within the array. In other words, the total number of 20 × 20 μm2 micropillars was 441 (i.e., 21 × 21 array) while that of 100 × 100 μm2 micropillars was 64 (i.e., 8 × 8 array). Field emission measurements of the samples were conducted using a parallel plate system.14 Figure 2a shows current density (J) versus the applied field (E) detected from these samples. In general, patterned CNTs exhibit much better field emission performance as compared to the unpatterned (as-grown) counterpart. Field emission enhancement due to such patterning effect shows best result from the CNTs array that was patterned into 40 × 40 μm2 square micropillars (i.e., sample III). Such improvement was followed by samples II, I, IV, and V. In general, two sets of experiments were conducted to address the issue of reproducibility of these results, and we found that the observations were reproducible. The enhancement factors of these samples were determined based on the Fowler−Nordheim (F−N) equation:

RESULTS AND DISCUSSION Pillared Array of CNTs for Field Emission Enhancement. In this work, CNTs samples comprising of array of micropillars were fabricated by postsynthesis laser pruning of aligned CNTs forest using a focused laser beam.13 By moving the CNT forest with respect to a fixed laser beam (Figure 1a), square micropillars of different dimensions were crafted from the CNT forest.

J=

⎛ Bϕ3/2 ⎞ AF 2 ⎟ exp⎜ − F ⎠ ϕ ⎝

(1) −1

where F (= βE) is local electric field (V cm ), β is the enhancement factor, ϕ is the work function (eV), and J (= Iγ−1) is current density, where I is the total current from the measurement and γ is the effective area of emission. A and B are constants with respective values being 1.4 × 10−6 A eV/V2 and 6.44 × 107 V eV−3/2/cm. By substituting the variables and rearranging eq 1, we arrive at the following relationship:

Figure 1. (a) Schematic of the laser patterning process to craft periodic array of square micropillars from the CNT forest. (b−d, h−j) Top view and (e−g, k−m) tilted 40° SEM images of the arrays of micropillars created within the 1000 × 1000 μm2 boundary. The dimensions of the micropillars changes from being unpatterned to 20 × 20, 30 × 30, 40 × 40, 50 × 50, and 100 × 100 μm2 square pillars.

⎛ Aγβ 2 ⎞ ⎛ I ⎞ ⎛ Bϕ3/2 ⎞ 1 ⎟ + ln⎜ ln⎜ 2 ⎟ = ⎜ − ⎟ ⎝E ⎠ ⎝ β ⎠E ⎝ ϕ ⎠

(2)

2

Plotting ln(I/E ) with respect to 1/E (termed as a FN plot) will allow information such as the enhancement factor or work function of the material to be obtained from the gradient of graph, while the intercept of the graph will generate the effective emission area after the enhancement factor has been determined. Figure 2b shows respective FN plots of the patterned CNT samples. Linearity of the FN plot is used as an indication that the emission current originates from the quantum tunneling effect.15 While the FN plots in the Figure 2b showed linear fit at higher electric field, deviations at lower electric field was observed. Such nonlinearity of the FN plots were frequently related to issues such as different enhancement factors within

Typically the height of the CNTs is 40 μm. Arrays of micropillars with different cross-sectional dimensions of 20 × 20 μm2, 30 × 30 μm2, 40 × 40 μm2, 50 × 50 μm2, and 100 × 100 μm2 were fabricated (Figure 1b−m). In this work, we shall denote these samples as sample I, sample II, sample III, sample IV, and sample V, respectively. To ensure similarity in terms of the quality as well as density of the CNTs, CNT samples used in comparison of the data shown within each individual figure were broken from a single larger piece of CNT sample. However, due to limitation in the size of the larger piece of CNT sample, different sets of samples were used to collect data shown across different figures presented in this article.

Figure 2. (a) Field emission measurements of the as-grown and patterned CNTs obtained at room temperature (inset is a fluorescence image obtained during field emission from a different sample with similar structures and dimensions as sample II) and (b) is the corresponding FN plots. (c) UPS data showing the work function of the CNTs at 4.9 eV as obtained by extrapolating the cut off point to the horizontal axis (inset). 14410

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emitters,16 space charge effect,17 and nonuniform local field near the tip of CNTs.18 Inset of Figure 2a shows fluorescence image obtained from a different sample with similar dimension as sample II. Under the same amount of applied field, regions with different degrees of illumination were observed. As such, the main contributing factor to the nonlinearity in our case is believed to be a consequence of different enhancement factors within the array of CNT emitters. Figure 2c shows an ultraviolet photoelectron (UPS) spectrum obtained for the CNTs. The work function of CNT was determined using the following equation:

With this as a basis, densification of the CNT square micropillars was carried out using the method illustrated in Figure 3a. A droplet (3 μL) of toluene was first deposited onto a piece of precleaned cover glass slip. Subsequently, the sample was submerged perpendicularly into the droplet. The sample was held in place for ∼10 s before it was removed and allowed to dry in ambient conditions. A longer period of submerging could result in the collapsing of the micropillars, which could be attributed to the pulling effect from the thicker solvent layer encompassing all the micropillars as the solvent evaporates. During this period of time, high affinity of toluene to both the silicon substrate as well as the CNTs allows it to be infiltrated from the silicon substrate into the square pillars via capillary forces. As toluene evaporates, the CNTs would be densified by elastocapillary action. van der Waals force or mechanical interlocking maintains the eventual outcome of such densification effect to the micropillars. Figure 3b shows a SEM image (taken at 40° tilt) of the CNT micropillars before densification. Inset shows the internanotubes distance of the micropillar. Densification of the same sample (Figure 3c) resulted in the formation of an hourglass-shaped micropillar with 2-fold reduction in internanotube spacing (inset of Figure 3c) at the constricted area. Mechanism for the Formation of Hourglass-Shaped Densified CNT Micropillars. The basic mechanism behind the densification of the CNT micropillar can be understood by considering the collective deformations and motions of the CNTs, due to both internal and external capillary forces on the CNT micropillar.21 In general, the mechanisms behind the formation of such hourglass-shaped microstructures will be divided into three sections: the base, the constriction, and the top (Figure 4a). At the base of the CNT micropillar after the densification process, the CNT strands were observed to adhere to the substrate by van der Waals forces before bending at a small

ϕ = hv − (binding energy of cutoff point − Fermi edge binding energy)

(3)

The Fermi edge binding energy was calibrated with a polycrystalline nickel sample and is already accounted for in all the UPS spectra shown in this report. This means that the work function would be equivalent to the cutoff point of the spectra in a plot of counts against kinetic energy. As such, we would be able to obtain the work function of CNT by extrapolating the cutoff point of the spectra obtained from the UPS spectrum as shown in Figure 2c. The inset of Figure 2c shows an enlarged view of the cutoff point. From the inset, the work function of CNTs was found to be 4.9 ± 0.1 eV. This value is similar to that reported by Shiraishi et al.19 Substituting the respective values into eq 2, together with the gradient obtained from the FN plot, shows that the enhancement factor from sample III was found to be 6239 ± 638. With the interpillar distance (R) to pillar height (h) (Figure 3b) ratio of sample III found to be 1.91 ± 0.03, this result is in good agreement with reported optimal ratio of 2.4,20

Figure 3. (a) Schematic of the densification process of the square micropillars. (b, c) Tilted 40° SEM images of sample II (b) before and (c) after densification with toluene. Insets show internanotube spacing of the CNTs from the respective micropillars. Labels R and h in (b) represent interpillar distance and pillar height.

Densified Pillared Array of CNTs. In the process of creating QDs−CNT hybrid microstructures, the fluid assisted assembly technique will be utilized for the assembly of QDs onto the CNT microstructures. This approach would inevitably lead to elastocapillary deformation of the CNTs into tight bundles (termed as densification). As such, it is necessary to carry out systematic study on the field emission properties of such densified CNT microstructures in the absence of the QDs.

Figure 4. SEM images (a) densified sample II obtained at 40° tilt. (b, c, f−h) Patterned and (d, e, i−k) densified micropillars of different dimensions as indicated in the images. 14411

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Figure 5. (a) Field emission measurements of the densified CNTs obtained at room temperature and (b) shows the corresponding FN plots. (c−f) COMSOL finite element analysis of the equipotential lines of the electrostatic field of the densified CNT micropillar (c) sample I, (d) sample II, (e) sample III, and (f) sample IV under an external electric potential of 400 V.

that the densified CNTs at the top of the micropillars are not perpendicular to the substrate. From the data presented, most notably densified sample II emerged as a superior field emitter with high emission current density and lowest turn-on field (corresponding to 10 μA/cm2) at 2.9 V/μm. From the FN plot (Figure 5b), the enhancement factor of densified sample II was determined to be 1997 ± 260. This result is 2.5 times better than that produced by the densified sample I. To elucidate how the different structures formed as a consequence of the densification process can exhibit different field emission effect, finite element analysis from the COMSOL Multiphysics electrostatic module software was used to simulate the influence of densification on the field emission effect of these CNT micropillars. Details of simulation setups are provided in the Supporting Information. In this analysis, the R/h ratio was not taken into consideration as the shape of the densified micropillars were not accounted for in the reported calculations by Nilsson et al.4 Results from the simulations are presented in Figure 5c−f. The simulations show the equipotential lines of the electrostatic field with the background color indicating the magnitude of the local electric field. These results were simulated under an external electric potential of 400 V. From these images, densified sample II produced the largest magnitude of electric field, followed by those from the densified samples III, IV, and I. The simulations excluded value from densified sample V due to folding from the top layer of the micropillar (Figure 4k). These results are in good agreement with the experimental results presented in Figure 5a,b. Since densified sample II emerged as the best field emitter, this became the dimension that was implemented in the creation of QDs infiltrated CNT hybrid micropillar. Quantum Dots Functionalized Densified CNT Micropillars. In the creation of the QDs−CNT hybrid microstructures, core−shell type QDs were utilized. Given that QDs without protective capping have insufficient long-term stability due to possible surface damage/oxidation, the use of core−shell type QDs, where the protective shell serves to protect the core

radius to form the remaining micropillar. Previous studies have shown that such buckling effect of the CNTs is in accordance with a classical Euler model, and these CNT strands are able to withstand large strains associated with the small radius of curvature.22 At the constriction, as toluene infiltrates into the CNT micropillar, a meniscus was formed around the boundary of the micropillar. Within the micropillar, surface tension would allow toluene to seep between the CNTs in a similar manner as those observed in previous work involving fluid induced aggregations of wet hair23 and microfibers.24 This internal capillary force would initiate elastocapillary coalescence of the CNT strands as the microstructure attempts to balance the capillary force exerted by the liquid in the spaces among the CNT strands with the elasticity of the CNT strands. As a result, the CNTs would move toward the centroid of the microstructure so as to minimize their overall elastic energy25,26 as toluene evaporates. Instead of packing together, the top portions of the CNT strands were observed to fan out, maintaining wider internanotube spacing compared to those at the constriction. This is due to the presence of tangled morphology at the top of the CNTs array, which forms as the CNTs self-organized at the start of the growth process.27 As a result of such morphology, lateral movements of the CNTs during densification were restricted, hence causing the top of the CNT strands to maintain wider internanotube spacing. The width of the CNT micropillar was varied at 20, 30, 40, 50, and 100 μm (Figure 4b,c,f−h). After densification, the micropillars exhibit different degree of bending at the constriction as seen in Figure 4d,e,i−k. Field Emission Effect of Densified CNT Micropillars. Field emission measurements from the densified samples are presented in Figure 5a. In general, the undensified micropillars appear to be better electron emitters in comparison to the densified counterpart. When the CNTs are densely packed, the screening effect from the CNTs would be very strong and thus local field may not be as high as undensified CNTs. Another possible explanation for the observed difference in the field emission performances of the two types of samples could be 14412

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Figure 6. (a) Fluorescence microscope (FM) image of sample II−QDs. Inset shows a typical photoluminescence (PL) spectrum obtained from one of the QD−CNT micropillar. (b) SEM and (c) FM of toppled, upright, and slightly off balanced QD−CNT micropillars taken from the same sample. (d) PL spectra obtained from the top, center, and base of the toppled QD−CNT micropillar indicated in (c). (e) Schematic of the proposed distributions of QDs within a QD−CNT micropillar.

QDs from undesirable oxidative deterioration, has emerged as an alternative. Detailed studies of the photoinduced charge transfer processes of the capped QDs have recently been reported by Makhal et al.28 Their work revealed the importance of thermalization of the excitons as criteria for electron transport. In addition, the rate of electron migration from the core (cadmium selenide, CdSe) through the zinc sulfide (ZnS) shell of the QDs to another indium tin oxide (TiO2) nanoparticle was found to be comparable to those reported for the bare CdSe QDs. As such core−shell CdSe/ZnS QDs were utilized in our work for the fabrication of QDs−CNT hybrid micropillars. Photoluminescence Spectroscopy and Fluorescence Microscopy Analysis. Figure 6a shows a fluorescence microscope (FM) image of the hybrid QD−CNT micropillars (sample II−QDs). The inset is a typical photoluminescence (PL) graph obtained from one of such micropillar. The peak position was located at 592.67 ± 0.02 nm, corresponding to the wavelength for orange emission. Detailed analysis of QDs distributions along the micropillar was conducted by toppling one of the QD−CNT micropillar. Figure 6b,c shows SEM and FM images taken from the same sample. The images comprise of toppled, upright, and slightly off balanced QD−CNT micropillars. From the toppled QD− CNT micropillars, PL spectra were (Figure 6d) obtained at the top, center, and base of the micropillar (as indicated in Figure 6c). The relative intensity of the peaks obtained at the base (Ibase) to that at the center (Icenter) of the micropillar was found to be ∼2, while that of the peaks obtained at the top (Itop) to Icenter was ∼3. Evidently, within a single micropillar, highest QDs density was observed at the top, followed by significant accumulation too at the base of the micropillar. This left the least amount of QDs to be allocated at the center of the micropillar (Figure 6e). The presence of tangled morphology at the top of the CNTs array restricted lateral movements of the CNTs during densification. The wider internanotube spacing provided more space for QDs to be packed between and around the CNT strands. Along the center of the micropillar, the CNTs are more closely packed and less intertwined. Hence, limiting the QDs to be packed only along the length of the CNTs. At the base of the micropillar, adhesion of the CNT strands to the substrate allows wider internanotube spacing to be preserved, hence providing more space for the QDs to be trapped between the CNT strands. However, the close contact between the CNT strands and the substrate prevented the QDs from packing around the CNTs. This resulted in the relatively

weaker PL signal in comparison to that obtained at the top of the micropillar. Transmission Electron Microscopy Analysis. Attachments of QDs to CNTs were verified using transmission electron microscopy (TEM) by scratching some of sample II− QDs onto copper (Cu) TEM grid. Figure 7a shows a TEM

Figure 7. (a) TEM image of QDs on CNTs. (b) HRTEM image of the region enclosed within the dotted box in (a). (c) EDX spectrum obtained from (b).

image of QDs on CNT strands. Average size of these QDs was 3.8 ± 0.5 nm. A higher resolution TEM (HRTEM) image of the area enclosed within the white box is presented in Figure 7b. From this image, nanoparticles with 3.49 Å lattice spacing were determined to be cadmium selenide (CdSe) nanoparticles with a cubic [111] orientation. The zinc sulfide (ZnS) capping of the QDs was not visible in the HRTEM image as it was only 1 or 2 monolayer in thickness. Through energy-dispersive X-ray (EDX) spectroscopy (Figure 7c) taken from the spot enclosed within the white dotted box in Figure 7a, trace elements of Zn, S, Cd, and Se were detected. The presence of Cu in the spectrum originated from the Cu TEM grid, while the element iron (Fe) came from the catalyst used to synthesize the CNTs. Evidences from both TEM imaging and EDX spectroscopy hence supported the claim that QDs had been successfully anchored onto the CNT strands. Hybrid Quantum Dots−CNT Micropillars for LaserEnhanced Field Emission Study. The field emission effect from sample II−QDs was measured and compared to that of densified sample II (Figure 8a). With a higher current density, sample II−QDs was a more superior field emitter than densified sample II. At an applied field of 2.8 V/μm, the current density supported by sample II−QDs is 2080 times 14413

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Figure 8. (a) Field emission measurements of the QD−CNT and densified CNT micropillars obtained in room temperature. Inset is the corresponding FN plots. (b) UPS data showing the work function of QDs, QD−CNT hybrid, and CNTs at 2.0, 3.9, and 4.9 eV as obtained by extrapolating the cutoff point to the horizontal axis (inset). (c) An energy band diagram (not to scale) illustrating the laser-enhanced field emission effect.

Figure 9. (a) Current density versus time graphs obtained from the QD−CNT micropillars with external excitations from laser beams of different wavelengths. (b) Power-dependent study of the QD−CNT micropillars using 405 nm laser beam excitation.

to the MWCNTs. During the process of field emission, the migrated electrons would further enhance the field emission effect from the hybrid CNT micropillars. Moreover, with the reduction in work function, these electrons would be able to tunnel through the potential barrier at the lower applied field. Figure 8c shows an illustration of the proposed band diagram for the hybrid system as it under goes laser enhanced field emission. Given the reduction in work function of the hybrid system, illuminating the sample with 405 nm laser would lead to the excitation of the electrons within the hybrid system with 3.1 eV of photon energy, E450 nm. With the bending of the potential barrier under an applied electric field, the excited electrons would find it relatively easier to tunnel through the potential barrier in comparison to the electrons that were being excited by a 660 nm laser (E660 nm = 1.9 eV). The results of studies on laser-enhanced field emission effects from sample II−QDs are presented in Figure 9a−d. From the changes in current density against time (ΔJ−t) graphs presented in Figure 9a, highest degree of improvements in electron emission was obtained by exciting the sample with a 405 nm laser beam. This is then followed by excitation with a 532 and 660 nm laser beam. Fitting the data to a square-wave function yields ∼2 mA/cm2 improvement in electron emissions upon excitation with the 660 nm laser. This comparison is taken in a dark environment and with respect to the electron emission when the laser was blocked off. Changing the excitation source to 532 nm resulted in ∼3 mA/cm 2 improvements in electron emission. Largest improvements (∼7 mA/cm2) in electron emission came from excitation by a 405 nm laser beam. Given the wavelength-dependent laser-

higher than densified sample II at the same field. Furthermore, the turn-on field (corresponding to 10 μA/cm2) from sample II−QDs was 1.3 V/μm, which was much lower than that obtained from densified sample II (2.4 V/μm). This value was also comparable to that reported on hybrid CNT/TiO2 nanoparticles composite film29 (turn-on was 1.3 V/μm). With an enhancement factor of 3014 ± 258, sample II−QDs exhibit much better field emission in comparison to densified sample II, whose enhancement factor was determined to be 1193 ± 108. Given the ease of exciting the QDs using a photon source, this hybrid structure offers the potential of further field emission enhancement upon irradiation with a laser beam. To understand how such hybrid microstructures could contribute to the enhancement of field emission measured, UPS analyses were conducted on three samples: QDs, QD− CNT hybrid, and CNTs (Figure 8b). By extrapolating the cutoff point to the horizontal axis (inset), work functions of QDs, QD−CNT hybrid, and CNTs were determined to be 2.0 ± 0.2, 3.9 ± 0.2, and 4.9 ± 0.1 eV, respectively. By assembling QDs onto the CNT strands, we observed a reduction in work function of CNTs by 1.0 eV from 4.9 to 3.9 eV. Similar lowering of CNTs work function had been reported by Kim et al.,5 where from their UPS analysis embedding of gold nanoparticles appears to reduce the work function of the CNTs, attributable to electron transfer from the nanoparticles. This was thought to be responsible for lowering of the threshold voltage for field emission. By combining multiwalled CNTs (MWCNTs) (which are metallic in nature) with semiconducting CdSe/ZnS QDs, photoinduced electrons are expected to migrate from the QDs 14414

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enhanced field emission effect, as well as the rapid rise in the graphs shown in Figure 9a, it can be justified that the electrons detected under these excitations were not thermionic electron emissions, and the results correspond to the proposal illustrated in Figure 8c. Since excitation with 405 nm laser beam produces the best result, a power-dependent study was conducted using this wavelength (Figure 9b). In this study, laser powers of 9 and 17 mW were used. In the plot, we have also included a graph obtained from densified sample II under periodic irradiation (5 s) with a 405 nm laser beam at 17 mW laser power. Fitting to a square-wave function, the change in current density from sample II−QDs was found to be 2 and 7 mA/cm2 compared to the change in current density without any laser irradiation. In contrast, no significant laser enhancement from densified sample II was detected. Thus, the power of laser irradiation contributes favorably to the eventual current density of sample II−QD structures. Laser-Enhanced Field Emission Initiated Patterning on Quantum Dots Hybridized CNT Micropillars. To understand the effect of field emission on the luminescence effect of the QDs on CNT strands, FM imaging and PL analysis were conducted on sample II−QDs after the field emission experiments. From the sample, an unexpected but yet exciting phenomenon was observed. Figure 10a shows the PL spectrum obtained from the same QD-CNT micropillar before and after field emission study. The peak position of the PL graphs lies approximately at 592 nm before (592.8 ± 0.02 nm) and after (592.4 ± 0.03 nm) field emission. The relative peak intensity shows 6-fold reduction after the sample has undergone field emission. Insets are the respective FM images of the micropillar. FM images of the entire sample before and after field emission are shown in Figure 10b,c. Results from the PL spectrum show that the field emission process has diminished the luminescence property of the QDs, which could be attributed to either reduction in electron−hole recombination in the QDs or through physical burning/destruction of the QDs. Higher magnification of the area enclosed within the dotted blue rectangle in Figure 10c is presented in Figure 10d. From this image, an interesting phenomenon was observeddark spots on specific sites were observed on some micropillars. The appearance of dark spots on selective micropillars could be attributed to higher local current distribution in this region. Figure 10e shows higher magnification of one of such micropillar enclosed within the dotted dark blue square in Figure 10d. From this image, five dark spots (diameter: ∼4 μm) forming a cross on the top surface of the micropillar were observed. The presence of these dark spots indicated complete lost of luminescence of the QDs. Figures 10f and 10g show the top view SEM image and EDX map (element Se) of the same pillar shown in Figure 10e. From Figure 10f, there is no visible burning/destructions on the QD−CNT micropillar. With the well-distributed white dots in Figure 10g representing elements of Se detected from the QD−CNT micropillar, it meant that the QDs were not destroyed at the dark spots. Hence, attributing migrations of electrons from the QDs to the CNT strands as the reason behind the lost of luminescence of the QDs and, as a result, further enhanced the field emission effect. In an attempt to understand if such selective contributions of electrons from the QDs to the CNT strands merely occurs at the top surface of the QD−CNT micropillar, confocal microscopy analysis was conducted on the sample. Figure 11

Figure 10. (a) PL spectrum of the same QD−CNT micropillar before and after field emission study. Insets are the respective FM images. (b, c) FM images of the entire samples (b) before and (c) after the field emission sample. (d) Higher magnification of the area enclosed within the dotted blue rectangle in (c). (e) Higher magnification of the isolated QD−CNT micropillar enclosed in the dotted dark blue square in (d). (f) SEM image and (g) EDX map (element Se) of the QD− CNT micropillar shown in (e).

Figure 11. Confocal microscopy imaging of individual QD−CNT micropillar (a) before and (b) after undergoing field emission. Inset in (b) shows a SEM image of the micropillar used for imaging in (b).

shows multisliced confocal images of a QD−CNT micropillar before and after the pillar has undergone field emission. These micropillars were obtained from two different samples that had undergone similar patterning and assembly process. The reason for using two different samples was attributed to photobleaching of QDs after prolong laser excitation during confocal imaging. Such bleaching effect was found to have significant 14415

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negative impact on the laser-enhanced field emission properties of the QD−CNT micropillars. Figure 11a shows top surface of a QD−CNT micropillar as well as two multisliced images along the XZ-plane and the ZYplane, through the center of the micropillar. From the multisliced images, distributions of the QDs within the micropillar appear to be rather uniform. After field emission study (Figure 11b), black dots were observed at the top surface of the micropillar. These black dots appeared along both the Xaxis (enclosed within the white dotted rectangle) and Y-axis (enclosed within the pink dotted rectangle), as well as at the center of the micropillar. Sliced image of the micropillar along the XZ-plane through the center of the micropillar shows significant lost of illumination from the QDs within the micropillar. In addition, three distinct black paths (indicated by the white arrows) running through the entire length of the micropillar were observed. Locations of these black paths corresponded to where the black spot were situated (enclosed within the white dotted rectangle). Taking a slice in the ZYplane revealed similar results. The inset of Figure 11b shows no observable destruction to the QD−CNT micropillar after field emission measurements had been carried out. While confocal microscopy analysis proves that such a phenomenon was not limited to just surface effect, the reason behind such site selective contributions of electrons from the QDs to the CNT strands was still unclear. As such, controlled removal of CNT strands was carried out. By removing the top crust of the QD−CNT micropillar, one would be able to see how the CNT strands were packed together within the micropillar. Two individual QD−CNT micropillars were selected from the same sample. Figure 12a,c shows the side

significantly lower than the surrounding area. This facilitates the flow of electrons and hence promoting local migrations of electrons from the QDs to the CNT strands within this region. As such, QDs located within the cross region would lose their luminosity earlier than the surrounding QDs. This phenomenon thus offers a means for us to understand and to determine the predominant 3D path of the emission of electrons from the sample down to a micrometer scale level. To prove this, we altered the cross section of the micropillar from square to rectangle. In doing so, the CNT strands within the rectangular wall would be packed differently. As a result, we would expect the black dots to be located in a different manner as those from the square-shaped micropillar. Using a similar method as those applied to the micropillar, FM (right inset of Figure 12e) shows two distinct black dots located along the horizontal bisector of the QD−CNT microwall. At the same time, the SEM image of the microwall shows no sign of destructions to the CNT strands where the black spots are located (left inset of Figure 12e). Confocal imaging reveals similar lost of luminosity of QDs located at the black dots across the entire height of the microwall (Figure 12e) as those presented in Figure 11b.

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CONCLUSIONS In summary, the patterning array of CNTs into micropillars has been determined to enhance the field emitting properties of the CNTs array. With successful creations of hybrid QD−CNT micropillars through a simple assisted self-assembly process, further enhancement in the field emitting property of the hybrid microstructures was detected. Analyses using fluorescence and confocal microscopy, as well as ultraviolet photoelectron spectroscopy, hinted at electron transfer from QDs to the CNT strands as the reason behind this phenomenon. In addition, evidence of lost of the QDs luminosity on the microstructures in specific patterns provided a new means for us to understand and to determine the path of the emission of electrons from the sample down to a micrometer scale level.

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ASSOCIATED CONTENT

S Supporting Information *

Finite element method simulation of densified CNT micropillars and analysis of internanotube spacing for densified QD− CNT micropillar. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 12. SEM image of the side (a, b) and top profile (c, d) of QD− CNT micropillars without and with laser patterning. (e) Confocal imaging of QD−CNT microwalls. Insets are SEM (left) and FM (right) images of the same microwall.

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AUTHOR INFORMATION

Corresponding Author

and top profile of the micropillar without undergoing any laser pruning. Using a focused laser beam with 24 mW power, slight trimming of the crust at the top of the micropillar was carried out (Figure 12b). A cross, as defined by the blue dotted lines, was sighted from the top profile of the same micropillar (Figure 12d). Detailed analysis using ImageJ shows internanotube spacing at region I, II, and III to be approximately 171, 41, and 135 nm, respectively (Supporting Information, Figure S2). Based on the above measurements, the cross region was made up of CNT strands that were packed more closely together as compared to those at region I and III. Overlapping Figure 10e with Figure 12d, the cross region corresponds to the locations of the black spots. On the basis of this result, we propose that capillary-induced packing of the CNTs gave rise to highly packed CNT in the region marked by the blue cross in Figure 12d. In this manner, resistance of the cross region would be

*Phone (+65) 65162957; Fax (+65) 67776126; e-mail [email protected] (C.-H.S.). Notes

The authors declare no competing financial interest.

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