The field emission (FE) device based on quantum dot (QD) films as a cathodoluminescent (CL) material has not emerged yet due to the relatively low quantum efficiency and weak photostability of nanocrystals (NCs). Here we improve film stability and luminescence yields by preparing neat films of well-packed core–multishell QDs using spray coating method and then using low-temperature atomic layer deposition (ALD) to infill the pores of these films with metal oxides to produce inorganic nanocomposites. The ALD coatings to protect oxidation and degradation by electrons prevent internal atomic and molecular diffusion and decrease surface trap densities of QD films. Furthermore, the CL of the core-multishell QD films is 2.4 times higher than before ALD infilling. We fabricate the FE device by combining cathode structure with carbon nanotube (CNT) emitters and anode plates with QD thin film and successfully can get brilliant images of the light-emitting FE device. Our research opens a way for developing new quantum optoelectronics with high-performance.
© 2013 OSA
Thin films of semiconductor QDs represent a novel class of optoelectronic materials of great promise for use in a variety of applications, including light-emitting diodes (LEDs) [1–7], ultrasensitive photodetectors [8–10], solar cells [11–16], and field-effect transistors (TFTs) [17–19]. However, there are several challenges regarding the practical realization of quantum devices, because these QD solids are inherently metastable materials and have a poor environmental stability under real operation conditions, which limits their applicability for commercial products. Another useful property of QDs, cathodoluminescence, which are emitted from QDs when struck by an electron energy, have been studied using a scanning electron microscope (SEM) and observed by the CL detector equipped in SEM [20–22]. QDs have been demonstrated to be more resistant to electron irradiation than quantum wells and epilayers due to the three-dimensional confinement of carriers [23–25]. However, no one has attempted to develop an optoelectronic QD device that can emit visible CL by field emission due to the relatively low quantum efficiency and reliability.
In this study, we attempted to overcome the instability of NCs related to luminescent properties by preparing neat films of random-packed CdSe/CdS/ZnS core–multishell QDs using spray coating and then infilling the pores of these films with amorphous Al2O3 by low-temperature ALD (at 100 °C). ALD is a stepwise thin film growth method that provides excellent conformality onto a complex topography, has good reproducibility, allows for control over submonolayer thickness, produces a uniform layer over large areas, and requires relatively low growth temperatures [26–28]. ALD infilling with an inorganic matrix offers excellent passivation to the NCs because the presence of defects, impurities, or localized charges in the vicinity of a QD will strongly influence its optoelectronic performance, and also acts to suppress the degradation of the NCs by electron irradiation. We have found that an increase in photoluminescence (PL), quantum yields (QY), and CL of the QD film are dependent of ALD coating. We first fabricated a FE device using a QD thin film and found that these devices were suitable as luminescent materials in regards to the realization of display images that are visible by the naked eye. This novel technique provides many opportunities and may open a way for developing new quantum optoelectronics.
2. Experimental section
The 624 nm red-emitting CdSe/CdS/ZnS core-multishell QDs were purchased from QD Solutions Co. (Nanodot HE-serise). Trimethylaluninum (97%), and anhydrous solvents were purchased from Aldrich and used as received. Thin multi-walled carbon nanotubes (MWCNTs) were synthesized to a high grade for field emission (CMA-1340F, Hanwha Nanotech).
2.2 QD film deposition
QD films were prepared by spray coating onto indium tin oxide (ITO) glass substrates using a home-built spraying system. Briefly, 5 × 5 cm2 ITO glass substrates were cleaned by acetone sonication and an ethanol rinse and 5 mL of a 0.1 wt% QDs in hexane solution was sprayed on the prepared ITO glass over an area of 3 × 3 cm2. The spraying conditions, such as N pressure, nozzle diameter, temperature, and distance between substrate and nozzle, were optimized to achieve uniform deposition of QDs. We fabricated QD thin films with a thickness of 110 ± 15 nm.
2.3 ALD infilling
QD films were coated with 5, 10, and 20 nm of amorphous Al2O3 using an ALD system (PLUS200, QUROS). Deposition was performed at 100 °C using alternating pulses of trimethylaluminum and O2 (50 ms pulse times, 10 s purge times, 0.1 Torr operating pressure).
2.4 Fabrication of FE device
The FE devices were fabricated by combining the cathode structure with CNT emitters and anode plates with QD thin film. To prepare CNT emitters, an ITO glass with an active area of 1 × 1 cm2 was coated with a CNT paste by screen printing. The organic additives in the paste were removed by firing at 410 °C in air. Finally, physical surface treatment was performed using adhesive tape (810 tape, 3M) to vertically align the CNTs for FE. Anode plates with QD films after ALD treatment (10 nm deposition) were used without futher modification.
Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F30 S-Twin model (FEI) at 300 kV. QDs were dispersed in hexane and dropped on 300-mesh holey carbon grids to prepare the sample. Cross-sectional TEM samples of QD films were prepared using a dual beam focused ion beam system (NOVA200, FEI). Energy dispersive x-ray spectroscopy (EDX) investigations were also performed at various locations across the entire particle shown in the TEM image. Optical absorption, PL, and QY of the QD solution and QD films were measured using a UV–vis spectrophotometer (SD-1000, Scinco), a fluorometer (Fluorolog, Horiba Jobin Yvon), and an absolute QY measurement system (C-9920-02, Hamamatsu) at room temperature. CL measurements were performed using an electron gun (PSI Co. Ltd.). The measurements were carried out at a current of 1, and 2 μA, and accelerating voltages of 3 kV measured in the range 380 ~780 nm. The FE characteristics of QD films with ALD infilling were measured in a vacuum chamber at a pressure of low 10−7 Torr at room temperature. A gap of 500 μm was maintained between a QD-coated ITO anode and a CNT cathode. Square high voltage pulses with a 1/60 duty ratio and a 500 Hz frequency were applied to the anode while the cathode was grounded. The FE image on the anode plates with QD films was taken using a charge-coupled device (CCD) camera.
3. Results and discussion
Figure 1(a) shows the TEM images of core–multishell QDs (CdSe/CdS/ZnS). It is quite clear that QDs are almost evenly distributed, without agglomeration. A well-defined lattice structure of the QD particles were observed in the high-resolution TEM image (inset of Fig. 1(a)), and the average diameter of the core–multishell QDs was about 7.5 ± 0.8 nm as manually determined by TEM images. Figure 1(b) shows typical optical absorption spectra and the corresponding PL spectra of the QD solutions, which corresponds to a QY of 72%. The first absorption feature was observed at 610 nm, and the emission peaks was observed at 624 nm with a full width at half-maximum (FWHM) of 32 nm, indicating that they were highly monodispersed with a narrow size distribution, as was observed in the TEM analysis. The photograph in Fig. 1(c) shows the color of the QD films on ITO glass using spray method under daylight and at 352 nm ultraviolet (UV) irradiation. The strong PL emission and nearly pure red-color emission indicate that the QD films are the novel fluorescent materials. Figure 1(d) shows typical cross-sectional TEM image of QD films before ALD treatment. Based on the TEM analysis, the QD films were found to be random-packed and uniform over all areas with a thickness of 110 ± 15 nm. The clear lattice fringe image and internal surface porosity is shown in the inset of Fig. 1(d) with high resolution TEM image. The EDX probe position and elemental intensity are shown in Fig. 1(e), indicating that the core-multishell QDs (CdSe/CdS/ZnS) are composed of Cd, Se, Zn, and S.
QD films suffer from the destruction of the crystalline structure, degradation, and oxidation by heat when exposed to the accelerated electrons. These unwelcome changes are usually detrimental to the performance of QD devices because they decrease the luminescence yields, scramble doping profiles , introduce trap sites [29, 30] and alter the electronic confinement on the QD surface . To improve film stability and luminescence yields, we used QD films that had been infilled with amorphous alumina deposited by ALD of Al(CH3)3 and O2 at 100 °C as shown in Fig. 2(a) . The initial few nanometers of alumina cover the accessible internal surface of the QD film to make a three-dimensional QDs/Al2O3 inorganic structure. Further ALD deposition ultimately seals off the internal pore network and forms an overcoating layer of thin alumina on the external surface of the QD films. The ALD infilling inhibits atomic and molecular diffusion leading to QD ripening and sintering, and the capping layer acts as a gas diffusion barrier to prevent thermal-induced QD oxidation and degradation . Figure 2(b) shows cross-sectional TEM images of QD films after deposition of 10 nm of ALD alumina. These TEM images clearly show a stack of two layers, the QD films infilled with alumina with a thickness of 110 ± 15 nm and the overcoating layer (~10 nm thick) of thin alumina covering the top of the QD films. The TEM images verified that our ALD infilling process yielded conformal and continuous amorphous alumina layers on the QD films. Although the deposited Al2O3 layer was amorphous, the crystalline structure of the QDs was maintained. It was also possible that the QDs were more close-packed toward the base of the QD films. From the Al element EDX mapping of QD films after ALD treatment as shown in Fig. 2(c), the Al2O3 was present throughout the QD films and homogeneously deposited on the internal and external surface that was free of pinholes. Figure 2(d) shows the EDX profile of the QD layers infilled with ALD alumina at the P2 region. The presence of aluminum and oxygen at the P2 region shows that alumina filled the nanoscale pore network of the internal surface of the QD films.
Figure 3(a) shows the optical absorption spectra of the QD films before and after ALD treatment. The first exciton peak of the QD films was slightly red-shifted, from 610 to 613 nm, relative to QDs in solution because of the increased dielectric constant and substantial electronic coupling during film formation . However, the absence of any shift of optical absorption spectra of the QD films before and after ALD infilling suggests that the encased QDs were not affected by the deposition of the alumina coating. These results also confirmed that ALD infilling with alumina can inhibit QD surface diffusion, oxidation, and degradation without deleteriously affecting the electrooptical behavior of complete QD devices. Figures 3(b) and 3(c) show the PL emission spectra and QY of the QD films before and after ALD treatment. The PL intensity, the PL spectral maximum, and QY of QD films before ALD infilling were different from those of the QD in solution. The red shifts in the PL spectral maximum, from 624 to 633 nm, arise from ripening and fusing, and the reduced PL intensity and QY result mainly from surface oxidation and degradation during the QD film fabrication process. After ALD infilling, the PL spectral maximum did not change, which was expected based on the optical absorption spectra. On the other hand, the PL intensity and QY increased slightly to an asymptotic value with an increase in the ALD deposition thickness. As a plausible reason of small increase of PL but large increase of CL by ALD infilling, specific effects for protection of the serious damage of QD due to the electron beam can be considered. At a low ALD deposition thickness, poorly infilled domains were present within the very narrow pore networks of the QD films. The improved efficiency probably results from an exiton mobility enhancement via trap passivation. ALD infilling prevents exitons from being captured by the presence of surface defects that act as nonradiative recombination sites.
CL is typically used to evaluate luminant materials in FE devices and provides information about the excitation and deexcitation mechanism involved during FE device operation. Figure 4(a) shows a schematic illustration of the emission mechanism of QD and the energy transfer in the region of electron injection. When a QD is exposed to the high energy electron beam, excitons (electron/hole pairs) are generated near the surface within a relatively small radius due to the inelastic scattering of electrons. At this time, emission loss (trapping of the ejected electrons, nonradiative recombination, scattering, etc.) and heat loss result from dissipation of that excitation energy through vibrations and translations occur in a QD. Simultaneously, excitons are deexcited by energy transfer to other nearby, unexcited agents and then the CL emission from the QD can be generated. Figure 4(b) shows the effect of ALD deposition thickness on the CL intensity of the QD films at an accelerating voltage of 3 kV and current density of 1 μA•cm−2. The CL was nearly identical to the PL, with a small blueshift (~6 nm) and wider linewidths (~46 nm). After ALD infilling, the CL intensity increases 1.9 times for ALD infilling of 5 nm and 2.4 times for ALD infilling of 10, and 20 nm when compared with that of no ALD infilling. The increase in CL signal with ALD infilling was related its effects on the properties of the QD, such as diffusion, oxidation, and degradation. As a result, we attributed the effective energy transfer after alumina infilling to trap passivation, which was similar to the increased PL and QY results. Also, whereas a saturated CL intensity was observed for the QD films infilled with 10 and 20 nm, a relatively low CL intensity was observed for the QD films infilled with 5 nm. This was likely due to incomplete infilling of the narrow interstitial spaces for the 5 nm QD films due to the difficulties associated with the diffusion of ALD precursors. Upon increasing the current density, the CL peak intensity increased significantly as shown in Fig. 4(c). This result indicates that the enhanced CL signal at higher current density (2 μA•cm−2) is related with interaction with more electrons per unit time, which is associated with an increase in the probability of deexcitation in QD films. The results obtained in the CL experiments show that our ALD infilling method can produce QD solids with enhanced film stability and luminescence yields and holds great promise for use in developing QD-based optoelectronic devices.
A FE device is a display technology that uses a large-area field electron emitter to provide electrons that strike colored phosphor to produce a visual color image. In a general sense, a FE device consists of a matrix of cathode ray tubes and cathodoluminescent phosphors as an anode plate. The structure of the device used in this study is shown in Fig. 5(a) . We applied QD films infilled with alumina deposited by ALD (10 nm deposition) as a cathodoluminescent material on a FE device. Details of the preparation method are provided in the Experimental. The resulting as-fabricated red-emitting FE device, which combines the cathode structure with CNT emitters and anode plates with QD film, is presented in Fig. 5(b). The color of the materials after combining onto a CNT emitter was mild grayish-brown, which was due to the combination of the pale black color of CNT emitters and the light yellow-orange color of CdSe/CdS/ZnS QDs. Figure 5(c) shows images of the red-light emitting FE device operated at an accelerating voltage of 1.6 kV of and current density of 150 μA•cm−2. Although the present FE device emits only a single color, it would be possible to realize a full-color display in a mode that utilizes a QD film with a different emission wavelength. There are still significant challenges for further improving the device performance of FE, through the optimization of QD structures as well as the design of the device structure and fabrication method.
In summary, we have developed ALD infilling method with alumina to make QD-based luminant materials that performed better than the conventional QDs. The enhanced electrooptical properties of this system were attributed to the inhibition of surface diffusion, oxidation, and degradation of QD films. As the result, the CL of the core-multishell QD films was 2.4 times higher after ALD infilling. We applied QD films infilled with alumina deposited by ALD as a cathodoluminescent material in a FE device, A FE device using the QD films was successfully demonstrated for the first time. This technique offers many exciting opportunities in regards to fabricating high-performance nanomaterials for new quantum optoelectronics.
This work is supported by QD phosphorous LED project (MKE), Center for Advanced Soft Electronics of Global Frontier, and Korea University Grant in Korea. We thank TEM support of KBSI (Seoul Center).
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