Name: enhanced electron injection and exciton confinement for pure blue quantum-dot light-emitting diodes by introducing partially oxidized aluminum cathode
Uploaded: 2018-05-31T15:00:00.000+00:00
Duration: 10 min 41 s
Description: Watch this Scientific Journal Video about Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode at JoVE

This work was supported by the NSFC (51573042), The National Key Basic Research Program of China (973 project, 2015CB932201), Fundamental Research Funds for the Central Universities, China (JB2015RCJ02, 2016YQ06, 2016MS50, 2016XS47).

1. Pattern Etching of Indium Tin Oxide (ITO) Glass
<ol>
	<li>Cut large pieces of ITO glass (12 cm &#215; 12 cm) into 15 mm wide strips. Clean the ITO glass surface using a dust-free cloth with alcohol.</li>
	<li>Check the conductive side of the ITO glass with a digital multimeter. Cover the active area of the ITO glass with adhesive tape, so that the active area is 2 mm wide in the middle.</li>
	<li>Pour the zinc powder on the ITO glass (to a thickness of about 0.5 mm).</li>
	<li>Pour the hydrochloric acid solution (36 wt%) onto the surface of the ITO glass and allow the ITO glass to completely soak in hydrochloric acid solution, then etch for 15 s.</li>
	<li>Pour out the hydrochloric acid solution after corrosion, then rinse the ITO glass with water immediately. Remove the adhesive tape on the ITO glass.</li>
</ol>
2. Cleaning the ITO Glass
<ol>
	<li>Immerse the ITO glass in a petri dish containing acetone solution for 15 min. Wipe off the glue on the ITO glass using a cotton ball.</li>
	<li>Cut the ITO glass into square pieces (approximately 15 mm &#215; 15 mm) with a glass cutter.</li>
	<li>Sonicate the ITO glass sequentially in detergent water, tap water, deionized water, acetone, and isopropyl alcohol for 15 min.</li>
	<li>Blow-dry the ITO glass with nitrogen gas followed by drying in an oven at 150 &#176;C for 5 min in air atmosphere.</li>
	<li>Put the cleaned ITO glass into an ultraviolet-ozone chamber and treat for 15 min.</li>
</ol>
3. Fabrication of a Hole Injection Layer
<ol>
	<li>Take out the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) solution from the refrigerator. Stir the PEDOT:PSS solution at room temperature for 20 min to obtain an evenly dispersed solution. 
	NOTE: The UV-Ozone treatment (Section 2.5) and agitation of PEDOT:PSS solution should be carried out simultaneously to avoid UV-Ozone treatment failure. Room temperature is maintained at 25 &#176;C.</li>
	<li>Take about 2 mL of PEDOT:PSS solution with a 10 mL syringe, and install a 0.45 &#181;m polyether sulfone (PES) filter on the syringe.</li>
	<li>Place the ITO glass at the center of the spin-coater. Pour two drops of the PEDOT:PSS solution onto the ITO glass.</li>
	<li>Spin-coat the filtered PEDOT:PSS solution on the ITO substrate at 3,000 rpm for 30 s and then anneal the PEDOT:PSS-coated ITO at 150 &#176;C for 15 min to give a 30 nm PEDOT:PSS film.</li>
</ol>
4. Fabrication of a Hole Transport Layer
NOTE: The hole transport layer and the emissive layer are fabricated in a nitrogen-filled glove-box with controlled oxygen and water concentration below 50 ppm.
<ol>
	<li>Dissolve poly(N,N&#8242;-bis(4-butylphenyl)-N,N&#8242;-bis(phenyl)-benzidine) (Poly-TPD) in o-dichlorobenzene (1 mL) with a concentration of 25 mg mL-1. Stir at room temperature overnight in the nitrogen-filled glove-box.</li>
	<li>Pour 35 &#181;L of Poly-TPD solution onto the PEDOT:PSS-coated ITO. Spin-coat the Poly-TPD solution at 3,000 rpm for 30 s to give a 40 nm Poly-TPD film, and then anneal at 150 &#176;C for 30 min.</li>
</ol>
5. Fabrication of an Emissive Layer
<ol>
	<li>Dissolve ZnCdS/ZnS quantum dots in toluene solution (300 &#181;L) with a concentration of 14.3 mg mL-1.</li>
	<li>Pour 35 &#181;L of the ZnCdS/ZnS solution onto the top of the Poly-TPD. Spin-coat the ZnCdS/ZnS solution at 3,000 rpm for 30 s to give a 40 nm film. 
	NOTE: Baking is not necessary.</li>
</ol>
6. Vacuum Deposition
<ol>
	<li>After a pressure of 10-4 Pa is reached, deposit the aluminum Tris(8-Hydroxyquinolinate) (Alq3) on the substrates with a rate of 0.3 &#197; s-1 in a thermal evaporation chamber to give a 15 nm Alq3 film. 
	NOTE: The Alq3 is deposited only for the device structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Alq3/Al.</li>
	<li>Scrape a 2 mm wide active layer with a scraper to expose the ITO glass substrate.</li>
	<li>Place the substrates in a metal mask and transfer them into a thermal evaporation chamber. Deposit the aluminum electrode with a rate of 1 &#197; s-1 to give a 100 nm Al film.</li>
</ol>
7. Autoxidation Procedure
<ol>
	<li>Transfer the as-prepared Al cathode into a vacuum oven, and then pump air out of the oven until it is almost a vacuum.</li>
	<li>Open the deflation valve and inject anhydrous compound air (O2=20%, N2=80%) into the vacuum oven with a pressure of 3 &#215; 104 Pa. Oxidize for 0, 4.5, 12, and 17 h at room temperature (in the oven).</li>
</ol>

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We have nothing to disclose.

The device architecture of the blue QD-LED consists of an ITO transparent anode, a PEDOT:PSS HIL (30 nm), a Poly-TPD HTL (40 nm), a ZnCdS/ZnS QDs EML (40 nm), and an Al:Al2O3 cathode (100 nm). Due to the porous character of the Al cathode, we obtained an oxidized Al cathode by exposing it to oxygen. Figure 2e and Figure 2f display the energy level alignment diagrams of QDs layer with Al and Al:Al2O3. When the QDs contact with the Al cathode, a Schottky contact is formed at the QDs/Al interface, which introduces large internal resistance to hinder the electron injection from the cathode (Figure 2e). When the QDs contact the Al:Al2O3 cathode, an ohmic contact is formed at the QDs/Al:Al2O3 interface (Figure 2f), which reduces the electron injection barrier and accelerates the electron injection. The decrease of the potential energy barrier at QDs/cathode interface improved the electron injection efficiency and reduced the energy consumption. To further investigate the exciton quenching effect at the QDs/cathode interface, the TRPL spectra were analyzed (Figure 3). The QDs/Al:Al2O3 sample showed a longer PL decay time (5.414 ns) compared to that of the QDs/Al sample (4.839 ns), indicating alleviation of the luminescence quenching at QD/cathode interface25,26. Therefore, the enhanced device performance can be ascribed to the reduced potential barrier and the suppression of luminescence quenching.
The Al:Al2O3 cathode can be prepared by easily controlled autoxidation procedures. It should be Noted that there is no additional ETL involved in the protocol. The device performance is expected to be further improved after combining the autoxidized Al:Al2O3 cathode with other ETL (e.g., ZnO)12,13,15.
In conclusion, we have presented a novel method to improve both the brightness and efficiency of blue QD-LEDs using autoxidized Al cathode. High color-saturated blue QD-LEDs have been demonstrated with a peak luminance of 13,002 cd m-2 and a peak current efficiency of 1.15 cd A-1, which is among the best performance reported in other works. The enhanced device performance can be ascribed to the improved electron injection and suppressed luminescence quenching. Therefore, the method presented in this protocol is an important step towards the realization of high-performance blue QD-LED devices.

Light-emitting diodes (LEDs) based on colloidal semiconductor quantum dots have attracted great interest due to their unique advantages, including solution processability, tunable emission wavelength, excellent color purity, flexible fabrication, and low processing cost1,2,3,4. Since the first demonstrations of QDs-based LEDs in 1994, tremendous efforts have been devoted to engineering the materials and device structures5,6,7. A typical QD-LED device is designed to have a three-layered sandwich architecture which consists of a hole transport layer (HTL), an emissive layer, and an electron transport layer (ETL). The choice of a suitable charge transport layer is critical to balance the injected holes and electrons into the emissive layer for radiative recombination. Currently, vacuum-deposited small molecules are widely used as ETL, for instance, bathocuproine (BCP), tris(8-Hydroxyquinolinate) (Alq3), and 3-(biphenyl-4-yl)-5-(4-tertbutylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ)8. However, the unbalanced carrier injection often causes the recombination region shift to ETL, making unwanted parasitic electroluminescence (EL) emission and deteriorating the device performance9.
To enhance the device efficiency and environmental stability, solution-processed ZnO nanoparticles were introduced as an electron transport layer instead of the vacuum-deposited small-molecule materials. Highly bright RGB QD-LEDs were demonstrated for conventional device architecture, showing luminance up to 31,000, 68,000, and 4,200 cd m-2 for emission of orange-red, green, and blue, respectively10. For an inverted device architecture, high performance RGB QD-LEDs with low turn on voltage were successfully demonstrated with brightness and external quantum efficiencies (EQE) of 23,040 cd m-2 and 7.3% for red, 218,800 cd m-2 and 5.8% for green, and 2,250 cd m-2 and 1.7% for blue, respectively11. To balance the injected charges and preserve the QDs emissive layer, an insulating poly(methylmethacrylate) (PMMA) thin film was inserted between the QDs and ZnO ETL. The optimized deep-red QD-LEDs exhibited high external quantum efficiencies up to 20.5% and a low turn-on voltage of only 1.7 V12.
Besides, optimizing the optoelectronic properties and nanostructures of QDs also plays a crucial role in boosting the device performance. For instance, highly fluorescent blue QDs with photoluminescence quantum yield (PLQE) up to 98% were synthesized through optimizing the ZnS shelling time13. Similarly, high-quality, violet-blue QDs with near 100% PLQE were synthesized by precisely controlling the reaction temperature. The violet-blue QDs-LED devices showed remarkable luminance and EQE up to 4,200 cd m-2 and 3.8%, respectively14. This synthesis method is also applicable to violet ZnSe/ZnS core/shell QDs, the QD-LEDs exhibited high luminance (2,632 cd m-2) and efficiency (EQE=7.83%) by using Cd-free QDs15. Since blue quantum dots with high PLQE have been demonstrated, high charge injection efficiency in the QDs layer plays another crucial role in fabricating high performance QD-LEDs. By substituting long chain oleic acid ligands to shorten 1-octanethiol ligands, the electron mobility of QDs film was increased two-fold, and a high EQE value over 10% was obtained16. The surface ligand exchange can also improve the morphology of QDs film and suppress the photoluminescence quenching among QDs. For instance, QDs-LED showed improved device performance by using chemically grafted QDs-semiconducting polymer hybrids17. Besides, high-performance QDs were prepared through reasonable optimization of the graded composition and thickness of the QDs shell, due to the enhanced charge injection, transport, and recombination18.
In this work, we introduced a partial autoxidized aluminum (Al) cathode to improve the performance of ZnCdS/ZnS graded core/shell-based blue QD-LEDs19. The change of the potential energy barrier of the Al cathode was confirmed by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Furthermore, the fast charge carrier dynamics at the QDs/Al and QDs/Al:Al2O3 interface were analyzed by time-resolved photoluminescence (TRPL) measurements. In order to further validate the influence of partially oxidized Al on device performance, QD-LEDs with different cathodes (Al only, Al:Al2O3, Al2O3/Al, Al2O3/Al:Al2O3, and Alq3/Al) were fabricated. As a result, high performance pure blue QD-LEDs were demonstrated by employing Al:Al2O3 cathodes, with a maximum luminance of 13,002 cd m-2 and a peak current efficiency of 1.15 cd A-1. Furthermore, there was no additional organic ETL involved in the device architecture, which can avoid unwanted parasitic EL to guarantee the color purity under different working voltages.

<table><tbody><tr><td>Indium Tin Oxide (ITO)-coated glass&lt;br/&gt; substrate</td><td>CSG Holding Co., Ltd.</td><td></td><td>Resistivity&amp;asymp;10 &amp;Omega;/sq</td></tr><tr><td>Zinc powder</td><td>Sigma-Aldrich</td><td>96454</td><td>Molecular Weight 65.38</td></tr><tr><td>Isopropyl alcohol</td><td>Beijing Chemical Reagent</td><td>67-63-0</td><td>Analytically pure</td></tr><tr><td>Toluene</td><td>Innochem</td><td>I01367</td><td>Analytically pure</td></tr><tr><td>Acetone</td><td>Innochem</td><td>I01366</td><td>Analytically pure</td></tr><tr><td>Hydrochloric acid</td><td>acros</td><td>124210025</td><td>1 N standard solution</td></tr><tr><td>O-dichlorobenzene</td><td>acros</td><td>396961000</td><td>98+%, Extra Dry</td></tr><tr><td>Poly(3,4-ethylenedioxythiophene) doped polystyrene sulfonate (PEDOT:PSS)</td><td>H. C.Stark</td><td>Clevious P VP Al 4083</td><td></td></tr><tr><td>Poly(N,N&amp;prime;-bis(4-butylphenyl)-N,N&amp;prime;-bis(phenyl)-benzidine) (Poly-TPD)</td><td>Luminescence Technology</td><td>LT-N149</td><td></td></tr><tr><td>Aluminum tris(8-Hydroxyquinolinate) (Alq&lt;sub&gt;3&lt;/sub&gt;)</td><td>Luminescence Technology</td><td>LT-E401</td><td></td></tr><tr><td>UV-O cleaner</td><td>Jelight Company</td><td>92618</td><td></td></tr><tr><td>Filter</td><td>Jinteng</td><td>JTSF0303/0304</td><td>Polyether sulfone (0.45 &amp;mu;m)</td></tr><tr><td>Ultrasonic cleaner</td><td>HECHUANG ULTRASONIC</td><td>KH-500DE</td><td></td></tr><tr><td>Digital multimeter</td><td>UNI-T</td><td>UT39A</td><td></td></tr><tr><td>Spin coater</td><td>IMECAS</td><td>KW-4A</td><td></td></tr><tr><td>Digital hotplate</td><td>Stuart</td><td>SD160</td><td></td></tr></tbody></table>

enhanced electron injection and exciton confinement for pure blue quantum-dot light-emitting diodes by introducing partially oxidized aluminum cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

UV-Vis absorption and photoluminescence (PL) spectra were used to record the optical properties of ZnCdS/ZnS graded core/shell-based blue QDs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were collected for the morphologies of QDs (Figure 1). X-ray photoelectron spectroscopy (XPS), electrochemical study, and ultraviolet photoelectron spectroscopy (UPS) were employed to detect the structural properties and energy levels of QDs (Figure 2). Time-resolved photoluminescence (TRPL) measurements were used to detect the fast charge carrier dynamics at the interface between QDs/Al and QDs/Al:Al2O3 (Figure 3). Finally, EL performance of QD-LEDs was performed (Figure 4, Figure 5, and Table 1).
Figure 1a shows the UV-vis absorption and PL emission spectra of the ZnCdS/ZnS graded core/shell-based blue QDs dispersed in toluene. The QDs solution showed an absorption edge at ~456 nm and a pure deep-blue emission peak located at 451 nm with a narrow full width at half maximum (FWHM) of only 17.8 nm. Sketch diagram of QDs in the inset show that the graded core/shell structure of QDs are capped with short-chain 1-octanethiol as surface ligands. By using a short-chain, 1-octanethiol molecule to replace long-chain organic molecules, the distance between QDs is shortened, which improves the charge injection properties in the QDs film. A representative transmission electron microscope (TEM) image of ZnCdS/ZnS blue QDs showed a relatively uniform size distribution, and the average diameter can be determined to be 14 &#177; 1.7 nm (Figure 1b). A continuous chemical composition gradient inside the QDs can be observed from the high resolution transmission electron microscope (HRTEM) image of a single ZnCdS/ZnS QD as shown in Figure 1c. Figure 1d shows the SEM image of the QDs layer spin-coated on the Poly-TPD substrate as used in device fabrication. A uniform and fully-covered QDs layer can be seen from the SEM image, which indicates the good film-forming properties of QDs.
The surface chemical characteristics of the autoxidized Al cathode were analyzed by XPS measurement. As shown in Figure 2a, the XPS spectrum of the Al 2p orbital can be fitted with three peaks. The peaks at 72.7, 74.3, and 75.5 eV correspond to metallic Al atoms, &#947; phase of Al2O3, and amorphous AlOx20. Electrochemical cyclic voltammetry (CV) measurement has been performed to determine the valence band (VB) and conduction band (CB) energy levels of the ZnCdS/ZnS QDs21. As illustrated in Figure 2b, the onset oxidation potential and the onset reduction potential were determined to be -1.13 and 1.69 V, respectively. As a result, the VB and CB values for ZnCdS/ZnS QDs were calculated to be 3.58 and 6.4 eV, respectively. We measured the work functions of the QDs layer and the partially oxidized Al cathode by UPS (Figure 2c). The work functions of QDs and Al:Al2O3 were 3.87 and 3.5 eV, respectively, which were calculated by subtracting the energies at the secondary electron edges from the energy of the ultraviolet source He I (21.22 eV). The energy level diagram of the device is given in Figure 2d. Figure 2e and Figure 2f display the energy level alignment diagrams of the QDs layer with Al and Al:Al2O3. The work function and Fermi level of the Al cathode were taken from the previous report22,23.
Next, we measured the TRPL spectra of the different QDs layers to determine the charge carrier dynamics (Figure 3). The TRPL curves were fitted by bi-exponential decays. For the QDs, QDs/Al and QDs/Al:Al2O3 samples, the average lifetimes were 8.945, 4.839, and 5.414 ns, respectively. Compared with the QDs/Al sample, the QDs/Al:Al2O3 sample exhibited a longer lifetime, which can be ascribed to the improvement of electron injection and the suppression of non-radiative recombination.
To further verify the effect of partially autoxidized Al cathode on the QD-LEDs device performance, QDs emissive layers contacted with different cathodes were fabricated. The performance parameters of QD-LEDs with different cathodes are summarized in Table 1. The device with pure Al cathode shows low brightness of 435 cd m-2 due to the inefficient charge injection (Figure 4a). In order to improve the electronic transmission ability, a 15 nm Alq3 layer was introduced as ETL to balance the injected charges. As a result, the brightness of the QDs-LED device improved to 1,300 cd m-2. After oxidation treatment, the QD-LEDs device (Al:Al2O3) exhibited a dramatically enhanced performance with a maximum luminance of 13,002 cd m-2 and a maximum current efficiency of 1.15 cd A-1. Further insight into the relationship between device performance and oxidation treatment was gained by inserting an ultrathin Al2O3 layer (1.5 nm) as described in&#160;24. However, the Al2O3/Al device exhibited a maximum luminance of 352 cd m-2, which has no enhancement compared with the bare Al-based device. The Al2O3/Al:Al2O3 device exhibited a modest level of luminance and efficiency (10,600 cd m-2 and 1.12 cd A-1), and a relatively high turn-on voltage (defined as 1 cd m-2) of 4.8 V. The CE-J curves of the QD-LEDs are shown in Figure 4b. The maximum efficiencies of the devices with Al:Al2O3 and Al2O3/Al:Al2O3 reached 1.15 and 1.12 cd A-1, respectively, which was much higher than that of devices with Al, Al2O3/Al, and Alq3/Al cathodes. In general, a high applied voltage will induce the spatial separation of electron and hole wave functions and should dramatically reduce the radiative recombination rate inside the QDs layer. The autoxidized Al:Al2O3 cathode can suppress exciton quenching at high current density, due to the enhanced electron injection and hole blocking abilities. Figure 4c depicts the J-V and L-V curves of QD-LEDs as a function of autoxidation time. The optimized device was achieved by using a 12 h autoxidized Al cathode.
Figure 5a shows the normalized EL spectra of QD-LEDs based on various cathodes. The EL peak of the Al:Al2O3 device was located at 457.3 nm with a FWHM of only 21.4 nm, which indicated a pure blue light without any parasitic EL emission. Contrastively, the EL spectrum based on Alq3/Al cathode shows a parasitic EL originated from Alq3. Stable EL emission can be observed from Al:Al2O3 based device under different applied biases (Figure 5b). As illustrated in Figure 5c, the coordinate of EL spectrum locates at very edge of the Commission Internationale de L&#39;Eclairage (CIE) chromaticity diagram, resulting a high saturated color. Figure 5d shows the operating photographs of QD-LEDs, which can be easily prepared with large area of 1 cm2.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57260/57260fig1.jpg" /> 
Figure 1: Optical properties and morphologies of ZnCdS/ZnS graded core/shell-based blue QDs. (a) Absorption and PL spectra of ZnCdS/ZnS QDs distributed in toluene solution. (b) TEM image and (c) HRTEM image of QDs. (d) SEM image of QDs film spin-coated on the Poly-TPD substrate as used in device fabrication. Reprinted with permission from 19. <a href="https://cloudflare.jove.com/files/ftp_upload/57260/57260fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57260/57260fig2.jpg" /> 
Figure 2: Use of X-ray photoelectron spectroscopy (XPS), electrochemical study, and ultraviolet photoelectron spectroscopy (UPS) to detect the structural properties and energy levels of QDs. (a) Al 2p orbital XPS spectra of autoxidized Al:Al2O3 cathode. (b) The cyclic voltammetry (CV) curves of the ZnCdS/ZnS QDs solution. (c) UPS spectra of the ZnCdS/ZnS QDs film and the autoxidized Al:Al2O3 cathode prepared on ITO substrates. Inset is the complete view of the spectra. (d) Flat band energy level diagram of the QD-LED device. (e) Energy level diagram at the QDs/Al interface and (f) the QDs/Al:Al2O3 interface. Reprinted with permission from 19. <a href="https://cloudflare.jove.com/files/ftp_upload/57260/57260fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57260/57260fig3v2.jpg" /> 
Figure 3: Time-resolved photoluminescence decay for the QDs films contacting with different layers. Reprinted with permission from reference19. The TRPL curves were fitted by bi-exponential decays. Compared with the QDs/Al sample, the QDs/Al:Al2O3 sample exhibits a longer lifetime, which can be ascribed to the improvement of electron injection and suppression of non-radiative recombination. <a href="/files/ftp_upload/57260/57260fig3v2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57260/57260fig4.jpg" /> 
Figure 4: EL performance of QD-LEDs based on different cathodes. (a) Current density-luminance-voltage (J-L-V) and (b) current efficiency-current density (CE-J) characteristic curves of devices with different cathodes. (c) The J-L-V curves of devices with different Al cathodes autoxidized time. Reprinted with permission from19. <a href="https://cloudflare.jove.com/files/ftp_upload/57260/57260fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57260/57260fig5.jpg" /> 
Figure 5: EL spectra, CIE coordinates and operating photographs of an Al:Al2O3 based QD-LEDs. (a) EL spectra of QD-LEDs based on Al, Al:Al2O3, Al2O3:Al, Al2O3/Al:Al2O3 and Alq3/Al cathodes. Inset is the operating photographs of QD-LEDs with active area of 4 mm2. (b) EL spectra of QD-LEDs derived from various biases. (c) CIE coordinates of the Al:Al2O3 based QD-LEDs. (d) Operating photographs of QD-LEDs with active area of 1 cm2. Reprinted with permission from 19. <a href="https://cloudflare.jove.com/files/ftp_upload/57260/57260fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Type of Sample</td>
			<td>Peak luminance</td>
			<td>Peak current efficiency</td>
			<td>Turn-on voltage</td>
			<td>Emission peak</td>
			<td>FWHM</td>
		</tr>
		<tr>
			<td></td>
			<td>(cd m-2)</td>
			<td>(cd A-1)</td>
			<td>(V)</td>
			<td>(nm)</td>
			<td>(nm)</td>
		</tr>
		<tr>
			<td>Al</td>
			<td>435</td>
			<td>0.11</td>
			<td>3.56</td>
			<td>459.1</td>
			<td>22.5</td>
		</tr>
		<tr>
			<td>Al:Al2O3</td>
			<td>13002</td>
			<td>1.15</td>
			<td>3.83</td>
			<td>457.3</td>
			<td>21.4</td>
		</tr>
		<tr>
			<td>Al2O3/Al</td>
			<td>352</td>
			<td>0.11</td>
			<td>3.70</td>
			<td>458.8</td>
			<td>20.9</td>
		</tr>
		<tr>
			<td>Al2O3/Al:Al2O3</td>
			<td>10600</td>
			<td>1.12</td>
			<td>4.80</td>
			<td>457.1</td>
			<td>20.8</td>
		</tr>
		<tr>
			<td>Alq3/Al</td>
			<td>1300</td>
			<td>0.57</td>
			<td>4.20</td>
			<td>455.9</td>
			<td>17.5</td>
		</tr>
	</tbody>
</table>
Table 1: EL parameters of QD-LEDs based on different cathodes. Reprinted with permission from reference19.

Watch this Scientific Journal Video about Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode at JoVE

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...

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8.7K Views.  North China Electric Power University. A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Video: Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space. The spin states of these electrons can then be used to implement the logical 0 and 1 of the quantum bit. Given the nanometer scale of these quantum dots, cleanroom facilities offering specialized equipment- such as scanning electron microscopes and e-beam evaporators- are required for their fabrication. Great care must be taken throughout the fabrication process to maintain cleanliness of the sample surface and to avoid damaging the fragile gates of the structure. This paper presents the detailed fabrication protocol of gate-defined lateral quantum dots from the wafer to a working device. Characterization methods and representative results are also briefly discussed. Although this paper concentrates on double quantum dots, the fabrication process remains the same for single or triple dots or even arrays of quantum dots. Moreover, the protocol can be adapted to fabricate lateral quantum dots on other substrates, such as Si/SiGe.

This paper presents a detailed fabrication protocol for gate-defined semiconductor lateral quantum dots on gallium arsenide heterostructures. These nanoscale devices are used to trap few electrons for use as quantum bits in quantum information processing or for other mesoscopic experiments such as coherent conductance measurements.

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and ...

Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by quantum mechanical effects such as tunneling through single dopants, scattering via interface defects, and discrete trap charge states. However, progress in silicon technology has shown that these phenomena can be harnessed and exploited for a new class of quantum-based electronics. Among others, multi-layer-gated silicon metal-oxide-semiconductor (MOS) technology can be used to control single charge or spin confined in electrostatically-defined quantum dots (QD). These QD-based devices are an excellent platform for quantum computing applications and, recently, it has been demonstrated that they can also be used as single-electron pumps, which are accurate sources of quantized current for metrological purposes. Here, we discuss in detail the fabrication protocol for silicon MOS QDs which is relevant to both quantum computing and quantum metrology applications. Moreover, we describe characterization methods to test the integrity of the devices after fabrication. Finally, we give a brief description of the measurement set-up used for charge pumping experiments and show representative results of electric current quantization.

The fabrication process and experimental characterization techniques relevant to single-electron pumps based on silicon metal-oxide-semiconductor quantum dots are discussed.

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by ...

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and flexibility of their emission properties can be achieved by controlling their composition and dopants. However, a lot of work is still required to improve their properties and to reduce the production cost. A possible approach is to correlate the luminescence properties of the Sialon particles with their local structural and chemical environment in order to optimize their growth parameters and find novel phosphors. For such a purpose, the low-voltage cathodoluminescence (CL) microscopy is a powerful technique. The use of electron as an excitation source allows detecting most of the luminescence centers, revealing their luminescence distribution spatially and in depth, directly comparing CL results with the other electron-based techniques, and investigating the stability of their luminescence properties under stress. Such advantages for phosphors characterization will be highlighted through examples of investigation on several Sialon phosphors by low-energy CL.

Low-energy Cathodoluminescence for OxyNitride Phosphors

An excellent chemical and luminescence stabilities of (oxy)nitride phosphors present it as an promising alternative to currently used sulfide and oxide phosphors. In this paper, we present the way to investigate its local luminescence properties using low-energy cathodoluminescence (CL).

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and ...

Chemistry

Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active layer which consists of blue-fluorescent poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and &#960;-conjugated triphenylamine molecules. This white light emission originates from exciplexes formed between PFD and amines in electronically excited states. A device containing PFD, 4,4&#39;,4&#39;&#39;-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), Poly(ethylene oxide) and K2CF3SO3 showed white light emission with Commission internationale de l&#39;&#233;clairage (CIE) coordinates of (0.33, 0.43) and a Color Rendering Index (CRI) of Ra = 73 at an applied voltage of 3.5 V. Constant voltage measurements showed that the CIE coordinates of (0.27, 0.37), Ra of 67, and the emission color observed immediately after application of a voltage of 5 V were nearly unchanged and stable after 300 sec.

The authors present a method for fabricating stable white-light-emitting electrochemical cells utilizing emission from exciplexes formed between a blue-emitting fluorene polymer and aromatic amines.

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active ...

Scale-up Chemical Synthesis of Thermally-activated Delayed Fluorescence Emitters Based on the Dibenzothiophene-S,S-Dioxide Core

We report a procedure to linearly scale-up the synthesis of 2,8-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)dibenzothiophene-S,S-dioxide (compound 4) and 2,8-bis(10H-phenothiazin-10-yl)dibenzothiophene-S,S-dioxide (compound 5) using Buchwald-Hartwig cross-coupling reaction conditions. In addition, we demonstrate a scaled-up synthesis of all non-commercially available starting materials that are required for the amination cross-coupling reaction. In the present article, we provide the detailed synthetic procedures for all of the described compounds, alongside their spectral characterization. This work shows the possibility to produce organic molecules for optoelectronic applications on a large scale, which facilitates their implementation into real world devices.

Scale-up synthesis of highly efficient thermally activated delayed fluorescence emitters is described in the presented article.

We report a procedure to linearly scale-up the synthesis of 2,8-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)dibenzothiophene-S,S-dioxide (compound 4) and ...

Time-resolved Photophysical Characterization of Triplet-harvesting Organic Compounds at an Oxygen-free Environment Using an iCCD Camera

Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables the acquisition of photoluminescence spectra covering the time regime from nanoseconds up to 0.1 s. This enables us to follow the changes in the intensity (decay) and emission of the spectra over time. Using this method, it is possible to study diverse photophysical phenomena, such as the emission of phosphorescence, and the contributions of prompt and delayed fluorescence in molecules showing thermally activated delayed fluorescence (TADF). Remarkably, all spectra and decays are obtained in a single experiment. This can be done for solids (thin film, powder, crystal) and liquid samples, where the only limitations are the spectral sensitivity of the camera and the excitation wavelength (532 nm, 355 nm, 337 nm, and 266 nm). This technique is, thus, very important when investigating the excited state dynamics in organic emitters for their application in organic light-emitting diodes and other areas where triplet harvesting is of paramount importance. Since triplet states are strongly quenched by oxygen, emitters with efficient TADF luminescence, or those showing room temperature phosphorescence (RTP), must be correctly prepared in order to remove any dissolved oxygen from solutions and films. Otherwise, no long-lived emission will be observed. The method of degassing solid samples as presented in this work is basic and simple, but the degassing of liquid samples creates additional difficulties and is particularly interesting. A method of minimizing solvent loss and changing the sample concentration, while still enabling to remove oxygen in a very efficient and a repeatable manner, is presented in this work.

Here, we present a method of the spectroscopic characterization of organic molecules by means of time-resolved photoluminescence spectroscopy on the nanosecond-to-millisecond timescale in oxygen-free conditions. Methods to efficiently remove oxygen from the samples and, thus, limit luminescence quenching are also described.

Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables ...

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection &#8211; for example, a differential transmission measurement &#8211; can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection &#8211; for example, above band-gap excitation &#8211; can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample&#39;s surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). ...

Production and Characterization of Vacuum Deposited Organic Light Emitting Diodes

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.

A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex ...

Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: butylammonium and octylammonium, A: methylammonium or formamidinium, B: lead, X: bromide and iodide, n: number of [BX6]4- octahedral layers in the direction of nanoplatelet thickness) via ligand-assisted reprecipitation. Individual perovskite precursor solutions are prepared by dissolving each nanoplatelet constituent salt in N,N-dimethylformamide (DMF), which is a polar organic solvent, and then mixing in specific ratios for targeted nanoplatelet thickness and composition. Once the mixed precursor solution is dropped into nonpolar toluene, the abrupt change in the solubility induces the instantaneous crystallization of nanoplatelets with surface-bound alkylammonium halide ligands providing colloidal stability. Photoluminescence and absorption spectra reveal emissive and strongly quantum-confined features. X-ray diffraction and transmission electron microscopy confirm the two-dimensional structure of the nanoplatelets. Furthermore, we demonstrate that the band gap of perovskite nanoplatelets can be continuously tuned in the visible range by varying the stoichiometry of the halide ion(s). Lastly, we demonstrate the flexibility of the ligand-assisted reprecipitation method by introducing multiple species as surface-capping ligands. This methodology represents a simple procedure for preparing dispersions of emissive 2D colloidal semiconductors.

This work demonstrates facile room-temperature synthesis of colloidal quantum-confined lead halide perovskite nanoplatelets by ligand-assisted reprecipitation method. Synthesized nanoplatelets show spectrally narrow optical features and continuous spectral tunability throughout the visible range by varying the composition and thicknesses.