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 
			substrate</td>
			<td>CSG Holding Co., Ltd.</td>
			<td></td>
			<td>Resistivity&#8776;10 &#937;/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&#8242;-bis(4-butylphenyl)-N,N&#8242;-bis(phenyl)-benzidine) (Poly-TPD)</td>
			<td>Luminescence Technology</td>
			<td>LT-N149</td>
			<td></td>
		</tr>
		<tr>
			<td>Aluminum tris(8-Hydroxyquinolinate) (Alq3)</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 &#956;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.remotexs.ntu.edu.sg/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.remotexs.ntu.edu.sg/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.remotexs.ntu.edu.sg/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.remotexs.ntu.edu.sg/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

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

Scalable Nanohelices for Predictive Studies and Enhanced 3D Visualization

Spring-like materials are ubiquitous in nature and of interest in nanotechnology for energy harvesting, hydrogen storage, and biological sensing applications.&#160; For predictive simulations, it has become increasingly important to be able to model the structure of nanohelices accurately.&#160; To study the effect of local structure on the properties of these complex geometries one must develop realistic models.&#160; To date, software packages are rather limited in creating atomistic helical models.&#160; This work focuses on producing atomistic models of silica glass (SiO2) nanoribbons and nanosprings for molecular dynamics (MD) simulations. Using an MD model of &#8220;bulk&#8221; silica glass, two computational procedures to precisely create the shape of nanoribbons and nanosprings are presented.&#160; The first method employs the AWK programming language and open-source software to effectively carve various shapes of silica nanoribbons from the initial bulk model, using desired dimensions and parametric equations to define a helix.&#160; With this method, accurate atomistic silica nanoribbons can be generated for a range of pitch values and dimensions.&#160; The second method involves a more robust code which allows flexibility in modeling nanohelical structures.&#160; This approach utilizes a C++ code particularly written to implement pre-screening methods as well as the mathematical equations for a helix, resulting in greater precision and efficiency when creating nanospring models.&#160; Using these codes, well-defined and scalable nanoribbons and nanosprings suited for atomistic simulations can be effectively created.&#160; An added value in both open-source codes is that they can be adapted to reproduce different helical structures, independent of material.&#160; In addition, a MATLAB graphical user interface (GUI) is used to enhance learning through visualization and interaction for a general user with the atomistic helical structures.&#160; One application of these methods is the recent study of nanohelices via MD simulations for mechanical energy harvesting purposes.

Accurate modeling of nanohelical structures is important for predictive simulation studies leading to novel nanotechnology applications.&#160; Currently, software packages and codes are limited in creating atomistic helical models.&#160; We present two procedures designed to create atomistic nanohelical models for simulations, and a graphical interface to enhance research through visualization.

Spring-like materials are ubiquitous in nature and of interest in nanotechnology for energy harvesting, hydrogen storage, and biological sensing ...

Casting Protocols for the Production of Open Cell Aluminum Foams by the Replication Technique and the Effect on Porosity

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many cases validated experimentally, for light weight or impact energy absorbing structures, as high surface area heat exchangers or electrodes, as implants to the body, and many more. Although great progress has been made in understanding their structure-properties relationships, the large number of different processing techniques, each producing material with different characteristics and structure, means that understanding of the individual effects of all aspects of structure is not complete. The replication process, where molten metal is infiltrated between grains of a removable preform material, allows a markedly high degree of control and has been used to good effect to elucidate some of these relationships. Nevertheless, the process has many steps that are dependent on individual &#8220;know-how&#8221;, and this paper aims to provide a detailed description of all stages of one embodiment of this processing method, using materials and equipment that would be relatively easy to set up in a research environment. The goal of this protocol and its variants is to produce metal foams in an effective and simple way, giving the possibility to tailor the outcome of the samples by modifying certain steps within the process. By following this, open cell aluminum foams with pore sizes of 1&#8211;2.36 mm diameter and 61% to 77% porosity can be obtained.

Replication is one of the processing techniques used for the production of porous metal sponges. In this paper one implementation of the method for the production of open celled porous aluminum is shown in detail.

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many ...

Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model

Analysis of the size distribution of nanocrystals is a critical requirement for the processing and optimization of their size-dependent properties. The common techniques used for the size analysis are transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence spectroscopy (PL). These techniques, however, are not suitable for analyzing the nanocrystal size distribution in a fast, non-destructive and a reliable manner at the same time. Our aim in this work is to demonstrate that size distribution of semiconductor nanocrystals that are subject to size-dependent phonon confinement effects, can be quantitatively estimated in a non-destructive, fast and reliable manner using Raman spectroscopy. Moreover, mixed size distributions can be separately probed, and their respective volumetric ratios can be estimated using this technique. In order to analyze the size distribution, we have formulized an analytical expression of one-particle PCM and projected it onto a generic distribution function that will represent the size distribution of analyzed nanocrystal. As a model experiment, we have analyzed the size distribution of free-standing silicon nanocrystals (Si-NCs) with multi-modal size distributions. The estimated size distributions are in excellent agreement with TEM and PL results, revealing the reliability of our model.

We demonstrate how to determine the size distribution of semiconductor nanocrystals in a quantitative manner using Raman spectroscopy employing an analytically defined multi-particle phonon confinement model. Results obtained are in excellent agreement with the other size analysis techniques like transmission electron microscopy and photoluminescence spectroscopy.

Analysis of the size distribution of nanocrystals is a critical requirement for the processing and optimization of their size-dependent properties. The ...

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 ...

Blue-hazard-free Candlelight OLED

A candlelight-style organic light emitting diode (OLED) is a human-friendly type of lighting because it is blue-hazard-free and has a low correlated color temperature (CCT) illumination. The low CCT lighting is deprived of high-energy blue radiation, and it can be used for a longer duration before causing retinal damage. This work presents the comprehensive protocols for the fabrication of blue-hazard-free candlelight OLEDs. The emission spectrum of the OLED was characterized by the maximum exposure time limit of the retina and the melatonin suppression sensitivity. The devices can be fabricated using dry and wet processes. The dry-processed OLED resulted in a CCT of 1,940 K and exhibited a maximum retinal exposure limit of 1,287 s at a brightness of 500 lx. It showed 2.61% melatonin suppression sensitivity relative to 480 nm blue light. The wet-processed OLED, where the spin coating is used to deposit hole injection, hole transport, and emissive layers, making fabrication fast and economical, produced a CCT of 1,922 K and showed a maximum retinal exposure limit of 7,092 at a brightness of 500 lx. The achieved relative melatonin suppression sensitivity of 1.05% is 86% and 96% less than that of the light emitting diode (LED) and compact fluorescent lamp (CFL), respectively. Wet-processed blue-hazard-free candlelight OLED exhibited a power efficiency of 30 lm/W, which is 2 times that of the incandescent bulb and 300 times that of the candle.

We present a protocol for the fabrication of a blue-hazard-free candlelight organic light emitting diode (OLED) for eye protection and melatonin secretion.

A candlelight-style organic light emitting diode (OLED) is a human-friendly type of lighting because it is blue-hazard-free and has a low correlated color ...

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated species. The synthesis approach inhibits the topochemical effect promoting rapid hydration when exposed to the relative humidity of ambient air. This stable, amorphous, nano-particle material was analyzed using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and showed an exothermic energy release at low temperature (i.e., 180 &#176;C) for the transformation of I4O9 into I2O5. This additional exothermic energy release contributes to an increase in overall reactivity of I4O9 when dry mixed with nano-aluminum (Al) powder, resulting in a minimum of 150% increase in flame speed compared to Al + I2O5. This study shows that as an oxidizer, I4O9 has more reactive potential than other forms of iodine(V) oxide when combined with Al, especially if I4O9 can be passivated to inhibit absorption of water from its surrounding environment.

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

A protocol for measuring flame speeds of a reactive mixture composed of tetraiodine nonoxide (I4O9) and aluminum (Al) is presented. A method for resolving reaction kinetics using differential scanning calorimetry (DSC) is also presented. It was found that I4O9 is 150% more reactive than other iodine(V) oxides.

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated ...

Experimental Procedure for Warm Spinning of Cast Aluminum Components

High performance, cast aluminum automotive wheels are increasingly being incrementally formed via flow forming/metal spinning at elevated temperatures to improve material properties. With a wide array of processing parameters which can affect both the shape attained and resulting material properties, this type of processing is notoriously difficult to commission. A simplified, light-duty version of the process has been designed and implemented for full-size automotive wheels. The apparatus is intended to assist in understanding the deformation mechanisms and the material response to this type of processing. An experimental protocol has been developed to prepare for, and subsequently perform forming trials and is described for as-cast A356 wheel blanks. The thermal profile attained, along with instrumentation details are provided. Similitude with full-scale forming operations which impart significantly more deformation at faster rates is discussed.

An experimental protocol for instrumented warm rotary forming of cast aluminum alloys employing a bespoke industrially scaled apparatus is presented. Experimental considerations including thermal and mechanical effects are discussed, as well as similitude with full-scale processing of automotive wheels.

High performance, cast aluminum automotive wheels are increasingly being incrementally formed via flow forming/metal spinning at elevated temperatures to ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

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). ...