Pb

Article Cite This: ACS Photonics 2019, 6, 1715−1727

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High-Efficient NUV Emission with Excellent Thermal Stability in Cd/ Pb-Free AS-ZnO QDs by ALD without Surface Passivation Jin Li, Youxing Yu,* and Xiaofang Bi*

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Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University (BUAA), Beijing 100191, China ABSTRACT: Recently, near-ultraviolet (NUV)-emitting quantum dots (QDs) have attracted considerable attention for their potential in next-generation NUV-driven white-lightemitting diodes (with color rendering index >90). However, their widespread application in lighting products is greatly hindered by the inferior NUV-emitting efficiency, poor temperature stability, and toxicity of currently available Cd-/ Pb-containing QD systems. In the present work, we report the synthesis of ecofriendly Al2O3-surrounded (AS)-ZnO QDs using a novel technique based on atomic layer deposition, which shows a superior quantum yield (QY) of 97.3% in the NUV band (395 nm), in contrast to arcsin n2 (denoted by

value is difficult to obtain. Hence, the QY of AS-ZnO QDs embedded in the solid film can be determined using the common solution-based approaches, neither relatively nor absolutely. To circumvent this problem, the QY in this work is derived based on the data of cryogenic PL experiments, as will be discussed in the following. According to the definition, the QY of QDs is equal to the radiative recombination efficiency relative to overall energy relaxation in recombination. In the PL process, after excited by the incident light, the electrons at excited states can return to the ground state through either (1) phonon-assisted NR processes or (2) emitting photons. By using NAbs, NEm, and NNR to denote the number of absorbed incident photons, emitted photons, and nonradiatively recombined electrons during PL processes, the QY can be written as NEm/NAbs. Considering that the probability of radiative and nonradiative recombination should add up to 1, i.e., NAbs = NEm + NNR, one has QY =

NEm NEm = NAbs NNR + NEm

(4)

Here, small probability events, e.g., multiphoton processes, have been neglected. Since the NR process is thermally activated, its probability would be nearly zero at sufficiently low temperature, as illustrated in Figure 11(a), and thus NNR + NEM at RT will be approximately equal to the NEM at cryogenic temperature. Consequently, the QY can be estimated by the ratio of PL intensity at RT to that at cryogenic temperature (IRT/Icryo),44,51−53 as illustrated in Figure 11(b). In principle, the Icryo should be measured at 0 K. However, as indicated by Figure 8 and Figure 11(a), NR probability has been negligible at Tc owing to the large ET in this work, below which the PL intensity is virtually invariant. Therefore, it is justified to use the PL intensity of nitrogen temperature (77 K) as the Icryo in this case. During the QY measurement, the conditions of optical excitation and sample position are fixed for RT and for cryogenic temperature, so that the η for IRT and Icryo measurement are considered identical. Hence, QY = (IRT/ η)/(Icryo/η) = IRT/Icryo will be little affected by the value of η and can more reliably reflect the intrinsic emitting properties of AS-ZnO QDs. Figure 11(c) summarizes the QY of AS-ZnO QDs with different luminescent colors, which is remarkably enhanced with decreasing the emitting wavelength. As discussed above, this is attributed to the increase of ET caused by the strengthened QC effect.

1

emission-1) will totally be reflected at the film surface, traveling in the QDs-ML like in an optical fiber, and mostly dissipate into thermal energy, as illustrated by Figure 10(b). Here, n1 and n2 are the refractive indexes for the QDs-ML film and surrounding medium (quartz or air). Only those with n incident angle θ < arcsin n2 (denoted by emission-2) can 1

escape and reach the detector. Therefore, the QY of AS-ZnO QDs is correlated with the EQE of the QDs-ML film by the relation EQE = QY × η, where η < 1 is the extraction factor. However, η is dependent on many extrinsic factors, such as the film thickness or the angle of the incident beam, whose exact 1724

DOI: 10.1021/acsphotonics.9b00382 ACS Photonics 2019, 6, 1715−1727

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The excellent optical properties, in conjunction with facile synthesis and nontoxicity, endow AS-ZnO QDs with great potential for widespread lighting applications.

The AS-ZnO QDs possess an NUV-emitting QY as high as 97.4% at RT, which represents significant improvement in comparison with previous results, as summarized in Table 1.

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Table 1. Comparison of Quantum Yield for NUV-Emitting QDs QD structure

emission peak (nm)

QY (%)

bandgap (eV)

origin of emission

AS-ZnO

395

97

5.08

Sa

ZnSe/ZnS Cs2Sb2Br9 CsPbCl3 FAPbCl3

409 410 405 415

63 46 10 1

3.12 3.31 3.10 3.16

Ib I I I

EXPERIMENTAL SECTION Detailed Deposition Conditions. Trimethylaluminum (TMA, Al(CH3)3, 99.999% purity, CAS: 75-24-1) and diethylzinc (DEZn, Zn(C2H5)2, 99.999% purity, CAS: 55720-0) were used as Al and Zn precursors, and deionized water (H2O) was used as the oxidant, respectively. High-purity nitrogen (N2) was used as carrier and purge gas with a flow rate of 30 sccm. During the deposition, the ALD chamber was fixed at 150 °C and precursors were at room temperature. Growth parameters for Al2O3 deposition were as follows: 0.3 s TMA exposure−40 s N2 purge−0.2 s H2O exposure−20 s N2 purge. Growth parameters for ZnO deposition were as follows: T s DEZn exposure−10 s N2 purge−0.02 s H2O exposure−10 s N2 purge, where T represents the exposure time for DEZn and is in the range of 0.01 to 0.03 s for depositing ZnO QDs of different sizes. The number of ZnO ALD cycles in a single layer of AS-ZnO QDs was fixed at 20. Characterization. Crystallinity was characterized by X-ray diffraction (XRD) measurement using a D/max 2500 X-ray diffraction spectrometer (Cu Kα, λ = 1.540 56 Å). Surface and cross-section morphology were observed using a transmission electron microscope (TEM) (JEM-2100F, JEM, Japan). Optical transmittance curves were measured using a UV3600 ultraviolet−visible spectrophotometer (UV-3600, Shimadzu, Japan), and luminescent behaviors were characterized by photoluminescence spectra using an FLS-980 fluorescence spectrometer with a cryogenic accessory (Edinburgh Instruments, UK) in the temperature range from 77 to 500 K. Chemical binding energies of the deposited films were inspected using an XPS and XAES spectroscope equipped with a monochromatic Al Kα radiation source of 1486.6 eV (Thermo Scientific Escalab 250Xi) at room temperature, and the obtained binding energies were corrected with respect to the C 1s binding energy of 284.5 eV. EPR measurements were performed at RT with a Bruker A300-10/12 spectrometer under dark and illumination conditions.

ref this work 54 16 6 5

a

S stands for surface state recombination. bI represents intrinsic transition.

Moreover, it is mentioned that the NUV emission has exceptional temperature stability, which can retain a high QY of 87% at 500 K, as shown in Figure 8c, without any irreversible structural deterioration. To date, it has been reported that luminescence of QDs was rapidly quenched at elevated temperatures. The surface structures of QDs are usually unstable to high temperature due to the degraded surface condition, such as the detachment of capped passivating ligands on QDs or deterioration of core/shell interface matching quality owing to the different thermal expansion between core/shell materials. These would lead to a large increase of surface state number as the temperature increases, which, in turn, causes intensified NR and irreversible damage to the luminescence of QDs.9,55,56 Therefore, the presented observation is of practical significance for lighting applications. Also mentioned is the advantages of AS-ZnO QDs in device implementation for their robustness to the environment and facile synthesis, which, for instance, are extremely stable toward oxygen and moisture due to the fullinorganic structure, showing no performance degradation with time. Besides, compared with spin-coating, ALD can grow very conformal QD thin films with precise thickness control on various substrates or the surface of nanostructures, which is highly desired in the fabrication of nanodevices.

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CONCLUSIONS In summary, we have developed a promising and facile technique to synthesize QDs through controlling exposure time of precursors during the ALD process, based on which AS-ZnO QD with ultrasmall average radius (2.1 nm) and highly enhanced bandgap (from 3.2 to 5.08 eV) was obtained. The AS-ZnO QDs exhibited a bright visible emission stemming from the peroxide surface state Zn*, whose energy can be enhanced from 2.11 eV to 3.14 eV (395 nm) into the NUV band, by reducing tep from 0.03 s to 0.01 s based on QC effects. The cryogenic PL experiments revealed that the ultrastrong QC in AS-ZnO QDs also remarkably increased the ET for NR and thermal quenching at surface states (from 65 to 168 meV), which effectively suppressed the NR probability. The simultaneous increase of emitting energy and radiative efficiency turned surface states into high-efficient NUV emitters with an ultrahigh QY of 97.4%. Besides, the ASZnO QDs possess extraordinary thermal stability, which demonstrated little efficiency degradation or structure deterioration above RT, retaining a QY of 87% at 500 K.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jin Li: 0000-0003-1099-6289 Youxing Yu: 0000-0002-3281-3765 Funding

This work was supported by National Natural Science Foundation of China under Grant No. 61671040 and the Academic Excellence Foundation of BUAA for PhD Students. Notes

The authors declare no competing financial interest.

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