An In-Situ Doping System to Improve Electric Field-Induced

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An In-Situ Doping System to Improve Electric Field-Induced Fluorescence Properties of CdZnS/ZnS Quantum Rods for Light Emitting Devices Wonseok Choi, Yonghee Lee, Hyunjin Cho, Moohyun Kim, Jeong Bin Shin, Young Cheol Seo, Jinwuk Kim, Inbyeong Kang, Kyung Cheol Choi, and Duk Young Jeon ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01015 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Applied Nano Materials

An In-situ Doping System to Improve Electric Field-Induced Fluorescence Properties of CdZnS/ZnS Quantum Rods for Light Emitting Devices Wonseok Choi†∥, Yonghee Lee†⊥∥, Hyunjin Cho†, Moohyun Kim†, Jeong Bin Shin‡, Young Cheol Seo‡, Jinwuk Kim§, In-Byeong Kang§, Kyung Cheol Choi‡ and Duk Young Jeon†* †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305–338, Republic of Korea. ‡

Department of Electrical Engineering, Korea Advanced Institute of Science and Technology,

291 Daehak-ro , Yuseong-gu , Daejeon 305–338, Republic of Korea. §

LG display, 245, LG-ro, Wollong-myeon, Paju-si, Gyeonggi-do, 413-779 Republic of

Korea.

KEYWORDS CdZnS/ZnS, Nanorods, In-situ doping, Nitrogen-doped, PL quenching.

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ABSTRACT.

The 1-D quantum rod (QR) has the property that the charge and the hole are separated under the e-field, the overlap of the wave-function decreases, and the photoluminescence (PL) quenching occurs. Because of this property, QR can be used in optical switching and future display applications. The CdZnS/ZnS QR is a material capable of emitting blue light. CdZnS/ZnS can easily separate carriers because the difference between the valence band of the core and the shell is small. However, CdS and ZnS have very low hole conductivity and can’t easily be separated. To solve this problem, we developed an in-situ doping system and demonstrated nitrogen doping. The in-situ doping system not only coats the ZnS on the CdZnS, but also proceeds with nitrogen doping. Previously studied doping methods additionally doped the synthesized nanomaterials and had no effect of doping because the dopant is not dispersed without subsequent heat treatment. However, the in-situ doping system grows the ZnS shell and uniformly dopes the nitrogen. This means that no additional heat treatment is required. The effect of doping gradually increases in proportion to the amount of dopant and the PL quenching increases, even though the aspect ratio is decreased.


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1. Introduction Increased attention has been paid to one-dimensional (1D) quantum rods (QRs) or nanorods (NRs) for their excellent properties compared to zero-dimensional (0D) quantum dots (QDs) including larger Stokes shift,

1-2

larger absorption cross sections,3-4 linearly

polarized light emissions,5-6 and longer multiexciton lifetimes.7-8 Furthermore, the QRs exhibit electric field-dependent photoluminescence (PL) characteristic which enables applications to an optical switching or a novel type of display.9-10 To boost versatility and usability of QR, it is required to develop QRs with improved ‘on/off characteristics’ which indicates how much PL is reduced upon an applied bias. Previously, we have reported CdZnS/ZnS core/shell QRs with improved PL efficiency and on/off characteristics through the control of the interfacial defects.11 So far, no other study has been reported regarding an effort to improve on/off characteristics of QRs, and a few articles only mentioned correlations of the on/off characteristics with aspect ratio (AR) of QRs; as QRs expand along c-axis, generated excitons are more easily dissociated upon an external electric field, resulting in the reduction of the PL intensity. 9-10 In this work, we report a facile route to improve on/off characteristics of QRs through nitrogen doping. An influence of the nitrogen doping on the electric field-induced PL quenching was investigated for CdZnS rod core with nitrogen-doped ZnS shell (CdZnS/ZnS:N). Wang et al. demonstrated nitrogen-doped p-type ZnS nanowires using solvothermal method.12 The sites of sulfur atoms in the ZnS lattice are replaced by nitrogen atoms under high pressure resulting in the increased holes conductivity.

12

To date, many

researchers have shown how a doping alters electrical properties of the nanocrystals,13-15 however, no previous research have dealt with the influence of doping on the electric fieldinduced PL quenching properties of QRs. This work presents how an optical property of QR 3 ACS Paragon Plus Environment


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is manipulated by a change in electrical property of it. We developed a novel doping method where nitrogen-doping and ZnS shell growth occurred simultaneously, and the resulting CdZnS/ZnS:N QRs exhibited the improved on/off characteristics as we increased the concentration of the substituted nitrogen.

2. Results and discussion The doping methods introduced in the previous studies conducted by other researchers led to the improvement of electrical properties, but subsequent heat treatment was essential for the even distribution of the dopant.16 We developed an in-situ doping method using a solvothermal process, which improves the electrical properties (E-field dependent optical properties) without any post-treatment. (Figure 1) To determine the optimal doping concentration, we prepared samples where we varied the amount of triethylamine (TEA) injected to 0 mL (S0), 3 mL (S3), 9 mL (S9) and 15 mL (S15), respectively. We used cyclic voltammetry (C-V) and UV-visible spectroscopy to analyze electric energy levels of each sample (Figure S1, Figure S2 and Table S1). It turned out that the HOMO and LUMO levels tended to deepen as p-type doping proceeded, indicating that the sulfur atoms in the ZnS shell was favorably substituted by nitrogen (Figure 2a). 17 In Figure 2(b), we show the XPS spectra corresponding to CdZnS NRs with undoped or nitrogen-doped ZnS shell. All XPS data were calibrated by the carbon 1s peak at 285 eV (Figure S3). Undoped ZnS lattice consisting of S-Zn-S is changed to S-Zn-N array, when N atoms are introduced. The binding energy was shifted to higher one in S-Zn-N linkage than in the S-Zn-S linkage. The shift might be caused by the difference in electronegativity between N (3.04) and S (2.58) resulting in the more positively charged S atoms in the S-Zn-N linkage than those in the S-Zn-S linkage.18 The binding energy of the S 2p peak gradually increased 4 ACS Paragon Plus Environment

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from 161.22 eV to 161.33 eV after heavily doped with nitrogen (S15). Furthermore, it is also seen from the N 1s peak that the atomic content of N increases when the doping amount increases. Atomic percent of N increases from 2.53% (S0) to 6.73% (S15). The recorded nitrogen atomic percentage was 2.53% for undoped sample, even though the sample was not treated with TEA. This was probably due to the oleylamine ligand present on the surface of QRs. Furthermore, to confirm the doping of nitrogen, S/Cd atomic ratio was calculated using XPS analysis (Figure S4). The atomic percent of S decreased, as we increased the amount of TEA, resulting in the decrease of S/Cd ratio from 13.62 to 9.15 depending on the amount of TEA. To observe the electrical characteristics of the samples, the vertical current was measured using c-AFM. The current continuously increases to 46.2 nA, 113.1 nA, 121.4 nA, and 203.2 nA, respectively. (Figure 2c, 2d and Table 1). This tendency is similar to the increase in the conductivity of the QR films (1.2E-6 Ω cm-1 for S0, and 1.4E-5 ohm cm-1 for S15) as shown in Figure S5. The vacancy for an electron is generated as sulfur atom with six valence electrons is replaced by nitrogen atom with five electrons, resulting in the increase in the hole conductivity.

12, 19

An on/off contrast efficiency, which refers to the decrease amount

of PL when a voltage of 100V (off-state) was applied, was changed according to the doping amount (Figure 3). The on/off contrast efficiency increased sharply from 9.3% (S0) to 25.3% (S15). According to the previous study of CdSe/CdS core/shell structure, the energy gap of the conduction band (CB) is smaller than the energy gap of the valence band (VB). When the electric field is applied to CdSe/CdS QRs, the electrons are easily separated and moving to the shell, while the holes are confined in the core. On the other hand, in a CdS/ZnS structure, the energy offset for CB is larger than that for VB, which implies that holes are likely to be separated while the electrons are likely to be confined in the core upon application of an electric field.20-21

In the mean time, widely used semiconductor materials such as CdSe,



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CdS and ZnS have poor hole conductivity,22-23 thus holes are not properly separated upon application of an electric field in the CdZnS/ZnS core/shell structure. Nitrogen as a p-type dopant has a great influence on the increase of the hole conductivity of ZnS. As a result, when the electric field is applied, the hole can move smoothly from the core to the shell, so that the overlapping of the hole and electron wave functions is reduced, and the on/off contrast efficiency is improved up to about 2.7 times compared with the undoped sample. Interestingly, the TEM image shows that the AR decreases as the amount of TEA increases (illustrated in Figure 3 and Figure S6). The increase in on/off contrast efficiency is reported by J. Muller et al.9 as the AR increases. However, the QR obtained through the in-situ doping system shows that the on/off contrast efficiency can be improved, even if the AR of NR decreases. The reason for the decrease of the AR seems to be that the ripening progressed due to the basic property of TEA.

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It was found that TEA causes ripening and at the same time

acts as a p-type dopant. HRTEM and FFT images of the nanocrystals are shown in Figure S7. It turned out that as we increased the amount of TEA injected, the host structures (hexagonal wurtzite Cd-Zn-S alloy) remained in the same, and the lattice spacing along c-axis increased, this was probably due to the intercalation of nitrogen atom along the c- axis of NRs. We confirmed that the morphology and on/off characteristics of the material vary with the amount of TEA. In order to justify the in-situ doping study, we applied the three-step method where a ZnS shell and nitrogen doping is proceeded in sequence followed by additional formation of a ZnS shell. The reason why additional ZnS shell growth is necessary is because the quantum efficiency is reduced by doping. For three-step method,

the CdZnS core was

placed in a flask and a ZnS shell was formed, the resulting CdZnS/ZnS core/shell QRs, TEA 6 ACS Paragon Plus Environment

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and OAm were placed in an autoclave and reacted at 200 °C to allow nitrogen to be doped. The quantum yield (PL QY) of the doped QRs significantly decreased after doping of nitrogen. The absorption spectra of Figure S8 showed that the peak near 300 nm corresponding to ZnS decreased, indicating that the ZnS shell was peeled during doping process. To recover the decrease in PL QY, an additional stacking of ZnS shell was carried out in a flask. The quantum efficiency of the CdZnS/ZnS:N/ZnS material formed in the threestep method was restored to a level similar to that of the undoped (CdZnS/ZnS), and the on/off contrast efficiency of CdZnS/ZnS:N/ZnS QRs increased from 39% to 54%. However, there is a problem in that a long process time is required due to the complicated three-step process, and ZnS shell is peeled off during doping, making it difficult to obtain reproducibility. In the in-situ doping system, doping is performed simultaneously with growth of the ZnS shell, which eliminates the need for an additional ZnS shell coating, thereby reducing the processing time. In addition, PL quenching up to 78.8% of 100V was observed for QRs obtained through in-situ doping process (Figure 4). Interestingly, the on/off characteristics of the in-situ method were found to be higher than those of the three-step method, even though the AR was smaller.

3. Conclusions We succeeded in synthesizing a CdZnS/ZnS:N quantum rod with a ZnS shell doped with the nitrogen as a p-type dopant. The on/off contrast efficiency gradually increased (around 2.7 times) from 9.34% to 25.28% as we increased the amount of dopants, despite decrease in AR of the QRs. It is worthy noting that the previous studies have reported that on/off contrast efficiency improve as the AR increases,9 on the other hands, our results show that the on/off contrast efficiency increases gradually even if the AR decreases. The on/off contrast 7 ACS Paragon Plus Environment


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efficiency of QRs was 54% (@ 100 V) for the control sample (three-step method), and was larger up to 79% (@ 100 V) for the sample obtained by in-situ doping method, even though the AR of the QR in the latter one was smaller.

Experimental Section 1. Materials

Trioctylphosphin oxide (99%, TOPO), cadmium oxide (≥99.99% trace metal basis, CdO), 1-octadecylphosphonic acid (97%, ODPA), hexylphosphonic acid (95%, HPA), zinc acetate(99.99% trace metal basis), Zinc sulfate heptahydrate (≥99%), Hexane (anhydrous 95%), oleylamine (70%, Oam), oleic acid (90%, OA), sulfur powder(99.998 trace metal basis, S) and triethylamine (TEA) were purchased from Sigma-Aldrich (Korea).

2. Synthesis of CdZnS/ZnS nanorod structure. 2.1. Preparation of CdZnS nanorod core. For synthesis of CdZnS core, 3 g of TOPO, 62.1 mg of CdO, 281 mg of ODPA, 83.1 mg of HPA, 45.9 mg of zinc acetate and 0.32 mL of OA were placed in a 3-necked flask and degassed under vacuum at 150 oC for 1 h. Then, the 3-necked flask was filled with argon gas and the reaction temperature increased to 330 oC. When increased the reaction temperature to 330 oC, the color of the mixture in the three-necked flask changes from a dark brown to a slightly yellowish translucent color. Afterwards, the stock solution consisting of 0.3 mL of 8 ACS Paragon Plus Environment

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ODE and 24.1 mg of S was swiftly injected into the 3-necked flask and reacted for 40 seconds. Samples were redispersed in chloroform after centrifugation using a solvent consisting of chloroform and ethanol in a volume ratio of 1: 4.

2.2. Preparing CdZnS/ZnS:N QRs through the in-situ doping system. For ZnS shell coating, 30 mg of washed CdZnS sample, 150 mg of zinc sulfate heptahydrate, 29.6 mg of S and 11 mL of OAm were placed in a 3-necked flask and heated at 120 °C for 1h under vacuum. This mixture was dissolved in both hexane and TEA, and then directly poured into the autoclave with a capacity of 50 mL. The amount of all the contents injected into the autoclave was fixed to 30 mL with decreasing amount of hexane as TEA increased. The autoclave was reacted at 200 °C for 30h. After completion of the reaction, Samples were washed in the same manner as the core cleaning method, and redispersed in chloroform. To develop CdZnS/ZnS:N/ZnS through three-step method, first, we coated ZnS shell according to the our previous work (ref). The synthesized CdZnS/ZnS QRs were added to the autoclave together with TEA and oleylamine in the glove box, and reacted at 150 °C for 1 hour to obtain CdZnS/ZnS:N QRs. After that, a ZnS shell was further grown through the above method to finally obtain CdZnS/ZnS:N/ZnS.

3. Characterization. Absorption spectra and photoluminescence (PL) spectra for optical analysis were obtained using UV-1800 spectrophotometer, SHIMADZU and F-7000 fluorescence spectrometer, and HITACHI, respectively. The energy levels were measured by cyclic voltammetry (C-V) 9 ACS Paragon Plus Environment


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analysis using Versastec, AMTEK. TEM images of the samples were obtained using JEM2100F HR, JEOL. The X-ray photoelectron spectroscopy (XPS) was used for the chemical composition analysis of the Sigma Probe and Thermo VG Scientific. Conductivity of the samples was obtained using HMS-3000, Ecopia. In order to analyze the E-field dependent PL quenching, samples dispersed in toluene were dropped on a Mo electrode having an electrode interval of 4 micro meters and aligned using a sine wave e-field of 100V. PL quenching was performed using AWG1000 (amplifier) of FTlab, CS-2000 (spectroradiometer) of KONICA MINOLTA and AWG1000P (arbitrary waveform generator) of FTlab. Conductive atomic microscopy (c-AFM) was measured with the NX20 Park Systems. The vertical current of films of the all samples were conducted in contact mode with CDT-contra cantilever under 0.08V sample bias.


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Figure 1. Schematic representation of CdZnS/nitrogen doped ZnS (core/shell) quantum rods using the In-situ doping system for improved electric field induced PL quenching efficiency.



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Figure 2. (a) Calculated energy diagram with both cyclic voltammetry and UV-visible spectroscopy, (b) XPS spectra from the S 2p region of samples that shows two peak, (c) cAFM images of samples, (d) Vertical current plots by c-AFM.


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Figure 3. E-field dependent PL quenching spectra and TEM images about samples



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Figure 4. Electric field-induced PL quenching properties of samples according to difference of doping method; (a) PL intensity depend on applied voltage with both three-step method and in−situ method (b) the on/off characteristics and TEM images with the three-step method, and (c) the in-situ method sample.


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Sample name

HOMO vs vacuum

LUMO vs vacuum

Band gap

/eV

/eV

/eV

Average vertical current /nA

On/off contrast efficiency (@100V) /%

S0

-6.12±0.03

-3.50±0.03

2.62

46.2

9.3

S3

-6.13±0.06

-3.51±0.06

2.62

113.1

15.1

S9

-6.15±0.05

-3.54±0.05

2.61

121.4

22.6

S15

-6.24±0.07

-3.68±0.07

2.56

203.2

25.3

Table 1. Energy levels and electrical properties of all the samples.

ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting UV visible spectroscopy and cyclic voltammetry characteristics, XPS spectra, Electrical conductivity, TEM and HR-TEM images. AUTHOR INFORMATION Corresponding Author *E-mail : [email protected] ORCID Duk Young Jeon : 0000-0002-9224-7769 Present Addresses ⊥

Current address: National NanoFab Center (NNFC), 291 Daehak-ro, Yuseong-gu, Daejeon

34141, Republic of Korea



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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ∥

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported and funded by LG Display Co., Ltd., and was supported by Global Research Development Center Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science and ICT(MSIT) (2015K1A4A3047100)

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An In-Situ Doping System to Improve Electric Field-Induced

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