Article pubs.acs.org/Langmuir
Electrostatic Stabilized InP Colloidal Quantum Dots with High Photoluminescence Efficiency Anush N. Mnoyan,† Artavazd Gh. Kirakosyan,† Hyunki Kim,† Ho Seong Jang,‡ and Duk Young Jeon*,† †
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Korea ‡ Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Korea S Supporting Information *
ABSTRACT: Electrostatically stabilized InP quantum dots (QDs) showing a high luminescence yield of 16% without any long alkyl chain coordinating ligands on their surface are demonstrated. This is achieved by UV-etching the QDs in the presence of fluoric and sulfuric acids. Fluoric acid plays a critical role in selectively etching nonradiative sites during the ligand-exchange process and in relieving the acidity of the solution to prevent destruction of the QDs. Given that the InP QDs show high luminescence without any electrical barriers, such as long alkyl ligands or inorganic shells, this method can be applied for QD treatment for application to highly efficient QD-based optoelectronic devices.
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INTRODUCTION Numerous studies recently have been conducted on semiconductor nanocrystals, so-called quantum dots (QDs), on the basis of their excellent size-dependent optical and electrical properties.1 In addition, many attempts to incorporate QDs into optoelectronic devices such as light-emitting diodes (LEDs), photovoltaics (PV), and transistors have been reported.2−4 However, as shown in the present research, in order to transport charge carriers between QDs and adjacent layers in a device, the electronic properties of surface ligands of the QDs should be considered thoroughly.5 Specifically, insulating alkyl ligands, which are required for controlling the growth of nanoparticles and providing colloidal stability, should be removed when the QDs are incorporated into an optoelectronic device. Several approaches to reduce the hindering effect of alkyl ligands and enhance charge carrier mobility in the aforementioned methods have been reported.6,7 Webber et al. reported that thermally unstable ligands which can be easily removed when QDs were annealed under 200 °C increased the electron mobility of a CdSe QD film by about 2 orders.6 In addition, Tang et al. recently reported that by using atomic ligands based on halide anions, high carrier mobility was achieved with good passivation of surface defects.7 De Angelis et al. presented uncapped InP QDs, with an average lateral size of 45 nm and a height of 4 nm, grown epitaxially on InGaP from a gas source.8 These epitaxial uncapped InP QDs showed luminescence sensitivity as methanol vapor was introduced on the surface. This is attributed to passivation of nonradiative trap states on the QD surface based on the electron donor ability of methanol. Although the aforementioned studies advanced the understanding of the electronic properties of QDs, the use of short and inorganic ligands typically reduces the luminescence © XXXX American Chemical Society
efficiency of core-only QDs. For example, it has been reported that capping by S2− decreased the photoluminescence (PL) quantum efficiency (QE) of CdSe core-only QDs from ∼13% to 2%.9 This likely stems from (a) the reduced quantum confinement effect (QCE) due to the absence of surface metalcoordinating ligands and/or (b) surface deterioration during ligand exchange. We have been seeking an alternative method to recover the luminescence efficiency by relieving surface deterioration, since the reduced effect of QCE cannot be completely avoided when ligands are replaced by electrically conductive ones. A process of eliminating surface deterioration for InP QDs by selectively etching of phosphorus dangling bonds with hydrofluoric acid (HF) has been reported, although the InP QDs contain long alkyl chain ligands coordinated on the surface.10 In this paper, we report for the first time that the coordinative alkylamine ligands of InP QDs can be removed at the same time as HF etching while attaining a high PL quantum yield (QY).
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EXPERIMENTAL DETAILS
All chemicals are purchased from Sigma-Aldrich and used without further purification: tris(dimethylamino)phosphine, P(N(CH3)2)3 (97%), indium(III) chloride, InCl3 (99.999%), oleylamine (OLA, 70%), toluene (99.8%, anhydrous), octane (99.0%), N,N-dimethylformamide (DMF, 99.8%, anhydrous), hydrofluoric acid (HF, 48% in water), and sulfuric acid (H2SO4, 95%). The InP QDs were synthesized by following a modified version of a previously reported solvothermal method.11 Briefly, 0.11 g of InCl3, 0.4 mL of P(N(CH3)2)3, and 10 mL of OLA were mixed with 5 mL of toluene in a glovebox under an argon atmosphere. The mixture was Received: March 6, 2015 Revised: May 6, 2015
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DOI: 10.1021/acs.langmuir.5b00847 Langmuir XXXX, XXX, XXX−XXX
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Langmuir tightly sealed in a 50 mL Teflon-lined autoclave. The autoclave was placed in a box furnace and then heated to 180 °C for 5 h and thereafter cooled slowly. The obtained InP QDs were separated from the nonreacted starting materials and impurities by centrifuging in a methanol/chloroform solution under 5000 rpm for 5 min (3000 rcf). After centrifugation, the precipitated QDs were redispersed in octane with a concentration of 10 mg mL−1. In order to investigate the combined process of ligand-exchange and etching of InP QDs, 3 mL of pristine InP QDs (InP-Pr) in octane was mixed with 3 mL of DMF, resulting in two phases of liquids (Figure 2a). Four types of InP samples were then prepared: (1) pristine-InP: as obtained, without acid solution; (2) InP with hydrofluoric acid (InP-F): 0.1 mL of HF was injected into the sample; (3) InP with sulfuric acid (InP-S): 0.1 mL of H2SO4 was added; and (4) InP with both hydrofluoric and sulfuric acids (InP-FS): here 0.1 mL of HF and 0.1 mL of H2SO4 were added. For the phase-transfer process of the QDs, samples were vigorously stirred for 10 min, and then the solutions were stored for 30 min in a dark place. Finally, all samples were irradiated under UV light. Furthermore, InP-FS QDs that were transferred into the DMF polar phase were selectively sampled and mixed with deinonized water (DI), and the obtained mixture was centrifuged under 5000 rpm for 5 min. The precipitated QDs were separated and redispersed in a DMF solvent. Additionally, a reference sample of HF etched InP QDs (InP-HF etch) has been prepared according to Adam et al.12 Photoluminescence spectra of the QDs were recorded by a Hitachi F-7000 FL spectrometer. A Shimadzu UV-3101 PC spectrophotometer was used to record the absorption spectra of the QDs. The QY of several types of InP QDs was measured by comparing absorption and integrated values of emission spectra with those of a “Coumarin 6” reference sample under excitation of 460 nm at a 1 mg/mL solution concentration.13,14 The particle size and the morphology of the InP QDs treated by different acid solutions were analyzed by means of transmission electron microscopy (TEM, JEM 3010, JEOL). The crystalline phase of the QDs was identified by a X-ray diffractometer (Rigaku D/max-RC, Rigaku), and the relative atomic ratios were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo/ MultiLab 2000). A luminescent decay time analysis of various types of InP QDs was carried out by acquiring the emission decay curves at 570 nm by using FL920 (Edinburgh Instruments) fluorescence decay analysis software and an EPL-470 picosecond pulsed diode laser (470 nm) as a light source. The average lifetime (τavg) values of conventionally HF etched (InP-HF etch) and our InP (InP-FS) QDs were determined as the average of a triexponential decay15 τavg =
α1τ12 + α2τ2 2 + α3τ32 = f1 τ1 + f2 τ2 + f3 τ3 α1τ1 + α2τ2 + α3τ3
Figure 1. Photo images of InP-Pr (#1, #2), InP-F (#3, #4), InP-S (#5, #6), and InP-FS (#7, #8) QDs taken under daylight (top) and UV light (bottom). The upper part of solution in each sample is the nonpolar phase of octane, and the bottom part is the polar phase of DMF.
#3). This phenomenon is assigned to the partial removal of stabilizing alkyl ligands. Weak light emission of the InP-F sample is defined by slightly etching InP-Pr. The QY of InP-F is determined to be about 1−2% (Figure 1, sample #4). In the third case, the InP QDs were etched out, and the solution became transparent when only sulfuric acid (H2SO4) was injected (Figure 1, samples #5 and #6). In contrast to all these cases, the InP QDs with simultaneous addition of HF and H2SO4 acids showed drastic differences: the InP QDs were completely transferred to the polar DMF phase (Figure 1, sample #7). Under UV illumination for 40 min, the QDs became actively luminescent with emission at 585 nm wavelength (Figure 1, sample #8). Figure 2 shows a schematic diagram of the proposed method, illustrating pristine (left InP-Pr) and electrostatically stabilized
(1)
where τ is the lifetime, α is the pre-exponential factor, and f is the fractional contribution of each decay component. Here, we assume that the intensity decay is given by the sum of individual single exponential decays: n
I(t ) =
⎛ t⎞ ⎟ i⎠
∑ αi exp⎜⎝− i=1
(2)
The goodness of fit was judged by χ (1 ± 0.2) values.
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2
Figure 2. Schematic diagram of the ligand exchange process; pristineInP QDs (left) undergo a ligand exchange process in hydrofluoric acid (HF) and sulfuric acid (H2SO4) solution mixture under UV irradiation, resulting in electrostatically coordinated QDs (right).
RESULTS AND DISCUSSION Figure 1 shows photo images of the samples under daylight (upper row) and irradiation at 365 nm wavelength UV light (bottom row). The InP-Pr QDs were well dispersed in the nonpolar octane phase of the octane-DMF mixture (Figure 1, sample #1). It is worth noting that the InP-Pr QDs exhibit poor emission under UV irradiation (Figure 1, sample #2). A low QY of InP QDs was previously observed by Li as well.11 However, when a hydrofluoric acid (HF) solution was injected into the solution, the QDs were agglomerated in the nonpolar phase and located at the interface of the two phases (Figure 1, sample
(right InP-FS) QDs. The InP-Pr QDs are coordinated by long alkyl ligands with nonradiative phosphorus dangling bonds at the QD surface. The InP-FS QDs are electrostatically stabilized by negatively charged sulfate (SO42−) ions with DMF ligands. In addition, surface phosphorus dangling bonds are removed, resulting in enhanced luminescence efficiency. The following sections clarify this scheme with a discussion of the results of B
DOI: 10.1021/acs.langmuir.5b00847 Langmuir XXXX, XXX, XXX−XXX
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Langmuir the etching process and an analysis of PL, UV/vis absorption, and XPS data of the samples. The successful formation of InP QDs with a zinc blende crystalline structure (space group: F43̅ m) was confirmed by the X-ray diffraction analysis, as shown in Figure 3a. Application of
Table 1. Acidity (pH) of InP QD Solution with Various Acid Additives samples pristine InP hydrofluoric acid−InP sulfuric acid−InP hydrofluoric and sulfuric acids−InP
pH 10.5 3.7 0.85 2.65
± ± ± ±
0.3 0.3 0.05 0.15
destructive etching process in the InP-S solution is mitigated by reducing the amount of sulfuric acid to make the solution less acidic, as shown in Supporting Information Table S1 and Figure S2. This indicates that addition of hydrofluoric acid stops the destructive etching process by making the DMF solution less acidic while attaining a solution that is acidic enough to detach the amine ligands. Figure 5 shows the atomic ratio obtained by a XPS analysis for three different samples. Indium (In), phosphorus (P), Figure 3. XRD pattern of InP-Pr (a) and TEM images of (b-1) InP-Pr and (b-2) InP-FS QDs.
the Debye−Scherrer equation to the detected XRD pattern suggests the particle size is in a range of 3−4 nm. This agrees with of the range obtained by the TEM investigation. Parts b-1 and b-2 of Figure 3 show the TEM images of InP-Pr and InPFS QDs. InP-Pr consists of well-dispersed spherical particles with a size of 2.7 ± 0.5 nm. The InP-FS sample also consists of spherical and dispersed particles, similar to InP-Pr, whereas the particles are etched and their size is reduced to 2.4 ± 0.5 nm. Figure 4 shows the absorption and emission spectra of InPPr, InP-FS, and reference InP-HF etched QDs. The absorption
Figure 5. Atomic ratio survey for InP-Pr, InP-F, InP-S, and InP-FS QDs by XPS analysis.
carbon (C), nitrogen (N), and sulfur (S) atoms were the focus of the analysis (a table of atomic contents of elements is presented in Supporting Information Table S2). The reduced P/In ratio for the InP-FS QDs compared with that of InPr QDs indicates that the surface phosphorus atoms of the QDs are etched out. The C/N ratio is changed from 18, which is appropriate to C/N ratio of oleylamine (C18H35NH2), to 4. Considering this fact in parallel to the difference of C/N ratio in DMF (3:1) and OLA (18:1), it is concluded that 93% of the surface ligands on InP-FS are composed of DMF, and the remaining 7% are OLA. This assumption is also supported by the reduced C/In ratio after the simultaneous hydrofluoric and sulfuric acid treatment when long alkyl ligands are replaced by shorter ones. The N/S ratio is 1 for InP-FS QDs, indicating that approximately equivalent amounts of DMF and SO42− molecules are coordinated on the surfaces of the QDs. Figure 6 shows a comparison of the PL decay curves of InP-Pr, HFetched InP, and our InP-FS QDs. The comparison of these three types of QDs shows that InP-Pr, which has the largest number of defect sites (dominantly P dangling bonds), undergoes the fastest decay. The other two cases, where the surface of the QDs is passivated by either method, show slower decay and higher QY. In particular, the combined treatment of InP QDs by HF and H2SO4 results in extended fluorescent decay with an average decay time of 75.5 ns, while the average
Figure 4. Absorption and PL spectra of InP-Pr, InP-HF etched, and InP-FS QDs.
features of all three types of QDs are similar, while only InP-FS and InP-HF etched QDs emitted light with luminescence yield of 16% and 15%, respectively. The hydrofluoric acid in this experiment is considered to have dual effects of (1) removing the nonradiative sites from the surface of the QDs and (2) terminating the destructive etching process of InP QDs by sulfuric acid. Regarding the first effect, it was previously reported that by etching InP QDs using HF and UV irradiation, the luminescence efficiency of InP QDs increases since fluorine ions can selectively remove nonradiative phosphorus dangling bonds.12 Moreover, the addition of the HF solution reduced the acidity of the DMF/sulfuric acid solution from pH = 0.85 to pH = 2.65, as shown in Table 1 and Supporting Information Figure S1. It is also confirmed that the C
DOI: 10.1021/acs.langmuir.5b00847 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Author Contributions
A.N.M., A.G.K., and H.K. have contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported in the scope of Korea-UK Collaboration in Plastic Electronics (N01150198).
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Figure 6. Transient PL emission decay curves of pristine InP-Pr, conventional HF-etched InP-HF, and our InP-FS QDs solutions detected at 580 nm emission wavelength.
decay time of HF etched InP-HF is 51.7 ns (a table of the decay time values is presented in Supporting Information Table S3). The extended emission decay corresponds to effective removal of dangling bonds and passivation of defect sites on the surface of the QDs, leading to a significant reduction of the nonradiative decay rate.16 The elimination of trapping states thus results in improved quantum yield values. Additionally, it was confirmed by a zeta-potential analysis that the InP-FS QDs are electrostatically stabilized showing a +14 mV zeta potential value when the QDs are dispersed in methanol. This means that surfaces of the QDs were positively charged by In cations, and most of them are electrically stabilized by negatively charged SO42− anions. Therefore, the overall scheme can be drawn as follows: the equivalent numbers of DMF and SO42− ions are bonded and electrically stabilized on the surface of the QDs, as shown in Figure 2.
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CONCLUSION In this report, we have demonstrated electrostatically stabilized InP QDs exhibiting high luminescence yield (∼16%) without any long alkyl surface ligands. It was shown that to synthesize electrostatically stabilized and luminescent InP QDs, the addition of fluoric acid with another strong acid is crucial to terminate the destructive etching process during ligand exchange and to initiate the UV-irradiative etching process. The proposed method to remove electrical barriers while attaining luminescence efficiency, it could open a new way to fabricate highly efficient QD-based optoelectronic devices.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1 and S2 and Tables S2−S3. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00847.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected], tel +82-42-350-3337, fax +82-42350-3310 (D.Y.J.). Present Addresses
A.G.K.: Advanced Technology Development Center, ELK Corporation, 687 Gwanpyung-dong, Yuseong-gu, Daejeon 305509, Korea. H.K.: Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01375. D
DOI: 10.1021/acs.langmuir.5b00847 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Influence of Intrinsic Defects and External Impurities. J. Appl. Phys. 2012, 111, 124314.
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DOI: 10.1021/acs.langmuir.5b00847 Langmuir XXXX, XXX, XXX−XXX
