Optical Characteristics of the Surface Defects in InP Colloidal

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Optical Characteristics of the Surface Defects in InP Colloidal Quantum Dots for Highly Efficient Light Emitting Applications Eunseog Cho, Taehyung Kim, Seon-myeong Choi, Hyosook Jang, Kyoungmin Min, and Eun Joo Jang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01947 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Nano Materials

Optical Characteristics of the Surface Defects in InP Colloidal Quantum Dots for Highly Efficient Light Emitting Applications Eunseog Cho*, Taehyung Kim, Seon-myeong Choi, Hyosook Jang, Kyoungmin Min, and Eunjoo Jang* Samsung Advanced Institute of Technology, Samsung Electronics, 130 Samsung-ro, Suwon, Gyeonggi-do, 16678, Republic of Korea.

Abstract The colloidal quantum dots (QDs) have inherent multiple dangling bonds (DBs) on the surface atoms due to the intrinsic weak bonding nature and steric hindrance of organic ligands. Such DBs can be the trap sites for charge carriers, leading to the reduction of luminescence efficiency, but their detailed characteristics are still unclear. In the present study, we disclose the electronic and optical features of the surface DBs of InP QDs via density functional calculations combined with experimental evidences. For InP core, both In-DB and P-DB create invariant DB energy levels with respect to the core size and their optical transition intensities exhibit an order of magnitude smaller than the band-edge transition. The InDB and P-DB generate a deep trap level at -3.947 eV and a shallow trap level at -5.717 eV respectively, and the deep trap level corresponds to the origin to induce the trap emissions. The passivation with ZnS shell on InP core significantly modifies the optical properties of both DBs to the radiative transition even when the passivating shell partially covers the InP surface. The ZnS shell growth pushes the energy levels of the In-DB and P-DB to near the band edges and makes the orbitals more delocalized. Such modified roles of the DBs significantly improve the optical intensities comparable to those of the band-edge transition, which is validated by the absorption calculations and luminescence measurements. Keywords: InP quantum dot, surface defect, dangling bond, optical property, density functional theory calculation, partial shell passivation *Corresponding author. E-mail address: [email protected] (E. Cho), [email protected] (E. Jang) ACS Paragon Plus Environment

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I. INTRODUCTION Quantum dots (QDs) have been proposed as useful light emitting materials in display applications because of their high quantum efficiency (QE), easy color tunability, and excellent color gamut.1 In particular, colloidal QDs have attracted significant attention because they are synthesized via simple, inexpensive, and scalable methods. Also, QDs allow practical engineering of electronic and optical structures by controlling surface properties.2,3 Recently, indium phosphide (InP) QD is attracting increasing attention as an environmentally benign material.4-14 Specifically, InP QD emits lights spanning entire visible ranges, but their relatively low QE14,15 have been an issue to be solved. For colloidal InP core QD, the surface In or P atoms should be well-passivated by organic ligands to obtain high QE, but the full passivation with the ligands is barely achieved owing to the intrinsic weak bonding nature and the steric hindrances of the ligands. Thus, insufficient surface coverage inevitably causes multiple dangling bonds (DBs) on the surface, which act as trap sites for charge carriers to reduce the luminescence efficiency. Although such a general picture of the DBs has been widely introduced2,16, the fundamental nature of the surface DBs and their electronic and optical characteristics have not been studied in detail because the actual surface structures of the InP QD are extremely complicated depending on the structure, size, shape, and the nature of the ligands. It is very challenging to synthesize InP QDs with controlled surface structures and to make correlations between surface defects and optical properties. Moreover, the surface oxidation caused by the consequence of the synthesis that involves viable organic ligands4 further generates complicated surface conditions. In this regard, theoretical studies are critically important to provide atomistic and fundamental information. Fu and Zunger theoretically examined the DBs of InP QD based on the semi-empirical pseudopotential method several years ago.17 However, to the best of our knowledge, subsequent calculations have not performed to elucidate the surface DBs and their correlation with optical properties of InP QDs due to the complexities in their surface defects. Our study is designed to clarify overall features in the optical behavior of DBs of InP QDs based on both theoretical calculation and experimental results. We performed density functional theory (DFT) 2 ACS Paragon Plus Environment

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calculations based on the hybrid functional approach combined with cluster models to describe both InP core and InP/ZnS core/shell QDs. It is known that the generalized gradient approximation (GGA) method produces considerably lower band gaps compared to the experimental ones. Specifically, the band gap of GGA (0.59 eV) for InP bulk gives only 40% of the experimental gap (1.41 eV at 70 K)18, which naturally leads to skewed estimation of the defect energy levels.23 On the contrary, the hybrid functional approach we adopted here has been proved to give accurate band gaps (1.401 eV13 for bulk InP), although it demands a large computational resource. Meanwhile, the slab model has been widely used for the defect calculations of QDs to understand the electronic structures affected by quantum confinement effects (QCE) in more simple way. However, this one-dimensional confinement is likely to reduce the amount of QCE and lead to unreliable defect energy levels. In this study, we resolved this type of error by adopting the cluster model that deals with all three-dimensional confinements. Based on our model, more accurate electronic and optical characteristics of DBs on the InP QD are disclosed, and their dramatic change in the optical properties during the shell coating are well explained. Furthermore, the calculated values for the surface DBs are fully supported with the experimental measurement of the photoluminescence (PL) of the synthesized InP QDs with specially designed structure at various temperatures.

II. Methods II.1. Construction of Cluster Models The InP clusters (diameter d = 1.07–2.29 nm) were generated by cutting out the zinc blende (ZB) bulk structure of InP, and fictitious hydrogen atoms with charges of 0.75e and 1.25e were selected to saturate the P and In atoms, respectively. In this study, we generated the stoichiometric clusters rather than In-rich ones to include various DB states. However, our model accurately describes the optical properties of the experimental QDs synthesized under In-rich condition, which will be addressed in the next section. In addition, considering that P-rich InP QD can also be synthesized under certain synthetic condition24, and 3 ACS Paragon Plus Environment


basically the stoichiometry strongly depends on the synthetic methods and precursors, the stoichiometric model is adequate to obtain the insight and also can be widely applied as a general model. The fictitious hydrogen atom was chosen as the simplest model ligand and it was proved to reproduce experimental band gaps of the InP QDs according to their sizes when combined with the hybrid functional approach proposed in the previous study.13 This type of ligand model is expected to clarify the atomistic origin of the complicated surface properties by allowing artificial control of the surface sites. A representative InP cluster model, (InP)119H158 (d = 2.29 nm) is shown in Fig. 1(a) and other cluster structures with different number of atoms are shown in Fig. S1, Supporting Information (SI). The surface structures of the clusters were distinguished by the four different facets, (100)In, (111)In, (100)P, and (111)P, based on the number of fictitious hydrogen atoms bonded on the surface atoms. The surface In/P atoms of the (100) facet denoted as (100)In/(100)P should be bonded to two fictitious atoms to saturate their surface DBs, but the surface In/P atoms exposed on the (111)/(111) facet denoted as (111)In/(111)P should be bonded to one fictitious atom for full passivation. It is noted that the (111) facet is different from the (111) facet in the ZB structure; on the (111) facet, bare In and P atoms on the surface have one and three DBs, respectively, while the bare In and P atoms on (111) surface have three and one DBs, respectively. Thus, for our cluster models, we assume that bare (111)In/(111)P atoms with three DBs are not exposed on the surface because the large number of DBs can make the clusters unstable. The structural features of the four facets are also shown in the form of the slab models with passivating atoms, as shown in Fig. S2. We intentionally produced surface defects of InP QDs by eliminating a fictitious hydrogen atom bonded to each In and P surface atom of pristine (fully passivated) cluster (denoted as In-DB and P-DB, respectively). For example, in order to construct the defective structure ((InP)119H157) with (111)In-DB, a fictitious atom randomly selected among a large number of H atoms bonded to the (111)In surface atoms is removed from the pristine (InP)119H158 cluster. The DB position dependency in energy level was tested with three defective structures per facet which were constructed by random H selection from the smallest (InP)13H30 pristine cluster. The DB energy levels of three structures were located within 0.1 eV, and the 4 ACS Paragon Plus Environment

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differences in their band gaps were also trivial (< 3%) for all facets, as shown in Table S1. Furthermore, even the small position discrepancy is expected to further decrease with increase in the cluster size because of the reduction in the curvature and position sensitivity. This indicates that random H selection is reasonable for the construction of defective structures. For the InP/ZnS core/shell model, S and Zn surfaces were passivated with the hydrogen atoms with charges of 0.5e and 1.5e, respectively, to satisfy the octet rule.

II.2 First-Principles Calculations The first-principles calculations for InP QDs with hybrid functional proposed by Heyd-ScuseriaErnzerhof (HSE06) were carried out using the Vienna ab initio simulation Package (VASP). The energy cutoff was selected as 280 eV (cutoff tests are shown in Fig. S3), and all atomic positions were fully relaxed until the Hellmann–Feynman force on each atom was lower than 0.02 eV/Å. The absorption coefficient (α) was obtained from the calculated dielectric constants that satisfy the same accuracy of the electronic structure calculations that are defined as follows: α=

4π 1 1/2 1/2 ― 𝜀1 + (𝜀21 + 𝜀22) ) ( 𝜆 2

(1)

where 𝜀1 and 𝜀2 denote the real and imaginary parts, respectively, of the dielectric constant, and 𝜆 denotes the wavelength of the light. In order to examine the DB contribution for the absorption, the energy levels that lead to the optical transitions with the DB levels were directly extracted from the imaginary part of the dielectric tensor. The additional details for the calculations can be found in SI.

II.3 Synthesis and Measurements The InP QD was synthesized based on the typical method.19 The 0.2 mmol of indium acetate and 0.6 mmol of palmitic acid were mixed with 10 mL of octadecene, and then degassed for 1 hour at 120 °C under vacuum. Subsequently, the temperature was increased to 280 °C, and 0.1 mmol of 5 ACS Paragon Plus Environment


tris(trimethylsilyl)phosphine, (P(SiMe3)3, TMS3P), was rapidly injected into the solution. The size of InP core was controlled via multiple injections of In and P precursors and the reaction time. For the temperature dependent PL measurement, the samples were prepared by drop-casting of the QD-toluene solution on a glass substrate and mounting it in a cryostat. Then, the samples were excited by using a 405 nm pulsed diode laser. For the growth of partial ZnS shell (p-shell), zinc oleate (0.12 mmol) and 0.4 M sulfur in trioctylphosphine (TOP, 0.075 mmol) were added to the reaction flask with the InP core and reacted at 280 °C for 30 min in a manner similar to the previously reported process.22 Scheme 1 shows the schematic illustration for synthetic process and PL measurement of the InP/ZnS core/p-shell QDs.

III. RESULTS III.1 Optical Properties of Surface DBs in InP Core Structures Figure 1(b) shows changes in the energy levels of surface defects per facet and of the band edges according to the InP cluster size. Each of the energy levels should be aligned to the vacuum level of 0 eV assigned by the stationary potential energy from the averaged electrostatic potentials over the radial direction of the clusters. When the size of the InP clusters increases, the level of the highest occupied molecular orbital (HOMO) increases and that of the lowest unoccupied molecular orbital (LUMO) decreases, as expected. The decrease in the LUMO level is approximately 1.5 times larger than the enhancement in the HOMO level when the size of QD increases. Although the band gaps after DB formations become slightly wider than the pristine gaps in all DB cases, the variances (

Optical Characteristics of the Surface Defects in InP Colloidal

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