Subscriber access provided by University of Newcastle, Australia
Communication
Mapping Charge Distribution in Single PbS Core – CdS Arm Nano-Multipod Heterostructures by Off-Axis Electron Holography Rajesh Chalasani, Alexander Pekin, Alexander Rabkin, Ran Eitan Abutbul, Oswaldo Dieguez,, Yaron Kauffmann, Yuval Golan, and Amit Kohn Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04957 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Mapping Charge Distribution in Single PbS Core – CdS Arm Nano-Multipod Heterostructures by Off-Axis Electron Holography Rajesh Chalasani†,ǁ, Alexander Pekin‡,§, Alexander Rabkin‡,§, Ran E. Abutbul‡,§, Oswaldo Diéguez†,ǂ, Yaron Kauffmann⊥, Yuval Golan*,‡,§ and Amit Kohn*,†,ǁ †
Department of Materials Science and Engineering, Tel Aviv University, Ramat Aviv, Tel Aviv
6997801, Israel. ǁ
Research Center for Nanoscience and Nanotechnology, Tel Aviv University, Ramat Aviv, Tel
Aviv 6997801, Israel. ǂ
The Raymond and Beverly Sackler Center for Computational Molecular and Materials Science,
Tel Aviv University, Ramat Aviv, Tel Aviv 6997801, Israel ‡
Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105,
Israel. §
Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev,
Beer-Sheva 84105, Israel. ⊥Department
of Materials Science and Engineering, Technion - Israel Institute of Technology,
Haifa 32000, Israel.
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ABSTRACT We synthesized PbS core – CdS arm nano-multipod heterostructures (NMHs) that exhibit PbS{111}/CdS{0002} epitaxial relations. The PbS‒CdS interface is chemically sharp as determined by aberration corrected transmission electron microscopy (TEM) and compared to density functional theory (DFT) calculations. Ensemble fluorescence measurements show quenching of the optical signal from the CdS arms indicating charge separation due to the heterojunction with PbS. A finite-element three-dimensional (3D) calculation of the Poisson equation shows a type-I heterojunction, which would prevent recombination in the CdS arm after optical excitation. In order to examine charge redistribution, we used off-axis electron holography (OAEH) in the TEM to map the electrostatic potential across an individual heterojunction. Indeed, a built-in potential of 500 mV is estimated across the junction though as opposed to the thermal equilibrium calculations, significant accumulation of positive charge at the CdS side of the interface is detected. We conclude that the NMH multipod geometry prevents efficient removal of generated charge carriers by the high energy electrons of the TEM. Simulations of generated electron-hole pairs in the insulated CdS arm of the NMH indeed show charge accumulation in agreement with the experimental measurements. Thus we show that OAEH can be used as a complementary methodology to ensemble measurements by mapping charge distribution in single NMHs with complex geometries. KEYWORDS. PbS core – CdS arm nano-multipod heterostructures, off-axis electron holography, electrostatic phase mapping, built-in potential, 3D electrostatic simulations, Focal series exit wave phase reconstruction.
ACS Paragon Plus Environment
Page 2 of 30
Page 3 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Quantum confinement in semiconducting crystals, which is achieved by controlling the size and shape of nanoscale crystals,1-4 is a topic of considerable scientific interest and technological applications, for example in optoelectronic devices.5 Heterostructures containing two or more different semiconducting nanocrystals add further control of both charge redistribution and the built in potential due to conduction and valence band offsets.6 Consequently, nanoscale semiconductors achieve additional novel optical properties in heterostructures for applications such as water splitting, single electron transistors and photovoltaic cells.7-9 The resulting charge separation and trapping across the junction depends on the abruptness of the structural interface, size, and morphology of each material.6,10 When characterizing nanoscale semiconductor heterostructures, a typical approach is ensemble measurements averaging the properties of many particles. However, differentiating between extrinsic (e.g. size dispersion) and intrinsic (e.g. surface states) properties is challenging. Thus, mapping the charge distribution in single nanostructures is a complementary measurement that contributes to their fundamental scientific understanding and subsequent technological development. Yet, to date, few studies have determined the band alignment and measured the built-in potential across heterojunctions in single nanostructures. Scanning probe microscopy has been used for characterization of heterostructured nanoscale semiconductors.11 For example, Kelvin probe force microscopy (KPFM) and Electric Force Microscopy (EFM) measured the built-in potential in single PbS‒CdS and CdS‒Cu2S nanorods, respectively.12,13 However, the characterization of nanoscale interfaces by probe methods is sensitive to the surface of the semiconductor, which often has different properties from the interior of the structure.14 Moreover, probe methods are limited to planar structures due to geometrical limitations.15 Such
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
studies are occasionally accompanied by TEM analysis which can determine the composition and crystal structure, even at atomic scale lateral resolution.13 An additional capability of TEM is OAEH, which can map variations in electrostatic potential at high lateral resolution, around 1 nm, and with a stated potential sensitivity of 30 mV.16,17 Both the phase and amplitude of the electron wave that transverse the sample are reconstructed from the electron hologram. The electrostatic potential can then be calculated from the relative phase shift of the electron wave. For non-magnetic samples, and assuming the absence of diffraction contrast, the phase change of the electron wave, ∆φ, is: 18 Δ, =
, , 1
where CE is the interaction constant, V is the electrostatic potential, and z is the direction of the optical axis. For semiconductors, the electrostatic potential is a combination of the mean inner potential (MIP) of the material,19 the thermal equilibrium of the redistributed charge,20 and excess generated charge carriers due to interaction with the electron beam.21 In the case of semiconducting materials prepared for TEM samples, significant stray electrostatic fields outside the sample have been reported,22,23 attributed to the reduced dimensions. Therefore, calculation of the phase change in nanoscale semiconductor heterostructures should be extended along the electron trajectory beyond the sample thickness. Thus, OAEH experiments enable to map the electrostatic potential for the entire thickness of the sample in addition to surface effects. OAEH has been applied to map dopant distribution in single Si nanowires, 50-100 nm in diameter.22-25 Moreover, for these relatively larger structures compared to the PbS core – CdS arm structures reported here, such measurements can be undertaken while applying external electric bias, for example to study its influence on the potential distribution across a silicon p-n junction.23 OAEH can also map three dimensional electrostatic potentials in combination with
ACS Paragon Plus Environment
Page 4 of 30
Page 5 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
electron tomography.26 Consequently, OAEH TEM studies of single nanoscale heterostructures complemented with TEM based structural analysis can contribute towards understanding of charge confinement and redistribution. Nanoscale semiconductor heterostructures can be prepared by wet, surfactant assisted chemical methods to achieve varied geometries, often as core-shell structures because the shell provides an efficient passivation of surface trap states of the core, giving rise to increased fluorescence quantum yield.27 Additional geometries achieved are rod, dot-rod, dumbbell, tetrapod and hyperbranched architectures. Tetrapod heterostructures have been reported for CdSe‒CdS, CdTe‒CdS and ZnTe‒CdS systems where CdSe, CdTe or ZnTe (zinc blende) are the core and CdS (wurtzite) is the arm structure.6,28,29 For wurtzite CdS, {0001} facets grow epitaxially on {111} facets of the zinc blende core. For these reported tetrapods, a tetrahedral symmetry occurs following a two-step sequential growth, which manipulates the crystal phase in those semiconductors with zinc blende–wurtzite polytypism along with anion-terminated {111} zinc blende surface forming a continuous polar/polar interface with the cation-terminated {0001} wurtzite surface.6 Here, we report on mapping charge distribution in a single PbS‒CdS heterostructure, which is a system with potential applications in photovoltaic cells. It has been shown that CdS increases power conversion efficiency and photo stability of PbS based solar cells.8,30 Chemical synthesis procedures for the preparation of PbS31-33and CdS34,35 nanocrystals have been established. Previous reports of PbS‒CdS heterostructures are core-shell and thin film structures but synthesis of heterostructures with rock salt‒wurtzite phases resulting in differing geometries than core‒ shell (rock salt‒zinc blende) has not been reported. Indications of the feasibility of such chemical synthesis is the report on rock salt PbSe nanocrystals grown on (0001) facets of wurtzite CdSe
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
nanorods.36 We present the synthesis of unique heterostructures with a PbS rock salt core and CdS wurtzite arms that exhibit well defined crystallographic orientations. Since ensemble photoluminescence (PL) measurements show that fluorescence emission from CdS is quenched in PbS–CdS core-arm structures, we studied the electrostatic potential distribution across the core-arm structures of single PbS‒CdS NMH using OAEH. We compared quantitatively our experimental results to finite-element 3D electrostatic simulations. We find that the potential variation in the NMH, around 2 V, is placed across the arm of the large bandgap CdS arm. We explain this measurement by excess electron-hole pairs generated by interaction with the electron beam. Due to the type I band alignment at thermal equilibrium, the excess holes in the CdS arm accumulate at the interface with the PbS cube. PbS‒CdS NMHs were prepared in two stages based on work reported by Efrima and coworkers.35 In the first stage, the precursors, lead-ethyl xanthate and cadmium-ethy xanthate were synthesized separately by dissolving potassium ethyl xanthogenate and metal (Cd or Pb) perchlorate hydrate in water. Metal (Cd or Pb) ethyl xanthates are insoluble in water and precipitate out from reaction mixture. After washing with water, both precipitates were dried and used for synthesis of NMHs. In the second stage, both precursors were decomposed simultaneously in a one-step process at 70 oC in octadecylamine (ODA) which serves as both solvent and capping agent. This procedure produced PbS‒CdS NMHs with a minor fraction of separate PbS and CdS nanoparticles. Figure 1a shows a bright field TEM image of nanoparticles with core-arm morphology of PbS‒CdS heterostructures. A high resolution transmission electron microscopy (HRTEM, phasecontrast) image of the heterostructure is shown in the Figure 1b. The reflections in the power spectrum of the PbS core (Figure 1c) and CdS arm (Figure 1d) are indexed to cubic rock salt PbS
ACS Paragon Plus Environment
Page 6 of 30
Page 7 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Figure 1. (a) Bright Field TEM image of PbS core ‒ CdS arm heterostructures. (b) Highresolution (phase-contrast) TEM image of single NMH. Inset shows a power spectrum from the interface region (region denoted by red square). (c) and (d) are the power spectra of PbS core and CdS arm, respectively. Zone axis (ZA) of PbS and CdS are [0-11] and [1120], respectively. (space group Fm3m) and hexagonal wurtzite CdS (space group P63mc), respectively. Figure 1b demonstrates stacking faults and defects in CdS arms. Stacking faults are attributed to the coexistence of zinc blende and wurtzite phases in the same CdS arm. Both phases are thermodynamically stable with a small energy difference of around 0.01 J/m2 according to our DFT calculations, as we report later. The inset in the Figure 1b shows that at the interface, (111) planes of PbS and the (0002) planes of CdS are parallel. These observations infer that the epitaxial relation between PbS core and CdS arm is PbS{111}||CdS{0002}, as reported for PbS films grown on CdS single crystals.37 At the interface, in-plane symmetry, similarity in atomic arrangement and a reduced misfit (
