Surface Traps in Colloidal Quantum Dots: A Combined Experimental

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Surface Traps in Colloidal Quantum Dots: A Combined Experimental and Theoretical Perspective Carlo Giansante, and Ivan Infante J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02193 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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The Journal of Physical Chemistry Letters

Surface Traps in Colloidal Quantum Dots: A Combined Experimental and Theoretical Perspective Carlo Giansante†,‡ and Ivan Infanteº †

Dipartimento di Matematica e Fisica ‘E. De Giorgi’, Università del Salento, via per Arnesano, 73100 Lecce, Italy ‡

NANOTEC-CNR Istituto di Nanotecnologia, via per Arnesano, 73100 Lecce, Italy

ºDepartment of Theoretical Chemistry, Faculty of Science, Vrije Universiteit Amsterdam, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

AUTHOR INFORMATION Corresponding Author * [email protected]

ACS Paragon Plus Environment

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AUTHORS' BIOGRAPHIES: Carlo Giansante, born and raised in Pescara (IT), studied chemistry at the Università di Bologna (IT) where, in 2008, he earned his doctoral degree dealing with discrete supramolecular systems and their photophysical and electrochemical properties in the group coordinated by Prof. Vincenzo Balzani. During this period, he spent six months at the University of California at Santa Barbara (US) working on non-linear optical properties of conjugated polymers. He then stepped up the ladder of matter complexity at the Université de Bordeaux (FR) studying self-assembled organic nanostructures and their photophysics down to the single molecule level. He’s now researcher at the Istituto di Nanotecnologia NANOTEC-CNR in Lecce (IT) where he has established his own research line on nanoscopic surfaces and interfaces using colloidal semiconductor nanocystals as soluble frameworks, with regard to optoelectronic and photonic applications. Carlo is co-author of some papers published in international peer-reviewed journals, raised little money, and got no awards.

Ivan Infante studied chemistry at the Universitá della Basilicata (IT). He then moved to the Vrije Universiteit in Amsterdam (NL) where he earned his PhD in October 2006 under the supervision of Lucas Visscher and Evert Jan Baerends, with a thesis aimed at the spectroscopic characterization of actinide molecules using highly precise ab initio computational methodologies. After a two-years postdoc experience under the supervision of Laura Gagliardi at the University of Geneva (CH), he was bestowed a Juan de la Cierva grant and moved to the University of the Basque Country (ES) where he could pursue his independent research career. In May 2014, he was awarded a prestigious Vidi grant from The Netherlands organization for scientific research. This grant has enabled him to establish his own research group as an Assistant Professor at the Vrije Universiteit in Amsterdam (NL). His current research is in the field of colloidal semiconductor nanocrystals, with a special emphasis in characterizing trap states combining theoretical calculations and experiments through his many collaborations.


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ABSTRACT

Surface traps are ubiquitous to nanoscopic semiconductor materials. Understanding their atomistic origin and manipulating them chemically have capital importance to design defect-free colloidal quantum dots and make a leap forward in the development of efficient optoelectronic devices. Recent advances in computing power established computational chemistry as a powerful tool to describe accurately complex chemical species and nowadays it became conceivable to model colloidal quantum dots with realistic sizes and shapes. In this perspective, we combine the knowledge gathered in recent experimental findings with the computation of quantum dot electronic structures. We analyze three different systems: namely, CdSe, PbS, and CsPbI3 as benchmark semiconductor nanocrystals showing how different types of trap states can form at their surface. In addition, we suggest experimental healing of such traps according to their chemical origin and nanocrystal composition.

TOC GRAPHICS

KEYWORDS. Quantum Dots – Colloidal Semiconductor Nanocrystals – Surface Chemistry – Trap States – Density Functional Theory


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Colloidal semiconductor nanocrystals (also known as quantum dots, QDs) show size-dependent optoelectronic properties governed by quantum confinement effects, which are a key feature for QD-based applications, especially in (electro-)luminescent devices.1–3 Colloidal QDs usually span a 2-10 nm diameter thus displaying a large number of atoms at the surface, as opposed to bulk semiconductors in which the surface is only a tiny fraction of the whole solid. The QD surface is a highly dynamic region, coordinated by chemical species (ligands) and exposed to the surroundings (solvents and other species in solution, matrices, etc.) that can have drastic effects on the QD properties. These effects are typically ascribed to the lower coordination of surface atoms compared to bulk atoms, which may lead to localized electronic states or highly reactive sites, which are prone to chemical and redox processes. In these conditions, it is highly likely the formation of shallow, or deep, mid-gap states as surface traps that provide pathways for non-radiative exciton recombination, detrimental for QD-based optoelectronic applications. The most exploited approach to eliminate surface traps relies on the growth of a (thick) shell of a wider band gap material around the (photo-)active core, thus moving surface defects to the outer region of the inorganic shell.4 In this configuration, the exciton is localized in the core, thus leading to almost unitary photoluminescence quantum yields (PLQY), as signature of the low probability of trapping photo-generated charge carriers in the outermost surface sites.5,6 Core/shell hetero-structures have however drawbacks: charge carrier localization in the core hinders transport in core/shell QD solids and lattice mismatch between the core and the shell materials may induce strain, ultimately deteriorating the overall optoelectronic properties.


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The actual elimination of trap states in core-only QDs has therefore paramount importance towards application and to this aim it is mandatory to deeply understand their atomistic origin. In this framework, the main questions we address are the following: (1) What are surface traps? (2) How do they originate? (3) How can we eliminate them? Pull Quote 1: “What are surface traps in colloidal QDs? How do they originate? How can we eliminate them?” To answer these questions, we believe that it is very important to combine the knowledge on the QD surface chemistry attained in recent experiments7–10 with modern computational chemistry tools, which are now at the stage of accurately simulating realistic QD systems thus providing a radical advancement of description towards control of the optoelectronic properties of colloidal QDs. The goal of this perspective is to provide the basis for understanding the surface chemistry of QDs by merging the most recent discoveries in both experiments and theory. To this aim, we will discuss three benchmark QD systems with different structural and electronic features (CdSe, PbS, and CsPbI3) and identify the processes leading to surface trap states, suggesting feasible post-synthesis treatments for their elimination. QD classification and notation. Careful analytical studies have recently disclosed structure and composition of colloidal QDs, demonstrating (size-dependent) metal-rich stoichiometries of II-


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VI8,11 and IV-VI metal chalcogenides,12–14 whereas metal halide perovskites seem not to show relevant off-stoichiometry probably due to their relatively large size, with diameters usually above 10 nm.15–17 Pull Quote 2: “QD Stoichiometry: the stoichiometric ratio between elements constituting the inorganic core” Chemical species at the QD surface have been extensively characterized by determining ligand type, surface coverage, and their eventual lability.8,14,16 In addition, such surface ligands guarantee overall QD charge neutrality, which is mandatory in low polarity solvents commonly used to disperse as-synthesized colloidal QDs. Usually, charge neutral QDs can be regarded as intrinsic, or doped, whether they satisfy, or not, the charge balance condition, which can be conveniently expressed according to the charge-orbital balance model (CBM), first introduced for computational purposes by Voznyy et al.18 Such model counts the number of excess/deficient valence electrons of a neutral QD by assuming that each core atom and surface ligand is in its thermodynamically most favorable oxidation state (as an example, +2 for Pb and -2 for S in PbS) and charge contribution (e.g., -1 for iodide, but valid also for carboxylates or thiolates; 0 for amines or phosphines, not considered here), respectively. It is thus possible to derive a simple formula that provides a quantitative expression for charge balance in neutral QDs: :

𝑁!"# = −

𝑁! 𝑞! !

where 𝑁!"# is the number of excess/deficient electrons in the QD and 𝑁! is the number of core atoms/surface ligands of type i with oxidation state/charge contribution 𝑞! . When the chosen QD model satisfies the condition of 𝑁!"# = 0, it means that the QD is intrinsic and exhibits a closed-


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shell configuration, i.e. an effectively charge-balanced QD. Usually, this is a necessary, but not sufficient condition for a clean band gap material. Suitable QD models thus may satisfy the condition 𝑁!"# = 0, upon assuming that the core atoms and the ligands are in their thermodynamically more stable oxidation state and charge contribution, respectively. Pull Quote 3: “Charge Balance: QD charge neutrality is attained when all constituents, i.e., core atoms and surface ligands, are in their thermodynamically most stable oxidation state and charge contribution, respectively” When the QD is non-charge balanced (𝑁!"# ≠ 0), then the QD can be regarded as doped. For 𝑁!"# < 0, excess electrons fill the conduction band (CB), raising the energy levels compared to vacuum and n-doping the QD; conversely, for 𝑁!"# > 0 electrons are removed from the valence band (VB), lowering the energy levels and p-doping the QD. As an example, upon removing two iodide ligands at the PbS QD surface to form molecular iodine would lead to 𝑁!"# = −2, with excess electrons filling the CB and forming reduced, n-doped PbS QDs. This information has led to infer atomic-level description of the QDs and sets the standards for building reliable QD models, which are nowadays computable with any available quantum chemistry (QM) package and go beyond the widely used models such as [CdSe]33: a review that covers the most successful outcomes of these cluster models was recently published and will not be covered here.19 Pull Quote 4: “Doping: excess/deficiency of electrons in an overall charge neutral QD achieved by redox processes; this leads to charge unbalanced QDs.”


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In order to describe the chemical bonding at the QD surface, the covalent bond classification (CBC) scheme mutuated from organometallic chemistry is commonly adopted20. Within this framework, both the metal chalcogenide core, hereafter referred to as [MmEn], and the surface ligands are considered as neutral species, with the latter named after the number of electrons that they donate to the surface atoms of the inorganic core: L-type for 2-electron donors, X-type for 1-electron donors and Z-type for 0-electron donors.8 A non-stoichiometric [MmEn] core, with 𝑚 > 𝑛 commonly attained experimentally, provides a source of (𝑚 − 𝑛) ∙ 𝑣 unpaired electrons (with 𝑣 as the number of valence electrons of each neutral M atom). Here, charge-balance is obtained when such unpaired electrons are saturated with one-electron donor species (X-type ligands), ultimately leading to a general molecular formula for the QD (including also L-type ligands), as 𝑀! 𝐸! 𝑋(!!!)∙! 𝐿! . Such QD chemical formula can alternatively be written as 𝑀𝐸 ! (𝑀𝑋! )!!! 𝐿! , thus highlighting the M stoichiometric excess and the presence of Z-type ligands (the electron-withdrawing (𝑀𝑋! ) moiety that also includes X-type ligands), which have been demonstrated to be active elements at the QD surface. Important Note. At this point, we must draw attention on the possible confusion that may arise by the use of both CBM and CBC models, as the former deals with ionic species and the latter with neutral species. This contradiction is evident in 𝑀𝐸 ! (𝑀𝑋! )!!! 𝐿! QDs when discussing the role of X surface species that can be regarded either as anions, which balance charges of excess M cations in an ionic representation of the QD (CBM), and as radicals that saturate valences of excess M atoms in a covalent representation of chemical bonding at the QD surface (CBC).

On the other hand, CBM and CBC models can be considered as analogous for the

(𝑀𝑋! ) and L motifs (Z-type and L-type ligands in the CBC), because they are charged neutral in both representations.


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As a result, we hereafter refer to either CBM or CBC when discussing QD doping or surface chemistry, respectively: CBM is indeed more intuitive for evaluating charge balance in those QDs regarded as mostly ionic, while CBC is more comprehensive to analyze the chemical bonding at the QD surface21. Pull Quote 5: “The use of both CBM and CBC models may induce confusion, as the former deals with ionic species and the latter with neutral species.” QD model. On this basis, we constructed our QD models as follows. For II-VI QDs, we adopted zinc-blende structure and charge-balanced chemical formula as 𝑀𝐸

!

𝑀𝑋! ! . We omitted

additional Z- and L-type ligands at the chalcogenide- and metal terminated [111] facets, because bare under-coordinated surface atoms in those facets do not lead to localized states and affect only little the electronic structure.22 Pull Quote 6: “Under-coordinated atoms: inorganic atoms at the QD surface with coordination below that expected from the crystal structure.” In terms of size, the [CdSe]55(CdX2)13 model (~2.0 nm) represents a good starting point for many simulations based on Density Functional Theory (DFT), because it already exhibits a dense set of states around the band edges and is still small enough to be computationally affordable. Similarly, for IV-VI QDs we adopted models with rock salt crystal structure and analogous charge-balanced chemical formula 𝑀𝐸

!

𝑀𝑋! ! , with M as Pb divalent cation. Reliable

models for metal halide perovskites ABX3 (A=Cs, methylammonium, formamidinium; B= Sn, Pb; X = halide) include distorted cubic/orthorhombic crystal structures and chemical formula as 𝐴𝐵𝑋!

!

𝐴𝑋

!

to satisfy charge balance. We note that perovskite QDs of experimental sizes


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(above ~10nm) exhibits n

Surface Traps in Colloidal Quantum Dots: A Combined Experimental

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