Investigation of Quantum Dot - Metal Halide Interactions and Their

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C: Physical Processes in Nanomaterials and Nanostructures

Investigation of Quantum Dot - Metal Halide Interactions and Their Effects on Optical Properties Finn Purcell-Milton, Maxime Chiffoleau, and Yurii K. Gun'ko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08256 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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The Journal of Physical Chemistry

Investigation of Quantum Dot - Metal Halide Interactions and their Effects on Optical Properties Finn Purcell-Milton1*, Maxime Chiffoleau2 and Yurii Gun’ko1* 1 School

of Chemistry and CRANN, University of Dublin, Trinity College, Dublin 2, Ireland. Département SGM, Institut National Des Sciences Appliiquées, Lyon *Email: [email protected], [email protected]

2

Abstract Quantum dots (QDs) are a class of important light emitting nanomaterials which have shown considerable potential for a range of applications. Here, we report detailed studies of the interactions between cadmium halide salts and II-VI -based quantum dots affecting their optical properties. A specific set of experiments have been utilized to better understand these effects using a range of core and core-shell quantum dots as model systems, examining CdSe, CdS, CdS/CdSe, CdTe/CdSe, CdSe/CdS, CdSe/ZnS QDs and CdSe/CdS dot in rod nanostructures. In our studies we have demonstrated that significant increases in the key property of photoluminescent (PL) and photoluminescent quantum yields (PLQY) can be achieved: producing a 1.5 to 4fold increase for CdSe QDs and 1.4 to 1.8 for a number of other Cd based core shell nanostructures. To explain these phenomena, the interaction’s efficiency for three alternative ligand capped CdSe QDs have been examined, with results showing a weak dependency upon capping ligand. By contrast, we have demonstrated that variation of the halide anion of the cadmium salt shows a strong dependence, with decreasing effectiveness found when comparing Cl- to Br- and I-. In addition, we have been able to show a large increase of PLQY for reverse Type I (CdS/CdSe) and Type II (CdTe/CdSe) QDs, both nanostructures which display strong surface sensitive PL properties, while Type I (CdSe/CdS) nanostructures showed a weaker effect, with an

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inverse relationship relative to shell thickness. Finally, it was also found that ZnS, or ZnS shelled QDs, show the onset of cation exchange, causing PL red shifting and a significant reduction of PLQY. Therefore, the culmination of these results can be best explained using standard covalent ligand classification, and points to this treatment working via a three-pronged approach in which surface passivation takes place through the presence of the L-type ligand oleylamine, the Z-type ligand, Cd(oleylamine) and the X-type ligand Cl- anion, the combination of which produce the total optimal effect observed. Overall, this study presents important approaches to increase quantum yields in a range of widely utilised QDs and provides important insights to the underlying interactions of this type of surface treatments as means to improve the resulting optical properties. .

1

Introduction

Colloidal quantum dots (QDs) are extensively investigated luminophores, finding applications in a diverse range of fields such as optoelectronics,1 bio-imaging/sensing2 and optics,3 due to their unique optical properties originating from the quantum confinement effects. A particularly important feature of QDs is their strong photoluminescence (PL), which can be varied from the UV to the IR range via variation in size and composition. Further optimization of these optical properties has led to the development of core-shell quantum dots, in which a single core quantum dot is coated in a shell of another material, usually another semiconductor, and can be used to increase quantum yields4, photostability5, eliminate emission blinking6-7 and enable further emission wavelength modulation8 and charge injection9.


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A large amount of work has also been focused upon increasing the PL efficiency of the vast range of QDs and it has been found that photoluminescent quantum yields (PLQYs) are strongly dependent upon the band structure,10 crystallinity,11 purity (oxidation, contamination) and surface passivation of the nanoparticles. In particular, the type of surface ligand is very important and plays an essential role in defining its ability to passivate the surface. A vast range of ligands have been tested in quantum dots, with the best results reported in non-polar media, and include alkyl -amines12, phosphines13, -phosphonic acids14, -carboxylates15, -thiols16 and -dithiocarbamate17. These different functional groups coordinate with the quantum dot surface in a range of ways and can be labelled via application of the covalent bond method, assigning them as either X (one electron donors, anions), L (two electron donors, neutral Lewis bases) or Z (two electron accepting, neutral Lewis acid) type ligands.18-19 Alkyl amines are L type ligands, which are among the most useful to date,20 with primary amines reported to give the highest increase in PLQY relative to secondary and ternary as post synthetic treatments12. A reason for this is the ligand’s intermediate binding energy, enabling coordination of the ligand at room temperature, in addition, it has been shown that this ligand has some of the highest coverage approaching 100 % level of any ligand, therefore enabling it to greatly increase surface passivation, with some reports additionally pointing towards greater photostability21. Other important but less


Figure 1. A scheme showing the approach applied to colloidal QD solutions in this study .


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explored ligands which have shown some effective results in post treatments are alkyl -phosphonic acids.22 An alternative well-known route to greater surface passivation is via increased concentration of cation (e.g. Cd

2+)

relative to chalcogenide at the QD surface.23-25 This has been explained via the

excited state of the QD becoming more core dominated, and less sensitive to surface states,19 with calculations showing the presence of mid band gaps states in chalcogenide rich surfaces,24, 26 which are eliminated by transitioning to a cation dominated surface.

24

It has also been demonstrated

through the application of Z type ligands such as MX2 (X= O2CR, Cl, SR) post synthesis the PLQY can be greatly increased for a given metal chalcogenide QD. 23 In contrast, an alternative approach evolving at present is the replacement of organic ligands with inorganic alternatives, such as halide anions,27-30 metal chalcogenides31-33 or metal halide perovskites34-36. This method enabled the production of films with minimal inter-dot spacing and excellent passivation, and as a result with outstanding charge transport properties, with some reports on complete ligand exchange producing quantum dot films of purely inorganic material.

31-33, 37

In

other approaches, inorganic ligand addition has been used to complement an organic ligand shell.2930, 38

Especially interesting are the studies on the application of halides or halogens

39

to a range of Cd

and Pb based QDs, since this research has shown the potential to greatly inhibit surface trap states in QDs.40 The exact mechanism of surface interaction seems to depend upon the conditions and source of halide/halogen used, with some cases showing definitive proof of formation of a CdCl2 or PbCl239 layer upon the surface of the QDs tested, termed chemisorption,39 while in other cases the interaction seems to be more of a physisorption process.40 This has the effect of increasing charge


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transport


29

and in some cases increasing photoluminescent quantum yield.28, 40 Therefore, these

approaches have shown substantial success in enabling highly efficient QD based photovoltaic solar cells and LEDs. Thus, due to these trends in surface passivation, this has led to the emergence of a combined approach using a long chain primary amine ligand, such as oleylamine (Olam), in combination with cadmium halide salts,30, 41-42 43producing a highly effective treatment in the organic phase. In addition theoretical modelling has been carried out predicting these type of treatments effectiveness due to its ability to terminate surface dangling bonds of cation rich (100) and (111) rich surfaces.44 In particular this has previously been investigated for PbS QDs,45 reporting particular success with increasing QD stability and luminescent quantum yields in solid films.41-42, 45-46 In our work, we focus on post synthetic treatments and the passivation of surface trap states responsible for non-emissive de-excitation routes, the elimination of which leads to significant increases in PL quantum yields (PLQY). Our aim is to investigate the precise conditions paramount to fostering this approach and its effectiveness across a range of II-IV based quantum dots. Our methodology is based upon a highly successful approach using a solution of CdCl2 dissolved in Olam, a long chain amine and the addition of tetradecylphosphonic acid (TDPA).45 This treatment involved four active elements, which could provide the enhanced surface passivation results. Their interaction with the surface can be most easily explained via covalent bond theory, categorising Olam and Tetradecylphosphonic acid (TDPA) as L-type ligands (a two electron donor, a neutral Lewis basis), which therefore enables coordinating to cations on the surface, while Cl - anions acted as a X-type ligand (one electron donor, an anion) and therefore could coordinate to the cations



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upon the surface, or Cd 2+ ions that can passivate anions present on the surface of QD, acting as a Ztype ligand (a two electron acceptor, a Lewis acid).

Therefore, in this study we have tested a range of treatments upon a model system, CdSe QDs, a commonly investigated and utilised luminophore, to determine the keys elements of this treatment. Firstly, the effects of varying the halide anion of Cd2+ salts from Cl- to Br- and I was examined, since definite trends have been reported within this chemical group concerning other approaches utilising halides or halogen treatments.28-29 Furthermore, Cd(stearate)2 dissolved in Olam was also tested, to determine the effect of an alternative source of Cd+2. To determine the effect of TDPA, solutions of CdCl2 + TDPA in Olam were compared, to solutions of just CdCl2 in Olam. In addition, to generalise this approach, we also have investigated a range of core -shell structures with the aim to understand how this affects the surface treatment interactions. Finally, we have investigated various ions effects upon wide band gap semiconductors of CdS and ZnS to evaluate whether the band gap of the QD plays a key role. UV-Vis absorption, PL and PL lifetimes and determination of the resulting PLQYs was then utilised to determine the effects of these treatments.

2

Experimental

2.1

Starting materials

BiCl3 (Bismuth chloride), CdCl2 (cadmium chloride, 99.99 %); CdBr2 (cadmium bromide, 99.99%), CdI2 (cadmium iodide, 99.999%), Coumarin 153 (99 %); Cd(st)2, (cadmium stearate, min 90%, Strem Chemicals); Olam (oleylamine, 98 %); Oxazine 170 perchlorate (95%); PbCl2 (lead chloride, 99.99%);


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HgCl2 (Mercury Chloride); QS (quinine sulfate dihydrate, 99%); Rhodamine 6G (99 %); Se (99.99 %); TDPA (tetradecylphosphonic acid, 97 %); and ZnCl2 (Zinc chloride, 99.999 %).

2.2

Halide oleylamine treatment stock solutions

A 0.15 M solution was prepared by dissolving 1.5 mmol of the appropriate metal salt (BiCl3, CdCl2, CdBr2, CdI2, Cd(stearate)2, HgCl2, PbCl2, TDPA, and ZnCl2) in 10.0 ml of Olam. This was carried out by adding the specific salt to Olam, which was then degassed at 50 ˚C for 1 hour. This mixture was then heated to 140 ˚C and the salt was dissolved over a period of 2 hours under an Ar atmosphere and was then allowed to cool, ready for use. When testing TDPA, 0.1 g of this was added to the solution, before heating begins, giving a conc of 10 mg /ml.

2.3

Halide oleylamine solution treatment

Firstly, 100 µl of the halide Olam solution was added to solution the QDs at a concentration between 1 x 10-4 to 1 x 10-5 M in 1.0 ml of toluene, giving a concentration of 0.015 M. The solution was then left in darkness for 24 hours at 2 ˚C. After that the solution was diluted to the required concentrations to achieve an absorption of between 0.5 to 1 at the 1st excitonic absorption peak of the QD in a 1 cm path length cuvette. Following this, the solution was diluted by a factor of ten in toluene to produce a total of 5.0 ml of QD solution. To this solution, was then added 50 µl of the specific solution which this sample had previously been treated with, so as to account for the effects of dilution. The samples were then left for 3 days in darkness at 2 °C, to allow the samples to reach a steady state, after which spectroscopic analysis was carried out.



2.4

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Quantum yield measurements

The relative comparison approach was utilised to determine quantum yields were calculated by the comparison of PL intensity between a sample and a standard dye, which have well-known quantum yields. Four standards were used, QS (quinine sulphate dihydrate) 0.105 M HClO4 in water (58.7%47), Coumarin 153 in ethanol (QY 53%48),Rhodamine 6G in ethanol (QY 95%48) and Oxazine 170 in ethanol (QY 57.9%49). The specific standard used depended on the position of the QDs exciton peak relative to the dye absorption peak. Samples absorption were firstly measured to be between 0.25 and 0.5 at peak exciton value, followed by a 1:10 dilution, after which sample’s PL was measured; this was done to minimize self-absorption of fluorescence, with the same approach applied QY dye standards.

2.5

Instrumentation

Transmission electron microscopy (TEM) images were obtained using an FEI Titan TEM at an operating voltage of 300 kV. Samples for TEM were prepared by dropping a small aliquot of a solution of the QDs in hexane onto a lacey carbon 300 mesh Cu TEM grid followed by drying at 70 °C for 2 hours. UV-Vis absorption spectra were recorded using a Varian Cary 60 UV-Visible Spectrophotometer, while PL spectra were recorded using a PerkinElmer LS 55 Fluorescence spectrometer. All spectra were corrected using calibration data provided by the manufacturer. PL lifetime measurements were performed using a time correlated single photon counting (TCSPC) spectrometer equipped with a PCS900 plug-in PC card (Fluorolog-3 Horiba Jobin Yvon) and a semiconductor diode “NanoLED” excitation source (455 nm, Horiba Jobin Yvon) with pulse duration


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shorter than 1 ns. Lifetimes were obtained by a deconvolution fit and the quality of fit judged by minimization of reduced chi-squared and the residuals. In addition, average lifetimes were calculated using the amplitude averaging approach,50 with the equation given in supporting information, Equation S1.

3

Results and discussion

To carry out our analysis, a series of quantum dots were initially synthesised in house using a range of modified methods reported in literature (see ESI, section 1. Synthetic methods), using standard hot injection approaches,51 producing the samples which were to then to be analysed. The second step synthetically was to produce a range of solutions to treat the range of QDs which is described in the experimental section. Following this, treated QD samples were then tested with the results given under specific sections below with a summary of QD samples and solutions tested given in Table 1, with the overall scheme of the experiment shown in Figure 1.

Table 1. The QD samples synthesised and studied in this work with the listed solutions that they were tested with.



Samples Tested

3.1

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Solutions Tested (0.015 M)

CdSe (ODPA capped)

CdCl2, CdCl2 +TDPA, CdBr2, CdI2, Olam, Cd(st)2

CdSe (OA capped)


CdSe (Olam capped)


CdS

CdCl2, CdCl2 +TDPA, CdBr2, CdI2, Olam,

CdSe/CdS 1

CdCl2, CdBr2, CdI2, Olam

CdSe/CdS 2

CdCl2, CdBr2, CdI2, Cd(st)2

CdSe/CdS 3

CdCl2, CdBr2, CdI2, Olam

CdSe/ZnS

CdCl2, CdBr2, CdI2, ZnCl2, Olam

CdS/CdSe

CdCl2, Olam, Cd(st)2

CdTe/CdSe

CdCl2, Olam, Cd(st)2

CdSe QDs

The first step of this work was the detailed investigation of the model cadmium-based quantum dots. CdSe QDs were chosen; as these QDs are among the most well understood and highly investigated QD materials. In our work, these QDs were treated with 0.015 M solutions of CdCl2, CdBr2 and CdI2 in Olam. In addition, we also investigated three other interactions to give us a more in-depth insight into the system this involved Olam on its own as a control, Cd(st)2 (cadmium stearate) in Olam, which acted as a source of organically soluble Cd2+ ions, and a combination of CdCl2/ Olam complex to which TDPA was added (10 mg/ml) since TDPA has been included in other previous studies in addition to Olam. 45


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A

B

C

D

E

F

Figure 2. PL spectra of CdSe QDs treated with a range of solutions (0.015 M), originally capped with either ODPA (A), OA (B), and ODA (C) with PLQY yields shown for each given in D, E and F, respectively.

To further develop the study, three different samples of CdSe QDs, each sample produced through an alternative synthetic method involving an alternative capping ligand, Octadecylamine, (ODA), oleic acid, (OA) and octadecylphosphonic acid, (ODPA), were tested (see ESI for UV-Vis absorption spectra in figure S1, synthetic procedure). This therefore allowed us to account for the possible effects that capping ligands may have upon the resulting PLQYs achieved from this treatment. Absorption spectra were measured for the samples and are shown in supporting information figure S2, which demonstrated little to no effect upon the QDs. PL was then measured with the resulting spectra of the CdSe sample shown in figure 2, along with calculated PLQYs. Our initial studies have demonstrated that across all three samples, CdCl2 and Olam treatments showed the strongest enhancement in emission across all of our samples, yielding a substantial increase of PLQY of 15.9 % to 26.8% for ODPA acid capped, 3.8 % to 11.2 % for OA capped and 2.5 % to 16.8 % for Olam capped CdSe QDs. Observing the trends across the samples, no large deviation in effect was found for any of the treatments dependent on capping ligands prior to treatment,



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therefore proving the universality of this approach for CdSe QD samples. When substituting the accompanying halide anion from Cl- to Br- to I- in the cadmium salt, the treatment became far less effective, with bromide showing a moderate increase in PLQY, while the iodide instead produced a quenching effect. This quenching effect has been previously reported with other iodide sources and has been explained by redox interactions with the surface of QDs taking place.52-54 We also found that substituting CdCl2 with Cd(stearate)2 as an alternative source of Cd+2 ions, did not yield comparable results either, indicating that the halide anions play an important role also. This trend of CdCl2 higher effectiveness has been previously reported in relation to the enhancement of photocurrent in CdSe films, though PL measurements in this study reported strong quenching in all cases.29 Also examining the effects of TDPA addition, across all three samples, we found the PLQY was reduced relative to samples without its addition, therefore clearly illustrating its ineffectiveness in this treatment. Furthermore, Olam (oleylamine) did show a positive effect, but not to the extent as when CdCl2 was involved, a definitive sign that both components, the long chain amine and cadmium halide salt contribute to the overall increase in PL observed. Following these results, we then investigated the positive effects in relation to the concentration of CdCl2/ Olam added to a 5 ml solution. The results (see ESI, Figure S4 A) showed no strong dependence for the CdSe QD samples, across the range of 5 µl to 100 µl, with the maximum addition set to 100 µl, since volumes above this displayed solubility issues. We also found that PLQY increases using CdCl2 / Olam treatment were most pronounced in toluene (see ESI Figure S4 B), relative to hexane and chloroform, which are other commonly used non-polar solvents.


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A 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

B

Figure 3. PL decay lifetimes of CdSe QDs capped in OA (A) and ODPA (B).

Furthermore, we examined the effects upon the PL lifetime for the OA capped and phosphonic acid capped CdSe QDs tested, with the resulting spectra shown in figure 3, comparing untreated, CdCl2 and Olam treated QDs. The data was fit with a multiexponential, with the average lifetimes results of the analysis giving for OA capped CdSe QDs gave 16.8 ns for untreated, 24.3 ns for CdCl2 and 28.6 ns for Olam, while ODPA acid capped CdSe QDs gave 5.13 ns for untreated, 11.1 ns for CdCl2 treated and 14.8 ns for Olam treated. Thus, according to these results, we find the same trend across both samples, with the untreated sample showing a shorter lifetime, while when treated with CdCl2 a notable increase was achieved. Also, worth noting is that surprisingly the Olam in fact gives the strongest increase in PL lifetime. In both cases this further backs up our determination of surface passivation eliminating non-emissive deexcitation, and therefore gives rise to longer fluorescent lifetimes and higher PLQYs. 3.2

CdS QDs

Subsequently the study was expanded to OA capped CdS QDs, (see ESI for UV-Vis absorption spectra in figure S5, TEM analysis in figure S20 and synthetic procedure). The overall photoluminescence study results are shown in figure 4. The untreated CdS QDs showed emission which was composed of a sharp excitonic emission peak, which contributed to the majority of emission, with a small amount of emission occurring in the form of a broad emission peak which stretches to 700 nm, and which has been assigned as surface deep trap state emission as reported in literature 55. These QDs



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showed an efficient emission, giving a 55 % PLQY prior to treatments. Upon addition of Olam solutions, the emission of the CdS QDs strongly shifted from excitonic in origin to nearly exclusively surface deep trap state emission, which was accompanied with a large decrease in PLQY. The largest loss in luminescence occurred with the addition of Olam, nearly completely quenching the excitonic emission, giving a PLQY of 9.3 %, with the same pattern occurring for CdCl2 + TDPA based solutions, giving a PLQY of 11.5%. Interestingly, a different pattern is seen in CdCl2 solutions, which also showed strong quenching in excitonic luminescence, but was also accompanied by a noteworthy increase in trap state emission, producing PLQY of 23.3% for CdCl2, while CdBr2 in fact even showed a stronger increase in the resulting trap state emission, giving a PLQY of 32.5 %. Therefore, this post synthesis treatment shows a distinctly different behaviour than CdSe QDs and hence it is not an effective tool for optimizing the emission of CdS QDs. The strong decrease in emission due to the addition of Olam and other amine-based ligands have been previously reported for CdS QDs56. This has been explained using DFT calculation to be due to the effects of net charge transfer between the ligand and the CdS QDs, effecting the carrier recombination dynamics by controlling the radiative recombination surface state centres and therefore lowering the resulting PLQY and can produce an increase in the surface deep trap state emission. This analysis is an excellent match with the results observed in this study.

A

B

ACS Paragon Environment Figure 4. PL spectra of CdS QDs (A) treated with a range of solutions with insertPlus showing the defective emission more clearly and PLQY yields (B) shown for each given.

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3.3


Effects Upon Band Alignments

Quantum dot optical properties (Stokes shift, PLQY, resistance to photobleaching) can be better optimized via a core-shell structured QD and have been widely reported.10 In these structures, a core QD is coated in a shell of another semiconductor, with the precise resulting properties dependent upon the resulting band alignment between the semiconductors utilised. 51 The most widely applied version of this is a type I band aligned core-shell QD, in which a larger band gap semiconducting shell is deposited upon a core QD, causing confinement of the QD’s exciton to the core quantum dot and therefore reducing the sensitivity of the quantum dot to the effects of surface interaction. Therefore for these reasons, we decided to investigate the effects of these treatments upon the commonly studied CdSe/CdS QDs9, 57 and CdSe/ZnS QDs. 58

3.4

CdSe/CdS nanostructures

The treatment of three different CdSe/CdS QDs samples was carried out to investigate the effects relative to the morphology of the structure. Therefore we tested two CdSe/CdS core-shell QDs, CdSe/CdS 1, measuring 3.8 +/- 0.4 nm in diameter (CdSe core, 2.8 nm, CdS shell thickness of 0.5 nm ) (see ESI, figure S18 for TEM data and figure S6 for UV-Vis spectra ), while CdSe/CdS 2 measured 4.8 +/- 0.6 nm in diameter (CdSe core, 3.6 nm, CdS shell thickness of 0.6 nm ) (see ESI figure S19 for TEM data and figure S7 for UV-Vis spectra) and a third dot in rod structure, CdSe/CdS 3, composed of a CdSe core of 3.32 nm and a CdS shell of rod shape, giving a length of 27.1 +/- 4.2 and width of 6.6 +/0.9 (see ESI, figure S21 for TEM data and figure S8 for UV-Vis spectra). The investigation of these three samples added a further aspect to the study relative to core QD, shell dimensions and morphology.



A

C

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B

E

D

Figure 5. PL spectra, PLQYs and PL lifetimes of CdSe/CdS QDs treated with a range of solutions, graph A and B referring to CdSe/CdS QDs 1, while graph D, E and F to CdSe/CdS QD 2.

The results for CdSe/CdS 1 and 2 samples is given in Figure 5. From this it was found that the CdCl2 treatment gave substantial PLQY increase, with results for CdSe/CdS 1 showing the most pronounced effect, giving an increase from 14.6% to 27.3%, while CdSe/CdS 2 showed a little more moderate increase from 28.1 % to 39.5 %. The same trend of CdBr2 and CdI2 was found for both samples, with CdBr2 giving PLQY within error of the untreated samples, while CdI2 shows strong quenching. Cd(stearate) is also shown to give no noticeable effect while Olam gave a moderate increase for CdSe/CdS 1 QD sample. We also investigated the fluorescence lifetime of the CdSe/CdS 2 QD samples with results shown in Figure 5 E and fit with a multiexponential decay (see ESI table s2). From this we compared the PL lifetimes of the untreated sample, CdCl2 treated and Olam treated, which gave average lifetimes of 14.1 ns (untreated), 19.7 ns (CdCl2) and 18.6 ns (Olam), displaying an increase in fluorescence lifetime due to the treatment as was found for the CdSe QD samples.


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The third sample, CdSe/CdS 3, a dot in rod sample, was then analysed with the results shown in figure 6, from which we find a modest increase in PL due to Olam treatment and CdCl2, while a minor quenching effect was noticed when testing CdBr2 and CdI2, with Olam increasing from the untreated to 34.2 % from 28.8%, while CdCl2 increase it to 31.4 %. Therefore, we find a significantly lower positive effect for this sample relative to the other two tested. A

B

Figure 6. PL spectra of CdSe/CdS dot in rods (A) treated with a range of solutions, with PLQY yields (B) shown for each given.

Overall all type I CdSe/CdS samples investigated demonstrate a positive PLQY increase occurring from CdCl2 treatment, but this effect does depend upon the level of surface sensitivity of the structure, with thicker shelled samples of CdSe/CdS such as CdSe/CdS 3 showing a much-decreased effect upon the resulting PLQY measured. This result is due to the surface passivation effect as the origin for the PLQY increase produced, and therefore with greater shell thickness being more effective at isolating the CdSe QD core from surface states and hence the effects of ligand intereaction.59 60 3.5

CdSe/ZnS

CdSe/ZnS is another very commonly used QD, 51 due to its high PLQY and lower toxicity due to the use of a biocompatible ZnS shell forming the type I band alignment. This sample was tested with the same Cd halide salts, and in addition ZnCl2 was tested, with the results shown in Figure 7. Firstly, we



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do not find a Zn based salt shows any positive effects upon PLQY with results within margin of error of the untreated sample, while a clear blue shift can be seen to take place, which is assigned to be A

B

Figure 7. PL spectra of CdSe/ZnS QDs (A) treated with a range of solutions, with PLQY yields (B) shown for each tested.

due to minor Cd to Zn exchange taking place. Aside from this, the CdSe/ZnS QDs show a distinctly different interaction with cadmium halide salts, with treatments showing quenching. This is proposed to be due to the onset of cation exchange, causing a loss in the type I band alignment present. This is supported by literature data which describes a cation exchange process which has been reported to take place with other treatments of Zn based QDs with cadmium salts.61 In addition we also investigated the effects of treatments upon ZnS QDs, with the results given in ESI (figure S12). From these studies it was also found that a substantial cation exchange took place, causing strong red shifting in the fluorescence and PL spectra, which further supports our reasoning of this treatment effects upon CdSe/ZnS QDs. Therefore, overall this treatment is not effective for quantum dots of alternative cation character such as Zn, due to the effects of cation exchange. 3.6

Reverse Type I and Type II QDs


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To further understand the effects of these treatments, we selected to investigate the salt effects upon other alternative band alignments of core-shell QDs, we therefore investigated a reverse type I band CdS/CdSe QD, of 4.53 +/- 0.66 nm in diameter (see ESI for synthesis, TEM figure S22 and UVVis spectra figure S10) and a Type II CdTe/CdSe QD, (see ESI for synthesis, TEM figure S23 and UVVis spectra figure S11) with the PL results shown in Figure 8. Interestingly, both structures are coreshell QDs in which the exciton of the QD is much more sensitive to passivation of the surface states present 62-63 and therefore we see this manifested in their interaction with the CdCl2 treatments, with CdS/CdSe QDs showing an increase in PLQY from 9.2 % to 14.9 %, while CdTe/CdSe gives an increase in PLQY of 28% to 52.5%. A

C

B

D

Figure 8. PL spectra of CdS/CdSe QDs (A) and CdTe/CdSe QDs (C) with PLQY yields (B) and (D) respectively treated with Olam, CdCl2, and Cd(st)2 solutions.



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In addition, PL lifetimes were measured and are shown in Figure 9, with data fit to a multiexponential (see ESI table s2), which show a substantial increase for the CdS/CdSe samples giving an increase of average lifetime of 7.5 ns to 44.5 ns due to CdCl2. In contrast, no substantial change takes place when analysing CdTe/CdSe QDs, which show an average time of 34.8 ns of the untreated sample which increases to 36.9 ns upon CdCl2 treatment. B

A

Figure 9. PL lifetimes of CdTe/CdSe QDs (A) and CdS/CdSe QDs (B) treated with CdCl2 and Olam solutions.

3.7

Cation exchange effects

Following the results of CdSe/ZnS QD and ZnS QD post synthesise treatments, which both showed a distinctly different effect due to the onset of cation replacement of Zn with Cd, the study was extended to other chloride-based transition metals also used in quantum dot synthesis. Therefore, the formation of Olam complexes of PbCl2, ZnCl2, and HgCl2 was carried out by dissolving in Olam under the same conditions as mentioned previously in the experimental section. These were tested with several of the QD samples mentioned already studied (CdSe QDs, CdSe/CdS QDs, CdTe/CdSe QDs and CdS/CdSe QDs). With all samples investigated, near identical trends emerged for all salts, with each showing a moderate to strong quenching from their addition (see ESI for S13 and S14). This was proposed to be due to the onset of cation exchange, which is supported by strong shifting of PL wavelength from Pb and to a lesser extent with Hg treatments and a less pronounced blue shifting


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due to Zn. Therefore, we concluded to use this approach to successfully produce a beneficial effect upon the PLQY, cadmium halide salts are the only effective treatment.

Overall the results obtained across all of the QD samples clearly illustrates that three elements of the post-synthetic treatment play a crucial role to produce the optimal increase in PLQY. This is best explained using the standard covalent bond classification18-19 method to describe the interaction of the various components of the post synthetic treatments upon the QD surface. Firstly, the use of Xtype ligands (one electron donors, anions), enables the passivation of cations on the QD surface through a lattice terminating interaction. Secondly the choice of cations must be selected so as to be able to act as an effective Z-type ligand (a ligand that accepts two electrons from the metal centre, a Lewis acid) and in this case refers to a MX2 ligand, a neutral acceptor, where M= metal, which enables it to passivate the anions present at the surface. Thirdly, the addition of L-type ligands (two electron donor,

a

neutral Lewis base) is also necessary to produce the most optimal effect, enabling passivation of cations present on the surface additionally. Due to these three differing interactions, each component must be selected so as to enable this with the QD being treated. Therefore, from our results, it is clear that the metal cation selected must be the metal cation present on the surface of the QD to enable effective surface passivation and to avoid unwanted cation exchange effects, which is Cd2+ in the majority of our systems studies. In addition, the presence and character of the halide anion of the metal salt is also important, with the chloride providing universally the optimal results, due to the higher compatibility of chloride relative to bromide or iodide, while cadmium stearate showed poor results under the same conditions due to the absence of the halide. The increase in



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near all QD samples with Olam proves the effectiveness of the addition of L-type ligands to increase PLQY, though the case of strong CdS QD Pl quenching also makes the point that the precise L-type ligand must be chosen for the QD so as to avoid unwanted charge transfer effects. Aside from the composition of the halide anion treatments and the QD which is being treated, we have also shown clearly that from the results of the core-shell QD cadmium structures examined, the type of band alignment is also important to understand if post synthetic treatment can further improve PLQYS. Our results clearly illustrate that understanding of the excited state of the QD’s interaction with the surface states strongly dictates if the is treatment is an effective strategy at increasing PLQY in QD samples, with reverse type I and type II QDs showing a much-enhanced effect, relative to the much more common type I QDs.

4

Conclusions

Thus, we have shown that an Olam /CdCl2 treatment is a highly efficient approach to increase the PLQY for a range of cadmium-based quantum dots, irrespective of the ligands present or morphology. In contrast, we have found that the effects on PL are strongly defined by the relevant band alignment of the quantum dots when examining core-shell structures, with reverse type I QDs, CdS/CdSe, or type II CdSe/CdTe QDs, showing a much stronger PLQY enhancement than type I aligned QDs, which are far less sensitive to surface trap states. We have also demonstrated that this treatment can induce an unintended cation exchange process, causing a large loss in PLQY when tested with CdSe/ZnS QDs or strong red shifting when tested with ZnS QDs. The same applies to alternative cation based (ZnCl2, PbCl2, HgCl2, etc) halide salts, which were also tested with cadmium based QDs. These overall trends clearly make the case for the need to choose the same metal cation already present on the QD


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surface, and also to use the chlorides in order to achieve the best overall effect of this treatment. This study presents a clear route to optimising one of the most important parameters of a colloidal QDs, their PLQY, in an easily applied post synthesis method. In addition, we believe that this work makes an important contribution to the further understanding of effective surface state control in colloidal QDs and therefore the production of high quality QD based materials and devices.

Supporting Information

Supporting information contains the synthetic methods used to synthesis the range of core and core-shell QDs studied, additional spectra of samples tested including absorption, emission, photoluminescence lifetime decay and a range of results from cation exchange and transmission electron microscopy of samples analysed.

Acknowledgements The authors gratefully acknowledge financial support from the Science Foundation of Ireland (16/TIDA/4122 and 12/IA/1300).

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Investigation of Quantum Dot - Metal Halide Interactions and Their

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