Highly Luminescent and Temperature Stable Quantum Dot Thin Films

May 19, 2012 - Joel van Embden , Anthony S. R. Chesman , and Jacek J. Jasieniak ... Anthony S. R. Chesman , Joel van Embden , Noel W. Duffy , Nathan A...
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Highly Luminescent and Temperature Stable Quantum Dot Thin Films Based on a ZnS Composite Francesco Todescato,‡,∥ Anthony S. R. Chesman,†,∥ Alessandro Martucci,§ Raffaella Signorini,‡ and Jacek J. Jasieniak†,∥,* †

Materials Science and Engineering, CSIRO, Bayview Avenue, Clayton, Victoria, 3168, Australia Department of Industrial Engineering and U.R. INSTM, University of Padova, Via Marzolo, 9, I-35131 Padova, Italy § Department of Mechanical Engineering, Material Sector and U.R. INSTM, University of Padova, Via Marzolo, 9, I-35131 Padova, Italy ‡

S Supporting Information *

ABSTRACT: A solution processable Zn(EtXn)2(octylamine) precursor has been used to deposit nanocrystalline ZnS thin films which can effectively host CdSe−CdS−ZnS quantum dots (QDs) with their native surface chemistry intact. The formation of such hybrid QD:ZnS composites proceeds through the initial decomposition of the octylamine stabilized zinc xanthate precursor to form nanocrystalline ZnS. To gain insight into this decomposition process we have utilized headspace gas chromatography− mass spectrometry (HS GC-MS), thermogravimetric analysis coupled with mass spectrometry (TGA-MS), grazing angle attenuated total reflectance Fourier transform infrared spectroscopy (GAATR FTIR), and grazing angle X-ray diffraction (GAXRD). Through these characterizations we identify that the major decomposition route of Zn(EtXn)2(octylamine) to form ZnS begins at 100 °C, generating predominantly CO2, COS, CS2, and ethanol as gaseous products. The octylamine used to solubilize the metal complex is found to remain adsorbed within the ZnS matrix up to temperatures of above 200 °C; however, due to its ability to favorably passivate the QDs, its presence only aids to increase the fluorescence thermal stability of the composite. Through the study of various alkyl amines passivating the QD surface within the ZnS composite, we identify that the role of the ZnS is to permit good chemical passivation of the QD surface and to create an electronically passivating host. Each of these factors and the additional presence of residual alkyl amines enables such composites to exhibit significantly higher fluorescence stability factors compared to neat QD films. To exemplify the highly lucrative fluorescence stability offered by these QD:ZnS composites, we study the amplified-spontaneous emission (ASE) properties of the resulting thin films and find unprecedented stability of the optical gain states. KEYWORDS: quantum dots, ZnS, xanthate, ASE, photoluminescence

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INTRODUCTION The chemical and optical properties of colloidal quantum dots (QDs) in solution are highly sensitive to the nature of their surface passivation.1−4 When incorporated into a solid-state matrix, the host’s chemical, structural, and electronic properties act to potentially modify those of the quantum dots. This enables the formation of versatile QD based composite materials that are suitable for a variety of applications, including luminescent down-converters,5 lasers,6−8 and solar cells.9 For each of these applications, an appropriate pairing of the host to the specific QDs is required to ensure optimum © 2012 American Chemical Society

synergy. In our continuing effort to develop solution processed optically pumped quantum dot lasers,10 we require composites that retain maximum luminescence, possess sufficiently high refractive indices (preferably >1.6), and can homogeneously host QDs at volume fractions of greater than 1%.11 In addition, the composites must possess long-term optical and chemical stability. Remarkably, despite the significant advances in the Received: February 29, 2012 Revised: May 15, 2012 Published: May 19, 2012 2117

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known that metal hydroxyl groups can effectively accept electrons.27,28 The unlikely condensation of all surface hydroxyl groups at the temperatures employed for such composites suggests that, even for ZrOx hosts, instability of the QD dopants can result. For the development of highly fluorescent QD composites, the host must provide favorable surface stabilization and be electronically passive with respect to the quantum dots. As both of these criteria are difficult to achieve with oxide based matrices, the development of nonoxide based hosts is thus a potential means to developing more efficient and stable composites. Organic polymer matrices are a viable candidate. The ability of such matrices to stabilize QDs without inducing large scale aggregation relies on favorable steric interactions with the passivating surface ligands. Early work in this area showed that through the free radical polymerization of lauryl methacrylate, with ethyleneglycol dimethacrylate as a cross-linker, luminescent composites containing tri-n-octylphosphine/tri-n-octylphosphine oxide coated CdSe−ZnS could be obtained.5 A number of authors have modified routes that utilize for instance ionic liquids29 for stabilization or employ copolymer systems30 that, again, harness lauryl methacrylates. Such polymer based composites are ideal for applications which require flexibility and low volume fractions of quantum dots, for example, fluorescent down-converters. In our work, we continue progress into the development of solution processed quantum dot lasers.8,24,31 For such applications, high volume fractions of quantum dots with maximum fluorescence yields are required to ensure sufficiently low stimulated emission build-up times.11 In addition, high refractive indices are required to ensure any propagating waves are efficiently confined to thin film based waveguides. As the refractive indices of organics are on average lower than those of inorganics, these combined factors inhibit the use of the above polymeric composite based materials. A secondary route to the development of nonoxide based composites is to utilize large band gap metal sulphide, selenide, arsenide, or phosphide host matrices. Danek and co-workers were among the first to exemplify this concept for CdSe QDs.32 In that work, electrospray organometallic chemical vapor deposition was used to grow a ZnSe matrix containing CdSe based QDs. The results highlighted that the ZnSe was effective in chemically stabilizing and electronically passivating the QD inclusions up to temperatures of 250 °C. In a more recent study, Mashford et al. used an all nanoparticle route to develop composite thin films based on ZnS doped with CdSe−ZnS QDs.22 In analogy with the work on ZnSe, the stabilizing nature of the ZnS on the chemical and electronic properties of the CdSe−ZnS was also evident up to temperatures of 250 °C. In that work, both CdSe−ZnS and ZnS QDs were stabilized in alcohol solutions. The necessary modification of the original core−shell QD surface chemistry to afford solubilization within the alcohol resulted in a significant reduction to the absolute quantum yield. In accordance with the requirements for developing high PL composites, this need for surface modification is an unfavorable aspect of this route. Notably, the use of aliphatic stabilized ZnS nanoparticles may obviate this issue; however, such a strategy is yet to be demonstrated. An all together different approach for developing ZnS matrices is through the use of zinc xanthate or carbamate as a single-source precursor.33,34 Such metal sulphide precursors

areas of surface chemistry and host matrix development, it is still unclear how to achieve all of these conditions within a single composite. Among the many fields which utilize quantum dots, research into biolabels12 and single molecule spectroscopy13 provide the greatest hints to developing an ideal composite. Original works on QD bioconjugates used direct ligand exchange routes to repassivate the QD surface with chemistries that could couple to proteins or antibodies.14,15 Through gradual advances, it was realized that QDs stabilized through ambipolar polymeric micelles provided more versatility for bioconjugation and maximized the fluorescence yield of the original nanoparticles.12 This latter factor indicates that preserving the original surface chemistry of the nanoparticles is critical for ensuring maximum fluorescence yields of any quantum dot composite. The related field of spectroscopy of single quantum dots delves into the role of temporal charge transfer between various acceptor states within a given system.16−18 Characteristic on and off events, which are termed “blinking”, represent fluorescence events from neutral and charged species, respectively.19 The formation of such charged species is believed to arise from charge transfer from within the QD to trap states at the core−shell interface or its surface or to the matrix.20,21 In low lattice mismatch core−shell QDs, such as those based on CdSe−CdS−ZnS heterostructures, appropriate surface passivation can suppress the extent of surface trapping, thus providing scope for blinking-free temporal characteristics.13 Furthermore, when the QDs are dispersed in a host matrix, the use of an electronically passivating matrix is also required to reduce the probability of ionization into acceptor states.22 Based on these two fields of research, we can state that an ideal composite for fluorescence based applications would preserve the original surface chemistry of the quantum dots and also provide adequate electronic passivation to prevent photoexcited charges from being ionized into the matrix. In many of the reports on QD based optical gain in the solidstate, sol−gel based inorganic composites have been primarily employed.5,23,24 The sol−gel process is favorable for composite formation as the structural nature of the sol is easily controlled through chelating agents, acid or base catalysts, and water.25 Through the use of appropriately modified quantum dots, for example, through 5-aminopentanol23,24 or octylamine-modified poly(acrylic acid) micelles,26 composite sol-solutions can be easily prepared which then enable the formation of QD/sol− gel thin films. The importance of matrix selection to ensure high photoluminescence (PL) has been demonstrated through a comparison of CdSe−ZnS quantum dots dispersed within two sol−gel hosts, (i) an insulating ZrOx matrix and (ii) a semiconducting TiOx host.24 The significantly higher fluorescence yields and temporal photostability of the QDs in the ZrOx were correlated to the type-I electronic structure between this system and a type-II electronic structure with the TiOx matrix. Despite the favorable electronic structure of ZrOx for a wide variety of quantum dots, to ensure adequate condensation for multilayer deposition, it is necessary to thermally anneal the composites at temperatures of typically 200−250 °C. This heating step is inherently detrimental to both the absolute fluorescence yield and chemical stability of the QDs due to oxidation of the surface, alloying, or ligand loss. In addition, it is 2118

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solvents were used as received without further purification. Zn(EtXn)2 was synthesized according to a literature procedure.37 QDs Synthesis. CdSe cores were prepared following the method described by van Embden and Mulvaney.38 CdSe−CdS−ZnS graded core−shell quantum dots were prepared according to a modified SILAR protocol that has been previously published. As synthesized core−shell QDs were purified through multiple extractions using MeOH, EtOH, and acetone, prior to being dispersed in toluene. For studies involving amines of various alkyl chain lengths, we employed a two-step method that included an initial pyridine surface exchange followed by a secondary alkyl amine surface repassivation. To accomplish this, QDs dispersed in toluene were precipitated with EtOH and then dispersed in pyridine at ∼30 μM concentration. The solution was allowed to stir under N2 at 80 °C for 24 h. At this point, the QDs were precipitated with minimum hexane and then redispersed in CHCl3 containing the appropriate amino ligand at a 10−100 mM concentration. The surface repassivation was permitted to occur for 24−48 h at room temperature. Prior to use, the QDs were precipitated from this stock solution using MeOH or EtOH and redissolved in chlorobenzene to make a solution with a QD concentration of 80 μM. Thin Film Deposition. Precursor solutions were prepared by mixing an 80 μM solution of QDs dispersed in chlorobenzene with a 325 mM solution of Zn(EtXn)2 in chlorobenzene. A totale of 1.4 equiv of octylamine was added to the Zn(EtXn)2 solution to aid in solubilization. Precursor solutions of QD:Zn(EtXn)2 with molar ratios in the range of 1:2000 to 1:16000 were prepared. Thin film samples were deposited by spin coating the precursor solution onto a substrate at 1000 rpm and annealing for 10 min under an atmosphere of N2. Thickness measurements were performed using a Veeco Dektak 6 M profilometer. X-ray diffraction (XRD) was performed on a Philips PW1710 diffractometer equipped with grazing-incidence X-ray optics. The analysis was performed at 0.5° incidence using Cu Kα Ni-filtered radiation at 30 kV and 40 mA. The average crystallites size of the crystalline phases has been evaluated from the diffraction patterns using the Scherrer formula. Zn(EtXn)2 Thermal Decomposition Study. For headspace gas chromatography−mass spectrometry (HS GC-MS) a solution of Zn(EtXn)2 and octylamine in chlorobenzene was spin coated onto a glass substrate (1 cm2), which was then immediately broken into smaller shards and placed into a 20 mL headspace vial that was then sealed with a Teflon seal. The vial was heated in an oven at 200 °C for 30 min, resulting in the decomposition of the complex, and a sample of the headspace was removed by syringe from the sealed vial. The headspace was analyzed on an Agilent 6890 GC and a 5971 mass selective detector (MSD) in full scan mode and a HP PLOT Q column (39 m × 0.32 mm i.d., 20 μm film thickness). After headspace analysis the glass shards were soaked in ethanol (∼1 mL), which was also analyzed by GC-MS. Spectra were obtained with a ThermoQuest TRACE DSQ GC mass spectrometer, with gas chromatography that was performed with a SGE BPX5 column (15 m × 0.1 mm i.d., 0.1 μm film thickness). Thermogravimetric analysis coupled to mass spectrometry (TGA-MS) was performed on a Setaram Setsys Evolution Thermal Analyzer coupled with a Pfeiffer - Thermostar 300 amu Mass Spectrometer, which was used to monitor the evolved gases emitted. Infrared data were collected on a Thermo Scientific Nicolet 6700 FT-IR spectrometer on a laminated diamond mounted in a stainless steel plate in the 4000−600 cm−1 range with a resolution of 4 cm−1. Spectroscopic Study. Absorption and fluorescence spectra of all samples were collected at room temperature using a Varian Cary 5 spectrophotometer and a Perkin-Elmer LS 50 fluorometer, respectively. All spectra were measured on thin films of properly passivated QDs, neat or mixed with the ZnS matrix. Amplified spontaneous emission (ASE) and variable stripe length (VSL) measurements were performed with an amplified Ti:Sapphire laser system. ASE measurements allowed for the determination of amplified emission thresholds, whereas with VSL optical gain coefficient values were measured. The Ti:Sapphire laser delivered 150 fs pulses, with a maximum of 0.7 mJ per pulse energy at 800 nm with a repetition rate of 1 kHz. A BBO doubling crystal was used with a cutoff filter to obtain a 400 nm

have been successfully employed for the growth of metal sulphide nanoparticles, epitaxial shells over existing metal chalcogenide quantum dots, neat thin films, and thin films incorporating conducting polymers.35,36 What has made these precursors so attractive is their ability to decompose at temperatures as low as 150 °C to form metal sulphides and volatile coproducts. The exploitation of such precursors to develop stabilizing host matrices for core−shell nanoparticles has, to our knowledge, not been demonstrated. In this work we present a route for developing a ZnS based matrix that is suitable for CdSe−ZnS core−shell QDs and does not require any modification of their original surface chemistry (Figure 1). It is based on using zinc xanthate complexes as a

Figure 1. Schematic illustration of the formation of a QD:ZnS matrix thin film from a single QD/Zn(EtXn)2(octylamine) precursor solution.

single source ZnS precursor. By selectively modifying the surface chemistry on the QDs with a variety of aliphatic surface ligands, we further identify key ligand selection criteria for ensuring high thermal stability in any QD based composite. The applicability of these composites for optical gain applications is exemplified through the study of their amplified spontaneous emission properties.

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EXPERIMENTAL SECTION

Materials. 5-Aminopentanol (95%), CdO (99.99%), decylamine (99%), dodecylamine (99%), hexadecylamine (95%), 1-octadecene (ODE) (90%), octadecylamine (90%), octylamine (99%), oleylamine (70%), pyridine (99.8%), sulfur flakes (99.99%), selenium powder (99.99%), tetradecylamine (95%), and trioctylphosphine oxide (TOPO) (90%) were purchased from Aldrich. Chlorobenzene (99.8%) was purchased from Ajax Chemicals. Toluene (99.5%) and Zn(OAc)2·2H2O (99.5%) were purchased from AnalR. Acetone (99.8%), chloroform (99.8%), ethanol (99.5%), hexane (95%), and methanol (99.8%) were purchased from Merck. All reagents and 2119

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wavelength beam. The intensity of the input beam was continuously varied in two ways: with a half-wave plate and polarizer with a set of neutral density filters. The input beam was focused with a 200 mm focal length cylindrical lens onto slide samples. The samples’ edge emitted beam was detected in a lateral configuration by an optical fiber connected to a microspectrometer (Ocean Optics HR2000). The spatial profile of the beam was measured using a CCD camera (Tecnova DF-2112), and pulse energies were sampled with a pyroelectric detector (Molectron J3-05). Transmittance at normal incidence and ellipsometry quantities Ψ and Δ have been measured using a J.A. Woollam V-VASE Spectroscopic Ellipsometer in vertical configuration, at three different angles of incidence (65°, 70°, 75°) in the wavelength range 400−1700 nm. Optical constants n and k and film thicknesses were evaluated from Ψ, Δ, and transmittance data using WVASE32 ellipsometry data analysis software. The molar ratio of the QD:ZnS matrix was fixed to 1:4000 for ASE studies. The 240 nm thick thin films used in these measurements were deposited onto quartz substrates with four subsequent layer depositions using spin-coating. After the deposition of each layer, the samples were heated at 200 °C for 10 min under N2.

monitored by TGA-MS (Figure 2). The generation of CO2 and other well-known products of xanthate decomposition, such as

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RESULTS AND DISCUSSION The low decomposition temperature of most transition metal xanthates makes them an ideal precursor for transition metal sulphides.34,39−41 Unfortunately, their polymeric nature severely limits their solubility in most common solvents, thus restricting their viability for depositing thin films or using them for composite film formation. To enhance the solubility of such precursors in organic solvents, alkyl amines can be added directly to the metal xanthate solution in order to generate highly soluble, mononuclear adduct species in situ.42−44 In this study we adopted this approach, using octylamine as the solubilizing alkyl amine species for Zn(EtXn)2 suspended in chlorobenzene. The addition of alkyl amines to the solutions makes them compatible with most high quality quantum dots. Such colloidal systems are typically synthesized in the presence of aliphatic amines, which act to increase the reactivity of metal carboxylates during the growth as well as providing favorable surface passivation. This compatibility ensures that the above precursor solutions can be deposited either as neat films or mixed with quantum dots in their native solvent environment to form hybrid thin films. The transformation of such thin films to form either ZnS or QD:ZnS matrices requires the thermal decomposition of the zinc xanthate within the films. In light of the numerous decomposition pathways and ultimately residues that can arise from xanthate decomposition39,45−51 we begin this report by presenting the chemical evolution of the zinc xanthate precursor during thermolysis and the physical properties of the resultant ZnS matrix. Decomposition of Octylamine-Zinc Xanthate Adducts. To gain insight into the decomposition mechanism of Zn(EtXn)2(octylamine) we first performed HS GC-MS analysis on the gases evolved during the annealing of a thin film of the precursor at 200 °C. This revealed that the primary volatile products were carbon disulfide (CS2), ethanol, and carbonyl sulfide (COS) (Supporting Information, Figure S1). Trace amounts of acetaldehyde, ethanethiol, diethyl sulphide, and diethyl disulfide were also detected. The detection of carbon dioxide (CO2) and ethene, noted coproducts of metal xanthate decomposition,45,52 was not possible in the HS GC-MS experimental setup used here. Having identified the volatile coproducts at 200 °C, their evolution as a function of temperature during the decomposition of a bulk sample of Zn(EtXn)2(octylamine) was

Figure 2. (a) TGA of Zn(EtXn)2(octylamine) and Zn(EtXn)2; (b) evolution of key gases during the decomposition of Zn(EtXn)2(octylamine).

S-ethyl O-ethyl xanthate, was also monitored.45,46,52 The evolution of ethene could not be monitored as it has the same molecular weight as the nitrogen carrier gas and could not be adequately resolved from the background signal. While TGA-MS analysis is quantitative for the triatomic species, fragmentation may result in a weaker than expected signal for the detection of larger molecules. Therefore, the TGA-MS analysis for species such as ethanol and octylamine should be viewed as qualitative and can only be used to determine the temperature range over which they are evolved. Hence, the signal resulting from the detection of the molecular ion of octylamine has been normalized to that of ethanol for clarity in Figure 2. TGA-MS analysis revealed that a multistep decomposition begins at approximately 100 °C, with the simultaneous evolution of CO2, CS2, EtOH, and a trace amount of COS. The CO2 that was detected at temperatures below the decomposition of the precursor is attributed to desorption from the crucible.45,52 The decomposition process, which is directly responsible for the release of the above constituents, appeared to be largely completed by 200 °C. Beyond this temperature, TGA-MS demonstrated that octylamine was evolved between 200 and 230 °C. As alkyl amines coordinated to metal ions may be evolved at temperatures much higher than their typical boiling points,53 2120

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this would explain why we did not observed octylamine in the HS GC-MS, despite it possessing a boiling point of 176 °C in its pure state. Furthermore, any residual, noncoordinated octylamine that was evolved at temperatures below 200 °C was probably present at concentrations below the limit of detection for GC-MS and TGA-MS instruments. Octylamine is also seen to be evolved at ∼300 °C. As this occurs as an event distinctly separate from the loss of octylamine coordinated to the ZnS matrix, it may be attributed to the thermal decomposition of the unidentifiable organic residue that remains on the surface after thermolysis (see below). Overall, the above results indicate that the decomposition of the Zn(EtXn)2(octylamine) is a multistep process in which the xanthate decomposes first, followed by the thermalization of the residual octylamine and its reacted byproduct. As HS GCMS and TGA-MS analysis has enabled us to only understand which gaseous species are generated during thermolysis of the Zn(EtXn)2(octylamine), to characterize the non volatile species that remain within the ZnS matrix following annealing at 200 °C, the residual ZnS matrix was soaked in ethanol for 30 min and then this solution was analyzed by GC-MS. Two species could be detected, one of which was identified as octylisothiocyanate (CH3(CH2)7NCS). A second, unidentifiable, species had a molecular weight higher than octylamine, and the fragmentation pattern resulting from EI ionization was indicative of a molecule with one or two large alkyl chains, although the molecular weight (282 Da) could not be matched to any xanthate derivative containing two octyl moieties. The very low concentration of the two species detected and the fact that no other species were observed in the solution indicate that there is little labile or soluble organic material left within the ZnS matrix after annealing at 200 °C. It is has been well documented that metal xanthate adducts provide a reduced decomposition temperature compared to their parent complex.41,54 Indeed, a comparison of the TGA of Zn(EtXn)2(octylamine) and the polymeric Zn(EtXn)2 shows this clearly (Figure 2A). The modification of the decomposition profile indicates that the presence of coordinating octylamine significantly alters the decomposition pathway for the zinc xanthate parent complex. To our knowledge there have been no reported decomposition studies which have examined the decomposition mechanism of zinc xanthate alkyl amine adducts in the solid state. Decomposition studies made on solid-state samples of the parent complex and mononuclear complexes that employed co-ligands other than alkyl amines have, however, been performed. These have suggested that both Sethyl O-ethyl xanthate and hydrogen sulfide should be detected during the decomposition process.34,36,39,46−48 As neither of these products were detected, it suggests that the decomposition mechanism of alkylamine adducts differs from what has been widely reported in the literature. Therefore, an absolute determination on the mechanism for the decomposition of the precursor lies beyond the immediate scope of this investigation, although further discussion regarding this subject can be found in the Supporting Information. Physical and Chemical Properties of ZnS Thin Films. In order to confirm the presence of the ZnS phase in the thin films, grazing incidence X-ray diffraction was performed on samples thermally annealed between 150 and 400 °C. The results indicate that annealing of the Zn(EtXn)2(octylamine) within this temperature range yields cubic phased ZnS crystallites (Figure 3). Through the use of Scherrer analysis on the (220) diffraction peak, the crystallite sizes were found to

Figure 3. GIXRD measurements of ZnS thin films annealed at various temperatures. The diffraction peaks have been assigned to cubic ZnS (JCPDS No. 05-0566). An estimation of the crystallite size is also included based on Scherrer analysis of the (220) diffraction peak.

increase from an average of 3 nm at 150 °C to 8 nm at 400 °C (see Figure 3). Notably, amorphous ZnS is also likely to be a co-constituent at the lower temperatures, as evidenced by the prominent scattering contribution which overlaps with the (100) diffraction peak. The existence of a cubic ZnS phase at temperatures as low as 150 °C proves that the Zn(EtXn)2(octylamine) adducts decompose below this temperature. To concurrently assess the chemical composition of the films during and above this decomposition point, we performed attenuated total reflectance (ATR) Fourier transform infrared spectroscopy (FTIR). In Figure 4 the FTIR spectra of neat Zn(EtXn)2, octylamine, and the Zn(EtXn)2(octylamine) adduct annealed at different temperatures in nitrogen are shown.

Figure 4. ATR-FTIR measurements of neat octylamine, Zn(EtXn)2, and thin films of the Zn(EtXn)2(octylamine) adduct annealed at various temperatures.

Neat Zn(EtXn)2 exhibits dominant stretches arising from the asymmetric and symmetric C−S−C and C−O−C stretches in the 1000−1250 cm−1 region.55 The expected alkyl stretches between 2800−3000 cm−1 and 1200−1500 cm−1 are observed under higher magnification (not shown). The FTIR spectrum of octylamine is dominated by the alkyl stretches, with weak contributions due to N−H stretching at ∼3300 cm−1 and NH2 bending at ∼1600 cm−1 also being observed. 2121

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Figure 5. Contour plots of the relative and normalized integrated fluorescence yields of neat QD thin films (a and b, respectively) and QD:ZnS composite thin films (c and d, respectively) featuring QDs passivated with amines of increasing boiling points. The corresponding alkyl amines are depicted on the right y-axis of each panel. To normalize the fluorescence data, the maximum fluorescence yield across the entire annealing temperature range studied was used as the normalizing factor. The dashed lines shown in (c) and (d) are intended as guides to the eye, demonstrating the average trend in the annealing temperature at which half of the maximum fluorescence is observed.

MS measurement that desorption of octylamine species is the major contributor to this event. The small difference between the spectra of the thin films annealed at 250 and 400 °C can be attributed to the thermolysis of the unidentifiable organic residue that was a minor byproduct of the precursor decomposition, an event that was also observed by TGA-MS. PL Properties of QD Composites. The low-temperature and relatively clean decomposition of the amine chelated zinc xanthate provides a facile mechanism by which to develop hybrid materials with QDs. For this study, we used ∼7 nm CdSe−CdS−ZnS core−shell QDs that were prepared under standard lyophobic conditions. Such QDs could be mixed directly with the zinc xanthate solution, thus providing a means to deposit composite material with the original surface chemistry intact. Motivated to understand the role of surface ligands in such matrices, we did however modify the surface chemistry with varying alkyl chain length amines, which are in any case chemically similar in nature to the original surface passivation. This was accomplished by initially stripping off the existing surface ligands with a pyridine exchange and then repassivating these labile surfaces with the alkyl amines. This type of approach has been well characterized and thus provides the most routine methodology for surface chemistry modification.58 By selectively choosing amines with varying alkyl lengths, the boiling points of surface ligands spanned between 176 °C (octylamine) and 351 °C (octadecylamine). This large variation in the boiling points of the ligands enables the influence of ligand desorption to be decoupled from matrix effects following thermal treatment. The repassivated QDs were mixed with an octylamine stabilized zinc xanthate solution at a QD:Zn molar

Formation of the Zn(EtXn)2(octylamine) species and gentle heating to 100 °C is found to result in the formation of a broad band at ∼3240 cm−1, a minor peak at 3080 cm−1, a shoulder at ∼1570 cm−1, and a peak at 1515 cm−1. In addition, an ensemble of vibrational contributions at wavenumbers between 1000−1500 cm−1 is observed. These can be assigned to the various stretches and bends arising from the amine or the xanthate ligands present in the film, but due to the convoluted nature of this spectral region they will not be further analyzed here. We will however note that the broad band at 3240 cm−1 and minor peak at 3080 cm−1 can be assigned to N−H stretches that arise from the amine coordinating to the zinc metal center.56 This is supported by the characteristic bending frequency of amines, which is known to shift downfield from ∼1600 cm−1 to values of ∼1570 cm−1 upon coordination to metals. At temperatures of between 150 and 200 °C the coordination of the amine changes as is evidenced by an increased absorbance of the 1570 cm−1 and 3080 cm−1 resonances. On the basis of the TGA and GIXRD, we assign this to the octylamine within the film now coordinating the nanocrystalline ZnS. The octylisothiocyanate that was detected on the surface of the matrix should give a strong absorption around 2100 cm−1 due to an asymmetric NCS vibration, but as this is not observed we can conclude it is not present in significant quantities and thus should be treated as only a minor impurity of the decomposition process.57 At temperatures of 250 °C and above, the major contributions arising from the organic content in the film are removed. While we cannot identify minor organic components within our films, the FTIR supports the findings of the TGA2122

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ratio of 1:4000 in toluene. These films were spin-cast onto quartz substrates and annealed at temperatures of between 100 and 400 °C under nitrogen. To understand the role of the surface ligands and the ZnS matrix on the fluorescence stability of the QDs, we compared the fluorescence yields of neat QD films with the QD:ZnS matrix thin films as a function of annealing temperature and surface ligand boiling point. These results are presented in Figure 5 through contour plots of the relative (parts a and c) and normalized (parts b and 5) fluorescence yields, respectively. Beginning with neat QD films, it is apparent that the fluorescence yields of QDs which possess longer chained alkyl amines are higher (Figure 5a). This trend reflects the quantum yields in solution (not shown), where the lower solubility of the longer chain length amines ensures that a greater extent of surface passivation is maintained following the purification step.1 To appreciate the absolute values in the thin film, the typical QY for the longer chained amine coated QDs in solution was 50−60%. Increasing the annealing temperature for all samples exhibits a concomitant decrease in the fluorescence yields. This trend is more clearly observed when comparing the normalized fluorescence data which are shown in Figure 5b. Here, we observe that the fluorescence yields reach half of their original value at an annealing temperature of between 120 and 180 °C, with the higher boiling point ligands providing a greater stabilization (see dashed line in Figure 5b). Accompanying the fluorescence degradation process is a slight red-shift, with no observable broadening of the fluorescence peak (Supporting Information Figure S2). Inspection of the absorption spectra further indicate that little change to the absorption features occurs at these temperatures (Supporting Information Figure S3). These observations suggest that the degradation process of the fluorescence is not associated with an Ostwald ripening event. Instead, it is believed that gradual ligand desorption and the consequent formation of surface dangling bonds are a major cause of this degradation process. The picture is in qualitative agreement with the slight increase to the stability of the longer alkyl amine passivated QDs. QDs embedded within the hybrid ZnS matrix exhibit vastly different fluorescence behaviors compared to neat QD films (Figure 5c,d). At low temperatures, all QDs possessed similar QYs. This contrasting behavior to neat QD solutions is believed to arise due to the slight excess of octylamine in the matrix solution. The octylamine forces the ligand equilibrium between the surface and solution toward the surface, thus ensuring a higher extent of surface passivation and consequently a higher fluorescence.1 As a result of this equilibrium, the surface passivation is most likely a mixture of both octylamine and the original alkyl amine passivating the QDs. As the annealing temperature is increased to 200 °C, the fluorescence yields of all the samples are increased by values up to 100%. The samples with QDs that are passivated with longer alkyl chain amines exhibit a progressively higher fluorescence. The temperature range at which the fluorescence increases is consistent with the decomposition of the zinc xanthate and the consequent formation of ZnS within the film. The vastly different fluorescence trends in these hybrid thin films compared to neat QD films can therefore be directly correlated to the favorable passivation of the QDs by the ZnS. Importantly, the ZnS matrix is acting not only as a type-I host matrix but can additionally supplement the existing amine

based surface ligands as they are gradually thermalized. Both of these factors ensure maximum surface passivation of the QDs within the composite. While passivation of the QDs through the formation of the ZnS phase in the composites can explain why the fluorescence yields of QDs are maximized in the hybrid films, it cannot account for why the relative fluorescence yields are increased for the higher chained amines following the ZnS formation. A comparison between the absorption spectra suggests that the extent of ZnS formation is similar regardless of the amine employed in passivating the QDs (Supporting Information Figure S4). Therefore, while we can state that the longer chained amines enable a more conducive surface passivation of the QDs during the ZnS formation, the exact origins of their enhancement remains at present unclear. At temperatures of between 250 and 300 °C the fluorescence of all samples reaches half of their maximum values. Beyond this range the fluorescence is found to rapidly decline, being accompanied by significant broadening and red-shifting of the fluorescence maximum (Supporting Information Figure S2). Both of these phenomena are consistent with an Ostwald ripening event. Unlike for neat QD films, where such a process is limited until temperatures of >350 °C, the ripening of the 3− 4 nm ZnS particulates (see above) onto the 6−7 nm sized QDs would be expected to occur at comparatively lower temperatures. The effect of this growth stage would be an effective increase in the shell thicknesses surrounding the QD cores. Consistent with the dependence of CdSe QD fluorescence yields on ZnS shell thickness,2 at these larger shell thicknesses, the large lattice mismatch between the CdSe and the ZnS matrix would induce the formation of interfacial core−shell defects, thus resulting in a drastic reduction in the fluorescence yields. Interestingly, unlike for QDs deposited within ZnSe matrices,32 the lack of any fluorescence blue-shift at increasing annealing temperatures suggests that alloying is not a dominant process in this system within the temperature range studied. The reason for this may also be related to the large lattice mismatch between the CdSe cores and the ZnS matrix. The above studies were performed on a fixed QD:Zn molar ratio of 1:4000. To further explore the influence of the matrix on the QDs, we varied the relative QD:ZnS molar ratio between 1:2000 and 1:16000 using QDs passivated with dodecylamine. The effect of increasing the molar ratio between the QDs and the Zn(EtXn)2 precursor was to increase the relative fluorescence intensity while decreasing its thermal stability (Supporting Information Figure S5). These observations not only confirm the stabilizing nature of the ZnS component but also suggest that for such matrices to be used as functional fluorescence materials, a compromise between fluorescence intensity and thermal stability has to be made. Dodecylamine passivated QDs demonstrate excellent thermal stability at higher annealing temperatures and have good absolute and relative PL values when annealed at 200 °C. To strengthen their appeal, by nature of the effective medium effect, thin films of QDs passivated with lower molecular weight amines will present higher refractive indices. This makes such ligands more conducive toward waveguiding applications than their longer alkyl chained counterparts. Spectroscopic ellipsometry of thin films composed of dodecylamine passivated QDs within the ZnS matrix indicate that such composites exhibit real refractive index values of >1.7 when annealed at temperatures of 150 °C and above (Supporting Information 2123

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Figure 6. Atomic force micrographs of thin films fabricated from CdSe−CdS−ZnS QDs dispersed in an octylamine stabilized zinc sulfide matrix that was annealed at 100, 150, and 300 °C in nitrogen. The root-mean-square roughness calculated for each of the temperatures was 0.93, 0.97, and 1.12 nm, respectively. The scan size for each micrograph is 2 × 2 μm.

Figure 7. (a) Absorption and steady-state fluorescence profiles of the CdSe−CdS−ZnS quantum dots employed in this study. Included are also the optical pump beam profiles used to generate amplified spontaneous emission under one-photon (400 nm) and two-photon (800 nm) optical pumping. (b) The stability of the ASE peak under both one-photon (400 nm) and two-photon (800 nm) optical pumping mechanisms in air at room temperature. (Inset) A depiction of the ASE measurement, showing the edge-emission stemming from the optical pumping process.

amicable to the formation of smooth thin films, a necessary criterion for the development of waveguides. ASE Measurements. To assess the viability of the QD:ZnS matrix thin films for optical gain applications, we deposited thin films of 240 nm on quartz substrates using a dodecylamine QD surface chemistry. Amplified spontaneous emission (ASE) experiments were performed on these waveguides using a fixed-stripe-length configuration59 under optical pumping within the one- (at 400 nm) and two-photon (at 800 nm) absorption ranges of the QDs. In Figure 7a we summarize the excitation, absorption, and emission sources for these ASE experiments. For both optical pumping mechanisms, only spontaneous emission was detected at low excitation fluences. When the fluences exceeded the optical gain threshold (0.094 ± 0.013 mJ cm−2 for one-photon and 12.40 ± 0.28 mJ cm−2 for two-photon excitation), a narrower ASE contribution was observed at ∼654 nm (fwhm ≈ 10−13 nm) (Supporting Information Figure S7). The approximately 360 cm−1 (40 meV) bathochromic shift of this band with respect to the maximum of the spontaneous emission is consistent with its origin being that from a biexcitonic state.60 Using experimentally determined one- (∼1.8 × 106 cm−1 M−1 at 400 nm) and two-photon (∼39500 GM at 800 nm) absorption cross

Figure S6). These values are directly comparable to sol−gel ZrO2:QD composites and are sufficiently high to observe waveguiding on a variety of substrates, for example, glass or quartz. The balance of PL properties and refractive index suggest that from all the surface chemistries studied here, dodecylamine passivated QDs are the most favorable for use within optically pumped laser devices. To appreciate the film forming properties of such hybrid matrices, atomic force microscopy (AFM) measurements were made on QD:ZnS matrix thin films with a dodecylamine QD surface chemistry. In Figure 6 we exemplify the topographic surface profiles of this matrix deposited as thin films and then annealed at 100, 150, and 300 °C. All films demonstrate nanoparticulate morphologies. At the lower annealing temperatures, the topographies appear similar with no evidence of significant grain growth. In comparison, an appreciable increase in particle size and rms roughness (0.97 nm at 150 °C vs 1.12 nm at 300 °C) is observed for the matrix annealed at 300 °C. This finding is in support of the gradual ripening of the ZnS matrix onto the existing QDs in the film. Overall, the AFM measurements highlight that the QD:ZnS matrix is not only capable of preserving high fluorescence stability but is also 2124

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passivation of the QDs by the ZnS matrix. To exemplify the lucrative properties offered by such a composite, we studied their amplified spontaneous emission properties. Unprecedented stability factors of the optical gain state stemming from the QD inclusions were observed.

sections, we calculate the average number of electron−hole pairs at the threshold to be 1.3 and 6.5, respectively. By varying the stripe length of the optical pump beam in this experimental configuration (see Figure 7b inset), the effective gain coefficient of the sample can be estimated.59 Using an Auger limited VSL model61 we found that the roomtemperature optical gain coefficient of our sample was ∼130 cm−1 under both excitation mechanisms. This value is comparable to those of previous reports that used neat or sol−gel based QD films and sufficiently high to be used within microcavity laser systems.8,10,62 Another requirement for such QD composites to be harnessed within optoelectronic devices is that they possess sufficiently high photostability. In Figure 7b we present ASE stability measurements of our QD/ZnS composites under continuous irradiation at a 1 kHz repetition rate in air. Excitation at 400 nm is found to exhibit an unprecedented stability,10,24 with no appreciable loss in ASE even after 6 h of optical pumping (2.2 × 107 pulses). Under two-photon excitation the ASE has a lower stability, exhibiting a half-life of ∼90 min. In addition, in contrast to its one-photon analogue, it also experiences a gradual red-shift of the ASE maximum (see Supporting Information). The overall difference in behavior between the one- and two-photon excitations can be attributed to the ∼100-fold higher energy density incident on the sample in the latter case. This is believed to cause significant local heating, far greater than the annealing temperatures used for the thin films, which causes degradation. Despite this factor, a comparison to previous reports of two-photon ASE shows that a significant improvement in the stability factor has been achieved through the use of a ZnS based matrix. Naturally, encapsulation against oxygen and moisture would further improve the photostability of these systems. The high optical gain coefficients, low ASE thresholds, and high optical gain stabilities observed for our QD/ZnS composites highlight the effectiveness of the ZnS matrix as a suitable host for QDs. We attribute these factors to (i) the electronically passivating nature of the ZnS and (ii) the favorable surface passivation offered through both the amine and the ZnS in the composite. In comparison to traditional systems which utilize sol−gel based host matrices,8,31 the lack of electron-trapping metal hydroxyl groups within the matrix and the ability to thermally anneal the systems up to temperatures of 250 °C without QD degradation provide the competitive advantage for this type of ZnS matrix to be used for developing electronically passive QD composites.

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ASSOCIATED CONTENT

S Supporting Information *

Discussion of the mechanism of thermoylsis of Zn(EtXn)2(octylamine), HS GC-MS spectrum of coproducts of Zn(EtXn)2 thermolysis, thicknesses of thin films deposited under varying conditions, PL features of neat QD thin films and QD:ZnS thin films, and extinction coefficients and refractive indices of ZnS thin films (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

These authors contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded through the Flexible Electronics Theme of the Future Manufacturing Flagship as part of an Office of the Chief Executive Postdoctoral Fellowship to A.S.R.C. Ross Wearne is thanked for performing HS GC-MS measurements. F.T. thanks PRAT_2010 prot. CPDA104332/10 of the University of Padova. J.J.J. wishes to acknowledge financial support through CSIRO, Australian Research Council for support through the APD Grant DP110105341, the Australian Solar Institute USASEC research exchange program, and the Fulbright Postdoctoral Fellowship Scheme.

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REFERENCES

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CONCLUSION In this work we have described the development of QD:ZnS composite thin films that are based on the decomposition of the single-source ZnS precursor, Zn(EtXn)2(octylamine), in the presence of aliphatically passivated colloidal quantum dots. By possessing a high solubility in organic solvents, the use of this precursor enables for its direct mixing with as-synthesized quantum dots, thus providing a facile route to fabricating inorganic based QD composites. As the decomposition temperature of the Zn(EtXn)2(octylamine) is as low as 100 °C and the majority of its decomposition products are volatile, an alkyl amine passivated nanocrystalline QD:ZnS hybrid film is formed at temperatures below 200 °C. In comparison to neat QD thin films, the resulting hybrid matrices exhibit vastly superior thermal stability factors of their fluorescence. This stems directly from the favorable chemical and electronic 2125

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