Controlling the Surface of Semiconductor Nanocrystals for Efficient

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Controlling the Surface of Semiconductor Nanocrystals for Efficient Light Emission from Single Excitons to Multiexcitons Brenna Walsh, Jonathan Saari, Michael M Krause, Robert Nick, Seth Coe-Sullivan, and Patanjali Kambhampati J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015

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The Journal of Physical Chemistry

Controlling the Surface of Semiconductor Nanocrystals for Efficient Light Emission from Single Excitons to Multiexcitons Brenna R. Walsh†, Jonathan I. Saari†, Michael M. Krause†, Robert Nick‡, Seth Coe-Sullivan‡, and Patanjali Kambhampati†* †

Department of Chemistry, McGill University, Montreal, QC, H3A 0B8 Canada ‡

QD Vision Inc., Watertown, MA, 02472, USA

Abstract: Semiconductor nanostructures have shown promise for light emission across various intensity regimes. Desired performance objectives of photoluminescence efficiency, low gain threshold, large gain lifetime and bandwidth have not been met by any one nanocrystal. A physical understanding of the design principles governing these objectives is also lacking. We show that a carefully engineered CdSe/Cd,Zn,S core/shell nanocrystal uniquely meets all criteria. The key factor allowing for these improvements is the gradual core/shell boundary, which decouples the surface electronic states.

Keywords: Interface, broadband gain, threshold, exciton-photon coupling

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Semiconductor nanocrystals (NCs) have been under intense investigation since their inception as active elements for applications involving light emission. These applications span simple photoluminescence (PL), wherein the NCs serve as efficient light emitters based upon single excitons (X), from light emitting diodes (LED) to displays, to fluorescence labels1-4. Nanocrystals are also unique in their ability to support multiple excitons per NC when illuminated at high intensity5-6. Upon formation of multiexcitons (MX), NCs can support optical gain via stimulated emission7-9, function as all-optical logic systems10 and enable LED function at increasingly high carrier concentration11. Researchers have identified several aspects of the physics which govern light emission in nanocrystals6, 12, and in concert, have made considerable progress on devising new materials with improved performance4. When looking to optimize emission from single excitons, the objective is to create high PL quantum yield, typically by variations on inorganic shells that passivate the NC core13. Multiexcitons are the relevant excitation in applications such as optical gain and alloptical switching6,

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. Creating a NC with low gain threshold, long gain lifetimes, broad

bandwidth, and size (color) universality has proven challenging. To generate optical gain for lasing applications, CdSe NCs require biexciton (XX) formation to enable stimulated emission in the simplest picture7. Biexcitons and higher MX have fast recombination rates due to Auger processes5-6, which limit their gain lifetimes and bandwidths. Initial experiments showed a reasonable gain threshold (Nth) of ~ 2 for large CdSe NCs. However, these experiments showed that the smaller the NC the larger the threshold, suggesting some poorly understood parasitic loss processes, which were assumed to arise from the surface7.


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Our group has explored the physics of optical gain in NCs to understand the driving processes involved9, 14. Through state-resolved optical pumping, it has been shown that surface trapping is a key process to manage when aiming to produce NCs for optical gain and related applications6, 12, 15-16

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Surface trapped excitons lead to photoinduced absorption that increases the gain threshold and the rate of Auger recombination, which limits gain performance6, 12, 15-16. Considerable effort has been put forth to create improved surface passivation with new confinement structures (Type I vs Type II) 8, and new shapes (rods, platelets)

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. Each of these systems has advantages and

disadvantages, from low oscillator strength for Type II systems, to short MX lifetimes for many core/shell systems, to narrow gain bandwidths in extended systems. A full understanding of the physics that dictates performance for light emission in this large variety of semiconductor NCs has not been realized. Some work has been done on the effects of adding a third shell component to the type 1 CdSe/ZnS material, looking at CdSe/CdxZn1-xS dots with varying shell thickness20 and effects of alloying the core and the shell21. The synthesis of the alloyed structure creates a structure without a step function transition from core to shell material. Photoluminescence quantum yield22, blinking21 and lifetime20 have been studied for these materials. A transient absorption (TA) experiment has also been done which looks at ps hole cooling rates for CdSe/CdZnS by Liu23. The potential of these materials as an efficient lasing media has been explored24, but the more fundamental growth, bandwidth and lifetime of optical gain for CdSe/CdZnS type materials has not yet been. Here a CdSe/Cd,Zn,S sample with some mixing of the core and shell material is examined. Optical gain signatures are seen that suggest that these NCs have a unique surface electronic structure which renders the formerly ubiquitous surface trapping processes largely


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irrelevant. Absent of a surface state, these NCs are ideal for efficient generation of light in one platform – from single to multiexcitons.

Experimental

CdSe NCs were over-coated with a shell of Cd, Zn and S, creating what we refer to as the CdSe/Cd,Zn,S system in the remainder of this publication. The CdSe/Cd,Zn,S NCs were passivated with oleic acid ligands and dispersed in toluene. This sample was received from an industrial collaborator and therefore the precise composition of the dots as they go from a CdSe core to the Cd, Zn, S shell is unknown; however, it is known that there is not a distinct boundary between core and shell, with some mixing of core shell material at the boundary. All steady state and ultrafast experiments were done on different aliquots of a single sample. Linear absorption spectra were taken on a Varian Cary UV/vis spectrophotometer and PL spectra were taken on a Horiba Jobin-Yvon Fluoromax2 spectrofluoremeter, both in a 1 cm path length quartz cuvette. Excitonic state-resolved femtosecond pump/probe spectroscopy was performed on these NCs, the experimental details are described in our previous work9-10, 14. To briefly elaborate, the pump/probe experiments were performed on an amplified Ti:sapphire laser system (Regen, Coherent) which produces pulses of 800 nm wavelength at a pulse length of ~70 fs with 2.2 mJ/pulse at 1 kHz. The beam was then divided and sent to two optical parametric amplifiers, which were each tuned to a specific pump wavelength, corresponding to the excitonic transitions of the CdSe/Cd,Zn,S QDs, seen in the absorption spectrum. All ultrafast experiments were performed on a sample in a 1 mm path length flow cell and repeated on the same sample on


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a second day. Absorption spectra were taken before and after each set of ultrafast measurements and no changes in the spectra due to sample degradation or optical heating were observed.

Results and Discussion

A. CdSe/Cd,Zn,S NCs

A transmission electron microscopy (TEM) image of the CdSe/Cd,Zn,S NCs is shown in Figure 1a. The particles have absorption into the band-edge exciton at 604 nm, which can be seen from the absorption spectra in Figure 1b. The core/shell particles are based upon a spherical CdSe core, but the final particles have a pyramidal shape, as deduced from the 2D projections seen in TEM, so some deviation from the common effective mass approximation is expected. If you bisect the triangles seen in the TEM image, the particles are ~ 10 nm across. The shells enable high PL quantum yields (~60 %), and single exponential PL lifetime as is anticipated for a homogeneously emitting core/shell NC system (inset Figure 1a, fits seen in Supplemental Figure S2). These simple measures do not guarantee multiexcitonic performance nor do they reveal any insight into nature of light emission from these nanocrystals, which is dictated by the core/shell interface. To explore this further, we will look at optical gain of the system through state-resolved transient absorption spectroscopy.


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Figure 1. a) Transmission electron microscope image of CdSe/Cd,Zn,S nanocrystals (NCs). Inset PL decay of CdSe, CdSe/ZnS and CdSe/Cd,Zn,S. b) Absorption (OD0), PL and non-linear spectra of CdSe/Cd,Zn,S c) Schematic of absorption features corresponding to core and surface excitons, X1-X3 representing the first three excitonic transitions specified in the absorption spectra and XS being the surface trap state. d) Schematic of exciton and biexciton level structure, with symbols on the left specifying absorption and emission events and symbols on the right specifying Stokes shifts (δ) and binding energies (∆) between specific states.

Figure 1b shows the linear and representative non-linear spectra of these NCs. The linear spectra reveal the excitonic structure via absorption (OD0) and PL (pumping details in Supplemental Figure S1). The non-linear optical density (ODNL) spectra, which is calculated by adding the OD0 obtained from the absorption spectra to the change in optical density (∆OD) obtained from the pump/probe measurement, reveal the exciton and multiexciton dynamics. An absorption spectrum is equivalent to a probe only measurement. ∆OD is the change in absorption of the sample induced by the pump pulse, when the pump precedes the probe. The non-linear optical density spectra are measured at a given pump/probe time delay, with some pump energy


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corresponding to a mean exciton occupancy, , with the pump resonant to a specific initial excitonic state. Figure 1b shows a representative ODNL spectrum that features some optical gain or stimulate emission (SE), where the non-linear absorption goes below zero (for other pump powers see Supplement Figure S3). The sample is pumped at the X1 transition at a fluence above the gain threshold and with a pump probe time delay of 2 ps, when all carriers have relaxed to the ground state. Both the linear and non-linear spectroscopic measurements can be interpreted based upon simple energy level diagrams. Figure 1c shows a schematic coarse energy level diagram in the exciton representation of spherical NCs, each excitonic state (Xi) corresponds to a feature in the linear absorption spectra. These excitonic states have been discussed in detail for spherical CdSe NCs12,

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, upon which these core/shell NCs are based. Though these nanocrystals are not

spherical, we will use this labeling scheme to simplify comparison to the literature. Figure 1d shows a schematic fine energy level diagram. The single exciton (X) fine structure has been discussed by Bawendi26. They have denoted separation between the absorbing (XA) and emitting (XE) levels, as corresponds to the single exciton Stokes shift (δX). Since multiexcitons (MX) are central to NC gain, the biexciton (XX) fine structure is also included6,

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. Here, XXA corresponds to the absorptive state of the biexciton and XXE

corresponds to the emissive state of the biexciton. Hence there is also a biexciton Stokes shift (δXX). The presence of δX yields a minimal level scheme for NCs as a three level system which should enable single exciton gain as in molecular systems. However, NCs absorb multiple photons, creating MX. Due to attractive multiexciton interactions, absorption into the biexciton is redshifted with respect to absorption by the biexciton binding energy, ∆XXA. This redshift can be similar in magnitude to δX, thereby producing absorptions which block single exciton gain.


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Hence gain is enabled by producing biexcitons, in the case of a doubly degenerate band edge exciton as in CdSe NCs, though this biexciton absorption can also block the gain signature. The biexciton gain is redshifted via δXX. In general, gain for single or multiexcitons becomes blocked by new absorptive transitions which produce loss instead of gain (specific case mentioned above). In most NCs, excitons undergo surface trapping6,

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. Surface trapping produces photoinduced absorptions which

compete with gain6, 9, 14. Surface trapping results in creation of new states lower in energy than the core excitonic levels. The surface state is shown schematically in both Figures 1c and 1d as the lowest lying state, in green. These surface trap states can block the biexciton emission, XXE, thereby blocking biexciton gain which increases the gain threshold. Surface trapping is also to be avoided as it increases the rate of Auger recombination thereby further hastening the gain lifetimes, whether via hot or cold exciton trapping15-16. In all NCs, it is the interplay between these levels that controls the optical gain performance. While Type I NCs are the primary optical gain materials, newer geometries have been developed. For example, Type II confinement can lower the theoretical gain threshold to Nth ~ 0.7, but at the cost of oscillator strength. Similarly nanorods and more recently nanoplatelets have shown good optical gain performance, but at the cost of gain bandwidth. Ultimately each system may offer its unique advantages and disadvantages in terms of metrics such as: threshold, oscillator strength, lifetime, bandwidth or even polarization. For any system, however, there are the common excitonic processes which must be understood and controlled.


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B. Optical gain performance of CdSe/Cd,Zn,S NCs

Towards such control of excitonic processes, we focus here upon the surface/interface of the CdSe/Cd,Zn,S nanocrystal, and how it effects optical gain. We have recently discussed in detail both the kinetics and thermodynamics of exciton surface trapping15 and the implication upon optical gain6, 9, 14. The role of the surface/interface in controlling the optical properties of NCs is still in its early stages. We hypothesize that the more gradual core/shell interface in these CdSe/Cd,Zn,S nanocrystals will reduce surface trapping and we will see increased gain performance as much optimization of interplay between levels is present in this system. These CdSe/Cd,Zn,S core/shell NCs are benchmarked against standard CdSe and CdSe/ZnS NCs. Figure 2a shows the optical gain threshold for these NCs with CdSe and CdSe/ZnS for comparison, the CdSe and CdSe/ZnS, having Nth = 1.5 and CdSe/Cd,Zn,S having Nth = 1.0, consistent with prior demonstration28. Samples are pumped at the X1 transition to avoid hot exciton trapping effects with different pump fluences at a pump/probe time delay of t = 2 ps. For the 1S state, as intraband rearrangement to higher states is negligible, the average occupancy of the 1S state should be equivalent to the average number of excitations per particle, , and is therefore calculated: ∆

< > = −2( )

1

where ∆OD is the change in optical density at a given pump fluence at the bandedge and OD0 is the optical density (from the linear absorption spectrum) at the bandedge29. The gain threshold is ODNL = 0.


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Figure 2. a) The ODNL magnitude as a function of exciton concentration, , with pumping into the band edge exciton (X1). The gain threshold is 1.5 for CdSe and CdSe/ZnS and single exciton gain is achieved under conditions of band edge pumping for CdSe/Cd,Zn,S dots. The pump fluence dependence of the stimulated emission spectra under conditions of pumping into


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X1 (b) and X3 (c), with regions of single excitonic (X) biexcitonic (XX) and higher multiexcitonic states (MX).

The decrease in threshold for the CdSe/Cd,Zn,S NCs with respect to CdSe and CdSe/ZnS arises from the balance between single exciton Stokes shift and biexciton binding energy. With band edge pumping into X1, the gain threshold is dictated by the exciton and biexciton level structure6, 27, which, here produce the low threshold gain of N = 1.0. There is little overlap with the photoinduced absorption signal for these dots as it is significantly displaced from the bandedge and is very short lived (this will be explored further in a future publication). However, the more significant contributor to increased optical gain is the lack of surface-core biexciton seen in these samples, which is allowed by the Cd,Zn,S shell structure, which significantly reduces hot exciton trapping effects. This biexcitonic state is known to depopulate the core state as excitons are trapped to the surface30. Both of these effects result in less potential overlap of the stimulated emission region with potential absorbing regions, allowing for a lower gain threshold. Trivially a lower gain threshold is desirable for the efficiency of pumping, but is not the only desired performance target. A Type II NC has lower theoretical threshold at Nth = 0.7 than a Type I NC with Nth = 1.0. However, this decrease in threshold comes at the cost of oscillator strength for Type II systems. The nontrivial importance of single exciton gain is that multiexcitons have shorter lifetimes due to enhanced Auger recombination rates. Biexcitons have lifetimes of 10 – 100 ps in typical NCs, thereby placing constraints upon the pumping rates. However, increasing the MX lifetimes enables one to exploit their interactions for gain bandwidth control9, 14 and all-optical modulation using NCs10. A NC with single exciton gain


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threshold, in conjunction with large multiexciton interaction energies and long multiexciton lifetimes has not yet been realized. Figure 2b-c show gain bandwidth control of unprecedented magnitude, Figure 2b shows the stimulated emission (ODNL < 0) of the ODNL spectra upon pumping into X1, and into X3 in Figure 2c. The SE spectra were obtained at a pump/probe time delay of 2 ps. The y-axis is scaled by energy per pump pulse and the color axis indicates the amount of stimulated emission obtained at a particular pump energy and wavelength, purple being maximum stimulated emission for a given pump. With X1 pumping SE is possible from single (X) and biexcitons (XX). With increasing fluence, there are minimal gain spectral changes and SE bandwidth is similar to what has been previously reported29. With X3 pumping, dramatic changes to the gain spectra can be seen to the red and the blue of the main SE band at 626 nm, with gain bandwidth of > 120 nm at high fluence. As the X3 state is theoretically six-fold degenerate, we should be able to access higher multiexcitonic states than the biexciton. The growth in gain bandwidth both below and above the wavelengths where gain is observed when pumping into the X1 state must be due excitation into these higher multiexcitonic states. The gain bandwidth control arises from two effects present in these NCs. The gain at additional long wavelength (>650 nm) arise from MX addition energies. The gain at additional short wavelength (

Controlling the Surface of Semiconductor Nanocrystals for Efficient

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