Understanding and Exploiting the Interface of Semiconductor

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Review

Understanding and exploiting the interface of semiconductor nanocrystals for light emissive applications Patanjali Kambhampati, Timothy Mack, and Lakshay Jethi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00951 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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Understanding and exploiting the interface of semiconductor nanocrystals for light emissive applications Patanjali Kambhampati*, Timothy Mack, Lakshay Jethi Department of Chemistry, McGill University, Montreal, Quebec, H3A 0B8, Canada

Abstract. Semiconductor nanocrystals have been extensively studied for optoelectronic applications including light emission, the focus of this review. Historically the core of the nanocrystal was the main aspect of the system as it gives rise to the confined excitons and multiexcitons which absorb and emit light. In addition to the core, the surface or interface of these nanocrystals is also important by virtue of their small size. Yet our understanding of the surface is in its early stages in terms of both chemical structure and electronic structure. Here, we review the ways in which the interface can control the excitonics which gives rise to optoelectronic function. We focus our discussion on the ways in which the interface can control optical gain for nanocrystal lasers and white light generation for nanocrystal based light emitting diodes. These processes are connected to different interfacial structures. Finally, we discuss two new applications based upon surface electronic structure control: optical switching and optical thermometry. The work here suggests that the interface of nanocrystals should be a profitable route for chemical control of the electronic structure which can yield both performance enhancements as well as qualitatively different functional behavior. Keywords: nanocrystal, quantum dot, exciton, multiexciton, optical gain, lasing, light emitting diodes, white light, optical switching, optical thermometry, surface chemistry *[email protected]

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Semiconductor nanocrystals hold promise for optical applications involving light emission1-4 by virtue of their unique electronic structure and dynamics – their excitonics5. These nanocrystals can be grown via solution phase methods to yield colloidal quantum dots, or via vacuum based methods to create self-organized quantum dots6. Based upon the presence of strong confinement, ease of solution phase processing, and chemically specific interfaces, the colloidal quantum dots are distinct and will form the basis for this review. The term nanocrystal notes that the issues are broader than simply quantum dots, as non-spherical geometries have been developed for some time. The aim of this review is to examine how specific aspects of nanocrystals, their surface/interface, dictates their function in light emissive applications. Semiconductor quantum dots were initially conceived in the 1980s for optoelectronic applications, in extension to the then developed quantum well systems6. The idea was that the density of electronic states in a continuum system would result in discrete transitions with favorably properties, such as large oscillator strength. In the past few decades this vision has been realized in a wide variety of systems, whether epitaxial or colloidal. Focusing on colloidal quantum dots, and related nanocrystals such as rods, these systems have been shown to support efficient light absorption and emission and optical gain. These basic processes gave rise to realization of nanocrystal based lasing2, light emitting diodes (LED)7-9, sensing10, photovoltaics11, 12, and photodetectors13. The reader is referred to the many excellent reviews on the general topic of optoelectronics of semiconductor nanocrystals1, 3, 4, 14. There are two key point of semiconductor nanocrystals for optoelectronics that are intimately coupled: their ability to be processed by solution phase methods, and their surface. The colloidal chemistry approach is what results in the flexibility that is enabled by solution phase methods. This flexibility in processing has enabled nanocrystal based photovoltaics, 2 ACS Paragon Plus Environment

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photodetectors, light emitting diodes, and other devices based upon use in large area devices. Conversely these processing techniques can be exploited to make small form factor structures such as lasers, light harvesters, and sensors. With such processing, the nanocrystal may be sensitive to the host matrix based upon the large surface to volume ratio of particles that are 1 – 10 nm in diameter. Hence the nature of the surface or interface of the nanocrystal is a central aspect to their function. Yet this aspect of their excitonics remains in its infancy15-17. Fig. 1 depicts how many aspects of the interface of semiconductor nanocrystals connects to the nature of the surface or interface. Fig. 1a shows a typical transmission electron microscope (TEM) image of CdSe nanocrystal. They key point is that the nanocrystals are clearly not perfect spheres. Moreover, they are more realistically some form of terminated crystal, with specific crystallographic faces. Hence the surface and/or interface should be a central aspect of their electronic structure as well as optoelectronic function. Fig. 1b shows how the nanocrystal is an enabling material for a wide variety of optical applications. Yet, the surface/interface will be a key parameter which controls how well these applications may be realized. Fig. 1c shows how CdSe can be tailored with organic ligands so as to controllably produce white light. Fig. 1d shows how the relative amount of surface photoluminescence can be controlled either by composition of ligands or by temperature thereby enabling new strategies for exploiting the surface science of nanocrystals. In recent years, the surface of semiconductor nanocrystals has emerged as a topic of great interest. Our group has reviewed the spectroscopy and electronic structure of the surface15, 16. Recently the Talapin group has reviewed chemical aspects of the surface17. We suggest that many of the ideas of classical bulk surface science may be a path forwards in understanding, controlling, and ultimately exploiting the surfaces and interfaces of these nanocrystals. 3 ACS Paragon Plus Environment

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The earliest semiconductor nanocrystals were passivated by organic ligands as surfactants. In order to achieve higher photoluminescence (photoluminescence) quantum yields (quantum yield), Guyot-Sionnest first developed the inorganic core/shell system of CdSe/ZnS18. In the last 20 years, these methods have been extended to a wide variety of other core/shell systems with an emphasis on maximizing the photoluminescence quantum yield of the core, as well as stability of single particle photoluminescence2,

17, 19-21

. For broad application in

optoelectronics, the simple metric of maximizing photoluminescence quantum yield for the single exciton (X) is merely one goal among many. Given the wide variety of applications for light emission, one asks what the other metrics may be, and how they may be controlled via materials design. For example, the photoluminescence quantum yield for the single exciton (X) may be high, but the photoluminescence quantum yield for multiexcitons (MX) may be low. Such a situation will impact the performance of LEDs at high carrier concentration. For lasing, one finds that the gain threshold and lifetime also has no clear relation to the photoluminescence quantum yield of the single exciton. For example, our work60 on CdSe showed that the lowest gain threshold were obtained with nanocrystals with low quantum yields of ~10%. Finally, the photoluminescence from X is merely the first form of light emission discussed in the nanocrystal community. Indeed, there has been realization of a number of dual emitter schemes. The value of dual emission is towards increased spectral bandwidth, and for ratiometric sensing. In short, one aims to discuss the plurality of strategies for controlling the manifold of emissive processes in nanocrystals.


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There are four essential types of interfacial control for nanocrystals. The initial is the core/shell system with an abrupt interface. This system has led to the development of radially graded alloys which result in a smoother interface between the nominally core and shell phases22. These structures are optimized for photoluminescence from the core of the nanocrystal. The photoluminescence may be from X or MX, but always from the core. In contrast, structures have been created via complex shell structures that enable dual emission23. The dual emission arises from the nominally core and shell phases. A variety of examples exist, which have been discussed elsewhere24, 25. The final example of dual emission is from the core and the surface of the nanocrystal. The reader is referred to the excellent review by Talapin regarding the chemistry of these nanocrystal17, 26, 27. Here, we describe how each of these strategies connects to specific applications involving light emission. Fig. 2 shows a schematic illustration of three strategies for surface/interfacial control of electronic structure. The original nanocrystals were passivated by organic ligands. These nanocrystals did not have a large photoluminescence quantum yield which was inferred to arise from non-radiative relaxation processes at the surface. Hence Guyot-Sionnest was the first to develop an epitaxial inorganic shell on CdSe, to form the CdSe/ZnS system18, Fig. 2a-b. Indeed, the photoluminescence quantum yield at room temperature increased with better surface passivation. In the following years, variations on such passivation schemes have been developed. Therein the objective was merely to increase the photoluminescence quantum yield for the core of the nanocrystal. The newer schemes also had the goal of minimizing photoluminescence “blinking” of the individual nanocrystal, a process which is still not well understood28-31. The most recent developments for optimizing the core for brightness involve better epitaxial growth of inorganic shells, use of inorganic or hybrid ligand passivation, giant shells, and radially


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graded alloys. In these newer schemes the idea is to also increase the photoluminescence quantum yield for the higher MX. In lieu of photoluminescence from the core of the nanocrystal, one can create more complex shell structures that support dual emission. One such example is the CdSe/ZnS/CdSe core/barrier/shell nanocrystal that was developed by Peng and co-workers23. This system supports emission from the core phase as well as the nominally shell-like phase, Fig. 2c-d. More detailed spectroscopic work showed that there are higher excitations to the blue of the band edge exciton32-35. These excitations are localized radially in the shell phase. With such localization, there is slow electron cooling which enables dual emission. There have been other strategies to create intrinsic dual emission from a single nanocrystal36-38. The third broad strategy for surface control of photoluminescence is to exploit the intrinsic photoluminescence from the surface, Fig. 2e-f. Here, we exploit the long standing observation that the surface of the nanocrystal can also emit light. This surface emission was long considered a nuisance to be eliminated. Rosenthal was perhaps the first to focus on the surface emission and to exploit it for white light generation39-41. The roadblock to this strategy had been that the surface was historically considered to be a source of random uncontrollable defects. Our group showed that the surface photoluminescence could be controlled, and the electronic structure understood in a simple electron transfer based theory16, 42-46. The main result is that the surface chemistry of the nanocrystal itself is now an aspect for materials control for light emission. The quantum dot was initially conceived for lasing applications. Indeed, the quantum “box” for electrons was conceived by the epitaxial growth community as natural extension to the semiconductor quantum well6. Since then, both the epitaxial and the colloidal quantum dot 6 ACS Paragon Plus Environment

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communities have developed various forms of nanocrystals for lasing applications. The reader is referred to other reviews on lasing, with emphasis on colloidal quantum dots2, 5, 6, 47, 48. Here, we will focus the discussion on the ways in which the nanocrystal interface can control the optical gain properties. The suitability of these nanocrystals for lasing is benchmarked in terms of different metrics: the gain threshold, bandwidth, lifetime, and cross section. Each of these metrics is controlled by quantum confinement effects as discussed in the literature. But the way in which the interface controls these specific metrics is in its infancy. These metrics will briefly be discussed on a general level, which will then set the stage for evaluating the specifics of the performance of various nanocrystal systems. The gain condition can be evaluated by the standard level systems used in laser theory, Fig 3. The two level system will not show gain as one cannot invert population. In contrast the three and four level systems support gain, with the four level system being the more efficient. Laser dyes are common examples of the four level system. The details of the electronic level structure describe the lasing threshold, and therefore the efficiency of the material. The gain bandwidth is determined by homogeneous linewidth of the transitions, and the number of transitions that may support stimulated emission. In the case of nanocrystals, the emission usually arises from the core, which has narrow linewidths (~ 30 meV) due to small homogeneous linewidths from the weak exciton-phonon couplings49. However, since nanocrystals can support emission from multiexcitons, these additional transitions may also contribute to the total gain bandwidth. A key feature of these strongly confined colloidal quantum dots is that they have large level spacings within the exciton manifold as well as large multiexciton interaction strengths so as to give rise to large gain bandwidth. The total gain cross 7 ACS Paragon Plus Environment

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section will be proportional to the oscillator strength of the lasing transition. In the case of strongly confined quantum dots, the oscillator strengths are large, and thereby advantageous. Variations such as nanoplatelets 50, 51 52 or Type II nanocrystals 53 can alter the threshold, cross section, and bandwidth, typically in a competing fashion. Hence there is no one ideal nanocrystal for gain. In the measurement of optical gain there are two commonly implemented methods. These methods connect to the gain lifetime in a way that can result in some confusion. The most direct way to measure the gain metrics is via pump/probe spectroscopy. In these measurements, an intense pump pulse excites the sample, and a variably time delayed probe pulse interrogates the excited sample. The probe pulse is typically attenuated via absorption. But in the case of population inversion at some transition, there will be gain instead of loss. In such a measurement, one can obtain the gain threshold, bandwidth, cross section, and lifetime in a direct manner. In this review we show excitonic state-resolved pump/probe spectroscopy which yields these observables. In contrast to the pump/probe measurements, one can alternatively perform amplified spontaneous emission (ASE) measurements. ASE measurements involve only a single excitation beam and no timing of pulses thereby simplifying the measurement. But this simpler measurement can obfuscate the processes and parameters of interest. The ASE measurement uses an excitation strip of variable length – hence is called the variable strip length approach, Fig. 3b. In a solid phase sample, a strip is excited from 1 – 10 mm length, for example. Spontaneous emission will then undergo amplification in the excitation volume. The total emitted light will experience exponential gain over some length of this strip.


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In order to explore how these ASE measurements are sensitive to the lifetime, we model the ASE of a nanocrystal film based upon realistic multiexciton decay parameters in Fig. 3c. The calculation assumes a pump pulse that is shorter than any of the relevant dynamics, as in the case of most VSL measurements with pumping from fs lasers. A spontaneous emission event then seeds the amplification through the strip. Assuming the excitation is infinitely long lived, the seed emission will undergo exponential amplification, characterized by a modal gain parameter. Of course the exciton and multiexciton (MX) lifetimes can be from ns – ps, hence the lifetime is important in the VSL measurements. These VSL measurements typically involve a strip of ~ 10 mm, which has a photon propagation time of ~ 30 ps. Hence the initial seed photon will experience the full gain of the pumped system whereas the concentration of excited nanocrystals will decay in time as the pulse propagates through the strip. We model the ASE in the strip with a pulse propagating down the strip, with decay of the multiexcitons over time. The seed event is averaged over the length of the strip. The parameters in the calculation are the lifetimes of the MX. We assume an initial population of N = 3 excitons per nanocrystal. We take MX recombination rates from the literature for three different nanocrystal samples: CdSe, CdSe/ZnS with an abrupt interface, and CdSe/CdZnS with a soft interface. The simulations reveal that these commonly used VSL measurements of ASE are connected to the total gain due to the lifetime of the MX. A longer MX lifetime will indeed yield more amplification over a 10 mm stip. However, provided the ASE took place in a 1 mm strip there would be no dependence upon lifetime. Indeed, one has dye lasers with mm long gain media and gas lasers with meter long gain media. Hence the use of these VSL measurements with strip propagation times that are similar to the MX lifetimes will cause misinterpretation of the results. A longer (Auger) lifetime does not make the gain more efficient. A longer lifetime


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merely enables a longer cavity to be used, which can enable more total gain. But the gain efficiency of a nanocrystal is not related to the lifetime of the multiexcitons. The efficiency is based upon how the real nanocrystal connects to the standard level systems in laser theory. Having established some of the generalities of optical gain, we focus on the specifics in the context of semiconductor nanocrystals. The main metrics upon which to focus are the gain threshold, lifetime, and bandwidth. The gain threshold gives the efficiency of the system for lasing. Ideally the threshold should be as small as possible on a per-particle basis, like the classic four level system of a laser dye. The semiconductor nanocrystal has a well known electronic structure of the single exciton (X) 54-57. The key spectroscopic observation was a Stokes shift between the energies of the lowest absorbing and emitting bands. The Stokes shift has values of 30 - 50 meV. This energy shift arises from electronic fine structure, although the details are still being discussed. This level structure is represented by the three level system in laser theory, in the absence of any other effects. The problem arises by unwanted excited state absorption which may be resonant with the desired stimulated emission. In nanocrystal this excited state absorption takes place from absorption into the biexciton (XX) 5, 48, 57. The XX binding energy is 5 - 30 meV, similar to the Stokes shift in X. We showed that there is a XX SS as well 58. This electronic structure of X and XX governs the gain threshold as illustrated in Fig. 4 48, 59. This minimal model for the electronic structure of X and XX rationalizes the experimental observation of the way in which the gain threshold depends upon various surface passivation schemes, Fig. 4. The gain threshold is shown for CdSe, CdSe/ZnS, and CdSe/CdZnS nanocrystal

59-61

. The CdSe nanocrystal is passivated with organic ligands and has

photoluminescence quantum yield ~10%. The CdSe/ZnS is a standard core/shell system with an 10 ACS Paragon Plus Environment

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abrupt interface. The photoluminescence quantum yield is ~50%. The CdSe/CdZnS nanocrystal is designed to have a radially graded interfacial composition which should yield a less abrupt potential interface. The photoluminescence quantum yield is 60%. The photoluminescence quantum yield, however, has no relation to the gain threshold. The data shows that both CdSe and CdSe/ZnS have the same gain threshold of N = 1.5. Note that this gain threshold requires the use of state-resolved optical pumping in order to circumvent hot exciton surface trapping. In contrast the CdSe/CdZnS nanocrystal has a threshold of N = 1, first shown by Nurmikko using different methods

62

. The thresholds arise between the interplay of Stokes shift for the single

exciton, and binding energy for absorption into the biexciton. In this specific case, it was reduction of the biexciton binding energy that largely gave rise to the differences shown. The general discussion of how the gain lifetime appears in the simpler measurements of ASE by the VSL method. One of the aims of interfacial control of the nanocrystal is to increase the lifetimes and photoluminescence quantum yield for the MX as well. It is some times discussed that the longer lifetime yields more efficient gain, or stronger ASE. This point was clarified in the general discussion based upon rate equations. We measured the SE from the three nanocrystal systems, Fig. 5. The SE lifetimes are 3 ps for CdSe, 50 ps for CdSe/ZnS, and 200 ps for CdSe/CdZnS. The lifetime for CdSe is controlled by surface trapping which increases the biexciton Auger recombination rate. And the slower lifetimes for the other nanocrystal shows how the Auger rates change when going from an abrupt to a graded interface. In the case of the graded interface nanocrystal, one sees SE from higher MX, a point discussed below. The higher the MX multiplicity, the shorter the lifetimes. Fig. 6 explores the gain from higher MX, and how it gives rise to gain bandwidth control 60, 61, 63, 64

. nanocrystal support formation of MX based upon the multiplicity of the excitonic 11 ACS Paragon Plus Environment

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levels. The 1S type exciton supports biexciton formation. Whereas the 1P type exciton supports a six-fold MX. Using excitonic state-resolved optical pumping in three different nanocrystal interfacial systems, we show how the stimulated emission spectral bandwidth can be controlled. The strongest effects are shown in the data from CdSe/CdZnS shown in Fig 6b. The same effects were first shown in CdSe and then in CdSe/ZnS/CdSe. Fig. 6a shows how MX formation enables SE from different bands48, 65. The SE takes place from a MX of multiplicity N to one of (N-1). Since XX is redshifted from X by the binding energy, a new transition arises. This process keeps building to produce new transitions to the red of the photoluminescence from X, up to the multiplicity of the pumped state. In the case of the graded interface nanocrystal, we were able to observe SE to the blue of the photoluminescence from x, due to excited configurations. In the case of XXX X, there are two pathways. The red edge SE pathway is 1S21P1 1S11P1, while the blue edge pathway is 1S21P1 1S2. One of the first light emission applications for nanocrystal was for biological imaging 66. In this specific application, the colloidal form of the QD was particularly useful by virtue of solution phase processing. Beyond this initial application, a large amount of interest lies in using nanocrystal for lighting and displays. There are many excellent reviews to which the reader is referred, on the topic of nanocrystal for lighting and displays1, 3, 4. Here, we focus specifically on aspects of the interface which connect to their suitability for LEDs and related devices. The main characteristics for nanocrystal that give them such suitability is their narrow emission linewidth, high photoluminescence quantum yield, and high photostability. In the case of colloidal nanocrystals, they are easily processed which can offer some benefits. These nanocrystal can also be tuned across the visible spectrum by either particle size of composition. The emission spectra can be recast into Commission Internationale de l’Eclairage (CIE) 12 ACS Paragon Plus Environment

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diagrams used in lighting analysis. Intense, saturated colors are obtained at the extrema by narrow spectra. The narrow linewidths (30 meV) of nanocrystal enable them to form viable active elements in LEDs. The recent years have seen much activity in making and optimizing new forms of nanocrystal LED. The practice of developing nanocrystal LED involves creation of narrow colors, efficient electrical pumping, and a brightness that does not droop with increasing current. We assess how these and related phenomena connect to the interface of the nanocrystal. Prior works have discussed how the photoluminescence quantum yield is optimized, as well as the fabrication of efficient devices. The more recent point of examination is how the brightness scales with drive current. Assuming the photoluminescence quantum yield were constant, increasing current should increase the number of excitons which should be linearly proportional to the emitted light. But this observation is not the case. The empirical observation of QD/NC LED is that they suffer from an efficiency droop 6769

. At higher drive currents, the LED efficiency rolls off. This droop has been proposed to arise

from Auger based non-radiative recombination of multiexcitons. Indeed, it is well known that higher MX in nanocrystal have faster non-radiative recombination rates due to Auger processes. In standard CdSe nanocrystal these decay times are on the 10 – 100 ps timescale based upon size of the nanocrystal, and multiplicity of the MX. Notably these MX decay rates do not connect to the linear properties such as photoluminescence quantum yield of the single exciton. As a result, even nanocrystal with high quantum yield for X will still experience efficiency droop. Fig 7 shows the efficiency droop of a model nanocrystal system based purely upon Auger recombination for the same three nanocrystals as in the VSL simulation. Higher excitation levels will create larger exciton concentrations. These higher MX have faster non-radiative decay rates 13 ACS Paragon Plus Environment

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via Auger recombination. Hence the higher MX have lower emission quantum yield. Fig 7a shows the predicted efficiency droop based upon MX recombination rates, and Fig 7b shows the relative emission from each MX within the ensemble. As a result of this increase in the MX recombination rates of nanocrystal, there have been strategies to slow the Auger recombination rate which is the primary process that limits device performance in the high carrier concentration regime. As noted in the section on lasing, the MX lifetimes can be increased by strategies such as radially graded alloys. The very same measurements of MX recombination rates and stimulated emission lifetimes also demonstrate how the nanocrystal will respond under high excitation conditions in an LED environment. Focusing on the spectral quality of the nanocrystal, the primary objective had historically been to generate saturated colors by exploiting the narrow photoluminescence linewidths of the excitons in nanocrystal3, 8. To that end, there has been effort to generate nanocrystal with narrower linewidths by virtue of either reductions of the polydispersity or by changes to the electronic structure of the exciton. Conversely, one might aim to exploit other aspects of quantum size effects to generate broad bandwidth emitters. The spectral response of these emitters is characterized using CIE diagrams. Fig. 8 shows such diagrams for a variety of nanocrystal systems. Shown is a standard CdSe nanocrystal which produces a saturated red spectrum for larger nanocrystal, Fig. 8a. This saturated response is standard for nanocrystal of varying size and composition. In contrast to photoluminescence from the nanocrystal core which produces these saturated colors, one can create multicomponent structures which support dual emission. The core/barrier/shell nanocrystal system of CdSe/ZnS/CdSe is one such variation, initially


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developed by Peng. This system has blue photoluminescence from higher lying shell phases 34. There have been variations on this dual emission scheme by several groups

32, 33, 70 35, 71

. The

main idea is slow electronic relaxation from higher lying states. Provided this cooling is slow enough to compete with radiative recombination (on the ns timescale) one will see dual emission. Such schemes can be used to control the spectral bandwidth and the CIE coordinates for white light generation. In contrast to dual emission from hybrid nanostructures, the intrinsic QD nanocrystal supports dual emission from the surface as well as the ubiquitous core photoluminescence. This emission from the surface is a topic we have discussed in detail elsewhere

43-46, 72

. The main

observation is a broad and redshifted photoluminescence band from the surface. Our group has proposed a theoretical model to rationalize the spectroscopic observables in light of the semiclassical electron transfer theory of Marcus and Jortner. Fig 8 shows how the intrinsic surface photoluminescence produces white light of excellent spectral character. Fig8a shows that this strategy can produce near perfect white light. One of the historic problems with exploiting the surface photoluminescence for lighting was that the surface was viewed as an uncontrollable source of defects. Our work showed this picture to be false15, 16, 43. Indeed, the surface can be chemically controlled so as to design the emission spectra as shown in Fig8b. Specific ligands can control the surface photoluminescence both in terms of spectrum and spectral amplitude relative to the core. Use of three different ligands on the same size of CdSe nanocrystal illustrates how the CIE coordinates can be tuned by the chemical composition of the ligands. Finally, the CIE coordinates can also be tuned by temperature. We have shown how temperature controls the relative fractions of core and surface


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photoluminescence. Hence the total photoluminescence will have a CIE coordinate that depends upon temperature as shown in Fig.8c. The main point of these studies on white light generation is that strategies are available for creating broadband emission from a single nanocrystal. In particular, the intrinsic surface emission from nanocrystal enables creation of white light with good spectral characteristics as revealed by their CIE coordinates. This strategy may be promising for generating white LEDs using surface emitting nanocrystal. Indeed, there have been some early studies to this effect. Indeed, these white LED using a single nanocrystal are not as efficient as the most advanced of modern nanocrystal LED systems. But here the aim is not efficiency yet. The aim is merely to show that the strategy works and can reliably and controllably generate white light. With further work, one aims to generalize this approach and to optimize it for efficiency and robustness. Many of the primary applications of nanocrystal involve light absorption, such as photovoltaics and photodetectors. Following absorption, applications involving light emission are a subset, including the main line topics of lasing, lighting, and displays. Here we briefly discuss two additional areas in which light emission from nanocrystal can enable new strategies for longstanding areas if technological interest. The prior works on optical gain and lasing from nanocrystal has shown that the process has both advantages and disadvantages from the quantum confinement effects in these systems. The main advantages have been discussed above. Whilst the main disadvantage is the fast MX recombination rates due to Auger processes. Indeed, the nanocrystal community has been working on strategies to increase MX lifetimes as discussed here and elsewhere 22, 27, 59, 63, 73-76. The surface/interface of the nanocrystal is now understood to be an important factor in dictating the MX recombination rates. 16 ACS Paragon Plus Environment

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Rather than try to solve the problem of slowing the MX recombination rates, our group exploited the intrinsic speed of the system in order to enable wholly new applications for ultrafast all-optical switching and logic

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. Fig. 9 summarizes this work. Fig9a illustrates the

concept of switching and logic gates using MX in nanocrystal. Briefly, one pulse is used to initialize the nanocrystal for stimulated emission. This initialization pulse is resonant with the 1S band edge exciton. It generates a biexciton (XX). A weak modulator pulse is resonant with the 1P exciton and then modulates the nanocrystal multiplicity from N = 2 to N = 3. The signal from the “on” state is from the N = 3 state of the MX (triexciton). Due to the gain bandwidth control from MX, the stimulated emission from the XXX is at a different wavelength than the emission from the XX state. Hence the signal from the XXX state is read out based upon timing of the pulses, Fig. 9b. The XXX state has its stimulated emission further to the red of the SE from XX. This is the signal which is read-out as function of the optical parameters such as pulse energy, pumped state, pulse timings. The pulse timings are measured in-situ by simultaneously monitoring the band edge bleach signals. The presence of the two input pulses, one of which is modulated enables creation of an optical switch, Fig. 9c. The switching signal is the stimulated emission from the higher MX. From this signal one can measure a modulation depth and bandwidth. This work reveals that MX in nanocrystal can be a strategy for creating ultrafast all-optical switches with fast switching speeds. Since the switch takes place on the level of single photons / excitons / nanocrystals, there may be promise in this platform for single particle types of measurements. Finally, the specific pulse sequences and signals here constitute a simple optical AND gate. One can imagine constructing more complex logic gates using this approach.


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One of the early applications for nanocrystal was in sensing. Based upon emission energies and intensities one can perform optical thermometry or chemical sensing. Essentially temperature or the local chemical environment can modulate the emission properties of the nanocrystal. Using such approaches, nanocrystal have seen much use in sensing. For sensing, one aims for a ratiometric measurement in order to increase precision and accuracy. Hence a single nanocrystal that functions as a dual emitter enables these criteria to be met. Fig. 10 shows a ratiometric temperature sensor using the intrinsic dual emission from the core and the surface of the nanocrystal

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. Our prior works on surface photoluminescence in

nanocrystal has shown that the integrated areas of the surface photoluminescence relative to the core photoluminescence has a rich temperature dependence 43-45. In most nanocrystal, this ratio is not a monotonic function of temperatures. In order to make a ratiometric temperature sensor using dual emission from nanocrystal, one needs to design a nanocrystal system in which the photoluminescence ratios change in a monotonic matter with respect to temperature. Fig10a illustrates one such nanocrystal/ligand system which was optimized for this application. This specific system was chosen to maximize surface photoluminescence over the desired temperature range, and to make the surface/total photoluminescence ratio a monotonic function of temperature. Indeed, the sensor is able to optically read out temperature with good precision and accuracy. Much like the use of surface emission in LED, the use of surface emission for sensing has a main problem in whether the surface photoluminescence is suitable for device applications. Is the surface photoluminescence reproducible? Is it stable? These questions are addressed in Fig10. These results show that the surface emission can produce reproducible films with small


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hysteresis and lifetime of at least a month. We hope that this proof of concept will be expanded into new strategies for optical sensing using nanocrystal. In summary, semiconductor nanocrystals are excellent materials for applications involving light emission. By virtue of their small size, the surface or interface is an important aspect of their structure and function. While the excitonics of the core of nanocrystals is now well understood, the behavior of the surface and interface is in its early stages. We discuss several applications of light emission from nanocrystals, and how the performance of the nanocrystals connects to the surface or interface. The specific nanocrystal interfaces we focus upon are: CdSe/ligands, CdSe/ZnS with a hard interface, CdSe/CdZnS with a soft interface, and CdSe/ZnS/CdSe. The interface is shown to be a controlling factor in optical gain threshold, lifetime, and bandwidth. The same factors which govern optical gain lifetimes also control efficiency droop in LEDs. The interface moreover enables a rational strategy for white LEDs. While such systems have already seen use in white LED, the work here illustrates how the surface chemistry yields a specific interfacial electronic structure that gives rise to controllable white light. Finally, we show that this interfacial control can be used to create fast optical switches and as well perform optical thermometry. We propose that the interface of nanocrystals will be a central area of developing and optimizing these materials for optoelectronic applications. The present work focuses upon the relevant spectroscopic features which arise from the electronic structure of the interface. Other works have focused upon the chemical structure of atomistic interactions at the interface. One aims to connect the two so as to better enable interfacial control of nanocrystals for an array of applications involving light emission.


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Acknowledgements. The financial support of CFI and NSERC is gratefully acknowledged.


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Fig 1. Transmission electron microscope image of CdSe nanocrystal, a). Excitons in nanocrystals are an enabling platform for a wide variety of optoelectronic applications, b). The surface of nanocrystal can be rationally designed to create white light from a single emitter, c). Nanocrystals support intrinsic dual emission from the core and the surface, d). The yields of surface and core emission depend upon temperature, which yields a CIE coordinate that may be temperature controlled, inset.


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Fig 2. Demonstration of three approaches to controlling the interface of nanocrystals for light emission. Shown are representative emission spectra nanocrystals of standard core/shell form, a), core/surface form, c), and core/barrier/shell form, e). The insets show schematics of the geometry and wavefunctions. Each of these types of interfaces results in electronic structure which gives rise to the observed spectra. The salient aspects of the electronic structure are schematically illustrated for the core/shell, core/surface, and core/barrier/shell systems, b), d), f). 22 ACS Paragon Plus Environment

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Fig 3. Optical gain studies may employ measurements of either stimulated emission or amplified spontaneous emission. The former is specific, whereas the latter depends upon a number of parameters that may obfuscate the measurement. Schematic illustration of the standard level structures for optical gain, a). The process of stimulated emission may be measured via 24 ACS Paragon Plus Environment

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pump/probe spectroscopy. Schematic illustration of the process of amplified spontaneous emission (ASE) with a pulsed source and a variable stripe length (VSL) measurement, b). A model calculation shows how differences in the multiexciton recombination rates gives rise to the appearance of changes to the threshold and modal gain/cross section, c). The legend refers to the biexciton lifetime.


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Fig 4. Schematic illustration of the minimal level structure for the band edge biexciton, a). The band edge single exciton (X), has an absorbing and an emitting state. The energy difference between these states is the Stokes shift (δX). The band edge biexciton (XX) also has absorbing and emitting states, separated by a biexciton Stokes shift (δXX). The biexciton binding energy may be measured in either absorptive or emissive experiments, yielding different energies. The interplay of these levels gives rise to the gain threshold. Pump/probe measurements of gain thresholds in different CdSe based nanocrystal, b).


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Fig 5. Stimulated emission dynamics. Shown are the time resolved stimulated emission spectra for CdSe nanocrystal of three different interfacial conditions, CdSe/ZnS, a), CdSe, b), and CdSe/CdZnS, c). These three spectra were obtained with pumping directly into the band edge (1S) exciton, whereas pumping into the higher (1P) exciton in CdSe/CdZnS is shown in d). The interfacial structure controls the stimulated emission lifetimes from the band edge, and enable emission from higher MX in the blue.


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Fig 6. The electronic structure of the interface extends the optical gain bandwidth of nanocrystal. Stimulated emission spectra are shown as a function of exciton occupancy, , for CdSe/CdZnS nanocrystal, a). The upper panel shows 1S pumping and the lower panel shows 1P pumping. Strong multiexciton interactions gives rise to gain bandwidth control. The multiexcitonic recombination pathways are schematically illustrated in, b).


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Fig 7. Auger recombination of multiexcitons gives rise to efficiency droop in LEDs. Simulation of Auger recombination on light emission for three interfacial conditions, a). The total emitted light is decomposed into the contributions from excitons of different multiplicity, b).


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Fig 8. Demonstration of how interfacial structure connects to lighting and display applications. Shown is a CIE diagram with coordinates for the three types of structures, a). Standard nanocrystals emit saturated colors at the extrema of the diagram. White light can be generated via dual emission from either the surface state or from a higher lying shell state. For surface emitting nanocrystals, the CIE coordinate is highly sensitive to choice in ligand, b). For surface emitting nanocrystals, the CIE coordinate is highly sensitive to temperature, c).


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Fig 9. Application of fast Auger processes in nanocrystal to make a fast optical switch. Modulating the MX population enables stimulated emission to be switched. Schematic illustration of the use of multiexcitons to perform all-optical switching, a). Measurement of switching of stimulated emission in a CdSe nanocrystal, b). The upper panel shows the readout of the stimulated emission (gain) signal. The lower panel shows the in-situ measurement of timings. Switching experiments on different size nanocrystal with different MX readout yields a switching response curve, c). . 31 ACS Paragon Plus Environment

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Fig 10. The intrinsic dual emission from semiconductor nanocrystals can be used for optical thermometry. Temperature dependent emission spectra of CdSe nanocrystal, a). The integrated areas of surface/core emission changes monotonically with temperature for this nanocrystal/ligand system, b). Two different nanocrystal films maintain a similar temperature profile, c). The nanocrystal film shows no measurable hysteresis, d).


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ACS Photonics

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Understanding and Exploiting the Interface of Semiconductor

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