Influence of Polytypism on the Electronic Structure of CdSe/CdS and CdSe/CdSe Core/Shell Nanocrystals

We address theoretically differences and similarities on the electronic structure of CdSe/CdS dot-in-dot nanocrystals (NCs) with wurtzite/wurtzite (WZ/WZ), zinc-blende/zinc-blende (ZB/ZB) and polytype ZB/WZ crystalline phases, as they are currently being synthesized and used in optoelectronic devices. We show that the electronic structure of polytypic CdSe/CdS NCs closely resembles that of WZ or ZB NCs with regard to quantum confinement and strain, resulting in similar single-exciton wave functions. The main differences arise in the nature and magnitude of built-in electric fields. We predict that these fields are stronger in polytypes than in pure WZ or ZB NCs due to the sharp spontaneous polarization mismatch between the cubic core and the hexagonal shell lattices. Polarization in NCs is currently believed to be screened by several surface effects. In polytypical structures, however, the polarization mismatch at the interface may create effective charges that are sufficiently far from the outer surface to be quenched. To make a definitive assessment on this controversial issue, we propose experiments in polytypic ZB/WZ NCs where both core and shell are made of CdSe. In such a case, band offsets are small, strain is absent, and our calculations predict pyroelectricity should become the driving force, inducing transitions from type-I to type-II excitons with increasing core or shell size.We thank I. Moreels and P. Guyot-Sionnest for useful discussions. Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 is acknowledged

Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots.

Conventional colloidal quantum dots (QDs) suffer from rapid energy losses by nonradiative (Auger) processes, leading to sub-ns lifetimes in all excited states but the lowest-energy single exciton. Suppression of interband Auger decay, such as biexciton Auger recombination, has been achieved with the design of heterostructured core-shell QDs. Auger-like processes are also believed to be responsible for rapid intraband hot-electron cooling in QDs. However, the simultaneous effect of shell growth on interband Auger recombination and intraband hot-electron cooling has not been addressed. Here we investigate how the growth of a CdS shell affects these two relaxation processes in CdSe/CdS core-shell QDs. Using a combination of ultrafast pump-push-probe spectroscopy on the QD ensemble and analysis of the photon statistics from single QDs, we find that Auger losses in the biexciton state are suppressed with increasing shell thickness, while hot-electron cooling remains unaffected. Calculations conducted within an eight-band k·p model confirm the experimental dependence of the biexciton Auger decay on the shell thickness, and provide insights into the factors determining the cooling rate of hot carriers.This work is part of the research program of the ”Stichting voor Fundamenteel Onderzoek der Materie (FOM)”, which is financially supported by the ”Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)”. A.L.E. acknowledges the financial support of the Office of Naval Research (ONR) through the Naval Research Laboratory Basic Research Program.This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acsnano.5b0449

Photocatalytic Activity Of Core/shell Semiconductor Nanocrystals Featuring Spatial Separation Of Charges

The present study investigates the photocatalytic activity of ZnSe/CdS core/shell semiconductor nanocrystals. These nanoparticles exhibit a spatial separation of photoinduced charges between the core and the shell domains, which makes them potentially viable for photocatalytic applications. Unfortunately, one of the excited charges remains inside the core semiconductor and thus cannot efficiently react with the external environment. Here, we explore this issue by investigating the mechanisms of hole extraction from the ZnSe core to the surface of the CdS shell. In particular, the effect of shell thickness in ZnSe/CdS core/shell nanocrystals on the ability of core-localized charges to perform oxidative reactions was determined. By using a combination of time-resolved spectroscopy and electrochemical techniques, we demonstrate that the use of hole-scavenging surfactants facilitates an efficient transfer of core-localized holes to the surface even in the case of shells exceeding 7 nm in thickness. These measurements further demonstrate that photoinduced holes can be extracted from the core faster than they recombine with shell-localized electrons, indicating that most of the absorbed energy in ZnSe/CdS nanocrystals can be used to drive catalytic reactions

Giant-Shell CdSe/CdS Nanocrystals: Exciton Coupling to Shell Phonons Investigated by Resonant Raman Spectroscopy

The interaction between excitons and phonons in semiconductor nanocrystals plays a crucial role in the exciton energy spectrum and dynamics, and thus in their optical properties. We investigate the exciton2 phonon coupling in giant-shell CdSe/CdS core-shell nanocrystals via resonant Raman spectroscopy. The Huang-Rhys parameter is evaluated by the intensity ratio of the longitudinal-optical (LO) phonon of CdS with its first multiscattering (2LO) replica. We used four different excitation wavelengths in the range from the onset of the CdS shell absorption to well above the CdS shell band edge to get insight into resonance effects of the CdS LO phonon with high energy excitonic transitions. The isotropic spherical giant-shell nanocrystals show consistently stronger exciton-phonon coupling as compared to the anisotropic rod-shaped dot-in-rod (DiR) architecture, and the 2LO/LO intensity ratio decreases for excitation wavelengths approaching the CdS band edge. The strong exciton-phonon coupling in the spherical giant-shell nanocrystals can be related to the delocalization of the electronic wave functions. Furthermore, we observe the radial breathing modes of the GS nanocrystals and their overtones by ultralow frequency Raman spectroscopy with nonresonant excitation, using laser energies well below the band gap of the heteronanocrystals, and highlight the differences between higher orderComment: 16 pages, 3 figure

Optical and X-ray Photo-emission Spectroscopies of Core/Shell Colloidal CdSe/CdS Quantum Dots: Modeling and Experimental Determination of Band Alignment

Optical properties of multilayer semi-conductor nano-emitters are crucially dependent on the relative energy levelsof their different components. For core/shell quantum dots, the relative energy difference between conduction bandedge of core and shell materials induces, depending on its value, either a confinement of the electron within the coreor a delocalization of its wave function within the whole quantum dot. This results in drastic consequences on theenergy and the oscillator strength of the fundamental transition. Surprisingly, the literature currently lacks a definitivevalue for the energy difference between CdSe and CdS conduction band edges as most of the experimental studiesprovide values corresponding to specific geometries of quantum dots. Here, we develop a full theoretical modelexpressing energy levels considering core/shell interface pressure, ligands and enabling the accurate prediction ofthe bandgap value with the nanocrystal size. It allows to reliably determine the energy difference between theconduction band edge of CdSe and CdS materials, known as the conduction band offset, in such a way that this valuecan later be used to model quantum dots of any geometry. This value is determined using our model and two differentexperimental methods: optical spectroscopy and X-ray photoemission (XPS) experiments

Nano-engineered electron–hole exchange interaction controls exciton dynamics in core–shell semiconductor nanocrystals

A strong electron–hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active ('bright') and optically passive ('dark') excitons. This dark–bright splitting has a significant effect on the optical properties of band-edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark–bright splitting can be widely tuned by controlling the electron–hole spatial overlap in core–shell CdSe/CdS NCs with a variable shell width. In thick-shell samples, the EI energy reduces to <250 μeV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI-manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron–hole overlap

친환경 합금 코어쉘 이종접합구조 양자점의 합성과 특성분석

학위논문(박사) -- 서울대학교대학원 : 공과대학 화학생물공학부, 2023. 8. 차국헌.콜로이드 양자점은 좁은 발광선폭, 높은 발광효율, 크기에 따라 조절되는 밴드갭과 같은 독특하고 우수한 광특성으로 광변환층, 전계발광소자, 레이저 등의 다양한 응용분야에서 각광받고 있다. 하지만 이러한 우수한 성과들은 대부분 CdSe 기반 양자점 기반 소재로 달성되었는데, 카드뮴계 양자점의 독성으로 인하여 산업계에 적용되기 불가능하다. 이에 대한 해답으로 InP 기반 양자점 소재가 개발되었지만 청색영역에서의 낮은 흡광특성, 넓은 발광반치폭, 청색 영역 발광 양자점의 합성 불가능 등의 이유로 새로운 친환경 양자점 소재의 필요성이 대두되었다. 본 학위논문에서는 전체 가시광 영역에서 높은 양자 발광효율을 보이는 양자우물구조 ZnSe/ZnSe1-XTeX/ZnSe 양자점과 코어쉘 AgInXGa1-XS2/AgGaS2 양자점의 합성법과 특성분석에 대해 논의한다. 심층적인 구조, 광, 광물리적 특성의 분석과 양자계산을 통해 합성 메커니즘과 발광 특성을 밝혀내었고 이의 다양한 응용을 소개한다. 제1 장에서는 기본적인 양자점의 특성과 친환경 양자점의 역사에 대해 간략히 소개하였다. 기존 양자점의 이종접합구조와 밴드갭 조절을 위한 합금 코어, 오제 재결합 특성을 요약하고 친환경 양자점을 소개하며 현 시점에서 친환경 양자점의 단점에 대해 논의하였다. 제2 장에서는 양자우물구조로 인한 격자비일치 차이를 완화하여 100 %의 발광효율을 가지는 ZnSe/ZnSe1-XTeX/ZnSe 양자점을 합성하는 방법을 소개한다. 뿐만 아니라 quasi-type Ⅱ의 밴드구조를 가지는 ZnSe/ZnSe1-XTeX/ZnSe 양자점은 쉘 두께 조절을 통하여서도 밴드갭을 조절할 수 있어 더욱 섬세한 조절이 가능하다. 이러한 특성으로 인해 청색부터 적색까지 넓은 발광파장대에서 높은 ㅂ라광효율을 구현할 수 있는 양자우물구조 양자점의 특성을 이용하여 2색성 (청색+주황색)의 백색 발광 전계발광 소자를 제작하여 해당 소재의 우수성을 증명하였다. 제3, 4 장에서는 2 장에서 제시한 ZnSe/ZnSe1-XTeX/ZnSe 청색 양자점에서 관측되는 비대칭적인 양자점의 발광스펙트럼과 녹색 양자점의 다중전하엑시톤(multicarrier)의 발광특성에 대해 분석하였다. 군집, 개별 양자점의 광 및 광물리학적 특성분석과 양자계산을 통하여 청색 발광 ZnSe/ZnSe1-XTeX/ZnSe 양자점의 비대칭 발광 스펙트럼이 양자점 발광층 내 개별 Te과 Te2 간의 에너지 준위 차이로 인해 발생하는 것을 밝혀내었다. 또한 새로운 광-전계측정법을 개발하여 기존 방법으로는 확인할 수 없었던 녹색 발광 ZnSe/ZnSe1-XTeX/ZnSe 양자점의 포지티브, 네거티브 트라이온 발광 거동을 측정하였다. 제5 장에서는 신규 친환경 양자점 조성으로 Ⅰ-Ⅲ-Ⅵ2/Ⅰ-Ⅲ-Ⅵ2 코어/쉘 양자점인 AgInXGa1-XS2/AgGaS2을 최초로 합성하였다. 본 연구진은 Ag-S-Ga(OA)2 중간체의 도입을 통하여 Ag와 Ga간 반응성 차이를 완화하여 균일한 AIGS 코어와 AGS 쉘 성장을 달성하였다. 이렇게 합성된 AIGS/AGS 양자점은 가시광 영역에서 높은 흡광계수, 발광효율, 좁은 발광선폭을 가져 InP 기반 양자점을 대체할 수 있을 것으로 기대된다. 실제 태양전지 광변환층으로 이용하였을 때 InP/ZnSeS 양자점에 비해 절반의 농도로 더 높은 광변환효율을 달성하였다. 본 학위논문으로 개발된 신규 친환경 양자점은 우수한 발광특성을 지녀 디스플레이용 양자점 소재로 적합하다. 추후 이러한 소재는 광변환층, 전계발광소자, 레이저 등 다양한 분야로 적용되어 양자점 산업 및 연구에 크게 기여할 것으로 예상된다.Colloidal semiconductor nanocrystals (NCs) have garnered interests for their remarkable optical properties such as broad absorption but narrow emission linewidth, tunable band gap and near-unity photoluminescence quantum yield (PL QY). These properties have facilitated the successful practical use of NCs in the luminescent solar concentrator (LSC), light-emitting-diodes (LEDs) and lasers. However, the extensive use of CdSe-based NCs, which have been instrumental in these advancements, is hampered by stringent environmental regulations. To address these regulatory concerns, both academia and industrial fields have developed environmentally benign NCs, notably InP-based NCs. Nevertheless, InP-based NCs suffer from significant drawbacks that hinder their use in the industrial fields. such as low absorption coefficient at blue spectral regime, broad emission linewidth, and the challenge of realizing blue-emitting InP-based NCs. Herein, we reported synthesis and characterization of spherical quantum well ZnSe/ZnSe1-XTeX/ZnSe NCs and core/shell AgInXGa1-XS2/AgGaS2 NCs, which represent new class of environmentally benign NCs bearing near-unity PL QYs across full visible range (450 nm to 630 nm). Through structural, optical, photophysical analyses with computational calculations, we unveil synthetic processes and emission mechanism of these NCs. Our findings expand the Cd-free material envelope to the ZnSe1-XTeX and AgInXGa1-XS2 NCs, thereby offering promising prospects for a wide range of light emitting applications. In Chapter 1, fundamental properties of NCs and history of environmentally benign NCs are introduced briefly. We also highlight the necessity of developing alternatives of InP-based NCs and the present challenges faced in developing environmentally benign NCs. In Chapter 2, we demonstrate a new class of quasi-type II nano-emitters formulated in ZnSe/ZnSe1-XTeX/ZnSe seed/spherical quantum well/shell heterostructures (SQW) whose emission wavelength ranges from blue to orange. In a given geometry, ZnSe1-XTeX emissive layers grown between the ZnSe seed and shell layer are strained to fit into the surrounding media, and thus the lattice mismatch between ZnSe1-XTeX and ZnSe is effectively alleviated. In addition, composition of ZnSe1-XTeX emissive layer and the dimension of ZnSe shell layer are engineered to tailor the distribution and energy of electron and hole wavefunctions. Benefited from the capabilities to tune the charge carriers on demand and to form defect-free heterojunctions, ZnSe/ZnSe1-XTeX/ZnSe/ZnS NCs show near-unity photoluminescence quantum yield (PL QY > 90 %) in a broad range of emission wavelength (peak PL from 450 nm to 600 nm). Finally, we exemplify dichromatic white NC-LEDs employing the mixed layer of blue- and yellow- emitting ZnSe/ZnSe1-XTeX/ZnSe/ZnS SQW NCs. In Chapter 3, we explore the excitonic states in ZnSe1-XTeX NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis on a series of ZnSe1-XTeX NC samples with varying Te ratios coupled with computational calculations show that, due to the distinct electronegativity between Se versus Te, nearest-neighbor Te pairs in ZnSe1-XTeX alloys create localized hole states spectrally distributed approximately 130 meV above 1Sh level of homogenous ZnSe1-XTeX NCs. This forms spatially separated excitons (delocalized electron + localized hole in trap), accounting for both inhomogeneous and homogeneous linewidth broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implication of photonic applications. In Chapter 4, Multicarrier dynamics of ZnSe1-XTeX NCs are undetermined yet due to their strange behavior that positive trion is prevailing, not negative trion. Thus, we demonstrate an opto-electrical method that enables us to precisely assess AR rates of X+ and X- in core/shell heterostructured NCs. Specifically, we devise electron-only-devices and hole-only-devices to inject extra charge carriers into NCs without unwanted side reactions or degradation of NCs, and probe AR characteristics of these charged NCs via time-resolved photoluminescence measurements. We find that AR rates of charged excitons, both X+ and X-, gained from the present method agree well with those attained from conventional approaches and the superposition principle, corroborating the validity of the present approach. This present method permits to comprehend multicarrier dynamics in NCs, prompting the use of NCs in light-emitting diodes and laser devices based on NCs. In Chapter 5, we present the heteroepitaxy for AIGS-AgGaS2 (AIGS-AGS) core-shell NCs bearing near-unity PL QYs in almost full visible range (460 nm to 620 nm) and enhanced photochemical stability. Key to the successful growth of AIGS-AGS NCs is the use of the Ag-S-Ga(OA)2 complex, which complements the reactivities among cations for both homogeneous AIGS cores in various compositions and uniform AGS shell growth. The heteroepitaxy between AIGS and AGS results in the Type I heterojunction that effectively confines charge carriers within the emissive core without optically active interfacial defects. AIGS-AGS NCs show remarkably higher extinction coefficient and narrower spectral linewidth compared to state-of-the-art heavy metal-free NCs, prompting their immediate use in practicable applications including displays and luminescent solar concentrators.Chapter 1. Introduction 1 1.1 Colloidal semiconductor nanocrystals 1 1.1.1 Fundamental properties of nanocrystals 1 1.1.2 Core/shell heterostructure 3 1.1.3 Alloyed core 6 1.1.4 Auger recombination 8 1.2 Environmentally benign quantum dos 12 Chapter 2. ZnSe/ZnSe1-XTeX/ZnSe spherical quantum well nanocrystals 15 2.1 Introduction 15 2.2 Experimental Section 18 2.3 Results and Discussion 21 2.3.1 Structural characteristics and optical properties 25 2.3.2 Lattice strain relaxation in spherical quantum well structures 27 2.3.3 Electronic features and photophysical properties 31 2.3.4 Device characteristics of dichromatic white NC-LEDs employing blue- and yellow-emitting nanocrystals 36 2.4 Summary 38 Chapter 3. Impact of morphological inhomogeneity on excitonic states in highly mismatched alloy ZnSe1-XTeX nanocrystals 40 3.1 Introduction 40 3.2 Experimental Section 42 3.3 Results and Discussion 46 3.3.1 Structural and optical characteristics of ZnSe/ZnSe1-XTeX/ZnSe/ZnS NCs 46 3.3.2 Photophysical characterisctiscs of ZnSe/ZnSe1-XTeX/ZnSe/ZnS ensemble NCs 52 3.3.3 Photophysical characteristics of ZnSe/ZnSe1-XTeX/ZnSe/ZnS individual NCs 57 3.3.4 Impact of nearest-neighbor pairs of Te atoms on the optical properties of ZnSe/ZnSe1-XTeX/ZnSe/ZnS NCs 62 3.4 Summary 66 Chapter 4. Direct Assessment of Auger Recombination Rates of Charged Excitons via Opto-Electrical Measurements 68 4.1 Introduction 68 4.2 Experimental Section 71 4.3 Results and Discussion 77 4.3.1 Schemes for direct measurement of Auger recombination characteristics of charged nanocrystals 77 4.3.2 Measurement of negative trion (X-) decay dynamics of NCs in EOD 82 4.3.3 Measurement of positive trion (X+) decay dynamics of NCs in HOD 86 4.3.4 Directly measured multicarrier decay dynamics in various NCs 90 4.4 Summary 95 Chapter 5. AgInGaS2/AgGaS2 Ⅰ-Ⅲ-Ⅵ2/Ⅰ-Ⅲ-Ⅵ2 core/shell heterostructured nanocrystals 96 5.1 Introduction 96 5.2 Experimental Section 97 5.3 Results and Discussion 100 5.3.1 Ag(In,Ga)S2-AgGaS2 (AIGS-AGS) core-shell NCs 103 5.3.2 AIGS-AGS NCs with variable core compositions and shell dimensions 107 5.3.3 Impact of AGS heteroepitaxy on photophysical and photochemical properties of individual AIGS-AGS NCs 111 5.3.4 Competitive advantages of AIGS-AGS NCs and their application to luminescent solar concentrator 116 5.4 Summary 121 Conclusion 123 Bibliography 125 국문 초록 140박

Type-II Colloidal Quantum Wells: CdSe/CdTe Core/Crown Heteronanoplatelets

Solution-processed quantum wells, also known as colloidal nanoplatelets (NPLs), are emerging as promising materials for colloidal optoelectronics. In this work, we report the synthesis and characterization of CdSe/CdTe core/crown NPLs exhibiting a Type-II electronic structure and Type-II specific optical properties. Here, based on a core-seeded approach, the CdSe/CdTe core/crown NPLs were synthesized with well-controlled CdTe crown coatings. Uniform and epitaxial growth of CdTe crown region was verified by using structural characterization techniques including transmission electron microscopy (TEM) with quantitative EDX analysis and X-ray diffraction (XRD). Also the optical properties were systematically studied in these Type-II NPLs that reveal strongly red-shifted photoluminescence (up to similar to 150 nm) along with 2 orders of magnitude longer fluorescence lifetimes (up to 190 ns) compared to the Type-I NPLs owing to spatially indirect excitons at the Type-II interface between the CdSe core and the CdTe crown regions. Photoluminescence excitation spectroscopy confirms that this strongly red-shifted emission actually arises from the CdSe/CdTe NPLs. In addition, temperature-dependent time-resolved fluorescence spectroscopy was performed to reveal the temperature-dependent fluorescence decay kinetics of the Type-II NPLs exhibiting interesting behavior. Also, water-soluble Type-II NPLs were achieved via ligand exchange of the CdSe/CdTe core/crown NPLs by using 3-mercaptopropionic acid (MPA), which allows for enhanced charge extraction efficiency owing to their shorter chain length and enables high quality film formation by layer-by-layer (LBL) assembly. With all of these appealing properties, the CdSe/CdTe core/crown heterostructures having Type-II electronic structure presented here are highly promising for light-harvesting applications

Internal atomic-scale structure and photothermal dynamics of heterostructured nanomaterials

In this thesis we adopt a multimodal materials characterization approach to unravel the internal structure and the photoexcited electronic and geometric structural dynamics of CdZn1−Te/CdSe core/shell quantum dots (QDs). These QDs belong to a broader class of heterostructured II-VI semiconducting that are known for their applications in optoelectronics and biomedical imaging. The foundation of this thesis is the determination of the internal structure of II-VI core/shell quantum dots (CSQDs) using a combination of several characterization modes, with X-ray absorption spectroscopy (XAS) as a crucial element-specific technique. Through a combination of optical spectroscopy, electron microscopy, elemental analysis, and the global analysis of the extended X-ray absorption fine structure (EXAFS) spectrum, we show that the intended ZnTe/CdSe CSQDs, that are synthesized using a common one-pot synthesis procedure, are in actuality nanoparticles with an alloyed core and a patchy CdSe shell. Electronic structure calculations show that the CSQDs essentially behave as one-component QDs with a direct band gap. Cation exchange and the unintended reaction of molecular precursors prevent the formation of a sharp type-II ZnTe/CdSe interface with small lattice mismatch. Instead, the large interfacial strain between CdZn1−Te ( = 0.8) and pure-phase CdSe leads to the growth of islands on the QD surface. Our results corroborate the challenges associated with the synthesis of Zn/Cd chalcogenide type-II heterostructures due to facile ion exchange. This study is an example of how the assessment of heterogeneous nanomaterials on the basis of spectroscopy or size analysis alone is not always sufficient. While our XAS data were obtained at a large-scale synchrotron X-ray facility with specialized infrastructure and limited access, the advent of tunable high-brightness table-top X-ray sources will enable characterization studies on heterostructured photovoltaic and photocatalytic nanomaterials with much higher throughput and more experimental flexibility. We use density functional theory in combination with state-of-the art theoretical XAS codes to demonstrate the sensitivity of the X-ray absorption near-edge structure (XANES) to the local structure beyond the first coordination shell. In this way, we are able to corroborate the structural characterization of the alloyed CdZn1−Te ( = 0.8) core as determined by EXAFS analysis. This work underscores the power of XAS, in both experiment and simulation, for understanding the internal structure of heterogeneous nanoparticles. Ultrafast XAS is a powerful tool to unravel the electronic and geometric structures of photoexcited materials with femtosecond (fs)-nanosecond (ns) resolution. Using systematic DFT-based XAS simulations, we show that the time-resolved XANES spectra of nanoparticles at early time delays after photoexcitation (90 picoseconds, ps) are dominated by thermal effects, such as a 0.2% lattice expansion and disorder, while spectra at later times (2.5 ns) have clear signatures of excited carriers. In combination with heat diffusion simulations we derive the heat interface conductance 7-15 MW/m2/K for the colloidal CdZn1−Te/CdSe nanoparticle sample. Application of ultrafast XAS and the data analysis methods to other nanomaterials is an exciting perspective; in particular, in view of the recent development of intense free electron laser sources delivering v100 fs X-ray pulses. These state-of-the-art facilities open new opportunities for exploring photoinduced electronic properties of semiconductor nanomaterials on the ultrafast time scale

Extending the near infrared emission range of indium phosphide quantum dots for multiplexed 'In Vivo' imaging

This report of the reddest emitting indium phosphide quantum dots (InP QDs) to date demonstrates tunable, near infrared (NIR) photoluminescence and fluorescence multiplexing in the first optical tissue window with a material that avoids toxic constituents. This synthesis overcomes the InP synthesis “growth bottleneck” and extends the emission peak of InP QDs deeper into the first optical tissue window using an inverted QD heterostructure. The ZnSe/InP/ZnS core/shell/shell structure is designed to produce emission from excitons with heavy holes confined in InP shells wrapped around larger-bandgap ZnSe cores and protected by a second shell of ZnS. The InP QDs exhibit InP shell thickness-dependent tunable emission with peaks ranging from 515 – 845 nm. The high absorptivity of InP leads to effective absorbance and photoexcitation of the QDs with UV, visible, and NIR wavelengths in particles with diameters of eight nanometers or less. These nanoparticles extend the range of tunable direct-bandgap emission from InP-based nanostructures, effectively overcoming a synthetic barrier that has prevented InP-based QDs from reaching their full potential as NIR imaging agents. Multiplexed lymph node imaging in a mouse model shows the potential of the NIR-emitting InP particles for in vivo imaging.First author draf