Tuning of electronic properties in IV–VI colloidal nanostructures by alloy composition and...

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FEATURE ARTICLE

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Tuning of electron

APfPAHiipcapU

1998 she joined the opto-electronshitz in the Technion-Israel InstResearcher. She is a co-author of mbook chapters, 72 USSR Inventor CInt. Appl.). Her current research inof nanostructured materialsoptoelectronics.

Schulich Faculty of Chemistry, Russell Berr

Institute, Technion-Israel Institute of Tec

[email protected]

Cite this: DOI: 10.1039/c3nr02141f

Received 29th April 2013Accepted 4th June 2013

DOI: 10.1039/c3nr02141f

www.rsc.org/nanoscale

This journal is ª The Royal Society of

ic properties in IV–VI colloidalnanostructures by alloy composition and architecture

Aldona Sashchiuk, Diana Yanover, Anna Rubin-Brusilovski, Georgy I. Maikov,Richard K. Capek, Roman Vaxenburg, Jenya Tilchin, Gary Zaiats and Efrat Lifshitz*

Colloidal lead chalcogenide (IV–VI) quantum dots and rods are of widespread scientific and technological

interest, owing to their size tunable energy band gap at the near-infrared optical regime. This article

reviews the development and investigation of IV–VI derivatives, consisting of a core (dot or rod) coated

with an epitaxial shell, when either the core or the shell (or both) has an alloy composition, so the

entire structure has the chemical formula PbSexS1�x/PbSeyS1�y (0 # x(y) # 1). The article describes

synthesis procedures and an examination of the structures' chemical and temperature stability. The

investigation of the optical properties revealed information about the quantum yield, radiative lifetime,

emission's Stokes shift and electron–phonon interaction, on the variation of composition, core-to-shell

division, temperature and environment. The study reflected the unique properties of core–shell

heterostructures, offering fine electronic tuning (at a fixed size) by changing their architecture. The

optical observations are supported by the electronic band structure theoretical model. The challenges

related to potential applications of the colloidal lead chalcogenide quantum dots and rods are also

briefly addressed in the article.

ldona Sashchiuk received herh.D. in Semiconductors Physicsrom the Semiconductorshysics Institute, Lithuaniancademy of Sciences in Vilnius.er research was focused onnvestigation of magnetic eldnuence on the electronicroperties of Ge and Si semi-onductors for microelectronicpplications. She has receivedrizes as USSR Inventor, and atSSR Exhibition in Moscow. Inic laboratory of Prof. Efrat Lif-itute of Technology as Seniorore than 80 scientic papers, 5erticates, and 3 patents (PCTterests include the developmentfor high performance in

ie Nanotechnology Institute, Solid State

hnology, Haifa 32000, Israel. E-mail:

Chemistry 2013

1 Introduction

IV–VI (PbTe, PbSe, PbS) colloidal nanostructures, such asquantum dots (QDs) and quantum rods (QRs), are the focus ofwidespread scientic and technological interest.1–4 They have a

Efrat Lifshitz is the FullProfessor of Chemistry of Mat-wei Gunsbuourgh AcademicChair, Schulich Faculty ofChemistry, Solid State Institute,Russell Berrie NanotechnologyInstitute, Technion, Israel. Sheearned her M.Sc in Chemistryand her Ph.D. in PhysicalChemistry from the University ofMichigan Ann Arbor, USA anddid postdoctoral research at theWeizmann Institute of Science,

Israel. Her research interest is the development and magneto-optical characterization of chemically prepared semiconductornanocrystals (NCs). She has contributed substantially to thesynthesis of near-infrared active NCs, had a particular strongimpact on the understanding of the basic magneto-optical prop-erties of manmade solids and already utilized the accumulatedknowledge in the development of opto-electronic devices and bio-logical tagging based on the mentioned NCs.

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http://dx.doi.org/10.1039/C3NR02141F

http://pubs.rsc.org/en/journals/journal/NR

Fig. 1 Schematic representation of the different kind of core/shell hetero-structures (top row). Illustration of possible electron and hole wave functiondistributions in core/shell heterostructures (bottom row).

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rock-salt crystal structure (space group Fm�3m), and exhibit asize-tunable energy band gap in the range of 0.3–1.7 eV, with thebroad band absorption prole ranging from near-infrared (NIR)to the visible range.5–7 Furthermore, they possess small electronand hole effective masses,8 a large dielectric constant9 and arelatively large exciton effective Bohr radius.10 These propertiesmake them applicable in NIR gain devices,6,7 biologicalmarkers,11–13 photovoltaic cells,14–20 Q-switches,21,22 and ther-moelectric devices.23–25 For example, the broad band absorptionenables light harvesting in photovoltaic cells, as recent effortsshowed IV–VI QD based solar cells with a power conversionefficiency of up to 8%.14–20 Investigations carried out during thepast decade have explored other characteristic propertiesrelated to the density of states,26 electron–hole exchange inter-actions,27 radiative lifetimes,28,29 Auger relaxations30 andmultiple exciton generation.31–41 Previous studies have demon-strated variations in the physical properties of IV–VI nano-structures, by variation of their shape (e.g., rods,42–44 wires,45–47

cubes,48,49 platelets50 or polypods.51–53) Additionally, it wasdemonstrated that surface properties, determined by the crystalfacet energy and surfactants, have a signicant inuence on themorphology,54,55 as well as on the physical properties of thenanostructures.56–58

Despite the advanced development of various IV–VI QDnanostructures, their implementation in various optoelec-tronic devices has been hampered due to the sensitivity ofPbTe and PbSe QDs to oxygen.59,60 Several research groupsinvestigated the inuence of the cation/anion ratio on thesurface properties and showed that excess Pb2+ in IV–VI QDsor Cd2+ in II–VI QDs at the exterior surfaces may increase thenumber of bound surfactant molecules, thus improving thesurface passivation.61–65 Other approaches included exchangeof surface ligands for control of surface coverage.66,67 Forexample, recent studies discussed the immediate inuence ofoxygen exposure on the luminescence intensity of grown oleicacid-capped PbSe QDs,60,68 and suppression of the oxidationprocess by an exchange of the surfactants with alkylselenidemolecules.69 Other studies demonstrated stabilization of theQDs' surface by a post-synthetic treatment, exchanging theorganic surfactants with inorganic shells70 or halideanions.16,71

Alternatively, efficient passivation is achieved by an epitaxialgrowth of another semiconductor onto IV–VI core QDs, leadingto the formation of core–shell heterostructures, where theexterior shell surface is still covered by organic molecules. Thisarticle discusses the formation of PbSe cores covered by anotherIV–VI semiconductor shell, i.e., PbS. Other reports showedcapping of PbSe cores by a cation exchange procedure, with a II–VI CdSe semiconductor shell.72–74 The PbSe/CdSe hetero-structures can be further covered by the high-potential barrierand relatively chemically inert ZnSe(S) epitaxial shell.72 Encap-sulation bymetal oxide compounds (e.g., ZnO) for the formationof air-stable QDs was also demonstrated.75 As discussed in thefollowing, besides achieving chemical and photochemicalstability, a shell growth might render an overall heterostructurewith new physical properties, depending on the core-to-shellband-edge offset, which leads to the generation of either type-I

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heterostructures (when the conduction and valence bands ofthe shell wrap those of the core) or type-II heterostructures(when the band-edge of the constituents has a staggeredalignment).76 Type-I heterostructures conne the carriersmainly into the core region, while type-II heterostructuresallocate one carrier in the shell and retain the second carrierwithin the core. The intermediate alignment, the so-calledquasi-type-II heterostructures, permits the delocalization of onecarrier over the entire core/shell structure, while the other isconned either in the core or in the shell. The blue-coloredshapes in Fig. 1 show a schematic drawing of the differentheterostructures (top row) and their anticipated band-edgealignment with the carriers' distribution curves (bottom row).Type-II and quasi-type-II heterostructures induce partial chargeseparation thereby inuencing the strength of direct Coulomband exchange interactions,77,78 which leads to an immediatechange of the radiative lifetime of a single79 exciton andmultiple excitons80–84 and their exciton bright-to-dark energysplit.77,85–87

In recent years, chemical stability as well as tuning of phys-ical properties has been successfully achieved by integratingalloy composition in colloidal II–VI QDs88–90 and in a fewexamples of colloidal IV–VI QDs.47,91 Interestingly, our work6,92 isshowing the advantage of alloy composition combined withcore/shell heterostructures. For example, the presence ofalloying in PbSe/PbSexS1�x or PbSexS1�x/PbS providessmoothing of the core-to-shell boundary potential (see Fig. 1red-colored shape), and induces considerable chemical andphoto-chemical stability.6,79,93 An epitaxially grown PbS orPbSexS1�x shell has an extremely small crystallographicmismatch (#3%) with the PbSe cores, thereby reducing core/shell interface defects.6,92 Recently developed alloyed QD het-erostructures, such as CdxZn1�xSe/ZnSe,94 CuInS2/ZnS,95 andCdTe/CdTexSe1�x,62,96 showed exceptionally high spectralstability (blinking-free),62,96 and sustained a relatively longbiexciton lifetime (0.5 ns).97 We recently published theoreticaland experimental work exploring the properties of PbSexS1�x

and PbSexS1�x/PbSeyS1�y (0 # x( y) # 1) alloyed colloidal QDs,showing the variability of the electronic properties with theelemental composition,92,98 and modication of exciton–phonon interaction, direct Coulomb and exchange interactionwhenever an alloy composition was involved.92,98 Additionaldetails of this work are the focus of this review.

This journal is ª The Royal Society of Chemistry 2013


Fig. 2 (a) A schematic drawing of a QD heterostructure with alloy compositionand general chemical formula PbSexS1�x/PbSeyS1�y, where 0# x, y# 1. Rc and Rsare the core and core/shell radius, respectively, and W is the shell thickness; (b) aplot of the remote band contributions to the anisotropic conduction and valenceband effective masses, meff/m0 (right axis, m0 is the free electron mass), m�,and m+ (left axis) versus the radial coordinate of a heterostructure shown in (a)with Rc ¼ 2 nm and Rs ¼ 3 nm. The band-edge energies are scaled with respect tozero vacuum energy.

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Shape control has recently received renewed attention, in thegeneration of alloyed anisotropic structures or heterostructures.These structures experience tuning of the electronic propertiesvia their unique shape and composition, anticipating polariza-tion-sensitive light emission,99 large absorption cross-sections,partial charge separation,100 transport along the major growthaxis, as well as possible efficient multiple exciton generation.41

Preliminary efforts in this direction, including the synthesis ofcolloidal PbSe quantum rods (QRs)52 and PbSexS1�x wires of afew microns in length, had been shown.47 Theoretical electronicband structure calculations of pure PbSe and PbS nanowiressuggested that the ground exciton binding energy in the elon-gated rod-like structures is substantially larger than that of QDswith a diameter equivalent to the rod's width.101

This article focuses on the synthesis, development, andcharacterization of structural and electronic properties in QDand QR core/shell heterostructures, having a general chemicalformula PbSeyS1�y/PbSexS1�x (0 # x( y) # 1). Section 2 providesthe theoretical model of the electronic band structure, using themultiband k$p envelope function method. The model empha-sizes the type of electronic conguration (e.g., type-I or quasi-type-II), the dependence on size, internal architecture, shapeand composition. Section 3 contains a summary of the appliedsynthetic methods, as well as description of the crystallographicand morphological characterization of the nanostructures.Section 4 presents the study of the steady-state and time-resolved optical properties of the discussed heterostructures,measured at various temperatures and recorded with/withoutoxygen exposure. The results revealed the formation of core/shell heterostructures with a quantum yield (QY) of up to 68%,relatively narrow emission bands with a long excited-state life-time, tolerance to oxygen environment for a restricted timeduration, peculiar Stokes shi behavior, and overall, a domi-nant tuning of band-edge properties by composition and core-to-shell division. Finally, representative applications of dis-cussed IV–VI QD heterostructures are given in Section 5.

2 Theoretical prediction of the electronicproperties of QD and QR heterostructures2.1a Electronic structure of the PbSexS1�x/PbSeyS1�y QDheterostructures

The electronic band structure of the QD heterostructures (with/without alloy composition) was evaluated, using the four-bandk$p envelope function method1,26 that revolves around thesolution of the effective Schrodinger equation of the formHk$p(�iV)F(r) ¼ EF(r), in which F(r) are the four-componentenvelope eigenfunctions. The model considered specicfeatures related to the discontinuity of the effective mass, crystalpotential, and dielectric constant of the constituents at the core/shell and at the shell/surroundings interface,98 covering allcases, when either the core, the shell (or both) has an alloycomposition. A schematic drawing of a PbSe/PbS core/shellheterostructure is shown in Fig. 2(a). Rc and Rs are the radii of acore QD heterostructure and a core/shell QD heterostructure,respectively;W is the shell thickness. The Hamiltonian Hkp(�iV)was adjusted to the discontinuity at the PbSe/PbS core/shell


heterostructure interface by the appropriate choice of thekinetic energy term, ensuring probability current conservationand continuity of the envelope functions.102

The theoretical treatment further considered a few specialpoints: anisotropy in effective mass (typical for IV–VI semi-conductors), expressed via the remote band contributions to thelongitudinal (k) and transverse (t) conduction (�) and valence(+) band effective masses, m�

k,t; the dependence of each phys-ical parameter on a position (r) across the dot (Fig. 2(b)), and itssmooth variability across the core/shell and the shell/surrounding interfaces, with a smoothing factor g. The smoothpotential prole reects the nature of the interface region inalloyed materials with a gradient composition. The overall bandoffset was chosen as that of the corresponding bulk materials(where the valence band maximum of bulk PbS lies 0.025 eVabove that of PbSe, while the conduction band minimum lies0.155 eV above that of PbSe). Then, the values of the conductionand valence band-edge energies of the QDs are given by: V� ¼�(4.6 + 0.155x) eV and V + ¼ �(5.01 + 0.025x) eV, respectively.These relations are based on the PbS bulk electron affinity,103

the calculated PbS to PbSe band offset,104 and the composition-dependent bulk PbSexS1�x band-gap energy.105 The vacuumlevel is selected as the zero reference energy. The height of theouter barrier (at r¼ Rs) of both electrons and holes is taken to beequivalent to the corresponding bulk electron affinity. It isworth noting that the ligand and solvent surroundingmay lowerthe barrier height by a limited amount; however, it does notchange the trends shown in Fig. 2. The carriers' mass outsidethe QD is considered as the free electron mass. The typicalradial variation of the effective masses and band-edge energiesis illustrated in Fig. 2(b). The QD heterostructures investigatedare ternary core or core/shell QDs, having a general formulaPbSexS1�x/PbSeyS1�y, covering the following cases: (a) x ¼ y ¼ 1or x ¼ y ¼ 0 refers to a simple core PbSe or PbS, respectively; (b)0 < x ¼ y < 1 is a homogenous alloy core; (c) x ¼ 1 and y ¼ 0 aresimple PbSe/PbS QDs; and (d) x¼ 1 ( y¼ 1) and ys 0 (xs 0) arecomplex core/shell QD heterostructures, when either the core orthe shell has a homogenous alloyed composition.

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Fig. 3 Variation of the ground-state exciton energy, Eg, versus (a) core radius (Rc)and shell thickness (W) in PbSe/PbS QD heterostructures, and (b) shell thickness(W) and composition (x) in PbSe/PbSexS1�x core/alloyed-shell QD hetero-structures with Rc ¼ 1.5 nm.

Fig. 4 (Top) Charge density distribution of electron (blue) and hole (red), shownas half a sphere of concentric isosurfaces, given in units of e/nm3 (e is the electroncharge). (Middle) Charge density difference, Dr, in PbSe, PbSe0.5S0.5, and PbS coreQDs. (Bottom) Charge density difference, Dr, in PbSe/PbS, PbSe/PbSe0.5S0.5 andPbSe0.5S0.5/PbS core/shell QD heterostructures. Red color corresponds to excesshole density, while the blue color designates excess electron density. The longi-tudinal [111] crystallographic direction is marked in the upper panel.


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In most cases, the value of the ground-state exciton energy(Eg) is of major practical and experimental importance. Fig. 3(a)presents the dependence of Eg on Rc and W in PbSe/PbS QDheterostructures and shows a decrease of the values of Eg incore/shell QDs with respect to those in pure core QDs of thesame overall size Rs, which is in agreement with previousexperimental observations.92 Fig. 3(b) presents the dependenceof Eg on W and x in PbSe/PbSexS1�x core/alloyed shell QD het-erostructures with Rc ¼ 1.5 nm, emphasizing that the tunabilityof Eg can be controlled not only by varying the core/shell size butalso by a change in composition or/and internal structure.Therefore, the diagrams given in Fig. 3 can be used as a prac-tical working tool, predicting the alloyed core/shell QD hetero-structures design with ground-state exciton energy on demand.

2.1b Spatial charge distributions in PbSexS1�x andPbSexS1�x/PbSeyS1�y QD heterostructures

The envelope functions are used as a base for the calculation ofthe probability density distribution (|F(r)|2) yielding knowledgeabout the spatial delocalization of the carriers over the entireQDs structure. Fig. 4 (top row) displays concentric chargedensity isosurfaces of the lowest electron and hole states inPbSe QDs with Rc ¼ 3 nm, framed within half spheres that arecut along the [1�10] crystallographic plane. The inner spheredesignates the larger density (in units of e nm�3, where e is theelectron charge), which gradually decreases toward the externalsurface (red/blue colors designate a positive/negative charge, asshown in the legend). The electron distribution was found to benearly isotropic around the QD's center in all the samples dis-cussed. However, an ellipsoid shape, slightly elongated in thetransverse direction, is seen in the density distribution of ahole, stemming from the anisotropy in its effective mass. Thesecond and third rows in Fig. 4 display charge differencefunctions, dened as Dr(r) ¼ e(|Fhole|

2 � |Felectron|2) of a few

core and core/shell QD heterostructures. The isosurfaces inthe second row show a ring-like displacement of the hole towardthe QD's equator in pure PbSe QDs that decreases gradually onthe increase of sulfur content in PbSexS1�x QDs and nearlydiminishes in pure PbS core QDs. Isosurfaces shown in thethird row reveal an obvious symmetric hole delocalization inPbSe/PbS core/shell QD heterostructures and anisotropic

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mutual electron–hole distribution displacement in PbSe/PbSe0.5S0.5 and PbSe0.5S0.5/PbS QD heterostructures. The iso-surfaces here are associated with QDs with an identical overallsize of Rs ¼ 3 nm. Charge density distribution of QDs of othersizes shows similar behavior (not shown). The probabilitydensity distribution of carriers in remote states is not includedin the current review. However, control over charge distributionof band-edge carriers shown here suggests an option to accu-mulate charges at different regions of the QD volume. A ring-shaped net positive charge is visualized at the equator in corePbSexS1�x QDs originating from a relatively larger anisotropy ineffective mass of the valence band with respect to that of theconduction band. In core/shell QD heterostructures botheffective mass anisotropy and the presence of the band offset,have a considerable effect, resulting in either a symmetric or anasymmetric total charge distribution, depending on the coreand shell composition.

The step-like band alignment of the heterostructure altersthe conning potential felt by the charge carriers. As explained(see Fig. 2(b)), the bulk band offsets between the core and theshell materials are functions of composition. As mentionedabove, in the case of the PbSe/PbS system, these values are 0.155eV and 0.025 eV for the conduction band and the valence band,respectively. This kind of alignment can be regarded as quasi-type-II, since the valence band offset is small, permittingasymmetric delocalization of the carriers over the entire core/shell structure. Moreover, for relatively small QDs, the typicalconnement energies largely exceed the energy of the bandoffsets. Consequently, the lowest electronic levels lie abovethe band offset, so they are weakly inuenced by the



Fig. 6 (a) Radial probability density of the ground-state electron and hole in theindicated QDs of dimensions Rc ¼ 1 nm, Rs ¼ 2 nm (top row) and Rc ¼ 6 nm, Rs ¼12 nm (bottom row). Solid vertical lines designate the external QD boundaries,and dashed lines represent the core/shell interface position. Probability oflocating the electron (b) and the hole (c) in the core region of various QD het-erostructures (indicated in c) as a function of overall radius, Rs, with a constantratio Rs/Rc ¼ 2.


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heterostructure architecture. However, in sufficiently large core/shell QDs, the conned electronic levels are allocated below theoffset energy, enforcing localization of at least one carrierwithin a selective regime. In contrast, the hole is less affected bythe small valence band offset and remains more delocalized asthe QDs size changes.106 This concept is illustrated in Fig. 5,where the gradual decrease of the lowest electron energy withincreasing size of the core/shell QD heterostructures can beseen. Note the strong pronounced advance of the density ofstate with the growth of the overall size. Since the band offsetsare functions of composition and the connement energies arefunctions of size, the degree of the electron localization in thecore (or, equivalently, its location within the system) can becontrolled by varying these two degrees of freedom.

An estimate of electron's connement within the core can beobtained by integrating the probability density (P) over the coreregion. The extent of the electron and hole separation inPbSexS1�x/PbSeyS1�y QD heterostructures can be furtheranalyzed by evaluating the probability of nding a carrier in thecore, Pcore, or in the shell, Pshell, when

PeðhÞcore ¼

ð

core

X4

j¼1

��F eðhÞj ðrÞ

��2d3r and Pshell z 1� Pcore: (1)

Fig. 6(a) presents radial probability density of the ground-state electron and hole in the indicated PbSe/PbS, PbSe/PbSe0.75S0.25, QD heterostructures of dimensions Rc ¼ 1 nm,Rs ¼ 2 nm (top row) and Rc ¼ 6 nm, Rs ¼ 12 nm (bottom row).The gure shows that the electron–hole charge separationbecomesmore pronounced with the growth of the overall size ofthe QDs, while retaining a constant Rc/Rs ratio. In addition, acharge separation seems to be slightly more efficient in PbSe/PbS with respect to that found in the alloyed PbSe/PbSe0.75S0.25QDs because of larger band offsets in PbSe/PbS (see the bottomrow). Fig. 6 displays plots of Pe

core, in (b), and Phcore, in (c), versus

values of Rs in a few different QD heterostructures (see legend b).The values of Pe

core span a range from 0.2 to 1.0, while those ofP hcore are grouped in a narrower range of 0.25–0.40. This is a

direct consequence of the quasi-type-II band alignment with verysmall valence band offset, emphasizing the option to engineerthe charge distribution between the core and the shellconstituents.

Fig. 5 Schematic drawing of the energy levels of electrons and holes (blacklines) in PbSe/PbS heterostructures of three different sizes, having a constant ratioRs/Rc ¼ 2. Blue curves represent the band alignment profile.


2.2 The electronic structure of the PbSexS1�x and PbSe/PbSQRs

The electronic structure of ternary PbSexS1�x QRs was calcu-lated using the framework of the four-band k$p envelopefunction method, in a similar manner to the procedure dis-cussed in Section 2.1a. However, a cylindrically symmetric partof the full Hamiltonian, Hkp(�iV), describing electrons andholes in the vicinity of a single L point of the rst Brillouinzone, is used in the present calculation.101 In general, thefundamental band gap in IV–VI semiconductor materialsoccurs at four different L-points, being energy degenerate inisotropic QDs. However, this degeneracy is lied in asym-metric QRs, so each electronic band extreme (L valley)possesses an individual set of direction-dependent materialparameters. Fig. 7(a) shows a representative plot of calculatedEg (ignoring the Coulomb interaction) of the lowest energyvalley in alloyed PbSexS1�x QRs with x ¼ 0.5, grown in theh110i directions. The results presented in the gure reveal theconsiderable inuence of the radius on the value of Eg;however, the contribution of the length becomes negligibleabove about 10 nm. Fig. 7(b) displays the dependence of Eg forQRs grown along the h110i directions versus the radius andcomposition of the QRs (with a xed length of 10 nm). Theplot exposes important exibility in tailoring Eg with control ofcomposition, when a xed length is demanded. It should bementioned that the current model is applicable mostly for QRswith a radius >3.0 nm, when evaluation by an effective massmodel deviates from measured values for very small structures(<2.5 nm) with an extreme quantum connement in at leastone spatial dimension.

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Fig. 7 Calculated values of Eg versus: (a) radius and length of ternary PbSexS1�x QRs with x ¼ 0.5 and (b) radius and composition of ternary PbSexS1�x QRs at a fixedlength of 10 nm.


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3 Synthesis, structural and compositioncharacterization of PbSexS1�x/PbSeyS1�y QDand QR core/shell colloidal heterostructures

PbSexS1�x/PbSeyS1�y alloyed core/shell QD and QR hetero-structures were formed by one-step or two-step synthesis,carried out at various temperatures. General terms of thesynthesis conditions are presented schematically in Fig. 8 anddiscussed in Sections 3.1 through 3.3.

3.1 PbSe/PbS core/shell colloidal QDs with band-edge energyin the range of 1.0–1.4 eV

The synthesis of spherical shaped PbSe/PbS core/shell colloidalQDs with Rs < 2.0 nm and band-edge energy in the range of

Fig. 8 Schematic presentation of four different core/shell colloidal hetero-structures, prepared by adjustable colloidal procedures, as described in the text.Pb(OA)2 ¼ lead oleate, TOP ¼ trioctylphosphine, TMS2S ¼ bis(trimethylsilyl)sulfide precursors.

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1.0–1.4 eV was described earlier.107 The preparation procedureincluded two steps. The rst step focused on the preparation ofultra-small PbSe core QDs with the band-edge energy in therange of 1.5–1.7 eV. The main reaction mixture was comprisedof lead oleate (Pb(OA)2), stearyl alcohol and hexadecane (HDC).The mixture was heated to 100 �C under vacuum for one hour. Aselenium precursor solution, containing trioctylphosphine(TOP)Se, diphenylphosphine (DPP) and hexadecane (HDC), wasprepared separately and was injected aer the rst hour into thereaction mixture at 80 �C. Next, the temperature was reduced to70 �C and selenium precursor solution was injected into themain reaction mixture. To quench the reaction, the mixture waspoured into a mixture of ethanol, acetonitrile, and toluene.

Fig. 9 (a) HR-TEM image of PbSe QDs with Rc ¼ 1.0 � 0.2 nm. Inset: FFT patternof the TEM of (a). (b) Absorption spectra of PbSe QDs shown in (a) recorded afterthermal annealing at 60–120 �C. Inset: dependence of the lowest absorptionband-edge energy on the annealing temperature. (c) Time-evolution of absorp-tion spectra of PbSe/PbS core/shell QDs during drop-wise injections of TMS2S intothe reactionmixture at 70 �C. Time interval between the spectra is 10 min. (d) XRDpattern of PbSe (blue) with Rc ¼ 1.2 � 0.2 nm and PbSe/PbS (green) core/shellQDs with a Rs/Rc ratio of 1.8 nm/1.1 nm indexed to the bulk rock-salt crystalstructure of PbSe (blue line) and PbS (red line).



Fig. 10 (a) HR-TEM of PbSe/PbS core/shell QDs with a Rs/Rc ratio of 1.8 nm/1.1 nm (scale bar ¼ 5 nm). Inset: FFT pattern of the HR-TEM of (a). (b) XPSspectra of PbSe/PbS core/shell (top) and of PbSe core (bottom) QDs. Theassignment of the bands is shown in the figure. (c) Dependence of the cation/anion stoichiometric ratio on the QD size in PbSe core (blue) and in PbSe/PbScore/shell (red) QDs.


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Then, the QDs were separated by centrifugation. A representa-tive high-resolution transmission electron microscopy (HR-TEM) image of small PbSe QDs is presented in Fig. 9(a). Thegure shows QDs with a nearly spherical shape having anaverage radius of 1.2 � 0.2 nm. The inset provides a fast Fouriertransform (FFT) pattern of the HR-TEM, revealing a cubic singlecrystal with a rock-salt structure and a space group Fm�3m, asexpected from IV–VI semiconductor structures.

The shell growth was carried out as a second step of thereaction, aer the small PbSe cores were precipitated from therst reaction mixture in order to eliminate the presence ofunreacted selenium precursor. The temperature stability ofsmall-sized PbSe QDs (1.0 � 0.2 nm in radius) was examined inthe reaction mixture, at the temperatures above 60 �C byfollowing the changes of the absorption spectrum. Fig. 9(b)shows the absorption spectra of the PbSe QDs dissolved intetrachloroethylene (TCE) and recorded aer thermal annealingprocesses carried out at various temperatures from 60 �C to100 �C. The inset of the gure shows a plot of the shi in theband-edge energy with increasing annealing temperature,having a drastic change beyond 70 �C. Thus, the shell coatingwas carried out at temperature #70 �C, which is substantiallylower than other previously reported procedures.108–110 The shellgrowth involved injection of diluted bis(trimethylsilyl) sulde(TMS2S) in a drop-wise manner into the reaction mixture con-taining PbSe QDs, Pb(OA)2 and diphenylether (DPE). The drop-wise injection of sulfur precursor prevented instantaneoussuper-saturation and co-nucleation of PbS QDs and made itpossible to coat the PbSe QDs with a shell of desired thickness.

The growth of the PbS shell was monitored by a gradual redshi of the absorption, as shown in Fig. 9(c). The X-raydiffraction (XRD) patterns of the PbSe/PbS core/shell QDs(green) and the corresponding PbSe QDs (blue) are shown inFig. 9(d), with pronounced (111), (200) and (220) diffractionpeaks at a 2q of 25.44�, 29.21� and 41.80�, respectively. Thediffraction lines of bulk PbSe and PbS are also displayed in thegure with blue and red lines, respectively. The diffractionpattern of the core/shell QDs exhibits only a minor change withrespect to that of the pure PbSe core, due to the close crystal-lographic similarities of the PbSe and PbS constituents.

The evidence for the formation of PbSe/PbS core/shell het-erostructures is also provided by HR-TEM and X-ray photo-electron spectroscopy (XPS). A representative HR-TEM image ofPbSe/PbS core/shell QDs with a Rs/Rc ratio of 1.8 nm/1.1 nm isgiven in Fig. 10(a), while the FFT pattern of the HR-TEM isshown in the inset. These measurements reveal that the PbSe/PbS QDs have the same structure as the initial PbSe core QDs(see Fig. 9(a)). The representative XPS spectra of PbSe/PbS andPbSe QDs prove the presence of both Se and S in core/shell QDs.The dependence of the cation/anion ratio on the sizes is plottedin Fig. 10(c), demonstrating excess of Pb in the ultra-small cores(blue symbols), similar to previous observations.69,111–113 On theother hand, a nearly stoichiometric Pb/Se(S) ratio is achieved inPbSe/PbS core/shell QDs (red symbols in Fig. 10(c)). The PbSshell coating process results in immediate lling of anionvacancies at the exterior surfaces, which is followed by furtherepitaxial growth of a full shell.


3.2 PbSexS1�x/PbSeyS1�y core/shell colloidal QDheterostructures with the band-gap energy in the range of0.62–1.0 eV

The synthesis of large-sized (4.2–10 nm) PbSe/PbS core/shellcolloidal QDs, with the tunable band-gap energy in the range of0.62–1.0 eV, is described in detail elsewhere.6 However, a briefdescription is given here. The preparation involves a two-stepprocess,6 starting with the formation of PbSe cores and theirisolation from the initial reaction solution. The cores wereprepared by injection of (TOP)Se into a main reaction mixturecontaining Pb(OA)2 and DPE at 180 �C, while the growthoccurred at 120 �C.3,114,115 Fig. 11(a) shows absorbance curvesrecorded in situ during the QDs' growth with a time interval ofone second. The spectra are characterized by the rst 1S-excitonabsorption peak, whose energy changes from 1.2 to 0.8 eV,associated with the formation of PbSe QDs with a radius of1.5� 0.2 nm to 2.6� 0.2 nm.113,116 The second step included thegrowth of an epitaxial PbS shell onto the pre-prepared PbSecores, by the injection of stoichiometric amounts of Pb and Sprecursors into a new, Se-free solution containing suspendedPbSe QDs. The coating reaction was performed at 120 �C using(TOP)S as the sulfur source. The PbS coating of totally 1–2monolayers (MLs) was obtained within the rst 15–30 minutesof the reaction.

Fig. 11(b) shows a HR-TEM image of PbSe/PbS core/shellQDs with a Rs/Rc ratio of 2.1 nm/1.5 nm. This image reveals theformation of spherical QDs with well-resolved crystal fringesand without distinct boundaries at the core/shell interface, dueto close matching (3% mismatch) of the PbSe and PbS crys-tallographic parameters. The inset shows a selective area

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Fig. 11 (a) Representative in situ absorbance of PbSe colloidal QDs monitoredin situ during the reaction, recorded one after the other with a time delay of onesecond, and leading to the growth of QDs with the size ranging from 3.0 �0.4 nm to 5.2 � 0.4 nm. (b) HR-TEM image of PbSe/PbS core/shell colloidal QDheterostructures with a Rs/Rc ratio of 2.1 nm/1.5 nm (scale bar ¼ 2 nm). Inset:SAED pattern of the same QDs. (c) HR-TEM image of PbSe/PbS core/shell QDswith a Rs/Rc ratio of 3.0 nm/1.5 nm (scale bar ¼ 5 nm). (d) A STM image ofPbSe/PbS core/shell QDs with a Rs/Rc ratio of 4.0 nm/1.5 nm spread over a goldsubstrate (scale bar ¼ 10 nm). A height profile of a single QD is shown in theinset. (e) TEM image of PbSe/PbSexS1�x core/alloyed-shell QDs with a Rs/Rc ratioof 2.5 nm/1.5 nm. Inset: HR-TEM image of the corresponding single QD (scalebar ¼ 4 nm).


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electron diffraction (SAED) image, disclosing a cubic singlecrystal, with a lattice spacing of 6.12 A associated with a rock-salt structure of the space group Fm�3m that was preserved inthe core/shell heterostructures. The increase in the overall sizeof core/shell QDs by 1.2 nm, compared to the preliminarycores, is consistent with a PbS shell thickness of one ML. PbSe/PbS core/shell QDs with a shell thickness >2 MLs requiredsynthesis with repeated injections (2–4 times) until the desiredshell thickness was achieved. Fig. 11(c) presents a HR-TEMimage of PbSe/PbS core/shell QDs with a Rs/Rc ratio of 3.0 nm/1.5 nm, while Fig. 11(d) shows a scanning tunneling micros-copy (STM) image of PbSe/PbS core/shell QDs with a Rs/Rc ratioof 4.0 nm/1.5 nm. The STM image was recorded with a biasvoltage of 2 V and a set-point current of 5 pA.117 The insetpresents a height prole of a single QD. The lateral dimen-sions of the prole are substantially larger than the actual sizedue to a broadening of the tip width, thus, the height aloneindicates the QD's size. The synthesis of PbSexS1�x/PbSeyS1�y

alloyed-core/alloyed-shell QDs involved a one-step procedure.6

Alloy core/shell QDs were produced by simultaneous injectionof Pb(OA)2, (TOP)Se and (TOP)S precursors into the

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preliminary prepared reaction solution. The composition andthickness of the shell were controlled by the growth parame-ters, such as the monomer concentration, temperature, andduration of the reaction. Fig. 11(e) shows a TEM image of thePbSe/PbSexS1�x core/alloyed-shell QDs with a Rs/Rc ratio of2.5 nm/1.5 nm; a HR-TEM image of a selective single QD isdisplayed in the inset. The images show the formation ofspherical QDs with a high crystallinity and without a distinctboundary at the core/shell interface. The composition of thePbSeyS1�y/PbSexS1�x QDs was determined by XPS and energydispersive X-ray (EDX) analysis by extracting aliquots from thereaction mixture at varied time intervals. In fact, the alloycomponent (y or x) refers to average composition at an inter-mediate time aer injection. It is likely that a graded compo-sition has been generated between adjacent aliquots and onlyan average composition can be assigned. Thus, a generalnotation of PbSeyS1�y/PbSexS1�x is adopted in the current case.The EDX analysis indicated that the use of Se/S [ 1 elementratio enabled the formation of PbSe/PbSexS1�x composition,while the use of Se/S < 1.2 caused an introduction of sulfurinto the core, leading to the formation of the so-calledPbSeyS1�y/PbSexS1�x QDs.

3.3 Structural properties of ternary PbSexS1�x QRs

The growth of elongated structures can occur in the presenceof templating ligands (e.g., amines, phosphonic acids) thatbind to selective crystallographic facets with high surfaceenergy and density of atoms. It was recently shown118 that thetris(diethylamino)phosphine (TDP) ligands, used in the growthof PbSe QRs, undergo chemical decomposition and releasediethylamines,118 which have a critical inuence on the nalshape of the nanostructure. We examined the structuralproperties of recently proposed ternary PbSexS1�x and PbSe/PbSexS1�x core/alloyed-shell QR heterostructures. Thesynthesis of binary PbSe QRs was based on the procedurereported earlier52 with small variations. PbSexS1�x QRs wereprepared from Pb(OA)2, (TDP)Se and (TOP)S in squalane. Thegrowth temperature was 170 �C and reaction time variedbetween 1 and 10 minutes.43

The synthesis of PbSe/PbSexS1�x QRs began with theformation of PbSe rods, and then continued with an epitaxialgrowth of the ternary shell. Fig. 12(a) and (b) show HR-TEMimages of ternary PbSexS1�x QRs with a length/radius ratio of15.1 nm/2.0 nm and 19.9 nm/1.3 nm, respectively, preparedunder the growth conditions described. The images presentdistinct uniform crystal fringes, identied with the [220] crys-tallographic planes (as marked in Fig. 12). The growth of therods takes place along the h110i (a) or h100i (b) crystallographicdirections, dominated by the appearance of a single crystal.Fig. 12(a) refers to QRs with a growth duration of 1.5 minutes,attaining a radius, Rc, of about 2.0 nm, while Fig. 12(b) presentsa QR, which grew in length, but its width has been reduced withtime (growth time > 1.5 minutes), accompanied by the forma-tion of radius uctuation along the QR. An unusual twist of acrystallographic plane rarely appears along a small fragment ofa QR, but the main crystallographic direction is preserved. It



Fig. 12 Representative HR-TEM images of ternary PbSexS1�x QRs with alength/radius ratio of (a) 15.0 nm/2.0 nm, (b) 19.9 nm/1.5 nm. Inset in (a) isthe FFT pattern of the HR-TEM. (c) HR-TEM image of an ensemble of QRsshown in (a). (d) In situ absorbance monitoring of the PbSe/PbSexS1�x core/alloyed-shell QRs synthesis recorded with a time delay of one second. (e)HR-TEM image of PbSe/PbSexS1�x core/alloyed-shell QR heterostructures withx ¼ 0.27.

Fig. 13 Absorption and cw-PL spectra of small-sized PbSe/PbS core/shell QDheterostructures and the corresponding different ultra-small PbSe core QDs.


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should be noted that the growth in length is delicately balancedby a change of the reaction temperature within �20 �C. Arepresentative fast Fourier transform of a TEM image of ternaryQRs is shown in the inset of (a), conrming a rock-salt crystal-lographic structure. Similar rock-salt structures appeared in allthe QRs investigated.

A TEM image of PbSexS1�x QRs of Fig. 12(a) deposited on aTEM grid is displayed in Fig. 12(c), showing relatively uniformdimensions, with an average size deviation of �5% in radiusand �15% in length of the QRs. Experimental evidence of QRscreation is given by the absorption line prole, which is notsymmetric. The spectra recorded by monitoring the absorbanceduring the reaction, with a time delay of one second, are pre-sented in Fig. 12(d). The spectra are initially characterized bythe existence of an exciton band, related to the band-edgetransition and considered as Eg. This band shis steadily toa longer wavelength with the growth progression; however, at acertain stage, the energy shi decelerates, together with achange in the curve prole, resembling typical rod-like absor-bance and the additional appearance of a weak ne structure. Itshould be noted that the value of Eg is predominantly controlledby the radius of the QRs, and mildly inuenced by their lengthabove 10 nm.

Fig. 12(e) gives an image of QRs with a PbSexS1�x composi-tion (typical length/radius ratio of 25 nm/1.5 nm). The Pb/Se/Satomic ratio within the QRs was measured by EDX, monitoringan ensemble of structures. The local composition along themain axes of a single QR was measured by high-angle annulardark-eld scanning transmission electron microscopy (HAADF-STEM) with a spatial resolution of 0.1 nm (not shown), revealinga homogeneous distribution of Pb, Se, and S atoms through thewhole volume of a QR and conrming the formation of theternary PbSexS1�x compound in the QRs.


4 Optical properties of PbSexS1�x/PbSeyS1�y (0# x(y)# 1) QD heterostructures4.1 Continuous-wave photoluminescence measurements atroom temperature of PbSexS1�x/PbSeyS1�y QDs

This section emphasizes the optical properties of PbSe/PbS, PbSe/PbSexS1�x and PbSexS1�x/PbSeyS1�y heterostructures, with vari-able core-to-shell divisionanda radial gradientcomposition (when0 # x( y) # 1). A thorough investigation of the optical propertieswas performed by continuous-wave (cw) photoluminescence (PL)spectroscopy, recorded under oxygen-free environment at roomtemperature (RT). The variation of the properties with a change intemperature is addressed in Section 4.2.

Representative absorption (red lines) and cw-PL (blue lines)spectra of PbSe/PbS QDs with an average total radius of 1.7 �0.2 nm (with a core radius of 1.1 nm) are shown in Fig. 13. Thesamples are dispersed in 3-(trimethoxysilyl)propyl methacrylateglass solution (GS). The core/shell spectra exhibit a systematicred shi of the absorption and emission bands, when comparedwith the corresponding cores. The band-edge energies of thesePbSe/PbS heterostructures center around 1.4 eV, while those ofPbSe QDs are centered between 1.55 and 1.7 eV. The sizedependence of the cw-PL bands is indicative of their associationwith band-edge (1Se–1Sh) transitions.

The cw-PL bands exhibit an energy Stokes shi from its rstabsorption bands (as marked on the gure), which graduallyincreases when the PbSe core radius drops. The energy Stokesshi in PbSe/PbS QDs (top curves in Fig. 13) was found to bearound 140 meV, which is smaller than that observed in theinitial PbSe cores (bottom curves in Fig. 13). The observationssuggest that the Stokes shi depends on the overall QD diam-eter. The theoretical calculations presented in Fig. 5 show thatin the strong connement regime, the band-gap states lie abovethe core/shell interface potential barrier. As a result, the carriersare delocalized over the entire core/shell structure and, conse-quently, the overall diameter appears to be the main factordetermining the decrease of the Stokes shi. The shi of140–460 meV seen in Fig. 13 is signicantly larger than that

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Fig. 14 (a) Absorption (dashed lines) and emission (solid lines) spectra of PbSecore QDs, and various core/shell QD heterostructures with the Rs./Rc ratio, asgiven in the legend. The exact composition and the radius of QDs are labeled inthe figure. (b) Calculated (empty circles) and experimental (filled circles) energyband gap values of the QDs given in the legend. (c) Stokes shift energy (ES) of thevarious QDs as indicated in the legend.


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expected119,120 for spherical PbSe QDs and may be explained by:(a) an exceptionally large exchange interaction in small-sizedIV–VI QDs; (b) existence of a Franck–Condon shi121,122 that canbe manipulated by off-resonance excitation or/and size distri-bution; (c) the li of four-fold degeneracy at the L-point of theBrillouin zone.42,98,123 Previous theoretical calculations esti-mated the existence of exchange splitting86,92,120 and a Franck–Condon shi121,122 in the range of 20–30 meV, which is smallerthan the Stokes shi observed in the present study. Also, theobserved Stokes shi was not affected by a change in the exci-tation energy. Liing of the four-fold degeneracy allows excita-tion in one valley and emission from another valley, whichoccurs in nanostructures with a minor deviation from a spher-ical shape that might be undetectable in a TEM image.42 Mostlikely, a slight shape distortion of the PbSe cores is maintainedaer homogenous covering of a PbS shell. A theoreticalapproach evaluating the inter-valley splitting both in PbSe andPbSe/PbS core/shell QDs was discussed in Section 2.1. Thistheoretical approach supports a large Stokes shi due to a li ofband-edge degeneracy, if a small elliptical shape distortion(with an aspect ratio of �1.1) could be assumed.107

The discussion below compares optical properties of large-sized QDs with different dimensions and compositions, exhib-iting band-gap energy in the range of 1.0–0.92 eV. Representa-tive absorption (dashed lines) and cw-PL (solid lines) spectra ofa few QDs are shown in Fig. 14(a). The cw-PL spectra werepumped by a non-resonant excitation (Eex ¼ 1.54 eV). Thebottom curve and top curve in Fig. 14(a) correspond to spectraof PbSe core QDs with average radii of Rc ¼ 1.5 � 0.2 nm and Rc

¼ 2.4 � 0.2 nm, respectively. The middle curves representdifferent QD heterostructures (core/shell, core/alloyed-shell andalloyed-core/alloyed-shell), each with a composition as indi-cated in the legend, when Rc ¼ 1.5 nm and Rs is either 2.1 nm or2.4 nm. This set of curves suggests the occurrence of a red shiof the absorption and emission spectra of the core/shell heter-ostructures with respect to those of their primary cores, but theyare blue-shied with respect to cores with a radius of 2.4 nm.This midway shi is related to a quantum size effect combinedwith composition tuning of the band gap.

The measured Eg, estimated by the rst exciton absorptionband, and the corresponding calculated values includingcorrection for the Coulomb interaction (discussed in Section 2)are compared in Fig. 14(b), showing qualitative agreementbetween the theoretical and experimental results. Table 1includes comparison of the experimental and theoretical valuesof Eg of various IV–VI QDs with a few other relevant parameters,i.e. quantum yield (h), radiative lifetime (srad), and emissionenergy Stokes shi (ES). The value of the cw-PL QY (h) wasmeasured using the integrating sphere technique at RT.124

Systematic improvement of h (up to 68%) was found in core/shell, core/alloyed-shell and alloyed-core/alloyed-shell hetero-structures, in contrast to a typical value of �48% found in theprimary core QDs. The relatively high h might be related to theimproved quality of surfaces, e.g., close crystallographic matchbetween the core and the shell constituent. Moreover, theincrease of the sulfur content at the exterior surface creates alower oxidation outcome.

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The cw-PL spectra are displayed in Fig. 14(a), eachshowing an asymmetric band whose maximal point is Stokesshied from the corresponding rst exciton absorption bandby energy, ES. Fig. 14(c) displays a plot of ES versus the valuesof Eg of the QDs listed in the legend. The emission energyStokes shi in the listed QD heterostructures may be relatedto the following factors: (i) an increase of the overall size ofthe QDs by the growth of the shell width; (ii) a variation ofthe valley–valley and electron–hole exchange interactions withrespect to that found in pure PbSe cores, due to a quasi-type-II band alignment. It is important to note that the value of ESin core/alloyed-shell and alloyed-core/alloyed-shell QDs issmaller than that found in pure cores; however, it is slightlylarger than that found in pure core/shell QDs of the sameoverall size (see Table 1). The variation in ES has also beenfound in the past in alloyed II–VI125 and III–V QDs,126

uniquely explained in those materials, as a consequent effectof optical bowing.127



Table 1 Relevant parameters of the investigated QDs at RT: Pb, Se, S percentages; core radius (Rc); overall radius (Rs); band gap energy (Eg); QY (h); radiative lifetime(srad); and Stokes shift (ES)

Molecular formula of QDs Pb [%] Se [%] S [%]Rc[nm]

Rs[nm]

Eg exp.[eV]

Eg calc.[eV]

h

[%]srad[ms]

ES[meV]

PbSe 77 22 0.0 1.1 1.1 1.65 1.9 58 6.1 280PbSe 51 48 0.0 1.5 1.5 1.17 1.03 48 3.0 112PbSe/PbS 55 17 27 1.1 1.7 1.32 1.50 62 6.6 140PbSe/PbS 55 17 27 1.5 2.1 1.03 1.10 55 4.5 55PbSe/PbSe0.68S0.32 50 40 10 1.5 2.1 1.00 1.14 68 4.2 75PbSe0.5S0.5 51 25 24 1.6 1.6 1.18 1.30 27 5.5 103PbSe0.5S0.5/PbSe0.24S0.76 49 20 30 1.6 1.9 1.09 1.40 46 4.4 73PbSe0.5S0.5/PbSe0.27S0.73 48 17 33 1.6 2.4 1.02 0.97 65 4.2 45


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4.2 Thermally activated processes in PbSeyS1�y/PbSexS1�x

QDs

Thermally activated processes in the QDs were studied byfollowing variation in the emission spectrum as a function oftemperature. Fig. 15(a–c) illustrate a set of cw-PL spectra ofPbSe, PbSe/PbSe0.7S0.3 and PbSe/PbS QDs, recorded at varioustemperatures from 1.4 to 300 K. The QDs had a similar coresize; they were dispersed in GS, were kept under oxygen-freeconditions, and were excited at 1.54 eV. The cw-PL spectraexhibit an energy red-shi with a decrease in temperature,resembling a trend found in low energy band-gap semi-conductors.128 Also, a majority of the PL curves have anasymmetric emission band, presumably consisting of twooverlapping recombination events, where the energy split var-ies from 55 meV to 80 meV, and it is the smallest for thelargest shell width and S/Se ratio. Interestingly, the split energyclosely retains its value with the increase in the temperature,although the high-energy component becomes the dominantband at elevated temperatures. The nature of the emissionbands was examined by a variation of the pumping intensity.We found that the spectra maintain the relative intensitybetween the red side and the blue side of the emission bands.We excluded existence of multiple excitons emission understrong pumping conditions due to the long lifetime of theentire emission bands (vide intra). The possibility that a trap-ped carrier recombination occurs in the present case was alsoexcluded, due to the lack of the saturation effect at the highestpumping power. More likely, the entire emission spectrum isrelated to a band-edge recombination. It is important tomention that the appearance of an asymmetric band or eventwo distinct emission bands in non-resonant excited cw-PLspectra of colloidal IV–VI QDs has already been reported inprevious studies,2,68,92,129,130 proposing the following routes,based on either theoretical42,86 or experimentalevidence:68,79,92,130–133 (a) exciton ne structure near the bandgap; (b) overlap of two events, one related to band-edgerecombination and the second to trap-to-band recombina-tion;69 (c) an inter-valley coupling at the L-points of the Bril-louin zone.5,86,126,129 Further, uorescence line narrowing ofbinary PbSe QDs showed Stokes and anti-Stokes energy shiemission bands with respect to the absorption edge,


associated with the existence of two bands.119,120,134 Rarely, wehave seen a split-exciton absorption band (unpublishedresults), which is unexpected in spherical structures unless ageometric distortion is present. Thus, the presence of twobands may be related to the degeneracy split of band-edgestates due to a slight shape anisotropy of the QDs.

Fig. 15(d) shows a plot of the normalized cw-PL integratedintensity (IPL) versus temperature (T) of a few oxygen-free core/shell and core IV–VI QDs. The values of IPL(T) include inte-gration over the entire emission spectrum, and IPL at eachtemperature is normalized with respect to the maximal valueat RT as observed under air-free conditions. The plots arecharacterized by a mild increase of IPL(T) between 5 K and 50K, correlated with a phonon-assisted process from a dark-to-bright exciton emission,135 followed by small quenching thatmay be associated with carrier trapping. Eventually >150 K, asharper intensity growth is achieved up to RT. The optimalintensity at RT is correlated with the recombination naturefrom a bright exciton state,135 which is presumably enhancedby a phonon-assisted de-trapping from shallow non-radiativedefect states.

Fig. 16 displays a set of measurements reecting plausibledeterioration of IV–VI QDs on exposure to air. The cw-PLspectra of PbSe core QDs covered with oleic acid (in GS) areshown in Fig. 16(a), illustrating reduction of the lumines-cence intensity with the increase of the temperature, aer airexposure over 20 minutes. Such a drastic effect may beassociated with thermally activated trapping at elevatedtemperature into defect states associated with oxidation sitesat the exterior surfaces.132 Fig. 16(b) exhibits cw-PL curves ofPbSe/PbS QDs with a diameter of 2.7 nm, recorded at asimilar temperature range and exposure time as in Fig. 16(a),but the air environment did not quench the luminescenceintensity in the shell coated QDs. In contrast, someenhancement of IPL is observed at RT. Fig. 16(c) displays thechange in normalized IPL versus T of the QDs discussed inFig. 16(b), with a variation in exposure time from 20 to 200minutes. The plot indicates that a limited exposure time ispossible without dramatic change of the luminescence QY,above which an oxidation process may occur, with a subse-quent quenching of the luminescence intensity.68,132 Althoughthe chemical stability of the QDs is restricted to a limited

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Fig. 15 cw-PL spectra of (a) PbSe QDs, (b) PbSe/PbS core/shell QDs, (c) PbSe/PbSe0.7S0.3 core/alloyed-shell QDs with the Rs/Rc ratio as indicated in the legend.(d) Plot of the normalized cw-PL integrated intensity (IPL) versus temperature (T) ofvarious colloidal QDs as given in the legend.

Fig. 16 cw-PL spectra of (a) PbSe QDs, (b) PbSe/PbS core/shell QDs with theRs/Rc ratio, as given in the figure. The samples in (a) and (b) were dispersed in GSand were recorded after an exposure to air atmosphere for a time duration of 20minutes. (c) Plot of the normalized IPL versus T of a QD shown in (b), with avariation in exposure time (from 20 to 200 minutes) to air environment.

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duration, it can be benecial for certain processes in theQD-based device fabrication.

4.3 Time-resolved photoluminescence investigations ofPbSexS1�x/PbSeyS1�y (0 # x(y) # 1) QDs at variabletemperatures

Time-resolved PL (tr-PL) measurements at various tempera-tures were used to investigate the nature of the radiativeprocesses, quantum efficiency and for further support of thedark-to-bright activation. The tr-PL spectra were measured byexciting the sample at 1.17 eV and following the PL intensity



Fig. 17 (a) Representative PL decay curves of PbSe core and core/shell QDs withRc and the Rs/Rc ratio, as given in the legend, measured when dispersed in GS. (b)srad dependence on the size and composition of the QDs listed in the legend,dispersed in GS. (c) PL decay curves of PbSe core QDs, dispersed in different media(see the legend).

Table 2 Room temperature values of the measured lifetime (s0), quantumefficiency (h) and radiative lifetime (srad) of PbSe QDs with Rc ¼ 2.2 nm, dispersedin different media

Solvent Size (nm) s0 (ms) h (%) srad (ms)

CH3Cl 4.7 1.07 39 2.74GS 4.7 1.17 48 1.67Hexane 4.7 1.14 60 1.29PMMA 4.7 1.28 7 4.03H2O 4.7 0.78 9 8.69


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decay process over time. A low power excitation ensured theformation of a single exciton, best tted to a single exponent,I(t) ¼ Aexp(�t/s0), with measured intrinsic PL decay time s0.Fig. 17(a) displays PL decay curves of PbSe and PbSe/PbS QDswith Rc and the Rs/Rc ratio as indicated in the legend. For themost part, the value of s0 decreased from 3.5 ms to 1.4 ms, withthe growth of the core radius from Rc ¼ 1.1 to 2.4 nm, andwith the variation of core-to-shell division.60,136,137 The values ofs0 at each temperature depend on the radiative (srad) andnonradiative (sNrad) processes, according to the followingrelation:138,139

1

s0ðTÞ ¼1

sradðTÞ þ1

sNradðTÞ (2)


The integrated PL intensity (IPL) and quantum yield, h, arerelated to each other in the following manner:

hðTÞ ¼ IPLðTÞI0

¼ s0ðTÞsradðTÞ (3)

where I0 is the exciting photon ux, in the current case assumedto be close to the maximal PL intensity at RT when measuredunder air-free conditions. srad is the meaningful quantity toconsider when comparing the behavior of core/shell versus coreQDs or versus literature reports.60,87

Fig. 17(b) shows a plot of srad versus the size of a few PbSeQDs,PbSe/PbS QDs, and PbSeyS1�y/PbSexS1�x QDs, as listed in thelegend.Theguredesignates that the valueof srad is reduced from6.5 ms to 2.7 ms with the growth of a PbSe core radius (See alsoTable 1). The origin of this trend is still questionable. However,someelucidation is gained from the theoreticalworkdescribed inSection 2.1b, suggesting partial electron–hole separation (seeFig. 4, middle row) in pure core PbSe QDs – a separation thatincreases with the decrease of the QD size (unpublished results).The values of srad are substantially longer (a few ms) than thosefound in II–VI semiconductor QDs (�20 ns), a fact that can beconnected to a relatively high dielectric constant of the PbSesemiconductor (3N ¼ 24) or to a possible contribution fromadjacent S and K Brillouin zone extremes.85 The arrows in (b)designate the increase of srad from core QDs (black symbols) tothat of the corresponding core/shell (red symbols) QDs. Thisincreaseof the srad canbeassociatedwithpotential eliminationofthe trapping site by the growth of the crystalline-matchedepitaxial shell, by the quasi-type-II character of the hetero-structures, or/and by the total increase of the QDs diameter.

The IV–VI QDs discussed can be employed in various opto-electronic devices, requiring dispersion in different media, suchas a poly(methyl methacrylate) (PMMA) polymer21,28 or waterenvironment.28 Representative tr-PL decay curves of 4.4 �0.4 nm PbSe QDs dissolved in several different solutions(hexane, chloroform, GS, water) or dispersed in the PMMApolymer lm are presented in Fig. 17(c), all showing singleexponential behavior. Table 2 summarizes the values of the sradand h of PbSe QDs having the same diameter while dispersed indifferent media. The pronounced differences given in the tablecan be related to a different surface quality, on variation of thesurrounding ligands and solvent, as well as to a small variationof the dielectric constant of the surrounding medium.29

The value of s0 for PbSe QDs dissolved in hexane, chloro-form, and GS is approximately 1.1 ms, while s0 values for thesame QDs dissolved in water or embedded in the PMMA

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polymer are <1.0 ms. Destruction of the organic capping by acertain medium might induce a substantial reduction in thevalue of h. Also, it should be noted that water soluble QDsunderwent a ligand exchange due to thiol-amine moleculeshaving less protection capability, which falls below that ofthe oleic acid. Additionally, water and PMMA media inducea surrounding with a relatively high dielectric constant(3N(water) ¼ 5.5, 3N(PMMA) ¼ 4.0) differing from other organicmedia (3N(hexane) ¼ 1.94). Indeed, the lifetime of the QDsembedded in PMMA and water solution is scaled by a factor of[3 3i/(23i + 3QD)]

�2 when 3i corresponds to the dielectric constantof the surrounding.29 The value of srad of QDs dissolved in wateris scaled by a factor of 3.2–4.5 with respect to QDs dissolved inorganic media; see Table 2. However, the value of srad of QDsdispersed in PMMA is scaled by a factor of only 1.45–2.0 withrespect that of QDs dissolved in organic media. It can beconcluded that the QD surface-capping quality with thiol-amine molecules has a bigger inuence on the radiative processin QDs.

Representative tr-PL curves of PbSe/PbS core/shell QDsrecorded under air-free conditions at various temperatures areshown in Fig. 18(a). Low power excitation measurements of PLdecay curves were tted to single exponent or two exponentfunctions, showing an obvious change with a variation of thetemperature. The values of srad derived from eqn (2) and (3) wereplotted versus themeasured T in Fig. 18(b) for the QDs indicatedin the legend. h was measured at RT by integrating sphere(discussed above). The plots reveal a long srad at cryogenictemperatures, which changes at elevated temperatures. Such a

Fig. 18 (a) Representative transient PL curves of PbSe/PbS core/shell QDs withthe Rs/Rc ratio as indicated in the legend. QDs dispersed in GS and recorded atvarious temperatures, as indicated in the legend. (b) Variation of the srad versusthe temperature of the various sizes and compositions of QDs with the Rs/Rc ratioas given in the legend.

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change is explained by the existence of electron–hole exchangeinteractions, which split exciton electronic states into brightand dark components. The dark component is the lowest inenergy and dominates the emission at cryogenic temperatures.However, srad changes dramatically at elevated temperatures,when bright state recombination becomes a dominant event.The dependence of srad on the dark–bright thermalization isgiven in the following relation:87,92,140

1

srad¼ 3=sd þ ð1=sbÞexpð � Dex=kBTÞ

3þ expð � Dex=kBTÞ (4)

where Dex is an exchange split energy; sb and sd correspond to aradiative process from a bright and a dark state, respectively.79 Atthe lowest temperature (1.4 K), sd¼ srad(1.4 K), and is viewed as a“frozen exciton” with a lifetime of �10 to 40 ms. The best tprocess, using eqn (4), to the curves shown in Fig. 18(b) revealedvalues for Dex from 13 to 18 meV and sb values in the range of100–650 ns, consistent with results reported previously.79 Thevalues of sb decreased with the increase of the QDs size, associ-ated with the reduction of Dex, and the enhancement of brightand dark states mixing in the larger QDs. The bright-dark energygap depends on the QDs size being 10–30 meV in sphericalQDs,120 and <1meV in elliptical or elongated structures (e.g., rodsor wires).42 The results shown in Fig. 18 reveal a longer srad inPbSe/PbS core/shell QDs with respect to that of PbSe core QDs.92

Interestingly, the values of srad vary only slightly across the wholetemperature range under examination in core/alloyed-shell QDs.A small variation is a consequence of a small exchange split darkand bright states in the alloyed QDs.141

4.4 Optical properties of PbSexS1�x and PbSe/PbSexS1�x QRsat different temperatures

The optical properties of alloyed QRs were examined in a similarmanner to the study of the analogous QDs. Fig. 19(a) illustratesRT absorption and cw-PL spectra of PbSe0.6S0.4 QRs with avariable size, dissolved in GS. The cw-PL bands show an energyStokes shi relative to the rst absorption band, whichdecreases gradually with the increase of the QRs' size. Fig. 19(b)displays a plot of ES versus the band gap energy of variousalloyed PbSexS1�x QRs, compared to an emission energy Stokesshi found in relevant binary QRs and QDs and observed at RT.In general, the Stokes shi in QRs is larger than that of QDswith similar Eg values, suggesting a larger binding energy of anexciton in elongated structures.

Representative cw-PL spectra of alloyed PbSexS1�x (x ¼ 0.6)QRs with an average length/radius ratio of 10.4 nm/2.1 nm areshown in Fig. 20(a); the spectra were recorded at varioustemperatures and under oxygen-free conditions. The cw-PLspectra in Fig. 20(a) are shied to higher energies withincreasing temperature. The cw-PL band always has an asym-metric appearance and may occasionally show a second weakband, depending on the dimension of the QRs.

The characteristic appearance of an asymmetric or satelliteband may be related to a break of degeneracy of extreme pointsat the L point of the Brillouin zone, which was discussed abovein the case of elliptical distortion of QDs. However, the QRs



Fig. 19 (a) RT absorption and cw-PL spectra of a few PbSe0.6S0.4 QRs withdifferent sizes as given in the legend, dissolved in GS. (b) Plots of the energy Stokesshift observed at RT of QDs and QRs as listed in the legend.

Fig. 20 (a) Representative cw-PL spectra of ternary PbSe0.6S0.4 QRs with anaverage length/radius ratio of 10.4 nm/2.05 nm recorded at various tempera-tures. The dashed line indicates the absorbance of the QRs, recorded at RT. (b) Plotof the FWHM of the cw-PL emission band versus T of various QDs and QRs. Therepresentative best-fit curve (see the text) is shown by a solid line. QDs and QRshave the same first-exciton absorption energy �0.925 eV. (c) Plot of thenormalized IPL versus temperature for PbSe and PbSe/PbSe0.7S0.3 core/alloyed-shell QR heterostructures.


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exhibit a larger aspect ratio (length/width). Fig. 20(b) exhibits aplot of the full-width-half-maximum (FWHM) of the entire(anisotropic) emission band versus the temperature (T) of theQDs and QRs listed in the legend. The symbols designateexperimental points. A best-t to these points was implementedusing the Bose–Einstein relation:142,143

G(T) ¼ Ginh + sT + GLO/(eELO/kBT � 1) (5)

where G(T) is the emission band FWHM, Ginh is the inhomo-geneous broadening parameter at T ¼ 0, s is an acousticphonon coupling, GLO is an optical phonon coupling, and ELO isthe longitudinal optics phonon energy. It should be noted thatthe value of ELO varies with size,79 shape,144 and composition(unpublished results) of the QRs; however, a constant value of16.8 meV was used in the above relation for a set of QRs withrelatively narrow size dispersion. The solid line shows a repre-sentative best-t curve. The theoretical t for all samples (notshown here) revealed the following values s ¼ 0.02, 0.04, 0.03,and 0.02 meV K�1 and GLO ¼ 26, 22, 59, and 41 meV forPbSexS1�x QRs, PbSe QRs, PbSe QDs, and PbSexS1�x QDs,respectively. The values reect reduction of optical phononcoupling in QR structures with alloy composition, suggestinglocalization of phonons in alloyed materials, as oen found inbulk semiconductors.145

Fig. 20(c) displays a plot of the normalized IPL versus T for theoxygen-free PbSe and PbSe/PbSe0.7S0.3 QR heterostructures. Theplots suggest a minor change in IPL over the 4–150 K tempera-ture range. However, above that temperature, a sharp intensityincrease is seen in the temperature-dependent curve of bothsamples only under oxygen-free conditions. The tolerance to a


short exposure time to air is similar to that found in IV–VI QDs(see Section 4.3).

Representative tr-PL decay curves of PbSe0.6S0.4 QRs, moni-tored at the peak emission energy and at various temperatures,are given in Fig. 21(a). These decay curves were tted to singleexponent or two exponent functions, always with a dominantdecay component, showing lifetime increase with decreasingtemperature. The relationship between s0 and srad was deter-mined by examining normalized IPL(T) at various T using asimilar procedure previously described in the analysis of similarcolloidal QDs. Fig. 21(b) displays a plot of the values of srad versusthe temperature of alloyed PbSe0.6S0.4 QRs and PbSe/PbSe0.7S0.3QR heterostructures. Over the entire temperature range, there isonly a minor change in srad with respect to those of core/shellQDs (Fig. 18(b)). The relatively large exchange interaction inIV–VI QDs implies the occurrence of an emission process from a

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Fig. 21 (a) Representative PL decay curves of PbSe0.6S0.4 QRs at varioustemperatures. (b) Plot of srad versus temperature for ternary PbSe0.6S0.4 QRs andPbSe/PbSe0.7S0.3 core/alloyed-shell QR heterostructures.


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dark state at cryogenic temperatures, but a dominant emissionfrom a bright state at room temperature (see Fig. 18(b)). Incontrast, these results showed that the variation of the emissionlifetime is minor in QRs, in agreement with the small exchangesplit (�180 meV) calculated by the theoreticalmodel described inSection 2.2; hence, both dark and bright states are thermallypopulated over the wide temperature range under investigation.The relatively low values of srad in QRs are in correlation withtheoretical considerations, suggesting reduction of the lifetimedue to anisotropic dielectric screening in elongated structures.101

The inuence of alloying on the lifetime of the QRs is negligiblewith respect to that of reduced screening in QRs.

5 Applications

PbSe, PbSe/PbS core/shell and PbSe/PbSexS1�x core/alloyed-shell QD heterostructures with a band gap of 0.8 eV, dispersedin the PMMA polymer, were used in the eye-safe laser ofEr:glass. The absorber saturation investigations revealed arelatively large ground state cross-section of absorption (sgs y10�15 cm2) and behavior of a “fast” absorber with an effectivelifetime (seff y 4.0 ps). The seff was associated with the forma-tion of multi-excitons at the measured pumping power. Theproduct of sgs and seff enables a sufficient Q-switching perfor-mance and tunability in the near-infrared spectral regime. Theimproved properties were achieved using PbSe/PbSexS1�x core/alloyed-shell QD heterostructures.21,22,120

The gain devices of the discussed colloidal QDs were exam-ined, showing an amplied spontaneous emission (ASE) underconditions that are suitable for technological devices, such asoptical pumping by a continuous diode laser under RT condi-tions. The optical gain and the ASE properties were determinedwith variable pump intensity on a 4.2mmthick,QD-dopedPMMAlm (arranged on a Si wafer) with a variable (0.01–0.2 cm) stripelength, using a cw laser diode at 980 nmas the pump source. Thepump beam is concentrated with a cylindrical lens, and a narrowstripe of uniform intensity is selected using a 300 mm wide slitmade of an aluminum foil directly deposited on the lm surface.The ASE band is clearly narrower than the spontaneous emissionband, but its width is independent of the pumping power, sug-gesting the absence of inhomogeneous broadening due to theQD size distribution. The results revealed a relatively large gainparameter (g ¼ 2.63–6.67 cm�1).120

Solid state high efficiency QDs/TiO2 heterojunction solarcells, which used small-sized PbSe/PbS core/shell QD

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heterostructures with a band gap in the range of 1.1–1.3 eV,achieved improved parameters, such as a power conversionefficiency (h) of 4.5% with a short circuit photocurrent ( Jsc) of17.3 mA cm�2. The PbS shell covering the PbSe core should offersufficient protection from a fast-oxygen penetration, providingthe PbSe/PbS QD heterostructures with low surface trapping,chemical and photo-chemical stability, and contribute to thesuccess of their integration into solar cell devices.20

6 Summary

The benets of the IV–VI nano-heterostructures discussedextensively in the current review could be of signicant impor-tance in applications where the material size is restricted.Examples appear in biological platforms, demanding QDs ofsmall dimensions, or in various opto-electronic applications,demanding closely packed self-assembled QDs of relativelysmall dimensions. However, all applications still have stringentrequirements regarding the optical tunability. We showed thatrestrictions can be overcome by the new strategies which gaincontrol of QDs' properties by using (a) alloyed ternary orquaternary compounds, with homogeneous or graded compo-sition along a selective direction; and (b) core/shell hetero-structures, consisting of a semiconductor core, covered by ashell of another semiconductor, when the band-edge offset atthe core/shell interface can be tuned from a type-I throughquasi-type-II to a type-II alignment and one of the constituents(core or shell) may have an alloy composition.

Unique alloyed colloidal core/shell heterostructures, such asPbSexS1�x/PbSeyS1�y, (0# x( y)# 1) QDs and QRs, were featuredin this document. They possess tunable band-gap energy and ahigh crystallographic and dielectric match between the core andthe shell. Moreover, the overall electronic properties and carrierdistribution functions can be regulated by the core/shell archi-tecture. Further, these heterostructures show exceptionally highemission QY, chemical and photochemical stability. The elec-tronic band structure calculation of the discussed hetero-structures was determined by using a k$pmodel, covering a widerange of physical aspects, including an effectivemass anisotropyand dielectric variation between the constituents, indicatingpossibilities for tailoring heterostructures with the relevantcomposition and optical properties. A thorough investigation ofthe optical properties was performed by recording variable-temperature continuous-wave and time-resolved PL spectra.Energy shis and band-edge temperature stability were exploredby alleviating dark–bright splitting by exchange and/or valley–valley interactions, and by emission QYs and radiative lifetimes.The results were compared to the existing known properties ofprimaryPbSe structures. The results reect theuniqueness of theelectronic properties of the heterostructures, which arecontrolled by the shell thickness and the alloy composition.

Acknowledgements

We thank G. Kventsel for helpful discussions, providing manyinsightful comments; A. Bartnik and F. Wise for helpfuldiscussions and guidance on the theoretical model; A. Efros for




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useful scientic discussions; and O. Solomeshch for assistancein the QY measurements. We thank D. Kolan for the prepara-tion of colloidal QRs, and Y. Kauffmann and D. Kolan for thestructural characterization of the rods. We thank M. Saraf forcarrying out the scanning tunneling spectroscopy measure-ments. We thank L. Etgar for implementing the QDs in photo-voltaic cells. We thank A. Q. Le Quang, I. Ledoux-Rak, and J. Zyssfor assisting in the gain device measurements. We acknowledgethe support from the Israel Science Foundation (project no.1009/07 and 1425/04), the USA-Israel Binational Science Foun-dation (no. 2006-225), the Israel Ministry of Science (no. 3-896),and the European FP7 SANS and Nanospec projects.

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Tuning of electronic properties in IV–VI colloidal nanostructures by alloy composition and...

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