Z. Phys. Chem. 2015; 229(1–2): 85–107Angshuman Nag, Hao Zhang, Eric Janke, and Dmitri V. Talapin*Inorganic Surface Ligands for ColloidalNanomaterialsAbstract:Since the discovery of metal chalcogenide complexes (MCCs) as cap-ping ligands for colloidal nanocrystals (NCs) in 2009, the chemistry of inorganicligands for NCs has provided a new paradigm for surface design of nanomateri-als. Various inorganic anions including MCCs, metal-free chalcogenides, oxoan-ions/oxometallates, and halides/pseudohalides/halometallates have been em-ployed to replace the original long-chain organic ligands on NCs. This ligand ex-change can also be achieved through a two-step route using ligands strippingagents like HBF4. This review outlines recent advances in inorganically-cappedcolloidal NCs and details the ligand exchange process for NCs using MCCs andmetal-freechalcogenides.ThebindingaffinitiesofligandstoNCsurfacehavebeenrationalized in terms of Pearson’s hard and soft acids and bases (HSAB) princi-ple. We also demonstrate that inorganic ligands broaden the functionality of NCsby tailoring their electro-optical properties or generating new inorganic phasesthrough chemical reactions between nanomaterials and their surface ligands. Es-pecially promising are the electronic, optoelectronic, and thermoelectric applica-tions of solution-processed, inorganically-capped colloidal NCs, which substan-tially outperform their organically-capped couterparts.Keywords:Inorganic Ligands, Nanocrystals, Surface Chemistry, Semiconductors,Electron Transport.DOI 10.1515/zpch-2014-0604Received September 3, 2014; accepted October 23, 2014||Dedicated toHorst Weller on the occasion of his 60thbirthday*Corresponding author: Dmitri V. Talapin,Department of Chemistry and James Franck Institute,University of Chicago, Chicago, IL 60637, USA; and Center for Nanoscale Materials, ArgonneNational Laboratory, Argonne, IL 60439, USA, e-mail: dvtalapin@uchicago.eduHao Zhang, Eric Janke:Department of Chemistry and James Franck Institute, University ofChicago, Chicago, IL 60637, USAAngshuman Nag:Department of Chemistry and James Franck Institute, University of Chicago,Chicago, IL 60637, USA; and Department of Chemistry, Indian Institute of Science Educationand Research (IISER), Pune, 411008, India
86| A. Nag et al.1 IntroductionNanocrystals (NCs) are the smallest functional units of inorganic solids and ex-hibit many intriguing size-dependent phenomena. Colloidal synthesis [1–3]ofNCs provides a route to inorganic functional materials dispersed into a desiredsolvent for solution-processed and low-cost device fabrication [4,5], variousbiomedical applications [6], and formation of composites [7]. Remarkably, thisapproach allows design of novel materials based on nano-heterostructures [8],which integrate materials with desired bulk properties on nano-scales. Col-loidally grown structures that have been realized to date include: semiconduc-tor in semiconductor core/shell (e. g., CdSe/ZnS [9]), metal in semiconductorcore/shell(e. g.,Au/PbS[10]),magnetinsemiconductorcore/shell(e. g.,FePt/PbSor FePt/CdSe [11,12]), or shape-controlled CdSe spherical core/CdS tetrapodshell [13]. Assembly of these nano-scale building blocks into three dimensionallattices offers a route to functional materials that are designed and synthesizedthrough solution-phase chemistry [14].Synthesis of high-quality colloidal NCs generally involves reacting precursorcompounds in the presence of surfactant species containing long-chain hydrocar-bons. These molecules bind as ligands to surface atoms of the NCs thereby medi-ating the growth of NCs during the synthesis and stabilizing NCs in a nonpolarreaction medium after synthesis. Due to their labile nature, these ligands can beexchanged for new stabilizing layers with tailored properties. For example, nativehydrophobic ligands can be exchanged for hydrophilic ones to achieve a biocom-patible aqueous dispersion [15]. The use of site-selective bio-targeting moleculesas ligands allows for a general method of bio-functionalization of the NC surface.In additionto affecting the interaction between NCs and the surrounding mediumor other species, ligands can alter the electro-optical properties of NCs throughtheir binding interactions with NC surface. Modifications in (i) luminescence [16],(ii) doping of charge carries in NCs [17], and (iii) modulation of exciton confine-ment [16] have been demonstrated by tuning the ligands of NCs.Cappingligands play a crucialrole in functionalizing colloidal inorganic NCs,complementing their inherent size-dependent behavior. However, the insulat-ing nature of long-chain organic ligands renders a disadvantage in interparticlecharge transport between NCs. Films cast from metallic or semiconducting NCsthat retain native organic ligands are highly insulating [18]. This inefficient injec-tion or extraction of charge carriers hinders the integration of NCs in electronicand optoelectronic devices such as solar cells, light-emitting-diodes (LEDs), pho-todetectors, and printable electronic circuits.
Inorganic Surface Ligands for Colloidal Nanomaterials |87To address this issue, we introduced a new concept of inorganic ligandswhich included chalcogenidometallates (MCCs, Sn2S64−,In2Se42−,etc.)[18–20], metal-free chalcogenides (S2−,Se2−,Te2−,SCN−,etc.)[21,22], oxoan-ions/polyoxometallates (POMs) [23], halides, pseudohalides, and halometal-lates [24,25]. These inorganic species replace the native organic ligands on col-loidal NC surfaces through a post-synthesis exchange process (for example, Fig-ure1a). The inorganic ligands provide colloidal stability for NCs in polar solvents,can passivate surface traps, and are electronically transparent. NCs capped withinorganic ligands show dramatically enhanced electronic coupling and chargetransport properties [26,27]. Furthermore, a solid-state reaction between the NCand inorganic ligandscanbedesigned toform semiconductor thinfilmsofvariousdesired compositions, starting from soluble precursors. The use of inorganic lig-ands provides a novel route to prepare functional NC solids. Here, we summarizethe major advances in the chemical design of inorganic ligand capped colloidalNCs and NC solids for various electronic, optoelectronic, and thermoelectric ap-plications.2 Diversity of inorganic ligands2.1 Metal-free ligands (e. g., chalcogenide ions, SCN−)A typical ligand exchange process involves the phase transfer of colloidal NCsacross the boundary of two immiscible solutions. In a typical example, a solu-tion of organically-capped CdSe NCs is dispersed in a nonpolar solvent (toluene),and floated on a solution of an inorganic ligand such as K2S, dissolved in a po-lar solvent (e. g., formamide, FA), as shown in Figure1b[21]. NCs transfer into theFA phase after5minof stirring, leaving a colorless toluene phase containing dis-placed organic molecules. In FA, S2−ligands bind to the NC surfaces, displacingthe original organic ligands and producing an electrostatically stabilized colloid(Figure1a). Counter ions, like K+in K2S, do not bind to the NC surface in polar sol-vent colloids. They form a diffuse ionic layer surrounding the NCs and maintainoverallchargeneutrality. TheS2−ligandsimparta netnegativecharge(𝜁-potentialabout−30mV) to CdSe NC surface and lead to mutual electrostatic repulsion be-tween NCs. This effect prevents agglomeration and results in colloidal stability ofNCs. In general, solvents with higher dielectric constants provide better colloidalstability.Theuseofshortinorganicanionstostabilizecolloidshasbeenknownforabout100years(for example, I−ion can provide colloidal stability to AgI micro-particles in a polar solvent), but this strategy had not been used for modern, well-
88| A. Nag et al.Figure1:(a) Proposed mechanism of exchange of original long-chain hydrocarbon ligands withmetal-free chalcogenides (for example, K2S) on NC surface. (b) A red-colored colloidal solutionof CdSe NCs undergoes a phase transfer from toluene (upper) to FA (bottom) in the presence ofK2S. (c) UV-visible absorption and PL spectra of CdSe NCs capped with organic and S2−ligandsdispersed in toluene and FA, respectively. Reproduced from [21] with permission from AmericanChemical Society.defined nanomaterials [28]. Figure1c shows the unaltered UV-visible absorptionspectra of CdSe NCs before and after ligand exchange. This implies that the sizeand size distribution of NCs are preserved during ligand exchange. Observation ofexcitonic emission(Figure1c)forS2−capped NCssuggeststhattheS2−ligandscanreasonably passivate the surface trap states. Fourier transformed infra-red spec-tra confirm complete removal of the organic ligand after the ligand exchange pro-cess [21]. Dynamic light scattering (DLS) and transmission electron microscopy(TEM) analysis also provide the evidence of a complete removal of the long-chainorganic ligands from NC surface [21]. S2−capped CdSe NCs are colloidally sta-ble for months in inert atmosphere. However, the colloidal stability is maintainedfor only weeks in an ambient environment, likely due to oxidation of the ligand.Nonetheless, S2−is probably one of the smallest, cheapest, and simplest ligandsfor colloidal NCs.Other chalcogenides (Se2−and Te2−) and hydrochalcogenides (HS−,HSe−,HTe−) can also act as inorganic ligands for NCs [21]. Mixed chalcogenides, suchas TeS32−((NH4)2TeS3),canactasligandsforcolloidalNCsaswell[21]. Other sim-ple inorganic bases like KOH and NaNH2can provide colloidal stability to cer-tain NCs [21]. Dissolution of these strongly basic salts into FA establishes an equi-librium between OH−or NH2−and deprotonated FA. These species all representpotential ligands to coordinate to the metal sites of NC surface. Generally, FTIRspectra of KOH and NaNH2treated NCs show∼90%removal of organic species.The resulting colloids consist of negatively charged NCs as expected. In additionto these chalcogenide ligands, Kagan and Murray group discovered that SCN−
Inorganic Surface Ligands for Colloidal Nanomaterials |89(NH4SCN) anions can also behave as inorganic ligands for various NCs (Au, CdSe,PbS, etc. [22]).In our experiments, we found that Pearson’s hard and soft acids and bases(HSAB) principle [29] can be utilized to explain the selective affinity of specificinorganic ligands to a given NC surface [21]. The HSAB principle states that softLewisacidsformstablecomplexeswithsoftLewisbaseswhereashardacidspreferhard bases. Highly polarizable chalcogenide and hydrochalcogenide ligands areexamples of soft Lewis bases, while FA solutions of NaNH2and KOH produce hardbase rich environments. A positively charged binding site (e. g., Cd𝛿+on CdSe) onan NC surface can be considered as a Lewis acid. Soft S2−can stabilize Au andCdTe NCs containing soft acids (Au𝛿+/Au0or Cd𝛿+), but hard bases cannot. Incontrast, ZnSe and ZnO NCs with harder Lewis acid surface sites (Zn𝛿+)canbestabilized with solutions of KOH but not with sulfide salts. Similarly, SCN−bindsto Cd-chalcogenide (softer Cd𝛿+) NCs through S-sites (soft base), whereas it bindsto harder Pb𝛿+sites on Pb-chalcogenideNCs through harder N-sites [22]. However,one should remember that Pearson’s classification of hard and soft categories pri-marily applies to free ions. A metal ion on a NC surface can have very differenthardness/softness compared to corresponding free ions. Despite this difference,the HSAB principle still explains a large number of ligand exchange reactions andis very useful for the development of novel ligands for a given NC.2.2 Metal chalcogenide complexes (MCC) as inorganic surfaceligandsTypical MCC ligands are chalcogenidometallate molecules such as Sn2S64−,In2Se42−and AsS33−[18,19]. The positive counter ions are generally Na+,K+,NH4+and N2H5+. The MCC ligands were the first category of modern inorganicligands discovered for colloidal NCs, though they are structurally more complexcompared to metal-free ligands.MCCs have been used as precursors for mesoporous metal chalcogenides [30,31] and semiconductor films with high carrier mobility [32,33]. Mitzi et al. devel-oped a generic method for the preparation of various MCCs, which entailed dis-solving bulk metal chalcogenides in hydrazine (N2H4) in the presence of excesschalcogen [32]. A typical reaction for the synthesis of In2Se42−MCC ligands is de-scribed in (1).5N2H4+2Se+2In2Se3→N2+4N2H5++2In2Se42−(1)Ambient atmosphere, hydrazine-free preparation of metal sulfide based MCCligands wasalso demonstrated inenvironmentally benign solvents likewater [19].
90| A. Nag et al.A typical reaction for the synthesis of Sn2S64−ligands is given in (2).2SnS2+2(NH4)2S→(NH4)4Sn2S6(2)Using a mild thermal treatment, MCCs can transform into correspondingamorphous or crystalline metal chalcogenides. For example, (N2H5)4Sn2S6de-composes into electronic grade SnS2after heating to180∘C,asshownin(3)[32].Such a transformation can be utilized for solution-processed device fabricationand will be discussed later.(N2H5)4Sn2S6→SnS2+4N2H4↑+2H2S↑(3)The mechanism for colloidal stabilization of NCs by MCC ligands is simi-lar to the previously discussed case of chalcogenide ligands and is representedschematically in Figure2a. Figures2b,c show the reduction in inter-NC spacingthat results from the replacement of the long-chain dodecanethiol by Sn2S64−onthe surface of Au NCs. Consequently, the Sn2S64−capped arrays of Au NCs showaconductivityof200Scm−1, an enhancement by a factor of 1011relative to thedodecanethiol capped NCs [18].Using UV-visible absorption spectroscopy, we observed the enhancementof electronic coupling between the adjacent MCC capped NCs. Dodecanethiolcapped Au NCs show a strong plasmonic band in both solution samples andthin films (Figure2d). The solution of Sn2S64−capped Au NCs shows a similarplasmonic band, which completely disappears in the film (Figure2e), suggestinga strong delocalization of electronic states among the Au NCs [18].Thesameef-fect is observed for quantum confined semiconductor NCs as well [18]. Figure2fshows the absorption spectra measured in solution and thin film for CdSe NCswith organic and Sn2S64−ligands, respectively. In the case of thin films made ofinorganically-capped NCs, interparticle distance is sufficiently short to result inelectronic coupling between neighboring NCs, and a concomitant red-shift andbroadening of the excitonic feature is observed compared to those in the otherthree spectra. The Sn2S64−capped CdSe NCs in the film can be redispersed intoa polar solvent, resulting in recovery of the original excitonic feature that is ob-served in well separated NCs. This reversible red-shift and broadening of the ab-sorption features can be explained by wavefunction overlap between adjacentNCsina closelypackedfilmofSn2S64−capped NCs. AnnealingtheSn2S64−cappedNC film at180∘Cleads to the decomposition of the ligand into SnS2(reaction3),which acts as an electronic glue between adjacent NCs and enhances interparti-cle coupling (Figure2f). Though electronic delocalization significantly increasesin these NC films, their absorption onset remains substantially blue-shifted withrespect to bulk CdSe. Characteristic excitonic features are also retained in thin
Inorganic Surface Ligands for Colloidal Nanomaterials |91Figure 2:(a) Sketch of a CdSe NC capped with Sn2S64−ions. (b) and (c) TEM images of an arrayof5nmAu NCs capped with dodecanethiol and (N2H5)4Sn2S6ligands, respectively. (d) and (e)Absorption spectra of5nmAu NCs stabilized with (d) dodecanethiol and (e) (N2H5)4Sn2S6,insolutions and in thin films. (f) Absorption spectra collected using an integrating sphere for thinfilms composed of 4.6-nm CdSe NCs capped with original organic ligands (red, solid line) andwith (N2H5)4Sn2S6ligands before (blue, dash line) and after (green, dash-dot line) annealing at180∘C. Reproduced from [18] with permission from American Association for Advancement ofScience.films even after mild annealing. These observations suggest preservation of thesize-dependent quantum confinement effect of individual NCs in the electroni-cally coupled NC film.2.3 Differences between the MCC and metal-free ligandsThough ligand chemistries of MCCs and metal-free species are largely similar,there are several specific differences. For instance, while N2H4is a convenient sol-vent for MCC capped NCs, ligand exchange with metal-free chalcogenides mostlyfailed in N2H4.S2−capped CdSe NCs dispersed in FA exhibit a𝜁-potential of about−30mV, while the same NCs capped with SnS44−or AsS33−show𝜁-potentials inthe range of−50to−80mV.Thelower𝜁-potential of S2−capped NCs is proba-bly the reason for their poor solubility in N2H4with a dielectric constant (𝜀=51)but a good solubility in FA (𝜀=108). In addition, chalcogenide ligands are moreprone to oxidation. Thus, MCC ligands provide better air stability of colloidal NCs.Metal-free ligands and MCCs have different binding affinities to NCs. S2−is a bet-ter ligand for highly luminescent CdSe/ZnS NCs, however, it is inferior to MCCs forcapping Pb chalcogenide NCs.
92| A. Nag et al.2.4 “Ligand-free” colloidal nanocrystalsInorganic NCs typically have electrophilic metal-rich surfaces that result from thebinding of nucleophilic organic ligands on the NC surface during synthesis. Donget al. demonstrated the removal of organic ligands (oleic acid and oleylamine) us-ing nitrosonium tetrafluoroborate (NOBF4) and aryldiazonium tetrafluoroborate(ArN2BF4)[34]. After treatment with NOBF4and ArN2BF4, NCs can be dispersed ina polar solvent like dimethylformamide(DMF). FTIR and thermogravimetric anal-ysis (TGA) show that no ligands are attached to the NC surface other than weaklycoordinated DMF solvent molecules [34]. Consequently, the metal-rich NC surfaceexhibits a positive charge that electrostatically stabilizes NCs in a polar solvent.Unliketheabovementionedinorganicligands,BF4−doesnotbindtothepositivelycharged NC surface due to its weak nucleophilicity [35].Itremainsasacounterionin the diffuse layer of the “Ligand-free” (or bare) NCs. The cations of tetrafluorob-orate (BF4−) play an important role in the stripping of organic ligands and thecolloidal stability of bare NCs. For instance, the NOBF4treatment was success-fully demonstrated for Fe3O4,FePt,Bi2S3and NaYF4NCs [34] but failed for manyimportant semiconductor NCs like CdSe and ZnO. This is because NO+is a strongone-electron oxidant and Lewis acid [36] that quickly etches NC surfaces [21,34].We employed a phase-transfer process using HBF4or HPF6as anon-oxidizingligand-stripping agent in FA and oleic acid/oleylamine capped CdSe NCs intoluene [21]. H+cleavedtheCd–NandCd–Obonds(schemeshowninFigure3),leaving positively charged bare NCs dispersed in FA. The HBF4treatment was un-successful for alkylphosphonic acid capped NCs. In addition, the usage of HBF4results ina veryacidicenvironment, limitingtheir applicationasa general ligand-stripping agent.Recently, Rosen et al. reported a more generic approach for achieving barecolloidal NCs in a polar solvent: organic ligands are removed using a nonoxidiz-ing and nonacidicreagent [37]. They deployed the superior alkylating characterofMeerwein’s salt (Et3OBF4) and other trialkyloxonium salts for a rapid and efficientremoval of the native organic ligands. Since trialkyloxonium salts are unreac-tive toward most of phases that constitute NC core, the ligand-stripping reactionswere successful for various categories of NCs including indium tin oxide (ITO),doped ZnO, TiO2,𝛼-Fe2O3, CdSebased heterostructures, PbSe, NaYF4:Yb/Tm, andAg. Very recently, we demonstrated that triphenylcarbenium tetrafluoroborate(Ph3CBF4), where Ph3C+is a strong electrophile, can also be a nonoxidizing andlessacidic ligand-strippingagentfor colloidalNCs[23]. OneadvantageofPh3CBF4compared to all the previous ligand-stripping ligands is its high hydrophobicity,which allows a convenient separation of excessive Ph3CBF4from bare NCs dis-persed in polar solvents.
Inorganic Surface Ligands for Colloidal Nanomaterials |93Figure 3:Removal of native organic ligands from the surface of CdSe NCs by HBF4treatment.Caboxylate binding is depicted as monodentate for simplicity. The acidic proton of HBF4releases carboxylic acid from the NC surface and BF4−remains as a non-binding anion.Reproduced from [21] with permission from American Chemical Society.All the above mentioned methodology involves post-synthesis ligand re-moval from NC surface. Recently, a single-step reaction preparing various ligand-free metal chalcogenide NCs has been demonstrated [38,39]. The surfaces ofthese ligand-free NCs were designed to be sulfide anion-rich exhibiting negativecharges. Therefore, electrostatic repulsion between NCs results in colloidal dis-persion of NCs in a polar solvent. While this single-step method is simple, the sizedistribution of the obtained NCs is broader compared to that of NCs synthesizedusing typical organic ligands and solvents.2.5 Beyond metal-free chalcogenide and MCC ligandsMetal-free chalcogenides and MCCs were the first classes of inorganic ligandsdeployed to stabilize colloidal NCs in polar solvents. Recently, nonchalcogen-based ligands, including oxoanions/polyoxometallates (POMs), halides, pseudo-halides, and halometallates [23–25], were developed to enrich the toolbox of sur-face chemistries for rational design of NCs. Besides the high stability in ambi-ent atmosphere, these ligands broaden the functionality of NCs. For instance,oxo-ligands like MoO42−allow the intercalation of Li+into Fe2O3NCs, forminga prototype cathode material for Li-ion batteries. The incorporation of POMslike Rb8K2[{Ru4O4(OH)2(H2O)4}(𝛾-SiW10O36)2]⋅25H2O as inorganic ligands intro-duces catalytic activity to NCs in water splitting [23]. The halide ligands can pas-sivate NC surface, leading to high PL efficiency (20%–30%quantum yield forPbS NCs/CH3NH3PbI3)[25] and decent carrier mobility (∼12cm2V−1s−1for CdSeNCs/NH4I) [24].In addition,azide(N3−) isthe first example of“group VA” (or pnic-tide) ligands, allowing for potential fabrication of “all III-V NC solids” with III-VNCs [24].
94| A. Nag et al.3 Tailoring the properties of all-inorganic NCs3.1 Cation treatment of inorganically-capped NCs as a way totune NC propertiesInorganic ligands (e.g., S2−,Sn2S64−) discussed above are anionic in nature andbind to NC surfaces forming a negatively charged Stern layer. Counter cations thattypically remain in the diffuse layer are alkali metal ions, NH4+,orN2H5+.Thesecations exhibit a high solvation energy and low affinity toward the NC surface.Choice of cations with different binding affinities to the anion-terminated NC sur-facecanengineertheoptical, electronic, magnetic, andcatalyticpropertiesofNCsboth in colloidal solution and in NC solids [40]. The cationic treatment discussedhere is limited only to the surface of NCs and does not alter its core composition.Figure4a shows the scheme of binding certain cations on the S2−capped NCsurface. First, K2S capped CdSe NCs were obtained by the ligand exchange pro-cess discussed in Section 2.1. When a nitrate solution of a suitable cation (Cd2+,Zn2+,Ca2+,Al3+and In3+) in FA is added to the negatively charged S2−cappedCdSe NC solution (in FA), a charge inversion immediately occurs on the NC sur-face. For example, the𝜁-potential of S2−capped NCs is−35mV, which is con-verted to +28mVafter treated with Cd2+ions [40]. The charge inversion is alwaysassociated with a sharp increase in PL efficiency. For example, the PL quantumyield of S2−capped CdSe NCs in FA is0.7%,andincreasesby∼25times up to17%by treating the NC solution with Cd2+ions. Similarly, the quantum efficiencyof S2−capped CdSe/ZnS NCs (12%)risesto58%after treatment with Cd2+ions,resulting in the highly luminescent all-inorganic NCs. The underlying chemistrythat is responsible for the observed increase in PL efficiency is still to be unrav-eled. However, an obvious reduction in non-radiative surface traps was observedin the PL decay profiles after cationic treatment [40]. This phenomenon of lumi-nescence enhancement was later employed to develop luminescent sensors fortoxic metal ions like Cd2+[41].TheseultrasensitiveS2−capped CdSe/CdSeS/CdScore/gradient-shell/shell NCs can detect Cd2+with a low concentration of about110pM. Theorganic-free nature of NCsurfacefacilitatesinteractions between theanalyte and the sensor NCs, resulting into a superior sensitivity. The colloidal sta-bility of the cation treated NCs are limited only to dilute solutions (∼0.3mg/mL,5mMof cations). Adding excess cations to a concentrated (∼20mg/mL)S2−capped NC solution leads to the precipitation of NCs, suggesting the binding ofa single cationic species with multiple negatively charged NCs (“bridging of NCs”,as shown in Figure4b).
Inorganic Surface Ligands for Colloidal Nanomaterials |95Figure 4:(a) Scheme of the change of the surface charge of K2S capped CdSe NCs before andafter the treatment with strongly binding metal cations, Mn+. (b) Scheme of bridging S2−cappedNCs with Mn+. (c) Schematic representation of a NC coated with an inorganic (AsS33−)-organic(DDA+) hybrid pair. (d) A TEM image of the hexagonal close-packed lattices of DDA+-AsS33−capped3.8nmFePt NCs. Reproduced from [40](aandb)and[42] (c and d) with permission fromAmerican Chemical Society.In order to explore the possible bridging of NCs using cations, we performeda solid-state cation treatment on thin films made from K2S capped CdSe NCs.Elemental analysis showed a complete exchange of the K+ion with the foreigncations after treatment. Moreover, S2−capped CdSe NC films treated with Cd2+,Zn2+,Mn2+and In3+ions show a red-shift in excitonic absorption, along witha dramatic enhancement in electron mobility estimated from Field Effect Transis-tor (FET) performance. For example, the linear electron mobility (𝜇lin)ofathinfilmmadefromK2S capped CdSe NCs is0.01cm2V−1s−1, which increases byafactorof100(upto1.3cm2V−1s−1)afterCd2+treatment. The red-shift in theexcitonic absorption and increase in electron mobility can result from enhancedelectronic communicationthrough the linking of adjacent NCs by the cations (Fig-ure4b). In fact,the polarity of CdTe based transistors can be tuned from𝑝-type toambi-polar to𝑛-type by choice of inorganic ligand and cation treatment [40]. Bydecorating the surface of S2−capped CdSe NCs with Mn2+and Pt2+ions, we wereable to further impart magnetic and catalytic properties to the NC solid. [40].Self-assembly of inorganically-capped colloidal NCs into ordered arrays waspartly successful for Au NCs [18] but so far failed as a generic approach for otherNC systems. This can be attributed to the electrostatic repulsion between simi-larly charged NCs and to the incompatibility of all-inorganic NCs with low boilingpoint nonpolar solvents commonly used in self-assembly. Kovalenko et al. solvedboth issues by using an inorganic-organic hybrid ion pair as the capping ligand(Figure4c) [42]. The counter cations (Na+or NH4+)ofAsS33−capped NCs werereplaced with hydrophobic tertiary alkylammonium ions like didodecyldimethy-
96| A. Nag et al.lammonium (DDA+), forming an AsS33−-DDA+ion pair capping layer (Figure4c).Thishybridionpairreducedmutualelectrostaticrepulsionandprovidedcolloidalstability for NCs in a variety of low boiling point nonpolar solvents. Various NCsystems with this hybrid capping could easily form long-range self-assembledstructures (Figure4d), including binary superlattices. Interestingly, the cationicsurfactant in the hybrid inorganic-organic coating can be thermally decomposedat significantly lower temperatures compared to the traditional organic ligandswhich bind covalently to the NC surface. This thermal decomposition of hybridcoatings results in the formationof electronically coupled andlong-range orderedNC solids.3.2 Integration of NCs into inorganic hosts: bright near-IRluminescence from PbS/CdS NCs embedded in an As2S3matrixColloidal synthesis can provide organically-capped NCs with a tunable near-IRemission. InNC films, theIRemissiongetspartially quenched via excitonic energytransfer from the NC core to the vibrational modes of the organic ligands (mainlyC–Hstretching), and alsoto nearby NCs with PLquenching defects. In order to ad-dress these issues, we replaced the organic ligands from the surface of the near-IRemitting PbS/CdS NCs with inorganic As2S33−ligands. The NCs were then incor-porated into an amorphous As2S3matrix to increase the inter-NC distance [43].Since amorphous As2S3is transparent in the near- to mid-IR region, this NC/hostcompositeprovidesa brightand stableIRluminescenceinthetelecommunicationwavelength region.4 Designing novel electronic and optoelectronicdevices using colloidal NCsThe properties of a NC film depend on not only the properties of individual NCs,but also on the interfaces between adjacent NCs in a film. As mentioned above,long-chain organic ligands typically used in colloidal synthesis impede facilecharge transport between NCs. In previous reports, these organic ligands were re-moved by treating NC thin films with small molecules like 1,2-ethanedithiol [44],hydrazine [45,46], methylamine [45], NaOH [47,48], halide ions [49], or formicacid [50]. Such solid-state treatments have shown a significant improvement incharge transport, but are also susceptible to crack-formation in thin films, and
Inorganic Surface Ligands for Colloidal Nanomaterials |97the incomplete removal of insulating organic species [51]. Instead, our approachensures a complete exchange of organic ligands with inorganic ligands in solu-tion, followed by casting the all-inorganic NCs into films applicable for variouselectronic and optoelectronic devices.4.1 Field effect transistors (FETs)FET devices provide a convenient way to study charge transport in a NC film. Typi-cally,thehighestelectronmobility(𝜇)achievedinNCfilmstreatedwiththeabove-mentioned small molecules is∼1cm2V−1s−1[48]. Similar carrier mobilities canalso be obtained from a film prepared from all-inorganic colloidal NCs, for ex-ample, S2−capped CdSe NCs [21]. With proper combinations of NCs, inorganicligands, counter ions, and electrodes, the electron mobility can be significantlyimproved to∼30cm2V−1s−1[24,26,27,52–54]. Figure5ashowstheschematicdiagram of an FET device with the source and drain on top of the NC film. Fig-ure5b shows the scanning electron microscopic (SEM) image of the cross-sectionof the device, showing Si substrate, SiO2dielectric layer, NC film and Al elec-trode. Figure5c shows drain current (𝐼D)vsdrain-source voltage (𝑉DS)atdiffer-ent gate voltages (𝑉G)foradevicemadeof3.9nmIn2Se42−capped CdSe NCs.𝐼Dincreases with𝑉G, indicating𝑛-type transport. The linear regime field-effectelectron mobility,𝜇lin,iscalculatedas15.3cm2V−1s−1from the transistor trans-fer characteristics (𝐼Dvs𝐼G,Figure5d) at𝑉DS=4V.𝐼ON/𝐼OFFis desirably high,on the order of 104. Mobility measured in the saturation limit (𝜇sat)wasslightlylower (8–10cm2V−1s−1) than the𝜇lin, which is unusual for disordered semicon-ductors like NC films, organic polymers, and amorphous semiconductors [46,55].The observation that𝜇lin>𝜇sattypical for high-quality inorganic semiconductorssuggests the In2Se42−capped CdSe NC film has a low density of trap states. Tem-perature dependent studies, in range of220Kto340K, show that both𝜇linand𝜇satdecreases as temperature increases, suggesting a band-like transport throughextended electronic states [56].Typical operation voltages of NC FETs are in the range of20–40V,whichistoo high for realistic applications of NC FETs in electronic circuits and portabledevices. Low-voltage operation requires a strong capacitive coupling between thechannel and the gate, and can be achieved by using a thinner dielectric layer witha higher dielectric constant (𝜀). We demonstrated a bottom-gate, top source-drainFET similar to Figure5a, however, replaced100nmSiO2dielectric layer (𝜀∼3.9)with6nmZrO𝑥(𝜀∼9)[27]. The thin dielectric layer was deposited using a sol-gelprocess. These FETs are operational at a voltage as low as 1 V, and still show highelectron mobilities (>10cm2V−1s−1)forIn2Se42−capped CdSe NC FETs.Another
98| A. Nag et al.Figure5:(a) Schematic of a FET with the channel made of inorganically-capped NCs. (b) SEMimage of the cross-section of an FET device made of In2Se42−capped CdSe NCs. (c) Output (𝐼Dvs.𝑉DSas a function of𝑉G) and (d) Transfer (𝐼Dvs.𝑉Gat𝑉DS=4V) characteristics of a devicemade of In2Se42−capped3.9nmCdSe NCs. The length and width of the FET channel are50and450μm, respectively. The current modulation and linear-regime field-effect mobility(𝜇lin=15.3cm2V−1s−1) was calculated based on (d). Reproduced from [26] with permissionfrom Nature Publishing Group.important parameter of FETs is their switching behavior under alternating on-to-off sweeps of the gate voltage. Frequently, unwanted hysteresis behavior arisesfrom deep states within the forbidden gap of a semiconductor that can accumu-latechargecarriersinthebeginningoftheoff-to-onsweepofthegatevoltage.For example, ZrO𝑥or SiO2dielectric layers have hydroxyl groups at the surfacethat can trap electrons at the channel-dielectric interface. To address this, we em-ployed a hydroxyl-free and highly insulating fluoropolymer, Cytop (𝜀∼2), as thegate dielectric layer in a top-gated device geometry [27].Inorganicligand(In2Se42−and S2−) capped CdSe NC FETs with Cytop as the dielectric layer exhibited nearlyzero-hysteresis𝐼–𝑉plots without compromising high mobility at low operationvoltage.4.2 PhotodetectorsThe photocurrent (𝑖ph) through the NC film at a wavelength𝜆,canbegivenas𝑖ph=𝜂𝑒𝑁𝐺i,where𝜂is the probability of carrier generation per exciton,𝑒is theelementary charge,𝑁is the number of absorbed photons of wavelength𝜆perunit time, and𝐺iis the internal photoconductive gain [57]. A sensitive photode-tector requires a high photoconductive gain. To achieve this, the mobility of oneof the charge carriers (electron or hole) should be significantly greater than thatof the other. For this purpose, we used CdSe/CdS core/shell NCs [26], where the
Inorganic Surface Ligands for Colloidal Nanomaterials |99Figure 6:Schematic of a photodetector made from CdSe/CdS core-shell heterostructures.Energy levels align such that holes are trapped in the CdSe cores while electrons move freely.Reproduced from [26] with permission from Nature Publishing Group.band offsets of core and shell materials confine the hole inside the CdSe core,while the electron wavefunctions spread over both the core and shell [58,59].AfilmmadeofIn2Se42−capped CdSe/CdS NCs was sandwiched between ITO andAl electrodes [26]. Absorption of a photon within the film of core/shell structurescreates anelectron-hole pair. The photogenerated electrons spread over the entireNC solid and generate a photocurrent. Meanwhile, the holes are trapped insidethe core of a single NC, facilitating electron injection from the Al electrode (Fig-ure6). Asa result, theabsorptionofonephotonleadstomultipleelectronscyclingthrough the circuit as new electrons are injected to maintainnet charge neutralityunder constant electron extraction at the Al electrode. This process continues un-til the trapped hole recombines with a traveling electron. The ability of such a de-vice to transmit more than one electron for each photogenerated pair of chargecarriers allows for greater than unity photoconductive gain (𝐺i).AfilmofIn2Se42−capped CdSe/CdS NCs shows a very high gain,𝐺i≥6.5×103, formonochromatic550nmillumination,assuming𝜂≤1[26]. Thehigh𝐺ivalue is attributed to two factors: (i) the high electron mobility provided by theinorganicligand; and(ii)the optimized core/shell NCstructure that trapsthe pho-togenrated minority carrier (holes). The high𝐺ileads to a normalized detectivity,𝐷∗>1013Jones (1Jones=1cmHz1/2W−1), which is a record for II–VI NCs.
100| A. Nag et al.5 Formation of semiconductor films throughsolid-state reactions between NCs andinorganic ligandsThe sintering of NC solids can lead to the formation of a bulk phase that main-tains the composition of the solids [60]. Here we discuss the possibility of solid-state reactions between NCs and inorganic ligands that form a new bulk phase.These reactions provide a promising solution-processed route for technologicallyimportant semiconductor thin films. Apart from low-cost and simple processing,this solution-processed fabrication strategy can enable new applications such asflexible electronic devices.5.1 CuInSe2, CuIn1−𝑥Ga𝑥Se2,andCu2ZnSn(S,Se)4thin films forphotovoltaic applicationsCuInSe2(CIS), CuIn1−𝑥Ga𝑥Se2(CIGS), and Cu2ZnSn(S,Se)4(CZTS) are promisingmaterials for photovoltaic applications [61,62]. Soluble molecular precursorssuch as organometallic compounds and hydrazinium chalcogenidometallates(hydrazinium MCCs), have previously been used in preparation of films of cor-responding semiconductors [63–65]. For instance, MCC precursors decompose atan elevated temperature, forming desired inorganic phases in thin films. How-ever, a large weight loss (≥25%) typically occurs during the decomposition ofprecursors, generating cracks and resulting in undesirable film morphology. Asan alternative to these molecular precursors, we used the combination of col-loidal NCs and their inorganic ligand molecules as precursors for desired inor-ganic phases through solid-state reactions [66]. To prepare CIS from soluble pre-cursors, colloidal Cu2−𝑥Se NCs (𝑥=0–0.2) were capped with the surface ligandsof In2Se42−(asshownintheschemeinFigure7a).Additionalfree In2Se42−ligandswereaddedtomaintainastoichiometricratioofCu:In(1:1).AsolutionofCu2−𝑥SeNCsalongwithexcessIn2Se42−ligandswasthencasted intoa film via spincoatingor spray coating. After annealing the film at500∘C, the mixture transformed intoapureCISphaseasevidencedbytheXRDpatterns(Figure7a). The compositionof the inorganic phase can be varied from CIS to CuIn1−𝑥Ga𝑥Se2(CIGS) througha partial replacement of In2Se42−ligands with a corresponding amount of Ga2Se3-MCC. If Cu2S NCs were used instead of Cu2−𝑥Se NCs, a CuInS0.5Se1.5phase is ob-tained instead of CIS. The counterions, N2H5+of In2Se42−ligand, decompose intogaseous species during the annealing process resulting in a∼21%weight loss.
Inorganic Surface Ligands for Colloidal Nanomaterials |101Figure 7:New type of soluble precursors to inorganic semiconductors. (a) A solid-state reactionbetween Cu2−𝑥Se NCs and In2Se42−generates CuInSe2phase, confirmed by XRD. (b) XRDpatterns of annealed Sn2S64−capped Cu2−𝑥Se NCs, Sn2S64−capped ZnS NCs, and a mixture ofthese NCs where solid-state reaction results in the formation of Cu2ZnSn(S,Se)4(CZTS) thinfilm. (c) The phase purity of CZTS phase is confirmed by Raman spectroscopy showing all CZTSRaman peaks at172cm−1,194cm−1and232cm−1and not showing Raman shifts of commoncompeting phases. Reproduced from [66] with permission from American Chemical Society.This is slightly lower than the weight loss in the molecular precursor approach(≥25%), but still large enough for the formation of discontinuous thin films andsmall grain sizes.In order to improve the “atom economy” and increase the grain size of semi-conductors, there needs to be a reduction in the amount of MCC ligands in theprecursor solution to reduce the weight loss. To achieve this, CuInSe2NCs weremixed with only0.1molarequivalent of {In2Cu2Se4S3}3−ligands as a soluble pre-cursor for growing CIS thin film. After annealing the film at500∘C,amere2.6%weight loss was observed accompanied with the formation of the CIS phase withlarge grain sizes [66]. Smaller amounts of In2Se32−and Ga2Se3-MCCs canalso pro-videcolloidalstabilityofCuInSe2NCs,constitutingasolubleprecursorforCISandCIGS thin films.To prepare CZTS thin films, a solution precursor was prepared by mixingSn2S64−cappedCu2−𝑥SeNCs,Sn2S64−cappedZnSNCs, anda smallamountof freeSn2S64−ligands to maintain a molar ratio ofCu:Zn:Sn=2:1:1.Thefilmcon-taining the above components was annealed at500∘Cfor just20min,andtrans-formed into a CZTS phase (Figure7b,c). The weight loss during annealing was∼9%. The formed CZTS phase exhibits sharp XRD patterns with a1.1%increasein the lattice parameters compared to Cu2ZnSnS4, indicating the incorporation of
102| A. Nag et al.Se from the Cu2−𝑥Se NCs in the final phase. Further tuning of CZTS compositionscanbeachieved bya proper choiceofprecursor NCsand ligands. Forexample, theCu2ZnSnSe4phasecanbepreparedbyusingCu2−𝑥SeNCs,ZnSeNCs,andSn2Se64−ligands as precursors. Charge transport studies show a𝑝-type characteristics inbothCIS and CZTS films witha sufficientFET holemobility (𝜇lin=0.8cm2V−1s−1)for photovoltaic applications.5.2 Bi2−𝑥Sb𝑥Te3alloys and biphase PbTe/Sb2Te3thermoelectric (TE) materialsThefigureofmeritofTEmaterial(ZT)isdefinedasZT=𝑆2𝜎𝑇/𝜅,where𝑆,𝜎,and𝜅areSeebeck coefficient, electricalconductivity, thermalconductivity, respectively.ZTvalue∼4is desired for commercial deployment beyond some niche appli-cations [67]. Nanostructured materials are expected to possess highZTthrough(i) the lowering of𝜅because of phonon scattering at the grain boundaries whilenot significantly decrease𝜎(“electron crystal-phonon glass”) [68], and (ii) the en-hancement of𝑆due to quantum confinement [69] or through energy filtering [70].Nanostructured TE materials are generally prepared using molecular beam epi-taxy [71] and high temperature solid-state process or reactions [72,73]withop-timizedZT=2.2[74]. Here we discuss the formation of TE materials involvingsolid-state reactions of NCs and their ligands, starting from their colloidal solu-tions [20].Alloying, that adds disorder to the unit cell, generally reduces thermal con-ductivity. Bi2−𝑥Sb𝑥Te3alloy films with a control over grain size were preparedusing a solution precursor containing Bi2S3nanorods and Sb2Te3-MCC ligands.AN2H4solution of Sb2Te3-MCC capped Bi2S3nanorods along with a calculatedexcess offree Sb2Te3-MCC ligands was cast into a0.1–0.4μmthickfilm.Mildther-mal annealing resulted in the following reactions:Sb2Te3−MCC→Sb2Te3+Te+N2H4↑(4)Bi2S3nanorods+3Te→Bi2Te3+3S↑(5)𝑥Sb2Te3+(2−𝑥)Bi2Te3→2Bi2−xSbxTe3(6)Sb2Te3-MCC decomposes into pure Sb2Te3and releases excess Te as soon asthe film is dried (reaction 4). The evaporation of S facilitates the formation ofapureBi2Te3phase by consumption of excess Te that is liberated from the Sb2Te3-MCC decomposition (5). The alloying (reaction 6) takes place at180∘Cor highertemperatures. The XRD patterns in Figure8show a clear evidence of these solid-state reactions. XRD reflections from (Bi0.5Sb0.5)2Te3are broader than the Sb2Te3
Inorganic Surface Ligands for Colloidal Nanomaterials |103Figure 8:(a) A combination of Bi2S3nanorods with Sb2Te3MCC ligands provides a convenientsoluble precursor for Bi2−𝑥Sb𝑥Te3thermoelectric material. (b) The NC-MCC precursor is used tomake thermoelectric films and pellets. (c) Tracking the growth of (Bi0.5Sb0.5)2Te3nanocompositephase through solid-state reaction of Bi2S3nanorods and Sb2Te3-MCCs. Powder X-raydiffraction patterns of oleate capped Bi2S3nanorods (bottom), Sb2Te3-MCCs, dried andannealed at300∘C(middle), and pure phase (Bi0.5Sb0.5)2Te3alloy (top), annealed at300∘C.(d)Combination of PbTe NCs and Sb2Te3MCC ligands results in nanostructured PbTe/Sb2Te3composites. Reproduced from [20] with permission from American Chemical Society.reflections (obtained by reaction 4), suggesting a desired control over the grainsize in forming a nanostructured (domain size30–50nm)(Bi0.5Sb0.5)2Te3phase.The composition of Bi2−𝑥Sb𝑥Te3can be precisely controlled by the ratio of Bi2S3nanorods and Sb2Te3-MCC. Moreover, high thermoelectric performances were ob-tainedintheseBi2−𝑥Sb𝑥Te3phasesprepared from solubleprecursors. After300∘Cannealing, a spray-coated Bi1.2Sb0.8Te3film exhibits a high electrical conductiv-ity (200–450Scm−1) at room temperature and has a linear I–V characteristic.Films show𝑝-type transport behavior with a Seebeck coefficient in the range of170–250μV/K. Pellets prepared from the same precursor solution followed byannealing show a𝜅of∼0.89W/(m K) at room temperature, significantly smallerthan that for the commercial BiSbTe ingots (𝜅∼1.4W/(m K)) [75]. The smaller𝜅for nanostructured Bi2−𝑥Sb𝑥Te3is attributed to much more significant phononscattering at the grain boundaries. The obtainedZTvalue is 0.7 at room temper-
104| A. Nag et al.ature and remains almost temperature-independent in a wide range of tempera-tures (ZT∼0.63at523K). Further optimizations of this system have led to aZT∼1.2at room temperature. The TE modules require both𝑛-and𝑝-type materialswith similar TE properties. To prepare𝑛-type materials, we doped the Bi2−𝑥Sb𝑥Te3phase with Se. The doping was carried out through the addition of Sb2Se3-MCCsin a precursor solution of Bi2−𝑥Sb𝑥Te3.Biphase PbTe/Sb2Te3TE materials can also be prepared from solution pre-cursors. The Sb2Te3-MCC capped PbTe NC film exhibited a𝑝-type behavior witha Seebeck coefficient up to+750μV/K.Onlyasmallamount(<20%)ofSb2Te3-MCCs was adopted to provide sufficient colloidal stability of the PbTe NCs. ThisSb2Te3-MCC capped PbS NCs along with a calculated excess amount of freeSb2Te3-MCC ligands can also be employed as the precursor solution. The solu-tion was cast into a film and annealed at300∘C. PbS NCs converted to PbTe NCs(PbS+Te=PbTe+S↑) by reacting with the excess Te, similar to what occurs inreaction 5. The consumption of excess Te ensures the formation of the stoichio-metric Sb2Te3phase. Immiscibility of the PbTe and Sb2Te3phases leads to a filmof biphase PbTe/Sb2Te3with nanostructured grain sizes (Figure8b).6 Conclusions and outlookLigand chemistry of NC surface offers a promising route to a wide variety of novelmaterials and potential applications. From a scientific perspective, a broad scopeof inorganic anions (chalcogen-, oxo-, halogen-, and even pnicogen-based) canserve as surfactants for NCs, allowing tunable surface properties (charge, hy-drophilicity, interaction between NCs and between NCs and the medium, etc.).The binding affinities of inorganic ligands to NC surface can be qualitatively pre-dicted by the HSAB theory. Moreover, the introduction of inorganic ligands to NCsallows for a rational design of nano- and micro-scale materials through doping orsolid-state synthesis. For instance, annealing Sb2Te3-MCC cappedBi2S3nanorodsleads to the formation of Bi2−𝑥Sb𝑥Te3phase, and a further doping of Bi2−𝑥Sb𝑥Te3with Se can be accomplished using Sb2Se3-MCCs. In addition, the improved cou-pling in inorganically capped NC solids facilitates the study of collective behaviorand interfacial properties of adjacent NCs, complementing prior body of knowl-edge about the interfacial behavior of bulk and micro-scale solids. From a practi-cal perspective, exchange of organic ligands for short inorganic species is particu-larly advantageous for applications requiring efficient electronic communicationbetween neighboring NCs, as evidenced by the muchimproved charge carrier mo-bility in thin films made of inorganically capped NCs. Furthermore, the integra-
Inorganic Surface Ligands for Colloidal Nanomaterials |105tion of NCs and inorganic ligands broadens the range of functionality of NCs be-sides their size-dependent electronic and optical properties. The functionalizedNCs have represented promising performance in various applications, includingphotovoltaics, photodetection, thermoelectrics, and photocatalysis.Acknowledgement:D. V.T. thanksProf. Dr. HorstWeller for providingintellectualleadership andcreative environment in the Weller group.We also thank allthe co-authors of the publicationsdiscussedin this review, their names appearin the ref-erences. We thank the II–VI Foundation and DOE SunShot program Award Num-ber DE-EE0005312 for financial support. D. V.T. also thanks the Keck Foundation.The work atthe Center for NanoscaleMaterials (ANL) wassupported bythe USDe-partment of Energy under Contract No. DE-AC02-06CH11357. A. N. acknowledgesScience and Engineering Research Board (SERB) Govt. of India for RamanujanFellowship (SR/S2/RJN-61/2012)References1. C. B. Murray, D. J. Norris, and M. G. Bawendi, J. Am. Chem. Soc.115(1993) 8706.2. Y. Yin and A. P. Alivisatos, Nature437(2005) 664.3. C. Burda, X. B. Chen, R. Narayanan, and M. A. El-Sayed, Chem. Rev.105(2005) 1025.4. E. H. Sargent, Nat. Photonics6(2012) 133.5. D. V. Talapin, J. S. Lee, M. V. Kovalenko, and E. V. Shevchenko, Chem. Rev.110(2010) 389.6. N. L. Rosi and C. A. Mirkin, Chem. Rev.105(2005) 1547.7. C. L. Choi and A. P. Alivisatos, Annu. Rev. Phys. Chem.61(2010) 369.8. A. Nag, J. Jundu, and A. Hazarika, Crystengcomm.16(2014) 9391.9. M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem.100(1996) 468.10. J. S. Lee, E. V. Shevchenko, and D. V. Talapin, J. Am. Chem. Soc.130(2008) 9673.11. J. S. Lee, M. I. Bodnarchuk, E. V. Shevchenko, and D. V. Talapin, J. Am. Chem. Soc.132(2010) 6382.12. J. S. Son, J. S. Lee, E. V. Shevchenko, and D. V. Talapin, J. Phys. Chem. Lett.4(2013) 1918.13. D. V. Talapin, J. H. Nelson, E. V. Shevchenko, S. Aloni, B. Sadtler, and A. P. Alivisatos, NanoLett.7(2007) 2951.14. D. V. Talapin, ACS Nano2(2008) 1097.15. I. L. Medintz, H. T. Uyeda, E. R. Goldman, and H. Mattoussi, Nat. Mater.4(2005) 435.16. K. E. Knowles, M. T. Frederick, D. B. Tice, A. J. Morris-Cohen, and E. A. Weiss, J. Phys.Chem. Lett.3(2012) 18.17. M. Shim and P. Guyot-Sionnest, Nature407(2000) 981.18. M. V. Kovalenko, M. Scheele, and D. V. Talapin, Science324(2009) 1417.19. M. V. Kovalenko, M. I. Bodnarchuk, J. Zaumseil, J. S. Lee, and D. V. Talapin, J. Am. Chem.Soc.132(2010) 10085.20. M. V. Kovalenko, B. Spokoyny, J. S. Lee, M. Scheele, A. Weber, S. Perera, D. Landry, andD. V. Talapin, J. Am. Chem. Soc.132(2010) 6686.
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