mn-doped multinary cizs and aizs nanocrystals

Mn-Doped Multinary CIZS and AIZS NanocrystalsGoutam Manna, Santanu Jana, Riya Bose, and Narayan Pradhan*

Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata700032, India

*S Supporting Information

ABSTRACT: Multinary nanocrystals (CuInS2, CIS, and AgInS2, AIS) are widelyknown for their strong defect state emission. On alloying with Zn (CIZS and AIZS),stable and intense emission tunable in visible and NIR windows has already beenachieved. In these nanocrystals, the photogenerated hole efficiently moves to the defect-induced state and recombines with the electron in the conduction band. As a result, thedefect state emission is predominantly observed without any band edge excitonicemission. Herein, we report the doping of the transition-metal ion Mn in thesenanocrystals, which in certain compositions of the host nanocrystals quenches thisstrong defect state emission and predominantly shows the spin−flip Mn emission.Though several Mn-doped semiconductor nanocrystals are reported in the literature,these nanocrystals are of its first kind that can be excited in the visible window, do notcontain the toxic element Cd, and provide efficient emission. Hence, when Mn emissionis required, these multinary nanocrystals can be the ideal versatile materials forwidespread technological applications.

SECTION: Plasmonics, Optical Materials, and Hard Matter

Generation of the charge carriers and their recombinationor separation in quantum confined semiconductor

nanocrystals remains one of the most important and leadingfields of research in recent days.1−5 Much effort has been madeto achieve control over the movements of these charge carriersand implement them in solving the current standing problem ofenergy, for light generation and light harvesting.6−10 One suchprocess is doping in semiconductor nanocrystals, which canpromptly transfer either or both of the charge carriers to thenewly induced state(s) or transfer the exciton energy to theinserted impurity state.11−18 Significant progress has been madein designing such nanocrystals, and among them, doping of thetransition-metal ion Mn2+ in various group II−VI semi-conductor hosts is one of the most widely studied systemstill date, which can lead to stable and intense visibleemission.11,13,18−28 Importantly, the larger Stokes shift andhigher excited-state lifetime of these nanocrystals distinguishthem from the band edge emitting nanocrystals and categorizethem in a different class of materials for various leadingapplications both in biology as well as in optoelectronics (LEDand photovoltaics).29−32 Among other transition-metal ions,doping of copper has also been studied for generating tunablelight-emitting materials, but the exact recombination mecha-nism in these nanocrystals is still under investigation.15,33−37 Incomparison, doping of Mn is the most optimized system, andits recombination mechanism has also been successfullyestablished to a greater extent.From the literature reports, it is revealed that once the bound

exciton energy of the host nanocrystal is transferred to the Mn6A1 state, one of the electrons inverts its spin, and it changes tothe excited 4T1 state.38−40 When it returns to the non-

degenerate ground state again, the released energy gives rise tothe highly efficient emission with a longer radiative excited-statelifetime. This emission also remains independent of particle sizeand the host band gap and shows a wide Stokes shift in severalhost nanocrystals.19−21 Till date, the high efficiency of the Mnemission has been mostly observed in ZnS, ZnSe, CdSe (or S),and different binary alloyed group II−VI semiconductornanocrystals.19−28 These hosts either have high energyexcitation or contain the toxic element Cd. The requiredband gap of the host, the placement of the Mn ground state,and the energy gap between the 4T1 and

6A1 restrict the wideselection of hosts for obtaining this Mn d-state emission.Moreover, the doping itself is also found to be selective on thesize and phase of the host nanocrystals. However, on successfuldoping, it has been observed that the dopant emission intensitycan go above 50% efficiency in selective hosts.21,27,28 Till date,the hosts used for Mn doping to obtain the efficient Mnemission either have the band edge excitonic or surface stateemission. Evolution of Mn emission can quench both of theseemissions.However, there are several other nanocrystals that emit

neither the band edge exciton nor the surface state emission(under room-temperature steady-state measurement). Onesuch example is multinary nanocrystals like CuInS2 (CIS),AgInS2 (AIS), and their alloyed nanocrystals (mainly with Zn),where the photogenerated hole moves permanently to theinternal trap/defect state placed within the band gap before its

Received: July 18, 2012Accepted: August 27, 2012Published: August 27, 2012

Letter

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direct recombination.41−49 The emission intensity of thesenanocrystals is also reported to be above 50%,41−43 and unlikethe unpredictable surface state emission, both its position andintensity are reproducible. Moreover, these nanocrystals do notcontain toxic elements like Cd and can be excited with visibleexcitation. Exploring these multinary nanocrystals as ideal hosts,we dope them with Mn, and in certain compositions, efficientMn emission is obtained, quenching the strong internal defectstate emission. It also favors the excitation window that thebiological samples require and also contains the greenermaterials that the community favors. Details of the selectionof host composition, doping strategy, switching over the holetransfer to exciton energy transfer emission, spectral character-izations, water solubility, and emission stability are studied andreported in this Letter.Multinary CIS and AIS nanocrytstals are known for their

size-dependent absorption variation. With the increase of size,the band edge absorption red shifts, but the compositionremains unchanged. The emission of these nanocrystalsoriginates from the involvement of the defect state, and itsintensity remains poor. However, treating these nanocrystalswith Zn significantly enhances the intensity and also stabilizesthe emission.41,43 Zn mostly replaces the Cu and In or Ag andIn in CIS and AIS nanocrystals, respectively, and this widensthe band gap and blue shifts both the absorption and emissionbands.41,42,47−49 These are known as Zn alloyed CIS (CIZS)and AIS (AIZS) nanocrystals, and their size remains almost thesame during the cation replacement, but the optical absorptionand emission vary with the amount of Zn in the composition.

To obtain Mn emission, these CIZS and AIZS nanocrystals areexplored here with introduction of Mn at different stages of theion exchange process to study the possibility of quenching thetrap state and obtaining the Mn d-state emission.CIS and AIS nanocrystals are synthesized following a

modified literature method.48 To our knowledge, there aretwo possible synthetic protocols reported till date.42,48,50 In onecase, CIS or AIS nanocrystals are synthesized first, and then, Znis added in a later stage, which replaces the Cu (or Ag) and Infrom the surface of CIS or AIS and forms the alloy CIZS orAIZS. In this case, both the optical absorption and emissionblue tunes. In the second approach, all of the precursors areloaded together, and alloyed CIZS or AIZS nanocrystals aresynthesized in a one-step process. In this case, long-rangetunability of the emission window is not observed like in thefirst case. However, to obtain the emission at differentpositions, the initial composition ratio of the precursors isvaried. For doping, the dopant Mn precursor has beenintroduced with or after the Zn addition in the first case andtogether with all of the precursors in the second case. Figure 1shows the optical spectra of undoped and doped CIZS andAIZS nanocrystals following both of these two-step (upperpanel) and one-step (bottom panel) processes. In both cases,the undoped nanocrystals of CIZS and AIZS show the puretrap state emissions, and on doping, these are quenched forcertain compositions of the host nanocrystals with evolution ofpure Mn d-state emission at ∼600 nm. It has been observedthat a minimum concentration of Zn is required in thecomposition of the nanocrystals above which this Mn emission

Figure 1. Photoluminescence (PL) spectra of undoped and doped CIZS and AIZS nanocrystals. (a,c) Typical PL spectra of CIZS and Mn-dopedCIZS nanocrystals. (b,d) Those of AIZS nanocrystals. The UV−visible spectra corresponding to these PL spectra are shown in Figure S1(Supporting Information). (e,f) Digital images of tunable CIZS and AIZS and their Mn-doped nanocrystals obtained under UV irradiation. Thesenanocrystals are synthesized following two-step synthesis methods where Zn has been introduced to presynthesized CIS or AIS nanocrystals. (g,h)UV−visible and PL spectra of CIZS and AIZS nanocrystals synthesized following an all loading together one-step synthetic protocol. Correspondingspectra of Mn-doped nanocrystals are shown in (i) and (j). The initial composition ratio of the reactants of the doped nanocrystals remains the sameas that of the undoped nanocrystals. The excitation wavelength used in all cases is 390 nm.

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appears (labeled in the right panels of Figure 1), and once itevolves, the emission is not further tuned; rather, its intensityenhances with the progress of the reaction (Figure 1c and d).These nanocrystals can be excited even with 450 nm excitation,and this suggests that the emission is coming from the ternarynanocrystal host and not from any side products (such as Mn-doped ZnS).These nanocrystals are further characterized by transmission

electron microscopy (TEM) and X-ray powder diffraction(XRD). Figure 2a,b and d,e show the TEM images of dopedCIZS and AIZS nanocrystals, respectively, and these remainspherical in shape. The size remains almost the samethroughout the alloying process in both of our syntheticapproaches, and only their compositions vary. Due to the use ofthiol ligands in our adopted reaction protocol, the particle sizeremains ∼3−4 nm throughout the reaction. XRD shows thezinc blende phase for both doped and undoped nanocrystals(Figure 2c and f). ICP suggests the presence of Mn within 0.5−1% (Supporting Information, Tables S2 and S3) in thesenanocrystals. The electron paramagnetic resonance (EPR)spectra also support the presence of Mn in the dopednanocrystals. Figure 3a and b show the EPR spectra of theMn-doped CIZS and AIZS nanocrystals. The hyperfineconstant (A) value of 68 gauss suggests that Mn has beendoped in a cubic lattice field in CIZS nanocrystals.27 The samehas also been observed for Mn-doped AIZS. However, thebroadness of the peak rather than the six clear hyperfine

splittings is expected due to the complex environment of theMn in the presence of copper or silver. Similar EPR spectrahave been observed in previously reported such alloyed systemsand also in Cu-codoped nanocrystals.37

The Mn emission obtained has been observed to be intense,and it is comparable with the best-available Mn-dopedsemiconductor nanocrystals.20,21,27 For undoped CIZS andAIZS, the quantum yield (QY) of the trap emission remainswithin 45 and 55% respectively. However, after doping, the Mndopant emissions show a QY of ∼43 and 55%, respectively.With the increase of Zn in the composition, the Mn emissionintensity is enhanced (Figure 1c,d). Details of the QYmeasurement have been provided in the SupportingInformation (Figures S2 and S4).The important observation here is the quenching of the

intense trap state emission with the appearance of Mn d-stateemission. The control reaction suggests that under similarcomposition, the trap state emission of undoped nanocrystalsalways remains at higher energy than the doped nanocrystals’emission. However, in a different composition, these ternaryundoped nanocrystals can also emit in the same window as thatobtained in doped nanocrystals. Hence, to distinguish them,their nature and spectral properties are studied in detail andcompared. The full-width at half maxima (fwhm) of the Mnemission remains ∼50−60 nm, which is narrower than the trap-induced emission and is typical of Mn d-state emission.20,21,27

The excited-state lifetime of the emission also shows significantdifference in comparison to the undoped nanocrystals. Figure 4shows the lifetime plots of undoped and doped CIZS and AIZSnanocrystals emitting the trap state and Mn d-state emission,respectively. The millisecond lifetime in Mn emissiondistinguishes it from the trap state emission of the undopednanocrystals (Figure 4a and c). Further, the lifetime has beenmeasured at different emission positions obtained with differentcompositions. As Mn emission is nontunable, the lifetime hasbeen measured at different positions of the emission spectrumof any composition showing the Mn emission. Figure 4c showsthe lifetime plots at different emission positions of Mn-dopedAIZS nanocrystals (PL spectra shown in the inset of Figure 4c).The lifetime values remain almost close in all cases, and the

Figure 2. TEM and HRTEM images of Mn/CIZS (a,b) and Mn/AIZS (d,e) nanocrystals in different resolutions. (c,f) XRD of the CIZS and AIZSnanocrystals, respectively. The XRD of the doped and undoped nanocrystals remains the same.

Figure 3. EPR spectrum of the Mn-doped CIZS (a) and AIZS (b)nanocrystals measured at liquid nitrogen temperature.



differences are within the error ranges. On the other hand, theemission of undoped AIZS is tunable, and the lifetime of theemission in different compositions of nanocrystals varies(Figure 4d). Similar observations have also been obtainedfrom doped and undoped CIZS nanocrystals. This fixed lifetimeof the Mn emission further distinguishes it from the tunablelifetimes of trap state emission. Moreover, the time delayspectra in Figure 5 with different delay times behave differentlyin these two types of emissions. With the delay of 0.03 ms, theemission from the undoped nanocrystals almost remains flat(Figure 5c for CIZS and 5d for AIZS), but under similarconditions, the emission from the Mn-doped nanocrystalsremains unchanged (Figure 5a and b). This observation alsodistinguishes the Mn emission from the trap state emission.Importantly, the composition of these nanocrystals provides

more information about the differences of these two emissions.Figure 6a shows the PL spectra of the doped and the undopedAIZS nanocrystals obtained almost from the same composi-tions of the reactants (all loading together approach). The PLEspectra (Figure 6b and c) of both emissions also support thatthey are obtained from same composition of nanocrystals.However, a wide difference in the emission windows of thesedoped and undoped nanocrystals clearly suggests that they areobtained from different processes. Similar observations havealso been obtained from the doped and undoped CIZS

nanocrystals. In addition, we have also observed here that thePLE spectra obtained in different positions of each emissionoverlap with each other (insets of Figure 6b and c). Thissuggests that the fwhm for both emissions are independent ofparticle size/composition. For Mn d-state emission it isknown,51 but for the ternary nanocrystals, this providesadditional information.Finally, we have discussed the recombination mechanism and

the evolution of the Mn d-state emission from the CIZS andAIZS nanocrystals. Using both of our synthetic protocols, wehave observed that the concentration of Zn should be higherthan 55% in CIZS and 60% for AIZS to obtain the pure Mn d-state emission. Further, the synthetic procedures suggest thatMn can be incorporated at the beginning of the reactions. Toverify this, we have analyzed the ICP and EPR of the samples inall compositions of the multinary nanocrystals after introduc-tion of the dopant Mn precursors. The results suggest that Mnis incorporated almost in all compositions of Cu, In, and Zn inthe nanocrystals, but the Mn emission appears above a certainlimiting composition, as stated above. Hence, the nanocrystalsbehave like Mn-doped CdSe, where, at certain size or belowthat, the Mn emission predominantly evolves, but with largersize, the band ege emission of CdSe predominates.24 Hence, itis the size- or composition-dependent band gap of thenanocrystals that determines the possible exciton energytransfer to the Mn d-state for obtaining the Mn spin−flipemission. However, in CIZS and AIZS nanocrystals, it is thetrap state emission rather than the excitonic emission thatcompetes with the Mn d-state emission with the compositionor band gap variations. Under steady-state measurement and atroom temperature, these nanocrystals can produce only onetype of emission. It is clear from Figure 6 that under a certaincomposition (the same PLE), the nanocrystals when undopedresult in the trap state emission, but when doped, they emit thedopant emission. Similarly, for a low concentration of Zn in thenanocrystals, irrespective of the presence of Mn, only trap state

Figure 4. Excited-state lifetime plots (Y axis in log scale) of doped andundoped CIZS and AIZS nanocrystals. (a,b) Lifetime plots of dopedand undoped CIZS nanocrystals emitting Mn and trap state emission,respectively, with the emission at 600 nm. Insets show correspondingPL spectra of the nanocrystals. (c,d) Lifetime plots of the doped andundoped AIZS nanocrystals, respectively. For the case of Mn dopantemission, the lifetime has been measured at different positions of onespectrum, but for trap state emission, it is measured at differentpositions obtained from different samples. The different positions atwhich lifetimes have been measured are marked by color bars in theinset PL spectra of Mn/AIZS in (c). The PL spectra of the samplecorresponding to the lifetime plots in (d) are shown in Figure 1b.Details of the calculations of the lifetime values are provided in theSupporting Information (Table S4).

Figure 5. (a,b) Time delay PL spectra of the doped CIZS and AIZSnanocrystals emitting the Mn d-state emission, respectively, and (c,d)trap state emission of their respective undoped nanocrystals. Thespectra scanned at different time delays have been labeled inside ofeach panel.



emission is obtained. This clearly indicates that under thefavorable condition, one emission channel supersedes other.On the basis of these observations, we propose here the most

plausible mechanism of the recombination process in the dopedmultinary nanocrystals. Figure 7 shows the schematicpresentation of both Mn d-states and the internal defect statein the multinary nanocrystals and their possible alignments forobtaining the Mn or trap state emission. Figure 7a shows atypical case of the hole transfer and obtaining the trap or defectstate emission in the multinary nanocrystals. Figure 7b presentsthe case where the Mn ground state remains above the trapstate as well as the host valence band, and this is possible whenthe Zn concentration is higher (high band gap). The otherpossible arrangement is the position of the trap state above theMn 6A1 state (Figure 7c), and this is typically observed for thecase when the concentration of Zn is lower than the limitingconcentration. It is known that in multinary nanocrystals, theStokes shift varies with composition48, and hence, the positionof the trap state moves with variation of the Zn percentage ineither CIZS or AIZS nanocrystals. Accordingly, the relativealignment of the trap state with the Mn state determines theallowed recombination process.Further, the question arises here whether the energy of the

exciton confined within the valence and conduction band or

within the trap state and conduction band transfers the energyto the Mn d-state for the spin inversion. To answer this,certainly a detailed study of the ultrafast spectroscopy isneeded, but in our case, as the nanocrystals do not show theband edge excitonic emission at all, we can only assume herethat the hole is promptly moved to the internal defect state andit is the exciton confined within the trap state and conductionband that transfers the energy to the Mn state for obtaining theMn d-state emission.These doped nanocrystals are further surface-ligand-

exchanged with mercaptopropionic acid (MPA) and dispersedin aqueous solution. It has been observed that the Mn emissioneven remains stable in water for weeks. Figure S3 (SupportingInformation) shows the optical absorption and emission spectraof Mn-doped CIZS nanocrystals emitting Mn emission inwater. The quantum efficiency has been observed to be ∼25%in water, and details of the surface ligand modification havebeen provided in the Supporting Information.In conclusion, we report here the doping of Mn in multinary

CIZS and AIZS nanocrystals and obtain the pure Mn d−demission. Appearance of this emission has been observed undercertain conditions where the Mn ground state remains abovethe valence band and the internal trap state of the respectiveternary nanocrystals. For the high lifetime and stability, this Mn

Figure 6. PLE spectra of the doped and undoped AIZS nanocrystals. (a) PL spectra of the doped and undoped nanocrystals; color bars are markedat different positions at which PLEs have been measured. (b,c) PLE of the doped and undoped nanocrystals, respectively, at different emissionpositions. Insets in (b) and (c) are the normalized PLE spectra.

Figure 7. Schematic presentation of the allowed photorecombination process in Mn-doped multinary nanocrystals. (a) A typical case of the holetransfer process in the ternary nanocrystals that emit the trap state emission. (b) The case having a higher amount of Zn concentration in CIZS orAIZS nanocrystals where the trap state (ts) and the host valence band remain below the Mn ground state (6A1). In this case, on excitation, the Mnemission is predominantly observed quenching the trap state emission. (c) The case of having a lower percentage of Zn where the trap state remainsabove the Mn d-state. This predominantly results in the trap state emission quenching the Mn d-state emission.



d-state emission is widely known. Doping in ternary hosts, itcan be excited in the visible window; it can be free from theelements Cd and Se and can generate comparable emissionintensity to several known Mn-doped nanomaterials. Moreover,we also report here the condition of obtaining the Mn and trapstate emission. We believe that the new materials presentedhere would be helpful for the community and that the newphysical insights represented here would help for betterunderstanding of the photophysical aspects of doping.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental, instrumentation, and supporting figures. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSDST and CSIR of India are acknowledged for funding andfellowship. N.P. acknowledges a DST Swarnajayanti Fellowship(DST/SJF/CSA-01/2010-2011).

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Emitting Cu Doped ZnSe Nanocrystals. Chem. Commun. 2010, 46,2853−2855.(35) Corrado, C.; Jiang, Y.; Oba, F.; Kozina, M.; Bridges, F.; Zhang, J.Z. Synthesis, Structural, and Optical Properties of Stable ZnS:Cu,ClNanocrystals. J. Phys. Chem. A 2009, 113, 3830−3839.(36) Peka, P.; Schulz, H. J. Empirical One-Electron Model of OpticalTransitions in Cu-Doped ZnS and CdS. Physica B 1994, 193, 57−65.(37) Jana, S.; Srivastava, B. B.; Pradhan, N. Correlation of DopantStates and Host Bandgap in Dual-Doped Semiconductor Nanocrystals.J. Phys. Chem. Lett. 2011, 2, 1747−1752.(38) Chen, H. Y.; Maiti, S.; Son, D. H. Doping Location-DependentEnergy Transfer Dynamics in Mn-Doped CdS/ZnS Nanocrystals. ACSNano 2012, 6, 583−591.(39) Chen, H.-Y.; Chen, T.-Y.; Son, D. H. Measurement of EnergyTransfer Time in Colloidal Mn-Doped Semiconductor Nanocrystals. J.Phys. Chem. C 2010, 114, 4418−4423.(40) Kuroda, T.; Ito, H.; Minami, F.; Akinaga, H. Ultrafast EnergyTransfer in Manganese-Doped ZnSe. J. Lumin. 1997, 72, 106−107.(41) Li, L.; Pandey, A.; Werder Donald, J.; Khanal Bishnu, P.;Pietryga Jeffrey, M.; Klimov Victor, I. Efficient Synthesis of HighlyLuminescent Copper Indium Sulfide-Based Core/Shell Nanocrystalswith Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133,1176−9.(42) Torimoto, T.; Adachi, T.; Okazaki, K.-I.; Sakuraoka, M.;Shibayama, T.; Ohtani, B.; Kudo, A.; Kuwabata, S. Facile Synthesis ofZnS−AgInS2 Solid Solution Nanoparticles for a Color-AdjustableLuminophore. J. Am. Chem. Soc. 2007, 129, 12388−12389.(43) Xie, R.; Rutherford, M.; Peng, X. Formation of High-Quality I−III−VI Semiconductor Nanocrystals by Tuning Relative Reactivity ofCationic Precursors. J. Am. Chem. Soc. 2009, 131, 5691−5697.(44) Zhong, H.; Lo, S. S.; Mirkovic, T.; Li, Y.; Ding, Y.; Li, Y.;Scholes, G. D. Noninjection Gram-Scale Synthesis of MonodispersePyramidal CuInS2 Nanocrystals and Their Size-Dependent Properties.ACS Nano 2010, 4, 5253−5262.(45) Li, L.; Daou, T. J.; Texier, I.; Kim Chi, T. T.; Liem, N. Q.; Reiss,P. Highly Luminescent CuInS2/ZnS Core/Shell Nanocrystals:Cadmium-Free Quantum Dots for In Vivo Imaging. Chem. Mater.2009, 21, 2422−2429.(46) Hamanaka, Y.; Ogawa, T.; Tsuzuki, M.; Kuzuya, T. Photo-luminescence Properties and Its Origin of AgInS2 Quantum Dots withChalcopyrite Structure. J. Phys. Chem. C 2011, 115, 1786−1792.(47) Mao, B.; Chuang, C.-H.; Wang, J.; Burda, C. Synthesis andPhotophysical Properties of Ternary I−III−VI AgInS2 Nanocrystals:Intrinsic versus Surface States. J. Phys. Chem. C 2011, 115, 8945−8954.(48) Zhang, J.; Xie, R.; Yang, W. A Simple Route for HighlyLuminescent Quaternary Cu−Zn−In−S Nanocrystal Emitters. Chem.Mater. 2011, 23, 3357−3361.(49) Nakamura, H.; Kato, W.; Uehara, M.; Nose, K.; Omata, T.;Otsuka-Yao-Matsuo, S.; Miyazaki, M.; Maeda, H. Tunable Photo-luminescence Wavelength of Chalcopyrite CuInS2-Based Semi-conductor Nanocrystals Synthesized in a Colloidal System. Chem.Mater. 2006, 18, 3330−3335.(50) Tang, X.; Yu, K.; Xu, Q.; Choo, E. S. G.; Goh, G. K. L.; Xue, J.Synthesis and Characterization of AgInS2−ZnS Heterodimers withTunable Photoluminescence. J. Mater. Chem. 2011, 21, 11239−11243.(51) Pradhan, N.; Peng, X. Efficient and Color-Tunable Mn-DopedZnSe Nanocrystal Emitters: Control of Optical Performance viaGreener Synthetic Chemistry. J. Am. Chem. Soc. 2007, 129, 3339−3347.



mn-doped multinary cizs and aizs nanocrystals

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