Tuning Optical Activity of IVâVI Colloidal Quantum Dots in the Short

Subscriber access provided by UNIV OF CAMBRIDGE

Article

Tuning optical activity of IV-VI colloidal quantum dots in the short-wave infrared (SWIR) spectral regime Arthur Shapiro, Youngjin Jang, Anna Rubin-Brusilovski, Adam K. Budniak, Faris Horani, Aldona Sashchiuk, and Efrat Lifshitz Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 12, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Tuning optical activity of IV-VI colloidal quantum dots in the shortwave infrared (SWIR) spectral regime Arthur Shapiro,† Youngjin Jang,† Anna Rubin-Brusilovski, Adam K. Budniak, Faris Horani, Aldona Sashchiuk, and Efrat Lifshitz* Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Nancy and Stephen Grand Technion Energy Program, Technion, Haifa 3200003, Israel

ABSTRACT: The achievement of tunable optical properties across a wide spectral range, along with an efficient surface passivation of lead chalcogenide (PbSe) colloidal quantum dots (CQDs), has significant importance for scientific research and for technological applications. This paper describes two comprehensive pathways to tune optical activities of PbSe CQDs in the near-infrared (NIR, 0.75-1.4 µm) and the short-wave infrared (SWIR, 1.4-3 µm) ranges. A one-pot procedure enabled the growth of relatively large PbSe CQDs (with average sizes up to 14 nm) exploiting programmable temperature control during the growth process. These CQDs showed optical activity up to 3.2 µm. In addition, PbSe/PbS core/shell CQDs prepared by an orderly injection rate led to an energy red-shift of the absorption edge with the increase of the shell thickness, while a post-annealing treatment further extended the bandedge energy toward the SWIR regime. A better chemical stability of the CQDs with respect to that of PbSe core CQDs was attained by shelling of PbSe by epitaxial layers of PbS, but limited to a short duration (< 1 day). However, air stability of the relatively large PbSe as well as the PbSe/PbS CQDs over a prolonged period of time (weeks) was achieved after a post-synthesis chlorination treatment.

SWIR PbSe CQDs used multiple injections in a two-pot synthesis,14,27 involving complexity derived from multiple reaction stages. Hence, seeking a simple procedure to produce large IV-VI core or core/shell CQDs with absorption at the SWIR spectral range is still a challenge. Furthermore, chemical instability of PbSe CQDs under ambient conditions23,28,29 impedes developing reliable optoelectronic devices due to loss of optical properties. Various approaches such as controlled passivation of the CQD surface by using inorganic shells4,8,13,21-23,30 and ligands5,17,27,31-38 have been proposed. Formation of PbSe/PbS via shell coating4,8,13,21-23 or generation of PbSe/CdSe via cation exchange process39-41 were reported in recent years as a way to increase the emission quantum efficiency. These core/shell structures also exhibit a (quasi)-type II alignment at the core-shell interface, involving partial distribution of one carrier (electron or hole) over the entire core/shell structure with potential influence on nonradiative processes.2 Recently, halide treatment was reported as a good approach to improve surface properties without changing the electronic configuration.5,27,32-35,42 This paper describes comprehensive approaches to obtain tunable optical activity in the NIR and SWIR regime, and at the same time to sustain high surface stability of CQDs based on PbSe cores. Two different systems (relatively large PbSe CQDs and PbSe/PbS core/shell CQDs) were explored to achieve optical activity in the SWIR regime. Relatively largesized PbSe CQDs were prepared by controlling the reaction agents and temperature program. PbSe/PbS core/shell CQDs

Introduction IV-VI colloidal quantum dots (CQDs) have attracted scientific and technological interest during the past 20 years, due to their size-tunable optical properties in infrared spectral ranges.1-3 Lead selenide (PbSe), one of the most studied CQDs, possesses a bulk narrow bandgap (0.26 eV at room temperature), a large exciton Bohr radius (46 nm), small electron and hole effective masses (me,h ≤ 0.1 m0), and a high dielectric constant (εm = 23),4 being attractive properties for various optoelectronic applications, such as photovoltaics,5,6 near-infrared (NIR) gain devices,7,8 field-effect transistors,9 thermoelectrics,10,11 biological markers,12 and Q-switches.13 The IV-VI CQDs have been synthesized in the nano regime, exploiting colloidal technique know-how, regulating the optical tunability via the change of size, shape, and composition.1420 Recently, Čapek et al. investigated the formation rate of PbSe-solute in a hot-injection colloidal procedure by varying the concentration of the reacting agents in addition to adaptation of the temperature, and demonstrated an exceptionally high level of size control of the CQDs with high chemical yields.19 The same group has also developed the formation of PbSe/PbS CQDs with a relatively high emission quantum yields.4,8,13,21-23 The previous studies cited were mainly focused on CQDs active in the NIR regime. However, optical activity of the IV-VI CQDs with absorption/emission14 at the shortwave infrared (SWIR) wavelength was studied to a lesser extent, although interest in this spectral regime is gradually growing.24-26 A few pioneering attempts for the synthesis of

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

were formed by a well controlled shell coating procedure. The synthesized CQDs followed by post-annealing treatment showed the desired bandgap tunability, relatively narrow excitonic peaks and small emission Stokes shift when compared with the corresponding PbSe CQDs. Further, a final halide treatment provided air stability of the CQDs under ambient conditions for prolonged period of time.

Page 2 of 9

tion was stopped by cooling down to room temperature. The purification of the product was performed by adding ethanol and centrifuging the suspension twice. The resulting precipitate was re-dissolved in hexane for further usage and characterization. Halide treatment. Halide treatment for as-synthesized CQDs was achieved by modifying the previously reported method.34 For chloride treatment, 0.5 ml of 0.2 M of ammonium chloride solution (in methanol) was added to the reaction mixture after the reaction solution cooled down to 60 oC and the mixture solution was kept for 10 min. The CQDs were isolated by centrifugation. For in situ treatment, PbX2 (X=Br and Cl) and OLA are used as a Pb precursor and solvent instead of Pb oleate and ODE. Annealing treatment. The annealing of PbSe/PbS CQDs was performed in a glove box. A certain amount of PbSe/PbS solution in hexane was dissolved in HDC after evaporating hexane. The solution containing CQDs was heated to 80 oC and maintained for 15 minutes. Then, the temperature was increased in increments of 10 oC until 150 oC and maintained at each annealing temperature for 15 minutes. After annealing for 15 minutes, aliquots were taken and washed by adding ethanol. The CQDs were dissolved in hexane for further characterization. Characterization. Transmission electron microscopy (TEM) images, high resolution TEM (HR-TEM) images, and energydispersive X-ray spectroscopy (EDS) spectrum were taken by using a FEI Tecnai T20 operated at 200 keV and a FEI Titan at 300 keV. Samples for TEM measurement were prepared by dropping the solution containing QDs on a carbon-coated copper grid at room temperature. A Rigaku SmartLab diffractometer with Cu Kα radiation (λ = 1.5418 Å) was used for X-ray powder diffraction (XRD) measurements. The samples were prepared by depositing the CQDs onto a glass substrate. UVVIS absorption spectra of the CQDs were obtained by using a JASCO V-570 UV-VIS-NIR spectrometer and Agilent Cary 5000 UV-Vis-NIR spectrophotometer. Continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (PL) decay measurements were carried out at room temperature. The CQDs (embedded in hexane) were excited either by a continuous-wave 532 nm laser diode or by a pulsed Nd:YAG laser with 532 nm wavelength. The emission was detected by an Acton Spectrapro 2300i monochromator equipped with a photomultiplier tube (Hamamatsu NIR-PMT H10330-75) operating in the NIR spectral region. X-ray photoelectron spectroscopy (XPS) was performed in a Thermo VG Scientific Sigma Probe fitted with a monochromatic X-ray Al Kα (1486.6 eV) source. The samples for XPS analysis were prepared by depositing the CQDs onto a gold substrate. The binding energy (BE) of the elements was determined by calibrating with respect to the C 1s peak (284.5 eV) as a reference. The XPS peaks were fitted by using XPSPEAK 4.1 software after subtraction of the Shirley background.

Experimental Section Materials. Lead(II) oxide (PbO; 99.999%), lead(II) bromide (PbBr2; 99.999%), selenium (Se; 99.99%), sulfur (S; 99.99%), bis(trimethylsilyl) sulfide (TMS2S; synthesis grade), nhexadecane (HDC; 99%), 1-octadecene (ODE; tech. grade, 90%), oleic acid (OA; 90%), oleyamine (OLA; tech. grade, 70%), and acetonitrile (≥99.9%) were purchased from SigmaAldrich. Ammonium chloride (A.C.S. grade) was purchased from Spectrum Chemicals. Trioctylphosphine (TOP; 97%) and diphenylphosphine (DPP; 99%) were purchased from Strem. Lead(II) chloride (PbCl2; ≥98%) and tetrachloroethylene (TCE; spectroscopic grade) were purchased from Merck. Methanol (absolute), toluene (analytical), hexane (analytical), and ethanol (absolute) were purchased from Bio-Lab Ltd. Acetone (absolute) was purchased from Gadot. These chemicals were used without further purification. PbSe CQDs synthesis. The PbSe CQDs were synthesized by modifying the method established by Čapek et al.19 For the synthesis of PbSe CQDs, PbO (89 mg, 0.4 mmol), OA (339 mg, 1.2 mmol), and HDC (the total mass of mixture was 8 g by adjusting the amount of HDC) were mixed in a three-neck flask. Then, the mixture was heated to 100 °C under vacuum for 1 hour. During this time the reaction mixture became clear, indicating the formation of Pb oleate from the reaction between PbO and OA. The mixture solution was heated to designated temperature under nitrogen. Next, the chalcogenide precursor solution containing TOP (1.19 g, 3.2 mmol), 1 M TOPSe (0.4 mmol), DPP (0.15 g, 0.8 mmol), and HDC (the total volume of injection solution was 2 ml) was swiftly injected into the Pb oleate solution under nitrogen. Then, the temperature was reduced to the growth temperature and maintained throughout the reaction until appropriate growth was obtained. After a specified time, the reaction mixture cooled down and the CQDs were isolated by centrifuging, using acetonitrile and ethanol, repeated twice. Afterward, the CQDs were dried and then re-dissolved in organic solvent such as hexane, toluene, or TCE for further characterization. PbSe/PbS core/shell CQDs synthesis. PbSe/PbS CQDs were synthesized by using a modification of the procedure developed by Yanover et al.4 The amount of the S precursors was calculated by considering the concentration of PbSe CQDs and the desired number of PbS shell layers. The required mass of Pb precursors was estimated using 8:1 of Pb:S molar ratio. In a typical synthesis, PbO, OA (PbO:OA=1:8), and ODE (5.84 g) were prepared in a three-neck flask. The solution was heated to 100 oC under vacuum for at least 1 hour and then cooled down to room temperature under nitrogen. A hexane solution of PbSe core CQDs (3.7 × 10-7 mol) was injected into the Pb oleate solution. The solution was degassed for 30 minutes until hexane was evaporated. Afterward, the reaction mixture was heated to 80 oC and diluted TMS2S solution prepared by mixing 0.1 mL of TMS2S and 4 mL of ODE was injected dropwise into the reaction mixture using a syringe pump (0.1 mL/min). After obtaining the desired shell thickness, the reac-

Result and discussion Synthesis of Relatively Large PbSe CQDs. Relatively large PbSe CQDs for achieving absorption over NIR and SWIR range were synthesized by injecting a stock solution mixture containing TOPSe/TOP/DPP into Pb oleate solution prepared by the reaction of PbO with OA (see Experimental section for specific details). The size control of PbSe CQDs was achieved by varying injection and growth temperatures. The effect of


Page 3 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials taken from the reaction vessel at time intervals given in the legend. (B) Plot of absorption peak energy (solid squares, in eV) and FWHM (open circles, in meV) versus the reaction time at injection/growth temperatures as in (A) (color coded with the curves in A). (C) Series of absorption spectra of PbSe CQDs covering the absorption range of 1100-3200 nm, prepared at temperatures between 145 oC and 240 oC. (D) Development of the first excitonic absorption peak wavelength relative to the reaction time, for different reaction temperatures as indicated in the legend (Injection/growth temperature).

other conditions (e.g. Pb:OA and TOPSe:DPP:TOP ratios) on the size of CQDs can be found in our previous work.19 To follow size evolution during the reaction, aliquots were taken from the reaction vessel at defined time intervals and the precipitate was collected by centrifugation. No additional sizeselection techniques were performed. Figure 1A displays absorption spectra of PbSe CQDs prepared at injection/growth temperatures of 145 oC/130 oC for varying reaction times from 0.5 min to 16 min as indicated in the legend. These absorption spectra exhibit at least four resolved excitonic features, designating the formation of high-quality crystallites (see Figure S1 of Supporting Information). A plot of absorption peak energy (solid squares) and full width at half maximum (FWHM, open circles) of the first excitonic bands from Figure 1A is presented in Figure 1B. The plot shows that the growth of PbSe CQDs progresses with the reaction time and reaches a plateau after 16 minutes. Moreover, the FWHM in the beginning of the growth is relatively large (~120 meV), but becomes smaller as the reaction progresses, indicating narrowing of the size distribution due to size-focusing effect. The gradual change of the injection/growth temperatures in the synthesis of PbSe CQDs up to 240 oC /230 oC led to a steady growth of the CQDs’ size. Representative absorption spectra of the PbSe CQDs prepared under various temperatures and reaction times are displayed in Figure 1C. The collection of spectra reveals that the tunability of the excitonic peak up to 3200 nm (0.39 eV) was achieved. It is important to note that sharp peaks around 2700 nm are associated with the ligand vibrational modes. Figure 1D depicts the absorption peak wavelength evolution of PbSe CQDs versus reaction temperatures and duration, showing the expected energy red-shift of the excitonic absorption peak toward the SWIR range with the increase of QD’s size. Figure 1D also shows that the synthesis at the highest reaction temperatures induces a faster reaction rate of PbSe CQDs, while equilibrium absorption peak wavelength is reached within a short reaction time. These results reveal that growth to sizes that endow optical activity in the SWIR is feasible via a reaction in one-pot synthesis by a single injection process, which is definitely a simple and practical procedure. This is in contradiction to other studies reported in the past, employing multiple injections or seed-mediated growth via two-pot synthesis.14,27

Transmission electron microscopy (TEM) analysis was performed to determine the size of CQDs. Figure 2 presents representative TEM images of PbSe CQDs synthesized at various reaction temperatures. Figure 2A is a TEM image of 4.2 ± 0.3 nm-sized spherical PbSe CQDs taken from the reaction mixture at 4 min after injection at 145 oC. When the reaction temperature is raised, the CQDs’ size is increased (see Figure 2B and 2C). By injection at 190 oC and growth duration of 16 min, PbSe CQDs with average diameter of 8.6 ± 0.5 nm were obtained (see Figure 2C). When the injection temperature increased to 200 oC, truncated cubic-shaped PbSe CQDs with a mean size of 11.9 ± 0.7 nm were observed (Figure 2D). Furthermore, increasing the injection temperature beyond 200 oC led to the formation of cubic PbSe CQDs with sizes larger than 12 nm (Figure 2E and 2F). At low temperatures, spherical PbSe CQDs are prepared because a sphere has the lowest surface energy owing to minimum surface to volume ratio.14 As the size of CQDs increases at higher reaction temperature, a faceted cubic shape with an intrinsic rock-salt crystal structure of the PbSe is favored.14,43 This observation of the shape transition at larger sizes is in agreement with previous reports about IV-VI chalcogenides (e.g. PbSe,14,15,44 PbTe,45 and SnTe43).

Figure 2. TEM images of PbSe CQDs synthesized at different injection temperatures. (A) 145 oC, (B) 180 oC, (C) 190 oC, (D) 200 oC, (E) 220 oC, and (F) 240 oC.

The X-ray diffraction (XRD) measurement was carried out to confirm the crystal structure of as-synthesized PbSe CQDs. Figure 3A indicates that PbSe CQDs have a rock-salt crystal structure and all peaks of XRD pattern are well matched with (111), (200), (220), (311), (222), (400), (420), and (422) planes of bulk PbSe (JCPDS 06-0354). Weak peaks in the XRD pattern are attributed to oxides such as SeO2 and Pb3O4. Energy dispersive X-ray spectroscopy (EDS) analysis confirmed the atomic ratio of our CQDs. Figure 3B shows that our

Figure 1. (A) Absorption spectra of PbSe CQDs synthesized at injection/growth temperatures of 145 oC /130 oC. Aliquots were



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CQDs have slight cation (Pb) excess (Pb:Se=1.17:1) stoichiometry, consistent with previous studies.46 The nonstoichiometry may be associated with Pb-rich exterior surface of the CQDs.4,46

Page 4 of 9

XPS peak (right (iii) of Figure 4E) also shows the presence of Cl in CQDs.

Figure 3. Characteristics of 7.0 ± 0.3 nm-sized PbSe CQDs synthesized at injection/growth temperatures of 145 oC /130 oC. (A) XRD pattern. (B) EDS spectrum. The vertical lines in (A) show the peak positions of the reflections for bulk PbSe.

Figure 4. (A) TEM (left and right) and HRTEM (middle) images of PbSe CQDs before and after Cl treatment. (B) Absorption spectra of Cl-treated PbSe CQDs measured after air exposure according to the indicated times. (C) EDS spectrum of Cl-treated PbSe CQDs. (D) XRD patterns of as-synthesized (blue line) and Cltreated PbSe CQDs (red line) as shown in (A). (E) XPS spectra of Pb 4f (i), Se 3d (ii), and Cl 2p (iii) peaks of Cl-treated PbSe CQD. The location of Se oxides (e.g. SeO2 and SeO32-) is marked by an arrow.

Chloride-Capped PbSe CQDs. Previous studies revealed that PbSe CQDs are readily oxidized in ambient conditions, resulting in formation of oxide layer on the surface and significant enhancement of the carrier trapping efficiency.4,8,23 OA-capped PbSe CQDs showed a significant blue shift of the first excitonic feature on air exposure (see Figure S2 of Supporting Information), indicating that the as-synthesized PbSe CQDs are susceptible to oxidation, supported by the result in Figure 3A. This instability of PbSe CQDs leads to the loss of their optical and electronic properties, hindering their use in many optoelectronic applications. To prevent surface oxidation, halide passivation was considered.5,17,27,31-36,42 The chloride passivation was performed by adding ammonium chloride (NH4Cl) solution into the reaction mixture to get enhanced air stability of the CQDs. After Cl treatment, unreacted reagents were removed by centrifugation and the CQDs were collected. TEM images of PbSe CQDs before and after Cl treatment are presented in Figure 4A, indicating that there is almost no visible change of PbSe CQDs after the treatment, in accordance with previous reports.34 The HRTEM image shows the good crystallinity of PbSe CQDs is maintained after Cl treatment. Figure 4B shows the spectra monitored on consecutive days after air exposure. The spectra remain the same for more than 10 days, depicting enhanced air-stability of PbSe CQDs by Cl treatment. Previous studies reported that the improved stability results from the formation of PbClx adlayer on the surface of CQDs.32, 34 EDS spectrum shows the presence of Cl in the treated PbSe CQDs (Figure 4C). XRD patterns (Figure 4D) indicate that the crystal structure of Cl-treated PbSe CQDs is unchanged (see red and blue lines). Figure 4E presents Pb 4f, Se 3d, and Cl 2P XPS spectra of Cltreated PbSe CQDs. In the Pb 4f spectrum (left (i) of Figure 4E), two features were resolved into the Pb 4f5/2 and 4f7/2 core levels. XPS fitting was further conducted to ascertain the surface chemistry of PbSe CQDs, displaying the presence of PbCl (138.8 eV), Pb-Se (137.3 eV), and Pb-O (138.0 eV) from Pb oleate conjugation.34 Se 3d XPS spectrum (middle (ii) of Figure 4E) shows that the additional peaks from Se oxides (e.g. SeO2 and SeO32-; ~59 eV)23 are not seen, showing good air stability of these CQDs. In addition, the detection of Cl 2p

PbSe/PbS Core/Shell CQDs. Another approach for tuning optical activities is forming core/shell structures. The formation of core/shell heterostructures renders new electron and hold wave function distribution (e.g. type-I and type-II), depending on core and shell’s band offset as well as shell thickness. In the case of PbSe/PbS core/shell CQDs, the electron in the core is confined due to relatively larger conduction band offset (~0.155 eV),2 while the hole is delocalized because of the small valence band energy’s difference of bulk PbS and PbSe (~ 0.025 eV),2 leading to a red-shift of excitonic transition.22 Our previous reports using small sized PbSe CQDs presented that a PbS shell provided a red-shift of the excitonic trantision.4,23 PbS is a good candidate semiconductor as a shell material since both PbSe and PbS have the same rock-salt crystal structure and a similar lattice parameter (6.12 Å and 5.94 Å for bulk PbSe and PbS, respectively),47-49 leading to a small crystallographic mismatch (~3%). PbS shell formation on the PbSe CQDs was achieved by slowly injecting highly reactive sulfur precursors, TMS2S in the presence of Pb oleate precursor and PbSe CQDs. More details for the synthesis PbSe/PbS core/shell CQDs are given in the Experimental Section. Controlled slow injection by a syringe pump was employed to prevent the formation of PbS nucleation. Consistent injection of sulfur precursors is important to make uniform PbSe/PbS CQDs, whereas manual injection could lead to an asymmetrical shape of the first excitonic peak, suggesting non-uniform PbS growth on the surface of PbSe CQDs. Moreover, it was found that the uniformity of the core/shell CQDs depends on the type of Pb precursor and its concentration. While an asymmetric excitonic peak with long tail at longer wavelength was observed when Pb acetate was used as a precursor, the PbO precursor was found to be optimal for obtaining PbSe/PbS core/shell with symmetric absorption peak. This may be explained by the effect of acetate remained in reaction mixture.50 A high concentration of PbO (e.g. PbO:TMS2S=10:1) led to secondary PbS nucleation,


Page 5 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


proven by the appearance of two emission peaks (not shown here), whereas an extreme low amount of PbO caused nearly no shell growth. Thus, the regulation of the amount of TMS2S precursor was critical for the control of the shell growth to achieve different shell thicknesses. Figure 5A shows representative TEM images of the PbSe core (3.9 ± 0.3 nm) and the corresponding PbSe/PbS core/shell CQDs with average diameter of 5.3 ± 0.3 nm (0.7 nm of shell thickness). The shell thickness was calculated by comparing the sizes of PbSe core and PbSe/PbS core/shell CQDs in the TEM analysis. A HRTEM image of the PbSe/PbS core/shell CQDs is shown in the middle of Figure 5A, revealing a high crystallinity and no distinct indications for core/shell interface due to the close crystallographic matching between PbSe core and PbS shell, in good agreement with previous results.8,22 A typical development of the absorption spectra during PbS shell growth is presented in Figure 5B, showing the first exciton transition shifts toward longer wavelength with increasing injection time. Figure 5C gives the corresponding change of position and FWHM value of the absorption peak during the shell growth, showing that the FWHM decreases as PbS shell grows. The absorption (dash line) and emission (solid line) spectra of the PbSe core and the corresponding PbSe/PbS core/shell CQDs are displayed in Figure 5D, presenting a small Stokes shift (~ 20 meV), close to previous theoretical calculation.51 FWHM values of absorption and emission bands are estimated to be 71.7 meV and 87.9 meV, smaller relative to the corresponding PbSe core CQDs (94.0 meV and 98.6 meV, respectively). XRD spectra of PbSe core (black line) and PbSe/PbS core/shell (red line) CQDs are shown in Figure 5E. XRD patterns present all peaks corresponding to the rock-salt crystal structure (see pink and green vertical lines for bulk PbS and PbSe, respectively). Peaks of core/shell CQDs shift towards bulk PbS line (see pink marks in Figure), in fair agreement with previous results.30

Alloyed PbSe/PbS Core/Shell CQDs. Although a PbS is considered as a good shell material due to close crystallographic parameter (6.12 Ǻ and 5.94 Ǻ for bulk PbSe and PbS, respectively), a small lattice mismatch cannot be avoided. The crystallographic mismatch can give the strain (large strain is experienced in the interface of core and shell), leading to the formation of defect. Annealing process can release the strain by forming alloyed layer between core and shell. The reduction of strain leads to influence of energy band offset as well as band structure. The previous results revealed that an annealing process of CQDs may induce red-shift of optical properties.52-54 The theoretical calculation55 also suggested that alloy composition in CQDs could tune the electronic properties (e.g. band gap energy). Post-annealing treatment of PbSe/PbS core/shell CQDs was performed by heating the CQDs solution at various temperatures in inert conditions. Representative HRTEM images of the CQDs annealed at 100 oC and 130 oC are displayed in Figure S3 of the Supporting Information, showing wellresolved lattice planes. The absorption (dash lines) and emission (bold lines) spectra of annealed PbSe/PbS core/shell CQDs with 5.3 nm of mean diameter are given in Figure 6A, showing a red shift of the peak with the increase of annealing temperature. Figure 6B presents the plot of absorption and emission transition versus the annealing temperatures, indicating that post-annealing can lead to an additional red shift. A more detailed discussion on alloyed CQD’s electron structures will be reported elsewhere.

Figure 6. (A) Absorption (dash lines) and emission (bold lines) spectra of PbSe/PbS CQDs after annealing at the indicated temperatures. (B) Plot of absorption peak (black squares) and emission (red circles) wavelength of PbSe/PbS and the corresponding alloyed CQDs prepared at 6 different temperatures given in x-axis.

Chloride-Capped PbSe/PbS Core/Shell CQDs. The airstability of PbSe CQDs was improved for a short time by forming PbS shell.23 However, blue shift (~30 nm) of the absorption spectrum was observed after air exposure for 1 day (see Figure S4 of the Supporting Information), indicating the surface oxidation on PbSe/PbS core/shell CQDs. This implies that PbS shell may slow the oxidation process of PbSe CQDs, rather than prevent the formation of oxides. Cl treatment was carried out for PbSe/PbS core/shell CQDs. Figure 7A-D shows representative TEM images of PbSe core and the corresponding Cl-treated PbSe/PbS core/shell CQDs with different shell thickness, prepared by varying the amount of sulfur precursors and adding Cl agents. Figure 7A presents 3.9 ± 0.3 nm PbSe core CQDs; Figure 7B-D displays the sizes of the corresponding core/shell CQDs: 4.5 ± 0.3 nm, 4.7 ± 0.3 nm, and 5.3 ± 0.4 nm. The PbS shell thicknesses were measured to be 0.3 nm, 0.4 nm, and 0.7 nm, respectively.

Figure 5. (A) TEM (left and right) and HRTEM (middle) images of PbSe core and PbSe/PbS core/shell CQDs. (B) Absorption spectra of aliquots taken at the defined times during injection of sulfur precursors. (C) Plot of absorption peak energy (black squares, in eV) and FWHM (red circles, in meV) versus injection times as measured from the spectra shown in (B). (D) Absorption (dash lines) and emission (bold lines) spectra of PbSe core and PbSe/PbS core/shell CQDs in (A). (E) XRD patterns of PbSe core (black line) and PbSe/PbS core/shell CQDs (red line). Bulk PbSe (green vertical lines) and PbS (pink vertical lines) peak positions are presented in bottom.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 9

reaction temperature enabled formation of PbSe CQDs exhibiting the deep SWIR regime up to 3.2 µm by increasing the CQDsʹ size up to 14 nm. Another pathway, PbSe/PbS core/shell CQDs, was explored. A controlled injection method led to tuning the shell thickness, resulting in a red-shift of excitonic transition. These CQDs exhibited relatively narrow excitonic peaks and a small Stokes shift with respect to PbSe core CQDs. Furthermore, a post-annealing process provided further bandgap tunability. The PbSe/PbS core/shell CQDs provided better air stability when compared with original PbSe CQDs, but after a short time ( < 1 day), optical property was degraded. A post- or insitu halogenation (e.g. chlorination and bromination) rendered high surface stability to relatively large PbSe and the PbSe/PbS core/shell CQDs. Our investigation provides an insight to tuning IV-VI CQDsʹ optical property as well as to maintaining sustainable surface stability. It is expected that these CQDs can be employed for spectroscopy and imaging applications using SWIR region and fundamental studies of photophysics.

Figure 7. TEM images of (A) PbSe core and (B-D) Cl-treated PbSe/PbS core/shell CQDs with different PbS shell thickness (B: 0.3 nm, C: 0.4 nm and D: 0.7 nm). (E) Absorption (dash lines) and emission (bold lines) spectra of same CQDs as in (A-D). (F) Absorption spectra of Cl-treated PbSe/PbS CQDs (core: 3.9 nm, core/shell: 4.8 nm) at air-exposure as indicated in the legend.

The absorption and emission spectra of PbSe core CQDs with the size of 3.9 nm and the corresponding PbSe/PbS core/shell CQDs with the diameter of 4.5-5.3 nm are shown in Figure 7E. It displays the continuous red-shift of the first excitonic transition with respect to that of the PbSe CQDs as PbS shell was grown. The red-shift values of absorption spectra were estimated to be 69.1 meV (93 nm), 118 meV (167 nm), and 162 meV (243 nm) for PbSe/PbS core/shell CQDs with 0.3 nm, 0.4 nm, and 0.7 nm of the shell thickness, respectively. FWHM values of Cl-treated PbSe/PbS core/shell CQDs in first absorption band were estimated to be 85.8 meV, 88.8 meV, and 78.0 meV, indicating a narrower absorption edge of the PbSe/PbS core/shell CQDs than that of the PbSe core sample (94.0 meV). In addition, all Stokes shifts of Cl-treated PbSe/PbS core/shell CQDs (22-24 meV) are smaller than that of the initial PbSe cores (37.2 meV). It should be noted that efforts using in situ halide (e.g. Br and Cl) treatment were carried out, giving similar result to that of post treatment. Replacing Pb precursor with PbX2 (X=Br or Cl) provided the preparation of halide-capped PbSe/PbS CQDs by in situ passivation, corresponding previous reports about PbS and PbSe CQDs.17,18,56,57 Our preliminary results showed that using PbBr2 led to a bigger red-shift as well as larger FWHM value than PbCl2, due to faster growth from higher diffusion coefficient (D(PbBr2) > D(PbCl2)).17 (see Figure S5 of the Supporting Information). The stability test of Cl-treated PbSe/PbS core/shell CQDs is presented in Figure 7F, showing that the excitonic peak has nearly the same position for at least up to three weeks. Comparing the monitoring result of PbSe/PbS core/shell CQDs (Figure S4), it is confirmed that Cl passivation provides improved stability.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the ACS Publication website at http://pubs.acs.org. Absorption spectrum, stability test, TEM, and HRTEM images.

AUTHOR INFORMATION Corresponding Author E-mails: [email protected]

Author Contributions †A.S. and Y.J. contributed equally to this work.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the Israel Council for Higher Education-Focal Area Technology (Project No. 872967), the Volkswagen Stiftung (Project No.88116), the Israel Ministry of Defense (Project No. 4440665406), the Israel Ministry of Trade (Maymad Project No. 54662), the Israel Science Foundation Bikura (Project No. 1508/14), the Israel Science Foundation (Project No. 985/11 and 914/15), and the Niedersachsen-Deutsche Technion Gesellschaft E.V (Project No. ZN2916). A.K.B. thanks to Marie Curie project PHONSI for the fellowship support.

REFERENCES (1) Antunez, P. D.; Buckley, J. J.; Brutchey, R. L., Tin and Germanium Monochalcogenide IV-VI Semiconductor Nanocrystals for Use in Solar Cells. Nanoscale 2011, 3, 2399-2411. (2) Sashchiuk, A.; Yanover, D.; Rubin-Brusilovski, A.; Maikov, G. I.; Čapek, R. K.; Vaxenburg, R.; Tilchin, J.; Zaiats, G.; Lifshitz, E., Tuning of Electronic Properties in IV-VI Colloidal Nanostructures by Alloy Composition and Architecture. Nanoscale 2013, 5, 7724-7745. (3) Xiao, G.; Wang, Y.; Ning, J.; Wei, Y.; Liu, B.; Yu, W. W.; Zou, G.; Zou, B., Recent Advances in IV–VI Semiconductor

Conclusion In summary, this work presents comprehensive study for obtaining tunable optical activity in the NIR and SWIR range and high surface stability of PbSe CQDs. Two different approaches (i.e. relatively large PbSe and PbSe/PbS core/shell CQDs) were investigated. Relatively large PbSe CQDs were synthesized by a controlled one-pot procedure. Control of the


Page 7 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials Significance of Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots for Optoelectronic Applications. J. Phys. Chem. C 2014, 118, 17001-17009.

Nanocrystals: Synthesis, Mechanism, and Applications. RSC Adv. 2013, 3, 8104-8130. (4) Yanover, D.; Čapek, R. K.; Rubin-Brusilovski, A.; Vaxenburg, R.; Grumbach, N.; Maikov, G. I.; Solomeshch, O.; Sashchiuk, A.; Lifshitz, E., Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots. Chem. Mater. 2012, 24, 4417-4423. (5) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E. H., Colloidal-Quantum-Dot Photovoltaics using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765-771. (6) Etgar, L.; Yanover, D.; Čapek, R. K.; Vaxenburg, R.; Xue, Z. S.; Liu, B.; Nazeeruddin, M. K.; Lifshitz, E.; Grätzel, M., Core/Shell PbSe/PbS QDs TiO2 Heterojunction Solar Cell. Adv. Funct. Mater. 2013, 23, 2736-2741. (7) Schaller, R. D.; Petruska, M. A.; Klimov, V. I., Tunable Near-Infrared Optical Gain and Amplified Spontaneous Emission Using PbSe Nanocrystals. J. Phys. Chem. B 2003, 107, 13765-13768. (8) Lifshitz, E.; Brumer, M.; Kigel, A.; Sashchiuk, A.; Bashouti, M.; Sirota, M.; Galun, E.; Burshtein, Z.; Le Quang, A. Q.; Ledoux-Rak, I.; Zyss, J., Air-Stable PbSe/PbS and PbSe/PbSexS1-x Core/Shell Nanocrystal Quantum Dots and Their Applications. J. Phys. Chem. B 2006, 110, 25356-25365. (9) Talapin, D. V.; Murray, C. B., PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors. Science 2005, 310, 86-89. (10) Snyder, G. J.; Toberer, E. S., Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105-114. (11) Korkosz, R. J.; Chasapis, T. C.; Lo, S.-h.; Doak, J. W.; Kim, Y. J.; Wu, C.-I.; Hatzikraniotis, E.; Hogan, T. P.; Seidman, D. N.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G., High ZT in pType (PbTe)1–2x(PbSe)x(PbS)x Thermoelectric Materials. J. Am. Chem. Soc. 2014, 136, 3225-3237. (12) Alivisatos, A. P., The Use of Nanocrystals in Biological Detection. Nat. Biotechnol. 2004, 22, 47-52. (13) Brumer, M.; Sirota, M.; Kigel, A.; Sashchiuk, A.; Galun, E.; Burshtein, Z.; Lifshitz, E., Nanocrystals of PbSe Core, PbSe/PbS, and PbSe/PbSexS1-x Core/Shell as Saturable Absorbers in Passively QSwitched Near-Infrared Lasers. Appl. Opt. 2006, 45, 7488-7497. (14) Pietryga, J. M.; Schaller, R. D.; Werder, D.; Stewart, M. H.; Klimov, V. I.; Hollingsworth, J. A., Pushing the Band Gap Envelope: Mid-Infrared Emitting Colloidal PbSe Quantum Dots. J. Am. Chem. Soc. 2004, 126, 11752-11753. (15) Lu, W. G.; Fang, J. Y.; Ding, Y.; Wang, Z. L., Formation of PbSe Nanocrystals: A Growth toward Nanocubes. J. Phys. Chem. B 2005, 109, 19219-19222. (16) Li, H.; Chen, D.; Li, L.; Tang, F.; Zhang, L.; Ren, J., Sizeand Shape-Controlled Synthesis of PbSe and PbS Nanocrystalsvia a Facile Method. CrystEngComm 2010, 12, 1127-1133. (17) Zhang, J.; Gao, J.; Miller, E. M.; Luther, J. M.; Beard, M. C., Diffusion-Controlled Synthesis of PbS and PbSe Quantum Dots with in Situ Halide Passivation for Quantum Dot Solar Cells. ACS Nano 2014, 8, 614-622. (18) Weidman, M. C.; Beck, M. E.; Hoffman, R. S.; Prins, F.; Tisdale, W. A., Monodisperse, Air-Stable PbS Nanocrystals via Precursor Stoichiometry Control. ACS Nano 2014, 8, 6363-6371. (19) Čapek, R. K.; Yanover, D.; Lifshitz, E., Size Control by Rate Control in Colloidal PbSe Quantum Dot Synthesis. Nanoscale 2015, 7, 5299-5310. (20) Bashouti, M.; Lifshitz, E., PbS Sub-Micrometer Structures with Anisotropic Shape: Ribbons, Wires, Octapods, and Hollowed Cubes. Inorg. Chem. 2008, 47, 678-682. (21) Maikov, G. I.; Vaxenburg, R.; Sashchiuk, A.; Lifshitz, E., Composition-Tunable Optical Properties of Colloidal IV-VI Quantum Dots, Composed of Core/Shell Heterostructures with Alloy Components. ACS Nano 2010, 4, 6547-6556. (22) Brumer, M.; Kigel, A.; Amirav, L.; Sashchiuk, A.; Solomesch, O.; Tessler, N.; Lifshitz, E., PbSe/PbS and PbSe/PbSexS1x Core/Shell Nanocrystals. Adv. Funct. Mater. 2005, 15, 1111-1116. (23) Yanover, D.; Vaxenburg, R.; Tilchin, J.; RubinBrusilovski, A.; Zaiats, G.; Čapek, R. K.; Sashchiuk, A.; Lifshitz, E.,

(24) Kovalenko, M. V.; Kaufmann, E.; Pachinger, D.; Roither, J.; Huber, M.; Stangl, J.; Hesser, G.; Schäffler, F.; Heiss, W., Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap Energies: From Telecommunications to Molecular Vibrations. J. Am. Chem. Soc. 2006, 128, 3516-3517. (25) Kovalenko, M. V.; Heiss, W.; Shevchenko, E. V.; Lee, J.S.; Schwinghammer, H.; Alivisatos, A. P.; Talapin, D. V., SnTe Nanocrystals: A New Example of Narrow-Gap Semiconductor Quantum Dots. J. Am. Chem. Soc. 2007, 129, 11354-11355. (26) Hu, C.; Gassenq, A.; Justo, Y.; Devloo-Casier, K.; Chen, H.; Detavernier, C.; Hens, Z.; Roelkens, G., Air-Stable Short-Wave Infrared PbS Colloidal Quantum Dot Photoconductors Passivated with Al2O3 Atomic Layer Deposition. Appl. Phys. Lett. 2014, 105, 171110. (27) Sayevich, V.; Gaponik, N.; Plötner, M.; Kruszynska, M.; Gemming, T.; Dzhagan, V. M.; Akhavan, S.; Zahn, D. R. T.; Demir, H. V.; Eychmüller, A., Stable Dispersion of Iodide-Capped PbSe Quantum Dots for High-Performance Low-Temperature Processed Electronics and Optoelectronics. Chem. Mater. 2015, 27, 4328-4337. (28) Sykora, M.; Koposov, A. Y.; McGuire, J. A.; Schulze, R. K.; Tretiak, O.; Pietryga, J. M.; Klimov, V. I., Effect of Air Exposure on Surface Properties, Electronic Structure, and Carrier Relaxation in PbSe Nanocrystals. ACS Nano 2010, 4, 2021-2034. (29) Chappell, H. E.; Hughes, B. K.; Beard, M. C.; Nozik, A. J.; Johnson, J. C., Emission Quenching in PbSe Quantum Dot Arrays by Short-Term Air Exposure. J. Phys. Chem. Lett. 2011, 2, 889-893. (30) Choi, H.; Song, J. H.; Jang, J.; Mai, X. D.; Kim, S.; Jeong, S., High Performance of PbSe/PbS Core/Shell Quantum Dot Heterojunction Solar Cells: Short Circuit Current Enhancement without the Loss of Open Circuit Voltage by Shell Thickness Control. Nanoscale 2015, 7, 17473-17481. (31) Zhang, J.; Gao, J.; Church, C. P.; Miller, E. M.; Luther, J. M.; Klimov, V. I.; Beard, M. C., PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient Atmosphere. Nano Lett. 2014, 14, 6010-6015. (32) Bae, W. K.; Joo, J.; Padilha, L. A.; Won, J.; Lee, D. C.; Lin, Q.; Koh, W.-k.; Luo, H.; Klimov, V. I.; Pietryga, J. M., Highly Effective Surface Passivation of PbSe Quantum Dots through Reaction with Molecular Chlorine. J. Am. Chem. Soc. 2012, 134, 20160-20168. (33) Ning, Z.; Ren, Y.; Hoogland, S.; Voznyy, O.; Levina, L.; Stadler, P.; Lan, X.; Zhitomirsky, D.; Sargent, E. H., All-Inorganic Colloidal Quantum Dot Photovoltaics Employing Solution-Phase Halide Passivation. Adv. Mater. 2012, 24, 6295-6299. (34) Woo, J. Y.; Ko, J.-H.; Song, J. H.; Kim, K.; Choi, H.; Kim, Y.-H.; Lee, D. C.; Jeong, S., Ultrastable PbSe Nanocrystal Quantum Dots via in Situ Formation of Atomically Thin Halide Adlayers on PbSe(100). J. Am. Chem. Soc. 2014, 136, 8883-8886. (35) Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J. P.; Minor, J.; Kemp, K. W.; Dong, H.; Rollny, L.; Labelle, A.; Carey, G.; Sutherland, B.; Hill, I.; Amassian, A.; Liu, H.; Tang, J.; Bakr, O. M.; Sargent, E. H., AirStable n-Type Colloidal Quantum Dot Solids. Nat. Mater. 2014, 13, 822-828. (36) Kim, S.; Marshall, A. R.; Kroupa, D. M.; Miller, E. M.; Luther, J. M.; Jeong, S.; Beard, M. C., Air-Stable and Efficient PbSe Quantum-Dot Solar Cells Based upon ZnSe to PbSe CationExchanged Quantum Dots. ACS Nano 2015, 9, 8157-8164. (37) Chuang, C. H.; Brown, P. R.; Bulovic, V.; Bawendi, M. G., Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796-801. (38) Woo, J. Y.; Lee, S.; Lee, S.; Kim, W. D.; Lee, K.; Kim, K.; An, H. J.; Lee, D. C.; Jeong, S., Air-Stable PbSe Nanocrystals Passivated by Phosphonic Acids. J. Am. Chem. Soc. 2016, 138, 876883.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 9

(48) Delin, A.; Ravindran, P.; Eriksson, O.; Wills, J. M., FullPotential Optical Calculations of Lead Chalcogenides. Int. J. Quantum Chem. 1998, 69, 349-358. (49) Zaiats, G.; Yanover, D.; Vaxenburg, R.; Tilchin, J.; Sashchiuk, A.; Lifshitz, E., PbSe-Based Colloidal Core/Shell Heterostructures for Optoelectronic Applications. Materials 2014, 7, 7243-7275. (50) Houtepen, A. J.; Koole, R.; Vanmaekelbergh, D.; Meeldijk, J.; Hickey, S. G., The Hidden Role of Acetate in the PbSe Nanocrystal Synthesis. J. Am. Chem. Soc. 2006, 128, 6792-6793. (51) Leitsmann, R.; Bechstedt, F., Characteristic Energies and Shifts in Optical Spectra of Colloidal IV-VI Semiconductor Nanocrystals. ACS Nano 2009, 3, 3505-3512. (52) Grumbach, N.; Capek, R. K.; Tilchin, E.; RubinBrusilovski, A.; Yang, J.; Ein-Eli, Y.; Lifshitz, E., Comprehensive Route to the Formation of Alloy Interface in Core/Shell Colloidal Quantum Dots. J. Phys. Chem. C 2015, 119, 12749-12756. (53) Isarov, M.; Grumbach, N.; Maikov, G. I.; Tilchin, J.; Jang, Y.; Sashchiuk, A.; Lifshitz, E., The Effect of Low Temperature Coating and Annealing on Structural and Optical Properties of CdSe/CdS Core/Shell QDs. Lith. J. Phys. 2016, 55, 292-305. (54) Tschirner, N.; Lange, H.; Schliwa, A.; Biermann, A.; Thomsen, C.; Lambert, K.; Gomes, R.; Hens, Z., Interfacial Alloying in CdSe/CdS Heteronanocrystals: A Raman Spectroscopy Analysis. Chem. Mater. 2011, 24, 311-318. (55) Vaxenburg, R.; Lifshitz, E., Alloy and Heterostructure Architectures as Promising Tools for Controlling Electronic Properties of Semiconductor Quantum Dots. Phys. Rev. B 2012, 85, 075304. (56) Yuan, L.; Patterson, R.; Cao, W.; Zhang, Z.; Zhang, Z.; Stride, J. A.; Reece, P.; Conibeer, G.; Huang, S., Air-Stable PbS Quantum Dots Synthesized with Slow Reaction Kinetics via a PbBr2 Precursor. RSC Adv. 2015, 5, 68579-68586. (57) Moreels, I.; Justo, Y.; De Geyter, B.; Haustraete, K.; Martins, J. C.; Hens, Z., Size-Tunable, Bright, and Stable PbS Quantum Dots: A Surface Chemistry Study. ACS Nano 2011, 5, 20042012.

(39) Zaiats, G.; Yanover, D.; Vaxenburg, R.; Shapiro, A.; Safran, A.; Hesseg, I.; Sashchiuk, A.; Lifshitz, E., PbSe/CdSe ThinShell Colloidal Quantum Dots. Z. Phys. Chem. 2015, 229, 3-21. (40) Zaiats, G.; Shapiro, A.; Yanover, D.; Kauffmann, Y.; Sashchiuk, A.; Lifshitz, E., Optical and Electronic Properties of Nonconcentric PbSe/CdSe Colloidal Quantum Dots. J. Phys. Chem. Lett. 2015, 6, 2444-2448. (41) Pietryga, J. M.; Werder, D. J.; Williams, D. J.; Casson, J. L.; Schaller, R. D.; Klimov, V. I.; Hollingsworth, J. A., Utilizing the Lability of Lead Selenide to Produce Heterostructured Nanocrystals with Bright, Stable Infrared Emission. J. Am. Chem. Soc. 2008, 130, 4879-4885. (42) Zhang, Z.; Yang, J.; Wen, X.; Yuan, L.; Shrestha, S.; Stride, J. A.; Conibeer, G. J.; Patterson, R. J.; Huang, S., Effect of Halide Treatments on PbSe Quantum Dot Thin Films: Stability, Hot Carrier Lifetime, and Application to Photovoltaics. J. Phys. Chem. C 2015, 119, 24149-24155. (43) Guo, S.; Fidler, A. F.; He, K.; Su, D.; Chen, G.; Lin, Q.; Pietryga, J. M.; Klimov, V. I., Shape-Controlled Narrow-Gap SnTe Nanostructures: From Nanocubes to Nanorods and Nanowires. J. Am. Chem. Soc. 2015, 137, 15074-15077. (44) Tan, T. T.; Selvan, S. T.; Zhao, L.; Gao, S. J.; Ying, J. Y., Size control, Shape Evolution, and Silica Coating of Near-InfraredEmitting PbSe Quantum Dots. Chem. Mater. 2007, 19, 3112-3117. (45) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P.; Mićić, O. I.; Ellingson, R. J.; Nozik, A. J., PbTe Colloidal Nanocrystals: Synthesis, Characterization, and Multiple Exciton Generation. J. Am. Chem. Soc. 2006, 128, 32413247. (46) Dai, Q.; Wang, Y.; Li, X.; Zhang, Y.; Pellegrino, D. J.; Zhao, M.; Zou, B.; Seo, J.; Wang, Y.; Yu, W. W., Size-Dependent Composition and Molar Extinction Coefﬁcient of PbSe Semiconductor Nanocrystals. ACS Nano 2009, 3, 1518-1524. (47) Fan, Z.; Koster, R. S.; Wang, S.; Fang, C.; Yalcin, A. O.; Tichelaar, F. D.; Zandbergen, H. W.; van Huis, M. A.; Vlugt, T. J., A Transferable Force Field for CdS-CdSe-PbS-PbSe Solid Systems. J. Chem. Phys. 2014, 141, 244503.


Page 9 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Insert Table of Contents artwork here

9 Environment ACS Paragon Plus

Tuning Optical Activity of IVâVI Colloidal Quantum Dots in the Short

Recommend Documents

Tuning Optical Activity of IVâVI Colloidal Quantum Dots in the Short