Near-infrared Colloidal Manganese Doped Quantum Dots

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Near-infrared Colloidal Manganese Doped Quantum Dots: Photoluminescence mechanism and Temperature Response Hui Zhang, Jiabin Liu, Chao Wang, Gurpreet S. Selopal, David Barba, Zhiming M. Wang, Shuhui Sun, Haiguang Zhao, and Federico Rosei ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00491 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Near-infrared Colloidal Manganese Doped Quantum Dots: Photoluminescence mechanism and Temperature Response Hui Zhang,† Jiabin Liu,† Chao Wang,† Gurpreet S. Selopal,†, § David Barba,† Zhiming M. Wang,§ Shuhui Sun,† Haiguang Zhao,,‡ and Federico Rosei,†

†Centre

for Energy, Materials and Telecommunications, Institut National de la Recherche Scientifique,

1650 Boulevard Lionel-Boulet Varennes, Québec J3X 1S2, Canada ‡College

of Physics & State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University, No. 308

Ningxia Road, Qingdao 266071, P. R. China §Institute

of Fundamental and Frontier Science, University of Electronic Science and Technology of

China, Chengdu 610054, P. R. China

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Abstract Doping in semiconductor quantum dots (QDs) is a promising approach for introducing unique properties compared to pure QDs. Although Manganese (Mn) ions doped wide band gap QDs have been widely studied, the optical properties of Mn-doped narrow band gap semiconductors are not well understood. Here, we report the synthesis of oil-soluble Mn-doped lead sulfide (PbS) QDs of identical size with different Mn contents. Compared to pure PbS QDs, the photoluminescence (PL) peak positions of Mndoped PbS QDs exhibit a significant red-shift, resulting in a larger Stokes shift. The large Stokes shift of Mn-doped QDs is due to the electronic state of Mn ions, in which the photogenerated electron is transferred to the energy states of Mn ions and then recombined with holes. Mn-doped PbS QDs exhibit a faster temperature-dependent PL response compared to pure PbS QDs, demonstrating that the Mn-doped PbS QDs are promising alternatives for use as thermal sensors.

Keywords: manganese doped, quantum dots, photoluminescence mechanism, lead sulfide, temperaturedependent optical properties


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The controlled synthesis of semiconductor quantum dots (QDs) has defined new opportunities to use them as building blocks in emerging technologies, such as optoelectronic, photocatalyst and thermal sensing devices.1-7 Among various types of QDs (bare QDs, core/shell QDs, alloyed QDs etc.), doped QDs exhibit unique features as they allow to introduce magnetic, optical, and electronic properties originating from the confined interaction between the charge carriers of the semiconductor host and the dopant ions.8 Doping with transition metal ions such as manganese (Mn) and copper (Cu) allows to modify the electronic, magnetic and photophysical properties of QDs.8-15 For instance, Mn doping significantly changes the electronic and magnetic properties of Cu2ZnSnS4 and perovskite (CsPbX3, CsGeX3) nanocrystals.8, 16-20 Many types of optoelectronic devices use doped QDs as light harvesters and exhibit improved performance compared to that of pure QDs, such as QDs sensitized solar cells using Mn-doped CdS/CdSe QDs9 and QDs based luminescent solar concentrators (LSCs) using Mn-doped ZnSe QDs21. In addition, doped QDs have also been used for nanoscale optical thermometry. For example, Vlaskin et al. reported a dual-emission temperature sensor based on Mn-doped Zn1-xMnxSe/ZnCdSe QDs.22 Besides the case of quasi-zero dimensional QDs, Mn doping in one dimensional nanowires and two dimensional nanoplatelets has been widely explored in recent years.19, 23-25 Dopants in semiconductor QDs can create electronic states that allow to alter the exciton recombination dynamics.9, 12 In general, most reported exciton dynamic mechanisms belong to either band edge emission (also named exciton emission) or dopant emission (d-emission). An electron-hole pair is generated when the intrinsic QD is excited by a photon of energy higher than the semiconductor band gap. The band edge emission occurs by the direct recombination of the electron-hole pair, which is “quantum confined”.26 The d-emission occurs in particular cases of the transition metal ion doped QDs, which is fundamentally different from band edge emission. For instance, the dopant could create electronic states in the intragap region of the QDs, thus altering the exciton recombination dynamics. Specifically, the photoexcited electrons are transferred to the electronic levels of the transition metal ions and subsequently recombination in the transition metal ion center results in a characteristic d-emission from this metal ion.27 Generally, the above-mentioned d-emission only occurs when the energy levels of doped transition metal ions are located between the conduction band (CB) and valence band (VB) of the wide bandgap host semiconductor QDs. To date, Mn-doped semiconductors were investigated for their photoluminescence (PL) properties. Mn ions exhibits exchange interactions with band edge carriers in several semiconductor QDs, such as CdS,28 CdSe,12 ZnS

29

and ZnSe

21, 26, 30-31

QDs. For example, Irvine

et al.30 reported that in Mn-doped ZnSe QDs electrons are photoexcited from VB to CB of ZnSe QDs, and subsequently, the excited electrons are transferred to the 4T1 upper florescent state of Mn ions within a short time (picosecond time-scale) and decay radiatively to the 6A1 state within a microsecond time-scale. These studies focused on Mn-doped wide band gap semiconductor hosts, in which the excitation energy is 3 ACS Paragon Plus Environment

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transferred to a Mn d-state, resulting in short-range d-emission. The PL of these reported Mn-doped QDs was found to change significantly due to the presence of atomic-like Mn d-emission that arises due to 4T16A

1

radiative transition. So far, few reports have described Mn-doped narrow band gap materials. Two

groups reported Mn-doped PbS QDs. Gen et al. reported the organic soluble Mn-doped PbS QDs and focused on their carrier spin polarization measured by circularly polarized magneto-PL, while the synthetic procedure was not detailed.32 Turyanska et al. reported the synthesis of Mn-doped PbS QDs with large particle size of 5 nm in aqueous solution and showed their promising application in toxicology and medical imaging.33-34 To date, the temperature-dependent optical properties and emission mechanism of Mn-doped PbS QDs are still not well understood, in particular concerning small sized Mn-doped PbS QDs. Here we report the synthesis of oil-soluble small sized Mn-doped PbS QDs in organic phase with uniform size distribution and well-controlled Mn content. The optical, magnetic properties, including absorption, PL, PL decay and Electron paramagnetic resonance (EPR) spectra of the colloidal QDs, have been studied for Mn-doped PbS QDs with identical size (∼2.8 ± 0.4 nm in diameter) and different Mn content of 0.00%, 0.70% and 1.66% (given in molar concentration). The absorption peaks remain similar for PbS and Mn-doped PbS QDs, whereas the PL peaks reveal a significant red-shift after the incorporation of Mn, resulting in larger Stokes shifts compared to that in pure PbS QDs. We propose a schematic illustration for the PL mechanism in the emission process, which does not correspond to the well-known exciton emission in intrinsic pure QDs nor to the reported d-emission in doped QDs. Subsequently we study the PL behavior of as-prepared pure PbS and Mn-doped PbS QDs in the 96–296 K temperature range. The temperature dependence of PL is investigated by exploring both the PL peak energy shifts and the PL intensity variations. The effect of temperature on the PL peak position in Mn-doped PbS QDs exhibits a different behavior compared to that in pure QDs, which is caused by the different emission process. The device based on Mn-doped PbS QDs was found to be more sensitive to temperature compared to pure PbS QDs, which makes it a potential candidate for applications in thermal sensing.

Experimental Materials: Aniline, 3,5-Bis(trifluoromethyl)phenyl isothiocyanate, toluene, lead (II) acetate trihydrate [Pb(CH3CO2)2·3H2O], manganese acetate trihydrate [Mn(CH3CO2)2·3H2O], oleic acid (OA), hexadecane, diphenyl

ether,

acetone,

ethanol,

methanol,

1-octadecene

(ODE),

chloroform,

and

poly(methylmethacrylate) (PMMA). All chemicals were bought from Sigma-Aldrich Co. and used as purchased without any purification. 4 ACS Paragon Plus Environment

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Synthesis of pure PbS QDs and Mn-doped PbS QDs: Our synthesis approach of PbS QDs is as follows: first, for the synthesis of substituted thiourea, a solution of 1.7 mL aniline in 6 mL toluene was added to a solution of 5 g 3,5-Bis(trifluoromethyl)phenyl isothiocyanate in 6 mL toluene. The mixture was stirred for 30 minutes before the exothermic reaction ended. The product was thoroughly dried under vacuum to remove trace toluene, resulting in white powder of substituted thiourea.35 Second, for the synthesis of undoped PbS QDs, 227.6 mg Pb(CH3CO2)2·3H2O and 0.4 mL OA were mixed with 10 mL hexadecane in a 3-neck flask and flushed with nitrogen gas flow for 5 minutes, then transferred in oil bath with stir bar at 160 °C held for 1h. The temperature was then decreased to 95 °C. During this period, a mixture of 145.7 mg of as-prepared thiourea and 1.61 g diphenyl ether was heated to 95 °C, then this clear solution was injected quickly into the flask containing lead oleate at 95 °C. The solution was maintained at the latter temperature for one minute, then cooled down to room temperature with cold water. Finally, the QDs were washed three times with ethanol/acetone (5:1 in volume) and re-dispersed in toluene. The synthesis of Mn-doped PbS QDs was achieved by a hot injection method. In detail, three different doped PbS QDs with different Mn incorporation content were synthesized. The sample synthesized with 432.0 mg Pb(CH3CO2)2·3H2O, 14.7 mg Mn(CH3CO2)2·3H2O was denoted as Mn0.007PbS in the following. The sample synthesized with 387 mg Pb(CH3CO2)2·3H2O, 44.1 mg Mn(CH3CO2)2·3H2O was denoted as Mn0.0115PbS. The sample synthesized with 364.0 mg Pb(CH3CO2)2·3H2O, 58.8 mg Mn(CH3CO2)2·3H2O was denoted as Mn0.0166PbS. The precursors of Pb, Mn and 0.8 mL OA were mixed with 18 mL ODE in a 3-neck flask and flushed with nitrogen gas flow for 5 minutes, then transferred in oil bath with stir bar at 160 °C held for 1h. Then thiourea (291.5 mg in 3.22 g diphenyl ether) was injected into the Mn and Pb mixed precursors at 95 °C. The solution was maintained for one minute, then cooled down to room temperature with cold water. Finally, the QDs were washed three times with ethanol/acetone (5:1 in volume) and re-dispersed in toluene. QDs film preparation: The QDs in toluene solution were mixed with PMMA in chloroform, then spincoated on a glass substrate at 300 rpm for one minute. The concentration of QDs in this mixture solution was around 1 μM, and the concentration of PMMA polymer was 1 wt %. Characterization: The morphologies of PbS and Mn-doped PbS QDs were characterized by using a JEOL 2100F transmission electron microscope (TEM). The Pb/Mn ratio was measured by using inductively coupled plasma optical emission spectrometry (ICP-OES) (PerkinElmer Model Optima 7300 DV). Scanning Electron Microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were employed to investigate the elemental analysis of the as-synthesized QDs by using a Tescan Vega3 equipment. Xray diffraction (XRD) of extensively purified as synthesized QDs were carried out using a Philips X’pert 5 ACS Paragon Plus Environment

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diffractometer equipped with a Cu Kα radiation source ( = 0.15418 nm). EPR spectra were collected on an X-band EPR spectrometer (Bruker Elexsys 580 FT/CW EPR console). The modulation frequency was 100 kHz. Measurements at lower temperature (114 K) were performed using liquid nitrogen. The size distribution of the QDs was estimated by measuring about 100−200 QDs for each sample and further analyzed using a Gaussian distribution. Absorption spectra were acquired with a Cary 5000 UV−vis−NIR spectrophotometer (Varian) with a scan speed of 600 nm/min. Fluorescence spectra were recorded with a Fluorolog-3 system (Horiba Jobin Yvon) equipped with a temperature controller. The excitation wavelength was set at 450 nm. The optical properties of pure PbS and Mn-doped PbS QDs were measured in the PMMA matrix by recording the variation of their PL spectra in the 96–296 K temperature range using THMS 600 temperature-controlled stages. Results and Discussion Synthesis and structural characterization. PbS QDs and Mn-doped PbS QDs were synthesized using an organometallic hot injection method in which different contents of Mn (manganese acetate trihydrate) were introduced into the reaction mixture, to facilitate the incorporation of the Mn atoms during the nucleation and growth of the QDs. We used substituted thiourea as sulfur precursor due to its inexpensive and air-stable advantages compared to the widely used bis(trimethylsilyl) sulfide and higher reaction activity with respect to elemental sulfur precursor.6, 35 By controlling the Pb/Mn stoichiometry during the reaction, different concentrations of Mn dopant were introduced in the QDs. Figure 1 displays the representative TEM images of PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs. The as-prepared QDs have a uniform size distribution with average diameter of approximately 2.8 nm, according to the size distribution curves shown in Figure S1 of supporting information. A high-resolution TEM (HRTEM) image is shown in the inset of Figure 1. The crystal lattice fringes with spacing of 0.34 nm corresponds to the interplanar distance of the (111) plane of PbS, which confirms that the Mn-doped PbS QDs retain the highly crystalline rock-salt structure of bulk lead sulfide.6, 36 The sizes of pure PbS and Mn-doped PbS QDs as a function of band gap are shown in Figure S2 and are consistent with the following empirical equation: 37-38

where Eg is the band gap of PbS QDs as determined by the first-excitonic peak feature in the absorption spectrum and d is the diameter of QDs in nm. This result also indicates that the size of the QDs does not change after the incorporation of Mn, as observed in the TEM micrographs presented in Figure 1. 6 ACS Paragon Plus Environment

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Figure 1. Representative TEM images of (a) pure PbS QDs, (b) Mn0.007PbS, (c) Mn0.0115PbS and (d) Mn0.0166PbS. The inset images are the HRTEM images of each QDs. The molar concentrations of Mn in the synthesized QDs were measured by ICP-OES. As shown in Table 1, the measured Mn molar concentrations were 0.00 %, 0.70 %, 1.15 % and 1.66 % in the PbS QDs, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS samples, respectively. As shown in Figure S3, EDX spectra show a characteristic Mn peak at 5.9 keV in Mn0.0166PbS and Mn0.0115PbS samples. However, no clear Mn peak is observed for Mn0.007PbS samples, since the Mn concentration is lower than the detection limit of our instrument. The Mn concentration in the QDs is controlled by tuning the reaction parameters and adjusting the molar feeding ratio of Pb/Mn precursors. To make sure the pure PbS and Mn doped PbS QDs are similar in size, we used the same reaction temperature and reaction time. We found that it is difficult to further improve the doping concentration of Mn under the selected reaction parameters. To analyze how many Mn ions are doped into one dot with an average QD size, we make a simple assumption that each dot has a standard spherical shape. We calculated the Mn doped number in each dot by using the following equation, *4/3*R3= M*nPbS/NA, where  is the density of PbS (7.60 g/cm3), R is the radius of the dot, M is the molar mass of PbS (239.30 g/mol), NA is the Avogadro constant (6.02 × 1023 mol-1), nPbS is the average PbS molecule number of one dot. The average Mn atom number of each dot, nMn, is therefore nMn= n*nPbS, i.e., nMn= 4nNAR3/3M, where n is the concentration of Mn in each sample. We calculated that nPbS is around 220 in our as-prepared QDs with 2.8 nm in dimeter. As a result, the average number of Mn ions was estimated as 0, 1.54, 2.53, 3.65 for PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS, respectively. Figure 2a shows the XRD patterns of as synthesized PbS (black) and Mn-PbS samples (red and blue). For pure PbS QDs sample, all the diffraction peaks are indexed to a face centered cubic rock-salt structure of PbS in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) No. 05-0592. The peaks with the four highest intensity correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes,


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respectively. The XRD spectra of Mn doped PbS samples demonstrate similar patterns compared to pure PbS QDs, indicating that the rock-salt crystal structure is still maintained despite the incorporation of Mn ions into QDs.

39

As the Mn ion concentration is very low, we cannot detect the XRD signal from MnS.

However, the characteristic peaks show slight shifts toward high diffraction angle values with the incorporation of Mn, as shown in Figure 2a (right pattern). It indicates a decrease in the lattice constant, which is caused by the lattice contraction due to the substitution of larger Pb2+ ions (133 pm) by smaller dopant Mn2+ ions (97 pm).8, 25 This result suggests successful incorporation of Mn2+ ions inside the host PbS lattice, which is consistent with previous report.40 EPR spectra for pure PbS and Mn doped PbS QDs containing different Mn ions concentrations were measured at two different temperatures (298 and 114 K). EPR spectroscopy is very sensitive to unpaired electrons and was used to explore the local environment of Mn ions in the doped QDs.41 For pure PbS QDs, the EPR spectra in Figure S4 show no noticeable signals at both room temperature and at 114 K. In Figure 2c-d, all the EPR lines show sextet signals superimposed on a broad background, which is attributed to the hyperfine interaction between d electrons. The hyperfine interaction constant (A) values are used to indicate the dopant location in Mn doped nanocrystals.31, 42 The EPR spectra lead to A values of 9.12, 9.14 and 9.18 mT for Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS samples respectively. These values are very close to the reported 9.22 mT reported in the literature, corresponding to the surface doped Mn.43 However, the broad background in the EPR spectra and asymmetric shape of EPR lines suggest that part of Mn ions are also located in the near surface or the core of QDs.33-34,

44

The EPR

spectra of Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS samples (Figure 2c-d) clearly show significant six line hyperfine structures of Mn at 298 K. At low temperature of 114 K, the EPR spectra remain the same, but exhibit obvious peak intensity enhancements compared to that at 298 K. Compared to the three samples, higher Mn concentration demonstrates a decreased resolution of six peaks in the spectra at each temperature, which is attributed to an increasing strength of Mn-Mn dipolar interactions and consistent with previous work.41


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Figure 2. (a) XRD spectra of the as synthesized pure and Mn doped PbS QDs. The right pattern shows the magnified view of the XRD spectra from 24º to 32º. The JCPDS card data for PbS (No. 05-0592, black line) is shown for identification. EPR spectra of the as synthesized (b) Mn0.007PbS sample, (c) Mn0.0115PbS sample and (d) Mn0.0166PbS sample at 298 K and 114 K, respectively.

Optical properties of pure and Mn-doped PbS QDs. The optical properties of as-prepared QDs were investigated by using absorption and PL spectroscopy. The normalized absorption and PL spectra of PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs are shown in Figure 3a. The absorption spectra show the characteristic PbS QDs absorbance. 45 Similar results were obtained for the absorption spectra of the Mndoped PbS QDs. With identical first-excitonic absorption peak positions (same size of PbS QDs), PL spectra indicate that the as-prepared PbS and Mn-doped PbS QDs exhibit different PL emissions. Specifically, as shown in Table 1, the absorption peak positions are found to be at 870, 866, 879 and 850 nm for PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs, respectively. A single PL peak was observed 9 ACS Paragon Plus Environment

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for both PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs samples, and their corresponding PL peak positions are 1000, 1039, 1052 and 1044 nm, respectively. With similar size, pure PbS QDs reveal a Stokes shift of 180 meV, smaller than the values of 240 meV for Mn0.007PbS QDs, 232 meV for Mn0.0115PbS and 269 meV for Mn0.0166PbS, respectively. Hence, we find that the incorporation of Mn in PbS QDs enlarges the Stokes shift. The time-resolved PL decay curves are reported in Figure 3b. The decay shows that the lifetime decreases continuously with the increase of Mn dopant content. The intensity-weighted average lifetime (τ) is calculated from the fitting of the PL decay, according to the following equation: 46-51

Where ai (i=1, 2, 3) are fitting coefficients and τi (i=1, 2, 3) are characteristic lifetimes. The measured lifetimes are 2.37 s, 1.29 s, 0.62 s and 0.36 s for PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs, respectively (Table 1). The decrease of PL decay lifetime was observed with the increase of Mn incorporation content in PbS QDs. By controlling the Mn concentration in the as-prepared QDs, the PL lifetime can be tuned regularly from hundreds of nanoseconds to several microseconds.


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Figure 3. Optical properties of PbS QDs and Mn-doped PbS QDs in toluene: (a) Absorption and PL spectra. The excitation wavelength is ex = 450 nm; (b) PL decay curves. The excitation wavelength is ex = 444 nm. Table 1. Size, absorption peak, PL peak and Stokes shift, Mn molar concentration and lifetime of asprepared PbS QDs and Mn-doped PbS QDs in toluene. Samples

Size (nm)

Absorption

PL peak

Stokes

Mn

Life time

peak (nm)

(nm)

shift (eV)

concentration

(s)

(by molar) PbS

2.81  0.40

870

1000

0.18

0.00 %

2.37

Mn0.007PbS

2.77  0.35

866

1039

0.24

0.70 %

1.29

Mn0.0115PbS

2.84  0.40

879

1052

0.23

1.15 %

0.62

Mn0.0166PbS

2.75  0.35

850

1044

0.27

1.66 %

0.36

The average lifetime of the PL decay is commonly used to compare PL quantum yield (QY) upon a certain treatment of an ensemble of QDs based on the following equations: 52

where Ket, Knet and τ are the radiative decay rate, the non-radiative decay rate and the experimentally detected lifetime extracted from Eq. (2), respectively. The PL QYs of as-prepared QDs were measured and values of Ket and Knet are calculated from the abovementioned equations (3) and (4), which are reported in Table 2. The PL QYs are 71.0%, 7.6%, 3.5% and 2.1% for PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs, respectively. The results show that the QYs decrease after the incorporation of Mn ions, as well as Ket. We also find that the Ket values are very similar (namely, 0.589×105 s-1 for Mn0.007PbS, 0.572×105 s-1 for Mn0.0115PbS and 0.583×105 s-1 for Mn0.0166PbS) for Mn-doped PbS QDs with different Mn dopant concentrations. However, Ket of PbS (2.996×105 s-1) is significantly higher than that of Mndoped PbS QDs. These results demonstrate that there is intrinsic difference of the emission behaviors between pure and Mn-doped PbS QDs. The Knet values increase with the increasing concentration of incorporated Mn ions, which could be attributed to the traps. 11 ACS Paragon Plus Environment

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Table 2. PL QY, Ket and Knet of as-prepared PbS and Mn-doped PbS QDs in toluene. Samples

PL QY (%)

Ket (105 s-1)

Knet (105 s-1)

PbS

71.0±7.0

2.996

1.22

Mn0.007PbS

7.6±0.8

0.589

7.16

Mn0.0115PbS

3.5±0.4

0.572

15.76

Mn0.0166PbS

2.1±0.2

0.583

27.19

To further investigate the variation of Stokes shift before and after Mn incorporation at different QD sizes, we synthesized PbS and Mn-doped PbS QDs of different sizes and studied their optical properties. In Figure S5, we report the summarized Stokes shifts that were measured as a function of the absorption peak of PbS and Mn-doped PbS QDs. These values indicate that Mn-doped PbS QDs show consistently increased Stokes shifts at different sizes, whereas value of the difference decreases with the red-shift of the absorption peak position. Emission mechanism of pure and Mn-doped PbS QDs. The difference in PL spectra between Mndoped PbS QDs and Mn-doped wide band gap QDs is revealed by the following two aspects: (i) The 4T1 6A

1

d-d transition of Mn ions was not observed in Mn-doped PbS QDs, while this emission was widely

reported in other Mn-doped wide band gap semiconductors 31, 53; (ii) A significant red-shift was observed for the PL peak of doped QDs compared to pure host materials at identical size and same temperature. Such results cannot be explained from the schematic diagrams of Mn-doped in wide band gap QDs previously reported in the literature.30, 53 PbS is a narrow band gap semiconductor, with a bulk band gap of 0.41 eV and a large exciton Bohr radius of 18 nm.37 To date, most reported PbS QDs reveal a firstexcitonic absorption peak ranging from 800 to 2000 nm, with tunable band gaps from 0.6-1.5 eV.35, 54 Figure 4a shows the band energy levels of PbS QDs and d-state of Mn ions. For PbS QDs with absorption peak around 850 nm, the band gap is around 1.4 eV, which is much lower than the 4T1 - 6A1 gap energy (2.12 eV). Generally speaking, depending on the bandgap of the host semiconductor, the ligand field states of 4T1 and 6A1 of the doped ions could be involved in the PL emission.55 The band edge positions of PbS QDs, Mn 4T1 and 6A1 state positions were given according to previous reports.53, 56-57 In our case, it is different from other Mn-doped in wide band gap QDs (for instance, ZnSe and CdSe QDs), in which the 4T 1

and 6A1 Mn d-state are located between the VB and CB of QDs. As shown in Figure 4a, the 4T1

energy state is located between the VB and CB of PbS QDs, while 6A1 energy state lies below the VB of 12 ACS Paragon Plus Environment

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PbS QDs. Radiative processes are indicated as straight lines and non-radiative processes as wavy lines. The rate of energy transferred from the photoexcited nanocrystal to Mn ions (~ps, 21 shown in green wavy lines in Figure 4a) significantly exceeds that of exciton recombination (~s), enabling the energy transfer towards Mn ions. Schematic diagrams of the electronic transitions involved in light-modulated fluorescence from PbS and Mn-doped PbS QDs are shown in Figure 4b (left). When a pure PbS QD is excited by photons with energy higher than its band gap, the direct recombination of the electron-hole pairs displays the general exciton emission. The schematic diagram of the electronic transitions path of Mn-doped PbS QDs is also shown in Figure 4b (right). Initially, electrons are pumped from the VB to the CB of the PbS host, and subsequently transferred to the 4T1 level of the Mn dopant non-radiatively. As the 6A1 level lies below the VB of the PbS host, the photogenerated electrons will radiatively relax towards the VB of QDs instead of 6A

1

level of Mn dopant, resulting in a ‘Mn-PbS’ emission. This is the reason why no 580 nm PL peak

associated with Mn 4T1 - 6A1 d-emission was observed (alternatively, its intensity may be below the detection limit), whereas this peak was reported by other wide band gap QDs doped with Mn.30 Meanwhile, the unique exciton behavior results in a large red-shift of the PL peak position, consistent with the PL spectra shown in Figure 3a. When the size of Mn-doped PbS QDs increases, the band gap decreases, so that the CB of PbS becomes lower, and the energy difference between exciton emission and ‘Mn-PbS’ emission decreases. This explains why the difference of Stokes shift between pure PbS and the similar sized Mn-doped PbS QDs shows a decreasing trend with the increase of QDs size, as shown in Figure S5. On the other hand, it was reported that the Mn 4T1 - 6A1 d-emission is spin forbidden resulting in a very long lifetime in the range of several hundreds of microseconds.9 As shown in table 1, the lifetime of Mn-doped PbS QDs is very different from those of the d-emission and band-edge emission in PbS QDs, thus indicating the occurrence of a different emission mechanism in Mn-doped PbS QDs.


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Figure 4. (a) Band energy levels of PbS QDs and Mn d-state (left). Radiative processes are indicated as straight lines and non-radiative processes as wavy lines. (b) Schematic illustration of the change in emitting excited state of PbS QDs (left) and with the incorporation of Mn in PbS QDs (right). Temperature response of as-synthesized QDs. PMMA is widely used in fabricating QDs based devices due to its excellent optical properties such as high transparency, low reabsorption losses as well as good environmental stability.45, 58 The prepared QDs/PMMA films were used for the temperature-dependent PL measurements. The temperature-dependence of the PL of as-prepared PbS and Mn-doped PbS QDs was investigated by controlling the temperature of the QDs during PL measurements. Figure 5a exhibits the evolution of PL spectra of PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS QDs upon excitation at 450 nm for a broad temperature window from 96 to 296 K. For both PbS and Mn-doped PbS QDs, the PL spectra show markedly temperature dependence. The spectral variations show that decreasing the temperature systematically increases the PL intensity and shifts the emission peak towards longer wavelengths.


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Figure 5. (a) Temperature-related PL quenching. Normalized PL spectra of PbS QDs and Mn-doped PbS QDs (from left to right, corresponding to samples of PbS, Mn0.007PbS, Mn0.0115PbS and Mn0.0166PbS, respectively) recorded at different temperatures. From top to bottom: 96, 106, 116, 126, 136, 146, 156, 166, 176, 186, 196, 206, 216, 226, 236, 246, 256, 266, 276, 286 and 296 K. The excitation wavelength is ex = 450 nm for the three samples of QDs. (b) Normalized integrated area of PL peak as a function of 1000/T for pure and Mn-doped PbS QDs, where T is temperature. (c) PL peak position as a function of temperature for pure and Mn-doped PbS QDs. The PL emission spectra were fitted using a single Gaussian function. The integrated PL intensities were calculated at different temperatures for each sample. In Figure 5b, we show the integrated PL intensity of QDs versus inverse temperature (1000/T) for PbS QDs and Mn-doped PbS QDs. The plots show a similar trend for all samples, consisting in an increase of the PL intensity with temperature decrease. At higher temperature (296 K-156 K) the increasing rate is faster than that at lower temperature, and the increase rate become relatively slower at lower temperature (156 K-96 K). The temperature dependence of the exciton PL intensity is well understood. Typically, such temperature-dependent increase of PL intensity in different types of QDs has been mainly attributed to the suppression of carrier trapping by defects/traps and phonon-assisted thermal escape.45,

59-60

In general, since the charge carrier trapping process is a

thermally activated process, it becomes faster at higher temperatures, resulting in deterioration of the PL intensity with increasing temperature. For higher temperatures, the PL intensity decreases as a result of an increase in charge carrier trapping, which is consistent with the behavior observed in other types of QDs.55 In Figure 5a and b, the increase of the integrated area is faster for Mn-doped PbS QDs than pure PbS QDs, indicating that the Mn-doped QDs are more sensitive to temperature effects. For instance, compared to pure PbS QDs, the Mn0.0166PbS displays enhancement in integrated PL area of 1.95 times at 126 K and 2.45 times at 96 K. For Mn-doped PbS QDs, the Knet is higher than that of pure PbS QDs, thus the traps in Mn-doped PbS QDs is more than pure one. As there is higher concentration of defects in the Mn-doped QDs as indicated by their higher values of Knet, at lower temperature, the PL quenching due to defects can be limited, leading to the increase of PL intensity compared to bare QDs. This explains the higher PL temperature sensitivity in Mn-doped QDs. In Figure S6, we reported the full width at halfmaximum (FWHM) of the PL peak for the as-synthesized pure PbS and Mn doped PbS QDs by varying the temperature. The FWHM decreases gradually from 296 to 96 K, which is mainly due to phonon broadening.41 The temperature-dependence of the PL peak energy of pure and Mn-doped PbS QDs is reported in Figure 5c. In pure PbS QDs, the red-shift to longer wavelengths as the temperature decreases can be associated with the decrease in energy gap. Previous works reported that the PL peak energy of CdSe/ZnS core/shell QDs 61 and ZnCuInS/ZnSe/ZnS QDs 62 reveal a linear decrease with the increase of temperature from 96 16 ACS Paragon Plus Environment

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to 296 K. The effect of the temperature on the PL peak energy of PbS QDs was already reported by Zhao et al.45. In this study, the PL peak energy shifts of the spectra were found to vary almost linearly with the temperature for pure PbS QDs samples, in agreement with our previous results.45 However, for Mn-doped PbS QDs, strong discrepancies were observed. Once the temperature decreases, the Mn0.007PbS bandgap remains almost unchanged up to 250 K and then gradually decreases. Meanwhile, Mn0.0115PbS and Mn0.0166PbS shows more obviously stable behavior at the beginning of the cooling cycle, then gradually decreases as same as pure PbS QDs. The different temperature behavior between pure and Mn-doped PbS QDs could be related to the schematic illustration of band alignment of VB and CB of PbS and Mn ions shown in Figure 4a. Previous work indicates that the incorporation of Mn ions altered the temperaturedependence behavior of the PL energy in colloidal QDs.63 Several reports demonstrated that Mn-doped QDs exhibit a significant temperature dependence.22, 55, 64 For example, Park et al. 55 reported that the Mn 4T 1

- 6A1 transition shows a blue-shift at higher temperatures. The effect of the temperature on the

bandgap of PbS host and energy of Mn 4T1 - 6A1 of the ligand field transition is ascribed to the thermal expansion of the crystalline lattice that directly influences the electronic structure of the semiconductor host and the strength of the ligand field on the Mn ions.64 In this work, the ‘Mn-PbS’ emission involves both the VB energy of PbS host and the 4T1 energy state of Mn ions, which dominate the PL process. The combined shifts of the PbS band edges and Mn 4T1 state result in the different temperature-dependence of PL peak energy shifts in Mn-doped PbS QDs with respect to pure PbS QDs. MacKay et al.65 reported that the Mn emission exhibited a shift towards lower energy when the temperature decreased from 300 to 100 K. At the higher temperatures of this range, the shifting rate is much slower. When further decreasing the temperature, the shifting rate accelerates at lower temperatures. Considering the different variation trend of the temperature dependent PL peak position between pure and Mn doped PbS QDs observed in Figure 5c, we conclude that the Mn 4T1 energy level makes dominant effect on the ‘Mn-PbS’ emission in Mn doped PbS QDs at higher temperature. This explains the difference between pure and Mn-doped PbS QDs in the temperature dependent PL peak variation at a higher temperature, as shown in Figure 5c. As the temperature-dependent PL response of Mn-doped PbS QDs is significantly improved compared to the pure QDs, our work suggests that Mn-doped PbS QDs holds potential for application in thermal sensors, which make it possible to precisely monitor the local temperature at the nanoscale.

Conclusions and Perspectives We report the synthesis of oil soluble Mn-doped PbS QDs with well-controlled Mn concentration and uniform size distribution. The optical performance indicates that Mn-doped PbS QDs reveals a larger Stokes shift than pure PbS QDs samples with identical size. The schematic illustration of ‘Mn-PbS’ 17 ACS Paragon Plus Environment

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emission was proposed by investigating the band alignment of band edges of PbS QDs and Mn d-state. The midgap states created by Mn doping cause electrons to be transferred from the CB of the PbS host to Mn 4T1 state and inhibit direct recombination with holes in the VB, which leads to a ‘Mn-PbS’ emission. The temperature-dependent optical properties were investigated for pure PbS and Mn-doped PbS QDs by exploring the PL peak energy shift and PL intensity variation in the 96-296 K temperature range. The temperature response of Mn-doped PbS QDs improves significantly as compared to the pure PbS QDs. Our work demonstrates that Mn-doped PbS QDs are potential candidates for application in thermal sensors and provides new fundamental knowledge of the emission mechanism and temperature dependent optical properties on Mn-doped QDs. We provide a strategy to tune the Stokes shift by Mn doping in PbS QDs. This strategy can be further developed in other elements (Mn, Ag, Cu et al.) doped narrow band gap materials (PbSe, InAs et al.). Moreover, the unique properties also make Mn-doped PbS QDs a potential candidate for future application in optoelectronics.

Supporting Information Supporting Information. Additional data: Size distributions, Sizes as a function of band gaps, EDX spectra, EPR spectra, Stokes shift as a function of absorption peak, Temperature dependence of FWHM. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements F.R. acknowledges NSERC for funding through an individual Discovery Grant and is grateful to the Canada Research Chairs program for funding and partial salary support. H.G.Z. acknowledges funding from Qingdao University and the Natural Science Foundation of Shandong Province (ZR2018MB001). G.S.S acknowledges the UNESCO Chair in Materials and Technologies for Energy Conversion, Saving 18 ACS Paragon Plus Environment

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and Storage (MATECSS) for a PDF Excellence Scholarship and funding from the University of Electronic Science and Technology of China. H.Z. acknowledges his doctoral Excellence Scholarships from the UNESCO Chair MATECSS and the Fonds de recherche du Québec Nature et technologies (FRQNT).

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Near-infrared Colloidal Manganese Doped Quantum Dots: Photoluminescence mechanism and Temperature Response Hui Zhang, Jiabin Liu, Chao Wang, Gurpreet S. Selopal, David Barba, Zhiming M. Wang, Shuhui Sun, Haiguang Zhao, and Federico Rosei

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A novel photoluminescence mechanism is proposed in Mn doped PbS QDs. The midgap states created by Mn doping cause photoexcited electrons to be transferred from the CB of the PbS host to Mn 4T1 state and then recombination with holes in the VB, which leads to a ‘Mn-PbS’ emission. This emission process is different from the well-known exciton emission in intrinsic pure QDs and the reported d-emission in doped QDs.


Near-infrared Colloidal Manganese Doped Quantum Dots

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