Higher Quantum State Transitions in Colloidal Quantum Dot with

Aug 30, 2016 - The observation of the second intraband transition (1Pe-De) allows us to utilize the higher quantized states that were hidden for the l...
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Higher Quantum State Transitions in Colloidal Quantum Dot with Heavy Electron Doping Bitna Yoon, Juyeon Jeong, and Kwang Seob Jeong* Department of Chemistry, Korea University, Seoul, Republic of Korea 02841 S Supporting Information *

ABSTRACT: Electron occupation in the lowest quantized state of the conduction band (1Se) in the colloidal quantum dot leads to the intraband transition in steady-state (1Se-1Pe). The intraband transition, solely originating from the quantum confinement effect, is the unique property of semiconducting nanocrystals. To achieve the electron occupation in 1Se state in the absence of impurity ions, nonthiol ligand passivated HgS colloidal quantum dots are synthesized. The nonthiol ligand passivated HgS quantum dot exhibits strong steady-state intraband transition in ambient condition and enables a versatile ligand replacement to oxide, acid, and halide functional ligands, which was not achievable from conventional HgS or HgSe quantum dots. Surprisingly, the atomic ligand passivation to HgS colloidal quantum dot solution efficiently maintains the electron occupation at 1Se of HgS CQDs in ambient condition. The electron occupation in 1Se of HgS CQD solid film is controlled by surface treatment with charged ions, which is confirmed by the mid-IR intraband absorption (1Se-1Pe) intensity imaged by the FTIR microscope. Furthermore, a novel second intraband transition (1Pe-1De) is observed from the HgS CQD solid. The observation of the second intraband transition (1Pe-De) allows us to utilize the higher quantized states that were hidden for the last three decades. The use of the intraband transition with narrow bandwidth in mid-IR would enable to choose an optimal electronic transition occurring in the nanocrystal for a number of applications: wavelengthselective low-energy consuming electronics, space-communication light source, mid-infrared energy sensitized electrode and catalyst, infrared photodetector, and infrared filter.

1. INTRODUCTION Colloidal quantum dots (CQD) have been of great interest due to the tunable bandgap which is a prominent property of colloidal semiconducting nanocrystals.1−4 The size tunable bandgap of semiconducting nanocrystals is estimated by adding the quantum confinement energy to the native bulk bandgap energy, ECQD = E Bulk +

{( )( π 2ℏ2 2R2

1 e meff

+

1 h meff

= 18.2), exhibits a steady-state intraband transition in the midIR regime, providing a platform to utilize higher quantized states in the conduction band of CQDs. The steady-state intraband transition measured by infrared spectrometer was first reported by Jeong et al. in 2014.12 The intraband transition is a direct evidence of electron occupation at 1Se state in the conduction band, indicating that the nanocrystal is heavily doped with electrons. The optoelectronic device based on the heavily doped semiconducting nanocrystal has been rapidly developed with better optimization.17,18 Although it was proved that the intraband transition is feasible to harness for infrared optoelectronic applications, some questions still remain such as what conditions (e.g., surface, solvent, ligand, metal composition, etc.) are involved to maintain the electron occupation at 1Se state. In order to clearly understand the steady-state intraband transition, it is important to scrutinize how this transition is controlled in the molecular level. In regards to the surface environment, the previously reported mercury chalcogenide CQDs are passivated with thiol molecules or sulfide.12,17,19 The thiol is well-known to serve as a hole

)}, implying that

the bandgap of colloidal quantum dots is always larger than that of the bulk bandgap.5 Therefore, there is no way to produce a bandgap energy that is smaller than the bulk bandgap energy of a material. However, by using the quantized states of the nanocrystal either in the conduction band or valence band, absorption energies less than the bulk bandgap energy can be obtained. Specifically, the intraband transition occurring in the conduction band of heavily doped nanocrystals is one way to achieve absorption energies smaller than the bulk bandgap.6−10 For example, the bulk bandgap energy of CdSe is 1.7 eV, and the intraband transition energy is smaller than 0.35 eV.11 To our knowledge, it is difficult to harness the intraband energy due to the fast hot electron relaxation except for the mercury chalcogenide CQDs with the estimated intraband radiative lifetime of 0.64 μs.12−16 The heavily doped mercury chalcogenide CQD, strongly quantum confined nanocrystal due to the large exciton Bohr radius (β-HgS aB = c.a. 28 nm, ε © XXXX American Chemical Society

Received: July 21, 2016 Revised: August 30, 2016

A

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Figure 1. Nonthiol ligand passivated HgS CQD: (A) Ligand versatility of oleylamine passivated HgS CQDs from amine to-thiol, amine to oxide and amine to halide atomic ligands. (B) FT-IR spectra of oleylamine passivated HgS CQDs of different size (inset) TEM image (scale bar = 20 nm) of HgS CQD. (C) NMR spectra of oleylamine passivated HgS CQD (Black) and free oleylamine in CDCl3 (red). (D) FT-IR spectra of oleylamine passivated HgS CQD (black) and DDT capped HgS CQD (blue) under air. The inset shows the drop in intensity of the intraband transition after ligand exchange.

2. EXPERIMENTAL METHODS

quencher (scavenger) that complicates the mechanisms of electronic transitions.20,21 For instance, the mid-infrared photoluminescence of CdSe CQDs and CdSe/ZnSe CQDs recently reported by Jeong et al. is exhibited by employing a thiol hole quencher ligand through multiphoton excitation with fast hole quenching process.22 For HgS CQD, however, it is not clear if the thiol ligand contributes to the generation of the intraband transition. Furthermore, it is worth to note that the strong binding strength of the thiol to nanocrystal makes it harder to exchange to other ligands with weak binding strength such as amine. Thus, it is necessary to synthesize nonthiol ligand passivated HgS CQDs for clear understanding of the photophysical mechanism of generating the steady-state intraband transition and for development of advanced optoelectronic devices in the mid-IR. Here, we report nonthiol organic ligand based mercury chalcogenide CQD showing size-tunable intraband transition at 10.4−3.1 μm, suggesting that heavy doping in nanocrystals can be achieved without thiol ligands. The nonthiol ligands with weaker binding strength help ease the ligand exchange process that is required for further optoelectronic applications. Surprisingly, atomic ligand passivated HgS CQDs dispersed in polar solvent still efficiently maintain the electron occupation in 1Se state, which also demonstrates intraband transition. Furthermore, the controlled local doping density of the HgS CQD solid film is optically measured by FTIR microscope and not observable by Raman microscope, indicating that the intraband transition of nanocrystal is IR active and Raman inactive.

The following chemicals were purchased from Sigma-Aldrich: bis (trimethylsilyl) sulfide ((TMS)2S, synthesis grade), 1octadecene (technical grade, 90%), oleylamine (technical grade, 70%), and tetrachloroethylene (TCE, ACS reagent, > 99.0%). Ammonium chloride (NH4Cl, 98+ %), chloroform-d (CDCl3, 99.8% (isotopic), contains 0.03% v/v TMS), dimethyl sulfoxide-d6 (DMSO-d6, 99.9% (isotopic)), and mercury(II) chloride (HgCl2, ACS, 99.5% min) were obtained from Alfa Aesar. Synthesis of HgS CQD. The synthesis of oleylamine passivated HgS CQD is adapted from that of dodecanethiol passivated HgS CQD.12 160 mg of HgCl2 powder was dissolved in 4 mL of oleylamine. The HgCl2 solution was stirred and degassed at 85 °C for 35 min under vacuum. The HgCl2 solution was then heated at 120 °C for 1 h under argon atmosphere. 0.2 mL of 0.43 mM bis(trimethylsilyl) sulfide in 1octadecene solution was quickly injected into the HgCl2 solution at 90 °C, and the solution immediately turned black, indicating formation of HgS CQDs. As compared to the other sulfur precursors such as thioaceamide, the product obtained by using TMS shows improved colloidal stability. The reaction time spans from 1 to 32 min. The nanocrystal growth reaction was stopped by addition of 8 mL of oleylamine in TCE solution and the 50 mL round-bottom flask with the product was moved into an ice bath. HgS CQDs were precipitated from the solution by adding a polar solvent such as ethanol to destabilize the QDs dispersion. The product was precipitated once again with chloroform/ethanol to remove residual organic molecules. B

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exchange. Figure 1B shows the mid-IR intraband absorption spectra of oleylamine passivated HgS CQDs of different nanocrystal size. Sharp peaks at 2850, 2926, and 3200 cm−1 correspond to symmetric, asymmetric stretching modes of CH and stretching mode of amine, respectively. The 1Se-1Pe absorption peak position red-shifts as an increase of reaction time. The absorption peak range is from 958 to 3240 cm−1. As the nanocrystal size increases, the fwhm gradually decreases to 375 cm−1. The reduction of the fwhm may result from the large electron density in a nanocrystal, possibly leading to the change from the intraband transition to the localized surface plasmon resonance (LSPR) with narrow bandwidth. However, the change of the fwhm needs to be thoroughly investigated.19,23 The synthetic method is described in the experiment part but briefly, mercury-oleylamine precursor is reacted with bis(trimethlysilyl)sulfide (TMS) in octadecene solution injected at 120 °C under argon atmosphere. Depending on the desired ligand, various precursors were used. For instance, the mercury oleate precursor was added for oleate passivated HgS CQD synthesis. The nanocrystal size is determined by the reaction time (30 s to 32 min) and temperature (90−120 °C). The TEM image of oleylamine passivated HgS CQDs with a diameter of 5.7 ± 1.3 nm is shown in Figure 1B. The density of the electrons in the conduction band for the 5.7 nm size nanocrystal is estimated as 1.8 × 1019 cm−3 which is in the heavy doping range when one compares it with the doping density of bulk semiconductor.24 The density of the electrons in the conduction band in steady state for the HgS nanocrystal estimated is in the range of 1.1 × 1019 cm−3 to 8.1 × 1019 cm−3 depending on the nanocrystal size assuming there are two electrons in the 1Se of nanocrystal in stable. The XRD results confirms that the crystal structure of the β-HgS CQD is zinc blende (Figure S1).12,25−27 The peak narrows by increasing the nanocrystal size as more crystal lattice units are uniformed due to the reduction of the surface-to-volume ratio. Figure 1C shows the 1H NMR spectra of the oleylamine passivated HgS CQD, indicating that the oleylamine molecule is the only chemical species bonded to the CQD (black). The red spectrum corresponds to the free oleylamine in CDCl3 (l). A broad peak at 5.3 ppm corresponds to the carbon double bond of the bound oleylamine (CH 3 (CH 2 ) 7 CH = CH(CH2)7CH2NH2).28 Interestingly, the intensity of proton at b (δ = 2.18 ppm) is abnormally smaller than that of the free oleylamine. This is probably because the proton at b is replaced by other chemical species such as deuterium considering the small shift. The oleylamine bound to the HgS CQD surface is successfully exchanged by the addition of thiol molecule or phosphine oxide molecule into the CQD solution (Figure 1D, S2). Based on the NMR and FT-IR results, the ligand exchange is efficient, so that there is no residual oleylamine ligand after ligand exchange to thiol and oxide organic molecule. The optical density, however, is dropped to a third of the original intensity, implying that the electron doping density of CQD film is significantly reduced during the thiol or oxide ligand exchange process. In order to sustain the heavy doping, an efficient ligand exchange is necessary such as atomic ligand passivation in solution. Surprisingly, the chloride passivated HgS CQD produces intraband transition in colloid under ambient conditions. The intraband absorption appears at 2350 cm−1, indicating that electrons still reside in 1Se state under polar solvent (NMF). The intraband transition of the atomic ligand passivation

The oleylamine-passivated HgS CQDs were dried under vacuum and redispersed in tetrachloroethylene. Infrared Spectra of Intraband Transitions. Mid-IR absorption of HgS CQDs was measured by FTIR (Nicolet iS10, Thermoscientific) with the resolution of 0.482 cm−1. Tetrachloroetyhlene was used for solvent due to the IR transparency. A demountable liquid cell with CaF2 windows was used for CQD sample. X-ray Diffraction Spectrum. The crystal structure of HgS quantum dot was obtained by Rigaku D/Max Ultima III X-ray Diffractometer with a graphite-monochromatized Cu Kα (λ = 1.54056 Å) at power settings 40 kV and 30 mA. The HgS CQD solution was dropped on a silicon sample stage and left for 10 min to dry the solvent. The XRD patterns were measured from 20° to 80° with 0.01° sampling width (step size). Transmission Electron Microscopy. The nanocrystal size was determined by taking the transmission electron microscopy image. The model Tecnai G2 F30ST (FEI) microscope at 300 kV was utilized to obtain the nanocrystal images. 1 H NMR. 1H NMR spectra of ligand-passivated HgS were obtained by using Varian Mercury 400 MHz spectrometer. CDCl3 and DMSO-d6 were used for solvents. XPS. XPS measurement was performed by a PHI X-tool system (ULVAC-PHI) using monochromatic Al Kα X-rays as the excitation source. The pass energy was fixed at 280 eV for survey scans. Ligand Exchange. Oleylamine to Chloride. A total of 1.5 mL of oleylamine-passivated HgS CQD solution (5 mg/mL in octane) and 1.5 mL of 0.4 M NH4Cl solution in methanol were placed in a 4 mL vial. The solution was stirred for 2 min. As oleylamine is exchanged by chloride, the nanocrystals transfer to polar phase. The nonpolar phase solution was decanted and polar phase was washed with NH4Cl solution to remove residual oleylamine completely. Chloride-passivated HgS CQD were redispersed in NMF and DMSO-d6, for optical and 1H NMR spectroscopy, respectively. For thiol or oxide ligand, dodecantiol and trioctylphosphine oxide were used. Raman Microscopy. Raman image was obtained by using DXRxi Raman microscope (Thermoscientific). A cw-Nd:YAG SHG 532 nm laser (1.4 mW/μm2) was irradiated on the sample for excitation. The image (1 μm pixel size) was scanned 40 times with 2.5 ms exposure time and 100× objective lens and a 50 μm confocal pinhole aperture. FTIR Microscopy. IR images were obtained by Thermoscientific Nicolet iS50 equipped with Continuum Infrared Microscope. CQD films on a gold substrate were prepared for the reflectance mode measurement. The images were obtained by scanning 16 times under a resolution of 8 cm−1. Sample Preparation: Control Local Area Doping. HgS CQD dispersed in hexane/octane (9:1) solution was dropped onto a CaF2 window and the solvent was dried under ambient conditions. Sulfide solution was prepared by adding 100 μL of ammonium sulfide (40−48 wt % in H2O) to 1 mL of ethanol and the solution was treated on the CQD film. Residual organic molecules were removed by ethanol rinsing. IR images of HgS CQD film were obtained by using Spotlight 400 FTIR Imaging System (PerkinElmer) in transmission mode.

3. RESULTS AND DISCUSSION The scheme in Figure 1A represents the ligand exchange from amine to organic and inorganic molecule. The oleylamine passivated HgS CQD is an excellent platform for intraband based CQD optoelectronic applications due to its ease of ligand C

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Figure 2. Inorganic ligand passivated CQD in solution and local doping control in CQD film. (A) The absorption spectra of two different size HgS CQDs before and after chloride atomic ligand exchange. Due to the solvent, NMF, the frequency range at CH vibrational and bending modes are saturated. (B) The image of HgS CQD in a vial before and after the inorganic ligand exchange. (C) 1H NMR spectrum of ammonium chloride passivated HgS CQD (bottom, red) showing no residual oleylamine ligand except solvent peaks (starred). The top spectrum corresponds to the NMR spectrum of oleylamine passivated HgS CQD for reference. (D) Monitoring electron doping density of two different ligand passivated HgS CQD solid film by FT-IR microscope: optical image (top) and FTIR image (bottom). The bottom left-handed area (red-yellow) corresponds to heavily doped HgS CQD film showing mid-IR intraband absorption at 2275 cm−1 and the bottom right-handed area (blue-purple) is sulfide treated HgS CQD film showing no mid-IR intraband absorption The geometry of the film is an analogy of a p-n junction. The image size is 8356 × 700 μm2.

The left-handed area and the right-handed area of the FTIR image (bottom) correspond to the heavily doped oleylamine passivated HgS CQD film and the undoped HgS CQD film treated with ammonium sulfide solution, respectively. The FTIR image (bottom) of both sides also exhibits different intensities at 2275 cm−1 where the intraband absorption appears when untreated with ammonium sulfide. The false color indicates the optical density of the intraband absorption peak at 2275 cm−1. This image shows that the solution treatment will be a promising method to control the doping density of semiconducting nanomaterials by tuning the surface charge.12,29,30 The method can be improved further by masking or patterning methods for optoelectronic devices. Quenching of the intraband absorption is mostly attributed to the negative charge on the surface of nanocrystal as reported by Jeong et al. According to the result reported, a positive charge on the surface of CQD leads to the intraband transition that is a direct evidence of electron occupation at 1Se. In contrast, the negative charge on the surface forbids the intraband transition, and therefore, the bandgap transition is recovered. Interestingly, based on the result in Figure 2A, the halide passivation maintains the oscillator strength of the intraband transition, implying that the electrons remain at 1Se state under polar NMF solvent conditions. The difference between sulfide and chloride is the amount of negative charges. Sulfide has a residual negative charge, whereas the single negative charge of

suggests that the intraband exciton is still confined in the absence of the large potential barrier created by organic ligands. The ligand exchange from the native oleylamine ligand to chloride was feasible to achieve, while conventional thiol ligands are not efficiently replaced by chloride due to the large binding strength of thiols to the surface. It is worth noting that the ligand exchange process was done under ambient condition by vigorous stirring for 1 min, implying that the HgS CQD passivated with the inorganic halide ligand is still air-stable. The 1 H NMR spectrum in Figure 2C proves the efficient ligand exchange from oleylamine to chloride. The two peaks correspond to the DMSO and H2O solvent peaks, respectively. Therefore, the combination of air-stable CQD and halide atomic ligand provides a new method to prevent electrons from oxidation even under ambient conditions whereas the thiol molecule is prone to oxidation. This is the first report of steadystate intraband transition of atomic and inorganic ligand passivated colloidal quantum dots. The doping density of CQD deposited on a micron scale substrate was proved to be controllable solely by drop casting as shown in Figure 2D. The ammonium sulfide in methanol solution was treated on the oleylamine passivated HgS CQD film, which resulted in the removal of heavy doping of the oleylamine passivated HgS CQD film. The treated side and untreated side show different optical images (top). D

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Figure 3. (A) FTIR image of the identical HgS CQD solid (inset: optical image). (B) The intraband absorption spectrum at position a in A, exhibiting two absorption peaks at 1600 and 2550 cm−1. (C) Intra- and interbandgap correlation based on k·p model.

intraband transition of HgS CQD does not require the use of thiol in mercury chalcogenide CQDs. The HgS CQD film was imaged by using FTIR microscope to confirm whether the heavy doping is constant throughout the naturally dried CQD film (Figure 3). A uniformly made film produces the same FTIR spectrum no matter what part of the film is measured. However, the film made by drop-casting method results in inconstant thickness on the substrate, and the mid-IR absorption is different depending on the area where it is measured. Surprisingly, at position a in Figure 3A, the FTIR absorption spectrum shows two different mid-IR absorption peaks at 1600 and 2550 cm−1 (Figure 3B). It is known that the 1Se-1Pe transition appears at 2550 cm−1 in HgS nanocrystals of this size. The absorption peak shown at 1600 cm−1 is only observed at b and a positions. The image was obtained by the reflective geometry where the IR light is reflected by the CQD deposited gold substrate. The 1Pe-1De transition is distinctly observable as the CQD film becomes thicker (ca. < 1 μm). Figure S5 shows the evolution of intraband absorption as the thickness of the CQD film increases. The fwhm of the mid-IR absorption peak at 1600 cm−1 is similar to that of the lowest intraband transition (1Se1Pe), and the peak position fits well to the predicted 1Pe-1De intraband transition energy by using the k·p approximation model. (Figure 3C). The conduction band with the non-

halide ion is all consumed to bind to the surface, resulting in the neutral surface charge. Thiol and Intraband Transition. An interesting property of mercury chalcogenide CQDs is the air-stable electron doping in colloidal phase. It is known that HgS CQDs that produce intraband transition are metal rich, which can be the first reason for the heavy doping as shown in other CQDs. The Hg-to-S ratio is 1.5 based on the XPS result (Figure S4.) Also, as described in ref 13, the positive charge on the surface is another reason for the heavy doping, which is frequently referred in recent CQD device results. Lastly, it is possible that the thiol ligand helps to retain electrons at 1Se state via hole quenching when the nanocrystal is exposed to radiative energy. Jeong et al. observed the mid-infrared photoluminescence by using a thiol ligand (dodecanethiol) to pull out the hole created during photoexcitation to surface. Due to the fast hole quenching process occurring within a few ps, it is possible that the hole quenching process is the origin for the steady-state intraband transition. The observation of intraband transition with oleylamine ligand passivation, however, proves that the hole quenching process is not involved in the generation of the intraband transition in HgS nanocrystal. Furthermore, even chloride passivated HgS CQDs exhibit steady-state intraband transition as well. Thus, it can be concluded that the steady-state E

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Figure 4. (A) Raman image of HgS CQD film. (B) Raman spectra of HgS CQD film at two different thick spots (red and blue).

parabolic energy dispersion, E =

−E G 2

+

EG 2 2

( )

4. CONCLUSION

+ A2 k 2 was

In conclusion, the synthesis of oleylamine passivated HgS CQDs provides a new direction for synthesizing thiol-free nanocrystals with electron occupation in the lowest quantum state in the conduction band, preventing unnecessary oxidation that leads to reduction of the doping density of nanocrystal. The air-stable nonthiol ligand based HgS CQD synthesis offers ligand versatility, which facilitates further applications. Atomic ligand passivation with chloride efficiently conserves the electron occupation at the 1Se state without loss in oscillator strength of the intraband transition in colloidal phase. It turns out that the thiol ligand exchange process reduces the oscillator strength of the intraband transition by a factor of 3, implying that the thiol molecule does not facilitate the generation of the intraband transition. Reduction of doping density by anion treatment of the CQD solid is performed, explicitly showing controllability of heavy doping by solution treatment confirmed by FTIR image and optical image. The intraband off/on area of the HgS CQD film is an analogy to homogeneous semiconducting CQD p−n junction, but further optimization will be needed. The intraband transition is Raman inactive, and higher intraband (or quanta, 1Pe-1De) transition is observed by FTIR microscope. The combined features of the novel oleylamine passivated HgS CQD synthesis, atomic ligand exchange and the second lowest intraband transition hold promise for the use of the colloidal quantum dot for solution erasable memory, infrared optoelectronics, and infrared free-space optical communications.

⎛ 0 Ak ⎞ ⎟⎟ of a 2-band obtained by using a Hamiltonian H = ⎜⎜ ⎝ Ak −EG ⎠ k·p model. In a spherical box of radius R, k of 1Se, 1Pe and 1De are k1S = π , k1P = 4.49 and k1D = 5.76 , respectively. R

R

R

The intraband energies for 1Se-1Pe and 1Pe-1De are ESP = E1Pe-E1Se and EPD = E1Pe-E1De, respectively. The Kane parameter is implied in A and the bandgap is the only parameter. The model neglects the valence band dispersion and Coulombic interactions. The second lowest intraband transition (1Pe-1De) is generated due to the reflectance geometry of the microscope. The reported lifetime of electron at 1Pe state of mercury chalcogenide CQD is in the order of few hundred picoseconds,17 but further experimental proof is required. Considering the excitation light source, it is highly possible to re-excite the electron by using the continuous wave IR light. Furthermore, thermal hotspot on Au substrate under infrared radiation may assist the second transition. To note, the peak shown at 1600 cm−1 does not correspond to the localized surface plasmon showing asymmetric scattering feature in midIR regime and narrow line width.31 The fwhm of the lowest intraband (1Se-1Pe) spectrum becomes narrow as the second lowest intraband transition (1Pe-1De) arises, implying that relatively large sized nanocrystal is more readily excited to show the second lowest intraband transition, which is consistent with the size dependent doping density of the semiconducting nanocrystal. The second lowest intraband has not been observed by transmission mode with an IR-transparent window, which supports the interpretation. Observation of the second lowest intraband transition is very reproducible using the reflective mode of FTIR microscope, but we have not obtained a spectrum showing only the 1Pe-1De transition yet, which needs further investigation. Optical and Raman images of a dried HgS CQD solid are shown in Figure 4A. The intraband transition is Raman-inactive throughout all films regardless of the thickness, and only the A1 optical phonon mode of HgS is measured at 262 cm−1 that is slightly blue-shifted compared to the bulk HgS.32−34 The blueshift is probably attributed to the strain of the crystal structure of CQD, that may make it harder for the crystal of the CQD to vibrate. To obtain the Raman image, a cw-Nd:YAG SHG 532 nm laser (1.4 mW/μm2) was irradiated on the HgS CQD film for excitation. The image (1 μm pixel size) was scanned 40 times with 2.5 ms exposure time and 100× objective lens.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07331. XRD spectra, Ligand exchange method, FTIR spectra of intraband modulated HgS CQD solid film, X-ray photoelectron spectrum, FTIR spectra at different positions of HgS CQD solid film, TEM images of different size HgS CQDs. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-3290-3127. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work is supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (NRF-2016R1C1B2013416) and the Ministry of Education (NRF20100020209). The authors used the facilities in the institute and companies (Korea Basic Science Institute Seoul Center, Scinco, PerkinElmer Korea).

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DOI: 10.1021/acs.jpcc.6b07331 J. Phys. Chem. C XXXX, XXX, XXX−XXX