Major Electronic Transition Shift from Bandgap to Localized Surface

Sep 12, 2017 - Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 02841, Republic of Korea. ‡ Korea Basic Sci...
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Major Electronic Transition Shift from Bandgap to Localized Surface Plasmon Resonance in CdxHg1−xSe Alloy Nanocrystals Dongsun Choi,†,§ Bitna Yoon,†,§ Dae-Kyu Kim,† Hionsuck Baik,‡ Jong-Ho Choi,† and Kwang Seob Jeong*,† †

Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 02841, Republic of Korea Korea Basic Science Institute (KBSI), Seoul 02841, Korea

‡

S Supporting Information *

ABSTRACT: CdxHg1−xSe alloy nanocrystals are obtained from CdSe semiconductor nanocrystals via cation exchange. By varying the composition during the exchange process, the CdxHg1−xSe alloy nanocrystals offer a widely tunable electronic transition from visible to NIR and even to mid-IR range. The visible bandgap transition of the CdSe colloidal quantum dot gradually redshifts to the near-IR with the addition of the Hg precursor, and then the steadystate intraband (or intersub-band) transition of the CdxHg1−xSe alloy nanocrystals appears. Finally, as the electron density is increased by successive addition of metal precursor, localized surface plasmon resonances (LSPRs) appear as a major electronic transition in the mid-IR regime. The shift of the major electronic transition from the bandgap to LSPRs infers that the exciton spatially moves to the surface from the inside of the nanocrystal through the cation change and further crystal growth. The corresponding variance of the nanocrystals’ structural, compositional, optical, electrical, and magnetic properties was carefully monitored by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron-dispersive X-ray (EDS) spectroscopy, time-resolved photoluminescence, photocurrent measurement, and electron paramagnetic resonance (EPR) spectroscopy, respectively. While a shift in only the bandgap has been observed in conventional quantum dots when cation-exchanged, the major oscillating transition transfers from the bandgap to the higher quantum states in CdxHg1−xSe alloy nanocrystal formed by the cation-exchange in this report. The compositional change expanding the optical range of nanocrystals from visible to mid-IR regime will provide a useful means of optimally tuning the electronic transition of nanocrystal-based applications along with improved optical selectivity demonstrated by a single intraband or LSPR peak.

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nanocrystal films enhances the carrier mobility in a solid-state nanocrystal transistor. The post-treatment on the nanocrystal surface also expedites the radiative recombination, resulting in the higher quantum yield for emitting applications. Furthermore, by careful control of the surface atoms with the posttreatment, the air-stability of nanocrystals can be achieved as well.11,12 Thus, compositional change through the metal ion treatment onto the nanocrystal has a number of advantages for device applications. Nevertheless, only limited information has been disclosed for monitoring the transition from a semiconductor to a metal nanocrystal. Investigating the transition from semiconductors to metals with careful control at the atomic level during synthesis will provide a clear understanding of the optical, electrical, and magnetic properties of the semiconductor colloidal quantum dots. Also, the transition from the semiconductor nanocrystal to the metal nanocrystal exhibits interesting optical properties such as the intraband transition

INTRODUCTION The transition from semiconductor to metal has been of great interest in nanomaterials because, depending on the electron density, the quantum confinement effect is differently reflected in the properties of nanomaterials.1−4 For example, localized surface plasmon resonances (LSPRs) are insensitive to the nanocrystal size, whereas the bandgap energy of a semiconductor nanocrystal such as CdSe colloidal quantum dot (CQD) increases with decreasing nanocrystal size. Recent reports show that it is possible to transfer the major electronic transition from bandgap to intraband transition, or from the intraband transition to localized surface plasmon resonances (LSPRs) through surface treatment with chemicals.5−10 The increase of the electron density, in principle, leads to the change of the major electronic transition from the bandgap to the LSPRs of a nanocrystal, inferring the spatial shift of the major electronic transition from the inside to the outside, here surface, of a nanocrystal through the phase change from the semiconductor to metal. Controlling the carrier density is crucial for not only the fundamental science, but also the nanocrystal-based device applications. For instance, the metal ion treatment onto the © 2017 American Chemical Society

Received: September 8, 2017 Revised: September 12, 2017 Published: September 12, 2017 8548

DOI: 10.1021/acs.chemmater.7b03813 Chem. Mater. 2017, 29, 8548−8554

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Chemistry of Materials

sonication. At 280 °C, Se precursor was quickly injected at a vigorous stirring rate, and the temperature temporarily dropped to 230−240 °C. The reaction proceeded for 2 min and was cooled by air blowing. CdSe QDs were separated from residual organic materials by centrifugation with acetone, methanol, and chloroform. Five milliliters of CdSe CQD in ODE (6 mg/mL) was prepared in a 50 mL roundbottom flask and degassed at 85 °C for 1 h. Zero to 1 mL of 80 mM Hg-oleylamine was injected drop by drop at 150 °C. The stirring rate was lowered so that Hg2+ can efficiently diffuse into CdSe nanocrystals. Cation exchange was held for 2 min, and then the temperature gradually cooled to room temperature. Nanocrystals were collected from the excess organic ligands and cations by adding nonpolar solvent. UV/Vis and IR Optical Measurements. UV/Visible absorption spectra were taken with an Agilent 8453 diode array spectrophotometer (Agilent technologies, U.S.). The excitation sources were tungsten/deuterium lamps, which were monochromatized by using a grating and slits. The fluorescence measurements were performed with a Hitach F-7000 fluorescence spectrometer (Hitachi. Ltd., Japan). The emission spectra were collected using a bandwidth of 5 nm and PMT voltage of 400 V. A 1× 1 cm2 quartz cuvette was used for all UV/ visible optical measurements. The intraband transition of the CdxHg1−xSe alloy and the vibrational modes of the surface ligands were obtained by using a Nicolet iS10 FTIR (Thermoscientific) with a resolution of 0.482 cm−1. The colloidal NCs were loaded in a demountable liquid sample cell with a 500-μL spacer between the ZnS windows (25.4 × 2 mm). Tetrachloroethylene, which is an infraredtransparent solvent, was used to disperse NCs for IR absorption. Time-Resolved Fluorescence Spectroscopy. The lifetime of each sample was measured using the time-correlated single photon counting (TCSPC) method. The samples were excited using a 520 nm pulsed laser (LDH-P-C-520). Fluorescent irradiation was measured at a wavelength of the highest emission intensity observed by steady-state fluorescence and collected using a PMT detector. The instrumental response function is about 570 ps in fwhm. Mid-IR Emission. For infrared emission on/off measurement, the CQD films were irradiated by a 532 nm pulsed laser with a laser power of 1.84 W/cm2. The pulsed laser was split into the CQD film and a beam dump to achieve a constant beam diameter and pulse time. The intraband photoluminescence was collected by a HgCdTe (MCT) detector. The germanium (Ge) window was placed in front of the detector to block the visible light. X-ray Diffraction. A Rigaku D/Max Ultima III X-ray diffractometer equipped with graphite-monochromatized Cu Kα (l = 1.54056 Å) was employed to identify the lattice structure of the alloys. The irradiation power was set to 40 kV and 30 mA. The diffraction pattern was recorded with a 0.01° sampling width. TEM Images/EDS Analysis/HAADF-STEM. The size and elemental analysis of the NCs were performed using a Tecnai G2 F30ST (FEI) microscope with energy-dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 300 kV. The high-angle annual dark-field (HAADF)-STEM image was obtained by using a FEI Double Cs Corrected Titan3 G2 60-300 S/TEM instrument with Chemi-STEM technology. The EDS elemental mapping data were collected using a Super-X detector with an XFEG higher efficiency detection system, which integrates four FEI-designed silicon drift detectors very close to the sample area. 1 H NMR Spectroscopy. 1H NMR spectra were recorded on a Bruker 500 MHz spectrometer. The NMR solvent, CDCl3 containing 0.03% tetramethylsilane, was used as an internal standard. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) measurement was carried out on an X-tool system (ULVAC-PHI) with monochromatic Al Kα X-rays as an excitation source. The pass energy was fixed at 280 eV for survey scans. Electron Paramagnetic Resonance Spectroscopy (EPR). The magnetism of CdxHg1−xSe alloy CQDs was detected using an X-band EPR spectrometer (ELEXSYS E500, Bruker, Germany) under room temperature. The alloy CQDs samples were placed in an EPR tube with an inside diameter of 5 mm. The solvent was removed under

and LSPR, providing an opportunity to access the energy smaller than the bulk bandgap, which has not been achievable with the bandgap transition in principle. The simple cation exchange method allows one to use the mid-IR and far-IR range wavelength through the intraband transition, where three atmospheric windows used for telecommunication are present. By optimizing the electronic wavelength through the cation exchange, one can fully harness many possible electronic transitions of the nanocrystals as well as the bandgap transition for photovoltaic, electrical, optical, and biological applications. It has been studied that heterogeneous nanocrystals such as alloys or core−shells provide versatility to control the electronic transitions of the material. Eychmüller and co-workers treated CdS nanocrystals with mercury precursors in aqueous solution to develop the CdS/HgS/CdS quantum dot/quantum-well structures.13,14 Guyot-Sionnest and co-workers synthesized HgSe/ZnS core− shell structures with improved emission quantum yield of the mid-IR luminescence and greater thermal stability.15 Rogach and co-workers demonstrated a CdHgTe quantum dot solid NIR light emitting device.16 Shuming and co-workers tuned the bandgap transition of alloy nanocrystals down to the near-IR regime.17,18 Alivisatos and co-workers reported the versatility of the cation exchange from CdSe to Ag2Se or CuSe.19 The wavelength tunability of the studies reported, however, was limited to only a short-range of wavelength of the bandgap transitions with a small variation in the carrier density. Also, the electronic transition observed for the heterogeneous nanomaterials was merely the bandgap transition. Here, we present CdxHg1−xSe II−II−VI alloy nanocrystals with wide wavelength-tunable electronic transitions ranging from visible to mid-IR by varying the nanocrystal composition through the cation exchange process. The major electronic transition changes from the bandgap of the CdSe CQDs to the mid-IR intraband (intersub-band) transition along with shell growth, and finally turns to localized surface plasmon resonance (LSPR) in the end. The change of the visible bandgap absorption feature to the mid-IR intraband single peak feature is a unique property of cation-exchanged colloidal nanocrystals. Moreover, the LSPR arising from the increase of the electron density in nanocrystals provides colloidal nanocrystals with versatility to cover a wide optical range while maintaining frequency selectivity and narrow bandwidth. The bandgap transition spatially delocalized inside a nanocrystal has changed into the LSPR mainly arising from the surface of a nanocrystal, inferring that the transition has spatially moved from the inside to the outside, the surface, of the nanocrystal. In addition, the corresponding magnetic properties were monitored during the cation exchange process, providing invaluable information to understand the underlying mechanism of the observed changes of the semiconductor nanocrystal by the cation exchange and the shell-growth with heterogeneous material.

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EXPERIMENTAL SECTION

CdSe Synthesis, Cation Exchange, and Shell Growth. 1.25 mmol of CdO and 0.5 mmol of stearic acid were placed in a 100 mL round-bottom flask with 10 mL of ODE. The mixture was vacuumed at 85 °C for 30 min (∼700 mTorr). The temperature was then increased to 200 °C and remained for 2 h under Ar to form a clear transparent solution. The heating source was removed from the reaction flask, and 1 g of TOPO and 1 g of octadecylamine (ODA) were injected into the flask at 85 °C. The mixture was degassed for 30 min at 85 °C, and the temperature was set to 280 °C for the reaction. For Se precursor, 0.25 mmol of Se was dissolved in 4 mL of TOP by 8549

DOI: 10.1021/acs.chemmater.7b03813 Chem. Mater. 2017, 29, 8548−8554

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Figure 1. (A) Schematic diagram of cation exchange of CdSe (yellow) to CdxHg1−xSe (red to black) nanocrystals and (B,C) the evolution of absorption spectra from CdSe to CdxHg1−xSe colloidal quantum dots in the visible and infrared region. vacuum before the EPR measurements. The microwave frequency was set to 9.84 GHz with a modulation frequency of 100 kHz. Photocurrent Measurement. The current−voltage (I−V) characteristics and photocurrent of the CdSe and CdHg(6)Se CQD film on interdigitated electrodes (IDA) were measured using a microscope probe station attached with a HP4145B semiconductor parameter analyzer. For the I−V characteristic measurement, a sweeping bias of 0−9 V is applied to the samples by a semiconductor parameter analyzer, and the current is measured. The infrared element, Newport 6580, was connected to an external DC power source for infrared irradiation. The incident light was collimated using an off-axis gold mirror and filtered with a Ge window to remove visible photons. The infrared element was completely blocked while measuring the IR light-off IV curve, and the block was removed when measuring the IR light-on. For the photocurrent measurement, a constant bias of 9 V is applied to each sample, and the current of the source and drain is measured while irradiating infrared element. The films were prepared by drop-casting on the IDA electrode and dried in ambient condition. The dried films are dipped into 2% 1,2-ethanedithiol and washed with methanol.

(CdSe)n + m(Hg(RNH)2 ) → (CdSe)n − m (HgSe)m + m(Cd(RNH)2 )

The cation exchange process at the surface may contribute to the blue-shift of the excitonic absorption peak by reducing the core CdSe size. However, the mercury selenide layer would produce a red-shift due to its narrow bandgap (or lower bandgap energy). Therefore, the inverted type I structure induced by the narrow bandgap of the HgSe could result in canceling out the blue-shift of the peak arising from the reduced core CdSe size, leading to a negligible shift of the absorption peak. The second possible explanation is the partial coverage of the nanocrystal surface with mercury ions as suggested by Eychmüller et al.21 The partial surface coverage can also explain the zero shift of the bandgap transition. Both the first and the second cases are still effective to understand the enhancement of the photoluminescence intensity with respect to the air stability of nanocrystal surface. The PL intensity is enhanced by the addition of mercury precursor by ca. 45 (±10)%. The measured PL quantum yield of the initial CdSe CQD is ca. 8.0 × 10−2, and the quantum yield of the CdHg(1)Se CQD is ca. 1.2 × 10−1. The PL enhancement of the first few samples of CdHg(1)Se and CdHg(2)Se is observed with high reproducibility. The surface of mercury treated nanocrystal is relatively more inert to moisture than that of CdSe when considering the reduction potential of water (H2/H2O, −4.5 eV vs vacuum). The lowest occupied electronic state of the CdSe CQD (1Se) is above the reduction potential of H2/H2O, and therefore photoexcitation can easily lead to the loss of the photogenerated electron, which is reflected by the reduction of PL intensity, whereas the photogenerated electron of HgSe CQD is inert to oxidation due to the lower electronic state of 1Se than the reduction potential of H 2 /H2 O. Another possible interpretation is the change of surface dipole by the addition of the mercury precursor. Because positive charges are created from the mercury precursor, the whole electronic band structure becomes more negative (vs vacuum), and the nanocrystal becomes more n-type leading to an increase in the bandgap PL intensity. The first two additions of mercury precursors, CdHg(1)Se and CdHg(2)Se, cause a broadening of

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RESULTS AND DISCUSSION Figure 1B and C shows the evolution of absorption spectra in the visible and infrared regime. The starting material is CdSe CQD with a bandgap absorption at 542 nm (=2.29 eV) with ca. 140 meV of fwhm. To synthesize the cadmium mercury selenide (CdxHg1−xSe) alloy nanocrystal, the mercury precursor (Hg−NH(CH2)8CHCH(CH2)7CH3) was first prepared by dissolving mercury(II) chloride with oleylamine at 120 °C under argon atmosphere, and then sequentially injected into the CdSe CQD solution at 150 °C under argon. 0−1.00 mL of mercury precursor (liq) was consequently added into 5 mL of 25 mM CdSe CQD solution and vigorously stirred in the flask. The number of the addition of the mercury precursor is indicated in the parentheses in the legend (Figure 1C). There are two possible processes that can occur during the first mercury treatment of the CdSe CQDs. On the basis of ref 17 describing the cation exchange of CdSe to HgSe colloidal nanoplatelet, the first layer of the CdSe nanocrystal surface can be efficiently replaced with HgSe.20 The possible mechanism may be the following: 8550

DOI: 10.1021/acs.chemmater.7b03813 Chem. Mater. 2017, 29, 8548−8554

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Figure 2. (A) Steady-state photoluminescence of CdxHg1−xSe alloy nanocrystals. (B) Time-resolved photoluminescence spectra. (C) Image of CdxHg1−xSe CQDs emitting PL under UV irradiation. (D) Metal-to-chalcogenide ratio of CdxHg1−xSe alloy nanocrystals based on the EDS analysis.

as shown in Figure S4. Two different mercury precursors were explored for controlling the shift of the electronic transition, and both successively tune the color of the CdxHg1−xSe nanocrystals. The time-resolved photoluminescence spectra were measured for the alloy nanocrystals at the center of the PL emission spectrum at 562, 564, 573, 668, 679, 730, and 730 nm with a photoexcitation wavelength of 520 nm (Figure 2A and B). As the electron wave function penetrates the shell, the spatial overlap of the electron and hole wave functions becomes inefficient, leading to the increase of emission lifetime from CdSe to CdHg(5)Se in consecutive order. The abrupt decrease of the lifetime from CdHg(5)Se to CdHg(6)Se is consistent with the suppression of the bandgap transition due to the electron occupation of the lowest quantum state of nanocrystal. In other words, the electron occupation at the lowest electronic state of the alloy nanocrystal forbids the bandgap transition, which is reflected in the efficient reduction of the bandgap emission. Simultaneously, the intraband transition occurring in the conduction band is produced due to the electron occupation as shown in Figure 1C. The mid-IR emission signal of the CdHg(7)Se nanocrystal in Figure S3 demonstrates the electron occupation of the lowest quantum state of the alloy nanocrystal. The time-resolved PL decays measured at room temperature were analyzed by two exponential components. The first time constant with τ1 = 5.5−6.3 ns corresponds to the radiative recombination of the bandgap transition, and the second time constant with τ2 = 31−42 ns is attributed to the trap emission (Figure S8).24 With increased mercury precursor addition, the contribution of the pre-exponential factor, α2, increases from 0.3 to 0.45, implying that the density of the defects increases by the cation exchange. The average lifetime τavg obtained by the equation below is from 26.6−47.1 ns with different mercury portion.

the excitonic peak in the visible regime and a red-shift of the exciton emission to the NIR regime. The broadening of the NIR emission spectrum results from the broadening and separation of the electron wave function in the conduction band. By lowering the electronic states of the conduction band (vs vacuum), the electron wave function is efficiently delocalized to the shell materials, giving rise to the red-shift of the emission spectrum with reduced intensity at the center of the emission peak due to the less efficient overlap with the hole wave function in the valence band. A strong mid-IR transition of the CdHg(6)Se appears in the IR absorption spectra. The mid-IR transitions are observed after the CdHg(5)Se and shift to longer wavelength under consecutive addition of the mercury precursor. As shown in previous reports, the mid-IR transition is designated to the intraband transition of the CdxHg1−xSe nanocrystal. The absorption coefficient of the intraband transition of the CdxHg1−xSe CQD is comparable to that of the bandgap transition of the initial nanocrystal, CdSe CQDs, which is also consistent with previous reports.22,23 Broad absorption features are observed from the CdHg(3)Se to the CdHg(5)Se samples in the NIR regime, inferring that the major electronic transition transfers to the bandgap transition of CdxHg1−xSe nanocrystal with a large portion of mercury. On the basis of the results of the X-ray photoelectron spectra, the CdSe core is not perfectly replaced by HgSe. Residual Cd ions of ca. 2.0 atomic % in the CdHg(8)Se CQDs are present. Accordingly, the steady-state intraband transition in Figure 1C does not originate from the pure HgSe nanocrystal but from the CdxHg1−xSe nanocrystal with residual Cd inside the nanocrystal. Both visible PL spectra and the intraband absorption spectra with weak intensity are simultaneously observed in the CdHg(4)Se and CdHg(5)Se samples. The transition from the intraband transition to the LSPR is also observed in the CdHg(6)Se CQDs. The mid-IR absorption peak fits well with the Lorentzian fitting (Figure S2), the property of LSPR, and has a fwhm of 677 cm−1 (84 meV). In addition, the red-shift of the alloy nanocrystal bandgap transition can be controlled by the shell growth method. Successive shell growth with mercury and selenide precursors slows the shift of the electronic transitions

n

τavg =

∑i (αiτi2) n

∑i αiτi

Surprisingly, the integrated area of the steady-state photoluminescence spectra in Figure 2A is pseudoanticorrelated with 8551

DOI: 10.1021/acs.chemmater.7b03813 Chem. Mater. 2017, 29, 8548−8554

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spherical shape. The XPS spectra show the variance of each composition in the alloy nanocrystal. The 67% Cd ratio of CdSe nanocrystal is reduced to 51% in CdHg(1)Se, and the portion becomes smaller with increased number of mercury precursor addition. Because of the negligible lattice mismatch between zinc blende crystal structures of CdSe and HgSe, the alloy nanocrystal epitaxially grows (or Schottky).25 Accordingly, the peaks exhibit no significant shift but become narrower due to the size-growth. The peaks shown at 25°, 42°, and 49° correspond to the (111), (220), and (311) facets of CdSe and HgSe nanocrystals, respectively, and are consistent with the results previously reported.5,6,22,23 The zinc-blende reference peaks for the HgSe and CdSe are provided at the top and bottom in Figure 3D. This zinc-blende structure of the CdxHg1−xSe alloy nanocrystal is apparently different from the recent result reported by Torres et al., showing wurtzite polymorph of HgSe.26 The elemental distribution of the CdxHg1−xSe alloy nanocrystals was successfully analyzed by EDS elemental analysis along with the high-angle annual dark-field (HAADF)-STEM in Figure 4. When comparing CdHg(5)Se (Figure 4C,D) and CdHg(8)Se (Figure 4H,I) CQDs, one can readily identify the loss of Cd elements and gain of Hg elements by the successive addition of Hg precursor. These elemental mapping images provide direct evidence for the mercury ion effectively exchanging the cadmium ion, and for the successive growth of the CdxHg1−xSe alloy nanocrystal. 1 H NMR spectroscopy provides information on the surrounding of the nanocrystal before and after the cation exchange. Trioctylphosphine oxide (TOPO) molecules are the native ligands of the CdSe nanocrystal and are efficiently replaced by oleylamine ligands via the addition of Hgoleylamine precursor. The ligands bonded to the nanocrystal surface generally exhibit a broad bandwidth due to the effect of metal atoms on the surface. Interestingly, sharp peaks at δ = 5.84, 5.01, and 4.95 are shown in Figure S11. Although it is possible that a heterogeneous catalytic reaction can occur at the metal-rich surface, it is more reasonable to understand it resulting from the residual octadecene molecules.

the average lifetime, inferring that the nonradiative recombination, trapping, becomes more significant with increasing mercury portion in the CdxHg1−xSe alloy nanocrystal (Figure S10).

Figure 3. (A,B) Transmission electron microscopy (TEM) images of CdSe and CdxHg1−xSe alloy nanocrystals (scale bar = 10 nm). (C) HRTEM image of the CdxHg1−xSe alloy nanocrystals (scale bar = 5 nm). (D) X-ray diffraction spectra of CdSe (orange) and CdxHg1−xSe alloy nanocrystals (red to black). Top and bottom peaks correspond to the reference zinc-blende data of HgSe and CdSe.

The size growth of the CdxHg1−xSe alloy nanocrystal was confirmed by TEM images, XRD, and XPS spectra in Figure 3 and Figure S6. The first three additions of the mercury precursor interestingly do not substantially increase the nanocrystal size because of the exchange of the cations. With consecutive addition of the mercury precursor, the alloy nanocrystal grows up to 11.3 nm (±1.3 while maintaining the

Figure 4. High-angle annual dark-field (HAADF)-STEM image (A,F) and elemental mapping of CdHg(5)Se CQDs (B−E) and CdHg(8)Se CQDs (G−J). Cd (green), Hg (blue), and Se (red). 8552

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Figure 6. Mid-IR photocurrent measurement. (A) Schematic diagram of the photocurrent measurement setup. (B) Photocurrent response of CdxHg1−xSe CQD solid and CdSe CQD solid (inset) under mid-IR light.

source. The ligand exchange was conducted by using the ethanedithiol cross-linkers. Apparently, the photocurrent of the CdSe CQD film is negligible under the cw-mid-IR light (inset, 5.18 × 10−11 A) filtered with a Ge window, whereas the CdxHg1−xSe alloy nanocrystal promptly responds to the mid-IR light with 7.66 × 10−5 A. Surprisingly, the CdHg(7)Se and CdHg(8)Se alloy nanocrystals do not produce discernible photocurrent. This is probably because of the large metal-like character arising from continuous electronic states producing the large off-current. The CdHg(6)Se alloy film also shows the off-current probably arising from the environmental thermal radiation. Thus, the photocurrent results successively demonstrate the presence of the mid-IR transition of CdHg(6)Se alloy nanocrystals, showing great promise for optoelectronic applications.

Figure 5. Electron paramagnetic resonance (EPR) spectra of CdSe (yellow) and CdxHg1−xSe (red to black) nanocrystals measured at room temperature.

Figure 5 represents the electron paramagnetic resonance (EPR) spectra obtained at room temperature for the series of CdxHg1−xSe nanocrystals. Interestingly, a distinct peak is shown at g = 2.37 in CdHg(1)Se, which is likely to be the unpaired electron at the surface as shown in a previous report with HgSe nanocrystals. Surprisingly, the single peak swiftly disappears, and a peak is shown at g = 2.39 with a large bandwidth. The broad feature disappears again, and a new feature appears at g = 4.44.27 Bliek et al. reported that the g-factor of the conduction electrons in bulk HgSe at the Fermi energy is close to 4 when the electron density is 7.0 × 1018 cm−3.28 Although we do not have the exact number of electrons occupying the quantized electronic states in the conduction band of the CdxHg1−xSe alloy nanocrystal ensemble, assuming two electrons, which is the smallest number in the conduction band per nanocrystal showing the intraband transition, the least doping density of CdxHg1−xSe nanocrystal with 11.3 nm (±1.3) diameter and 1.0 nm ligand length is estimated to ca. 2.1 × 1018 cm−3 (±0.8 × 1018). Therefore, the g-factor estimated is close to the range of the value obtained by the equation:

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CONCLUSION Varying the composition of a semiconductor nanocrystal can shift the major electronic transition from the bandgap transition occurring in the delocalized states of a nanocrystal to the intraband (or intersubband), and even to the mid-IR LSPR mainly related to the localized states at the surface. The corresponding magnetic properties were carefully investigated by EPR spectroscopy, showing the appearance and disappearance of the paramagnetism depending on the portion of Hg in CdxHg1−xSe alloy nanocrystal. The change of nanocrystal structure, elemental replacement, and the ligand exchange were thoroughly monitored by XRD, XPS, 1H NMR, HAADFSTEM, and EDS elemental mapping, providing invaluable structural and compositional information to understand the formation of the alloy nanocrystal via cation exchange and further nanocrystal growth. The modulated photocurrent signal of CdxHg1−xSe alloy quantum dot solid device obtained under a mid-IR light source demonstrated the quantum confined states of CdxHg1−xSe in the mid-IR regime, whereas the starting material, CdSe CQDs, exhibits no mid-IR light-induced photoresponse. Furthermore, the CdxHg1−xSe exhibiting LSPR nanocrystal does not have significant photoresponse due to continuously degenerated states arising from the metallic property of excess Hg. Controlling the composition of the alloy semiconductor nanocrystal will be a promising method to tune the optical, electrical, and magnetic properties of the nanocrystals with tunable electron density in the nanocrystal. Furthermore, this method would make it possible to expand the range of electronic transition from the UV to far-IR, and down to terahertz (THz) transition with an optimum combination of metal oxide and narrow bandgap materials.

⎫ ⎧ ⎞ ⎛ m0 Δ ⎬ − 1⎟ g (k) = 2⎨1 − ⎜ ⎠ 3ε′(k) + 2Δ ⎭ ⎝ m*(k) ⎩ ⎪

⎪

where g(k), m0, m*(k), Δ, and ε′(k) are g-factor, the free electron mass, the effective mass, the spin−orbit splitting, and ε′(k) = ε(k) −

ε 2k 2 , 2m 0

respectively.29

To demonstrate the change of the optical wavelength from visible to mid-IR regime and explore the potential for optoelectronic applications, we performed photocurrent measurements with an IR globar source and germanium optical filter under ambient condition. Because the germanium optical filter blocks light shorter than 2.0 μm, only the mid-IR light was allowed to transmit. Figure 6 represents the photocurrent result of CdSe (inset) and CdxHg1−xSe nanocrystal films deposited onto the interdigitated electrodes (IDA) under the mid-IR light 8553

DOI: 10.1021/acs.chemmater.7b03813 Chem. Mater. 2017, 29, 8548−8554

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Chemistry of Materials

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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.chemmater.7b03813. Mid-IR emission signal, EPR, PL, UV/vis, FTIR spectra, mercury precursor-dependent optical results, and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kwang Seob Jeong: 0000-0003-3246-7599 Author Contributions §

D.C. and B.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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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 (NRF-20100020209). We thank the Korea Institute of Radiological & Medical Sciences (KIRAMS), Korea Basic Science Institute (KBSI), and Prof. Han Young Woo for the usage of their TEM, HRTEM, HAADF-STEM, and spectrometers.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b03813 Chem. Mater. 2017, 29, 8548−8554