Singly and Doubly Occupied Higher Quantum States in Nanocrystals

Jan 23, 2017 - ... and vice versa by varying the number of electrons through postchemical treatment. The switchable magnetic property will be very use...
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Singly and Doubly Occupied Higher Quantum States in Nanocrystals Juyeon Jeong,† Bitna Yoon,† Young-Wan Kwon,*,‡ Dongsun Choi,† and Kwang Seob Jeong*,† †

Department of Chemistry, Research Institute for Natural Sciences and ‡KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841 Korea S Supporting Information *

ABSTRACT: Filling the lowest quantum state of the conduction band of colloidal nanocrystals with a single electron, which is analogous to the filling the lowest unoccupied molecular orbital in a molecule with a single electron, has attracted much attention due to the possibility of harnessing the electron spin for potential spin-based applications. The quantized energy levels of the artificial atom, in principle, make it possible for a nanocrystal to be filled with an electron if the Fermi-energy level is optimally tuned during the nanocrystal growth. Here, we report the singly occupied quantum state (SOQS) and doubly occupied quantum state (DOQS) of a colloidal nanocrystal in steady state under ambient conditions. The number of electrons occupying the lowest quantum state can be controlled to be zero, one (unpaired), and two (paired) depending on the nanocrystal growth time via changing the stoichiometry of the nanocrystal. Electron paramagnetic resonance spectroscopy proved the nanocrystals with single electron to show superparamagnetic behavior, which is a direct evidence of the SOQS, whereas the DOQS of the two- or zero-electron occupied nanocrystals in the 1Se exhibit diamagnetic behavior. In combination with the superconducting quantum interference device measurement, it turns out that the SOQS of the HgSe colloidal quantum dots has superparamagnetic property. The appearance and change of the steady-state mid-IR intraband absorption spectrum reflect the sequential occupation of the 1Se state with electrons. The magnetic property of the colloidal quantum dot, initially determined by the chemical synthesis, can be tuned from diamagnetic to superparamagnetic and vice versa by varying the number of electrons through postchemical treatment. The switchable magnetic property will be very useful for further applications such as colloidal nanocrystal based spintronics, nonvolatile memory, infrared optoelectronics, catalyst, imaging, and quantum computing. KEYWORDS: superparamagnetic, switchable magnetic property, HgSe, colloidal nanocrystal, infrared

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unoccupied molecular orbital (LUMO) with an electron in a molecule, which is called singly occupied molecular orbital (SOMO). As the application of SOMO has brought enormous benefit to the field of chemistry such as catalytic reaction using SOMO activation process, it is expected that the ability to fill the lowest quantum state of a colloidal nanocrystal with a single electron would open up new opportunities for nanomaterials research both in fundamental science and technology.9 Therefore, the controllability of the electron occupation of the 1Se states has been of a great interest in semiconducting nanomaterials due to the numerous potentials. The steady-state intraband transition arising from filling the 1Se with electrons has been recently achieved by Jeong et al.10 In the work, it was assumed that the mercury chalcogenide CQD showing the mid-IR optical transition is filled with two electrons in the 1Se state.11,12 However, there has been no direct evidence for monitoring the process of the electron

olloidal quantum dots (CQDs) have been rigorously studied for the last three decades and nowadays are used for a number of applications such as imaging, display, sensor, solar cell, detector, spectrometer. and so forth.1−8 Optimizing the doping density of the colloidal semiconducting nanocrystal is critical to the electrical, optical, and magnetic properties that determine the CQD-based device performance. To increase the carrier concentration, heterogeneous impurities are usually embedded into nanocrystal structure during synthesis or after. The addition of the metal impurity to the semiconducting materials leads to the elevation of the Fermi level close to the conduction band. Filling the lowest quantum state (1Se) of the conduction band (CB) with the addition of the metal impurities, however, has not been successful yet because the metal impurities virtually serve as carrier recombination centers (or trap) that locate below the 1Se state in energy (versus vacuum). If the 1Se state is stably occupied with electrons in steady state, it is possible to utilize higher quantum transitions beyond the bandgap transition (Figure 1A). Filling the 1Se of nanocrystal with electrons is conceptually analogous to filling the lowest © 2017 American Chemical Society

Received: November 25, 2016 Revised: January 17, 2017 Published: January 23, 2017 1187

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Figure 1. (A) Schematic diagram of electron occupation in the lowest quantum state in the conduction band (1Se) of CQD with different size. (B) TEM image of HgSe colloidal quantum dot with 6.0 (±0.6) nm diameter (scale bar = 20 nm). Inset is the HRTEM image of the HgSe CQD showing HgSe(111) lattice structure with 3.45 Å distance. (C) Evolution of the 300 K X-band EPR spectra with nanocrystal size growth: 3.4 (±0.3) nm (15 s growth time); 4.0 (±0.4) nm (30 s); 4.3 (±0.5) nm (1 min); 4.6 (±0.4) nm (2 min); 6.0 (±0.6) nm (8 min). (D) Formation and change of the steady-state intraband transition (1Se-1Pe electronic) of HgSe colloidal quantum dot with an increase of nanocrystal growth time.

occupation in the 1Se state. Especially, if the 1Se state is filled with two electrons after the chemical synthesis, an intermediate state with a singly (electron) occupied quantum state (SOQS) in the CB should exist that would produce superparamagnetic or ferromagnetic properties (Figure 1A). The magnetic property of CQDs has been intensively studied for the past decade in conjunction with the size tunable bandgap of CQD due to its potential for many optomagnetic applications13−16 Chemical synthesis has enabled embedding of the impurity dopants into the semiconductor nanomaterials.17 The magnetic property of the impurity-doped nanocrystals mainly results from the electrons of the metal impurity embedded in the nanocrystal. Strictly speaking, the magnetic property of the impurity-doped CQD does not originate from the nanocrystal itself but from the impurity dopant such as the transition metal ions. The photoelectronic doping method is known to be capable of creating unpaired electrons in ZnO nanocrystal under the irradiation of cw-UV light source with a reducing agent. However, in the absence of photoexcitation or reducing agents, the magnetic property of the nanocrystal disappears.18 Also, recent advances in diluted magnetic semiconductors (DMS) such as Au nanocrystal suggest that there is unexpected magnetism that is created when the metal cluster size is in the range of a few nanometer where the nonbonding electrons or the ligand on the surface are not

negligible.19−21 Unfortunately, the origin of the magnetic property of the metal nanocrystal has not been clearly understood yet, and hence there is indeed a demand for understanding and controlling the magnetism of nanocrystals. Here, we report the intrinsic superparamagnetic and diamagnetic properties of colloidal nanocrystals depending on the number of electron(s) (0, 1, or 2) in the 1Se state controlled by chemical synthesis. The electron occupation process is monitored by magnetic and optical measurements. The steady-state mid-IR optical absorption of the 1Se-1Pe intraband transition is created and gains strength as the magnetic property of the nanocrystal varies from dia-, superpara-, diamagnetic in order during the nanocrystal growth. This is because the origins of the optical and magnetic properties are the increase of the number of electrons in the 1Se. The electron paramagnetic resonance (EPR) spectra confirmed that the numbers of electrons are zero, one, and two in the 1Se state as the nanocrystals form. The SQUID measurement reveals the superparamagnetism of the solid HgSe CQD with the SOQS. Furthermore, the number of electrons in the 1Se state is tunable by postchemical treatment, which will be a promising method to control the optical and magnetic property of the colloidal nanocrystal based devices. The electron digitized colloidal quantum dot will pave the way for the development of spin-based applications.22−24 1188

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Nano Letters Experimental Methods. Synthesis. Mercury(II) chloride (HgCl2, ACS, 99.5% min) was purchased from Alfa Aesar. Selenourea (98%). Oleylamine (OLA, technical grade, 70%), tetrachloroethylene (TCE, ACS reagent, ≥99.0%), and ammonium sulfide solution (40−48 wt % in H2O) were purchased from Sigma-Aldrich. HgSe CQD Synthesis. A 12.6 mg sample of selenourea (0.1 mmol) was dissolved in 1 mL of oleylamine. The selenourea solution was degassed at 90 °C for 1 h under vacuum (100 mTorr), yielding a clear solution. The selenium precursor solution gradually turns dark brown under heating for 2 h at 180 °C in argon atmosphere. A 27.2 mg sample of mercury(II) chloride (HgCl2, 0.1 mmol) was dissolved in oleylamine, degassed at 90 °C for 1 h, and heated to 110 °C for 1 h under argon. The selenourea solution was injected into the HgCl2 solution and the mixture turned black immediately. The nanocrystal size was controlled by varying the chemical reaction time. Five milliliters of oleyamine in tetrachloroethylene solution was quickly added to the flask to stop the nanocrystal growth. The product was precipitated with methanol and centrifuged. The precipitate was dried and redispersed in tetrachloroethylene. Chalcogenide Treatment. A 0.5 mL sample of oleylamine and 150 μL of 0.1 M (NH4)2S in methanol were mixed with 1 mL of 10 mg/mL HgSe CQD solution in TCE. This mixture was stirred for 5 min at 25 °C, precipitated with ethanol, and centrifuged. The precipitate was dried and redispersed in TCE. Infrared Spectra of Intraband Transitions. The mid-IR absorption spectra were obtained by FTIR (Nicolet iS10, Thermoscientific) with the resolution of 4 cm−1. Tetrachloroethylene, which is relatively transparent in the mid-IR regime, was used as a solvent. Electron Paramagnetic Resonance Spectroscopy (EPR). EPR spectroscopy was performed over the temperature range from 4 to 300 K using a Jeol JES-FA200 EPR spectrometer (Xband; 8.75−9.65 GHz; Japan). The EPR signals were detected at 0.9980 mW incident microwave power. The modulation width was 10 G at 100 kHz modulation frequency and the sweep magnetic field width was from 0 to 10000 G. The HgSe CQD sample was loaded in a 5 mm quartz tube (Wilmad Lab Glass, U.S.A.). For the solid sample, the CQD solution sample was evacuated by applying rough vacuum (10−2 Torr) first for 1 h and then high vacuum (10−5 Torr) was applied again for additional 4 h. Therefore, the CQD solid was well dried under vacuum inside the quartz holder. The sealed tube with the solid sample was loaded into a cylinder cavity equipped with a liquid nitrogen cryostat system. For the solution sample, the measurement was performed without further drying. Superconducting Quantum Interference Device (SQUID). The magnetic susceptibility was measured using a SQUID magnetometer (Quantum Design, mpms 7.5, U.S.A.) over the temperature range of 4−300 K. The HgSe QD sample was loaded within a gelatin capsule, the magnetization−magnetic field (M-H) dependence was obtained at room temperature from −40 000 G to +40 000 G and was also measured at 4 K after cooling down to 4 K under zero magnetic field. The temperature-dependent magnetic susceptibility was collected from 4 to 300 K at 1000 and 10 000 G. X-ray Photoelectron Spectroscopy (XPS). XPS analyses were performed using VG ESCALAB (220i). Spectra were acquired using a monochromatic Al Kα1,2 X-ray source (hν = 1486.6 eV). 50 eV pass energy and 20 eV were used to obtain high intensity for a wide scan and for narrow scans to achieve

high-energy resolution, respectively. Sample charging was corrected by setting the C 1s region to 284.5 eV X-ray Diffraction Spectrum (XRD). The X-ray diffraction patterns were acquired by using Rigaku/D/Max UltimaIII X-ray diffractometer with a graphite-monochromatized Cu Kα (λ = 1.54056 Å) at power settings of 40 kV and 30 mA. The patterns were measured from 20° to 80° with 0.01° sampling width. Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed using a TECNAI G2 F30ST (FEI) microscope at 300 kV. 1 H Nuclear Magnetic Resonance (1H NMR). All 1H NMR spectra were obtained by using the BRUKER 500 MHz spectrometer. Result and Discussion. SOQS 1Se. Figure 1B shows the TEM and the high-resolution TEM (inset) images of HgSe CQD. The TEM, HRTEM, and STEM images confirm the monodispersed and spherical HgSe CQD (Figures S1 and S2). The magnetic properties of HgSe CQD, either diamagnetic (N = 0, 2) or superparamagnetic (N = 1), are determined by the number of electrons and their pairing, where N is the number of electrons in 1Se state. The EPR spectra of HgSe nanocrystals with different size reveal the evolution of electron occupation in the 1Se state in Figure 1C during the nanocrystal formation. The legend indicates the size of nanocrystal with the chemical reaction time of nanocrystal growth after selenium precursor injection. The longer the reaction time, the larger the nanocrystal size. The red spectrum (4.0 nm ± 0.4, 30 s growth time) in Figure 1C exhibits strong unpaired electron signal at approximately 3000 G (g = 2.2), indicating the presence of unpaired electrons in nanocrystals whereas the black spectrum (3.4 nm ± 0.3, 15 s growth time) in Figure 1C exhibits a featureless background. By increasing the nanocrystal growth time, the paramagnetic feature shown from the sample of 30 s growth time swiftly disappears, and the featureless curve recovers (blue), inferring that the unpaired electron no longer exists. Under the magnetic field of above 6000−12000 G, there is no feature observed from the SOQS sample in Figure S4, meaning that a hole trap is not created in the nanocrystal, which was observed from the bandgap excited InP CQD under photoexcitation.25 Because of the steady-state occupation at the 1Se state with electrons, the HgSe nanocrystal is indeed free of the hole trap located in midgap state. The mid-IR electronic absorption spectra for the identical batches of HgSe nanocrystals are shown in Figure 1D. The spectrum of the HgSe CQD with the size of 3.4 nm (±0.3 nm) mainly shows vibrational features of oleylamine mixed with a broad feature that may be the onset of the intraband electronic transition occurring in the CB. As the nanocrystal size increases, the intraband electronic transition absorption peak in the mid-IR arises and gradually redshifts although the intensity correlation is not promptly reflected due to the overlap with the CH vibrational modes of the oleylamine ligands. The sample concentration was kept constant for all samples in order to compare the mid-IR optical density. The appearance of electronic transition occurring between 1Se and 1Pe in the CB suggest that the 1Se is filled with at least one electron. The absorption peak shifts by approximately 750 cm−1 due to the nanocrystal size growth from 3.4 to 6.0 nm. The absorption coefficient of HgSe CQD is 2.0 × 105 M−1cm−1 when the 1Se state is fully filled with electrons corresponding to the 6.0 nm HgSe CQD sample in this work. 1189

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Figure 2. (A) X-ray diffraction spectra of HgSe CQD solid of different size. (B) X-ray photoelectron spectra of three different size HgSe colloidal quantum dots. (C) EPR spectra of HgSe CQDs (4.0 nm, 30 s) before (red, N = 1) and after (blue, N = 0) the sulfide treatment, and of the assynthesized HgSe CQDs for comparison (3.4 nm, black). The EPR spectrum of the sulfide-treated sample is well overlapped with that of the assynthesized HgSe CQDs (black) showing diamagnetism. (D) EPR spectrum of CQD solid (black) and solution (red).

electrons, which is reflected by the featureless EPR spectrum (blue, Figure 1C). The loss of unpaired electron feature in the EPR spectrum and the steady-state mid-IR intraband absorption transition demonstrate that two electrons are indeed paired with opposite spin momentum in the 1Se whereas in the photodoped ZnO nanocrystal only unpaired electrons are accumulated under UV irradiation.27 The X-ray diffraction spectra also offer information on where the excess metal ions are placed. No peak shift in the XRD spectra indicates that no significant strain is in the crystal due to the excess metal ions, inferring that the excess Hg ions bind to the surface of nanocrystal. Interestingly, the HgSe(111) facet dominates when metal-to-chalcogenide ratio exceeds unity based on the XRD spectra and XPS analysis data. This observation is consistent with the result reported by Woo et al. that the metal rich air-stable n-type PbS CQDs shows enhanced intensity in the PbS(111) facet compared to other facets.28 The high intensity of the HgSe(111) in XRD spectrum is a clue to finding the origin of the appearance of intraband transition with increasing nanocrystal size. The intraband transition is an intermediate optical property having both semiconductor and metal nanocrystal characters. When the metal-to-chalcogenide ratio is above unity, the intraband absorption gains strength due to the increased metal character arising from the increase of the number of electrons on certain facets. Assuming an ideal spherical nanocrystal, the difference between the number of metal ions and chalcogenide are roughly estimated to be approximately 20−55 each with excess Se for 3.4 nm (±0.5 nm) HgSe CQD, approximately 48−86 each with excess Hg for 4.0 nm (±0.4 nm) HgSe CQD and approximately 180−300 each with excess Hg for the 4.6 nm (±0.5 nm) CQD. The estimation is based on counting the atom numbers of a unit cell

In regards to the synthesis of HgSe CQD, HgCl2, and selenourea were used as the mercury and selenium precursors. A nonthiol ligand, oleylamine, was employed for capping the HgSe CQD surface in a similar way to the oleylamine passivated HgS nanocrystal synthesis.26 The nonthiol ligand renders ligand versatility for future optoelectronic applications that require further ligand exchange process. The NMR spectra of the as-synthesized HgSe CQD passivated with oleylamine are in Figure S3. The X-ray diffraction result confirms the zinc blende crystal structure of the HgSe CQDs in Figure 2A. The peaks become narrow with increasing nanocrystal size. Interestingly, the (111) peak intensity increases as the nanocrystal size increases. Three CQD samples of different sizes were analyzed by X-ray photoelectron spectroscopy (XPS). The 0.9 Hg/Se ratio of the HgSe CQD with 3.4 nm (15 s growth time) lies in the chalcogenide rich regime (47.56/52.44 = 0.9 Hg/Se, black). The HgSe CQDs with 4.0 nm (30 s) are slightly metal rich (52/47 = 1.1). Thus, the unity Hg/Se ratio is expected to lie between the 15 and 30 s growth time. Surprisingly, the combination of the XPS results with the EPR spectra revealed that the unpaired electron feature of the SOQS in EPR is related to the stoichiometry of the nanocrystal. As the Hg/Se ratio exceeds unity, the SOQS property is created at approximately 3000 G, and the optical transition (oscillator) strength is presumed to gain strength although it is difficult to deconvolute the minute intraband broad absorption from the infrared absorption spectrum due to the overlapping with redundant organic molecules. The change in stoichiometry suggests that the metal rich in the nanocrystal contributes to the filling of 1Se with an electron(s). As the metal portion sufficiently increases, the 1Se state ends up being filled with two 1190

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Figure 3. (A) EPR spectra of SOQS of HgSe CQD solid at various temperature. (B) EPR spectra of SOQS of HgSe CQD solid at different angles of sample. (C) Magnetization curves of HgSe CQD solid at 300 and 4 K. (D) The magnetization curves of HgSe CQD solid showing superparamagnetic components at 300 and 4 K after a linear background subtraction.

for the zincblende structure of HgSe depending on the metalto-chalcogenide ratio obtained by the XPS result. The hypothesis is also supported by the quenching of steadystate intraband absorption by surface treatment with chalcogenide (S2−) in ref 10. The chalcogenide treatment substantially attenuates the intraband transition intensity and recovers the bandgap transition by reducing the metal-tochalcogenide ratio. The consecutive metal ion treatment leads to the recovery of the intraband transition that is red-shifted due to the size growth. This observation infers that the metal ions are on the surface and not inside of the nanocrystal and serve as a platform for further growth. The change of surface dipole by alternating deposition of metal ion and chalcogenide on the surface can also explain the on/off control of the steadystate intraband transition because in terms of stoichiometry the surface dipole of the nanocrystals and the metal-tochalcogenide ratio are interrelated as discussed. Furthermore, Shen et al. recently reported that the intraband transition gains more plasmon character with increasing nanocrystal size based on the fwhm analysis.29 The large HgSe(111) area of large size nanocrystals may contribute to the conversion of the intraband transition to plasmon resonance because more metal Hg on HgSe(111) surface are involved in the electronic transitions. The change of the number of electrons in the CB by postchemical treatment is monitored by EPR spectroscopy. As the HgSe CQD is treated with sulfide solution, the metal-tochalcogenide ratio decreases due to the sulfide deposition on the Hg-end of the surface. As a result, the magnetic feature of the SOQS disappears due to the loss of an electron, which is

reflected in the EPR spectra in Figure 2C. Similarly, the DOQS of the HgSe CQD with two electrons (1 min, 4.3 nm) in 1Se loses one electron and becomes paramagnetic (Figure S5). The appearance of the unpaired electron peak in EPR by sulfide treatment provides invaluable information to assign the origin of the EPR peak at 3000 G. If the peak results from “trapped” electrons in a partially positive charge site on the surface, the sulfide (S2−) treatment should only quench the EPR peak and cannot recover the peak. However, the sulfide treatment to the 4.3 nm HgSe (1 min) sample initially showing featureless EPR spectrum before the sulfide treatment produces a strong EPR peak corresponding to the unpaired electron. Therefore, it is concluded that the EPR peak at approximately 3000 G (g = 2.2) originates from delocalized electrons in the 1Se state. The successful controllability of the number of electron in the 1Se state via postchemical treatment is of importance in terms of solution accessible spin control method. Moreover, the method can be utilized for further optoelectronic application such as minimal energy-required nonvolatile erasable memory. The EPR measurement of HgSe nanocrystals were performed in two different phases (Figure 2D). Solid sample has a smaller g-value (g = 2.09) than the solution sample (g = 2.54). The shift of the peak to the lower g-value possibly arises from magnetic dipole interactions between aligned neighboring nanocrystals.30 The alignment of the magnetic dipole of nanocrystal solid is identified by the angle-dependent EPR spectrum in Figure 3B. In addition, the cyclotron resonance (CR) is observed in the lower magnetic field regime from 0 to 500 G. The cyclotron resonance is known to appear when the optical intraband transition is allowed in low-dimensional 1191

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show strong absorption strength in Figure 1D. The SOQS of the large size HgSe nanocrystal may be in agreement with the second intraband transition recently reported by Yoon et al.26 The rationale to assign the second SOQS to the unpaired electrons in 1Pe state is the featureless EPR spectra of 4.3, 4.6, and 6.0 nm size of HgSe nanocrystals in Figure 1C, implying the lack of density of states between 1Se and 1Pe discrete energy levels. During the formation of nanocrystal, paired electrons are observed before the creation of the unpaired electrons at higher electronic state. Further study is necessary to identify the origin of SOQS in higher quantum states. Conclusion. In conclusion, we present the superparamagnetic-(Ne = 1) and diamagnetic-(Ne = 0, 2) behavior of colloidal mercury selenide quantum dots with no heterogeneous metal impurity (Figure 4). The magnetic property of the

epitaxy quantum well or quantum dots under microwave irradiation.31 Therefore, the cyclotron resonance in the EPR spectra confirms the presence of the intraband transition for the colloidal nanocrystal with SOQS. The EPR spectra for the 4.0 nm (30 s) sample demonstrate the presence of both the unpaired electron and the cyclotron resonance at approximately 500 G, which was not clearly identifiable by the infrared spectrum due to the organic vibrational peaks. The cyclotron resonance appears when the EPR signal exists but the intensity varies from batch to batch. It will be worth studying further to find out the correlation between the higher quantum states and the cyclotron resonance by elevating the Fermi level step by step. Temperature-Dependent EPR. The EPR spectra monitoring the unpaired electrons at different temperature from 4 to 294 K are presented in Figure 3A and Figure S7. As the temperature decreases from 294 to 4 K, the EPR peak intensity loses strength probably due to the creation of the antiferromagnetism although the Boltzmann distribution between the two states (N+/N−= exp(−E/kBT)) predicted the enhancement of EPR signal intensity, which is opposite to the observation, where E (= gβH), kB, g, β, and H are the energy difference of the Zeeman splitting, Boltzmann constant, g-value, Bohr magneton, and the applied magnetic field, respectively. The reduction of the EPR signal is caused by the spin−spin correlation enhanced by reducing interparticle distance as the temperature approaches the Neel temperature. The power of the microwave (∼1 mW) used is insufficient to saturate the signal through all the temperature range scanned. Figure S8 provides information on the microwave power saturation. On the basis of the result of the angle-dependent EPR spectra, the HgSe nanocrystals are formed with a preferred direction due to the superparamanetic components that will be discussed soon. By rotating the sample holder from 0° to 360°, the EPR spectrum shifts to lower field at 90° and 270° and recovers the initial spectrum when the sample is aligned with the magnetic field at 180° and 360° (Figure 3B). The shift and reduction of the EPR peak indicates the presence of local nanocrystal ensemble structure (or assembly) with aligned magnetic dipole, offering potential to finely control the arrangement of electron spin of the SOQS. Superparamagnetism. The magnetization of nanocrystal solid as a function of the magnetic field and temperature have been examined by superconducting quantum interference device (SQUID) measurement. Figure 3C represents the SQUID curves of the as-synthesized HgSe CQD solid at 300 and 4 K. Surprisingly, the results demonstrate the superparamagnetic property of the HgSe CQDs. The SQUID measures both paired and unpaired electrons in the nanocrystal, and the overall result would produce diamagnetism of the nanocrystal solid, which can be understood when considering the ratio between unpaired electrons and the paired electrons in a nanocrystal. By subtracting a linear diamagnetic component, the sigmoidal curve across the zero point, a direct evidence of the superparamagnetism, is clearly observed in Figure 3D. The Second SOQS. Lastly, another SOQS frequently appears from the large size HgSe nanocrystal above 6.0 nm. Figure S9 seems to show the onset of filling the 1Pe state of nanocrystal, leading to the single electron peak on the EPR spectrum. The corresponding intraband absorption was not observed from the sample, implying that the onset of the electron occupation is not sufficient to have strong optical transition in the lower midIR regime just as the onset of the SOQS of the 1Se does not

Figure 4. Solution-processable nonvolatile magnetic multistep memory of nanocrystal.

nanocrystal is switchable and can be finely controlled by varying the metal-to-chalcogenide ratio via kinetic control of the nanocrystal growth time during and after chemical synthesis. Furthermore, the steady-state intraband mid-IR absorption proves that the increase of the number of electrons monitored by EPR spectroscopy directly corresponds to electron occupation of the lowest quantum state of the CB (1Se) of the nanocrystal. The hydrogen atom-like electronic structure of the colloidal quantum dot allows for unpaired electron and the paired electrons in the energetically discrete 1Se state. The EPR spectroscopy and the SQUID measurement prove the superparamagnetism of HgSe CQDs. In addition, EPR spectrum suggests the existence of unpaired electrons even in large size HgSe CQDs where the location of these unpaired electrons is tentatively assigned to 1Pe state, offering the potential for the tunable magnetic property of higher quantum states beyond the 1Se. Further study based on SOQS in the nanocrystal may open the possibility to harness the colloid nanocrystal for low-cost spintronics, nonvolatile memory, infrared optoelectronics, catalyst, imaging, and quantum computing.

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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.nanolett.6b04915. Additional STEM and TEM images of HgSe CQDs and the anion treated. The NMR spectrum of the HgSe CQDs passivated with oleylamine ligands. The SOQS EPR at higher magnetic field up to 12500 G. The EPR spectra of HgSe CQD solid before and after anion 1192

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treatment. X-ray diffraction spectra of HgSe CQD solid. EPR saturation power range (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kwang Seob Jeong: 0000-0003-3246-7599 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, and Future Planning (NRF-2016R1C1B2013416), the Ministry of Education (NRF20100020209), and a Korea University Grant (NRF2013R1A1A2062323). The TEM images and SQUID data were obtained by using the facilities in Korea Basic Science Institute (KBSI).

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ABBREVIATIONS CQD: colloidal quantum dot VB: valence band CB: conduction band EPR: electron paramagnetic resonance spectroscopy SOQS: singly occupied quantum state DOQS: doubly occupied quantum state SQUID: superconducting quantum interference device

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DOI: 10.1021/acs.nanolett.6b04915 Nano Lett. 2017, 17, 1187−1193