Heterovalent-Doping-Enabled Efficient Dopant Luminescence and Controllable Electronic Impurity Via a New Strategy of Preparing II-VI Nanocrystals.

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Heterovalent-Doping-Enabled Efficient Dopant Luminescence and Controllable Electronic Impurity Via a New Strategy of Preparing II−VI Nanocrystals Jian Liu, Qian Zhao, Jia-Long Liu, Yi-Shi Wu, Yan Cheng, Mu-Wei Ji, Hong-Mei Qian, Wei-Chang Hao, Lin-Juan Zhang, Xiang-Jun Wei, Shou-Guo Wang,* Jia-Tao Zhang,* Yi Du, Shi-Xue Dou, and He-Sun Zhu

Doped semiconductor nanocrystals (NCs) that are formed by intentional insertion of transition metal impurities into materials or by nonstoichiometry-induced self-doping have been proved to control their optical, electronic, transport, and magnetic properties efficiently.[1–13] Light-emitting doped nanocrystals exhibit advantages in comparison to undoped ones due to the elimination of self-quenching and reabsorption from the enlarged Stokes shift, and they are insensitive to thermal, chemical, and photochemical disturbances.[14,15] Heterovalent doping, in particular, can provide extra electrons (n-type doping) or extra holes (p-type doping) to enrich their electronic applications. Although much progress has been made,[16–18] an efficient method to achieve substitutional heterovalent doping into deep positions within the NCs to obtain stable dopant levels together with efficient dopant luminescence and p/n electronic-impurity control still needs to be developed to overcome intrinsic J. Liu, Q. Zhao, M.-W. Ji, H.-M. Qian, Prof. J.-T. Zhang, Prof. H. S. Zhu Research Center of Materials Science School of Materials Science and Engineering Beijing Institute of Technology Beijing 100081, P.R. China E-mail: [email protected] J.-L. Liu, Y. Cheng, Prof. W.-C. Hao Department of Physics Beihang University Beijing 100191, P.R. China Dr. Y.-S. Wu Beijing National Laboratory for Molecular Science Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P.R. China L.-J. Zhang X.-J. Wei Shanghai Synchrotron Radiation Facility Shanghai Institute of Applied Physics Shanghai 201800, P.R. China Prof. S.-G. Wang Department of Materials Physics and Chemistry University of Science and Technology Beijing Beijing 100083, P.R. China E-mail: [email protected] Y. Du, Prof. S.-X. Dou Institute for Superconducting and Electronic Materials University of Wollongong Wollongong, NSW 2500, Australia

DOI: 10.1002/adma.201500247

Adv. Mater. 2015, DOI: 10.1002/adma.201500247

self-purification, self-quenching, and self-compensation effects in II−VI NCs.[19–22] For example, it is difficult to prepare p-type CdS NCs because of the self-compensation effects of sulfur vacancies in CdS.[20] Therefore, it is greatly important to find a facile strategy to solve these problems synergistically, to speed up their optoelectronic applications. Monovalent doping, for example with Ag+ and Cu+ ions, has been the subject of research by many groups, because it provides not only color-center impurities, but also extra holes for electronic-impurity doping.[3,8,21–27] It could be concluded that different synthesis and doping methods usually enabled different kinds of doping level and dopant location in the NC matrix. This may result in different doping luminescence and electronic impurities. Norris and co-workers, for example, utilized the cation-exchange reaction between CdSe and PbSe nanocrystals and ethanolic Ag+ in solution to achieve electronic n-type and p-type Ag doping based on different doping levels.[22,23] Pradhan and co-workers used a growth-doping method to realize p-type Ag doping and blue-color dopant luminescence in CdZnS NCs.[24] Mocatta et al. realized a heavy p-type Ag-doping level of Ag into InAs NCs by cation exchange between InAs NCs and Ag+ in toluene.[25] However, strong bandgap fluorescence coexists with weak dopant fluorescence in most reported doped NCs and could not ensure the efficient elimination of the self-quenching and reabsorption.[13–17,22] Substitutional doping into deep positions of NCs and a dominant, strong, and stable dopant fluorescence are necessary for effective applications of doped NCs. Meanwhile, substitutional monovalent doping induced efficient p-type and n-type electronic impurity tailoring in II−VI NCs should be flexibly tailored to satisfy their electronic applications. In this work, we develop a new versatile strategy to prepare II− VI NCs with substitutional heterovalent doping in deep positions. Based on this prerequisite, efficient dopant luminescence and tailored p/n conduction have been realized. Even in the case of the cation-exchange method, different cation-exchange processes lead to different dopant-related behaviors.[22–24,28] Here, we use a new cation-exchange process (Figure 1) to dope M+ into CdX NCs, namely the cation exchange between M+ in an amorphous/crystalline M2X matrix (where M is a metal and X a chalcogen) and Cd2+ in solution. Instead of complete cation-exchange reactions from exterior to interior, as published before,[28,29] trace tributylphosphine (TBP) initiates Reaction (1) and generates single-crystalline CdX NCs with a trace M+ residue deep inside, as shown in Figure 1. By

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Figure 1. Schematic illustration of the preparation process to achieve deep monovalent doping in II−VI NCs.

fine-tuning the reaction temperature (25−60 °C), the reaction time, and the concentration of Cd2+ and TBP in Reaction (1), the trace M+ ions can act as substitutional dopants in CdX NCs with controllable concentrations, if their coordination locations to X are similar to those of the Cd ions. M2 X + Cd(TBP)x → CdX + 2M(TBP)x /2 + ΔG

(1)

Applying this new strategy, Reaction (1), monodisperse amorphous Ag2X nanoparticles (NPs) (see also Figure S2 in the Supporting Information and the X-ray diffraction (XRD) pattern in Figure S3) have been chosen as the starting material to obtain CdX NCs with Ag+ doping.[30] For example, to increase the Ag doping in CdS NCs from 1% to 2%, the amount of a Cd(NO3)2·4H2O methanol solution (0.1 g) was changed from 1 mL to 0.1 mL, and the reaction temperature was decreased from 60 °C to room temperature (RT). For 3% Ag doping, 0.05 mL of the Cd(NO3)2·4H2O methanol solution (0.1 g mL−1) was used, and the reaction temperature was kept at RT. The high uniformity of as-prepared CdS, CdS0.58Se0.42 (where the atomic ratio of S to Se is determined by energy dispersive X-ray (EDX) spectroscopy measurements), and CdSe NCs has been confirmed by the low-resolution transmission electron

Figure 2. LRTEM images of as-prepared Ag-doped CdS (a) and CdS0.58Se0.42 (b) NCs and Cu-doped CdS (c) NCs. d) The initiative largescale self-assembly of CdSSe:Ag NCs into a superlattice, with the insets showing the corresponding fast Fourier transform and the bulk-size film on a flexible substrate.

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microscopy (LRTEM) images shown in Figure 2a and b and in Figure S1. The size distribution of the Ag2X NPs in the starting material and the chemical thermodynamics controlling Reaction (1) affect the crystallization and monodispersity of the resulting NCs. The S to Se ratio in CdSxSe1−x NCs can be controlled easily by tuning the quantity of S and Se in the AgSxSe1−x NPs. When starting from monodisperse Cu2S NCs (Figure S2 and S3), Cu+-dopant-dominated CdS NCs can be obtained easily by Reaction (1) with high monodispersity and a large-scale self-assembly, as illustrated by the LRTEM image in Figure 2c. The high monodispersity of the NCs and the appropriate van der Waals forces of the capping ligands enable the self-assembly of NCs into a superlattice.[31,32] As a result, confirmed by Figure 2d, a large-scale multilayer superlattices of as-prepared CdX NCs with hexagonal close packing order is easily estabilished. This effective bottom-up self-assembly has laid the foundation for the use of CdX NCs in large-area film applications. The method is also suitable for preparing doped NCs with different sizes. As confirmed by an energy dispersive spectroscopy (EDS) line scan, elemental mapping, and the X-ray photoelectron spectroscopy (XPS) depth profile shown in Figure S4, micrometer-size quasi-single-crystalline CdS plates with Ag doping have been successfully prepared through this strategy. It is difficult to identify Ag and Cu in a CdS lattice matrix directly by TEM imaging because of the adjacent atomic numbers of Ag and Cu dopants to Cd. Herein, applying a correction based on a high-resolution TEM (HRTEM) image of CdS without defects (Figure S5), strain mapping of the HRTEM images by geometric phase analysis (GPA)[33] and image intensity analysis by Digital Micrograph were used to characterize the substitutional-doping-induced CdS lattice strain, as shown in Figure 3a−d, Figure S6 (Cu dopant), and Figure S7 (Ag dopant). The detailed GPA process with correction is explained in SI-1 in the Supporting Information. Doped CdS has a hexagonal structure and the binding force of S and Cd differs with that of S and the doping element, leading to lattice distortion and, thus, a contrast variation pixel by pixel. By using the GPA method, the overall distortion can be decoupled into main projected components. Based on a reference area (where Dxx =Dyy = 1.00, Rxy = 0°, Sxy = 1.00) selected arbitrarily, the percentage degrees of Dxx, Dyy, Rxy, and Sxy are calculated to construct corresponding topological maps of strain distribution. For the HRTEM image and the corresponding data of Dxx, Dyy, Rxy, and Sxy for representative Cu-doped CdS NCs in Figure 3a andb and Figure S6, the analysis of Dxx and Dyy show a contraction near the center of the NCs and an expansion around this area. This indicates that Cu is doped deeply into the CdS NCs. The HRTEM images and inserted intensity line profiles of Cuand Ag-doped CdS NCs in Figure 3c and d and Figure S7 also illustrate lattice distortions in the central part, which supports the dominant substitutional Cu and Ag doping into the lattice matrix of CdS NCs. Single-crystalline CdS NCs without dopant should have no defect but a perfectly consistent intensity profile, as shown in Figure S8. The indexed lattice spacings of 0.33 nm and 0.355 nm in Figure 3e are well matched with the interplanar distances of the wurtzite CdS (002) and (100) planes. The XRD patterns in Figure 3f confirm the wurtzite CdS and CdSSe matrix after cation exchange. As shown in Figure 3g and




COMMUNICATION Figure 3. a,b) HRTEM for Cu-doped CdS NCs along and the corresponding strain mappings for Dxx, Dyy, Rxy, and Sxy by GPA. c,d) HRTEM images of representative Cu-doped CdS NCs with line-profile analysis in the direction of [001]. e) TEM image of Ag-doped CdS NCs; inset: higher magnification with indexed lattice spacing (scale bar:6 nm). f) XRD patterns of Ag-doped CdS and CdS0.58Se0.42NCs. g) EDS elemental mapping of a Ag-doped CdS NC (2% dopant) with a size of ∼15 nm. h) Line scan elemental analysis of a Ag-doped CdS NC with a size of ∼10 nm (2% dopant).

h, EDS elemental mapping of a Ag-doped CdS NC with a size of ∼15 nm and line-scan analysis of a Ag-doped CdS NC with a size of ∼10 nm by scanning transmission electron microscopy (STEM) also confirm a deep doping rather than a surface doping. To characterize the Ag- or Cu-dopant location and concentration obtained by this strategy, XPS, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy on as-prepared CdS NCs were carried out. The sampling depth of XPS analysis (from 3 to 10 nm in semiconductor inorganics) is sufficient to characterize the composition of as-prepared CdS NCs (∼4.5 nm). XPS analysis of as-prepared CdS NCs with different Ag doping is presented in Figure 4a-c and in Table S1. The Cd3d peak is split into 3d5/2 (405.1 eV) and 3d3/2 (411.9 eV) peaks, and the peaks at 161.3 eV and 162.5 eV correspond to S2p transitions. These observed binding energies are in agreement with reported data on CdS NCs.[34,35] The features and the positions of Cd3d, S2p peaks remain almost the same regardless of changes in the Ag-dopant concentration. These XPS results confirm the intrinsic CdS lattice framework under different Ag-doping levels. The clear Ag3d peaks in Figure 4c and the corresponding atomic concentrations in Table S1 confirm the existence of Ag+ dopant with controllable concentrations (according to the fitting results of the Ag3d5/2 and Ag3d3/2 peaks). The doublet structures of Cd3d, S2p, and Ag3d are a consequence of spin−orbit splitting effects, which take place in p and higher orbitals.[34,35] XANES is element specific and highly sensitive to the local structure around the absorbing atoms due to single and multiple scattering of ejected photoelectrons. EXAFS can determine the nearest-neighbor coordination profiles of absorber atoms.[21,36] As shown in the XANES spectra (Figure 4d), the


features of the Ag K-edge in CdS NCs (∼25520 eV) are the same for different concentrations (1% and 3%). In contrast to Ag foil, the K-edge of the Ag dopant is slightly shifted to a lower energy, close to that of Ag2S. Therefore, the Ag impurity in CdS NCs is Ag+ rather than metallic Ag. Figure 4e shows the Fourier transforms of the EXAFS spectra of the absorbing Ag and Cd atoms in Ag-doped CdS NC samples. The peak positions of the Ag dopant (at ∼2Å) for different doping levels are very similar to the position of the Cd atoms in CdS, but different from those in Ag foil and Ag2S. This accurate atom-specific evidence confirms that Ag dopants, in the form of Ag+ rather than Ag0, occupy the position of Cd atoms in the wurtzite CdS lattice and, thus, constitutes substitutional doping,[37] as shown schematically in Figure 4f. The first peak next to the Ag or Cd peak should be attributed to the coordination to S atoms. The slightly different distances to the central atoms are derived from Ag−S and Cd−S bond-length-difference-induced lattice distortions. This is consistent with the HRTEM analysis in Figure 3a-d. Through this unprecedented out-to-in cation-exchange process, substitutional deep doping of Ag+ is feasible because of the same tetrahedral location of Ag−S in Ag2S and Cd−S in the CdS matrix. In the case of Cu doping, compared with CuS and Cu2S, the K-edge of the Cu dopant is slightly shifted to a lower energy, as shown in Figure 4g. In comparison to Cu+ in Cu2S and Cu2+ in CuS (EXAFS spectra in Figure 4h), the central absorbing Cudopant peak position is closer to that of Cu2S and Cd−S.[38,39] It should be noted that, in contrast to the pure tetrahedral location of Ag−S in Ag2S, the coordination varieties of Cu atoms in chalcocite Cu2S coexist in a disordered manner in four-fold, threefold, and two-fold modes.[20,40] Therefore, by means of the verification of XANES and EXAFS spectra (Figure 4g and 4h), both substitutional Cu+, interstitial Cu+, and even trace amounts of



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Figure 4. XPS spectra of Cd3d (a), S2p (b), and Ag3d (c) in as-prepared CdS NCs with different Ag-dopant concentrations: 1%, 2%, and 3%, respectively. The inset in (c) shows the fitting curve for Ag peaks with 3% concentration. All peaks were calibrated by using C1s (284.8 eV) as a reference. d) Ag K-edge XANES spectra of Ag foil, Ag2S, and CdS NCs with lower (1%) and higher (3%) Ag-ion doping; the inset shows an enlargement of the indicated region. e) Magnitude of the Fourier transforms of the k3-weighted Ag K-edge and Cd K-edge EXAFS functions in Ag-doped CdS NCs, Ag2S NCs, and Ag foil. f) Schematic illustration of the lattice of wurtzite CdS with substitutional Ag doping. g) Cu K-edge XANES spectra of CuS, Cu2S, and Cu dopant in CdS NCs; the inset shows an enlargement of the indicated region. h) Magnitude of the Fourier transforms of k3-weighted Cu K-edge and Cd K-edge EXAFS functions in Cu-doped CdS NCs, Cu2S NCs, and CuS. i) XPS spectra of Cu2p in CdS NCs with different Cu-dopant concentrations.

Cu2+ may exist in the CdS lattice after cation exchange from Cu2S to CdS.[26] From the EXAFS evidence of the Cu dopant in Figure 4h, however, the radical distributions of neighbor coordination profiles around the Cu dopant is almost the same as for Cd atoms, but different from those for Cu+ in Cu2S and Cu2+ in CuS. This evidence supports the dominance of substitutional Cu doping. Furthermore, the similar tetrahedral location of Cu in Cu2S to that of Cd in the CdS matrix, and the same hexagonal close-packed matrix of sulfur atoms in chalcocite Cu2S and CdS provide the prerequisites for substitutional Cu doping. From Figure 4i, the binding energy of Cu2p3/2 and Cu2p1/2 in CdS NCs locates at 931 and 951 eV, respectively, which is in good agreement with the 2p signals of Cu+ in Cu2S.[36] Hence, the oxidation state of Cu dopant here is mainly +1. In 4


particular, consistent with reported complete cation exchange from Ag2Se NCs to CdSe NCs,[28] 0% Cu dopant was also obtained when Cu2S NCs were used to carry out the cation exchange. It is reasonable to conclude that, by controlling the kinetics of Reaction (1) and the crystallinity of M2X, the cation exchange between M+ and Cd2+ can either be executed completely or with a certain concentrations of M+ residue. II−VI NCs with or without dopant, reported by many groups, such as Peng, Bawendi, Alivisatos, Weller, Brus and their coworkers, exhibit strong quantum confinement, i.e., a strong confinement of electrons and holes when the radius of a particle is below the exciton Bohr radius.[7,16,22,41–46] As a result, a strong exciton-absorption- (1Se/1S3/2) and exciton-recombination-induced emission is expected in their absorption and




COMMUNICATION Figure 5. Ag-doped CdS NCs: a) UV−vis−NIR absorption spectra with different Ag dopant concentrations (black: 1%, red: 2%, blue: 3%). b) RT steady-state fluorescence spectra with different Ag-dopant concentrations (λex = 420 nm). The inset shows a digital photograph of fluorescence under 365 nm UV irradiation. c) Dopant emission dynamics monitored at the peaks of 640 nm by time-resolved fluorescence lifetime (τ) measurements. d,e) Femtosecond transient absorption spectra for Ag-doped CdS NCs in toluene at the indicated time delays. Kinetic traces at representative wavelengths are also shown (pump laser wavelength: 390 nm). f) Schematic diagram of the electronic structure responsible for dominant Ag-dopant PL. Efficient energy transfer (KET) quenches excitonic emission and sensitizes dopant-related luminescence.

photoluminescence (PL) spectra. Different from the above phenomena, owing to new doping strategy towards II−VI CdX NCs used here, the morphologies remain highly uniform with different levels of Ag doping (as confirmed by the TEM images in Figure 2), but the exciton-related absorption or emission peaks are very weak or cannot be observed. Interestingly, only the defect-related red fluorescence peak at about 630 nm is strong and dominant at RT, as shown in Figure 5a and b. As a result, here, the self-quenching problem is solved, due to the large Stokes shift of 0.71 eV (Figure S9), with benefits for the potential applications of doped II−VI CdX NCs in LEDs and bioimaging. The absence of any absorption features in the nearinfrared (NIR) region (800−1400 nm), as shown in the UV− vis−NIR absorption spectra in Figure 5a, confirms that no Ag2S phase residue remains using this cation-exchange strategy.[47] Furthermore, as shown in Figure 5b, in contrast to reported dopant emissions,[7] the lower the Ag-dopant concentration, the stronger the defect-related fluorescence. When the Ag-dopant concentration is only 1%, as-prepared CdS NCs exhibit a very strong defect-related fluorescence with an absolute quantum yield (QY) of 42%. The QY is several times higher than that of CdS NCs without doping.[48] This kind of dominant red-color emission is mainly due to deep M+ doping. The very low dopant concentration in combination with dominant, strong, and stable dopant emission confirms the novelty and advantages of deep,


substitutional doping through this unprecedented strategy. This kind of dopant emission could be stable for more than one year without any fading. In order to understand these new kinds of PL features, Figure S10 and Figure 5c present the fluorescence lifetime spectra of weak, broad, bandgap-related PL (∼480 nm) and strong dopant-related PL (∼640 nm), respectively, which were collected to determine the carrier dynamics in as-prepared CdS NCs. Consistent with reported II−VI NCs without doping, the weak bandgap PL exhibits a very short lifetime (τ = 1.3 ns) with single-term exponential behavior. The dominant fluorescence peak at ∼640 nm exhibits ultralong lifetime decay behavior. The 640-nm kinetic curve can be fitted well with a two-term exponential model, resulting in two lifetimes of 0.2 µs and 1.3 µs, respectively. The long-lived red-color fluorescence further supports the dominance of Ag-dopant-related emission. Femtosecond transient absorption (TA) spectroscopy was used to probe the relaxation process. The TA spectra of Agdoped CdS NCs in toluene are shown in Figure 5d and e. Upon photoexcitation, a broad absorption band covering the 560−770 nm range is shown. This band is followed by an intense negative signal at ∼470 nm. The positive absorption is attributed to the exciton absorption, whereas the strong negative one is due to ground-state bleaching (GSB). As the time proceeds, the exciton-absorption band decays rapidly. At the same time, another absorption band appears at the blue



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Figure 6. a,b) Ag-doped CdSSe and CdSe NCs: Typical UV−vis absorption and PL spectra comparisons with different Ag-doping concentrations. The black absorption curves correspond to black PL curves. The inset in (a) shows a digital photo of the fluorescence of CdSSe NCs film on a substrate at 365 nm UV irradiation. c) Cu-doped CdS NCs: UV−vis absorption spectra with different Cu-dopant concentrations. d) Cu-doped CdS NCs: RT steadystate fluorescence spectra with different Cu-dopant concentrations (λex = 390 nm). The inset shows a digital photo of the fluorescence at 365 nm UV irradiation.

edge of the GSB band. The GSB band shows a dynamically bathochromatic shift during this decaying course, indicating an energy “cooling” process in this nanometer-size NC system. Kinetic traces at representative wavelenths of the sample are shown in Figure 5e. The 760-nm kinetic curve can be fitted well with a three-term exponential function, resulting in time constants of 2.5 ± 0.5 ps, 50 ± 20 ps, and 3200 ± 900 ns, respectively. The 412-nm kinetic curve is nicely described by a fast-rise component of 2.4 ± 0.2 ps and a slow-decay component of 4000 ± 1000 ns. The analysis on the 470-nm experimental points results in three decay components of 20 ± 3 ps, 190 ± 30 ps, and 5500 ± 700 ns, respectively. The similar 2.4 ps decay and rise time constants reveal a close correlation between the exciton absorption decay and a new band formation. The absence of the 2.4 ps component in the decay of GSB also proves the highly efficient conversion between the two transient species involved. Therefore, we assign this new absorption band at 412 nm to the dopant state. The energy transfer occurs very fast with arate constant of 4.2 × 1011 s−1. Based on the above characterization of the dynamics, the schematic representation of the energy transfer is summarized in Figure 5f. When the carriers are excited to the conduction band (CB), the efficient energy transfer (KET) from the excited state to the Ag-defect states quenches excitonic emission and inspires the dopant-related emission from the Ag-dopant level to the valence band (VB).[49] This efficient energy transfer from the CB to the lower dopant energy level will be beneficial for obtaining efficient photoexcited electron/hole separation and collection and light harvesting.[50,51] Besides the CdS example, similar optical properties were found for CdSe and CdSSe NCs prepared by this strategy.

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The efficient energy transfer (KET) from the excited state to the Ag-defect states leads to dominant dopant fluorescence at RT, as shown in Figure 6a and b. Similarly, almost no excitonabsorption (1Se/1S3/2) peaks are observed in UV−vis absorption spectra and the exciton-recombination-induced emission is very weak, but the Ag-dopant fluorescence is very strong and dominant. Furthermore, the lower Ag-dopant concentration leads to a stronger dopant emission. The Gauss fitting results for a typical PL spectrum of CdSe NCs in Figure S11 confirms the dominant Ag-dopant PL (632 nm) with weak bandgap PL (565 nm) and weak surface-state PL (707 nm).[22,52] As shown in Figure 2e, a Ag-doped CdSSe NC film on a flexible substrate can be prepared by large-scale self-assembly of the NCs. As shown in Figure 6a, the solid NCs film demonstrates strong fluorescence. Figure 6c shows the UV−vis absorption spetra of Cudoped CdS NCs with different doping levels. In contrast to the Ag-doped CdS NCs, the exciton-absorption peaks assigned to CdS still exist and change with different doping concentrations. Figure 6d shows CdS NCs with ∼0.5% Cu doping that exhibit dominant dopant-related luminescence. The absolute QY can reach up to 29%. However, the luminescence mechanism here should be different from the one in Figure 5f, because of the complex coordination varieties of Cu atoms, coexistence of Cu+ and Cu2+, and their different doping levels.[26,40] Further research is required to fully describe the relaxation process in Cu-doped CdX NCs. The CdS NCs with 0% Cu dopant received after cation exchange using this strategy also show both exciton emission and emissions from deep defects (such as dislocations).[28] The various types of M+ doping here provide an alternative way to control the electronic impurities in II−VI NCs.[22] It is




COMMUNICATION Figure 7. a,b) Typical Mott–Schottky plots of Ag-doped CdS (a) and Cu-doped CdS (b) NC films in 0.1 M Na2S and 0.1 M NaOH at 1 kHz. The potentials are measured with respect to a standard calomel electrode (SCE). c) UPS spectrum of a Cu-doped CdS NC film, demonstrating the calculation of the work function. d) Schematic representation of the calculated band-level positions.

well known that, due to the high self-compensating process derived from the intrinsic donor defects, such as sulfur vacancies (Vs), n-type CdS or CdSe are formed naturally, but p-type CdS is difficult to be formed.[20] Here, by Cu+ doping using this new strategy, p-type CdS NCs were prepared successfully. Typical Mott–Schottky plots of doped CdS NC films (1/Csc2 versus applied potential, where Csc is the space charge region capacitance) are shown in Figure 6, from which the n- or p-type conductivity can be identified (see also the Mott−Schottky impedance spectroscopy analysis in SI-2).[53] As shown in Figure 7a, the positive slope of all linear parts in the Mott–Schottky impedance curve of the as-prepared Ag-doped CdS NC film confirmes the n-type conductivity. The substitutional Ag doping increases S vacancies (Vs) when charge compensations take places during the process of Ag+ substitution for Cd2+, and the increased number of Vs leads to n-type electron impurity. The negative slope of all the linear parts in Figure 7b, however, confirmes the p-type conductivity of as-prepared Cu-doped CdS NCs. Consistent with reported p-type CdS films created by ion implantation of Cu acceptors,[20] a large proportion of the Cu dopants substituted into Cd sites and act as acceptors. They are compensated with electrons from Vs centers and accommodated on Cd sites as Cu+ ions. This is the origin of the p-type conduction in CdS NCs. UV photoelectron spectroscopy (UPS) was used to further confirm the bandgap positions and the Fermi level of as-prepared Cu-doped CdS NCs film (see also the detailed analysis in SI-3).[54] As shown in Figure 7c, Figure S12, and Figure S13, the work function (ϕ), and the band positions of VB and CB could be defined by UPS. Subsequently, the schematic bandgap positions of as-prepared Cu-doped CdS NCs film were calculated, as Figure 7d, from which the p-type acvitity could be confirmed.


In summary, this study provides an innovative strategy for the production of high-quality II−VI NCs with deep substitutional M+ doping. Enabled by this method, the efficient energy transfer from the intrinsic CB to the deep dopant energy level quenches the exciton emission and inspires stable and strong dopant emission. The Ag+ or Cu+ doping in CdS NCs provides an alternative way to control electronic impurity in II−VI NCs. The high uniformity and the appropriate capping ligands produced by this strategy afford a simple bottom-up approach for self-assembly into multilayer superlattices. This method is useful for solving the self-purification, self-quenching, and p/nelectronic-impurity-control issues in NCs. Besides monovalent doping, this versatile strategy will also find potential applications in other types of heterovalent doping, with subsequent dopant luminescence and control of electronic impurities.

Experimental Section All chemicals were used as received without further processing. Monodisperse Amorphous Ag2X (Ag2S, Ag2Se, and Ag2SxSe1-x) NPs: Monodisperse Ag NPs with a size of about 4 nm were prepared according to Wang and co-workers.[30] Amorphous Ag2S and Ag2Se NPs were prepared by the reaction of colloidal Ag NPs with twice the quantity of a S or Se organic precursor in the molar ratio of 1: 5 at 50 °C. Here, the S precursor was prepared by the reaction of 2 mmol S powder with 5 mL oleylamine and 10 mL oleic acid at 100 °C. The Se precursor was prepared from 1 mmol Se powder with 7 mL octadecylene at 270 °C. CdX NCs with Ag Doping: Typically, 0.035 mmol Ag2X NPs were dispersed in 6 mL toluene with 0.2 mL oleic acid and 0.1 mL oleylamine. Then, 1 mL Cd(NO3)2·4H2O methanol solution (0.1 g mL−1) was added. After stirring for 1 min, 0.1 mL TBP was added, and the mixture was stirred at 60 °C for 2 h. The resulting CdX NCs contained



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www.MaterialsViews.com 1% Ag dopant. To increase the Ag doping in CdS NCs from 1% to 2%, the amount of a Cd(NO3)2·4H2O methanol solution (0.1 g ) was changed from 1 mL to 0.1 mL, and the reaction temperature was decreased from 60 °C to room temperature (RT). For 3% Ag doping, 0.05 mL of the Cd(NO3)2·4H2O methanol solution (0.1 g mL−1) was used, and the reaction temperature was kept at RT. CdS NCs with Cu Doping: First, monodisperse Cu2S NCs were prepared according to Li and co-workers.[31] Typically, 0.125 mmol Cu2S NCs were dispersed in 5 mL toluene, and 0.5 mL Cd(NO3)2·4H2O methanol solution (0.2 g mL−1) was added. After stirring for 1 min, 0.1 mL TBP was added, and the mixture was stirred at 60 °C for 2 h. The resulting CdS NCs contained 0.5% Cu dopant. Then, similar to the Ag doping, different levels of Cu doping were realized. Comparably, when Cd(NO3)2·4H2O methanol solution (0.2 g mL−1) was changed to 1mL and the mixture was stirred at 60 °C for 4 h, CdS NCs with 0% Cu doping was achieved. When Cd(NO3)2·4H2O methanol solution (0.2 g mL−1) was still 0.5 mL and the reaction was carried out at room temparature for 2h, CdS NCs with 4% Cu doing was achieved. LRTEM and HRTEM: Samples for TEM characterization were prepared by placing one drop of toluene solution, together with the product, onto a 300 mesh copper grid with a carbon support film. A JEOL JEM 1200EX working at 100 kV and a HRTEM (FEI Tecnai G2 F20 S-Twin working at 200 kV) were utilized to characterize the morphology, crystal lattice,EDS elemental mapping, and line scane elemental analysis. The HRTEM image intensity was analyzed by DigitalMicrograph (Gatan Co. LTD.). Strain Mapping: To investigate the location and extent of lattice distortion of CdS nanodots induced by Cu doping, the in-plane strain was calculated by STEM_CELL software based on GPA, which enables us to construct false-colour strain maps of fringe deformation in two reference directions (Dxx and Dyy), fringe rotation (Rxy), and shear strain (Sxy). UV−vis−NIR Absorption and Luminescence Spectroscopy: The UV−vis− NIR) absorption spectra of as-prepared CdS, CdSSe, and CdSe NCs toluene colloids were recorded on a Shimadzu UV3600 UV−vis−NIR spectrophotometer at RT. The steady-state luminescence spectra of the colloidal samples were collected on a Hitachi F-4500 fluorescence spectrophotometer at RT. The absolute fluorescence quantum yield was measured on a Hamamatsu Absolute Quantum Yield Spectrometer C11347 with λex = 360 nm. XPS and UPS Analysis: The XPS spectra were obtained with a PHI Quantera II X-ray photoelectron spectrometer using Al Kα nonmonochromatic radiation. The colloidal NCs were dip-coated onto glass substrate for XPS characterization. The measurement parameters were: light spot size: 100 µm; power: 100W; voltage: 20 kV. An energy correction was applied to account for sample charging based on the carbon (1s) peak at 284.8 eV. The elemental concentrations are reported relative to carbon, calculated from the XPS spectra based on the area of the characteristic photoelectron peaks after correcting for atomic sensitivity. The UPS spectra were obtained with an ESCALAB250XI photoelectron spectrometer, Thermo Scientific. The colloidal NCs were dip-coated onto ITO substrates to form a 1 cm × 1 cm film for UPS characterization. XANES and EXAFS: The Cd K-edge, Ag K-edge, and Cu K-edge XANES and EXAFS spectra of CdS NCs doped with Ag and Cu were collected in fluorescence mode, due to the lower concentration, while the Ag K-edge spectra of Ag foil and Ag2S NPs, and the Cu K-edge of CuS and Cu2S were collected in transmission mode on a beamline BL14W1 at the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics (SINAP), China. The electron-beam energy was 3.5 GeV, and the maximum stored current was 300 mA. The raw data analysis was performed using the FEFF6 software package according to the standard data-analysis procedures. Fluorescence Lifetime Measurements: A ps time-resolved fluorescence apparatus was used. Excitation laser pulses (450 nm) were supplied by an optical parametric amplifier (OPA-800CF, Spectra Physics), which was pumped by a regenerative amplifier (Spitfire, Spectra Physics). The excitation energy at the sample was 50 nJ pulse−1. Fluorescence collected at 90°-geometry was dispersed by a polychromator (250is, Chromex) and detected with a streak camera (C5680, Hamamatsu Photonics). The spectral resolution was 0.2 nm, and the temporal resolution was

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about 30 ns on the measured delay-time-range setting of 5 µs. Analysis of the kinetic traces derived from time-resolved spectra was performed individually using nonlinear least-squares fitting to a general sum-ofexponentials function after deconvolution of the instrument response function (IRF). All spectroscopic measurements were carried out at RT. Femtosecond Transient Absorption Spectroscopy: A Ti:sapphire femtosecond laser system provided laser pulses for the femtosecond transient absorption measurements. A regenerative amplifier (Spitfire, Spectra Physics) seeded with a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics) delivered laser pulses at 800 nm (120 fs, 1 kHz), which were then divided into two components by using a 9:1 beam splitter. The major component was sent to an optical parametric amplifier (OPA800CF, Spectra Physics) to generate the pump pulses (615 nm, 130 fs, 1 kHz). The minor component was further attenuated and focused into a 3-mm sapphire plate to generate the probe pulses. A band-pass filter (SPF-750, CVI) was inserted into the probe beam to select the visible probe (630−790 nm). The time delay between pump and probe beams were regulated with a computer-controlled motorized translation stage in the probe beam. A magic-angle scheme was adopted in the pump− probe measurement. The temporal resolution between the pump and the probe pulses was determined to be ∼150 fs full width at half maximum (FWHM). The transmitted light was detected by either a CMOS linear image sensor (S8377–512Q, Hamamatsu) or an InGaAs linear image sensor (G9203–256D, Hamamatsu) when necessary. The excitation-pulse energy was 0.2 µJ pulse−1, as measured at the rotating sample cell (optical path length: 1 mm). The stability of the solutions was spectrophotometrically checked before and after each experiment. Mott−Schottky Impedance−Potential Curve Measurements: Mott− Schottky impedance−potential curves were measured on an IM6e electrochemical workstation (Zahner, Germany) with a standard three-electrode system, namely, the working electrode, the Pt counter electrode, and the Ag/AgCl reference electrode. An as-prepared CdS NC film (dried under vacuum at 100 °C for 3 h) on indium tin oxide (ITO) glass was used as the working electrode. The electrolyte was a 200 mL solution of 0.1 M NaOH and 0.1 M Na2S.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements J.L. and Q.Z. contributed equally to this work. This work was supported by the Natural Science Foundation of China (51372025, 21322105, 21373239, 51272015, and 11274371), the Research Fund for the Doctoral Program of Higher Education of China (2011101120016), and the Program for New Century Excellent Talents in University (NCET11–0793). The authors would like to thank Prof. Y. S. Zhao and Dr. W. Zhang from the key laboratory of photochemistry of CAS for absolute QY measurements and helpful discussion and Dr. W. X. Li from the University of Western of Sydney in Australia for finalizing the manuscript. Received: January 16, 2015 Revised: February 12, 2015 Published online:

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