Heterovalent Doping in Colloidal Semiconductor Nanocrystals: Cation

Perspective pubs.acs.org/JPCL

Heterovalent Doping in Colloidal Semiconductor Nanocrystals: Cation-Exchange-Enabled New Accesses to Tuning Dopant Luminescence and Electronic Impurities Jiatao Zhang,* Qiumei Di, Jia Liu, Bing Bai, Jian Liu, Meng Xu, and Jiajia Liu Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *

ABSTRACT: Heterovalent doping in colloidal semiconductor nanocrystals (CSNCs), with provisions of extra electrons (n-type doping) or extra holes (p-type doping), could enhance their performance of optical and electronical properties. In view of the challenges imposed by the intrinsic self-purification, self-quenching, and self-compensation effects of CSNCs, we outline the progress on heterovalent doping in CSNCs, with particular focus on the cation-exchange-enabled tuning of dopant luminescence and electronic impurities. Thus, the well-defined substitutional or interstitial heterovalent doping in a deep position of an isolated nanocrystal has been fulfilled. We also envision that new coordination ligand-initiated cation exchange would bring about more choices of heterovalent dopants. With the aid of high-resolution characterization methods, the accurate atom-specific dopant location and distribution could be confirmed clearly. Finally, new applications, some of the remaining unanswered questions, and future directions of this field are presented.

T

Self-Purif ication Effect Due to the Size Ef fect. Due to the size effect, the introduction of a few impurity atoms into CSNCs that contain only hundreds or thousands of atoms may lead to their expulsion to the surface, namely, the well-known self-purification effect. This obstinate effect has been well-studied by several groups, such as the Chelikowsky group and the Norris group.20,21 They proposed that self-purification was an intrinsic feature of CSNCs arising from the increase in the formation energy of defects and impurities. The increase in the formation energies of doped impurities is a predominant reason that doping of CSNCs was more difficult than doping bulk. Furthermore, the formation energy of defects in CSNCs increases with decreasing NC size. The Peng group reported that, considering the self-purification effect, the key for successful deep-position doping is to identify the critical temperature of the elementary processes, which involve surface adsorption, lattice incorporation, lattice diffusion, and lattice ejection.22 In order to get efficient and stable doping to speed up device applications, deep-position doping, other than the surficial doping in CSNCs to decrease side effects and achieve stable dopant level, is obviously important. However, after decades of efforts, this is still a challenge for researchers in this field. Heterovalent Doping in CSNCs. While progress has been made with isovalent substitutional impurities in CSNCs, for example, Mn impurities in II−VI NCs, no additional carriers have been introduced.1,2,21,23 Heterovalent doping in CSNCs, however, can

he semiconductor industry annually spends billions of dollars to deliberately add atomic impurities, called dopants, into a pure semiconductor matrix for device applications.1−5 Doping enables manipulation of both the optical and electrical properties of semiconductors, and this is considered the key to device functionality.6,7 Colloidal semiconductor nanocrystals (CSNCs) have emerged as a family of materials with sizedependent optical and electronic properties.8−12 Such NCs are a form of “artificial atoms” that have presented potential applications in a wide range of optoelectronic devices, such as light-emitting diodes (LEDs),13 photovoltaic cells,14−16 flexible electronic circuitry, and other future new energy devices.17,18 In CSNC systems, controllable heterovalent doping could increase the number of negatively (n) or positively (p) charged mobile carriers and enhance their electronic conductivity.1−7 In particular, it has been reported that dopants, including nonstoichiometry-induced self-dopants, could influence the charge carrier density of the host CSNCs and thus generate metallic-type localized surface plasmon resonance in these NCs.10,19

Heterovalent doping in colloidal semiconductor nanocrystals could enhance their performance of optical and electronical properties.

Received: February 14, 2017 Accepted: September 19, 2017 Published: September 19, 2017 © 2017 American Chemical Society

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DOI: 10.1021/acs.jpclett.7b00351 J. Phys. Chem. Lett. 2017, 8, 4943−4953

The Journal of Physical Chemistry Letters

Perspective

Table 1. Summary of the Heterovalent Doped CSNCs by Different Synthesis Methods dopant Cu+

host CSNCs

synthesis methods

dopant position

photostability

Ag+ Ag+, Cu+

ZnS/Zn1−xCdxS, ZnInSe alloy CdS CdS, CdSe, CdSSe alloy

Tb3+, Eu3+

ZnS

growth doping cation exchange between Ag2X (X = S, Se) and Cd2+ cation exchange between dopant and host

Er3+, Eu3+

TiO2, ZnO, SnO2

growth doping

not reported

good

Ag+

CdSe, PbTe

not reported

not so good

Ag+, Cu+

InAs InAs, Bi2S3

substitutional and interstitial doping not well studied

not reported

Mn2+, Cu2+ Li+, Na+

cation exchange between Ag ions and CdSe host cation exchange between dopant and InAs host nucleation doping

not reported

ZnO

nucleation doping

interstitial and shallow doping

not reported

Cu+

In2S3

cation exchange between dopant and host

not reported

not reported

Bi3+

PbS

nucleation doping

not reported

not reported

Mn2+

NaGdF4

cation exchange between dopant and host

surficial doping

good

Cd2+

InAs

growth doping

not reported

not reported

Cu2+ Cu+, Ag+

InP ZnSe

growth doping growth doping

not reported not reported

good good

quantum yield

ref

growth doping nucleation doping

not reported

good

20−30%

30, 31

center doping deep position

not reported excellent

58% 40−50%

32 33

not well studied

good

not reported not reported not reported not reported not reported not reported not reported not reported not reported not reported 40% 3−8%

34 35−37 25, 38 5 39, 40 41 42 43 44 45 46 29, 47, 48

occurrence of deep-position doping instead of surficial doping, as illustrated in Figure 1A. Cation-Exchange-Enabled Heterovalent Doping. The in situ conversion of preformed NCs has become a new promising way to achieve heterovalent doping, including both cation and anion doping.5,25,33,52 Through the use of binding ligands on the nanoparticle surface (which can donate carriers) or electrochemical carrier injection, n-type doping in CSNCs superlattices or films has been realized.53,54 Cation exchange, which involves the reaction of dopant cations with preformed host NCs, has become increasing appealing in recent years.5,25,33,34,38,42,44 As for the synthesis of pure phased NCs, metastable NCs, single-crystalline porous NCs, and hybrid composite NCs, cation-exchange reactions, in which the cations ligated within a NC host lattice are substituted with those in solution, have emerged as particularly powerful tools for fine control over NC composition and phases.55−63 From traditional inorganic chemistry of cation elements in the periodic table, the formation energies for a number of binary chalcogenides, oxides, and pnictides can be obtained and the aqueous redox potentials are available for virtually every metal ion. The reaction thermodynamics and kinetics of any given cation-exchange reaction can be readily calculated.64,65 On the basis of these advantages, a cationexchange strategy could be even more powerful in ongoing development, in particular, toward heterovalent doping. Cation exchange could make full use of the fundamental thermodynamic and kinetic basis of inserted cations and host cations in NC matrixes. Even in the case of the cation-exchange method, different cation-exchange processes lead to different dopant-related behaviors.5,25,33 As shown in Table 1, there are many emerging synthetic protocols to get different kinds of heterovalent doping by cation-exchange reactions either in the organic phase or aqueous phase.5,25,33,44 Transition metal ions and lanthanide have been doped into II−VI and III−V CSNCs successfully.

provide extra electrons (n-type doping) or extra holes (p-type doping) to enrich their electronic applications.24−26 The properties of the doped semiconductors strongly depend on the electronic levels introduced by the heterovalent dopants.27 Even after decades of research, such progress in CSNCs could hardly satisfy the device applications. Heterovalent doping can cause not only dramatic changes but also increased complexity in host lattice distortion. What’s more, there are intrinsic selfcompensation effects in II−VI NCs.28,29 For example, it is difficult to prepare p-type CdS NCs because of the self-compensation effects of sulfur vacancies in CdS.28 Therefore, heterovalent deep-position doping based on atomic-level confirmation of substitutional or interstitial location and distribution in small-size CSNCs is highly desirable to get flexible n-type- and p-typedoped levels for the electronic device applications discussed in the following section. Many efforts have been devoted to incorporate heterovalent impurities that can provide these carriers. Various strategies, such as nucleation doping, growth doping (including the layer-by-layer method), and recently developed cation-exchange methods, have been used to get versatile heterovalent doping.29−48 The recently developed heterovalent doping synthesis methods are summarized in Table 1. In general, these methods could be classified into two kinds of protocols; one is introducing heterovalent impurities during the growth of a host NC matrix,31,32,35−37,39−41,43,45−48 and the other is the in situ conversion of preformed NCs.5,25,33,34,38,42,44 The former could realize deep-position doping when performing nucleation doping or layer-by-layer overcoating during the growth of NCs. In particular, the Cao group, Peng group, Narayan group, Klimov group, and so forth have carried out such a growth doping process.29,49−51 Specifically, the dopants were initially bounded on the surface of host NCs, and then temperature-dependent lattice incorporation was allowed to take place, or host shells were overcoated again to ensure the 4944



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Figure 1. (A) Illustration of surface absorption and lattice incorporation of dopant ions to host NCs by temperature-dependent dopant lattice diffusion. Adapted with permission from ref 29. Copyright 2009, American Chemical Society. (B) Schematic illustration of the cation-exchange reaction between M+ in an amorphous/crystalline M2X matrix (where M is a metal and X a chalcogen) and Cd2+ in solution to achieve deep monovalent doping in II−VI NCs. Adapted with permission from ref 33. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. (C) Extended prospect of heterovalent deep-position doping based on versatile well-defined metal chalcogenides, oxides, and pnictides reaction thermodynamics and various ligands’ coordination-initialized cation-exchange thermodynamics and kinetics.

Norris and co-workers, for example, utilized the cation-exchange reaction between CdSe and PbSe NCs with ethanolic Ag+ in solution to achieve electronic n-type and p-type Ag doping based on different doping levels.25,26 Banin and co-workers reported room-temperature cation-exchange synthesis of metallically doped InAs CSNCs by Cu+ and Ag+ dopants reacted with preformed InAs NCs. They presented strong evidence that both n- and p-type NCs can be formed and also gave insights into the electronic and optical effects of doping small NCs.5 They doped colloidal InAs NCs with metallic impurities, copper (Cu) or silver (Ag), by cation-exchange-enabled ion diffusion (see Figure 2). As shown in Figure 2, although Cu and Ag impurities showed similar diffusion properties inside of the InAs crystal lattice, opposite electronic doping effects were observed. That is, copper is an n-type interstitial impurity and silver is a p-type substitutional impurity. It is worth mentioning that they used scanning tunneling spectroscopy (STS) to characterize the doped InAs NCs and directly measure the electronic energy levels with respect to the Fermi level. The exact dopant location in NC matrixes for these listed examples so far has not been convincingly characterized. Considering the normal cation-exchange process, these kinds of “outside-to-inside” processes usually lead to surface doping because the original cation dopant would interact with the surface of the host preferably.

The doping effects on the electronic properties of CSNCs have been incompletely investigated because of the lack of robust synthetic methods for doping. Taking into account the thermodynamic considerations for guiding the development of new cation-exchange syntheses,64 we put forward a new cationexchange process (Figure 1B) to dope M+ into CdX NCs. It involves cation exchange between M+ in an amorphous/ crystalline M2X matrix (where M is a metal and X a chalcogen) and Cd2+ in solution.33 In contrast to complete cation-exchange reactions from exterior to interior as published before,55,66 trace phosphine could initiate different cation-exchange reactions and generate single-crystalline CdX NCs with a trace amount of M+ residue deep inside, as shown in Figure 1B,C. In such a way, the location for the deep-position dopant could be better controlled. On account of various kinds of phosphine-initiated cation exchange reactions,65,66 the dopant concentration could be tailored more flexibly.67 What’s more, seeing that there are welldefined reaction thermodynamics for abundant metal chalcogenides, oxides, and pnictides64,65 and well-established thermodynamics and kinetics for various ligand coordination-initiated cation-exchange processes,67 additional kinds of heterovalentdoped CSNCs, such as III−V group NCs and some metal oxides, could be expected, as scheme illustrated in Figure 1C. Materials properties, encompassing optical and electronic responses, can be greatly enhanced by isolated single dopants. 4945



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Figure 2. Heterovalent doping in InAs NCs through metal ion diffusion: (a) 2D simplified representation of the InAs lattice with tetrahedral bonding; (b) InAs lattice with a Cu atom at an interstitial site donating valence electrons to the crystal leading to n-type doping; (c) substitutional Au impurity on an In site in the InAs lattice; and (d) substitutional Ag impurity on an In site in the InAs lattice. (e) Scanning tunneling microscopy (STM) tunneling spectra of undoped (black trace), Au-doped (green trace), Cu-doped (blue trace), and Ag-doped (red trace) InAs NCs, highlighting the relative shifts of the band edges in the doped samples. The inset shows the STM image of a single NC. Adapted with permission from ref 5. Reproduced with permission from The American Association for the Advancement of Science, Copyright 2011.

“invisible” Mn impurities inside of ZnSe NCs (Figure 3B).69 The Demie Kepaptsoglou and Quentin M. Ramasse group used a combination of the HAADF-STEM image, EELS, and ab initio calculations to describe the electronic structure modifications incurred by free-standing graphene through B and N single-atom p- and n-type doping (Figure 3C).70 Rossell et al. used STEMEELS as well, combined with multivariate statistical analysis and image simulations, to detect single Ba atoms in strontium titanate NCs (Figure 3D).71 T. Mizoguchi’s group achieved direct observation of individual Er atoms in an optical glass fiber using aberration-corrected HAADF-STEM.72 These reports demonstrated that high-resolution TEM and related spectroscopies together with image simulations have become powerful for reconstructing three-dimensional information from a single, two-dimensional image. XAFS Spectroscopic Techniques for Dopant Local Structural Environment Characterization. XAFS is a spectroscopic technique that uses X-rays to probe the physical and chemical structure of matter at an atomic scale. XAFS is element-specific, in which X-rays are chosen to be at and above the binding energy of a particular core electronic level of a particular atomic species. Because all but the lightest elements have core-level binding energies in the X-ray regime, nearly all elements can be studied with XAFS. Using EXAFS, alteration of the local structural environment of the impurity with the size of elemental ions could be distinguished by exploiting the computer lattice simulation studies.29,33,73,74 The XANES spectra are highly sensitive to the local structure around the absorbing atoms due to single and multiple scattering of ejected photoelectrons, as shown in Figure 4A. With regard to the unique cation-exchange process in Figure 1B, the Ag K-edge XANES spectra of Ag-doped CdS relative to Ag foil, Ag2S, and CdS have been studied (Figure 4B). Moreover, the Fourier transforms of the EXAFS spectra of the

The doping effects on the electronic properties of CSNCs have been incompletely investigated because of the lack of robust synthetic methods for doping, but we put forward a new cationexchange process to dope M+ into CdX NCs. The three-dimensional single-dopant defect structure and spatial distribution are therefore critical to understanding and adequately tuning functional properties. Accurate atom-specific evidence of dopant location and distribution is still very necessary to understand the doping quality and confirm the induced band gap, Fermi energy, and p-type- and n-type-doped level. With the tens of years of efforts conducted on doped CSNC synthesis, the development of atomic-resolution characterization, in particular, the spherical aberration correction electron microscopy and extended X-ray absorption fine structure (EXAFS) measurements, has provided opportunities to understand the dopant location and distribution more precisely. The first outstanding progress was made by directly imaging dopant atoms in specific systems with high-resolution TEM.68−72 As shown in Figure 3A, Ryo Ishikawa et al. determined the three-dimensional location of a single-atom Ce dopant embedded within wurtzite-type AlN with atomic precision by aberration-corrected STEM combining quantitative Z-contrast STEM and associated image simulations.68 The Mkhoyan group and the Norris group combined electron energy loss spectroscopy (EELS) with annular dark-field scanning transmission electron microscopy (ADF-STEM) to image individual 4946



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Figure 3. (A) Three-dimensional location of a Ce dopant embedded within wurtzite-type AlN with atomic precision by aberration-corrected STEM: Atomic-resolution ADF STEM images averaged over the frames of (a) and (b). The Ce-containing atomic columns are marked as i and ii, and the corresponding structure model is shown in (c). (d) Atomic-resolution thickness map. (B) ADF-STEM image and EELS to image “invisible” individual Mn impurities inside of ZnSe NCs: (e) Atomic-resolution ADF-STEM image of Mn-doped ZnSe NCs suspended on an ultrathin amorphous carbon film. (f) Extracted core-level EELS map (shown as pixels on a grid) for the Mn L2,3-edge along with the corresponding ADF-STEM image of a Mn-doped ZnSe NC. (g) Overlap of the Mn L2,3-edge intensity map and the ADF-STEM image. (C) A combination of STEM, EELS, and ab initio calculations is used to describe the electronic structure modifications incurred by free-standing graphene through n- and p-type single-atom doping: (h) HAADFSTEM image of a B-implanted graphene sample; (i) HAADF signal acquired simultaneously with the EELS spectrum image (B1 and C1−4 mean boron and carbon elements); (j) map of the B K EELS signal shown in (k). (D) Spatial distribution of chemically identified Ba atoms in Ba-doped STO nanoparticles: (l) HAADF-STEM image of Ba-doped SrTiO3 NCs along the [011] direction; (m) simultaneous HAADF images and elemental atomicresolution maps calculated from the EELS images obtained from areas outlined with white squares in (a). The RGB maps were generated using red for Ba, green for Ti, and blue for Sr. Adapted from refs68−71. Reproduced with permission from The American Chemical Society, Copyright 2011, 2012, 2014, and 2015.

absorbing Ag and Cd atoms in Ag-doped CdS NC samples have been demonstrated (Figure 4C). Compared to the Ag species in Ag2S and Ag foil and Cd ions in a CdS matrix, the almost same local coordination environment of Ag dopant to that of Cd atoms confirmed that Ag dopants, in the form of Ag+ rather than Ag0, occupied the position of Cd atoms in the wurtzite CdS lattice and thus constituted substitutional doping.33 Furthermore, the oxidation state of the dopant could be characterized by XANES and Fourier filtered EXAFS spectra. Pavle V. Radovanovic’s group performed Fe K-edge XANES and Fourier filtered EXAFS spectroscopy measurements to elucidate the oxidation state of iron dopant ions in In2O3 and SnO2 NCs.75 Besides accurate atom-specific characterization of dopant location (substitutional or interstitial) by XAFS spectroscopic techniques, the sampling depth X-ray photoelectron spectroscopy (XPS) analysis and inductively coupled plasma−optical emission spectroscopy (ICP-OES) analysis could also be utilized to probe the dopant concentration precisely.33,76

In summary, based on well-defined cation-exchange reaction thermodynamics and kinetics for various metal chalcogenides, oxides, and pnictides and the novel cation-exchange protocols illustrated in Figure 1B,C, the deep-position heterovalent-doped CSNCs could be synthesized in a robust way. High-resolution TEM spectroscopy, such as HAADF-STEM and EELS, could be utilized to determine the three-dimensional location of isolated dopant’s location. Element-specific XANES, EXAFS, XPS, and ICP-OES analysis could provide information on the dopant substitutional or interstitial location and concentrations. Femtosecond Transient Absorption (TA) Spectroscopy for Probing the Relaxation Process. Heterovalent doping in II−VI CSNCs has been a hot subject of research in recent years because it provides not only color-centered impurities but also extra holes for electronic-impurity doping.5,25,30−33 It could be concluded that different synthesis and doping methods usually enabled different kinds of doping levels and dopant locations in the NC matrix. This may result in different doping luminescence and electronic impurities. The doping luminescence has a larger Stokes shift 4947



Perspective

Figure 4. (A) XANES and EXAFS spectra characterizations for the doped element coordination environment and near-neighbor distances. (B) Ag K-edge XANES spectra of Ag foil, Ag2S, and CdS NCs with lower (1%) and higher (3%) doping of Ag ions; the inset shows an enlargement of the indicated region. (C) 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. Adapted from ref 33 with permission. Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA.

than intrinsic band gap luminescence, which has been verified by many previous reports.23,29−33 In order to understand the electrons relaxation process after excitation, different characterization measurements have been carried out. The Klimov group conducted spectroelectrochemical measurements on Cu-doped core/shell ZnSe/CdSe NCs to determine the distribution of PL intensity between the intrinsic and the impurity bands (Figure 5A).77 The Gamelin group carried out variable-temperature photoluminescence measurements over doped NCs under different temperatures and showed efficient energy transfer (KET) between excitonic bands and the doped level (Figure 5B).78 Femtosecond TA spectral characterizations could effectively investigate the different kinds of interface induced charge transfer or energytransfer processes. In our group, we exploited femtosecond TA spectroscopy to probe the relaxation process of synthesized doped CSNCs.33 Figure 5C,D shows the TA spectra of Ag-doped CdS NCs in toluene. On the basis of such dynamics characterizations together with ultraviolet photoelectron spectroscopy (UPS),76 the schematic representation of the energy transfer between the excited state and doped level could be deduced clearly, as shown in Figure 5E. Regarding the deepposition substitutional Ag doping in CdS NCs, after the carriers are excited to the conduction band (CB), efficient energy transfer (KET) from the excited state to the Ag defect states quenches excitonic emission and induces dopant-related emission from the Ag-dopant level to the valence band (VB). Furthermore, femtosecond TA spectroscopy could be used to study the relaxation process at the atomic or nanoscaled interface in both doped NCs and hetero-CSNCs, facilitating the study of energy transfer and photoexcited electron/hole separation in such systems.

Different synthesis and doping methods usually enabled different kinds of doping levels and dopant locations in the NC matrix, which may result in different doping luminescence and electronic impurities. Larger Stokes Shift-Enabled New Potential Applications. Lightemitting doped NCs exhibit advantages in comparison to undoped ones due to the elimination of self-quenching and reabsorption because of the enlarged Stokes shift, and they are insensitive to thermal, chemical, and photochemical disturbances.23,79,80 Two additional advantages of doped CSNCs, namely, longer dopant emission lifetime and potentially lower cytotoxicity (such as doped ZnS CSNCs and lanthanide ion-doped CSNCs) also make contributions to their attractive chemo/ biosensing and bioimaging applications perspective. Yan’s group has conducted a series of attempts for biological applications of doped ZnS CSNCs.23 The Chen and Liu groups have made some progress in bioimaging applications of lanthanide ion-doped CSNCs.81 the Hyeon group has fulfilled high-resolution threephoton biomedical imaging by using doped ZnS NCs.82 For bioimaging applications, novel design and fabrication of highly sensitive and selective sensing systems utilizing long-lived dopant emission are highly desired to efficiently eliminate interference from biological background fluorescence and scattering light. Luminescent solar concentrators (LSCs) composed by homogeneous dispersion of colloidal CSNCs in poly-(methyl methacrylate) (PMMA) matrix are cost-effective complements to 4948



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Figure 5. (A) Spectroelectrochemical measurements of Cu-doped core/shell ZnSe/CdSe NCs were conducted under a negative electrochemical potential to study the Cu-doped PL and doped level. (B) Schematic representation of electronic structures related to photoluminescence in colloidal Mn2+-doped wide-gap CSNCs. When Mn2+ states reside within the semiconductor gap, efficient energy transfer (kET) quenches excitonic emission and sensitizes doped luminescence. Adapted from refs77 and 78 with permission. Copyright 2010 and 2012, American Chemical Society. (C,D) 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). (E) Schematic diagram of the electronic structure responsible for dominant Ag-dopant PL. Efficient energy transfer (KET) quenches excitonic emission and sensitizes dopant-related luminescence. Adapted from ref 33 with permission. Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA.

Summary and Outlook. Intentional introduction of heterovalent impurities in CSNCs with controlled location and concentration is still a challenging task. In this Perspective, we have shown the research progress on cation-exchange reactionbased heterovalent doping. Via cation-exchange-enabled heterovalent doping, a substitutional or interstitial located dopant in a deep position of the host matrix has been achieved, and this in turn enables flexible n- and p-type impurity control. However, many fundamental questions still remain open. For example, what are the well-defined location and distribution of more kinds of transition metal heterovalent dopants in more individual kinds of CSNCs, such as not only II−VI but also, III−V, metal oxide, and how can this be flexibly controlled?85−87 For example, the Alivisatos group utilized monodisperse Cd3As2 and Cd3P2 NCs as host anion lattices to prepared III−V CSNCs by treatment with In3+ or Ga3+ ions at elevated temperature.88 What would happen if there were co-doping of two different kinds of heterovalent cations? For example, the Zhang group studied the co-doping of Cu+ and Al3+ in ZnSe CSNCs.29 Furthermore, the local structure of the Cu dopant in this case, studied by EXAFS, indicated that Cu is in ZnSe: Cu NCs occupy a site that is neither substitutional nor interstitial and is adjacent to a Se vacancy. Additionally, Al3+ co-doping aided in Cu doping by accounting for the charge imbalance originating from Cu+ doping and consequently reduced surface Cu doping. Further studies on doping chemistry associated with thermodynamics of cation-exchange reactions and the surface functionalization by ligand-coordinated

photovoltaics that can boost the output of solar cells and allow for photovoltaic windows applications.83 CSNCs without doping mostly have small Stokes shifts and large reabsorption losses that hinder the realization of large-area devices. the Brovelli and Klimov groups have fabricated “Stokes shift-engineered” CdSe/CdS giant quantum dots dispersed in PMMA to create LSCs without reabsorption losses even for devices with dimensions up to tens of centimeters.80,84 These results demonstrated the significant promise of larger Stokes shift-engineered CSNCs for large-area LSC applications. The heterovalent doped CSNCs attained from cation exchange also have the features of an enlarged Stokes shift. For example, Ag+- or Cu+- doped CdX CSNCs by cation-exchange strategy in Figure 1B could afford a Stokes shift larger than 0.7 eV (on average, more than 3 times larger than CdX CSNCs without doping), as shown in Figure 6. In particular, these kinds of heterovalent and substitutional doped CdX CSNCs have stable doped luminescence (Ag+-doped CdS CSNCs here are stable for more than 1 year, with the absolute quantum yield maintained at 42%) because of the unprecendented high-purity deep-location doping.33 These advantages could meet the requirements for LSC applications. So far, we could functionalize such doped CSNCs with MMA ligands and use an in situ polymerization method to realize a tens of centimeters-sized PMMA solid with uniform dispersion. Besides, the efficient luminescence collection behaviors have been realized successfully. This research progress is believed to boost their practical applications in constructing LSC devices. 4949



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Figure 6. Outlook on heterovalent doped CSNC applications profiting from the advantages of a larger Stokes shift and flexible n- and p-type electronic impurity tailoring. On the basis of these advantages, heterovalent doped CSNCs and their thin-film solids or dispersion in a polymer matrix could be used in some kinds of optoelectronic or electronic devices, such as a luminescent solar concentrator, field-effect transistor, photoresistor, light-emitting diode, and quantum dot solar cell. Reproduced according to refs 17, 80, and 84. Copyright 2011, The American Association for the Advancement of Science, and 2014 and 2015, Nature Publishing Group.

CNSCs with ultrafast optical spectroscopy (including timeresolved Faraday rotation (TRFR) studies), which represents an important but previously unexplored aspect of nanoscale magnetism in colloidal doped NCs.93

cations are strongly needed. Moreover, the cation-exchange strategy has enabled oriented attachment of heterovalent doped CSNCs and improved their contact with rigid and flexible substrates synergistically.89 Thus, the dimension-span 3D selfassembly from doped CSNCs to bulk film with enhanced conductivity is anticipated for new functional device design.17,90 Regarding the conductivity of the doped CSNCs film, to fully deploy the potential of heterovalent doped CSNC films as lowcost electronic materials, improved determination of dopant concentration is required to get satisfactory conductivity, and also, high enough free carrier concentrations are needed to achieve metallic conductivity.6,7,19,91 Typically, in metal oxide NCs, substitutional doping with heterovalent metal ions has recently been used to introduce large free-electron populations that support SPR in the infrared region.19 Interface manipulation between neighboring doped CSNCs when assembled into a film reserves more considerations in future research. For example, very recently, the Shklovskii group studied the metal−insulator transition in films of phosphorus-doped, ligand-free silicon NCs.91 Advanced characterization methods, such as high-resolution TEM direct imaging, XANES, EXAFS, XPS, and femtosecond TA spectroscopies have been employed to characterize the exact location, concentration, and electron relaxation process of heterovalent dopants. These advanced tools pave the way for fundamental studies of heterovalent doped CSNCs. With the variety of introduced dopants, more powerful analytical techniques are clearly desired. For example, electron paramagnetic resonance (EPR) spectroscopy is well-suited to probe the nature of the extra electrons introduced by heterovalent doping in CSNCs.92The Klimov and Crooker groups studied the doped

Superior control of the surfaces and interfaces in heterovalent doped CSNC films is of particular importance, and this can promote the solution-based fabrication of low-cost, large-area, flexible, optical, and electronical devices. For bulk device applications, including those composed by NC dispersion in polymer solid, flexible surface functionalization and film-scale assembly of heterovalent doped CSNCs have become very desirable. For example, congruent with the statements in the review of The Sargent and Talapin group in 2016,17 exploration of the surface chemistries and the self-assembly of heterovalent doped CSNCs to be superstructures or integration of diverse hetero-NCs in devices remain to be challenges. For electronic device applications, aside from the above prerequisites on flexible dopant luminescence and electronical impurity control, the critical step is to create n-type and p-type conductive solid films and a p−n heterojunction film.6,7,15,17 Such n-type and p-type heterovalent doped CSNC films would underpin their applications in fabricating field-effect transistors, LEDs, solar cells, and photodetector devices, as shown in Figure 6. Ultimately, it can be expected that, in the future, superior control of the surfaces and 4950



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interfaces in heterovalent doped CSNC films is of particular importance, and this can promote the solution-based fabrication of low-cost, large-area, flexible, optical, and electronical devices. Compared to undoped systems, the achievements of future research efforts will exhibit more distinct advantages.

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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.jpclett.7b00351. Optical properties comparison for Ag-doped CdSe and CdS CSNCs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiatao Zhang: 0000-0001-7414-4902 Notes

The authors declare no competing financial interest. Biographies Jiatao Zhang earned his Ph.D. in 2006 from Prof. Yadong Li’s group of Tsinghua University, China. Then, he worked as a postdoctoral research fellow in Prof. Dieter Fenske’s group of Karlsruhe Institute of Technology from 2006 to 2007. From 2008 to 2011, he worked as a research associate in Min Ouyang’s group of the physics department, University of Maryland, College Park. From 2011 to now, he has been working as Xu Teli Professor in the School of Materials Science and Engineering, Beijing Institute of Technology, China. In 2011, he was awarded Program of New Century Excellent Talents, Ministry of Education of China. In 2012, he was awarded Program of Excellent Talents, Beijing Government. In 2013, he was awarded Excellent Young Scientist foundation of NSFC. Since 2015, he has been the director of the Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications. His current interest is focused on colloidal metal/semiconductor hetero-nanocrystals and doped nanocrystals to assess their novel optical and electronic properties for applications in optoelectronics, energy conversion and storage, catalysis, and biology. Qiumei Di is a Ph.D. candidate in the School of Materials Science and Engineering, Beijing Institute of Technology. She joined the Zhang group after completing her Master’s degree at Beijing Technology and Business University. Her current research is heterovalent doping in semiconductor nanocrystals by cation exchange.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51372025, 91323301, 51631001, 21322105, 51501010). The authors would like to thank Prof. Gregory D. Scholes from the department of chemistry, Princeton University for helpful discussions on doped semiconductor nanocrystals.

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Heterovalent Doping in Colloidal Semiconductor Nanocrystals: Cation

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