Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. These materials were discovered when liquid-phase chemical syntheses of spherical nanocrystals were modified. However, despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remained unclear, and even counter-intuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead halide perovskites). Here we describe an intrinsic instability in growth kinetics that can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface, and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

Two-dimensional (2D) semiconductors are finding a renewed interest in recent years, due to their combination of physical properties, e.g., mobility, fluorescence and spin selectivity, with a potential implementation in new and emerging opto-electronic and spin-electronic devices. The current work discusses various magneto-optical phenomena, found in two different 2D systems: The transition metal phosphorous trichalcogenides, indium chalcogenides and magnetically doped colloidal nanoplatelets. The magneto-optical properties are investigated by following optical polarization in the presence of an external magnetic field, as various temperatures, as well as implementing the use of optically detected magnetic resonance spectroscopy.

The transition metal phorphorous trichalcogenides resemble the most common layered dichalcogenides, but one-third of the metals are replaced with a P-P pair; hence, the chemical formula is written as M2/3(P-P)1/3X2 or M2P2X6. The dilution of metal site by non-magnetic atoms, endows a column like arrangement of the remaining metals, leading to a special magnetic properties, from a full antiferromagnetic Neel, through antiferromagnetic zigzag to ferromagnetic character. The various arrangements can be tuned by variation of the metal cations (among the first row of transitions metal atoms). The work focuses on the influence of the created magnetism on the magneto-optical properties of the M2P2X6 semiconductors. In addition, the talk will report about the synthesis and characterization of In2S3 layered compounds, exploring the various possible structural phases and their magnetically doped derivatives.

Transition metal dopant embedded in colloidal semiconductor nanoplatelets (NPLs) exhibit special magnetic properties, resemble the bulk diluted magnetic semiconductors. However, the NPLs confined thickness induces an extremely intense spin-exchange interaction between the resident photo-excited carriers and the guest magnetic spins. Such an interaction leads to a giant magnetization and g-factor, and consequently endows the materials with special magneto-optical properties. The work emphases the investigation of the spin-exchange interaction while varying the magnetic dopants, by following variation in the magneto-optical properties, when detecting either an ensemble of NPLs or a focus on a single platelet. **

** Magnetically doped NPLs project was carried out in collaboration with Prof. Volkan Hilmi Demir and his groups from Nanyang Technological University – NTU Singapore 639798, Singapore and from Bilkent University, Ankara 06800, Turkey.

We introduce the electronic and exciton dynamic properties of these nanoplatelets and their hetero structures and demonstrate for example that ligand induced strain in these colloidal quantum wells with finite size results in a strong alteration of the transition energies. We show that intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy efficient display technology and propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic and directed photoluminescence.  Our analysis of the two-dimensional k-space distribution of the nanoplatelet absorption and emission reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters for photonic applications. We also demonstrate by 2D k-space spectroscopy that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with extremely large cross sections.

References:                                                                                                                     1. Scott and Achtstein et al., Nature Nanotechnology 12, 1155 (2017)                        2. Heckmann and Achtstein et al., Nano Letters 17, 6321, (2017)                                3. Antanovich and Achtstein et al., Nanoscale, (2017); DOI: 10.1039/c7nr05065h

Solution-processed semiconductor nanocrystals have attracted great interest in photonics including high-purity color conversion and enrichment in quality lighting and display backlighting. These nanocrystals span different types and heterostructures of semiconductors in the forms of colloidal quantum dots and rods to a more recently emerging class of colloidal quantum wells. Here we will talk about colloidal photonics using the family of quasi-2D, tightly-confined, atomically flat nanocrystals. Here we will show that custom-design 2D heteronanoplatelets uniquely offer record high optical gain coefficients and ultra-low threshold stimulated emission. In addition, we will show that controlled stacking of these nanoplatelets provides us with the ability to further tune and master their excitonic properties. Also, we will discuss doping of these nanoplatelets with Cu for high-flux solar concentration properties and with Mn for precise wavefunction-engineered magnetic properties. Given their most recent accelerating progress, these solution-processed quantum materials hold great promise to challenge their epitaxial counterparts in semiconductor optoelectronics in the near future.

Colloidal PbS nanosheets represent an important infrared 2D nanomaterial. They have tunable energy gaps and much higher charge carrier mobility than quantum-dot films, which makes them ideal for optoelectronic and electronic applications including photovoltaic devices and field-effect transistors.

The growth of PbS nanosheets is confirmed by electron microscopy and photoluminescence spectroscopy. Their thickness can be tuned by changing the reaction temperature during the synthesis. Recent research also demonstrates that the lateral size of the nanosheets can be systematically tuned between 20 nm to a few hundred nanometers. Core/shell PbS/CdS nanosheets can be synthesized using a cation exchange method. This method can protect the PbS core as well as further tuning the core thickness. Surface passivation of the nanosheets using organic molecules is also proven to be very effective to suppress the surface defects and improve the optical properties.

The energy gap of the PbS nanosheets can be tuned by changing their thickness. The thickness dependent energy gap is a unique feature of the exciton under one-dimensional confinement. In contrast to quantum dots, the confinement energy can be achieved in a typical nanosheet is smaller than a quantum dot. However, the maximum energy gap of the nanosheet can still reach 1 eV which is about more than twice of the energy gap of the bulk PbS.

The optical absorption of typical PbS nanosheets shows a step-like spectrum. The step edge is nearly coincident with the photoluminescence peak, indicating a negligible Stokes shift. The absorption spectrum of the nanosheets of 20 nm in lateral size shows an excitonic peak similar to quantum dots, which is possibly due to the additional lateral confinement.

The absolute photoluminescence quantum yield of the PbS nanosheets can be measured accurately by using an integrating-sphere technique. Well-passivated PbS nanosheets typically show 20% to 30% photoluminescence quantum yield. The maximum photoluminescence quantum yield has reached 60%, which is about twice of the quantum yield from PbS quantum dots of the same energy gap.

The time-resolved photoluminescence of the PbS nanosheets shows a distinct fast decay followed by a slow decay. The fast decay accounts for more than 90% of the total luminescence intensity. The exciton radiative lifetime derived from the time-resolved photoluminescence and the quantum yield is much shorter than the PbS quantum dots. It is an indication of giant oscillation strength transition which is a consequence of large exciton coherence volume in a nanosheet.

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Tin(II)sulfide nanosheets with lateral sizes in the range of several micrometers and thicknesses of only tens of nanometers are synthesized by injection of S-oleylamine into a hot solution of oleylamine, oleic acid, HMDS and tin chloride. We will present structural data to follow the nanosheets growth and show how additional octadecene can influence the lateral growth direction to form either rectangular or hexagonal nanosheets.

In the second part we focus on the electrical properties of individual hexagonal SnS-nanosheets. Here we present data from combined Kelvin Probe Force Microscopy (KPFM) with simultaneous Scanning Photocurrent Measurements (SPCM) to attribute the electro-optical properties to mutual interaction between locally generated charge carriers and electric potentials. For example the occurrence of zero-bias photocurrent can directly be attributed to changes of the band bending due to the optically generated charge carriers.

The surface ligation of semiconductor nanocrystals determines their optical and transport properties, and purposeful control of surface chemistry is under active investigation.  Colloidal 2D CdSe nanoribbons or quantum belts provide an ideal system for the study of rapid, reversible, and complete exchange of surface ligation at room temperature.  L-type primary-amine ligation is readily replaced by Z-type Lewis acid ligation of type MX₂ (M = Zn, Cd; X = carboxylate or halide).  Re-exposure to primary amines removes the Z-type ligands and restores L-type ligation.  Primary-amine ligation is readily converted to bound-ion-pair X-type ligation upon exposure of the quantum belts to acids HX (X = halide, carboxylate, or nitrate).  Salts of type R₄NX or NaX (X = carboxylate or halide) are unable to displace L-type ligation, but readily substitute for Z-type ligation, forming bound-ion-pair X-type ligation.  This bound-ion-pair X-type ligation is readily exchanged to L-type ligation upon exposure to primary amines.  The surface exchanges are characterized by a variety of means, and the absorption and emission spectra of the quantum belts are particularly sensitive to surface ligation.  Such surface exchanges promise to optimize the properties of the 2D nanocrystals.

One important feature of two-dimensional semiconductors (e.g., transition metal
dichalcogenide monolayers or phosphorene) is the strong Coulomb and Exchange interactions between photo-generated charge carriers resulting from the unusual carrier screening and its environmental sensitivity.   Particular interesting is the formation of bound states involving multiple carriers, such as positively and negatively charged trions (3 particles),  biexcitons and more. The low screening situation leads to quasiparticles that can remain stable even near-room temperature. I will address the corresponding situation describe some of the atomistic theoretical approaches available. En emphasis will be placed on the effects of the environment on the quasiparticle binding energies (exciton binding energies and the shift of exciton transitions relative to trions and biexcitons). Empirical theoretical approaches will be contrasted to more ab-initio descriptions, which still suffer from the high computational demand. Finally, the results will be compared to —the rather well established— situation in semiconductor quantum dots where screening effects tend to be rather well described by bulk screening.

The nanocrystal-ligand interface of colloidal semiconductors has attracted significant research interest owing to their dominant role in tuning and tailoring the physio-chemical properties of such functional nanomaterials. For metal chalcogenide quantum dots (QDs), a comprehensive picture has emerged over the past decade on surface passivation and photoluminescence relationship. The surface of QDs is metal rich passivated by X-type ligands. Displacing them from the surface results in a marked drop of PLQY, which could be reversed by readsorption of ligands. DFT studies indicate that this is linked to the formation of under-coordinated surface Se which behave as traps for electrons or holes. QDs are nanometre-size crystallites with crystal facets of few square nanometres. This differs markedly from 2-D nanocrystals which are few monolayers thick with atomically flat top and bottom surfaces measuring hundreds of square nanometre e.g. CdSe nanoplatelets. A pronounced interplay between the ligand capping and optical properties of nanoplatelets has been reported. But, research on CdSe nanoplatelets is often hampered by their limited colloidal stability and unwanted stacking in bundles, two aspects that are linked to nanoplatelet-ligand interface. This combination of promising properties and cumbersome processing calls for an in-depth study of their surface chemistry. Here, the question as to whether concepts developed for multifaceted QDs can be transferred to nanoplatelets, which are terminated solely by atomically flat interfaces, stands out.

In this work, detailed study of the surface-chemistry of CdSe nanoplatelets is reported. We first introduce an improved synthesis strategy for nanoplatelets that yields colloidally stable and aggregation free nanoplatelets suspensions. Despite large surface area, these core-only nanoplatelets have a PLQY as high as 55%. Elemental analysis show nanoplatelets are Cd rich.¹H NMR analysis show that Cd excess comes with a surface termination by X-type carboxylate ligands, a binding motif similar to QDs. Addition of an L-type ligand displaces cadmium-carboxylate complexes from surface. The displacement isotherm unravelled that the surface features adsorption sites with different binding energies for CdX₂. We further emphasize the validity of a multiple-site model by DFT calculations, which yield a variation in binding sites from edges to the centre of the facet. Moreover, we analysed different types of mid-gap trap states and site dependent surface reconstruction the nanoplatelet undergoes with displacement of the ligands. Finally, this surface-model for CdSe nanoplatelets is most likely not restricted to CdSe only and should be considered when analysing the surface reactions of another 2-D nanoplatelet system.

We present our recent results on the optical and magneto-optical studies of charged and neutral excitons in ensemble of CdSe-based colloidal nanoplatets (NPLs). The influence of the temperature and external magnetic field on the time-resolved dynamics of photoluminescence (PL) allows us to distinguish between the recombination originating from neutral and changed excitons. We compare the dependence of the fine structure energy splitting of the neutral exciton on the NPLs thickness determined by different experimental methods with the theoretical calculations. We show that at low temperatures the PL in CdSe bare NPLs is determined by the radiative recombination of the lowest dark (spin-forbidden) exciton. We propose an effective mechanism of the dark exciton activation via the exchange interaction with surface dangling bonds. We discuss the mechanisms of the dangling bond spins alignment in external magnetic field and its effect on the exciton spin polarization [3].

The sign of the circular polarization of the PL induced in magnetic field allows us to distinguish between the recombination originating from negative or positive trions. The theoretical analysis of the magnetic field dependences of the degree of circular polarization (DCP) and of the Spin Flip Raman Scattering (SFRS) energy shift allows us to determine the effective g-factor controlling the Zeeman splitting of the spin sublevels of resident electrons, excitons and holes in the negative trion in external magnetic field. The maximum value of the DCP and the SFRS intensity allow us to extract an information about the preferable orientation of the NPLs in the ensemble.

This work was supported in part by the Russian Foundation for Basic Research (Grant No. 17-02-01063 and Grant No. 15-52-12015).

[1] E. V. Shornikova, L. Biadala, D. R. Yakovlev, V. F. Sapega, Y. G. Kusrayev, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, V. V. Belykh, M. V. Kochiev, N. N. Sibeldin, A. A. Golovatenko, A. V. Rodina, N. A. Gippius, A. Kuntzmann, Ye Jiang, M. Nasilowski, B. Dubertret, M.Bayer, Nanoscale 10, 646 (2018).

[2] E. V. Shornikova, L. Biadala, D. R. Yakovlev, D. H. Feng, V. F. Sapega, N. Flipo, A. A. Golovatenko, M. A. Semina, A. V. Rodina, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, Y. G. Kusrayev, M. Nasilowski, B. Dubertret, M. Bayer, Nano Letters 18, 373 (2018).

[3] A.V Rodina, A.A. Golovatenko, E.V. Shornikova,  D.R. Yakovlev, Al.L. Efros, Journal of Electronic Materials 4, 2018

It has been reported that drop casting of a suspension of PbX (X=S, Se, Te) nanocrystals on an ethylene glycol liquid substrate results in the formation of atomically coherent 2-D sheets of a nanocrystal monolayer in thickness. The Klinke group reported the formation of PbS NC sheets. We reported PbSe (S, Te) sheets with a superimposed honeycomb and square geometry.

I will present here our recent progress in this field which is based on extremely slow solvent evaporation under a constant (nearly saturated) solvent vapor phase. We present silicene type honeycomb sheets with lateral dimensions in the 100 micrometer range. We also prepared silicene structures that extend in the vertical dimension over several unit cells. Finally, we studied the "atomic-like" and orientation defects in these systems.

The mechanism of this remarkable self-assembly process was studied by in-situ GISAX and GIWAX and by molecular dynamic simulations.

Low-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal Attachment." Nano Letters 13(6): 2317-2323.

Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices." Science 344(6190): 1377-1380.

In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals." Nature Materials 15(12): 1248-1254.

Mono- and multilayer silicene-type honeycomb lattices by oriented attachment of PbSe nanocrystals: synthesis, structural characterization, and analysis of the disorder.

Submitted

The rapid development of colloidally synthesized, two-dimensional nanoplatelets with precise thickness-tunable, narrow band-edge absorption and photoluminescence unlock several classes of investigations. We have examined charge and energy transfer involving these structures for purposes of energy capture and conversion and lighting. Particularly fast transfer of excitations between self-assembled, co-facial arrangements of thinner donor and thicker acceptor structures is found and modeled. Electron transfer rates for four isoenergetic donor–acceptor pairs comprising a well-known molecular electron acceptor and controlled lateral extents of nanoparticles, examined via ultrafast photoluminescence, relate a dependence of charge transfer rate on the spatial extent of the electron–hole pair wave function explicitly. A nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

The rapid development of colloidally synthesized, two-dimensional nanoplatelets with precise thickness-tunable, narrow band-edge absorption and photoluminescence unlock several classes of investigations. We have examined charge and energy transfer involving these structures for purposes of energy capture and conversion and lighting. Particularly fast transfer of excitations between self-assembled, co-facial arrangements of thinner donor and thicker acceptor structures is found and modeled. Electron transfer rates for four isoenergetic donor–acceptor pairs comprising a well-known molecular electron acceptor and controlled lateral extents of nanoparticles, examined via ultrafast photoluminescence, relate a dependence of charge transfer rate on the spatial extent of the electron–hole pair wave function explicitly. A nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

A solid-state assembly of nanoparticles for the use as a thermoelectric generator¹ is presented. The colloidal synthesis of the two-dimensional lead sulphide nanosheets was performed by oriented attachment of PbS nanoparticles². A self-supporting network of these anisotropic nanoplatelets was formed by slow destabilization of the colloidal solution by altering the pH value. Supercritical drying of such gel-type networks resulted in aerogels. These aerogel materials exhibit extremely low density, high surface to mass ratio and mesoporosity. The physical properties were characterized by transmission electron microscopy, scanning electron microscopy and N₂-physisorption measurements. The thermoelectric properties around room temperature were measured with a hot probe setup utilising multifunctional probes. Thus, the application of a temperature difference and the simultaneous measurement of the temperature difference and the induced thermovoltage was possible³. The material features the nanostructural benefits of aerogels while retaining the Seebeck coefficient of the bulk phase, enabling promising approaches for the use in thermoelectrical devices.

Semiconductor nanoparticle (NP) based photoelectrochemical sensors exhibit several advantages over other sensor types, such as a wide analyte range, fast responses, and high sensitivies.¹ For the development of systems for multi-analyte detection, not only a spatial structuring of the photoelectrodes, but also the improvement of their sensitivity and their detection range is necessary. Due to their high surface-to-volume ratio, semiconductor nanoplatelets (NPLs) are promising candidates for the application in (multi-analyte) photoelectrochemical sensing devices. The high surface area of the NPLs potentially increases the absolute number of surface trap states per particle, which may lead to an enhanced photoresponse of the NP covered electrode.³ In addition, CdSe NPLs were already shown to assemble into highly porous non-ordered network structures with large surface areas.² Our work reports on the preparation of different CdSe NPL based 3D assemblies on conductive glass electrodes and the photoelectrochemical characterization of the charge transfer processes across these structures. Electron microscopy revealed that porous NPL gels with different morphologies were obtained via the applied gelation processes. Photocurrents more than one magnitude larger than for simple particle monolayers were detected and the transport of charge carriers was proven by means of intensity modulated photocurrent spectroscopy (IMPS).⁴

References:

(1) Yue Z. et al., ACS Appl. Mater. Interfaces 2013, 5, 2800−2814.

(2) Naskar, S. et al., Chem. Mater. 2016, 28 (7), 2089–2099.

(3) Spittel, D. et al., ACS Nano 2017, 11 (12), 12174–12184.

(4) Jan F. Miethe, Anja Schlosser et al., submitted.

Colloidal nanoplatelets (NPLs) are quasi-two-dimensional nanocrystals with atomically precise thickness in one dimension. Due to their highly anisotropic shape, they offer favorable optical properties such as narrow emission linewidths and large absorption cross-sections.^1,2 However, as-synthesized, NPLs exhibit poor photo- and chemical stability. Thus, strategies have been sought to improve their properties by adding a shell on the NPLs. For spherical quantum dots, recently developed recipes for growing high-quality shells are performed at high temperatures. However, to date, this strategy has not been extended to NPLs because of their low thermal stability compared to quantum dots.³ Here, we present a method for obtaining CdSe/CdS core/shell NPLs in which the shell is added at high temperatures (~300 °C).⁴ This enables the growth of uniform and thick CdS shells, which is not possible with existing continuous-growth protocols. We further extend this protocol to produce alloyed Cd_xZn_1-xS shells with varying composition and thickness. We obtain high-quality monodisperse and stable core/shell NPLs with narrow emission linewidths, high QYs exceeding 70-80%, and suppressed blinking. Such samples exhibit tunable emission peaks that can result in improvements for a wide range of applications in optics and optoelectronics relying on efficient and narrow emitters.

1)  S. Ithurria et al., Nat. Mater., 10, 936 (2011)

2)  A. Yeltik et al., J. Phys. Chem. C, 119, 26768 (2015)

3)  A. Riedinger et al., Nat. Mater., 16, 743 (2017)

4)  A. A. Rossinelli et al., Chem. Commun., 53, 9938 (2017)

One of the most attractive areas of research in Nanoscience is the combination of 2D materials through van der Waals forces to form heterostructures (Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. Science 2016, 353, DOI: 10.1126/science.aac9439). Here, we present strategies beyond dispersion forces to interface 0D (molecules) and 2D materials.

We will show examples of 0D-2D heterostructures connected through covalent and noncovalent chemistry. In the first example, we will show a method to functionalize graphene covalently with exquisite atomic selectivity and yield (NanoLett. 2016, Chem. Commun. 2017). In the second example, we will show that the photoresponse of MoS2 photodetectors can be enhanced by simple and reversible supramolecular functionalization with organic dyes (Chem. Commun. 2016). We will also show the example of a naturally occurring van der Waals heterostructure, franckeite, an air stable, p-doped semiconductor with photoresponse in the far IR (Nat. Comms. 2017). Finally, we will present strategies for the self assembly of liquid-phase exfoliated 2D materials from suspension into functioning electronic devices (Chem. Commun. 2017 and Nanoscale 2018).

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).

Scattering-type scanning near-field optical microscopy (s-SNOM) has emerged as a key technology to gain chemical information, study structural properties and observe charge carrier distributions on the 10 nm length-scale. s-SNOM employs a metallic AFM tip which is illuminated to create a 10 nm small optical hot-spot at the tip apex independent from the wavelength of incident light. The concentrated light in the optical hotspot interacts with the sample surface below the tip and is modified by the local dielectric properties (absorption, reflection) of the sample. Detection of the elastically scattered light simultaneous to AFM imaging yields near-field images and broadband near-field spectra (nano-FTIR) with <20 nm spatial resolution. [1]

Using infrared s-SNOM imaging, the free carrier distribution in Bi2Se3 plates could be determined and it could be revealed that polyol-synthesized plates feature a non-uniform distribution of Se vacancies which cannot be cured by post-annealing. CVD-grown Bi2Se3 plates on the contrary don’t show such inhomogeneities. [2]

Furthermore s-SNOM images, recorded using mid-IR light, reveals the highly symmetric optical pattern of solvothermally grown Sb2Te3 hexagonal nanoplatelets. The superordinate optical patterns of the spiral growth patterns are shown to represent domains of distinct electronic properties, resulting from so-called growth twins with different densities of antisite defects. [3]

References:

[1] F. Huth et al., Nano Lett. 2012, 12, 3973

[2] X. Lu et al., Adv. Electron. Mater., 2018, 4, 1700377

[3] B. Hauer et al., Nano Lett., 2015, 15 (5), 2787

Colloidal two-dimensional (2D) tin (II) sulfide (SnS) nanocrystals are now emerging as potential and promising nanomaterials for electronic and optoelectronic applications. This is due to lower toxicity compared to other metal chalcogenides, such as PbS, PbSe, CdS, CdSe. We present a new and simple method for the preparation of colloidal 2D SnS nanosheets with large size, tunable thickness and single-crystallinity. The synthesis is performed by using tin (II) acetate as precursor to replace the common used tin halides (e.g. tin chloride) and harmful precursor (bis[bis(trimethylsilyl)amino] tin(II). The lateral size of synthesized square nanosheets can be tuned from 150 nm to 500 nm, and the thickness in a range of 25 to 30 nm. In addition, hexagonal shaped SnS nanosheets can also be synthesized (lateral size: 200-1700 nm, thickness: 15-50 nm). We control the shape and size by varying the amounts of ligands and precursors, which is also supported by DFT simulations. The crystal phase can also be optimized from the mixture of pseudotetragonal structure (PT, from nanoparticle byproducts) and orthorhombic structure (OR, from main product nanosheets) into single-crystalline OR structure. The optoelectronic measurements show their impressive conductivity and highly sensitivity to light. These materials are thus promising regarding electronic and optoelectronic applications.

Two-dimensional metal chalcogenide (CdSe, PbSe, PbS) materials were synthesized by processing from solution. Typical thicknesses range from one to more than ten nanometers. We studied the photogeneration, mobility and decay dynamics of charge carriers and excitons in: 1) CdSe nanosheets, 2) superlattices of connected PbSe QDs with square or honeycomb geometry, and 3) PbS nanosheets. The studies were performed using ultrafast pump-probe laser spectroscopy with optical or terahertz conductivity detection.

The composition and nanogeometry of the material were found to have pronounced effects on the relative yield of free mobile charges and neutral excitons. The relative yield of excitons was found to increase with excitation density. This effect could be described on the basis of the Saha equation, which accounts for more charge recombination at higher photoexcitation density.

The mobility of charge carriers depends strongly on the nanogeometry and material composition. In PbSe honeycomb superlattices mobilities are of the order of 1 cm2/Vs, while in square superlattices and PbS nanosheets values as high as a few hundred cm2/Vs were found. The frequency dependence of the mobility could be described theoretically by the Drude-Smith model, which includes effects of charge scattering on phonons as well as static defects.

Colloidal CdSe nanoplatelets (NPLs), also known as colloidal quantum wells, exhibit interesting properties such as narrow emission lines and atomic controlled thicknesses. To selectively tune the electrical and optical properties of these highly interesting nanomaterials, several methods have been developed to yield either core/shell or core/crown NPLs. Moreover, it has recently been shown that those NPLs can also be converted into nanorings with a toroidal topology.¹ It is hypothesized that this new geometry exhibits interesting properties such as the presence of magnetoexcitonic states and terahertz absorption. Until know, not much knowledge has been obtained about this new geometric shape of CdSe.

In our research, we optimize the synthesis procedure of NPLs and subsequently etch the NPLs to obtain the characteristic ring-like geometry of nanorings. To reveal information about the obtained particles, we use a wide range of techniques such as time-resolved PL spectroscopy, single dot spectroscopy, AFM and TEM. From these experiments, the electrical and optical properties can be determined. As such, we take the first steps to extend the knowledge about this unexplored geometry of CdSe.

1.             Fedin, I.; Talapin, D. V., Colloidal CdSe Quantum Rings. J Am Chem Soc 2016, 138 (31), 9771-4.

Nanocrystal (NC) solids are commonly prepared from non-polar organic suspensions, with preservation of the original capping. On the other hand, NC superstructures with a direct crystalline connection between the NCs have been reported too. Here, we present large-scale uniform superstructures of attached PbSe NCs with a silicene-type honeycomb geometry, resulting from solvent evaporation under nearly reversible conditions. Those superstructures were prepared by self-assembly of the nanocrystals on a liquid substrate. The nanocrystals are subsequently oriented attached to form a superstructure with preferred geometry. Besides  the hoenycomb lattice, we also prepared multilayered silicene honeycomb structures by using larger amounts of PbSe NCs. Using HAADF-STEM tomography we show that the bilayer and multilayered silicene structures of attached PbSe nanocrystals form slices of the simple cubic superstructure. We describe the disorder in the silicene lattices in terms of the nanocrystals position and their atomic alignment. The silicene honeycomb sheets are large enough to be used in transistors and opto-electronic devices.

Semiconducting monolayers hold many promises for the development of optoelectronic and spintronic devices, but are comparatively less explored for light harvesting. This situation is partly due to the lack of adequate materials: current production strategies (exfoliation and epitaxial growths) are limited to explore the wide range of potential two-dimensional (2D) materials for light conversion applications. Using colloidal chemistry to synthesize monolayers opens up the possibility to achieve an unprecedented control over the size, edges nature, composition and structure of the nanosheets. This control would allow to precisely engineer their electronic and optical properties to produce highly efficient monolayer-based nanomaterials for light conversion applications.

Using tungsten disulfide as a prototype material, we explored the transition metal dichalcogenide (TMDC) colloidal synthesis, through a strategy relying on precursors decomposition in high boiling point organic solvent. After having identified conditions to reproducibly synthetize colloidal WS₂ monolayers in the metallic 1T crystal structure,¹ we systematically modify the reaction parameters. The obtained protocols allow us to tune the crystal structure as well as the shape and the aggregation of WS₂ nanosheets. These first steps demonstrate the potential of colloidal synthesis to prepare TMDCs nanosheets of controlled crystal structure, shape and thickness. Other materials and alloys can also be prepared through this approach, as recently demonstrated by other groups for WSe₂² and Mo_xW_1-xS₂.³

(1)           Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. J. Am. Chem. Soc. 2014, 136, 14121.

(2)           Jung, W.; Lee, S.; Yoo, D.; Jeong, S.; Miró, P.; Kuc, A.; Heine, T.; Cheon, J. J. Am. Chem. Soc. 2015, 137, 7266.

(3)           Sun, Y.; Fujisawa, K.; Lin, Z.; Lei, Y.; Mondschein, J. S.; Terrones, M.; Schaak, R. E. J. Am. Chem. Soc. 2017, 139, 11096.

Following the discovery of graphene, a vibrant research area on two-dimensional (2D) transition metal chalcogenides (TMDs) layered materials has emerged in recent years due to their exciting and diverse properties.^1-4 The existing approaches to make TMDs are mainly exfoliation, substrate growth and colloidal synthesis. Among them, colloidal wet-chemistry methods are particularly promising as the as-synthesized colloidal dispersions are directly fit for solution-based processing, opening a gateway for a straightforward and cost-efficient introduction into various technology platforms. However, compared to the other two preparation schemes, colloidal synthesis is relatively underdeveloped with only a few examples being known.^2-4

In this contribution, we describe the preparation of WSe₂ nanocrystals using a hot injection colloidal synthesis in the presence of a capping ligand as the solvent. We find that the choice of capping ligands influences the shape of the nanocrystals to great extent. Nano-flowers were obtained in the presence of oleic acid as capping ligand, whereas, nanosheets were obtained in the presence of oleylamine. Moreover, modification of the reaction temperature allowed to control the size of the nanocrystals. A thorough investigation of the reaction yield by means of quantitative XRF analysis enabled us to rationalize the reaction process. This synthesis strategy might provide a versatile approach to synthesize a wide range of TMDs such as WS₂ and MoSe₂.

[1] Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., ... & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306(5696), 666-669.

[2] Mahler, B., Hoepfner, V., Liao, K., & Ozin, G. A. (2014). Colloidal synthesis of 1T-WS₂ and 2H-WS₂ nanosheets: applications for photocatalytic hydrogen evolution. Journal of the American Chemical Society, 136(40), 14121-14127.

[3] Jung, W., Lee, S., Yoo, D., Jeong, S., Miró, P., Kuc, A., ... & Cheon, J. (2015). Colloidal synthesis of single-layer MSe₂ (M= Mo, W) nanosheets via anisotropic solution-phase growth approach. Journal of the American Chemical Society, 137(23), 7266-7269.

[4] Jin, H., Ahn, M., Jeong, S., Han, J. H., Yoo, D., Son, D. H., & Cheon, J. (2016). Colloidal single-layer quantum dots with lateral confinement effects on 2D exciton. Journal of the American Chemical Society, 138(40), 13253-13259.

We have developed new families of highly ordered two dimensional covalent organic framework (2D-COF) materials. The materials have a hexagonal aromatic backbone with nanopores with controlled sizes that can be functionalized with any number of functional groups. We make them in high yields from condensation reactions from easy to obtain starting materials that in some cases produce very sharp x-ray diffraction patterns as well as showing triangular shaped crystallites with sharp diffraction spots in the TEM indicating a high degree of order. Single metal binding sites can also be designed within the framework structures. We have initially concentrated on putting charged functional groups in the pores to use as membrane materials and have demonstrated very specific ion charge and size selectivity with membranes made from these 2D-COFs. We have also produced 2D-COF materials that have less order but selectively absorb and release hydrogen at near room temperature. Other applications such as size exclusion filtration will also be shown.

In recent years, monolayers of transition metal dichalcogenides have attracted a great deal of research interest. These two-dimensional (2D) materials typically have direct bandgaps in the visible or near-infrared, making them attractive for both classical and quantum optoelectronic applications [1]. Many different techniques have been developed to isolate monolayers from bulk; among them liquid assisted exfoliation is preferred by many, as it is simple, scalable, convenient and cost-effective [2]. However, the flakes produced using this class of technique are often not suitable for many applications, having poor uniformity and optical properties due to unintentional doping and other impurities.

In this work we employ ultrasonic bath sonication of bulk MoS₂, starting with powders of grain sizes ranging from 6 to 40 µm, in an isopropanol/water mixture (70/30 vol %), following the procedure according to [3]. This produces suspensions of few-layer MoS₂ flakes. Photoluminescence (PL) measurements from drop-cast samples on silicon substrates are found to have full widths at half maxima of 200 meV, indicating an average thickness of 2-3 layers. This agrees with independent atomic force microscopy measurements [4]. However, PL intensities are found to be weak – a typical result for flakes produced with this technique.

Super acid treatment of dry mechanically-exfoliated monolayers of MoS₂ has been shown to vastly improve internal quantum efficiency, and with it, PL intensity [5] But it impractical to implement in a manufacturing process. In this work we will detail a new all-wet superacid treatment process for MoS₂ flakes. We show that it enhances PL emission intensity by over 200 times. It thus represents a promising technique for the practical application of 2D inks for optoelectronics.

Reference:

1. Ryou et al. Scientific Reports 6, 29184 (2016)

2. Huo et al. Science Bulletin 60, 1994 (2015)

3. Bernal et al. 2D Materials, 3, 035014 (2016)

4. Bissett et al. ACS Omega, 2, 738 (2017)

5. Amani et al. Science, 350, 1065 (2015).

Colloidal 2D hetero-nanoplatelets offer new possibilities in terms of heterostructure engineering. The growth can be done either in the confined direction² or perpendicular to the confined direction³. This second type of heterostruture is called core/crown. It gives the opportunity to tune the composition and the lateral dimensions of each material while keeping a constant confinement (thickness).  We show that synthesizing CdSe/CdSe_1-xTe_x core/crown nanoplatelets with the right composition and lateral extension enables bicolor emission at the single-nanoparticle level. The first transition at low energy comes from core/crown interface recombination (Xint) and is comparable to the one observed in CdSe/CdTe⁴. The second one, at higher energy, originates from a direct recombination of the exciton in the crown. It is only visible for Te compositions x close to 60% and more likely occurs as the crown dimensions increase. This bicolor emission results from a competition between the conduction band offset that attracts the electron in the core material and the exciton binding energy that retains it in the crown. For 60% of Te those energies are similar (~250meV) and allows the coexistence of the two recombination processes.

(1)         Dufour, M.; Steinmetz, V.; Izquierdo, E.; Pons, T.; Lequeux, N.; Lhuillier, E.; Legrand, L.; Chamarro, M.; Barisien, T.; Ithurria, S. Engineering Bicolor Emission in 2D Core/crown CdSe/CdSe1-xTex nanoplatelet Heterostructures Using Band-Offset Tuning. J. Phys. Chem. C 2017, 121, 24816–24823.

(2)         Ithurria, S.; Talapin, D. V. Colloidal Atomic Layer Deposition (c-ALD) Using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media. J. Am. Chem. Soc. 2012, 134, 18585–18590.

(3)         Tessier, D.; Spinicelli, P.; Dupont, D.; Patriarche, G.; Ithurria, S.; Dubertret, B. Efficient ExcitonConcentrators Built from Colloidal Core/Crown CdSe/CdS Semiconductor Nanoplatelets. 2014.

(4)         Pedetti, S.; Ithurria, S.; Heuclin, H.; Patriarche, G.; Dubertret, B. Type-II CdSe/CdTe Core/crown Semiconductor Nanoplatelets. J. Am. Chem. Soc. 2014, 136, 16430–16438.

Solution-processed two-dimensional (2D) semiconductors with tunable band gaps represent highly promising materials for next generation ultrathin electronics. Their dimensionality-dependent optoelectronic properties differ significantly from their zero-, one- and three-dimensional counterparts and can be tuned by colloidal chemistry methods for controlling the structures’ thickness.

We use optical pump-terahertz probe (OPTHzP) spectroscopy as a non-contact method to determine the thickness-dependent transient charge carrier mobility in 2D PbS nanosheets of different thickness (4 – 16 nm) and find high values ranging from 231 cm²/Vs in the thinnest, 4 nm thick sheets, up to 472 cm²/Vs and 427 cm²/Vs in 6 nm and 16 nm thick PbS nanosheets, respectively. Furthermore, we model the frequency dependent charge carrier mobility of 2D PbS nanosheets with a Drude-Smith behavior and reveal a growing contribution of photoexcited excitons in thinner PbS nanosheets due to their increased exciton binding energy.[1] 

We find that by carefully controlling the reaction kinetics, the thickness of colloidal 2D PbS layers can be reduced to < 2 nm to the formation of ultrathin PbS nanoplatelets. Ultrathin 2D PbS layers are particularly interesting due to their increasing carrier multiplication (CM) efficiency with decreasing nanosheet/nanoplatelet thickness.[2,3] We show that in thicker PbS nanosheets, free and mobile charges are generated under photoexcitation, whereas in ultrathin PbS nanoplatelets bound excitons are formed. A photoluminescence quantum yield of up to 20 % is obtained by surface passivation of the significantly blue-shifted PbS nanoplatelets (Abs: 683 nm, 1.8 eV, PL: 705 nm, 1.75 eV) and underpins their potential for NIR light-emitting applications.[4] Our work emphasizes the excellent usability of colloidal chemistry and spectroscopy methods for producing 2D tailor-made band gap materials for high mobility AND light emitting optoelectronics.

[1] Lauth, J., Failla, M, Klein, E., Klinke, C., Kinge, S., Siebbeles, L. D. A., submitted 2018.

[2] Bielewicz, T.; Dogan, S.; Klinke, C., Small 2015,11, 826-833.

[3] Aerts, M.; Bielewicz, T.; Klinke, C.; Grozema, F. C.; Houtepen, A. J.; Schins, J. M.; Siebbeles, L. D. A., Nat. Commun. 2014,5, 3789.

[4] Manteiga Vazquéz, F., Yu, Q., Crisp, R., Kinge, S., Houtepen, A. J., Siebbeles, L. D. A., Lauth, J., in preparation.

Ultrathin colloidal semiconductor nanosheets with thickness in the strong quantum confinement regime are of particular interest, since they combine the extraordinary properties of 2D nanomaterials with versatility in terms of composition, size, shape, and surface control, and the prospects of solution processability. However, synthesis procedures for materials other than the prototypical Pb- and Cd- chalcogenides are still underdeveloped. Compound Cu-chalcogenides are an interesting class of materials, which have been attracting increasing attention as alternatives to Cd- and Pb-chalcogenides, since they have low toxicity, potentially lower costs, and a very wide range of compositions. This latter point makes them extremely versatile, capable not only of offering similar properties to those already demonstrated by Cd- and Pb-chalcogenide nanocrystals (such as PL tunability in the visible to NIR spectral range and high absorption coefficients), but also unprecedented features, such as plasmon resonances.

In this talk, we discuss recent work by our group on ultrathin (~2nm thick) nanosheets of both binary (Cu_2-xA and Cu_2-xA, A= S, Se) and ternary (CuInA₂ and CuInA₂)  Cu-chalcogenides, with well-defined shape (triangular or hexagonal) and dimensions in the ~100 nm to ~1 µm range. The binary nanosheets form through 2D-constrained stack-templated nucleation and growth. The 2D-constraints are imposed by halide-stabilized lamellar Cu-thiolate (or Cu-selenoate) supramolecular complexes that act as soft-templates. Covellite Cu-rich CuInS₂ nanosheets form via self-organization and oriented attachment of chalcopyrite CuInS₂ nanocrystals, which is induced by a sudden change in the composition of the nanocrystal building blocks due to preferential extraction of In³⁺ by in-situ generated H₂S. Primary amines play several essential roles in the formation of these nanosheets. CuInA₂ nanosheets are also obtained by partial self-limited cation exchange in Cu_2-xSe or In₂S₃ template nanosheets. Moreover, the charge carrier dynamics in ultrathin Cu_2-xS nanosheets is studied with THz spectroscopy.

Hybrid organic/inorganic perovskites have emerged as efficient semiconductor materials
for applications in photovoltaic solar cells with conversion efficiency above
20 %. In addition, recent experiments have shown the possibility of synthesizing ultra-thin two-dimensional (2D) organic perovskites. These 2D structures would have similar optical properties to others layered semiconductors such as the single-layer
transition metal dichalcogenides (MoS₂, WSe₂, etc.). For instance, a large exciton
binding energy, together with advantages such as a simple fabrication process with potentially low-cost and large-scale manufacture, and the possibility of having a wide range of optical bandgap values and exciton binding energies by changing the chemical identity of the constituent atoms.

Up to now, state-of-the-art simulations of the excitonic states have been limited
to the study of bulk organic perovskites. A large number of atoms in the unit cell together with the complex role of the molecules makes difficult and inefficient the use
of ab initio methods and the research on the excitonic states in 2D
perovskites have been mainly addressed with semi-empirical methods. In this work, we propose to define a simplified crystal structure to describe
2D perovskites, by replacing the molecular cations with inorganic atoms. Our intention is to apply state-of-the-art, parameter-free and predictive ab initio methods like the GW method and the Bethe-Salpeter equation to obtain the excitonic states of a simple unit cell which resembles a classic 2D material.

We find that inorganic 2D perovskites are stable and hence ultra-thin
2D materials based in all-inorganic perovskites could be synthesized. Moreover,
the optical activity  (like absorption or photoluminescence) is carried out at the bromine and lead atoms and therefore
the conclusions can be qualitatively exported to organic 2D perovskites. For
instance, optical properties of all-inorganic 2D
perovskites are strongly influenced by excitonic effects, with
binding energies up to 0.6 eV. Besides of conceiving a simple material to interpret optical
experiments in more complex 2D organic perovskites, we propose a new set of materials to
increase the family of 2D semiconductors.

Quantum confinement in semiconductor nanocrystals leads to size-dependent optical properties. Conventionally, the growth of nanocrystal results in a gradual red-shift of the features in absorption spectra. However, in certain cases, the absorption features shift in discrete energetic steps with growth. Such discrete growth is often assigned to nanoplatelets or magic-sized clusters. Although the existence and growth of nanoplatelets is proven^1,2, the understanding for the latter remains elusive. This is due in part to complicated synthetic routes that involve multiple ligands and coordinating solvents^3,4 or specific complex pathways⁵ (e.g. templates), which make the investigation of such species difficult.  Here, we introduce a simple route to synthesize and isolate stable CdSe nanocrystals showing discrete evolution of absorption features, a subset of which have been previously assigned to magic-sized clusters. We grow them in a template-free regime using only one type of ligand in a non-coordinating solvent up to a size of 2.8 nm. Their isolation is confirmed by absorption, photoluminescence, photoluminescence excitation, and NMR studies. The particles are further analyzed using electron microscopy to evaluate their size and monodispersity. Finally, we provide mechanistic insight into the evolution of different sizes by conducting parametric studies and studying the growth in precursor-free conditions. This synthetic protocol allows us to investigate the early stages of nucleation and growth of nanocrystals in greater detail.

1.         Ithurria, S. et al. Colloidal nanoplatelets with two-dimensional electronic structure. Nat. Mater. 10, 936–941 (2011).

2.         Riedinger, A. et al. An intrinsic growth instability in isotropic materials leads to quasi-two-dimensional nanoplatelets. Nat. Mater. 16, 743–748 (2017).

3.         Kudera, S. et al. Sequential Growth of Magic-Size CdSe Nanocrystals. Adv. Mater. 19, 548–552 (2007).

4.         Cossairt, B. M. et al. CdSe Clusters: At the Interface of Small Molecules and Quantum Dots. Chem. Mater. 23, 3114–3119 (2011).

5.         Wang, Y. et al. Magic-Size II–VI Nanoclusters as Synthons for Flat Colloidal Nanocrystals. Inorg. Chem. 54, 1165–1177 (2015).