CdSe Nanoplatelets Research Articles

From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfiguration.

The variation in the shape of colloidal semiconductor nanocrystals (NCs) remains intriguing. This interest goes beyond crystallography as the shape of the NC determines its energy levels and optoelectronic properties. While thermodynamic arguments point to a few or just a single shape(s), terminated by the most stable crystal facets, a remarkable variation in NC shape has been reported for many different compounds. For instance, for the well-studied case of CdSe, close-to-spherical quantum dots, rods, two-dimensional nanoplatelets, and quantum rings have been reported. Here, we report how two-dimensional CdSe nanoplatelets reshape into quantum rings. We monitor the reshaping in real time by combining atomically resolved structural characterization with optical absorption and photoluminescence spectroscopy. We observe that CdSe units leave the vertical sides of the edges and recrystallize on the top and bottom edges of the nanoplatelets, resulting in a thickening of the rims. The formation of a central hole, rendering the shape into a ring, only occurs at a more elevated temperature.

Targeted transfer of self-assembled CdSe nanoplatelet film onto WS2 flakes to construct hybrid heterostructures

Colloidal CdSe nanoplatelets are thin semiconductor materials with atomic flatness surfaces and one-dimensional strong quantum confinement, and hence they own very narrow and anisotropic emission. Here, we present a polydimethylsiloxane (PDMS) assisted transferring method that can pick up single layer CdSe nanoplatelet films self-assembled on a liquid surface and then precisely transfer to a target. By layer-by-layer picking up and transferring, multiple layers of CdSe films can be built up to form CdSe stacks with each single layer having dominant in-plane transition dipole distribution, which both material and energic structures are analogous to traditional multiple quantum wells grown by molecular-beam epitaxy. Additionally, with the great flexibility of colloidal nanoplatelets and this transferring method, CdSe nanoplatelets films can be combined with other materials to form hybrid heterostructures. We transferred a single-layer CdSe film onto WS2 flakes, and precisely studied the fast energy transfer rate with controlled CdSe nanoplatelet orientation and by using a streak camera with a ps time resolution.

Optically Active CdSe/CdS Nanoplatelets Exhibiting Both Circular Dichroism and Circularly Polarized Luminescence

AbstractLigand‐induced chirality in colloidal semiconductor nanocrystals attracts attention because of their tunable chiroptical properties. Here, the induced chirality and circularly polarized luminescence (CPL) are investigated as a function of the CdS shell growth in a range of 2D CdSe/CdS nanoplatelets (NPLs) capped with chiral ligands. Five samples of CdSe/CdS NPLs are synthesized by a one‐pot approach to vary the island‐like shell on a four‐monolayer (4 ML) CdSe NPLs core, which effectively reduces the interfacial strain energy. The successful preparation of L‐/D‐Cysteine‐capped CdSe/CdS NPLs with both tunable circular dichroism (CD) and CPL behaviors and a maximum anisotropic luminance factor (glum) of 5.29 × 10−4 is described. The induced chiroptical response shows a direct relationship with the formation of island‐like shell in the first and second stages and shows a clear signal evolution. In the third stage with a full coating shell, the CD and CPL signals are inversely proportional to the CdS shell thickness. The island‐like shell gives birth to the CPL signal, while the formation of full coating shell decreases the induced chirality. Such chiral and emissive NPLs provide an ideal platform for the rational design of semiconductor nanocrystals with chiroptical properties in areas of biomedicine, polarizers, and new generation of display devices.

Comparison between Trion and Exciton Electronic Properties in CdSe and PbS Nanoplatelets

The optoelectronic properties of metal chalcogenide colloidal nanoplatelets are often interpreted in terms of excitonic states. However, recent spectroscopic experiments evidence the presence of trion states, enabled by the slow Auger recombination in these structures. We analyze how the presence of an additional charge in trions modifies the emission energy and oscillator strength as compared to neutral excitons. These properties are very sensitive to dielectric confinement and electronic correlations, which we describe accurately using image-charge and variational Quantum Monte Carlo methods in effective mass Hamiltonians. We observe that the giant oscillator strength of neutral excitons is largely suppressedin trions. Both negative and positive trions are redshifted with respect to the exciton, and their emission energy increases with increasing dielectric mismatch between the platelet and its surroundings, which is a consequence of the self-energy potential. Our results are consistent with experiments in the literature, and assess on the validity of previous theoretical approximations.

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

Driven by the unique optoelectronic features arising from the strong quantum confinement along the thickness direction, rapid development of CdSe‐based nanoplatelets (NPL) has accomplished facile bandgap tunability over a broad spectrum via different preparation protocols or various postsynthesis treatments. The anisotropic geometry of NPLs also stimulates the exploitation of self‐assembled CdSe NPL superstructures to enhance light extraction efficiency in display technologies and achieves polarized lasing performance or even electrically pumped laser by adjusting the transition dipole orientation. Although several review articles about the general optical properties of CdSe NPLs have been published, none of them focus on the ability of spectral tunability and precise control of orientations in the solid‐state phase of CdSe NPLs. Herein, the band structure engineering approaches in the CdSe NPL family are systematically summarized and the optical properties and device performance metrics of these deliberately engineered NPLs are compared. Then, the recent advanced studies of the motivation and assembly methods of geometrically anisotropic CdSe NPL superlattices are discussed. Finally, the current challenges and future outlook on the controlled modification of optical features and the influences of the approaches in the aspect of CdSe NPL–based devices’ performance are highlighted.

Shape-Dependent Linear Dichroism Spectra of Colloidal Semiconductor Nanocrystals.

Semiconductor nanocrystals are normally dispersed in the solvent for property studies as well as practical applications. However, rare attention has been paid to their orientation status in the colloidal solution. Herein, with the help of linear dichroism (LD) spectroscopy, we demonstrate that isotropic NCs of high symmetry (i.e., quantum dots, QDs) and anisotropic NCs (e.g., quantum rods, QRs and nanoplates, NPLs) but under diluted concentration are randomly dispersed without any preferential orientation. Meanwhile, anisotropic NCs under a high concentration can behave with some net orientation along a certain direction. For example, CdSe quantum rods (QRs) and nanoplatelets (NPLs) both show an obviously preferred orientation along the vertical direction in solution when their solution absorbances increase to certain values. An in-depth analysis of QRs' LD spectrum shows that the first excitonic transition of QRs is strongly quantumly confined while its higher-energy excitonic transitions are weakly quantumly confined. In contrast, the NPLs' LD spectrum indicates that their excitonic transitions are isotropic in the spatial space. This work provides a new viewpoint of the real status of anisotropic semiconductor NCs in solution.

Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect

2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm2. Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.

Simultaneous Ligand and Cation Exchange of Colloidal CdSe Nanoplatelets toward PbSe Nanoplatelets for Application in Photodetectors.

Cation exchange emerged as a versatile tool to obtain a variety of nanocrystals not yet available via a direct synthesis. Reduced reaction times and moderate temperatures make the method compatible with anisotropic nanoplatelets (NPLs). However, the subtle thermodynamic and kinetic factors governing the exchange require careful control over the reaction parameters to prevent unwanted restructuring. Here, we capitalize on the research success of CdSe NPLs by transforming them into PbSe NPLs suitable for optoelectronic applications. In a two-phase mixture of hexane/N-methylformamide, the oleate-capped CdSe NPLs simultaneously undergo a ligand exchange to NH4I and a cation exchange reaction to PbSe. Their morphology and crystal structure are well-preserved as evidenced by electron microscopy and powder X-ray diffraction. We demonstrate the successful ligand exchange and associated electronic coupling of individual NPLs by fabricating a simple photodetector via spray-coating on a commercial substrate. Its optoelectronic characterization reveals a fast light response at low operational voltages.

Molecular Polaritons Generated from Strong Coupling between CdSe Nanoplatelets and a Dielectric Optical Cavity.

We demonstrate the formation of CdSe nanoplatelet (NPL) exciton-polaritons in a distributed Bragg reflector (DBR) cavity. The molecule-cavity hybrid system is in the strong coupling regime with an 83 meV Rabi splitting, characterized from angle-resolved reflectance and photoluminescence measurements. Mixed quantum-classical dynamics simulations are used to investigate the polariton photophysics of the hybrid system by treating the electronic and photonic degrees of freedom (DOF) quantum mechanically and the nuclear phononic DOF classically. Our numerical simulations of the angle-resolved photoluminescence (PL) agree extremely well with the experimental data, providing a fundamental explanation of the asymmetric intensity distribution of the upper and lower polariton branches. Our results also provide mechanistic insights into the importance of phonon-assisted nonadiabatic transitions among polariton states, which are reflected in the various features of the PL spectra. This work proves the feasibility of coupling nanoplatelet electronic states with the photon states of a dielectric cavity to form a hybrid system and provides a new platform for investigating cavity-mediated physical and chemical processes.

Effect of PMMA polymer matrix on optical properties of CdSe nanoplatelets

Colloidal cadmium chalcogenide nanoplatelets (NPLs) possess unique properties such as ultrapure emission and giant-oscillator strength originating from their atomically-flat surfaces and one-dimensional carrier confinement. However, unlike quantum dots, the NPLs do not provide continuously tunable absorption and emission, which restricts the range of their possible applications. In this paper, we report a new approach for tuning opto-electronic properties of cadmium chalcogenide NPLs. We demonstrate that incorporation of CdSe NPLs into a poly(methyl methacrylate) (PMMA) matrix leads to a red-shift in all the low energy excitonic transitions and band-edge emission of the NPLs. The polymer matrix induced red-shift of the band-edge emission depends on thickness of the NPLs and ranges from 109 to 253 meV for 4.5 and 2.5 monolayer thick CdSe NPLs, respectively. In the case of CdSe/CdS core-shell NPLs, the polymer matrix induced red-shift of emission peak strongly depends on thickness of the shell and becomes negligible for thick shell CdSe/CdS core-shell NPLs. Possible explanations for the observed effect are presented. The demonstrated here simple approach allows one to tune optical and electronic properties of cadmium chalcogenide NPLs without altering their sizes and composition, which makes it interesting for different practical applications.

Single Trap States in Single CdSe Nanoplatelets.

Trap states can strongly affect semiconductor nanocrystals, by quenching, delaying, and spectrally shifting the photoluminescence (PL). Trap states have proven elusive and difficult to study in detail at the ensemble level, let alone in the single-trap regime. CdSe nanoplatelets (NPLs) exhibit significant fractions of long-lived “delayed emission” and near-infrared “trap emission”. We use these two spectroscopic handles to study trap states at the ensemble and the single-particle level. We find that reversible hole trapping leads to both delayed and trap PL, involving the same trap states. At the single-particle level, reversible trapping induces exponential delayed PL and trap PL, with lifetimes ranging from 40 to 1300 ns. In contrast with exciton PL, single-trap PL is broad and shows spectral diffusion and strictly single-photon emission. Our results highlight the large inhomogeneity of trap states, even at the single-particle level.

Light-Induced Paramagnetism in Colloidal Ag+-Doped CdSe Nanoplatelets.

We describe a study of the magneto-optical properties of Ag+-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures doped with nominally nonmagnetic Ag+ impurities. The excitonic circular polarization (PX) and the exciton Zeeman splitting (ΔEZ) were recorded as a function of the magnetic field (B) and temperature (T). Both ΔEZ and PX have a Brillouin-function-like dependence on B and T, verifying the presence of paramagnetism in Ag+-doped CdSe NPLs. The observed light-induced magnetism is attributed to the transformation of nonmagnetic Ag+ ions into Ag2+, which have a nonzero magnetic moment. This work points to the possibility of incorporating these nanoplatelets into spintronic devices, in which light can be used to control the spin injection.

Chemo- and stereoselective intermolecular [2 + 2] photocycloaddition of conjugated dienes using colloidal nanocrystal photocatalysts

The use of visible-light photosensitizers to power [2+2] photocycloadditions that produce complex tetrasubstituted cyclobutanes is a true success of photochemistry, but the scope of this reaction has been limited to activated α, β-unsaturated carbonyls. This paper describes selective intermolecular homo- and hetero-[2+2] photocycloadditions of terminal and internal aryl conjugated dienes - substrates historically unsuited for this reaction because of their multiple possible reaction pathways and product configurations - through triplet-triplet energy transfer from CdSe nanocrystal photocatalysts, to generate valuable and elusive syn-trans aryl vinylcyclobutanes. The negligible singlet-triplet splitting of nanocrystals' excited states allows them to drive the [2+2] pathway over the competing [4+2] photoredox pathway, a chemoselectivity not achievable with any known molecular photosensitizer. Reversible tethering of the cyclobutane product to the nanocrystal surface results in near quantitative yield of the syn-trans product. Flat colloidal CdSe nanoplatelets produce cyclobutanes coupled at the terminal alkenes of component dienes with up to 89% regioselectivity.

Determining the Structure-Property Relationships of Quasi-Two-Dimensional Semiconductor Nanoplatelets.

We report a theoretical study of CdSe nanoplatelets aimed at identifying the main factors determining their photophysical properties. Using atomic configurations optimized with density functional theory calculations, we computed quasiparticle and exciton binding energies of nanoplatelets with two to seven monolayers. We employed many body perturbation theory at the GW level and solved the Bethe-Salpeter equation to obtain absorption spectra and excitonic properties. Our results, which agree well with recent experiments, were then used to design a model that allows us to disentangle the effects of quantum confinement, strain induced by passivating ligands, and dielectric environment on the electronic properties of nanoplatelets. We found that, for the model to accurately reproduce our first principle results, it is critical to account for surface stress and consider a finite potential barrier and energy-dependent effective masses when describing quantum confinement. Our findings call into question previous assumptions on the validity of an infinite barrier to describe carrier confinement in nanoplatelets, suggesting that it may be possible to optimize interfacial charge transfer and extraction by appropriately choosing passivating ligands. The model developed here is generalizable to core-shell platelets and enables the description of system sizes not yet directly treatable by first-principles calculations.

Van Hove Singularities and Trap States in Two-Dimensional CdSe Nanoplatelets.

Semiconductor nanoplatelets, which offer a compelling combination of the flatness of two-dimensional semiconductors and the inherent richness brought about by colloidal nanostructure synthesis, form an ideal and general testbed to investigate fundamental physical effects related to the dimensionality of semiconductors. With low temperature scanning tunnelling spectroscopy and tight binding calculations, we investigate the conduction band density of states of individual CdSe nanoplatelets. We find an occurrence of peaks instead of the typical steplike function associated with a quantum well, that rule out a free in-plane electron motion, in agreement with the theoretical density of states. This finding, along with the detection of deep trap states located on the edge facets, which also restrict the electron motion, provides a detailed picture of the actual lateral confinement in quantum wells with finite length and width.

Anisotropic Transient Disordering of Colloidal, Two-Dimensional CdSe Nanoplatelets upon Optical Excitation.

Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.

Correction: Tuning trion binding energy and oscillator strength in a laterally finite 2D system: CdSe nanoplatelets as a model system for trion properties.

Correction for 'Tuning trion binding energy and oscillator strength in a laterally finite 2D system: CdSe nanoplatelets as a model system for trion properties' by Sabrine Ayari et al., Nanoscale, 2020, 12, 14448-14458, DOI: .

Theory of single and double electron spin-flip Raman scattering in semiconductor nanoplatelets

A theory of electron spin-flip Raman scattering (SFRS) is presented that describes the Raman spectral signals shifted by both single and twice the electron Zeeman energy under nearly resonant excitation of the heavy hole excitons in semiconductor nanoplatelets. We analyze the spin structure of photoexcited intermediate states, derive compound matrix elements of the spin-flip scattering and obtain polarization properties of the one- and two-electron SFRS common for all the intermediate states. We show that, in the resonant scattering process under consideration, the complexes "exciton plus localized resident electrons" play the role of main intermediate states rather than tightly bound trion states. It is demonstrated that, in addition to the direct photoexcitation (and similar photorecombination) channel, there is another indirect channel contributing to the SFRS process. In the indirect channel, the photohole forms the exciton state with the resident electron removed from the localization site while the photoelectron becomes localized on this site. The theoretical results are compared with recent experimental findings for ensembles of CdSe nanoplatelets.

Temperature-Dependent Photoluminescent Properties of PbSe Nanoplatelets.

Semiconductor colloidal nanoplatelets (NPLs) are a promising new class of nanostructures that can bring much impact on lightning technologies, light-emitting diodes (LED), and laser fabrication. Indeed, great progress has been made in optimizing the optical properties of the NPLs for the visible spectral range, which has already made the implementation of a number of effective devices on their basis possible. To date, state-of-the-art near-infrared (NIR)-emitting NPLs are significantly inferior to their visible-range counterparts, although it would be fair to say that they received significantly less research attention so far. In this study, we report a comprehensive analysis of steady-state and time-dependent photoluminescence (PL) properties of four monolayered (ML) PbSe NPLs. The PL measurements are performed in a temperature range of 78–300 K, and their results are compared to those obtained for CdSe NPLs and PbSe quantum dots (QDs). We show that multiple emissive states, both band-edge and trap-related, are responsible for the formation of the NPLs’ PL band. We demonstrate that the widening of the PL band is caused by the inhomogeneous broadening rather than homogeneous one, and analyze the possible contributions to PL broadening.

A simple variational quantum Monte Carlo-effective mass approach for excitons and trions in quantum dots

A computational model is presented to calculate the ground state energy of neutral and charged excitons confined in semiconductor quantum dots. The model is based on the variational Quantum Monte Carlo method and effective mass Hamiltonians. Through an iterative Newton–Raphson process, minimizing the local energy, and (optional) parallelization of random walkers, fast and accurate estimates of both confinement and Coulomb binding energies can be obtained in standard desktop computers. To illustrate the reach of the model, we provide Fortran programs and illustrative calculations for colloidal CdSe nanoplatelets with large lateral dimensions and dielectric confinement, where electronic correlations are strong. The results compare well with exact variational calculations and largely outperform configuration interaction calculations in computational efficiency. Program summaryProgram title: vqmc-emaCPC Library link to program files:https://doi.org/10.17632/dtpyc2pffg.1Licensing provisions: GPLv3Programming language: Fortran 90Nature of problem: Calculation of either exciton or trion ground state energy and wave function in a cuboidal semiconductor nanoplatelet. Hard wall quantum confinement coexists with dielectric confinement (self-energy and Coulomb polarization terms).Solution method: Variational quantum Monte Carlo with effective mass Hamiltonians, integrated into a Newton–Raphson solver. OMP parallelization library can be (optionally) linked.