II-VI semiconductor nanoplatelets have attracted much interest due to their narrow optical properties controlled by the thickness of these 2D particles. Thanks to the synthesis of heterostructures, it is possible to grow NPLs with more than one emission thus breaking the Kasha’s rule. With core/crown/crown CdSe/CdTe/CdSe NPLs composed at 95% of CdSe, a dual emission is obtained where a green emission arises from the recombination of the exciton through the band edge in 4 ML CdSe NPLs while a red emission arises from the type II band alignment between CdSe and CdTe. To improve their quantum yield a shell needs to be grown however this also leads to a red shift of their optical features. So, we propose a new heterostructure with blue-shifted dual emission which shell growth induces two emissions in a range with potential applications in displays. These NPLs can be introduced in light emitting diodes which exhibit dual NPLs as in the active materials.

The direct wet-chemical synthesis of two-dimensional (2D) lead chalcogenide semiconductors yields photoluminescent materials with strong excitonic contribution at room temperature. ^[1,2,3] In the spirit of the recent Nobel prize for the discovery and synthesis of quantum dots (QDs) we report herein on our studies of strongly confined wet-chemically synthesized flat lead selenide (PbSe) QDs. These 2D nanocrystals possess dimensions of e.g. 6 x 5 x 0.8 nm³ and exhibit PL in the near-infrared region between 860 – 1510 nm with a PL quantum yield of up to 60 %, which is mainly determined by their lateral dimensions. ^[1,2] Their highly efficient photoluminescence (PL) at fiber-optics-relevant telecommunication wavelengths renders colloidal lead chalcogenide 2D semiconductors intriguing materials for future solution-processable optics.

Scanning tunnelling spectroscopy of single PbSe NCs reveals a conduction and valence band density of states that is typical for QDs rather than a steplike function linked to 2D nanoplatelets and substantiates the strong confinement in the flat PbSe QDs. These experimental observations are substantiated by theoretical calculations of the electronic band structure using the tight-binding approach.

Our results paint a comprehensive picture of the optical and electronic properties of near-infrared active 2D PbSe QDs.

The discovery of the atomically flat two-dimensional cadmium chalcogenide nanoplatelets (NPLs) is considered to be one of the notable milestones in the field of semiconductor nanocrystals, due to their remarkable electronic and optical features and anisotropic shape. One of the important synthetic aspects of such NPLs is the control over their thickness and lateral dimensions, since they determine NPLs’ optical properties and surface area, which, in turn, are crucial parameters for photovoltaic devices. At the same time, despite great progress in new synthesis methods, the production of NPLs with a thickness larger than 5 monolayers (MLs) still remains a challenging task and thus the optical range covered by absorption and emission bands remains limited, which hinders the wider use of planar semiconductor NPLs.

In this work, we demonstrate straightforward methods to prepare six and seven ML-thick zinc-blende CdSe nanoplatelets (NPLs). The synthesized laterally small CdSe NPLs show emission maxima at 582 nm (6 ML) and 596 nm (7 ML), respectively. Moreover, our study shows that CdS crowns can be grown on both six and seven ML-thick CdSe NPLs, as evidenced by the increase of their lateral size and the appearance of new absorption features that correlate with the absorption features of CdS NPLs of the same thickness. Our novel and optimized synthesis procedures and post-synthetic crown growth demonstrate the direct fabrication of 6+ ML thick CdSe NPLs and their core/crown alternatives, allowing for further investigation of these mostly unstudied two-dimensional (2D) nanomaterials.

Macroscopic Bi₂Se₃ is a layered material consisting of a high number (>10)  Se/Bi/Se/Bi/Se quintuple layers, separated by van der Waals gaps. The material has been characterized as a topological quantum spin Hall insulator, with an inverted bandgap of 300 meV, and protected helical surface states. Now we consider the high-energy electron-hole excitations involving bands below - and further above theFermi-level: In bulk Bi2Se3, the absorption spectrum shows intriguing optical transitions in the 0.5 – 3.5 eV energy region: a strong optical transition at 2.8 eV located at the surface, followed by luminescence in the region of 2.3 eV, preserving chirality (1,2).

We used colloidal two-dimensional Bi2Se3 nanoplatelets as a model system to monitor the transition from bulk Bi2Se3 to two-dimensional Bi2Se3, to finally a single quintuple layer. We studied the optical transitions in the 1-3 eV region with absorption and transient absorption quenching spectroscopy (3). We observe more than 10 strong optical transitions. Comparison with DFT-GW theory allowed us to identify these transitions, group them as surface and interior transitions, and locate them in the two-dimensional Brillouin zone. In the time region below 10 ps, the electron and hole cooling and recombination dynamics are very specific for the bands involved, with in some cases signatures of electron-hole separation in momentum space by cooling.   

1. Observation of chiral surface excitons in a topological insulator Bi2Se₃. Proceedings of the National Academy of Sciences of the United States of America 116, 4006-4011 (2021)

2. Complex optical conductivity of Bi2Se3 thin film: Approaching two-dimensional limit Applied Physics Letters 118, DOI: 10.1063/5.0049170 (2021)

3. Identification of high-energy excitations in two-dimensional topological Bi2Se3 platelets. Jara Vliem, Ricardo Reho, Servet Ataberk Cayan*, Pieter Geiregat*, Zeila Zanolli, and D. Vanmaekelbergh

Quasi-two-dimensional (2D) semiconductor nanocrystals (NCs) are captivating due to their improved optical properties, as compared to their spherical counterparts, such as extremely narrow emission line widths, suppressed Auger recombination, and giant oscillator strength. However, procedures for the direct synthesis of 2D nanoplatelets (NPLs) are not yet developed for a broad range of materials, and even well understood systems, such as CdSe, are still limited due to limited thicknesses that are readily available. At the same time, extending the optical properties into the near-infrared region (NIR) is desirable for applications in telecommunications, photovoltaics, and photodetectors, where light sources with photoluminescence (PL) inside the optical windows of glass fibers, as well as active light-absorbing and charge-transporting layers are necessary to produce high-performance components at the low cost associated with solution-processing techniques. To capitalize on the research success of the CdSe system, one can employ postsynthetic cation exchange reactions to obtain narrow band-gap materials such as lead and mercury chalcogenides, which show optical activity in the NIR. Control of the chemical composition as well as the size of the NPLs allows for broad and fine tuning of their absorption and emission spectra.

In this presentation, our recent results on the synthesis, characterisation, and application of NIR NPLs will be summarized. Two main cation exchange reactions Cd²⁺-to-Hg²⁺ and -to-Pb²⁺ have been employed to synthesize corresponding Cd_xHg_1–xSe and PbSe NPLs. Already small inclusions of mercury ions into template CdSe NPLs result in a pronounced shift of their PL to longer wavelengths.[1] A more extended partial Cd²⁺-to-Hg²⁺ cation exchange yields Cd_xHg_1–xSe NPLs emitting in the range of 700–1100 nm with quantum yields reaching 55%.[2] The resulting NPLs possess broad PL spectra due to inhomogeneous distribution of HgSe domains within CdSe cores. By optimizing the synthesis conditions, we achieved much narrower PL tunable from 1300 to 1500 nm with bandwidths down to 102 meV for Cd_xHg_1–xSe/Zn_yCd_1–yS core/shell NPLs, thus, covering the first and the second telecommunication windows.[3] A further shift of the PL can be realised by complete Cd²⁺-to-Pb²⁺ cation exchange, depending on the thickness of initial CdSe NPLs.[4] Resulting PbSe NPLs exhibit extremely narrow PL spectra with bandwidths down to 80 meV with quantum yields reaching 15%. Thus synthesized NPLs were successfully applied in photodetectors [5] and efficient NIR LEDs [3].

Quantum dots are nanometer-sized crystallites of semiconductor that have a roughly spherical shape. Due to extensive research, quantum dots are now commercially used as a robust fluorescent material in displays and lighting. However, even with our best procedures, state-of-the-art samples still contain particles with a distribution in size and shape. Because this causes variations in their optical properties, their performance for applications is reduced. This leads to a fundamental question: can we achieve a sample of semiconductor nanocrystals in which all the particles are exactly the same? In this talk we will discuss this possibility by examining two classes of nanomaterials. First, we will consider thin rectangular particles known as semiconductor nanoplatelets. Amazingly, nanoplatelet samples can be synthesized in which all crystallites have the same atomic-scale thickness (e.g., 4 monolayers). This uniformity in one dimension suggests that routes to monodisperse samples might exist. After describing the underlying growth mechanism for nanoplatelets, we will then move to a much older nanomaterial—magic-sized clusters (MSCs). Such species are believed to be molecular-scale arrangements (i.e., clusters) of semiconductor atoms with a specific (“magic”) structure with enhanced stability compared to particles slightly smaller or larger. Their existence implies that MSC samples can in principle be the same size and shape. Unfortunately, despite three decades of research, the formation mechanism of MSCs remains unclear, especially considering recent experiments that track the evolution of MSCs to sizes well beyond the “cluster” regime. Again, we will discuss the underlying growth mechanism and its implications for nanocrystal synthesis. Finally, we will present an outlook if perfect nanomaterials can be obtained.

Semiconductor nanocrystals (SNCs) have become the most important material for colloidal nanophotonics – a rapidly advancing research field that evolves into a powerful technological platform for lighting, biomedicine, lasing, photovoltaics, etc. Over the years major strides in this field were made for II-VI and IV-VI SNCs, due to the availability of precursors and relative simplicity of synthetic protocols for obtaining high-quality SNCs with varying shape, tunable composition and structure, and surface properties. However recently their attractiveness was severely undercut by tightening restrictions on the use of elements comprising such crystals (e.g. Cd, Pb and Hg) due to their toxicity. This highlights the importance of alternative materials among which III-V semiconductors stand out: The nature of their composing elements is relatively benign and they exhibit improved chemical stability due to a higher covalent bond share in the materials.

Although the first syntheses of III-V colloidal NCs can be traced back to the early 90s, their development was much slower than that of their II-VI and IV-VI counterparts, and many unresolved challenges still exist. One such attractive research avenue is the realization of shape control III-V NCs, in particular the synthesis of two-dimensional (2D) nanoplatelets (NPls). The interest in such nanocrystals is mainly driven by the studies of cadmium chalcogenide NPls which showed that they exhibit much superior properties to their counterparts with other shapes, i.e., among others, narrow emission and absorbance bands, high absorption coefficients directed emission, etc. Regardless of the recent advancements in understanding the driving forces of the directed growth of cadmium chalcogenide NPls [1,2], the proposed models have limited predictive power and currently, it is impossible to straightforwardly use or adapt existing protocols to the direct synthesis of III-V NPls.

In this work, we investigate a more general indirect approach consisting in the Cu-for-In cation exchange in pre-synthesised Cu_3-xP NPls. This approach benefits from the inherent structural anisotropy of Cu_3-xP driving their 2D growth into NPls, which can subsequently serve as templates for InP NPls. In addition, this approach can be potentially further expanded to other indium pnictides as well as a broad range of heterostructured NPls, which are hard or impossible to obtain using direct synthesis. A few reports have already demonstrated the possibility of cation exchange in Cu_3-xP, however, they mostly focused on the process itself and have not yielded InP NPls with pronounced absorbance features and noticeable emission, which was related to the excessive amounts of the residual copper atoms. [3,4] In this work, we further optimize the cation exchange conditions, such as the effect of ligands, and complexing additives to tune the balance between incoming and outgoing cations, which is important to avoid a considerable amount of vacancies and domains that act as centers for nonradiative recombination of charge carriers.

MXenes have recently shown promising properties in a variety of device applications. In this talk, I will present recent results on using MXenes as gate materials in thin film transistors using MoS₂ and GaN semiconductor channels. Gate controllability is a key factor that determines the performance of GaN high electron mobility transistors (HEMTs). However, at the traditional metal-GaN interface, the direct chemical interaction between metal and GaN can result in fixed charges and traps, which can significantly deteriorate the gate controllability. We show that Ti₃C₂T_x MXene films integrated into GaN HEMTs as the gate contact induce van der Waals heterojunctions between MXene films and GaN without direct chemical bonding. The GaN HEMTs with enhanced gate controllability exhibited an extremely low off-state current (I_OFF) of 10⁻⁷ mA/mm, a record high I_ON/I_OFF current ratio of ~10¹³ (which is six orders of magnitude higher than conventional Ni/Au contact), a high off-state drain breakdown voltage of 1085 V, and a near-ideal subthreshold swing of 61 mV/dec. However, MXenes do not fare as well when used as contacts in MoS₂ transistors. The origin in this difference will be discussed in the  context of the interface between MXene and GaN and MoS₂ channel materials.

We studied excitons, charge carriers and many-body complexes thereof in CdSe nanoplatelets with thickness of a few atomic layers and lateral sizes of tens of nanometers. Excitons and charge carriers were generated by photoexcitation with ultrashort laser pulses and detected by time-resolved optical absorption and terahertz conductivity measurements [1].

The shape of photoluminescence and absorption spectra varies with the lateral sizes of the nanoplatelets. We attribute this to quantum-confinement effects on the center-of-mass motion of excitons in the plane of the nanoplatelets. The spectra can be reproduced by a theoretical description of excitons based on the quantum mechanical particle-in-a-box model [2].

The initial photogeneration quantum yields of free charge carriers versus excitons were found to increase with photon energy [3]. Biexcitons and trions were observed due to formation of an exciton by a probe photon near an already present exciton or charge carrier left after the pump laser pulse. Initially hot excitons and charges were found to relax to the same energy distribution in about one picosecond for different pump photon energies. We found that excitons are stable even at high densities where they start to exhibit spatial overlap. A crossover to an electron-hole plasma of uncorrelated free electrons and holes was not observed. This counter intuitive result can be understood theoretically from the fact that the Coulomb screening length, and thus the exciton binding energy, remain non-zero even at high density [4,5].

References

[1] R. Tomar et al., J. Phys. Chem. C 123, 9640 (2019).

[2] M. Failla et al., Phys. Rev. B., 102, 195405 (2020).

[3] M. Failla et al., J. Phys. Chem. C, 127, 1899 (2023).

[4] F. García Flórez et al., Phys. Rev. B 100, 245302 (2019).

[5] F. García Flórez et al., Phys. Rev. B 102, 115302 (2020).

Transition metal dichalcogenides (TMDCs) such as MoS₂ and WS₂ are well studied van der Waals materials forming stable layers without dangling bonds.
Recently, we obtained colloidal, phase-pure mono- and few-layered semiconducting MoS₂ and WS₂ nanoplatelets (NPLs).[1] The wet-chemical approach allows for the formation of scalable amounts of TMDC NPLs with different lateral sizes, each with a highly monodisperse size distribution.[1,2] By confining the 2D NPLs not only in z-direction but also in the lateral dimension, the excitonic features can be shifted to higher energy, allowing a limited extend of band tuning.[1] Expanding on these findings, we show, that besides changing the morphology of the NPLs, it is possible to further tune the band structure of colloidal TMDCs via the composition of MoS₂ and WS₂ content in the structure. The study of the optical transitions shifting as a function of the elemental composition is supported by microscopic and macroscopic structural information from scanning transmission electron spectroscopy (STEM) and Raman spectroscopy. This wet chemical synthesis protocol is designed to be adapted also for other combinations of TMDCs. Alloying is the next step for tailoring colloidal TMDCs towards optoelectronics, catalysis and sensing.[3]

[1] Niebur, A.; Söll, A.; Haizmann, P.; Strolka, O.; Rudolph, D.; Tran, K.; Renz, F.; Frauendorf, A.; Hübner, J.; Peisert, H.; Scheele, M.; Lauth, J. Untangling the intertwined: metallic to semiconducting phase transition of colloidal MoS2 nanoplatelets and nanosheets. Nanoscale, 2023,15, 5679-5688.
[2] Frauendorf, A.; Niebur, A.; Harms, L.; Shree, S.; Urbaszek, B.; Oestreich, M.; Hübner, J.; Lauth, J. Room Temperature Micro-Photoluminescence Studies of Colloidal WS2 Nanosheets. J. Phys. Chem. C 2021, 125, 34, 18841–18848.
[3] Xie, L. Two-dimensional transition metal dichalcogenide alloys: preparation, characterization and applications. Nanoscale, 2015,7, 18392-18401.

Optical studies of colloidal CdSe nanoplatelets at cryogenic temperatures (typically 1.7–4 K) and in high magnetic fields (typically above 10 T) provide valuable information about their emission properties. Importantly, magnetooptics allows for distinguishing the emission from excitons and trions (charged excitons). These particles have very different photoluminescence kinetics, additionally, their emission lifetimes are affected differently by magnetic fields and temperatures [1]. The exciton fine structure can be visualized by time-resolved polarized photoluminescence and fluorescence line narrowing, and exciton binding energy can be obtained by measuring one- and two-photon absorption [2]. The trion charge, positive or negative, can be identified by studying the photoluminescence polarization in magnetic fields [1].

Interestingly, the surface spins in CdSe nanoplatelets at cryogenic temperatures can be detected by optical techniques [3]. In applied magnetic field, the surface spins orientation results in large Zeeman splitting for the exciton interacting with these spins, similar to the known effect of the giant Zeeman splitting in diluted magnetic semiconductors, like (Cd,Mn)Se. The circular polarization degree about 50% in a magnetic field of 3 T is observed in bare CdSe nanoplatelets.

Dopant emission in colloidal ZnX nanoplatelets

Ultrathin, atomically flat, two-dimensional semiconductor nanocrystals receive a rapidly increasing attention due to their unique physicochemical properties. I will show that ZnS and ZnSe nanoplatelets with low toxicity exhibit sharp excitonic absorption and narrow excitonic emission. Further, I will demonstrate that direct manganese doping leads to tunable emission and photoluminescence lifetime. The initially low dopant PL quantum yield can be dramatically enhanced by passivating the surface trap states of the samples. Using time-resolved PL spectroscopy and density functional theory calculations, a connection between (magnetic) coupling and PL kinetics of Mn ions can be established. Using a descriptive mathematical model, we recognized the dominant role of the Mn2+ nonradiative relaxation channels in the energy-transfer route from the matrix absorption to the luminescence of Mn2+ ions.

We believe that the presented doping strategy and simulation methodology of the Mn-doped ZnS system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors and might be an interesting system for catalysis.