Tellurium Precursor for Nanocrystal Synthesis: Tris(dimethylamino)phosphine Telluride

Nucleation and Growth of Monodisperse CdTe and CdTe/ZnSe Core/shell Nanocrystals: Roles of Cationic Precursors, Ligands, and Solvents

With CdTe nanocrystals as a model system, we discover a new synthetic strategy for control of size and size distribution of colloidal semiconductor nanocrystals in both nucleation and growth stages. Especially in the nucleation stage, an in situ-formed cadmium complex with approximately one alkanoate and one alkylphosphonate ligand enables both high-yield nucleation by reacting the reactive cadmium-carboxylate bond with Te precursors and efficient size control by immediate passivation with the close-proximity alkylphosphonate ligand from the same complex. Conversely, control on size distribution during either homoepitaxial or heteroepitaxial growth requires reactive cadmium (or zinc) alkanoates as the cationic precursors with a minimum concentration of alkylphosphonate ligands in the novel synergistic solvents. This new strategy not only yields monodisperse CdTe and CdTe/ZnSe core/shell nanocrystals with unprecedented optical quality but also provides a much-needed alternative route for synthesizing monodisperse semiconductor nanocrystals, which is commonly hindered by the growth barrier of the dense ligand monolayer.

Heteroconfinement in Single CdTe Nanoplatelets

Dimension-engineered synthesis of atomically thin II-VI nanoplatelets (NPLs) remains an open challenge. While CdSe NPLs have been made with confinement ranging from 2 to 11 monolayers (ML), CdTe NPLs have been significantly more challenging to synthesize and separate. Here we provide detailed mechanistic insight into the layer-by-layer growth kinetics of the CdTe NPLs. Combining ensemble and single-particle spectroscopic and microscopic tools, our work suggests that beyond 2 ML CdTe NPLs, higher ML structures initially appear as heteroconfined materials with colocalized multilayer structures. In particular, we observe strongly colocalized 3 and 4 ML emissions, accompanied by a broad trap emission. Accompanying transient absorption, single-particle optical, and atomic force microscopy analyses suggest islands of different MLs on the same NPL. To explain the nonstandard nucleation and growth of these heteroconfined structures, we simulated the growth conditions of NPLs and quantified how the monomer binding energy modifies the kinetics and permits single NPLs with multi-ML structures. Our findings suggest that the lower bond energy associated with CdTe relative to CdSe limits higher ML syntheses and explains the observed differences between CdTe and CdSe growth.

Revealing Anion Exchange in Two-Dimensional Nanocrystals

Ion exchange is a powerful postsynthesis tool for the design of functional nanomaterials. However, achieving anion exchange while maintaining the original morphology and crystal structure, as well as elucidating the mechanism, remains challenging. Here, we developed an anion-exchange strategy under mild conditions and revealed an unusual ion-exchange mechanism in the semiconductor nanoplatelets. Kinetic studies have demonstrated that the transformation follows first-order kinetics, with the ligand restricting the guest anion from diffusing only in one-dimensional directions. By monitoring the reaction process, we demonstrated that the anion exchange reaction occurs selectively on the polar surface of the NPLs and exhibits asymmetry at the two polar end faces. Theoretical simulations further confirmed that anion exchange began from the chalcogenide-dominated facet. The thermodynamic data suggest that guest ions diffuse into the crystal interior via a direct exchange mechanism. This study provides a pathway for anion exchange and the construction of functional nanocrystals and a platform for studying the optoelectronic behavior of single-sheet heterojunctions.

Alloyed Zinc Chalcogenide Magic-Sized Nanoclusters and Their Transformation to Alloyed Quantum Dots

Alloying provides the opportunity to widen the physical and chemical properties of quantum dots (QDs); however, the precise controlled composition of alloyed QDs is still a challenge. In this work, a few quaternary alloyed zinc chalcogenide magic-sized nanoclusters (MSCs) were synthesized using the active chalcogen precursors of tri(dimethylamine)phosphine chalcogen, such as Zn21S4Se3Te4 (MSCs-348), Zn14S4Se4Te7 (MSCs-350), Zn15S1Se4Te6 (MSCs-349), and Zn17S2Se2Te7 (MSCs-355) MSCs. The composition of alloyed zinc chalcogenide MSCs was tuned with the different amounts of added chalcogen precursors. Finally, the produced alloyed zinc chalcogenide MSCs can be used as precursors to synthesize alloyed zinc chalcogenide QDs, and the composition of zinc chalcogenide QDs can be adjusted with different alloyed MSCs. This work provides methods to alloy MSCs with controlled composition, providing efficient precursors for alloyed QDs.

Organoborane Se and Te Precursors for Controlled Modulation of Reactivity in Nanomaterial Synthesis

To exploit the distinctive optoelectrical properties of nanomaterials, precise control over the size, morphology, and interface structure is essential. Achieving a controlled synthesis demands precursors with tailored reactivity and optimal reaction temperatures. Here, we introduce organoborane-based selenium and tellurium precursors borabicyclononane-selenol (BBN-SeH) and tellurol (BBN-TeH). The reactivity of these precursors can be modified by commercially available additives, covering a wide range of intermediate reactivity and filling significant reactivity gaps in existing options. By allowing systematic adjustment of growth conditions, they achieve the controlled growth of quantum dots of various sizes and materials. Operating via a surface-assisted conversion mechanism, these precursors rely on surface coordination for activation and undergo quantitative deposition on coordinating surfaces. These properties allow precise control over the radial distribution and density of different chalcogenide atoms within the nanoparticles. Diborabicyclononanyl selane ((BBN)2Se), an intermediate from the BBN-SeH synthesis, can also serve as a selenium precursor. While BBN-SeH suppresses nucleation, (BBN)2Se exhibits efficient nucleation under specific conditions. By leveraging these distinct activation behaviors, we achieved a controlled synthesis of thermally stable nanoplates with different thicknesses. This study not only bridges critical reactivity gaps but also provides a systematic methodology for precise nanomaterial synthesis.

Understanding the Growth Mechanism of HgTe Colloidal Quantum Dots through Bilateral Injection

As potential low-cost alternatives of traditional bulk HgCdTe crystals, HgTe colloidal quantum dots (CQDs) synthesized through reactions between HgCl2 and trioctylphosphine-telluride in hot oleylamine have shown promising performances in mid-wave infrared photodetectors. Tetrapodic or tetrahedral HgTe CQDs have been obtained by tuning the reaction conditions such as temperature, reaction time, concentrations, and ratios of the two precursors. However, the principles governing the growth dynamics and the mechanism behind the transitions between tetrapodic and tetrahedral HgTe CQDs have not been sufficiently understood. In this work, synthesis of HgTe CQDs through bilateral injection is introduced to study the growth mechanism. It suggests that tetrahedral HgTe CQDs usually result from the breaks of tetrapodic HgTe CQDs after their legs grow thick enough. The fundamental factor determining whether the growth makes their legs longer or thicker is the effective concentration of the Te precursor during the growth, rather than temperature, Hg-rich environment, or reactivity of precursors. A chemical model is proposed to illustrate the principles governing the growth dynamics, which provides valuable guidelines for tuning the material properties of HgTe CQDs according to the needs of applications.

Atomically flat semiconductor nanoplatelets for light-emitting applications

The last decade has witnessed extensive breakthroughs and significant progress in atomically flat two-dimensional (2D) semiconductor nanoplatelets (NPLs) in terms of synthesis, growth mechanisms, optical and electronic properties and practical applications. Such NPLs have electronic structures similar to those of quantum wells in which excitons are predominantly confined along the vertical direction, while electrons are free to move in the lateral directions, resulting in unique optical properties, such as extremely narrow emission line width, short photoluminescence (PL) lifetime, high gain coefficient, and giant oscillator strength transition (GOST). These unique optical properties make NPLs favorable for high color purity light-emitting applications, in particular in light-emitting diodes (LEDs), backlights for liquid crystal displays (LCDs) and lasers. This review article first introduces the intrinsic characteristics of 2D semiconductor NPLs with atomic flatness. Subsequently, the approaches and mechanisms for the controlled synthesis of atomically flat NPLs are summarized followed by an insight on recent progress in the mediation of core/shell, core/crown and core/crown@shell structures by selective epitaxial growth of passivation layers on different planes of NPLs. Moreover, an overview of the unique optical properties and the associated light-emitting applications is elaborated. Despite great progress in this research field, there are some issues relating to heavy metal elements such as Cd²⁺ in NPLs, and the ambiguous gain mechanisms of NPLs and others are the main obstacles that prevent NPLs from widespread applications. Therefore, a perspective is included at the end of this review article, in which the current challenges in this stimulating research field are discussed and possible solutions to tackle these challenges are proposed.

Growth kinetics determine the polydispersity and size of PbS and PbSe nanocrystals

A library of thio- and selenourea derivatives is used to adjust the kinetics of PbE (E = S, Se) nanocrystal formation across a 1000-fold range (k r = 10-1 to 10-4 s-1), at several temperatures (80-120 °C), under a standard set of conditions (Pb : E = 1.2 : 1, [Pb(oleate)2] = 10.8 mM, [chalcogenourea] = 9.0 mM). An induction delay (t ind) is observed prior to the onset of nanocrystal absorption during which PbE solute is observed using in situ X-ray total scattering. Density functional theory models fit to the X-ray pair distribution function (PDF) support a Pb2(μ2-S)2(Pb(O2CR)2)2 structure. Absorption spectra of aliquots reveal a continuous increase in the number of nanocrystals over more than half of the total reaction time at low temperatures. A strong correlation between the width of the nucleation phase and reaction temperature is observed that does not correlate with the polydispersity. These findings are antithetical to the critical concentration dependence of nucleation that underpins the La Mer hypothesis and demonstrates that the duration of the nucleation period has a minor influence on the size distribution. The results can be explained by growth kinetics that are size dependent, more rapid at high temperature, and self limiting at low temperatures.

Laser-assisted micropyramid patterned PDMS encapsulation of 1D tellurium nanowires on cellulose paper for highly sensitive strain sensor and its photodetection studies

This work demonstrates the fabrication of tellurium-nanowires (Te-NWs)/paper based device encapsulated using laser assisted mircopyramid patterned polydimethylsiloxane (PDMS) films. Although there are multiple reports published on 1D Te, most of them are limited to establishing its properties and studying its behavior as a sensor and research on the utilization of Te-NWs for physical sensors remain unexplored. Further, reports on p-type photodetectors also remain scarce. The fabricated Te-NWs/paper with micropyramid structured PDMS films encapsulation was used as a strain sensor, and it exhibited considerable improvement (∼60%) in sensitivity compared to smooth PDMS films. The gauge factor of the developed strain sensor was found to be ∼15.3. In addition, fabricated Te-NWs/paper device with contacts was used as a photodetector and it showed photoresponsivity of ∼22.5 mA W^-1and ∼14.5 mA W^-1in visible and NIR regions, respectively. Furthermore, the device exhibited long-term mechanical stability under harsh deformations. Fabricated 1D Te-NWs/paper device was utilized as a strain sensor to monitor the angular movements in the human body and successfully monitored various human motions, including wrist bending, finger knuckle, elbow joint, and knee joint. The successful demonstration of Te-NWs based physical sensors and utilization in broadband photodetectors opens avenues of research for tellurium based flexible and wearable devices.

Synthesis of Atomically Thin CdTe Nanoplatelets by Using Polytelluride Tellurium Precursors

Colloidal two-dimensional (2D) semiconductor nanocrystals are of great importance due to their remarkable optical and electronic properties. Herein, shape-controllable synthesis of 2D wurtzite CdTe nanoplatelets (NPLs) by simply tailoring the reactivity of a tellurium (Te) precursor is reported. Ribbon-, shield-, and bullet-like 2D CdTe NPLs were prepared by a stepwise conversion from CdTe magic-size nanoclusters (MSNCs) by using Te32–, Te22–, and Te2– polytellurides as the tellurium precursor, respectively. This work not only develops a synthetic strategy capable of synthesising wurtzite CdTe nanoplatelets with controlled shapes by tailoring the reactivity of tellurium precursors but also gives insights into the growth mechanisms of colloidal 2D semiconductor nanocrystals.

Atomically thin heavy-metal-free ZnTe nanoplatelets formed from magic-size nanoclusters

Atomically thin colloidal quasi-two-dimensional (2D) semiconductor nanoplatelets (NPLs) have attracted tremendous attention due to their excellent properties and stimulating applications. Although some advances have been achieved in Cd- and Pb-based semiconductor NPLs, research into heavy-metal-free NPLs has been reported less due to the difficulties in the synthesis and the knowledge gap in the understanding of the growth mechanism. Herein wurtzite ZnTe NPLs with an atomic thickness of about 1.5 nm have been successfully synthesized by using Superhydride (LiEt₃BH) reduced tributylphosphine-Te (TBP-Te) as the tellurium precursor. Mechanistic studies, both experimentally and theoretically, elucidate the transformation from metastable ZnTe MSC-323 magic-size nanoclusters (MSCs) to metastable ZnTe MSC-398, which then forms wurtzite ZnTe NPLs via an oriented attachment mechanism along the [100] and [002] directions of the wurtzite structure. This work not only provides insightful views into the growth mechanism of 2D NPLs but also opens an avenue for their applications in optoelectronics.

Core-Shell Cadmium Telluride Quantum Platelets with Absorptions Spanning the Visible Spectrum

CdS and CdSe shells are deposited on wurtzite CdTe quantum platelets (nanoplatelets) by exchanging the initial primary-amine ligation to Cd(OAc)₂ ligation, with subsequent reaction of the Cd(OAc)₂ ligand shell and thiourea or selenourea, respectively. Shell deposition is conducted in a cyclic manner, with 0.21-0.34 monolayers of CdS and 0.99-1.20 monolayers of CdSe being deposited in each cycle. The CdTe quantum platelets having an initial thickness of 1.9 nm are converted to CdTe-CdS and CdTe-CdSe core-shell quantum platelets having maximum thicknesses of 3.0 and 6.3 nm, respectively. The morphologies and wurtzite structure of the initial CdTe quantum platelets are retained upon shell deposition. The absorption spectrum of the CdTe quantum platelets is progressively shifted to lower energy with increasing shell thickness, across the entire visible spectrum. The spectral shifts observed scale with the inverse square of the total core-shell thickness.

Why Do Colloidal Wurtzite Semiconductor Nanoplatelets Have an Atomically Uniform Thickness of Eight Monolayers?

Herein we employed a first-principles method based on density functional theory to investigate the surface energy and growth kinetics of wurtzite nanoplatelets to elucidate why nanoplatelets exhibit a uniform thickness of eight monolayers. We synthesized a series of wurtzite nanoplatelets (ZnSe, ZnS, ZnTe, and CdSe) with an atomically uniform thickness of eight monolayers. As a representative example, the growth mechanism of 1.39 nm thick (eight monolayers) wurtzite ZnSe nanoplatelets was studied to substantiate the proposed growth kinetics. The results show that the growth of the seventh and eighth layers along the [112̅0] direction of 0.99 nm (six monolayers) ZnSe magic-size nanoclusters is accessible, whereas the growth of the ninth layer is unlikely to occur because the formation energy is large. This work not only gives insights into the synthesis of atomically uniform thick wurtzite semiconductor nanoplatelets but also opens up new avenues to their applications in light-emitting diodes, catalysis, detectors, and lasers.