Dopants in semiconductor nanostructures offer tremendous control over electronic, optical, and magnetic properties beyond what is achievable in bulk materials. We demonstrate that the broad dopant emission in semiconductor nanoplatelets effectively maps the electron wave function across the nanoplatelet thickness. Both the emission energy and lifetime of the dopant transition depend strongly on the depth of the dopant within the nanoplatelet. This dependence arises from the electrostatic self-interaction of the charged dopant, which varies with proximity to the dielectric discontinuity at the nanoplatelet surface. Through comprehensive single-particle spectroscopy of silver-doped CdSe nanoplatelets, we verify that acceptors near the center emit at higher energies with shorter lifetimes, while those near the surface emit at lower energies with longer lifetimes. This spatial mapping also reveals unusual two-color emission from individual nanoplatelets, with enhanced Auger recombination yielding exceptional photon antibunching (>90% purity) at room temperature, suggesting potential applications in quantum information technologies.

The growth of atomically flat CdSe nanoplatelets (NPLs) thicker than 5 monolayers (ML) remains a major challenge in colloidal semiconductor synthesis, particularly for core/crown heterostructures. Here we report the successful synthesis of zinc-blende CdSe NPLs with unprecedented thicknesses of 6 and 7 ML, exhibiting sharp photoluminescence at 579 and 596 nm, respectively. We demonstrate that these thick NPLs can serve as cores for CdSe/CdS core/crown heterostructures, confirmed by lateral size expansion and the emergence of characteristic CdS absorption features. Through density-functional theory calculations, we uncover a critical relationship between NPL shape and crown growth: fluoride species bind three times stronger to {100} facets of rectangular NPLs compared to {110} facets of square NPLs, effectively poisoning crown growth on rectangular species. This mechanistic insight explains the shape-dependent success of crown formation and provides a framework for controlling two-dimensional (2D) semiconductor growth. Our optimized synthesis of thick core NPLs and their crown-enhanced derivatives significantly expands the spectral range of CdSe-based NPLs, advancing their development as narrow-line width red emitters.

Atom manipulation by scanning tunneling microscopy was used to construct quantum dots on the InAs(111)A surface. Each dot comprised six ionized indium adatoms. The positively charged adatoms create a confining potential acting on surface-state electrons, leading to the emergence of a bound state associated with the dot. By lining up the dots into N-dot chains with alternating tunnel coupling between them, quantum-dot molecules were constructed that revealed electronic boundary states as predicted by the Su-Schrieffer-Heeger (SSH) model of one-dimensional topological phases. Dot chains with odd N were constructed such that they host a single end or domain-wall state, allowing one to probe the localization of the boundary state on a given sublattice by scanning tunneling spectroscopy. We found probability density also on the forbidden sublattice together with an asymmetric energy spectrum of the chain-confined states. This deviation from the SSH model arises because the dots are charged and create a variation in onsite potential along the chain - which does not remove the boundary states but shifts their energy away from the midgap position. Our results demonstrate that topological boundary states can be created in quantum-dot arrays engineered with atomic-scale precision.

Doping can tailor the electronic, optical, and magnetic properties of colloidal nanocrystals for optimal performance in devices. We present a novel synthesis of copper (Cu)-doped PbS colloidal nanocrystals based on Cu-to-Pb cation exchange. These nanocrystals exhibit both a blueshift of the energy of the lowest exciton peak, relative to undoped PbS nanocrystals, and photoluminescence. This blueshift of the lowest exciton energy is unique, as only redshifts are observed in Cu-doped II-VI and III-V nanocrystals. Furthermore, retention of the strong infrared photoluminescence is noteworthy because the photoluminescence in other doped PbS nanocrystals is quenched. Three possible reasons for these results are discussed. Cu-doping is confirmed using inductively coupled plasma optical emission spectroscopy (ICP-OES), in addition to high-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS). Coupled with photoluminescence quantum yields (PLQYs) comparable to undoped PbS, the shorter photoluminescence lifetimes measured from time-correlated single photon counting (TCSPC) of the Cu-doped PbS nanocrystals show an increase in the radiative and nonradiative rates by 3-4x as compared to pure oleate-capped PbS nanocrystals. These improved radiative rates could lead to brighter nanocrystal infrared-emitting devices such as single-photon emitters.

Semiconductor nanoplatelets are atomically flat nanocrystals which emit light with high spectral purity at wavelengths controlled by their thickness. Despite their technological potential, efforts to further sharpen the emission lines of nanoplatelets have generally failed for unknown reasons. Here, we demonstrate theoretically that the linewidth is controlled by surface chemistry-specifically, inhomogeneities in the ligand layer on the nanoplatelet surface lead to a spatially fluctuating potential that localizes excitons. This localization leads to increased scattering and optical broadening. Importantly, localization also reduces the rate of radiative emission. Our model explains the observed linewidth and predicts that a more uniform ligand layer will sharpen the lines and increase the emission rates. These findings demonstrate that light emission from nanoplatelets can be controlled by optimizing their surface chemistry, an important advantage for their eventual use in optical technologies. Semiconductor nanoplatelets emit light in narrow spectral ranges. Here, the authors establish a theoretical model showing this behavior is controlled by the inhomogeneities of the ligand layer on the nanoplatelet surface.

Cryogenic scanning tunneling microscopy was employed in combination with density-functional theory calculations to explore quantum dots made of In adatoms on the InAs(110) surface. Each adatom adsorbs at a surface site coordinated by one cation and two anions, and transfers one electron to the substrate, creating an attractive quantum well for electrons in surface states. We used the scanning-probe tip to assemble the positively charged adatoms into precisely defined quantum dots exhibiting a bound state roughly 0.1 eV below the Fermi level at an intrinsic linewidth of only ~4 meV, as revealed by scanning tunneling spectroscopy. For quantum-dot dimers, we observed the emergence of a bonding and an antibonding state with even and odd wave-function character, respectively, demonstrating the capability to engineer quasi-molecular electronic states. InAs(110) constitutes a promising platform in this respect because highly perfect surfaces can be readily prepared by cleavage and charged adatoms can be generated in-situ by the scanning-probe tip.

Cadmium chalcogenide nanoplatelets (NPLs) are established as promising materials for a wide variety of optoelectronic applications due to their properties surpassing in many aspects counterpart nanocrystals (NCs) with other shapes. Most of these features arise from strong quantum confinement in the direction of thickness which can be tuned with precision down to one monolayer. However, atomic smoothness of their basal planes and hence the ability to change the NPL thickness only in discrete steps prevent precise tuning of absorption and photoluminescence spectra unlike in the case of quantum dots. Preparation of alloyed NCs provides a potential solution to this problem, but it is complicated by the different reactivity of chalcogenide sources, which becomes even more restrictive in the case of NPLs, since they are more sensitive to alterations of reaction conditions. In this work, we overcome this obstacle by employing highly reactive stearoyl sulfide and selenide as chalcogen sources, which enable straightforward variation of the NPLs composition and thickness by changing the ratio of chalcogen precursors and reaction temperature, respectively. Alloyed CdSexS1-x NPLs obtained exhibit tunable absorption and photoluminescence bands covering the blue-green region from 380 to 520 nm with bright band-edge emission and quantum yields of ~30–50 % due to their relatively small lateral size enabled by a much finer control of the lateral growth.

We investigate theoretically the band transport of electrons and holes in a "quantum-dot-in-perovskite" solid, a periodic array of semiconductor nanocrystal quantum dots embedded in a matrix of lead halide perovskite. For concreteness we focus on PbS quantum dots passivated by inorganic halogen ligands and embedded in a matrix of CsPbI3. We find that the halogen ligands play a decisive role in determining the band offset between the dot and matrix and may therefore provide a straightforward way to control transport experimentally. The model and analysis developed here may readily be generalized to analyze band transport in a broader class of dot-in-solid materials.

Topological electronic phases exist in a variety of naturally occurring materials but can also be created artificially. We used a cryogenic scanning tunneling microscope to create dimerized chains of identical quantum dots on a semiconductor surface and to demonstrate that these chains give rise to one-dimensional topological phases. The dots were assembled from charged adatoms, creating a confining potential with single-atom precision acting on electrons in surface states of the semiconductor. Coupling between the dots leads to electronic states localized at the ends of the chains, as well as at deliberately created internal domain walls, in agreement with the predictions of the Su-Schrieffer-Heeger model. Scanning tunneling spectroscopy also reveals deviations from this well-established model manifested in an asymmetric level spectrum and energy shifts of the boundary states. The deviations arise because the dots are charged and hence lead to an onsite potential that varies along the chain. We show that this variation can be mitigated by electrostatic gating using auxiliary charged adatoms, enabling fine-tuning of the boundary states and control of their superposition. The experimental data, which are complemented by theoretical modeling of the potential and the resulting eigenstates, reveal the important role of electrostatics in these engineered quantum structures.

We develop a microscopic theoretical model of AlN, GaN, and InN film growth by atomic layer epitaxy. To make the model realistic, we take into account the atomic hydrogen that is created by the hydrogen plasma commonly used in plasma-assisted atomic layer epitaxy. This growth technique relies on separate deposition steps for nitrogen and the group-III cation. Our model addresses the processes that occur after a complete monolayer of nitrogen has formed, that is, the deposition, adsorption, surface diffusion, island nucleation, and island growth of group-III cations. According to our model, the three nitrides grow in a standard and qualitatively similar manner: during a brief nucleation phase, a modest attractive interaction leads to stable island nuclei consisting of just a few atoms. These then grow in place at a nearly constant island density and with an island size distribution which obeys an expected universal scaling relationship that depends only on the critical island size.

The self-assembly of submonolayer amounts of Au on the densely stepped Si(553) surface creates an array of closely spaced "atomic wires" separated by 1.5 nm. At low temperature, charge transfer between the terraces and the row of silicon dangling bonds at the step edges leads to a charge-ordered state within the row of dangling bonds with ×3 periodicity. Interactions between the dangling bonds lead to their ordering into a fully two-dimensional (2D) array with centered registry between adjacent steps. We show that as the temperature is raised, soliton defects are created within each step edge. The concentration of solitons rises with increasing temperature and eventually destroys the 2D order by decoupling the step edges, reducing the effective dimensionality of the system to 1D. This crossover from higher to lower dimensionality is unexpected and, indeed, opposite to the behavior in other systems.

We investigate theoretically the transport of electrons and holes in crystalline solids consisting of three-dimensional arrays of semiconductor nanocrystals passivated by two types of organic ligands-linear chain carboxylates and functionalized aromatic cinnamates. We focus on a critical quantity in transport: the quantum-mechanical overlap of the strongly confined electron and hole wavefunctions on neighboring nanocrystals. Using results from density-functional-theory (DFT) calculations, we construct a one-dimensional model system whose analytic wavefunctions reproduce the full DFT numerical overlap values. By investigating the analytic behavior of this model, we reveal several important features of electron transport. The most significant is that the wavefunction overlap decays exponentially with ligand length, with a characteristic decay length that depends primarily on properties of the ligand and is almost independent of the size and type of nanocrystal. Functionalization of the ligands can also affect the overlap by changing the height of the tunneling barrier. The physically transparent analytic expressions we obtain for the wavefunction overlap and its decay length should be useful for future efforts to control transport in nanocrystal solids.

Atomically flat colloidal semiconductor CdSe nanoplatelets (NPLs) with precisely controlled thickness possess a range of unique optoelectronic properties. Here, we study the growth of CdSe, CdTe, and CdS NPLs with the aim of synthesizing thicker NPLs in order to extend their optical activity further into the lower energy/larger wavelength range. We employ cadmium halides, which lead to faster reaction kinetics as confirmed by control experiments with cadmium hydroxide as a Cd-precursor. Addition of halides in all cases led to the formation of thicker NPL species, as compared with the corresponding syntheses without these additives. Analysis of a recent theoretical model of the platelet growth mechanism confirms an earlier suggestion that reducing both the reaction enthalpy and the surface energy of CdSe, by replacing acetate ligands with chloride ions, should indeed lead to thicker NPLs as observed. We noticed a formation of Cd-0-metal nanoparticles in the first stage of the synthesis by preparing the Cd-precursor, which is another key finding of our work. We assume that these particles can serve as an active cadmium source facilitating the growth of the NPLs. The resulting 6 ML CdSe NPLs exhibited bright photoluminescence with quantum yield of up to 50%, exceptionally narrow spectrum centered at 582 nm with full width at half-maximum of approx. 11 nm, and small Stokes shift of 2 nm. Moreover, we demonstrated the synthesis of heterostructured core/shell CdSe/CdS NPLs based on 6 ML CdSe platelets, which also exhibited bright fluorescence. This work shows the possibility to overcome energetic barrier limiting the size (thickness) control by using appropriate promoters of the growth of CdSe, CdTe, and CdS 2D structures.

Indium phosphide based quantum dots have emerged in recent years as alternatives to traditional heavy metal (cadmium, lead) based materials suitable for biomedical application due to their non-toxic nature. The major barrier to this application, is their low photoluminescent quantum yield in aqueous environments (typically < 5%). Here we present a synthetic method for InP/ZnS quantum dots, utilizing a controlled cooling step for equilibration of zinc sulfide across the core, resulting in a photoluminescent quantum yield as high as 85% in organic solvent and 57% in aqueous media. To the best of our knowledge, this is the highest reported for indium phosphide quantum dots. DFT calculations reveal the enhancement in quantum yield is achieved by redistribution of zinc sulfide across the indium phosphide core through thermal diffusion. By eliminating the need for a glove box and relying on Schlenk line techniques, we introduce a widely accessible method for quantum dots with a realistic potential for improved biomedical applications.

A straightforward, rapid method to create colloidally stable and brightly luminescent core/shell CdSe‐based nanoplatelets (NPLs) with fluorescence quantum yields (QYs) up to 50% is demonstrated. A layer‐by‐layer deposition technique is used which is based on a two‐phase mixture—consisting of a nonpolar phase, which includes the NPLs, and a saturated ionic polar phase—to separate the reagents and hinder the nucleation of the shell material. The deposition of the first sulfur layer leads to a significant redshift (by more than 100 nm) of the optical absorption and emission of the NPLs. Hence, by varying either the sulfur precursor content or the reaction time, one can precisely and continuously tune the absorption and emission maxima from 520 to 630 nm. This evolution of the absorption onset during the shell growth is explained quantitatively using density‐functional theory and atomistic statistical simulations. The emission can be further enhanced by exposure of the NPL solution to ambient sunlight. Finally, it is demonstrated that the core/shell NPLs can be transferred from the organic solution to aqueous media with no reduction of their QY, which opens the door to a broad range of practical applications.

Fe$_{3}$Si/Ge(Fe,Si)/Fe$_{3}$Si thin film stacks were grown by a combination of molecular beam epitaxy and solid phase epitaxy (Ge on Fe$_{3}$Si). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron X-ray diffraction. The Ge(Fe,Si) films crystallize in the well oriented, layered tetragonal structure FeGe$_{2}$ with space group P4mm. This kind of structure does not exist as a bulk material and is stabilized by solid phase epitaxy of Ge on Fe$_{3}$Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.

We have demonstrated a reduction in the prevalence of surface defect states in FeS2 iron pyrite thin films by encapsulating them with ZnS. Thin films of FeS2 were grown and encapsulated, without exposure to atmosphere, by a variety of films. X-ray photoelectron spectroscopy (XPS) measurements, using a novel technique that permits us to selectively probe the FeS2-capping layer interface, show a reduction in the surface defect state characterized by the sulfur 2p doublet for ZnS-encapsulated films. We further present an atomistic density functional theory (DFT) model that explains this effect based on epitaxial bonding of FeS2 and ZnS across their interface.

Atomistic control over the growth of semiconductor thin films, such as aluminum nitride, is a long-sought goal in materials physics. One promising approach is plasma-assisted atomic layer epitaxy, in which separate reactant precursors are employed to grow the cation and anion layers in alternating deposition steps. The use of a plasma during the growth—most often a hydrogen plasma—is now routine and generally considered critical, but the precise role of the plasma is not well-understood. We propose a theoretical atomistic model and elucidate its consequences using analytical rate equations, density functional theory, and kinetic Monte Carlo statistical simulations. We show that using a plasma has two important consequences, one beneficial and one detrimental. The plasma produces atomic hydrogen in the gas phase, which is important for removing methyl radicals left over from the aluminum precursor molecules. However, atomic hydrogen also leads to atomic carbon on the surface and, moreover, opens a channel for trapping these carbon atoms as impurities in the subsurface region, where they remain as unwanted contaminants. Understanding this dual role leads us to propose a solution for the carbon contamination problem which leaves the main benefit of the plasma largely unaffected.

Ostwald ripening describes how the size distribution of colloidal particles evolves with time due to thermodynamic driving forces. Typically, small particles shrink and provide material to larger particles, which leads to size defocusing. Semiconductor nanoplatelets, thin quasi-two-dimensional (2D) particles with thicknesses of only a few atomic layers but larger lateral dimensions, offer a unique system to investigate this phenomenon. Experiments show that the distribution of nanoplatelet thicknesses does not defocus during ripening, but instead jumps sequentially from m to (m + 1) monolayers, allowing precise thickness control. We investigate how this counterintuitive process occurs in CdSe nanoplatelets. We develop a microscopic model that treats the kinetics and thermodynamics of attachment and detachment of monomers as a function of their concentration. We then simulate the growth process from nucleation through ripening. For a given thickness, we observe Ostwald ripening in the lateral direction, but none perpendicular. Thicker populations arise instead from nuclei that capture material from thinner nanoplatelets as they dissolve laterally. Optical experiments that attempt to track the thickness and lateral extent of nanoplatelets during ripening appear consistent with these conclusions. Understanding such effects can lead to better synthetic control, enabling further exploration of quasi-2D nanomaterials.