Session S09: Nanoparticles and Nanoplatelets: Structural and Optoelectronic Properties and Phenomena

Second harmonic generation from nanoparticles of noncentrosymmetric geometry

The nonlinear polarization resulting in generation of second harmonic (SH) radiation from centrosymmetrical (CS) media mostly takes place at the surface where the inversion symmetry is lost. This local disruption of symmetry disappears globally for a symmetrical particle, suppressing its total SH polarization. In this work, we explore SHG from particles of CS materials with non-CS geometry. We choose an isolated cylindrical particle with a deformed circular cross-section in the presence of an external field with a long wavelength. Assuming a small deformation parameter, a perturbative scheme is employed to analytically solve the linear and quadratic field equations, employing the dipolium model [1] to calculate the SH bulk and surface polarization. We report the SH dipolar and quadrupolar hyperpolarizability tensors, their spectra, the radiation patterns, and the conversion efficiency.

[1] B. S. Mendoza and W. L. Mochán, Phys. Rev. B 53, 4999 (1996).   

Quantum Dot Supercrystals: Assembly and properties

The assembly of semiconductor nanoparticles, quantum dots (QD), into dense crystalline nanostructures holds great promise for future optoelectronic devices. Emulsion-templated assembly has recently been shown to provide three-dimensional quantum-dot supercrystals of excellent crystal quality. We combine emulsion-templated assembly with in situ small-angle X-ray scattering to obtain direct insight into the nanoscale interactions underlying the nucleation, growth and densification of quantum dot supercrystals. We show that while hard-sphere behaviour governs their initial nucleation, at later stages the ligands play a crucial role in balancing steric repulsion against attractive van der Waals attraction. We also elucidate the optoelectronic properties of these supercrystals. We show that the supercrystals exhibit Mie resonances that can lead to absorption efficiencies larger than one, beneficial for photovoltaic applications. Ultrafast spectroscopy reveals energy transfer processes due to quantum-dot coupling in these supercrystals.   

Electronic, Optical and Transport Properties of PbS Nanocrystal Superlattices

Optoelectronic devices made from colloidal quantum dots (CQDs) often take advantage of the combination of tunable quantum confined optical and electronic properties and carrier mobilities of strongly coupled systems. For lattices of connected CQDs, of interest is the interplay between confinement effects and band-like behavior. In this work, first-principles calculations are applied to investigate the electronic, optical and transport properties of PbS CQD superlattices. Our results show that even in the regime of strong necking between CQDs, quantum confinement can be preserved. In the bandlike regime, computed carrier mobilities for simple cubic and 2D square lattices fused along the {100} facets are 2-4 orders of magnitude larger than those of superlattices fused along {110} and {111} facets. The relative magnitude of the electron and hole mobilities strongly depends on the crystal and electronic structures. We also find that the carrier mobilities of CQD solids increases as the size of the quantum dot increases due to the stronger coupling between neighboring CQDs. Our results illustrate the importance of understanding the effects of crystal structure and connectivity of CQD films.   

Nanoscale Core-Shell Hyperbolic Metamaterial for Ultralow Threshold Laser Action

We develop the nanoscale core-shell hyperbolic metamaterial (HMM) possessing a remarkably coupling effect in between the multishell components, which is due to the formation of higher density of states and longer time of collective oscillations of the electrons than the plasmonic-based pure metal nanoparticles. Subsequently, a giant localized electromagnetic wave of surface plasmon resonance is formed at the surface causing the pronounced out-coupling effect. The nanoscale core-shell HMM confines the energy well without being decayed, reducing the propagation loss and then achieving the stimulated emission (e.g., random lasing action by dye molecule) with an ultralow lasing threshold (~30 μJ/cm²). We believe that the nanoscale core-shell HMM paves a way to enlarge the development of applications, such as highly optoelectronic conversion efficiency of solar cells, great power extraction of light emitting diodes and wide spectra photodetectors.   

Hot-electron generation and energy transfer in plasmonic metastructures with hot spots: Quantum and Classical mechanisms

Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications, which often involve excited electrons. However, the central questions in the problem of plasmonic hot electrons are quantifying the number of optically excited energetic electrons in a nanocrystal and finding out how to extract such electrons. Here we develop a theory describing the generation rates and the energy distributions of hot electrons in nanocrystals with various geometries. Hot-electron generation [1] together with non-radiative plasmonic transfer [2] represent efficient mechanisms to transport and localize optical and photo-chemical energies.

1. A.O. Govorov, H. Zhang, V. Demir, and Y. K. Gun’ko, Nano Today, 9, 85 (2014).
2. E.-M. Roller, L.V. Besteiro, C. Pupp, L. Khosravi Khorashad, A. O. Govorov, T.Liedl, Nature Physics, 13, 761-765 (2017).   

Solid-state diffusion in Cu core/Ni shell nanoparticles

Cu core/Ni shell nanoparticles with nominal Cu/Ni atomic ratios of 2/1, 1/1,1/2, and 1/3 have been prepared in a microwowave reactor by the sequential reduction of Cu acetate (at 100 ^○C) and Ni acetate (at 175 ^○C) in ethylene glycol using sodium hypophosphite as the reducing agent. The measured room temperature lattice parameters of the as-prepared samples are slightly smaller/larger than the corresponding bulk Cu and Ni lattice parameters indicating a small amount of diffusional mixing at the Cu/Ni interface during sample preparation. Ex-situ vacuum annealing experiments followed by room temperature x-ray diffraction measurements and in-situ high temperature x-ray diffraction measurements have been used to follow the conversion of the as-prepared core/shell nanoparticles to a homogeneous Cu-Ni alloy as a function of both the temperature and the time. These measurements reveal that significant diffusional mixing occurs starting at a temperature between 300 and 400 ^○C and complete mixing with the formation of a homogeneous Cu-Ni alloy occurs at a temperature of 600 ^○C.   

Growth and Shape Stability of Bi-metallic Nano-particles

Controlling the morphology of non-noble bi-metallic nano-crystals has become the focus of several studies as these nano-stuructures can provide a great opportunity to improve their performance and activity in catalytic reactions. Although much of the focus has been on the overall macroscopic description of synthesis processes, it is still a great challenge to identify the leading factors in a typical crystal growth process at atomic-scale. Here we report results of atomic scale calculations on shape evolution and stability of grown bi-metallic Cu-Ni nano-particles using molecular static and dynamic simulations. Our growth simulations on bare Cu and Ni nano-cubes reveals that single-atom diffusion characteristics play an important role in utilizing the particle with specific morphology and architecture   

Interface Modification in Type-II ZnCdSe/(Zn)CdTe QDs for High Efficiency Intermediate Band Solar Cells

Intermediate band solar cells have been proposed as being able to overcome the Shockley-Queisser limit for single junction solar cells.¹ Our group has been devolving a II-VI based material system using type-II submonlayer quantum dots (QDs) composed of (Zn)CdTe embedded in a ZnCdSe host. These materials, when lattice matched to InP closely match the theoretically predicted requirements. However, do to the dissimilar group VI elements and Cd desorption that occurs during the formation of the quantum dots a highly strained interfacial layer is formed.² A new growth process is proposed to avoid formation of a strain-inducing ZnSe interfacial layer. We show that the new growth sequence allows for improved control of the interfacial composition and simplifies the fabrication of the intermediate band solar cell device structure based on these QDs, since additional strain balancing schemes are no longer required to grow stress-free structures.
[1] A. Luque, and A. Marti, Phys. Rev. Lett., 78, 5014 – 5017 (1997).
[2] S. Dhomkar, et al., Journal of Vacuum Science & Technology B, Nano. and Micro.: Mat., Process., Meas., and Phen., 31(3), 03C119 (2013).   

Observation of optical referigeration on CdSe/CdS quantum dots

Optical refrigeration on CdSe/CdS (core/shell) quantum dots (QDs) was observed for the first time. From the observed temperature drop and the experimental conditions, the effective cooling power in our experiment was estimated to be ~10^-14 W per dot. Experiments were done with samples (colloidal QDs suspension) synthesized in our lab following the method recently developed at Peng's group (DOI: 10.1021/jacs.6b02909), which can effectively eliminate the excitonic non-radiative decay path inside the QDs. The cooling effect comes from the significant up-conversion of the mean emission energy (typical up-conversion of the mean emission energy is >25 meV) observed in the photoluminescence spectra of our samples under sub-band excitation (excitation energy is much lower than the energy band gap of the QDs). Details of the experiments and the observation will be discussed in the presentation.   

Confined excitons in CdSe/CdMnS and CdSe/ZnMnS nanoplatelets

We have used absorption and PL spectroscopies to study the magneto-optical properties of colloidal CdSe/CdMnS and CdSe/ZnMnS core-shell nanoplatelets. We observe three absorption features tentatively attributed to the e₁h₁, e₁l₁, and e₂h₂ confined excitons. In the presence of a magnetic field B applied normal to the substrate surface, the PL from e₁h₁ becomes circularly polarized as σ₊, indicating the presence of exchange interaction between the carrier spins and the spins of the Mn²⁺ ions. The circular polarization initially increases with B and saturates at 4T. At a fixed B the circular polarization decreases with increasing temperature and vanishes around 50K. The circular polarization peak, unlike previously studied samples [1], coincides with the e₁h₁ exciton. This is attributed to the smaller thickness (2ML) of the CdSe core compared to the previously studied samples (5ML).
We also measured the e₁h₁ exciton magnetic splitting from the magneto-transmission spectra. Saturation splittings up to 5 meV have been observed.
1- J.R. Murphy et. al., Applied Physics Letters 108,242406 (2016).   

First-Principles Calculations of Nanoplatelet Heterostructures: Surface Ligands and Strain Fields

Nanoplatelets (NPLs) – highly anisotropic, quasi-two-dimensional semiconductor nanostructures – exhibit optoelectronic properties that are governed by their precisely tunable thickness of a few atomic layers. However, the atomistic structure of NPLs is not completely known and is expected to depend on the passivating ligands. Here we determine the structure of both CdSe-core and CdSe/CdS-core/shell NPLs, using Density Functional Theory (DFT) and the Qbox code[1]. For both core and core/shell NPLs with different thicknesses and ligands, we determined the equilibrium positions and the in-plane strain. We found that NPL thickness, different ligands and the presence of a shell are all factors affecting the axial atomistic structure and they are responsible for inducing an in-plane strain field, which in many studies has been neglected. We show that, compared to CdSe NPL of the same thickness, the lattice constants of the core-shell NPLs expand in the axial direction, in agreement with recent experiments[2]. We rationalize our first principles findings by a continuum elastic model including surface-stress terms that account for the surface passivation of NPLs.

[1] Gygi F, IBM J. Res. Dev. 52.1.2 (2008): 137-144.
[2] Hazarika et al. Submitted.   

First Principles Calculations of Nanoplatelet Heterostructures: Optoelectronic Properties

Two-dimensional, atomically precise core-shell nanoplatelets (NPLs) have recently been synthesized with no sample inhomogeneity, leading to remarkable photoluminescence quantum efficiencies and tunable electronic properties dependent on NPL thickness [1]. Here we use Density Functional Theory and the Qbox code [2] to investigate the unique optoelectronic properties of CdS/CdSe core-shell NPLs. We show that, compared to a pure CdSe NPL of the same thickness, the band gap and dielectric constant of the core-shell NPLs decrease consistent with experimental results, corresponding to a larger exciton binding energy by approximately 30%. We attribute our findings to an expansion of the lattice constant of the core-shell NPLs in the axial direction, leading to a strain-induced modification of the electronic properties of the system. In contrast to nanoparticles, where the optoelectronic properties are governed by quantum confinement, we find that the properties of NPLs depend on a subtle interplay between quantum confinement and strain induced in the NPLs by the heterostructured interfaces.

[1] Hazarika, Fedin, Hong, Guo, Srivastava, Cho, Coropceanu, Portner, Diroll, Greenwood, Mazzotti, Galli, Klie, Talapin, Submitted.
[2] www.qboxcode.org   

Low-temperature photoluminescence spectroscopy of single core/shell nanoplatelets

With promising applications in lasing, light-emitting diodes, as well as quantum photonics, semiconducting nanoplatelets (NPLs) with atomically controlled thickness and giant oscillator strength has attracted considerable interest in the past few years. Our previous studies¹ have revealed that with excitons in NPLs being strongly confined in the thickness direction, an increase in their lateral size and a weakening in the in-plane confinement has complex effects on their photon emission statistics and carrier dynamics. These properties are unique to NPLs due to their quasi-two-dimensional structures.

While room temperature studies of single NPLs provide valuable information, the spectrum is broadened by carrier-phonon interactions, making the assignment of optical transitions and the related electronic structures challenging. Here, we perform single NPL spectroscopic measurements at cryogenic temperatures. Spectral linewidth and diffusion are investigated systematically. In addition, we observe rich spectral fine structures from the NPLs. These results will shed new light on the intrinsic electronic structures of the NPLs, and help understand the origin of the much debated side emission peak observed in ensemble measurements.
1. X. Ma et al. ACS Nano, 2017, 11, 9119 – 9127   

Tuning the Photophysical Properties of Colloidal Two-Dimensional Nanoplatelets

Two-dimensional colloidal semiconductor nanoplatelets (NPLs) are promising optoelectronic materials with outstanding photophysical properties, such as large optical cross-sections and high photoluminescence quantum yield.
Using first-principles density functional theory calculations, we demonstrate strong tunability of NPL band edge energies through surface passivation by common organic molecules. We develop a simple quantitative electrostatic theory describing this effect through dipole-dipole interactions mediated by platelet-ligand interactions and ligand-dependent dielectric function. Finally, using parameter-free self-energies and an effective mass model of the excitons, we show that the band-edge tunability of NPLs together with the strong dependence of the optical bandgap of NPL on thickness can lead to favorable, and controlled tunability of photochemical and optoelectronic properties.