Search query: Electronic structure of nanocrystals

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abstract: We review the state of the art of determining the electronic structure of nanocrystals in thin films by electrochemistry. Our core conclusion, the necessity of combining electrochemical with spectroscopic techniques, is illustrated with the holistic analysis of thin films of CdSe nanocrystals cross linked with electroactive metal b tetraaminophthalocyanines by differential pulse voltammetry, optical spectroscopy and potential modulated absorption spectroscopy. We show that the same nanocrystals cross linked with phthalocyanines of different metal centers exhibit rather similar electrochemical signatures, but behave distinctly different in spectroelectrochemical investigations. We argue that in one case, namely CdSe nanocrystals cross linked with Co b tetraaminophthalocyanine, we find supporting evidence for the hybridization of energy levels at the organic/inorganic interface. This work suggests that spectroelectrochemistry is capable of revealing the electronic structure of complex nanomaterials, such as semiconductor nanocrystals functionalized with organic pi systems.
author_names: Michelle Weber

Sophia Westendorf

Bjorn Marker

Kai Braun

Marcus Scheele
title: Opportunities and challenges for electrochemistry in studying the electronic structure of nanocrystals.
venue: Physical chemistry chemical physics PCCP
year: 2019

abstract: This Review discusses the electronic structure of plasmonic resonances in metal nanostructures, clarifying existing misconceptions on the topic. Here we underscore the key property of the plasmonic response in metal nanocrystals: the plasmon and its wave function are mostly composed of a large number of low energy excitations, which involve electrons near the Fermi level. Simultaneously, some number of high energy hot electrons are excited in a nanocrystal due to the scattering of electrons by surfaces and in hot spots. It is an established fact that plasmon excitations are well described by classical frameworks, considering the collective oscillation of low energy carriers moving as the result of classical acceleration. This classical motion is intrinsically dissipative and leads to heating. On the other hand, the generation of hot electrons in nanocrystals is a quantum surface effect. The energy efficiency of such hot electron processes is always limited. However, there are interesting possibilities for.
author_names: Le Chang

Lucas V Besteiro

Jiachen Sun

Eva Yazmin Santiago

Stephen K Gray

Zhiming M Wang

Alexander O Govorov
title: Electronic Structure of the Plasmons in Metal Nanocrystals: Fundamental Limitations for the Energy Efficiency of Hot Electron Generation
venue: ACS Energy Letters
year: 2019

abstract: The electronic structures of a variety of experimentally identified gold and silver nanoclusters from 20 to 246 atoms, either unprotected or protected by several types of ligands, are characterized by using point group specific symmetry analysis. The delocalized electron states around the HOMO LUMO energy gap, originating from the metal s electrons in the cluster core, show symmetry characteristics according to the point group that describes best the atomic arrangement of the core. This indicates strong effects of the lattice structure and overall shape of the metal core to the electronic structure, which cannot be captured by the conventional analysis based on identification of spherical angular momentum shells in the "superatom" model. The symmetry analysis discussed in this paper is free from any restrictions regarding shape or structure of the metal core, and is shown to be superior to the conventional spherical harmonics analysis for any symmetry that is lower than Ih. As an immediate application, we also demonstrate that it is possible to reach considerable savings in computational time by using the symmetry information inside a conventional linear response calculation for the optical absorption spectrum of the Ag55 cluster anion, without any loss in accuracy of the computed spectrum. Our work demonstrates an efficient way to analyze the electronic structure of nonspherical, but atomically ordered nanocrystals and ligand protected clusters with nanocrystal metal cores, and it can be viewed as the generalization of the superatom model demonstrated for spherical shapes 10 years ago (Walter, M. et al. Proc. Natl. Acad. Sci. U. S. A.2008, 105, 9157 916218599443)
author_names: Sami Kaappa

Sami Malola

Hannu Hakkinen
title: Point Group Symmetry Analysis of the Electronic Structure of Bare and Protected Metal Nanocrystals
venue: The journal of physical chemistry. A
year: 2018

abstract: Semiconductor nanocrystals, such as quantum dots (QDs) and wires (QWs) often contain from a few thousands to more than 106 atoms. It has been a great challenge to calculate the electronic structure of these large nanosystems using self consistent first principles method. In this chapter, recent development of calculations of nanocrystal physical properties using the large scale ab initio pseudopotential or charge patching methods is reviewed. The calculated size dependent exciton energies and absorption spectra of QDs and QWs are in good agreement with experiments. The calculated ratios of bandgap increases between QWs and QDs are found to be material dependent, and for most direct bandgap materials, this ratio is close to 0.586, as predicted by simple effective mass approximation. We show that the electronic structure of a nanocrystal can be tuned not only by its size, but also by its shape. Therefore, the shape can be used as an efficient way to control the electronic structure of the nanocrystals. Changing the shape is expected to be more flexible and provides more variety of the electronic states than simply changing the size of the system. The special features of the electronic states obtained in different shapes of the nanocrystals can be used in various device applications. We also show that defect properties in QDs could be significantly different from those in bulk semiconductors. For example, although negatively charged DX center is unstable in bulk GaAs:Si with respect to the tetrahedral coordinated SiGa when the dot size is small enough, it becomes stable.
author_names: Jingbo Li

Suhuai Wei
title: Large Scale Ab Initio Study of Size, Shape, and Doping Effects on Electronic Structure of Nanocrystals
year: 2009

author_names: P M Yakibchuk

O V Bovgyra

L R Toporovska

I V Kutsa
title: Nonlocal Model Pseudopotential Calculations of the Electronic Structure of AIIBVI (CdS, CdSe) Bulk Crystals and Nanocrystals
year: 2017

abstract: Bi2S3 nanocrystals with different shapes and sizes are obtained by a hot injection approach using bismuth neodecanoate and thiolamine as bismuth and sulfur precursors. The colloid morphology, from nanodots to nanorods, with sizes ranging from 3 4 nm to 40 50 nm strongly depends on the preparation conditions such as injection temperature and ratio of Bi/S precursors and ultimately impacts the optical and electrical properties of the final nanocrystals. The resulting products are analyzed using X ray powder diffraction (XRD) transmission electron microscope (TEM) UV vis absorption spectroscopy, and photoluminescence spectroscopy (PL) A blue shift in the band gap is observed at 1.87, 1.89, and 2.04 eV as we go from nanodots to nanorods with aspect ratios of 3, 5, and 1, respectively. These observations indicate quantum confinement effects due to the different diameters of nanocrystals. The crystallinity and morphology of nanocrystals influence significantly the PL emission, decreasing for nanodots and inc.
author_names: Peilin Han

Agustin Mihi

Josep Ferre-Borrull

Josep Pallares

Lluis F Marsal
title: Interplay Between Morphology, Optical Properties, and Electronic Structure of Solution Processed Bi2S3 Colloidal Nanocrystals
year: 2015

abstract: We carry out an accurate computational analysis on the nature and distribution of electronic trap states in shape tailored anatase TiO2 structures, investigating the effect of the morphology on the electronic structure. Linear nanocrystal models up to 6 nm in length with various morphologies, reproducing both flattened and elongated rod shaped TiO2 nanocrystals, have been investigated by DFT calculations, to clarify the effect of the crystal facet percentage on the nanocrystal electronic structure, with particular reference to the energetics and distribution of trap states. The calculated densities of states below the conduction band edge have been very well fitted assuming an exponential distribution of energies and have been correlated with experimental capacitance data. In good agreement with the experimental phenomenology our calculations show that elongated rod shaped nanocrystals with higher values of the ratio between (100) and (101) facets exhibit a relatively deeper distribution of trap states. Our results point at the crucial role of the nanocrystal morphology on the trap state density, highlighting the importance of a balance between the low energy (101) and high energy (100)(001) surface facets in individual TiO2 nanocrystals.
author_names: Francesca Nunzi

Loriano Storchi

Michele Manca

Roberto Giannuzzi

Giuseppe Gigli

Filippo De Angelis
title: Shape and morphology effects on the electronic structure of TiO(2) nanostructures: from nanocrystals to nanorods.
venue: ACS applied materials interfaces
year: 2014

abstract: Abstract The electronic structure and surface composition of perovskite films after incorporation of different concentration of CsPbBr3 nanocrystals (NCs) have been investigated with ultraviolet photoelectron spectroscopy (UPS) and X ray, and angle resolved photoelectron spectroscopy (XPS, AR XPS) The electronic structure, surface composition and morphology of perovskite film have been changed through the interaction between CsPbBr3 NCs and perovskite film. The band can be shifted upward by in incorporation of CsPbBr3 NCs and it is independent of the concentration of CsPbBr3 NCs. The surface component and its distribution can be adjusted by changing the concentration of CsPbBr3 NCs and the proper concentration of CsPbBr3 NCs would improve the stoichiometric ratio of perovskite film.
author_names: Yuquan Liu

Huachao Zai

Haipeng Xie

Baoxing Liu

Shitan Wang

Yuan Zhao

Dongmei Niu

Han Huang

Qi Chen

Yongli Gao
title: Effects of CsPbBr3 nanocrystals concentration on electronic structure and surface composition of perovskite films
venue: Organic Electronics
year: 2019

abstract: The effect of Cr dopant on the structural, optical, magnetic properties and local electronic structure of aqueous synthesis derived Zn1 xCrxS diluted magnetic semiconductor nano crystals have systematically investigated. The nano crystalline structure and crystallite size have been estimated by X ray diffraction measurements with Rietveld refinement and high resolution transmission electron microscopy. Effective increase of the lattice parameter has been observed in doped samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder and defects in the host lattice. The tetrahedral coordination of the sulfur ions surrounding the zinc ions has been studied by FTIR analysis. The decrease of energy band gap for Cr doped samples has observed. Blue emission has been observed by photo luminescence spectroscopy due to defect formation (Cri) in Cr doped samples. The local electronic structures of Zn and Cr sites are thoroughly studied by synchrotron based X ray absorption spectroscopy comprising of both X ray Absorption Near Edge Structure (XANES) and Extended X ray Absorption Fine Structure (EXAFS) EXAFS studies indicate the presence of secondary phase within the sphalerite lattice of diluted Zn1 xCrxS nanocrystals while XANES studies exhibit single pre edge feature. The amplitude of such pre edge is found to be independent of Cr amount on doped ZnS nanocrystals. The results demonstrated that diluted Cr3+ ions are substituted on the host ZnS nanocrystal. The Cr doped ZnS sample shows paramagnetism at room (300 K) and at low (5 K) temperature. The Cr S bonds are the crucial premise for paramagnetic ordering.
author_names: Soumendra Ghorai

N Patra

Dibyendu Bhattacharyya

S N Jha

Bishwajit Ray

Sandip Chatterjee

Anup K Ghosh
title: Influence of chromium concentration on the structural, electronic structure, optical and temperature dependent magnetic properties of ZnS nanocrystals
venue: Journal of Materials Science: Materials in Electronics
year: 2019

abstract: Since reports that silicon nanocrystals (Si NCs) can exhibit direct transition emission, the silicon laser field is at a juncture where the importance of this discovery needs to be evaluated. Most theoretical models predicted a monotonic increase in the bandgap and experimental information currently available on the electronic structure at the G valley of these promising materials is circumstantial as it is obtained from emission measurements where competing non radiative relaxation and recombination processes only provide an incomplete picture of the electronic structure of Si NCs. Optical absorption, the most immediate probe of the electronic structure beyond the band edges, showing the evolution of the G valley states with nanocrystal size has not been measured. Here, we show such measurements, performed with high dynamic range, allowing us to observe directly the effect of crystal size on the G valley splitting far above the band edges. We show that the splitting is 100 s of meV more pronounced than predicted by pseudo potential calculations and Luttinger Kohn model. We also show that ultrafast red shifting emission can be observed in plasma enhanced chemical vapor deposition prepared Si NCs.
author_names: Waqas Mustafeez

Arka Majumdar

Jelena Vuckovic

Alberto Salleo
title: A direct measurement of the electronic structure of Si nanocrystals and its effect on optoelectronic properties
year: 2014