Yunhua Xing


Last Name

Xing

First Name

Yunhua

ORCID

0000-0002-8741-6337

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2017 - 201912020 - 20253
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Publications 1 - 4 of 4
  • Effect of Positional Disorders on Charge Transport in Nanocrystal Quantum Dot Thin Films
    Item type: Journal Article
    Xing, Yunhua; Yazdani, Nuri; Lin, Weyde M.M.; et al. (2022)
    ACS Applied Electronic Materials
    Understanding the impact of positional and energetic disorders in nanocrystal (NC) quantum dot thin films on charge transport is crucial to determine what to prioritize in terms of the synthesis and fabrication of these materials and to accelerate their development for electronics. Here, we computationally construct realistic NC thin films with different types of disorders and apply a density functional theory (DFT)-parameterized, kinetic Monte Carlo simulation to systematically study the effects of disorders on transport. We obtain statistics on the carrier transit pathways through the NC films and carrier residence times on individual NCs. This provides insights into the distribution of transit times across the thin films and the effective mobility. We conclude that the impact of positional disorders on charge transport depends on the type of disorder and how it affects the spacing between neighboring NCs. The formation of transport paths with short inter-NC distances can enhance mobility. Meanwhile, random packing (RP) of NCs and energetic disorders due to a distribution of NC sizes decreases mobility 2- to 4-fold. Because of the large reorganization energy of small NCs, increasing the electric field has little influence on the median residence time of a charge carrier on an NC; however, an electric field straightens the transport path of the charge carrier and reduces the average number of hops a carrier makes, which can slightly enhance mobility. Deep electronic trap states are especially detrimental to carrier mobility, particularly at low fields and when the films are otherwise highly ordered.
  • Mapping the Atomistic Structure of Graded Core/Shell Colloidal Nanocrystals
    Item type: Journal Article
    Yarema, Maksym; Xing, Yunhua; Lechner, Rainer T.; et al. (2017)
    Scientific Reports
    Engineering the compositional gradient for core/shell semiconductor nanocrystals improves their optical properties. To date, however, the structure of graded core/shell nanocrystal emitters has only been qualitatively described. In this paper, we demonstrate an approach to quantify nanocrystal structure, selecting graded Ag-In-Se/ZnSe core/shell nanocrystals as a proof-of-concept material. A combination of multi-energy small-angle X-ray scattering and electron microscopy techniques enables us to establish the radial distribution of ZnSe with sub-nanometer resolution. Using ab initio shape-retrieval analysis of X-ray scattering spectra, we further determine the average shape of nanocrystals. These results allow us to generate three-dimensional, atomistic reconstructions of graded core/shell nanocrystals. We use these reconstructions to calculate solid-state Zn diffusion in the Ag-In-Se nanocrystals and the lattice mismatch between nanocrystal monolayers. Finally, we apply these findings to propose design rules for optimal shell structure and record-luminescent core/shell nanocrystals.
  • Density Functional Theory (DFT)-Parameterized Multiscale Modelling on Properties and Electronic Transport in Metal Chalcogenide Nanocrystals (NC) and NC Solids
    Item type: Doctoral Thesis
    Xing, Yunhua (2025)
    Benefiting from the low cost in manufacturing and high tunability in material properties, rapid development and enormous progress have been made over the past few decades from material synthesis to device fabrication for solution-synthesized semiconductor nanocrystals (NC), rendering them the unmatched choice for a wide range of advanced applications in electronic, optoelectronic, and thermoelectronic devices. By precisely controlling the constituent elements, size, shape, and surface termination of an individual NC, and engineering packing structures (ordered/disordered) of NC assemblies/solids, the optical/electronic properties and the performance of the devices incorporating NC solids can be readily adjusted. For applications requiring controllable charge carrier mobilities, further advancing the device performance requires a decent knowledge of impacts from NC composition and morphology as well as NC packing nonideality on the charge carrier dynamics and transport mechanism in NC solids determined by electronic NC-NC communication and charge-NC interaction. Recent advancements in computational studies have been empowering a better understanding of structural/optical/electronic properties covering a length scale from individual NCs to NC solids. Among these methods, density functional theory (DFT)-based calculations and simulations on atomistic models of NCs prevail through the inclusion of atomistic complexity on the NC surfaces. In Chapter 1, we review the various atomistic DFT-based methods that have been proposed to study NCs and their assemblies, highlighting the insights they provide for understanding the static and dynamic structural and elec- tronic/optical properties of individual NCs and NC solids. Supported by the new experimental and more advanced DFT techniques, promising future perspectives can be expected for a better understanding in more NC systems and empowering their applications in all fields. In this thesis, we focus on investigating charge carrier transport in NC solids. Choosing from a wide library of metal chalcogenide NCs, we pick out lead sulfide (PbS) and mercury telluride (HgTe) NCs as two model systems. Both NC systems are widely used and proved great success in IR photodetecting and imaging, where high charge carrier mobility is an essential metric for good device performance. By applying the same DFT-parameterized multiscale methodology armed with precise and robust atomistic models, we unveil how the size, shape, surface chemistry and packing condition of the NCs can impact on charge carrier transport in different NC solids, provide insights into the occurrence of complexity in charge transport mechanism (band-like or hopping), and prove the possible wide-applicability of this model on more other NC systems. In chapter 2, with PbS NC as model system, where charge carrier transport limited to phonon-assisted hopping regime, we perform a DFT-parameterized kinetic Monte Carlo simulation on computationally constructed NC thin films with a range of disorders, including positional disorder, energetic disorder and deep electronic traps. We contribute by extracting and analysing statistics of the charge transients under a range of electric field biases to understand the impact of different types of disorder on charge carrier transport in NC solids. Based on the results, we propose several guidelines from NC synthesis to device fabrication for controlling charge carrier mobility. We further endeavor to transfer this model to exploit NC systems where charge transport mechanism is still unclear and debatable. Taken HgTe NC as an example, presence of both band-like and hopping transport are reported in the NCs above 10 nm in size. In order to elucidate the mechanism complexity in HgTe NC solids, we firstly build atomistic-precise models for HgTe NCs covering variety in NC shape, size and surface termination. As reported in Chapter 3, our atomistic model reproduce mid-gap-trap-free electronic structures of HgTe NCs, with the resulting structural, optical and electronic properties in line with experimental measurements. In Chapter 4, based on this atomistic model, we further contribute by explaining NC-charge interaction and showing evidence of charge delocalization among several HgTe NCs through DFT calculations of NC reorganization energy λ introduced by atomic rearrangement on NC surface upon charging (polaron formation) and electronic coupling Vc between neighboring NCs. Under the influence of NC size, interparticle aligning orientation and facet-to-facet distance ∆ff, criteria for the occurrence and geometrical landscape of charge delocalization in randomly packed NC solids (implying band-like charge transport) are derived. Possible ways to improve charge carrier mobillity are then suggested accordingly. This model is proven to be well transferable to studying other mercury chalcogenide NCs.
  • Earth-abundant Ni-Zn nanocrystals for efficient alkyne semihydrogenation catalysis
    Item type: Journal Article
    Clarysse, Jasper; De Jesus Silva, Jordan; Xing, Yunhua; et al. (2025)
    Nature Communications
    The development of catalysts that are based on earth-abundant metals remains a grand challenge. Alloy nanocrystals (NCs) form an emerging class of heterogeneous catalysts, offering the promise of small, uniform catalysts with composition-control. Here, we report the synthesis of small Ni and bimetallic Ni-X (X= Zn, Ga, In) NCs for alkyne semihydrogenation catalysis. We show that Ni3Zn NCs are particularly reactive and selective under mild reaction conditions and at low loadings. While bimetallic NCs are all more selective than pure Ni NCs, Ni-Zn NCs also maintain excellent reactivity compared to Ni-Ga and Ni-In alloys. Ab-initio calculations can explain the differences in reactivity, indicating that, unlike Ga and In, Zn atoms interact with the substrates. We further show that Ni3Zn NCs are robust and tolerate a broad range of substrates, which may be linked to the favorable amine-terminated surface.
Publications 1 - 4 of 4