Control Over Cadmium Chalcogenide Nanocrystal Heterostructures via Precursor Conversion Kinetics

Semiconductor nanocrystals have immense potential to make an impact in consumer products due to their narrow, tunable emission linewidths. One factor limiting their use is the ease and reproducibility of core/shell nanocrystal syntheses. This thesis aims to address this issue by providing chemical control over the formation of core/shell nanostructures by replacing engineering controls with kinetic controls. Chapter 1 contextualizes our study on nanoparticle synthesis with a brief discussion on the physics of quantum confinement and the importance of narrow size dispersities, core/shell band alignments, and low lattice mismatches and strain at core/shell nanocrystal interfaces. Next, the evolution of cadmium chalcogenide nanocrystal reagents is described, ranging from the original organometallic reagents used in the 1980s to modern approaches involving cadmium phosphonates and carboxylates. This is followed by a description of chalcogen precursors, highlighting the recent introduction of molecules whose well-controlled and tunable reaction rates allow for the size tuning of nanocrystals at 100% yield, and accompanying theories on nanocrystal nucleation. Chapter 2 covers work to expand the library of available sulfur precursors to a wider range of molecules relevant for the synthesis of cadmium sulfide nanocrystals. Using thioureas alone, only very fast or very slow precursor conversion rates can be accessed. This limits the accessible sizes of cadmium sulfide nanocrystals using a single hot injection of precursor at standardized reaction conditions. We observe that thiocarbonate and thiocarbamate precursors with varying electronic substituents allow access to intermediate precursor conversion rates and cadmium sulfide nanocrystal sizes. Interestingly, we note that these new precursor classes nucleate particles with higher monodispersity than ones synthesized from thioureas. These results indicate that in addition to precursor structure controlling precursor conversion rate, precursor structure additionally impacts nanocrystal monodispersity. Chapter 3 expands the library of sulfur and selenium precursors to include cyclic thiones and selenones which extends chemical control of precursor conversion kinetics to cover five orders of magnitude. This unprecedented breadth of rate control allows for the simultaneous hot injection of multiple precursors to generate core/shell or alloyed nanoparticles using precursor reactivity. Using this new synthetic strategy, we observe that kinetic control runs into several issues which we partially attribute to differences in cadmium sulfide and cadmium selenide critical concentrations and growth rates. Nevertheless, combined with a syringe pump shelling method, we are able to access core/shell and alloyed nanocrystals with photoluminescence quantum yields of 67-81%. Chapter 4 applies the concept of nanostructure control via precursor conversion kinetics to a better model system: two-dimensional nanoplatelets. Cadmium chalcogenide nanoplatelets are highly desirable materials due to their exceptionally narrow emission full width half max (FWHM) values which make them pure emitters relative to quantum dots or organic dyes. We synthesize 3 monolayer thick nanoplatelets whose lateral dimensions vary from 10 nm x 10 nm to 186 x 100 nm and demonstrate compositional control on the smallest platelet sizes with STEM EELS

DESIGNING METAL-HALIDE PEROVSKITES WITH ENHANCED OPTICAL PROPERTIES AND STABILITY USING SURFACE LIGANDS

Metal-halide perovskites (MHPs), with formula ABX3 (A = methylammonium, formamidinium, or Cs+; B = Sn2+ or Pb2+; and X = Cl-, Br-, or I-) are versatile and attractive materials because of their tunable optical and electronic properties. These optical and electronic properties include tunable direct band gaps, high absorption coefficients, low exciton binding energies, relatively high electron and hole mobilities, narrow emission line-widths, and high photoluminescence (PL) quantum yields (ΦPL). Much of the initial excitement around organic metal-halide perovskites focused on their application in photovoltaics (PVs) based on thin polycrystalline films, whereas colloidal metal-halide perovskite nanocrystals (NCs) are now a subject of intense interest due to their highly desirable emission properties and low rate of non-radiative recombination for light emitting applications. However, both polycrystalline MHP thin films and their NC counterparts suffer from poor stability and are highly moisture sensitive. In this thesis, facile and rapid anion exchange and surface modification routes of MHP NCs are discussed using alkyltrichlorosilane, alkanethiols, and alkanethiol-aluminum trihalide combinations. In addition, similar approaches are employed to modify solution processed MHP thin films for fabricating efficient and stable photovoltaic devices. Rapid anion exchange and surface modification reactions with MHP NCs readily proceed via coupling and/or hydrolysis reactions of the surface ligands at room temperature. Both NCs, thin films, and thin film-based PV devices demonstrate significantly enhanced performance and stability upon surface modification. It is shown that alkyltrichlorosilanes (RSiCl3) can be used as Cl- sources for rapid anion exchange with host CsPbBr3 NCs during hydrolysis of alkytrichlorosilanes in the colloidal dispersion of CsPbBr3 NCs. Hydrolysis of alkyltrichlorosilanes leads to the formation of siloxane coated CsPbCl3 NCs with significantly improved ΦPL of up to 12% and improved long-term stability. In another study of surface modification, dodecanethiol modification of CsPbBr3 NCs is demonstrated to significantly enhance the stability and ΦPL of CsPbBr3 NCs, with ΦPL of near 100%. This surface modification can be expedited through exposure to UV light, which also induces thiol-ene reactions. A mixture of dodecanethiol (DDT) and AlX3 (X = Cl, Br, I) can be used to increase the applicability of alkanethiol treatment to all NC compositions. Here, DDT and AlX3 (X = Cl, Br, I) treatment transforms CsPbCl3 nanocubes into 4-15 monolayer thick CsPbX3 nanoplates (NPs) with high ΦPL (up to 47% and 65% for violet and blue emitting NPs, respectively, near 100% for green emitting NPs, and 81% for red emitting NPs) while maintaining good long-term stability at room temperature. NC modifications do not directly translate to their thin film counterparts because of variations in surface properties. However, with some ligand engineering, thiol derived surface ligand modified polycrystalline Cs0.15FA0.85PbI3 photovoltaics show power conversion efficiency of near 17% with enhanced stability. These findings will help pave the way towards efficient and stable future optoelectronic devices

Make selenium reactive again: Activating elemental selenium for synthesis of metal selenides ranging from nanocrystals to large single crystals

The inertness of elemental selenium is a significant obstacle in the synthesis of selenium-containing materials at low reaction temperatures. Over the years, several recipes have been developed to overcome this hurdle, however, most of these developed methods are associated with the use of highly toxic, expensive, and environmentally harmful reagents. As such, there is an increasing demand for the design of cheap, stable, and non-toxic reactive selenium precursors usable in the low-temperature synthesis of transition metal selenides with vast applications in nanotechnology, thermoelectrics, and superconductors. Herein, a novel synthetic route has been developed for activating elemental selenium using a solvothermal approach. By comprehensive 77Se NMR, Raman, and infrared spectroscopies, and gas chromatography-mass spectrometry, we show that the activated Se solution contained HSe–, [Se-Se]2–, and Se2– ions, as well as dialkyl selenide (R2Se) and dialkyl diselenide (R-Se-Se-R) species in dynamic equilibrium. This also corresponded with the first observation of naked [Se-Se]2– in solution. The versatility of the developed Se precursor was demonstrated by the successful synthesis of (i) the polycrystalline room-temperature modification of the β-Ag2Se thermoelectric material; (ii) large single crystals of superconducting β-FeSe; (iii) CdSe nanocrystals with different particle sizes (3-10 nm); (iv) nanosheets of PtSe2; and (v) mono- and dibenzyl selenides and diselenides at room temperature. The simplicity and diversity of the developed Se activation method holds promise for applied and fundamental research.This is a manuscript of an article published as Abusa, Yao, Philip Yox, Sarah D. Cady, Gayatri Viswanathan, Jemima Opare-Addo, Emily A. Smith, Yaroslav Mudryk, Oleg I. Lebedev, Frédéric A. Perras, and Kirill Kovnir. "Make Selenium Reactive Again: Activating Elemental Selenium for Synthesis of Metal Selenides Ranging from Nanocrystals to Large Single Crystals." Journal of the American Chemical Society 145, no. 41 (2023): 22762-22775. doi: https://doi.org/10.1021/jacs.3c08637

Probing the stability and solution processability of metal chalcogenide semiconducting materials

Metal chalcogenide (MCh) semiconductors have long research history due to their earth abundance, easy synthesizability, and accessible band gap tunability. People have realized their functionality as p-type semiconductors that provide good hole mobility and conductivity within the materials. However, MChs have easier reaction pathway with gas molecules (H2O and O2) compared to other semiconductors such as metal oxides. Also, poor solution processability of all MChs make low-cost thin film fabrication methods hard to achieve. In this thesis, we target to understand both the chemical stability and solution processability of MCh materials. ZnSe based inorganic organic layered hybrids (LHs) were exfoliated by solution-based method the first time. The study suggests under optimum conditions, two materials, ZnSe(butylamine) and ZnSe(octylamine) can be exfoliated down to bilayers. Possible defects of the exfoliated sheets were detected by TEM electron diffraction mapping which originate from the edge of the materials. Further surface analysis by XPS and Raman confirmed the composition change (degradation) at the surface. Also, the extent of degradation is corelated with the intrinsic stability of the materials, which ZnSe(octylamine) that coated with longer organic ligands tend to have slower degradation rate than ZnSe(butylamine). A series of ZnSe-LHs with different types of aliphatic amine ligands coated on ZnSe layers were synthesized next and the chemical stability of all these materials under exposure to atmospheric environment were carefully examined. The result revealed materials with the longest alkylamine show best resistivity toward reacting with gas molecules. To confirm the reaction pathway of ZnSe-LHs with gas molecules and provide an insight into how 2D materials in general can be degraded, H218O and 18O2 isotopic study combined with density function theory calculation provide a H2O initiated degradation pathway of ZnSe-LHs. Dissolution mechanism of pure metals in thiol-amine cosolvent system (alkahest) was studied at the end of the thesis with the aim to synthesize MChs by solution processing method. By mixing ethylenediamine (en) with 2-mercaptoethanol (mer), a wide range of metals can be dissolved well in this semi-eutectic like system that has ionic conductivity. Although mer/en alkahest shows good stability in inert condition, it will form disulfide product when mixing in air. Several tris(ethylenediamine) metal (II) bis(2-hydroxylethanethiolate) crystals have been isolated from this cosolvent system in inert environment. All metals are oxidized into 2+ and coordinated with three neutral en ligands, the counter ions are two deprotonated mer ligands, which show great instability and will be further oxidized upon exposure to air. Also, in this alkahest system, solutions of redox active metals such as Mn, Co and Fe showed reversible color change between inert and air condition. Studies carried out by uv-vis and EPR study showed change of metal oxidation state and potential formation of superoxide ions within the solution when exposed to air

Electrochemical Gelation Of Metal Chalcogenide Quantum Dots

Quantum dots (QDs) are attractive because of their unique size-dependent optical and electronic properties and high surface area. They are tested in research for diverse applications, including energy conversion, catalysis, and sensing. Assembling QDs into functional solid-state devices while preserving their attractive properties is a challenge. Methods currently under the research are not effective in directly fabricating QDs onto devices, making large area assemblies, maintaining the high surface area by forming 3D porous structures, and conducting electricity for applications such as sensing. QD gels are an example of QD assemblies that consist of a 3D porous interconnected QD network. They are ideal candidates for gas sensing due to two main reasons. First, their extremely high surface area and the accessibility through porous openings provide a large number of interactions per unit volume of the material. Second, the partial removal of surface ligands during the gelation increases the active sites and, therefore, the number of signals generated compared to QDs covered by ligands. Preparation of QD gels was conventionally carried out by directly adding oxidizing agents to a stable QD dispersion.9 Dimensions and shapes of chemical gels are defined by a mold, so it does not allow the flexibility to introduce fine detail to the QD gel form. Using chemical gels in applications such as sensing or catalysis often requires a gel deposition on an electrode by following a technique such as drop-casting. This dissertation aims to develop and understand the electrochemical techniques to assemble QDs into porous networks. QD assemblies are prepared using two new methods: oxidative electrochemical gelation (OE-gelation) and metal-mediated electrogelation (ME-gelation). Material properties, mechanisms, and applications of the two gelation techniques are studied in detail. OE-gelation is the first use of electrochemical techniques for QD gel synthesis. This technique offers the ability to produce QD gels within minutes and tunable gelation by selecting different electrochemical parameters. The kinetics and thermodynamics studies have revealed that the electrogelation of metal chalcogenide QDs proceeds via a two-step mechanism: the electrochemical removal of the organic capping agents followed by the oxidative dichalcogenide bonds formation. More interestingly, we have found that the gelation process is reversible by applying a negative potential to reduce the dichalcogenide bonds that connect the QD in the gel network. The facile electrogelation of QDs significantly simplifies the preparation of gel-based sensors. We demonstrated the one-step fabrication of CdS xerogel sensors for NO2 gas sensing. The resulting CdS xerogel sensors exhibit an outstanding performance toward NO2 gas sensing at room temperature. While the OE-gelation technique offers unique advantages for applications such as gas sensing, OE-gels are unstable in reducing conditions as their disulfide bonds can go back to sulfide form. Therefore, electrochemistry was used to control the in-situ generation of metal ions, making QD ME-gels via metal-ligand bonding. ME-gelation was demonstrated using Co, Zn, and Cu electrodes. TEM images and inductively coupled plasma mass spectrometer (ICP-MS) analysis revealed that QDs in a ME-gel are connected via QD-metal ion-QD bonding. Gel growth was controlled by the charge employed in the oxidation of metal electrodes. The optical and physical properties of the formed gels were characterized, which confirmed the formation of quantum confined macroscale 3-D architectures of QDs. The use of the ME-gelation method in fast QD pattering was demonstrated using printed circuit board electrodes. Finally, a competition between ME- and OE-gelations was studied to understand the interplay of the two electrogelation methods. Elemental compositions and the microscopic connectivity in produced gels suggested the OE-gelation dominates the QD assembly. A byproduct formed during the OE-gelation blocked the electrode surface from further producing ME-gels

Semiconductor Nanocrystals: From Quantum Dots to Quantum Disks

The bottom-up colloidal synthesis opened up the possibility of finely tuning and tailoring the semiconductor nanocrystals. Numerous recipes were developed for the preparation of colloidal semiconductor nanocrystals, especially the traditional quantum dots. However, due to the lack of thorough understanding to those systems, the synthesis chemistry is still on the empirical level. CdS quantum dots synthesis in non-coordinating solvent were taken as a model system to investigate its molecular mechanism and formation process, ODE was identified as the reducing agent for the preparation of CdS nanocrystals, non-injection and low-temperature synthesis methods developed. In this model system, we not only proved it\u27s possible to systematically study the formation procedure of semiconductor nanocrystals, the insight learned during the research but also enhanced our understanding to this delicate system and promoted the development of synthetic chemistry. Although quantum dots could be routinely prepared in the lab with mature recipes, the colloidal semiconductor quantum well type materials are still hard to fathom. CdSe quantum disks structure was thoroughly analyzed with polar axes as the growth direction along the thickness direction, with both basal planes ended with Cd atom layer, which was coordinated with carboxylate ligands. Besides, four different thickness CdS quantum disks were prepared, its size-dependent lattice dilation, extremely sharp band-edge emission, and two-order of magnitude faster photoluminescence decay compared to quantum dots was investigated

SURFACE PROPERTIES AND ELECTRONIC STRUCTURE OF SEMICONDUCTOR NANOCRYSTALS

Semiconductor nanomaterials, including quantum dots and nanoplatelets, are poised to address pressing global energy challenges due to their unique optical and electronic properties. These optoelectronic properties arise due to quantum confinement of charge carriers below the Bohr radius. Due to the nanoscale nature of these materials, they exhibit a high surface-to-volume ratio, and nearly all desirable and undesirable properties are influenced by the surface. Notably, the QD surface is highly susceptible to defectivity that can hinder the utility of the material for targeted applications. For example, undercoordinated surface ions can localize excited state charge carriers thereby quenching the photoluminescence of the material — a key property of semiconductor nanomaterials for display technologies. Alternatively, the same surface defects can impact charge transfer reactions necessary for photocatalytic reactions.Material optimization by defect mitigation requires a holistic understanding of their origin, impact, and methods to eliminate defects. This compendium of research on quantum dots and nanoplatelets leverages complementary spectroscopic techniques to deliver molecular-level insight to the surface properties of the material and demonstrates a unique application of nanomaterials for polariton-mediated chemical reactivity. Defect elimination by post-synthetic treatment of defect-rich QDs is monitored by photoluminescence spectroscopy and 1H NMR spectroscopy which allows for quantitative characterization of the QD surface. These techniques are used to establish an effective method of defect remediation by controlled chemical reactions. These concepts are expanded to applications of QDs including photocatalysis. Precisely tuned defects are controllably introduced to the QD surface and their impact on interfacial charge transfer is monitored by cyclic voltammetry. Complementary work to advance NMR spectroscopy and diffusiometry methods for surface characterization are presented to guide highly controlled ligand exchange reactions. In an alternative area of research, semiconductor nanoplatelets are integrated in a surface lattice resonance cavity to generate exciton polaritons and in-situ control of exciton polaritons is achieved by an applied electrochemical potential. Ultimately, a foundational understanding of QD surface properties is crucial to realizing the full potential of QD-based devices.Doctor of Philosoph

Perovskite-inspired materials for photovoltaics and beyond—from design to devices

Funder: Ministry of Education, TaiwanAbstract: Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed

Perovskite-inspired materials for photovoltaics and beyond—from design to devices

Funder: Ministry of Education, TaiwanAbstract: Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed

Halide perovskite nanocrystal-based light emitting diodes

Lead halide perovskite (LHP)-based optoelectronic device has been a hot research topic in the last years, owing to the versatile optoelectronic properties of this class of materials, such as large light absorption coefficient, long-range charge carrier mobility, high defect tolerance, direct bandgap, and to the facile synthesis process. Since their synthesis by Kovalenko group in 2015, LHP nanocrystals (NCs) gained increasing attention also for application in light-emitting diodes (LEDs), owing to the possibility to easily tune their emission wavelength across the whole range of visible light and their high color purity. A detailed introduction on the chemistry and physics of LHP NCs, some basic knowledge about LEDs and the development of LEDs based on the LHP NCs are provided in Chapter 1. In Chapter 2, the chemicals, the basic experiments and the characterization techniques adopted in the thesis are introduced. In Chapter 3, I identified four issues that need to be solved before the colloidal LHP NCs could be efficiently applied into the fabrication of high efficient and stable LEDs. Given that long-chain and insulating ligands are used during the NC synthesis to ensure their good solubility and stability in solution, conductivity of LHP NCs films is too low to transport carriers efficiently. Secondly, post-treatments on the LHP NCs, often aimed at improving the aforementioned transport properties, are easy to introduce new defects and compromise the optical properties of the LHP NCs. Thirdly, since LHP NCs are ionic crystals and sensitive to humid air as well as to post-treatments, their long-term stability is still a big issue. Finally, it is difficult to obtain smooth and compact LHP NCs films, which are necessary for low current leakage and high-efficient devices. I addressed the above four issues step-by-step and obtained highly stable LHP NCs films. A combination of benzoyl bromide, ammonium thiocyanate and ethyl acetate was used to treat the pristine LHP NC solution. The method demonstrated to be able to improve the conductivity of the LHP NCs films and well retain the optical properties and morphology even after ten months. In Chapter 4, I employed the obtained NCs as emissive layer in green LEDs based on the proposed treatment method in Chapter 3. The champion device showed high efficiency of 1.2% at 518 nm with a maximum brightness near 3000 cd/m2 and high stability during operation with a half-lifetime of 27 min at a constant bias of 5 V as well as during storage (23 days in air). Furthermore, I conducted a mechanism study on the efficiency roll-off of the NCs-based LEDs using conductive atomic force microscopy (c-AFM). Morphology and current distribution of the NCs films under increasing bias were collected and a new insight about the efficiency roll-off was proposed. In Chapter 5, I further focused on the improvement of the efficiency of blue LEDs based on LHP NCs, which is still lower than that of green and red ones. In this context, I studied the effect of addition of various metal halides during the synthesis of LHP NCs on their optical properties. I found that the post-synthesis addition of CuCl2 leads to the formation of NCs with sky-blue emission and high stability in air. I applied the obtained NCs in the fabrication of sky-blue LEDs. The champion device, based on NCs with further optimized ligands, produces the up-to-date highest external quantum efficiency (EQE) of 5.02% and the highest luminance of 130 cd/m2 at the maximum EQE. In summary, this thesis firstly provides a promising route and proposes a possible mechanism to achieve high stable LHP NCs film. Secondly, efficient green LHP LEDs were obtained. Thirdly, a possible mechanism of the efficiency roll-off in LHP NC LED was proposed and may give guides for the design of NC LEDs with suppressed efficiency roll-off in the future. Last but not least, high efficient and stable sky-blue LED was fabricated based on CuCl2-treated LHP NCs, paving a promising way towards the fabrication of highly efficient LHP NC-based blue LEDs