School of Chemistry - Research Publications
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ItemSedimentation of C60 and C70: Testing the Limits of Stokes' Law(AMER CHEMICAL SOC, 2018-11-01)Virtually all dynamic methods for determining particle size on the nanoscale use the Stokes-Einstein-Sutherland (SES) equation to convert the diffusion coefficient into a hydrodynamic radius. The validity of this equation on the nanoscale has not been rigorously validated by experiment. Here we measure the sedimentation rates and diffusion coefficients of C60 and C70 in toluene using analytical ultracentrifugation and compare the results to the SES equation. We find that the SES equation for the drag force (nonslip boundary condition) works down to 1 nm length scales.
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ItemFabrication of a Three-Dimensional Plasmon Ruler Using an Atomic Force Microscope(AMER CHEMICAL SOC, 2019-08-15)We have assembled a three-dimensional (3D) plasmon ruler using an atomic force microscope (AFM) tip to manipulate single gold nanocrystals on top of electron beam lithography fabricated base layers. The 3D structures exhibit several polarization-dependent surface plasmon scattering peaks, including symmetric and asymmetric Fano resonances. We map these resonances as a function of the degree of asymmetry of the structure. We show that the coupled surface plasmon resonances are extremely sensitive to the position of the upper particle and that the resonances can be engineered and tuned using an AFM tip to move the upper nanocrystal just a few Angstroms.
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ItemTuning Single Quantum Dot Emission with a Micromirror(AMER CHEMICAL SOC, 2018-02)The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.
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ItemAqueous Synthesis of High-Quality Cu2ZnSnS4 Nanocrystals and Their Thermal Annealing Characteristics(AMER CHEMICAL SOC, 2018-01-30)Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by Sn2S64- and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes.
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ItemThe Degradation and Blinking of Single CsPbl3 Perovskite Quantum Dots(AMER CHEMICAL SOC, 2018-06-28)We demonstrate using single molecule spectroscopy that inorganic CsPbI3 perovskite quantum dots (PQDs) undergo an irreversible, photoaccelerated reaction with water that results in a blue-shift of the photoluminescence (PL) and ultimately to complete quenching of the emission. We find that decomposition does not take place in the presence of oxygen alone but that it requires light and water. We also analyze the blinking for some stable PQDs and find a continuous distribution of emission states with a linear correlation between intensity and lifetime. We postulate that, in addition to charging and discharging processes, blinking arises from the activation and deactivation of nonradiative recombination centers in the PQDs.
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ItemNo Preview AvailableImpact of Surface Functionalization on the Quantum Coherence of Nitrogen-Vacancy Centers in Nanodiamonds(AMER CHEMICAL SOC, 2018-04-18)Nanoscale quantum probes such as the nitrogen-vacancy (NV) center in diamonds have demonstrated remarkable sensing capabilities over the past decade as control over fabrication and manipulation of these systems has evolved. The biocompatibility and rich surface chemistry of diamonds has added to the utility of these probes but, as the size of these nanoscale systems is reduced, the surface chemistry of diamond begins to impact the quantum properties of the NV center. In this work, we systematically study the effect of the diamond surface chemistry on the quantum coherence of the NV center in nanodiamonds (NDs) 50 nm in size. Our results show that a borane-reduced diamond surface can on average double the spin relaxation time of individual NV centers in nanodiamonds when compared to thermally oxidized surfaces. Using a combination of infrared and X-ray absorption spectroscopy techniques, we correlate the changes in quantum relaxation rates with the conversion of sp2 carbon to C-O and C-H bonds on the diamond surface. These findings implicate double-bonded carbon species as a dominant source of spin noise for near surface NV centers. The link between the surface chemistry and quantum coherence indicates that through tailored engineering of the surface, the quantum properties and magnetic sensitivity of these nanoscale systems may approach that observed in bulk diamond.
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ItemElectronic Structure Engineering in ZnSe/CdS Type-II Nanoparticles by Interface Alloying(American Chemical Society, 2014-06-19)We report the synthesis and characterization of type-II ZnSe/CdS semiconductor nanocrystals that exhibit strong charge separation, high photoluminescence quantum yields, low optical gain thresholds, and alloyed core–shell interfaces. Shell growth rates and the degree of alloying both depend strongly on the shelling temperature. The core–shell NCs exhibit band edge PL with emission wavelengths spanning the blue to orange region of the electromagnetic spectrum (380–562 nm). Fluorescence quantum yields up to 75% can be obtained by deposition of an additional ZnS layer. Transient absorption spectroscopy reveals that the population of the first two exciton states (1Se–1Sh, 1Se–2Sh) in the type-II structures can be controlled by alloying. Increased alloying leads to a greater population of the 2S hole state exciton.
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ItemDirect Assembly of Large Area Nanoparticle Arrays(AMER CHEMICAL SOC, 2018-08)A major goal of nanotechnology is the assembly of nanoscale building blocks into functional optical, electrical, or chemical devices. Many of these applications depend on an ability to optically or electrically address single nanoparticles. However, positioning large numbers of single nanocrystals with nanometer precision on a substrate for integration into solid-state devices remains a fundamental roadblock. Here, we report fast, scalable assembly of thousands of single nanoparticles using electrophoretic deposition. We demonstrate that gold nanospheres down to 30 nm in size and gold nanorods <100 nm in length can be assembled into predefined patterns on transparent conductive substrates within a few seconds. We find that rod orientation can be preserved during deposition. As proof of high fidelity scale-up, we have created centimeter scale patterns comprising more than 1 million gold nanorods.
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ItemAn Optically Responsive Soft Etalon Based on Ultrathin Cellulose Hydrogels(WILEY-V C H VERLAG GMBH, 2019-10)Abstract Stimuli‐responsive optical etalons are an exciting class of next‐generation sensors because they are scalable, cost‐effective and offer tunability of their optical response across the entire UV–vis–NIR wavelength range. In this study, double‐network cellulose hydrogels are used as a soft, responsive medium and are incorporated into Fabry–Pérot optical etalons. The thin cellulose hydrogel layer can be solution processed. The hygroscopic hydrogel undergoes both refractive index and thickness changes in response to changes in humidity. This leads to strong changes in reflection due to optical interference within the metal–insulator–metal (MIM) cavity. The response can be optimized by adjusting the chemical crosslinker ratio. These flexible MIM structures provide a robust platform for optically based chemical sensors.
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ItemDirected Chemical Assembly of Single and Clustered Nanoparticles with Silanized Templates(AMER CHEMICAL SOC, 2018-06-26)The assembly of nanoscale materials into arbitrary, organized structures remains a major challenge in nanotechnology. Herein, we report a general method for creating 2D structures by combining top-down lithography with bottom-up chemical assembly. Under optimal conditions, the assembly of gold nanoparticles was achieved in less than 30 min. Single gold nanoparticles, from 10 to 100 nm, can be placed in predetermined patterns with high fidelity, and higher-order structures can be generated consisting of dimers or trimers. It is shown that the nanoparticle arrays can be transferred to, and embedded within, polymer films. This provides a new method for the large-scale fabrication of nanoparticle arrays onto diverse substrates using wet chemistry.