Publications

Lanthanide-doped nanocrystals are of particular interest for the research community not only due to their ability to shape light by downshifting, quantum cutting, and upconversion but also because novel optical properties can be found by the precise engineering of core-shell nanocrystals. Because of the large surface area-to-volume ratio of nanocrystals, the luminescence is typically suppressed by surface quenching. Here, we demonstrate a mechanism that exploits surface quenching processes to improve the luminescence of our core-shell lanthanide-doped nanocrystals. By carefully tuning the shell thickness of inert β-NaLuF₄ around β-NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺, we unravel the relationship between quantum yield and shell thickness, and quantify surface quenching rates for the relevant Er³⁺ and Yb³⁺ energy levels. This enhanced understanding of the system's dynamics allowed us to design nanocrystals with a surface quenching-assisted mechanism for bright NIR to NIR downshifting with a distinctive efficiency peak for an optimized shell thickness.

We present electrical measurements of single Au and CdSe nanocrystals. The devices are fabricated using a hybrid scheme which combines electron beam lithography and wet chemistry to bind nanocrystals in tunneling contact between two closely spaced metallic leads. The current–voltage characteristics of these devices exhibit a Coulomb staircase with a charging energy of ∼50 meV. This technique is readily adapted to the study of a host of nanocrystals made by solution chemistry.

A phase transition within the ligand shell of core/shell quantum dots is studied in the prototypical system of colloidal CdSe/CdS quantum dots with a ligand shell composed of bound oleate (OA) and octadecylphosphonate (ODPA). The ligand shell composition is tuned using a ligand exchange procedure and quantified through proton NMR spectroscopy. Temperature-dependent photoluminescence spectroscopy reveals a signature of a phase transition within the organic ligand shell. Surprisingly, the ligand order to disorder phase transition triggers an abrupt <i>increase</i> in the photoluminescence quantum yield (PLQY) and full-width at half-maximum (FWHM) with increasing temperature. The temperature and width of the phase transition show a clear dependence on ligand shell composition, such that QDs with higher ODPA fractions have sharper phase transitions that occur at higher temperatures. In order to gain a molecular understanding of the changes in ligand ordering, Fourier transform infrared and vibrational sum frequency generation spectroscopies are performed. These measurements confirm that an order/disorder transition in the ligand shell tracks with the photoluminescence changes that accompany the ligand phase transition. The phase transition is simulated through a lattice model that suggests that the ligand shell is well-mixed and does not have completely segregated domains of OA and ODPA. Furthermore, we show that the unsaturated chains of OA seed disorder within the ligand shell.

Liquid Nanocrystals In high-resolution transmission electron microscopy, grid materials are used to support solid samples while providing a means for preventing a build-up of static charge. Liquids are difficult to study at the same atomic resolution and require encapsulation to prevent excess sample movement, sample damage, or evaporation. Materials that have been used for liquid cells, like silicon nitride or silicon oxide, need thick layers and have poor electron transmittance at the thicknesses required because they contain high atomic number elements. Yuk et al. (p. 61 ; see the Perspective by Colliex ) show that liquids can be encapsulated in graphene sheets, and through this technique, they studied the formation of platinum nanocrystals with atomic resolution. The crystals could be tracked as they selectively coalesced, modified their shape, and formed surface facets.

Plasmon rulers, consisting of pairs of gold nanoparticles, allow single-molecule analysis without photobleaching or blinking; however, current plasmon rulers are irreversible, restricting detection to only single events. Here, we present a reversible plasmon ruler, comprised of coupled gold nanoparticles linked by a single aptamer, capable of binding individual secreted molecules with high specificity. We show that the binding of target secreted molecules to the reversible plasmon ruler is characterized by single-molecule sensitivity, high specificity, and reversibility. Such reversible plasmon rulers should enable dynamic and adaptive live-cell measurement of secreted single molecules in their local microenvironment.

Significance Concentrator photovoltaic (CPV) systems, wherein light focuses onto multijunction solar cells, offer the highest efficiencies in converting sunlight to electricity. The performance is intrinsically limited, however, by an inability to capture diffuse illumination, due to narrow acceptance angles of the concentrator optics. Here we demonstrate concepts where flat-plate solar cells mount onto the backplanes of the most sophisticated CPV modules to yield an additive contribution to the overall output. Outdoor testing results with two different hybrid module designs demonstrate absolute gains in average daily efficiencies of between 1.02% and 8.45% depending on weather conditions. The findings suggest pathways to significant improvements in the efficiencies, with economics that could potentially expand their deployment to a wide range of geographic locations.

We investigate the quantum size effects in the pressure-induced direct-to-indirect band gap transition in InP nanocrystals. Hydrostatic pressures of up to 13 GPa are applied to two different sizes of InP nanocrystal samples in a diamond anvil cell. The band gap pressure dependence and the nature of the emitting states are studied by photoluminescence (PL) and fluorescence line narrowing (FLN) techniques at 10 K. Pressure-dependent FLN spectra show that the nature of the emitting states at pressures up to 9 GPa is similar to that at ambient pressure, suggesting that no direct-to-indirect transition happens below 9 GPa. For both sizes, the PL peak energy exhibits a strong blueshift with rising pressure until approximately 9 to 10 GPa. Above this pressure, the PL peak position slightly shifts red. Beyond 12 GPa, the band gap emission intensity becomes extremely weak and trap emission dominates the PL spectra. As the pressure is released, both the luminescence intensity and the peak position recover in a fully reversible manner. The change in the sign of the band gap energy pressure dependence and the disappearance of the band edge luminescence indicate the pressure-induced direct-to-indirect band gap transition. Contrary to theoretical calculations, no substantial reduction of the transition pressure is observed in the nanocrystal cases compared to the bulk transition pressure.

Liquid-phase transmission electron microscopy (TEM) can probe and visualize dynamic events with structural or functional details at the nanoscale in a liquid medium. Earlier efforts have focused on the growth and transformation kinetics of hard material systems, relying on their stability under electron beam. Our recently developed graphene liquid cell technique pushed the spatial resolution of such imaging to the atomic scale but still focused on growth trajectories of metallic nanocrystals. Here, we adopt this technique to imaging three-dimensional (3D) dynamics of soft materials instead, double strand (dsDNA) connecting Au nanocrystals as one example, at nanometer resolution. We demonstrate first that a graphene liquid cell can seal an aqueous sample solution of a lower vapor pressure than previously investigated well against the high vacuum in TEM. Then, from quantitative analysis of real time nanocrystal trajectories, we show that the status and configuration of dsDNA dictate the motions of linked nanocrystals throughout the imaging time of minutes. This sustained connecting ability of dsDNA enables this unprecedented continuous imaging of its dynamics via TEM. Furthermore, the inert graphene surface minimizes sample-substrate interaction and allows the whole nanostructure to rotate freely in the liquid environment; we thus develop and implement the reconstruction of 3D configuration and motions of the nanostructure from the series of 2D projected TEM images captured while it rotates. In addition to further proving the nanoconjugate structural stability, this reconstruction demonstrates 3D dynamic imaging by TEM beyond its conventional use in seeing a flattened and dry sample. Altogether, we foresee the new and exciting use of graphene liquid cell TEM in imaging 3D biomolecular transformations or interaction dynamics at nanometer resolution.

Images of individual Te chiral nanoparticles and labels of their handedness for classifier training.

In the present work, we analyze the geometry and composition of the nanostructures obtained from the oxidation of iron nanoparticles. The initial oxidation of iron takes place by outward diffusion of cations through the growing oxide shell. This net material flow is balanced by an opposite flow of vacancies, which coalesce at the metal/oxide interface. Thus, the partial oxidation of colloidal iron nanoparticles leads to the formation of core−void−shell nanostructures. Furthermore, the complete oxidation of iron nanoparticles in the 3−8 nm size range leads to the formation of hollow iron oxide nanoparticles. We analyze the size and temperature range in which vacancy coalescence during oxidation of amine-stabilized iron nanoparticles takes place. Maghemite is the crystallographic structure obtained from the complete oxidation of iron nanoparticles under our synthetic conditions.