V Volkov

Long Electron−Hole Separation of ZnO-CdS Core−Shell Quantum Dots

The tunability of electronic and optical properties of semiconductor nanocrystal quantum dots (QDs) has been an important subject in nanotechnology. While control of the emission property of QDs in wavelength has been studied extensively, control of the emission lifetime of QDs has not been explored in depth. In this report, ZnO-CdS core-shell QDs were synthesized in a two-step process, in which we initially synthesized ZnO core particles, and then stepwise slow growth of CdS shells followed. The coating of a CdS shell on a ZnO core increased the exciton lifetime more than 100 times that of the core ZnO QD, and the lifetime was further extended as the thickness of shell increased. This long electron-hole recombination lifetime is due to a unique staggered band alignment between the ZnO core and CdS shell, so-called type II band alignment, where the carrier excitation holes and electrons are spatially separated at the core and shell, and the exciton lifetime becomes extremely sensitive to the thickness of the shell. Here, we demonstrated that the emission lifetime becomes controllable with the thickness of the shell in ZnO-CdS core-shell QDs. The longer excitonic lifetime of type II QDs could be beneficial in fluorescence-based sensors, medical imaging, solar cells photovoltaics, and lasers.

The newly installed aberration corrected dedicated STEM (Hitachi HD2700C) at Brookhaven National Laboratory

The Hitachi HD2700C was recently successfully installed at the newly established Center for Functional Nanomaterials, Brookhaven National Lab (BNL). It was the first commercial aberration corrected electron microscope manufactured by Hitachi. The instrument is based on HD2300, a dedicated STEM developed a few years ago to complete with the VG STEMs [1]. The BNL HD2700C has a cold-field-emission electron source with high brightness and small energy spread, ideal for atomically resolved STEM imaging and EELS. The microscope has two condenser lenses and an objective lens with a gap that is slightly smaller than that of the HD2300, but with the same ±30° sample tilts capability. The projector system consists of two lenses that provide more flexibility in choosing various camera lengths and collection angles for imaging and spectroscopy. There are seven fixed and retractable detectors in the microscope. Above the objective lens is the secondary electron detector to image surface morphology of the sample. Below are the Hitachi HAADF and BF detector for STEM, and a Hitachi TV rate (30frame/sec) CCD camera for fast observations and alignment. The Gatan 14bit 2.6k×2.6k CCD camera located further down is for diffraction (both convergent and parallel illumination) and Ronchigram analysis. The Gatan ADF detector and EELS spectrometer (a specially modified high energy resolution Enfina spectrometer incorporating full 2nd and dominant 3rd order corrected optics and low drift electronics, a 16bit 100×1340 pixel CCD) are located at the bottom of the instrument. The CEOS probe corrector has been modified and optimized for this instrument.

Double-resolution electron holography with simple Fourier transform of fringe-shifted holograms

We propose a fringe-shifting holographic method with an appropriate image wave recovery algorithm leading to exact solution of holographic equations. With this new method the complex object image wave recovered from holograms appears to have much less traditional artifacts caused by the autocorrelation band present practically in all Fourier transformed holograms. The new analytical solutions make possible a double-resolution electron holography free from autocorrelation band artifacts and thus push the limits for phase resolution. The new image wave recovery algorithm uses a popular Fourier solution of the side band-pass filter technique, while the fringe-shifting holographic method is simple to implement in practice.

Detection Limits and Resolution for Quantitative Phase Retrieval through Transport-of-Intensity

Phase retrieval based on the transport-of-intensity formalism developed for light optics has been extended to electron optics, and seems especially suitable for studying magnetic structures [1]. The heart of the technique is based on the transport-of-intensity equation (TIE) that relates transverse derivatives (normal to direction of electron beam propagation) of the electron phase to the longitudinal derivative (along the beam direction) of the electron wave intensity [2]. The TIE arises from the image-wave spreading due to Fresnel propagation, and essentially requires two images recorded with different focus in order to estimate the longitudinal derivative of the electron intensity. Solving the TIE (a second-order differential equation) allows the electron phase to be recovered. The TIE technique does not require special instrumentation and, ostensibly, may be applied to an arbitrarily large (or small) field of view. The technique is not overly demanding in terms of sample preparation or data acquisition. While the TIE approach suffers certain limitations, either fundamentally (e.g., specifying proper boundary conditions in solving the equation), or technically (e.g., image alignment and defocus calibration), an important obstacle to the approach lies in proper interpretation of details in the recovered (projected) phase map. In order to quantitatively relate the results obtained with the TIE technique to meaningful materials properties, electromagnetic calculations must be performed, and, in this context, a good understanding of the limits and resolution of the technique is needed.

Imaging and Spectroscopy of Energy-Related Nanomaterials

energy technologies has become one of the most important missions of our nation. The scientific challenge is to discover new ways to efficiently generate, transport, store, and use energies. In the past decade, our group has devoted significant efforts on energy related materials, including superconductors [1], thermoelectrics [2], photovoltaics, fuel cells, and batteries to understand their structure and property relationship. In this presentation, we report our recent work on Li-ion batteries [3] and core-shell nanocatalysts for hydrogen fuel-cell applications [4].

TEM Characterization and Optical Properties of Hetero-Structured ZnO/CdS Quantum Dots with Long Exciton Lifetime

The tunability of electronic and optical properties of semiconductor nano-crystalline quantum dots (QDs) has been an important subject in nanotechnology. While the control of emission properties of QDs on different wavelengths has been studied extensively, the control of emission lifetimes for many types of QDs has not been explored in depth. In this work we provide combined TEM/ED/EDX characterization in parallel with optical properties measurement for the new ZnOCdS member from family of AII-BVI hetero-structured QDs, especially important for visible optical range and solar applications. Such novel QDs were synthesized in two-step process [1], starting with synthesis of core ZnO nanoparticles followed by the slow stepwise growth of CdS shells. The energy-band diagram ZnO-CdS (Fig.1a) is specific for type-II heterostructures and favors effective electron-hole spatial separation at hetero-junction interfaces ZnO-CdS upon QDs photon’s excitation, potentially leading to long-living excitons and long photoluminescence (PL) decay times.

On Almost Double Resolution Improvement for Off-Axis Electron Holography

Off-axis electron holography (EH) is currently the most popular and direct experimental technique for precise phase measurements of complex object exit-wave functions created by scattering of coherent electron beams on complex nano-objects studied by the EH in transmission electron microscopy (TEM). Usually, practical resolution of reconstructed phase map is limited by an appropriate Fourier-filter window size applied to one of side-bands of the Fourier transform applied to experimental holograms recorded for different nanoobjects in TEM. The presence of strong autocorrelation spot in typical Fourier-transformed holograms is setting an optimal choice of the Fourier window and, whence, resolution limit, which typically does not exceed 1/2-1/3 of the distance ω (EH-carrier frequency) between the sideband and autocorrelation maximum in reciprocal space. Even for these optimized conditions the residual “cross-talking” effect between the sideband and central autocorrelation spot may still have a negative effect on the quality of reconstructed phase maps, if the autocorrelation spot appears to be strong and/or carrier frequency too low. Recently, similar problem was examined in light optics and novel solution [1] was suggested on resolution improvement for optical holograms without changing experimental setup.

Uranium single atom imaging and EELS mapping using aberration corrected STEM and LN2 cold stage

Single heavy atoms on a thin carbon substrate represent a nearly ideal test specimen to evaluate STEM performance [1,2]. The single atoms approximate point scatters when imaged with the STEM large angle annular detector. (This is not necessarily true when using small angle scattering in TEM to make a phase contrast image.) The high scattering power relative to the substrate gives a high signal-to-noise ratio, even with relatively low beam current. The thinness of the sample eliminates any issues regarding depth of focus or channelling effects. The specimen was prepared in a manner similar to negative staining, except with a much lower concentration of Uranyl Acetate. The sample shown consisted of tobacco mosaic virus (TMV) on a 2nm thick carbon film substrate supported by holey film. The sample was rinsed several times with 0.01% Uranyl Acetate (compare to 2% normally used for negative staining) and air dried.

Electron Microscopy Study of Layered Misfit Cobalt Oxide [Ca2CoO3]0.62CoO2

Layered cobaltates are of great interest due to its unique thermoelectric and magnetoresistance properties and practical thermal power applications. In order to understand the origin of physical properties of layered cobaltates and, in particular, of [Ca2CoO3]0.62CoO2 cobalt oxide that exhibits high thermoelectric power, an accurate determination of the crystal structure is required. There are several ambiguities in the structure analysis of this compound performed by X-ray diffraction (see [1] and [2]) and electron microscopy studies [3] especially the models of the cobalt disorder and distribution of vacancies. Direct observation of vacancies is a very difficult task. In case of their periodic distribution it becomes possible to investigate this problem using electron microscopy, which stimulated us to re-examine the structure of [Ca2CoO3]0.62CoO2 by the electron diffraction (ED) and high resolution electron microscopy (HREM) methods.

Ferromagnetic Domains and Phase Separation in La5/8-yPryCa3/8MnO3 Manganites: Observations by Lorentz Electron Microscopy

La 5/8-y Pr y Ca 3/8 MnO 3 (LPCMO) manganese oxides (space group Pnma and lattice parameters a≈0.546 nm, b≈0.772 nm, c≈0.548 nm) have attracted great scientific interest due to their unusual electronic transport and magnetic properties, such as colossal magnetoresistance (CMR) and charge ordering. To utilize these materials for practical applications, it is vital to understand their structureproperty relationships, especially the role of ferromagnetic (FM) domain structures and phase separation (PS). [1, 2] Here, we present some of our recent work using in-situ electron microscopy and magnetic phase retrieval using a field-free electron microscope (JEOL 2100F-M).