Johnny Pham

Bio: Johnny Pham is an academic researcher from Lawrence Berkeley National Laboratory. The author has contributed to research in topics: Vapor–liquid–solid method & Nano-. The author has an hindex of 3, co-authored 4 publications receiving 2371 citations. Previous affiliations of Johnny Pham include University of California, Berkeley.

Controlled growth of ZnO nanowires and their optical properties

Abstract: This article surveys recent developments in the rational synthesis of single-crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power-dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short-wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 1010 cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single-crystalline, well-facetted nanowires can function as self-contained optical resonance cavities. This argument is further supported by our recent near-field scanning optical microscopy (NSOM) studies on single nanowires.

Morphogenesis of One‐Dimensional ZnO Nano‐ and Microcrystals

Abstract: timescales of hundreds of seconds (0.00 lm 2 s ±1). It is interesting that in the neutral MEA experiment, in which some of the surface amine groups are protonated, the diffusion coefficient was slightly lower than in the other cases. A surprising result was obtained when measuring the diffusion coefficients of the three sizes of MESA particles on the neutral surface derivatized with hexadecanethiol (C 16 H 33 SH). Comparing D values to those found on the MESA surface, we found near agreement for all but the small-diameter particles (6 lm ” 90 nm), which reproducibly stuck to the substrate immediately on contact. For larger particles, the long-range elec-trostatic force (presumably between the negatively charged sulfonate groups and an image charge in the substrate) is expected to dominate, [22] but for smaller ones the short-range van der Waals force is more important. Apparently, the two are closely balanced for the C 16 H 33 SH functionalized surface with rods of the size investigated here. While the scaling of these interactions is not completely understood at present, these initial studies provide some guidance as to the conditions that are desirable for nanorod assembly experiments. The observation of pH-dependent diffusion may, for example, be useful for affixing particles to specific areas or in specific conformations on basic surfaces. More importantly, the particle tracking method described here provides a simple and convenient method for quantifying the surface diffusion of non-spherical particles under arbitrary conditions. Experimental Whatman Al 2 O 3 filter membranes that contain 300±350 nm diameter internal pores were used as a template material. For 90 nm internal pore diameter, Al 2 O 3 membranes were prepared in-house by the electrochemical anodization of an Al plate [23]. In both cases, one face of the membranes was coated with approximately 150 nm of thermally evaporated Ag. More Ag was electrodepos-ited (Silver 1024, Technic, Inc.) directly onto the evaporated Ag in order to close any open pores. This Ag layer was then used as the back contact in the electrochemical cell, and more Ag was deposited, further filling-in the pores. The membrane and cell were rinsed with deionized H 2 O, and Au solution was added (Orotemp, Technic, Inc.). Plating was stopped with the desired rod length was reached. The Ag backing was removed by dissolving in 2 mL of 50 % HNO 3 , and the Al 2 O 3 template was dissolved …

ZnO Nanoribbon Microcavity Lasers

Abstract: Exploration of one-dimensional semiconductor nanostruc-tures has led to great progress in the areas of optoelectronics in the past few years. [1] Nanolasers, [2±4] waveguides, [5] frequency converters (second or third harmonic generators), [6] photoconductive optical switches, [7] and sensors [8,9] have been developed based on oxide nanowires. The single crystalline nature of these nanowires makes them ideal candidates for probing size-dependent and dimensionality-controlled physical phenomena. In particular, the transverse nanoscale and longitudinal microscale dimensions (i.e., large aspect ratio) as well as well-defined faceting nature of such nanostructures enable the observation of unique optical confinement and mi-crocavity effects. [2±4] Previously, hexagonal cylindrical ZnO nanowires have been examined as a laser gain medium. These nanocylinders indeed can serve as miniaturized Fabry± Perot optical cavities in the ultraviolet (UV) region with high gain and quality factor. [10] Based on classical waveguide theory , different transverse optical modes can be sustained within waveguides of different cross-sections. [11] It is thus fundamentally interesting to examine the optical cavity effects within nanowires with cross-sections other than hexagonal. Herein, we examine the lasing phenomenon from ZnO nanoribbons with pseudo-rectangular cross-section. Cavity-length dependent optical mode analysis reveals different cavity effects from those of hexagonal nanocylinders. ZnO, being environmentally benign and having a large bandgap (3.37 eV) and exciton binding energy (60 meV), has been considered as a promising candidate for UV light-emitting diodes and laser diodes. It has also displayed an astonishing series of nanostructures with different morphologies. Among many others, the hexagonal nanocylinders, [2±4] nano-ribbons, [5,13] tetrapods, [12] and comb-like nanowire arrays [14] are highly interesting for their fundamental significance in revealing microcavity effects as well as near-field optical coupling phenomena. The ZnO nanoribbons in this work were synthesized using two methodologies. One is a carbon thermal reduction process at 900 C. [12] The other is the high temperature (1350 C) approach developed by Wang and co-workers. [13] The two methods involve different growth mechanisms. The low temperature process utilized Au as the growth initiator. The observation of Au nanoparticles on the nanoribbon tips in the transmission electron microscope (TEM, Figure 1B) suggests that this is a vapor±liquid±solid (VLS) growth process. In contrast , the high-temperature approach is a vapor±solid (VS) growth process without any foreign metal initiation. Regardless of these different growth mechanisms, the growth directions and side facets of the produced nanoribbons are identical for the two sets of the samples. The length of the …

Colloidal nanocrystal synthesis and the organic–inorganic interface

Abstract: Colloidal nanocrystals are solution-grown, nanometre-sized, inorganic particles that are stabilized by a layer of surfactants attached to their surface. The inorganic cores possess useful properties that are controlled by their composition, size and shape, and the surfactant coating ensures that these structures are easy to fabricate and process further into more complex structures. This combination of features makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. Chemists are achieving ever more exquisite control over the composition, size, shape, crystal structure and surface properties of nanocrystals, thus setting the stage for fully exploiting the potential of these remarkable materials.

Colloidal nanocrystal synthesis and the organic-inorganic interface - eScholarship

Abstract: Colloidal nanocrystals are nanometer-sized, solution-grown inorganic particles stabilized by a layer of surfactants attached to their surface. The inorganic cores exhibit useful properties controlled by composition as well as size and shape, while the surfactant coating ensures that these structures are easy to fabricate and process. It is this combination of features that makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. But their full potential can only be exploited if we achieve exquisite control over their composition, size, shape, crystal structure and surface properties. Here we review what is known about nanocrystal growth and outline strategies for controlling it.

Catalytic Nanomotors: Autonomous Movement of Striped Nanorods

Abstract: Rod-shaped particles, 370 nm in diameter and consisting of 1 μm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2−3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10-14 N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR2γ/μDL, where S is the area-normalized oxygen evolution rate, γ is the liquid−vapor interfacial tension, R is the rod radius, μ is the viscosity, D is the diffusion coefficient of oxygen, and L is the le...

Optical properties of ZnO nanostructures.

Abstract: We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.