Cezhou Dong

Doping Monolayer Graphene with Single Atom Substitutions

Functionalized graphene has been extensively studied with the aim of tailoring properties for gas sensors, superconductors, supercapacitors, nanoelectronics, and spintronics. A bottleneck is the capability to control the carrier type and density by doping. We demonstrate that a two-step process is an efficient way to dope graphene: create vacancies by high-energy atom/ion bombardment and fill these vacancies with desired dopants. Different elements (Pt, Co, and In) have been successfully doped in the single-atom form. The high binding energy of the metal-vacancy complex ensures its stability and is consistent with in situ observation by an aberration-corrected and monochromated transmission electron microscope.

Atomic-scale observation of lithiation reaction front in nanoscale SnO2 materials.

In the present work, taking advantage of aberration-corrected scanning transmission electron microscopy, we show that the dynamic lithiation process of anode materials can be revealed in an unprecedented resolution. Atomically resolved imaging of the lithiation process in SnO2 nanowires illustrated that the movement, reaction, and generation of b = [1[overline]1[overline]1] mixed dislocations leading the lithiated stripes effectively facilitated lithium-ion insertion into the crystalline interior. The geometric phase analysis and density functional theory simulations indicated that lithium ions initial preference to diffuse along the [001] direction in the {200} planes of SnO2 nanowires introduced the lattice expansion and such dislocation behaviors. At the later stages of lithiation, the Li-induced amorphization of rutile SnO2 and the formation of crystalline Sn and LixSn particles in the Li2O matrix were observed.

Epitaxial TiO2/SnO2 core–shell heterostructure by atomic layer deposition

Taking TiO2/SnO2 core–shell nanowires (NWs) as a model system, we systematically investigate the structure and the morphological evolution of this heterostructure synthesized by atomic layer deposition/epitaxy (ALD/ALE). All characterizations, by X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction and Raman spectra, reveal that single crystalline rutile TiO2 shells can be epitaxially grown on SnO2 NWs with an atomically sharp interface at low temperature (250 °C). The growth behavior of the TiO2 shells highly depends on the surface orientations and the geometrical shape of the core SnO2 NW cross-section. Atomically smooth surfaces are found for growth on the {110} surface. Rough surfaces develop on {100} surfaces due to (100) − (1 × 3) reconstruction, by introducing steps in the [010] direction as a continuation of {110} facets. Lattice mismatch induces superlattice structures in the TiO2 shell and misfit dislocations along the interface. Conformal epitaxial growth has been observed for SnO2 NW cores with an octagonal cross-section ({100} and {110} surfaces). However, for a rectangular core ({10} and {010} surfaces), the shell also derives an octagonal shape from the epitaxial growth, which was explained by a proposed model based on ALD kinetics. The surface steps and defects induced by the lattice mismatch likely lead to improved photoluminescence (PL) performance for the yellow emission. Compared to the pure SnO2 NWs, the PL spectrum of the core–shell nanostructures exhibits a stronger emission peak, which suggests potential applications in optoelectronics.

Synthesis and stress relaxation of ZnO/Al-doped ZnO core–shell nanowires

Doping nanostructures is an effective method to tune their electrical and photoelectric properties. Taking ZnO nanowires (NWs) as a model system, we demonstrate that atomic layer deposition (ALD) can be adopted for the realization of a doping process by the homo-epitaxial growth of a doped shell on the NW core. The Al-doped ZnO NWs have a layered superlattice structure with dopants mainly occupying the interstitial positions. After annealing, Al(3+) ions diffuse into the ZnO matrix and occupy substitutional locations, which is desirable for dopant activation. The stress accumulated during epitaxial growth is relaxed by the nucleation of dislocations, dislocation dipoles and anti-phase boundaries. We note that the proposed method can be easily adopted for doping different types of nanostructures, and fabricating superlattices and multiple quantum wells on NWs in a controllable way.

Electrical failure behaviors of semiconductor oxide nanowires.

Electrical failure studies on semiconductor oxide nanowires (NWs) were performed in situ inside a transmission electron microscope (TEM). A high driven current leads to a sudden fracture of the SnO(2) NW and creates ultra-sharp and high aspect ratio tips at the broken ends, which provides a simple and reliable way for in situ nanoprobe fabrication. As a comparison, the TiO(2) NW fails due to Joule-heating-induced melting and retracts back into a nanosphere. The distinct behaviors are rooted in the different bonding nature. The strong ionic bonding between titanium and oxygen ions preserves the stoichiometry, while the covalently bonded SnO(2) NW decomposes before melting. The decomposition process is observed by resistively heating an SnO(2)/TiO(2) core-shell structure. It has been demonstrated that the needle-like geometry greatly enhanced field emission properties of SnO(2) NWs.

Microstructure-dependent conformal atomic layer deposition on 3D nanotopography.

The capability of atomic layer deposition (ALD) to coat conformally complex 3D nanotopography has been examined by depositing amorphous, polycrystalline, and single-crystal TiO(2) films over SnO(2) nanowires (NWs). Structural characterizations reveal a strong correlation between the surface morphology and the microstructures of ALD films. Conformal growth can only be rigorously achieved in amorphous phase with circular sectors developed at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e., sequential and self-limiting surface reactions result in smooth and conformal films. Orientation-dependent growth and surface reconstruction generally lead to nonconformal coating in polycrystalline and single-crystal films. Especially, an octagonal single-crystal TiO(2) shell was derived from a rectangular SnO(2) NW core, which was the consequence of both self-limited growth kinetics and surface reconstruction. Models were proposed to explain the conformality of ALD deposition over 3D nanostructures by taking account of the underlying microstructures. Besides the surface morphologies, the microstructures also have significant consequence to the surface electronic states, characterized by the broad band photoluminescence. The comparison study suggests that ALD process is determined by the interplay of both thermodynamic and kinetic factors.

Evolution of Pt Clusters on Graphene Induced by Electron Irradiation

In situ low-voltage transmission electron microscopy (TEM) was performed to study the evolution of small Pt clusters on suspended graphene. Pt clusters, trapped by the edge of holes, generally take a stable shape of truncated octahedron for sizes ranging from sub-1 to � 5nm. The interaction to the graphene dots takes in charge when they form composite nanostructures embedded in graphene. The Pt clusters are slowly flattened due to hole enlargement under electron irradiation. The planar structure is maintained by the peripheral Pt-C bonds and instantly collapses into a three-dimensional (3D) cluster if one side is detached from the edge. Based on the heat transfer model, the thermal effect can be excluded under the experimental condition. Atomistic evolution can be attributed to the electron irradiation. Molecular dynamics simulations revealed that the evolution kinetics was found to be dominated by the surface diffusion (characterized by the migration barrier Em), the temperature (the thermal activation energy � 5kBT), and the scattering from electrons (the maximum transferred energy Emax). The corresponding energies are comparable for the Pt cluster system, leading to similar evolution behaviors. A different scenario in graphene systems is due to the large difference in agitations, i.e., Emax � Em � 5kBT at 3000K. This unique behavior comes from TEM observation, implying that electron beam irradiation can be utilized as a unique tool in shaping carbon nanostructures. [DOI: 10.1115/1.4024168]