Huigao Duan

High-performance and ultra-stable lithium-ion batteries based on MOF-derived ZnO@ZnO quantum dots/C core-shell nanorod arrays on a carbon cloth anode.

MOF-derived ZnO@ZnO Quantum Dots/C core-shell nanorod arrays grown on flexible carbon cloth are successfully fabricated as a binder-free anode for Li-ion storage. In combination with the advantages from the ZnO/C core-shell architecture and the 3D nanorod arrays, this material satisfies both efficient ion and fast electron transport, and thus shows superior rate capability and excellent cycling stability.

Nanoplasmonics: Classical down to the Nanometer Scale

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light-matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms.

Resolution Limits of Electron-Beam Lithography toward the Atomic Scale

We investigated electron-beam lithography with an aberration-corrected scanning transmission electron microscope. We achieved 2 nm isolated feature size and 5 nm half-pitch in hydrogen silsesquioxane resist. We also analyzed the resolution limits of this technique by measuring the point-spread function at 200 keV. Furthermore, we measured the energy loss in the resist using electron-energy-loss spectroscopy.

Direct and Reliable Patterning of Plasmonic Nanostructures with Sub-10-nm Gaps

Nanoscale gaps in metal films enable strong field enhancements in plasmonic structures. However, the reliable fabrication of ultrasmall gaps (<10 nm) for real applications is still challenging. In this work, we report a method to directly and reliably fabricate sub-10-nm gaps in plasmonic structures without restrictions on pattern design. This method is based on a lift-off process using high-resolution electron-beam lithography with a negative-tone hydrogen silsesquioxane (HSQ) resist, where the resulting nanogap size is determined by the width of the patterned HSQ structure, which could be written at less than 10 nm. With this method, we fabricated densely packed gold nanostructures of varying geometries separated by ultrasmall gaps. By controlling structure sizes during lithography with nanometer precision, the plasmon resonances of the resulting patterns could be accurately tuned. Optical and surface-enhanced Raman scattering (SERS) measurements on the patterned structures show that this technique has promising applications in the fabrication of passively tunable plasmonic nanostructures with ultrasmall gaps.

Understanding of hydrogen silsesquioxane electron resist for sub-5-nm-half-pitch lithography

The authors, demonstrated that 4.5-nm-half-pitch structures could be achieved using electron-beam lithography, followed by salty development. They also hypothesized a development mechanism for hydrogen silsesquioxane, wherein screening of the resist surface charge is crucial in achieving a high initial development rate, which might be a more accurate assessment of developer performance than developer contrast. Finally, they showed that with a high-development-rate process, a short duration development of 15s was sufficient to resolve high-resolution structures in 15-nm-thick resist, while a longer development degraded the quality of the structures with no improvement in the resolution.

Plasmon-modulated photoluminescence of individual gold nanostructures

In this work, we performed a systematic study on the photoluminescence and scattering spectra of individual gold nanostructures that were lithographically defined. We identify the role of plasmons in photoluminescence as modulating the energy transfer between excited electrons and emitted photons. By comparing photoluminescence spectra with scattering spectra, we observed that the photoluminescence of individual gold nanostructures showed the same dependencies on shape, size, and plasmon coupling as the particle plasmon resonances. Our results provide conclusive evidence that the photoluminescence in gold nanostructures indeed occurs via radiative damping of plasmon resonances driven by excited electrons in the metal itself. Moreover, we provide new insight on the underlying mechanism based on our analysis of a reproducible blue shift of the photoluminescence peak (relative to the scattering peak) and observation of an incomplete depolarization of the photoluminescence.

Directed Self-Assembly at the 10 nm Scale by Using Capillary Force-Induced Nanocohesion

We demonstrated a new nanoassembly strategy based on capillary force-induced cohesion of high-aspect ratio nanostructures made by electron-beam lithography. Using this strategy, ordered complex pattern were fabricated from individual nanostructures at the 10 nm length scale. This method enables the formation of complex designed networks from a sparse array of nanostructures, suggesting a number of potential applications in fabrication of nanodevices, nanopatterning, and fluid-flow investigations.

Fowler-nordheim tunneling induced charge transfer plasmons between nearly touching nanoparticles

Summary form only given. A plasmon resonant mode is the collective oscillation of free electrons in a structure stimulated by incident light. Reducing the gap between two metal nanoparticles down to atomic dimensions uncovers novel plasmon resonant modes. Of particular interest is a mode known as the charge transfer plasmon (CTP). This mode has been experimentally observed in touching nanoparticles, where charges can shuttle between the nanoparticles via a conductive path. However, the CTP mode for nearly touching nanoparticles has only been predicted theoretically to occur via direct tunneling when the gap is reduced to ~0.4 nm2. Because of challenges in fabricating and characterizing gaps at these dimensions, experiments have been unable to provide evidence for this plasmon mode that is supported by tunneling. In this work, we consider an alternative tunneling process, that is, the well-known Fowler-Nordheim (FN) tunneling that occurs at high electric fields, and apply it for the first time in the theoretical investigation of plasmon resonances between nearly touching nanoparticles. This new approach relaxes the requirements on gap dimensions, and intuitively suggests that with a sufficiently high-intensity irradiation, the CTP can be excited via FN tunneling for a range of subnanometer gaps. For instance, a plasmonic gap field of 1010 V/m (or an incident power of 3×1010 W/cm2) is needed when the gap length is 0.8 nm. The unique feature of FN tunneling induced CTP is the ability to turn on and off the charge transfer by varying the intensity of an external light source, and this could inspire the development of novel quantum devices, such as high speed switches and modulators.

Sub-10-nm half-pitch electron-beam lithography by using poly(methyl methacrylate) as a negative resist

Developing high-resolution resists and processes for electron-beam lithography is of great importance for high-density magnetic storage, integrated circuits, and nanoelectronic and nanophotonic devices. Until now, hydrogen silsesquioxane (HSQ) and calixarene were the only two reported negative resists that could approach sub-10-nm half-pitch resolution for electron-beam lithography. Here, the authors report that 10-nm half-pitch dense nanostructures can also be readily fabricated using the well known poly(methyl methacrylate) (PMMA) resist operating in negative tone, even at exposure energies as low as 2 keV. In addition to scanning electron microscopy metrology, transmission electron microscopy metrology was done to confirm the high-resolution capability of negative-tone PMMA. This process was compared to HSQ with salty development and showed similar ultimate resolution, so it could be used as an alternative for applications incompatible with HSQ.

Construction of hierarchical CoS nanowire@NiCo2S4 nanosheet arrays via one-step ion exchange for high-performance supercapacitors

In this paper, using CoS@NiCo2S4 as an example, we report the successful design and synthesis of a novel hierarchical core/shell metal sulfide with conductive CoS on nickel foam via a facile ion exchange process. The prepared sulfide core/shell nanoarrays exhibit superior redox reactivity for supercapacitors. High areal capacitances of 7.62 F cm−2 (almost a 2.5-fold enhancement compared with the CoO nanowire@NiCo2O4 sheet arrays) at a scan rate of 5 mA cm−2 and even 5.52 F cm−2 at 30 mA cm−2 were achieved. The superior electrochemical performance demonstrates that the CoS nanowire@NiCo2S4 sheet arrays are promising for high-performance supercapacitor applications.