Hongbing Cai

Tuning Chemical Enhancement of SERS by Controlling the Chemical Reduction of Graphene Oxide Nanosheets

Chemical enhancement is an important mechanism in surface-enhanced Raman spectroscopy. It is found that mildly reduced graphene oxide (MR-GO) nanosheets can significantly increase the chemical enhancement of the main peaks by up to 1 order of magnitude for adsorbed Rhodamine B (RhB) molecules, in comparison with the mechanically exfoliated graphene. The observed enhancement factors can be as large as ∼10(3) and show clear dependence on the reduction time of graphene oxide, indicating that the chemical enhancement can be steadily controlled by specific chemical groups. With the help of X-ray photoelectron spectra, these chemical species are identified and the origin of the observed large chemical enhancement can thus be revealed. It is shown that the highly electronegative oxygen species, which can introduce a strong local electric field on the adsorbed molecules, are responsible for the large enhancement. In contrast, the local defects generated by the chemical reduction show no positive correlation with the enhancement. Most importantly, the dramatically enhanced Raman spectra of RhB molecules on MR-GO nanosheets reproduce all important spectral fingerprints of the molecule with a negligible frequency shift. Such a unique noninvasive feature, along with the other intrinsic advantages, such as low cost, light weight, easy availability, and flexibility, makes the MR-GO nanosheets very attractive to a variety of practical applications.

Improving the photovoltaic performance of solid-state ZnO/CdTe core–shell nanorod array solar cells using a thin CdS interfacial layer

The properties of the electron donor–acceptor interface play a crucial role in the photovoltaic performance of the core–shell nanorod array solar cells (NRASCs). In this paper, all-inorganic solid-state ZnO/CdTe and ZnO/CdS/CdTe core–shell NRASCs have been fabricated by a simple low temperature and low cost solution-based process. We investigate the influence of the CdS interfacial layer with different thicknesses on the performance of the solar cells. It is found that inserting such an interfacial layer can significantly improve the short-circuit current density and the open-circuit voltage of the device. The overall power conversion efficiency of the ZnO/CdS/CdTe core–shell NRASC with a 4 nm thick CdS interfacial layer can reach 0.72% under AM 1.5G illumination (100 mW cm−2), which is three times that of the ZnO/CdTe NRASC. The improvement in the performance is attributed to the designed graded energy band alignment of ZnO/CdS/CdTe and the passivation of surface defects of the ZnO nanorod by the CdS interfacial layer, which can result in the enhanced carrier separation and collection. The result clearly demonstrates that the performance of all-inorganic core–shell photovoltaic devices can be greatly improved with uncomplicated interface engineering.

Tailoring the coupling between localized and propagating surface plasmons: realizing Fano-like interference and high-performance sensor

Surface plasmon modes originated from various metallic nanostructures possess unique features of strong nanoscale light confinement and enhancement with tunable energy, which make them attractive and promising for a variety of applications such as sensing, solar cell, and lasing. Here, we have investigated the interaction between localized and propagating surface plasmons in a structure consisting of a gold nanobar array and a thick gold film, separated by a silica dielectric spacer layer. It is found that the reflection spectrum of the designed plasmonic structure can be readily tailored by changing the gold nanobar size, array period and the spacer layer thickness. Moreover, the strong coupling between the localized and propagating modes can result in an anticrossing behavior and even induce a Fano-like asymmetric lineshape. Importantly, the sensitivity and the figure of merit (FoM) of this plasmonic system can reach as high as 936 nm/RIU and 112, respectively. Our study offers a new, simple, efficient and controllable way to design the plasmonic systems with desired modes coupling and spectral lineshapes for different applications.

Realizing full visible spectrum metamaterial half-wave plates with patterned metal nanoarray/insulator/metal film structure.

Abrupt phase shift introduced by plasmonic resonances has been frequently used to design subwavelength wave plates for optical integration. Here, with the sandwich structure consisting of a top periodic patterned silver nanopatch, an in-between insulator layer and a bottom thick Au film, we realize a broadband half-wave plate which is capable to cover entire visible light spectrum ranging from 400 to 780 nm. Moreover, when the top layer is replaced with a periodic array of composite super unit cell comprised of two nanopatches with different sizes, the operation bandwidth can be further improved to exceed an octave (400-830 nm). In particular, we demonstrate that the designed half-wave plate can be used efficiently to rotate the polarization state of an ultra-fast light pulse with reserved pulse width. Our result offers a new strategy to design and construct broadband high efficiency phase-response based optical components using patterned metal nanoarray/insulator/metal structure.

Maximizing Integrated Optical and Electrical Properties of a Single ZnO Nanowire through Native Interfacial Doping

A native interfacial doping layer introduced in core-shell type ZnO nano-wires by a simple vapor phase re-growth procedure endows the produced nano-wires with both excellent electrical and optical performances compared to conventional homogeneous ZnO nanowires. The unique Zn-rich interfacial structure in the core-shell nanowires plays a crucial role in the outstanding performances.

Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphene with Urea

Chemical doping is an effective method to intrinsically modify the chemical and electronic property of graphene. We propose a novel approach to synthesize the nitrogen-doped graphene via thermal annealing graphene with urea, in which the nitrogen source can be controllably released from the urea by varying the annealed temperature and time. The doped N content and the configuration N as well as the thermal stabilities are also evaluated with X-ray photoelectron spectroscopy and Raman spectra. Electrical measurements indicate that the conductivity of doped graphene can be well regulated with the N content. The method is expected to produce large scale and controllable N-doped graphene sheets for a variety of potential applications.

Wafer scale fabrication of highly dense and uniform array of sub-5 nm nanogaps for surface enhanced Raman scatting substrates

Metallic nanogap is very important for a verity of applications in plasmonics. Although several fabrication techniques have been proposed in the last decades, it is still a challenge to produce uniform nanogaps with a few nanometers gap distance and high throughput. Here we present a simple, yet robust method based on the atomic layer deposition (ALD) and lift-off technique for patterning ultranarrow nanogaps array. The ability to accurately control the thickness of the ALD spacer layer enables us to precisely define the gap size, down to sub-5 nm scale. Moreover, this new method allows to fabricate uniform nanogaps array along different directions densely arranged on the wafer-scale substrate. It is demonstrated that the fabricated array can be used as an excellent substrate for surface enhanced Raman scatting (SERS) measurements of molecules, even on flexible substrates. This uniform nanogaps array would also find its applications for the trace detection and biosensors.

Near-infrared quarter-waveplate with near-unity polarization conversion efficiency based on silicon nanowire array

Metasurfaces made of subwavelength resonators can modify the wave front of light within the thickness much less than free space wavelength, showing great promises in integrated optics. In this paper, we theoretically show that electric and magnetic resonances supported simultaneously by a subwavelength nanowire with high refractive-index can be utilized to design metasurfaces with near-unity transmittance. Taking silicon nanowire for instance, we design numerically a near-infrared quarter-waveplate with high transmittance using a subwavelength nanowire array. The operation bandwidth of the waveplate is 0.14 μm around the center wavelength of 1.71 μm. The waveplate can convert a 45° linearly polarized incident light to circularly polarized light with conversion efficiency ranging from 94% to 98% over the operation band. The performance of quarter waveplate can in principle be tuned and improved through optimizing the parameters of nanowire arrays. Its compatibility to microelectronic technologies opens up a distinct possibility to integrate nanophotonics into the current silicon-based electronic devices.

Highly efficient and controllable method to fabricate ultrafine metallic nanostructures

We report a highly efficient, controllable and scalable method to fabricate various ultrafine metallic nanostructures in this paper. The method starts with the negative poly-methyl-methacrylate (PMMA) resist pattern with line-width superior to 20 nm, which is obtained from overexposing of the conventionally positive PMMA under a low energy electron beam. The pattern is further shrunk to sub-10 nm line-width through reactive ion etching. Using the patter as a mask, we can fabricate various ultrafine metallic nanostructures with the line-width even less than 10 nm. This ion tailored mask lithography (ITML) method enriches the top-down fabrication strategy and provides potential opportunity for studying quantum effects in a variety of materials.

One-step CVD fabrication and optoelectronic properties of SnS2/SnS vertical heterostructures

Heterostructures constructed by two-dimensional (2D) material layers, which are usually prepared via a transfer/stacking method or van der Waals epitaxy, have achieved significant success in various optoelectronic devices including solar cells, light-emitting diodes and photodetectors. However, to date, most of these heterostructures comprise 2D materials with a similar crystal structure. Thus, preparation of heterostructures with different crystal structures is desirable but still a great challenge. Herein, we report a one-step CVD strategy to successfully grow SnS2/SnS vertical heterostructures on a mica substrate. Raman spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM) characterizations reveal that the heterostructure is formed by stacking of pyramid-shaped SnS2 of the hexagonal structure onto the rhombus SnS flake of the orthorhombic structure. The photodetector based on the SnS2/SnS heterostructure demonstrates high optoelectronic performance: a 27.7 A W−1 photoresponsivity, 2.2 × 103 on/off ratio, less than 10 ms response time and 2.1 × 1010 jones specific detectivity. The superior performance originates from the high crystal quality of the as-grown heterostructure and its vertical device architecture. This study can expand our capability to fabricate a variety of two-dimensional heterostructures and make these heterostructures highly desirable as novel building blocks for potential applications in electronic and optoelectronic devices.