Hot carrier multiplication on graphene/TiO2 Schottky nanodiodes

Plasmonic hot carrier-driven photoelectrochemical water splitting on antenna–reactor Pt/Ag/TiO2 Schottky nanodiodes

Plasmonic photoelectrochemical (PEC) water splitting has excited immense interest, as it can overcome the intrinsic limitations of semiconductors, in terms of light absorption, by the localized-surface plasmon resonances effect. Here, to get insight into the role of plasmonic hot carriers in plasmonic water splitting, a rational design of an antenna–reactor type Pt/Ag/TiO2 metal–semiconductor Schottky nanodiode was fabricated and used as a photoanode. Using the designed PEC cell system combined with the Pt/Ag/TiO2 nanodiode, we show that the plasmonic hot carriers excited from Ag were utilized for the oxygen (O2) evolution reaction and, consequently, had a decisive role in the enhancement of the photocatalytic efficiency. These results were supported by finite-difference time-domain simulations, and the faradaic efficiency was measured by the amount of actual gas produced. Therefore, this study provides a deep understanding of the dynamics and mechanisms of plasmonic hot carriers in plasmonic-assisted PEC water splitting.

Light-induced osteogenic differentiation of BMSCs with graphene/TiO2 composite coating on Ti implant

Light-induced surface potential have been demonstrated as an effective bone marrow mesenchymal stem cells (BMSCs) osteogenic differentiation regulator. However, traditional bone repair implants almost were weak or no light-responsive. Fortunately, surface modification was a feasible strategy to realize its light functionalization for bone implants. Herein, a graphene oxide (GO)/titanium dioxide (TiO₂) nanodots composite coating on the surface of titanium (Ti) implant was constructed, and GO was reduced to reduced graphene oxide (rGO) with the method of UV-assisted photocatalytic reduction. After rGO deposited on the surface of TiO₂, a heterojunction formed at the interface of rGO and TiO₂. With visible light illumination, positive charges accumulated on the surface of rGO/TiO₂ film, and performed as a positive surface potential change. The light-induced surface potential which was generated under proper light intensity is harmless to the cell adhesion and proliferation behavior, but presented a good BMSCs osteogenic differentiation promoting effect, and the activation of the voltage-gated calcium channels through surface potential and the promotion of the adsorption of osteogenic growth factors could be the reason. This work given a new insight of the modification for Ti implant with a light-induced surface potential, and shows potential application for bone regeneration on the clinical practice through light stimulation.

Nanobowls-assisted broadband absorber for unbiased Si-based infrared photodetection

Hot electrons from the nonradiative decay of surface plasmons have drawn extensive attention due to the outstanding performance in realizing below-bandgap photodetection. However, the widely employed metallic nanostructures are normally complex and delicate with a great challenge in large-area fabrication, and there is a great limitation to achieve substantial photoresponse at relatively long wavelengths (e.g., 2000nm) with polarization- and incident-angle independence. In this study, we theoretically and experimentally demonstrate a broadband, omnidirectional, and polarization-insensitive absorber based on wafer-scale silicon honeycomb nanobowls with 20-nm-thick gold overlayer. The average absorption across the long wave near infrared band (LW-NIR, i.e., 1100-2500 nm) is higher than 82%, which is contributed from the random nature and multimode localized plasmonic resonances excited on the side walls of nanobowls. Benefitted from the well-connected thin Au film and relatively low Schottky barrier, the generated hot electrons have a high transport probability to reach Schottky interface and participate in the interfacial charge transfer process. As a result, the hot-electron photodetector under no bias realizes a broadband photodetection up to 2000nm wavelength with a responsivity of 0.145 mA/W, and its cutoff wavelength is predicted up to 3300 nm by fitting the experimental result with Fowler theory. Our proposed Au/Si nanobowls photodetector could open a pathway to further extend the detection wavelength of Si-based photodetectors with a large-area and low-cost fabrication process, which promotes practical hot-electron applications.

Effects of doping graphene on the performance of graphene-silicon hybrid photoconductive detectors

The photoconductive detector based on a graphene-silicon heterostructure retains excellent optoelectrical properties, in which the graphene plays an indispensable role, acting as the carrier transporting channel. Herein, we systematically investigate by simulation and experiment how doping graphene will affect the performance of graphene-silicon hybrid photoconductors. Compared with lightly p-doped graphene devices, the responsivity can be made nine times better through increasing the p-type doping level. In addition, the net photocurrent can also be enhanced by about four times through increasing the n-type doping level of graphene. We attribute this improvement to the barrier height change adjusted by doping graphene, which can optimize the lifetime and transport of photocarriers. Such a graphene-doping method, that manipulates the junction region, could offer useful guidance for achieving high-performance graphene photodetectors.

High-Throughput Electrical Characterization of Nanomaterials from Room to Cryogenic Temperatures

We present multiplexer methodology and hardware for nanoelectronic device characterization. This high-throughput and scalable approach to testing large arrays of nanodevices operates from room temperature to milli-Kelvin temperatures and is universally compatible with different materials and integration techniques. We demonstrate the applicability of our approach on two archetypal nanomaterials-graphene and semiconductor nanowires-integrated with a GaAs-based multiplexer using wet or dry transfer methods. A graphene film grown by chemical vapor deposition is transferred and patterned into an array of individual devices, achieving 94% yield. Device performance is evaluated using data fitting methods to obtain electrical transport metrics, showing mobilities comparable to nonmultiplexed devices fabricated on oxide substrates using wet transfer techniques. Separate arrays of indium-arsenide nanowires and micromechanically exfoliated monolayer graphene flakes are transferred using pick-and-place techniques. For the nanowire array mean values for mobility μ_FE = 880/3180 cm² V^-1 s^-1 (lower/upper bound), subthreshold swing 430 mV dec^-1, and on/off ratio 3.1 decades are extracted, similar to nonmultiplexed devices. In another array, eight mechanically exfoliated graphene flakes are transferred using techniques compatible with fabrication of two-dimensional superlattices, with 75% yield. Our results are a proof-of-concept demonstration of a versatile platform for scalable fabrication and cryogenic characterization of nanomaterial device arrays, which is compatible with a broad range of nanomaterials, transfer techniques, and device integration strategies from the forefront of quantum technology research.

Tunable Onset of Hydrogen Evolution in Graphene with Hot Electrons

Here, we show that the turn-on voltage for the hydrogen evolution reaction on a graphene surface can be tuned in a semiconductor-insulator-graphene (SIG) device immersed in a solution. Specifically, it is shown that the hydrogen evolution reaction (HER) onset for the graphene can shift by >0.8 V by application of a voltage across a graphene-Al₂O₃-silicon junction. We show that this shift occurs due to the creation of a hot electron population in graphene due to tunneling from the Si to graphene. Through control experiments, we show that the presence of the graphene is necessary for this behavior. By analyzing the silicon, graphene, and solution current components individually, we find an increase in the silicon current despite a fixed graphene-silicon voltage, corresponding to an increase in the HER current. This additional silicon current appears to directly drive the electrochemical reaction, without modifying the graphene current. We term this current "direct injection current" and hypothesize that this current occurs due to electrons injected from the silicon into graphene that drives the HER before any electron-electron scattering occurs in the graphene. To further determine whether hot electrons injected at different energies could explain the observed total solution current, the nonequilibrium electron dynamics was studied using a 2D ensemble Monte Carlo Boltzmann transport equation (MCBTE) solver. By rigorously considering the key scattering mechanisms, we show that the injected hot electrons can significantly increase the available electron flux at high energies. These results show that semiconductor-insulator-graphene devices are a platform which can tune the electrochemical reaction rate via multiple mechanisms.

Hybrid graphene heterojunction photodetector with high infrared responsivity through barrier tailoring

The graphene/Si heterojunction is attractive for high gain and broadband photodetection through photogating effect. However, the photoresponsivity in these devices are still limited to under 1 A W^-1 if no narrowband absorption-enhanced nanostructures were used. In this paper, the effects of barriers on photoresponse are systematically studied at 1550 nm wavelength. Different barrier heights are obtained through selection of substrates, graphene doping and electrical tuning. Lower barrier height for graphene side and higher barrier height for silicon side are found to be beneficial for better infrared photoresponse. Through Polyetherimide doping of graphene and back-gated electrical modulation, the responsivity finally reached 5.71 A W^-1, which to our knowledge is among the best results for graphene-based infrared photodetectors with graphene adopted as a light-absorption material. It is found that the thermionic emission efficiency of indirect transition in graphene is related to the difference in emissioin barrier height, and the lifetime of photoinduced carriers in the channel can be enhanced by built-in potential. These results lay the foundation for the photodetection applicatioins of graphene/Si heterojunction in the longer-wavelength infrared region.

WO3/p-Type-GR Layered Materials for Promoted Photocatalytic Antibiotic Degradation and Device for Mechanism Insight

Graphene enhanced WO₃ has recently become a promising material for various applications. The understanding of the transfer of charge carriers during the photocatalytic processes remains unclear because of their complexity. In this study, the characteristics of the deposited WO₃/graphene layered materials were investigated by Raman spectroscopy, UV-vis spectroscopy, and SEM. According to the results, p-graphene exhibits and enhances the characteristics of the WO₃/graphene film. The photocatalytic activities of WO₃/graphene layered materials were assessed by the photocatalytic degradation of oxytetracycline antibiotics as irradiated by UV light. Here, a higher current of cyclic voltammetry and a higher resistance of impedance spectra were obtained with the as-grown WO₃/graphene directly synthesized on Cu foils under UV light using an electrochemical method, which was different from traditional WO₃ catalysts. Thus, it is urgent to explore the underlying mechanism in depth. In this study, a large layered material WO₃/graphene was fabricated on a Si substrate using a modified CVD method, and a WO₃/graphene device was developed by depositing a gold electrode material and compared with a WO₃ device. Due to photo-induced doping effects, the current-voltage test suggested that the photo-resistance is larger than dark-resistance, and the photo-current is less than the dark current based on WO₃/graphene layered materials, which are significantly different from the characteristics of the WO₃ layered material. A new pathway was developed here to analyze the transfer properties of carriers in the photocatalytic process.

Ultra-sensitive graphene-bismuth telluride nano-wire hybrids for infrared detection

The myriad technological applications of infrared radiation sensors make the search for ultra-sensitive detectors extremely crucial. Materials such as bismuth telluride (Bi2Te3), having a small bulk band gap of 0.17 eV, are ideal infrared detectors. However, due to the high recombination rate of photo-generated charge carriers in the bulk, the electrical response under optical illumination is typically very weak in these materials. We have circumnavigated this by sensitizing graphene with Bi2Te3 nano-wires. These hybrid devices show an ultra-high sensitivity of ∼106 A W-1, under incident electromagnetic radiation from 940 nm to 1720 nm. The theoretical limit of the noise equivalent power and specific detectivity in these devices are ∼10-18 W Hz-1/2 and ∼1011 Jones respectively, which are comparable to those of some of the best known detectors.

Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity

Graphene's unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 μm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications.