Tu Hong

Plasmonic Hot Electron Induced Photocurrent Response at MoS2-Metal Junctions

We investigate the wavelength- and polarization-dependence of photocurrent signals generated at few-layer MoS2-metal junctions through spatially resolved photocurrent measurements. When incident photon energy is above the direct bandgap of few-layer MoS2, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, which can be attributed to photovoltaic effects. In contrast, if incident photon energy is below the direct bandgap of MoS2, the photocurrent response is maximized when the incident light is polarized in the direction perpendicular to the electrode edge, indicating different photocurrent generation mechanisms. Further studies show that this polarized photocurrent response can be interpreted in terms of the polarized absorption of light by the plasmonic metal electrode, its conversion into hot electron-hole pairs, and subsequent injection into MoS2. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in metal-semiconductor junctions, opening the door for engineering future two-dimensional materials based optoelectronics through surface plasmon resonances.

Anisotropic photocurrent response at black phosphorus–MoS2 p–n heterojunctions

We investigate the photocurrent generation mechanisms at a vertical p-n heterojunction between black phosphorus (BP) and molybdenum disulfide (MoS2) flakes through polarization-, wavelength-, and gate-dependent scanning photocurrent measurements. When incident photon energy is above the direct band gap of MoS2, the photocurrent response demonstrates a competitive effect between MoS2 and BP in the junction region. In contrast, if the incident photon energy is below the band gap of MoS2 but above the band gap of BP, the photocurrent response at the p-n junction exhibits the same polarization dependence as that at the BP-metal junction, which is nearly parallel to the MoS2 channel. This result indicates that the photocurrent signals at the MoS2-BP junction primarily result from the direct band gap transition in BP. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in vertical 2D semiconductor heterojunctions, offering a new way of engineering future two-dimensional materials based optoelectronic devices.

Dual-modality photothermal optical coherence tomography and magnetic-resonance imaging of carbon nanotubes

We demonstrate polyethylene-glycol-coated single-walled carbon nanotubes (CNTs) as contrast agents for both photothermal optical coherence tomography (OCT) and magnetic-resonance imaging (MRI). Photothermal OCT was accomplished with a spectral domain OCT system with an amplitude-modulated 750 nm pump beam using 10 mW of power, and T(2) MRI was achieved with a 4.7 T animal system. Photothermal OCT and T(2) MRI achieved sensitivities of nanomolar concentrations to CNTs dispersed in amine-terminated polyethylene glycol, thus establishing the potential for dual-modality molecular imaging with CNTs.

Visualizing Light Scattering in Silicon Waveguides with Black Phosphorus Photodetectors

A black phosphorus photodetector is utilized to investigate the light-scattering patterns of a silicon waveguide through wavelength- and polarization-dependent scanning photocurrent measurements. The photocurrent signals exhibit similar patterns to the light-intensity distribution of the waveguide calculated by finite-difference time-domain simulations, suggesting that photoexcited electron-hole pairs in the silicon waveguide can be injected into phosphorene to induce its photoresponse.

Single-Walled Carbon Nanotube-Mediated Small Interfering RNA Delivery for Gastrin-Releasing Peptide Receptor Silencing in Human Neuroblastoma

Small interfering RNA (siRNA) has the potential to influence gene expression with a high degree of target gene specificity. However, the clinical application of siRNA therapeutics has proven to be less promising as evidenced by its poor intracellular uptake, instability in vivo, and nonspecific immune stimulations. Recently, we have demonstrated that single-walled carbon nanotube (SWNT)-mediated siRNA delivery can enhance the efficiency of siRNA-mediated gastrin-releasing peptide receptor (GRP-R) gene silencing by stabilizing siRNA while selectively targeting tumor tissues. Based on our recent findings, we introduce a novel technique to silence specific gene(s) in human neuroblastoma through SWNT-mediated siRNA delivery in vitro and in vivo.

Ultrathin single-walled carbon nanotube network framed graphene hybrids.

Graphene and single-walled carbon nanotubes (SWNTs) have shown superior potential in electronics and optoelectronics because of their excellent thermal, mechanical, electronic, and optical properties. Here, a simple method is developed to synthesize ultrathin SWNT-graphene films through chemical vapor deposition. These novel two-dimensional hybrids show enhanced mechanical strength that allows them to float on water without polymer supporting layers. Characterizations by Raman spectroscopy and transmission electron microscopy indicate that SWNTs can interlace as a concrete backbone for the subsequent growth of monolayer graphene. Optical and electrical transport measurements further show that SWNT-graphene hybrids inherit high optical transparency and superior electrical conductivity from monolayer graphene. We also explore the local optoelectronic properties of SWNT-graphene hybrids through spatially resolved photocurrent microscopy and find that the interactions between SWNTs and graphene can induce a strong photocurrent response in the areas where SWNTs link different graphene domains together. These fundamental studies may open a door for engineering optoelectronic properties of SWNT-graphene hybrids by controlling the morphologies of the SWNT frames.

Thermal and optical properties of freestanding flat and stacked single-layer graphene in aqueous media

Graphene, a two-dimensional atomic layer of carbon atoms, represents a class of nanostructures whose physical properties are strongly dependent on their morphology as well as the environment in which they exist. Aqueous media is one of the most common environments that play an important role in influencing the performance of these materials. Here, we investigate the thermal and optical properties of suspended flat and stacked graphene ribbons that are typical structures in aqueous media. We demonstrate that stacked graphene structures thermalize much more rapidly than flat graphene and display unequilibrated electron and phonon temperatures upon laser excitation. The interface thermal conductivity between graphene and water of (7.2 + 1.4/−5.5) × 105 W m−2 K−1 is also obtained. We also show that graphene hot electron luminescence not only depends on Fermi energy, but also exhibit dramatic differences between flat and stacked regions. This indicates the morphology of a graphene structure may affect its opti...

High-Performance WSe2 Phototransistors with 2D/2D Ohmic Contacts

We report high-performance WSe2 phototransistors with two-dimensional (2D) contacts formed between degenerately p-doped WSe2 and undoped WSe2 channel. A photoresponsivity of ∼600 mA/W with a high external quantum efficiency up to 100% and a fast response time (both rise and decay times) shorter than 8 μs have been achieved concurrently. More importantly, our WSe2 phototransistor exhibits a high specific detectivity (∼1013 Jones) in vacuum, comparable or higher than commercial Si- and InGaAs-based photodetectors. Further studies have shown that the high photoresponsivity and short response time of our WSe2 phototransistor are mainly attributed to the lack of Schottky-barriers between degenerately p-doped WSe2 source/drain contacts and undoped WSe2 channel, which can reduce the RC time constant and carrier transit time of a photodetector. Our experimental results provide an accessible strategy to achieve high-performance WSe2 phototransistor architectures by improving their electrical transport and photocurrent generation simultaneously, opening up new avenues for engineering future 2D optoelectronic devices.

Membrane cholesterol mediates the cellular effects of monolayer graphene substrates

Graphene possesses extraordinary properties that promise great potential in biomedicine. However, fully leveraging these properties requires close contact with the cell surface, raising the concern of unexpected biological consequences. Computational models have demonstrated that graphene preferentially interacts with cholesterol, a multifunctional lipid unique to eukaryotic membranes. Here we demonstrate an interaction between graphene and cholesterol. We find that graphene increases cell membrane cholesterol and potentiates neurotransmission, which is mediated by increases in the number, release probability, and recycling rate of synaptic vesicles. In fibroblasts grown on graphene, we also find an increase in cholesterol, which promotes the activation of P2Y receptors, a family of receptor regulated by cholesterol. In both cases, direct manipulation of cholesterol levels elucidates that a graphene-induced cholesterol increase underlies the observed potentiation of each cell signaling pathway. These findings identify cholesterol as a mediator of graphene’s cellular effects, providing insight into the biological impact of graphene.Understanding the biological role of graphene in eukaryotic cells is essential for future biomedicine applications. Here, the authors investigate the interaction of neurons and fibroblasts with graphene substrates, which increase cell membrane cholesterol and potentiate neurotransmitter release and receptor signaling.

Probing photoresponse of aligned single-walled carbon nanotube doped ultrathin MoS2

We report a facile method to produce ultrathin molybdenum disulfide (MoS2) hybrids with polarized near-infrared (NIR) photoresponses, in which horizontally-aligned single-walled carbon nanotubes (SWNTs) are integrated with single- and few-layer MoS2 through a two-step chemical vapor deposition process. The photocurrent generation mechanisms in SWNT-MoS2 hybrids are systematically investigated through wavelength- and polarization-dependent scanning photocurrent measurements. When the incident photon energy is above the direct bandgap of MoS2, isotropic photocurrent signals are observed, which can be primarily attributed to the direct bandgap transition in MoS2. In contrast, if the incident photon energy in the NIR region is below the direct bandgap of MoS2, the maximum photocurrent response occurs when the incident light is polarized in the direction along the SWNTs, indicating that photocurrent signals mainly result from the anisotropic absorption of SWNTs. More importantly, these two-dimensional (2D) hybrid structures inherit the electrical transport properties from MoS2, displaying n-type characteristics at a zero gate voltage. These fundamental studies provide a new way to produce ultrathin MoS2 hybrids with inherited electrical properties and polarized NIR photoresponses, opening doors for engineering various 2D hybrid materials for future broadband optoelectronic applications.