Tuning On–Off Current Ratio and Field-Effect Mobility in a MoS₂–Graphene Heterostructure <i>via</i> Schottky Barrier Modulation

Field-effect transistor (FET) devices composed of a MoS₂–graphene heterostructure can combine the advantages of high carrier mobility in graphene with the permanent band gap of MoS₂ for digital applications. Herein, we investigate the electron transfer, photoluminescence, and gate-controlled carrier transport in such a heterostructure. We show that the junction is a Schottky barrier, whose height can be artificially controlled by gating or doping graphene. When the applied gate voltage (or the doping level) is zero, the photoexcited electron–hole pairs in monolayer MoS₂ can be split by the heterojunction, significantly reducing the photoluminescence. By applying negative gate voltage (or <i>p</i>-doping) in graphene, the interlayer impedance formed between MoS₂ and graphene exhibits an 100-fold increase. For the first time, we show that the gate-controlled interlayer Schottky impedance can be utilized to modulate carrier transport in graphene, significantly depleting the hole transport, but preserving the electron transport. Accordingly, we demonstrate a new type of FET device, which enables a controllable transition from NMOS digital to bipolar characteristics. In the NMOS digital regime, we report a very high room temperature on/off current ratio (<i>I</i>_ON/<i>I</i>_OFF ∼ 36) in comparison to graphene-based FET devices without sacrificing the field-effect electron mobilities in graphene. By engineering the source/drain contact area, we further estimate that a higher value of <i>I</i>_ON/<i>I</i>_OFF up to 100 can be obtained in the device architecture considered. The device architecture presented here may enable semiconducting behavior in graphene for digital and analogue electronics

Observation of Switchable Photoresponse of a Monolayer WSe₂–MoS₂ Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging

In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p–n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe₂–MoS₂ lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe₂ and MoS₂, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS₂ (or WSe₂) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures

Lipid Exchange Envelope Penetration (LEEP) of Nanoparticles for Plant Engineering: A Universal Localization Mechanism

Nanoparticles offer clear advantages for both passive and active penetration into biologically important membranes. However, the uptake and localization mechanism of nanoparticles within living plants, plant cells, and organelles has yet to be elucidated. Here, we examine the subcellular uptake and kinetic trapping of a wide range of nanoparticles for the first time, using the plant chloroplast as a model system, but validated in vivo in living plants. Confocal visible and near-infrared fluorescent microscopy and single particle tracking of gold-cysteine-AF405 (GNP-Cys-AF405), streptavidin-quantum dot (SA-QD), dextran and poly­(acrylic acid) nanoceria, and various polymer-wrapped single-walled carbon nanotubes (SWCNTs), including lipid-PEG-SWCNT, chitosan-SWCNT and 30-base (dAdT) sequence of ssDNA (AT)₁₅ wrapped SWCNTs (hereafter referred to as ss­(AT)₁₅-SWCNT), are used to demonstrate that particle size and the magnitude, but not the sign, of the zeta potential are key in determining whether a particle is spontaneously and kinetically trapped within the organelle, despite the negative zeta potential of the envelope. We develop a mathematical model of this lipid exchange envelope and penetration (LEEP) mechanism, which agrees well with observations of this size and zeta potential dependence. The theory predicts a critical particle size below which the mechanism fails at all zeta potentials, explaining why nanoparticles are critical for this process. LEEP constitutes a powerful particulate transport and localization mechanism for nanoparticles within the plant system

Layer Number Dependence of MoS₂ Photoconductivity Using Photocurrent Spectral Atomic Force Microscopic Imaging

Atomically thin MoS₂ is of great interest for electronic and optoelectronic applications because of its unique two-dimensional (2D) quantum confinement; however, the scaling of optoelectronic properties of MoS₂ and its junctions with metals as a function of layer number as well the spatial variation of these properties remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy (PCS-AFM) to image the current (in the dark) and photocurrent (under illumination) generated between a biased PtIr tip and MoS₂ nanosheets with thickness ranging between <i>n</i> = 1 to 20 layers. Dark current measurements in both forward and reverse bias reveal characteristic diode behavior well-described by Fowler–Nordheim tunneling with a monolayer barrier energy of 0.61 eV and an effective barrier scaling linearly with layer number. Under illumination at 600 nm, the photocurrent response shows a marked decrease for layers up to <i>n</i> = 4 but increasing thereafter, which we describe using a model that accounts for the linear barrier increase at low <i>n</i>, but increased light absorption at larger <i>n</i> creating a minimum at <i>n</i> = 4. Comparative 2D Fourier analysis of physical height and photocurrent images shows high spatial frequency spatial variations in substrate/MoS₂ contact that exceed the frequencies imposed by the underlying substrates. These results should aid in the design and understanding of optoelectronic devices based on quantum confined atomically thin MoS₂

Experimental Observation of Real Time Molecular Dynamics Using Electromigrated Tunnel Junctions

Single molecule tunnel junctions (SMTJs) can provide important physical insights into electronic and vibrational phenomena at the molecular scale. However, observations and analysis are typically confined to sufficiently low temperatures as to suppress molecular motion and the resulting stochastic fluctuations in the tunneling current. In this work, we introduce and experimentally validate a methodology whereby a slightly higher temperature (9 K) compared to a typical SMTJ study can be used to induce sparse fluctuations in the inelastic tunneling current and provide the fingerprints of dynamics between the conformational states of the molecule. Two examples of benzene dithiol and cysteine are studied in electromigratively formed W/Au nanowire SMTJs on SiO₂ at 9 K. The second-order transform of the tunneling current reveals the expected vibrational spectra. However, we show that temporal fluctuations can be analyzed using a hidden Markov Model to reveal dynamics assigned to millisecond rearrangements of the molecule, with apparent energy barriers ranging from 35 to 66 meV, consistent with theoretical predictions. The observed transitions are consistent with a model of lateral migration of the thiol-anchored molecules in an asymmetric junction. The use of temperature in SMTJs in this way can provide new insights into molecule dynamics in confined volumes and at electrode interfaces