Plasmon resonance in individual nanogap electrodes studied using graphene nanoconstrictions as photodetectors

Low-Dimensional-Materials-Based Photodetectors for Next-Generation Polarized Detection and Imaging

The vector characteristics of light and the vectorial transformations during its transmission lay a foundation for polarized photodetection of objects, which broadens the applications of related detectors in complex environments. With the breakthrough of low-dimensional materials (LDMs) in optics and electronics over the past few years, the combination of these novel LDMs and traditional working modes is expected to bring new development opportunities in this field. Here, the state-of-the-art progress of LDMs, as polarization-sensitive components in polarized photodetection and even the imaging, is the main focus, with emphasis on the relationship between traditional working principle of polarized photodetectors (PPs) and photoresponse mechanisms of LDMs. Particularly, from the view of constitutive equations, the existing works are reorganized, reclassified, and reviewed. Perspectives on the opportunities and challenges are also discussed. It is hoped that this work can provide a more general overview in the use of LDMs in this field, sorting out the way of related devices for "more than Moore" or even the "beyond Moore" research.

Optical rectification and thermal currents in optical tunneling gap antennas

Electrically-contacted optical gap antennas are nanoscale interface devices enabling the transduction between photons and electrons. This new generation of device, usually constituted of metal elements (e.g. gold), captures visible to near infrared electromagnetic radiation and rectifies the incident energy in a direct-current (DC) electrical signal. However, light absorption by the metal may lead to additional thermal effects which need to be taken into account to understand the complete photo-response of the devices. The purpose of this communication is to discriminate the contribution of laser-induced thermo-electric effects in the photo-assisted electronic transport. We show case our analysis with the help of electromigrated devices.

Plasmonic sensors based on graphene and graphene hybrid materials

The past decade has witnessed a rapid growth of graphene plasmonics and their applications in different fields. Compared with conventional plasmonic materials, graphene enables highly confined plasmons with much longer lifetimes. Moreover, graphene plasmons work in an extended wavelength range, i.e., mid-infrared and terahertz regime, overlapping with the fingerprints of most organic and biomolecules, and have broadened their applications towards plasmonic biological and chemical sensors. In this review, we discuss intrinsic plasmonic properties of graphene and strategies both for tuning graphene plasmons as well as achieving higher performance by integrating graphene with plasmonic nanostructures. Next, we survey applications of graphene and graphene-hybrid materials in biosensors, chemical sensors, optical sensors, and sensors in other fields. Lastly, we conclude this review by providing a brief outlook and challenges of the field. Through this review, we aim to provide an overall picture of graphene plasmonic sensing and to suggest future trends of development of graphene plasmonics.

High-Performance 3D Vertically Oriented Graphene Photodetector Using a Floating Indium Tin Oxide Channel

Vertically oriented graphene (VG), owing to its sharp edges, non-stacking morphology, and high surface-to-volume ratio structure, is promising as a consummate material for the application of photoelectric detection. However, owing to high defect and fast photocarrier recombination, VG-absorption-based detectors inherently suffer from poor responsivity, severely limiting their viability for light detection. Herein, we report a high-performance photodetector based on a VG/indium tin oxide (ITO) composite structure, where the VG layer serves as the light absorption layer while ITO works as the carrier conduction channel, thus achieving the broadband and high response nature of a photodetector. Under the illumination of infrared light, photoinduced carriers generated in VG could transfer to the floating ITO layer, which makes them separate and diffuse to electrodes quickly, finally realizing large photocurrent detectivity. This kind of composite structure photodetector possesses a room temperature photoresponsivity as high as ~0.7 A/W at a wavelength of 980 nm, and it still maintains an acceptable performance at temperatures as low as 87 K. In addition, a response time of 5.8 s is observed, ~10 s faster than VG photodetectors. Owing to the unique three-dimensional morphology structure of the as-prepared VG, the photoresponsivity of the VG/ITO composite photodetector also presented selectivity of incidence angles. These findings demonstrate that our novel composite structure VG device is attractive and promising in highly sensitive, fast, and broadband photodetection technology.

Quantum Tunneling Induced Optical Rectification and Plasmon-Enhanced Photocurrent in Nanocavity Molecular Junctions

Molecular junctions offer the opportunity for downscaling optoelectronic devices. Separating two electrodes with a single layer of molecules accesses the quantum-tunneling regime at low voltages (<1 V), where tunneling currents become highly sensitive to local nanometer-scale geometric features of the electrodes. These features generate asymmetries in the electrical response of the junction which combine with the incident oscillating optical fields to produce optical rectification and photocurrents. Maximizing photocurrents requires accurate control of the overall junction geometry and a large confined optical field in the optimal location. Plasmonic nanostructures such as metallic nanoparticles are prime candidates for this application, because their size and shape dictate a consistent junction geometry while strongly enhancing the optical field from incident light. Here we demonstrate a robust lithography-free molecular optoelectronic device geometry, where a metallic nanoparticle on a self-assembled molecular monolayer is sandwiched between planar bottom and semitransparent top electrodes, to create molecular junctions with reproducible morphology and electrical response. The well-defined geometry enables predictable and intense plasmonic localization, which we show creates optical-frequency voltages ∼ 30 mV in the molecular junction from 100 μW incident light, generating photocurrent by optical rectification (>10 μA/W) from only a few hundred molecules. Quantitative agreement is thus obtained between DC- and optical-frequency quantum-tunneling currents, predicted by a simple analytic equation. By measuring the degree of junction asymmetry for different molecular monolayers, we find that molecules with a large DC rectification ratio also boost zero-bias electrical asymmetry, making them good candidates for sensing and energy harvesting applications in combination with plasmonic nanomaterials.

Electrostatic gating of single-molecule junctions based on the STM-BJ technique

The gating of charge transport through single-molecule junctions is considered a critical step towards molecular circuits but remains challenging. In this work, we report an electrostatic gating method to tune the conductance of single-molecule junctions using the scanning tunneling microscope break junction (STM-BJ) technique incorporated with a back-gated chip as a substrate. We demonstrated that the conductance varied at different applied gating voltages (Vgs). The HOMO-dominated molecules show a decrease in conductance with an increase in Vg, and the LUMO-dominated molecules show the opposite trend. The measured conductance trends with Vg are consistent with the transition voltage spectroscopy measurements. Moreover, the transmission functions simulated from density functional theory (DFT) calculations and the finite element analysis all suggest that Vg changed the energy alignment of the molecular junction. This work provides a simple method for modulating the molecular orbitals' alignment relative to the Fermi energy (Ef) of metal electrodes to explore the charge transport properties at the single-molecule scale.

Graphene Plasmonics in Sensor Applications: A Review

Surface plasmon polaritons (SPPs) can be generated in graphene at frequencies in the mid-infrared to terahertz range, which is not possible using conventional plasmonic materials such as noble metals. Moreover, the lifetime and confinement volume of such SPPs are much longer and smaller, respectively, than those in metals. For these reasons, graphene plasmonics has potential applications in novel plasmonic sensors and various concepts have been proposed. This review paper examines the potential of such graphene plasmonics with regard to the development of novel high-performance sensors. The theoretical background is summarized and the intrinsic nature of graphene plasmons, interactions between graphene and SPPs induced by metallic nanostructures and the electrical control of SPPs by adjusting the Fermi level of graphene are discussed. Subsequently, the development of optical sensors, biological sensors and important components such as absorbers/emitters and reconfigurable optical mirrors for use in new sensor systems are reviewed. Finally, future challenges related to the fabrication of graphene-based devices as well as various advanced optical devices incorporating other two-dimensional materials are examined. This review is intended to assist researchers in both industry and academia in the design and development of novel sensors based on graphene plasmonics.

Graphene-Based Mixed-Dimensional van der Waals Heterostructures for Advanced Optoelectronics

Although the library of 2D atomic crystals has greatly expanded over the past years, research into graphene is still one of the focuses for both academia and business communities. Due to its unique electronic structure, graphene offers a powerful platform for exploration of novel 2D physics, and has significantly impacted a wide range of fields including energy, electronics, and photonics. Moreover, the versatility of combining graphene with other functional components provides a powerful strategy to design artificial van der Waals (vdWs) heterostructures. Aside from the stacked 2D-2D vdWs heterostructure, in a broad sense graphene can hybridize with other non-2D materials through vdWs interactions. Such mixed-dimensional vdWs (MDWs) structures allow considerable freedom in material selection and help to harness the synergistic advantage of different dimensionalities, which may compensate for graphene's intrinsic shortcomings. A succinct overview of representative advances in graphene-based MDWs heterostructures is presented, ranging from assembly strategies to applications in optoelectronics. The scientific merit and application advantages of these hybrid structures are particularly emphasized. Moreover, considering possible breakthroughs in new physics and application potential on an industrial scale, the challenges and future prospects in this active research field are highlighted.

Photoinduced Doping To Enable Tunable and High-Performance Anti-Ambipolar MoTe2/MoS2 Heterotransistors

van der Waals (vdW) p-n heterojunctions formed by two-dimensional nanomaterials exhibit many physical properties and deliver functionalities to enable future electronic and optoelectronic devices. In this report, we demonstrate a tunable and high-performance anti-ambipolar transistor based on MoTe₂/MoS₂ heterojunction through in situ photoinduced doping. The device demonstrates a high on/off ratio of 10⁵ with a large on-state current of several micro-amps. The peak position of the drain-source current in the transfer curve can be adjusted through the doping level across a large dynamic range. In addition, we have fabricated a tunable multivalue inverter based on the heterojunction that demonstrates precise control over its output logic states and window of midlogic through source-drain bias adjustment. The heterojunction also exhibits excellent photodetection and photovoltaic performances. Dynamic and precise modulation of the anti-ambipolar transport properties may inspire functional devices and applications of two-dimensional nanomaterials and their heterostructures of various kinds.

Self-Folded Three-Dimensional Graphene with a Tunable Shape and Conductivity

Three-dimensional (3D) graphene architectures are of great interest as applications in flexible electronics and biointerfaces. In this study, we demonstrate the facile formation of predetermined 3D polymeric microstructures simply by transferring monolayer graphene. The graphene adheres to the surface of polymeric films via noncovalent π-π stacking bonding and induces a sloped internal strain, leading to the self-rolling of 3D microscale architectures. Micropatterns and varied thicknesses of the 2D films prior to the self-rolling allows for control over the resulting 3D geometries. The strain then present on the hexagonal unit cell of the graphene produces a nonlinear electrical conductivity across the device. The driving force behind the self-folding process arises from the reconfiguration of the molecules within the crystalline materials. We believe that this effective and versatile way of realizing a 3D graphene structure is potentially applicable to alternative 2D layered materials as well as other flexible polymeric templates.

Photothermoelectric Detection of Gold Oxide Nonthermal Decomposition

A thin coating of gold oxide, metastable at room temperature, can be formed by placing gold in a strongly oxidizing environment such as an oxygen plasma. We report scanning photovoltage measurements of lithographically defined gold nanowires subsequent to oxygen plasma exposure. Photovoltages are detected during the first optical scan of the devices that are several times larger than those mapped on subsequent scans. The first-scan enhanced photovoltage correlates with a reduction of the electrical resistance of the nanostructure back to preoxygen-exposure levels. Repeating oxygen plasma exposure "reinitializes" the devices. These combined photovoltage and transport measurements imply that the enhanced photovoltage results from the photothermoelectric response of a junction between Au and oxidized Au, with an optically driven decomposition of the oxide. Comparisons with the known temperature-dependent kinetics of AuOx decomposition suggest that the light-driven decomposition is not a purely thermal effect. These experiments demonstrate that combined optical and electronic measurements can provide a window on surface-sensitive photochemical processes.

Electromigrated electrical optical antennas for transducing electrons and photons at the nanoscale

Background: Electrically controlled optical metal antennas are an emerging class of nanodevices enabling a bilateral transduction between electrons and photons. At the heart of the device is a tunnel junction that may either emit light upon injection of electrons or generate an electrical current when excited by a light wave. The current study explores a technological route for producing these functional units based upon the electromigration of metal constrictions. Results: We combine multiple nanofabrication steps to realize in-plane tunneling junctions made of two gold electrodes, separated by a sub-nanometer gap acting as the feedgap of an optical antenna. We electrically characterize the transport properties of the junctions in the light of the Fowler-Nordheim representation and the Simmons model for electron tunneling. We demonstrate light emission from the feedgap upon electron injection and show examples of how this nanoscale light source can be coupled to waveguiding structures. Conclusion: Electromigrated in-plane tunneling optical antennas feature interesting properties with their unique functionality enabling interfacing electrons and photons at the atomic scale and with the same device. This technology may open new routes for device-to-device communication and for interconnecting an electronic control layer to a photonic architecture.

Enhancing electronic and optoelectronic performances of tungsten diselenide by plasma treatment

Transition metal dichalcogenides (TMDCs) have recently become spotlighted as nanomaterials for future electronic and optoelectronic devices. In this work, we develop an effective approach to enhance the electronic and optoelectronic performances of WSe2-based devices by N2O plasma treatment. The hole mobility and sheet density increase by 2 and 5 orders of magnitude, reaching 110 cm2 V-1 s-1 and 2.2 × 1012 cm-2, respectively, after the treatment. At the same time, the contact resistance (Rc) between WSe2 and its metal electrode drop by 5 orders of magnitude from 1.0 GΩ μm to 28.4 kΩ μm. The WSe2 photoconductor exhibits superior performance with high responsivity (1.5 × 105 A W-1), short response time (<2 ms), high detectivity (3.6 × 1013 Jones) and very large photoconductive gain (>106). We have also built a lateral p-n junction on a single piece of WSe2 flake by selective plasma exposure. The junction reaches an exceedingly high rectifying ratio of 106, an excellent photoresponsivity of 2.49 A W-1 and a fast response of 8 ms. The enhanced optoelectronic performance is attributed to band-engineering through the N2O plasma treatment, which can potentially serve as an effective and versatile approach for device engineering and optimization in a wide range of electronic and optoelectronic devices based on 2D materials.

Gate-Controlled Graphene-Silicon Schottky Junction Photodetector

Various photodetectors showing extremely high photoresponsivity have been frequently reported, but many of these photodetectors could not avoid the simultaneous amplification of dark current. A gate-controlled graphene-silicon Schottky junction photodetector that exhibits a high on/off photoswitching ratio (≈10⁴ ), a very high photoresponsivity (≈70 A W^-1 ), and a low dark current in the order of µA cm^-2 in a wide wavelength range (395-850 nm) is demonstrated. The photoresponsivity is ≈100 times higher than that of existing commercial photodetectors, and 7000 times higher than that of graphene-field-effect transistor-based photodetectors, while the dark current is similar to or lower than that of commercial photodetectors. This result can be explained by a unique gain mechanism originating from the difference in carrier transport characteristics of silicon and graphene.

Electrical and SERS detection of disulfide-mediated dimerization in single-molecule benzene-1,4-dithiol junctions

Electrical and in situ SERS characterization of the benzene-1,4-dithiol (BDT) junction suggested that dimerization of BDT contributed to the low conductance.

We applied a combination of mechanically controllable break junction (MCBJ) and in situ surface enhanced Raman spectroscopy (SERS) methods to investigate the long-standing single-molecule conductance discrepancy of prototypical benzene-1,4-dithiol (BDT) junctions. Single-molecule conductance characterization, together with configuration analysis of the molecular junction, suggested that disulfide-mediated dimerization of BDT contributed to the low conductance feature, which was further verified by the detection of S–S bond formation through in situ SERS characterization. Control experiments demonstrated that the disulfide-mediated dimerization could be tuned via the chemical inhibitor. Our findings suggest that a combined electrical and SERS method is capable of probing chemical reactions at the single-molecule level.

Building nanogapped graphene electrode arrays by electroburning

Carbon nanoelectrodes with nanogap are reliable platforms for achieving ultra-small electronic devices. One of the main challenges in fabricating nanogapped carbon electrodes is precise control of the gap size. Herein, we put forward an electroburning approach for controllable fabrication of graphene nanoelectrodes from preprocessed nanoconstriction arrays. The electroburning behavior was investigated in detail, which revealed a dependence on the size of nanoconstriction units. The electroburnt nanoscale electrodes showed the capacity to build molecular devices. The methodology and mechanism presented in this study provide significant guidance for the fabrication of proper graphene and other carbon nanoelectrodes.

An approach for the efficient fabrication of graphene nanoelectrodes through the combination of dash-line lithography and electroburning is demonstrated in detail.

Facile synthesis of AgBiS2 nanocrystals for high responsivity infrared detectors

Solution-processable AgBiS₂ nanocrystals are emerging materials for near-infrared photodetectors.

Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based hybrid phototransistors. However, the resulting QD/graphene hybrid phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron-transition-based optical absorption of B-doped Si QDs in the ultraviolet (UV) to near-infrared (NIR) region additionally leads to photogating for graphene. The resulting UV-to-MIR ultrabroadband photodetection of our QD/graphene hybrid phototransistors features ultrahigh responsivity (up to ∼10⁹ A/W), gain (up to ∼10¹²), and specific detectivity (up to ∼10¹³ Jones).

Ultrathin thermoresponsive self-folding 3D graphene

Graphene and other two-dimensional materials have unique physical and chemical properties of broad relevance. It has been suggested that the transformation of these atomically planar materials to three-dimensional (3D) geometries by bending, wrinkling, or folding could significantly alter their properties and lead to novel structures and devices with compact form factors, but strategies to enable this shape change remain limited. We report a benign thermally responsive method to fold and unfold monolayer graphene into predesigned, ordered 3D structures. The methodology involves the surface functionalization of monolayer graphene using ultrathin noncovalently bonded mussel-inspired polydopamine and thermoresponsive poly(N-isopropylacrylamide) brushes. The functionalized graphene is micropatterned and self-folds into ordered 3D structures with reversible deformation under a full control by temperature. The structures are characterized using spectroscopy and microscopy, and self-folding is rationalized using a multiscale molecular dynamics model. Our work demonstrates the potential to design and fabricate ordered 3D graphene structures with predictable shape and dynamics. We highlight applicability by encapsulating live cells and creating nonlinear resistor and creased transistor devices.

Substantial local variation of the Seebeck coefficient in gold nanowires

Nanoscale structuring holds promise to improve the thermoelectric properties of materials for energy conversion and photodetection. We report a study of the spatial distribution of the photothermoelectric voltage in thin-film nanowire devices fabricated from a single metal. A focused laser beam is used to locally heat the metal nanostructure via a combination of direct absorption and excitation of a plasmon resonance in Au devices. As seen previously, in nanowires shorter than the spot size of the laser, we observe a thermoelectric voltage distribution that is consistent with the local Seebeck coefficient being spatially dependent on the width of the nanostructure. In longer structures, we observe extreme variability of the net thermoelectric voltage as the laser spot is scanned along the length of the nanowire. The sign and magnitude of the thermoelectric voltage is sensitive to the structural defects, metal grain structure, and surface passivation of the nanowire. This finding opens the possibility of improved local control of the thermoelectric properties at the nanoscale.

Photothermoelectric Effects and Large Photovoltages in Plasmonic Au Nanowires with Nanogaps

Nanostructured metals subject to local optical interrogation can generate open-circuit photovoltages potentially useful for energy conversion and photodetection. We report a study of the photovoltage as a function of illumination position in single-metal Au nanowires and nanowires with nanogaps formed by electromigration. We use a laser scanning microscope to locally heat the metal nanostructures via excitation of a local plasmon resonance and direct absorption. In nanowires without nanogaps, where charge transport is diffusive, we observe voltage distributions consistent with thermoelectricity, with the local Seebeck coefficient depending on the width of the nanowire. In the nanowires with nanogaps, where charge transport is by tunneling, we observe large photovoltages up to tens of mV, with magnitude, polarization dependence, and spatial localization that follow the plasmon resonance in the nanogap. This is consistent with a model of photocurrent across the nanogap carried by the nonequilibrium, "hot" carriers generated upon plasmon excitation.

Plasmonic Enhanced Performance of an Infrared Detector Based on Carbon Nanotube Films

The carbon nanotube (CNT) has been proved to be a promising material in infrared detection, due to its many advantages of high mobility, strong infrared light absorption, and carrier collection efficiency. However, the absorption restriction from the single layer limits its effective utilization of incident light. In this paper, we introduce a plasmonic electrode structure in a CNT thin-film photodetector based on random deposited high-purity semiconducting CNTs, which can collect photoinduced carriers effectively and enhance light absorption at the same time. The largest enhancement of photocurrents can be achieved at 1650 nm wavelength with suitable plasmonic structure size. Especially, we further discuss the influence of plasmonic structures on the performance of devices. We demonstrate that the best performance improvement of the carbon nanotube detector with plasmonic structure can be enhanced by 13.7 times for photocurrent mode and 5.62 times for photovoltage mode compared to those devices without structure at 1650 nm resonant wavelength. At last, the plasmonic structures are applied on tandem photodetectors with nine virtual contacts, and both the photocurrent and photovoltage are increased. The application of plasmonic electrodes can improve detector performance and retain compact device structures, which shows great potential for optimizing infrared detectors based on nanomaterials.

Chemical Interface Damping Depends on Electrons Reaching the Surface

Metallic nanoparticles show extraordinary strong light absorption near their plasmon resonance, orders of magnitude larger compared to nonmetallic nanoparticles. This "antenna" effect has recently been exploited to transfer electrons into empty states of an attached material, for example to create electric currents in photovoltaic devices or to induce chemical reactions. It is generally assumed that plasmons decay into hot electrons, which then transfer to the attached material. Ultrafast electron-electron scattering reduces the lifetime of hot electrons drastically in metals and therefore strongly limits the efficiency of plasmon induced hot electron transfer. However, recent work has revived the concept of plasmons decaying directly into an interfacial charge transfer state, thus avoiding the intermediate creation of hot electrons. This direct decay mechanism has mostly been neglected, and has been termed chemical interface damping (CID). CID manifests itself as an additional damping contribution to the homogeneous plasmon line width. In this study, we investigate the size dependence of CID by following the plasmon line width of gold nanorods during the adsorption process of thiols on the gold surface with single particle spectroscopy. We show that CID scales inversely with the effective path length of electrons, i.e., the average distance of electrons to the surface. Moreover, we compare the contribution of CID to other competing plasmon decay channels and predict that CID becomes the dominating plasmon energy decay mechanism for very small gold nanorods.

Ultrasensitive Near-Infrared Photodetectors Based on a Graphene-MoTe2-Graphene Vertical van der Waals Heterostructure

Graphene and other layered materials, such as transition metal dichalcogenides, have rapidly established themselves as exceptional building blocks for optoelectronic applications because of their unique properties and atomically thin nature. The ability to stack them into van der Waals (vdWs) heterostructures with new functionality has opened a new platform for fundamental research and device applications. Nevertheless, near-infrared (NIR) photodetectors based on layered semiconductors are rarely realized. In this work, we fabricate a graphene-MoTe₂-graphene vertical vdWs heterostructure on a SiO₂/p⁺-Si substrate by a facile and reliable site-controllable transfer method and apply it for photodetection from the visible to NIR wavelength range. Compared to the layered semiconductor photodetectors reported thus far, the graphene-MoTe₂-graphene photodetector has a superior performance, including high photoresponsivity (∼110 mA W^-1 at 1064 nm and 205 mA W^-1 at 473 nm), high external quantum efficiency (EQE; ∼12.9% at 1064 nm and ∼53.8% at 473 nm), rapid response and recovery processes (a rise time of 24 μs and a fall time of 46 μs under 1064 nm illumination), and free from an external source-drain power supply. We have employed scanning photocurrent microscopy to investigate the photocurrent generation in this heterostructure under various back-gate voltages and found that the two Schottky barriers between the graphenes and MoTe₂ play an important role in the photocurrent generation. In addition, the vdWs heterostructure has a uniform photoresponsive area. The photoresponsivity and EQE of the photodetector can be modulated by the back-gate (p⁺-Si) voltage. We compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. The heterostructure has promising applications in future novel optoelectronic devices, enabling next-generation high-responsivity, high-speed, flexible, and transparent NIR devices.

Free-Standing Monolayered Metallic Nanoparticle Networks as Building Blocks for Plasmonic Nanoelectronic Junctions

The effective coupling of optical surface plasmons (SPs) and electron transport in a plasmonic-electronic device is one of the fundamental issues in nanoelectronics and the emerging field of plasmonics, and offer promise in providing a solution to next generation nanocircuits in which all coupling is in the near field. Attempts toward this end, however, are limited because of the integration challenge to compatible nanodevices. To date, direct electrical detection of SP-electron coupling from metallic nanostructures alone are not reported, and thus it remains a great experimental challenge. In this paper, we succeed in preparing a new suspended-film-type nanoelectronic junction, in which free-standing 2D fractal nanoparticle networks act as plasmonically active nanocomponents. Direct electrical detection of optical collective SPs was evidenced by photocurrent response of the junction upon illumination. Room-temperature I-V characteristics, differing from nonlinear to Ohmic behaviors, are found to be sensitive to the nanometer-scale morphology changes of the nanomembranes. The finding and approach may enable the development of advanced plasmonic nanocircuits and new nanoelectronics, nanophotonics, and (solar) energy applications.

Electroluminescence Dynamics across Grain Boundary Regions of Monolayer Tungsten Disulfide

We study how grain boundaries (GB) in chemical vapor deposition (CVD) grown monolayer WS2 influence the electroluminescence (EL) behavior in lateral source-drain devices under bias. Real time imaging of the WS2 EL shows arcing between the electrodes when probing across a GB, which then localizes at the GB region as it erodes under high bias conditions. In contrast, single crystal WS2 domains showed no signs of arcing or localized EL. Analysis of the eroded GB region shows the formation of micro- and nanoribbons across the monolayer WS2 domains. Comparison of the EL spectrum with the photoluminescence spectrum from the monolayer WS2 shows close agreement, indicating the EL emission comes from direct band gap excitonic recombination. These results provide important insights into EL devices that utilize CVD grown monolayer transition metal dichalcogenides when GBs are present in the active device region.

Nonlinear Ballistic Transport in an Atomically Thin Material

Ultrashort devices that incorporate atomically thin components have the potential to be the smallest electronics. Such extremely scaled atomically thin devices are expected to show ballistic nonlinear behavior that could make them tremendously useful for ultrafast applications. While nonlinear diffusive electron transport has been widely reported, clear evidence for intrinsic nonlinear ballistic transport in the growing array of atomically thin conductors has so far been elusive. Here we report nonlinear electron transport of an ultrashort single-layer graphene channel that shows quantitative agreement with intrinsic ballistic transport. This behavior is shown to be distinctly different than that observed in similarly prepared ultrashort devices consisting, instead, of bilayer graphene channels. These results suggest that the addition of only one extra layer of an atomically thin material can make a significant impact on the nonlinear ballistic behavior of ultrashort devices, which is possibly due to the very different chiral tunneling of their charge carriers. The fact that we observe the nonlinear ballistic response at room temperature, with zero applied magnetic field, in non-ultrahigh vacuum conditions and directly on a readily accessible oxide substrate makes the nanogap technology we utilize of great potential for achieving extremely scaled high-speed atomically thin devices.

Coherent Generation of Photo-Thermo-Acoustic Wave from Graphene Sheets

Many remarkable properties of graphene are derived from its large energy window for Dirac-like electronic states and have been explored for applications in electronics and photonics. In addition, strong electron-phonon interaction in graphene has led to efficient photo-thermo energy conversions, which has been harnessed for energy applications. By combining the wavelength independent absorption property and the efficient photo-thermo energy conversion, here we report a new type of applications in sound wave generation underlined by a photo-thermo-acoustic energy conversion mechanism. Most significantly, by utilizing ultrafast optical pulses, we demonstrate the ability to control the phase of sound waves generated by the photo-thermal-acoustic process. Our finding paves the way for new types of applications for graphene, such as remote non-contact speakers, optical-switching acoustic devices, etc.

Tailoring 10 nm scale suspended graphene junctions and quantum dots

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials.

Impact of optical antennas on active optoelectronic devices

Remarkable progress has been made in the fabrication and characterization of optical antennas that are integrated with optoelectronic devices. Herein, we describe the fundamental reasons for and experimental evidence of the dramatic improvements that can be achieved by enhancing the light-matter interaction via an optical antenna in both photon-emitting and -detecting devices. In addition, integration of optical antennas with optoelectronic devices can lead to the realization of highly compact multifunctional platforms for future integrated photonics, such as low-cost lab-on-chip systems. In this review paper, we further focus on the effect of optical antennas on the detectivity of infrared photodetectors. One particular finding is that the antenna can have a dual effect on the specific detectivity, while it can elevate light absorption efficiency of sub-wavelength detectors, it can potentially increase the noise of the detectors due to the enhanced spontaneous emission rate. In particular, we predict that the detectivity of interband photon detectors can be negatively affected by the presence of optical antennas across a wide wavelength region covering visible to long wavelength infrared bands. In contrast, the detectivity of intersubband detectors could be generally improved with a properly designed optical antenna.

Lateral and temporal dependence of the transport through an atomic gold contact under light irradiation: signature of propagating surface plasmon polaritons

Metallic point contacts (MPCs) with dimensions comparable to the Fermi wavelength of conduction electrons act as electronic waveguides and might operate as plasmon transmitters. Here we present a correlated study of optical and conductance response of MPCs under irradiation with laser light. For elucidating the role of surface plasmon polaritons (SPPs), we integrate line gratings into the leads that increase the SPP excitation efficiency. By analyzing spatial, polarization, and time dependence, we identify two SPP contributions that we attribute to transmitted and decaying SPPs, respectively. The results demonstrate the role of SPPs for optically controlling the transport in metallic nanostructures and are important for designing opto-nanoelectronic devices.

Photosensitive graphene transistors

High performance photodetectors play important roles in the development of innovative technologies in many fields, including medicine, display and imaging, military, optical communication, environment monitoring, security check, scientific research and industrial processing control. Graphene, the most fascinating two-dimensional material, has demonstrated promising applications in various types of photodetectors from terahertz to ultraviolet, due to its ultrahigh carrier mobility and light absorption in broad wavelength range. Graphene field effect transistors are recognized as a type of excellent transducers for photodetection thanks to the inherent amplification function of the transistors, the feasibility of miniaturization and the unique properties of graphene. In this review, we will introduce the applications of graphene transistors as photodetectors in different wavelength ranges including terahertz, infrared, visible, and ultraviolet, focusing on the device design, physics and photosensitive performance. Since the device properties are closely related to the quality of graphene, the devices based on graphene prepared with different methods will be addressed separately with a view to demonstrating more clearly their advantages and shortcomings in practical applications. It is expected that highly sensitive photodetectors based on graphene transistors will find important applications in many emerging areas especially flexible, wearable, printable or transparent electronics and high frequency communications.

High-yield fabrication of nm-size gaps in monolayer CVD graphene

Herein we demonstrate the controlled and reproducible fabrication of sub-5 nm wide gaps in single-layer graphene electrodes. The process is implemented for graphene grown via chemical vapor deposition using an electroburning process at room temperature and in vacuum. A yield of over 95% for the gap formation is obtained. This approach allows producing single-layer graphene electrodes for molecular electronics at a large scale. Additionally, from Raman spectroscopy and electroburning carried out simultaneously, we can follow the heating process and infer the temperature at which the gap formation happens.

Graphene and graphene-like two-dimensional materials in photodetection: mechanisms and methodology

Graphene and graphene-like two-dimensional (2D) materials have attracted much attention due to its extraordinary electronic and optical properties, which accommodate a large potential in optoelectronic applications such as photodetection. However, although much progress has been made, many challenges exist in fundamental and practical aspects hindering graphene and graphene-like 2D materials from photodetector and other photonic and optoelectronic applications. Here, we review the recent progress in photodetection based on graphene and graphene-like 2D materials and start with the summary of some most important physical mechanisms, including photoelectric, photo-thermoelectric, and photo-bolometric regimes. Then methodology-level discussions are given from viewpoints of state-of-the-art designs in device geometry and materials. It is worth emphasizing that emerging photodetection and photodetectors based on graphene-like 2D materials such as metal chalcogenide nanosheets are reviewed systematically. Finally, we conclude this review in a brief discussion with remaining challenges in photodetection of two-dimensional photonics and optoelectronics (2D POE) and note that complete understandings of 2D materials and 2D POE may inspire solar energy conversion and other new applications.

Nonlinear photon-assisted tunneling transport in optical gap antennas

We introduce strongly coupled optical gap antennas to interface optical radiation with current-carrying electrons at the nanoscale. The transducer relies on the nonlinear optical and electrical properties of an optical gap antenna operating in the tunneling regime. We discuss the underlying physical mechanisms controlling the conversion involving d-band electrons and demonstrate that a simple two-wire optical antenna can provide advanced optoelectronic functionalities beyond tailoring the electromagnetic response of a single emitter. Interfacing an electronic command layer with a nanoscale optical device may thus be facilitated by the optical rectennas discussed here.

Using the plasmon linewidth to calculate the time and efficiency of electron transfer between gold nanorods and graphene

We present a quantitative analysis of the electron transfer between single gold nanorods and monolayer graphene under no electrical bias. Using single-particle dark-field scattering and photoluminescence spectroscopy to access the homogeneous linewidth, we observe broadening of the surface plasmon resonance for gold nanorods on graphene compared to nanorods on a quartz substrate. Because of the absence of spectral plasmon shifts, dielectric interactions between the gold nanorods and graphene are not important and we instead assign the plasmon damping to charge transfer between plasmon-generated hot electrons and the graphene that acts as an efficient acceptor. Analysis of the plasmon linewidth yields an average electron transfer time of 160 ± 30 fs, which is otherwise difficult to measure directly in the time domain with single-particle sensitivity. In comparison to intrinsic hot electron decay and radiative relaxation, we furthermore calculate from the plasmon linewidth that charge transfer between the gold nanorods and the graphene support occurs with an efficiency of ∼10%. Our results are important for future applications of light harvesting with metal nanoparticle plasmons and efficient hot electron acceptors as well as for understanding hot electron transfer in plasmon-assisted chemical reactions.

Fast and efficient photodetection in nanoscale quantum-dot junctions

We report on a photodetector in which colloidal quantum dots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another.

Purely coherent nonlinear optical response in solution dispersions of graphene sheets

We have developed an efficient chemical exfoliation approach for the high-throughput synthesis of solution-processable, high-quality graphene sheets that are noncovalently functionalized by alkylamine. Purely coherent nonlinear optical response of these graphene sheets has been investigated, using near-infrared, visible, and ultraviolet continous wave and ultrafast laser beams. Spatial self-phase modulation has been unambiguously observed in the solution dispersions. Our results suggest that this coherent light scattering is due to a broadband, ultrafast, and remarkably huge third-order optical nonlinearity χ(3), which is a manifestation of the graphene's cone-shaped large-energy-scale band structure. Our experimental findings endow graphene new potentials in nonlinear optical applications.

Plasmon resonance enhanced multicolour photodetection by graphene

Graphene has the potential for high-speed, wide-band photodetection, but only with very low external quantum efficiency and no spectral selectivity. Here we report a dramatic enhancement of the overall quantum efficiency and spectral selectivity that enables multicolour photodetection, by coupling graphene with plasmonic nanostructures. We show that metallic plasmonic nanostructures can be integrated with graphene photodetectors to greatly enhance the photocurrent and external quantum efficiency by up to 1,500%. Plasmonic nanostructures of variable resonance frequencies selectively amplify the photoresponse of graphene to light of different wavelengths, enabling highly specific detection of multicolours. Being atomically thin, graphene photodetectors effectively exploit the local plasmonic enhancement effect to achieve a significant enhancement factor not normally possible with traditional planar semiconductor materials.

Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes

We report on a method to fabricate and measure gateable molecular junctions that are stable at room temperature. The devices are made by depositing molecules inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. The molecular junctions display gateable I-V-characteristics at room temperature.

Accurate determination of plasmonic fields in molecular junctions by current rectification at optical frequencies

Current rectification, i.e., induction of dc current by oscillating electromagnetic fields, is demonstrated in molecular junctions at an optical frequency. The magnitude of rectification is used to accurately determine the effective oscillating potentials in the junctions induced by the irradiating laser. Since the gap size of the junctions used in this study is precisely determined by the length of the embedded molecules, the oscillating potential can be used to calculate the plasmonic enhancement of the electromagnetic field in the junctions. With a set of junctions based on alkyl thiolated molecules with identical HOMO-LUMO gap and different lengths, an exponential dependence of the plasmonic field enhancement on gap size is observed.