Resonant Tunneling-Enhanced Photoresponsivity in a Twisted Graphene van der Waals Heterostructure.

Leveraging its ultrahigh carrier mobility, zero-bandgap linear dispersion, and extremely short response time, graphene exhibits remarkable potential in ultrafast broad-band photodetection. Nonetheless, the inherently low responsivity of graphene photodetectors, due to the low photogenerated carrier density, significantly impedes the development of practical devices. In this study, we present an improved photoresponse within a graphene-hexagonal boron nitride-graphene vertical tunnel junction device, where the crystallographic orientation of the two graphene electrodes is aligned. Through meticulous device structure design and the adjustment of bias and gate voltages, we observe a 2 orders of magnitude increase in tunneling photocurrent, which is attributed to the momentum-conserving resonant electron tunneling. The enhanced external photoresponsivity is evident across a wide temperature and spectral range and achieves 0.7 A/W for visible light excitation. This characteristic, coupled with the device's negative differential conductance, suggests a novel avenue for highly efficient photodetection and high-frequency, logic-based optoelectronics using van der Waals heterostructures.

Strong In-Plane Optoelectronic Anisotropy and Polarization Sensitivity in Low-Symmetry 2D Violet Phosphorus.

Anisotropic optoelectronics based on low-symmetry two-dimensional (2D) materials hold immense potential for enabling multidimensional visual perception with improved miniaturization and integration capabilities, which has attracted extensive interest in optical communication, high-gain photoswitching circuits, and polarization imaging fields. However, the reported in-plane anisotropic photocurrent and polarized dichroic ratios are limited, hindering the achievement of high-performance anisotropic optoelectronics. In this study, we introduce novel low-symmetry violet phosphorus (VP) with a unique tubular cross-linked structure into this realm, and the corresponding anisotropic optical and optoelectronic properties are investigated both experimentally and theoretically for the first time. Remarkably, our prepared VP-based van der Waals phototransistor exhibits significant optoelectronic anisotropies with a giant in-plane anisotropic photocurrent ratio exceeding 10 and a comparable polarized dichroic ratio of 2.16, which is superior to those of most reported 2D counterparts. Our findings establish VP as an exceptional candidate for anisotropic optoelectronics, paving the way for future multifunctional applications.

Probing the photocurrent in two-dimensional titanium disulfide.

Generating photocurrent in a condensed matter system involves the excitation, relaxation, and transportation of charge carriers. As such, it is viewed a potent method for probing the dynamics of non-equilibrium carriers and the electronic band structure of solid state materials. In this research, we analyze the photoresponse of the mechanically exfoliated titanium disulfide (TiS2), a transition metal dichalcogenide whose classification as either a semimetal or a semiconductor has been the subject of debate for years. The scanning photocurrent microscopy and the temperature-dependent photoresponse characterization expose the appearance of a photovoltaic current primarily from the metal/TiS2junction in an unbiased sample, while negative photoconductivity due to the bolometric effect is observed in the conductive TiS2channel. The optoelectronic experimental results, combined with electrical transport characterization and angle-resolved photoemission spectroscopy measurements, indicate that the TiS2employed in this study is likely a heavily-doped semiconductor. Our findings unveil the photocurrent generation mechanism of two dimensional TiS2, highlighting its prospective optoelectronic applications in the future.

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure.

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Tunable Photoresponse in a Two-Dimensional Superconducting Heterostructure.

The photo-induced superconducting phase transition is widely used in probing the physical properties of correlated electronic systems and to realize broadband photodetection with extremely high responsivity. However, such photoresponse is usually insensitive to electrostatic doping due to the high carrier density of the superconductor, restricting its applications in tunable optoelectronic devices. In this work, we demonstrate the gate voltage modulation to the photoresponsivity in a two-dimensional NbSe2-graphene heterojunction. The superconducting critical current of the NbSe2 relies on the gate-dependent hot carrier generation in graphene via the Joule heating effect, leading to the observed shift of both the magnitude and peak position of the photoresponsivity spectra as the gate voltage changes. This heating effect is further confirmed by the temperature and laser-power-dependent characterization of the photoresponse. In addition, we investigate the spatially-resolved photocurrent, finding that the superconductivity is inhomogeneous across the junction area. Our results provide a new platform for designing tunable superconducting photodetector and indicate that the photoresponse could be a powerful tool in studying the local electronic properties and phase transitions in low-dimensional superconducting systems.

Controlled Synthesis of a Two-Dimensional Non-van der Waals Ferromagnet toward a Magnetic Moiré Superlattice.

Two-dimensional (2D) magnetic materials provide an ideal platform for spintronics, magnetoelectrics, and numerous intriguing physical phenomena in 2D limits. Moiré superlattices based on 2D magnets offer an avenue for controlling the spin degree of freedom and engineering magnetic properties. However, the synthesis of high-quality, large-grain, and stable 2D magnets, much less obtaining a magnetic moiré superlattice, is still challenging. We synthesize 2D ferromagnets (trigonal Cr5Te8) with controlled thickness and robust stability through chemical vapor deposition. Single-unit-cell-thick flakes with lateral sizes of tens of micrometers are obtained. We observe the layer-by-layer growth mode for the crystal formation in non-van der Waals Cr5Te8. The robust anomalous Hall signal confirms that Cr5Te8 of varying thickness have a long-range ferromagnetic order with an out-of-plane easy axis. There is no obvious change of the Curie temperature when the thickness of Cr5Te8 decreases from 52.1 to 7.2 nm. Here, we construct diverse 2D non-van der Waals/van der Waals vertical heterostructures (Cr5Te8/graphene, Cr5Te8/h-BN, Cr5Te8/MoS2). A uniform moiré superlattice is formed in the heterostructure through a lattice mismatch. The successful growth of 2D Cr5Te8 and a related moiré superlattice introduces 2D non-van der Waals ferromagnets into moiré superlattice research, thus highlighting prospects for property investigation of a non-van der Waals magnetic moiré superlattice and massive applications which require a scalable approach to magnetic moiré superlattices.

Atomically Thin CrCl3: An In-Plane Layered Antiferromagnetic Insulator.

The recent discovery of magnetism in atomically thin layers of van der Waals (vdW) crystals has created new opportunities for exploring magnetic phenomena in the two-dimensional (2D) limit. In most 2D magnets studied to date, the c-axis is an easy axis, so that at zero applied field the polarization of each layer is perpendicular to the plane. Here, we demonstrate that atomically thin CrCl3 is a layered antiferromagnetic insulator with an easy-plane normal to the c-axis, that is, the polarization is in the plane of each layer and has no preferred direction within it. Ligand-field photoluminescence at 870 nm is observed down to the monolayer limit, demonstrating its insulating properties. We investigate the in-plane magnetic order using tunneling magnetoresistance in graphene/CrCl3/graphene tunnel junctions, establishing that the interlayer coupling is antiferromagnetic down to the bilayer. From the temperature dependence of the magnetoresistance, we obtain an effective magnetic phase diagram for the bilayer. Our result shows that CrCl3 should be useful for studying the physics of 2D phase transitions and for making new kinds of vdW spintronic devices.

Imaging quantum spin Hall edges in monolayer WTe2.

A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of the QSH effect have recently been reported in an atomically thin material, monolayer WTe2. Here, we directly image the local conductivity of monolayer WTe2 using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe2. Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform.

Voltage Control of a van der Waals Spin-Filter Magnetic Tunnel Junction.

Atomically thin chromium triiodide (CrI3) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled. This unusual magnetic structure naturally comprises a series of antialigned spin filters, which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR). Here we report voltage control of TMR formed by four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of 10 or more). In addition, without switching the state, the TMR can be continuously modulated between 17,000% and 57,000%, due to the combination of spin-dependent tunnel barrier with changing carrier distributions in the graphene contacts. Our work demonstrates new kinds of magnetically moderated transistor action and opens up possibilities for voltage-controlled van der Waals spintronic devices.

Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures.

Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here, we report multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance that is drastically enhanced with increasing CrI3 layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. Using magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of the atomically thin CrI3 Our work reveals the possibility to push magnetic information storage to the atomically thin limit and highlights CrI3 as a superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.

Tunable Ultrafast Thermal Relaxation in Graphene Measured by Continuous-Wave Photomixing.

Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the ultrafast cooling of hot carriers with an optical fluence that is orders of magnitude smaller than in conventional time-domain methods, allowing for accurate characterization of electron-phonon cooling near charge neutrality. Our measurements reveal that near the charge neutral point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling prevails in the nonlinear dependence. The relative contribution of these competing mechanisms can be electrostatically tuned through the application of a gate voltage-an effect that is unique to graphene.

Electronic transport properties of Ir-decorated graphene.

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature, and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir.

Pulsed Near-IR Photoresponse in a Bi-metal Contacted Graphene Photodetector.

We use an ultra-fast near-infrared pulse coincidence technique to study the time, temperature, and power dependence of the photoresponse of a bi-metal contacted graphene photodetector. We observe two components of the photovoltage signal. One component is gate-voltage dependent, linear in power at room temperature and sub-linear at low temperature-consistent with the hot-electron photothermoelectric effect due to absorption in the graphene. The power dependence is consistent with supercollision-dominated cooling in graphene. The other component is gate-voltage independent and linear in temperature and power, which we interpret as due to thermoelectricity of the metal electrodes due to differential light absorption.

Plasmon-Enhanced Terahertz Photodetection in Graphene.

We report a large area terahertz detector utilizing a tunable plasmonic resonance in subwavelength graphene microribbons on SiC(0001) to increase the absorption efficiency. By tailoring the orientation of the graphene ribbons with respect to an array of subwavelength bimetallic electrodes, we achieve a condition in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array, while the resulting photothermal voltage can be observed between the outermost electrodes.

Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene.

Terahertz radiation has uses in applications ranging from security to medicine. However, sensitive room-temperature detection of terahertz radiation is notoriously difficult. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism; photoexcited carriers rapidly thermalize due to strong electron-electron interactions, but lose energy to the lattice more slowly. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating or dissimilar contact metals produces a net current via the thermoelectric effect. Here, we demonstrate a graphene thermoelectric terahertz photodetector with sensitivity exceeding 10 V W(-1) (700 V W(-1)) at room temperature and noise-equivalent power less than 1,100 pW Hz(-1/2) (20 pW Hz(-1/2)), referenced to the incident (absorbed) power. This implies a performance that is competitive with the best room-temperature terahertz detectors for an optimally coupled device, and time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.

Approaching the limits of transparency and conductivity in graphitic materials through lithium intercalation.

Various band structure engineering methods have been studied to improve the performance of graphitic transparent conductors; however, none has demonstrated an increase of optical transmittance in the visible range. Here we measure in situ optical transmittance spectra and electrical transport properties of ultrathin graphite (3-60 graphene layers) simultaneously during electrochemical lithiation/delithiation. On intercalation, we observe an increase of both optical transmittance (up to twofold) and electrical conductivity (up to two orders of magnitude), strikingly different from other materials. Transmission as high as 91.7% with a sheet resistance of 3.0 Ω per square is achieved for 19-layer LiC6, which corresponds to a figure of merit σ(dc)/σ(opt) = 1,400, significantly higher than any other continuous transparent electrodes. The unconventional modification of ultrathin graphite optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques enable investigation of other aspects of intercalation in nanostructures.